Page 45 - Metalforming Magazine April 2022

In 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-partition- ing 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, steel- makers introduced hot stamping grades that do not harden during heat treat- ment. 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, appli- cations 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 absorp- tion. Mercedes used PQS to ensure pre- cise part dimensions for geometric ref- erencing. 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 elon- gation, 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 tem-
Fig. 2—Work- and bake-hardening effects on TBF980. After 5-percent pre-strain and paint baking, yield strength of TBF980 could exceed 1000 MPa, but the residual elonga- tion would be 11 percent.
Cutting Edge
0% PS +BH
As-delivered
2% PS +BH 5%
PS +BH
10% PS
+BH
1200
1000 800 600 400 200 0
0 0.02 0.04
0.06 0.08 0.1
True Strain (ɛ) [-]
0.12 0.14 0.16 0.18
1800 1600 1400 1200 1000
800 600 400 200
0
0 2 4 6 8 10 12 14 16 18 20 22 24
Engineering Strain (e) [%]
PHS 1500
PHS 1000
Q&P 1500
DP 980
PQS
550
DP
590
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MetalForming/April 2022 43
Fig. 3—Comparing hot- and cold-stamped steels at similar strength levels. Note that the properties of hot stamping grades result from forming and quenching, and properties of cold-forming grades are “as-delivered,” and may get stronger and less ductile from cold stamping.
pered flange from PQS steel provides improved crashworthiness, due to opti- mized weld quality. PHS 1500 also can- not 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 steelmak- ers have introduced new hot-formable grades, including a composite sand- wich material combining layers of PHS
1500 and PQS 550, and coating-free PHS1700. New hot-forming technolo- gies, developed to help reduce costs and improve productivity, include pre- cooled direct forming, multistep hot forming and vacuum hot forming.
Cold forming of even higher- strength steels also continues to devel- op, 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 Smart- form technology for example). MF
Engineering Stress (ơe) [MPa] True Stress (ơ) [MPa]