Strip steel with tensile strength of 440 MPa and excellent low-temperature secondary machining performance and production method

Abstract

The invention discloses a strip steel with the tensile strength of 440 MPa and excellent low-temperature secondary machining performance. The strip steel comprises the components of, in parts by weight, 0.0008%-0.0028% of C, 0.07%-0.1% of Si, 1.4%-1.8% of Mn, 0.06%-0.09% of P, less than or equal to 0.008% of S, 0.02%-0.05% of Al, 0.02%-0.06% of Ti, 0.006%-0.022% of Nb, 0.0005%-0.0015% of B and less than or equal to 0.002% of N. The production method comprises the following steps of smelting and casting into a blank; heating a casting blank; rough rolling; finish rolling; coiling; cold rollingafter pickling; carrying out continuous annealing treatment; slowly cooling; rapidly cooling; aging treatment; and leveling. According to the strip steel and the production method thereof, on the premise that the mechanical property is guaranteed, the plastic strain ratio is larger than or equal to 1.5 <r90>, and the strain hardening index is larger than or equal to 1.0 <n90>; and the secondary machining brittle transition temperature is stabilized below minus 50 DEG C, the amount of elements Ti and Nb is reduced, so that the cost is reduced by not less than 5%, the surface of the steel plateis free of carbonization edges and bonding defects, and therefore the steel plate can be used for preparing automobile outer covering parts applied to more severe cold areas.

Description

technical field [0001] The invention relates to a steel for automobiles and a production method, in particular to a strip steel with a tensile strength of 440 MPa and a production method with excellent secondary processing performance at low temperature, which is especially suitable for high-strength automobile outer covering parts used in northern severe cold regions steel. Background technique [0002] With the development of automobile lightweight, the proportion of cold-rolled high-strength steel in automobile outer covering materials is getting higher and higher. Studies have shown that about 75% of fuel consumption is related to vehicle quality, and reducing vehicle quality can effectively reduce fuel consumption and emissions. For every 10% reduction in vehicle quality, fuel consumption will drop by 8%, and emissions will drop by 4%. With the use of high-strength steel plates, the body steel plates with an original thickness of 1.0-1.2 mm can be reduced to 0.7-0.8 m...

AI Technical Summary

Problems solved by technology

[0004] Deficiencies in the prior art: some elements are not matched with C elements and Ti and Nb elements, and a large amount of FePTiNb composite precipitates are easily precipitated on the grain boundaries during hot rolling, thereby weakening the secondary processing of the product performance, so that the secondary processing embrittlement temperature can not reach -60 ℃ or lower

Although this document can obtain strength ≥ 440 MPa, yield ratio less than 0.6, elongation 33-36%, r value ≥ 1.7, good deep drawability, but the resistance to secondary processing brittleness has not been paid attention to, and has not revealed the impact of ductile-brittle transition. The ratio relationship between elements that have a great influence on temperature, especially the effect of the addition of Ti and Nb elements on the ductile-brittle transition temperature

In the literature, the Nb element is 0.08-0.12%, far exceeding the ratio relationship with the C element. The excessive addition of Nb directly causes a large amount of Nb composite phase to precipitate in hot rolling, weakens the resistance to secondary processing brittleness, and increases the ductile-brittle transition temperature. Adding excess Nb element increases manufacturing cost

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