Method of producing an austenitic steel

Abstract

A method of producing an austenitic steel strip or sheet excellent in resistance to delayed cracking and a steel produced thereby.

1 . A method of producing an austenitic steel strip or sheet excellent in resistance to delayed cracking, comprising
 casting a workpiece selected from an ingot, or a continuously cast slab, or a continuously cast thin slab or a strip-cast strip, the workpiece composition comprising, in weight:   0.50%-0.80% C   10-17% Mn   1.0-5% Al   at most 0.5% Si   at most 0.020% S   at most 0.050% P   50-200 ppm N   0.050-0.25% V   
       the remainder being iron and unavoidable impurities inherent to fabrication;
 providing a hot-rolled strip or sheet by hot rolling the work piece selected from the ingot, the continuously cast slab, the continuously cast thin slab or the strip-cast strip to the desired hot rolled thickness. 
 cold-rolling the hot-rolled strip or sheet to the desired final thickness, 
 continuous annealing of the cold-rolled strip or sheet, the continuous annealing comprising heating up the strip or sheet at a heating rate V h  of between 3 and 60° C./s to an annealing temperature T a  for an annealing time t a  of between 15 and 300 seconds followed by cooling at a cooling rate V c  of between 10 and 100° C./s and wherein T a  is from 750 to 850° C. 
 
     
     
         2 . Process according to  claim 1 , wherein the aluminium content of the work piece is at least 1.25 and/or at most 3.5%. 
     
     
         3 . Process according to  claim 1 , wherein during the cooling at a cooling rate V c  after the continuous annealing the strip or sheet is led through a hot dip bath for providing a metallic coating by hot dipping the strip or sheet into a molten bath of the metal making up the metallic coating. 
     
     
         4 . Process according to  claim 1 , wherein the strip or sheet is pickled after continuous annealing and wherein the strip or sheet is provided with a metallic coating by pickling after annealing followed by heating to a temperature below the continuous annealing temperature before the strip or sheet is led through a hot dip bath for providing a metallic coating by hot dipping the strip or sheet into a molten bath of the metal making up the metallic coating. 
     
     
         5 . Process according to  claim 1 , wherein reduction of the cold-rolling is between 10 to 90%. 
     
     
         6 . Process according to  claim 1 , wherein the annealed strip or sheet is temper rolled with a reduction of from 0.5 to 10% prior to or after the metallic coating has been provided to the strip or sheet. 
     
     
         7 . Process according to  claim 1 , wherein the Vanadium content is between 0.06 and 0.22%. 
     
     
         8 . Process according to  claim 1 , wherein the cooling rate V c  is between 20 and 80° C./s. 
     
     
         9 . Process according to  claim 1 , wherein the strip or sheet is pickled after continuous annealing and wherein the strip or sheet is provided with a metallic coating, by pickling after continuous annealing followed by heating to a temperature between 400 and 600° C. before the strip or sheet is led through a hot dip bath for providing a metallic coating by hot dipping the strip or sheet into a molten bath of the metal making up the metallic coating. 
     
     
         10 . Process according to  claim 9 , wherein the Fe in the strip or sheet material is reduced during or after the heating to a temperature below the continuous annealing temperature and before the hot dip galvanizing. 
     
     
         11 . Process according to  claim 10 , wherein an excess amount of O 2  is provided in the atmosphere during or after the heating of the strip or sheet material and before the reduction of the strip or sheet material. 
     
     
         12 . Process according to  claim 1 , wherein the coiling temperature after hot rolling is kept below 500° C. and/or wherein the annealing temperature T a  is 785° C.±10° C. 
     
     
         13 . Strip or sheet produced by a method according to  claim 1 , wherein the steel is provided with a metallic coating. 
     
     
         14 . A method of use of a strip or sheet according to  claim 13 , comprising producing automotive inner or outer parts or wheels from the strip or sheet. 
     
     
         15 . A method of use of a strip or sheet according to  claim 13 , comprising hydroforming the strip or sheet. 
     
     
         16 . Process according to  claim 1 , wherein reduction of the cold-rolling is between 30 and 85% 
     
     
         17 . Process according to  claim 1 , wherein reduction of the cold-rolling is between 45 and 80%. 
     
     
         18 . Process according to  claim 9 , wherein the Fe in the strip or sheet material is reduced during or after the heating to a temperature below the continuous annealing temperature and before the hot dip galvanizing, wherein the reduction is performed using H 2 N 2  in the reducing atmosphere. 
     
     
         19 . Process according to  claim 9 , wherein the Fe in the strip or sheet material is reduced during or after the heating to a temperature below the continuous annealing temperature and before the hot dip galvanizing, wherein the reduction is performed using 5-30% H 2 N 2  in the reducing atmosphere. 
     
     
         20 . Process according to  claim 10 , wherein an excess amount of O 2  is provided in the atmosphere during or after the heating of the strip or sheet material and before the reduction of the strip or sheet material, wherein the excess amount of O 2  is provided in an amount of 0.05-5% O 2 .