Patent Number: 7,731,808

Title: Hot-rolled steel sheet and cold-rolled steel sheet and manufacturing method thereof

Abstract: A steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances and machine structures and of constructing buildings, and a manufacturing method thereof are provided. The steel sheet is a hot-rolled steel sheet of carbon steel or low-alloy steel, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D (.mu.m) at the depth of 1/4 of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by the formulas (1) and (2) given below and the increase rate X (.mu.m/min) in average ferrite crystal grain diameter at 700.degree. C. at the depth of 1/4 of the sheet thickness from the steel sheet surface and said average crystal diameter D (.mu.m) satisfy the relation defined by the formula (3) given below: 1.2.ltoreq.D.ltoreq.7 formula (1) D.ltoreq.2.7+5000/(5+350C+40Mn).sup.2 formula (2) DX.ltoreq.0.1 formula (3) wherein C and Mn represent the contents (in % by mass) of the respective elements in the steel.

Inventors: Tomida; Toshirou (Hyogo, JP), Imai; Norio (Hyogo, JP), Yoshida; Mitsuru (Sanda, JP), Kawano; Kaori (Neyagawa, JP), Wakita; Masayuki (Nishinomiya, JP), Toki; Tamotsu (Hyogo, JP), Yasuyama; Masanori (Hyogo, JP), Nishibata; Hitomi (Hyogo, JP)

Assignee: Sumitomo Metal Industries, Ltd.

International Classification: C22C 38/00 (20060101); C21D 8/02 (20060101); C21D 8/04 (20060101)

Expiration Date: 2022-06-08 0:00:00