Method for producing molten steel having high wear resistance and steel having said characteristics

Abstract

The invention refers to a method for producing a molten steel having high wear resistance having a mainly bainitic microstructure and a suitable balance of tensile strength and hardness for pieces of large size in mining operations such as milling and grinding, the chemical composition of which, expressed in weight percentage, comprises: 0.30-0.40% of C, 0.50-1.30% of Si, 0.60-1.40% of Mn, 2.30-3.20% of Cr, 0.0-1.00% of Ni, 0.25-0.70% of Mo, 0.0-0.50% of Cu, 0.0-0.10% of A, 0.0-0.10% of Ti, 0.0-0.10% of Zr, less than 0.050% of P, less than 0.050% of S, less than 0.030% of N, optionally less than 0.050% of Nb, optionally 0.0005-0.005% of B, optionally 0.015-0.080% of rare earth metals, and residual contents of W, V, Sn, Sb, Pb and Zn of less than 0.020%, and the balance in iron. The method for producing the molten steel includes smelting and heat treatment. The smelting can be carried out in an electric arc furnace having basic or acid refractory or an electric induction furnace. Smelting in an electric arc furnace as a normal operation includes melting, oxygen insufflation, blocking, refining and deoxidation. Smelting in an electric induction furnace includes melting, refining, control of nitrogen in solution and deoxidation. Heat treatment comprises settling and tempering. The molten steel described in the invention exhibits a suitable balance of the chemical composition, tensile strength and hardenability to assure complete hardening in cast pieces of large size, typically up to 17 inches in thickness, with Brinell hardness preferably in the range of 385-495 BHN throughout the section of the piece and excellent resistance to wear by abrasion impact.

1 . A method for producing cast steel having high wear resistance, with predominantly bainite microstructure and a suitable balance of toughness and hardness for mining applications such as grinding, crushing and all those applications that require large components with high resistance to wear by abrasion and impact, wherein the chemical composition used, expressed in percentage by weight, comprises at least:
 0.30-0.40% w/w C;   0.50-1.30% w/w Si;   0.60-1.40% w/w Mn;   2.30-3.20% w/w Cr;   0.00-1.00% w/w Ni;   0.25-0.70% w/w Mo;   0.00-0.50% w/w Cu;   0.00-0.10% w/w Al;   0.00-0.10% w/w Ti;   0.00-0.10% w/w Zr;   less than 0.050% w/w P;   less than 0.050% w/w S;   less than 0.030% w/w N;   the remainder is iron;   
       where the method comprises: 
       a) completely melting the steel of the aforementioned composition; 
       b) normalizing heat treatment at a temperature between 950 and 1050° C., for a time of between 3 and 10 hours; followed by cooling from the normalizing temperature to a temperature between 500 and 80° C., at a rate in the range from 0.05 to 0.5° C./s; 
       c) annealing heat treatment at a temperature in the range from 450 to 630° C., for a time of between 3 and 10 hours. 
     
     
         2 . The method as claimed in  claim 1 , wherein the percentage by weight of chromium in the chemical composition of the steel is preferably 2.40-3.00% w/w. 
     
     
         3 . The method as claimed in  claim 1 , wherein the chemical composition of the steel further comprises less than 0.050% w/w of niobium. 
     
     
         4 . The method as claimed in  claim 1 , wherein the chemical composition of the steel further comprises boron in the range 0.0005-0.005% w/w. 
     
     
         5 . The method as claimed in  claim 1 , wherein the chemical composition of the steel further comprises rare earths in the range 0.015-0.080% w/w. 
     
     
         6 . The method as claimed in  claim 5 , wherein the rare earths correspond to commercial mixtures of cerium and lanthanum. 
     
     
         7 . The method as claimed in  claim 1 , wherein the chemical composition of the steel further comprises residual contents of tungsten, vanadium, tin, antimony, lead and zinc of less than 0.020% w/w. 
     
     
         8 . The method as claimed in  claim 1 , wherein the melting step (a) is carried out in an arc furnace. 
     
     
         9 . The method as claimed in  claim 8 , wherein the arc furnace has a basic refractory or an acid refractory. 
     
     
         10 . The method as claimed in  claim 1 , wherein the melting step is carried out in an induction furnace. 
     
     
         11 . The method as claimed in  claim 10 , wherein the melting step (a) is carried out at a maximum temperature of 1700° C. 
     
     
         12 . The method as claimed in  claim 1 , wherein the cooling in the normalizing heat treatment step (b) is carried out until a temperature of between 500 and 150° C. is reached. 
     
     
         13 . The method as claimed in  claim 1 , wherein the cooling in the normalizing heat treatment step (b) is carried out in still air. 
     
     
         14 . The method as claimed in  claim 1 , wherein the cooling in the normalizing heat treatment step (b) is carried out in direct or indirect forced air. 
     
     
         15 . The method as claimed in  claim 1 , wherein the cooling in the normalizing heat treatment step (b) is carried out by a sequence of substeps in still air and in indirect forced air. 
     
     
         16 . Cast steel having high wear resistance, with predominantly bainite microstructure and a suitable balance of toughness and hardness for mining applications such as grinding, crushing and all those applications that require large components with high resistance to wear by abrasion and impact, wherein it is produced by the method as claimed in  claim 1 . 
     
     
         17 . Cast steel having high wear resistance and a suitable balance of toughness and hardness for mining applications such as grinding, crushing and all those applications that require large components with high resistance to wear by abrasion and impact, wherein it comprises at least:
 0.30-0.40% w/w C;   0.50-1.30% w/w Si;   0.60-1.40% w/w Mn;   2.30-3.20% w/w Cr;   0.00-1.00% w/w Ni;   0.25-0.70% w/w Mo;   0.00-0.50% w/w Cu;   0.00-0.10% w/w Al;   0.00-0.10% w/w Ti;   0.00-0.10% w/w Zr;   less than 0.050% w/w P;   less than 0.050% w/w S;   less than 0.030% w/w N; and   the remainder is iron;   
       and in that said steel has a predominantly bainite structure. 
     
     
         18 . The cast steel as claimed in  claim 17 , wherein the percentage by weight of chromium in the chemical composition of the steel is preferably 2.40-3.00% w/w. 
     
     
         19 . The cast steel as claimed in  claim 17 , wherein the chemical composition of the steel further comprises less than 0.050% w/w of niobium. 
     
     
         20 . The cast steel as claimed in  claim 17 , wherein the chemical composition of the steel further comprises boron in the range 0.0005-0.005% w/w. 
     
     
         21 . The cast steel as claimed in  claim 17 , wherein the chemical composition of the steel further comprises rare earths in the range 0.015-0.080% w/w. 
     
     
         22 . The cast steel as claimed in  claim 21 , wherein the rare earths correspond to commercial mixtures of cerium and lanthanum. 
     
     
         23 . The cast steel as claimed in  claim 17 , wherein the chemical composition of the steel further comprises residual contents of tungsten, vanadium, tin, antimony, lead and zinc of less than 0.020% w/w.