Eren Billur Eren Billur
Technical Manager

To Heat or Not to Heat

April 1, 2022

For many years, metal forming professionals have discussed hot stamping vs. cold, and where Tier suppliers should invest. Hot stamping requires high capital expenditure, including typically a high-tonnage (800 to 1200 tons) hydraulic press with 3 by 2-m bolster, a roller-hearth furnace more than 30 m long or a multi-chamber furnace, and associated automation equipment. These lines are energy-intensive with installed power capacities often exceeding 2 MW. Recently, even wider furnaces and higher-tonnage presses with larger bolster dimensions have become the norm, to accommodate large laser-welded parts such as door rings. Typical cycle times can range from 8 to 20 sec.—considered inefficient by some. However, with two- or four-out systems, productivity caCutting Edge Fig. 1n approach that of a cold-stamping line. 

On the other side of the spectrum, cold forming typically occurs in a progressive- or transfer-die press, servo or conventional mechanical drive. 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). Recently, we’ve seen European companies installing transfer presses of 3000 tons or higher capacity. Steels for cold forming can range from the softest interstitial-free grades with ultimate tensile strength (UTS) as low as 250 MPa, to 3rd Gen advanced high-strength steels (AHSS) with 1500-MPa UTS. 

In early 2000s, the only available steel for hot stamping was Type 22MnB5, commonly referred to as PHS 1500. Initially, the grade was nominated with a minimum 1000-MPa yield strength (YS) and a minimum 1500-MPa UTS, although many OEMs now classify this material with a minimum YS of 950 MPa and UTS of 1300 or 1350 MPa. 

PHS Unrivaled, Until Cold-Formable Giga-Pascal Steels

Cutting Edge Fig. 2When the 7th-generation Honda Civic rolled out in 2001, it was the first Honda to contain DP590 AHSS in its body-in-white. In 2005, the Honda Odyssey featured a rollformed martensitic rear bumper beam with a UTS of 1300 MPa. In 2007, several Honda and Acura models started using TRIP780 and DP980 steels. That same year Honda began using hot-stamped steel. During the mid-2000s to early 2010s, many OEMs sought alternative cold-formable grades at relatively high strength to replace hot-stamped components. 

Ten years ago, I counted approximately 160 hot stamping lines around the world. Stamping-industry professionals were discussing the more-formable AHSS grades on the horizon, promising tensile strength of 1.0 to 1.2 GPa. Around this time, Volvo presented a study in Germany showing that a DP980 steel could have a higher YS than PHS 1500 after cold forming and bake hardening, perhaps making hot stamping no longer feasible. However, during the last 10 years, the number of hot stamping lines worldwide actually has more than quadrupled. 

In 2017, I, along with a group of researchers, completed a study similar to Volvo’s with a 3rd Generation TBF (TRIP-aided bainitic ferrite). Since the material had nearly twice the elongation of DP980, we strained it to 10 percent and bake-hardened it. With 2 percent or more pre-strain (Fig. 2), the material could easily have a yield strength of 1000 MPa after paint baking, almost on par with PHS 1500. 

Cuting Edge Fig. 3In 2013, Nissan became the first automaker to use 3rd Generation AHSS in its body-in-whites, including use of TBF1180 in several components of the Infiniti Q50. In 2019 Mazda used 1310-MPa cold-stamped steel, surpassed by Nissan in 2021 when it used 1500-MPa cold-stamped quenching-and-partitioning steel (Fig. 3). (Controlling springback with these high-strength cold-formed grades required significant investment.)

Meanwhile, Mazda became the first to use hot-stamped PHS 1800, as early as 2011. Around that same time, steelmakers introduced hot stamping grades that do not harden during heat treatment. These press-quenched steels (PQS), when hot-stamped, do not have a fully martensitic microstructure. Initial uses were limited to laser-welded blanks in B-pillars and front or rear rails, applications requiring strength and ductility in different areas of the part. 

PQS Replaces Dual Phase

In 2014, Mercedes rolled out its 4th generation C-Class vehicle, with several components made of PHS 1500 and laser-welded blanks of PHS 1500 and PQS550. Until this time, many OEMs and Tier suppliers seemingly used PHS for strength and PQS for energy absorption. Mercedes used PQS to ensure precise part dimensions for geometric referencing. When using DP or TRIP steels, mechanical properties of the steel would vary, affecting springback and geometry. When using PQS550, parts had very high repeatability. In addition, cold-forming DP steels reduces their residual elongation (Fig. 2). As-delivered TBF980 steel has 16-percent total elongation, while adding 5-percent pre-strain results in 12-percent residual elongation. However, a hot formed and quenched PQS has all of the elongation shown in Fig. 3 nearly everywhere in the formed part. Finally, hot forming PQS550 optimizes ductility for crash-energy absorption (Fig. 2).

PQS grades also provide improved weldability compared to PHS 1500, which has a high carbon equivalent that makes resistance welding difficult. Many studies have shown that a tempered flange from PQS steel provides improved crashworthiness, due to optimized weld quality. PHS 1500 also cannot be mechanically fastened using rivets or flow-drill screws, providing another application for PQS. Case in point: the Jaguar I-PACE, with a B-pillar made of a patchwork blank with a PQS450 master—easily fastened mechanically to the aluminum BIW—and a PHS 1500 patch blank. Similar strategies also are used with PHS 1000 by some OEMs on newer models. 

What May be Next?

During the last few years steelmakers have introduced new hot-formable grades, including a composite sandwich material combining layers of PHS 1500 and PQS 550, and coating-free PHS1700. New hot-forming technologies, developed to help reduce costs and improve productivity, include precooled direct forming, multistep hot forming and vacuum hot forming. 

Cold forming of even higher-strength steels also continues to develop, as many Tier One suppliers have invested in larger and higher-tonnage transfer presses. Many studies illustrate success with cold forming of 1180- and 1500-MPa UTS steels. 3rd Generation AHSS, especially the Q&P processed grade, offer improved formability; and new cold-forming methods have been developed to control or even eliminate springback (ThyssenKrupp’s Smartform technology for example). MF

Industry-Related Terms: Blank, Case, Ductility, Flange, Forming, Hydraulic Press, Lines, Point, Spectrum, Stroke, Tensile Strength, Transfer, Weldability
View Glossary of Metalforming Terms

Technologies: Stamping Presses


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