Muscle 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: 0.0010-0.0250 mass% and Ti: 0.010-0.100 mass%, with the balance being Fe and inevitable impurities, where the standard deviation σTi of the Ti segregation degree (CTi / C0Ti) fulfills σTi≤72.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to 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 for a variety of parts, including automotive gears. Because the carburizing process is performed at high temperatures for a long period of time, the austenite grains can grow coarse. Since coarsening of austenite grains has a negative effect on fatigue properties, it is important to prevent this.

[0003] One technique for preventing austenite grain coarsening is to disperse fine precipitates in steel to prevent grain boundary migration. For example, Patent Document 1 proposes a case-hardened steel that can suppress abnormal grain growth during carburizing by specifying the density of carbides and carbonitrides containing Ti and Nb with an equivalent circle diameter of less than 10 nm and the density of precipitates containing Ti and S with an equivalent circle diameter of 200 nm or more. Patent Document 2 also proposes a method for manufacturing case-hardened steel that specifies heat treatment conditions to diffuse elements such as Cr and Mo, thereby reducing segregation in the structure after hot working, and thus preventing grain coarsening even when carburized at high temperatures. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-160979 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-74951 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, in order to manufacture parts efficiently, there has been a shift from hot forging to cold forging, an increase in the amount of cold forging formed, and a reduction in the carburizing time due to an increase in the temperature of the carburizing treatment, and as a result, there are an increasing number of cases in which 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 carburizing. [Means for solving the problem]

[0007] In order to achieve the above object, the inventors conducted extensive research into the effect of pre-carburizing structure on grain growth during carburizing of steel, and as a result discovered that the smaller the segregation of alloying elements that form precipitates with a pinning effect in the steel structure before carburizing, the more effectively grain coarsening during carburizing can be suppressed. The present invention is based on this discovery and is summarized 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: 0.010~0.090% by mass, N: 0.0010 to 0.0250 mass% and Ti:0.010~0.100% by mass The composition is composed of the remainder Fe and impurities, and the Ti segregation degree (C Ti / C0 Ti ) standard deviation σ Ti Case hardening steel that satisfies the following formula (1). σ Ti ≦72 …(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 mass% or less and B: 0.0100% by mass or less 2. The case-hardening steel according to 1 above, 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 1 or 2 above, 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 4. The case-hardening steel according to any one of 1 to 3, containing one or more selected from the following: [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a case-hardening steel that can suppress grain coarsening during carburizing. DETAILED DESCRIPTION OF THE INVENTION

[0013] The case-hardening steel of the present invention will be described below in order, starting with its chemical composition. C: 0.10~0.35% by mass C needs to be 0.10% by mass or more 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 the range of 0.10 to 0.35% by mass. Preferably, it is in the range of 0.13 to 0.27% by mass. More preferably, it is in the range of 0.15 to 0.25% by mass.

[0014] Si:0.01~2.00% by mass Si is necessary as a deoxidizer, and must be added in an amount of at least 0.01% by mass. However, excessive addition of Si increases the deformation resistance of steel and adversely affects cold forgeability, so the upper limit is set to 2.00% by mass. The content is preferably 0.04 to 0.70% by mass, and more preferably 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%. The preferred range is 0.40 to 1.30 mass%.

[0016] P: 0.1% by mass or less P segregates at grain boundaries and reduces toughness, so its content is preferably as low as possible, but up to 0.1% by mass is acceptable. It is preferably 0.02% by mass or less. While there is no problem if the lower limit is not particularly set, unnecessary reduction in P content increases refining time and costs, so it is preferable to set it at 0.003% by mass or more.

[0017] S: 0.5% by mass or less S exists as sulfide-based inclusions and is an element effective in improving machinability, but excessive addition leads to a decrease in cold forgeability, so the upper limit is set to 0.5% by mass. There is no particular lower limit, but since excessively low S content increases refining costs, it is recommended to set it to 0.003% by mass or more. The content is preferably 0.004 to 0.3% by mass, and more preferably 0.005 to 0.09% by 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. However, if the content is less than 0.20% by mass, the effect of adding Cr is poor. On the other hand, if the content exceeds 2.00% by mass, excessive carburization and the formation of retained austenite are promoted, adversely affecting fatigue strength. Therefore, the Cr content is limited to the range of 0.20 to 2.00% by mass. Preferably, it is in the range of 0.7 to 1.9% by 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 pinning role. It also forms oxides and is effective for deoxidation, but if its content is less than 0.010 mass%, its addition effect is poor. However, excessive addition leads to an increase in inclusions, which increases the number of fatigue fracture initiation sites and reduces fatigue strength, so the upper limit is set at 0.090 mass%. The preferred range is 0.015 to 0.080 mass%, and more preferably 0.015 to 0.060 mass%. Furthermore, when B is added, the solute B contributes to improving hardenability and fatigue strength, and in this case, a range of 0.045 to 0.075 mass% is suitable.

[0020] N:0.0010~0.0250% by mass N combines with Al to form nitride (AlN). Such AlN precipitates finely, refining the crystal grains during carburizing heating and improving fatigue properties. However, adding excessive N excessively improves the thermodynamic stability of AlN, resulting in the formation of coarse AlN. The presence of coarse AlN reduces the volume fraction of fine AlN, preventing the grain refinement effect. For this reason, the upper limit of N is set to 0.0250 mass%. The lower limit of N is set to 0.0010 mass%, preferably 0.0050 to 0.0180 mass%, and more preferably 0.0100 to 0.0170 mass%.

[0021] Ti:0.010~0.100% by mass Ti combines with C to form carbide (TiC). The more finely and uniformly dispersed TiC is, the more grain coarsening during carburizing heating is suppressed. To achieve this effect, at least 0.010% by mass must be added. On the other hand, if the addition exceeds 0.100% by mass, the effect saturates, so the upper limit is set to 0.100% by mass. The content is preferably 0.015 to 0.065% by mass. More preferably, it is 0.020 to 0.055% by mass.

[0022] The basic components of the present invention have been described above, but the following components may be further added as needed. Mo: 0.35% by mass or less, Cu: 1.00% by mass or less, Ni: 1.00% by mass or less, V: 0.10 mass% or less and B: 0.0100% by mass or less One or more types selected from

[0023] 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 zone. Being a useful element, it may be added. However, if the content exceeds 0.35% by 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% by mass or less. To achieve the aforementioned effects of Mo on improving hardenability and temper softening resistance, and reducing the carburized abnormal zone, it is preferable that Mo be contained in an amount of 0.01% by mass or more. More preferably, it is contained in a range of 0.03 to 0.25% by mass. More preferably, it is contained in a range of 0.05 to 0.22% by mass.

[0024] Cu: 1.00% by mass or less Cu is an element that contributes to improving hardenability. To achieve this effect, the Cu content is preferably 0.01% by mass or more. On the other hand, if the Cu content exceeds 1.00% by 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% by mass or less. More preferably, it is in the range of 0.015 to 0.500% by mass. Even more preferably, it is in the range of 0.030 to 0.300% by mass.

[0025] Ni: 1.00% by mass or less Ni contributes to improving hardenability and is a useful element for improving toughness. To achieve these effects, Ni is preferably contained in an amount of 0.01% by mass or more. On the other hand, even if Ni is contained in an amount exceeding 1.00% by mass, the above effects saturate. Therefore, the Ni content is preferably limited to a range of 1.00% by mass or less. More preferably, it is in the range of 0.015 to 0.500% by mass. Even more preferably, it is in the range of 0.030 to 0.300% by mass.

[0026] V: 0.10% by mass or less V dissolves in solid solution during carburizing heating and has the effect of improving hardenability during rapid cooling. To achieve this effect, it is preferable to contain at least 0.003 mass% of V. However, even if added in excess of 0.1 mass%, the effect saturates, so the upper limit is preferably set to 0.10 mass%. A more preferred range is 0.005 to 0.08 mass%, and even more preferred is 0.01 to 0.06 mass%.

[0027] B: 0.0100% by mass or less B segregates at grain boundaries and suppresses diffusional transformation, thereby effectively improving hardenability. It also strengthens grain boundaries, suppressing the initiation and propagation of fatigue cracks and improving fatigue strength. To achieve this effect, B is preferably contained in an amount of 0.0003% by mass or more. However, if the B content exceeds 0.0100% by mass, toughness decreases, so the B content is preferably limited to a range of 0.0100% by mass or less. A more preferred range is 0.0005 to 0.0050% by mass. An even more preferred range is 0.0007 to 0.0020% by mass.

[0028] Furthermore, the following components may be added as needed. 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 that is 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, excessive addition of Sb deteriorates forgeability, so the Sb content is preferably 0.100 mass% or less. More preferably, it is 0.001 to 0.050 mass%, and even more preferably, it is 0.0015 to 0.035 mass%.

[0029] 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 the Sn content be 0.0003 mass% or more. On the other hand, excessive addition of Sn deteriorates forgeability, so the Sn content is preferably 0.100 mass% or less. More preferably, it is 0.001 to 0.050 mass%, and even more preferably, it is 0.0015 to 0.035 mass%.

[0030] Furthermore, the following components may be added as needed. Ca: 0.300% by mass or less Se: 0.300% by mass or less Te: 0.300 mass% or less Pb: 0.30 mass% or less and Bi: 0.30 mass% or less one or more selected from these Ca, Se, Te, Pb, and Bi are free-cutting elements that improve the machinability of steel and may be added as needed. To obtain this effect, it is preferable to add Ca, Se, Te, Pb, and Bi at 0.010 mass% or more respectively. However, even if these elements are added excessively, the effect of improving machinability saturates. Therefore, to suppress an increase in alloy cost, the upper limit values of Ca, Se, and Te are set to 0.300 mass% and the upper limit values of Pb and Bi are set to 0.30 mass%. Preferably, it is 0.010 - 0.200 mass%. More preferably, it is 0.03 - 0.100 mass%.

[0031] The remainder other than the elements described above is Fe and impurities. Impurities are those mixed in from ore, scrap, or the manufacturing environment as raw materials when steel materials are industrially manufactured and are allowed within a range that does not adversely affect the properties of the skin-burning steel of the present invention.

[0032] <Ti segregation degree (C Ti / C0 Ti ) standard deviation σ Ti satisfies the following formula (1)> σ Ti ≦72 …(1) Ti can exert a pinning effect that blocks the movement of grain boundaries by combining with C to form carbides (TiC). For this, it is advantageous that the standard deviation of the Ti segregation degree is small. That is, the fact that the standard deviation of the Ti segregation degree is as small as 72 or less means that Ti 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 grain growth driven by the grain size difference is suppressed.

[0033] Here, Ti segregation degree (C Ti / C0 Ti) in C Ti and C0 Ti can be determined using an electron probe microanalyzer (EPMA). Ti is the average Ti concentration (mass%) in the EPMA analysis area, and C Ti is the maximum Ti concentration (mass%) within the same range. The EPMA analysis range is 10 different locations at a depth of 1 mm or more from the surface of the steel material. That is, the Ti segregation degree (C Ti / C0 Ti ) and calculate the standard deviation.

[0034] To reduce the standard deviation of Ti segregation to 72 or less, the diffusion of Ti in the steel can be promoted. For example, increasing the heating temperature and time during hot rolling is effective. Furthermore, increasing the number of heating cycles during hot rolling is also effective in terms of refining the grain size and promoting the grain boundary diffusion of Ti. That is, after austenitizing the steel in the first heating cycle, the steel is cooled to induce a phase transformation, and then austenitized again in the second heating cycle before starting rolling. In this case, two heating cycles are performed. Therefore, the number of heating cycles indicates the number of times the steel is heated to a temperature in the austenite single-phase region and held at that temperature before starting rolling. Rolling after multiple heating cycles can be performed under normal conditions. Thus, as long as the diffusion of Ti is promoted by performing multiple heat treatments at a predetermined heating temperature and holding time during hot rolling, the heating rate to the predetermined heating temperature and the cooling rate to the cooling temperature during the first cooling cycle can be any rate and can be adjusted appropriately according to the equipment capacity. The temperature during the initial cooling varies depending on the cooling rate, and can be adjusted appropriately according to the equipment capacity, but a good guideline for phase transformation is to set the upper limit at 650°C.

[0035] In the field of construction and industrial machinery, examples of parts that can use the case-hardened steel of the present invention include gears for travel reducers (gears such as planetary gears and sun gears), gears for large reducers, valve plates for hydraulic pumps, nuts for ball screws, curved plates and pins for cyclone reducers, and blocks for linear bearings.Similarly, in the field of automobiles, examples of parts that can use the case-hardened steel of the present invention include various bearings, piston pins for engines, camshafts and timing gears, gears for transmissions (missing gears, ring gears, sun gears, planetary gears, etc.), and differential bevel gears, tripods, inner gears, and balls for drive systems.In addition to the fields of construction and industrial machinery and automobiles, examples of parts that can be used include bearings and reduction gears for wind turbine generators in the field of electrical equipment. [Example]

[0036] The following examples will explain the structure and effects of the present invention in more detail. 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 such modifications are within the technical scope of the present invention.

[0037] A 150 kg ingot of steel with the chemical composition shown in Table 1 was melted and heated at 1200°C for 1 hour, then hot forged and formed into a 70 mm diameter round bar. To vary the degree of Ti segregation, the obtained round bar was heated to various temperatures above 1000°C and held for various periods of time above 1 hour 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.

[0038] [Table 1]

[0039] An EPMA test piece was taken from the obtained round bar at 1 / 4 of the diameter in the radial direction from the surface, and the degree of segregation of Ti in the test piece was investigated under the EPMA conditions of an acceleration voltage of 15.0 kV and a probe current of 5.0 × 10 -7The beam shape was a 2.0 μm circle, and measurements were made in a 1 mm x 1 mm area in 2 μm steps. The time for one step was 50 ms. The quantitative value of Ti was calculated by converting the concentration using the calibration curve method. The above measurements were made at 10 different points in the field of view (at a depth of 1 mm or more from the surface of a cross section perpendicular to the longitudinal (axial) direction of the rod, with each analysis area separated by 1 mm or more). In this way, C Ti (Maximum Ti concentration) and C0 Ti (average Ti concentration) was measured in the same way at 10 different visual fields, and the Ti segregation degree (C Ti / C0 Ti ) and then calculate its standard deviation σ Ti was evaluated.

[0040] To evaluate the ability to suppress abnormal grain growth during carburizing, a φ15 × 22.5 mm test piece was taken from the obtained round bar at d / 2 (d: round bar diameter) and cold-compressed to a cumulative height reduction of 70%. The cold-compressed test piece was then subjected to a carburizing heat treatment in which it was heated to 1020°C for 7 hours and then water-cooled. The prior austenite grain size was observed at the center of the heat-treated test piece, 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.

[0041] Table 2 shows the standard deviation σ Ti The evaluation results of the prior austenite grain size are shown in Table 2. Here, the prior austenite grain size was evaluated such that abnormal grain growth occurred when grains with a maximum diameter of more than 45 μm were observed, and this was marked with "x" 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 was suppressed, and this was marked with "o" in the table. As shown in Table 2, by following the present invention, a case-hardening steel capable of suppressing grain coarsening during carburizing can be obtained.

[0042] [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% by mass, N: 0.0010 to 0.0250 mass% and Ti: 0.010 to 0.100% by mass and the balance being Fe and impurities, and the standard deviation σ of the ratio C Ti / C0 Ti of the maximum Ti concentration C Ti (mass%) within the EPMA analysis range to the average Ti concentration C0 Ti (mass%) within the range Ti Case hardening steel that satisfies the following formula (1). s Ti ≦72 …(1)

2. The case-hardening steel according to claim 1, wherein the chemical composition further contains one or more elements 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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