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.

1 . A process for manufacturing a high-strength, high-ductility alloy carbon steel, said process comprising:
 (a) cooling a solid carbon alloy steel having a homogeneous austenite phase with all alloying elements in solid solution so as to form a carbon steel alloy having a microstructure consisting of laths of martensite alternating with from about 0.5% to about 15% by volume of films of retained austenite, and   (b) cold working the carbon steel alloy from step (a) without intermediate heat treatment to a reduction sufficient to achieve a tensile strength of at least about 150 ksi.   
     
     
         2 . The process of  claim 1  wherein 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 . The process of  claim 1  wherein step (b) comprises cold working said carbon steel alloy to a cross-sectional area reduction of at least about 20% per pass. 
     
     
         4 . The process of  claim 1  wherein step (b) comprises cold working said steel alloy to a cross-sectional area reduction of at least about 25% per pass 
     
     
         5 . The process of  claim 1  wherein 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 . The process of  claim 1  wherein step (b) comprises cold working said carbon steel alloy in a series of passes without heat treatment between passes. 
     
     
         7 . The process of  claim 1  wherein step (b) is performed at a temperature of about 10° C. or below. 
     
     
         8 . The process of  claim 1  wherein step (b) is performed within approximately 25 C of ambient temperature. 
     
     
         9 . The process of  claim 1  wherein 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 . The process of  claim 1  wherein said carbon steel alloy is in the form of a sheet, and step (b) comprises rolling said carbon steel alloy. 
     
     
         11 . The process of  claim 1  wherein step (a) further 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 . The process of  claim 11  wherein said carbon steel alloy composition having a martensite start temperature of at least about 350 C. 
     
     
         13 . The process of  claim 11  wherein said retained austenite films are of a uniform orientation. 
     
     
         14 . The process of  claim 11  wherein 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 . The process of  claim 11  wherein said temperature of step (ii) is from about 800 C to about 1150 C. 
     
     
         16 . (canceled) 
     
     
         17 . The process of  claim 11  wherein step (iii) comprises cooling said homogeneous austenite phase to a temperature of from about 800 C to about 1,000 C. 
     
     
         18 . The process of  claim 11  wherein 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 . The process of  claim 11  wherein 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. 
     
     
         20 . The process of  claim 1  wherein said films of retained austenite constitute from about 3% to about 10% by volume of said microstructure. 
     
     
         21 . The process of  claim 1  wherein said films of retained austenite constitute from about 0.5% to about 5% by volume of said microstructure.