Non-quenched and tempered steel having ultrafine grained pearlite structure and method of manufacturing the same

Abstract

This invention relates to a method of manufacturing non-quenched and tempered steel having an ultrafine grained pearlite structure, including hot forging a steel material so as to be high-temperature compression deformed, thus obtaining a hot forged body; rapidly cooling the hot forged body to a low-temperature pearlite transformation range, thus obtaining a supercooled hot forged body; isothermally holding the supercooled hot forged body in the low-temperature pearlite transformation range so as to be isothermally transformed; and air-cooling the hot forged body; and to non-quenched and tempered steel manufactured thereby.

1 . A method of manufacturing non-quenched and tempered steel having an ultrafine grained pearlite structure, the method comprising:
 hot forging a steel material so as to be high-temperature compression deformed, thus obtaining a hot forged body;   rapidly cooling the hot forged body to a low-temperature pearlite transformation range, thus obtaining a supercooled hot forged body;   isothermally holding the supercooled hot forged body in the low-temperature pearlite transformation range, thus isothermally transforming it; and   air-cooling the hot forged body.   
     
     
         2 . The method of  claim 1 , wherein the hot forging is performed at about 1000˜1250° C. 
     
     
         3 . The method of  claim 1 , wherein the rapidly cooling is performed by cooling the hot forged body to about 500˜600° C. at a rate of about 10° C./s or more. 
     
     
         4 . The method of  claim 1 , wherein the isothermally transforming is performed by isothermally holding the supercooled hot forged body at about 500˜600° C. for about 5˜30 minutes. 
     
     
         5 . The method of  claim 1 , wherein the rapidly cooling is performed by cooling the hot forged body to about 500˜600° C. at a rate of about 10° C./s or more, and the isothermally transforming is performed by isothermally holding the supercooled hot forged body at about 500˜600° C. for about 5˜30 minutes. 
     
     
         6 . The method of  claim 1 , wherein the step of air-cooling the hot forged body provides a non-quenched and tempered steel having an ultrafine grained pearlite structure composed mainly of Fe and, further, about 0.43˜0.47 wt % of C, about 0.15˜0.35 wt % of Si, about 1.1˜1.3 wt % of Mn, greater than 0 wt % and up to about 0.03 wt % of P, greater than 0 wt % and up to about 0.04 wt % of S, greater than 0 wt % and up to about 0.3 wt % of Cu, greater than 0 wt % and up to about 0.2 wt % of Ni, about 0.1˜0.2 wt % of Cr, greater than 0 wt % and up to about 0.05 wt % of Mo, about 0.08˜0.15 wt % of V, greater than 0 wt % and up to about 0.02 wt % of Al. 
     
     
         7 . A non-quenched and tempered steel having an ultrafine grained pearlite structure, produced by subjecting a hot forged steel material at about 1000˜1250° C. to rapid cooling to a pearlite transformation range of about 500˜600° C., isothermal holding at the corresponding temperature for about 5˜30 minutes, and then air-cooling. 
     
     
         8 . The non-quenched and tempered steel of  claim 7 , wherein the hot forged steel material is rapidly cooled to the pearlite transformation range of about 500˜600° C. at a rate of about 10° C./s or more. 
     
     
         9 . The non-quenched and tempered steel of  claim 7 , wherein the non-quenched and tempered steel formed after air-cooling has a proeutectoid ferrite fraction of about 5 vol % or less and pearlite colonies having a size of about 5˜10 μm.