KR101578094B1 - A ductile cast iron for low temperature atmosphere and the production method thereof - Google Patents

Abstract

The present invention relates to a spheroidal graphite cast iron for low temperature and a method for producing the same. More specifically, the present invention relates to a spheroidal graphite cast iron for low temperature, To 0.02% by weight of Mg, 0.02 to 0.07% by weight of Mg and the balance of iron and unavoidable impurities is subjected to heat treatment for 3 to 5 hours and then to 700 DEG C by lowering the temperature to 780 to 810 DEG C According to the present invention, it is possible to remarkably improve the low-temperature characteristics of a conventional product using a continuous casting process even by a simple heat treatment, and to provide a cast iron containing a conventional nickel It is possible to provide a spheroidal graphite cast iron for low temperature which exhibits physical properties equal to or higher than those of the alloying material.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a spheroidal graphite cast iron,

The present invention relates to a spheroidal graphite cast iron for low temperature and a method for producing the same. More specifically, the present invention relates to a spheroidal graphite cast iron for low temperature, To 0.02% by weight of Mg, 0.02 to 0.07% by weight of Mg and the balance of iron and unavoidable impurities is subjected to heat treatment for 3 to 5 hours and then to 700 DEG C by lowering the temperature to 780 to 810 DEG C And a method for producing the same.

The steel industry is developing with the development of machinery such as automobiles, ships and various heavy equipment. In particular, recently, spheroidal graphite cast iron has attracted attention as a material for mechanical devices. Since ordinary graphite is a graphite structure, graphite is prone to cracking along with graphite when subjected to stress, and has brittleness and small strength. In contrast, graphite cast iron usually has graphite, which appears in the texture of cast iron, To prevent cracking in the graphite, and thus it has excellent strength and ductility. Therefore, it is an excellent material that overcomes both low strength and brittleness which are major limit in engineering use of cast iron. I have. In particular, spheroidal graphite cast iron has excellent mechanical properties and is widely used as a base material for various internal combustion engine parts. Spheroidal graphite cast iron produced by continuous casting with the use of ordinary steel has a high cooling rate compared to the material produced by sand casting casting, which is dense in internal structure and excellent in tensile strength and hardness It is widely used in industrial machinery, hydraulic valves, ships, and automobile industries.

In recent years, the number of transportation vessels capable of operating in extremely low temperature and ice zones has increased due to the recent development of poles and the reduction of operating costs, and in the case of cast iron and cast steel parts used for polar service vessels, Special alloys considered are in the spotlight. The demand of the present market continues to demand new materials that can withstand more extreme conditions. Especially, in the polar ship industry, once the polar regions are operated, the durability of products is low enough to replace related parts. Therefore, due to the high quality of the materials constituting the parts, the life of the ship is ultimately extended by prolonging the life of the parts. In this case, the existing material has an excellent internal defect and hardness strength in comparison with the sandblasted parts under the same conditions, but a new material is needed to prolong the life of the part due to the extreme situation.

Thus, there have been many attempts to meet the recent demands for physical properties. Korean Registered Patent No. 681270 discloses an alloy containing 3.1 to 3.8 wt% of C, 2.0 to 2.6 wt% of Si, 0.02 to 0.5 wt% of Mn, 0.05 wt% or less of P, 0.01 wt% or less of S, 0.6 wt% or less of Cu, To 0.06% by weight of Ni and 2.4 to 2.95% by weight of Ni and the balance of Fe and inevitable impurities, the spheroidal graphite cast iron has a spheroidization ratio of 90% by weight or more of graphite, a graphite size of 5 to 7 μm, Of a high-strength, high-elongation spheroidal graphite cast iron which is formed in the form of a ferrite structure surrounded by a ferrite structure of 20 wt% or less, and has a pearlite matrix structure of 80 wt% or more. In the patent publication of Korean Patent No. 613938, there is disclosed a steel sheet comprising 2.0-3.0 wt% of C, 1.52-2.5 wt% of Si, 0.1-0.3 wt% of Mn, 0.045 wt% or less of P, 0.020 wt% or less of S, 1.0 to 4.0 wt% of Mo, 1.0 to 5.0 wt% of V, and the balance of Fe containing slight impurities, wherein the shape coefficient of graphite is 0.2-0.5 And a CV graphitization rate of 80% or more; charging the cast iron and scrap iron with low thermal expansion and heating the cast iron to a temperature of 1500 ° C or higher to form a molten metal; Adjusting the molten metal composition so that the final composition is 2.0-3.0 wt% of C, 1.52-2.5 wt% of Si, 0.1-0.3 wt% of Mn, 0.045 wt% or less of P, and 0.020 wt% or less of S; After heating up to 1500-1600 캜, the content of the alloy element in the final composition is 34-36 wt% of Ni, 0.012-0.023 wt% of Mg, 1.0-4.0 wt% of Mo, 1.0-5.0 wt% of V, Adding an alloy so as to be composed of Fe containing some impurities; Heating the additive alloy element for a predetermined time so that the additive alloy element is in a molten state; And a step of casting after performing the CV conversion treatment and the seeding treatment in the ladle at the hot water boiling point. Japanese Patent Application Laid-Open No. 61-157655 discloses a steel sheet comprising 3.0 to 7.0% of C, 5.0% or less of Si, 3.0% or less of Mn, 0.5 to 40.0% of Ni and 0.5 to 20.0% A steel sheet having a composition of 30.0% Co, 0.1-30.0% Co, 0.1-10.0% Mo, 0.1-10.0% W, 0.05-5.0% V, 0.01-3.0% Nb, 0.01-3.0% %, The balance being substantially Fe, and an area ratio of graphite of not less than 5.0% and an area ratio of carbide or carbonitride of not less than 1.0% .

However, in the case of the spheroidal graphite cast iron disclosed in the above document, mainly the characteristics at high temperature are reinforced, and in the case of a special ship operating in the polar region, a low-temperature brittleness problem occurs, and research and development thereof are required. Particularly, the existing products contain a high content of silicon (Si) and have excellent mechanical properties at room temperature, but the impact strength and elongation at low temperature are relatively weak. Efforts have been made to solve these problems. Korean Patent No. 1040658 discloses a method for producing a cerium-zirconium composite oxide comprising 3.37 to 3.97 wt% of carbon, 0.100 to 0.149 wt% of manganese, 0.025 to 0.033 wt% of phosphorus, 0.001 to 0.01 wt% of sulfur, 0.01 to 0.05 wt% of chromium, 0.030 to 0.044 wt% Austenitizing the spheroidal graphite cast iron comprising 1.69 to 2.09 wt% of silicon and 0.850 to 0.900 wt% of nickel and the balance of iron and unavoidable impurities at a temperature of 800 to 840 DEG C for 9 to 11 hours A process; Dropping the spheroidal graphite cast iron treated by the austenitizing process at a temperature of 800 to 840 占 폚 for 6 hours to 660 占 폚 to 700 占 폚 for 3 hours; Maintaining the spheroidal graphite cast iron treated by the austenitizing process at a temperature of 660 to 700 ° C for 1 to 3 hours; And a ferritization step of slowly cooling the spheroidal graphite cast iron maintained at a constant temperature for 60 hours to 200 ° C .; the spheroidal graphite cast iron has a ferrite matrix structure of 90 to 100% and a spheroidization ratio of 90 to 100% Of the cryogenic graphite cast iron for cryogenic use. Although the ductile iron of the above-mentioned document has lowered the silicon content to improve the low temperature property, the mechanical properties are lowered and the nickel and chromium, which are expensive noble metals, are contained in a large amount, and the production cost is high and the heat treatment process is difficult.

Korean Patent No. 681270 Korea Patent No. 613938 Japanese Patent Laid-Open No. 61-157655 Korean Patent No. 1040658 Korean Patent No. 1024358

Therefore, in order to overcome the above disadvantages, the present invention has been made to overcome the above-mentioned disadvantages, and it is an object of the present invention to improve the low-temperature characteristics remarkably by a simple heat treatment only on a conventional product using a continuous casting process, The present invention provides a low temperature spheroidal graphite cast iron exhibiting properties equivalent to or superior to those of a material and a method for producing the same.

In order to accomplish the above object, the present invention provides a method of manufacturing a semiconductor device, comprising: 3.3-3.7 wt% of C, 2.0-2.3 wt% of Si, 0.25 wt% or less of Mn, 0.03 wt% or less of P, 0.005 wt% Wherein the spheroidal graphite cast iron having the composition including the weight% and the balance of iron and unavoidable impurities is subjected to heat treatment for 3 to 5 hours while being maintained at a temperature of 780 to 810 캜, followed by low cooling to 700 캜. do.

Further, the present invention provides a low-temperature spheroidal graphite cast iron having an elongation of at least 20%, a tensile strength of at least 400 N / mm < 2 >, and an impact strength at minus 20 & .

The present invention also provides a low-temperature spheroidal graphite cast iron produced by the continuous casting process of the spheroidal graphite cast iron.

Still another aspect of the present invention is a process for the production of a ferritic stainless steel comprising the steps of: i) providing a mixture of 3.3 to 3.7% by weight of C, 2.0 to 2.3% by weight of Si, up to 0.25% by weight of Mn, up to 0.03% Dissolving a composition containing impurities in a melting furnace; Ii) transferring the molten material dissolved in the melting furnace to the warming furnace; Iii) Mg is added so as to maintain the sludge at a temperature in the range of 1230 to 1350 ° C in the above-mentioned heating furnace so that the sintering temperature is in the range of 0.02 to 0.07% by weight of the total composition, and the sludge from the heating furnace is cooled, ; Iv) heating the spheroidal graphite cast iron at a temperature raising rate of 1.5 to 2.0 ° C / min to maintain the temperature at 780 to 810 ° C for 3 to 5 hours and then cooling to 700 ° C; ≪ / RTI >

According to the present invention, it is possible to remarkably improve the low-temperature characteristics even by a simple heat treatment in a conventional product using the continuous casting method, and to provide a low-temperature spheroidal graphite which exhibits equivalent or superior physical properties to conventional alloying materials including nickel, Cast iron can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a system diagram for schematically explaining a continuous casting method, which is a part of the process for producing the low temperature spheroidal graphite cast iron of the present invention
2 is a graph showing the heat treatment conditions of the spheroidal graphite cast iron produced by the continuous casting method

Hereinafter, the present invention will be described in detail.

The low-temperature spheroidal graphite cast iron according to the present invention is characterized in that it contains 3.3-3.7% by weight of C, 2.0-2.3% by weight of Si, 0.25% by weight or less of Mn, 0.03% by weight or less of P, 0.005-0.02% And the balance iron and unavoidable impurities are subjected to heat treatment for 3 to 5 hours and then to 700 ° C after the heat treatment is carried out at a temperature of 780 to 810 ° C.

In the present specification, 'low temperature' means the temperature level below the lowest temperature environment of the polar regions including the Antarctic or North Pole.

Hereinafter, the compositional components constituting the low-temperature spheroidal graphite cast iron of the present invention will be described.

C (carbon) is present as spherical graphite particles in spheroidal graphite cast iron, thereby improving the mechanical properties such as strength, elongation and impact characteristics as compared with long ribbon-shaped flake graphite existing in ordinary cast iron, This is an element which improves the flowability of the molten metal and also purifies the graphite during casting. C is preferably added in the range of 3.3 to 3.7% by weight based on the total composition of the spheroidal graphite cast iron. If the content of C is less than 3.3 wt%, the flowability of the molten metal is low and the number of spherical phases of the graphite to be poured decreases, resulting in deteriorated mechanical properties. On the other hand, when the content of C exceeds 3.7 wt%, coarse graphite increases to cause carbon dross and coagulation This is because the carbon flotation is formed in the portion, and defects are likely to occur.

Si (silicon) contributes to the crystallization of graphite during casting, prevents chill, and adjusts the ferrite structure and pearlite structure of the stable structure. Further, by the inclusion of Si, when the cast iron is placed in a high-temperature oxidizing gas, a dense oxide film is easily formed on the surface, and oxidation resistance is improved. Further, Si reacts with the oxidizing gas intruding from the outside to generate a protective coating at the boundary between the graphite and the base, and the oxidation of graphite and its surrounding matrix due to the penetration of oxidizing gas into graphite and the diffusion of C from graphite Which is known to improve the function of inhibiting the cell growth. However, as described above, the impact strength and elongation are lowered in a low-temperature environment, which is not suitable for use in the polar regions. Therefore, it is preferable that Si is contained in the spheroidal graphite cast iron of the present invention in the range of 2.0 to 2.3 wt%. When the Si content is less than 2.0%, the cementite structure, which is a chill structure at the outer portion due to quenching, is promoted due to the characteristics of the performance process. When the Si content is less than 2.0%, the elongation rate is lowered, .

Mn (manganese) is usually known to exist in the form of MnS in combination with sulfur. In the spheroidal graphite cast iron of the present invention, ferrite formation is prevented and pearlite is formed. Manganese is preferably added in an amount of 0.25% by weight or less based on the total composition. This is due to segregation in the grain boundaries of the structure even after the heat treatment in the case of 0.25% or more, and also low temperature impact property due to promotion of pearlite.

P (phosphorus) is incorporated in the range of 0.03 wt% or less as an element for increasing the brittleness of the spheroidal graphite cast iron. If the phosphorus content is more than 0.03 wt%, the intermetallic compound Fe ₃ P is formed in the crystal grain boundary and mechanical properties are deteriorated.

S (sulfur) is preferably incorporated in the range of 0.005 wt.% To 0.02 wt.% Of the composition of the total spheroidal graphite cast iron. If less than 0.005 wt.% Sulfur is present, carbides are produced, while if more than 0.02 wt.%, This is because MgS slag is formed in the casting and a dross defect occurs.

Mg (magnesium) is preferably incorporated in the range of 0.02 to 0.07% by weight of the total composition of the nodular graphite cast iron. If the content of magnesium is less than 0.02% by weight, spheroidization is difficult to achieve and mechanical properties are deteriorated. On the other hand, when the content of magnesium exceeds 0.07% by weight, the formation of inverse chill in the material is accelerated. .

The production method of the low temperature spheroidal graphite cast iron of the present invention is not particularly limited, but is preferably produced by the continuous casting process and the subsequent heat treatment. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a system configuration diagram schematically illustrating a continuous casting method, which is a partial process of the process for producing the low temperature spheroidal graphite cast iron of the present invention. FIG. In casting casting, which is a typical production method of producing cast iron, it is possible to lower the silicon content without any problem in the solidifying property, but it is suitable to be used for ship or vehicle parts parts operating in polar poles not. Therefore, the continuous casting method is more preferable. On the other hand, in the case of the continuous casting method, the structure of the cast iron is relatively higher than that of the casting casting, and the mechanical properties are better than that of the casting casting. However, the content of silicon in the cast iron is relatively high because the coagulation method must be produced by a continuous process by quenching. Degrade the low-temperature characteristics of the material. Therefore, the low-temperature spheroidal graphite cast iron of the present invention needs to be changed in the continuous casting process as compared with the general case.

The low temperature spheroidal graphite cast iron of the present invention comprises i) from 3.3 to 3.7% by weight of C, from 2.0 to 2.3% by weight of Si, up to 0.25% by weight of Mn, up to 0.03% by weight of P, from 0.005 to 0.02% by weight of S, Dissolving a composition containing impurities in a melting furnace; Ii) transferring the molten material dissolved in the melting furnace to the warming furnace; Iii) Mg is added so as to maintain the sludge at a temperature in the range of 1230 to 1350 ° C in the above-mentioned heating furnace so that the sintering temperature is in the range of 0.02 to 0.07% by weight of the total composition, and the sludge from the heating furnace is cooled, ; Iv) heating the spheroidal graphite cast iron at a temperature raising rate of 1.5 to 2.0 캜 / min and maintaining the temperature at 780 to 810 캜 for 3 to 5 hours followed by cooling to 700 캜 .

The Applicant has already found that the amount of sulfur (S) is not more than 0.1% by weight, the content of carbon (C) is in the range of 3.1 to 3.5% by weight, the content of silicon is 3.2 to 3.8% : 0.01% by weight or less, magnesium (Mg): 0.065% by weight or less and the balance being impurities and iron is melted in an electric melting furnace at a melting temperature in the range of 1450 to 1600 ° C, The molten metal dissolved in the electric melting furnace is introduced into a warming furnace at a ladle charging temperature in the range of 1270 to 1400 ° C and the temperature of the molten metal charged into the heating furnace is maintained at a warming furnace temperature in the range of 1230 to 1350 ° C, The drawing speed is set to 1.0 to 26.0 seconds in the drawing apparatus for maintaining the cooling water temperature of the cooling apparatus for cooling the molten metal from the furnace at a maximum of 120 DEG C or less and controlling the length and speed at which the molten metal from the cooling apparatus is drawn, To 10 to 100 mm She received registration to apply for a patent with respect to the continuous casting method of the cast iron to nodular cast iron, it characterized that comprises the information (see Republic of Korea Patent No. 1024358). Compared with the above-mentioned Korean Patent No. 1024358 of the present invention, the continuous casting process is performed in the same category, but the silicon content is small, and the drawing speed and drawing It is preferable that the length is set to be somewhat faster and longer. In the continuous casting process of the low temperature spheroidal graphite cast iron manufacturing method of the present invention, the drawing speed and drawing length are preferably 1.0 to 26.0 sec, 10 to 100 mm, more preferably 2.0 to 20.0 sec, and 40 to 100 mm, Of 5.0 to 12.0 sec and 45 to 65 mm.

2 is a graph showing heat treatment conditions of the spheroidal graphite cast iron produced by the continuous casting method. As can be seen from FIG. 2, the spheroidal graphite cast iron of the present invention can significantly improve the low temperature characteristics of the spheroidal graphite cast iron produced by the continuous casting method only by a simple heat treatment.

The low-temperature spheroidal graphite cast iron of the present invention thus produced has an elongation of 20% or more, a tensile strength of 400 N / mm 2 or more, and an impact strength at -20 ° C of minus 15 J or more. The physical properties of the low-temperature spheroidal graphite cast iron according to the present invention means that the parts for marine products made of the spheroidal graphite cast iron for low temperature according to the present invention can exhibit sufficient durability and physical properties such as mechanical strength in a low-temperature environment such as the polar regions.

Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are for the purpose of understanding the present invention and the scope of the present invention is not limited to the following examples.

The compositions of Comparative Examples and Examples are summarized in Table 1 below.

C Si Mn P S Mg Ni Comparative Example 1 3.48 2.54 0.19 0.029 0.005 0.039 Comparative Example 2 3.51 2.28 0.17 0.029 0.007 0.041 0.45 Example 1 3.46 2.22 0.18 0.030 0.008 0.044

The raw material having been previously blended so as to have the above composition except for the magnesium component is added to the melting furnace and melted, and then transferred to a warming furnace. The molten iron is maintained at a temperature in the range of 1250 to 1350 ° C, Spheroidal graphite cast iron was prepared by continuous casting method with the speed and draw length of 7.8 sec and 50 mm, respectively.

In addition, heat treatment was performed under the same conditions, and the physical properties before and after the heat treatment were measured. The heat treatment was carried out at a heating rate of 1.78 ° C / min. To 800 ° C, maintained at 800 ° C for 4 hours, and then cooled to 700 ° C. In Table 2, three samples of spheroidal graphite cast iron prepared in Comparative Examples and Examples were sampled, their mechanical properties were measured, and the average values and ranges were described.

Comparative Example 1 Comparative Example 2 Example 1 Before heat treatment After heat treatment Before heat treatment After heat treatment Before heat treatment After heat treatment Impact strength (J)
at-20 3 13 6.7 15.7 7.7 15.3 Hardness (HB) 179-207 156-163 179-192 156-163 189-217 149-156 Tensile strength (N /) side 558 438 546 430 542 420 center 605 431 603 425 566 410 yield point
(N /) side 400 304 394 300 374 282 center 437 300 424 299 397 277 Elongation
(%) side 16 25 11 18 12 25 center 10 26 13 27 17 26

As can be seen from the above table, the low-temperature spheroidal graphite cast iron of the present invention shows equivalent or superior properties to nickel-containing alloys in a low-temperature environment without containing expensive alloying materials such as nickel, Can be sufficiently used as a component material of

The embodiments of the present invention described above should not be construed as limiting the technical idea of the present invention. The scope of protection of the present invention is limited only by the matters described in the claims, and those skilled in the art will be able to modify the technical idea of the present invention in various forms. Accordingly, such improvements and modifications will fall within the scope of the present invention as long as they are obvious to those skilled in the art.

Claims (4)

A composition containing 3.3 to 3.7 wt.% Of C, 2.0 to 2.3 wt.% Of Si, 0.25 wt.% Or less of Mn, 0.03 wt.% Or less of P, 0.005 wt.% To 0.02 wt.% Of S and 0.02 to 0.07 wt.% Of Mg and balance iron and unavoidable impurities Wherein said spheroidal graphite cast iron is annealed at a temperature of 780 to 810 캜 for 3 to 5 hours and then cooled to 700 캜. The method according to claim 1,
Wherein the low-temperature spheroidal graphite cast iron has an elongation of 20% or more, a tensile strength of 400 N / mm2 or more, and an impact strength at -20 占 폚 of 15J or more. The method according to claim 1,
Wherein the spheroidal graphite cast iron is produced by a continuous casting method. I) dissolving a composition comprising 3.3 to 3.7% by weight of C, 2.0 to 2.3% by weight of Si, 0.25% by weight or less of Mn, 0.03% by weight or less of P, 0.005 to 0.02% by weight of S and balance of iron and unavoidable impurities in a melting furnace ,;
Ii) transferring the molten material dissolved in the melting furnace to the warming furnace;
Iii) Mg is added so as to maintain the sludge at a temperature in the range of 1230 to 1350 ° C in the above-mentioned heating furnace so that the sintering temperature is in the range of 0.02 to 0.07% by weight of the total composition, and the sludge from the heating furnace is cooled, ;
Iv) heating the spheroidal graphite cast iron at a temperature raising rate of 1.5 to 2.0 ° C / min to maintain the temperature at 780 to 810 ° C for 3 to 5 hours and then cooling to 700 ° C; Way.

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