STEEL AND METHOD FOR MANUFACTURING STEEL

Abstract

The present disclosure relates to a steel and a method for manufacturing the steel. Specifically, the present disclosure is characterized by providing a method for providing optimal alloy components that can reduce carburizing thermal deformation by reducing contents of Cr and Mo elements compared to the existing ones, and improving the physical property with fatigue resistance of a carburized steel by adjusting a rolling thermal treatment temperature, a carburizing thermal treatment condition, and the like to increase a fraction of a fine MX precipitate and improve a precipitation strengthening effect.

Claims

1. A steel comprising: C: 0.17 to 0.23 wt %, Si: 0.6 to 0.8 wt %, Mn: 0.35 to 0.65 wt %, Cr: 1.35 to 1.65 wt %, Ni: 0.25 to 0.45 wt %, Mo: 0.15 to 0.25 wt %, Nb: 0.015 to 0.035 wt %, and V: 0.02 to 0.04 wt %; and residues comprising iron and other inevitably mixed impurities, wherein the steel satisfies the following formula:
6.7≤5[C]+2[Si]+2[Mn]+2[Cr]+2[Ni]+5[Mo]≤9.5 wherein [C], [Si], [Mn], [Cr], [Ni], and [Mo] are amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively, and wherein a fraction of a precipitate satisfies 0.025% to 0.045%.

2. The steel of claim 1, wherein a contact fatigue L10 life of the steel satisfies 2,980,000 cycles or more and a bending fatigue life satisfies 31,000 cycles or more.

3. A method for manufacturing a steel, the method comprising: preparing a carburized steel comprising C, Si, Mn, Cr, Ni, Mo, Nb, and V; rolling the carburized steel at a temperature of 1180 to 1460° C.; carburizing and hardening the rolled carburized steel at a temperature of 850 to 920° C. for 150 to 300 minutes; and precipitating an MX precipitate comprising V and Nb at a fraction of 0.025 to 0.045%.

4. The method of claim 3, wherein the carburized steel comprises: C: 0.15 to 0.25 wt %. Si: 0.5 to 0.9 wt %, Mn: 0.3 to 0.7 wt %, Cr: 1.3 to 1.7 wt %, Ni: 0.2 to 0.5 wt %, Mo: 0.1 to 0.4 wt %, Nb: 0.01 to 0.04 wt %, and V: 0.03 to 0.05 wt %; and residues including iron and other inevitably mixed impurities, and wherein the carburized steel satisfies the following formula:
6.7≤5[C]+2[Si]+2[Mn]+2[Cr]+2[Ni]+5[Mo]≤9.5 wherein [C], [Si], [Mn], [Cr], [Ni], and [Mo] are amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other features of the present disclosure are described in detail with reference to certain examples thereof illustrated in the accompanying drawings which are given herein below by way of illustration only, and thus are not limitative of the present disclosure, and wherein:

[0020] FIG. 1 shows measuring the amount of carburizing thermal deformation according to the present disclosure.

[0021] It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the disclosure. The specific design features of the present disclosure as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in section by the particular intended application and use environment.

[0022] In the figures, reference numbers refer to the same or equivalent sections of the present disclosure throughout the several figures of the drawing.

DETAILED DESCRIPTION

[0023] The above objects, other objects, features, and advantages of the present disclosure should be readily understood through the following embodiments related to the accompanying drawings. However, the present disclosure is not limited to the embodiments described herein and can also be specified in other forms. Rather, the embodiments described herein are provided so that the disclosed contents can be thorough and complete and the spirit of the present disclosure can be sufficiently conveyed to those having ordinary skill in the art.

[0024] In the present specification, it should be understood that the term “include” or “have” is intended to specify the presence of features, numbers, steps, operations, components, parts or combinations thereof described in the specification, and does not preclude the possibility of the presence or addition of one or more other features or numbers, steps, operations, components, parts or combinations thereof in advance. In addition, if an element such as a layer, a membrane, a region, a plate, and the like is said to be “on” another portion, this includes not only a case where it is “directly above” another portion, but also a case where it has other parts interposed therebetween. Conversely, if an element such as a layer, a membrane, a region, a plate, and the like is said to be “under” another portion, this includes not only a case where it is “directly under” another portion, but also a case where it has other portions interposed therebetween.

[0025] Unless otherwise specified, since all numbers, values, and/or expressions representing components, reaction conditions, polymer compositions, and an amount of mixtures used in the present specification are approximations reflecting various uncertainties of measurements that essentially occur in obtaining these values from the others, it should be understood that all cases are expressed by the term “about”. In addition, if the numerical range is disclosed in the present disclosure, this range is continuous, and includes all values from the minimum value to the maximum value in this range unless indicated otherwise. Furthermore, if this range refers to an integer, all integers including the minimum value to the maximum value are included unless otherwise indicated.

[0026] The present disclosure relates to a method for manufacturing a steel and a carburized steel manufactured by the manufacturing method.

[0027] Method for Manufacturing the Steel

[0028] The present disclosure is characterized by providing a method that can adjust a rolling thermal treatment temperature, a carburizing thermal treatment condition, and the like to increase a fraction of a fine MX precipitate and improve a precipitation strengthening effect, thereby improving the physical property with fatigue resistance of a carburized steel.

[0029] Specifically, the method for manufacturing the steel according to the present disclosure includes preparing a carburized steel including C, Si, Mn, Cr, Ni, Mo, Nb, and V, rolling the carburized steel, and carburizing and hardening the rolled carburized steel. The method may also include precipitating an MX precipitate.

[0030] Preparing the Carburized Steel

[0031] This is a step of preparing the carburized steel including C, Si, Mn, Cr, Ni, Mo, Nb, and V.

[0032] The carburized steel is, in one example, characterized by including C: 0.17 to 0.23 wt %, Si: 0.6 to 0.8 wt %, Mn: 0.35 to 0.65 wt %, Cr: 1.35 to 1.65 wt %, Ni: 0.25 to 0.45 wt %, Mo: 0.15 to 0.25 wt %, Nb: 0.015 to 0.035 wt %, and V: 0.02 to 0.04 wt % based on the entire composition of 100 wt %. The carburized steel is also characterized in that the residues include iron and other inevitably mixed impurities.

[0033] Hereinafter, the reason of limiting the composition is described in detail.

[0034] Carbon (C) is an element essential to form an MX precipitate after carburizing thermal treatment and to improve hardenability to improve the strength. Therefore, the content of C is added in an amount of 0.17 wt % or more to increase the strength for improving the physical property of fatigue resistance after the carburizing thermal treatment. However, when the content of C is added in an amount of 0.23 wt % or more, a toughness is reduced. Therefore, the content of C is limited to a range of 0.17 wt % to 0.23 wt % to increase the strength of the carburizing thermal treatment material and secure the toughness.

[0035] Silicon (Si) is an element for strengthening a base tissue and increasing a tempering softening resistance through solid solution strengthening. Therefore, for the purpose of improving the physical property of the fatigue resistance and improving the softening resistance when a gear is operated, the content of Si is added in an amount of 0.65 wt % or more. However, when the content of Si is excessively added, there occurs a problem of interfering the diffusion of carbon and reducing hardness as an oxide is formed on a carburized surface during the atmospheric carburizing thermal treatment. Therefore, the content of Si is limited to a range of 0.6 to 0.8 wt % to improve the physical property with the fatigue resistance and to prevent the oxide from being formed on the surface during the atmospheric carburizing thermal treatment.

[0036] Manganese (Mn) is an element being useful for deoxidation in the process of manufacturing the steel and strengthening the base tissue by strengthening solid solution. The content of Mn needs to be added in an amount of 0.35 wt % or more. However, when Mn is excessively added, there occurs a problem of reducing processability as the hardness of the base increases. Therefore, the content of Mn is limited to a range of 0.35 to 0.65 wt % to strengthen the base tissue and prevent the problem of reducing processability.

[0037] Chromium (Cr) is an element for strengthening the base tissue according to the increase in hardenability upon quenching after the carburizing thermal treatment. The content of Cr is added in an amount of 1.35 wt % or more to improve the physical property with the fatigue resistance. However, as hardenability and thus thermal deformation increase when the content of Cr is excessively added, the content of Cr is limited to a range of 1.35 to 1.65 wt % to secure vehicle quietness and assembly stability.

[0038] Nickel (Ni) is an element for improving toughness of the base tissue of the steel and the content of Ni is added in an amount of 0.25 wt % or more to improve the physical property with the fatigue resistance. However, since the excessive addition of the Ni element increases a manufacturing cost, the content of Ni is limited to a range of 0.25 to 0.45 wt %.

[0039] Molybdenum (Mo) is an element for increasing hardenability similarly to the Cr element and is added in an amount of 0.15 wt % or more to secure the physical property with the fatigue resistance after the carburizing thermal treatment. However, when the Mo element is excessively added in an amount of a certain range or more, the effect of increasing the strength is insignificant. As the thermal deformation due to the increase in hardenability is increased, the content of Mo is limited to a range of 0.15 to 0.25 wt %.

[0040] Vanadium (V) is an element for forming a composite MX precipitate like the Nb element during the carburizing thermal treatment. The fine MX precipitate prevents precipitation strengthening and grain boundary coarsening of steel to improve the physical property with the fatigue resistance and make hardenability uniform, thereby preventing the thermal deformation from unevenly occurring. However, when the V element is excessively added in an amount of a certain range or more, the effect of increasing the precipitation strengthening is insignificant and therefore is not effective. Therefore, the content of V is limited to a range of 0.02 to 0.04 wt %.

[0041] Niobium (Nb) is an element of forming a composite MX precipitate like the V element and is added in an amount of 0.015 wt % or more to improve the physical property with the fatigue resistance and prevent the uneven thermal deformation. However, the increase in the Nb element increases the rolling solid solution temperature and the non-solid solution Nb element forms a coarse MX precipitate and therefore, the effect of improving the physical property with the fatigue resistance is insignificant. Therefore, the content of Nb is limited to a range of 0.015 to 0.035 wt % to form a fine MX precipitate by dissolving the Nb element as much as possible while rolling and thermally treating the raw material to form the fine MX precipitate.

[0042] Rolling Step

[0043] This is a step of thermally treating and rolling the carburized steel.

[0044] The present disclosure is characterized by dissolving Nb and V included in the carburized steel in the rolling step as much as possible to form a fine MX precipitate in the carburizing and hardening step. In other words, by dissolving the Nb and V elements in the rolling step, it is possible to increase the number of fine MX precipitates in the carburizing and hardening step.

[0045] The rolling may be performed at a temperature of 1180 to 1460° C.

[0046] Carburizing and Hardening Step

[0047] This is a step of carburizing and hardening the rolled carburized steel by carburizing and thermally treating it. More specifically, this is a step of manufacturing the steel by carburizing and thermally treating the thermally treated carburized steel in the rolling step.

[0048] The carburizing and thermally treating gives a diffusion temperature and a time in order for the solid solution element to form the fine MX precipitate as much as possible.

[0049] The carburizing and thermally treating can be performed at a temperature of 850 to 920° C., and the carburizing and thermally treating can be performed for 150 to 300 minutes.

[0050] After the carburizing and thermally treating, by maximizing the fraction of the fine MX precipitate, it is possible to improve the physical property with the fatigue resistance of the steel as the effect of strengthening the precipitation.

[0051] Steel

[0052] The present disclosure is characterized by providing the steel including the optimal alloy components capable of reducing the carburizing thermal deformation by reducing the contents of the Cr and Mo elements compared to the existing ones.

[0053] The steel manufactured by the method for manufacturing the steel according to the present disclosure is, in one example, characterized by including C: 0.17 to 0.23 wt %, Si: 0.6 to 0.8 wt %, Mn: 0.35 to 0.65 wt %, Cr: 1.35 to 1.65 wt %, Ni: 0.25 to 0.45 wt %, Mo: 0.15 to 0.25 wt %, Nb: 0.015 to 0.035 wt %, and V: 0.02 to 0.04 wt % based on the entire composition of 100 wt %. The steel is also characterized in that the residues include iron and other inevitably mixed impurities.

[0054] The composition included in the steel satisfies Formula 1 below,

6.7≤5[C]+2[Si]+2[Mn]+2[Cr]+2[Ni]+5[Mo]≤9.5  Formula 1

[0055] (where [C], [Si], [Mn], [Cr], [Ni], and [Mo] mean amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively).

[0056] When the above value is smaller than 6.7, the life can be reduced by the fatigue resistance as the quenchability is reduced after the carburizing thermal treatment. When the above value is larger than 9.5, there can be a concern about the increase in a part noise and an assembly quality problem as the thermal deformation is increased after the carburizing thermal treatment.

[0057] Hereinafter, the present disclosure is described in more detail through specific examples. However, these examples are for illustrating the present disclosure, and the scope of the present disclosure is not limited thereto.

Manufacturing Example

[0058] Cylindrical test pieces [10 mm (circle diameter)×100 mm (height)] were manufactured to compare the amounts of the thermal deformation of the carburized hardened material performed under the carburizing thermal treatment condition after applying the rolling thermal treatment temperature conditions. The rolling thermal treatment was performed at 1200° C., and the carburizing thermal treatment performed an oil quenching after maintaining 920° C. for 200 minutes in the atmospheric carburizing thermal treatment furnace.

[0059] Measuring Method (Amount of the Thermal Deformation)

[0060] As shown in FIG. 1, the amount of the thermal deformation was measured by fixing the test piece to the jig before and after the carburizing thermal treatment to measure the out-of-roundness of the edge end thereof using a micrometer.

Examples 1 to 10

[0061] Test pieces of Examples 1 to 10 having compositions in Table 1 below were manufactured in the method of the manufacturing example.

TABLE-US-00001 TABLE 1 C Si Mn Cr Ni Mo Nb V Example 1 0.17 0.60 0.35 1.35 0.25 0.15 0.015 0.020 Example 2 0.23 0.80 0.65 1.65 0.45 0.25 0.035 0.040 Example 3 0.20 0.70 0.50 1.50 0.35 0.20 0.025 0.030 Example 4 0.17 0.80 0.65 1.35 0.25 0.20 0.030 0.025 Example 5 0.19 0.75 0.40 1.40 0.35 0.20 0.035 0.025 Example 6 0.18 0.60 0.65 1.35 0.45 0.15 0.030 0.020 Example 7 0.18 0.80 0.35 1.65 0.25 0.25 0.030 0.030 Example 8 0.19 0.76 0.42 1.49 0.38 0.22 0.030 0.040 Example 9 0.17 0.65 0.45 1.38 0.41 0.24 0.030 0.025 Example 10 0.21 0.78 0.63 1.62 0.40 0.23 0.030 0.025

Comparative Examples 1 to 18

[0062] Test pieces of Comparative examples 1 to 18 having composition in Tables 2 and 3 below were manufactured in the method of the manufacturing example.

TABLE-US-00002 TABLE 2 C Si Mn Cr Ni Mo Nb V Comparative 0.17 0.06 0.70 1.30 0.10 0.55 0.025 0 example 1 Comparative 0.20 0.63 0.57 2.4 0.10 0.35 0.025 0 example 2 Comparative 0.20 0.70 0.50 2.00 0.35 0.55 0.025 0 example 3 Comparative 0.20 0.70 0.50 1.50 0.35 0.20 0 0 example 4 Comparative 0.24 0.80 0.65 1.65 0.45 0.25 0.025 0 example 5 Comparative 0.16 0.60 0.35 1.35 0.25 0.15 0.025 0 example 6 Comparative 0.23 0.85 0.65 1.65 0.45 0.25 0.025 0 example 7 Comparative 0.17 0.55 0.35 1.35 0.25 0.15 0.025 0 example 8 Comparative 0.23 0.80 0.70 1.65 0.45 0.25 0.025 0 example 9 Comparative 0.17 0.60 0.30 1.35 0.25 0.15 0.025 0 example 10

TABLE-US-00003 TABLE 3 C Si Mn Cr Ni Mo Nb V Comparative 0.23 0.80 0.65 1.70 0.45 0.25 0.025 0 example 11 Comparative 0.17 0.60 0.35 1.30 0.25 0.15 0.025 0 example 12 Comparative 0.23 0.80 0.65 1.65 0.50 0.25 0.025 0 example 13 Comparative 0.17 0.60 0.35 1.35 0.20 0.15 0.025 0 example 14 Comparative 0.23 0.80 0.65 1.65 0.45 0.30 0.025 0 example 15 Comparative 0.17 0.60 0.35 1.35 0.25 0.10 0.025 0 example 16 Comparative 0.20 0.70 0.50 1.50 0.20 0.20 0.025 0.010 example 17 Comparative 0.20 0.70 0.50 1.50 0.35 0.20 0.010 0.030 example 18

Comparative Example 19

[0063] A test piece was manufactured in the same method as that of Example 3 except that the rolling thermal treatment was performed at 1100° C.

Comparative Example 20

[0064] A test piece was manufactured in the same method as that of Example 3 except that the rolling thermal treatment was performed at 1150° C.

Comparative Example 21

[0065] A test piece was manufactured in the same method as that of Example 3 except that the carburizing thermal treatment was performed at 830° C.

Experimental Example

[0066] The test pieces manufactured in Examples and Comparative examples were inspected for the amount of the thermal deformation, the fraction of the precipitate, the contact fatigue L10 life, the bending fatigue, and whether to satisfy Formula 1. The results thereof are expressed in Table 4 below.

TABLE-US-00004 TABLE 4 Amount of Fraction Contact Whether thermal of fatigue Bending to deformation precipitate L10 life fatigue satisfy (um) (%) (cycles) (cycles) Formula 1 Example 1 13 0.025 2,980,471 31,071 ◯ Example 2 107 0.041 3,104,155 40,050 ◯ Example 3 60 0.036 3,097,460 35,888 ◯ Example 4 59 0.034 3,041,052 35,538 ◯ Example 5 49 0.033 3,019,905 34,502 ◯ Example 6 42 0.030 3,008,106 33,501 ◯ Example 7 71 0.037 3,101,077 37,580 ◯ Example 8 61 0.043 3,110,920 36,108 ◯ Example 9 47 0.034 3,061,120 34,390 ◯ Example 10 94 0.032 3,010,012 39,401 ◯ Comparative 49 0.020 2,832,015 34,542 ◯ example 1 Comparative 116 0.017 2,802,938 40,278 ◯ example 2 Comparative 158 0.021 2,849,199 41,053 X example 3 Comparative 62 0 2,016,903 31,807 ◯ example 4 Comparative 112 0.014 2,401,801 40,091 X example 5 Comparative 13 0.018 2,819,201 30,992 X example 6 Comparative 114 0.015 2,539,630 40,208 X example 7 Comparative 10 0.018 2,814,382 30,651 X example 8 Comparative 113 0.015 2,653,017 40,130 X example 9 Comparative 11 0.018 2,810,153 30,902 X example 10 Comparative 113 0.015 2,710,823 40,149 X example 11 Comparative 11 0.017 2,807,109 30,722 X example 12 Comparative 112 0.015 2,558,941 40,108 X example 13 Comparative 12 0.018 2,812,109 30,945 X example 14 Comparative 118 0.015 2,691,105 40,458 X example 15 Comparative 6 0.018 2,810,710 30,260 X example 16 Comparative 56 0.022 2,854,372 34,988 ◯ example 17 Comparative 60 0.022 2,853,191 36,003 ◯ example 18 Formula 1* 6.7 ≤ 5[C] + 2[Si] + 2[Mn] + 2[Cr] + 2[Ni] + 5[Mo] ≤ 9.5 (where [C], [Si], [Mn], [Cr], [Ni], and [Mo] mean the amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively.)

Experimental Example (Thermal Treatment Temperature)

[0067] The test pieces manufactured in Example 3 and Comparative examples 19 to 21 were inspected for the fraction of the precipitate and the results thereof are expressed in Table 5 below.

TABLE-US-00005 TABLE 5 Fraction of precipitate (%) Example 3 0.036 Comparative example 19 0.018 Comparative example 20 0.026 Comparative example 21 0.012