DE10029062C2 - Hypoeutectic nodular cast iron - Google Patents

Abstract

A hypoeutectic nodular cast iron includes: DOLLAR A 1.5 to 3.0% C; 1.0 to 5.5% Si; 0.008 to 0.25% REM; 0.005 to 0.015% BN (B equivalent) and the remainder essentially Fe by weight. The cast iron contains BN with the same hexagonal graphite-like crystal structure as graphite, so that graphite is formed using BN as the core in a eutectic reaction, and a structure in which fine spherical graphite is dispersed can be obtained.

Description

Background of the Invention 1. Technical field

The present invention relates to hypoeutectic or hypoeutectic cast iron with spheroidal graphite with improved mechanical properties such as strength, toughness, stiffness ability and the like in the as-cast state.

2. State of the art

In recent years, cast iron has components used, have a high strength due to the need for weight loss and high stiffness ability to suppress vibrations and noise. Therefore was made various improvements to cast iron. E.g. discloses the Japanese patent application, First Publ chung No. 112753/87 a technology in the mechanical egg properties and workability are improved by a Intermediate stage compensation on cast steel with nodular graphite to be led. Japanese patent applications, First publ fentlichungs no. 108342/89 and 152240/89 disclose cast iron with nodular graphite, in which bainite and residual austenite in one metallic structure formed by intermediate stage compensation become. These cast steels are said to have high rigidity, by providing a lower carbon content than in conventional cast iron to hypereutectoid compositions to form and it should have a high strength by Zwi rule gradually compensation.

However, in the conventional techniques mentioned above, the Production costs are relatively high because of an intermediate stage compensation treatment is required. In addition, the pouring temp rature high, since it is always hypereutectoidal cast steels  deals in spheroidal graphite with low carbon content and raise these problems since using molds and smelting plants high heat resistance are required and the produc on costs for this are further increased. In addition, at Guss steel with spheroidal graphite, in which residual austenite in the me metallic structure is formed, the residual austenite in Ver Formungsmartensit is converted, which problems caused by Ver reduction in workability. Therefore, are cast iron and nodular cast iron with improved mechanical Properties and improved processability in the Cast condition very desirable.

Japanese Patent Application, First Publication No. 145444/95 discloses a spheroidal graphite cast iron in the cast condition. The cast iron has a high rigidity (modulus of elasticity 169 to 171 mm ² ) in the as-cast state by alloying a cast iron with elements such as Ni, Mo and Cu and the like. However, since the proposed spheroidal graphite cast iron has an approximately eutectic composition with a high carbon content, the amount of graphite is large and therefore there is naturally a limit to high rigidity.

Japanese Patent Application, First Publication No. 339675/93 discloses a ferritic cast iron with Kugelgra phosphite. The proposed cast steel has high rigidity, because it has a lower carbon content than usual Cast iron to give a hypereutectoid composition ten, and has high strength and high toughness, a graphite spheroidizing agent that is a rare Contains earth metal as the main component, is added to to obtain a spheroidal graphite structure in the as-cast state. Since ever but the cast steel is a spheroidal graphite cast iron with a is hypereutectoid composition, is the carbon ge stop low and the casting temperature is high, making it a Problem exists because molds and melting plants with high heat resistance  are necessary and therefore the production costs are increased.

This produces the hypereutectoid cast steel with spheroidal graphite (JP '339675/83) and the eutectic cast steel with Kugelgra phit (JP '145444/85) the problems mentioned above due to the Carbon content and a hypoeutectic cast iron with Ku gel graphite with a Koh lying between these steels Oil content was therefore hoped for. However, it was accepted men that graphite spheroidization in hypoeutectic Ductile iron is difficult to cast.

Although Mg has so far mainly been used for the graphite phase roidization was added, it was shown that by the Graphite spheroidization can easily form carbides. If Mg is added to a hypoeutectic composition the graphitization is inhibited and a large amount of car biden formed and therefore it can be practically tough in the casting condition can not be maintained. The reason for this is given that impurities such as Pb, Sb, Te, Cs and As that inhibit graphitization in a molten metal due to the growth of the primary crystallized austenite directly before the eutectic reaction by cooling during casting be concentrated so that Mg is not intense for the Gra phit formation is used in the eutectic reaction and the formation of carbides is promoted.

It must also be considered that the graphite spheroidizing agent that is considered a rare earth metal Main component contains that in Japanese patent application dung, First Publication No. 339675/93 and the Japani patent application, First Publication No. 103049/88 for hypoeutectic compositions is applied. In this case, however, it is slightly coarser Graphite and carbides are formed and toughness can be practical table will not be preserved. It is believed that the  The reason for this is that the eutectic reaction in a hypoeutectic composition takes place, though eutectic reactions for the crystallization of carbides and Graphite does not appear in a hypereutectoid composition kick, and the concentration of Verunreini mentioned above conditions promotes the formation of carbides.

Summary of the invention

Therefore, the formation of a spheroidal graphite structure in a hypoeutectic composition in the as-cast state actually unrealized. It is therefore an object of the invention a hypoeutectic spheroidal graphite cast iron or hypoeutecti to provide neat spherulitic cast iron Heat treatment requires a high Fe when cast can convey stability and high toughness and the increase the casting temperature can prevent, thereby producing using common molds and smelting directions becomes possible.

There are different theories that explain the function, in the case of graphite when solidifying spheroidal graphite cast egg spheroidized. According to one of these theories, Bla sinks made in molten iron due to addition Elements such as SEM and graphite are found in the bubble cores formed during consolidation. The inventors have again Holt examines whether spheroidization by solid nuclei in addition to the usual bladder cores may or may not. The inventors have considered one small amount of boron nitride (hereinafter referred to as "BN" net) in addition to REM and have numerous versu che carried out. As a result, they found that BN serves as a solid core and the reaction of graphite formation in a hardening reaction in the hypoeutectic range promotes and inhibits the formation of carbides, so that  Spheroidization of fine graphite is promoted and the Matrix can be ferrite.

The present invention has been accomplished based on the above knowledge. The invention provides a hypoeutectic Spheroidal graphite cast iron with 1.5 to 3.0% C; 1.0 to 5.5% Si; 0.008 to 0.25% REM; 0.008 to 0.25% BN (B equivalent) and Remainder essentially Fe, based on weight.

The BN used according to the invention has the same hexagonal graphite-like crystal structure and the same electroni properties like graphite and has a melting point of about 3000 ° C, which makes it in molten iron when Ab set temperature (about 1600 ° C) is stable. Hence graphite formed with BN as the nucleus in a eutectic reaction serves. As a result, the matrix of the final solidification structure that had previously been made of cementite and pearlite stands, ferrite, which is ductile iron, and a metallic one Structure be in the fine spherical graphite in the matrix is dispersed. The elongation and toughness of the material can be improved since the matrix is made of ferrite, which Processability, especially the stiffness, of the material can be improved since there are no adverse effects due to coarse graphite. Therefore, the fiction according hypoeutectic spheroidal graphite cast iron fine graphite disperse even in the as-cast state and can mechanical Improve properties. It should be noted that heat acts, such as an intermediate stage remuneration, according to the invention can be applied.

To the formation of spheroidal graphite and its refinement promote, the cast iron preferably still contains one or several of the following: 0.0005 to 0.02% Bi; 0.005 up to 0.08% Al; 0.002 to 0.04% approx. If such elements ent the effects of the cooling rate can be improved and a fine spherical graphite structure can be reliable  can be obtained even if bodies with complicated shapes or large bodies are poured that cool the ver would slow down. The reason for the numerical restrictions gene is explained below. In the explanation below "%" refers to "% by weight".

C: 0.5 to 3.0%

C is essential to the formation of graphite and contributes to one improved ductility of the matrix by using it as a sphere graphite is dispersed therein. The start temperature of the ver consolidation is determined by the C concentration and the Start temperature of the hardening decreases when the C-Kon concentration in the hypoeutectic area increases. The graphite concentration increases as the C concentration increases, this causes a decrease in rigidity (elasticity mo dul). If the C content is less than 1.5%, the start is Solidification temperature high and for molding and melting high heat resistance is required. On the contrary, if the C content exceeds 3.0%, it can required modulus of elasticity are not conveyed. because The C content is from 1.5 to 3.0% limits.

Si: 1.0 to 5.5%

Si has the effect of promoting the formation of graphite. If the Si content is less than 1.0%, this effect cannot can be obtained and good workability cannot mediated. In contrast, if the Si content is 5.5% exceeds, silicon ferrite is formed excessively and the hardness of the material increases, causing stretch and stiffness lose weight. Therefore, the Si content is within a range limited from 1.0 to 5.5%.

SEM: 0.008 to 0.25%

REM promotes the formation of graphite and the effect can cannot be obtained if his salary is less than 0.008%  is. In contrast, if the SEM content exceeds 0.25%, not only is the effect of graphite formation not achieved, but graphite formation is also inhibited. thats why the SEM content ranges from 0.008 to 0.25% limits. It should be noted that REM is on rare earth Metals comes from La with an atomic number of 57 to Lu an atomic number of 71 and one or more species applied can be.

BN: 0.005 to 0.015% (B equivalent)

BN can be a core to the formation of fine nodular graphite promote and can inhibit the formation of carbides. If the BN content is less than 0.005% as B content (B equivalent), this effect cannot be obtained. In contrast to cannot expect further effects on graphite formation even if the BN content (B equivalent) is 0.015% exceeds. Therefore, the BN content is in a range of 0.005 to 0.015% limited. It should be noted that BN contains by adding B molten iron and B merges with N there.

One or more types of the elements mentioned below can optionally added to the formation and ver to further refine spheroidal graphite.

Bi: 0.0005 to 0.02%

Bi promotes the fine dispersion of graphite and the effect cannot be expected if the Bi content is less than Is 0.0005%. In contrast, if the Bi content exceeds 0.02% increases, the graphite spheroidization is inhibited and the shape of the graphite does not become spherical and the strength and the rigidity of the material will be affected. Therefore the Bi content is preferably in a range from 0.0005 to 0.02%.  

Al: 0.005 to 0.08%

Al is added as a deoxidizer. Deoxidation is not effective if its content is less than 0.005% and the graphitization by SEM is at a certain Cooling rate inhibited during solidification. If contrary the Al content exceeds 0.08%, the graphite spheroid inhibition and the shape of the graphite does not become spherical shaped so that the strength and rigidity of the cast iron be affected. Therefore, the Al content is preferably in a range of 0.005 to 0.08%.

Ca: 0.002 to 0.04%

Ca is an element that supports the formation and refinement of Gra phit promotes and the effect is not expected if the Ca content is less than 0.002%. In contrast, if the Ca content exceeds 0.04%, not only the effect of Formation and refinement of graphite not preserved, but the graphitization is adversely affected and the bil The formation of coarse graphite is promoted and the rigidity the material is therefore impaired. Therefore the ca Content preferably in a range of 0.002 to 0.04%.

Brief explanation of the drawings

FIGS. 1A to 1C are photomicrographs that metalli specific structures of cast iron show, where Fig. 1A is a case of game and Figs. 1B and 1C Comparative Examples.

Detailed description of the invention example

An example of the invention is explained below.

Table 1 shows the chemical composition (% by weight) for the cast iron of the example and the comparative examples. FIGS. 1A to 1C are photomicrographs showing the metal structures which are formed of cast molten iron with the chemical composition of Table 1. As shown in FIGS. 1A to 1C show clearly that hypoeutek diagram cast iron (B) has a structure composed of cementite and pearlite and there is only one ge rings amount of graphite of Comparative Example 1. This means that in Fig. 1B the part that appears as a white network is cementite and the gray part is pearlite. The proportion in which black dots are dispersed in the white matrix consists of ferrite and graphite. In Comparative Example 2 ( Fig. 1C), which is a common spheroidal graphite cast iron, a large amount of graphite is formed because it has a eutectic composition, and its shape is coarse. In contrast, the hypoeutectic spheroidal graphite cast iron ( Fig. 1A) of the example has a structure in which fine spheroidal graphite is dispersed in a ferritic matrix. Therefore, the hy spherical graphite cast iron of the present invention can provide a structure in which fine spheroidal graphite is dispersed without the formation of carbides, even in the as-cast state, by containing BN in addition to SEM.

Table 1

The mechanical properties of the hypoeutectic Kugelgra The cast iron of the example was measured and the results nisse are shown in Table 2. The mechanical properties ten of a common nodular cast iron are as  Data for Comparative Example 2 shown in Table 2. As in Shown in Table 2 is hypoeutectic nodular cast iron iron have a better modulus of elasticity and a better tensile strength strength compared to eutectic spheroidal graphite cast iron of comparative example 2 and the effect of the dispersion of fine nodular graphite was confirmed. The cast iron from Ver same example 1 was very hard and brittle and the stretch was practically zero. Hence the other mechanical ones Properties to be impaired without further testing.

Table 2

Claims (3)

1. Hypoeutectic spheroidal graphite cast iron comprising:
1.5 to 3.0% C; 1.0 to 5.5% Si; 0.008 to 0.25% REM;
0.005 to 0.015% BN (B equivalent) and the rest essentially Fe, based on weight. 2. Hypoeutectic spheroidal graphite cast iron according to claim 1, characterized in that the cast iron further comprises one or more of the following elements:
0.0005 to 0.02% Bi; 0.005 to 0.08% Al; 0.002 to 0.04% approx. 3. Hypoeutectic spheroidal graphite cast iron according to claim 1 or 2, characterized in that the cast iron white contains up to 0.3% Mn.