Cold rolled steel sheet with relationship between inclusion separation distance and yield strength

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Cold-rolled steel sheets, the first comprising ferrite and a martensite-containing second phase, has a composition (by weight) : 0.05-0.3 % carbon, 1.5-3.5 % manganese, 0-3.0 % silicon, 0-0.1 phosphorous, 0-0.5 % sulphur, 0-0.15 % aluminium, 0-1 % chromium, 0-0.5 % molybdenum, 0-0.2 % titanium, 0-0.2 % vanadium, 0-0.3 % niobium, 0-0.5 % copper, 0-0.5 % nickel, 0-0.010 % calcium, 0-0.010 % magnesium, 0-005 % rare earth elements, with the balance being iron and impurities. The second, comprising a martensite single phase structure, has a composition (by weight) : 0.12-0.3 % carbon, 1.5-3.0 % manganese, 0-0.5 % silicon, 0-0.02 phosphorous, 0-0.01 % sulphur, 0-0.15 % aluminium, 0-0.01 % nitrogen, 0-2.0 % chromium, 0-0.01 % boron, 0-0.5 % copper, 0-0.5 % nickel, 0-0.2 % titanium, 0-0.1 % vanadium, 0-0.1 % niobium, with the balance being iron and impurities. For each steel the distribution of inclusions is controlled, for example by RH degassing the melt and having a rolling reduction performed at 1000 °C or lower of less than 98 % with the total reduction being at least 90 % for the first steel and by aluminium killing and RH degassing the melt and having a rolling reduction performed at 950 °C or lower of less than 97 % with the total reduction being at least 90 % for the second steel.