| 摘要 |
PROBLEM TO BE SOLVED: To reduce the variation of strength in a steel sheet without adding special alloy elements thereto by previously finding the relation valid among strength, sheet thickness, carbon equivalent and water quenching starting temp. to a prescribed strength level and finding the water quenching starting temp. from the actual sheet thickness and the objective strength. SOLUTION: Previously, the formula expressing the relation valid among the strength TS (kg f/mm2), sheet thickness (t) (mm), carbon equivalent Ceq (mass %) and water quenching starting temp. TQ ( deg.C) to a prescribed strength level has been found. The carbon equivalent calculated from the actual tapping components, actual sheet thickness and the objective strength are substituted for the formula to find the water quenching starting temp., and, from this temp., water quenching is executed. In principle, a high strength cold rolled steel sheet free from variation in strength can be produced; actually, by the errors of the constants (a), (b), (c) and (d) in the formula, the variation in the strength can not be made zero, but it can be reduced to <=5 kgf/mm2. Desirably, the tempering temp. after the water quenching is controlled to <=300 deg.C to prevent the low temp. tempering embrittlement therein.
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