Hot dip galvanized high strength steel sheet having excellent plating adhesion and hole expansibility, and its production method

Abstract

The present invention provides hot dip galvanized high strength steel sheet excellent in plating adhesion and hole expandability and a method of production of the same, that is, hot dip galvanization steel sheet excellent in plating adhesion and hole expandability containing, by mass%, C: 0.08 to 0.35%, Si: 1.0% or less, Mn: 0.8 to 3.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.25 to 1.8%, Mo: 0.05 to 0.35%, and N: 0.010% or less and having a balance of Fe and unavoidable impurities, said hot dip galvanized high strength steel characterized in that the steel sheet has a metal structure having ferrite, bainite, by area percent, 0.5% to 10% of tempered martensite, and, by volume percent, 5% or more of residual austenite, and a method of production comprising annealing by a continuous annealing process at 680 to 930 DEG C in temperature, then cooling to the martensite transformation point or less, then hot dip galvanizing the steel during which heating the steel to 250 to 600 DEG C, then hot dip galvanizing it.

Description

technical field [0001] The present invention relates to a hot-dip galvanized high-strength steel sheet excellent in coating adhesion and hole expandability and a manufacturing method thereof. Background technique [0002] In recent years, the fuel efficiency of automobiles has been improved, and the demand for weight reduction of the vehicle body has been increasing. In order to achieve weight reduction, there is an increasing demand for high-strength steel sheets with excellent tensile strength and yield strength. However, forming such a high-strength steel sheet becomes difficult as the strength increases, and in particular, the elongation of the steel material decreases. In response to this, recently, TRIP steel (high retained austenitic steel) having both high strength and elongation has been gradually used for structural members of automobiles. [0003] However, since the conventional TRIP steel contains more than 1% of Si, there is a problem that it is difficult to un...

AI Technical Summary

Problems solved by technology

However, in the case of high-temperature annealed materials and high-strength steel sheets used to manufacture the above-mentioned hot-dip galvanized steel sheets, the yield is low due to high-temperature annealing. In the case of ordering / production, there is a problem that the production is concentrated on the hot-dip galvanizing line with its own annealing furnace, which cannot be dealt with

[0005] On the other hand, although the usual continuous annealing line for annealing cold-rolled steel sheets and electro-galvanized steel sheets generally has high speed and high productivity, the production load may decrease due to the same production changes as described above, depending on the situation. Different from the problem that there is no plate material to pass, the production line is temporarily stopped, and there is a profound problem of surplus production capacity