PRODUCTION OF COLD ROLLED STEEL SHEET EXCELLENT IN AGING RESISTANCE BY CONTINUOUS ANNEALING

摘要

A cold rolled steel sheet having a good ageing resistance is produced by subjecting a cold rolled steel sheet to continuous annealing including recrystallization, grain growth, quenching, supercooling, reheating and overageing according to inclinatory cooling, where after the recrystallization and the grain growth, the steel sheet is quenched at a cooling rate of 50 to 250 DEG C/sec from 720 - 600 DEG C to 200 - 310 DEG C; after retaining the steel sheet at the same temperature for 0 to 15 seconds, the steel sheet is reheated by at least 40 DEG C up to 320 - 400 DEG C; then the steel sheet is cooled from or retained at the same temperature at a rate of not more than 0.7 DEG C/sec including the time for retaining the steel sheet at the same temperature; and then the steel sheet is cooled at an average cooling rate of not more than 10 DEG C/sec in a temperature zone of higher than 350 DEG C, at a specific average cooling rate in a temperature zone of 350 DEG C to 300 DEG C and at a specific average cooling rate down to 285 - 220 DEG C in a temperature zone of lower than 300 DEG C.