JP5503417B2 - Case-hardened steel parts with excellent sliding characteristics - Google Patents

Description

  The present invention relates to a case-hardened steel part having a hardened layer of carburizing or carbonitriding on the surface and having a low coefficient of friction and excellent sliding properties. The case-hardened steel parts of the present invention are suitably used for mechanical structural parts such as transmissions such as CVT pulleys used in automobiles, bearings, gears and shafts used in industrial machines.

  In mechanical structural parts such as automobile transmissions, industrial machine bearings, gears, and shafts, energy loss due to friction is a major cause of reduced energy efficiency, and hence a reduction in friction coefficient is required. For machine structural parts, forging and other processing are performed on case-hardened steel represented by JIS standard SCM420H, SCr420H, etc., and finished to the final part shape by carburizing or carbonitriding Manufactured by performing a surface hardening treatment (including treatment under atmospheric pressure, low pressure, vacuum, and plasma atmosphere).

  For example, Patent Literature 1 and Patent Literature 2 can be cited as techniques for improving the sliding characteristics by reducing the friction loss of mechanical structural parts. Among them, in Patent Document 1, the friction coefficient is reduced by coating the surface of the gear with a DLC film (diamond-like carbon film), and a low friction coefficient is obtained even in the lubricating oil by controlling the amount of hydrogen in the DLC film. Obtaining techniques are disclosed. However, the cost of the DLC film is high, and depending on the use conditions of the DLC film, there is a problem that the DLC film peels off due to insufficient adhesion.

  Patent Document 2 discloses a technique for reducing the friction coefficient by controlling the height and length of the irregularities on the surface of the sliding member. However, in the above method, since the uneven shape is controlled using a special tool or a precision processing method, the cost is remarkably increased and it is not suitable for mass production on an industrial scale.

JP 2008-32150 A JP 2001-304267 A

  With increasing needs for improving fuel efficiency of automobiles and the like, demands for reducing friction loss are increasing, and it is desired to provide case-hardened steel parts having a low friction coefficient and excellent sliding characteristics.

  The present invention has been made in view of the above circumstances, and its purpose is to perform new coating processing, special precision processing, etc., as in the above-mentioned patent document, etc., for mechanical structural parts that are required to reduce loss due to friction. The object is to provide a case-hardened steel part that is low in cost, has a low coefficient of friction, and has excellent sliding characteristics.

The case-hardened steel part having excellent sliding characteristics according to the present invention that can solve the above-mentioned problems is a case-hardened steel part having a carburized layer or a carbonitrided layer on its surface, and the component in the steel is C: 0. 1 to 0.5% (mass%, the same applies hereinafter), Si: 0.03 to 2%, Mn: 0.2 to 1.8%, Cr: 3% or less (excluding 0%), P: 0 0.03% or less (excluding 0%), S: 0.2% or less (excluding 0%), Al: 0.16-0.5%, B: 0.0005-0.008%, N : 0.002 to 0.015%, O: 0.002% or less, balance: iron and inevitable impurities, satisfying the following formulas (1) and (2), and surface roughness Ra It has a gist where it is 0.16 μm or less.
[N] -1.3 × [B] ≧ 0.0010% (1)
In the formula, [] is the content (%) of each element.
Surface solid solution Al ≧ 0.15% (2)

  In a preferred embodiment of the present invention, the case-hardened steel part further contains Mo: 1% or less.

  In a preferred embodiment of the present invention, the case-hardened steel part further contains Cu: 1% or less and / or Ni: 2.5% or less.

  In a preferred embodiment of the present invention, the case-hardened steel part further contains 1% or less of at least one element selected from the group consisting of Nb, Ti, and V.

  In a preferred embodiment of the present invention, the case-hardened steel part further contains 0.1% or less of at least one element selected from the group consisting of Ca, Zr, Sb, Pb and Bi.

  According to the present invention, the loss due to friction is reduced, and the fuel efficiency of automobiles and the energy efficiency of industrial machines and the like are improved. Therefore, the case-hardened steel parts of the present invention can be used for shifting gears such as CVT pulleys used in automobiles. It is suitably used for bearings, gears, shafts, etc. used in machines and industrial machines.

FIG. 1 is a diagram showing an outline of a friction coefficient measurement test in Example 1. FIG. 2 is a schematic cross-sectional view showing the shape of the test roller used for measuring the friction coefficient in Example 1. FIG. 3 is a schematic cross-sectional view showing the shape of the mating roller used in the measurement of the friction coefficient in Example 1.

  The present inventors have studied to provide a case-hardened steel part having a small friction coefficient and excellent sliding characteristics. As a result, in order to reduce the friction coefficient, it was determined that BN precipitates and the solid solution Al on the surface (specifically, the region from the outermost surface of the component to a depth of 100 μm) act effectively. Equations (1) and (2) were derived. Furthermore, it has been found that control of the surface roughness Ra is indispensable for reducing the friction coefficient. Moreover, in order to ensure desired BN, using the steel in which the component was controlled appropriately, after heating to 1100 degreeC or more, it hold | maintains in the temperature range of 900-1050 degreeC for 150 seconds or more, and then it cools to 900 degreeC It was also found that the method of setting the average cooling rate from 0.05 to 700 ° C. to 0.05 to 10 ° C./second is effective, and this reduces the amount of AlN in the steel and increases the desired amount of BN. Furthermore, it has been found that it is effective to appropriately control the cooling rate after the surface hardening treatment in order to prevent the BN thus precipitated from re-dissolving. Specifically, after the above method, after cutting into a predetermined part shape, carburizing treatment or carbonitriding treatment is performed, and then the average cooling rate from 900 ° C. to 800 ° C. is set to 1 ° C./second or more when cooling. The present inventors have found that it is sufficient to control the present invention and completed the present invention.

  The case-hardened steel parts controlled as described above have a coating effective for reducing the friction coefficient on the sliding surface (sliding surface), so that the friction coefficient is lowered and the sliding characteristics are improved. Inferred.

As described above, the case-hardened steel part of the present invention is a case-hardened steel part having a carburized layer or a carbonitrided layer on the surface, and the component in the steel is C: 0.1 to 0.5% (mass%, The same applies hereinafter), Si: 0.03 to 2%, Mn: 0.2 to 1.8%, Cr: 3% or less (not including 0%), P: 0.03% or less (not including 0%) ), S: 0.2% or less (excluding 0%), Al: 0.16-0.5%, B: 0.0005-0.008%, N: 0.002-0.015%, O: 0.002% or less, balance: iron and inevitable impurities, satisfying the following formulas (1) and (2), and the surface roughness Ra of the sliding part is 0.16 μm or less There is a feature.
[N] -1.3 × [B] ≧ 0.0010% (1)
In the formula, [] is the content (%) of each element.
Surface solid solution Al ≧ 0.15% (2)

  First, the components in steel will be described. In the present invention, the composition of the case-hardened steel can be representatively exemplified, but Si and Cr exceeding those upper limits can be used. Specifically, it is as follows.

C: 0.1 to 0.5%
C is an essential element for ensuring the strength of the case-hardened steel part, and for that purpose, the C content is 0.1% or more. However, if added excessively, the hardness before the surface treatment of carburizing and carbonitriding becomes too high and the life of the tool or mold is reduced during cutting or forging, so the upper limit of C amount is 0.5% To do. A preferable amount of C is 0.15% or more and 0.22% or less, and more preferably 0.18% or more and 0.20% or less.

Si: 0.03 to 2%
In addition to being effective as a deoxidizing element during steelmaking, Si is an element effective for improving surface fatigue strength in gears and the like. In order to effectively exhibit such an action, the lower limit of the Si amount is set to 0.03%. However, if added in excess, the machinability and forgeability deteriorate, so the upper limit of Si content is 2%. In addition, although the upper limit of Si amount in normal case-hardened steel is about 0.9%, in the present invention, it has been confirmed by experiment that it can be tolerated up to 2%. A preferable Si amount is 0.35% or more and 1.0% or less, and more preferably 0.4% or more and 0.8% or less.

Mn: 0.2 to 1.8%
Mn is an element that is effective in generating retained austenite that contributes to improvement in rolling fatigue life, and is also an element that contributes to improvement in hardenability. In order to effectively exhibit such an action, the lower limit of the amount of Mn is set to 0.2%. However, if added excessively, the hardness of the material becomes too high and the machinability and the like deteriorate, so the upper limit of the Mn amount is set to 1.8%. A preferable amount of Mn is 0.4% or more and 1.2% or less, and more preferably 0.9% or less.

Cr: 3% or less (excluding 0%)
Cr is an element that improves hardenability and improves the hardness after carburizing or carbonitriding, and in order to effectively exhibit such action, the Cr content is preferably 0.5% or more. . However, if excessively added, the machinability at the time of processing such as cutting performed before carburizing or carbonitriding is lowered, so the upper limit of Cr content is set to 3%. In addition, although the upper limit of the Cr content in ordinary case-hardened steel is about 1.2%, in the present invention, it has been confirmed by experiments that it is acceptable up to 3%. A preferable Cr amount is 0.8% or more and 1.5% or less, and more preferably 1.0% or more and 1.2% or less.

P: 0.03% or less (excluding 0%)
P is an element (impurity) inevitably contained in the steel, and promotes cracking during hot working, so it is preferably reduced as much as possible. The amount of P is 0.03% or less, preferably 0.02% or less, more preferably 0.01% or less. In addition, it is industrially difficult to make P amount 0%.

S: 0.2% or less (excluding 0%)
S is an element that improves machinability, but if contained excessively, the ductility and toughness of the steel material decrease, so the upper limit is made 0.2%. In particular, when the amount of S is excessive, the amount of MnS inclusions reacting with Mn increases, and these inclusions extend in the rolling direction during rolling to deteriorate the toughness in the direction perpendicular to the rolling (lateral toughness). However, S is an impurity inevitably contained in steel, and it is industrially difficult to reduce the amount to 0%.

Al: 0.16-0.5%
Al is one of the components that characterize the present invention, and is a supply component of surface solid solution Al that contributes to the reduction of the friction coefficient. In addition, Al combines with N to refine crystal grains during carburization and also has an effect of increasing fatigue strength. In the present invention, in order to effectively exhibit the above-described action, the amount of Al (lower limit 0.16%) larger than the amount of Al normally contained in the case-hardened steel (generally about 0.01 to 0.03%) is specified. doing. On the other hand, even if the Al content exceeds 0.5%, the friction coefficient reducing effect is saturated, and problems such as nozzle clogging during casting and cracking during rolling occur in the steel manufacturing process. 0.5%. A preferable Al amount is 0.18% or more and 0.40% or less, and more preferably 0.20% or more and 0.30% or less.

B: 0.0005 to 0.008%
B is also one of the components that characterize the present invention, and is a component that combines with N to form BN and contributes to a reduction in the friction coefficient. In order to effectively exhibit such an action, it is necessary that the B amount is 0.0005% or more and that the formula (1) that defines the relationship between the N amount and the B amount is satisfied. Formula (1) will be described later. However, even if B is added excessively, the above effect is saturated, and the hardenability of the steel material becomes excessively high, causing variation in the strength of the parts, so the upper limit is made 0.008%. A preferable amount of B is 0.0010% or more and 0.006% or less, and more preferably 0.0015% or more and 0.004% or less.

N: 0.002 to 0.015%
N is a supply component for precipitating BN which combines with B described above and contributes to the reduction of the friction coefficient. Further, N combines with Al to produce an AlN compound, and has an effect of increasing the fatigue strength by refining crystal grains during carburization. In order to effectively exhibit such an action, it is necessary that the N amount is 0.002% or more and that the formula (1) that defines the relationship between the N amount and the B amount is satisfied. Formula (1) will be described later. However, if N is added excessively, cracking easily occurs during hot working, so the upper limit of N content is 0.015%. The lower limit of the preferable N amount is 0.005% or more, more preferably 0.007% or more. Moreover, the upper limit of preferable N amount is 0.012% or less.

O: 0.002% or less (excluding 0%)
O is an element inevitably contained in the steel, and is an element that forms coarse oxide inclusions and adversely affects machinability, ductility, toughness, hot workability, and the like. In particular, in the present invention, the formation of oxides such as Al ₂ O ₃ is prevented as much as possible in order to secure the solid solution Al that contributes to the reduction of the friction coefficient and AlN that contributes to the improvement of the fatigue strength due to the refinement of the crystal grains during carburization. There is a need. From such a viewpoint, the amount of O is made 0.002% or less. Preferably it is 0.0015% or less. In addition, it is industrially difficult to make O amount 0%.

  The components in steel in the present invention include the above components, and the balance: iron and inevitable impurities, but the following selected components are added for the purpose of imparting other characteristics and the like within a range not impairing the action of the present invention. be able to.

Mo: 1% or less Mo is an element that improves hardenability and effectively acts to suppress the formation of an incompletely quenched structure in a carburized or carbonitrided layer. In order to effectively exhibit such an effect, the lower limit of the Mo amount is preferably set to 0.1%. However, if added in excess, the machinability and forgeability deteriorate, so the upper limit of Mo is preferably 1%. A more preferable amount of Mo is 0.2% or more and 0.45% or less, and further preferably 0.3% or more and 0.4% or less.

Cu: 1% or less and / or Ni: 2.5% or less Cu and Ni are both elements for improving corrosion resistance, and Ni further has an effect of improving toughness. However, when the Cu content exceeds 1%, the hot workability deteriorates, so the upper limit of the Cu content is preferably 1%. Further, if the Ni content exceeds 2.5%, the amount of retained austenite increases excessively and causes a problem of hardness reduction due to quenching, so the upper limit of the Ni content is preferably set to 2.5%. A more preferable range of the amount of Cu is 0.05% or more and 0.6% or less, and further preferably 0.1% or more and 0.4% or less. Further, a more preferable lower limit of the Ni amount is 0.1% or more, and more preferably 0.2% or more. Further, the more preferable upper limit of the Ni amount is 1.0% or less, more preferably 0.8% or less, and still more preferably 0.6% or less. Cu and Ni may be added either alone or in combination.

1% or less each of at least one element selected from the group consisting of Nb, Ti, and V. These elements are all carbonitride-forming elements, and the fatigue strength is improved by refining the austenite grain size during heating. Contribute to. In order to effectively exhibit such an action, 0.05% or more should be added when Nb is added, 0.05% or more when Ti is added, and 0.05% or more when V is added. preferable. However, if added excessively, coarse carbonitrides are formed, and conversely, the fatigue strength is lowered. Therefore, the upper limit of the Nb amount is 1% (more preferably 0.5%), and the upper limit of the Ti amount is 1%. (More preferably 0.5%), the upper limit of Ti amount is preferably 1% (more preferably 0.5%), and the upper limit of V amount is preferably 1% (more preferably 0.5%). These elements may be added alone or in combination of two or more.

0.1% or less of at least one element selected from the group consisting of Ca, Zr, Sb, Pb and Bi, each of which is useful as a machinability improving element during cutting. In order to effectively exhibit such an action, the lower limit of the preferable content of Ca, Zr, Sb, Pb and Bi is set to Ca: 0.0005%, Zr: 0.001%, Sb: 0.0005%, Pb: 0.001%, Bi: 0.001%. More preferably, they are Ca: 0.001%, Zr: 0.005%, Sb: 0.002%, Pb: 0.01%, Bi: 0.005%. However, if added excessively, the fatigue life is reduced, so the upper limit of each element is preferably 0.1% (more preferably 0.05%). These elements may be added alone or in combination of two or more.

  Furthermore, in the present invention, it is necessary to satisfy the following expressions (1) and (2).

[N] -1.3 × [B] ≧ 0.0010% (1)
In the formula, [] is the content (%) of each element.

  As described above, in the present invention, the friction coefficient is reduced by precipitation of BN, and further, the effect of suppressing abnormal grain growth at the time of carburizing heating is achieved by precipitation of AlN. Therefore, N is the desired BN and AlN. It is necessary to make the content as much as possible. That is, even if all B in the steel is combined with N, it is necessary to generate AlN with surplus N. From such a viewpoint, in the present invention, the above formula (1) is defined for the relationship between the N content [N] and the B content [B]. In the above formula (1), the coefficient (1.3) is the ratio of the atomic weight of N (about 14.0) to the atomic weight of B (about 10.8). This shows that N of 0010% or more remains. The value on the left side of the above formula (1) is preferably 0.0012%, more preferably 0.0014%. In addition, the upper limit of the value on the left side of the above formula (1) is preferably 0.020%, more preferably 0.015%, considering that the hot workability is reduced by excessive solute N. is there.

Surface solid solution Al ≧ 0.15% (2)
As described above, in the present invention, the friction coefficient is reduced by the solid solution Al on the surface of the component. In order to effectively exhibit such an action, the surface solid solution Al amount is set to 0.15% or more. . Preferably it is 0.18% or more, More preferably, it is 0.20% or more. However, if the surface solid solution Al amount is excessive, cracking occurs during hot working, so the upper limit is preferably made 0.5%.

  Here, the “surface” means a region from the outermost surface of the component to a position at a depth of 100 μm. The method for measuring the surface solid solution Al amount will be described in the Examples section.

  Furthermore, in this invention, surface roughness Ra shall be 0.16 micrometer or less. In order to increase the sliding characteristics by reducing the friction coefficient, it is necessary to appropriately control the surface roughness in addition to the above-described components in steel and the above formulas (1) and (2). If the surface roughness Ra exceeds 0.16 μm, seizure due to contact between metals tends to occur, and it has been confirmed by experiments that the desired low friction cannot be obtained even if the components in the steel are appropriately controlled. (See the Examples section below). The surface roughness Ra is preferably as small as possible, preferably 0.14 μm or less, and more preferably 0.12 μm or less. Note that the lower limit of the surface roughness Ra is not particularly limited, but a low coefficient of friction can be stably obtained regardless of the steel composition, and is generally 0.02 μm or more in consideration of the balance with productivity. It is preferable.

  Here, the surface roughness Ra means the arithmetic average roughness Ra specified in JIS B0601 2001. In the present invention, Ra having a line segment length of 3.0 mm (reference length) was measured using a surface roughness meter (surface shape measuring device DEKTAK6M manufactured by ULVAC).

  Next, a method for obtaining such a case-hardened steel part will be described.

  In the present invention, in order to secure BN useful for reducing the coefficient of friction, the steel having appropriately controlled components as described above is used, and before the surface hardening treatment of the carburizing treatment or carbonitriding treatment (that is, in the part) Before processing) and after the surface hardening treatment, it is necessary to appropriately control the cooling rate particularly in the vicinity of 900 to 700 ° C. or in the vicinity of 900 to 800 ° C. Thereby, it is possible to secure the BN deposited before the surface hardening treatment without re-dissolving in the cooling process after the surface hardening treatment. Furthermore, in order to ensure surface-dissolved Al useful for reducing the friction coefficient, it is necessary to use steel with appropriately controlled components as described above, and particularly to control the atmosphere during the surface hardening treatment to a reducing atmosphere. There is. Thereby, the penetration | invasion of oxygen at the time of a surface hardening process is suppressed, and predetermined surface solid solution Al amount can be ensured.

  Hereinafter, it demonstrates in detail in order of a process.

  First, steel having the above chemical components is prepared, heated to 1100 ° C. or higher, and precipitates such as AlN and BN contained in the steel are once again solid-dissolved. That is, steel containing a large amount of Al of 0.16% or more as in the present invention has a large change in the solid solution state and precipitation state of Al, B, and N depending on the production conditions. Thus, AlN and BN contained in the steel can be re-dissolved in the steel.

  Next, it hold | maintains for 150 second or more in a 900-1050 degreeC temperature range. Thereby, BN can be selectively deposited. If the holding time is less than 150 seconds, the precipitation of BN does not proceed sufficiently and the BN becomes insufficient. A preferable holding time is 170 seconds or more, and more preferably 200 seconds or more. The upper limit of the holding time is not particularly limited, but 600 seconds or less is preferable because productivity deteriorates when held for a long time.

  The above-mentioned holding in the high temperature range of 900 to 1050 ° C. may be performed at a constant temperature, and may be heated and / or cooled within this temperature range, and the holding time in this temperature range is 150 seconds or more. I just need it.

  After keeping BN at 900 to 1050 ° C. as described above, cooling is performed by controlling the average cooling rate from 900 to 700 ° C. to 0.05 to 10 ° C./second. By controlling the passage time in the above temperature range in this way, it is possible to suppress the precipitation of AlN, prevent BN from changing to AlN, and secure the amount of BN precipitated. That is, since AlN is thermodynamically more stable than BN in the temperature range of 900 to 700 ° C., even if BN is selectively precipitated in the high temperature range of 900 to 1050 ° C. as described above, When the time for passing through the low temperature region of 700 ° C. becomes longer, BN changes to AlN, and the amount of BN deposited decreases. As described above, AlN contributes to the improvement of fatigue characteristics, but acts in the direction of deteriorating the machinability. Therefore, the average cooling rate in the low temperature range is set to 0.05 ° C./second or more. Preferably it is 0.1 degree-C / second or more, More preferably, it is 0.5 degree-C / second or more, More preferably, it is 1 degree-C / second or more. However, if the average cooling rate in the low temperature region is too large, a supercooled structure such as martensite or bainite is generated, and the machinability deteriorates. Therefore, the average cooling rate from 900 ° C. to 700 ° C. is set to 10 ° C./second or less. It is preferably 9.5 ° C./second or less, more preferably 8 ° C./second or less, further preferably 5 ° C./second or less, and particularly preferably 3 ° C./second or less.

  After cooling as described above, the surface is hardened by carburizing or carbonitriding after cutting into a part shape.

  The surface hardening treatment is preferably performed in a reducing atmosphere so as to ensure a predetermined amount of surface-dissolved Al. For example, under normal carburizing conditions, oxygen enters the outermost surface, so that the solid solution Al becomes an oxide such as alumina, and the amount of solid solution Al is greatly reduced. Therefore, surface hardening treatment is performed in a reducing atmosphere for the purpose of suppressing oxygen intrusion. Specifically, for example, by performing carburization of CP (carbon potential) of 0.85 or more under a mixed gas containing CO modified from propane gas, or by vacuum carburizing in the presence of acetylene gas, It is preferable to perform a curing treatment.

  Moreover, it is preferable to control the temperature at the time of a surface hardening process to about 900-1000 degreeC. When the above temperature exceeds 1000 ° C., AlN is liable to be dissolved, which may cause abnormal grain growth and deteriorate fatigue characteristics. The holding time at the above temperature is preferably about 30 minutes to 8 hours, for example.

  The type of carburizing or carbonitriding is not particularly limited, and known methods such as gas carburizing (gas carbonitriding), vacuum carburizing (vacuum carbonitriding), and high concentration carburizing (high carbon carburizing) can be employed. The degree of vacuum when vacuum carburizing (vacuum carbonitriding) is preferably about 0.01 MPa or less, for example.

  It cools after the said surface hardening process. In cooling, the average cooling rate from 900 ° C. to 800 ° C. is set to 1 ° C./second or more so that the precipitated BN does not re-dissolve.

  That is, the precipitation temperature of AlN is approximately 750 to 900 ° C., and the precipitation temperature of BN is approximately 600 to 1050 ° C. In the temperature range of 800 to 900 ° C., AlN is thermodynamically more stable than BN. Therefore, by shortening the time when passing through this temperature range, it is possible to suppress the BN precipitated in the steel from changing to AlN, and the desired BN amount can be maintained. Therefore, in this invention, the average cooling rate from 900 degreeC to 800 degreeC shall be 1 degree-C / sec or more. Preferably it is 2 degrees C / second or more, More preferably, it is 5 degrees C / second or more. In addition, although the upper limit of the average cooling rate in the said temperature range is not specifically limited, Considering the ease of temperature control etc., it is preferable that it is about 10 degrees C / sec or less in general.

  In cooling from 900 ° C. to 800 ° C., the above temperature range may be cooled at a constant rate, or the cooling rate may be changed in the middle. In short, the temperature range from 900 ° C. to 800 ° C. It is sufficient that the average cooling rate is 1 ° C./second or more.

  After cooling as described above, a quenching and tempering treatment is performed. Quenching and tempering conditions can be selected from conditions normally employed when manufacturing machine structural parts. For example, after quenching, it is preferable to perform tempering by holding at about 150 to 400 ° C. for about 20 minutes to 1 hour.

  After quenching and tempering as described above, finishing is performed by polishing the surface. Thereby, the surface roughness Ra of the final part can be made 0.16 μm or less. Moreover, since the outermost surface region into which oxygen easily penetrates is removed by the above-described finishing process, the amount of surface solid solution Al can be reliably ensured. The polishing method is not particularly limited, and examples thereof include polishing with a grindstone or abrasive grains. Further, the method of controlling the surface roughness Ra is not limited to the above polishing process, and for example, a finishing process such as electrolytic polishing may be employed.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. All of these are possible within the scope of the present invention.

Example 1
150 kg of steel having the chemical composition shown in Table 1 below (the balance: iron and inevitable impurities other than P, S, and O) is melted in a vacuum induction furnace, and the upper surface: φ245 mm × lower surface: φ210 mm × length: 480 mm ingot , Forging (soaking: about 1250 ° C. × about 3 hours, forging heating: about 1100 ° C. × about 1 hour) and cutting, and forging (φ80 mm, long) via a square shape of 150 mm on each side × length of 680 mm 350 mm round bar).

  The forged material thus obtained was heated to a predetermined temperature and then cooled. Table 2 shows the heating temperature (° C.), the holding time (second) in the temperature range of 900 to 1050 ° C., and the average cooling rate (° C./second) from 900 ° C. to 700 ° C. after holding.

  Next, the cooled round bar was cut into the shape of the test piece shown in FIG. 1, and then carburized or carbonitrided to give a case-hardened steel part.

(Gas carburizing)
No. obtained by cutting. Test specimens 1-7, 9, 12-15 were heated to 920 ° C., held at this temperature for 5 hours, and mixed with carbon potential (CP) 0.85 and CO modified with propane gas Gas carburization was performed in a reducing atmosphere of gas (propane-modified gas). Then, after hold | maintaining at 850 degreeC for 30 minutes, it put into the oil bath of 70 degreeC, quenched, and after tempering at 190 degreeC for 30 minutes, the surface was grind | polished with the grindstone and the finishing process was performed. After gas carburizing, the average cooling rate from 900 ° C. to 800 ° C. is shown in Table 2 above. In this example, in order to investigate the influence of the surface roughness Ra on the friction coefficient, the surface roughness Ra was changed by changing the roughness of the grindstone.

(Vacuum carburizing)
No. obtained by cutting. The test piece No. 11 was heated to 930 ° C., held at this temperature for 4 hours, and gas carburized in a reducing atmosphere of propane gas at a carbon potential (CP) of 0.85 and a pressure of 0.005 MPa or less. It was. Then, after hold | maintaining at 850 degreeC for 30 minutes, it put into the oil bath of 70 degreeC, quenched, and after tempering at 190 degreeC for 30 minutes, the surface was grind | polished with the grindstone and the finishing process was performed. Table 2 shows the average cooling rate from 900 ° C. to 800 ° C. after vacuum carburization.

(High concentration carburization)
No. obtained by cutting. Ten test pieces were heated to 1000 ° C., held at this temperature for 5 hours, and subjected to high-concentration carburization (high carbon carburization) at a carbon potential (CP) of 1.5 under a reducing atmosphere of propane gas. It was. Then, after hold | maintaining at 850 degreeC for 30 minutes, it put into the oil bath of 70 degreeC, quenched, and after tempering at 190 degreeC for 30 minutes, the surface was grind | polished with the grindstone and the finishing process was performed. Table 2 shows the average cooling rate from 900 ° C. to 800 ° C. after high concentration carburization.

(Carbonitriding)
No. obtained by cutting. The test piece 8 was heated to 960 ° C., held at this temperature for 5 hours, and carbonitriding was performed in a reducing atmosphere of a mixed gas of propane shift gas and ammonia at a carbon potential (CP) of 0.5. I did it.
On the other hand, no. No. 19 test piece was heated to 960 ° C. and held at this temperature for 10 minutes. Carbonitriding similar to that of No. 8 was performed.
Then, the above No. The test pieces 8 and 19 were both held at 850 ° C. for 30 minutes, then placed in a 70 ° C. oil bath, quenched, and tempered at 190 ° C. for 30 minutes, and then the surface was polished with a grindstone and finished. . After carbonitriding, the average cooling rate from 900 ° C. to 800 ° C. is shown in Table 2 above.

(Measurement of surface solid solution Al content)
In order to measure the amount of solid solution Al present on the surface of the case-hardened steel part obtained in this way, a portion between the surface and a depth of 100 μm is cut off with a lathe, and the chips are collected to obtain the amount of solid solution Al. Was calculated. Specifically, the total amount of Al and the amount of Al compound (Al ₂ O ₃ ) in the chips were measured by the following method, and the amount of surface solid solution Al was calculated based on the following formula. These results are also shown in Table 1.

Total Al amount: ICP emission spectroscopic analysis method Al compound amount: Halogen / methanol extraction (using 10% bromine methanol solution, filter size: 0.1 μm) → ICP emission spectroscopic analysis method Solid solution Al amount = (total Al amount)-( Al compound amount)

(Measurement of surface roughness Ra)
The surface roughness Ra of the case-hardened steel part obtained as described above was measured based on the method described above. These results are also shown in Table 1.

(Measurement of friction coefficient)
Using the case-hardened steel part obtained as described above, the friction measurement test shown in FIG. 1 was performed using a traction tester. Specifically, using the test roller processed into the shape shown in FIG. 2 and the mating roller shown in FIG. 3, the commercial roller oil (MTF-S made by Catrol) was heated to 110 ° C. and lubricated, and the test roller The friction coefficient at the time of sliding for 30 minutes at a rotational speed of 764 rpm (circumferential speed of 1.0 m / s), a counter roller rotational speed of 764 rpm (circumferential speed of 2.0 m / s), and a surface pressure of 2.0 GPa was measured. did. These results are also shown in Table 2.

  No. 1 to 15 are examples that satisfy the requirements defined in the present invention, and the friction coefficient is kept low regardless of the type of surface hardening treatment.

  On the other hand, the following examples that do not satisfy any of the requirements defined in the present invention have higher friction coefficients than the above-described examples of the present invention.

  No. No. 16 is an example in which the amount of Al in steel (total Al amount) is small, and below the value of Formula (1) (0.0001% or more), and a predetermined surface solid solution Al amount cannot be ensured. It became high.

  No. No. 17 is a component in steel, and the formulas (1) and (2) satisfy the requirements of the present invention, but the surface roughness Ra is large, and the friction coefficient is high.

  No. No. 18 is an example in which the amount of surface solid solution Al is small because the cooling rate after carburizing is slow, and the friction coefficient is high.

  No. No. 19 is an example in which the amount of surface solid solution Al is small because the carbonitriding time is too long, and the friction coefficient is high.

Claims (5)

A case-hardened steel part having a carburized layer or a carbonitrided layer on its surface,
Components in steel are
C: 0.1 to 0.5% (mass%, the same shall apply hereinafter)
Si: 0.03 to 2%,
Mn: 0.2-1.8%
Cr: 3% or less (excluding 0%),
P: 0.03% or less (excluding 0%),
S: 0.2% or less (excluding 0%),
Al: 0.16 to 0.5%,
B: 0.0005 to 0.008%,
N: 0.002 to 0.015%,
O: 0.002% or less,
The rest: iron and inevitable impurities
While satisfying the following formulas (1) and (2),
[N] -1.3 × [B] ≧ 0.0010% (1)
In the formula, [] is the content (%) of each element.
Solid solution Al ≧ 0.15% in the region from the outermost surface of the component to the depth of 100 μm (2)
A case-hardened steel part having excellent sliding characteristics, characterized in that the surface roughness Ra is 0.16 μm or less.   Furthermore, Mo is hardened steel part of Claim 1 containing 1% or less.   Furthermore, the case hardening steel part of Claim 1 or 2 containing Cu: 1% or less and / or Ni: 2.5% or less.   The case-hardened steel part according to any one of claims 1 to 3, further containing 1% or less of at least one element selected from the group consisting of Nb, Ti, and V.   The case-hardened steel part according to any one of claims 1 to 4, further comprising at least 0.1% of at least one element selected from the group consisting of Ca, Zr, Sb, Pb and Bi.