| 摘要 |
High tensile strength steel sheets having high press formability and excellent shape-fixability in which an average roughness in surface Ha is 0.4-1.8 mu m, PPI value is more than 80 at a cut off level of 0.5 mu m, the proportional limit stress is less than 20 kg/mm2 and the microstructure consists of ferrite grains dispersed with fine martensite islands, are produced by cold rolling a hot rolled steel sheet containing 0.005-0.15% of carbon, and manganese, if necessary boron, molybdenum, chromium, silicon, nickel and copper within a range of 0.27-3% of Mneq. shown by the following formula (1) Mneq.=Mn+124B+3Mo+3/2Cr+1/3Si+1/3Ni+1/2Cu (1) so as to obtain the cold rolled steel sheet having the above described surface property and then heating the cold rolled steel sheet to an intercritical temperature between A1 and A3, after which cooling the heated steel sheet at a particularly defined cooling rate.
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