KR101438825B1 - Ferritic nodular cast iron - Google Patents

Abstract

(0 is not included), P is not more than 0.05% (0 is excluded), S is not more than 0.03% (including 0 is not included), the content of C is 3.0 to 3.6%, the content of Si is 4.3 to 5.0% , Mo: 0.5 to 1.5%, V: 0.5 to 1.5%, spheroidizing treatment agent: 0.1% or less (0 is excluded), remaining Fe And other unavoidable impurities are introduced into the ferrite based spheroidal graphite cast iron. These main steel materials are excellent in strength, heat resistance and oxidation resistance at high temperatures.

Spheroidal graphite cast iron, high temperature strength, oxidation resistance, high temperature heat resistance

Description

FERRITIC NODULAR CAST IRON}

The present invention relates to a high silicon-vanadium-molybdenum ferrite spheroidal graphite core material excellent in strength and oxidation resistance under high temperature use conditions.

Materials used for exhaust manifolds, turbine housings of turbochargers, and the like, which are components of exhaust systems of a vehicle engine that undergo repeated heating and cooling processes, should have high temperature characteristics.

For example, in recent years, the exhaust gas temperature has been continuously rising to meet the increase of the output of the vehicle and the regulation of the exhaust gas, and accordingly, the durability and quality have been strengthened and the load exerted by the exhaust system has been increasing more and more .

Currently, low-silicon molybdenum, high-silicon molybdenum spheroidal graphite cast iron, Ni-resist cast iron and cast steel are used as the material of the casting exhaust manifold. However, such exhaust manifold materials should have excellent high temperature characteristics, .

DISCLOSURE Technical Problem The present invention has been proposed in order to solve the above-mentioned problems, and it is an object of the present invention to provide a ferrite spheroidal graphite steel material having high temperature strength and oxidation resistance and being relatively inexpensive.

In order to accomplish the above object, the ferritic spheroidal graphite primary iron according to the present invention comprises 3.0 to 3.6% of C, 4.3 to 5.0% of Si, 0.5% or less of Mn (0 is not included), P: Mo: 0.5-1.5%, Ni: 1.0% or less (0 is not included), Cr: 0.1% or less (inclusive) (0 is not included), 0.1% or less (0 is excluded) of a spheroidizing treatment agent, the balance Fe and other unavoidable impurities.

The ferrite based spheroidal graphite main iron as described above is excellent in strength, heat resistance and oxidation resistance at high temperature and low in manufacturing cost as compared with conventional spheroidal graphite main iron.

Therefore, the spheroidal graphite cast iron can be used as a cast alloy requiring excellent high-temperature characteristics, and is particularly applicable to an exhaust manifold of a high-output engine, a turbine housing, and the like.

The ferritic spheroidal graphite main iron material according to the present invention as described in the above-mentioned task solution will be described with reference to specific examples with reference to the accompanying drawings.

The above-mentioned spherical graphite iron-based material is a high silicon-vanadium-molybdenum alloy, and exhibits a ferrite matrix structure in spherical graphite, and Si, V, and Mo are particularly added in the composition, thereby exhibiting excellent high-temperature characteristics.

Carbon (C) is included in the range of 3.0 to 3.6 wt% considering the fluidity (lowering) during casting and the crystallization of fine graphite (coarse).

Silicon (Si) contributes 4.3 to 5.0 wt% as an element contributing to graphite crystallization. Particularly, Si is produced by forming a large amount of Fe ₂ SiO ₄ oxide film having excellent compatibility with the matrix to improve the oxidation resistance. In consideration of the ferrite action of the matrix, the austenite transformation effect in ferrite, , Preferably about 5.0 wt%. When the content of Si is less than 4.3 wt%, the oxidation resistance improving effect is lowered.

Manganese (Mn) is segregated at the process cell boundary and lowers the ferrite → austenite transformation temperature at this part. The content is preferably 0.5 wt% or less (0 is not included).

Phosphorus (P) forms steadite and its content needs to be controlled to 0.05 wt% or less (0 is not included).

Sulfur (S) is harmful to graphitization and is limited to 0.03 wt% or less (0 is not included).

Chromium (Cr) improves oxidation resistance by forming a C-based oxide. When it contains a large amount of chromium (Cr), machinability decreases, so it is controlled to 0.1 wt% or less (0 is not included). This level of Cr content is usually of the order of magnitude naturally included in scrap.

Nickel (Ni) improves the room temperature elongation and is not more than 1.0 wt% (0 is not included). If it exceeds 1.0 wt%, the base pelletization becomes too strong.

Molybdenum (Mo) improves the high-temperature strength by the effect of solid solution strengthening in the ferrite. In particular, it forms a precipitate carbide by bonding with the above-mentioned composition C to lower the average thermal expansion coefficient and lower the generation of thermal stress in the high temperature region, . However, when added in a large amount, grain boundary carbides increase to reduce the machinability and the room temperature elongation, so that it is included in the range of 0.5 to 1.5 wt%. When the content is less than 0.5 wt%, the effect of improving the high-temperature characteristics of 750 to 800 ° C or more is deteriorated.

Vanadium (V) improves high temperature strength through precipitation strengthening by VC, and is contained in 0.5 ~ 1.5 wt%. If it is contained in an amount exceeding 1.5 wt%, the oxidation resistance is lowered due to the increase of the decarburization reaction, and when the content is less than 0.5 wt%, the effect of improving the high temperature characteristics of 750 to 800 ° C or more is deteriorated.

The spheroidizing treatment agent is contained in an amount of 0.1 wt% or less (0 is not included). The use of Mg is preferable. For example, about 0.08 wt% is added and the content after graphitization is about 0.06 wt%. When the spheroidizing treatment agent is contained in an amount exceeding 0.1 wt%, carbide is generated, and graphite and oxide dross are mixed into the molten metal during injection, resulting in product defects.

The high silicon-vanadium-molybdenum ferrite spheroidal graphite core material having the composition as described above is improved in oxidation resistance at high temperature by the addition of Si and improves high-temperature characteristics due to solid solution strengthening of Mo, and furthermore, (V, Mo) composite carbide is precipitated, the high temperature strength and the heat resistance characteristics are further improved.

The excellent high-temperature characteristics of the spheroidal graphite cast iron as described above can be confirmed from the following experimental examples. As an example, test results of measurement of tensile strength and oxidation resistance at high temperature for comparison with the example having the composition shown in the following [Table 1] will be described. Here, the composition of Comparative Example 1 corresponds to a high silicon-based heat-resistant spherical graphite cast iron article used as an exhaust manifold material, and the composition of Comparative Example 2 corresponds to a low silicon-based heat-resistant spheroidal graphite cast iron article.

division
Chemical composition (wt%) C Si Mn P S Cr Ni Mo V Mg Example 3.18 4.44 0.24 0.05 0.01 0.08 0.36 1.22 0.90 0.041 Comparison 1 3.31 4.16 0.32 0.048 0.03 0.02 0.03 0.65 - 0.026 Comparison 2 3.69 2.84 0.26 0.042 0.02 0.06 0.13 0.45 - 0.034

The high temperature tensile test was carried out in accordance with normal tensile strength test conditions. As a result, as shown in FIG. 1, the highest tensile strength was obtained in the example at a surface temperature of 800 deg. C, which is a severe exhaust system mode. It is considered that this is due to the enhancement of high temperature strength by solid solution strengthening by Mo addition, VC precipitation strengthening by V addition, and (V, Mo) complex carbide precipitation by addition of V and Mo.

On the other hand, in the high-temperature oxidation test, square specimens having a composition of 20 mm in width, 20 mm in length and 2 mm in height according to Examples and Comparative Examples 1 and 2 were prepared, and each specimen was kept in the atmosphere for 300 hours in a heating / (G / mm ₂ ) per unit surface area before and after the oxidation test after removing the oxide scale by performing a shot blast treatment and then cooling it by taking out from the heating / heating furnace. As a result of this experiment, as shown in Fig. 2, the embodiment showed the lowest oxidation loss. This is probably due to the fact that the oxidation resistance is improved by proper addition of Si, and Fe ₂ SiO ₄ is closely formed at the interface with the matrix.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. It is to be understood that the invention may be variously modified and changed.

1 is a graph showing the results of a high-temperature tensile test according to an embodiment of the present invention and a conventional example,

2 is a graph showing the results of a high temperature oxidation test according to an embodiment of the present invention and a conventional example.

Claims (1)

(0 is not included), P is not more than 0.05% (0 is excluded), S is not more than 0.03% (including 0 is not included), the content of C is 3.0 to 3.6%, the content of Si is 4.3 to 5.0% Mo: 1.1 to 1.5%, V: 0.5 to 1.5%, a spheroidizing treatment agent of 0.1% or less (0 is excluded), the balance Fe And other unavoidable impurities.

Images