JP5589646B2 - Spheroidal graphite cast iron products with excellent wear resistance - Google Patents

Description

  The present invention relates to a spheroidal graphite cast iron product suitable for a structural member for machine tools, a structural member for civil engineering / architecture, a machine part, etc., and particularly wear resistance of a spheroidal graphite cast iron product having a high ductility of about 5 to 10%. Regarding improvement.

  Spheroidal graphite cast iron has good castability and high strength, and is widely used as a substitute for forged steel and cast steel, especially for structural members for machine tools, structural members for civil engineering, and mechanical structural components. However, in recent years, from the viewpoint of extending the life of various structural members, structural parts, etc., and economically, it is an inexpensive cast iron with improved wear resistance while maintaining the desired high strength and ductility as it is. Goods are desired.

In response to such a request, for example, Patent Document 1 discloses, in weight ratio, C: 3.20 to 4.00%, Si: 2.00 to 3.20%, Mn: 0.05 to 3.00%, P: 0.10% or less, S: 0.120. % Or less, Cu: 0.40 to 2.00%, Mg: 0.02 to 0.08%, rare earth: 0.005 to 0.300%, and a spheroidal graphite cast iron product composed of the remaining Fe has been proposed. According to the technique described in Patent Document 1, by adding rare earth elements together with Mn and Cu to make pearlite dense, tensile strength: 700N / mm ² or more high strength and 2% or more elongation. Can be secured.

Further, Patent Document 2 discloses a high hardness part and a tough part which are cast after casting and have a hardness difference, and in weight percent, TC: 3.0 to 4.0%, Si: 2.0 to 3.0%, Mg: 0.03 to 0.06. %, Mn: 0.2 to 0.8%, Cu: 0.5 to 2.0%, Sn: 0.03 to 0.1% and Bi: 0.002 to 0.05% have been proposed. According to the technique described in Patent Document 2, tensile strength can be obtained as-cast by containing an appropriate amount of pearlite stabilizing elements Mn, Cu, Sn, densifying pearlite, and further containing Bi. : A cast iron product having a high strength of 600 N / mm ^{2 to} 700 N / mm ² or more can be obtained.

Patent Document 3 includes C: 3.2 to 3.9%, Si: 2.0 to 2.6%, Mn: 0.6% or less, P, S, and Mg are adjusted to appropriate amounts, and Cu: 2.4 to Spheroidal graphite cast iron containing 3.3%, Sn: 0.01 to 0.05%, and remaining Fe has been proposed. Spheroidal graphite cast iron manufactured by the technique described in Patent Document 3 contains Cu and Sn in combination, so that high tensile strength: 900 N / mm ² or higher and elongation: high ductility of 4% or higher Can be secured.

  Patent Document 4 contains, in mass%, C: 3.4 to 4.0%, Si: 1.5 to 2.0%, Mn: 0.35 to 0.5%, Ni: more than 1.0 to 2.0%, Cu: 0.2 to 0.5%. There has been proposed a spheroidal graphite cast iron having a structure containing the balance of Fe and inevitable impurities, and containing 60 to 90% pearlite and 10 to 40% ferrite. According to the technique described in Patent Document 4, a spheroidal graphite cast iron member having a tensile strength of 770 MPa or more and an elongation of 8.0% or more can be obtained.

JP 2000-26932 A JP 2000-345279 A JP 2001-131678 A JP 2009-001865 A

  However, in the above-described prior art, it is said that the strength can be increased to a certain level mainly by increasing and densifying pearlite, and the improvement of the wear resistance to a certain level can be ensured. However, in the above-described conventional technology, desired wear resistance is secured by increasing and densifying pearlite, so that it is difficult to significantly improve the wear resistance while maintaining the required strength and ductility. There was a problem.

  The present invention solves such problems of the prior art, and provides a spheroidal graphite cast iron product having strength and ductility similar to those of conventional materials, and having excellent wear resistance of 1.4 times or more that of conventional materials. Objective.

  In order to achieve the above-mentioned object, the present inventors diligently studied various factors that affect wear resistance and strength improvement. As a result, both the strength and wear resistance of spheroidal graphite cast iron can be improved to a certain level by containing pearlite in the cast iron base structure containing Cu and / or Ni. In order to improve, the present inventors have found that a large amount of carbide forming elements such as Cr, Mo, W, and V is required. However, containing a large amount of carbide forming elements improves the strength, but decreases ductility and leads to hard embrittlement of the cast iron product. Furthermore, a large amount of carbide-forming elements leads to crystallization of eutectic carbides, resulting in a problem that the cast iron product is made of a very brittle material.

  Therefore, the present inventors have conducted further studies, and in order to further improve the wear resistance while maintaining the desired high strength and high ductility, fine extremely hard granular carbides are precipitated in the base. It has been realized that it is important that the hard carbides that can be dispersed and have little influence on the surrounding matrix structure. The present inventors paid attention to Nb as an alloying element that can realize this. According to the study of the present inventors, Nb combines with C to form the hardest granular MC carbide in cast iron, but Nb hardly dissolves in the matrix, so the change in matrix structure due to Nb content And the formation of eutectic carbides is also suppressed.

For this reason, the present inventors have combined the appropriate amount of Cu and Nb, thereby maintaining the desired range of strength and high ductility of 5% to 10% while maintaining wear resistance. Has obtained a novel finding that a spheroidal graphite cast iron with a marked improvement can be obtained.
First, the experimental results on which the present invention is based will be described.
It is composed of 3.4 to 3.9% C-2.2 to 2.8% Si-0.3 to 1.1% Mn-0 to 1.5% Nb-0.1 to 1.5% Cu-0.029 to 0.051% Mg-0 to 1.5% Ni-balance Fe. Test pieces were collected from spheroidal graphite cast iron Y-shaped keel blocks (parallel wall thickness: 15 mm) of each composition and tested.

In addition, the conventional example was made into 0.38% C-2.7% Si-0.6% Mn-0.5% Cu-0.05% Mg-balance Fe spheroidal graphite cast iron in mass%.
FIG. 1 shows the relationship between tensile strength and elongation. In FIG. 1, the relationship between the tensile strength and elongation of spheroidal graphite cast iron defined in JIS G 5502 is shown together as a reference.
The decrease in elongation (ductility) with increasing strength, shown for reference, is very common, as in the spheroidal graphite cast iron specified in JIS, but Nb-Cu and Cu are The elongation (ductility) is significantly improved compared with the same strength.

For these Cu-based and Nb-Cu-based, based on the results of a two-disk rolling-slip wear test, the wear ratio = (wear amount of the conventional example) / (wear amount of each cast iron product) based on the wear amount of the conventional example. FIG. 2 shows the relationship between the wear ratio and elongation.
From Fig. 2, compared with the same elongation, Nb-Cu system (marked with ●) with the appropriate amount of Nb and Cu added, marked wear resistance compared to the conventional example, Cu system, and Nb-Cu system outside the appropriate range. The knowledge that the improvement of sex was recognized was acquired.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 3.0 to 4.0%, Si: 1.8 to 3.0%, Mn: 0.2 to 1.2%, S: 0.001 to 0.03%, Cu: 0.2 to 1.1 % (excluding 1.1%) , nb: 0.05~ 0.41% (excluding 0.3%), Mg: contains 0.01 to 0.06%, having a composition the balance being Fe and unavoidable impurities, cast, characterized in that excellent wear resistance As-is, spheroidal graphite cast iron product.
(2) (1), in addition to the composition, in mass%, Ni: 0.1 to 1.5%, characterized in that a composition containing as cast remain spheroidal graphite cast iron.

  According to the present invention, it has a tensile strength of 700 MPa or more, a high ductility of 5% or more, and an excellent wear resistance that is 1.4 times or more that of conventional materials. Spheroidal graphite cast iron products suitable for members, civil engineering / construction structural members, machine parts and the like can be easily produced, and have remarkable industrial effects. In addition, the spheroidal graphite cast iron according to the present invention is a cast iron product that has moderately high strength and excellent ductility and is excellent in wear resistance, and is provided with mechanical parts such as metal fittings, nails, floor boards, and fasteners. There is also an effect that it can be applied to a landscape member, a manhole cover, a stop for civil engineering, and other various cast members.

It is a graph which shows the relationship between tensile strength and elongation. It is a graph which shows the relationship between a wear ratio and elongation.

First, the reasons for limiting the composition of the spheroidal graphite cast iron product of the present invention will be described. Hereinafter, the mass% in the composition is simply expressed as%.
C: 3.0-4.0%
C is an important element that affects the amount of spheroidal graphite crystallization, the amount of layered cementite in pearlite and the amount of MC carbide precipitated, and the fluidity of the molten metal. If the C content is less than 3.0%, the fluidity is particularly insufficient, shrinkage cavities are easily generated, and the graphite content is insufficient, making it difficult to obtain a spheroidal graphite cast iron that secures desired strength and ductility. On the other hand, if the content exceeds 4.0%, the amount of graphite becomes excessive and the strength decreases. For this reason, C was limited to 3.0-4.0%. In addition, Preferably, it is 3.3 to 3.9%.

Si: 1.8-3.0%
Si is an element that affects the fluidity and whitening of the molten metal and further promotes the formation of graphite. In the present invention, Si is required to be contained in an amount of 1.8% or more. If Si is less than 1.8%, the fluidity is lowered, making it difficult to fill the thin portion with molten metal, and whitening also occurs. On the other hand, if the content exceeds 3.0%, the graphite shape is disturbed, and the graphite tends to be deformed, making it difficult to increase the strength. For this reason, Si was limited to the range of 1.8 to 3.0%. In addition, Preferably, it is 2.2 to 2.9%.

Mn: 0.2-1.2%
Mn is a useful element that dissolves in the base and contributes to increasing the strength of the base. In order to obtain such an effect, the content of 0.2% or more is required. If Mn is less than 0.2%, the strength decreases, and the desired high strength cannot be ensured. On the other hand, if the Mn content exceeds 1.2%, the Mn segregates at the grain boundaries of the solidification cell and embrittles the material. For this reason, Mn was limited to the range of 0.2 to 1.2%. In addition, Preferably, it is 0.3 to 0.8%.

Cu: 0.2% to 1.1 % (excluding 1.1%)
Cu is an element that has the effect of densifying the pearlite structure and increasing the strength of the matrix. In order to obtain such an effect, the content of 0.2% or more is required. On the other hand, the content exceeding 1.3% causes a decrease in ductility. In the present invention , Cu is limited to the range of 0.2% to 1.1 % (excluding 1.1%) .

Nb: 0.05 to 0.41 % (excluding 0.3%)
Nb is the most important element in the present invention, and forms a hard granular MC-type carbide, maintaining moderately high strength and excellent ductility, and effectively contributing to further improvement in wear resistance. It is. Nb also makes the solidified structure finer and contributes effectively to higher strength. Such an effect is recognized when the content is 0.05% or more. However, when the content exceeds 1.0%, the MC-type carbide is coarsened and ductility is reduced. In the present invention , Nb is limited to a range of 0.05 to 0.41 % (excluding 0.3%) . In addition, Preferably, it is 0.1 to 0.41 % (however, except 0.3%) . Nb is preferably added using ferroniobium or an Fe—Nb alloy. However, it tends to be difficult to dissolve in the molten metal, and it is preferable to add Nb at an early stage of melting. Recently, Nb alloys with a flux coated or mixed with the surface of Nb-containing alloys are commercially available. If these Nb alloys are used, dissolution of the Nb-containing alloy into the molten metal is promoted. There is an advantage.

Mg: 0.01-0.06%
Mg has a function of spheroidizing graphite, and is an essential element in spheroidal graphite cast iron. In order to ensure such an effect, the content of 0.01% or more is required. On the other hand, if the content exceeds 0.06%, Mg oxide generates a large amount of dross and increases surface defects. For this reason, Mg was limited to the range of 0.01 to 0.06%.

The above-described components are basic components, but in addition to the basic composition described above, Ni and S may be further included as selective elements.
Ni: 0.1-1.5%
Ni is an element that has the effect of dissolving in the matrix and suppressing the diffusion of carbon, promoting the pearlite transformation of the matrix, and increasing the strength, and can be contained as necessary. Such an effect becomes remarkable when the content is 0.1% or more. On the other hand, if the content exceeds 1.5%, austenite is stabilized, and part of the base structure is made bainite or martensite, which has an adverse effect of increasing strength variation. For this reason, Ni is preferably limited to a range of 0.1 to 1.5%. More preferably, it is 0.1 to 1.0%.

S: 0.001 to 0.03%
S is basically an impurity element, but is said to be an element that also has a function of forming a nucleus of graphite by forming a compound with Mg, Si, etc., and promoting graphitization. In order to acquire such an effect, it is preferable to make it contain 0.001% or more in this invention. On the other hand, a content exceeding 0.03% lowers the graphite shape. For this reason, it is preferable to limit S to 0.001 to 0.03% of range.

The balance other than the above components is Fe and inevitable impurities. As unavoidable impurities, it is preferable to adjust P to less than 0.04%, Cr to less than 0.1%, Ti to less than 0.05%, and W to less than 0.1%.
P is an element that has an effect of increasing the number of nests or segregating at the grain boundaries of the solidification cell to embrittle the material. In the present invention, P is preferably reduced as much as possible. Above 0.04%, the above-mentioned adverse effects become significant. For this reason, it is preferable to adjust P to less than 0.04%.

  Cr, Ti, and W are all elements that promote whitening, and are preferably reduced in order to suppress whitening. If Cr: less than 0.1%, Ti: less than 0.05%, W: less than 0.1%, the adverse effect is small and acceptable. In order to make it within the above-mentioned range, it is important to use dissolved raw materials that do not contain a large amount of these elements. And not. More preferably, Cr is less than 0.05%, Ti is less than 0.03%, and W is less than 0.05%.

Moreover, Al, Ca, Ba, Bi, and REM as inevitable impurities are Al: less than 0.05%, Ca: less than 0.008%, Ba: less than 0.002%, Bi: less than 0.02%, REM: less than 0.05%. acceptable.
Al, Ca, Ba, Bi, and REM are usually contained in Fe-Si-Mg alloys that are used as graphite spheroids, and Fe-Si alloys and Ca-Si alloys that are usually used as inoculating agents. The For this reason, Al, Ca, Ba, Bi, and REM are impurities inevitably contained in the spheroidal graphite cast iron. However, if Al is contained in an amount of 0.05% or more, it adversely affects the spheroidization of graphite and causes a decrease in strength. Further, when Ca is contained in an amount of 0.008% or more and Ba is contained in an amount of 0.002% or more, dross increases and the occurrence of surface defects increases. Further, when Bi is contained in an amount of 0.02% or more, it may adversely affect the spheroidization of graphite and may cause crystallization of carbides. Further, when REM is contained in an amount of 0.05% or more, a chill structure is generated or the graphite shape is deteriorated when casting a thin cast iron product.

There is N as an inevitable impurity other than the above-described impurities, but in the case of a normal molten metal melting method, the N content is about 0.002 to 0.03%. If the content is within this range, there is no particular adverse effect.
The spheroidal graphite cast iron product of the present invention has the above-described composition, has a high strength of about 700 MPa or more and a high ductility of 5% or more as it is, and has an excellent strength-ductility balance. It has an excellent wear resistance of 1.4 times or more.

Next, a preferred method for producing the spheroidal graphite cast iron product of the present invention will be described.
After melting the mother molten metal by a conventional cast iron melting method such as a high-frequency furnace, adding a graphite spheroidizing agent such as a conventional Si-Mg alloy to the mother molten metal, It is preferable to inject with a conventional mold such as a sand mold or a mold formed in a desired shape by inoculating with an inoculant such as Fe-Si alloy or Ca-Si alloy. In the present invention, it goes without saying that the inoculation may be carried out by any of the usual methods, such as a method of transferring to a ladle or a method of performing in a mold such as a runner (in-mold inoculation). .

  The present invention will be further described below based on examples.

An alloying element was added to the molten mother metal melted using a high-frequency furnace so that the alloy composition shown in Table 1 was obtained. The maximum temperature of the molten metal after addition of the alloy elements was 1490 to 1580 ° C. Graphite spheroidizing treatment by sandwich method using commercially available Si- Mg alloy (Fe-45 mass% Si-5 mass% Mg-1.8 mass% REM-2 mass% Ca alloy) to the molten metal after addition of alloying elements Went. Subsequently, the molten metal was transferred to a pan, and inoculated with Fe-75 mass% Si-0.2 mass% Al alloy. Immediately after inoculation, a sample for chemical analysis was collected, and immediately the molten metal was poured into a sand mold (casting) to form a Y-type keel block (parallel part thickness 15 mm). The casting temperature was 1360 ° C to 1480 ° C.

After casting, the mold was brushed after standing for 18 hours or more, and test pieces were collected from the Y-type keel block in an as-cast state, and a tensile test and a wear test were performed. The test method was as follows.
(1) Tensile test A JIS 14A tensile test piece (parallel part diameter: 10mmφ x GL50mm) was collected from a Y-type keel block and subjected to a tensile test at room temperature (25 ° C) in accordance with the provisions of JIS Z 2241. The tensile strength TS and the elongation El were measured.
(2) Wear test A wear test piece (disk-shaped test piece: outer diameter φ60 mm × thickness 10 mm) was collected from the Y-type keel block. The abrasion test was a two-spin rolling method. The mating material was a disk-shaped test piece made of S45C material (outer diameter φ190 mm × thickness 15 mm). In the abrasion test, the number of revolutions of the test piece was 700 rpm, the sliding rate was 5%, the load was 100 kgf (980 N), and the test time was 60 min. In order to avoid heat generation of the test piece, an abrasion test was performed while cooling the test piece with water. The weight of the test piece was measured before and after the wear test, and the wear loss (wear amount) of the test piece was measured. The wear resistance of each cast iron product was evaluated by the ratio to the wear amount of the conventional example (cast iron product No. M), wear ratio = (wear amount of conventional example) / (wear amount of each cast iron product (test piece)). . Higher wear ratio means better wear resistance.

  The obtained results are shown in Table 2.

  Each of the examples of the present invention is a spheroidal graphite cast iron product that has the same strength and elongation as the conventional example and has a wear resistance 1.4 times or more that of the conventional example. On the other hand, in the comparative example that is out of the scope of the present invention, at least one of strength, ductility, and wear resistance cannot secure desired characteristics.

Claims (2)

% By mass
C: 3.0-4.0%
Si: 1.8-3.0%,
Mn: 0.2-1.2%
S: 0.001 to 0.03%,
Cu: 0.2 to 1.1 % (excluding 1.1%) ,
Nb: 0.05~ 0.41% (excluding 0.3%),
Mg: 0.01-0.06%,
Containing, having a composition the balance being Fe and inevitable impurities, as-cast remain spheroidal graphite cast iron, characterized in that excellent wear resistance. In addition to the composition, in mass%, Ni: cast remain spheroidal graphite cast iron according to claim 1, characterized in that 0.1 to 1.5% is a composition containing.

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