High-strength hot rolled steel sheet being free from peeling and excelling in surface and burring properties and process for manufacturing the same

Abstract

A hot rolled steel sheet comprised of, by mass, 0.01 to 0.1% C, 0.01 to 0.1% Si, 0.1 to 3% Mn, 0.1% or less P, 0.03% or less S, 0.001 to 1% Al, 0.01% or less N, 0.005 to 0.08% Nb, 0.001 to 0.2% Ti andthe balance Fe and unavoidable impurities, satisfying the formula ¢Nb! x ¢C!=4.34 x 10<-3>, wherein the density of solid solution C grain boundary quantity is in the range of 1 to 4.5 grain boundaries / nm<2>, and wherein the diameter of cementite grains precipitated on the grain boundaries within the steel sheet is 1 ¢mu!m or less. This hot rolled steel sheet is manufactured according to the following process. A steel piece with the above composition of the hot rolled steel sheet is heated at temperature SRTmin ( DEG C) to 1170 DEG C and subjected to rough rolling in the condition of 1080 DEGto 1150 DEG C finalization temperature. Thereafter, finishing rolling is started at 1000 DEG to below 1080 DEG C within 30 to 150 sec and terminated at temperature ranging from the Ar3 transformation point temperature to 950 DEG C so that the draft upon final pass is in the range of 3 to 15%. The resultant steel sheet is subjected to cooling from the start to the temperature range of 450 DEG to550 DEG C at a cooling rate of over 15 DEG C / sec and wound up.

Description

technical field [0001] The present invention relates to a high-strength hot-rolled steel sheet excellent in surface properties and hole expansion and burring properties, and to a method for producing the same. [0002] This application claims the priority of Japanese Patent Application No. 2007-82567 filed on March 27, 2007, the contents of which are incorporated herein by reference. Background technique [0003] In recent years, in order to reduce the weight of various steel sheets represented by improving the fuel consumption of automobiles, the strength of steel sheets such as ferroalloys has been increased or light metals such as Al alloys have been used. However, although light metals such as Al alloys have the advantage of high specific strength compared with heavy metals such as steel, they have the disadvantage of being significantly expensive, so their application is limited to special purposes. Therefore, in order to reduce the weight of various steel sheets more ...

AI Technical Summary

Problems solved by technology

[0016] Therefore, among the techniques disclosed in Patent Documents 2 and 3, there is no disclosure at all of techniques for suppressing flaws or microcracks in end faces formed by shearing or punching.

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