KR20090132798A - Ferritic nodular cast iron - Google Patents

Abstract

PURPOSE: Ferritic spheroidal graphite cast iron to secure oxidation resistance, heat resistance and intensity is provided to be used as the casted alloy demanding the high temperature characteristics and to be used for an exhaust manifold and turbine housing of a large-output engine. CONSTITUTION: Ferritic spheroidal graphite cast iron is made of C(Carbon) 3.0~3.6wt.%, Si(Silicon) 4.3~5.0wt.%, Mo(Molybdenum) 0.5~1.5wt.%, Ni(Nickel) less than 1.0wt.%, Cu(Copper) less than 0.1wt.%, S(Sulfur) less than 0.03wt.%, P(Phosphorus) less than 0.05wt.%, Mn(Manganese) less than 0.5wt.%, V(Vanadium) less than 0.5~1.5wt.%, residual Fe(Iron), spheroidizing treatment agent less than 0.1wt.% and inevitable impurities.

Description

Ferritic spheroidal graphite cast iron {FERRITIC NODULAR CAST IRON}

The present invention relates to a high silicon-vanadium-molybdenum-based ferrite spheroidal graphite cast iron having excellent strength and oxidation resistance under high temperature use conditions.

Materials used in exhaust manifolds, which are exhaust components of vehicle engines, and turbine housings of turbochargers, which undergo repeated heating and cooling processes, should have good high temperature characteristics.

As an example, in recent years, the exhaust gas temperature is continuously increasing to satisfy the increase of the output of the vehicle and the tightening of the exhaust regulations. Accordingly, the load received by the exhaust system is increasing as durability and quality are enhanced. .

Currently, low silicon molybdenum, high silicon molybdenum nodular cast iron, Ni-resist cast iron, and cast steel are used as casting exhaust manifold materials. However, these exhaust manifold materials must be excellent in high temperature properties, but the cost must also be considered. There is.

The present invention has been proposed to solve the above problems, and an object of the present invention is to provide a ferrite spherical graphite cast iron material having excellent high temperature strength and oxidation resistance and having a relatively low cost.

Ferritic spherical graphite cast iron according to the present invention for achieving the above object, in weight%, C: 3.0 ~ 3.6%, Si: 4.3 ~ 5.0%, Mn: 0.5% or less, P: 0.05t% or less, S: 0.03% or less, V: 0.5-1.5%, Mo: 0.5-1.5%, Ni: 1.0% or less, Cr: 0.1% or less, 0.1% or less of spheroidizing agent, and a composition containing remaining Fe and other unavoidable impurities.

As described above, the ferritic spheroidal graphite cast iron is superior in strength, heat resistance, and oxidation resistance at high temperatures and low in manufacturing cost as compared with the existing spherical graphite cast iron.

Therefore, the spherical graphite cast iron may be used as a casting alloy requiring excellent high temperature characteristics, and in particular, it may be used for exhaust manifolds, turbine housings, and the like of high power engines.

The ferritic spheroidal graphite cast iron material according to the present invention as described in the above-mentioned problem solving means will be described with reference to specific examples with reference to the accompanying drawings.

The spheroidal graphite cast iron material described above is a high silicon-vanadium-molybdenum-based, exhibiting a ferrite matrix structure in the spheroidal graphite, and Si, V, and Mo are added in the composition, so that the high temperature characteristics are excellent.

Carbon (C) is included in the range of 3.0 to 3.6 wt% in consideration of fluidity (deterioration) and primary graphite crystallization (coarsening) during casting.

Silicon (Si) is contained in the range of 4.3 to 5.0wt% as an element contributing to the crystallization of graphite. In particular, Si produces a lot of Fe ₂ SiO ₄ oxide film with excellent compatibility with the base to improve the oxidation resistance, considering the ferrite effect of the matrix structure, synergistic effect of austenite transformation in the ferrite and melt flow and cutting properties during casting At this time, it is preferably included up to about 5.0wt%. When the content of Si is less than 4.3wt%, the effect of improving oxidation resistance is lowered.

Manganese (Mn) segregates at the process cell boundaries, lowering the ferrite-austenite transformation temperature at this region. The content is preferably 0.5 wt% or less.

Phosphorus (P) forms steadite, so its content needs to be managed at 0.05 wt% or less.

Sulfur (S) is limited to 0.03wt% or less because it is harmful to graphite spheroidization.

Chromium (Cr) forms a C-based oxide to improve oxidation resistance, and when contained in a large amount, the machinability is lowered so that it is managed at 0.1 wt% or less. This amount of Cr is usually an amount that is naturally contained in the scrap.

Nickel (Ni) improves room temperature elongation and is included in an amount of 1.0wt% or less. If it exceeds 1.0 wt%, known perliteization is too strong.

Molybdenum (Mo) improves the high temperature strength due to the effect of solid solution strengthening in the ferrite, in particular, combined with C of the composition to form precipitated carbides, lower the average coefficient of thermal expansion and lower the occurrence of thermal stress in the high temperature region, To improve. However, when a large amount is added, the grain boundary carbide increases, which reduces the machinability and the elongation at room temperature. If the content is less than 0.5wt%, the effect of improving the high temperature property of 750 ~ 800 ℃ or more is inferior.

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

The spheroidizing agent is included at 0.1 wt% or less. Mg (magnesium), Ce (cerium), Ca (calcium) and the like can be used. Mg is preferably used. For example, about 0.08 wt% is added and the content after graphite spheroidization is about 0.06 wt%. When more than 0.1 wt% of the spheroidizing agent is included, carbides are generated and graphite and oxide dross are incorporated during melt injection to cause product defects.

The high silicon-vanadium-molybdenum ferrite spheroidal graphite cast iron material having the composition as described above improves the high temperature oxidation resistance by the addition of Si, and the high temperature property is improved by the solid solution strengthening of Mo. By the precipitation of the (V, Mo) composite carbide according to the high temperature strength and heat resistance properties are further improved.

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

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 according to the normal tensile strength test conditions, as a result, as shown in Figure 1, the embodiment showed the highest high temperature tensile strength at the material surface temperature of 800 ℃ in the harsh exhaust system mode. This may be due to the enhanced high temperature strength due to the strengthening of VC precipitation by the addition of Mo, the addition of V, and the precipitation of the (V, Mo) composite carbide by the addition of V and Mo.

On the other hand, the high temperature oxidation test produced square test pieces having a composition of 20mm in width, 20mm in height, and 2mm in height according to Examples and Comparative Examples 1 and 2, and maintained each test piece in the air for 300 hours in a heating thermostat with an ambient temperature of 700 ° C. After removing the scale by performing a short blast treatment, removing the scale from the heating, and then cooling it, the mass change per unit area before and after the oxidation test, that is, the oxidation loss (g / mm ₂ ) was calculated. As a result of these experiments, as shown in Figure 2, the embodiment showed the lowest oxidation loss. We suggest in particular, to be due to the oxidation resistance is improved by the proper addition of Si, generate a lot of Fe ₂ SiO ₄ close to the interface with the base.

While specific embodiments of the present invention have been shown and described, those of ordinary skill in the art will appreciate that the present invention may be made without departing from the spirit and scope of the invention as set forth in the claims below. It is to be understood that various modifications and changes can be made.

1 is a graph showing the results of high temperature tensile test of one embodiment of the present invention and the conventional example,

Figure 2 is a graph showing the results of the high temperature oxidation test of one embodiment of the present invention and the conventional example.

Claims (1)

By weight%, C: 3.0-3.6%, Si: 4.3-5.0%, Mn: 0.5% or less, P: 0.05t% or less, S: 0.03% or less, Cr: 0.1% or less, Ni: 1.0% or less, Mo : 0.5-1.5%, V: 0.5-1.5%, Ferritic spheroidal graphite cast iron material having a composition containing not more than 0.1% of spheroidizing agent and remaining Fe and other unavoidable impurities.

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