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Ultra Fine-Grained Advanced High Strength Steel Sheet Having Superior Formability

a technology of formability and high strength steel, applied in the field of advanced high strength steel, to achieve the effect of reducing the hardness of steel and easing the requirements of cold rolling

Active Publication Date: 2017-04-27
ARCELORMITTAL INVESTIGACION Y DESARROLLO SL
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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Benefits of technology

[0006]The inventors have found that a superior combination of strength and ductility in such steels can be obtained after short time annealing and a significant amount of austenite was retained regardless of cooling rate without any isothermal holding in the bainite region. The stability of austenite appears to be a result of a combination of fine grain size, and high Mn / carbon content in the austenite at specific intercritical temperature. Therefore the control of grain size and Mn / C partitioning during annealing is a key process which helps to achieve superior properties in the inventive steels.
[0008]The cold rolled TRIP steel sheet may be formed by the steps of: 1) forming an ingot of said composition; 2) hot rolling said ingot to form a plate; 3) hot rolling said plate to form a hot rolled sheet; 4) optionally annealing said hot rolled sheet to reduce the hardness thereof, thereby easing the cold rolling requirements; 5) cold rolling said hot rolled sheet to form a cold rolled sheet; and 6) annealing said cold rolled sheet to form the microstructure and mechanical properties of said cold rolled TRIP steel sheet.

Problems solved by technology

% Mn) and extremely ductile TWIP (TWinning Induced Plasticity) steels having up to 50% elongation at 1000 MPa tensile strength, although they have not found commercial application yet.

Method used

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  • Ultra Fine-Grained Advanced High Strength Steel Sheet Having Superior Formability

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examples

[0013]50 kg ingots of steel were produced by vacuum induction melting. The ingots were reheated to 1230° C. and hot rolled to 20 mm thick plates. The plates were then reheated to 1230° C. again for homogenization for 3 hours, followed by 3-pass secondary hot rolling to the gauge of 4 mm with finishing temperature of 900° C. Accelerated cooling to a coiling temperature (CT) of 750° C. was employed to simulate the Run-Out-Table of hot strip mills. The hot rolled sheets were held for 1 hr at 750 ° C., and then cooled to room temperature for coiling simulation in a furnace.

[0014]Since the as-rolled Mn steels displayed very high strength, a part of as-rolled material was subjected to batch annealing at various temperatures to soften the materials. The surface oxides and decarburized layers were mechanically removed by grinding 0.5 mm from each side. The ground as-hot rolled and batch annealed sheets were cold rolled to 1.5 mm at a cold reduction of 50%. Cold rolled sheets were annealed u...

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Abstract

A cold rolled, annealed TRIP steel sheet which has a composition including (in wt. %): C: 0.1-0.3; Mn: 4-10, Al: 0.05-5, Si: 0.05-5; and Nb: 0.008-0.1, the remainder being iron and inevitable residuals. The cold rolled sheet has an ultimate tensile strength of at least 1000 MPa, and a total elongation of at least 15%. The cold rolled sheet may have at least 20% retained austenite in its microstructure and may have greater than 50% lath-type annealed ferrite structure. The cold rolled sheet may have an ultra fine grain size of less than 5 micron for the retained austenite and ferrite.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This Application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 61 / 588,393 filed Jan. 19, 2012.FIELD OF THE INVENTION[0002]The present invention relates generally to Advanced High Strength Steels (AHSS). More specifically the present invention relates to (transformation induced plasticity) TRIP steel having a medium manganese content ranging between 4-10 wt. %. Most specifically the present invention relates to medium manganese bearing TRIP steels with high strength and good total elongation providing superior formability.BACKGROUND OF THE INVENTION[0003]As the use of high strength steels increases in automotive applications, there is a growing demand for steels of increased strength without sacrificing formability. Growing demands for weight saving and safety requirement motivate intensive elaborations of new concepts of automotive steels that can achieve higher ductility simultaneously with higher strength...

Claims

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Application Information

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Patent Type & Authority Applications(United States)
IPC IPC(8): C22C38/14C22C38/08C22C38/00C22C38/04C22C38/02C22C38/12C22C38/06
CPCC22C38/14C22C38/12C22C38/08C22C38/001C22C38/04C22C38/02C22C38/06C22C38/00
Inventor JUN, HYUN JOPOTTORE, NARAYAN SUBRAMANIAMPONSTEIN, NINA MICHAILOVNAYAKUBOVSKY, OLEG N.
Owner ARCELORMITTAL INVESTIGACION Y DESARROLLO SL
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