Title of the invention: Crankshaft and method for manufacturing crankshaft material
Technical field
[0001]
The present invention relates to a method of manufacturing a crankshaft and a cast material for a crankshaft.
Background technology
[0002]
Crankshafts are manufactured by hot-forging steel into raw materials, then machining such as cutting, grinding, and drilling, and if necessary, surface hardening such as induction hardening.
[0003]
In order to improve the fatigue strength of crankshafts that are induction hardened and used, not only the parts that are induction hardened (hereinafter referred to as "induction hardened parts") but also the parts that are not induction hardened (hereinafter "non-induction hardened parts"). It is necessary to improve the hardness of ). 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 is a problem that the machinability is deteriorated and the processing cost is increased.
[0004]
As a method of improving hardness without increasing the C content, it is known to add V to steel materials and use precipitation strengthening by VC. However, since V is a relatively expensive element and has a high risk of price fluctuation, it is preferable not to use V from a commercial point of view.
[0005]
WO 2010/140596 discloses steel for machine structural use with improved machinability and hot workability by balancing the four elements N, Ti, B, and Al so as to satisfy a specific relationship. disclosed. Further, International Publication No. 2011/155605 discloses a high-strength steel with improved machinability by appropriately controlling the area ratio of bainite contained in the metal structure according to the amount of C contained in the steel. ing.
[0006]
Japanese Patent Application Laid-Open No. 2009-30160 discloses mechanical structural steel containing a predetermined amount of Al. WO2010-116670 discloses a carburized steel component containing a predetermined amount of Al. Japanese Unexamined Patent Application Publication No. 2012-162780 discloses a method for manufacturing a forged component, which can form a high-strength portion and a low-strength portion in a non-thermal refining manner in one component.
prior art documents
patent literature
[0007]
Patent Document 1: International Publication No. 2010/140596
Patent Document 2: International Publication No. 2011/155605
Patent Document 3: JP-A-2009-30160
Patent Document 4: International Publication No. 2010-116670
Patent document 5: JP 2012-162780 A
Non-patent literature
[0008]
Non-patent document 1: Satoru Nakana et al., "Steel for high-strength induction hardening with excellent machinability", Sanyo Technical Report Vol. 11 (2004) No.1, pp57-60
Non-Patent Document 2: Masanao Fujiwara et al., "Material Control Forging Technology Using Thermomechanical Heat Treatment", Daido Steel Technical Report, Vol. 82, No. 2 (2011), pp.157-163
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0009]
The technologies of WO 2010/140596 and WO 2011/155605 described above assume gears as a specific application target, and the fatigue strength (bending fatigue strength) required for crankshafts is sufficient. have not been considered.
[0010]
An object of the present invention is to provide a crankshaft with excellent fatigue strength and machinability.
Means to solve problems
[0011]
A crankshaft according to one embodiment of the present invention is a crankshaft having a pin and a journal, and has a chemical composition, in mass %, of C: 0.40 to 0.60%, Si: 0.01 to 1.50. %, Mn: 0.4-2.0%, Cr: 0.01-0.50%, Al: 0.20-0.50%, N: 0.001-0.02%, P: 0. 03% or less, S: 0.005 to 0.20%, Nb: 0.005 to 0.060%, Ti: 0 to 0.060%, the balance: Fe and impurities, in each of the pins and journals , The hardness at a depth position of 1/4 of each diameter from the surface layer is higher than that of HV245, the structure at the same position is a structure mainly composed of ferrite and pearlite, and the ferrite fraction is 16% or more. is.
[0012]
A method for producing a crankshaft cast material according to one embodiment of the present invention has a chemical composition, in mass %, of C: 0.40 to 0.60%, Si: 0.01 to 1.50%, Mn: 0.4 to 2.0%, Cr: 0.01 to 0.50%, Al: 0.20 to 0.50%, N: 0.001 to 0.02%, P: 0.03% or less, S: 0.005 to 0.20%, Nb: 0.005 to 0.060%, Ti: 0 to 0.060%, balance: Fe and impurities A step of preparing a steel material and a temperature just before finish forging A step of hot forging the steel material so that the temperature is more than 800 ° C. and less than 1100 ° C., and after the hot forging, the average cooling rate in the temperature range of 800 to 650 ° C. is 2.5 ° C./sec or less. and cooling the steel material.
[0013]
A method for producing a crankshaft cast material according to one embodiment of the present invention has a chemical composition, in mass %, of C: 0.40 to 0.60%, Si: 0.01 to 1.50%, Mn: 0.4 to 2.0%, Cr: 0.01 to 0.50%, Al: 0.20 to 0.50%, N: 0.001 to 0.02%, P: 0.03% or less, S: 0.005 to 0.20%, Nb: 0.005 to 0.060%, Ti: 0.005 to 0.060%, the balance: a step of preparing steel materials that are Fe and impurities, and immediately before finish forging A step of hot forging the steel material so that the temperature of the steel material is above 800 ° C. and 1180 ° C. or less, and after the hot forging, the average cooling rate in the temperature range of 800 to 650 ° C. is 0.07 ° C./sec or less. and cooling the steel material so as to cool the steel material.
Effect of the invention
[0014]
According to the present invention, a crankshaft with excellent fatigue strength and machinability can be obtained.
Brief description of the drawing
[0015]
1] FIG. 1 is a flowchart of a method for manufacturing a crankshaft blank according to an embodiment of the present invention. [FIG.
[Fig. 2] Fig. 2 shows one of the heat patterns of a simulated hot forging experiment.
[Fig. 3] Fig. 3 shows one of the heat patterns of a simulated hot forging experiment.
[Fig. 4] Fig. 4 shows one of the heat patterns of a simulated hot forging experiment.
[Fig. 5] Fig. 5 shows one of the heat patterns of the hot forging simulation test.
[Fig. 6] Fig. 6 shows one of the heat patterns of the hot forging simulation test.
[FIG. 7] FIG. 7 shows one of the heat patterns of the hot forging simulation test.
MODE FOR CARRYING OUT THE INVENTION
[0016]
The present inventors studied means for improving the fatigue strength and machinability of crankshafts and obtained the following findings.
[0017]
As described above, a crankshaft that is induction hardened and used 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.
[0018]
The decrease in machinability due to the increase in C is due not only to the increase in hardness due to the increase in C, but also to the decrease in the ferrite fraction in the ferrite/pearlite. On the other hand, when comparing steels with the same C content, it has been reported that even if the ferrite fraction is increased, the fatigue strength is the same or even improved (Satoru Nakana et al. Steel for Induction Hardening”, Sanyo Technical Report Vol. 11 (2004) No.1, pp57-60). This is probably because the crystal grains are substantially refined by increasing the ferrite fraction.
[0019]
Therefore, both machinability and fatigue strength can be improved by increasing the ferrite fraction compared to ordinary ferrite/pearlite steel with the same C content. If the C content is 0.40 to 0.60% by mass and the ferrite fraction is 16% or more, a steel material with excellent balance between fatigue strength and machinability can be obtained.
[0020]
It has been reported that the ferrite fraction can be increased by lowering the finish forging temperature in the hot forging process (Masanao Fujiwara et al., "Material Control Forging Technology Using Thermomechanical Processing," Daido Steel Technical Report, Vol. 82, No. 2 (2011), pp.157-163). However, lowering the forging temperature significantly reduces the life of the die. From the viewpoint of productivity, it is preferable to be able to increase the ferrite fraction without excessively lowering the forging temperature.
[0021]
The inventors have found that by adding appropriate amounts of Al and Nb to the steel material, the ferrite fraction can be increased without excessively lowering the forging temperature. This is considered to be due to the following mechanism.
[0022]
Austenite grains that have been processed by hot forging (hereinafter referred to as "γ grains") recrystallize to release the strain introduced by processing. At this time, NbC, NbN, and Nb(CN) precipitated in the γ grains suppress grain growth of the γ grains after recrystallization. This makes it possible to refine the γ grains. As the γ grains become finer, the number of grain boundaries per unit area that serve as ferrite nucleation sites increases, and the ferrite fraction increases.
[0023]
Al is a ferrite-forming element, which significantly increases the A3 point and expands the region where pro-eutectoid ferrite is generated to the high temperature side. Al also has the effect of increasing the eutectoid carbon concentration, increasing the maximum pro-eutectoid ferrite fraction predicted from equilibrium. Steel containing an appropriate amount of Al has a wide pro-eutectoid ferrite precipitation region in the cooling process after hot forging, and the maximum pro-eutectoid ferrite fraction predicted from the equilibrium state is also high. higher rate.
[0024]
In this way, Nb increases the ferrite fraction by refining the γ grains, and Al increases the ferrite fraction by enlarging the precipitation region of pro-eutectoid ferrite and by Al itself increasing the pro-eutectoid ferrite. By adding Al and Nb in combination, these effects are superimposed, and the ferrite fraction can be significantly increased.
[0025]
The present inventors also found that the ferrite fraction can be further increased by adding an appropriate amount of Ti to the steel material in addition to Al and Nb, and by reducing the average cooling rate in the temperature range of 800 to 650 ° C. I found Then, by adding appropriate amounts of Al, Nb, and Ti to the steel material in combination and setting the average cooling rate in the temperature range of 800 to 650°C to 0.07°C/sec or less, even if the forging temperature is further increased, It was found that a predetermined amount of ferrite can be secured.
[0026]
The present invention was completed based on the above findings. Hereinafter, a method for manufacturing a crankshaft and a crankshaft preform according to an embodiment of the present invention will be described in detail.
[0027]
[Crankshaft]
[Chemical composition]
The crankshaft according to this embodiment has the chemical composition described below. In the following description, "%" of element content means % by mass.
[0028]
C: 0.40-0.60%
Carbon (C) improves the hardness of the induction hardened part and the non-induction hardened part and contributes to the improvement of fatigue strength. On the other hand, if the C content is too high, the quench crack resistance and machinability will deteriorate. Therefore, the C content is 0.40-0.60%. The lower limit of the C content is preferably 0.45%, more preferably 0.48%. The upper limit of the C content is preferably 0.55%, more preferably 0.52%.
[0029]
Si: 0.01-1.50%
Silicon (Si) has a deoxidizing effect and an effect of strengthening ferrite. On the other hand, if the Si content is too high, the machinability will deteriorate. Therefore, the Si content is 0.01-1.50%. 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%.
[0030]
　Mn: 0.4 to 2.0%
Manganese (Mn) increases the hardenability of steel and contributes to improving the hardness of induction hardened parts. On the other hand, if the Mn content is too high, during the cooling process after hot forging, Bainite is formed in the steel, and the machinability is lowered. Therefore, the Mn content is 0.4-2.0%. 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%.
[0031]
Cr: 0.01-0.50%
Chromium (Cr) increases the hardenability of steel and contributes to improving the hardness of induction hardened parts. On the other hand, if the Cr content is too high, bainite is formed in the cooling process after hot forging, resulting in deterioration of machinability. Therefore, the Cr content is 0.01-0.50%. The lower limit of 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%.
[0032]
Al: 0.20-0.50%
Aluminum (Al) is a ferrite-forming element, which significantly increases the A3 point and expands the region where pro-eutectoid ferrite is generated to the high temperature side. Al also has the effect of increasing the eutectoid carbon concentration, increasing the maximum pro-eutectoid ferrite fraction predicted from equilibrium. On the other hand, if the Al content is too high, an excessive amount of alumina-based inclusions is produced, resulting in a decrease in machinability. Therefore, the Al content is 0.20-0.50%. The lower limit of Al content is preferably 0.25%. The upper limit of the Al content is preferably 0.45%, more preferably 0.40%.
[0033]
N: 0.001-0.02%
Nitrogen (N) forms nitrides and carbonitrides and contributes to 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 0.001-0.02%. 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%.
[0034]
P: 0.03% or less
Phosphorus (P) is an impurity. P lowers the quench cracking resistance of steel. Therefore, the P content is 0.03% or less. The P content is preferably 0.025% or less, more preferably 0.02% or less.
[0035]
S: 0.005-0.20%
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 deteriorates. Therefore, the S content is 0.005-0.20%. The lower limit of the S content is preferably 0.010%, more preferably 0.030%, still more preferably 0.035%. The upper limit of the S content is preferably 0.15%, more preferably 0.10%.
[0036]
Nb: 0.005-0.060%
Niobium (Nb) forms NbC, NbN, and Nb(CN) to refine γ grains. As a result, the grain boundaries per unit area that serve as ferrite nucleation sites are increased, and the ferrite fraction is increased. Nb also contributes to refinement of the structure after induction hardening, that is, the structure of the induction hardened portion. On the other hand, even if the Nb content is excessively high, the Nb that cannot be dissolved in the matrix during heating for hot forging forms coarse undissolved NbC, which does not contribute to grain refinement. Also, excessive Nb addition causes cracking during casting. Therefore, the Nb content is 0.005-0.060%. 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%.
[0037]
The rest of the chemical composition of the crankshaft according to this embodiment is Fe and impurities. The term "impurities" as used herein refers to elements mixed in from ores and scraps used as raw materials for steel, or elements mixed in from the environment during the manufacturing process.
[0038]
The chemical composition of the crankshaft according to this embodiment may contain Ti instead of part of Fe. Ti is an element of choice. That is, the chemical composition of the crankshaft according to this embodiment does not have to contain Ti.
[0039]
Ti: 0-0.060%
Titanium (Ti) forms TiC, TiN, and Ti(CN) to refine γ grains. As a result, the grain boundaries per unit area that serve as ferrite nucleation sites are increased, and the ferrite fraction is increased. In particular, when it is contained together with Nb, the effect of refining γ grains is increased. On the other hand, even if the Ti content is excessively increased, the effect is saturated. Therefore, the Ti content is 0-0.060%. The lower limit of Ti content is preferably 0.005%, more preferably 0.020%. The upper limit of the Ti content is preferably 0.050%, more preferably 0.030%.
[0040]
[Tissue and hardness]
In the crankshaft according to the present embodiment, the hardness of each pin and journal at a depth position of 1/4 of the diameter from the surface layer (hereinafter referred to as "1/4 depth position") is higher than that of HV245. , The structure at the same position is a structure mainly composed of ferrite and pearlite, and the ferrite fraction is 16% or more. The reason why the 1/4 depth position is used as the measurement position is that it is suitable for defining the hardness and structure of the base material that is not affected by induction hardening.
[0041]
　The hardness at the 1/4 depth position is higher than that of HV245. If the hardness at the quarter depth position is HV245 or less, it becomes difficult to obtain the required fatigue strength. The lower limit of the hardness at the quarter depth position is preferably HV250, more preferably HV255. On the other hand, if the hardness at the 1/4 depth position is too high, the machinability will deteriorate. The upper limit of the hardness at the quarter depth position is preferably HV350, more preferably HV300, and still more preferably HV280.
[0042]
The hardness at the 1/4 depth position is measured according to JIS Z 2244 (2009) by taking a sample from the pin and journal so that the surface perpendicular to the axial direction is the test surface. The test force shall be 300 gf (2.942 N).
[0043]
The structure at the 1/4 depth position is a structure mainly composed of ferrite and pearlite. The area ratio of ferrite/pearlite at the 1/4 depth position is preferably 90% or more, more preferably 95% or more.
[0044]
The structure at the 1/4 depth position further has a ferrite fraction of 16% or more. The lower limit of the ferrite fraction in the structure at the 1/4 depth position is preferably 18%, more preferably 20%, and even more preferably 22%. Although no particular upper limit is set for the ferrite fraction, it is considered that if the ferrite fraction is too high, the required fatigue strength cannot be obtained. The upper limit of the ferrite fraction in the structure at the 1/4 depth position is preferably 30%.
[0045]
The ferrite fraction of the structure at the 1/4 depth position is measured as follows. A sample is taken from the pin and journal so that the surface perpendicular to the axial direction is the observation surface. The observation surface is polished and etched using a mixed solution of ethanol and nitric acid (nital). Using an optical microscope (observation magnification of 100 to 200 times), the area ratio of ferrite on the etched surface is measured by image analysis. The measured area ratio (%) of ferrite is defined as the ferrite fraction.
[0046]
The crankshaft according to this embodiment preferably has an induction hardened layer having a structure mainly composed of martensite or tempered martensite on the surfaces of the pin and journal. The area ratio of martensite or tempered martensite in the induction hardened layer is preferably 90% or more, more preferably 95% or more. The thickness of the induction hardened layer is preferably 2 mm or more, more preferably 4 mm or more.
[0047]
[Crankshaft manufacturing method]
Although not limited to this, the crankshaft according to the present embodiment can be manufactured by subjecting the crankshaft material described below to machining such as cutting, grinding, and drilling. After machining, induction hardening may be applied if necessary. Further, after induction hardening, tempering may be applied as necessary.
[0048]
[Manufacturing method of cast material for crankshaft]
A method of manufacturing a crankshaft material suitable for the crankshaft according to the present embodiment will be described below.
[0049]
FIG. 1 is a flow diagram of a method for manufacturing a crankshaft blank according to this embodiment. This manufacturing method includes a step of preparing steel (step S1), a step of hot forging the steel (step S2), and a step of cooling the hot forged steel (step S3).
[0050]
First, a steel material having the chemical composition described above is prepared (step S1). For example, a steel having the chemical composition described above is melted and subjected to continuous casting or blooming rolling to form a steel slab. The billet may be subjected to hot working, cold working, heat treatment, etc. in addition to continuous casting or blooming rolling.
[0051]
Next, the steel material is hot-forged and processed into the rough shape of the crankshaft (step S2).
[0052]
The heating 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. The heating temperature is preferably 1220-1280°C, more preferably 1240-1260°C.
[0053]
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 at more than 800°C and less than 1100°C. Under certain conditions, the temperature immediately before finish forging can be made even higher, but this will be described later, and the case where the temperature immediately before finish forging is set to more than 800 ° C. and less than 1100 ° C. will be described. .
[0054]
The hot forging may be performed in multiple steps. In this case, the temperature just before the final finish forging should be more than 800°C and less than 1100°C.
[0055]
When the temperature immediately before finish forging (hereinafter simply referred to as the "finish forging temperature") reaches 1100°C or higher, the γ grains coarsen, making it difficult to obtain a structure with a high ferrite fraction after cooling. On the other hand, when the finish forging temperature is 800° C. or less, the deformation resistance is significantly increased, and the life of the die is significantly shortened, making industrial production difficult, if not impossible. Moreover, since the pearlite transformation temperature rises and the lamellar spacing increases, the required hardness may not be obtained. 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.
[0056]
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 2.5°C/sec or less. If the average cooling rate in the temperature range of 800 to 650° C. is higher than 2.5° C./sec, bainite may form and a structure mainly composed of ferrite/pearlite may not be obtained. If the average cooling rate in the temperature range of 800 to 650° C. is set to 2.5° C./sec or less, a structure mainly composed of ferrite/pearlite and having a ferrite fraction of 16% or more can be obtained.
[0057]
It is preferable not to reheat the steel before hot forging and cooling. When the steel material after hot forging is reheated, the γ grains refined by hot forging are coarsened. As a result, the number of crystal grain boundaries per unit area, which serve as ferrite nucleation sites, is reduced, and a structure having a ferrite fraction of 16% or more may not be obtained.
[0058]
As the average cooling rate in the temperature range of 800 to 650°C is decreased, the amount of ferrite precipitated increases, and the ferrite fraction after cooling can be increased. In this case, the average cooling rate may be reduced by slowly cooling the temperature range of 800 to 650 ° C., or the average cooling rate may be reduced by holding the steel material at an arbitrary temperature of 800 to 650 ° C. for a predetermined time. You can slow it down. The average cooling rate in the temperature range of 800 to 650° C. is preferably 1.0° C./second or less, more preferably 0.5° C./second or less, and still more preferably 0.07.°C/sec or less. The cooling rate in the temperature range lower than 650°C is arbitrary.
[0059]
If the steel material contains 0.005 to 0.060% Ti and the average cooling rate in the temperature range of 800 to 650 ° C. is 0.07 ° C./sec or less, even if the finish forging temperature is further increased , the ferrite fraction of the structure after cooling can be 16% or more. Specifically, even if the temperature immediately before finish forging is set to 1100° C. or more and 1180° C. or less, the ferrite fraction of the structure after cooling can be 16% or more. When the steel material contains 0.005 to 0.060% Ti and the average cooling rate in the temperature range of 800 to 650 ° C. is set to 0.07 ° C./sec or less, the upper limit of the temperature immediately before finish forging is It is preferably 1150°C, more preferably 1120°C.
[0060]
Through the above processes, the raw material for the crankshaft is manufactured. The cast material for crankshafts manufactured according to the present embodiment has a hardness higher than that of HV245, a structure mainly composed of ferrite and pearlite, and a ferrite fraction of 16% or more.
[0061]
The method of manufacturing the crankshaft and the crankshaft preform according to one embodiment of the present invention has been described above. According to this embodiment, a crankshaft having excellent fatigue strength and machinability can be obtained.
Example
[0062]
The present invention will be described in more detail below with reference to examples. The invention is not limited to these examples.
[0063]
A 150 kg vacuum induction melting furnace (VIM) was used to melt steel having the chemical composition shown in Table 1 to produce an ingot. This ingot was processed into a round bar with an outer diameter of 35 mm by hot forging. After holding this round bar at 950° C. for 30 minutes, it was subjected to a normalizing treatment of air cooling and used as a test material. "-" in Table 1 indicates that the content of the corresponding element is at the impurity level.
[0064]
[table 1]

[0065]
A test piece with an outer diameter of 8 mm and a height of 12 mm was taken from this material, and a simulated hot forging experiment was performed using a processing former master. 2 to 6 show heat patterns of hot forging simulation experiments.
[0066]
The heat pattern in Figure 2 simulates general forging conditions. In this heat pattern, the test piece was held at 1250° C. for 10 seconds, then subjected to hot compression simulating forging at 1100° C. to a height of 6 mm, and air-cooled to room temperature.
[0067]
The heat pattern in Figure 3 is obtained by adding retention treatment at 700°C after hot forging in Figure 2. In this heat pattern, after the hot forging shown in FIG. 2, the steel was held at 700° C. for 30 minutes and then air-cooled to room temperature.
[0068]
The heat pattern in Fig. 4 is obtained by lowering the finish forging temperature. In this heat pattern, after holding the test piece at 1250 ° C. for 10 seconds, the first step hot compression processing simulating rough forging at 1100 ° C. was performed to a height of 9 mm, and further simulating finish forging at 1000 ° C. Then, the second-stage hot compression processing was performed and processed to a height of 6 mm.
[0069]
The heat pattern in Fig. 5 is obtained by lowering the finish forging temperature and adding a retention treatment at 700°C. In this heat pattern, after holding the test piece at 1250 ° C. for 10 seconds, the first step hot compression processing simulating rough forging was performed at 1100 ° C. to a height of 9 mm, and further finished at 1000 ° C. or 780 ° C. A second-stage hot compression process simulating forging was carried out to a height of 6 mm. After that, it was air-cooled to room temperature after being subjected to retention treatment at 700° C. for 30 minutes.
[0070]
The heat pattern in Fig. 6 is obtained by increasing the cooling rate after hot forging in Fig. 4.
[0071]
In the heat pattern of FIG. 7, after holding the test piece at 1250 ° C. for 10 seconds, the first stage hot compression processing simulating rough forging was performed at 1200 ° C. to a height of 9 mm, and then finish forging was performed at 1150 ° C. A second step of hot compression processing simulating was performed and processed to a height of 6 mm. After that, it was air-cooled to room temperature after being subjected to retention treatment at 700° C. for 30 minutes.
[0072]
Table 2 shows the conditions of the forging simulation test.
[0073]
[Table 2]

[0074]
A sample was taken from the test piece after cooling, and the ferrite fraction and Vickers hardness near the center of the test piece were measured. Table 3 shows the test results.
[0075]
[Table 3]

[0076]
"F/P" in the "structure" column of Table 3 indicates that the structure of the test piece was mainly composed of ferrite/pearlite. "F/P/B" in the same column indicates that the structure of the test piece was a mixed structure of ferrite/pearlite and bainite. The numerical values ​​in the "Fα" column of Table 3 are the ferrite fractions in the structure of the test piece.
[0077]
The value of "estimated drill life" in Table 3 is obtained by drilling with an SKH51 drill with an outer diameter of 5 mm under the conditions of a cutting speed of 50 m/min, a feed of 0.2 mm/rev, no cutting oil, and a drilling depth of 15 mm. It is an estimate of the number of holes to failure. This drill life estimate is an extrapolation from other experimental results.
[0078]
The test pieces with test symbols 1A, 1B, 2A, 2B, 2D, 1G, and 2H had a higher hardness than HV245 and a ferrite fraction of 16% or more. In particular, the test pieces with test symbols 2D and 2H had a structure with a ferrite fraction of 16% or more even though the temperatures immediately before the finish forging were relatively high, 1100 ° C. and 1150 ° C., respectively. was
[0079]
The test pieces with test symbols 3B, 4B, 5B, 6A, 7B, and 8A had a ferrite fraction lower than 16%. This is probably because at least one of the Al content and the Nb content of the steels with steel numbers 3 to 8 was too low.
[0080]
The test pieces with test symbols 9B, 10A, 10B, and 9C had a hardness of HV245 or less. This is believed to be due to the C content of steels Nos. 9 and 10 being too low.
[0081]
The test pieces with test symbols 1C, 1D, and 2C had a ferrite fraction lower than 16%. This is probably because the temperature just before finish forging was too high.
[0082]
The test pieces with test symbols 3C and 7C had a ferrite fraction lower than 16%. This is probably because the Al content and Nb content of the steel materials of Steel Nos. 3 and 7 were too low and the temperature immediately before the finish forging was too high.
[0083]
The hardness of the test piece with test symbol 1E was HV245 or less. This is probably because the temperature just before finish forging was too low.
[0084]
The test piece of test number 1F had a ferrite fraction lower than 16%, and bainite was mixed in the structure. This is probably because the average cooling rate in the temperature range of 850 to 600°C was too high.
[0085]
Although one embodiment of the present invention has been described above, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and it is possible to modify the above-described embodiment as appropriate without departing from the scope of the invention.
The scope of the claims
[Claim 1]
A crankshaft with pins and journals,
The chemical composition, in mass%,
C: 0.40-0.60%,
Si: 0.01 to 1.50%,
Mn: 0.4-2.0%,
Cr: 0.01 to 0.50%,
Al: 0.20-0.50%,
N: 0.001-0.02%,
P: 0.03% or less,
S: 0.005-0.20%,
Nb: 0.005 to 0.060%,
Ti: 0 to 0.060%,
The balance: Fe and impurities,
In each of the pins and journals, the hardness at a depth position of 1/4 of the diameter from the surface layer is higher than HV245, and the structure at the position is a structure mainly composed of ferrite and pearlite, and A crankshaft having a ferrite fraction of 16% or more.
[Claim 2]
The crankshaft according to claim 1,
The chemical composition, in % by mass,
Ti: 0.005 to 0.060%,
A crankshaft that contains
[Claim 3]
The chemical composition is mass %, C: 0.40 to 0.60%, Si: 0.01 to 1.50%, Mn: 0.4 to 2.0%, Cr: 0.01 to 0.50 %, Al: 0.20 to 0.50%, N: 0.001 to 0.02%, P: 0.03% or less, S: 0.005 to 0.20%, Nb: 0.005 to 0 .060%, Ti: 0 to 0.060%, balance: Fe and a step of preparing a steel material that is impurities;
A step of hot forging the steel material so that the temperature immediately before finish forging is more than 800°C and less than 1100°C;
After the hot forging, the steel material is cooled so that the average cooling rate in the temperature range of 800 to 650°C is 2.5°C/sec or less.
[Claim 4]
The chemical composition is mass %, C: 0.40 to 0.60%, Si: 0.01 to 1.50%, Mn: 0.4 to 2.0%, Cr: 0.01 to 0.50 %, Al: 0.20 to 0.50%, N: 0.001 to 0.02%, P: 0.03% or less, S: 0.005 to 0.20%, Nb: 0.005 to 0 .060%, Ti: 0.005 to 0.060%, the balance: a step of preparing a steel material that is Fe and impurities;
a step of hot forging the steel material so that the temperature immediately before finish forging is over 800°C and 1180°C or less;
A method for manufacturing a crankshaft formed material, comprising a step of cooling the steel material after the hot forging so that the average cooling rate in the temperature range of 800 to 650°C is 0.07°C/sec or less.
[Claim 5]
A method for manufacturing a crankshaft formed material according to claim 4,
A method for manufacturing a cast material for a crankshaft, wherein the temperature immediately before the finish forging is 1100°C or higher and 1180°C or lower.
[Claim 6]
A method for manufacturing a crankshaft formed material according to any one of claims 3 to 5,
The cast crankshaft has a hardness higher than that of HV245, a structure mainly composed of ferrite and pearlite, and a ferrite fraction of 16% or more. Production method.