Austenitic stainless steel and method for producing same

Abstract

An austenitic stainless steel according to one embodiment of the present invention contains, in wt%, C: 0.005-0.07%, Si: 0.1-1.0%, Mn: 0.1-2.0%, Ni: 6.0-9.0%, Cr: 16.0-19.0%, Nb: 0.01-0.30%, N: 0.01-0.20%, and the balance Fe and unavoidable impurities, satisfies formula (1): Nb # imgabs0 # N > = 0.015 (where Nb and N represent the wt% of each element), contains a (Cr, Fe)-Nb-N-based Z-phase precipitated phase, and has the balance of Fe and unavoidable impurities. The average crystal grain size in the thickness center portion is 2 [mu] m or less.

Description

Technical Field

[0001] The present invention relates to an austenitic stainless steel, and more particularly to an austenitic stainless steel in which the grain size is refined by controlling the alloy composition and the precipitation phase to ensure excellent yield strength, and a method for manufacturing the same. Background Art

[0002] Austenitic stainless steel is used in various applications such as transportation parts and construction parts due to its excellent formability, work hardening ability and weldability. However, the yield strength of 304 series stainless steel or 301 series stainless steel is at the level of 200-350MPa, which limits its application in structures. In order to make this general 300 series stainless steel obtain a higher yield strength, it is usually necessary to go through a temper rolling process. However, the temper rolling process may have the problem of increased cost and extremely low elongation of the material.

[0003] Therefore, there is a need for an austenitic stainless steel having high yield strength by a method that does not require a temper rolling process.

[0004] Patent document 0001 describes a method for manufacturing 300 series stainless steel, which flat-rolls a cold-rolled annealed material used as a laser metal mask for photolithography, and then performs a secondary stress relief (SR) heat treatment to maintain a small curvature even after half etching. However, in the case of the disclosed patent, as a manufacturing technology for controlling etching properties and curvature after etching, technical content regarding structural parts having a thickness of 0.3-3.0 mm is not included.

[0005] Patent document 0002 proposes a method for manufacturing a component for nuclear power, which is to perform a long-term heat treatment at a temperature of 600-700°C for more than 48 hours to reduce the average grain size to less than 10 μm. In the case of this method, productivity is reduced when it is implemented in an actual production line, and there is a problem of increased manufacturing cost due to the long-term heat treatment method.

[0006] [Prior art literature]

[0007] (Patent Document 1) International Patent Publication No. 2016-043125 (Publication Date: March 24, 2016)

[0008] (Patent Document 2) Japanese Patent Application Publication No. 2020-050940 (Publication Date: April 2, 2020) Summary of the invention

[0009] 1. Technical issues to be resolved

[0010] An object of the present invention for solving the above problems is to provide an austenitic stainless steel having a high yield strength suitable for use in structural members and a method for manufacturing the same.

[0011] The technical problems to be solved by the present invention are not limited to the above technical problems, and those skilled in the art can clearly understand other technical problems not mentioned from the following description.

[0012] (II) Technical Solution

[0013] As a means for achieving the above object, the austenitic stainless steel according to an example of the present invention may contain, by weight %: C: 0.005 - 0.07%, Si: 0.1 - 1.0%, Mn: 0.1 - 2.0%, Ni: 6.0 - 9.0%, Cr: 16.0 - 19.0%, Nb: 0.01 - 0.30%, N: 0.01 - 0.20%, the balance being Fe and inevitable impurities. The austenitic stainless steel may satisfy the following formula (1), and the austenitic stainless steel may contain a (Cr,Fe)-Nb-N-based Z-phase precipitation phase, and the average grain size at the center of the thickness may be 2 μm or less. Formula (1): Nb N ≥ 0.015 (wherein, Nb and N represent the weight % of each element)

[0014] The austenitic stainless steel according to an example of the present invention may be an austenitic stainless steel in which the formation temperature of the (Cr,Fe)-Nb-N-based Z-phase precipitation phase is 1150 °C or higher. In addition, the diameter of the Z-phase precipitation phase may be 50 - 300 nm.

[0015] The pitting potential of the austenitic stainless steel according to an example of the present invention in a 3.5% NaCl solution at 30 °C may be 250 mV or higher.

[0016] The thickness of the austenitic stainless steel according to an example of the present invention may be 0.3 mm or more and less than 3.0 mm.

[0017] The yield strength of the austenitic stainless steel according to an example of the present invention may be 930 MPa or higher.

[0018] The austenitic stainless steel hot-rolled and annealed material according to an example of the present invention may contain, by weight %: C: 0.005 - 0.07%, Si: 0.1 - 1.0%, Mn: 0.1 - 2.0%, Ni: 6.0 - 9.0%, Cr: 16.0 - 19.0%, Nb: 0.01 - 0.30%, N: 0.01 - 0.20%, the balance being Fe and inevitable impurities. The austenitic stainless steel hot-rolled and annealed material may satisfy the following formula (1), and the austenitic stainless steel hot-rolled and annealed material may contain a (Cr,Fe)-Nb-N based Z-phase precipitation phase, and the average grain size at the thickness center may be 10 μm or less.

[0019] Formula (1): Nb N ≥ 0.015 (wherein, Nb and N represent the weight % of each element)

[0020] The formation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitation phase in the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention may be 1150 °C or higher.

[0021] In addition, the thickness of the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention may be 0.3 mm or more, and the diameter of the Z-phase precipitation phase may be 2 μm or less.

[0022] The manufacturing method of the austenitic stainless steel according to an example of the present invention may include the following steps: casting a slab which, by weight %, contains: C: 0.005 - 0.07%, Si: 0.1 - 1.0%, Mn: 0.1 - 2.0%, Ni: 6.0 - 9.0%, Cr: 16.0 - 19.0%, Nb: 0.01 - 0.30%, N: 0.01 - 0.20%, the balance being Fe and inevitable impurities, and satisfies the following formula (1); hot-rolling the slab; performing hot-rolled annealing; performing cold-rolling; and performing cold-rolled annealing at 700 - 850 °C, wherein after the step of hot-rolled annealing and after the step of cold-rolled annealing, a (Cr,Fe)-Nb-N based Z-phase precipitation phase may be included. Formula (1): Nb N ≥ 0.015 (wherein, Nb and N represent the weight % of each element)

[0023] In the manufacturing method of the austenitic stainless steel according to an example of the present invention, the formation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitation phase may be 1150 °C or higher.

[0024] In the manufacturing method of the austenitic stainless steel according to an example of the present invention, after the step of hot-rolled annealing, the average grain size at the thickness center may be 10 μm or less.

[0025] In the method for manufacturing austenitic stainless steel according to an example of the present invention, after the hot rolling annealing step, the thickness of the hot rolled and annealed material can be 3.0 mm or more.

[0026] In the method for manufacturing austenitic stainless steel according to an example of the present invention, after the cold rolling annealing step, the average grain size at the thickness center portion can be 2 μm or less.

[0027] In the method for manufacturing austenitic stainless steel according to an example of the present invention, after the cold rolling annealing step, the thickness of the cold rolled and annealed material can be 0.3 mm or more and less than 3.0 mm.

[0028] In the method for manufacturing austenitic stainless steel according to an example of the present invention, the pitting potential in a 3.5% NaCl solution at 30 °C can be 250 mV or more.

[0029] In the method for manufacturing austenitic stainless steel according to an example of the present invention, the yield strength can be 930 MPa or more.

[0030] (III) Beneficial effects

[0031] According to an embodiment of the present invention, an austenitic stainless steel having a high yield strength and a method for manufacturing the same can be provided. The austenitic stainless steel controls the alloy composition and precipitation phases to obtain a fine grain size, and thus can be applied to structural components. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A view showing the precipitates of the hot rolled and annealed material of Invention Example 2.

[0033] Figure 2 A view showing the precipitates of the hot rolled and annealed material of Comparative Example 1.

[0034] Figure 3 A view showing the precipitates of the cold rolled and annealed material of Invention Example 2.

[0035] Figure 4 A view showing the precipitates of the cold rolled and annealed material of Comparative Example 1.

[0036] Figure 5 A view showing the microstructure of the hot rolled and annealed material of Invention Example 2.

[0037] Figure 6 A view showing the microstructure of the hot rolled and annealed material of Comparative Example 1.

[0038] Figure 7 A view showing the microstructure of the cold rolled and annealed material of Invention Example 2.

[0039] Figure 8A figure showing the microstructure of the cold-rolled and annealed material of Comparative Example 1. Detailed Description of the Invention

[0040] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0041] The terms used in this application are only for explaining specific examples. Therefore, unless it is clearly indicated in the context that it must be singular, the singular expression includes the plural expression. In addition, it should be noted that terms such as "comprising", "including" or "having" used in this application are used to clearly specify the existence of the features, steps, functions, components or combinations thereof described in the specification, rather than to preclude the existence of other features or steps, functions, components or combinations thereof in advance.

[0042] In addition, unless otherwise defined, all terms used in this specification should be regarded as having the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. Therefore, in this specification, unless otherwise clearly defined, specific terms should not be interpreted in an overly ideal or formal meaning.

[0043] In addition, with respect to "about", "substantially", etc. in this specification, when allowing for manufacturing and material tolerances inherent in the meanings mentioned, they are used in the sense of that value or close to that value, and are used to prevent unethical infringers from illegally using the mention of exact or absolute numerical values to help understand the disclosure of the present invention.

[0044] An austenitic stainless steel according to an example of the present invention will be described below.

[0045] The austenitic stainless steel according to an example of the present invention may contain, by weight %: C: 0.005 - 0.07%, Si: 0.1 - 1.0%, Mn: 0.1 - 2.0%, Ni: 6.0 - 9.0%, Cr: 16.0 - 19.0%, Nb: 0.01 - 0.30%, N: 0.01 - 0.20%, the balance being Fe and inevitable impurities.

[0046] The reasons for limiting the composition ranges of the respective alloying elements will be described below.

[0047] The content of C may be 0.005 - 0.07 wt%.

[0048] C is an austenite phase stabilizing element. The higher the addition amount, the better the effect of stabilizing the austenite phase. Considering this, the content of C added needs to reach more than 0.005%. However, when the content of C is 0.07% or more, chromium carbides (chromium carbide) are formed during low-temperature annealing, which may lead to the problem of reducing the corrosion resistance of the grain boundary. Considering this, the content of C can be 0.005 - 0.07 wt%.

[0049] The content of Si can be 0.1 - 1.0 wt%.

[0050] Si is a component added as a deoxidizer in the steelmaking process. When a certain amount of Si is added, silicon oxide (Si-Oxide) is formed on the passivation film during the bright annealing process, thereby having the effect of improving the corrosion resistance of the steel. Considering this, the content of Si added needs to reach 0.1% or more. However, when the content of Si exceeds 1.0%, it may cause the problem of reducing the ductility of the steel. Considering this, the content of Si can be 0.1 - 1.0 wt%.

[0051] The content of Mn can be 0.1 - 2.0 wt%.

[0052] Mn is an austenite phase stabilizing element. The more the addition amount, the better the effect of stabilizing the austenite phase. Considering this, the content of Mn added needs to reach 0.1% or more. However, when an excessive amount of Mn is added, it may hinder the corrosion resistance. Therefore, the content of Mn can be 2.0% or less. Considering this, the content of Mn can be 0.1 - 2.0 wt%.

[0053] The content of Ni can be 6.0 - 9.0 wt%.

[0054] Ni is an austenite phase stabilizing element. As the addition amount increases, it stabilizes the austenite phase. In order to soften the material, Ni of 6.0% or more can be added. However, excessive addition of Ni will cause the problem of increased cost. Therefore, Ni of 9.0% or less can be added. Considering this, the content of Ni can be 6.0 - 9.0 wt%.

[0055] The content of Cr can be 16.0 - 19.0 wt%.

[0056] Cr is an element that must be added to improve the corrosion resistance. Considering this, the content of Cr added needs to reach 16.0% or more. However, when the content of Cr is too high, the material may harden, and the problem of suppressing strain-induced martensite may occur during cold rolling. Therefore, the content can be set to 19.0% or less. Considering this, the content of Cr can be 16.0 - 19.0 wt%.

[0057] The content of Nb may be 0.01 - 0.30 wt%.

[0058] Nb is crucial for forming the Z-phase precipitation corresponding to the (Cr,Fe)-Nb-N system. By including Nb, the Z-phase precipitation can be formed, thereby suppressing grain growth. Considering this, the content of added Nb needs to reach more than 0.01%. However, when the content of Nb is too high, defects such as the formation of nitrogen pores occur during the continuous casting process, so the content can be set below 0.30%. Considering this, the content of Nb may be 0.01 - 0.30 wt%.

[0059] The content of N may be 0.01 - 0.20 wt%.

[0060] N is an austenite phase stabilizing element. As the addition amount increases, it can stabilize the austenite phase and increase the strength. In addition, N can form the Z-phase precipitation together with Nb. Considering this, the content of N can be set above 0.01%. However, when the content of N is too high, problems such as hardening and reduced hot workability may occur, so the content of N can be set below 0.20%. Considering this, the content of N may be 0.01 - 0.20 wt%.

[0061] In addition, the austenitic stainless steel according to an example of the present invention can satisfy the following formula (1).

[0062] Formula (1): Nb N ≥ 0.015

[0063] Wherein, Nb and N represent the wt% of each element. From the reasons for including the elements of Nb and N, in order to form the Z-phase precipitation corresponding to the (Cr,Fe)-Nb-N system, Nb corresponding to formula (1) N can be set above 0.015. When Nb N is above 0.015, the Z-phase formation temperature can be increased. By increasing the Z-phase formation temperature, it will not melt even at high hot rolling annealing temperatures and cold rolling annealing temperatures, so the Z-phase precipitation can exist in the hot rolled material and the cold rolled material. Nb N is preferably above 0.020, more preferably above 0.025, and more preferably above 0.030.

[0064] The remaining component is iron (Fe). However, in the conventional manufacturing process, it is inevitable to mix in undesirable impurities from raw materials or the surrounding environment, so these impurities cannot be excluded. These impurities are well known to those skilled in the conventional manufacturing process, so not all of their contents are specifically described in this specification.

[0065] Austenitic stainless steel according to an example of the present invention may contain a (Cr,Fe)-Nb-N-based Z-phase precipitation phase.

[0066] When the Z-phase precipitation phase is present, grain growth at high temperatures can be suppressed. The Z-phase precipitation phase is generated during the heat treatment process and does not melt while undergoing the steps of hot rolling annealing and cold rolling annealing, so grain coarsening can be prevented. In contrast, precipitation phases such as chromium carbides (chromium carbide) and chromium nitrides (chromium nitride) are not formed before the heat treatment. At the normal hot rolling annealing temperature, precipitation phases such as chromium carbide and chromium nitride melt and dissolve in the matrix. Therefore, different from the Z-phase precipitation phase, they precipitate during the heat treatment process, and it may be difficult to obtain the effect of suppressing grain growth.

[0067] The type of precipitation phase of austenitic stainless steel according to an example of the present invention is observed in the thickness center part of the austenitic stainless steel by the transmission electron microscope (TEM) replica method. For austenitic stainless steel according to an example of the present invention, when the thickness of the austenitic stainless steel is set to t, the thickness center part refers to 1 / 4t to 3 / 4t.

[0068] The formation temperature of the (Cr,Fe)-Nb-N-based Z-phase precipitation phase of austenitic stainless steel according to an example of the present invention may be 1150 °C or higher, and the diameter of the precipitation phase may be 50 - 300 nm.

[0069] The formation temperature of the precipitation phase of austenitic stainless steel according to an example of the present invention refers to the formation temperature of the Z-phase as the (Cr,Fe)-Nb-N-based precipitation phase calculated in the ThermoCalc precipitation phase analysis program.

[0070] The diameter of the precipitation phase of austenitic stainless steel according to an example of the present invention can be measured by observing the thickness center part of the austenitic stainless steel by the transmission electron microscope (TEM) replica method.

[0071] The thickness of austenitic stainless steel according to an example of the present invention may be 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 - 2.0 mm.

[0072] The austenitic stainless steel according to an example of the present invention can be applied to structures such as automotive outer panels and building components. In the case of structural components, the material thickness range can be 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 - 2.0 mm. In the case of the present invention, excellent yield strength can also be obtained when the thickness of the austenitic stainless steel is 0.3 mm or more and less than 3.0 mm.

[0073] The average grain size of the thickness center part of the austenitic stainless steel according to an example of the present invention can be 2 μm or less.

[0074] In the past, in order to ensure excellent yield strength, the austenite phase was transformed into the martensite phase by cold rolling, and ultrafine grains were achieved by low-temperature annealing. In the present invention, by controlling the alloy composition and precipitation phase, a fine grain size can be ensured, and an austenitic stainless steel with excellent yield strength can be provided. The fine grain size can be expressed, for example, as an average grain size of 3 μm or less, and more specifically, it can be 2 μm or less.

[0075] When the thickness of the austenitic stainless steel is set to t, the thickness center part of the austenitic stainless steel according to an example of the present invention refers to 1 / 4t to 3 / 4t. In addition, the average value of the thickness center part refers to the average value of the values measured at any three positions in the region of 1 / 4t to 3 / 4t. The grain size of any three positions in the thickness center part is observed and measured by the transmission electron microscope (TEM) thin foil method.

[0076] The pitting potential of the austenitic stainless steel according to an example of the present invention in a 3.5% NaCl solution at 30 °C can be 250 mV or more.

[0077] The yield strength of the austenitic stainless steel according to an example of the present invention can be 930 MPa or more. An austenitic stainless steel can be provided, and the austenitic stainless steel can be used as a structural component such as an automotive outer panel or a building component by ensuring a high yield strength of 930 MPa or more.

[0078] The austenitic stainless steel according to an example of the present invention can correspond to a cold-rolled annealed material.

[0079] Hereinafter, the hot-rolled annealed material of the austenitic stainless steel according to an example of the present invention will be described.

[0080] The austenitic stainless steel hot-rolled and annealed material according to an example of the present invention may contain, by weight %: C: 0.005 - 0.07%, Si: 0.1 - 1.0%, Mn: 0.1 - 2.0%, Ni: 6.0 - 9.0%, Cr: 16.0 - 19.0%, Nb: 0.01 - 0.30%, N: 0.01 - 0.20%, the balance Fe and inevitable impurities.

[0081] In addition, the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention may satisfy the following formula (1).

[0082] Formula (1): Nb N ≥ 0.015

[0083] Wherein, Nb and N represent the weight % of each element.

[0084] The reasons for limiting the composition ranges of the respective alloying elements and formula (1) are as described in the austenitic stainless steel according to an example of the present invention.

[0085] The austenitic stainless steel hot-rolled and annealed material according to an example of the present invention may contain a (Cr,Fe)-Nb-N based Z-phase precipitation phase.

[0086] The description of the (Cr,Fe)-Nb-N based Z-phase precipitation phase is as described in the austenitic stainless steel according to an example of the present invention.

[0087] The formation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitation phase in the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention may be 1150 °C or higher, and the diameter of the precipitation phase may be 0.2 μm or less.

[0088] By controlling the formation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitation phase in the hot-rolled and annealed material to 1150 °C or higher, which is the same as that of the austenitic stainless steel, the Z-phase precipitation phase can exist and not dissolve even in the hot-rolling and annealing step. By controlling the formation temperature, the Z-phase precipitation phase can be made to exist in the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention, thereby preventing grain coarsening during the annealing step.

[0089] The measuring method for the type, formation temperature, and diameter of the precipitation phase in the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention is as described in the austenitic stainless steel according to an example of the present invention.

[0090] The thickness of the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention can be 3.0 mm or more. By controlling the thickness of the hot-rolled and annealed material to be 3.0 mm or more, the austenitic stainless steel made from the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention can be used as structures such as automotive outer panels and building components.

[0091] The average grain size of the thickness center part of the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention can be 10 μm or less.

[0092] The fine grain size in the hot-rolled and annealed material may mean, for example, that the average grain size is 10 μm or less.

[0093] When the thickness of the austenitic stainless steel hot-rolled and annealed material is set as t, the thickness center part of the austenitic stainless steel hot-rolled and annealed material according to an example of the present invention means 1 / 4t to 3 / 4t. In addition, the average value of the thickness center part means the average value of the values measured at any 3 positions in the region of 1 / 4t to 3 / 4t. For the grain size, observe and measure at any 3 positions in the thickness center part by means of an Optical Microscope (OM).

[0094] Hereinafter, a method for manufacturing an austenitic stainless steel according to an example of the present invention will be described.

[0095] The method for manufacturing an austenitic stainless steel according to an example of the present invention may include the following steps: casting a slab which, by weight%, contains: C: 0.005 - 0.07%, Si: 0.1 - 1.0%, Mn: 0.1 - 2.0%, Ni: 6.0 - 9.0%, Cr: 16.0 - 19.0%, Nb: 0.01 - 0.30%, N: 0.01 - 0.20%, the balance being Fe and inevitable impurities, and satisfying the following formula (1); hot-rolling the slab; performing hot-rolling annealing; performing cold-rolling; and performing cold-rolling annealing at 700 - 850 °C.

[0096] Formula (1): Nb N ≥ 0.015, where Nb and N represent the weight% of each element.

[0097] In the method for manufacturing an austenitic stainless steel according to an example of the present invention, the hot-rolled and annealed material after the hot-rolling annealing step may contain a (Cr,Fe)-Nb-N based Z-phase precipitation phase, and the cold-rolled and annealed material after the cold-rolling annealing step may contain a (Cr,Fe)-Nb-N based Z-phase precipitation phase.

[0098] The reasons for limiting the composition ranges of the respective alloying elements and of formula (1) are as described for the austenitic stainless steel according to an example of the present invention. Hereinafter, the manufacturing method and the Z-phase precipitation phase will be examined in more detail.

[0099] The austenitic stainless steel corresponding to the hot-rolled annealed material and the cold-rolled annealed material of the austenitic stainless steel according to an example of the present invention may contain a (Cr,Fe)-Nb-N-based Z-phase precipitation phase. At this time, the formation temperature of the Z-phase as the (Cr,Fe)-Nb-N-based precipitation phase calculated in the Thermo Calc precipitation phase analysis program may be 1150 °C or higher.

[0100] In the manufacturing method of the austenitic stainless steel according to an example of the present invention, the hot-rolling annealing step may be carried out at 1000 - 1150 °C. When the hot-rolling annealing temperature is set to 1000 - 1150 °C, recrystallization occurs. This can be regarded as the normal hot-rolling annealing temperature.

[0101] The Z-phase as the (Cr,Fe)-Nb-N-based precipitation phase is generated before the hot-rolling annealing step. Therefore, by controlling the formation temperature of the Z-phase as the (Cr,Fe)-Nb-N-based precipitation phase to 1150 °C or higher, the Z-phase is generated before the hot-rolling annealing step and may remain undissolved during the hot-rolling annealing step. By controlling the presence of the Z-phase in the hot-rolled annealed material, grain coarsening can be prevented in subsequent steps.

[0102] In the manufacturing method of the austenitic stainless steel according to an example of the present invention, the reduction ratio in the cold-rolling step may be 40% or more. When the cold-rolling reduction ratio is 40% or more, a TRIP phase transformation can be induced. The reduction ratio may be 40% or more, and more specifically 50% or more.

[0103] In the manufacturing method of the austenitic stainless steel according to an example of the present invention, after the hot-rolling annealing step, the thickness of the hot-rolled annealed material may be 3.0 mm or more, and after the cold-rolling annealing step, the thickness of the cold-rolled annealed material may be 0.3 mm or more and less than 3.0 mm.

[0104] In the manufacturing method of the austenitic stainless steel according to an example of the present invention, the cold-rolling annealing step may be carried out at 700 - 850 °C. When the cold-rolling annealing temperature is 700 °C or higher, new reverted austenite nuclei can be generated. When the cold-rolling annealing temperature is 850 °C or higher, it is beneficial for grain refinement. Taking this into account, the cold-rolling annealing temperature in the present invention can be controlled at 700 - 850 °C.

[0105] Similar to the hot-rolled annealed material, after the cold-rolled annealing step, the austenitic stainless steel corresponding to the cold-rolled annealed material contains the Z phase as a (Cr,Fe)-Nb-N-based precipitation phase, thereby ensuring a fine grain size.

[0106] In the method for manufacturing an austenitic stainless steel according to an example of the present invention, after the hot-rolled annealing step, the average grain size of the thickness center portion of the hot-rolled annealed material can be 10 μm or less.

[0107] In the method for manufacturing an austenitic stainless steel according to an example of the present invention, after the cold-rolled annealing step, the average grain size of the thickness center portion of the stainless steel corresponding to the cold-rolled annealed material can be 2 μm or less.

[0108] The meanings of the thickness center portion and the thickness center are as described in the austenitic stainless steel according to an example of the present invention and the hot-rolled annealed material of the austenitic stainless steel according to an example of the present invention.

[0109] In addition, by ensuring fine grains, an austenitic stainless steel with excellent yield strength can be obtained.

[0110] The pitting potential of the austenitic stainless steel manufactured by the method for manufacturing an austenitic stainless steel according to an example of the present invention can be 250 mV or more in a 3.5% NaCl solution at 30 °C. In addition, the yield strength can be 930 MPa or more. An austenitic stainless steel that ensures corrosion resistance while ensuring high yield strength can be obtained.

[0111] Hereinafter, the present invention will be described in more detail with reference to examples and drawings. However, the description of such examples and drawings is only for illustrating the implementation of the present invention, and the present invention is not limited to the description of such examples. This is because the scope of the rights of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0112] {Examples}

[0113] Table 1 below shows the alloy compositions and types of precipitation phases of the inventive examples and comparative examples.

[0114] The slab having the alloy composition according to Table 1 below was hot-rolled and then hot-rolled annealed at 1050 °C, and the types of precipitation phases of the hot-rolled annealed material were observed.

[0115] In addition, cold rolling was performed at a reduction rate of 40%, and after cold-rolled annealing at 800 °C, the types of precipitation phases of the cold-rolled annealed material with a thickness of 0.8 mm were observed.

[0116] Figure 1 A figure showing the precipitates of the hot-rolled annealed material of Inventive Example 2, Figure 2A figure showing the precipitates of the hot-rolled and annealed material of Comparative Example 1. Figure 3 A figure showing the precipitates of the cold-rolled and annealed material of Invention Example 2, Figure 4 A figure showing the precipitates of the cold-rolled and annealed material of Comparative Example 1.

[0117] The types and sizes of the precipitated phases in the thickness center parts of the hot-rolled and annealed materials and the cold-rolled and annealed materials were observed by the TEM replica method.

[0118] [Table 1]

[0119]

[0120] The alloy compositions of Invention Examples 1 to 4 satisfy the scope of the present invention. In particular, the content of Nb N is 0.015 or more. In Invention Examples 1 to 4, it was confirmed that not only in the cold-rolled and annealed materials, but also in the hot-rolled and annealed materials, the types of precipitated phases observed included the Z phase. Figure 1 A figure showing the precipitates of the hot-rolled and annealed material of Invention Example 2, Figure 3 A figure showing the precipitates of the cold-rolled and annealed material of Invention Example 2. By Figure 1 and Figure 3 It was possible to confirm the types of precipitates and the elements contained in the precipitates of Invention Example 2. It was confirmed that the precipitates of the hot-rolled and annealed material and the cold-rolled and annealed material of Invention Example 2 corresponded to precipitates composed of Cr, Fe, N, and Nb. From this, it was confirmed that the precipitates of the hot-rolled and annealed material and the cold-rolled and annealed material in Invention Example 2 were Z-phase precipitated phases.

[0121] On the contrary, Comparative Example 1 contained no Nb at all. Since the Z phase corresponds to the (Cr,Fe)-Nb-N system, it was confirmed that in Comparative Example 1 that contained no Nb at all, the Z phase was not formed at all in the cold-rolled and annealed materials and the hot-rolled and annealed materials.

[0122] Figure 2 A figure showing the precipitates of the hot-rolled and annealed material of Comparative Example 1, Figure 4 A figure showing the precipitates of the cold-rolled and annealed material of Comparative Example 1.

[0123] Referring to Figure 2 , it was confirmed that no precipitated phase was formed. In addition, since no precipitated phase was formed, the elements constituting the precipitate could not be confirmed. From this, it was confirmed that when Nb was not contained, not only the Z-phase precipitated phase was not obtained, but also precipitates such as chromium carbide and chromium nitride were dissolved before hot rolling and were not formed in the form of precipitates.

[0124] Referring to Figure 4, it is possible to confirm the precipitates in the cold-rolled annealed material of Comparative Example 1 and the types of elements contained in the precipitates. It is possible to confirm that Comparative Example 1 corresponds to precipitates composed of Cr, Fe, and N. From this, it can be confirmed that the precipitates in the cold-rolled annealed material of Comparative Example 1 are chromium nitride precipitate phases.

[0125] The alloy compositions of Comparative Examples 2 to 4 satisfy the scope of the present invention, but the value of Nb N is less than 0.015. Therefore, the Z-phase precipitate formation temperature cannot be ensured, and thus no Z-phase precipitate can be observed in either the cold-rolled annealed material or the hot-rolled annealed material.

[0126] See Figure 1 and Figure 2 , it is possible to confirm that the diameter of the precipitates in the hot-rolled annealed material of Invention Example 2 according to an example of the present invention is much smaller than that of Comparative Example 1. In addition, in Invention Example 2, it is possible to confirm that since Cr, Fe, Nb, and N are observed, a Z-phase precipitate is obtained in the hot-rolled annealed material. Comparative Example 1 does not contain Nb, so it can be confirmed that no precipitate is observed in the hot-rolled annealed material.

[0127] See Figure 3 and Figure 4 , in Invention Example 2 according to an example of the present invention, Cr, Fe, Nb, and N are observed, so it can be confirmed that a Z-phase precipitate is obtained in the hot-rolled annealed material. Comparative Example 1 does not contain Nb, so Cr, Fe, and N are observed in the cold-rolled annealed material, and thus it can be confirmed that chromium nitride precipitate phases are formed.

[0128] Table 2 below shows the Z-phase formation temperature, the average grain size of the cold-rolled annealed material, the pitting potential, and the yield strength of the invention examples and the comparative examples.

[0129] The Z-phase formation temperature refers to the formation temperature of the Z-phase as a (Cr,Fe)-Nb-N system precipitate calculated in the Thermo Calc precipitate analysis program.

[0130] The average grain size of the cold-rolled annealed material refers to the average value of the values observed and measured at three arbitrary positions in the thickness center using the transmission electron microscope (TEM) thin foil method. When the thickness of the cold-rolled annealed material is set to t, the thickness center refers to 1 / 4t to 3 / 4t.

[0131] The pitting potential represents the value measured in a 30°C and 3.5% NaCl solution.

[0132] The yield strength refers to the yield strength obtained after performing a tensile test on a JIS 13B tensile test piece at a crosshead in the range of 10 mm / min to 20 mm / min at room temperature.

[0133] Figure 5 A diagram showing the microstructure of the hot-rolled and annealed material of Invention Example 2 Figure 6 A diagram showing the microstructure of the hot-rolled and annealed material of Comparative Example 1 Figure 7 A diagram showing the microstructure of the cold-rolled and annealed material of Invention Example 2 Figure 8 A diagram showing the microstructure of the cold-rolled and annealed material of Comparative Example 1. The microstructure of the hot-rolled and annealed material was observed by the optical microscope (OM) method, and the microstructure of the cold-rolled and annealed material was observed by the transmission electron microscope (TEM) thin foil method

[0134] [Table 2]

[0135]

[0136] In Invention Examples 1 to 4, it was confirmed that the Z-phase formation temperature was 1150 °C or higher. From this, it was confirmed that this was because when the alloy composition and the value of formula (1) were satisfied, Z-phase precipitates could be observed in both the cold-rolled and annealed material and the hot-rolled and annealed material, but corresponding to a Z-phase formation temperature of 1150 °C or higher, the Z-phase precipitates remained even after hot rolling and annealing and could not be melted. The average grain size of the cold-rolled and annealed material in Invention Examples 1 to 4 satisfied 2 μm or less. From this, it was confirmed that the pitting potential was 250 mV or higher and the yield strength was 930 MPa or higher. Contrary to austenitic stainless steel that can provide high yield strength and excellent corrosion resistance, in the case of Comparative Example 1 that did not contain Nb at all, it was a condition where the Z-phase could not be generated at all, so it was meaningless to measure the formation temperature. It was confirmed that the average grain size of the cold-rolled and annealed material in Comparative Example 1 was 6.7 μm and was very coarse, and the yield strength was 545 MPa, and high yield strength could not be ensured

[0137] In addition, in Comparative Examples 2 to 4, the Z-phase precipitate formation temperature was 1150 °C or lower. Therefore, all the precipitates melted before the hot-rolling and annealing step, so as shown in Table 1, precipitates in the hot-rolled and annealed material could not be observed. In addition, even after cold rolling and annealing, no melted Z-phase precipitates were observed

[0138] It was confirmed that Comparative Examples 2 to 4 instead obtained chromium carbide and / or chromium nitride precipitates. Chromium carbide and / or chromium nitride precipitates are not precipitates that can refine the grains. Therefore, it was confirmed that Comparative Examples 2 to 4 had a coarse average grain size of 3.2 μm or more, and in addition, it was confirmed that the yield strength was only 672 MPa or lower and was poor

[0139] See Figure 5 and Figure 6, it can be confirmed that the grain size of the fine structure in the hot-rolled annealed material of Invention Example 2 according to an example of the present invention is very fine compared to Comparative Example 1.

[0140] See Figure 7 and Figure 8 , it can be confirmed that the grain size of the fine structure in the cold-rolled annealed material of Invention Example 2 according to an example of the present invention is very fine compared to Comparative Example 1.

[0141] From this, it can be known that by controlling the alloy composition of the present invention and Nb corresponding to formula (1) N, ensuring the precipitation temperature of the Z-phase precipitation phase, when the Z-phase precipitation phase is ensured in the cold-rolled annealed material and the hot-rolled annealed material, an austenitic stainless steel can be provided, and the austenitic stainless steel ensures an average grain size of the fine cold-rolled annealed material of 2 μm or less and ensures a high yield strength of 930 MPa or more.

Claims

1. An austenitic stainless steel, by weight %, the austenitic stainless steel comprises: C: 0.005 - 0.07%, Si: 0.1 - 1.0%, Mn: 0.1 - 2.0%, Ni: 6.0 - 9.0%, Cr: 16.0 - 19.0%, Nb: 0.01 - 0.30%, N: 0.01 - 0.20%, the balance Fe and inevitable impurities, the austenitic stainless steel satisfies the following formula (1), the austenitic stainless steel contains a (Cr,Fe)-Nb-N based Z-phase precipitation phase, the average grain size at the center of the thickness is 2 μm or less, Formula (1): Nb N ≥ 0.015 Among them, Nb and N represent the weight % of each element.

2. The austenitic stainless steel according to claim 1, wherein, the generation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitation phase is 1150 °C or higher.

3. The austenitic stainless steel according to claim 1, wherein, the diameter of the (Cr,Fe)-Nb-N based Z-phase precipitation phase is 50 - 300 nm.

4. The austenitic stainless steel according to claim 1, wherein, the pitting potential in 3.5% NaCl solution at 30 °C is 250 mV or higher.

5. The austenitic stainless steel according to claim 1, wherein, the yield strength is 930 MPa or higher.

6. The austenitic stainless steel according to claim 1, wherein, 0.3 mm or more and less than 3.0 mm.

7. An austenitic stainless steel hot-rolled and annealed material, by weight %, the austenitic stainless steel hot-rolled and annealed material comprises: C: 0.005 - 0.07%, Si: 0.1 - 1.0%, Mn: 0.1 - 2.0%, Ni: 6.0 - 9.0%, Cr: 16.0 - 19.0%, Nb: 0.01 - 0.30%, N: 0.01 - 0.20%, the balance Fe and inevitable impurities, the austenitic stainless steel hot-rolled and annealed material satisfies the following formula (1), the austenitic stainless steel hot-rolled and annealed material contains a (Cr,Fe)-Nb-N based Z-phase precipitation phase, the average grain size at the center of the thickness is 10 μm or less, Formula (1): Nb N ≥ 0.015 Among them, Nb and N represent the weight % of each element.

8. The austenitic stainless steel hot-rolled and annealed material according to claim 7, wherein, the generation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitation phase is 1150 °C or higher.

9. The austenitic stainless steel hot-rolled and annealed material according to claim 7, wherein, the thickness is 3.0 mm or more.

10. The austenitic stainless steel hot-rolled and annealed material according to claim 7, wherein, the diameter of the (Cr,Fe)-Nb-N based Z-phase precipitation phase is 2 μm or less.

11. A manufacturing method of an austenitic stainless steel, which comprises the following steps: casting a slab, by weight %, the slab comprises: C: 0.005 - 0.07%, Si: 0.1 - 1.0%, Mn: 0.1 - 2.0%, Ni: 6.0 - 9.0%, Cr: 16.0 - 19.0%, Nb: 0.01 - 0.30%, N: 0.01 - 0.20%, the balance Fe and inevitable impurities, and satisfies the following formula (1); hot-rolling the slab; performing hot-rolling annealing; performing cold-rolling; and performing cold-rolling annealing at 700 - 850 °C, wherein, after the step of hot-rolling annealing and after the step of cold-rolling annealing, they respectively contain a (Cr,Fe)-Nb-N based Z-phase precipitation phase, Formula (1): Nb N ≥ 0.015 wherein, Nb and N represent the weight % of each element.

12. The method for manufacturing austenitic stainless steel according to claim 11, wherein, the generation temperature of the (Cr,Fe)-Nb-N based Z-phase precipitation phase is 1150 °C or higher.

13. The manufacturing method of the austenitic stainless steel according to claim 11, wherein, After the hot rolling annealing step, the thickness of the hot rolling annealed material is 3.0 mm or more, and the average grain size at the center of the thickness is 10 μm or less.

14. The method for manufacturing austenitic stainless steel according to claim 11, wherein, After the cold rolling annealing step, the average grain size at the center of the thickness is 2 μm or less.

15. The manufacturing method of the austenitic stainless steel according to claim 11, wherein, After the cold rolling annealing step, the thickness of the cold rolling annealed material is 0.3 mm or more and less than 3.0 mm.

16. The method for manufacturing austenitic stainless steel according to claim 11, wherein, The pitting potential is 250 mV or more in a 3.5% NaCl solution at 30 °C.

17. The method for manufacturing austenitic stainless steel according to claim 11, wherein, The yield strength is 930 MPa or more.

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