Patent ID: 9200343
Filing Date: 2015-12-01
Classification: B32B,C21D,C22C,C23C

Abstract:
1. A high strength steel sheet having a tensile strength of at least 1470 MPa and (tensile strength (TS)×total elongation (T. EL)) of at least 29000 MPa·%, comprising a composition including by mass %, C: 0.55% to 0.73%, Si: 3.0% or less, Al: 3.0% or less, Si+Al: at least 0.7%, Cr: 0.2% to 8.0%, Mn: 10.0% or less, Cr+Mn: at least 1.0%, P: 0.1% or less, S: 0.07% or less N: 0.010% or less, and remainder as Fe and incidental impurities, and having a microstructure satisfying conditions: area ratio of martensite with respect to the entire microstructure of 15% to 90%; content of retained austenite of 10% to 50%, wherein at least 50% of said martensite is constituted of tempered martensite and an area ratio of said tempered martensite with respect to the entire microstructure of the steel sheet is at least 10%; and an area ratio of polygonal ferrite with respect to the entire microstructure of the steel sheet is 10% or less, wherein average carbon concentration in said retained austenite is at least 0.7mass %.