JP6301694B2 - Steel material for vacuum carburizing and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material for vacuum carburizing, and more particularly to a steel material for obtaining a carburized part excellent in surface fatigue characteristics and bending fatigue characteristics after vacuum carburizing, and further a manufacturing method thereof, a carburized part using the steel material, and The present invention relates to a method for manufacturing the component. The steel material of the present invention is useful as a material for gears and shafts used in automobiles, construction machines, and other various industrial machines, and will be described below by taking an example of application to automobile gears. However, it is not intended to be limited to this.

  The environment surrounding automobiles, construction machinery, and other various industrial machines is demanded by society to save energy and improve performance. In recent years, efforts to reduce the weight of automobile bodies and increase engine output have been increasingly promoted. ing. For this reason, the use environment of the gear used for a motor vehicle, a construction machine, etc., especially the gear used for the drive-train transmission part has become severer, and the gear provided with the outstanding fatigue strength is requested | required.

  Conventional gears are case-hardened steel such as JIS-SCr420 steel (including SCr420H steel) that is chromium steel or JIS-SCM420 steel (including SCM420H steel) that is chromium molybdenum steel. Is adopted. These case-hardened steels are formed into gear shapes and then subjected to carburizing, quenching and tempering treatment (hereinafter, carburizing, quenching and tempering may be collectively referred to as “carburizing treatment”), so-called carburized gears. Used as

  However, the following problems have been pointed out in the conventional gear described above. In other words, in recent years, car body gears that are required for automobiles, construction machines, etc., have become more and more demanding for lightweight car bodies and high output of engines. The surface fatigue strength and bending fatigue strength cannot be satisfied.

  For example, Patent Document 1 discloses a carburized component or a carbonitrided component that satisfies a predetermined chemical composition, is shot peened after carburizing or carbonitriding, and has a predetermined surface layer hardness and hardened layer depth. Has been. However, the improvement of the softening properties of the surface layer is not sufficient, and the surface hardening technology cannot provide the surface fatigue strength and bending fatigue strength that can sufficiently cope with the downsizing of parts and the high stress load that are required recently. .

  Patent Document 2 discloses a case-hardened steel for high-strength gears that satisfies a predetermined chemical composition. The case-hardened steel describes that surface hardening treatment such as gas carburizing, vacuum carburizing, carbonitriding, high-concentration carburizing (hypereutectoid carburizing), and shot peening may be performed. In this technique, it is considered that the softening property of the surface layer is not sufficiently improved. Therefore, even with the technique of Patent Document 2, it is not possible to obtain the surface fatigue strength and bending fatigue strength that can sufficiently cope with the downsizing of parts and the high stress load that are required recently.

JP 2008-261037 A JP 2005-163148 A

  The present invention has been made in view of the problems as described above, and the object thereof is to obtain a carburized part having sufficient surface fatigue strength, and further to obtain a carburized part having surface fatigue strength and bending fatigue strength, and the carburized part. It is to obtain a steel material for obtaining.

The present invention that has achieved the above problems
C: 0.15 to 0.35% (meaning mass%, hereinafter the same for chemical composition)
Si: 0.6-2.0%,
Mn: 0.3 to 1.3%
S: 0.020% or less (excluding 0%),
P: 0.015% or less (excluding 0%),
Cr: 0.7 to 1.7%,
Mo: 0.3-0.8%
V: 0.10 to 0.4%,
Al: 0.005 to 0.05%,
N: 0.004 to 0.025%
The balance is iron and inevitable impurities,
It is a steel material for vacuum carburizing characterized in that the average equivalent circle diameter of vanadium carbide is 25 nm or less.

  The present invention further includes at least one of Nb: 0.06% or less (not including 0%) and Ti: 0.2% or less (not including 0%), or B: 0.005% or less (0% It is also preferable to contain.

The present invention also includes a method for producing the above-described steel material, and specifically, the production method includes:
Steel having the chemical composition described in any of the above,
Hold for 30 to 300 minutes at 1200 ° C. or higher, and perform batch rolling.
A method for producing a steel material for vacuum carburizing, characterized by performing hot rolling at a heating temperature before hot rolling of 950 ° C. or higher and a heating holding time of 30 minutes to 5 hours.

The present invention also includes a carburized part obtained from the above steel for vacuum carburizing, and specifically, the carburized part is:
Having any of the chemical component compositions described above,
The surface grain boundary oxide layer depth is 3 μm or less,
It is a carburized part whose surface hardness when tempered at 400 ° C. is 600 or more in terms of Vickers hardness. The carburized parts are excellent in surface fatigue strength.

Furthermore, a part obtained by further shot peening the carburized part is also included in the present invention.
Having any of the chemical component compositions described above,
The surface grain boundary oxide layer depth is 3 μm or less,
The residual stress integral value from the surface to the 30 μm depth position is 40 MPa · mm or more,
It is a carburized part whose surface hardness when tempered at 400 ° C. is 600 or more in terms of Vickers hardness. The part is excellent in surface fatigue strength and bending fatigue strength.

The present invention also includes a method of manufacturing a part subjected to the above-described shot peening, and specifically the manufacturing method,
A steel material for vacuum carburizing according to any one of the above, vacuum carburizing, quenching and tempering and shot peening carburizing parts manufacturing method,
The particle size of the shot peening projection material is 0.10 to 0.5 mm,
It is a manufacturing method of the carburized component excellent in the surface fatigue strength and bending fatigue strength whose hardness of the said projection material is 800-1000 in Vickers hardness.

  According to the steel material for vacuum carburizing of the present invention, the chemical component composition is appropriately adjusted and the average equivalent circle diameter of vanadium carbide is set to a predetermined value or less, so that the surface carburizing strength after vacuum carburizing treatment is excellent, and the vacuum carburizing is performed. And carburized parts excellent in bending fatigue strength after shot peening can be obtained.

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

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

  (I) In recent years, in an environment where low oil viscosity and high surface pressure load are applied to parts due to the reduction in fuel consumption of automobiles, in order to improve surface fatigue strength, softening resistance of contact surfaces on parts It has been found that it is important to improve the properties, and in particular, it is effective to improve the tempering hardness at 400 ° C. In particular, by setting the tempering hardness of the component surface at 400 ° C. to HV600 or more in terms of Vickers hardness, the surface fatigue strength can be greatly improved.

  The carburized component of the present invention is also characterized in that it is obtained by vacuum carburizing. When vacuum carburization is not performed and gas carburization, gas carbonitriding, or the like is performed, a grain boundary oxide layer is generated on the surface, and surface fatigue strength and bending fatigue strength described later are reduced. In the carburized component of the present invention obtained by vacuum carburizing, the surface grain boundary oxide layer depth can be 3 μm or less.

(Ii) In order to set the tempering hardness at 400 ° C. of the component surface to HV600 or more, it is necessary to adjust Si, Mo and V to a predetermined range and to adjust the size of vanadium carbide in the steel material before carburizing. There is. Si suppresses the formation of carbides such as ε carbide, χ carbide, and η carbide during tempering, and Mo and V precipitate Mo ₂ C and VC during tempering and contribute to secondary hardening. The amounts of Si, Mo, and V in the steel material before carburizing are Si: 0.6 to 2.0%, Mo: 0.3 to 1.3%, and V: 0.10 to 0.4%, respectively.

  Furthermore, in the steel material before carburizing, the average equivalent circle diameter of vanadium carbide needs to be 25 nm or less. By setting the average equivalent circle diameter of vanadium carbide to 25 nm or less, vanadium carbide can be sufficiently dissolved during vacuum carburizing treatment, and vanadium carbide is precipitated during tempering or when the part is used, and the part is secondarily cured. As a result, the surface fatigue strength can be increased.

  (Iii) In order to set the average equivalent circle diameter of vanadium carbide to 25 nm or less in the steel material before carburizing, it is necessary to appropriately adjust the heating conditions before rolling. That is, by setting the heating temperature and holding time before rolling to a predetermined value or more, vanadium carbide precipitated before rolling can be sufficiently dissolved, and fine vanadium carbide is ensured by cooling after rolling (that is, The average equivalent circle diameter of the vanadium carbide is 25 nm or less).

  (Iv) In order to further improve the bending fatigue strength in addition to the surface fatigue strength of the carburized component, shot peening is applied to the component obtained by vacuum carburizing the steel material to give a predetermined residual stress. It is valid. Specifically, by setting the residual stress integral value from the surface of the carburized part to a depth position of 30 μm to 40 MPa · mm or more, the occurrence of initial cracks and crack propagation can be suppressed, and the bending fatigue strength can be greatly improved.

  (V) In order to set the residual stress integral value from the surface of the carburized part to a depth of 30 μm to 40 MPa · mm or more, the size and hardness of the projection material are appropriately adjusted by shot peening performed after vacuum carburizing. There is a need. The particle diameter of the projection material is 0.10 to 0.5 mm, and the hardness is HV800 to 1000 in terms of Vickers hardness.

  The steel material of the present invention (steel material after hot rolling and before vacuum carburizing) is characterized in that the size of the vanadium carbide in the steel is defined as described above, but the basic characteristics as a carburized part. Therefore, it is necessary to appropriately adjust the chemical composition of the steel material. Below, the chemical component composition of the steel material of this invention is demonstrated.

C: 0.15-0.35%
C is an element that can impart strength to the steel material. In order to obtain the required strength, the C content is 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 reduced. Therefore, the C content is set to 0.35% or less. The amount of C is preferably 0.33% or less, and more preferably 0.31% or less.

Si: 0.6-2.0%
Si acts as a temper softening resistance improving element, and when the temperature of the contact part rises during driving in gears, etc., it maintains hardness by suppressing softening, and improves fatigue strength such as pitching strength and wear resistance. Contribute. In order to exhibit such an effect effectively, the Si amount was determined to be 0.6% or more. The amount of Si is preferably 0.8% or more, and more preferably 1.0% or more. However, when the amount of Si becomes excessive, the strength rises remarkably and cold workability and machinability deteriorate. Therefore, the Si amount is 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 deoxidizer, desulfurizer, and hardenability improving element. In order to exhibit such an effect effectively, the amount of Mn was determined to be 0.3% or more. The amount of Mn is preferably 0.4% or more, more preferably 0.5% or more. However, when the amount of Mn is excessive, cold forgeability and toughness are lowered and machinability is also deteriorated. Therefore, the amount of Mn is set to 1.3% or less. The amount of Mn is preferably 1.2% or less, more preferably 1.1% or less.

S: 0.020% or less (excluding 0%)
S is an element contained in steel as an unavoidable impurity, and it is desirable to reduce it as much as possible because it precipitates as MnS and deteriorates fatigue characteristics and impact characteristics. However, extremely reducing causes an increase in steelmaking cost. From this point of view, the S content is determined to be 0.020% or less. The amount of S is preferably 0.015% or less, more preferably 0.010% or less. As described above, S is an unavoidable impurity, and it is difficult to make the amount 0% in industrial production, and the lower limit of the amount of S is about 0.0005%.

P: 0.015% or less (excluding 0%)
P is an element contained in steel as an inevitable impurity, and it is desirable to reduce it as much as possible because it segregates at the grain boundary and deteriorates workability and fatigue characteristics. However, extremely reducing causes an increase in steelmaking cost. From this point of view, the P content is set to 0.015% or less. The amount of P is preferably 0.010% or less, and more preferably 0.008% or less. As described above, P is an unavoidable impurity, and it is difficult to make the amount 0% in industrial production, and the lower limit of the P amount is about 0.0005%.

Cr: 0.7-1.7%
Cr, like Mn, is added as a hardenability improving element and acts as a temper softening resistance element. In order to exhibit such an effect effectively, the Cr content is set to 0.7% or more. The amount of C is preferably 0.8% or more, and more preferably 0.9% or more. However, when the amount of Cr becomes excessive, cold forgeability and toughness are lowered and machinability is also deteriorated. From this point of view, the Cr content is 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 the effect of improving the softening resistance by precipitating Mo ₂ C during tempering. When the temperature of the contact part rises during driving in a gear or the like, the hardness is maintained by suppressing the softening and pitching. Contributes to improving fatigue strength such as strength. Mo also has the effect of improving toughness. In order to effectively exhibit such an effect, the Mo amount 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 strength rises remarkably and cold workability and machinability deteriorate. Therefore, the Mo amount is set to 0.8% or less. Mo amount becomes like this. Preferably it is 0.75% or less, More preferably, it is 0.7% or less.

V: 0.10 to 0.4%
V has the effect of improving the softening resistance by precipitating vanadium carbide during tempering. When the temperature of the contact part rises during driving in a gear or the like, the hardness is maintained by suppressing the softening, and the pitching strength This contributes to improving fatigue strength. In order to exhibit such an effect effectively, the V amount 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 excessive, the strength increases remarkably, cold workability and machinability decrease, and coarse vanadium carbide precipitates after rolling, which does not contribute to the improvement of softening resistance after vacuum carburizing treatment. . Therefore, the V amount is set to 0.4% or less. V amount becomes like this. Preferably it is 0.35% or less, More preferably, it is 0.3% or less.

Al: 0.005 to 0.05%
At the same time as Al is a deoxidizer, it also has the effect of reducing crystal grains and improving toughness by forming fine Al-based nitrides. In order to exhibit such an effect effectively, 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, when the amount of Al is excessive, the machinability is adversely affected, the workability is lowered, and coarse nitrides are generated. Therefore, the Al content does not contribute as pinning particles and causes coarsening of crystal grains. From this point of view, the Al content is determined to be 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 forms nitrides with Al and the like, refines crystal grains, and exhibits the effect of improving toughness. In order to exhibit such an effect effectively, the N content is set to 0.004% or more. The amount of N is preferably 0.0060% or more, and more preferably 0.010% or more. However, when the amount of N is excessive, coarse nitrides (particularly Al-based nitrides) are generated and do not contribute as pinning particles, which causes crystal grain coarsening. From this point of view, the N content is determined to be 0.025% or less. The N amount is preferably 0.020% or less, more preferably 0.017% or less.

  The basic components of the steel for vacuum carburization of the present invention are as described above, and the balance is substantially iron. However, as a matter of course, it is permissible for steel to contain inevitable impurities other than P and S, which are brought in depending on the situation of raw materials, materials, manufacturing facilities, and the like. Furthermore, in this invention, you may contain the following arbitrary elements as needed in the range which does not inhibit the effect | action of this invention. The properties of the steel are further improved according to the types of the following elements.

One or more types of Nb: 0.06% or less (excluding 0%) and Ti: 0.2% or less (not including 0%) Nb and Ti refine crystal grains after carburizing, and toughness of steel materials This is useful for improving the bending fatigue strength. These elements exhibit the above effects by containing either one or two of them as necessary. In order to effectively exhibit these effects, the Nb content is preferably 0.01% or more, and the Ti content is preferably 0.005% or more. The preferable Nb amount and Ti amount are both 0.015% or more. However, when these elements become excessive, not only the effect is saturated, but also coarse precipitates are formed and the strength is lowered. Therefore, the Nb content is preferably 0.06% or less, and the Ti content is preferably 0.2% or less. The Nb content is more preferably 0.05% or less, the Ti content is more preferably 0.1% or less, and even more preferably 0.08% or less.

B: 0.005% or less (excluding 0%)
B is an element that has the effect of enhancing the hardenability in the carburizing treatment and also enhances the bending fatigue strength by strengthening the grain boundary. Since B can improve the hardenability by adding a trace amount, the influence on workability and the like is low. In order to effectively exhibit these actions, the B content is preferably 0.0005% or more, more preferably 0.0008% or more. However, when the amount of B is excessive, BN is generated by the combination with N, and the strength of the carburized component is lowered. Accordingly, the B content is preferably 0.005% or less, more preferably 0.0045% or less, and still more preferably 0.0040% or less.

  In the steel material for vacuum carburization of the present invention, the average equivalent circle diameter of vanadium carbide is 25 nm or less. Vanadium carbide is precipitated by heating during tempering and by sliding heat generated during use of parts, and has the effect of improving softening resistance. That is, when the temperature of the contact portion rises during driving in a gear or the like, the hardness is maintained by suppressing softening by vanadium carbide, which contributes to improvement of fatigue strength such as pitching strength. In order to exert such an effect, it is necessary to finely disperse vanadium carbide in the steel material after hot rolling and before vacuum carburizing, and to dissolve it at the time of 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 carburization treatment, so the average equivalent circle diameter of the vanadium carbide is 25 nm or less. The average equivalent circle diameter of vanadium carbide is preferably 20 nm or less, and more preferably 15 nm or less. The lower limit of the average equivalent circle diameter of vanadium carbide is not particularly limited, but is usually about 1 nm. In addition, the vanadium carbide in this invention means the precipitate from which V (vanadium) and C (carbon) are detected, and includes the case where elements other than V and C are contained.

  In order to adjust the vanadium carbide described above, the heating conditions before hot rolling are adjusted in a series of manufacturing steps in which the steel is melted in accordance with a normal melting method, subjected to ingot rolling, and then hot rolled. This is very important. The heating temperature before hot rolling is preferably 950 ° C. or higher. When the heating temperature before hot rolling is less than 950 ° C., the vanadium carbide existing before rolling cannot be sufficiently dissolved, and the undissolved vanadium carbide becomes coarse, and the vanadium carbide after rolling The average equivalent circle diameter cannot be made 25 nm or less. The heating temperature is more preferably 1000 ° C. or higher, and still more preferably 1050 ° C. or higher. The upper limit of the heating temperature is preferably 1250 ° C. or less, more preferably 1200 ° C. or less 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 cannot be sufficiently dissolved, and the undissolved vanadium carbide becomes coarse, and the average equivalent circle diameter of the vanadium carbide after rolling. Cannot be 25 nm or less. The heating and holding time is more preferably 1 hour or longer, and further preferably 1.5 hours or longer. On the other hand, if the heating and holding time exceeds 5 hours, the vanadium carbide becomes coarse due to Ostwald growth, and the average equivalent circle diameter of the vanadium carbide after rolling cannot be made 25 nm or less. The heating and holding time is more preferably 4.5 hours or less, and further preferably 4 hours or less.

  In addition, the above-described conditions of the partial rolling are not particularly limited, and for example, the partial rolling may be performed by holding at 1200 ° C. or higher (preferably 1250 ° C. or higher) for 30 to 300 minutes. Although the upper limit of the heating temperature of a block rolling is not specifically limited, For example, it is 1300 degrees C or less.

  A part having excellent surface fatigue strength can be obtained by vacuum carburizing the steel material of the present invention in which the chemical component composition and the vanadium carbide size are adjusted as described above, and a shot having predetermined conditions after vacuum carburizing. Carburized parts having excellent bending fatigue strength can be obtained by peening.

  In the present invention, vacuum carburizing is adopted as the carburizing treatment. In the carburizing steel material of the present invention, as described above, the Si content is increased to 0.6% or more. When such steel is carburized by gas carburizing or gas carbonitriding other than vacuum carburizing, a grain boundary oxide layer is formed on the surface, reducing the surface fatigue strength of the component, and further reducing the bending fatigue strength of the component. To do. The parts of the present invention obtained by vacuum carburization have a surface grain boundary oxide layer depth of 3 μm or less. The surface grain boundary oxide layer depth is preferably 2 μm or less, more preferably 1 μm or less, and most preferably 0 μm. The conditions for the vacuum carburizing treatment are not particularly limited. For example, the carburizing temperature may be 900 to 1000 ° C. (preferably 930 to 980 ° C.). After carburizing, (a) quenching may be performed directly, or (b) carburizing and cooling may be performed followed by reheating and quenching. Further, in both cases (a) and (b), after being put into an oil bath of about 50 to 150 ° C. (preferably 60 to 130 ° C.) and quenched, for example, 150 to 200 ° C. (preferably 160 Tempering may be performed at about ~ 180 ° C. In the case of (a), after vacuum carburization, the furnace is cooled to 750 to 900 ° C. (preferably 780 to 880 ° C.), and then quenched and tempered.

  In order to increase the fatigue strength such as the pitching strength of the carburized component, it is effective to increase the hardness. However, since the hardness decreases when the temperature of the contact part rises during driving in gears, etc., increasing the hardness near the heat generation temperature (400 ° C), not the initial hardness, is effective in improving the fatigue strength. It is. The parts of the present invention obtained by vacuum carburizing the steel for vacuum carburization of the present invention can have a surface hardness of HV600 or higher in terms of Vickers hardness when tempered at 400 ° C. The surface hardness is preferably HV620 or more, and more preferably HV650 or more. The upper limit of the surface hardness is not particularly limited, but is usually about HV900.

  Parts obtained by vacuum carburizing can be further imparted with compressive residual stress by performing shot peening. The compressive residual stress can suppress the occurrence of initial cracks and crack propagation when a repeated stress is applied, and can greatly improve the bending fatigue strength. In order to exert such an effect, it is necessary that the residual stress integrated value from the surface to the 30 μm depth position 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 integral value is not particularly limited, but is usually about 100 MPa · mm.

  In order to give the above-mentioned compressive residual stress to the carburized component, it is necessary to appropriately control the particle size and hardness of the projection material used in shot peening. The particle size of the projection material is 0.10 to 0.5 mm. When the particle size is less than 0.10 mm, compressive residual stress is imparted only to the surface layer, so that the compressive residual stress from the surface to a depth of 30 μm cannot be increased. If the particle size exceeds 0.5 mm, compressive residual stress is applied to the inner side, and the compressive residual stress from the surface to a depth of 30 μm cannot be set in the above range.

  The projection material has a Vickers hardness of HV 800 to 1000. When the hardness is less than HV800, the compressive residual stress is not sufficiently applied, and the compressive residual stress from the surface to the 30 μm depth position cannot be within the above range. The hardness of a projection material becomes like this. Preferably it is HV820 or more, More preferably, it is HV850 or more. If the hardness exceeds HV1000, the amount of corrosion of the steel material increases, and a predetermined part shape cannot be obtained. The hardness of a projection material becomes like this. Preferably it is HV980 or less, More preferably, it is HV950 or less.

The steel material for vacuum carburizing of the present invention can obtain a part having excellent surface fatigue strength by vacuum carburizing, and further obtaining a part having excellent bending fatigue strength by shot peening after vacuum carburizing. it can. The surface fatigue strength can be set to 3.3 GPa or more (preferably 3.4 GPa or more), for example, with a 1,000,000 times strength (maximum stress that does not break when tested 1,000,000 times) in a roller pitching test, and a bending fatigue strength of 4 It can be set to 1260 MPa or more (preferably 1300 MPa or more) at 100,000 times strength (maximum stress that does not break when tested 100,000 times) in the point bending fatigue test. Therefore, such components are suitable for gears and shafts used in automobiles, construction machines, and other various industrial machines, and are industrially useful.

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

  Steel having the chemical composition shown in the following Table 1 (the balance is iron and inevitable impurities) was melted and held at 1250 ° C. for 30 minutes to 300 minutes, and then rolled into pieces. Thereafter, as shown in Tables 2 and 3, hot rolling was performed at a heating temperature before rolling of 920 ° C. to 1100 ° C. and a heating holding time of 0.3 to 6 hours to obtain a hot rolled material (bar steel) having a diameter of 32 mm. . The steel No. 1 shown in Table 1 was used. Reference numeral 1 denotes a conventional steel, SCr420H equivalent steel.

About each obtained hot-rolled material, the magnitude | size of vanadium carbide was measured by the method of following (1). Moreover, after carburizing said hot-rolled material on the carburizing conditions (vacuum carburizing or gas carburizing) shown in Tables 2 and 3, it was tested No. About 26-34, the shot peening was further performed using the projection material of the particle size and hardness shown in Table 3, and the test piece was produced. The vacuum carburizing treatment is performed in the temperature range of 930 to 980 ° C. shown in Tables 2 and 3, and then cooled to 780 to 880 ° C. and then poured into oil at 60 to 130 ° C. and quenched. And tempering by reheating to 170 ° C. Shot peening was performed using the projection material shown in Table 3 at a projection pressure of 0.4 MPa and a coverage of 400% or more. The blasting material was classified by sieving, in a range of 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. Particle size was used.
In addition, the gas carburizing used for comparison was Cp (carbon potential: carburizing gas atmosphere of 0.8% carburizing treatment at 930 ° C., oil cooling, and further tempering treatment at 170 ° C. for 2 hours.

  For these specimens, (2) 400 ° C. tempering hardness, (3) residual stress integral value at a depth of 30 μm from the surface, and (4) depth of the surface grain boundary oxide layer are measured ( 5) Roller pitting fatigue characteristics and (6) bending fatigue characteristics were evaluated.

(1) Measurement of vanadium carbide size in hot-rolled material After cutting and polishing the D / 4 position (D is the diameter of the rolled material) of the hot-rolled material in a cross section, carbon deposition is performed, and FE- Replica observation by TEM (Field-Emission Transmission Electron Microscope) was performed. At this time, precipitates in which V and C were detected were identified by TEM EDX (Energy Dispersive X-ray Analysis), and a 1.0 μm × 1.2 μm field of view was observed at a magnification of 100,000 times. Observation was performed for three arbitrary visual fields, and the arithmetic average value of the observed equivalent circle diameter of vanadium carbide was defined as the average equivalent circle diameter of vanadium carbide. In addition, from the measurement limit of FE-TEM, the minimum of the magnitude | size of the vanadium carbide used as the measuring object is about 1 nm in a circle equivalent diameter.

(2) Measurement of tempering hardness at 400 ° C. The surface of the hot-rolled material described above was polished to φ26.02 mm, carburized, and polished again to φ26 mm. Test No. 26 to 34 were further shot peened to obtain test pieces for measuring 400 ° C. tempering hardness. Test No. For Nos. 1 to 25, test pieces after carburization, No. About 26-34, about the test piece after carburizing and shot peening, tempering was performed at 400 degreeC for 3 hours, and hardness was measured with the Vickers hardness meter about the 50 micrometer position from the surface in the cross section. The test load of the Vickers hardness tester was 300 gf, and five points were measured to obtain the arithmetic average value. This was the 400 ° C. tempered hardness of each test piece.

(3) Measurement of integrated value of residual stress at 30 μm depth from the surface A 4-point bending test piece of FIG. 26 to 34 were further subjected to shot peening and used as test pieces for residual stress measurement. Test No. For Nos. 1 to 25, test pieces after carburization, No. For samples 26 to 34, the specimens after carburization and shot peening were measured at positions of 10 μm, 20 μm, and 30 μm from the notch bottom surface of the specimen using a PSPC (Position-Sensitive Proportional Counter) micro part X-ray stress measurement device, respectively. Residual stress was measured, and the integrated value of residual stress from the surface to the 30 μm depth position was calculated by the following formula. The measurement conditions of the PSPC minute part X-ray stress measurement apparatus are collimator diameter: φ1 mm, measurement site: axial center position, measurement direction: circumferential direction.
Residual stress integrated value from surface to 30 μm depth σ
= {Σ (0 mm) + σ (0.01 mm)} / 2 × 0.01 mm
+ {Σ (0.01 mm) + σ (0.02 mm)} / 2 × 0.01 mm
+ {Σ (0.02 mm) + σ (0.03 mm)} / 2 × 0.01 mm
However, (sigma) (Xmm) means the value of the residual stress in the position of Xmm from the surface.

(4) Measurement of depth of surface grain boundary oxide layer The surface of the hot-rolled material described above was polished to φ26.02 mm, carburized, and polished again to φ26 mm. Test No. 26 to 34 were further shot peened to obtain test pieces. Test No. For Nos. 1 to 25, test pieces after carburization, No. For No. 26 to No. 34, after carburizing and shot peening, the test piece was cut out perpendicular to the rolling direction, embedded in the resin and polished, and then the outermost surface of the test piece was observed with an optical microscope (magnification: 1000 times). The depth of the deepest position of the oxide layer was measured.

(5) Evaluation of roller pitting fatigue characteristics A test piece similar to the above (4) was prepared, and the obtained test piece was subjected to surface pressure: 2.7, 3.0, 3.3 GPa, rotation speed: 1500 rpm, and slip. Rate: -40%, roller pitching test was performed under the condition of using automatic oil (oil temperature: 80 ° C), and stress S-repetition number N diagram (hereinafter referred to as S-N diagram) was created, 1 million times Pitting strength was evaluated by strength (meaning the maximum stress that does not break when tested 1 million times). The mating roller used at this time was a tempered product (surface hardness: HV700, crowning R: 150 mm) made of SUJ2.

(6) Evaluation of bending fatigue characteristics A test piece having the shape shown in FIG. The samples 26 to 34 were further shot peened to obtain test pieces for a bending fatigue test. Using this test piece, as shown in FIG. 2, an S—N diagram was created using a four-point support jig under the conditions of frequency 20 Hz, maximum stress (repeated load stress): 1371, 1523, 1675, and 1828 MPa. Then, based on this SN diagram, the strength was determined 100,000 times 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, 14-18, 22-29, 31-34 are steel materials that satisfy the chemical composition defined in the present invention and obtained under appropriate hot rolling conditions. Therefore, the average equivalent circle diameter of VC is 25 nm or less, and the steel obtained by vacuum carburizing or the steel obtained by vacuum carburizing and shot peening has a surface hardness when tempered at 400 ° C. The surface fatigue strength (1,000,000 times strength) is 3.3 GPa or more. Compared to 1, the surface fatigue strength was 1.20 times or more. Among these, in particular, Test No. Nos. 26, 31 to 34 are examples in which shot peening was performed under appropriate conditions after vacuum carburization, and since compressive residual stress could be sufficiently applied, bending fatigue strength (100,000 times strength) was 1260 MPa or more. Test No. Compared to 1, it was possible to achieve a bending fatigue strength of 1.20 times or more. In addition, the characteristic of the projection material of 3, 5, 14-18, 22-25 which did not perform shot peening, and shot peening was not adjusted appropriately. Nos. 27 to 29 had good surface fatigue strength as described above, but the bending fatigue strength was No. 27. Compared with 26, 31-34, the result was inferior.

  Test No. Nos. 1 and 2 are examples in which the amount of Si, V and Mo was small, vanadium carbide was not formed, and the 400 ° C. tempering hardness after carburization was low, resulting in poor surface fatigue strength (million times strength). It was. No. No. 1 employs gas carburizing as the carburizing treatment, and therefore a grain boundary oxide layer is formed. The surface fatigue strength was inferior to 2. No. Since No. 4 employs gas carburization, a grain boundary oxide layer was formed, and the surface fatigue strength was inferior.

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

No. No. 19 is an example in which the heating temperature before hot rolling was low, No. 20 is an example in which the heating and holding time before hot rolling was short. No. 21 is an example in which the heating and holding time before hot rolling was long. In either case, the average equivalent circle diameter of vanadium carbide was increased, resulting in poor surface fatigue strength. No. 30 is an example in which gas carburization was performed, and a grain boundary oxide layer was formed, resulting in poor surface fatigue strength.

Claims (7)

C: 0.15-0.35% (meaning mass%; hereinafter the same for chemical component composition), Si: 0.6-2.0%,
Mn: 0.3 to 1.3%
S: 0.020% or less (excluding 0%),
P: 0.015% or less (excluding 0%),
Cr: 0.7 to 1.7%,
Mo: 0.3-0.8%
V: 0.10 to 0.4%,
Al: 0.005 to 0.05%,
N: 0.004 to 0.025%
The balance is iron and inevitable impurities,
A steel material for vacuum carburizing, wherein an average equivalent circle diameter of vanadium carbide is 25 nm or less. Furthermore,
The steel material for vacuum carburizing according to claim 1, comprising at least one of Nb: 0.06% or less (not including 0%) and Ti: 0.2% or less (not including 0%). Furthermore,
B: Steel material for vacuum carburizing according to claim 1 or 2 containing 0.005% or less (not including 0%). Steel having the chemical composition according to any one of claims 1 to 3,
Hold for 30 to 300 minutes at 1200 ° C. or higher, and perform batch rolling.
The method for producing a steel material for vacuum carburizing according to any one of claims 1 to 3, wherein hot rolling is performed at a heating temperature before hot rolling of 950 ° C or higher and a heating holding time of 30 minutes to 5 hours. The chemical component composition according to claim 1,
The surface grain boundary oxide layer depth is 3 μm or less,
A vacuum carburized component having excellent surface fatigue strength, wherein the surface hardness when tempered at 400 ° C. is 600 or more in terms of Vickers hardness. The chemical component composition according to claim 1,
The surface grain boundary oxide layer depth is 3 μm or less,
The residual stress integral value from the surface to the 30 μm depth position is 40 MPa · mm or more,
A vacuum carburized component excellent in surface fatigue strength and bending fatigue strength, characterized in that the surface hardness when tempered at 400 ° C. is 600 or more in terms of Vickers hardness. A steel material according to any one of claims 1 to 3, which is a carburized part manufacturing method for vacuum carburizing, quenching and tempering, and shot peening,
The particle size of the shot peening projection material is 0.10 to 0.5 mm,
The method for producing a carburized part excellent in surface fatigue strength and bending fatigue strength, wherein the projection material has a Vickers hardness of 800 to 1000.