CAST IRON MATERIAL

Abstract

Provided is a cast iron material from which excellent friction characteristics can be obtained. Provided is the cast iron material containing C and Fe as a composition, and further containing Cr as the composition in 1.0 to 3.5% in terms of mass %. The cast iron material is used in a sliding component sliding under an environment of lubricating oil to which an additive containing Mo as a constituent element, such as MoDTC, is added. Cr contained in the cast iron material promotes a decomposition reaction of the additive containing Mo added to the lubricating oil to form a film of molybdenum disulfide, the film having low friction. Thus, the fiction can be reduced.

Description

TECHNICAL FIELD

The invention relates to a cast iron material having excellent friction characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based upon and claims the benefit of priority to Japanese Application No. 2017-221363, filed Nov. 16, 2017, the entire contents of which are incorporated herein by reference.

BACKGROUND ART

Cast iron has been widely used as a sliding material for a sliding component of an internal combustion engine, or the like because of having favorable wear resistance and seizure resistance.

For example, Patent Literature 1 discloses, as cast iron for a cylinder liner in which wear resistance is improved, a material which has a composition containing C: 3.0 to 3.5%, Si: 1.5 to 2.5%, Mn: 0.5 to 1.0%, P: 0.2 to 0.5%, S: 0.12% or less, Cr: 0.1 to 0.5%, B: 0.09 to 0.18%, Cu: 0.4 to 1.0%, and Mo: 0.1 to 0.5% in terms of mass %, and a remainder composed of Fe and inevitable impurities, and is formed into a structure configured by including a base phase mainly composed of pearlite, dispersing a hard phase composed of steadite and a boron compound in 14 to 22% in terms of an area proportion, and simultaneously dispersing flake graphite so as to have an average graphite distance of 9 to 15 μm. According to the cast iron for the cylinder liner, a corrosion loss can be reduced to a half in comparison with conventional cast iron, and a seizure limit load can be improved to about one and a half times larger than the conventional cast iron.

In addition, for example, Patent Literature 2 discloses high wear-resistant Cr cast iron that has a chemical composition containing C: 2.7 to 3.3%, Si: 0.2 to 1.0%, Mn: 0.4 to 2.0%, Cr: 18 to 25%, Mo: 0.5 to 4%, Ni: 0.5 to 3%, and N: less than 0.2% in terms of mass %, and a remainder composed of Fe and impurities; and has a structure composed of 30 to 40 area % of crystallized carbide and a matrix surrounding the carbide, in which the matrix is mainly formed of martensite, and a quenched structure partially containing retained austenite is tempered, and fine precipitated carbide having a particle size of 1 rpm or less in an equivalent circular diameter is dispersed in the matrix, and a total amount of the fine precipitated carbide is adjusted to 3.0 to 14 area % based on the total structure. According to the high wear-resistant Cr cast iron, a wear loss can be reduced to a half in comparison with the conventional cast iron.

In addition, for example, Patent Literature 3 discloses flake graphite cast iron containing A-type graphite including an existence form in which graphite is distributed disorderly and uniformly with no direction, and having a chemical composition containing C: 2.8 to 4.0%, Si: 1.2 to 3.0%, Mn: 1.1 to 3.0%, P: 0.01 to 0.6%, and S: 0.01 to 0.30% in terms of mass %, and a remainder composed of Fe and inevitable impurities, in which a ratio of a Mn content to a S content (Mn/S) is in a range of 3 to 300. According to the flake graphite cast iron, tensile strength can be improved to about 1.2 to 2 times larger than the conventional cast iron, and can also obtain favorable machinability.

Further, for example, Patent Literature 4 discloses flake graphite cast iron which has a composition containing C: 2.4 to 3.6%, Si: 0.8% or more and less than 2.8%, Mn: 1.1 to 3.0%, and further P: 0.01 to 0.6% and B: 0.001 to 0.2%, or further S: more than 0.01% and 0.15% or less, and one kind or two or more kinds selected from Cu, Cr, Mo, and Ni in 0.1 to 6.0% in total, one kind or two or more kinds selected from W, V, and Nb in 0.01 to 5.0% in total, and one kind or two or more kinds selected from Sn: 0.3% or less and Sb: 0.3% or less, in terms of mass %, and further has a structure in which carbide is dispersed in 8% or less in terms of area %. According to the flake graphite cast iron, tensile strength can be improved to about 1.5 times larger than the conventional cast iron.

CITATION LIST

Patent Literature

• Patent Literature 1: JP 2006-206986 A
• Patent Literature 2: JP 2009-007597A
• Patent Literature 3: JP 2013-117071 A
• Patent Literature 4: JP 2014-062318 A

Non-Patent Literature

• Ushioda et al, Effect of Low Viscosity Passenger Car Motor Oils on Fuel Economy Engine Tests, SAE international, 2013-01-2606

SUMMARY OF INVENTION

Technical Problem

However, reduction of viscosity of engine oil has progressed in recent years in order to reduce stirring resistance of the engine oil, thereby causing an increase of direct contact of metal with each other according to high load operating conditions and the like, and a rise of friction (see, Non-Patent Literature 1, for example), which has a possibility of causing seizure or the like. Therefore, a friction modifier is added to the engine oil to reduce the friction. As the friction modifier that has been most frequently used so far, MoDTC (molybdenum dithiocarbamate or molybdenum dialkyldithiocarbamate) is available. However, an action mechanism of the friction modifier containing Mo (molybdenum) as an additive, or a reason why an effect of the friction modifier is different depending on a material has not been specified yet. Therefore, it is desired to elucidate the mechanisms, thereby developing the effect of the friction modifier at maximum to reduce the fiction. Further, a large amount of materials in which hardness is increased in order to improve the characteristics such as wear resistance has been provided so far to have a problem of having difficulty in working and poor productivity.

The invention has been made based on such problems, and contemplated for providing a cast iron material from which excellent fiction characteristics can be obtained.

Solution to Problem

A cast iron material according to the invention contains C and Fe as a composition, and Cr as the composition in 1.0 to 3.5% in terms of mass %, and includes graphite as a structure, and is used in a sliding component sliding under an environment of lubricating oil containing Mo.

Advantageous Effects of Invention

According to a cast iron material of the invention, the cast iron material contains Cr in 1.0% or more in terms of mass %, and therefore active Cr exposed onto a surface due to sliding promotes a decomposition reaction of an additive contained in lubricating oil, and a film of molybdenum disulfide, the film having low friction, can be formed. Accordingly, friction can be reduced, and wear can also be reduced, and seizure or the like can also be suppressed. In addition, Cr is adjusted to 3.5% or less in terms of mass %, and therefore the cast iron material is suppressed from becoming unnecessarily hard, and can be easily worked.

Moreover, the cast iron material is adjusted to contain at least Si from the group consisting of Si, Cu and Ni as the composition to adjust a content thereof to 2 to 6.5% of Si, 0 to 1.5% of Cu, and 0 to 1.5% of Ni in terms of mass %, and therefore the cast iron material can be adjusted to suitable hardness, and can be easily worked.

Further, if the cast iron material is arranged to be used in a sliding component of the engine, the friction can be reduced and fuel efficiency can also be improved.

BRIEF DESCRIPTION OF DRAWINGS

This application file contains at least one drawing executed in color. Copies of the patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 shows one example of a structure of a cast iron material according to one embodiment of the invention, and is an optical micrograph of the structure, showing the structure including flake graphite.

FIG. 2 shows another example of a structure of a cast iron material according to one embodiment of the invention, and is an optical micrograph of the structure, showing the structure including spheroidal graphite.

FIG. 3 is a transmission electron micrograph showing one example of a cross-sectional structure of friction marks after conducting a friction test on a cast iron material according to the invention.

FIG. 4 is an enlarged diagram of a frame portion in FIG. 3, and a transmission electron micrograph showing formed molybdenum disulfide.

FIG. 5 is a characteristic diagram showing, in comparison, a friction coefficient under an environment of lubricating oil to which MoDTC is added with regard to the cast iron material in each of Examples and Comparative Examples.

FIG. 6 is another characteristic diagram showing, in comparison, a friction coefficient under an environment of lubricating oil to which MoDTC is added with regard to the cast iron material in each of Examples and Comparative Examples.

FIG. 7 is a list showing a difference in friction characteristics depending on a composition of a cast iron material under an environment of lubricating oil to which MoDTC is added with regard to the cast iron material in each of Examples and Comparative Examples.

FIG. 8 is a list showing results obtained after preparing a test piece having a circular arc cross section by using the cast iron material in each of Examples and Comparative Examples, and conducting a friction test under an environment of lubricating oil to which MoDTC is added.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the invention will be described in detail with reference to drawings.

FIG. 1 and FIG. 2 each show one example of a structure of a cast iron material according to one embodiment of the invention. FIG. 1 shows the structure including flake graphite as graphite, and FIG. 2 shows the structure including spheroidal graphite as the graphite. The cast iron material is used in a sliding component sliding under an environment of lubricating oil containing molybdenum (Mo), and in particular, can be preferably used in the sliding component of an engine. In addition, a term “the lubricating oil containing Mo” means the lubricating oil to which an additive containing Mo as a constituent element is added, and means the lubricating oil to which organic molybdenum such as MoDTC is added as the additive, for example. The additive containing Mo as the constituent element in this manner is used as a friction modifier, for example.

The cast iron material according to the present embodiment contains carbon (C) and iron (Fe), and further chromium (Cr) as the composition. C reinforces a pearlite base in a base material, and simultaneously crystallizes the graphite to improve slidability, wear resistance and seizure resistance. A content of C is preferably from 2.0 to 6.5% in terms of mass %, for example. The reason is that, if the content of C is small, the flake graphite is not crystallized to affect workability or the like.

Cr is a material for promoting a decomposition reaction of the additive containing Mo as the constituent element, such as MoDTC, added to the lubricating oil to form a larger amount of a film of molybdenum disulfide, the film having low friction. If the film of molybdenum disulfide is formed, friction can be reduced, and wear can also be reduced, and seizure or the like can also be suppressed.

A mechanism according to which Cr decomposes the additive containing Mo as the constituent element, such as MoDTC, is estimated as described below. First, if an oxide layer existing in a surface layer of the cast iron material is scraped by the friction caused by sliding, active metal (such as Fe and Cr) is exposed. In addition, the additive such as MoDTC contained in the lubricating oil is decomposed by heat, and molybdenum oxysulfide (MoS_2-xO_x) being an intermediate product exists in the lubricating oil. Ionization tendency of the metal is specified by a series: Cr>Fe≈Mo, and therefore Cr which is further easily oxidized than Fe deprives molybdenum oxysulfide existing in the lubricating oil of oxygen into chromium oxide. Meanwhile, molybdenum oxysulfide which is deprived of oxygen is formed into molybdenum disulfide (MoS₂) to form the film on the cast iron material. In addition, when the friction test is conducted on the cast iron material according to the present embodiment under the environment of lubricating oil to which MoDTC is added, it has been confirmed that a reactional film as shown in FIG. 3 is formed on the surface of the cast iron material after testing, and layer films of molybdenum disulfide are formed as shown in FIG. 4.

A content of Cr is preferably in the range from 1.0 to 3.5% in terms of mass %. The reason is that, if the content thereof is less than 1.0% by mass, the effect of forming the film of molybdenum disulfide by Cr cannot be sufficiently obtained, and if the content thereof becomes more than 3.5% by mass, the cast iron material becomes unnecessarily hard, thereby being difficult in being easily worked.

Further, the cast iron material according to the present embodiment preferably contains at least silicon (Si) from the group consisting of silicon (Si), copper (Cu) and nickel (Ni) as the composition. Si is a material having an effect on suppressing Cr from forming carbide to suppressing the cast ion material from being hard, thereby facilitating working. A content of Si is preferably adjusted to 2 to 6.5% in terms of mass %. The reason is that, if the content thereof is less than 2% by mass, the cast iron material becomes excessively hard in several cases, and if the content thereof becomes more than 6.5% by mass, the cast iron material becomes brittle and has a possibility of losing strength.

In a manner similar to Si, Cu and Ni also have an effect on suppressing Cr from forming carbide; but the cast iron material may contain or need not contain Cu and Ni. A content of Cu is preferably adjusted to 0 to 1.5%, and a content of Ni is preferably adjusted to 0 to 1.5% in terms of mass %. The reason is that, within the above ranges, an effect of suppressing Cr from forming carbide, and avoiding being excessively hard, thereby facilitating working can be obtained.

In addition, the hardness of the cast iron material is preferably adjusted within the range from HB200 to HB380 in terms of Brinell hardness. The reason is that, if the harness thereof becomes more than HP380, machinability is deteriorated, and working becomes difficult.

As the structure of the cast iron material according to the present embodiment, the base material is composed of the pearlite base, and flake graphite or spheroidal graphite, and crystallized carbide are dispersed in the base material. Further, Cr is dispersed in the base material, thereby promoting a reaction of the friction modifier containing Mo to form molybdenum disulfide. Thus, excellent friction characteristics can be obtained.

The cast iron material can be obtained by melt-forming a melt having the above-described composition by an ordinary melt-forming method using a cupola, an electric furnace or the like, casting the resultant material by using a publicly-known casting method and solidifying the resultant material, for example.

Thus, according to the present embodiment, the cast iron material contains Cr in 1.0% or more in terms of mass %, and therefore active Cr exposed on the surface by sliding can promote the decomposition reaction of the friction modifier contained in the lubricating oil to form the film of molybdenum disulfide. Accordingly, the friction can be reduced, the wear can also be reduced, and the seizure or the like can also be suppressed. In addition, Cr is adjusted to 3.5% or less in terms of mass %, and therefore the cast iron material can be suppressed from becoming unnecessarily hard, and can be easily worked.

Further, the cast iron material is adjusted to contain at least Si from the group consisting of Si, Cu and Ni as the composition, and the content is adjusted to 2 to 6.5% of Si, 0 to 1.5% of Cu; and 0 to 1.5% of Ni in terms of mass %. Therefore, the cast iron material can be adjusted to suitable hardness and can be easily worked.

Further, if the cast iron material is arranged to be used in a sliding portion of an engine part or a driving part, the friction can be reduced and fuel efficiency can be improved.

EXAMPLES

Examples 1-1 to 1-6 and Comparative Examples 1-1 to 1-6

A cast iron material was prepared, in which the cast ion material had a composition containing 2.6% of C, 2.6% of Cr, 4.4% of Si, and 1.0% of Cu in terms of mass %, and a remainder composed of Fe and impurities, and had a structure in which a base material was formed of a pearlite base, and flake graphite and crystallized carbide were dispersed in the base material. On the thus obtained cast iron material, a reciprocating sliding friction test was conducted under an environment of lubricating oil. As the lubricating oil, test oil to which MoDTC was added was used for all, and kinds thereof were changed in Examples 1-1 to 1-6. As a mating material, bearing steel (SUJ2) was used. The test was conducted for 30 minutes by adjusting a load to 80 N, a frequency to 10 Hz, and a temperature to 80° C., and a friction coefficient during stabilization was measured.

As Comparative Examples 1-1 to 1-6, a friction test similar to the test in the present Example was conducted by using a cast iron material having a composition without containing Cr and containing 3.0% of C, and 2.2% of Si in terms of mass %, and a remainder composed of Fe and impurities, and having a structure in which a base material was formed of a pearlite base, and flake graphite and crystallized carbide were dispersed in the base material, and a function coefficient was measured. As lubricating oil, the same lubricating oil was used between Example 1-1 and Comparative Example 1-1, between Example 1-2 and Comparative Example 1-2, between Example 1-3 and Comparative Example 1-3, between Example 1-4 and Comparative Example 1-4, between Example 1-5 and Comparative Example 1-5, and between Example 1-6 and Comparative Example 1-6, respectively.

The t results obtained are shown in FIG. 5. As shown in FIG. 5, according to the present Examples, the friction coefficient was able to be reduced for all. More specifically, it was found that the friction can be reduced in the cast iron material containing Cr as the composition.

Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-4

A cast iron material was prepared, in which the cast ion material had a composition containing 3.2% of C, 2.5% of Cr, and 4.9% of Si in terms of mass %, and a remainder composed of Fe and impurities, and had a structure in which a base material was formed of a pearlite base, and spheroidal graphite and crystallized carbide were dispersed in the base material. On the thus obtained cast iron material, a reciprocating sliding friction test was conducted under an environment of lubricating oil. As the lubricating oil, test oil to which MoDTC was added was used for all, and kinds thereof were changed in Examples 2-1 to 2-4. As a mating material, bearing steel (SUJ2) was used. The test was conducted for 30 minutes by adjusting a load to 80 N, a frequency to 10 Hz, and a temperature to 80° C., and a friction coefficient during stabilization was measured.

As Comparative Examples 2-1 to 2-4, a friction test similar to the test in the present Example was conducted by using a cast iron material having a composition without containing Cr and containing 3.5% of C, and 2.4% of Si in terms of mass %, and a remainder composed of Fe and impurities, and having a structure in which a base material was formed of a pearlite base, and spheroidal graphite and crystallized carbide were dispersed in the base material, and a friction coefficient was measured. As lubricating oil, the same lubricating oil was used between Example 2-1 and Comparative Example 2-1, between Example 2-2 and Comparative Example 2-2, between Example 2-3 and Comparative Example 2-3, and between Example 2-4 and Comparative Example 2-4, respectively.

The results obtained are shown in FIG. 6. As shown in FIG. 6, according to the present Examples, the friction coefficient was able to be reduced for all. More specifically, it was found that the friction can be reduced in the cast iron material containing Cr as the composition.

Examples 3-1 to 3-4 and Comparative Examples 3-1 to 3-3

A cast iron material having a composition in which an amount of Cr, an amount of Si, an amount of Ni, and an amount of Cu were different from each other, and a remainder was composed of Fe and impurities was prepared. On the thus obtained cast iron material, a friction test was conducted under an environment of lubricating oil containing MoDTC. As the friction test, a ball-on-disk type friction test was conducted, and as a mating material, bearing steel (SUJ2) was used. The test was conducted for 30 minutes by adjusting a load to 80 N, a frictional speed to 0.5 m/s, and a temperature to 80° C., and a friction coefficient during stabilization was measured.

As Comparative Examples, in Comparative Example 3-1, a material containing Cr in an amount less than 1.0% was used, and in Comparative Example 3-2 and 3-3, materials each containing Cr in an amount more than 3.5% were used. In Examples 3-1 and 3-2, materials each containing Cr in about 1.0% were used, and in Examples 3-3 and 3-4, materials each containing Cr in 2.3 to 2.55% were used. An amount of Si and an amount of Cu were different from each other between Examples 3-1 and 3-2 and between Examples 3-3 and 3-4.

The results obtained are shown in FIG. 7. As shown in FIG. 7, it was found that, while the friction coefficient is high in Comparative Example 3-1 in which the amount of Cr is low, the friction coefficient was low to be a half or less in Examples 3-1 to 3-4. It was found that, while the friction coefficient is low in Comparative Examples 3-2 and 3-3 in which the amount of Cr is large, the cast iron material becomes excessively hard.

Examples 4-1 to 4-3 and Comparative Examples 4-1 and 4-2

A test piece having a circular arc cross section was prepared by using a cast iron material having a composition in which an amount of Cr, an amount of Si, an amount of Ni, and an amount of Cu were different from each other, and a remainder was composed of Fe and impurities. On the thus obtained test piece, a function test was conducted under an environment of lubricating oil containing MoDTC. The test was conducted for 30 minutes by adjusting a load to 80 N, a frequency to 10 Hz, and a temperature to 80° C., and a friction coefficient during stabilization was measured.

As Comparative Examples 4-1 and 4-2, a material containing Cr in an amount less than 1.0% was used, and as Examples 4-1 to 4-3, a material containing Cr in 1.0 to 3.5% was used. In Examples 4-1 to 4-3, the amount of Si and the amount of Cu were different from each other.

The results obtained are shown in FIG. 8. As shown in FIG. 8, it was found that, in comparison with Comparative Examples 4-1 and 4-2 in which the amount of Cr is less than 1.0%, the friction coefficient is reduced by about 20% in Examples 4-1 to 4-3 in which the amount of Cr is from 1.0 to 3.5%. From the results, it was revealed that the friction is reduced by use of the cast iron material according to the invention also in a cylinder liner sliding along a piston ring as a mating material in assuming an automobile engine.

As described above, the invention is described by exemplifying the embodiments, but the invention is not limited to the embodiments described above, and can be modified in various manners. For example, the composition of the cast iron is specifically described in the embodiments described above, but the composition may contain any other element. For example, any other element includes Mg, Mn, S, P or the like.

The invention is also understood as a sliding mechanism including a pair of sliding members each having a sliding surface sliding with each other, and lubricating oil interposed between the sliding surfaces facing each other, in which at least one of the sliding surfaces is formed of a cast iron material containing carbon (C) and iron (Fe) as a composition, and further chromium (Cr) in 1.0 to 3.5% in terms of mass %, and including graphite as a structure, and the lubricating oil contains molybdenum (Mo) as an additive.

Claims

1. A cast iron material, comprising:

carbon (C) and iron (Fe) as a composition; and

chromium (Cr) as the composition in 1.0 to 3.5% in terms of mass %; and

including graphite as a structure, wherein

the cast iron material is used in a sliding component sliding under an environment of lubricating oil containing molybdenum (Mo) as an additive.

2. The cast iron material according to claim 1, comprising at least silicon (Si) from the group consisting of silicon (Si), copper (Cu) and nickel (Ni) as the composition, wherein a content thereof is from 2 to 6.5% of Si, from 0 to 1.5% of Cu, and from 0 to 1.5% of Ni in terms of mass %.

3. The cast iron material according to claim 1, wherein Brinell hardness is from HB200 to HB380.

4. The cast iron material according to claim 1, wherein a film containing molybdenum disulfide is formed on a surface by sliding.

5. The cast iron material according to claim 1, wherein the cast iron material is used in a sliding portion of an engine part and a driving part.