Austenitic steel having high strength and formability, method of producing said steel and use thereof

Abstract

Substantially austenitic steel having high strength and good formability for cold rolling including (in weight percent): 0.05 to 1.0% C; 11.0 to 14.9% Mn; 1.0 to 5.0% Al; O to 2.5% Ni the remainder being iron and unavoidable impurities, wherein the microstructure includes at least 75% in volume of austenite, and wherein (Ni+Mn) is from 11.0 to 15.9%.

1 . Substantially austenitic steel strip having high strength and good formability for cold rolling comprising (in weight percent):
 0.05 to 0.78% C   11.0 to 14.9% Mn   1.0 to 5.0% Al   0 to 2.5% Ni   
     the remainder being iron and unavoidable impurities, wherein the microstructure comprises at least 80% in volume of austenite, and wherein (Ni+Mn) is from 11.0 to 15.9%. 
   
   
       2 . Steel according to  claim 1 , wherein the microstructure comprises at least 85%. 
   
   
       3 . Steel according to  claim 1 , wherein the carbon content is between 0.30 and 0.75%. 
   
   
       4 . Steel according to  claim 1 , wherein the nickel content is at most 0.05%. 
   
   
       5 . Steel according to  claim 1 , wherein the aluminium content is at most 4.0%. 
   
   
       6 . Steel according to  claim 1 , wherein the manganese content is at least 11.5%. 
   
   
       7 . Steel according to  claim 1 , wherein the manganese content is at most 14.7%. 
   
   
       8 . Steel according to  claim 1 , provided in the form of a hot rolled steel having a thickness between 0.5 and 20 mm. 
   
   
       9 . Steel according to  claim 1 , wherein the steel is provided in the form of a cold-rolled strip, or in the form of a cold-rolled and continuously annealed or batch-annealed strip, optionally coated with a coating system comprising one or more metallic and/or organic layer or layers. 
   
   
       10 . Cold rolled steel according to  claim 9 , wherein the microstructure after rolling and annealing comprises at least 80% in volume of austenite. 
   
   
       11 . Method of producing austenitic steel strip, having an austenite content according to  claim 1 , comprising the steps of:
 providing molten steel having a composition comprising (in weight percent):   0.05 to 0.78% C   11.0 to 14.9% Mn   1.0 to 5.0% Al   0 to 2.5% Ni   
     the remainder being iron and unavoidable impurities wherein the microstructure comprises at least 80% in volume of austenite, and wherein (Ni+Mn) is from 11.0 to 15.9%;
 casting said steel into an ingot, or a continuously cast slab, or a continuously cast thin slab or a strip-cast strip; 
 providing a hot-rolled strip by hot rolling the ingot, the continuously cast slab, the continuously cast thin slab or the strip-cast strip to the desired hot rolled thickness. 
 
   
   
       12 . Method according to  claim 11 , wherein the hot-rolled strip is cold-rolled to the desired final thickness. 
   
   
       13 . Method according to  claim 12 , wherein the cold-rolled strip is annealed after cold rolling to the desired final thickness in a continuous or batch annealing process. 
   
   
       14 . Method according to  claim 11 , wherein the strip-cast strip is obtained after strip casting using a twin-roll casting, belt-casting or drum-casting device. 
   
   
       15 . Strip or sheet produced from a steel comprising (in weight percent):
 0.05 to 0.78% C   11.0 to 14.9% Mn   1.0 to 5.0% Al   0 to 2.5% Ni   
     the remainder being iron and unavoidable impurities wherein the microstructure comprises at least 80% in volume of austenite, and wherein (Ni+Mn) is from 11.0 to 15.9%, by a method according to  claim 11 . 
   
   
       16 . A method comprising producing automotive inner or outer parts or wheels from a steel according to  claim 1 . 
   
   
       17 . A method comprising hydroforming a steel according to  claim 1 . 
   
   
       18 . Steel according to  claim 1 , wherein the microstructure comprises at least 90% in volume of austenite. 
   
   
       19 . Steel according to  claim 1 , wherein the microstructure comprises at least 95% in volume of austenite. 
   
   
       20 . Steel according to  claim 1 , wherein the manganese content is at least 12.0%. 
   
   
       21 . Steel according to  claim 1 , provided in the form of a hot rolled steel having a thickness between 0.7 and 10 mm 
   
   
       22 . Steel according to  claim 1 , provided in the form of a hot rolled steel strip having a strip thickness is at most 8 mm. 
   
   
       23 . Steel according to  claim 1 , provided in the form of a hot rolled steel strip having a strip thickness between 0.8 and 5 mm. 
   
   
       24 . Cold rolled steel according to  claim 9 , wherein the microstructure after rolling and annealing comprises at least 85% in volume of austenite. 
   
   
       25 . Cold rolled steel according to  claim 9 , wherein the microstructure after rolling and annealing comprises at least 90% in volume of austenite. 
   
   
       26 . Cold rolled steel according to  claim 9 , wherein the microstructure after rolling and annealing comprises at least 95% in volume of austenite. 
   
   
       27 . Method according to  claim 11 , wherein the hot-rolled strip is cold-rolled to the desired final thickness, wherein the cold-rolling reduction is between 10 to 90% 
   
   
       28 . Method according to  claim 11 , wherein the hot-rolled strip is cold-rolled to the desired final thickness, wherein the cold-rolling reduction is between 30 and 85. 
   
   
       29 . Method according to  claim 11 , wherein the hot-rolled strip is cold-rolled to the desired final thickness, wherein the cold-rolling reduction is between 45 and 80%. 
   
   
       30 . The strip or sheet of  claim 15 , wherein the steel is galvanized. 
   
   
       31 . A method comprising producing automotive inner or outer parts or wheels from a steel strip or sheet of  claim 15 . 
   
   
       32 . A method comprising hydroforming a strip or sheet according to  claim 15 .