JP5712560B2 - 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%. Related to 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 2 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

  According to the above-described prior art, it is said that an increase in strength to a certain degree can be achieved mainly by increasing and densifying pearlite, and an improvement in wear resistance to a certain level can be ensured. However, in the above-described prior art, since the desired wear resistance is secured by increasing the amount of pearlite and densifying, it is difficult to significantly improve the wear resistance while maintaining the desired strength and ductility. There was a problem.

  The present invention solves the 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 an excellent wear resistance of 1.5 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, the inclusion of the carbide forming element increases the pearlite and densifies it to improve the strength, but decreases the 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 came to mind 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 alloy element that can realize such a thing. 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.

  Furthermore, according to the study by the present inventors, when Mo is compounded in addition to Nb, Mo is dissolved in MC carbide, MC carbide is strengthened, and wear resistance is not reduced without deteriorating strength properties. It was found that the cast iron product can be further improved. According to further studies by the present inventors, it has been newly found out that a cast iron product having further improved wear resistance can be obtained by adding a small amount of V after adding Mo to Nb. It was. This is considered to be because V was dissolved in MC carbide and MC carbide was further strengthened.

For this reason, the present inventors combined the appropriate amounts of Nb, Mo, and V to maintain a desired high strength and a high ductility of 5% to 10% while maintaining the resistance to resistance. The inventors obtained a new finding that a spheroidal graphite cast iron with significantly improved wearability can be obtained.
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%, Cu: 0.2 to 1.3%, Nb: 0.05 to 1.0%, Mo: 0.05 to 0.5%, V : 0.05 to 1.5%, Mg: 0.01 to 0.06% , S: 0.001 to 0.03% , a composition composed of the balance Fe and inevitable impurities, and a spheroidal graphite cast iron product characterized by excellent wear resistance .
(2) The spheroidal graphite cast iron product according to (1), wherein in addition to the above composition, the composition further contains Ni: 0.1 to 1.5% by mass% .

  According to the present invention, a tensile strength of 700 MPa or more, a high ductility of 5% or more, and an excellent wear resistance that is 1.5 times or more that of conventional materials, are structural members for machine tools. Further, it is possible to easily produce a spheroidal graphite cast iron product suitable for civil engineering / architectural structural members, machine parts, and the like, and has a remarkable industrial effect. 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.

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%, fluidity is particularly insufficient, shrinkage cavities are likely to occur, and the graphite content is insufficient, making it difficult to obtain a spheroidal graphite cast iron that secures desired high strength and high 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 has an influence on the fluidity and whitening of the molten metal, and further contributes to promoting the formation of graphite and precipitation of ferrite and improving the ductility. In the present invention, it is 1.8% or more. Containing is required. 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 1.0%.

Cu: 0.2 to 1.3%
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. For this reason, Cu was limited to the range of 0.2 to 1.3%. In addition, Preferably it is 0.3 to 1.1%.

Nb: 0.05-1.0%
Nb is the most important element in the present invention, and forms a hard granular MC type carbide, while 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, but when the content exceeds 1.0%, the MC-type carbide is coarsened and ductility is reduced. For this reason, Nb was limited to the range of 0.05 to 1.0%. In addition, Preferably, it is 0.1 to 0.8%. 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. In addition, recently, Nb alloys, etc., in which the surface of the Nb alloy is coated with flux or mixed with flux are commercially available, and the use of these Nb alloys promotes dissolution of the Nb-containing alloy into the molten metal. There are advantages.

Mo: 0.05-0.5%
Mo is an important element in the present invention, and when mixed with Nb, Mo dissolves in the hard granular MC type carbide, strengthens the MC carbide, and does not deteriorate the strength characteristics. It has the effect of further improving the wear properties. Mo that could not be completely dissolved in the MC type carbide has the effect of strengthening the base. Such an effect becomes remarkable when the content is 0.05% or more. However, when the content exceeds 0.5%, shrinkage nests (zaku nests) tend to appear and the soundness of the casting is lowered. For this reason, Mo was limited to a range of 0.05 to 0.5%. In addition, Preferably it is 0.05 to 0.4%.

V: 0.05-1.5%
When V is contained alone without containing Nb, elongated V carbide (VC) appears, and the ductility is remarkably lowered. In the case of the present invention containing V together with Nb and Mo, V is solid-solved with Mo in MC carbide that appears when Nb is contained, further strengthening and increasing the amount of MC carbide, and further improving wear resistance. Have the effect of V, which could not be completely dissolved in MC carbide, contributes to dense strengthening of pearlite by dissolving in the base. In order to acquire such an effect, V needs to contain 0.05% or more. On the other hand, when the content exceeds 1.5%, the tendency of MC carbides to become elongated and elongated increases, and ductility decreases. For this reason, V is limited to a range of 0.05 to 1.5%. In addition, Preferably, it is 0.1 to 1.0%.

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%. In addition, Preferably it is 0.02 to 0.05%.

The above-described components are basic components, but in addition to the basic composition described above, Ni may be further included as a selective element.
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%.

S: 0.001 to 0.03%
S is basically an impurity element, but is said to be an element having an action of promoting the graphitization by forming a nucleus of graphite by forming a compound with Mg, Si and the like. 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. On the other hand, when Bi is contained in an amount of 0.02% or more, it may adversely affect the spheroidization of graphite and may cause carbides to crystallize. 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, and as-cast, has a high tensile strength of approximately 700 MPa or more and a high ductility of 5% or more, with a strength-ductility balance. Excellent and more than 1.5 times the wear resistance of conventional materials.

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 spheroidization by sandwich method using commercially available Si-Mg alloy (Fe-45 mass% Si-5 mass% Mg-1.8 mass% REM-2 mass% Ca alloy) for 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 120 kgf (1176 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 having the same strength and ductility (elongation) as the conventional example and having 1.5 times or more wear resistance 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%, Cu: 0.2-1.3%
Nb: 0.05 to 1.0%, Mo: 0.05 to 0.5%,
V: 0.05-1.5%, Mg: 0.02-0.06%,
S: 0.001 to 0.03%
Nodular cast iron product characterized by having a composition composed of the balance Fe and inevitable impurities and having excellent wear resistance. 2. The spheroidal graphite cast iron product according to claim 1, further comprising Ni: 0.1 to 1.5% by mass% in addition to the composition .