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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.

Hood in 180 BH

Door in 260 BH

Front longitudinal beam in 300 BH

Designation and standard

 EuronormsVDA 239-100
180 BHHC180B (+ZE) / HX180BD (+Z, +ZF, +ZM)CR180BH (-UNC,-EG,-GI,-GA,-ZM)
195 BH
220 BHHC220B (+ZE) / HX220BD (+Z, +ZF, +ZM)CR210BH (-UNC,-EG,-GI,-GA,-ZM)
260 BHHC260B (+ZE) / HX260BD (+Z, +ZM)CR240BH (-UNC,-EG,-GI,-ZM)
300 BHHC300B (+ZE) / HX300BD (+Z)CR270BH (-UNC,-EG,-GI)
 

Euronorms
Uncoated ( EN 10268: 2006+A1: 2013): Steel grade name
Electrogalvanized (EN 10268 :2006+A1: 2013 + EN 10152 :2009): 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

 
 Hot rolled     Cold rolled

While the ArcelorMittal grades conform perfectly well to the indicated EN standards, ArcelorMittal grades generally offer tighter mechanical properties (see table below).

The above indicative table summarizes the European and VDA standards corresponding to the ArcelorMittal product range.

The 195 BH grade corresponds to Japanese standards.

Guaranteed for ISO 20X80 test specimen in the transverse direction

 YS (MPa)UTS (MPa)ef (%)
L0 = 80 mm
th < 3 mm
rnBH2 (MPa)
 180 BH180 - 230300 - 360≥ 34≥ 1,6≥ 0,17≥ 35
 195 BH195 - 270340 - 400≥ 32≥ 1,3≥ 0,16≥ 35
 220 BH220 - 270340 - 400≥ 32≥ 1,5≥ 0,16≥ 35
 260 BH260 - 300370 - 430≥ 30≥ 0,15≥ 35
 300 BH300 - 360420 - 480≥ 28≥ 0,14≥ 40
 
 Hot rolled     Cold rolled

Chemical composition (%)

 CMnSi
 MaxMaxMax
180 BH0,040,700,50
195 BH0,060,700,50
220 BH0,060,700,50
260 BH0,080,800,50
300 BH0,100,700,50
 
 Hot rolled     Cold rolled

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

Available coatings and surface conditions

 UncoatedElectrogalvanizedExtragal®GalvannealedZagnelis®
 EURNAMSAMRSACHIEURNAMSAMRSACHIEURNAMSAMRSACHIEURNAMSAMRSACHIEURNAMSAMRSACHI
180 BH          
195 BH                  
220 BH           
260 BH            
300 BH                

 Hot rolled     Cold rolled

Available in non-visible part quality Undergoing customer testing Under development Available in visible and non-visible part quality (Z)

EUR : Europe Region - NAM : North America Region - SAM : South America Region - RSA : South Africa Region - CHI : China

For additional information about the availability of thin organic coatings for the ArcelorMittal range of BH steels, please refer to the coating technical sheets. Curing of these organic coatings may lead to changes in mechanical properties. 

Microstructure of grade 180 BH

Recommendations for use and secondary processing

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 bake hardening family of steels (thickness: 1.0 mm)

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

Weldability is determined by means of the method set out in the ISO 18278-2 standard.

Examples of resistance spot welding parameters for the bake hardening range of products:

 Welding range (kA)
180 BH Ez (thickness: 0.75 mm)1,6
180 BH nu (thickness: 0.75 mm)1,4
 
 Hot rolled     Cold rolled

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

ArcelorMittal can provide technical support in adjusting the welding parameters for any other product in its Bake Hardening range.

Fatigue strength

Fatigue strength can be expressed as an endurance limit (maximum stress).

 σD A 5.106 cycles (MPa)
180 BH uncoated after curing (thickness: 0.8 mm)334
260 BH uncoated after curing (thickness: 1.2 mm)384
 
 Hot rolled     Cold rolled

Examples of endurance limits at 5.106 cycles under tension-compressionfor a loading ratio of R = 0.1:

Wöhler curves for 180 BH and 260 BH steels after 2% elongation and heat treatment

ArcelorMittal can make available a comprehensive database covering the fatigue performance of its bake hardening steels.

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