High strength cold-rolled steel sheet excellent in ductility, hole-forming property and surface treatment property, molten galvanized steel sheet, and method for manufacturing same

Abstract

Provided are a high-strength cold-rolled steel sheet excellent in ductility, boring workability and surface treatment characteristics, a molten galvanized steel sheet, and a method for manufacturing the same. The cold-rolled steel sheet of the present invention comprises in % by weight: 0.05 to 0.3% of carbon (C); 0.6 to 2.5% of silicon (Si); 0.01 to 0.5% of aluminum (Al); 1.5 to 3.0% of manganese(Mn); and the remainder being Fe and unavoidable impurities, wherein the steel microstructure contains, in an area fraction, ferrite of 60% or less, acicular bainite of 25% or more, martensite of 5%or more, and acicular retained austenite of 5%, wherein the ferrite has an average diameter of 2 [mu]m or less and the ferrite satisfies that Fn2 defined by relational expression 1 is 89% or more andFa5 defined by relational expression 2 is 70% or less.

Description

technical field [0001] The present invention relates to a high-strength steel plate used for structural parts of automobiles, and more specifically, to a high-strength cold-rolled steel plate, a hot-dip galvanized steel plate and a manufacturing method thereof, the hole expandability and elongation of the steel plate Since it is excellent, it is very excellent in press formability, and also excellent in phosphate processability and spot weldability. Background technique [0002] In order to reduce the weight of automobiles, efforts are being made to increase the strength and reduce the thickness of steel plates used in structural parts. However, when the strength of the steel sheet is increased, there is a problem that the press formability is relatively lowered. In order to improve the press formability, high hole expandability is required in addition to the elongation of the steel, and therefore, a steel with a transformation structure using martensite, bainite, and retai...

AI Technical Summary

Problems solved by technology

Therefore, the phosphate treatment property of the cold-rolled steel sheet is poor, and the coating quality of the hot-dip galvanized steel sheet is deteriorated and cracks are generated at the spot welds

[0003] In order to solve the above problems, there is a method (JP2002-47535A) that reduces the composition of the alloy, and ensures a structure with excellent workability through two annealing heat treatments, and, after annealing, the surface of the steel plate is formed at a rate of 5 to 70 mg / m 2 However, since the cooling rate during the first annealing is 30°C / s or more, the shape of the plate is poor, so when plating Ni and other metals after the first annealing, due to Partial plating failure due to uneven plating and poor water cut-off

[0004] And, also proposed a kind of method (KR1998-7002926A), this method produces internal oxidation during annealing, reduces the amount of Si and Mn concentrated on the surface, thereby ensures the quality of hot-dip galvanizing, but, this method ensures excellent There are limitations in elongation and hole expandability, and the amount of alloy used to ensure retained austenite increases

[0005] In addition, the Si and Mn surface oxides formed during annealing hinder the phosphate treatment of cold-rolled steel sheets, and then reduce the adhesion of the electrophoretic coating, so the corrosion of the steel sheet is caused by the peeling of the electrophoretic coating caused by chips, etc. Reduced durability of automotive components

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