Processes for producing thicker gage products of niobium microalloyed steel

Abstract

A process for controlling austenite grain size in austenite processing through nano-scale precipitate engineering of TiN—NbC composites to produce thicker gage product of niobium microalloyed steel includes controlling the base chemical composition of a steel product to include 0.003-0.004 wt. percent nitrogen, 0.012-0.015 wt. percent titanium, 0.03-0.07 wt. percent carbon, and 0.07-0.15 wt. percent nobium; lowering the temperature of roughening to end the roughening operation in the temperature range of from about 980° C. to 1030° C.; retaining greater than about 0.03 wt. percent niobium in solution in the matrix by rapid cooling of the product to enter the finish rolling operation below the temperature of no recrystallization, with an austenite grain size of about 30 microns; and applying reduced rolling reduction in the finish rolling operation.

What is claimed is: 
     
         1 . A process for controlling austenite grain size in austenite processing through nano-scale precipitate engineering of TiN—NbC composites to produce thicker gage product of niobium microalloyed steel, comprising:
 (i) controlling the base chemical composition of a steel product to include 
 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   Element 
                   Amount (wt %) 
                 
                     
                     
                 
                     
                   N 
                   0.003-0.004 
                 
                     
                   Ti 
                   0.012-0.015 
                 
                     
                   C 
                   0.03-0.07 
                 
                     
                   Nb 
                   0.07-0.15 
                 
                     
                     
                 
             
                
                
                
               
               
                
                
                
                
                
               
            
           
         
         (ii) lowering the temperature of roughening to end the roughening operation in the temperature range of from about 980° C. to 1030° C. to prevent grain refined austenite from coarsening above about 30 microns by formation of TiN—NbC composite precipitates; 
         (iii) retaining greater than about 0.03 wt % niobium in solution in the matrix by rapid cooling of the product to enter the finish rolling operation below the temperature of no recrystallization, with an austenite grain size of about 30 microns; and 
         (iv) applying reduced rolling reduction in the finish rolling operation to pancake the fine austenite grain size of about 30 microns to obtain a sufficient surface to volume ratio to produce thicker gage resulting steel product. 
       
     
     
         2 . A process as recited in  claim 1 , wherein greater than about 0.04 wt % niobium is retained in solution in the matrix. 
     
     
         3 . A process as recited in  claim 1 , wherein austenite grain size is controlled in the range of about 20-40 microns at entry to the finish rolling operation. 
     
     
         4 . A process as recited in  claim 1 , wherein TiN precipitates are in the range of about 10-20 nm and the inter-particle spacing is about 200-300 nm. 
     
     
         5 . A process as recited in  claim 1  wherein thermodynamic potential for precipitation of NbC occurs towards the end of the roughing operation at temperatures ranging from about 980° C. to about 1030° C. 
     
     
         6 . A process as recited in  claim 1 , wherein TiN—NbC composites are in the size range of about 20-50 nm. 
     
     
         7 . A process as recited in  claim 1 , further comprising applying accelerated cooling upstream between the end of the roughing operation and the start of finish rolling to avoid depletion of solute niobium from the matrix to less than 0.03 wt percent. 
     
     
         8 . A process as recited in  claim 7 , further comprising applying accelerated cooling upstream between the end of the roughing operation and the start of finish rolling to avoid depletion of solute niobium from the matrix to less than 0.03 wt percent and enter finish rolling at or below the temperature of no recrystallization. 
     
     
         9 . A process as recited in  claim 1 , further comprising applying accelerated cooling of the steel product to avoid rolling in the partial recrystallization regime and to enter finish rolling below the temperature of no recrystallization. 
     
     
         10 . A process as recited in  claim 1 , further comprising controlling nitrogen at or below about 40 ppm, and making a titanium addition to meet the stoichiometric requirement to combine with all nitrogen to form high number density of TiN precipitate in about the 10-20 nm size range. 
     
     
         11 . A process as recited in  claim 1 , further comprising processing the steel product by at least one of conventional plate rolling, conventional hot strip rolling, steckel mill rolling, or near net shape processing. 
     
     
         12 . A process as recited in  claim 1 , wherein the steel product is line pipe steel. 
     
     
         13 . A process as recited in  claim 1 , wherein the steel product is infra-structure steel. 
     
     
         14 . A process as recited in  claim 1 , wherein the steel product is supermartensitic stainless steel. 
     
     
         15 . A process as recited in  claim 1 , wherein the crystallographic texture-related anisotropic properties of the resulting steel product are minimized. 
     
     
         16 . A process as recited in  claim 1 , further comprising substituting titanium partially or fully in the base chemistry with a member of the group consisting of Zr, Hf, Ta, W, V, Cr, Mo, Al and mixtures thereof, each with high affinity for nitrogen to form nano-scale precipitates on which NbC can grow epitaxially to give composite precipitates. 
     
     
         17 . A process as recited in  claim 1 , further comprising partially substituting niobium in the base chemistry with other microalloying elements with high affinity for carbon selected from the group consisting of Zr, Hf, Ta, W, V, Cr, Mo, and mixtures thereof, each to give composite precipitates. 
     
     
         18 . A process as recited in  claim 1 , further comprising substituting solute niobium on entry to finish rolling with other elements, which exhibit solute drag comparable to niobium. 
     
     
         19 . A process as recited in  claim 1 , further comprising rapidly cooling the steel product to enter finish rolling at a temperature at or below about 920° C. 
     
     
         20 . A process as recited in  claim 1 , wherein the rolling reduction in said finish rolling operation is reduced substantially more than 15%. 
     
     
         21 . A process as recited in  claim 1 , wherein the steel product exhibits a gage thickness of about 17-30 mm. 
     
     
         22 . A steel product obtained by the process of  claim 1 .