Page 48 - MetalForming February 2022 Special Automotive Issue
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Cutting Edge
Bake-Hardening Effect
LSS
HSS
AHSS
3rd Gen
Mild
HS-IF
DP
DP-HD (DH)
IF
HSLA
CP
TBF (CFB)
CMn
FB Q&P
BH TRIP
MART
PHS / PQS
Table 4—Steels that exhibit bake-hardening effect. Contrary to common belief, bake- hardening effect is not exclusive to BH steels.
tion hardening, bake hardening or microalloying (in HSLA).
2. 1st Gen AHSS are martensitic and multiphase, and strength-ductility depends on the phase fractions and composition.
3. There is no clear consensus on a UHSS definition. Previously, SAE defined DP600 (and yes, it used the 600 designation) as UHSS. Currently, the widely accepted value for UHSS is minimum 980-MPa tensile strength.
Note that the main criterion for a steel to be classified as AHSS is not minimum yield or tensile strength, but the microstructure. For example, in the VDA239-100 standard HR700LA—a fer- ritic steel with 700-MPa minimum yield and 750-MPa minimum tensile strength—should be classified as HSS. And, CR290Y490T-DP has 290-MPa minimum yield and 490-MPa mini- mum tensile strength but is classified as AHSS because it is multiphase (fer- ritic and martensitic).
Bake Hardening and TRIP Effects
Bake-hardening (BH) steels is a baf- fling name, as most people in the industry and academia believe that only BH steels harden during paint baking after deformation. BH steels (Table 4) are the only grades in the HSS family to exhibit the bake-hardening effect. All AHSS, including Gen 3 steels, have even higher (between 40 and 100 MPa) bake-hardening indexes, com- pared to BH steels (typically 25 to 40 MPa).
Within the 1st Gen AHSS, retained austenite is only found in TRIP steels. When austenite is deformed, it may transform to martensite. This increases the strain-hardening rate, and thus improves formability. This is called TRIP effect. In 2nd Gen AHSS, austenitic stainless steel can be nearly entirely austenitic. 3rd Gen AHSS also rely on the TRIP effect for improved formability. So far, all commercially available 3rd Gen AHSS have retained austenite and thus the TRIP effect, as shown in Table 5. MF
TRIP effect
LSS
HSS
AHSS
3rd Gen
Mild
HS-IF
DP
DP-HD (DH)
IF
HSLA
CP
TBF (CFB)
BH
FB
Q&P
TRIP
MART
PHS / PQS
Table 5—Steels that exhibit TRIP effect. Some 2nd Gen steels and all 3rd Gen steels have TRIP effect.
Also note in Table 3 that n and r val- ues may be in different directions: 0 deg. means longitudinal to rolling direction while 90 deg. means trans- verse. rm is the average of all r values:
rm = (r0 + 2r45 + r90)/4
Creating a mill certificate for the steels listed in Table 3 may require dif- ferent specimen types cut from differ- ent directions. To prepare certs for JFS and VDA, the steelmaker would need to perform at least three tests from all directions specified in the equation above—rolling (0 deg.), diagonal (45 deg.) and transverse (90 deg.). Without these required experimental values, you cannot say that an HC180B is equivalent to CR180BH.
VDA239-100 specified tensile testing of HSLA grades in the longitudinal direction, whereas the EuroNorm stan- dard called for testing in the transverse direction. One German steelmaker sells HX340LAD (coated HSLA with mini- mum 340-MPa yield strength in the transverse direction) in EuroNorm as
CR300LA in VDA239-100 (minimum 300-MPa yield strength in the longitu- dinal direction). Thus, EN 10346 HX340LAD is not equivalent to VDA239-100 CR340LA, as most equiv- alency spreadsheets claim.
As seen in several examples, I do not advise using the steel-equivalency tables found online, as most of them fail to catch some important details. Rather than “equivalent,” the correct word should be “similar.”
Deciding if a Grade is HSS, AHSS or UHSS
I hear this question regularly. In the past, several steel authorities developed global formability diagrams (common- ly known as banana curves), with threshold yield- and/or tensile-strength values for HSS, AHSS and UHSS. This created confusion among users.
Steels typically are classified based on their microstructure and the hard- ening mechanisms used:
1. Conventional HSS are ferritic only and are hardened through solid solu-
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