| 摘要 |
The high carbon hot-rolled steel sheet contains, in terms of percentages of mass, 0.10 to 0.7% C, 2.0% or less Si, 0.20 to 2.0% Mn, 0.03% or less P, 0.03% or less S, 0.1% or less Sol.Al, 0.01% or less N, and the balance being Fe and inevitable impurities, and has a structure of ferrite having 6 mum or less average grain size and carbide having 0.10 mum or more and less than 1.2 mum of average grain size. The volume ratio of the carbide having 2.0 mum or more of grain size is 10% or less. The volume ratio of the ferrite containing no carbide is 5% or less. The manufacturing method thereof has the steps of hot-rolling, primary cooling, holding, coiling, acid washing, and annealing. The primary cooling step is to cool the hot-rolled steel sheet down to cooling termination temperatures ranging from 450° C. to 600° C. at cooling rates of higher than 120 ° C./sec. The holding step is to apply secondary cooling to hold the primarily cooled hot-rolled steel sheet at a temperature range from 450° C. to 650° C. until coiling. |
