Steel for machine structural use excellent in friability of chips

Abstract

Disclosed is a free cutting steel for machine structural use having excellent chip-breakability. The steel consists essentially of, by wt. %. C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Ca or Ca+Mg; 0.0005-0.02%, Ti: 0.002-0.010% and/or Zr: 0.002-0.025%, O: 0.0005-0.010%, and the balance of impurities and Fe. At least five MnS inclusion particles having averaged particles sizes of 1.0 μm or more exists per mm 2 per 0.01% of S-content in the steel. The steel satisfies the condition that, in the microscopic fields, (area[μm 2 ]/aspect ratio)≧10, and that the the area percentage of Ca-containing sulfide inclusions containing at least 1.0 wt. % of Ca is in the range of 15-40% of the area of all the sulfide inclusions.

1 . A steel for machine structural use having excellent chip-breakability, comprising alloying elements necessary for a machine structural steel except for Pb and Bi, at least five MnS-inclusion particles having averaged particle size of 1.0 μm or more existing per mm 2  per S-content 0.01%, in the microscopic field, the condition (area [μm 2 ]/aspect ratio)≧10 being met, the area percentage of Ca-containing sulfide inclusion particles containing at least 1.0 wt. % of Ca being in the range of 15-40% of the area of all the sulfide inclusion particles, and film of sulfide inclusions being formed on the tool surface during turning thereby to minimize curl diameter of chips.  
     
     
         2 . The steel for machine structural use having excellent chip-breakability according to  claim 1 , wherein the steel consists essentially of, by wt. %, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Ca alone or both Ca and Mg (in case of the both, the total amount): 0.0005-0.02%, one or both of Ti: 0.002-0.010% and Zr: 0.002-0.025%, O: 0.0005-0.010%, and the balance of inevitable impurities and Fe.  
     
     
         3 . The steel for machine structural use having excellent chip-breakability according to  claim 2 , wherein the steel further contains, one or more of Se: up to 0.4%, Te: up to 0.2% and REM: up to 0.05%.  
     
     
         4 . The steel for machine structural use having excellent chip-breakability according to one of claims  2  and  3 , wherein the steel further contains, one or more of Cr: up to 3.5%, Mo: up to 2.0%, Cu: up to 2.0%, Ni: up to 4.0% and B: 0.0005-0.01%.  
     
     
         5 . The steel for machine structural use having excellent chip-breakability according to one of  claims 2  to  3 , wherein the steel further contains, one or both of Nb: up to 0.2% and V: up to 0.5%.