Raw material for crankshaft and method for manufacturing raw material for crankshaft

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

Technical Field

[0001] The present invention relates to a blank for a crankshaft and a method for manufacturing the blank for a crankshaft. In this specification, the "blank for a crankshaft" means an intermediate product processed into a shape close to the shape of the final crankshaft product.

Background Art

[0002] A crankshaft used for automotive engine parts and the like is generally used after being subjected to induction hardening on a pin portion or a journal portion which are sliding portions. By performing induction hardening, a high-hardness structure such as martensite is formed on the surface of the pin portion or the journal portion, and the wear resistance of the pin portion or the journal portion is improved. On the other hand, a crankshaft that omits induction hardening is also used mainly for cost reduction. In carbon steel for machine structures and alloy steel for machine structures, steel that omits heat treatment such as quenching and tempering is called "abnormalized steel".

[0003] Japanese Patent Laid-Open No. 8-92687 discloses a high-strength and high-toughness abnormalized steel. This abnormalized steel has a predetermined chemical composition and contains inclusions having an average particle size of 0.1 to 5 μm at 1×10 2 ~1×10 6 pieces / mm 2 and is characterized in that the inclusions are Ti acid / nitrides, MnS, and composite compounds mainly composed of Ti acid / nitrides and MnS.

[0004] Japanese Patent No. 3235442 discloses a high-strength and low-ductility abnormalized steel. This abnormalized 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] The rough blank for the crankshaft is subjected to machining such as grinding and drilling. Therefore, the rough blank for the crankshaft preferably has excellent machinability. Although it is not related to the rough blank for the crankshaft, regarding steel with excellent machinability, Japanese Patent No. 3437079 discloses a mechanical structure steel excellent in chip disposal property. This mechanical structure steel has sulfides with a length of 20 μm or more alone, or a sulfide group with a length of 20 μm or more in which a plurality of sulfides are connected in a substantially straight line, and 30 or more are present within a visual field of a cross-section of 1 mm in the rolling direction 2 of the cross-section.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] As a method for improving the wear resistance of steel, it is conceivable to increase the C content to improve the hardness of the steel. However, increasing the C content is in a trade-off relationship with the machinability of the steel material. In addition, as a method for improving the mechanical properties of non-quenched and tempered steel, precipitation hardening elements such as V are also added, but since V is a relatively expensive element, it is preferable that the mechanical properties of the steel can be improved by a method other than adding V.

[0008] <000,094>An object of the present invention is to provide a rough blank for a crankshaft that has excellent wear resistance and excellent machinability even when used without high-frequency quenching.

Means for Solving the Problems

[0009] The semifinished product for a crankshaft according to an 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 to 0.010%, and the balance: Fe and impurities, and 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 primary ferrite, and the balance is pearlite. At a position 50 μm deep from the surface of the portion corresponding to the pin portion of the crankshaft, the Mn content Mn θ and the C content C θ ratio 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 C, Si, Mn, Cr, and Ti are substituted in mass %. The energy-dispersive X-ray analysis targets C, Mn, Cr, V, and Fe, and each of the Mn θ and C θ is a value obtained in mass % with the total of C, Mn, Cr, V, and Fe as the denominator.

[0010] The method for manufacturing a semifinished product for a crankshaft according to an embodiment of the present invention is a method for manufacturing the above-described semifinished product for a crankshaft, and includes a billet forging step of subjecting a slab manufactured by a continuous casting method to hot forging to obtain a billet, and a die forging step of subjecting the billet to hot forging. 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.

Effects of the Invention

[0011] According to the present invention, a crankshaft material is obtained that has excellent wear resistance and excellent machinability, even when used without high-frequency induction hardening. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a scatter plot showing the relationship between Fn1 and the volume fraction Fα of protereminate ferrite. [Figure 2] Figure 2 is a scatter plot showing the relationship between Fn1 and flank wear. [Modes for carrying out the invention]

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

[0014] Crankshaft raw materials are manufactured by hot forging billets of a predetermined size. Crankshaft raw materials manufactured by hot forging have a structure consisting of ferrite and pearlite. 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 solid solution in the cementite within the pearlite.

[0015] Billets, which are the raw materials for crankshafts, are manufactured, for example, by hot forging or hot rolling from slabs produced by continuous casting. In slabs produced by continuous casting, Mn tends to be concentrated near the center of the slab due to the solidification of molten steel from the surface inward. Therefore, Mn may also be concentrated near the center in billets produced from these slabs, and Mn may also be concentrated near the center in crankshaft raw materials produced from these billets.

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

[0017] To ensure a sufficient amount of Mn is present near the surface of the crankshaft material, it is effective to perform prolonged heating during the billet manufacturing stage to sufficiently diffuse the Mn, followed by hot forging to eliminate central segregation. From the perspective of diffusing Mn, it might be considered to increase the heating time when manufacturing the crankshaft material from the billet, but as will be discussed later, this prolonged heating time will not result in the desired microstructure.

[0018] To improve the machinability of crankshaft raw materials, it is effective to increase the volume fraction of protereminate ferrite in the ferrite-pearlite structure. To increase the volume fraction of protereminate ferrite, it is effective to control the chemical composition of the steel and to increase the heating rate before hot forging when manufacturing crankshaft raw materials from billets. Specifically, it is effective to rapidly heat the material using high-frequency induction heating and then immediately start hot forging. This suppresses the grain growth of austenite grains in the high-temperature range. By making the austenite grains smaller, the area of ​​the grain boundaries that serve as precipitation sites for protereminate ferrite increases, thereby increasing the amount of protereminate ferrite.

[0019] The present invention was completed based on the above findings. Below, a crankshaft material according to one embodiment of the present invention will be described in detail.

[0020] [Crankshaft material] [Chemical composition] The crankshaft material according to this embodiment has the chemical composition described below. In the following description, the "%" for elemental content refers to mass percent.

[0021] C: 0.35~0.50% Carbon (C) improves the hardness of steel and contributes to improved 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: 0.50~1.00% Silicon (Si) has deoxidizing and ferrite-enhancing properties. However, 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: 1.00~1.50% Manganese (Mn) improves the hardness of steel and contributes to improved fatigue strength and wear resistance. Mn also dissolves in cementite within pearlite, strengthening 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 improved 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 should be 0.30% or less. The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, and still more preferably 0.08%. The upper limit of the Cr content is preferably 0.25%, and still 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. Preferably, the P content is 0.025% or less, and more preferably 0.020% or less.

[0026] S: 0.020~0.100% 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 decreases. Therefore, the S content is 0.020 to 0.100%. The lower limit of the S content is preferably 0.030%, and more preferably 0.040%. The upper limit of the S content is preferably 0.080%, and 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 still more preferably 0.020%. The upper limit of the Al content is preferably 0.200%, more preferably 0.100%, and still more preferably 0.050%.

[0028] Ti: 0.001~0.050% Titanium (Ti) forms nitrides and carbonitrides, contributing to the refinement of crystal grains (austenite grains) at high temperatures. This refinement of austenite grains increases the area of ​​grain boundaries, which serve as precipitation sites for protereminate ferrite, resulting in an increase in protereminate ferrite. However, excessive addition of Ti saturates the effect and increases 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%.

[0029] N: 0.002~0.020% Nitrogen (N) forms nitrides and carbonitrides, contributing to the refinement of crystal grains (austenite grains) at high temperatures. This refinement of austenite grains increases the area of ​​grain boundaries, which serve as precipitation sites for protereminate ferrite, resulting in an increase in protereminate ferrite. Furthermore, excessively reducing N increases manufacturing costs. On the other hand, excessively high N content reduces the hot workability of the steel. Therefore, the N content is preferably 0.002-0.020%. 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%.

[0030] The remainder of the chemical composition of the crankshaft material according to this embodiment consists of Fe and impurities. These impurities refer to elements introduced from the ore or scrap used as raw materials for steel, or elements introduced from the environment during the manufacturing process.

[0031] The chemical composition of the crankshaft material 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 arbitrary elements. In other words, the chemical composition of the crankshaft material according to this embodiment may not contain either or both of V and Ca.

[0032] V: 0~0.20% Vanadium (V) enhances the wear resistance of steel through precipitation hardening. This effect can be obtained even if only a small amount of V is present. The crankshaft material according to this embodiment improves wear resistance by means other than adding V. Therefore, the crankshaft material according to this embodiment does not need to contain V. Omitting V is advantageous from a cost perspective. 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 still preferably 0.05%. From the viewpoint of manufacturing cost, it is most preferable not to include V.

[0033] Ca: 0~0.010% Calcium (Ca) improves the hot workability of steel. This effect can be obtained even if only a small amount of Ca is present. Therefore, Ca may be included 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] [About Fn1] The crankshaft material according to this embodiment has an Fn1 defined by the following formula (1) that is between 3.80 and 5.00. Fn1 = 7C + 0.5Si + Mn + 3Cr - 10Ti (1) In equation (1), the content of the corresponding elements in mass percent is substituted for C, Si, Mn, Cr, and Ti.

[0035] Fn1 is a parameter that correlates with the volume fraction of protereminate ferrite. The smaller Fn1, the larger the volume fraction of protereminate ferrite. When Fn1 is less than 3.80 or greater than 5.00, it becomes difficult to keep the volume fraction of protereminate ferrite within an appropriate range. The lower limit of Fn1 is preferably 4.00, more preferably 4.20, and still more preferably 4.40. The upper limit of Fn1 is preferably 4.95, more preferably 4.80, and still more preferably 4.60.

[0036] [Organization] The crankshaft material according to this embodiment has a structure that contains 12 to 40 volume percent of protereminate ferrite, with the remainder being pearlite.

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

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

[0039] The crankshaft material 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 equations (2) and (3), Fα is substituted with the volume fraction of the proterite ferrite in %,.

[0040] [Mn θ / C θ ] The crankshaft material according to this embodiment has a Mn content obtained by analyzing the cementite in pearlite at a depth of 50 μm from the surface of the portion corresponding to the pin portion of the crankshaft using energy-dispersive X-ray analysis. θ and C content C θ The ratio of Mn θ / C θ The value is 0.360 or higher. Here, energy-dispersive X-ray analysis targets C, Mn, Cr, V, and Fe, and Mn θ and C θ Each of these values ​​is calculated as a mass percent, with the sum of C, Mn, Cr, V, and Fe as the denominator.

[0041] To improve the wear resistance of the ferrite-pearlite structure, it is effective to increase the amount of Mn dissolved in the cementite within the pearlite. The wear resistance is most significantly affected by the surface-level portion of the crankshaft material. Therefore, to improve wear resistance, it is preferable to increase the amount of Mn dissolved in the cementite within the pearlite near the surface.

[0042] In this embodiment, the evaluation target is a depth of 50 μm from the surface of the part corresponding to the pin portion of the crankshaft, and the cementite in the pearlite present at this location is analyzed by energy-dispersive X-ray spectroscopy (EDX). Specifically, the part corresponding to the pin portion of the crankshaft is cut so that the plane perpendicular to the axial direction of the pin portion is the cut surface, and the cut surface is observed with a scanning electron microscope (SEM) at a magnification of approximately 10,000x. While confirming the SEM image, the cementite in the pearlite is irradiated with an electron beam, and the chemical composition of the cementite is analyzed by EDX. C, Mn, Cr, V, and Fe are targeted for analysis because the possibility of other elements being solid-dissolved in the cementite is low, and even if they are present, they are thought to be in small amounts and not affect the properties.

[0043] In this embodiment, the Mn content obtained from the analysis results is Mn θ and C content C θ The ratio of Mn θ / C θ This method evaluates the amount of Mn dissolved in cementite within the pearlite near the surface. θ / C θ If the value is 0.360 or higher, excellent wear resistance can be obtained. θ / C θ The lower limit is preferably 0.380, more preferably 0.400, more preferably 0.450, more preferably 0.500, and still more preferably 0.550. θ / C θ The upper limit is not particularly limited, but is, for example, 2.000, preferably 1.500, and more preferably 1.200.

[0044] [Manufacturing method for crankshaft material] Next, an example of a method for manufacturing crankshaft raw materials according to this embodiment will be described. The manufacturing method described below is merely illustrative and does not limit the method of manufacturing crankshaft raw materials according to this embodiment.

[0045] The crankshaft material according to this embodiment can be manufactured by hot forging a slab produced by a continuous casting method to create a billet, and then hot forging this billet. Hereinafter, the hot forging process 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 process used to produce a crankshaft material 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 chemical composition described above is subjected to hot forging to create a steel billet of a size suitable for die forging (billet forging process).

[0047] In the hot forging (billet forging) process of billet forging, the heating temperature before forging is set to 1150-1300°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 time before forging diffuses the Mn segregated in the center of the slab. If the heating temperature is too low or the holding time is too short, the diffusion of Mn 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, and 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, and more preferably 4.5 hours. The upper limit of the holding time at the heating temperature is not particularly limited, but for example it is 8.0 hours, and preferably 6.0 hours.

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

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

[0050] [Die forging process] Next, the billets produced in the billet forging process are hot-forged to become raw materials for crankshafts (die forging process).

[0051] In hot forging (die forging) in the die forging process, the heating temperature before forging is set to 1200-1350°C. If the heating temperature is too low, it may be difficult to process into complex shapes. 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, and more preferably 1250°C. The upper limit of the heating temperature is preferably 1325°C, and more preferably 1300°C.

[0052] In die forging, the heating time to the heating temperature (the time from the start of heating until the heating temperature is reached) should be 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 making the austenite grains smaller, the area of ​​the grain boundaries that become the precipitation sites for protereminate ferrite increases, and the amount of protereminate ferrite can be increased. The upper limit of the heating time to the heating temperature is preferably 400 seconds, and 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, and preferably 200 seconds.

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

[0054] The billet is forged and shaped to resemble the final product, the crankshaft. A finish forging may be performed after this rough forging.

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

[0056] The raw materials for the crankshaft are manufactured through the above process.

[0057] The above describes an example of a crankshaft raw material and a method for manufacturing the same according to one embodiment of the present invention. The crankshaft raw material according to this embodiment is subjected to machining such as grinding and drilling to become a final product, a crankshaft. The crankshaft raw material according to this embodiment has excellent machinability. Furthermore, the crankshaft raw material according to this embodiment has excellent wear resistance even when used without high-frequency induction hardening.

[0058] [crankshaft] A crankshaft according to one embodiment of the present invention is a crankshaft manufactured from the crankshaft raw material described above. Among the crankshafts manufactured from the crankshaft raw material described above, those used without high-frequency induction hardening are particularly preferred. That is, a crankshaft according to one embodiment of the present invention is a crankshaft used without high-frequency induction hardening, and its chemical composition is, in mass%, 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%, remainder: Fe and impurities, Fn1 as defined by formula (1) below is 3.80 or more and 5.00 or less, the microstructure contains 12-40 volume% proterite ferrite and the remainder is pearlite, and the Mn content obtained by analyzing the cementite in the pearlite at a depth of 50 μm from the surface of the part corresponding to the pin portion of the crankshaft by energy-dispersive X-ray analysis. θ and C content C θ The ratio of Mnθ / C θ The value is 0.360 or higher. Fn1 = 7C + 0.5Si + Mn + 3Cr - 10Ti (1) In equation (1), the content of the corresponding elements in mass percent is substituted for C, Si, Mn, Cr, and Ti. The aforementioned energy-dispersive X-ray analysis targets C, Mn, Cr, V, and Fe, and the aforementioned Mn θ and C θ Each of these values ​​is calculated as a mass percent, with the sum of C, Mn, Cr, V, and Fe as the denominator. [Examples]

[0059] The present invention will be described more specifically below with reference to examples. The present invention is not limited to these examples.

[0060] A slab with a cross-section of 400 mm × 530 mm was manufactured by casting molten steel having the chemical composition shown in Table 1. The "hardness" in Table 1 refers to the Vickers hardness of the manufactured slab.

[0061] [Table 1]

[0062] These slabs were hot-forged (billet forged) to produce round billets with a diameter of 85 mm. The billet forging conditions are shown in Table 2. Process symbol c1 involved heating only, without forging, and then machining to produce round billets with a diameter of 85 mm. Process symbol d1 involved machining to produce round billets with a diameter of 85 mm from the slabs.

[0063] [Table 2]

[0064] The manufactured billets were hot forged (die forging) to produce raw materials for crankshafts. The die forging conditions are shown in Table 3. In Table 3, "heating time" refers to the time from the start of heating until the heating temperature is reached. After the forging described in Table 3, finish forging was performed at a starting temperature of 1100°C and an ending temperature of 1070°C.

[0065] [Table 3]

[0066] The portion of the manufactured crankshaft material corresponding to the crankshaft pin (hereinafter simply referred to as the "pin portion") was cut in the middle of the parallel section 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). The area ratio of protereminate ferrite was measured by observing the surface at a depth of 50 μm with a 500x optical microscope. The measured area ratio of protereminate ferrite was considered to be the volume ratio of protereminate ferrite.

[0067] Similarly, the pin portion of the crankshaft material manufactured was cut in the middle of the parallel section 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). The surface was observed at a depth of 50 μm from the surface using a scanning electron microscope (SEM) at a magnification of approximately 10,000x. While confirming the SEM image, the cementite in the pearlite was irradiated with an electron beam and the chemical composition of the cementite was analyzed by EDX, and Mn θ and C θ EDX was used to analyze C, Mn, Cr, V, and Fe, and Mn θ and C θ Each of these was calculated as a mass percent, with the sum of C, Mn, Cr, V, and Fe as the denominator. θ and C θ This was calculated as the average value obtained from 8 locations.

[0068] EDX was performed using a JEOL JED-2300. The measurement conditions were an acceleration voltage of 15kV.

[0069] The wear resistance of crankshaft raw materials was evaluated by the wear diameter of the pin portion after wear testing. Specifically, crankshafts manufactured by machining crankshaft raw materials were mounted on an engine bench testing machine, and a wear test was conducted by rotating the engine for 20 hours at 7000 rpm, oil temperature 110°C, oil pressure 200 kPa, and low viscosity oil (OW-16). The change in the diameter of the pin portion was measured. Before and after the wear test, the diameter was measured at three points on the circumference (0°, 120°, and 240° positions) using a micrometer, and the average was calculated. The wear diameter was defined as the difference between the diameter before the wear test and the diameter after the wear test. A wear diameter of 1.00 μm or less was considered to indicate excellent wear resistance.

[0070] The fatigue strength of the crankshaft material was measured as follows: Rotary bending fatigue test specimens were taken from the crankshaft material using the Ono method, and a rotary bending fatigue test was performed in accordance with JIS Z 2274. Number of cycles: 10 7 The fatigue strength was defined as the maximum stress experienced by the material that did not fracture up to that point. The target fatigue strength was 350 MPa or higher.

[0071] The machinability of the crankshaft material was evaluated as follows: A guide hole with a depth of 22 mm was drilled into the crankshaft material using a 5 mm diameter guide drill, and then an oil hole with a depth of 91 mm was drilled using a 4.98 mm diameter long drill. Both the guide drill and the long drill were solid drills coated with TiN and had internal lubrication holes. After drilling 600 times under predetermined conditions while lubricating using the MQL method, the machinability was evaluated by measuring the amount of wear on the flank surface of the long drill. Specifically, a magnified photograph of the outside of the flank surface was taken with a microscope, and the width of the area showing metallic luster (the part where the coating has peeled off) between the coated portion and the cutting edge was measured, and this width was defined as the amount of flank surface wear. If the amount of flank surface wear was 0.020 mm or less, it was evaluated as having excellent machinability.

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

[0073] [Table 4]

[0074] As shown in Table 4, crankshaft profiles No. 3, 7, 9, 10, 12, and 13 showed excellent wear resistance, with the pin portion wear diameter after wear testing being 1.00 μm or less. These crankshaft profiles also showed excellent machinability, with the flank surface wear of the long drill being 0.020 mm or less.

[0075] The crankshaft raw materials No. 1, 2, 8, and 11 had poor machinability. This is thought to be due to a low volume fraction of protereminate ferrite. The low volume fraction of protereminate ferrite is thought to be due to an excessively large Fn1.

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

[0077] The raw materials for crankshafts No. 5 and 6 had poor wear resistance. This is because Mn θ / C θ This is thought to be because the value was low. θ / C θ The low quality is thought to be due to the fact that forging was not performed during the manufacturing of the billet.

[0078] The raw material for crankshaft No. 14 had poor wear resistance. This is because Mn θ / C θ This is thought to be because the value was low. θ / C θ The low performance was likely due to an insufficient holding time during billet forging.

[0079] The crankshaft material No. 15 had poor machinability. This is thought to be due to a low volume fraction of protereminate ferrite. The low volume fraction of protereminate ferrite is thought to be due to an excessively long heating time during die forging.

[0080] Figure 1 is a scatter plot showing the relationship between Fn1 and the progenitized ferrite volume fraction Fα, created from nine data points (No. 1-3 and 8-13) where the billet forging process symbol is "a1" and the die forging process symbol is "a2". Figure 2 is a scatter plot showing the relationship between Fn1 and flank wear, created from eight of the nine data points in Figure 1, excluding No. 1 where the cutting tool broke. From Figures 1 and 2, it can be seen that there is a strong correlation between Fn1 and the progenitized ferrite volume fraction, and between Fn1 and 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 the above-described embodiment by appropriately modifying it within the scope of the invention.

Claims

1. A raw material for a crankshaft, The chemical composition is expressed in mass percent. 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 to 0.050%, N: 0.002 to 0.020%, V: 0 to 0.20%, Ca: 0-0.010%, The remainder consists of Fe and impurities. Fn1, as defined by the following formula (1), is between 3.80 and 5.00, The structure contains 12-40 volume percent proterite ferrite, with the remainder being pearlite. The Mn content obtained by analyzing the cementite in the pearlite at a depth of 50 μm from the surface of the part corresponding to the pin portion of the crankshaft using energy-dispersive X-ray spectroscopy. θ and C content C θ The ratio of Mn θ / C θ A crankshaft material having a coefficient of 0.360 or higher. Fn1=7C+0.5Si+Mn+3Cr-10Ti (1) In equation (1), the content of the corresponding elements in mass percent is substituted for C, Si, Mn, Cr, and Ti. The energy-dispersive X-ray analysis mentioned above analyzes C, Mn, Cr, V, and Fe, and the Mn θ and C θ Each of these values ​​is calculated as a mass percent, with the sum of C, Mn, Cr, V, and Fe as the denominator.

2. A crankshaft material according to claim 1, wherein the crankshaft material satisfies the following formulas (2) and (3). Fα>-21.364×Fn1+112.0 (2) Fα<-21.364×Fn1+130.0 (3) In equations (2) and (3), Fα is substituted with the volume fraction of the proterite ferrite in %,.

3. A method for manufacturing a crankshaft material according to claim 1 or 2, The billet forging process involves hot forging a slab manufactured by a continuous casting method to create a billet, The process includes a die forging step in which the billet is subjected to hot forging, In the hot forging process described above, 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. A method for manufacturing a crankshaft material, wherein the hot forging process in the die forging step is characterized by a heating temperature of 1200 to 1350°C before hot forging, and a heating time of 500 seconds or less to reach the aforementioned heating temperature.

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