Process for producing high-strength cold rolled steel sheet with low yield strength and with less material quality fluctuation

Abstract

A process for producing a high-strength cold rolled steel sheet of low YP with less material quality fluctuation, comprising sequentially subjecting a steel containing, by mass, more than 0.01 to less than 0.08% C, 0.2% or less Si, 0.8 to less than 1.7% Mn, 0.03% or less P, 0.02% or less S, 0.3% or less sol. Al, 0.01% or less N and more than 0.4 to 2% Cr so that the relationships 1.9<([%Mn]+1.3x[%Cr])<3 and 0.34=[%Cr] / [%Mn] are satisfied and containing the balance iron and unavoidable impurities to hot rolling and cold rolling, heating at an average heating rate of less than 3 DEG C / sec between 680 DEG and 740 DEG C, annealing at an annealing temperature of higher than 740 DEG to lower than 820 DEG C and cooling at an average cooling rate of 2 DEG to 30 DEG C / sec between the annealing temperature and 650 DEG C, at an average cooling rate of 10 DEG C / sec or greater between 650 DEG C and the following Tc and at an average cooling rate of 0.2 DEG to 10 DEG C / sec between the Tc and 200 DEG C. Tc = 410-40x[%Mn]-30x[%Cr] ([%Mn] and [%Cr] represent the contents of Mn and Cr, respectively).

Description

technical field [0001] The present invention relates to a method for producing a high-strength cold-rolled steel sheet for press forming used in applications such as automobiles and home appliances through a press forming process. Background technique [0002] For a long time, for automotive panels that require dent resistance, such as hoods, doors, trunk lids, rear doors, and fenders, it is possible to apply carbonitrides such as Nb and Ti to form carbonitrides based on ultra-low carbon steel. TS: 340MPa grade BH steel plate (sinter hardened steel plate, hereinafter referred to as 340BH), TS: 270MPa grade IF steel plate (Interstitial Free steel plate, hereinafter referred to as 270IF). In recent years, due to the further increase in the demand for vehicle body weight reduction, studies have been advanced to further increase the strength of the above-mentioned covering to improve the dent resistance and to reduce the thickness of the steel plate. In addition, studies are be...

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Problems solved by technology

[0003] However, if it is desired to increase the strength by adding solid-solution strengthening elements such as Mn and P based on 340BH with yield strength YP: 230MPa and 270IF with YP: 180MPa, and to make the steel plate thinner, the problem of surface deformation will occur.

Here, surface deformation refers to minute wrinkles and wavy patterns on the press-formed surface caused by an increase in YP. If this surface deformation occurs, the appearance and design of the door, trunk lid, etc. will be significantly impaired.

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