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Eren Billur Eren Billur
Technical Manager

Heating Helps Forming, Part 1: Hot Stamping of High-Strength Steels

July 29, 2020

Hot stamping (commonly known as press hardening in Europe or hot press forming in Asia) is nothing new. During the process, a heated steel blank is formed at a relatively soft stage and then is quenched in a die. Hot stamping is performed using one of four processes:

  1. Fig. 1Direct process—The most common hot stamping process, direct hot stamping, or press hardening, finds use on Al-Si-coated or uncoated blanks. The undeformed blank is heated in a furnace over its austenitization temperature (Ac3), approximately 1450-1550 F (780-840 C). The heated blank then is formed (at ➋ in Fig. 1) and subsequently quenched in the same die.  
  2. Indirect process—Typically used for galvanized blanks, indirect hot stamping, or form hardening, starts by cold forming the part (at ❶ in Fig. 1). The part then is pierced and trimmed, almost to the final shape, heated in a special furnace and quenched in a second die set. 
  3. Hybrid process—Here, some of the forming occurs at the cold stage (❶). The semi-formed part then is heated in the furnace, significantly deformed to a final shape (➋) and quenched subsequently in the same die. This process typically has found use in Europe, with a special varnish-type coating. Since the early 2010s, the process has been abandoned and replaced by the direct process on Al-Si-coated blanks. 
  4. Multistep process—For use on air-hardening steels, multistep hot stamping occurs in a transfer press (preferably servo-driven) and then the formed part is air-quenched. At least one OEM in Germany uses this process on galvannealed parts. This is the most recent process route in hot stamping.

Hot Stamping of 22MnB5

The uncoated ultra-high-strength steel originally designed for the hot forming process is 22MnB5 (also called PHS 1500 or HPF 1470). It has a yield strength of approximately 1000 MPa, and a 1500-MPa tensile (145,000 and 215,000 psi respectively) strength after hot stamping. These grades have a minimum 5-percent total elongation, depending on the type of tensile test.

Until 1997, stampers could only obtain 22MnB5 as uncoated bare steel. Then, steelmakers introduced an aluminum-silicon coated grade, and since then, the use of hot stamped components has increased exponentially. During the 1990s, hot stamping in the automotive industry only found use for bumper beams and door beams. However, by 2010 many car models featured hot stamped B-pillars, and some European models had more than 10 percent of their body-in-white hot stamped (in terms of mass, excluding doors and closures). Today, several models, including the latest-generation Ford Focus, have more than 30 percent of their body-in-white mass hot stamped. 

Recent Steel Developments

  • Fig. 2Energy-absorbing steel grades—Since their first use, press-hardening steel (PHS) grades were used in the safety cage and in areas requiring high yield strength. Car bodies require deformation resistance (i.e., high yield strength) and the ability to absorb energy (i.e., elongation times strength). Initially, steelmakers focused on developing grades with 450-600-MPa tensile strength (65,000-87,000 psi), with a minimum 13 to 16-percent total elongation after quenching, depending on the strength level. Since these grades were not hardened in the press, they became known as press-quenched steels (PQS).
  • Since the late 2010s, steelmakers have introduced a second PQS family: grades with a yield strength greater than 800 MPa and as high as 1000 MPa (116,000 to 145,000 psi), after bake-hardening treatment. Their tensile strength, on the other hand, ranges from 1000 to 1300 MPa (145,000 to 188,000 psi), and they have a slightly higher total elongation (minimum of 6 percent), compared to PHS1500.New standards for PHS/PQS.

One may ask: What is the benefit of using PQS1000 or PQS1300, compared to PHS1500, since it offers only a slight upgrade in total elongation? In 2010, the German Association of the Automotive Industry (VDA) published a new test standard, VDA 238-100, later updated in 2017. This test calls for bending hardened or quenched samples with a very tight radius, until fracture. The outer angle of bending (α in Fig. 2) provides a measure of energy-absorbing capacity. So, while PQS1000 grades have a yield strength similar to that of PHS1500, they have a higher bending angle.

VDA, in 2016, standardized cold-forming steel grades with the publication of VDA 239-100, “Sheet Steel for Cold Forming,” and some European OEMs switched to this designation. VDA has begun working on a draft of VDA 239-500, “Sheet Steel for Hot Forming,” to standardize PQS and PHS grades.

  • Higher-strength gradesSince 2010, a Japanese OEM has been using 1800-MPa (260,000-psi) tensile-strength steel (30MnB5) in its bumper beams. Compared to PHS1500, 30MnB5 steel can reduce part weight by 12.5 percent. However, these steels can exhibit delayed cracking, or hydrogen embrittlement, and steelmakers are developing high-carbon grades with special coatings and using process parameters designed to reduce hydrogen diffusion to the base steel.

In 2019, an electric car debuted in Germany with 1900-MPa (275,000-psi) tensile-strength steel components in its underbody, around the battery. And, some studies show that carmakers can use 2000-MPa (290,000-psi) grades to protect the battery in pole-crash conditions.

A Look at the Future

Most experts in higher-strength hot stamped steels predict that the use of PHS and PQS grades will continue to increase.  With electrification (plug-in hybrid or battery-electric vehicle), lightweighting has become increasingly important. Although third-generation cold-formable advanced high-strength steels (AHSS) seem to challenge PHS grades in some applications, in most cases PHS still offers better formability in the press shop, and higher strength after quenching.

Some OEMs are trying to reduce the use of PHS through roll forming or cold forming of AHSS grades with tensile strengths exceeding 1200 MPa (174,000 psi). However, in both cases, attempts are limited to simpler geometries such as roof rails and rocker reinforcements, while PHS and PQS get the call for complex components such as A- and B-pillars and front and rear rails.

Most OEMs continue to increase their use of hot stamped components, and with electrification we may see even more components hot stamped. When PQS1000-1300 grades become available, they will offer improved spot weldability, as well as added energy-absorbing capacity, compared to PHS1500, at a similar yield strength. And, PHS1800-2000 grades may offer weight savings as great as 15 percent compared to PHS1500.

For more information on hot stamping, refer to the author’s book, Hot Stamping of Ultra High-Strength Steels;

Industry-Related Terms: Roll Forming, Tensile Strength, Transfer, Weldability, Hydrogen Embrittlement, Quenching, Bending, Blank, Die, Form, Forming
View Glossary of Metalforming Terms


See also: Billur Metal Form

Technologies: Materials


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