Method of making low yield point cold-reduced steel sheet by continuous annealing process

Claim 1

In a process of making a low carbon steel sheet for press-forming through a continuous annealing, an improved method substantially comprising the following treatment: a. In steelmaking stage, the chemical composition of said low carbon steel is controlled as follows, C, 0.03% to 0.10% Mn, 0.10% to 0.60% P, 0.04% or less S, 0.05% or less Si, 0.2% or less Sol.Al, 0.005% or less O 2 , 0.02% or less, by addition of Si and Al. b. In hot-rolling stage after ordinary slabbing, a hot-roller steel strip is finished at 800° C. or more and is coiled at a temperature of 650° C. to 800° C. c. In continuous annealing stage after ordinary pickling and cold reducing, a cold-reduced steel strip is subjected to the full continuous annealing process comprising the following steps, 1. heating up within the range of 680° C. to 900° C. and holding for 30 to 180 seconds, 2. cooling to 500° C. or less with a rate of 10° C./second or more,