Crankshaft and method for manufacturing crankshaft

Abstract

Provided is a crankshaft having excellent crack resistance and fatigue strength. The crankshaft has a chemical composition satisfying the following formula (1), 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%, and the remainder being Fe and impurities, and has a hardened layer on at least a portion of the surface of the crankshaft, the hardened layer having a structure containing 9.0 vol% or more of ferrite with the remainder being martensite and / or bainite, and the thickness of the hardened layer being less than or equal to 0.5 [mu] m. The Vickers hardness of the hardened layer is 520 or more. ([C]-0.05) / [N]-300 * [Ti] < = 30.0 (1), and the C content and the N content are substituted in mass%, respectively, at [C] and [N] in formula (1).

Description

Technical Field

[0001] The present invention relates to a crankshaft and a method for manufacturing the same. Background Art

[0002] Generally, a crankshaft is manufactured in the following manner: after hot forging steel into a blank, machining such as cutting and grinding is performed, and after high-frequency quenching to improve fatigue strength, finish machining is performed to manufacture the crankshaft. At this time, if it is left for a long time between high-frequency quenching and finish machining, cracks may sometimes occur, or cracks may occur during finish machining, and the crack generation susceptibility of the material becomes a problem.

[0003] A high-frequency quenched crankshaft is disclosed in International Publication No. 2020 / 004060. The structure of the non-high-frequency quenched part of this high-frequency quenched crankshaft is composed of a structure mainly composed of ferrite and pearlite, the structure of the high-frequency quenched part is composed of a structure mainly composed of martensite or tempered martensite, and the original austenite grain diameter is 30 μm or less.

[0004] A crankshaft with high-frequency quenched surface is disclosed in Japanese Patent Application Laid-Open No. 2018-112222. In this high-frequency quenched crankshaft, when the distance from the connection position between the arc-shaped fillet part of the pin or journal and the thrust part to the top of the peak part is set as H (mm), the distance from the connection position to the quenched hardened layer on the shoulder surface of the pin or journal is set as L (mm), and the diameter of the pin or journal is set as D (mm), the distance L on the peak part side of the shoulder of the pin or journal is (-0.032D + 6.6521)×H 1 / 3 (mm) or more.

[0005] A crankshaft having a quenched hardened layer on at least the surface of the crankpin is disclosed in Japanese Patent Application Laid-Open No. 2008-127620. The surface compressive residual stress at the bottom fillet part of the crankpin of this crankshaft is 600 MPa or more.

[0006] Although not related to a crankshaft, a rolled wire rod that effectively suppresses crack generation during cold forging even when spheroidizing annealing before cold forging is omitted or the spheroidizing annealing before cold forging is shortened is disclosed in International Publication No. 2018 / 008703. The structure of this rolled wire rod is a mixed structure of ferrite and pearlite, and the average area of sulfides present in the range from the outermost surface to the D / 8 position (D is the diameter of the rolled wire rod) is 6 μm 2 Hereinafter, the average aspect ratio of the sulfides is 5 or less.

[0007] Prior art documents

[0008] Patent documents

[0009] Patent Document 1: International Publication No. 2020 / 004060

[0010] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2018-112222

[0011] Patent Document 3: Japanese Unexamined Patent Application Publication No. 2008-127620

[0012] Patent Document 4: International Publication No. 2018 / 008703 Summary of the Invention

[0013] Problems to be Solved by the Invention

[0014] The following is disclosed in International Publication No. 2020 / 004060: By containing a predetermined amount of Nb, the grains of the quenched structure can be refined, and quenching cracks can be suppressed. On the other hand, regarding cracks generated when stress is applied in manufacturing processes such as grinding, there is no research in the publication of International Publication No. 2020 / 004060.

[0015] The subject of the present invention is to provide a crankshaft with excellent crack resistance and fatigue strength.

[0016] Solutions to Solve the Problems

[0017] The chemical composition of the crankshaft according to an embodiment of the present invention 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%, and the balance: Fe and impurities. The chemical composition satisfies the following formula (1). At least a part of the surface of the crankshaft has a hardened layer. 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. The Vickers hardness of the hardened layer is 520 or more, ([C] - 0.05) / [N] - 300 × [Ti] ≤ 30.0 (1), and the C content and N content are substituted for [C] and [N] in formula (1) by mass%, respectively.

[0018] Alternatively, in a chemical composition of a crankshaft according to an embodiment of the present invention, 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%, balance: Fe and impurities, the chemical composition satisfies the following formula (1), at least a part of a surface of the crankshaft has a hardened layer, 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, and a Vickers hardness of the hardened layer is 520 or more.

[0019] ([C] - 0.05) / [N] ≤ 30.0 (1)

[0020] In formula (1), the C content and the N content are substituted for [C] and [N] by mass %, respectively.

[0021] Alternatively, in a chemical composition of a crankshaft according to an embodiment of the present invention, 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%, it further contains Ti: 0.020% or less, balance: Fe and impurities, the chemical composition satisfies the following formula (1), at least a part of a surface of the crankshaft has a hardened layer, 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, and a Vickers hardness of the hardened layer is 520 or more.

[0022] ([C] - 0.05) / [N] - 300×[Ti] ≤ 30.0 (1)

[0023] In formula (1), the C content and the N content are substituted for [C] and [N] by mass %, respectively.

[0024] A method for manufacturing a crankshaft according to an embodiment of the present invention is a method for manufacturing the above-described crankshaft, and includes: a step of preparing an intermediate product of the crankshaft; a step of heating an object area, which is an area to form the hardened layer of the intermediate product, to a temperature of 920 to 980°C, i.e., a heating temperature; a step of cooling the object area from the heating temperature to a temperature of 710 to 760°C, i.e., an isothermal holding temperature, at a cooling rate of 80°C / second or more and holding at the isothermal holding temperature for 80 seconds or more; and a step of cooling the object area from the isothermal holding temperature to a temperature below the Ms point at a cooling rate of 80°C / second or more.

[0025] Advantages of the Invention

[0026] According to the present invention, a crankshaft excellent in crack resistance and fatigue strength can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the heating mode of the heat treatment performed in the examples.

[0028] Figure 2 is a binarized image of the structure of the steel material of No. 2 in Table 2.

[0029] Figure 3 is a binarized image of the structure of the steel material of No. 4 in Table 2.

[0030] Figure 4 is a scatter diagram showing the relationship between hardness and the volume fraction of ferrite.

[0031] Figure 5 is a scatter diagram showing the relationship between the bending fatigue strength and the volume fraction of ferrite. DETAILED DESCRIPTION OF THE INVENTION

[0032] In order to develop a crankshaft excellent in crack resistance, the present inventors investigated the relationship between the structure and hardness of steel materials and crack resistance. In addition, "crack resistance" in the present specification means cracks generated during grinding or the like after forming a hardened layer (delayed cracks), rather than cracks caused by fatigue.

[0033] Generally, the fatigue strength of a crankshaft is important, and therefore, high hardness is required to ensure fatigue strength. On the other hand, from the viewpoint of crack resistance, it is not preferable for the hardness to be too high. The present inventors studied whether it is possible to suppress delayed cracks by controlling the structure of steel materials.

[0034] Specifically, a four-point bending test was performed on various steel materials under dilute hydrochloric acid immersion to evaluate crack resistance, and a structure excellent in crack resistance was explored. As a result, it was found that a structure in which an appropriate amount of ferrite precipitates in a structure mainly composed of martensite or bainite has excellent crack resistance. When the structure is composed only of high-hardness structures such as martensite and bainite, or even when ferrite precipitates, the amount is small, stress concentrates on the high-hardness structures. High-hardness structures are sensitive to cracks and are likely to generate cracks due to an increase in stress load. By precipitating an appropriate amount of ferrite, the ferrite in the structure can be made uniform, stress concentration on the high-hardness structures can be prevented, and crack resistance can be improved. In addition, by optimizing the balance with hardness, crack resistance can be improved while maintaining the fatigue strength unchanged.

[0035] In order to ensure crack resistance, it is also effective to make the grains of the high-hardness structure finer. In order to make the grains finer, it is effective to increase the N content.

[0036] The present invention has been completed based on the above insights. Hereinafter, a crankshaft according to an embodiment of the present invention and a method for manufacturing the same will be described in detail.

[0037] [Chemical composition]

[0038] The crankshaft of the present embodiment has the chemical composition described below. In the following description, "%" of the content of an element means mass %.

[0039] C: 0.35 to 0.65%

[0040] Carbon (C) increases the hardness of steel and contributes to an increase in fatigue strength. On the other hand, if the C content is too high, the 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%.

[0041] Si: 0.01 to 0.60%

[0042] Silicon (Si) has a deoxidizing effect and a role in strengthening ferrite. On the other hand, if the Si content is too high, the 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%, still more preferably 0.10%. The upper limit of the Si content is preferably 0.58%, more preferably 0.55%.

[0043] Mn: 1.00 to 2.00%

[0044] Manganese (Mn) increases the hardenability of steel and contributes to an increase in 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, and the machinability decreases. 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%.

[0045] Cr: 0.01 to 0.50%

[0046] Chromium (Cr) increases the hardenability of steel and contributes to an increase in 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, and the machinability decreases. 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%.

[0047] Al: 0.001 to 0.050%

[0048] 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 too large, and the machinability decreases. 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%.

[0049] S: 0.010 to 0.100%

[0050] Sulfur (S) forms MnS, improving the machinability of the 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%, more preferably 0.020%. The upper limit of the S content is preferably 0.090%, more preferably 0.080%.

[0051] N: 0.010 to 0.030%

[0052] Nitrogen (N) forms nitrides and carbonitrides, contributing to the refinement of grains and improving the crack resistance. In addition, not only through the refinement of grains, but also through the fine dispersion of nitrides and carbonitrides themselves, the strength of the steel is increased and the crack resistance is improved. 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%, more preferably 0.012%. The upper limit of the N content is preferably 0.020%, more preferably 0.018%.

[0053] The balance of the chemical composition of the crankshaft of the present embodiment is Fe and impurities. Here, the so-called impurities refer to elements mixed in from ores and scraps used as raw materials for steel, or elements mixed in from the manufacturing process environment, etc.

[0054] The chemical composition of the crankshaft of the present embodiment may also contain Ti: 0.020% or less to replace a part of Fe. Ti is an optional element. That is, the crankshaft of the present embodiment may not contain Ti.

[0055] Ti: 0 to 0.020%

[0056] Titanium (Ti) forms nitrides and carbonitrides, contributing to the refinement of grains. As long as a small amount of Ti is contained, this effect can be obtained. On the other hand, even if the Ti content is increased excessively, the effect saturates. Therefore, the Ti content is 0 to 0.020%. The lower limit of the Ti content is preferably 0.005%, more preferably 0.010%. The upper limit of the Ti content is preferably 0.018%.

[0057] [For formula (1)]

[0058] The higher the C content, the higher the hardness of the steel. On the other hand, there is a tendency to easily cause delayed cracks. Therefore, it is necessary to adjust the N content and Ti content according to the C content. Specifically, the chemical composition of the crankshaft of the present embodiment satisfies the following formula (1).

[0059] ([C] - 0.05) / [N] - 300×[Ti] ≤ 30.0 (1)

[0060] Substitute the C content, N content, and Ti content in mass% for [C], [N], and [Ti] in formula (1), respectively.

[0061] When the crankshaft does not contain Ti, substitute 0 for [Ti] in formula (1). That is, when not containing Ti, formula (1) becomes as follows.

[0062] ([C] - 0.05) / [N] ≤ 30.0 (1)

[0063] Substitute the C content and N content in mass% for [C] and [N] in formula (1), respectively.

[0064] 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, more preferably 26.0. The lower limit of the left side of formula (1) is not particularly limited, for example, it is 20.0.

[0065] [Structure]

[0066] The crankshaft of the present embodiment has a hardened layer (quenched and hardened layer) on at least a part of the surface. The hardened layer is formed by, for example, high-frequency quenching. The parts where the hardened layer is formed are, for example, the pin part and the journal part of the crankshaft. The hardened layer can be formed only on one of the pin part and the journal part, or on both. The hardened layer can be formed on parts other than the pin part and the journal part, or on the entire surface. In addition, it can be that the hardened layer is formed not only on the surface of the crankshaft but also into the core part.

[0067] This 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.

[0068] When the structure consists only of high-hardness structures such as martensite and bainite, or when the amount of ferrite precipitation is small even if it occurs, stress concentrates on the high-hardness structures. High-hardness structures are sensitive to cracking and are prone to crack generation due to increased stress loading. By allowing an appropriate amount of ferrite to precipitate, the ferrite in the structure can be homogenized, preventing stress concentration on 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, there is a possibility of reducing the fatigue strength. The upper limit of the volume fraction of ferrite in the structure of the hardened layer is preferably 16.0%, more preferably 14.0%.

[0069] The balance of the structure of the hardened layer other than ferrite is at least one of martensite and bainite. That is, the balance of the hardened layer is any one of martensite, bainite, and a mixed structure of martensite and bainite.

[0070] In this hardened layer, preferably, the prior austenite grain diameter of martensite and bainite is 30 μm or less. If the prior austenite grain diameter of martensite and bainite is 30 μm or less, more excellent fatigue strength and crack resistance can be obtained. The upper limit of the prior austenite grain diameter is more preferably 28 μm, further preferably 26 μm. The lower limit of the prior austenite grain diameter is not particularly limited and is, for example, 15 μm.

[0071] The Vickers hardness of the hardened layer is 520 Hv or more. By setting the Vickers hardness to 520 Hv or more, the fatigue strength is further improved. The lower limit of the Vickers hardness of the hardened layer is preferably 530 Hv, more preferably 540 Hv, further preferably 550 Hv. On the other hand, if the Vickers hardness of the hardened layer is too high, cracking is likely to occur. The upper limit of the Vickers hardness of the hardened layer is preferably 750 Hv, more preferably 700 Hv, further preferably 650 Hv.

[0072] The higher the Vickers hardness of the hardened layer, the greater the fatigue strength of the steel. On the other hand, there is a tendency for delayed cracking to occur more easily. Therefore, it is preferable to adjust the volume fraction of ferrite in the hardened layer according to the magnitude of the Vickers hardness. Specifically, the Vickers hardness and the volume fraction of ferrite of the hardened layer preferably satisfy the following formula (2).

[0073] [α]≥0.0259×Hv - 4.36 (2)

[0074] In formula (2), substitute the volume fraction of ferrite in % for [α], and substitute the Vickers hardness of the hardened layer for Hv.

[0075] Similarly, the greater the fatigue strength, the greater the tendency to cause delayed cracking. Therefore, it is preferable to adjust the volume ratio of ferrite in the hardened layer according to the magnitude of the fatigue strength. Specifically, the bending fatigue strength of the hardened layer and the volume ratio of ferrite preferably satisfy the following formula (3).

[0076] [α]≥0.0028×[M]+6.86(3)

[0077] In formula (3), substitute the volume ratio of ferrite in [%] for [α], and substitute the bending fatigue strength in [MPa] for [M].

[0078] In the crankshaft of the present embodiment, the structure of the portion other than the hardened layer is arbitrary. Within the range of the chemical composition of the crankshaft of the present embodiment, the structure other than the hardened layer generally becomes a structure mainly composed of ferrite and pearlite. The structure of the portion other than the hardened layer of the crankshaft of the present embodiment preferably has 90% by volume or more of ferrite and pearlite, and more preferably 95% by volume or more.

[0079] [Manufacturing method]

[0080] An example of the manufacturing method of the crankshaft of the present embodiment will be described. The manufacturing method described below is also merely an example, and the manufacturing method of the crankshaft of the present embodiment is not limited thereto.

[0081] Prepare an intermediate product of the crankshaft. For example, an intermediate product of the crankshaft can be manufactured as follows.

[0082] Smelt the steel having the above chemical composition, and perform continuous casting or blooming to obtain a steel billet. The steel billet is hot forged into a rough shape of the crankshaft. The conditions of hot forging are not limited thereto. The heating temperature is, for example, 1000 to 1300 °C, and the holding time is, for example, 1 second to 20 minutes. The hot forging can also be carried out in multiple steps. In addition, heat treatment such as annealing can be performed before and after hot forging. After hot forging, machining is performed as needed. Thus, an intermediate product of the crankshaft is manufactured.

[0083] The intermediate product of the crankshaft is subjected to the heat treatment described in detail below to form a hardened layer (quenched hardened layer). The hardened layer can be formed only at a specific portion of the intermediate product of the crankshaft, or can be formed on the entire intermediate product of the crankshaft. In the following description, the region where the hardened layer is formed is referred to as the "target region".

[0084] First, heat the object area to a temperature of 920 to 980 °C, i.e., the heating temperature. Austenitize the structure of the object area by this heating. If the heating temperature is too low, a uniform austenite structure will not be formed, and a uniform structure cannot 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 diameter of the structure after cooling will increase. The lower limit of the heating temperature is preferably 930 °C, more preferably 940 °C. The upper limit of the heating temperature is preferably 970 °C, more preferably 960 °C. The holding time at the heating temperature is not particularly limited, for example, it is 10 seconds to 30 minutes.

[0085] After heating the object area to the heating temperature, cool it from the heating temperature to a temperature of 710 to 760 °C, i.e., the isothermal holding temperature, at a cooling rate of 80 °C / second or more, and hold it at the isothermal holding temperature for 80 seconds or more. Then, cool it from the isothermal holding temperature to a temperature below the Ms point (martensite transformation start temperature) at a temperature of 80 °C / second or more.

[0086] By holding at the isothermal holding temperature for 80 seconds or more, ferrite precipitates in the austenite. If the isothermal holding temperature deviates from the range of 710 to 760 °C, or the holding time at the isothermal holding temperature is too short, there is a case where a sufficient amount of ferrite cannot be obtained. The lower limit of the holding time at the isothermal holding temperature is preferably 90 seconds, more preferably 100 seconds.

[0087] If the cooling rate from the heating temperature to the isothermal holding temperature is too small, there is a case where a structure other than ferrite is formed, or a sufficient amount of ferrite cannot be obtained. The lower limit of the cooling rate from the heating temperature to the isothermal holding temperature is preferably 100 °C / second, 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 large, it is difficult to hold at the isothermal holding temperature. The upper limit of the cooling rate from the heating temperature to the isothermal holding temperature is preferably 250 °C, more preferably 200 °C.

[0088] Similarly, if the cooling rate from the isothermal holding temperature to the Ms point is too small, there is a case where a structure other than ferrite (such as pearlite) is formed. The lower limit of the cooling rate from the isothermal holding temperature to the Ms point is preferably 100 °C / second, 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, for example, it is 400 °C / second.

[0089] Thus, a hardened layer having a structure containing 9.0% by volume or more of ferrite and the balance being at least one of martensite and bainite can be obtained. After forming the hardened layer, finish machining such as grinding is performed as needed. A crankshaft is manufactured using the above process.

[0090] As described above, the crankshaft according to an embodiment of the present invention and a method for manufacturing the same have been described. According to this embodiment, a crankshaft excellent in crack resistance and fatigue strength can be obtained.

[0091] Examples

[0092] Hereinafter, the present invention will be described in more detail using examples. The present invention is not limited to these examples.

[0093] Steel having the chemical composition shown in Table 1 was melted using a 50 kg vacuum induction melting furnace to produce an ingot. The ingot was hot forged at a temperature of 1000°C or higher. After being set to a thickness of 30 mm, a width of 90 mm, and a length of 2000 mm, it was cut into lengths of 100 mm to produce steel billets. The steel billets were hot rolled at a temperature of 1000°C or higher and air cooled to produce billets having a thickness of 10 mm and a width of 100 mm. All of these billets had a structure mainly composed of ferrite and pearlite.

[0094] [Table 1]

[0095] Table 1

[0096]

[0097] Fn1 = ([C] - 0.05) / [N] - 300 × [Ti]

[0098] The following heat treatment was performed on the billets. Specifically, after heating the billets to the heating temperature T1, they were cooled to the isothermal holding temperature T2 at a cooling rate CR1. After holding for the holding time t1 at the isothermal holding temperature T2, they were cooled to room temperature at a cooling rate CR2. Figure 1 The Vickers hardness of the steel after the heat treatment was measured. The Vickers hardness was measured at 5 points with a 1 kg load, and the average value was calculated.

[0099] Test pieces for microstructural observation were collected from the steel after the heat treatment. After mirror finishing the surface of the test pieces for microstructural observation, nitric acid ethanol etching was performed, and SEM observation was carried out. For the volume fraction of the microstructure, the uneven images (3 fields of view for each test piece) obtained by SEM observation were color-coded using drawing software, the images were binarized using the image analysis software ImageJ, and the particles were detected using the particle analysis function of the image analysis software ImageJ, thereby calculating the area ratio, and the area ratio was regarded as the volume ratio.

[0100] is a binarized image (magnification 1000 times) of the microstructure of the steel No. 2 in Table 2 below, Figure 2 is a binarized image (magnification 1000 times) of the microstructure of No. 4. In Figure 3 and Figure 2 andFigure 3 In the figure, the white part is ferrite, and the black part is martensite and / or bainite.

[0101] The prior austenite grain size was measured as follows. After mirror finishing the surface of a test piece collected from a heat-treated steel material, the prior austenite grain boundary was revealed by etching with a saturated aqueous solution of picric acid, and the prior austenite grain size was calculated using the linear intercept method. Specifically, a straight line with a total length L was drawn, and the number of grains n that the straight line crossed was calculated. L , find the intercept length (L / n L ). For more than 5 straight lines, find the intercept length (L / n L ), and its arithmetic mean is set as the original austenite grain diameter.

[0102] The heat treatment conditions and the hardness, original austenite grain size (original γ grain size) and structure of the steel after the heat treatment are shown in Table 2. 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. In addition, in No. 3, 4, and 11, isothermal holding was not performed, and cooling was performed from the heating temperature T1 to room temperature at a cooling rate CR1.

[0103] [Table 2]

[0104] Table 2

[0105]

[0106] A plurality of test pieces of 10 mm×75 mm×2 mm were collected from the heat-treated steel material and subjected to a hydrochloric acid immersion four-point bending stress corrosion test to evaluate the crack resistance. The test conditions were as follows.

[0107] Test method: 4-point bending, stress load measurement using full-bridge strain gauge method

[0108] Solution: 4.1 mass% hydrochloric acid solution

[0109] Temperature: Room temperature

[0110] Test time: 24 hours

[0111] Two tests were conducted under each stress load condition. If cracks occurred twice in the two tests, the test was considered unqualified. If cracks did not occur even in one test, the test was considered qualified. The test was conducted instead of the applied stress, and the maximum stress that would be qualified was set as the "crack test critical stress". The crack test critical stress of 750MPa or more was considered qualified.

[0112] The bending fatigue strength of a rotating bending fatigue test piece is measured. The test piece is fabricated as follows: a steel billet before hot rolling (thickness 30 mm, width 90 mm, length 2000 mm) is cut and processed into the shape of a test piece, and after performing the same heat treatment as in Table 2, finish machining is carried out. The test conditions are as follows. A fatigue strength (fatigue limit) of 700 MPa or more is considered qualified.

[0113] Test method: Ono type fatigue test

[0114] Test piece size: Test piece with φ12 mm and a notched part of φ8 mm

[0115] Cut-off number: 1×10 7 times

[0116] Temperature: Room temperature

[0117] Rotation speed: 3600 rpm

[0118] The results are shown in Table 3. The relationship between hardness and the volume fraction of ferrite is shown in Figure 4 , and the relationship between bending fatigue strength and the volume fraction of ferrite is shown in Figure 5 . In Figure 4 and Figure 5 , hollow markers indicate that the critical stress of the crack test is 750 MPa or more, and solid markers indicate that the critical stress of the crack test is less than 750 MPa.

[0119] [Table 3]

[0120] Table 3

[0121]

[0122] Fn1 = ([C] - 0.05) / [N] - 300×[Ti]

[0123] Fn2 = 0.0259×Hv - 4.36

[0124] Fn3 = 0.0028×[M] + 6.86

[0125] As shown in Table 3, the critical stress of the crack test for the steel materials of No.1, 2, and 7 - 10 is 750 MPa or more, and the bending fatigue strength is 700 MPa or more.

[0126] Although the bending fatigue strength of the steel materials of No.3, No.4, and No.11 is relatively high, the critical stress of the crack test is relatively low. This is considered to be due to the relatively low volume fraction of ferrite. The relatively low volume fraction of ferrite is considered to be due to the absence of isothermal holding. In addition, the original austenite grain diameter of the steel material of No.3 is also relatively large, which is considered to be due to the excessively high heating temperature T1.

[0127] The bending fatigue strength of the steel No.5 is low. It is considered that this is due to the too low C content.

[0128] Both the bending fatigue strength and the critical stress in the crack test of the steel No.12 are low. It is considered that this is due to the low volume fraction of ferrite. It is considered that the low volume fraction of ferrite is due to the low isothermal holding temperature.

[0129] The steel No.13 has a high bending fatigue strength, but the critical stress in the crack test is low. It is considered that this is due to the low volume fraction of ferrite. It is considered that the low volume fraction of ferrite is due to the short holding time at the isothermal holding temperature.

[0130] The steel No.14 has a high bending fatigue strength, but the critical stress in the crack test is low. It is considered that this is due to the large prior austenite grain diameter. It is considered that the large prior austenite grain diameter is due to the too low N content.

[0131] The steels No.15 and No.16 have high bending fatigue strengths, but the critical stresses in the crack tests are low. It is considered that this is because the formula (1) is not satisfied.

[0132] As described above, the embodiments of the present invention have been described, but the above embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and the above embodiments can be appropriately deformed within the scope of the invention for implementation.

Claims

1. A crankshaft, the chemical composition of which is, by 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％、 Ti: 0 - 0.020%, balance: Fe and impurities, the chemical composition satisfies the following formula (1), at least a part of the surface of the crankshaft has a hardened layer, 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, the Vickers hardness of the hardened layer is 520 or more, ([C] - 0.05) / [N] - 300×[Ti] ≤ 30.0 (1), in formula (1), the C content and N content are substituted for [C] and [N] respectively by mass%.

2. The 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 formula (2), the volume fraction of the ferrite is substituted for [α] in %, and the Vickers hardness of the hardened layer is substituted for Hv.

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

4. The 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 formula (3), the volume fraction of the ferrite is substituted for [α] in %, and the bending fatigue strength is substituted for [M] in MPa.

5. A method for manufacturing a crankshaft, which is a method for manufacturing the crankshaft according to any one of claims 1 to 4, and includes: a step of preparing an intermediate product of the crankshaft; a step of heating an object area, which is an area to form the hardened layer of the intermediate product, to a temperature of 920 - 980°C, i.e., the heating temperature; a step of cooling the object area from the heating temperature to a temperature of 710 - 760°C, i.e., the isothermal holding temperature, at a cooling rate of 80°C / second or more and holding at the isothermal holding temperature for 80 seconds or more; and a step of cooling the object area 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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