A strip steel with a tensile strength of 440mpa grade and excellent secondary processing performance at low temperature and its production method

Abstract

A strip steel with a tensile strength of 440MPa and excellent secondary processing performance at low temperature, its composition and wt% are: C: 0.0008-0.0028%, Si: 0.07-0.1%, Mn: 1.4-1.8%, P: 0.06~0.09%, S≤0.008%, Al: 0.02~0.05%, Ti: 0.02~0.06%, Nb: 0.006~0.022%, B: 0.0005~0.0015%, N≤0.002%; production steps: smelting and casting billet; billet heating; rough rolling; finish rolling; coiling; cold rolling after pickling; continuous annealing treatment; slow cooling; rapid cooling; aging treatment; leveling. Under the premise of ensuring the mechanical properties, the present invention makes the plastic strain ratio ≥ 1.5r 90 , strain hardening index ≥ 1.0n 90 ;The brittle transition temperature of the secondary processing is stable below ‑50°C, the reduction in the amount of Ti and Nb reduces the cost by no less than 5%, and there are no carbonized edges and bonding defects on the surface of the steel plate, which is used to prepare automotive outer coverings for applications in colder regions pieces.

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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