Carburized and nitrided steel material

Abstract

To provide a carburized and nitrided steel material that exhibits superior resistance to surface peeling.SOLUTION: A carburized and nitrided steel material having a predetermined chemical composition, wherein Fx1 determined by formula (1) is 0.36 or more and 0.83 or less, the surface nitrogen content Ns satisfies formula (2), and the surface nitride area ratio is 3.00% or more. Fx1=30×{0.008×(Si%+Mn%)-0.003×(Cr%)} (1). Ns (mass%)≥0.43×e0.96×Cr% (2). In formulas (1) and (2), Si%, Mn%, and Cr% respectively represent the contents (expressed in mass%) of Si, Mn, and Cr in the steel.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to carbonitrided steel materials. [Background technology]

[0002] Conventionally, machine structural steels such as SCr, SCM, and SNCM, so-called case-hardened steels, have been used for machine structural parts obtained by carburizing and carbonitriding shafts, gears, etc. Machine structural parts are obtained by forming this case-hardened steel into the desired part shape by machining such as forging and cutting, and then performing a surface hardening treatment such as carburizing or carbonitriding. The following technologies have been proposed for carbonitrided steels or machine structural parts obtained using them:

[0003] For example, Patent Document 1 discloses a gear with excellent pitting resistance. The gear is forged or machined into a gear shape, then carburized in a vacuum, cooled in a furnace, and quenched. Nitriding is performed in the furnace between cooling after carburizing and holding at a temperature before quenching, and tempering is performed after quenching. Patent Document 1 also discloses that the gear has a predetermined chemical composition, and that a temper softening resistance parameter HSiCrN, which is determined by the Si and Cr in the chemical composition and the maximum amount of nitrogen that penetrates into the surface layer due to the nitriding, satisfies the predetermined formula (1).

[0004] Patent Document 2 discloses a steel product having a carbonitrided layer that can ensure excellent wear resistance and high pitting strength. Patent Document 2 discloses that the steel product has a carbonitrided layer, the base steel material has predetermined components, has a chemical composition in which fn, expressed by formula (1): fn = (Si + Mn) / Cr, is 2 or more, with the balance being Fe and impurities, and in a region from the surface of the carbonitrided layer to a depth of 50 μm, the only alloy nitride dispersed is MnSiN2, and the amount of austenite on the surface of the carbonitrided layer is 30% or more and 40% or less by volume.

[0005] Patent Document 3 discloses a carbonitrided bearing steel that has an excellent rolling fatigue life in an environment containing foreign matter. Patent Document 3 also discloses that the carbonitrided bearing steel has a predetermined composition, a surface hardness of 58 HRC or more after carbonitriding or the subsequent secondary quenching and tempering, and an amount of surface retained austenite of 20 to 50%.

[0006] Patent Document 4 discloses a steel material for carbonitriding, which has a predetermined chemical composition and is used to manufacture carbonitrided parts with improved surface fatigue life. Patent Document 5 discloses a carbonitrided induction-hardened part and a nitrided induction-hardened part, in which the base material has a predetermined chemical composition, and in a region from the surface to a depth of 0.1 mm, the average C concentration Cave is 0.005 to 0.30%, the average N concentration Nave is 0.45 to 0.80%, and Cave + Nave is 0.60 to 1.00%, and the minimum Vickers hardness in the region from the surface to a depth of 0.1 mm is 550 HV or more when measured with a load of 20 g. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-112827 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-168820 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-042188 [Patent Document 4] Japanese Patent Application Publication No. 2017-125232 [Patent Document 5] Japanese Patent Application Laid-Open No. 2016-102253 Summary of the Invention [Problem to be solved by the invention]

[0008] In Patent Documents 1 to 5, pitting resistance and the like are improved by controlling the chemical composition and the morphology of the surface layer after carbonitriding. However, in gears, for example, various surface spalling can occur, such as tooth surface scuffing caused by temperature increases on the tooth surface and pitting spalling originating on the surface. Therefore, there is a need for a carbonitrided steel material that can suppress various surface spalling, i.e., has excellent surface spalling resistance. Furthermore, in recent years, there has been strict demand for improved fuel efficiency in automobiles, and the miniaturization of parts to reduce vehicle body weight has become increasingly strict. As a result, the sliding load of power transmission parts such as gears increases, and there is a need for a carbonitrided steel material that is even more excellent in surface spalling resistance for use in such parts. The present disclosure has been made in consideration of the above circumstances, and its purpose is to provide a carbonitrided steel material that has excellent surface spalling resistance. [Means for solving the problem]

[0009] Aspect 1 of the present invention is The chemical composition is C: 0.10% by mass or more, 0.30% by mass or less, Si: 0.50 mass% or more, 2.00 mass% or less, Mn: 1.00% by mass or more, 2.00% by mass or less, Cr: 0.10% by mass or more, 0.60% by mass or less, N: 0.0010% by mass or more, 0.0500% by mass or less, Al: 0.010 mass% or more, 0.100 mass% or less, P: more than 0 mass% and 0.100 mass% or less, and S: More than 0% by mass, 0.100% by mass or less Including, A carbonitrided steel material having the balance being Fe and unavoidable impurities, Fx1 calculated by the following formula (1) is 0.36 or more and 0.83 or less, The surface nitrogen content Ns satisfies the following formula (2), This is a carbonitrided steel material with a surface nitride area ratio of 3.00% or more. Fx1=30×{0.008×(Si%+Mn%)-0.003×(Cr%)} (1) Ns (mass%)≧0.43×e0.96×Cr% (2) In formula (1) and formula (2), Si%, Mn%, and Cr% respectively represent the contents of Si, Mn, and Cr in the steel expressed in mass%.

[0010] Aspect 2 of the present invention is Aspect 1 is a carbonitrided steel material according to aspect 1, wherein the chemical composition satisfies one or more of the following (a) to (e): (a) It further contains one or more elements selected from the group consisting of Mo: more than 0.05 mass % and 1.00 mass % or less, and B: 0.0005 mass % or more and 0.0100 mass % or less. (b) Further containing one or more elements selected from the group consisting of Ti: 0.005% by mass or more and 0.1000% by mass or less, V: 0.005% by mass or more and 0.500% by mass or less, and Nb: 0.005% by mass or more and 0.100% by mass or less. (c) Further containing one or more elements selected from the group consisting of Cu: more than 0.01% by mass and 2.00% by mass or less, and Ni: more than 0.01% by mass and 2.00% by mass or less. (d) Sn: More than 0 mass % and 0.100 mass % or less. (e) Further containing one or more elements selected from the group consisting of Bi: 0.0001% by mass or more and 0.0100% by mass or less, Se: 0.0001% by mass or more and 0.0100% by mass or less, Ca: 0.0001% by mass or more and 0.0100% by mass or less, Mg: 0.0001% by mass or more and 0.0100% by mass or less, and REM: 0.0001% by mass or more and 0.0100% by mass or less.

[0011] Aspect 3 of the present invention is A carbonitrided steel material according to aspect 1 or 2, having a Vickers hardness of 700 HV or more at a depth of 0.025 mm from the outermost surface. [Effects of the Invention]

[0012] According to the present disclosure, it is possible to provide a carbonitrided steel material having excellent resistance to surface spalling. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a graph showing the relationship between the nitride area ratio of the surface layer of a carbonitrided steel material and the friction coefficient. [Figure 2] FIG. 2 is a graph showing the relationship between the surface layer nitrogen content Ns and the surface layer nitride area ratio. [Figure 3] FIG. 3 is a graph showing the relationship between the Cr content in the steel material and the surface layer nitrogen content Ns for each nitride area ratio in the surface layer. [Figure 4] FIG. 4 is a diagram showing the shape of a small roller test piece used in the roller pitching test in the examples. [Figure 5] FIG. 5 shows the heat treatment pattern of the carbonitriding treatment in the example. [Figure 6] FIG. 6 is a diagram showing the shape of a large roller test piece (counterpart) used in the roller pitching test in the examples. [Figure 7] FIG. 7 is a diagram illustrating the roller pitching test method in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0014] As mentioned above, various types of surface flaking can occur in gears, such as tooth flank scuffing caused by temperature increases on the tooth surface and pitting flaking that originates on the surface. The lubrication conditions within the meshing tooth surface are thought to have a significant impact on the tooth surface's lifespan before failure. The lubrication conditions of the tooth surface can be organized by the friction coefficient, as shown in the Stribeck curve, where the horizontal axis is (viscosity x speed) / load and the vertical axis is the friction coefficient. By reducing the friction coefficient, various types of surface flaking can be suppressed, which means that excellent resistance to surface flaking can be achieved.

[0015] The inventors have investigated means for reducing the coefficient of friction and have found that it is possible to do so by adjusting the chemical composition of the carbonitrided steel, the amount of nitrogen in the surface layer of the carbonitrided steel, and the nitride area ratio in the surface layer. Details are explained below.

[0016] First, Figure 1 is a graph showing the relationship between the nitride area ratio of the surface layer of a carbonitrided steel and the friction coefficient, compiled using data from the Examples described below. As shown in Figure 1, the friction coefficient can be calculated using the nitride area ratio of the surface layer of the carbonitrided steel. We have found that increasing the nitride area ratio of the surface layer of the carbonitrided steel is effective in reducing the friction coefficient. Specifically, Figure 1 shows that to achieve a friction coefficient of 2.40 or less, which can suppress the various surface spalling problems described above, the nitride area ratio of the surface layer of the carbonitrided steel (hereinafter sometimes simply referred to as the "nitride area ratio") should be 3.00% or more. The nitride area ratio is preferably 3.50% or more, and more preferably 4.00% or more. Note that if the nitride area ratio is too high, precipitates can become crack initiation sites, reducing fatigue strength. Therefore, the nitride area ratio is preferably 10.00% or less.

[0017] Next, we investigated the use of parameters related to the chemical composition of carbonitrided steel to achieve a nitride area ratio of 3.00% or more in the surface layer of carbonitrided steel. After carbonitriding, surface nitrides are formed, primarily consisting of Si+Mn and Cr. Si+Mn-based nitrides have large precipitate diameters, ensuring a sufficient amount of nitride. On the other hand, Cr-based nitrides have small precipitate diameters, making it difficult to ensure a sufficient amount of nitride. Based on these findings, we developed a parameter Fx1, expressed by the following formula (1), to increase the amount of nitride precipitation by controlling the amounts of Si, Mn, and Cr and adjusting the ratio of these components. We then first derived that the nitride area ratio can be roughly expressed using this parameter Fx1. To achieve a predetermined nitride area ratio, the parameter Fx1 must be 0.36 or greater, preferably 0.40 or greater. On the other hand, if the parameter Fx1 is large, the Si+Mn nitrides become coarse and may become the starting point of cracks, so the parameter Fx1 must be 0.83 or less, and preferably 0.75 or less. Fx1=30×{0.008×(Si%+Mn%)-0.003×(Cr%)} (1) In formula (1), Si%, Mn%, and Cr% represent the respective contents of Si, Mn, and Cr in the steel expressed in mass%.

[0018] However, to reliably achieve the specified nitride area ratio, further measures are required in addition to parameter Fx1. Further investigation revealed that, since the amount of nitrogen in the surface layer of carbonitrided steel significantly affects the nitride area ratio, it is advisable to control the amount of nitrogen in the surface layer as well. Specifically, the amount of nitrogen in the surface layer Ns should satisfy the following formula (2). Formula (2) is described in detail below. Surface layer nitrogen amount Ns (mass%) ≧0.43×e 0.96×Cr% (2) In formula (2), Cr% represents the Cr content in the steel expressed in mass%.

[0019] FIG. 2 is a graph showing the relationship between the surface layer nitrogen content Ns and the nitride area ratio, compiled using data from the Examples described later. In FIG. 2, ◆ indicates data for steel grades A to C and G, △ indicates data for steel grade E, and □ indicates data for steel grade D. Although it depends on the chemical composition of the steel, it has been found that the nitride area ratio can be increased by increasing the surface layer nitrogen content Ns. In the carbonitrided steels A to C and G having the chemical composition according to this embodiment, it has been found that the nitride area ratio can be sufficiently increased by increasing the surface layer nitrogen content Ns. Furthermore, as will be explained below, it has been found that the gradient in FIG. 2 can be organized by the Cr content in the steel. FIG. 3 is a graph showing the relationship between the Cr content in the steel and the surface layer nitrogen content Ns, compiled using data from the Examples described later, for each nitride area ratio (3.0%, 2.5%, 2.0%). From FIG. 3, it can be seen that to achieve a target nitride area ratio of 3.00% or more, the surface layer nitrogen content Ns (mass%) needs to be adjusted to 0.43×e depending on the Cr content. 0.96×Cr% (In this specification, the "0.43 × e" in formula (2) 0.96×Cr%" is sometimes referred to as "Fx2"). This graph also shows that, to achieve the same nitride area ratio, the greater the Cr content, the easier it is for Cr-based nitrides with smaller precipitate diameters to form, and the Si+Mn-based nitrides with larger precipitate diameters to relatively decrease, so the amount of nitrogen required in the surface layer also increases. In this way, in the present disclosure, by adjusting the steel composition to increase the amount of nitrogen in the surface layer and ensuring a sufficient nitride area ratio in the surface layer, it is possible to obtain a carbonitrided steel material with excellent surface spalling resistance. The surface layer nitrogen content Ns and the surface nitride area ratio are calculated by the method described in the Examples below.

[0020] In addition, the conventional technologies of Patent Documents 1 to 5 do not take into consideration the control of the nitride area ratio, and these conventional technologies have a low amount of nitrogen in the surface layer, and are unable to secure the amount of nitrogen in the surface layer necessary to ensure a sufficient amount of nitride, which is thought to result in low surface peeling resistance.

[0021] Next, the chemical composition of the carbonitrided steel will be described.

[0022] [Chemical composition of carbonitrided steel] [C: 0.10% by mass or more, 0.30% by mass or less] C is an element effective in ensuring the hardness of the steel material. If the C content is too low, the hardness will be insufficient, resulting in insufficient static strength for the carburized part. Therefore, the C content is set to 0.10 mass% or more, preferably 0.11 mass% or more, and more preferably 0.12 mass% or more. On the other hand, if it is contained in excess, cold forgeability will decrease. Therefore, the C content is set to 0.30 mass% or less, preferably 0.27 mass% or less, and more preferably 0.25 mass% or less.

[0023] [Si: 0.50 mass% or more, 2.00 mass% or less] Si is an element effective in improving temper softening characteristics. Furthermore, during carbonitriding, it bonds with N to form nitrides that are effective in reducing the friction coefficient. To effectively exert this effect, the Si content is set to 0.50% by mass or more, preferably 0.55% by mass or more, and more preferably 0.60% by mass or more. On the other hand, if Si is contained in excess, machinability during part manufacturing decreases, so the Si content is set to 2.00% by mass or less, preferably 1.90% by mass or less, and more preferably 1.80% by mass or less.

[0024] [Mn: 1.00 mass% or more, 2.00 mass% or less] Mn is an element necessary for improving hardenability during carburizing. Mn also acts as a deoxidizer, reducing the amount of oxide-based inclusions in steel and improving internal quality. Mn also bonds with N, along with Si, to form nitrides that are effective in reducing the friction coefficient. To effectively utilize these functions, the Mn content is set to 1.00% by mass or more. The Mn content is preferably 1.05% by mass or more, and more preferably 1.10% by mass or more. On the other hand, excessive Mn content reduces machinability during part manufacturing, so the Mn content is set to 2.00% by mass or less. The Mn content is preferably 1.90% by mass or less, and more preferably 1.80% by mass or less.

[0025] [Cr: 0.10 mass% or more, 0.60 mass% or less] Cr is an element that improves the hardenability of steel materials and ensures a stable hardened layer depth and sufficient core hardness, thereby effectively ensuring the static strength and fatigue strength of mechanical components. To effectively exert these effects, the Cr content is set to 0.10% by mass or more. The Cr content is preferably 0.15% by mass or more, and more preferably 0.20% by mass or more. On the other hand, if Cr is contained in excess, as described above, the amount of Cr-based nitrides with small precipitate diameters increases, making it difficult to ensure the amount of Si+Mn-based nitrides with relatively large precipitate diameters. This makes it difficult to achieve a certain nitride area ratio in the surface layer of the carbonitrided steel material. Furthermore, the increase in Cr-based nitrides reduces cold forgeability. Therefore, in this embodiment, the Cr content is limited to 0.60% by mass or less. The Cr content is preferably 0.55% by mass or less, and more preferably 0.50% by mass or less.

[0026] [N: 0.0010 mass% or more, 0.0500 mass% or less] N is an important element because it forms AlN, which is effective as pinning particles for suppressing grain coarsening. Therefore, the N content is set to 0.0010% by mass or more. The N content is preferably 0.0020% by mass or more, and more preferably 0.0030% by mass or more. On the other hand, excessive N content can cause cracks during casting, so the N content is set to 0.0500% by mass or less. The N content is preferably 0.0450% by mass or less, and more preferably 0.0400% by mass or less.

[0027] [Al: 0.010 mass% or more, 0.100 mass% or less] Al acts as a deoxidizer, reducing the amount of oxide-based inclusions and improving the internal quality of steel. To achieve this effect, it is preferable to add an appropriate amount of Al. To achieve this effect, the Al content is set to 0.010% by mass or more. The Al content is preferably 0.015% by mass or more, and more preferably 0.020% by mass or more. On the other hand, excessive Al content reduces machinability, so the Al content is set to 0.100% by mass or less. The Al content is preferably 0.090% by mass or less, and more preferably 0.080% by mass or less.

[0028] [P: More than 0 mass%, 0.100 mass% or less] P segregates at grain boundaries and reduces the impact properties of mechanical parts. Therefore, the P content is set to 0.100% by mass or less. The P content is preferably 0.095% by mass or less, and more preferably 0.090% by mass or less. On the other hand, since P is an element inevitably contained in steel and the higher the purity, the higher the production cost, so the lower limit of the P content can be about 0.001% by mass. The P content can further be 0.003% by mass or more, and even more preferably 0.005% by mass or more.

[0029] [S: More than 0 mass%, 0.100 mass% or less] S segregates at grain boundaries and reduces the impact properties of mechanical parts. Therefore, the S content is set to 0.100% by mass or less. The S content is preferably 0.095% by mass or less, and more preferably 0.090% by mass or less. On the other hand, since S is an element inevitably contained in steel and the higher the purity, the higher the production cost, so the lower limit of the S content can be about 0.001% by mass. The S content can further be 0.003% by mass or more, and even more preferably 0.005% by mass or more.

[0030] [Remainder: Fe and unavoidable impurities] In a preferred embodiment, the balance is Fe and unavoidable impurities. A trace element (e.g., As, Sb, Sn, etc.) introduced due to the conditions of raw materials, materials, manufacturing facilities, etc. is permitted as an unavoidable impurity. For example, there are elements such as P and S, whose content is usually the lower the better, and therefore they are unavoidable impurities, but whose composition ranges are separately specified as described above. Furthermore, the "unavoidable impurities" in the above "Fe and unavoidable impurities" may also include any element at the unavoidable impurity level described below. Mo: 0.05% by mass or less, B: less than 0.0005% by mass, Ti: less than 0.005% by mass, V: less than 0.005% by mass, Nb: less than 0.005% by mass, Cu: 0.01% by mass or less, Ni: 0.01% by mass or less, Sn: less than 0.001% by mass, Bi: less than 0.0001% by mass, Se: less than 0.0001% by mass, Ca: less than 0.0001% by mass, Mg: less than 0.0001% by mass, REM: Less than 0.0001% by mass

[0031] The chemical composition in this embodiment does not necessarily contain any of the optional elements described below. As long as the desired properties can be maintained, the optional elements described below may also be contained. By including the optional elements described below as necessary, the properties can be further improved.

[0032] [(a) One or more elements selected from the group consisting of Mo: more than 0.05 mass% and 1.00 mass% or less, and B: 0.0005 mass% or more and 0.0100 mass% or less]

[0033] Mo is an element that is effective in suppressing the formation of a soft, incompletely hardened structure after carburizing and increasing softening resistance. To achieve this effect, the Mo content is preferably more than 0.05 mass%, more preferably 0.08 mass% or more, and even more preferably 0.10 mass% or more. However, since an excessive Mo content leads to a decrease in workability and an increase in steel costs, the Mo content is preferably 1.00 mass% or less, more preferably 0.90 mass% or less, and even more preferably 0.80 mass% or less.

[0034] B is an element effective in improving hardenability. To achieve this effect, the B content is preferably 0.0005% by mass or more, more preferably 0.0006% by mass or more, and even more preferably 0.0007% by mass or more. However, even if B is contained in excess, the effect saturates, so the B content is preferably 0.0100% by mass or less, more preferably 0.0090% by mass or less, and even more preferably 0.0080% by mass or less.

[0035] [(b) one or more elements selected from the group consisting of Ti: 0.005% by mass or more and 0.1000% by mass or less, V: 0.005% by mass or more and 0.500% by mass or less, and Nb: 0.005% by mass or more and 0.100% by mass or less]

[0036] Ti may be added because it combines with C and N in the steel to form carbonitrides, which are effective in preventing grain coarsening during carburizing. To achieve this effect, the Ti content is preferably 0.005% by mass or more. The Ti content is more preferably 0.008% by mass or more, and even more preferably 0.010% by mass or more. On the other hand, an excessive Ti content increases costs and causes the precipitation of coarse TiN, which reduces machinability, so the Ti content is preferably 0.1000% by mass or less. The Ti content is more preferably 0.0900% by mass or less, and even more preferably 0.0800% by mass or less.

[0037] V may be added because it combines with C and N in the steel to form carbonitrides, which are effective in preventing grain coarsening during carburization. To achieve this effect, the V content is preferably 0.005% by mass or more. The V content is more preferably 0.008% by mass or more, and even more preferably 0.010% by mass or more. However, since an excessive V content saturates the grain coarsening prevention property and leads to an increase in steel costs, the V content is preferably 0.500% by mass or less. The V content is more preferably 0.490% by mass or less, and even more preferably 0.480% by mass or less.

[0038] Nb may be added because it combines with C and N in the steel to form carbonitrides and is effective in preventing grain coarsening during carburization. To achieve this effect, the Nb content is preferably 0.005% by mass or more. The Nb content is more preferably 0.008% by mass or more, and even more preferably 0.010% by mass or more. However, even if Nb is contained in excess, the grain coarsening prevention property saturates and the steel cost increases, so the Nb content is preferably 0.100% by mass or less. The Nb content is more preferably 0.090% by mass or less, and even more preferably 0.080% by mass or less.

[0039] [(c) one or more elements selected from the group consisting of Cu: more than 0.01% by mass and 2.00% by mass or less, and Ni: more than 0.01% by mass and 2.00% by mass or less] Both Cu and Ni are effective elements for improving the hardenability of steel, and therefore may be added. To achieve the above effect, the Cu and Ni contents are preferably more than 0.01 mass%. However, in both cases, excessive amounts of Cu and Ni increase the cost of the steel. Therefore, the Cu content is preferably 2.00 mass% or less, more preferably 1.70 mass% or less, and even more preferably 1.50 mass% or less. The Ni content is preferably 2.00 mass% or less, more preferably 1.90 mass% or less, and even more preferably 1.80 mass% or less.

[0040] [(d)Sn: more than 0 mass%, 0.100 mass% or less] Sn segregates at grain boundaries and reduces the impact properties of mechanical components. Therefore, the Sn content is preferably limited to 0.100% by mass or less, more preferably 0.095% by mass or less, and even more preferably 0.090% by mass or less. The Sn content may be 0% by mass, but since Sn is an element that is inevitably contained in steel and the higher the purity, the higher the production cost. Therefore, the lower limit of the Sn content may be greater than 0% by mass, and the lower limit of the Sn content may even be about 0.001% by mass. The Sn content may even be 0.003% by mass or more, or even 0.005% by mass or more.

[0041] [(e) one or more elements selected from the group consisting of Bi: 0.0001% by mass or more and 0.0100% by mass or less, Se: 0.0001% by mass or more and 0.0100% by mass or less, Ca: 0.0001% by mass or more and 0.0100% by mass or less, Mg: 0.0001% by mass or more and 0.0100% by mass or less, and REM: 0.0001% by mass or more and 0.0100% by mass or less] Since Bi, Se, Ca, Mg, and REM are all elements that improve machinability, increasing their content can improve machinability. To achieve this effect, the Bi content is preferably 0.0001% by mass or more, more preferably 0.0002% by mass or more, and even more preferably 0.0005% by mass or more. The Se content is preferably 0.0001% by mass or more, more preferably 0.0002% by mass or more, and even more preferably 0.0005% by mass or more. The Ca content is preferably 0.0001% by mass or more, more preferably 0.0002% by mass or more, and even more preferably 0.0005% by mass or more. The Mg content is preferably 0.0001% by mass or more, more preferably 0.0002% by mass or more, and even more preferably 0.0005% by mass or more. In the case of REM, the content is preferably 0.0001% by mass or more, more preferably 0.0002% by mass or more, and even more preferably 0.0005% by mass or more.

[0042] However, excessive content of these elements may result in the formation of coarse oxides, resulting in a deterioration in impact properties. Therefore, in the case of Bi, the content is preferably 0.0100% by mass or less, more preferably 0.0090% by mass or less, and even more preferably 0.0080% by mass or less. In the case of Se, the content is preferably 0.0100% by mass or less, more preferably 0.0090% by mass or less, and even more preferably 0.0080% by mass or less. In the case of Ca, the content is preferably 0.0100% by mass or less, more preferably 0.0090% by mass or less, and even more preferably 0.0080% by mass or less. In the case of Mg, the content is preferably 0.0100% by mass or less, more preferably 0.0090% by mass or less, and even more preferably 0.0080% by mass or less. In the case of REM, the content is preferably 0.0100% by mass or less, more preferably 0.0090% by mass or less, and even more preferably 0.0080% by mass or less. In the present invention, REM includes lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[0043] The carbonitrided steel material according to this embodiment may be any steel material that has undergone carbonitriding, and includes, for example, carbonitrided steel parts obtained by performing rough forming, carbonitriding, and then finish processing. Specific examples of carbonitrided steel parts include parts that are used after surface hardening, particularly power transmission parts such as gears, shafts, and other constant velocity joint parts, bearings, and continuously variable transmission (CVT) pulleys.

[0044] [Characteristics of carbonitrided steel] The carbonitrided steel material according to this embodiment has excellent resistance to surface spalling. In particular, it has high hardness and excellent resistance to surface spalling. These properties of the carbonitrided steel material according to this embodiment will be described in detail below.

[0045] (1) Hardness The carbonitrided steel material according to this embodiment preferably has a Vickers hardness of 700 HV or more at a depth of 0.025 mm from the outermost surface, more preferably 720 HV or more, and even more preferably 750 HV or more.

[0046] (2) Friction coefficient The carbonitrided steel material according to this embodiment has a small friction coefficient of 2.40 or less, as determined by the method described in the examples below, and has excellent resistance to surface spalling. The friction coefficient is preferably 2.20 or less, more preferably 2.00 or less, and even more preferably 1.60 or less.

[0047] [Method for manufacturing carbonitrided steel] The conditions for producing the steel material for carbonitriding to be subjected to carbonitriding treatment are not particularly limited, and it may be produced under conventional conditions. For example, it can be obtained by melting in a converter or the like, casting to obtain a slab having the above-mentioned chemical composition, and then performing hot rolling such as blooming or bar rolling. For example, the cast slab may be heated and held at 1100 to 1300°C for 30 minutes to 5 hours, followed by blooming. After blooming, it may be cooled to a temperature below the A1 point at an average cooling rate of 0.01 to 5°C / second. It may then be rolled into a bar while still heated to 800 to 1100°C. For example, it may be possible to obtain a wire rod or a steel bar having a diameter of 20 to 80 mm.

[0048] The steel material for carbonitriding may be one that has been subjected to rough forming by one or more methods selected from the group consisting of cutting, cold forging, and hot forging in accordance with a conventional method after the above-mentioned rolling, and may also be one that has been subjected to annealing treatment, solution treatment, and normalizing treatment in accordance with conventional methods as necessary before carbonitriding treatment.

[0049] The steel material for carbonitriding, such as the steel bar, is subjected to a carbonitriding treatment. In order to ensure that the surface nitrogen content of the carbonitrided steel material of this embodiment is at least a certain level, the carbonitriding period is extended in the production of the carbonitrided steel material. While the carbonitriding period has conventionally been typically 120 minutes, in this embodiment it is longer, exceeding 120 minutes, preferably 200 minutes or more, and more preferably 300 minutes or more. The carbonitriding period may be 500 minutes or more. However, if the carbonitriding period is too long, grain boundary oxidation increases, leading to a decrease in fatigue strength, and therefore the upper limit of the carbonitriding period is approximately 900 minutes.

[0050] Furthermore, the amount of NH3 supplied during the carbonitriding period must be relatively high to ensure a certain level of nitrogen in the surface layer of the carbonitrided steel. The amount of NH3 supplied is 10% or more, preferably 12% or more. However, since poor carburization may occur if the amount of NH3 supplied is too high, the upper limit of the amount of NH3 supplied is approximately 30%. The amount of NH3 supplied during the carbonitriding process can be adjusted by changing the ammonia flow rate in the ammonia-containing propane gas atmosphere.

[0051] The carbon potential Cp during carbonitriding can be, for example, 0.5% or more and 1.0% or less. The temperature during carbonitriding can be, for example, 800 to 900°C. The carbonitriding method is not particularly limited, and known methods such as gas carbonitriding and vacuum carbonitriding can be used. The degree of vacuum during vacuum carbonitriding can be, for example, about 0.01 MPa or less.

[0052] In the production of carbonitrided steel, there are no particular limitations on the procedures other than the carbonitriding treatment. As performed in the examples described below, carburizing treatment may be performed before the carbonitriding treatment. For example, the carburizing treatment may be performed by setting the carbon potential Cp to 0.5 to 1.0 mass % and holding the temperature at 850 to 1000°C for 30 minutes to 3 hours, followed by the carbonitriding treatment. The carburizing treatment may be performed in two or more stages.

[0053] Furthermore, in obtaining the carbonitrided steel material of this embodiment, after the carbonitriding treatment, quenching such as oil quenching and subsequent tempering may be performed, and if necessary, finishing treatment such as polishing, lubricating coating treatment, or shot peening may be performed in accordance with a conventional method. [Example]

[0054] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can be practiced with appropriate modifications within the scope of the above-mentioned and below-mentioned aims, and all such modifications are included in the technical scope of the present invention.

[0055] 1. Preparation of steel for carbonitriding Steels having the chemical compositions shown in Table 1 were melted using a VIF, and then ingots were obtained using the ingot method. In Table 1, a line (-) indicates that the content is at the unavoidable impurity level described above. Note that 0.01% by mass of Cu and Ni shown in Table 1 are unavoidable impurities. The "unavoidable impurities" shown in Table 1 may also include optional elements at the unavoidable impurity level described above. Note that in Tables 1 and 2, underlined values ​​indicate values ​​outside the range of the present disclosure.

[0056] The resulting ingot was heated at 1200-1300°C for 60 minutes or more, then hot rolled or hot forged to simulate blooming, and cooled to a temperature below the A1 point at an average cooling rate of 0.01-5°C / s. The ingot was then heated to 800-1000°C in an atmospheric furnace to simulate the thermal history of bar rolling, yielding a steel bar with a diameter of 32 mm.

[0057] [Table 1]

[0058] 2-1. Measurement of surface nitrogen content (1) Sample preparation From the 32 mm diameter steel bar, samples were prepared with the same shape (Fig. 4) as the small roller test pieces used in the roller pitting test described below. Specifically, the bars were roughly machined on a lathe to a size slightly larger than the specified diameter shown in Fig. 4. Next, carbonitriding was performed using the heat treatment pattern shown in Fig. 5. In order to obtain samples with various precipitate amounts, the carbonitriding time was varied within the range of 120 to 600 minutes. The bars were then tempered (conditions: heated at 170°C for 2 hours, then allowed to cool), and then finish-machined to the size shown in Fig. 4 to obtain the samples.

[0059] The test specimen was cut so that a cross section, which was a cross section perpendicular to the longitudinal direction of the obtained test specimen, could be observed. Then, using an EPMA (electron beam microanalyzer, manufacturer: JEOL Ltd., serial number: ISP100), the N content (mass%) was measured from the outermost surface to 0.035 mm from the outermost surface toward the center of the cross section at a measurement pitch of 0.005 mm (5 μm) under the condition of an acceleration voltage of 15 kV. The average value of five points from 0.015 to 0.035 mm from the outermost surface was taken as the surface nitrogen content (mass%).

[0060] 2-2.Evaluation of surface deposits The test pieces were prepared in the same manner as in the measurement of the surface nitrogen content, and the test pieces were cut so that the cross section, which is a cross section perpendicular to the longitudinal direction, could be observed. The cross section was then corroded, and a position 0.025 mm from the outermost surface toward the center of the cross section was observed using an SEM, and image analysis was performed. All granular precipitates observed within the observation field were determined to be carbonitrides. Detailed evaluation conditions are as follows: (Evaluation conditions) Precipitates to be evaluated: Precipitates with a circle equivalent diameter of 0.01 to 2.00 μm N number: 1 (3 images were taken at random, and the amount of precipitate was calculated for the image that was considered to be in the most average position.) SEM equipment: JEOL JXA-ISP100 electron microscope analyzer ·Magnification: 10,000x (Field of view: 106μm 2 ) Polishing: OPS (Oxide Polishing Suspensions), manufactured by Struers Image analysis software: "Particle Analysis Ver. 3.0" by Sumitomo Metal Technology Co., Ltd.

[0061] 2-3. Hardness measurement The test specimens were prepared in the same manner as in the measurement of the surface nitrogen content, and the test specimens were cut so that the cross section, which is a cross section perpendicular to the longitudinal direction, could be observed. The Vickers hardness was measured at a position 0.025 mm from the outermost surface toward the center of the cross section. Three different N numbers were measured in the circumferential direction, and the average value of the three points was calculated. A Vickers hardness of 700 HV or more was evaluated as high hardness. The results are shown in Table 2.

[0062] 2-4. Roller pitching test (1) Preparation of specimens for roller pitting test The small roller test pieces shown in Figure 4 were prepared from the steel bars with a diameter of 32 mm. Specifically, they were roughly machined on a lathe to a size slightly larger than the specified diameter shown in Figure 4. Next, as described above, they were carbonitrided using the heat treatment pattern shown in Figure 5. In order to obtain samples with various amounts of precipitates, the carbonitriding time was varied within the range of 120 to 600 minutes. After that, they were tempered (conditions: heated at 170°C for 2 hours, then allowed to cool), and then finish-machined to the size shown in Figure 4.

[0063] The mating material used in the roller pitting test was obtained by processing a tempered high-carbon chromium bearing steel SUJ2 into the large roller shown in Figure 6. A small roller and a large roller were prepared for each example, and the roller pitting test was carried out under the test conditions listed below. Figure 7 shows the appearance during the test, with the small roller test piece 1 and the large roller 2 in contact and rolling while sliding. 3 in Figure 7 indicates the sliding part.

[0064] (Roller pitching test conditions) Roller pitching tester: Komatsu type roller pitching tester Test conditions: surface pressure 1.4GPa, rotation speed 1000rpm, sliding speed 9.5m / s, oil temperature 100℃, oil flow rate 2L / min

[0065] In the roller pitting test, the test was performed at 2000 revolutions per minute, and the highest torque value Td was measured. The torque Tx used to calculate the coefficient of friction was then calculated using the following formula (3). The obtained torque Tx was used to calculate the coefficient of friction using the following formula (4). A friction coefficient of 2.40 or less was evaluated as having excellent surface peeling resistance. The results are shown in Table 2. Torque Tx = Maximum torque Td - No-load torque T0 (3) In equation (3), the no-load torque T0 indicates the torque value when the test pieces are not in contact with each other (no load). Friction coefficient = Torque Tx / Test piece radius / Applied load (4)

[0066] [Table 2]

[0067] The following can be seen from Tables 1 and 2. Steel materials Nos. 2 to 6 are inventive examples that satisfy all of the requirements for the embodiments of the present invention. That is, each component and Fx1 calculated by formula (1) are within a predetermined range, the surface layer nitrogen content Ns satisfies formula (2), and further the area ratio of surface layer nitrides is 3.00% or more, so that the surface hardness is high, the friction coefficient is low, and excellent surface spalling resistance is exhibited.

[0068] In contrast, steel Nos. 1 and 7 to 11 did not satisfy any of the requirements of the embodiment of the present invention, resulting in poor surface spalling resistance. Specifically, steel No. 1 had a short carbonitriding treatment time, the surface layer nitrogen content Ns did not satisfy formula (2), and the nitride area ratio was insufficient. As a result, the friction coefficient was high.

[0069] Steel No. 7 had an insufficient Mn content and an excessive Cr content, and the carbonitriding treatment time was short, so Fx1 fell below the specified range, and the surface layer nitrogen content Ns did not satisfy formula (2), resulting in an insufficient nitride area ratio, which resulted in a high friction coefficient.

[0070] Steel No. 8 had an insufficient Mn content and an excessive Cr content, so Fx1 was below the specified range, and the surface layer nitrogen content Ns did not satisfy formula (2), resulting in an insufficient nitride area ratio, which resulted in a high friction coefficient.

[0071] Steel No. 9 had insufficient Si and Mn contents and excessive Cr content, and the carbonitriding treatment time was short, so Fx1 fell below the specified range, and the surface layer nitrogen content Ns did not satisfy formula (2), resulting in an insufficient nitride area ratio, which resulted in a high friction coefficient.

[0072] Steel No. 10 had insufficient Si and Mn contents and excessive Cr content, so Fx1 was below the specified range, and the surface layer nitrogen content Ns did not satisfy formula (2), resulting in an insufficient nitride area ratio, which resulted in a high friction coefficient.

[0073] Steel No. 11 corresponds to the conventional SCr420H standard steel. This steel No. 11 had insufficient Si and Mn contents and an excessive Cr content. Furthermore, it was only carburized without carbonitriding. As a result, Fx1 fell below the specified range, and the surface layer nitrogen content Ns did not satisfy formula (2), resulting in an insufficient nitride area ratio. As a result, the friction coefficient was significantly increased.

Claims

1. The chemical composition is C: 0.10% by mass or more, 0.30% by mass or less, Si: 0.50 mass% or more, 2.00 mass% or less, Mn: 1.00% by mass or more, 2.00% by mass or less, Cr: 0.10% by mass or more, 0.60% by mass or less, N: 0.0010% by mass or more, 0.0500% by mass or less, Al: 0.010% by mass or more, 0.100% by mass or less, P: more than 0% by mass and 0.100% by mass or less, and S: More than 0% by mass, 0.100% by mass or less Including, A carbonitrided steel material having the balance being Fe and unavoidable impurities, Fx1 calculated by the following formula (1) is 0.36 or more and 0.83 or less, The surface layer nitrogen content Ns satisfies the following formula (2), A carbonitrided steel material having a nitride area ratio of the surface layer of 3.00% or more. Fx1=30×{0.008×(Si%+Mn%)-0.003×(Cr%)} (1) Ns (mass %) ≥ 0.43 × e ０．９６×Ｃｒ％ (2) In formulas (1) and (2), Si%, Mn%, and Cr% represent the contents of Si, Mn, and Cr in the steel, respectively, expressed in mass%.

2. 2. The carbonitrided steel material according to claim 1, wherein the chemical composition satisfies one or more of the following (a) to (e): (a) The alloy further contains one or more elements selected from the group consisting of Mo: more than 0.05 mass % and 1.00 mass % or less, and B: 0.0005 mass % or more and 0.0100 mass % or less. (b) Further containing one or more elements selected from the group consisting of Ti: 0.005% by mass or more and 0.1000% by mass or less, V: 0.005% by mass or more and 0.500% by mass or less, and Nb: 0.005% by mass or more and 0.100% by mass or less. (c) Further containing one or more elements selected from the group consisting of Cu: more than 0.01% by mass and not more than 2.00% by mass, and Ni: more than 0.01% by mass and not more than 2.00% by mass. (d) Sn: more than 0 mass % and 0.100 mass % or less. (e) Further containing one or more elements selected from the group consisting of Bi: 0.0001% by mass or more and 0.0100% by mass or less, Se: 0.0001% by mass or more and 0.0100% by mass or less, Ca: 0.0001% by mass or more and 0.0100% by mass or less, Mg: 0.0001% by mass or more and 0.0100% by mass or less, and REM: 0.0001% by mass or more and 0.0100% by mass or less.

3. 3. The carbonitrided steel material according to claim 1, wherein the Vickers hardness at a depth of 0.025 mm from the outermost surface is 700 HV or more.

Images