Carbide-free bainite and retained austenite steels, producing method and applications of same

Abstract

One aspect of this invention relates to a method for producing a carbide-free bainite and retained austenite steel. The method includes providing an iron alloy containing a composition designed according to property objectives of the carbide-free bainite and retained austenite steel; heat-treating the alloy to a temperature above a temperature at which a transformation from ferrite into austenite is finished; succeedingly quenching the heat-treated alloy to a bainite region at a temperature between a first temperature at which a martensitic transformation starts in the alloy and a second temperature at which a coupled diffusional/displacive bainitic transformation starts the alloy; and optimally cooling the quenched alloy to form to form the carbide-free bainite and retained austenite steel that meets the property objectives, with a cooling ratio precisely controlled so that the temperature of the alloy continues to be slightly above the first temperature which keeps decreasing during the optimally cooling step.

What is claimed is: 
     
         1 . A method for producing a carbide-free bainite and retained austenite steel, comprising:
 providing an iron (Fe) alloy containing a composition designed according to property objectives of the carbide-free bainite and retained austenite steel, wherein the property objectives are design specifications of the carbide-free bainite and retained austenite steel;   heat-treating the alloy to a temperature above A c3 , wherein A c3  is a temperature at which a transformation from ferrite into austenite is finished;   succeedingly quenching the heat-treated alloy to a bainite region at a temperature between M s  and B s , wherein M s  is a temperature at which a martensitic transformation starts in the alloy, and B s  is a temperature at which a coupled diffusional/displacive bainitic transformation starts the alloy; and   optimally cooling the quenched alloy to form to form the carbide-free bainite and retained austenite steel that meets the property objectives, wherein a cooling ratio of the optimally cooling step is precisely controlled so that the temperature of the alloy continues to be slightly above the M s  temperature which keeps decreasing during the optimally cooling step.   
     
     
         2 . The method of  claim 1 , wherein the heat-treating step is performed with an austenization or hot rolling treatment. 
     
     
         3 . The method of  claim 1 , wherein the heat-treating step is performed with a hot rolling treatment and subsequently a cold rolling treatment and a solution treatment. 
     
     
         4 . The method of  claim 1 , wherein the optimally cooling step is performed with gradually cooling or step-wise cooling. 
     
     
         5 . The method of  claim 1 , wherein the property objectives comprise a yield strength in a range of about 1000-2000 MPa, a uniform ductility, a desired total elongation and hole-expansion ratio, a desired level of weldability and an austenite stability designed to have an austenite start temperature M s   σ  to be equal to an application temperature in range from about 50° C. to −50° C. 
     
     
         6 . The method of  claim 1 , wherein the composition comprises carbon (C) no more than 0.4 wt %, silicon (Si) no less than 1.0 wt %, and iron (Fe) in balance. 
     
     
         7 . The method of  claim 6 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and molybdenum (Mo) in a range of about 0.4-0.8 wt. %. 
     
     
         8 . The method of  claim 6 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and chromium (Cr) in a range of about 0.1-0.9 wt. %. 
     
     
         9 . A method for designing a carbide-free bainite and retained austenite steel, comprising:
 defining property objectives of the carbide-free bainite and retained austenite steel, wherein the property objectives are design specifications of the carbide-free bainite and retained austenite steel;   designing a composition of the carbide-free bainite and retained austenite steel according to the property objectives; and   processing the composition to form the carbide-free bainite and retained austenite steel that meets the property objectives, wherein the processing step is performed with a cooling and partitioning process.   
     
     
         10 . The method of  claim 9 , wherein the processing step comprises solidifying the composition to form an alloy; and reheating the alloy. 
     
     
         11 . The method of  claim 10 , wherein the cooling and partitioning process comprises:
 heat-treating the alloy to a temperature above A c3 , wherein A c3  is a temperature at which a transformation from ferrite into austenite is finished;   succeedingly quenching the heat-treated alloy to a bainite region at a temperature between M s  and B s , wherein M s  is a temperature at which a martensitic transformation starts in the alloy, and B s  is a temperature at which a coupled diffusional/displacive bainitic transformation starts the alloy; and   optimally cooling the quenched alloy to form to form the carbide-free bainite and retained austenite steel that meets the property objectives, wherein a cooling ratio of the optimally cooling step is precisely controlled so that the temperature of the alloy continues to be slightly above the M s  temperature which keeps decreasing during the optimally cooling step.   
     
     
         12 . The method of  claim 11 , wherein the heat-treating step is performed with an austenization or hot rolling treatment. 
     
     
         13 . The method of  claim 11 , wherein the heat-treating step is performed with a hot rolling treatment and subsequently a cold rolling treatment and a solution treatment. 
     
     
         14 . The method of  claim 11 , wherein the optimally cooling step is performed with gradually cooling or step-wise cooling. 
     
     
         15 . The method of  claim 9 , wherein the property objectives comprise a yield strength in a range of about 1000-2000 MPa, a uniform ductility, a desired total elongation and hole-expansion ratio, a desired level of weldability and an austenite stability designed to have an austenite start temperature M s   σ  to be equal to an application temperature in range from about 50° C. to −50° C. 
     
     
         16 . The method of  claim 9 , wherein the composition comprises carbon (C) no more than 0.4 wt %, silicon (Si) no less than 1.0 wt %, and iron (Fe) in balance. 
     
     
         17 . The method of  claim 16 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and molybdenum (Mo) in a range of about 0.4-0.8 wt. %. 
     
     
         18 . The method of  claim 16 , wherein the composition further comprises manganese (Mn) in a range of about 0.2-1.0 wt. %, and chromium (Cr) in a range of about 0.1-0.9 wt. %. 
     
     
         19 . A method for designing a carbide-free bainite and retained austenite steel, comprising:
 determining a composition, and producing a trial alloy from the trial composition, wherein the trial alloy has substantially high hardenability to avoid formation of ferrite, and contains carbon no more than 0.4 wt % for weldability and silicon no less than 1.0 wt % for carbide prohibition;   performing a cooling and partitioning treatment to the trial alloy, and experimentally evaluating the trial alloy at an initial state, a transitional path, and an end state of the cooling and partitioning treatment to obtain trial parameters comprising at least a quenching temperature, a descent of a M s  temperature, and a final partitioning temperature; and   refining the composition by computational material engineering models using the trial parameters, such that an alloy formed of the refined composition meets property objectives of the carbide-free bainite and retained austenite steel, wherein the property objectives are design specifications of the carbide-free bainite and retained austenite steel.   
     
     
         20 . The method of  claim 19 , wherein at the initial state, M s  temperature, B s  temperature used to identify a quenching temperature, and a bainite start time at different temperatures are measured; at the transitional path, the M s  temperature is measured at different process times so as to determine a descent of the M s  temperature; and at the end state, a final partitioning temperature, mechanical performance, austenite stability, and microstructures are measured. 
     
     
         21 . The method of  claim 19 , wherein the cooling and partitioning treatment comprises:
 heat-treating the alloy to a temperature above A c3 , wherein A c3  is a temperature at which a transformation from ferrite into austenite is finished;   succeedingly quenching the heat-treated alloy to a bainite region at a temperature between M s  and B s , wherein M s  is a temperature at which a martensitic transformation starts in the alloy, and B s  is a temperature at which a coupled diffusional/displacive bainitic transformation starts the alloy; and   optimally cooling the quenched alloy to form to form the carbide-free bainite and retained austenite steel that meets the property objectives, wherein a cooling ratio of the optimally cooling step is precisely controlled so that the temperature of the alloy continues to be slightly above the M s  temperature which keeps decreasing during the optimally cooling step.   
     
     
         22 . The method of  claim 21 , wherein the optimally cooling step is performed with gradually cooling or step-wise cooling. 
     
     
         23 . The method of  claim 19 , wherein the property objectives comprise a yield strength in a range of about 1000-2000 MPa, a uniform ductility, a desired total elongation and hole-expansion ratio, a desired level of weldability and an austenite stability designed to have an austenite start temperature M s   σ  to be equal to an application temperature in range from about 50° C. to −50° C.