How Steel Properties Influence the Roll Forming Process

How Steel Properties Influence the Roll Forming Process

You’ll find this content as part of our page on Roll Forming, but this month, we want to highlight it in our AHSS Insights blog. Thanks to Brian Oxley, Product Manager, Shape Corporation, and Dr. Daniel Schaeffler, President, Engineering Quality Solutions, Inc., and Technical Editor – Metallurgy and Forming, AHSS Application Guidelines, for this case study.

Roll forming is no longer limited to producing simple circular, oval, or rectangular profiles. Advanced cross sections, such as those shown in Figure 1, highlight some profile designs that aid in body structure stiffness and packaging space reductions.

Roll Forming Profile Diagrams

Figure 1 – Roll forming profile design possibilities. Courtesy of Shape Corporation.

 

Optimizing the use of roll forming requires understanding how the sheet metal behaves through the process. Making a bend in a roll-formed part occurs only when forming forces exceed the metal’s yield strength, causing plastic deformation to occur. Higher-strength sheet metals increase forming force requirements, leading to the need to have larger shaft diameters in the roll forming mill. Each pass must have greater overbend to compensate for the increasing springback associated with the higher strength.

Although a high-strength material requires greater forming loads, grades with higher yield strength can resist stretching of the strip edge and prevent longitudinal deformations such as twisting or bowing.

Force requirements for piercing operations are a function of the sheet tensile strength. High strains in the part design exceeding uniform elongation resulting from loads in excess of the tensile strength produce local necking, representing a structural weak point. However, assuming the design does not produce these high strains, the tensile strength has only an indirect influence on the roll forming characteristics.

Yield strength and flow stress are the most critical steel characteristics for roll forming dimensional control. Receiving metal with limited yield strength variability results in consistent part dimensions and stable locations for pre-pierced features.

Flow stress represents the strength after some amount of deformation and is therefore directly related to the degree of work hardening: starting at the same yield strength, a higher work hardening steel will have a higher flow stress at the same deformation.

Two grades are shown in Figure 2: ZE 550 and CR420Y780T-DP. ZE 500, represented by the red curve, is a recovery annealed grade made by Bilstein having a yield strength range of 550 to 625 MPa and a minimum tensile strength of 600 MPa, while CR420Y780T-DP, represented by the blue curve, is a conventional dual phase steel with a minimum yield strength of 420 MPa and a minimum tensile strength of 780 MPa. For the samples tested, ZE 550 has a yield strength of approximately 565 MPa, where that for CR420Y780T-DP is much lower at about 485 MPa. Due to the higher work hardening (n-value) of the DP steel, its flow stress at 5% strain is 775 MPa, while the flow stress for the HSLA grade at 5% strain is 620 MPa.

 

Stress-strain curves for CR420Y780T-DP (blue) and ZE 550 (red). See text for description of the grades

Figure 2 – Stress-strain curves for CR420Y780T-DP (blue) and ZE 550 (red). See text for description of the grades.

 

In conventional stamping operations, this work hardening is beneficial to delay the onset of necking. However, the use of dual-phase steels and other grades with high n-value can lead to dimensional issues in roll-formed parts. Flow stress in a given area is a function of the local strain. Each roll station induces additional strain on the overall part, and strains vary within the part and along the edge. This strength variation is responsible for differing springback and edge wave across a roll formed part.

Unlike conventional stamping, grades with a high yield/tensile ratio where the yield strength is close to the tensile strength are better suited to produce straight parts via roll forming.

Total elongation to fracture is the strain at which the steel breaks during tensile testing, and is a value commonly reported on certified metal property documents (cert sheets). As observed on the colloquially called “banana diagram”, elongation generally decreases as the strength of the steel increases.

For lower-strength steels, total elongation is a good indicator of a metal’s bendability. Bend severity is described by the r/t ratio or the ratio of the inner bend radius to the sheet thickness. The metal’s ability to withstand a given bend can be approximated by the tensile test elongation since, during a bend, the outermost fibers elongate like a tensile test.

In higher-strength steels where the phase balance between martensite, bainite, austenite, and ferrite plays a much larger role in developing strength and ductility than in other steels, microstructural uniformity usually limits bendability. Dual-phase steels, for example, have excellent uniform elongation and resistance to necking coming from the hardness difference between ferrite and martensite. However, this large hardness difference is also responsible for relatively poor edge stretchability and bendability. In roll-forming applications, those grades with a uniform microstructure will typically have superior performance. As an example, refer to Figure 2. The dual-phase steel shown in blue can be bent to a 2T radius before cracking, but the recovery annealed ZE 550 grade with noticeably higher yield strength and lower elongation can be bent to a ½T radius.

Remember that each roll-forming station only incrementally deforms the sheet, with subsequent stations working on a different region. Roll-formed parts do not need to use grades associated with high total elongation, especially since these typically have a bigger gap between yield and tensile strength.

We encourage you to visit https://ahssinsights.org/forming/roll-forming/roll-forming/ to learn more about roll forming and the types of coil shape that influence roll forming.Thank you to Brian Oxley and Dr. Daniel Schaeffler for providing this case study.

Photo of Brian Oxley

Brian Oxley, Product Manager, Shape Corporation, is a Product Manager in the Core Engineering team at Shape Corp. Shape Corp. is a global, full-service supplier of lightweight steel, aluminum, plastic, composite and hybrid engineered solutions for the automotive industry. Brian leads a team responsible for developing next generation products and materials in the upper body and closures space that complement Shape’s core competency in roll forming. Brian has a Bachelor of Science degree in Material Science and Engineering from Michigan State University.

 

Danny Schaeffler is the Metallurgy and Forming Technical Editor of the AHSS Applications Guidelines available from WorldAutoSteel. He is founder and President of Engineering Quality Solutions (EQS). Danny wrote the monthly “Science of Forming” and “Metal Matters” column for Metalforming Magazine, and provides seminars on sheet metal formability for Auto/Steel Partnership and the Precision Metalforming Association. He has written for Stamping Journal and The Fabricator, and has lectured at FabTech. Danny is passionate about training new and experienced employees at manufacturing companies about how sheet metal properties impact their forming success.

 

Stronger AHSS Knowledge Required for Metal Stampers

Stronger AHSS Knowledge Required for Metal Stampers

This month’s blog was contributed by Peter Ulintz, Precision Metalforming Association. This content originally appeared in the September 2023 issue of MetalForming Magazine under the title Stronger AHSS Knowledge Required for Metal Stampers” and has been reproduced with the permission of MetalForming Magazine.

Metal stampers and die shops experienced with mild and HSLA steels often have problems making parts from AHSS grades. The higher initial yield strengths and increased work hardening of these steels can require as much as four times the working loads of mild steel. Some AHSS grades also have hardness levels approaching the dies used to form them.

Dies Get Tougher

Metal stampers and die shops experienced with mild and HSLA steels often have problems making parts from AHSS grades. The higher initial yield strengths and increased work hardening of these steels can require as much as four times the working loads of mild steel. Some AHSS grades also have hardness levels approaching the dies used to form them.

The higher stresses required to penetrate higher-strength materials require increased punch-to-die clearances compared to mild steels and HSLA grades. Why? This clearance acts as leverage to bend and break the slug out of the sheet metal. Stronger materials need longer levers to bend the slug. The required clearance is a function of the steel grade and tensile strength, and sheet thickness.

Increasing cutting clearance can result in punch cracking and head breakage due to higher snapthrough loads and reverse-unloading forces within the die. Adding shear angles to the punch face helps reduce punch forces and reverse unloading.

Tight-cutting clearances increase the tendency for die galling and chipping. The severity of galling depends on the surface finish and microstructure of both the tool steel and work material. Chipping can occur when process stresses are high enough to cause low-cycle fatigue of the tooling material, indicating that the material lacks toughness.

Stamping Tool Failure Modes (Citations T-20 and U-7)

 

Tempering of tools and dies represents a critical heat-treatment step and serves more than one purpose, but of primary concern is the need to relieve residual stresses and impart toughness. Dies placed in service without proper tempering likely will experience early failure.

Dies made from the higher-alloy tool-steel grades (D, M or T grades) require more than one tempering step. These grades contain large amounts of retained austenite and untempered martensite after the first tempering step and require at least one more temper to relieve internal stresses, and sometimes a third temper for even greater toughness.

Unfortunately, heat treatment remains a “black-box” process for most die shops and manufacturing companies, which send soft die details to the local heat treat facility, with hardened details returned. A cursory Rockwell hardness test may be conducted at the die shop when the parts return. If they meet hardness requirements, the parts usually are accepted, regardless of how they may have been processed—a problem, as hardness alone does not adequately measure impact toughness.

Machines Get Stronger

The increased forces needed to form, cut and trim higher-strength steels create significant challenges for pressroom equipment and tooling. These include excessive tooling deflections, damaging tipping-moments, and amplified vibrations and snapthrough forces that can shock and break dies—and sometimes presses. Stamping AHSS materials can affect the size, strength, power and overall configuration of every major piece of the press line, including material-handling equipment, coil straighteners, feed systems and presses. 

Here is what every stamper should know about higher-strength materials:

  • Because higher-strength steels require more stress to deform, additional servo motor power and torque capability may be needed to pull the coil material through the straightener. Additional back tension between the coil feed and straightening equipment also may be required due to the higher yield strength of the material in the loop as the material tries to push back against the straightener and feed system. 
  • Higher-strength materials, due to their greater yield strengths, have a greater tendency to retain coil set. This requires greater horsepower to straighten the material to an acceptable level of flatness. Straightening higher-strength coils requires larger-diameter rolls and wider roll spacing in order to work the stronger material more effectively. But increasing roll diameter and center distances on straighteners to accommodate higher-strength steels limits the range of materials that can effectively be straightened. A straightener capable of processing 600-mm-wide coils to 10 mm thick in mild steel may still straighten 1.5-mm-thick material successfully. But a straightener sized to run the same width and thickness of DP steel might only be capable of straightening 2.5 mm or 3.0-mm thick mild steel. This limitation is primarily due to the larger rolls and broadly spaced centers necessary to run AHSS materials. The larger rolls, journals and broader center distances safeguard the straightener from potential damage caused by the higher stresses. 
  • Because higher-strength materials require greater stress to blank and punch as compared to HSLA or mild steel, they generate proportionally increased snapthrough and reverse-unloading forces. High-tensile snapthrough forces introduce large downward accelerations to the upper die half. These forces work to separate the upper die from the bottom of the ram on every stroke. Insufficient die-clamping force could cause the upper-die half to separate from the bottom of the ram on each stroke, causing fatigue to the upper-die mounting fasteners. 
  • Because energy is expended with each stroke of the press—and this energy must be replaced—critical attention must focus on the size (horsepower) of the main drive motor and the rotational speed of the flywheel in higher-strength-steel applications. The main motor, with its electrical connection, provides the only source of energy for the press and it must generate sufficient power to meet the demands of the stamping operation. The motor must be properly sized to replace the increased energy expended during each press stroke. For these reasons, some stampers consider the benefits of servo-driven presses for these applications.

As steels becomes stronger, a corresponding increase in process knowledge is required in terms of die design, construction and maintenance, and equipment selection.

You can read more about these topics at these links:

Tooling and Die Wear
Coil Processing Straightening and Leveling
Press Requirements

Peter Ulintz

Thanks go to Peter Ulintz, of the Precision Metalforming Association (PMA) for authoring this article. Ulintz was employed in the metal stamping and tool & die industries for 38 years before joining Precision Metalforming Association (PMA) in 2015. He provides industry-related training and seminars in Stamping Press Operation and Setup; Designing and Building Metal Stamping Dies; Die Maintenance and Troubleshooting; Metal Stamping Design for Manufacturability; Deep Draw Tooling and Process Technology; Stamping Higher Strength Steels; and Problem Solving in the Press Shop. Peter is a contributor to ASM Handbook, Volume 14B, Metalworking: Sheet Forming (2006) and writes the monthly column, Tooling by Design, for PMA’s monthly publication, MetalForming Magazine.

 
Steel E-Motive: Autonomous Vehicles That Only Steel Can Make Real

Steel E-Motive: Autonomous Vehicles That Only Steel Can Make Real

The Steel E-Motive program–commissioned by WorldAutoSteel in partnership with Ricardo plc–has developed the world’s first fully autonomous electric vehicle body structure concept purpose-fit for ride-sharing. This global steel industry initiative showcases the strength and durability of steel with an eye on playing a pivotal role in reaching net zero emissions targets.

Download the Steel E-Motive Engineering Report

Here, we break down the many benefits of the Steel E-Motive concept that only Advanced High-Strength Steel (AHSS) can enable.

Steel E-Motive Was Conceived as a Level 5 Autonomous Vehicle

The Steel E-Motive concept is designed to be a Level 5 autonomous vehicle, so it does not include any driver interfaces. The design features a spacious, airy cabin with rear-facing front-passenger seat configurations. The B-pillarless structure and unique battery system design offer easy ingress and egress.

Steel E-Motive Vehicle is shown in a brick paved area with greenery

The Steel E-Motive concept is designed to be a Level 5 autonomous vehicle.

Designed to Exceed Future Mobility Safety Standards

Modern Advanced High-Strength Steels innovations allow the Steel E-Motive autonomous vehicle to exceed current global high-speed crashworthiness standards. By using AHSS, the Steel E-Motive vehicle is the first to acknowledge compliance with NHTSA and IIHS safety standards publicly.

For example, the 4-passenger B-sized urban concept SEM1 introduced a new front-end passenger protection zone. This design features the small overlap Glance Beam, which forces the car to “glance” off the barrier and reduces passenger cabin intrusion. It also lowers the crash pulse and ultimately minimizes passenger injury. Advanced High-Strength Steels also offer strong battery protection and preserve door ring integrity in this autonomous vehicle.

The Evolution of Advanced High-Strength Steel

Over the past quarter century, vehicle concept projects have showcased the continuous advancement of steel. In 1998, global steelmakers introduced the Ultralight Steel Auto Body, which used one of the earliest forms of AHSS. This project demonstrated steel’s ability to reduce weight without compromising safety.

By 2010, we introduced the Future Steel Vehicle concept. Using 27 AHSS materials, the body structure design reduced mass by over 35%. Steel materials enable these massive reductions while allowing the design to meet global crash and durability requirements.

The Steel E-Motive concepts benefit from no fewer than 64 materials under the AHSS umbrella. The “infinite tunability” of AHSS allows product customization by designers and engineers to select exactly the right steel for every need and purpose in the vehicle.

Key Attributes of the Steel E-Motive Autonomous Vehicle

From lowering the carbon footprint to massively reducing weight, the Steel E-Motive vehicle offers first-of-its-kind benefits for future mobility made possible by AHSS.

Steel allows the vehicle to reduce weight without sacrificing strength. For example, 66% of the Steel E-Motive autonomous vehicle structures’ materials have an Ultimate Tensile Strength of at least 1,000 MPa, and these materials’ weighted average tensile strength is 1259 MPa.

By using 33% Press Hardened Steels and 11% 3rd Generation AHSS, the design includes complex geometries fully formed by hot and cold-stamped gigapascal steels.

In another example, 43% of the Steel E-Motive structure is fabricated from material-efficient processes such as press hardening, hydroforming, roll forming, and roll stamping. With these processes, the steel body design maximizes material utilization and minimizes scrap rate. This means less material is produced, lowering the structure’s carbon footprint. These achievements reduce manufacturing costs to support a profitable margin both for the vehicle manufacturer and the mobility service provider.

Using AHSS, the Steel E-Motive autonomous vehicle’s body structure mass is 25% lower than benchmark vehicles of a similar volumetric footprint. Additionally, Steel E-Motive realizes a 27% lower battery frame cost than a fully enclosed battery design, with 37% mass savings.

In conclusion, the Steel E-Motive program stands as a remarkable testament to the innovative potential of steel in shaping the future of mobility and autonomous vehicles. With its groundbreaking design, the Steel E-Motive concept paves the way for Level 5 autonomous electric vehicles prioritizing safety, sustainability, and efficiency.

Harnessing the unique attributes of AHSS, this global steel industry initiative also showcases the remarkable evolution of steel materials over the years. From Ultralight Steel Auto Body to Future Steel Vehicle, the journey of AHSS has been one of continuous improvement, leading to Steel E-Motive’s exceptional achievements in weight reduction, enhanced safety, and minimized environmental impact.

As we venture into an era of net-zero emissions and advanced mobility solutions, the Steel E-Motive concept proudly positions steel as a driving force in shaping a cleaner, safer, and more connected future. 

Download the Steel E-Motive Engineering Report

Talk Like a Metallurgist

Talk Like a Metallurgist

Every industry has its own jargon. In certain settings, these words might be necessary – you wouldn’t want a cardiologist talking to a gastroenterologist about boo-boos and upset tummies. But when these professionals talk with their patients, it’s sometimes necessary for them to use much simpler words. That is, assuming the goal is to actually communicate the issues and concerns.

The steel industry is no different. We use words that have precise meanings in our daily discussions, and we forget that many people we work with don’t have exposure to the terminology that we are accustomed to using. What follows is a brief tour of the words and phrases you are likely to hear when speaking with your metallurgical representative.

Let’s start with the most common word: steel. Simply, steel is just an alloy of iron with up to about 2% carbon. Of course, other elements are in the composition. These fall into two categories: those intentionally added to improve one or more properties (called alloying elements), and those remaining from the steelmaking process that are too costly to remove relative to the benefit the removal would provide (called residual elements). High residuals are usually bad, typically because they lower ductility. But remember high is a relative term. The value may be higher than the standard to which you ordered (which is a cause for rejection), or just higher than what you’ve received in the past. If they are within the tolerance allowed within the standard, the product should still meet your strength and ductility requirements.

I’ve worked with metal formers who believe “steel is steel” and that all grades should behave the same way. According to the World Steel Association, there are more than 3,500 different grades of steel, each with unique properties and characteristics, 75% of which were developed in the past 20 years. Certainly, not all of these are sheet steels, but even within this category, there are sizable numbers. When it comes to just advanced high-strength sheet steels, more than 60 unique grades are available today.

The most common sheet steel grade is routinely called mild steel. Mild steels are low-carbon steels with no alloying elements added for substantial strengthening, and for that reason, they are characterized by relatively lower yield strength. However, there is no single grade or chemistry that meets this definition. Grade definitions require the steelmaker to meet certain chemistry or property limits. These grades are ordered to a standard usually written by the steel producer, a pertinent industry society (like ASTM, Euronorm, or JFS), or the end-user OEM. What is generally thought of as mild steel has chemistry, strength, and ductility overlapping many defined grades. Steel users should order to standards that define and constrain important properties like strength and ductility.

If you hang out with enough metallurgists, you are bound to hear passionate discussions about the iron-carbon phase diagram. (Why you are hanging around metallurgists is another topic entirely.) Before explaining the purpose of a phase diagram, it’s important to understand that a phase is a region of a material that is physically distinct, chemically uniform, and can be seen as different from the rest of the material. Ice and water are two phases that exist in my beverage. You’ll find a chocolate chip phase in my vanilla ice cream. And you’ll find ferrite in my steel – tasty! The properties of each of these change if you increase temperature (converting H2O from a solid to a liquid and eventually a gas) or if you add more alloying elements (chocolate chips or carbon). If you add a lot of that alloying element, you can get something entirely different like ripple or pearlite.

A phase diagram is a graphical representation of composition on the horizontal axis and temperature on the vertical axis. Two important phase diagrams are shown below. The far-left side of each represents 100% vanilla or 100% iron. Different phases exist as the temperature increases, or as the product is alloyed with increasing amounts of either chocolate or carbon.

 

Vanilla Chocolate Phase Diagram

Figure 1: Vanilla-Chocolate Phase Diagram A-77

 

Iron Carbon Phase Diagram

     Figure 2: Iron Carbon Phase DiagramA-78

 

Atoms arrange themselves in three-dimensional patterns called lattices. Think about billiard balls in multiple layers. The balls can be one layer directly above the prior one, or they can be shifted and rest in the crevice formed by adjacent balls in the layer below. The balls are all the same material, but the gap size changes with different arrangements. This is what happens with steel. At lower temperatures, only up to 0.02% carbon fits in the gap. This orientation is called ferrite. At higher temperatures, a different atomic orientation is stable, which we call austenite. Up to 2% carbon can fit into this arrangement of atoms. For low-carbon steels under normal conditions, austenite cannot exist at room temperature – when the steel is slowly cooled, it changes from austenite to a combination of ferrite and a mixture of phases called pearlite. However, heating a certain chemistry to the austenitic zone followed by rapidly cooling just right bypasses the natural conversion to ferrite and pearlite, and creates a structure that contains austenite at room temperature. This leads to the term retained austenite, which is the phase that gives TRIP and 3rd Generation Steels excellent ductility. More on these later.

100% iron is very soft. As a matter of fact, 100% of any element is very soft. As an example, think about gold. 24-carat gold is pure gold. You might think that a wedding ring, as a symbol of long-lasting love and devotion, should be made from 100% gold. In reality, many gold bands are made from 12-carat gold, which is half gold and half impurities. (Showing your love by giving something 50% impure perhaps is not the best marketing approach.) Adding alloying elements to gold is done to improve certain characteristics, like strength, making the alloy appropriate for the applications it serves.

When we talk about ferrite at room temperature, that’s iron with no more than 80 parts per million carbon. That’s really close to pure iron, so when we hear the term ferrite, we should think of something that is really soft, low-strength, and very ductile.

If additional strength is needed, then more alloying elements must be used in addition to carbon. The next most cost-effective alloying element is manganese which produces higher-strength steels called carbon-manganese steels. These are substitutional solid solutions strengthened, where the atoms of manganese swap into where atoms of iron would otherwise go. These grades have limited ductility, especially at higher carbon and manganese contents, so they are used in structural applications that do not need a lot of formability and are therefore also called structural steels (SS). In the ASTM standard specification covering many sheet steels, ASTM A1008/A1008M, these grades are grouped in the SS category.

Around 1980, steelmakers rolled out a new approach to getting higher strength levels while minimizing the loss of elongation usually seen with higher strengths. They do this by strengthening the ferrite through the addition of very small quantities of titanium, niobium, and vanadium to form carbide and nitride precipitates. These microalloying additions are used in precipitation hardening of the ferrite to create High Strength Low Alloy (HSLA) steels.

Switching gears a bit to discuss something unrelated to sheet steel but a process with which we might be familiar: forged gears. We want forged gears to be hard and high strength. Typical production of gears involves heating up a steel alloy of certain chemistry, followed by rapid cooling (quenching) them faster than a critical cooling rate. The structure that’s produced is called martensite. If the quench rate is only a little too slow, a different phase called bainite can be produced. While martensite is the highest strength phase, it has limited elongation. Bainite is a little lower in strength but has higher elongation and toughness compared with martensite. Bainite shines in applications needing cut-edge ductility during stretch flanging.

Martensite wasn’t commonly found as a microstructural component during most of the history of automotive sheet steels due to the limited number of companies having an annealing line with appropriate quenching capabilities. This started to change around the turn of the millennium when newer annealing lines were installed with the ability to hold at a specific temperature which may be lower than the annealing temperature followed by quenching to another much lower temperature. This led to greater production of the first generation of Advanced High-Strength Steels (AHSS), including grades that have a microstructure of only martensite

Dual Phase steels are the most common AHSS. As you might guess, there are two phases in Dual-Phase steels. Ferrite and martensite are the two phases: ferrite is super-soft and comprises the majority of the microstructure, while martensite is super-hard and takes up 10% (590DP) to 40% (980DP) of the microstructure. The more martensite, the stronger the steel. Elongation is the ductility measured in a tensile test, and since most of the structure is ferrite, these steels have exceptional elongation for the strength level. However, there is a large hardness difference between ferrite and martensite, leading to crack initiation sites and resulting in poor cut-edge ductility during stretch flanging.

[A brief digression on testing. Tensile testing takes a standard sample shape, typically looking like a bone you might give a dog to chew on, and pulls it in tension from the edges. The test results include yield strength, tensile strength, and total elongation, commonly called the YTEs or TYEs based on the initials. More information comes out of the tensile test, covered elsewhere. However, the tensile test is usually not used to measure cut-edge ductility. Cut edge ductility is typically characterized by the hole expansion test, where a punched hole is expanded with a conical punch until a through-thickness crack forms.]

Ferrite-bainite steels have a combination of decent elongation (from the ferrite) and excellent cut-edge ductility (from the bainite). Yes, your assumption is correct that there are only two phases in these steels, with ferrite being about 85% of the microstructure. Due to the way these are produced, ferrite-bainite steels are available as hot-rolled products only. That’s in contrast with Complex Phase (CP) steels, which can be found either at hot-rolled or cold-rolled thicknesses.

Soft ferrite is the primary microstructural component in DP steels and the soon-to-be-discussed TRIP steels, which results in low yield strength and relatively high elongation. On the other hand, the primary microstructural components of complex phase steels are bainite and precipitation-strengthened ferrite, with martensite and retained austenite also present in lower amounts. Lacking soft ferrite, these steels have relatively high yield strength and low elongation as measured in a tensile test, but the bainite leads to exceptional cut-edge ductility as measured in a hole expansion test. Multi-phase steels are a related product. Some OEMs group CP and MP steels in the same category, while others say that CP steels are engineered to favor improved bendability and cut edge extension over tensile elongation at the same tensile strength and that MP steels target balancing the fracture resistance needed for better bendability and hole expansion with the necking resistance found with higher uniform elongation and n-value.

TRIP steels contain mostly ferrite surrounding islands of martensite, as well as some bainite and retained austenite. The amount of bainite is pretty low, so it doesn’t add much to the cut-edge ductility. But the magic is in the retained austenite. Austenite is a very ductile phase. What makes this a special phase is that as austenite-containing steels deform, the atoms rearrange and the austenite transforms into martensite, giving the steel enhanced ductility. (Jargon alert: Another word for ductility used by professionals is plasticity.) A quick review: this enhanced ductility comes from austenite transforming to martensite. In other words, these steels have Transformation Induced Plasticity (TRIP).

Wouldn’t it be great to have an alloy that was just austenite? We’d have a high-strength, high-ductility product. There are two types of steels that are in this category. First are austenitic stainless steels in the 3XX family, like SS304 and SS316. In these alloys, austenite is stable at room temperature, but these require approximately 18% chromium and 8% nickel. Next are TWIP steels. These may look like TRIP steels from how they are written, but these steels get their plasticity differently. TWIP steels deform by a mechanism known as twinning, so they are described as Twinning Induced Plasticity Steels (TWIP). Of course, there are no free lunches. To get fantastic formability properties, a lot of alloying is necessary. This drives up the steelmaking complexity and cost. The alloying elements also make welding much more challenging. TWIP steels are called second-generation advanced high-strength steels.

The 3rd Generation Advanced High-Strength Steels (3rd Gen AHSS or 3rd Gen) are made possible by another advance in annealing technology, allowing steelmakers to produce a refined microstructure. Nearly all 3rd Gen steels have retained austenite in the microstructure and therefore benefit from a high strength, high ductility combination. The latest annealing lines used to make these steels come equipped to not just hold and quench to defined temperatures but have reheating capability followed by another hold and quench to different temperature targets. This allows for the creation of an engineered balance and distribution of ferrite, bainite, martensite, and austenite in the microstructure.

The resultant tensile property ranges from 3rd Gen steels produced at different companies may be similar, but their methods of getting those properties are a function of chemistry and the capabilities and characteristics of the equipment used to produce them. A different chemistry approach may result in different weldability, for example, so users are encouraged to perform thorough due diligence before switching suppliers. The days of steel being simply a commodity are in the past as it relates to these highly engineered higher strength steels.

 

Final thought 1: What’s an MPa?

This note may have a global readership, but this answer is focused on the countries that haven’t embraced the metric system. Megapascals, abbreviated MPa, is a measure of strength, just like pounds per square inch (psi) or force per area. Like Celsius and Fahrenheit or inches and millimeters, we can convert between them easily enough. There are 1000 psi in a ksi, with k being the abbreviation for kilopounds. And there are 6.895 ksi in an MPa. Make your life easier and focus on a 7:1 difference. 100 ksi is about 700 MPa.

 

Final thought 2: What about Press Hardening Steels?

Press hardening steel for hot stamping is a separate topic with a lot of nuances. One of the biggest differences is how the properties develop. For cold stamping operations, the stamping company is responsible for creating the formed part from sheet metal supplied to the necessary strength. With press hardening steels, the stamping company creates both the shape and the strength. Different grades come from a combination of different chemistries from the steelmaker and different heating and cooling profiles at the stamping location. The chosen corrosion protection approach impacts the various options. Learn more at the Press Hardening Primer on this site.

 

Final thought 3: Don’t hesitate to ask questions.

If your metallurgical representative says something that you don’t understand, ask for clarification. Your suppliers want to be your valued partner for more than just a simple transaction. Quite likely, your met rep is passionate about their offerings and would love to talk about them. If you get a deeper understanding of what makes one product different from another, then you’ll be in a better position to weigh the benefits against the inevitable constraints, leading to an optimized material selection. Remember, communication is the key to success for all parties.

 

Thanks go to author Daniel J. Schaeffler, Ph.D., President, Engineering Quality Solutions, Inc.

Danny Schaeffler is the Metallurgy and Forming Technical Editor of the AHSS Applications Guidelines available from WorldAutoSteel.  He is founder and President of Engineering Quality Solutions (EQS).  Danny writes the monthly “Metal Matters” column for Metalforming Magazine, and provides seminars on sheet metal formability for the Precision Metalforming Association.  He has written for Stamping Journal and The Fabricator, and has lectured at FabTech.  Danny is passionate about training new and experienced employees at manufacturing companies about how sheet metal properties impact their forming success.

 

 

Transformation Induced Plasticity (TRIP)

Transformation Induced Plasticity (TRIP)

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The microstructure of Transformation Induced Plasticity (TRIP) steels contains a matrix of ferrite, with retained austenite, martensite, and bainite present in varying amounts. Production of TRIP steels typically requires the use of an isothermal hold at an intermediate temperature, which produces some bainite. Higher silicon and carbon content of TRIP steels result in significant volume fractions of retained austenite in the final microstructure. Figure 1 shows a schematic of TRIP steel microstructure, with Figure 2 showing a micrograph of an actual sample of TRIP steel. Figure 3 compares the engineering stress-strain curve for HSLA steel to a TRIP steel curve of similar yield strength.

 

Figure 1: Schematic of a TRIP steel microstructure showing a matrix of ferrite, with martensite, bainite and retained austenite as the additional phases.

Figure 1: Schematic of a TRIP steel microstructure showing a matrix of ferrite, with martensite, bainite and retained austenite as the additional phases.

 

Figure 2: Micrograph of Transformation Induced Plasticity steel.

Figure 2: Micrograph of Transformation Induced Plasticity steel.

 

Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and TRIP 350/600.K-1

Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and TRIP 350/600.K-1

 

 

During deformation, the dispersion of hard second phases in soft ferrite creates a high work hardening rate, as observed in the DP steels. However, in TRIP steels the retained austenite also progressively transforms to martensite with increasing strain, thereby increasing the work hardening rate at higher strain levels. This is known as the TRIP Effect. This is illustrated in Figure 4, which compares the engineering stress-strain behavior of HSLA, DP and TRIP steels of nominally the same yield strength. The TRIP steel has a lower initial work hardening rate than the DP steel, but the hardening rate persists at higher strains where work hardening of the DP begins to diminish. Additional engineering and true stress-strain curves for TRIP steel grades are shown in Figure 5.

 

Figure 4: TRIP 350/600 with a greater total elongation than DP 350/600 and HSLA 350/450 Reference K-1

Figure 4: TRIP 350/600 with a greater total elongation than DP 350/600 and HSLA 350/450. K-1

 

Figure 5: Engineering stress-strain (left graphic) and true stress-strain (right graphic) curves for a series of TRIP steel grades. Sheet thickness: TRIP 350/600 = 1.2mm, TRIP 450/700 = 1.5mm, TRIP 500/750 = 2.0mm, and Mild Steel = approx. 1.9mm. V-1

Figure 5: Engineering stress-strain (left graphic) and true stress-strain (right graphic) curves for a series of TRIP steel grades. Sheet thickness: TRIP 350/600 = 1.2mm, TRIP 450/700 = 1.5mm, TRIP 500/750 = 2.0mm, and Mild Steel = approx. 1.9mm. V-1

 

 

The strain hardening response of TRIP steels are substantially higher than for conventional HSS, resulting in significantly improved formability in stretch deformation. This response is indicated by a comparison of the n-value for the grades. The improvement in stretch formability is particularly useful when designers take advantage of the improved strain hardening response to design a part utilizing the as-formed mechanical properties. High n-value persists to higher strains in TRIP steels, providing a slight advantage over DP in the most severe stretch forming applications.

Austenite is a higher temperature phase and is not stable at room temperature under equilibrium conditions. Along with a specific thermal cycle, carbon content greater than that used in DP steels are needed in TRIP steels to promote room-temperature stabilization of austenite. Retained austenite is the term given to the austenitic phase that is stable at room temperature.

Higher contents of silicon and/or aluminum accelerate the ferrite/bainite formation. These elements assist in maintaining the necessary carbon content within the retained austenite. Suppressing the carbide precipitation during bainitic transformation appears to be crucial for TRIP steels. Silicon and aluminum are used to avoid carbide precipitation in the bainite region.

The carbon level of the TRIP alloy alters the strain level at which the TRIP Effect  occurs. The strain level at which retained austenite begins to transform to martensite is controlled by adjusting the carbon content. At lower carbon levels, retained austenite begins to transform almost immediately upon deformation, increasing the work hardening rate and formability during the stamping process. At higher carbon contents, retained austenite is more stable and begins to transform only at strain levels beyond those produced during forming. At these carbon levels, retained austenite transforms to martensite during subsequent deformation, such as a crash event.

TRIP steels therefore can be engineered to provide excellent formability for manufacturing complex AHSS parts or to exhibit high strain hardening during crash deformation resulting in excellent crash energy absorption.

The additional alloying requirements of TRIP steels degrade their resistance spot-welding behavior. This can be addressed through weld cycle modification, such as the use of pulsating welding or dilution welding.  Table 1 provides a list of current production grades of TRIP steels and example automotive applications:

Table 1: Current Production Grades Of TRIP Steels And Example Automotive Applications.

Table 1: Current Production Grades Of TRIP Steels And Example Automotive Applications.

 

Some of the specifications describing uncoated cold rolled 1st Generation transformation induced plasticity (TRIP) steel are included below, with the grades typically listed in order of increasing minimum tensile strength and ductility. Different specifications may exist which describe hot or cold rolled, uncoated or coated, or steels of different strengths. Many automakers have proprietary specifications which encompass their requirements.
• ASTM A1088, with the terms Transformation induced plasticity (TRIP) steel Grades 690T/410Y and 780T/440YA-22
• JFS A2001, with the terms JSC590T and JSC780TJ-23
• EN 10338, with the terms HCT690T and HCT780TD-18
• VDA 239-100, with the terms CR400Y690T-TR and CR450Y780T-TRV-3
• SAE J2745, with terms Transformation Induced Plasticity (TRIP) 590T/380Y, 690T/400Y, and 780T/420YS-18

 

Transformation Induced Plasticity Effect

Austenite is not stable at room temperature under equilibrium conditions. An austenitic microstructure is retained at room temperature with the combined use of a specific chemistry and controlled thermal cycle. Deformation from sheet forming provides the necessary energy to allow the crystallographic structure to change from austenite to martensite. There is insufficient time and temperature for substantial diffusion of carbon to occur from carbon-rich austenite, which results in a high-carbon (high strength) martensite after transformation. Transformation to high strength martensite continues as deformation increases, as long as retained austenite is still available to be transformed.

Alloys capable of the TRIP effect are characterized by a high ductility – high strength combination. Such alloys include 1st Gen AHSS TRIP steels, as well as several 3rd Gen AHSS grades like TRIP-Assisted Bainitic Ferrite, Carbide Free Bainite, and Quench & Partition Steels.

 

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3rd Generation Steels

 

First Generation Advanced High-Strength Steels (AHSS) are based on a ferrite matrix for baseline ductility, with varying amounts of other microstructural components like martensite, bainite, and retained austenite providing strength and additional ductility. These grades have enhanced global formability compared with conventional high strength steels at the same strength level. However, local formability challenges may arise in some applications due to wide hardness differences between the microstructural components.

The Second Generation AHSS grades have essentially a fully austenitic microstructure and rely on a twinning deformation mechanism for strength and ductility. Austenitic stainless steels have similar characteristics, so they are sometimes grouped in this category as well. 2nd Gen AHSS grades are typically higher-cost grades due to the complex mill processing to produce them as well as being highly alloyed, the latter of which leads to welding challenges.

Third Generation (or 3rd Gen) AHSS are multi-phase steels engineered to develop enhanced formability as measured in tensile, sheared edge, and/or bending tests. Typically, these steels rely on retained austenite in a bainite or martensite matrix and potentially some amount of ferrite and/or precipitates, all in specific proportions and distributions, to develop these enhanced properties.

Individual automakers may have proprietary definitions of 3rd Gen AHSS grades containing minimum levels of strength and ductility, or specific balances of microstructural components. However, such globally accepted standards do not exist. Prior to 2010, one steelmaker had limited production runs of a product reaching 18% elongation at 1000 MPa tensile strength. Starting around 2010, several international consortia formed with the hopes of achieving the next-level properties associated with 3rd Gen steels in a production environment. One effortU-11, S-95 targeted the development of two products: a high strength grade having 25% elongation and 1500 MPa tensile strength and a high ductility grade targeting 30% elongation at 1200 MPa tensile strength. The “exceptional-strength/high-ductility” steel achieved 1538 MPa tensile strength and 19% elongation with a 3% manganese steel processed with a QP cycle. The 1200 MPa target of the “exceptional-ductility/high-strength” was met with a 10% Mn alloy, and exceeded the ductility target by achieving 37% elongation. Another effort based in EuropeR-22 produced many alloys with the QP process, including one which reached 1943 MPa tensile strength with 8% elongation. Higher ductility was possible, at the expense of lower strength.

3rd Gen steels have improved ductility in cold forming operations compared with other steels at the same strength level. As such, they may offer a cold forming alternative to press hardening steels in some applications. Also, while 3rd Gen steels are intended for cold forming, some are appropriate for the hot stamping process.

Like all steel products, 3rd Gen properties are a function of the chemistry and mill processing conditions. There is no one unique way to reach the properties associated with 3rd Gen steels – steelmakers use their available production equipment with different characteristics, constraints, and control capabilities. Even when attempting to meet the same OEM specification, steelmakers will take different routes to achieve those requirements. This may lead to each approved supplier having properties which fall into different portions of the allowable range. Manufacturers should use caution when switching between suppliers, since dies and processes tuned for one set of properties may not behave the same when switching to another set, even when both meet the OEM specification.

There are three general types of 3rd Gen steels currently available or under evaluation. All rely on the TRIP effect. Applying the QP process to the other grades below may create additional high-performance grades.

  • TRIP-Assisted Bainitic Ferrite (TBF) and Carbide-Free Bainite (CFB)
    • TRIP-Assisted Bainitic Ferrite (TBF) and Carbide-Free Bainite (CFB) are descriptions of essentially the same grade. Some organizations group Dual Phase – High Ductility (DP-HD, or DH) in with these. Their production approach leads to an ultra-fine bainitic ferrite grain size, resulting in higher strength. The austenite in the microstructure allows for a transformation induced plasticity effect leading to enhanced ductility.
  • Quenched and Partitioned Grades, Q&P or simply QP
    • Quenching and Partitioning (Q&P) describes the processing route resulting in a structure containing martensite as well as significant amounts of retained austenite. The quenching temperature helps define the relative percentages of martensite and austenite while the partitioning temperature promotes an increased percentage of austenite stabile room temperature after cooling.
  • Medium Manganese Steels, Medium-Mn, or Med-Mn
    • Medium Manganese steels have a Mn content of approximately 3% to 12%, along with silicon, aluminum, and microalloying additions. This alloying approach allows for austenite to be stable at room temperature, leading to the TRIP Effect for enhanced ductility during stamping. These grades are not yet widely commercialized.

 

TRIP Assisted Grades, like TRIP-Assisted Bainitic Ferrite (TBF)

and Carbide-Free Bainite (CFB)

During the slow cooling of conventional steels, austenite transforms into a microstructure containing alternating regions of ferrite and cementite. Note that cementite is the name given to iron carbide with the composition Fe3C. At higher magnification, this microstructure looks like Mother-of-Pearl, leading to its name of pearlite.

Depending on the chemistry and thermal profile, rapid controlled cooling produces new microstructures which are not achievable with slow cooling, including martensite, austenite, and bainite. Bainite consists of regions of dislocation-rich (higher strength) ferrite separated by austenite, martensite, and/or cementite. These phases within bainite have relatively small hardness differences, leading to improved local formability compared with conventional dual phase or TRIP steels. Producing a fully-bainitic microstructure is challenging, so bainite is usually accompanied by other phases, resulting in ferrite-bainite steels or complex phase.

With an appropriate chemistry and use of specific thermal profiles capable of holding at specific temperatures and even reheating after quenching further reduces the size of these microstructural components, and essentially eliminates the production of the low-ductility cementite (iron carbide). Large “blocky” austenite, characteristic of 1st Generation TRIP steels, is minimized and instead thin fine submicron austenitic laths form (Figure 1).

Figure 1: On the left, the typical bainitic structure showing bainitic ferrite laths with interlath carbideS-96; On the right is the microstructure of TRIP Assisted Bainitic Ferrite / Carbide Free Bainite showing bainitic ferrite laths interwoven with thin films of untransformed retained austeniteC-31. ab is bainitic ferrite and y is retained austenite. Note the slightly different magnification.

Figure 1: On the left, the typical bainitic structure showing bainitic ferrite laths with interlath carbideS-96; On the right is the microstructure of TRIP Assisted Bainitic Ferrite / Carbide Free Bainite showing bainitic ferrite laths interwoven with thin films of untransformed retained austenite.C-31  αb is bainitic ferrite and γ is retained austenite. Note the slightly different magnification.

 

The fine components result in higher strength, similar to fine grain size being associated with increased strength. Since the ferrite is higher strength than conventional bainite due to the fine component size and even greater dislocation density, the component hardness difference is further minimized, leading to additional improvements in local formability. The austenite promotes the TRIP effect, resulting in greater uniform elongation and enhanced global formability. Combined, these features result in calling this microstructure either TRIP Assisted Bainitic Ferrite (TBF) or Carbide Free Bainite (CFB). Some sources suggest this is the same product as “Dual Phase with High Ductility,” abbreviated as DP-HD or simply DH.H-18, A-70, R-23, B-58  TBF, CFB, DP-HD, and DH are used interchangeably.

One potential processing route (Figure 2) may involve intercritically annealing in the two-phase austenite+ferrite region, cooling slightly to promote ferrite formation (1→2), and then quenching (2→3) to a temperature below the start of bainite formation (Bs) while remaining above the Ms temperature, the start of martensitic transformation. Once the targeted amount of bainite has formed in an isothermal overaging step (3→4), the steel is then quenched to room temperature (4→5).

Figure 2: Potential thermal cycle to produce TRIP assisted Bainitic Ferrite (Carbide-Free Bainite).

Figure 2: Potential thermal cycle to produce TRIP assisted Bainitic Ferrite (Carbide-Free Bainite).

 

These steels are characterized by a good balance of strength and global formability (as measured by high TSxEL, uniform elongation, and total elongation combined with low YS/TS) against local formability (as measured by bend angle and hole expansion ratio).C-31  A YS/TS ratio of approximately 0.7, similar to DP steels, is a characteristic of these grades.H-59, C-31

These steels exhibit a significant bake hardening response. One study found a BH kick of over 200 MPa after a 4% prestrain and a bake cycle of 30 minutes at 200 °C. The total hardening response (strain hardening plus bake hardening) was almost 800 MPa.T-41 However, in production, this paint bake cycle is not likely to be practical due to paint over curing and the preference for faster cycle times. A different study evaluated TBF700Y/1050T and found after 15 minutes at 195 °C, samples prestrained to 4.5% had a BH kick of 150 MPa, with a total hardening response in excess of 350 MPa.B-60

Challenges exist when producing these grades with a galvanized or galvannealed coating. The relatively higher silicon content needed to suppress carbide formation may lead to difficulties galvanizing and with galvanized surface quality. Replacing silicon with aluminum helps with the coating issues, but makes the thermal cycle more complex. The chosen thermal cycle needs to be appropriate for the selected chemistry and targeted properties, and constrained by the capabilities of the existing mill equipment. Descriptions of the capabilities of equipment used in the production of cold rolled and galvanized AHSS are found elsewhere.K-43, B-59

The 2013 Infiniti Q50 is one of the earliest production applications for TBF 1180, where it formed 4% of the Body-In-White mass. Applications included A- and B-pillar reinforcements, sill reinforcements, and roof rail and side reinforcements. Adjusted welding techniques resulted in the same stress concentration as seen when welding mild steels.I-22, K-44  The same grade applied on the 2015 Nissan Murano in the A-Pillar Inner and reinforcements allowed numerous components to be downgauged from 1.6 mm to 1.2 mm compared with the prior version.C-32 1180TBF represented over 6% of the mass of the 2016 Nissan Maxima body-in-white, primarily applied in the A- and B-Pillar Reinforcements. Typically, 1.4 mm thick 980 grade steel was downgauged to 1.2 mm.C-33

A sample of commercially available TBF1180 was shown to have 946 MPa yield strength, 1222 MPa tensile strength, 18% elongation (JIS sample) , with a 40% hole expansion ratioM-54, which is consistent with the minimum properties listed by one automotive OEM: YS: 850 MPa minimum, TS: 1180 MPa minimum, elongation: 14% JIS minimum, and 30% minimum hole expansion ratio.F-36  Stretch formability as tested using a dome height evaluation was shown to be comparable to a conventional DP980 product, with deep drawability characterized by forming height in a cup draw test being superior to both conventional DP980 and DP1180.

Stress-strain curves of TBF700Y/1050T are found in the literature and presented in Figure 3 for reference. Note that these are random samples from a commercially available product tested at different laboratories, and therefore may not be representative of all products of this grade.

Figure 3: Stress strain curves of commercially available TBF 700Y/1050T. A) YS=775 MPa, TS = 1235 MPa, EL = 10%G-44; B) YS=751 MPa, TS = 1035 MPa, EL = 17%. Also shown is the pre-strain and bake hardening response for 1.0 mm thick blanks, tested after a 20 minute dwell time in a 170°C furnace.B-60

Figure 3: Stress strain curves of commercially available TBF 700Y/1050T. A) YS=775 MPa, TS = 1235 MPa, EL = 10%G-44; B) YS=751 MPa, TS = 1035 MPa, EL = 17%. Also shown is the pre-strain and bake hardening response for 1.0 mm thick blanks, tested after a 20 minute dwell time in a 170 °C furnace.B-60

 

The 2018 Infiniti QX50 SUV is an example of a vehicle believed to have TBF980 in the body structure.I-23  The product shown is called SHF980, and has a microstructure of approximately 50% ferrite, approximately 10% retained austenite, with the remainder as martensite/bainite, which is consistent with expectations for a TBF product. The thermal processing route to achieve this microstructural balance is consistent with a Quenching & Partitioning process (Figure 4). Both SHF980 and the reference DP980 are shown to have 660 MPa yield strength and 1000 MPa tensile strength. However, where DP980 has 15% elongation, SHF980 has 23% elongation. In addition, SHF980 is capable of 10% greater energy absorption over DP980 at the same thickness.I-23

Figure 4: Production and properties of SHF980, possessing a TBF microstructure.I-23

Figure 4: Production and properties of SHF980, possessing a TBF microstructure.I-23

 

The highest strength TBF grade commercially available has 1,470MPa minimum tensile strength. Properties in Table 1 are compared with DP1470.

Table 1: Tensile properties of 1.2mm steels with 1470 MPa minimum tensile strength.M-55

Table 1: Tensile properties of 1.2mm steels with 1470 MPa minimum tensile strength.M-55

 

Case Study: Production Application Where 3rd Gen Steels

Reduced Weight and Improved Performance

Toyota Motor Europe designed a part requiring a minimum tensile strength of 980 MPa, but when stamped using a conventional AHSS grade, experienced both global formability (necking) failures and local formability (sheared edge) failures (Figure 5). In the search for a grade which blended the high elongation of dual phase grades and the high hole expansion of complex phase grades, Toyota chose TBF980, a TRIP-assisted bainitic ferrite grade with the same yield and tensile strength of a conventional 980 grade but with improved elongation of approximately 14% and hole expansion of approximately 65%.A-1

Also reported were grade and design changes in a production vehicle where the strength of TBF980 allows for a 20% thickness reduction over the prior model. The improved formability of TBF980 facilitated a reduction in packaging space of the component, with the new design being 6% narrower and 20% shorter.  Combined, these improvements reduced the vehicle weight by 1 kg.A-1

Figure 5: 980 MPa part with global and local formability failures.  Converting the steel to TBF980 eliminated both types of splits.  Image adapted from Citation A-1.

Figure 5: 980 MPa part with global and local formability failures.  Converting the steel to TBF980 eliminated both types of splits.  Image adapted from Citation A-1.

 

Quenched and Partitioned Grades, Q&P or simply QP

Quenching and partitioning (Q&P, or QP) describes a multi-step heat treatment which produces high tensile strength, high global ductility (total elongation) and high local ductility (hole expansion and bendability), compared with other similar strength steels. The QP process was first explained in 2003 by Speer et al.S-97, S-98, S-99

Among the unique aspects of the required thermal cycle is that after the first quench from the fully austenitized or intercritical annealing temperature, the steel may be reheated to a higher temperature, and then quenched to room temperature.

Figure 6 provides a general overview of the QP thermal cycle. After austenitization in either the single phase austenite region or the two-phase ferrite+austentite (intercritical annealing), the steel is quenched to a temperature below the start of martensitic transformation (Ms) but above the Mf (temperature at which all austenite has transformed to martensite), as indicated by segment 1→2. In the two-step QP process, the temperature is raised above Ms, shown in segment 2→3. No temperature increase is seen in the one-step QP process, meaning 2=3. Then the steel is held at this partitioning temperature for the appropriate time to generate the targeted microstructure and properties, segment 3→4. Once reached, the steel is quenched again (4→5), this time to a temperature below Mf, the temperature below which all transformation to martensite has occurred.

Figure 5: Thermal cycle for the Quenching and Partitioning Process.

Figure 6: Thermal cycle for the Quenching and Partitioning Process.

 

The QP microstructure contains martensite and austenite. Ferrite is also present if intercritical annealing in the two-phase region is employed rather than in the single-phase austenitic region. The first quench forms a controlled volume fraction of martensite. With a QP chemistry containing C between 0.15 and 0.4%, Mn between 1.5 and 2.5%,and (Al + Si) around 1.5 wt.%, the quenching temperature usually lies in the range 200 to 350 °C.S-100  After raising to the partitioning temperature typically between 300 to 500 °CS-100, an isothermal hold allows carbon from the carbon-supersaturated martensite to diffuse into the untransformed austenite. This enriches the austenite with carbon while similarly depleting the martensite. The carbon enriched austenite increases its room temperature stability. Since the partitioning temperature above that required for martensite formation, some of the martensite transforms to tempered martensite. Tempered martensite provides high strength with more ductility than untempered martensite. After the partitioning step, the final quench results in the formation of fresh martensite.

When stamping parts from this steel, the austenite transforms to newly formed martensite through the TRIP effect, enhancing the ductility and strength. Adjusting the chemistry, quenching temperature, partitioning temperature, and partitioning time affects the amount, morphology, and stability of the retained austenite, leading to a wide range of potential properties.D-32  The microstructure of commercial Q&P steels is composed of martensite (50–80%) formed during quenching, ferrite (20–40%) formed as austenite slowly cools, and dispersed retained austenite (5–10%) stabilized by carbon enrichment during partitioning. Higher strength QP steels will have reduced amounts of ferrite.W-35 This is mostly consistent with a study highlighting commercially produced QP980 and QP1180 which showed that both products have approximately 10-12% retained austenite, with QP980 containing 56% ferrite / 32% martensite and QP1180 containing 21% ferrite / 69% martensite.W-36

There is no standard processing route with defined chemistry and temperatures. The complex thermal cycle needs to be appropriate for the selected chemistry and targeted properties, and constrained by the capabilities of the existing mill equipment. Citation K-43 presents descriptions of the equipment and capabilities used at one location. Process variants exist, such as a one-step approach using the same temperature for the initial quench and the partitioning.S-98  Other modifications allow for production of a Carbide-Free Bainitic structure during the first quench, improving the damage resistance due to additional strain-hardening capacity within the local plasticity deformation zone near the tips of micro-cracks.G-45

The Q&P process is applicable to other products as well, including stainless steelsM-56, M-57, S-101 and Press Hardening Steels.A-71, A-72, X-1  A one-step Q&P approach was applied to a laser welded blank with 22MnB5 and TRIP components, resulting in tailored properties to improve the intrusion resistance and energy-absorption capabilities in the pertinent regions.K-46

Complex phase steels with High Ductility (CP-HD, or CH) have similar microstructural constituents, along with bainite. Although CH steels reach high hole expansion values, they do not have the elongation levels typically associated with QP steels. Still, some sources equate CH and QP steels.H-18

Two levels of Quenched & Partitioned steels are in global production, 980 MPa and 1180 MPa. The enhanced properties of QP steels offer benefits over similar-strength steels of other microstructures. Compared against Dual Phase steel with similar yield and tensile strength, a Quenched & Partitioned steel showed higher uniform elongation, total elongation, work hardening index, and FLC0, highlighted in Table 2 and Figure 7.C-34  A different production supplier of QP980 reports similar strength and elongation properties, with a targeted 23% hole expansion ratio.G-46

Table 2: Tensile properties of production DP980 and QP980.C-34

Table 2: Tensile properties of production DP980 and QP980.C-34

 

Figure 6: Comparison of Forming Limit Curves of production DP980 and QP980.C-34

Figure 7: Comparison of Forming Limit Curves of production DP980 and QP980.C-34

 

QP980 is seeing expanded use in automotive production. The 2016 Chevrolet Sail from SAIC-GM represented the first application at General Motors.H-60  The 2021 Ford Bronco uses hot dip galvanized QP980 in five components of the front and rear floor assemblies.S-102  Sixty percent of the body structure of the 2021 Jeep Grand Cherokee L is made from AHSS, with some parts stamped from 3rd Gen steels.F-37

Table 3 contains typical mechanical property ranges for industrially produced QP980 and QP1180.W-35  A typical strain–stress curve of QP980 is shown in Figure 8.

Table 3: Typical mechanical property ranges for industrially produced QP980 and QP1180.W-35

Table 3: Typical mechanical property ranges for industrially produced QP980 and QP1180.W-35

 

Figure 7: Stress-strain curve of industrially produced QP980.W-35

Figure 8: Stress-strain curve of industrially produced QP980.W-35

 

Of course, there are additional characteristics beyond strength and elongation that impact successful use in manufactured products. Typical forming-limit curves for cold rolled QP980, DP780, and DP 980 steels are shown in Figure 9, highlighting that the formability of QP980 is comparable to that of DP780.

Figure 8: Forming-limit curves for 1 mm thick Q&P 980, DP 780, and DP 980.W-35

Figure 9: Forming-limit curves for 1 mm thick Q&P 980, DP 780, and DP 980.W-35

 

Figure 10 contains the results of high strain rate tensile testing, confirming that QP980 has positive strain rate sensitivity and therefore has the potential for improved crash energy absorption.

Figure 9: True stress-strain curves for QP980 generated at different strain rates.W-35

Figure 10: True stress-strain curves for QP980 generated at different strain rates.W-35

 

Sheared-edge ductility is also a concern in AHSS grades. Hole expansion of QP1180, QP980, and DP980 is compared in Figure 11, with similar results seen in QP980 and DP980. QP1180 had the highest hole expansion, possibly because of its microstructure containing components of relatively uniform hardness.

Figure 10: Hole expansion of QP1180, QP980, and DP980, generated from either punched or machined holes.W-35 

Figure 11: Hole expansion of QP1180, QP980, and DP980, generated from either punched or machined holes.W-35

 

The bending under tension test was used to determine the critical R/t value below which the risk for shear fracture increases. These experiments showed that critical R/t values of QP980 were close to those of other steels having 600 MPa tensile strength.W-35

Similar springback was observed in QP980 and DP980 when a 5 mm radius was used in the bending-under-tension test, with QP980 exhibiting less springback when a 12.7 radius die was used instead.W-35

General Motors provided stress-strain curves for production QP700/1180 tested at different strain rates (Figure 12), showing increases in strength and ductility as strain rates increase.H-60

Figure 11: Engineering stress-strain curves for QP700Y/1180T at different strain rates.H-60

Figure 12: Engineering stress-strain curves for QP700Y/1180T at different strain rates.H-60

 

A recent conference highlighted several applications (Figure 13) where thinner gauge QP980 replaced DP590 in General Motors vehicles.W-37

Figure 15: Replacing DP590 with QP980 allows for downgauging.W-37

Figure 13: Replacing DP590 with QP980 allows for downgauging.W-37

 

The same presentationW-37 showed the example of QP980 replacing press hardening steels in B-pillar reinforcements and door anti-intrusion beams in a First Auto Works vehicle, Figure 14.

Figure 16: QP980 may replace press hardening steels in some safety applications.W-37

Figure 14: QP980 may replace press hardening steels in some safety applications.W-37

 

 

Medium Manganese Steels, Medium-Mn, or Med-Mn

Manganese has a lower density than iron, so using alloys with higher amounts of manganese truly creates lightweight products. 1st Generation steels typically contain no more than around 2% Mn. 2nd Generation TWIP steels have about 20% Mn. Lean medium-manganese (MedMn) steels typically use between 3% and 12% manganese along with silicon, aluminum, and microalloying additions.R-16, D-27, S-80, K-35  Aluminum in these steels further lowers the density.

No standard chemistry or processing route exists, but several studies use a thermal cycle similar to that seen with Q&P steels. This approach leads to a complex multiphase fine-grained microstructure. Compared with QP steels at the same strength levels, the higher manganese levels of Med-Mn steels promote greater amounts of retained austenite, and therefore greater ductility through the TRIP Effect. One study showed a combination of 1400 MPa tensile strength and a total elongation of 18%.S-103

One difference from the thermal cycle to produce QP steels used by some researchers to process Med-Mn steels is that after intercritical (two-phase) annealing, the quench is to room temperature rather than simply below Ms, the start of martensitic transformation.S-80 This is facilitated by the high levels of manganese, which adjusts the Mf below room temperature. Quenching a steel containing 0.25% C, 8.23% Mn, 1.87% Si, 0.05% Ni, and 0.24% Mo to room temperature and subsequently partitioning at 300 °C led to tensile strengths greater than 1800 MPa combined with total elongations of approximately 15%.S-80

In addition to lowering the Mf (martensite finish) level below room temperature, the manganese levels are sufficiently high enough that the coils after hot rolling may be either partially or fully martensitic. This phenomenon means that it may be possible to produce hot rolled Med-Mn steels.

Another production method called Austenite-Reverted Transformation (ART) annealing results in a large percentage of retained austenite in medium manganese steels. The fully or partially martensitic hot or cold rolled coil is heated to the single phase austenite region or the intercritical two phase austenite+ferrite region where the martensite reverts to austenite – hence the name of the process. The austenite nucleates on the former sites of fine martensitic features. This approach results in a final product with extremely fine features. During annealing, diffusion of both carbon and manganese occurs, which determines both the phase fraction and stability of the retained austenite. Processing of Fe–0.3C–11.5Mn–5.8Al resulted in a microstructure with 60% retained austenite.B-59

Multi-step thermal treatments are one approach to control the relative proportions of martensite, ferrite, and austenite. One example, termed “double-soaking” (DS), aims for substantial Mn-enrichment of austenite in a first soaking step followed by a second soaking step at a higher temperature which leads to a greater fraction of martensite in the final product. The brief second soak is long enough to allow the carbon to partition, but not long enough for manganese partitioning to occur, producing regions of higher and lower Mn within the austenite. The higher-Mn regions allow for greater amounts of austenite in the final product, while the lower-Mn regions transform to martensite, leading to TRIP-effect ductility and high strength.S-80, G-47 In an industrial environment, the initial soak may be done in a batch anneal furnace, with the brief second soak targeted for the time and temperature available in continuous annealing or galvanizing lines.

Still another production method proposed is known as Deforming and Partitioning (D&P). This route uses a warm rolling followed by cold rolling to generate an extremely high dislocation density. A subsequent partitioning treatment relieves the residual stresses from rolling and stabilizes the retained austenite via carbon enrichment. Figure 15 shows a representative Deforming and Partitioning thermal cycle. A D&P MedMn steel with a composition of 0.47C–10Mn–2Al–0.7V reached a yield strength of 2.2 GPa (2,200 MPa) and a uniform elongation of 16%.H-65

Figure 15: Representative Deforming and Partitioning (D&P) thermal cycle

Figure 15: Representative Deforming and Partitioning (D&P) thermal cycle.H-65

 

Medium-manganese steels with Mn contents between 3 wt.% and 10 wt.% have a microstructure consisting of an ultra-fine grained ferritic matrix (grain size < 1 μm) with up to 40 vol.% retained austenite.K-47  A chemistry of Fe-7.9Mn-0.14Si-0.05Al-0.07C resulted in 39% retained austenite with the processing route evaluated.Z-10

Properties are dependent on all aspects of the chosen chemistry and thermal cycle. With an appropriate approach, the steel may exhibit both a transformation-induced plasticity (TRIP) effect and a twinning-induced plasticity (TWIP) effect.

Studies indicate that Medium Manganese steels are also suitable for use in press hardening applications. A studyL-63 indicates that an alloy with 0.14 %C – 7.0 %Mn rivals conventional 22MnB5 PHS1500 in strength, but has more ductility. After hot forming and processing through a typical paint bake cycle, 22MnB5 exhibited a tensile strength of 1510 MPa, a uniform elongation of 4.6%, and a total elongation of 7.3%.  The MedMn steel showed values of 1565 MPa, 9.6% and 11.7% respectively.  These enhanced properties are suspected to be associated with the high volume fraction (15%) of retained austenite found in the Medium Manganese steels.

Figure 16: Engineering stress–strain curves of the medium-Mn martensitic steel and 22MnB5.

Figure 16: Engineering stress–strain curves of the medium-Mn martensitic steel and 22MnB5. L-63

 

Unlike TBF and QP steels, Medium-Manganese steels may exhibit discontinuous yielding, also known as yield point elongation or Lüders bands. Depending on chemistry and processing, these may extend beyond 5% engineering strain.

Medium manganese steels are not yet widely commercialized. They were the focus of an entire issue of a technical journal.M-58  The lead Editorial presents an overview of prior studies and highlights areas of interest.R-16

 

Consortia Activities

The European Commission through the European Research Executive Agency has funded a multi-year study called WarP-AHSS, which stands for Warm Press-Formed Zinc-Coated Third Generation Advanced High Strength Steels with High Crash and Corrosion Resistance and Minimized Microcracking.

The WarP-AHSS project seeks to develop end-to-end processing of warm press-formed parts from zinc-coated Medium Manganese Steels. This approach is expected to reduce the reheating and warm press-forming temperatures, making the process greener and energy-efficient, while allowing the use of zinc-coated sheets without liquid metal embrittlement-induced micro-cracking during warm-forming.

Partners in the WarP-AHSS research project include a steelmaker, an automaker, a university, and two research institutions. The project runs from October 2023 through March 2027.

The European Research Executive Agency has funded another multi-year study, Sup3rForm, that seeks to optimize the production and use of both 3rd Generation Q&P and medium-Mn steels. The Sup3rForm consortium, coordinated by Eurecat Technology Centre, is made up of eight partners, including steelmakers, an automaker, tier suppliers, and universities. Sup3rForm runs from July 2023 through December 2026.

 

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