| 摘要 |
This hot- rolled steel sheet includes , in mass% 0.05- 0.15% of C , 0- 0.2% of Si, 0.5- 3.0% of Al , 1.2- 2.5% of Mn , not more than 0.1% of P, not more than 0.01% of S , not more than 0.007% of N, 0.03 -0.10% of Ti, 0.008- 0.06% of Nb , 0- 0.12% of V, a total of 0- 2.0% of one or more from among Cr , Cu , Ni , and Mo 0- 0.005% of B, a total of 0- 0.01% of one or more from among Ca , Mg , La ,and Ce, a total of 0.8x(Mn -1)% or more of Si and Al, and a total of 0.04- 0.14% of Ti and Nb, the remainder being Fe and unavoidable impurities. In the steel structure , the total surface area percentage of martensite and retained austenite is in the range of 3 -20% , the surface area percentage of ferrite is in the range 50- 96% , and the surface area percentage of pearlite is not more than 3%. In a surface layer portion, the thickness of an area in which network oxides are present, in the plate thickness direction , is less than 0.5µm. The maximum tensile strength is at least 720MPa. |
