JP5282546B2 - High-strength, thick-walled spheroidal graphite cast iron with excellent wear resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick high-strength spherical graphite cast iron product with a thickness of &ge;100 mm, as cast, which has high strength satisfying the tensile strength of &ge;750 MPa and high ductility satisfying the elongation of &ge;1.5%, and further has excellent wear resistance. <P>SOLUTION: The thick high-strength spherical graphite cast iron product has a composition containing, by mass%, 3 to 4% C, 1.8 to 3.0% Si, 0.5 to 2% Mn, 0.003 to 0.03% S, 0.1 to 1.5% Nb, 1.2 to 3.0% Cu, 0.6 to 1.5% Ni, 0.1 to 1.5% V and 0.02 to 0.06% Mg, and the balance Fe with inevitable impurities. In this way, the thick high-strength spherical graphite cast product, as cast, having high strength of &ge;750 MP and high ductility satisfying the elongation of &ge;1.5%, and further having excellent wear resistance can be obtained. Further, as impurities, preferably, the content of P is controlled to &lt;0.04%, and the content of Cr is controlled to &lt;0.1%. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a spheroidal graphite cast iron product suitable for various machine parts such as a press machine, various civil engineering and building materials, and in particular, has a tensile strength exceeding 750 MPa and a high wear resistance. The present invention relates to a strength thick spheroidal graphite cast iron product. The “thick cast iron product” here refers to a cast iron product having a wall thickness of 100 mm or more.

  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 parts for machine tools and structural members for civil engineering and construction. In recent years, from the viewpoints of improving the life of structural members and structural parts, and further from the viewpoint of economy, there is a tendency to use inexpensive cast iron products that have high strength and excellent wear resistance. Therefore, there is a demand for a high-strength spheroidal graphite cast iron product having a high strength exceeding tensile strength: 700 MPa and excellent wear resistance in an as-cast state without performing heat treatment.

In response to such a request, for example, Patent Document 1 includes C: 3.20 to 4.00%, Si: 2.00 to 3.20%, Mn: 0.05 to 3.00% by weight ratio, and P, S, and Mg in appropriate amounts. There has been proposed a spheroidal graphite cast iron product that is adjusted and contains Cu: 0.40 to 2.00%, rare earth: 0.005 to 0.300%, and the balance being Fe. Spheroidal graphite cast iron manufactured by the technique described in Patent Document 1 contains high strength with a tensile strength of 700 N / mm ² or more and high ductility of 2% or more by containing Cu and a rare earth element in combination. Can be secured.

Patent Document 2 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 produced by the technique described in Patent Document 2, by composite containing Cu and Sn, tensile strength: 900 N / mm ² near or with 900 N / mm ² or more high strength, elongation : High ductility of 4% or more can be secured.

  Patent Document 3 includes, by weight%, C: 3.0 to 4.5%, Si: 1.6 to 2.5%, Mn: 0.2 to 0.5%, and P, S, and Mg adjusted to appropriate amounts, Zr: High strength spheroidal graphite cast iron containing 0.0005 to 0.09%, one or two of Sn and Cu, 0.03 to 0.11% in terms of Sn, and the balance consisting of Fe and inevitable impurities has been proposed. Yes. The spheroidal graphite cast iron product produced by the technique described in Patent Document 3 is said to have a tensile strength of 900 MPa or more as it is as cast and has good machinability.

Patent Document 4 includes, by weight ratio, C: 3.20 to 4.00%, Si: 2.00 to 3.20%, Mn: 0.30 to 2.50%, and P, S, and Mg adjusted to appropriate amounts. Cu: 0.30 to 3.50%, rare earth element: 0.005 to 0.30%, promotes cooling of the molten metal consisting of the remaining Fe, cast iron products, and petals with ferrite or ferrite and pearlite around graphite And a spheroidal graphite cast iron product having an as-cast tensile strength of 800 N / mm ² or more, preferably 900 to 1000 N / mm ² and an elongation of 2% or more, preferably 3% or more. A method for producing a spheroidal graphite cast iron product has been proposed.

  Patent Document 5 includes, by weight%, C: 2.0 to 4.0%, Si: 1.5 to 4.5%, Mn: 2.0% or less, and P, S, and Mg adjusted to appropriate amounts, and Cu : 1.8-4.0%, or even Sn: 0.08% or less, and / or Mo: 0.5% or less, Ni: 0.5% or less, one or two of them, the balance consisting of Fe and inevitable impurities, high Strength spheroidal graphite cast iron has been proposed. Spheroidal graphite cast iron products produced by the technique described in Patent Document 5 have a high tensile strength of 800 MPa or more, as-cast, can remarkably suppress water embrittlement, and machinability. Is going to improve.

  In Patent Document 6, Mg is appropriate in terms of weight ratio: C: 3.0 to 4.0%, Si: 1.6 to 3.3%, Mn: 0.2 to 1.0%, Ni: 0.5 to 2.0%, Mo: 0.2 to 1.5% A high-strength ductile containing Cu: 1.0 to 3.0%, V: 0.03 to 0.2%, the balance being Fe and unavoidable impurities, and the base structure being pearlite or pearlite and bainite. Cast iron has been proposed. A high-strength spheroidal graphite cast iron product manufactured by the technique described in Patent Document 6 is said to have a tensile strength exceeding 1000 MPa and an elongation of 2% or more.

Patent Document 7 describes spheroidal graphite cast iron containing, by mass%, Ni: 2.0 to 4.0%, Mn: 0.4% or less, Cu: 0.2% or less, and the total amount of Mn and Cu being 0.5% or less. Yes. With the technique described in Patent Document 7, high strength of 700 MPa or more and elongation of 7% or more can be adjusted to a high level in a balanced manner by devising the cooling rate in graphite cast iron having a thickness of about 20 to 50 mm. It is said.
JP 2000-26932 A JP 2001-131678 A JP 2002-275575 A JP 2002-317219 A Japanese Patent Laid-Open No. 2003-13170 JP 2004-99923 A JP 2004-124225 A

  However, although the above-described conventional technology can ensure a high strength of a certain level or more, there is a problem that the wear resistance is inferior or no consideration is given to the wear resistance. There was still a problem with the demand for improvement in performance. Furthermore, the above-described prior art targets a thin cast iron product having a thickness of about 50 mm at most, and there is a problem that the strength is remarkably lowered as the thickness is increased.

  The present invention solves the above-mentioned problems of the prior art, and until cast, tensile strength: high strength of 750 MPa or more, elongation: high ductility of 1.5% or more, and superior to conventional materials by 1.5 times or more An object of the present invention is to provide a high-strength thick graphite cast iron product having a thickness of 100 mm or more, which also has wear resistance.

  In order to achieve the above-described object, the present invention first made extensive studies on various factors that affect the strength and wear resistance of an as-cast thick-walled spheroidal graphite cast iron product. Conventionally, it is known that inclusion of Cu is effective for improving the strength of spheroidal graphite cast iron. However, in order to improve wear resistance, especially when a large amount of carbide forming elements such as Cr, Mo, V, W, etc. is contained, eutectic carbides crystallize, both strength and ductility decrease, and thick cast iron There is a problem that the desired high strength cannot be secured in the product.

  Therefore, the present inventors, based on the spheroidal graphite cast iron composition, contain Cu for increasing the strength of the base, and by adding an element that forms hard carbide but does not form eutectic carbide. Thus, it was conceived that fine hard carbides can be dispersed in the base, and a thick cast iron product having high strength, high ductility, and excellent wear resistance can be obtained. Furthermore, it has been thought that further strengthening of thick cast iron products can be achieved by including an element that suppresses the diffusion of carbon to promote pearlite transformation. As a result of further studies by the present inventors, Nb is suitable as an element that forms the above-mentioned hard carbide but does not form a eutectic carbide. Further, by adding V in addition to Nb, V becomes MC. It has been found that while the solid carbide is solid-dissolved in the type carbide (hard carbide) and strengthens the carbide, the remaining V strengthens the base and contributes to improvement in strength as well as wear resistance. Further, it has been found that the addition of an appropriate amount of Ni suppresses the diffusion of carbon and contributes to further improvement in strength of the thick cast iron product.

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: 3-4%, Si: 1.8-3.0%, Mn: 0.5-2%, S: 0.003-0.03%, Nb: 0.1-1.5%, Cu: 1.2-3.0%, Ni : 0.6 to 1.5%, V: 0.1 to 1.5%, Mg: 0.02 to 0.06%, a composition comprising the balance Fe and inevitable impurities, and having high wear resistance Spheroidal graphite cast iron product.
(2) A high-strength thick spheroidal graphite cast iron product according to (1), wherein P is adjusted to less than 0.04% and Cr is adjusted to less than 0.1% as the inevitable impurities.

  According to the present invention, the as-cast high strength spheroidal graphite cast iron having a high strength of about 750 MPa or more, high ductility, and excellent wear resistance, and a thickness of 100 mm or more. The product can be manufactured easily, and it has a remarkable industrial effect. Further, the cast iron product according to the present invention is a cast iron product having excellent wear resistance and high strength and high ductility. In addition, there is also an effect that it can be applied to a stopper for civil engineering and other various wear-resistant cast iron 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: 3-4%
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. When the C content is less than 3%, 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 product that ensures desired high strength and high ductility. On the other hand, if the content exceeds 4%, the amount of graphite becomes excessive and the strength decreases. For this reason, C was limited to 3-4%. 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 in the present invention, it needs 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 2.0 to 2.6%.

Mn: 0.5-2%
Mn is a useful element that dissolves in the base and contributes to increasing the strength of the base. In order to acquire such an effect, 0.5% or more of content is required. If Mn is less than 0.5%, the strength decreases, and the desired high strength cannot be secured. On the other hand, if Mn content exceeds 2%, Mn segregates at the grain boundary of the solidification cell, and the material becomes brittle. For this reason, Mn was limited to the range of 0.5 to 2%. In addition, Preferably, it is 0.5 to 1.4%.

S: 0.003-0.03%
S is an element having an action of forming a compound with Mg, Si, etc. to form graphite nuclei and promoting graphitization. In order to acquire such an effect, it contains 0.003% or more in this invention. On the other hand, a content exceeding 0.03% lowers the graphite shape. For this reason, S was limited to the range of 0.003 to 0.03%. In addition, Preferably it is 0.003 to 0.02%.

Nb: 0.1-1.5%
Nb is a very important element in the present invention, which forms a hard MC type carbide and contributes effectively to improving the wear resistance, and also makes the solidified structure finer and contributes effectively to increasing the strength. Such an effect is recognized when the content is 0.1% or more, but when the content exceeds 1.5%, the MC type carbide is coarsened and the strength is reduced. For this reason, Nb was limited to the range of 0.1 to 1.5%. In addition, Preferably, it is 0.1 to 1.2%.

Cu: 1.2-3.0%
Cu is an important element in the present invention, which has the effect of densifying the pearlite structure and increasing the strength of the matrix until it is cast. In order to obtain such an effect, a content of 1.2% or more is required. On the other hand, if the content exceeds 3.0%, a large amount of Cu is segregated at the grain boundaries of the solidification cell, leading to a decrease in strength. For this reason, Cu was limited to the range of 1.2 to 3.0%. In addition, Preferably it is 1.2 to 2.7%.

Ni: 0.6-1.5%
Ni suppresses carbon diffusion, especially in the base phase of thick cast iron products with a low cooling rate after solidification, promotes pearlite transformation, increases strength, and contributes greatly to increasing the strength of thick cast iron products. It is an element and is an important element in the present invention. Such an effect becomes remarkable when the content is 0.6% 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 was limited to the range of 0.6 to 1.5%. The Ni content has an appropriate value depending on the casting shape, wall thickness, mold used, casting conditions, etc., and the required characteristics also vary. Therefore, adjust the Ni content within the above range according to the purpose. Is preferred.

V: 0.1-1.5%
V is an important element in the present invention that has the effect of strengthening the MC type carbide by dissolving in the MC type carbide that appears due to the inclusion of Nb, and contributes to an increase in strength and an improvement in wear resistance. The remainder of V that cannot be completely dissolved in the MC type carbide also has a function of forming a solid solution in the matrix to densify and strengthen the pearlite structure. In order to obtain such an effect, a content of 0.1% or more is required. On the other hand, if the content exceeds 1.5%, the amount of carbide is excessively increased and the strength and ductility are lowered. For this reason, V was limited to the range of 0.1 to 1.5%.

Mg: 0.02-0.06%
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 0.02% 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 0.02 to 0.06%. In addition, Preferably it is 0.02 to 0.05%.

The balance other than the above components is Fe and inevitable impurities. It is preferable to adjust P to less than 0.04% and Cr to less than 0.1% as inevitable impurities.
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%. More preferably, it is 0.03% or less.

  Cr is an element that promotes whitening and is preferably reduced in order to suppress whitening. If Cr is less than 0.1%, the adverse effect is small and acceptable. For this reason, it is preferable to adjust to Cr: less than 0.1%. In order to keep Cr within the above-mentioned range, it is important to use a melting raw material that does not contain a large amount of these elements. Do not need. More preferably, Cr is less than 0.05%.

As inevitable impurities other than P and Cr, Ti: less than 0.03% and W: less than 0.1% are acceptable.
Ti and W are elements that promote whitening and deteriorate the graphite shape, and the content of these elements is preferably low in order to suppress whitening or to suppress deterioration of the graphite shape. If Ti: less than 0.03% and W: less than 0.1%, the adverse effect is small and acceptable. For this reason, it is preferable to adjust to Ti: less than 0.03% and W: less than 0.1%. In order to make Ti and W within the above ranges, it is important to use a melting raw material that does not contain a large amount of these elements. Does not require careful selection. More preferably, Ti is less than 0.02% and W is less than 0.04%.

Further, Al, Ca, Ba, and Bi, which are inevitable impurities, can be allowed to be Al: less than 0.05%, Ca: less than 0.008%, Ba: less than 0.002%, and Bi: less than 0.02%.
Al and Ca are usually contained in an Fe-Si-Mg alloy used as a graphite spheroidizing agent, and Ca, Al, Ba and Bi are usually Fe-Si alloys and Ca- Contained in Si alloy. For this reason, Al, Ca, Ba, and Bi 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. For this reason, it is preferable to adjust to Al: less than 0.05%. 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. For this reason, it is preferable to adjust to Ca: less than 0.008% and Ba: less than 0.002%. 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. For this reason, Bi is preferably adjusted to less than 0.02%.

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 0.01%. 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, as-cast, spheroidized graphite, and the base is preferably a densified layered pearlite, and the base is a hard carbide. Thick cast iron product with a thickness of 100 mm or more with a structure in which (MC type carbide) is dispersed. Tensile strength as cast, high strength of 750 MPa or more, high ductility of 1.5% or more, conventional materials It has an excellent wear resistance of 1.5 times or more.

Next, a preferred method for producing the high strength thick 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, and performing a graphite spheroidizing treatment by adding a graphite spheroidizing agent such as a conventional Mg alloy to the mother molten metal, a normal Fe It is preferable to inoculate with an inoculant such as a -Si alloy or Ca-Si alloy to have the above-described composition, and to pour (cast) it into a conventional mold such as a sand mold or a mold formed in a desired shape. 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. The molten metal after addition of the alloy elements was then subjected to a graphite spheroidization process by a sandwich method using a commercially available Mg alloy (Fe-45 mass% Si-5 mass% Mg-1 mass% Ca alloy). Next, the molten metal was transferred to a pan and inoculated with an Fe-75% Si alloy. Immediately after inoculation, a sample for chemical analysis was collected, and immediately the molten metal was poured (cast) into a sand mold to form a Y-type keel block (parallel portion thickness 110 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. 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. 9), wear ratio = (wear amount of the 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.

  All examples of the present invention have high tensile strength of TS: 750 MPa or more and high ductility of elongation El: 1.5% or more, and more than 1.5 times higher wear resistance than the conventional example (cast iron product No. 9). It is a thick-walled spheroidal graphite cast iron product having excellent properties. On the other hand, a comparative example out of the scope of the present invention is whether the tensile strength is less than 750 MPa, the elongation is less than 1.5%, or the wear resistance is about 1.1 times that of the conventional one. . The comparative example (cast iron product No. 10) containing Cr does not have Nb-free addition and the Cu content is out of the range of the present invention, and none of the desired properties of strength, ductility, and wear resistance can be secured. In addition, the comparative example (cast iron product No. 11) in which Ni deviates from the scope of the present invention does not ensure the desired properties of strength and wear resistance. In addition, the comparative example (cast iron product No. 12) in which Ni deviates from the scope of the present invention and Ni deviates from the scope of the present invention does not ensure desired properties in strength and ductility.

Claims (2)

% By mass
C: 3 to 4%, Si: 1.8 to 3.0%,
Mn: 0.5-2%, S: 0.003-0.03%,
Nb: 0.1-1.5%, Cu: 1.2-3.0%,
Ni: 0.6-1.5%, V: 0.1-1.5%,
Mg: 0.02-0.06%
A high-strength thick-walled spheroidal graphite cast iron product having a composition comprising the balance Fe and inevitable impurities and having excellent wear resistance.   2. The high-strength thick-walled spheroidal graphite cast product according to claim 1, wherein P is adjusted to less than 0.04% and Cr is less than 0.1% as the inevitable impurities.