Crankshaft and method for manufacturing a 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

Technical Field

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

Background Art

[0002] Generally, a crankshaft is manufactured by first forming a blank from steel by hot forging, then performing machining such as cutting and grinding, and then performing quenching by high-frequency heating to improve fatigue strength, and finally performing finishing. At this time, if it is left for a long time between high-frequency quenching and finishing, cracking may occur, or cracking may occur during finishing, and the ease of cracking of the material has become a problem.

[0003] International Publication No. 2020 / 004060 discloses a high-frequency quenched crankshaft. In this high-frequency quenched crankshaft, the structure of the non-high-frequency quenched part is mainly composed of ferrite and pearlite, the structure of the high-frequency quenched part 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 No. 2018-112222 discloses a crankshaft with a high-frequency quenched surface. In this high-frequency quenched crankshaft, when the distance from the connection position between the fillet R part of the pin or journal and the thrust part to the top part of the top part is H (mm), the distance from the connection position to the hardened layer by quenching on the shoulder surface of the pin or journal is L (mm), and the diameter of the pin or journal is D (mm), the distance L on the top part side of the shoulder of the pin or journal is (-0.032D + 6.6521)×H 1 / 3 (mm) or more.

[0005] Japanese Patent Application Laid-Open No. 2008-127620 discloses a crankshaft having a hardened layer by quenching on at least the surface of the crank pin. In this crankshaft, the surface compressive residual stress at the bottom R part of the crank pin is 600 MPa or more.

[0006] Although not relating to crankshafts, International Publication No. 2018 / 008703 discloses a rolled wire in which crack formation during cold forging is effectively suppressed even when spheroidizing annealing before cold forging is omitted or shortened. This rolled wire has a mixed structure of ferrite and pearlite, and the average area of ​​sulfides present from the outermost layer to the D / 8 position (where D is the diameter of the rolled wire) is 6 μm². 2 The following conditions apply, and the average aspect ratio of the sulfides is 5 or less. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2020 / 004060 [Patent Document 2] Japanese Patent Publication No. 2018-112222 [Patent Document 3] Japanese Patent Publication No. 2008-127620 [Patent Document 4] International Publication No. 2018 / 008703 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] International Publication No. 2020 / 004060 discloses that quench cracking can be suppressed by refining the crystal grains of the quenched structure by incorporating a predetermined amount of Nb. On the other hand, cracking that occurs when stress is applied during manufacturing processes such as grinding is not considered in the same publication.

[0009] The object of the present invention is to provide a crankshaft with excellent crack resistance and fatigue strength. [Means for solving the problem]

[0010] A crankshaft according to one embodiment of the present invention has a chemical composition in mass%, of C: 0.35-0.65%, Si: 0.01-0.60%, Mn: 1.00-2.00%, Cr: 0.01-0.50%, Al: 0.001-0.050%, S: 0.010-0.100%, N: 0.010-0.030%, Ti: 0-0.020%, the remainder being Fe and impurities, the chemical composition satisfies the following formula (1), and has a hardened layer on at least a portion of its surface, the hardened layer having a structure in which it contains 9.0 volume% or more of ferrite and the remainder being at least one of martensite and bainite, and the Vickers hardness of the hardened layer is 520 or higher. ([C]-0.05) / [N]-300×[Ti]≦30.0 (1) In equation (1), the C content and N content are substituted for [C] and [N] respectively, in mass percent.

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

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

[0013] A method for manufacturing a crankshaft according to one embodiment of the present invention is a method for manufacturing the above-mentioned crankshaft, comprising the steps of: preparing an intermediate product of a crankshaft; heating a target region, which is a region for forming a 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 it at the isothermal holding temperature for 80 seconds or more; 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. [Effects of the Invention]

[0014] According to the present invention, a crankshaft with excellent crack resistance and fatigue strength can be obtained. [Brief explanation of the drawing]

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

Embodiments for Carrying Out the Invention

[0016] In order to develop a crankshaft with excellent crack resistance, the inventor investigated the relationship between the structure and hardness of the steel material and crack resistance. In this specification, "crack resistance" means not the crack due to fatigue but the crack (delayed crack) that occurs when grinding or the like is performed after forming a hardened layer.

[0017] Since the fatigue strength is generally important for a crankshaft, a high hardness is required to ensure the fatigue strength. On the other hand, from the viewpoint of crack resistance, it is not preferable that the hardness is too high. The inventor investigated whether the delayed crack can be suppressed by controlling the structure of the steel material.

[0018] Specifically, four-point bending tests were carried out on various steel materials under hydrochloric acid immersion to evaluate the crack resistance, and a structure with excellent crack resistance was explored. As a result, it was found that a structure in which an appropriate amount of ferrite is precipitated in a structure mainly composed of martensite or bainite has excellent crack resistance. When the structure consists only of high-hardness structures such as martensite and bainite, or when the amount of ferrite precipitated is small, stress concentrates on the high-hardness structure. High-hardness structures are sensitive to cracks and are likely to crack due to an increase in stress load. By precipitating an appropriate amount of ferrite, the ferrite in the structure is made uniform, preventing stress concentration on the high-hardness structure and improving the crack resistance. Also, by optimizing the balance with the hardness, the crack resistance can be improved while maintaining the fatigue strength.

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

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

[0021] [Chemical composition] The crankshaft according to this embodiment has the chemical composition described below. In the following description, the "%" for elemental content refers to mass percent.

[0022] C: 0.35~0.65% Carbon (C) improves the hardness of steel and contributes to improved 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%, and more preferably 0.40%. The upper limit of the C content is preferably 0.60%, and more preferably 0.55%.

[0023] Si: 0.01~0.60% Silicon (Si) has deoxidizing and ferrite-enhancing properties. However, 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 still more preferably 0.10%. The upper limit of the Si content is preferably 0.58%, and still more preferably 0.55%.

[0024] Mn: 1.00~2.00% Manganese (Mn) enhances the hardenability of steel and contributes to improving its hardness. However, if the Mn content is too high, bainite will form 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%, and more preferably 1.20%. The upper limit of the Mn content is preferably 1.80%, and more preferably 1.60%.

[0025] Cr: 0.01~0.50% Chromium (Cr) enhances the hardenability of steel and contributes to improving its hardness. However, if the Cr content is too high, bainite will form 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%, and more preferably 0.08%. The upper limit of the Cr content is preferably 0.30%, and more preferably 0.20%.

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

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

[0028] N: 0.010~0.030% Nitrogen (N) forms nitrides and carbonitrides, contributing to grain refinement and improving crack resistance. Furthermore, not only grain refinement, but also the fine dispersion of nitrides and carbonitrides themselves increases the strength and improves crack resistance of the steel. 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 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 manufacturing environment.

[0030] The chemical composition of the crankshaft according to this embodiment may include Ti: 0.020% or less in place of some of the Fe. Ti is an arbitrary element. In other words, the crankshaft according to this embodiment does not need to contain Ti.

[0031] Ti: 0~0.020% Titanium (Ti) forms nitrides and carbonitrides, contributing to grain refinement. This effect can be obtained even with a small amount of Ti present. On the other hand, the effect saturates if the Ti content is excessively high. 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 equation (1)] The higher the carbon content, the higher the hardness and fatigue strength of the steel, but the more prone it is to delayed cracking. Therefore, it is necessary to adjust the nitrogen and titanium content according to the carbon content. Specifically, the chemical composition of the crankshaft according to this embodiment satisfies the following equation (1). ([C]-0.05) / [N]-300×[Ti]≦30.0 (1) In equation (1), the C content, N content, and Ti content are substituted for [C], [N], and [Ti], respectively, in mass percent.

[0033] If the crankshaft does not contain Ti, then 0 is substituted for [Ti] in equation (1). That is, if it does not contain Ti, equation (1) becomes as follows: ([C]-0.05) / [N]≦30.0 (1) In equation (1), the C content and N content are substituted for [C] and [N] respectively, in mass percent.

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

[0035] [Organization] The crankshaft according to this embodiment has a hardened layer (quenched hardened layer) on at least a portion of its surface. The hardened layer is formed, for example, by high-frequency induction hardening. The locations where the hardened layer is formed are, for example, the pin portion and the journal portion of the crankshaft. The hardened layer may be formed on only one of the pin portion and the journal portion, or on both. The hardened layer may be formed in locations other than the pin portion and the journal portion, or it 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 that contains 9.0 volume% or more of ferrite, with the remainder being at least one of martensite and bainite.

[0037] If the microstructure consists only of high-hardness structures such as martensite or bainite, or if ferrite precipitates but in small quantities, stress concentrates in the high-hardness structures. High-hardness structures are sensitive to cracking, and increased stress loads make them prone to cracking. By precipitating an appropriate amount of ferrite, the ferrite in the microstructure becomes more uniform, preventing stress concentration in the high-hardness structures and improving crack resistance. The lower limit of the ferrite volume fraction in the hardened layer microstructure is preferably 10.0%, and more preferably 10.5%. On the other hand, if the ferrite volume fraction is too high, the fatigue strength may decrease. The upper limit of the ferrite volume fraction in the hardened layer microstructure is preferably 16.0%, and more preferably 14.0%.

[0038] The remaining portion of the hardened layer structure, excluding the ferrite, is at least one of martensite and bainite. That is, the remaining portion 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. A prior austenite grain size of 30 μm or less provides superior fatigue strength and crack resistance. 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 higher. Achieving a Vickers hardness of 520 Hv or higher further improves fatigue strength. 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 becomes more likely. 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 prone it is to delayed cracking. 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 equation (2). [α]≧0.0259×Hv-4.36 (2) In equation (2), the volume fraction of ferrite is substituted for [α] in percentage, and the Vickers hardness of the hardened layer is substituted for Hv.

[0042] Similarly, the greater the fatigue strength, the more likely delayed cracking is to occur. Therefore, it is preferable to adjust the volume fraction of ferrite in the hardened layer according to 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 equation (3). [α]≧0.0028×[M]+6.86 (3) In equation (3), the volume fraction of ferrite is substituted for [α] in percent, and the bending fatigue strength is substituted for [M] in MPa.

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

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

[0045] Prepare an intermediate crankshaft. An intermediate crankshaft can be manufactured, for example, as follows:

[0046] Steel having the chemical composition described above is melted and formed into a steel billet by continuous casting or bract rolling. The steel billet is hot forged to form the rough shape of a crankshaft. The conditions for hot forging are not limited to these, 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 carried out in multiple stages. Heat treatment such as annealing may also be performed before or after hot forging. After hot forging, machining is performed as needed. This produces an intermediate crankshaft.

[0047] A hardened layer (quenched hardened layer) is formed on the intermediate crankshaft by performing the heat treatment described below. The hardened layer may be formed only on specific parts of the intermediate crankshaft, or it may be formed on the entire intermediate crankshaft. In the following description, the area on which the hardened layer is 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 causes the structure of the target region to austenitize. If the heating temperature is too low, a uniform austenite structure will not be formed, 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 coarseen, and the prior austenite grain size of the structure after cooling will be large. 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 for example, it is 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 710-760°C at a cooling rate of 80°C / second or more, and held at the isothermal holding temperature for 80 seconds or more. Subsequently, it is cooled from the isothermal holding temperature to a temperature below the Ms point (martensitic transformation initiation temperature) at a temperature of 80°C / second or more.

[0050] By holding the mixture at an isothermal temperature for 80 seconds or more, ferrite precipitates in the austenite. If the isothermal temperature falls outside the range of 710 to 760°C, or if the holding time at the isothermal temperature is too short, a sufficient amount of ferrite may not be obtained. The lower limit of the holding time at the isothermal 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 low, structures other than ferrite may be formed, 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 / second, and more preferably 120°C / second. 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 be 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 low, other structures (e.g., pearlite) may form. The lower limit of the cooling rate from the isothermal holding temperature to the Ms point is preferably 100°C / second, and more preferably 120°C / second. 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 / second.

[0053] This process yields a quenched layer having a structure containing 9.0 volume% or more of ferrite, with the remainder being at least one of martensite and bainite. After forming the quenched layer, finishing processes such as grinding are performed as needed. The crankshaft is manufactured through these steps.

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

[0055] The present invention will be described more specifically below with reference to examples. The present invention is not limited to these examples.

[0056] Steel with the chemical composition shown in Table 1 was melted in a 50 kg vacuum induction melting furnace to produce an ingot. This ingot was hot forged at a temperature of over 1000°C to a thickness of 30 mm, a width of 90 mm, and a length of 2000 mm, and then cut into 100 mm lengths to produce steel billets. These steel billets were hot rolled at a temperature of over 1000°C and air-cooled to produce materials with 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] [Table 1]

[0058] The material was subjected to the heat treatment shown in Figure 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 heat-treated steel was measured. Vickers hardness was measured at five points under a 1 kg load, and the average was calculated.

[0060] Test specimens for microstructure observation were taken from heat-treated steel materials. After mirror-finishing the surface of the test specimens, nital etching was performed, followed by SEM observation. The volume fraction of the microstructure was calculated by coloring the topographic images (3 fields of view for each specimen) obtained by SEM observation using paint software, binarizing the images using the image analysis software ImageJ, and detecting particles using the particle analysis function of the same software to calculate the area fraction, which was then considered as the volume fraction. Figure 2 is the binarized image (magnification 1000x) of the microstructure of steel material No. 2 in Table 2 shown below, and Figure 3 is the binarized image (magnification 1000x) of the microstructure of No. 4. In Figures 2 and 3, the white areas are ferrite, and the black areas are martensite and / or bainite.

[0061] The prior austenite grain size was measured as follows: After mirror-finishing the surface of a specimen taken from heat-treated steel, the prior austenite grain boundaries were exposed by etching with a saturated picric acid solution, and the prior austenite grain size was calculated using the section method. Specifically, a straight line was drawn along the total length L, and the number of crystal grains that crossed this line n was calculated. L Find the intercept length (L / n L The intercept length (L / n) was calculated for five or more straight lines. L The arithmetic mean of the prior austenite grain size was calculated and used as the prior austenite grain size.

[0062] Table 2 shows a list of heat treatment conditions, as well as the hardness, prior austenite grain size (prior γ grain size), and microstructure of the steel after heat treatment. In Table 2, "M+B fraction" is the sum of the volume fractions of martensite and bainite, "P fraction" is the volume fraction of pearlite, and "F fraction" is the volume fraction of ferrite. For samples No. 3, 4, and 11, isothermal holding was not performed, and the samples were cooled from the heating temperature T1 to room temperature at a cooling rate of CR1.

[0063] [Table 2]

[0064] Multiple 10mm x 75mm x 2mm test specimens were taken from heat-treated steel materials, and their crack resistance was evaluated by a hydrochloric acid immersion four-point bending stress corrosion test. The test conditions were as follows: Testing method: Stress loading by 4-point bending and 100% gauge method. Solution: 4.1% by mass hydrochloric acid solution Temperature: room temperature Exam duration: 24 hours

[0065] Two tests were conducted under each stress loading condition. If the material cracked in both tests, it was considered a failure; if it cracked even once, it was considered a pass. The tests were repeated with different applied stresses, and the maximum stress at which a pass was achieved was defined as the "critical stress for cracking." A critical stress of 750 MPa or higher was considered a pass.

[0066] Bending fatigue strength was measured using rotary bending fatigue test specimens. The test specimens were prepared by cutting out a steel billet (30 mm thick, 90 mm wide, 2000 mm long) before hot rolling, processing it into the desired shape, applying the same heat treatment as shown in Table 2, and then performing finishing processes. The test conditions were as follows. A fatigue strength (fatigue limit) of 700 MPa or higher was considered acceptable. Testing method: Ono fatigue test Test specimen size: φ12mm, with a notched section of φ8mm. Number of cancellations: 1 x 10 7 times Temperature: room temperature Rotation speed: 3600 rpm

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

[0068] [Table 3]

[0069] As shown in Table 3, steel materials No. 1, 2, and 7-10 had a cracking critical stress of 750 MPa or higher and a bending fatigue strength of 700 MPa or higher.

[0070] Steel materials No. 3, No. 4, and No. 11 exhibited high bending fatigue strength, but low critical stress in crack testing. This is thought to be due to a low ferrite volume fraction. The low ferrite volume fraction is likely due to the lack of isothermal holding. Furthermore, steel material No. 3 also had a large prior austenite grain size, which is thought to be due to an excessively high heating temperature T1.

[0071] Steel material No. 5 exhibited low bending fatigue strength. This is thought to be due to an excessively low carbon content.

[0072] Steel material No. 12 exhibited low bending fatigue strength and low critical stress in crack testing. This is thought to be due to a low ferrite volume fraction. The low ferrite volume fraction is thought to be due to a low isothermal holding temperature.

[0073] Steel material No. 13 exhibited high bending fatigue strength, but low critical stress in crack testing. This is thought to be due to a low ferrite volume fraction. The low ferrite volume fraction is likely due to a short holding time at the isothermal holding temperature.

[0074] Steel material No. 14 exhibited high bending fatigue strength, but low critical stress in crack testing. This is thought to be due to a large prior austenite grain size. The large prior austenite grain size is thought to be due to an excessively low nitrogen content.

[0075] Although steel materials No. 15 and No. 16 exhibited high bending fatigue strength, their critical stress in cracking tests was low. This is thought to be because they did not satisfy equation (1).

[0076] Although embodiments of the present invention have been described above, the embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and it is possible to carry out the present invention by appropriately modifying the embodiments described above within the scope of the invention.

Claims

1. The chemical composition is expressed in mass percent. C: 0.35-0.65%, Si: 0.01 to 0.60%, Mn: 1.00-2.00%, Cr: 0.01-0.50%, Al: 0.001-0.050%, S: 0.010-0.100%, N: 0.010-0.030%, Ti: 0 to 0.020%, The remainder consists of Fe and impurities. The aforementioned chemical composition satisfies the following formula (1), Having a hardened layer on at least a portion of the surface, The hardened layer has a structure that contains 9.0 volume% or more of ferrite, with the remainder being at least one of martensite and bainite. The Vickers hardness of the hardened layer is 520 or higher. A crankshaft in which the prior austenite grain size of the martensite and bainite is 30 μm or less. ([C]-0.05) / [N]-300×[Ti]≦30.0 (1) In equation (1), the C content, N content, and Ti content are substituted for [C], [N], and [Ti], respectively, in mass percent.

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

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

4. A method for manufacturing a crankshaft according to any one of claims 1 to 3, The process of preparing intermediate parts for the crankshaft, A step of heating the target region, which is the region where the hardened layer of the intermediate product is formed, to a heating temperature of 920 to 980°C, The process involves 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 it at the isothermal holding temperature for 80 seconds or more. A method for manufacturing a crankshaft, comprising the step of 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.

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