Methods of forming steel sheets with enhanced flatness

Abstract

The present disclosure provides a method for preparing a steel alloy sheet to enhance flatness. The method includes, inter alia, heating a steel alloy material to a first temperature that is greater than a full-austenitization point for the steel alloy material; holding steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet; air quenching the precursor steel sheet to a second temperature that is less than the first temperature and greater than martensitic transformation starting temperature for the steel alloy material; and cooling the precursor steel sheet to room temperature to prepare the steel alloy sheet. The steel alloy material includes greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium.

What is claimed is: 
     
         1 . A method for preparing a steel alloy sheet to enhance flatness, the method comprising:
 heating a steel alloy material to a first temperature that is greater than a full-austenitization point for the steel alloy material;   holding steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet;   air quenching the precursor steel sheet to a second temperature that is less than the first temperature and greater than martensitic transformation starting temperature for the steel alloy material; and   cooling the precursor steel sheet to room temperature to prepare the steel alloy sheet, the room temperature being greater than or equal to about 15° C. to less than or equal to about 25° C.   
     
     
         2 . The method of  claim 1 , wherein the first temperature is greater than or equal to about 800° C. to less than or equal to about 950° C., the second temperature is greater than or equal to about 300° C. to less than or equal to about 500° C., and the cooling rate during the air quenching is greater than or equal to about 2° C./s to less than or equal to about 15° C./s. 
     
     
         3 . The method of  claim 1 , wherein the air quenching is a first air quenching step, and the method further comprises a second air quenching step, the second air quenching step comprising air quenching the precursor steel sheet to a third temperature less than the second temperature. 
     
     
         4 . The method of  claim 3 , wherein the second air quenching step is a continuation of the first air quenching step. 
     
     
         5 . The method of  claim 3 , wherein the cooling rate during the second air quenching step is greater than or equal to about 0.1 C./s to less than or equal to about 15° C./s, and the third temperature is less than or equal to about 400° C. 
     
     
         6 . The method of  claim 3 , wherein the steel alloy sheet has a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1600 MPa, and a total elongation greater than or equal to about 3%, and
 wherein the steel alloy sheet has a microstructure that comprises greater than or equal to about 80 vol. % to less than or equal to about 99 vol. % of martensite phase;   greater than or equal to about 1 vol. % to less than or equal to about 10 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.   
     
     
         7 . The method of  claim 3 , wherein the method comprises holding the precursor steel sheet at the third temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds. 
     
     
         8 . The method of  claim 6 , wherein the method further comprises heating the precursor steel sheet from the third temperature to a fourth temperature that is less than the first temperature. 
     
     
         9 . The method of  claim 8 , wherein the fourth temperature is greater than or equal to about 300° C. to less than or equal to about 500° C. 
     
     
         10 . The method of  claim 8 , wherein the method further comprises holding the precursor steel sheet at the fourth temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds. 
     
     
         11 . The method of  claim 10 , wherein the steel alloy sheet has a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1500 MPa, a total elongation greater than or equal to about 7%, and a bending angle greater than or equal to about 50 degrees, and
 wherein the steel alloy sheet has a microstructure that comprises greater than or equal to about 50 vol. % to less than or equal to about 95 vol. % of martensite constituents; greater than or equal to about 5 vol. % to less than or equal to about 17 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 25 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.   
     
     
         12 . The method of  claim 1 , wherein the method further comprises holding the precursor steel sheet at the second temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds. 
     
     
         13 . The method of  claim 12 , wherein the steel alloy sheet has a yield strength greater than or equal to about 1100 MPa, an ultimate tensile strength greater than or equal to about 1550 MPa, a total elongation greater than or equal to about 7%, and bending angle greater than or equal to about 50 degrees, and
 wherein the steel alloy has a microstructure that comprises greater than or equal to about 30 vol. % to less than or equal to about 97 vol. % of martensite constituents;   greater than or equal to about 3 vol. % to less than or equal to about 15 vol. % of retained austenite phase; greater than or equal to about 0 vol. % to less than or equal to about 45 vol. % of bainite phase; and greater than or equal to about 0 vol. % to less than or equal to about 10 vol. % of ferrite phase.   
     
     
         14 . The method of  claim 1 , wherein the steel alloy material comprises:
 greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon;   greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium;   greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon;   greater than 0 wt. % to less than or equal to about 4.5 wt. % of manganese;   greater than 0 wt. % to less than or equal to about 2 wt. % of aluminum, wherein a chromium to aluminum ratio is greater than or equal to about 1.7, and a sum of the aluminum and silicon is greater than or equal to about 0.7 wt. %; and   a balance of iron.   
     
     
         15 . The method of  claim 1 , wherein the steel alloy material further comprises:
 greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium;   greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium; and   greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium.   
     
     
         16 . A method for preparing a steel alloy sheet to enhance flatness, the method comprising:
 heating a steel alloy material to a first temperature greater than or equal to about 800° C. to less than or equal to about 950° C.;   holding steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet;   a first air quenching of the precursor steel sheet from the first temperature to a second temperature greater than or equal to about 300° C. to less than or equal to about 500° C. at a first cooling rate greater than or equal to about 2° C./s to less than or equal to about 15° C./s;   a second air quenching of the precursor steel sheet from the second temperature to a third temperature less than or equal to about 400° C. at a second cooling rate greater than or equal to about 0.1 C./s to less than or equal to about 15° C./s; and   cooling the precursor steel sheet to room temperature to prepare the steel alloy sheet, the room temperature being greater than or equal to about 15° C. to less than or equal to about 25° C.   
     
     
         17 . The method of  claim 16 , wherein the method further comprises holding the precursor steel sheet at the third temperature for a holding period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, after the holding period heating the precursor steel sheet from the third temperature to a fourth temperature, and holding the precursor steel sheet at the fourth temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, the fourth temperature being greater than or equal to about 300° C. to less than or equal to about 500° C. 
     
     
         18 . The method of  claim 16 , wherein the steel alloy material comprises:
 greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon;   greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium;   greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon;   greater than 0 wt. % to less than or equal to about 4.5 wt. % of manganese;   greater than 0 wt. % to less than or equal to about 2 wt. % of aluminum, wherein a chromium to aluminum ratio is greater than or equal to about 1.7, and a sum of the aluminum and silicon is greater than or equal to about 0.7 wt. %;   greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium;   greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium;   greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium; and   a balance of iron.   
     
     
         19 . A method for preparing a steel alloy sheet to enhance flatness, the method comprising:
 heating a steel alloy material to a first temperature greater than or equal to about 800° C. to less than or equal to about 950° C.;   holding the steel alloy material at the first temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to from a precursor steel sheet;   a first air quenching of the precursor steel sheet from the first temperature to a second temperature greater than or equal to about 300° C. to less than or equal to about 500° C. at a first cooling rate greater than or equal to about 2° C./s to less than or equal to about 15° C./s;   holding the precursor steel sheet at the second temperature for a period greater than or equal to about 1 second to less than or equal to about 10,000 seconds; and   cooling the precursor steel sheet from the second temperature to room temperature to prepare the steel alloy sheet, the room temperature being greater than or equal to about 15° C. to less than or equal to about 25° C.   
     
     
         20 . The method of  claim 19 , wherein the steel alloy material comprises:
 greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % of carbon;   greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % of chromium;   greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % of silicon;   greater than 0 wt. % to less than or equal to about 4.5 wt. % of manganese;   greater than 0 wt. % to less than or equal to about 2 wt. % of aluminum, wherein a chromium to aluminum ratio is greater than or equal to about 1.7, and a sum of the aluminum and silicon is greater than or equal to about 0.7 wt. %;   greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % of vanadium;   greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % of niobium;   greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % of titanium; and   a balance of iron.