High-carbon hot-rolled steel sheet and method for manufacturing the same

Abstract

A high-carbon hot-rolled steel sheet and a method for manufacturing the steel sheet are provided. The high-carbon hot-rolled steel sheet has a particular chemical composition. The microstructure of the steel sheet includes ferrite, cementite, and pearlite that accounts for 6.5% or less of the entire microstructure by area fraction. The proportion of the number of cementite grains having an equivalent circle diameter of 0.1 μm or less to the total number of cementite grains is 20% or less, the average cementite grain size is 2.5 μm or less, and the cementite accounts for 3.5% or more and 10.0% or less of the entire microstructure by area fraction. The average concentration of solute B in a region extending from a surface layer to a depth of 100 μm is 10 mass ppm or more. The average concentration of N present as AlN in the region is 70 mass ppm or less.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This is the U.S. National Phase application of PCT / JP2020 / 000783, filed Jan. 14, 2020, which claims priority to Japanese Patent Application No. 2019-013957, filed Jan. 30, 2019, the disclosures 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 high cold workability and high hardenability (immersion-quench hardenability and carburizing hardenability) and a method for manufacturing the high-carbon hot-rolled steel sheet.BACKGROUND OF THE INVENTION[0003]Currently, automotive parts such as transmissions and seat recliners are often produced by processing hot-rolled steel sheets (high-carbon hot-rolled steel sheets) which are carbon steels for machine structural use specified in JIS G4051 and alloy steels for machine structural use into desired shapes through cold working and then subjecting t...

AI Technical Summary

Benefits of technology

The present invention relates to a high-carbon hot-rolled steel sheet that has high cold workability and high hardenability (immersion-quench hardenability and carburizing hardenability). By controlling the average carbide grain size and the proportion of carbide grains with a size of 0.3 μm or less, the technique helps improve the workability of the steel sheet. The chemical composition of the steel is also adjusted to prevent nitrogen infiltration and enhance hardenability. Additionally, the invention provides a method for annealing the steel sheet in a nitrogen atmosphere to prevent nitrogen infiltration and maintain higher hardenability. The high-carbon hot-rolled steel sheet can be used as a material steel sheet for automotive parts that require stable quality and excellent effects in industry production.

Problems solved by technology

However, Patent Literature 1 does not discuss cold workability and does not describe carburizing hardenability.

However, Patent Literature 3 does not describe immersion-quench hardenability or carburizing hardenability.

However, Patent Literature 4 specifies the hydrogen concentration in an atmosphere in the annealing step as 95% or more and does not describe whether nitrogen absorption can be suppressed to ensure solute B in an annealing step in a nitrogen atmosphere.