EP4613907A1

EP4613907A1 - Austenitic stainless steel and method for manufacturing same

Info

Publication number: EP4613907A1
Application number: EP23907278.8A
Authority: EP
Inventors: Sangseok KIM; Minam PARK
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-20
Filing date: 2023-08-18
Publication date: 2025-09-10
Also published as: EP4613907A4; WO2024135988A1; KR20240097192A; JP2025541929A; TW202428907A; CN120344705A

Abstract

An austenitic stainless steel according to one embodiment of the present invention comprises, in wt%, 0.005-0.07% of C, 0.1-1.0% of Si, 0.1-2.0% of Mn, 6.0-9.0% of Ni, 16.0-19.0% of Cr, 0.01-0.30% of Nb, 0.01-0.20% of N, and the remainder in Fe and unavoidable impurities, wherein the steel satisfies formula (1) of Nb*N>0.015 (here, Nb and N refer to the weight percentages of the respective elements), comprises a (Cr,Fe)-Nb-N-based Z-phase precipitate, and has an average particle size of at most 2 µm at the center of thickness.

Description

[Technical Field]

The present invention relates to an austenitic stainless steel, and more specifically, to an austenitic stainless steel having excellent yield strength by controlling the alloy composition and precipitates to refine the grain size, and a method for manufacturing the same.

[Background Art]

Austenitic stainless steels are used for various applications such as transportation parts and construction parts due to their excellent formability, work hardenability, and weldability. However, the yield strength of 304 stainless steels or 301 stainless steels is typically in a range of 200 to 350 MPa, which limits the use in structural applications. In order to obtain higher yield strength in the general-purpose 300-series stainless steels, a temper rolling process is generally used. However, the temper rolling process may lead to an increased cost and may significantly degrade elongation of the material.
Accordingly, there is a demand for austenitic stainless steels having high yield strength without using a temper rolling process.
Patent document 0001 describes a method for manufacturing a 300 series stainless steel having minimal warpage even after half-etching, by performing temper rolling on cold rolled annealed material for use as a laser metal mask for photo etching and then performing stress relief (SR) heat treatment twice. However, the patent application does not include any technical details on structural parts having a thickness of 0.3 to 3.0 mm in relation to a manufacturing technology for controlling etching performance and warpage after etching.
Patent document 0002 suggests a method of manufacturing parts for nuclear power by performing long-term heat treatment for 48 hours or more in a range of 600 to 700°C, in order to achieve an average grain size of 10 µm or less. This method is difficult to implement in an actual production line due to low productivity, and also leads to increased manufacturing costs due to the long-term heat treatment method.

[Related Art Document]

(Patent Document 1) International Patent Publication No. 2016-043125 (Publication Date: 2016.03.24)
(Patent Document 2) Japanese Patent Publication No. 2020-050940 (Publication Date: 2020.04.02)

[Disclosure]

[Technical Problem]

To resolve the above-described issues, the present invention is directed to providing an austenitic stainless steel having high yield strength suitable for application to structural members and a method for manufacturing the same.
The technical objectives of the present invention are not limited to the above, and other objectives that are not described above will be clearly understood by those skilled in the art from the above detailed description.

[Technical Solution]

An austenitic stainless steel according to an example of the present invention may include, in percent by weight (wt%), 0.005 to 0.07% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 9.0% of Ni, 16.0 to 19.0% of Cr, 0.01 to 0.30% of Nb, 0.01 to 0.20% of N, the remainder being Fe and unavoidable impurities, and satisfy Formula (1) below, include a (Cr,Fe)-Nb-N based Z-phase precipitate, and have an average grain size of 2 µm or less at a thickness center region. Formula (1): Nb * N ≥ 0.015, wherein Nb and N represent the weight percent (wt %) of each element.
The austenitic stainless steel according to an example of the present invention may be an austenitic stainless steel in which the (Cr,Fe)-Nb-N based Z-phase precipitate has a formation temperature of 1150°C or higher. In addition, the Z-phase precipitate may have a diameter of 50 to 300 nm.
The austenitic stainless steel according to an example of the present invention may have a pitting potential of 250 mV or higher in a 3.5% NaCl solution at 30°C.
The austenitic stainless steel according to an example of the present invention may have a thickness of 0.3 mm or more and less than 3.0 mm.
The austenitic stainless steel according to an example of the present invention may have a yield strength of 930 MPa or higher.
A hot-rolled annealed austenitic stainless steel material according to an example of the present invention may include, in percent by weight (wt%), 0.005 to 0.07% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 9.0% of Ni, 16.0 to 19.0% of Cr, 0.01 to 0.30% of Nb, 0.01 to 0.20% of N, the remainder being Fe and unavoidable impurities, and satisfying Formula (1) below, and include a (Cr,Fe)-Nb-N based Z-phase precipitate, and have an average grain size of 10 µm or less at a thickness center region. $Nb * N \geq 0.015$ , wherein Nb and N represent the weight percent (wt %) of each element.
In the hot-rolled annealed austenitic stainless steel material according to an example of the present invention, the (Cr,Fe)-Nb-N based Z-phase precipitate may have a formation temperature of 1150°C or higher.
The hot-rolled annealed austenitic stainless steel material according to an example of the present invention may have a thickness of 3.0 mm or more, and the (Cr,Fe)-Nb-N based Z-phase precipitate may have a diameter of 2 µm or less.
A method of manufacturing an austenitic stainless steel according to an example of the present invention may include: casting a slab comprising, in percent by weight (wt%), 0.005 to 0.07% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 9.0% of Ni, 16.0 to 19.0% of Cr, 0.01 to 0.30% of Nb, 0.01 to 0.20% of N, the remainder being Fe and unavoidable impurities, and satisfying Formula (1) below; hot rolling the slab; hot-rolled annealing; cold rolling; and cold-rolled annealing at a temperature of 700 to 850°C, wherein the method includes a (Cr,Fe)-Nb-N based Z-phase precipitate after each of the hot-rolling annealing and the cold-rolling annealing. Formula (1): Nb * N ≥ 0.015, wherein Nb and N represent the weight percent (wt %) of each element.
In the method according to an example of the present invention, the (Cr,Fe)-Nb-N based Z-phase precipitate may have a formation temperature of 1150°C or higher.
In the method according to an example of the present invention, after the hot-rolled annealing, an average grain size at a thickness center region may be 10 µm or less.
In the method according to an example of the present invention, after the hot-rolled annealing, a hot-rolled annealed material may have a thickness of 3.0 mm or more,
In the method according to an example of the present invention, after the cold-rolling annealing, an average grain size at a thickness center region may be 2 µm or less.
In the method according to an example of the present invention, after the cold-rolling annealing, a cold-rolled annealed material may have a thickness of 0.3 mm or more and less than 3.0 mm.
In the method according to an example of the present invention, a pitting potential in a 3.5% NaCl solution at 30°C may be 250 mV or higher.
In the method according to an example of the present invention, a yield strength may be 930 MPa or higher.

[Advantageous Effects]

According to an embodiment of the present invention, precipitate an austenitic stainless steel having high yield strength applicable to structural members and a method for manufacturing the same can be provided by refining the grain size through control of the alloy composition and precipitates.

[Description of Drawings]

FIG. 1 is a view showing precipitates of a hot-rolled annealed material of Inventive Example 2.
FIG. 2 is a view showing precipitates of a hot-rolled annealed material of Comparative Example 1.
FIG. 3 is a view showing precipitates of a cold-rolled annealed material of Inventive Example 2.
FIG. 4 is a view showing precipitates of a cold-rolled annealed material of Comparative Example 1.
FIG. 5 is a view showing the microstructure of a hot-rolled annealed material of Inventive Example 2.
FIG. 6 is a view showing the microstructure of a hot-rolled annealed material of Comparative Example 1.
FIG. 7 is a view showing the microstructure of a cold-rolled annealed material of Inventive Example 2.
FIG. 8 is a view showing the microstructure of a cold-rolled annealed material of Comparative Example 1.

[Modes of the Invention]

The following describes preferred embodiments of the present invention. However, the embodiment of the present invention may be modified in various other forms, and the technical idea of the present invention is not limited to the embodiment described below. In addition, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.
Terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression unless the context clearly requires it to be singular. In addition, terms such as "comprises" or "including" as used in the present application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, and other features It should be noted that it is not used to preliminarily exclude the existence of elements, steps, functions, components, or combinations thereof.
On the other hand, unless otherwise defined, all terms used herein should be regarded as having the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Accordingly, unless explicitly defined herein, specific terms should not be construed in an unduly idealistic or formal sense. For example, the singular expression herein includes the plural expression unless the context clearly dictates otherwise.
In addition, in this specification, "about", "substantially", etc. are used in or close to the numerical value when manufacturing and material tolerances inherent in the stated meaning are presented, and to help the understanding of the present invention Accurate or absolute figures are used to prevent unfair use by unscrupulous infringers of the stated disclosure.
Hereinafter, an austenitic stainless steel according to an example of the present invention will be described.
An austenitic stainless steel according to an example of the present invention may include, in percent by weight (wt%), 0.005 to 0.07% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 9.0% of Ni, 16.0 to 19.0% of Cr, 0.01 to 0.30% of Nb, 0.01 to 0.20% of N, the remainder being Fe and unavoidable impurities.
The reason for limiting the composition range of each alloy element is described below.
The content of C may be 0.005 to 0.07 wt%.
C is an austenite phase stabilizing element, and the more it is added, the more effective it is in stabilizing the austenite phase. Considering this, it is required to add a content of C of 0.005% or more. However, when C is contained at 0.07% or more, it may lead to the formation of Cr-carbide during low-temperature annealing, which may lower the intergranular corrosion resistance. Considering this, the C content may be set to 0.005 to 0.07 wt%.
The content of Si may be 0.1 to 1.0 wt%.
Si is an element added as a deoxidizer during the steelmaking process. Addition of a certain amount of Si may contribute to formation a Si-Oxide on the passive film when a bright annealing process is performed, thereby improving the corrosion resistance of the steel. Considering this, the Si content needs to be added in an amount of 0.1% or more. However, when Si is contained at more than 1.0%, it may lowering the ductility of the steel. Considering this, the Si content may be set to 0.1 to 1.0 wt%.
The content of Mn may be 0.1 to 2.0 wt%.
Mn is an austenite phase stabilizing element, and the more it is added, the more it stabilizes the austenite phase. Considering this, the Mn content needs to be added in an amount of 0.1% or more. However, since excessive addition of Mn may impair corrosion resistance, the Mn content may be limited to 2.0% or less. Considering this, the Mn content may be set to 0.1 to 2.0 wt%.
The content of Ni may be 6.0 to 9.0 wt%.
Ni is an austenite phase stabilizing element, and since the more it is added, the more the austenite phase becomes stabilized, Ni may be added in an amount of 6.0% or more to soften the material. However, since excessive addition of Ni increases production costs, N may be added in an amount of 9.0% or less. Considering this, the Ni content may be set to 6.0 to 9.0 wt%.
The content of Cr may be 16.0 to 19.0 wt%.
Cr is an element that is essential for improving corrosion resistance. Considering this, Cr needs to be added in an amount of 16.0% or more. However, excessive Cr content may harden the material and suppress strain-induced martensite during cold rolling. Therefor, the content of Cr may be limited to 19.0% or less. Considering this, the Cr content may be set to 16.0 to 19.0 wt%.
The content of Nb may be 0.01 to 0.30 wt%.
Nb is essential for forming a Z-phase precipitate corresponding to (Cr,Fe)-Nb-N system. The formation of Z-phase precipitate by including Nb may suppress grain growth. Considering this, the Nb content needs to be added in an amount of 0.01% or more. However, when the content is excessive, defects such as nitrogen pores being formed during the casting process may occur. Therefore, the Nb content may be limited to 0.30% or less. Considering this, the content of Nb may be set to 0.01 to 0.30 wt%.
The content of N may be 0.01 to 0.20 wt%.
N is an austenite phase stabilizing element, and the more it is added, the more it may stabilize the austenite phase and improve the strength. In addition, N may form a Z-phase precipitate together with Nb. Considering this, the content of N may be 0.01% or more. However, when the content is excessive, it may cause hardening and degrade hot workability. Therefore, the N content may be limited to 0.20% or less. Considering this, the content of N may be set to 0.01 to 0.20 wt%.
In addition, the austenitic stainless steel according to an example of the present invention may satisfy Formula (1) below. $Nb * N \geq 0.015$
Here, Nb and N represent the weight percent (wt %) of each component. As described above from the reason for including Nb and N elements, in order to form a Z-phase precipitate corresponding to the (Cr,Fe)-Nb-N system, Nb * N corresponding to Formula (1) may be set to 0.015 or more. When Nb * N is 0.015 or more, the formation temperature of the Z-phase may be increased. By increasing the formation temperature of the Z-phase, the precipitate is not dissolved even at high hot-rolled annealing temperatures and cold-rolled annealing temperatures, allowing the Z-phase precipitate to be present in both a hot-rolled material and a cold-rolled material. Preferably, Nb * N may be 0.020 or more, more preferably 0.025 or more, and even more preferably 0.030 or more.
The remaining component(s) of the disclosed invention is iron (Fe). However, unintended impurities may inevitably be introduced from raw materials or the surrounding environment in a typical manufacturing process, and thus cannot be excluded. Since such impurities may be well known to those skilled in the art of conventional manufacturing processes, details thereof are not described in this specification.
An austenitic stainless steel according to an example of the present invention may include a (Cr,Fe)-Nb-N based Z-phase precipitate.
When the Z-phase precipitate exists, grain growth at high temperatures may be suppressed. The Z-phase precipitates are generated during heat treatment and not dissolved during hot-rolled annealing and cold-rolled annealing, thereby preventing grains from coarsening. In contrast, precipitates such as chromium carbides (Cr-carbides) and chromium nitrides (Cr-nitrides) are not formed before heat treatment. At conventional hot-rolled annealing temperature, precipitates such as Cr-carbide and Cr-nitride are dissolved and become solid-solutioned into the matrix. Therefore, unlike the Z-phase precipitate, it may be difficult to obtain the effect of suppressing grain growth through precipitation during heat treatment.
The type of precipitates of the austenitic stainless steel according to an example of the present invention was observed using the transmission electron microscope (TEM) replica method at a thickness center region of the austenitic stainless steel. The thickness center region of the austenitic stainless steel according to an example of the present invention refers to a region between 1/4t and 3/4t when the thickness of the austenitic stainless steel is t.
The formation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitate of the austenitic stainless steel according to an example of the present invention may be 1150°C or higher, and the diameter of the (Cr,Fe)-Nb-N based Z-phase precipitate of the austenitic stainless steel may be 50 to 300 nm.
The formation temperature of the precipitate of the austenitic stainless steel according to an example of the present invention refers to the formation temperature of the Z-phase, which is a (Cr,Fe)-Nb-N series precipitate calculated by the ThermoCalc precipitate analysis program.
The diameter of the precipitate of the austenitic stainless steel according to an example of the present invention was measured by observing the thickness center region of the austenitic stainless steel using the TEM replica method.
The austenitic stainless steel according to an example of the present invention may have a thickness of 0.3 mm or more and less than 3.0 mm, preferably 0.3 mm or more and 2.5 mm or less, and more preferably 0.4 to 2.0 mm.
The austenitic stainless steel according to an example of the present invention may be applied to structural uses such as automobile exterior panels and architectural parts. In the case of structural parts, the applicable material thickness may range from 0.3 mm or more and less than 3.0 mm, preferably 0.3 mm or more and less than 2.5 mm, and more preferably 0.4 to 2.0 mm. In the case of the present invention, even when the thickness of the austenitic stainless steel is 0.3 mm or more and less than 3.0 mm, excellent yield strength may be obtained.
The austenitic stainless steel according to an example of the present invention may have an average grain size of 2 µm or less at the thickness center region.
Conventionally, in order to ensure excellent yield strength, the austenitic phase has been transformed into a martensitic phase through cold rolling, followed by low-temperature annealing to achieve ultrafine grains. In the present invention, an austenitic stainless steel having excellent yield strength may be provided by ensuring a fine grain size through control of the alloy composition and precipitates. For example, a fine grain size may refer to an average grain size of 3 µm or less, and more specifically, 2 µm or less.
The center thickness of the austenitic stainless steel according to an example of the present invention refers to a region between 1/4t and 3/4t when the thickness of the austenitic stainless steel is t. In addition, the average at the thickness center region refers to the average value of measurements taken at three arbitrary points in the region between 1/4t and 3/4t. The grain size was observed and measured at three arbitrary points in the thickness center region using a TEM thin foil method.
The austenitic stainless steel according to an example of the present invention may have a pitting potential of 250 mV or higher in a 3.5% NaCl solution at 30°C.
The austenitic stainless steel according to an example of the present invention may have a yield strength of 930 MPa or more. It is possible to provide an austenitic stainless steel that may be used as a structural component for automobile exterior panels, building components, and the like by ensuring a high yield strength of 930 MPa or more.
The austenitic stainless steel according to an example of the present invention may correspond to a cold-rolled annealed material.
Hereinafter, a hot-rolled annealed austenitic stainless steel material according to an example of the present invention will be described.
The hot-rolled annealed austenitic stainless steel material according to an example of the present invention may include, in percent by weight (wt%), 0.005 to 0.07% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 9.0% of Ni, 16.0 to 19.0% of Cr, 0.01 to 0.30% of Nb, 0.01 to 0.20% of N, the remainder being Fe and unavoidable impurities.
In addition, the hot-rolled annealed austenitic stainless steel material according to an example of the present invention may satisfy Formula (1) below. $Nb * N \geq 0.015$
Here, Nb and N represent the weight percent (wt %) of each component.
The reasons for limiting the composition range of each alloy element and the value of Formula (1) are as described above with respect to the austenitic stainless steel according to the example of the present invention.
The hot-rolled annealed austenitic stainless steel material according to an example of the present invention may include a (Cr,Fe)-Nb-N based Z-phase precipitate.
The description of the (Cr,Fe)-Nb-N based Z-phase precipitate is as described above with respect to the austenitic stainless steel according to an example of the present invention.
The formation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitate of the hot-rolled annealed austenitic stainless steel material according to an example of the present invention may be 1150°C or higher, and the diameter of the precipitate may be 0.2 µm or lower.
By controlling the formation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitate of the hot-rolled annealed material to 1150°C or higher, similar to the austenitic stainless steel, the Z-phase precipitate may be present without being dissolved even during the hot-rolled annealing. Through the control, the presence of a Z-phase precipitate may be ensured even in the hot-rolled annealed austenitic stainless steel material according to an example of the present invention, thereby preventing grains from coarsening during the annealing.
The types of precipitates, the formation temperature of the precipitates, and the measurement method of the diameter of the precipitates of the hot-rolled annealed austenitic stainless steel material according to an example of the present invention are as described above with respect to the austenitic stainless steel according to an example of the present invention.
The hot-rolled annealed austenitic stainless steel material according to an example of the present invention may have a thickness of 3.0 mm or more. By controlling the thickness of the hot-rolled annealed material to 3.0 mm or more, the austenitic stainless steel manufactured from the hot-rolled annealed austenitic stainless steel material according to an example of the present invention may be applied to structural uses such as automobile exterior panels and building parts.
The hot-rolled annealed austenitic stainless steel material according to an example of the present invention may have an average grain size of 10 µm or less at the thickness center region.
In the hot-rolled annealed material, a fine grain size may refer, for example, to an average grain size of 10 µm or less.
The thickness center region of a hot-rolled annealed austenitic stainless steel material according to an example of the present invention refers to a region between 1/4t and 3/4t when the thickness of the hot-rolled annealed austenitic stainless steel material is t. In addition, the average grain size at the thickness center region refers to the average value of measurements taken at three arbitrary points in the region between 1/4t and 3/4t. The grain size was observed and measured at three arbitrary points in the thickness center region using an optical microscope (OM) method.
Hereinafter, a method of manufacturing an austenitic stainless steel according to an example of the present invention will be described.
According to an example of the present invention, a method for manufacturing an austenitic stainless steel include: casting a slab comprising, in percent by weight (wt%), 0.005 to 0.07% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 9.0% of Ni, 16.0 to 19.0% of Cr, 0.01 to 0.30% of Nb, 0.01 to 0.20% of N, the remainder being Fe and unavoidable impurities, and satisfying Formula (1) below; hot rolling the slab; hot-rolled annealing; cold rolling; and cold-rolled annealing at a temperature of 700 to 850°C. $Nb * N \geq 0.015,$ wherein, Nb and N represent the weight percent (wt %) of each component.
In the method of manufacturing an austenitic stainless steel according to an example of the present invention, a hot-rolled annealed material after the hot-rolled annealing may include a (Cr,Fe)-Nb-N based Z-phase precipitate, and a cold-rolled annealed material after the cold-rolled annealing may include a (Cr,Fe)-Nb-N based Z-phase precipitate .
The reasons for limiting the component range of each alloy element and the value of Formula (1) are as described with respect to the austenitic stainless steel according to an example of the present invention. Hereinafter, the manufacturing method and the Z-phase precipitate will be described in more detail.
The austenitic stainless steel corresponding to a hot-rolled annealed austenitic stainless steel material and a cold-rolled annealed austenitic stainless steel material according to an example of the present invention may include a (Cr,Fe)-Nb-N based Z-phase precipitate . In this case, the formation temperature of the Z-phase, which is a (Cr,Fe)-Nb-N series precipitate, calculated by the ThermoCalc precipitate analysis program, may be 1150°C or higher.
In the method of manufacturing an austenitic stainless steel according to an example of the present invention, the hot-rolled annealing may be an operation performed at 1000 to 1150°C. Recrystallization occurs only when the hot-rolled annealing temperature is 1000 to 1150°C. This may be considered a typical hot-rolled annealing temperature.
The Z-phase, which is a (Cr,Fe)-Nb-N series precipitate, is generated before the hot-rolled annealing. Therefore, by controlling the formation temperature of the Z-phase, which is a (Cr,Fe)-Nb-N series precipitate, to 1150°C or higher, the Z-phase may be generated before the hot-rolled annealing and may remain without being dissolved during the hot-rolled annealing. By controlling the Z-phase to be present in the hot-rolled annealed material, grain coarsening may be prevented in a subsequent processing.
In the method of manufacturing an austenitic stainless steel according to an example of the present invention, the cold rolling may have a reduction ratio of 40% or more. When the cold rolling reduction ratio is 40% or more, TRIP transformation may be induced. The reduction ratio may be 40% or more, and more specifically, 50% or more.
In the method of manufacturing an austenitic stainless steel according to an example of the present invention, after the hot rolled annealing, the hot rolled annealed material may have a thickness of 3.0 mm or more, and after the cold rolled annealing, the cold rolled annealed material may have a thickness of 0.3 mm or more and less than 3.0 mm.
In the method of manufacturing an austenitic stainless steel according to an example of the present invention, the cold rolled annealing may be an operation performed at a temperature of 700 to 850°C. When the cold rolled annealing temperature is 700°C or more, new nucleation of reverted austenite may occur. When the cold rolled annealing temperature is 850°C or higher, it is favorable for grain refinement. Considering this, the cold rolled annealing temperature in the present invention may be controlled to 700 to 850°C.
Similar to the hot rolled annealed material, the austenitic stainless steel corresponding to the cold rolled annealed material after the cold rolled annealing may ensure a fine grain size by including the Z-phase, which is a (Cr,Fe)-Nb-N series precipitate .
In the method of manufacturing an austenitic stainless steel according to an example of the present invention, after the hot rolled annealing, the average grain size at the thickness center region of the hot rolled annealed material may be 10 µm or less.
In the method of manufacturing an austenitic stainless steel according to an example of the present invention, after the cold rolled annealing, the average grain size at the thickness center region of the stainless steel corresponding to the cold rolled annealed material may be 2 µm or less.
The thickness center region and the meaning of the thickness center region are as described with respect to the austenitic stainless steel according to the above-described example of the present invention and the hot-rolled annealed austenitic stainless steel material according to the above-described example of the present invention.
In addition, an austenitic stainless steel having excellent yield strength may be obtained with the refined grains.
An austenitic stainless steel manufactured by a method of manufacturing an austenitic stainless steel according to the above-described example of the present invention may have a pitting potential of 250 mV or more in a 3.5% NaCl solution at 30°C. In addition, the yield strength may be 930 MPa or more. An austenitic stainless steel having high yield strength while ensuring corrosion resistance may be obtained.
Hereinafter, the present invention will be described in more detail through embodiments. However, the descriptions of the embodiments are only for illustrating the implementation of the present invention, and the present invention is not limited by the descriptions of the embodiments. This is because the scope of the rights of the present invention is determined by matters described in the scope of claims and matters reasonably inferred therefrom.

{Examples}

Table 1 below shows the alloy compositions and types of precipitates of the Inventive Examples and Comparative Examples.
A slab having an alloy composition according to Table 1 below was subjected to hot-rolling and then subjected to hot-rolled annealing at 1050°C, after which the types of precipitates of the hot-rolled annealed material were observed.
In addition, cold rolling with a reduction ratio of 40% and cold rolled annealing at a temperature of 800°C were performed, and the types of precipitates of the cold-rolled annealed material having a thickness of 0.8 mm were observed.
FIG. 1 is a view showing precipitates of a hot-rolled annealed material of Inventive Example 2. FIG. 2 is a view showing precipitates of a hot-rolled annealed material of Comparative Example 1. FIG. 3 is a view showing precipitates of a cold-rolled annealed material of Inventive Example 2. FIG. 4 is a view showing precipitates of a cold-rolled annealed material of Comparative Example 1.

The types and sizes of the precipitates were observed using TEM Replica at the thickness center region of the hot-rolled annealed material and the cold-rolled annealed material.

[Table 1]

Class.	C	Si	Mn	Cr	Ni	N	Nb	Formul a(1)	Types of precipitates
Class.	C	Si	Mn	Cr	Ni	N	Nb	Formul a(1)	Hot-rolled annealed material	Cold-rolled annealed material
Inventive Example 1	0.03	0.53	1.24	17.5	6.7	0.17	0.21	0.0357	Z-phase	Z-phase and Cr nitride
Inventive Example 2	0.04	0.53	1.24	17.5	6.8	0.17	0.25	0.0425	Z-phase	Z-phase and Cr nitride
Inventive Example 3	0.03	0.30	0.46	17.3	7.2	0.15	0.26	0.0390	Z-phase	Z-phase and Cr nitride
Inventive Example 4	0.05	0.30	1.21	18.1	8.1	0.15	0.28	0.0420	Z-phase	Z-phase and Cr nitride
Comparative Example 1	0.04	0.53	1.24	17.5	6.7	0.10	0	0	none	Cr nitride
Comparative Example 2	0.03	0.30	0.46	17.3	6.8	0.15	0.01	0.0015	none	Cr nitride
Comparative Example 3	0.03	0.32	1.79	18.2	8.2	0.17	0.01	0.0017	none	Cr nitride
Comparative Example 4	0.04	0.45	0.51	18.1	8.1	0.04	0.02	0.0008	none	Cr carbide and Cr nitride

The alloy compositions of Inventive Examples 1 to 4 satisfy the range of the present invention. In particular, the content of Nb * N corresponds to 0.015 or more. In Inventive Examples 1 to 4, it may be confirmed that the type of precipitate observed includes the Z-phase, not only in the cold-rolled annealed material but also in the hot-rolled annealed material. FIG. 1 is a view showing the precipitate of the hot-rolled annealed material of Inventive Example 2, and FIG. 3 is a view showing the precipitate of the cold-rolled annealed material of Inventive Example 2. From FIGS. 1 and 3, the precipitates of Inventive Example 2 and the types of elements included in the precipitates may be identified. It may be confirmed that the precipitates of the hot-rolled annealed material and the cold-rolled annealed material of Inventive Example 2 correspond to precipitates composed of Cr, Fe, N, and Nb. Through this, it may be confirmed that the precipitates of the hot-rolled annealed material and the cold-rolled annealed material of Inventive Example 2 are Z-phase precipitates.
In contrast, Comparative Example 1 does not contain Nb at all. Since Z-phase corresponds to the (Cr,Fe)-Nb-N system, it may be confirmed that Comparative Example 1, which does not contain Nb at all, does not form Z-phase at all in both the cold-rolled annealed material and the hot-rolled annealed material.
FIG. 2 is a view showing the precipitates of the hot-rolled annealed material of Comparative Example 1, and FIG. 4 is a view showing the precipitates of the cold-rolled annealed material of Comparative Example 1.
Referring to FIG. 2, it may be confirmed that no precipitates were formed. In addition, since not precipitates were formed, the elements forming the precipitates may not be identified. Through this, it may be confirmed that when Nb is not included, Z-phase precipitates are not obtained, and also precipitates such as Cr-carbides and Cr-nitrides are dissolved before the hot-rolled annealing and therefore are not formed as precipitates.
Referring to FIG. 4, precipitates of the cold-rolled annealed material of Comparative Example 1 and the types of elements included in the precipitates are identified. It may be confirmed that precipitates in Comparative Example 1 are composed of Cr, Fe, and N. Through this, it may be confirmed that the precipitates in the cold-rolled annealed material of Comparative Example 1 are Cr-nitride precipitates.
Comparative Examples 2 to 4 have alloy compositions that satisfy the range of the present invention, but the value of Nb * N is less than 0.015. Therefore, the Z-phase precipitate formation temperature is not ensured, and the Z-phase precipitates may not be observed in the cold-rolled annealed material as well as the hot-rolled annealed material.
Referring to FIGS. 1 and 2, Inventive Example 2 according to an example of the present invention may have a significantly smaller diameter of precipitate in the hot-rolled annealed material of compared to Comparative Example 1. In addition, the presence of Cr, Fe, Nb, and N observed in Inventive Example 2 confirms that Z-phase precipitates were obtained in the hot-rolled annealed material. Since Comparative Example 1 does not include Nb, no precipitates were observed in the hot-rolled annealed material.
Referring to FIGS. 3 and 4, Inventive Example 2 according to an example of the present invention exhibits the presence of Cr, Fe, Nb, and N, confirming that Z-phase precipitates are obtained in the hot-rolled annealed material. In contrast, since Comparative Example 1 does not include Nb, and Cr, Fe, and N were observed in a cold-rolled annealed material, confirming that Cr nitride precipitates were formed.
Table 2 below shows the Z-phase formation temperature, average grain size, pitting potential, and yield strength of the cold-rolled annealed material according to the Inventive Examples and Comparative Examples.
The Z-phase formation temperature refers to the formation temperature of the Z-phase, which is a (Cr, Fe)-Nb-N series precipitate, calculated by the ThermoCalc precipitate analysis program.
The average grain size of the cold-rolled annealed material refers to the average value of values observed and measured at three arbitrary points in the thickness center region using a TEM thin foil method. The thickness center region refers to a region between 1/4t and 3/4t, when the thickness of the cold rolled annealed material is t.
The pitting potential refers to the value measuredin a 3.5% NaCl solution at 30°C.
The yield strength refers to the yield strength obtained after performing a tensile test at room temperature at a crosshead speed in a range of 10 mm/min to 20 mm/min using a JIS13B tensile test specimen.

FIG. 5 is a view showing the microstructure of a hot-rolled annealed material of Inventive Example 2, FIG. 6 is a view showing the microstructure of a hot-rolled annealed material of Comparative Example 1, FIG. 7 is a view showing the microstructure of a cold-rolled annealed material of Inventive Example 2, and FIG. 8 is a view showing the microstructure of a cold-rolled annealed material of Comparative Example 1. The microstructure of the hot-rolled annealed material was observed using an optical microscope (OM) method, and the microstructure of the cold-rolled annealed material was observed using a transmission electron microscope (TEM) thin foil.

[Table 2]

Class.	Z-phase formation temperature (°C)	Average grain size of cold rolled annealed material (µm)	Pitting Potential (mV)	Yield strength (MPa)
Inventive Example 1	1324	1.2	380	975
Inventive Example 2	1340	0.8	360	1002
Inventive Example 3	1347	1.5	340	958
Inventive Example 4	1352	0.9	390	994
Comparative Example 1	-	6.7	320	545
Comparative Example 2	1026	3.2	320	672
Comparative Example 3	1041	5.2	345	659
Comparative Example 4	989	7.2	290	453

Inventive Examples 1 to 4 have a Z-phase formation temperature of 1150°C or higher. Based on this, it was confirmed that when the alloy composition and the value of Formula (1) are satisfied, Z-phase precipitates may be observed not only in the cold-rolled annealed material but also in the hot-rolled annealed material. This is because the Z-phase formation temperature is 1150°C or higher, allowing Z-phase precipitates to remain undissolved even after hot-rolled annealing. Inventive Examples 1 to 4 satisfy an average grain size of 2 µm or less in the cold-rolled annealed materials. Based on this, it was confirmed that the pitting potential is 250 mV or higher and the yield strength is 930 MPa or higher. It is possible to provide an austenitic stainless steel having excellent corrosion resistance while enabling high yield strength. In contrast, in Comparative Example 1, which does not contain Nb at all, Z-phase precipitates may not be formed at all, and thus measurement of the formation temperature is not meaningful. The average grain size of the cold-rolled annealed material in Comparative Example 1 was found to be very large as 6.7 µm, and the yield strength was 545 MPa, indicating that high yield strength was not achieved.
In addition, Comparative Examples 2 to 4 have Z-phase precipitate formation temperatures of 1150 °C or lower. Therefore, all precipitates are completely dissolved before the hot-rolled annealing, and as shown in Table 1, no precipitates were observed in the hot-rolled annealed materials. In addition, the already dissolved Z-phase precipitates were not observed even after cold rolled annealing.
In Comparative Examples 2 to 4, Cr carbide and/or Cr nitride precipitates were observed instead. Cr carbide and/or Cr nitride precipitates are not precipitates that may refine grains. Therefore, it may be confirmed that Comparative Examples 2 to 4 have a coarse average grain size of 3.2 µm or more, and the yield strength was found to be only 672 MPa or less, indicating an inferior yield strength.
Referring to FIGS. 5 and 6, Inventive Example 2 according to an example of the present invention exhibits a significant finer grain size in the microstructure of the hot-rolled annealed material compared to Comparative Example 1.
Referring to FIGS. 7 and 8, Inventive Example 2 according to an example of the present invention exhibits a significant finer grain size in the microstructure of the cold-rolled annealed material compared to Comparative Example 1.
Based on this, it may be confirmed that by controlling the alloy composition of the present invention and Nb * N corresponding to Formula (1) to ensure the Z-phase precipitate formation temperature, Z-phase precipitates may be retained not only in the cold-rolled annealed material but also in the hot-rolled annealed material, and thus a fine average grain size of 2 µm or less is achieved in the cold-rolled annealed material, thereby providing an austenitic stainless steel having a high yield strength of 930 MPa or higher.

Claims

An austenitic stainless steel, comprising, in percent by weight (wt%),
0.005 to 0.07% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 9.0% of Ni, 16.0 to 19.0% of Cr, 0.01 to 0.30% of Nb, 0.01 to 0.20% of N, the remainder being Fe and unavoidable impurities, and

satisfying Formula (1) below,

including a (Cr,Fe)-Nb-N based Z-phase precipitate,

having an average grain size of 2 µm or less at a thickness center region, $Nb * N \geq 0.015$

wherein Nb and N represent the weight percent (wt %) of each component.
The austenitic stainless steel of claim 1, wherein the (Cr,Fe)-Nb-N based Z-phase precipitate has a formation temperature of 1150°C or higher.
The austenitic stainless steel of claim 1, wherein the (Cr,Fe)-Nb-N based Z-phase precipitate has a diameter of 50 to 300 nm.
The austenitic stainless steel of claim 1, having a pitting potential of 250 mV or higher in a 3.5% NaCl solution at 30°C.
The austenitic stainless steel of claim 1, having a yield strength of 930 MPa or higher.
The austenitic stainless steel of claim 1, having a thickness of 0.3 mm or more and less than 3.0 mm.
A hot-rolled annealed austenitic stainless steel material, comprising, in percent by weight (wt%), 0.005 to 0.07% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 9.0% of Ni, 16.0 to 19.0% of Cr, 0.01 to 0.30% of Nb, 0.01 to 0.20% of N, the remainder being Fe and unavoidable impurities,
satisfying Formula (1) below,

including a (Cr,Fe)-Nb-N based Z-phase precipitate, and

having an average grain size of 10 µm or less at a thickness center region, $Nb * N \geq 0.015$

wherein Nb and N represent the weight percent (wt %) of each component.
The hot-rolled annealed austenitic stainless steel material of claim 7, wherein the (Cr,Fe)-Nb-N based Z-phase precipitate has a formation temperature of 1150°C or higher.
The hot-rolled annealed austenitic stainless steel material of claim 7, having a thickness of 3.0 mm or more.
The hot-rolled annealed austenitic stainless steel material of claim 7, wherein the (Cr,Fe)-Nb-N based Z-phase precipitate has a diameter of 2 µm or less.
A method of manufacturing an austenitic stainless steel, the method comprising:
casting a slab comprising, in percent by weight (wt%), 0.005 to 0.07% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 9.0% of Ni, 16.0 to 19.0% of Cr, 0.01 to 0.30% of Nb, 0.01 to 0.20% of N, the remainder being Fe and unavoidable impurities, and satisfying Formula (1) below;

hot rolling the slab;

hot-rolling annealing;

cold rolling; and

cold-rolling annealing at a temperature of 700 to 850°C,

wherein the method includes a (Cr,Fe)-Nb-N based Z-phase precipitate after each of the hot-rolling annealing and the cold-rolling annealing, $Nb * N \geq 0.015$

wherein Nb and N represent the weight percent (wt %) of each component.
The method of claim 11, wherein the (Cr,Fe)-Nb-N based Z-phase precipitate has a formation temperature of 1150°C or higher.
The method of claim 11, wherein, after the hot-rolling annealing,
a hot-rolled annealed material has a thickness of 3.0 mm or more, and an average grain size of 10 µm or less at a thickness center region.
The method of claim 11, wherein, after the cold-rolling annealing,
the austenitic stainless steel has an average grain size of 2 µm or less at a thickness center region.
The method of claim 11, wherein, after the cold-rolling annealing,
a cold-rolled annealed material has a thickness of 0.3 mm or more and less than 3.0 mm.
The method of claim 11, wherein a pitting potential in a 3.5% NaCl solution at 30°C is 250 mV or higher.
The method of claim 11, wherein a yield strength is 930 MPa or higher.