WO2015147067A1

WO2015147067A1 - Steel component for high-temperature carburizing with excellent spalling strength and low-cycle fatigue strength

Info

Publication number: WO2015147067A1
Application number: PCT/JP2015/059152
Authority: WO
Inventors: 武浩酒道; 新堂　陽介; 藤田　学
Original assignee: 株式会社神戸製鋼所
Priority date: 2014-03-28
Filing date: 2015-03-25
Publication date: 2015-10-01
Also published as: JP2015193929A; TWI545205B; TW201542841A

Abstract

This steel component for high-temperature carburizing which has excellent spalling strength and low-cycle fatigue strength and which is useful as a material for power transmission components, etc., is provided by suitably adjusting the chemical component composition, setting the average C concentration up to a depth of 0.05mm from the surface to 0.50% or greater, and controlling the average circle equivalent diameter Dm(μm) and the number density Nm (number/mm²) of MnS in a non-carburized portion, the average circle equivalent diameter Dt(μm) and the number density Nt (number/mm²) of TiN in the non-carburized portion, the Vickers hardness Hs (HV) at a position 0.05mm deep from the surface and the Vickers hardness Hi (HV) at the non-carburized portion measured with a test force of 300gf, and the effective hardened layer depth ECD (mm) so as to satisfy the expressions A=exp{Dm/10+Nm^1/2/50}×exp{Dt/15+Nt^1/2/50}, B=exp{Hs/650+ECD^1/2+Hi/250}, C=exp[Hs/650-ECD-{(0.89×Hi-202.16)/250}²], 3.7×A-0.47×B+1.14≦0, and C≧0.66.

Description

High temperature carburizing steel parts with excellent spalling strength and low cycle fatigue strength

The present invention relates to a steel part for high-temperature carburizing excellent in spalling strength and low cycle fatigue strength. Specifically, steel parts used after high-temperature carburizing treatment, especially carburizing steel parts useful as materials for gears, bearings, shafts and power transmission parts such as CVT (Continuously Variable Transmission) pulleys. About.

The carburized steel parts used for the various applications described above are required to have excellent strength against spalling (this is referred to as “spalling strength”). In spalling, when there is little or no sliding between parts, a shearing stress is generated by sliding contact load, and a crack is generated in the inside, and it develops, and fatigue failure that leads to delamination It is a phenomenon. In recent years, the damage form is changing from the conventional surface-origin type damage (pitching) to the spalling due to the increase in the load load accompanying the increase in output and miniaturization of the power transmission component.

In general, increasing the effective hardened layer depth (carburized hardened layer depth) and improving the core hardness (improving hardenability) are effective as measures for improving the spalling strength. However, on the other hand, low cycle fatigue strength and part workability are reduced. In particular, steel parts used in the above-described applications are required to have excellent fatigue strength in a low cycle with a cycle number of about 10 ⁴ to 10 ⁵ times.

In recent years, increasing the temperature of carburizing is aimed at improving productivity and reducing parts costs. By increasing the temperature of the carburizing process, the rate at which C (carbon) penetrates and diffuses into the steel surface increases, and the processing time required for obtaining the specified material properties (surface hardness, effective hardened layer depth) is shortened. Is possible. However, when the austenite crystal grains become coarse due to high temperature, the low cycle fatigue strength tends to decrease.

As measures from the material side to improve the properties of steel parts obtained by high-temperature carburizing, (i) control the precipitation state of inclusions, (ii) control the material after carburizing, (iii) ) For example, the addition of crystal grain coarsening preventing elements such as Nb and Ti is the main countermeasure.

From this point of view, various techniques have been proposed so far for improving the characteristics of steel parts. For example, in Patent Document 1, while containing a relatively large amount of S and controlling the atomic ratio of Mn and S within a predetermined range, 5,000 sulfide inclusions per unit area of MnS as a main component are contained per mm. ^A technique for improving the strength of parts by providing ^two or more has been proposed.

Moreover, in patent document 2, from the viewpoint of implement | achieving the case hardening steel excellent in cold forgeability and a crystal grain coarsening prevention characteristic, while containing predetermined amount Ti, by prescribing | regulating the content ratio of Ti and N A technique for precipitating Ti as TiC as much as possible has been proposed.

In Patent Document 3, the Nb content is reduced to 0.04% or less, and the relationship between the content of Mo, Ni, B, Si, P, and V, the carburization concentration of the steel surface layer, and the surface hardness is a predetermined relationship. It has been proposed to achieve carburized and hardened steel and carburized parts that are excellent in low cycle fatigue properties by defining the formula so as to satisfy the equation.

On the other hand, Patent Document 4 proposes a gear component that achieves both tooth root strength and tooth surface strength by appropriately controlling the chemical composition while reducing the Ti content to 0.05% or less. Yes.

JP 2005-105390 A JP 2008-81841 A JP 2011-63886 A JP 2010-1527 A

In the technique of Patent Document 1, since the S content is large, the spalling strength is decreased. As described in the above-mentioned Patent Document 2, if the content of Ti or N is only specified, the coarsening of crystal grains is inevitable, and the strength of the steel material tends to decrease. Further, Ti remaining without being bonded to C tends to precipitate as coarse TiN in a normal manufacturing method, and the spalling strength tends to decrease. The technique of the above-mentioned Patent Document 3 basically has a low Nb content, so that coarsening of crystal grains cannot be avoided, and there is a problem that the strength of the steel material is reduced. In the technique of Patent Document 4 described above, since the Ti content is basically small, it is impossible to prevent the coarsening of crystal grains, and the strength of the gear itself is reduced. Moreover, in this technique, BN tends to be formed, and the spalling strength tends to decrease due to a decrease in core hardness due to a decrease in hardenability.

Thus, various techniques for improving the properties of steel parts have been proposed so far, and some results have been recognized in terms of improving either steel strength or low cycle fatigue strength. Yes. However, no technology has been established to achieve both spalling strength and low cycle fatigue strength.

The present invention has been made in view of the above circumstances, and its purpose is to exhibit excellent low cycle fatigue strength without reducing spalling strength, such as gears, bearings, shafts, and CVT pulleys. The object is to provide steel parts for high-temperature carburizing that are useful as materials for power transmission parts and the like.

The steel parts for high-temperature carburizing of the present invention that can solve the above-mentioned problems are, by mass%, C: 0.10 to 0.3%, Si: 0.03 to 1.50%, Mn: 0.2 to 1.8%, P: more than 0% and 0.03% or less, S: more than 0% and 0.03% or less, Cr: 0.30 to 2.50%, Al: more than 0% and 0.08% or less, N : More than 0% and 0.0150% or less, Nb: 0.05 to 0.3%, Ti: 0.05 to 0.1% and B: 0.0005 to 0.005%, respectively, the balance being Iron and inevitable impurities, the average C concentration from the surface to a depth of 0.05 mm is 0.50% or more, and the following A, B and C are defined by the following formulas (1) to (3), respectively. These are characterized by satisfying the following expressions (4) and (5).
A = exp {Dm / 10 + Nm ^1/2 / 50} × exp {Dt / 15 + Nt ^1/2 / 50} (1)
B = exp {Hs / 650 + ECD ^1/2 + Hi / 250} (2)
C = exp [Hs / 650−ECD − {(0.89 × Hi−202.16) / 250} ² ] (3)
3.7 × A−0.47 × B + 1.14 ≦ 0 (4)
C ≧ 0.66 (5)
However, Dm is the average equivalent circle diameter (μm) of MnS in the non-carburized part, Nm is the number density (pieces / mm ² ) of MnS in the non-carburized part, and Dt is the average circle of TiN in the non-carburized part. Equivalent diameter (μm), Nt is the number density of TiN in non-carburized part (pieces / mm ² ), Hs is Vickers hardness (HV) at a depth of 0.05 mm from the surface measured with a test force of 300 gf , ECD indicates the effective hardened layer depth (mm), and Hi indicates the Vickers hardness (HV) at the non-carburized portion measured at a test force of 300 gf.

The average equivalent circle diameter of MnS and TiN means the average of the diameters (equivalent circle diameters) when the sizes of MnS and TiN are converted into circles of the same area. The “non-carburized portion” means a portion 0.5 to 2.0 mm deeper than the effective hardened layer depth ECD (position opposite to the hardened layer). It means the inner side (center side) from the 1/4 position. Furthermore, the effective hardened layer depth ECD means the carburized hardened layer depth specified in JIS G 0557 (2006).

In the steel parts for high-temperature carburizing of the present invention, the Vickers hardness Hs at a depth of 0.05 mm from the surface measured with a test force of 300 gf is 650 HV or more, the effective hardened layer depth ECD is 0.4 mm or more, and the test force is 300 gf. It is preferable that the Vickers hardness Hi in the non-carburized part measured by (1) is 300 HV or more.

The steel part for high-temperature carburizing of the present invention may further contain at least one of Ni: more than 0% and not more than 2.0% and Mo: more than 0% and not more than 1.00% by mass as necessary. preferable.

According to the present invention, the chemical composition is appropriately defined, the average C concentration from the surface to a depth of 0.05 mm is 0.50% or more, and the size and number density of MnS and TiN are different at each part of the component. By controlling so as to satisfy the predetermined formula in relation to the Vickers hardness, a steel part for high-temperature carburizing excellent in spalling strength and low cycle fatigue strength can be obtained. Such steel parts for high-temperature carburizing are useful as materials for power transmission parts such as gears, bearings, shafts, and CVT pulleys.

FIG. 1 is a pattern diagram showing carburizing heat treatment conditions in the examples. FIG. 2 is a pattern diagram showing tempering process conditions. FIG. 3 is a graph showing the relationship between the precipitation ratio of TiN and MnS and the temperature. FIG. 4 is a schematic explanatory view showing the shape of the test piece used in the roller pitching test. FIG. 5 is a schematic explanatory diagram showing the implementation status of the roller pitching test. FIG. 6 is a schematic explanatory view showing the shape of a test piece used in the four-point bending fatigue test. FIG. 7 is a schematic explanatory view showing an implementation status of a four-point bending fatigue test. FIG. 8 is a graph showing the relationship between the value (3.7 × A−0.47 × B + 1.14) on the left side of equation (4) and the spalling intensity in the first embodiment. FIG. 9 is a graph showing the relationship between the value of C defined by the equation (3) and the low cycle fatigue strength in Example 1. FIG. 10 is a graph showing the relationship between the value (3.7 × A−0.47 × B + 1.14) on the left side of equation (4) and the spalling intensity in the second embodiment. FIG. 11 is a graph showing the relationship between the value of C defined by the expression (3) and the low cycle fatigue strength in Example 2.

The present inventors have studied from various angles in order to realize a steel part for high-temperature carburizing excellent in both properties of spalling strength and low cycle fatigue strength. In particular, intensive investigations were made on the influence of the size and number density of MnS and TiN on the above-mentioned properties of parts. As a result, the chemical composition is appropriately defined, the average C concentration from the surface to a depth of 0.05 mm is 0.50% or more, and the sizes (Dm, Dt) of MnS and TiN in the non-carburized part Based on the number density (Nm, Nt), when A to C are defined by the above equations (1) to (3), respectively, these satisfy the above equations (4) and (5) The inventors have found that the above object can be achieved by controlling the present invention and completed the present invention.

First, the reason for defining the above equations (1) to (5) will be described.

If inclusions are present in the steel material, when stress is applied, the stress concentrates in the vicinity of the inclusions and the generation and propagation of internal cracks are promoted, so that the spalling strength is reduced. The value of A defined by the above equation (1) indicates the sensitivity of the occurrence and propagation of internal cracks by inclusions, and the value of A increases as the size of inclusions increases and the number density increases. The low cycle fatigue strength decreases. The above formula (1) defines such a relationship.

As for the spalling, as the effective hardened layer depth ECD is shallower, the Vickers hardness Hs at a depth of 0.05 mm from the surface (hereinafter sometimes referred to as “surface hardness Hs”) and the non-carburized portion. The lower the hardness Hi (hereinafter sometimes referred to as “core hardness Hi”), the easier it is to occur and the spalling strength decreases. The value of B defined by the above equation (2) indicates the resistance to the occurrence and propagation of internal cracks due to shear stress. The higher the surface hardness Hs and the core hardness Hi, and the effective hardened layer depth. The deeper the ECD, the greater the value of B. The above formula (2) defines such a relationship.

Further, when the surface hardness Hs is less than 650 HV, the core hardness Hi is less than 300 HV, or the effective hardened layer depth ECD is less than 0.4 mm, spalling does not occur. The surface of the part may sink and the low cycle fatigue strength may further decrease. For these reasons, the surface hardness Hs is preferably 650 HV or more, the core hardness Hi is 300 HV or more, and the effective hardened layer depth ECD is preferably 0.4 mm or more. The surface hardness Hs is more preferably 680 HV or more, and still more preferably 700 HV or more. From the viewpoint of reduction in machinability after carburizing, the surface hardness Hs is preferably 850 HV or less. Moreover, core part hardness Hi becomes like this. More preferably, it is 350 HV or more, More preferably, it is 400 HV or more. From the viewpoint that the spalling strength is saturated by the promotion of internal crack propagation, the core hardness Hi is preferably 500 HV or less. The effective hardened layer depth ECD is more preferably 0.7 mm or more, and further preferably 1.0 mm or more. From the viewpoint of an increase in cost associated with a longer carburizing process, the effective hardened layer depth ECD is preferably 1.5 mm or less.

Under a load environment in a low cycle, an impact load is applied to the power transmission component. Therefore, when the effective hardened layer depth ECD is deep or the core hardness Hi is high, the generation and propagation of cracks are promoted. Therefore, the low cycle fatigue strength decreases. The value of C defined by the above equation (3) indicates the resistance to crack initiation and propagation due to a load in a low cycle, and increases as the surface hardness Hs increases, while the effective hardened layer depth is increased. The deeper the ECD and the smaller the core hardness Hi, the smaller it becomes. Further, when the surface hardness Hs is lowered or the core hardness Hi is lowered, plastic deformation may occur before the occurrence of cracks, and the low cycle fatigue strength may further decrease. The above equation (3) defines such a relationship. In the formula (3), the preferable range of the surface hardness Hs is the same as described above. On the other hand, the core hardness Hi is preferably 250 HV or higher, more preferably 300 HV or higher, from the viewpoint of not causing “plastic deformation” (described later). From the viewpoint of preventing the occurrence of “brittle fracture” as described later, the core hardness Hi is preferably 450 HV or less. The effective hardened layer depth ECD is preferably 0.25 mm or more, more preferably 0.5 mm or more. From the viewpoint of not causing “brittle fracture”, the effective hardened layer depth ECD is preferably 1.30 mm or less.

In this specification, the surface hardness Hs and the core hardness Hi are values when measured with a test force of 300 gf, that is, 300 × 9.8 N.

By combining the above parameters A, B and C, it was found that a high correlation was obtained with the spalling strength and the low cycle fatigue strength. The inclusion form and material were optimized, and the following formula (4) and ( By satisfying the formula (5), it is possible to realize a steel part for high-temperature carburizing excellent in both spalling strength and low cycle fatigue strength. The value on the left side of the formula (4) is preferably −5.0 or less, more preferably −10.0 or less. The lower limit of the value on the left side of the equation (4) is determined based on the A value and the B value, but is preferably −20 or more, more preferably −15 or more. The value of C is preferably 0.80 or more, and more preferably 1.00 or more. The upper limit of the value of C is preferably 2.00 or less, more preferably 1.50 or less.
3.7 × A−0.47 × B + 1.14 ≦ 0 (4)
C ≧ 0.66 (5)

Regarding the structure of steel parts for high-temperature carburizing (structure after carburizing and quenching and tempering), the carburized layer is composed of martensite, retained austenite and partly troostite or bainite structure, and the non-carburized layer is composed of martensite and partly bainite or It consists of a ferrite structure.

The steel component for high-temperature carburizing of the present invention has an appropriate chemical composition in order to exhibit required mechanical properties when applied to materials for power transmission components such as gears, bearings, shafts, and CVT pulleys. It is necessary to adjust to. The basic chemical composition is as follows.

(C: 0.10 to 0.3%)
C is an element necessary for ensuring the core hardness Hi of the final product. However, if contained excessively, the workability is lowered and the low cycle fatigue strength is lowered. Therefore, it is necessary to be 0.3% or less. If the amount of C is less than 0.10%, the core hardness Hi becomes too low, and sufficient spalling strength cannot be obtained. From this point of view, the C content is 0.10 to 0.3%. The minimum with preferable C amount is 0.13% or more, More preferably, it is 0.15% or more. Moreover, the upper limit with the preferable amount of C is 0.27% or less, More preferably, it is 0.25% or less.

(Si: 0.03-1.50%)
Si is an element effective for suppressing the hardness reduction during the tempering treatment and improving the hardenability of the steel material to ensure the core hardness Hi of the final product. However, if contained excessively, C penetration during carburization is inhibited, carburization failure is caused, and workability is reduced by strengthening ferrite, so the upper limit was made 1.50% or less. If the amount of Si is less than 0.03%, it is insufficient for improving the core hardness Hi. From this point of view, the Si content is set to 0.03 to 1.50%. The minimum with the preferable amount of Si is 0.05% or more, More preferably, it is 0.07% or more. Moreover, the upper limit with the preferable amount of Si is 1.0% or less, More preferably, it is 0.60% or less.

(Mn: 0.2-1.8%)
Mn is an element effective for securing the core hardness of the final product by combining with S to suppress the formation of FeS and to suppress the deterioration of forgeability during rolling and to enhance the hardenability of the steel material. However, excessive addition reveals material variations due to striped segregation, and lowers the surface hardness by increasing the amount of retained austenite after carburization by lowering the martensitic transformation start temperature (Ms point). . For these reasons, the Mn content is set to 1.8% or less. On the other hand, if the amount of Mn is less than 0.2%, the formation of FeS and the core hardness are insufficient. The minimum with the preferable amount of Mn is 0.30% or more, More preferably, it is 0.35% or more. Moreover, the upper limit with the preferable amount of Mn is 1.70% or less, More preferably, it is 1.60% or less.

(P: more than 0% and 0.03% or less)
P is segregated at the grain boundaries to lower the low cycle fatigue strength, so the smaller the content, the better. From this point of view, the P content is 0.03% or less. The amount of P is preferably 0.020% or less, and more preferably 0.015% or less. On the other hand, P is an element inevitably contained in steel, and the production cost increases as the purity is increased. Therefore, the amount of P is preferably 0.001% or more, more preferably 0.005%. That's it.

(S: more than 0% and 0.03% or less)
Since S combines with Mn to become MnS inclusions and lowers the low cycle fatigue strength, it is desirable to reduce it as much as possible. From this viewpoint, the upper limit of the amount of S is set to 0.03% or less. The amount of S is preferably 0.025% or less, and more preferably 0.020% or less. On the other hand, S is an element inevitably contained in the steel, and the production amount increases as the purity is increased, and in addition, the machinability is lowered, so the S amount is preferably 0.001% or more, More preferably, it is 0.005% or more.

(Cr: 0.30-2.50%)
Cr, like Mn, is an element effective for improving the hardenability of the steel material and ensuring the core hardness Hi of the final product. However, if excessively contained, the formation of coarse carbides is promoted during carburizing, and the low cycle fatigue strength is lowered. On the other hand, if the Cr content is less than 0.30%, the core hardness Hi of the final product cannot be sufficiently obtained. The minimum with the preferable amount of Cr is 0.50% or more, More preferably, it is 0.80% or more. Moreover, the upper limit with the preferable amount of Cr is 2.00% or less, More preferably, it is 1.80% or less.

(Al: more than 0% and 0.08% or less)
Al couple | bonds with N in steel materials, produces | generates AlN, and suppresses the crystal grain coarsening at the time of carburizing. However, excessive addition produces Al ₂ O ₃ and reduces workability. From such a viewpoint, the Al content is set to 0.08% or less. The amount of Al is preferably 0.060% or less, more preferably 0.050% or less. On the other hand, Al is useful as a deoxidizing action, and is an element inevitably contained in the steel material. As the purity is increased, the manufacturing cost increases, and further the coarsening of crystal grains is promoted. The content is preferably 001% or more, more preferably 0.005% or more.

(N: more than 0% and 0.0150% or less)
N combines with Al and Ti in the steel material to form a nitride, and serves as a starting point for spalling to lower the low cycle fatigue strength. Further, when the amount of N is more than 0.3 times the amount of Ti, N that could not be combined with Ti combines with B to form BN, not only lowering the hardenability of the steel material, but also pinning. Since TiC which is a particle | grain does not precipitate, a crystal grain coarsens and a spalling intensity | strength falls. From such a viewpoint, the N content is set to 0.0150% or less. The amount of N is preferably 0.010% or less, more preferably 0.008% or less. N is an element inevitably contained in the steel material, and the production cost increases as the purity increases. Therefore, the N content is preferably 0.001% or more, more preferably 0.002% or more. .

(Nb: 0.05-0.3%)
Nb combines with C and N in the steel material to form Nb (CN), which acts as pinning particles, and is an element effective for preventing crystal grain coarsening during carburization. However, even if contained excessively, the crystal grain coarsening prevention characteristic is saturated, the steel material cost is increased, and forgeability is lowered due to the coarse crystallized product, so the Nb amount is set to 0.3% or less. The Nb amount is preferably 0.20% or less, and more preferably 0.10% or less. On the other hand, if the Nb content is less than 0.05%, a sufficient pinning force cannot be obtained and the crystal grains become coarse, and accordingly, the spalling strength and the low cycle fatigue strength are lowered. From such a viewpoint, the Nb amount is set to 0.05% or more. The Nb amount is preferably 0.06% or more, and more preferably 0.07% or more.

(Ti: 0.05-0.1%)
Ti combines with C in the steel material to form TiC and acts as pinning particles in the same manner as Nb (CN), and is an element effective in preventing crystal grain coarsening during carburization. However, even if contained excessively, the crystal grain coarsening prevention characteristics are saturated, and steel costs increase and coarse crystallized products are generated, resulting in a decrease in forgeability. Therefore, the Ti amount is 0.1% or less. did. The amount of Ti is preferably 0.09% or less, and more preferably 0.08% or less. On the other hand, if the amount of Ti is less than 0.05%, a sufficient pinning action cannot be obtained, and the crystal grains become coarse, resulting in a decrease in strength. Further, the remaining N which cannot be combined with Ti is produced as BN, and the hardenability of the steel material is remarkably lowered and the strength is lowered. Therefore, the Ti amount is set to 0.05% or more. The amount of Ti is preferably 0.06% or more, and more preferably 0.07% or more.

(B: 0.0005-0.005%)
B is an element effective for improving the impact strength while greatly improving the hardenability of the steel material in a small amount. However, when the amount of B exceeds 0.005%, the effect is saturated and the workability of the parts is lowered. The amount of B is preferably 0.004% or less, and more preferably 0.003% or less. On the other hand, if the B content is less than 0.0005%, the above effect cannot be obtained. The amount of B is preferably 0.0010% or more, more preferably 0.0015% or more.

The basic components in the steel parts for high-temperature carburizing of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel.

Moreover, in order to further improve the characteristics as a steel part, the steel part for high-temperature carburizing according to the present invention may further include, if necessary, mass%, Ni: more than 0% and not more than 2.0% and Mo: 0. It is also preferable to contain at least one of more than% and 1.00% or less. The reason for setting the range when these are contained is as follows.

(Ni: more than 0% and 2.0% or less)
Ni has the effect of improving the toughness of the carburized layer. However, if it is contained excessively, the steel material cost is increased, and the low cycle fatigue strength is reduced due to a decrease in workability and an increase in core hardness Hi. From such a viewpoint, when Ni is contained, the upper limit is preferably made 2.0% or less. The amount of Ni is more preferably 1.80% or less, and still more preferably 1.60% or less. In addition, in order to exhibit the said effect by Ni, it is preferable that Ni amount is 0.01% or more, More preferably, it is 0.05% or more.

(Mo: more than 0% and 1.00% or less)
Mo is an element that improves the hardenability of the steel material, and has the effect of improving the toughness of the carburized layer. However, if it is contained excessively, the steel material cost is increased, and the low cycle fatigue strength is reduced due to a decrease in workability and an increase in core hardness Hi. From such a viewpoint, when Mo is contained, the upper limit is preferably made 1.00% or less. The amount of Mo is more preferably 0.80% or less, and still more preferably 0.60% or less. In addition, in order to exhibit the said effect by Mo, it is preferable that Mo amount is 0.01% or more, More preferably, it is 0.05% or more.

Ni and Mo are both effective elements for improving the toughness of steel parts, and each of them may be used alone or in combination of two kinds.

In the steel parts for high-temperature carburizing of the present invention, in order to increase the surface hardness Hs and increase the spalling strength, it is also necessary that the average C concentration from the surface to a depth of 0.05 mm is 0.50% or more. It is. When the average C concentration is lower than 0.50%, the surface hardness Hs is increased, and good spalling strength cannot be secured. The average C concentration is preferably 0.60% or more, more preferably 0.70% or more. The upper limit of the average C concentration is naturally determined by carburizing conditions such as carbon potential.

In the steel parts for high-temperature carburizing of the present invention, the size and number density of MnS and TiN satisfy the above formulas (4) and (5) in relation to the hardness (Vickers hardness) at each part of the parts. It is characterized by controlling as follows. In order to satisfy such requirements, it is preferable to appropriately control the conditions in the steel material production stage and the carburizing and quenching treatment conditions.

As conditions at the steel production stage,
(A) Average cooling rate when solidifying molten steel,
(B) Forging pressure ratio when rolling or forging from a slab to a predetermined size,
(C) Soaking process before soaking (soaking process)
These conditions are preferably controlled as follows. However, it is not necessary to satisfy all of these conditions (a) to (c), and the steel part for high-temperature carburizing of the present invention can be obtained by combining one or more conditions if necessary. It is done.

(A) Average cooling rate when solidifying molten steel The average cooling rate when solidifying molten steel is preferably 0.06 ° C / second or more. MnS and TiN crystallize in the molten steel and become finer as the average cooling rate when the molten steel is solidified is higher. From such a viewpoint, the average cooling rate when solidifying molten steel is more preferably 0.08 ° C./second or more, and further preferably 0.10 ° C./second or more. The upper limit of the average cooling rate when solidifying the molten steel is not particularly limited, but is usually about 0.01 ° C./second.

(B) Forging pressure ratio when rolling or forging from a slab to a predetermined size The forging pressure ratio is preferably 25 or more. When rolling or forging from a slab to a predetermined size, MnS and TiN become finer as the final size (diameter for shaft parts) is smaller, that is, the forging pressure ratio is larger, and the value of A is smaller. Become. From such a viewpoint, the forging pressure ratio is more preferably 50 or more. The upper limit of the forging pressure ratio is not particularly limited, but is about 500, for example. The forging pressure ratio means “cross-sectional area perpendicular to the casting direction of the slab / cross-sectional area perpendicular to the processing direction of the rolled material or forged material”.

(C) Soaking process before soaking (soaking process)
The soaking temperature is preferably 1100 to 1300 ° C. When a soaking process is performed before the block rolling, the crystallized MnS and TiN are refined, and the value A is reduced. If the soaking temperature is less than 1100 ° C., MnS and TiN are not dissolved, whereas if the soaking temperature exceeds 1300 ° C., the slab surface may be melted during holding. From such a viewpoint, the more preferable lower limit of the soaking temperature is 1150 ° C. or more, and the more preferable upper limit is 1250 ° C. or less.

The soaking time is preferably about 0.5 to 3 hours. If the holding time is less than 0.5 hour, the miniaturization of MnS and TiN becomes insufficient. On the other hand, if the holding time exceeds 3 hours, productivity is lowered and cost is increased. The holding time is more preferably 1 hour or more and 2 hours or less.

In addition, although the above-mentioned soaking process is performed before the partial rolling, if the partial rolling is not performed, the soaking process may be performed before rolling or forging.

The steel material obtained by rolling or forging may be carburized and quenched in a carburizing furnace after being processed into a predetermined shape.

As carburizing and quenching treatment conditions, for example, the average C concentration from the surface to 0.05 mm depth is 0.50% or more, and the effective hardened layer depth ECD (EffectiveffCase. Depth) is 0.40 to 1.5 mm. In addition, the carbon potential CP, the carburizing temperature, and the carburizing time in the atmosphere may be appropriately controlled. Specifically, the carbon potential CP in the atmosphere is adjusted to 0.50 or more and held at 800 to 1000 ° C. for 0.5 hours or more and less than 5 hours, and then the carbon potential CP is kept at 0.50 or more. And quenching immediately after holding at 800 to 900 ° C. for 0.5 to 2 hours.

As the carburizing gas, for example, a mixed gas of modified gas (RX gas) and propane gas may be used.

After carburizing and quenching, holding at 100 to 250 ° C. for 0.5 to 3 hours, allowing to cool, and then performing tempering treatment.

This application claims the benefit of priority based on Japanese Patent Application No. 2014-0669866 filed on Mar. 28, 2014. The entire contents of the specification of Japanese Patent Application No. 2014-069866 are incorporated herein by reference.

Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

Example 1
A steel ingot having the chemical composition shown in Table 1 below was melted in a converter and a small melting furnace having a capacity of 50 kg or 150 kg. In Table 1, “-” means no addition.

For these steel ingots, the production condition No. shown in Table 2 below. Test Nos. 1 and 2 in Table 3 and Table 4 below are combined. 1 to 27 steel materials were obtained.

At this time, in the converter material melted in the converter, the slab was heated and held in a heating furnace at 1250 ° C. for 60 minutes to perform a soaking process, or subjected to block rolling without heating and holding. Further, hot rolling was performed to a predetermined diameter of 32 to 80 mm at a forging pressure ratio shown in Table 2 below.

In addition, even in a small amount of smelting material melted in a small melting furnace (in Table 2 below, 50 kg of “small amount 1” and 150 kg of “

small amount

2, 3”) are also heated in a heating furnace at 1250 ° C. The soaking process was carried out by heating for 60 minutes, or hot forging was carried out at a forging ratio shown in Table 2 below to a predetermined diameter of 32 to 80 mm without heating and holding. Although the small volume 2 and the small volume 3 have the same capacity, the average cooling rate during solidification was changed as shown in Table 2 below.

The obtained steel material (hot rolled material or hot forged material) was processed into a predetermined shape, and then carburized and quenched in a gas carburizing furnace. As the carburizing gas, a mixed gas of RX gas and propane gas was used. At this time, an average C concentration from the surface to a depth of 0.05 mm (hereinafter sometimes referred to as “surface layer C concentration Cs”): 0.60%, effective hardened layer depth ECD: 0.75 mm The carbon potential CP in the atmosphere was adjusted to 0.65, held at 950 ° C. for 0.4 to 3.0 hours, and then held at 850 ° C. for 0.5 hours without changing the carbon potential CP. Immediately after quenching. Table 5 below shows specific carburizing conditions. Furthermore, after holding at 170 degreeC for 2 hours, it stood to cool and tempered. FIG. 1 is a pattern diagram showing the carburizing heat treatment conditions at this time. FIG. 2 is a pattern diagram showing tempering processing conditions. The general carburizing temperature is about 920 ° C., and the above 950 ° C. is a holding temperature at which the crystal grains tend to become coarse.

The longitudinal section of the carburized part obtained above was investigated, and a sample of 10 mm (radial direction) × 10 mm (longitudinal direction) was cut out in the longitudinal direction (corresponding to the rolling direction) from 1/4 position of the diameter D of each sample. The cross section was polished. The composition analysis is performed with EPMA (Electron Probe Micro Analyzer) for inclusions with a minor axis of 3 μm or more existing in an area of 15 μm ² scanned from an arbitrary position on the polished surface, and MnS and TiN are determined by using EPMA (Electron Probe Micro Analyzer). Identified. Further, the area and number of MnS and TiN were measured, the number density (number per 1 mm ² : Nm, Nt) was evaluated by (each detected number / scanning area), and the average equivalent circle diameter (Dm, Dt) was Each was calculated from the average area per piece.

The investigation targets are rolled and forged materials, but MnS and TiN are crystallized products, and their melting points are very high, about 1500-2000 ° C, and they do not dissolve in the carburizing temperature range. There is no change in the diameter and number density. As a reference, FIG. 3 shows the results of calculating the temperature change of the precipitation ratio of MnS and TiN using thermodynamic calculation software “Thermo-calc” (trade name: manufactured by Thermo-calc software). This result supports that it does not dissolve in the temperature range of the carburizing treatment. The measurement conditions of EPMA at this time are as follows (1) to (4).
(1) EPMA device: JXA 8500F (trade name: manufactured by JEOL Ltd.)
(2) EDS analysis (energy dispersive X-ray analysis): Thermo Fisher Scientific system six
(3) Acceleration voltage: 15 kV
(4) Scanning current: 1.76 nA

For the carburized parts obtained above, the spalling strength and low cycle fatigue strength were evaluated by the following methods, and the hardness, the effective hardened layer depth ECD, and the surface layer C concentration Cs were measured. As the hardness, surface hardness Hs and core hardness Hi were measured.

(Evaluation of spalling strength)
The test was conducted using a “RP-201” roller pitching tester (trade name: manufactured by Komatsu Engineering Co., Ltd.). At this time, the life until spalling occurs at a surface pressure of 2.0 to 6.3 GPa is measured, and the test surface pressure when the life reaches 2 million cycles by single regression is defined as the spalling strength (GPa). Calculated. The test piece used in this test was manufactured by processing it into the shape shown in FIG. 4, subjecting it to carburizing and quenching, and polishing the sliding surface. FIG. 5 shows a state in which the test piece 1 (roller) and the load roller 2 roll in contact with each other as a test appearance (implementation status of a roller pitching test). The test conditions are as follows.
(1) Rotation speed: 2000rpm
(2) Slip rate: 0% (life is judged to be due to spalling only)
(3) Load roller: JIS G4805 (2013) high carbon chrome bearing steel SUJ2
(4) Test oil temperature: 120 ° C

(Low cycle fatigue strength)
Using a hydraulic servo tester (manufactured by Shimadzu Corporation) and a jig that supports 4-point bending, the life until the specimen breaks at a stress of 1.0 to 3.0 GPa is measured. The strength to break at 2000 cycles was calculated as low cycle fatigue strength (GPa). The test piece was fabricated by carburizing and quenching after processing into the shape shown in FIG. FIG. 7 shows the external appearance of a four-point bending test (the portion indicated by ◯ is a support point). In FIG. 7, 3 is a test piece, 4 is a jig, and 5 is a direction of load. A single swing load was applied from the back while two horizontal portions on the test piece notch side were supported. The frequency at this time was 20 Hz.

(Evaluation of hardness (surface hardness Hs, core hardness Hi), and effective hardened layer depth ECD)
The roller pitching test piece was cut so as to cross the center of the sliding portion, the cut surface was polished, and the hardness was measured. Hardness measured Vickers hardness HV by test force: 300gf (300 * 9.8N). The surface hardness Hs was an average value obtained by measuring five points from the surface in a depth direction of 0.05 mm. Moreover, core part hardness Hi was made into the average value which measured the 1/4 position of the diameter D 5 points | pieces in the roller pitching test piece. The effective hardened layer depth ECD was calculated according to JIS G0557.

(Measurement of surface layer C concentration Cs)
After depositing gold on the cross-sectional sample, the surface layer C concentration Cs was measured by EPMA analysis. At this time, measurement was performed at a pitch of 0.005 mm from the surface to a position of 0.05 mm in the depth direction, and the average value was defined as the surface layer C concentration Cs.

Table 3 shows the average equivalent circle diameter Dm of MnS, the average equivalent circle diameter Dt of TiN, the number density Nm of MnS, and the number density Nt of TiN, together with the steel types used and the production conditions. In addition, the spalling strength and low cycle fatigue strength, steel type used, manufacturing conditions, carburizing conditions, carburized parts material (surface C concentration Cs, surface hardness Hs, core hardness Hi, effective hardened layer depth ECD ), Parameters (values of A, B and C), and the value on the left side of equation (4) (3.7 × A−0.47 × B + 1.14) are shown in Table 4 below.

From these results, it can be considered as follows. First, test no. Examples 1 to 3, 5 to 9, 12 to 14, 16 to 19, and 21 are examples that satisfy the requirements defined in the present invention. It can be seen that the spalling strength is 3.70 GPa or more, the low cycle fatigue strength is 2.10 GPa or more, and both the spalling strength and the low cycle fatigue strength are excellent.

In contrast, test no. 4, 10, 11, 15, 20, 22 to 27 are comparative examples in which any of the requirements of the present invention was not satisfied, and it can be seen that at least the spalling strength is reduced.

Test No. In 4, 11, 15, 20, and 22, the manufacturing conditions deviate from the conditions recommended in the present invention, the relationship of the expression (4) cannot be satisfied, and the spalling strength is deteriorated.

Test No. No. 10, carburizing conditions deviate from the conditions recommended in the present invention, the relationship of the formula (4) cannot be satisfied, and the spalling strength is deteriorated.

Test No. Nos. 23 to 27 are steel grades Nos. It is a comparative example using the steel materials 6 and 7, regardless of the manufacturing conditions and carburizing conditions, the relationship of the formula (4) is not satisfied, and the spalling strength is deteriorated.

Based on these results, FIG. 8 shows the relationship between the value on the left side of equation (4) (3.7 × A−0.47 × B + 1.14) and the spalling intensity. Moreover, the relationship between the value of C prescribed | regulated by (3) Formula and low cycle fatigue strength is shown in FIG. 8 and 9, the test No. Examples 1-3, 5-9, 12-14, 16-19, 21 Comparative examples of 4, 10, 11, 15, 20, 22 to 27 are indicated by ◆. As is clear from these results, it is understood that satisfying the expression (4) is effective in improving the spalling strength. It can also be seen that setting the value of C defined by equation (3) to 0.66 or more is effective in securing low cycle fatigue strength.

(Example 2)
Steel ingots having chemical composition shown in Table 6 below were melted, and the production conditions No. 1 shown in Table 2 above were obtained. In 4 the steel material was obtained. Further, hot rolling was performed to a predetermined diameter of 32 to 80 mm. In addition, the steel type No. shown in Table 6 below. 1 to 5 are steel types No. 1 shown in Table 1 above. Same as 1-5.

After the obtained steel material (hot rolled material) was processed into a predetermined shape, carburizing and quenching treatment was performed in a gas carburizing furnace (carburizing gas: RX gas + propane gas) in the same manner as in Example 1 above. As the carburizing gas, a mixed gas of RX gas and propane gas was used. At this time, the carbon potential CP in the atmosphere is adjusted so that the surface layer C concentration Cs from the surface to a depth of 0.05 mm is 0.44 to 0.80% and the effective hardened layer depth ECD is 0.29 to 1.33 mm. Is adjusted to a range of 0.45 to 0.75, held at 930 to 960 ° C. for 0.3 to 6.0 hours, and then held at 850 ° C. for 0.5 hours without changing the carbon potential CP. Quenched immediately after. Table 5 also shows specific carburizing conditions. Furthermore, after holding at 170 degreeC for 2 hours, it stood to cool and tempered.

About the carburized parts obtained above, in the same manner as in Example 1 above, evaluation of the form of MnS and TiN, and evaluation of spalling strength and low cycle fatigue strength, as well as hardness (surface hardness Hs, core portion) Hardness Hi), effective hardened layer depth ECD, and surface layer C concentration Cs from the surface to a depth of 0.05 mm were measured.

Table 7 shows the average circle equivalent diameter Dm of MnS, the average equivalent circle diameter Dt of TiN, the number density Nm of MnS, and the number density Nt of TiN together with the steel types used. In addition, the spalling strength and low cycle fatigue strength, steel type used, carburizing conditions, carburized parts material (surface C concentration Cs, surface hardness Hs, core hardness Hi, effective hardened layer depth ECD), parameters ( Table 8 below shows the values of A, B, and C) and the value on the left side of equation (4) (3.7 × A−0.47 × B + 1.14). “Depression”, “plastic deformation”, “brittle fracture” and “GG” shown in the “Remarks” section of Table 8 below mean that the following phenomenon occurred.

(Depression)
In a roller pitching test piece, when the surface hardness Hs measured with a test force of 300 gf is lower than 650 HV, the sliding surface is depressed during the test and the entire surface is prematurely peeled even though the equation (4) is satisfied. There are things to do.

(Plastic deformation)
When the surface hardness Hs measured with a test force of 300 gf is lower than 650 HV or the core hardness Hi measured with a test force of 300 gf is lower than 300 HV, Although the value of C to be defined is 0.66 or more and the equation (5) is satisfied, the test piece undergoes plastic deformation in the load direction and breaks early during the test. Usually, cracks are generated on the surface without plastic deformation, progress in the direction of the core, and finally break.

(Brittle fracture)
In the 4-point bending test piece, when the core hardness Hi measured with a test force of 300 gf is harder than 450 HV, the value of C defined by the expression (3) is 0.66 or more, and the expression (5) is satisfied. Nevertheless, it breaks early and the fracture surface at that time becomes a brittle fracture surface.

(GG: grain coarsening)
When the maximum grain size of the test piece after the carburizing treatment is 6.0 or less is defined as grain coarsening (GG), the grain is coarsened. Such a test piece breaks early despite satisfying the equations (4) and (5).

From these results, it can be considered as follows. First, test no. 32, 34, 35, 37 to 39, 42 to 45, 48, and 49 satisfy all of the requirements defined in the present invention. It can be seen that the spalling strength is 3.70 GP or more, the low cycle fatigue strength is 2.10 GPa or more, and both the spalling strength and the low cycle fatigue strength are excellent.

In contrast, test no. Reference numerals 31, 33, 36, 40, 41, 46, 47, and 50 to 70 are examples in which any of the requirements of the present invention is not satisfied.

Test No. No. 31 has a low surface layer C concentration Cs and a surface hardness Hs lower than the preferred lower limit for the low cycle fatigue strength, causing the above-mentioned “depression” and “plastic deformation”, and the spalling strength and low cycle fatigue strength. Both are falling.

Test No. In No. 33, the value of C defined by equation (3) is small and does not satisfy equation (5), and the low cycle fatigue strength is reduced.

Test No. No. 36, the surface layer C concentration Cs decreases, the surface hardness Hs is lower than the preferred lower limit for the low cycle fatigue strength, the value of C defined by the formula (3) is small, and the low cycle fatigue strength decreases. is doing. Test No. In No. 36, the surface hardness Hs is low, but no depression occurs because the core hardness Hi is relatively high.

Test No. No. 40 has a lower surface layer C concentration Cs and a lower surface hardness Hs than the preferred lower limit for the low cycle fatigue strength, and does not satisfy the formula (4). Both are falling.

Test No. In Nos. 41 and 47, the effective hardened layer depth ECD is shallower than the preferred lower limit for the low cycle fatigue strength and does not satisfy the formula (4), and the spalling strength is lowered.

Test No. In 46, the value of C defined by the expression (3) is small, and the low cycle fatigue strength is low.

Test No. Nos. 50 to 70 are steel grades Nos. This is an example using 8 to 17 steel materials. Of these, test no. No. 50, the surface layer C concentration Cs decreases and the surface hardness Hs is lower than the preferred lower limit for the low cycle fatigue strength, the value of C defined by the equation (3) is small, and the low cycle fatigue strength decreases. ing. Test No. No. 50 has a surface hardness Hs lower than the preferred lower limit for the low cycle fatigue strength, but no depression occurs because the core hardness Hi is much higher than the preferred upper limit for the low cycle fatigue strength.

Test No. No. 51, the effective hardened layer depth ECD becomes shallower than the preferred lower limit for the low cycle fatigue strength. No. 52 has a small C value defined by the expression (3), and the core hardness Hi is higher than the preferred upper limit for the low cycle fatigue strength. As a result, all have low cycle fatigue strength.

Test No. Nos. 53 to 55 are steel types Nos. Having a low C content. In this example, the core hardness Hi is lower than the preferred lower limit for the low cycle fatigue strength. Of these, test no. 53, the effective hardened layer depth ECD is shallower than the preferred lower limit for the low cycle fatigue strength, does not satisfy the formula (4), and the spalling strength is lowered. Test No. No. 54 does not satisfy the formula (4), and the spalling strength is lowered. Test No. In No. 55, the effective hardened layer depth ECD is deeper than the preferred upper limit for the low cycle fatigue strength, and the low cycle fatigue strength is reduced. In addition, brittle fracture occurred.

Test No. Nos. 56 and 57 are steel types having excessive Si amounts. In this example, the surface layer C concentration Cs is lowered due to poor carburization. Of these, test no. 56, the effective hardened layer depth ECD is shallower than the preferred lower limit for the low cycle fatigue strength, and does not satisfy the formula (4), and the spalling strength and the low cycle fatigue strength are reduced. Sinking and plastic deformation also occurred. Test No. In 57, the value of C defined by the expression (3) is small, and the low cycle fatigue strength is low.

Test No. Nos. 58 and 59 are steel types with excessive Mn amounts. This is an example using 11 steel materials. Test No. 58, the effective hardened layer depth ECD becomes shallower than the preferred lower limit for the low cycle fatigue strength. No. 59 has a small C value defined by the expression (3), and the low cycle fatigue strength is low in all cases.

Test No. Nos. 60 to 62 are steel types Nos. Having excessive amounts of Ni as preferred components. This is an example using 12 steel materials. Of these, test no. In No. 60, since the core hardness Hi is higher than the preferred upper limit for the low cycle fatigue strength, the low cycle fatigue strength is reduced due to brittle fracture. Test No. In Nos. 61 and 62, the core hardness Hi is higher than the preferable upper limit for the low cycle fatigue strength, and the value of C defined by the equation (3) is small, and the low cycle fatigue strength is reduced.

Test No. Nos. 63 and 64 are steel types with excessive Cr content. This is an example using 13 steel materials. Test No. No. 63, the effective hardened layer depth ECD becomes shallower than the preferred lower limit for the low cycle fatigue strength. In 64, the value of C defined by the expression (3) is small, and the low cycle fatigue strength is low in all cases.

Test No. Nos. 65 and 66 are steel type Nos. With a small amount of Cr. This is an example using 14 steel materials. Of these, test no. No. 65 does not satisfy the formula (4), and the low cycle fatigue strength is lowered. Test No. In 66, the effective hardened layer depth ECD is deeper than the preferred upper limit for the low cycle fatigue strength, and the low cycle fatigue strength is reduced. In addition, brittle fracture occurred.

Test No. Nos. 67 and 68 are steel grades No. 6 and No. 6 that have an excessive amount of Mo as a preferred component. This is an example using 15 steel materials. Test No. No. 67, the effective hardened layer depth ECD becomes shallower than the preferred lower limit for the low cycle fatigue strength. In 68, the value of C defined by the expression (3) is small, and the low cycle fatigue strength is reduced in all cases.

Test No. No. 69 is a steel type No. with a small amount of Ti. Although it is an example using 16 steel materials, the coarsening (GG) of a crystal grain generate | occur | produces and both the spalling strength and the low cycle fatigue strength are falling.

Test No. No. 70 is a steel type No. with a small amount of Nb. Although it is an example using 17 steel materials, crystal grain coarsening (GG) generate | occur | produces and the low cycle fatigue strength is falling.

Based on these results, FIG. 10 shows the relationship between the value on the left side of equation (4) (3.7 × A−0.47 × B + 1.14) and the spalling intensity. In FIG. Examples Nos. 32, 34, 35, 37 to 39, 42 to 45, 48, 49 Comparative examples of 31, 33, 36, 40, 41, 46, 47, and 50 to 70 are indicated by ◆.

Also, FIG. 11 shows the relationship between the value of C defined by equation (3) and the low cycle fatigue strength. In FIG. Examples Nos. 32, 34, 35, 37 to 39, 42 to 45, 48, 49 Comparative examples of 31, 33, 36, 40, 41, 46, 47, and 50 to 70 are indicated by ◆.

As is clear from these results, it can be seen that satisfying the expression (4) is effective in improving the spalling strength. It can also be seen that setting the value of C defined by equation (3) to 0.66 or more is effective in securing low cycle fatigue strength.

1 Test piece 2 Load roller 3 Test piece 4 Jig 5 Load direction

Claims

% By mass
C: 0.10 to 0.3%,
Si: 0.03-1.50%,
Mn: 0.2-1.8%
P: more than 0% and 0.03% or less,
S: more than 0% and 0.03% or less,
Cr: 0.30 to 2.50%,
Al: more than 0% and 0.08% or less,
N: more than 0% and 0.0150% or less,
Nb: 0.05 to 0.3%,
Ti: 0.05 to 0.1% and B: 0.0005 to 0.005%,
Each of which is iron and inevitable impurities,
When the average C concentration from the surface to a depth of 0.05 mm is 0.50% or more, and the following A, B and C are defined by the following formulas (1) to (3), these are the following (4 Steel parts for high-temperature carburizing excellent in spalling strength and low cycle fatigue strength, characterized by satisfying the formulas (5) and (5).
A = exp {Dm / 10 + Nm 1/2 / 50} × exp {Dt / 15 + Nt 1/2 / 50} (1)
B = exp {Hs / 650 + ECD 1/2 + Hi / 250} (2)
C = exp [Hs / 650−ECD − {(0.89 × Hi−202.16) / 250} 2 ] (3)
3.7 × A−0.47 × B + 1.14 ≦ 0 (4)
C ≧ 0.66 (5)
However, Dm is the average equivalent circle diameter (μm) of MnS in the non-carburized part, Nm is the number density (pieces / mm 2 ) of MnS in the non-carburized part, and Dt is the average circle of TiN in the non-carburized part. Equivalent diameter (μm), Nt is the number density of TiN in non-carburized part (pieces / mm 2 ), Hs is Vickers hardness (HV) at a depth of 0.05 mm from the surface measured with a test force of 300 gf , ECD indicates the effective hardened layer depth (mm), and Hi indicates the Vickers hardness (HV) at the non-carburized portion measured at a test force of 300 gf.
Vickers hardness Hs at a depth of 0.05 mm from the surface measured with a test force of 300 gf is 650 HV or more,
Effective hardened layer depth ECD is 0.4 mm or more,
The steel part for high-temperature carburizing according to claim 1, wherein the Vickers hardness Hi at the non-carburized portion measured at a test force of 300 gf is 300 HV or more.
Furthermore, in mass%,
The steel part for high-temperature carburizing according to claim 1 or 2, comprising at least one of Ni: more than 0% and 2.0% or less and Mo: more than 0% and 1.00% or less.