| 摘要 |
PROBLEM TO BE SOLVED: To produce a hot rolled high tensile strength steel plate excellent in fatigue characteristics and workability by subjecting a steel having a specified compsn. to rough rolling under specified temp. conditions, removing scales, subjecting it to finish rolling and thereafter executing heat treatment and coiling. SOLUTION: A steel slab having a compsn. contg., by weight, <0.18% C and specified amounts of one or two kinds among Si, Mn, P, S and Al and satisfying C>=0.03+ Ti/4+Nb/4 is cast, and after that, or after heating at 1050 to 1200 deg.C, rough rolling is executed, and successively, surface scales are removed by the injection of ultrahigh pressure water under >2.5MPa collision pressure and >0.002l/cm<2> liq. content density. Immediately after that, finish rolling is started and is finished in the temp. range of (the Ar3 +150 deg.C) to (Ar3 +50 deg.C). Within 2sec, it is cooled to 800 deg.C at a cooling rate of >=20 deg.C/sec, is held to 800 to 700 deg.C for 5 to 20sec and is subsequently coiled at 500 to 350 deg.C. This steel plate has a mixed structure of ferrite, bainite, martensite or the like contg. the carbides of Ti and Nb, the surface roughness thereof is regulated to <1.5μm Ra, and it is composed of ferrite having <=5μm average grain size to 20μm from the surface.
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