Material for sliding parts having self lubricity and wire material for piston ring

Abstract

A material for use as self-lubricating sliding parts, which consists of a steel containing, by mass, from not less than 0.4% to less than 1.5% of C (carbon), 0.1 to 3.0% of Si, 0.1 to 3.0% of Mn, from inclusive zero to 0.5% of Cr, 0.05 to 3.0% of Ni, 0.3 to 2.0% of Al, 0.3 to 20% in total (Mo+W+V) of at least one element selected from the group consisting of Mo, W (tungsten) and V (vanadium), and 0.05 to 3.0% of Cu, wherein there can be observed graphite particles having an average particle size of not more than 3 μm in a section of the metal structure of the steel. The steel is also used as piston rings. In the steel, the graphite particles observed in the structural section occupy an area rate of not less than 1% in the overall area of the structural section, and have an average particle size of not more than 3 μm. The steel may further contain not more than 0.3% of S and / or not more than 0.01% of Ca. The steel is subjected to nitriding treatment for use.

Description

TECHNICAL FIELD [0001] The present invention relates to material for use as sliding parts, e.g. piston rings, cylinder liners and vanes, which are incorporated in automotive engines or other internal combustion engines, as well as usual plain bearings, roller bearings, ball bearings, gears and dies. BACKGROUND ART [0002] Materials having excellent wear resistance property have been applied so far to sliding parts such as cylinder liners and vanes. Materials for piston rings used in internal combustion engines, especially automotive engines, have been changed from cast steel to steel wire, which is processed to ring forms. In usual, the piston rings are produced in such a manner that an ingot with a predetermined composition is hot-worked to wire by forging, hot-rolling or the like, the thus obtained wire is further formed to have a given cross-sectional form corresponding to a small sectional form of a piston ring by drawing or the like, the formed wire is conditioned to have a give...

AI Technical Summary

Benefits of technology

[0020] A key aspect of the invention is realization of the particular metal structure of steel, in which fine graphite particles are precipitated in a steel matrix by a proper rate, in order to improve seizure resistance property of steeland lower a friction coefficient of sliding parts such as piston rings. Namely, the present invention aims at improvement of durability fully taking peculiar frictional motions between piston rings and cylinders into consideration. According to the invention metal structure, it is possible to attain satisfactory advantageous effects in the above posed subjects which are to improve seizure resistance property of steel and lower a friction coefficient, and on which the prior art has been unsatisfactory even in connection with non-surface-treatment material or nitrided material which is advantageous in a cost of surface-treatment.

[0021] With regard to the invention metal structure with graphite precipitates, the present inventors sought for chemical components which enable fully rapid precipitation of fine graphite particles and also be effective in improvement of workability and machinability of steel in order to establish the metal structure as industrially applicable means. Fruitful results are that further improvement in the above effects could be attained by addition of a single element of sulfur or of sulfur and Ca to the steel. This is another key feature of the present invention.

[0022] First, the invention metal structure with graphite precipitates will be described.

[0023] A sliding part is mainly designed for fluid lubrication, wherein a fluid film such as oil or water is constantly formed between mechanical elements which are brought into sliding contact with each other under severe conditions. Formation of the fluid film leads to application of buoyancy to a relatively moving fluid, as noted by relative motion of an airplane in the air. The fluid film between sliding parts becomes thicker as viscosity rise of the fluid or an increase of a relative velocity, so that mechanical elements are protected from abrasion.

[0024] However, most of internal combustion engines are the reciprocation type that a relative velocity between a piston ring and a cylinder becomes zero in the vicinity of upper and lower dead points, so that sliding parts suffer often wear and / or seizure due to break of the fluid film, resulting in abnormal motion of internal combustion engines. Furthermore, since inhibition of oil inflow into a combustion chamber, so-called as “oil scraping”, becomes an important factor for advanced performance of piston rings, it is difficult to maintain the fluid film in a state suitable for the purpose.

[0025] Nevertheless, the inventors have reviewed various fluid lubricating modes during sliding motion for effective use of fluid lubrication in order to overcome the above problems. In short, the fluid lubricating mode comprises three actions, i.e. (1) wedging, (2) expansion / contraction, and (3) squeezing. The squeezing action (3) is effective even in the state that a relative velocity becomes zero. The squeezing action can be explained as follows, on the presumption that a solid plate is sliding on a base plate in presence of a fluid. On such a presumption, distribution of a pressure, which acts on a surface of the solid plate facing to the base plate, is under the boundary condition that a pressure becomes zero at an edge of the solid plate. The pressure distribution shall be varied according to a domed function in order to generate a positive pressure distribution necessary for maintenance of lubrication. Such pressure distribution is represented by the formula of: ∂2∂x2⁢P+∂2∂y2⁢P<0Formula⁢ ⁢1

Problems solved by technology

Steel piston rings are outstandingly excellent in mechanical properties and wear resistance property than those of cast iron rings, but have poor seizure resistance property, which is one of reasons that second rings of steel have not been made good progress.

But, such unsolved problems still remain as a cost for surface-treatment and aluminum adhesion which occurs at the interface between a piston and a piston ring.

As countermeasures against such disadvantages, the patent literature No. 2 proposes surface-treatments such as PVD, whereas an increase of a manufacturing cost cannot be avoided according to such surface-treatment.

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