Cold Forming Archives - AHSS Guidelines

Cold Stamped or Hot Formed? Part II

Grade Options and Corrosion Protection Considerations When Deciding How A Part Gets Formed

Automakers contemplating whether a part is cold stamped or hot formed must consider numerous ramifications impacting multiple departments. Our prior blog on this topic highlighted the equipment differences and the property development differences between each approach. We continue this blog series, now focusing on grade options and corrosion protection.

The discussions below relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid-formed.

Grade Options for Cold Stamped or Hot Formed Steel

There are two types of parts needed for vehicle safety cage applications: those with the highest strength that prevent intrusion, and those with some additional ductility that can help with energy absorption. Each of these types can be achieved via cold stamping or hot stamping.

When it comes to cold stamped parts, many grade options exist at 1000 MPa that also have decent ductility. The advent of the 3rd Generation Advanced High Strength Steels adds to the tally. Most of these top out at 1200 MPa, with some companies offering cold-formable Advanced High Strength Steels with 1400 or 1500 MPa tensile strength. The chemistry of AHSS grades is a function of the specific characteristics of each production mill, meaning that OEMs must exercise diligence when changing suppliers.

Figure 1: Stress-strain curve of industrially produced QP980.^W-35

Martensitic grades from the steel mill have been in commercial production for many years, with minimum strength levels typically ranging from 900 MPa to 1470 MPa, depending on the grade. These products are typically destined for roll forming, except for possibly those at the lower strengths, due to limited ductility. Until recently, MS1470, a martensitic steel with 1470 MPa minimum tensile strength, was the highest strength cold formable option available. New offerings from global steelmakers now include MS1700, with a 1700 MPa minimum tensile strength, as well as MS 1470 with sufficient ductility to allow for cold stamping. Automakers have deployed these grades in cold stamped applications such as crossmembers and roof reinforcements.

Figure 2: Cold-Stamped Martensitic Steel with 1500 MPa Tensile Strength used in the Nissan B-Segment Hatchback.^K-57

Until these recent developments, hot stamping was the primary option to reach the highest strength levels in part shapes having even mild complexity. Under proper conditions, a chemistry of 22MnB5 could routinely reach a nominal or aim strength of 1500 MPa, which led to this grade being described as PHS1500, CR1500T-MB, or with similar nomenclature. Note that in this terminology, 1500 MPa nominal strength typically corresponds to a minimum strength of 1300 MPa.

The 22MnB5 chemistry is globally available, but the coating approaches discussed below may be company-specific.

Newer PHS options with a modified chemistry and subsequent processing differences can reach nominal strength levels of 2000 MPa. Other options are available with additional ductility at strength levels of 1000 MPa or 1200 MPa. A special class called Press Quenched Steels have even higher ductility with strength as low as 450 MPa.

The spectrum of grades available for cold-stamped and hot formed steel parts allows automakers to fine-tune the crash energy management features within a body structure, contributing to steel’s “infinite tune-ability” capability which gives automotive engineers design flexibility and freedoms not available from other structural materials.

Corrosion Protection

Uncoated versions of a grade must take a different chemistry approach than the hot dip galvanized (GI) or hot dipped galvannealed (HDGA) versions since the hot dip galvanizing process acts as a heat treatment cycle that changes the properties of the base steel. Steelmakers adjust the base steel chemistry to account for this heat treatment to ensure the resultant properties fall within the grade requirements.

Figure 3: Schematic of a typical hot-dipped galvanizing line with galvanneal capability.

This strategy has limitations as it relates to grades with increasing amounts of martensite in the microstructure. Complex thermal cycles are needed to produce the highly engineered microstructures seen in advanced steels. Above a certain strength level, it is not possible to create a GI or HDGA version of that grade.

For example, when discussing fully martensitic grades from the steel mill, hot dip galvanizing is not an option. If a martensitic grade needs corrosion protection, then electrogalvanizing is the common approach since an EG coating is applied at ambient temperature, which is low enough to avoid negatively impacting the properties. Automakers might choose to forgo a galvanized coating if the intended application is in a dry area that is not exposed to road salt.

Figure 4: Schematic of an electrogalvanizing line.

For press hardening steels, coatings serve multiple purposes. Without a coating, uncoated steels will oxidize in the austenitizing furnace and develop scale on the surface. During hot stamping, this scale layer limits efficient thermal transfer and may prevent the critical cooling rate from being reached. Furthermore, scale may flake off in the tooling, leading to tool surface damage. Finally, scale remaining after hot stamping is typically removed by shot blasting, an off-line operation that may induce additional issues.

Using a hot dip galvanized steel in a conventional direct press hardening process (blank -> heat -> form/quench) may contribute to liquid metal embrittlement (LME). Getting around this requires either changing the steel chemistry from the conventional 22MnB5 or using an indirect press hardening process that sees the bulk of the part shape formed at ambient temperatures followed by heating and quenching.

Those companies wishing to use the direct press hardening process can use a base steel having an aluminum-silicon (Al-Si) coating, providing that the heating cycle in the austenitizing furnace is such that there is sufficient time for alloying between the coating and the base steel. Welding practices using these coated steels need to account for the aluminum in the coating, but robust practices have been developed and are in widespread use.

For more information about PHS grades and processing, see our Press Hardened Steel Primer.

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.

Cold Stamped or Hot Formed? Part 1

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Equipment, Responsibilities, and Property Development Considerations When Deciding How A Part Gets Formed

Automakers contemplating whether a part is cold stamped or hot formed must consider numerous ramifications impacting multiple departments. Over a series of blogs, we’ll cover some of the considerations that must enter the discussion.

The discussions relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid-formed.

Equipment

There is a well-established infrastructure for cold stamping. New grades benefit from servo presses, especially for those grades where press force and press energy must be considered. Larger press beds may be necessary to accommodate larger parts. As long as these factors are considered, the existing infrastructure is likely sufficient.

Progressive-die presses have tonnage ratings commonly in the range of 630 to 1250 tons at relatively high stroke rates. Transfer presses, typically ranging from 800 to 2500 tons, operate at relatively lower stroke rates. Power requirements can vary between 75 kW (630 tons) to 350 kW (2500 tons). Recent transfer press installations of approximately 3000 tons capacity allow for processing of an expanded range of higher strength steels.

Hot stamping requires a high-tonnage servo-driven press (approximately 1000 ton force capacity) with a 3 meter by 2 meter bolster, fed by either a roller-hearth furnace more than 30 m long or a multi-chamber furnace. Press hardened steels need to be heated to 900 °C for full austenitization in order to achieve a uniform consistent phase, and this contributes to energy requirements often exceeding 2 MW.

Integrating multiple functions into fewer parts leads to part consolidation. Accommodating large laser-welded parts such as combined front and rear door rings expands the need for even wider furnaces, higher-tonnage presses, and larger bolster dimensions.

Blanking of coils used in the PHS process occurs before the hardening step, so forces are low. Post-hardening trimming usually requires laser cutting, or possibly mechanical cutting if some processing was done to soften the areas of interest.

That contrasts with the blanking and trimming of high strength cold-forming grades. Except for the highest strength cold forming grades, both blanking and trimming tonnage requirements are sufficiently low that conventional mechanical cutting is used on the vast majority of parts. Cut edge quality and uniformity greatly impact the edge stretchability that may lead to unexpected fracture.

Responsibilities

Most cold stamped parts going into a given body-in-white are formed by a tier supplier. In contrast, some automakers create the vast majority of their hot stamped parts in-house, while others rely on their tier suppliers to provide hot stamped components. The number of qualified suppliers capable of producing hot stamped parts is markedly smaller than the number of cold stamping part suppliers.

Hot stamping is more complex than just adding heat to a cold stamping process. Suppliers of cold stamped parts are responsible for forming a dimensionally accurate part, assuming the steel supplier provides sheet metal with the required tensile properties achieved with a targeted microstructure.

Suppliers of hot stamped parts are also responsible for producing a dimensionally accurate part, but have additional responsibility for developing the microstructure and tensile properties of that part from a general steel chemistry typically described as 22MnB5.

Property Development

Independent of which company creates the hot formed part, appropriate quality assurance practices must be in place. With cold stamped parts, steel is produced to meet the minimum requirements for that grade, so routine property testing of the formed part is usually not performed. This is in contrast to hot stamped parts, where the local quench rate has a direct effect on tensile properties after forming. If any portion of the part is not quenched faster than the critical cooling rate, the targeted mechanical properties will not be met and part performance can be compromised. Many companies have a standard practice of testing multiple areas on samples pulled every run. It’s critical that these tested areas are representative of the entire part. For example, on the top of a hat-section profile where there is good contact between the punch and cavity, heat extraction is likely uniform and consistent. However, on the vertical sidewalls, getting sufficient contact between the sheet metal and the tooling is more challenging. As a result, the reduced heat extraction may limit the strengthening effect due to an insufficient quench rate.

For more information, see our Press Hardened Steel Primer to learn more about PHS grades and processing!

Thanks are given to Eren Billur, Ph.D., Billur MetalForm for his contributions to the Equipment section, as well as many of the webpages relating to Press Hardening Steels at www.AHSSinsights.org.