Patent ID: 9085816
Filing Date: 2015-07-21
Classification: B23K,C21C,C21D,C22C,Y10T

Abstract:
1. A steel sheet, consisting of Si: 0.20-2% (the term “%” herein means “mass %”, the same is true hereinbelow), Mn: 1.5-2.5%, and O: 0.002% or less (exclusive of 0%), C: 0.02-0.25%, P: 0.1% or less (exclusive of 0%), S: 0.05% or less (exclusive of 0%), Al: 0.02-0.2%, N: 0.0015-0.015%, and at least one element selected from the group consisting of Cr: 0.03-2% and B: 0.0003-0.005%, optionally at least one element selected from the group consisting of Nb: 0.005-0.1%, Ti: 0.005-0.3%, V: 0.005-5%, W: 0.005-0.5%, Cu: 0.03-0.5%, Ca: 0.0005-0.005%, REM (rare earth elements): 0.0005-0.005%, Zr and Mg: 0.0005-0.005% in total, Co: 0.03-1% and Ni: 0.015-0.5%, and the balance being iron and inevitable impurities, wherein a thickness of an inner oxide layer existing in a surface layer of the steel sheet is 5 μm or less (inclusive of 0 μm), which steel sheet has a tensile strength of not less than 720 MPa.