Cold-worked steels with packet-lath martensite/austenite microstructure

Abstract

Strain-hardened steel alloys having a high tensile strength are prepared by cold working of alloys whose microstructure includes grains in which laths of martensite alternate with thin films of stabilized austenite. Due to the high dislocation density of this microstructure and the tendency of the strains to move between the martensite and austenite phases, the strains created by cold working provide the microstructure with unique mechanical properties including a high tensile strength. Surprisingly, this is achieved without the need for intermediate heat treatments (patenting, in the case of steel wire) of the steel between cold working reductions.

What is claimed is:  
     
         1 . A process for manufacturing a high-strength, high-ductility alloy carbon steel, said process comprising: 
 (a) forming a carbon steel alloy having a microstructure comprising laths of martensite alternating with films of retained austenite, and    (b) cold working said carbon steel alloy to a reduction sufficient to achieve a tensile strength of at least about 150 ksi.    
     
     
         2 . A process in accordance with  claim 1  in which step (b) comprises cold working said carbon steel alloy to a reduction sufficient to achieve a tensile strength of from about 150 ksi to about 500 ksi.  
     
     
         3 . A process in accordance with  claim 1  in which step (b) comprises cold working said carbon steel alloy to a cross-sectional area reduction of at least about 20% per pass.  
     
     
         4 . A process in accordance with  claim 1  in which step (b) comprises cold working said steel alloy to a cross-sectional area reduction of at least about 25% per pass  
     
     
         5 . A process in accordance with  claim 1  in which step (b) comprises cold working said carbon steel alloy to a cross-sectional area reduction of from about 25% to about 50% per pass.  
     
     
         6 . A process in accordance with  claim 1  in which step (b) comprises cold working said carbon steel alloy in a series of passes without heat treatment between passes.  
     
     
         7 . A process in accordance with  claim 1  in which step (b) is performed at a temperature of about 100° C. or below.  
     
     
         8 . A process in accordance with  claim 1  in which step (b) is performed within approximately 25° C. of ambient temperature.  
     
     
         9 . A process in accordance with  claim 1  in which said carbon steel alloy is in the form of a rod or wire, and step (b) comprises drawing said carbon steel alloy through a die.  
     
     
         10 . A process in accordance with  claim 1  in which said carbon steel alloy is in the form of a sheet, and step (b) comprises rolling said carbon steel alloy.  
     
     
         11 . A process in accordance with  claim 1  in which step (a) comprises 
 (i) forming a carbon steel alloy composition having a martensite start temperature of at least about 300° C.,  
 (ii) heating said carbon steel alloy composition to a temperature sufficiently high to cause austenitization thereof, to produce a homogeneous austenite phase with all alloying elements in solution, and  
 (iii) cooling said homogeneous austenite phase through said martensite transition range at a cooling rate sufficiently fast to achieve said microstructure substantially avoiding carbide formation at interfaces between said laths of martensite and said films of retained austenite.  
 
     
     
         12 . A process in accordance with  claim 11  in which said carbon steel alloy composition having a martensite start temperature of at least about 350° C.  
     
     
         13 . A process in accordance with  claim 11  in which said retained austenite films are of a uniform orientation.  
     
     
         14 . A process in accordance with  claim 11  in which said carbon steel alloy composition consists of iron and alloying elements comprising from about 0.04% to about 0.12% carbon, from 0% to about 11% chromium, from 0% to about 2.0% manganese, and from 0% to about 2.0% silicon, all by weight.  
     
     
         15 . A process in accordance with  claim 11  in which said temperature of step (ii) is from about 800° C. to about 1150° C.  
     
     
         16 . A process in accordance with  claim 1  in which step (a) comprises 
 (i) forming a carbon steel alloy composition having a martensite start temperature of at least about 300° C.,  
 (ii) heating said carbon steel alloy composition to a temperature sufficiently high to cause austenitization thereof, to produce a homogeneous austenite phase with all alloying elements in solution,  
 (iii) cooling said homogeneous austenite phase to transform a portion of said austenite phase to ferrite crystals, thereby forming a two-phase microstructure comprising ferrite crystals fused with austenite crystals, and  
 (iv) cooling said two-phase microstructure through said martensite transition range under conditions causing conversion of said austenite crystals to a microstructure containing laths of martensite alternating with films of retained austenite.  
 
     
     
         17 . A process in accordance with  claim 16  in which step (iii) comprises cooling said homogeneous austenite phase to a temperature of from about 800° C. to about 1,000° C.  
     
     
         18 . A process in accordance with  claim 16  in which step (ii) comprises heating said carbon steel alloy composition to a temperature of from about 1,050° C. to about 1, 170° C., and step (iii) comprises cooling said homogeneous austenite phase to a temperature of from about 800° C. to about 1,000° C.  
     
     
         19 . A process in accordance with  claim 16  in which said carbon steel alloy composition consists of iron and alloying elements comprising from about 0.02% to about 0.14% carbon, from 0% to about 3.0% silicon, from 0% to about 1.5% manganese, and from 0% to about 1.5% aluminum, all by weight.