JP2008179844A - Two-phase stainless steel and casting of two-phase stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a two-phase stainless steel which does not contain Mo, is inexpensive and has sufficient strength and corrosion resistance for practical use in a balanced manner. <P>SOLUTION: The two-phase stainless steel comprises 0.08% or less C, 0.5 to 1.5% Si, 1.0% or less Mn, 4.0 to 8.0% Ni, 23 to 27% Cr, 2.0 to 6.0% Cu, 0.05 to 0.3% N and the balance Fe with unavoidable impurities; and has a ferrite phase and an austenite phase, in which the ferrite phase occupies 20 to 60% by an area ratio. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a duplex stainless steel having a ferrite phase and an austenite phase, and a cast product using the duplex stainless steel.

  As stainless steel, duplex stainless steel having a ferrite phase and an austenite phase is used as a material for various members. It is known that the duplex stainless steel has excellent corrosion resistance due to the austenite phase.

  As such a duplex stainless steel, in the following Patent Document 1, C is 0.02 mass% or less, Si is 2.0 mass% or less, Mn is 2.0 mass% or less, and P is 0.04 mass% or less. 0.04% or less of S, 3-7% by mass of Ni, 17-27% by mass of Cr, 0.5-6.0% by mass of Mo, 1-5% by mass of Cu, 3% by mass of W Hereinafter, materials for large stainless steel products containing 0.05 to 0.3% by mass of N, 0.0005 to 0.0015% by mass of B, and the balance being Fe have been proposed.

  Here, in thick cast products such as large seawater propellers, when duplex stainless steel is used, the sigma phase and carbonitride produced due to the slow cooling rate adversely affect corrosion resistance and toughness. Such adverse effects are prevented by adjusting the content.

Moreover, in the following patent document 2, C is 0.06 mass% or less, Si is 1.5 to 3.5 mass%, Mn is 0.1 to 3.0 mass%, Ni is 2 to 8 mass%, Cr 18 to 28% by mass, Mo 0.1 to 0.9% by mass, N 0.03 to 0.2% by mass, the balance is made of Fe, and has corrosion resistance against seawater and high strength. Duplex stainless steel has been proposed.
Japanese Patent No. 3270498 JP-A-9-302446

  However, in the conventional duplex stainless steel, an austenite phase and a ferrite phase are formed by containing a large number of component elements in addition to Cr, and all of the large number of component elements must be present within a predetermined range. The austenite phase and the ferrite phase could not be present in a desired range, and there were many essential component elements.

  Among these component elements, Mo is an essential component for improving the corrosion resistance, particularly the corrosion resistance in a reducing environment. In stainless steel, it is indispensable like Cr. However, this Mo has been pointed out as having an adverse effect on strength such as toughness when the amount added is large, and is expensive. For this reason, in Patent Document 2 and the like, although attempts have been made to reduce the Mo content, it is necessary to add a small amount of Mo.

  Further, conventionally, a cast product having a thin wall portion such as a screw used in a propulsion device for a small vessel has been cast using stainless steel. When casting such a thin cast product, at the time of casting, since the thin portion is easily cooled by the mold, the molten metal flowability is likely to be lowered. For this reason, when the temperature difference from the casting temperature to the melting point of stainless steel is small, the molten metal cannot be sufficiently injected into the thin-walled portion, and a defective portion is easily generated.

  However, if the casting temperature is increased to widen the temperature difference between the casting temperature and the melting point of stainless steel to prevent the occurrence of defects, the temperature of the molten metal introduced into the mold is increased accordingly. As a result, the thermal load is increased, and gas generation is likely to occur.

  In view of this, an object of the present invention is to provide a duplex stainless steel that ensures a good balance between strength and corrosion resistance that can withstand practical use without adding Mo and by reducing the types of component elements.

  It is another object of the present invention to provide a duplex stainless steel that can ensure a wide flowable temperature range and easily form a thin portion without increasing the casting temperature.

  The duplex stainless steel according to claim 1, which solves the above problem, is composed of Fe as a main component, contains C, Si, Mn, Ni, Cr, Cu, N, and inevitable impurities, and includes a ferrite phase and an austenite phase. And the ferrite phase area ratio is 20% or more and 60% or less.

  In the duplex stainless steel according to claim 2, C is 0.08 mass% or less, Si is 0.5 mass% or more and 1.5 mass% or less, Mn is 1.0 mass% or less, and Ni is 4.0. % By mass to 8.0% by mass, Cr from 23% by mass to 27% by mass, Cu from 2.0% by mass to 6.0% by mass, and N from 0.05% by mass to 0.3% by mass And the balance consists of Fe and unavoidable impurities, has a ferrite phase and an austenite phase, and has a ferrite phase area ratio of 20% to 60%.

  The duplex stainless steel according to claim 3 is used for casting of a cast product having a thin portion of 3 mm or less in addition to the configuration according to claim 1 or 2, and has a melting point of 1450 ° C. or less. Features.

  A duplex stainless steel casting according to claim 4 is made of the duplex stainless steel according to any one of claims 1 to 3.

  A cast product made of the duplex stainless steel according to claim 5 is a screw for a propulsion device of a small vessel made of the duplex stainless steel according to claim 3.

  According to the duplex stainless steel according to claim 1 or 2, C, Si, Mn, Ni, Cr, Cu, N, and inevitable impurities are contained in the main component Fe, and the amount of each component element is determined. By adjusting, since it has a ferrite phase and an austenite phase and the ferrite phase area ratio is 20% or more and 60% or less, strength and corrosion resistance can be ensured in a balanced manner without adding Mo. Therefore, it is possible to provide inexpensive duplex stainless steel that can be practically used as a material for various members such as screws that come into contact with water or seawater while suppressing the types of component elements.

  According to the duplex stainless steel according to claim 3, the duplex stainless steel according to claim 1 having a melting point of 1450 ° C. or lower is used for casting a cast product having a thin portion of 3 mm or less. It is easy to ensure a wide flowable temperature range without increasing the casting temperature. Therefore, it is possible to improve the flow of molten metal in the mold without increasing the thermal load applied to the mold, and even if a thin part having a thickness of 3 mm or less is molded, a defective part is formed in the thin part. Is difficult to form.

  According to the duplex stainless steel casting according to claim 4, since it is made of the duplex stainless steel according to claim 1 or 2, it is sufficient to be brought into contact with water or seawater using a material to which Mo is not added. It is possible to provide a cast product having corrosion resistance and strength that can be used in the present invention.

  According to the duplex stainless steel casting according to claim 5, since it is a screw for a propulsion device of a small vessel, it has a thin blade portion in a wide range, but from the duplex stainless steel according to claim 2 Therefore, at the time of casting, the hot water flowability is good, and the defective portion is hardly formed in the blade portion, so that it is easy to manufacture. Moreover, at the time of use, sufficient corrosion resistance and intensity | strength can be ensured in the environment used by receiving the stress corresponding to the driving force which generate | occur | produces in normal temperature water or seawater. Therefore, it is possible to provide a screw that is easy to manufacture and has corrosion resistance and strength that are sufficiently practical.

  Hereinafter, embodiments of the present invention will be described.

  FIG. 1 shows a screw for a small vessel propulsion device, which is a cast product of this embodiment.

  The screw 10 is made of a specific duplex stainless steel, and has a central portion 11 that is a center of rotation and a blade portion 12 that is provided integrally with the central portion 11 and is rotated. . The blade portion 12 is formed to be thin in a wide area, and the thickness of the thinnest portion is preferably 3 mm or less and 2 mm or less.

  Such a screw 10 is cast using a mold in which a hollow part for forming the central part 11 and a hollow part for forming the blade part 12 are integrally communicated. The molten metal to be poured into the mold at the time of casting is prepared by adjusting the component elements so that a specific duplex stainless steel can be formed with the screw 10 formed. For example, the molten metal is poured into the mold at a casting temperature of 1550 to 1650 ° C. Thus, it is manufactured by dissipating heat.

  The duplex stainless steel that is the material constituting the screw 10 contains C, Si, Mn, Ni, Cr, Cu, N, the balance is Fe and inevitable impurities, and expensive Mo is added. There is no stainless steel. In this invention, by adjusting the content of each component element, a duplex stainless steel having a ferrite phase and an austenite phase is formed without adding Mo, and at the same time, the melting point of the steel is lowered.

  The reason for adjusting the content of each component element is as follows.

  C ensures the strength of the duplex stainless steel and is a strong austenite stabilizing element. When the content is excessively increased, Cr carbide is precipitated to lower the corrosion resistance, and the duplex stainless steel tends to be brittle. If the C content is increased, the melting point tends to be lowered, but the influence on the strength is large. Therefore, it is preferably suppressed here, and C is set to 0.08% by mass or less.

  Si serves as a deoxidizer for duplex stainless steel and is a somewhat strong ferrite stabilizing element. If the content increases excessively, it tends to become brittle. If the Si content is increased, the melting point can be lowered. Here, since Mo which is a ferrite stabilizing element is not added, it is preferable to increase the content of Si that can achieve both effects of stabilizing the ferrite phase and lowering the melting point. .5% by mass or less.

  Mn becomes a deoxidizer for duplex stainless steel, is similar to Ni, contributes to an increase in the solid solution amount of N, and is a slightly weak austenite stabilizing element. If the Mn content is increased, the melting point can be lowered. However, if the content is increased, production problems are likely to occur, and corrosion resistance such as pitting corrosion resistance is likely to be lowered. Therefore, Mn is set to a range of 1.0% by mass or less.

  Ni is an austenite stabilizing element that improves mechanical properties, formability, and the like and contributes to maintenance of corrosion resistance. Although Ni has a different effect on the melting point depending on the content range, the effect is small. Here, since Mo which is a ferrite stabilizing element is not added, it is preferable to suppress the content of Ni, which is an austenite stabilizing element, to have a small influence on the melting point, and is preferably 4.0% by mass or more and 8.0% by mass. The range is as follows.

  Cr is a main component contributing to the corrosion resistance of the duplex stainless steel and is a ferrite stabilizing element. The corrosion resistance can be improved by increasing the stability of the non-moving body coating as the Cr content is increased. Although Cr has a different effect on the melting point depending on the content range, the effect is sufficiently small. Since Mo that contributes to corrosion resistance as a ferrite stabilizing element is not added, it is preferable to add a large amount of Cr that contributes to stabilization of the ferrite phase and corrosion resistance. However, an excessive amount impairs mechanical properties, formability, and the like. Therefore, Cr is in the range of 23 mass% or more and 27 mass% or less.

  Cu is a component that imparts corrosion resistance to the duplex stainless steel. If the Cu content is increased, the melting point can be lowered. Since Mo that contributes to corrosion resistance is not added, it is preferable to add a large amount of Cu in the present invention, but an excessive amount tends to be brittle. Therefore, Cu is in the range of 2.0 mass% or more and 6.0 mass% or less.

  N can increase the strength of the duplex stainless steel in a small amount and is a strong austenite stabilizing element. The influence of the N content on the melting point is small. Since nitride precipitates when it is added excessively, N is in the range of 0.05 mass% or more and 0.3 mass% or less.

  Further, in addition to these component elements, the balance of this duplex stainless steel is made of Fe and inevitable impurities. As unavoidable impurities, P, S, etc. are contained, and Mo may be contained together with the material used, for example, 0.3 mass% or less. In the present invention, Mo contained as an inevitable impurity may be further removed, but it can be used as it is without being removed because the content is small.

  In the duplex stainless steel of the present invention, by adjusting the content of the component elements as described above within the above range, first, the ratio of the ferrite phase to the austenite phase is adjusted, and the ferrite phase area ratio is set to 20%. % To 60%. As a result, the corrosion resistance and strength of the steel are secured in a well-balanced manner. Here, the pitting resistance is particularly improved as the corrosion resistance.

Although this ferrite area ratio varies depending on various conditions such as cooling rate, it is classified into a ferrite stabilizing element and an austenite stabilizing element, for example, using the following formulas (1), (2), etc. A chromium equivalent and a nickel equivalent may be calculated and used as an index.
[Equation 1]

  Cr equivalent =% Cr +% Mo + 1.5 ×% Si + 0.5 ×% Nb (1)

Ni equivalent =% Ni + 30 ×% C + 0.5 ×% Mn + 30 ×% N (2)
(In each formula,% component element is the content of the component element expressed by mass percent. In formula (1), the term “% Nb” is a term added when it is contained. )

  A schematic phase structure in the case of using such chromium equivalent and nickel equivalent is shown in the state diagram of FIG. In this invention, the chromium equivalent and the nickel equivalent are adjusted to the region S in which the ferrite phase area ratio is 20% or more and 60% or less. This is because if the ferrite area ratio is too small, the strength tends to be insufficient, whereas if it is too large, the corrosion resistance tends to be insufficient.

  Furthermore, in the duplex stainless steel of this embodiment, the melting point is 1450 ° C. or lower by adjusting the content of the component elements as described above at the same time as adjusting the ratio of the ferrite phase and the austenite phase as described above. Particularly preferably, the temperature is adjusted to 1430 ° C. or lower. This melting point is preferably as low as possible, and if it is too high, the fluidity of the molten metal tends to be insufficient and the molding of thin-walled parts tends to be difficult unless the temperature of the molten metal is raised during casting. is there.

  According to the duplex stainless steel as described above, C, Si, Mn, Ni, Cr, Cu, and N are contained in specific contents, respectively, and the balance consists of Fe and inevitable impurities, and the ferrite phase and austenite Since it has a phase and the ferrite phase area ratio is 20% or more and 60% or less, strength and corrosion resistance can be secured in a well-balanced manner without adding Mo. Therefore, it is possible to provide a duplex stainless steel that is inexpensive and can be practically used as the screw 10 that comes into contact with water or seawater while suppressing the types of component elements.

  Moreover, since this duplex stainless steel has a melting point of 1450 ° C. or lower, it is easy to ensure a wide flowable temperature range without increasing the casting temperature during casting. Therefore, the hot water flow in the mold can be improved without increasing the thermal load applied to the mold, and as a result, the blade portion 12 of the screw 10 has a thick portion of 3 mm or less. However, it is difficult to form a defective portion in the blade portion 12.

  Furthermore, according to the screw 10 made of such a duplex stainless steel, at the time of manufacture, it can be manufactured at a low cost with few kinds of component elements, and the hot water flowability of the material is good, and the vane portion 12 has a defect portion. It is difficult to form and easy to manufacture. Moreover, at the time of use, sufficient corrosion resistance and intensity | strength can be ensured in the environment used by receiving the stress corresponding to the driving force which generate | occur | produces in normal temperature water or seawater.

  Examples of the present invention will be described below.

  A stainless steel containing the components as shown in Table 1 and the balance being Fe and inevitable impurities is formed, the liquidus temperature as its melting point, the ferrite phase (α phase) and the austenite phase (γ phase) The area ratio was determined by measurement or simulation.

［強度試験］ The results are shown in Table 1.

[Strength test]

  A tensile test and an impact test were performed using the test pieces obtained in Comparative Example 1 and Example 6.

  The tensile test was performed using the test pieces formed in the same shape under the same conditions in accordance with the JIS Z 2371 metal material tensile test method.

  Moreover, the impact test was performed on the same conditions based on the JIS Z 2371 metal material impact test method using the test piece formed in the same shape.

The results are shown in Table 2.

As is clear from the results in Table 2, the duplex stainless steel of Example 6 has the same or higher results of the tensile test and the impact test than the duplex stainless steel of Comparative Example 1 containing Mo. Was confirmed.
[Casting test]

  A small boat screw as shown in FIG. 1 was manufactured using a mold using the materials having the component ratios of Comparative Example 1 and Example 6.

  In the screw 10, the thickness of the thinnest wall portion of the blade portion 12 was 1.6 mm.

As a result, in Example 6, the casting can be performed satisfactorily, whereas in Comparative Example 1, since the melting point is high, when a molten metal having the same temperature as in Example 1 is used, the temperature range is narrow, and the vane portion 12 has a casting defect. It was easy to occur. Therefore, it was confirmed that the material of Example 6 was easier to cast a thin product than the material of Comparative Example 1.
[Corrosion resistance test]

  Next, a corrosion resistance test was performed on each screw 10 manufactured from the materials of Comparative Example 1 and Example 6 in a casting test.

  In the corrosion resistance test, the presence or absence of rust was visually confirmed by spraying 5% by weight saline solution at 35 ° C. for 4 days under the same conditions based on the JIS Z 2371 salt spray test method.

  As a result, neither the screw 10 of Example 6 nor the screw 10 of Comparative Example 1 produced rust color on the surface. As a result, it was confirmed that the material of Example 6 had the same corrosion resistance as the material of Comparative Example 1.

1 is a schematic plan view of a screw according to an embodiment of the present invention. It is a state figure showing an outline phase composition using chromium equivalent and nickel equivalent of a component element.

Explanation of symbols

10 Screw 11 Central part 12 Blade part (thin part)

Claims (5)

  The main component is Fe, contains C, Si, Mn, Ni, Cr, Cu, N, and inevitable impurities, has a ferrite phase and an austenite phase, and has a ferrite phase area ratio of 20% to 60%. Duplex stainless steel characterized by   C is 0.08 mass% or less, Si is 0.5 mass% or more and 1.5 mass% or less, Mn is 1.0 mass% or less, Ni is 4.0 mass% or more and 8.0 mass% or less, Cr is 23 mass% or more and 27 mass% or less, Cu is contained in a content of 2.0 mass% or more and 6.0 mass% or less, N is contained in a content of 0.05 mass% or more and 0.3 mass% or less, and the balance is Fe. And a ferrite phase and an austenite phase, and a ferrite phase area ratio of 20% or more and 60% or less.   3. The duplex stainless steel according to claim 1, wherein the duplex stainless steel has a melting point of 1450 ° C. or less and is used for casting of a cast product having a thin portion of 3 mm or less.   A duplex stainless steel casting comprising the duplex stainless steel according to any one of claims 1 to 3.   A casting made of a duplex stainless steel, characterized in that it is a screw for a small vessel propulsion device made of the duplex stainless steel according to claim 3.

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