Cold-rolled steel sheet for press-forming

Claim 2

A PROCESS FOR PREPARING A COLD-ROLLED STEEL SHEET FOR A PRESS FORMING, WHICH PROCESS COMPRISES HEATING A COLD-ROLLED STEEL SHEET CONSISTING ESSENTIALLY OF LESS THAN 0.08 WT. PERCENT CARBON 0.05 TO 0.4 WT. PERCENT MANGANESE 0.025 TO 0.10 WT. PERCENT SILICON 0.003 TO 0.915 WT. PERCENT NITROGEN 0.01 TO 0.10 WT. PERCENT ACID-SOLUBLE ALUMINUM THE REMAINDER BEING IRN AND UNAVOIDABLE IMPURITIES, TO A TEMPERATURE IN A RANGE FROM 680* TO 900*C AND HOLDING THE SAME AT THIS TEMPERATURE FOR LESS THAN 10 MINUTES, SUBSEQUENTLY COOLING THE STEEL SHEET FROM THE SAID TEMPERATURE WITH A COOLDING RATE OF 5 TO 54*C/SEC. TO THE TEMPERATURE OF AN OVERAGING TREATMENT, THEN SUBJECTING THE STEEL SHEET TO SAID OVERAGING TREATMENT, IN WHICH THE STEEL SHEET IS HELD AT A TEMPERATUE OF 300 TO 500*C FOR LESS THAN 10 MINUTES AND THEREAFTER SUBJECTING THE STEEL SHEET TO A CONTINUOUS ANNEALING TREATMENT, IN WHICH THE STEEL SHEET IS QUENCHED TO ROOM TEMPERATURE.

Claim 3

The process of claim 1 wherein the overaging treatment is for 2 to 5 minutes.

Claim 4

A process for preparing a cold-rolled steel sheet for press forming, which includes vacuum degassing, whereby prior to cold rolling, the initial carbon content of a molten steel of 0.02 to 0.08 wt. percent is reduced to range of 0.001 to 0.009 wt. percent, which process comprises heating a cold-rolled steel sheet consisting essentially of less than 0.08 wt. percent carbon 0.05 to 0.4 wt. percent manganese 0.015 to 0.10 wt. percent silicon 0.003 to 0.015 wt. percent nitrogen 0.01 to 0.10 wt. percent acid-soluble aluminum the remainder being iron and unavoidable impurities, to a temperature in a range from 680* to 900*C and holding the same at this temperature for less than 10 minutes, subsequently cooling the steel sheet from the said temperature with a cooling rate of 5* to 54*C/sec. to the temperature of an overaging treatment, then subjecting the steel sheet to said overaging treatment, in which the steel sheet is held at a temperature of 300* to 500*C for less than 10 minutes and thereafter subjecting the steel sheet to a continuous annealing treatment, in which steel sheet is quenched to room temperature.

Claim 5

A process for preparing a cold-rolled steel sheet for press forming, which includes vacuum degassing, whereby prior to cold rolling, the initial carbon content of a molten steel of 0.02 to 0.08 wt. percent is reduced to range of 0.001 to 0.009 wt. percent, which process comprises heating a cold-rolled steel sheet consisting essentially of less than 0.08 wt. percent carbon 0.05 to 0.4 wt. percent manganese 0.015 to 0.10 wt. percent silicon 0.003 to 0.015 wt. percent nitrogen 0.01 to 0.10 wt. percent acid-soluble aluminum the remainder being iron and unavoidable impurities, to a temperature in a range from 680* to 900*C and holding the same at this temperature for less than 10 minutes, subsequently cooling the steel sheet from the said temperature with a cooling rate of 5* to 54*C/sec. to the temperature of an overaging treatment, then subjecting the steel sheet to said overaging treatment, in which the steel sheet is held at a temperature of 300* to 500*C for less than 10 minutes and thereafter subjecting the steel sheet to a continuous annealing treatment, in which steel sheet is quenched to room temperature.

Claim 6

The product of the process of claim 1.

Claim 7

The product of the process of claim 3.