Steel sheet, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and production methods therefor

Abstract

A steel sheet according to one embodiment of the present invention has a prescribed chemical composition. The structure of a portion comprising 1 / 4 of the sheet thickness includes, expressed as volumefractions, 30-70% of tempered martensite, and a total of at least 20% but not more than 70% of ferrite and / or bainite. The structure of the portion comprising 1 / 4 of the sheet thickness includes, expressed as volume fractions, less than 10% of retained austenite, not more than 10% of fresh martensite, and not more than 10% of pearlite. The total volume fraction of retained austenite, fresh martensite, and pearlite in said structure is not more than 15%. The number density of iron-based carbides having a longitudinal diameter of at least 5 nm in the tempered martensite in the portion comprising 1 / 4 of the sheet thickness is at least 5*107 per mm<2>. The ratio of the number of [Epsilon] carbides to the number of the iron-based carbides having a longitudinal diameter of at least 5 nm in theportion comprising 1 / 4 of the sheet thickness is at least 20%. The tensile strength is at least 780 MPa.

Description

Technical field [0001] The present invention relates to steel sheets, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that are preferred as structural members for automobiles, buildings, household appliances, etc., and have a tensile strength of 780 MPa or more and excellent delayed fracture resistance properties, and their Production method. Background technique [0002] In recent years, steel sheets used as structural members for automobiles, buildings, home appliances, and the like are required to have excellent delayed fracture resistance in addition to required strength and formability. Delayed fracture is a phenomenon in which hydrogen that has penetrated into the steel material accumulates in the stress concentration area to destroy the steel material. [0003] It has long been known that delayed fracture occurs in high-strength steel such as high-strength bolts, PC steel wires, and line pipes. Various measures to improve the delayed fracture re...

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 intended to improve formability and delayed fracture resistance (see Non-Patent Document 2)

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

Images