Page 47 - MetalForming February 2022 Special Automotive Issue

Cutting Edge
Strength kgf/mm2
Strength, MPa, rounded to nearest 10
Strength, MPa, rounded to nearest 50
Approximate Strength, ksi
Example grades
45
440
450
65
FB440 / FB450
60
590
600
85
DP590 / DP600
70
690
700
100
TRIP690 / TRIP700
80
780
800
115
DH780 / DP800HD
100
980
1000
145
CP980 / CP1000
120
1180
1200
170
MS1180 / MS1200
highly alloyed with a nearly 100-per- cent austenitic microstructure as- delivered. Note that the use of 2nd Gen AHSS has been low and continues to diminish over time. Automotive austenitic stainless steels are named based on their yield strength, while the names of TWIP steels reflect their tensile strength.
Lastly, 3rd Gen AHSS, still being developed, include TRIP-aided bainitic ferrite (TBF, or carbide-free bainite, CFB), quenching and partitioning (Q & P) steels, dual-phase high-ductility (DP-HD or DH), and complex phase- high ductility (CP-HD or CH). Steel- makers and automotive OEMs use these terms interchangeably. But note: The abbreviation DH was used in the SAE standard for 1st Gen DP steels with high yield strength, so be cautious when referring to these grades.
Table 1 shows the naming systems for various grades used by automotive OEMs and standards-writing bodies. For example, the generic DP590 steel may be named:
• SAE J2745 DP590T/340Y (where T signifies tensile strength and Y yield strength);
• EN 10338 HCT590X (H signifies high strength, C cold rolled, T tensile strength, X dual phase;
• VDA239-100 CR330Y590T-DP (cold rolled 330-MPa minimum yield, 590- MPa minimum tensile strength, dual phase; or
• JFS A2001 JSC590R (cold rolled, 590-MPa minimum tensile strength, and R signifies dual phase).
590 or 600? 780 or 800? Why?
Some people refer to DP590 steels as DP600, and some refer to DP780 as DP800. This rounding occurs because the earlier SAE norm ( J2340-1999) referred to strength levels of 600, 800 and 1000 MPa. Now, most steel stan- dards list strength levels as 590, 780, 980 and 1180 MPa. One may argue that DP590 and DP600 are not the same, claiming theoretically that a coil with 595-MPa tensile strength would qualify to be named DP590 but not DP600. Even though some OEMs still use 600,
Table 2—AHSS strength levels and the logic behind their naming (a similar table can be found in AHSS Guidelines V7.0.)
Standard
Name
Yield
Strength
(Rp0.2 or ReL), MPa
UTS (Rm), MPa
Total Elongation (percent)
n-value
r-value
ISO (A80)
ASTM or JIS (A50)
EN10268
HC180B
180-230
290-360
≥34
-
n90≥0.17
r90≥1.6
VDA239-100
CR180B H
180-240
290-370
≥34
≥35
n0≥0.17
r0≥1.1 rm≥1.3
SAE J2340
180B
≥180
>300
-
-
n0≥0.19
-
JFS A2001
JSC340H
≥165
>340
-
>34
-
rm ≥ 1.4
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Table 3—Bake-hardenable steel with approximately 180-MPa yield and 300-MPa tensile strength, from different parts of the world.
800 and 1000 in their steel names, on the spec sheet their minimum tensile- strength requirements still are 590, 780 and 980 MPa.
So, in practice it still is correct to use 590, 690, 780, 980 and 1180. They seem arbitrarily chosen, but when divided into the gravity constant (9.81 N/kg) the logic behind these numbers emerges (Table 2). In some parts of Asia, steels are referred to using a num- ber based on kgf/mm2. For example, a TRIP 980 steel may be called 100K TRIP. This is especially confusing for Americans, as it may be understood as 100-ksi TRIP steel, equivalent to TRIP 690.
Steel Equivalency Tables: Avoid Using Them!
Note that for material testing, dif- ferent standards bodies (SAE, EuroNorm [EN], VDA, JFS or OEMs)
may use different specimen types and experimental conditions. Therefore, the total elongation of a European steel-mill certificate may not be com- parable to one from the United States ( Table 3). Europeans typically use ISO Type II tensile specimens, with 80-mm gauge length and 20-mm specimen width. In the United States, ASTM E8 dictates 50-mm gauge length with 12.5- mm width. And in Japan, JIS specimens call for 50-mm gauge and 25-mm spec- imen width.
Several global OEMs specify total elongation requirements based on dif- ferent standards, typically abbreviated as A80 and A50. A80 is used for ISO or EN standards, whereas A50 may be used for JIS and ASTM, using 50-mm gauge length. Since the specimen geometries differ, the total elongation measured from an ASTM specimen may differ from that of a JIS specimen.