Two-step hot forming of steels

Abstract

Methods for press hardening steel alloys comprised of medium-Mn are provided. The press-hardened steel alloy may have an ultimate tensile strength (UTS) of at least 1,700 MPa and a tensile elongation of at least 8%. The press-hardened steel alloy may be formed in two forming steps above the martensitic finish temperature. The press-hardened steel may have a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

Description

INTRODUCTION[0001]This section provides background information related to the present disclosure which is not necessarily prior art.[0002]The present disclosure relates to two-step forming methods of press-hardening steel alloys to form high-strength press-hardened components.[0003]Press-hardened steel (PHS), also referred to as “hot-stamped steel,” is one of the strongest steels used for automotive body structural applications, having tensile strength properties on the order of about 1,500 mega-Pascal (MPa) and total elongation on the order of about 5% to 6%. Such steel has many desirable properties and uses, including forming steel components with significant increases in strength-to-weight ratios. Further, PHS components have become increasingly prevalent in various industries and applications, including general manufacturing, construction equipment, automotive or other transportation industries, home or industrial structures, and the like. For example, in automotive manufacturin...

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Benefits of technology

[0056]In another example, the process may consist essentially of the following steps. A blank comprised of a medium-Mn steel alloy is heated to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is ten press hardened to form a press-hardened component. The press-hardened component is formed a second time at a temperature of greater than or equal to about the martensitic finish temperature. The further forming includes at least one of trimming, punching, or re-striking the press-hardened component. The press-hardened component is cooled at a controlled rate between the martensitic start temperature and the martensitic finish temperature such that the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. In certain other variations, such a process may be further limited as further consisting essentially of, any combination of, or all of the following: (1) heating the blank to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy; (2) having a steel alloy comprised of carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %; (3) a PHS component having an ultimate tensile strength of greater than or equal to about 1,700 MPa; (5) air cooling the press-hardened component to the martensitic finish temperature after further forming the press-hardened component; and (6) galvanizing the medium-Mn steel alloy blank before heating. Notably, such a process excludes any laser cutting that are often required in conventional processes and quenching the press-hardened component, which can result in time, energy, and cost savi...

Problems solved by technology

After shot blasting, further forming of the PHS component is typically accomplished with las...

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