JP4688727B2 - Carburized parts and manufacturing method thereof - Google Patents

Description

  The present invention relates to a carburized part (machine part) used by carburizing in a transport device such as an automobile, a construction machine, and other various industrial machines, and a manufacturing method thereof, and more particularly to gears, shafts, shafts, and the like. The present invention relates to attached gears, bearings, and the like.

  Conventionally, carburized parts that have undergone surface hardening heat treatment such as carburizing treatment, nitriding treatment, or carbonitriding treatment are required for machine parts that require wear resistance and fatigue characteristics in transportation equipment, construction machinery, and other various industrial machines. Is used. In order to exhibit the effect of such surface hardening heat treatment, as the material steel (base material) of the above-mentioned parts, structural steel materials (H steel) with guaranteed hardenability as defined in JIS G4052 are usually used. It has been. Among H steels, SMn420H, SCr415H, SCr420H, SCM415H, SCM418H, SCM420H, SNC415H, SNCM220H, SNCM420H and the like that contain Si in a range of 0.35% or less are generally used.

  Among these, as a technique related to the carburizing heat treatment method, for example, in Patent Document 1, the carbon potential is set to a high carbon range of 0.9 to 1.2%, carburizing treatment is performed, and after cooling to Ar1 transformation or less, 850 ° C. The method is described in which the steel is reheated to 200 ° C. to 230 ° C. and kept at a temperature of 200 ° C. to 230 ° C. for a predetermined time to prevent a decrease in strength due to high carbon martensite of the matrix and to improve the pitting resistance and bending fatigue limit. .

By the way, parts used for automobiles, construction machines, industrial machines, etc. have recently been increasingly demanded for high stress and small size and light weight. The stress applied is also increasing, and there is a need for further improvement in bending fatigue characteristics.
JP-A-7-34218 (Claims, paragraph 0011, etc.)

  This invention is made | formed in view of such a condition, The objective is to provide the carburized component which was extremely excellent in the bending fatigue characteristic. Another object of the present invention is to provide a method capable of manufacturing such a carburized part.

  The carburized parts according to the present invention that have been able to solve the above-mentioned problems are based on mass%, C: 0.05 to 0.3%, Si: more than 0.35%, 2% or less, Mn: 3% or less (not including 0%), Cr: 2% or less (not including 0%), Al: 0.1% or less (not including 0%), N: 0.03% or less (0% Not contained), P: 0.03% or less, S: 0.03% or less, O: 0.002% or less are satisfied, the balance is composed of Fe and inevitable impurities, and the C concentration of the carburized hardened layer Is 0.9% or more, the structure contains bainite and cementite, and the gist is that the maximum hardness is 650 Hv or more.

The base material is still another element,
(A) Cu: 1% or less (not including 0%) and / or Ni: 3% or less (not including 0%),
(B) Mo: 1% or less (excluding 0%),
(C) A group consisting of Ti: 0.2% or less (not including 0%), V: 0.3% or less (not including 0%), and Nb: 0.2% or less (not including 0%) One or more selected from
(D) B: 0.003% or less (excluding 0%),
(E) A group consisting of Ca: 0.02% or less (not including 0%), Mg: 0.02% or less (not including 0%), and Te: 0.05% or less (not including 0%) One or more selected from
Etc. may be contained.

The carburized parts of the present invention include a step of carburizing steel satisfying the above chemical components at a carbon concentration of 0.9 to 1.5% at a high concentration, a step of holding at 820 to 890 ° C. for 30 minutes or more, and the following (1 And the step of maintaining for 30 minutes or more in the temperature range of Ms point to Ms point + 90 ° C. calculated by the formula can be manufactured in this order.
Ms point (° C.) = 550−350 × [C] −40 × [Mn] −20 × [Cr] + 30 × [Al] −10 × [Cu] −17 × [Ni] −10 × [Mo] −35 × [V] (1)
In the formula, [] means the content (% by mass) of a chemical component contained in austenite on the surface of the part after high-concentration carburization.

  In the carburized parts of the present invention, the base material is composed of steel with a large amount of Si, the C concentration of the carburized hard layer on the surface is as high as 0.9% or more, and the structure is a mixed structure of bainite and cementite. Since it has and the surface hardness is raised moderately, it is very excellent in bending fatigue characteristics.

  The present inventors have repeatedly studied from the viewpoint of providing a carburized part that is particularly excellent in bending fatigue characteristics among the characteristics required for carburized parts. As a result, if the component composition of the material steel (base material) constituting the carburized component and the structure and hardness of the carburized hardened layer on the surface are appropriately adjusted, a desired carburized component can be obtained. The present invention was completed by finding that it can be obtained by carburizing at a high concentration using steel having a higher Si content than H steel, which is widely used as raw material steel, and then performing appropriate heat treatment.

  First, the bending fatigue characteristics will be described in detail. In general, it is known that when a metal material is subjected to repeated stress, it breaks at a stress considerably lower than a normal fracture stress. This failure is called fatigue of a metal material, and the stress at the time of fatigue failure is greatly different between a smooth test piece without a notch and a test piece with a notch. In a smooth specimen without notch, fatigue failure does not occur until a stress of about 1/3 to 1/5 of the tensile strength is applied, whereas in a specimen with notch, the tensile strength Fatigue failure occurs only by applying a stress of about 1/10 or less. This is called a notch effect. For example, in parts such as gears where the root of teeth (tooth root) is a stress-concentrated part, it is required to have excellent fatigue characteristics against notches. Such a smooth part is considered to have a different approach for improving fatigue characteristics.

  Therefore, for example, in Patent Document 1 described above, a carburizing heat treatment method excellent in bending fatigue limit is described, but the present inventors performed the above method using H steel which is widely used as material steel. However, as shown in the examples described later, the fatigue characteristics evaluated using notched test pieces (four-point bending test pieces shown in FIG. 1 of the examples described later) are good, but smooth test pieces. It was found that the fatigue characteristics evaluated using (rotary bending test piece shown in FIG. 2 of an example described later) were inferior. Moreover, as described in Patent Document 1, after carrying out the high-concentration carburizing treatment, if a high-concentration carburizing heat treatment method in which the temperature is uniformly maintained at a temperature of 200 to 230 ° C. regardless of the composition of the base material, It was found that, depending on the steel component, the carburized hardened layer is softened and the surface hardness is lowered, so that the bending fatigue strength of a part having a smooth surface is lowered. That is, it has been found that the method of Patent Document 1 has a large variation in characteristics due to the base material and cannot have desired characteristics.

  Accordingly, the inventors of the present invention pay particular attention to the chemical composition of the base material in order to provide carburized parts with improved fatigue characteristics for both notched parts such as gears and smooth parts such as shafts and shaft parts of shafted gears. And have been repeatedly studied. As a result, in the case of a rotating bending test piece, if the hardness is increased, the tensile strength can be improved and the fatigue strength can be increased. On the other hand, in the case of a four-point bending test piece, the hardness can be increased. The effect was small, and it was found that increasing the toughness of the steel material is more effective.

  Based on such a viewpoint, the present inventors have further studied. As a result, when using a steel having a larger amount of Si than that specified by JIS as a base material, the heating and holding process after high-concentration carburization (bainite forming process shown in FIG. 4 described later) is controlled in relation to the Ms point. The inventors have found that the decrease in surface hardness can be suppressed and the bending fatigue characteristics can be improved, and the present invention has been completed.

  First, the carburized component of the present invention will be described. As shown below, the carburized component is divided into a base material and a carburized hardened layer. The base material means a material steel that is a part other than the carburized hardened layer and is used for manufacturing a carburized part.

  First, the base material will be described. In the base material constituting the carburized part of the present invention, Si is contained in a range of more than 0.35% and less than 2%, exceeding the range of the component composition defined as H steel in JIS G 4052. Therefore, it is possible to prevent a reduction in the hardness of the carburized hardened layer in the heating and holding step after high-concentration carburizing. By ensuring the surface hardness, the bending fatigue strength of parts having a smooth surface can be particularly increased. Si is preferably 0.4% or more, more preferably 0.45% or more. However, when Si is excessive, the hardness of the base material becomes extremely high and the machinability and forgeability are deteriorated. Therefore, Si is 2% or less. Preferably it is 1% or less, More preferably, it is 0.6% or less.

  The chemical components of the base material other than Si are not particularly limited, and may be the same as, for example, the structural steel material defined as H steel in JIS G 4052. Specifically, it contains C, Mn, Cr, Al, N as basic elements, and may further contain various selection elements (for example, Cu, Ni, etc.) as necessary. The addition amount of each element and the reason for addition are as follows.

C: 0.05-0.3%
C is an important element in securing the core hardness necessary for carburized parts. If C is less than 0.05%, the hardness is insufficient, and the static strength as a carburized component is insufficient. Therefore, C is 0.05% or more. Preferably it is 0.1% or more, More preferably, it is 0.15% or more, More preferably, it is 0.17% or more. However, when there is too much C, hardness will become high too much and machinability and forgeability will deteriorate. Therefore, C is 0.3% or less. Preferably it is 0.26% or less, More preferably, it is 0.25% or less, More preferably, it is 0.23% or less.

Mn: 3% or less (excluding 0%)
Mn acts as a deoxidizer, has the effect of reducing the amount of oxide inclusions and improving the internal quality of the base material, and also has the effect of significantly improving the hardenability during carburizing and quenching. Mn is an element that lowers the martensitic transformation point (Ms point), and can produce a bainite structure even at a relatively low temperature. However, if Mn exceeds 3%, center segregation occurs remarkably, deteriorating the internal quality. In addition, a striped structure is remarkably generated, causing variations in the material and deteriorating impact characteristics. Therefore, Mn is 3% or less. Preferably it is 2% or less, More preferably, it is 1.6% or less. In addition, although Mn contains about 0.2% on manufacture, in order to exhibit the said effect | action effectively, it is more preferable that Mn is 0.3% or more, More preferably, it is 0.4% or more, Particularly preferably, it is 0.5% or more.

Cr: 2% or less (excluding 0%)
Cr is an element that improves the hardness of the carbide by solid solution in the carbide (cementite), and thus effectively acts to improve the wear resistance. However, if the content exceeds 2%, the base material becomes too hard and the machinability and forgeability deteriorate. Therefore, Cr is 2% or less. Preferably it is 1.5% or less, More preferably, it is 1.4% or less. Although the minimum of Cr is not specifically limited, For example, it is 0.4% or more, and is 0.9% or more especially.

Al: 0.1% or less (excluding 0%)
Al is an element that acts to refine the crystal grains of the base material. Moreover, Al combines with N in the steel to produce AlN, and this nitride suppresses the growth of austenite grains during high-concentration carburization. However, when Al is contained excessively, coarse and hard non-metallic inclusions (Al ₂ O ₃ ) are generated, and impact strength and rolling fatigue strength are lowered. Therefore, Al is 0.1% or less. Preferably it is 0.05% or less, More preferably, it is 0.03% or less. Al is unavoidably contained in steel in an amount of about 0.001%, but in order to effectively exhibit the above action, it is preferably 0.005% or more, in particular 0.01%. More preferably. When Al is added to molten steel as a deoxidizer, the base material contains about 0.01 to 0.03%.

N: 0.03% or less (excluding 0%)
N is an element that forms nitrides or carbonitrides in combination with Al, Ti, Nb, etc., and suppresses the growth of austenite grains during high-concentration carburization. However, when N is contained excessively, the hot workability is lowered, making the production difficult, and the impact characteristics of the base material are lowered. Therefore, N is 0.03% or less. Preferably it is 0.025% or less, More preferably, it is 0.02% or less. N is inevitably contained in steel by about 0.001%. Although the minimum of N is not specifically limited, For example, it is 0.003% or more, and is 0.005% or more especially.

  As described above, the base material subjected to high-concentration carburization in the present invention contains C, Mn, Cr, Al, and N as basic elements, but P, S, and O (oxygen) contained in the base material are The following range should be satisfied.

P: 0.03% or less P is an element inevitably contained in the steel, but P is 0.03% or less because it segregates at the grain boundaries and lowers the impact characteristics of the carburized parts. Is desirable. Preferably it is 0.015% or less, More preferably, it is 0.01% or less. P is preferably reduced as much as possible, but unavoidably contains about 0.001%. Although the minimum of P is not specifically limited, For example, it is 0.003% or more.

S: 0.03% or less S is an element that is inevitably contained in the steel, but combines with Mn to form MnS inclusions and lower the impact strength of the carburized component. % Or less is desirable. Preferably it is 0.02% or less, More preferably, it is 0.015% or less, More preferably, it is 0.01% or less. S is preferably reduced as much as possible, but unavoidably contains about 0.001%. Although the minimum of S is not specifically limited, For example, it is 0.003% or more.

O: 0.002% or less O is an element inevitably contained in the steel, but O is preferably 0.002% or less in order to reduce the strength of the base material. Preferably it is 0.0015% or less, More preferably, it is 0.001% or less. O is preferably reduced as much as possible, but unavoidably contains about 0.0001%. Although the minimum of O is not specifically limited, For example, it is 0.0003% or more.

  The chemical components of the base material subjected to high-concentration carburization in the present invention are as described above, and the balance may be Fe and inevitable impurities. Further, in order to further improve the characteristics of the carburized component, as another element, one or more selected from the group consisting of (a) Cu and / or Ni, (b) Mo, (c) Ti, V and Nb (D) B, (e) One or more elements selected from the group consisting of Ca, Mg and Te may be contained. Hereinafter, preferable ranges of these elements will be described.

(A) Cu: 1% or less (not including 0%) and / or Ni: 3% or less (not including 0%)
Cu and Ni are elements that improve the corrosion resistance of the base material. However, if the Cu content exceeds 1%, the hot ductility of the base material is reduced, and cracking is likely to occur. Therefore, Cu is preferably 1% or less. More preferably, it is 0.8% or less, More preferably, it is 0.6% or less. Although the minimum of Cu is not specifically limited, For example, it is 0.1% or more, Especially 0.2% or more.

  Ni is an element that has the effect of improving the impact resistance of a steel material as well as improving the corrosion resistance of the steel material in the same manner as Cu. Ni, like Mn, acts to lower the martensitic transformation point (Ms point) and can produce a bainite structure even at a relatively low temperature. However, if Ni is excessive, the cost increases, so the upper limit is preferably 3%. More preferably, it is 2% or less, More preferably, it is 1.5% or less. Although the minimum of Ni is not specifically limited, For example, it is 0.1% or more, especially 0.3% or more.

  Cu and Ni can be used alone or in combination. A preferable upper limit when Cu and Ni are used in combination is 3%, and a more preferable upper limit is 2%.

(B) Mo: 1% or less (excluding 0%)
Mo is an element that acts to keep the hardness of the carburized hardened layer from decreasing when the parts after high-temperature carburizing are held in the temperature range of Ms point to Ms point + 90 ° C. is there. Mo not only has the effect of remarkably improving the hardenability during high-concentration carburization, but also effectively acts to improve hydrogen embrittlement resistance. However, when it contains excessively, a base material will become hard too much and machinability will worsen. Moreover, even if it contains excessively, the addition effect will be saturated and it will become expensive. Therefore, it is recommended that Mo is 1% or less. Preferably it is 0.8% or less, More preferably, it is 0.45% or less. Although the minimum of Mo is not specifically limited, For example, it is 0.1% or more, Especially 0.15% or more.

(C) A group consisting of Ti: 0.2% or less (not including 0%), V: 0.3% or less (not including 0%), and Nb: 0.2% or less (not including 0%) One or more selected from Ti, V, and Nb are elements that combine with N and C in steel to form nitrides, carbides, or carbonitrides, and suppress the growth of austenite grains during high-concentration carburizing. It is. However, if it is contained excessively, coarse metal inclusions are produced, and the impact strength and rolling fatigue strength may be reduced. Therefore, Ti is preferably 0.2% or less, V is 0.3% or less, and Nb is 0.2% or less.

  Ti is preferably 0.15% or less, and more preferably 0.1% or less. Although the minimum of Ti is not specifically limited, For example, it is 0.01% or more, especially 0.02% or more. V is preferably 0.2% or less, and more preferably 0.15% or less. Although the minimum of V is not specifically limited, For example, it is 0.03% or more, Especially 0.05% or more. Nb is preferably 0.15% or less, more preferably 0.1% or less. Although the minimum of Nb is not specifically limited, For example, it is 0.01% or more, Especially 0.02% or more.

  Ti, V and Nb can be used alone or in combination. The total when Ti, V and Nb are all contained is preferably 0.2% or less.

(D) B: 0.003% or less (excluding 0%)
B is an element that greatly improves the hardenability during high-concentration carburizing. Moreover, it has the effect | action which strengthens a crystal grain boundary and raises impact strength. However, if it exceeds 0.003%, B nitride is likely to be generated, and B nitride may deteriorate cold workability and hot workability. Therefore, B is preferably 0.003% or less. Preferably it is 0.0025% or less, More preferably, it is 0.002% or less. The lower limit of B is not particularly limited, but is, for example, 0.0005% or more, particularly 0.0008% or more.

(E) A group consisting of Ca: 0.02% or less (not including 0%), Mg: 0.02% or less (not including 0%), and Te: 0.05% or less (not including 0%) At least one selected from the group consisting of Ca, Mg, and Te is an element that suppresses the spreading of sulfides generated in the steel during rolling and improves the impact characteristics.

  However, when Ca and Mg are contained excessively, coarse oxides are generated, and the strength of the base material may be decreased. Therefore, Ca is preferably 0.02% or less, and Mg is preferably 0.02% or less. Ca is preferably 0.01% or less, and more preferably 0.005% or less. Mg is preferably 0.01% or less, more preferably 0.005% or less. In order to effectively exhibit the above action, Ca is preferably 0.0005% or more, particularly 0.001% or more, for example. Similarly to Ca, Mg is, for example, preferably 0.0005% or more, particularly preferably 0.001% or more.

  On the other hand, when Te is contained excessively, the hot ductility is remarkably deteriorated, so that it is difficult to manufacture the base material itself or to form the part shape. Therefore, Te is preferably 0.05% or less. Te is preferably 0.03% or less, more preferably 0.02% or less, and still more preferably 0.015% or less. In order to effectively exhibit the above action, Te is preferably, for example, 0.001% or more, particularly 0.005% or more.

  Ca, Mg and Te can be used alone or in any combination. A preferable combination of these elements is Ca and Mg, which may be used at a ratio of 1: 1 on a mass basis. The total when Ca, Mg and Te are all contained is preferably 0.02% or less.

  Such a base material structure only needs to be mainly composed of martensite and bainite.

  Next, the carburized hardened layer will be described. The C concentration of the carburized hardened layer (the sum of bainite and cementite) is 0.9% or more. When the C concentration is less than 0.9%, cementite is not generated, so that the hardness is insufficient, and the bending fatigue strength of a part having a smooth surface is lowered. Therefore, the C concentration is 0.9% or more, preferably 0.95% or more, more preferably 1% or more. The upper limit of the C concentration is not particularly limited, but is, for example, 1.5%.

  The structure of the carburized hardened layer includes bainite and cementite, and in detail, bainite is mainly used as long as cementite is precipitated. The bainite main body means that the ratio of bainite (bainite fraction) in the carburized hardened layer is 50% by volume or more. Preferably it is 55 volume% or more, More preferably, it is 60 volume% or more. The upper limit of the bainite fraction is determined by the balance with the cementite fraction, and is about 95% by volume at the maximum.

  The proportion of cementite in the carburized hardened layer is preferably 5% by volume or more in order to ensure the hardness of the carburized hardened layer. More preferably, it is 7 volume% or more. The cementite fraction is determined by the balance with bainite and is, for example, 15% by volume.

  The carburized hardened layer should just contain the bainite and cementite of the said range at least, may be comprised only from bainite and cementite, and may have another structure | tissue (for example, retained austenite). The proportion of retained austenite in the carburized parts is not particularly limited, but if a large amount of retained austenite is generated, the bainite fraction is reduced, the hardness of the carburized hardened layer is reduced, and the part surface is smooth. Bending fatigue strength decreases. Therefore, the retained austenite fraction is preferably 30% by volume at the maximum, more preferably 25% by volume or less, and still more preferably 20% by volume or less. Of course, it may be 0% by volume.

  As described above, the carburized hardened layer of the carburized component of the present invention has a C concentration of 0.9% or more and a structure containing bainite and cementite, but when the hardness of the carburized hardened layer is measured, Needs to be 650 Hv or more. When the maximum hardness is less than 650 Hv, the hardness is insufficient, and the bending fatigue strength of a part having a smooth surface such as a shaft or a shaft part of a shaft-equipped gear becomes low (see examples described later). Therefore, the maximum hardness should just be 650 Hv or more, Preferably it is 660 Hv or more, More preferably, it is 670 Hv or more. However, if the surface becomes too hard, the fatigue characteristics deteriorate on the contrary, so the upper limit is about 900 Hv. Preferably it is 850 Hv or less. The method for measuring the hardness of the carburized hardened layer will be described in detail in the column of Examples described later.

  The thickness of the carburized hardened layer formed on the surface of the carburized component is not particularly limited, but at least 0.3 mm is necessary to effectively exhibit the effect of carburizing. Preferably it is 0.35 mm or more, more preferably 0.4 mm or more. The upper limit of the thickness of the carburized hardened layer is not particularly limited, but is generally about 1 mm. A preferable upper limit is 0.8 mm.

  In the present invention, the thickness of the carburized hardened layer means the effective hardened layer depth defined in JIS G0557, and when the Vickers hardness of the surface layer portion of the carburized part is measured, the hardness becomes 550 HV or more. Refers to the thickness of the region.

Next, a method for producing the carburized part of the present invention will be described. In the manufacturing method of the present invention, the base material (material steel) having the above-described component composition is carburized at a high concentration with [1] carbon potential of 0.9 to 1.5%, thereby carburizing the surface of the base material. By dissolving C in the matrix of the base material, the surface hardness of the carburized component is increased, and it is easy to generate cementite in the step [2] described later, and then [2] 820 after high concentration carburization. By holding at ~ 890 ° C for 30 minutes or longer, cementite is generated in the carburized hardened layer, and then [3] held at a temperature range of Ms point to Ms point + 90 ° C calculated by the following formula (1) for 30 minutes or longer. Thus, it can be produced by converting the structure of the carburized hardened layer to bainite. Hereinafter, it demonstrates along the process of said [1]-[3], referring FIG. 3 and FIG.
Ms point (° C.) = 550−350 × [C] −40 × [Mn] −20 × [Cr] + 30 × [Al] −10 × [Cu] −17 × [Ni] −10 × [Mo] −35 × [V] (1)
In the formula, [] means the content (% by mass) of a chemical component contained in austenite on the surface of the part after high-concentration carburization.

[1]
First, as shown in FIG. 3, the base material having the above component composition is subjected to high-concentration carburization at a temperature T1 (° C.), a time t1, and a carbon potential (Cp) of 0.9 to 1.5%. Form. If the carbon potential is less than 0.9%, the amount of C dissolved in the matrix phase is small, so that a predetermined amount of cementite cannot be generated in the carburized hardened layer in the step [2] described later. If the amount of cementite is small, the hardness of the carburized hardened layer cannot be secured, and the bending fatigue strength of a part having a particularly smooth surface cannot be sufficiently increased. Therefore, the carbon potential when high-concentration carburizing is 0.9% or more. Preferably it is 0.95% or more, More preferably, it is 1% or more. However, if the carbon potential exceeds 1.5%, excessive carburization occurs and coarse cementite remains, which adversely affects fatigue characteristics and causes variation.

  A method for carburizing at a high concentration is not particularly limited, and for example, a method such as gas carburizing, vacuum carburizing, or ion carburizing (plasma carburizing) may be employed. The conditions for high-concentration carburizing are not particularly limited. In the case of gas carburizing, the high-concentration carburizing temperature T1 may be about 900 to 950 ° C. and the high-concentration carburizing time t1 may be about 1 to 8 hours. In the case of vacuum carburizing or ion carburizing (plasma carburizing), processing at a higher temperature and shorter time than gas carburizing is possible and may be performed at about 950 to 1100 ° C.

[2]
After high-concentration carburization in [1] above, as shown in FIG. 4, the temperature t2 (° C.) is maintained for a time t2. The holding temperature T2 is set to a temperature range of 820 to 890 ° C., and the holding time t2 is set to 30 minutes or more. By maintaining the temperature in this temperature range for a predetermined time, cementite can be precipitated in the carburized hardened layer, the carburized hardened layer becomes hard, and the bending fatigue strength of a part having a particularly smooth surface is improved.

  The holding temperature T2 is preferably 830 ° C. or higher, more preferably 840 ° C. or higher. On the other hand, the holding temperature T2 is preferably 880 ° C. or lower, more preferably 870 ° C. or lower.

  Further, when the holding time t2 is less than 30 minutes, cementite is not sufficiently precipitated. Accordingly, the holding time is 30 minutes or more. Preferably it is 1 hour or more, More preferably, it is 2 hours or more. The upper limit of the holding time is not particularly limited, but even if it is held for a long time, the precipitation amount of cementite does not increase and the productivity is lowered, so the upper limit may be about 3 hours.

  The atmosphere when holding in the temperature range T2 is not particularly limited, but may be an inert gas atmosphere, for example. Moreover, a carburizing atmosphere may be sufficient and the carbon potential (Cp) at this time is not specifically limited.

After high concentration carburizing, when is kept at the temperature range T2, after carburization, once, after cooling to A ₁ below the transformation point (for example, about 600 ° C.), and cementite to precipitate by reheating to the temperature range T2 may be, or after the high-concentration carburization, once, without cooling below the a ₁ transformation point, in the middle cooling may be held at the above temperature range T2. Preferably prevent coarsening of cementite by that the former method is adopted may be cooled below the A ₁ transformation point.

  The conditions for cooling are not particularly limited, but the cooling rate should be increased. This is because increasing the cooling rate can prevent coarsening of the precipitated cementite.

[3]
After holding for the predetermined time t2 in the temperature range T2 of [2], as shown in FIG. 4, the temperature is held at the temperature T3 (° C.) for the time t3. The holding temperature T3 (° C.) is a temperature range of Ms point to Ms point calculated by the above formula (1) to 90 ° C., and the holding time t3 is 30 minutes or more. This step is important for obtaining a predetermined bainite structure, and thereby the fatigue strength of a notched part such as a gear tooth base is particularly improved. That is, after high-concentration carburizing, the process of [2] and [3] is not performed and the cooling is performed as it is, or after high-concentration carburizing, heating is performed in the temperature range T2 as described in [2]. When it is held and cooled to below the Ms point, the carburized hardened layer is not bainite but martensite. Since martensite is harder than bainite, the part itself becomes too hard, and the bending fatigue strength when stress is concentrated becomes low. On the other hand, by maintaining the temperature range T3, the structure of the carburized hardened layer can be bainite, and thereby the bending fatigue strength when stress is concentrated can be increased.

  The present invention is characterized in that the holding temperature T3 after quenching is set using the Ms point determined based on the components in the steel, as shown by the following equation (1). This is because by setting the holding temperature using the Ms point, it is possible to suppress variations in bending fatigue characteristics due to the composition of the raw steel. That is, the reason why the temperature is maintained at a temperature equal to or higher than the Ms point is that the austenite is left as it is without martensitic transformation of the surface structure, and bainite is formed when cooled. The holding temperature is preferably Ms point + 10 ° C. or higher, more preferably Ms point + 20 ° C. or higher.

  The upper limit of the holding temperature is Ms point + 90 ° C. Even if kept at a temperature exceeding the Ms point + 90 ° C., the carburized hardened layer becomes bainite, but the hardness of the entire carburized component tends to be low, and the bending fatigue strength when stress is concentrated is low. . In other words, the C concentration on the outermost surface of the carburized component is increased by high-concentration carburization, but the C concentration is decreased as it goes into the carburized component. When the C concentration decreases, the martensite transformation temperature (Ms point) calculated based on the following formula (1) increases, so that martensite is easily generated. If this martensite structure is maintained at a temperature exceeding the Ms point + 90 ° C., supersaturated carbon in the martensite precipitates and is tempered to reduce the internal hardness, resulting in the hardness of the carburized hardened layer. This also decreases the bending fatigue strength when stress is concentrated. Therefore, in this invention, holding temperature shall be Ms point +90 degrees C or less, Preferably it shall be Ms point +80 degrees C or less.

The Ms point can be calculated from the following equation (1).
Ms point (° C.) = 550−350 × [C] −40 × [Mn] −20 × [Cr] + 30 × [Al] −10 × [Cu] −17 × [Ni] −10 × [Mo] −35 × [V] (1)
In the formula (1), [] is substituted with the content (% by mass) of a chemical component contained in austenite on the part surface after high-concentration carburizing. When the base material is carburized at a high concentration with a carbon potential of 0.9 to 1.5% as in the present invention, the carburized hardened layer after carburizing has a two-phase structure of austenite and cementite, and the chemical composition of the base material containing C is In the present invention, the Ms point is calculated based only on the concentration of chemical components contained in austenite that can undergo martensite transformation.

Here, the amount of C dissolved in austenite (X mass%) can be calculated using the temperature at the Acm point of the equilibrium diagram of the steel material. In the component composition of the steel material used in the present invention, the following equation (2) It can be calculated from (2) In formula, T2 means the heating temperature (degreeC) when it heat-holds after high temperature carburizing (refer FIG. 2).
X = (T2-623.05) /273.17 (2)
For example, when carbon potential (Cp) is carburized at a high concentration of 1.1% and then held at 860 ° C. (= T2) for 1 hour (= t2), the amount of C contained in austenite after high concentration carburizing is As shown in the following formula (3), it becomes 0.87%. Therefore, the amount of C dissolved in cementite is 0.23%.
X = (860−623.05) /273.17=0.87 (%) (3)
On the other hand, when normal carburizing is performed with the carbon potential (Cp) being less than 0.9%, cementite is not generated, so the amount of C dissolved in austenite is the same value as the carbon potential during carburizing. For example, when carburizing with a carbon potential of 0.8%, the amount of C dissolved in austenite is 0.8%.

  On the other hand, for the content of chemical components other than C, the amount contained in the raw steel may be substituted as it is without using the Acm point as calculated for the amount of C described above.

  By the way, even if it hold | maintains in the said temperature range T3, if the holding time t3 is less than 30 minutes, the austenite of a carburized hardened layer will not be bainite, but a martensite will produce | generate. Accordingly, the holding time is 30 minutes or more. Preferably it is 1 hour or more, More preferably, it is 1.5 hours or more. The upper limit of the holding time is not particularly limited, but the upper limit may be about 8 hours in order to ensure the hardness of the carburized part and increase the bending fatigue strength of the carburized part having a stress concentration portion.

The conditions for cooling and quenching in the temperature range of Ms point to Ms point + 90 ° C. calculated by the above formula (1) after being held in the temperature range T2 are not particularly limited. For example, the solvent used for the quenching layer For example, quenching oil or a salt bath may be used. Examples of the salt bath include KNO ₃ —NaNO ₂ which is generally used as a low temperature salt bath.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  Steels A to E having the composition shown in Table 1 (the balance being Fe and inevitable impurities) were melted in a vacuum induction melting furnace to produce a 150 kg slab. The cast slab was heated to 1200 ° C. and then hot worked to obtain a round bar with a diameter of 30 mm. The obtained round bar was subjected to a solution treatment (1250 ° C. × 30 minutes → air cooling), and then subjected to a normalizing treatment under conditions (900 ° C. × 1 hour) assuming rolling in an actual machine.

  The normalized round bar was machined into a structure observation / hardness measurement specimen, a four-point bending fatigue test specimen, or a rotating bending fatigue test specimen as shown below. Of these, the four-point bending fatigue test is a test for measuring the fatigue strength when stress is concentrated on the carburized part, and is a test that simulates the stress state of the tooth base of the shaft gear. On the other hand, the rotating bending fatigue test is a test for measuring the fatigue strength of a carburized part having a smooth surface, and is a test simulating a shaft portion and a shaft of a gear with a shaft.

  As a specimen for tissue observation / hardness measurement, a round bar machined to φ26 mm × 80 mm was used. As a test piece for a four-point bending fatigue test, as shown in FIG. 1 (a), the one provided with a notch shown in FIG. 1 (b) at the center of a 13 mm × 100 mm square bar was used. As the test piece for the rotating bending fatigue test, a round bar machined to φ12 mm × 80 mm and machined into the shape shown in FIG. 2 was used.

  The structure observation / hardness measurement specimen, the four-point bending fatigue test specimen, or the rotational bending fatigue test specimen processed as described above was heated at a temperature T1 of 930 ° C. as shown in FIG. After carburizing (high concentration carburizing shown in Table 2 or normal carburizing), oil quenching was performed at 50 ° C. Specifically, the high-concentration carburization was performed at a carbon potential (Cp) of 1.1% and a time t1 of 2 hours, and the normal carburization was performed at a carbon potential (Cp) of 0.8% and a time t1 of 2 hours. . The cooling rate during oil quenching is approximately 180 ° C./min.

  The Ms point of each test piece subjected to high-concentration carburization or normal carburization as described above was calculated based on the method described above. The calculation results are also shown in Table 2 below.

  Next, after oil quenching, as shown in FIG. 4, T2 was heated again at 860 ° C., and at this temperature, time t2 was maintained as 1 hour, followed by salt bath quenching, and various salt bath temperatures shown in Table 2 At T3, time t3 was maintained as 2 hours.

  About the test piece which carried out the salt bath hardening, C amount of the carburized hardened layer was measured based on the method mentioned above. In the example of the present invention, taking into account the machining allowance (usually about 10 to 30 μm by finishing polishing) when machining into parts such as gears, the C amount measurement position is 50 μm deep from the surface of the test piece. The position. From the same viewpoint as described above, the structure observation (described later) of the carburized hardened layer was also performed at a depth of 50 μm from the surface of the test piece.

  Next, the hardness in the area | region from the surface of a test piece to 2 mm in depth was measured in the following procedure using the test piece for structure | tissue observation and hardness measurement. The center part in the longitudinal direction of the test piece for tissue observation / hardness measurement was wet-cut and the hardness of the cut surface was measured with a Vickers hardness meter (load 300 gf). The hardness was measured from the surface to a depth of 100 μm at a pitch of 25 μm, and from the depth of 100 μm to 2 mm at a pitch of 100 μm. However, the hardness at the 25 μm position and the 50 μm position from the surface was measured for each of 5 points, and the average value was used. The maximum hardness of the hardness at each depth position is obtained, and the results are shown in Table 2 below.

  In addition, the center part in the longitudinal direction of the test piece for tissue observation / hardness measurement is wet-cut, and the hardness of the cut surface is measured in a region from the surface of the test piece to a depth of 3 mm with a Vickers hardness meter (load 300 gf). did. At this time, the hardness is measured at a position of 0.05 mm depth from the surface and a region with a depth of 0.1 mm to 3 mm at a pitch of 0.1 mm, and a region having a hardness of 550 HV or more is a carburized hardened layer, The thickness (effective carburized layer depth) was measured. The results are shown in Table 2 below.

  Moreover, using the test piece for structure | tissue observation and hardness measurement, the metal structure in the 50-micrometer depth position from the surface of the test piece was observed in the following procedure. The center part in the longitudinal direction of the test piece for tissue observation / hardness measurement is wet-cut, and the cut-off surface of the structure at a depth of 50 μm from the surface of the test piece is 1600 times using a scanning electron microscope (SEM). Observe and determine the structure (bainite or martensite). The distinction between bainite and martensite was determined with reference to “Steel Baynite Photobook-1” (edited by the Japan Iron and Steel Institute Basic Research Group, Bainite Research Group).

  For cementite, the same part as described above was observed at a magnification of 8000 using a scanning electron microscope (SEM), the cementite part was identified with image analysis software, and the cementite area ratio was measured. Further, the same portion as above was analyzed with a micro X-ray analyzer (“RINT 2500V” manufactured by Rigaku) to measure the residual austenite area ratio. The area ratio of bainite or martensite was a value obtained by subtracting the cementite area ratio and the residual austenite area ratio from 100. The area ratio of each tissue is shown in Table 3 below. In this example, a case where the total area ratio of bainite and cementite was 70% or more was evaluated as pass (◯), and the other cases were evaluated as fail (x), and the results are shown in Table 3 below. In Table 3, the column of “structure” shows the main metal structure, B means bainite, M means martensite, θ means cementite, and residual γ means retained austenite. The area% may be considered to be equal to the volume fraction (volume%).

Next, the four-point bending fatigue test specimen or the rotational bending fatigue test specimen was shot peened under the conditions shown below, and a fatigue strength test was performed.
<Shot peening conditions>
・ Shot system: Air nozzle system ・ Shot grain: CW0.6 (700HV)
・ Projection pressure: 3.0 kg / cm ³
・ Projection distance: 170mm
・ Projection time: 9 seconds ・ Arc height: 0.7 mmA
・ Turntable is fixed (no rotation)

  The four-point bending fatigue test is performed at 8 levels by changing the load between a load of 10000 N (stress 1523 MPa) to 20000 N (3026 MPa) at a frequency of 20 Hz, and a stress corresponding to 20,000 times life is obtained. It was used as an index. In this example, the stress was evaluated as pass (◯) when the stress was 2000 MPa or more, and reject (x) when it was less than 2000 MPa. The measurement results and evaluation results are shown in Table 3 below.

  The rotating bending fatigue test was performed using an Ono type rotating bending fatigue testing machine in accordance with JIS Z2274 “Rotating bending fatigue testing method for metal materials”. In this example, the stress was evaluated as pass (◯) when the stress was 1000 MPa or more, and reject (x) when less than 1000 MPa. The measurement results and evaluation results are shown in Table 3 below.

  In the column of “Comprehensive evaluation” in Table 3, “○” is given to those satisfying all the requirements of the present invention, and “X” is marked to those not satisfying any of the requirements of the present invention.

  From Tables 1 to 3, it can be considered as follows. No. 2 to 4, 14 to 16, 21 to 23, 26, and 27 to 28 are excellent in both the four-point bending fatigue characteristics and the rotational bending fatigue characteristics.

  Other than the above, all are comparative examples that do not meet any of the requirements defined in the present invention. Of these, No. 1, 13, 19, 20, 25, and 31 are comparative examples in which the holding temperature T3 is too high, the maximum hardness is insufficient, and the rotational bending fatigue characteristics are inferior. No. 5, 6, 11, 12, 17, 18, 24, 29, 30, 35, 36 are all comparative examples in which the holding temperature T3 is too low, and a predetermined bainite is not generated in the carburized hardened layer. Bending fatigue properties are inferior. No. Nos. 7 to 12 are comparative examples in which normal carburization was performed with a carbon potential of 0.8%, and carbide (cementite) was not generated in the carburized hardened layer, and the rotational bending fatigue characteristics were inferior. No. Reference numerals 31 to 34 are examples of simulating the heat treatment pattern of Patent Document 1 described above. Since these steel types that contain a small amount of Si contained in the base material were used, the maximum hardness decreased and the rotation decreased even though it was controlled within the predetermined temperature range within the holding temperature T3 specified in the present invention. Bending fatigue properties are degraded.

1A and 1B are schematic views of a test piece for a four-point bending test used in the examples. FIG. 1A is a schematic view of the test piece, and FIG. 1B is a shape of a notch. FIG. 2 shows a schematic view of the test piece for the rotary bending test used in the examples. FIG. 3 is a diagram showing a heat pattern of the step [1] of the present invention. FIG. 4 is a diagram showing a heat pattern of steps [2] and [3] of the present invention.

Claims (7)

The base material is mass%.
C: 0.05 to 0.3%
Si: more than 0.35%, 2% or less,
Mn: 3% or less (excluding 0%),
Cr: 2% or less (excluding 0%),
Al: 0.1% or less (excluding 0%),
N: 0.03% or less (not including 0%)
P: 0.03% or less,
S: 0.03% or less,
O: satisfies 0.002% or less,
The balance consists of Fe and inevitable impurities,
The C concentration of the carburized hardened layer is 0.9% or more, and its structure is excellent in bending fatigue characteristics characterized by containing bainite and 50% by volume and cementite and having a maximum hardness of 650 Hv or more . Carburized parts. The base material is still another element,
The carburized part according to claim 1, containing Cu: 1% or less (not including 0%) and / or Ni: 3% or less (not including 0%). The base material is still another element,
The carburized part according to claim 1 or 2, containing Mo: 1% or less (not including 0%). The base material is still another element,
Ti: 0.2% or less (excluding 0%),
4. One or more selected from the group consisting of V: 0.3% or less (not including 0%) and Nb: 0.2% or less (not including 0%) Carburized parts as described. The base material is still another element,
B: The carburized component according to any one of claims 1 to 4, containing 0.003% or less (excluding 0%). The base material is still another element,
Ca: 0.02% or less (excluding 0%),
Any one or more selected from the group consisting of Mg: 0.02% or less (excluding 0%) and Te: 0.05% or less (not including 0%) is provided. Carburized parts as described. A method for producing a carburized part according to claim 1,
A step of carburizing the steel satisfying the chemical component according to any one of claims 1 to 6 at a carbon concentration of 0.9 to 1.5% at a high concentration;
Holding at 820-890 ° C. for 30 minutes or more;
A step of maintaining for 30 minutes or more in a temperature range of Ms point to Ms point + 90 ° C. calculated by the following equation (1);
In this order, a method for manufacturing carburized parts.
Ms point (° C.) = 550−350 × [C] −40 × [Mn] −20 × [Cr] + 30 × [Al] −10 × [Cu] −17 × [Ni] −10 × [Mo] −35 × [V] (1)
In the formula, [] means the content (% by mass) of a chemical component contained in austenite on the surface of the part after high-concentration carburization.

Images