Title of the invention: Method for manufacturing induction material for induction-hardened crankshaft and induction-hardened crankshaft
Technical field
[0001]
　The present invention relates to a method for manufacturing an induction-hardened crankshaft and an induction material for an induction-hardened crankshaft.
Background technology
[0002]
　The crankshaft is manufactured by hot forging a steel material into a raw material, then performing machining such as cutting, grinding, and drilling, and further subjecting it to surface hardening treatment such as induction hardening as necessary.
[0003]
　Hereinafter, a crankshaft that has been surface-hardened by induction hardening is referred to as an "induction-hardened crankshaft", and a crankshaft material used for an induction-hardened crankshaft is referred to as an "induction-hardened crankshaft base material".
[0004]
　In order to improve the fatigue strength of an induction-hardened crankshaft, not only the induction-hardened part (hereinafter referred to as "induction-hardened part") but also the part not induction-hardened (hereinafter referred to as "non-induction-hardened part"). ) Needs to be improved. In order to improve the hardness of both the induction hardened portion and the non-induction hardened portion, it is effective to increase the C content of the steel material. However, when the C content is increased, there are problems that the machinability is lowered and shrinkage is likely to occur.
[0005]
　As a method of improving the hardness without increasing the C content, a method of adding V to the steel material and utilizing precipitation strengthening by VC is known. However, since V is a relatively expensive element and the risk of price fluctuations is high, it is preferable not to use V from a commercial point of view.
[0006]
　In Japanese Patent No. 4699341 and Japanese Patent No. 4699342, it is possible to improve the tensile strength and fatigue limit ratio of steel parts by precipitating ultrafine precipitates (particle size 15 nm or less) of Nb, Ti, and V. Are listed.
[0007]
　As a method for producing this ultrafine precipitate, Japanese Patent No. 4699341 described above states that after hot forging, the range up to 650 ° C. is cooled at an average cooling rate of 60 ° C./min or more, and the temperature is from 650 ° C. to 500 ° C. It is described that the range of is cooled at an average cooling rate of 10 ° C./min or less. Similarly, in Japanese Patent No. 4699342 described above, the range up to 650 ° C. after hot rolling is cooled at an average cooling rate of 120 ° C./min or more, and the range from 650 ° C. to 500 ° C. is 60 ° C./min or less. It is stated that it cools at an average cooling rate.
Disclosure of invention
[0008]
　Japanese Patent No. 4699341 and Japanese Patent No. 4699342 relate to non-tampered steel parts, and shrink resistance is not considered.
[0009]
　An object of the present invention is to provide an induction hardened crankshaft having an excellent balance of fatigue strength, machinability, and quench crack resistance. Another object of the present invention is to provide a method for producing a material for an induction hardened crankshaft, which has an excellent balance between fatigue strength, machinability, and quench cracking resistance when induction hardening is performed.
[0010]
　The induction-hardened crankshaft according to one embodiment of the present invention is an induction-hardened crankshaft having a non-induction-hardened portion and an induction-hardened portion, and has a chemical composition of mass% and C: 0.30 to 0.60%. , Si: 0.01 to 1.50%, Mn: 0.4 to 2.0%, Cr: 0.01 to 0.50%, Al: 0.001 to 0.06%, N: 0.001 ~ 0.02%, P: 0.03% or less, S: 0.005 to 0.20%, Nb: 0.005 to 0.060%, balance: Fe and impurities of the non-induction hardening part. The structure is composed mainly of ferrite pearlite, the ferrite fraction Fα satisfies the following formula (1), and the structure of the induction hardening portion is mainly composed of martensite or tempered martensite. And the old austenite particle size is 30 μm or less.
　　In Fα ≧ −150 × [C%] +84 (1)
　[C%], the C content of the induction hardening crankshaft is substituted by mass%.
[0011]
　The method for producing a base material for a high-frequency forged crank shaft according to an embodiment of the present invention has a chemical composition of mass%, C: 0.30 to 0.60%, Si: 0.01 to 1.50%, and so on. Mn: 0.4 to 2.0%, Cr: 0.01 to 0.50%, Al: 0.001 to 0.06%, N: 0.001 to 0.02%, P: 0.03% Below, S: 0.005 to 0.20%, Nb: 0.005 to 0.060%, balance: Fe and the process of preparing steel materials that are impurities, and the temperature immediately before finish forging is more than 800 ° C and less than 1100 ° C. A step of hot forging the steel material so as to be, and a step of cooling the steel material so that the average cooling rate in the temperature range of 800 to 650 ° C. is 0.07 ° C./sec or less after the hot forging. To be equipped.
[0012]
　According to the present invention, an induction hardening crankshaft having excellent fatigue strength, machinability, and quench crack resistance can be obtained.
A brief description of the drawing
[0013]
FIG. 1 is a flow chart of a method for manufacturing an induction material for an induction hardened crankshaft according to an embodiment of the present invention.
FIG. 2 is a heat pattern of a hot forging simulation experiment by Machining Formaster.
FIG. 3 is another heat pattern of a hot forging simulation experiment by Machining Formaster.
FIG. 4A is a microstructure of a test piece of a tissue observation test.
FIG. 4B is a microstructure of a test piece of a tissue observation test.
FIG. 4C is a microstructure of a test piece of a tissue observation test.
FIG. 5A is a microstructure of a test piece of a tissue observation test.
FIG. 5B is a microstructure of a test piece of a tissue observation test.
FIG. 5C is a microstructure of a test piece of a tissue observation test.
FIG. 6A is a graph showing the relationship between the finish forging temperature and the ferrite fraction in steel type C.
FIG. 6B is a graph showing the relationship between the finish forging temperature and the ferrite fraction in the steel type D.
FIG. 6C is a graph showing the relationship between the finish forging temperature and the ferrite fraction in the steel type E.
FIG. 7A is a graph showing the relationship between the finish forging temperature and the Vickers hardness in the steel type C.
FIG. 7B is a graph showing the relationship between the finish forging temperature and the Vickers hardness in the steel type D.
FIG. 7C is a graph showing the relationship between the finish forging temperature and the Vickers hardness in the steel type E.
FIG. 8 is a graph showing the relationship between Vickers hardness and durability ratio.
FIG. 9A is a microstructure of a test piece in which steel type C is hot forged at 1100 ° C. after induction hardening simulated heat treatment.
FIG. 9B is a microstructure of a test piece obtained by hot forging steel type C at 1000 ° C. after induction hardening simulated heat treatment.
FIG. 9C is a microstructure of a test piece obtained by hot forging steel type C at 900 ° C. after induction hardening simulated heat treatment.
FIG. 9D is a microstructure of a test piece obtained by hot forging steel type C at 800 ° C. after induction hardening simulated heat treatment.
FIG. 10A is a microstructure of a test piece obtained by hot forging steel grade D at 1100 ° C. after induction hardening simulated heat treatment.
FIG. 10B is a microstructure of a test piece obtained by hot forging steel type D at 1000 ° C. after induction hardening simulated heat treatment.
FIG. 10C is a microstructure of a test piece obtained by hot forging steel type D at 900 ° C. after induction hardening simulated heat treatment.
FIG. 10D is a microstructure of a test piece obtained by hot forging steel type D at 800 ° C. after induction hardening simulated heat treatment.
FIG. 11A is a microstructure of a test piece in which steel type E is hot forged at 1100 ° C. after induction hardening simulated heat treatment.
FIG. 11B is a microstructure of a test piece obtained by hot forging steel type E at 1000 ° C. after induction hardening simulated heat treatment.
FIG. 11C is a microstructure of a test piece in which steel type E is hot forged at 900 ° C. after induction hardening simulated heat treatment.
FIG. 11D is a microstructure of a test piece in which steel type E is hot forged at 800 ° C. after induction hardening simulated heat treatment.
Mode for carrying out the invention
[0014]
　The present inventors have investigated means for improving the fatigue strength, machinability, and quench crack resistance of induction-hardened crankshafts, and obtained the following findings.
[0015]
　The induction-hardened crankshaft has an induction-hardened portion and a non-induction-hardened portion (base material). The induction hardened part is composed of a structure mainly composed of martensite or tempered martensite, and the non-induction hardened part is composed of a structure mainly composed of ferrite pearlite.
[0016]
　The lowering of machinability with higher C is due to the fact that the hardness is improved with higher C and the ferrite fraction in ferrite pearlite is lowered. On the other hand, when comparing steel materials with the same C content, it has been reported that even if the ferrite content is increased, the fatigue strength is equal to or rather improved (Satoru Nakana et al. Steel for induction hardening ”, Sanyo Technical Report Vol. 11 (2004) No.1, pp57-60). It is considered that this is because the crystal grains are substantially refined by increasing the ferrite fraction.
[0017]
　Therefore, if the ferrite fraction is increased as compared with ordinary ferrite pearlite when the C content is the same, both machinability and fatigue strength can be improved. Specifically, if the ferrite fraction Fα satisfies the following formula (1), a steel material having an excellent balance between fatigue strength and machinability can be obtained.
　　In Fα ≧ −150 × [C%] +84 (1)
　[C%], the C content of the induction hardening crankshaft is substituted by mass%.
[0018]
　It has been reported that the ferrite component can be increased by lowering the finish forging temperature in the hot forging process (Masahisa Fujiwara et al., "Material Control Forging Technology Using Machining Heat Treatment", Daido Special Steel Technical Report, No. Vol. 82, No. 2 (2011), pp.157-163). However, when the forging temperature is lowered, the life of the die is significantly shortened. From the viewpoint of productivity, it is preferable that the ferrite fraction can be increased without lowering the forging temperature excessively.
[0019]
　The present inventors have found that the ferrite fraction can be increased by containing an appropriate amount of Nb in the steel material without lowering the forging temperature excessively. This is considered to be due to the following mechanism.
[0020]
　Austenite grains processed by hot forging (hereinafter referred to as "γ grains") undergo recrystallization in order to release the strain introduced by the processing. At this time, the Nb (C, N) precipitated in the γ grains suppresses the grain growth of the γ grains after recrystallization. As a result, the γ grains can be miniaturized. As the γ grains become finer, the grain boundaries per unit area, which is the nucleation site of ferrite, increase, and the ferrite fraction increases.
[0021]
　Nb also contributes to the miniaturization of the structure after induction hardening. That is, by containing an appropriate amount of Nb, the structure of the induction hardening portion can also be miniaturized. Thereby, the fatigue strength and the quench crack resistance of the induction hardened portion can also be improved.
[0022]
　The present inventors have further found that the ferrite fraction can be further increased by setting the average cooling rate in the temperature range of 800 to 650 ° C. to 0.07 ° C./sec or less after hot forging.
[0023]
　The present invention has been completed based on the above findings. Hereinafter, a method for manufacturing an induction-hardened crankshaft and an induction material for an induction-hardened crankshaft according to an embodiment of the present invention will be described in detail.
[0024]
　[Induction Hardened Crankshaft]
　[Chemical Composition]
　The induction hardened crankshaft according to the present embodiment has the chemical composition described below. In the following description, "%" of the element content means mass%.
[0025]
　C: 0.30 to 0.60%
　carbon (C) improves the hardness of the induction hardened portion and the non-induction hardened portion, and contributes to the improvement of fatigue strength. On the other hand, if the C content is too high, the shrinkage resistance and machinability are lowered. Therefore, the C content is 0.30 to 0.60%. The lower limit of the C content is preferably 0.35%, more preferably 0.37%. The upper limit of the C content is preferably 0.55%, more preferably 0.51%.
[0026]
　Si: 0.01 to 1.50%
　silicon (Si) has a deoxidizing action and a ferrite strengthening action. On the other hand, if the Si content is too high, the machinability is lowered. Therefore, the Si content is 0.01 to 1.50%. The lower limit of the Si content is preferably 0.05%, more preferably 0.40%. The upper limit of the Si content is preferably 1.00%, more preferably 0.60%.
[0027]
　Mn: 0.4 to 2.0%
　manganese (Mn) enhances the hardenability of steel and contributes to the improvement of the hardness of the induction hardened portion. On the other hand, if the Mn content is too high, bainite is generated in the cooling process after hot forging, and the machinability is lowered. Therefore, the Mn content is 0.4 to 2.0%. The lower limit of the Mn content is preferably 1.0%, more preferably 1.2%. The upper limit of the Mn content is preferably 1.8%, more preferably 1.6%.
[0028]
　Cr: 0.01 to 0.50%
　chromium (Cr) enhances the hardenability of steel and contributes to the improvement of the hardness of the induction hardened portion. On the other hand, if the Cr content is too high, bainite is generated in the cooling process after hot forging, and the machinability is lowered. Therefore, the Cr content is 0.01 to 0.50%. The lower limit of the Cr content is preferably 0.05%, more preferably 0.10%. The upper limit of the Cr content is preferably 0.30%, more preferably 0.20%.
[0029]
　Al: 0.001 to 0.06%
　aluminum (Al) has a deoxidizing action. On the other hand, if the Al content is too high, the amount of alumina-based inclusions produced becomes excessive, and the machinability is lowered. Therefore, the Al content is 0.001 to 0.06%. The lower limit of the Al content is preferably 0.002%. The upper limit of the Al content is preferably 0.05%, more preferably 0.04%.
[0030]
　N: 0.001 to 0.02%
　nitrogen (N) forms nitrides and carbonitrides and contributes to the refinement of crystal grains. On the other hand, if the N content is too high, the hot ductility of the steel will decrease. Therefore, the N content is 0.001 to 0.02%. The lower limit of the N content is preferably 0.002%. The upper limit of the N content is preferably 0.015%, more preferably 0.01%.
[0031]
　P: 0.03% or less
　Phosphorus (P) is an impurity. P lowers the corrosion resistance of the steel. Therefore, the P content is 0.03% or less. The P content is preferably 0.025% or less, and more preferably 0.02% or less.
[0032]
　S: 0.005 to 0.20%
　sulfur (S) forms MnS and enhances the machinability of steel. On the other hand, if the S content is too high, the hot workability of the steel is lowered. Therefore, the S content is 0.005 to 0.20%. The lower limit of the S content is preferably 0.010%, more preferably 0.030%. The upper limit of the S content is preferably 0.15%, more preferably 0.10%.
[0033]
　Nb: 0.005 to 0.060%
　niobium (Nb) forms Nb (C, N) to refine γ grains. As a result, the grain boundaries per unit area, which is the nucleation site of ferrite, increase, and the ferrite fraction increases. As a result, the fatigue strength and machinability of the non-induction hardened portion are improved. Nb also contributes to the miniaturization of the structure after induction hardening, that is, the structure of the induction hardening portion. As a result, the fatigue strength and quench crack resistance of the induction hardened portion are improved. On the other hand, even if the Nb content is excessively increased, Nb that cannot be solid-solved in the matrix during hot forging heating forms coarse unsolid-dissolved NbC, which does not contribute to granulation. In addition, the addition of excess Nb causes cracks in the casting stage. Therefore, the Nb content is 0.005 to 0.060%. The lower limit of the Nb content is preferably 0.008%, more preferably 0.010%. The upper limit of the Nb content is preferably 0.050%, more preferably 0.030%.
[0034]
　The rest of the chemical composition of the induction hardened crankshaft according to this embodiment is Fe and impurities. The impurities referred to here refer to elements mixed from ores and scraps used as raw materials for steel, or elements mixed from the environment of the manufacturing process.
[0035]
　[Structure]
　The induction-hardened crankshaft according to the present embodiment has an induction-hardened portion and a non-induction-hardened portion.
[0036]
　Induction hardening of the crankshaft is generally applied only to the surface layer of the crankshaft. That is, the core of the crankshaft usually remains non-quenched. In addition, heat treatment for induction hardening may be applied only to places where fatigue strength and abrasion resistance are particularly required (journal part, etc.), and the surface layer part is also not applied to places where heat treatment is not performed. It remains a hardened structure. The "non-induction hardened portion" in the present embodiment shall mean both of these.
[0037]
　The non-induction hardening portion is composed mainly of ferrite pearlite. The area ratio of ferrite pearlite in the non-induction hardened portion is preferably 90% or more, more preferably 95% or more.
[0038]
　In the induction hardening crankshaft according to the present embodiment, the ferrite fraction Fα in ferrite pearlite satisfies the following equation (1).
　　In Fα ≧ −150 × [C%] +84 (1)
　[C%], the C content of the induction hardening crankshaft is substituted by mass%.
[0039]
　The ferrite fraction is measured as follows. A sample is taken from the non-induction hardening part so that the cross section including the direction perpendicular to the surface of the crankshaft is the observation surface. The observation surface is polished and etched with a mixed solution of ethanol and nitric acid (Nital). Using an optical microscope (observation magnification 100 to 200 times), the area ratio of ferrite on the etched surface is measured using image analysis. The measured area ratio (%) of ferrite is defined as the ferrite fraction.
[0040]
　The induction hardened portion is composed of a structure mainly composed of martensite or tempered martensite. The area ratio of martensite or tempered martensite in the induction hardened portion is preferably 90% or more, more preferably 95% or more.
[0041]
　The induction-hardened crankshaft according to the present embodiment has an old austenite particle size (hereinafter referred to as "former γ particle size") of martensite or tempered martensite of 30 μm or less. When the old γ particle size is 30 μm or less, excellent fatigue strength and corrosion resistance can be obtained. The old γ particle size is preferably 25 μm or less, and more preferably 20 μm or less.
[0042]
　The old γ particle size is measured as follows. A sample is taken from the induction hardening part so that the cross section including the direction perpendicular to the surface of the crankshaft is the observation surface. The observation surface is polished and etched with a saturated aqueous solution of picric acid to reveal the old austenite grain boundaries. The average particle size is calculated by the intercept method. Specifically, a straight line having a total length L is drawn, the number n L of crystal grains crossing this straight line is obtained, and the intercept length (L / n L ) is obtained. Obtain the intercept length (L / n L ) for 5 or more straight lines , and use the arithmetic mean as the average particle size.
[0043]
　[Manufacturing method of induction-hardened crankshaft]
　The induction-hardened crankshaft according to the present embodiment is not limited to this, but the material of the crankshaft described later is machined by cutting, grinding, drilling, etc. It can be manufactured by hardening. After induction hardening, tempering may be performed if necessary.
[0044]
　[Method for Manufacturing Induction Hardened Crankshaft Base Material]
　Hereinafter, a method for manufacturing a material for induction hardening crankshaft suitable for the induction hardening crankshaft according to the present embodiment will be described.
[0045]
　FIG. 1 is a flow chart of a method for manufacturing an induction material for an induction hardened crankshaft according to the present embodiment. This manufacturing method includes a step of preparing the steel material (step S1), a step of hot forging the steel material (step S2), and a step of cooling the hot forged steel material (step S3).
[0046]
　First, a steel material having the above-mentioned chemical composition is prepared (step S1). For example, steel having the above-mentioned chemical composition is melted and continuously cast or block-rolled to make steel pieces. The steel piece may be one that has been subjected to hot working, cold working, heat treatment, or the like in addition to continuous casting or block rolling.
[0047]
　Next, the steel material is hot forged and processed into a rough shape of the crankshaft (step S2).
[0048]
　The heating conditions for hot forging are not limited to this, but the heating temperature is, for example, 1000 to 1300 ° C., and the holding time is, for example, 1 second to 20 minutes. The heating temperature is preferably 1220 to 1280 ° C, more preferably 1240 to 1260 ° C.
[0049]
　In the present embodiment, the temperature immediately before finish forging (more specifically, the surface temperature of the steel material immediately before finish forging) is set to more than 800 ° C and less than 1100 ° C. Hot forging may be carried out in a plurality of times. In this case, the temperature immediately before the final hot forging for finishing may be set to more than 800 ° C and less than 1100 ° C.
[0050]
　When the temperature immediately before the finish forging (hereinafter, simply referred to as “finish forging temperature”) becomes 1100 ° C. or higher, the γ grains become coarse and a structure having a high ferrite fraction cannot be obtained after cooling. On the other hand, when the finish forging temperature is 800 ° C. or lower, the deformation resistance is remarkably increased, so that the life of the die is remarkably shortened, which makes industrial production difficult, if not impossible. The lower limit of the finish forging temperature is preferably 850 ° C, more preferably 900 ° C. The upper limit of the finish forging temperature is preferably 1075 ° C, more preferably 1025 ° C.
[0051]
　The steel material after hot forging is cooled (step S3). At this time, the average cooling rate in the temperature range of 800 to 650 ° C. is set to 0.07 ° C./sec or less. As a result, ferrite is precipitated at the austenite grain boundaries, and the ferrite fraction after cooling can be increased.
[0052]
　This cooling may be performed as long as the average cooling rate in the temperature range of 800 to 650 ° C. is 0.07 ° C./sec or less, the temperature range of 800 to 650 ° C. may be slowly cooled, or 800 to 650 ° C. is arbitrary. The steel material may be held at the temperature of the above for a predetermined time. Further, the cooling rate in the temperature range lower than 650 ° C. is arbitrary.
[0053]
　Through the above steps, an induction material for an induction hardened crankshaft is manufactured. The induction material for induction-hardened crankshafts produced by the present embodiment has a structure mainly composed of ferrite and pearlite, and has a high ferrite fraction.
[0054]
　The method for manufacturing an induction-hardened crankshaft and an induction material for an induction-hardened crankshaft according to an embodiment of the present invention has been described above. According to this embodiment, an induction hardened crankshaft having an excellent balance of fatigue strength, machinability, and quench crack resistance can be obtained.
Example
[0055]
　Hereinafter, the present invention will be described in more detail with reference to Examples. The present invention is not limited to these examples.
[0056]
　[Structural observation test]
　First, the relationship between the chemical composition and forging conditions of the steel material and the structure of the steel material was investigated.
[0057]
　Steels having the chemical compositions shown in Table 1 were melted in a 150 kg vacuum induction melting furnace (VIM) to prepare an ingot. This ingot was processed into a round bar having an outer diameter of 35 mm by hot forging. This round bar was held at 950 ° C. for 30 minutes and then subjected to an air-cooled normalizing treatment to prepare a test material.
[0058]
[table 1]

[0059]
　A test piece having an outer diameter of 8 mm and a height of 12 mm was collected from this material, and a hot forging simulation experiment was conducted by Machining Formaster. 2 and 3 show the heat pattern of the hot forging simulation experiment by Machining Formaster.
[0060]
　The heat pattern of FIG. 2 simulates general forging conditions. In this heat pattern, the test piece was held at 1250 ° C. for 10 seconds, then hot-compressed at 1100 ° C. to simulate forging, processed to a height of 6 mm, and air-cooled to room temperature.
[0061]
　The heat pattern of FIG. 3 is obtained by lowering the finish forging temperature and applying a retention treatment at 700 ° C. or 650 ° C. In this heat pattern, the test piece is held at 1250 ° C. for 10 seconds, and then hot compression processing is performed in the first stage at 1100 ° C. to simulate rough forging to a height of 9 mm, and then 1000 ° C., 900 ° C. or 800 A second stage hot compression process simulating finish forging was performed at ° C. to a height of 6 mm. Then, after holding at 700 degreeC or 650 degreeC for 30 minutes, it was air-cooled to room temperature.
[0062]
　All of the test pieces after cooling had a structure mainly composed of ferrite pearlite. Specifically, the area ratio of ferrite pearlite was 95% or more.
[0063]
　An observation test piece was taken from the cooled test piece, and the ferrite fraction and Vickers hardness near the center of the test piece were measured. The test results are shown in Tables 2 and 3. The numerical values ​​in the column of "old γ grain size after induction hardening" in Tables 2 and 3 are expected values ​​from the test results (described later) of the same type of steel.
[0064]
[Table 2]

[0065]
[Table 3]

[0066]
　As shown in Table 2, No. The ferrite fraction of each of the test pieces 1 to 12 satisfied the formula (1).
[0067]
　No. 13, 23, 29, 30, and 35 are test pieces to which the heat pattern of FIG. 2 is applied. All of these test pieces had a low ferrite fraction and did not satisfy the formula (1).
[0068]
　No. The test pieces of 15, 17, 27, 31, 33, and 37 all had a low ferrite fraction and did not satisfy the formula (1). It is probable that this is because the finish forging temperature was too high.
[0069]
　No. The test pieces of 20, 21, 22, 32, 34, 36, 38, and 39 all had a ferrite fraction satisfying the formula (1). However, since the finish forging temperature is low, it is considered difficult, if not impossible, to apply to actual production.
[0070]
　No. Since the Nb content of the test pieces 13 to 18 and 23 to 28 is too low, it is expected that the old γ grain size after induction hardening will be larger than 30 μm.
[0071]
　FIG. 4A shows No. It is a microstructure of 23 test pieces. FIG. 4B is a microstructure of a test piece made of the same steel material as in FIG. 4A, having a finish forging temperature of 800 ° C. and holding at 700 ° C. for 30 minutes. FIG. 4C shows No. It is a microstructure of 9 test pieces. In the figure, the part that looks white is ferrite.
[0072]
　Comparing FIGS. 4A and 4B, it can be seen that the ferrite fraction can be increased by lowering the finish forging temperature. Further, as shown in FIG. 4C, by containing Nb in the steel material, even if the finish forging temperature is raised to 1000 ° C., a ferrite fraction equivalent to that of the test piece of FIG. 4B in which the finish forging temperature is 800 ° C. can be obtained. It turns out that it can be done.
[0073]
　FIG. 5A shows No. It is a microstructure of 13 test pieces. FIG. 5B is a microstructure of a test piece made of the same steel material as in FIG. 5A, having a finish forging temperature of 800 ° C. and holding at 700 ° C. for 30 minutes. FIG. 5C shows No. It is the microstructure of the test piece of 1. As in the case of FIGS. 4A to 4C, the portion that looks white is ferrite. In this case as well, it can be seen that by adding Nb to the steel material, even if the finish forging temperature is raised to 1000 ° C., a ferrite fraction equivalent to that of the test piece when the finish forging temperature is 800 ° C. can be obtained.
[0074]
　6A to 6C are graphs showing the relationship between the finish forging temperature and the ferrite fraction in the steel types C to E, respectively. From FIGS. 6A to 6C, it can be seen that by increasing the Nb content, the finish forging temperature at which a large ferrite fraction can be obtained shifts to the higher temperature side.
[0075]
　In the steel types D and E, when the finish forging temperature is set to 800 ° C., the ferrite fraction is lower than when the finish forging temperature is 900 ° C. This is thought to be due to an increase in unrecrystallized austenite. The recrystallized austenite grains are finer than the austenite grains before finish forging. On the other hand, since unrecrystallized austenite inherits the structural units of the original coarse austenite grains, the grain boundaries per unit area, which is the main nucleation site of ferrite, do not increase, so that the ferrite fraction decreases.
[0076]
　7A to 7C are graphs showing the relationship between the finish forging temperature and the Vickers hardness in the steel types C to E, respectively. From FIGS. 7A to 7C, it can be seen that the Vickers hardness is greatly affected by the retention temperature. The softening due to retention at 700 ° C. is considered to be due to an increase in the ferrite fraction. The softening due to retention at 650 ° C. is considered to be due to an increase in the lamella spacing of pearlite in addition to an increase in the ferrite fraction.
[0077]
　From FIGS. 6A to 6C and FIGS. 7A to 7C, it can be seen that the ferrite fraction and the Vickers hardness can be controlled to some extent independently by selecting a combination of the chemical composition, the finish forging temperature, and the retention temperature.
[0078]
　From the above results, it was confirmed that by containing Nb, a structure having a high ferrite fraction can be obtained without excessively lowering the finish forging temperature.
[0079]
　[Fatigue test]
　Next, the relationship between the structure of steel materials and fatigue characteristics was investigated.
[0080]
　Steels having the chemical compositions shown in Table 4 were melted in a 150 kg vacuum induction melting furnace (VIM) to prepare an ingot.
[0081]
[Table 4]

[0082]
　This ingot was processed into a plate-shaped rolling material having a thickness of 40 mm by hot forging. This rolling material was hot-rolled under the conditions shown in Table 5.
[0083]
[Table 5]

[0084]
　Specifically, under condition 1, after heating to 1250 ° C., rough rolling was started from 1100 ° C., processed to a thickness of 20 mm in 5 passes, and then air-cooled to room temperature. Under condition 2, after heating to 1250 ° C., rough rolling was started from 1100 ° C., processing was performed to a thickness of 30 mm in 3 passes, finish rolling was started from 1000 ° C., and processing was performed to a thickness of 20 mm in 4 passes. Then, it was held at 700 ° C. for 30 minutes, and then air-cooled to room temperature. Condition 3 is the same as condition 2 except that the finish rolling start temperature is 850 ° C.
[0085]
　Observation test pieces were taken from the rolled steel sheet, and the ferrite fraction and Vickers hardness were measured.
[0086]
　A No. 14A test piece (outer diameter 8 mm, reference point distance 40 mm) specified in JIS Z 2241 was collected from the rolled steel sheet and subjected to a tensile test.
[0087]
　Ono-type rotary bending fatigue test pieces (length 106 mm, parallel portion outer diameter 8 mm, grip portion outer diameter 15 mm) were collected from the rolled steel sheet and subjected to a rotary bending fatigue test.
[0088]
　The results are shown in Table 6. In Table 6, "0.2% PS" represents 0.2% proof stress, and "TS" represents tensile strength. “-” In Table 6 indicates that the corresponding steel sheet has not been subjected to a fatigue test.
[0089]
[Table 6]

[0090]
　FIG. 8 is a graph showing the relationship between Vickers hardness and durability ratio (fatigue strength / tensile strength). From FIG. 8, No. 1 in which the pearlite fraction satisfies the formula (1). The steel plates of Nos. 3, 6 and 8 do not satisfy the formula (1). It can be seen that it has a high durability ratio as compared with the steel plates 1, 4, and 7.
[0091]
　From the above results, it was confirmed that when the pearlite fraction satisfies the formula (1), a steel material having an excellent balance between fatigue strength and machinability can be obtained.
[0092]
　[Induction hardening mock test]
　Finally, the relationship between the chemical composition of steel and the structure after induction hardening was investigated.
[0093]
　Steels having the same chemical composition as the steel types C to E in Table 1 were melted in a 150 kg vacuum induction melting furnace (VIM) to prepare an ingot. This ingot was processed into a round bar having an outer diameter of 35 mm by hot forging. This round bar was held at 950 ° C. for 30 minutes and then subjected to an air-cooled normalizing treatment to obtain a steel material.
[0094]
　A test piece having an outer diameter of 8 mm and a height of 12 mm was collected from this steel material, and a hot forging simulation experiment was conducted by Machining Formaster. Specifically, after holding the test piece at 1250 ° C. for 10 minutes, hot compression processing simulating forging is performed at 1100 ° C., 1000 ° C., 900 ° C., or 800 ° C. to a height of 6 mm, and air cooling to room temperature. did. In this test, no retention or slow cooling was performed after hot compression processing. This is because the effect on the tissue after induction hardening is considered to be small.
[0095]
　Then, simulating induction hardening, it was heated to 1000 ° C. at a heating rate of 40 ° C./sec, held at 1000 ° C. for 40 seconds, and then heat-treated to cool to room temperature at a cooling rate of about 40 ° C./sec. ..
[0096]
　9A-9D, 10A-10D, and 11A-11D are microstructures of the test piece after induction hardening simulated heat treatment.
[0097]
　From FIGS. 9A to 9D, it can be seen that the test piece having the forging temperature of 800 ° C. has an old γ particle size of about 30 μm and is slightly finer than the other test pieces. On the other hand, no significant difference was observed between the test pieces whose forging temperatures were set to 1100 ° C., 1000 ° C., and 900 ° C., and it can be seen that the old γ grain size was coarsened to 30 μm or more.
[0098]
　From FIGS. 10A to 10D, it can be seen that by adding Nb, the old γ particle size becomes 30 μm or less, and the size is significantly reduced. Further, it can be seen that in the test piece containing Nb, the structure tends to become finer as the forging temperature becomes lower.
[0099]
　From FIGS. 11A to 11D, it can be seen that by increasing the Nb content, the structure becomes finer than in FIGS. 10A to 10D. Further, as in the case of FIGS. 10A to 10D, it can be seen that the lower the forging temperature, the finer the structure tends to be. In particular, when the forging temperature is 1000 ° C. or lower, it can be seen that the old γ grain size is refined to 20 μm or less.
[0100]
　From the above results, it was confirmed that the particle size of the old austenite in the induction hardening portion can be made finer by containing Nb.
[0101]
　Although one embodiment of the present invention has been described above, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.
The scope of the claims
[Claim 1]
　An induction-hardened crank shaft having an induction-hardened portion and an induction-hardened portion, the
　chemical composition of which is mass%,
　C: 0.30 to 0.60%,
　Si: 0.01 to 1.50%,
　Mn. : 0.4 to 2.0%,
　Cr: 0.01 to 0.50%,
　Al: 0.001 to 0.06%,
　N: 0.001 to 0.02%,
　P: 0.03% or less ,
　S: 0.005 to 0.20%,
　Nb: 0.005 to 0.060%,
　balance: Fe and impurities, and the
　structure of the non-induction hardening portion is composed mainly of ferrite pearlite. In addition, the ferrite fraction Fα satisfies the following formula (1), the
　structure of the induction hardening portion is composed mainly of martensite or tempered martensite, and the old austenite particle size is 30 μm or less. Hardened crank shaft.
　　In Fα ≧ −150 × [C%] +84 (1)
　[C%], the C content of the induction hardening crankshaft is substituted by mass%.
[Claim 2]
　The chemical composition is mass%, C: 0.30 to 0.60%, Si: 0.01 to 1.50%, Mn: 0.4 to 2.0%, Cr: 0.01 to 0.50. %, Al: 0.001 to 0.06%, N: 0.001 to 0.02%, P: 0.03% or less, S: 0.005 to 0.20%, Nb: 0.005 to 0 .060%, remainder: a step of preparing a steel is Fe and impurities,
　a step of forging the steel material so that the temperature of the finish forging immediately before is 800 ° C. ultra 1100 below ° C. heat,
　after the hot forging, A method for producing a base material for a high-frequency forged crank shaft, comprising a step of cooling the steel material so that the average cooling rate in the temperature range of 800 to 650 ° C. is 0.07 ° C./sec or less.