Crankshaft material and method for producing crankshaft material

Abstract

The present invention provides a crankshaft material that has excellent machinability and that has excellent wear resistance even when used without high frequency hardening. The crankshaft material has a prescribed chemical composition, wherein: Fn1 defined by formula (1) is 3.80-5.00; the composition contains 12-40 vol% pro-eutectoid ferrite, with the remainder being pearlite; and at a position at a depth of 50 μm from the surface of a portion corresponding to a pin part of a crankshaft, the ratio Mnθ / Cθ of Mn content Mnθ to C content Cθ obtained by analyzing cementite in the pearlite via energy-dispersive X-ray analysis is not less than 0.360.　(1): Fn1=7C+0.5Si+Mn+3Cr-10Ti In formula (1), C, Si, Mn, Cr, and Ti are substituted by the content of the respective elements in mass%.

Description

Crankshaft base material and manufacturing method for crankshaft base material

[0001] The present invention relates to a crankshaft preform and a method for manufacturing the crankshaft preform. In this specification, the term "crankshaft preform" refers to an intermediate product that has been machined into a shape similar to that of a crankshaft, which is the final product.

[0002] Crankshafts used in automobile engine parts, etc., are generally induction hardened on the sliding parts, such as the pins and journals. Induction hardening forms a high-hardness structure, such as martensite, on the surface of the pins and journals, improving their wear resistance. However, crankshafts that do not undergo induction hardening are also used, mainly for the purpose of reducing costs. When it comes to mechanical structural carbon steels and mechanical structural alloy steels, steels that do not undergo thermal refining processes such as quenching and tempering are called "non-thermal-refined steels."

[0003] Japanese Patent Laid-Open Publication No. 8-92687 discloses a high-strength, high-toughness, non-heat treated steel. This non-heat treated steel has a predetermined chemical composition and contains 1×10 inclusions with an average grain size of 0.1 to 5 μm. ２ ~1 x 10 ６ pieces / mm ２ The inclusions are characterized in that they are Ti oxynitride, MnS, and composite compounds mainly composed of Ti oxynitride and MnS.

[0004] Japanese Patent No. 3235442 discloses a high-strength, low-ductility, non-heat treated steel. This non-heat treated 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 preforms are subjected to machining processes such as grinding and drilling. Therefore, it is preferable that crankshaft preforms have excellent machinability. Although not related to crankshaft preforms, Japanese Patent No. 3437079 discloses a steel for machine structural use with excellent chip disposal properties, as a steel with excellent machinability. This steel for machine structural use has sulfides with a length of 20 μm or more, or sulfide groups with a length of 20 μm or more, each of which is a series of multiple sulfides, within 1 mm of the cross section in the rolling direction. ２ It is characterized by the presence of 30 or more within the field of view.

[0006] JP-A-8-92687 Patent No. 3235442 Patent No. 3437079

[0007] One method for improving the wear resistance of steel is to increase the C content to increase the hardness of the steel. However, increasing the C content is in a trade-off relationship with the machinability of the steel. Furthermore, adding precipitation-hardening elements such as V has been used to improve the mechanical properties of non-heat-treated steel. However, since V is a relatively expensive element, it is preferable to improve the mechanical properties of steel by a method other than adding V.

[0008] An object of the present invention is to provide a crankshaft preform that has excellent wear resistance and excellent machinability even when used without induction hardening.

[0009] A crankshaft preform according to one embodiment of the present invention has a chemical composition, in mass %, of C: 0.35 to 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 to 0.100%, Al: 0.500% or less, Ti: 0.001 to 0.050%, N: 0.002 to 0.020%, V: 0 to 0.20%, Ca: 0. 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 volume % of pro-eutectoid ferrite and the balance is pearlite, and the Mn content Mn obtained by analyzing cementite in the pearlite at a position at a depth of 50 μm from the surface of a portion corresponding to a pin portion of a crankshaft by energy dispersive X-ray analysis is θ and C content C θ Ratio of Mn θ / C θ is 0.360 or more. Fn1=7C+0.5Si+Mn+3Cr-10Ti (1) In formula (1), the contents of the corresponding elements are substituted in mass% for C, Si, Mn, Cr, and Ti. The energy dispersive X-ray analysis targets C, Mn, Cr, V, and Fe, and the Mn θ and C θ Each of the above is a value calculated in mass % with the sum of C, Mn, Cr, V and Fe as the denominator.

[0010] A method for producing a crankshaft preform according to one embodiment of the present invention is a method for producing the above-mentioned crankshaft preform, and includes a billet forging step in which a slab produced by a continuous casting method is hot forged to form a billet, and a die forging step in which the billet is hot forged. In the hot forging in the billet forging step, the heating temperature before hot forging is 1150 to 1300°C and the holding time at the heating temperature is 3.0 hours or more. In the hot forging in the die forging step, the heating temperature before hot forging is 1200 to 1350°C and the heating time to the heating temperature is 500 seconds or less.

[0011] According to the present invention, a crankshaft preform having excellent wear resistance and excellent machinability can be obtained even when used without induction hardening.

[0012] Fig. 1 is a scatter diagram showing the relationship between Fn1 and the pro-eutectoid ferrite volume fraction Fα. Fig. 2 is a scatter diagram showing the relationship between Fn1 and the flank wear amount.

[0013] The present inventors have conducted various studies to solve the above problems and have come to the following findings.

[0014] Crankshaft preforms are produced by hot forging billets of a predetermined size. Hot forged crankshaft preforms have a structure consisting of ferrite and pearlite. In order to improve the wear resistance of the as-hot-forged structure, i.e., the ferrite-pearlite structure, it is effective to increase the amount of Mn dissolved in cementite in pearlite.

[0015] Billets that are used as raw materials for crankshaft preforms are produced, for example, by hot forging or hot rolling a slab produced by a continuous casting method. In slabs produced by a continuous casting method, Mn tends to be concentrated near the center of the slab due to the molten steel solidifying from the surface. Therefore, Mn may also be concentrated near the center in billets produced from such slabs, and Mn may also be concentrated near the center in crankshaft preforms produced from such billets.

[0016] On the other hand, since it is the properties near the surface that contribute to wear resistance, it is preferable that a sufficient amount of Mn be present near the surface of the crankshaft preform in order to improve the wear resistance of the crankshaft preform.

[0017] In order to ensure that a sufficient amount of Mn is present near the surface of a crankshaft preform, it is effective to heat the billet for a long period of time to sufficiently diffuse the Mn, and then perform hot forging to eliminate center segregation. From the viewpoint of diffusing Mn, it is possible to extend the heating time when producing a crankshaft preform from the billet, but as will be described later, extending this heating time makes it difficult to obtain the desired structure.

[0018] Increasing the volume fraction of pro-eutectoid ferrite in the ferrite-pearlite structure is effective for improving the machinability of crankshaft preforms. Increasing the volume fraction of pro-eutectoid ferrite is achieved by controlling the chemical composition of the steel and increasing the heating rate before hot forging when producing crankshaft preforms from billets. Specifically, rapid heating by high-frequency induction heating followed by prompt initiation of hot forging is effective. This suppresses the growth of austenite grains in the high-temperature range. By reducing the austenite grain size, the area of ​​grain boundaries that serve as pro-eutectoid ferrite precipitation sites increases, allowing for an increase in the amount of pro-eutectoid ferrite.

[0019] The present invention has been completed based on the above findings. A crankshaft preform according to one embodiment of the present invention will now be described in detail.

[0020] [Crankshaft Preform] [Chemical Composition] The crankshaft preform according to this embodiment has the chemical composition described below. In the following description, "%" for the content of an element means mass %.

[0021] C: 0.35 to 0.50% Carbon (C) improves the hardness of steel and contributes to improving fatigue strength and wear resistance. On the other hand, if the C content is too high, machinability decreases. Therefore, the C content is 0.35 to 0.50%. The lower limit of the C content is preferably 0.37%. The upper limit of the C content is preferably 0.45%.

[0022] Si: 0.50 to 1.00% Silicon (Si) has the effects of deoxidizing and strengthening ferrite. On the other hand, if the Si content is too high, the machinability of the steel decreases. Therefore, the Si content is 0.50 to 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%.

[0023] Mn: 1.00 to 1.50% Manganese (Mn) improves the hardness of steel and contributes to improving fatigue strength and wear resistance. Mn also dissolves in cementite in pearlite and strengthens the pearlite. On the other hand, if the Mn content is too high, the machinability of the steel decreases. Therefore, the Mn content is 1.00 to 1.50%. The lower limit of the Mn content is preferably 1.10%, and more preferably 1.20%. The upper limit of the Mn content is preferably 1.40%, and more preferably 1.35%.

[0024] Cr: 0.30% or less Chromium (Cr) improves the hardness of steel and contributes to improving fatigue strength and wear resistance. On the other hand, if the Cr content is too high, the machinability of the steel decreases. 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%.

[0025] P: 0.030% or less 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.

[0026] S: 0.020 to 0.100% Sulfur (S) forms MnS and improves the machinability of steel. On the other hand, if the S content is too high, the hot workability of the steel decreases. Therefore, the S content is 0.020 to 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%.

[0027] Al: 0.500% or less Aluminum (Al) has a deoxidizing effect. On the other hand, if the Al content is too high, the machinability of the steel decreases. 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%.

[0028] Ti: 0.001 to 0.050% Titanium (Ti) forms nitrides and carbonitrides, contributing to the refinement of crystal grains (austenite grains) in high temperature ranges. By refining the austenite grains, the area of ​​the grain boundaries that become the precipitation sites for pro-eutectoid ferrite increases, resulting in an increase in the amount of pro-eutectoid ferrite. On the other hand, even if excessive Ti is added, the effect saturates and causes an increase in costs. Therefore, the Ti content is 0.001 to 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%.

[0029] N: 0.002 to 0.020% Nitrogen (N) forms nitrides and carbonitrides, contributing to the refinement of crystal grains (austenite grains) in high temperature ranges. Refining austenite grains increases the area of ​​grain boundaries that serve as precipitation sites for pro-eutectoid ferrite, resulting in an increase in the amount of pro-eutectoid ferrite. Furthermore, excessive reduction of N increases manufacturing costs. On the other hand, if the N content is too high, the hot workability of the steel deteriorates. 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%, more preferably 0.010%.

[0030] The balance of the chemical composition of the crankshaft preform according to this embodiment is Fe and impurities, which refer to elements that are mixed in from ores or scraps used as raw materials for steel, or from the manufacturing environment, etc.

[0031] The chemical composition of the crankshaft preform according to this embodiment may contain one or two elements selected from V and Ca in place of a portion of Fe. V and Ca are optional elements. That is, the chemical composition of the crankshaft preform according to this embodiment may not contain either or both of V and Ca.

[0032] V: 0 to 0.20% Vanadium (V) improves the wear resistance of steel through precipitation hardening. This effect can be achieved even if even a small amount of V is contained. The crankshaft preform according to this embodiment improves wear resistance by a method other than the addition of V. Therefore, the crankshaft preform according to this embodiment does not need to contain V. Not including V is advantageous in terms of cost. On the other hand, V may be contained as needed. However, if the V content is too high, the machinability of the steel decreases. 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 production cost, the upper limit of the V content is preferably 0.10%, more preferably 0.08%, and even more preferably 0.05%. From the viewpoint of production cost, it is most preferable to not include V.

[0033] Ca: 0 to 0.010% Calcium (Ca) improves the hot workability of steel. This effect can be achieved even if even a small amount of Ca is contained. Therefore, Ca may be contained as needed. On the other hand, if the Ca content is too high, the toughness of the steel decreases. 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%.

[0034] [Regarding Fn1] The crankshaft preform according to this embodiment has Fn1 defined by the following formula (1) of 3.80 or more and 5.00 or less: Fn1=7C+0.5Si+Mn+3Cr-10Ti (1) In formula (1), the contents of the corresponding elements, in mass %, are substituted for C, Si, Mn, Cr, and Ti.

[0035] Fn1 is a parameter correlated with the volume fraction of pro-eutectoid ferrite. The smaller Fn1, the larger the volume fraction of pro-eutectoid ferrite. If Fn1 is less than 3.80 or more than 5.00, it becomes difficult to keep the volume fraction of pro-eutectoid 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.

[0036] [Structure] The crankshaft preform according to this embodiment has a structure containing 12 to 40 volume % of pro-eutectoid ferrite, with the remainder being pearlite.

[0037] The crankshaft preform according to this embodiment has a ferrite-pearlite structure. That is, the crankshaft preform according to this embodiment has a structure consisting of pro-eutectoid ferrite and pearlite. Pearlite is a structure in which ferrite and cementite form a fine layer structure (pearlite lamellae), and pro-eutectoid ferrite refers to ferrite that precipitates from prior austenite grain boundaries, separate from the ferrite in the pearlite lamellae.

[0038] The structure of the crankshaft preform according to this embodiment has a pro-eutectoid ferrite volume fraction of 12 to 40%. If the pro-eutectoid ferrite volume fraction is too low, good machinability cannot be obtained. On the other hand, if the pro-eutectoid ferrite volume fraction is too high, fatigue strength may decrease. The lower limit of the pro-eutectoid ferrite volume fraction is preferably 14%, more preferably 18%, and even more preferably 22%. The upper limit of the pro-eutectoid ferrite volume fraction is preferably 36%, more preferably 32%, and even more preferably 30%.

[0039] The crankshaft preform according to this embodiment preferably satisfies the following formulas (2) and (3): Fα>-21.364×Fn1+112.0 (2) Fα<-21.364×Fn1+130.0 (3) In formulas (2) and (3), the volume fraction of pro-eutectoid ferrite is substituted for Fα in percentage.

[0040] [Mn θ / C θ The crankshaft preform according to this embodiment has a Mn content of Mn obtained by analyzing cementite in pearlite by energy dispersive X-ray analysis at a depth of 50 μm from the surface of a portion corresponding to the pin portion of the crankshaft. θ and C content C θ Ratio of Mn θ / C θ Here, the energy dispersive X-ray analysis is performed to analyze C, Mn, Cr, V, and Fe, and Mn θ and C θ Each of the above is a value calculated in mass % with the sum of C, Mn, Cr, V and Fe as the denominator.

[0041] In order to improve the wear resistance of a ferrite-pearlite structure, it is effective to increase the amount of Mn dissolved in cementite in pearlite. The portion near the surface of a crankshaft preform contributes to wear resistance. Therefore, in order to improve wear resistance, it is preferable to increase the amount of Mn dissolved in cementite in pearlite near the surface.

[0042] In this embodiment, the target of evaluation is a position 50 μm deep from the surface of the portion corresponding to the pin portion of the crankshaft, and the cementite in the pearlite present at this position is analyzed using energy dispersive X-ray analysis (EDX). Specifically, the portion corresponding to the pin portion of the crankshaft is cut so that the cut surface is perpendicular to the axial direction of the pin portion, and the cut surface is observed using a scanning electron microscope (SEM) at approximately 10,000x magnification. While observing the SEM image, the cementite in the pearlite is irradiated with an electron beam and the chemical composition of the cementite is analyzed using EDX. C, Mn, Cr, V, and Fe are the targets of analysis because other elements are unlikely to be solid-solubilized in cementite, and even if they are present, the amount is so small that it is thought that they would not affect the properties.

[0043] In this embodiment, the Mn content Mn obtained from the analysis results θ and C content C θ Ratio of Mnθ / C θ The amount of Mn dissolved in cementite in pearlite near the surface is evaluated by the following formula. θ / C θ If Mn is 0.360 or more, excellent wear resistance can be obtained. θ / C θ The lower limit of Mn is preferably 0.380, more preferably 0.400, even more preferably 0.450, even more preferably 0.500, and even more preferably 0.550. θ / C θ The upper limit of is not particularly limited, but is, for example, 2.000, preferably 1.500, and more preferably 1.200.

[0044] [Method of Manufacturing a Crankshaft Preform] Next, an example of a method of manufacturing a crankshaft preform according to this embodiment will be described. The manufacturing method described below is merely an example and does not limit the method of manufacturing a crankshaft preform according to this embodiment.

[0045] The crankshaft preform according to this embodiment can be manufactured by hot forging a slab produced by continuous casting to form a billet, and then hot forging the billet. Hereinafter, the hot forging used to produce a billet from a slab will be referred to as "billet forging," and this process will be referred to as the billet forging process. Furthermore, the hot forging used to produce a crankshaft preform from a billet will be referred to as "die forging," and this process will be referred to as the "die forging process."

[0046] [Billet Forging Process] First, a steel slab produced by continuous casting of molten steel having the above-described chemical composition is hot forged into a steel billet of a size suitable for die forging (billet forging process).

[0047] In the hot forging (billet forging) in the billet forging process, the heating temperature before forging is set to 1150 to 1300°C, and the holding time at the heating temperature is set to 3.0 hours or more. Holding at a high temperature for a long period of time before forging diffuses Mn segregated at the center of the slab. If the heating temperature is too low or the holding time is too short, Mn diffusion may be 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, but is, for example, 8.0 hours, preferably 6.0 hours.

[0048] After being held at the heating temperature, the slab is forged into a billet of a predetermined size. At this time, it is preferable that the final area reduction rate ({(cross-sectional area of ​​slab) - (cross-sectional area of ​​billet)} / (cross-sectional area of ​​slab)) is 90% or more. The area reduction rate is more preferably 92% or more, and even more preferably 94% or more. The billet to be produced is preferably a billet with a circular cross section (round billet).

[0049] After forging, the material is cooled to room temperature. The cooling rate is not particularly limited, but is preferably 300°C / hour or less, and more preferably 250°C / hour or less.

[0050] [Die Forging Step] Next, the billet produced in the billet forging step is hot forged to form a crankshaft blank (die forging step).

[0051] In the hot forging (die forging) in the die forging process, the heating temperature before forging is set to 1200 to 1350°C. If the heating temperature is too low, it may be difficult to process into a complex shape. On the other hand, if the heating temperature is too high, the billet may partially melt. The lower limit of the heating temperature is preferably 1230°C, more preferably 1250°C. The upper limit of the heating temperature is preferably 1325°C, more preferably 1300°C.

[0052] In die forging, the heating time to the heating temperature (the time from the start of heating to 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 grain growth of austenite grains is suppressed. By reducing the austenite grain size, the area of ​​the grain boundaries that serve as precipitation sites for pro-eutectoid ferrite increases, allowing the amount of pro-eutectoid ferrite to increase. The upper limit of the heating time to the heating temperature is preferably 400 seconds, more preferably 320 seconds. The lower limit of the heating time to the heating temperature is not particularly limited, but is, for example, 100 seconds, preferably 200 seconds.

[0053] Forging is started promptly after the heating temperature is reached. The forging start temperature (surface temperature of the billet immediately before the start of forging) is preferably (heating temperature - 150°C) or higher, and more preferably (heating temperature - 100°C) or higher.

[0054] The billet is forged into a shape that approximates the shape of the final crankshaft. This forging (rough forging) may be followed by finish forging.

[0055] After forging, the material is cooled to room temperature. The cooling rate is not particularly limited, but is preferably 5°C / sec or less, and more preferably 2°C / sec or less.

[0056] Through the above steps, a crankshaft blank is manufactured.

[0057] The above describes an example of a crankshaft preform according to one embodiment of the present invention and a method for manufacturing the same. The crankshaft preform according to this embodiment is machined, such as by grinding and drilling, to form a crankshaft, which is the final product. The crankshaft preform according to this embodiment has excellent machinability. Furthermore, the crankshaft preform according to this embodiment has excellent wear resistance even when used without induction hardening.

[0058] [Crankshaft] A crankshaft according to one embodiment of the present invention is manufactured from the above-described crankshaft preform. Among the crankshafts manufactured from the above-described crankshaft preform, those that are used without induction hardening are particularly preferred. That is, a crankshaft according to one embodiment of the present invention is a crankshaft that is used without induction hardening, and has a chemical composition, in mass %, of C: 0.35 to 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 to 0.100%, Al: 0.500% or less, Ti: 0.001 to 0.050%, N: 0.002 to 0.020%, V: 0 to 0.20%, Ca: 0 to 0.010%, balance: Fe and impurities, Fn1 defined by the following formula (1) is 3.80 to 5.00, the structure contains 12 to 40 volume % of pro-eutectoid ferrite and the balance is pearlite, and the Mn content Mn obtained by analyzing cementite in the pearlite at a position at a depth of 50 μm from the surface of a portion corresponding to a pin portion of a crankshaft by energy dispersive X-ray analysis is θ and C content C θ Ratio of Mn θ / C θ is 0.360 or more. Fn1=7C+0.5Si+Mn+3Cr-10Ti (1) In formula (1), the contents of the corresponding elements are substituted in mass% for C, Si, Mn, Cr, and Ti. The energy dispersive X-ray analysis targets C, Mn, Cr, V, and Fe, and the Mn θ and C θ Each of the above is a value calculated in mass % with the sum of C, Mn, Cr, V and Fe as the denominator.

[0059] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0060] Slabs with a cross section of 400 mm x 530 mm were produced by casting molten steel having the chemical composition shown in Table 1. "Hardness" in Table 1 is the Vickers hardness of the produced slabs.

[0061]

[0062] These slabs were hot forged (billet forging) to produce round billets with a diameter of 85 mm. The conditions for billet forging are shown in Table 2. In process code c1, only heating was performed without forging, and round billets with a diameter of 85 mm were produced by machining. In process code d1, round billets with a diameter of 85 mm were produced by machining the slabs.

[0063]

[0064] The produced billets were hot forged (die forged) to produce crankshaft blanks. The die forging conditions are shown in Table 3. The "heating time" in Table 3 refers to the time from the start of heating until the heating temperature is reached. After the forging shown in Table 3, finish forging was carried out at a starting temperature of 1100°C and a finishing temperature of 1070°C.

[0065]

[0066] The portion of the produced crankshaft preform corresponding to the pin portion of the crankshaft (hereinafter simply referred to as the "pin portion") was cut at the center of the parallel portion so that the cut surface was a plane perpendicular to the axial direction of the pin portion. The cut surface was polished and etched using a mixed solution of ethanol and nitric acid (nital). A position 50 μm deep from the surface was observed with an optical microscope at 500x magnification to measure the area fraction of pro-eutectoid ferrite. The measured area fraction of pro-eutectoid ferrite was considered to be the volume fraction of pro-eutectoid ferrite.

[0067] The pin portion of the crankshaft blank was also cut at the center of the parallel portion so that the cut surface was perpendicular to the axial direction of the pin portion. The cut surface was polished and etched using a mixed solution of ethanol and nitric acid (nital). A position 50 μm deep from the surface was observed with a scanning electron microscope (SEM) at a magnification of approximately 10,000 times. While checking the SEM image, cementite in pearlite was irradiated with an electron beam, and the chemical composition of the cementite was analyzed by EDX to determine Mn. θ and C θ The EDX was used to analyze C, Mn, Cr, V, and Fe, and Mn θ and C θEach of the above was calculated in mass % using the sum of C, Mn, Cr, V and Fe as the denominator. θ and C θ was the average value obtained at eight locations.

[0068] For EDX, a JED-2300 manufactured by JEOL was used, and the measurement conditions were an acceleration voltage of 15 kV.

[0069] The wear resistance of the crankshaft preform was evaluated based on the wear diameter of the pin portion after the wear test. Specifically, a crankshaft manufactured by machining the crankshaft preform was mounted on an engine bench tester, and a wear test was performed in which the engine was rotated for 20 hours at 7,000 rpm, an oil temperature of 110°C, an oil pressure of 200 kPa, and low viscosity oil (OW-16), and the change in the diameter of the pin portion was measured. Before and after the wear test, measurements were taken at three points on the circumference (0°, 120°, and 240° positions) using a micrometer, and the average was calculated. The diameter before the wear test minus the diameter after the wear test was defined as the wear diameter. A wear diameter of 1.00 μm or less was evaluated as having excellent wear resistance.

[0070] The fatigue strength of the crankshaft preform was measured as follows. Ono-type rotating bending fatigue test specimens were taken from the crankshaft preform, and a rotating bending fatigue test was carried out in accordance with JIS Z 2274. The number of repetitions was 10. ７ The maximum stress among those that did not break up until that point was taken as the fatigue strength. The target fatigue strength was 350 MPa or more.

[0071] The machinability of the crankshaft preform was evaluated as follows. A 22 mm deep guide hole was drilled in the crankshaft preform using a 5 mm diameter guide drill, and then a 91 mm deep oil hole was drilled using a 4.98 mm diameter long drill. Both the guide drill and the long drill were solid drills with a TiN coating and an internal oil supply hole. After drilling 600 times under specified conditions while supplying oil using the MQL method, the machinability was evaluated by measuring the flank wear of the long drill. Specifically, a magnified photograph of the outer flank was taken with a microscope, and the width of the area exhibiting metallic luster (the area where the coating had peeled off) between the coating and the cutting edge was measured. This width was defined as the flank wear. A flank wear of 0.020 mm or less was evaluated as having excellent machinability.

[0072] The results are shown in Table 4. In Table 4, "Fα" represents the volume fraction of pro-eutectoid ferrite.

[0073]

[0074] As shown in Table 4, the crankshaft preforms Nos. 3, 7, 9, 10, 12, and 13 showed excellent wear resistance, with pin wear diameters of 1.00 μm or less after the wear test. These crankshaft preforms showed excellent machinability, with flank wear amounts of long drills of 0.020 mm or less.

[0075] The crankshaft preforms Nos. 1, 2, 8, and 11 had poor machinability. This is thought to be due to the low volume fraction of pro-eutectoid ferrite. The low volume fraction of pro-eutectoid ferrite is thought to be due to an excessively large Fn1.

[0076] The crankshaft material No. 4 had poor wear resistance. This was due to the presence of Mn θ / C θ This is thought to be because Mn θ / C θ The reason for the low value is thought to be that the heating temperature during billet forging was too low.

[0077] The crankshaft preforms Nos. 5 and 6 had poor wear resistance. This was due to the presence of Mn θ / Cθ This is thought to be because Mn θ / C θ The reason for the low value is thought to be that no forging was performed when manufacturing the billet.

[0078] The crankshaft material No. 14 had poor wear resistance. This was due to the presence of Mn θ / C θ This is thought to be because Mn θ / C θ The reason for the low value is thought to be that the holding time during billet forging was too short.

[0079] The crankshaft preform No. 15 had poor machinability. This was thought to be due to the low volume fraction of pro-eutectoid ferrite. The low volume fraction of pro-eutectoid ferrite was thought to be due to the heating time during die forging being too long.

[0080] Fig. 1 is a scatter diagram showing the relationship between Fn1 and the pro-eutectoid ferrite volume fraction Fα, created from nine data sets (Nos. 1 to 3 and 8 to 13) for which the billet forging process code is "a1" and the die forging process code is "a2." Fig. 2 is a scatter diagram showing the relationship between Fn1 and the amount of flank wear, created from eight of the nine data sets in Fig. 1, excluding No. 1 for which the cutting tool broke. Figs. 1 and 2 show that there is a strong correlation between Fn1 and the pro-eutectoid ferrite volume fraction, and between Fn1 and the amount of flank wear.

[0081] Although one embodiment of the present invention has been described above, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and it is possible to carry out appropriate modifications of the above-described embodiment within the scope of the invention.

Claims

1. A crankshaft material having a chemical composition, in mass%, of 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, Fn1 defined by the following formula (1) is 3.80 or more and 5.00 or less, the structure contains 12-40% by volume of pro-eutectoid ferrite and the balance is pearlite, The Mn content Mn obtained by analyzing the cementite in the pearlite by energy dispersive X-ray analysis at a depth of 50 μm from the surface of the part corresponding to the pin part of the crankshaft θ and C content C θ Ratio of Mn θ / C θ A crankshaft blank having a mass fraction of 0.360 or more. Fn1=7C+0.5Si+Mn+3Cr-10Ti (1) In formula (1), C, Si, Mn, Cr and Ti are substituted with the contents of the corresponding elements in mass%. The energy dispersive X-ray analysis is performed on C, Mn, Cr, V and Fe, and the Mn θ and C θ Each of the above is a value calculated in mass % with the sum of C, Mn, Cr, V and Fe as the denominator.

2. A crankshaft preform according to claim 1, which satisfies the following formulas (2) and (3): Fα>-21.364×Fn1+112.0 (2) Fα<-21.364×Fn1+130.0 (3) In formulas (2) and (3), the volume fraction of pro-eutectoid ferrite is substituted for Fα in percentage.

3. A method for producing a crankshaft blank as claimed in claim 1 or 2, comprising: a billet forging process in which a slab produced by a continuous casting method is hot forged to form a billet; and a die forging process in which the billet is hot forged, wherein the hot forging in the billet forging process has a heating temperature before hot forging of 1150 to 1300°C and a holding time at the heating temperature is 3.0 hours or more; and the hot forging in the die forging process has a heating temperature before hot forging of 1200 to 1350°C and a heating time up to the heating temperature is 500 seconds or less.

Images