Stainless Steel Castings

Abstract

To provide a ferritic stainless steel casting that has excellent anticorrosiveness in an atmosphere including a corrosive gas and a molten salt at high temperatures, and also has excellent ductility, and can be used as a component for incinerators.SOLUTION: The inventive ferritic stainless steel casting contains, in mass%, C: 0.08% or less, Si: 1.50-3.50%, Mn: 0.30-2.00%, Cr: 20.00-33.00%, Nb: 0.10-1.50%, and Al: 0.30-2.50% with the balance being Fe and inevitable impurities.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to ferritic stainless steel castings. [Background technology]

[0002] For example, components such as nozzles and heat exchangers used in waste incinerators and biomass boilers, which are used in high-temperature environments where corrosive gases and molten salts are present, require high corrosion resistance.

[0003] Patent Document 1 describes a ferritic stainless steel having excellent corrosion resistance and high-temperature creep strength in a high-temperature corrosive combustion gas environment, which contains, by mass%, C: 0.040% or less, Si: 0.40 to 1.20%, Mn: 0.01 to 0.50%, P: 0.040% or less, S: 0.030% or less, Cr: 22.50 to 27.50%, Al: 0.60 to 1.40%, Ti: 0.10 to 0.90%, N: 0.0500% or less, and Nb: 0.10 to 0.90%, with these chemical components being (Ti / 48+Nb / 93)-(C / 12+N / 14): 0.004 to 0.025%, with the remainder being Fe and unavoidable impurities. This publication discloses a ferritic stainless steel with excellent high-temperature corrosion resistance and high-temperature creep strength, characterized in that the increase in precipitation of the Laves phase until rupture is 0.20 volume % or more when a creep rupture test is carried out at 850°C and 9 MPa.

[0004] Patent Document 2 discloses a heat-resistant steel that can be used as a grate material for incinerators exposed to high temperatures, and is characterized by being mainly composed of Fe with the addition of, by weight percent, 0.04 to 0.16 C, 24.0 to 31.0 Cr, 1 to 4.9 Al, and 0.6 to 4.9 Si. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2017-71829 A [Patent Document 2] Japanese Patent Application Publication No. 55-128567 Summary of the Invention [Problem to be solved by the invention]

[0006] The alloy structure of castings differs from that created by plastic processing, and is a residual solidification structure from the time of casting. Castings have the advantage of being easier to manufacture than plastically processed products, since any shape can be obtained by pouring molten metal into a mold. On the other hand, the solidification structure of castings has large microsegregation and coarse crystal grains. Large segregation makes the matrix more susceptible to corrosion, and large crystal grains make the grain boundaries more susceptible to corrosion, so it is said that the corrosion resistance of castings is lower than that of plastically processed products.

[0007] In the case of castings, there is still room for improvement in terms of corrosion resistance in high-temperature environments where both corrosive gases and molten salts are present, and in terms of resistance to stress cracking caused by thermal deformation due to repeated heating and cooling during operation of incinerators, etc.

[0008] An object of the present invention is to provide a ferritic stainless steel casting that has excellent corrosion resistance in an atmosphere containing corrosive gases and molten salts at high temperatures, and further has excellent ductility, and can be used for incinerator parts. [Means for solving the problem]

[0009] The present inventors have conducted extensive research into ferritic stainless steel castings that have high ductility and corrosion resistance in high-temperature environments containing corrosive gases and molten salts. As a result, they have found that high corrosion resistance can be obtained by increasing the Si content and forming a protective coating of Al2O3, and that high ductility can be obtained by adjusting the amount of Al added and fixing C with Nb, thereby suppressing intergranular corrosion and further improving corrosion resistance.

[0010] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0011] (1) A ferritic stainless steel casting characterized by containing, by mass%, C: 0.08% or less, Si: 1.50-3.50%, Mn: 0.30-2.00%, Cr: 20.00-33.00%, Nb: 0.10-1.50%, and Al: 0.30-2.50%, with the remainder being Fe and unavoidable impurities.

[0012] (2) The ferritic stainless steel casting according to (1) above, characterized in that it contains Ni: 0 to 10.00% in place of a portion of the Fe.

[0013] (3) The ferritic stainless steel casting according to (1) or (2) above, characterized in that it contains Mo: 0 to 5.00% in place of a portion of the Fe.

[0014] (4) The ferritic stainless steel casting according to any one of (1) to (3) above, characterized in that it contains W: 0 to 3.00% in place of a portion of the Fe.

[0015] (5) The ferritic stainless steel casting according to any one of (1) to (4) above, characterized in that it contains Ti: 0 to 2.00% in place of a portion of the Fe. Effect of the Invention

[0016] According to the present invention, it is possible to provide a ferritic stainless steel casting that has excellent corrosion resistance in an atmosphere containing corrosive gases and molten salts at high temperatures and also has excellent ductility and can be used for incinerator parts. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The chemical components of the ferritic stainless steel casting of the present invention will be described. In the following, "%" in the content means "mass %".

[0018] C: 0.08% or less C contributes to increasing the strength of steel, so it may be included as necessary. This effect can be obtained even with a small amount, but it is effective and preferable to set the C content to 0.01% or more, or 0.02% or more. If the C content is high, the ductility decreases and casting cracks become more likely to occur. In addition, C combines with Cr in the steel and precipitates as Cr carbides at the grain boundaries, which reduces the Cr concentration in the base, leading to a decrease in oxidation resistance and corrosion resistance at high temperatures. Therefore, the C content is set to 0.08% or less, preferably 0.06% or less, and more preferably 0.05% or less. In the ferritic stainless steel casting of the present invention, C is not an essential element, and the content may be 0.

[0019] Silicon: 1.50-3.50% Silicon is effective in improving oxidation resistance, particularly in improving oxidation resistance and corrosion resistance at high temperatures. In order to obtain the effect of improving oxidation resistance and corrosion resistance, 1.50% or more of silicon is necessary, preferably 2.00% or more, more preferably 2.50% or more, and even more preferably 2.70% or more. In addition, silicon also promotes the formation of a sigma layer, and there is a concern that embrittlement may occur when used at high temperatures for a long time, so the silicon content must be 3.50% or less, preferably 3.30% or less, more preferably 3.20% or less, and even more preferably 3.00% or less.

[0020] Mn: 0.30-1.50% Mn is an austenite forming element and is also a useful component as a deoxidizing agent and a desulfurizing agent. In order to fully obtain the effects, the Mn content must be 0.30% or more, preferably 0.40% or more, and more preferably 0.50% or more. From the viewpoint of preventing embrittlement and preventing deterioration of creep strength and corrosion resistance, the Mn content must be 1.50% or less, preferably 1.00% or less, and more preferably 0.80% or less.

[0021] Cr: 20.00~33.00% Cr is an element that improves high-temperature strength, and oxidation resistance and corrosion resistance at high temperatures. To fully obtain this effect, the Cr content must be 20.00% or more, preferably 23.00% or more, and more preferably 26.00% or more. For example, in an environment where molten chlorides adhere, such as a biomass boiler, if the Cr content is too high, volatile Cr2O2Cl2 is formed, which reduces oxidation resistance and corrosion resistance at high temperatures, so the Cr content must be 33.00% or less, preferably 32.00% or less, and more preferably 31.00% or less.

[0022] Nb: 0.10-1.50% Nb easily forms carbides, and by fixing the C in the steel and suppressing the precipitation of Cr carbides, it is possible to prevent a decrease in high-temperature strength and ductility. To obtain this effect, the content must be 0.1% or more, preferably 0.20% or more, and more preferably 0.40% or more. If the content exceeds 1.50%, the effect is saturated and creep strength, oxidation resistance, and corrosion resistance decrease, so the content must be 1.50% or less, preferably 1.30% or less, and more preferably 1.20% or less.

[0023] In the ferritic stainless steel casting of the present invention, Cr carbide (M 23 C6) is hardly precipitated, and the area ratio is preferably 1.0% or less, more preferably 0.5% or less, and further preferably 0.1% or less.

[0024] Al: 0.30-2.50% Al is an element with high deoxidizing ability, and is used as a deoxidizing agent during steelmaking. It also improves corrosion resistance by forming a protective coating of Al2O3 that is resistant to molten salt corrosion. To obtain this effect, the content must be 0.30% or more, preferably 0.50% or more, and more preferably 0.80% or more. If the content exceeds 2.50%, the effect is saturated and creep strength, oxidation resistance, and corrosion resistance decrease, so the content is set to 2.50% or less, preferably 2.30% or less, and more preferably 2.10% or less.

[0025] The balance of the chemical components is Fe and unavoidable impurities. However, instead of a part of Fe, the elements described below may be contained. The inclusion of the elements described below is optional and not essential.

[0026] Ni: 0-10.00% Ni is an element that improves room temperature ductility and may be contained in the range of 10.00% or less. This effect can be obtained even with a small amount of Ni, but it is most effective when it is contained in an amount of 0.10% or more.

[0027] Mo: 0 to 5.00% Mo has the effect of increasing corrosion resistance, and may be contained in the range of 5.00% or less. This effect can be obtained even with a small amount of Mo, but it is most effective when it is contained in the range of 0.50% or more.

[0028] W:0~3.00% W, like Nb, easily forms carbides and can fix C in the steel to suppress the precipitation of Cr carbides, preventing a decrease in high-temperature strength. It may be contained in the range of 3.00% or less. This effect can be obtained even with a small amount of content, but it is most effective when it is contained in a range of 0.20% or more.

[0029] Ti: 0 to 2.00% Ti, like Nb, easily forms carbides and can fix C in the steel to suppress the precipitation of Cr carbides, preventing a decrease in high-temperature strength. It may be contained in the range of 2.00% or less. This effect can be obtained even with a small amount of content, but it is most effective when it is contained in a range of 0.10% or more.

[0030] Next, a method for producing the ferritic stainless steel casting of the present invention will be described.

[0031] The manufacturing method of the present invention is not particularly limited, and may be a conventional method. First, a molten metal having the above-mentioned composition is prepared, and the molten metal is poured into a mold, and the poured molten metal is cooled and solidified. The mold, the device for pouring the molten steel into the mold, and the method for pouring are not particularly limited, and known devices and methods may be used.

[0032] By casting a molten metal having the chemical composition of the present invention, it is possible to obtain a ferritic stainless steel casting having excellent corrosion resistance and ductility in the as-cast state without using any special manufacturing method.

[0033] The castings of the present invention are in principle used as cast, but can be subjected to solution treatment by heating to 900 to 1200° C. and holding for 2 to 24 hours, if necessary.

[0034] The ferritic stainless steel casting of the present invention is a steel that is not subjected to plastic processing after being cast into a mold of a predetermined shape, and is distinguished from alloys that have been subjected to plastic processing such as hot rolling or forging. That is, the casting has the solidification structure as cast as the alloy structure, whereas the alloy that has been subjected to plastic processing has an alloy structure generated by processing, and the structures are significantly different. In the casting of the present invention, the size of the crystal grains (area equivalent circle diameter) is about 0.2 to 10.0 mm. EXAMPLES

[0035] The present invention will be described in detail below with reference to examples.

[0036] Ferritic stainless steel castings having the composition shown in Table 1 were prepared and subjected to a corrosion test. The corrosion test was carried out under the test conditions shown in Table 2 by taking test pieces of 20 × 20 × 3 mm from the prepared ferritic stainless steel castings. The corrosion resistance was evaluated in terms of corrosion weight loss (mg / cm 2 ) and 70mg / cm 2 If the resistance was less than this, it was judged to be excellent in corrosion resistance.

[0037] In addition, test pieces having a gauge length of φ10 × 50L were taken, and the room temperature ductility was evaluated by measuring the reduction in area at room temperature in accordance with JIS Z2241:2011. A reduction in area of ​​2.5% or more was determined to be excellent room temperature ductility.

[0038] [Table 1]

[0039] [Table 2]

[0040] It has been confirmed that the present invention makes it possible to obtain ferritic stainless steel castings that have excellent corrosion resistance and ductility in an atmosphere containing high-temperature corrosive gases and molten salts, and that are suitable for incinerator parts.

[0041] The comparative examples not satisfying the requirements of the present invention were inferior in either or both of corrosion resistance and ductility.

Claims

1. In mass percent, C: 0.08% or less, Si: 2.70-3.50%, Mn: 0.30-2.00%, Cr: 20.00-33.00%, Nb: 0.10 to 1.50%, and Al: 0.30~2.50% and the balance being Fe and unavoidable impurities.

2. 2. The stainless steel casting according to claim 1, further comprising: 0 to 10.00% Ni instead of a portion of the Fe.

3. 3. The stainless steel casting according to claim 1, further comprising: Mo: 0 to 5.00% in place of a portion of said Fe.

4. 4. The stainless steel casting according to claim 1, further comprising W: 0 to 3.00% in place of a portion of the Fe.

5. 5. The stainless steel casting according to claim 1, further comprising: 0 to 2.00% Ti instead of a portion of the Fe.

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