JP7136320B2 - Crankshaft - Google Patents

Description

The present invention relates to crankshafts, and more particularly to crankshafts having a hardened layer.

A known crankshaft has a hardened layer formed on its surface in order to improve wear resistance and fatigue strength. For example, Japanese Patent No. 4910654 discloses a crankshaft having a quench-hardened layer on the surface of the crank pin, wherein the surface residual compressive stress at the bottom R portion of the crank pin is 600 MPa or more. .

Although it does not relate to crankshafts, as a document related to steel materials that are induction hardened and used, Japanese Patent Application Laid-Open No. 11-217649 discloses a steel material for induction hardening that has both cold workability and high strength characteristics. there is In addition, International Publication No. 2004/035848 discloses a steel material for machine structural use which is excellent in formability, quench cracking resistance and torsional properties.

In recent years, against the backdrop of the need for fuel-saving automobiles, weight reduction of crankshafts and reduction of the viscosity of lubricating oils are progressing. Therefore, the sliding conditions between the crankshaft and the bearings are severe, and the crankshaft is required to have better seizure resistance than before.

It is known that by increasing the thermal conductivity of the crankshaft, it is possible to suppress the decrease in the viscosity of the lubricating oil as the temperature rises, thereby improving the seizure resistance. It is also known that the thermal conductivity of the crankshaft can be increased by reducing the Si content.

In Japanese Patent No. 4589885, Si: 0.15% or less, Mn: more than 1.0% and 2.0% or less, etc., thermal conductivity is 40 W / mK or more, and induction hardening A crankshaft is disclosed in which the surface hardness Hv and the thermal conductivity κ satisfy the relationship of Hv>2.7×κ+420.

International Publication No. 2012/029395 contains Si: 0.20% or less, Mn: 0.75 to 2.0%, Cr: 0.05 to 1.2%, etc., and further Mn and Cr content is disclosed as a steel for induction hardening with a total of 1.20 to 2.10%.

JP 2017-61747 contains Si: less than 0.50%, Mn: less than 0.70%, Mo: 0.5 to 3.5%, S: 0.05 to 0.1%, etc. A sliding component is disclosed.

Further weight reduction is required for crankshafts, and sliding conditions are expected to become even more severe. SUMMARY OF THE INVENTION An object of the present invention is to provide a crankshaft having excellent seizure resistance.

The crankshaft according to one embodiment of the present invention has a chemical composition in mass% of C: 0.35 to 0.50%, Si: 0.55% or less, Mn: 0.01 to 0.55%, Cr : 0.10 to 0.50%, P: 0.03% or less, S: 0.10% or less, Mo: 0.10 to 0.30%, B: 0.0020 to 0.0050%, Ti: 0.020 to 0.10%, N: 0.020% or less, Al: 0.10% or less, the balance: Fe and impurities, having a hardened layer on the surface and dissolved in the hardened layer Si, Mn, Cr, and Mo satisfy the following formula (1).
_1.960SiM + _1.002MnM + _1.059CrM + _{0.574MoM≤0.75} (1)
Si _M , Mn _M , Cr _M , and Mo _M in formula (1) are respectively substituted with the amounts of Si, Mn, Cr, and Mo solid-soluted in the quench-hardened layer in mass %.

According to the present invention, a crankshaft having excellent seizure resistance can be obtained.

FIG. 1 is a flow diagram of a method for analyzing Mn, Cr, Mo, Ti, and Si from electro-extraction residue. FIG. 2 is a flow diagram of a method for analyzing S from the electro-extraction residue. FIG. 3 is a flow diagram of a method for analyzing B from the electrolytic extraction residue. FIG. 4 is a flow diagram of a method for analyzing N from the electrolytic extraction residue. FIG. 5 is a flow diagram of a method for analyzing C from the electrolytic extraction residue. FIG. 6 is a heat pattern of the test shaft produced in Example. FIG. 7 is a schematic diagram of an evaluation device used in the seizure test. FIG. 8 is a schematic diagram of the change over time of the surface pressure applied to the test shaft. FIG. 9 is a graph showing the relationship between the seizure contact pressure of the test shaft and the thermal conductivity of the hardened layer at room temperature.

The present inventors have studied how to further increase the thermal conductivity of the crankshaft to improve the seizure resistance. The inventors of the present invention focused on the fact that the smaller the amount of solid-solution elements, the better the thermal conductivity. It was clarified that excellent seizure resistance can be obtained.

On the other hand, since Mn is an element that improves the hardenability of steel materials, if the Mn content is reduced, the fatigue strength cannot be guaranteed. The present inventors have found that, instead of reducing the Mn content, by containing a small amount of Mo and B and further containing an appropriate amount of Ti, it is possible to obtain excellent seizure resistance while ensuring hardenability. I found

The present invention has been completed based on the above findings. A crankshaft according to one embodiment of the present invention will be described in detail below.

[Chemical composition]
The crankshaft according to this embodiment has the chemical composition described below. In the following description, "%" of the content of elements means % by mass unless otherwise specified.

C: 0.35-0.50%
Carbon (C) improves the hardness of steel 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 0.35-0.50%. The lower limit of the C content is preferably 0.36%, more preferably 0.37%. The upper limit of the C content is preferably 0.45%, more preferably 0.40%.

Si: 0.55% or less Silicon (Si) forms a solid solution in the matrix and lowers the thermal conductivity of steel. The Si content is preferably as low as possible, and is limited to 0.55% or less in this embodiment. Since Si has a smaller atomic weight than Mn, Si has a greater effect on thermal conductivity than Mn. The Si content is preferably 0.20% or less, more preferably 0.10% or less, more preferably 0.05% or less, still more preferably 0.02% or less.

Mn: 0.01-0.55%
Manganese (Mn) forms a solid solution in the matrix and lowers the thermal conductivity of steel. The Mn content is preferably as low as possible to improve thermal conductivity. On the other hand, if the Mn content is too low, it becomes difficult to form austenite, making it difficult to ensure hardenability, so there is a possibility that a hardened layer will not be formed. Also, if the Mn content is too low, S can react with Fe and cause brittle cracking. Therefore, the Mn content is 0.01-0.55%. The lower limit of the Mn content is preferably 0.20%, more preferably 0.30%, still more preferably 0.40%. The upper limit of the Mn content is preferably 0.52%, more preferably 0.50%, still more preferably 0.48%, still more preferably 0.45%.

Cr: 0.10-0.50%
Chromium (Cr) increases the hardenability of steel. On the other hand, if the Cr content is too high, the thermal conductivity of the steel will decrease. Therefore, the Cr content is 0.10-0.50%. The lower limit of Cr content is preferably 0.05%, more preferably 0.10%. The upper limit of the Cr content is preferably 0.40%, more preferably 0.30%, still more preferably 0.25%, still more preferably 0.20%.

P: 0.03% or less Phosphorus (P) is an impurity. P lowers the quench cracking resistance of steel. Therefore, the P content is preferably as low as possible, and is limited to 0.03% or less in this embodiment. The P content is preferably 0.025% or less, more preferably 0.02% or less.

S: 0.10% or less Sulfur (S) reduces the hot workability of steel. Also, if the Mn content is too low, S can react with Fe and cause brittle cracking. Therefore, in this embodiment, the S content is limited to 0.10% or less. On the other hand, S forms MnS and _MnS2 in steel and reduces the amount of dissolved Mn. Furthermore, since MnS is soft and brittle, it has the effect of improving the machinability of the crankshaft, especially the machinability. Therefore, depending on the Mn content, a small amount of S may be positively included. The lower limit of the S content is preferably 0.010%, more preferably 0.020%, still more preferably 0.030%, still more preferably 0.040%, still more preferably 0 0.045%. The upper limit of the S content is preferably 0.070%, more preferably 0.065%.

Mo: 0.10-0.30%
Molybdenum (Mo) improves the hardenability of steel. In this embodiment, 0.10% or more Mo is contained in order to compensate for the deterioration of hardenability due to the restriction of the Mn content. Since Mo has a larger atomic weight than Mn, it has less influence on thermal conductivity than Mn. However, if the content exceeds 0.30%, it becomes difficult to ensure the necessary thermal conductivity. Therefore, the Mo content is 0.10-0.30%. The lower limit of Mo content is preferably 0.12%, more preferably 0.14%. The upper limit of the Mo content is preferably 0.25%, more preferably 0.20%, still more preferably 0.18%.

B: 0.0020 to 0.0050%
Boron (B) improves the hardenability of steel. In this embodiment, 0.0020% or more of B is contained in order to compensate for the decrease in hardenability due to the restriction of the Mn content. On the other hand, even if the content exceeds 0.0050%, the effect of improving the hardenability is saturated and the hot workability of the steel deteriorates. Therefore, the B content is 0.0020-0.0050%. The lower limit of the B content is preferably 0.0023%, more preferably 0.0025%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0030%.

Ti: 0.020-0.10%
Titanium (Ti) forms TiN to fix N in steel. As a result, the formation of BN by B is suppressed, and the hardenability improvement effect of B is ensured. On the other hand, if the Ti content is too high, excessive TiN is formed and machinability deteriorates. Therefore, the Ti content is 0.020-0.10%. The lower limit of the Ti content is preferably 0.023%, more preferably 0.025%. The upper limit of the Ti content is preferably 0.050%, more preferably 0.040%, still more preferably 0.030%.

N: 0.020% or less Nitrogen (N) is an impurity. N reacts with B to hinder the effect of B on improving the hardenability. N further forms nitrides to reduce machinability and fatigue strength. Therefore, the lower the N content, the better, and in this embodiment, it is limited to 0.020% or less. The upper limit of the N content is preferably 0.010%, more preferably 0.005%, still more preferably 0.004%. On the other hand, an attempt to extremely reduce the N content leads to an increase in cost. The lower limit of the N content is preferably 0.001%.

Al: 0.10% or less Aluminum (Al) is an impurity. Al forms inclusions and nitrides and lowers machinability and fatigue strength. Therefore, the smaller the Al content, the better, and in the present embodiment, the Al content is limited to 0.10% or less. The upper limit of the Al content is preferably 0.010%, more preferably 0.005%, still more preferably 0.004%. On the other hand, an attempt to extremely reduce the Al content leads to an increase in cost. The lower limit of Al content is preferably 0.001%.

The balance 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.

[Quenching Hardened Layer]
The crankshaft according to this embodiment has a hardened layer on its surface. The quench-hardened layer preferably has a martensite ratio of 80% by volume or more. The martensite referred to here includes tempered martensite.

The crankshaft according to the present embodiment may have a quench-hardened layer formed only on the surface, or may be quenched to the core. When the quench-hardened layer is formed only on the surface, the thickness of the region having a martensite ratio of 80% by volume or more is preferably 1.0 mm or more. In this case, the ratio of martensite in the region from the surface of the crankshaft to 1.0 mm is preferably 90% by volume or more, more preferably 95% by volume or more, and still more preferably 99% by volume or more. The thickness of the region in which the proportion of martensite is 80% by volume or more is more preferably 2.0 mm or more, and still more preferably 3.0 mm or more. The quench-hardened layer need not be formed on the entire crankshaft, and may be formed only on a portion (for example, a pin or journal portion).

[Amount of solid solution in quench hardened layer]
In the crankshaft according to the present embodiment, Si, Mn, Cr, and Mo dissolved in the quench-hardened layer satisfy the following formula (1).
_1.960SiM + _1.002MnM + _1.059CrM + _{0.574MoM≤0.75} (1)
Si _M , Mn _M , Cr _M , and Mo _M in formula (1) are respectively substituted with the amounts of Si, Mn, Cr, and Mo dissolved in the quench-hardened layer in mass %.

In the crankshaft according to the present embodiment, Si, Mn, Cr, Mo, C, S, B, Ti, and N dissolved in the quench-hardened layer preferably satisfy the following formula (2).
1.960 Si _M + 1.002 Mn _M + 1.059 Cr _M + 0.574 Mo _M + 4.585 CM + 1.717 SM + 5.094 B _M + 1.150 Ti _M + _{3.930 N M ≤ 2.00} (2)
_SiM , _MnM , _CrM , _MoM , CM, _SM , _BM , _TiM , and _NM in formula (2) respectively contain Si, _Mn , and Cr dissolved in the quench-hardened layer. , Mo, C, S, B, Ti, and N are substituted in mass %.

The greater the amount of elements dissolved in the quench-hardened layer, the lower the thermal conductivity of the quench-hardened layer. The effect of solid solution on thermal conductivity is proportional to the number of atoms in solid solution. Therefore, even if the mass is the same, the smaller the atomic weight of the element, the greater the effect on the thermal conductivity. Therefore, it is appropriate to evaluate the amount of dissolved elements in terms of atomic percent rather than mass percent. 1.960, 1.002, 1.059, 0.574, 4.585, 1.717, 5.094, 1.150, and 3.930 in the above formulas are weight percent to atomic percent is the conversion factor.

That is, the solid solution amounts of Si, _Mn , Cr, _Mo , C, S, B, Ti, and N expressed in mass% SiM, _MnM , _CrM , MoM, _CM , _SM , _BM , Ti _M , and NM, and the solid solution amounts of Si, Mn, Cr, Mo, C, S, B, Ti, and N expressed in atomic % Si _A , _{Mn A} , Cr _A , Mo _A , C The relationship between _A , S _A , _{BA , Ti A and NA} can be approximately expressed as follows.
_SiA + _MnA + _CrA + _MoA = _1.960SiM + _1.002MnM + _1.059CrM + _0.574MoM
Si _A + Mn _A + Cr _A + Mo _A + CA + SA + B _A + Ti _A + NA = _1.960 Si _M + 1.002 _{Mn M + 1.059 Cr M + 0.574 Mo M + 4.585 CM + 1.717} SM + 5.094 _{B M} Ti + 1.150 _M + 3.930 N _M

Strictly speaking, in order to calculate the amount of solid solution represented by atomic %, it is necessary to use the total amount of substances of all the elements contained in the steel material as the denominator. However, in this embodiment, most of the chemical composition is Fe, and even if the content of other elements changes, the total amount of substances does not change greatly. In obtaining the above coefficients, the total amount of substances contained in 100 g of steel is fixed at 1.816, and [Amount of solid solution expressed in atomic %] = 100/{[Atomic weight of each element] x 1.816. 816}×[Amount of solid solution represented by mass %]. In addition, the atomic weights of Si, Mn, Cr, Mo, C, S, B, Ti, and N are 28.09, 54.94, 52.00, 95.94, 12.01, 32.07, and 10, respectively. .81, 47.88, and 14.01.

If the value of the left side of formula (1) is set to 0.75 or less, high thermal conductivity can be obtained, and excellent seizure resistance can be obtained. The value of the left side of formula (1) is preferably 0.73 or less, more preferably 0.71 or less, even more preferably 0.70 or less, and even more preferably 0.68 or less. In the crankshaft according to this embodiment, it is more preferable to set the left side of equation (2) to 2.00 or less. The value of the left side of formula (2) is more preferably 1.98 or less, still more preferably 1.95 or less, still more preferably 1.93 or less, still more preferably 1.90 or less.

_SiM , _MnM , _CrM , _MoM , _CM , _SM , _BM , _TiM , and _NM are measured as follows.

First, the ratio (% by mass) of Si, Mn, Cr, Mo, C, S, B, Ti, and N in a non-dissolved state in the quench hardened layer is determined by electrolytic extraction residue measurement. 1 to 5 are flow diagrams of the analysis of electrolytic extraction residue.

FIG. 1 is a flow diagram of a method for analyzing Mn, Cr, Mo, Ti, and Si from electro-extraction residue. First, a sample for electrolytically extracted residue measurement is taken from a portion where a quench hardened layer is formed. For example, if the steel material to be measured has a cylindrical shape with a diameter of 47.9 mm, the sample to be collected can be a cylindrical shape with a height of 12.0 mm and a diameter of 47.9 mm obtained by cutting a part of the steel material. The cross section of this sample is masked to dissolve only the surface layer. Specifically, a constant current electrolysis method using a 10% by mass acetylacetone-1% by mass tetramethylammonium chloride/methanol solution (AA solution) as an electrolytic solution was performed at a current density of 20 mA/cm ² to a portion from the surface to 40 μm. is anodically dissolved (AA electrolysis). The solution is filtered through a filter with a pore size of 0.2 μm. Dissolve the filter with acid and collect the residue.

In order to determine the ratio (% by mass) of Mn, Cr, Mo, and Ti in a non-dissolved state, the recovered residue is decomposed by an acid decomposition method. Specifically, mixed acid containing H ₂ SO ₄ , HNO ₃ , H ₃ PO ₄ , HClO ₄ and H ₂ O is used and heated at 350° C. for 1 hour. Masses of Mn, Cr, Mo, and Ti are determined by ICP emission spectroscopy.

In order to obtain the proportion (% by mass) of Si in a non-dissolved state, the recovered residue is heated to 1000° C. or higher with a gas burner to melt it. The mass of Si in the melted residue is quantified by ICP emission spectroscopy.

FIG. 2 is a flow diagram of a method for analyzing S from the electro-extraction residue. The process of filtering the AA-electrolyzed solution through a filter with a pore size of 0.2 μm to recover the residue, and subjecting the recovered residue to acid decomposition is the same as the measurement of Mn, Cr, Mo, and Ti. In the determination of S, the acid-decomposed solution is further filtered through a filter with a pore size of 0.2 μm, the residue not decomposed by acid decomposition is recovered, and the mass of S is quantified by distillation separation methylene blue spectrophotometry. Distillation separation methylene blue absorption spectrophotometry shall be performed in accordance with JIS G 1215.

FIG. 3 is a flow diagram of a method for analyzing B from the electrolytic extraction residue. The process of filtering the AA-electrolyzed solution through a filter with a pore size of 0.2 μm to recover the residue, and subjecting the recovered residue to acid decomposition is the same as the measurement of Mn, Cr, Mo, and Ti. For determination of B, the acidolyzed solution is further distilled. Distillation shall be performed in accordance with JIS G 1227 Annex 2. After distillation, the mass of B is quantified by ICP emission spectrometry, not in accordance with JIS G 1227.

FIG. 4 is a flow diagram of a method for analyzing N from the electrolytic extraction residue. The process of filtering the AA-electrolyzed solution through a filter with a pore size of 0.2 μm to recover the residue, and subjecting the recovered residue to acid decomposition is the same as the measurement of Mn, Cr, Mo, and Ti. In the determination of N, the mass of N is determined by distillative separation amidosulfuric acid titration of the acid-decomposed solution. This method shall comply with JIS G 1228 Annex 1.

FIG. 5 is a flow diagram of a method for analyzing C from the electrolytic extraction residue. In the measurement of C, the AA electrolyzed solution is filtered through a glass filter with a pore size of 0.2 μm to recover the residue, and the mass of C is quantified by the high-frequency combustion infrared absorption method. The high-frequency combustion infrared absorption method is performed in accordance with JIS G 1211-3:2018 (Iron and steel-Carbon quantification method-) 8.2 (Quantitative operation) b). Specifically, the glass filter is placed in a furnace and a combustion improver (a mixture of tungsten and tin (tungsten:tin = 9:1) is added. Add pure iron.) and burn the carbon on the glass filter to CO and _CO2 . The detected amounts of CO and CO ₂ are integrated, and the amount of carbon is obtained by comparing the integrated amount with a previously obtained calibration curve.

These masses are divided by the mass of the electrolyzed sample to obtain the proportions (% by mass) of Si, Mn, Cr, Mo, C, S, B, Ti, and N in non-dissolved states. From the difference between the respective contents (% by mass) of Si, Mn, Cr, Mo, C, S, B, Ti, and N in the chemical composition of the steel and the above values, Si _M , Mn _M , Cr _M , Determine _MoM , _CM , _SM , _BM , _TiM , and _NM .

_SiM , _MnM , _CrM , _MoM , _CM , _SM , _BM , _TiM , and _NM can each be adjusted by the chemical composition of the steel material. That is, if the contents of Si, _Mn , Cr, _Mo , C, S, B, Ti, and N in the steel are reduced, SiM, _MnM , _CrM , MoM, _CM , _SM , B _M , Ti _M , and _NM also become smaller.

_SiM , _MnM , _CrM , and _MoM also vary with precipitates and inclusions. Therefore, it varies depending on the content of C, N, and S that react with these elements and the heat treatment. For example, when the S content is high, _Mn tends to react with S to precipitate MnS and MnS ₂ and MnM tends to decrease. Also, for example, if the residence time at around 600° C. is lengthened during cooling from hot working, or if tempering is performed in this temperature range, carbides precipitate and Cr _M and Mo _M decrease. Further, when cementite is precipitated, Mn and Cr form a solid solution in cementite, so the values of Mn 2 _M and Cr 2 _M become smaller.

[Ideal critical diameter]
The crankshaft according to this embodiment preferably has an ideal critical diameter Di calculated from the chemical composition according to ASTM A255-67 of 1.7 or more.

The ideal critical diameter Di is the critical diameter (50% martensitic The diameter of the steel material that becomes (unit: inch)), and is used as an index of hardenability. If the ideal critical diameter Di is 1.7 or more, the hardenability required to secure the fatigue strength required for crankshafts can be obtained. The ideal critical diameter Di is more preferably 1.9 or greater, more preferably 2.0 or greater, more preferably 2.1 or greater, and more preferably 2.2 or greater.

The ideal critical diameter Di is calculated from the chemical composition of the crankshaft according to ASTM A255-67 as follows.

From the table of ASTM A255-67, numerical values C _D , Mn _D , Si _D , Cr _D , corresponding to each content (% by mass) of C, Mn, Si, Cr, Mo, P, S, Al, and Ti Determine Mo _D , P _D , S _D , Al _D , and Ti _D . For carbon, grain size no. 8 is used. The numbers in the ASTM A255-67 table are interpolated by applying a first order approximation. The interpolation formula is as follows.
C _D =0.6×C+0.056
_MnD = 0.352 x Mn + 0.274
SiD = _0.229 x Si + 0.009
_CrD = 0.727 x Cr + 0.013
_MoD = 0.9 x Mo + 0.026
P _D =1.1×P
S _D =−0.3×S+0.001
Al _D =2×Al
Ti _D = −0.85 × Ti
Here, the content of the corresponding element is substituted for C, Mn, Si, Cr, Mo, P, S, Al, and Ti in each of the above formulas in mass %.

A total sum SUM of the calculated numerical values C _D , Mn _D , Si _D , Cr _D , Mo _D , P _D , S _D , Al _D and Ti _D is obtained. From the table of ASTM A255-67, obtain Di (provisional) corresponding to the total sum SUM. The numbers in the ASTM A255-67 table are interpolated by applying a first order approximation. The interpolation formula is as follows.
Di (provisional) = 2.408 × SUM - 1.30

For crankshafts with a B content of 0.0010% or more, Di is the product of Di (provisional) and the following boron factor (Bolon F). For crankshafts with a B content of less than 0.0010%, Di (temporary) is used as Di.
Bolon F=2.616-1.6×C
The C content is substituted for C in the above formula in terms of % by mass.

[Thermal conductivity]
In the crankshaft according to the present embodiment, the heat conductivity of the quench-hardened layer at room temperature is preferably 45 W/m·K or more.

Thermal conductivity is measured as follows. A disc-shaped sample having an outer diameter of 10 mm and a thickness of 1 mm is taken from the hardened layer. The density of the sample is calculated from the volume and mass of the sample. The specific heat and thermal diffusivity of the sample are determined by the laser flash method specified in JIS R 1611. The thermal conductivity of the sample is determined as the product of density, specific heat, and thermal diffusivity.

[Crankshaft manufacturing method]
An example of a method for manufacturing a crankshaft according to this embodiment will be described below. The manufacturing method described below is merely an example, and does not limit the manufacturing method of the crankshaft according to the present embodiment.

A material having the chemical composition described above is prepared. The material is, for example, a steel bar. The raw material can be produced, for example, by continuously casting or blooming molten steel having the above chemical composition.

The material is hot forged into the rough shape of the crankshaft. Hot forging may be divided into rough forging and finish forging. If necessary, heat treatment such as normalizing may be performed on the crankshaft crude product manufactured by hot forging.

Machine the crankshaft blank. Machining includes cutting, grinding, drilling, and the like. This process produces an intermediate product having a shape close to the final product.

Quench the intermediate part of the machined crankshaft. This forms a hardened layer on the surface. Specifically, it is rapidly cooled after being heated to a predetermined heating temperature. At this time, the intermediate product may be locally heated by a high-frequency induction heating device, or the entire intermediate product may be heated by a heat treatment furnace. The heating temperature is preferably Ac ₃ or higher, more preferably 900° C. or higher. After quenching, tempering may be performed as necessary. Quenching and tempering may be performed multiple times, and other heat treatments may be performed in addition to quenching and tempering.

The intermediate product on which the quench-hardened layer is formed is subjected to finishing as necessary. For example, crankshaft journals and pins are ground or lapped to adjust their surface profile. Through the steps described above, the crankshaft according to the present embodiment can be manufactured.

A crankshaft according to one embodiment of the present invention has been described above. According to this embodiment, a crankshaft having excellent seizure resistance can be obtained.

EXAMPLES The present invention will be described in more detail below with reference to examples. The invention is not limited to these examples.

Using steel having the chemical composition shown in Table 1 as a raw material, a test shaft for a seizure test was produced. FIG. 6 shows the heat pattern when the test shaft was produced.

Specifically, after heating the raw material at 1250° C. for 1 hour, hot forging was performed at 1100 to 850° C. After completion of forging, the material was allowed to cool to room temperature. Then, after normalizing by heating at 1250° C. for 20 minutes and air cooling, the outer diameter was reduced to 47.9 mm by machining (cutting). After that, induction hardening was performed to form a hardened layer having a surface hardness of HV650.

The hardenability of the material was evaluated by the ideal critical diameter Di. Specifically, if the ideal critical diameter Di is 1.7 or more, which is about 90% of the conventional steel (Di≈1.98), it is determined that the crankshaft has the necessary hardenability. In calculating Di, less than 0.002% by mass of Si, less than 0.002% by mass of Mn, and less than 0.002% by mass of Mo were treated as 0%. Table 2 shows the progress of calculation of Di.

According to the method described in the embodiment, the amounts of Si, Mn, Cr, Mo, C, S, B, Ti, and N (Si _M , Mn _M , Cr _M , Mo _M , C _M , _{SM , BM , Ti M , and NM} ) were measured. Those with a Si content of less than 0.002 mass% have 0% SiM, those with a _Mn content of less than 0.002 mass% have 0% _MnM , and those with a Mo content of less than 0.002 mass%. were treated as 0% MoM, those with a B content of less than _0.0002 % by mass were treated as having 0% _BM , and those with a Ti content of less than 0.002% by mass were treated as having 0% _TiM . On the other hand, regarding N with a small atomic weight, NM was obtained without treating _0.0040 % by mass as 0%.

The thermal conductivity of the base material (part other than the quench-hardened layer) and the quench-hardened layer was measured according to the method described in the embodiment.

Table 3 shows the results. Columns F1 and F2 in Table 3 describe the values of the left sides of equations (1) and (2), respectively. In addition, "-" in each column means that the corresponding measurement was not performed.

As shown in Table 3, No. The test shafts of 4 and 5 had a high thermal conductivity of 45 W/mK or more while having the same ideal critical diameter Di as conventional steel (No. 1).

No. The hardenability of test shaft No. 2 was within the allowable range, but no. Thermal conductivity was inferior compared to 4 and 5. This is probably because the amount of dissolved Mn was too large.

No. The test shaft of No. 3 had high thermal conductivity but poor hardenability. This is the No. This is probably because the test shaft of No. 6 did not contain Mo and B.

No. The test shaft of No. 6 had high thermal conductivity but poor hardenability. This is No. This is probably because the test shaft of 6 did not contain B.

No. The test shaft of No. 7 had high thermal conductivity but poor hardenability. This is No. This is probably because the test shaft of No. 7 did not contain Mo.

No. The test shaft of No. 8 had high thermal conductivity but poor hardenability. This is the No. This is probably because the Mn content of the test shaft of No. 8 was too low.

Next, No. Seizure tests were performed using 2, 4, and 5 test shafts. As a finishing process, the test shafts subjected to the seizure test were subjected to grinding and lapping so that the arithmetic mean height Pa of the cross-sectional curve was 0.07 to 0.09 μm. The outer diameter of the test shaft was adjusted so that the clearance with the bearing used in the seizure test described later was about 0.090 mm.

The seizure test was performed using a crank metal abrasion resistance seizure evaluation device manufactured by Shinko Engineering Co., Ltd. A schematic diagram of the evaluation device 20 is shown in FIG. The test shaft TP was inserted into a plurality of bearings 21 , and the test shaft TP was rotated at 8000 rpm by a motor (not shown) while lubricating the bearings 21 . Al alloy of HV40-50 was used for the metal of the bearing. The lubricating oil was 0W-20, the oil supply temperature was 140° C., and the oil pressure was 0.8 MPa.

In this state, one of the bearings 21 was pulled down to gradually increase the surface pressure applied to the test shaft TP, and the test shaft TP was operated until seizure occurred. FIG. 8 schematically shows changes over time in the surface pressure applied to the test shaft TP. The holding time at the same surface pressure was 3 minutes, the surface pressure increase width per step was 4.35 MPa, and the surface pressure increase time was 15 seconds. It was determined that seizure occurred when the surface temperature of the test shaft TP reached 280° C. or higher, or when the torque applied to the test shaft reached 25 Nm or higher. Three tests were performed for each test shaft, and the average value was taken as the seizure surface pressure.

The results are shown in Table 3 and FIG. 9 above. FIG. 9 is a graph showing the relationship between the seizure contact pressure of the test shaft and the thermal conductivity of the hardened layer at room temperature. As shown in FIG. 4 and 5 test axes are no. It was confirmed to have excellent seizure resistance compared to the test shaft of No. 2.

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 can be implemented by appropriately modifying the above-described embodiment without departing from the scope of the invention.

Claims (4)

The chemical composition, in mass %,
C: 0.35 to 0.50%,
Si: 0.55% or less,
Mn: 0.01-0.55%,
Cr: 0.10 to 0.50%,
P: 0.03% or less,
S: 0.10% or less,
Mo: 0.10-0.30%,
B: 0.0020 to 0.0050%,
Ti: 0.020 to 0.10%,
N: 0.020% or less,
Al: 0.10% or less,
Balance: Fe and impurities,
Having a hardened layer on the surface,
A crankshaft in which Si, Mn, Cr, and Mo dissolved in the quench-hardened layer satisfy the following formula (1).
_1.960SiM + _1.002MnM + _1.059CrM + _{0.574MoM≤0.75} (1)
Si _M , Mn _M , Cr _M , and Mo _M in formula (1) are respectively substituted with the amounts of Si, Mn, Cr, and Mo solid-soluted in the quench-hardened layer in mass %. A crankshaft according to claim 1,
A crankshaft, wherein Si, Mn, Cr, Mo, C, S, B, Ti, and N dissolved in the quench-hardened layer satisfy the following formula (2).
1.960 Si _M + 1.002 Mn _M + 1.059 Cr _M + 0.574 Mo _M + 4.585 CM + 1.717 SM + 5.094 B _M + 1.150 Ti _M + _{3.930 N M ≤ 2.00} (2)
_SiM , _MnM , _CrM , _MoM , CM, _SM , _BM , _TiM , and _NM in formula (2) respectively contain Si, _Mn , and Cr dissolved in the quench-hardened layer. , Mo, C, S, B, Ti, and N are substituted in mass %. 3. A crankshaft according to claim 1 or 2,
A crankshaft having an ideal critical diameter Di calculated from the chemical composition according to ASTM A255-67 of 1.7 or more. The crankshaft according to any one of claims 1 to 3,
A crankshaft, wherein the quench-hardened layer has a thermal conductivity of 45 W/mK or more at room temperature.

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