Ductile Ultra High Strength Medium Manganese Steel Produced Through Continuous Annealing and Hot Stamping

Abstract

The present invention relates to the use of medium manganese steels for hot stamping to produce ultra high strength steel blanks with improved residual ductility of hot formed steels (as compared to boron-added steels such as 22MnB5 grade). The medium manganese steel sheets require much lower processing temperatures (<700° C. vs. >900° C.) prior to forming operation, allowing reduction of energy consumption during manufacturing.

What is claimed is: 
     
         1 . A method for producing ultra high strength steel alloy, comprising:
 providing a ferrous alloy comprising carbon and greater than 2.5 mass % manganese;   annealing the ferrous alloy at a temperature to form an annealed alloy comprising an austenitic microstructure; and   cooling the steel comprising the austenitic microstructure to room temperature to form an ultra high strength steel comprising the austenitic microstructure, and at least one microstructure of ferrite or martensite.   
     
     
         2 . The method of  claim 1 , wherein the annealing the ferrous alloys is performed at a temperature of between about 600° C. and about 800° C. 
     
     
         3 . The method of  claim 2 , wherein the ultra high strength steel comprises about 5 vol. % to about 65 vol. % austenite, between about 35 vol. % and about 90 vol. % ferrite, and the balance martensite. 
     
     
         4 . The method of  claim 1 , wherein the temperature is between about 750° C. and about 900° C. 
     
     
         5 . The method of  claim 4 , wherein the ultra high strength steel comprises about 5 vol. % to about 65 vol. % austenite, between about 35 vol. % and about 90 vol. % martensite. 
     
     
         6 . The method of  claim 1 , wherein the ferrous alloy comprises between about 3 mass % and about 15 mass % manganese. 
     
     
         7 . The method of  claim 1 , wherein a cooling rate following the hot forming step is substantially less than about 30° C./s. 
     
     
         8 . A ductile, ultra high strength steel product, comprising between about 2.5 mass % and 15 mass % manganese and wherein the microstructure comprises about 5 vol. % to about 65 vol. % austenite, about 0 vol. % to about 90 vol. % ferrite, and the balance being martensite. 
     
     
         9 . The ultra high strength steel product of  claim 8 , wherein the steel has a grain size of between about 0.1 micron and about 5 micron. 
     
     
         10 . The ultra high strength steel product of  claim 8 , wherein the dislocation density is between about 10 9  m −2  and about 10 10  m −2 . 
     
     
         11 . The ultra high strength steel product of  claim 8 , wherein the steel product has a tensile strength of between about 1000 MPa and about 1700 MPa and has a residual total elongation of between about 10% and about 30%. 
     
     
         12 . The ultra high strength steel product of  claim 8 , wherein the ultra high strength steel product has an x-ray diffraction pattern and wherein the x-ray diffraction pattern shows an average FWHM of all ferrite peaks of between about 0.25° and about 1°. 
     
     
         13 . The ultra high steel product of  claim 8 , further comprising an additive selected from the group consisting of an aluminum, a silicon, a carbon, a titanium, a niobium, a vanadium, and combinations thereof. 
     
     
         14 . A method for producing ultra high strength steel, comprising:
 providing a cold-rolled medium manganese steel comprising between about 2.5 mass % and about 12 mass % manganese;   hot forming the steel at a temperature between about 600° C. and about 900° C. to form a hot formed steel; and   quenching the hot formed steel to form the ultra high strength steel.   
     
     
         15 . The method of  claim 14 , further comprising annealing the steel prior to the hot forming step. 
     
     
         16 . The method of  claim 15 , wherein an annealing temperature is between about 600° C. and about 900° C. 
     
     
         17 . The method of  claim 14 , wherein the ultra high strength steel comprises at least 10 vol. % austenite. 
     
     
         18 . The method of  claim 14 , wherein a cooling rate following the hot forming step is less than about 30° C./s. 
     
     
         19 . The method of  claim 14 , wherein the temperature is between about 600° C. and about 750° C., and wherein the ultra high strength steel comprises about 5 vol. % to about 65 vol. % austenite, between about 35 vol. % and about 90 vol. % ferrite, and the balance martensite. 
     
     
         20 . The method of  claim 14 , wherein the temperature is between about 750° C. and about 900° C., and wherein the ultra high strength steel comprises about 5 vol. % to about 65 vol. % austenite, and the balance martensite.