JP2018150582A - Forging component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a forging component excellent in seizure resistance.SOLUTION: A forging component consists of a steel material, which has a chemical composition containing, by mass%, C:0.35 to 0.65%, Si:less than 0.50%, Mn:0.10% or more and less than 0.80%, P:over 0.01% and 0.1% or less, S:0.06% or less, Cr:0.2 to 1.5%, Al:0.005 to 0.05%, N:0.001 to 0.02%, V:0 to 0.2% and the balance:Fe with impurities, and satisfying the following formula (1), a structure containing perlite of 90% or more by area percentage, average lamella interval of the perlite of less than 50 nm and dislocation density of 10mor more. [Si]+[Mn]≤1.2 Formula (1), wherein [Si] and [Mn] is assigned by each content of Si and Mn by mass%.SELECTED DRAWING: None

Description

  The present invention relates to forged parts.

  A sliding component represented by a crankshaft is required to have wear resistance and fatigue strength.

  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. For this reason, the sliding condition between the crankshaft and the bearing is 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. 4140283 Japanese Patent No. 4589885

  As a result of the investigation by the present inventors, the sliding component after sliding for a long time is affected by heat more than conventionally thought, such as tempered structure and recrystallized structure observed on the surface. all right. When the recrystallized structure is formed, the local softened region is selectively worn, and the seizure resistance is significantly reduced.

  An object of the present invention is to provide a forged part excellent in seizure resistance.

  The inventors reduced the nano hardness of the sliding parts after sliding for a long time by lowering the Si and Mn contents of the steel material constituting the hot forged parts and increasing the Cr and P contents. The following invention was filed as Japanese Patent Application No. 2015-192588 as a hot forged part that can be used even if the surface hardening treatment is omitted, and that it can be maintained at a high level and is excellent in thermal stability of the sliding surface.

Chemical composition is mass%, C: 0.35 to 0.65%, Si: less than 0.50%, Mn: 0.10% or more and less than 0.80%, P: more than 0.01% and 0.00. 1% or less, S: 0.06% or less, Cr: 0.2-1.5%, Al: 0.005-0.05%, N: 0.001-0.02%, V: 0-0 .2%, balance: Fe and impurities, the chemical composition satisfies the following formula (1), has a structure containing pearlite of 90% or more in area ratio, and the average lamella spacing of the pearlite is 50 to 50%. Hot forged parts made of steel with a thickness of 140 nm.
[Si] + [Mn] ≦ 1.2 Formula (1)
Here, each content of Si and Mn is substituted by mass% for [Si] and [Mn].

  As a result of repeated studies, the present inventors have obtained the knowledge that when the dislocation density of the surface layer of the hot forged part is increased, higher seizure resistance can be expected, and the present invention has been achieved.

The forged part according to one embodiment of the present invention has a chemical composition of mass%, C: 0.35 to 0.65%, Si: less than 0.50%, Mn: 0.10% or more and less than 0.80%. , P: more than 0.01% and 0.1% or less, S: 0.06% or less, Cr: 0.2-1.5%, Al: 0.005-0.05%, N: 0.001 -0.02%, V: 0-0.2%, balance: Fe and impurities, and the chemical composition satisfies the following formula (1) and has a structure containing pearlite of 90% or more in area ratio. And the average lamella space | interval of the said pearlite is less than 50 nm, Comprising: It consists of steel materials whose dislocation density is 10 ^{< 15} > m ^<-2 > or more.
[Si] + [Mn] ≦ 1.2 Formula (1)
Here, each content of Si and Mn is substituted by mass% for [Si] and [Mn].

  According to the present invention, a forged part having excellent seizure resistance can be 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 forged part according to an embodiment of the present invention. FIG. 5 is a schematic diagram of a pin-on-disk wear test. FIG. 6 is a schematic diagram of nanohardness measurement by the nanoindentation method.

  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.

  Therefore, in order to improve the nano hardness of the sliding surface, (1) improvement of work hardening, (2) improvement of thermal stability, (3) frictional heat dissipation due to improvement of thermal conductivity is effective. is there.

(1) Improvement of work hardenability The present inventors have found that a steel material having pearlite whose structure is appropriately controlled is greatly work hardened by sliding and exhibits excellent nano hardness.

  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.

  In addition, the finer the pearlite lamella spacing, the easier it is to crystallize and the higher the work curability.

(2) Improvement of thermal stability Perlite has higher thermal stability because it has less accumulated strain than martensite and bainite. Therefore, from the viewpoint of improving the thermal stability of the sliding surface, a structure having a high pearlite fraction is preferable.

  The nanocrystallized layer is thermally unstable because it is in a non-equilibrium state where carbon is dissolved in supersaturation and has a very high dislocation density. When the temperature rises by sliding, a large structural change occurs even in a temperature range of 500 ° C. or lower, and cementite precipitation, coarsening, and recrystallization proceed in a short time.

  Cr controls the precipitation form of cementite and suppresses coarsening of cementite. Therefore, in order to suppress cementite coarsening, it is preferable to contain an appropriate amount of Cr.

  P suppresses the coarsening of the crystal grains of the nanocrystallized layer. Therefore, in order to suppress the coarsening of the crystal grains of the nanocrystallized layer, it is preferable to increase the P content.

  Conventionally, there has been a problem that carbide is easily coarsened on the sliding surface due to the influence of frictional heat. However, it was confirmed that the coarsening of the carbide was suppressed by the reduction of Si and Mn, and as a result, the coarsening of the crystal grains of the nanocrystallized layer was suppressed by the pinning effect by the fine carbide.

(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, the content of P is increased, the appropriate amount of Cr is included, and the content of Si and Mn is decreased. Furthermore, a structure having a high pearlite fraction and a pearlite lamella spacing may be reduced. More specifically, the chemical composition of the steel material constituting the forged part satisfies the following formula (1), has a structure containing pearlite with an area ratio of 90% or more, and the average lamella spacing of pearlite is less than 50 nm. As a result, a forged part having excellent seizure resistance can be obtained.
[Si] + [Mn] ≦ 1.2 Formula (1)
Here, each content of Si and Mn is substituted by mass% for [Si] and [Mn].

(4) Suppression of peeling on sliding surface As described above, an appropriate amount of the nanocrystallized layer contributes to 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.

However, if the dislocation density on the sliding surface is increased, even if the average lamella spacing of the pearlite is small, cracking and peeling in the nanocrystallized layer can be suppressed, and higher nanohardness can be obtained. More specifically, if the dislocation density is 10 ¹⁵ m ⁻² or more, cracking and peeling in the nanocrystallized layer can be suppressed even if the average lamella spacing of pearlite is less than 50 nm.

In order to achieve both a dislocation density of 10 ¹⁵ m ⁻² or more and an average pearlite lamellar spacing of less than 50 nm, it is preferable to impart cold working to the pearlite structure.

  Based on the above findings, the present invention has been completed. Hereinafter, a forged part according to an embodiment of the present invention will be described in detail.

[Chemical composition]
The steel material constituting the forged part according to the present embodiment has a chemical composition described below. In the following description, “%” of the element content means mass%.

C: 0.35-0.65%
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 0.65%, the machinability of the steel decreases. Therefore, the C content is 0.35 to 0.65%. The lower limit of the C content is preferably 0.4%, more preferably 0.5%. The upper limit of the C content is preferably 0.6%, more preferably 0.58%.

Si: Less than 0.50% Silicon (Si) is contained in steel as an impurity. Si can be positively contained in order to obtain a pearlite structure. Si also strengthens pearlite by dissolving in ferrite in pearlite. On the other hand, Si reduces the thermal conductivity of steel. Therefore, when the Si content is 0.50% or more, sufficient seizure resistance cannot be obtained. Accordingly, the Si content is less than 0.50%. The lower limit of the Si content is preferably 0.10%. The upper limit of the Si content is preferably 0.40%, more preferably 0.30%.

Mn: 0.10% or more and less than 0.80% Manganese (Mn) is an element necessary for obtaining a pearlite structure. When the Mn content is less than 0.10%, a structure having a high pearlite fraction cannot be obtained. On the other hand, Mn reduces the thermal conductivity of steel. Therefore, if the Mn content is 0.80% or more, sufficient seizure resistance cannot be obtained. Therefore, the Mn content is 0.10% or more and less than 0.80%. The lower limit of the Mn content is preferably 0.30%, more preferably 0.40%, and further preferably 0.50%. The upper limit of the Mn content is preferably 0.78%.

P: More than 0.01% and 0.1% or less Phosphorus (P) strengthens pearlite by dissolving in ferrite in pearlite. P also improves the work stability of pearlite and improves the thermal stability of the tissue. If the P content is 0.01% or less, these effects cannot be obtained. On the other hand, if the P content exceeds 0.1%, excessive P segregates at the grain boundaries, and the fatigue strength of the steel decreases. Therefore, the P content exceeds 0.01% and is 0.1% or less. From the viewpoint of the lower limit, the P content is preferably 0.02% or more, and more preferably higher than 0.02%. The upper limit of the P content is preferably 0.08%, and more preferably 0.05%.

S: 0.06% or less Sulfur (S) is contained in steel as an impurity. When S is positively incorporated, 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. The lower limit of the S content is preferably 0.01%, more preferably 0.02%. The upper limit of the S content is preferably 0.055%.

Cr: 0.2 to 1.5%
Chromium (Cr) increases the strength of the steel. Cr further controls the precipitation form of cementite and improves the thermal stability of the nanocrystallized layer. If the Cr content is less than 0.2%, this effect cannot be sufficiently obtained. On the other hand, when the Cr content exceeds 1.5%, martensite and bainite are likely to be generated. Therefore, the Cr content is 0.2 to 1.5%. The lower limit of the Cr content is preferably 0.25%. The upper limit of the Cr content is preferably 1.4%, more preferably 1.3%.

Al: 0.005 to 0.05%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, this effect cannot be sufficiently obtained. On the other hand, if the Al content exceeds 0.05%, the machinability of the steel decreases. Therefore, the Al content is 0.005 to 0.05%. The lower limit of the Al content is preferably 0.010%, more preferably 0.020%. The upper limit of the Al content is preferably 0.045%, more preferably 0.04%.

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%, more preferably 0.01%.

  The balance of the chemical composition of the steel material constituting the 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 forged part according to the present embodiment may contain vanadium (V) instead of part of Fe. V increases the strength of the steel. 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 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 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 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. Perlite is also excellent in thermal stability. If the steel material contains martensite, bainite, or the like, the thermal stability of the structure decreases. 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 forged part to include a position at a depth of 100 μm from the surface. At this time, a sample is taken so that a surface perpendicular to the surface of the 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 forged part according to the present embodiment, the average lamella spacing of pearlite is less than 50 nm. When the average lamella spacing is 50 nm or more, cementite divided during sliding tends to be coarsened, and the thermal stability of the sliding surface decreases. The lower limit of the average lamella interval is preferably 10 nm. The upper limit of the average lamella spacing is preferably 35 nm.

  The average lamella spacing of pearlite is measured as follows.

  Similarly to the measurement of the area ratio of pearlite, a sample is taken so that the surface perpendicular to the surface of the 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 steel material constituting the forged part according to this embodiment has a dislocation density of 10 ¹⁵ m ⁻² or more. The dislocation density is preferably 1.5 × 10 ¹⁵ m ⁻² or more, and more preferably 2.0 × 10 ¹⁵ m ⁻² or more.

  The dislocation density is measured as follows.

A sample is taken from the forged part and mechanically and electrolytically polished. Using an X-ray generator using Cu-Kα as a radiation source, four or more diffraction peaks of the ferrite phase are measured. The measured diffraction peak is separated into Kα1 and Kα2 by removing the background using dedicated analysis software. The half width (full width at half maximum) Δ2θ [rad] at each diffraction angle θ [rad] is measured for the separated Kα1 diffraction peak. The half width is corrected by measuring the half width derived from the apparatus using a single crystal of LaB ₆ (lanthanum hexaboride) and subtracting it from the measured half width.

The dislocation density ρ [m ⁻² ] is obtained from the modified Williamson-Hall equation shown below.

Where ΔK _FWHM = Δ2θ cos θ / λ, K = 2 sin θ / λ, λ is the X-ray wavelength (0.154 nm), D is the crystallite size, b is the ferrite Burgers vector (0.249 nm), and O is the higher order term. It is.

  C is a constant indicating crystal anisotropy as described below.

  M is a dimensionless coefficient indicating a dislocation array, and here, M = 1.

In the formula (*), if high-order terms are ignored, ΔK _FHHM and K show a linear relationship. By plotting ΔK _FHHM and K, the crystallite size D can be obtained from the intercept, and the dislocation density ρ can be obtained from the slope.

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

  FIG. 4 is a flowchart showing an example of a method for producing a 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. A step of cooling (step S4) and a step of cold working the sliding surface (step S5) are provided.

  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 formula (2).
CI = 10 ^x Expression (2)
However, x = 0.1 * [Al]-[Si]-[Mn] -0.5 * [Cr] -5.8 * [V] +2.2
Here, the contents of Al, Si, Mn, Cr, and V of the material are substituted by mass% for [Al], [Si], [Mn], [Cr], and [V]. The unit of the cooling rate index CI is ° C./second.

  If the material having the chemical composition specified 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, a structure having an area ratio of pearlite of 90% or more can be obtained. it can.

In addition to the above, it is preferable to make the average cooling rate from 900 ° C. to 400 ° C. larger than the pearlite cooling rate index PI defined by the following formula (3).
PI = 500 × 10 ^−y Formula (3)
However, y = 0.6 * [Si] + 1.7 * [Mn] + 46 * [P] + 66 * [S] + 17 * [V] + 1.2 * [Al] + 0.2 * [Cr] -1. 9
Here, [Al], [Si], [Mn], [Cr], [V], [P], and [S] contain each of the materials Al, Si, Mn, Cr, V, P, and S The quantity is substituted in mass%. The unit of the pearlite cooling rate index PI is ° C./second.

  If the average cooling rate from 900 ° C to 400 ° C is equal to or less than the pearlite cooling rate index PI, the average lamella spacing of the pearlite becomes too large, and the average lamella spacing of the pearlite is less than 50 nm even if cold processing described later is performed May be difficult to do.

  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.

Cold processing is performed on the cooled material (step S5). The cold working is, for example, cold roll working, cold rolling, cold forging, or the like. The cold work may be a local cold work such as a fillet process. In the cold working, for example, in the case of cold rolling, the area reduction rate is set to 30% or more. If the degree of cold working is small, the dislocation density is low. This is because recrystallization occurs locally due to the heat generated during cold working. When locally recrystallized, the recrystallized grains are selectively abraded, resulting in irregularities on the surface, which causes further wear and seizure. If the degree of cold working is increased and the area reduction rate is 30% or more, the dislocation density can be 10 ¹⁵ m ⁻² or more and the pearlite lamella spacing can be less than 50 nm. The area reduction rate in the case of cold rolling is preferably 40% or more.

  The embodiments of the present invention have been described above. According to this embodiment, a 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.

  Cold rolling was performed on a test material having a thickness of 7.2 mm cooled to room temperature, simulating cold rolling. Specifically, after both surfaces of a 7.2 mm thick material were polished, cold rolling was performed so that the area reduction rate was 30 to 50%. A plurality of blocks having dimensions of 2.7 mm × 2.0 mm × 2.0 mm were collected from the surface of the test material after cold rolling so that the sliding surface was a surface of 2.7 mm × 2.0 mm. Using this block, the area ratio of pearlite, the average lamella spacing of pearlite, the dislocation density, and the nano hardness after the wear test were measured.

  Table 2 shows the relationship between the average cooling rate from 900 ° C. to 400 ° C. and the structure. In Table 2, “P” indicates that the area ratio of pearlite was 90% or more. “Δ” indicates that the area ratio of pearlite was less than 90%. “-” Indicates that measurement has not been performed.

  As shown in Table 2, it was found that when the average cooling rate from 900 ° C. to 400 ° C. was the cooling rate index CI or less, a structure having a pearlite area ratio of 90% or more was 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. 5 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. .

  A thermocouple (not shown) was attached to the surface of the test piece 30 to measure the surface temperature during the wear test. The surface temperature rose to about 50 ° C. within a few seconds after the start of the abrasion test, but it was confirmed that the temperature change thereafter was small.

  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. 6, the 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 3 summarizes the cooling conditions, the structure, the area ratio of pearlite, the average lamella spacing, the dislocation density, and the nano hardness at a position within 20 nm from the surface layer (hereinafter referred to as surface layer nano hardness) for each specimen. .

  The test materials of test numbers 5 to 8 are the cold of the surface reduction rate described in the column of “Cold rolling surface reduction rate (%)” in the test materials of test numbers 1, 2, 4, and 1, respectively. It is the one with processing. The measured values after cold working are described in the columns of “average lamella spacing”, “dislocation density”, and “surface nanohardness” of these test materials.

  In the “structure” column of Table 3, “P” indicates a pearlite structure.

As shown in Table 3, the test materials of Test Nos. 5 and 6 have a structure containing pearlite with an area ratio of 90% or more, a dislocation density of 10 ¹⁵ m ⁻² or more, and an average lamella of pearlite. The interval was less than 50 nm. The test materials of test numbers 5 and 6 showed excellent nano hardness.

  The test materials of Test Nos. 1 to 4 were inferior in surface layer nano hardness as compared with the test materials of Test Nos. 5 and 6. This is presumably because the dislocation density was too low.

  The test materials of Test Nos. 7 and 8 were inferior in surface layer nano hardness as compared with the test materials of Test Nos. 5 and 6. This is probably because the dislocation density was too low, or the average lamella spacing of pearlite was too large. The test material of test number 7 had an average lamellar spacing of the steel before cold working (test material 4) that was too large, so even if cold working was applied, it should be adjusted to the prescribed dislocation density and average lamellar spacing. I could not.

  From the above results, it was confirmed that a forged part excellent in seizure resistance was obtained.

Claims (2)

Chemical composition is mass%,
C: 0.35-0.65%,
Si: less than 0.50%,
Mn: 0.10% or more and less than 0.80%,
P: more than 0.01% and 0.1% or less,
S: 0.06% or less,
Cr: 0.2 to 1.5%
Al: 0.005 to 0.05%,
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,
An average lamellar spacing of the pearlite is less than 50 nm,
A forged part made of a steel material having a dislocation density of 10 ¹⁵ m ⁻² or more.
[Si] + [Mn] ≦ 1.2 Formula (1)
Here, each content of Si and Mn is substituted by mass% for [Si] and [Mn]. The forged part according to claim 1,
The chemical composition is mass%,
V: 0.05-0.2%
Forged parts containing.