Manufacturing method for high-concentration carburized steel

Abstract

This invention provides a process for producing a high-concentration carburized steel which can realize dispersion of a large amount of fine and spherical carbides without causing lowered service life of a furnace, deformed steel products, and lowered working efficiency. The production process comprises (i) a primary carburization step for carburizing a steel product having a predetermined composition at a primary carburization temperature (T 1 ) (° C.) until the surface carbon concentration (C) reaches a predetermined carbon concentration, (ii) a cooling step of, after the completion of the primary carburization step, cooling the steel product at a cooling rate of not less than 1° C./min to Ar 1 point or less, (iii) a secondary carburization initial step of raising the temperature of the steel product to a secondary carburization start temperature (T 2 s ), which is at least 100° C. below the primary carburization temperature (T 1 ), and carburizing the steel product at a secondary carburization temperature (T 2 ), (iv) a secondary carburization late step of, subsequent to the completion of the secondary carburization initial step, raising the temperature of the steel product to a quench hardening temperature (Tq) and further carburizing the steel product at the quench hardening temperature (Tq), and (v) a quench hardening step of, after the completion of the secondary carburization late step, quench hardening the steel product.

1 . A manufacturing method for high-concentration carburized steel, comprising:
 (i) a primary carburization step of carburizing a steel material having C of 0.15-0.30 mass %, Si of 0.40-0.80 mass %, Mn of 0.3-0.8 mass %, Cr of 1.25-2.00 mass %, and balance of Fe and unavoidable impurities at a primary carburization temperature T 1 (° C.) till a surface carbon concentration C becomes Ceu<C≦C(Acm), wherein Ceu is an eutectoid carbon concentration of the steel material, and C(Acm) is a carbon concentration corresponding to a Acm line of the aforesaid steel material at the primary carburization temperature T 1 ;   (ii) a cooling step of cooling the steel material to 700° C. or lower at a cooling rate of 1° C./min or more after completion of the primary carburization step;   (iii) a secondary carburization initial step of raising the temperature of the steel material to a secondary carburization start temperature T 2   s  to carburize the steel material at a secondary carburization temperature T 2 , wherein Ac 1  point (° C.)≦T 2   s  (° C.)≦primary carburization temperature T 1 −100° C.≦Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material immediately after initiation of the secondary carburization, and T 2   s ≦T 2 ≦Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material;   (iv) a secondary carburization late step of raising the temperature, after completion of the secondary carburization initial step, to a quenching temperature Tq (° C.) to further carburize at the quenching temperature of Tq, wherein Tq≦Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material); and   (v) a step of quenching the steel material after completion of the secondary carburization.   
   
   
       2 . The manufacturing method for high-concentration carburized steel as set forth in  claim 1 , wherein the primary carburization temperature T 1  is 1,100° C. or lower. 
   
   
       3 . The manufacturing method for high-concentration carburized steel as set forth in  claim 1 , wherein the secondary carburization initial step is a step of performing the carburization for a period of 15 minutes or longer with retaining the secondary carburization temperature T 2  at the secondary carburization start temperature T 2   s.    
   
   
       4 . The manufacturing method for high-concentration carburized steel as set forth in  claim 1 , wherein the secondary carburization initial step is a step of performing the carburization with raising the secondary carburization temperature T 2  stepwisely or continuously from the secondary carburization start temperature T 2   s  to the quenching temperature Tq. 
   
   
       5 . The manufacturing method for high-concentration carburized steel as set forth in  claim 1 , wherein the secondary carburization late step is a step of performing the carburization for a period of 15 minutes or longer at the quenching temperature Tq. 
   
   
       6 . The manufacturing method for high-concentration carburized steel as set forth in  claim 2 , wherein the secondary carburization late step is a step of performing the carburization for a period of 15 minutes or longer at the quenching temperature Tq. 
   
   
       7 . The manufacturing method for high-concentration carburized steel as set forth in  claim 3 , wherein the secondary carburization late step is a step of performing the carburization for a period of 15 minutes or longer at the quenching temperature Tq. 
   
   
       8 . The manufacturing method for high-concentration carburized steel as set forth in  claim 4 , wherein the secondary carburization late step is a step of performing the carburization for a period of 15 minutes or longer at the quenching temperature Tq. 
   
   
       9 . The manufacturing method for high-concentration carburized steel as set forth in  claim 2 , wherein the secondary carburization initial step is a step of performing the carburization for a period of 15 minutes or longer with retaining the secondary carburization temperature T 2  at the secondary carburization start temperature T 2   s.    
   
   
       10 . The manufacturing method for high-concentration carburized steel as set forth in  claim 9 , wherein the secondary carburization late step is a step of performing the carburization for a period of 15 minutes or longer at the quenching temperature Tq. 
   
   
       11 . The manufacturing method for high-concentration carburized steel as set forth in  claim 2 , wherein the secondary carburization initial step is a step of performing the carburization with raising the secondary carburization temperature T 2  stepwisely or continuously from the secondary carburization start temperature T 2   s  to the quenching temperature Tq. 
   
   
       12 . The manufacturing method for high-concentration carburized steel as set forth in  claim 11 , wherein the secondary carburization late step is a step of performing the carburization for a period of 15 minutes or longer at the quenching temperature Tq.