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.

 

Resistance Spot Welding: 5T Dissimilar Steel Stack-ups for Automotive Applications

Resistance Spot Welding: 5T Dissimilar Steel Stack-ups for Automotive Applications

Urbanization and waning interest in vehicle ownership point to new transport opportunities in megacities around the world. Mobility as a Service (MaaS) – characterized by autonomous, ride-sharing-friendly EVs – can be the comfortable, economical, sustainable transport solution of choice thanks to the benefits that today’s steel offers.

The WorldAutoSteel organization is working on the Steel E-Motive program, which delivers autonomous ride-sharing vehicle concepts enabled by Advanced High-Strength Steel (AHSS) products and technologies.

The Body structure design for this vehicle is shown in Figure 1. It also indicates the specific joint configuration of 5 layers AHSS sheet stack-up as shown in Table 1. Resistance spot welding parameters were developed to allow this joint to be made by a single weld. (The previous solution for this welded joint is to create one spot weld with the bottom 3 sheets indicated in the table and a second weld to join the top 2 sheets, combining the two-layer groups to 5T stack-up.)

NOTE: Click this link to read a previous AHSS Insights blog that summarizes development work and recommendations for resistance spot welding 3T and 4T AHSS stack-ups: https://bit.ly/42Alib8

 

 

Table 1. Provided materials organized in stack-up formation showing part number, name, grade, gauge in mm, and coating type. Total thickness = 6.8 mm

 

The same approach of utilizing multiple current pulses with short cool time in between the pulses was shown to be most effective in this case of 5T stack-up. It is important to note that in some cases, the application of a secondary force was shown to be beneficial, however, it was not used in this example.

To establish initial welding parameters simulations were conducted using the Simufact software by Hexagon. As shown in Figure 2, the final setup included a set of welding electrodes that clamped the 5-layer AHSS stack-up. Several simulations were created with a designated set of welding parameters of current, time, number of pulses, and electrode force.

Figure 2. Example of simulation and experimental results showing acceptable 5T resistance spot weld (Meets AWS Automotive specifications)

 

 

Thanks is given to Menachem Kimchi, Associate Professor-Practice, Dept of Materials Science, Ohio State University and Technical Editor – Joining, AHSS Application Guidelines, for this article.

 

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: A Future Mobility Concept Paving the Way to Net Zero Emissions

Steel E-Motive: A Future Mobility Concept Paving the Way to Net Zero Emissions

Net Zero Emissions by 2050 – it’s a goal for future mobility that can seem distant and daunting. But over the past five years, WorldAutoSteel’s global automotive steel suppliers have conducted extensive research that illuminates a path forward. The Steel E-Motive concept – borne of this research – can be a catalyst for reaching the Net Zero goal.

Urbanization and changing attitudes towards vehicle ownership point to new transport opportunities in megacities worldwide. Mobility as a Service (MaaS) – characterized by autonomous, ride-sharing-friendly EVs – can be the comfortable, economical, and sustainable transportation solution of choice thanks to the benefits that modern steels offer, which will foster the higher vehicle occupancy that is critical to Net Zero ambitions.

Here, we break down the many benefits of the Steel E-Motive vehicle.

The Key Steel E-Motive Vehicle Features for Future Mobility

The Steel E-Motive Vehicle features seven key Advanced High-Strength Steel structural innovations to create a safe, economical vehicle.

  1. A B-Pillarless open-body structure offers excellent comfort, accessibility and easy ingress/egress.
  2. The Short Front Crash Zone design meets all global high-speed frontal crash requirements.
  3. The AHSS Extended Front Passenger Protection Zone provides excellent cabin intrusion protection for occupants.
  4. The Small Offset Crash Glance Beam minimizes the energy pulse into the occupant cabin, reducing the potential for passenger injuries.
  5. Hex beam energy absorbers provide superior battery protection for both side pole and deformable barrier crashes.
  6. The Scissor Door with Virtual B-Pillars offers excellent passenger visibility while saving mass and costs.
  7. The Coverless Battery Carrier Frame concept rewards 37% mass savings over benchmarks and 27% cost reduction; it also affords enhanced battery protection from road debris and other floor impacts.

The Steel E-Motive vehicle is created to meet Level 5 autonomy, meaning it is void of driver interfaces and does not require any human attention. With all of these features and more, the SEM architecture affords a spacious, safe, and comfortable cabin for occupants.

Three Steel E-Motive vehicles stopped at a crosswalk while pedestrians cross, overhead view

Steel E-Motive concepts are designed to help pave the way to a Net Zero future.

Exceeds Crash Guidelines

The Steel E-Motive vehicle is one of the world’s first autonomous vehicle concepts to validate and report excellent performance measured against the most stringent global crash requirements, which aligns with an IIHS “Good” rating. Modern Advanced High-Strength Steel product and fabrication process innovations enable the vehicle design to exceed these stringent crashworthiness standards while minimizing overall mass and production emissions.

Created to Be Affordable

Considering both production and life cycle costs, Steel E-Motive concepts have low maintenance requirements and are designed to be manufacturable using the world’s global manufacturing infrastructure at costs that support profitable margins, both for the vehicle manufacturer and the mobility service providers. Steel E-Motive is a fully engineered vehicle program that start-up companies can use to significantly reduce their cost and time to market.

Designed with Sustainability in Mind

The viability of any MaaS disrupter is contingent on cost competitiveness versus existing solutions, such as private ownership or taxis.

Moreover, our designs minimize steel thicknesses for lower mass while maximizing material utilization for lower steel production and emissions. Overall, the vehicle design offers the potential for ~86% CO2 emissions reduction when all factors contributing to sustainability are optimized. Autonomy further reduces operating emissions due to drive cycle smoothing.

To achieve our Net Zero future, high-occupancy vehicle usage is crucial and must be appealing for riders and profitable for providers.

Steel E-Motive concepts play a vital role in enabling Future Mobility Solutions THAT ONLY STEEL CAN MAKE REAL. Learn more about the program: https://steelemotive.world/

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.