PURPOSE: To produce a steel plate well balanced between strength and toughness by continuously casting a molten steel having a composition in which respective contents of C, Mn, and S are specified under specific conditions, applying slight-degree working in which total amount of strain is specified to the resulting cast slab, and then controlling cooling.
CONSTITUTION: A steel having a composition consisting of, by weight, ≤0.20% C, 0.6-3.0% Mn, 0.002-0.1% S, and the balance Fe with inevitable impurities is refined. The resulting molten steel is continuously cast while controlling average cooling rate in the range from solidification to the Ar3 transformation point to 1-30°C/sec. The resulting cast slab is subjected, in the as-rolled state or in a temp. region of the Ar3 transformation point or above, to slight- degree working, such as light draft rolling of ≥one pass in which total amount of strain is regulated to ≤0.1 by logarithmic strain. Then, cooling is carried out while controlling average cooling rate from 900 down to 600°C to ≥10°C/sec. By this method, the formation of ferrite can be inhibited and a fine ferritic bainite structure can be formed by means of slight-degree working alone. Moreover, if Ti is added to the above steel composition so that the relationship between Ti content and the content of inevitably contained N satisfies Ti>(48/14).N and further 0.0003-0.01% B is added, the intergranular nucleation of ferrite can be inhibited.
AKAMATSU SATOSHI
YADA HIROSHI
