PROCESS FOR MANUFACTURING HIGH-STRENGTH HOT-ROLLED STEEL PLATE SHOWING EXCELLENT MATERIAL UNIFORMITY AND BORE EXPANDABILITY AND STEEL PLATE

摘要

PROBLEM TO BE SOLVED: To provide a hot rolled steel plate having both a strength of ≥720 N/m² and a bore expandability of ≥70% without adding any Ti, Nb or Cr.;SOLUTION: Upon hot rolling a steel comprising 0.06-0.13 mass% C, 0.5-1.0 mass% Si, 2.1-3.0 mass% Mn, ≤0.030 mass% P, ≤0.003 mass% S, 0.02-0.06 mass% Al, 0.002-0.006 mass% N, ≤0.0030 mass% O, and, if required, at least one chosen from ≤0.0050 mass% B and ≤0.0050 mass% Ca and the balance being Fe and unavoidable impurities to obtain a coil, the steel is subjected to hydraulic descaling at an impinging pressure of ≥2,0 kgf/cm² before finish hot rolling. Here, the average cooling rate from a final pass temperature T1 within 800-950°C to T2 is adjusted to ≥80°C/sec, and the average cooling rate from T2 to a coiling temperature T3 within 250-450°C is adjusted to ≤40°C/sec, provided that T2(°C)=673×C-37×Si+52×Mn-4284×S+442.;COPYRIGHT: (C)2003,JPO