EP4606920A1

EP4606920A1 - Ferritic stainless steel

Info

Publication number: EP4606920A1
Application number: EP23903681.7A
Authority: EP
Inventors: Junghyun KONG; Jonghee Kim; Bosung Seo; Jinsuk Kim; Youngjin Kwon
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-16
Filing date: 2023-09-04
Publication date: 2025-08-27
Also published as: KR20240094684A; CN120344694A; EP4606920A4; JP2025540848A; WO2024128477A1

Abstract

Disclosed herein is a ferritic stainless steel, including, in percent by weight (wt%), 0.003 to 0.02% of carbon (C), 0.003 to 0.02% of nitrogen (N), 0.05 to 0.5% of silicon (Si), 0.1 to 1.5% of manganese (Mn), 19.0 to 25.0% of chromium (Cr), 0.01 to 2.0% of molybdenum (Mo), 0.05 to 0.7% of niobium (Nb), 0.01 to 0.2% of titanium (Ti), the remainder of iron (Fe), and inevitable impurities, and satisfying Formula (1): 35≤ Cr / Mn≤ 60, wherein Cr and Mn represent the content (wt%) of the respective elements.

Description

[Technical Field]

The present disclosure relates to a ferritic stainless steel, and more particularly, to a ferritic stainless steel having excellent high-temperature conductivity by controlling a ratio of Cr and Mn or a thickness of an oxide scale.

[Background Art]

Stainless steel is applied in various fields from room temperature to high temperature due to its excellent corrosion resistance and oxidation resistance. Many studies have been conducted to manufacture a stainless steel component such as a separator plate of a fuel cell operating in a high-temperature environment.
To apply stainless steel to high-temperature fuel cells, the thickness of the scale formed on the surface of the stainless steel in a high-temperature oxidizing environment should not become excessively thick, and electrical conductivity should not be reduced. In the case where the thickness of the scale becomes thicker than a predetermined level, the scale may be exfoliated, damaging the material, and in the case where the electrical conductivity is low, the efficiency of the fuel cell may be reduced.
When stainless steel is oxidized, chromium oxide (Cr₂O₃) is formed on the surface, and corrosion resistance may be achieved due to the oxide scale composed of chromium oxide. However, the formed scale has excellent corrosion resistance but low electrical conductivity. Accordingly, stainless steel with excellent electrical conductivity is required.

[Disclosure]

[Technical Problem]

The present disclosure provides a ferritic stainless steel having excellent high-temperature conductivity by controlling a ratio of Cr and Mn and/or a thickness of an oxide scale.
The technical aspects that may be achieved by the disclosure are not limited to the above-mentioned aspects, and other technical aspects not mentioned will be clearly understood by one of ordinary skill in the technical art to which the disclosure belongs from the following description.

[Technical Solution]

According to an embodiment of the present disclosure, a ferritic stainless steel may include, in percent by weight (wt%), 0.003 to 0.02% of carbon (C), 0.003 to 0.02% of nitrogen (N), 0.05 to 0.5% of silicon (Si), 0.1 to 1.5% of manganese (Mn), 19.0 to 25.0% of chromium (Cr), 0.01 to 2.0% of molybdenum (Mo), 0.05 to 0.7% of niobium (Nb), 0.01 to 0.2% of titanium (Ti), the remainder of iron (Fe), and inevitable impurities, and satisfying Formula (1): 35≤ Cr / Mn≤ 60, wherein Cr and Mn represent the content (wt%) of the respective elements.
The ferritic stainless steel according to an embodiment of the present disclosure may have an 800°C high-temperature interface conductivity of 40 mΩcm² or less.
The ferritic stainless steel according to an embodiment of the present disclosure may have a thickness of an oxide scale of 3.5µm or less.
The ferritic stainless steel according to an embodiment of the present disclosure may have a diameter of an oxide scale of 3.0µm or less.
The ferritic stainless steel according to an embodiment of the present disclosure may have the oxide scale mainly including Cr, Mn, and O.

[Advantageous Effects]

According to an embodiment of the present disclosure, a ferritic stainless steel may obtain high-temperature interface conductivity by considering a correlation between a ratio of Cr and Mn and electrical conductivity, and forming a fine and uniform oxide scale in a high-temperature oxidizing environment.

[Description of Drawings]

FIG. 1 is a diagram showing a composition of an oxide scale of Inventive Example 1.
FIG. 2 is a view showing shapes of oxide scales of Inventive Example 1 and Inventive Example 2.
FIG. 3 is a view showing shapes of oxide scales of Comparative Example 2 and Comparative Example 3.

[Mode for Invention]

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. However, the embodiments of the present disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.
The terms used herein are merely used to describe particular embodiments. An expression used in the singular encompasses the expression of the plural, unless otherwise indicated. Throughout the specification, the terms such as "including" or "having" are intended to indicate the existence of features, operations, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, operations, functions, components, or combinations thereof may exist or may be added.
Meanwhile, unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Thus, these terms should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.
In addition, the terms "about", "substantially", etc. used throughout the specification means that when a natural manufacturing and a substance allowable error are suggested, such an allowable error corresponds the value or is similar to the value, and such values are intended for the sake of clear understanding of the present disclosure or to prevent an unconscious infringer from illegally using the disclosure of the present disclosure.
According to an embodiment of the present disclosure, a ferritic stainless steel may include, in percent by weight (wt%), 0.003 to 0.02% of carbon (C), 0.003 to 0.02% of nitrogen (N), 0.05 to 0.5% of silicon (Si), 0.1 to 1.5% of manganese (Mn), 19.0 to 25.0% of chromium (Cr), 0.01 to 2.0% of molybdenum (Mo), 0.05 to 0.7% of niobium (Nb), 0.01 to 0.2% of titanium (Ti), the remainder of iron (Fe), and inevitable impurities.
The reasons for limiting the compositional range of each alloying element are described below.
The content of C may be 0.003 to 0.02 wt%.
C is an element essentially contained in a stainless steel manufacturing process. An excess of C may form precipitates such as chromium carbides, adversely affecting the composition and oxidizing properties of the base material. Considering the above, the content of C may be 0.02% or less. However, controlling the content of C to an extremely low level may cause excessive cost increases. Accordingly, the content of C may be 0.003% or more.
The content of N may be 0.003 to 0.02 wt%.
In the case where the content of N increases excessively, various nitrides may precipitate or the occurrence of pores may impair the quality. Considering the above, the content of N may be 0.02% or less. However, controlling the content of N to an extremely low level may cause excessive cost increases. Accordingly, the content of N may be 0.003% or more.
The content of Si may be 0.05 to 0.5 wt%.
Si requires to be strictly limited, because when the material is exposed to a high temperature, film-type precipitates are formed at the interface between the scale and the base material, and an insulating film is formed. Considering the above, the content of Si may be 0.5% or less. However, reducing the content of Si to less than 0.05% may require high-cost processes such as vacuum melting. Accordingly, the content of Si may be 0.05% or more.
The content of Mn may be 0.1 to 1.5 wt%.
Mn diffuses rapidly when stainless steel is oxidized at high temperatures, forming dense manganese/chromium oxides in the outer layer of the scale. Considering the above, Mn may be added in an amount of 0.1% or more. However, excessive addition of Mn may excessively promote the growth of the scale, causing exfoliation of the scale. Accordingly, the content of Mn may be 1.5% or less. According to the present disclosure, it is ensured that Mn has a correlation with electrical conductivity together with Cr, and electrical conductivity may be obtained by controlling Mn together with Cr.
The content of Cr may be 19.0 to 25.0 wt%.
Cr is an essential element for obtaining a corrosion resistance of stainless steel. Cr depletion due to oxidation over long periods in high-temperature oxidizing environments is to be prevented. Considering the above, Cr may be added in an amount of 19.0% or more. However, an increase in manufacturing costs and precipitation of chromium carbides, intermetallic compounds, etc., are to be prevented. Accordingly, the content of Cr may be 25.0% or less. According to the present disclosure, it is ensured that Cr has a correlation with electrical conductivity together with Mn, and electrical conductivity may be obtained by controlling Cr together with Mn.
The content of Mo may be 0.01 to 2.0 wt%.
Mo is an element that may increase the strength of the material in a high-temperature environment. Accordingly, Mo may be added in an amount of 0.01% or more. However, since Mo is a high-priced element, an increase in manufacturing cost require to be suppressed. Considering the above, the content of Mo may be 2.0% or less.
The content of Nb may be 0.05 to 0.7 wt%.
Due to its excellent oxidizing properties, Nb oxidizes at the scale/base material interface to form oxides, thereby suppressing the formation of insulating silicon oxides. Considering the above, the content of Nb may be 0.05% or more. However, adding excessive amounts of Nb may impair hot workability and increase manufacturing cost. Accordingly, the content of Nb may be 0.7% or less.
The content of Ti may be 0.01 to 0.2 wt%.
Ti may increase the strength of the material by forming internal oxides just below the interface between the base material and the scale, i.e., near the surface of the base material, at high temperatures. Considering the above, the content of Ti may be 0.01% or more. However, adding excessive amounts of Ti may increase manufacturing cost and form titanium oxides outside the scale. Accordingly, the content of Ti may be 0.2% or less.
The remaining component is iron (Fe). However, since unintended impurities may inevitably be introduced from raw materials or the surrounding environment during a typical manufacturing process, this may not be excluded. Since such impurities may be well known to those skilled in the art during a typical manufacturing process, details thereof are not described in this specification.
The ferritic stainless steel according to an embodiment of the present disclosure may be a ferritic stainless steel satisfying Formula (1): 35≤ Cr / Mn≤ 60. Here, Cr and Mn represent the content (wt%) of the respective elements.
The ferritic stainless steel according to an embodiment of the present disclosure may have an 800°C high-temperature interface conductivity of 40 mΩcm² or less.
In the case where the ratio of Cr and Mn is 35 or more and 60 or less, electrical conductivity, i.e., an 800°C high-temperature interface conductivity of 40 mΩcm² or less, may be obtained. From the above, the ratio of Cr and Mn has a correlation with electrical conductivity, and by controlling the ratio of Cr and Mn to 35 or more and 60 or less, excellent electrical conductivity, i.e., an 800°C high-temperature interface conductivity of 40 mΩcm² or less, may be obtained.
The ferritic stainless steel according to an embodiment of the present disclosure may have a thickness of an oxide scale of 3.5µm or less, and may have a diameter of an oxide scale of 3.0µm or less. The main components of the oxide scale of the present disclosure may be Cr, Mn, and O.
The oxide scale of the present disclosure may be CrMn oxide. The CrMn oxide may have excellent electrical conductivity due to its spinel structure compared to a thick Cr oxide formed on the surface of general stainless steel.
The oxide scale thickness of the present disclosure may be 3.5µm or less, and the oxide scale diameter of the present disclosure may be 3.0µm or less. Here, the oxide scale may be considered as the thickness and diameter of the CrMn oxide by controlling the ratio of Cr and Mn.
The oxide scale thickness of 3.5µm or less and the oxide scale diameter of 3.0µm or less indicate that a plate-like CrMn oxide is uniformly formed on the surface layer of the ferritic stainless steel. When CrMn oxide is uniformly formed, excellent electrical conductivity may be obtained. Even though the scale is uniformly formed such that the thickness of CrMn oxide is 3.5µm or less and/or the diameter is 3.0µm or less, the ratio of Cr and Mn should be 35 or more and 60 or less to achieve excellent electrical conductivity, i.e., an 800°C high-temperature interface conductivity of 40 mΩcm² or less.
The ferritic stainless steel may be exposed to an oxidizing environment of 300 ~ 900°C to form CrMn oxide on the surface of the ferritic stainless steel. In this instance, in the case where the component composition of the ferritic stainless steel corresponds to the range of the present disclosure and Cr/Mn satisfies 35 ~ 60, a fine and uniform CrMn oxide may be formed on the surface layer. By controlling the above, excellent electrical conductivity at high temperatures may be obtained, enough to obtain an 800°C high-temperature interface conductivity of 40 mΩcm² or less. Also, the fine and uniform CrMn oxide, as an oxide scale, may have a thickness of 3.5µm or less, and a diameter of 3.0µm or less.
The ferritic stainless steel of the present disclosure may be manufactured by referring to known methods for manufacturing ferritic stainless steel and is not limited to the manufacturing method described in the examples below. For example, an annealing temperature, a reduction ratio, and the like, in the following examples may be changed as required.
Hereinafter, the present disclosure is described in more detail through embodiments and drawings. However, the descriptions of the embodiments are only for illustrating the implementation of the present disclosure, and the present disclosure is not limited by the descriptions of the embodiments. This is because the scope of the rights of the present disclosure is determined by matters described in the scope of claims and matters reasonably inferred therefrom.

{Embodiments}

Table 1 below shows whether a ferritic stainless steel having the alloy compositions of Inventive Examples and Comparative Examples and a value of Formula (1) obtains a high-temperature interface conductivity.
The alloy compositions in Table 1 below were manufactured by 50kg ingot casting, reheated to a temperature of about 1240°C, and hot-rolled to a thickness of about 5.5mm. The hot-rolled material was annealed at a temperature of about 1050°C and cold-rolled to a thickness of about 2.0mm. The cold-rolled material was annealed at a temperature of about 1050°C, and 15mm * 15mm samples were prepared to produce ferritic stainless steel specimens.

The high-temperature interface conductivity of 40 mΩcm² or less is marked as "O" and the high-temperature interface conductivity exceeding 40 mΩcm² is marked as "X" to distinguish conductivity. The conductivity was determined by measuring an area specific resistance (ASR) for 500 hours at a temperature of 800°C on samples that had completed pre-heat treatment for 100 hours at 800°C.

[Table 1]

	C	Si	Mn	Cr	Mo	Ti	Nb	N	Formul a (1)	high-temper ature interfac e conduct ivity
Inventive Example 1	0.0060	0.11	0.46	22.4	0.20	0.048	0.50	0.0072	48.6	O
Inventive Example 2	0.0050	0.14	0.49	21.5	0.20	0.069	0.51	0.0060	43.9	O
Comparative Example 1	0.0060	0.12	0.29	21.6	0.20	0.035	0.50	0.0049	74.5	X
Comparative Example 2	0.0069	0.13	0.36	22.7	0.175	0.067	0.44	0.0102	63.7	X
Comparative Example 3	0.0050	0.11	0.66	22.5	0.20	0.050	0.49	0.0056	34.1	X
Comparative Example 4	0.0050	0.13	0.67	21.5	0.20	0.043	0.51	0.0060	32.1	X

In Inventive Example 1 and Inventive Example 2, the alloy composition and the value of Formula (1) satisfy the range of the present disclosure. It may be confirmed that 800°C high-temperature interface conductivity of 40 mΩcm² or less, i.e., high-temperature interface conductivity was obtained. In Comparative Example 1 to Comparative Example 4, the alloy composition satisfies the range of the present disclosure, but the value of Formula (1) is less than 35 or exceeds 60. Specifically, in Comparative Example 1 and Comparative Example 2, the values of Formula (1) are 74.5 and 63.7, respectively, which are cases where Cr is added in excess compared to Mn. In Comparative Example 3 and Comparative Example 4, the values of Formula (1) are 34.1 and 32.1, respectively, which are cases where Cr is added in a smaller amount compared to Mn. From the above, it may be confirmed that even though the alloy composition satisfies the range of the present disclosure, when the ratio of Cr and Mn deviates from the range of the present disclosure, the 800°C high-temperature interface conductivity, i.e., high-temperature conductivity, may not be obtained.

Table 2 below shows a thickness and a diameter of CrMn oxide of Inventive Example 1 and Inventive Example 2, and Comparative Example 2 and Comparative Example 3. The thickness of the CrMn oxide of the present disclosure was processed to a width of about 30µm and a depth of 10µm using Focused Ion Beam (FIB) and was observed by Scanning Electron Microscopy (SEM). The thickness was measured 3 times at arbitrary positions on the processed steel, and it was confirmed whether an average value was 3.5µm or less. The diameter of the CrMn oxide of the present disclosure was measured by observation with SEM. When the diameter of the CrMn oxide was 3.0µm or less, a plate-like CrMn oxide was observed, and when the diameter of the CrMn oxide exceeded 3.0µm, a single-type CrMn oxide was observed.

[Table 2]

	Thickness of CrMn oxide				Shape according to diameter of CrMn oxide
	1st	2nd	3rd	Average	Shape according to diameter of CrMn oxide
Inventive Example 1	2.60	1.74	2.03	2.12	plate-like
Inventive Example 2	1.86	2.67	2.20	2.24	plate-like
Comparative Example 2	4.65	3.48	2.90	3.67	single-type
Comparative Example 3	2.90	3.60	1.74	2.74	single-type

In Inventive Example 1 and Inventive Example 2 where the alloy composition and the value of Formula (1) satisfy the range of the present disclosure, the thicknesses of CrMn oxide are 2.12µm and 2.24µm, respectively, which are less than 3.5µm. In addition, the diameter of CrMn oxide is less than or equal to 3.0µm, and the CrMn oxide layer is observed to be plate-like. In the case of Comparative Example 2, which has a value of Formula (1) of 63.7 that exceeds 60, the thickness of the CrMn oxide is 3.67µm. In addition, the diameter of the CrMn oxide exceeds 3.0µm, resulting in a single-type CrMn oxide. Reviewing Table 1 and Table 2, it may be confirmed that when Formula (1) is not satisfied and the thickness of the CrMn oxide exceeds 3.5µm, conductivity may not be obtained at high temperatures.
In the case of Comparative Example 3, which has a value of Formula (1) of 34.1 that is less than 35, the thickness of the CrMn oxide is 2.74µm. In addition, the diameter of the CrMn oxide exceeds 3.0µm, resulting in a single-type CrMn oxide. Reviewing Table 1 and Table 2, even though Comparative Example 3 shows a thickness of 3.5µm or less, which corresponds to the CrMn oxide thickness range of the present disclosure, the ratio of Cr and Mn is less than 35, and thus the CrMn oxide layer is not formed densely and soundly, i.e., internal pores are formed. Accordingly, Comparative Example 3 does not obtain conductivity at high temperatures. In other words, even when the thickness of the CrMn oxide is 3.5µm or less, i.e., the thickness of the CrMn oxide is relatively thin, high-temperature conductivity may not be obtained in the case where the value of Formula (1) is less than 35. Thus, according to the present disclosure, the thickness of the CrMn oxide may be controlled by the alloy composition together with by determining whether Formula (1), which controls the ratio of Cr and Mn, is satisfied, and it may be confirmed that whether Formula (1) is satisfied is the critical factor for high-temperature conductivity.
Hereinafter, the present disclosure is described in more detail with reference to Tables 1 and 2 and the drawings.
FIG. 1 is a diagram showing an elemental composition of the CrMn oxide scale of Inventive Example 1, FIG. 2 is a diagram showing the surface and cross-sectional views of the CrMn oxide scale of Inventive Example 1 and Inventive Example 2, and FIG. 3 is a diagram showing the surface and cross-sectional views of the CrMn oxide scale of Comparative Example 2 and Comparative Example 3.
The elemental composition of the CrMn oxide of the present disclosure was analyzed using FIB SEM/EDS (Energy Dispersive Spectroscopy).
Referring to FIG. 1, it may be confirmed that the oxide scale of the present disclosure corresponds to CrMn oxide, which may obtain conductivity, rather than Cr oxide, and that the main components are Cr, Mn, and O.
The thickness of the CrMn oxide of the present disclosure was observed by SEM after processing to a width of about 30µm and a depth of 10µm using FIB. The surface and cross-section of the CrMn oxide were also observed by SEM.
Referring to FIG. 2 and FIG. 3, the CrMn oxide on the surface and cross-section of the Inventive Examples and Comparative Examples of the present disclosure may be seen. FIG. 2 is a photograph of the surface and cross-section of a stainless steel sample having the alloy composition corresponding to the Inventive Example, as observed by SEM after high-temperature oxidation at 800°C for 500 hours. FIG. 3 is a photograph of the surface and cross-section of a stainless steel sample having the alloy composition corresponding to the Inventive Example, as observed by SEM after high-temperature oxidation at 800°C for 500 hours.
In FIG. 2, in the case of Inventive Example 1 and Inventive Example 2 which satisfy the range of Formula (1) of the present disclosure, CrMn oxide with a diameter of 3.0µm or less is uniformly formed on the surface layer, and the plate-like CrMn oxide is observed. In addition, it may be confirmed that the thickness of the CrMn oxide is 3.5µm or less. Accordingly, it may be confirmed that the value of Formula (1) is to satisfy the range of the present disclosure and the CrMn oxide is to be finely and uniformly distributed in order to obtain excellent high-temperature electrical conductivity.
In FIG. 3, in the case of Comparative Example 2 and Comparative Example 3 which do not satisfy the range of Formula (1) of the present disclosure, CrMn oxide with a diameter exceeding 3.0µm is irregularly formed on the surface layer, and a single-type CrMn oxide is observed. However, in the case of Comparative Example 3, even though the thickness of the CrMn oxide is 3.5µm or less, it may be confirmed that high-temperature electrical conductivity was not obtained due to the CrMn oxide which is not formed densely and soundly.
In the ferritic stainless steel of the present disclosure, CrMn oxide is formed on the surface layer by exposure to a high-temperature oxidizing environment of 300 ~ 900°C, and in the case where the ratio of Cr and Mn satisfies 35 ~ 60, a fine and uniform CrMn oxide is formed on the surface layer, and thus it may be confirmed that the 800°C high-temperature interface conductivity of 40 mΩcm² or less is obtained.

Claims

A ferritic stainless steel, comprising, in percent by weight (wt%), 0.003 to 0.02% of carbon (C), 0.003 to 0.02% of nitrogen (N), 0.05 to 0.5% of silicon (Si), 0.1 to 1.5% of manganese (Mn), 19.0 to 25.0% of chromium (Cr), 0.01 to 2.0% of molybdenum (Mo), 0.05 to 0.7% of niobium (Nb), 0.01 to 0.2% of titanium (Ti), the remainder of iron (Fe), and inevitable impurities, and satisfying Formula (1) below, $35 \leq Cr / Mn \leq 60$
(wherein Cr and Mn represent the content (wt%) of the respective elements).
The ferritic stainless steel of claim 1, having an 800°C high-temperature interface conductivity of 40 mΩcm² or less.
The ferritic stainless steel of claim 1, having a thickness of an oxide scale of 3.5µm or less.
The ferritic stainless steel of claim 1, having a diameter of an oxide scale of 3.0µm or less.
The ferritic stainless steel of claim 3 or claim 4, wherein the oxide scale mainly includes Cr, Mn, and O.