JPWO2012132679A1 - Austenitic cast stainless steel - Google Patents

Abstract

An austenitic stainless cast steel having a ferrite phase volume ratio of 0.1 to 5.0%.

Description

  The present invention relates to an austenitic stainless cast steel.

  Since austenitic stainless cast steel is particularly excellent in corrosion resistance, strength, weldability and the like, it is widely used in piping and valves of chemical plants and power plants. Austenitic stainless cast steel is formed, for example, from the metallurgical viewpoint of two phases of about 10 to 20% α phase and about 90 to 80% γ phase (austenite phase).

  CF8C is known as a cast steel product of austenitic stainless steel. For example, a CF8C austenitic stainless steel cast product has a maximum of 0.08 mass% C (carbon), a maximum of 2.0 mass% Si (silicon), a maximum of 1.5 mass% Mn (manganese), 18.0 to 21.0 mass% Cr (chromium), 9.0 to 12.0 mass% Ni (nickel), and 1.0 mass% Nb (niobium) at the maximum.

  CF8C contains about 12.0% ferrite phase. For example, the ferrite phase can be calculated from a Schaeffler phase diagram based on the constituent elements by measuring the ferrite content in the austenitic stainless steel with a known ferrite scope, and is displayed in volume ratio (percent (%)).

  The ferrite phase is said to be effective in preventing weld cracking and reducing stress corrosion cracking. However, when the content of the ferrite phase increases, for example, when CF8C is exposed to a high temperature for a long period of time, the ferrite phase may be transformed into a sigma phase (σ phase) that is a compound of iron and chromium and embrittled.

  Patent Document 1 discloses CF8C-Plus, which is an alloy obtained by modifying CF8C, and describes that the CF8C-Plus does not contain a ferrite phase. In patent document 1, CF8C-Plus is 0.05-0.15 mass% C, 0.2-1.0 mass% Si, 0.5-10.0 mass% Mn, 18.0 25.0 mass% Cr, 10.0-15.0 mass% Ni, 0.1-1.5 mass% Nb, 0.05-0.5 mass% N .

  The absence of a ferrite phase in CF8C-Plus is considered important to maintain the properties of the material during casting for the life of the component made from the material.

Special table 2009-545675

  When CF8C is exposed to a high temperature for a long period of time in a use environment, the sigma phase is precipitated to cause aging embrittlement and the aging ductility may be insufficient. Further, in CF8C-Plus described in Patent Document 1, further improvement in oxidation resistance has been demanded.

  Accordingly, an object of the present invention is to provide an austenitic stainless cast steel excellent in aging ductility and oxidation resistance.

In order to achieve the above object, the following inventions [1] to [6] are provided.
[1] Austenitic stainless cast steel having a ferrite phase volume ratio of 0.1 to 5.0%.
[2] C: 0.01 to 0.10% by mass, Si: 0.6 to 1.0% by mass, Mn: 2.0 to 2.8% by mass, N: 0.1 to 0.4% by mass The austenitic stainless cast steel according to [1] above, containing
[3] In the above [1] or [2] containing Cr: 18.0 to 24.0 mass%, Ni: 8.0 to 15.0 mass%, and Nb: 0.2 to 0.7 mass% The austenitic stainless cast steel described.
[4] The volume fraction of the ferrite phase is 0.1 to 5.0%,
C: 0.01-0.10 mass%, Si: 0.6-1.0 mass%, Mn: 2.0-2.8 mass%, N: 0.1-0.4 mass%, Cr: 18.0 to 24.0 mass%, Ni: 8.0 to 15.0 mass%, Nb: 0.2 to 0.7 mass%,
An austenitic stainless cast steel with the balance being Fe and inevitable impurities.
[5] In any one of the above [1] to [4] obtained by cooling from a temperature range of 1150 to 1350 ° C. to a temperature range of 600 to 800 ° C. at a cooling temperature of 30 ° C./min or more. The austenitic stainless cast steel described.
[6] A valve formed using the austenitic stainless cast steel according to any one of [1] to [5].

  The austenitic stainless cast steel of the present invention is excellent in aging ductility, tensile strength, and oxidation resistance, for example, as shown in Examples described later. In particular, the aging ductility of the examples of the present invention was about 2.4 times that of the comparative examples. Similarly, regarding the oxidation resistance, it was recognized that the example of the present invention was improved to about 9.5 times the comparative example.

  The reason why the austenitic stainless cast steel has such excellent characteristics is that the volume fraction of the ferrite phase is 0.1 to 5.0%, and the contained components C, Si, Mn, Cr, Ni , Nb, N content is considered important. Each component will be described in detail.

  By setting the volume fraction of the ferrite phase to 0.1 to 5.0%, the amount of sigma phase deposited can be reduced even when exposed to high temperatures for a long period of time. By reducing the precipitation amount of the sigma phase, the austenitic stainless cast steel becomes difficult to become brittle, and an austenitic stainless cast steel excellent in aging ductility is obtained.

  C has the effect of lowering the melting point and improving the fluidity of the molten metal, that is, the castability. Further, C is preferably as low as possible from the viewpoint of corrosion resistance, and when added in a large amount, the corrosion resistance of the base material is lowered. Based on these, in the present invention, in order to improve high temperature ductility, the amount of C added is set to 0.01 to 0.10% by mass.

  Si is an element effective for improving the deoxidizer and flowability, oxidation resistance, and weldability of the molten metal. However, if added excessively, the austenite structure becomes unstable, which causes deterioration of castability, and promotes hindrance to workability and weldability and weld cracking. Therefore, in the present invention, the addition amount of Si is set to 0.6 to 1.0 mass%.

  Mn is effective as a deoxidizer for molten metal, and improves the flowability during casting to improve productivity. Furthermore, it is effective in reducing weld cracking. Since excessive addition impairs oxidation resistance, the amount of Mn added is set to 2.0 to 2.8% by mass in the present invention. If Mn is in this range, as shown in the examples described later, austenitic stainless cast steel having excellent oxidation resistance can be obtained.

  N is an element that improves high-temperature strength and heat fatigue resistance, is a strong austenite-forming element, and stabilizes the austenite base. It is also an effective element for grain refinement. This refinement of crystal grains makes it possible to ensure the ductility of materials that are important as structures, and to improve the disadvantage of poor machinability that is characteristic of austenitic cast stainless steel. In particular, in a member that is subjected to a drilling process for connecting parts, the drilling processability is improved. When a large amount of N is added, embrittlement is promoted, while an effective amount of Cr is reduced and oxidation resistance is deteriorated. Therefore, in this invention, the addition amount of N is 0.1-0.4 mass%.

  Cr is an element that improves the oxidation resistance and stabilizes the ferrite structure, but is made 18.0% by mass or more in order to ensure the effect. On the other hand, the addition of a large amount causes a decrease in the aging ductility of the steel due to excessive precipitation of Cr carbide in the course of high temperature use, so the upper limit is 24.0% by mass.

  Ni forms a stable austenite base, stabilizes the austenite phase, and improves the high temperature strength and oxidation resistance of the steel. In consideration of good castability / corrosion resistance and weldability, in the present invention, the addition amount of Ni is set to 8.0 to 15.0 mass%.

  Nb combines with C to form fine carbides and improves high temperature strength. Moreover, oxidation resistance is improved by suppressing the production | generation of Cr carbide. In order to exhibit these effects effectively, the content must be 0.2% or more. However, when added in a large amount, the hot cracking sensitivity is remarkably increased and the internal quality is deteriorated. Therefore, in the present invention, the amount of Nb added is set to 0.2 to 0.7% by mass.

The austenitic stainless cast steel of the present invention can be produced by cooling from a temperature range of 1150 to 1350 ° C. to a temperature range of 600 to 800 ° C. at a cooling temperature of 30 ° C./min or more. By producing the austenitic stainless cast steel of the present invention under the above-mentioned conditions, since it has excellent strength characteristics even as cast, the solution heat treatment can be omitted.
The produced austenitic stainless cast steel is used, for example, as a material for piping and valves of chemical plants and power plants.

It is a graph which shows the result of having investigated oxidation resistance (mm / year) in austenitic stainless cast steel.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The austenitic stainless cast steel of the present invention is configured so that the volume fraction of the ferrite phase is 0.1 to 5.0%, preferably 0.5 to 3.0%. The austenitic stainless cast steel of the present invention contains C, Si, Mn, Cr, Ni, Nb, N and the like as components.
C is 0.01 to 0.10% by mass, preferably 0.02 to 0.04% by mass,
Si is 0.6 to 1.0 mass%, preferably 0.7 to 0.9 mass%,
Mn is 2.0 to 2.8% by mass, preferably 2.2 to 2.4% by mass,
N: 0.1 to 0.4% by mass, preferably 0.15 to 0.25% by mass,
Cr: 18.0 to 24.0 mass%, preferably 19.5 to 21.5 mass%,
Ni: 8.0 to 15.0% by mass, preferably 10.5 to 12.5% by mass,
Nb: 0.2 to 0.7 mass%, preferably 0.2 to 0.4 mass%.
Table 1 shows the range of the composition (mass%) of the above components in the austenitic stainless cast steel of the present invention and, as a reference, CF8C and CF8C-Plus.

By setting the volume fraction of the ferrite phase to 0.1 to 5.0% in the austenitic stainless cast steel of the present invention, the amount of precipitation of the sigma phase can be reduced even when exposed to high temperatures for a long time. . Therefore, the austenitic stainless cast steel of the present invention is not easily embrittled and is excellent in aging ductility.
In the austenitic stainless cast steel of the present invention, the Mn content is set higher than that of CF8C, and the C content is set lower. Thereby, strength and oxidation resistance at high temperatures can be improved.

  The austenitic stainless cast steel of the present invention contains W, B, Al, Mo, Co, Ti, Zr, Cu, rare earth elements (La, Ce, Y, Pd, Nd, etc.) and the like in addition to the above-described composition. The balance is Fe and inevitable impurities.

  The above-mentioned metal component is melted in a melting furnace and cooled to a temperature range of 600 to 800 ° C. at a cooling temperature of 1150 to 1350 ° C. or more at a cooling temperature of 30 ° C./min or more. Stainless cast steel can be produced. By producing the austenitic stainless cast steel of the present invention under these conditions, the solution has excellent strength characteristics even when cast, so that the solution heat treatment can be omitted.

  The produced austenitic stainless cast steel is used, for example, in piping and valves of chemical plants and power plants.

[Example 1]
Examples of the present invention will be described.
The compositions (mass%) of the main components of the austenitic stainless cast steel of the present invention (Examples 1-1 to 1-6) and CF8C (Comparative Examples 1-1 to 1-5) are shown in Tables 2 and 3, respectively.

  For these examples and comparative examples, aging ductility (700 ° C.-620 hours), tensile strength (900 ° C.), 0.2% proof stress (900 ° C.), oxidation resistance (1000 ° C.) were examined, and high temperature low cycle fatigue The test (double oscillation triangle wave, strain rate 0.1% / second, 700 ° C., total strain 0.5%) was performed.

  In both the examples and the comparative examples, casting was performed by a conventional in-place casting method. Regarding heat treatment, Examples 1 and 2 were performed by as cast (as cast), and other Examples and Comparative Examples were performed by SHT (Solution Solution Heat Treatment). Table 4 shows the results of examining the aging ductility, tensile strength, 0.2% proof stress, and oxidation resistance.

As a result, the aging ductility was 20.4% or more in the example, whereas it was 17.2% or less in the comparative example.
About the tensile strength, it was 113-134 Mpa in the Example, and it was 93-127 Mpa in the comparative example.
About 0.2% yield strength, it was 87-91 Mpa in the Example, and it was 70-84 Mpa in the comparative example.
The oxidation resistance was 0.489 mm / year or less in the example and 1.278 mm / year or more in the comparative example.

That is, regarding the 0.2% proof stress, although no significant difference was observed between the examples and the comparative examples, it was found that the examples were superior in terms of aging ductility, tensile strength, and oxidation resistance. In particular, regarding the aging ductility, the average value of the example was 24.8%, the average value of the comparative example was 10.4%, and the example was about 2.4 times that of the comparative example. Similarly, regarding oxidation resistance, the average value of the example is 0.290 mm / year and the average value of the comparative example is 2.770 mm / year, and the example is improved to about 9.5 times the comparative example. It was recognized that
The above-described results are obtained when the volume fraction of the ferrite phase of the austenitic stainless cast steel of the present invention is 0.2%, but the same applies even when the lower limit of the volume fraction of the ferrite phase is 0.1%. It is thought that the result of is obtained.

[Example 2]
In Example 1, the volume fraction of the ferrite phase of the austenitic stainless cast steel of the present invention was 0.2% (Examples 1-1 to 1-6), but not limited thereto, the volume fraction of the ferrite phase. Also in the case of 1 to 3%, aging ductility, tensile strength, 0.2% proof stress, and oxidation resistance were examined (Examples 2-1 to 2-4). These were performed under the same conditions as in Example 1. Each component of Examples 2-1 to 2-4 is shown in Table 5, and the results are shown in Table 6.

  As a result, the average value of aging ductility in Examples 2-1 to 2-4 was 24.6%, and the average value of oxidation resistance was 0.159 mm / year. These values were the same as in Example 1. It was recognized that it was superior to the comparative example. Even if the upper limit of the volume fraction of the ferrite phase of the austenitic stainless cast steel of the present invention is 5%, it is considered that the same result can be obtained.

Example 3
The oxidation resistance (mm / year) of austenitic stainless cast steel having a Mn content of about 1.0 to 4.5% by mass was examined. The austenitic stainless cast steel of the present invention was examined for an embodiment in which the Mn content was 2.26% by mass (Example 3-1) and 2.33% by mass (Example 3-2). The austenitic stainless cast steel of the comparative example has a Mn content of 1.04% by mass (Comparative Example 3-1), 1.17% by mass (Comparative Example 3-2), and 1.81% by mass (Comparative Example 3- 3) The aspect of 4.37 mass% (Comparative Example 3-4) and 4.48 mass% (Comparative Example 3-5) was examined. In these Examples and Comparative Examples, the respective components are shown in Table 7. The results are shown in Table 8 and FIG.

  From FIG. 1, in the austenitic stainless cast steel of the present invention, it was recognized that the oxidation resistance could be suppressed to 1 mm / year or less if the Mn content was 2.0 to 2.8% by mass.

  The present invention can be used for producing austenitic cast stainless steel.

Claims (6)

  An austenitic stainless cast steel having a ferrite phase volume ratio of 0.1 to 5.0%.   C: 0.01 to 0.10% by mass, Si: 0.6 to 1.0% by mass, Mn: 2.0 to 2.8% by mass, N: 0.1 to 0.4% by mass The austenitic stainless cast steel according to claim 1.   The austenitic stainless cast steel according to claim 1 or 2, containing Cr: 18.0 to 24.0 mass%, Ni: 8.0 to 15.0 mass%, and Nb: 0.2 to 0.7 mass%. . The volume fraction of the ferrite phase is 0.1 to 5.0%,
C: 0.01-0.10 mass%, Si: 0.6-1.0 mass%, Mn: 2.0-2.8 mass%, N: 0.1-0.4 mass%, Cr: 18.0 to 24.0 mass%, Ni: 8.0 to 15.0 mass%, Nb: 0.2 to 0.7 mass%,
An austenitic stainless cast steel with the balance being Fe and inevitable impurities.   The austenitic stainless cast steel according to any one of claims 1 to 4, obtained by cooling from a temperature range of 1150 to 1350 ° C to a temperature range of 600 to 800 ° C at a cooling temperature of 30 ° C / min or more. . The valve | bulb formed using the austenitic stainless cast steel as described in any one of Claims 1-5.

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