HIGH-STRENGTH COLD-ROLLED STEEL SHEET, HIGH-STRENGTH GALVANIZED STEEL SHEET, AND HIGH-STRENGTH ALLOYED HOT-DIP GALVANIZED STEEL SHEET HAVING EXCELLENT FORMABILITY AND WELDABILITY,AND METHODS FOR MANUFACTURING THE SAME

摘要

This cold-rolled steel sheet includes, in terms of mass %, C: not less than0.05%and not more than 0.095%, Cr: not less than 0.15% and not more than 2.0%, B:not lessthan 0.0003% and not more than 0.01%, Si: not less than 0.3% and not more than2.0%,Mn: not less than 1.7% and not more than 2.6%, Ti: not less than 0.005% andnot morethan 0.14%, P: not more than 0.03%, S: not more than 0.01 %, Al: not more than0.1 %, N:less than 0.005%, 0: not less than 0.0005% and not more than 0.005%, andcontains as theremainder, iron and unavoidable impurities, wherein the microstructure of thesteel sheetincludes mainly polygonal ferrite having a crystal grain size of not more than4 µm, andhard microstructures of bainite and martensite, the block size of themartensite is not morethan 0.9 µm, the Cr content within the martensite is 1.1 to 1.5 times theCr content withinthe polygonal ferrite, and the tensile strength is at least 880 MPa.