HIGH-STRENGTH GALVANIZED STEEL SHEET, HIGH-STRENGTH ALLOYED HOT-DIP GALVANIZED SHEET, AND HIGH-STRENGTH COLD-ROLLED STEEL SHEET WHICH EXCEL IN MOLDABILITY AND WELDABILITY, AND MANUFACTURING METHOD FOR THE SAME

摘要

Disclosed is a cold-rolled steel sheet which contains, by mass, 0.05-0.095% C, 0.15-2.0% Cr, 0.0003-0.01% B, 0.3-2.0% Si, 1.7-2.6% Mn, 0.005-0.14% Ti, 0.03% or less P, 0.01% or less S, 0.1% or less Al, less than 0.005% N, and 0.0005-0.005% O, with the remainder comprising iron and unavoidable impurities. The steel sheet composition has a polygonal ferrite mainly with a grain size of 4µm or less and hard compositions of bainite and martensite, wherein the block size of the martensite is no more than 0.9µm and the volume of Cr contained in the martensite is 1.1-1.5 times the amount of Cr contained in the polygonal ferrite, and has a tension strength of 880MPa and higher.