High-strength austenitic stainless steel and manufacturing method thereof

Abstract

A high-strength austenitic stainless steel according to one embodiment of the present invention comprises, by wt%, 0.01-0.1% of C, 0.1-1% of Si, less than 0.05% of P, less than 0.03% of S, 1-5% of Mn, 1-5% of Ni, 15.0-18.0% of Cr, 0.1-2% of Cu, 0.1-0.2% of N, 0.15-0.25% of Nb, and the balance of Fe and inevitable impurities, and can satisfy relation (1) and relation (2). Relation (1): [Ni]+0.8[Cu]+0.5([Mn]+[Cr])+16([C]+[N]) ≤ 20, Relation (2): ([N]+1.8[Nb]) / (0.01[Cr]) ≥ 2.6 (Here, [Ni], [Cu], [Mn], [Cr], [C], [N] and [Nb] represent the wt% of each element)

Description

High-strength austenitic stainless steel and its manufacturing method

[0001] The present invention relates to a high-strength austenitic stainless steel and a method for manufacturing the same, which increases cost competitiveness by lowering the amount of expensive Ni, has high elongation by utilizing a conventional grain refinement process, and secures excellent yield strength by controlling the microstructure of the final cold-annealed material through component and temperature control.

[0002] Austenitic stainless steels are widely used as structural materials due to their excellent strength and corrosion resistance. However, with the recent rise and volatility of nickel (Ni) prices, demand is increasing for low-Ni austenitic stainless steels with lower nickel content. Furthermore, high-strength austenitic stainless steels, which are advantageous for weight reduction and energy savings, are gaining attention. Work hardening through additional cold rolling is typically used to enhance the strength of austenitic stainless steels. However, while work-hardened products produced through additional cold rolling offer excellent strength, their elongation decreases significantly, which reduces their formability and poses a disadvantage when used in structural applications.

[0003] The present invention relates to a high-strength austenitic stainless steel having high yield strength and elongation through component control and micro-grain control, and a method for manufacturing the same.

[0004] In addition, the present invention reduces process costs by omitting additional cold rolling and degreasing processes for manufacturing existing high-strength steel, and reduces the cold rolling reduction rate, which was a limitation in manufacturing post-process materials by utilizing precipitates, thereby reducing the load during the process, thereby securing high yield strength and elongation, thereby securing excellent properties as a structural material.

[0005] The problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0006] As a means for achieving the above-described purpose, a high-strength austenitic stainless steel according to an example of the present invention includes, in wt%, C: 0.01% or more and 0.1% or less, Si: 0.1% or more and 1% or less, P: less than 0.05%, S: less than 0.03%, Mn: 1% or more and 5% or less, Ni: 1% or more and 5% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2% or less, N: 0.1% or more and 0.2% or less, Nb: 0.15% or more and 0.25% or less, the remainder being Fe and unavoidable impurities.

[0007] Satisfies the following equation (1),

[0008] The following equation (2) can be satisfied.

[0009] Equation (1): [Ni]+0.8[Cu]+0.5([Mn]+[Cr])+16([C]+[N]) ≤ 20

[0010] Equation (2): ([N]+1.8 [Nb]) / (0.01[Cr]) ≥ 2.6

[0011] (Here, [Ni], [Cu], [Mn], [Cr], [C], [N] and [Nb] represent the weight percent of each element)

[0012] In addition, the high-strength austenitic stainless steel according to one example of the present invention may have an average grain size of the austenite phase at the center of the thickness of the final coil of 1 ㎛ or less.

[0013] In addition, the high-strength austenitic stainless steel according to one example of the present invention may have a standard deviation of the average grain size of the austenite phase at the center of the thickness of the final coil of 0.5 ㎛ or less.

[0014] In addition, the high-strength austenitic stainless steel according to one example of the present invention may have a yield strength of the final coil of 800 MPa or more.

[0015] In addition, the high-strength austenitic stainless steel according to one example of the present invention may have an elongation of the final coil of 30% or more.

[0016] In addition, the high-strength austenitic stainless steel according to one example of the present invention has a change in Gibbs free energy from the austenite phase to the martensite phase at room temperature (ΔG γ-α (25℃)) may be less than -2.1 kJ / mol.

[0017] Additionally, the high-strength austenitic stainless steel according to one example of the present invention may have a final coil thickness of 2.5 mm or more and 3.5 mm or less.

[0018] In addition, a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention comprises the steps of manufacturing an ingot by melting a material including C: 0.01% or more and 0.1% or less, Si: 0.1% or more and 1% or less, P: less than 0.05%, S: less than 0.03%, Mn: 1% or more and 5% or less, Ni: 1% or more and 5% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2% or less, N: 0.1% or more and 0.2% or less, Nb: 0.15% or more and 0.25% or less, and the remainder Fe and unavoidable impurities;

[0019] A step of preparing a slab by heating the above ingot at a temperature of 1150°C or higher and 1300°C or lower for 1 hour or higher and 3 hours or lower;

[0020] A step of hot rolling the above slab to a thickness of 4 mm or more and 6 mm or less;

[0021] A step of winding into a coil at a temperature of 200℃ or higher and 700℃ or lower after the above hot rolling;

[0022] A step of subjecting the above-mentioned coil to a solution treatment at a temperature of 1050°C or higher and 1200°C or lower for 8 minutes or longer and 10 minutes or shorter, and then subjecting the coil to a water cooling treatment;

[0023] A step of manufacturing a cold-rolled material by cold-rolling the above water-cooled coil at a cold reduction ratio of 30% or more and 50% or less; and

[0024] It may include a step of manufacturing a final coil by annealing the cold-rolled material at 800°C or higher and 900°C or lower.

[0025] In addition, a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention can satisfy the following equations (1) and (2).

[0026] Equation (1): [Ni]+0.8[Cu]+0.5([Mn]+[Cr])+16([C]+[N]) ≤ 20

[0027] Equation (2): ([N]+1.8[Nb]) / (0.01[Cr]) ≥ 2.6

[0028] (Here, [Ni], [Cu], [Mn], [Cr], [C], [N] and [Nb] represent the weight percent of each element)

[0029] In addition, in a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the cold rolling reduction ratio in the step of manufacturing the cold-rolled material may be 35% or more and 45% or less.

[0030] In addition, in a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the annealing heat treatment temperature in the step of manufacturing the final coil may be 800°C or more and 900°C or less.

[0031] In addition, in the method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the thickness of the final coil may be 2.5 mm or more and 3.5 mm or less.

[0032] In addition, in a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the average grain size of the austenite phase at the center of the thickness of the final coil may be 1 µm or less.

[0033] In addition, in a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the standard deviation of the average grain size of the austenite phase at the center of the thickness of the final coil may be 0.5 ㎛ or less.

[0034] In addition, the method for manufacturing high-strength austenitic stainless steel according to an example of the present invention may have a yield strength of the final coil of 800 MPa or more.

[0035] In addition, the method for manufacturing high-strength austenitic stainless steel according to an example of the present invention may have an elongation of the final coil of 30% or more.

[0036] In addition, a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention is capable of measuring the change in Gibbs free energy from the austenite phase to the martensite phase at room temperature (ΔG γ-α (25℃)) may be less than -2.1 kJ / mol.

[0037] According to an embodiment of the present invention, a high-strength austenitic stainless steel having a yield strength of 800 MPa or more and an elongation of 30% or more can be manufactured through control of components and microstructure.

[0038] Figure 1 shows the results of measuring the microstructure of an austenitic stainless steel according to one embodiment of the present invention using a scanning electron microscope (SEM).

[0039] Preferred embodiments of the present invention are described below. However, the embodiments of the present invention may be modified in various ways, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more fully explain the present invention to those of ordinary skill in the art.

[0040] The terminology used in this application is solely for the purpose of describing specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Additionally, it should be noted that terms such as "comprise" or "have" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0041] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, specific terms should not be interpreted in an overly idealistic or formal sense. For example, singular expressions herein include plural expressions unless the context clearly indicates otherwise.

[0042] In addition, the terms "about", "substantially", etc. in this specification are used in the sense of or close to the numerical value when manufacturing and material tolerances inherent to the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly using the disclosure contents in which exact or absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0043] Recently, to overcome the shortcomings of work-hardened steels manufactured through additional cold rolling, a method has been proposed that utilizes an ultra-fine graining process to refine grains and improve strength and elongation. The ultra-fine graining process is advantageous in producing austenitic stainless steels with excellent strength and formability by controlling the cold reduction rate and annealing temperature. Ultra-fine grained steels are typically characterized by the induction of a large amount of martensitic transformation through high cold reduction rates. However, thick steels, primarily used for structural materials, require high cold reduction rates to utilize the aforementioned general ultra-fine graining process. However, in actual production, the high cold rolling resistance often exceeds equipment capacity. Therefore, the present invention proposes a high-strength austenitic stainless steel capable of achieving ultra-fine grains even at low cold reduction rates through microstructural control of a reduced-Ni content composition with excellent cost competitiveness.

[0044] Before explaining the high-strength austenitic stainless steel below, let's explain the drawing.

[0045] Fig. 1 is a photograph of a material that was hot-rolled at a cold reduction ratio of 30% to 50% after manufacturing a hot-rolled steel sheet for the steel grades corresponding to Examples 1 to 4, and then subjected to a hot-rolled annealing treatment at a temperature of 800°C to 900°C, observed through a scanning electron microscope (SEM). Referring to Fig. 1, it can be confirmed through the austenitic grain size of the final coil that went through the above process that when the steel grades corresponding to Examples 1 to 4 in Table 1 are manufactured according to the above process, the microstructure is refined to have a grain size of 1 μm or less and a grain standard deviation of 0.5 or less.

[0046] Below, high-strength austenitic stainless steels are described in detail.

[0047] A high-strength austenitic stainless steel according to an example of the present invention may include, in wt%, C: 0.01% or more and 0.1% or less, Si: 0.1% or more and 1% or less, P: less than 0.05%, S: less than 0.03%, Mn: 1% or more and 5% or less, Ni: 1% or more and 5% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2% or less, N: 0.1% or more and 0.2% or less, Nb: 0.15% or more and 0.25% or less, the remainder being Fe and unavoidable impurities.

[0048] The reasons for limiting the composition range of each alloy element are described below. Unless otherwise specified, the unit is weight percent.

[0049] The content of C may be 0.01% or more and 0.1% or less.

[0050] C is an element added to secure the strength of the product, and is very effective in stabilizing the austenite system and is inexpensive. In addition, C is an interstitial element that increases strength through the solid solution strengthening effect. However, if the C content is less than 0.01%, sufficient strength cannot be secured, and if the C content exceeds 0.1%, Cr may be present in the heat affected zone after welding. 23 The sensitization phenomenon caused by grain boundary precipitation of carbides such as C6 may occur, which may reduce the ductility, toughness, and corrosion resistance of the product. Therefore, it is desirable to keep the C content to 0.01% or more and 0.1% or less.

[0051] The Si content may be 0.1% or more and 1% or less.

[0052] Silicon (Si), a representative substitutional element, is advantageous for securing strength through deoxidation and solid solution strengthening. Therefore, to ensure sufficient strength in products, as well as to act as a deoxidizer and enhance corrosion resistance during the steelmaking process, Si is added at a content of 0.1% or more. However, if the Si content exceeds 1%, the delta-ferrite phase may form due to the peritectic reaction during casting, which may reduce hot workability. Therefore, it is recommended to keep the Si content between 0.1% and 1%.

[0053] The content of P may be less than 0.05% and the content of S may be less than 0.03%.

[0054] P and S should be added in as little as possible as they reduce corrosion resistance and hot workability. Therefore, it is desirable to keep the P and S contents to less than 0.05% and 0.03%, respectively.

[0055] The content of Mn may be 1% or more and 5% or less.

[0056] Manganese (Mn) is an inexpensive element that stabilizes the austenite phase. Furthermore, Mn enhances the stability of the austenitic phase against induced martensite, so Mn is added at a concentration of 1% or more. However, if the Mn content exceeds 5%, inclusions (MnS) increase, which can reduce the corrosion resistance of the steel. Therefore, it is recommended that the Mn content be between 1% and 5%.

[0057] The Ni content can be 1% or more and 5% or less.

[0058] Ni is a powerful element that stabilizes the austenite phase. Furthermore, Ni is a key element that enhances the stability of the austenitic system. It effectively prevents thermal and process-induced martensitic transformations, thereby preventing toughness degradation at extremely low temperatures. Therefore, Ni is added at a content of 1% or more. However, if the Ni content exceeds 5%, it acts as a factor that reduces grain refinement, and as an expensive element, Ni can lead to increased raw material costs. Therefore, it is recommended that the Ni content be between 1% and 5%.

[0059] The Cr content may be 15.0% or more and 18.0% or less.

[0060] Cr is an essential element for ensuring corrosion resistance and phase stability. Adding Cr above 15.0% increases the solubility of nitrogen (N), facilitating the production of high-nitrogen austenitic stainless steels. However, if the Cr content exceeds 18%, the formation of delta-ferrite phases by peritectic reaction during casting can reduce hot workability. Therefore, it is preferable to keep the Cr content between 15.0% and 18.0%.

[0061] The content of Cu may be 0.1% or more and 2% or less.

[0062] Copper (Cu) is an element that stabilizes the austenite phase. Furthermore, Cu inhibits heat-induced and process-induced martensitic transformations. Adding Cu at 0.1% or more effectively improves corrosion resistance in reducing environments. However, if the Cu content exceeds 2%, solidification segregation of Cu can deteriorate hot workability. Therefore, it is preferable to keep the Cu content between 0.1% and 2%.

[0063] The content of N may be 0.1% or more and 0.2% or less.

[0064] Nitrogen is a very effective element for stabilizing the austenite phase and is inexpensive. Adding 0.1% or more of Ni can increase strength and improve corrosion resistance through solid solution strengthening. However, if the Ni content exceeds 0.2%, hot workability can deteriorate. Therefore, it is desirable to keep the Ni content between 0.1% and 0.2%.

[0065] The content of Nb may be 0.15% or more and 0.25% or less.

[0066] Nb is a favorable element for the formation of precipitates within the austenite phase. When Nb is added in an amount of 0.15% or more, it is very effective in suppressing grain growth, but when the Nb content exceeds 0.25%, Cr is formed during the annealing step. 23 The precipitation temperature of carbides such as C6 also increases, raising concerns about the generation of carbides during the grain refinement process, which may ultimately deteriorate the hot workability of the product. Therefore, it is desirable to keep the Nb content to 0.15% or more and 0.25% or less.

[0067] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment can inevitably be mixed in during the typical manufacturing process, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0068] Hereinafter, a high-strength austenitic stainless steel according to one embodiment of the present invention having the above-described alloy composition will be described in detail.

[0069] A high-strength austenitic stainless steel according to an example of the present invention includes, in wt%, C: 0.01% or more and 0.1% or less, Si: 0.1% or more and 1% or less, P: less than 0.05%, S: less than 0.03%, Mn: 1% or more and 5% or less, Ni: 1% or more and 5% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2% or less, N: 0.1% or more and 0.2% or less, Nb: 0.15% or more and 0.25% or less, the remainder being Fe and unavoidable impurities, and can satisfy the following formulas (1) and (2).

[0070] Equation (1): [Ni]+0.8[Cu]+0.5([Mn]+[Cr])+16([C]+[N]) ≤ 20

[0071] Equation (2): ([N]+1.8[Nb]) / (0.01[Cr]) ≥ 2.6

[0072] (Here, [Ni], [Cu], [Mn], [Cr], [C], [N] and [Nb] represent the weight percent of each element)

[0073] In the present invention, temperature-dependent phase stability evaluation of austenite phase and martensite phase (ΔG γ-α (25℃)) and precipitate behavior were evaluated using the thermodynamic analysis program Thermo-Calc. TCFE 12.0 thermodynamic database. The free energy changes (G) of the austenite and martensite phases according to the content of alloying elements and temperature changes were analyzed. γ ,G α ) was measured and the change in Gibbs free energy from the austenite phase to the martensite phase at room temperature (ΔG γ-α (25℃)) was calculated. In addition, the phase stability index at room temperature (Equation (1)) and the recrystallization index at high temperature (Equation (2)) were derived through this.

[0074] SP = [Ni]+0.8[Cu]+0.5([Mn]+[Cr])+16([C]+[N]) ≤ 20 (Equation (1))

[0075] RP = ([N]+1.5[Nb]) / (0.01[Cr]) ≥ 2.6 (Equation (2))

[0076] The value of SP (Stability Parameter) (Equation (1)), which is an indicator of phase stability of the austenitic system, is 20 or less, and the change in free energy from the austenite phase to the martensite phase at room temperature (ΔG γ-α (25℃)) is -2.1 kJ / mol or less, when the cold reduction ratio is 30% to 50%, grain refinement occurs through recrystallization during reverse transformation, and a large amount of work-induced martensite is formed due to the high cold reduction ratio, providing numerous recrystallization sites in the matrix, thereby controlling grain growth. Therefore, martensite phase transformation occurs by 20% or more, promoting grain refinement.

[0077] On the other hand, if the value of equation (1) exceeds 20, the change in free energy from the austenite phase to the martensite phase at room temperature (ΔG γ-α (25℃)) exceeds -2.1 kJ / mol and the cold reduction ratio is less than 30%, the growth of crystal grains cannot be controlled, making it impossible to realize the fine grain structure required by the present invention.

[0078] In addition, the Recrystallization Parameter (RP) (Formula (2)), which is an index of recrystallization of the austenite phase during cold rolling annealing, was made to be 2.6 or higher, thereby reducing the deviation in the grain size of austenitic structures deformed during the annealing process due to the formation of precipitates by the Z-phase during recrystallization.

[0079] This is to solve the problem that the austenitic grains remaining as a deformed structure during cold rolling become coarser than the austenitic structure recrystallized through reverse transformation in the work-induced martensite after the recovery stage and recrystallization are completed. The purpose is to reduce coarse grains by suppressing grain growth during the recrystallization process using micro-precipitates such as Z-phase.

[0080] On the other hand, when the value of equation (2) is less than 2.6, the formation of precipitates by Z-phase during recrystallization is suppressed, so the deviation in the grain size of the austenitic structures transformed during the annealing process increases.

[0081] In addition, the high-strength austenitic stainless steel according to one example of the present invention may have a standard deviation of the average grain size of the austenite phase in the center of the thickness of the final coil of 0.5 µm or less. Here, the center of the thickness means an area of ​​40% to 60% in the thickness direction.

[0082] In addition, in the present invention, the crystal grain size means the diameter of a circle assuming a virtual circle having the same area as the crystal grain area. In addition, the average crystal grain size is (measured area x number of crystal grains). 0.5 can be calculated. The grain size can be measured based on the plane parallel to the cross-section in the vertical direction of the rolling (TD plane). At this time, the grain size was evaluated using an image analyzer with a built-in analysis program that assumes that multiple hexagons are connected (ASTM E112 grain measurement method).

[0083] In addition, the center of thickness in the present invention means the center in the thickness direction with respect to the surface of the cross-section.

[0084] In addition, the high-strength austenitic stainless steel according to an example of the present invention may have a yield strength of the cold-rolled material of 800 MPa or more.

[0085] In addition, the high-strength austenitic stainless steel according to an example of the present invention may have an elongation of the cold-rolled material of 30% or more.

[0086] In addition, the high-strength austenitic stainless steel according to one example of the present invention has a change in Gibbs free energy from the austenite phase to the martensite phase at room temperature (ΔG γ-α (25℃)) can be greater than -2.1 kJ / mol.

[0087] In order to produce a low-cost austenitic steel with a lower content of expensive Ni in the present invention, the phase stability of austenite must be controlled by utilizing other austenite phase stabilizing elements (Mn, Cu, C, N, etc.) in addition to Ni. That is, the free energy change from the austenite phase to the martensite phase at room temperature (|ΔG γ-α (25℃)|) value must be controlled. This is because maximizing the transformation of induced martensite is advantageous for ultra-fine grains.

[0088] Hereinafter, a method for manufacturing high-strength austenitic stainless steel according to an embodiment of the present invention will be described in detail.

[0089] In addition, a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention comprises the steps of: melting a material including, in wt%, C: 0.01% or more and 0.1% or less, Si: 0.1% or more and 1% or less, P: less than 0.05%, S: less than 0.03%, Mn: 1% or more and 5% or less, Ni: 1% or more and 5% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2% or less, N: 0.1% or more and 0.2% or less, Nb: 0.15% or more and 0.25% or less, and the remainder Fe and unavoidable impurities; heating the ingot at 1150°C or more and 1300°C or less for 1 hour or more and 3 hours or less to prepare a slab; hot-rolling the slab to a thickness of 4 mm or more and 6 mm or less; The method may include: a step of coiling the hot-rolled material at a temperature of 200°C or more and 700°C or less; a step of solution-treating the coiled material at a temperature of 1050°C or more and 1200°C or less for 8 minutes or more and 10 minutes or less, and a step of water-cooling the coil; a step of cold-rolling the water-cooled material at a cold reduction ratio of 30% or more and 50% or less to produce a cold-rolled material; and a step of annealing the cold-rolled material at a temperature of 800°C or more and 900°C or less to produce a final coil.

[0090] Typically, the ultra-fine graining process is achieved by controlling the microstructural changes and annealing temperature during cold rolling of unstable austenitic stainless steel.

[0091] That is, during cold rolling of austenitic stainless steel, the development of deformation-induced martensite phase (deformation-induced α´-martensite) is induced, and during annealing, a reverse transformation occurs from the deformation-induced martensite phase and the deformed austenite structure to the recrystallized austenite phase.

[0092] At this time, the induced martensite phase generally shows a tendency to develop differently depending on the stability of the austenite phase. In austenitic stainless steels with low phase stability, ε-martensite bands develop at the initial stage of deformation, and as the amount of deformation increases, α´-martensite is generated from the intersections within the bands.

[0093] Here, when a cold reduction ratio (thickness reduction) of 60% or more is generally applied, α´-martensite is generated with a volume fraction of 80% or more within the entire microstructure. When a large amount of martensite is generated, it provides numerous recrystallization sites during reverse transformation, which facilitates recrystallization during the annealing process, and is therefore effective in creating a finer-grained austenitic structure in the final structure.

[0094] However, when manufacturing thick materials for actual structural use, it is difficult to manufacture a large amount of α´-martensite by applying a cold reduction ratio of 60% or more. In addition, when the cold reduction ratio is less than 30%, the process-induced martensite phase and the transformed austenitic structure that are easy to recrystallize are not sufficiently formed, making it difficult to form a uniform recrystallized austenitic structure. Therefore, when manufacturing thick materials for structural use, it is preferable to set the cold reduction ratio to 30% or more and 50% or less. More preferably, the cold reduction ratio may be 35% or more and 45% or less.

[0095] In addition, grain refinement occurs through recrystallization during reverse transformation, and when a high cold reduction ratio is applied, a large amount of work-induced martensite provides a large number of recrystallization sites in the matrix, which controls grain growth. On the other hand, when the cold reduction ratio is low, grain growth cannot be controlled. Therefore, a method to compensate for this is needed. Accordingly, the present invention utilized a method of suppressing grain growth during the annealing process using precipitates. Specifically, when grain growth is suppressed with precipitates alone, there is a problem that the deviation in grain size increases during recrystallization as some grains become coarser, so an ultra-fine grain process was utilized together to reduce this.

[0096] In addition, in order to achieve ultra-fine grains, the annealing temperature must be 800°C or higher so that the reverse transformation from the work-induced martensite to the austenite system occurs, and if the annealing temperature is lower than 800°C, some work-induced martensite remains. However, if the annealing temperature exceeds 900°C, some crystal grains grow unevenly, making it difficult to form an ultra-fine microstructure of 1 μm or less. Therefore, it is preferable to manufacture the final coil by annealing the cold-rolled material at 800°C or higher and 900°C or lower. More preferably, the annealing heat treatment temperature may be 800°C or higher and 900°C or lower.

[0097] In addition, the method for manufacturing high-strength austenitic stainless steel according to an example of the present invention can cause the slab to satisfy the following equations (1) and (2).

[0098] Equation (1): [Ni]+0.8[Cu]+0.5([Mn]+[Cr])+16([C]+[N]) ≤ 20

[0099] Equation (2): ([N]+1.8[Nb]) / (0.01[Cr]) ≥ 2.6

[0100] (Here, [Ni], [Cu], [Mn], [Cr], [C], [N] and [Nb] represent the weight percent of each element)

[0101] In addition, in a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the cold rolling reduction ratio in the step of manufacturing the cold-rolled material may be 35% or more and 45% or less.

[0102] In addition, in a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the annealing heat treatment temperature in the step of manufacturing the final coil may be 800°C or more and 900°C or less.

[0103] In addition, in the method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the thickness of the final coil may be 2.5 mm or more and 3.5 mm or less.

[0104] In addition, in a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the average grain size of the austenite phase at the center of the thickness of the final coil may be 1 µm or less.

[0105] In addition, in a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention, the standard deviation of the average grain size of the austenite phase at the center of the thickness of the final coil may be 0.5 ㎛ or less.

[0106] In addition, the method for manufacturing high-strength austenitic stainless steel according to an example of the present invention may have a yield strength of the final coil of 800 MPa or more.

[0107] In addition, the method for manufacturing high-strength austenitic stainless steel according to an example of the present invention may have an elongation of the final coil of 30% or more.

[0108] In addition, a method for manufacturing high-strength austenitic stainless steel according to an example of the present invention is capable of measuring the change in Gibbs free energy from the austenite phase to the martensite phase at room temperature (ΔG γ-α (25℃)) can be greater than -2.1 kJ / mol.

[0109] The high-strength austenitic stainless steel of the present invention can provide a high-strength stainless steel and a method for manufacturing the same, which increases cost competitiveness by lowering the expensive Ni content, has high elongation by utilizing a conventional grain refinement process, and secures excellent yield strength by controlling the microstructure of the final cold-annealed material through control of components and temperature.

[0110] {Example}

[0111] Hereinafter, the present invention will be described in more detail through examples.

[0112] Here, for materials having various alloy composition ranges shown in [Table 1] below, solution treatment was performed at 1250°C for 10 minutes after ingot melting, followed by water cooling. Cold rolling was performed at a thickness reduction rate of 30% to 50% on the hot-rolled annealed material to produce a cold-rolled material with a thickness of 2.5 mm to 3.5 mm. Then, annealing heat treatment was performed on the cold-rolled material at 800°C to 900°C.

[0113] Steel grade (wt%) CSiMnPSCrNiNbCuNExample 10.0180.051.100.0030.00516.513.960.190.130.11Example 20.0580.973.790.0020.00316.214.150.221.720.17Example 30 .0250.731.630.0030.00515.524.950.231.110.2Example 40.0480.010.750.0030.00517.534.520.211.670.17Comparative Example 10.0850.74.100.0030.00317.953.27 0.181.630.2Comparative Example 20.0900.539.710.0030.00317.442.000.21.360.16Comparative Example 30.0320.924.430.0030.00317.452.750.112.000.16Comparative Example 40.0160.632.290 .0030.00317.624.30.160.480.05Comparative Example 50.0480.010.750.0030.00517.534.520.211.670.17Comparative Example 60.0480.010.750.0030.00517.534.520.211.670.17

[0114] [Table 2] below shows the exponent values ​​and thermodynamic free energy change ΔG applied to SP (Equation (1)) and RP (Equation (2)) for the steel types (Examples 1 to 4 and Comparative Examples 1 to 6) specified in [Table 1]. γ-α Calculated values ​​of (25℃), grain size, and yield strength (YS) from tensile test 0.2 ) and elongation (El). For each example and comparative example, a tensile test was performed after manufacturing a plate-shaped sub-size tensile specimen according to the JIS13B standard for the annealed cold-rolled material, and the yield strength (YS) was measured accordingly. 0.2 ), MPa) and elongation (Elongation (EL), %) are listed in [Table 2] below, and the phase stability evaluation (|ΔG) of the austenite phase and martensite phase by temperature γ-α(25℃)|) and precipitate behavior were evaluated using the thermodynamic analysis program Thermo-Calc. TCFE 12.0 thermodynamic database. The change in Gibbs free energy (ΔG) from austenite to martensite at room temperature according to the content of alloying elements and temperature changes was calculated. γ-α (25℃)) was calculated.

[0115] Steel SP (Formula (1)) RP (Formula (2)) ΔG γ-α (25℃) (kJ / mol) Cold rolling reduction rate (%) Annealing temperature (℃) Grain size (㎛) Grain deviation YS 0.2 (MPa)El(%)Example 114.92.7-3106309000.70.181532Example 219.23.5-2318408500.60.282531Example 318.04.0-2538408500.60.383032Example 418.53.1-2679508000.60.285030Comparative Example 120.22.9-2052408501.50.48 2031Comparative Example 220.73.0-1482508502.00.581030Comparative Example 318.42.1-2196409001.00.779032Comparative Example 415.71.9-2964309000.80.978038Comparative Example 518.53.1-26795010002.00.575040Comparative Example 618.53.1-2679507003.01.0105025

[0116] First, for grain refinement during the cold pressing stage, sufficient process-induced martensite must be formed within the structure. In this regard, the change in Gibbs free energy from the austenite phase to the martensite phase, ΔG, is γ-αIn the case of steels having a value of (25℃) of -2.1 kJ / mol or more, it was advantageous to sufficiently form induced martensite at room temperature. This is an example corresponding to SP (Formula (1)) ≤ 20, and it was found through Examples 1 to 4 and Comparative Examples 3 to 4 in Table 2 that when the value of SP (Formula (1)) is 20 or less, the average grain size of the austenite phase at the center of the thickness of the final coil was generated to be 1 ㎛ or less. In addition, in order to prevent some structures from coarsening during the process of reversely transforming the martensite phase into the austenite phase in the cold rolling annealing step, micro-precipitates that contribute to reducing the deviation in grain size due to recrystallization, such as Z-phase, were utilized. This is an example corresponding to RP (Formula (2)) ≥ 2.6, and it was found through Examples 1 to 4 and Comparative Examples 1 to 2 in Table 2 that the standard deviation of the average grain size of the austenite phase in the center of the thickness of the final coil can be controlled to 0.5 ㎛ or less when the value of RP (Formula (2)) satisfies 2.6 or more. Consequently, in the case of Examples 1 to 4, which are steel grades that satisfy both SP (Formula (1)) ≤ 20 and RP (Formula (2)) ≥ 2.6, it was confirmed that not only was the average grain size of the austenite phase in the center of the thickness of the final coil 1 ㎛ or less and the standard deviation of the average grain size of the austenite phase in the center of the thickness of the final coil 0.5 ㎛ or less, but also the yield strength of the final coil was 800 MPa or more and the elongation of the final coil was 30% or more.

[0117] On the other hand, Comparative Examples 1 and 2 are examples corresponding to SP (Formula (1)) > 20, which are examples in which process-induced martensite is not sufficiently formed. In this case, since the value of SP (Formula (1)) exceeds 20, a site for recrystallization is not sufficiently provided, and as a result, the grain size becomes coarser, resulting in a grain size exceeding 1 ㎛.

[0118] In addition, Comparative Examples 3 and 4 are examples corresponding to RP (Formula (1)) < 2.6, which are examples in which the deviation in the size between grains increases due to coarsening of some grains as the Z-phase is not sufficiently uniformly precipitated. As a result, the size of other grains decreases due to some coarsened grains, thereby reducing the strength strengthening effect and reducing the yield strength to less than 800 MPa.

[0119] Comparative Examples 5 and 6 are examples where the annealing temperature was not controlled. When the annealing temperature is low, some recrystallization is not completed, resulting in an increase in yield strength and a decrease in elongation below 30%. When the annealing temperature is high, some grain growth causes the yield strength to decrease below 800 MPa.

[0120] Meanwhile, although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims described below.

Claims

1. Contains C: 0.01% or more and 0.1% or less in weight%, Si: 0.1% or more and 1% or less, P: less than 0.05%, S: less than 0.03%, Mn: 1% or more and 5% or less, Ni: 1% or more and 5% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2% or less, N: 0.1% or more and 0.2% or less, Nb: 0.15% or more and 0.25% or less, the remainder including Fe and unavoidable impurities, Satisfies the following equation (1), High-strength austenitic stainless steel satisfying the following equation (2). Equation (1): [Ni]+0.8[Cu]+0.5([Mn]+[Cr])+16([C]+[N]) ≤ 20 Equation (2): ([N]+1.8[Nb]) / (0.01[Cr]) ≥ 2.6 (Here, [Ni], [Cu], [Mn], [Cr], [C], [N] and [Nb] represent the weight% of each element) 2. In claim 1, High-strength austenitic stainless steel having an average grain size of austenite phase of 1 ㎛ or less in the center of the thickness of the final coil.

3. In claim 1, High-strength austenitic stainless steel having a standard deviation of the average grain size of the austenite phase in the center of the thickness of the final coil of 0.5 ㎛ or less.

4. In claim 1, High-strength austenitic stainless steel with a final coil yield strength of 800 MPa or more.

5. In claim 1, High-strength austenitic stainless steel with a final coil elongation of 30% or more.

6. In claim 1, Change in Gibbs free energy from austenite to martensite at room temperature (ΔG γ-α High-strength austenitic stainless steel having a hardness of -2.1 kJ / mol or less (at 25℃).

7. In claim 1, High-strength austenitic stainless steel with a final coil thickness of 2.5 mm or more and 3.5 mm or less.

8. A step of manufacturing an ingot by melting a material including C: 0.01% or more and 0.1% or less, Si: 0.1% or more and 1% or less, P: less than 0.05%, S: less than 0.03%, Mn: 1% or more and 5% or less, Ni: 1% or more and 5% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2% or less, N: 0.1% or more and 0.2% or less, Nb: 0.15% or more and 0.25% or less, and the remainder Fe and unavoidable impurities; A step of preparing a slab by heating the above ingot at a temperature of 1150℃ or higher and 1300℃ or lower for 1 hour or longer and 3 hours or less; A step of hot rolling the above slab to a thickness of 4 mm or more and 6 mm or less; A step of winding into a coil at a temperature of 200℃ or higher and 700℃ or lower after the above hot rolling; A step of performing a solution treatment on the above-mentioned coil at a temperature of 1050℃ or higher and 1200℃ or lower for 8 minutes or longer and 10 minutes or shorter, and then performing a water cooling treatment; A step of manufacturing a cold rolled material by cold rolling the above water-cooled coil at a cold rolling reduction ratio of 30% or more and 50% or less; and A method for manufacturing high-strength austenitic stainless steel, comprising: a step of manufacturing a final coil by annealing the cold-rolled material at 800°C or higher and 900°C or lower.

9. In claim 8, A method for manufacturing high-strength austenitic stainless steel satisfying the following equations (1) and (2). Equation (1): [Ni]+0.8[Cu]+0.5([Mn]+[Cr])+16([C]+[N]) ≤ 20 Equation (2): ([N]+1.8[Nb]) / (0.01[Cr]) ≥ 2.6 (Here, [Ni], [Cu], [Mn], [Cr], [C], [N] and [Nb] represent the weight% of each element) 10. In claim 8, A method for manufacturing high-strength austenitic stainless steel, wherein the cold rolling reduction ratio in the step of manufacturing the above cold-rolled material is 35% or more and 45% or less.

11. In claim 8, A method for manufacturing high-strength austenitic stainless steel, wherein the annealing heat treatment temperature in the step of manufacturing the final coil is 800°C or higher and 900°C or lower.

12. In claim 8, A method for manufacturing high-strength austenitic stainless steel, wherein the thickness of the final coil is 2.5 mm or more and 3.5 mm or less.

13. In claim 8, A method for manufacturing high-strength austenitic stainless steel having an average grain size of austenite phase at the center of the thickness of the final coil of 1 ㎛ or less.

14. In claim 8, A method for manufacturing high-strength austenitic stainless steel having a standard deviation of the average grain size of the austenite phase at the center of the thickness of the final coil of 0.5 ㎛ or less.

15. In claim 8, The yield strength of the above final coil is 800 MPa or more, The elongation of the final coil is 30% or more, Change in Gibbs free energy from austenite to martensite at room temperature (ΔG γ-α A method for manufacturing high-strength austenitic stainless steel having a thermal conductivity of (25℃) of -2.1 kJ / mol or less.

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