Austenitic stainless steel and its manufacturing method

Abstract

SUMMARY OF THE INVENTION In order to solve the above problems, an object of the present invention is to provide an austenitic stainless steel having high yield strength suitable for use in structural members, and a method for producing the same. [Solution] The austenitic stainless steel of the present invention is characterized by comprising, 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%, with the remainder being Fe and unavoidable impurities, satisfying formula (1): Nb*N≧0.015 (where Nb and N represent the weight percentages of each element), containing a (Cr, Fe)-Nb-N series Z-phase precipitate phase, and having an average crystal grain size at the thickness center of 2 μm or less.

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel, and more particularly to an austenitic stainless steel capable of ensuring excellent yield strength by controlling the alloy composition and precipitate phases to thereby refine the crystal grain size, and a method for producing the same. [Background technology]

[0002] Austenitic stainless steels have been used in a variety of applications, including transportation and construction components, due to their excellent formability, work hardening ability, and weldability. However, the yield strength of 304 and 301 stainless steels is in the 200-350 MPa range, limiting their practical application in structures. To obtain higher yield strength in general-purpose 300 series stainless steels, a typical method is to use a temper rolling process. However, temper rolling can have problems such as increased costs and extremely poor elongation of the material.

[0003] Therefore, there is a demand for austenitic stainless steels that have high yield strength and can be produced without undergoing a temper rolling process.

[0004] Patent Document 1 describes a method for manufacturing 300 series stainless steel for laser metal masks for photoetching, in which cold-rolled and annealed material is subjected to temper rolling, followed by two SR (Stress Relief) heat treatments to achieve small warpage even after half-etching. However, Patent Document 1 is a manufacturing technology for controlling etchability and warpage after etching, and does not include technical content related to structural parts with a thickness of 0.3 to 3.0 mm.

[0005] Patent Document 2 discloses that a long-term heat treatment is carried out at a temperature of 600-700°C for 48 hours or more to manufacture components for nuclear power plants with an average crystal grain size of 10 μm or less. However, this method has problems in that productivity is reduced when implemented on an actual production line, and the long-term heat treatment method increases manufacturing costs. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2016 / 043125 [Patent Document 2] Japanese Patent Publication No. 2020-050940 Summary of the Invention [Problem to be solved by the invention]

[0007] SUMMARY OF THE INVENTION In order to solve the above problems, an object of the present invention is to provide an austenitic stainless steel having high yield strength suitable for use in structural members, and a method for producing the same.

[0008] The problems to be solved by the present invention are not limited to the above problems, and other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0009] As a means for achieving the above object, the austenitic stainless steel of the present invention is characterized in that it contains, by weight%, 0.005 to 0.07% C, 0.1 to 1.0% Si, 0.1 to 2.0% Mn, 6.0 to 9.0% Ni, 16.0 to 19.0% Cr, 0.01 to 0.30% Nb, 0.01 to 0.20% N, with the remainder being Fe and unavoidable impurities, satisfies the following formula (1), contains a (Cr, Fe)-Nb-N series Z-phase precipitate phase, and has an average crystal grain size at the thickness center of 2 μm or less.

[0010] Formula (1): Nb*N≧0.015 (Here, Nb and N mean the weight percentage of each element.)

[0011] The austenitic stainless steel of the present invention may be an austenitic stainless steel in which the (Cr, Fe)-Nb-N series Z-phase precipitate phase is formed at a temperature of 1150°C or higher. Furthermore, the diameter of the Z-phase precipitate can be set to 50 to 300 nm.

[0012] The austenitic stainless steel of the present invention may have a pitting potential of 250 mV or more in a 3.5% NaCl solution at 30°C.

[0013] The austenitic stainless steel of the present invention may have a thickness of 0.3 mm or more and less than 3.0 mm.

[0014] The austenitic stainless steel of the present invention may have a yield strength of 930 MPa or more.

[0015] The hot-rolled and annealed austenitic stainless steel of the present invention is characterized in that it contains, by weight, 0.005-0.07% C, 0.1-1.0% Si, 0.1-2.0% Mn, 6.0-9.0% Ni, 16.0-19.0% Cr, 0.01-0.30% Nb, 0.01-0.20% N, with the remainder being Fe and unavoidable impurities, satisfies the following formula (1), contains a (Cr, Fe)-Nb-N series Z-phase precipitate phase, and has an average crystal grain size at the thickness center of 10 μm or less.

[0016] Formula (1): Nb*N≧0.015 (Here, Nb and N mean the weight percentage of each element.)

[0017] In the hot-rolled and annealed austenitic stainless steel of the present invention, the (Cr, Fe)-Nb-N series Z-phase precipitation phase may be formed at a temperature of 1150°C or higher.

[0018] The hot-rolled and annealed austenitic stainless steel of the present invention may have a thickness of 0.3 mm or more, and the diameter of the Z-phase precipitate may be 2 μm or less.

[0019] The method for producing austenitic stainless steel of the present invention includes the steps of casting a slab consisting of, by weight, 0.005-0.07% C, 0.1-1.0% Si, 0.1-2.0% Mn, 6.0-9.0% Ni, 16.0-19.0% Cr, 0.01-0.30% Nb, 0.01-0.20% N, with the remainder being Fe and unavoidable impurities, and satisfying the following formula (1); hot rolling the slab, hot-rolling annealing the slab, cold-rolling the slab, and cold-rolling annealing the slab at 700-850°C, wherein the slab contains a (Cr, Fe)-Nb-N series Z-phase precipitate phase after the hot-rolling annealing and cold-rolling annealing steps.

[0020] Formula (1): Nb*N≧0.015 (Here, Nb and N mean the weight percentage of each element.)

[0021] In the method for producing an austenitic stainless steel of the present invention, the (Cr, Fe)-Nb-N series Z-phase precipitate phase may be formed at a temperature of 1150°C or higher.

[0022] In the method for producing austenitic stainless steel of the present invention, after the hot rolling and annealing step, the average grain size at the thickness center may be 10 μm or less.

[0023] In the method for producing austenitic stainless steel of the present invention, after the hot rolling and annealing step, the thickness of the hot rolled and annealed material may be 3.0 mm or more.

[0024] In the method for producing austenitic stainless steel of the present invention, after the cold rolling and annealing step, the average grain size at the thickness center may be 2 μm or less.

[0025] In the method for producing austenitic stainless steel of the present invention, the thickness of the cold-rolled and annealed material after the cold-rolling and annealing step may be 0.3 mm or more and less than 3.0 mm.

[0026] In the method for producing an austenitic stainless steel of the present invention, the pitting potential in a 3.5% NaCl solution at 30°C may be 250 mV or more.

[0027] In the method for producing an austenitic stainless steel of the present invention, the yield strength may be 930 MPa or more. [Effects of the Invention]

[0028] According to the present invention, by controlling the alloy composition and precipitate phase, it is possible to obtain a fine grain size and provide an austenitic stainless steel having high yield strength suitable for use in structural members, and a method for producing the same. [Brief explanation of the drawings]

[0029] [Figure 1] 1 is a diagram showing precipitates in a hot-rolled annealed material of Example 2. [Figure 2] 1 is a diagram showing precipitates in the hot-rolled annealed material of Comparative Example 1. [Figure 3] 1 is a diagram showing precipitates in a cold-rolled annealed material of Example 2. [Figure 4] 1 is a diagram showing precipitates in the cold-rolled annealed material of Comparative Example 1. [Figure 5] 1 is a diagram showing the microstructure of a hot-rolled and annealed material of Example 2. [Figure 6] 1 is a diagram showing the microstructure of the hot-rolled annealed material of Comparative Example 1. [Figure 7] 1 is a diagram showing the microstructure of a cold-rolled annealed material of Example 2. [Figure 8] 1 is a diagram showing the microstructure of the cold-rolled annealed material of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0030] The following describes preferred embodiments of the present invention. However, the embodiments of the present invention can be modified into various other forms, 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 completely explain the present invention to those with average knowledge in the art. The terms used in this application are merely used to describe specific examples. Thus, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. In addition, the terms "comprise" or "use" as 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 intended to preclude the presence of other features, steps, functions, components, or combinations thereof. On the other hand, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Therefore, unless clearly defined herein, specific terms should not be construed in an overly ideal or formal sense.

[0031] Furthermore, in this specification, the terms "about," "substantially," and the like are used to mean from or near a numerical value when manufacturing and material tolerances inherent in the stated meaning are given, and are used to prevent unscrupulous infringers from unfairly taking advantage of disclosures in which precise or absolute numerical values ​​are stated to aid in the understanding of the present invention.

[0032] An austenitic stainless steel according to an example of the present invention will now be described.

[0033] An austenitic stainless steel according to one example of the present invention may consist, in weight percent, of 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%, with the remainder being Fe and unavoidable impurities.

[0034] The reasons for limiting the range of each alloying element are described below.

[0035] The C content may be 0.005 to 0.07% by weight.

[0036] C is an austenite phase stabilizing element, and the more it is added, the more effective it is in stabilizing the austenite phase. Taking this into consideration, the C content must be 0.005% or more. However, if C is added in an amount of 0.07% or more, it may form Cr-carbide during low-temperature annealing, causing problems with reduced intergranular corrosion resistance. Taking this into consideration, the C content can be set to 0.005 to 0.07% by weight.

[0037] The Si content may be 0.1 to 1.0% by weight.

[0038] Silicon is an element added as a deoxidizer during steelmaking. When a certain amount of silicon is added, it forms silicon oxide in the passive film during bright annealing, improving the corrosion resistance of steel. Considering this, the silicon content must be 0.1% or more. However, if silicon is added in amounts exceeding 1.0%, it can cause problems such as a decrease in the ductility of steel. Taking this into account, the silicon content can be set to 0.1 to 1.0 wt%.

[0039] The Mn content may be 0.1 to 2.0% by weight.

[0040] Mn is an austenite phase stabilizing element, and the more it is added, the more effective it is in stabilizing the austenite phase. Taking this into consideration, the Mn content must be 0.1% or more. However, since excessive addition of Mn may impair corrosion resistance, the content can be set to 2.0% or less. Taking this into consideration, the Mn content can be set to 0.1 to 2.0% by weight.

[0041] The Ni content may be 6.0 to 9.0 wt %.

[0042] Ni is an austenite phase stabilizing element, and the more Ni is added, the more the austenite phase is stabilized, and to soften the material, 6.0% or more can be added. However, since adding too much Ni can cause cost increases, it can be added up to 9.0%. Taking this into consideration, the Ni content can be set to 6.0 to 9.0 wt%.

[0043] The Cr content may be 16.0 to 19.0 wt %.

[0044] Cr is an essential element for improving corrosion resistance. Taking this into consideration, the Cr content must be 16.0% or more. However, if the Cr content is excessive, it can harden the material and cause problems with suppressing deformation-induced martensite during cold rolling, so the Cr content can be 19.0% or less. Taking this into consideration, the Cr content can be 16.0 to 19.0 wt%.

[0045] The Nb content may be 0.01 to 0.30% by weight.

[0046] Nb is essential for forming the Z-phase precipitation phase, which corresponds to the (Cr, Fe)-Nb-N system. The inclusion of Nb can suppress grain growth by forming the Z-phase precipitation phase. Taking this into consideration, the Nb content must be 0.01% or more. However, since excessive Nb content can cause defects such as nitrogen pore formation during the continuous casting process, the content can be kept below 0.30%. Taking this into consideration, the Nb content can be 0.01 to 0.30 wt%.

[0047] The N content may be 0.01 to 0.20% by weight.

[0048] N is an austenite phase stabilizing element, and the more it is added, the more the austenite phase can be stabilized and the greater the strength can be. Furthermore, N can form a Z-phase precipitation phase together with Nb. Taking this into consideration, the N content can be 0.01% or more. However, since an excessive N content can cause problems of hardening and reduced hot workability, the N content can be 0.20% or less. Taking this into consideration, the N content can be 0.01 to 0.20 wt%.

[0049] Furthermore, the austenitic stainless steel according to one example of the present invention can satisfy the following formula (1).

[0050] Formula (1): Nb*N≧0.015

[0051] Here, Nb and N mean the weight percentage of each element.

[0052] Considering the reasons for including the elements Nb and N, Nb*N in formula (1) can be set to 0.015 or more to form a Z-phase precipitate corresponding to the (Cr,Fe)-Nb-N series. When Nb*N is 0.015 or more, the Z-phase generation temperature can be increased. By increasing the Z-phase generation temperature, the Z-phase does not dissolve even at high hot rolling annealing temperatures and cold rolling annealing temperatures, so that the Z-phase precipitate can exist in both the hot rolled material and the cold rolled material. Preferably, Nb*N is 0.020 or more, more preferably 0.025 or more, and even more preferably 0.030 or more.

[0053] The remaining component is iron (Fe). However, in the normal manufacturing process, unintentional impurities may be inevitably mixed in from the raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, not all of their contents will be specifically mentioned in this specification.

[0054] An austenitic stainless steel according to an example of the present invention may include a (Cr, Fe)-Nb-N series Z-phase precipitate phase.

[0055] The presence of Z-phase precipitates can suppress grain growth at high temperatures. Z-phase precipitates are formed during heat treatment and remain soluble throughout the hot and cold rolling annealing stages, preventing grain coarsening. In contrast, precipitates such as chromium carbide and chromium nitride are not formed prior to heat treatment. At typical hot rolling annealing temperatures, precipitates such as chromium carbide and chromium nitride are dissolved and solid-solved within the matrix. Therefore, unlike Z-phase precipitates, it may be difficult to achieve the effect of suppressing grain growth by precipitation during heat treatment.

[0056] The type of precipitated phase in the austenitic stainless steel according to the example of the present invention was observed at the center of the thickness of the austenitic stainless steel using a TEM (Transmission Electron Microscope) Replica method. The thickness center portion of the austenitic stainless steel according to one example of the present invention means 1 / 4t to 3 / 4t, where t is the thickness of the austenitic stainless steel.

[0057] In an austenitic stainless steel according to one example of the present invention, the (Cr, Fe)-Nb-N series Z-phase precipitate phase may be formed at a temperature of 1150° C. or higher, and the diameter of the precipitate phase may be 50 to 300 nm.

[0058] The formation temperature of the precipitate phase of the austenitic stainless steel according to one example of the present invention means the formation temperature of the Z-phase, which is a (Cr, Fe)-Nb-N series precipitate phase, calculated by the ThermoCalc precipitate phase analysis program.

[0059] The diameter of the precipitated phase of the austenitic stainless steel according to the embodiment of the present invention was measured by observing the center of the thickness of the austenitic stainless steel using a TEM (Transmission Electron Microscope) Replica method.

[0060] The austenitic stainless steel according to one 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.

[0061] The austenitic stainless steel according to one embodiment of the present invention can be used for structural applications such as automobile exterior panels and building components. In the case of structural components, the material thickness may be in the range 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. In the case of the present invention, excellent yield strength can be obtained even when the thickness of the austenitic stainless steel is 0.3 mm or more and less than 3.0 mm.

[0062] In the austenitic stainless steel according to one example of the present invention, the average crystal grain size at the thickness center may be 2 μm or less.

[0063] In the past, to ensure excellent yield strength, cold rolling was used to transform the austenite phase into the martensite phase, followed by low-temperature annealing to achieve ultrafine grains. The present invention provides an austenitic stainless steel that achieves excellent yield strength by controlling the alloy composition and precipitation phases to ensure fine grain size. "Fine grain size" can mean, for example, an average grain size of 3 μm or less, and more specifically, 2 μm or less.

[0064] The thickness center of the austenitic stainless steel according to one example of the present invention refers to 1 / 4t to 3 / 4t, where t is the thickness of the austenitic stainless steel. Furthermore, the thickness center average refers to the average of measurements taken at three arbitrary locations in the 1 / 4t to 3 / 4t region. The grain size was measured by observing three arbitrary locations in the thickness center using a thin foil TEM (Transmission Electron Microscope).

[0065] The austenitic stainless steel according to one example of the present invention may have a pitting potential of 250 mV or more in a 3.5% NaCl solution at 30°C.

[0066] The austenitic stainless steel according to one embodiment of the present invention may have a yield strength of 930 MPa or more. By ensuring a high yield strength of 930 MPa or more, it is possible to provide an austenitic stainless steel that can be used for structural parts such as automotive exterior panels and building components.

[0067] The austenitic stainless steel according to one embodiment of the present invention may be a cold-rolled and annealed material.

[0068] Hereinafter, an austenitic stainless steel hot-rolled and annealed material according to one embodiment of the present invention will be described. An austenitic stainless steel hot-rolled and annealed material according to one example of the present invention can consist, in weight percent, of 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%, with the remainder being Fe and unavoidable impurities. Furthermore, the hot-rolled and annealed austenitic stainless steel according to one embodiment of the present invention can satisfy the following formula (1).

[0069] Formula (1): Nb*N≧0.015 Here, Nb and N mean the weight percentage of each element.

[0070] The reasons for limiting the range of each alloying element and formula (1) are as described above in the austenitic stainless steel according to the example of the present invention.

[0071] The hot-rolled and annealed austenitic stainless steel according to one example of the present invention may contain a (Cr, Fe)-Nb-N series Z-phase precipitate phase.

[0072] The (Cr, Fe)-Nb-N series Z-phase precipitate phase is as described above in the austenitic stainless steel according to the example of the present invention.

[0073] In one example of the present invention, the (Cr, Fe)-Nb-N series Z-phase precipitate phase of the hot-rolled and annealed austenitic stainless steel may be formed at a temperature of 1150°C or higher, and the diameter of the precipitate phase may be 0.2 μm or less.

[0074] By controlling the formation temperature of the (Cr,Fe)-Nb-N series Z-phase precipitate phase in the hot-rolled and annealed material to 1150°C or higher, as in austenitic stainless steel, the Z-phase precipitate phase can remain undissolved even during the hot-rolled and annealed stage. By controlling this, the Z-phase precipitate phase can be controlled to remain even during the hot-rolled and annealed austenitic stainless steel according to one example of the present invention, which further prevents the crystal grains from becoming coarse during the annealing stage.

[0075] The type of precipitated phase, the temperature at which the precipitated phase is formed, and the method for measuring the diameter of the precipitated phase in the hot-rolled and annealed austenitic stainless steel according to one example of the present invention are the same as those described above for the austenitic stainless steel according to one example of the present invention.

[0076] The hot-rolled and annealed austenitic stainless steel according to one embodiment of the present invention may have a thickness of 3.0 mm or more. By controlling the thickness of the hot-rolled and annealed austenitic stainless steel according to one embodiment of the present invention to 3.0 mm or more, the austenitic stainless steel produced from the hot-rolled and annealed austenitic stainless steel according to one embodiment of the present invention can be used for structural applications such as automotive exterior panels and building components.

[0077] In the hot-rolled and annealed austenitic stainless steel according to one example of the present invention, the average crystal grain size at the thickness center may be 10 μm or less.

[0078] The fact that the grain size of the hot rolled and annealed material is fine can mean, for example, that the average grain size is 10 μm or less.

[0079] The thickness center of the hot-rolled and annealed austenitic stainless steel according to one example of the present invention refers to the region between 1 / 4t and 3 / 4t, where t is the thickness of the hot-rolled and annealed austenitic stainless steel. Furthermore, the thickness center average refers to the average of measurements taken at three arbitrary locations in the 1 / 4t to 3 / 4t region. The grain size was measured by observing and measuring three arbitrary locations in the thickness center using an OM (Optical Microscope).

[0080] An example of a method for producing austenitic stainless steel according to the present invention will now be described.

[0081] One example of the present invention can be a method for producing austenitic stainless steel, comprising the steps of: casting a slab consisting of, by weight, 0.005-0.07% C, 0.1-1.0% Si, 0.1-2.0% Mn, 6.0-9.0% Ni, 16.0-19.0% Cr, 0.01-0.30% Nb, 0.01-0.20% N, with the remainder being Fe and unavoidable impurities, and satisfying the following formula (1); hot rolling the slab; hot rolling annealing; cold rolling; and cold rolling annealing at 700-850°C.

[0082] Formula (1): Nb*N≧0.015 Here, Nb and N mean the weight percentage of each element.

[0083] In a method for producing austenitic stainless steel according to an embodiment of the present invention, the hot-rolled and annealed material after the hot-rolling and annealing step may contain a (Cr, Fe)-Nb-N series Z-phase precipitation phase, and the cold-rolled and annealed material after the cold-rolling and annealing step may contain a (Cr, Fe)-Nb-N series Z-phase precipitation phase.

[0084] The reasons for limiting the ranges of the alloying elements and formula (1) are as described above for the austenitic stainless steel according to the present invention. Below, we will look more specifically at the manufacturing method and the Z-phase precipitate.

[0085] The austenitic stainless steel corresponding to the hot-rolled and annealed austenitic stainless steel and the cold-rolled and annealed austenitic stainless steel according to an embodiment of the present invention may contain a (Cr, Fe)-Nb-N series Z-phase precipitate phase. In this case, the formation temperature of the Z-phase, which is a (Cr, Fe)-Nb-N series precipitate phase, calculated using the ThermoCalc precipitate phase analysis program may be 1150°C or higher.

[0086] In the method for producing austenitic stainless steel according to one embodiment of the present invention, the hot rolling and annealing step may be performed at a temperature of 1000 to 1150°C. By setting the hot rolling and annealing temperature at 1000 to 1150°C, recrystallization can occur. This can be considered as a normal hot rolling and annealing temperature.

[0087] The Z-phase, a (Cr, Fe)-Nb-N series precipitation phase, is generated before the hot rolling and annealing stage. Therefore, by controlling the generation temperature of the Z-phase, a (Cr, Fe)-Nb-N series precipitation phase, to 1150°C or higher, the Z-phase is generated before the hot rolling and annealing stage and may remain undissolved during the hot rolling and annealing stage. By controlling the Z-phase to exist in the hot rolling and annealing material, it is possible to prevent coarsening of the crystal grains in later stages.

[0088] In a method for producing austenitic stainless steel according to an embodiment of the present invention, the cold rolling step may have a reduction rate of 40% or more. When the cold rolling reduction rate is 40% or more, TRIP transformation can be induced. The reduction rate may be 40% or more, more specifically, 50% or more.

[0089] In the method for producing austenitic stainless steel according to one embodiment of the present invention, the hot-rolled and annealed material may have a thickness of 3.0 mm or more after the hot-rolled and annealed step, and the cold-rolled and annealed material may have a thickness of 0.3 mm or more but less than 3.0 mm after the cold-rolled and annealed step.

[0090] In a method for producing austenitic stainless steel according to one embodiment of the present invention, the cold rolling annealing step may be performed at a temperature of 700 to 850°C. When the cold rolling annealing temperature is 700°C or higher, new reverted austenite nucleation can occur. When the cold rolling annealing temperature is 850°C or higher, it is advantageous for grain refinement. In consideration of this, the cold rolling annealing temperature in the present invention may be controlled to be 700 to 850°C.

[0091] Similar to hot-rolled and annealed materials, austenitic stainless steels corresponding to cold-rolled and annealed materials after the cold-rolled and annealed stage contain Z-phase, which is a (Cr, Fe)-Nb-N series precipitate phase, thereby ensuring fine grain size.

[0092] In the method for producing austenitic stainless steel according to an embodiment of the present invention, after the hot rolling and annealing step, the average grain size at the thickness center of the hot rolled and annealed material may be 10 μm or less.

[0093] In the method for producing austenitic stainless steel according to an embodiment of the present invention, after the cold rolling and annealing step, the average grain size at the thickness center of the stainless steel corresponding to the cold rolled and annealed material may be 2 μm or less.

[0094] The meanings of the thickness center and the thickness center are the same as those described above in connection with the austenitic stainless steel according to one example of the present invention and the hot-rolled annealed austenitic stainless steel according to one example of the present invention.

[0095] Furthermore, it is possible to obtain an austenitic stainless steel that has excellent yield strength due to the finely secured crystal grains.

[0096] The austenitic stainless steel produced by the austenitic stainless steel production method according to one embodiment of the present invention may have a pitting potential of 250 mV or more in a 3.5% NaCl solution at 30°C. The austenitic stainless steel may also have a yield strength of 930 MPa or more. This makes it possible to obtain austenitic stainless steel that has high yield strength while maintaining corrosion resistance.

[0097] The present invention will be described in more detail below with reference to examples and drawings. However, the description of such embodiments and drawings is intended to illustrate the implementation of the present invention, and the present invention is not limited by the description of such embodiments. The scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom. [Example]

[0098] Table 1 below shows the alloy compositions and types of precipitated phases of the invention examples and comparative examples. Slabs having the alloy compositions shown in Table 1 below were hot rolled, and then hot rolled and annealed at 1050°C, and the types of precipitated phases in the hot rolled and annealed materials were observed. In addition, the material was cold rolled at a reduction ratio of 40% and cold-rolled and annealed at 800°C, and the type of precipitated phase in the cold-rolled and annealed material with a thickness of 0.8 mm was observed.

[0099] Fig. 1 is a diagram showing precipitates in the hot-rolled and annealed material of Example 2, and Fig. 2 is a diagram showing precipitates in the hot-rolled and annealed material of Comparative Example 1. Fig. 3 is a diagram showing precipitates in the cold-rolled and annealed material of Example 2, and Fig. 4 is a diagram showing precipitates in the cold-rolled and annealed material of Comparative Example 1.

[0100] The type and size of the precipitated phases were observed at the center of the thickness of the hot-rolled and annealed material using a TEM replica method.

[0101] [Table 1]

[0102] The alloy compositions of Examples 1 to 4 satisfy the range of the present invention. In particular, the Nb*N content falls within the range of 0.015 or more. In Examples 1 to 4, it can be confirmed that the types of precipitated phases observed in both the cold-rolled and annealed material and the hot-rolled and annealed material include Z-phase.

[0103] FIG. 1 is a diagram showing the precipitates in the hot-rolled and annealed material of Example 2, and FIG. 3 is a diagram showing the precipitates in the cold-rolled and annealed material of Example 2. The precipitates in Example 2 and the types of elements contained therein can be confirmed from FIGS. 1 and 3. It can be confirmed that the precipitates in the hot-rolled and annealed material of Example 2 correspond to precipitates consisting of Cr, Fe, N, and Nb. This confirms that the precipitates in the hot-rolled and annealed material of Example 2 are Z-phase precipitates.

[0104] In contrast, Comparative Example 1 does not contain any Nb. Since the Z-phase corresponds to the (Cr, Fe)-Nb-N series, it can be confirmed that Comparative Example 1, which does not contain any Nb, was unable to form any Z-phase at all in either the cold-rolled and annealed material or the hot-rolled and annealed material.

[0105] FIG. 2 is a diagram showing precipitates in the hot-rolled and annealed material of Comparative Example 1, and FIG. 4 is a diagram showing precipitates in the cold-rolled and annealed material of Comparative Example 1.

[0106] As shown in Figure 2, it can be seen that no precipitate phase is formed. Furthermore, since no precipitate phase is formed, the elements forming the precipitates cannot be identified. This confirms that not only is it impossible to obtain a Z-phase precipitate phase when Nb is not included, but also that precipitates such as Cr-carbide and Cr-nitride are dissolved before hot rolling and annealing and are not formed as precipitates.

[0107] As shown in Fig. 4, the precipitates of the cold-rolled and annealed material of Comparative Example 1 and the types of elements contained in the precipitates can be confirmed. It can be confirmed that the precipitates of Comparative Example 1 correspond to precipitates consisting of Cr, Fe, and N. This confirms that the precipitates of the cold-rolled and annealed material of Comparative Example 1 are Cr-nitride precipitate phases.

[0108] Although the alloy compositions of Comparative Examples 2 to 4 satisfy the range of the present invention, the Nb*N value is less than 0.015. Therefore, the Z-phase precipitation temperature cannot be ensured, and the Z-phase precipitation cannot be observed in either the cold-rolled and annealed material or the hot-rolled and annealed material.

[0109] 1 and 2, it can be seen that the diameter of precipitates in the hot-rolled annealed steel of Example 2 according to one embodiment of the present invention is much smaller than that of Comparative Example 1. In addition, it can be seen that Z-phase precipitates were obtained in the hot-rolled annealed steel of Example 2, since Cr, Fe, Nb, and N were observed. It can be seen that no precipitates were observed in the hot-rolled annealed steel of Comparative Example 1, since it did not contain Nb.

[0110] 3 and 4, it can be seen that a Z-phase precipitate was formed in the hot-rolled and annealed material in Example 2, which is an example of the present invention, because Cr, Fe, Nb, and N were observed. Since Comparative Example 1 does not contain Nb, it can be seen that a Cr nitride precipitate was formed in the cold-rolled and annealed material, because Cr, Fe, and N were observed.

[0111] Table 2 below shows the Z-phase generation temperature, average grain size, pitting potential and yield strength of the cold-rolled and annealed materials of the invention examples and comparative examples.

[0112] The Z-phase formation temperature refers to the formation temperature of the Z-phase, which is a (Cr, Fe)-Nb-N series precipitate phase, calculated using the ThermoCalc precipitate phase analysis program.

[0113] 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 using a thin foil TEM (Transmission Electron Microscope). The thickness center refers to 1 / 4t to 3 / 4t, where t is the thickness of the cold-rolled annealed material.

[0114] The pitting potential is a value measured in a 3.5% NaCl solution at 30°C.

[0115] The yield strength means the yield strength obtained after a tensile test was carried out on a JIS13B tensile test piece at room temperature with a crosshead speed in the range of 10 mm / min to 20 mm / min.

[0116] Fig. 5 is a diagram showing the microstructure of the hot-rolled and annealed material of Example 2, and Fig. 6 is a diagram showing the microstructure of the hot-rolled and annealed material of Comparative Example 1. Fig. 7 is a diagram showing the microstructure of the cold-rolled and annealed material of Example 2, and Fig. 8 is a diagram showing the microstructure of the cold-rolled and annealed material of Comparative Example 1.

[0117] The microstructure of the hot-rolled and annealed material was observed using an OM (Optical Microscope), and the microstructure of the cold-rolled and annealed material was observed using a TEM (Transmission Electron Microscope) thin foil.

[0118] [Table 2]

[0119] Inventive Examples 1 to 4, it was confirmed that the Z-phase formation temperature was 1150°C or higher. Therefore, when the alloy composition and the value of Equation (1) were satisfied, Z-phase precipitates were observed in both cold-rolled and annealed materials as well as hot-rolled and annealed materials. This was confirmed to be because the Z-phase formation temperature was 1150°C or higher, and the Z-phase precipitates remained unmelted even after hot-rolled and annealed. Inventive Examples 1 to 4, the average grain size of the cold-rolled and annealed materials was 2 μm or less. This confirmed that the pitting potential was 250 mV or higher and the yield strength was 930 MPa or higher. This provides an austenitic stainless steel that can achieve high yield strength and excellent corrosion resistance.

[0120] In contrast, in the case of Comparative Example 1, which does not contain any Nb, the conditions are such that the Z-phase cannot be generated at all, so there is no point in measuring the generation temperature. The average grain size of the cold-rolled and annealed material of Comparative Example 1 was very coarse at 6.7 μm, and it was confirmed that the yield strength was 545 MPa, which indicated that a high yield strength could not be achieved.

[0121] In Comparative Examples 2 to 4, the Z-phase precipitation phase generation temperature was 1150°C or lower. Therefore, all of the precipitation phase was melted before the hot rolling and annealing stage, and therefore no precipitation phase was observed in the hot-rolled and annealed material, as shown in Table 1. Furthermore, the Z-phase precipitation phase, which had already melted, could not be observed even after cold rolling and annealing.

[0122] It can be confirmed that Cr carbide and / or Cr nitride precipitate phases were obtained in Comparative Examples 2 to 4. The Cr carbide and / or Cr nitride precipitate phases are not precipitate phases that can refine crystal grains. Therefore, it can be confirmed that Comparative Examples 2 to 4 have coarse average crystal grain sizes of 3.2 μm or more, and at the same time, the yield strength is only 672 MPa or less, which is inferior.

[0123] As shown in FIGS. 5 and 6, it can be seen that the grain size of the microstructure in the hot-rolled annealed material of Example 2 according to the present invention is much finer than that of Comparative Example 1.

[0124] As shown in FIGS. 7 and 8, it can be seen that the crystal grain size of the microstructure in the cold rolled and annealed material of Example 2 according to the present invention is much finer than that of Comparative Example 1.

[0125] Therefore, by controlling the alloy composition of the present invention and Nb*N corresponding to formula (1) to ensure the Z-phase precipitation temperature, it is possible to ensure the Z-phase precipitation phase in not only the cold-rolled and annealed material but also the hot-rolled and annealed material, and it is possible to provide an austenitic stainless steel with a high yield strength of 930 MPa or more by ensuring the average crystal grain size of the cold-rolled and annealed material to be fine at 2 μm or less.

Claims

1. In weight percent, C: 0.005 to 0.07%, Si: 0.1 to 1.0%, Mn: 0.1 to 2.0%, Ni: 6.0 to 9.0%, Cr: 16.0 to 19.0%, Nb: 0.01 to 0.30%, N: 0.01 to 0.20%, the balance being Fe and unavoidable impurities, The following formula (1) is satisfied: Contains a (Cr, Fe)-Nb-N series Z-phase precipitate phase, An austenitic stainless steel characterized in that the average crystal grain size at the center of the thickness is 2 μm or less. Formula (1): Nb*N≧0.015 (Here, Nb and N mean the weight percentage of each element.)

2. 2. The austenitic stainless steel according to claim 1, wherein the (Cr, Fe)-Nb-N series Z-phase precipitate phase is formed at a temperature of 1150° C. or higher.

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

4. 2. The austenitic stainless steel according to claim 1, characterized in that it has a pitting potential of 250 mV or more in a 3.5% NaCl solution at 30°C.

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

6. 2. The austenitic stainless steel according to claim 1, characterized in that the grain size is 0.3 mm or more and less than 3.0 mm.

7. In weight percent, C: 0.005 to 0.07%, Si: 0.1 to 1.0%, Mn: 0.1 to 2.0%, Ni: 6.0 to 9.0%, Cr: 16.0 to 19.0%, Nb: 0.01 to 0.30%, N: 0.01 to 0.20%, the balance being Fe and unavoidable impurities, The following formula (1) is satisfied: Contains a (Cr, Fe)-Nb-N series Z-phase precipitate phase, A hot-rolled and annealed austenitic stainless steel material, characterized in that the average crystal grain size at the center of the thickness is 10 μm or less. Formula (1): Nb*N≧0.015 (Here, Nb and N mean the weight percentage of each element.)

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

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

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

11. A step of casting a slab consisting of, in weight percent, C: 0.005 to 0.07%, Si: 0.1 to 1.0%, Mn: 0.1 to 2.0%, Ni: 6.0 to 9.0%, Cr: 16.0 to 19.0%, Nb: 0.01 to 0.30%, N: 0.01 to 0.20%, the remainder being Fe and unavoidable impurities, and satisfying the following formula (1): hot rolling the slab; hot rolling and annealing; cold rolling; and cold rolling annealing at 700 to 850 ° C; The method for producing austenitic stainless steel, wherein the steel contains a (Cr, Fe)-Nb-N series Z-phase precipitate phase after the hot rolling and annealing steps and after the cold rolling and annealing steps. Formula (1): Nb*N≧0.015 (Here, Nb and N mean the weight percentage of each element)

12. 12. The method for producing austenitic stainless steel according to claim 11, wherein the (Cr, Fe)-Nb-N series Z-phase precipitate phase is formed at a temperature of 1150° C. or higher.

13. After the hot rolling annealing step, The thickness of the hot-rolled annealed material is 3.0 mm or more, 12. The method for producing austenitic stainless steel according to claim 11, wherein the average crystal grain size at the center of the thickness is 10 μm or less.

14. After the cold rolling annealing step, 12. The method for producing austenitic stainless steel according to claim 11, wherein the average crystal grain size at the center of the thickness is 2 μm or less.

15. After the cold rolling annealing step, 12. The method for producing austenitic stainless steel according to claim 11, wherein the thickness of the cold-rolled annealed material is 0.3 mm or more and less than 3.0 mm.

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

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

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