TW201843317A - Cement steel component and steel material having excellent stability of rolling fatigue life, and method for manufacturing same - Google Patents

Abstract

A steel material having excellent stability of rolling fatigue life, characterized by containing, in terms of mass%, 0.15-0.25% C, 0.35-0.75% Si, 0.2-1% Mn, 1.2-1.7% Cr, 0.3-0.6% Mo, more than 0% to 0.05% P, more than 0% to 0.05% S, 0.005-0.2% Al, more than 0% to 0.05% N, more than 0% to 0.005% O, and more than 0% to 0.014% Ti, the remainder comprising iron and unavoidable impurities, the Cr segregation rate calculated by measurement under the conditions below being 2.0 or less. (i) Measurement position: A total of 4 locations for each 90 DEG on a line from the periphery to the center of the steel material in an arbitrary cut section perpendicular to the rolling direction of the steel material/A total of 4 locations across a length of 5 mm for each 90 DEG with the center of the steel material as the starting point on a line positioned 1/4 the diameter of the steel material in an arbitrary cut section parallel to the rolling direction of the steel material. (ii) Measurement method: At each of the above measurement positions, linear analysis of the Cr concentration is performed using EPMA, the minimum value [Cr]min and the maximum value [Cr]max of the Cr concentration are obtained, [Cr]max/[Cr]min is calculated, and the average at a total of 8 locations is used as the Cr segregation rate.

Description

 Steel material excellent in rolling fatigue life, carburized steel parts, and manufacturing method thereof  

The contents disclosed in the present invention relate to steel materials, carburized steel parts, and the like, which are excellent in rolling fatigue life stability.

Bearing parts for bearings, shafts, gears, and the like used in automobiles and various industrial machines, or parts for mechanical construction, are, for example, a method of obtaining a sufficient strength by performing a quench hardening treatment on a high carbon steel material; The case hardened steel is produced by a method of surface hardening such as carburization treatment or carburizing treatment to harden the surface. The above-mentioned case-hardened steel material is premised on the necessity of performing carburization treatment or carburizing and nitriding treatment, for example, chrome steel (Japanese industrial standard JIS G4053 SCr steel), chrome molybdenum steel (Japanese industrial standard JIS G4053) Specification of SCM steel), nickel-chromium-molybdenum steel (Japanese industrial standard JIS G4053 SNCM steel).

However, in recent years, with the high performance and weight reduction of machinery, the above-mentioned components have been used in a harsh environment where the contact surface pressure is high and the external force is changed. Therefore, there is a problem that it is easy to generate fatigue cracks from very fine defects (inclusions or incompletely hardened structures, etc.). Further, since the above-mentioned defects are distributed in a biased manner, there is a problem that the "rolling fatigue life is unstable". In particular, the parts manufactured by the surface hardening treatment of the case hardened steel have an inconsistent structure of the surface hardened layer, and thus the rolling fatigue life is more unstable, and the inconsistency between the parts is large. Therefore, if such a part is used, based on the safety considerations, it is impossible to use the average of the life of the parts with large inconsistency, and only the lower limit of the life of the parts can be used to replace or inspect the parts, etc., which may result in A lot of unnecessary waste.

For example, the case-hardened steel disclosed in Patent Document 1 does not have a technical problem to be solved by improving the life of rolling fatigue, but it can reduce the carburization treatment or carburizing of the gear formed body. After the treatment, the deformation caused by hardening is performed, and the surface hardened steel of the gear with high precision can be manufactured. Patent Document 1 stands for: "If you want to obtain the dimensional accuracy required for the nearest gear, it is not enough to measure the center segregation degree of C in the cast piece. It is important to eliminate the casting. From the viewpoint of the segregation state of the microscopic C and Mn in the radial cross section of the sheet, the microsegregation degree of C and Mn is controlled within a predetermined range.

Further, the technique disclosed in Patent Document 2 is a technique for improving the anti-coking property of a gear by controlling the nitrogen concentration of the surface up to a depth of 20 μm within a predetermined range.

 [Previous Technical Literature]    [Patent Document]  

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2006-097066

[Patent Document 2] Japanese Laid-Open Patent Publication No. 2013-227674

An object of an embodiment of the present invention is to provide a steel material, a carburized steel part, and a method for producing the same, which are excellent in rolling fatigue life stability.

The steel material excellent in the stability of the rolling fatigue life of the embodiment of the present invention which solves the above-described problems is intended to contain, in mass%, C: 0.15 to 0.25%, Si: 0.35 to 0.75%, and Mn: 0.2 to 1 %, Cr: 1.2~1.7%, Mo: 0.3~0.6%, P: higher than 0% and 0.05% or less, S: higher than 0% and 0.05% or less, Al: 0.005 to 0.2%, N: higher than 0 % and 0.05% or less, O: more than 0% and 0.005% or less, and Ti: more than 0% and 0.014% or less, the balance being iron and unavoidable impurities, Cr segregation determined by the following conditions The rate is below 2.0.

 (i) Measuring position  

In a cross section perpendicular to the rolling direction of the steel material, four straight places are taken at intervals of 90° on a straight line from the outer peripheral portion of the outer peripheral portion of the steel material.

In any cross section parallel to the rolling direction of the steel material, the center of the steel material is used as a starting point, and the center of the steel material is taken as a starting point, and each of the four points is separated by 90 degrees. And the length is 5mm.

 (ii) Method of measurement  

For each of the above measurement positions, the line analysis of the Cr concentration is performed using EPMA to obtain the lowest value [Cr] _min and the maximum value [Cr] _{max of} the Cr concentration, and [Cr] _max / [Cr] _{min is} calculated, and the total is 8 The average value of each place is taken as the Cr segregation rate.

In a preferred embodiment of the present invention, the steel material further contains, in mass%, from Cu: more than 0% and less than 1%, Ni: more than 0% and less than 1%, and B: more than 0%. 0.005% or less, V: more than 0% and less than 1%, W: more than 0% and 0.5% or less, and Nb: one or more selected from the group of more than 0% and 0.1% or less.

The carburized steel component which is excellent in the rolling fatigue life and the rolling fatigue life stability of the embodiment of the present invention which solves the above-described problems is intended to contain C: 0.15 to 0.25% and Si: 0.35 to 0.75% by mass%. Mn: 0.2~1%, Cr: 1.2~1.7%, Mo: 0.3~0.6%, P: higher than 0% and 0.05% or less, S: higher than 0% and 0.05% or less, Al: 0.005~0.2% , N: above 0% and below 0.05%, O: above 0% and below 0.005%, and Ti: above 0% and below 0.014%, the balance being iron and inevitable impurities, present from the surface The ratio of the maximum value to the minimum value of the number density of carbides, nitrides, and carbonitrides having an equivalent circle diameter of 0.1 to 1.0 μm in the surface layer up to a depth of 50 μm is 2.0 or less.

In a preferred embodiment of the present invention, the above-mentioned parts, in mass%, further contain from Cu: more than 0% and less than 1%, Ni: more than 0% and less than 1%, and B: more than 0%. 0.005% or less, V: more than 0% and less than 1%, W: more than 0% and 0.5% or less, and Nb: one or more selected from the group of more than 0% and 0.1% or less.

In a preferred embodiment of the present invention, the average number density of the parts is 0.5 to 3.0 / μm ² .

The method for producing the steel material according to the embodiment of the present invention, which is capable of solving the above problems, is intended to be a temperature range from the solidification start temperature of the molten steel to the solidification end temperature, and is cooled at an average cooling rate of 150 ° C /hr or more. Thereafter, the mixture is heated to 1,100 to 1,300 ° C, and then subjected to a soaking treatment for 1.0 to 40 hours.

The method for producing the above-described component according to the embodiment of the present invention, which is capable of the above-described problems, is intended to be a temperature range from the solidification start temperature of the molten steel to the solidification end temperature, and is cooled after an average cooling rate of 150 ° C /hr or more. After heating to 1100 to 1300 ° C, and then performing a soaking treatment for 1.0 to 40 hours to produce a steel material, a carburizing treatment or a carburizing treatment is performed.

According to the embodiment of the present invention, the steel material and the carburized steel part excellent in the stability of the rolling contact fatigue life can be obtained by adopting the above-described configuration.

1, 2, 3, 4‧‧‧ Where the Cr concentration is measured in any section perpendicular to the rolling direction

5,6,7,8‧‧‧Measurement of Cr concentration in any section parallel to the rolling direction

Fig. 1 is an explanatory view for explaining a measurement site of a Cr concentration in a cross section perpendicular to the rolling direction of the steel material.

Fig. 2 is an explanatory view for explaining a measurement site of a Cr concentration in a cross section parallel to the rolling direction of the steel material.

The inventors of the present invention have conducted many reviews in order to provide steel materials and carburized steel parts (parts subjected to carburizing treatment or carburizing and nitriding treatment on steel materials) which can stably ensure excellent rolling fatigue life. In particular, in order to solve the above problems, it is necessary to suppress the inconsistency of precipitates (carbides, nitrides, carbonitrides) generated when the steel material is subjected to surface hardening treatment, and the distribution (segregation) of the precipitates is The view that causes the instability of rolling fatigue life is constantly being reviewed.

As a result, it has been found that, in order to obtain a steel material which can stably ensure the rolling fatigue life, it is effective to improve the distribution state (segregation) of the formation element of the precipitate, that is, the Cr having the largest content in the steel material. Moreover, a kind of originality was found: the steel material is obtained by appropriately controlling the Cr content and appropriately controlling the cooling conditions at the time of casting and the soaking conditions thereafter.

In addition, it is also known that if a carburized steel part capable of stably ensuring rolling fatigue life is required, it must be in the stage of steel, and the formation element of the above precipitate is improved, that is, the Cr having the most content in the steel. Distribution state (segregation). Moreover, a kind of originality is found: the steel with the Cr segregation rate is appropriately controlled, and if the carburizing treatment or the carburizing and nitriding treatment is performed, the maximum and minimum values of the fine precipitates in the surface layer can be obtained. A carburized steel part excellent in stability of rolling fatigue life can be obtained by controlling the ratio within a suitable range. In addition, a kind of originality was also found: if the carburized steel part is to be obtained, the Cr content is appropriately controlled, and the cooling conditions during casting and the subsequent soaking conditions are appropriately controlled. The steel material can be subjected to carburization or carburizing and nitriding treatment, and the present invention has been completed.

Further, in Patent Document 1 described above, a technique for controlling the degree of segregation of C and Mn in a steel material is disclosed. However, unlike the embodiment of the present invention, the segregation state of Cr is not considered at all. It is considered that it is impossible to stably obtain excellent rolling fatigue life. Further, in the above-described Patent Document 1, when the degree of segregation of C and Mn is measured, as shown in the first drawing of Patent Document 1, the radial section of the cast piece is divided into 4 at intervals of 90°. Each of the aliquots is divided into a plurality of places at equal intervals in the radial direction, and the content of C and Mn is measured. This measurement is equivalent to the rolling direction of the steel in the embodiment of the present invention. The measurement position in the vertical section. However, in the embodiment of the present invention, not only the cross section perpendicular to the rolling direction of the steel material but also the Cr concentration in the cross section parallel to the rolling direction of the steel material is measured and calculated. Since the Cr segregation ratio is controlled more closely to the Cr distribution state in the steel material, the inconsistency in the rolling contact fatigue life characteristics can be remarkably suppressed. In the actual situation, as shown in No. 4 of Table 2 and No. 18 of Table 3 of the examples to be described later, although the Cr segregation ratio of the cross section of the steel material perpendicular to the rolling direction is controlled, However, if the Cr segregation rate of the cross section of the steel material parallel to the rolling direction is not controlled, the inconsistency of the rolling contact fatigue life characteristics cannot be suppressed.

In the present specification, the term "carburizing part" refers to a part obtained by carburizing or carburizing a steel material, and examples thereof include a bearing component, a sliding component, and a mechanical structural component.

Hereinafter, a steel material according to an embodiment of the present invention which can be used as a material of a carburized steel part according to an embodiment of the present invention will be described in detail.

First, the Cr segregation ratio which is the most characteristic for the present invention will be described. In the embodiment of the present invention, the Cr segregation ratio calculated by the following conditions is 2.0 or less.

 (i) Measuring position  

‧ In a cross section perpendicular to the rolling direction of the steel material, four straight places are separated by 90° on a straight line from the center of the outer peripheral portion of the steel material.

‧ In any cross-section that is parallel to the rolling direction of the steel material, and on the straight line at the 1/4 position of the diameter of the steel material, the center of the steel material is used as a starting point, and each of the phases is divided by 90°. Place and length is 5mm.

 (ii) Method of measurement  

For each of the above measurement positions, the line analysis of the Cr concentration is performed using EPMA to obtain the lowest value [Cr] _min and the maximum value [Cr] _{max of} the Cr concentration, and [Cr] _max / [Cr] _{min is} calculated, and the total is 8 The average value of each place is taken as the Cr segregation rate.

Hereinafter, a method of measuring the Cr segregation rate will be described in detail with reference to FIGS. 1 and 2 . First, the steel material was cut into a test piece for measuring the Cr concentration, and the Cr segregation rate was measured in accordance with the following procedure.

First, the Cr concentration in an arbitrary cross section perpendicular to the rolling direction of the steel material is as shown in Fig. 1, and is separated from the outer peripheral portion from the outer peripheral portion of the test piece. The measurement was carried out at 90° in total and taken in four places (1 to 4 in Fig. 1). 1 to 4 in Fig. 1 correspond to the radius of the test piece for measuring the Cr concentration. That is, in the cross section perpendicular to the rolling direction of the steel material, the Cr concentration was measured over the entire radius of the test piece.

On the other hand, the Cr concentration in any cross-section parallel to the rolling direction of the steel material is as shown in Fig. 2, on the straight line at the position of 1/4 of the diameter of the steel material, The center was used as a starting point, and the length was measured by taking a total length of 5 mm from 5 points (5 to 8 in Fig. 2) of 5 mm.

The line analysis of the Cr concentration was performed using EPMA for each of the above-described measurement sites (1 to 4 in the first drawing and 5 to 8 in the second drawing). The measurement conditions of EPMA are as follows:

‧JXA-8500F made by Japan Electronics Standard Co., Ltd.

‧ Accelerating voltage: 15kV

‧Measurement spacing: 10μm

The Cr concentration of each of the measurement points 1 to 8 was measured, and the lowest value [Cr] _min and the maximum value [Cr] _max of the Cr concentration were determined, and the ratio of [Cr] _max / [Cr] _min was calculated. In the embodiment of the present invention, the average relative concentration of [Cr] _max / [Cr] _min in a total of eight places is taken as the Cr segregation ratio.

The steel segregation ratio calculated by the above method has a Cr segregation ratio of 2.0 or less. When the steel material whose Cr segregation rate is controlled within the above range is subjected to surface hardening treatment, the distribution of precipitates formed on the surface layer of the carburized steel part can be suppressed, and the carburized steel having high rolling fatigue life and inconsistent life can be obtained. Components. In order to effectively exhibit such an effect, the Cr segregation ratio is preferably as small as possible, and the upper limit of the Cr segregation ratio is preferably 1.9 or less, more preferably 1.8 or less, and still more preferably 1.7 or less. On the other hand, the lower limit of the Cr segregation ratio is not particularly limited, but is preferably 1.2 or more, and more preferably 1.3 or more in consideration of factors such as manufacturability.

Next, the composition of the steel in the steel will be described. Moreover, the steel components of the carburized steel parts are also the same as the steel.

The "%" indicated in the following description of the ingredients means "% by mass" unless otherwise stated.

 C: 0.15~0.25%  

C is an element for ensuring the hardness of the core portion of the bearing component or the like. Therefore, the lower limit of the C content is set to 0.15% or more. The lower limit of the C content is preferably 0.16% or more, more preferably 0.17% or more. However, when the C content is more than 0.25%, the machinability and cold forgeability of the steel material are deteriorated, and the toughness of the bearing component or the like is deteriorated. Therefore, the upper limit of the C content is set to 0.25% or less. The upper limit of the C content is preferably 0.24% or less, more preferably 0.23% or less.

 Si: 0.35~0.75%  

Si is an effective element for enhancing the solid solution strengthening, quench hardenability, and temper softening resistance of the base material. Therefore, the lower limit of the Si content is set to 0.35% or more. The lower limit of the Si content is preferably 0.38% or more, more preferably 0.40% or more. However, if the Si content is too large, the machinability and cold forgeability of the steel material are remarkably deteriorated. Therefore, the upper limit of the Si content is set to 0.75% or less. The upper limit of the Si content is preferably 0.70% or less, more preferably 0.60% or less.

 Mn: 0.2~1%  

Mn is an element effective for improving the solid solution strengthening and quench hardenability of the base material. Therefore, the lower limit of the Mn content is set to 0.2% or more. The lower limit of the Mn content is preferably 0.25% or more, more preferably 0.30% or more. However, when the Mn content is too large, the machinability and the cold forgeability of the steel material are deteriorated, and after the carburization is performed, a large amount of residual Worthite iron is generated, and the strength of the part is deteriorated. Therefore, the upper limit of the Mn content is set to 1% or less. The upper limit of the Mn content is preferably 0.80% or less, more preferably 0.60% or less.

 Cr: 1.2~1.7%  

Cr is an important element in the embodiment of the present invention, and can improve quench hardenability, and by performing surface hardening treatment, precipitates such as carbides, nitrides, and carbonitrides can be formed in the surface hardened layer. The element of rolling fatigue life is helpful. In addition, Cr is also an element that contributes greatly to the stability of rolling fatigue life. Therefore, the lower limit of the Cr content is set to 1.2% or more. The lower limit of the Cr content is preferably 1.3% or more, more preferably 1.35% or more. However, when the Cr content is too large, the machinability and cold forgeability of the steel material are deteriorated, and coarse precipitates are precipitated to deteriorate the rolling fatigue life and the stability of the rolling fatigue life. Therefore, the upper limit of the Cr content is set to 1.7% or less. The upper limit of the Cr content is preferably 1.6% or less, more preferably 1.5% or less.

 Mo: 0.3~0.6%  

Mo can significantly improve quench hardenability and is an effective element for improving the punching strength. Therefore, the lower limit of the Mo content is set to 0.3% or more. The lower limit of the Mo content is preferably 0.35% or more, more preferably 0.40% or more. However, if the Mo content is too large, the machinability will be deteriorated and the cost will increase. Therefore, the upper limit of the Mo content is set to 0.6% or less. The upper limit of the Mo content is preferably 0.55% or less, more preferably 0.50% or less.

 P: higher than 0% and less than 0.05%  

P is an element which is inevitably contained as an impurity, and segregates at the grain boundary to deteriorate workability. Therefore, the upper limit of the P content is set to 0.05% or less. The upper limit of the P content is preferably 0.04% or less, more preferably 0.03% or less. However, it is a substantial difficulty to change the P content to 0%, and if it is excessively reduced, the steelmaking cost will increase. Therefore, the lower limit of the P content is preferably 0.001% or more.

 S: higher than 0% and less than 0.05%  

S is an element which is inevitably contained as an impurity. When the S content is too large, it is precipitated in the form of MnS, and becomes a starting point of fine cracks, resulting in deterioration of wear resistance. Therefore, the upper limit of the S content is set to 0.05% or less. The upper limit of the S content is preferably 0.04% or less, more preferably 0.03% or less. However, it is a substantial difficulty to change the S content to 0%, and if it is excessively reduced, the steelmaking cost will increase. Therefore, the lower limit of the S content is preferably 0.001% or more.

 Al: 0.005~0.2%  

Al has a strong deoxidation effect, and combines with N to form a nitride, which can refine the crystal grains, and is an element that contributes to the improvement of rolling fatigue life. Therefore, the lower limit of the Al content is set to 0.005% or more. The lower limit of the Al content is preferably 0.010% or more, more preferably 0.015% or more. However, if the Al content exceeds 0.2%, the effect tends to be saturated, so the upper limit of the Al content is set to 0.2% or less. The upper limit of the Al content is preferably 0.1% or less, more preferably 0.05% or less.

 N: higher than 0% and less than 0.05%  

N forms a nitride with Al and suppresses the growth of Worthite iron crystal grains, and can refine the crystal grains, which is an element that contributes to the improvement of rolling fatigue life. Therefore, the lower limit of the N content is preferably 0.0010% or more, more preferably 0.0015% or more, and still more preferably 0.0020% or more. However, if the N content is too large, coarse Al or Ti nitride will be formed and become a starting point of fine cracks. Therefore, the upper limit of the N content is set to 0.05% or less. The upper limit of the N content is preferably 0.040% or less, more preferably 0.020% or less.

 O: above 0% and below 0.005%  

O is an element which combines with Al and Si to form oxide-based inclusions, which adversely affects rolling fatigue life and further adversely affects cold workability. Therefore, the upper limit of the O content is set to 0.005% or less. The upper limit of the O content is preferably 0.004% or less, more preferably 0.003% or less. However, it is a substantial difficulty to change the O content to 0%, and if it is excessively reduced, the steelmaking cost will increase. Therefore, the lower limit of the O content is preferably 0.0001% or more.

 Ti: above 0% and below 0.014%  

Ti is an element which is inevitably contained as an impurity, and is easily combined with N in steel to form coarse TiN, which is a harmful element which adversely affects the surface properties during polishing. Therefore, the upper limit of the Ti content is set to be 0.014% or less. The upper limit of the Ti content is preferably 0.008% or less, more preferably 0.005% or less. However, if the Ti content is to be changed to 0%, there is a substantial difficulty, and if it is excessively reduced, the steelmaking cost will increase. Therefore, the lower limit of the Ti content is preferably 0.0001% or more.

The elements in the steel used in the embodiment of the present invention are as described above, and the remainder are iron and unavoidable impurities. The unavoidable impurities are impurities mixed in the raw materials, materials, manufacturing equipment, and the like, and examples thereof include As, H, and the like.

Further, the steel material according to the embodiment of the present invention may contain the following optional elements.

One or more selected from the group consisting of Cu: more than 0% and less than 1%, Ni: more than 0% and less than 1%, and B: more than 0% and 0.005% or less

Cu, Ni, and B all have an action as a lifting element capable of improving the hardenability of the mother phase, and are elements which can improve the hardness and contribute to the improvement of the rolling fatigue life. These elements may be added alone or in combination of two or more.

In order to effectively exert the above-described effects, the lower limit of the Cu content and the Ni content are preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.03% or more. The lower limit of the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more. However, if the content of each element is excessive, the manufacturability of the steel material will be deteriorated. Therefore, the upper limit of each of the Cu content and the Ni content is preferably 1% or less, more preferably 0.20% or less, still more preferably 0.15% or less. The upper limit of the B content is preferably 0.005% or less, more preferably 0.0040% or less, still more preferably 0.0030% or less.

From V: more than 0% and less than 1%, W: more than 0% and less than 0.5%, and Nb: more than 0% and less than 0.1%

V, W, and Nb are elements that form a hard carbonitride and are useful for improving rolling fatigue life. These elements may be added alone or in combination of two or more.

In order to effectively exert the above effects, the lower limit of the V content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.03% or more. The lower limit of the W content is preferably 0.005% or more, more preferably 0.007% or more, and still more preferably 0.010% or more. The lower limit of the Nb content is preferably 0.01% or more, more preferably 0.02% or more, still more preferably 0.03% or more. However, if the content of each element is excessive, the machinability and cold forgeability of the steel material may deteriorate. Therefore, the upper limit of the V content is preferably 1% or less, more preferably 0.9% or less, still more preferably 0.8% or less. The upper limit of the W content is preferably 0.5% or less, more preferably 0.4% or less, still more preferably 0.3% or less. The upper limit of the Nb content is preferably 0.1% or less, more preferably 0.08% or less, still more preferably 0.07% or less.

Next, a carburized steel part according to an embodiment of the present invention will be described.

The carburized steel part according to the embodiment of the present invention is characterized in that the ratio of the maximum value to the minimum value of the number density of precipitates having an equivalent circle diameter of 0.1 to 1.0 μm in the surface layer up to a depth of 50 μm from the surface is obtained. It is 2.0 or less. Thereby, excellent stability of rolling fatigue life can be obtained.

Moreover, it is preferable that the average number density of the precipitates having an equivalent circle diameter of 0.1 to 1.0 μm in the surface layer up to a depth of 50 μm from the surface is 0.5 to 3.0/μm ² . Thereby, excellent rolling fatigue life can be obtained.

In the embodiment of the present invention, the term "precipitate" refers to all of the carbides in which the carbide-forming elements described below are bonded to carbon, and all of the nitrides in which the nitride-forming elements are combined with nitrogen. And the meaning of carbonitride after the combination of the two.

Carbides [(Fe,Cr) ₃ C, (Fe,Cr) ₇ C ₃ , Mo ₂ C, VC, etc.]

Nitride [(Cr, V, Al) N, etc.]

Carbonitride [(Fe,Cr) ₃ (C,N), (Fe,Cr) ₇ (C,N) ₃ , Mo ₂ (C,N), V(C,N), etc.]

When the average number density of the precipitates is less than 0.5/μm ² , an excellent rolling fatigue life cannot be obtained. Therefore, it is preferable to set the lower limit to 0.5/μm ² or more. The lower limit of the average of the number density is more preferably 0.6 / μm ² or more, still more preferably 0.7 / μm ² or more. On the other hand, when the average number density of the precipitates exceeds 3.0/μm ² , Cr, Mo, etc. will be solid-melted in the precipitate, and the concentration of the hardened lifting element of the mother phase becomes low to cause quench hardenability. Deterioration, rolling fatigue characteristics deteriorate. Therefore, the upper limit of the average of the number density is preferably 3.0/μm ² or less. The upper limit of the average of the number density is more preferably 2.8 / μm ² or less, still more preferably 2.6 / μm ² or less.

On the other hand, reducing the ratio of the maximum value to the minimum value of the number density of the above precipitates is important for stably obtaining an excellent rolling fatigue life. If the above ratio exceeds 2.0, rolling fatigue characteristics cannot be stably obtained. Therefore, the upper limit of the above ratio is set to 2.0 or less. The upper limit of the above ratio is preferably 1.9 or less, more preferably 1.8 or less. Although the lower limit of the above ratio is not particularly limited, it is preferably 1.2 or more in consideration of manufacturability.

Next, a method of producing a steel material according to an embodiment of the present invention will be described.

The manufacturing method according to the embodiment of the present invention is characterized in that the temperature range from the solidification start temperature of the molten steel satisfying the composition component to the solidification end temperature is cooled at an average cooling rate of 150 ° C /hr or more, and then heated. It is heated to 1100~1300°C and then subjected to a soaking treatment for 1.0~40 hours. In this way, the segregation ratio of Cr can be suppressed to 2.0 or less.

First, the average cooling rate in the temperature range from the solidification start temperature of the molten steel to the solidification end temperature is set to 150 ° C /hr or more. In the embodiment of the present invention, in order to suppress segregation of Cr generated during the casting process, it is necessary to appropriately control the cooling conditions of the molten steel. The term "temperature range from the solidification start temperature of the molten steel to the end of the solidification end temperature" as used herein means the temperature range from the liquidus temperature of the molten steel to the solidus temperature, and the average cooling in the above temperature range. Speed means the meaning of the average solidification speed of the cast piece. When the average cooling rate is too slow, the solidification becomes too slow, so that a concentrated portion of Cr is formed and the Cr segregation rate is increased. The faster the above average cooling rate, the smaller the Cr segregation rate will be. The above average cooling rate of the conventional steel material is about 50 ° C / hour, so it is presumed that the Cr segregation rate is very high. The lower limit of the average cooling rate is preferably 160 ° C /hr or more, more preferably 170 ° C / h or more. The upper limit of the average cooling rate is not particularly limited. However, in consideration of manufacturability and the like, it is preferably 300 ° C /hr or less, more preferably 250 ° C /hr or less.

The above average cooling rate was measured by the following method using a thermocouple type temperature measuring device having a temperature detecting portion at the tip end portion. The temperature detecting portion is placed at a position 1/2 of the height h of the mold for casting into the molten steel and is at a position 1/4 of the diameter D to directly measure the temperature of the molten steel, and determine the solidification starting temperature of the molten steel. The average cooling rate was calculated from the time from the start of the solidification end temperature. The solidification start temperature and the solidification end temperature of the molten steel are: the integrated thermodynamic calculation system (THREMO-CALC SOFTWARE Ver.R; manufactured by Itochu Technology Solutions Co., Ltd.), specifying C content, Si content, Cr content, Mn content, Mo Calculated by content and Al content. Further, the range of the composition components shown in Table 1 does not greatly change between the solidification start temperature and the solidification end temperature, and in the examples described later, the solidification start temperature of the calculated value of the steel type A is 1507 ° C, The solidification end temperature: 1463 ° C was used to calculate the average cooling rate.

Next, the slab obtained by cooling to the solidification end temperature of the molten steel in the above manner is heated to 1,100 to 1,300 ° C, and then subjected to a soaking treatment for 1.0 to 40 hours.

Here, if the heating temperature (soaking temperature) is less than 1,100 ° C, the diffusion of Cr is insufficient, so that the Cr segregation rate cannot be lowered. Therefore, the lower limit of the above heating temperature is set to 1100 ° C or higher. The lower limit of the heating temperature is preferably 1150 ° C or higher, more preferably 1170 ° C or higher. From the viewpoint of lowering the Cr segregation rate, the heating temperature is preferably as high as possible, but if it is too high, the manufacturability and the like are lowered. Therefore, the upper limit is set to 1300 ° C or lower. The upper limit of the heating temperature is preferably 1280 ° C or lower, more preferably 1270 ° C or lower.

Further, if the soaking time is less than 1.0 hour, the diffusion of Cr is insufficient, so that the Cr segregation rate cannot be lowered. Therefore, the lower limit of the soaking time is 1.0 hours or more. Conventional steels are cooled immediately after heating (the soaking time is zero), and therefore, the Cr segregation rate of the conventional steel is predicted to significantly exceed 2.0. The lower limit of the soaking time is preferably 5 hours or longer, more preferably 8 hours or longer. From the viewpoint of reducing the Cr segregation rate, the above-mentioned soaking time is as long as possible, but if it is too long, the manufacturing property and the like are deteriorated, so the upper limit is 40 hours or shorter. The upper limit of the soaking time is preferably 25 hours or shorter, more preferably 20 hours or shorter.

The above is the most characteristic process in the manufacturing method of the embodiment of the present invention. The production method of the embodiment of the present invention is characterized by the above steps, and the steps other than the steps are not particularly limited, and a usual method can be employed.

The steel material according to the embodiment of the present invention includes a steel material having a linear shape and a rod shape. In order to control such a shape, after performing the above-described soaking treatment, hot forging (hot forging) is performed according to a general usual method, followed by Further, hot intercalation processing such as hot rolling (hot rolling) is performed. Melt treatment and normalizing treatment may be further carried out as necessary.

The hot forging in the hot working is preferably carried out in a temperature range of 1,100 to 1,300 °C. When the hot forging temperature is too low, the cast piece is not easily deformed and the manufacturability is deteriorated. A better temperature is above 1150 °C. On the other hand, when the hot forging temperature is too high, the time required for heating to a high temperature is long, and fuel or the like is consumed, so that the manufacturability is deteriorated. A better temperature is below 1250 °C.

For hot rolling, for example, it is preferred to carry out rolling after heating to a range of 850 to 1300 °C. When the heating temperature of the hot rolling is too low, the steel sheet is not easily deformed and the manufacturability is deteriorated. A better temperature is above 900 °C. On the other hand, when the heating temperature is too high, the time required for heating to a high temperature is long, and fuel or the like is consumed, so that the manufacturability is deteriorated. A better temperature is below 1200 °C.

After hot rolling, it is preferred to carry out cooling to room temperature at an average cooling rate of 0.01 to 10 ° C / sec. If the average cooling rate is too slow, the manufacturability will be deteriorated. A better average cooling rate is 0.05 ° C / sec or more. On the other hand, if the average cooling rate is too fast, cracks or flaws may occur. A better average cooling rate is 8 ° C / sec or less.

The purpose of the melt treatment is to solidify the coarse precipitates formed during hot forging and hot rolling. Specifically, it is preferably heated to 1100 to 1300 ° C for 1 to 5 hours, and then cooled at an average cooling rate of 0.5 to 20 ° C / sec.

When the heating temperature of the melt treatment is too low, the precipitate cannot be solid-melted. A better heating temperature is above 1150 °C. On the other hand, if the heating temperature is too high, the manufacturability is deteriorated. A better heating temperature is below 1250 °C.

Further, when the holding time of the melt treatment is too short, the precipitate cannot be solid-melted. A better hold time is more than 2 hours. On the other hand, if the holding time is too long, the manufacturability will be deteriorated. A better hold time is less than 4 hours.

The average cooling rate after the melt treatment is preferably 0.5 to 20 ° C / sec. When the average cooling rate is too slow, coarse precipitates are formed and it is impossible to solidify. A better average cooling rate is 1.0 ° C / sec or more. On the other hand, if the average cooling rate is too fast, cracks or flaws may occur. A better average cooling rate is 10 ° C / sec or less.

The purpose of normalizing is to obtain: a multiphase structure of ferrite iron, a single phase of Borne iron, a complex phase of ferrite and ferrite, or a complex phase of snow and iron. A uniform tissue that is constructed. Specifically, the normalizing treatment is performed by heating to 750 to 1100 ° C for 10 minutes or more and 5 hours or less, and then cooling to room temperature at an average cooling rate of 0.01 to 10 ° C / sec.

If the heating temperature of the normalizing treatment is too low, the heating is insufficient, and the above-described normalizing treatment effect cannot be obtained. A better heating temperature is above 760 °C. On the other hand, if the heating temperature is too high, the manufacturability is deteriorated. A better heating temperature is below 1050 °C.

Further, if the holding time of the normalizing treatment is too short, the heating is insufficient, and the above-described normalizing treatment effect cannot be obtained. A better hold time is 20 minutes or more. On the other hand, if the holding time is too long, the manufacturability will be deteriorated. A better hold time is less than 4 hours.

The average cooling rate of the normalizing treatment is preferably 0.01 to 10 ° C / sec. When the average cooling rate is too slow, the manufacturability is deteriorated. A better average cooling rate is 0.02 ° C / sec or more. On the other hand, if the average cooling rate is too fast, cracks or flaws may occur. A better average cooling rate is 8 ° C / sec or less.

The steel material according to the embodiment of the present invention obtained in this manner is controlled to have a Cr segregation ratio of 2.0 or less. Therefore, the rolling fatigue life is excellent in stability.

The steel material according to the embodiment of the present invention is suitable as a material for use in a bearing component, a sliding component, a mechanical structural component, or the like, which is used in automobiles and various industrial machines. Examples of the above-mentioned components include a roller bearing such as a roller bearing and a ball bearing; an inner wheel and an outer wheel of the rolling bearing; a rolling element of the rolling bearing; and a rolling contact member such as a shaft and a gear.

Next, a method of producing a carburized steel part according to an embodiment of the present invention will be described.

In the carburized steel part according to the embodiment of the present invention, the steel material obtained in the above-described manner is subjected to cold-working processing such as cutting and cold forging according to a general conventional method to form a predetermined part shape, and then the osmosis is performed. Manufactured by carbon treatment or carburizing and nitriding treatment.

The carburization treatment in the surface hardening treatment may be, for example, a carburization treatment in which the carbon potential (Cp; Carbon Potential) of 6 hours or longer and 6 hours or less is 0.6 to 1.4% at a temperature of 850 to 950 °C. Thereafter, a method of hardening using a refrigerant such as oil or water is carried out. The cooling at this time is preferably up to 680 ° C, more preferably up to 650 ° C, and is preferably 50 ° C / sec or more and 150 ° C / sec or less, more preferably 70 ° C / sec or more and 130 ° C / sec or less. The average cooling rate is carried out.

For example, a method of performing a two-stage carburization treatment as described below; a method of performing carbonitriding by nitriding after carburizing may be suitably employed.

 (1) Two-stage carburizing treatment  

The above two-stage carburization treatment preferably comprises: at a temperature range of 900 to 950 ° C and after maintaining for 2 to 6 hours in a gas phase atmosphere having a carbon potential (Cp; Carbon Potential) of 1.0 to 1.4%, 50 a first carburization step of cooling to 680 ° C at an average cooling rate of ° C /sec or more; and heating to 800 to 880 ° C at an average temperature increase rate of 25 ° C / min or more, and at the above temperature and carbon potential (Cp After maintaining for 0.5 to 8 hours in a gas phase atmosphere of 0.8 to 1.2%, a second carburization step of hardening is performed.

When the holding temperature in the first carburization step is less than 900 ° C, the amount of carburization of the surface layer is insufficient, and even if the second carburizing step is performed, the amount of fine precipitates cannot be sufficiently ensured. Therefore, the lower limit of the temperature is preferably 900 ° C or higher, more preferably 930 ° C or higher. On the other hand, if it is higher than 950 ° C, the amount of carbon in the surface layer is too large, and in the second carburization step, the amount of precipitates is too large. When the amount of precipitates is large, the unevenness of the precipitates can be reduced, but the concentration of the hardened lifting elements of the mother phase is lowered, the quench hardenability is deteriorated, and the rolling fatigue characteristics are also deteriorated. Therefore, the upper limit of the holding temperature is preferably 950 ° C or lower, more preferably 940 ° C or lower.

When the Cp in the first carburization step is less than 1.0%, the amount of carburization of the surface layer is insufficient, and the amount of fine precipitates cannot be sufficiently ensured even if the second carburization step is performed. Therefore, the lower limit of Cp is preferably 1.0% or more, more preferably 1.1% or more. On the other hand, if Cp exceeds 1.4%, the surface layer has too much carburization. Therefore, the upper limit of Cp is preferably 1.4% or less, more preferably 1.3% or less.

When the holding time in the first carburization step is less than 2 hours, the amount of carburization of the surface layer is insufficient, and even if the second carburization step is performed, the amount of sufficient fine precipitates cannot be ensured. Therefore, the lower limit of the holding time is preferably 2 hours or longer, more preferably 3 hours or longer. On the other hand, if the holding time exceeds 6 hours, the amount of carburization of the surface layer is excessive. Therefore, the upper limit of the holding time is preferably 6 hours or shorter, more preferably 5 hours or shorter.

When the average cooling rate in the first carburization step is too slow, coarse precipitates are formed during the cooling process, and the amount of the desired fine precipitates will be insufficient. Therefore, it is preferably at most 680 ° C, more preferably at 650 ° C, and preferably at 50 ° C / sec or more, more preferably at an average cooling rate of 70 ° C / sec or more. The upper limit of the average cooling rate is not particularly limited. However, in view of manufacturability, it is preferably 150 ° C / sec or less, more preferably 130 ° C / sec or less.

The cooling method in the first carburization step may be hardened by using a refrigerant such as oil or water, or may be blown and cooled.

When the average temperature increase rate in the second carburization step is too slow, coarse precipitates will be formed during the heating process, and the amount of the desired fine precipitates will be insufficient. Therefore, it is preferably from 800 to 880 ° C, more preferably from 820 to 860 ° C, and preferably at 25 ° C / min or more, more preferably at a temperature increase rate of 30 ° C / min or more. The upper limit of the average temperature increase rate is not particularly limited. However, in consideration of manufacturability and the like, it is preferably 100 ° C / min or less.

When the holding temperature in the second carburization step is less than 800 ° C, the amount of carburization of the surface layer is insufficient, and the amount of fine precipitates cannot be sufficiently ensured. Therefore, the lower limit of the temperature is preferably 800 ° C or higher, more preferably 820 ° C or higher. On the other hand, if it exceeds 880 ° C, carbon will be solid-melted in the matrix phase, and the amount of fine precipitates cannot be sufficiently ensured. Therefore, the upper limit of the holding temperature is preferably 880 ° C or lower, more preferably 860 ° C or lower.

When Cp in the second carburization step is less than 0.8%, the amount of carburization of the surface layer is insufficient, and the amount of fine precipitates cannot be sufficiently ensured. Therefore, the lower limit of Cp is preferably 0.8% or more, more preferably 0.9% or more. On the other hand, if Cp exceeds 1.2%, the amount of precipitates will be excessive. Therefore, the upper limit of Cp is preferably 1.2% or less, more preferably 1.1% or less.

When the holding time in the second carburization step is less than 0.5 hours, the amount of carburization of the surface layer is insufficient, and the amount of fine precipitates cannot be sufficiently ensured. Therefore, the lower limit of the holding time is preferably 0.5 hours or longer, more preferably 2 hours or longer. On the other hand, if the holding time exceeds 8 hours, the amount of precipitates will be excessive. Therefore, the upper limit of the holding time is preferably 8 hours or shorter, more preferably 7 hours or shorter.

After the above holding, the fine precipitates can be dispersed by hardening with a refrigerant such as oil or water.

In addition, the measurement of the carbon potential (Cp) can be carried out by a generally used method such as an O ₂ sensor method, a CO ₂ method using an infrared analyzer, a dew point measurement method, or a carbon potential meter using an iron wire. Determination. Among these measurement methods, from the viewpoint of measurement accuracy, the best method is to place an iron wire called a "Cp coil" in a gas phase atmosphere in a furnace, and use this Cp coil by infrared absorption method. Wait for the method of quantitative analysis.

 (2) Carburizing and nitriding treatment  

The carburizing and nitriding treatment includes a carburizing step of cooling to 800 to 880 ° C after holding in a gas phase atmosphere at a temperature of 900 to 950 ° C and a Cp of 0.7 to 1.2% for 2 to 6 hours; The nitriding step of hardening is preferably carried out after maintaining the cooling temperature in a gas phase atmosphere having a Cp of 0.5 to 0.9% and an amount of NH ₃ of 6 to 12 vol% for 2 to 8 hours.

The preferable conditions of the carburization step in these steps are the first carburization in the two-stage carburization treatment described above except for the conditions of Cp of 0.7 to 1.2% and cooling to 800 to 880 °C. The conditions of the process are the same.

When the Cp in the carburization step is less than 0.7%, the amount of carburization of the surface layer is insufficient, and the amount of fine precipitates cannot be sufficiently ensured. Therefore, the lower limit of Cp is preferably 0.7% or more, more preferably 0.8% or more. On the other hand, when Cp exceeds 1.2%, the amount of carburization of the surface layer is too large, and the amount of precipitates is too large. Therefore, the upper limit of Cp is preferably 1.2% or less, more preferably 1.1% or less.

Cooling from 900 to 950 ° C to 800 to 880 ° C can be performed by furnace cooling.

When the holding temperature in the nitriding step is less than 800 ° C, the amount of carburization and the amount of nitriding in the surface layer are insufficient, and the amount of fine precipitates cannot be sufficiently ensured. Therefore, the lower limit of the temperature is preferably 800 ° C or higher, more preferably 820 ° C or higher. On the other hand, when it exceeds 880 ° C, carbon and nitrogen will solidify in the mother phase, and the amount of fine precipitates cannot be sufficiently ensured. Therefore, the upper limit of the holding temperature is preferably 880 ° C or lower, more preferably 860 ° C or lower.

When the Cp in the nitriding step is less than 0.5%, the amount of carburization and the amount of nitriding in the surface layer are insufficient, and the amount of fine precipitates cannot be sufficiently ensured. Therefore, the lower limit of Cp is preferably 0.5% or more, more preferably 0.6% or more. On the other hand, if Cp exceeds 0.9%, the amount of precipitates is too large. Therefore, the upper limit of Cp is preferably 0.9% or less, more preferably 0.8% or less.

When the amount of NH ₃ in the nitriding step is less than 6% by volume, the amount of carburization and the amount of nitriding in the surface layer are insufficient, and the amount of fine precipitates cannot be sufficiently ensured. Therefore, the lower limit of the amount of NH ₃ is preferably 6% by volume or more, more preferably 7% by volume or more. On the other hand, if the amount of NH ₃ exceeds 12% by volume, the amount of precipitates is too large. Therefore, the upper limit of the amount of NH ₃ is preferably 12% by volume or less, more preferably 10% by volume or less.

After the above holding, the fine precipitates can be dispersed by hardening with a refrigerant such as oil or water.

After the above surface hardening treatment, tempering treatment may be performed as necessary. Tempering treatment, for example, is preferably carried out at 80 to 250 ° C for 30 to 240 minutes.

The carburized steel part according to the embodiment of the present invention which is obtained by this method is suitable as a material for bearing parts, sliding parts, parts for machine structural use, etc. used in automobiles and various industrial machines. Examples of the above-mentioned components include a roller bearing such as a roller bearing and a ball bearing; an inner wheel and an outer wheel of the rolling bearing; a rolling element of the rolling bearing; and a rolling contact member such as a shaft and a gear.

 [Examples]  

In the following, the present invention will be specifically described by way of examples, but the present invention is not limited to the following examples, and may be modified and implemented within the scope of the gist of the invention described below. It is within the technical scope of the present invention.

 Example 1  

In Example 1, the rolling fatigue life after the carburization of the steel material was measured.

 Production of test strips  

Using a small melting furnace having a capacity of 150 kg/1ch, the test steels of various chemical compositions as shown in Table 1 were melted, and after cooling at the average cooling rate shown in Table 2, the conditions shown in Table 2 were carried out. The mixture is heat treated to form a cast piece. The cast piece obtained in this manner was heated to 1,250 ° C, and then hot forged at a temperature of 1,200 ° C, and then cooled to room temperature. Subsequently, after heating to 1,100 ° C, hot rolling was performed, and the mixture was cooled to room temperature at an average cooling rate of 0.5 ° C / sec, whereby a round bar having a diameter D of 70 mm was produced. Further, the above average cooling rate is calculated in accordance with the aforementioned method.

Next, a test piece having a diameter D of 70 mm in which the following measurement points were observed was cut out from the round bar steel, and the Cr segregation rate was calculated according to the above method. The above-mentioned Cr segregation ratio (average value of 8 places in total) is indicated in Table 2, and for reference, the Cr segregation rate in any cross section perpendicular to the rolling direction of the steel material is also indicated. (the average value of the measurement points 1 to 4) and the Cr segregation rate (the average value of the measurement places 5 to 8) in any cross-section parallel to the rolling direction of the steel material.

 Determination of rolling fatigue life after carburizing  

A disk-shaped test piece having a diameter of 60 mm and a thickness of 5 mm was cut out from the above-mentioned round bar steel. Next, carburization treatment was carried out at a temperature of 940 ° C for 3 hours with a Cp of 0.85%, and oil hardening was performed. Then, tempering treatment was performed for 120 minutes at a temperature of 160 °C. Further, the average cooling rate at the time of cooling after carburization was cooled to 680 ° C at 70 ° C / sec. Further, the carburizing substrate gas is: RX gas, and the carburizing gas for controlling Cp is: propane gas. And Cp coil was used to measure Cp.

For each test No., 16 tempered test pieces were used, and by a thrust type rolling fatigue tester, the number of times of repetition was 1500 rpm, the surface pressure was 5.3 GPa, and the number of suspensions was 2 × 10 ⁸ times. To determine the rolling fatigue life. The index of the stability of rolling fatigue life is: Weber coefficient m. The Weber coefficient m is a slope of an approximate curve obtained by plotting the test results of rolling fatigue life on Weber's accuracy paper. The larger the Weber coefficient m is, the better the stability of the rolling fatigue life is. In the present embodiment, the stability of the rolling contact fatigue life when the Weber coefficient m is 0.6 or more is judged to be excellent. In addition, for reference, Table 2 also shows together the L ₁₀ lifetime calculated when calculating the Weber coefficient m, that is, the number of stress repetitions before the fatigue failure is reached when the cumulative damage is 10%. .

From these results, the following investigations can be made.

Test Nos. 1, 2, and 6 to 13 in Table 2 are the steel grades A to F, L, and O in Table 1 which are in accordance with the compositional components specified in the embodiment of the present invention, and in accordance with the embodiment of the present invention. An example of manufacturing conditions of Test Nos. 1, 2, and 6 to 13 in Table 2 of the specified requirements. As can be seen from these examples, the Weber coefficient m is 0.6 or more, and the stability of the rolling fatigue life after the carburizing treatment is excellent.

On the other hand, the following test No. is one of the requirements that do not conform to the requirements specified in the embodiment of the present invention.

In Test No. 3 in Table 2, although the composition is in accordance with the requirements of the embodiment of the present invention, the average cooling rate is too slow, so that the Cr segregation rate becomes large, and the stability of rolling fatigue life is lowered. In addition, the life of L ₁₀ is also low.

In Test No. 4 in Table 2, although the composition is in accordance with the requirements of the embodiment of the present invention, the soaking temperature is too low, so that the Cr segregation rate becomes large, and the stability of rolling fatigue life is lowered. In addition, the life of L ₁₀ is also low. More specifically, as shown in Table 2, Test No. 4, although the Cr segregation rate in the surface perpendicular to the rolling direction is small, the Cr segregation rate in the surface parallel to the rolling direction is very high. Large, and thus the average Cr segregation rate becomes large, so that the stability of rolling fatigue life is lowered. From this, it is understood that the stability of the rolling fatigue life cannot be improved simply by lowering the Cr segregation rate in the surface perpendicular to the rolling direction.

In Test No. 5 in Table 2, the composition of the composition is in accordance with the requirements of the embodiment of the present invention, but the soaking time is too short, so that the Cr segregation rate becomes large, and the stability of rolling fatigue life is lowered. In addition, the life of L ₁₀ is also low.

In Test No. 14 in Table 2, an example of the steel type P of Table 1 having a small Cr content was used, and the Cr segregation rate was increased and the stability of the rolling fatigue life was lowered. In addition, the life of L ₁₀ is also low.

In Test No. 15 in Table 2, an example of the steel type Q of Table 1 having a large Cr content was used, and the Cr segregation rate was increased and the stability of the rolling fatigue life was lowered. In addition, the life of L ₁₀ is also low.

 Example 2  

Example 2 is a measurement of the rolling fatigue life after the carburizing treatment of the steel material.

In this Example 2, except that the cooling was carried out to the solidification end temperature at the average cooling rate shown in Table 3, and the soaking treatment was carried out under the conditions shown in Table 3, the other portions were utilized as described above. In the same manner as in Example 1, after the round bar steel was produced, the Cr segregation ratio was measured.

Next, the round bar steel was subjected to a carburization treatment in the same manner as in the above Example 1, and then cooled to 860 ° C. At this temperature, Cp was 0.7% for 2 hours and the amount of NH ₃ was 4% by volume. Nitriding treatment, followed by oil cooling hardening. Then, tempering treatment was performed at 160 ° C for 120 minutes. Further, the nitriding gas was NH ₃ gas, and the ratio with respect to the base gas (RX gas) was set to 4% by volume. Cp was measured using a Cp coil.

Using the obtained test piece, the rolling fatigue life was measured in the same manner as in Example 1. When the Weber coefficient m was 0.6 or more, it was judged that the rolling fatigue life was excellent in stability.

These results are shown in Table 3.

From these results, the following investigations can be made.

Test Nos. 16, 19, 20, and 22 to 28 in Table 3 are the steel grades A, G~K, M, and N in Table 1 which are in accordance with the compositional components specified in the embodiment of the present invention, and are in accordance with Examples of the manufacturing conditions of Test Nos. 16, 19, 20, and 22 to 28 in Table 3 of the requirements specified in the embodiments of the present invention are manufactured. It can be seen that these are all Weber coefficient m of 0.6 or more, and have excellent rolling fatigue life stability after carburizing and nitriding treatment.

On the other hand, the following test No. are any of the requirements that do not conform to the requirements specified in the embodiment of the present invention.

In Test No. 17 in Table 3, although the composition is in accordance with the requirements of the embodiment of the present invention, the average cooling rate is too slow, so that the Cr segregation rate becomes large, and the stability of rolling fatigue life is lowered. In addition, the life of L ₁₀ is also low.

In Test No. 18 in Table 3, although the composition is in accordance with the requirements of the embodiment of the present invention, the soaking temperature is too low, so that the Cr segregation rate becomes large, and the stability of rolling fatigue life is lowered. In addition, the life of L ₁₀ is also low. More specifically, as shown in Table 3, in Test No. 18, the Cr segregation rate in the surface perpendicular to the rolling direction was small, but the Cr segregation rate in the surface parallel to the rolling direction was very high. Large, and thus the average Cr segregation rate becomes large, so that the stability of rolling fatigue life is lowered. From this, it is understood that the stability of the rolling fatigue life cannot be improved simply by lowering the Cr segregation rate in the surface perpendicular to the rolling direction.

In Test No. 21 in Table 3, the composition of the composition was in accordance with the requirements of the embodiment of the present invention, but the heat treatment time was too short, so that the Cr segregation rate became large, and the stability of the rolling fatigue life was lowered. In addition, the life of L ₁₀ is also low.

As is apparent from the results of the above-described first and second embodiments, as long as the steel material according to the embodiment of the present invention is used, the parts after the carburization treatment and the carburizing and nitriding treatment can stably ensure excellent rolling fatigue life.

 Example 3  

In Example 3, the rolling fatigue life after the two-stage carburization treatment was applied to the steel material.

 Production of test strips  

Using a small melting furnace having a capacity of 150 kg/1ch, the test steels of various chemical compositions shown in Table 4 were melted, and after cooling at the average cooling rate shown in Table 5, the conditions shown in Table 5 were carried out. The mixture is heat treated to form a cast piece. The cast piece obtained in this way was heated to 1,250 ° C, and then hot forged at a temperature of 1,200 ° C, and then cooled to room temperature. Subsequently, after heating to 1,100 ° C, hot rolling was performed, and the mixture was cooled to room temperature at an average cooling rate of 0.5 ° C / sec, whereby a round bar having a diameter D of 70 mm was produced. Further, the above average cooling rate is calculated in accordance with the aforementioned method.

A disk-shaped test piece having a diameter of 60 mm and a thickness of 5 mm was cut out from the above-mentioned round bar steel. Next, a two-stage carburization treatment was carried out under the conditions shown in Table 5. Then, tempering treatment was performed at 160 ° C for 120 minutes. Further, the first carburization step and the second carburization step are cooled by oil cooling. The cooling up to 650 ° C in the first carburization step was carried out at an average cooling rate of 70 ° C / sec. Further, the temperature rise to 820 to 860 ° C in the second carburization step was carried out at an average temperature increase rate of 30 ° C /min. Further, the carburized substrate gas is RX gas, and propane gas is used as a carburizing gas for controlling Cp. In addition, Cp was measured using a Cp coil.

Determination of carbides, nitrides, and carbonitrides having an equivalent circle diameter of 0.1 to 1.0 μm in the surface layer

Using the test piece after the tempering treatment, the measurement was carried out in accordance with the following procedure: a precipitate having an equivalent circle diameter of 0.1 to 1.0 μm in the surface layer having a depth of 50 μm from the surface. The cutting was performed in such a manner that the test piece was observed to have a depth of 50 μm from the surface, and after the cross-section was polished, a scanning electron microscope (SEM, Scanning Electron Microscope) was used at a magnification of 8000 times. Observe and define the precipitate. Among the above-mentioned precipitates, the definition of the carbide, the nitride, and the carbonitride which are the targets of the embodiment of the present invention is an electron micro-probe X-ray analyzer manufactured by JEOL Ltd. The components (C, N, Cr, Mo, V, Al) in the precipitate were analyzed and completed. For the carbides, nitrides, and carbonitrides defined by the above method, the particle analysis software [particle analysis III for Windows. Version 3.00 SUMITOMO METAL TECHNOLOGY (trade name)] is used for 10 observation fields. (The area of one field of view was 108 μm ² ), and the equivalent circle diameter was measured. Further, the number of carbides, nitrides, and carbonitrides having an equivalent circle diameter of 0.1 to 1.0 μm among these is measured, and is converted into an average value of the number density per 1 μm ² and is shown in Table 6. . In addition, the number density of each field of view is also calculated. In Table 6 below, the maximum and minimum values of the number density in the 10 observation fields (that is, the number density of each field of view in 10 fields) are also indicated. ratio.

 Determination of rolling fatigue life after carburizing  

For each test No., 16 tempered test pieces were used, and by a thrust type rolling fatigue tester, the number of times of repetition was 1500 rpm, the surface pressure was 5.3 GPa, and the number of suspensions was 2 × 10 ⁸ times. To determine the rolling fatigue life. Further, in the case where the cumulative damage is 10%, the number of stresses before reaching the fatigue failure, that is, the rolling fatigue life L ₁₀ (L ₁₀ life) is 1.0 × 10 ⁷ or more, is judged to have excellent rolling. Fatigue life. The index of the stability of rolling fatigue life is: Weber coefficient m. The Weber coefficient m is a slope of an approximate curve obtained by plotting the test results of rolling fatigue life on Weber's accuracy paper. The larger the Weber coefficient m is, the better the stability of the rolling fatigue life is. In the present embodiment, the stability of the rolling contact fatigue life when the Weber coefficient m is 0.6 or more is judged to be excellent.

From these results, the following investigations can be made.

In Test Nos. 1 to 3, 8 to 17, 20, and 21 in Table 6, the steel types A, B, D, F to H, Q in Table 4 which are in accordance with the composition of the embodiment defined by the embodiment of the present invention are used. T is an example of manufacturing according to the manufacturing conditions of Test Nos. 1 to 3, 8 to 17, 20, and 21 in Table 5 of the requirements specified in the embodiment of the present invention. It can be seen that the ratio of the maximum value to the minimum value of the number density of the precipitates in the surface layer of the carburized steel part is appropriately controlled as a result of suppression of the Cr segregation rate of these steel materials, and the Weber coefficient m is 0.6. As described above, it has excellent stability in rolling fatigue life after carburization treatment.

Among these examples, Test Nos. 1, 3, 9, 10, 12, 14 to 16, 20, 21, and the carburization treatment shown in Table 5 are based on preferred carburization treatment conditions of the embodiment of the present invention. As a result, it can be seen that its L ₁₀ lifetime is 1.0 × 10 ⁷ or more, and it has excellent rolling fatigue life.

On the other hand, the following test No. are any of the requirements that do not conform to the requirements specified in the embodiment of the present invention.

In Test No. 4 in Table 6, although the composition is in accordance with the requirements of the embodiment of the present invention, the average cooling rate at the time of casting is too slow, and therefore, the degree of inconsistency in the number density of fine precipitates becomes large, and the rolling fatigue life is increased. The stability is low. In addition, the life of L ₁₀ is also low.

In Test No. 5 in Table 6, although the composition is in accordance with the requirements of the embodiment of the present invention, the soaking temperature is too low, and therefore, the degree of inconsistency in the number density of fine precipitates becomes large, and the stability of rolling fatigue life is stabilized. Go low. In addition, the life of L ₁₀ is also low.

In Test No. 6 in Table 6, although the composition is in accordance with the requirements of the embodiment of the present invention, the soaking time is too short, and therefore, the degree of inconsistency in the number density of fine precipitates becomes large, and the stability of rolling fatigue life is stabilized. Go low. In addition, the life of L ₁₀ is also low.

In Test No. 7 in Table 6, although the composition is in accordance with the requirements of the embodiment of the present invention, the average cooling rate at the time of casting is too slow, the soaking temperature is too low, and the soaking time is too short, so that fine precipitates are The degree of inconsistency in the number density becomes large, and the stability of the rolling fatigue life becomes low.

Test No. 18 in Table 6 is an example in which the steel type U in Table 4 having a small Cr content is used, and the number density of fine precipitates is small, and the life of L ₁₀ is lowered.

Test No. 19 in Table 6 is an example in which the steel type V in Table 4 having a large Cr content is used, and the number density of fine precipitates is increased, and the life of L ₁₀ is lowered.

 Example 4  

Example 4 is a measurement of the rolling fatigue life after the carburizing and nitriding treatment of the steel material.

Specifically, in the third embodiment described above, in addition to the cooling at the average cooling rate shown in Table 7, the soaking treatment was carried out under the conditions shown in Table 7, and the other portions were used as described above. The round bar steel was produced in the same manner as in Example 3.

Next, in the same manner as in the above-described Example 3, a disk-shaped test piece was cut out from the round bar steel, and after carburizing treatment and nitriding treatment under the conditions shown in Table 7, the temperature was 160 ° C. The temperature was tempered for 120 minutes. Further, the cooling of the carburizing step and the nitriding step is performed by oil-cooling and hardening. Further, the carburized substrate gas was RX gas, and propane gas was used as a carburizing gas for controlling Cp, and Cp was measured using a Cp coil. The nitriding gas was NH ₃ gas, and the ratio with respect to the base gas (RX gas) was set to 4% by volume.

Using the obtained test piece, the rolling fatigue life was measured by the same method as in Example 3, and the example of the L ₁₀ life of 1.0 × 10 ⁷ or more was evaluated as follows: the rolling fatigue life was excellent, and the Weber coefficient m was 0.6. In the above, the evaluation was as follows: the rolling fatigue life was excellent in stability.

These results are shown in Table 8.

From these results, the following investigations can be made.

Test Nos. 22, 26, 27, and 29 to 38 in Table 8 are the steel grades A, K, M~O, and R in Table 4 which are in accordance with the compositional components specified in the embodiment of the present invention, and are in accordance with Examples of the manufacturing conditions of Test Nos. 22, 26, 27, and 29 to 38 in Table 7 of the requirements specified in the embodiments of the present invention are manufactured. It can be seen that the ratio of the maximum value to the minimum value of the number density of the precipitates in the surface layer of the carburized steel part is appropriately controlled as a result of suppression of the Cr segregation rate of these steel materials. Therefore, the Weber coefficient m is At 0.6 or more, it has excellent stability of rolling fatigue life after carburizing and nitriding treatment and stability thereof.

Among these, Test Nos. 22, 26, 27, 31, 33 to 35, 37, and the carburizing and nitriding treatment shown in Table 7 were carried out by using the preferred carbonitriding treatment conditions of the embodiment of the present invention. It can be seen that its L ₁₀ life is 1.0×10 ⁷ times or more, and it has excellent rolling fatigue life.

On the other hand, the following test No. are any of the requirements that do not conform to the requirements specified in the embodiment of the present invention.

In Test No. 23 in Table 8, although the composition is in accordance with the requirements of the embodiment of the present invention, the average cooling rate at the time of casting is too slow, and therefore, the degree of inconsistency in the number density of fine precipitates becomes large, and rolling fatigue The stability of life is reduced. In addition, the life of L ₁₀ is also low.

In Test No. 24 in Table 8, although the composition is in accordance with the requirements of the embodiment of the present invention, the soaking temperature is too low, so that the degree of inconsistency in the number density of fine precipitates is large, and the rolling fatigue life is stabilized. Sexuality is low. In addition, the life of L ₁₀ is also low.

In Test No. 25 in Table 8, although the composition is in accordance with the requirements of the embodiment of the present invention, the average cooling rate at the time of casting is too slow, the soaking temperature is too low, and the soaking time is too short, therefore, fine precipitates The degree of inconsistency in the number density becomes large, and the stability of the rolling fatigue life becomes low.

In Test No. 28 in Table 8, although the composition is in accordance with the requirements of the embodiment of the present invention, the soaking time is too short, and therefore, the degree of inconsistency in the number density of fine precipitates becomes large, and the rolling fatigue life is stabilized. Sexuality is low. In addition, the life of L ₁₀ is also low.

Further, although not shown in the table, after measuring the Cr segregation ratio for the round bars used in these Examples 3 and 4, it was confirmed that the compositional parts according to the embodiment of the present invention and the cooling conditions at the time of casting were used. And examples of the subsequent soaking conditions, the Cr segregation ratio is 2.0 or less.

The present invention encompasses the following aspects.

 Aspect 1  

A steel material excellent in rolling fatigue life stability, characterized by containing C: 0.15 to 0.25%, Si: 0.35 to 0.75%, Mn: 0.2 to 1%, Cr: 1.2 to 1.7%, and Mo by mass% : 0.3~0.6%, P: higher than 0% and 0.05% or less, S: higher than 0% and 0.05% or less, Al: 0.005 to 0.2%, N: higher than 0% and 0.05% or less, O: higher than 0% and 0.005% or less, and Ti: more than 0% and 0.014% or less, the balance being iron and unavoidable impurities, and the Cr segregation ratio determined by the following conditions is 2.0 or less.

 (i) Measuring position  

In an arbitrary cross-section perpendicular to the rolling direction of the steel material, four straight places are separated from each other by a distance of 90° on a straight line from the outer peripheral portion of the steel material; and the steel material is rolled. The direction is parallel in any cross-section, and is on the straight line at the 1/4 position of the diameter of the steel material, taking the center of the steel material as a starting point, each of which is separated by 90° and takes four places, and the length is 5 mm;

 (ii) Method of measurement  

For each of the above measurement positions, the line analysis of the Cr concentration is performed using EPMA to obtain the lowest value [Cr] _min and the maximum value [Cr] _{max of} the Cr concentration, and [Cr] _max / [Cr] _{min is} calculated, and the total is 8 The average value of each place is taken as the Cr segregation rate.

 Aspect 2:  

A steel material according to aspect 1, wherein, in mass%, further contains from Cu: more than 0% and less than 1%, Ni: more than 0% and less than 1%, and B: more than 0%. More than one of the groups selected below 0.005%.

 Aspect 3:  

A steel material according to aspect 1 or 2, wherein, in mass%, further contains from V: more than 0% and less than 1%, W: more than 0% and less than 0.5%, and Nb: higher than 0. One or more selected from the group of % and less than 0.1%.

 Aspect 4:  

A method for producing a steel material excellent in rolling fatigue life stability, which is a method for producing a steel material according to any one of the aspects 1 to 3, characterized in that: from the solidification start temperature of the molten steel The temperature range until the solidification end temperature is cooled at an average cooling rate of 150 ° C /hr or more, and then heated to 1,100 to 1300 ° C, and then subjected to a soaking treatment for 1.0 to 40 hours.

 Aspect 5:  

A carburized steel part excellent in rolling fatigue life and stability, characterized by: C: 0.15 to 0.25%, Si: 0.35 to 0.75%, Mn: 0.2 to 1%, Cr: 1.2% by mass% 1.7%, Mo: 0.3 to 0.6%, P: higher than 0% and 0.05% or less, S: higher than 0% and 0.05% or less, Al: 0.005 to 0.2%, N: higher than 0% and 0.05% or less, O: more than 0% and less than 0.005%, and Ti: more than 0% and less than 0.014%, the balance being iron and unavoidable impurities, and the equivalent circle diameter in the surface layer at a depth of 50 μm from the surface The ratio of the maximum value to the minimum value of the number density of carbides, nitrides, and carbonitrides of 0.1 to 1.0 μm is 2.0 or less.

 Aspect 6:  

A carburized steel part according to aspect 5, wherein the average number density is 0.5 to 3.0 pieces/μm ² .

 Aspect 7:  

A carburized steel part according to aspect 5 or 6, wherein, in mass%, further contains from Cu: more than 0% and less than 1%, Ni: more than 0% and less than 1%, and B: More than one selected from the group of more than 0% and less than 0.005%.

 Aspect 8:  

A carburized steel part according to any one of the aspects 5 to 7, wherein, in mass%, it further contains from V: more than 0% and less than 1%, and W: more than 0% and 0.5%. Hereinafter, and Nb: one or more selected from the group of more than 0% and 0.1% or less.

 Aspect 9:  

A method for producing a carburized steel part excellent in rolling fatigue life and stability thereof, which is a method for producing a carburized steel part according to any one of aspects 5 to 8, characterized in that The temperature range from the solidification start temperature of the molten steel to the solidification end temperature is cooled at an average cooling rate of 150 ° C / h or more, and after heating to 1,100 to 1300 ° C, a soaking treatment is performed for 1.0 to 40 hours to produce a steel. Thereafter, carburization treatment or carburizing treatment is performed.

This application is based on the Japanese Patent Application No. 2016-033663, which is filed on February 24, 2016, and the Japanese Patent Application (Patent Application) on February 24, 2016. Priority No. 2016-245766, which is the basis of the Japanese Patent Application No. 2016-245766, which is filed on December 19, 2016, is the basic application. Therefore, the contents of the Japanese Patent Application No. Hei. No. Hei. No. Hei.

Claims (7)

A steel material excellent in rolling fatigue life stability, characterized by containing C: 0.15 to 0.25%, Si: 0.35 to 0.75%, Mn: 0.2 to 1%, Cr: 1.2 to 1.7%, and Mo by mass% : 0.3~0.6%, P: higher than 0% and 0.05% or less, S: higher than 0% and 0.05% or less, Al: 0.005 to 0.2%, N: higher than 0% and 0.05% or less, O: higher than 0% and 0.005% or less, and Ti: more than 0% and 0.014% or less, the balance being iron and unavoidable impurities, and the Cr segregation ratio determined by the following conditions is 2.0 or less, (i) measurement The position is in an arbitrary cross section perpendicular to the rolling direction of the steel material, and is a line on the outer circumference of the steel material from the center to the center, and each of the phases is divided into four places by 90°; The rolling direction is parallel in any cross-section, and is on the straight line at the 1/4 position of the diameter of the steel material, taking the center of the steel material as a starting point, each of the four points separated by 90°, and the length is 5 mm; (ii) method for measuring the position of each measurement, for the use of EPMA line analysis of the Cr concentration of the Cr concentration determined lowest value of [Cr] _min, the maximum [Cr] _max, calculated _{[Cr] max / [Cr]} min, and the average total of eight locations as Cr segregation ratio.  The steel material according to claim 1, wherein, in mass%, further contains from Cu: more than 0% and less than 1%, Ni: more than 0% and less than 1%, and B: more than 0% and 0.005%. Hereinafter, V: more than 0% and 1% or less, W: more than 0% and 0.5% or less, and Nb: one or more selected from the group of more than 0% and 0.1% or less.    A method for producing a steel material excellent in rolling fatigue life stability, which is a method for producing a steel material according to claim 1 or 2, which is characterized in that: from the solidification start temperature of the molten steel to the solidification end temperature The temperature range is cooled at an average cooling rate of 150 ° C / hour or more, and then heated to 1100 to 1300 ° C, and then subjected to a soaking treatment for 1.0 to 40 hours.    A carburized steel part excellent in rolling fatigue life and stability, characterized by: C: 0.15 to 0.25%, Si: 0.35 to 0.75%, Mn: 0.2 to 1%, Cr: 1.2% by mass% 1.7%, Mo: 0.3 to 0.6%, P: higher than 0% and 0.05% or less, S: higher than 0% and 0.05% or less, Al: 0.005 to 0.2%, N: higher than 0% and 0.05% or less, O: more than 0% and less than 0.005%, and Ti: more than 0% and less than 0.014%, the balance being iron and unavoidable impurities, and the equivalent circle diameter in the surface layer at a depth of 50 μm from the surface The ratio of the maximum value to the minimum value of the number density of carbides, nitrides, and carbonitrides of 0.1 to 1.0 μm is 2.0 or less.    The carburized steel part according to claim 4, wherein, in mass%, further contains from Cu: more than 0% and less than 1%, Ni: more than 0% and less than 1%, and B: more than 0% And more than 0.005%, V: more than 0% and 1% or less, W: more than 0% and 0.5% or less, and Nb: one or more selected from the group of more than 0% and 0.1% or less.   The carburized steel part according to claim 4, wherein the average number density is 0.5 to 3.0 pieces/μm ² .  A method for producing a carburized steel part excellent in rolling fatigue life and stability thereof, which is a method for producing a carburized steel part according to claim 4 or 5, characterized in that the solidification starting temperature from the molten steel The temperature range from the start of the solidification end temperature is cooled at an average cooling rate of 150 ° C / h or more, and after heating to 1,100 to 1300 ° C, a soaking treatment is performed for 1.0 to 40 hours to produce a steel material, and then carburizing treatment is performed. Or carburizing and nitriding treatment.