Case-hardening steel

Abstract

To propose a case-hardening steel with the ability to inhibit crystal grain coarsening during carburization.SOLUTION: A case-hardening steel contains C: 0.10-0.35 mass%, Si: 0.01-2.00 mass%, Mn: 0.20-1.90 mass%, P: 0.1 mass% or less, S: 0.5 mass% or less, Cr: 0.20-2.00 mass%, Al: 0.010-0.090 mass%, N: 0.0040-0.0250 mass% and Al: 0.010-0.100 mass%, with the balance being Fe and inevitable impurities, where the standard deviation σAl of the Al segregation degree (CAl / C0Al) fulfills σAl≤68.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a case-hardened steel for use in machine structural parts used in the fields of construction and industrial machinery and automobiles. [Background technology]

[0002] Carburizing, quenching and tempering is a heat treatment that improves the fatigue properties of steel parts, and is used in a variety of parts, including automotive gears. The above carburizing process is carried out at high temperatures for a long period of time, which can cause the austenite grains to grow coarse. The coarsening of austenite grains has a negative effect on fatigue properties, so it is important to prevent this.

[0003] As a technique for preventing the coarsening of austenite grains, there is a method of dispersing fine precipitates in steel to prevent the movement of grain boundaries. For example, Patent Document 1 proposes a case-hardened steel capable of suppressing abnormal grain growth during carburizing treatment, in which the density of carbides and carbonitrides containing Ti and Nb with a circle equivalent diameter of less than 10 nm and the density of precipitates containing Ti and S with a circle equivalent diameter of 200 nm or more are specified. In addition, Patent Document 2 proposes a method for manufacturing a case-hardened steel in which elements such as Cr and Mo are diffused by specifying heat treatment conditions to reduce segregation in the structure after hot working, and the grains do not coarsen even when carburized at high temperatures. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-160979 A [Patent Document 2] JP 2016-74951 A Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, for efficient parts manufacturing, there has been a shift from hot forging to cold forging, an increase in the amount of cold forging, and a reduction in the carburizing time due to an increase in the carburizing temperature, and as a result, there are an increasing number of cases where it is difficult to suppress the coarsening of crystal grains. In this context, the techniques described in Patent Documents 1 and 2 have left an issue in that it is difficult to stably suppress the coarsening of crystal grains.

[0006] The present invention has been developed in view of the above circumstances, and aims to propose a case-hardening steel that can suppress grain coarsening during carburization. [Means for solving the problem]

[0007] In order to achieve the above object, the inventors have intensively studied the effect of pre-carburization structure on grain growth during carburization of steel, and as a result have found that the smaller the segregation of alloying elements that form precipitates with a pinning effect in the steel structure before carburization, the more easily grain coarsening during carburization can be suppressed. The present invention is based on this finding, and its gist is as follows.

[0008] 1.C:0.10~0.35% by mass, Si:0.01~2.00% by mass, Mn: 0.20~1.90% by mass, P: 0.1% by mass or less, S: 0.5% by mass or less, Cr:0.20~2.00% by mass, Al: 0.010 to 0.090 mass% and N:0.0040~0.0250% by mass The composition is composed of Fe and unavoidable impurities, and the degree of Al segregation (C Al / C0 Al ) standard deviation σ Al Case-hardening steel that satisfies the following formula (1). σ Al ≦68 …(1)

[0009] 2. The composition further comprises: Mo: 0.35% by mass or less, Cu: 1.00% by mass or less, Ni: 1.00% by mass or less, V: 0.10% by mass or less and B: 0.0100% by mass or less The case-hardening steel according to claim 1, containing one or more selected from the following:

[0010] 3. The composition further comprises: Sn: 0.100 mass% or less and Sb: 0.100% by mass or less 3. The case-hardening steel according to claim 1 or 2, containing one or two selected from the following:

[0011] 4. The composition further comprises: Ca: 0.300% by mass or less, Se: 0.300% by mass or less, Te: 0.300% by mass or less, Pb: 0.30% by mass or less and Bi: 0.30% by mass or less The case-hardening steel according to any one of 1 to 3, containing one or more selected from the following: Effect of the Invention

[0012] According to the present invention, it is possible to provide a case-hardened steel capable of suppressing grain coarsening during carburization. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, the case-hardening steel of the present invention will be described in order, starting with its chemical composition. C: 0.10~0.35% by mass C needs to be 0.10% by mass or more in order to increase the hardness of the center by quenching after carburizing heat treatment. On the other hand, if the C content exceeds 0.35% by mass, the toughness of the core after quenching decreases, so the C content is limited to a range of 0.10 to 0.35% by mass. The range is preferably 0.13 to 0.27% by mass. The range is more preferably 0.15 to 0.25% by mass.

[0014] Si:0.01~2.00% by mass Silicon is necessary as a deoxidizer, and must be added in an amount of at least 0.01% by mass. However, excessive addition of silicon increases the deformation resistance of steel and adversely affects cold forgeability, so the upper limit is set at 2.00% by mass. The content is preferably 0.04 to 0.70% by mass. More preferably, the content is 0.05 to 0.35% by mass.

[0015] Mn:0.20~1.90% by mass Mn is an element effective in improving hardenability, and must be added in an amount of at least 0.20 mass %. However, excessive addition of Mn increases the deformation resistance of the steel, so the upper limit is set at 1.90 mass %, and preferably 0.40 to 1.30 mass %.

[0016] P: 0.1% by mass or less P segregates at grain boundaries and reduces toughness, so the lower the P content, the more desirable it is, but up to 0.1 mass% is acceptable. It is preferably 0.02 mass% or less. There is no problem if there is no particular limit to the P content, but unnecessary reduction in P content increases refining time and refining costs, so it is better to set it at 0.003 mass% or more.

[0017] S: 0.5% by mass or less S exists as sulfide-based inclusions and is an effective element for improving machinability, but excessive addition leads to a decrease in cold forgeability, so the upper limit is set to 0.5 mass%. There is no particular lower limit, but excessively low S content increases refining costs, so it is preferable to set it to 0.003 mass% or more. It is preferably 0.004 to 0.3 mass%, and more preferably 0.005 to 0.09 mass%.

[0018] Cr:0.20~2.00% by mass Cr contributes to improving hardenability and temper softening resistance, and is also useful for promoting spheroidization of carbides, but if the content is less than 0.20 mass%, the effect of adding it is poor. On the other hand, if the content exceeds 2.00 mass%, it promotes excessive carburization and the formation of retained austenite, which has a negative effect on fatigue strength. Therefore, the Cr content is limited to the range of 0.20 to 2.00 mass%, and preferably to the range of 0.7 to 1.9 mass%.

[0019] Al:0.010~0.090% by mass Al is a useful element that combines with N to form nitride (AlN) and plays a role of pinning. It also forms oxides and is an effective element for deoxidization, but if the content is less than 0.010 mass%, the effect of adding it is poor. However, excessive addition leads to an increase in inclusions, which increases the starting points of fatigue fracture and causes a decrease in fatigue strength, so the upper limit is set to 0.090 mass%. It is preferably 0.015 to 0.080 mass%, and more preferably 0.015 to 0.060 mass%. When B is added, it contributes to improving hardenability and fatigue strength due to solid solution B, and in that case, the range of 0.045 to 0.075 mass% is suitable.

[0020] N:0.0040~0.0250% by mass N combines with Al to form nitride (AlN). Such AlN precipitates finely and refines the crystal grains during carburizing heating, improving fatigue properties. However, the addition of excessive N improves the thermodynamic stability of AlN too much, resulting in the formation of coarse AlN. If coarse AlN is present, the volume fraction of fine AlN decreases, and the grain refinement effect is not achieved. For this reason, the upper limit of N is set to 0.0250 mass%. The lower limit of N is set to 0.0040 mass% in order to obtain a sufficient grain refinement effect. It is preferably 0.0050 to 0.0180 mass%. More preferably, it is 0.0100 to 0.0170 mass%.

[0021] The basic components of the present invention have been described above, but the following components may be further added as required. Mo: 0.35% by mass or less, Cu: 1.00% by mass or less, Ni: 1.00% by mass or less, V: 0.10% by mass or less and B: 0.0100% by mass or less One or more selected from

[0022] Mo: 0.35% by mass or less Mo contributes to improving hardenability and temper softening resistance, and furthermore has the effect of reducing the carburized abnormal layer, and is therefore a useful element that may be added. However, if the content exceeds 0.35 mass%, the hardenability becomes excessive, the hardness after rolling increases, and there is a concern that forgeability and machinability may decrease. Therefore, it is preferable to limit the Mo content to a range of 0.35 mass% or less. In order to realize the above-mentioned effects of Mo on improving hardenability and temper softening resistance, and reducing the carburized abnormal layer, it is preferable that Mo be contained in an amount of 0.01 mass% or more. More preferably, it is in the range of 0.03 to 0.25 mass%. More preferably, it is 0.05 to 0.22 mass%.

[0023] Cu: 1.00% by mass or less Cu is an element that contributes to improving hardenability. To obtain this effect, Cu is preferably contained in an amount of 0.01 mass% or more. On the other hand, if the Cu content exceeds 1.00 mass%, the surface of the rolled material becomes rough, and there is a concern that these may remain as defects. Therefore, the Cu content is preferably limited to a range of 1.00 mass% or less. More preferably, it is in the range of 0.015 to 0.500 mass%. Even more preferably, it is in the range of 0.030 to 0.300 mass%.

[0024] Ni: 1.00% by mass or less Ni is an element that contributes to improving hardenability and is useful for improving toughness. In order to obtain these effects, Ni is preferably contained in an amount of 0.01 mass% or more. On the other hand, even if Ni is contained in an amount exceeding 1.00 mass%, the above effects are saturated. Therefore, the Ni content is preferably limited to a range of 1.00 mass% or less. More preferably, it is in the range of 0.015 to 0.500 mass%. Even more preferably, it is in the range of 0.030 to 0.300 mass%.

[0025] V: 0.10% by mass or less V has the effect of forming a solid solution during carburizing heating and improving hardenability during rapid cooling. To obtain this effect, it is preferable to contain at least 0.003 mass% V. On the other hand, even if it is added in excess of 0.1 mass%, the effect is saturated, so the upper limit is preferably 0.10 mass%. More preferably, it is 0.005 to 0.08 mass%, and even more preferably, it is 0.01 to 0.06 mass%.

[0026] B: 0.0100% by mass or less B segregates at grain boundaries and inhibits diffusion-type transformation, which is effective in improving hardenability, and also has the effect of strengthening grain boundaries, inhibiting the occurrence and progression of fatigue cracks, and improving fatigue strength. In order to obtain this effect of B, it is preferable to contain 0.0003 mass% or more of B. On the other hand, if the content exceeds 0.0100 mass%, toughness decreases, so the B content is preferably limited to a range of 0.0100 mass% or less. More preferably, it is in the range of 0.0005 to 0.0050 mass%. Even more preferably, it is in the range of 0.0007 to 0.0020 mass%.

[0027] Furthermore, the following components may be added as necessary. Sn: 0.100 mass% or less and Sb: 0.100% by mass or less One or two types selected from Sb: 0.100% by mass or less Sb is an element effective in suppressing decarburization of the steel surface and preventing a decrease in surface hardness. To achieve this effect, it is preferable to contain 0.0003 mass% or more of Sb. On the other hand, since excessive addition of Sb deteriorates forgeability, the Sb content is preferably 0.100 mass% or less. More preferably, it is 0.001 to 0.050 mass%. Even more preferably, it is 0.0015 to 0.035 mass%.

[0028] Sn: 0.100% by mass or less Sn is an element effective for improving the corrosion resistance of the steel surface. From the viewpoint of improving corrosion resistance, it is preferable that Sn is contained in an amount of 0.0003 mass% or more. On the other hand, since excessive addition of Sn deteriorates forgeability, the Sn content is preferably 0.100 mass% or less. More preferably, it is 0.001 to 0.050 mass%. Further preferably, it is 0.0015 to 0.035 mass%.

[0029] Furthermore, the following components may be added as necessary. Ca: 0.300% by mass or less Se: 0.300% by mass or less Te: 0.300% by mass or less Pb: 0.30% by mass or less and Bi: 0.30% by mass or less One or more selected from Ca, Se, Te, Pb and Bi are free-cutting elements that improve the machinability of steel and may be added as necessary. To obtain this effect, it is preferable to add 0.010 mass% or more of each of Ca, Se, Te, Pb and Bi. However, even if these elements are added in excess, the effect of improving machinability is saturated. Therefore, in order to prevent an increase in alloy cost, the upper limit of Ca, Se and Te is set to 0.300 mass%, and the upper limit of Pb and Bi is set to 0.30 mass%. It is preferably 0.010 to 0.200 mass%, and more preferably 0.03 to 0.100 mass%.

[0030] The remainder other than the elements described above is Fe and impurities. Impurities are those that are mixed in from ores, scraps, or manufacturing environments as raw materials during the industrial production of steel materials, and are tolerated within a range that does not adversely affect the properties of the skin-burned steel of the present invention.

[0031] <Degree of Al segregation (C Al / C0 Al ) with a standard deviation σ Al satisfies the following formula (1)> σ Al ≦68 …(1) Al can exert a pinning effect that prevents the movement of grain boundaries by combining with N to form nitrides (AlN). For this purpose, it is advantageous that the standard deviation of the Al segregation degree is small. That is, the fact that the standard deviation of the Al segregation degree is as small as 68 or less means that Al atoms are more uniformly dispersed in the steel and exert a uniform pinning force. The crystal grain size of the steel changes corresponding to the pinning force, and the larger the pinning force, the finer it becomes. Therefore, when the pinning force is uniform, it means that the distribution of the crystal grain size is uniform, and the grain growth driven by the grain size difference is suppressed.

[0032] Here, C Al / C0 Al ) in the Al segregation degree, C Al and C0 Al can be determined by an electron probe microanalyzer (EPMA). That is, C0 Al is the average Al concentration (mass%) in the EPMA analysis range, and C Al is the maximum Al concentration (mass%) within the same range. And this EPMA analysis range is set to 10 different locations at a depth of 1 mm or more from the surface of the steel material. That is, at a depth of 1 mm or more from the surface of the steel material, the Al segregation degree (C Al / C0 Al ) is measured at 10 locations where each analysis range is separated from each other by 1 mm or more, and the standard deviation of these is obtained.

[0033] In order to make the standard deviation of the Al segregation degree 68 or less, it is sufficient to promote the diffusion of Al in the steel, and for example, in hot rolling, it is effective to increase the heating temperature and time. In addition, in terms of refining the crystal grain size and promoting the grain boundary diffusion of Al, it is also effective to increase the number of heatings in hot rolling. That is, after austenitizing in the first heating, it is preferable to cause a phase transformation by cooling once, and to austenitize again in the second heating and start rolling. In this case, two heatings are performed. Therefore, the number of heatings indicates how many times the steel is heated to a temperature in the austenite single phase region and held before the start of rolling, and rolling after multiple heatings can be performed under normal conditions. In this way, it is sufficient to promote the diffusion of Al by performing heat treatment at a predetermined heating temperature and holding time multiple times during hot rolling, and the heating rate to the predetermined heating temperature and the cooling rate to the cooling temperature during the first cooling can be any rate and can be appropriately adjusted according to the equipment capacity. The temperature during the initial cooling varies depending on the cooling rate and can therefore be adjusted appropriately according to the capacity of the equipment. However, a good guideline for inducing phase transformation is to set the upper limit at 650°C.

[0034] In the field of construction and industrial machinery, examples of parts using the case hardening steel of the present invention include gears of travel reducers (gears such as planetary gears and sun gears), gears of large reducers, valve plates of hydraulic pumps, nuts of ball screws, curved plates and pins of cyclone reducers, and blocks of linear bearings, while in the field of automobiles, examples include various bearings, piston pins of engines, camshafts and timing gears, gears of transmissions (missing gears, ring gears, sun gears and planetary gears, etc.), and differential bevel gears, tripods, inner gears and balls of drive systems. In addition to the fields of construction and industrial machinery and automobiles, examples of parts include bearings and reduction gears for wind power generators in the field of electrical equipment. EXAMPLES

[0035] The configuration and effects of the present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples, and appropriate modifications can be made within the scope of the present invention, and all of these modifications are included in the technical scope of the present invention.

[0036] A 150 kg ingot of steel having the composition shown in Table 1 was melted, heated at 1200°C for 1 hour, and then hot forged and formed into a round bar with a diameter of 70 mm. In order to change the degree of segregation of Al, the obtained round bar was heated to various temperatures of 1000°C or higher, and then held for various times of 1 hour or more, multiple times, and finally hot rolled into round bars with various diameters of 50 mm or less. The hot rolling conditions here are as shown in Table 2.

[0037] [Table 1]

[0038] A test piece for EPMA was taken from the obtained round bar at 1 / 4 of the diameter in the radial direction from the surface, and the segregation degree of Al in the test piece was investigated. The EPMA conditions were an acceleration voltage of 15.0 kV and a probe current of 5.0 × 10 -7 A, the beam shape was a 2.0 μm circle, and a 1 mm x 1 mm range was measured in 2 μm steps. The time for one step was 50 ms. The quantitative value of Al was obtained by concentration conversion using the calibration curve method. The above measurements were performed at 10 different points in the field of view (at a depth of 1 mm or more from the surface of the cross section perpendicular to the longitudinal (axial) direction of the round bar, with each analysis range separated by 1 mm or more). In this way, C Al (Maximum Al concentration) and C0 Al (average Al concentration) was calculated from the results of similar measurements at 10 different visual fields. Al / C0 Al ) and then calculate its standard deviation σ Al was evaluated.

[0039] In addition, to evaluate the ability to suppress abnormal grain growth during carburization, a test piece with a diameter of φ15 × 22.5 mm was taken from d / 2 (d: round bar diameter) of the obtained round bar, and cold compression was performed on the test piece with a cumulative height reduction rate of 75%. The test piece after cold compression was subjected to a carburizing heat treatment in which it was heated to 900°C for 7.5 hours and then cooled in water. The prior austenite grain size was observed at the center of the test piece after the heat treatment, and its maximum diameter was evaluated. The maximum diameter was determined by image analysis of a micrograph to determine the maximum width of the prior austenite grain.

[0040] Table 2 shows the standard deviation σ Al The evaluation results of the prior austenite grain size are shown in Table 2. Here, the prior austenite grain size was evaluated such that when grains with a maximum diameter of more than 45 μm were observed, it was determined that abnormal grain growth had occurred, and this was indicated as × in Table 2. On the other hand, when grains with a maximum diameter of 45 μm or less were observed, it was determined that abnormal grain growth had been suppressed, and this was indicated as ◯ in the table. As shown in Table 2, by following the present invention, a case-hardened steel capable of suppressing grain coarsening during carburizing can be obtained.

[0041] [Table 2]

Claims

1. C: 0.10 to 0.35% by mass, Si: 0.01 to 2.00% by mass, Mn: 0.20 to 1.90% by mass, P: 0.1% by mass or less, S: 0.5% by mass or less, Cr: 0.20 to 2.00% by mass, Al: 0.010 to 0.090 mass% and N: 0.0040 to 0.0250% by mass The composition of the alloy is composed of the remainder Fe and impurities, and the degree of Al segregation (C Al / C0 Al ) standard deviation σ Al A case-hardening steel that satisfies the following formula (1). s Al ≦68 …(1)

2. The case-hardened steel according to claim 1, wherein the component composition further contains one or more selected from any one or more of the following groups A to C. Group A Mo: 0.35% by mass or less, Cu: 1.00% by mass or less, Ni: 1.00% by mass or less, V: 0.10% by mass or less, B: 0.0100% by mass or less Group B Sn: 0.100% by mass or less, Sb: 0.100% by mass or less Group C Ca: 0.300% by mass or less, Se: 0.300% by mass or less, Te: 0.300% by mass or less, Pb: 0.30% by mass or less, Bi: 0.30% by mass or less

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