JP2017066474A - Hot forging component and manufacturing method of hot forging component - Google Patents

Abstract

A hot forged part having excellent seizure resistance is provided. A hot forged part has a chemical composition of mass%, C: 0.35 to 0.6%, Si: 0.1% to less than 1.2%, Mn: 0.1% to 1%. Less than 2%, P: more than 0.02% and 0.05% or less, S: 0.06% or less, Al: more than 0.05% and 0.8% or less, N: 0.001 to 0.02 %, V: 0 to 0.2%, balance: Fe and impurities, the chemical composition satisfies the following formula (1), and has a structure containing pearlite of 90% or more by area ratio, and the average of pearlite It consists of a steel material having a lamellar spacing of more than 35 nm and not more than 100 nm. [Si] + [Mn] ≦ 1.2 Formula (1) Here, each content of Si and Mn is substituted by mass% for [Si] and [Mn]. [Selection] Figure 8

Description

  The present invention relates to a hot forged part suitable as a sliding part and a method for producing a hot forged part.

  A sliding component represented by a crankshaft is required to have wear resistance and fatigue strength. In order to increase the wear resistance and fatigue strength, the sliding parts are often subjected to surface hardening treatment such as induction hardening and soft nitriding. On the other hand, sliding parts that can be used even if the surface hardening treatment is omitted are demanded from the demand for cost reduction.

  Japanese Patent No. 5370281 discloses a forged crankshaft made of a ferrite-pearlite structure in which the area ratio of pro-eutectoid ferrite is less than 10%, or a non-heat treated steel material having a pearlite structure. This document describes that if the pro-eutectoid ferrite area ratio is less than 10%, excellent wear resistance can be obtained even if the surface hardening treatment is omitted.

  In Japanese Patent No. 4140283, the structure in the region from the surface layer to 200 nm is a mixed structure of proeutectoid ferrite with an area ratio of 5% or less and pearlite with a lamellar spacing of 30 nm or less. A steel crankshaft is disclosed. In this document, it is the hardness in the region from the surface layer to 200 nm that affects the wear resistance, and if the hardness measured in this region using a nanoindentation apparatus is 10 GPa or more, excellent wear resistance It is described that the sex can be obtained.

  Sliding parts are required to have excellent seizure resistance in addition to the above characteristics. In response to the need for lighter automobiles, there is an increasing demand for narrower and narrower crankshafts. Therefore, the sliding conditions between the crankshaft and the bearing are severe. Therefore, the crankshaft is required to have better seizure resistance than before.

  Japanese Patent No. 4589885 discloses a crankshaft characterized in that the thermal conductivity κ is 40 W / mK or more and the surface hardness Hv after induction hardening is larger than (2.7 × κ + 420). Has been. This document describes that the seizing factor is the temperature rise of the sliding surface. It is described that it is effective to increase the thermal conductivity and reduce the friction coefficient in order to suppress the temperature rise.

Japanese Patent No. 5370281 Japanese Patent No. 4140283 Japanese Patent No. 4589885

  As described in the above-mentioned patent document, in order to improve the seizure resistance, the hardness (hereinafter referred to as “nano hardness”) of a region within a few hundreds of nanometers from the surface (sliding surface). It is effective to increase the Vickers hardness of the base material in order to distinguish it from nano hardness. If the nano hardness is increased, excellent seizure resistance can be obtained even if the base material hardness is low.

  The present inventors have found that the nano hardness of the sliding component changes by sliding for a long time. In the above-mentioned patent document, the nano hardness after sliding for a long time is not examined.

  The subject of this invention is providing the hot forging components excellent in the seizure resistance which can be used even if a surface hardening process is abbreviate | omitted.

The hot forged part according to one embodiment of the present invention has a chemical composition of mass%, C: 0.35 to 0.6%, Si: 0.1% or more and less than 1.2%, Mn: 0.1 % To less than 1.2%, P: more than 0.02% to 0.05% or less, S: 0.06% or less, Al: more than 0.05% to 0.8% or less, N: 0.001 to 0.02%, V: 0 to 0.2%, balance: Fe and impurities, the chemical composition satisfies the following formula (1), and has a structure containing pearlite of 90% or more in area ratio The pearlite is made of a steel material having an average lamella spacing of more than 35 nm and not more than 100 nm.
[Si] + [Mn] ≦ 1.2 Formula (1)
Here, each content of Si and Mn is substituted by mass% for [Si] and [Mn].

In the method for producing a hot forged part according to an embodiment of the present invention, the chemical composition is mass%, C: 0.35 to 0.6%, Si: 0.1% or more and less than 1.2%, Mn: 0.1% or more and less than 1.2%, P: more than 0.02% and 0.05% or less, S: 0.06% or less, Al: more than 0.05% and 0.8% or less, N: 0 0.001 to 0.02%, V: 0 to 0.2%, balance: Fe and a step of preparing materials that are impurities, a step of heating the materials to 1000 to 1250 ° C, and the heated materials A step of hot forging and a step of cooling the hot forged material. The chemical composition satisfies the following formula (1), and in the cooling step, an average cooling rate from 900 ° C. to 400 ° C. is set to a cooling rate index CI defined by the following formula (2), and , Larger than the pearlite cooling rate index PI defined by the following formula (3).
[Si] + [Mn] ≦ 1.2 Formula (1)
CI = 10 ^x Expression (2)
PI = 500 × 10 ^−y Formula (3)
However, x = 0.1 * [Al]-[Si]-[Mn] -5.8 * [V] +2.2
y = 0.6 * [Si] + 1.7 * [Mn] + 46 * [P] + 66 * [S] + 22 * [V] + 1.2 * [Al] -1.9
Here, [Si], [Mn], [Al], [V], [P], and [S] each contain the contents of Si, Mn, Al, V, P, and S in the material in mass%. Assigned. The unit of the cooling rate index CI and the pearlite cooling rate index PI is ° C./second.

  ADVANTAGE OF THE INVENTION According to this invention, the hot forging components excellent in the seizure resistance which can be used even if a surface hardening process is abbreviate | omitted are obtained.

FIG. 1 is a cross-sectional view showing the structure of the sliding surface of pearlite steel. FIG. 2 is a graph showing the relationship between the amount of alloy element added and the thermal conductivity. FIG. 3 is a specific example of how to draw a test line for measuring the average lamella spacing of pearlite. FIG. 4 is a flowchart showing an example of a method for producing a hot forged part according to an embodiment of the present invention. FIG. 5A is a scatter diagram showing the relationship between the average cooling rate from 900 ° C. to 400 ° C., the structure and the base material hardness. FIG. 5B is a scatter diagram showing the relationship between the average cooling rate from 900 ° C. to 400 ° C., the structure and the base material hardness. FIG. 6 is a schematic diagram of a pin-on-disk wear test. FIG. 7 is a schematic diagram of nano hardness measurement by the nano indentation method. FIG. 8 is a graph showing the nano hardness distribution after the wear test.

  The present inventors examined the seizure phenomenon mainly for a crankshaft for an automobile. Usually, the crankshaft and the bearing slide in a state of fluid lubrication facing each other across an oil film. However, as a result of the investigation by the present inventors, it has been found that there are not a few phases from fluid lubrication to boundary lubrication during long-time sliding. In general, when steel is subjected to sliding for a long time in a boundary lubrication environment, it has been found that work hardening and softening due to frictional heat occur, and the hardness near the sliding surface (within 20 nm from the surface) changes. .

  As a result of the structure observation, it was found that the formation of the processed layer by sliding and the alteration of the structure (tempering, recovery, and recrystallization) by frictional heat occurred on the sliding surface in a superimposed manner. Among these, the formation of the processed layer by sliding is a factor that improves the nano hardness, and the alteration of the structure due to frictional heat is a factor that decreases the nano hardness. Moreover, when peeling occurs on the sliding surface, wear is promoted and frictional heat increases, which causes a decrease in nano hardness.

  Therefore, in order to improve the nano hardness of the sliding surface, (1) improvement of work hardening, (2) suppression of peeling on the sliding surface, (3) frictional heat dissipation due to improvement of thermal conductivity. It is valid.

(1) Improvement of work hardenability The present inventors show that a steel material having appropriately controlled pearlite is greatly work hardened by sliding and exhibits a nanohardness equivalent to or higher than that of a surface hardened material. I found.

  FIG. 1 is a cross-sectional view showing the structure of the sliding surface of pearlite steel. As shown in FIG. 1, the pearlite lamella is greatly plastically deformed on the sliding surface. In the outermost layer, cementite forming pearlite is divided and refined, and pearlite is nanocrystallized. In the nanocrystallized layer (hereinafter referred to as nanocrystallized layer), part of cementite is decomposed and dissolved in ferrite. Factors that indicate high nano-hardness of pearlite after sliding are that cementite is refined by strong processing by sliding, and that the amount of solid solution carbon in ferrite increases due to decomposition of refined cementite, It is thought that there is.

  Si and Mn suppress the decomposition of cementite. Therefore, Si and Mn reduce the work hardening of pearlite. Therefore, it is preferable to reduce the contents of Si and Mn in order to improve work curability.

  P dissolves in the ferrite in the pearlite and enhances the work hardening of the pearlite. P generally segregates at the grain boundaries to reduce the strength of the steel. However, since the grain boundary area is large in the nanocrystallized layer, P hardly segregates. Therefore, in order to improve work curability, it is preferable to increase the P content.

  The finer the pearlite lamella spacing, the easier it is to crystallize and the higher the work curability. On the other hand, if the steel material contains a hard structure such as martensite or bainite, the plastic deformation of pearlite is hindered and the work hardenability is lowered. Therefore, in order to improve work curability, it is preferable to have a structure having a fine lamella and a high pearlite fraction.

(2) Suppression of peeling on sliding surface As described above, an appropriate amount of the nanocrystallized layer contributes to the improvement of the nano hardness of the sliding surface. On the other hand, if the nanocrystallized layer becomes too thick, cracks and peeling are likely to occur in the nanocrystallized layer, and the nanohardness is reduced.

  If the pearlite lamella spacing is too small, the nanocrystallized layer becomes too thick, and cracking and peeling are likely to occur in the nanocrystallized layer. In order to suppress the peeling on the sliding surface while obtaining the work hardening effect, it is necessary to appropriately control the pearlite lamella spacing.

  Excessive development of the nanocrystallized layer can be suppressed by adding Al or V. Although this mechanism is not clear, V suppresses an increase in the amount of solid solution carbon in the ferrite by generating carbide, and Al dissolves in the ferrite to change the deformation form of the ferrite to change the nanocrystallization layer. This is thought to be due to the effect of suppressing the generation of.

(3) Friction heat dissipation by improving thermal conductivity It has been conventionally known that reduction of Si content is effective for improving thermal conductivity. In order to clarify the influence of other elements, the inventors measured the thermal conductivity of the medium-high carbon steel while changing the addition amount of the alloy element. The results are shown in FIG. As shown in FIG. 2, in addition to C and Si, Mn is an element that greatly affects the thermal conductivity, and it was found that the thermal conductivity can be improved by reducing the Mn content. Therefore, in order to improve thermal conductivity, it is preferable to reduce the contents of Si and Mn.

As described above, in order to improve the nano hardness of the sliding surface by the above (1) to (3), the P and Al contents are increased, the Si and Mn contents are decreased, and the pearlite content is increased. What is necessary is just to set it as a structure | tissue with a high rate, and to control the lamella space | interval of a pearlite appropriately. More specifically, the chemical composition of the steel material constituting the hot forged part satisfies the following formula (1), has a structure containing pearlite of 90% or more in area ratio, and the average lamella spacing of pearlite exceeds 35 nm. If the thickness is 100 nm or less, nanohardness equivalent to or higher than that of the surface-cured material can be obtained without performing the surface-curing treatment.
[Si] + [Mn] ≦ 1.2 Formula (1)
Here, each content of Si and Mn is substituted by mass% for [Si] and [Mn].

  The surface hardening treatment has an effect of suppressing fatigue failure by giving not only surface hardening but also compressive strain to the surface. In order to obtain a sliding component that can be used even if the surface hardening treatment is omitted, it is preferable to increase not only the nano hardness of the sliding surface but also the strength of the base material. In order to obtain high-strength pearlite, (4) it is necessary to select cooling conditions according to the chemical composition.

(4) Selection of cooling conditions according to chemical composition In order to increase the strength of pearlite, the cooling rate after hot forging may be increased. On the other hand, if the cooling rate after hot forging is too high, depending on the chemical composition, martensitic transformation or bainite transformation precedes and a structure with a high pearlite fraction cannot be obtained. Therefore, in order to obtain a structure having high strength and a high pearlite fraction, it is necessary to select an appropriate chemical composition and a cooling condition corresponding to the chemical composition.

As a result of various studies, in the step of cooling after hot forging, the average cooling rate from 900 ° C. to 400 ° C. is set to a cooling rate index CI or less defined by the following equation (2), and the following equation ( It was found that if the pearlite cooling rate index PI defined in 3) is made larger, the steel material can have a structure containing pearlite with an area ratio of 90% or more, and the average lamella spacing of pearlite can be more than 35 nm and less than 100 nm. .
CI = 10 ^x Expression (2)
PI = 500 × 10 ^−y Formula (3)
However, x = 0.1 * [Al]-[Si]-[Mn] -5.8 * [V] +2.2
y = 0.6 * [Si] + 1.7 * [Mn] + 46 * [P] + 66 * [S] + 22 * [V] + 1.2 * [Al] -1.9
Here, [Si], [Mn], [Al], [V], [P], and [S] are substituted for each content of Si, Mn, Al, V, P, and S by mass%. . The unit of the cooling rate index CI and the pearlite cooling rate index PI is ° C./second.

  Based on the above findings, the present invention has been completed. Hereinafter, the hot forged part by one Embodiment of this invention is explained in full detail.

[Chemical composition]
The steel material which comprises the hot forging components by this embodiment has the chemical composition demonstrated below. In the following description, “%” of the element content means mass%.

C: 0.35-0.6%
Carbon (C) is an element necessary for obtaining a pearlite structure. When the C content is less than 0.35%, a structure having a high pearlite fraction cannot be obtained. On the other hand, if the C content exceeds 0.6%, the machinability of the steel decreases. Therefore, the C content is 0.35 to 0.6%. The lower limit of the C content is preferably 0.4%, more preferably 0.5%. The upper limit of the C content is preferably 0.58%.

Si: 0.1% or more and less than 1.2% Silicon (Si) is an element necessary for obtaining a pearlite structure. Si also strengthens pearlite by dissolving in ferrite in pearlite. When the Si content is less than 0.1%, a structure having a high pearlite fraction cannot be obtained. On the other hand, Si suppresses the solid solution of cementite and lowers the work hardening property of pearlite. Si also reduces the thermal conductivity of the steel. Therefore, when the Si content is 1.2% or more, sufficient seizure resistance cannot be obtained. Therefore, the Si content is 0.1% or more and less than 1.2%. The lower limit of the Si content is preferably 0.2%. The upper limit of the Si content is preferably 1.0%, and more preferably 0.9%.

Mn: 0.1% or more and less than 1.2% Manganese (Mn) is an element necessary for obtaining a pearlite structure. When the Mn content is less than 0.1%, a structure having a high pearlite fraction cannot be obtained. On the other hand, Mn suppresses solid solution of cementite and lowers the work hardening property of pearlite. Mn also reduces the thermal conductivity of the steel. Therefore, if the Mn content is 1.2% or more, sufficient seizure resistance cannot be obtained. Therefore, the Mn content is 0.1% or more and less than 1.2%. The lower limit of the Mn content is preferably 0.2%. The upper limit of the Mn content is preferably 1.1%, more preferably 1.0%.

P: more than 0.02% and 0.05% or less Phosphorus (P) strengthens pearlite by dissolving in ferrite in pearlite. P also improves the work hardening of pearlite. If the P content is 0.02% or less, these effects cannot be obtained. On the other hand, if the P content exceeds 0.05%, excess P segregates at the grain boundaries, and the fatigue strength of the steel decreases. Therefore, the P content exceeds 0.02% and is 0.05% or less. The lower limit of the P content is preferably 0.022%. The upper limit of the P content is preferably 0.045%, and more preferably 0.04%.

S: 0.06% or less Sulfur (S) is contained in steel as an impurity. Further, when S is positively contained, sulfide inclusions are formed and the machinability of the steel is improved. On the other hand, when the S content exceeds 0.06%, the hot workability decreases. Accordingly, the S content is 0.06% or less. In order to obtain the machinability improving effect, the lower limit of the S content is preferably 0.01%, more preferably 0.02%. The upper limit of the S content is preferably 0.05%, more preferably 0.04%.

Al: more than 0.05% and 0.8% or less Aluminum (Al) deoxidizes steel. Al further suppresses excessive development of the nanocrystallized layer. When the Al content is 0.05% or less, these effects cannot be obtained sufficiently. On the other hand, if the Al content exceeds 0.8%, the thermodynamic phase stability between austenite and ferrite changes, resulting in an increase in pearlite lamellar spacing and a decrease in pearlite strength. Therefore, the Al content is more than 0.05% and 0.8% or less. The lower limit of the Al content is preferably 0.055%. The upper limit of the Al content is preferably 0.7%, more preferably 0.6%.

N: 0.001 to 0.02%
Nitrogen (N) increases the strength of the steel. If the N content is less than 0.001%, this effect cannot be sufficiently obtained. On the other hand, if the N content exceeds 0.02%, the toughness of the steel decreases. Therefore, the N content is 0.001 to 0.02%. The lower limit of the N content is preferably 0.0015%, more preferably 0.002%. The upper limit of the N content is preferably 0.015%.

  The balance of the chemical composition of the steel material constituting the hot forged part according to the present embodiment is Fe and impurities. The impurity here refers to an element mixed from ore and scrap used as a raw material of steel, or an element mixed from the environment of the manufacturing process.

V: 0 to 0.2%
The chemical composition of the steel material constituting the hot forged part according to the present embodiment may contain vanadium (V) instead of part of Fe. V increases the strength of the steel. V further suppresses excessive development of the nanocrystallized layer. On the other hand, if the V content exceeds 0.2%, the machinability of the steel decreases. Therefore, the V content is 0 to 0.2%. If V content is 0.05% or more, said effect will be acquired notably. The lower limit of the V content is more preferably 0.08%. The upper limit of the V content is preferably 0.15%, more preferably 0.12%.

[Regarding Formula (1)]
The chemical composition of the steel material constituting the hot forged part according to the present embodiment satisfies the following formula (1).
[Si] + [Mn] ≦ 1.2 Formula (1)
Here, each content of Si and Mn is substituted by mass% for [Si] and [Mn].

  Si and Mn suppress the decomposition of cementite and reduce the work hardening of pearlite. Si and Mn also reduce the thermal conductivity of the steel. Therefore, when there is too much content of these elements, sufficient seizure resistance cannot be obtained. In addition to limiting the respective upper limits, the total amount of Si and Mn is also limited to 1.2% or less. The total content of Si and Mn is preferably 1.1% or less.

[Organization]
The steel material constituting the hot forged part according to the present embodiment has a structure including pearlite with an area ratio of 90% or more. Pearlite is greatly hardened by sliding. If the steel material contains a hard structure such as martensite or bainite, the plastic deformation of pearlite is hindered and the work hardenability decreases. Moreover, when there is much proeutectoid ferrite, intensity | strength will be insufficient. The area ratio of pearlite is preferably 95% or more.

  The area ratio of pearlite is measured as follows.

  A sample is taken from the hot forged part to include a position 100 μm deep from the surface. At this time, a sample is taken so that a surface perpendicular to the surface of the hot forged part becomes an observation surface. After the observation surface is mechanically polished, the structure is revealed by electrolytic polishing. The observation surface is observed with a scanning electron microscope (SEM) at five magnifications at a magnification of 1000. The perlite area ratio of each visual field is obtained by image processing. The average of the five fields of view is the area ratio of the pearlite of the steel material.

  In the steel material constituting the hot forged part according to this embodiment, the average lamella spacing of pearlite is more than 35 nm and not more than 100 nm. If the average lamella spacing exceeds 100 nm, the work curability is lowered. Moreover, sufficient base material hardness cannot be obtained. On the other hand, when the average lamella spacing is 35 nm or less, the nanocrystallized layer becomes thick, and cracks and peeling easily occur in the nanocrystallized layer. The lower limit of the average lamella interval is preferably 40 nm. The upper limit of the average lamella interval is preferably 90 nm.

  The average lamella spacing of pearlite is measured as follows.

  Similar to the measurement of the area ratio of pearlite, a sample is taken so that the surface perpendicular to the surface of the hot forged part becomes the observation surface. After the observation surface is mechanically polished, the structure is revealed by electrolytic polishing. The observation surface is observed by SEM at five magnifications at a magnification of 5000 times.

In each field of view, the number n of cementite that intersects the test line of length L is determined. FIG. 3 is a specific example of how to draw a test line. As shown in FIG. 3, in each field of view, three test lines parallel and perpendicular to the surface are drawn at intervals of 5 μm, and the average lamella spacing lav is obtained from the following formula (A) for each test line. Furthermore, let the average value of 5 visual fields x 6 test lines be the average lamella spacing of pearlite.
lav = 0.5 × (L / n) (A)

  The hot forged part according to the present embodiment preferably has a base material hardness of Hv300 or higher.

[Production method]
Hereinafter, an example of the manufacturing method of the hot forged part by this embodiment is demonstrated. The manufacturing method of the hot forged part by this embodiment is not limited to this.

  FIG. 4 is a flowchart showing an example of a method for producing a hot forged part according to the present embodiment. This manufacturing method includes a step of preparing a material (step S1), a step of heating the material (step S2), a step of hot forging the heated material (step S3), and a hot forged material. And a step of cooling (step S4).

  First, a material having the above-described chemical composition is prepared (step S1). For example, the steel having the above-described chemical composition is melted and billets are produced by continuous casting and ingot rolling.

  The material is heated to 1000 to 1250 ° C. (step S2). Subsequently, the heated material is hot forged (step S3). The material is processed into the rough shape of the product by hot forging. The hot forging conditions are not particularly limited, but the finishing temperature is, for example, 900 ° C.

The hot forged material is cooled (step S4). At this time, the average cooling rate from 900 ° C. to 400 ° C. is made equal to or lower than the cooling rate index CI defined by the following equation (2), and from the pearlite cooling rate index PI defined by the following equation (3): Also make it bigger.
CI = 10 ^x Expression (2)
PI = 500 × 10 ^−y Formula (3)
However, x = 0.1 * [Al]-[Si]-[Mn] -5.8 * [V] +2.2
y = 0.6 * [Si] + 1.7 * [Mn] + 46 * [P] + 66 * [S] + 22 * [V] + 1.2 * [Al] -1.9
Here, [Al], [Si], [Mn], [V], [P], and [S] are substituted with the contents of Al, Si, Mn, V, P, and S of the material in mass%. Is done. The unit of the cooling rate index CI is ° C./second.

  When the material having the chemical composition defined in the present embodiment is cooled so that the average cooling rate from 900 ° C. to 400 ° C. is equal to or lower than the cooling rate index CI, the pearlite area ratio is 90% or more, and the average pearlite A tissue with a lamella spacing larger than 35 nm can be obtained.

  In addition to the above, the average cooling rate from 900 ° C. to 400 ° C. needs to be larger than the pearlite cooling rate index PI in order to set the average lamella spacing of pearlite to 100 nm or less.

  Although the cooling means is not particularly limited, slow air cooling using a heat insulating material, normal air cooling, forced air cooling using a blower, oil cooling, water cooling, ice water cooling, or the like can be used depending on the cooling rate.

  The embodiments of the present invention have been described above. According to this embodiment, a hot forged part having excellent seizure resistance can be obtained. The above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these examples.

  Steel having the chemical composition shown in Table 1 was melted by a 150 kg vacuum induction melting furnace (VIM) to obtain a raw material. Each material was heated to 1200 ° C., and hot forging was performed at 1100 ° C. with a reduction in area of 40%. The thickness of the hot forged material was 7.2 mm. The hot forged material was cooled to room temperature by changing the cooling rate to obtain a test material. Note that “-” in Table 1 indicates that the content of the corresponding element is at the impurity level.

  A plurality of blocks having dimensions of 2.7 mm × 2.0 mm × 2.0 mm were collected from a material having a thickness of 7.2 mm. Using this block, the area ratio of pearlite of each specimen and the average lamella spacing of pearlite were measured by the method described in the embodiment.

  Table 2 shows the relationship between the average cooling rate from 900 ° C. to 400 ° C. and the structure. In Table 2, “P / number” indicates that the area ratio of pearlite was 90% or more, and the number indicates the average lamella spacing (nm) of pearlite. “P / −” indicates that the area ratio of pearlite was 90% or more, and that the average lamella spacing of pearlite was not measured. “P + B” indicates that the area ratio of pearlite is 60% or more and less than 90%, and the balance is a structure containing bainite. “P + α” indicates that the area ratio of pearlite is 60% or more and less than 90%, and the balance is a structure containing ferrite. “X” indicates that the area ratio of pearlite was less than 60%. “-” Indicates that measurement has not been performed.

  As shown in Table 2, in steel types C, E, and I, if the average cooling rate from 900 ° C. to 400 ° C. is equal to or less than the cooling rate index CI and greater than the pearlite cooling rate index PI, the area ratio of pearlite is It was found that a structure having 90% or more and an average lamella spacing of more than 35 nm and not more than 100 nm can be obtained.

  Using the block, the base material hardness (Vickers hardness) of each test material was measured. Table 3 shows the relationship between the average cooling rate from 900 ° C. to 400 ° C. and the base material hardness. In Table 3, “-” indicates that measurement has not been performed.

  From Table 3, it can be seen that if the average cooling rate from 900 ° C. to 400 ° C. is larger than the pearlite cooling rate index PI, the base material hardness can be increased to Hv300 or more.

  5A and 5B are scatter diagrams showing the relationship between the average cooling rate from 900 ° C. to 400 ° C., the structure and the base material hardness. 5A and 5B, solid marks (♦, ■, ●, etc.) indicate that the area ratio of pearlite is 90% or more, and the average lamella spacing is greater than 35 nm and less than 100 nm. White marks (◇, □, ○, etc.) indicate that the area ratio of pearlite is less than 90%, or the average lamella spacing is 35 nm or less or more than 100 nm.

  As described above, if the material having the chemical composition defined in the present embodiment is cooled at a cooling rate with an average cooling rate from 900 ° C. to 400 ° C. equal to or lower than the cooling rate index CI and larger than the pearlite cooling rate index PI. Thus, a hot forged part having a base material hardness of Hv300 or more, a base material structure having a pearlite area ratio of 90% or more, and an average lamella spacing of greater than 35 nm and less than 100 nm is obtained.

[Evaluation of seizure resistance]
In order to evaluate the nano hardness after sliding for a long time, as described below, a wear test was performed on each specimen, and the nano hardness after the wear test was measured.

  First, in order to remove unevenness on the surface, paper polishing and polishing using colloidal silica were performed on a 2.7 mm × 2.0 mm surface of the block. Furthermore, in order to remove the processing layer produced | generated by the grinding | polishing using paper grinding | polishing and colloidal silica, electrolytic polishing was implemented. The thickness of the removed processed layer was about 3 μm. A pin-on-disk wear test shown in FIG. 6 was performed on the test piece from which the processed layer was removed.

  More specifically, the pin-on-disk wear test was performed as follows. The 800-th emery paper 20 was affixed on the surface of the rotating disk 10 of the pin-on-disk tester. Then, while the 2.7 mm × 2.0 mm surface of the test piece (block) 30 was pressed against the emery paper 20 with a surface pressure of 0.3 MPa, the rotating disk 10 was rotated so that the sliding distance was 2000 m. . After the abrasion test, the nano hardness of the sliding surface (2.7 mm × 2.0 mm surface) was measured by the nanoindentation method.

  The nano hardness was measured using a nanoindenter, XP / DCM type, manufactured by Agilent Technology. As shown in FIG. 7, measurement was performed by bringing the probe 40 of the nanoindenter into contact with the sliding surface 30 a of the test piece 30. More specifically, a diamond Barkovic needle was pushed into the sliding surface 30a by a continuous stiffness method (CSM method). The conditions for the continuous stiffness method were a frequency of 45 Hz, an amplitude of 2 nm, and an indentation depth of 250 nm.

  Table 4 summarizes the cooling conditions, the structure, the area ratio of pearlite, the base material hardness, and the nano hardness at a position within 20 nm from the surface layer (hereinafter referred to as surface layer nano hardness). In addition, the test material of the test number 1 implements the heat processing equivalent to induction hardening after hot forging, and is made into the martensitic structure.

  In the “structure” column of Table 4, “M” indicates a martensite structure. “P + α” indicates a ferrite pearlite structure, “P” indicates a pearlite structure, “B + M” indicates a mixed structure of bainite and martensite, and “P + B” indicates a mixed structure of pearlite and bainite.

  As shown in Table 4, the test materials of Test Nos. 5, 9 to 11 and 17 had an area ratio of pearlite of 90% or more and an average lamella spacing of pearlite of more than 35 nm and 100 nm or less. These specimens had a base metal hardness of Hv300 or higher, and showed a nanohardness equivalent to or higher than that of test specimen No. 1 that was subjected to heat treatment equivalent to induction hardening.

  The test material of Test No. 2 had a low base material hardness and a surface layer nano hardness. This is probably because the area ratio of pearlite was low.

  The test materials of test numbers 3, 4, 6, and 8 had low base material hardness and surface layer nano hardness. This is probably because the average lamella spacing of pearlite was too large.

  In Test Nos. 7, 12 to 14, and 18, although the base material hardness was high, the surface layer nano hardness was low. This is presumably because the area ratio of pearlite was low and work hardening by nanocrystallization did not proceed.

  In test numbers 15 and 16, although the base material hardness was high, the surface nano hardness was low. This is thought to be because the average lamella spacing of pearlite was too small, and cracking and peeling occurred on the sliding surface.

  Table 5 and FIG. 8 show the nano hardness distribution in some of the test materials.

  Although the test materials of test numbers 5 and 10 have a base material hardness of about half of the base material hardness of the induction-hardened material (test material of test number 1), the work hardening amount of the surface layer is small. It is large and shows a surface layer nano hardness equal to or higher than that of the induction-hardened material. On the other hand, the test material of test number 13 has an insufficient work-hardening amount, and the surface nanohardness is low.

  From the above results, it was confirmed that even if the surface hardening treatment is omitted, a hot forged part having seizure resistance equivalent to or higher than that of the induction hardening material can be obtained.

Claims (3)

Chemical composition is mass%,
C: 0.35-0.6%,
Si: 0.1% or more and less than 1.2%,
Mn: 0.1% or more and less than 1.2%,
P: more than 0.02% and 0.05% or less,
S: 0.06% or less,
Al: more than 0.05% and 0.8% or less,
N: 0.001 to 0.02%,
V: 0 to 0.2%,
Balance: Fe and impurities,
The chemical composition satisfies the following formula (1):
It has a structure containing pearlite of 90% or more in area ratio,
A hot forged part made of steel, wherein the average lamella spacing of the pearlite is more than 35 nm and not more than 100 nm.
[Si] + [Mn] ≦ 1.2 Formula (1)
Here, each content of Si and Mn is substituted by mass% for [Si] and [Mn]. The hot forged part according to claim 1,
The chemical composition is mass%,
V: 0.05-0.2%
Hot forged parts containing. Chemical composition is mass%, C: 0.35-0.6%, Si: 0.1% or more and less than 1.2%, Mn: 0.1% or more and less than 1.2%, P: 0.02 %: 0.05% or less, S: 0.06% or less, Al: more than 0.05% and 0.8% or less, N: 0.001 to 0.02%, V: 0 to 0.2% The remainder: a step of preparing a material that is Fe and impurities;
Heating the material to 1000-1250 ° C .;
Hot forging the heated material;
Cooling the hot forged material,
The chemical composition satisfies the following formula (1):
In the cooling step, the average cooling rate from 900 ° C. to 400 ° C. is made the cooling rate index CI defined by the following formula (2) or less, and the pearlite cooling rate defined by the following formula (3) A method for manufacturing a hot forged part that is larger than the index PI.
[Si] + [Mn] ≦ 1.2 Formula (1)
CI = 10 ^x Expression (2)
PI = 500 × 10 ^−y Formula (3)
However, x = 0.1 * [Al]-[Si]-[Mn] -5.8 * [V] +2.2
y = 0.6 * [Si] + 1.7 * [Mn] + 46 * [P] + 66 * [S] + 22 * [V] + 1.2 * [Al] -1.9
Here, [Si], [Mn], [Al], [V], [P], and [S] each contain the contents of Si, Mn, Al, V, P, and S in the material in mass%. Assigned. The unit of the cooling rate index CI and the pearlite cooling rate index PI is ° C./second.