Thin production manner null of the high tension steel sheet where the fatigue limit by the cast band

摘要

PURPOSE:To produce a high tensile strength steel plate having high fatigue limit at a low cost by directly and continuously casting a thin slab from a molten steel, applying heat treat ment to this thin cast slab under specific conditions, and carrying out hot rolling and coiling. CONSTITUTION:A molten steel having a composition consisting of, by weight, 0.02-0.4% C, 0.2-3% Mn, and 0.003-0.05% S is directly and continuously cast into a thin slab by a continuous casting method, and this thin cast slab is cooled from solidification down to the Ae3 transformation point at 0.5-30 deg.C/sec average cooling rate and further cooled from the Ae3 transformation point down to the higher temp. between 550 deg.C and a temp. T1 deg.C represented by an equation I or below at =10 deg.C/sec cooling rate. Successively, the above thin cast slab is heated again up to a temp. in the region between the Ae3 transformation point and 1100 deg.C at =5 deg.C/sec average temp. rise rate and hot-rolled in the austenite region at 20% reduction of area, and the resulting plate is cooled down to =650 deg.C at =20 deg.C/sec average cooling rate and coiled. By this method, the formation of lamellar pearlite can be inhibited and ferrite structure can be refined, and the high tensile strength steel plate having high fatigue limit can be produced at a low cost.