High-strength cold-rolled steel sheet, high-strength plated steel sheet, and production methods therefor

Abstract

The present invention addresses the problem of: obtaining a high-strength cold-rolled steel sheet and a high-strength plated steel sheet having a tensile strength (TS) of 780 MPa or higher and excelling in ductility, stretch-flangeability, and in-plane stability of stretch-flangeability; and providing effective production methods for same. This high-strength cold-rolled steel sheet has a steel structure containing a specific component composition, ferrite at an area ratio of 50-80%, martensite having an average grain size of 2.5 [mu]m or less at an area ratio of 8% or less, retained austeniteat an area ratio of 6-15%, and tempered martensite at an area ratio of 3-40%, and where the value of the ratio fM / fM+TM of the area ratio fM of the martensite and the total area ratio fM+TM of the martensite and the tempered martensite is 50% or less and the standard deviation of crystal grain sizes of the martensite is 0.7 [mu]m or less across a total of five sites, namely, a width-center part located at the center in the sheet width direction, both edge parts that extend for 50 mm toward the sheet width center from both sheet width direction edges, and center parts between the width-center part and said both edge parts.

Description

technical field [0001] The present invention relates to a high-strength cold-rolled steel sheet, a high-strength plated steel sheet, and a method for producing them, which are excellent in formability and are mainly suitable for structural members of automobiles. In particular, it relates to a high-strength cold-rolled steel sheet having a tensile strength (TS) of 780 MPa or more, excellent in ductility, stretch-flangeability, and in-plane stability of stretch-flangeability, a high-strength plated steel sheet, and their Manufacturing method. Background technique [0002] In recent years, high-strength steels have been widely used due to increasing demands for crash safety and fuel costs of automobiles. In addition, since thin steel sheets for automobiles are formed into automobile parts by press working, flanging, etc., excellent formability is required. Therefore, steel sheets for automobiles need to maintain high strength, and also need to have excellent ductility and st...

AI Technical Summary

Problems solved by technology

However, as a result of increasing the content of alloying elements for high strength, in-plane variations in formability, especially stretch flangeability, occurred, and there was a problem that it was not possible to provide a raw material with sufficient properties.

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