High-strength hot-dip galvanized steel sheet and process for producing same

Abstract

A hot-dip galvannealed steel sheet which comprises a steel sheet that has a composition containing, in terms of mass%, 0.02-0.30% C, 0.01-2.5% Si, 0.1-3.0% Mn, 0.003-0.08% P, up to 0.01% S, 0.001-0.20% Al, and 0.03-0.40% Ti, with the remainder comprising Fe and unavoidable impurities, and a zinc coating layer formed on each of the surfaces thereof in an amount of 20-120 g / m2 per surface, wherein each coating layer contains carbide grains having an average grain diameter of 10 nm or smaller in a number of 5-50 per section and oxide grains having an average grain diameter of 50 nm or larger in a number of 5-50 per section. The section means an area (t11 ([mu]m2)) defined by the coating-layer thickness (t1 [mu]m) and segments obtained by dividing a cross-section of the coating layer in the direction perpendicular to the thickness direction at an interval of 1 [mu]m.

Description

technical field [0001] The invention relates to an alloyed hot-dip galvanized steel sheet suitable as an antirust surface-treated steel sheet for automobiles and a manufacturing method thereof. Background technique [0002] TS440MPa or lower hot-rolled steel sheets have been used in the frame and running system of automobiles and trucks. However, recently, in order to improve the crash resistance of automobiles and protect the global environment, high-strength and thin-walled steel sheets for automobiles are being promoted, and studies have begun to use high-strength hot-rolled steel sheets of TS590MPa, TS780MPa, and TS980MPa or higher. . [0003] Automobile components are mostly complex-shaped components obtained by press molding, and materials with high strength and excellent processability are required. On the other hand, from the viewpoint of ensuring the antirust performance of the vehicle body while reducing the thickness of the steel sheet, surface-treated steel she...

AI Technical Summary

Problems solved by technology

When coiling a hot-rolled base material containing Si, Mn, and other elements that are more easily oxidized than Fe (hereinafter also referred to as easily oxidizable elements) under such high CT conditions, the surface layer of the steel plate base The formation of internal oxides containing easily oxidizable elements will excessively promote the Zn-Fe alloying reaction in the subsequent hot-dip galvanizing treatment and alloying treatment, resulting in a problem that the adhesion of the plating layer will deteriorate.

In addition, when a large amount of internal oxides are present in the surface layer of the base steel sheet, there is a problem that the internal oxides serve as starting points during stretch flange processing, and microcracks occur in the surface layer of the steel sheet and the coating, and the stretch flanged portion Deterioration of corrosion resistance after painting

[0007] On the other hand, when the coiling treatment is performed by lowering the CT in order to suppress internal oxides generated during hot rolling, the strength and workability decrease due to insufficient precipitation of carbides and growth of structures such as pearlite. In addition, when the steel sheet is annealed in the subsequent continuous hot-dip galvanizing equipment, hydrogen gas is absorbed, which causes the problem of deterioration of hydrogen embrittlement resistance

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