Hot-rolled steel sheet exhibiting excellent cold formability and excellent surface hardness after forming

Abstract

Disclosed is a hot-rolled steel sheet that has a thickness of 3 to 20 mm and includes, in a chemical composition in mass percent, C of 0.3% or less (excluding 0%), Si of 0.5% or less (excluding 0%), Mn of 0.2% to 1%, P of 0.05% or less (excluding 0%), S of 0.05% or less (excluding 0%), Al of 0.01% to 0.1%, and N of 0.008% to 0.025%, with the remainder consisting of iron and inevitable impurities. A solute nitrogen content is 0.007% or more, and the carbon and nitrogen contents meet a condition as specified by 10C+N≦3.0. The microstructure of the steel sheet includes pearlite of less than 20% in area percentage, with the remainder approximately consisting of ferrite. The average grain size of the ferrite is 3 to 35 μm. The steel sheet has good cold formability during forming and still has predetermined surface hardness after forming.

Description

TECHNICAL FIELD[0001]The present invention relates to a hot-rolled steel sheet that has good cold formability during forming and still has predetermined surface hardness after forming.BACKGROUND ART[0002]Recently, better fuel efficiency is required in automobiles from the viewpoint of environmental protection. To meet the requirement, steels for use in automobile parts such as gears and other gearbox unit parts and casings more and more require lighter weights, i.e., higher strength. To meet the requirement of lighter weights and higher strength, hot-forged steels prepared from steel bars by hot forging have been generally used. Instead of the hot forged gears and other parts, demands are increasingly made to provide these parts by cold forging so as to reduce CO2 emission in parts production processes.[0003]Advantageously, the cold forming (cold forging) offers higher productivity and provides both good dimensional accuracy and good steel yield as compared with hot forming and warm...

AI Technical Summary

Benefits of technology

[0024]According to the present invention, a microstructure mainly containing ferrite having a predetermined average grain size is controlled so that the microstructure contains solute nitrogen in a certain amount, and the carbon content and the nitrogen content meet a predete

Problems solved by technology

Disadvantageously, however, the cold forming, when employed to produce parts, has to essentially use steels having high strength, i.e., high deformation resistance so as to allow the cold-worked parts to surely have strength at predetermined levels or higher.

Unfortunately, steels with increasing deformation resistance may more readily invite shorter lives of cold-forming tools and more readily cause fracture/cracking upon cold forming.

However, the parts inevitably change their dimensions in the heat treatment after cold forging and thereby require s