Manufacturing Method of Alloy Steel For Cold-Forging, Alloy Steel For Cold-Forging, and Gear For Cold-Forging

Abstract

A manufacturing method of an alloy steel for cold-forging according to an embodiment of the present disclosure comprises homogenizing a steel material, furnace-cooling the steel material, and segmentation-annealing the steel material, in which the segmentation-annealing may be performed at least once and is optionally repeated one or more times. An alloy steel for cold-forging according to an embodiment of the present disclosure may comprise segmented cementite. A gear for cold-forging according to an embodiment of the present disclosure may have a gear surface grain boundary oxide layer of 19 μm or less.

What is claimed is: 
     
         1 . A manufacturing method of an alloy steel for cold-forging, the manufacturing method comprising:
 homogenizing a steel material;   furnace-cooling the steel material; and   segmentation-annealing the steel material,   wherein the segmentation-annealing is performed at least once and is optionally repeated one or more times.   
     
     
         2 . The manufacturing method of the alloy steel for cold-forging of  claim 1 , wherein the homogenizing is performed at or above an austenitizing temperature of the steel material. 
     
     
         3 . The manufacturing method of the alloy steel for cold-forging of  claim 1 , wherein the homogenizing is maintained at a temperature of 800° C. to 900° C. for 2 hours to 3 hours. 
     
     
         4 . The manufacturing method of the alloy steel for cold-forging of  claim 1 , wherein the furnace-cooling is performed at a temperature of 550° C. to 700° C. 
     
     
         5 . The manufacturing method of the alloy steel for cold-forging of  claim 1 , wherein the segmentation-annealing is performed by segmenting cementite. 
     
     
         6 . The manufacturing method of the alloy steel for cold-forging of  claim 1 , wherein the segmentation-annealing comprises
 annealing at a temperature of 740° C. to 760° C.; and   stabilizing at a temperature of 680° C. to 700° C.   
     
     
         7 . The manufacturing method of the alloy steel for cold-forging of  claim 1 , wherein the segmentation-annealing is performed one to three times. 
     
     
         8 . The manufacturing method of the alloy steel for cold-forging of  claim 1 , wherein the homogenizing, the furnace-cooling, and the segmentation-annealing comprises an annealing time of 8 hours or less. 
     
     
         9 . An alloy steel for cold-forging comprising segmented cementite. 
     
     
         10 . The alloy steel for cold-forging of  claim 9 , wherein the segmented cementite has a size of 0.1 μm to 3 μm in a longitudinal direction. 
     
     
         11 . The alloy steel for cold-forging of  claim 9 , wherein the alloy steel comprises a hardness from 100 HV to 200 HV. 
     
     
         12 . The alloy steel for cold-forging of  claim 9 , wherein the alloy steel comprises carbon (C): 0.17 to 0.23 wt %, silicon (Si): more than 0 to 0.7 wt % or less, manganese (Mn): 0.45 to 0.9 wt %, phosphorus (P): more than 0 to 0.020 wt % or less, sulfur (S): more than 0 to 0.030 wt % or less, chromium (Cr): 0.85 to 2.25 wt %, nickel (Ni): more than 0 to 0.25 wt % or less, and niobium (Nb): 0.015 to 0.035 wt %. 
     
     
         13 . A gear for cold-forging comprising a gear surface grain boundary oxide layer of 19 μm or less.