JP4557731B2 - Cast iron with excellent corrosion resistance and wear resistance - Google Patents

Description

  The present invention relates to cast iron suitable for a cylinder liner of an internal combustion engine, and more particularly to improvement of corrosion resistance and wear resistance.

Cylinder liners for internal combustion engines are strongly required to have excellent wear resistance and scuffing resistance because the piston ring slides on the inner peripheral surface to maintain airtightness. Conventionally, graphite and carbide have been dispersed. Special alloy cast iron with a texture has been used.
For example, in Patent Document 1, as an inner layer of a composite cylinder liner, by weight, C: 2.5-4.0%, Si: 0.8-2.5%, Mn: 0.3-1.5%, P: 0.05-1.5%, S: 0.3 % Or less, Ni: 2.5% or less, Cr: 1.5% or less, Mo: 0.8% or less, Sn: 0.5% or less, Cu: 4.0% or less, one or two of B, Ti, V, Nb, Zr Alloy cast iron containing one or more of the above total 1.0% or less, Al, Ca, Ba, Sr, or one or more rare earth elements total 0.2% or less, and the balance being substantially Fe. Is disclosed. However, due to demands for higher output of internal combustion engines and exhaust gas regulations, the alloy cast iron described in Patent Document 1 has a problem that the wear resistance and scuffing resistance of the inner peripheral surface of the cylinder liner is insufficient. It was. Further, in the case of a wet cylinder liner equipped with a water cooling jacket, cavitation erosion on the outer peripheral surface of the cylinder liner becomes a problem, and improvement in corrosion resistance is required.

  For such a problem, for example, Patent Document 2 proposes an alloy cast iron cylinder liner in which a nitride layer is formed on the inner surface and a hard chromium plating layer is formed on the outer surface. According to the technique described in Patent Document 2, the wear resistance of the inner surface, the scuffing resistance, and the corrosion resistance of the outer surface can be improved at the same time. In Patent Document 2, as an example of alloy cast iron for cylinder liners, C: 2.8 to 3.5%, Si: 1.5 to 2.5%, Mn: 0.5 to 1.0%, P: 0.2 to 0.5%, S : 0.12% or less, B: 0.07 to 0.11%, Mo: 0.1 to 0.5%, Cu: 0.2 to 0.6%, and the balance substantially consisting of Fe is described. However, according to the technique described in Patent Document 2, the wear resistance and corrosion resistance are certainly improved by forming a nitride layer or a plating layer on the surface, but the effect until the nitride layer or the plating layer disappears. There is a problem that it is difficult to maintain wear resistance and corrosion resistance thereafter.

For example, Japanese Patent Application Laid-Open No. H10-228867 discloses that mass ratios include C: 2.5 to 3.6%, Si: 1.4 to 2.6%, Mn: 0.5 to 1.0%, P: 0.1 to 0.4%, and S: 0.12. %: Cr: 0.1 to 0.4%, B: 0.03 to 0.12%, Cu: 0.2 to 2.0%, Co: 1.0 to 10%, the composition comprising the balance Fe and inevitable impurities, and the matrix comprising pearlite A cast iron having a structure in which a hard phase and flake graphite are dispersed has been proposed. The cast iron described in Patent Document 3 has both wear resistance and corrosion resistance, and in particular, the corrosion wear resistance is improved.
JP-A-60-121254 Japanese Patent Laid-Open No. 5-86966 Japanese Patent No. 3297150

However, the cast iron described in Patent Document 3 needs to contain a large amount of expensive Co, which causes an increase in manufacturing cost and is economically disadvantageous. Was leaving.
The present invention solves the above-mentioned problems of the prior art, and does not require a large amount of expensive alloy elements, and is inexpensive and suitable for use as a cylinder liner, and has excellent corrosion resistance and wear resistance. The purpose is to provide.

  In order to achieve the above-described problems, the present inventors diligently studied various factors that affect the wear resistance and corrosion resistance of cast iron. As a result, it has been found that by adding Cr, Cu, B, and Mo to the cast iron containing no Co, the base pearlite becomes finer, the strength is increased, and the sliding characteristics are remarkably improved. . In addition, the present inventors have found that the average graphite interval of the newly defined flake graphite greatly affects the strength and corrosion resistance of the cast iron, and the strength is adjusted by adjusting the graphite interval of the flake graphite to an appropriate range. It has been found that the wear resistance is improved and the corrosion resistance is also improved.

In addition, the present inventors have combined Cr, Cu, B, and Mo, further increased the B content as compared with the prior art, and increased the abundance ratio of the hard phase containing steadite and boron carbide. It has been found that wear resistance and corrosion resistance can be simultaneously improved at a low cost by using together adjustment of the average graphite interval of glassy graphite within an appropriate range.
The present invention has been completed based on the above findings and further studies. That is, the present invention is in mass%, C: 3.0 to 3.5%, Si: 1.5 to 2.5%, Mn: 0.5 to 1.0%, P: 0.2 to 0.5%, S: 0.12% or less, Cr: 0.1 to 0.5% , B: 0.09 to 0.18%, Cu: 0.4 to 1.0%, Mo: 0.1 to 0.5%, a composition composed of the balance Fe and inevitable impurities, a base phase mainly composed of pearlite, and a steadyte in the base phase And a hard phase composed of boron compound and flake graphite are dispersed, the hard phase is 14 to 22% in area ratio, and the mean graphite interval of flake graphite is 9 to 15 μm. Cast iron with excellent corrosion resistance and wear resistance. Moreover, this invention is a cylinder liner manufactured using the cast iron which has an above-described composition and structure | tissue.

  According to the present invention, the corrosion resistance and wear resistance of cast iron can be improved at the same time, and a cylinder liner excellent in durability can be manufactured at a low cost, and a remarkable industrial effect can be achieved. The present invention has an effect that it can be effectively applied to uses other than the cylinder liner, such as a valve guide that requires wear resistance and corrosion resistance.

First, the reasons for limiting the composition of the cast iron of the present invention will be described. Unless otherwise specified, mass% is simply expressed as%.
C: 3.0-3.5%
C is an element that strengthens the base as a structure mainly composed of pearlite and crystallizes graphite to improve wear resistance and scuffing resistance. In order to obtain such an effect, 3.0% or more. Containing is required. On the other hand, if the content exceeds 3.5%, the amount of graphite and carbides become too large, which promotes embrittlement. For this reason, C was limited to the range of 3.0 to 3.5%.

Si: 1.5-2.5%
Si is one of the basic elements of cast iron, and at least 1.5% or more is required for crystallization of graphite. On the other hand, if the content exceeds 2.5%, the content becomes excessive and the strength decreases. For this reason, Si was limited to the range of 1.5 to 2.5%.
Mn: 0.5-1.0%
Mn is an element that increases the strength of cast iron and needs to contain 0.5% or more. On the other hand, if it exceeds 1.0%, cementite is precipitated and the toughness is lowered. For this reason, Mn was limited to the range of 0.5 to 1.0%.

P: 0.2-0.5%
P is an element that crystallizes steadite (phosphorus eutectic) and disperses it as a hard phase in the matrix to improve the wear resistance, and needs to contain 0.2% or more. On the other hand, if the content exceeds 0.5%, the material becomes brittle. For this reason, P was limited to the range of 0.2 to 0.5%.
S: 0.12% or less S is an impurity element that embrittles material properties and is preferably reduced as much as possible. However, if it is 0.12% or less, MnS is formed, which is advantageous for workability. On the other hand, if it exceeds 0.12%, the material properties become brittle. For this reason, S was limited to 0.12% or less.

Cr: 0.1-0.5%
Cr is an important element that dissolves in the matrix and strengthens the matrix, and is contained in the carbide to increase the hardness of the carbide and improve the wear resistance, and needs to be contained in an amount of 0.1% or more. On the other hand, if the content exceeds 0.5%, the amount of carbide increases and the shape of graphite is broken, so that the machinability is lowered. For this reason, Cr was limited to the range of 0.1 to 0.5%. In addition, Preferably it is 0.2 to 0.4%.

B: 0.09 to 0.18%
B is an important element that forms boron carbide, forms a hard phase with the phosphorus eutectic, increases the hardness, and improves wear resistance and scuffing resistance like Cr. In the present invention, B is a hard phase. In order to increase the area ratio and improve the wear resistance, it is necessary to contain 0.09% or more. On the other hand, if it exceeds 0.18%, the hard phase becomes excessive and the toughness decreases. For this reason, B was limited to the range of 0.09 to 0.18%. In addition, Preferably it is 0.12-0.15%.

Cu: 0.4-1.0%
Cu is an element that solidifies in the matrix and strengthens the matrix to improve wear resistance and corrosion resistance. In the present invention, Cu is an important element together with Cr and B. Such an effect becomes remarkable when the content is 0.4% or more, but even if the content exceeds 1.0%, the effect is saturated, and an effect commensurate with the content cannot be expected. For this reason, Cu was limited to the range of 0.4 to 1.0%. In addition, Preferably, it is 0.5 to 0.8%.

Mo: 0.1-0.5%
Mo is an element that dissolves in the matrix, strengthens the matrix, and improves the material strength, and needs to be contained in an amount of 0.1% or more. On the other hand, if the content exceeds 0.5%, the base strength becomes too high and the machinability deteriorates. For this reason, Mo was limited to the range of 0.1 to 0.5%.
The balance other than the above components is Fe and inevitable impurities.

  Further, the cast iron of the present invention has a matrix in which a matrix phase, a hard phase composed of steadite and a boron compound, and flake graphite are dispersed in the matrix phase. The base phase consists mainly of perlite. By using the above-described composition, especially a composition containing Mo, Cr and Cu in combination, the pearlite is refined and the strength is increased. It should be noted that there is no problem even if the base phase contains 2% by volume or less of ferrite phase other than the main pearlite.

    The hard phase dispersed in the matrix phase contains Fe, P, B, and C. It has a state in which steadite and boron compound are mixed in a complicated manner and has high hardness, and improves the wear resistance and scuffing resistance of cast iron. Contributes to improved properties, increased hardness, and improved corrosion resistance. In the present invention, the B content is increased so that the hard phase dispersion ratio is 14 to 22% in terms of area ratio. If the hard phase is less than 14% in area ratio, the scuffing resistance and wear resistance are lowered, and the sliding characteristics are deteriorated. On the other hand, if the hard phase exceeds 22% in area ratio, the sliding characteristics are improved, but the workability of cast iron is lowered. In addition, the hard phase as used in the field of this invention shall point out the phase observed in white by the optical microscope structure of nital corrosion.

  In the cast iron of the present invention, the flake graphite dispersed in the matrix phase is adjusted so that the average graphite interval is 9 to 15 μm. If the average graphite interval is less than 9 μm, the strength decreases and the corrosion resistance also decreases. On the other hand, when the average graphite interval exceeds 15 μm, the amount of graphite decreases and the scuffing resistance decreases. By using the above average graphite interval, both an increase in strength and an improvement in corrosion resistance can be achieved. The average graphite interval of flake graphite can be adjusted by adjusting the type (composition) of the inoculum and the amount added. As the inoculum, Mg, Ca, Se, Si and the like are preferable.

In addition, the value (average value) measured by the following procedure is used as the “average graphite interval” of flake graphite in the present invention.
(1) Polish the cast iron to be measured so that it can be examined.
(2) Without corrosion, the graphite structure is imaged using an optical microscope (magnification: 100 times) to produce a graphite structure photograph (size: 73 mm × 95 mm).
(3) Enlarge this graphite structure photograph twice. (Figure 2)
(4) In the enlarged structure photograph, two diagonal lines are entered as shown in FIG. 2, and graphite grains 11-1i and 21-2i respectively intersecting the two diagonal lines are marked (FIG. 3). Graphite grains with a thickness of less than 0.5 mm (2.5 μm) on the photograph are excluded from the objects to be marked.
(5) For each of the marked graphite particles 11 to 1i and 21 to 2i, the shortest distance Δd _i from the tip of each graphite particle to a graphite particle having a thickness of 0.5 mm (2.5 μm) or more on the photograph Is measured (FIG. 4).

The measurement is performed at both ends of each graphite grain. In addition, the graphite grains connected at a portion having a thickness of less than 0.5 mm (2.5 μm) on the photograph are separate graphite grains. In addition, graphite that is partially out of the photograph is excluded from measurement.
(6) The shortest distance Δd _i in the obtained graphite grains is averaged to obtain an average graphite interval Δd at the measurement location.

Next, the preferable manufacturing method of this invention cast iron is demonstrated.
It is preferable that the molten metal having the above composition is melted by a normal melting method such as a cupola or an electric furnace, and cast and solidified by a known casting method such as a mold to obtain a cast iron product having a predetermined size. In addition, an appropriate amount of a predetermined inoculant is introduced into the molten metal so as to obtain a target graphite interval at the time of melting or immediately before casting. The obtained cast product is preferably subjected to cutting or the like as necessary to obtain a cylinder liner having a desired size and shape, for example.

The molten metal having the composition shown in Table 1 is melted with a cupola, and after adding a predetermined type and amount of inoculum, the molten metal is poured into a horizontal mold, and the size is: outer diameter φ140 mm × inner diameter φ135 mm × height 260 mm Cylinder liner shaped cast iron product.
From the obtained cast iron product, a test piece was collected from the vicinity of the inner peripheral sliding surface, polished, and polished (no corrosion), and the average graphite interval of the flake graphite was measured according to the measurement procedure described above. Further, the collected specimen was polished and subjected to Nital corrosion, and the base phase structure was observed using an optical microscope (magnification: 100 times), and the type of the base phase structure and the area ratio of the hard phase were measured. The area ratio of the hard phase is the area ratio of the hard phase (white area) in each field of view with an optical microscope (magnification: 100 times) for 10 specimens or more of the specimen that had undergone nital corrosion. Were measured and averaged to obtain the hard phase area ratio of the cast iron product. An example of an optical microscope structure (Example of the present invention) that has undergone Nital corrosion is shown in FIG.

Further, the obtained cast iron product was subjected to a corrosion resistance test, an abrasion resistance test, a scuffing resistance test, and a machinability test. The test method was as follows.
(1) Corrosion resistance test A test piece (size: length 90mm x width 10mm x thickness 5mm) is taken from the obtained cast iron product, exposed only on the upper surface, and the other surface is protected with Teflon tape and 2.5% nitric acid solution. (Liquid temperature: 70 ° C.) was immersed for 20 minutes. After the test, the scale on the surface of the test piece was removed, washed with an ultrasonic cleaner, dried, and the weight of the test piece was measured. The difference in weight before and after the test was divided by the surface area, and corrosion resistance was evaluated as corrosion weight loss (mg / cm ² ).
(2) Abrasion resistance test A donut-shaped test piece (size: outer diameter 40 mmφ x inner diameter 16 mmφ x thickness 10 mm) was sampled from the cast iron product obtained and tested with the roller chip type wear tester shown in Fig. 5. did. A test piece 53 is attached to the testing machine 51, and one end is rotated while being immersed in a lubricating liquid. A plate-like mating material 52 (size: 8 mm × 7 mm × 5 mm) as a fixed piece is set for a predetermined time (3 h). During this time, contact is made while applying a load. After a predetermined time, the test piece was removed, the step profile of the test piece was measured with a roughness meter, the amount of wear (μm) was determined, and the wear resistance was evaluated.

The test conditions used were as follows.
Load P on the test piece P: 80kgf
Number of rotations of test piece 53: 478 rpm
Material of mating material 52: PVD coated steel (Cr-N series, hardness: 1400HV0.05)
Lubricating liquid: SAE # 30 (Liquid temperature: 80 ° C)
(3) Scuffing resistance test From the obtained cast iron product, a donut-shaped test piece 53 having the same size and shape as the test piece subjected to the wear resistance test was sampled and tested with a roller tip type wear tester 51. The test piece 53 is rotated at a constant speed, the plate-like mating member 51 that is a fixed piece is pressed against the test piece 53 with a predetermined load (P), and the surface pressure when the scuff is generated is defined as the limit surface pressure. Scuffing properties were evaluated. The load (P) increased with a slope of 49 N / min, assuming an initial load of 98 N.

The test conditions are as follows.
Rotation speed: 478rpm
Lubricating oil: SAE30 + white kerosene (1: 1)
Oil amount: Oil-free (only initial application)
Fixed piece (mating material) material: PVD coated steel (Cr-N series, hardness: 1400HV0.05)
(4) Machinability test The surface layer part of the obtained cast iron product was cut and the cutting tool life of the used processing tool was measured. Based on the tool life of the cast iron product No. P (comparative example) of the conventional composition as a reference, calculate the tool tool life ratio (= conventional machining number (standard) / (number of machining)) of the machining tool used in each cast iron product, The machinability was evaluated. The tool life was the number of tools that could be processed with a single tool. The blade life ratio was evaluated as ○ when the tool life ratio was less than 1.0 and × when 1.0 or more.

  The obtained results are shown in Table 2.

  In all of the examples of the present invention, the corrosion weight loss is small and the corrosion resistance is excellent, the wear weight loss is small and the abrasion resistance is excellent, the scuffing load is high, and the scuffing resistance is excellent. Is also excellent. On the other hand, in comparative examples that are outside the scope of the present invention, any or all of corrosion resistance, wear resistance, scuffing resistance, and machinability are deteriorated.

It is an optical microscope structure | tissue photograph which shows an example of the structure | tissue of this invention cast iron. It is an optical microscope structure photograph which shows an example of the graphite structure of cast iron. It is explanatory drawing explaining the procedure which measures a graphite space | interval from a graphite structure | tissue photograph. It is explanatory drawing explaining the measuring method of a graphite space | interval. It is explanatory drawing which shows the outline of a roller chip | tip type | mold abrasion tester typically.

Explanation of symbols

51 Roller tip wear tester
52 Counterpart
53 Specimens
54 Lubricant

Claims (2)

% By mass
C: 3.0-3.5%, Si: 1.5-2.5%,
Mn: 0.5 to 1.0%, P: 0.2 to 0.5%,
S: 0.12% or less, Cr: 0.1-0.5%,
B: 0.09 to 0.18%, Cu: 0.4 to 1.0%,
Mo: 0.1-0.5%
Including a composition composed of the balance Fe and inevitable impurities, a matrix phase mainly composed of pearlite, a hard phase composed of steadite and a boron compound, and flake graphite dispersed in the matrix phase, A cast iron excellent in corrosion resistance and wear resistance, characterized in that the hard phase has an area ratio of 14 to 22% and the flake graphite has an average graphite interval of 9 to 15 µm.   A cylinder liner comprising the cast iron according to claim 1.