Crankshaft and method for producing crankshaft

Abstract

Provided is a crankshaft excellent in terms of crack resistance and fatigue strength. The crankshaft has a chemical composition comprising, in terms of mass%, 0.35-0.65% C, 0.01-0.60% Si, 1.00-2.00% Mn, 0.01-0.50% Cr, 0.001-0.050% Al, 0.010-0.100% S, 0.010-0.030% N, 0-0.020% Ti, and Fe and impurities as the remainder, the chemical composition satisfying expression (1). The crankshaft has a hardened layer in at least some of the surface. The hardened layer has a structure including 9.0 vol% or more ferrite, the remainder being martensite and / or bainite. The hardened layer has a Vickers hardness of 520 or greater.　(1): ([C]-0.05) / [N]-300×[Ti]≤30.0 In expression (1), the C content and the N content, in mass%, are substituted respectively for [C] and [N].

Description

Crankshaft and manufacturing method thereof

[0001] The present invention relates to a crankshaft and a method for manufacturing a crankshaft.

[0002] Crankshafts are generally manufactured by hot forging steel into a preform, then machining it with cutting, grinding, and other processes, then induction hardening it to improve its fatigue strength, and finally finishing it. However, if the preform is left for a long time between induction hardening and finishing, or if it cracks during finishing, the material's susceptibility to cracking is an issue.

[0003] International Publication No. 2020 / 004060 discloses an induction-hardened crankshaft, in which the structure of the non-induction-hardened portion is mainly composed of ferrite and pearlite, the structure of the induction-hardened portion is mainly composed of martensite or tempered martensite, and the prior austenite grain size is 30 μm or less.

[0004] Japanese Patent Application Laid-Open Publication No. 2018-112222 discloses a crankshaft whose surface has been induction hardened. In this induction hardened crankshaft, the distance L on the top side of the shoulder of the pin or journal is (-0.032D+6.6521)×H, where H (mm) is the distance from the connection position of the fillet R portion and thrust portion of the pin or journal to the top of the apex, L (mm) is the distance from the connection position to the quench-hardened layer on the shoulder surface of the pin or journal, and D (mm) is the diameter of the pin or journal. １／３ (mm) or more.

[0005] Japanese Patent Application Laid-Open No. 2008-127620 discloses a crankshaft having a quench-hardened layer on at least the surface of the crankpin, in which the surface compressive residual stress in the bottom R portion of the crankpin is 600 MPa or more.

[0006] Although not related to crankshafts, WO 2018 / 008703 discloses a rolled wire rod in which cracking during cold forging is effectively suppressed even when spheroidizing annealing before cold forging is omitted or shortened. This rolled wire rod has a mixed structure of ferrite and pearlite, and the average area of ​​sulfides present in the range from the outermost layer to the D / 8 position (D is the diameter of the rolled wire rod) is 6 μm ２ and the average aspect ratio of the sulfides is 5 or less.

[0007] International Publication No. 2020 / 004060 Japanese Patent Application Laid-Open No. 2018-112222 Japanese Patent Application Laid-Open No. 2008-127620 International Publication No. 2018 / 008703

[0008] International Publication No. 2020 / 004060 discloses that quench cracking can be suppressed by adding a predetermined amount of Nb to refine the grain size of the quenched structure. However, this publication does not discuss cracking that occurs when stress is applied during manufacturing processes such as grinding.

[0009] An object of the present invention is to provide a crankshaft that is excellent in crack resistance and fatigue strength.

[0010] A crankshaft according to one embodiment of the present invention has a chemical composition, in mass %, of C: 0.35 to 0.65%, Si: 0.01 to 0.60%, Mn: 1.00 to 2.00%, Cr: 0.01 to 0.50%, Al: 0.001 to 0.050%, S: 0.010 to 0.100%, N: 0.010 to 0.030%, Ti: 0 to 0.020%, and the balance: Fe and impurities, wherein the chemical composition satisfies the following formula (1): ([C]-0.05) / [N]-300×[Ti]≦30.0 (1) In formula (1), the C content and N content are substituted in mass % for [C] and [N], respectively.

[0011] A crankshaft according to one embodiment of the present invention may have a chemical composition, in mass%, of C: 0.35 to 0.65%, Si: 0.01 to 0.60%, Mn: 1.00 to 2.00%, Cr: 0.01 to 0.50%, Al: 0.001 to 0.050%, S: 0.010 to 0.100%, N: 0.010 to 0.030%, and the balance: Fe and impurities, wherein the chemical composition satisfies the following formula (1), and the crankshaft has a hardened layer on at least a portion of its surface, the hardened layer containing 9.0 volume% or more of ferrite and the balance having a structure of at least one of martensite and bainite, and the hardened layer may have a Vickers hardness of 520 or more: ([C] - 0.05) / [N] ≦ 30.0 (1) where [C] and [N] in formula (1) are substituted with the C content and the N content, in mass%, respectively.

[0012] A crankshaft according to one embodiment of the present invention may have a chemical composition, in mass %, of C: 0.35 to 0.65%, Si: 0.01 to 0.60%, Mn: 1.00 to 2.00%, Cr: 0.01 to 0.50%, Al: 0.001 to 0.050%, S: 0.010 to 0.100%, N: 0.010 to 0.030%, and further containing 0.020% or less Ti, the balance being Fe and impurities, wherein the chemical composition satisfies the following formula (1), the crankshaft has a hardened layer on at least a portion of its surface, the hardened layer having a structure containing 9.0% or more by volume of ferrite and the balance being at least one of martensite and bainite, and the hardened layer has a Vickers hardness of 520 or more. ([C]-0.05) / [N]-300×[Ti]≦30.0 (1) In formula (1), the C content and N content are substituted in mass % for [C] and [N], respectively.

[0013] A method for manufacturing a crankshaft according to one embodiment of the present invention is a method for manufacturing the above-mentioned crankshaft, and includes the steps of: preparing an intermediate crankshaft; heating a target region of the intermediate, which is a region where a hardened layer is to be formed, to a heating temperature of 920 to 980°C; cooling the target region from the heating temperature to an isothermal holding temperature of 710 to 760°C at a cooling rate of 80°C / sec or more and holding the target region at the isothermal holding temperature for 80 seconds or more; and cooling the target region from the isothermal holding temperature to a temperature not exceeding the Ms point at a cooling rate of 80°C / sec or more.

[0014] According to the present invention, a crankshaft having excellent crack resistance and fatigue strength can be obtained.

[0015] Fig. 1 shows the heat pattern of the heat treatment performed in the examples. Fig. 2 shows a binarized image of the structure of steel No. 2 in Table 2. Fig. 3 shows a binarized image of the structure of steel No. 4 in Table 2. Fig. 4 is a scatter diagram showing the relationship between hardness and ferrite volume fraction. Fig. 5 is a scatter diagram showing the relationship between bending fatigue strength and ferrite volume fraction.

[0016] The inventors of the present invention investigated the relationship between the structure and hardness of a steel material and cracking resistance in order to develop a crankshaft with excellent cracking resistance. Note that "crack resistance" in this specification does not refer to cracking due to fatigue, but rather to cracking (delayed cracking) that occurs when grinding or the like is performed after the hardened layer is formed.

[0017] Since fatigue strength is generally important for crankshafts, high hardness is required to ensure fatigue strength. On the other hand, from the viewpoint of crack resistance, too high hardness is undesirable. The inventors investigated whether delayed cracking could be suppressed by controlling the structure of the steel material.

[0018] Specifically, we performed four-point bending tests on various steels immersed in dilute hydrochloric acid to evaluate their crack resistance and search for a microstructure with excellent crack resistance. As a result, we found that a microstructure with an appropriate amount of ferrite precipitated in a microstructure primarily composed of martensite or bainite exhibits excellent crack resistance. When a microstructure consists solely of high-hardness structures such as martensite or bainite, or when ferrite is precipitated in small amounts, stress concentrates in the high-hardness structures. High-hardness structures are sensitive to cracks and become more susceptible to cracking due to increased stress load. Precipitating an appropriate amount of ferrite homogenizes the ferrite in the microstructure, preventing stress concentration in the high-hardness structures and improving crack resistance. Furthermore, optimizing the balance with hardness can improve crack resistance while maintaining fatigue strength.

[0019] In order to ensure crack resistance, it is also effective to refine the crystal grains of the high hardness structure, and in order to refine the crystal grains, it is effective to increase the N content.

[0020] The present invention has been completed based on the above findings. A crankshaft and a method for manufacturing the same according to one embodiment of the present invention will now be described in detail.

[0021] [Chemical Composition] The crankshaft according to this embodiment has the chemical composition described below. In the following description, "%" for the content of an element means mass %.

[0022] C: 0.35 to 0.65% Carbon (C) improves the hardness of steel and contributes to improving fatigue strength. On the other hand, if the C content is too high, crack resistance and machinability decrease. Therefore, the C content is 0.35 to 0.65%. The lower limit of the C content is preferably 0.37%, more preferably 0.40%. The upper limit of the C content is preferably 0.60%, more preferably 0.55%.

[0023] Si: 0.01 to 0.60% Silicon (Si) has the effects of deoxidizing and strengthening ferrite. On the other hand, if the Si content is too high, machinability decreases. Therefore, the Si content is 0.01 to 0.60%. The lower limit of the Si content is preferably 0.02%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Si content is preferably 0.58%, and even more preferably 0.55%.

[0024] Mn: 1.00 to 2.00% Manganese (Mn) improves the hardenability of steel and contributes to improving the hardness of steel. On the other hand, if the Mn content is too high, bainite is formed during the cooling process after hot forging, reducing machinability. Therefore, the Mn content is 1.00 to 2.00%. The lower limit of the Mn content is preferably 1.10%, more preferably 1.20%. The upper limit of the Mn content is preferably 1.80%, more preferably 1.60%.

[0025] Cr: 0.01 to 0.50% Chromium (Cr) improves the hardenability of steel and contributes to improving the hardness of steel. On the other hand, if the Cr content is too high, bainite is formed during the cooling process after hot forging, reducing machinability. Therefore, the Cr content is 0.01 to 0.50%. The lower limit of the Cr content is preferably 0.05%, more preferably 0.08%. The upper limit of the Cr content is preferably 0.30%, more preferably 0.20%.

[0026] Al: 0.001 to 0.050% Aluminum (Al) has a deoxidizing effect. On the other hand, if the Al content is too high, the amount of alumina-based inclusions formed becomes excessive, resulting in a decrease in machinability. Therefore, the Al content is 0.001 to 0.050%. The lower limit of the Al content is preferably 0.002%, more preferably 0.005%. The upper limit of the Al content is preferably 0.040%, more preferably 0.030%.

[0027] S: 0.010 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 steel decreases. Therefore, the S content is 0.010 to 0.100%. The lower limit of the S content is preferably 0.015%, more preferably 0.020%. The upper limit of the S content is preferably 0.090%, more preferably 0.080%.

[0028] N: 0.010 to 0.030% Nitrogen (N) forms nitrides and carbonitrides, contributing to grain refinement and improving crack resistance. In addition to grain refinement, the nitrides and carbonitrides themselves are finely dispersed, which increases the strength of the steel and improves crack resistance. On the other hand, if the N content is too high, the hot ductility of the steel decreases. Therefore, the N content is 0.010 to 0.030%. The lower limit of the N content is preferably 0.011%, and more preferably 0.012%. The upper limit of the N content is preferably 0.020%, and more preferably 0.018%.

[0029] The remainder of the chemical composition of the crankshaft according to this embodiment is Fe and impurities. The impurities here refer to elements that are mixed in from ores or scrap used as raw materials for steel, or from the environment during the manufacturing process.

[0030] The chemical composition of the crankshaft according to this embodiment may contain 0.020% or less of Ti instead of a portion of Fe. Ti is an optional element. In other words, the crankshaft according to this embodiment does not necessarily need to contain Ti.

[0031] Ti: 0 to 0.020% Titanium (Ti) forms nitrides and carbonitrides, contributing to the refinement of crystal grains. This effect can be obtained even if even a small amount of Ti is contained. On the other hand, even if the Ti content is excessively high, the effect saturates. Therefore, the Ti content is 0 to 0.020%. The lower limit of the Ti content is preferably 0.005%, and more preferably 0.010%. The upper limit of the Ti content is preferably 0.018%.

[0032] [Regarding Formula (1)] As the C content increases, the hardness of the steel increases and the fatigue strength increases, but delayed cracking also tends to occur more easily. Therefore, the N content and Ti content must be adjusted according to the C content. Specifically, the chemical composition of the crankshaft according to this embodiment satisfies the following formula (1): ([C] - 0.05) / [N] - 300 x [Ti] ≤ 30.0 (1) The C content, N content, and Ti content in mass % are substituted for [C], [N], and [Ti] in formula (1), respectively.

[0033] When the crankshaft does not contain Ti, 0 is substituted for [Ti] in formula (1). That is, when Ti is not contained, formula (1) becomes as follows: ([C] - 0.05) / [N] ≤ 30.0 (1) The C content and N content in mass % are substituted for [C] and [N] in formula (1), respectively.

[0034] If the left side of formula (1) is 30.0 or less, a crankshaft with higher crack resistance can be obtained. The upper limit of the left side of formula (1) is preferably 28.0, and more preferably 26.0. The lower limit of the left side of formula (1) is not particularly limited, but is, for example, 20.0.

[0035] [Structure] The crankshaft according to this embodiment has a hardened layer (quench-hardened layer) on at least a portion of the surface. The hardened layer is formed, for example, by induction hardening. The hardened layer is formed, for example, on the pin portion or journal portion of the crankshaft. The hardened layer may be formed on only one of the pin portion or journal portion, or on both. The hardened layer may be formed in a location other than the pin portion or journal portion, or may be formed on the entire surface. The hardened layer may also be formed not only on the surface of the crankshaft, but also in the core portion.

[0036] This hardened layer has a structure containing 9.0% by volume or more of ferrite, with the remainder being at least one of martensite and bainite.

[0037] When the structure consists only of high-hardness structures such as martensite or bainite, or when ferrite is precipitated but in small amounts, stress concentrates in the high-hardness structures. High-hardness structures are sensitive to cracks and are prone to cracking as stress load increases. Precipitating an appropriate amount of ferrite homogenizes the ferrite in the structure, preventing stress concentration in the high-hardness structures and improving crack resistance. The lower limit of the volume fraction of ferrite in the structure of the hardened layer is preferably 10.0%, more preferably 10.5%. On the other hand, if the volume fraction of ferrite is too high, fatigue strength may decrease. The upper limit of the volume fraction of ferrite in the structure of the hardened layer is preferably 16.0%, more preferably 14.0%.

[0038] The remainder of the structure of the hardened layer, excluding ferrite, is at least one of martensite and bainite, i.e., the remainder of the hardened layer is either martensite, bainite, or a mixed structure of martensite and bainite.

[0039] In this hardened layer, the prior austenite grain size of martensite and bainite is preferably 30 μm or less. If the prior austenite grain size of martensite and bainite is 30 μm or less, better fatigue strength and crack resistance can be obtained. The upper limit of the prior austenite grain size is more preferably 28 μm, and even more preferably 26 μm. The lower limit of the prior austenite grain size is not particularly limited, but is, for example, 15 μm.

[0040] The hardened layer has a Vickers hardness of 520 Hv or more. By making the Vickers hardness 520 Hv or more, fatigue strength is further improved. The lower limit of the Vickers hardness of the hardened layer is preferably 530 Hv, more preferably 540 Hv, and even more preferably 550 Hv. On the other hand, if the Vickers hardness of the hardened layer is too high, cracking is more likely to occur. The upper limit of the Vickers hardness of the hardened layer is preferably 750 Hv, more preferably 700 Hv, and even more preferably 650 Hv.

[0041] The higher the Vickers hardness of the hardened layer, the greater the fatigue strength of the steel, but the more likely it is that delayed cracking will occur. Therefore, it is preferable to adjust the volume fraction of ferrite in the hardened layer according to the magnitude of the Vickers hardness. Specifically, it is preferable that the Vickers hardness of the hardened layer and the volume fraction of ferrite satisfy the following formula (2): [α]≧0.0259×Hv−4.36 (2) In formula (2), [α] is substituted with the volume fraction of ferrite in percent, and Hv is substituted with the Vickers hardness of the hardened layer.

[0042] Similarly, the higher the fatigue strength, the more likely delayed cracking tends to occur. Therefore, it is preferable to adjust the volume fraction of ferrite in the hardened layer depending on the magnitude of the fatigue strength. Specifically, it is preferable that the bending fatigue strength of the hardened layer and the volume fraction of ferrite satisfy the following formula (3): [α]≧0.0028×[M]+6.86 (3) In formula (3), [α] is substituted with the volume fraction of ferrite in %, and [M] is substituted with the bending fatigue strength in MPa.

[0043] In the crankshaft according to this embodiment, the structure of the portion other than the hardened layer is optional. Within the range of chemical composition of the crankshaft according to this embodiment, the structure of the portion other than the hardened layer is usually a structure mainly composed of ferrite and pearlite. The structure of the portion other than the hardened layer of the crankshaft according to this embodiment is preferably 90% by volume or more, and more preferably 95% by volume or more, of ferrite and pearlite.

[0044] [Manufacturing Method] An example of a manufacturing method for the crankshaft according to this embodiment will be described. The manufacturing method described below is merely an example, and the manufacturing method for the crankshaft according to this embodiment is not limited to this.

[0045] 1. Preparing a crankshaft intermediate product The crankshaft intermediate product can be manufactured, for example, as follows.

[0046] Steel having the above-mentioned chemical composition is melted and then subjected to continuous casting or blooming to produce a billet. The billet is then hot forged to form the rough shape of the crankshaft. The hot forging conditions are not limited to the above, but the heating temperature is, for example, 1000 to 1300°C, and the holding time is, for example, 1 second to 20 minutes. Hot forging may be performed in multiple steps. Heat treatment such as annealing may also be performed before or after hot forging. After hot forging, machining is performed as necessary. This produces an intermediate crankshaft.

[0047] The intermediate crankshaft is subjected to a heat treatment, which will be described in detail below, to form a hardened layer (quench hardened layer). The hardened layer may be formed only in specific locations on the intermediate crankshaft, or may be formed over the entire intermediate crankshaft. In the following description, the area where the hardened layer is to be formed is referred to as the "target area."

[0048] First, the target region is heated to a heating temperature of 920 to 980°C. This heating austenitizes the structure of the target region. If the heating temperature is too low, the austenite structure will not be uniform, and a uniform structure will not be obtained after cooling. On the other hand, if the heating temperature is too high, the austenite grains will coarsen, and the prior austenite grain size in the structure after cooling will increase. The lower limit of the heating temperature is preferably 930°C, and more preferably 940°C. The upper limit of the heating temperature is preferably 970°C, and more preferably 960°C. The holding time at the heating temperature is not particularly limited, but is, for example, 10 seconds to 30 minutes.

[0049] After heating the target region to the heating temperature, it is cooled from the heating temperature to an isothermal holding temperature of 710 to 760°C at a cooling rate of 80°C / sec or more and held at the isothermal holding temperature for 80 seconds or more. Thereafter, it is cooled from the isothermal holding temperature to a temperature below the Ms point (martensitic transformation start temperature) at a rate of 80°C / sec or more.

[0050] By holding at the isothermal holding temperature for 80 seconds or more, ferrite precipitates in the austenite. If the isothermal holding temperature is outside the range of 710 to 760°C or if the holding time at the isothermal holding temperature is too short, a sufficient amount of ferrite may not be obtained. The lower limit of the holding time at the isothermal holding temperature is preferably 90 seconds, and more preferably 100 seconds.

[0051] If the cooling rate from the heating temperature to the isothermal holding temperature is too slow, structures other than ferrite may be generated, or a sufficient amount of ferrite may not be obtained. The lower limit of the cooling rate from the heating temperature to the isothermal holding temperature is preferably 100°C / sec, and more preferably 120°C / sec. The upper limit of the cooling rate from the heating temperature to the isothermal holding temperature is not particularly limited, but if the cooling rate is too high, it may become difficult to maintain the isothermal holding temperature. The upper limit of the cooling rate from the heating temperature to the isothermal holding temperature is preferably 250°C, and more preferably 200°C.

[0052] Similarly, if the cooling rate from the isothermal holding temperature to the Ms point is too slow, a structure other than ferrite (e.g., pearlite) may be formed. The lower limit of the cooling rate from the isothermal holding temperature to the Ms point is preferably 100°C / sec, more preferably 120°C / sec. The upper limit of the cooling rate from the isothermal holding temperature to the Ms point is not particularly limited, but is, for example, 400°C / sec.

[0053] This produces a hardened layer containing 9.0 volume % or more of ferrite, with the remainder being at least one of martensite and bainite. After the hardened layer is formed, finishing such as grinding is carried out as necessary. Through the above steps, a crankshaft is manufactured.

[0054] The crankshaft and its manufacturing method according to one embodiment of the present invention have been described above. According to this embodiment, a crankshaft with excellent crack resistance and fatigue strength can be obtained.

[0055] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0056] A 50 kg steel having the chemical composition shown in Table 1 was melted in a vacuum induction melting furnace to produce an ingot. This ingot was hot forged at a temperature of 1000°C or higher to a thickness of 30 mm, width of 90 mm, and length of 2000 mm, and then cut into a length of 100 mm to produce a steel billet. This steel billet was hot rolled at a temperature of 1000°C or higher and air-cooled to produce a material having a thickness of 10 mm and a width of 100 mm. All of these materials had a structure mainly composed of ferrite and pearlite.

[0057]

[0058] This material was subjected to the heat treatment shown in Fig. 1. Specifically, the material was heated to a heating temperature T1, then cooled to an isothermal holding temperature T2 at a cooling rate CR1, held at the isothermal holding temperature T2 for a holding time t1, and then cooled to room temperature at a cooling rate CR2.

[0059] The Vickers hardness of the heat-treated steel was measured. The Vickers hardness was measured at five points under a load of 1 kg, and the average was calculated.

[0060] Test specimens for microstructure observation were collected from the heat-treated steel. The surfaces of the test specimens for microstructure observation were mirror-finished, then subjected to nital etching and SEM observation. The volume fraction of the microstructure was determined by coloring the topographical images (three fields of view for each test specimen) obtained by SEM observation using paint software, binarizing the images using image analysis software ImageJ, and detecting particles using the software's particle analysis function to calculate the area fraction, which was then considered to be the volume fraction. Figure 2 shows a binarized image (1000x magnification) of the microstructure of steel No. 2 in Table 2 below, and Figure 3 shows a binarized image (1000x magnification) of the microstructure of steel No. 4. In Figures 2 and 3, the white areas represent ferrite, and the black areas represent martensite and / or bainite.

[0061] The prior austenite grain size was measured as follows. The surface of a test piece taken from the heat-treated steel was mirror-finished, and then etched with a saturated aqueous solution of picric acid to reveal the prior austenite grain boundaries. The prior austenite grain size was calculated by the intercept method. Specifically, a straight line with a total length L was drawn, and the number of crystal grains that intersected this line, n, was calculated. Ｌ Calculate the intercept length (L / nＬ The intercept length (L / n Ｌ ) was calculated, and the arithmetic mean was taken as the prior austenite grain size.

[0062] Table 2 shows a list of the heat treatment conditions, as well as the hardness, prior austenite grain size (prior γ grain size), and structure of the steel material after the heat treatment. In Table 2, "M+B fraction" is the sum of the volume fraction of martensite and the volume fraction of bainite, "P fraction" is the volume fraction of pearlite, and "F fraction" is the volume fraction of ferrite. Note that Nos. 3, 4, and 11 were not subjected to isothermal holding, but were cooled from the heating temperature T1 to room temperature at a cooling rate CR1.

[0063]

[0064] A number of test pieces measuring 10 mm x 75 mm x 2 mm were taken from the heat-treated steel material and subjected to a hydrochloric acid immersion four-point bending stress corrosion test to evaluate crack resistance. The test conditions were as follows: Test method: Four-point bending, stress applied by the full gauge method; Solution: 4.1 mass% hydrochloric acid solution; Temperature: Room temperature; Test time: 24 hours

[0065] The test was conducted twice under each stress load condition, and if the specimen cracked twice out of the two times, it was deemed to have failed, and if it did not crack even once, it was deemed to have passed. The test was conducted by changing the applied stress, and the maximum stress that resulted in passing was designated as the "critical stress for the cracking test." A critical stress for the cracking test of 750 MPa or more was deemed to have passed.

[0066] The bending fatigue strength was measured using a rotating bending fatigue test specimen. The test specimen was prepared by cutting out a steel billet (thickness 30 mm, width 90 mm, length 2000 mm) before hot rolling, processing it into the shape of the test specimen, and then subjecting it to the same heat treatment as in Table 2, followed by finish processing. The test conditions were as follows. A fatigue strength (fatigue limit) of 700 MPa or more was considered to be acceptable. Test method: Ono type fatigue test Test specimen size: φ12 mm, test specimen with notched portion φ8 mm Number of interruptions: 1 × 10 ７ Temperature: Room temperature Rotation speed: 3600 rpm

[0067] The results are shown in Table 3. Figure 4 shows the relationship between hardness and ferrite volume fraction, and Figure 5 shows the relationship between bending fatigue strength and ferrite volume fraction. In Figures 4 and 5, open marks indicate that the critical stress for the cracking test is 750 MPa or more, and solid marks indicate that the critical stress for the cracking test is less than 750 MPa.

[0068]

[0069] As shown in Table 3, the steel materials Nos. 1, 2, and 7 to 10 had a critical stress for the cracking test of 750 MPa or more and a bending fatigue strength of 700 MPa or more.

[0070] Steels No. 3, No. 4, and No. 11 had high bending fatigue strength but low critical stress for cracking tests. This is thought to be due to the low volume fraction of ferrite. The low volume fraction of ferrite is thought to be due to the lack of isothermal holding. Steel No. 3 also had a large prior austenite grain size, which is thought to be due to the heating temperature T1 being too high.

[0071] Steel No. 5 had low bending fatigue strength, which is thought to be due to its too low C content.

[0072] Steel No. 12 had low bending fatigue strength and critical stress for cracking test. This is thought to be due to the low volume fraction of ferrite. The low volume fraction of ferrite is thought to be due to the low isothermal holding temperature.

[0073] Although the No. 13 steel had high bending fatigue strength, the critical stress for the cracking test was low. This is thought to be due to the low volume fraction of ferrite. The low volume fraction of ferrite is thought to be due to the short holding time at the isothermal holding temperature.

[0074] Although the bending fatigue strength of steel No. 14 was high, the critical stress for the cracking test was low. This is thought to be due to the large prior austenite grain size. The large prior austenite grain size is thought to be due to the N content being too low.

[0075] Although the steel materials No. 15 and No. 16 had high bending fatigue strength, the critical stress for the cracking test was low. This is thought to be because they did not satisfy formula (1).

[0076] Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out within the scope of the invention.

Claims

1. The chemical composition is by mass%, C: 0.35 to 0.65%, Si: 0.01 to 0.60%, Mn: 1.00 to 2.00%, Cr: 0.01 to 0.50%, Al: 0.001 to 0.050%, S: 0.010 to 0.100%, N: 0.010 to 0.030%, Ti: 0 to 0.020%, the balance being Fe and impurities, the chemical composition satisfies the following formula (1), it has a hardened layer on at least a part of the surface, the hardened layer has a structure containing 9.0% by volume or more of ferrite, and the balance being at least one of martensite and bainite, a crankshaft in which the Vickers hardness of the hardened layer is 520 or more. ([C] - 0.05) / [N] - 300×[Ti] ≤ 30.0 (1) In [C], [N], and [Ti] in formula (1), the C content, N content, and Ti content are substituted in mass%, respectively.

2. A crankshaft according to claim 1, wherein the Vickers hardness of the hardened layer and the volume fraction of the ferrite satisfy the following formula (2). [α] ≥ 0.0259×Hv - 4.36 (2) In [α] in formula (2), the volume fraction of the ferrite is substituted in %, and in Hv, the Vickers hardness of the hardened layer is substituted.

3. A crankshaft according to claim 1, wherein the prior austenite grain size of the martensite and bainite is 30 μm or less.

4. A crankshaft according to claim 1, wherein the bending fatigue strength of the hardened layer and the volume fraction of the ferrite satisfy the following formula (3). [α] ≥ 0.0028×[M] + 6.86 (3) In [α] in formula (3), the volume fraction of the ferrite is substituted in %, and in [M], the bending fatigue strength is substituted in MPa.

5. A method for manufacturing a crankshaft according to any one of claims 1 to 4, comprising the steps of preparing an intermediate product of the crankshaft, heating a target region, which is a region for forming the hardened layer of the intermediate product, to a heating temperature of 920 to 980°C, cooling the target region from the heating temperature to an isothermal holding temperature of 710 to 760°C at a cooling rate of 80°C / second or more and holding for 80 seconds or more at the isothermal holding temperature, and cooling the target region from the isothermal holding temperature to a temperature below the Ms point at a cooling rate of 80°C / second or more.