800 MPa-level cold rolled steel with hardening index value n larger than or equal to 0.155 and production method

Abstract

The invention provides 800 MPa-level cold rolled steel with a hardening index value n larger than or equal to 0.155. The cold rolled steel comprises components in percentage by weight as follows: 0.16%-0.19% of C, 0.15%-0.20% of Si, 1.5%-2.5% of Mn, 0.40%-0.50% of Cr, 0.040%-0.05% of Ni, 0.010%-0.020% of Nb, smaller than 0.005% of S, smaller than 0.008% of P and smaller than 0.006% of N. A technology comprises steps as follows: hot rolling of a casting blank; coiling; cold rolling; continuous annealing; slow cooling; fast cooling; overaging treatment; natural cooling to room temperature. The yield strength Rp0.2 of 540 MPa-670 MPa and the tensile strength Rm of 870 MPa-895 MPa are guaranteed, the hardening index n is enabled to be larger than or equal to 0.155, the elongation A80mm is enabled to be larger than or equal to 17%, so that the grain size of the metallographic structure reaches level 15.5, and the preparation demand of more complicated parts can be completely met.

Description

technical field [0001] The invention relates to a cold-rolled deep-drawing steel and a production method thereof, in particular to a cold-rolled steel with a tensile strength of 800MPa and a production method thereof. Background technique [0002] According to the different forming methods of automobile stamping parts, it can be divided into five categories: bulging-drawing forming, deep drawing forming, shallow drawing forming, bending forming and rummaging forming. At the same time, many parts with different degrees of difficulty in forming have different performance requirements for materials, and a one-to-one correspondence relationship is established. For example, for bulging-drawing forming, the important indicators are n value and r value. The main indicators are elongation and yield strength. n is the work hardening exponent or strain hardening exponent, and its physical meaning is the amount of deformation when uniaxial stretching occurs necking. The larger the n ...

AI Technical Summary

Problems solved by technology

What is more prominent is that with the further improvement of steel plate strength, all relevant standards have not put forward clear performance index requirements for the n value of 800MPa grade steel plates, resulting in low n values ​​or large fluctuations of 800MPa grade related products, generally only 0.10-0.14

The lower n value causes severe local necking and aggravated cracking tendency of the material during the stamping process

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