Vehicle part having high strength and excellent durability, and manufacturing method therefor

Abstract

Provided are a part for vehicle having high strength and excellent durability, and a manufacturing method therefor. The part for vehicle comprises, by a weight ratio, a composition comprising 0.20-0.50% of C, 0.5% or less of Si, 1.0-2.0% of Mn, 0.01-0.1% of Al, 0.010% or less of P, 0.003% or less of S, 0.01-0.1% of Ti, 0.05-0.5% of Cr, 0.05-0.3% of Mo, 0.01% or less of N, and the remainder being Fe and other inevitable impurities, and the part for vehicle can have, by an area ratio, a microstructure comprising 90% or more of tempered martensite, 4% or less of retained austenite, and the remainder being one type or both of two types selected from among the ferrite and bainite structures.

1 .- 20 . (canceled) 
     
     
         21 . A part for vehicle, comprising a composition including, by weight ratio, 0.20% to 0.50% of carbon (C), 0.5% or less of silicon (Si), 1.0% to 2.0% of manganese (Mn), 0.01% to 0.1% of aluminum (Al), 0.010% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.01% to 0.1% of titanium (Ti), 0.05% to 0.5% of chromium (Cr), 0.05% to 0.3% of molybdenum (Mo), 0.01% or less of nitrogen (N), and a remainder of iron (Fe) and other inevitable impurities, and
 comprising a microstructure including, by an area ratio, 90% or more of tempered martensite, 4% or less of retained austenite, and a remainder of one or both of two selected from among ferrite and bainite,   wherein epsilon carbide is contained in the tempered martensite as a precipitate.   
     
     
         22 . The part for vehicle of  claim 21 , wherein the composition further includes, by weight ratio, 0.0005% to 0.005% of boron (B). 
     
     
         23 . The part for vehicle of  claim 21 , wherein the composition further includes, by weight ratio, one or both of two selected from among 0.05% to 0.5% of copper (Cu) and 0.05% to 0.5% of nickel (Ni). 
     
     
         24 . The part for vehicle of  claim 21 , wherein the composition further includes, by weight ratio, one or both of two selected from among 0.01% to 0.07% of niobium (Nb) and 0.05% to 0.3% of vanadium (V). 
     
     
         25 . The part for vehicle of  claim 21 , wherein the composition satisfies the relationship of Mo/P>10,
 where each of Mo and P indicates the content (weight %) of the corresponding element.   
     
     
         26 . The part for vehicle of  claim 21 , wherein the epsilon carbide exists in a numerical ratio of 70% or more to a total precipitate in the tempered martensite. 
     
     
         27 . The part for vehicle of  claim 21 , wherein a yield ratio is 0.7 to 0.85. 
     
     
         28 . A method for manufacturing a part for vehicle, comprising:
 preparing a material having a composition including, by weight ratio, 0.20% to 0.50% of C, 0.5% or less of Si, 1.0% to 2.0% of Mn, 0.01% to 0.1% of Al, 0.010% or less of P, 0.003% or less of S, 0.01% to 0.1% of Ti, 0.05% to 0.5% of Cr, 0.05% to 0.3% of Mo, 0.01% or less of N, and a remainder of Fe and other inevitable impurities;   heating the material to a temperature at which the material is transformed into austenite;   obtaining an intermediate product by cooling while forming the material having been heated in a mold; and   tempering the intermediate product at a temperature of 150° C. to 250° C.   
     
     
         29 . The method for manufacturing a part for vehicle of  claim 28 , wherein the composition further includes, by weight ratio, at least one selected from the group of 0.0005% to 0.005% of B, one or both of two selected from among 0.05% to 0.5% of Cu and 0.05% to 0.5% of Ni, one or both of two selected from among 0.01% to 0.07% of Nb and 0.05% to 0.3% of V. 
     
     
         30 . The method for manufacturing a part for vehicle of  claim 28 , wherein the composition satisfies the relationship of Mo/P>10,
 where each of Mo and P indicates the content (weight %) of the corresponding element.   
     
     
         31 . The method for manufacturing a part for vehicle of  claim 28 , wherein the intermediate product has a microstructure including, by an area ratio, 90% or more of martensite, 4% or less of retained austenite, and a remainder of one or both of two selected from among of ferrite and bainite. 
     
     
         32 . The method for manufacturing a part for vehicle of  claim 28 , wherein the temperature at which the material is transformed into austenite is 850° C. to 960° C. 
     
     
         33 . The method for manufacturing a part for vehicle of  claim 28 , wherein the time for holding at the temperature at which the material is transformed into austenite is 100 seconds to 1000 seconds. 
     
     
         34 . The method for manufacturing a part for vehicle of  claim 28 , wherein a cooling rate in the obtaining an intermediate product by cooling is equal to or higher than a martensite critical cooling rate and wherein a cooling stop temperature in the obtaining an intermediate product by cooling is equal to or lower than 100° C. 
     
     
         35 . The method for manufacturing a part for vehicle of  claim 28 , wherein the holding time in the tempering is equal to 10 minutes or more. 
     
     
         36 . A method for manufacturing a part for vehicle, comprising:
 preparing a material having a composition including, by weight ratio, 0.20% to 0.50% of C, 0.5% or less of Si, 1.0% to 2.0% of Mn, 0.01% to 0.1% of Al, 0.010% or less of P, 0.003% or less of S, 0.01% to 0.1% of Ti, 0.05% to 0.5% of Cr, 0.05% to 0.3% of Mo, 0.01% or less of N, and a remainder of Fe and other inevitable impurities;   cold forming the material;   heating the material having been cold formed to a temperature at which the material is transformed into austenite;   obtaining an intermediate product by cooling the material having been heated; and   tempering the intermediate product at a temperature of 150° C. to 250° C.   
     
     
         37 . The method for manufacturing a part for vehicle of  claim 36 , wherein the composition further includes, by weight ratio, at least one selected from the group of 0.0005% to 0.005% of B, one or both of two selected from among 0.05% to 0.5% of Cu and 0.05% to 0.5% of Ni, one or both of two selected from among 0.01% to 0.07% of Nb and 0.05% to 0.3% of V. 
     
     
         38 . The method for manufacturing a part for vehicle of  claim 36 , wherein the composition satisfies the relationship of Mo/P>10,
 where each of Mo and P indicates the content (weight %) of the corresponding element.   
     
     
         39 . The method for manufacturing a part for vehicle of  claim 36 , wherein the intermediate product has a microstructure including, by an area ratio, 90% or more of martensite, 4% or less of retained austenite, and a remainder of one or both of two selected from among of ferrite and bainite. 
     
     
         40 . The method for manufacturing a part for vehicle of  claim 36 , wherein the temperature at which the material is transformed into austenite is 850° C. to 960° C. 
     
     
         41 . The method for manufacturing a part for vehicle of  claim 36 , wherein the time for holding at the temperature at which the material is transformed into austenite is 100 seconds to 1000 seconds. 
     
     
         42 . The method for manufacturing a part for vehicle of  claim 36 , wherein a cooling rate in the obtaining an intermediate product by cooling is equal to or higher than a martensite critical cooling rate and wherein a cooling stop temperature in the obtaining an intermediate product by cooling is equal to or lower than 100° C. 
     
     
         43 . The method for manufacturing a part for vehicle of  claim 36 , wherein the holding time in the tempering is equal to 10 minutes or more.