Crankshaft profile and method for manufacturing crankshaft profile

Abstract

Provided is a profile for crankshafts, which has excellent wear resistance and excellent machinability even when used without high-frequency hardening. The crankshaft profile has a prescribed chemical composition, Fn1 defined by formula (1) is 3.80-5.00, the structure contains 12-40 vol% of proeutectoid ferrite, the balance being pearlite, and the thickness of the crankshaft profile at a position 50 [mu] m deep from the surface of a portion corresponding to the pin portion of the crankshaft is 10-30 [mu] m. The ratio (Mn [theta] / C [theta]) of the Mn content (Mn [theta]) to the C content (C [theta]) obtained by analyzing cementite in the pearlite by energy dispersive X-ray analysis is 0.360 or more. Fn1 = 7C + 0.5 Si + Mn + 3Cr-10Ti (1) In formula (1), the contents of the corresponding elements are substituted in mass% at C, Si, Mn, Cr, and Ti.

Description

Technical Field

[0001] This invention relates to crankshaft profiles and a method for manufacturing crankshaft profiles. In this specification, "crankshaft profile" refers to an intermediate product processed into a shape similar to that of the crankshaft as the final product. Background Technology

[0002] Crankshafts used in automotive engine components typically employ high-frequency quenching of the pins and / or journals, which serve as sliding parts. High-frequency quenching forms a high-hardness structure, such as martensite, on the surface of the pins and / or journals, improving their wear resistance. On the other hand, crankshafts that omit high-frequency quenching are also used primarily to reduce costs. In carbon steel and / or alloy steel for mechanical structures, steel that omits quenching and tempering treatments is called "non-quenched and tempered steel."

[0003] Japanese Patent Application Publication No. 8-92687 discloses a high-strength, high-toughness non-quenched and tempered steel. This non-quenched and tempered steel is characterized by having a specified chemical composition and containing 1×10⁻⁶... 2 ~1×10 6 pcs / mm 2 The inclusions have an average particle size of 0.1~5μm and are Ti oxide / nitride, MnS, and composite compounds mainly composed of Ti oxide / nitride and MnS.

[0004] Japanese Patent No. 3235442 discloses a high-strength, low-ductility non-quenched and tempered steel. This non-quenched and tempered steel is characterized by satisfying fn2 (=Si+2V+5P-0.8)≥0 and fn61 (=C+(Si / 10)+(Mn / 5)+(5Cr / 22)+1.65V-0.8)≥0.

[0005] Crankshaft profiles are machined by grinding and / or drilling. Therefore, crankshaft profiles preferably have excellent machinability. As for steels that, while not for crankshaft profiles, have excellent machinability, Japanese Patent No. 3437079 discloses a mechanical structural steel with excellent chip handling properties. This mechanical structural steel is characterized by a cross-section of 1 mm in the rolling direction. 2 The field of view contains more than 30 individual sulfides with a length of more than 20 μm, or groups of sulfides with a length of more than 20 μm formed by multiple sulfides connected in a roughly tandem manner.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 8-92687

[0009] Patent Document 2: Japanese Patent No. 3235442

[0010] Patent Document 3: Japanese Patent No. 3437079 Summary of the Invention

[0011] The problem the invention aims to solve

[0012] As a method to improve the wear resistance of steel, increasing the carbon content to increase the hardness of the steel is considered. However, there is a trade-off between increasing the carbon content and the machinability of the steel. Additionally, as a method to improve the mechanical properties of non-quenched and tempered steel, precipitation-hardening elements such as v are added. However, since v is a relatively expensive element, it is preferable to improve the mechanical properties of the steel through methods other than adding v.

[0013] The objective of this invention is to provide a crankshaft profile that exhibits excellent wear resistance and machinability even when used without high-frequency quenching.

[0014] Solution for solving the problem

[0015] The chemical composition of the crankshaft profile according to one embodiment of the present invention, by mass%, is as follows: C: 0.35~0.50%, Si: 0.50~1.00%, Mn: 1.00~1.50%, Cr: 0.30% or less, P: 0.030% or less, S: 0.020~0.100%, Al: 0.500% or less, Ti: 0.001~0.050%, N: 0.002~0.020%, V: 0~0.20%, Ca: 0~0.010%, balance: Fe and impurities. The Fn1 defined by the following formula (1) is 3.80 or more and 5.00 or less. The microstructure contains 12~40 vol% proeutectoid ferrite and the balance is pearlite. The Mn content is obtained by analyzing the cementite in the pearlite at a depth of 50 μm from the surface at a position corresponding to the pin portion of the crankshaft using energy dispersive X-ray analysis. θ With C content C θ The ratio of Mn θ / C θ It is above 0.360.

[0016] Fn1=7C+0.5Si+Mn+3Cr-10Ti (1)

[0017] In equation (1), the mass percentage of the corresponding element is substituted for C, Si, Mn, Cr and Ti.

[0018] The above energy-dispersive X-ray analysis used C, Mn, Cr, V, and Fe as analytical objects, and the Mn... θ and C θThese are values ​​calculated by using the sum of C, Mn, Cr, V, and Fe as the denominator and calculating by mass% respectively.

[0019] One embodiment of the present invention provides a method for manufacturing crankshaft profiles, comprising: a billet forging process in which a slab manufactured by continuous casting is hot-forged to form a billet; and a die forging process in which the billet is hot-forged. The heating temperature before hot forging in the billet forging process is 1150~1300°C, and the holding time at the heating temperature is 3.0 hours or more. The heating temperature before hot forging in the die forging process is 1200~1350°C, and the heating time up to the heating temperature is 500 seconds or less.

[0020] The effects of the invention

[0021] According to the present invention, crankshaft profiles with excellent wear resistance and excellent machinability can be obtained even when used without high-frequency quenching. Attached Figure Description

[0022] Figure 1 This is a scatter plot showing the relationship between Fn1 and the proeutectoid ferrite volume fraction Fα.

[0023] Figure 2 This is a scatter plot showing the relationship between Fn1 and the amount of wear on the back face. Detailed Implementation

[0024] In order to solve the above-mentioned problems, the inventors have conducted various studies and obtained the following insights.

[0025] Crankshaft profiles are manufactured by hot forging steel billets of a specified size. Hot-forged crankshaft profiles have a microstructure composed of ferrite and pearlite. To improve the wear resistance of the hot-forged microstructure, i.e., the ferrite-pearlite microstructure, increasing the amount of cementite dissolved in pearlite by Mn is effective.

[0026] The steel billets that become the raw materials for crankshaft profiles are manufactured, for example, from slabs produced by continuous casting through hot forging or hot rolling. In slabs produced by continuous casting, due to the solidification of molten steel from the surface, manganese (Mn) tends to accumulate near the center of the slab. Therefore, in steel billets manufactured from such slabs, Mn sometimes also accumulates near the center, and in crankshaft profiles manufactured from such steel billets, Mn sometimes also accumulates near the center.

[0027] On the other hand, since the properties near the surface contribute to wear resistance, it is preferable to have a sufficient amount of Mn near the surface of the crankshaft profile in order to improve the wear resistance of the crankshaft profile.

[0028] To ensure a sufficient amount of Mn near the surface of the crankshaft profile, it is effective to perform prolonged heating during the billet manufacturing stage to allow for sufficient Mn diffusion, followed by hot forging to eliminate center segregation. It should be noted that, from the perspective of Mn diffusion, extending the heating time during the manufacturing of the crankshaft profile from the billet has also been considered; however, as will be discussed later, extending this heating time will not yield the desired microstructure.

[0029] To improve the machinability of crankshaft profiles, increasing the volume fraction of proeutectoid ferrite in the ferrite-pearlite microstructure is effective. To increase the volume fraction of proeutectoid ferrite, controlling the chemical composition of the steel and increasing the heating rate before hot forging when manufacturing crankshaft profiles from billets is effective. Specifically, rapidly initiating hot forging after rapid heating via high-frequency induction heating is effective. This suppresses austenite grain growth in the high-temperature region. By reducing austenite grain size, the area of ​​grain boundaries that become precipitation sites for proeutectoid ferrite increases, thereby increasing the amount of proeutectoid ferrite.

[0030] This invention is based on the above insights. Hereinafter, a crankshaft profile according to one embodiment of the invention will be described in detail.

[0031] Crankshaft Profiles

[0032] [Chemical Composition]

[0033] The crankshaft profile of this embodiment has the chemical composition described below. In the following description, the "%" of the element content refers to mass%.

[0034] C: 0.35~0.50%

[0035] Carbon (C) increases the hardness of steel, contributing to improved fatigue strength and wear resistance. However, excessively high C content reduces machinability. Therefore, the C content is typically 0.35% to 0.50%. The lower limit of the C content is preferably 0.37%, and the upper limit is preferably 0.45%.

[0036] Si: 0.50~1.00%

[0037] Silicon (Si) has deoxidizing and ferrite-strengthening effects. On the other hand, if the Si content is too high, the machinability of the steel will decrease. Therefore, the Si content is 0.50~1.00%. The lower limit of the Si content is preferably 0.55%. The upper limit of the Si content is preferably 0.90%, and more preferably 0.70%.

[0038] Mn: 1.00~1.50%

[0039] Manganese (Mn) increases the hardness of steel, contributing to improved fatigue strength and wear resistance. Furthermore, Mn dissolved in cementite within pearlite strengthens the pearlite. On the other hand, excessively high Mn content reduces the machinability of the steel. Therefore, the Mn content is typically 1.00–1.50%. The lower limit of the Mn content is preferably 1.10%, more preferably 1.20%. The upper limit of the Mn content is preferably 1.40%, more preferably 1.35%.

[0040] Cr: less than 0.30%

[0041] Chromium (Cr) increases the hardness of steel, contributing to improved fatigue strength and wear resistance. However, excessively high Cr content reduces the machinability of the steel. Therefore, the Cr content is 0.30% or less. The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Cr content is preferably 0.25%, and even more preferably 0.20%.

[0042] P: below 0.030%

[0043] Phosphorus (P) is an impurity. P reduces the hot workability of steel. Therefore, the P content is 0.030% or less. The P content is preferably 0.025% or less, and more preferably 0.020% or less.

[0044] S: 0.020~0.100%

[0045] Sulfur (S) forms MnS, which improves the machinability of steel. On the other hand, if the S content is too high, the hot workability of the steel will decrease. Therefore, the S content is 0.020~0.100%. The lower limit of the S content is preferably 0.030%, more preferably 0.040%. The upper limit of the S content is preferably 0.080%, more preferably 0.070%.

[0046] Al: Below 0.500%

[0047] Aluminum (Al) has a deoxidizing effect. On the other hand, if the Al content is too high, the machinability of the steel will decrease. Therefore, the Al content is 0.500% or less. The lower limit of the Al content is preferably 0.005%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Al content is preferably 0.200%, more preferably 0.100%, and even more preferably 0.050%.

[0048] Ti: 0.001~0.050%

[0049] Titanium (Ti) forms nitrides and / or carbonitrides, which contribute to grain refinement (austenite grains) in high-temperature regions. By refining the austenite grains, the area of ​​grain boundaries that become precipitation sites for proeutectoid ferrite increases, resulting in an increase in proeutectoid ferrite. On the other hand, even with excessive Ti addition, the effect saturates, leading to increased costs. Therefore, the Ti content is 0.001~0.050%. The lower limit of the Ti content is preferably 0.005%. The upper limit of the Ti content is preferably 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0050] N: 0.002~0.020%

[0051] Nitrogen (N) forms nitrides and / or carbonitrides, which contribute to the refinement of grains (austenite grains) in high-temperature regions. By refining the austenite grains, the area of ​​grain boundaries that become precipitation sites for proeutectoid ferrite increases, resulting in an increase in proeutectoid ferrite. However, excessively reducing N content increases manufacturing costs. On the other hand, if the N content is too high, the hot workability of the steel decreases. Therefore, the N content is 0.002% to 0.020%. The lower limit of the N content is preferably 0.005%. The upper limit of the N content is preferably 0.015%, and more preferably 0.010%.

[0052] The crankshaft profile of this embodiment has a chemical composition balance of Fe and impurities. Impurities, as referred to here, are elements mixed in from ores and / or waste used as raw materials for steel, or elements mixed in from the environment during the manufacturing process.

[0053] The chemical composition of the crankshaft profile in this embodiment may also contain one or two elements selected from V and Ca to replace a portion of Fe. V and Ca are arbitrary elements. That is, the chemical composition of the crankshaft profile in this embodiment may also not contain one or both of V and Ca.

[0054] V: 0~0.20%

[0055] Vanadium (V) improves the wear resistance of steel through precipitation hardening. This effect can be achieved with only a small amount of V. The crankshaft profile of this embodiment improves wear resistance by means other than the addition of V. Therefore, the crankshaft profile of this embodiment may also be V-free. The absence of V becomes advantageous in terms of cost. On the other hand, V may be included as needed. However, if the V content is too high, the machinability of the steel will decrease. Therefore, the V content is 0 to 0.20%. The lower limit of the V content is preferably 0.02%, more preferably 0.08%. The upper limit of the V content is preferably 0.15%, more preferably 0.12%. From the viewpoint of manufacturing cost, the upper limit of the V content is preferably 0.10%, more preferably 0.08%, and more preferably 0.05%. From the viewpoint of manufacturing cost, it is most preferable to have no V.

[0056] Ca: 0~0.010%

[0057] Calcium (Ca) improves the hot workability of steel. This effect can be achieved with only a small amount of Ca. Therefore, Ca can be included as needed. On the other hand, if the Ca content is too high, the toughness of the steel will decrease. Therefore, the Ca content is 0 to 0.010%. The lower limit of the Ca content is preferably 0.001%. The upper limit of the Ca content is preferably 0.008%, and more preferably 0.006%.

[0058] [About Fn1]

[0059] In this embodiment, the Fn1 of the crankshaft profile, as defined by the following formula (1), is 3.80 or more and 5.00 or less.

[0060] Fn1=7C+0.5Si+Mn+3Cr-10Ti (1)

[0061] In equation (1), the mass percentage of the corresponding element is substituted for C, Si, Mn, Cr and Ti.

[0062] Fn1 is a parameter related to the volume fraction of proeutectoid ferrite. The smaller Fn1 is, the larger the volume fraction of proeutectoid ferrite becomes. If Fn1 is less than 3.80 or more than 5.00, it is difficult to keep the volume fraction of proeutectoid ferrite within an appropriate range. The lower limit of Fn1 is preferably 4.00, more preferably 4.20, and even more preferably 4.40. The upper limit of Fn1 is preferably 4.95, more preferably 4.80, and even more preferably 4.60.

[0063] [organize]

[0064] The microstructure of the crankshaft profile in this embodiment comprises 12 to 40% by volume of proeutectoid ferrite, with the balance being pearlite.

[0065] The crankshaft profile of this embodiment has a ferrite-pearlite microstructure. That is, the crankshaft profile of this embodiment has a microstructure composed of proeutectoid ferrite and pearlite. Pearlite refers to a microstructure in which ferrite and cementite form a fine layered structure (pearlite lamellae), and proeutectoid ferrite refers to ferrite that precipitates from the original austenite grain boundaries, unlike the ferrite in the pearlite lamellae.

[0066] In the crankshaft profile of this embodiment, the volume fraction of proeutectoid ferrite is 12-40%. If the volume fraction of proeutectoid ferrite is too low, good machinability will not be obtained. On the other hand, if the volume fraction of proeutectoid ferrite is too high, fatigue strength may decrease. The lower limit of the volume fraction of proeutectoid ferrite is preferably 14%, more preferably 18%, and more preferably 22%. The upper limit of the volume fraction of proeutectoid ferrite is preferably 36%, more preferably 32%, and more preferably 30%.

[0067] The crankshaft profile of this embodiment preferably satisfies the following formulas (2) and (3).

[0068] Fα>-21.364×Fn1+112.0 (2)

[0069] Fα<-21.364×Fn1+130.0 (3)

[0070] In equations (2) and (3), the volume fraction of proeutectoid ferrite is substituted at Fα with %.

[0071] [Mn θ / C θ ]

[0072] Regarding the crankshaft profile of this embodiment, the Mn content (Mn) was obtained by analyzing the cementite in the pearlite at a depth of 50 μm from the surface in the portion corresponding to the crankshaft pin section using energy-dispersive X-ray diffraction (EDXRD). θ With C content C θ The ratio of Mn θ / C θ The value is above 0.360. Here, energy-dispersive X-ray analysis uses C, Mn, Cr, V, and Fe as analytical objects, with Mn... θ and C θ These are values ​​calculated by using the sum of C, Mn, Cr, V, and Fe as the denominator and calculating by mass% respectively.

[0073] To improve the wear resistance of the ferrite-pearlite microstructure, increasing the amount of cementite dissolved in the pearlite with Mn is effective. The portion near the surface of the crankshaft profile contributes to wear resistance. Therefore, to improve wear resistance, it is preferable to increase the amount of cementite dissolved in the pearlite near the surface with Mn.

[0074] In this embodiment, the portion corresponding to the crankshaft pin, at a depth of 50 μm from the surface, is used as the evaluation target. Energy-dispersive X-ray diffraction (EDX) analysis is employed to analyze the cementite within the pearlite at this location. Specifically, the portion corresponding to the crankshaft pin is cut with a plane perpendicular to the pin's axial direction as the cut surface, and the cut surface is observed using a scanning electron microscope (SEM) at approximately 10,000x magnification. While confirming the SEM image, electron beams are irradiated onto the cementite within the pearlite, and the chemical composition of the cementite is analyzed using EDX. C, Mn, Cr, V, and Fe are selected for analysis because elements other than these are considered to have a low probability of solid dissolving in cementite, and even if present, they are minimal and would not affect the properties.

[0075] In this embodiment, the Mn content (Mn) obtained from the analysis results is used as the basis for the Mn content. θ With C content C θ The ratio of Mn θ / C θ The solid solution amount of Mn into cementite in pearlite near the surface was evaluated. If Mn θ / C θ A value above 0.360 indicates excellent wear resistance. Mn θ / C θ The lower limit is preferably 0.380, more preferably 0.400, more preferably 0.450, more preferably 0.500, and more preferably 0.550. Mn θ / C θ There is no particular upper limit, for example, 2.000, preferably 1.500, and more preferably 1.200.

[0076] [Manufacturing method for crankshaft profiles]

[0077] Next, an example of the manufacturing method of the crankshaft profile according to this embodiment will be described. The manufacturing method described below is merely illustrative and does not limit the manufacturing method of the crankshaft profile according to this embodiment.

[0078] The crankshaft profile of this embodiment can be manufactured by hot forging a slab produced by continuous casting to form a steel billet, and then hot forging the steel billet to produce the crankshaft profile. Hereinafter, the hot forging process of manufacturing a steel billet from a slab will be referred to as "blanking forging," and this process will be referred to as the blanking forging process. In addition, the hot forging process of manufacturing a crankshaft profile from a steel billet will be referred to as "die forging," and this process will be referred to as the "die forging process."

[0079] [Blank forging process]

[0080] First, the steel slab obtained by continuous casting of molten steel with the above chemical composition is hot forged to produce steel sheets (steel billets) of a size suitable for die forging (open forging process).

[0081] In the hot forging process of the billet forging, the pre-forging heating temperature is set to 1150~1300°C, and the holding time at the heating temperature is set to 3.0 hours or more. By holding at a high temperature for a long time before forging, Mn segregated in the center of the slab diffuses. If the heating temperature is too low or the holding time is too short, the diffusion of Mn may become insufficient. On the other hand, if the heating temperature is too high, the slab may partially melt. The lower limit of the heating temperature in the billet forging process is preferably 1180°C. The upper limit of the heating temperature in the billet forging process is preferably 1280°C, more preferably 1260°C. The lower limit of the holding time at the heating temperature in the billet forging process is preferably 4.0 hours, more preferably 4.5 hours. The upper limit of the holding time at the heating temperature is not particularly limited, for example, it is 8.0 hours, preferably 6.0 hours.

[0082] After holding at the heating temperature, the slab is forged to produce a steel billet of a specified size. At this point, it is preferable to set the final cross-sectional area reduction rate ({(cross-sectional area of ​​slab) - (cross-sectional area of ​​steel billet)} / (cross-sectional area of ​​slab)) to 90% or more. More preferably, the cross-sectional area reduction rate is 92% or more, and even more preferably 94% or more. The manufactured steel billet is preferably a circular cross-section billet (round billet).

[0083] After forging, cool to room temperature. The cooling rate is not particularly limited at this time, but is preferably below 300°C / hour, and more preferably below 250°C / hour.

[0084] [Forging process]

[0085] Next, the steel billet obtained in the billet forging process is hot forged to produce crankshaft profiles (die forging process).

[0086] In hot forging (die forging) during the die forging process, the heating temperature before forging is set to 1200~1350°C. If the heating temperature is too low, it is sometimes difficult to process into complex shapes. On the other hand, if the heating temperature is too high, the steel billet may partially melt. The lower limit of the heating temperature is preferably 1230°C, and more preferably 1250°C. The upper limit of the heating temperature is preferably 1325°C, and more preferably 1300°C.

[0087] In die forging, the heating time up to the heating temperature (the time from the start of heating to reaching the heating temperature) is set to 500 seconds or less. Such rapid heating can be achieved, for example, by a high-frequency induction heating device. By heating the billet to the heating temperature in a short time, the growth of austenite grains is suppressed. By reducing the austenite grain size, the area of ​​the grain boundaries that become precipitation sites for proeutectoid ferrite increases, thereby increasing the amount of proeutectoid ferrite. The upper limit of the heating time up to the heating temperature is preferably 400 seconds, and more preferably 320 seconds. The lower limit of the heating time up to the heating temperature is not particularly limited, for example, it is 100 seconds, and preferably 200 seconds.

[0088] Once the heating temperature is reached, forging shall begin rapidly. The forging start temperature (the surface temperature of the billet just before forging begins) is preferably above (heating temperature - 150°C), and more preferably above (heating temperature - 100°C).

[0089] The steel billet is forged to a shape that closely resembles the crankshaft that becomes the final product. Fine forging can also be performed after this forging (rough forging).

[0090] After forging, cool to room temperature. The cooling rate is not particularly limited at this time, but is preferably below 5°C / second, and more preferably below 2°C / second.

[0091] The crankshaft profiles are manufactured through the above processes.

[0092] The above describes an example of a crankshaft profile and its manufacturing method according to one embodiment of the present invention. The crankshaft profile of this embodiment, after undergoing machining such as grinding and / or drilling, becomes a crankshaft as the final product. The crankshaft profile of this embodiment exhibits excellent machinability. Furthermore, the crankshaft profile of this embodiment exhibits excellent wear resistance even when used without high-frequency hardening.

[0093] [crankshaft]

[0094] One embodiment of the present invention provides a crankshaft manufactured from the aforementioned crankshaft profile. Among crankshafts manufactured from the aforementioned crankshaft profile, crankshafts that are not subjected to high-frequency quenching are particularly preferred. That is, the crankshaft of one embodiment of the present invention is a crankshaft used without high-frequency quenching, and its chemical composition, in mass %, is: C: 0.35~0.50%, Si: 0.50~1.00%, Mn: 1.00~1.50%, Cr: 0.30% or less, P: 0.030% or less, S: 0.020~0.100%, Al: 0.500% or less, Ti: 0.001~0.050%, N: 0.002~0. 0.020%, V: 0~0.20%, Ca: 0~0.010%, balance: Fe and impurities, Fn1 as defined by the following formula (1) is 3.80 or more and 5.00 or less, the microstructure contains 12~40 vol% proeutectoid ferrite, the balance is pearlite, and the Mn content is obtained by analyzing the cementite in the pearlite at a depth of 50 μm from the surface in the part corresponding to the pin of the crankshaft by energy dispersive X-ray analysis. θ With C content C θ The ratio of Mn θ / C θ It is above 0.360.

[0095] Fn1=7C+0.5Si+Mn+3Cr-10Ti (1)

[0096] In equation (1), the mass percentage of the corresponding element is substituted for C, Si, Mn, Cr and Ti.

[0097] The above energy-dispersive X-ray analysis used C, Mn, Cr, V, and Fe as analytical objects, and the Mn... θ and C θ These are values ​​calculated by using the sum of C, Mn, Cr, V, and Fe as the denominator and calculating by mass% respectively.

[0098] Example

[0099] The present invention will now be described in more detail through examples. The present invention is not limited to these examples.

[0100] Molten steel with the chemical composition shown in Table 1 was cast to produce slabs with a cross-section of 400 mm × 530 mm. The "hardness" in Table 1 refers to the Vickers hardness of the resulting slabs.

[0101] [Table 1]

[0102]

[0103] These slabs were hot forged (open forging) to produce round billets with a diameter of 85 mm. The conditions for open forging are shown in Table 2. It should be noted that for process symbol c1, no forging was performed, only heating was carried out, and a round billet with a diameter of 85 mm was manufactured by machining. For process symbol d1, a round billet with a diameter of 85 mm was manufactured from the slab by machining.

[0104] [Table 2]

[0105]

[0106] The obtained steel billets were hot forged (die forging) to produce crankshaft profiles. The die forging conditions are shown in Table 3. The "heating time" in Table 3 refers to the time from the start of heating to reaching the heating temperature. After the forging in Table 3, finish forging was performed at a starting temperature of 1100℃ and an ending temperature of 1070℃.

[0107] [Table 3]

[0108]

[0109] The portion of the crankshaft profile corresponding to the crankshaft pin (hereinafter referred to as the "pin") is cut off at the center of the parallel section with a cut surface perpendicular to the axial direction of the pin. The cut surface is ground and etched using a mixture of ethanol and nitric acid (nitroethanol). The area fraction of proeutectoid ferrite is measured at a depth of 50 μm from the surface using an optical microscope with 500x magnification. The measured area fraction of proeutectoid ferrite is regarded as the volume fraction of proeutectoid ferrite.

[0110] The same crankshaft profile was cut at the center of the parallel section with the cut surface perpendicular to the pin's axis. The cut surface was ground and etched using a mixture of ethanol and nitric acid (nitroethanol). A scanning electron microscope (SEM) was used to observe a depth of 50 μm from the surface at approximately 10,000x magnification. While confirming the SEM image, the cementite in the pearlite was irradiated with electron beams, and the chemical composition of the cementite was analyzed by EDX to determine Mn. θ and C θ EDX uses C, Mn, Cr, V, and Fe as analytical elements, with Mn... θ and C θ The values ​​are calculated by mass percentage, with the sum of C, Mn, Cr, V, and Fe as the denominator. The average value obtained at the eight locations is taken as Mn. θ and C θ .

[0111] The EDX was performed using a JEOL JED-2300. The measurement conditions were set to an accelerating voltage of 15kV.

[0112] The wear resistance of crankshaft profiles is evaluated by the wear diameter of the pins after a wear test. Specifically, a crankshaft manufactured by machining the crankshaft profiles is mounted on an engine bench tester and subjected to a wear test at 7000 rpm, oil temperature 110°C, oil pressure 200 kPa, and low-viscosity oil (OW-16) for 20 hours. The change in the pin diameter is measured. Before and after the wear test, measurements are taken at three points on the circumference (0°, 120°, and 240°) using a micrometer, and the average is calculated. The difference between the diameter before and after the wear test is taken as the wear diameter. If the wear diameter is less than 1.00 μm, the wear resistance is considered excellent.

[0113] The fatigue strength of crankshaft profiles was determined as follows. Rotational bending fatigue test specimens (Ono-style) were collected from crankshaft profiles and subjected to rotational bending fatigue tests according to JIS Z 2274. The test was conducted up to a maximum of 10 replicates. 7 The maximum stress in the unbroken test piece is taken as the fatigue strength. A fatigue strength of 350 MPa or higher is taken as the target.

[0114] The machinability of crankshaft profiles was evaluated as follows. A 22mm deep pilot hole was drilled in the crankshaft profile using a 5mm diameter guide drill, followed by a 91mm deep oil hole using a 4.98mm diameter long drill. Both the guide drill and the long drill were integral drills with a TiN coating and internal oil supply holes. After drilling 600 times under specified conditions while supplying oil using the MQL method, the flank wear of the long drill was measured to evaluate machinability. Specifically, a magnified photograph of the outer side of the flank was taken under a microscope, and the width of the area showing metallic luster (the area where the coating has peeled off) formed from the coated portion to the tip was measured. This width was taken as the flank wear. If the flank wear was less than 0.020mm, the machinability was rated as excellent.

[0115] The results are shown in Table 4. In Table 4, “Fα” represents the volume fraction of proeutectoid ferrite.

[0116] [Table 4]

[0117]

[0118] As shown in Table 4, crankshaft profiles No. 3, 7, 9, 10, 12, and 13 exhibited excellent wear resistance with pin wear diameters below 1.00 μm after wear tests. The long drill bits made from these crankshaft profiles also showed excellent machinability with flank wear below 0.020 mm.

[0119] The crankshaft profiles No. 1, 2, 8, and 11 exhibit poor machinability. This is attributed to the low volume fraction of proeutectoid ferrite. The low volume fraction of proeutectoid ferrite is further attributed to an excessively high Fn1.

[0120] The crankshaft profile No. 4 has poor wear resistance. This is believed to be due to the poor wear resistance of Mn. θ / C θ Low. It is believed that Mn... θ / C θ The low temperature is due to the excessively low heating temperature during the initial forging process.

[0121] The crankshaft profiles No. 5 and 6 exhibit poor wear resistance. This is believed to be due to the presence of Mn. θ / C θ Low. It is believed that Mn... θ / C θ The low value is because the steel billet was not forged during manufacturing.

[0122] The crankshaft profile No. 14 has poor wear resistance. This is believed to be due to the poor wear resistance of Mn. θ / C θ Low. It is believed that Mn... θ / C θ The low value is due to the short holding time during the initial forging process.

[0123] The crankshaft profile No. 15 has poor machinability. This is believed to be due to the low volume fraction of proeutectoid ferrite. The low volume fraction of proeutectoid ferrite is further attributed to excessively long heating time during die forging.

[0124] Figure 1 It is a scatter plot showing the relationship between Fn1 and the proeutectoid ferrite volume fraction Fα, made from nine data (No. 1~3 and 8~13) of the process symbol "a1" for billet forging and "a2" for die forging. Figure 2 It means by Figure 1 A scatter plot showing the relationship between Fn1 and flank wear was created from eight of the nine data points, excluding the No. 1 data point indicating tool breakage. Figure 1 and Figure 2 It can be seen that there is a strong correlation between Fn1 and the volume fraction of proeutectoid ferrite, and between Fn1 and the amount of flank wear.

[0125] The above describes one embodiment of the present invention, but the above embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above embodiment, and can be implemented by appropriate modifications within the scope of the invention.

Claims

1. A crankshaft profile material, the chemical composition of which is, in mass %, C：0.35~0.50%、 Si: 0.50 to 1.00%, Mn: 1.00 to 1.50%, Cr: 0.30% or less, P: 0.030% or less, S：0.020~0.100%、 Al: 0.500% or less, Ti: 0.001 to 0.050%, N：0.002~0.020%、 V：0~0.20%、 Ca: 0 to 0.010%, balance: Fe and impurities, Fn1 defined by the following formula (1) is 3.80 or more and 5.00 or less, the structure contains 12 to 40% by volume of proeutectoid ferrite, and the balance is pearlite, The ratio Mn / C of the Mn content Mn in the pearlite obtained by analyzing the cementite in the pearlite by energy dispersive X-ray analysis at a position 50 μm deep from the surface of the portion corresponding to the pin portion of the crankshaft θ to the C content C θ was 0.360 or greater. θ / C θ ​ Fn1 = 7C + 0.5Si + Mn + 3Cr - 10Ti (1) in formula (1), the contents of the respective elements are substituted for C, Si, Mn, Cr, and Ti in mass %; The energy dispersive X-ray analysis takes C, Mn, Cr, V, and Fe as analysis objects, and the Mn θ and C θ are values obtained in mass% with the total of C, Mn, Cr, V, and Fe as a denominator, respectively.

2. The crankshaft profile material according to claim 1, which is a crankshaft profile material satisfying the following formulae (2) and (3), Fα > -21.364 x Fn1 + 112.0 (2) Fα < -21.364 x Fn1 + 130.0 (3) in formulae (2) and (3), the volume fraction of proeutectoid ferrite is substituted for Fα in %.

3. A method for manufacturing a crankshaft profile material according to claim 1 or 2, the method comprising: a blooming forging step of hot forging a slab manufactured by a continuous casting method to produce a steel billet; and a die forging step of hot forging the steel billet, the heating temperature before the hot forging in the blooming forging step is 1150 to 1300°C, and the holding time at the heating temperature is 3.0 hours or more, the heating temperature before the hot forging in the die forging step is 1200 to 1350°C, and the heating time to the heating temperature is 500 seconds or less.

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