High strength steel product with improved formability and steel manufacturing process

Abstract

A process and apparatus are disclosed for manufacturing high strength low alloy (HSLA) flat rolled steel products having high yield strength and formability. A preferred process comprises the formation of an as-cast product; reducing the thickness of the as cast product in a roughing mill; holding the temperature of the rough-reduced product at a temperature sufficient to permit substantially complete recrystallization of austenite grains in the rough-reduced product; followed by a final reduction step in a hot rolling strip mill. In this process, there is little or no precipitation of microalloy until after the material passes through the strip mill and therefore the material being rolled is relatively soft compared to known processes. Therefore, less power is required to roll the material to its final dimensions, with a corresponding improvement in dimensional control. The material produced according to this process has a strength of at least about 70 ksi, more preferably at least about 80 ksi, and a formability as measured by n-value within the range of from about 0.1 to about 0.15.

Description

FIELD OF THE INVENTION [0001] The present invention relates to high strength steel products, and more particularly to high strength low alloy (HSLA) flat rolled steel products having high yield strength and formability. The invention also relates to manufacturing processes for producing flat rolled steel products having high yield strength and formability. BACKGROUND OF THE INVENTION [0002] Most HSLA steels are produced in conventional processes where molten steel from a basic oxygen furnace (BOF) or an electric arc furnace (EAF) is cast, cooled, reheated and reduced in thickness while still hot in a rolling mill. The rolling mill reduces the thickness of the slab to produce thin gauge steel sheet or strip material having high strength characteristics. Some HSLA steels are produced by modern thin-slab or medium-slab casting processes in which slabs of steel, still hot from the caster, are transferred directly to a reheating or equalizing furnace prior to thickness reduction in the h...

AI Technical Summary

Benefits of technology

[0012] (e) holding the rough-reduced product at a temperature above the recrystallization stop temperature and above the precipitation temperature of the microalloy for a time sufficient to permit substantially complete recrystallization of the austenite and thereby reduce a grain size of the austenite;

Problems solved by technology

Due to the steel microstructure and metallurgical transformations taking place in the material during hot rolling, reducing the gauge of the material also causes the material to become harder.

As the hardness increases, further thickness reduction by rolling becomes more difficult, and the rolling mill must operate with increasing power levels to reduce the material thickness to the desired level at a particular width.

This makes shaping of the material more difficult.

Thus, rolling the HSLA material to light gauges interferes with the ability to shape the material, limiting its utility for many applications requiring high strength, light weight and good formability, such as automotive applications.

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