High yield strength steel

Abstract

This disclosure is related to high yield strength steel where yield strength can be increased without significantly affecting ultimate tensile strength (UTS) and in some cases, higher yield strength can be obtained without significant decrease in ultimate tensile strength and total elongation.

1 . A method to increase yield strength in a metallic alloy comprising:
 a. supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C, melting said alloy, cooling at a rate of 10 −4 K/sec to 10 3  K/sec and solidifying to a thickness of >5.0 mm to 500 mm;   b. processing said alloy into a first sheet form with thickness from 0.5 to 5.0 mm with the first sheet having a total elongation of X 1  (%), an ultimate tensile strength of Y 1  (MPa), and a yield strength of Z 1  (MPa);   c. permanently deforming said alloy in the temperature range of 150° C. to 400° C. into a second sheet form exhibiting one of the following tensile property combinations A or B:   A. (1) total elongation X 2 =X 1 ±7.5%;
 (2) ultimate tensile strength Y 2 =Y 1 ±100 MPa; and 
 (3) yield strength Z 2 ≧Z 1 +100 MPa. 
   B. (1) ultimate tensile strength Y 3 =Y 1 ±100 MPa; and
 (2) yield strength Z 3 ≧Z 1 +200 MPa 
   
     
     
         2 . The method of  claim 1 , wherein said alloy contains at least 70 atomic % iron and five or more elements selected from Si, Mn, Cr, Ni, Cu or C. 
     
     
         3 . The method of  1  wherein said alloy contains at least 70 atomic % iron and Si, Mn, Cr, Ni, Cu and C. 
     
     
         4 . The method of  claim 1  wherein said alloy formed in step (b) exhibits X 1  values of 10.0 to 70.0%, Y 1  values of 900 MPa to 2050 MPa and Z 1  values of 200 MPa to 750 MPa. 
     
     
         5 . The method of  claim 1  wherein said tensile property combination A is as follows: X 2 =2.5% to 77.5%, Y 2 =800 MPa to 2150 MPa and Z 2 ≧300 MPa. 
     
     
         6 . The method of  claim 1  wherein said tensile property combination B is as follows: Y 3 =800 MPa to 2150 MPa and Z 3 ≧300 MPa. 
     
     
         7 . The method of  claim 1  wherein said first sheet formed in step (c) is permanently deformed into said second alloy sheet by reducing thickness of said first alloy sheet. 
     
     
         8 . The method of  claim 1  wherein step (b) is carried out at a temperature of 700° C. to a temperature below the melting point (Tm) of said alloy. 
     
     
         9 . The method of  claim 1 , wherein after step (b) the alloy is heat treated at a temperature of 650° C. to a temperature below the melting point (Tm) of the alloy. 
     
     
         10 . The method of  claim 1  wherein in step (c) the alloy is permanently deformed with a reduction in thickness of greater than 20% before failure. 
     
     
         11 . The method of  claim 1  wherein said first sheet formed in step (c) is permanently deformed into said second alloy sheet by the process of roll forming, metal stamping, metal drawing, or hydroforming. 
     
     
         12 . The method of  claim 1  wherein said alloy formed at a thickness of >5.0 mm to 500 mm contains greater than 10 volume percent of austenite. 
     
     
         13 . The method of  claim 1  wherein after permanently deforming said alloy in the temperature range of 150° C. to 400° C. in step (c) into a second sheet, said second sheet is permanently deformed at a temperature range of ≦150° C. 
     
     
         14 . A method to increase yield strength in a metallic alloy comprising:
 a. supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C, melting said alloy, cooling at a rate of 10 −4 K/sec to 10 3  K/sec and solidifying to a thickness of >5.0 mm to 500 mm;   b. processing said alloy into a first sheet form with thickness from 5.0 to 0.5 mm;   c. permanently deforming said alloy in the temperature range of 150° C. to 400° C. into a second sheet form;   d. permanently deforming said alloy in a temperature of <150° C. into a second sheet form exhibiting the following tensile property combinations:
 (1) total elongation=10.0 to 40.0%; 
 (2) ultimate tensile strength=1150 to 2000 MPa; 
 (3) yield strength=550 to 1600 MPa. 
   
     
     
         15 . The method of  claim 14  wherein step (b) is carried out at a temperature of 700° C. to a temperature below the melting point (Tm) of said alloy. 
     
     
         16 . The method of  claim 14 , wherein after step (b) the alloy is heat treated at a temperature of 650° C. to a temperature below the melting point (Tm) of the alloy. 
     
     
         17 . The method of  claim 14  wherein said step of permanently deforming comprises the process of roll forming, metal stamping, metal drawing, or hydroforming. 
     
     
         18 . The method of  claim 14  wherein said permanently deformed part formed in step (d) is positioned in a vehicular frame, vehicular chassis, or vehicular panel. 
     
     
         19 . The method of  claim 14  wherein said permanently deformed part in step (d) is positioned in one of a drill collar, drill pipe, pipe casing, tool joint, wellhead, compressed gas storage tank, railway tank car/tank wagon or liquefied natural gas canister.