JP2023001684A - Cast iron-made cylindrical member and method for producing the same - Google Patents

Abstract

To provide a cast iron-made cylindrical member that allows for increased strength while ensuring high slidability of the inner peripheral part and a method for producing the same.SOLUTION: To a molten metal that contains C: 3.5-3.85%, Si: 2.3-2.7%, Mn: 0.5-1.5%, and S: 0.005-0.015% with the balance being Fe and has a CE value of 4.45-4.70, added are a graphite spheroidizing agent containing Mg: 2.0-4.0% and RE: 4.0-6.0% and an Fe-Si-Bi inoculating agent; the molten metal is injected into a die for centrifugal casting; and a first cooling is performed to cool the temperature at injection to an eutectic temperature and then a second cooling is performed to cool the eutectic temperature to an eutectoid temperature at a cooling rate lower than in the first cooling. Thus, a cast iron-made cylindrical member can be produced in which granular or spherical graphite particles of less than 50 μm exist by 1000 or more per mm2; the number of graphite particles increases toward the inner peripheral face of the member; the graphite area ratio is 5% or more; and the graphite area ratio increases toward the inner peripheral face of the member.SELECTED DRAWING: Figure 12

Description

TECHNICAL FIELD The present invention relates to a cast iron cylindrical member and a manufacturing method thereof.

Many long, thin-walled cast-iron cylindrical members are manufactured by a centrifugal casting method in which molten metal is poured into the inner periphery of a cylindrical mold that is rotated at high speed. Typical items manufactured by this centrifugal casting method include cast iron pipes for infrastructure and rolling rolls for plastic working. It is widely used because cast iron cylindrical members of stable quality can be obtained without using sand cores. Cylinder sleeves are also manufactured by the green casting method, which is highly productive. is used. The reason for this is that the flake-crystallized graphite has excellent damping properties, thermal conductivity, self-lubricating properties, and the like.

For example, Patent Literature 1 describes a method for manufacturing a cylinder liner (cylinder sleeve) for an internal combustion engine having both slidability and high strength at low cost by setting simple conditions. Molten cast iron with a hypereutectic composition is cast into a centrifugal casting mold, and the mold is rotated at a predetermined speed to use centrifugal force and fading phenomenon to form a similar to flake graphite in the inner part near the center of the mold. It is described that an inner cast iron layer with crystallized shaped graphite is formed, and an outer cast iron layer with crystallized spheroidal graphite is formed on the outer part in contact with the mold wall.

Japanese Patent Application Laid-Open No. 2001-227405

Cylinder sleeves for automobile engines are required to have sliding friction performance (low friction), high vibration damping performance, high thermal conductivity, and machinability in order to improve fuel efficiency and quietness of the engine. FC cast iron that satisfies is used. However, FC cast iron has a Young's modulus as low as about half that of steel (210 GPa). Therefore, as shown in FIG. 1, which schematically shows an automobile engine, a cylinder block 10 in which a thin-walled cylindrical cylinder sleeve 11 is encapsulated in a cylinder barrel 12 by die-casting has a high axis by a cylinder head 20 and steel bolts. When fastened with force, the cylinder sleeve 11 with low rigidity is likely to undergo bore deformation (strain) due to a strong compressive load (arrow 1 in FIG. 1) acting in the direction of the cylinder axis. In addition, the cylinder sleeve 11 tends to be strained (or deformed) so as to protrude in the radial direction due to the bore internal pressure load (arrow 2 in FIG. 1) due to the combustion/explosion pressure during operation. Furthermore, in the thin-walled cylinder sleeve 11 with low rigidity, residual stress (arrow 3 in FIG. 1) on the side of the aluminum barrel 12 introduced during die casting of the cylinder block 10 is released over time as the engine runs. Therefore, there is a risk of causing bore deformation. These deformation behaviors of the bore shape have the problem of causing an increase in friction and blow-by when the piston ring slides, resulting in a decrease in fuel consumption.

When the cylinder sleeve cast-iron cylindrical member manufactured by the method disclosed in Patent Document 1 is used for the cylinder sleeve of an automobile engine, the inner peripheral portion (that is, the sliding surface side) of the cast-iron cylindrical member has a flake shape. Graphite having a shape similar to graphite is crystallized, and there is a problem that it is difficult to achieve both the slidability and strength required for the inner peripheral surface.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a cast iron cylindrical member capable of increasing strength while ensuring high slidability in the inner peripheral portion, and a method for manufacturing the same.

In order to achieve the above object, as one aspect of the present invention, in mass%, C: 3.5 to 3.85%, Si: 2.3 to 2.7%, Mn: 0.5 to 1.5%, S: 0.005 to 0.015%, the balance consists of Fe and unavoidable impurities, and the carbon equivalent defined by the following formula 1 (hereinafter referred to as “CE value”) is 4 .45 to 4.70 cast iron cylindrical member,
CE value = total carbon (hereinafter referred to as "TC") + (Si content + P content) / 3 (Formula 1)
In the cross section of the cast iron cylindrical member, the number of graphite particles of granular or spherical graphite with a particle size of 1 μm or more and less than 50 μm is 1000 or more per 1 mm ² , and the number of graphite particles is the outer circumference of the cast iron cylindrical member. Granular or spherical graphite with a particle size of 1 μm or more and less than 50 μm has a graphite area ratio of 5% or more in the cross section of the cast iron cylindrical member, and the graphite increases from the surface to the inner peripheral surface. The area ratio increases from the outer peripheral surface toward the inner peripheral surface of the cast iron cylindrical member.

Another aspect of the present invention is a method for producing a cast iron cylindrical member by centrifugal casting. ~ 2.7%, Mn: 0.5 to 1.5%, S: 0.005 to 0.015%, the balance consisting of Fe and unavoidable impurities, CE defined by the above formula 1 a step of preparing a molten metal having a value of 4.45 to 4.70; adding a graphite spheroidizing agent made of Fe--Si--Mg--RE based alloy containing 0-6.0%; adding an Fe--Si--Bi based inoculant to the molten metal; A step of pouring the molten metal to which the spheroidizing agent and the inoculant are added into a centrifugal casting mold, and performing a first cooling to precipitate a solid phase by cooling from the pouring to the eutectic temperature. and performing a second cooling that cools the solid phase from the eutectic temperature to the eutectoid temperature at a cooling rate lower than that of the first cooling to form a rough material of the cast iron cylindrical member; and cutting the inner peripheral surface of the material to form an inner peripheral machined surface.

As described above, according to the cast iron cylindrical member of the present invention, 1000 or more fine granular or spherical graphite particles are present per 1 mm ² in the inner peripheral portion of the cast iron circular motion member having a predetermined composition, and this graphite The number of particles is increased from the outer peripheral surface to the inner peripheral surface, the graphite area ratio of fine granular or spherical graphite is 5% or more, and the graphite area ratio is increased from the outer peripheral surface to the inner peripheral surface. By doing so, it is possible to provide a cast iron cylindrical member that can be increased in strength while ensuring high slidability.

Further, according to the production method of the present invention, a molten metal having a predetermined composition is reacted with a graphite spheroidizing agent having a predetermined composition, and an inoculant having a predetermined composition is used to inoculate the molten metal. After pouring into the casting mold, first cooling is performed to cool from the pouring to the eutectic temperature to precipitate a solid phase, and the solid phase is precipitated at a cooling rate lower than that of the first cooling. By performing the second cooling from the eutectic temperature to the eutectoid temperature, the cast iron cylindrical member having the above characteristics can be manufactured.

1 is a cross-sectional view schematically showing an example of an internal combustion engine in which a cast-iron cylindrical member is used as a cylinder sleeve; FIG. BRIEF DESCRIPTION OF THE DRAWINGS It is a flowchart which shows typically one Embodiment of the manufacturing method of the cast-iron cylindrical member which concerns on this invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view schematically showing one embodiment of a cast-iron cylindrical member according to the present invention; FIG. 4 is a schematic cross-sectional view of the cast-iron cylindrical member shown in FIG. 3 taken along line III-III. 5 is a schematic cross-sectional view showing an enlarged part of the cross section of the cast-iron cylindrical member shown in FIG. 4. FIG. It is an optical microscope photograph of the cross section of the cast-iron cylindrical member (rough material) of an Example. 4 is a graph showing the number of graphite particles for each minimum particle size immediately below the outer peripheral surface of cast-iron cylindrical members (coarse materials) of Examples and Comparative Examples. 4 is a graph showing the number of graphite particles for each minimum particle size at a distance of 1.0 mm from the outer peripheral surface of cast iron cylindrical members (coarse materials) of Examples and Comparative Examples. 4 is a graph showing the number of graphite particles for each minimum particle size at a distance of 2.0 mm from the outer peripheral surface of cast iron cylindrical members (coarse materials) of Examples and Comparative Examples. 4 is a graph showing the number of graphite particles for each minimum particle size at a distance of 3.0 mm from the outer peripheral surface of cast iron cylindrical members (coarse materials) of Examples and Comparative Examples. 4 is a graph showing the number of graphite particles for each minimum particle size at a distance of 3.5 mm from the outer peripheral surface of the cast iron cylindrical member (coarse material) of the example. 4 is a graph showing changes in the number of graphite particles having a particle size of 1 μm or more and less than 50 μm with respect to the distance from the outer peripheral surface of cast iron cylindrical members (coarse materials) of Examples and Comparative Examples. 4 is a graph showing the graphite spheroidization rate for each minimum particle size for each distance from the outer peripheral surface of the cast iron cylindrical member (coarse material) of the example. 4 is a graph showing changes in the area ratio of graphite with a particle size of 1 μm or more and less than 50 μm with respect to the distance from the outer peripheral surface of cast iron cylindrical members (coarse materials) of Examples and Comparative Examples.

An embodiment of a cast-iron cylindrical member and a method for manufacturing the same according to the present invention will be described below with reference to the accompanying drawings. The drawings are drawn with priority given to ease of understanding, and are not drawn to scale.

First, an embodiment of a method for manufacturing a cast-iron cylindrical member will be described with reference to FIG. The method of manufacturing a cast iron cylindrical sliding member according to the present embodiment includes the step of preparing a molten metal 41 having a predetermined composition as shown in FIG. and adding an inoculant 43A having a predetermined composition to the molten metal 41 as shown in FIG. 2(b) or (c); As shown in FIG. 2(c), a step of pouring a molten metal 44 containing a graphite spheroidizing agent and an inoculant into a cylindrical mold 35 for centrifugal force casting after further adding an inoculant 43B; to the eutectic temperature to crystallize the solid phase; A step of forming a rough material for a cast iron cylindrical sliding member by cooling, and a step of cutting the inner peripheral surface of the rough material to form an inner peripheral machined surface that will be the sliding surface, although not shown including. Each step is described below.

(1) Molten metal preparation step In the centrifugal casting method using a cylindrical mold, the solidification speed of the poured molten metal is extremely fast, so in order to stably crystallize graphite from the molten metal, the following high C, A molten metal with a high Si content is essential. The composition of the molten metal 41 for manufacturing the cast iron cylindrical sliding member is, in mass%, C: 3.5 to 3.85%, Si: 2.3 to 2.7%, Mn: 0.5 to 1. .5%, S: 0.005-0.015%, the balance being Fe and unavoidable impurities, and the CE value defined by the following formula 1 is 4.45-4.70. A cast-iron cylindrical sliding member also has such a composition.
CE value = TC + (Si content + P content) / 3 (Formula 1)

In order to melt the molten metal having the above composition, it is preferable to use a high-frequency induction melting furnace as shown in FIG. 2(a). In a melting furnace or holding furnace (hereinafter referred to as a melting furnace) 31 equipped with an induction coil 32, raw materials are melted and adjusted so as to have the above composition to obtain a molten metal 41 (original melt). Each component of the above composition and its content will be described below.

Si is a strong graphitization promoting element, and has the effect of preferentially crystallizing graphite from the molten metal. When Si is less than 2.3% by mass, in the centrifugal casting method with a high solidification rate, chill (= eutectic of austenite and cementite, also called ledeburite) is likely to crystallize instead of graphite, resulting in embrittlement and deterioration of machinability. Invite. On the other hand, as the Si content increases, the solid solubility of C decreases. Therefore, when the Si content exceeds 2.7% by mass, C in a supersaturated state causes significant carbon dross. Since carbon dross has a smaller specific gravity than molten iron, it segregates in the form of a film on the inner surface of the cylindrical coarse material before the completion of solidification. provokes Therefore, the Si content is set to 2.3 to 2.7% by mass.

C is the first major element of cast iron and is an essential element for crystallizing graphite in the structure, which leads to unique material properties. Since the Si content is 2.3 to 2.7% by mass, if the C content is less than 3.5% by mass, the CE value decreases to the eutectic composition (that is, 4.3) or less. put away. When the CE value decreases to the eutectic composition, the molten metal centrifugally cast into the cylindrical mold has a high solidification rate, so chill is likely to crystallize due to supercooling, and the number of graphite grains decreases and the graphite spheroidization rate decreases. Let On the other hand, when the C content exceeds 3.85% by mass, the CE value may exceed 5.0, and the presence of excess C crystallizes carbon dross, chunky graphite, explosive graphite, etc., Degrades mechanical properties. Therefore, the C content should be 3.5 to 3.85% by mass.

The CE value is the effect of the tertiary elements (i.e., Si and P) added to the Fe—C alloy on the solubility of carbon, as well as the increase or decrease in carbon activity, i.e., the crystallization of carbon as graphite or cementite. It expresses the ease of release, and the degree of influence due to the combination of C, Si, and P in the melting and solidification of cast iron is expressed by converting only the amount of C. By setting the CE value in the range of 4.45 to 4.70, which is the hypereutectic range, even when producing a thin long cylindrical rough material by centrifugal casting with a high solidification rate, in solidification of cast iron There is an effect that primary crystal graphite of stable shape and size can be crystallized from the liquid phase and casting defects can be suppressed. TC in Formula 1 that defines the CE value is measured by a solid carbon/sulfur analyzer, a total organic carbon meter, or the like. Examples of solid carbon/sulfur analyzers include model EMIA series manufactured by Horiba, Ltd., and the like.

Mn has the effect of promoting the formation of carbides and promoting the precipitation of pearlite in the matrix. Excessive addition of Mn further promotes the precipitation of pearlite in the matrix and increases segregation to grain boundaries, which may increase hardness and increase brittleness. Conversely, if the Mn content is low, more ferrite precipitates in the matrix, resulting in lower wear resistance. Therefore, the content of Mn is set to 0.5 to 1.5% by mass.

P is one of graphite spheroidization-inhibiting elements. At the final stage of solidification, P is concentrated in the liquid phase near the eutectic cell grain boundary, and hard Fe ₃ P is formed at the eutectic cell grain boundary after solidification. segregate. Therefore, in the present invention, P addition is unnecessary, and it is preferable to suppress the amount of P as an unavoidable impurity to 0.02% by mass or less.

Although S is an unavoidable impurity, S is said to form sulfides such as MnS in the molten metal when it coexists with Mn and RE, and act as nuclei for graphite crystallization. That is, addition of an appropriate amount of S increases the number of graphite grains. However, excessive addition of S inhibits graphite spheroidization, resulting in a decrease in elongation of the cast iron cylindrical sliding member. Therefore, the S content should be 0.005 to 0.015% by mass.

(2) Step of adding a graphite spheroidizing agent As the graphite spheroidizing agent 42, Fe—Si containing Mg: 2.0 to 4.0% and RE: 4.0 to 6.0% by mass% -Mg-RE alloy is used. Fe--Si--Mg--RE alloys may optionally contain Ca. That is, an Fe--Si--Mg--Ca--RE alloy may be used.

Mg is an element that does not form a solid solution with Fe, and since it has a low boiling point, it generates fine bubbles in high-temperature molten cast iron and dissipates into the atmosphere. The fine bubbles described above are said to be crystallization sites for spheroidal graphite, and in order to obtain spheroidal graphite cast iron, Fe--Si--Mg alloys are mostly used as graphite spheroidizing agents. Here, in the method shown in this proposal, graphite spheroidizing treatment is performed in a small ladle 34 for each shot, but if a graphite spheroidizing agent with a large Mg content used in conventional FCD cast iron casting is used, , the violent reaction with the molten metal in the small ladle continues for a long time, and the lead time from receiving the molten metal to the start of pouring into the small ladle becomes longer, resulting in a longer casting cycle time. Furthermore, since the yield of Mg in the molten metal is not stable due to the severity of the reaction, variations in the graphite spheroidization ratio of the cast coarse material also increase. Conversely, when an Fe—Si—Mg alloy with a low Mg content is used as a graphite spheroidizing agent, the number of microbubbles generated in the molten metal is reduced and the fading time is shortened. A decrease in the number of particles and a decrease in graphite spheroidization rate are likely to occur. Therefore, a graphite spheroidizing agent containing 2.0 to 4.0% by mass of Mg is used.

RE, which is a graphite spheroidizing agent, strongly promotes graphitization by suppressing elements that inhibit graphite spheroidization, preventing fading, and forming nucleation sites for spheroidal graphite through sulfide formation. It is an essential element in large centrifugal casting processes. Therefore, the amount of RE contained in the graphite spheroidizing agent used is set to 1.5% by mass or more, preferably 4.0% by mass or more. On the other hand, when RE exceeds 6.0% by mass, spheroidization of graphite is inhibited, and chunky graphite tends to crystallize. Therefore, RE is set to 1.5 to 6.0% by mass.

Although the contents of Si and Ca are not particularly limited, it is preferable that the Si content is 40 to 70% by mass. Ca is preferably contained in an amount of 1.5 to 5.0% by mass. Ca can strongly deoxidize the molten metal, and can promote crystallization of graphite from the molten metal in the same manner as Si.

The balance is Fe. These components are alloyed and used as the graphite spheroidizing agent 42 . The graphite spheroidizing agent 42 is preferably in the form of lumps, for example, the size is preferably 10 mm or less, more preferably 5 mm or less. The addition amount is preferably in the range of 0.2 to 2% by mass with respect to the molten metal.

The graphite spheroidizing agent 42 is preferably added to the molten metal 41 by a pouring method, and as shown in FIG. By pouring the molten metal 41, the molten metal 41 is subjected to graphite spheroidizing treatment. To the small ladle 34, the molten metal (original molten metal) 41 for one pouring is poured out from the melting furnace 31 in small amounts. The hot water temperature at this time is preferably 1500 to 1550°C. This is because the temperature of the molten metal when it is poured from the small ladle 34 into the cylindrical mold 35 varies depending on the distribution of the molten metal to the small ladle 34 and the reaction with the graphite spheroidizing agent 42 and the inoculant 43 at the time of pouring. Since the temperature of the hot water is lowered by about 100°C at the maximum, the temperature of the hot water at the time of pouring can be set to the desired 1400 to 1450°C by setting the temperature of the hot water at the time of tapping to the above range.

(3) Inoculant Addition Step Inoculation is performed together with graphite spheroidizing treatment because the inoculation enhances the graphitization ability, prevents chilling, improves the shape of graphite, and the like. As an inoculant, an Fe—Si alloy is generally used, but in the present embodiment, when the molten metal 41 is received in the small ladle 34, Bi is contained together with the graphite spheroidizing agent 42. 0.2 to 0.8% by mass of the inoculant 43A is added to the weight of the molten metal to be poured, and the molten metal 44 is inoculated by the pouring method. When the molten metal is poured into the rotary mold, the Fe—Si—Bi-based inoculant 43B containing Bi is inoculated by pouring 0.1% by mass to 0.3% by mass of the weight of the molten metal (late inoculation). . As a result, the synergistic effect with the predetermined composition of the graphite spheroidizing agent 42 described above significantly increases the number of graphite grains and prevents the occurrence of chill. Therefore, the number of fine graphite particles can be increased to 1000 or more per 1 mm ² . The inoculant containing Bi described above can be directly used as the inoculant 43A in FIG. The number of graphite grains is reduced compared to the case of flow inoculation.

The content of each component in the Fe—Si—Bi-based inoculant is not particularly limited, but Si is preferably 50 to 75% by mass, and Bi is preferably 0.5 to 2% by mass. Optionally, 0.5 to 2.0 wt% RE may be included. The balance is Fe and unavoidable impurities such as Ca. The Fe—Si—Bi-based inoculant is preferably used in a granular form. For example, when it is used for pouring inoculation such as the inoculant 43B in FIG. 2(c), the particle size is 1 mm or less. is preferred. Also, if the inoculant 43A of FIG. 2(b) is to be used for pouring, a size of 3 to 10 mm is preferable. As for the amount to be added, it is preferable to add in the range of 0.1 to 1% by mass based on the molten metal because the effect can be obtained with a small amount. In addition, other inoculants may be used in combination with the Fe—Si—Bi inoculant. After inoculating in the pan 34 by the pouring method, the Fe--Si--Bi inoculant may be used only for pouring inoculation. When an Fe--Si--Sr-based inoculant is used, Sr reacts with RE (the main element is Ce) to reduce the mutual graphitization effect and promote chilling.

As described above, the Fe—Si—Bi based inoculant is placed in the bottom of the small ladle 34 as the primary inoculant 43A, as shown in FIG. The molten metal 41 is inoculated by pouring the molten metal 41 into it. In addition, in the pouring flow inoculation, as shown in FIG. 2(c), Fe—Si—Bi system inoculum is added as a secondary inoculant 43B into the molten metal 44 poured into the cylindrical mold 35 from the small ladle 34. Inoculation treatment is performed by adding the agent. Since the number of graphite grains can be increased significantly even when the amount is 0.1% by mass based on the weight of the molten metal, the Fe—Si—Bi inoculant can be added to the molten metal during pouring to perform pouring inoculation. more preferred.

(4) Pouring Step into Cylindrical Mold As shown in FIG. It is This coating layer 36 is for forming a plurality of convex projections on the outer peripheral surface of the cast-iron cylindrical sliding member in order to increase the adhesion to the surroundings when casting the cast-iron cylindrical sliding member. is. As a method for forming the mold coating layer 36, a mold coating slurry is applied to the inner peripheral surface of the rotating cylindrical mold 35. By drying and solidifying, the mold coating layer 36 having concave portions corresponding to the convex protrusions can be formed. The thickness of the coating layer 36 is preferably about 1 mm.

Then, the molten metal 44 subjected to graphite spheroidization and inoculation is poured from a small ladle 34 into a rotating cylindrical mold 35 for centrifugal casting. It is preferable that the rotational speed of the cylindrical mold 35 is such that the centrifugal acceleration on the inner peripheral surface is equivalent to 110 to 130G. The amount of the molten metal 44 poured into the cylindrical mold 35 is preferably such that the cast iron cylindrical sliding member (rough material) has a thickness of 6.0 to 8.0 mm. Although details will be described later, since the inner peripheral surface of the cast iron cylindrical sliding member (rough material) is machined, it should be 2.5 to 4.0 mm thicker than the desired thickness of the cast iron cylindrical sliding member. is preferred.

(5) First and Second Cooling Steps By cooling while rotating the cylindrical mold 35, the molten metal solidifies and the rough material of the cast iron cylindrical sliding member is formed. A first cooling step in which a solid phase (primary graphite and eutectic graphite + eutectic austenite) is crystallized by cooling from hot water to the eutectic temperature, and a cooling rate lower than that of the first cooling step to form a solid phase. By performing the second cooling step of cooling from the eutectic temperature to just below the eutectoid temperature, from the outer peripheral surface to the inner peripheral surface of the coarse material, from only primary crystal graphite to primary crystal graphite + eutectic graphite A graphite distribution can be formed, and the graphite area ratio can be increased from the outer peripheral surface to the inner peripheral surface.

As such different cooling rates, for example, the first cooling step is performed in the range of 5 ° C./sec or more and 15 ° C./sec or less, and the second cooling step is 1 ° C./sec or more and less than 5 ° C./sec. can be done within the range of As a first cooling method, for example, by water-cooling the rotary mold 35 from the outer periphery, it is possible to cool at the above-described cooling rate, and the solidification direction of the coarse material is oriented from the outer peripheral surface to the inner peripheral surface. can also As a second cooling method, the cooling rate can be moderated by stopping the water cooling and allowing the rough material to cool in the mold. In particular, the cooling rate of the inner peripheral surface of the rough material is more moderated by the radiant heat.

In particular, in the cast iron cylindrical sliding member, the number of graphite particles and the graphite area ratio gradually increase toward the inner peripheral surface, and the point of inflection occurs because the temperature gradient ΔT in the solid-liquid coexistence interval from the primary crystal to the eutectic This is thought to be due to For example, if the water-cooling time of the cylindrical mold 35 is extremely short, the temperature gradient of ΔT from the start of the primary crystal to the eutectic is small, and the solid-liquid coexistence time increases. Here, since the molten metal has a hypereutectic composition from its CE value, the first solid crystallized from the liquid is primary crystal graphite. When the centrifugal force acts in a state where the primary crystal graphite and the liquid phase coexist, solidification proceeds from the outer peripheral surface of the rough material in contact with the cylindrical mold 35 toward the inner peripheral surface, but the primary crystal graphite is significantly stronger than the liquid phase. Since the specific gravity is small, it grows while moving from the outer peripheral surface side of the rough material toward the inner peripheral surface direction. This is considered to be the mechanism of having an inflection point at which the number of graphite particles and the graphite area ratio gradually increase toward the inner peripheral surface from a position at a certain distance from the outer peripheral surface. Conversely, if the water cooling time is long, the temperature gradient of ΔT is large and the solid-liquid coexistence time is short. It is thought that the increasing trend of the number of graphite particles and the graphite area ratio will be difficult to recognize.

(6) Cutting Step The rough material of the cast-iron cylindrical sliding member obtained by the above-described cooling is subjected to cutting or grinding to form an inner peripheral machined surface that serves as a sliding surface. For example, after cutting with a lathe or the like, the machined surface can be honed to obtain an inner peripheral machined surface. The thickness of the cast-iron cylindrical sliding member can be, for example, 3.5 to 4.0 mm.

Further, the inner peripheral processed surface obtained by cutting may be further subjected to laser irradiation to vaporize and disappear the graphite exposed from the inner peripheral processed surface, thereby forming fine concave dimples on the inner peripheral processed surface. . As a result, since a hard martensite structure can be formed on the surface layer of the inner peripheral machined surface, surface pressure resistance performance can be improved. Furthermore, since 90% or more of the graphite on the inner peripheral processed surface has a particle size of 1 to 10 μm, a large number of fine concave dimples are formed corresponding to the portions where the graphite particles have been removed. As a result, lubricating oil accumulates, and a uniform oil film can be formed over the entire surface, so that further reduction of friction can be expected.

The cast iron cylindrical sliding member thus obtained is shown in FIGS. 3 to 5. FIG. As shown in FIGS. 3 to 5, the cast iron cylindrical sliding member 11 has an outer peripheral surface 13 formed with a plurality of convex projections 15 corresponding to the concave portions of the coating layer provided on the inner peripheral surface of the cylindrical mold. It is The inner peripheral surface 14 of the cast-iron cylindrical sliding member 11 is cut to form a sliding surface. The direction from the outer peripheral surface to the inner peripheral surface is called the radial direction of the cast iron cylindrical sliding member 11 . Also, the distance from the outer peripheral surface to the inner peripheral surface is called the thickness of the cast iron cylindrical sliding member 11 .

The composition of the cast iron cylindrical sliding member 11 is, in mass %, C: 3.5 to 3.85%, Si: 2.3 to 2.7%, Mn: 0.5 to 1.5%, S: It contains 0.005-0.015%, the balance being Fe and unavoidable impurities, and the CE value is 4.45-4.70. The composition of the molten metal is substantially the same as described above, because the added graphite spheroidizing agent and inoculant are in small amounts relative to the molten metal, and most of the components of the graphite spheroidizing agent and inoculant evaporate from their boiling points. It is from. However, not all Mg in the graphite spheroidizing agent evaporates, and is contained in the cast iron cylindrical sliding member 11 as an unavoidable impurity at 0.005 to 0.04%. Moreover, not all of the Bi in the inoculant evaporates, and is contained in the cast-iron cylindrical sliding member 11 as an unavoidable impurity at 0.1 to 30 ppm. Bi can be quantified by high frequency inductively coupled plasma atomic emission spectrometry (ICP-MS method).

In the cross section of the cast-iron cylindrical sliding member 11, there are 1000 or more granular or spherical graphite particles per 1 mm ² having a particle size of 1 μm or more and less than 50 μm. Due to the presence of a large number of such fine granular or spherical graphite, the Young's modulus and strength of the cast iron cylindrical sliding member 11 can be improved to the same level as spheroidal graphite cast iron (FCD cast iron), which is superior to FC cast iron. . In addition, the graphite area ratio of granular or spherical graphite having a particle size of 1 μm or more and less than 50 μm is 5% or more, and the number of graphite particles is from the outer peripheral surface 13 to the inner peripheral surface 14 of the cast iron cylindrical sliding member 11. , and the graphite area ratio also increases from the outer peripheral surface 13 toward the inner peripheral surface 14 of the cast iron cylindrical sliding member 11 . As a result, the distance between particles of graphite is shortened, so that the inner peripheral surface 14 can exhibit excellent sliding properties due to the solid lubricating effect of graphite. Thus, the cast-iron cylindrical sliding member 11 of the present embodiment has high strength while ensuring high slidability on the inner peripheral surface 14 .

EXAMPLES Examples and comparative examples of the present invention will be described below.

First, a cylindrical mold for a cast-iron cylindrical sliding member was produced. A preheated cylindrical mold with an inner diameter of about 80 mm was rotated, and the mold coating slurry was applied at a rotation speed corresponding to a centrifugal acceleration of about 15 G on the inner peripheral surface, and dried and solidified while maintaining the mold rotation. As a result, a mold coating layer having a uniform thickness of about 1 mm was formed on the inner peripheral surface of the mold to complete a cylindrical mold for the member.

Next, as a molten metal for manufacturing a cast iron cylindrical sliding member, in a high-frequency induction melting furnace, C: 3.75%, Si: 2.1%, Mn: 0.8%, P : 0.02%, S: 0.010%.

[Comparative Example 1]
At the bottom of a small ladle, Fe-Si-Mg-Ca-RE alloy (grain size: 1 to 5 mm, addition amount: 0.85% by mass based on the amount of hot water) as a graphite spheroidizing agent, and Fe- as a primary inoculant Si—Sr-based inoculant (composition in mass %, Si: 75%, Sr: 1%, Fe: balance, grain size: 1 to 6 mm, amount added: 0.6% by mass based on the amount of hot water) After that, the original hot water for one pouring from the melting furnace was poured into a small ladle. The hot water temperature at this time was 1518°C. Then, the molten metal subjected to the graphite spheroidization treatment and the inoculation treatment was poured into the cylindrical mold rotated at a rotation speed corresponding to a centrifugal acceleration of about 120 G, and centrifugal casting was performed. The water cooling time of the outer peripheral surface of the mold was performed for 40 seconds immediately after the start of pouring, and thereafter the water cooling was stopped to moderate the cooling rate. As a result, a long and thin cast iron cylindrical sliding member (coarse material) was obtained.

[Example 1]
S, which is one of the main elements of cast iron, has the effect of significantly promoting graphite crystallization by reaction with RE (main component is Ce) contained in the graphite spheroidizing agent added in the subsequent process. Since Ce was preferentially consumed, in Comparative Example 1 using an inoculant containing Sr, a significant increase in the number of graphite grains was not obtained, and chill was crystallized in the matrix.

Therefore, in Example 1, which does not use an inoculant containing Sr, an Fe—S alloy was added to the residual melt in the melting furnace to adjust the S content to 0.012%, and then used as the base melt. Further, in Example 1, instead of the Fe--Si--Sr-based inoculant of Comparative Example 1 as the primary inoculant, an Fe--Si--Al-based inoculant (composition, in mass %, Si: 75%, Al: 1 0.5%, Fe: balance, grain size of 8 mm or less, amount added: 0.6% by mass based on the amount of molten metal). Furthermore, in Example 1, as a secondary inoculant, an Fe—Si—Bi inoculant (composition, in mass %, Si: 73%, Bi: 1%, RE: 1%, Fe: balance, grain size : 0.2 to 0.8 mm, addition amount: 0.2% by mass of the amount of molten metal to be poured) was added to the molten metal poured into the cylindrical mold from a small ladle to perform pouring flow inoculation. In addition to the above, the centrifugation was performed in the same manner as in Comparative Example 1 except that the temperature of the hot water was 1516 ° C. when it was discharged from the melting furnace to the small ladle, and that the water cooling time was 50 seconds immediately after the start of pouring. Casting was performed to obtain a long, thin-walled cast-iron cylindrical sliding member (coarse material).

[Comparative Example 2]
In Comparative Example 2, instead of the Fe--Si--Bi-based inoculant of Example 1 as the secondary inoculant, an Fe--Si--Al-based inoculant (composition, in mass %, Si: 75%, Al: 1 .5%, Fe: balance, grain size is 0.1 to 0.8 mm, addition amount is 0.2% by mass based on the amount of molten metal poured), and the hot water temperature when tapping from the melting furnace to the small ladle Centrifugal casting was performed in the same manner as in Example 1, except that the temperature was 1507° C., to obtain a long and thin cast iron cylindrical sliding member (rough material).

As described above, cast iron cylindrical sliding members (coarse materials) were manufactured in the order of Comparative Example 1, Example 1, and Comparative Example 2 based on the relationship of the S content in the molten metal. Table 1 summarizes the graphite spheroidization treatment, inoculation treatment, and cooling conditions of these examples and comparative examples.

Each cast iron cylindrical sliding member (rough material) of Comparative Example 1, Example 1, and Comparative Example 2 was cut, and the cross section was observed using an optical microscope (OLYMPUS GX51 manufactured by Olympus Corporation), and image analysis was performed. Using software (OLYMPUS Stream Basic manufactured by Olympus Corporation), the number of graphite particles, the graphite spheroidization rate, and the graphite area ratio of granular or spherical graphite were measured.

For the specular surface, the cut surface was wet-polished with emery paper and then buffed. No corrosion was observed on the speculum surface. The observation magnification was 100 times, and the observation field was five fields per sample. The detected size of granular or spherical graphite particles was measured every ≧1 μm, ≧3 μm, ≧5 μm, ≧7.5 μm, ≧10 μm, ≧15 μm, ≧20 μm, ≧25 μm, ≧35 μm, ≧50 μm.

FIG. 6 shows an optical microscope photograph of a cross section of the cast-iron cylindrical sliding member (coarse material) of Example 1. As shown in FIG. As shown in FIG. 6, on the outer peripheral surface of the cast-iron cylindrical sliding member (coarse material), convex net-like projections are formed, and the distance from the base surface 13B is 0.5 mm to 4.0 mm. A mark is attached every 5 mm. For example, when the member is used as a cylinder sleeve, the rough material is processed to remove the portion after 3.5 mm from the base surface 13B, so the area that can be used as the sliding surface of the cylinder sleeve is The range is 1.5 to 3.5 mm from the base surface 13B. As can be seen from the optical microscope photograph in FIG. 6, in the cast iron cylindrical sliding member (coarse material) obtained by the above method, minute granular or spherical graphite particles are crystallized from the outer peripheral surface to the inner peripheral surface. rice field.

Immediately below the base surface of each cast iron cylindrical sliding member (rough material) of Comparative Example 1, Example 1, and Comparative Example 2, and at positions at distances of 1.0 mm, 2.0 mm, and 3.0 mm from the base surface Graphs of graphite particle counts for granular or spherical graphite are shown in FIGS. The number of graphite particles is the number per 1 mm ² of cross section of the member. As legends for these graphs, the Sr-based inoculant is used for Comparative Example 1, the Bi-based inoculant is used for Example 1, and the Al-based inoculant is used for Comparative Example 2, because of the difference in the inoculants used. In addition, only for Example 1, a graph of the number of graphite particles at a distance of 3.5 mm from the basal plane is shown in FIG.

As shown in FIGS. 7 to 11, Example 1 using the Bi-based inoculant was compared to Comparative Example 1 using the Sr-based inoculant and Comparative Example 2 using the Al-based inoculant at any position. It can be seen that the number of graphite particles in granular or spherical graphite is remarkably large. In particular, the number of graphite particles of graphite having a particle size of 1 µm or more and less than 15 µm is large.

FIG. 12 is a graph showing changes in the number of graphite particles of granular or spherical graphite having a particle size of 1 μm or more with respect to the distance from the basal plane. As shown in FIG. 12, in Example 1 using the Bi-based inoculant, the number of granular or spherical graphite particles having a particle size of 1 μm or more was 1000 or more per 1 mm ² . Further, in Example 1 using the Bi-based inoculant and Comparative Example 1 using the Sr-based inoculant, the number of granular or spherical graphite particles having a particle size of 1 μm or more increases toward the inner peripheral surface. showed a trend. In particular, it gradually increased from a position at a distance of 2.0 mm from the base surface toward the inner peripheral surface. Among them, the number of graphite particles in Example 1 using a Bi-based inoculant was directed from a position at a distance of 3.0 mm to a position at a distance of 3.5 mm from the base surface, which is likely to be the sliding surface of the cylinder sleeve. increased significantly.

FIG. 13 shows the measurement results of the graphite spheroidization rate of Example 1 using the Bi-based inoculant. In the graph shown in FIG. 13, the graphite spheroidization rate for each of the minimum graphite particle sizes is shown at positions immediately below the basal plane and at distances of 1.0 mm, 2.0 mm, 3.0 mm, and 3.5 mm from the basal plane. showed that. As shown in FIG. 13 , the graphite spheroidization rate of graphite with a particle size of 5 μm or more was 50% or more in the region from immediately below the base surface to a position at a distance of 3.5 mm from the base surface. Further, in the same region, the graphite spheroidization ratio of graphite having a particle size of 15 μm or more was 10 to 20%.

FIG. 14 shows a graph showing the change in the graphite area ratio with respect to the distance from the base surface for the cast iron cylindrical sliding members (rough materials) of Comparative Example 1, Example 1, and Comparative Example 2. The graphite area ratio is the ratio of the area of granular or spherical graphite having a particle size of 1 μm or more to the cross-sectional area of the member. As shown in FIG. 14, in Example 1 using the Bi-based inoculant, the graphite area ratio was 5% or more in the region from directly below the base surface to a position at a distance of 3.5 mm from the base surface. . Moreover, in Example 1 using the Bi-based inoculant and Comparative Example 1 using the Sr-based inoculant, the graphite area ratio tended to increase toward the inner peripheral surface. In particular, in Example 1 using a Bi-based inoculant, it gradually increased from a position at a distance of 3.0 mm from the base surface toward the inner peripheral surface, and in Comparative Example 1 using an Sr-based inoculant, the It gradually increased from a position 2.0 mm away from the surface toward the inner peripheral surface. This is because in Example 1 using the Bi-based inoculant, the cooling time was longer than in Comparative Example 1 using the Sr-based inoculant, so the inflection point of gradual increase was closer to the inner peripheral surface of the coarse material. presumably shifted.

10 cylinder block 11 cylinder sleeve (cylindrical sliding member made of cast iron)
12 cylinder barrel 13 outer peripheral surface 14 inner peripheral surface 15 convex projection 20 cylinder head 21 combustion chamber 31 melting furnace (or holding furnace)
32 Induction coil 34 Small ladle 35 Cylindrical mold 36 Coating mold 41 Molten metal (original hot water)
42 Graphite spheroidizing agent 43 Inoculant 44 Molten metal (after treatment)

Claims (6)

C: 3.5 to 3.85%, Si: 2.3 to 2.7%, Mn: 0.5 to 1.5%, S: 0.005 to 0.015% by mass% , the balance being Fe and unavoidable impurities, and a cast iron cylindrical member having a carbon equivalent (hereinafter referred to as "CE value") defined by the following formula 1 of 4.45 to 4.70,
CE value = total carbon + (Si content + P content) / 3 (Formula 1)
In the cross section of the cast iron cylindrical member, the number of graphite particles of granular or spherical graphite with a particle size of 1 μm or more and less than 50 μm is 1000 or more per 1 mm ² , and the number of graphite particles is the outer circumference of the cast iron cylindrical member. It increases from the surface to the inner peripheral surface,
In the cross section of the cast iron cylindrical member, granular or spherical graphite with a particle size of 1 μm or more and less than 50 μm has a graphite area ratio of 5% or more, and the graphite area ratio is inward from the outer peripheral surface of the cast iron cylindrical member. Cast iron cylindrical member increasing towards the circumference. 1. In the cross section of the cast iron cylindrical member, graphite having a particle size of 5 μm or more has a graphite spheroidization rate of 50% or more, and graphite having a particle size of 15 μm or more has a graphite spheroidization rate of 10% or more. The cast iron cylindrical member according to . The cast iron cylindrical member according to claim 1 or 2, which contains 0.1 to 30 ppm of Bi as the unavoidable impurity. The cast iron cylindrical member according to any one of claims 1 to 3, containing 0.005 to 0.04% Mg as the unavoidable impurity. A method for manufacturing a cast iron cylindrical member by centrifugal casting,
C: 3.5 to 3.85%, Si: 2.3 to 2.7%, Mn: 0.5 to 1.5%, S: 0.005 to 0.015% by mass% , the balance being Fe and unavoidable impurities, and preparing a molten metal having a carbon equivalent (hereinafter referred to as "CE value") defined by the following formula 1 of 4.45 to 4.70;
CE value = total carbon + (Si content + P content) / 3 (Formula 1)
Fe—Si—Mg—RE alloy containing 2.0 to 4.0% by mass of Mg and 4.0 to 6.0% by mass of rare earth metal (hereinafter referred to as “RE”) in the molten metal adding a graphite spheroidizing agent consisting of
adding an Fe—Si—Bi based inoculant to the molten metal;
pouring the molten metal containing the graphite spheroidizing agent and the inoculant into a centrifugal casting mold;
A first cooling step of cooling from the pouring to the eutectic temperature to precipitate a solid phase;
a step of performing a second cooling of cooling the solid phase from the eutectic temperature to the eutectoid temperature at a cooling rate lower than that of the first cooling to form a rough material for a cast iron cylindrical member;
A method of manufacturing a cast iron cylindrical member, comprising: cutting the inner peripheral surface of the rough material to form an inner peripheral machined surface. The cast iron according to claim 5, further comprising a step of decomposing and removing graphite exposed from the inner peripheral processed surface by applying laser irradiation to the inner peripheral processed surface of the cast iron cylindrical member to form a minute recess. A manufacturing method for cylindrical members.

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