Surface hardening steel

Abstract

The present invention relates to a surface-hardened steel containing 0.10 mass% < = C < = 0.20 mass%, 0.05 mass% < = Si < = 2.00 mass%, 0.30 mass% < = Mn < = 2.00 mass%, P < = 0.030 mass%, S < = 0.030 mass%, 0.01 mass% < = Cu < = 1.00 mass%, 0.01 mass% < = Ni < = 1.00 mass%, 0.30 mass% < = Cr < = 3.00 mass%, 0.0001 mass% < = Ti < = 0.2000 mass%, 0.040 mass% < = Nb < = 0.080 mass%, 0.002 mass% < = Al < = 0.060 mass%, and 0.003 mass% < = N < = 0.040 mass%, with the balance being Fe and unavoidable impurities, the pinned Nb content > = 0.020 mass%, and 2.0 * 10 <-6 > < = [Ti] * [N] < = 2.0 * 10 <-3 > are satisfied.

Description

Technical Field

[0001] This invention relates to a surface-hardening steel, and more specifically to a surface-hardening steel capable of suppressing austenite grain coarsening during carburizing. Background Technology

[0002] The term "surface-hardened steel (or carburized steel)" refers to steel suitable for carburizing or carbonitriding treatments (hereinafter collectively referred to as "carburizing"). When carburized surface-hardened steel is quenched, only the surface layer can be hardened. Therefore, surface-hardened steel has been used in various components requiring wear resistance, such as gears and continuously variable transmissions (CVTs).

[0003] In recent years, from the perspectives of energy conservation and shortening manufacturing time, surface-hardened steel has often undergone cold forging.

[0004] However, in cold-forged products, the austenite grains tend to coarsen during the carburizing process.

[0005] To suppress austenite grain coarsening, a reduction in carburizing temperature is considered. However, when the carburizing temperature is reduced, it becomes necessary to extend the carburizing time, which leads to a decrease in production efficiency.

[0006] Traditionally, fine particles such as AlN and NbC are known as pinning particles that suppress austenite grain growth by pinning austenite grain boundaries.

[0007] By dispersing and precipitating these fine particles, the movement of austenite grain boundaries is hindered, effectively suppressing austenite grain growth.

[0008] Several proposals have been put forward as a method to suppress austenite grain growth using such fine particles.

[0009] For example, Patent Document 1 discloses a surface-hardening steel comprising (in mass %): C: 0.10%-0.35%, Si: 0.01%-0.50%, Mn: 0.30%-1.50%, P: ≤0.02%, S: ≤0.03%, Al: 0.04%-0.10%, Cr: 0.5%-2.5%, B: 0.0005%-0.0050%, Nb: 0.015%-0.10%, Ti: ≤0.003%, Mo: ≤0.01%, N: <0.0080%, with the remainder being Fe and unavoidable impurities. This surface-hardening steel has a microstructure in which ferrite and pearlite account for more than 80%, and precipitates containing Nb with a diameter of less than 50 nm are present at 30 particles / μm. 2 The above-mentioned precipitates containing Nb with a diameter of 50 nm or more and less than 100 nm are 3 particles / μm. 2 The following is the number n of precipitates containing Nb with a diameter of less than 50 nm.A and the number n of Nb-containing precipitates with a diameter of 50 nm or more and less than 100 nm. B Satisfying relation n A - 5n B > 30.

[0010] Patent document 1 describes a surface-hardening steel that simultaneously possesses excellent cold forging properties and grain coarsening suppression capabilities by specifying the above-mentioned steel composition, setting the area ratio of ferrite to pearlite microstructure to the aforementioned microstructure, and controlling the amount of Nb-containing precipitates as described above.

[0011] When surface-hardened steel containing NbC or precipitates composed of compounds such as Ti and N dissolved in NbC (hereinafter, these may be referred to as "NbC-based particles") is hot-worked, coarse NbC-based particles can be dissolved in the steel by heating the surface-hardened steel to a predetermined temperature or above. When the surface-hardened steel is cooled at a predetermined cooling rate after hot working, fine NbC-based particles can precipitate in the steel. As described in Patent Document 1, when a large number of fine NbC-based particles precipitate, these fine NbC-based particles act as pinning particles, which can suppress the coarsening of austenite grains during carburizing.

[0012] However, when the amount of Nb is too high, coarse NbC-based particles precipitate and grow during smelting and casting. Therefore, even if the surface-hardened steel is subsequently heated to a predetermined temperature, it may be difficult to completely dissolve the coarse NbC-based particles in the steel.

[0013] When too much coarse NbC-based particle is precipitated, the amount of Nb dissolved in the steel becomes less, and therefore the amount of fine NbC-based particles precipitated becomes less in precipitation steps (e.g., cooling steps after hot working) that cause the NbC-based particles to precipitate finely.

[0014] In other words, when the amount of Nb increases, only the amount of coarse NbC-based particles increases, while the amount of fine NbC-based particles decreases. This may lead to coarsening of austenite grains during the carburizing process.

[0015] Furthermore, Patent Document 1 describes a method in several μm 2 The precipitation technique for nanoscale NbC-based particles was observed within a certain range. However, even these fine NbC-based particles may coarsen during carburizing, losing their function as pinning particles.

[0016] Therefore, the method in Patent Document 1 has limitations in suppressing the coarsening of austenite grains during the carburizing process.

[0017] Coarse NbC-based particles typically grow using TiN-based or AlN-based particles as nuclei. In cases where the number of TiN-based and / or AlN-based particles is insufficient, coarse NbC-based particles are more likely to form. When this surface-hardened steel is heated to a specific temperature, these coarse NbC-based particles may grow further.

[0018] As mentioned above, the total amount of fine NbC-based particles is related to the total amount of coarse NbC-based particles.

[0019] In other words, in order to determine the total amount of fine NbC-based particles, it is necessary to determine the total amount of coarse NbC-based particles.

[0020] However, there is still no target for approximately 10 6 μm 2 The example presented is the total amount of coarse NbC-based particles precipitated over a wide area in surface-hardened steel.

[0021] Patent Document 1: JP2015-166495A Summary of the Invention

[0022] The purpose of this invention is to provide a surface-hardening steel that can suppress austenite grain coarsening during carburizing.

[0023] To solve the above problems, the surface-hardened steel according to the present invention comprises:

[0024] 0.10% by mass ≤ C ≤ 0.20% by mass

[0025] 0.05% by mass ≤ Si ≤ 2.00% by mass

[0026] 0.30% by mass ≤ Mn ≤ 2.00% by mass

[0027] P ≤ 0.030% by mass

[0028] S ≤ 0.030% by mass

[0029] 0.01% by mass ≤ Cu ≤ 1.00% by mass

[0030] 0.01% by mass ≤ Ni ≤ 1.00% by mass

[0031] 0.30% by mass ≤ Cr ≤ 3.00% by mass

[0032] 0.0001% by mass ≤ Ti ≤ 0.2000% by mass

[0033] 0.040% by mass ≤ Nb ≤ 0.080% by mass

[0034] 0.002% by mass ≤ Al ≤ 0.060% by mass, and

[0035] 0.003% mass ≤ N ≤ 0.040% mass

[0036] The remainder consists of Fe and unavoidable impurities, and

[0037] It satisfies the following formulas (1) and (2):

[0038] Nb content in pinning ≥ 0.020% by mass (1)

[0039] 2.0 × 10 -6 ≤ [Ti] × [N] ≤ 2.0 × 10 -3 (2)

[0040] Here,

[0041] Pinned Nb content = [Nb] - NbC-based inclusion area ratio (area%) × 0.892

[0042] [X] represents the content (mass%) of element X.

[0043] "NbC-based inclusions" refer to NbC-based particles with a Nb content of 50% or more by mass and a maximum length of 0.1 μm or more.

[0044] "NbC-based inclusion area ratio" refers to the ratio of the areas of NbC-based inclusions within a 4000 μm × 4000 μm field of view in the measurement range.

[0045] In a cross section perpendicular to the longitudinal direction of rolled or forged material, the length of the line connecting the centroid of the cross section to the boundary of the cross section closest to the centroid is d / 2. The closed curve on the cross section at a distance of d / 4 from the boundary is called the d / 4 portion. The "measurement range" refers to the range of length d / 4 extending from the boundary to the d / 4 portion.

[0046] Specifically, in such Figure 1A When the cross-sectional shape is circular, the center of the circle is the centroid G, and the radius of the circle is d / 2. In this case, since the d / 4 portion is a closed curve at a distance of d / 4 from the boundary of the cross-section, the d / 4 portion forms a circle with a radius of d / 4 around the centroid G. The length d / 4 extending from the boundary to the d / 4 portion is the measurement range. In this case, the extension line of the measurement range towards the interior of the cross-section passes through the centroid G.

[0047] In such Figure 1BWhen the cross-sectional shape is a square, the intersection of the diagonals of the square is the centroid G. Here, the length of the line connecting the centroid to the boundary of the cross-section closest to the centroid is d / 2, and the length of a single side of the cross-section is d. Furthermore, since the d / 4 portion is a closed curve at a distance of d / 4 from the boundary of the cross-section, the d / 4 portion forms concentric squares with a side length of d / 2 and the centroid G. The length d / 4 range connecting the boundary to the d / 4 portion is the measurement range. In this case, the direction perpendicular to the d / 4 portion in the cross-section is the measurement range.

[0048] Under optimized manufacturing conditions for surface-hardened steel and with [Ti] × [N], TiN-based particles can be moderately dispersed in the steel during the cooling process after casting. When TiN-based particles are moderately dispersed in the steel, fine NbC-based particles (i.e., pinned particles) disperse and grow, which can suppress the formation of coarse NbC-based particles (i.e., NbC-based inclusions).

[0049] Correspondingly, when the amount of NbC-based inclusions is low, the NbC-based inclusions are easily dissolved in subsequent heating steps.

[0050] Therefore, in the precarburized state, the pinned Nb content can be above 0.020% by mass.

[0051] When surface-hardened steel with this structure undergoes carburizing treatment, the coarsening of austenite grains is suppressed by the large number of pinned particles dispersed in the steel. Attached Figure Description

[0052] Figure 1A This is a schematic diagram of the measurement range when the cross-sectional shape is circular.

[0053] Figure 1B This is a schematic diagram of the measurement range when the cross-sectional shape is square. Detailed Implementation

[0054] Embodiments of the present invention will be described in detail below.

[0055] [1. Surface-hardened steel]

[0056] [1.1 ingredients]

[0057] [1.1.1 Main Constituent Elements]

[0058] The surface-hardened steel according to the present invention comprises the following elements, with the remainder being Fe and unavoidable impurities. The types of added elements, their composition ranges, and the reasons for their limitations are as follows.

[0059] (1) 0.10% mass ≤ C ≤ 0.20% mass:

[0060] C combines with Nb to form NbC-based particles.

[0061] In this invention, “NbC-based particles” refers to particles composed of any of the following: (a) NbC; (b) a solid solution of another element (e.g., Ti, N, etc.) dissolved in NbC; (c) NbC grown using TiN-based particles or AlN-based particles as nuclei.

[0062] In this invention, the fine NbC-based particles that help suppress the coarsening of austenite grains during the carburizing process can be referred to as "pinning particles".

[0063] In this invention, coarse NbC-based particles that do not help suppress austenite grain coarsening during the carburizing process can be referred to as "NbC-based inclusions".

[0064] Carbon (C) is an essential element to ensure hardness and strength.

[0065] If the amount of carbon is too low, it may be impossible to ensure the hardness of the core (center) of the carburized area.

[0066] Therefore, the C content needs to be above 0.10% by mass.

[0067] On the other hand, if the amount of C is too high, incompletely dissolving NbC-based inclusions may form during the heating step prior to the pinning particle precipitation step (hereinafter, this step may be referred to as the "precipitation step"). Therefore, the amount of pinning particles that can precipitate in the precipitation step may be insufficient. As a result, the number of pinning particles capable of pinning austenite grain boundaries in the carburizing step may be reduced, and austenite grain coarsening may not be suppressed.

[0068] Therefore, the C content needs to be below 0.20% by mass. Preferably, the C content is below 0.18% by mass.

[0069] (2) 0.05% by mass ≤ Si ≤ 2.00% by mass:

[0070] Si is an element that effectively improves hardenability and enhances strength.

[0071] To achieve this effect, the Si content needs to be above 0.05% by mass.

[0072] Si is an element that strengthens ferrite through solid solution.

[0073] Therefore, if the amount of Si is too high, the hardness may increase and the cold workability may decrease.

[0074] Therefore, the Si content needs to be 2.00% by mass or less. Preferably, the Si content is 0.50% by mass or less, and more preferably 0.30% by mass or less.

[0075] (3) 0.30% by mass ≤ Mn ≤ 2.00% by mass:

[0076] Mn is an element that effectively improves hardenability and enhances strength.

[0077] To achieve this effect, the Mn content needs to be above 0.30% by mass.

[0078] Mn is an element that strengthens ferrite through solid solution.

[0079] Therefore, if the amount of Mn is too high, the hardness may increase and the cold workability may decrease.

[0080] Therefore, the Mn content needs to be 2.00% by mass or less. Preferably, the Mn content is 1.00% by mass or less, and more preferably 0.70% by mass or less.

[0081] (4) P ≤ 0.030 mass%

[0082] Excessive phosphorus (P) content may promote grain boundary embrittlement. Therefore, a low P content is preferable.

[0083] Therefore, the P content needs to be below 0.030% by mass.

[0084] (5) S ≤ 0.030% (mass):

[0085] S combines with Mn, increasing the amount of MnS and improving machinability. However, if the amount of S is too high, fatigue strength may decrease.

[0086] Therefore, the sulfur content needs to be below 0.030% by mass.

[0087] (6) 0.01% by mass ≤ Cu ≤ 1.00% by mass:

[0088] Cu is an effective element for improving hardenability and increasing strength. In particular, when the amount of C is low, Cu is an effective element for ensuring the hardness of the core part (central part) of the carburized area.

[0089] To achieve this effect, the Cu content needs to be above 0.01% by mass.

[0090] On the other hand, if the amount of Cu is too high, hot forgeability may decrease.

[0091] Therefore, the Cu content needs to be below 1.00% by mass. Preferably, the Cu content is below 0.30% by mass.

[0092] (7) 0.01% by mass ≤ Ni ≤ 1.00% by mass:

[0093] Ni is an effective element for improving hardenability and increasing strength. Ni is also an effective element for improving ductility. In particular, when the amount of C is low, Ni is an effective element for ensuring the hardness of the core part (central part) of the carburized area.

[0094] To achieve this effect, the Ni content needs to be above 0.01% by mass.

[0095] On the other hand, if the amount of Ni is too high, bainitic structure will form, which may reduce cold workability.

[0096] Therefore, the Ni content needs to be below 1.00% by mass. Preferably, the Ni content is below 0.30% by mass.

[0097] (8) 0.30% by mass ≤ Cr ≤ 3.00% by mass:

[0098] Cr is an element that effectively improves hardenability and enhances strength.

[0099] To achieve this effect, the Cr content needs to be 0.30% by mass or higher. Preferably, the Cr content is 0.80% by mass or higher.

[0100] On the other hand, if the amount of Cr is too high, the machinability may decrease.

[0101] Therefore, the Cr content needs to be below 3.00% by mass. Preferably, the Cr content is below 2.00% by mass.

[0102] (9) 0.0001 mass% ≤ Ti ≤ 0.2000 mass%:

[0103] When the amount of Ti is too low, the number of TiN-based particles decreases, and the number of NbC-based particles using TiN-based particles as nuclei is also too small. Therefore, the NbC-based particles in each nucleus become too large. As a result, the NbC-based particles may continue to grow during the solidification process after dissolution, forming NbC-based inclusions.

[0104] Therefore, the Ti content needs to be 0.0001% or more. The Ti content is preferably 0.0030% by mass or more, and more preferably 0.0035% by mass or more.

[0105] On the other hand, if the amount of Ti is too high, Ti dissolves in the NbC-based particles and increases the solid solution temperature of the NbC-based particles. As a result, even when heat treatment is performed, coarse NbC-based particles may remain undissolved.

[0106] Therefore, the Ti content needs to be below 0.2000% by mass.

[0107] (10) 0.040% mass ≤ Nb ≤ 0.080% mass:

[0108] Nb has the function of precipitating pinned particles in the precipitation step and inhibits austenite grain coarsening during carburizing.

[0109] To achieve this effect, the Nb content needs to be 0.040% by mass or higher. Preferably, the Nb content is 0.045% by mass or higher.

[0110] On the other hand, excessive Nb content may increase NbC-based inclusions. Furthermore, excessive NbC-based inclusions may reduce cold workability.

[0111] Therefore, the Nb content needs to be below 0.080% by mass. Preferably, the Nb content is below 0.070% by mass.

[0112] (11) 0.002 mass% ≤ Al ≤ 0.060 mass%:

[0113] Al not only forms tiny AlN-based particles with a pinning effect, but also acts as a deoxidizer.

[0114] If the amount of Al is too low, these functions may not be fully utilized.

[0115] Therefore, the Al content needs to be above 0.002% by mass.

[0116] On the other hand, if the amount of Al is too high, aluminum-based compounds may form as the core of NbC-based inclusions.

[0117] Therefore, the Al content needs to be below 0.060% by mass.

[0118] (12) 0.003% mass ≤ N ≤ 0.040% mass:

[0119] When the amount of N is appropriate, it can combine with Al to form fine AlN-based particles with a pinning effect, which can suppress austenite grain coarsening.

[0120] To achieve this effect, the nitrogen content needs to be above 0.003% by mass.

[0121] On the other hand, when the amount of N is too high, N dissolves in the NbC-based particles and increases the solid solution temperature of the NbC-based particles. As a result, even when heat treatment is performed, coarse NbC-based particles may remain undissolved.

[0122] Therefore, the nitrogen content needs to be below 0.040% by mass.

[0123] (13) Unavoidable impurities:

[0124] The surface-hardened steel according to the present invention may contain unavoidable impurities. Here, "unavoidable impurities" refers to components introduced during the industrial production of surface-hardened steel due to various factors such as raw materials and production processes, the content of which is within a range that does not adversely affect the performance of the surface-hardened steel according to the present invention.

[0125] [1.1.2 Secondary Constituent Elements]

[0126] In addition to the main constituent elements mentioned above, the steel according to the present invention may further include the following elements. The types of added elements, their composition ranges, and the reasons for their limitations are as follows.

[0127] (14) 0.0003% mass ≤ B ≤ 0.010% mass:

[0128] Boron (B) is an effective element for improving hardenability and enhancing strength. In particular, when the amount of carbon (C) is low, B is an effective element for ensuring the hardness of the core portion (central part) of the carburized area. Furthermore, B segregates at grain boundaries to strengthen them, thereby increasing strength. To achieve this effect, the B content is preferably 0.0003% by mass or more.

[0129] On the other hand, if the amount of boron is too high, boron reacts with nitrogen in the steel to form boron nanotubes (BN), which may reduce toughness.

[0130] Therefore, the B content is preferably below 0.010% by mass.

[0131] (15) Mo ≤ 1.00% by mass:

[0132] Mo is an element that improves hardenability, increases strength, and enhances wear resistance.

[0133] To achieve this effect, the Mo content is preferably 0.01% by mass or more.

[0134] On the other hand, if the amount of Mo is too high, bainitic structure will form, which may reduce cold workability. Since Mo is also an expensive element, it is preferable to reduce the amount of Mo used.

[0135] Therefore, the Mo content is preferably 1.00% by mass or less, and more preferably 0.10% by mass or less.

[0136] [1.2 Composition Balance]

[0137] The surface-hardened steel according to the present invention needs to satisfy the following expressions (1) and (2):

[0138] Nb content in pinning ≥ 0.020% by mass (1)

[0139] 2.0×10 -6 ≤ [Ti] × [N] ≤ 2.0 × 10 -3 (2)

[0140] Here,

[0141] Pinned Nb content = [Nb] - NbC-based inclusion area ratio (area%) × 0.892

[0142] [X] represents the content (mass%) of element X.

[0143] "NbC-based inclusions" refer to NbC-based particles with an Nb content of 50% by mass or more and a maximum length of 0.1 μm or more.

[0144] "NbC-based inclusion area ratio" refers to the ratio of the areas of NbC-based inclusions included in a 4000 μm × 4000 μm field of view within the measurement range.

[0145] Here, "maximum length" refers to the maximum length of a single particle.

[0146] Furthermore, in a cross section perpendicular to the longitudinal direction of the rolled or forged material, the length of the line connecting the centroid of the cross section to the boundary closest to the centroid is d / 2, and the closed curve on the cross section with a distance of d / 4 from the boundary is called the d / 4 portion. The "measurement range" refers to the range of length d / 4 extending from the boundary to the d / 4 portion.

[0147] [1.2.1 Expression (1)]

[0148] "Nb content in pinning" represents the percentage (by mass) of Nb added to steel that can precipitate as pinning particles.

[0149] In other words, "pinning Nb content" is the sum of the amount of Nb that can dissolve in steel and precipitate as pinning particles and the amount of Nb that has already precipitated as pinning particles.

[0150] "Pinned particles" refer to NbC-based particles with an Nb content of 50% or more by mass and a maximum length of less than 0.1 μm.

[0151] If the amount of pinned particles (fine NbC-based particles) can be estimated, it can be determined whether austenite grain coarsening can be suppressed during the carburizing process.

[0152] In calculating the amount of pinned particles, one can first consider methods that detect and integrate the area of ​​pinned particles in the plane. However, detecting small objects from images or similar sources is often difficult, and false positives and errors frequently increase.

[0153] Therefore, the amount of NbC-based inclusions is calculated from NbC-based inclusions (coarse NbC-based particles) that are relatively easy to detect from images, etc. The amount of Nb in the NbC-based inclusions is subtracted from the amount of Nb added, thereby indirectly calculating the amount of Nb in the pinned particles (pinned Nb content).

[0154] More specifically, the content (mass%) of NbC-based inclusions in Fe can be calculated by multiplying the area ratio (area%) of NbC-based inclusions by [density of NbC] / [density of Fe] = 7.82 / 7.76.

[0155] NbC-based inclusions are primarily composed of Nb and C, with an atomic ratio of approximately 1:1. Therefore, assuming an atomic ratio of 1:1, the amount of Nb in an NbC-based inclusion can be calculated by multiplying the NbC inclusion content (mass%) by [atomic weight of Nb = 92.9] / [molecular weight of NbC = 92.9 + 12 = 104.9].

[0156] The amount of pinned Nb that can be precipitated as pinned particles can be determined by subtracting the amount of Nb required to precipitate NbC-based inclusions from the amount of Nb added.

[0157] Generally, as the amount of pinned Nb increases, more pinned particles are formed, which makes it easier to suppress austenite grain coarsening during carburizing.

[0158] Therefore, the pinned Nb content needs to be 0.020% by mass or more. Preferably, the pinned Nb content is 0.025% by mass or more, and more preferably 0.030% by mass or more.

[0159] [1.2.2 Expression (2)]

[0160] The value of [Ti] × [N] is related to the ease of TiN formation and the amount of TiN formed. When TiN-based particles are moderately dispersed, NbC-based particles also disperse and grow, which can suppress the formation of NbC-based inclusions.

[0161] When the value of [Ti] × [N] is too low, the number of TiN-based particles becomes too small, and the absolute number of NbC-based particles using TiN-based particles as nuclei becomes too small. Therefore, the NbC-based particles per nucleus become too large. As a result, NbC-based particles may continue to grow during the solidification process after dissolution, forming NbC-based inclusions.

[0162] Therefore, the value of [Ti] × [N] needs to be 2.0 × 10 -6 The preferred value for [Ti] × [N] is 1.0 × 10⁻⁶. -5 above.

[0163] On the other hand, when the value of [Ti] × [N] is too large, in other words, when the amount of Ti and / or N is too high, these elements may dissolve in NbC-based particles, leading to an increase in solid solution temperature and making it difficult to dissolve NbC-based inclusions.

[0164] Therefore, the value of [Ti] × [N] needs to be 2.0 × 10 -3 The preferred value for [Ti] × [N] is 5.0 × 10⁻⁶. -4 the following.

[0165] [2. Method for manufacturing surface-hardened steel]

[0166] The surface-hardened steel according to the present invention can be manufactured according to the following steps. The method includes: (a) a “melting and casting step” – melting and casting raw materials mixed according to a predetermined composition to obtain a steel ingot; (b) a “hot forging step” – hot forging the obtained steel ingot to obtain a hot forging; (c) a “normalizing step” – normalizing the hot forging if necessary to obtain a normalized part; (d) a “spheroidizing and annealing step” – spheroidizing and annealing the hot forging or normalized part if necessary to obtain a spheroidized and annealed part; and (e) a “cold working step” – cold working the hot forging, normalized part, or spheroidized and annealed part if necessary.

[0167] [2.1 Smelting and Casting Steps]

[0168] First, raw materials mixed according to a predetermined composition are smelted and cast to obtain steel ingots. The methods and conditions for smelting and casting are not particularly restricted, and the optimal methods and conditions can be selected according to the purpose.

[0169] For example, electric furnaces, vacuum high-frequency induction melting furnaces, etc., can be used to produce molten steel.

[0170] [2.2 Hot Forging Steps]

[0171] The obtained steel ingots are then hot-forged. The purpose of hot forging is to: (a) break down the coarse casting structure and refine the structure, (b) process the steel ingots into steel products with shapes suitable for subsequent steps, such as slabs, billets, or small billets, and (c) dissolve NbC-based inclusions and precipitate pinned particles.

[0172] The solution temperature of NbC-based inclusions depends on the amount of carbon in the steel. If the heating temperature during hot forging is too low, not only may the deformation resistance increase, but the NbC-based inclusions may also fail to dissolve sufficiently.

[0173] Therefore, the heating temperature is preferably 900 + 1500 × [C] ℃ or higher. More preferably, it is 930 + 1500 × [C] ℃ or higher. The upper limit of the heating temperature during the hot forging step is not particularly limited, and is, for example, 1350 ℃ or lower.

[0174] If the heating time is too short, NbC-based inclusions may not dissolve sufficiently.

[0175] Therefore, the heating time is preferably 0.5 hours or more.

[0176] On the other hand, even when the heating time is extended beyond what is necessary, there is no difference in effect and no practical benefit.

[0177] Therefore, the heating time is preferably less than 20 hours.

[0178] After hot forging, the hot forging is cooled to room temperature. At this time, the spiked particles precipitate in the steel during the cooling process.

[0179] The final forging temperature for hot forging is preferably below 1050°C. This is because if the final forging temperature exceeds 1050°C, hard bainite may form, causing manufacturability problems. Furthermore, in subsequent steps, the bainite may form fine austenite grains, resulting in an uneven microstructure and coarsening of the austenite grains.

[0180] The cooling rate is preferably below 2 °C / s. This is because if the cooling rate exceeds 2 °C / s, hard bainite may form, causing manufacturability problems. Furthermore, in subsequent steps, the bainite may form fine austenite grains, resulting in an uneven microstructure and coarsening of the austenite grains.

[0181] [2.3 Normalizing Steps]

[0182] Subsequently, the hot forgings are normalized if necessary.

[0183] When the austenite grain size of ferrite and pearlite in a hot forging is uneven, its cold workability may decrease. In this case, it is preferable to normalize the hot forging to adjust the austenite grain size.

[0184] If the austenite grains in the hot forging are uniform, normalizing can be omitted.

[0185] Normalizing is performed by heating a hot forging to a predetermined temperature and holding it, followed by cooling.

[0186] If the normalizing temperature is too low, the uniformity of the austenite grains may be insufficient.

[0187] Therefore, the preferred normalizing temperature is above 850℃.

[0188] On the other hand, if the normalizing temperature becomes too high, there is a problem of pinning particles growing.

[0189] Therefore, the normalizing temperature is preferably below 1050℃.

[0190] If the normalizing time is too short, the uniformity of austenite grains may be insufficient.

[0191] Therefore, the optimal normalizing time is 0.5 hours or more.

[0192] On the other hand, even if the duration of the heat treatment is extended beyond what is necessary, the effect is the same and there is no practical benefit.

[0193] Therefore, the heating time is preferably less than 5 hours.

[0194] After normalizing, the normalized part is cooled to room temperature. During this cooling process, studded particles may precipitate in the steel.

[0195] [2.4 Spheroidizing and Annealing Steps]

[0196] Subsequently, if necessary, the hot forgings or normalized parts are spheroidized and annealed.

[0197] In the presence of strip-shaped carbides in hot forgings or normalized parts, cold workability may decrease. In such cases, it is preferable to spheroidize and anneal the hot forgings or normalized parts to spheroidize the carbides and soften the material.

[0198] If the hardness of hot forgings or normalized parts is sufficient for cold working, spheroidizing and annealing can be omitted.

[0199] When spheroidizing and annealing are performed, local pinning due to spheroidized carbides may occur, potentially leading to significant coarsening of the austenite grains. Therefore, from the perspective of suppressing austenite grain coarsening, it may be preferable not to perform spheroidizing and annealing.

[0200] In this invention, the methods of spheroidizing and annealing are not particularly limited, and the optimal method can be selected according to the purpose.

[0201] After spheroidizing and annealing, the spheroidized and annealed part is cooled to room temperature. At this time, pinning particles may precipitate in the steel during the cooling process.

[0202] [2.5 Cold working steps]

[0203] Subsequently, if necessary, the hot forgings, normalized parts, or spheroidized annealed parts are cold-worked.

[0204] Cold working is performed to form hot forgings, normalized parts, or spheroidized annealed parts into the final product shape.

[0205] Cold working methods are not particularly restricted, and the optimal method can be selected according to the purpose.

[0206] The obtained cold-worked parts are then subjected to carburizing treatment.

[0207] [3. Effect]

[0208] When the production conditions and [Ti] × [N] of surface-hardened steel are optimized, TiN-based particles can be moderately dispersed in the steel during the cooling process after casting. When TiN-based particles are moderately dispersed in the steel, fine NbC-based particles (i.e., pinned particles) disperse and grow, while the formation of coarse NbC-based particles (i.e., NbC-based inclusions) can be suppressed.

[0209] Accordingly, when the amount of NbC-based inclusions is low, the NbC-based inclusions are easily dissolved in subsequent heating steps.

[0210] Therefore, in the state before carburizing, the pinning Nb content can be above 0.020% by mass.

[0211] When surface-hardened steel with this structure undergoes carburizing treatment, the coarsening of austenite grains is suppressed by the dispersion of a large number of pinned particles in the steel.

[0212] In calculating the amount of pinned particles, the detection and integration methods for pinned particles in a plane are first considered. However, detecting small pinned particles from images and other sources is often difficult, leading to misjudgments and increased errors in many cases.

[0213] Therefore, in this invention, the amount of NbC-based inclusions is calculated from coarse NbC-based inclusions that are relatively easy to detect from images, etc., and the amount of Nb in the NbC-based inclusions is subtracted from the amount of Nb added, thereby indirectly calculating the amount of Nb in the pinned particles (pinned Nb content).

[0214] In other words, this method estimates the amount of pinned particles by focusing on NbC-based inclusions, which is significantly different from existing techniques.

[0215] Example

[0216] Example 1-23 and Comparative Example 1-6

[0217] [1. Sample Preparation]

[0218] [1.1 Smelting and Casting Steps]

[0219] First, 50 kg of steel with the composition shown in Table 1 is melted and cast in a vacuum high-frequency induction melting furnace to produce steel ingots with a diameter of 100 mm.

[0220] [1.2 Hot working steps]

[0221] Subsequently, the steel ingot is heated at 1200℃ or 1170℃ for 4 hours, and then hot forged to a diameter of 30 mm to obtain a bar-shaped steel strip.

[0222] [1.3 Normalizing Steps]

[0223] Then, each steel bar was normalized.

[0224] Normalizing is performed by holding the steel bar at 920°C for 1 hour and then allowing it to cool.

[0225] [1.4 Spheroidizing and Annealing Steps]

[0226] Subsequently, each steel bar undergoes spheroidization and annealing after normalizing.

[0227] Spheroidization and annealing are performed by holding the steel bar at 765°C for 6 hours, then gradually cooling it to 670°C at a rate of 9.5°C / h, and then cooling it in a furnace.

[0228] [1.5 Cold working steps]

[0229] Subsequently, cylindrical steel with a diameter of 15 mm and a height of 22.5 mm was machined from each spheroidized and annealed steel bar.

[0230] The cylindrical steel was cold-headed to obtain a cylindrical steel with a height of 6.75 mm.

[0231] [1.6 Simulated Carburizing Step]

[0232] Each obtained cylindrical steel was subjected to a heat treatment simulating carburizing.

[0233] Heat treatment is performed by holding cylindrical steel in an atmospheric furnace at each temperature of 940°C to 1040°C for 3 hours, followed by water cooling.

[0234] [Table 1]

[0235]

[0236] [Table 1 (continued)]

[0237]

[0238] [2. Testing Method]

[0239] [2.1. Amount of NbC-based inclusions]

[0240] Specifically, the NbC-based inclusion area ratio was determined according to the following steps: (1) A sample for scanning electron microscopy (SEM) observation was cut from a hot forging, with the boundary and d / 4 portion located at the end. (2) A 4000 × 4000 μm sample at the center was obtained by SEM. 2 (3) By contrast, inclusions with a maximum length of 0.1 μm or more are extracted from the cross-sectional images. (4) Energy dispersive spectroscopy (EDS) is performed on the center of each inclusion, and inclusions with a Nb content of 50% by mass or more are identified as NbC-based inclusions. (5) The area of ​​the NbC-based inclusions is integrated. (6) The integrated area of ​​the NbC-based inclusions is divided by 4000 × 4000 μm. 2 To calculate the area ratio (area %) of NbC-based inclusions.

[0241] [2.2 Nb content in pinning]

[0242] The pinned Nb content was calculated by using the calculated NbC-based inclusion area ratio and the amount of Nb added.

[0243] [2.3 Austenite grain coarsening]

[0244] Austenite grain coarsening was determined according to the following steps: (1) The central portion of the simulated carburized part was cut along the longitudinal direction, and the austenite grain boundaries of the cut surface were exposed using a known etching solution. (2) The entire cut surface was observed using an optical microscope, and the equivalent circle diameter (the diameter of a circle with the same area) of the austenite grains in the observed area was calculated by image analysis. (3) Grains with an equivalent circle diameter of 100 μm or more were determined to be austenite grain coarsening. For each simulated carburized part, the lowest temperature at which coarsened grains were observed was defined as the "temperature at which coarsened grains were observed".

[0245] [3. Results]

[0246] The results are shown in Table 2. Table 2 also shows (a) the amount of C added (mass%), (b) the lower limit required for forging heating temperature (900 + 1500 × [C] (°C)), (c) the actual forging heating temperature (°C), (d) the amount of Nb added (mass%), and (e) the amount of Nb in the NbC-based inclusions as expressed by the NbC-based inclusion area ratio (area%) × 0.892. Table 2 is shown below.

[0247] (1) In Comparative Example 1, coarsening of grains was observed when the simulated carburizing temperature was 980°C. This was attributed to the pinned Nb content being less than 0.020% by mass (0.018% by mass).

[0248] The reason for the low Nb content in the pinning is believed to be due to the excessive amount of C. The actual forging heating temperature (1200℃) did not reach the lower limit of the required forging heating temperature (900 + 1500 × [C] (℃) = 1235℃), so the NbC-based inclusions could not be completely dissolved and remained in large quantities.

[0249] (2) In Comparative Example 2, coarsened grains were observed when the simulated carburizing temperature was 940°C. This was considered to be due to pinning Nb content below 0.020% by mass (0.005% by mass).

[0250] The reason for the lower Nb content in pinning is believed to be the lower amount of Nb added.

[0251] (3) In Comparative Example 3, coarsened grains were observed when the simulated carburizing temperature was 980°C. This was attributed to the pinned Nb content being less than 0.020% by mass (0.019% by mass).

[0252] The reason for the low Nb content in the pinning is believed to be due to the excessive amount of Al, which formed a large number of Al-based compounds and a large number of NbC-based inclusions that used Al-based compounds as crystal nuclei.

[0253] (4) In Comparative Example 4, coarsening of grains was observed when the simulated carburizing temperature was 960°C. This was attributed to the pinned Nb content being less than 0.020% by mass (0.019% by mass).

[0254] The reason for the low Nb content is believed to be due to the excessive amount of Nb, which forms a large number of NbC-based inclusions.

[0255] (5) In Comparative Example 5, coarsening of grains was observed when the simulated carburizing temperature was 980°C. This was attributed to the pinned Nb content being less than 0.020% by mass (0.019% by mass).

[0256] The reason for the low Nb content in the pinning is believed to be that the actual forging heating temperature (1170℃) did not reach the lower limit of the required forging heating temperature (900 + 1500 × [C] (℃) = 1184℃), so the NbC-based inclusions could not be completely dissolved and remained in large quantities.

[0257] (6) In Comparative Example 6, coarsening of grains was observed when the simulated carburizing temperature was 980°C. This was attributed to the pinning of Nb content to be less than 0.020% by mass (0.018% by mass).

[0258] The reason for the low Nb content in the pinning is believed to be that the actual forging heating temperature (1170℃) did not reach the lower limit of the required forging heating temperature (900 + 1500 × [C] (℃) = 1196℃), so the NbC-based inclusions could not be completely dissolved and remained in large quantities.

[0259] (7) In Comparative Example 7, coarsened grains were observed when the simulated carburizing temperature was 960°C. This was attributed to the pinned Nb content being less than 0.020% by mass (0.016% by mass).

[0260] The reason for the low Nb content in the pinning is believed to be due to the excessive amount of C. The actual forging heating temperature (1170℃) did not reach the lower limit of the required forging heating temperature (900 + 1500 × [C] (℃) = 1235℃), so the NbC-based inclusions could not be completely dissolved and remained in large quantities.

[0261] (8) In Comparative Example 8, coarsening of grains was observed when the simulated carburizing temperature was 940°C. This was attributed to the pinned Nb content being less than 0.020% by mass (0.005% by mass).

[0262] The reason for the lower Nb content in pinning is believed to be the lower amount of Nb added.

[0263] (9) In Examples 1-21, the temperature of the coarsened particles was observed to be above 1000°C. This is believed to be because the pinned Nb content was above 0.020% by mass (0.021-0.050% by mass).

[0264] [Table 2]

[0265]

[0266] [Table 2 (continued)]

[0267]

[0268] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention.

[0269] This application is based on Japanese Patent Application No. 2024-176281 filed on October 8, 2024 and Japanese Patent Application No. 2025-124688 filed on July 25, 2025, the contents of which are incorporated herein by reference.

[0270] Industrial applicability

[0271] The surface-hardened steel according to the present invention can be used in components such as gears and continuously variable transmissions (CVTs).

Claims

1. A case hardening steel comprising: 0.10 mass% ≤ C ≤ 0.20 mass%, 0.05 mass% ≤ Si ≤ 2.00 mass%, 0.30 mass% ≤ Mn ≤ 2.00 mass%, P ≤ 0.030 mass%, S ≤ 0.030 mass%, 0.01 mass% ≤ Cu ≤ 1.00 mass%, 0.01 mass% ≤ Ni ≤ 1.00 mass%, 0.30 mass% ≤ Cr ≤ 3.00 mass%, 0.0001 mass% ≤ Ti ≤ 0.2000 mass%, 0.040 mass% ≤ Nb ≤ 0.080 mass%, 0.002 mass% ≤ Al ≤ 0.060 mass%, and 0.003 mass% ≤ N ≤ 0.040 mass%, with the balance being Fe and unavoidable impurities, and the following equations (1) and (2) are satisfied: Pinning Nb content ≥ 0.020 mass% (1) 2.0 × 10 -6 ≤ [Ti] × [N] ≤ 2.0 × 10 -3 (2) wherein, Pinning Nb content = [Nb] - NbC-based inclusion area ratio (area%) x 0.892, [X] is the content of element X (mass%), the "NbC-based inclusion" refers to an NbC-based particle having a Nb content of 50 mass% or more and a maximum length of 0.1 μm or more, the "NbC-based inclusion area ratio" refers to the ratio of the area of the NbC-based inclusion included in a field area of 4000 μm x 4000 μm in the measurement range, in a cross section perpendicular to the longitudinal direction of a rolled material or a forged material, the length of a line connecting the center of gravity of the cross section to the boundary of the cross section closest to the center of gravity is d / 2, and a closed curve on the cross section at a distance of d / 4 from the boundary is referred to as a d / 4 portion, and the "measurement range" refers to a range extending from the boundary to a length of d / 4 of the d / 4 portion.

2. The case hardening steel according to claim 1, further comprising: B ≤ 0.010 mass%, and / or Mo ≤ 1.00 mass%.

3. A production method of a case hardening steel, comprising: a melting and casting step of melting and casting a mixed raw material having the following composition, thereby obtaining an ingot, and a hot forging step of hot forging the ingot, thereby obtaining a hot forged piece, and optionally, at least one of the following steps: a normalizing step of normalizing the hot forged piece, thereby obtaining a normalized piece, a spheroidizing and annealing step of spheroidizing and annealing the hot forged piece or the normalized piece, thereby obtaining a spheroidized and annealed piece, and a cold working step of cold working the hot forged piece, the normalized piece, or the spheroidized and annealed piece.

4. The production method of a case hardening steel according to claim 3, wherein the hot forged piece, the normalized piece, the spheroidized and annealed piece, or the cold worked piece is subjected to a machining step, thereby obtaining a machined piece. wherein the composition includes: 0.10 mass% ≤ C ≤ 0.20 mass%, 0.05 mass% ≤ Si ≤ 2.00 mass%, 0.30 mass% ≤ Mn ≤ 2.00 mass%, P ≤ 0.030 mass%, S ≤ 0.030 mass%, 0.01 mass% ≤ Cu ≤ 1.00 mass%, 0.01 mass% ≤ Ni ≤ 1.00 mass%, 0.30 mass% ≤ Cr ≤ 3.00 mass%, 0.0001 mass% ≤ Ti ≤ 0.2000 mass%, 0.040 mass% ≤ Nb ≤ 0.080 mass%, 0.002 mass% ≤ Al ≤ 0.060 mass%, and 0.003 mass% ≤ N ≤ 0.040 mass%, with the balance being Fe and unavoidable impurities, and wherein, in the hot forging step, the ingot is heated to a heating temperature of 900 + 1500 × [C] °C or higher, where [X] is the content of element X (mass%).

4. The method of claim 3, wherein, The composition further includes: B ≤ 0.010 mass%, and / or Mo ≤ 1.00 mass%.

Images