Muscle steel

Abstract

To provide a case-hardening steel that can suppress the coarsening of austenite grains during carburizing. [Solution] The case-hardening steel contains 0.10≦C≦0.20mass%, 0.05≦Si≦2.00mass%, 0.30≦Mn≦2.00mass%, P≦0.030mass%, S≦0.030mass%, 0.01≦Cu≦1.00mass%, 0.01≦Ni≦1.00mass%, 0.30≦Cr≦3.00mass%, 0.0001≦Ti≦0.2000mass%, 0.040≦Nb≦0.080mass%, 0.002≦Al≦0.060mass%, and 0.003≦N≦0.040mass%, with the remainder being Fe and unavoidable impurities. The case-hardening steel further contains pinned Nb amount ≥ 0.020mass%, and 2.0×10 -6 ≤[Ti]×[N]≦2.0×10 -3 It satisfies the relationship.

Description

[Technical Field]

[0001] The present invention relates to case-hardening steel, and more particularly to case-hardening steel capable of suppressing the coarsening of austenite crystal grains during carburizing treatment. [Background technology]

[0002] "Case-hardened steel (or carburized steel)" refers to steel suitable for carburizing or carbonitriding treatment (hereinafter, these are collectively referred to as "carburizing treatment"). When carburized case-hardened steel is quenched, only the surface layer can be hardened. For this reason, case-hardened steel is used in various parts that require wear resistance, such as gears and components of continuously variable transmissions (CVTs).

[0003] In recent years, the use of cold forging for case-hardened steel has been increasing from the perspective of energy conservation and reduction of manufacturing time. However, cold-forged products are prone to coarsening of austenite grains during the carburizing process. Therefore, one possible measure to suppress the coarsening of austenite grains is to lower the carburizing temperature. However, lowering the carburizing temperature necessitates a longer carburizing time, which reduces production efficiency.

[0004] Traditionally, fine particles such as AlN and NbC have been known as pinning particles that suppress grain growth by pinning grain boundaries. By dispersing and precipitating these fine particles, the movement of grain boundaries is inhibited, and the growth of crystal grains is effectively suppressed. Various methods have been proposed for suppressing grain growth using such fine particles.

[0005] For example, Patent Document 1 describes a material with the following composition by 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%, and N: <0.0080%, with the remainder being Fe and unavoidable impurities. It has a structure in which ferrite and pearlite fractions are 80% or more, and contains 30 precipitates / μm with a diameter of less than 50 nm and containing Nb. 2 In addition, there are 3 precipitates per μm containing Nb with a diameter of 50 nm to 100 nm. 2 The following conditions apply, and the number of precipitates containing Nb with a diameter of less than 50 nm is n. A , the number of precipitates containing Nb with a diameter of 50 nm to 100 nm n B However, n A -5n B A case-hardening steel that satisfies the relationship >30 is disclosed.

[0006] The document states that by specifying the component composition of the steel as described above, setting the microstructure fraction of ferrite and pearlite as described above, and further regulating the number of Nb-containing precipitates as described above, the case-hardened steel possesses excellent cold forgeability and the ability to suppress grain coarsening.

[0007] When hot working a case-hardening steel containing precipitates made of NbC, or compounds in which Ti, N, etc., are solid-dissolved (hereinafter, these may be referred to as "NbC-based particles"), heating the case-hardening steel above a predetermined temperature allows coarse NbC-based particles to be solid-dissolved in the steel. Furthermore, cooling at a predetermined cooling rate after hot working allows fine NbC-based particles to be precipitated in the steel. As described in Patent Document 1, when a large amount of fine NbC-based particles are precipitated, these fine NbC-based particles function as pinning particles, suppressing the coarsening of austenite crystal grains during carburizing.

[0008] However, when the amount of Nb becomes excessive, coarse NbC-based particles precipitate and grow during melting and casting. Therefore, even if the skin-burned steel is heated to a predetermined temperature thereafter, it may be difficult to completely dissolve the coarse NbC-based particles in the steel. When the precipitation amount of coarse NbC-based particles becomes excessive, the amount of Nb dissolved in the steel decreases. Therefore, in the precipitation process for finely precipitating NbC-based particles (for example, the cooling process after hot working), the precipitation amount of fine NbC-based particles decreases. That is, simply increasing the amount of Nb only increases the coarse NbC-based particles, but on the contrary, the precipitation amount of fine NbC-based particles decreases, which may cause coarsening of austenite grain boundaries during carburizing treatment.

[0009] Also, Patent Document 1 describes a technique for precipitating nano-level fine NbC-based particles in an observation within the range of several μm. 2 However, even fine NbC-based particles may coarsen during carburizing treatment and lose their function as pinning particles. Therefore, the method of Patent Document 1 has limitations in suppressing the coarsening of austenite grain boundaries during carburizing treatment.

[0010] Coarse NbC-based particles often grow with TiN-based particles or AlN-based particles as nuclei. At this time, if the number of TiN-based particles and / or AlN-based particles is excessively small, coarse NbC-based particles are likely to be generated. When such skin-burned steel is heated to a predetermined temperature, the coarse NbC-based particles may further grow.

[0011] As described above, the total amount of fine NbC-based particles is correlated with the total amount of coarse NbC-based particles. In other words, to grasp the total amount of fine NbC-based particles, it is necessary to grasp the total amount of coarse NbC-based particles. However, there has been no prior example of skin-burned steel that focuses on the total precipitation amount of coarse NbC-based particles in a relatively wide area of about 10 6 μm 2 extent.

Prior Art Documents

Patent Document

[0012]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0013] The problem to be solved by the present invention is to provide a skin-burning steel capable of suppressing the coarsening of austenite crystal grains during carburizing treatment.

Means for Solving the Problems

[0016] More specifically, as shown in Figure 1(a), when the cross-sectional shape is a circle, the center of the circle is the centroid G, and the radius of the circle is d / 2. Furthermore, the d / 4 section is a closed curve with a distance of d / 4 from the boundary of the cross-section, and is therefore a circle with radius d / 4 centered at the centroid G. The measurement range is the range of length d / 4 connecting the boundary and the d / 4 section. In this case, the extension of the measurement range into the cross-section will pass through the centroid G.

[0017] Furthermore, as shown in Figure 1(b), when the cross-sectional shape is a square, the centroid G is the intersection of the diagonals of the square. Here, since the length of the line connecting the centroid to the boundary of the cross-section closest to it is d / 2, the length of one side of the cross-section is d. Also, the d / 4 section is a closed curve with a distance of d / 4 from the boundary of the cross-section, so it is a square with a side length of d / 2 and the centroid G concentric. The measurement range is the range of length d / 4 connecting the boundary and the d / 4 section. In this case, the measurement range is the direction perpendicular to the d / 4 section within the cross-section. [Effects of the Invention]

[0018] In case-hardening steel, optimizing the manufacturing conditions and the [Ti]×[N] ratio allows for a moderate dispersion of TiN-based particles within the steel during the cooling process after casting. When TiN-based particles are moderately dispersed within the steel, fine NbC-based particles (=pinned particles) can be dispersed and grown, while the formation of coarse NbC-based particles (=NbC-based inclusions) can be suppressed. As a result, if the amount of NbC-based inclusions is small, it becomes easier to solidify the NbC-based inclusions during the subsequent heating process. Therefore, the amount of pinned Nb can be increased to 0.020 mass% or more before carburizing. When carburizing is performed on case-hardened steel exhibiting this type of structure, the coarsening of austenite grains is suppressed by the pinned particles dispersed in large quantities within the steel. [Brief explanation of the drawing]

[0019] [Figure 1] (a) An explanatory diagram of the measurement range when the cross-sectional shape is a circle, and (b) an explanatory diagram of the measurement range when the cross-sectional shape is a square.

[0020] One embodiment of the present invention will be described in detail below. [1. Case hardened steel] [1.1. Composition] [1.1.1. Main constituent elements] The case-hardening steel according to the present invention contains the following elements, with the remainder being Fe and unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows.

[0021] (1) 0.10 ≤ C ≤ 0.20 mass%: C combines with Nb to form NbC particles. In this invention, "NbC-based particles" means, (a) NbC, (b) A solid solution in which other elements (e.g., Ti, N, etc.) are dissolved in NbC, or (c) NbC grown with TiN-based particles or AlN-based particles as nuclei It refers to a particle made up of any of the following. In this invention, fine NbC-based particles that contribute to suppressing the coarsening of austenite grains during carburizing treatment are sometimes referred to as "pinning particles." In this invention, coarse NbC-based particles that do not contribute to suppressing the coarsening of austenite grains during carburizing treatment are sometimes referred to as "NbC-based inclusions."

[0022] C is an essential element for ensuring hardness and strength. If the carbon content becomes too low, the hardness of the core (center) of the carburized part may not be ensured. Therefore, the amount of C must be 0.10 mass% or more.

[0023] On the other hand, if the amount of carbon becomes too high, NbC-based inclusions that cannot be completely dissolved may form during the heating process prior to the process of precipitating pinning particles (hereinafter sometimes referred to as the "precipitation process"). As a result, the amount of pinning particles that can precipitate during the precipitation process becomes insufficient. Consequently, the number of pinning particles that can pin the grain boundaries during the carburizing process decreases, and it may become impossible to suppress the coarsening of austenite grains. Therefore, the amount of carbon (C) must be 0.20 mass% or less. Preferably, the amount of carbon (C) is 0.18 mass% or less.

[0024] (2) 0.05 ≤ Si ≤ 2.00 mass%: Si is an effective element for improving hardenability and increasing strength. To achieve this effect, the Si content must be 0.05 mass% or higher.

[0025] Si is an element that strengthens ferrite through solid solution. Therefore, if the Si content becomes too high, it can lead to an increase in hardness and a deterioration in cold workability. Therefore, the Si content must be 2.00 mass% or less. Preferably, the Si content is 0.50 mass% or less, and more preferably 0.30 mass% or less.

[0026] (3) 0.30 ≤ Mn ≤ 2.00 mass%: Mn is an effective element for improving hardenability and increasing strength. To achieve this effect, the amount of Mn must be 0.30 mass% or higher.

[0027] Mn is an element that strengthens ferrite through solid solution. Therefore, if the amount of Mn becomes too high, it can lead to an increase in hardness and a deterioration in cold workability. Therefore, the amount of Mn must be 2.00 mass% or less. Preferably, the amount of Mn is 1.00 mass% or less, and more preferably 0.70 mass% or less.

[0028] (4) P ≤ 0.030 mass%: If the amount of phosphorus (P) becomes too high, grain boundary embrittlement may be promoted. Therefore, a lower amount of P is preferable. Therefore, the amount of P must be 0.030 mass% or less.

[0029] (5) S ≤ 0.030 mass %: S combines with Mn to increase the MnS content, improving machinability. However, too much S can reduce fatigue strength. Therefore, the amount of sulfur (S) must be 0.030 mass% or less.

[0030] (6) 0.01 ≤ Cu ≤ 1.00 mass%: Cu is an effective element for improving hardenability and increasing strength. In particular, it is an effective element for ensuring the hardness of the core (center) of carburized parts when the carbon content is low. To achieve this effect, the amount of Cu must be 0.01 mass% or more.

[0031] On the other hand, if the amount of copper becomes too high, it can lead to a decrease in hot forgeability. Therefore, the amount of Cu must be 1.00 mass% or less. Preferably, the amount of Cu is 0.30 mass% or less.

[0032] (7) 0.01 ≤ Ni ≤ 1.00 mass%: Ni is an effective element for improving hardenability and increasing strength. It is also an effective element for improving ductility. In particular, it is an effective element for ensuring hardness in the core (center) of carburized parts when the carbon content is low. To achieve this effect, the amount of Ni must be 0.01 mass% or higher.

[0033] On the other hand, if the amount of nickel becomes too high, it can lead to the formation of a bainite structure, which can reduce cold workability. Therefore, the amount of Ni must be 1.00 mass% or less. Preferably, the amount of Ni is 0.30 mass% or less.

[0034] (8) 0.30 ≤ Cr ≤ 3.00 mass%: Cr is an effective element for improving hardenability and increasing strength. To obtain such an effect, the amount of Cr must be 0.30 mass% or more. Preferably, the amount of Cr is 0.80 mass% or more.

[0035] On the other hand, if the amount of chromium becomes too high, the machinability may deteriorate. Therefore, the amount of Cr must be 3.00 mass% or less. Preferably, the amount of Cr is 2.00 mass% or less.

[0036] (9) 0.0001 ≤ Ti ≤ 0.2000 mass%: If the amount of Ti becomes too low, the number of TiN-based particles decreases, and the number of NbC-based particles using these as nuclei becomes too low. As a result, the NbC-based particles per nucleus become too large. Consequently, the NbC-based particles continue to grow during the solidification process after dissolution, which can lead to the formation of NbC-based inclusions. Therefore, the Ti content must be 0.0001% or more. Preferably, the Ti content is 0.0030 mass% or more, and more preferably 0.0035 mass% or more.

[0037] On the other hand, if the amount of Ti becomes too high, it will dissolve in the NbC-based particles, raising the solid solution temperature of the NbC-based particles. As a result, even after heat treatment, coarse NbC-based particles may not dissolve and may remain. Therefore, the amount of Ti must be 0.2000 mass% or less.

[0038] (10) 0.040 ≤ Nb ≤ 0.080 mass%: Nb has the function of precipitating pinned particles during the precipitation process and suppressing the coarsening of austenite crystal grains during the carburizing treatment. To obtain such an effect, the Nb content must be 0.040 mass% or higher. Preferably, the Nb content is 0.045 mass% or higher.

[0039] On the other hand, if the amount of Nb becomes too high, it may increase the amount of NbC-based inclusions. Furthermore, excessive NbC-based inclusions may reduce cold workability. Therefore, the Nb content must be 0.080 mass% or less. Preferably, the Nb content is 0.070 mass% or less.

[0040] (11) 0.002 ≤ Al ≤ 0.060 mass%: Al not only forms fine AlN-based particles that provide a pinning effect, but also functions as a deoxidizing agent. If the amount of Al becomes too low, these functions may not be performed. Therefore, the amount of Al must be 0.002 mass% or more.

[0041] On the other hand, if the amount of Al becomes too high, it can form Al-based compounds that act as nuclei for NbC-based inclusions. Therefore, the amount of Al must be 0.060 mass% or less.

[0042] (12) 0.003 ≤ N ≤ 0.040 mass%: When the amount of N is appropriate, it can bond with Al to form fine AlN-based particles that exhibit a pinning effect, thereby suppressing grain coarsening. To achieve this effect, the amount of N must be 0.003 mass% or higher.

[0043] On the other hand, if the amount of nitrogen becomes too high, it will dissolve in the NbC-based particles, raising the solid solution temperature of the NbC-based particles. As a result, even after heat treatment, coarse NbC-based particles may not dissolve and may remain. Therefore, the amount of N must be 0.040 mass% or less.

[0044] (13) Inevitable impurities: The case-hardening steel according to the present invention may contain unavoidable impurities. Here, "unavoidable impurities" refers to components that are mixed in during the industrial production of case-hardening steel due to various factors such as raw materials and manufacturing processes, and whose content is within a range that does not adversely affect the properties of the case-hardening steel according to the present invention.

[0045] [1.1.2. Sub-constituent elements] The steel material according to the present invention may further contain the following elements in addition to the main constituent elements described above. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

[0046] (14) 0.0003 ≤ B ≤ 0.010 mass%: B is an effective element for improving hardenability and increasing strength. In particular, it is an effective element for ensuring the hardness of the core (center) of carburized parts when the carbon content is low. Furthermore, it also has the effect of reinforcing grain boundaries by segregating at the grain boundaries, thereby improving strength. To obtain these effects, the amount of B is preferably 0.0003 mass% or more.

[0047] On the other hand, if the amount of B becomes too high, it can react with N in the steel to form BN, which can reduce toughness. Therefore, the amount of B is preferably 0.010 mass% or less.

[0048] (15) Mo ≤ 1.00 mass %: Mo is an element that improves hardenability, increases strength, and enhances wear resistance. To obtain such an effect, the amount of Mo is preferably 0.01 mass% or more.

[0049] On the other hand, if the amount of Mo becomes too high, it can lead to the formation of a bainite structure, which can worsen cold workability. Also, Mo is an expensive element, so it is preferable to reduce the amount used. Therefore, the amount of Mo is preferably 1.00 mass% or less, and more preferably 0.10 mass% or less.

[0050] [1.2. Ingredient Balance] The case-hardening steel according to the present invention must satisfy the following formulas (1) and (2). Pinning Nb amount ≥ 0.020 mass% …(1) 2.0×10 -6 ≤[Ti]×[N]≦2.0×10 -3 …(2)

[0051] however, Pinning Nb amount = [Nb] - Area ratio of NbC-based inclusions (area%) × 0.892 [X] is the mass %) of element X. "NbC-based inclusions" refers to NbC-based particles having an Nb content of 50 mass% or more and a maximum length of 0.1 μm or more. "Area ratio of NbC-based inclusions" refers to the proportion of the area of ​​NbC-based inclusions contained within a 4000 μm × 4000 μm field of view within the measurement range. Here, "maximum length" refers to the longest possible length in a single particle. Furthermore, the "measurement range" refers to the range of length d / 4 connecting the boundary and the d / 4 section, where d / 2 is the length of the line connecting the centroid of the cross-section and the boundary of the cross-section closest to the centroid, and d / 4 is the distance from the boundary to the closed curve on the cross-section.

[0052] [1.2.1. Formula (1)] "Pinned Nb amount" refers to the amount (mass%) of Nb added to the steel that can precipitate as pinned particles. In other words, the "pinned Nb amount" is the sum of the amount of Nb that can dissolve in the steel and precipitate as pinned particles, and the amount of Nb that has already precipitated as pinned particles. Furthermore, "pinned particles" refer to NbC-based particles with an Nb content of 50 mass% or more and a maximum length of less than 0.1 μm.

[0053] If the amount of pinning particles (fine NbC-based particles) can be estimated, it is possible to determine whether or not the coarsening of austenite crystal grains can be suppressed during the carburizing process. Here, regarding the calculation of the pinned particle quantity, one possible method is to detect and integrate the area of ​​pinned particles within a plane. However, detecting minute particles from images is generally difficult, and misjudgments and errors are often large. Therefore, by calculating the amount of NbC-based inclusions (coarse NbC-based particles) that are relatively easy to detect from images, etc., and then subtracting the amount of Nb in the NbC-based inclusions from the amount of added Nb, the amount of Nb in the pinned particles (pinned Nb amount) can be calculated indirectly.

[0054] More specifically, the mass (mass%) of NbC-based inclusions in Fe can be calculated by multiplying the area fraction (area%) of NbC-based inclusions by [density of NbC] / [density of Fe] = 7.82 / 7.76. Furthermore, NbC inclusions are formed almost entirely 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 the NbC inclusion can be calculated by multiplying the mass (mass%) of the NbC inclusion by [atomic weight of Nb = 92.9] / [molecular weight of NbC = 92.9 + 12 = 104.9]. Then, by subtracting the amount of Nb required for the precipitation of NbC-based inclusions from the amount of Nb added, the amount of pinning Nb that can precipitate as pinning particles can be determined.

[0055] Generally, the higher the amount of pinning Nb, the more pinning particles are formed, making it easier to suppress the coarsening of austenite grains during the carburizing process. Therefore, the amount of pinned Nb must be 0.020 mass% or more. Preferably, the amount of pinned Nb is 0.025 mass% or more, and more preferably 0.030 mass% or more.

[0056] [1.2.2. Formula (2)] The [Ti]×[N] value correlates with the ease and amount of TiN formation. When TiN-based particles are properly dispersed, NbC-based particles also disperse and grow, suppressing the formation of NbC-based inclusions.

[0057] If the value of [Ti]×[N] becomes too low, the number of TiN-based particles becomes excessively small, and the absolute number of NbC-based particles using them as nuclei becomes too small. As a result, the NbC-based particles per nucleus become too large. Consequently, the NbC-based particles continue to grow during the solidification process after dissolution, sometimes forming NbC-based inclusions. Therefore, the value of [Ti] × [N] is 2.0 × 10 -6 The above is required. The value of [Ti] × [N] is preferably 1.0 × 10 -5 That's all.

[0058] On the other hand, if the value of [Ti] × [N] becomes too large, in other words, if the amount of Ti and / or N is too high, it may dissolve in the NbC-based particles, raising the solid solution temperature and making it difficult to dissolve the NbC-based inclusions. Therefore, the value of [Ti] × [N] is 2.0 × 10 -3 The following is required: The value of [Ti] × [N] is preferably 5.0 × 10 -4 The following applies:

[0059] [2. Method for manufacturing case-hardened steel] The case-hardening steel according to the present invention is manufactured according to the following procedure. (a) A "melting and casting process" in which raw materials that have been blended to have a predetermined composition are melted and cast, (b) The obtained ingot is subjected to a "hot forging process," (c) A "normalization process" in which the hot forged body is normalized as needed, (d) A "spheroidizing annealing process" in which spheroidizing annealing is performed on the hot forged body or the normalized body as needed, (e) A "cold working process" in which a hot forged body, a normalized body, or a spheroidized annealed body is cold-worked as needed. It is equipped with.

[0060] [2.1. Melting and Casting Process] First, the raw materials, which have been blended to achieve a predetermined composition, are melted and cast. The method and conditions of melting and casting are not particularly limited, and the most suitable method and conditions can be selected according to the purpose. For the production of molten steel, for example, electric furnaces, vacuum high-frequency induction melting furnaces, etc., can be used.

[0061] [2.2. Hot Forging Process] Next, the resulting ingot is subjected to hot forging. Hot forging is, (a) In order to break down the coarse cast structure and refine the structure, (b) To process the ingot into a steel material of a shape suitable for subsequent processes such as slabs, blooms, and billets, (c) To solid-solve NbC-based inclusions and precipitate pinned particles It will continue.

[0062] The solid 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 will the deformation resistance increase, but the solid solution of NbC-based inclusions may also be insufficient. Therefore, a heating temperature of 900 + 1500 × [C]°C or higher is preferable. More preferably, the heating temperature is 930 + 1500 × [C]°C or higher.

[0063] Furthermore, if the heating time is too short, the solid solution of NbC-based inclusions may not be sufficiently achieved. Therefore, a heating time of 0.5 hours or more is preferable. On the other hand, extending the heating time unnecessarily does not make any difference in effectiveness and offers no practical benefit. Therefore, a heating time of 20 hours or less is preferable. After hot forging, the hot-forged body is cooled to room temperature. During this cooling process, pinned particles precipitate in the steel.

[0064] Furthermore, the finishing temperature for hot forging is preferably 1050°C or lower. This is because if the finishing temperature exceeds 1050°C, a hard bainite structure may form, causing problems with manufacturability, and the bainite structure may lead to the formation of fine grains in the next process, resulting in an uneven structure and coarsening of the grains. Furthermore, the cooling rate is preferably 2°C / s or less. This is because if the cooling rate exceeds 2°C / s, a hard bainite structure may form, causing problems with manufacturability, and the bainite structure may lead to the formation of fine crystal grains in the next process, resulting in an uneven structure and coarsening of the crystal grains.

[0065] [2.3. Standardizing process] Next, if necessary, the hot forged body is subjected to normalization. If the ferrite and pearlite grain sizes in a hot-forged body are uneven, the cold workability may decrease. In such cases, it is preferable to normalize the hot-forged body to make the grains uniform. Furthermore, if the crystal grains of the hot-forged body are uniform, the normalizing process can be omitted.

[0066] Normalization is performed by heating and holding a hot-forged body to a predetermined temperature, and then cooling it. If the annealing temperature is too low, the uniformity of the crystal grains may be insufficient. Therefore, a preservation temperature of 850°C or higher is preferable. On the other hand, if the calcination temperature is too high, there is a problem in that pinned particles will grow. Therefore, a preservation temperature of 1050°C or lower is preferable.

[0067] Furthermore, if the quenching time is too short, the uniformity of the crystal grains may be insufficient. Therefore, a pre-seasoning time of 0.5 hours or more is preferable. On the other hand, extending the pre-cooking time unnecessarily does not make any difference in effectiveness and offers no practical benefit. Therefore, a heating time of 5 hours or less is preferable. After normalization, the normalized material is cooled to room temperature. During this cooling process, pinned particles may precipitate in the steel.

[0068] [2.4. Spheroidizing Annealing Process] Next, if necessary, spheroidizing annealing is performed on the hot-forged or normalized body. When hot-forged or normalized bodies contain striated carbides, their cold workability may be reduced. In such cases, it is preferable to perform spheroidizing annealing on the hot-forged or normalized body to spheroidize the carbides and soften the material. Furthermore, if the hardness of the hot-forged or normalized body allows for cold working, spheroidizing annealing can be omitted. Furthermore, spheroidizing annealing can sometimes lead to localized pinning by spheroidized carbides, resulting in grain coarsening. Therefore, from the viewpoint of suppressing grain coarsening, it may be preferable not to perform spheroidizing annealing.

[0069] In the present invention, the method of spheroidizing annealing is not particularly limited, and the most suitable method can be selected depending on the purpose. After spheroidizing annealing, the spheroidized annealed body is cooled to room temperature. During this cooling process, pinned particles may precipitate in the steel.

[0070] [2.5. Cold working process] Next, if necessary, cold working is performed on the hot-forged body, normalized body, or spheroidized annealed body. Cold working is performed to shape hot-forged, normalized, or spheroidized annealed bodies into the final product shape. The method of cold working is not particularly limited, and the most suitable method can be selected according to the purpose. The resulting cold-worked body is subjected to a carburizing treatment.

[0071] [3. Effect] In case-hardening steel, optimizing the manufacturing conditions and the [Ti]×[N] ratio allows for a moderate dispersion of TiN-based particles within the steel during the cooling process after casting. When TiN-based particles are moderately dispersed within the steel, fine NbC-based particles (=pinned particles) can be dispersed and grown, while the formation of coarse NbC-based particles (=NbC-based inclusions) can be suppressed. This means that if the amount of NbC inclusions is small, it becomes easier to solidify the NbC inclusions during the subsequent heating process. Therefore, the amount of pinned Nb can be increased to 0.020 mass% or more before carburizing. When carburizing is performed on case-hardened steel exhibiting this type of structure, the coarsening of austenite grains is suppressed by the pinned particles dispersed in large quantities within the steel.

[0072] Regarding the calculation of pinned particle quantity, one possible method is to detect and accumulate pinned particles within a plane. However, detecting minute pinned particles from images is generally difficult, and misjudgments and errors are often large. Therefore, in this invention, the amount of NbC inclusions is calculated from coarse NbC inclusions that are relatively easy to detect from images, etc., and the amount of Nb in the pinned particles (pinned Nb amount) is indirectly calculated by subtracting the amount of Nb in the NbC inclusions from the amount of added Nb. In other words, by focusing on NbC-based inclusions, the amount of pinned particles is estimated, and this is a major difference from conventional methods. [Examples]

[0073] (Examples 1-23, Comparative Examples 1-6) [1. Sample Preparation] [1.1. Melting and Casting Process] First, 50 kg of steel with the composition shown in Table 1 was melted in a vacuum high-frequency induction melting furnace to form an ingot with a diameter of 100 mm.

[0074] [1.2. Hot Working Process] Next, the ingot was heated at 1200°C or 1170°C for 4 hours, then hot-forged to a diameter of 30 mm to form a rod-shaped steel bar.

[0075] [1.3. Standardizing process] Next, each steel bar was subjected to a hardening treatment. The steel bars were normalized by holding them at 920°C for 1 hour, followed by air cooling.

[0076] [1.4. Spheroidizing Annealing Process] Next, each of the steel bars that had been normalized was subjected to spheroidizing annealing. Spheroidizing annealing was performed by holding at 765°C for 6 hours, followed by slow cooling to 670°C at a rate of 9.5°C / hr, and then in-furnace cooling.

[0077] [1.5. Cold Working Process] Next, cylindrical steel rods with a diameter of 15 mm and a height of 22.5 mm were fabricated from each steel rod after spheroidizing annealing. This cylindrical steel was cold-upscaled to obtain a cylindrical steel with a height of 6.75 mm.

[0078] [1.6. Simulated carburizing process] Each of the resulting cylindrical steel samples was subjected to a heat treatment simulating carburizing. The heat treatment was performed by holding the cylindrical steel in an atmospheric furnace at temperatures ranging from 940°C to 1040°C for 3 hours, followed by water cooling.

[0079] [Table 1]

[0080] [2. Test Method] [2.1. Amount of NbC inclusions] The area ratio of NbC-based inclusions was specifically determined by following the procedure below. (1) A sample for SEM observation was cut from the hot-forged body, with the boundary and the d / 4 section designated as the end. (2) SEM of the 4000 × 4000 μm area at the center of the sample 2 Cross-sectional images were obtained for the specified range. (3) Inclusions with a maximum length of 0.1 μm or more were extracted from the cross-sectional images based on the contrast between light and dark areas. (4) The center of the inclusion was subjected to EDS measurement, and those in which an Nb content of 50 mass% or more was detected were classified as NbC-based inclusions. (5) The area of ​​NbC-based inclusions was calculated. (6) The cumulative area of ​​NbC-based inclusions is 4000 × 4000 μm 2 The area percentage of NbC-based inclusions was calculated by dividing by [amount].

[0081] [2.2. Pinning Nb amount] The amount of pinned Nb was calculated using the calculated area ratio of NbC-based inclusions and the amount of Nb added.

[0082] [2.3. Grain Coarsening] The determination of grain coarsening was performed according to the following procedure. (1) The central part of the simulated carburized body was cut along its longitudinal direction, and the grain boundaries of the cut surface were exposed using a known etching solution. (2) The entire cross-section was observed with an optical microscope, and the equivalent diameter of the crystal grains in the observed area (the diameter of a circle with the same area) was calculated by image analysis. (3) If crystal grains with an equivalent diameter of 100 μm or more were observed, it was determined that the crystal grains had coarsened. The lowest temperature at which coarse grains were observed in the simulated carburized material was defined as the "coarse grain observation temperature".

[0083] [3. Results] The results are shown in Table 2. Note that Table 2 contains the following: (a) Amount of C added (mass%) (b) The lower limit of the required forging heating temperature (900 + 1500 × [C](°C)), (c) Actual forging heating temperature (°C) (d) Amount of Nb added (mass%), and (e) The amount of Nb in the NbC-based inclusions expressed by the area fraction (area%) of NbC-based inclusions × 0.892 The following can be seen from Table 2.

[0084] (1) In Comparative Example 1, coarse grains were observed when the simulated carburizing temperature was 980°C. This is thought to be because the pinning Nb amount was less than 0.020 mass% (0.018 mass%). The low amount of pinned Nb is thought to be because the amount of C was too high, causing the actual forging heating temperature (1200°C) to not reach the lower limit of the required forging heating temperature (900 + 1500 × [C](°C) = 1235°C). As a result, the NbC-based inclusions could not be completely dissolved and remained in large quantities.

[0085] (2) In Comparative Example 2, coarse grains were observed when the simulated carburizing temperature was 940°C. This is thought to be because the pinning Nb amount was less than 0.020 mass% (0.005 mass%). The low amount of pinned Nb is thought to be due to the small amount of Nb added.

[0086] (3) In Comparative Example 3, coarse grains were observed when the simulated carburizing temperature was 980°C. This is thought to be because the pinning Nb amount was less than 0.020 mass% (0.019 mass%). The low amount of pinned Nb is thought to be due to the excessive amount of Al, which leads to the formation of numerous Al-based compounds, and consequently, a large amount of NbC-based inclusions using these as nuclei.

[0087] (4) In Comparative Example 4, coarse grains were observed when the simulated carburizing temperature was 960°C. This is thought to be because the pinning Nb amount was less than 0.020 mass% (0.019 mass%). The low amount of pinned Nb is thought to be due to the excessive amount of Nb, which led to the formation of a large amount of NbC-based inclusions.

[0088] (5) In Comparative Example 5, coarse grains were observed when the simulated carburizing temperature was 980°C. This is thought to be because the pinning Nb amount was less than 0.020 mass% (0.019 mass%). The low amount of pinned Nb is thought to be because the actual forging heating temperature (1170°C) did not reach the lower limit of the required forging heating temperature (900 + 1500 × [C](°C) = 1184°C), resulting in a large amount of NbC-based inclusions remaining undissolved.

[0089] (6) In Comparative Example 6, coarse grains were observed when the simulated carburizing temperature was 980°C. This is thought to be because the pinning Nb amount was less than 0.020 mass% (0.018 mass%). The low amount of pinned Nb is thought to be because the actual forging heating temperature (1170°C) did not reach the lower limit of the required forging heating temperature (900 + 1500 × [C](°C) = 1196°C), resulting in a large amount of NbC-based inclusions remaining undissolved.

[0090] (7) In Comparative Example 7, coarse grains were observed when the simulated carburizing temperature was 960°C. This is thought to be because the pinning Nb amount was less than 0.020 mass% (0.016 mass%). The low amount of pinned Nb is thought to be because the amount of C was too high, causing the actual forging heating temperature (1200°C) to not reach the lower limit of the required forging heating temperature (900 + 1500 × [C](°C) = 1235°C). As a result, the NbC-based inclusions could not be completely dissolved and remained in large quantities.

[0091] (8) In Comparative Example 8, coarse grains were observed when the simulated carburizing temperature was 940°C. This is thought to be because the pinning Nb amount was less than 0.020 mass% (0.005 mass%). The low amount of pinned Nb is thought to be due to the small amount of Nb added.

[0092] (9) In all of Examples 1 to 21, the temperature at which coarse grains were observed was 1000°C or higher. This is thought to be because the pinning Nb content was 0.02 mass% or higher (0.021 to 0.050 mass%).

[0093] [Table 2]

[0094] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0095] The case-hardened steel according to the present invention can be used in components of gears and continuously variable transmissions (CVTs), etc.

Claims

1. 0.10≦C≦0.20 mass%, 0.05≦Si≦2.00mass%, 0.30≦Mn≦2.00mass%, P≦0.030mass%, S≦0.030mass%, 0.01≦Cu≦1.00 mass%, 0.01≦Ni≦1.00mass%, 0.30≦Cr≦3.00mass%, 0.0001≦Ti≦0.2000mass%, 0.040≦Nb≦0.080 mass%, 0.002 ≤ Al ≤ 0.060 mass%, and, 0.003≦N≦0.040mass%, It contains, with the remainder consisting of Fe and unavoidable impurities. A case-hardening steel that satisfies the following equations (1) and (2). Pinning Nb amount ≥ 0.020 mass% … (1) 2.0×10 －６ ≦[Ti]×[N]≦2.0×10 －３ …(2) however, Pinning Nb amount = [Nb] - Area ratio of NbC-based inclusions (area%) × 0.892 [X] is the mass %) content of element X. The aforementioned "NbC-based inclusions" refer to NbC-based particles having an Nb content of 50 mass% or more and a maximum length of 0.1 μm or more. The aforementioned "area ratio of NbC-based inclusions" refers to the ratio of the area of ​​the NbC-based inclusions included in a 4000 μm × 4000 μm field of view within the measurement range. The "measurement range" refers to the range of length d / 4 connecting the boundary and the d / 4 section, where, in a cross section perpendicular to the longitudinal direction of a rolled or forged material, the length of the line connecting the centroid of the cross section and the boundary of the cross section 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 defined as section d / 4.

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

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