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

This patent is about controlling a microstructure containing ferrite to have a certain amount of solute nitrogen, and to also meet certain conditions for carbon and nitrogen content. This results in a hot-rolled steel sheet that is easier to form and has a longer life for tools used in the process. The steel sheet also resists fracture and cracking, and after forming, produces a part with a certain surface hardness.

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 secondary correction by machining such as cutting.

This technique, however, controls the strain aging hardening by the solute carbon content alone and hardly gives a steel that has both sufficient cold formability and required level of hardness / strength after forming.

Disadvantageously, however, the steel is a hot-forged steel as with the wire rod / steel bar disclosed in PTL 1 and suffers from high production cost.

Disadvantageously, however, the technique requires nitriding as an extra process after cold forming and fails to offer sufficiently low cost.