High carbon hot rolled steel sheet and method for manufacturing the same (as amended)

Abstract

A high carbon hot rolled steel sheet having a chemical composition containing by mass %, C: 0.20% to 0.48%, Si: 0.1% or less, Mn: 0.5% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.1% to 0.6%, Cr: 0.05% to 0.5%, B: 0.0005% to 0.0050%, Ca: 0.0010% to 0.0050%, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, where the average amount of N in a surface layer portion from the surface to the position at a depth of 0.1 mm in thickness direction is 0.1% or more and the average amount of N in the central portion in thickness is 0.01% or less, and a microstructure composed of ferrite and carbides, wherein the average grain size of the ferrite is 10 to 20 and the spheroidization ratio of the carbides is 90% or more.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This is the U.S. National Phase application of PCT / JP2012 / 008319, filed Dec. 26, 2012, which claims priority to Japanese Patent Application No. 2012-000913, filed Jan. 6, 2012, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes.FIELD OF THE INVENTION[0002]The present invention relates to a high carbon hot rolled steel sheet having excellent press formability and hardenability and a method for manufacturing the same.BACKGROUND OF THE INVENTION[0003]At present, automotive parts, e.g., gears, transmissions, and seat recliners, have been produced by forming hot rolled steel sheets, which are carbon steels for machine structural use specified in JIS G 4051, through cold press forming into predetermined shapes and applying a hardening treatment to ensure a predetermined hardness. Consequently, a steel sheet as a material is required to have excellent press formability and ha...

AI Technical Summary

Benefits of technology

This patent is about a high carbon hot rolled steel sheet with excellent press formability and hardenability, particularly on its surface layers. The steel is made by creating a microstructure of ferrite and carbides, with a specific average grain size and spheroidization ratio. The surface layer of the steel contains a minimum of 0.1% N, and annealing in a nitrogen gas atmosphere helps to stabilize the hardness of the surface layer after hardening. This high carbon hot rolled steel is useful for making parts like gears, transmissions, and seat recliners for automobiles.

Problems solved by technology

However, as for the high carbon steel sheets described in Patent Literatures 1 to 3, degradation in the ductility and decrease in the hardness of a surface layer portions of the steel sheet after hardening are observed, and it is difficult to provide excellent press formability and excellent hardenability stably.