Low-carbon resulfurized free-machining steel excellent in machinability

Abstract

A low-carbon resulfurized free-machining steel is excellent in machinability and contains 0.02% to 0.15% by mass of C; 0.004% by mass or less (exclusive of 0%) of Si; 0.6% to 3% by mass of Mn; 0.02% to 0.2% by mass of P; 0.35% to 1% by mass of S; 0.005% by mass or less (exclusive of 0% by mass) of Al; 0.008% to 0.03% by mass of 0; and 0.007% to 0.03% by mass of N, with the remainder being iron and inevitable impurities, in which the ratio [Mn]/[S] of the manganese content [Mn] to the sulfur content [S] is within the range of 3 to 4, and the carbon content [C], the manganese content [Mn] and the nitrogen content [N] satisfy the following Expression (1): 10[C]×[Mn] −0.94 +1226 [N] 2 ≦1.2, wherein [C], [Mn] and [N] represent the contents on the percent by mass basis of carbon, manganese, and nitrogen, respectively.

1 - 4 . (canceled) 
     
     
         5 . A low-carbon resulfurized free-machining steel, comprising:
 0.02 to 0.15 percent by mass of carbon (C);   0.004 percent by mass or less, exclusive of 0 percent, of silicon (Si);   0.6 to 3 percent by mass of manganese (Mn);   0.02 to 0.2 percent by mass of phosphorus (P);   0.35 to 1 percent by mass of sulfur (S);   0.005 percent by mass or less, exclusive of 0 percent by mass, of aluminum (Al);   0.008 to 0.03 percent by mass of oxygen (O); and   0.007 to 0.03 percent by mass of nitrogen (N), with the remainder being iron and inevitable impurities and containing no B,   wherein a ratio [Mn]/[S] of the manganese content [Mn] to the sulfur content [S] is within the range of from 3.4 to 3.8, and   wherein the carbon content [C], the manganese content [Mn] and the nitrogen content [N] satisfy following Expression (1):
   10[C]×[Mn] −0.94 +1226[N] 2 ≦1.2   (1)
 
   wherein [C], [Mn] and [N] represent the contents on the percent by mass basis of carbon, manganese, and nitrogen, respectively.   
     
     
         6 . The low-carbon resulfurized free-machining steel according to  claim 17 , wherein the content of soluble nitrogen is 0.002 to 0.02 percent by mass. 
     
     
         7 . The low-carbon resulfurized free-machining steel according to  claim 18 , wherein the total content of at least one element selected from the group consisting of Ti, Cr, Nb, V, and Zr is 0.02 percent by mass or less, exclusive of 0 percent by mass. 
     
     
         8 . The low-carbon resulfurized free-machining steel according to  claim 17  produced by a process comprising electromagnetic stirring the steel during casting, wherein a magnetic field of 100 to 500 gausses is applied during casting. 
     
     
         9 . The low-carbon resulfurized free-machining steel according to  claim 17 , wherein the total content of at least one element selected from the group consisting of Ti, Cr, Nb, V, and Zr is 0.02 percent by mass or less, exclusive of 0 percent by mass. 
     
     
         10 . The low-carbon resulfurized free-machining steel according to  claim 17 , wherein the steel has no surface defects detected by an automatic defect detector. 
     
     
         11 . The low-carbon resulfurized free-machining steel according to  claim 20 , wherein the steel has no surface defects detected by an automatic defect detector. 
     
     
         12 . The low-carbon resulfurized free-machining steel according to  claim 17 , wherein the nitrogen content is in a range of from 0.007 to 0.025 percent by mass. 
     
     
         13 . The low-carbon resulfurized free-machining steel according to  claim 17 , wherein the S content is in a range of from 0.35 to 0.8 percent by mass. 
     
     
         14 . The low-carbon resulfurized free-machining steel according to  claim 17 , wherein the Mn content is in a range of from 1 to 2 percent by mass. 
     
     
         15 . The low-carbon resulfurized free-machining steel according to  claim 17 , wherein the Si content is less than 0.003 percent by mass. 
     
     
         16 . The low-carbon resulfurized free-machining steel according to  claim 20 , wherein the Si content is less than 0.003 percent by mass. 
     
     
         17 . The low-carbon resulfurized free-machining steel according to  claim 17 , wherein the C content is in a range of from 0.05 to 0.12 percent by mass. 
     
     
         18 . The low-carbon resulfurized free-machining steel according to  claim 17 , consisting of Fe and inevitable impurities and:
 0.02 to 0.15 percent by mass of carbon (C);   0.004 percent by mass or less, exclusive of 0 percent, of silicon (Si);   0.6 to 3 percent by mass of manganese (Mn);   0.02 to 0.2 percent by mass of phosphorus (P);   0.35 to 1 percent by mass of sulfur (S);   0.005 percent by mass or less, exclusive of 0 percent by mass, of aluminum (Al);   0.008 to 0.03 percent by mass of oxygen (O);   0.007 to 0.03 percent by mass of nitrogen (N), and   optionally at least one element from the group consisting of Ti, Cr, Nb, V and Zr,   wherein a ratio [Mn]/[S] of the manganese content [Mn] to the sulfur content [S] is within the range of from 3 to 4, and   wherein the carbon content [C], the manganese content [Mn] and the nitrogen content [N] satisfy following Expression (1):
   10[C]×[Mn] −0.94 +1226[N] 2 ≦1.2   ( 1 )
 
   wherein [C], [Mn] and [N] represent the contents on the percent by mass basis of carbon, manganese, and nitrogen, respectively.   
     
     
         19 . The low-carbon resulfurized free-machining steel according to  claim 17 , wherein the steel has no surface defect. 
     
     
         20 . The low-carbon resulfurized free-machining steel according to claim  30 , wherein the steel has no surface defect.