Process for producing steel products having improved grain size properties and machinability

Abstract

Disclosed is a production process of a steel product for induction hardening and a steel product for carburizing, having improved grain size properties and machinability. This production process comprises the steps of: providing an ingot or bloom comprising a steel comprising, by weight, carbon (C): 0.10 to 0.45% or 0.25 to 0.70%, silicon (Si): 0.03 to 1.0%, manganese (Mn): 0.2 to 2.0%, titanium (Ti): 0.05 to 0.2%, aluminum (Al): 0.005 to 0.05%, and nitrogen (N): not more than 0.01% with the balance consisting of iron (Fe) and unavoidable impurities; and subjecting the steel ingot or bloom to a series of hot working steps including the step of rolling the steel ingot or bloom into a semi-finished steel product, the step of rolling the semi-finished steel product into a steel bar or wire rod, and the step of forging the steel bar or wire rod into a product. In the above series of hot working steps, the steel is given a thermal history in which said steel is at least once heated to 1,250 ° C. or above, is cooled to room temperature, and is then reheated to a temperature range of 800 to 1,100 ° C.

1 . A steel product for carburizing, comprising a steel comprising, by weight, carbon (C): 0.10 to 0.45%, silicon (Si): 0.03 to 1.0%, manganese (Mn): 0.2 to 2.0%, titanium (Ti): 0.05 to 0.2%, aluminum (Al): 0.005 to 0.05%, and nitrogen (N): not more than 0.01% with the balance consisting of iron (Fe) and unavoidable impurities, wherein 
 said steel    contains not more than 10,000 dispersedly precipitated titanium compounds per μm 2 ,    has a hardness of not more than 100 HRB, and,    when carburized, permits the number of dispersedly precipitated titanium compounds to be not less than 50 per μm 2 .    
   
   
       2 . A steel product for induction hardening, comprising a steel comprising, by weight, carbon (C): 0.25 to 0.70%, silicon (Si): 0.03 to 1.0%, manganese (Mn): 0.2 to 2.0%, titanium (Ti): 0.05 to 0.2%, aluminum (Al): 0.005 to 0.05%, and nitrogen (N): not more than 0.01% with the balance consisting of iron (Fe) and unavoidable impurities, wherein 
 said steel    contains not more than 10,000 dispersedly precipitated titanium compounds per μm 2 ,    has a hardness of not more than 103 HRB, and,    when subjected to high frequency induction heating, permits the number of dispersedly precipitated titanium compounds to be not less than 50 per μm 2 .