High-Hardness Steel Product and Method of Manufacturing the Same

Abstract

Described is a hot-rolled steel strip product that includes a composition consisting of, in terms of weight percentages, 0.17% to 0.38% C, 0% to 0.5% Si, 0.1% to 0.4% Mn, 0.015% to 0.15% Al, 0.1% to 0.6% Cu, 0.2% to 0.8% Ni, 0.1% to 1% Cr, 0.01% to 0.3% Mo, 0% to 0.005% Nb, 0% to 0.05% Ti, 0% to 0.2% V, 0.0008% to 0.005% B, 0% to 0.025% P, 0.008% or less S, 0.01% or less N, 0% to 0.01% Ca, and the remainder being Fe and inevitable impurities, wherein the steel product has a Brinell hardness in the range of 420-580 HBW, and a corrosion index (ASTM G101-04) of at least 5.

1 . A hot-rolled steel strip product comprising a composition consisting of, in terms of weight percentages (wt. %): 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   C 
                   0.17-0.38,     
                 
                     
                   Si 
                   0-0.5, 
                 
                     
                   Mn 
                   0.1-0.4,     
                 
                     
                   Al 
                   0.015-0.15    
                 
                     
                   Cu 
                   0.1-0.6,     
                 
                     
                   Ni 
                   0-0.8, 
                 
                     
                   Cr 
                   0.1-1,    
                 
                     
                   Mo 
                   0.01-0.3,   
                 
                     
                   Nb 
                       0-0.005 
                 
                     
                   Ti 
                    0-0.05, 
                 
                     
                   V 
                   0-0.2, 
                 
                     
                   B 
                   0.0005-0.005,   
                 
                     
                   P 
                    0-0.025, 
                 
                     
                   S 
                    0-0.008, 
                 
                     
                   N 
                    0-0.01, 
                 
                     
                   Ca 
                    0-0.01, 
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
               
            
           
         
       
       remainder Fe and inevitable impurities, wherein the steel product has a Brinell hardness in the range of 420-580 HBW, and a corrosion index (ASTM G101-04) of at least 5. 
     
     
         2 . The steel product according to  claim 1 , wherein the amount of Ti is in the range of 0-0.005 wt. % when the amount of N is in the range of 0-0.003 wt. %. 
     
     
         3 . The steel product according to  claim 1 , wherein the amount of Ti is more than 0.005 wt. % and not more than 0.05 wt. % when the amount of N is more than 0.003 wt. % and not more than 0.01 wt. %. 
     
     
         4 . The steel product according to  claim 1 , wherein
 [Ni]>[Cu]/3, and wherein   [Ni] is the amount of Ni in the composition,   [Cu] is the amount of Cu in the composition.   
     
     
         5 . The steel product according to  claim 1 , wherein the Ca/S ratio is in the range of 1-2. 
     
     
         6 . The steel product according to  claim 1 , wherein the steel product has a Brinell hardness in the range of 450-550 HBW. 
     
     
         7 . The steel product according to  claim 1 , wherein the steel product has a corrosion index (ASTM G101-04) of at least 5.5. 
     
     
         8 . The steel product according to  claim 1 , wherein the steel product has a Charpy-V impact toughness of at least 34 J/cm 2  at a temperature of −20° C. or −40° C. in transversal and/or longitudinal direction. 
     
     
         9 . The steel product according to  claim 1 , wherein the steel product has a minimum bending radius of 3.4 t or less in a measurement direction longitudinal to the rolling direction; a minimum bending radius of 2.7 t or less in a measurement direction transversal to the rolling direction; and wherein t is the thickness of the steel strip product. 
     
     
         10 . The steel product according to  claim 1 , wherein the steel product has a microstructure consisting of, in terms of volume percentages (vol. %), 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   martensite 
                   ≥90, 
                 
                     
                   residual austenite 
                   0-1, 
                 
                     
                     
                 
             
                
               
               
                
                
                
               
            
           
         
         remainder bainite, ferrite and/or pearlite. 
       
     
     
         11 . The steel product according to  claim 1 , wherein the steel product has a prior austenite grain size of 50 μm or less. 
     
     
         12 . The steel product according to  claim 1 , wherein the steel product has a prior austenite grain structure with an aspect ratio in the range of 1.5-7. 
     
     
         13 . The steel product according to  claim 1 , wherein the steel strip product has a thickness of 10 mm. 
     
     
         14 . A method for manufacturing the steel product according to  claim 1  comprising the following steps of
 providing a steel slab consisting of the chemical composition according to any one of the  claims 1  to  5 ; 
 heating the steel slab to the austenitizing temperature of 1200-1350° C.; 
 hot-rolling to the desired thickness at a temperature in the range of Ar3 to 1300° C., wherein the finish rolling temperature is in the range of 800° C. to 960° C.; 
 direct quenching the hot-rolled steel strip product to a cooling end and coiling temperature of 450° C. or less; and 
 optionally, temper annealing at a temperature in the range of 150° C.-250° C.