Pre-Coated or Uncoated Low Carbon, Heat-Treatable Boron Steel
|Publication Date:||1 June 2011|
This specification covers the requirements for low carbon, heat-treatable boron steel used in the manufacture of structural parts requiring heat treatments to achieve high strength through a press hardening process in a water quenched die. Press-hardened steel (PHS) can be formed by either direct or indirect stamping processes. In the commonly used direct process, the heated blank is formed in the hot stamping die and quenched "in-situ" in a water cooled die. With the indirect process, a part is pre-stamped at room temperature and subsequently hot stamped and die quenched. Both processes result in a part with a microstructure consisting of essentially 100% tempered martensite or martensite with trace amounts of bainite. Typical applications include door impact beams, bumper impact beams, and structural reinforcements for side impact and roof crush.
Note: Nothing in this standard supercedes applicable laws and regulations.
Note: In the event of conflict between the English and domestic language, the English language shall take precedence.
Material Description. The material designation is defined by the GMW base metal specification number, material category (Sheet or Tubular product), steel product type, grade, and finish type. The substrates used in these applications may be cold rolled or hot rolled but the product type is designated as heat treated (HT) or hot stamped (HS), as appropriate. The steel grades will correspond to the chemical and mechanical requirements as shown in Table 1 and Table A1 through Table A3. The surface quality, represented as an alpha character, shall be designated with the surface quality. Only the surface quality designation of U, representing an unexposed application, shall be used unless otherwise approved by Materials Engineering. The above descriptive items are used together to form the coding system. Examples of the coding system for engineering part drawings, electronic math data files, and/or manufacturing engineering documents are shown in Section 8.
Resistance Welded Mechanical Tubing. Tubes supplied to this specification are roll formed from a steel strip to the desired shape and dimension, electrical resistance welded into a tube, and then heat treated to achieve the final mechanical property requirements.
Induction Heat Treated Steel. Part sections formed with this operation and supplied under this standard shall be made by stamping in the as-rolled condition. The raw stamping can be selectively coil induction hardened to achieve the required strength requirements. Al-Si, Zn, Zn-Fe, or Zn-Ni pre-coated products shall not be used in these applications.
Uncoated Products. Parts formed with a direct or an indirect hot stamping operation with uncoated steel and supplied under this standard shall be subsequently blasted to remove oxide from the surface as described in section 188.8.131.52.2.
Pre-Coated Products. Parts formed with a hot stamping operation may also utilize pre-coated steels furnished according to this specification in order to eliminate the need for blasting and/or improve corrosion performance.
Aluminum-Silicon (Al-Si) Pre-Coatings: Parts with Al-Si coatings shall not be cold-stamped prior to the hot stamping operation, but slight roll-forming operations may be allowable if the coating is not damaged. Blasting is not permitted for Al-Si pre-coated parts to prevent damage of the coating.
Zinc (Zn) Pre-Coatings: Parts must be cold-stamped or roll formed (indirect hot stamping) prior to the hot stamping operation, in order to avoid significant deformation at high temperature. After the indirect hot stamping operation, the part shall be blasted to remove the oxide layer from the surface as described in section 184.108.40.206.2.
Zinc-Iron (Zn-Fe) or Zinc-Nickel (Zn-Ni) Alloy Pre-Coatings: Parts can be hot stamped in a direct or indirect hot stamping operation. After hot forming and cooling the part shall be blasted to remove the oxide layer from the surface as described in section 220.127.116.11.2.
Symbols. Not applicable.
Typical Applications. Typical applications are uncoated or pre-coated sheet metal and thin gauge tubular structural parts requiring yield and tensile strengths exceeding 950 MPa and 1250 MPa respectively.
Remarks. See Table A1 in Appendix A for