WO2009025456A1

WO2009025456A1 - Method for fabricating cast iron for turbine housing/manifold

Info

Publication number: WO2009025456A1
Application number: PCT/KR2008/004370
Authority: WO
Inventors: Je Won Han; Yun Taek Kim
Original assignee: Borgwarner Inc.
Priority date: 2007-08-17
Filing date: 2008-07-25
Publication date: 2009-02-26
Also published as: KR20090018462A

Abstract

The method for fabricating cast iron for the turbine housing/manifold in accordance with the present invention includes the steps of: providing an initial melt; providing a ladle and performing CV inoculation to the ladle; pouring out the initial melt with the ladle; adding a nodularizer to the initial melt; performing first inoculation with the initial melt, to which the nodularizer has been added, thereby forming a molten metal; and performing second inoculation while pouring the molten metal into a mold. Therefore, the method for fabricating cast iron for the turbine housing/manifold in accordance with the present invention has the advantageous effect of satisfying the required physical properties even for high-temperature operating conditions, i.e., thermal conductivity, thermal expansion, thermal impact, thermal deformation, etc.

Description

METHOD FOR FABRICATING CAST IRON FOR TURBINE

HOUSING/MANIFOLD

Technical Field

[1] The present invention relates to a method for fabricating cast iron for the turbine housing/manifold of an automobile, and more particularly to a method for fabricating cast iron for the turbine housing/manifold having improved physical properties required even for high-temperature operating conditions and having improved high- temperature oxidation resistance, by improving the method for fabricating cast iron for the turbine housing/manifold which is used at high temperature. Background Art

[2] Most of the materials presently used as the automobile exhaust system material are the FCD-H and FCD-50HS materials; and to date, such materials have been suitably used in the current situation where the output and exhaust gas temperatures are not very high.

[3] However, due to the increases in the recent automobile engine displacement, output, and the like, the exhaust gas temperature has increased sharply, and as a consequence thereof, when a gasoline engine is operated at the maximum temperature of about 1000 0C, or when a diesel engine is operated at about 830 ⁰C, the thermal load on such parts as the exhaust manifold and the turbine housing becomes very significant.

[4] The biggest problem caused by the exhaust gas temperature rise is the problem of thermal deformation of the material, in which, as the exhaust manifold, which alternates between the regions of high temperature and low temperature, is accompanied by thermal expansion and shrinkage depending on situation, surface oxidation wrinkles may be caused by such thermal deformation, which can progress and develop into a penetration crack.

[5] Such oxidation wrinkles and penetration cracks frequently occur at the port section or the merging section, which are relatively weak sections, and this is determined to be partly affected by the thin port section and the structural imbalance due to cooling.

[6] As such, as the high-speed performance improves through the exhaust temperature increase, there have been attempts to gradually raise the exhaust temperature of the engine, and accordingly, there is an urgent need to develop heat-resistant cast iron material having superior thermal deformation and oxidation resistance characteristics that can support such attempts. Disclosure of Invention Technical Problem [7] The present invention was devised by taking into consideration the aforementioned concerns, and its object is to provide a method for fabricating cast iron for the turbine housing/manifold of an automobile having improved high-temperature oxidation resistance, by improving the method for fabricating cast iron for the turbine housing/ manifold which is used at high temperature. Technical Solution

[8] In order to achieve the above-stated object, the method for fabricating cast iron for the turbine housing/manifold in accordance with the present invention includes the steps of: providing an initial melt; providing a ladle and performing CV inoculation to the ladle; pouring out the initial melt with the ladle; adding a nodularizer to the initial melt; performing first inoculation with the initial melt, to which the nodularizer has been added, thereby forming a molten metal; and performing second inoculation while pouring the molten metal into a mold.

[9] Characteristically, said initial melt is formed by melting pig iron, return scrap and/or steel scrap, in an electric furnace.

[10] Further characteristically, the temperature of said initial melt is from 1480 ⁰C to 1540

⁰C.

[11] Further characteristically, the composition which is admixed in said initial melt includes from 3.0 to 3.6% of carbon (C) by weight, from 4 to 4.6% of silicon (Si) by weight, less than 0.3% of manganese (Mn) by weight, less than 0.047% of phosphorus (P) by weight, less than 0.02% of sulfur (S) by weight, less than 0.05% of magnesium (Mg) by weight, from 0.5 to 3% of molybdenum (Mo) by weight, from 0.1 to 2% of nickel (Ni) by weight, and from 0.5 to 2% of vanadium (V) by weight.

[12] Further characteristically, the CV inoculant in said step of performing CV inoculation adds from 0.5 to 0.7% of silicon carbide (SiC) by weight.

[13] Further characteristically, said nodularizer adds from 0.3 to 0.5% of magnesium

(Mg) by weight.

[14] Further characteristically, said nodularizer optionally further adds from 0.6 to 0.8% of denodul by weight and/or from 0.05 to 0.15% of Fe-Si (ferro-silicon) by weight.

[15] Further characteristically, in said step of performing first inoculation, said first inoculant uses from 0.1 to 0.2% of a calcium-based compound by weight, or from 0.1 to 0.2% of SRF75 by weight.

[16] Further characteristically, in said step of performing second inoculation, said second inoculant uses barium.

[17] Further characteristically, in the process of pouring said molten metal into a mold, the injection temperature of said molten metal is from 1390 ⁰C to 1430 ⁰C.

[18] Further characteristically, the cast iron for the turbine housing/manifold in ac- cordance with the present invention is fabricated by the method for fabricating cast iron for the turbine housing/manifold as described above. [19] Further characteristically, the structure of said cast iron for the turbine housing/ manifold is at least 60% vermicular. [20] Further characteristically, the structure of said cast iron for the turbine housing/ manifold is at least 90% ferrite.

Advantageous Effects

[21] The method for fabricating cast iron for the turbine housing/manifold in accordance with the present invention has the advantageous effect of satisfying the required physical properties even for high-temperature operating conditions, i.e., thermal conductivity, thermal expansion, thermal impact, thermal deformation, etc.

Brief Description of the Drawings

[22] Figs. 1-6 are drawings illustrating the method for fabricating cast iron for the turbine housing/manifold in accordance with the present invention.

[23] Fig. 7 is a drawing showing the microscopic structure of the cast iron for the turbine housing/manifold formed by the fabrication method in accordance with the present invention.

[24] Figs. 8 and 9 are drawings showing the images of the microscopic structure of the cast iron for the turbine housing/manifold formed by the fabrication method in accordance with the present invention. Mode for the Invention

[25] Hereinbelow, the aspect of the present invention that the cast iron for the turbine housing/manifold fabricated by the method for fabricating cast iron for the turbine housing/manifold according to the examples of the present invention, has improved physical properties required at high temperature, shall be described by referring to the specific examples and the comparative examples.

[26] Some information has been omitted from the description of the present invention, because such information is deemed to be within the common knowledge of a person of ordinary skill in the relevant art and as such easily inferable.

[27] Figs. 1-6 are drawings illustrating the method for fabricating cast iron for the turbine housing/manifold in accordance with the present invention.

[28] As shown in Fig. 1, an electric furnace (500) is provided for fabricating the cast iron for the turbine housing/manifold in accordance with the present invention; and then, pig iron, return scrap and/or steel scrap, which are the raw materials of the initial melt (310), are provided.

[29] And then, the raw materials of the initial melt (310) are poured into the electric furnace (500), and are melted therein. [30] Referring to Fig. 2, as described above, the raw materials of the initial melt (310) are melted to form the initial melt (110); and the temperature of the initial melt (110) is maintained between 1480 ⁰C and 1540 ⁰C.

[31] Next, the composition (320) is admixed to the initial melt (110). [32] Here, the contents by weight of the components of the composition (320) are organized in the below Table 1.

[33] Table 1 [Table 1] [Table ]

[34] As recorded in Table 1, the composition (320) includes from 3.0 to 3.6% of carbon (C) by weight, from 4 to 4.6% of silicon (Si) by weight, less than 0.3% of manganese (Mn) by weight, less than 0.047% of phosphorus (P) by weight, less than 0.02% of sulfur (S) by weight, less than 0.05% of magnesium (Mg) by weight, from 0.5 to 3% of molybdenum (Mo) by weight, from 0.1 to 2% of nickel (Ni) by weight, and from 0.5 to 2% of vanadium (V) by weight.

[35] The composition (320) is described in greater detail hereinbelow. [36] 1) Molybdenum (Mo) 0.5 - 3 weight % [37] From 0.3 to 3.5% of molybdenum (Mo) by weight may be added to the composition of the present invention, and preferably from 0.5 to 3 weight % may be added.

[38] As such, the molybdenum added to the composition may improve the high- temperature tensile strength, the creep properties and the rupture strength of the composition, as well as improve the oxidation resistance by forming a protective coating at high temperature.

[39] However, the toughness and impact resistance at low temperature may be decreased. Accordingly, the content of molybdenum in the composition is preferably from 0.5 to 3% by weight.

[40] 2) Vanadium (V) 0.5 - 2 weight % [41] From 0.3 to 2.5% of vanadium (V) by weight may be added to the composition of the present invention, and more preferably from 0.5 to 2 weight % may be added. Such vanadium may prevent oxidation at high temperature as well as prevent deformation at high temperature, of the composition of the present invention.

[42] 3) Silicon (Si) 4 - 4.6 weight % [43] From 3 to 5% of the silicon component by weight may be added to the composition according to the present invention, and preferably from 4 to 4.6 weight % may be added.

[44] As more of the silicon (Si) component is added to the composition, it is possible to obtain such effects as the heat resistance of the composition and improved characteristics of the structure.

[45] In addition, it has an important effect on the properties of the ferrite type spheroidal graphite cast iron.

[46] By raising the transition temperature of ferrite/austenite, together with carbon, it is possible to improve the dimensional stability of the composition; and by forming a protective coating on the surface of the composition according to the reaction Si + 02 → SiO2 of the silicon, it is possible to improve the oxidation resistance.

[47] On the other hand, if the content of the silicon (Si) is insufficient, then the matrix structure of the composition cannot become ferritized and pearlite may exist. Accordingly, pearlite dissociates at the rising temperature of higher than 650 ⁰C, which may be the cause of thermal expansion of the composition.

[48] Accordingly, for the reasons stated above, in the event that too much silicon (Si) is added, the brittleness is increased, and therefore it is preferable to limit it within the range of from 4 to 4.6 weight %.

[49] More specifically, in the event that more than 4% of the silicon (Si) component by weight is added, there is formed an Fe2SiO4 layer inside the iron oxide (FeO) oxided layer, and this layer is very fine whereby it is possible to decrease the progression of oxidation, and therefore in the event that the silicon (Si) component is added in the range of from 4 to 4.6 weight %, it has an effect of mitigating the catalytic attack from the high-temperature oxidation scale.

[50] 4) Carbon (C) 3 ~ 3.6 weight %

[51] From 2.5 to 4.0% of carbon by weight may be added to the composition according to the present invention, and preferably from 3 to 3.6 weight % may be added.

[52] In view of the fact that much silicon component is added to the composition, in the event that the content of the carbon component exceeds the range of from 3 to 3.6 weight %, there is a risk of causing overprocessing.

[53] Accordingly, although the carbon content added to the composition may improve the hardness of the composition, since it may deteriorate the corrosion resistance of the composition, it is preferable to add from 3 to 3.6 weight %.

[54] 5) Nickel (Ni) 0.1 ~ 2.0 weight %

[55] From 0.1 to 2.5% of the nickel component by weight may be added to the composition according to the present invention, and preferably from 0.1 to 2.0 weight % may be added.

[56] The nickel (Ni) component added to the composition may improve the corrosion re- sistance of the material, and it can toughen the material of the composition. Moreover, when the nickel component in the amount in excess of the prescribed amount is added, it may reduce the thermal expansion coefficient of the composition. [57] Accordingly, it is preferable to add from 0.1 to 2.0% of the nickel component by weight to the composition.

[58] 6) Manganese (Mn) less than 0.3 weight %

[59] Less than 0.5% of the manganese (Mn) component by weight may be added to the composition according to the present invention, and preferably less than 0.3 % of the manganese component by weight may be added. [60] The manganese component may improve the hardness and high-temperature strength of the composition, and it may improve the toughness of the composition. [61] Accordingly, it is preferable to add less than 0.3 % of the manganese component by weight to the composition. [62] In addition, a number of impurities may be included in the composition, which may be difficult to completely eliminate, and it is important to minimize the content of such impurities. [63] Therefore, it is preferable to minimize the contents of phosphorus and sulfur which can decrease the corrosion resistance and strength of the cast iron, and which can be the cause of all kinds of brittleness. [64] Accordingly, the phosphorus component added to the cast iron is preferably less than

0.047 weight %, and the sulfur component is preferably less than 0.02 weight %. [65] As shown in Fig. 3, the ladle (200) is provided and said ladle (200) is CV (compacted vermicular) inoculated. [66] Such CV inoculation is an inoculation process for changing the structure of the cast iron to that of CV (compacted vermicular), which is a middle step from the flake graphite cast iron (grey iron) to the spheroidal graphite cast iron (ductile iron), whereby the thermal conductivity of the flake graphite cast iron and the strength of the spheroidal graphite cast iron are obtained. [67] At this point, the CV inoculant (330) used at the time of performing CV inoculation, may use from 0.5 to 0.7% of silicon carbide (SiC) by weight. [68] The CV inoculant (330) may promote the nucleation of graphite, and it may prevent defects such as chill that are harmful in casting. [69] Furthermore, the CV inoculant (330) may improve machine processibility, it may improve graphite shape and matrix structure, and it may improve microscopic contraction and the like. [70] As shown in Fig. 4, the initial melt (110) is poured out of the electric furnace (500) and is poured into the ladle (200). And, the nodularizer (340) is added to the initial melt (110) in the ladle (200). [71] The nodularizer (340) may be used for the purpose of spherifying the graphite structure inside the initial melt thereby obtaining superior mechanical properties.

[72] The nodularizer (340) may add from 0.3 to 0.5% of magnesium (Mg) by weight.

And, from 0.6 to 0.8% of denodul by weight and/or from 0.05 to 0.15% of Fe-Si (ferro- silicon) by weight, may optionally be added as the nodularizer (340).

[73] As such, the nodularizer (340) inside the initial melt (110) not only neutralizes the detrimental elements and increases the graphite availability, but it also may decrease violent reaction.

[74] As shown in Fig. 5, the molten metal (120) is formed by adding the first inoculant

(350) to the initial melt (110) to which the nodularizer (340) has been added.

[75] At this point, the first inoculant (350) may use from 0.1 to 0.2% of a calcium-based compound based on weight and from 0.1 to 0.2% of SRF75 based on weight.

[76] The first inoculant (350) may achieve uniformization of the graphite structure.

[77] As shown in Fig. 6, the step of performing the second inoculation is performed by pouring the molten metal (120) into the mold (520).

[78] At this point, the second inoculant (360), which is admixed in the second inoculation, may use barium (Ba). The second inoculant (360) can stabilize the graphite structure of cast iron.

[79] At this point, when the molten metal (120) is poured into the mold (520), the injection temperature of the molten metal (120) is preferably maintained in the range of from 1390 ⁰C to 1430 ⁰C.

[80] As such, the method for fabricating cast iron for the turbine housing/manifold in accordance with the present invention has the advantageous effects of improving the thermal deformation properties of cast iron for the turbine housing/manifold used at high temperature, as well as enhancing the oxidation resistance and mechanical properties at high temperature.

[81] Fig. 7 is a drawing showing the microscopic structure of the cast iron for the turbine housing/manifold formed by the fabrication method in accordance with the present invention, and Fig. 3a and Fig. 3b are drawings showing the images of the microscopic structure of the cast iron for the turbine housing/manifold formed by the fabrication method in accordance with the present invention.

[82] Referring to Fig. 7, Fig. 8 and Fig. 9, it can be confirmed that the graphite component of the cast iron for the turbine housing/manifold of the present invention in formed in vermicular shape.

[83] Moreover, it can be confirmed that the ratio of said vermicular shape is at least 60%, and that the ratio of the ferrite is at least 90%.

[84] As such, the cast iron for the turbine housing/manifold formed by the fabrication method in accordance with the present invention, as seen from the microscopic structure as described above, can satisfy the required physical properties even for high- temperature operating conditions, i.e., thermal conductivity, thermal expansion, thermal impact, thermal deformation, etc.

Claims

[1] A method for fabricating cast iron for the turbine housing/manifold, comprising the steps of: providing an initial melt; providing a ladle and performing CV inoculation to the ladle; pouring out the initial melt with the ladle, and adding a nodularizer to the initial melt; performing first inoculation with the initial melt, to which the nodularizer has been added, thereby forming a molten metal; and performing second inoculation while pouring the molten metal into a mold. [2] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 1, characterized in that said initial melt is formed by melting pig iron, return scrap and/or steel scrap, in an electric furnace. [3] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 1, characterized in that the temperature of said initial melt is from 1480

⁰C to 1540 ⁰C. [4] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 1, characterized in that the composition which is admixed in said initial melt includes from 3.0 to 3.6% of carbon (C) by weight, from 4 to 4.6% of silicon (Si) by weight, less than 0.3% of manganese (Mn) by weight, less than

0.047% of phosphorus (P) by weight, less than 0.02% of sulfur (S) by weight, less than 0.05% of magnesium (Mg) by weight, from 0.5 to 3% of molybdenum

(Mo) by weight, from 0.1 to 2% of nickel (Ni) by weight, and from 0.5 to 2% of vanadium (V) by weight. [5] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 1, characterized in that the CV inoculant in said step of performing CV inoculation, adds from 0.5 to 0.7% of silicon carbide (SiC) by weight. [6] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 1, characterized in that said nodularizer adds from 0.3 to 0.5% of magnesium (Mg) by weight. [7] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 6, characterized in that said nodularizer optionally further adds from 0.6 to 0.8% of denodul by weight and/or from 0.05 to 0.15% of Fe-Si (ferro-silicon) by weight. [8] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 1, characterized in that said first inoculant in said step of performing first inoculation, uses from 0.1 to 0.2% of a calcium-based compound by weight, or from 0.1 to 0.2% of SRF75 by weight. [9] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 1, characterized in that said second inoculant in said step of performing second inoculation uses barium. [10] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 1, characterized in that, in the process of pouring said molten metal into a mold, the injection temperature of said molten metal is from 1390 ⁰C to 1430

⁰C. [11] Cast iron for the turbine housing/manifold fabricated by the method as claimed in claims 1-10. [12] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 11, characterized in that the structure of said cast iron for the turbine housing/manifold is at least 60% vermicular. [13] The method for fabricating cast iron for the turbine housing/manifold as claimed in claim 11, characterized in that the structure of said cast iron for the turbine housing/manifold is at least 90% ferrite.