Patent Application
20240392401
The present invention relates to a steel material for a sliding part and a method of manufacturing a steel material for a sliding part.
Steel materials are widely used in industrial products such as automotive parts, railroad vehicle parts, construction components, and pipes. In particular, carbon steel materials for mechanical structures and alloy steel materials for mechanical structures are often used as materials for sliding parts such as power transmission system parts, like gears and shafts, due to their high mechanical strengths.
The biggest problem with sliding parts is friction/wear between the parts, which is considered to cause failures and reductions in efficiency in the entire mechanical system. Environments for sliding parts are expected to become even harsher, as mechanical systems become smaller and lighter. For example, in crankshafts, which are automobile engine parts, improving the seizure resistance of the rotating sliding portion is a permanent issue, along with downsizing and weight reduction. To solve these problems, it is necessary to develop a steel material for a sliding part with higher slidability than the current ones, and to prepare for the downsizing and weight reduction of entire mechanical systems.
One of the problems with steel materials for sliding parts that are to be solved is how to improve wear resistance to extend part life and improve reliability. To improve the wear resistance of a steel material, it is considered to be effective to increase its hardness. However, increasing the hardness of a steel material impairs its workability which poses a risk when the part is to be mass produced. Accordingly to improve the slidability of a sliding part, it is effective to selectively adjust its microstructure by targeting only the surface layer to harden only this portion.
For example, JP H1-230746 A discloses a sliding part including a fixed member made of cast iron and a slidable member made of a material with a higher hardness than the cast iron, where the microstructure of the surface layer of the fixed member is composed of (a) a hardened layer including martensite or a mixed-phase structure of martensite, pearlite, ferrite and graphite, and (b) oxides.
Instead of regulating the hardness of a steel material, its seizure resistance may be improved by adjusting precipitates therein to prevent adhesion. JP 2013-227674 A discloses a gear including a surface layer having a steel microstructure in which retained austenite is present in tempered martensite and/or tempered bainite in an area ratio ranging from 1 to 10% with carbide precipitates in an area ratio not lower than 5%, and where the nitrogen concentration at a depth of 20 μm from the surface is 2.0 to 6.0%.
JP 2010-100881 A discloses a carburized or carbonitrided sliding part including a surface layer from the top of the sliding surface down to the depth of 10 μm, where the Vickers hardness at the depth of 10 μm from the top of the sliding surface is 700 or higher, the average particle diameter of cementite particles is 0.6 μm or smaller, the number density of cementite particles in a cross section perpendicular to the sliding surface is 1/μm2 or higher.
In sliding parts such as rotating shafts or crankshafts, it is important that the sliding parts function with reduced friction and wear on their surfaces and reduced damage thereto such as excessive load-induced mechanical damage or thermal cracking. Hardness may be increased to improve wear resistance, but this could impair workability. In addition, to prevent seizure in a sliding part, it is important to reduce the likelihood of adhesion.
An object of the present invention is to provide a steel material for a sliding part with improved slidability and workability. Another object of the present invention is to provide a method of manufacturing a steel material for a sliding part with improved slidability and workability.
A steel material for a sliding part according to an embodiment of the present invention is a steel material for a sliding part including a steel material with a C content of 0.30 to 0.60 mass %, having a microstructure including (a) at least one of tempered martensite and bainite and (b) an iron carbide, a volume fraction of the tempered martensite and the bainite combined being not lower than 80% and a volume fraction of the iron carbide being not lower than 2.0%, a Vickers hardness being not lower than 300 and not higher than 600, the volume fraction of the iron carbide, X, and the Vickers hardness, Hv, satisfy the following relational expression, (1):
where X is in % and Hv is in Hv.
A steel material for a sliding part according to an embodiment of the present invention may have a chemical composition of, in mass %: 0.30 to 0.60% C; 0.01 to 2.00% Si; 0.10 to 2.00% Mn; up to 0.060% Al; up to 0.020% N; up to 0.10% P; up to 0.20% S; 0 to 0.50% Cr; and balance Fe and impurities.
A method of manufacturing a steel material for a sliding part according to an embodiment of the present invention is a method of manufacturing any one of the above-described steel materials for a sliding part, including: holding a material at a temperature not lower than 830° C. and not higher than 1100° C. and then quenching the material by cooling at a cooling rate not lower than 300° C./s in a range from the holding temperature down to 300° C.; and tempering the quenched material by holding at a temperature not lower than 200° C. and not higher than 600° C.
The present invention provides a steel material for a sliding part with improved slidability and workability.
To develop a steel material with good slidability and workability the present inventors investigated the slidability and workability of steel materials, and obtained the following findings.
The topographic image of
This suggests that increasing the volume fraction of iron carbides will improve seizure resistance. On the other hand, increasing the volume fraction of iron carbides may decrease the hardness of the steel material and decrease its wear resistance.
In
where X is in % and Hv is in Hv.
The present invention was made based on the above-described findings. Now, a steel material for a sliding part according to embodiments of the present invention will be described in detail.
A steel material for a sliding part according to the present embodiments includes a steel material with a C content of 0.30 to 0.60 mass %. There is a tendency that the higher the C content, the higher the volume fraction of carbides. Further, there is a tendency that the higher the C content, the higher the Vickers hardness of the steel for a sliding part. If C content is outside the range of 0.30 to 0.60 mass %, it will be difficult to satisfy the relational expression (1) for the volume fraction of iron carbides and Vickers hardness or, even if the relational expression (1) is satisfied, it may not be possible to obtain a steel material with the right balance between slidability and workability. A lower limit of C content of the steel material for a sliding part according to the present embodiments is preferably 0.32 mass %, more preferably 0.35 mass %, yet more preferably 0.38 mass %, and still more preferably 0.40 mass %. An upper limit of C content of the steel material for a sliding part according to the present embodiments is preferably 0.58 mass %, and more preferably 0.55 mass %.
The chemical composition of the steel material for a sliding part according to the present embodiments is only required to have a C content of 0.30 to 0.60 mass % and there are no other limitations, although the material may, for example, have such a chemical composition as described below. In the following description, “%” for the content of an element means mass %.
Carbon (C) increases the hardenability of steel. As mentioned above, if C content is outside the appropriate range, it will be difficult to satisfy the relational expression (1) for the volume fraction of iron carbides and Vickers hardness or, even if the relational expression (1) is satisfied, it may not be possible to obtain a steel material with the right balance between slidability and workability. In view of this, C content is to be 0.30 to 0.60%. A lower limit of C content is preferably 0.32%, more preferably 0.35%, yet more preferably 0.38%, and still more preferably 0.40%. An upper limit of C content is preferably 0.58%, and more preferably 0.55%.
Silicon (Si) deoxidizes steel. On the other hand, if Si content is too high, this decreases the workability of the steel. In view of this, Si content may be 0.01 to 2.00%. A lower limit of Si content is preferably 0.02%, more preferably 0.05%, and yet more preferably 0.10%. An upper limit of Si content is preferably 1.50%, more preferably 1.20%, yet more preferably 0.80%, still more preferably 0.60%, and yet more preferably 0.40%.
Manganese (Mn) increases the hardenability of steel. On the other hand, if Mn content is too high, this decreases the workability of the steel. In view of this, Mn content may be 0.10 to 2.00%. A lower limit of Mn content is preferably 0.20%, more preferably 0.40%, and yet more preferably 0.60%. An upper limit of Mn content is preferably 1.80%, more preferably 1.60%, yet more preferably 1.50%, still more preferably 1.00%, and yet more preferably 0.90%.
Aluminum (Al) deoxidizes steel. On the other hand, if Al content is too high, this decreases the workability of the steel. In view of this, Al content may be not higher than 0.060%. An upper limit of Al content is preferably 0.050%, more preferably 0.040%, and yet more preferably 0.030%. To produce Al's deoxidizing effect, Al content may be not lower than 0.020%.
Nitrogen (N) decreases the hot workability of steel. In view of this, N content may be not higher than 0.020%. An upper limit of N content is preferably 0.018%, more preferably 0.015%, yet more preferably 0.010%, and still more preferably 0.005%. On the other hand, excessively restricting N content leads to increased manufacturing costs. In view of this, a lower limit of N content may be 0.0010%.
Phosphorus (P) is an impurity. P segregates on the grain boundaries, thus decreasing the hot workability and/or toughness of the steel. In view of this, P content may be not higher than 0.10%. P content is preferably not higher than 0.03%, and more preferably not higher than 0.02%. Preferably P content is as low as possible.
Sulfur (S) may be added to steel to increase its workability (i.e., machinability). On the other hand, if S content is too high, this decreases the quench cracking resistance of the steel. In view of this, S content may be not higher than 0.20%. An upper limit of S content is preferably 0.12%, more preferably 0.08%, and yet more preferably 0.06%. To produce S's effect of improving workability S content may be not lower than 0.020%.
Chromium (Cr) is an optional element. In other words, the steel material for a sliding part according to the present embodiments need not contain Cr. Cr increases the hardenability of steel. This effect is produced if a small amount of Cr is contained in the steel. On the other hand, if Cr content is too high, this decreases the workability of the steel. In view of this, Cr content may be 0 to 0.50%. A lower limit of Cr content is preferably 0.01%, and more preferably 0.05%. An upper limit of Cr content is preferably 0.20%.
The balance of the chemical composition of the steel material for a sliding part according to the present embodiments may be Fe and impurities. “Impurity” as used herein means an element originating from ore or scrap used as raw material for steel or an element that has entered from the environment or the like during the manufacturing process.
The steel material for a sliding part according to the present embodiments may be made of a carbon steel material for mechanical structures or an alloy steel material for mechanical structures. The steel material for a sliding part according to the present embodiments is preferably made of a carbon steel material for mechanical structures defined in JIS G 4051:2016 or an alloy steel material for mechanical structures defined in JIS G 4053:2016. S45C and S50C in JIS G 4051:2016 and SMn438 in JIS G 4053:2016 are particularly preferable. Furthermore, such a steel material may further contain up to 0.20 mass % S to improve workability (i.e., machinability).
The microstructure of the steel material for a sliding part according to the present embodiments includes (a) at least one of tempered martensite and bainite (including bainite that has undergone tempering; the same applies hereinafter) and (b) an iron carbide, the volume fraction of the tempered martensite and bainite combined being not lower than 80% and the volume fraction of the iron carbide being not lower than 2.0%.
In the microstructure of the steel material for a sliding part according to the present embodiments, the sum of the volume fraction of tempered martensite and the volume fraction of bainite is not lower than 80%. The microstructure of the steel material for a sliding part according to the present embodiments is only required to include at least one of tempered martensite and bainite.
According to the present embodiments, the steel material for a sliding part is tempered to produce a microstructure containing a predetermined amount of iron carbides to provide sufficient workability to the steel material for a sliding part. In contrast, if a steel material for a sliding part had a microstructure made up of an as-quenched microstructure (i.e., microstructure mainly composed of as-quenched martensite), it would be difficult to ensure good workability. The steel material for a sliding part according to the present embodiments preferably contains tempered martensite.
If the sum of the volume fraction of tempered martensite and the volume fraction of bainite is lower than 80%, it is difficult to provide good wear resistance. The sum of the volume fraction of tempered martensite and the volume fraction of bainite is preferably not lower than 85%, more preferably not lower than 90%, and yet more preferably not lower than 95%.
For the purposes of the present embodiments, in the calculation of the volume fractions of structures, an iron carbide is treated as an independent structure, i.e., an iron carbide is distinguished from tempered martensite and bainite. That is, the volume of iron-carbide precipitates is not included in the volume of tempered martensite or bainite.
In the microstructure of the steel material for a sliding part according to the present embodiments, the volume fraction of the iron carbide is not lower than 2.0%. Specifically the iron carbide of the steel material for a sliding part according to the present embodiments is at least one of s-carbide and cementite. One iron carbide or a plurality of iron carbides may be contained in the steel material for a sliding part. If a plurality of iron carbides are contained in the material, the volume fraction of the iron carbide is the sum of the volume fractions of those iron carbides.
If the volume fraction of iron carbides is lower than 2.0%, it is difficult to provide good wear resistance. A lower limit of the volume fraction of iron carbides is preferably 3.0%, more preferably 5.0%, and yet more preferably 7.0%. An upper limit of the volume fraction of iron carbides is preferably 18.0%, more preferably 15.0%, yet more preferably 12.0%, still more preferably 10.0%, and yet more preferably 8.0%.
The volume fraction of iron carbides may be regulated by changing the C content of the steel material and tempering conditions. Specifically there is a tendency that the higher the C content, the higher the volume fraction of iron carbides. Further, regarding tempering conditions, there is a tendency that the higher the holding temperature and longer the holding time, the higher the volume fraction of iron carbides.
The microstructure of the steel material for a sliding part according to the present embodiments may include small amounts of structures other than tempered martensite, bainite, and iron carbides. Examples of structures other than tempered martensite, bainite and iron carbides include ferrite, pearlite, retained austenite, and MnS. In the microstructure of the steel material for a sliding part according to the present embodiments, the total volume fraction of the structures other than tempered martensite, bainite and carbides is preferably not higher than 5.0%, more preferably not higher than 3.0%, yet more preferably not higher than 2.0%, and still more preferably not higher than 1.0%.
The steel material for a sliding part according to the present embodiments has a Vickers hardness not lower than 300 and not higher than 600. If Vickers hardness is lower than 300, it is difficult to provide good wear resistance. On the other hand, if Vickers hardness is higher than 600, this decreases workability. To provide wear resistance, a lower limit of Vickers hardness is preferably 350, more preferably 400, yet more preferably 450, still more preferably 500, and yet more preferably 530. To provide workability an upper limit of Vickers hardness is preferably 580, more preferably 560, yet more preferably 550, still more preferably 530, and yet more preferably 520.
The Vickers hardness of the steel material for a sliding part may be regulated by changing the C content of the steel material, quenching conditions and tempering conditions. Specifically there is a tendency that the higher the C content, the higher the Vickers hardness. Regarding quenching conditions, there is tendency that the higher the cooling rate, the higher the Vickers hardness. Regarding tempering conditions, there is a tendency that the lower the holding temperature and shorter the holding time, the higher the Vickers hardness.
In the steel material for a sliding part according to the present embodiments, the volume fraction of iron carbides, X, and the Vickers hardness of the steel material, Hv, satisfy the following relational expression, (1). Satisfying the relational expression (1) provides good wear resistance.
where X is in % and Hv is in Hv.
In the steel material for a sliding part according to the present embodiments, the average minor-axis length of iron-carbide particles is preferably not larger than 0.027 μm. Distributing iron-carbide particles of such shapes enables retaining the hardness of the entire steel material. If the iron-carbide particles are too large, this increases the effects of the softness of the matrix, potentially decreasing wear resistance. The average minor-axis length of iron-carbide particles is preferably not larger than 0.025 μm.
The steel material for a sliding part according to the present embodiments preferably has none of a nitriding layer, a carburization layer and a carbonitriding layer on its surface.
In the steel material for a sliding part according to the present embodiments, it is preferable that the Vickers hardness at the surface is not lower than 300 and not higher than 600, and the volume fraction X of iron carbides and the Vickers hardness Hv at the surface satisfy the above relational expression (1).
“Vickers hardness at the surface” mentioned above means, more specifically, the Vickers hardness in a region from the top surface of the steel material for a sliding part down to the depth of 100 μm. “Volume fraction of iron carbides at the surface” means, more specifically the volume fraction of iron carbides in the microstructure in a region from the top surface of the steel material for a sliding part down to the depth of 100 μm.
A method of manufacturing a steel material for a sliding part according to the present embodiments will be described below.
A material having such a chemical composition as specified above is prepared. The material may be a hot-forged product, for example. For example, a steel having such a chemical composition as specified above may be smelted and subjected to continuous casting or blooming to produce a billet, which may be hot forged to produce a roughly shaped sliding part, which may be used as the material. The hot-forged material may be subjected to machining, for example.
The material is held at a temperature not lower than 830° C. and not higher than 1100° C. before being quenched by cooling in such a manner that the cooling rate from the holding temperature to 300° C. is not lower than 300° C./s. If the holding temperature is too low, a uniform microstructure may not be provided. On the other hand, if the holding temperature is too high, the crystal grains may coarsen. If the cooling rate is too low, a desired microstructure may not be provided. It is to be noted that there is a tendency that the higher the cooling rate during quenching, the higher the Vickers hardness of the eventual steel material for a sliding part.
The quenched material is tempered by holding it at a temperature not lower than 200° C. and not higher than 600° C. There is a tendency that the higher the holding temperature during tempering and the longer the holding time, the lower the Vickers hardness of the eventual steel material for a sliding part. There is also a tendency that the higher the holding temperature during tempering and the longer the holding time, the higher the volume fraction of iron carbides in the microstructure of the eventual steel material for a sliding part. If the holding temperature during tempering is outside of that range, it is difficult to provide iron carbides in a volume fraction and with a Vickers hardness that fall within desired ranges.
The quenching and tempering conditions are regulated depending on the chemical composition of the steel material, for example, such that the volume fraction X and Vickers hardness Hv of the iron carbides satisfy the relational expression (1). This results in a steel material for a sliding part according to the present embodiments.
A steel material for a sliding part according to embodiments of the present invention has been described. The steel material for a sliding part according to the present embodiments has improved slidability and workability. Thus, the steel material for a sliding part according to the present embodiments is suitable as a material for a sliding part. A sliding part may be, for example, a crankshaft.
The present invention will now be described more specifically with reference to examples. The present invention is not limited to these examples.
Steels having the chemical compositions shown in Table 1 were prepared, and each smelted in a 10 kg vacuum induction melting furnace to produce an ingot.
The resulting ingot was hot forged at 950 to 1200° C. so as to have a thickness of 30 mm, a width of 100 mm and a length of 290 mm, before being rolled to have a thickness of 7 mm and a width of 110 mm. The rolled material was cut into a width of 15 mm, a length of 60 to 120 mm and a thickness of 7 mm; then, a heat treatment shown in Table 2 was performed. The microstructure of each of the materials before the heat treatment was ferrite/pearlite (F+P). A value in the column labeled “Cooling rate” for “Quenching” in Table 2 is a cooling rate from the holding temperature for quenching to 300° C.
From each of the materials after the heat treatment, a plurality of test specimens, each 20 mm square and 2 mm thick, were taken. These test specimens were used to observe their microstructure, measure Vickers hardness and evaluate slidability.
For each test specimen for microstructure observation, the surface was treated by Ar-ion milling. An Ar-ion beam was directed to a sample at an angle of 800 or larger relative to the vertical direction to abrade iron matrix, which is softer than iron carbides, such that the iron carbides remained as protrusions, making it possible to search for the iron carbides with atomic-force microscopy (AFM). An exemplary surface topographic image of a treated test specimen, obtained with AFM, is shown in
The volume fraction of iron carbides was calculated by obtaining topographic images of three areas of each test specimen, 2 μm by 2 μm each, by AFM and then using image analysis software for calculation. The image analysis software used was ImageJ. The contrast and resolution of each image were regulated to enable detection of grain size with the image analysis software before the image was binarized, and grains were detected using the grain-analysis function of the image analysis software.
Exemplary iron carbides detected by the image analysis software are shown in
The volume fraction of ferrite was calculated using a secondary electron image (i.e., topographic image) from SEM. The surface of each sample was nital etched to etch ferrite only to create recesses, and a secondary electron image was obtained at a magnification of 1000 times. The obtained image was loaded into the image analysis software ImageJ, and a relevant region was selected using the “freehand selection” and/or “polygon selection” functions, the selected area was masked before being binarized as with iron carbides, and the relevant region was captured using the grain analysis function of the image analysis software. The area ratio of ferrite was calculated for each of three observed areas and the average thereof was determined. The obtained area ratio was treated as the volume fraction of ferrite.
The volume fraction of retained austenite was measured by X-ray diffraction. The volume fraction of MnS was determined by photographing the surface of each test specimen by optical microscopy (at a magnification of 210 times and for a view area of 1218 μm by 1218 μm) and determining the area ratio of MnS, which was treated as the volume fraction. The sum of the volume fraction of tempered martensite and the volume fraction of bainite (or the sum of the volume fraction of martensite and the volume fraction of bainite) was determined by subtracting the sum of the volume fractions of iron carbides, retained austenite, ferrite and MnS from 100%.
The geometry of iron-carbide particles was determined by image analysis of the topographic images obtained for determining the volume fraction of iron carbides. Specifically, the image was binarized, and the particle-analysis function of the image analysis software was used to conduct elliptic approximation for all the particles in each viewing field being observed. The average minor-axis length and the average major-axis length for each of the three observed areas were determined, and the average thereof was determined.
The Vickers hardness was determined by measuring at five points with a test force of 1 kgf (9.807 N) and determining the average thereof.
The microstructure and Vickers hardness of each steel material after the heat treatment are shown in Table 3. In the columns for the volume fraction of the microstructure in Table 3, “M” means as-quenched martensite, “B” means bainite, “TM” means tempered martensite, and “Retained y” means retained austenite.
The surface of each test specimen for sliding testing was mirror finished. The sliding testing was conducted by ball-on-disc friction/wear testing equipment.
For each steel material, the Vickers hardness, the volume fraction of the carbides (i.e., iron carbides) and the result of sliding testing are shown in
As shown in Table 4, for each of the steel materials labeled Nos. 4 to 7, 9, 10 and 14 to 17, the Vickers hardness was 300 to 600, and the volume fraction X of iron carbides and the Vickers hardness Hv satisfied the relational expression (1). Each of these test specimens had a wear-mark width after sliding testing not larger than 160 μm, meaning good wear resistance. Further, each of these test specimens had a Vickers hardness Hv not higher than 600, meaning good workability.
For each of the steel materials labeled Nos. 1 to 3, 8, 12 and 13, the wear-mark width after sliding testing exceeded 160 μm. This is presumably because the volume fraction X of iron carbides and the Vickers hardness Hv did not satisfy the relational expression (1).
The steel material labeled No. 11 had an as-quenched microstructure. The steel material labeled No. 11 had good wear resistance but a Vickers hardness Hv exceeding 600, meaning poor workability.
Embodiments of the present invention have been described. The above-described embodiments are merely illustrative examples useful for carrying out the present invention. Thus, the present invention is not limited to the above-described embodiments, and the above-described embodiments, when carried out, may be modified as appropriate without departing from the spirit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-181882 | Nov 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/039894 | 10/26/2022 | WO |
