Cast iron cylindrical members

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]

[0001] The present invention relates to a cast iron cylindrical member and a method for manufacturing the same. [Background technology]

[0002] Many long, thin-walled cast iron cylindrical components are produced using centrifugal casting, in which molten metal is poured into the inner periphery of a rotating cylindrical mold at high speed. Typical examples of products manufactured using this method include cast iron pipes for infrastructure and rolling mill rolls for plastic processing. Centrifugal casting is also widely used for cylinder sleeves for automobile engines, which are produced in large quantities, because it produces cast iron cylindrical components of consistent quality without the need for expensive sand cores. Cylinder sleeves are also produced using the highly productive green sand casting method. Regardless of the casting method, cylinder sleeves are made from gray cast iron (FC cast iron), which contains uniform flake graphite in the cross-sectional metallographic structure. This is because the flake-crystallized graphite has excellent vibration damping, thermal conductivity, and self-lubrication properties.

[0003] For example, Patent Document 1 describes a method for manufacturing a cylinder liner (cylinder sleeve) for an internal combustion engine that has high strength properties in addition to sliding properties at low cost by simply setting conditions. The method involves pouring molten cast iron of a hypereutectic composition containing a graphite spheroidizing component into a centrifugal casting mold, rotating the mold at a predetermined speed, and using centrifugal force and the fading phenomenon to form an inner cast iron layer in which graphite similar in shape to flake graphite is crystallized in the inner part near the center of the mold, and an outer cast iron layer in which spheroidal graphite is crystallized in the outer part in contact with the mold wall. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-227405 Summary of the Invention [Problem to be solved by the invention]

[0005] Cylinder sleeves for automobile engines require low friction, high vibration damping, high thermal conductivity, and machinability to achieve fuel economy and quietness. FC cast iron is used to meet these requirements. However, FC cast iron's Young's modulus is low, approximately half that of steel (210 GPa). As shown in Figure 1, which shows a schematic representation of an automobile engine, when a cylinder block 10, consisting of a thin-walled cylindrical cylinder sleeve 11 encased in a cylinder barrel 12 by die-casting, is fastened to the cylinder head 20 with steel bolts under high axial force, the low-rigidity cylinder sleeve 11 is prone to bore deformation (distortion) due to the strong compressive load acting in the axial direction (arrow 1 in Figure 1). Furthermore, the cylinder sleeve 11 tends to distort (or deform) radially due to the internal bore pressure load (arrow 2 in Figure 1) caused by combustion and explosion pressure during operation. Furthermore, in the case of a thin, low-rigidity cylinder sleeve 11, residual stress (arrow 3 in Figure 1) introduced into the aluminum barrel 12 during die-casting of the cylinder block 10 runs the risk of causing bore deformation as it is released over time as the engine operates. This deformation behavior of the bore shape increases friction and blow-by during piston ring sliding, resulting in reduced fuel economy.

[0006] When a cylinder sleeve cast iron cylindrical member manufactured by the method disclosed in Patent Document 1 is used as a cylinder sleeve for an automobile engine, graphite with a shape similar to flake graphite crystallizes on the inner periphery of the cast iron cylindrical member (i.e., the sliding surface side), which poses a problem in that it is difficult to achieve both the sliding properties and strength required for the inner periphery.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a cylindrical cast iron member that can be strengthened while ensuring high sliding properties on the inner periphery, and a method for manufacturing the same. [Means for solving the problem]

[0008] In order to achieve the above object, one aspect of the present invention is a cast iron cylindrical member containing, by mass%, 3.5 to 3.85% C, 2.3 to 2.7% Si, 0.5 to 1.5% Mn, 0.005 to 0.015% S, with the remainder being Fe and unavoidable impurities, and 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 (hereinafter referred to as "TC") + (Si content + P content) / 3 (Equation 1) In the cross section of the cast iron cylindrical member, the number of granular or spherical graphite particles with a particle size of 1 μm or more and less than 50 μm is 1 mm 2 The number of graphite particles is 1,000 or more per 1000 particles, and the number of the graphite particles increases from the outer peripheral surface toward the inner peripheral surface of the cast iron cylindrical member. In the cross section of the cast iron cylindrical member, 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 graphite area ratio increases from the outer peripheral surface toward the inner peripheral surface of the cast iron cylindrical member.

[0009] In another aspect, the present invention provides a method for producing a cast iron cylindrical member by centrifugal casting, the method comprising the steps of: preparing a molten metal containing, by mass%, 3.5 to 3.85% C, 2.3 to 2.7% Si, 0.5 to 1.5% Mn, 0.005 to 0.015% S, the balance being Fe and unavoidable impurities, and having a CE value defined by the above-mentioned formula 1 of 4.45 to 4.70; and adding, to the molten metal, a Fe-Si-Mg-RE alloy containing, by mass%, 2.0 to 4.0% Mg and 4.0 to 6.0% rare earth metal (hereinafter referred to as "RE"). a step of adding a graphite spheroidizing agent consisting of Fe—Si—Bi-based inoculant to the molten metal; a step of pouring the molten metal to which the graphite spheroidizing agent and the inoculant have been added into a centrifugal casting mold; a first cooling step of cooling the molten metal from the pouring to a eutectic temperature to precipitate a solid phase; a second cooling step of cooling the solid phase from the eutectic temperature to a eutectoid temperature at a cooling rate slower than that of the first cooling step to form a raw material for a cast iron cylindrical member; and a step of cutting the inner peripheral surface of the raw material to form an inner peripheral machined surface. [Effects of the Invention]

[0010] Thus, according to the cast iron cylindrical member of the present invention, minute granular or spherical graphite particles are deposited on the inner periphery of the cast iron cylindrical member having a predetermined composition. 2 By making the number of graphite particles present 1,000 or more per 1000, and increasing the number of these graphite particles from the outer peripheral surface toward the inner peripheral surface, and further making the graphite area ratio of the fine granular or spherical graphite 5% or more, and increasing this graphite area ratio from the outer peripheral surface toward the inner peripheral surface, it is possible to provide a cast iron cylindrical member that can be made high in strength while ensuring high sliding properties.

[0011] Furthermore, according to the manufacturing 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 inoculation treatment is carried out with an inoculant having the predetermined composition. The molten metal is then poured into a mold for centrifugal casting, followed by a first cooling step in which the molten metal is cooled from the poured molten metal to a eutectic temperature to precipitate a solid phase, and then a second cooling step in which the solid phase is cooled from the eutectic temperature to the eutectoid temperature at a cooling rate slower than that of the first cooling step, thereby making it possible to manufacture a cast iron cylindrical member having the above characteristics. [Brief explanation of the drawings]

[0012] [Figure 1] 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. [Figure 2] 1 is a flow chart schematically showing one embodiment of a method for manufacturing a cast iron cylindrical member according to the present invention. FIG. [Figure 3] 1 is a perspective view schematically showing one embodiment of a cast iron cylindrical member according to the present invention. [Figure 4] FIG. 4 is a schematic cross-sectional view of the cast iron cylindrical member shown in FIG. 3 taken along line III-III. [Figure 5] 5 is a schematic cross-sectional view showing an enlarged portion of the cross section of the cast iron cylindrical member shown in FIG. 4. [Figure 6] 1 is an optical microscope photograph of a cross section of a cast iron cylindrical member (raw material) of an example. [Figure 7]1 is a graph showing the number of graphite particles for each minimum particle size immediately below the outer peripheral surface of a cast iron cylindrical member (raw material) of an example and a comparative example. [Figure 8] 1 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 a cast iron cylindrical member (raw material) of an example and a comparative example. [Figure 9] 1 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 a cast iron cylindrical member (raw material) of an example and a comparative example. [Figure 10] 1 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 a cast iron cylindrical member (raw material) of an example and a comparative example. [Figure 11] 1 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 a cast iron cylindrical member (raw material) of an example. [Figure 12] 1 is a graph showing the change 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 the cast iron cylindrical members (raw materials) of Examples and Comparative Examples. [Figure 13] 1 is a graph showing the graphite spheroidization rate for each minimum particle size at each distance from the outer peripheral surface of a cast iron cylindrical member (raw material) of an example. [Figure 14] 1 is a graph showing the change in graphite area ratio of particles of 1 μm or more and less than 50 μm in size with respect to the distance from the outer peripheral surface of cylindrical cast iron members (raw materials) of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of a cast iron cylindrical member and a manufacturing method thereof according to the present invention will be described with reference to the accompanying drawings. Note that the drawings are drawn with priority given to ease of understanding and are not drawn to scale.

[0014] First, one embodiment of a method for manufacturing a cast iron cylindrical member will be described with reference to FIG. The manufacturing method for the cast iron cylindrical sliding member of this embodiment includes the steps of preparing a molten metal 41 having a predetermined composition, as shown in Figure 2(a), adding a graphite spheroidizing agent 42 having a predetermined alloy composition to the molten metal 41, as shown in Figure 2(b), adding an inoculant 43A having a predetermined composition to the molten metal 41, as shown in Figure 2(b) or (c), pouring the molten metal 44 containing the graphite spheroidizing agent and the inoculant into a cylindrical mold 35 for centrifugal casting, further adding an inoculant 43B, as shown in Figure 2(c), performing a first cooling step to cool the poured molten metal to a eutectic temperature to crystallize a solid phase, performing a second cooling step to cool the solid phase from the eutectic temperature to just below the eutectoid temperature at a cooling rate slower than the first cooling step to form a raw material for the cast iron cylindrical sliding member, and (not shown) cutting the inner circumferential surface of the raw material to form an inner circumferential machined surface that will become the sliding surface. Each step is described below.

[0015] (1) Molten metal preparation step In centrifugal casting using a cylindrical mold, the solidification rate of the poured molten metal is extremely fast, so a molten metal with high C and Si content as shown below is essential to ensure stable crystallization of graphite from the molten metal. The composition of molten metal 41 for producing a cast iron cylindrical sliding member is, in mass%, 3.5 to 3.85% C, 2.3 to 2.7% Si, 0.5 to 1.5% Mn, and 0.005 to 0.015% S, with the remainder being Fe and unavoidable impurities, and the CE value defined by the following formula 1 is 4.45 to 4.70. The cast iron cylindrical sliding member also has this composition. CE value = TC + (Si content + P content) / 3 (Equation 1)

[0016] In order to produce a molten metal having the above composition, it is preferable to use a high-frequency induction melting furnace, as shown in Fig. 2(a). Raw materials are melted and adjusted in a melting furnace or holding furnace (hereinafter referred to as melting furnace) 31 equipped with an induction coil 32 so as to have the above composition, thereby obtaining a molten metal 41 (modified molten metal). Each component of the above composition and its content will be explained below.

[0017] Silicon is a powerful graphitization-promoting element, preferentially crystallizing graphite from the molten metal. When the Si content is less than 2.3% by mass, the rapid solidification rate of centrifugal casting tends to result in the crystallization of chill (a eutectic of austenite and cementite, also known as ledeburite) rather than graphite, leading to embrittlement and poor machinability. On the other hand, increasing the Si content reduces the solid solubility of carbon. Therefore, when the Si content exceeds 2.7% by mass, significant carbon dross is generated due to the supersaturated carbon. Because carbon dross has a lower specific gravity than molten iron, it segregates in a film-like form on the inner surface of the cylindrical blank before solidification is complete. This prevents the gas from escaping into the atmosphere and induces gas defects within the blank. Therefore, the Si content is limited to 2.3-2.7% by mass.

[0018] Carbon (C) is the primary element of cast iron and is essential for the crystallization of graphite within the structure, which contributes to its unique material properties. Because 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 drops below the eutectic composition (i.e., 4.3). When the CE value drops to the eutectic composition, the molten metal centrifuged into a cylindrical mold is prone to chill crystallization due to supercooling due to the high solidification rate, and this also reduces the number of graphite nodules and the graphite spheroidization rate. On the other hand, if the C content exceeds 3.85% by mass, the CE value may exceed 5.0. The presence of excess C causes the crystallization of carbon dross, chunky graphite, and explosive graphite, deteriorating mechanical properties. Therefore, the C content is limited to 3.5 to 3.85% by mass.

[0019] The CE value represents the effect of third elements (i.e., Si and P) added to Fe-C alloys on the solubility of carbon, as well as the increase or decrease in carbon activity, i.e., the ease with which carbon crystallizes as graphite or cementite. It expresses the degree of effect of the complexation of C, Si, and P during the melting and solidification of cast iron, converted into the amount of C alone. By maintaining the CE value within the hypereutectic range of 4.45 to 4.70, primary graphite of consistent shape and size can be crystallized from the liquid phase during cast iron solidification, even when thin-walled, long cylindrical blanks are produced using centrifugal casting, which has a high solidification rate, and casting defects can be reduced. The TC in Equation 1, which defines the CE value, is measured using a solid carbon / sulfur analyzer or a total organic carbon analyzer. Examples of solid carbon / sulfur analyzers include the EMIA series manufactured by Horiba, Ltd.

[0020] Mn has the effect of promoting the formation of carbides and promoting the precipitation of pearlite in the matrix. If excessive Mn is added, the precipitation of pearlite in the matrix is further promoted and segregation to grain boundaries increases, which can increase hardness and brittleness. Conversely, if the Mn content is low, ferrite precipitation increases in the matrix, reducing wear resistance. Therefore, the Mn content is set to 0.5 to 1.5 mass%.

[0021] P is one of the elements that inhibits graphite spheroidization, and causes P to concentrate in the liquid phase near the eutectic cell grain boundaries at the final stage of solidification, causing hard Fe3P to segregate at the eutectic cell grain boundaries after solidification. For this reason, the addition of P is unnecessary in this invention, and it is preferable to limit its content as an unavoidable impurity to 0.02 mass% or less.

[0022] S is an unavoidable impurity, but when it coexists with Mn or RE, it forms sulfides such as MnS in the molten metal, which are said to act as nuclei for graphite crystallization. In other words, adding an appropriate amount of S increases the number of graphite nodules. However, adding too much S inhibits graphite spheroidization, resulting in a decrease in the elongation of cast iron cylindrical sliding members. Therefore, the S content is set to 0.005 to 0.015 mass%.

[0023] (2) Adding the graphite spheroidizing agent An Fe-Si-Mg-RE alloy containing, by mass, 2.0 to 4.0% Mg and 4.0 to 6.0% RE is used as the graphite spheroidizing agent 42. Ca may be optionally added to the Fe-Si-Mg-RE alloy. That is, an Fe-Si-Mg-Ca-RE alloy may be used.

[0024] Mg is an element that does not dissolve in Fe. Because of its low boiling point, it generates tiny bubbles in high-temperature molten cast iron and dissipates into the atmosphere. These tiny bubbles are known to act as crystallization sites for spheroidal graphite. To obtain spheroidal graphite cast iron, Fe-Si-Mg alloys are typically used as graphite spheroidizing agents. In the method described here, spheroidizing is performed in a small ladle 34 after each shot. However, using a high-Mg-content graphite spheroidizing agent, as used in conventional FCD cast iron casting, results in a prolonged, vigorous reaction with the molten metal in the small ladle, lengthening the lead time between pouring into the small ladle and the start of pouring, thereby extending the casting cycle time. Furthermore, the vigorous reaction results in an unstable magnesium yield in the molten metal, resulting in significant variation in the graphite spheroidization rate of the cast raw material. Conversely, if an Fe-Si-Mg alloy with a low Mg content is used as a graphite spheroidizing agent, the number of micro-bubbles generated in the molten metal will be reduced and the fading time will also be shortened, which will likely result in a decrease in the number of graphite grains and a decrease in the graphite spheroidizing rate. Therefore, a graphite spheroidizing agent with a Mg content of 2.0 to 4.0 mass% is used.

[0025] The RE in the graphite spheroidizing agent is an essential element in centrifugal casting, which has a high solidification rate, because it strongly promotes graphitization by suppressing elements that inhibit graphite spheroidization, preventing fading, and forming nucleation sites for spheroidized graphite through sulfide formation. Therefore, the RE content 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, if the RE content exceeds 6.0% by mass, the spheroidization of graphite is inhibited and chunky graphite tends to crystallize. Therefore, the RE content is set to 1.5 to 6.0% by mass.

[0026] The contents of Si and Ca are not particularly limited, but the Si content is preferably 40 to 70 mass%, which can promote graphite crystallization from the molten metal in the same way as an inoculant. The Ca content is preferably 1.5 to 5.0 mass%, which can strongly deoxidize the molten metal and promote graphite crystallization from the molten metal in the same way as Si.

[0027] The remainder is Fe. These components are alloyed and used as graphite spheroidizing agent 42. It is preferable to use graphite spheroidizing agent 42 in a block form, for example, with a size of preferably 10 mm or less, and more preferably 5 mm or less. The amount added is preferably in the range of 0.2 to 2 mass % relative to the molten metal.

[0028] The graphite spheroidizing agent 42 is preferably added to the molten metal 41 by the pouring method. As shown in FIG. 2(b), the graphite spheroidizing agent 42 is placed at the bottom of a small ladle 34, and the molten metal 41 is poured into the small ladle 34 to perform the graphite spheroidizing process. A single pour of the molten metal (moist molten metal) 41 is tapped in small amounts from the melting furnace 31 into the small ladle 34. The molten metal temperature at this time is preferably set to 1500 to 1550°C. This is because the temperature of the molten metal when poured from the small ladle 34 into the cylindrical mold 35 drops by up to about 100°C from the temperature at the time of tapping due to the distribution of the molten metal to the small ladle 34 and reactions with the graphite spheroidizing agent 42 and the inoculant 43. Therefore, by setting the molten metal temperature at the time of tapping within the above range, the molten metal temperature at the time of pouring can be maintained at the desired temperature of 1400 to 1450°C.

[0029] (3) Inoculant addition step Inoculation is performed in conjunction with graphite spheroidization treatment because it enhances graphitization ability, prevents chilling, and improves graphite shape. While an Fe-Si alloy is generally used as the inoculant, in this embodiment, when the molten metal 41 is poured into the small ladle 34, a Bi-free inoculant 43A is added together with the graphite spheroidizing agent 42 in an amount of 0.2 to 0.8 mass% based on the pouring weight, and inoculation treatment is performed by the settling-pour method. Furthermore, when the molten metal 44 that has undergone the graphite spheroidization and inoculation treatment described above is poured into the rotary die, a Bi-containing Fe-Si-Bi inoculant 43B is poured in an amount of 0.1 to 0.3 mass% based on the molten metal weight (late inoculation). This significantly increases the number of graphite nodules and prevents chilling due to a synergistic effect with the predetermined composition of the graphite spheroidizing agent 42. Therefore, the number of fine graphite particles is reduced to 1 mm. 2 The number of graphite particles can be increased to 1000 or more per molten steel. The above-mentioned Bi-containing inoculant can be directly used as the inoculant 43A in Fig. 2(b) and the pouring flow inoculation of the inoculant 43B in Fig. 2(c) can be omitted, but the number of graphite particles will be lower than when the above-mentioned pouring flow inoculation is performed.

[0030] The content of each component in the Fe-Si-Bi-based inoculant is not particularly limited, but preferably 50 to 75% by mass of Si and 0.5 to 2% by mass of Bi. Optionally, RE may be contained in an amount of 0.5 to 2.0% by mass. The remainder is Fe and unavoidable impurities such as Ca. The Fe-Si-Bi-based inoculant is preferably used in granular form. For example, when used for pouring inoculation such as inoculant 43B in FIG. 2(c), the particle size is preferably 1 mm or less. When used for stationary pouring inoculation such as inoculant 43A in FIG. 2(b), the particle size is preferably 3 to 10 mm. Since a small amount is effective, it is preferable to add the inoculant in the range of 0.1 to 1% by mass relative to the molten metal. It is also possible to use other inoculants in combination with the Fe-Si-Bi-based inoculant. For example, an Fe-Si-Al-based inoculant or another Fe-Si-based inoculant not containing Sr may be inoculated into the small ladle 34 by the pouring method, and then the Fe-Si-Bi-based inoculant may be used only for pouring inoculant. However, when an Fe-Si-Sr-based inoculant is used, the graphitization effect of Sr and RE (main element is Ce) is reduced due to the reaction, and chilling is promoted.

[0031] As described above, in the inoculation method, as shown in FIG. 2(b), the Fe-Si-Bi-based inoculant is placed at the bottom of a small ladle 34 as a primary inoculant 43A, and the molten metal 41 is poured into the inoculant, thereby inoculating the molten metal 41. In the pouring inoculation method, as shown in FIG. 2(c), the Fe-Si-Bi-based inoculant is added as a secondary inoculant 43B to the molten metal 44 poured from the small ladle 34 into a cylindrical mold 35. Since a significant increase in the number of graphite nodules can be achieved even with an addition of 0.1 mass% of the molten metal weight, pouring inoculation in which the Fe-Si-Bi-based inoculant is added to the molten metal during pouring is more preferable.

[0032] (4) Pouring molten metal into the cylindrical mold As shown in FIG. 2(c), a cylindrical mold 35 for centrifugal casting of a cast iron cylindrical sliding member has a mold wash layer 36 formed on its inner peripheral surface. This mold wash layer 36 is intended to form multiple convex protrusions on the outer peripheral surface of the cast iron cylindrical sliding member in order to improve adhesion to the surrounding area when the cast iron cylindrical sliding member is inserted into the insert. The mold wash layer 36 can be formed by applying a mold wash slurry, which is a mixture of a binder such as bentonite and a refractory material in a predetermined ratio with water, to the inner peripheral surface of the rotating cylindrical mold 35, and then drying and solidifying the slurry to form the mold wash layer 36 with concave portions corresponding to the convex protrusions. The thickness of the mold wash layer 36 is preferably about 1 mm.

[0033] The molten metal 44 that has been subjected to the graphite spheroidizing treatment and inoculation treatment is then poured from a small ladle 34 into a rotating cylindrical mold 35, where it is centrifugal cast. The rotation speed of the cylindrical mold 35 is preferably set to a rotation speed that corresponds to a centrifugal acceleration of 110 to 130 G on the inner peripheral surface. The amount of molten metal 44 poured into the cylindrical mold 35 is preferably an amount that will result in a cast iron cylindrical sliding member (raw material) with a thickness of 6.0 to 8.0 mm. As will be described in detail later, because the inner peripheral surface of the cast iron cylindrical sliding member (raw material) is machined, it is preferable that the amount of molten metal 44 be 2.5 to 4.0 mm thicker than the desired thickness of the cast iron cylindrical sliding member.

[0034] (5) First and second cooling steps By cooling the cylindrical mold 35 while rotating, the molten metal solidifies and a raw material for the cast iron cylindrical sliding member is formed. This molten metal is cooled in a first cooling step, in which the metal is cooled from pouring to the eutectic temperature to crystallize a solid phase (primary graphite and eutectic graphite + eutectic austenite), and in a second cooling step, in which the solid phase is cooled from the eutectic temperature to just below the eutectic temperature at a cooling rate slower than that of the first cooling step. This allows a graphite distribution consisting of primary graphite + eutectic graphite to be formed from only primary graphite from the outer peripheral surface to the inner peripheral surface of the raw material, and also increases the graphite area ratio from the outer peripheral surface to the inner peripheral surface.

[0035] As examples of such different cooling rates, the first cooling step may be performed at a rate in the range of 5°C / sec or more and 15°C / sec or less, and the second cooling step may be performed at a rate in the range of 1°C / sec or more and less than 5°C / sec. As a first cooling method, for example, the rotating mold 35 may be water-cooled from the outer periphery, thereby achieving the above-mentioned cooling rate and directing the solidification direction of the coarse material from the outer periphery to the inner periphery. As a second cooling method, the water cooling may be stopped and the coarse material may be allowed to cool naturally within the mold, thereby slowing the cooling rate. In particular, the cooling rate on the inner periphery of the coarse material is further slowed by radiant heat.

[0036] In particular, the occurrence of an inflection point where the number of graphite particles and the graphite area ratio gradually increase toward the inner circumferential surface in cast iron cylindrical sliding components is thought to be due to the temperature gradient ΔT within the solid-liquid coexistence zone from primary crystallization to eutectic. For example, if the water-cooling time of the cylindrical mold 35 is extremely short, the temperature gradient ΔT between the start of primary crystallization and the eutectic phase is small, and the solid-liquid coexistence time increases. Here, since the molten metal has a hypereutectic composition based on its CE value, the first solid to crystallize from the liquid is primary graphite. When centrifugal force is applied in a state where primary graphite and liquid phases coexist, solidification proceeds from the outer circumferential surface of the coarse material in contact with the cylindrical mold 35 toward the inner circumferential surface. However, because the specific gravity of primary graphite is significantly lower than that of the liquid phase, it grows while moving from the outer circumferential surface of the coarse material toward the inner circumferential surface. This is thought to be the mechanism by which the inflection point where the number of graphite particles and the graphite area ratio gradually increase toward the inner circumferential surface occurs from a certain distance from the outer circumferential surface. Conversely, if the water cooling time is long, the temperature gradient ΔT becomes large and the time during which solid and liquid coexistence is possible becomes short, so that eutectic solidification is completed with little movement of primary graphite, and it is thought that the tendency for an increase in the number of graphite particles and graphite area ratio with the above-mentioned inflection point becomes difficult to observe.

[0037] (6) Cutting process step The inner circumferential surface of the cast iron cylindrical sliding member obtained by the above cooling is formed into an inner circumferential surface that will become the sliding surface by cutting or grinding. For example, after cutting using a lathe or the like, the cut surface can be honed to obtain the inner circumferential surface. The thickness of the cast iron cylindrical sliding member can be, for example, 3.5 to 4.0 mm.

[0038] Furthermore, the inner peripheral machined surface obtained by cutting may be further irradiated with a laser to vaporize and remove the graphite exposed on the inner peripheral machined surface, forming minute concave dimples on the inner peripheral machined surface. This allows a hard martensite structure to be formed on the surface layer of the inner peripheral machined surface, thereby improving surface pressure resistance. Furthermore, since 90% or more of the graphite on the inner peripheral machined surface has a particle size of 1 to 10 μm, numerous minute concave dimples are formed corresponding to the areas where the graphite particles have been removed. This forms an oil reservoir for the lubricating oil, allowing a uniform oil film to be formed throughout, which is expected to further reduce friction.

[0039] The cast iron cylindrical sliding member obtained in this manner is shown in Figs. 3 to 5. As shown in Figs. 3 to 5, a plurality of convex protrusions 15 corresponding to the recesses in the mold wash layer provided on the inner peripheral surface of the cylindrical mold are formed on the outer peripheral surface 13 of the cast iron cylindrical sliding member 11. A sliding surface is formed on the inner peripheral surface 14 of the cast iron cylindrical sliding member 11 by cutting. The direction from the outer peripheral surface toward the inner peripheral surface is called the radial direction of the cast iron cylindrical sliding member 11. The distance from the outer peripheral surface to the inner peripheral surface is called the thickness of the cast iron cylindrical sliding member 11.

[0040] The cast iron cylindrical sliding member 11 has a composition, by mass, of 3.5-3.85% C, 2.3-2.7% Si, 0.5-1.5% Mn, and 0.005-0.015% S, with the remainder consisting of Fe and unavoidable impurities. Its CE value is 4.45-4.70. This is essentially the same as the composition of the molten metal described above because the added graphite spheroidizing agent and inoculant are small amounts relative to the molten metal, and most of them evaporate due to the boiling points of the graphite spheroidizing agent and inoculant components. However, not all of the Mg in the graphite spheroidizing agent evaporates, and the cast iron cylindrical sliding member 11 contains 0.005-0.04% as an unavoidable impurity. Furthermore, not all of the Bi in the inoculant evaporates, and the cast iron cylindrical sliding member 11 contains 0.1-30 ppm as an unavoidable impurity. Bi can be quantitatively determined by high-frequency inductively coupled plasma atomic emission spectroscopy (ICP-MS).

[0041] The cross section of the cast iron cylindrical sliding member 11 has a particle size of 1 μm or more and less than 50 μm, and the number of granular or spherical graphite particles is 1 mm 2 There are 1,000 or more particles per 1000. The presence of such a large number of minute granular or spherical graphite particles can improve the Young's modulus and strength of the cast iron cylindrical sliding member 11 to the same level as spheroidal graphite cast iron (FCD cast iron), which is superior to FC cast iron. The graphite area ratio of the granular or spherical graphite particles having a particle size of 1 μm or more and less than 50 μm is 5% or more. The number of graphite particles increases 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 to the inner peripheral surface 14 of the cast iron cylindrical sliding member 11. This shortens the interparticle distance of the graphite particles, allowing the solid lubrication effect of the graphite to exhibit excellent sliding properties at the inner peripheral surface 14. Thus, the cast iron cylindrical sliding member 11 of this embodiment has high strength while ensuring high sliding properties at the inner peripheral surface 14. [Example]

[0042] Examples and comparative examples of the present invention will be described below.

[0043] First, a cylindrical mold for a cast iron cylindrical sliding component was prepared. A preheated cylindrical mold with an inner diameter of approximately 80 mm was rotated, and the wash slurry was applied to the inner surface at a rotation speed equivalent to a centrifugal acceleration of approximately 15 G. The slurry was then dried and solidified while the mold continued to rotate. This resulted in a wash layer of approximately 1 mm thickness being formed on the inner surface of the mold, completing the cylindrical mold for the component.

[0044] Next, raw materials were blended and melted in a high-frequency induction melting furnace to obtain a molten metal for producing a cast iron cylindrical sliding member, with the composition being, by mass%, C: 3.75%, Si: 2.1%, Mn: 0.8%, P: 0.02%, and S: 0.010%, and this was used as the base molten metal.

[0045] [Comparative Example 1] An Fe-Si-Mg-Ca-RE alloy (particle size: 1–5 mm, added in an amount of 0.85 mass% of the molten metal) was placed at the bottom of a small ladle as a graphite spheroidizing agent, and an Fe-Si-Sr-based primary inoculant (composition: Si: 75%, Sr: 1%, Fe: balance, particle size: 1–6 mm, added in an amount of 0.6 mass% of the molten metal) was placed at the bottom of the small ladle. A single pour of the molten metal was then poured into the small ladle. The molten metal temperature at this time was 1518°C. The molten metal, which had been subjected to the graphite spheroidizing and inoculation processes, was then poured into the cylindrical mold rotating at a centrifugal acceleration of approximately 120 G and centrifugal casting was performed. The outer surface of the mold was water-cooled for 40 seconds immediately after the start of pouring, after which the water cooling was stopped and the cooling rate was slowed down. This resulted in the production of a long, thin-walled cast iron cylindrical sliding component (raw material).

[0046] [Example 1] S, one of the main elements of cast iron, has the effect of significantly promoting graphite crystallization by reacting with RE (main component is Ce) contained in the graphite spheroidizing agent added in a later process. However, when Sr is present, Ce is preferentially consumed. Therefore, in Comparative Example 1, in which an inoculant containing Sr was used, no significant increase in the number of graphite nodules was obtained, and chill crystallized in the matrix.

[0047] Therefore, in Example 1, which did not use an inoculant containing Sr, an Fe-S alloy was added to the remaining molten metal in the melting furnace to adjust the S content to 0.012% before being used as the modifier. In Example 1, instead of the Fe-Si-Sr-based inoculant used in Comparative Example 1 as the primary inoculant, an Fe-Si-Al-based inoculant (composition, by mass, of Si: 75%, Al: 1.5%, Fe: balance, particle size: 8 mm or less, and amount added: 0.6% by mass relative to the amount of molten metal poured) was used. Furthermore, in Example 1, an Fe-Si-Bi-based inoculant (composition, by mass, of Si: 73%, Bi: 1%, RE: 1%, Fe: balance, particle size: 0.2 to 0.8 mm, and amount added: 0.2% by mass relative to the amount of molten metal poured) was added as the secondary inoculant to the molten metal poured from a small ladle into a cylindrical mold. This inoculant was used as a pouring inoculant. Then, centrifugal casting was carried out in the same manner as in Comparative Example 1, except that the molten metal temperature when poured from the melting furnace into the small ladle was 1516°C and the water cooling time was 50 seconds immediately after the start of pouring, to obtain a long, thin-walled cast iron cylindrical sliding member (raw material).

[0048] Comparative Example 2 In Comparative Example 2, instead of the Fe-Si-Bi-based inoculant used in Example 1, an Fe-Si-Al-based inoculant (composition, by mass, of Si: 75%, Al: 1.5%, Fe: balance, particle size: 0.1 to 0.8 mm, added amount: 0.2 mass% relative to the poured molten metal) was used as the secondary inoculant. Centrifugal casting was performed in the same manner as in Example 1, except that the molten metal temperature when poured from the melting furnace into the small ladle was 1507°C, and a long, thin-walled cast iron cylindrical sliding member (raw material) was obtained.

[0049] As described above, in consideration of the relationship between the S content in the molten metal, cast iron cylindrical sliding members (raw materials) were produced in the order of Comparative Example 1, Example 1, and Comparative Example 2. The conditions for the graphite spheroidizing treatment, inoculation treatment, and cooling for these Examples and Comparative Examples are summarized in Table 1.

[0050] [Table 1]

[0051] Each of the cast iron cylindrical sliding members (raw materials) 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 the number of graphite particles, graphite spheroidization rate, and graphite area rate of granular or spherical graphite were measured using image analysis software (OLYMPUS Stream Basic manufactured by Olympus Corporation).

[0052] The cut surface for the inspection was water-polished with emery paper and then buffed. No corrosion was observed on the inspection surface. The observation magnification was 100x, and five fields of view were observed per sample. The detected sizes of granular or spherical graphite particles were measured in increments of ≥1 μm, ≥3 μm, ≥5 μm, ≥7.5 μm, ≥10 μm, ≥15 μm, ≥20 μm, ≥25 μm, ≥35 μm, and ≥50 μm.

[0053] FIG. 6 shows an optical microscope photograph of the cross section of the cast iron cylindrical sliding member (raw material) of Example 1. As shown in FIG. 6, convex mesh-like protrusions are formed on the outer peripheral surface of the cast iron cylindrical sliding member (raw material), and their distance from the base surface 13B is marked every 0.5 mm from 0.5 mm to 4.0 mm. For example, when the member is used as a cylinder sleeve, the portion of the raw material 3.5 mm and beyond from the base surface 13B is removed by machining, and therefore the area usable as the sliding surface of the cylinder sleeve is in the range of 1.5 to 3.5 mm from the base surface 13B. As can be seen from the optical microscope photograph in FIG. 6, the cast iron cylindrical sliding member (raw material) obtained by the above method had fine granular or spherical graphite particles crystallized from the outer peripheral surface to the inner peripheral surface.

[0054] Graphs of the number of granular or spherical graphite particles at positions directly below the base surface and at distances of 1.0 mm, 2.0 mm, and 3.0 mm from the base surface of each of the cast iron cylindrical sliding members (raw materials) of Comparative Example 1, Example 1, and Comparative Example 2 are shown in Figures 7 to 10. The number of graphite particles was calculated for each 1 mm cross section of the member. 2The number of graphite particles per 1000 particles is shown in Fig. 11. The legends of these graphs indicate the difference in the inoculants used: Comparative Example 1 is an Sr-based inoculant, Example 1 is a Bi-based inoculant, and Comparative Example 2 is an Al-based inoculant. Also, for Example 1 only, a graph of the number of graphite particles at a position 3.5 mm away from the basal plane is shown in Fig. 11.

[0055] 7 to 11, Example 1 using a Bi-based inoculant has a significantly larger number of granular or spherical graphite particles at all positions than Comparative Example 1 using a Sr-based inoculant or Comparative Example 2 using an Al-based inoculant. In particular, the number of graphite particles with a particle size of 1 μm or more and less than 15 μm is larger.

[0056] 12 is a graph showing the change in the number of granular or spherical graphite particles with 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 a Bi-based inoculant, the number of granular or spherical graphite particles with a particle size of 1 μm or more increased by 1 mm. 2 The number of graphite particles was 1,000 or more per 1000. In addition, in Example 1 using a Bi-based inoculant and Comparative Example 1 using a Sr-based inoculant, the number of granular or spherical graphite particles with a particle size of 1 μm or more tended to increase toward the inner circumferential surface. In particular, the number gradually increased from a position 2.0 mm away from the base surface toward the inner circumferential surface. In particular, the number of graphite particles in Example 1 using a Bi-based inoculant significantly increased from a position 3.0 mm away from the base surface toward a position 3.5 mm away, which is likely to be the sliding surface of the cylinder sleeve.

[0057] The measurement results of the graphite spheroidization ratio in Example 1 using a Bi-based inoculant are shown in Figure 13. The graph in Figure 13 shows the graphite spheroidization ratio for each minimum particle size of graphite at positions directly 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. As shown in Figure 13, in the region from directly below the basal plane to a position 3.5 mm from the basal plane, the graphite spheroidization ratio of graphite with a particle size of 5 µm or more was 50% or more. Furthermore, in the same region, the graphite spheroidization ratio of graphite with a particle size of 15 µm or more was 10 to 20%.

[0058] Figure 14 shows a graph showing the change in graphite area ratio with distance from the basal surface for each of the cast iron cylindrical sliding members (raw materials) of Comparative Example 1, Example 1, and Comparative Example 2. The graphite area ratio is the ratio of granular or spherical graphite with a particle size of 1 μm or more to the cross-sectional area of the member. As shown in Figure 14, in Example 1, which used a Bi-based inoculant, the graphite area ratio was 5% or more in the region from just below the basal surface to a position 3.5 mm from the basal surface. Furthermore, in Example 1, which used a Bi-based inoculant, and Comparative Example 1, which used a Sr-based inoculant, the graphite area ratio tended to increase toward the inner circumferential surface. In particular, in Example 1, which used a Bi-based inoculant, the graphite area ratio gradually increased from a position 3.0 mm from the basal surface toward the inner circumferential surface, while in Comparative Example 1, which used a Sr-based inoculant, the graphite area ratio gradually increased from a position 2.0 mm from the basal surface toward the inner circumferential surface. This is thought to be because the cooling time in Example 1, which used a Bi-based inoculant, was longer than that in Comparative Example 1, which used a Sr-based inoculant, and therefore the inflection point of the gradual increase shifted closer to the inner surface of the raw material. [Explanation of symbols]

[0059] 10 Cylinder block 11 Cylinder sleeve (cast iron cylindrical sliding member) 12 cylinder barrel 13 Outer surface 14 Inner surface 15 Convex process 20 Cylinder head 21 Combustion chamber 31 Melting furnace (or holding furnace) 32 induction coil 34 Small ladle 35 Cylindrical mold 36 Coating 41 Molten metal (original metal) 42 Graphite spheroidizing agent 43 Inoculants 44 Molten metal (after processing)

Claims

1. A cylindrical cast iron member containing, by mass%, 3.5 to 3.85% C, 2.3 to 2.7% Si, 0.5 to 1.5% Mn, 0.005 to 0.015% S, with the balance being Fe and inevitable impurities, wherein the inevitable impurities include 0.1 to 30 ppm Bi, and the carbon equivalent defined by the following formula 1 (hereinafter referred to as "CE value") is 4.45 to 4.70, CE value = total carbon + (Si content + P content) / 3 (Equation 1) In the cross section of the cast iron cylindrical member, the number of granular or spherical graphite particles having a particle size of 1 μm or more and less than 50 μm is 1 mm ２ the number of graphite particles increases from a position of 0.5 mm thickness toward the inner circumferential surface of the cast iron cylindrical member in a radial direction from the inner circumferential surface, A cast iron cylindrical member in which, in a cross section of the cast iron cylindrical member, 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 graphite area ratio increases from the position of the cast iron cylindrical member toward the inner peripheral surface.

2. A cast iron cylindrical member as described in claim 1, wherein in a cross section of the cast iron cylindrical member, the graphite spheroidization rate of graphite having a particle size of 5 μm or more is 50% or more, and the graphite spheroidization rate of graphite having a particle size of 15 μm or more is 10% or more.

3. 3. The cast iron cylindrical member according to claim 1, wherein the inevitable impurities include 0.005 to 0.04% of Mg.

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