KR100507424B1 - Piston ring excellent in resistance to scuffing, cracking and fatigue and method for producing the same, and combination of piston ring and cylinder block - Google Patents

Abstract

Improves scuffing resistance, cracking resistance and fatigue resistance of high chromium martensitic stainless steels used in piston rings.

Stainless steel is C: 0.3-1.0%, Cr: 14.0-21.0%, N: 0.05-0.50%, total of at least one or more of Mo, V, W, and Nb: 0.03-3.0%, Si: 0.1-1.0%, Mn: 0.1-1.0%, P: 0.05% or less, S: 0.05% or less, the balance consists of Fe and unavoidable impurities, and the hard particles composed mainly of nitride on the surface of the sliding nitride layer have a mean particle diameter in the range of 0.5-2 µm, It is 7 micrometers or less in maximum particle diameter, and 5-30% of area ratio.

Description

Piston ring with excellent scuffing resistance, cracking resistance and fatigue resistance, and a method for manufacturing the same AND CYLINDER BLOCK}

The present invention relates to a piston ring for use in an internal combustion engine, and in particular, a high chromium martensitic stainless steel nitride piston ring having excellent scuffing resistance (sticking resistance), cracking resistance (fault resistance) and fatigue resistance and its manufacture It is about a method.

Recently, low fuel consumption, light weight, and high performance of internal combustion engines are required. Accordingly, in order to reduce the weight of piston rings due to light weight and high rotation, the piston ring needs to be improved in characteristics such as wear resistance, scuffing resistance, fatigue resistance, and the like. In view of heat resistance, conventional cast iron piston rings have been replaced by steel piston rings. Since steel piston rings are less inferior in scuffing resistance compared to cast iron piston rings, some surface treatment is usually applied to the sliding surfaces. Steels for piston rings are largely divided into carbon steel, silicon chromium steel, and martensitic stainless steel according to the type of surface treatment to be combined, mainly chromium plating in carbon steel and silicon chromium steel, and gas nitriding in martensitic stainless steel. In the conventional steel piston ring, chromium plating is mostly performed, but in recent years, nitriding piston rings have become the mainstream due to scuffing of the plating layer at high loads and environmental problems of waste liquid treatment.

In high chromium martensitic stainless steel, C: 0.80-0.95%, Cr: 17.0-18.0%, Si: 0.25-0.50%, Mn: 0.25-0.40%, Mo: 0.70-1.25%, V: 0.07-0.15%, Fe: JIS SUS440B equivalent material which has a composition of remainder is a main steel grade used for a nitride piston ring. When nitriding is performed on a steel having this composition, nitrogen atoms penetrate into the steel from the surface and diffuse to form a nitride layer. The nitride in the nitride layer is mainly a compound with Cr, V, Mo, or a compound thereof in which Fe is dissolved. In particular, Cr, which is contained in a lot of steel, exists as Cr carbide in addition to solid solution in the base.Since the affinity with nitrogen is greater than that of carbon, nitrogen and Cr carbide, which are diffused from the surface by nitriding, react to form Cr nitride. do. Since SUS440B equivalent material has Cr as many as 17.0-18.0%, the nitride layer of the comparatively high hardness by which hard Cr nitride was disperse | distributed by moderate area ratio can be obtained for the reason mentioned above, and shows the outstanding wear resistance and scuffing resistance.

Recently, as martensitic stainless nitride steel for piston rings, in Japanese Unexamined Patent Publication No. 11-80907, even if Cr is slightly lower than 5.0 to less than 12.0, Si: 0.25% or less, Mn: 0.30% or less, Mo, W, V, Nb It is known that excellent coffee resistance can be obtained by containing one or two or more kinds: 0.3-2.5% or Cu: 4.0% or less, Ni: 2.0% or less, Al: 1.5% or less. 106874 discloses that a piston ring material having excellent workability in addition to scuffing resistance can be obtained by setting the content of M ₇ C ₃ type carbide present in the structure to 4.0% or less in area%.

However, the nitride piston ring exhibiting such excellent wear resistance and coffee resistance also causes a problem of scuffing when used in an internal combustion engine with high rotation and high output load. In particular, in the case of diesel engines, the direction of changing the conventional liner from the method of inserting the conventional liner into the cylinder block from the viewpoint of weight reduction and cost reduction, and the combustion pressure from the viewpoint of purifying exhaust gas or increasing the output of the exhaust gas are increased. There is a direction to let. In the cast iron monoblock, the microscopic structure of the sliding surface with the piston ring has a large variation in the dispersion state of the graphite due to the nonuniformity of the cooling rate, and the soft ferrite phase that causes scuffing is ubiquitous. When the cylinder surface having such a microscopic structure and the martensitic stainless steel nitride piston ring are combined, scuffing occurs well at the beginning of operation. The causes are as follows. That is, when the cylinder surface is honed, the loading of the grindstone is easily caused by localized ferrite, and the roughness of the cylinder surface after honing is likely to be rough.The graphite is covered by the plastic flow, and consequently, the area ratio of graphite is lowered. Deterioration of lubrication and oil storage function and, in the case of high combustion pressure, back pressure on the piston ring also increases. Most of this scuffing is caused by cracks perpendicular to the sliding direction of the piston ring outer circumferential surface, and the cracks are almost parallel to the surface formed at the grain boundaries of the nitride layer of the piston ring sliding surface, and are relatively coarse grain boundary layered compounds. Is also known as the seagull phase).

These problems are further addressed by surface treatments such as TiN and CrN by ion plating, which have excellent wear resistance and scuffing resistance. However, the manufacturing cost is higher than that of nitriding, which does not satisfy the user in terms of cost efficiency.

Therefore, the object of the present invention is not to cause problems of wear, scuffing, cracking and fatigue, even when used in high-speed, high-combustion, high-load internal combustion engines, especially diesel engines employing lightweight cast iron monoblocks that are expected to increase in the future. The present invention also provides a high chromium martensitic stainless steel nitride piston ring which is excellent in cost efficiency. In particular, it is an object of the present invention to control the size and distribution of hard particles, especially nitrides, in nitride layers of high chromium martensitic stainless steel, and to reduce the size of grain boundary layered compounds to suppress cracking and propagation associated with these compounds. It is done. The present invention also provides a method for producing a high chromium martensitic stainless steel piston ring.

Fig. 1 is a reflection electron image of a scanning electron microscope on the surface of a sliding nitride layer, in which Fig. 1 (a) corresponds to Example 1 and Fig. 1 (b) corresponds to Comparative Example 1.

2 is an optical micrograph of a cross section of a nitride layer, in which FIG. 2A corresponds to Example 1, and FIG. 2B corresponds to Comparative Example 1. FIG.

It is a figure which shows the test piece of a scuffing test.

4 is a view showing an operating mechanism of the friction wear tester.

It is a figure which shows the operation mechanism of a piston ring fatigue tester.

6 is a graph of fatigue limit diagrams.

7 is a crack photograph generated on the sliding surface of Comparative Example 13. FIG.

Disclosure of the Invention

Piston Rings for Automobiles According to the Automotive Piston Rings editorial committee, Sankaido, p.188, 1997, the scuffing of piston rings places concentrated loads on the minute uneven protrusions (especially soft protrusions) of the sliding surface. This is explained by the fact that the temperature rises due to the high frictional heat and abnormal softening and melting occurs.

The nitride layer structure of high chromium martensitic stainless steel is generally in the form of hard nitrides dispersed in the matrix of tempered martensite. Scuffing is largely related to the fine irregularities of the sliding surface in the mechanism, that is, the size and dispersion state of the hard particles dispersed in the relatively soft matrix. When the surface layer having such a structure is observed in the cross section, the soft layer becomes relatively concave in contact with the sliding surface to which convex hard particles face. That is, the probability that the nitride steel is in direct contact with the counterpart is reduced, and a lubricating oil film is formed in the concave portion, and when sliding, pressure is generated in the oil film to reduce the contact pressure and lubricate the convex contact to prevent scuffing. can do. In order to exhibit the effect as a convex hard particle by such a mechanism, it is preferable that the particle size of submicron to several micron size is needed, and the dispersion amount is 5% or more by area ratio. In the case where the hard particles are extremely small or the amount of dispersion is small, the mechanism due to the action effect of the convex hard particles cannot be expected.

However, such a scuffing prevention mechanism also depends on the situation of the sliding surface of the counterpart. In the cast iron monoblock having an inhomogeneous structure as described above, the surface roughness of the cylinder surface tends to be roughened by grinding wheel, or graphite is often blocked by the plastic flow of ferrite phase. Such cast iron also causes the following phenomenon due to proper sliding (also referred to as "breaking in" among those skilled in the art). In other words, the rough surface of the inner circumferential surface of the cylinder is smoothed and the graphite blocked by the ferrite phase is opened. In the period until the break-in is completed, the oil film on the sliding surface breaks well, and a large friction force is repeatedly loaded on the outer circumferential surface of the piston ring. Thus, cracks are generated and enlarged in the nitride layer on the outer circumferential surface of the piston ring in a direction perpendicular to the sliding direction due to the cyclic stress caused by the frictional force. The stresses that are loaded with the break-in of the cylinder's inner circumference are reduced, but the cracks develop over time, causing local surface peeling or lack, and the flaws of the cylinder's inner circumference, which cause scuffing. The grain boundary compounds present in the nitride layer are very brittle and thus promote crack generation and growth. Therefore, in order to prevent such initial scuffing, by uniformly and widely dispersing hard particles mainly composed of Cr nitrides of the nitride layer in an appropriate size, the contact probability between the matrix and the cylinder is reduced, and in particular, the grain boundary compound produced by the nitriding treatment is removed. The present inventors have found that it is necessary to suppress the occurrence of cracks related to grain boundary compounds and to prevent the expansion by finely dividing the propagation even when cracks occur.

Also, in high chromium martensitic stainless steels, eutectic Cr carbide (η phase: (Cr, Fe) ₇ C ₃ ) precipitates at the primary austenite grain boundary when molten steel solidifies. do. Cr carbides with a maximum size (length) of more than 20 μm are observed even after hot rolling, spheroidizing heat treatment and final quenching / tempering heat treatment. Regarding the refinement of the coarse primary eutectic carbide, fine Cr carbide structures are obtained by adding 0.25% or more of nitrogen (N) to iron and steel, Vol. 82, No. 4, p. 309-314 (1996). Can be reported. According to this report, the process Cr carbide of the primary γ grain boundary is lost, and instead, lamellar phases M ₂₃ C ₆ and M ₂ N (M: Cr, Fe) are precipitated around the primary γ grain boundary and precipitated into these lamellar phases. M ₂₃ C ₆ and M ₂ N are finely divided by hot rolling, and in the subsequent spheroidization annealing, fine M ₂₃ C ₆ is newly precipitated at the site different from M ₂ N, so that it becomes a totally fine Cr carbide structure. It is. Heat treatment, Vol. 36, No. 4, p.234-238 (1996) also showed the highest quenching hardness with increasing N content for the mechanical properties of 16.5% Cr-0.65% C martensitic stainless steel with 0.25% N. It is reported that the indicated temperature is shifted to the low temperature side and the ductility is increased, and the reason is that the higher the quenching temperature, the higher the amount of N dissolved in the austenite phase and the austenite phase is stabilized.

Japanese Unexamined Patent Application Publication No. 9-289053 and Japanese Unexamined Patent Application Publication No. 9-287058 disclose a rolling bearing using a technique for miniaturizing Cr carbides by adding these N.

The present inventors have considered the above-mentioned scuffing mechanism, and the addition of N to the grain boundary layered compound that is substantially parallel to the surface formed at the grain boundary of the nitride layer of the piston ring sliding surface where cracks are observed. As a result of intensive research on the background of the micro-crystallization of Cr carbide, the microstructure of the nitride layer has a lot of nitrides, and the grain boundary layer compound in the nitride layer is fine. It has been found that high chromium martensitic stainless steel nitride piston rings excellent in abrasion resistance, scuffing resistance, cracking resistance, and fatigue resistance can be obtained even when used in high internal combustion engines, especially in light weight cast monoblock diesel engines.

That is, in the high chromium martensitic stainless steel nitride piston ring of the present invention, the high chromium martensitic stainless steel nitride ring has a weight percentage of C: 0.3-1.0%, Cr: 14.0-21.0%, N: 0.05-0.50%, Mo At least one sum of V, W, and Nb of at least 0.03-3.0%, Si: 0.1-1.0%, Mn: 0.1-1.0%, P: 0.05% or less, S: 0.05% or less, the balance of Fe and unavoidable impurities Hard particles of nitrides, carbides, and carbonitrides mainly composed of nitrides on the surface of the sliding nitride layer, having an average particle diameter in the range of 0.5-2 µm, a maximum particle diameter of 7 µm or less, and an area ratio of 5-30%. It is done. Moreover, the magnitude | size (length) of the grain boundary compound observed in the cross section of the nitride layer of the longitudinal direction of a piston ring is characterized by the maximum 20 micrometers or less. In addition, the hardness of the sliding surface nitride layer having the above-described structural characteristics is characterized in that the Vickers hardness is in the range of 900-1400, the depth of the nitride layer has a sufficient thickness on the surface subjected to the nitriding treatment.

In the method for producing a high chromium martensitic stainless steel nitride piston ring of the present invention, first, a steel having a predetermined composition is dissolved, added with nitrogen, and cast into an ingot, followed by hot rolling, annealing, cold wire drawing, and cold rolling to provide a predetermined method. Piston ring is close to the cross-sectional shape and hardened and tempered to make a wire. The piston ring is bent into a ring shape, and a piston ring is manufactured through stress relief heat treatment, side rough grinding, nitriding, surface compound layer removal, seam gap grinding, side finish grinding, and outer roughing. In the quenching process before bending to the ring, the high chromium martensitic stainless steel at 850-1000 ° C. is quenched at a relatively low temperature to obtain a fine and as much carbide-dispersed material structure. In addition, nitriding may use gas nitriding, ion nitriding, radical nitriding, and all are treated for 1-20 hours in the 450-600 ° C range.

Next, the present invention will be described in detail. Referring to the components of the high chromium martensitic stainless steel of the present invention, first, C is solid-dissolved into Fe to improve the known hardness, and at the same time easily combine with Cr, Mo, V, W, and Nb to form carbides. easy. By nitriding, carbides in the nitride layer are mainly converted to nitrides to improve wear resistance and scuffing resistance on the sliding surface of the piston ring. If C is less than 0.3%, there is little increase in hardness or formation of carbides. If it exceeds 1.0%, coarse molten steel is solidified and a large amount of eutectic Cr carbide (η phase: M ₇ C ₃ ) is determined to be used in subsequent wire production. C is in the range of 0.3-1.0% because workability is extremely degraded. Preferably it is 0.4-0.9% of range.

Since Cr is solid-dissolved in Fe, it improves heat sedimentation resistance by strengthening solid solution in addition to improving corrosion resistance. Here, the thermal sedimentation refers to a phenomenon in which the seal property deteriorates due to a decrease in tension caused by a creep phenomenon while the piston ring is used at a high temperature. It also reacts with C in the steel to form Cr carbide. This Cr carbide easily reacts with N which penetrates from the surface by nitriding treatment, and becomes CrN in the nitride layer and is dispersed as hard particles. These hard particles in the nitride layer significantly improve the wear resistance and scuffing resistance of the piston ring sliding surface. If the amount of Cr is less than 14%, the formation of Cr compound is small, and if it exceeds 21%, the toughness due to δ ferrite generation or the Cr concentration in the matrix becomes too high, leading to a decrease in Ms (martensite transformation start temperature) to obtain sufficient hardening hardness. Since there is a case where it is not, the amount of Cr is made into 14 to 21% of range. Preferably it is 16-19% of range.

N is solidly dissolved in Fe in the same manner as C. By adding N, the C concentration at the left end of the process line of the concentration cross section, such as 17% Cr in the Fe-Cr-C phase diagram, shifts to the concentration side higher than the C concentration of the thickened molten steel present at the initial grain boundary during the solidification process. Therefore, process reaction is suppressed and precipitation of (eta) phase is suppressed therefore. In the subsequent cooling, supersaturated C, N precipitates around the primary γ grain boundaries as lamellar phase M ₂₃ C ₆ and M ₂ N precipitates. When N is less than 0.05%, the η phase precipitates, and when it exceeds 0.50%, the amount of precipitation of rod-shaped M ₂ N increases and the toughness decreases, so that N is in the range of 0.05-0.50%. Preferably it is 0.10 to 0.30% range. In addition, employment of N base into inhibits the C diffusion in the base, and contributes to the refinement of the grain boundary compound (finally Fe ₃ C in the form of changes in Fe ₃ N). N is added requires a solvent in a pressurized N ₂ atmosphere, if may be added at normal pressure is less than 0.2% (常壓), exceeds 0.2%. That is, the range of 0.05-0.20% is preferable from the viewpoint of N addition.

Mo, V, W, and Nb are all carbide generating elements, improving abrasion resistance and scuffing resistance. In addition, Mo has a function of preventing softening in tempering and nitriding treatment, and plays an important role in the dimensional stability of the piston ring. V has the effect of improving the hardness of the nitride layer as a nitride promotion element. Therefore, it is useful because it improves all the performances required for any element or piston ring, but if the sum of at least one or more of Mo, V, W, and Nb is less than 0.03%, the effect is almost insignificant. Since it inhibits and reduces toughness, the sum total of at least 1 or more types of Mo, V, W, and Nb shall be 0.03-3.0% of range.

Si is added as a deoxidizer and solid-solution in Fe improves temper softening resistance and improves the so-called heat sedimentation resistance. If it is less than 0.1%, the effect is small. If it exceeds 1.0%, the toughness is lowered, so Si is in the range of 0.1 to 1.0%.

Mn is also added as a deoxidizer similarly to Si. If it is less than 0.1%, the effect is small. If it exceeds 1.0%, the workability is lowered, so Mn is in the range of 0.1-1.0%.

Since P forms inclusions such as Mn to lower the fatigue strength and lower the corrosion resistance, as few impurities as steel are preferable. Therefore, it is made into 0.05% or less from a practical viewpoint. Preferably it is 0.03% or less.

Since S reduces fatigue strength and corrosion resistance similarly to P, it is preferable to use as few impurities as steel. Therefore, it is made into 0.05% or less from a practical viewpoint. Preferably it is 0.03% or less.

In order to make the steel having the composition in the above range into a structure excellent in scuffing resistance, it is necessary to have a large amount of nitride in the nitride layer. That is, hard particles of nitrides, carbides, and carbonitrides mainly composed of Cr nitrides on the surface of the sliding nitride layer have a mean particle size in the range of 0.5-2 µm, a maximum particle diameter of 7 µm or less, and an area ratio of 5-30%. If the average particle diameter is 0.5 µm or less, the effect of the anti-scuffing convex hard particles cannot be expected. If the average particle diameter exceeds 2 µm, the scuffing problem remains when the load is high. Moreover, when the maximum particle diameter exceeds 7 micrometers, the uniformity of a tissue will fall, and also, when a load is high, a scuffing problem remains. If the area ratio is less than 5%, there is a problem in scuffing, and if it exceeds 30%, it becomes difficult to process the wire rod after the solvent and bend the wire rod into a ring shape. Preferably it is 10-25%. In addition, in order to make a structure excellent in cracking resistance in this invention, the magnitude | size (length) of the grain boundary compound observed in the cross section of the nitride layer of the piston ring longitudinal direction which consists of a matrix and hard particle is 20 micrometers or less at most. If the maximum length exceeds 20 mu m, problems related to cracking occur when the load is high.

The nitride layer structure of the present invention as described above originates in the microstructure of stainless steel. In this structure, first, after quenching / tempering through hot rolling, spheroidizing heat treatment, cold wire drawing, or the like, there is no η phase ((Cr, Fe) ₇ C ₃ ) of the coarse eutectic Cr carbide. This can be achieved by adding nitrogen.

Second, secondary carbides (ε phase: (Fe, Cr) ₂₃ C ₆ ), which are precipitated when maintained at the quenching temperature before nitriding, are finely present. Considering this point according to the Fe-Cr-C-based state diagram, in the (γ + ε) region, as the temperature decreases, more carbides are equilibrated, so that the low temperature region of the (γ + ε) region is quenched so that By doing so, it is possible to deposit as much epsilon carbide as possible. In addition, the quenching from the low temperature region suppresses the growth of the? Grains, so that the? Grains can be made fine, that is, the grain boundary compound phase formed in subsequent nitriding can be made fine. Preferred quenching temperatures in this respect are in the range of 850-1000 ° C. If it is less than 850 ° C, it is not quenched or a predetermined hardness cannot be obtained by precipitation of the α phase. At a quenching temperature exceeding 1000 ° C., agglomeration of carbides and coarsening of γ grains occur at the step of maintaining the quenching temperature, and as a result, the nitride and grain boundary phases formed in the subsequent nitriding treatment also coarsen. The high hardness of 900-1400 can be obtained in the nitride layer to a sufficient depth for a relatively short time, and thus, a relatively fine γ grain can be obtained by low quenching temperature, and the grain boundary which serves as a major diffusion path of N in the nitriding treatment is increased. Is caused by one. In the present invention, the nitriding treatment in the range of 450-600 ° C. was thought to be due to the maximum solubility of N in the α-Fe lattice at about 590 ° C. none. From the viewpoint of the shape stability of the piston ring, it is preferable to treat it at the lowest temperature, but from the practical point of view, it is 1-20 hours in the 450-600 degreeC range.

The present invention will be described in more detail with reference to the following specific examples.

Examples 1-11 (J 1-J 11), Comparative Examples 1-8 (H 1-H 8)

High chromium martensitic stainless steel having the chemical composition shown in Table 1 was dissolved in a 10 kg vacuum induction furnace. However, the steel is less than 0.2% N is added at normal pressure and nitrogen, more than 0.2% N steel were solvent from the pressurized N ₂ atmosphere. Next, a linear material having a diameter of 12 mm was subjected to hot working, and after acid pickling, spheroidization annealing was carried out at 750 ° C. for 10 hours, and then processed into a wire having a 3.5 mm × 5.0 mm rectangular cross section through a predetermined process. Here, quenching and tempering are performed by air-quenching the quenching furnace (Ar atmosphere) at 930 ° C. for about 10 minutes, and then performing the tempering furnace (Ar atmosphere) at 620 ° C. for about 25 minutes, and further nitriding. The wire rod was cut into 50 mm lengths, and gas nitridation was performed at 570 ° C. for 4 hours. However, about the quenching temperature of the comparative example 1 (H1), it implemented at 1100 degreeC conventionally practiced conventionally. Other conditions are the same as that of another Example and a comparative example.

In addition, each wire test piece was cut into a 10 mm length for microscopic structure observation, embedded in a resin, polished to a mirror surface, and the observation of the tissue and the quantification of the tissue were performed by an image analysis device. 1 and 2 show reflection electron micrographs (a, b) of the scanning electron microscope and the nitride layer of the sliding nitride layer surfaces of Example 1 (J 1) and Comparative Example 1 (H 1). The optical microscope photograph of a cross section ((a), (b) of FIG. 2) is shown. Hard particles are black in reflective electron micrographs and white in optical micrographs. In the present invention, it can be seen that the size of the hard particles is small and the size of the grain boundary compound in the cross section of the nitride layer is very small. Table 2 shows the average particle diameter, the maximum particle diameter, and the area ratio of the hard particles on the sliding surface nitride layer surface as a quantitative result of the structures of Examples 1-11 (J 1-J 11) and Comparative Examples 1-8 (H 1-H 8). (%) And the maximum length of the grain boundary compound of the nitride layer cross section, and the hardness of the sliding surface nitride layer surface.

Hard particles on the sliding surface nitride layer surface Maximum length of grain boundary compound on the surface of nitride layer (㎛) Vickers Hardness Average particle size (㎛) Particle size (㎛) Area rate (%) J1 1.6 5 17.2 16 1253 J2 1.3 4 13.0 15 1050 J3 1.0 5 22.5 13 1185 J4 1.7 6 15.9 12 1120 J5 1.6 5 17.1 15 1148 J6 1.5 4 10.7 14 955 J7 0.9 4 21.0 12 1219 J8 1.2 6 18.0 13 1193 J9 1.3 6 13.0 12 984 J10 1.8 5 14.2 17 1031 J11 1.2 5 16.2 14 1083 H1 2.7 15 13.6 28 1065 H2 * * * * * H3 1.5 5 7.5 15 830 H4 * * * * * H5 1.4 5 4 14 920 H6 2.2 8 9.1 14 874 H7 ** 1.6 5 16.5 16 1109 H8 * * * * *

* Comparative Examples 2, 4 and 8 (H 2, H 4 and H 8) were poor in workability and could not be wired.

** In Comparative Example 7 (H 7), the yield after nitriding was unstable.

The scuffing test is a U-shaped two-pin integrated test piece having a total length of 45 mm shown in FIG. 3 manufactured by a wire test piece, and a friction wear tester (a Riken manufactured by Riken) "Tribolic I"). The sliding surface at the tip of the pin (Fig. 4, reference numeral 1) is a convex shape having a radius of 20 mm, and the compound layer (white layer) having a thickness of 5 to 20 µm formed on the surface by gas nitriding is removed by grinding and polishing. Finish with a mirror. In addition, the disc (FIG. 4, 2) of FC250 used what adjusted the surface roughness Rz of the sliding surface to 1-2 micrometers. The operating mechanism of the friction wear tester is shown in Fig. 4 in the following scuffing test conditions.

Sliding Speed (Plate): 8m / sec

Pressurized Weighting: Increases every 1.0 MPa from initial 1.0 MPa, stepping up to scuffing

Lubricant oil: Motor oil (brand name, Nichishi Motor Oil P # 20)

Lube oil temperature: 80 ° C (near the exit)

Oil bath: 100 ℃

Lubricant Supply: 40cc / min

Scuffing surface pressure was calculated from the pressure weighting when scuffing occurred and the wear area of the sliding surface. Table 3 shows the scuffing surface pressures of Examples 1-11 (J 1-J 11) and Comparative Examples 1-8 (H 1-H 8).

Scuffing Surface Pressure (MPa) J 1 454 J 2 443 J 3 469 J 4 428 J 5 458 J 6 420 J 7 464 J 8 430 J 9 441 J 10 419 J 11 452 H 1 376 H 2 - H 3 340 H 4 - H 5 328 H 6 297 H 7 388 H 8 -

Examples 1-11 (J 1-J 11) according to the present invention can be seen that the scuffing resistance is improved compared to Comparative Examples 1, 3, 5-7 (H 1, H 3, H 5-H 7). .

Examples 12-14 (J 12-14) and Comparative Examples 9-11 (H 9-H 11)

The material of the chemical composition of Example 1 was quenched by air-quenching from the quenching temperature shown in Table 4 in the quenching process after wire rod processing, and quantified about the nitride layer structure which carried out the gas nitriding through the same process as Example 1. The results are shown in Table 4.

Quenching temperature (℃) Hard particles on the sliding nitride layer surface Maximum length of grain boundary compound of nitride layer cross section (㎛) Average particle size (㎛) Particle size (㎛) Area rate (%) H 9 * 800 0.3 5 15.4 14 J 12 870 0.5 5 19.4 11 J 13 920 1.3 6 18.5 15 J 14 980 1.8 6 17.4 18 H 10 1030 2.3 9 14.7 31 H 11 1080 2.8 11 11.5 49

In Comparative Example 9 (H 9), the nitride layer hardness was low at 860.

Example 15 and Comparative Example 12

Pressure ring of rectangular cross section of nominal diameter (d ₁ ) 95.0 mm, length (a ₁ ) 3.35 mm, width (h ₁ ) 2.3 mm from the steels of Example 1 and Comparative Example 1 through a predetermined process (Example 15, comparison Example 12) was processed. Here, quenching and tempering are performed by air cooling of the quenching furnace at about 930 ° C. for about 10 minutes, and then the tempering furnace is continuously passed at about 620 ° C. for about 25 minutes. Was carried out. However, about the quenching temperature of the comparative example 12, it implemented at 1100 degreeC conventionally practiced conventionally. Other conditions are the same as in Example 15.

The fatigue test was performed with the piston ring fatigue tester which has the operation mechanism shown in FIG. 5 using the produced pressure ring. That is, the product 3 which cuts the both ends of the joint and expands the free joint dimension is set to the tester in a state of being closed to the ring nominal diameter, and the stroke of the load stressed portion by the eccentric cam 4 in the direction of further closing in this state. The ring was broken by repeatedly giving 40 cycles / second, and the number of stress loads at the time of breaking was determined. This test was repeated while changing the load stress with respect to the sample of the same specification, the so-called S-N diagram was created, and finally the fatigue limit diagram was calculated | required.

Although the fatigue limit diagram is shown in FIG. 6, compared with the comparative example 12, it can be seen that it is greatly improved in Example 15 of this invention.

Examples 16-19 and Comparative Examples 13-14

From the steel materials of Example 1 (Examples 16 and 17), Example 7 (Examples 18 and 19) and Comparative Example 1 (Comparative Examples 13 and 14), the nominal diameter d ₁ was 99.2 mm and the thickness ( a ₁ ) A pressure ring (Examples 16 and 18 and Comparative Example 13) having a rectangular cross section of 3.8 mm and a width h ₁ and 2.5 mm and a nominal diameter (d ₁ ) 99.2 mm, thickness (a ₁ ) 2.5 mm and width ( h ₁ ) A main body (Examples 17 and 19 and Comparative Example 14) of a two-piece oil ring having an angled cross section of 3.0 mm was processed. The heat treatment of quenching / tempering and gas nitriding were carried out in the same manner as in Examples 15 to 19 and Comparative Examples 13 to 14, respectively.

The pressure ring and the oil ring thus prepared were subjected to a durability test for 100 hours under the following conditions using a 4 cylinder 3200cc cast iron monoblock diesel engine.

RPM: 3600rpm

Output: 75kW

Load: full load

Water temperature: 110 ℃

Oil temperature: 130 ℃

Comparative Example 13 caused scuffing at 2 hours and 10 minutes after the start of the test, and Comparative Example 14 occurred at 7 hours and 55 minutes after the start of the test, whereas in Examples 16 to 19, the test was completed without any problem. The crack photograph which generate | occur | produced on the sliding surface of the comparative example 13 is shown in FIG.

As described above, the high chromium martensitic stainless steel nitride piston ring according to the present invention has a fine amount of nitride in the nitride layer due to the finer technique of Cr carbide due to nitrogen addition and quenching from a relatively low temperature. Since the grain boundary layer compound in the layer becomes a fine microscopic structure and has excellent abrasion resistance, scuffing resistance, cracking resistance, and fatigue resistance, it can be used for internal combustion engines with high rotation and high power load, especially for light cast iron monoblock diesel engines. Will be. In addition, it can be effectively used for the fatigue of the piston ring when using the exhaust brake in a small truck. In addition to the pressure ring, the applicable piston ring can be suitably used in the body of the two-piece oil ring or the rail of the three-piece oil ring.

Claims (8)

In a piston ring made of high chromium martensitic stainless steel having a surface nitride layer, the high chromium martensitic stainless steel has a weight% of C: 0.3-1.0%, Cr: 14.0-21.0%, and N: 0.05-0.50. Total of at least one of%, Mo, V, W, and Nb: 0.03-3.0%, Si: 0.1-1.0%, Mn: 0.1-1.0%, P: 0.05% or less, S: 0.05% or less, the balance being Fe and Hard particles composed of nitrides, carbides and carbonitrides on the surface of the sliding nitride layer having an unavoidable impurity range from 0.5 to 2 μm in average particle size, 7 μm or less in maximum particle diameter, and 5 to 30% in area ratio. Piston rings with excellent scuffing, cracking and fatigue resistance. The piston ring according to claim 1, wherein the size (length) of the grain boundary compound observed in the cross section of the nitride layer in the longitudinal direction of the piston ring is at most 20 µm or less. The piston ring according to claim 1 or 2, wherein the nitrogen content of the high chromium martensitic stainless steel is in a weight% range of N: 0.05-0.20%. The piston ring according to claim 1 or 2, wherein the Vickers hardness of the sliding surface nitride layer is in the range of 900-1400. In the method for producing a piston ring for nitriding the surface of high chromium martensitic stainless steel, by weight% C: 0.3-1.0%, Cr: 14.0-21.0%, N: 0.05-0.50%, Mo, V, W, Nb Total of at least one or more of: 0.03-3.0%, Si: 0.1-1.0%, Mn: 0.1-1.0%, P: 0.05% or less, S: 0.05% or less, the balance being high chromium martensite composed of Fe and inevitable impurities A method for producing a piston ring having excellent scuffing resistance, cracking resistance and fatigue resistance, wherein the stainless steel is quenched at a temperature in the range of 850 to 1000 ° C. in the quenching process before bending the piston ring. The piston ring according to claim 1 or 2, wherein the piston ring is combined with a cast iron monoblock cylinder. 4. The piston ring of claim 3 wherein the Vickers hardness of the sliding surface nitride layer is in the range of 900-1400. 4. The piston ring of claim 3 wherein the piston ring is combined with a cast iron monoblock cylinder.