KR101054771B1 - Ferritic Spheroidal Graphite Cast Iron Castings - Google Patents

Abstract

By weight%, carbon 2.7-3.5%, silicon 4.5-5.0, manganese 0.35% or less, phosphorus 0.05 or less, sulfur 0.05% or less, molybdenum 0.6-0.8%, nickel 0.1% or less, copper 0.1% or less, magnesium 0.025% or more, Ferritic nodular cast iron having a composition containing the remaining iron and other unavoidable impurities is introduced. Such nodular cast iron has excellent heat resistance and high temperature properties at high temperatures, but is cheaper than conventional materials.

Spheroidal Graphite, Cast Iron, Heat Resistant, Oxidation Resistance

Description

Ferritic spherical graphite cast iron manufacturing method {METHOD FOR MANUFACTURING METAL CASTING OF FERRITIC NODULAR IRON}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to ferritic nodular cast iron having heat resistance, and more particularly, to high silicon ferritic nodular graphite cast iron having excellent strength and oxidation resistance at high temperatures, and a method for producing a cast of such a cast iron material.

Components such as exhaust manifolds and turbine housings in vehicle engine exhaust systems are repeatedly exposed to heating and cooling processes. Therefore, such parts require excellent high temperature strength and oxidation resistance.

Except cast steel, FCD500-SM1 and FCD550-SM1 cast iron are used for the current exhaust manifold, and Si, Ni, Mo, etc. are added to the existing spheroidal graphite cast iron to improve physical properties and oxidation resistance at high temperatures. It is characteristic to use. Such a material is not a problem for use at an exhaust system temperature of about 850 ° C. or lower.

However, in recent years, the exhaust gas temperature is rapidly increasing due to an increase in vehicle displacement and an increase in output, and there is an increasing demand for development of an exhaust system material that can withstand exhaust gases of 870 ° C or more. The main problem of current exhaust system materials caused by the rise of exhaust temperature is thermal deformation of the material due to rapid temperature change, cracks caused by this and catalyst attack by high temperature oxidation scale and damage to the carrier.

In general, the elements added to improve the high temperature strength of cast iron are Mo, V and the like to secure physical properties at high temperature through precipitation and solid solution strengthening of carbides. Conventional materials set the molybdenum content to about 0.8 to 1.2wt% to secure high temperature strength, but the amount of molybdenum is inevitably limited due to the recent increase in the cost of molybdenum, a valuable metal, and a rise in manufacturing cost.

On the other hand, silicon is added to improve ferrite and known oxidation resistance, but is limited to within 4.5wt% in consideration of deterioration in castability and machinability. Therefore, in the conventional materials, the increase in the oxidation release in the exhaust system and the damage to the catalyst carrier due to the increase in the exhaust temperature inevitably increases.

Therefore, there is an urgent need for the development of an optimal exhaust system material and its casting conditions in order to optimize the use of molybdenum, an essential element for improving the high temperature strength, to cope with the increase in manufacturing cost and to minimize the effect of castability even with a high silicon content.

The present invention has been proposed in order to solve the problems described above, and an object of the present invention is to provide a ferritic spheroidal graphite cast iron having excellent heat resistance and high temperature properties at high temperatures and a low cost compared to existing materials, and a method for manufacturing a cast of such cast iron materials. have.

In addition, the present invention adjusts the constituents of the alloy for exhaust manifolds used at high temperatures and derives the optimum casting conditions, thereby preventing the inclusion of Si in the matrix even when 4.5% or more of Si is added. It is an object of the present invention to provide ferrite heat-resistant cast iron having excellent high temperature strength and oxidation resistance and a method for producing a casting of such cast iron material by decomposing a predetermined amount of crystallized carbides such as and the like and exhibiting inherent characteristics of these elements without segregation of these elements.

Ferritic spherical graphite cast iron according to the present invention for achieving the above object, by weight%, carbon 2.7-3.5%, silicon 4.5-5.0%, manganese 0.35% or less, phosphorus 0.05% or less, sulfur 0.05% or less, molybdenum It consists of 0.6-0.8%, nickel 0.1% or less, copper 0.1% or less, magnesium 0.025% or more, and the remaining iron and other unavoidable impurities.

Casting method according to the present invention, in weight%, carbon 2.7-3.5%, silicon 4.5-5.0%, manganese 0.35% or less, phosphorus 0.05% or less, sulfur 0.05% or less, molybdenum 0.6-0.8%, nickel 0.1% or less, Preparing an alloy molten metal having a composition containing 0.1% or less of copper, 0.025% or more of magnesium, and remaining iron and other unavoidable impurities; And casting the melt at a tapping temperature of 1500 to 1600 ° C. and an injection temperature of 1420 to 1460 ° C. to produce a casting having a ferrite matrix structure of 80% or more, a graphite size of less than 100 μm, and a spheroidization rate of 70% or more. Include.

Ferrite-based nodular cast iron prepared as described above is superior in high temperature oxidation resistance as compared with conventional heat-resistant nodular cast iron.

In addition, while reducing the Mo content, the cost saving effect is great while maintaining the high-temperature tensile properties at an appropriate level.

In addition, since there is no brittleness due to the increase in the silicon content in cast iron, no additional heat treatment process is required.

The ferritic spheroidal graphite cast iron manufactured according to the present invention is excellent in high temperature mechanical properties such as high temperature tensile strength, together with the above characteristics, and can be used in a turbine housing of an exhaust manifold or a turbocharger of a high power engine.

Hereinafter, with reference to the accompanying drawings looks at in detail with respect to the ferritic spheroidal graphite iron according to the present invention.

Looking at the reason for the function and content limitation of the components contained in the ferritic nodular cast iron according to the present invention, carbon (C) is an essential element to increase the strength and hardness of the material and precipitate the carbides, the content is fluidity It is contained in the range of 2.7 to 3.5 wt% in consideration of deterioration and primary graphite crystallization.

Silicon (Si) is included 4.5 ~ 5.0wt%. Silicon increases the amount of ferrite formed during vacancy changes and improves oxidation resistance. In the casting, the silicon content of the spherical graphite cast iron is usually limited to less than 4.5 wt% in consideration of the melt flow and the machinability, but in the present invention, up to 5.0 wt% is added for the purpose of improving oxidation resistance.

Manganese (Mn) is limited to 0.35wt% or less because it lowers the austenite transformation temperature in ferrite. Manganese is usually introduced through scrap as a raw material, so only the upper limit of the process content is carefully controlled.

Phosphorus (P) is managed to 0.05 wt% or less (including 0%) because it forms steadite and affects brittleness and malleability.

Sulfur (S) increases the stability of Fe ₃ C but inhibits graphite crystallization is managed to 0.05wt% or less (including 0%).

Molybdenum (Mo) provides a solid-solution strengthening effect on the ferrite, and forms precipitated carbides to improve the high temperature strength, and also reduces the average thermal expansion coefficient to reduce the generation of thermal stress in the high temperature region. The content is managed at 0.6wt% or more, and because it is a valuable metal, it is controlled to 0.8wt% or less for cost reduction to maintain the required high temperature strength properties.

Nickel (Ni) is added in small amounts for the purpose of improving the room temperature elongation, the content is limited to 0.1wt% or less (0% not included). It is introduced through scrap as raw material.

Copper (Cu) is added to improve the hardness and wear resistance, the content is limited to 0.1wt% or less (0% not included) because it reduces the machinability when a large amount is added. It is introduced through scrap as raw material.

Magnesium (Mg) is added in small amounts to promote graphite spheroidization in the solidification process, the content of which should be 0.025wt% or more, preferably, 0.025 ~ 0.05wt%.

On the other hand, as described above, when a large amount of silicon is contained in the spheroidal graphite cast iron at 4.5wt% or more, Fe ₂ SiO _{4 which} is close to the interface between the oxide film and the matrix is generated, which is advantageous in improving oxidation resistance. However, castability becomes a problem due to an increase in the silicon content.
Therefore, according to the embodiment of the present invention, by weight%, carbon 2.7-3.5%, silicon 4.5-5.0%, manganese 0.35% or less, phosphorus 0.05% or less, sulfur 0.05% or less, molybdenum 0.6-0.8%, nickel 0.1% or less , An alloy molten metal having a composition containing 0.1% or less of copper, 0.025% or more of magnesium, and remaining iron and other unavoidable impurities, was prepared, and then maintained at a tapping temperature of 1500 to 1600 ° C (preferably 1500 ° C) and an injection temperature of 1420 to 1460 ° C. Casting.
This ensures excellent properties of oxidation resistance and high temperature strength even without the problem of castability even at 4.5 wt% or more of silicon. Spheroidal graphite cast iron produced is more than 80% ferrite matrix, graphite size is less than 100㎛, spheroidization rate is 70%, hardness is preferably 210 ~ 270HB.

As an example for confirming the performance of the nodular graphite cast iron according to the present invention, the specimens of Example 1 and Comparative Examples 1 and 2 having the composition shown in Table 1 below were prepared to test the tensile strength and oxidation at high temperature. Was carried out.

division
Composition (wt%) C Si Mn P S Mo Ni Cu Mg Example 1 3.05 5.00 0.20 0.035 0.0090 0.61 0.040 0.061 0.036 Example 2 3.01 5.00 0.20 0.035 0.0077 0.62 0.042 0.059 0.035 Comparative Example 1 3.74 3.60 0.17 0.04 0.0027 0.46 0.044 0.071 0.034 Comparative Example 2 3.18 4.33 0.21 0.036 0.0047 1.04 0.047 0.084 0.035

Example  One

Spheroidal graphite composed of carbon 3.05wt%, silicon 5.00wt%, manganese 0.20wt%, phosphorus 0.035wt%, sulfur 0.009wt%, molybdenum 0.61wt%, nickel 0.04wt%, copper 0.061wt%, magnesium 0.036wt% Cast iron specimens were fabricated using conventional casting methods.

Example  2

Carbon C 3.01wt%, Silicon 5.00wt%, Manganese 0.20wt%, Phosphorus 0.035wt%, Sulfur 0.0077wt%, Molybdenum 0.62wt%, Nickel 0.042wt% or less, Copper 0.059wt%, Magnesium 0.035wt% Spheroidal graphite cast iron specimens are fabricated by using a conventional casting method, but after 90 minutes at 930 ℃ in the furnace (furnace) to confirm the improvement effect on the brittleness due to the increase in the content of silicon, after the furnace is cooled to 500 ℃ Air-cooling heat treatment was performed.

Comparative example  One

Spheroidal graphite cast iron with the composition of carbon 3.74wt%, silicon 3.60wt%, manganese 0.17wt%, phosphorus 0.04wt%, sulfur 0.0027wt%, molybdenum 0.46wt%, nickel 0.044wt%, copper 0.071wt%, magnesium 0.034wt% Specimens were fabricated using conventional casting methods.

Comparative example  2

Spherical composition consisting of carbon C 3.18wt%, silicon 4.33wt%, manganese 0.21wt%, phosphorus 0.036wt%, sulfur 0.0047wt%, molybdenum 1.04wt%, nickel 0.047wt%, copper 0.084wt%, magnesium 0.035wt% Graphite cast iron specimens were fabricated using conventional casting methods.

Test 1 (high temperature The tensile strength  exam)

In order to compare the physical properties of Examples 1 and 2 and Comparative Examples 1 and 2, a high temperature tensile test was conducted using conventional equipment.

As a result, as shown in Figures 1a to 1c, Comparative Example 2 exhibited the highest high temperature tensile strength at the material surface temperature 700 ~ 800 ℃ of the harsh exhaust system mode, and then of Example 1,2 and Comparative Example 1 In order. From these results, it can be seen that the high temperature tensile strength is improved in proportion to the amount of Mo added to exhibit a solid solution strengthening effect. On the other hand, as shown in the test results of Examples 1 and 2 it can be seen that there is no change in tensile strength with or without heat treatment.

Test 2 (high temperature oxidation test)

The high temperature oxidation test was carried out for Example 1 and 2 and Comparative Examples 1 and 2 using conventional equipment. The test was carried out by placing each specimen in a furnace at 700 ° C. and then measuring the weight change of each specimen over time and measuring the thickness of the oxide scale through the microscope.

As a result, as shown in Figure 2, Examples 1 and 2 showed that the smallest weight change, the smallest scale of oxidation occurs. The increase in the Si content may be due to the generation of a large amount of Fe ₂ SiO ₄ close to the interface between the oxide film and the matrix to improve the oxidation resistance. Figure 3 shows the results of the measurement of the thickness of the oxide scale through a microscope, the thickness of the oxide scale is about 100㎛ or less in the case of the embodiment, compared with the oxidation resistance compared to the comparative examples of the thickness of the oxide scale is 130 ~ 200㎛ or more It is much better than the examples.

While specific embodiments of the present invention have been illustrated and described, those of ordinary skill in the art may vary the present invention without departing from the spirit of the invention as set forth in the following claims. It is to be understood that modifications and variations are possible.

Figure 1a to 1c is a graph showing the results of the high temperature tensile test for the Examples and Comparative Examples according to the present invention, Figure 1a is 700 ℃, Figure 1b is 750 ℃, Figure 1c is the test results at 800 ℃ conditions,

2 and 3 is a view showing the results of the high temperature (700 ℃) oxidation test for the Example and Comparative Example according to the present invention, Figure 2 is a graph showing the weight change amount, Figure 3 is a microscope showing the thickness of the surface oxide layer It is a photograph.

Claims (2)

By weight%, carbon 2.7-3.5%, silicon 4.5-5.0%, manganese 0.35% or less, phosphorus 0.05% or less, sulfur 0.05% or less, molybdenum 0.6-0.8%, nickel 0.1% or less, copper 0.1% or less, magnesium 0.025- Preparing an alloy molten metal having a composition containing 0.05%, remaining iron and other unavoidable impurities; And Casting the molten alloy at a tapping temperature of 1500 to 1600 ° C. and an injection temperature of 1420 to 1460 ° C. to produce a casting having a ferrite matrix of 80% or more, a graphite size of less than 100 μm, and a spheroidization rate of 70% or more. Ferrite-based nodular cast iron casting manufacturing method characterized in that. delete

Images