PURPOSE: To manufacture a steel plate with a favorable stretch flanging property by subjecting a steel contg. specified percentages of C, Mn and Si to hot rolling, slow cooling, rapid cooling and coiling under specified conditions to form a structure contg. a specified percentage by area of an upper bainite structure.
CONSTITUTION: A steel contg. 0.02W0.25% C, 0.3W2% Mn and 0.1W1.5% Si or further contg. ≥1 kind among 0.01W1% Cr, 0.005W0.06% Nb, 0.02W1.0% V, 0.01W0.08% Ti and 0.02W0.15% Zn is hot rolled at 700W900°C finish temp., slowly cooled for 3W20sec at 4W10°C/sec average cooling rate, rapidly cooled at 50W100°C/sec average cooling rate, and coiled at 370W575°C. Thus, a high strength hot rolled steel plate having a polygonal ferrite + upper bainite structure contg. 3W60% by area of an upper bainite structure is manufactured.
HASHIMOTO SHIYUNICHI
KANBE AKIFUMI
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