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.

Claims

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.

Description

EXAMPLE

Example 1

[0103] Hot rolling was performed using a steel slab having a composition illustrated in Table 1. In Table 1, an element marked as * is only illustrated in ppm unit, and the remaining elements are illustrated in weight % (the same is applied to the remaining tables).

TABLE-US-00001 TABLE 1 Classification C Si Mn P* S* Al Ti Cr B* Mo Nb V Cu Ni N* Mo/P Inventive 0.35 0.15 1.3 71 27 0.029 0.029 0.16 20 0.14 45 19.7 Steel 1

[0104] During hot rolling, the steel slab having the composition, described above, was uniformized by heating the steel slab in a range of 120020 C. for 180 minutes. Then, after rough rolling, hot rolling was completed with a target of a range of 88020 C. subsequently. Then, coiling was performed at a temperature of 65015 C., and a hot-rolled steel sheet having a thickness of 3.0 mm was manufactured. The hot-rolled steel sheet was pickled, and heated at a temperature of 93010 C. for 6 minutes (360 seconds). Then, rapid cooling was performed to allow a temperature of the steel sheet to be equal to 30 C. or less by immersing the steel sheet in a cooling water tank, maintained at 20 C. to 30 C., at a cooling rate of 60 C./sec to 80 C./sec, higher than a martensite critical cooling rate. Then, tempering heat treatment was performed for 30 minutes at the temperature illustrated in Table 2, and a part was manufactured. In order to manufacture the part, forming at a high temperature or in a cold state before heating is included. The forming process does not have any particular effect on a change of mechanical properties of the part. Thus, it is common to test mechanical properties of a part, obtained by simulating a hot press forming or post-heat treatment process, with omitting forming process. Regarding the part, having been obtained, a tensile test and low cycle fatigue life were evaluated. The tensile test was conducted using a JIS5 specimen, and a low cycle fatigue test was conducted under the strain rate control conditions of R=1 and /2=0.5%, with a specimen in which a length of an parallel portion is 150.01 mm and a width of an parallel portion is 12.50.01 mm. The results of the test described above are shown in Table 2. In Table 2, YS indicates yield strength, TS indicates tensile strength, EL indicates elongation, U-EL indicates uniform elongation, and T-El indicates total elongation. In Table 2, for example, 1-2 refers to Second Example of Inventive Steel 1. Moreover, PO, indicating the type of product, means that a steel sheet, on which hot rolling and pickling were performed, was targeted.

TABLE-US-00002 TABLE 2 Physical Low Properties of Physical Properties YS Cycle Coiling Material Tempering of Part U-El Fatigue Thickness Temperature TS T-El Temperature YS TS U-El T-El (Mpa Life No Type (mm) ( C.) YS (Mpa) (Mpa) (%) ( C.) (Mpa) (Mpa) (%) (%) %) (Cycle) Note 1-1 PO 3.0 650 428 620 22 1250 1960 4.7 9 5875 4006 Comparative Example 1-2 PO 3.0 650 428 620 22 160 1360 1850 5.2 9.6 7022 6237 Inventive Example 1-3 PO 3.0 650 428 620 22 220 1460 1800 5.2 10.1 7592 6445 Inventive Example 1-4 PO 3.0 650 428 620 22 250 1470 1730 4.1 10.1 6027 5780 Inventive Example 1-5 PO 3.0 650 428 620 22 330 1370 1500 3.3 9 4521 3300 Comparative Example 1-6 PO 3.0 650 428 620 22 500 1040 1100 4.7 9 4888 3580 Comparative Example 1-7 PO 3.0 650 428 620 22 550 960 1050 6.2 12 5952 4950 Comparative Example

[0105] As shown in Table 2, when a tempering temperature increased after quenching, tensile strength was continuously decreased, while yield strength was increased immediately after the quenching, was maximum at around 250 C. of a tempering temperature, and was then continuously decreased as in the same manner as the tensile strength. The uniform elongation has a maximum value at around 220 C., was decreased rapidly, has a minimum value at 330 C., and was then gradually increased again. As the yield strengthuniform elongation balance is compared to the change in tensile properties, the balance tends to decrease rapidly from 250 C. as a boundary. This result is almost identical to a change in a low cycle fatigue life. On the contrary, as the fatigue lives at 150 C. tempering and quenching states are compared, the fatigue life in the case of 150 C. tempering heat treatment is better as compared to that in the case of the as-quenched state.

[0106] Through the above example, when tempering temperature exceeded 250 C. after quenching, the uniform elongation and the total elongation were decreased, and the yield strengthuniform elongation value was also decreased, which was consistent with a low cycle fatigue life. Thus, when tempering heat treatment after quenching is performed in a temperature range of 150 C. to 250 C., as compared to conventional conditions of tempering heat treatment, such as 500 C. to 550 C., more excellent fatigue properties may be obtained.

[0107] Table 3 shows the results of examining a structure of a part obtained by each manufacturing method.

TABLE-US-00003 TABLE 3 Microstructure (area %) Tempered Retained Classification Martensite Ferrite austenite Bainite 1-1 93.1 1.0 2.7 3.2 1-2 95.3 2.6 2.1 0 1-3 92.0 2.4 2.0 3.6 1-4 96.0 0 2.0 2.0 1-5 93.4 1.5 1.9 3.2 1-6 96.0 1.0 2.2 0.8 1-7 94.7 3.3 0 2.0

[0108] As described previously, microstructures of Inventive Example and Comparative Example satisfy the conditions according to the present disclosure. However, as a result of examining the precipitate formed inside, in the case of Inventive Example, manufactured in conditions according to the present disclosure, 90% or more of a precipitate in a number ratio is present as epsilon carbide. However, in the case of Comparative Example, it was confirmed that most of the formed precipitate were cementite and did not satisfy conditions of precipitate according to the present disclosure. The difference in the precipitate affects significant difference in yield strength, tensile strength, and elongation behavior. As a result, the difference in the precipitate is determined to also affect a low cycle fatigue life.

Example 2

[0109] Hot rolling was performed using a steel slab having a composition of Inventive Steel 2 and 3 illustrated in Table 4, and pickling was performed. Here, Inventive Steel 2 corresponds to steel having tensile strength of 1500 MPa grade after tempering, while Inventive Steel 3 corresponds to steel having tensile strength of 2000 MPa grad after tempering.

TABLE-US-00004 TABLE 4 Classification C Si Mn P* S* Al Ti Cr B* Mo Nb V Cu Ni N* Mo/P Inventive 0.25 0.15 1.25 58 12 0.03 0.033 0.4 22 0.1 50 17.2 Steel 2 Inventive 0.42 0.15 1.3 67 11 0.035 0.04 0.1 10 0.11 42 16.4 Steel 3

[0110] Regarding Inventive Steel 2, during hot rolling, a slab having the composition, described above, was uniformized by heating the slab in a range of 120020 C. for 180 minutes. Then, after rough rolling, hot rolling was finished with a target of a range of 89020 C. successively. Then, coiling was performed at a temperature illustrated in Table 5, and a hot-rolled steel sheet having a thickness of 3.0 mm was manufactured. Moreover, pickling was performed to obtain a final hot-rolled steel sheet. Regarding Inventive Steel 3, after hot rolling and pickling in the same process as Inventive Steel 2, cold rolling was performed. Then, annealing was performed at a temperature of 80010 C., and overaging was performed at a temperature of 43010 C. in an overaging zone, to obtain a cold-rolled steel sheet. In the case of Inventive Steel 2, the hot-rolled steel sheet, heating was performed at 930 C. for 7 minutes. In the case of Inventive Steel 3, the cold-rolled steel sheet, heating was performed at 880 C. for 6 minutes. Then, rapid cooling was performed to 20 C. to 30 C. at a cooling rate of 50 C./sec, higher than a martensite critical cooling rate. Then, tempering heat treatment was performed at the temperature illustrated in Table 2 for 30 minutes. A tensile test and a low cycle fatigue test were conducted on the steel sheet, having been heat treated. The tensile test was conducted using a JIS5 specimen, while a low cycle fatigue test was conducted under the strain rate control conditions of R=1 and /2=0.5%, with a specimen in which a length of an parallel portion is 150.01 mm and a width of an parallel portion is 12.50.01 mm. The results of the test described above are shown in Table 5. In Table 5, YS indicates yield strength, TS indicates tensile strength, EL indicates elongation, U-EL indicates uniform elongation, and T-El indicates total elongation. Moreover, in Table 5, PO, indicating the type of product, means that a steel sheet, on which hot rolling and pickling were performed, was targeted, and CR means a steel sheet, on which cold rolling and annealing were performed. In Table 5, for example, 2-2 refers to Second Example of Inventive Steel 2.

[0111] As shown in Table 5, under conventional conditions of a tempering temperature, such as 500 C., a range of yield strength was 960 Mpa to 1180 Mpa, tensile strength was 1030 Mpa to 1290 Mpa, and a yield ratio was 0.91. When a tempering temperature is 250 C., it was confirmed that a range of yield strength was 1270 Mpa to 1630 Mpa, a range of tensile strength was 1605 Mpa to 1960 Mpa, and a yield ratio was 0.79 to 0.83. In other words, there is a significant difference in yield and tensile strength in a quenching state depending on the content of carbon. However, when a tempering temperature rises, the difference is significantly reduced. In this case, even when the content of carbon is changed, the difference in yield and tensile strength is not significant. Moreover, if tempering temperatures are 160 C. and 220 C., yield ratios are about 0.73 and 0.81, respectively, which were evaluated to be controlled in within the range according to the present disclosure.

[0112] When comparing materials of Inventive Steel 2 after tempering, the yield strengthuniform elongation value and low cycle fatigue life were significantly changed at 250 C. as a boundary. As compared with the case in which heat treating is performed at 330 C. (2-3) and 550 C. (2-4), in which a tempering temperature is 250 C. or more, in the case of 2-1 and 2-2, to which low temperature tempering heat treatment is applied, the yield strengthuniform elongation value was more excellent, and the low cycle fatigue life was also more excellent.

[0113] Regarding Inventive Steel 3, similarly, the yield strengthuniform elongation value and low cycle fatigue life were significantly changed at 250 C. as a boundary. As compared with the case in which heat treating is performed at 330 C. (3-3) and 550 C. (3-4), in which a tempering temperature is 250 C. or more, in the case of 3-1 and 3-2, to which low temperature tempering heat treatment is applied, the yield strengthuniform elongation value was more excellent, and the low cycle fatigue life was also more excellent.

[0114] Meanwhile, in a tempering temperature range of 200 C. to 250 C., when comparing the fatigue life of Inventive Steel 2 with the fatigue life of Inventive Steel 3, as the content of carbon is increased, yield strength and tensile strength were high. Moreover, in such a section, the yield strengthuniform elongation value was also increased. The result thereof is consistent with a result in which the low cycle fatigue properties are improved according to an increase in strength.

TABLE-US-00005 TABLE 5 Physical Low Properties of Physical Properties YS Cycle Coiling Material Tempering of Part U-El Fatigue Thickness Temperature TS T-El Temperature YS TS U-El T-El (Mpa Life No Type (mm) ( C.) YS (Mpa) (Mpa) (%) ( C.) (Mpa) (Mpa) (%) (%) %) (Cycle) Note 2-1 PO 3.0 680 410 570 27 220 1190 1650 4.9 11.5 5831 6200 Inventive Example 2-2 PO 3.0 250 1270 1605 4.7 11.6 5969 6320 Inventive Example 2-3 PO 3.0 330 1295 1480 3.5 9.5 4533 3710 Comparative Example 2-4 PO 3.0 500 960 1030 4.3 12.7 4128 5100 Comparative Example 3-1 CR 2.0 680 510 740 18 200 1690 2100 5.1 7.5 8619 6990 Inventive Example 3-2 CR 2.0 250 1630 1960 4.3 6.6 7009 6167 Inventive Example 3-3 CR 2.0 330 1560 1805 3.66 6 5710 3906 Comparative Example 3-4 CR 2.0 500 1180 1290 4.7 8 5546 5308 Comparative Example

[0115] Table 6 shows the result of examining a structure of a part obtained by each manufacturing method.

TABLE-US-00006 TABLE 6 Microstructure Tempered Retained Classification Martensite Ferrite austenite Bainite 2-1 91.0 3.8 1.6 3.6 2-2 92 3.4 1.1 3.5 2-3 92.5 3.3 1.3 2.9 2-4 90.5 3.0 1.7 4.8 3-1 94.1 0 3.3 2.6 3-2 95 1.0 3 1 3-3 94.2 0 3.9 1.9 3-4 93.6 1.0 3.6 1.8

[0116] As described previously, microstructures of Inventive Example and Comparative Example satisfy the conditions according to the present disclosure. However, as a result of analyzing the precipitate formed inside, in the case of Inventive Example, manufactured in conditions according to the present disclosure, 90% or more of a precipitate in a number ratio is present as epsilon carbide. However, in the case of Comparative Example, it was confirmed that most of the formed precipitate were cementite and did not satisfy conditions of precipitate according to the present disclosure. The difference in the precipitate affects significant difference in yield strength, tensile strength, and elongation behavior. As a result, the difference in the precipitate is determined to also affect a low cycle fatigue life.

Example 3

[0117] Hot rolling was performed using a steel slab having a composition illustrated in Table 7, and pickling was performed.

TABLE-US-00007 TABLE 7 Classification C Si Mn P* S* Al Ti Cr B* Mo Nb V Cu Ni N* Mo/P Inventive 0.35 0.15 1.3 71 27 0.029 0.029 0.16 20 0.14 45 19.7 Steel 4 Inventive 0.25 0.15 1.25 58 12 0.03 0.033 0.4 22 0.1 50 17.2 Steel 5 Inventive 0.42 0.15 1.3 67 11 0.035 0.04 0.1 10 0.11 42 16.4 Steel 6 Comparative 0.36 0.11 2.1 100 300 0.031 0.034 0.15 15 0.1 49 12.5 Steel 1 Comparative 0.35 0.1 1.3 140 29 0.034 0.032 0.16 20 0.1 43 7.1 Steel 2 Comparative 0.36 0.15 1.27 160 14 0.029 0.027 0.17 17 0.11 39 6.9 Steel 3 Inventive 0.34 0.2 1.8 69 27 0.03 0.03 0.11 16 0.15 38 21.7 Steel 7 Comparative 0.36 0.14 1.2 180 28 0.027 0.027 0.18 13 0.15 55 8.3 Steel 4 Inventive 0.37 0.11 1.3 96 22 0.029 0.029 0.2 15 0.15 60 15.6 Steel 8 Comparative 0.36 0.14 1.3 70 33 0.022 0.029 0.15 18 0.38 44 54.3 Steel 5 Inventive 0.35 0.15 1.3 70 27 0.031 0.025 0.17 19 0.15 0.05 42 21.4 Steel 9 Inventive 0.34 0.2 1.2 80 14 0.03 0.031 0.15 15 0.13 0.2 42 16.3 Steel 10 Inventive 0.35 0.2 1.4 71 25 0.025 0.023 0.17 19 0.15 0.2 38 21.1 Steel 11 Inventive 0.35 0.21 1.3 66 21 0.023 0.03 0.18 18 0.19 0.5 0.3 55 28.8 Steel 12 Inventive 0.23 0.18 1.25 62 10 0.026 0.031 0.2 17 0.1 45 16.1 Steel 13 Comparative 0.22 0.25 0.9 65 32 0.033 0.026 0.15 17 0.15 40 23.1 Stee 16 Comparative 0.2 0.11 1.3 80 15 0.031 0.029 0.4 26 0.21 57 26.3 Stee 17 Inventive 0.4 0.16 1.3 78 9 0.027 0.029 0.15 17 0.18 38 23.1 Steel 14 Inventive 0.46 0.2 1.2 65 10 0.025 0.02 0.1 13 0.1 43 15.4 Steel 15 Inventive 0.49 0.15 1.0 70 11 0.029 0.030 0.18 19 0.15 44 21.4 Steel 16 Comparative 0.53 0.20 1.3 82 15 0.031 0.027 0.15 15 0.20 52 14.6 Steel 8

[0118] During hot rolling, a slab having the composition, described above, was uniformized by heating the slab in a range of 120030 C. for 180 minutes. Then, after rough rolling, hot rolling was completed with a target of a range of 87020 C. subsequently. Then, coiling was performed at a temperature of 620 C. to 690 C., and a hot-rolled steel sheet having a thickness of 3.0 mm was manufactured. The hot-rolled steel sheet was pickled, to obtain a final hot-rolled steel sheet. In this case, a final thickness was 3.0 mm. In the case of CR material of Comparative Steel 2 in Table 2, 50% cold rolling was performed on the hot-rolled steel sheet, and a thickness of 1.5 mm is obtained. Then, annealing was performed at a temperature of 79010 C., and overaging was performed at 43010 C., to obtain a final cold-rolled steel sheet. The hot-rolled steel sheet or cold-rolled steel sheet, having been obtained, was heated in a temperature range of 880 C. to 960 C., and was then maintained for 5 minutes to 7 minutes. Then, rapid cooling was performed to 30 C. or less at a cooling rate of 60 C./sec to 80 C./sec, higher than a martensite critical cooling rate. After the part, having been rapid cooled, was heat-treated for one hour at a tempering temperature illustrated in Table 8, the tensile properties and fatigue life were evaluated and illustrated in Table 8. For a tensile test, a tensile test specimen was manufactured according to ASTM370. For a fatigue test, an hourglass type low cycle fatigue test piece was manufactured. Moreover, in the same manner as Example 1 or Example 2, the tensile test and fatigue life were evaluated.

TABLE-US-00008 TABLE 8 Physical Low Properties of Physical Properties Cycle Coiling Material Tempering of Part YS Fatigue Thickness Temperature TS T-El Temperature YS TS U-El T-El U-El Life Classification Type (mm) ( C.) YS (Mpa) (Mpa) (%) ( C.) (Mpa) (Mpa) (%) (%) (MPa %) (Cycle) Inventive PO 3.0 650 428 620 22 220 1460 1800 5.2 10.1 7592 6445 Steel 4 Inventive PO 3.0 680 410 570 27 250 1270 1605 4.7 11.6 5969 6320 Steel 5 Inventive PO 3.0 620 510 740 18 200 1610 2100 5.1 7.5 8211 6990 Steel 6 Comparative PO 3.0 620 601 840 16 Steel 1 Comparative CR 1.5 650 410 619 23 220 1450 1750 4.9 9.8 7105 5160 Steel 2 Comparative PO 3.0 650 440 638 23 220 1444 1804 4.8 9.7 6931 5007 Steel 3 Inventive PO 3.0 620 510 700 18 220 1510 1840 4.6 9.9 6946 6400 Steel 7 Comparative PO 3.0 620 465 650 19 220 1500 1850 4.5 8.6 6615 5060 Steel 4 Inventive PO 3.0 620 470 665 19 220 1470 1838 4.9 10 7203 6390 Steel 8 Comparative PO 3.0 620 550 810 17 Steel 5 Inventive PO 3.0 600 477 658 20 220 1490 1820 5.2 11 7748 6910 Steel 9 Inventive PO 3.0 620 480 660 21 220 1480 1810 5.1 11 7548 6670 Steel 10 Inventive PO 3.0 650 454 655 23 220 1445 1840 4.9 9.5 7081 6700 Steel 11 Inventive PO 3.0 650 448 637 24 220 1455 1820 5.1 9.9 7421 6819 Steel 12 Inventive PO 3.0 650 399 580 26 220 1290 1620 5 10 6450 6300 Steel 13 Comparative PO 3.0 650 390 550 27 220 1210 1490 5.3 10.1 6413 6200 Steel 6 Comparative PO 3.0 650 387 520 28 220 1168 1450 5.2 11.3 6074 6150 Steel 7 Inventive PO 3.0 650 472 688 20 200 1550 2070 4.9 8.8 7595 7006 Steel 14 Inventive PO 3.0 620 650 920 13 Steel 15 Inventive PO 3.0 690 565 781 18 210 1645 2212 4.0 9.0 8848 7110 Steel 16 Comparative PO 3.0 690 641 851 15 Steel 8

[0119] As shown in Table 8, a level of strength after tempering mainly depends on an amount of carbon, and tensile strength in a range of 1444 Mpa to 2212 Mpa is obtained. In the case of Comparative Steel 7, the content of C was low, so tempering strength of about 1450 Mpa was obtained. Thus, a level of strength was not sufficient. On the other hand, in the case of Inventive Steel 15, a composition according to the present disclosure was satisfied, but C was 0.46% and was a bit high. In this case, a coiling temperature is slightly low. Thus, it is difficult to form a steel pipe while material strength exceeds 800 MPa. In other words, tempering strength was 2100 Mpa and was excellent, but strength of a material state was a level of 920 MPa and was significant and mechanical property deviation in a width direction was also significantly high. In this case, Inventive Steel 15 is not suitable for quenching after cold forming or blanking for manufacturing a part for vehicle. As described above, the case, in which material strength exceeds a grade of 800 Mpa, was also confirmed in Comparative Steel 1, in which the content of Mn is significant, and Comparative Steel 5, in which Mo is contained in an amount of 0.38%. Moreover, an upper limit of Mn and Mo, hardenability elements, is determined based on the Example described above. However, if the content of Mn is significantly small, like Comparative Steel 6, strength after tempering heat treatment may be reduced to about 1490 MPa. In this regard, because formability should be secured during quenching after cold forming or manufacturing a steel pipe, but it is difficult to perform forming due to a reduction in elongation, when tensile strength exceeds 800 Mpa, in general. However, in the case of Comparative Steel 8, the content of C satisfies the range defined by the claim according to the present disclosure, but is slightly high. In this case, a method, in which a coiling temperature is controlled to be slightly high in order to reduce tensile strength of a material, may be used for forming. To confirm this, in the case of Inventive Steel 16, which has a composition similar to that of Comparative Steel 8, and in which the content of C is 0.49% and is a bit high, a hot-rolled steel sheet is manufactured after coiling at temperature of 690 C. and pickling, tensile strength of the steel sheet, having been obtained, was analyzed. As a result, the tensile strength was 781 MPa, a value suitable for cold forming. However, in the case of Comparative Steel 8, in which the content of C is further increased and is 0.53%, even though a hot-rolled steel sheet is manufactured by performing coiling at 690 C. in a similar manner as Inventive Steel 16, tensile strength of the steel sheet is 851 MPa, unsuitable for forming. Thus, it is confirmed that the content of C suitable in the present disclosure is 0.50% or less.

[0120] Meanwhile, in the present disclosure, P segregation, concentrated in a grain boundary during solution heat treatment of austenite, may reduce the fatigue life and may also reduce impact energy, so it may be problematic. Thus, it is required to control the content of P in steel to be low. Moreover, it is also effective to add Mo to allow a degree of P concentration in a grain boundary to be lowered. Therefore, it is required to control a ratio of Mo/P. In the case of Comparative Steel 3 and Comparative Steel 4, the content of P is high, so a ratio of Mo/P is less than 10. Moreover, in the case of Comparative Steel 2, an amount of Mo added is also low, so a ratio of Mo/P is also less than 10. When three cases described above are compared with Inventive Steel 4, Inventive Steel 7, Inventive Steel 8, and Inventive Steel 9 to 14, having the similar content of carbon, it was confirmed that the yield strengthuniform elongation balance is low and the fatigue life has also a low degree.

[0121] In the case of Comparative Steel 4, Inventive Steel 8, Comparative Steel 5, and Inventive Steel 11, the tensile properties and fatigue life are evaluated with respect to compositions, to which Nb, V, Cu, and CuNi are added, respectively. In this case, when low temperature tempering is performed, it is confirmed that good fatigue properties may be obtained.

[0122] While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.