JP6477129B2 - Product member manufacturing method and product member - Google Patents

Description

  The present invention relates to a method for manufacturing a product member, and more particularly, to a method for manufacturing a product member to be carburized and quenched. The present invention also relates to a product member.

  Some product members (for example, machine structural parts) used in automobiles and industrial machines are subjected to carburizing and quenching, which is a kind of surface hardening treatment.

  As a manufacturing method of the product member which performs carburizing quenching, the following method is mentioned, for example. That is, first, a rough member having a shape close to the final product is manufactured. The manufactured rough member is cut to produce an intermediate member closer to the final product. Carburizing and quenching and grinding are sequentially performed on the intermediate member to obtain a product member.

  Usually, a product member is required to have excellent fatigue strength. Techniques for increasing fatigue strength are disclosed in JP-A-3-120313 (Patent Document 1), JP-A-8-260125 (Patent Document 2), and JP-A-2007-21070 (Patent Document 3). .

  Patent Document 1 discloses a technique in which a steel member is subjected to shot peening for improving strength, and the extreme surface layer portion is further cut. In Patent Document 1, it is said that the fatigue characteristics and the machinability of the shot peening portion are improved by the technique.

  Patent Document 2 discloses a technique of grinding and removing a surface hardened layer after carburizing and tempering to form a surface hardened layer. In Patent Document 2, the fatigue strength is increased by the technique.

  Patent Document 3 discloses a technique for turning a metal member using a tool having a rake face inclined in a negative direction in a range of 30 to 50 °. In Patent Document 3, it is said that the fatigue strength can be increased by applying compressive residual stress without applying a process such as shot peening by the technique.

Japanese Patent Laid-Open No. 3-120313 JP-A-8-260125 Japanese Patent Laid-Open No. 2007-21070

  By the way, both excellent wear resistance and bending fatigue strength are required for recent product members. However, in the product member obtained by the techniques disclosed in Patent Documents 1 to 3, it may be difficult to achieve both wear resistance and fatigue strength because the surface layer structure is inappropriate. Furthermore, in consideration of cost, it is also desired to suppress tool wear during cutting.

  The present invention has been made in view of the above circumstances, and provides a manufacturing method of a product member that can achieve both wear resistance and fatigue strength and can suppress tool wear during cutting (excellent machinability). The purpose is to do. Another object of the present invention is to provide a product member having the above characteristics.

  The inventors of the present invention have intensively studied a manufacturing method capable of suppressing a cutting wear during cutting and obtaining a product member excellent in wear resistance, fatigue strength, and machinability. As a result, by controlling the chemical composition of the rough member that is the material of the product member, the metal structure and hardness after carburizing, and the metal structure and arithmetic average roughness after cutting, wear resistance, bending fatigue strength and We have obtained knowledge that we can manufacture product parts with excellent machinability. Based on the above findings, the present inventors have completed the invention. The summary is as follows.

[1] By mass%, C: 0.1 to 0.3%, Si: 0.25% or less, Mn: 0.4 to 2.0%, P: 0.050% or less, S: 0.005 -0.020%, Cr: 0.4-3.5%, Al: 0.010-0.050%, N: 0.010-0.025% and O: 0.003% or less, And the balance consists of Fe and unavoidable impurities, a process of obtaining a rough member by processing a steel material having a chemical composition satisfying the formulas (1) and (2),
Carburizing treatment, isothermal holding treatment, quenching treatment and tempering treatment for the rough member to obtain a carburized material,
The carburizing temperature is 900 ° C. or higher and 1050 ° C. or lower, the carbon potential during carburizing is 0.7% or higher and 1.1% or lower, the carburizing time is 60 minutes or longer and 240 minutes or shorter, and the constant temperature holding temperature is 820 ° C. or higher and 870 ° C. or lower. The carbon potential during the constant temperature holding treatment is 0.7% or more and 0.9% or less, the tempering temperature is 160 ° C. or more and 200 ° C. or less, and the tempering time is 60 minutes or more and 180 minutes or less.
In the carburized material, the structure at the reference position corresponding to the depth position of 20 μm from the position corresponding to the surface of the product member which is the final form contains martensite and residual austenite of 12 to 35% by volume ratio. , A step in which the phase other than the martensite and retained austenite is 3% or less by volume,
A process of obtaining a product member by cutting the carburized material,
The rake angle of the cutting tool is -30 ° to -5 ° or less, the tool nose is 0.4 mm to 1.2 mm, the feed is 0.1 mm / rev to 0.4 mm / rev and less, and the cutting speed is 50 m. / Min or more and 150 m / min or less, and the incision is 0.05 mm or more and 0.2 mm or less,
In the structure at the reference position, the volume fraction of retained austenite is 20% or less, and the residual austenite volume ratio (RI) before cutting and the residual austenite volume ratio (RF) after cutting are obtained by the formula (A). Austenite reduction rate Δγ is 35% or more, arithmetic mean roughness Ra of the surface is 0.8 μm or less, and further, tool wear during cutting is suppressed,
A method for manufacturing a product member.
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr ≧ 2.35 (1)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr ≦ 33.8 (2)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (1) and (2).
Δγ = (RI−RF) / RI × 100 (A)

  [2] The method for producing a product member according to the above [1], wherein the steel material contains Pb: 0.5% or less instead of part of Fe.

  [3] In the above [1] or [2], the steel material contains one or more selected from the group consisting of V and Ti instead of a part of Fe in a total content of 0.1% or less. The manufacturing method of the product member of description.

[4] The steel material contains at least one selected from the group consisting of Mo: 3.0% or less and Ni: 2.5% or less, instead of a part of Fe, and the formula (1) The method for producing a product member according to any one of [1] to [3], wherein the formula (3) and the formula (4) are satisfied instead of the formula (2).
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr + 1.77 × Mo + 0.63 × Ni ≧ 2.35 (3)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr + 6.7 × Mo + 6.2 × Ni ≦ 33.8 (4)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (3) and (4).

[5] By mass%, C: 0.1 to 0.3%, Si: 0.25% or less, Mn: 0.4 to 2.0%, P: 0.050% or less, S: 0.005 -0.020%, Cr: 0.4-3.5%, Al: 0.010-0.050%, N: 0.010-0.025% and O: 0.003% or less, And the balance consists of Fe and inevitable impurities, and has a chemical composition satisfying the formulas (5) and (6),
C content of the surface layer part is 0.6 to 1.0%,
The structure at a depth of 20 μm from the surface is 97% or more in total of martensite and retained austenite,
The maximum retained austenite volume fraction in the range from the surface to a depth of 200 μm is 10 to 30%,
The expected retained austenite reduction rate Δγp determined by the formula (B) from the retained austenite volume ratio (R2) and the maximum retained austenite volume ratio (R1) at the depth position is 20% or more,
It has a plastic flow structure with a thickness of 1 to 15 μm on the surface,
The arithmetic average roughness Ra of the surface is 0.8 μm or less,
A product member characterized by that.
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr ≧ 2.35 (5)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr ≦ 33.8 (6)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (5) and (6).
Δγp = (R1−R2) / R1 × 100 (B)

  [6] The product member according to the above [5], which contains Pb: 0.5% or less instead of part of Fe.

  [7] The product member according to the above [5] or [6], which contains, in place of a part of Fe, one or more selected from the group consisting of V and Ti in a total content of 0.1% or less .

[8] Instead of a part of Fe, containing at least one selected from the group consisting of Mo: 3.0% or less, Ni: 2.5% or less, and in formula (5) and formula (6) Instead, the product member according to any one of [5] to [7], which satisfies formula (7) and formula (8).
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr + 1.77 × Mo + 0.63 × Ni ≧ 2.35 (7)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr + 6.7 × Mo + 6.2 × Ni ≦ 33.8 (8)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (7) and (8).

  [9] The product member according to any one of [5] to [8] obtained by carburizing, quenching, and tempering and cutting the steel material.

  In the manufacturing method of the product member according to the present invention, on the assumption that the chemical composition of the rough member which is the material of the product member is adjusted, in particular, the metal structure and hardness of the carburized material after quenching, and the product member after cutting Improvements have been made to the metal structure and arithmetic average roughness. As a result, according to the product member manufacturing method of the present invention, a product member excellent in wear resistance, bending fatigue strength, and machinability can be obtained.

FIG. 1 is a scanning electron microscope image of a surface perpendicular to the axial direction of the product member. FIG. 2 is a side view of a wear test piece used in a two-cylinder wear test. FIG. 3 is a side view of a fatigue test piece used in the Ono type rotating bending fatigue test. FIG. 4 is a front view showing a two-cylinder wear test method.

  Hereinafter, embodiments of the present invention (hereinafter, simply referred to as “embodiments”) will be described in detail with reference to the drawings. These embodiments do not limit the present invention. The constituent elements of the above embodiment include those that can be easily replaced by those skilled in the art or those that are substantially the same. Furthermore, various forms included in the above-described embodiments can be arbitrarily combined within a range obvious to those skilled in the art. In the drawings, the same or corresponding members are denoted by the same reference numerals and description thereof will not be repeated.

<Production member manufacturing method>
The manufacturing method of the product member of the present embodiment includes a step of processing a steel material to obtain a rough member (a rough member manufacturing step), and carburizing, constant temperature holding, quenching, and tempering of the rough member. A process of obtaining a material (carburized material manufacturing process) and a process of cutting the carburized material to obtain a product member (product member manufacturing process).

[Coarse member manufacturing process]
In this step, a rough member having a desired shape close to the shape of the product member is manufactured. First, a steel material is prepared.

(Chemical composition of steel (required requirement))
The steel material has the following chemical composition. In addition, the ratio (%) of each element shown below means mass%.

C: 0.1 to 0.3%
Carbon (C) increases the strength of the product member (particularly the strength of the core). If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, the strength of the steel material becomes too high, and the machinability of the rough member decreases. Therefore, the C content is 0.1 to 0.3%. The minimum with preferable C content is 0.15%. The upper limit with preferable C content is 0.25%.

Si: 0.25% or less Silicon (Si) reacts with the tool and causes adhesive wear during cutting after carburizing and tempering. Therefore, Si increases tool wear. Therefore, the Si content is 0.25% or less. The upper limit with preferable Si content is 0.15%.

Mn: 0.4 to 2.0%
Manganese (Mn) increases the hardenability of the steel and increases retained austenite in the steel. Compared to austenite not containing Mn, austenite containing Mn is likely to undergo work-induced martensitic transformation during cutting. As a result, the wear resistance and bending fatigue strength of the product member are increased. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, residual austenite after carburizing and tempering becomes excessive. Therefore, sufficient work-induced martensitic transformation does not occur during cutting, and residual austenite is less likely to decrease after cutting. As a result, the wear resistance and bending fatigue strength of the product member after cutting are reduced. Therefore, the Mn content is 0.4 to 2.0%. The preferable lower limit of the Mn content is 0.8%. The upper limit with preferable Mn content is 1.8%.

P: 0.050% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and embrittles the steel. Therefore, the P content is 0.050% or less. The upper limit with preferable P content is 0.030%. The P content should be as low as possible.

S: 0.005-0.020%
Sulfur (S) combines with Mn to form MnS and enhances machinability. If the S content is too low, this effect cannot be obtained. On the other hand, if the S content is too high, coarse MnS is formed, and the hot workability, cold workability, wear resistance, and bending fatigue strength of the steel are reduced. Therefore, the S content is 0.005 to 0.020%. A preferable lower limit of the S content is 0.008%. The upper limit with preferable S content is 0.015%.

Cr: 0.4 to 3.5%
Chromium (Cr) increases the hardenability of the steel and further increases the retained austenite. If the Cr content is too low, this effect cannot be obtained. On the other hand, if the Cr content is too high, the retained austenite after carburizing and tempering becomes excessive. In this case, sufficient machining-induced martensite transformation does not occur during cutting in the product member manufacturing process, and residual austenite is less likely to decrease after cutting. As a result, the wear resistance and bending fatigue strength of the product member are reduced. Therefore, the Cr content is 0.4 to 3.5%. A preferable lower limit of the Cr content is 0.5%. The upper limit with preferable Cr content is 3.1%.

Al: 0.010 to 0.050%
Aluminum (Al) deoxidizes steel. Al is further combined with N to form AlN, and the crystal grains are refined. As a result, the strength of the steel is increased. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, hard and coarse Al ₂ O ₃ is produced, the machinability of the steel is lowered, and the wear resistance and bending fatigue strength are also lowered. Therefore, the Al content is 0.010 to 0.050%. The minimum with preferable Al content is 0.020%. The upper limit with preferable Al content is 0.040%.

N: 0.010 to 0.025%
Nitrogen (N) forms nitrides to refine crystal grains and increases the bending fatigue strength of steel. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, coarse nitrides are generated and the toughness of the steel is reduced. Therefore, the N content is 0.010 to 0025%. The minimum with preferable N content is 0.012%. The upper limit with preferable N content is 0.020%.

O: 0.003% or less Oxygen (O) is an impurity. O combines with Al to form hard oxide inclusions. Oxide inclusions reduce the machinability of steel and also reduce the wear resistance and bending fatigue strength. Accordingly, the O content is 0.003% or less. The lower the O content, the better.

  The balance of the chemical composition of the steel material is Fe and inevitable impurities. The inevitable impurities mean components that are mixed from ore and scrap used as a raw material of steel, or the environment of the manufacturing process, and are not components intentionally included in steel materials.

(Relationship of the content of each element (required requirement))
The relationship of the content of each element constituting the steel material satisfies the following expressions (1) and (2).
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr ≧ 2.35 (1)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr ≦ 33.8 (2)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (1) and (2).

About Formula (1) It defines as F1 = 1.54 * C + 0.81 * Si + 1.59 * Mn + 1.65 * Cr. F1 is a parameter representing the hardenability of steel. If F1 is too low, the hardenability of the steel will be low. In this case, ferrite and pearlite having low strength are generated, and the wear resistance and bending fatigue strength of the product member are lowered. Therefore, F1 is 2.35 or more. A preferred lower limit of F1 is 3.0. The upper limit of F1 is preferably set to 8.0 to ensure the toughness of the product member.

About Formula (2) It defines as F2 = -0.1 * Si + 15.2 * Mn + 7.0 * Cr. F2 is a parameter representing the stability of austenite. If F2 is too low, the wear resistance of the product member will be low. On the other hand, if F2 is too high, residual austenite after carburizing and tempering becomes excessive. In this case, processing-induced martensitic transformation does not occur during cutting. Therefore, the wear resistance and bending fatigue strength of the product member are reduced. Therefore, F2 is 11.3 or more and 33.8 or less. A preferred lower limit of F2 is 12.0. The preferable upper limit of F2 is 33.0.

(Chemical composition of steel (optional requirement))
The steel material may further contain Pb instead of part of Fe.

Pb: 0.5% or less Lead (Pb) is an optional element and may not be contained. When contained, reduction in tool wear and improvement in chip disposal are realized. However, if the Pb content is too high, the strength and toughness of the steel decrease, and the wear resistance and bending fatigue strength also decrease. Therefore, the Pb content is preferably 0.5% or less. A more preferable upper limit of the Pb content is 0.4%. In addition, in order to acquire said effect, it is preferable to make Pb content 0.03% or more.

  The steel material may further contain one or more selected from the group consisting of V and Ti instead of a part of Fe.

V and Ti: 0.1% or less in total content Vanadium (V) and titanium (Ti) are optional elements and may not be contained. These elements combine with C and N to form precipitates. Precipitates of these elements complement the grain refinement of the quenched part by AlN. Precipitates of these elements increase the wear resistance of the product member. However, if the total content of these elements exceeds 0.1%, the precipitates become coarse, and the bending fatigue strength and machinability decrease. Therefore, the total content of V and Ti is preferably 0.1% or less. If any one or more of V and Ti is contained as an optional element, the above effect can be obtained. A more preferable upper limit of the total content of V and Ti is 0.08%. The minimum with preferable total content of V and Ti is 0.01%.

  The steel material may further contain one or more selected from the group consisting of Mo and Ni instead of a part of Fe. All of these elements increase the hardenability of the steel and increase the retained austenite.

Mo: 3.0% or less Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo increases the hardenability of the steel and increases the retained austenite. Mo further increases temper softening resistance and wear resistance. However, if the Mo content is too high, residual austenite after carburizing and tempering becomes excessive. In this case, sufficient work-induced martensitic transformation does not occur during cutting. As a result, the wear resistance of the product member is reduced. Therefore, the Mo content is preferably 3.0% or less. A more preferable upper limit of the Mo content is 2.0%. A preferable lower limit of the Mo content is 0.1%.

Ni: 2.5% or less Nickel (Ni) is an optional element and may not be contained. When contained, Ni increases the hardenability of the steel and increases the retained austenite. Ni further increases the toughness of the steel. However, if the Ni content is too high, residual austenite after carburizing and tempering becomes excessive. In this case, sufficient work-induced martensitic transformation does not occur during cutting after tempering. As a result, the wear resistance of the product member is reduced. Therefore, the Ni content is preferably 2.5% or less. A more preferable upper limit of the Ni content is 2.0%. A preferable lower limit of the Ni content is 0.1%.

(Relationship of content of each element (optional requirement))
The relationship of the content of each element constituting the steel material preferably satisfies the following formulas (3) and (4).
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr + 1.77 × Mo + 0.63 × Ni ≧ 2.35 (3)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr + 6.7 × Mo + 6.2 × Ni ≦ 33.8 (4)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (3) and (4).

About Formula (3) It defines as F3 = 1.54 * C + 0.81 * Si + 1.59 * Mn + 1.65 * Cr + 1.77 * Mo + 0.63 * Ni. F3 is a parameter representing the hardenability of the steel. F3 is obtained by adding a Mo term and a Ni term to F1. If F3 is too low, the hardenability of the steel is lowered, and the wear resistance and bending fatigue strength of the product member are lowered. Therefore, F3 is preferably 2.35 or more. A more preferable lower limit of F3 is 3.0. In order to ensure the toughness of the product member, the preferable upper limit of F3 is 8.0.

About Formula (4) It defines as F4 = -0.1xSi + 15.2xMn + 7.0xCr + 6.7xMo + 6.2xNi. F4 is a parameter representing the stability of austenite. F4 is obtained by adding a Mo term and a Ni term to F2. Similar to F2, if F4 is too low or too high, the wear resistance and bending fatigue strength of the product member will decrease. Therefore, F4 is preferably 11.3 to 33.8. A more preferred lower limit of F4 is 12.0. A more preferable upper limit of F4 is 33.0.

(Manufacture of rough members)
A steel member having the above chemical composition is processed to obtain a rough member. A known method can be adopted as the processing method. Examples of the processing method include hot processing, cold processing, cutting processing, and the like. The rough member has a shape close to that of the product member.

[Carburized material manufacturing process]
The carburizing material is obtained by subjecting the coarse member obtained as described above to carburizing, isothermal holding, quenching and tempering. Thereby, in the carburized material, the structure at the reference position corresponding to the depth position of 20 μm from the position corresponding to the surface of the product member which is the final form is martensite and the retained austenite of 12 to 35% by volume ratio. In addition, other phases other than martensite and retained austenite are 3% or less by volume.

(Carburizing and quenching)
In the carburizing and quenching step, first, carburizing treatment is performed, and then a constant temperature holding treatment is performed. The carburizing process and the constant temperature holding process are performed under the following conditions.

(Carburization treatment)
Carburizing temperature (T1): 900-1050 ° C
If the carburizing temperature T1 is too low, the surface layer of the rough member is not sufficiently carburized. In this case, there is little retained austenite after carburizing and quenching, and the hardness of the surface layer is also low. Therefore, the wear resistance and bending fatigue strength of the product member are lowered. On the other hand, if the carburizing temperature T1 is too high, austenite grains become coarse and wear resistance and bending fatigue strength decrease. Therefore, the carburizing temperature T1 is 900 to 1050 ° C. A preferable lower limit of the carburizing temperature T1 is 910 ° C, and a preferable upper limit is 1000 ° C.

Carbon potential (Cp1): 0.7 to 1.1%
If the carbon potential Cp1 is too low, sufficient carburization is not performed. In this case, there is little retained austenite after carburizing and quenching, and the hardness of the surface layer is also low. Therefore, the wear resistance and bending fatigue strength of the product member are reduced. On the other hand, if the carbon potential is too high, hard pro-eutectoid cementite deposited during carburization remains over 3% even after carburizing and quenching. In this case, tool wear at the time of cutting in the product member manufacturing process increases, and the machinability of the carburized material decreases. Therefore, the carbon potential Cp1 is 0.7 to 1.1%. The carbon potential Cp1 may be varied within the above range during the carburizing process.

Carburizing time (t1): 60 to 240 minutes If the carburizing time (carburizing time) t1 is too short, sufficient carburizing is not performed. On the other hand, if the carburizing time t1 is too long, the productivity is lowered. Therefore, the carburizing time is 60 to 240 minutes.

(Constant temperature maintenance process)
After the carburizing process, a constant temperature holding process is performed. The constant temperature holding process is performed under the following conditions.

Constant temperature holding temperature (T2): 820-870 ° C.
If the constant temperature holding temperature T2 is too low, it becomes difficult to control the atmosphere such as the carbon potential. In this case, it is difficult to adjust the volume ratio of retained austenite. On the other hand, if the constant temperature holding temperature T2 is too high, the distortion generated at the time of quenching may increase, and a crack may occur. Accordingly, the constant temperature holding temperature T2 is 820 to 870 ° C.

Carbon potential (Cp2): 0.7-0.9%
If the carbon potential Cp2 during the constant temperature holding process is too low, C that has entered during carburizing is released to the outside again. In this case, there is little retained austenite after carburizing and quenching, and the surface layer hardness is also low. As a result, the wear resistance and bending fatigue strength of the product member are reduced. On the other hand, if the carbon potential Cp2 is too high, hard pro-eutectoid cementite precipitates. In this case, tool wear at the time of cutting in the product member manufacturing process increases, and the machinability of the carburized material decreases. Therefore, the carbon potential Cp2 is 0.7 to 0.9%.

Constant-temperature holding time (t2): 20 to 60 minutes If the constant-temperature holding time t2 is too short, the temperature of the rough member is not uniform, and the strain generated during quenching increases. In this case, burn cracking may occur in the carburized material. On the other hand, if the constant temperature holding time t2 is too long, the productivity is lowered. Therefore, the constant temperature holding time t2 is 20 to 60 minutes.

(Quenching process)
After the constant temperature holding treatment, a quenching treatment is performed by a well-known method. The quenching process can be, for example, oil quenching.

(Tempering treatment)
A tempering process is performed after carburizing and quenching. If the tempering treatment is performed, the toughness of the product member is increased. Furthermore, since C diffuses to produce a carbide precursor, the retained austenite becomes unstable, and processing-induced martensitic transformation is likely to occur during cutting, thereby improving wear resistance and bending fatigue strength. The tempering process is performed under the following conditions.

Tempering temperature (T3): 160-200 ° C
If the tempering temperature T3 is too low, the above-mentioned effect by tempering cannot be obtained. On the other hand, if the tempering temperature is too high, the retained austenite becomes extremely unstable, and the retained austenite decomposes during tempering. Furthermore, since the austenite remaining without being decomposed is released from the heat treatment strain and stabilized, sufficient work-induced martensitic transformation does not occur at the time of cutting and does not contribute to the improvement of wear resistance and bending fatigue strength. Accordingly, the tempering temperature T3 is 160 to 200 ° C.

Tempering time (t3): 60 to 180 minutes If the tempering time t3 is too short, the above tempering effect cannot be obtained. On the other hand, if the tempering time t3 is too long, the retained austenite becomes extremely unstable, and the retained austenite decomposes during tempering. Furthermore, since the austenite remaining without being decomposed is released from the heat treatment strain and stabilized, the processing-induced martensitic transformation does not occur sufficiently during the cutting process and does not contribute to the improvement of the wear resistance and the bending fatigue strength. Accordingly, the tempering time is 60 to 180 minutes.

(Carburized material structure and hardness after carburized material production process)
The position corresponding to the surface of the product member that is the final form of the carburized material obtained by performing the carburizing process, the constant temperature holding process, the quenching process, and the tempering process under the above-described conditions (hereinafter simply referred to as the “final surface position”) The structure at the reference position corresponding to the position of 20 μm deep contains martensite and 12 to 35% of retained austenite in volume ratio, and other phases other than martensite and retained austenite are in volume ratio. 3% or less. Here, the position corresponding to the surface of the product member which is the final form is a position obtained by subtracting a cut (thickness direction dimension) set at the time of cutting (product member manufacturing process) described later from the surface of the carburized material. Say.

  In addition, the structure observation of the reference position in the carburized material is performed by the following method. That is, in the carburized material, a test piece including a reference position that is 20 μm deep from a position corresponding to the surface of the product member as the final form is prepared. The specimen is corroded with a 5% nital solution. The corroded surface is observed with three optical fields using an optical microscope with a magnification of 1000 times. At this time, the reference position is set to the center of the visual field. 10 μm from the center of the field of view to the surface of the carburized material, 10 μm from the center of the field of view to the direction opposite to the surface of the carburized material, and a range of 20 μm × 100 μm, 50 μm from the center of the field to both directions perpendicular to the surface of the carburized material. In this plane, the area ratio of each phase is obtained by a normal image analysis method. The area ratio of each obtained phase is defined as the volume ratio of each phase.

  In structural observation with an optical microscope, retained austenite is contained in martensite. That is, the optical microscope cannot distinguish between martensite and retained austenite. Therefore, the volume ratio (RI) of the retained austenite of the carburized material is measured by the following method. In the carburized material, a sample including a reference position with a depth of 20 μm is taken from a position corresponding to the surface of the final product member. Electropolishing is performed on the surface including the reference position. An electrolytic solution containing 11.6% ammonium chloride, 35.1% glycerin, and 53.3% water is prepared. Using this electrolytic solution, electropolishing is performed at a voltage of 20 V on the surface including the reference position.

The electropolished surface is analyzed by the X-ray diffraction method. For X-ray diffraction, the trade name RINT-2500HL / PC manufactured by Rigaku Corporation is used. A Cr tube is used as the light source. Based on the integrated intensity ratio of the diffraction peaks of the (221) plane of the bcc structure and the (220) plane of the fcc structure obtained by X-ray diffraction, the volume ratio (RI) of retained austenite is measured.
(Iwasaki) The description about hardness was deleted.

  The volume ratio of retained austenite at the reference position after completion of the carburized material manufacturing process is 12 to 35%. Residual austenite undergoes a work-induced martensitic transformation during the cutting after carburizing and tempering. This increases the wear resistance of the product member. If the volume fraction of retained austenite at the reference position is too low, this effect cannot be obtained. On the other hand, if the volume fraction of retained austenite at the reference position is too high, a large amount of austenite remains even after cutting. In this case, improvement in wear resistance and bending fatigue strength of the product member cannot be expected.

The volume fraction of phases other than martensite and retained austenite (for example, ferrite, pearlite, proeutectoid cementite) at the reference position of the carburized material is 3% or less. If low-strength phases such as ferrite and pearlite are present at the reference position of the carburized material, these phases are maintained even after cutting, so cracks are likely to occur from these phases and wear resistance of product parts And bending fatigue strength are reduced. Moreover, if pro-eutectoid cementite exists, tool wear will increase and it will not be possible to ensure excellent machinability.
(Iwasaki) The description about hardness was deleted.

[Product member manufacturing process (cutting)]
Cutting is performed after carburizing, isothermal holding, quenching and tempering. The machining-induced martensitic transformation is generated while finishing the product member by cutting. This increases the wear resistance and bending fatigue strength of the product member. Cutting is performed under the following conditions.

Cutting tool rake angle α: −30 ° <α ≦ −5 °
If the rake angle α is larger than −5 °, the processing-induced martensitic transformation does not sufficiently occur during cutting. Therefore, the wear resistance of the product member is reduced. On the other hand, if the rake angle is −30 ° or less, the cutting resistance becomes too large, the machinability is lowered, the tool wear is increased, and the tool is lost in some cases. Accordingly, the rake angle α is −30 ° <α ≦ −5 °. A preferable rake angle α is −25 ° ≦ α ≦ −15 °.

Tool nose r: 0.4 to 1.2 mm
If the nose r of the tool is too small, the surface roughness becomes too large and the wear resistance of the product member is lowered. When the surface roughness increases, finish polishing must be performed to reduce the surface roughness. On the other hand, if the tool nose r is too large, the cutting resistance increases, so that the machinability decreases and the tool wear increases. Therefore, the nose r of the tool is 0.4 to 1.2 mm.

Feed f: more than 0.1 to 0.4 mm / rev (rotation)
If the feed f is too small, the cutting resistance, that is, the force with which the tool is pressed against the work material is too small. In this case, sufficient work-induced martensitic transformation does not occur. Therefore, the wear resistance of the product member is reduced. On the other hand, if the feed is too large, the cutting resistance increases. In this case, tool wear increases and machinability decreases. Accordingly, the feed f is more than 0.1 to 0.4 mm / rev. A preferable lower limit of the feed f is 0.2 mm / rev.

Cutting speed v: 50 to 150 m / min If the cutting speed v is too large, the cutting temperature rises and adhesive wear occurs. In this case, tool wear increases and machinability decreases. On the other hand, if the cutting speed is too low, the cutting efficiency is lowered and the production efficiency is lowered. Accordingly, the cutting speed v is 50 to 150 m / min.

Cutting depth d: 0.05 to 0.2 mm
If the notch d is too small, the cutting resistance becomes small. In this case, sufficient work-induced martensitic transformation does not occur. Therefore, the wear resistance and bending fatigue strength of the product member are reduced. On the other hand, if the cut v is too large, the cutting resistance increases. In this case, tool wear increases and machinability decreases. Accordingly, the cut d is 0.05 to 0.2 mm. The preferable lower limit of the cut d is 0.08 mm, and the preferable upper limit is 0.15 mm.

(Arithmetic mean roughness of the structure and surface of product parts)
A product member is obtained by the cutting process described above. At the reference position of the product member, the volume ratio of retained austenite is 20% or less. The volume reduction rate of retained austenite before and after cutting is 35% or more.

  The volume ratio (RF) of residual austenite of the product member after cutting is also measured by the same method as the volume ratio of residual austenite (RI) of the carburized material. For the product member, a sample having a surface including the reference position is taken. The same electrolytic polishing as that performed on the carburized material is performed on this sample. The sample after electropolishing is analyzed by an X-ray diffraction method, and the volume ratio (RF) of retained austenite is measured.

Based on the obtained volume ratios (RI) and (RF), the volume reduction rate Δγ of the retained austenite before and after cutting is obtained by the equation (A).
Reduction rate Δγ = (RI−RF) / RI × 100 (A)

  The volume fraction of retained austenite at the reference position of the product member is 20% or less. If the volume ratio of the retained austenite after cutting is too high, hard martensite cannot be obtained, and the wear resistance and bending fatigue strength of the product member are lowered.

  The volume reduction rate Δγ of retained austenite before and after cutting is 35% or more. As the retained austenite undergoes work-induced martensitic transformation by cutting, the wear resistance and bending fatigue strength are increased. If the volume reduction rate Δγ is too low, this effect cannot be obtained sufficiently.

  The arithmetic average roughness Ra of the surface of the product member is 0.8 μm or less. If the arithmetic average roughness Ra of the surface of the product member is too large, the frictional resistance when the product member slides increases, and the wear resistance of the product member decreases.

  The arithmetic average roughness Ra corresponds to the arithmetic average roughness Ra defined in JIS B0601 (2001) and conforms to this rule. The evaluation method of arithmetic mean roughness Ra and the specification of a measuring machine are based on the provisions of JIS B0633 (2001) and JIS B0651 (2001).

  When the arithmetic average roughness Ra of the surface of the product member is larger than 0.8 μm, finish polishing is performed to make the arithmetic average roughness Ra 0.8 μm or less. As final polishing, for example, lapping polishing can be employed.

<Product parts>
The product member of the present embodiment includes the above-described components and has a chemical composition satisfying the above formulas (1) and (2), and the C content of the surface layer portion is 0.6 to 1.0%. The structure at a depth of 20 μm from the surface is 97% or more in total of martensite and retained austenite (in other words, the other phase is 3% or less by volume), and in the range from the surface to a depth of 200 μm. The maximum residual austenite volume fraction (R1) is 10 to 30%, and the expected residual obtained by the formula (B) from the residual austenite volume fraction (R2) and the maximum residual austenite volume fraction (R1) at the above-mentioned depth position. The austenite reduction rate Δγp is 20% or more, the surface has a plastic flow structure with a thickness of 1 to 15 μm, and the arithmetic average roughness Ra of the surface is 0.8 μm or less. Here, the maximum retained austenite volume ratio R1 means the maximum value of the retained austenite volume ratio measured from the surface of the product member to a depth of 200 μm at a pitch of 10 μm.
Δγp = (R1−R2) / R1 × 100 (B)

[C content in surface layer: 0.6 to 1.0% or more]
C contained in the surface layer portion of the product member increases the wear resistance and (bending) fatigue strength of the product member. Here, the C content of the component surface layer was measured by the following method.

  A part surface layer of 50 μm was cut out by cutting, the C content in the chip was quantitatively measured by emission spectroscopic analysis, and the value was defined as the C content of the surface layer part. In addition, the C concentration in the component surface layer can be quantitatively analyzed using EPMA (electron beam microanalyzer).

  If the C content (Cs) contained in the surface layer portion is low, the above-described expected retained austenite reduction rate (Δγp) becomes small, and the hardness of the surface layer also becomes low. As a result, the wear resistance and bending fatigue strength of the product member are reduced. On the other hand, if (Cs) is high, hard pro-eutectoid cementite is generated in the surface layer portion of the product member. When Cs is excessively high and the pro-eutectoid cementite exceeds 3%, the cementite serves as a starting point for fatigue failure, and the bending fatigue strength decreases. Furthermore, tool wear during cutting increases, and machinability decreases. Accordingly, the C content (Cs) is 0.6 to 1.0%. The preferable lower limit of Cs is 0.65%. A preferable upper limit of Cs is 0.90%.

[Tissue at a depth of 20μm from the surface]
If there is a low-strength phase such as ferrite or pearlite as a structure at a depth of 20 μm from the surface of the product member, cracks are likely to occur based on these phases, and the wear resistance and bending fatigue of the product member Strength is lowered. Moreover, if pro-eutectoid cementite exists, the tool wear at the time of cutting in a product member manufacturing process will increase, and since it becomes a starting point of fatigue fracture, bending fatigue strength will fall. Accordingly, the total volume ratio of martensite and retained austenite is limited to 97% or more. In addition, the preferable range of the said volume ratio is 99% or more.

[Maximum retained austenite volume fraction (R1) in the range from the surface to a depth of 200 μm]
Residual austenite causes a processing-induced martensitic transformation during the finishing of the product member. As a result, the strength of the product member increases, and the wear resistance and bending fatigue strength increase. In order to obtain such an effect, the maximum retained austenite volume fraction (R1) must be 10% or more.

  On the other hand, since retained austenite is soft, (R1) exceeds 30%. On the contrary, the strength of the product member decreases. In addition, it is preferable that the range of R1 shall be 11 to 28%.

[Residual austenite volume ratio (R2) at the depth position and expected residual austenite reduction ratio Δγp determined from the maximum retained austenite volume ratio (R1)]
The maximum retained austenite volume ratio (R1) in the range from the surface to a depth of 200 μm is 8 to 30%. From the retained austenite volume ratio (R2) and the maximum retained austenite volume ratio (R1) The expected residual austenite reduction rate (Δγp) determined by (B) is 20% or more. Here, the residual austenite volume ratio (R2) at a depth of 20 μm from the surface is the above-described residual austenite volume ratio (RF) (described in the column of the manufacturing method of the product member). Further, the expected retained austenite reduction rate (Δγp) obtained from the maximum retained austenite volume ratio (R1) by the formula (B) is a value similar to the above-described Δγ (described in the column of the manufacturing method of the product member). These represent the degree of processing-induced transformation of austenite at the time of cutting in the product material manufacturing process. Therefore, Δγp increases as Δγ increases.

  The expected residual austenite reduction rate (Δγp) represents the degree of work-induced martensitic transformation during cutting. When Δγp is large, it means that more work-induced martensitic transformation has occurred during cutting, and wear resistance and bending fatigue strength are improved. In order to obtain such an effect, Δγp must be 20 or more. A preferable value of Δγp is 25 or more.

[Thickness of plastic flow structure on the surface: 1 to 15 μm]
The thickness of the plastic flow structure on the surface of the product member is measured by the following method. A test piece including the surface of the product member and having a surface (cross section) perpendicular to the axial direction of the product member (for example, the longitudinal direction in the case of a dumbbell-shaped test piece) as an observation surface is collected. The mirror polished specimen is corroded with 5% nital solution. The corroded surface is observed with a scanning electron microscope (SEM) at a magnification of 5000 times. An example of the obtained SEM image is shown in FIG. In the figure, a plastic flow structure 11 is a portion where the structure is curved in the circumferential direction of the product member (from the left direction to the right direction in FIG. 1) with respect to the center portion 12, and is curved from the surface of the product member. The distance to the edge of the texture was defined as the thickness of the plastic flow structure 11.

  At the time of cutting, a large deformation is generated in the surface layer portion of the carburized material, so that a plastic flow structure is formed. This plastic flow structure is hard, and when the thickness is 1 μm or more, the wear resistance and bending fatigue strength of the product member are improved. However, since the plastic flow structure is fragile, it can be deformed to some extent when its thickness is thin. However, if the thickness exceeds 15 μm, cracking occurs and becomes the starting point of crack generation. On the contrary, the fatigue strength decreases. Furthermore, when the thickness of the composition flow structure exceeds 15 μm, the machinability is lowered, the load on the tool during cutting is increased, and the tool life is significantly reduced. As described above, the thickness of the plastic flow structure of the surface layer of the product member is limited to 1 to 15 μm.

[Arithmetic mean surface roughness Ra: 0.8 μm or less]
The arithmetic average roughness Ra on the surface of the product member is as described above in the column of the method for manufacturing the product member.

  Using a vacuum melting furnace, 150 kg of molten steel A to S having the chemical composition shown in Table 1 was obtained.

  Ingots were obtained by the ingot-making method using molten steel of each steel type. Each ingot was heated at 1250 ° C. for 4 hours and then hot forged to obtain a round bar having a diameter of 35 mm. The finishing temperature during hot forging was 1000 ° C.

  Normalizing treatment was performed on each round bar. The normalizing treatment temperature was 925 ° C., and the normalizing treatment time was 2 hours. After the normalizing treatment, the round bar was allowed to cool to room temperature (25 ° C.).

  Machining was performed on the round bar after being allowed to cool, and a rough member that was the basis of the wear test piece 21 shown in FIG. 2 and the rotating bending fatigue test piece 31 shown in FIG. 3 was produced. A wear test piece 21 corresponding to a product member has a circular cross section, and includes a cylindrical test part 22 and a pair of cylindrical grip parts 23 arranged at both ends of the test part 22. As shown in FIG. 2, the outer diameter of the test part 22 was 26 mm, the length of the test part 22 was 28 mm, and the overall length of the wear test piece 21 was 130 mm. The bending fatigue test piece 31 has a circular cross section, and has a notch with a radius of curvature of 1 mm at the center. Moreover, it machined with respect to the round bar after standing_to_cool, and manufactured the rod-shaped machinability test piece (not shown) of diameter 30mm and length 300mm.

  Based on conditions a to l shown in Table 2, carburizing treatment, isothermal holding treatment, quenching treatment and tempering treatment are performed on the rough members of the wear test piece 21 and the bending fatigue test piece 31 and the machinability test piece. gave. A gas carburizing furnace was used for carburizing and quenching. After the tempering, the gripping part 23 was finished for the purpose of removing the heat treatment distortion. Cp1, Cp2, T1, T2, T3, t1, t2, and t3 shown in Table 2 are the above-described carbon potential, temperature, and time, respectively.

  The carburized material after carburizing and tempering (the rough members of the wear test piece 21 and the bending fatigue test piece 31 and the machinability test piece) is measured for the volume fraction of retained austenite by a method described later before cutting. The structure observation test and the Vickers hardness test were performed.

  The test part 22 of the wear test piece 21 and the notch of the bending fatigue test piece 31 which are carburized materials are subjected to cutting under the conditions shown in Table 3, so that the wear test piece 21 and the bending fatigue test piece corresponding to product members are obtained. 31 was obtained. However, for the bending fatigue test piece 31, the tool nose r was fixed to 0.4 mm. In the cutting process, one pass, that is, the cutting amount shown in Table 3 was applied only once.

  As the cutting tool, a cBN sintered tool in which a TiAlN-based ceramic coating was applied to the surface of a sintered material mainly composed of cBN particles and ceramics as a binder was used.

  And after performing said cutting, the abrasion test piece 21 and the bending fatigue test piece 31 were created. About the abrasion test piece 21, finish polishing was implemented using # 2000 emery paper. The arithmetic average roughness Ra of the surfaces of the wear test piece 21 of each test number after finish polishing and the bending fatigue test piece 31 after cutting was 0.8 μm or less.

[Measurement of volume fraction (RF) of retained austenite]
Electrolytic polishing was performed on the surface of the test piece (wear test piece 21) corresponding to the product member. Specifically, the test piece surface was masked with a hole diameter of 3 mm, and in an electrolytic solution containing 11.6% ammonium chloride, 35.1% glycerin, and 53.3% water, Using the test piece as an anode, electrolytic polishing was performed at a voltage of 20 V to expose a surface at a depth of 20 μm from the surface (hereinafter referred to as an observation surface).

  X-ray diffraction was performed on the observation surface by the above-described method, and the volume ratio (RF (R2)) of retained austenite at a position of 20 μm from the surface was obtained.

  Electrolytic polishing is performed on the observation surface by the same method, the depth of the hole is increased by 10 μm to 30 μm, and X-ray diffraction is performed on the surface by the above-described method. The volume fraction of austenite was determined. By repeating this process, the hole was deepened by 10 μm, and the volume ratio of retained austenite (RF (R2)) was measured each time until the hole depth reached 200 μm. And the largest retained austenite volume fraction obtained in that was made into R1.

[Tissue observation test]
In the test piece corresponding to the product member, the volume ratio of the structure other than the retained austenite was measured by the method described above.

[Two-cylinder wear test (RP test)]
A two-cylinder wear test (RP test) shown in FIG. 4 was performed using the wear test piece 21 shown in FIG. FIG. 4 is a front view showing the RP test method. As shown in FIG. 4, in the RP test, a wear test piece 21 and a large roller test piece 41 were prepared. The large roller test piece 41 was disk-shaped, had a diameter of 130 mm, a circumferential surface width of 18 mm, and a circumferential surface radius of curvature of 700 mm. The large roller test piece had a chemical composition corresponding to JIS standard SCM882 and was carburized and quenched. The circumferential surface of the large roller test piece 41 was brought into contact with the surface of the wear test piece 21, and the RP test was performed under the conditions shown in Table 4.

  After the test, the wear depth of the portion of the surface of the wear test piece 21 in contact with the large roller test piece 41 was measured. A stylus type surface roughness meter was used to measure the wear depth. The measurement length was 26 mm, and the stylus was scanned in the axial direction of the wear test piece 21 to obtain a cross-sectional curve. In each test piece, cross-sectional curves were measured at four locations in 90 ° increments in the circumferential direction. From the obtained cross-sectional curve, the difference in height between the portion where the large roller test piece 41 is not in contact and the most worn portion when the large roller test piece 41 is in contact was measured. The average value of the four measured height differences was defined as the wear depth (RP wear amount: μm) of the wear test piece 21 of each test number.

[Rotating bending fatigue test]
Using the fatigue test piece 31 shown in FIG. 3, the Ono type rotating bending fatigue test was performed by changing the test load at a 50 MPa pitch, and the fatigue limit was obtained. Note that the minimum stress (σ _{f, min} ) that broke before reaching 10 ⁷ repetitions and the maximum stress (σ _{r, max} ) at a stress lower than (σ _{f, min} ) The midpoint was the fatigue limit.

[Tool wear measurement]
The tool wear of the cutting tool was evaluated by the flank wear amount (μm). The method is as follows. That is, the machinability test piece after carburizing and tempering (equivalent to a carburized material) was cut by one pass per piece under the same cutting conditions as the rough member shown in Table 3. Cutting was repeated for a plurality of test pieces, and after cutting until the total cutting time was 5 minutes, the flank wear width of the cutting tool was measured. A microscope was used to measure the flank wear width. The tool was set so that the tool flank face was parallel to the measurement object, and the worn part was observed at a magnification of 200 times. At this time, the distance from the cutting edge of the portion where the wear was maximum near the center of the wear portion to the wear tip was measured, and was defined as the flank wear amount. In this measurement, the case where the flank wear amount is 40 μm or less is acceptable in that the tool wear during cutting can be suppressed as compared with the conventional technique.

[Test results]
Tables 5 and 6 show the results regarding the tests described above.

  As is apparent from Tables 5 and 6, each condition for the production method of the product member according to the present invention is satisfied (that is, on the premise that the chemical composition of the coarse member is adjusted, in particular, the carburized material after quenching) Test number 1, 2, 5, 10-20, 26-28, 45, 46, 48, and improvements are made on the metal structure and hardness, and the metal structure and arithmetic average roughness of the product member after cutting. As for 59, 60, 63, 65 and 66, it can be seen that excellent results are obtained for any of the structure of the carburized material and the structure of the product member and the arithmetic average roughness Ra of the surface. Therefore, according to the manufacturing methods of these test examples, it was proved that a product member excellent in wear resistance, bending fatigue strength and machinability can be obtained.

  On the other hand, as is clear from Tables 5 and 6, the conditions for the production method of the product member according to the present invention are not satisfied (that is, the chemical composition of the coarse member is adjusted and the metal structure of the carburized material after quenching) No improvement is made on at least one of the metal structure of the product member after cutting and the arithmetic average roughness of the product member) Test numbers 3, 4, 6-9, 21-25, 29-44, For 47, 49-58, 61, 62, 64, 67-71, at least one of the structure of the carburized material, the Vickers hardness of the member, the structure of the product member, and the arithmetic average roughness Ra of the surface of the member It can be seen that excellent results are not obtained. Therefore, according to the manufacturing methods of these test examples, it cannot be said that a product member excellent in wear resistance, bending fatigue strength and machinability can be obtained.

DESCRIPTION OF SYMBOLS 11 Plastic flow structure 12 Center part 21 Abrasion test piece 22 Test part 23 Grasp part 31 Bending fatigue test piece 41 Large roller

Claims (8)

C: 0.1-0.3%, Si: 0.25% or less, Mn: 0.4-2.0%, P: 0.050% or less, S: 0.005-0. 020%, Cr: 0.4-3.5%, Al: 0.010-0.050%, N: 0.010-0.025% and O: 0.003% or less, and the balance A process of obtaining a rough member by processing a steel material having a chemical composition satisfying the formula (1) and the formula (2), comprising Fe and inevitable impurities;
Carburizing treatment, isothermal holding treatment, quenching treatment and tempering treatment for the rough member to obtain a carburized material,
The carburizing temperature is 900 ° C. or higher and 1050 ° C. or lower, the carbon potential during carburizing is 0.7% or higher and 1.1% or lower, the carburizing time is 60 minutes or longer and 240 minutes or shorter, and the constant temperature holding temperature is 820 ° C. or higher and 870 ° C. or lower. The carbon potential during the constant temperature holding treatment is 0.7% or more and 0.9% or less, the tempering temperature is 160 ° C. or more and 200 ° C. or less, and the tempering time is 60 minutes or more and 180 minutes or less.
In the carburized material, the structure at the reference position corresponding to the depth position of 20 μm from the position corresponding to the surface of the product member which is the final form contains martensite and retained austenite having a volume ratio of 12 to 35%. A step in which the total volume ratio of the martensite and retained austenite is 97% or more ;
A step of cutting the carburized material to obtain a product member,
The rake angle of the cutting tool is -30 ° to -5 ° or less, the tool nose is 0.4 mm to 1.2 mm, the feed is 0.1 mm / rev to 0.4 mm / rev and less, and the cutting speed is 50 m. / Min or more and 150 m / min or less, and the incision is 0.05 mm or more and 0.2 mm or less,
In the structure at the reference position, the volume ratio of residual austenite is 20% or less, and the residual austenite volume ratio (RI) before cutting and the residual austenite volume ratio (RF) after cutting are obtained by the formula (A). Austenite reduction rate Δγ is 35% or more, arithmetic mean roughness Ra of the surface is 0.8 μm or less, and further, tool wear during finishing is suppressed,
Bei to give a,
The C content of the surface layer portion is 0.6 to 1.0%, the maximum retained austenite volume ratio (R1) in the range from the surface to a depth of 200 μm is 10 to 30%, and a plastic flow structure of 1 to 15 μm is obtained. Yusuke that method of manufacturing a product member.
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr ≧ 2.35 (1)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr ≦ 33.8 (2)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (1) and (2).
Δγ = (RI−RF) / RI × 100 (A)   The manufacturing method of the product member according to claim 1, wherein the steel material contains Pb: 0.5% or less in place of part of Fe.   3. The production of a product member according to claim 1, wherein the steel material contains at least one selected from the group consisting of V and Ti, in place of a part of Fe, in a total content of 0.1% or less. Method. The steel material contains at least one selected from the group consisting of Mo: 3.0% or less and Ni: 2.5% or less, instead of a part of Fe, and the formulas (1) and ( The manufacturing method of the product member of any one of Claim 1 to 3 which replaces with 2) and satisfy | fills Formula (3) and Formula (4).
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr + 1.77 × Mo + 0.63 × Ni ≧ 2.35 (3)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr + 6.7 × Mo + 6.2 × Ni ≦ 33.8 (4)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (3) and (4). C: 0.1-0.3%, Si: 0.25% or less, Mn: 0.4-2.0%, P: 0.050% or less, S: 0.005-0. 020%, Cr: 0.4-3.5%, Al: 0.010-0.050%, N: 0.010-0.025% and O: 0.003% or less, and the balance Consists of Fe and inevitable impurities, and has a chemical composition satisfying the formulas (5) and (6),
C content of the surface layer part is 0.6 to 1.0%,
The volume fraction of retained austenite at a depth position of 20 μm from the surface is 20% or less, and the structure at a depth position of 20 μm from the surface is 97% or more in terms of the total volume ratio of martensite and retained austenite,
The maximum retained austenite volume fraction in the range from the surface to a depth of 200 μm is 10 to 30%,
The expected retained austenite reduction rate Δγp determined by the formula (B) from the retained austenite volume fraction (R2) and the maximum retained austenite volume fraction (R1) at the depth position is 20% or more,
It has a plastic flow structure with a thickness of 1 to 15 μm on the surface,
The arithmetic average roughness Ra of the surface is 0.8 μm or less,
A product member characterized by that.
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr ≧ 2.35 (5)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr ≦ 33.8 (6)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (5) and (6).
Δγp = (R1−R2) / R1 × 100 (B)   The product member according to claim 5, which contains Pb: 0.5% or less instead of a part of Fe.   The product member according to claim 5 or 6, wherein, in place of a part of Fe, one or more selected from the group consisting of V and Ti are contained in a total content of 0.1% or less. In place of a part of Fe, containing at least one selected from the group consisting of Mo: 3.0% or less, Ni: 2.5% or less, instead of formula (5) and formula (6), The product member according to claim 5, wherein the product member satisfies Formula (7) and Formula (8).
1.54 × C + 0.81 × Si + 1.59 × Mn + 1.65 × Cr + 1.77 × Mo + 0.63 × Ni ≧ 2.35 (7)
11.3 ≦ −0.1 × Si + 15.2 × Mn + 7.0 × Cr + 6.7 × Mo + 6.2 × Ni ≦ 33.8 (8)
Here, the content (mass%) of each element is substituted for each element symbol in the formulas (7) and (8).