Method of heat treating steel

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 A 1 temperature, but less than its A 3 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.

What is claimed is: 
     
         1 . A method of heat treating a steel workpiece having a polycrystalline microstructure, the method comprising the following steps:
 (a) heating the steel workpiece to a first temperature greater than or equal to its lower austenite transformation temperature A 1 , but less than its upper austenite transformation temperature A 3  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;   (b) heating the steel workpiece to a second temperature greater than the first temperature to increase the average grain diameter of the grains of austenite in the steel workpiece; and   (c) cooling the steel workpiece to ambient temperature at a rate sufficient to retain a major portion of the grains of austenite obtained during steps (a) and (b) so that the microstructure of the steel workpiece comprises a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature.   
     
     
         2 . The method set forth in  claim 1  wherein the steel workpiece comprises, by weight, 5-12% manganese (Mn) and 0.1-0.3% carbon (C). 
     
     
         3 . The method set forth in  claim 1  wherein the steel workpiece is heated in step (a) to a first temperature less than or equal to 50° C. below the A 3  temperature of the steel workpiece. 
     
     
         4 . The method set forth in  claim 1  wherein the steel workpiece is heated in step (b) to a second temperature greater than or equal to 100° C. below the A 3  temperature of the steel workpiece and less than or equal to 20° C. above the A 3  temperature of the steel workpiece. 
     
     
         5 . The method set forth in  claim 1  wherein the steel workpiece is heated in step (a) for a duration in the range of one second to one-hundred hours, and wherein the steel workpiece is heated in step (b) for a duration in the range of one second to 1000 seconds. 
     
     
         6 . The method set forth in  claim 1  further comprising:
 between steps (a) and (b), cooling the steel workpiece to a third temperature less than the first temperature. 
 
     
     
         7 . The method set forth in  claim 1  wherein, after step (c), 10-100% of the grains of austenite in the steel workpiece have a diameter greater than an average grain diameter of the ferrite matrix phase. 
     
     
         8 . The method set forth in  claim 1  wherein, after step (c), the retained austenite phase comprises at least 30 vol % of the microstructure of the steel workpiece. 
     
     
         9 . The method set forth in  claim 1  wherein, after step (c), the ferrite matrix phase comprises at least 40 vol % of the microstructure of the steel workpiece. 
     
     
         10 . The method set forth in  claim 1  wherein, after step (c), the steel workpiece comprises ≤10 vol % martensite, bainite, pearlite, and/or cementite. 
     
     
         11 . The method set forth in  claim 1  wherein, after step (c), the steel workpiece is formed into a shaped part without exhibiting Lüders strain or yield point elongation during deformation thereof. 
     
     
         12 . The method set forth in  claim 1  wherein the steel workpiece is in the form of a hot-rolled and cold-rolled steel sheet. 
     
     
         13 . A method of manufacturing a steel part comprising:
 providing a hot-rolled and cold-rolled steel sheet having a polycrystalline microstructure and comprising, by weight, 0.1-0.3% carbon (C) and 5-12% manganese (Mn);   heating the steel sheet to a first temperature greater than or equal to its lower austenite transformation temperature A 1 , but less than its upper austenite transformation temperature A 3  to transform the microstructure of the steel sheet into a multiphase microstructure including grains of ferrite and grains of austenite having an average grain diameter;   heating the steel sheet to a second temperature greater than the first temperature to increase the average grain diameter of the grains of austenite in the steel sheet;   cooling the steel sheet to ambient temperature at a rate sufficient to retain a major portion of the grains of austenite within the microstructure of the steel sheet so that the steel sheet comprises a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature; and   forming the steel sheet into a shaped steel part.   
     
     
         14 . The method set forth in  claim 13  wherein the hot-rolled and cold-rolled steel sheet has a thickness in the range of 0.5 mm to 6 mm. 
     
     
         15 . The method set forth in  claim 13  wherein the steel sheet is formed into the shaped steel part without exhibiting Lüders strain or yield point elongation during deformation thereof. 
     
     
         16 . The method set forth in  claim 13  wherein an exterior surface of the shaped steel part does not exhibit Lüders bands or stretcher strain marks. 
     
     
         17 . The method set forth in  claim 13  wherein the shaped steel part comprises an automotive vehicle body panel or support structure.