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 line pipe steel that includes controlling the base chemistry of the steel to include 0.003-0.004 wt. % nitrogen, 0.012-0.015 wt. % titanium, 0.03-0.07 wt. % carbon, and 0.07-0.15 wt. % niobium; conducting a first stage of roughing above the temperature of dissolution of NbC to refine austenite grain size by static recrystallization below 30 microns, cooling so that the center of the thick rolled slab is about 1040° C., conducting a second stage of roughing to promote strain induced growth of NbC on pre-existing TiN to form TiN—NbC composites to increase Zener pinning pressure on austenite grain boundaries to about 0.1 MPa in order to prevent recrystallization and grain coarsening of austenite grains beyond 30 microns at the end of roughing, and conducting finish rolling below 920° C.

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 
 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                   Amount 
                 
                     
                   Element 
                   (wt %) 
                 
                     
                     
                 
                     
                   N 
                   0.003-0.004 
                 
                     
                   Ti 
                   0.012-0.015 
                 
                     
                   C 
                   0.03-0.07 
                 
                     
                   Nb 
                   0.07-0.15 
                 
                     
                     
                 
             
                
                
                
                
               
               
                
                
                
                
                
               
            
           
         
         wherein controlling the nitrogen to 0.003-0.004 wt % and controlling the Ti to 0.012-0.015 wt % in the steel product before rolling produces TiN precipitates with a mean interparticle spacing in the range of about 200-300 nm and a mean size in the range of about 10-20 nm; 
         (ii) conducting a first stage of roughing on the steel product with rolling passes in a temperature window ranging from 1080° C. to 1150° C. to promote refinement of austenite grains from 300 microns to less than 30 microns by repeated static recrystallization between the rolling passes; 
         (iii) cooling the steel product for 100-120 seconds to allow for temperature homogenization and for the center of the steel product to cool to a temperature of about 1040° C.; 
         (iv) conducting a second stage of roughing on the steel product, wherein the second stage of roughing includes conducting at least one heavy rough rolling pass reduction (>25%) wherein the center of the steel product is at about 1040° C. in order to promote strain induced growth of NbC on pre-existing TiN to form TiN—NbC composites having a mean size of 25-30 nm with a mean interparticle spacing of 200-300 nm pre-set by the interparticle spacing of the TiN to stop the static recrystallization in the center of a thick section of the steel product by pinning the austenite grain boundaries with a Zener pinning pressure of about 0.1 MPa, raising the temperature of no recrystallization in the roughing window to counteract the driving force of recrystallization and prevent grain coarsening of austenite grains beyond 30 microns in the steel product; 
         (v) cooling the center of the steel product to a temperature at or below about 920° C. to retain solute niobium (Nb>0.03 wt %) in the matrix for transformation hardening; 
         (vi) conducting finish rolling on the steel product to achieve a total rolling reduction of 70% to pancake the austenite grains of about 30 microns to target a pancaked austenite thickness of less than 10 microns; and 
         (vii) applying accelerated cooling to the steel product after finish rolling and its austenite grains having a large surface to volume ratio (Sv factor), large strain accumulation and solute niobium (>0.03 wt % Nb) in the matrix to produce 1-3 micron size acicular ferrite grains upon transformation to obtain high strength and toughness at low temperature. 
       
     
     
         2 . A process as recited in  claim 1 , wherein the niobium in the base chemical composition increases the hardenability in order to promote acicular ferrite transformation at low temperature by shear transformation to promote high angle boundaries to arrest microcracks, thereby suppressing brittle fracture. 
     
     
         3 . A process as recited in  claim 1 , wherein the second stage of roughing aids in strain induced growth of NbC on pre-existing TiN by accelerated diffusion due to dislocations generated by deformation from the rough rolling to form the TiN—NbC composite. 
     
     
         4 . A process as recited in  claim 1 , wherein the TiN—NbC composite size of 25-30 nm and interparticle spacing of 200-300 nm results in an austenite grain size ranging from 20-40 microns before conducting finish rolling, thereby increasing the surface to volume ratio of austenite grains (Sv factor) that enables production of thicker gage steel product. 
     
     
         5 . A process as recited in  claim 1 , wherein the TiN—NbC interparticle spacing of 200-300 nm is configured to suppress delayed strain induced precipitation of NbC, which renders it difficult to control solute niobium required in the matrix to control the transformation structure in downstream processing. 
     
     
         6 . A process as recited in  claim 1 , wherein the Zener pinning pressure of about 0.1 MPa results in austenite grains in the center of the steel product, which is at a higher temperature than the surface of the steel product by about 100° C., being fully pancaked to modify the cubic texture of recrystallized austenite grains to brass or copper texture before phase transformation in order to suppress the formation of undesirable rotated cube cleavage texture upon transformation, thereby suppressing brittle fracture in the final product. 
     
     
         7 . A process as recited in  claim 1 , wherein the dissolution temperature of NbC is more than 1040° C. 
     
     
         8 . 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. 
     
     
         9 . A process as recited in  claim 1 , wherein the steel product is at least one of line pipe steel, infra-structure steel, or super-martensitic stainless steel. 
     
     
         10 . A process as recited in  claim 1 , wherein the crystallographic texture-related anisotropic properties of the resulting steel product are minimized. 
     
     
         11 . 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. 
     
     
         12 . 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. 
     
     
         13 . 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. 
     
     
         14 . A process as recited in  claim 1 , wherein the steel product exhibits a gage thickness of about 17-40 mm. 
     
     
         15 . 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 
 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                   Amount 
                 
                     
                   Element 
                   (wt %) 
                 
                     
                     
                 
                     
                   N 
                   0.003-0.004 
                 
                     
                   Ti 
                   0.012-0.015 
                 
                     
                   C 
                   0.03-0.07 
                 
                     
                   Nb 
                   0.07-0.15 
                 
                     
                     
                 
             
                
                
                
                
               
               
                
                
                
                
                
               
            
           
         
         (ii) conducting a first stage of roughing on the steel product with rolling passes in a temperature window ranging from 1080° C. to 1150° C. to promote refinement of austenite grains from 300 microns to less than 30 microns by repeated static recrystallization between the rolling passes; 
         (iii) cooling the steel product for 100-120 seconds to allow for temperature homogenization and for the center of the steel product to cool to a temperature of about 1040° C.; 
         (iv) conducting a second stage of roughing on the steel product, wherein the second stage of roughing includes conducting at least one heavy rough rolling pass reduction (>25%) wherein the center of the steel product is at about 1040° C.; 
         (v) cooling the center of the steel product to a temperature at or below about 920° C. to retain solute niobium (Nb>0.03 wt %) in the matrix for transformation hardening; 
         (vi) conducting finish rolling on the steel product to achieve a total rolling reduction of 70% to pancake the austenite grains of about 30 microns to target a pancaked austenite thickness of less than 10 microns; and 
         (vii) applying accelerated cooling to the steel product after finish rolling to produce 1-3 micron size acicular ferrite grains upon transformation to obtain high strength and toughness at low temperature.