Heat treatable steel, product formed thereof having ultra high strength and excellent durability, and method for manufacturing same
The present invention relates to a formed product used in vehicle components and the like, and to a method for manufacturing the same. The present invention provides heat treatable steel, a formed product using the same having ultra-high strength and excellent durability, and a method for manufacturing the same, wherein the heat treatable steel contains, in wt %, C (0.22-0.42%), Si (0.05-0.3%), Mn (1.0-1.5%), Al (0.01-0.1%), P (0.01% or less (including 0), S (0.005% or less), Mo (0.05-0.3%), Ti (0.01-0.1%), Cr (0.05-0.5%), B (0.0005-0.005%), N (0.01% or less), the balance Fe, and other inevitable impurities, Mn and Si satisfying Relationship formula (1), below, Mo/p satisfying Relationship formula (2), below: [Relationship formula 1] Mn/Si≥5 [Relationship formula 2] Mo/P≥15.
1. A method for manufacturing a formed product having tensile-strength of 1500MPa or more and a frequency fatigue life of 5,000 cycles or more, where a number of cycles refers to a cycle number at which fracture occurs under a ± 0.5% strain application condition, the method comprising:
preparing heat treatable steel, the heat treatable steel comprising, by wt %, carbon (C): 0.22% to 0.42%, silicon (Si): 0.05% to 0.3%, manganese (Mn): 1.0% to 1.5%, aluminum (Al): 0.01% to 0.1%, phosphorus (P): 0.01% or less(including 0%), sulfur (S): 0.005% or less, molybdenum (Mo): 0.05% to 0.3%, titanium (Ti): 0.01% to 0.1%, chromium (Cr): 0.05% to 0.5%, boron (B): 0.0005% to 0.005%, nitrogen (N): 0.01% or less, and a balance of iron (Fe) and inevitable impurities, wherein Mn and Si in the heat treatable steel satisfy Formula 1, below, and Mo/P in the heat treatable steel satisfies Formula 2, below,
Mn/Si ≥5 [Formula 1]
Mo/P ≥15; [Formula 2]
forming the heat treatable steel to obtain a formed product;
wherein the forming of the heat treatable steel is performed by heating the heat treatable steel and then hot forming and cooling the heat treatable steel simultaneously, using a cooling die,
in the heating of the heat treatable steel before the hot forming of the heat treatable steel, the heat treatable steel is heated to a temperature of 850° C. to 950° C. and maintained at the temperature for 100 seconds to 1,000 seconds, and in the cooling of the heat treatable steel after the hot forming of the heat treatable steel, the heat treatable steel is cooled to a temperature of 200° C. or less at a cooling rate ranging from a critical cooling rate of martensite to 300° C./s; and
tempering the formed product, wherein the tempering of the formed product is performed by maintaining the formed product at a tempering temperature satisfying Formula 4, below, for 15 minutes to 60 minutes,
Tempering temperature(° C.)=Ttempering(° C.)±30° C.[where Ttempering (° C.)=111*[C.] −0.633] [Formula 4].
2. The method of claim 1 , wherein the heat treatable steel further comprises at least one or two selected from the group consisting of niobium (Nb): 0.01% to 0.07%, copper (Cu): 0.05% to 1.0%, and nickel (Ni): 0.05% to 1.0%.
3. The method of claim 1 , wherein the heat treatable steel comprises one selected from the group consisting of a hot-rolled steel sheet, a pickled and oiled steel sheet, and a cold-rolled steel sheet.
4. The method of claim 1 , wherein the heat treatable steel comprises a steel pipe.
5. The method of claim 1 , wherein a content of carbon in the heat treatable steel is adjusted 0.23% to 0.27% for 1500 MPa grade, 0.33% to 0.37% for 1800 MPa grade, and 0.38% to 0.42% for 2000 MPa grade.
6. The method of claim 1 , wherein the heat treatable steel further comprises niobium (Nb) in an amount of 0.01% to 0.07%.