Process for manufacturing stainless steel containing fine carbonitrides, and product obtained from this process

Abstract

The invention relates to a process for manufacturing a semi-finished product made of stabilized stainless steel, which includes a casting step by means of a hollow jet nozzle placed between a tundish and a continuous casting mould. The nozzle comprises, in its upper part, a distributing member for deflecting the liquid metal arriving at the inlet of the nozzle, thus defining an internal volume with no liquid metal in the hollow jet. A non-stabilized stainless steel containing no nitride, carbide and carbonitride precipitates, is delivered in liquid metal form into the tundish. The liquid metal is cast by means of the nozzle, simultaneously carrying out an addition of metal powder into an internal volume of the hollow jet. The metal powder contains at least one element for stabilizing the stainless steel, the addition being carried out at a temperature between T liquidus +10° C. and T liquidus +40° C. The liquid metal is solidified, the solidification starting less than 2 seconds after the addition, in order to obtain the semi-finished product.

1 . Process for manufacturing a semi-finished product made of stabilized stainless steel, which includes a casting step by means of a hollow jet nozzle placed between a tundish ( 1 ) and a continuous casting mould ( 10 ), said nozzle comprising, in its upper part, a distributing member ( 4 ) for deflecting the liquid metal arriving at the inlet of said nozzle, thus defining an internal volume with no liquid metal, characterized in that:
 a non-stabilized stainless steel containing no nitride, carbide and carbonitride precipitates, is delivered in liquid metal form into said tundish; then   said liquid metal is cast by means of said nozzle, simultaneously carrying out an addition of metal powder ( 6 ) into said internal volume of said hollow jet, said metal powder containing at least one element for stabilizing said stainless steel, said addition being carried out at a liquid steel temperature between T liquidus +10° C. and T liquidus +40° C.; and   said liquid metal is solidified, the solidification of said liquid metal starting less than 2 seconds after said addition, in order to obtain said semi-finished product.   
     
     
         2 . Process according to  claim 1 , characterized in that said stabilizing element is chosen from one or more of the following elements: titanium, niobium, zirconium and vanadium. 
     
     
         3 . Process according to  claim 2 , characterized in that said stabilizing element is titanium, the titanium, carbon and nitrogen contents of said stainless steel satisfying, the contents being expressed in percentages by weight:
   Ti≧0.15+4(C+N).   
     
     
         4 . Process according to any one of  claims 1  to  3 , characterized in that said steel is a ferritic stainless steel or an austenitic stainless steel or a martensitic stainless steel or an austeno-ferritic stainless steel. 
     
     
         5 . Semi-finished product manufactured by a process according to any one of  claims 1  to  4 , characterized in that it has a fully-equiaxed solidification structure. 
     
     
         6 . Stainless steel product manufactured from a semi-finished product produced by a process according to any one of  claims 1  to  4 , characterized in that the stabilizing element is titanium and in that the number of titanium nitrides or carbonitrides of size smaller than 2.5 microns is greater than 15 000/cm 2 . 
     
     
         7 . Product according to  claim 6 , characterized in that the number of titanium nitrides or carbonitrides of size greater than 10 microns is less than 50/cm 2 . 
     
     
         8 . Product according to  claim 6  or  7 , characterized in that the mean inter-precipitate distance is less than 15 microns.