CASE-HARDENING STEEL

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2024-176281 filed on Oct. 8, 2024 and Japanese Patent Application No. 2025-124688 filed on Jul. 25, 2025, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

[0002]The present invention relates to a case-hardening steel, and more particularly to a case-hardening steel capable of suppressing coarsening of austenite grains during carburizing treatment.

BACKGROUND ART

[0003]The term “case-hardening steel (or carburizing steel)” refers to steel suitable for carburizing treatment or carbonitriding treatment (hereinafter also collectively referred to as “carburizing treatment”). When the case-hardening steel subjected to the carburizing treatment is quenched, only the surface layer portion can be hardened. For this reason, case-hardening steels have been used for various components requiring wear resistance, such as components of gears, continuously variable transmission (CVT), or the like.

[0004]In recent years, case-hardening steel is often subjected to cold-forging from the viewpoint of energy saving, shortening of manufacturing time, and the like.

[0005]However, in the cold-forged product, austenite grains are likely to be coarsened during the carburizing treatment.

[0006]For suppressing coarsening of austenite grains, it is considered to lower the carburizing temperature. However, when the carburizing temperature is lowered, it is necessary to lengthen the carburizing time, which deteriorates production efficiency.

[0007]Conventionally, fine particles such as AlN and NbC have been known as pinning particles that suppress the growth of austenite grains by pinning austenite grain boundaries.

[0008]By dispersing and precipitating these fine particles, the movement of austenite grain boundaries is inhibited, and the growth of austenite grains is effectively suppressed.

[0009]As a method for suppressing the growth of austenite grains using such fine particles, various proposals have been made.

[0010]For example, Patent Literature 1 discloses a case-hardening steel containing, in mass %, C: 0.10% to 0.35%, Si: 0.01% to 0.50%, Mn: 0.30% to 1.50%, P: ≤0.02%, S: ≤0.03%, Al: 0.04% to 0.10%, Cr: 0.5% to 2.5%, B: 0.0005% to 0.0050%, Nb: 0.015% to 0.10%, Ti: ≤0.003%, Mo: ≤0.01%, and N: <0.0080%, with the balance being Fe and unavoidable impurities, and having a microstructure with a fraction of ferrite and pearlite of 80% or more, in which a precipitate having a diameter of less than 50 nm and containing Nb is 30 particles/μm² or more, a precipitate having a diameter of 50 nm or more and 100 nm or less and containing Nb is 3 particles/μm² or less, and a number n_A of the precipitates having a diameter of less than 50 nm and containing Nb and a number n_B of the precipitates having a diameter of 50 nm or more and 100 nm or less and containing Nb satisfies the relationship of n_A−5n_B>30.

[0011]Patent Literature 1 describes that the case-hardening steel has both excellent cold forgeability and grain coarsening suppression ability by specifying the component composition of the steel as described above, setting the area fraction of ferrite and pearlite microstructure to the above-described microstructure, and controlling the number of precipitates containing Nb as described above.

[0012]When a case-hardening steel containing NbC or a precipitate composed of a compound in which Ti, N, or the like is dissolved in NbC (hereinafter, these may be referred to as “NbC-based particles”) is subjected to hot-working, coarse NbC-based particles can be dissolved in the steel by heating the case-hardening steel to a predetermined temperature or higher. When the case-hardening steel is cooled at a predetermined cooling rate after hot-working, fine NbC-based particles can be precipitated in the steel. As described in Patent Literature 1, when a large amount of fine NbC-based particles are precipitated, the fine NbC-based particles function as pinning particles, and coarsening of austenite grains during carburizing treatment can be suppressed.

[0013]However, in the case where the amount of Nb is excessive, coarse NbC-based particles precipitate and grow during melting and casting. Therefore, even when the case-hardening steel is heated to a predetermined temperature thereafter, it may be difficult to completely dissolve the coarse NbC-based particles in the steel.

[0014]In the case where the precipitation amount of the coarse NbC-based particles becomes excessive, the amount of Nb dissolved in the steel becomes small, and thus the precipitation amount of fine NbC-based particles becomes small in a precipitation step for finely precipitating the NbC-based particles (for example, a cooling step after hot-working).

[0015]That is, when the Nb amount is simply increased, only coarse NbC-based particles are increased, and on the contrary, the precipitation amount of fine NbC-based particles is reduced, which may cause coarsening of austenite grains during the carburizing treatment.

[0016]In addition, Patent Literature 1 describes a technique of precipitating fine NbC-based particles at a nano level in observation within a range of several μm². However, even fine NbC-based particles may be coarsened during carburizing treatment and lose the function as pinning particles.

[0017]Therefore, in the method of Patent Literature 1, there is a limit to suppression of coarsening of austenite grains during carburizing treatment.

[0018]Coarse NbC-based particles often grow by using TiN-based particles or AlN-based particles as nuclei. At this time, in the case where the number of the TiN-based particles and/or the AlN-based particles is excessively small, coarse NbC-based particles are likely to be generated. When such a case-hardening steel is heated to a predetermined temperature, coarse NbC-based particles may further grow.

[0019]As described above, the total amount of the fine NbC-based particles correlates with the total amount of the coarse NbC-based particles.

[0020]In other words, in order to grasp the total amount of the fine NbC-based particles, it is necessary to grasp the total amount of the coarse NbC-based particles.

SUMMARY OF INVENTION

[0023]An object of the present invention is to provide a case-hardening steel capable of suppressing coarsening of austenite grains during carburizing treatment.

[0024]In order to solve the above problems, a case-hardening steel according to the present invention includes:

[0031]More specifically, in the case where the cross-sectional shape is a circle as illustrated in FIG. 1A, the center of the circle is the center of gravity G, and the radius of the circle is d/2. In this case, since the d/4 portion is a closed curve where the separation distance from the boundary of the cross-section is d/4, the d/4 portion forms a circle having a radius of d/4 around the center of gravity G. The range of the length d/4 extending the boundary to the d/4 portion is the measurement range. In this case, the extension line of the measurement range toward the inside of the cross-section passes through the center of gravity G.

[0032]In the case where the cross-sectional shape is a square as illustrated in FIG. 1B, the intersection of diagonal lines of the square is the center of gravity G. Here, since the length of the line connecting the center of gravity and the boundary of the cross-section closest to the center of gravity is d/2, the length of one side of the cross-section is d. In addition, since the d/4 portion is a closed curve where the separation distance from the boundary of the cross-section is d/4, the d/4 portion forms a concentric square with one side having a length of d/2 and the center of gravity G. The range of a length d/4 connecting the boundary and the d/4 portion is the measurement range. In this case, a direction perpendicular to the d/4 portion in the cross-section is the measurement range.

[0033]In the case where the production conditions and [Ti]×[N] are optimized in the case-hardening steel, the TiN-based particles can be moderately dispersed in the steel in the cooling process after casting. When the TiN-based particles are moderately dispersed in the steel, fine NbC-based particles (=pinning particles) are dispersed and grown, and formation of coarse NbC-based particles (=NbC-based inclusions) can be suppressed.

[0034]Accordingly, when the amount of NbC-based inclusions is small, the NbC-based inclusions are easily dissolved in the subsequent heating step.

[0035]Therefore, in the state before carburization, the pinning Nb content can be 0.020 mass % or more.

[0036]When a case-hardening steel having such a structure is subjected to carburizing treatment, coarsening of austenite grains is suppressed by a large amount of pinning particles dispersed in the steel.

BRIEF DESCRIPTION OF DRAWINGS

[0037]FIG. 1A is an explanatory view of the measurement range in the case where the cross-sectional shape is circle.

[0038]FIG. 1B is an explanatory view of the measurement range in the case where the cross-sectional shape is square.

[0039]Hereinafter, an embodiment of the present invention will be described in detail.

1. Case-Hardening Steel

[1.1 Composition]

[1.1.1 Main Constituent Elements]

[0040]The case-hardening steel according to the present invention contains the following elements, with the balance being Fe and inevitable impurities. The types of the additive elements, the component ranges thereof, and the reasons for limitation thereof are as follows.

[0042]In the present invention, the “NbC-based particles” refer to particles composed of either (a) NbC, (b) a solid solution in which another element (for example, Ti, N, or the like) is dissolved in NbC, and (c) NbC grown using TiN-based particles or AlN-based particles as nuclei.

[0043]In the present invention, fine NbC-based particles that contribute to suppression of coarsening of austenite grains during carburizing treatment may be referred to as “pinning particles”.

[0044]In the present invention, coarse NbC-based particles that do not contribute to suppression of coarsening of austenite grains during carburizing treatment may be referred to as “NbC-based inclusions”.

[0045]C is an element necessary for ensuring hardness and strength.

[0046]In the case where the amount of C is too small, the hardness at the core (center) of the carburized part may not be ensured.

[0047]Therefore, the C content needs to be 0.10 mass % or more.

[0048]On the other hand, in the case where the amount of C is too large, NbC-based inclusions that cannot be completely dissolved may be formed in a heating step before the step of precipitating the pinning particles (hereinafter, this step may be referred to as “precipitation step”). Therefore, the amount of pinning particles that can be precipitated in the precipitation step may be insufficient. As a result, the number of pinning particles capable of pinning the austenite grain boundaries in the carburizing step may decrease, and coarsening of austenite grains may not be suppressed.

[0049]Therefore, the C content needs to be 0.20 mass % or less. The C content is preferably 0.18 mass % or less.

$\begin{array}{cc}0.05\mathrm{mass}\%\le \mathrm{Si}\le 2.\mathrm{mass}\%:& \left(2\right)\end{array}$

[0050]Si is an element effective for improving hardenability and enhancing strength.

[0051]In order to obtain such an effect, the Si content needs to be 0.05 mass % or more.

[0052]Si is an element that solid-solution strengthens ferrite.

[0053]Therefore, in the case where the amount of Si is too large, the hardness may increase, and the cold workability may deteriorate.

[0054]Therefore, the Si content needs to be 2.00 mass % or less. The Si content is preferably 0.50 mass % or less, and more preferably 0.30 mass % or less.

$\begin{array}{cc}0.3\mathrm{mass}\%\le \mathrm{Mn}\le 2.\mathrm{mass}\%:& \left(3\right)\end{array}$

[0055]Mn is an element effective for improving hardenability and enhancing strength.

[0056]In order to obtain such an effect, the Mn content needs to be 0.30 mass % or more.

[0057]Mn is an element that solid-solution strengthens ferrite.

[0058]Therefore, in the case where the amount of Mn is too large, the hardness may increase, and the cold workability may deteriorate.

[0059]Therefore, the Mn content needs to be 2.00 mass % or less. The Mn content is preferably 1.00 mass % or less, and more preferably 0.70 mass % or less.

$\begin{array}{cc}P\le 0.03\mathrm{mass}\%:& \left(4\right)\end{array}$

[0060]In the case where the amount of P is too large, grain boundary embrittlement may be promoted. Therefore, the P content is preferably small.

[0061]Therefore, the P content needs to be 0.030 mass % or less.

$\begin{array}{cc}S\le 0.03\mathrm{mass}\%:& \left(5\right)\end{array}$

[0062]S combines with Mn, increases the amount of MnS, and improves machinability. However, in the case where the amount of S is too large, the fatigue strength may decrease.

[0063]Therefore, the S content needs to be 0.030 mass % or less.

$\begin{array}{cc}0.01\mathrm{mass}\%\le \mathrm{Cu}\le 1.\mathrm{mass}\%:& \left(6\right)\end{array}$

[0064]Cu is an element effective for improving hardenability and enhancing strength. In particular, in the case where the amount of C is low, Cu is an effective element for ensuring hardness at the core portion (central portion) of the carburized part.

[0065]In order to obtain such an effect, the Cu content needs to be 0.01 mass % or more.

[0066]On the other hand, in the case where the amount of Cu is too large, the hot forgeability may be deteriorated.

[0067]Therefore, the Cu content needs to be 1.00 mass % or less. The Cu content is preferably 0.30 mass % or less.

$\begin{array}{cc}0.01\mathrm{mass}\%\le \mathrm{Ni}\le 1.\mathrm{mass}\%:& \left(7\right)\end{array}$

[0068]Ni is an element effective for improving hardenability and enhancing strength. Ni is also an element effective for improving ductility. In particular, in the case where the amount of C is low, Ni is an effective element for ensuring hardness at the core portion (central portion) of the carburized part.

[0069]In order to obtain such an effect, the Ni content needs to be 0.01 mass % or more.

[0070]On the other hand, in the case where the amount of Ni is too large, a bainite structure is formed, and cold workability may be deteriorated.

[0071]Therefore, the Ni content needs to be 1.00 mass % or less. The Ni content is preferably 0.30 mass % or less.

$\begin{array}{cc}0.3\mathrm{mass}\%\le \mathrm{Cr}\le 3.\mathrm{mass}\%:& \left(8\right)\end{array}$

[0072]Cr is an element effective for improving hardenability and enhancing strength.

[0073]In order to obtain such an effect, the Cr content needs to be 0.30 mass % or more. The Cr content is preferably 0.80 mass % or more.

[0074]On the other hand, in the case where the amount of Cr is too large, machinability may deteriorate.

[0075]Therefore, the Cr content needs to be 3.00 mass % or less. The Cr content is preferably 2.00 mass % or less.

$\begin{array}{cc}0.0001\mathrm{mass}\%\le \mathrm{Ti}\le 0.2\mathrm{mass}\%:& \left(9\right)\end{array}$

[0076]In the case where the amount of Ti is too small, the number of TiN-based particles is decreased, and the number of NbC-based particles using the TiN-based particles as nuclei is too small. Therefore, the NbC-based particles per nucleus become too large. As a result, the NbC-based particles may continue to grow in the solidification process after dissolution to form NbC-based inclusions.

[0077]Therefore, the Ti content needs to be 0.0001% or more. The Ti content is preferably 0.0030 mass % or more, and more preferably 0.0035 mass % or more.

[0078]On the other hand, in the case where the amount of Ti is too large, Ti is dissolved in the NbC-based particles and increases the solid solution temperature of the NbC-based particles. As a result, even when a heat treatment is performed, coarse NbC-based particles may remain without being dissolved.

[0079]Therefore, the Ti content needs to be 0.2000 mass % or less.

$\begin{array}{cc}0.04\mathrm{mass}\%\le \mathrm{Nb}\le 0.08\mathrm{mass}\%:& \left(10\right)\end{array}$

[0080]Nb has a function of precipitating pinning particles in the precipitation step and suppressing coarsening of austenite grains during the carburizing treatment.

[0081]In order to obtain such an effect, the Nb content needs to be 0.040 mass % or more. The Nb content is preferably 0.045 mass % or more.

[0082]On the other hand, in the case where the amount of Nb is too large, NbC-based inclusions may be increased. In addition, excessive NbC-based inclusions may deteriorate cold workability.

[0083]Therefore, the Nb content needs to be 0.080 mass % or less. The Nb content is preferably 0.070 mass % or less.

$\begin{array}{cc}0.002\mathrm{mass}\%\le \mathrm{Al}\le 0.060\mathrm{mass}\%:& \left(11\right)\end{array}$

[0084]Al not only forms fine AlN-based particles exhibiting a pinning effect, but also functions as a deoxidizing agent.

[0085]In the case where the amount of Al is too small, these functions may not be fulfilled.

[0086]Therefore, the Al content needs to be 0.002 mass % or more.

[0087]On the other hand, in the case where the amount of Al is too large, an Al-based compound serving as a core of NbC-based inclusions may be formed.

[0088]Therefore, the Al content needs to be 0.060 mass % or less.

$\begin{array}{cc}0.003\mathrm{mass}\%\le N\le 0.040\mathrm{mass}\%:& \left(12\right)\end{array}$

[0089]In the case where the N amount is appropriate, fine AlN-based particles having a pinning effect can be formed by bonding with Al, and coarsening of austenite grains can be suppressed.

[0090]In order to obtain such an effect, the N content needs to be 0.003 mass % or more.

[0091]On the other hand, in the case where the amount of N is too large, N is dissolved in the NbC-based particles and increases the solid solution temperature of the NbC-based particles. As a result, even when a heat treatment is performed, coarse NbC-based particles may remain without being dissolved.

[0092]Therefore, the N content needs to be 0.040 mass % or less.

(13) Inevitable Impurities:

[0093]The case-hardening steel according to the present invention may contain unavoidable impurities. Here, the “inevitable impurities” are components mixed due to various factors such as a raw material and a production process when a case-hardening steel is industrially produced, and a content thereof is in a range in which the properties of the case-hardening steel according to the present invention are not adversely affected.

[1.1.2 Sub-Constituent Elements]

[0094]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 the additive elements, the component ranges thereof, and the reasons for limitation thereof are as follows.

$\begin{array}{cc}0.0003\mathrm{mass}\%\le B\le 0.010\mathrm{mass}\%:& \left(14\right)\end{array}$

[0095]B is an element effective for improving hardenability and enhancing strength. In particular, in the case where the amount of C is low, B is an effective element for ensuring hardness at the core portion (central portion) of the carburized part. Further, B segregates at grain boundaries to strengthen the grain boundaries, thereby improving strength. In order to obtain such an effect, the B content is preferably 0.0003 mass % or more.

[0096]On the other hand, in the case where the amount of B is too large, B reacts with N in the steel to form BN, which may reduce the toughness.

[0097]Therefore, the B content is preferably 0.010 mass % or less.

$\begin{array}{cc}\mathrm{Mo}\le 1.\mathrm{mass}\%:& \left(15\right)\end{array}$

[0098]Mo is an element that improves hardenability, increases strength, and improves wear resistance.

[0099]In order to obtain such an effect, the Mo content is preferably 0.01 mass % or more.

[0100]On the other hand, in the case where the amount of Mo is too large, a bainite structure is formed, and cold workability may deteriorate. Since Mo is also an expensive element, it is preferable to reduce the amount of Mo used.

[0101]Therefore, the Mo content is preferably 1.00 mass % or less, and more preferably 0.10 mass % or less.

[1.2 Component Balance]

[0102]The case-hardening steel according to the present invention needs to satisfy the following expressions (1) and (2).

[0106]Here, the “maximum length” refers to a maximum length in one particle.

[0107]In addition, in a cross-section perpendicular to the longitudinal direction of a rolled material or a forged material, where the length of the line connecting the center of gravity of the cross-section and the boundary of the cross-section closest to the center of gravity is d/2 and a closed curve on the cross-section at which the separation distance from the boundary is d/4 is referred to as a d/4 portion, the “measurement range” refers to the range of the length d/4 extending the boundary to the d/4 portion.

[1.2.1 Expression (1)]

[0108]The “Pinning Nb Content” represents the content (mass %) of Nb that can be precipitated as pinning particles among Nb added to steel.

[0109]In other words, the “Pinning Nb Content” is the sum of the amount of Nb that can be dissolved in steel and precipitated as pinning particles and the amount of Nb that has already precipitated as pinning particles.

[0110]The “pinning particles” refer to NbC-based particles having a Nb content of 50 mass % or more and a maximum length of less than 0.1 μm.

[0111]If the amount of pinning particles (fine NbC-based particles) can be estimated, it can be determined whether coarsening of austenite grains can be suppressed in the carburizing step.

[0112]Here, in the calculation of the amount of the pinning particles, a method of detecting and integrating the area of the pinning particles in a plane can be considered first. However, it is generally difficult to detect a fine object from an image or the like, and erroneous determination and error often increase.

[0113]Therefore, the amount of NbC-based inclusions is calculated from NbC-based inclusions (coarse NbC-based particles) that are relatively easily detected from an image or the like, and the amount of Nb in the NbC-based inclusions is subtracted from the added amount of Nb, whereby the amount of Nb in the pinning particles (Pinning Nb Content) can be indirectly calculated.

[0114]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.

[0115]The NbC-based inclusion is formed of substantially Nb and C, and the atomic ratio thereof is substantially 1:1. Therefore, assuming that the atomic ratio is 1:1, the amount of Nb in the NbC-based inclusions can be calculated by multiplying the content (mass %) of the NbC-based inclusions by [atomic weight of Nb=92.9]/[molecular weight of NbC=92.9+12=104.9].

[0116]By subtracting the amount of Nb required for precipitation of NbC-based inclusions from the added amount of Nb, the amount of pinning Nb that can be precipitated as pinning particles can be determined.

[0117]In general, as the amount of pinning Nb increases, more pinning particles are formed, and coarsening of austenite grains during carburizing treatment is easily suppressed. Therefore, the pinning Nb content needs to be 0.020 mass % or more. The pinning Nb content is preferably 0.025 mass % or more, and more preferably 0.030 mass % or more.

[1.2.2 Expression (2)]

[0118]The value of [Ti]×[N] correlates with the ease of forming TiN and the amount of TiN formed. When the TiN-based particles are appropriately dispersed, the NbC-based particles are also dispersed and grown, and the formation of NbC-based inclusions can be suppressed.

[0119]In the case where the value of [Ti]×[N] is too low, the number of TiN-based particles becomes excessively small, and the absolute number of NbC-based particles using the TiN-based particles as nuclei becomes excessively small. Therefore, the NbC-based particles per nucleus become too large. As a result, the NbC-based particles may continue to grow in the solidification process after dissolution to form NbC-based inclusions.

[0120]Therefore, the value of [Ti]×[N] needs to be 2.0×10⁻⁶ or more. The value of [Ti]×[N] is preferably 1.0×10⁻⁵ or more.

[0121]On the other hand, in the case where the value of [Ti]×[N] is too large, in other words, in the case where the amount of Ti and/or N is too large, it may be dissolved in the NbC-based particles to increase the solid solution temperature and make it difficult to dissolve the NbC-based inclusions.

[0122]Therefore, the value of [Ti]×[N] needs to be 2.0×10⁻³ or less. The value of [Ti]×[N] is preferably 5.0×10⁻⁴ or less.

2. Method for Producing Case-Hardening Steel

[0123]The case-hardening steel according to the present invention can be produced according to the following procedure. The method includes (a) a “melting and casting step” of melting and casting a raw material blended so as to have a predetermined composition, to thereby obtain an ingot, (b) a “hot-forging step” of performing hot-forging on the obtained ingot, to thereby obtain a hot-forged body, (c) a “normalizing step” of performing normalizing on the hot-forged body as necessary, to thereby obtain a normalized body, (d) a “spheroidizing and annealing step” of performing spheroidizing and annealing on the hot-forged body or the normalized body as necessary, to thereby obtain a spheroidized and annealed body, and (e) a “cold-working step” of performing cold-working on the hot-forged body, the normalized body, or the spheroidized and annealed body as necessary.

[2.1 Melting and Casting Step]

[0124]First, raw materials blended so as to have a predetermined composition are melted and cast, to thereby obtain an ingot. The method and conditions for melting and casting are not particularly limited, and the optimum method and conditions can be selected according to the purpose.

[0125]For example, an electric furnace, a vacuum high-frequency induction-melting furnace, or the like can be used to produce molten steel.

[2.2 Hot-Forging Step]

[0126]Next, the obtained ingot is subjected to hot-forging. The hot-forging is performed in order to (a) destroy a coarse cast structure and refine the structure, (b) process the ingot into a steel material having a shape suitable for a subsequent step, such as a slab, a bloom, or a billet, and (c) dissolve the NbC-based inclusions and precipitate the pinning particles.

[0127]The solid solution temperature of the NbC-based inclusions depends on the amount of C in the steel. In the case where the heating temperature during hot-forging is too low, not only the deformation resistance may increase, but also the NbC-based inclusions may not be dissolved sufficiently.

[0128]Therefore, the heating temperature is preferably 900+1500×[C] ° C. or higher. The heating temperature is more preferably 930+1500×[C] ° C. or higher. The upper limit of the heating temperature during the hot-forging step is not particularly limited, and for example, 1350° C. or lower.

[0129]In the case where the heating time is too short, the NbC-based inclusions may not be dissolved sufficiently.

[0130]Therefore, the heating time is preferably 0.5 hours or more.

[0131]On the other hand, even if the heating time is made longer than necessary, there is no difference in effect and there is no practical benefit.

[0132]Therefore, the heating time is preferably 20 hours or less.

[0133]After the hot-forging, the hot-forged body is cooled to room temperature. At this time, the pinning particles precipitate in the steel in the cooling process.

[0134]The finishing temperature of the hot-forging is preferably 1050° C. or less. This is because if the finishing temperature exceeds 1050° C., a hard bainite structure may be formed to cause a problem in manufacturability, and fine austenite grains may be formed due to the bainite structure in the next step, the structure may not be uniform, and the austenite grains may be coarsened.

[0135]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 be formed to cause a problem in manufacturability, and fine austenite grains may be formed due to the bainite structure in the next step, the structure may not be uniform, and the austenite grains may be coarsened.

[2.3 Normalizing Step]

[0136]Next, the hot-forged body is subjected to normalizing as necessary.

[0137]In the case where the sizes of the austenite grains of ferrite and pearlite contained in the hot-forged body are not uniform, cold workability may deteriorate. In such a case, it is preferable to perform normalizing on the hot-forged body to regulate the size of the austenite grains.

[0138]In the case where the austenite grains of the hot-forged body are uniform, normalizing can be omitted.

[0139]The normalizing is performed by heating and holding the hot-forged body at a predetermined temperature and then cooling.

[0140]If the normalizing temperature is too low, the uniformity of austenite grains may be insufficient.

[0141]Therefore, the normalizing temperature is preferably 850° C. or higher.

[0142]On the other hand, if the normalizing temperature becomes too high, there is a problem that pinning particles grow.

[0143]Therefore, the normalizing temperature is preferably 1050° C. or lower.

[0144]If the normalizing time is too short, the uniformity of austenite grains may be insufficient.

[0145]Therefore, the normalizing time is preferably 0.5 hours or more.

[0146]On the other hand, even if the normalizing time is made longer than necessary, there is no difference in effect and there is no practical benefit.

[0147]Therefore, the heating time is preferably 5 hours or less.

[0148]After normalizing, the normalized body is cooled to room temperature. At this time, pinning particles may precipitate in the steel in the cooling process.

[2.4 Spheroidizing and Annealing Step]

[0149]Next, if necessary, spheroidizing and annealing is performed on the hot-forged body or the normalized body.

[0150]In the case where the hot-forged body or the normalized body contains a streaky carbide, cold workability may decrease. In such a case, it is preferable to subject the hot-forged body or the normalized body to spheroidizing and annealing to spheroidize the carbide and soften the material.

[0151]In the case where the hot-forged body or the normalized body has a hardness capable of cold-working, the spheroidizing and annealing can be omitted.

[0152]When the spheroidizing and annealing is performed, local pinning due to the spheroidized carbide may occur, and austenite grains may be rather coarsened. Therefore, from the viewpoint of suppressing coarsening of austenite grains, it may be preferable not to perform spheroidizing and annealing.

[0153]In the present invention, the method of spheroidizing and annealing is not particularly limited, and an optimum method can be selected according to the purpose.

[0154]After the spheroidizing and annealing, the spheroidized and annealed body is cooled to room temperature. At this time, pinning particles may precipitate in the steel in the cooling process.

[2.5 Cold-Working Step]

[0155]Next, if necessary, cold-working is performed on the hot-forged body, the normalized body, or the spheroidized and annealed body.

[0156]The cold-working is performed to form the hot-forged body, the normalized body, or the spheroidized and annealed body into a final product shape.

[0157]The method of cold-working is not particularly limited, and an optimum method can be selected according to the purpose.

[0158]The obtained cold-worked body is subjected to a carburizing treatment.

3. Effect

[0159]When the production conditions and [Ti]×[N] are optimized in the case-hardening steel, the TiN-based particles can be moderately dispersed in the steel in the cooling process after casting. When the TiN-based particles are moderately dispersed in the steel, fine NbC-based particles (=pinning particles) are dispersed and grown, and formation of coarse NbC-based particles (=NbC-based inclusions) can be suppressed.

[0160]Accordingly, when the amount of NbC-based inclusions is small, the NbC-based inclusions are easily dissolved in the subsequent heating step.

[0161]Therefore, in the state before carburization, the pinning Nb content can be 0.020 mass % or more.

[0162]When a case-hardening steel having such a structure is subjected to carburizing treatment, coarsening of austenite grains is suppressed by a large amount of pinning particles dispersed in the steel.

[0163]In the calculation of the amount of the pinning particles, a method of detecting and integrating the pinning particles in a plane is considered first. However, it is generally difficult to detect fine pinning particles from an image or the like, and erroneous determination and error increase in many cases.

[0164]Therefore, in the present invention, the amount of NbC-based inclusions is calculated from coarse NbC-based inclusions that are relatively easily detected from an image or the like, and the amount of Nb in the NbC-based inclusions is subtracted from the added amount of Nb to indirectly calculate the amount of Nb in the pinning particles (pinning Nb content).

[0165]In other words, the amount of pinning particles is estimated by focusing on the NbC-based inclusions, and this point is greatly different from the related art.

EXAMPLE

Example 1-23 and Comparative Example 1-6

[1. Preparation of Sample]

[1.1 Melting and Casting Step]

[0166]First, 50 kg of steel having the composition shown in Table 1 was melted and cast in a vacuum high-frequency induction-melting furnace to produce an ingot having a diameter of 100 mm.

[1.2 Hot-Working Step]

[0167]Next, the ingot was heated at 1200° C. or 1170° C. for 4 hours, and then processed by hot-forging to a diameter of 30 mm, thereby obtaining a rod-shaped steel bar.

[1.3 Normalizing Step]

[0168]Next, normalizing was performed on each steel bar.

[0169]The normalizing was performed by holding the steel bar at 920° C. for 1 hour and then allowing the steel bar to cool.

[1.4 Spheroidizing and Annealing Step]

[0170]Next, each steel bar after normalizing was subjected to spheroidizing and annealing.

[0171]The spheroidizing and annealing was performed by holding the steel bar at 765° C. for 6 hours, then gradually cooling to 670° C. at a rate of 9.5° C./hr, and then cooling in the furnace.

[1.5 Cold-Working Step]

[0172]Next, from each steel bar after the spheroidizing and annealing was produced a cylindrical steel having a diameter of 15 mm and a height of 22.5 mm.

[0173]This cylindrical steel was subjected to cold-upsetting to obtain a cylindrical steel having a height of 6.75 mm.

[1.6 Simulated Carburizing Step]

[0174]Each of the obtained cylindrical steels was subjected to heat treatment simulating carburizing treatment.

[0175]The heat treatment was performed by holding the cylindrical steel in an atmospheric furnace at each temperature of 940° C. to 1040° C. for 3 hours and then water-cooling.

[2. Test Method]

[2.1. Amount of NbC-Based Inclusions]

[0176]Specifically, the area ratio of NbC-based inclusions was determined according to the following procedure. (1) A sample for s scanning electron microscopy (SEM) observation was cut out from the hot-forged body so that the boundary and the d/4 portion were located at the end. (2) A cross-sectional image in a range of 4000×4000 μm² at the center of the sample was acquired by SEM. (3) From the contrast of light and shade, inclusions having a maximum length of 0.1 μm or more were extracted from the cross-sectional image. (4) The center of each of the inclusions was subjected to an energy dispersive spectroscopy (EDS) measurement, and the inclusions in which the Nb content was detected to be 50 mass % or more were determined as NbC-based inclusions. (5) The area of NbC-based inclusions was integrated. (6) The area ratio (area %) of NbC-based inclusions was calculated by dividing the integrated area of NbC-based inclusions by 4000×4000 μm².

[2.2 Pinning Nb Content]

[0177]The pinning Nb content was calculated by using the calculated area ratio of NbC-based inclusions and the amount of Nb added.

[2.3 Austenite Grain Coarsening]

[0178]The austenite grain coarsening was determined according to the following procedure. (1) The central portion of the simulated carburized body was cut along the longitudinal direction, and the austenite grain boundaries of the cut surface were made to appear by using a known corrosive solution. (2) The entire cut surface was observed with an optical microscope, and the equivalent circle diameter (diameter of a circle having the same area) of the austenite grain in the observation region was calculated by image analysis. (3) Cases where austenite grains having an equivalent circle diameter of 100 μm or more were observed were determined that the austenite grains were coarsened. For each of the simulated carburized bodies, the minimum temperature at which coarse grains were observed was defined as the “coarse grain-observed temperature”.

[3. Results]

[0179]The results are shown in Table 2. Table 2 also shows (a) the amount of added C (mass %), (b) the lower limit value of the required forging heating temperature (900+1500×[C] (° C.), (c) the actual forging heating temperature (° C.), (d) the amount of added Nb (mass %), and (e) the amount of Nb in the NbC-based inclusions represented by the area ratio (area %) of the NbC-based inclusions×0.892. Table 2 shows the following.

[0180](1) In Comparative Example 1, coarse grains were observed when the simulated carburizing temperature was 980° C. This is considered to be because the pinning Nb content was less than 0.020 mass % (0.018 mass %).

[0181]It is considered that the reason why the pinning Nb content was small is that since the amount of C was too large, the actual forging heating temperature (1200° C.) did not reach the lower limit value (900+1500×[C] (C)=1235° C.) of the required forging heating temperature, and thus the NbC-based inclusions could not be completely dissolved and remained in a large amount.

[0182](2) In Comparative Example 2, coarse grains were observed when the simulated carburizing temperature was 940° C. This is considered to be because the pinning Nb content was less than 0.020 mass % (0.005 mass %).

[0183]The reason why the pinning Nb content was small is considered that the amount of Nb added was small.

[0184](3) In Comparative Example 3, coarse grains were observed when the simulated carburizing temperature was 980° C. This is considered to be because the pinning Nb content was less than 0.020 mass % (0.019 mass %).

[0185]It is considered that the reason why the pinning Nb content was small is that since the amount of Al was excessive, a large number of Al-based compounds were formed, and a large number of NbC-based inclusions using the Al-based compounds as nuclei were also formed.

[0186](4) In Comparative Example 4, coarse grains were observed when the simulated carburizing temperature was 960° C. This is considered to be because the pinning Nb content was less than 0.020 mass % (0.019 mass %).

[0187]The reason why the pinning Nb content was small is considered that a large amount of NbC-based inclusions were formed because the amount of Nb was excessive.

[0188](5) In Comparative Example 5, coarse grains were observed when the simulated carburizing temperature was 980° C. This is considered to be because the pinning Nb content was less than 0.020 mass % (0.019 mass %).

[0189]It is considered that the reason why the pinning Nb content was small is that the actual forging heating temperature (1170° C.) did not reach the lower limit value (900+1500×[C] (° C.)=1184° C.) of the required forging heating temperature, and thus the NbC-based inclusions could not be completely dissolved and remained in a large amount.

[0190](6) In Comparative Example 6, coarse grains were observed when the simulated carburizing temperature was 980° C. This is considered to be because the pinning Nb content was less than 0.020 mass % (0.018 mass %).

[0191]It is considered that the reason why the pinning Nb content was small is that the actual forging heating temperature (1170° C.) did not reach the lower limit value (900+1500×[C] (° C.)=1196° C.) of the required forging heating temperature, and thus the NbC-based inclusions could not be completely dissolved and remained in a large amount.

[0192](7) In Comparative Example 7, coarse grains were observed when the simulated carburizing temperature was 960° C. This is considered to be because the pinning Nb content was less than 0.020 mass % (0.016 mass %).

[0193]It is considered that the reason why the pinning Nb content was small is that since the amount of C was too large, the actual forging heating temperature (1170° C.) did not reach the lower limit value (900+1500×[C] (° C.)=1235° C.) of the required forging heating temperature, and thus the NbC-based inclusions could not be completely dissolved and remained in a large amount.

[0194](8) In Comparative Example 8, coarse grains were observed when the simulated carburizing temperature was 940° C. This is considered to be because the pinning Nb content was less than 0.020 mass % (0.005 mass %).

[0195]The reason why the pinning Nb content was small is considered that the amount of Nb added was small.

[0196](9) In Example 1-21, the temperature at which coarse particles were observed was 1000° C. or higher. This is considered to be because the pinning Nb content was 0.020 mass % or more (0.021-0.050 mass %).

[0197]Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention.

INDUSTRIAL APPLICABILITY

[0198]The case-hardening steel according to the present invention can be used for components of gears, continuously variable transmission (CVT), or the like.