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Method for producing high strength, high ductility steel strip

a high ductility, steel strip technology, applied in the field of high ductility steel strip production, can solve the problems of unfavorable strip surface oxidation, unfavorable strip surface removal, and insufficient ductility of conventional high strength steels to produce complex parts

Inactive Publication Date: 2006-07-27
USX CORP
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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Benefits of technology

[0008] In a preferred form of the invention, the hot rolled steel strip is cooled at a rate sufficient to ensure that the microstructure of the strip consists of greater than 90 percent by volume of martensite. In another preferred form of the invention, the annealing cycle time is controlled so that the maximum difference in temperature between an outer lap of the coil and an inner lap of the coil is less than 28° C. (50° F.) during the anneal cycle.
[0009]FIG. 1 is a plot of the product of ultimate tensile strength in MPa times the percent tota

Problems solved by technology

However, conventional high strength steels lack sufficient ductility to produce complex parts.
One disadvantage of this reference steel is that the relatively high Si content causes unfavorable oxidation of the strip surface during hot rolling and coiling.
Removal of the oxidized surface during hot rolling and in a subsequent pickling operation is difficult and requires extra processing time.
Failure to remove the oxidized strip surface results in rejection of the strip for automotive applications due to undesirable surface appearance or unremoved scale.

Method used

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  • Method for producing high strength, high ductility steel strip
  • Method for producing high strength, high ductility steel strip
  • Method for producing high strength, high ductility steel strip

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Embodiment Construction

[0012] Tests were conducted in the laboratory to determine whether a high strength, high ductility steel could be produced utilizing conventional hot rolling practices and box annealing techniques. Table 1 below presents the composition of the steel materials studied in these laboratory tests.

TABLE 1SteelCMnPSSiCuNiCrMoAlNA0.1005.18.015.008.12.03.03.04.02.03.009B0.0955.80.013.008.13.04.04.04.02.03.008C0.0997.09.015.008.13.03.03.04.01.03.008

[0013] Laboratory ingots of the above compositions were reheated to 1260° C. (2300° F.) and rolled from 178 mm (7) inch thickness to an intermediate slab thickness of 51 mm (2 inches). The slabs were reheated again to the same temperature and hot rolled in seven passes to 4 mm (0.16 inch) thickness to simulate a conventional hot rolling process. The hot rolled strips were then placed in a programmable furnace which was set to cool at 28° C. / hour (50° F. / hour) from the prescribed “coiling temperature” to simulate coiling on a commercial hot strip...

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Abstract

A method is provided for the manufacture of steel steel strip having high strength and high ductility. The method includes hot rolling steel consisting essentially of in weight percent C 0.05 / 0.20, Mn 3.0 / 8.0, Si less than 0.5, Al less than 0.1, the balance Fe and inevitable impurities at a minimum finishing temperature of Ar1+50 C, cooling the hot rolled steel strip at a rate sufficient to ensure that the microstructure of the strip consists of greater than 50 percent by volume of martensite, and then annealing the steel strip at a temperature within the range of Ac1 to Ac1+50° C. for a minimum time at temperature of one hour. The steel has a microstructure after annealing that consists essentially of ferrite and retained austenite.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a method for producing high strength, high ductility steel strip, and particularly to a method for producing steel strip containing 0.05 / 0.20% carbon, 3 to 8% manganese, less than 0.5% silicon, less than 0.1 aluminum, balance iron and incidental impurities by hot rolling, cooling to form a microstructure of which a major portion consists of martensite, and box annealing in the intercritical temperature range to form ferrite and significant amounts of austenite that is retained upon cooling to room temperature. BACKGROUND OF THE INVENTION [0002] Customers for automotive steel sheet and strip are interested in the development of steels that possess higher levels of strength than that of currently available steels, to enable the reduction of steel thickness for automotive applications. Higher strength steels enable the automotive designer to reduce vehicle weight while maintaining structural integrity. However, conventional...

Claims

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Application Information

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IPC IPC(8): C21D8/00
CPCC21D8/0205C21D8/0263C21D2211/001C21D2211/005C22C38/58
Inventor MERWIN, MATTHEW J.
Owner USX CORP
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