JP7656463B2 - Austenitic stainless steel - Google Patents

Description

The present invention relates to an austenitic stainless steel with excellent resistance to red scale.

When austenitic stainless steel is used for applications such as exhaust gas route components, stove combustion, fuel cell components, etc., it is usually heated to high temperatures of 300 to 900°C during use. Furthermore, in the above applications, the austenitic stainless steel is used in an environment that contains water vapor, which can cause red scale, an iron-based oxide, to form on the surface.

In some cases, this red scale may fly off and adhere to other parts, causing adverse effects, and there is also a risk of reducing high-temperature strength due to thinning caused by oxidation. Therefore, austenitic stainless steels that are resistant to red scale in high-temperature steam atmospheres are desired. Various methods have been known to improve high red scale resistance.

Patent Document 1 discloses an austenitic stainless steel material with excellent red scale resistance that is obtained by adjusting the total amount of Si and Al contained and performing a mechanical polishing finish.

JP 2004-043903 A

According to the technology described in Patent Document 1, red scale can be prevented by promoting Cr diffusion in the steel. However, to improve red scale resistance using such conventional technology, the amount of added elements must be increased or manufacturing processes must be added, resulting in increased manufacturing costs.

One aspect of the present invention aims to realize an austenitic stainless steel that can be manufactured at low cost and has excellent resistance to red scale.

In order to solve the above problems, an austenitic stainless steel according to one embodiment of the present invention contains, by mass%, C: 0.12% or less, Si: 0.20 to 4.0%, Mn: 0.05 to 2.5%, P: 0.05% or less, S: 0.005% or less, Ni: 8.0 to 17.0%, Cr: 15 to 25.0%, Cu: 2.5% or less, Al: 0.01 to 1.0%, N: 0.35% or less, Mo: 3.0% or less, Nb: 0.8% or less, and Ti: 1.0% or less, with the balance being Fe and unavoidable impurities, and an oxide containing Cr, Si, and Al and having a major axis of 0.1 to 5 μm is formed on the surface, and the sum of the concentration of Cr and the concentration of at least one of Si and Al in the oxide is 4.0 to 12.0% by mass.

In one embodiment of the austenitic stainless steel of the present invention, the total concentration of Cr, Si, and Al in the oxide may be 5.0 to 12.0 mass%.

The austenitic stainless steel according to one embodiment of the present invention may further contain, by mass%, one or more of B: 0.01% or less, Ca: 0.0002-0.0050%, Hf: 0.001-0.50%, Zr: 0.01-0.50%, Sb: 0.005-0.50%, Co: 0.01-1.50%, W: 0.01-2.0%, Ta: 0.001-1.0%, Sn: 0.002-0.5%, Ga: 0.0002-0.30%, Mg: 0.0003-0.0050%, and rare earth elements: 0.001-0.20%.

According to one aspect of the present invention, it is possible to realize an austenitic stainless steel that can be manufactured at low cost and has excellent resistance to red scale.

1 is a flowchart showing an example of a method for producing an austenitic stainless steel according to an embodiment of the present invention.

One embodiment of the present invention will be described below. Note that the following description is provided to allow a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. In this specification, "A-B" indicates A or more and B or less. In this specification, the term "stainless steel" refers to a stainless steel material whose specific shape is not limited. Examples of this stainless steel material include steel plates, steel pipes, and bar steel.

<Composition of austenitic stainless steel>
The composition of components contained in an austenitic stainless steel according to one embodiment of the present invention is as follows.

The austenitic stainless steel according to one embodiment contains, by mass%, C: 0.12% or less, Si: 0.20-4.0%, Mn: 0.05-2.5%, P: 0.05% or less, S: 0.005% or less, Ni: 8.0-17.0%, Cr: 15-25.0%, Cu: 2.5% or less, Al: 0.01-1.0%, N: 0.35% or less, Mo: 3.0% or less, Nb: 0.8% or less, and Ti: 1.0% or less. In addition to the components shown below, the austenitic stainless steel is made of iron (Fe) or small amounts of impurities that are inevitably mixed in (unavoidable impurities).

(Carbon: C)
C is an austenite-forming element that stabilizes the austenite phase and is effective in maintaining non-magnetic properties. C is also an element that is effective in solid solution strengthening of the austenite phase. However, if C is contained in excess, the amount of carbides increases and corrosion resistance decreases. Therefore, in the austenitic stainless steel according to one embodiment, the C content is 0.12 mass% or less, and preferably 0.005 to 0.08 mass%.

(Silicon: Si)
Silicon is an element effective in improving red scale resistance. However, excessive silicon content can cause deterioration in toughness and workability. Therefore, in an austenitic stainless steel according to one embodiment, the silicon content is 0.20 to 4.0 mass%, and preferably 0.30 to 3.5 mass%.

(Manganese: Mn)
Mn is an austenite-forming element in austenitic stainless steel and improves the adhesion of scale. However, excessive Mn content promotes the generation of MnS, which is a corrosion starting point. Therefore, in the austenitic stainless steel according to one embodiment, the Mn content is 0.05 to 2.5 mass%, preferably 0.1 to 2.0 mass%.

(Rin: P)
An excessive content of P reduces workability, so in the austenitic stainless steel according to one embodiment, the P content is 0.05 mass % or less, and preferably 0.01 to 0.04 mass %.

(Sulfur: S)
Excessive S content promotes the formation of corrosion initiation points in austenitic stainless steel, so the S content in the austenitic stainless steel according to one embodiment is 0.005 mass% or less, and preferably 0.0001 to 0.002 mass%.

(Nickel: Ni)
Ni is an element that improves the corrosion resistance of austenitic stainless steel. However, excessive Ni content increases material costs. Therefore, in the austenitic stainless steel according to one embodiment, the Ni content is 8.0 to 17.0 mass%, and preferably 9.0 to 15.0 mass%.

(Chromium: Cr)
Cr is an essential element for forming a passive film and ensuring corrosion resistance. It is also effective for ensuring red scale resistance. However, excessive Cr content increases material costs and causes a decrease in toughness. Therefore, in an austenitic stainless steel according to one embodiment, the Cr content is 15.0 to 25.0 mass%, and preferably 17.0 to 21.0 mass%.

(Copper: Cu)
Cu is an element added to ensure high-temperature strength. However, excessive Cu content increases material costs. Therefore, in the austenitic stainless steel according to one embodiment, the Cu content is 2.5 mass% or less, and preferably 0.02 to 2.2 mass%.

(Aluminum: Al)
Al is an element effective for improving the corrosion resistance of austenitic stainless steel and for improving red scale resistance. Furthermore, Al is an element effective as a deoxidizer during steelmaking. However, if an excessive amount of Al is contained, there is a possibility that the surface quality may deteriorate. Therefore, in an austenitic stainless steel according to one embodiment, the content of Al is 0.01 to 1.0 mass%, and preferably 0.02 to 0.10 mass%.

(Nitrogen: N)
N is an austenite-forming element and is effective in maintaining non-magnetic properties. However, excessive N content will form nitrides with other elements, leading to hardening. Therefore, in the austenitic stainless steel according to one embodiment, the N content is 0.35 mass% or less, and preferably 0.005 to 0.30 mass%.

(Molybdenum: Mo)
Mo is an element added to ensure high-temperature strength and red scale resistance. However, excessive Mo content hardens the material, reduces workability, and increases material costs. Therefore, in the austenitic stainless steel according to one embodiment, the Mo content is 3.0 mass% or less, and preferably 0.01 to 2.5 mass%.

(Niobium: Nb)
Nb is an element added to ensure high-temperature strength. However, excessive Nb content may deteriorate workability and toughness. Therefore, in the austenitic stainless steel according to one embodiment, the Nb content is 0.8 mass% or less, and preferably 0.01 to 0.5 mass%.

(Titanium: Ti)
Ti is an element that improves red scale resistance and workability. However, if Ti is contained excessively, workability and surface quality may deteriorate. Therefore, in the austenitic stainless steel according to one embodiment, the Ti content is 1.0 mass% or less, and preferably 0.01 to 0.5 mass%.

<Other ingredients>
An austenitic stainless steel according to one embodiment of the present invention may further contain, by mass%, one or more of B: 0.01% or less, Ca: 0.0002 to 0.0050%, Hf: 0.001 to 0.50%, Zr: 0.01 to 0.50%, Sb: 0.005 to 0.50%, Co: 0.01 to 1.50%, W: 0.01 to 2.0%, Ta: 0.001 to 1.0%, Sn: 0.002 to 0.5%, Ga: 0.0002 to 0.30%, Mg: 0.0003 to 0.0050%, and rare earth elements: 0.001 to 0.20%.

(Boron: B)
B is an element that improves the secondary workability of molded products manufactured using austenitic stainless steel. However, if B is contained in excess, compounds such as Cr ₂ B are likely to be formed, which may deteriorate the red scale resistance. Therefore, in the austenitic stainless steel according to one embodiment, 0.01 mass % or less of B may be added as necessary, and preferably 0.0001 to 0.0030 mass % of B may be added.

(Calcium: Ca)
Ca is an element that promotes high-temperature oxidation resistance. However, excessive addition of Ca leads to a decrease in corrosion resistance. Therefore, in the austenitic stainless steel according to one embodiment, 0.0002 to 0.0050 mass % of Ca may be added as necessary, and preferably 0.0003 to 0.0040 mass % of Ca may be added.

(Hafnium: Hf)
Hf is an element that improves corrosion resistance, high-temperature strength, and oxidation resistance. However, excessive addition of Hf may lead to deterioration of workability and manufacturability. Therefore, in the austenitic stainless steel according to one embodiment, 0.001 to 0.50 mass % of Hf may be added as necessary, and preferably 0.005 to 0.10 mass % of Hf may be added.

(Zirconium: Zr)
Zr is an element that improves high-temperature strength, corrosion resistance, and high-temperature oxidation resistance. However, excessive addition of Zr leads to deterioration of workability and manufacturability. Therefore, in the austenitic stainless steel according to one embodiment, 0.01 to 0.50 mass % of Zr may be added as necessary, and preferably 0.02 to 0.30 mass % of Zr may be added.

(Antimony: Sb)
Sb is an element that improves high-temperature strength. However, excessive addition of Sb reduces weldability and toughness. Therefore, in the austenitic stainless steel according to one embodiment, 0.005 to 0.50 mass % of Sb may be added as necessary, and preferably 0.010 to 0.25 mass % of Sb may be added.

(Cobalt: Co)
Co is an element that improves high-temperature strength. However, excessive addition of Co leads to a decrease in toughness and therefore to a decrease in manufacturability. Therefore, in the austenitic stainless steel according to one embodiment of the present invention, 0.01 to 1.50 mass % of Co may be added as necessary, and preferably 0.03 to 1.0 mass % of Co may be added.

(Tungsten: W)
W is an element added to ensure high-temperature strength. However, excessive W content increases material costs. Therefore, in the austenitic stainless steel according to one embodiment, 0.01 to 2.0 mass % of W may be added as necessary, and preferably 0.03 to 1.50 mass % of W may be added.

(Tantalum: Ta)
Ta is an element that improves high-temperature strength. However, excessive addition of Ta reduces weldability and toughness. Therefore, in the austenitic stainless steel according to one embodiment, 0.001 to 1.0 mass % of Ta may be added as necessary, and preferably 0.010 to 0.50 mass % of Ta may be added.

(Tin: Sn)
Sn is an element that improves corrosion resistance and high-temperature strength. However, excessive addition of Sn may lead to a decrease in toughness and manufacturability. Therefore, in the austenitic stainless steel according to one embodiment, 0.002 to 0.50 mass % of Sn may be added as necessary, and preferably 0.005 to 0.20 mass % of Sn may be added.

(Gallium: Ga)
Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance. However, excessive addition of Ga reduces weldability and toughness. Therefore, in the austenitic stainless steel according to one embodiment, 0.0002 to 0.30 mass % of Ga may be added as necessary, and preferably 0.0010 to 0.20 mass % of Ga may be added.

(Magnesium: Mg)
In addition to being a deoxidizing element, Mg also refines the structure of the slab and improves formability. However, excessive addition of Mg leads to deterioration of corrosion resistance, weldability, and surface quality. Therefore, in the austenitic stainless steel according to one embodiment, 0.0003 to 0.0050 mass % of Mg may be added as necessary, and preferably 0.0005 to 0.0030 mass % of Mg may be added.

(Rare earth element: REM)
REM refers to a collective term for scandium (Sc), yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). REM may be added as a single element or as a mixture of multiple elements. REM is an element that improves the cleanliness of stainless steel and also improves high-temperature oxidation resistance. However, excessive addition increases the alloy cost and reduces manufacturability. Therefore, in the austenitic stainless steel according to one embodiment, 0.001 to 0.20 mass % of REM may be added as necessary, and preferably 0.005 to 0.10 mass % of REM may be added.

<Oxide containing Cr, Al and Si>
In an austenitic stainless steel according to one embodiment of the present invention, oxides containing Cr, Al and Si and having a major axis of 0.1 to 5 μm are present on the surface. In this specification, such oxides present on the surface of the austenitic stainless steel may be referred to as "oxides of the present invention."

The longest diameter of an oxide refers to the longest distance between any two points on the oxide surface. The oxide of the present invention shows an improvement effect on red scale resistance if the longest diameter is 0.1 to 5 μm. If the longest diameter of the oxide of the present invention is less than 0.1 μm, the effect of the oxide of the present invention in improving red scale resistance is low. Furthermore, if the longest diameter of the oxide of the present invention is greater than 5 μm, a Cr-poor layer is more likely to form and the design of the surface of the austenitic stainless steel is also reduced. The Cr-poor layer will be described later.

In one embodiment, an austenitic stainless steel has an oxide formed on the surface, the oxide having a major axis of 0.1 to 5 μm and containing Cr, Si, and Al, and the sum of the concentration of Cr and the concentration of at least one of Si and Al in the oxide is 4.0 to 12.0 mass%.

The oxide of the present invention on the surface of austenitic stainless steel can be detected and analyzed, for example, by a scanning electron microscope (SEM). The content of Cr, Al or Si in the oxide on the surface of austenitic stainless steel can be measured, for example, by an energy dispersive X-ray spectroscopy (EDS).

The reasons why the oxide of the present invention improves red scale resistance are believed to be as follows. First, the oxide of the present invention acts as a protective coating. Second, the oxide of the present invention grows by heating, and the oxygen partial pressure around the oxide of the present invention decreases. Since Al, Si, Cr, and Fe are more likely to be oxidized in that order, Al, Si, and Cr are oxidized preferentially over Fe. Therefore, the growth of the oxide of the present invention reduces the formation of red scale, which is an Fe-based oxide, and the formation of an oxide with excellent protective properties mainly composed of Cr, Al, and Si (e.g., Cr ₂ O ₃ ) becomes more preferential.

However, if the sum of the Cr concentration and the Al concentration and/or Si concentration in the oxide of the present invention present on the surface of the austenitic stainless steel becomes excessive, firstly, a Cr-poor layer with little Cr is formed on the surface. This is because if the austenitic stainless steel is not pickled under appropriate conditions after the final annealing process, a Cr-poor layer (on the parent phase side) remains at the interface between the oxide formed on the surface and the parent phase. In this case, Fe-based oxides tend to remain on the surface of the austenitic stainless steel even after the nitric hydrofluoric acid immersion process, reducing the red scale resistance. Secondly, there is a risk that the weldability of the austenitic stainless steel will be reduced due to the oxide of the present invention. Therefore, it is preferable that the sum of the Cr concentration and the Al concentration and/or Si concentration of the oxide of the present invention on the surface of the austenitic stainless steel is 12.0 mass% or less.

If the sum of the Cr concentration, Al concentration and/or Si concentration of the oxide of the present invention on the surface of austenitic stainless steel is less than 4.0 mass%, the effect of the oxide of the present invention in improving red scale resistance is not sufficient. Therefore, it is preferable to control the sum of the Cr concentration, Al concentration and/or Si concentration of the oxide of the present invention on the surface of austenitic stainless steel to within the range of 4.0 to 12.0 mass%.

Moreover, it is more preferable that the total of the Cr concentration, Al concentration, and Si concentration on the surface of the austenitic stainless steel is 5.0 to 12.0 mass%.

Conventionally, the general approach to improving red scale resistance was to increase the amount of Cr and Si added or to increase the number of manufacturing steps (surface polishing, pre-oxidation, etc.), which resulted in increased manufacturing costs. The inventors focused on oxides containing Cr, Al and Si on the surface of austenitic stainless steel. They then discovered that by controlling the concentrations of Cr, Al and Si contained in such oxides on the surface of austenitic stainless steel within a specified range, it is possible to realize an austenitic stainless steel with excellent red scale resistance.

The inventors have found that, for example, by using the following manufacturing method, the total of the Cr concentration, Al concentration and/or Si concentration of the oxide of the present invention on the surface is 4.0 to 12.0 mass %, and an austenitic stainless steel with excellent red scale resistance can be obtained.

<Method of manufacturing austenitic stainless steel>
The austenitic stainless steel according to one embodiment of the present invention is obtained, for example, as an austenitic stainless steel strip. Fig. 1 is a flow chart showing an example of a method for producing an austenitic stainless steel according to one embodiment. As shown in Fig. 1, the method for producing an austenitic stainless steel strip according to this embodiment includes a pretreatment step S1, a hot rolling step S2, an annealing step S3, a pickling step S4, a cold rolling step S5, a final annealing step S6, a sulfuric acid immersion step S7, a nitric acid electrolysis step S8, and a nitric hydrofluoric acid immersion step S9.

In the pretreatment step S1, first, a melting furnace in a vacuum or argon atmosphere is used to melt steel whose composition is adjusted to fall within the range of the present invention, and the steel is cast to produce a slab (steel ingot). After that, a slab piece for hot rolling is cut out from the slab. The slab piece is then heated to a temperature range of 1100 to 1300°C in an air atmosphere. The time for which the slab piece is heated and held at this temperature is not limited. When the pretreatment step is carried out industrially, the casting may be continuous casting.

The hot rolling process S2 is a process in which the slab obtained in the pretreatment process S1 is hot rolled to produce a hot-rolled steel strip of a specified thickness.

The annealing step S3 is a step in which the hot-rolled steel strip obtained in the hot rolling step S2 is heated to soften the steel strip. The annealing step S3 is a step that is performed as necessary, but does not have to be performed.

The pickling process S4 is a process in which scale adhering to the steel strip surface is washed off using a pickling solution such as hydrochloric acid or a mixture of nitric acid and hydrofluoric acid.

The cold rolling process S5 is a process in which the steel strip that has been descaled in the pickling process S4 is rolled even thinner.

The final annealing step S6 is a step in which the steel strip rolled in the cold rolling step S5 is heated to remove distortion and soften the steel strip. The annealing in the final annealing step S6 is performed at a temperature of 1000 to 1100°C depending on the alloy composition.

The sulfuric acid immersion step S7, the nitric acid electrolysis step S8, and the nitric hydrofluoric acid immersion step S9 are steps for leaving the oxide of the present invention on the surface of the austenitic stainless steel being manufactured. In other words, the sulfuric acid immersion step S7, the nitric acid electrolysis step S8, and the nitric hydrofluoric acid immersion step S9 can also be called surface oxide remaining steps.

The sulfuric acid immersion step S7 is a step in which the steel strip obtained in the final annealing step S6 is immersed in an aqueous sulfuric acid solution. The sulfuric acid concentration of the aqueous sulfuric acid solution in the sulfuric acid immersion step S7 is preferably 50 g/L or more, more preferably 60 g/L or more, and even more preferably 70 g/L or more. The liquid temperature of the aqueous sulfuric acid solution in the sulfuric acid immersion step S7 is preferably 40°C or more, and even more preferably 50°C or more. The immersion time of the steel strip in the aqueous sulfuric acid solution in the sulfuric acid immersion step S7 is preferably 20 to 50 seconds, and even more preferably 25 to 35 seconds.

The nitric acid electrolysis step S8 is a step in which the steel strip after the sulfuric acid immersion step S7 is electrolytically treated in an aqueous nitric acid solution. The nitric acid concentration of the aqueous nitric acid solution in the nitric acid electrolysis step S8 is preferably 130 g/L or more, more preferably 150 g/L or more. The liquid temperature of the aqueous nitric acid solution in the nitric acid electrolysis step S8 is preferably 40 to 60°C. The current density in the nitric acid electrolysis step S8 is preferably 50 mA/ ^cm2 or more, more preferably 70 mA/ ^cm2 or more. The electrolysis time in the nitric acid electrolysis step S8 is preferably 60 seconds or more. The upper limits of the nitric acid concentration, current density, and electrolysis time in the nitric acid electrolysis step S8 are not particularly limited, but it is preferable that the nitric acid concentration is 250 g/L or less, the current density is 150 mA/cm2 or ^less , and the electrolysis time is 180 seconds or less.

The nitric hydrofluoric acid immersion step S9 is a step in which the steel strip after the nitric acid electrolysis step S8 is immersed in a nitric hydrofluoric acid aqueous solution, which is a mixture of an aqueous nitric acid solution and an aqueous hydrofluoric acid solution. The nitric acid concentration of the nitric hydrofluoric acid aqueous solution in the nitric hydrofluoric acid immersion step S9 is preferably 130 to 150 g/L, and the hydrofluoric acid concentration is preferably 10 to 15 g/L. The liquid temperature of the nitric hydrofluoric acid aqueous solution in the nitric hydrofluoric acid immersion step S9 is preferably 40 to 60°C. The immersion time of the steel strip in the nitric hydrofluoric acid aqueous solution in the nitric hydrofluoric acid immersion step S9 depends on the component composition of the austenitic stainless steel, but is preferably 40 to 60 seconds.

The inventors have found that the above manufacturing method makes it possible to leave the oxide of the present invention, which has a major axis of 0.1 to 5 μm and contains Cr, Si, and Al, on the surface of austenitic stainless steel.

In addition, the austenitic stainless steel according to one embodiment of the present invention has a total concentration of Cr and at least one of Si and Al contained in the oxide of the present invention formed on the surface of 4.0 to 12.0 mass %. As a result, the austenitic stainless steel according to one embodiment does not require treatment such as surface polishing to improve red scale resistance, and can be manufactured at low cost.

An embodiment of the present invention will be described below. As inventive or comparative examples of austenitic stainless steel, steel materials No. 1 to 56 were produced as follows. First, the components shown in Table 1 below were used as raw materials, and the above-mentioned manufacturing method was carried out up to the final annealing step S6 under the following conditions:
Atmosphere of melting furnace in pretreatment step S1: vacuum Mass of slab piece produced in pretreatment step S1: 30 kg
Heating temperature of slab piece in pretreatment step S1: 1230° C.
Heating time of the slab in the pretreatment process S1: 2 hours Plate thickness after the hot rolling process S2: 5 mm
Annealing step S3: Not performed Pickling solution used in pickling step S4: Nitric hydrofluoric acid aqueous solution (3% hydrofluoric acid, 10% nitric acid)
Temperature of pickling solution in pickling process S4: 60° C.
・ Sheet thickness after cold rolling process S5: 2 mm
Annealing temperature in the final annealing step S6: 1000 to 1100° C. (varies depending on the alloy composition).

In this embodiment, the composition shown in Table 1 is shown in mass %. The remainder other than each component shown in Table 1 is Fe or a small amount of impurities that are inevitably mixed in. The underlines in Table 1 indicate that the range of each component contained in each stainless steel according to the comparative example of the present invention is outside the range of the present invention.

Next, in order to leave the oxide of the present invention on the surface of the steel strip, a sulfuric acid immersion step S7, a nitric acid electrolysis step S8, and a nitric hydrofluoric acid immersion step S9 were carried out under the following conditions:
Sulfuric acid concentration in sulfuric acid immersion step S7: 80 g/L
Sulfuric acid temperature in sulfuric acid immersion step S7: 60° C.
Nitric acid concentration in nitric acid electrolysis step S8: 200 g / L
Nitric acid liquid temperature in nitric acid electrolysis step S8: 40 to 60° C.
Current density in nitric acid electrolysis step S8: 80 mA / cm ²
Nitric acid/hydrofluoric acid aqueous solution used in nitric acid/hydrofluoric acid immersion step S9: nitric acid 130-150 g/L, hydrofluoric acid 10-15 g/L
Nitric hydrofluoric acid liquid temperature in the nitric hydrofluoric acid immersion step S9: 40 to 60°C.

The immersion time in sulfuric acid in the sulfuric acid immersion step S7 was 30 seconds. The electrolysis time in the nitric acid electrolysis step S8 was 60 seconds. The immersion time in the nitric hydrofluoric acid aqueous solution in the nitric hydrofluoric acid immersion step S9 was 60 seconds. The hydrofluoric acid concentration (g/L) in the nitric hydrofluoric acid immersion step S9 for each steel material is shown in Table 2.

For steel materials No. 1 to 56, the concentrations of Cr, Al, and Si contained in the oxides on the steel surface were measured, and the oxidation weight gain and weldability were evaluated using the methods described below.

<Oxide concentration measurement>
The analysis of Cr, Si, and Al-based oxides on the surface of each steel material was carried out as follows. Using a scanning electron microscope (SEM) SU5000 (manufactured by Hitachi High-Technologies Corporation), the oxides dispersed on the steel surface were analyzed at a magnification of 10,000 times. In addition, elemental analysis of the composition of the oxides was carried out using an energy dispersive X-ray spectroscopy (EDS) for electron microscopes (manufactured by Horiba, Ltd.) at an acceleration voltage of 15 kV, an analysis time of 60 seconds, and a beam diameter of 0.2 μm.

The area analyzed was 6 μm × 4 μm = ²⁴ μm2, and the size of the oxides measured was in the range of 0.1 to 5 μm in major axis. The analysis range was the area on the surface of each steel material where 10 or more oxides of this size were present within a range of ²⁴ μm2. Five locations within the analysis range that contained oxides with major axes of 0.1 to 5 μm were randomly selected, and spot analysis was performed by EDS with the analysis target elements being Fe, Ni, Cr, Si, Al, and O. The analysis result for each steel material was the average value of the five spot analyses.

The analysis results, including the concentrations of Cr, Si, and Al in the oxides on the surface of the steel material, and the concentrations of "Cr+Si", "Cr+Al", and "Cr+Si+Al", are shown in Table 2. "Cr+Si" indicates the sum of the Cr concentration and the Si concentration, while "Cr+Al" indicates the sum of the Cr concentration and the Al concentration. "Cr+Si+Al" indicates the sum of the Cr concentration, the Si concentration, and the Al concentration.

If the "Cr+Si+Al" concentration was within the range of 5.0 to 12.0 mass%, it was rated as "○" (good); if it was not within the range, it was rated as "×" (bad). All steel products that were rated as having a good result for "Cr+Si+Al" also met the range of 4.0 to 12.0 mass% for at least one of the results for "Cr+Si" and "Cr+Al".

<Evaluation of oxidation gain>
The red scale resistance of each steel material was evaluated based on the oxidation mass gain after oxidation testing in accordance with JIS Z2281. A test piece measuring 20 mm x 25 mm was cut from each steel material, and only the end face of the test piece was dry polished with a #600 abrasive. The test piece was continuously heated at 800°C for 100 hours in a nitrogen atmosphere containing 20% water vapor, after which the oxidation mass gain (mg/ ^cm2 ) was measured.

The oxidation test was performed twice for each steel material, and the average oxidation weight gain was calculated. The calculated oxidation weight gains are shown in Table 2. Regarding red scale resistance, an oxidation weight gain of less than 0.3 mg/ ^cm2 was evaluated as ○ (good), and an oxidation weight gain of 0.3 mg/cm2 or ^more was evaluated as × (poor).

<Weldability evaluation>
The weldability of each steel material was evaluated based on the total crack length after arc strike welding. The test pieces used for the evaluation were obtained by cutting out 50 mm squares from each steel material with a plate thickness of 2 mm. Arc strike welding was performed with a TIG welding machine at a current of 120 A for 3 seconds, a shielding gas of Ar at 10 mL/min, an electrode with a diameter of 2.4 mm with a protrusion length of 5 mm, and an arc length of 1 mm. The welding hot cracking susceptibility of each steel material was evaluated based on the presence or absence of cracks in the test pieces and the total crack length (mm). Table 2 shows the measurement results of the total crack length as the evaluation result of weldability. Regarding weldability, a total crack length of less than 5 mm was evaluated as ○ (good), and a total crack length of 5 mm or more was evaluated as × (bad).

<Results>
Table 2 shows the overall evaluation results, in which the element concentration of the oxide of the present invention on the surface, the oxidation weight gain, and the weldability of each steel material were all evaluated as good (◯) and the results were shown as bad (×) when at least one of them was evaluated as bad.

As shown in Table 2, all of the steel materials produced using steel types A1 to A9 having the composition of the present invention and with a hydrofluoric acid concentration in the nitric hydrofluoric acid immersion step S9 in the range of 10 to 15 g/L had a specified element concentration of the oxide of the present invention on the surface of 4.0 to 12.0 mass%. The specified element concentration refers to the sum of the Cr concentration and the Al concentration and/or Si concentration. Furthermore, all of the steel materials according to these invention examples had good red scale resistance and weldability.

On the other hand, in the steel materials made using steel types A1 to A9 and with a hydrofluoric acid concentration of 5 g/L or 7.5 g/L in the nitric hydrofluoric acid immersion step S9, the specified element concentration of the oxide of the present invention on the surface exceeded 12.0 mass%. The steel materials according to these comparative examples were evaluated as having poor oxidation weight gain and weldability.

In addition, all of the steel materials produced using steel types B1 to B5, which have compositions that deviate from the composition of the present invention, were evaluated as poor in at least one of the oxidation weight gain and weldability.

[Additional Notes]
The present invention is not limited to the above-described embodiments or examples, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the different embodiments and examples are also included in the technical scope of the present invention.

Claims (2)

An austenitic stainless steel containing, by mass%, C: 0.12% or less, Si: 0.20 to 4.0%, Mn: 0.05 to 2.5%, P: 0.05% or less, S: 0.005% or less, Ni: 8.0 to 17.0%, Cr: 15 to 25.0%, Cu: 2.5% or less, Al: 0.01 to 1.0%, N: 0.35% or less, Mo: 3.0% or less, Nb: 0.8% or less, and Ti: 1.0% or less, with the balance being Fe and unavoidable impurities,
An oxide containing Cr, Si and Al and having a major axis of 0.1 to 5 μm is formed on the surface,
In the oxide, the total of the concentration of Cr and the concentration of at least one of Si and Al is 4.0 to 12.0 mass %,
In the oxide, the total of a Cr concentration, a Si concentration, and an Al concentration is 5.0 to 12.0 mass %,
A test piece measuring 20 mm x 25 mm is cut out from the austenitic stainless steel in accordance with JIS Z2281, and only the end faces of the test piece are dry-polished with a #600 abrasive. The test piece is continuously heated at 800°C for 100 hours in a nitrogen atmosphere containing 20% water vapor, and the oxidation mass gain measured is less than 0.3 mg/ ^cm2 . The austenitic stainless steel according to claim 1, further containing, by mass%, one or more of B: 0.01% or less, Ca: 0.0002-0.0050%, Hf: 0.001-0.50%, Zr: 0.01-0.50%, Sb: 0.005-0.50%, Co: 0.01-1.50%, W: 0.01-2.0%, Ta: 0.001-1.0%, Sn: 0.002-0.5%, Ga: 0.0002-0.30%, Mg: 0.0003-0.0050%, and rare earth elements: 0.001-0.20%.

Images