PURPOSE: To obtain a steel sheet having excellent boundary breakage resistance by specifying surface layer components and internal layer component at the time of casting, respectively and executing heat treatment to a double layer steel sheet containing Ti and Nb satisfying a specific inequality and having a transition layer specifying ratios of the thicknesses at one and both sides to the whole sheet thickness.
CONSTITUTION: The surface layer components at the time of casting are made to be ≤0.0800wt.% C, 3,0-30.0% Cr, ≤0.0500% N, and the interval layer components at the time of casting are made to be ≤0.0150% C and ≤0.0200% N. The double layer steel sheet containing either one or two kinds of Ti and Nb satisfying the inequality I and having the transition layer of the surface internal layer at 0.1-100 the one side thickness and 0.2-20% both side thicknesses to the whole sheet thickness and having 1-20% surface layer ratio at one side and 2-40% at both sides to the whole sheet thickness is produced by a cntinuous casting method. Thereafter, by excuting the heat treatment, the average components in the sheet thickness direction of C and N in the surface layer of the steel sheet are made to be ≤0.0100wt.% C and ≤0.0200wt.% N. Wherein, the inequality I is made to be 8.0≥Ti/(4C+3.4N)+Nb/7.7 C≥1.0.
SENUMA TAKEHIDE
KITAGAWA ITSURO
