Cold-rolled steel sheet and method for manufacturing same

Abstract

This cold-rolled steel sheet has a predetermined chemical composition, and has a metallographic structure containing, by area fraction, 40.0% to below 60.0% of polygonal ferrite, 30.0% or more of bainitic ferrite, 10.0-25.0% of retained austenite, and 15.0% or less of martensite. In the retained austenite, the proportion of retained austenite having an aspect ratio of 2.0 or less, a major axis length of 1.0 microns or less, and a minor axis length of 1.0 microns or less is 80.0% or more. In the bainitic ferrite, the proportion of bainitic ferrite which has an aspect ratio of 1.7 or less, and in which the average crystallographic misorientation is 0.5 degrees to below 3.0 degrees in a region surrounded by grain boundaries having a crystallographic misorientation of 15 degrees or more, is 80.0% or more. The connectivity D value between the martensite, the bainitic ferrite, and the retained austenite is 0.70 or less.

Description

technical field [0001] The present invention relates to a cold-rolled steel sheet and a method for producing the same, and more particularly to a high-strength cold-rolled steel sheet excellent in ductility, hole expandability, and punching fatigue properties mainly used in automobile parts and the like, and a method for manufacturing the same. This application is based on Japanese Patent Application No. 2015-034137 filed in Japan on February 24, 2015, Japanese Patent Application No. 2015-034234 filed in Japan on February 24, 2015, and Japanese Patent Application No. 2015 filed in Japan on July 13, 2015. -139888 and Japanese Patent Application No. 2015-139687 filed on Jul. 13, 2015 in Japan claim priority, and the contents thereof are incorporated herein by reference. Background technique [0002] In order to suppress the amount of carbon dioxide emitted from automobiles, weight reduction of suitable automobile bodies using high-strength steel sheets has been advanced. In a...

AI Technical Summary

Problems solved by technology

However, in the cold-rolled steel sheet of Patent Document 3, since the metal structure of the steel sheet is mainly composed of low-temperature transformation-generated phases, the boundary between the low-temperature transformation-generated phases and retained austenite is formed in the sheet end portion during punching. Voids are generated in the hole, and it is difficult to ensure high fatigue characteristics in the fatigue environment where stress is repeatedly applied to the punched hole

[0009] As described above, conventionally, it has been extremely difficult to simultaneously improve ductility and hole expandability in high-strength steel sheets, and to ensure fatigue characteristics (punching fatigue characteristics) in a fatigue environment where stress is repeatedly applied to punched holes.

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