Patent ID: 9644247
Date: 2017-05-09
CPC Classifications: C21D,C22C

Claim:
1. A method of manufacturing a high strength press-formed member, comprising: preparing a steel sheet having a composition including by mass %: heating the steel sheet to a temperature of 750° C. to 1000° C. and retaining the steel sheet in that state for 5 seconds to 1000 seconds; subjecting the steel sheet to hot press-forming at a temperature of 350° C. to 900° C.; cooling the steel sheet to a temperature of 50° C. to 350° C.; heating the steel sheet to a temperature in a temperature region of 350° C. to 490° C.; and retaining the steel sheet at the temperature in the temperature region for 5 seconds to 1000 seconds, wherein a microstructure of a steel sheet constituting the high strength press-formed member comprises martensite, retained austenite and bainite containing bainitic ferrite, an area ratio of said martensite with respect to the entire microstructure of the steel sheet is 10% to 57%, at least 25% of said martensite is tempered martensite, content of retained austenite is 5% to 40%, area ratio of said bainitic ferrite in said bainite with respect to the entire microstructure of the steel sheet is at least 5%, total of area ratios of said martensite, said retained austenite, and said bainitic ferrite in said bainite with respect to the entire microstructure of the steel sheet is at least 65%, area ratio of remaining microstructure with respect to the entire microstructure of the steel sheet is 20% or less, and average carbon concentration in the retained austenite is at least 0.65 mass %.