Bake-hardening hot-rolled steel sheet with excellent workability and process for producing the same

Abstract

A bake-hardening hot-rolled steel sheet which contains 0.01-0.2 mass% carbon, 0.01-2 mass% silicon, 0.1-2 mass% manganese, up to 0.1 mass% phosphorus, up to 0.03 mass% sulfur, 0.001-0.1 mass% aluminum, up to 0.01 mass% nitrogen, and 0.005-0.05 mass% niobium, the remainder comprising iron and unavoidable impurities. The microstructure of the sheet is a polygonal ferrite having an average particle diameter of 2-8 [mu]m and / or a continuous cooling transformation texture. The proportion of a solid solution of carbon and / or a solid solution of nitrogen present at the grain boundaries in the sheet is 0.28 or lower. Also provided is a process for producing the hot-rolled steel sheet which comprises heating a steel billet comprising those ingredients to at least a temperature satisfying the equation SRT ( DEG C) = 6670 / {2.26-log(%Nb)(%C-273 (A) rough-rolling the steel billet heated, finish-rolling the sheet under such conditions that the final temperature is in the range of from the Ar3 transformation point to (Ar3 transformation point + 100 DEG C), cooling it at a cooling rate of 80 DEG C / sec or higher for cooling from cooling initiation to 500 DEG C or lower, and winding it up.

Description

technical field [0001] The present invention relates to a sinter-hardening type hot-rolled steel sheet excellent in workability and a method for producing the same. Specifically, it relates to a steel plate having a microstructure of polygonal ferrite and / or continuous cooling transformation structure with an average grain size of 2 μm to 8 μm, whereby not only parts requiring strict processing can be easily formed, Moreover, even steel sheets with a tensile strength of 370 to 640 MPa can obtain stamping strength equivalent to the design strength of steel sheets of 540 to 780 MPa by introducing deformation by stamping and coating and sintering. [0002] This application claims priority to Japanese Patent Application No. 2005-010210 filed on January 18, 2005, the contents of which are incorporated herein by reference. Background technique [0003] In recent years, light metals such as Al alloys and high-strength steel sheets have been used in automobile parts for the purpose...

AI Technical Summary

Problems solved by technology

[0010] However, finer grains may degrade press formability

In addition, in the case of running parts and inner plate parts, although excellent rimming workability is required, the microstructure is ferrite-pearlite, so rimming workability is considered unsuitable

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