Austenitic stainless steel casting

Abstract

To provide a casting having excellent corrosion resistance and σ embrittlement resistance, which is not broken even when the casting is used in a composite gas atmosphere in a high temperature range and is repeatedly cooled to room temperature.SOLUTION: 0.08% or less of C, 2.50% or more and 4.00% or less of Si, 0.30% or more and 2.00% or less of Mn, 17.00% or more and 30.00% or less of Cr, more than 45.00% and 57.00% or less of Ni, 0% or more and 3.00% or less of Nb, 0% or more and 5.00% or less of Mo, 0% or more and 0.500% or less of Se, 0% or more and 0. 100% or less of Te, 0% or more and 0. 100% or less of Bi, 0% or more and 2.00% or less of W, 0% or more and 2.00% or less of Ti, and 0% or more and 2.00% or less of Al by mass%, the steel sheet has a composition comprising, by mass, 0 to 0.200% Mg and 0 to 0.30% N, and the balance Fe with impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel casting, and more particularly to an austenitic stainless steel casting having excellent σ embrittlement resistance. [Background technology]

[0002] In recent years, the use of biomass resources has been considered as an attempt to reduce the use of fossil fuels. Biomass resources also mean the reuse of resources that would otherwise be discarded, and their use for combustion is attracting attention as it leads to a reduction in the use of fossil fuels.

[0003] Biomass boilers are a type of boiler that is widely used, in which a furnace is filled with inert inorganic material such as sand as a bed material that acts as a heat transfer medium, and combustion gas is blown in from the hearth through an air outlet nozzle to agitate the bed material, allowing the material to be burned while maintaining the bed material at a desired temperature.

[0004] In recent years, as various biomass resources are used as fuel, problems have arisen such as oxidation by H2O, CO2, and O2, as well as chlorination caused by chlorine generated from plastic fuels and corrosion caused by sulfur dioxide generated from waste tires.Corrosion is a phenomenon in which corroded Cl, S, and O form a corrosion layer on the matrix of the air blowing nozzle, which then evaporates or peels off.

[0005] Therefore, high corrosion resistance is required for components such as nozzles, heat exchangers, and grates that are used in high-temperature environments where corrosive gases and molten salts are present, such as in the biomass boilers described above or waste incinerators.

[0006] Patent Document 1 discloses a high-alloy steel for waste heat boiler tubes in waste incinerators that produce molten salt, and proposes a high alloy steel for the waste heat boiler tubes, which has excellent resistance to stress corrosion cracking and intergranular corrosion.

[0007] Patent Document 2 discloses a nozzle material that is highly resistant to wear caused by sand.

[0008] Patent Document 3 discloses a casting that has excellent abrasion resistance and corrosion resistance in a high-temperature composite gas atmosphere and can be used for an air blowing nozzle for biomass.

[0009] Patent Document 4 discloses an austenitic stainless steel that is suitable as a structural material for a small hydrogen generator and that combines high levels of scale spallation resistance, carburization resistance, and σ embrittlement resistance. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 4-350149 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-78198 [Patent Document 3] Patent Publication No. 2021-025080 [Patent Document 4] Japanese Patent Application Laid-Open No. 2003-129192 Summary of the Invention [Problem to be solved by the invention]

[0011] Since the grates of waste incinerators are used in high-temperature environments, when manufacturing them using castings, it is important to take measures to prevent corrosion caused by chlorination, sulfidation, and oxidation.

[0012] In Patent Document 1, the plastically worked product is subjected to plastic working such as hot forging. In contrast, the alloy structure of a casting is different from the alloy structure formed by plastic working, and is the solidification structure at the time of casting that remains as it is. Castings can be formed into any shape by pouring molten metal into a mold, and therefore have the advantage of being easier to manufacture than plastically worked products.

[0013] Patent Document 2 discloses a cast iron that can be used for nozzles and has excellent resistance to wear caused by sand, but there is room for improvement in terms of corrosion resistance in a composite gas atmosphere.

[0014] Patent Document 3 is an invention made to improve the problems of Patent Documents 1 and 2. According to Patent Document 3, it is possible to obtain a casting that has excellent abrasion resistance and corrosion resistance in a high-temperature composite gas atmosphere and is suitable for an air blowing nozzle for biomass.

[0015] However, when the casting of Patent Document 3 is used, for example, as a grate in an incinerator such as a biomass boiler, and is fixed inside the incinerator in an environment where the temperature changes repeatedly between high temperatures during use and room temperature when the incinerator is not in use, there is room for improvement in resistance to σ embrittlement.

[0016] σ embrittlement is likely to occur in steels with high Cr and Mo content, and is a phenomenon caused by the precipitation of the σ phase at around 800°C. The formation of the σ phase adversely affects toughness after cooling, and can lead to breakage due to vibration or impact.

[0017] Patent Document 4 discloses an austenitic stainless steel that combines scale spallation resistance, carburization resistance, and σ embrittlement resistance, making it suitable as a structural material for small hydrogen generators. However, there is room for improvement when it is used as a material for incinerator grates.

[0018] In view of the above circumstances, an object of the present invention is to provide a casting that has excellent corrosion resistance and σ embrittlement resistance and will not break even when subjected to temperature changes such as repeated use in a complex gas atmosphere at high temperatures and cooling to room temperature, and that can be used, for example, as a fire grate for an incinerator. [Means for solving the problem]

[0019] The present inventors have conducted extensive research into the component composition of a steel casting that has excellent σ embrittlement resistance even when subjected to temperature changes such as repeated use in a composite gas atmosphere at high temperatures and cooling to room temperature.

[0020] In oxidizing atmospheres such as H2O and O2, Cr forms stable Cr2O3 or FeCr2O4, which is effective in improving high-temperature corrosion resistance. However, in combustion gas atmospheres containing SO2 or Cl2, it forms CrS or CrCl2. As the sulfurization reaction progresses, the amount of Cr in the matrix decreases, reducing corrosion resistance. Similarly, as the chlorination reaction progresses, the reaction products evaporate, reducing the amount of Cr in the matrix and reducing corrosion resistance. The addition of Si is known to be effective in suppressing this decrease in corrosion resistance due to a decrease in the amount of Cr in the matrix.

[0021] The present inventors have further found that adding a large amount of Ni stabilizes the austenite phase, suppresses the precipitation of the σ phase, and improves σ embrittlement resistance. They have also found that the Ni remains without corroding, thereby improving corrosion resistance.

[0022] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0023] (1) In mass%, C: 0.08% or less, Si: 2.50% or more and 4.00% or less, Mn: 0.30% or more and 2.00% or less, Cr: 17.00% or more and 30.00% or less, Ni: over 45.00% and 57.00% or less, Nb: 0% or more and 3.00% or less, Mo: 0% or more and 5.00% or less, Se: 0% or more and 0.50% or less, Te: 0% or more and 0.10% or less, Bi: 0% or more and 0.50% or less, W: 0% or more and 2.00% or less, Ti: 0% or more and 2.00% or less, Al: 0% or more and 2.00% or less 、M An austenitic stainless steel casting characterized by containing g: 0% or more and 0.20% or less, and N: 0% or more and 0.30% or less, with the remainder being Fe and impurities.

[0024] (2) The austenitic stainless steel casting of (1) above, characterized in that it contains, by mass%, Ni: 48.00% or more and 57.00% or less.

[0025] ( 3 ) (1) or (2) Incinerator grates made of austenitic stainless steel castings. [Effects of the Invention]

[0026] According to the present invention, it is possible to provide an austenitic stainless steel casting that has high corrosion resistance and σ-embrittlement resistance in a complex gas atmosphere at high temperatures and is suitable for use, for example, incinerator grates. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described in detail below. First, the chemical composition of the heat-resistant and wear-resistant cast iron of the present invention will be described. Hereinafter, "%" in the chemical composition will mean "% by mass."

[0028] C combines with Cr in the steel and precipitates as Cr carbides at grain boundaries. When Cr precipitates as carbides, the Cr concentration in the matrix decreases, resulting in poor high-temperature oxidation resistance. Therefore, a low C content is preferable. Therefore, the C content is set to 0.08% or less, preferably 0.05% or less.

[0029] Silicon acts as a deoxidizer and is necessary for improving oxidation resistance and corrosion resistance, especially at high temperatures. To fully achieve this effect, the silicon content is set to 2.50% or more, preferably 2.70% or more. In consideration of σ embrittlement, the silicon content is set to 4.00% or less, preferably 3.50% or less.

[0030] Mn is an austenite-forming element and is also a useful component as a deoxidizer and desulfurizer. To fully obtain these effects, the Mn content must be 0.30% or more. From the perspective of preventing embrittlement of cast iron and a decrease in creep strength, the upper limit of the Mn content is set to 2.00% or less.

[0031] Cr is an element that improves high-temperature strength and oxidation resistance at high temperatures. To fully obtain this effect, the Cr content is set to 17.00% or more, preferably 19.0% or more. In an environment where molten chlorides adhere, such as a biomass boiler, if the Cr content is too high, volatile Cr2O2Cl2 is formed and high-temperature oxidation resistance is reduced, so the Cr content is set to 30.00% or less, preferably 28.00% or less.

[0032] Ni is an austenite-forming element that suppresses the precipitation of the σ phase and improves σ embrittlement resistance. It also improves high-temperature strength and corrosion resistance at high temperatures. Furthermore, Ni remains uncorroded, improving corrosion resistance. It also increases the amount of work hardening and enhances wear resistance. To achieve this effect, the Ni content is set to more than 45.00%, preferably 46.00% or more, and more preferably 48.00% or more. Considering the balance between material cost and effect, the Ni content is set to 57.00% or less, preferably 56.00% or less, and more preferably 54.00% or less.

[0033] Nb easily forms carbides, so it can fix C in the steel and suppress the precipitation of Cr carbides, preventing a decrease in high-temperature strength. Even a small amount of Nb has the effect of suppressing the precipitation of Cr carbides, but to fully obtain the effect of its addition, it is preferable to add 0.20% or more, and more preferably 0.50% or more. For each element, the effect saturates when the content exceeds 3.00%, so the upper limit is set to 3.00%, preferably 2.50%.

[0034] The austenitic stainless steel castings of the present invention can further contain Mo. Mo dissolves in austenite and has the effect of increasing wear resistance. This effect can be obtained even with a small amount added, but to fully utilize the effect of the addition, it is preferable to keep the Mo content at 1.00% or more. Even if the Mo content is high, the effect saturates and there is a possibility of a decrease in toughness due to segregation, so the upper limit is set at 3.00%.

[0035] The austenitic stainless steel casting of the present invention may further contain one or more of Se, Te, and Bi.

[0036] Se is an element that improves the machinability of austenitic stainless steels, which are generally difficult to cut, and can be added as needed. This effect can be achieved with a small amount, but it is most effective when added in amounts of 0.001% or more. If the Se content is too high, the effect saturates, so the upper limit is set at 0.50%.

[0037] Te, like Se, is an element that improves machinability and can be added as needed. This effect can be obtained with a small amount, but it is most effective when added in amounts of 0.001% or more. If the Te content is too high, the effect saturates, so the upper limit is set at 0.10%.

[0038] Like Se and Te, Bi is an element that improves machinability and can be added as needed. This effect can be achieved with a small amount, but it is most effective when added in amounts of 0.001% or more. If the Bi content is too high, the effect saturates, so the upper limit is set at 0.50%.

[0039] Adding Se, Te or Bi has the effects of reducing cutting resistance, extending tool life, suppressing built-up edge, improving finished surface properties and improving chip breakability.

[0040] However, the addition of these elements reduces hot workability, making them unsuitable for forging or rolling. However, since the austenitic stainless steel castings of this invention are as-cast, the reduction in hot workability is not a major problem, and the addition of Se, Te, and Bi is effective in improving machinability.

[0041] The austenitic stainless steel casting of the present invention may further contain one or more of W, Ti, and Al.

[0042] W easily forms carbides, and can fix the C in the steel, suppressing the precipitation of Cr carbides and preventing a decrease in high-temperature strength. This effect can be achieved with a small amount of addition, and is effective when added in amounts of 0.001% or more. If the W content is too high, the effect will saturate, so the upper limit is set at 2.00%.

[0043] Like W, Ti is an element that can suppress the precipitation of Cr carbides and prevent a decrease in high-temperature strength. This effect can be obtained even with a small amount added, and it is effective when added in amounts of 0.001% or more. If the Ti content is too high, the effect will saturate, so the upper limit is set at 2.00%.

[0044] Like W and Ti, Al is an element that can suppress the precipitation of Cr carbides and prevent a decrease in high-temperature strength. This effect can be achieved even with a small amount added, and it is effective when added in amounts of 0.001% or more. If the Al content is too high, the effect will saturate, so the upper limit is set at 2.00%.

[0045] The austenitic stainless steel casting of the present invention further comprises , Mg It can be contained.

[0046] Mg Toughness Mg is an element that can suppress the deterioration of mechanical properties. This effect can be obtained even with a small amount of addition, and it is effective when added in an amount of 0.001% or more. If the Mg content is too high, the effect will saturate, so the upper limit is set at 0.20%.

[0047] The austenitic stainless steel castings of the present invention can further contain nitrogen. N has the effect of preventing stress corrosion cracking and is an element that improves the strength of steel through solid solution strengthening. N also stabilizes austenite. These effects can be obtained with small amounts, but to fully utilize the effects of addition, it is preferable to keep the N content at 0.01% or more. Higher N content can lead to defects due to blowholes in the steel ingot, so the upper limit is set at 0.30%. Even if N is not intentionally added, it may be present as an impurity in steel at up to 0.02%. Unintentional inclusion of N at 0.02% or less does not adversely affect the austenitic stainless steel castings of the present invention.

[0048] The remainder of the composition is Fe and inevitable impurities. The inevitable impurities are those that are inevitably mixed in from raw materials or the manufacturing environment during the industrial production of castings having the composition specified in the present invention, such as P and S. P and S are usually unavoidably mixed in castings at approximately 0.030% or less.

[0049] 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 component composition is prepared, the molten metal is poured into a mold, and the poured molten metal is cooled and solidified. In principle, the cast iron of the present invention is used as cast, but can be subjected to solution treatment as needed.

[0050] By casting molten metal with the chemical composition of the present invention, it is possible to obtain austenitic stainless steel castings that have excellent σ-embrittlement resistance in the as-cast state without using any special manufacturing process.

[0051] The austenitic stainless steel castings of the present invention are steels that are not subjected to plastic processing after being poured into a mold of a predetermined shape, and are therefore distinct from alloys that have undergone plastic processing such as hot rolling or forging. In other words, while castings retain the solidification structure as they are cast, alloys that have undergone plastic processing have an alloy structure that is created by processing, and the structures are significantly different. In the castings of the present invention, the grain size is approximately 0.2 to 10.0 mm. [Example]

[0052] The present invention will be described in more detail below using examples. The examples given below are examples of embodiments of the present invention, and it goes without saying that the present invention is not limited to the following examples.

[0053] Castings having the component compositions shown in Table 1 were manufactured and evaluated for corrosion resistance and σ embrittlement resistance. Note that blank spaces in Table 1 indicate that the element in question was not intentionally added.

[0054] Corrosion resistance was evaluated by taking a φ15mm x 20L round bar test piece from the manufactured casting and immersing it in 10g of molten salt (NaCl-KCl (1:1)) at 700℃ for 100 hours in a corrosion test, after which the test piece was cut and the corrosion depth of the cross section was measured. As a result, a corrosion depth of 0.60mm or less was judged to be excellent in corrosion resistance.

[0055] The σ-embrittlement resistance was evaluated by subjecting the produced castings to aging treatment at 800°C for 144 hours and then conducting a room-temperature tensile test. If the elongation in the room-temperature tensile test after aging treatment is less than 6%, there is a risk of cracking due to σ-embrittlement, so those with an elongation of 6% or more were judged to have excellent σ-embrittlement resistance.

[0056] The results are shown in Table 1.

[0057] No. 1 to No. 2 shown in Table 1 22、24、 25 is an example of the invention using the steel of the present invention, No. 23 is a reference example,Nos. 31 to 33 are comparative examples using common steel. As shown in Table 1, it was confirmed that the castings of the present invention are excellent in corrosion resistance and σ embrittlement resistance in the high temperature range.

[0058] [Table 1]

Claims

1. In mass%, C: 0.08% or less, Si: 2.50% or more, 4.00% or less, Mn: 0.30% or more, 2.00% or less, Cr: 17.00% or more, 30.00% or less, Ni: more than 45.00%, less than 57.00%, Nb: 0% or more, 3.00% or less, Mo: 0% or more, 5.00% or less, Se: 0% or more, 0.50% or less, Te: 0% or more, 0.10% or less, Bi: 0% or more, 0.50% or less, W: 0% or more, 2.00% or less, Ti: 0% or more, 2.00% or less, Al: 0% or more, 2.00% or less, Mg: 0% or more and 0.20% or less, and N: 0% or more, 0.30% or less and the balance being Fe and impurities.

1. An austenitic stainless steel casting characterized by:

2. An austenitic stainless steel casting as described in claim 1, characterized in that it contains, by mass%, Ni: 48.00% or more and 57.00% or less.

3. 3. An incinerator grate made of the austenitic stainless steel casting according to claim 1 or 2.

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