Steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets, and methods for their manufacture

Abstract

A steel sheet according to one embodiment of the present invention has a predetermined chemical composition, and the microstructure of a 1 / 4 portion of the sheet thickness of the steel sheet contains, by volume percentage, tempered martensite: 30 to 70%, and ferrite and bainite. One or two types of: the total amount is 20% or more and 70% or less, and in the structure of the above-mentioned 1 / 4 part of the plate thickness, the retained austenite is less than 10% by volume percentage, and the fresh martensite is 10% or less, Pearlite is 10% or less, their total volume percentage is 15% or less, and the number density of iron-based carbides with a major diameter of 5 nm or more in the tempered martensite at the 1 / 4 part of the plate thickness is 5×10 7 Pieces / mm 2 In the above, the ratio of the number of ε-based carbides in the 1 / 4 part of the sheet thickness to the number of the iron-based carbides with a major diameter of 5 nm or more is 20% or more, and the tensile strength of the steel sheet is 780 MPa or more. .

Description

technical field [0001] The present invention relates to a steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a steel sheet having a tensile strength of 780 MPa or more and excellent delayed fracture resistance, which are preferable as structural members of automobiles, buildings, and home appliances, and their Manufacturing method. Background technique [0002] In recent years, steel sheets used as structural members of automobiles, buildings, home appliances, etc. have been required to have excellent delayed fracture resistance in addition to required strength and formability. Delayed fracture is a phenomenon in which hydrogen infiltrated into the steel accumulates at stress-concentrated areas and destroys the steel. [0003] Delayed fracture has long been known to occur in high-strength steel such as high-strength bolts, PC steel wire, and line pipe. Various measures for improving the delayed fracture resistance characteristics ...

AI Technical Summary

Problems solved by technology

However, the martensite generated after forming promotes the occurrence of delayed fracture, so it is difficult to apply the TRIP effect to a steel sheet that is intended to improve formability and delayed fracture resistance (see Non-Patent Document 2)

Thus, it is difficult to improve both formability and delayed fracture resistance of the steel sheet.

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