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Method of heat treating steel

a technology of heat treatment and steel workpiece, which is applied in the direction of heat treatment apparatus, manufacturing tools, furnaces, etc., can solve the problems of reducing the uniform deformation behavior of steel workpieces, reducing the ductility and/or mechanical strength of steel workpieces, and reducing the ductility of steel workpieces. , to achieve the effect of increasing the average grain diameter of grains

Inactive Publication Date: 2019-03-07
GM GLOBAL TECH OPERATIONS LLC
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  • Summary
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  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

The patent describes a method for heat treating a steel workpiece with a polycrystalline microstructure to create a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature. The method involves heating the steel workpiece to a first temperature, then cooling it at a controlled rate to retain a majority of the grains of austenite. The resulting microstructure has improved mechanical properties and can be formed into shaped parts without exhibiting strain or yield point elongation during deformation. The heat treatment can be carried out on a hot-rolled and cold-rolled steel sheet, which can be used in automotive vehicles or support structures.

Problems solved by technology

Depending on its composition and microstructure, in some loading situations, the steel workpiece may experience heterogeneous plastic deformation, with the deformed regions of the workpiece undesirably manifesting themselves as depressions known as Lüders bands or stretcher strain marks on the surface of the steel workpiece.
However, such methods oftentimes result in a loss of ductility and / or mechanical strength and may only improve the uniform deformation behavior of the steel workpiece for a limited duration.

Method used

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  • Method of heat treating steel
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Examples

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example 1

[0047]A first one of the steel sheets was subjected to a single stage heat treatment process. Starting at ambient temperature, the steel sheet was heated at a suitable heating rate to a temperature T of about 620° C. (T=A3-90° C.), and maintained at such temperature for 3 minutes (180 seconds). Thereafter, the steel sheet was air cooled down to ambient temperature.

[0048]FIG. 5 is an electron backscattered diffraction (EBSD) map depicting the microstructure of the heat treated steel sheet. As shown, the heat treated steel sheet exhibits a multiphase microstructure including a plurality of dispersed austenite grains (white) and a matrix phase of ultrafine grains of ferrite (black). Austenite grains having high angle grain boundaries (>15°) are shown as black lines and ferrite grains having high angle grain boundaries (>15°) are shown as white lines. The EBSD map indicates that the heat treated steel sheet comprises about 20 vol. % retained austenite and about 80 vol. % ferrite. The au...

example 2

[0051]A second one of the steel sheets was subjected to a dual stage heat treatment process. Starting at ambient temperature, in the first heat treatment stage, the steel sheet was heated at a suitable heating rate to a first temperature T1 of about 620° C. (T1=A3−90° C.) and maintained at such temperature for 3 minutes (180 seconds). Immediately thereafter, in the second heat treatment stage, the steel sheet was heated to a second temperature T2 of about 710° C. (T2=A3) and maintained at such temperature for 3.25 minutes (195 seconds). The steel sheet was not cooled between the first and second heat treatment stages. After completion of the second heat treatment stage, the steel sheet was air cooled down to ambient temperature.

[0052]FIG. 6 is an electron backscattered diffraction (EBSD) map depicting the microstructure of the heat treated steel sheet. As shown, the heat treated steel sheet exhibits a multiphase microstructure including a plurality of dispersed austenite grains (whi...

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Abstract

A method of heat treating a steel workpiece may include two heat treatment stages. In the first stage, the steel workpiece may be heated to a first temperature greater than or equal to its A1 temperature, but less than its A3 temperature to transform the microstructure of the steel workpiece into a multiphase microstructure including grains of ferrite and grains of austenite having an average grain diameter. In the second stage, the steel workpiece may be heated to a second temperature greater than the first temperature to increase the average grain diameter of the grains of austenite. Thereafter, the steel workpiece may be cooled to ambient temperature at a rate sufficient to retain a major portion of the grains of austenite obtained during the first and second heat treatment stages. The as-heat treated steel workpiece may comprise a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature.

Description

TECHNICAL FIELD[0001]The present disclosure relates to methods of heat treating steel and, more specifically, to methods of heat treating steel workpieces to improve the deformation behavior thereof.INTRODUCTION[0002]Steel is commonly used in the manufacture of automotive vehicle body panels and support structures, including frames, rails, crossmembers, pillars, roofs, quarter panels, hoods, and deck lids, to name a few. Various forming processes, including drawing, stamping, and rolling, may be used to plastically deform a steel workpiece for the purpose of strengthening the workpiece and / or forming the into a desired shape. Depending on its composition and microstructure, in some loading situations, the steel workpiece may experience heterogeneous plastic deformation, with the deformed regions of the workpiece undesirably manifesting themselves as depressions known as Lüders bands or stretcher strain marks on the surface of the steel workpiece. Various methods of promoting the hom...

Claims

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Application Information

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IPC IPC(8): C21D9/46C22C38/04C22C38/02C21D6/00
CPCC21D9/46C22C38/04C22C38/02C21D2211/005C21D6/005C21D2211/001C21D6/008
Inventor MA, JIAWEILU, QISUN, LI
Owner GM GLOBAL TECH OPERATIONS LLC