Bake-hardenable cold rolled steel sheet with superior strength and aging resistance, gal-vannealed steel sheet using the cold rolled steel sheet and method for manufacturing the cold rolled steel shee

Abstract

A bake-hardenable cold rolled steel sheet with high strength and superior aging resistance used for outer panels of an automobile body, a galvannealed steel sheet using the cold-rolled steel sheet, and a method for manufacturing the cold-rolled steel sheet are disclosed. The steel sheet comprises, by weight%, C: 0.0016-0.0025%, Si: 0.02% or less, Mn: 0.2-1.2%, P: 0.05-0.11%, S: 0.01% or less, Sol. Al: 0.08-0.12%, N: 0.0025% or less, Ti: 0-0.003%, Nb: 0.003-0.011%, Mo: 0.01-0.1%, B: 0.0005-0.0015%, the balance of Fe and other unavoidable impurities. The steel sheet has superior bake hardenability, aging resistance at room temperature, and secondary work embrittlement resistance.

Description

technical field [0001] The present invention relates to a cold-rolled steel sheet used as an outer panel of a vehicle body, a galvannealed steel sheet using the galvanized layer of the cold-rolled steel sheet, and a method for manufacturing the cold-rolled steel sheet. More specifically, the present invention relates to a high-strength cold-rolled steel sheet having excellent bake hardenability, room-temperature aging resistance, and secondary work embrittlement resistance, and a galvanized layer diffusion annealing treatment of the cold-rolled steel sheet A steel plate, and a method for manufacturing the cold-rolled steel plate. Background technique [0002] In order to improve fuel efficiency and reduce the weight of automobiles, there is an increasing demand for high-strength steel sheets for vehicle bodies to improve dent resistance and reduce thickness of outer panels. [0003] For cold-rolled steel sheets used for body panels, excellent properties are required in term...

AI Technical Summary

Problems solved by technology

Whereas in the case of batch annealing, because P-added aluminum deoxidized steel is cooled at a relatively fast rate, it is easy to ensure bake hardenability, but there is a problem that formability is deteriorated because of rapid heating and a short annealing process

[0029] In the above method, although the addition of boron (B) is about 5ppm, and it is expected that boron can improve the resistance to secondary processing embrittlement, the excessively high P content limits the improvement of secondary embrittlement resistance by adding B Effect

[0030] In addition, if an excessive amount of B is added to the steel to improve the secondary processing embrittlement resistance, the secondary processing embrittlement resistance will be deteriorated due to the high B content

Therefore, there is a certain limit to the amount of B added to the steel

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