عملیات سخت کاری پخت فولاد،کتاب،Bh،BAKE HARDENING

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.

Designation and standard

	EN 10268 :2006 (uncoated)	EN 10268 :2006 + EN 10152 :2009 (electrogalvanized)	EN 10346 :2009 (Extragal®/Galvannealed)
180 BH	HC180B	HC180B+ZE	HX180BD+Z
195 BH
220 BH	HC220B	HC220B+ZE	HX220BD+Z
260 BH	HC260B	HC260B+ZE	HX260BD+Z
300 BH	HC300B	HC300B+ZE	HX300BD+Z
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 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	r		n		BH2 (MPa)
180 BH	180 - 230	300 - 360	≥ 34	≥ 1,6		≥ 0,17		≥ 35
195 BH	195 - 270	340 - 400	≥ 32	≥ 1,3		≥ 0,16		≥ 35
220 BH	220 - 270	340 - 400	≥ 32	≥ 1,5		≥ 0,16		≥ 35
260 BH	260 - 300	370 - 430	≥ 30			≥ 0,15		≥ 35
300 BH	300 - 360	420 - 480	≥ 28			≥ 0,14		≥ 40
Hot rolled     Cold rolled

Chemical composition (%)

	C	Mn	Si
	Max	Max	Max
180 BH	0,04	0,70	0,50
195 BH	0,06	0,70	0,50
220 BH	0,06	0,70	0,50
260 BH	0,08	0,70	0,50
300 BH	0,10	0,70	0,50
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.

Available coatings and surface conditions

	Uncoated	Electrogalvanized	Galvannealed	Extragal®	Ultragal®
180 BH	XX	XX		XX	XX
195 BH			XX
220 BH	XX	XX		XX	XX
260 BH	XX	XX		XX	XX
300 BH	X	X		X
Hot rolled     Cold rolled

X Available - XX Available in visible part quality

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.

لینک دانلود کل مطلب و تصاویر 168+8 صفحه

عملیات سخت کاری پخت فولاد،BAKE HARDENING