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Bake hardening steels

Description

The composition and processing of these steels are designed to promote a significant increase in yield strength during low-temperature heat treatment, particularly paint curing.

ArcelorMittal bake hardening steels can thus achieve higher strength in the finished part while retaining good forming performance. The gain in yield strength through the "bake hardening" (BH) effect is generally greater than 40 MPa. Thanks to this BH effect, ArcelorMittal steels offer two advantages compared to conventional drawing quality steels:

  • Improved dent resistance in all finished parts in the case of low forming strains (hood, roof, doors and wings)
  • Substantial weight reduction potential at equivalent dent resistance (the decrease in thickness is offset by increased yield strength resulting from the heat treatment process)

Bake hardening steels thus offer a suitable response to automotive bodywork requirements. By providing an excellent drawability-dent resistance combination, they enhance vehicle weight reduction and aesthetics.

Applications

Steels in the BH range are designed for visible (door, hood, tailgate, front wing, roof) and structural (underbody, reinforcement, cross member, lining) parts.

  • Bonnet in CR180BH

  • Door in CR240BH

  • Front longitudinal beam in CR270BH

Designation and standard

The table below summarizes the cross-references between ArcelorMittal product range, European standards and the VDA 239-100 standard.

The CR195BH grade refers to Japanese standards.

Show all
ArcelorMittal name Generic name Euronorms VDA 239-100 JFS ABNT NBR
CR180BH BH180 HC180B (+ZE) / HX180BD (+Z, +ZF, +ZM) CR180BH (-UNC,-EG,-GI,-GA,-ZM) BH-180 / ZARBH-180
CR195BH BH195
CR210BH BH210 / BH220 HC220B (+ZE) / HX220BD (+Z, +ZF, +ZM) CR210BH (-UNC,-EG,-GI,-GA,-ZM) JSC340H / JAC340H BH-210 / ZARBH-210
CR240BH BH240 / BH260 HC260B (+ZE) / HX260BD (+Z, +ZM) CR240BH (-UNC,-EG,-GI,-ZM) BH-240 / ZARBH-240
CR270BH BH270 / BH300 HC300B (+ZE) / HX300BD (+Z) CR270BH (-UNC,-EG,-GI) BH-270 - BH-300 / ZARBH-270 - ZARBH-300
  • Euronorms

    Uncoated (EN 10268: 2006 + A1: 2013): Steel grade name
    Electrogalvanized (EN 10268: 2006 + A1: 2013 + EN 10152: 2017): Steel grade name+ZE
    Galvannealed (EN 10346: 2015): Steel grade name+ZF
    Extragal®/Ultragal® for visible parts (EN 10346: 2015): Steel grade name+Z
    Zagnelis® (EN 10346: 2015): Steel grade name+ZM

  • VDA 239-100

    Uncoated: Steel grade name-UNC
    Electrogalvanized: Steel grade name-EG
    Galvannealed: Steel grade name-GA
    Extragal®/Ultragal® for visible parts: Steel grade name-GI
    Zagnelis®: Steel grade name-ZM

These cross-references are given for information only. ArcelorMittal BH grades generally offer guarantees of tighter mechanical properties (see table below).

Mechanical properties

Show all
ArcelorMittal name Generic name Direction Yield stress Rp0.2 (MPa) Tensile strength Rm (MPa) Type 1 Min. A50mm (%) Type 2 Min. A80mm (%) Type 3 Min. A50mm (%) Min. r0-20 Min. rm-20 Min. BH2 MPa n10-20/Ag
CR180BH BH180 RD 180 - 240 290 - 370 35 34 37 1.1 1.3 20/30 0.17
CR195BH BH195 TD 195 - 270 340 - 400 34 32 35 1.1 1.2 20/30 0.16
CR210BH BH210 / BH220 RD 210 - 270 320 - 400 34 32 35 1.1 1.2 20/30 0.16
CR240BH BH240 / BH260 RD 240 - 300 340 - 440 31 29 31 1 1.1 20/30 0.15
CR270BH BH270 / BH300 RD 270 - 330 360 - 460 29 27 29 20/30 0.13

A80mm %: Percentage elongation after fracture using a specimen with gauge length L0 = 80 mm (ISO 6892-1 type 2 (EN20x80))
A50mm %: Percentage elongation after fracture using a specimen with gauge length L0 = 50 mm (ISO 6892-1 type 1 (ASTM12.5x50) or type 3 (JIS25x50)
A%: Percentage elongation after fracture using a proportional specimen with L0 = 5.65 (So)1/2
Ag %: Percentage plastic extension at maximum force
BH2: Increase in yield strength between a reference condition after a 2% plastic pre-strain and the condition obtained after heat treatment (170°C-20minutes)

Microstructure of grade CR180BH

Microstructure of grade CR180BH

Chemical composition

Show all
ArcelorMittal name Generic name Max. C (%) Max. Si (%) Max. Mn (%) Max. P (%) Max. S (%) Min. Al (%) Max. Cu (%)
CR180BH BH180 0.06 0.5 0.7 0.06 0.025 ≥ 0.015 0.2
CR195BH BH195 0.06 0.5 0.7 0.08 0.025 ≥ 0.015 0.2
CR210BH BH210 / BH220 0.08 0.5 0.7 0.085 0.025 ≥ 0.015 0.2
CR240BH BH240 / BH260 0.1 0.5 1.0 0.1 0.03 ≥ 0.015 0.2
CR270BH BH270 / BH300 0.1 0.5 1.0 0.1 0.03 ≥ 0.015 0.2

Definition of BH2

"Bake hardening" is a controlled aging phenomenon related to the presence of carbon and/or nitrogen in solid solution in the steel. The BH2 parameter is used to evaluate the resulting increase in dent resistance. It is given by: BH2 = LYS - 2% PS, in which LYS is the lower yield stress measured after heat treatment and PS is the yield stress after initial 2% plastic pre-strain. BH2 measurement is a reliable and reproducible way to quantify the metal's ability to harden during cataphoresis.

The diagram below illustrates the Bake hardening mechanism and shows the displacement of carbon atoms in solution during heat treatment-typically 20 minutes at 170°C to block the dislocations generated by forming. This ultimately increases the metal's yield strength.

BH effect

Global availability


Under development     Customer trials     Commercial unexposed only     Commercial exposed and unexposed    

Show all
ArcelorMittal name Generic name Uncoated (-UNC) Extragal® (-GI) Ultragal® (-GI) Galvannealed (-GA) Zagnelis® Protect (-ZMP) Zagnelis® Surface (-ZMS) Electrogalvanized (-EG)
CR180BH BH180
CR195BH BH195
CR210BH BH210 / BH220
CR240BH BH240 / BH260
CR270BH BH270 / BH300
Show all
ArcelorMittal name Generic name Uncoated (-UNC) Extragal® (-GI) Ultragal® (-GI) Galvannealed (-GA) Zagnelis® Protect (-ZMP) Zagnelis® Surface (-ZMS) Electrogalvanized (-EG)
CR180BH BH180
CR195BH BH195
CR210BH BH210 / BH220
CR240BH BH240 / BH260
CR270BH BH270 / BH300
Show all
ArcelorMittal name Generic name Uncoated (-UNC) Extragal® (-GI) Galvannealed (-GA) Electrogalvanized (-EG)
CR180BH BH180
CR195BH BH195
CR210BH BH210 / BH220
CR240BH BH240 / BH260
CR270BH BH270 / BH300
Show all
ArcelorMittal name Generic name Uncoated (-UNC) Extragal® (-GI) Galvannealed (-GA)
CR180BH BH180
CR195BH BH195
CR210BH BH210 / BH220
CR240BH BH240 / BH260
CR270BH BH270 / BH300
Show all
ArcelorMittal name Generic name Galvannealed (-GA)
CR180BH BH180
CR195BH BH195
CR210BH BH210 / BH220
CR240BH BH240 / BH260
CR270BH BH270 / BH300
Show all
ArcelorMittal name Generic name Uncoated (-UNC) Extragal® (-GI) Galvannealed (-GA)
CR180BH BH180
CR195BH BH195
CR210BH BH210 / BH220
CR240BH BH240 / BH260
CR270BH BH270 / BH300

Forming

The bake hardening family of steels exhibits good drawability in all strain modes; its drawability is essentially equivalent to that of IF (interstitial-free) steels of similar yield strength.

The figure below shows examples of forming limit curves for the bake hardening family of steels in a thickness of 1.0 mm.

Forming limit curves for the BH steels family (ArcelorMittal model for Europe)

Forming limit curves for the BH steels family (Keeler model for North America)

ArcelorMittal has a database on the forming of BH grade steels. To integrate these steels at the design stage, a team of experts is available to perform specific studies based on modeling or shop-floor experience.

Welding

Due to their low alloy content, bake hardening steels can be readily welded by all conventional welding processes.

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