Eren Billur Eren Billur
Technical Manager

Cold Forming of 1.5-GPa Steel: Is it Feasible?

February 8, 2023

For many years, one of the strongest automotive steels was 22MnB5, a manganese-boron grade developed specifically for hot stamping applications. The steel also is known as PHS 1500 (press-hardened with 1500-MPa tensile strength), and in Europe a new nomenclature has been agreed upon by automakers to refer to the steel as CR1500T-MB—cold-rolled, 1500-MPa tensile strength, manganese-boron alloyed. 

Fig. 1 High strength cold stampedAs discussed in a previous Cutting Edge column (To Heat or Not to Heat, April 2022), automotive OEMs and Tier suppliers long have compared the benefits and the costs of the hot stamping and cold stamping processes—and some still do so. Until a decade ago, cold forming only applied to steels with a maximum tensile strength of 980 MPa. Higher-strength materials only could be stamped into simple shapes such as door beams, or rollformed to constant cross sections—roof crossmembers or rocker reinforcements, for example. 

Advancements in steelmaking have led to the development of the so-called 3rd Gen advanced high-strength steels (AHSS):

  • TBF—TRIP-aided bainitic ferrite 
  • DH/DP-HD—Dual-phase high-ductility
  • Q&P—Quenched and partitioned
  • TM—Tempered martensitic.

Hot and cold stamping comparedThese grades made cold forming of higher-strength steels possible. In 2013, stampers began to cold-stamp A-pillars from TBF1180 steel, a record surpassed in 2019 with 1320-MPa TM steel. (Note: You may have read this as 1310 MPa in several online sources, although this grade has a tensile strength of 135 kgf/mm2, which should be rounded to 1320 MPa; at least one steelmaker calls this grade 1320.)

In December 2020, the first use of cold-stamped 1470-MPa TM steel was seen in a seat crossmember, and by mid-2022 A-pillar reinforcements were being stamped from 1470-MPa TM steel. 

Advantages of Cold Stamping

Fig 3 Springback of stamped channelCold stamping gains favor for its (highly believed) lower costs. Most Tier suppliers and OEMs already have invested in cold stamping lines—transfer or tandem-transfer. Piercing and trimming traditionally occurs within the press, reducing the need for large capital investments and floorspace requirements for the laser trimming lines that typically accompany hot stamping lines. 

In recent applications with higher-strength steels, higher-tonnage presses get the call. For example, one supplier reports that it has stamped DP780 steel with 65 percent of its rated press-tonnage capacity, but when the part was modified to TM1320 steel, the press was not able to stamp the piece—doing so required 107-percent press capacity. 

In addition, a European press maker recently stated that its sales of transfer presses rated more than 2500 metric tons recently have been skyrocketing. These presses, whether flywheel or servomechanical, have high energy consumption. However, the overall energy required to run the line may still be significantly lower than for a hot stamping line when considering the energy consumption of the furnace.

Advantages of Hot Stamping, or Disadvantages of Cold Stamping

New cold-stamped steel grades require significant capital investment for the mill, along with a complicated thermomechanical processing route to achieve high strength and elongation. Hot-stamped steel, on the other hand, per ton can cost considerably less than 3rd Gen AHSS, especially if uncoated. 

When strength level increases, many believe that springback can become very hard to control, but most tool makers now rely on sheet metal-forming simulation for springback predictions that include the required kinematic hardening models. Even with the Yoshida-Uemori model (see earlier Cutting Edge articles for details), one OEM reports that only 55 percent of a simulated part fell within ±1-mm deviation with the real part. For 1180-MPa and higher-strength steels, many tool makers also consider the elastic deflections of the tools. When running a coupled simulation including elastic deformation of the tools, the match rate (the area within 1-mm deviation) increased from 55 percent to 93 percent. 

Part productivity between hot and cold stamping depends on the part dimensions and thickness. In an ideal case, hot stamping lines may run at speeds to 24 parts/min.—in the range of some cold stamping lines. In realistic conditions, most often cold stamping will be more productive. Lastly, die wear, galling and die-cracking problems may increase during cold stamping as steel strength rises. MF

Industry-Related Terms: Case, Die, Edge, Forming, LASER, Lines, Model, Piercing, Run, Tensile Strength, Thickness, Transfer
View Glossary of Metalforming Terms


See also: Billur Metal Form

Technologies: Materials


Must be logged in to post a comment.
There are no comments posted.

Subscribe to the Newsletter

Start receiving newsletters.