Steel sliding materials and sliding members with excellent high-temperature sliding characteristics and thermal shock resistance

Abstract

To obtain an iron-based sliding material capable of maintaining a high friction coefficient even under a high temperature environment.SOLUTION: An iron-based sliding material comprising: C: 0.15 mass% or more and 0.30 mass% or less, Si: 0.25 mass% or more and 1.0 mass% or less, Mn: 0.30 mass% or more and 1.0 mass% or less, Ni: less than or equal to 1.7 mass%, Cr: 0.7 mass% or more and less than 1.3 mass%, Mo: 0.4 mass% or more and 1.60 mass% or less, V: 0.40 mass% or less, Nb: 0.12 mass% or less, and Al: 0.09 mass% or less, a carbon equivalent Ceq represented by a following formula (1) is 0.65 mass% or more, the residue is made of iron and impurities, Ceq (% by mass)=C+Si / 24+Mn / 6+Cr / 5+Ni / 40+Mo / 4+V / 14 (1), and a total content of Mn and Ni is 1.4 mass% or more and 2.2 mass% or less is used.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a steel material for high-temperature sliding members such as brake discs used in railway vehicles with high braking loads and the like.

Background Art

[0002] A disc brake used for braking of railway vehicles, automobiles, airplanes, etc. presses a brake pad against a brake disc that rotates together with an axle and a wheel, and converts the kinetic energy of the wheel and the brake disc into thermal energy generated by friction. Therefore, since a high-temperature sliding material such as a brake disc receives a strong frictional force on its surface while rotating at high speed, it is required to have not only the strength and toughness generally required for vehicle structural materials, but also excellent wear resistance and heat shock resistance. As steel-based materials for brake discs, various materials added with many elements have been proposed in order to satisfy such characteristics in a well-balanced manner.

[0003] For example, there is a steel for a brake disc as described in Patent Document 1. This material contains, by mass%, 0.1 to 0.6% of C, 0.01 to 1.2% of Si, 0.2 to 2.0% of Mn, 0.8 to 3.0% of Ni, 1.3 to 5.0% of Cr, 0.1 to 3.0% of Mo, 0.005 to 0.5% of V, the balance being composed of iron and impurities, and the amounts of C, Cr, Mo, and V satisfy a predetermined relational expression for the purpose of controlling the amount of carbide.

[0004] Also, it is required to improve heat shock resistance. As such a material for a brake disc, there is a steel for a brake disc as described in Patent Document 2. It has C in the range of 0.1 to 0.6%, Si exceeding 1.2% and 2.0% or less, Mn in the range of 0.2 to 2.0%, Ni in the range of 0.8 to 3.0%, Cr in the range of 1.0 to 5.0%, Mo in the range of 0.5 to 2.0%, V in the range of 0.05 to 0.5%, Al in the range of 0.001 to 0.10%, has N in the range of 0.0015 to 0.020%, and the balance is composed of iron and impurities, A c3 The transformation temperature is 850exceeding ℃, and having an austenite grain size number of 6.0 or more and a fracture toughness value of 130 MPa·m 1 / 2 or more, which is characterized by having such a value.

[0005] Furthermore, as a material for a brake disk as described in Patent Document 3, C is 0.15% or more and 0.30% or less, Si is 0.25% or more and 1.3% or less, Mn is 0.3% or more, Ni is 0.25% or more and 1.0% or less, Cr is 0.6% or more and 1.0% or less, Mo is 0.4% or more, V is 0.05% or more and 0.22% or less, Al is 0.10% or less, and the carbon equivalent Ceq represented by the following formula (1) is 0.60% or more and 0.86% or less based on the content percentage values of each metal, the total of Mn and Ni is 1.3% or less, and the balance is a steel-based material composed of iron and impurities is used.

[0006] Ceq (mass%) = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14 ……(1)

[0007] The characteristics of the material according to Patent Document 3 are based on the fact that a material mainly composed of iron may undergo transformation into a martensite structure by quenching that repeats heating and cooling. The martensite structure is harder but more brittle than the structure that can be obtained by casting a steel-based material in a general method. Therefore, cracks are likely to occur at the locations where martensite transformation has occurred. In addition, since heat is concentrated on the part near the surface of the brake disk where it slides with the brake pad, transformation occurs only in a part near the surface of the disk, resulting in local plastic deformation and leading to crack generation. Therefore, if martensite transformation is likely to occur, the thermal shock resistance will be low. On the other hand, by finding the component ratio and other conditions in which transformation into this martensite structure is less likely to occur, the thermal shock resistance is enhanced.

[0008] In addition, Patent Document 4 discloses a hot-rolled steel excellent in scale removability by brush polishing, which has a component composition containing C: 0.1 to 1.30% by mass, Si: 2.0% by mass or less, Mn: less than 1.2% by mass, Cr: more than 0.20% by mass and less than 0.90% by mass, with the balance being Fe and inevitable impurities, has a scale layer on the surface of the steel sheet, and has a dissociation layer between the scale layer and the steel base.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0010] By the way, sliding members such as brake disks are heated locally to 900°C or higher due to so-called heat spots caused by friction with mating members such as brake pads during braking at high speeds, and are repeatedly oxidized by cooling to room temperature after braking. Therefore, it is required to have not only high-temperature strength but also high toughness at room temperature. Furthermore, during braking, the surface of the disk is exposed to high temperatures, so the surface of the sliding member is oxidized, and the friction coefficient tends to decrease due to the adhesion and remaining of the oxide film. This is considered to be because metal oxides have a lower thermal conductivity than metals, and the heat conduction is inhibited by the generated oxides, the surfaces of the sliding member and the mating member are exposed to high temperatures, resulting in the disintegration of the mating member and the decrease in the friction coefficient. Therefore, an object of the present invention is to not only impart high-temperature deformation resistance, heat shock resistance, and high toughness achieved by the material of Patent Document 3, but also maintain the friction coefficient at high temperatures, that is, the braking performance.

Means for Solving the Problems

[0011] In this invention, as a steel material for sliding members, with respect to the steel material described in Patent Document 3 having C, Si, Mn, Ni, Cr, and Mo, the contents of Si, Mn, Ni, and Cr involved in oxide film formation and its peeling are optimized. And for improving thermal shock resistance, by containing appropriate amounts of C, Si, Mn, Ni, Cr, Mo, V, and Nb, the above problems are solved.

[0012] Specifically, C is 0.15 mass% or more and 0.30 mass% or less, Si is 0.25 mass% or more and 1.0 mass% or less, Mn is 0.30 mass% or more and 1.0 mass% or less, Ni to 1 .7 mass% or less, Cr is 0.7 mass% or more and less than 1.3 mass%, Mo is 0.4 mass% or more and 1.6 mass% or less, V is 0.40 mass% or less, Nb is 0.12 mass% or less, Al is 0.09 mass% or less, and from the values of the mass content percentages of each element, the carbon equivalent Ceq represented by the following formula (1) is 0.65 mass% or more, the total of Mn and Ni is 1.4 mass% or more and 2.2 mass% or less, and the balance is composed of iron and impurities. With the steel-based sliding material, the target values could be satisfied. As this specific target, the friction coefficient at 800 °C is 0.30 or more, the hardness at room temperature is 300 HV or more, the tensile strength is 850 MPa or more, the tensile strength at 700 °C is 200 MPa or more, and the A c3 transformation temperature is 850 °C or more.

[0013] Ceq (mass%) = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14 ……(1)

[0014] Furthermore, by containing 0.010 mass% or more of Al, the deoxidation effect can be exerted, and a material less likely to have casting defects can be obtained.

[0015] First, regarding the maintenance of the friction coefficient at high temperatures required for braking performance, it is based on finding new behaviors and characteristics on the surface during sliding. It has been found that during sliding, an oxide film is formed by high-temperature oxidation, and the removal (peeling) of the oxide film is repeated by friction with the mating material, i.e., shear force. Based on this finding, it has been found that a higher oxidation resistance, making it less likely for an oxide film of the sliding material to form, or a higher peelability of the oxide film, increases the friction coefficient.

[0016] Furthermore, it has been found that there is a correlation between the thickness of the oxide film remaining after friction and the friction coefficient. Based on this correlation, it has been found that by determining the thickness of the oxide film remaining even after the friction test, it is possible to adjust to the range of the friction coefficient required for the steel-based sliding material. The target of the friction coefficient is 0.3 or more, and based on the thickness of the oxide film that shows a correlation, it has been possible to find the blending ratio determined to be appropriate as described above.

[0017] Also, regarding the maintenance of thermal shock resistance, it is based on the fact that thermal fatigue occurs due to the repetition of thermal stress and transformation stress by quenching that repeats heating and cooling, leading to crack generation. Excellent thermal fatigue strength, that is, high high-temperature strength, makes it less likely for thermal cracks to occur. Therefore, for high-temperature strength, elements such as Mo, which is essential, and V and Nb that may be appropriately added to generate fine carbides are contained to increase the strength.

[0018] Note that the above carbon equivalent Ceq is defined in JIS G 0203(5103), and for the contained elements other than carbon, it is a value obtained by converting the influence degree mainly improving hardness and strength into the influence degree of carbon. Generally, the higher it is, the higher the hardness becomes, and accordingly, the tensile strength also improves. The carbon equivalent corresponding to the room-temperature tensile strength required for the sliding material is set within the above range.

Advantages of the Invention

[0019] When a sliding member such as a brake disk is manufactured using the steel-based sliding material according to this invention, it becomes a suitable one with high braking performance and ensuring thermal shock resistance.

Brief Description of the Drawings

[0020]

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Figure 17

Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be specifically described. The steel-based sliding material according to the present invention mainly consists of iron and specifically has the following composition. Note that the following ratios are all in mass%, and they are not the values at the time of material blending before casting, but the values of the components of the sliding member made of the sliding material obtained after casting.

[0022] The above steel-based sliding material needs to contain 0.15 mass% or more of C. If it is less than 0.15 mass%, the hardness and tensile strength will be insufficient, and it is highly likely that the mechanical properties required for the sliding material cannot be ensured. On the other hand, the C content needs to be 0.30 mass% or less. When C exceeds 0.30 mass%, the decrease in thermal shock resistance becomes significant. To more reliably avoid the decrease in thermal shock resistance, it is preferably 0.25 mass% or less. In addition, there is a content limit considering the elements involved in the carbon equivalent Ceq.

[0023] The above-mentioned steel sliding material needs to contain 0.25% by mass or more of Si. Si has a deoxidizing effect during melting and has the effect of suppressing casting defects by removing oxygen, which is the cause of casting defects during casting. If it is less than 0.25% by mass, this effect becomes insufficient, and the possibility of casting defects occurring cannot be ignored. On the other hand, if there is too much Si, there is a risk of deterioration in thermal shock resistance. On the other hand, in order to correspond to the thinness of the residual oxide film thickness corresponding to the property of achieving the required friction coefficient value in the oxidation test described later, it needs to be 1.0% by mass or less. In addition, there are content restrictions in consideration of elements involved in the carbon equivalent Ceq described later.

[0024] The above-mentioned steel sliding material needs to contain 0.30% by mass or more of Mn. Mn contributes to improving hardenability, and if it contains about 0.1% by mass, it is possible to avoid a decrease in hardenability. However, if Mn is too low, the oxide film thickness tends to increase slightly, so it needs to be 0.30% by mass or more, preferably 0.40% by mass or more. On the other hand, if it exceeds 1.0% by mass, there is a risk that the hardenability becomes excessive, so it needs to be 1.0% by mass or less, preferably 0.8% by mass or less.

[0025] The above-mentioned steel sliding material may contain Ni. When contained, it is preferably 1.7% by mass or less. If it exceeds 1.7% by mass, the risk that the oxide film thickness remaining during sliding exceeds the allowable amount becomes high. Also, regarding the total amount with Mn described later, there are further limiting conditions depending on the allowable amount of the oxide film thickness remaining.

[0026] The above-mentioned steel sliding material needs to contain 0.7% by mass or more of Cr, preferably 0.8% by mass or more. If the amount of Cr is insufficient, the risk that the oxide film thickness remaining during sliding exceeds the allowable amount becomes high. On the other hand, it needs to be less than 1.3% by mass, preferably 1.2% by mass or less. Even if there is too much, the risk that the oxide film thickness remaining during sliding exceeds the allowable amount becomes high.

[0027] The above-described steel sliding material needs to contain 0.4 mass% or more of Mo. Mo forms fine carbides (Mo2C) and contributes to the improvement of hardness, tensile strength, especially high-temperature strength. However, if it is less than 0.4 mass%, the amount of carbides will be small and the strength will become insufficient. On the other hand, even if the content of Mo increases, there is little effect of increasing hardenability like Cr, so there is no particular upper limit for maintaining thermal shock resistance alone. However, since it has a slight effect similar to that of carbon, there is a de facto upper limit as one item of the above carbon equivalent. Also, in reality, even if it is contained in excess of 1.6 mass%, the effect of improving high-temperature strength hardly appears and it becomes wasteful, so it is preferably 1.6 mass% or less, and preferably 1.2 mass% or less.

[0028] It is preferable that the above-described steel sliding material contains 0.01 mass% or more of V. V forms fine carbides VC and mainly contributes to heat-resistant deformability, that is, improvement of high-temperature strength. If V is less than 0.01 mass%, the effect will be limited. Preferably it is 0.03 mass% or more. On the other hand, if it is too much, there is no cost advantage, and if it is kept at 0.40 mass% or less, a stable property improvement effect can be obtained.

[0029] The above-described steel sliding material may contain Nb. Similar to V, Nb forms fine carbides NbC and mainly contributes to heat-resistant deformability, that is, improvement of high-temperature strength. However, if it is too much, unexpected changes may occur, and if it is kept at 0.12 mass% or less, a stable property improvement effect can be obtained.

[0030] The above-described steel sliding material may contain Al. When contained, it is preferably 0.010 mass% or more. Al also has a deoxidizing effect and exhibits the effect with a relatively small amount compared to Si. If Al is less than 0.010 mass%, the deoxidizing effect will be insufficient and the possibility of casting defects occurring may become non-negligible. On the other hand, if the Al content increases too much, it will lead to a decrease in toughness, and also, even if it is contained excessively, the contribution to the deoxidizing effect hardly improves, so the content is preferably 0.09 mass% or less.

[0031] Next, among the elements constituting the above-described material for sliding members, for elements other than Nb, Al, and Fe, it is necessary to satisfy the relationship that the carbon equivalent Ceq represented by the following formula (1) is 0.65% or more. This carbon equivalent Ceq is a value obtained by converting the influence degree of elements other than carbon to the influence degree of carbon. If this is less than 0.65%, there is a risk that the strength required for the sliding material cannot be obtained.

[0032] Ceq (mass%) = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14 ……(1)

[0033] Furthermore, the total amount of Mn and Ni contained in the above-described material for brake disks needs to be 1.4 mass% or less and 2.2 mass% or less. Mn and Ni contribute integrally to the thermal cycle characteristics. This is because their chemical properties are similar and they are elements that lower the transformation temperature with respect to their involvement in the thermal cycle characteristics. If the total amount is 1.4 mass% or more, it becomes easier to maintain the remaining oxide film thickness. On the other hand, if it is too much, conversely, the oxide film thickness tends to increase.

[0034] The elements contained in the above-described steel-based sliding material, in addition to the above C, Si, Mn, Ni, Cr, Mo, V, Nb, and Al, are those in which iron occupies the main component. Other than that, elements that may be impurities and are not intentionally included but are allowed to be contained within a range that does not inhibit the performance required for the steel-based sliding material according to this invention. These impurities are those remaining in the crucible or mold for adjusting the molten metal for casting, those remaining in the ladle used when applying to large articles, those remaining elements contained in the molten metal used so far, or elements that cannot be completely excluded when using scrap as a material from the viewpoint of environmental protection.

[0035] The content of the elements that become the above impurities is preferably as follows. P is preferably 0.05% by mass or less. If it exceeds 0.05% by mass, P segregating at the grain boundaries will reduce the high-temperature strength, and the decrease in thermal shock resistance will become significant. S is also preferably 0.05% by mass or less. In addition to Cu and Ti, the content of other elements is preferably 0.05% by mass or less. This is because the inclusion of these elements may exert an unexpected effect and inhibit the properties of the steel-based sliding material according to the present invention. Also, the inclusion of these elements leads to an unnecessary cost increase. Note that the lower the content of these impurity elements, the better, and it is desirable that it is less than the detection limit.

[0036] The content ratio of the above elements is the content ratio of the actually obtained product after casting, not the mixing ratio of the elements as raw materials. The specific measurement method is spark discharge emission spectroscopy analysis method by the method (JIS G 1253).

[0037] With the steel-based sliding material having the above configuration, when a sliding material is manufactured by a general casting method, it has high thermal shock resistance, excellent mechanical properties such as hardness and tensile strength, improved heat resistance to deformation, has ductility at room temperature, and can maintain a high friction coefficient even during high-temperature sliding.

Examples

[0038] Hereinafter, examples specifically examined for this invention will be described. On the premise of this example, taking advantage of the significant correlation between the friction coefficient and the thickness of the oxide film, the thickness of the oxide film is used as an evaluation criterion for whether it corresponds to an example as a sliding material. A high friction coefficient is a desirable property for use in sliding members such as brake disks. However, measuring the friction coefficient is time-consuming, and it is not realistic to measure it in all examples. In contrast, in an environment similar to the steel-based sliding material according to the present invention, a correlation has been found between the friction coefficient and the thickness of the oxide film remaining during sliding. Furthermore, a significant correlation has been found between the thickness of the oxide film generated in the sliding test and the thickness of the oxide film generated by the thermal cycle test. That is, through the thickness of the remaining oxide film, it has been confirmed that the thickness of the oxide film generated by the thermal cycle test with a smaller measurement load has a correlation with the friction coefficient. Based on this correlation, an example where the thickness of the oxide film observed in the thermal cycle test falls within a predetermined range indicates that the material can maintain a high friction coefficient.

[0039] <Melting and Heat Treatment of Ingot> In each example and comparative example, after heating the adjusted raw materials to 1600 °C and melting them, the molten steel was cooled to 1550 °C and then cast into a sand mold with dimensions of 30 mm × 30 mm × 100 mm in height to produce an ingot. At the same time, the molten steel was cast into a split mold with dimensions of 23 mm × 37 mm × 30 mm in height to collect a sample for component analysis. After holding the ingot at 960 °C for 3 hours, it was once cooled in the furnace to room temperature, then heated to 950 °C and held for 3 hours, followed by water cooling to room temperature, and further heated to 650 °C and held for 5 hours, and then air cooled to room temperature for heat treatment and subjected to each test.

[0040] <Component Analysis> Using the above sample for component analysis, each contained element was measured by the spark discharge emission spectroscopy analysis method method specified in JIS G 1253. The components shown in the following tables indicate this value.

[0041] <Measurement of Friction Coefficient> The test material for each example was processed into a ring 1 as shown in Figure 1. Meanwhile, a copper-based sintered material was selected as the counterpart material and processed into a plate (disk 2). Tests were conducted in an electric furnace arranged as shown in Figure 2, with a pressing pressure of 0.6 MPa, a rotation speed of 1000 rpm, atmosphere: air, and temperature of 800°C, and the friction coefficient was calculated from the torque. Additionally, the thickness of the remaining oxide film for each example was measured at four points using an SEM, and the average was taken as the remaining oxide film thickness.

[0042] <Heat cycle test (oxidation test)> The test materials for each example were processed into oxidation test pieces as shown in Figure 3(a), 1 cm thick, 10 cm long, and 10 cm wide, with a hole in the center for hanging. The oxidation test pieces were introduced into an electric furnace maintained at 900°C, and heated for five minutes while suspended by a quartz tube as shown in Figure 3(b). After heating, they were removed from the electric furnace and air-cooled to room temperature in three minutes. After cooling, they were introduced back into the electric furnace and the heating and air-cooling cycle was repeated 50 times in the same way. The thickness of the oxide film was then measured using a digital microscope. Measurements were taken at 12 points for each example, and the average was calculated.

[0043] <Correlation between friction coefficient and remaining oxide film thickness> For the test pieces with the composition ratios of each example shown in Table 1, the friction coefficient and the thickness of the oxide film remaining during the friction test (friction test oxide The thickness of the oxide film in the thermal cycle test (oxide film thickness in the oxidation test) was measured. The results are shown in Table 1.

[0044] [Table 1]

[0045] Among these results, when the friction coefficient μ was plotted on the vertical axis and the friction test oxide film thickness (μm) was plotted on the horizontal axis as shown in Fig. 4, a negative correlation was shown, indicating a clear correlation particularly in the region where the friction becomes low. Next, when the oxide film thickness (μm) of the oxidation test (thermal cycle test) was plotted on the vertical axis and the friction test oxide film thickness (μm) was plotted on the horizontal axis and plotted as shown in Fig. 5, a positive correlation was shown. Therefore, connecting these, the friction coefficient μ was plotted on the vertical axis and the oxide film (μm) of the oxidation test (thermal cycle test) was plotted on the horizontal axis as shown in Fig. 6. The target for the friction coefficient μ is 0.30 or more when used as a material for the brake disk. However, it was shown that the oxide film thickness of the oxidation test (thermal cycle test) may be 200 μm or less to meet that condition. Therefore, in the subsequent judgment, the evaluation condition that can be used as the steel-based sliding material according to the present invention is set to be "the oxide film thickness of the oxidation test is 200 μm or less" in the above thermal cycle test (oxidation test). This means that an oxide film with a friction coefficient smaller than the original component hardly remains on the surface or is easily peeled off, indicating that it is easy to maintain a high friction coefficient of the material itself.

[0046] Also, among the above, for Reference Example 1j and Reference Example 1h, the cross section near the surface after the friction test was photographed by SEM. The photograph of Reference Example 1j is shown in Fig. 7(a), and the photograph of Reference Example 1h is shown in Fig. 7(b). In Reference Example 1j with a high friction coefficient of 0.40, the thickness of the friction test oxide film is as thin as 14.33 μm. On the other hand, it was confirmed that in Reference Example 1h with a low friction coefficient of 0.16, the thickness of the friction test oxide film is as thick as 47.23 μm. From this, it is considered that when the adhering and remaining oxide film is thin, heat exchange between the sliding material and the mating material is efficiently performed, and a high friction coefficient is obtained.

[0047] <Evaluation Test by Oxide Film> As shown in Table 2, test materials were prepared with the contents of the elements set as the main variable elements changed and the contents of the other elements adjusted so as not to vary as much as possible, and used as each example and comparative example. The following tests were carried out for each test material. However, "0.5 Si」 is Si an example group in which the content of Si is maintained at around 0.5% by mass and other elements are varied.

[0048]

Table 2

[0049] For groups of examples and comparative examples for each of the main variable elements, in each of FIGS. 8(a) to (d), with the corresponding element content on the horizontal axis and the oxide film thickness (μm) of the oxidation test on the vertical axis, the variations were plotted and smoothed. Those with the main variable element being Si are shown in FIG. 8(a), those with Mn are shown in FIG. 8(b), those with Ni are shown in FIG. 8(c), and those with Cr are shown in FIG. 8(d). Further, for each series, the case with the total amount of Mn and Ni on the horizontal axis is shown in FIG. 8(e). In each series for each of the main elements, examples were those with an oxide film thickness of 200 μm or less, and comparative examples were those exceeding it.

[0050] In FIG. 8(a), it was confirmed that the oxide film thickness becomes 200 μm or less in the range where Si is 0.25% by mass or more and 1.0% by mass or less. In FIG. 8(b), when Mn is 0.1% by mass, the oxide film thickness slightly exceeds 200 μm, but it was confirmed that it surely becomes 200 μm or less when Mn is 0.3% by mass or more. Note that Comparative Example 3c where Mn is 1.14% by mass is a favorable value in this evaluation of the oxide film thickness, but since it is not preferable to exceed 1.0% by mass in terms of quenching, it is a comparative example. In FIG. 8(c), it was confirmed that the oxide film thickness becomes 200 μm or less in the range where Ni is 1.7% by mass or less. In FIG. 8(d), it was confirmed that the oxide film becomes 200 μm or less in the range where Cr is 0.7% by mass or more and less than 1.3% by mass. In FIG. 8(e), although there are variations depending on the series, it was confirmed that the oxide film becomes 200 μm or less in any series as long as Mn + Ni is in the range of 1.4% by mass or more and 2.2% by mass or less.

[0051] <Evaluation test of oxide films for other elements> As shown in Table 3, examples were prepared for the elements related to strength (C, Mo, V, and Nb) and Al related to deoxidation, respectively, while keeping the contents of other elements as constant as possible. For any element, within the range shown in Table 3, it was confirmed that the oxide film thickness in the oxidation test was 200 μm or less.

[0052]

Table 3

[0053] <Tensile Strength and 0.2% Proof Stress Evaluation Test> Next, the following evaluations were carried out on the materials with the compositions shown in Table 4. As the evaluation at room temperature, the hardness was measured by JIS Z 2244 (Vickers hardness test), and the tensile strength and 0.2% proof stress at room temperature were measured in accordance with JIS Z 2241 (Test method for tensile test of metallic materials). As the evaluation at high temperature, in accordance with JIS G 0567, the tensile strength and 0.2% proof stress at 700 °C were measured. Each measurement was carried out twice, and the average value is shown. (HV0.10) at room temperature in accordance with (T.S.) and 0.2% proof stress (Y.S.) at room temperature. As the evaluation at high temperature, in accordance with JIS G 0567, the tensile strength (T.S.) and 0.2% proof stress (Y.S.) at 700 °C were measured. Each measurement was carried out twice, and the average value is shown.

[0054]

Table 4

[0055] All the materials with a carbon equivalent of 0.65 or more shown in Table 4 above were able to exhibit a target room temperature hardness of 300 HV or more and a tensile strength of 850 MPa or more. Among them, the evaluation results for the carbide-forming elements Mo, V, and Nb, which improve high-temperature strength, will be described together with the graphs shown in Figs. 9(a) to (c).

[0056] First, when the content of Mo was 0.4 mass% or more, the tensile strength at 700 °C reached above the target value of 200 MPa. Conversely, when the content was 1.2 mass% or more, the effect tended to decrease, but it was confirmed that there was still an effect compared to the case without Mo.

[0057] Next, even when V was contained in extremely trace amounts, the high-temperature strength could be improved. In the example where V was contained at 0.1% by mass, the tensile strength at 700 °C was 200 MPa or more. However, when the content was 0.3% by mass or more, the tensile strength was further improved, and the increase leveled off once more was added.

[0058] Furthermore, as shown in Examples 21a to e, even when Nb was contained in extremely trace amounts, an effect was observed when enhancing the high-temperature strength. It was confirmed that the effect could be expected up to 0.12% by mass.

[0059] <Thermal Shock Test and Transformation Temperature Measurement> For materials with the compositions shown in Table 5 below for the components, a thermal shock test was conducted to evaluate the thermal shock resistance of the steel sheets. Specifically, each material with the blending ratios shown in Table 5 was melted in the same manner as above, and cylindrical test pieces (height 20 mm, φ25 ± 0.2) having the notch shown in Fig. 10 were fabricated by machining. On the side surface, a pair of grooves with a width of 6 mm and a depth of 3 mm facing each other were provided in the vertical direction. A hole with a depth of 5 mm and a φ5 ± 0.2 was made in the center of the upper bottom surface. The arithmetic mean roughness of the side surface was approximately 2.0.

[0060] A schematic diagram of a thermal shock testing machine 20 for rapidly heating and cooling this cylindrical test piece is shown in Fig. 11, and a cross-sectional view is shown in Fig. 12. The main body 21 is a cylindrical shape with a tube thickness and an open upper end, and the above test piece 11 can be accommodated inside it. A high-frequency coil 31 is wound around the main body 21, and the stored test piece 11 can be heated to 1000 °C in 3 seconds by this high-frequency coil 31.

[0061] At the upper end ring portion of the main body 21, holes communicating to the inside are provided at equal intervals. Half of these are air holes 22 for supplying air, and the rest are cold water holes 23 for supplying cooling water. These are provided alternately. An air supply pipe 24 and a cold water supply pipe 25 are connected to each of them, and air and cold water are supplied to a large number of air nozzles 27 and cold water nozzles 28 formed on the inner peripheral surface of the main body 21 through the hole paths stretched inside the main body 21, so that the test piece 11 can be cooled. The supplied water is discharged through a discharge pipe 29.

[0062] With this thermal shock tester, the temperature change consisting of rapid heating and rapid cooling as shown in the example of FIG. 13 was repeated. This temperature change first performs rapid heating that raises the temperature to 1000° C. in about 3 seconds by heating from the high-frequency coil 31. Next, the heating by the high-frequency coil 31 is stopped, and cold water is supplied from the cold water ejection port 28 for 20 seconds, and after the cold water supply, air cooling is performed for 30 seconds. Thereby, the heated test piece 11 is rapidly cooled. Such a thermal cycle of rapid heating and rapid cooling was repeated 50 times, and the presence or absence of cracks was confirmed by visual inspection and penetrant testing. Each photograph is shown in FIG. 14.

[0063] The evaluation of cracks was specifically performed by observing the progress of cracks when the above thermal cycle was repeated 10 times, 20 times, 30 times, and 50 times. A test piece in which obvious cracks were observed up to 50 thermal cycles was evaluated as "×". In addition, an example evaluated as "○" is one in which no cracks or penetrant dye is observed at the end of 50 thermal cycles. The penetrant test was performed in accordance with JIS Z 2343.

[0064]

Table 5

[0065] Furthermore, for each example described in Table 5, the A C3 transformation temperature was measured. If the A C3 transformation temperature is high, the martensite transformation is suppressed, and the transformation stress tends to be reduced. In the examples of Table 5, it was confirmed that if the A C3 transformation temperature is 850° C. or higher, it has sufficient thermal shock resistance.

[0066] <Deoxidation effect confirmation test> When deoxidation is insufficient, the oxygen that could not be removed exists in the ingot as a casting defect due to bubbles (blow holes). By visually checking the inside of the sample for component analysis, it was confirmed whether the Si and Al contents exhibit a deoxidation effect.

[0067] Table 6 shows the components of the materials in which the deoxidation effect of Si was confirmed, and photographs of the respective ingots are shown in Fig. 15. In Comparative Examples 24a and 24b where Si was insufficient, casting defects were observed, but in Example 24 where Si was 0.25% by mass, no casting defects were observed, indicating that Si exerted a sufficient deoxidation effect.

[0068]

Table 6

[0069] Table 7 shows the components of the materials in which the deoxidation effect of Al was confirmed, and photographs of the respective ingots are shown in Fig. 16. In Comparative Example 25 where Al was insufficient, casting defects were observed, but in Examples 25a and 25b in which Al was contained at 0.010% by mass or more, the casting defects disappeared, indicating that a sufficient deoxidation effect was exerted.

[0070]

Table 7

[0071] <Hardening Evaluation Test of Mn> Table 8 shows the components of the materials in which the effect of the Mn content on hardenability was investigated. After hardening the steel-based sliding material having the components in Table 8, the ingot was cut, and the cross-sectional hardness from the hardened end (ingot surface) was measured. The results are shown in the graph of Fig. 17. When Mn is contained at 0.1% by mass or more, the hardenability does not decrease and the strength is not affected. Note that in the range of about 2 mm from the surface, the hardened hardness of any of the steel materials decreases, which is due to decarburization caused by being held at a high temperature during the heat treatment.

[0072]

Table 8

Explanation of Signs

[0073] 1 Ring 2 Disk 11 Test Piece 20 Thermal shock testing machine 21 Main body 22 Air hole 23 Cold water hole 24 Air supply pipe 25 Cold water supply pipe 27 Air outlet 28 Cold water outlet 29 Drain pipe 31 High-frequency coil

Claims

1. containing C of 0.15% by mass or more and 0.25% by mass or less, Si of 0.25% by mass or more and 0.55% by mass or less, Mn of 0.30% by mass or more and 0.80% by mass or less, Ni of 1.7% by mass or less, Cr of 0.7% by mass or more and less than 1.3% by mass, Mo of 0.4% by mass or more and 1.60% by mass or less, V of 0.40% by mass or less, Nb of 0.12% by mass or less, and Al of 0.010% by mass or more and 0.09% by mass or less, the carbon equivalent Ceq represented by the following formula (1) is 0.65% by mass or more based on the values of the mass percentage contents of the respective metals, and the balance consists of iron and impurities, Ceq (mass%) = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14 …… (1) and a steel-based sliding material in which the total of Mn and Ni is 1.4% by mass or more and 2.2% by mass or less.

2. A ｃ３ The steel-based sliding material according to claim 1, wherein the transformation temperature is 850°C or higher.

3. A sliding member comprising the steel-based sliding material according to Claim 1 or 2.

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