High carbon hot-rolled steel sheet and method for manufacturing the same

Abstract

The high carbon hot-rolled steel sheet contains, in terms of percentages of mass, 0.10 to 0.7% C, 2.0% or less Si, 0.20 to 2.0% Mn, 0.03% or less P, 0.03% or less S, 0.1% or less Sol.Al, 0.01% or less N, and the balance being Fe and inevitable impurities, and has a structure of ferrite having 6 μm or less average grain size and carbide having 0.10 μm or more and less than 1.2 μm of average grain size. The volume ratio of the carbide having 2.0 μm or more of grain size is 10% or less. The volume ratio of the ferrite containing no carbide is 5% or less. The manufacturing method thereof has the steps of hot-rolling, primary cooling, holding, coiling, acid washing, and annealing. The primary cooling step is to cool the hot-rolled steel sheet down to cooling termination temperatures ranging from 450° C. to 600° C. at cooling rates of higher than 120 ° C./sec. The holding step is to apply secondary cooling to hold the primarily cooled hot-rolled steel sheet at a temperature range from 450° C. to 650° C. until coiling.

1 . A high carbon hot-rolled steel sheet consisting essentially of: in terms of percentage of mass, 0.10 to 0.7% C, 2.0% or less Si, 0.20 to 2.0% Mn, 0.03% or less P, 0.03% or less S, 0.1% or less Sol.Al, 0.01% or less N, and the balance being Fe and inevitable impurities; 
 ferrite having an average grain size of 6 μm or less and carbide having an average grain size of 0.10 μm or more and less than 1.2 μm;    the carbide having a volume ratio of 10% or less regarding a grain size of 2.0 μm or more; and    the ferrite containing no carbide having a volume ratio of 5% or less.    
     
     
         2 . The high carbon hot-rolled steel sheet according to  claim 1 , further containing at least one element selected from the group consisting of, in terms of percentages of mass, 0.05 to 1.5% Cr and 0.01 to 0.5% Mo.  
     
     
         3 . The high carbon hot-rolled steel sheet according to  claim 1 , further containing at least one element selected from the group consisting of, in terms of percentages of mass, 0.005% or less B, 1.0% or less Cu, 1.0% or less Ni, and 0.5% or less W.  
     
     
         4 . The high carbon hot-rolled steel sheet according to  claim 2 , further containing at least one element selected from the group consisting of, in terms of percentages of mass, 0.005% or less B, 1.0% or less Cu, 1.0% or less Ni, and 0.5% or less W.  
     
     
         5 . The high carbon hot-rolled steel sheet according  claim 1 , further containing at least one element selected from the group consisting of, in terms of percentages of mass, 0.5% or less Ti, 0.5% or less Nb, 0.5% or less V, and 0.5% or less Zr.  
     
     
         6 . The high carbon hot-rolled steel sheet according to  claim 1 , wherein the content of Si is 0.005 to 2.0% by mass.  
     
     
         7 . The high carbon hot-rolled steel sheet according to  claim 6 , wherein the content of Si is 0.02 to 0.5% by mass.  
     
     
         8 . The high carbon hot-rolled steel sheet according to  claim 1 , wherein the content of Mn is 0.2 to 1.0% by mass.  
     
     
         9 . The high carbon hot-rolled steel sheet according to  claim 2 , wherein the content of Cr is 0.05 to 0.3% by mass.  
     
     
         10 . The high carbon hot-rolled steel sheet according to  claim 2 , wherein the content of Cr is 0.8 to 1.5% by mass.  
     
     
         11 . The high carbon hot-rolled steel sheet according to  claim 2 , wherein the content of Mo is from 0.05 to 0.5% by mass.  
     
     
         12 . A method for manufacturing a high carbon hot-rolled steel sheet, comprising the steps of: 
 hot-rolling a steel consisting essentially of, in terms of percentages of mass, 0.10 to 0.70% C, 2.0% or less Si, 0.20 to 2.0% Mn, 0.03% or less P, 0.03% or less S, 0.1% or less Sol.Al, 0.01% or less N, and the balance being Fe and inevitable impurities, at finishing temperatures of (Ar 3  transformation point −10° C.) or more;    applying primary cooling to the hot-rolled steel sheet down to cooling termination temperatures ranging from 450° C. to 600° C. at cooling rates of more than 120° C./sec;    applying secondary cooling to hold the primarily cooled hot-rolled steel sheet in a temperature range from 450° C. to 650° C. until coiling;    coiling the cooled hot-rolled steel sheet at coiling temperatures of 600° C. or less;    applying acid washing to the coiled hot-rolled steel sheet; and    annealing the acid-washed hot-rolled steel sheet at annealing temperatures ranging from 680° C. to Ac 1  transformation point.    
     
     
         13 . The method according  claim 12 , wherein the cooling rate in the primary cooling step is in a range from 120 to 700° C./sec.  
     
     
         14 . The method according to  claim 12 , wherein the coiling temperature is in a range from 200° C. to 600° C.  
     
     
         15 . The high carbon hot-rolled steel sheet according  claim 2 , further containing at least one element selected from the group consisting of, in terms of percentages of mass, 0.5% or less Ti, 0.5% or less Nb, 0.5% or less V, and 0.5% or less Zr.  
     
     
         16 . The high carbon hot-rolled steel sheet according  claim 3 , further containing at least one element selected from the group consisting of, in terms of percentages of mass, 0.5% or less Ti, 0.5% or less Nb, 0.5% or less V, and 0.5% or less Zr.  
     
     
         17 . The high carbon hot-rolled steel sheet according  claim 4 , further containing at least one element selected from the group consisting of, in terms of percentages of mass, 0.5% or less Ti, 0.5% or less Nb, 0.5% or less V, and 0.5% or less Zr.