Carburitizing-nitriding steel and carburitizing-nitriding steel

Abstract

By carburizing nitriding treatment with steel, can provide high surface fatigue strength and high temperature tempering hardness is sufficient high carburizing nitriding treatment with steel and carburizing nitriding steel made of carburizing nitriding treatment, the carburizing nitriding treatment with steel contains C, Si, Mn, P, S, Cu, Ni, Cr, Mo, Al, N, the balance is made of Fe and inevitable impurities, by implementing carburizing nitriding, thereby obtaining carburizing nitriding steel, the carburizing nitriding steel satisfies surface C concentration (mass %) + 12 / 14 x surface N concentration (mass %) = 0.6-1.4, and satisfies 129.7805 x [Cr (mass %)]-76.9797 x [Cr (mass %)] 2 + 339.3375 x [surface N concentration (mass %)]-539.345 x [surface N concentration (mass %)] 2 + 181.4983 x [Cr (mass %)] x [surface N concentration (mass %)]+437.6799>560, and in the case of implementing tempering treatment at 500 DEG C after the carburizing nitriding, the hardness of the part of 0.05 mm depth from the surface is 560 HV or more.

Description

Technical Field

[0001] This invention relates to steel for carburizing and nitriding treatment, and carburized and nitrided steel. Background Technology

[0002] For example, steel components such as gears and shafts in automobiles or industrial machinery require high surface fatigue strength. Therefore, carburized and nitrided components have traditionally been used for these components. It is well known that improving surface fatigue strength by carburizing and nitriding steel is a common practice.

[0003] As existing methods in this regard, for example, the methods described in Patent Documents 1 to 5 can be listed.

[0004] For example, Patent Documents 1 and 2 describe a carburized and nitrided steel component, characterized in that the blank is a steel with a specific composition, wherein in a region from the surface to a depth of 0.1 mm, the average C concentration (Cs) is 0.60–0.90%, the average N concentration (Ns) is 0.15–0.35%, and the Cs+Ns concentration is 0.80–1.10%, and each component satisfies a specific relationship. Furthermore, it is described that, according to such a carburized and nitrided component, it is possible to provide a carburized and nitrided steel component with significantly superior bending fatigue strength and surface fatigue strength compared to that manufactured by carburizing and quenching, which is the most representative surface hardening treatment, without significantly increasing costs, and which can meet the requirements of lightweight, miniaturized, and high-stress-load-bearing components.

[0005] Furthermore, Patent Document 3 discloses a carburizing and nitriding steel, characterized by being composed of a specific composition, with each component satisfying a specific relationship. It also describes that such a carburizing and nitriding steel provides excellent machinability for component shapes and excellent pitting life even without the grinding process following carburizing and nitriding treatment, as well as components using this steel.

[0006] Furthermore, Patent Document 4 describes a steel component characterized by a substrate being a steel material with a specific composition. In a region extending from the surface to a depth of 5 μm, the porosity ratio is less than 10%, and in a region extending from the surface to a depth of 100 μm, the average C concentration (Cave) is 0.005–0.80%, the average N concentration (Nave) is 0.30–0.70%, and the sum of Cave and Nave is 0.50–1.40%. It also states that such a steel component can provide excellent surface fatigue strength and wear resistance, making it suitable for use in automotive or industrial machinery components such as gears, crankshafts, and camshafts.

[0007] Furthermore, Patent Document 5 discloses a carburizing and nitriding component comprising: a surface portion having a flat portion and an edge portion, and a core portion further inward than the surface portion. The core portion has a specific composition: a carbon concentration CP1 of 0.70–0.89% and a nitrogen concentration of 0.10–0.80% in the region from the flat portion to a depth of 0.05 mm; a carbon concentration CP2 higher than the carbon concentration CP1 but less than 1.20% in the region from the edge portion to a depth of 0.05 mm; a Vickers hardness of HV650 or higher at a position 0.3 mm from the flat portion; a grain boundary oxide layer depth of less than 3.0 μm in the surface portion; and a Vickers hardness of HV260 or higher in the core portion. It is also disclosed that, according to such a carburizing and nitriding component, a carburizing and nitriding component with a surface having a flat portion and an edge portion, and excellent flexural fatigue strength and pitting resistance can be provided.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2010-70827

[0011] Patent Document 2: Japanese Patent Application Publication No. 2010-70831

[0012] Patent Document 3: Japanese Patent Application Publication No. 2016-186120

[0013] Patent Document 4: Japanese Patent Application Publication No. 2017-171951

[0014] Patent Document 5: Japanese Patent Application Publication No. 2017-171970 Summary of the Invention

[0015] The problem that the invention aims to solve

[0016] In recent years, automobiles have become increasingly electric, which means that the sliding temperature of some components may become higher than before (e.g., around 300-500°C). Therefore, some components require sufficient hardness even when exposed to high temperatures, i.e., high-temperature tempering hardness is required.

[0017] On the other hand, the inventors discovered that the formation of CrN clusters contributes significantly to the improvement of surface fatigue strength during carburizing and nitriding. Therefore, to fully enhance surface fatigue strength, a certain amount of Cr needs to be added. Specifically, it was found that adding N before 300°C and adding both Cr and N in the high-temperature region above 300°C is effective.

[0018] Furthermore, the inventors discovered that, depending on the amount of Cr and N added, the tempering hardness at high temperatures (around 300–500°C) decreases.

[0019] That is, it was found that it is difficult to improve both surface fatigue strength and high-temperature tempering hardness; they are in a compromise relationship.

[0020] The purpose of this invention is to solve the above-mentioned problems.

[0021] That is, the purpose of this invention is to provide carburizing and nitriding steel with high surface fatigue strength and sufficient high hardness after high temperature tempering, as well as carburizing and nitriding steel obtained by carburizing and nitriding.

[0022] Methods for solving problems

[0023] In order to solve the above-mentioned problems, the inventors conducted in-depth research and completed this invention.

[0024] The present invention is as follows (1) to (7).

[0025] (1) A steel for carburizing and nitriding treatment, comprising:

[0026] C: 0.1–0.3% by mass

[0027] Si: less than 0.3% by mass

[0028] Mn: 0.4–2.0% by mass

[0029] P: less than 0.03% by mass

[0030] S: less than 0.03% by mass

[0031] Cu: less than 0.3% by mass

[0032] Ni: less than 2.5% by mass

[0033] Cr: 0.5–3.0% by mass

[0034] Mo: 0.001–1.0% by mass

[0035] Al: 0.01–0.08% by mass

[0036] N: 0.005–0.03% by mass

[0037] The balance consists of Fe and unavoidable impurities.

[0038] Carburizing and nitriding are performed to obtain carburized and nitrided steel, which satisfies Equation 1: Surface C concentration (mass%) + 12 / 14 × Surface N concentration (mass%) = 0.6~1.4, and

[0039] Equation 2 is satisfied: 129.7805 × [Cr (mass%)] - 76.9797 × [Cr (mass%)] 2+339.3375 × [Surface N concentration (mass%)] - 539.345 × [Surface N concentration (mass%)] 2 +181.4983×[Cr(mass%)]×[surface N concentration(mass%)]+437.6799>560, and

[0040] When the carburizing and nitriding process is followed by tempering at 500°C, the hardness of the portion at a depth of 0.05 mm from the surface is 560 HV or higher.

[0041] (2) The carburizing and nitriding steel according to (1) above further comprises at least one selected from the group consisting of:

[0042] Nb: 0.001~0.08% by mass

[0043] V: less than 0.5% by mass

[0044] Ti: less than 0.05% by mass, and

[0045] B: 0.0005~0.003% by mass.

[0046] (3) The carburizing and nitriding steel according to (1) or (2) above, comprising:

[0047] C: 0.15–0.25% by mass

[0048] Si: 0.01–0.24% by mass

[0049] Mn: 0.5–1.8% by mass

[0050] Cu: 0.001–0.3% by mass

[0051] Ni: 0.01–0.6% by mass

[0052] Cr: 0.6–1.8% by mass

[0053] Mo: 0.01–0.8% by mass

[0054] Al: 0.02–0.05% by mass

[0055] N: 0.01–0.025% by mass

[0056] Further includes at least one selected from the group consisting of:

[0057] Nb: 0.0015–0.06% by mass

[0058] V: less than 0.25% by mass

[0059] Ti: 0.012–0.04% by mass, and

[0060] B: 0.0006~0.0025% by mass

[0061] The balance consists of Fe and unavoidable impurities.

[0062] By performing the aforementioned carburizing and nitriding, carburized and nitrided steel is obtained that satisfies Equation 1: Surface C concentration (mass%) + 12 / 14 × Surface N concentration (mass%) = 0.7 to 1.2.

[0063] (4) The carburizing and nitriding steel according to any one of (1) to (3) above contains Cu: 0.03 to 0.3% by mass.

[0064] (5) The carburizing and nitriding steel according to any one of (1) to (4) above contains Si: 0.01 to 0.17% by mass.

[0065] (6) The carburizing and nitriding steel according to any one of (1) to (5) above contains Cu: 0.03 to 0.25% by mass.

[0066] (7) A carburizing and nitriding steel, which is obtained by carburizing and nitriding the steel for carburizing and nitriding treatment described in any one of (1) to (6) above.

[0067] The carburized and nitrided steel satisfies Equation 1: Surface C concentration (mass%) + 12 / 14 × Surface N concentration (mass%) = 0.6~1.4, and

[0068] Equation 2 is satisfied: 129.7805 × [Cr (mass%)] - 76.9797 × [Cr (mass%)] 2 +339.3375 × [Surface N concentration (mass%)] - 539.345 × [Surface N concentration (mass%)] 2 +181.4983×[Cr(mass%)]×[surface N concentration(mass%)]+437.6799>560, and

[0069] When further tempered at 500°C, the hardness of the portion 0.05 mm deep from the surface becomes 560 HV or higher.

[0070] The effects of the invention

[0071] According to the present invention, carburizing and nitriding steel with high surface fatigue strength and sufficiently high hardness after high-temperature tempering can be provided, as well as carburizing and nitriding steel obtained by carburizing and nitriding. Attached Figure Description

[0072] [ Figure 1[Illustrated side view of the small roller used in the embodiment]

[0073] [ Figure 2 [1] is a graph showing the relationship between 500°C tempering hardness and fatigue strength-life ratio in the examples and comparative examples.

[0074] [ Figure 3 [1] is a graph showing the relationship between the 500°C tempering hardness and the sintering ultimate load ratio in the examples and comparative examples. Detailed Implementation

[0075] The present invention will now be described.

[0076] The carburizing and nitriding steel of the present invention contains C: 0.1-0.3% by mass, Si: less than 0.3% by mass, Mn: 0.4-2.0% by mass, P: less than 0.03% by mass, S: less than 0.03% by mass, Cu: less than 0.3% by mass, Ni: less than 2.5% by mass, Cr: 0.5-3.0% by mass, Mo: 0.001-1.0% by mass, Al: 0.01-0.08% by mass, and N: 0.005-0.03% by mass, with the balance consisting of Fe and unavoidable impurities. By performing carburizing and nitriding, carburized and nitrided steel is obtained. The carburized and nitrided steel satisfies Formula 1: Surface C concentration (by mass) + 12 / 14 × Surface N concentration (by mass) = 0.6-1.4, and Formula 2: 129.7805 × [Cr (by mass)] - 76.9797 × [Cr (by mass)] 2 +339.3375 × [Surface N concentration (mass%)] - 539.345 × [Surface N concentration (mass%)] 2 +181.4983×[Cr(mass%)]×[surface N concentration(mass%)]+437.6799>560, and when tempering is performed at 500°C after the aforementioned carburizing and nitriding, the hardness of the portion at a depth of 0.05 mm from the surface is 560 HV or higher.

[0077] Furthermore, the carburizing and nitriding steel of the present invention is obtained by carburizing and nitriding the steel for carburizing and nitriding treatment of the present invention. The carburizing and nitriding steel satisfies Formula 1: Surface C concentration (mass%) + 12 / 14 × Surface N concentration (mass%) = 0.6~1.4, and also satisfies Formula 2: 129.7805 × [Cr (mass%)] - 76.9797 × [Cr (mass%)]. 2 +339.3375 × [Surface N concentration (mass%)] - 539.345 × [Surface N concentration (mass%)] 2+181.4983×[Cr(mass%)]×[surface N concentration(mass%)]+437.6799>560, and when further tempered at 500°C, the hardness of the portion 0.05mm deep from the surface becomes 560HV or higher.

[0078] The composition of the steel used for carburizing and nitriding according to the present invention will be described.

[0079] <c>

[0080] The carbon content in the steel used for carburizing and nitriding treatment of the present invention is 0.1 to 0.3% by mass, preferably 0.15 to 0.25% by mass.

[0081] At this C content, the hardenability of the carburizing and nitriding steel of the present invention is improved, and the hardness of the surface and core is ensured. When the C content is too high, toughness and hot workability may decrease.

[0082] <si>

[0083] The Si content in the carburizing and nitriding steel of the present invention is 0.3% by mass or less, preferably 0.01 to 0.24% by mass, more preferably 0.01 to 0.17% by mass, and even more preferably 0.01 to 0.16% by mass.

[0084] When the Si content is such, the precipitation of nitrides (Si3N4, SiMnN2, etc.) in the steel for carburizing and nitriding treatment of the present invention can be suppressed, and the reduction of fatigue strength can be prevented.

[0085]

[0086] The Mn content in the steel used for carburizing and nitriding of the present invention is 0.4 to 2.0% by mass, preferably 0.5 to 2.0% by mass, more preferably 0.53 to 2.0% by mass, even more preferably 0.5 to 1.8% by mass, even more preferably 0.5 to 1.4% by mass, and even more preferably 0.53 to 1.5% by mass.

[0087] At this Mn content, the hardenability is improved, the core hardness is increased, and the fatigue strength is enhanced. Furthermore, machinability, hardenability, and manufacturability become better. When the Mn content is too low, the improved hardenability effect is not achieved. Conversely, when the Mn content is too high, manufacturability may be impaired.

[0088]

[0089] The P content in the carburizing and nitriding steel of the present invention is 0.03% by mass or less, preferably 0.020% by mass or less. P is an impurity contained in steel, which segregates at grain boundaries and causes embrittlement of the steel. In particular, when its content exceeds 0.030% by mass, the degree of embrittlement can sometimes become significant. Therefore, the P content in the carburizing and nitriding steel of the present invention is 0.03% by mass or less.

[0090] <s>

[0091] The sulfur content in the steel used for carburizing and nitriding treatment of the present invention is 0.03% by mass or less, preferably 0.020% by mass or less.

[0092] At this S content, it has the effect of forming MnS and improving machinability. On the other hand, when the S content exceeds 0.030% by mass, coarse MnS is formed, and hot forging performance and bending fatigue strength tend to decrease. Therefore, 0.005 to 0.030% by mass is preferred. When hot forging performance and bending fatigue strength are of greater importance, the S content is preferably 0.020% by mass or less.

[0093] <cu>

[0094] The Cu content in the steel used for carburizing and nitriding treatment of the present invention is 0.3% by mass or less, preferably 0.001 to 0.3% by mass, more preferably 0.03 to 0.3% by mass, and even more preferably 0.03 to 0.25% by mass.

[0095] At this Cu content, carbide formation is suppressed, and hardenability is improved. However, if the Cu content is too high, hot workability may decrease.

[0096] <ni>

[0097] The Ni content in the steel used for carburizing and nitriding treatment of the present invention is 2.5% by mass or less, preferably 0.01 to 0.6% by mass, and more preferably 0.05 to 0.6% by mass.

[0098] At this Ni content, hardenability and toughness are improved. Furthermore, Ni is a non-oxidizing element, which allows for surface strengthening and toughening of steel during carburizing without increasing the depth of the grain boundary oxide layer.

[0099] <cr>

[0100] The Cr content in the steel used for carburizing and nitriding treatment of the present invention is 0.5 to 3.0% by mass, preferably 0.5 to 2.5% by mass, and more preferably 0.6 to 1.8% by mass.

[0101] At this Cr content, hardenability is improved, machinability is ensured, pitting fatigue strength is increased, and toughness is also improved.

[0102] When the Cr content is too high, the hardness increases and the machinability decreases. In addition, due to the formation of coarse Cr carbides during carburizing, and the formation of coarse CrN along the grain boundaries during carburizing and nitriding, the flexural strength may decrease.

[0103]

[0104] The Mo content in the steel used for carburizing and nitriding of the present invention is 0.001 to 1.0% by mass, preferably 0.01 to 0.8% by mass, and more preferably 0.05 to 0.6% by mass.

[0105] With this level of Mo content, the core hardness and fatigue strength of the quenched component are increased due to improved hardenability. Additionally, surface hardness and hardened layer hardness are also improved.

[0106] When the Mo content is too low, the strength after hot forging increases, but the machinability may decrease. Conversely, when the Mo content is too high, it tends to form nucleation sites and promotes the formation of precipitates such as carbonitrides. Furthermore, undissolved, coarse carbonitrides remain in the steel, and during carburizing and quenching, these coarse carbonitrides further grow and coarsen, potentially reducing fatigue strength.

[0107] <al>

[0108] The Al content in the steel used for carburizing and nitriding treatment of the present invention is 0.01 to 0.08% by mass, preferably 0.02 to 0.05% by mass.

[0109] When the Al content is such, Al easily combines with N to form AlN, which can refine the grains and strengthen the steel.

[0110] When the Al content is too high, the machinability may be reduced due to the formation of hard and coarse Al2O3. In addition, Al2O3, as a large hard inclusion, becomes the starting point of fatigue failure and may be the cause of reduced bending fatigue strength or pitting strength.

[0111] <n>

[0112] The N content in the steel used for carburizing and nitriding treatment of the present invention is 0.005 to 0.03% by mass, preferably 0.01 to 0.025% by mass, more preferably 0.01 to 0.020% by mass, and even more preferably 0.01 to 0.015% by mass.

[0113] At this N content, due to nitride formation, grain refinement occurs, and bending fatigue strength is improved.

[0114] When the nitrogen content is too high, the toughness may be reduced due to the formation of coarse nitrides.

[0115] <nb>

[0116] The steel used for carburizing and nitriding of the present invention may contain Nb.

[0117] The Nb content in the steel used for carburizing and nitriding treatment according to the present invention is preferably 0.001 to 0.08% by mass, more preferably 0.0015 to 0.06% by mass.

[0118] When the Nb content is such, it is difficult to coarsen the grains during carburizing due to the formation of fine precipitates (NbC).

[0119] <v>

[0120] The steel used for carburizing and nitriding of the present invention may contain V.

[0121] The V content in the steel used for carburizing and nitriding treatment of the present invention is preferably 0.5% by mass or less, more preferably 0.25% by mass or less.

[0122] When the V content is such a level, V precipitates appear dispersedly, and the destructive properties are enhanced.

[0123] <ti>

[0124] The steel used for carburizing and nitriding of the present invention may contain Ti.

[0125] The Ti content in the steel used for carburizing and nitriding treatment of the present invention is preferably 0.05% by mass or less, more preferably 0.08% by mass or less, and even more preferably 0.012 to 0.04% by mass.

[0126] When the Ti content is such, the grain size is difficult to coarsen during carburizing due to the formation of fine precipitates (TiC).

[0127]

[0128] The steel used for carburizing and nitriding of the present invention may contain B.

[0129] The B content in the steel used for carburizing and nitriding treatment of the present invention is preferably 0.0005 to 0.003% by mass, more preferably 0.0006 to 0.0025% by mass.

[0130] When the B content is such, hardenability is greatly improved, and crack machinability is also improved.

[0131] When the boron content is too high, boron nuclei (BN) are formed, which reduces the effect of improving hardenability at deeper depths.

[0132] The carburizing and nitriding steel of the present invention may contain C, Si, Mn, P, S, Cu, Ni, Cr, Mo, Al, and N in the proportions described above, and may further contain at least one of the elements selected from the group consisting of Nb, V, Ti, and B as an arbitrary component in a specific proportion. Moreover, the balance consists of Fe and unavoidable impurities.

[0133] Here, unavoidable impurities refer to components that may be introduced from raw materials or during the manufacturing process, even if not intentionally added. Examples of unavoidable impurities include oxygen (O) and aspartate (As).

[0134] The content of each component in the steel used for carburizing and nitriding treatment of the present invention refers to the value obtained by the following method.

[0135] Si, Mn, P, Cu, Ni, Cr, Mo, V, Ti, and Nb refer to values ​​determined by X-ray fluorescence analysis; Al refers to values ​​determined by emission spectroscopy; O refers to values ​​determined by dissolution in an inert gas-infrared absorption method; and C and S refer to values ​​determined by combustion-infrared absorption method. Additionally, N refers to values ​​determined by melting in an inert gas-thermal conduction method, and B refers to values ​​determined by emission spectroscopy.

[0136] The method for manufacturing the carburizing and nitriding steel of the present invention is not particularly limited. For example, the carburizing and nitriding steel of the present invention can be manufactured by conventionally known methods.

[0137] By performing carburizing and nitriding treatment on the steel of the present invention having the above-described composition, the carburizing and nitriding steel of the present invention can be obtained.

[0138] Here, there is no particular limitation on the carburizing and nitriding treatment, as long as the carburizing and nitriding treatment of the present invention can be obtained from the carburizing and nitriding steel of the present invention, for example, it can be the following carburizing and nitriding treatment X.

[0139] The carburizing and nitriding treatment X can be either gas carburizing and nitriding or vacuum carburizing and nitriding. Furthermore, the conditions for carburizing and nitriding (carburizing temperature, type of carburizing gas, carburizing gas pressure, processing time in the carburizing process, processing time in the diffusion process, cooling rate in the cooling process, nitriding gas pressure, ammonia flow rate, processing time, quenching temperature, etc.) can be appropriately determined according to the required surface hardness and tempering hardness of the carburized and nitrided part, without particular limitation. For example, in gas carburizing and nitriding, the surface nitrogen concentration is typically controlled by adjusting CP = 0.5–1.0, using ammonia as the nitriding gas, and adjusting the ammonia flow rate, furnace ammonia concentration, diffusion time, and quenching temperature, followed by quenching. Then, the temperature is heated to 100°C–300°C and held for 1–3 hours for tempering.

[0140] The carburized and nitrided steel of the present invention will be described.

[0141] The carburizing and nitriding steel of the present invention can be obtained by performing a carburizing and nitriding treatment (e.g., the carburizing and nitriding treatment X described above) on the carburizing and nitriding steel of the present invention as described above.

[0142] <Surface C concentration>

[0143] In the carburized and nitrided steel of the present invention, the surface C concentration is preferably 0.4 to 0.8% by mass, more preferably 0.45 to 0.70% by mass.

[0144] Here, surface C concentration refers to the C concentration obtained by applying the combustion-infrared absorption method to the chips (dairy powder: chips) obtained by cutting the carburized and nitrided steel of the present invention from its surface to a depth of 100 μm.

[0145] <Surface N concentration>

[0146] In the carburized and nitrided steel of the present invention, the surface N concentration is preferably 0.25 to 0.8% by mass, more preferably 0.30 to 0.70% by mass.

[0147] Here, surface N concentration refers to the N concentration obtained by measuring the melt-thermal conductivity of the resulting chips (dairy powder) obtained by cutting the carburized and nitrided steel of the present invention from its surface to a depth of 100 μm.

[0148] In the carburized and nitrided steel of the present invention, the surface C concentration and surface N concentration as described above satisfy the following formula 1.

[0149] Formula 1: Surface C concentration (mass%) + 12 / 14 × Surface N concentration (mass%) = 0.6~1.4

[0150] The calculation result of Formula 1 is preferably 0.7 to 1.2.

[0151] In the carburized and nitrided steel of the present invention, the surface C concentration, surface N concentration and Cr content as described above satisfy the following formula 2.

[0152] Formula 2: 129.7805 × [Cr (mass%)] - 76.9797 × [Cr (mass%)] 2 +339.3375 × [Surface N concentration (mass%)] - 539.345 × [Surface N concentration (mass%)] 2 +181.4983 × [Cr (mass%)] × [Surface N concentration (mass%)] + 437.6799 > 560

[0153] The hardness of the portion of the carburized and nitrided steel of the present invention obtained by performing the above-described carburizing and nitriding treatment X on the carburizing and nitriding steel of the present invention at a depth of 0.05 mm from its surface is preferably 600 HV or more.

[0154] The carburized and nitrided steel of the present invention, obtained by performing the above-described carburized and nitrided treatment X on the carburized and nitrided steel of the present invention, has a hardness of 560 HV or higher at a depth of 0.05 mm from the surface when further tempered at 500°C.

[0155] Example

[0156] <Manufacturing of the Experimental Film>

[0157] The embodiments of the present invention will be described below.

[0158] For each of Examples 1 to 31 and Comparative Examples 1 to 19 shown in Table 1, the raw materials were mixed in such a manner as to become the composition shown in Tables 1 and 2 (in mass%, with the balance being Fe and unavoidable impurities), melted in a 150 kg high-frequency induction furnace, and cast to obtain steel block A.

[0159] Next, steel block A is hot rolled or hot forged to obtain a round bar with a cross-sectional diameter of 125 mm. Then, it is further hot forged to obtain a round bar with a cross-sectional diameter of 32 mm. Then, it is normalized (925℃×1HrAC) and a round bar with a cross-sectional diameter of 15 mm (length 210 mm) is cut from the obtained round bar.

[0160] Next, the round bars were subjected to carburizing and nitriding treatments to obtain test pieces. However, in some comparative examples, only carburizing treatment was performed, without nitriding treatment.

[0161] Here, carburizing treatment is the following process.

[0162] The round bar is placed in a gas carburizing and nitriding furnace, and carburizing gas (using propane gas as enrichment gas) is introduced at a temperature of 930°C. By adjusting the partial pressure of carbon monoxide and carbon dioxide, the CP (carbon potential) is controlled to 0.7, and carburizing is carried out.

[0163] In addition, nitriding is the following treatment.

[0164] The carburized round bars were cooled to 850°C, and ammonia gas was introduced as the nitriding gas while maintaining a constant concentration (CP) to perform nitriding. It should be noted that after nitriding, the bars were quenched in semi-hot oil at 120°C. Further, as the next step, the quenched round bars were heated in a furnace at 160°C for 2 hours, then removed from the furnace and subjected to tempering by cooling indoors.

[0165] <Surface C concentration, surface N concentration>

[0166] For test pieces that underwent the above carburizing and nitriding treatments (some of which only underwent carburizing treatment), the C and N concentrations in the resulting chips (drill powder) were measured by cutting to a depth of 100 μm from their surface. The C concentration was determined using the combustion-infrared absorption method, and the N concentration was determined using the melt-thermal conductivity method.

[0167] The results are shown in Table 1.

[0168] <Surface hardness at room temperature>

[0169] For the test pieces that underwent the above carburizing and nitriding treatments (some of which were carburized only), their surfaces were mirror-polished, and the hardness at a position 0.05 mm from the surface was measured with a load of 2.94 N based on JIS Z 2244.

[0170] The results are shown in Tables 1 and 2.

[0171] Surface hardness during tempering at 500℃

[0172] Test pieces that underwent the aforementioned carburizing and nitriding treatments (a portion of which were carburized only) were heated in a furnace at 500°C for 3 hours, then removed from the furnace and subjected to tempering treatment by cooling indoors. Subsequently, their surfaces were mirror-polished, and the hardness at a position 0.05 mm from the surface was measured under a load of 2.94 N, based on JIS Z 2244.

[0173] The results are shown in Tables 1 and 2.

[0174] <Fatigue strength-to-life ratio>

[0175] The steel block A described above is used to manufacture a round bar through the same process, which is then machined to obtain a small roller. Small roller 1 is made from... Figure 1 It consists of a contact portion 2 with a diameter of 26 mm and a width of 28 mm, and small diameter portions 4 with a diameter of 22 mm arranged on both sides thereon.

[0176] Then, the small roller was subjected to carburizing and nitriding treatments to obtain the test piece.

[0177] Next, the large roller on the opposite side, which will serve as the test piece, is prepared. The roller is made of SUJ2 material and undergoes quenching and tempering treatment to achieve HRC61 hardness. It should be noted that the radius of curvature of the roller is set to 150R.

[0178] Then, a roller pitting test is performed. In the roller pitting test, the test piece and the large roller on the hand side are brought into contact under various surface pressures ranging from 2.0 to 4.0 GPa at a rotation speed of 3000 rpm. Using a roller pitting tester, they are rotated at a slip rate of -100%. The test results are obtained after 10... 7 The load stress that does not produce pitting corrosion in each cycle is defined as the surface fatigue strength (pitting fatigue strength). Then, the surface fatigue strength of the vacuum carburized material relative to JIS SCR420 is determined for each test piece. That is, the fatigue strength-life ratio is (surface fatigue strength of the test piece / surface fatigue strength of the vacuum carburized material of JIS SCR420).

[0179] The results are shown in Tables 1 and 2.

[0180] <Sintering ultimate load ratio>

[0181] The steel block A was machined to obtain two test pieces, one on the load roll side and the other on the test roll side.

[0182] The test piece consists of a diameter of 78 mm and a width of 18 mm, with the radius of curvature of the load roller side set to 700R.

[0183] Then, the test piece was subjected to carburizing and nitriding treatments to obtain the test piece.

[0184] Then, sintering tests were conducted using a roller pitting tester.

[0185] In the sintering test, the two test pieces prepared above were subjected to a certain sliding speed (2.0~20.0m / s), and the load was increased by 0.05GPa every 60 seconds in stages.

[0186] The sintering determination is set at the moment when the torque of the torque meter set on the load side rises sharply, and the load at this moment is set as the sintering load.

[0187] Then, the surface fatigue strength of the vacuum carburized material relative to JIS SCR420 was determined in each test piece.

[0188] That is, the sintering limit load ratio refers to "the sintering load of the test piece / the sintering load of the vacuum carburized material of JIS SCR420".

[0189] The sintering limit load was measured for Examples 1-31 and Comparative Examples 1-19, respectively.

[0190] The results are shown in Tables 3 and 4.

[0191] [Table 1]

[0192]

[0193] [Table 2]

[0194]

[0195] [Table 3]

[0196] < / ti> < / v> < / nb> < / n> < / al> < / cr> < / ni> < / cu> < / s> Sintering ultimate load ratio Example 1 1.15 Example 2 1.22 Example 3 1.11 Example 4 1.27 Example 5 1.19 Example 6 1.30 Example 7 1.17 Example 8 1.14 Example 9 1.24 Example 10 1.25 Example 11 1.31 Example 12 1.38 Example 13 1.14 Example 14 1.17 Example 15 1.39 Example 16 1.30 Example 17 1.50 Example 18 1.11 Example 19 1.20 Example 20 1.25 Example 21 1.13 Example 22 1.12 Example 23 1.13 Example 24 1.13 Example 25 1.20 Example 26 1.11 Example 27 1.19 Example 28 1.13 Example 29 1.20 Example 30 1.13 Example 31 1.20 <s>

[0197] [Table 4]

[0198] Sintering ultimate load ratio Comparative Example 1 0.98 Comparative Example 2 1.06 Comparative Example 3 1.01 Comparative Example 4 0.98 Comparative Example 5 1.00 Comparative Example 6 0.93 Comparative Example 7 0.97 Comparative Example 8 0.99 Comparative Example 9 0.98 Comparative Example 10 1.03 Comparative Example 11 1.09 Comparative Example 12 0.95 Comparative Example 13 1.01 Comparative Example 14 1.04 Comparative Example 15 - Comparative Example 16 0.98 Comparative Example 17 1.09 Comparative Example 18 1.03 Comparative Example 19 1.00

[0199] Figure 2 The relationship between surface hardness and fatigue strength life ratio (life ratio under high-speed roll pitting conditions) when tempering is performed at 500℃ is shown in Tables 1 and 2.

[0200] from Figure 2 It can be confirmed that the embodiments are all improved compared to the comparative examples.

[0201] in addition, Figure 3 The relationship between the surface hardness when tempering at 500°C as shown in Tables 1 and 2 and the sintering limit load ratios shown in Tables 3 and 4 is presented.

[0202] from Figure 3 It can be confirmed that the embodiments are all improved compared to the comparative examples.

[0203] Industrial applicability

[0204] According to the present invention, carburizing and nitriding steel with high surface fatigue strength and sufficiently high hardness after high-temperature tempering can be provided, as well as carburizing and nitriding steel obtained by carburizing and nitriding.

[0205] This application is based on Japanese patent application (Japanese Patent Application No. 2021-071046) filed on April 20, 2021, the contents of which are incorporated herein by reference.

[0206] Explanation of symbols

[0207] 1 small roller

[0208] 2 contact part

[0209] 4. Small diameter section < / s> < / si> < / c>

Claims

1. A type of steel for carburizing and nitriding, comprising the following elements: C: 0.1–0.3% by mass Si: less than 0.3% by mass Mn: 0.4–2.0% by mass P: less than 0.03% by mass S: less than 0.03% by mass Cu: less than 0.3% by mass Ni: less than 2.5% by mass Cr: 0.5–3.0% by mass Mo: 0.001–1.0% by mass Al: 0.01–0.08% by mass N: 0.005~0.03% by mass Nb: less than 0.08% by mass Ti: less than 0.05% by mass, and B: Less than 0.003% by mass The balance consists of Fe and unavoidable impurities. By performing carburizing and nitriding, carburized and nitrided steel is obtained. The carburized and nitrided steel satisfies Formula 1: Surface C concentration (mass%) + 12 / 14 × Surface N concentration (mass%) = 0.6~1.4, and Equation 2 is satisfied: 129.7805 × [Cr (mass%)] - 76.9797 × [Cr (mass%)] 2 +339.3375 × [Surface N concentration (mass%)] - 539.345 × [Surface N concentration (mass%)] 2 +181.4983×[Cr (mass%)]×[surface N concentration (mass%)]+437.6799>560, and After the aforementioned carburizing and nitriding is performed and then tempered at 500°C, the hardness of the portion at a depth of 0.05 mm from the surface is 560 HV or higher. The surface C concentration of the carburized and nitrided steel is 0.4 to 0.7% by mass, and the surface N concentration is 0.36 to 0.8% by mass.

2. The carburizing and nitriding steel according to claim 1, wherein it satisfies the condition of being selected from at least one of the following groups: Nb: 0.001–0.08% by mass Ti: 0.012–0.05% by mass, and B: 0.0005~0.003% of mass.

3. The steel for carburizing and nitriding according to claim 1, comprising the following elements: C: 0.15–0.25% by mass Si: 0.01–0.24% by mass Mn: 0.5–1.8% by mass Cu: 0.001–0.3% by mass Ni: 0.01–0.6% by mass Cr: 0.6–1.8% by mass Mo: 0.01–0.8% by mass Al: 0.02–0.05% by mass N: 0.01~0.025% by mass and selected from at least one of the following groups: Nb: 0.0015–0.06% by mass Ti: 0.012–0.04% by mass, and B: 0.0006~0.0025% by mass The balance consists of Fe and unavoidable impurities. By performing the aforementioned carburizing and nitriding, carburized and nitrided steel is obtained that satisfies Formula 1: Surface C concentration (mass%) + 12 / 14 × Surface N concentration (mass%) = 0.7 to 1.

2.

4. The carburizing and nitriding steel according to claim 1, containing Cu: 0.03 to 0.3% by mass.

5. The carburizing and nitriding steel according to claim 1, containing Si: 0.01 to 0.17 by mass.

6. The carburizing and nitriding steel according to any one of claims 1 to 5, containing Cu: 0.03 to 0.25 by mass.

7. The carburizing and nitriding steel according to claim 1, containing Mn: 0.80-2.0 by mass.

8. A carburizing and nitriding steel, which is obtained by carburizing and nitriding the steel for carburizing and nitriding treatment as described in any one of claims 1 to 7. The carburized and nitrided steel satisfies Formula 1: Surface C concentration (mass%) + 12 / 14 × Surface N concentration (mass%) = 0.6~1.4, and Equation 2 is satisfied: 129.7805 × [Cr (mass%)] - 76.9797 × [Cr (mass%)] 2 +339.3375 × [Surface N concentration (mass%)] - 539.345 × [Surface N concentration (mass%)] 2 +181.4983×[Cr (mass%)]×[surface N concentration (mass%)]+437.6799>560, and When further tempered at 500℃, the hardness of the portion 0.05mm deep from the surface becomes above 560HV. The surface C concentration of the carburized and nitrided steel is 0.4 to 0.7% by mass, and the surface N concentration is 0.36 to 0.8% by mass.

Images