KR20160133549A - Steel material for vacuum carburizing and method for producing same - Google Patents

Abstract

It is an object of the present invention to obtain a carburizing component having a sufficient surface fatigue strength, that is, a carburizing component having a surface fatigue strength and a bending fatigue strength, and obtaining a steel material for obtaining the carburizing component. The present invention is a steel for vacuum carburization containing C, Si, Mn, S, P, Cr, Mo, V, Al and N and having an average circle equivalent diameter of vanadium carbide of 25 nm or less. The present invention also encompasses a method of manufacturing a steel material, and more specifically, a steel having a predetermined chemical composition is maintained at a temperature of 1,200 DEG C or more for 30 to 300 minutes to perform crushing, and a heating temperature before hot rolling is 950 DEG C or higher, And the hot-holding time is changed from 30 minutes to 5 hours.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel material for vacuum carburization,

More particularly, the present invention relates to a steel material for obtaining a carburizing part having excellent surface fatigue characteristics and bending fatigue characteristics after vacuum carburizing, and also relates to a manufacturing method thereof, a carburizing part using the steel material, And a manufacturing method of the component. The steel material of the present invention is useful as a material for gears, shafts and the like used in automobiles, construction machines, and various other industrial machines. Hereinafter, the present invention is applied to automotive gears. It is not the purpose.

In the environment surrounding automobiles, building machines, and various other industrial machines, there is a societal demand for energy saving and further improvement of performance. In recent years, efforts are being made to reduce the weight of automobile bodies and increase the engine output. Therefore, the use environment of the toothed wheels used in automobiles and construction machines, particularly, the toothed wheels used in the drive system transmitting portion becomes more severe, and a toothed wheel having excellent fatigue strength is required.

Conventional toothed wheels are made of JIS-SCr420 steel such as chromium steel, or JIS-SCM420 steel such as chromium molybdenum steel as gear steel for manufacturing the gears. The JIS-SCr420 steel contains SCr420H strength and the JIS-SCM420 steel contains SCM420H strength. These props steels are formed into a saw-toothed shape and then carburized, quenched and tempered (hereinafter sometimes referred to as " carburizing ") is used as a so-called carburized toothed wheel .

However, in the above-described conventional gear, the following problems are pointed out. That is, in recent years, since the weight of automobile bodies required for automobiles, construction machines, and the like are becoming stronger and the demand for high output of the engine becomes stronger, the carburizing gears, The fatigue strength can not be satisfied.

For example, Patent Document 1 discloses a carburizing component or a carbo-nitriding component that satisfies a predetermined chemical composition, performs shot peening after carburizing or carbo-nitriding, and has a hardness and a hardened layer depth at a predetermined surface layer portion. However, the improvement of the softening characteristics of the surface layer is not sufficient, and the surface hardening technique near the surface can not obtain the surface fatigue strength and the bending fatigue strength that can sufficiently cope with the miniaturization and high stress load required in recent years.

Further, Patent Document 2 discloses an indium steel for high-strength toothed wheels satisfying a predetermined chemical composition. There is described that surface hardening treatment such as gas carburization, vacuum carburization, carbo-nitriding, high-concentration carburization (superalloy carburization), or shot peening may be performed on the indigestion steel, but in the technology of Patent Document 2, The improvement of the characteristics is not considered to be sufficient. Therefore, according to the technique of Patent Document 2, the surface fatigue strength and the bending fatigue strength which can sufficiently cope with the miniaturization and high stress load of parts required in recent years can not be obtained.

Japanese Patent Application Laid-Open No. 2008-261037 Japanese Patent Application Laid-Open No. 2005-163148

The object of the present invention is to provide a carburizing component having a sufficient surface fatigue strength and a carburizing component having a surface fatigue strength and a bending fatigue strength in addition to a steel member for obtaining the carburizing component It is in obtaining.

According to the present invention,

In terms of% by mass,

C: 0.15 to 0.35%

Si: 0.6 to 2.0%

Mn: 0.3 to 1.3%

S: more than 0% and not more than 0.020%

P: more than 0% and not more than 0.015%

0.7-1.7% Cr,

Mo: 0.3 to 0.8%

V: 0.10 to 0.4%

Al: 0.005 to 0.05%

N: 0.004 to 0.025%

And the balance of iron and unavoidable impurities,

Characterized in that the mean circle equivalent diameter of the vanadium carbide is 25 nm or less.

It is also preferable that the present invention further contains at least one of the following (a) and (b).

(a) at least one of Nb: more than 0 mass% and not more than 0.06 mass% and Ti: not less than 0 mass% and not more than 0.2 mass%

(b) B: more than 0 mass% and less than 0.005 mass%

The present invention also includes a method of manufacturing the steel material described above,

A steel having the chemical composition described in any one of the above,

Milled at 1200 DEG C or higher for 30 to 300 minutes,

And the hot-rolled steel sheet is subjected to hot rolling at a heating temperature of 950 占 폚 or higher before the hot rolling and a heating holding time of 30 minutes to 5 hours.

The present invention also includes a carburizing component obtained from the aforementioned steel for vacuum carburizing,

Which has the chemical composition described in any one of the above,

The depth of the surface boundary oxide layer is 3 占 퐉 or less,

And the surface hardness when tempering at 400 占 폚 is 600 or more in Vickers hardness. The carburizing parts have excellent surface fatigue strength.

In addition, a part subjected to shot peening in addition to the carburizing part is also included in the present invention,

Which has the chemical composition described in any one of the above,

The depth of the surface boundary oxide layer is 3 占 퐉 or less,

The residual stress integral value from the surface to a depth position of 30 mu m is 40 MPa · mm or more,

And the surface hardness when tempering at 400 占 폚 is 600 or more in Vickers hardness. The parts are excellent in surface fatigue strength and bending fatigue strength.

The present invention also includes a method of manufacturing a part subjected to the shot peening described above,

A method for manufacturing a carburizing part for vacuum carburizing, quenching tempering and shot peening a steel material for vacuum carburization according to any one of the above,

The particle diameter of the shot material of shot peening is 0.10 to 0.5 mm,

And the hardness of the projected member is in the range of 800 to 1000 in terms of Vickers hardness, and the bending fatigue strength is excellent.

According to the steel material for vacuum carburization of the present invention, the chemical composition is appropriately adjusted and the average circle equivalent diameter of the vanadium carbide is set to a predetermined value or less. Therefore, the surface fatigue strength after the vacuum carburizing treatment is excellent, A carburizing part having excellent bending fatigue strength after peening can be obtained.

Fig. 1 is a view showing the shape of a test piece for bending fatigue test according to the embodiment described later.
Fig. 2 is a schematic view showing the procedure of the bending fatigue test in the embodiment described later.
Fig. 3 is a view for explaining the meaning of the strength of 100,000 times in the bending fatigue test in the embodiment described later.

The present inventors have studied from various angles in order to secure the surface fatigue strength and further the bending fatigue strength of the carburizing parts. As a result, the following findings (i) to (v) were obtained.

(i) Nowadays, in order to improve the surface fatigue strength in an environment in which a high-pressure load is applied to a low-viscosity point of oil or a component due to the fuel consumption of automobiles, the softening of the contact surface It is important to increase the resistance, and in particular, it is effective to improve the tempering hardness at 400 ° C. In particular, the surface fatigue strength can be greatly improved by setting the tempering hardness at 400 占 폚 of the component surface to HV600 or more at Vickers hardness.

Further, the carburizing part of the present invention is also characterized in that it is obtained by vacuum carburizing. When gas carburizing, gas carburizing, nitriding, or the like is performed without performing vacuum carburizing, a grain boundary oxide layer is formed on the surface, and the surface fatigue strength and the bending fatigue strength described below are lowered. In the carburizing part of the present invention obtained by vacuum carburizing, the depth of the surface boundary oxide layer can be made 3 μm or less.

(ii) In order to set the tempering hardness at 400 占 폚 of the component surface to HV 600 or higher, it is necessary to adjust Si, Mo and V in the predetermined range and adjust the size of the vanadium carbide in the steel before carburization. Si suppresses the formation of carbides such as ε carbide, χ carbide and η carbide at the time of tempering, and Mo and V contribute to secondary curing by precipitating Mo ₂ C and VC at the time of tempering. The amounts of Si, Mo and V in the steel before carburization are 0.6 to 2.0% of Si, 0.3 to 0.8% of Mo and 0.10 to 0.4% of V, respectively.

In the steel before carburizing, the average circle equivalent diameter of the vanadium carbide needs to be 25 nm or less. By setting the average circle equivalent diameter of the vanadium carbide to 25 nm or less, the vanadium carbide can be sufficiently solidified during the vacuum carburizing process, and the vanadium carbide is precipitated during the tempering or the component use to harden the component secondarily, The strength can be increased.

(iii) In order to make the average circle equivalent diameter of the vanadium carbide in the steel before carburization to be 25 nm or less, it is necessary to appropriately adjust the heating conditions before rolling. That is, by setting the heating temperature and the holding time before rolling to a predetermined value or more, vanadium carbide precipitated before rolling can be sufficiently solidified, and fine vanadium carbide can be secured by cooling after rolling. That is, the average circle equivalent diameter of the vanadium carbide can be 25 nm or less.

(iv) In order to further improve the bending fatigue strength in addition to the surface fatigue strength of the carburizing component, it is effective to subject the component obtained by vacuum carburizing the steel to shot peening to give a predetermined residual stress. Concretely, the residual stress integration value from the surface of the carburizing component to the depth position of 30 mu m is set to 40 MPa · mm or more, whereby generation of initial cracks and propagation of cracks can be suppressed and the bending fatigue strength can be greatly improved.

(v) In order to make the residual stress integral value from the surface of the carburizing component to the depth position of 30 탆 deep to be 40 MPa mm mm or more, it is necessary to appropriately adjust the size and hardness of the projection material by shot peening performed after vacuum carburization. The particle diameter of the projection material is 0.10 to 0.5 mm, and the hardness is Vickers hardness HV 800 to 1000.

The steel material of the present invention, that is, the steel material after hot rolling and before the vacuum carburization is characterized by specifying the size of the vanadium carbide in the steel as described above. However, in order to exhibit the basic characteristics as a carburizing part, It is necessary to adjust it appropriately. Hereinafter, the chemical composition of the steel material of the present invention will be described. In the present specification, the chemical composition means all% by mass.

C: 0.15 to 0.35%

C is an element capable of imparting strength to the steel. In order to obtain the required strength, the amount of C was set to 0.15% or more. The amount of C is preferably 0.17% or more, and more preferably 0.19% or more. On the other hand, when the amount of C is excessive, machinability and toughness are lowered. Therefore, the C content was set at 0.35% or less. The amount of C is preferably 0.33% or less, more preferably 0.31% or less.

Si: 0.6 to 2.0%

Si functions as a tempering softening resistance improving element and contributes to improvement in fatigue strength such as pitching strength and improvement in wear resistance by suppressing softening when the temperature of the contact portion rises during driving in a gear wheel or the like. In order to effectively exhibit such effects, the amount of Si is set to 0.6% or more. The amount of Si is preferably 0.8% or more, and more preferably 1.0% or more. However, if the amount of Si is excessive, the increase in strength becomes remarkable, and the cold workability and machinability are deteriorated. Therefore, the amount of Si was set to 2.0% or less. The amount of Si is preferably 1.8% or less, and more preferably 1.6% or less.

Mn: 0.3 to 1.3%

Mn is added as a deoxidizing agent, a desulfurizing agent and a hardenability improving element. In order to effectively exhibit such an effect, the Mn content is set to 0.3% or more. The amount of Mn is preferably 0.4% or more, and more preferably 0.5% or more. However, when the amount of Mn is excessive, the cold workability and toughness are lowered, and the machinability is also deteriorated. Therefore, the amount of Mn was set to 1.3% or less. The amount of Mn is preferably 1.2% or less, and more preferably 1.1% or less.

S: more than 0% and less than 0.020%

S is an inevitable impurity contained in the steel and precipitates as MnS to lower the fatigue characteristics and the impact characteristics, so that it is preferable to reduce it as much as possible. However, extreme reduction results in an increase in steelmaking costs. From this point of view, the amount of S is set to 0.020% or less. The amount of S is preferably 0.015% or less, and more preferably 0.010% or less. As described above, S is an impurity inevitably contained, so it is difficult to industrialize the amount of S to be 0%, and the lower limit of S amount is about 0.0005%.

P: more than 0% and not more than 0.015%

P is an inevitable impurity contained in the steel and is segregated at the grain boundaries to lower the workability and fatigue characteristics, so that it is desirable to reduce the maximum. However, extreme reduction results in an increase in steelmaking costs. From this point of view, the P content was set at 0.015% or less. The P content is preferably 0.010% or less, and more preferably 0.008% or less. As described above, P is an impurity inevitably contained, so that it is difficult for industrial production to make its amount to 0%, and the lower limit of P amount is about 0.0005%.

Cr: 0.7 to 1.7%

Cr, like Mn, is added as an element for improving hardenability and also serves as a tempering softening resistance element. In order to effectively exhibit such effects, the amount of Cr is set to 0.7% or more. The amount of Cr is preferably 0.8% or more, and more preferably 0.9% or more. However, if the amount of Cr is excessive, it causes deterioration of cold forging and toughness as well as machinability. From this viewpoint, the amount of Cr was set to 1.7% or less. The amount of Cr is preferably 1.6% or less, and more preferably 1.5% or less.

Mo: 0.3 to 0.8%

Mo has an effect of improving the softening resistance by precipitating Mo ₂ C at the time of tempering. When the temperature of the contact portion rises during driving in a gear or the like, hardness is maintained by softening suppression and fatigue It contributes to strength improvement. Mo also has an effect of improving toughness. In order to effectively exhibit such effects, the amount of Mo is set to 0.3% or more. The amount of Mo is preferably 0.35% or more, and more preferably 0.4% or more. On the other hand, when the amount of Mo becomes excessive, the increase in strength becomes remarkable, and the cold workability and machinability are deteriorated. Therefore, the amount of Mo was set to 0.8% or less. The amount of Mo is preferably 0.75% or less, and more preferably 0.7% or less.

V: 0.10 to 0.4%

V has the effect of improving the softening resistance by depositing vanadium carbide at the time of tempering. When the temperature of the contact portion rises during driving in the gears and the like, hardness is maintained by suppressing softening, and fatigue strength . In order to effectively exhibit such effects, the V content is set to 0.10% or more. The amount of V is preferably 0.15% or more, and more preferably 0.2% or more. However, when the amount of V is excessively large, the strength increase is remarkable, and cold workability and machinability are lowered. Further, coarse vanadium carbide precipitates after rolling, and does not contribute to improvement in softening resistance after vacuum carburizing treatment. Therefore, the V amount was set to 0.4% or less. The amount of V is preferably 0.35% or less, and more preferably 0.3% or less.

Al: 0.005 to 0.05%

Al is a deoxidizing agent and also has an effect of improving the toughness by forming a fine Al-based nitride by making the crystal grains finer. In order to effectively exhibit this effect, the Al content is set to 0.005% or more. The amount of Al is preferably 0.01% or more, and more preferably 0.012% or more. However, if the amount of Al is excessive, the workability is adversely affected and the workability is deteriorated. In addition, since coarse nitride is produced, it does not contribute as finning particles and causes crystal grain coarsening. From this point of view, the amount of Al is set to 0.05% or less. The amount of Al is preferably 0.045% or less, and more preferably 0.043% or less.

N: 0.004 to 0.025%

N exhibits an effect of improving the toughness by forming nitride with Al or the like and making crystal grains finer. In order to effectively exhibit such effects, the N content is set to 0.004% or more. The N content is preferably 0.0060% or more, and more preferably 0.010% or more. However, when the amount of N is excessive, a coarse nitride such as an Al-based nitride is generated and does not contribute as finning particles, causing crystal grain coarsening. From this point of view, the N content was set to 0.025% or less. The N content is preferably 0.020% or less, and more preferably 0.017% or less.

The basic component of the steel for vacuum carburization of the present invention is as described above, and the remainder is substantially iron. However, it is a matter of course that inevitable impurities other than P and S are included in the steel, which are mixed by the conditions of the raw materials, materials, and manufacturing facilities. In the present invention, as occasion demands, the following optional elements may be contained within a range not inhibiting the action of the present invention. The characteristics of the steel are further improved depending on the kinds of the following elements.

Nb: more than 0% and not more than 0.06%, and Ti: more than 0% and not more than 0.2%

Nb and Ti are useful for improving the toughness of the steel material and improving the bending fatigue strength by refining the crystal grains after carburizing. These effects can be exhibited by containing one or two kinds of elements as necessary. To effectively exhibit such effects, the Nb content is preferably 0.01% or more, and the Ti content is preferably 0.005% or more. The preferable amounts of Nb and Ti are all 0.015% or more. However, when these elements are excessive, not only the effect is saturated but also coarse precipitates are formed and the strength is lowered. Therefore, the amount of Nb is preferably 0.06% or less, and the amount of Ti is preferably 0.2% or less. The amount of Nb is more preferably 0.05% or less, more preferably 0.1% or less, and still more preferably 0.08% or less.

B: more than 0% and not more than 0.005%

B is an element that enhances the hardenability in the carburizing treatment and also strengthens the grain boundaries to improve the bending fatigue strength. B is capable of improving the hardenability by adding a trace amount, so that the effect on workability and the like is low. In order to effectively exhibit such action, the amount of B is preferably 0.0005% or more, and more preferably 0.0008% or more. However, when the amount of B becomes excessive, the bond with N generates BN, and the strength of the carburizing part is lowered. Therefore, the amount of B is preferably 0.005% or less, more preferably 0.0045% or less, and still more preferably 0.0040% or less.

In the steel for vacuum carburization of the present invention, the average circle equivalent diameter of vanadium carbide is 25 nm or less. The vanadium carbide is precipitated by heating at the time of tempering and by sliding heat generated at the time of use of the component, thereby improving the softening resistance. That is, when the temperature of the contact portion rises during driving in a gear or the like, hardness is maintained by inhibiting softening by vanadium carbide, contributing to improvement of fatigue strength such as pitching strength. In order to exhibit such an effect, it is necessary to finely disperse vanadium carbide in a steel material after hot rolling and before vacuum carburizing and to solidify it in vacuum carburizing. If the vanadium carbide of the steel for vacuum carburization is coarse, it does not contribute to the improvement of the softening resistance after the vacuum carburizing treatment. Therefore, the average circle equivalent diameter of the vanadium carbide is set to 25 nm or less. The average circle equivalent diameter of the vanadium carbide is preferably 20 nm or less, more preferably 15 nm or less. The lower limit of the mean circle equivalent diameter of the vanadium carbide is not particularly limited, but is usually about 1 nm. On the other hand, the vanadium carbide in the present invention means a precipitate in which V (vanadium) and C (carbon) are detected, and includes a case where elements other than V and C are included.

In order to adjust the above-described vanadium carbide, it is important to adjust the heating conditions before hot rolling in a series of production steps in which hot rolling is performed after the solvent is subjected to a conventional casting method and after crushing and rolling. The heating temperature before hot rolling is preferably 950 DEG C or higher. When the heating temperature before the hot rolling is less than 950 DEG C, the vanadium carbide existing before rolling can not be sufficiently solidified, and the unused vanadium carbide is coarsened, so that the average circle equivalent diameter of the vanadium carbide after rolling is made 25 nm or less I can not. The heating temperature is more preferably 1000 DEG C or more, and still more preferably 1050 DEG C or more. The upper limit of the heating temperature is preferably 1250 占 폚 or lower, more preferably 1200 占 폚 or lower, from the viewpoint of decarburization.

The heating and holding time before hot rolling is preferably 30 minutes to 5 hours. If the heating and holding time is less than 30 minutes, the vanadium carbide existing before rolling can not be sufficiently solidified, and the un-solidified vanadium carbide is coarsened, so that the average circle equivalent diameter of the vanadium carbide after rolling can not be 25 nm or less. The heating holding time is more preferably 1 hour or more, and still more preferably 1.5 hours or more. On the other hand, if the heating holding time exceeds 5 hours, the vanadium carbide is coarsened by oustow growth, and the average circle equivalent diameter of the vanadium carbide after rolling can not be 25 nm or less. The heating holding time is more preferably 4.5 hours or less, and still more preferably 4 hours or less.

On the other hand, the conditions of the above-mentioned crushing rolling are not particularly limited, and the crushing rolling may be carried out by holding it at 1200 DEG C or higher, preferably 1250 DEG C or higher, for 30 to 300 minutes. The upper limit of the heating temperature for crushing rolling is not particularly limited, but is, for example, 1300 占 폚 or less.

It is possible to obtain a component having excellent surface fatigue strength by subjecting the steel material of the present invention in which the aforementioned chemical composition and vanadium carbide size are adjusted to vacuum carburization and to perform shot peening with predetermined conditions after vacuum carburization It is possible to obtain a carburizing part having excellent bending fatigue strength.

In the present invention, vacuum carburization is employed as the carburization treatment. In the carburization steel of the present invention, the amount of Si is increased to 0.6% or more as described above. When such a steel material is carburized by gas carburization, gas carburization nitridation or the like other than vacuum carburization, a grain boundary oxide layer is formed on the surface, the surface fatigue strength of the component is lowered, and the bending fatigue strength of the component is also lowered. The component of the present invention obtained by vacuum carburizing has a depth of the surface boundary oxide layer of 3 탆 or less. The surface grain boundary oxide layer depth is preferably 2 탆 or less, more preferably 1 탆 or less, and most preferably 0 탆. The conditions of the vacuum carburizing treatment are not particularly limited, and for example, the carburizing temperature may be 900 to 1000 占 폚, preferably 930 to 980 占 폚. After carburization, (a) may be quenched directly, (b) carburized, and then reheated and quenched. In any of the above cases (a) and (b), after quenching is performed by introducing into an oil bath or the like at about 50 to 150 占 폚, preferably 60 to 130 占 폚, Tempering may be performed at 180 占 폚. In the case of (a), after vacuum carburizing, the steel sheet may be cooled to 750 to 900 占 폚, preferably 780 to 880 占 폚, and quenching tempering may be performed.

In order to increase the fatigue strength of the carburizing parts such as the pitching strength, it is effective to increase the hardness. However, since the hardness is lowered when the temperature of the contact portion rises during driving in the cogwheel or the like, it is effective to improve the fatigue strength by raising the hardness at about 400 deg. C around the exothermic temperature instead of the initial hardness. The component of the present invention obtained by vacuum carburizing the steel for vacuum carburization of the present invention can have a surface hardness of not less than HV 600 at Vickers hardness when tempered at 400 ° C. The surface hardness is preferably HV 620 or more, more preferably HV 650 or more. The upper limit of the surface hardness is not particularly limited, but is usually about HV 900.

The component obtained by the vacuum carburization can further be subjected to shot peening to impart compression residual stress. The compressive residual stress can suppress generation of initial cracks and propagation of cracks when the repeated stress is applied, and can significantly improve the bending fatigue strength. In order to exhibit such an effect, it is necessary to set the residual stress integral value from the surface to a depth position of 30 mu m to be 40 MPa · mm or more. The residual stress integral value is preferably 42 MPa · mm or more, and more preferably 45 MPa · mm or more. The upper limit of the residual stress integration value is not particularly limited, but is usually about 100 MPa · mm.

In order to impart the compression residual stress to the carburizing part as described above, it is necessary to appropriately control the particle diameter and the hardness of the projection material used in the shot peening. The particle size of the projection material is 0.10 to 0.5 mm. If the particle diameter is less than 0.10 mm, a compressive residual stress is applied only to the surface layer, so that the compressive residual stress from the surface to the depth position of 30 mu m can not be increased. Further, when the particle diameter exceeds 0.5 mm, a compressive residual stress is applied to the inner side, and the compressive residual stress from the surface to the depth position by 30 mu m can not be set within the above range.

The hardness of the projection material is Vickers hardness HV 800 to 1000. If the hardness is less than HV 800, the compressive residual stress is not sufficiently imparted, and the compressive residual stress from the surface to the depth position by 30 mu m can not be set within the above range. The hardness of the projection material is preferably HV 820 or more, more preferably HV 850 or more. Further, when the hardness exceeds HV 1000, the amount of cutting of steel is increased, so that a predetermined component shape can not be obtained. The hardness of the projection material is preferably HV 980 or less, more preferably HV 950 or less.

The steel for vacuum carburizing according to the present invention can obtain a component having excellent surface fatigue strength by carrying out vacuum carburizing. Further, by performing shot peening after vacuum carburizing, a component having excellent bending fatigue strength can be obtained. The surface fatigue strength can be set to, for example, 3.3 GPa or more, preferably 3.4 GPa or more at a maximum stress not to be broken when tested for one million cycles in the roller pitching test, Point strength in a four-point bending fatigue test, that is, a maximum stress that can not be broken when the test is conducted for 100,000 times, can be set to 1260 MPa or more, preferably 1,300 MPa or more.

This application claims the benefit of priority based on Japanese Patent Application No. 2014-060210 filed on March 24, 2014. The entire contents of the specification of Japanese Patent Application No. 2014-060210 filed on March 24, 2014 are incorporated herein by reference.

Example

Hereinafter, the present invention will be described more specifically by way of examples. The present invention is not limited to the following embodiments, and it is of course possible to carry out the present invention by appropriately modifying it within a range that is suitable for the purpose of the latter, and they are all included in the technical scope of the present invention.

The steel having the chemical composition shown in the following Table 1 was melted and maintained at 1250 占 폚 for 30 minutes to 300 minutes, followed by crushing and rolling. The remainder of the chemical composition shown in Table 1 is iron and unavoidable impurities. Thereafter, as shown in Tables 2 and 3, hot rolling was performed at a preheating temperature of 920 to 1100 占 폚 and a heating and holding time of 0.3 to 6 hours to obtain a hot rolled material of? 32 mm, that is, a steel strip. On the other hand, 1 is SCr420H equivalent steel of conventional steel.

For each obtained hot rolled material, the size of the vanadium carbide was measured by the following method (1). The hot rolled material was subjected to vacuum carburizing or gas carburizing under the carburizing conditions shown in Tables 2 and 3, Shot peening was carried out using the projectile of the particle diameter and hardness shown in Table 3 for 26 to 34 to prepare a test piece. In the vacuum carburizing treatment, the vacuum carburizing treatment is carried out in the temperature range of 930 to 980 캜 shown in Tables 2 and 3. Thereafter, the furnace is cooled to 780 to 880 캜 and then quenched into oil of 60 to 130 캜, Deg.] C to perform tempering. The shot peening was performed using the projection material described in Table 3, with a projection pressure of 0.4 MPa and a coverage of 400% or more. The projecting material used was 0.05 to 0.06 mm, 0.11 to 0.13 mm, 0.18 to 0.21 mm, 0.36 to 0.43 mm and 0.60 to 0.71 mm in particle size classified by a sieve.

On the other hand, gas carburization to be compared was carburized at 930 캜 in a carburizing gas atmosphere of carbon potential Cp: 0.8%, followed by oil cooling and further tempering at 170 캜 for 2 hours.

(2) 400 deg. C tempering hardness, (3) residual stress integration value at a depth of 30 mu m from the surface, and (4) depth of the surface grain boundary oxide layer were measured. (5) Roller Pitching Fatigue characteristics and (6) bending fatigue characteristics were evaluated.

(1) Measurement of the size of vanadium carbide in hot rolled material

The D / 4 position of the hot rolled material was cut out in a cross section, polished, carbon deposited, and replica observed by FE-TEM (Field-Emission Transmission Electron Microscope). D is the diameter of the rolled material. At this time, a precipitate in which V and C were detected by EDX (Energy Dispersive X-ray Analysis) of TEM was specified and a field of view of 1.0 mu m x 1.2 mu m was observed at a magnification of 100,000 times. Observation was carried out for any three fields of view, and the arithmetic average value of the circle equivalent spheres of the observed vanadium carbides was taken as the average circle equivalent diameter of the vanadium carbide. On the other hand, from the measurement limit of FE-TEM, the lower limit of the size of the vanadium carbide to be measured is approximately 1 nm in terms of the circle equivalent.

(2) Measurement of 400 ° C tempering hardness

The surface of the hot rolled material was polished to make a diameter of 26.02 mm, carburized, and then polished again to give a diameter of 26 mm. Test No. For 26 to 34, further shot peening was performed to obtain a test piece for measurement of 400 占 폚 tempering hardness. Test No. The test specimens after carburizing, No. For 26 to 34, the specimens after carburization and shot peening were tempered at 400 DEG C for 3 hours, and the hardness was measured with a Vickers hardness meter at a position 50 mu m from the surface in the transverse section. The test load of the Vickers hardness tester was set to 300 gf, and the arithmetic mean value was obtained at five places, and the tempering hardness of each test piece was set at 400 캜.

(3) Measurement of residual stress integral value at a depth of 30 μm from the surface

The four-point bending test piece of FIG. 1 to be described later was carburized. 26 to 34 were further subjected to shot peening to obtain test specimens for measuring residual stress. Test No. The test specimens after carburizing, No. For 26 to 34, the residual stresses at 10 μm, 20 μm, and 30 μm positions from the notch bottom surface of the test specimen, respectively, were measured using a PSPC (Position-Sensitive Proportional Counter) And the residual stress integral value from the surface to the depth position of 30 mu m was calculated by the following equation. The measurement conditions of the PSPC micro portion X-ray stress measuring apparatus are the collimator diameter:? 1 mm, the measurement site: the center position in the axial direction, and the measurement direction: the circumferential direction.

The residual stress integral value σ

= {σ (0 mm) + σ (0.01 mm)} / 2 × 0.01 mm

+ {sigma (0.01 mm) + sigma (0.02 mm)} / 2 x 0.01 mm

+ {σ (0.02 mm) + σ (0.03 mm)} / 2 × 0.01 mm

However,? (Xmm) means the value of the residual stress at the Xmm position from the surface.

(4) Measurement of the depth of surface boundary layer

The surface of the hot rolled material was polished to make a diameter of 26.02 mm, carburized, and then polished again to give a diameter of 26 mm. Test No. For 26 to 34, further shot peening was performed to obtain test specimens. Test No. The test specimens after carburizing, No. For 26 to 34, the specimens after carburizing and shot peening were cut perpendicularly to the rolling direction, embedded in a resin and polished, and then the outermost surface of the test piece was observed at a magnification of 1000 times using an optical microscope, The depth of deep position was measured.

(5) Evaluation of roller pitching fatigue characteristics

The same test piece as in the above (4) was prepared and the obtained test piece was subjected to a roller pitching test under conditions of surface pressure: 2.7, 3.0, 3.3 GPa, revolution: 1500 rpm, slip ratio: An S-repeat number N-line (hereinafter referred to as an SN chart) was prepared, and the pitching strength was evaluated by a one million-fold strength. On the other hand, the oil temperature of the above-mentioned automatic oil is 80 DEG C, and the above-mentioned one million times of the strength means the maximum stress which is not broken when it is tested for one million times. The counter roller used here was a rough product made of SUJ2, having a surface hardness of HV 700 and a crowning R of 150 mm.

(6) Evaluation of bending fatigue characteristics

A test piece having the shape shown in Fig. 1 was taken out from the hot rolled material and carburized. Shot-pinning was further carried out on 26 to 34 to obtain a test piece for bending fatigue test. Using this test piece, an SN diagram was created under the conditions of a maximum stress of 1371, 1523, 1675 and 1828 MPa at a frequency of 20 Hz and a repeated load stress by a jig having four points of support as shown in Fig. Based on the diagram, the strength of 100,000 times was determined as shown in Fig. 3, and the value was taken as the bending fatigue strength.

The results of the above (1) to (6) are shown in Tables 2 and 3.

Test No. 3, 5, 14 to 18, 22 to 29, and 31 to 34 are steels obtained by satisfying the chemical composition specified in the present invention and under appropriate hot rolling conditions. Therefore, they have an average circle equivalent diameter of VC of 25 nm or less, a steel obtained by vacuum carburization, a steel obtained by vacuum carburization and shot peening, a surface hardness when tempered at 400 캜 is Vickers hardness of 600 or more, And the surface fatigue strength was 3.3 GPa or more. 1, the surface fatigue strength of 1.20 times or more was achieved. Among them, in particular, Test No. 1. 26, and 31 to 34 are examples in which shot peening was carried out under appropriate conditions after vacuum carburizing. The bending fatigue strength indicated by the strength of 100,000 times was 1260 MPa or more because the compressive residual stress was sufficient. 1, the bending fatigue strength of 1.20 times or more was achieved. On the other hand, in the case of No. 3, No. 5, No. 14 to No. 18, No. 22 to No. 25, and No. 5 of No. 5 in which shot peening was not performed, 27 to 29, the surface fatigue strength was good as described above, but the bending fatigue strength was no. 26, and 31 to 34, respectively.

Test No. 1 and 2 are examples in which Si, V, and Mo were small, vanadium carbide was not formed, and the tempering hardness at 400 ° C after carburization was low. In addition, Since the gas carburization was adopted as the carburization treatment, the intergranular oxidation layer was formed. 2, the fatigue strength was inferior. No. 4, since the gas carburization was adopted, a grain boundary oxide layer was formed, and the surface fatigue strength was inferior.

No. 6 is an example in which the amount of Si is small; 7 is an example in which the Cr amount is small, and No. 8 is an example in which the amount of Mn is small, and No. 9 shows the case where the amount of P was large, and Fig. 10 shows the case where the amount of S was large, and Fig. 11 is an example in which the amount of V was small, and No. 12 is an example in which the amount of V is large; 13 was an example in which the amount of Mo was small, and all the surface fatigue strength was inferior.

No. 19 is an example in which the heating temperature before the hot rolling is low, and Fig. 20 is the example in which the heating and holding time before hot rolling was short, 21 shows an example in which the heating and holding time before hot rolling was long, and the average circle equivalent diameter of vanadium carbide was large in all cases, resulting in poor surface fatigue strength. No. 30 was an example in which gas carburization was carried out, and a grain boundary oxide layer was formed, resulting in poor surface fatigue strength.

INDUSTRIAL APPLICABILITY The carburizing parts obtained by using the steel for vacuum carburizing according to the present invention are suitable for industrial use because they are suitable for gear wheels and shafts used in automobiles, construction machines, and various other industrial machines.

Claims (6)

In terms of% by mass,
C: 0.15 to 0.35%
Si: 0.6 to 2.0%
Mn: 0.3 to 1.3%
S: more than 0% and not more than 0.020%
P: more than 0% and not more than 0.015%
0.7-1.7% Cr,
Mo: 0.3 to 0.8%
V: 0.10 to 0.4%
Al: 0.005 to 0.05%
N: 0.004 to 0.025%
And the balance of iron and unavoidable impurities,
Wherein the mean circle equivalent diameter of the vanadium carbide is 25 nm or less. The method according to claim 1,
A steel material for vacuum carburization further containing at least one of the following (a) and (b).
(a) at least one of Nb: more than 0 mass% and not more than 0.06 mass% and Ti: not less than 0 mass% and not more than 0.2 mass%
(b) B: more than 0 mass% and less than 0.005 mass% A method for producing a steel having the chemical composition according to claim 1 or 2,
Milled at 1200 DEG C or higher for 30 to 300 minutes,
Wherein the hot-rolled steel sheet is subjected to hot rolling at a heating temperature of 950 DEG C or higher before the hot rolling and a heating holding time of 30 minutes to 5 hours. A chemical mechanical polishing composition having chemical composition according to claim 1 or 2,
The depth of the surface boundary oxide layer is 3 占 퐉 or less,
Characterized in that the surface hardness when tempered at 400 占 폚 is 600 or more in terms of Vickers hardness. A chemical mechanical polishing composition having chemical composition according to claim 1 or 2,
The depth of the surface boundary oxide layer is 3 占 퐉 or less,
The residual stress integral value from the surface to a depth position of 30 mu m is 40 MPa · mm or more,
Characterized in that the surface hardness when tempered at 400 占 폚 is 600 or more in terms of Vickers hardness, and the carburizing part having excellent surface fatigue strength and bending fatigue strength. A method of manufacturing a carburizing part for vacuum carburizing, quenching and tempering the steel material as set forth in claim 1 or 2,
The particle diameter of the shot material of shot peening is 0.10 to 0.5 mm,
Wherein the hardness of the projection material is 800 to 1000 in terms of Vickers hardness, and wherein the projection fatigue strength and the bending fatigue strength are excellent.

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