High-strength austenitic stainless steel and method for producing the same

Abstract

The present invention provides a low-Ni austenitic stainless steel having high yield strength, excellent hot workability and corrosion resistance, and a method for producing the same. [Solution] The high-strength austenitic stainless steel comprises C: 0.05% or more and 0.1% or less, Si: 0.1% or more and 1.0% or less, Mn: 1.0% or more and 5.0% or less, P: less than 0.05%, S: less than 0.03%, Cr: 14.0% or more and 18.0% or less, Ni: 1.0% or more and 5.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.1% or more and 0.2% or less, the remainder being Fe and unavoidable impurities, and the diameter deviation between crystal grains in the thickness direction is 2 or less, The value of the following formula (1) is less than 1.0, and the formula (1): 0.18Si + 0.45C + 4.4N The value of the following formula (2) is 3.6 or less, i.e., formula (2): 0.02Si + 0.21Mn + 0.09Cr + 0.15Ni + 2.73(C + N); the value of the following formula (3) is 0.35 or more, i.e., formula (3): -0.04Si + 0.05Mn - 0.01Cr + 0.05Ni + 1.18(C + N).

Description

[Technical Field]

[0001] The present invention relates to a high-strength austenitic stainless steel and a method for producing the same, and more particularly to a high-strength austenitic stainless steel that has high yield strength, excellent hot workability, and corrosion resistance due to component control and grain refinement, and a method for producing the same. [Background technology]

[0002] In recent years, the rise in Ni prices and deepening volatility have led to an increase in demand for low-Ni austenitic stainless steels, but they have a problem of poor yield strength below 250 MPa.

[0003] In addition, attempts have been made to reduce the content of Ni, an expensive element, by substituting it with austenite stabilizing elements such as Mn and N, but this has led to the problem of poor corrosion resistance due to the formation of MnS.

[0004] To increase the strength of austenitic stainless steel, methods such as work hardening by temper rolling and adding large amounts of interstitial elements such as C and N are used. However, tempered materials have poor elongation and are less usable, high C additions reduce weldability, and high N additions reduce hot workability.

[0005] Patent Document 1 discloses a fine-grained austenitic stainless steel with excellent strength and ductility. However, Patent Document 1 does not disclose the difference in grain refinement due to differences in austenite phase stability depending on the components, or the change in hot workability due to grain refinement. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Publication No. 10-2007-0067905 Summary of the Invention [Problem to be solved by the invention]

[0007] In order to solve the above problems, an object of the present invention is to provide a low-Ni austenitic stainless steel having high yield strength, excellent hot workability and corrosion resistance through component control and grain refinement, and a method for producing the same. [Means for solving the problem]

[0008] The high-strength austenitic stainless steel is characterized by having C: 0.05% or more and 0.1% or less, Si: 0.1% or more and 1.0% or less, Mn: 1.0% or more and 5.0% or less, P: less than 0.05%, S: less than 0.03%, Cr: 14.0% or more and 18.0% or less, Ni: 1.0% or more and 5.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.1% or more and 0.2% or less, the remainder being Fe and unavoidable impurities, and having a diameter deviation between crystal grains in the thickness direction of 2 or less. Furthermore, the value of the following formula (1) may be less than 1.0.

[0009] Formula (1): 0.18Si+0.45C+4.4N In the above formula (1), Si, C, and N represent the content (wt %) of each element.

[0010] Furthermore, the value of the following formula (2) may be 3.6 or less.

[0011] Formula (2):0.02Si+0.21Mn+0.09Cr+0.15Ni+2.73(C+N) In the above formula (2), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.

[0012] Furthermore, the value of the following formula (3) may be 0.35 or more.

[0013] Formula (3):-0.04Si+0.05Mn-0.01Cr+0.05Ni+1.18(C+N) In the above formula (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.

[0014] The average crystal grain diameter at the center of the thickness may be 10 μm or less.

[0015] The yield strength may be 600 MPa or more.

[0016] The stretching ratio may be 35% or more.

[0017] The pitting potential may be 200 mV or more.

[0018] The thickness may be 0.5 to 2.0 mm.

[0019] The method for producing high-strength austenitic stainless steel includes the steps of producing a slab containing C: 0.05% to 0.1%, Si: 0.1% to 1.0%, Mn: 1.0% to 5.0%, P: less than 0.05%, S: less than 0.03%, Cr: 14.0% to 18.0%, Ni: 1.0% to 5.0%, Cu: 0.1% to 2.0%, N: 0.1% to 0.2%, and the remainder being Fe and unavoidable impurities; hot-rolling the slab and then reheating it to produce a hot-rolled material; and cold-rolling the hot-rolled material and then cold-rolling annealing it at 850 to 900°C to produce a cold-rolled material. The slab may have a value of less than 1.0 in the following formula (1):

[0020] Formula (1): 0.18Si+0.45C+4.4N In the above formula (1), Si, C, and N represent the content (wt %) of each element.

[0021] Furthermore, the slab may have a value of the following formula (2) of 3.6 or less.

[0022] Formula (2):0.02Si+0.21Mn+0.09Cr+0.15Ni+2.73(C+N) In the above formula (2), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.

[0023] Furthermore, the slab may have a value of the following formula (3) of 0.35 or more.

[0024] Formula (3):-0.04Si+0.05Mn-0.01Cr+0.05Ni+1.18(C+N) In the above formula (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.

[0025] The reheating may be carried out at a temperature of 1000 to 1150°C. The cold rolling may be carried out at a thickness reduction rate of 60% or more. [Effects of the Invention]

[0026] According to the present invention, it is possible to obtain a low-Ni austenitic stainless steel that has high yield strength, controlled phase stability, and excellent hot workability and corrosion resistance, and a method for producing the same. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is an image of the microstructure of the high-strength austenitic stainless steel of the present invention photographed by EBSD (Electron Backscatter Diffraction, a backscattered electron diffraction pattern analyzer). DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention will now be described in detail with reference to the accompanying drawings. The following examples are presented to fully convey the spirit of the disclosed invention to those skilled in the art to which the disclosed invention pertains. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. In the drawings, parts irrelevant to the description may be omitted to clarify the disclosed invention, and the sizes of components may be somewhat exaggerated to facilitate understanding. Throughout the specification, when a part is said to "comprise" certain elements, this means that it may further include other elements, rather than excluding other elements, unless specifically stated to the contrary. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0029] The reasons for limiting the alloying element contents in the examples of the present invention will be explained below. Unless otherwise specified, the units are % by weight.

[0030] High-strength austenitic stainless steel consists of C: 0.05% or more and 0.1% or less, Si: 0.1% or more and 1.0% or less, Mn: 1.0% or more and 5.0% or less, P: less than 0.05%, S: less than 0.03%, Cr: 14.0% or more and 18.0% or less, Ni: 1.0% or more and 5.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.1% or more and 0.2% or less, and the remainder being Fe and unavoidable impurities.

[0031] The C (carbon) content may be 0.05% or more and 0.1% or less.

[0032] C is an inexpensive element that is very effective in stabilizing austenite. As an interstitial element, C contributes to improving strength through the solid solution strengthening effect. Taking this into consideration, C can be added in an amount of 0.05% or more. However, if the C content is excessive, the Cr content in the heat-affected zone after welding will increase. 23 Sensitization may occur due to grain boundary precipitation of carbides such as C6, which may result in poor ductility, toughness, corrosion resistance, etc. In consideration of this, the upper limit of the C content may be limited to 0.1%. Preferably, the C content may be 0.058% or more and 0.096% or less.

[0033] The content of Si (silicon) may be 0.1% or more and 1.0% or less.

[0034] Silicon acts as a deoxidizer during the steelmaking process and is an effective element for improving corrosion resistance. Taking this into consideration, silicon can be added in an amount of 0.1% or more. However, excessive silicon content can lead to the formation of a delta-ferrite phase due to a peritectic reaction during casting, which can degrade hot workability. Taking this into consideration, the upper limit of silicon content can be set at 1.0%. Preferably, the silicon content is 0.34% or more and 0.97% or less.

[0035] The Mn (manganese) content may be 1.0% or more and 5.0% or less.

[0036] Mn is an inexpensive element that stabilizes the austenite phase. Furthermore, Mn is an effective element for suppressing thermally induced and strain-induced martensitic transformations and increasing low-temperature impact toughness. Taking this into consideration, Mn can be added in an amount of 1.0% or more. However, excessive Mn content can lead to an increase in inclusions (MnS), which can degrade the corrosion resistance and hot workability of the steel. Taking this into consideration, the upper limit of the Mn content can be set at 5.0%. Preferably, the Mn content may be 1.02% or more and 3.79% or less.

[0037] The P (phosphorus) content may be 0% to less than 0.05%, or more than 0% to less than 0.05%.

[0038] P is an element that is inevitably contained in steel and reduces corrosion resistance and hot workability. In consideration of this, the P content may be 0% or more than 0% and less than 0.05%. Preferably, the P content may be 0.02% or less. In this case, the effect of reducing process costs can be further improved while minimizing the effect of the P content on physical properties.

[0039] The content of S (sulfur) may be 0% to less than 0.03%, or more than 0% to less than 0.03%.

[0040] Like P, S is an element that is inevitably contained in steel as an impurity and reduces corrosion resistance and hot workability. In consideration of this, the S content may be 0% or more than 0% and less than 0.03%. Preferably, the S content may be 0.002% or less. In this case, the effect of reducing process costs can be further improved while minimizing the effect of the S content on physical properties.

[0041] The Cr (chromium) content may be 14.0% or more and 18.0% or less.

[0042] Cr is an essential element for ensuring corrosion resistance and phase stability. Taking this into consideration, Cr can be added in an amount of 14.0% or more. However, if the Cr content is excessive, a delta-ferrite phase may be formed by a peritectic reaction, which may result in a deterioration of hot workability. Taking this into consideration, the upper limit of the Cr content can be set at 18.0%.

[0043] The Ni (nickel) content may be 1.0% or more and 5.0% or less.

[0044] Ni is a powerful element that stabilizes the austenite phase. Ni also effectively suppresses thermally induced and strain-induced martensitic transformation, preventing a decrease in toughness at cryogenic temperatures. The addition of Ni also improves hot and cold workability. Taking this into consideration, Ni can be added in an amount of 1.0% or more. However, excessive Ni content can reduce grain refinement. Furthermore, excessive Ni content can increase raw material costs. Taking this into consideration, the upper limit of the Ni content can be set at 5.0%. Preferably, the Ni content can be 2.61% or more and 4.45% or less.

[0045] The content of Cu (copper) may be 0.1% or more and 2.0% or less.

[0046] Cu is an effective element for stabilizing the austenite phase. Cu is also effective for suppressing thermally induced and strain-induced martensitic transformation. Taking this into consideration, Cu can be added in an amount of 0.1% or more. However, an excessive Cu content can deteriorate hot workability due to Cu solidification segregation. Taking this into consideration, the upper limit of the Cu content can be limited to 2.0%. Preferably, the Cu content may be 1.19% or more and 1.88% or less.

[0047] The N (nitrogen) content may be 0.1% or more and 0.2% or less.

[0048] N is an inexpensive element that is very effective in stabilizing the austenite phase. N is also an element that is effective in increasing strength and corrosion resistance through solid solution strengthening. In consideration of this, N can be added in an amount of 0.1% or more. However, if the N content is excessive, hot workability may be deteriorated. In consideration of this, the upper limit of the N content may be limited to 0.2%. Preferably, the N content may be 0.173% or more and 0.189% or less.

[0049] 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 it is not possible to exclude these. Since these impurities are known to anyone skilled in the normal manufacturing process, not all of the contents thereof will be specifically mentioned in this specification.

[0050] Generally, mechanically-induced martensite (ε, α'-martensite) can develop in austenitic stainless steels during cold rolling. The mechanically-induced phase tends to develop differently depending on the austenite phase stability. In austenitic stainless steels with low phase stability, ε-martensite bands develop at the beginning of deformation, and as the amount of deformation increases, α'-martensite can form at the intersection points within the bands.

[0051] To reduce the amount of expensive Ni and increase cost competitiveness, it is necessary to control the phase stability by utilizing other austenite phase stabilizing elements other than Ni. To achieve this, the free energy change (ΔG γ-α ) values ​​need to be controlled.

[0052] On the other hand, when the rolled and tempered material is subjected to cold rolling and annealing, a reversion transformation from the deformation-induced martensite phase to the austenite phase can occur. The reversion transformation process can be mainly divided into diffusion reversion and shear reversion. The reversion transformation process occurs due to the free energy change (ΔG α-γ ) can proceed. In general, martensite shear reverse transformation requires a larger free energy change (ΔG α-γ ) is required.

[0053] Therefore, in the present invention, in order to suppress the decrease in hot workability due to the reduction in Ni, the alloy composition is controlled. γ-α ) to smooth the transformation from austenite to martensite. Also, the free energy change (ΔG α-γ ) to induce the reverse transformation and recrystallization from martensite to austenite during cold rolling and annealing to occur easily.

[0054] Furthermore, the present invention aims to ensure excellent strength and corrosion resistance by controlling the grain diameter and uniformity.

[0055] High-strength austenitic stainless steel has a value of less than 1.0 in the following formula (1), and may be preferably 0.48 or more and less than 1.0.

[0056] Formula (1): 0.18Si+0.45C+4.4N In the above formula (1), Si, C, and N represent the content (wt %) of each element. Formula (1) represents HRI (Hot Rolling Index) as a hot rollability index.

[0057] When the value of formula (1) is controlled to less than 1.0, hot workability is improved and slab edge cracking may not occur. Furthermore, when the value of formula (1) is controlled to 0.48 or more, the effect of grain refinement can be improved. Specifically, the value of formula (1) may be 0.49 or more but less than 1.0, more specifically 0.76 or more but less than 1.0, and even more specifically 0.86 or more but less than 1.0. Within these ranges, the high-strength austenitic stainless steel of the present invention has improved hot workability, no slab edge cracking, and a better balance between composition control and grain refinement, thereby achieving even better hot workability and corrosion resistance.

[0058] In the present invention, the phase stability of the austenite and martensite phases is evaluated using the thermodynamic database of the thermodynamic analysis program (Thermo-Calc. TCFE 6.0).The phase stability index is derived by calculating the free energy change of the austenite and ferrite phases depending on the alloy element content and temperature.

[0059] The high-strength austenitic stainless steel may have a value of 3.6 or less in the following formula (2), and preferably 2.0 or more and 3.6 or less.

[0060] Formula (2):0.02Si+0.21Mn+0.09Cr+0.15Ni+2.73(C+N) In the above formula (2), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element. Equation (2) represents the Austenite Stability Index (ASI) as an index of austenite phase stability.

[0061] When the value of equation (2) is controlled to 3.6 or less, the free energy change from the austenite phase to the martensite phase (ΔG γ-α ) may be -2.1 kJ / mol or less. Therefore, when the value of formula (2) is controlled to 3.6 or less, the material is easily transformed into martensite, compensating for the austenite phase stability due to the reduced Ni content and also favoring grain refinement. Furthermore, when the value of formula (2) is controlled to 3.6 or less, it is possible to minimize the residual austenite structure remaining as a band structure during cold rolling, thereby realizing uniform crystal grains. When the value of formula (2) is controlled to 2.0 or more, it is possible to improve the hot workability and the effect of grain refinement. Specifically, the value of formula (2) may be 2.6 or more and 3.6 or less, more specifically 2.9 or more and 3.6 or less, and even more specifically 3.1 or more and 3.6 or less. Within the above range, the high-strength austenitic stainless steel according to one embodiment of the present invention has a free energy change (ΔG γ-α ) is -2.1 kJ / mol or less, and the balance between component control and grain refinement is further improved, thereby achieving better hot workability and corrosion resistance.

[0062] In the high-strength austenitic stainless steel, the value of the following formula (3) may be 0.35 or more, and preferably 0.35 or more and 0.71 or less.

[0063] Formula (3):-0.04Si+0.05Mn-0.01Cr+0.05Ni+1.18(C+N) In the above formula (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element. Equation (3) represents the ARI (Austenite Recrystallization Index) as an index of recrystallization of the austenite phase.

[0064] When the value of equation (3) is 0.35 or more, the free energy change from the martensite phase to the austenite phase (ΔG α-γ) can be -0.44 kJ / mol or less. Therefore, when the value of formula (3) is 0.35 or more, recrystallization by diffusion reverse transformation during cold rolling annealing can be advantageous. Also, when the value of formula (3) is 0.35 or more, the residual martensite band structure can be minimized, thereby suppressing the generation of defects during forming and achieving good surface properties. When the value of formula (3) is 0.71 or less, the free energy change (ΔG α-γ ) is -0.44 kJ / mol or less, the effect of grain refinement can be further improved. Specifically, the value of formula (3) can be 0.4 or more and 0.71 or less, more specifically 0.44 or more and 0.71 or less, and even more specifically 0.44 or more and less than 0.6. Within the above range, the high-strength austenitic stainless steel according to one embodiment of the present invention has a free energy change (ΔG α-γ ) is -0.44 kJ / mol or less, and the balance between component control and grain refinement is further improved, thereby achieving better hot workability and corrosion resistance.

[0065] High-strength austenitic stainless steel has a free energy change (ΔG γ-α ) can be −2.1 kJ / mol or less, specifically −2.19 kJ / mol or less, more specifically −2.19 kJ / mol or less. In addition, the free energy change from the austenite phase to the martensite phase at 25° C. (ΔG γ-α The lower limit of Ni content may be, for example, -5.0 kJ / mol or more, or -3.0 kJ / mol or more. Within this range, the high-strength austenitic stainless steel according to one embodiment can compensate for the austenite phase stability due to the reduced Ni content, and at the same time, is advantageous for grain refinement, and can achieve high yield strength, excellent hot workability, and corrosion resistance.

[0066] High-strength austenitic stainless steel has a high free energy change (ΔG α-γ) can be −0.44 kJ / mol or less, specifically −0.5 kJ / mol or less, more specifically −0.55 kJ / mol or less. In addition, the free energy change from the martensite phase to the austenite phase at 850°C (ΔG α-γ The lower limit of ) may be, for example, -1.9 kJ / mol or more, or -0.9 kJ / mol or more. Within this range, the high-strength austenitic stainless steel according to an embodiment is more advantageous in recrystallization due to diffusion reverse transformation during cold rolling and annealing, is more advantageous in minimizing the residual martensite band structure, and can achieve higher yield strength, excellent hot workability, and corrosion resistance.

[0067] By controlling the above-mentioned alloy components, formulas (1), (2), and (3), and the manufacturing method described below, the high-strength austenitic stainless steel can have a grain diameter deviation in the thickness direction of 2 or less and an average grain diameter at the center of the thickness of 10 μm or less. In this case, it is possible to improve the balance between component control and grain refinement, and to provide a low-Ni austenitic stainless steel with high yield strength, excellent hot workability, and corrosion resistance.

[0068] Furthermore, the high-strength austenitic stainless steel may have a yield strength of 600 MPa or more, an elongation of 35% or more, and a pitting potential of 200 mV or more.

[0069] Furthermore, high-strength austenitic stainless steel can have sufficient strength and corrosion resistance, and therefore can have a thickness of 0.5 to 2.0 mm.

[0070] Next, the method for producing high strength austenitic stainless steel of the present invention will be described. A method for producing high-strength austenitic stainless steel includes the steps of producing a slab containing C: 0.05% to 0.1%, Si: 0.1% to 1.0%, Mn: 1.0% to 5.0%, P: less than 0.05%, S: less than 0.03%, Cr: 14.0% to 18.0%, Ni: 1.0% to 5.0%, Cu: 0.1% to 2.0%, N: 0.1% to 0.2%, and the remainder being Fe and unavoidable impurities; hot-rolling the slab and then reheating it to produce a hot-rolled material; and cold-rolling the hot-rolled material and then cold-rolling annealing it at 850 to 900°C to produce a cold-rolled material.

[0071] The slab may have a value of the following formula (1) that is less than 1.0, and preferably is 0.48 or more and less than 1.0.

[0072] Formula (1): 0.18Si+0.45C+4.4N In the above formula (1), Si, C, and N represent the content (wt %) of each element.

[0073] Furthermore, the slab may have a value of 3.6 or less in the following formula (2), and preferably 2.0 or more and 3.6 or less.

[0074] Formula (2):0.02Si+0.21Mn+0.09Cr+0.15Ni+2.73(C+N) In the above formula (2), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.

[0075] Furthermore, the slab may have a value of the following formula (3) of 0.35 or more, preferably 0.35 or more and 0.71 or less.

[0076] Formula (3):-0.04Si+0.05Mn-0.01Cr+0.05Ni+1.18(C+N) In the above formula (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.

[0077] The component ranges of each alloy composition and the reasons for limiting the values ​​of formulas (1), (2) and (3) are as described above, and each manufacturing step will be described in more detail below.

[0078] After the slab satisfying the alloy composition, formula (1), formula (2) and formula (3) is produced, it can undergo a series of hot rolling, reheating, cold rolling and cold roll annealing steps.

[0079] First, the slab is heated to 1200 to 1350°C, hot-rolled, and then reheated to 1000 to 1150°C to produce a hot-rolled material.

[0080] By reheating at 1000 to 1150°C, coarse precipitates formed during the production of the hot-rolled material can be re-decomposed, and the internal crystal grains can be controlled to an appropriate size.

[0081] Next, the hot-rolled material can be cold-rolled at a thickness reduction rate of 60% or more.

[0082] By cold rolling to a thickness reduction rate of 60% or more and transforming most of the structure into martensite, it is possible to compensate for the austenite phase stability and simultaneously realize the refinement of the crystal grains.

[0083] After cold rolling, cold rolling annealing can be performed at 850 to 900°C to produce a cold-rolled material.

[0084] Reverse transformation recrystallization from martensite to austenite can be easily carried out by cold rolling annealing at 850 to 900°C. Preferably, the cold rolling annealing can be carried out at 850 to 890°C.

[0085] The present invention will be described in more detail below through examples. However, the description of these examples is merely for the purpose of illustrating the implementation of the present invention, and the present invention is not limited by the description of these examples. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them. [Example]

[0086] Slabs were produced in a vacuum induction melting furnace for various alloy composition ranges shown in Table 1 below. The produced slabs were heated at 1250°C for 2 hours and then hot-rolled to a thickness of 3.0 mm to produce hot-rolled materials. The hot-rolled materials were reheated at 1100°C for 10 minutes and then water-cooled to produce hot-rolled materials. The hot-rolled materials were cold-rolled to a thickness reduction rate of 70% to produce cold-rolled materials with a thickness of 0.9 mm. The cold-rolled materials were cold-rolled and annealed at 850°C to produce cold-rolled materials.

[0087] [Table 1]

[0088] Table 2 below shows the values ​​of formula (1), formula (2), formula (3), and the thermodynamic free energy change ΔG γ-α (25℃) Calculated value and ΔG α-γ The calculated value (850°C) is shown. The formula (1) value was calculated using the following formula (1): Formula (1): 0.18Si + 0.45C + 4.4N

[0089] In the above formula (1), Si, C, and N represent the content (wt %) of each element.

[0090] The values ​​of formula (2) were calculated using the following formula (2).

[0091] Formula (2):0.02Si+0.21Mn+0.09Cr+0.15Ni+2.73(C+N) In the above formula (2), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.

[0092] The value of formula (3) was calculated using the following formula (3).

[0093] Formula (3):-0.04Si+0.05Mn-0.01Cr+0.05Ni+1.18(C+N) In the above formula (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.

[0094] Thermodynamic free energy change ΔG γ-α (25℃) Calculated value and ΔG α-γ (850°C) The calculated values ​​were calculated using the Thermo-Calc. TCFE 6.0 thermodynamic database to calculate the free energy changes of the austenite and ferrite phases depending on the alloying element content and temperature.

[0095] [Table 2]

[0096] Table 3 below shows the average grain diameter, intergrain diameter deviation, presence or absence of cracks in the hot-rolled material, presence or absence of recrystallization in the cold-rolled material, yield strength, elongation, and pitting potential. The average grain diameter was measured by photographing the thickness center of the cold-rolled material with a scanning electron microscope (SEM). Meanwhile, the term "average" in the present invention refers to the average of values ​​measured at five arbitrary points. Furthermore, the thickness center refers to the point between 1 / 4t and 3 / 4t, where t is the thickness.

[0097] The diameter deviation between crystal grains was calculated by photographing the center of the thickness of the cold-rolled material with a scanning electron microscope (SEM) and calculating the diameter deviation between crystal grains in the thickness direction.

[0098] On the other hand, deviations in the disclosed invention are calculated in the conventional manner as standard deviations.

[0099] The presence or absence of cracks in the hot-rolled material was judged based on the presence or absence of edge cracks in the hot-rolled material. When no edge cracks were present, the hot-rolled material was evaluated as "good," and when edge cracks were present, the hot-rolled material was evaluated as "poor."

[0100] On the other hand, in the present invention, when a crack of 3 mm or more appears in the width direction, it is considered that an edge crack has occurred.

[0101] The presence or absence of recrystallization in the cold-rolled material was determined by photographing the center of the thickness of the cold-rolled material with a scanning electron microscope (SEM) and judging based on the area fraction of the retained martensite band structure. The presence or absence of recrystallization in the cold-rolled material was evaluated as "good" when the retained martensite band structure was less than 3%, and as "poor" when the retained martensite band structure was 3% or more.

[0102] The yield strength and elongation were measured using a tensile tester manufactured by Zwick Roell, using a JIS13B tensile test piece at a tension rate of 20 mm per minute at room temperature.

[0103] The pitting potential was measured using a potentiostat in accordance with KS D 0238. The stainless steel was immersed in a NaCl solution, and a voltage of 20 mV / min was applied. The pitting potential was measured at which the current reached 100 μA. The NaCl solution was set at 30°C and its concentration was 3.5%. A higher pitting potential indicates better corrosion resistance.

[0104] [Table 3]

[0105] As shown in Tables 2 and 3, Examples 1 to 5 satisfied the alloying elements, values ​​of formulas (1) to (3), and manufacturing methods proposed in the present invention. Therefore, Examples 1 to 5 satisfied the requirements of a grain diameter deviation in the thickness direction of 2 or less, an average grain diameter at the thickness center of 10 μm or less, and were good in terms of the presence or absence of cracks in the hot-rolled material and the presence or absence of recrystallization in the cold-rolled material. Furthermore, Examples 1 to 5 satisfied the requirements of a yield strength of 600 MPa or more, an elongation of 35% or more, and a pitting potential of 200 mV or more. However, Comparative Examples 1, 2, and 5 did not satisfy the requirement of a formula (2) value of 3.6 or less, and ΔG γ-αThe (25°C) value of -2.1 kJ / mol or less was not met. Therefore, Comparative Examples 1, 2, and 5 did not satisfy the requirements of a grain diameter deviation in the thickness direction of 2 or less and an average grain diameter in the thickness center of 10 µm or less, and did not satisfy the yield strength of 600 MPa or more. That is, in Comparative Examples 1, 2, and 5, sufficient deformation-induced martensitic transformation did not occur, and the austenite retained structure remained as a coarse structure, resulting in poor strength.

[0106] Comparative Examples 3 to 5 did not satisfy the requirement that the value of formula (1) be less than 1.0, and therefore Comparative Examples 3 to 5 were poor in terms of the presence or absence of cracks in the hot-rolled materials, and the hot workability was reduced.

[0107] In Comparative Example 4, the value of formula (3) did not satisfy the requirement of 0.35 or more. Therefore, in Comparative Example 4, despite low austenite phase stability, a large amount of residual martensite remained, and the presence or absence of cold-rolled material recrystallization was poor. Therefore, although Comparative Example 4 had high yield strength, it was unable to achieve an elongation rate of 35% or more. In addition, in Comparative Example 4, the presence of a large amount of unrecrystallized structure induced defects during forming, resulting in poor quality.

[0108] Comparative Examples 2 and 5 contained excessive Mn, and therefore did not satisfy the pitting potential of 200 mV or more in Comparative Examples 2 and 5. Therefore, Comparative Examples 2 and 5 had poor corrosion resistance.

[0109] FIG. 1 is an image of the microstructure of the high-strength austenitic stainless steel of the present invention, taken with an EBSD (Electron Backscatter Diffraction) pattern analyzer.

[0110] Referring to FIG. 1, it can be seen that the present invention can achieve high yield strength and corrosion resistance by realizing grain refinement.

[0111] According to the present invention, it is possible to provide a low-cost austenitic stainless steel that realizes high yield strength and corrosion resistance by controlling phase stability and achieving grain refinement, and has improved manufacturability, and a manufacturing method thereof.

Claims

1. C: 0.05% or more and 0.1% or less, Si: 0.1% or more and 1.0% or less, Mn: 1.0% or more and 5.0% or less, P: less than 0.05%, S: less than 0.03%, Cr: 14.0% or more and 18.0% or less, Ni: 1.0% or more and 5.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.1% or more and 0.2% or less, and the balance being Fe and unavoidable impurities, A high-strength austenitic stainless steel characterized in that the diameter deviation between crystal grains in the thickness direction is 2 or less.

2. 2. The high-strength austenitic stainless steel according to claim 1, wherein the value of the following formula (1) is less than 1.0: Formula (1): 0.18Si+0.45C+4.4N (In the above formula (1), Si, C, and N represent the content (wt%) of each element.)

3. 2. The high-strength austenitic stainless steel according to claim 1, wherein the value of the following formula (2) is 3.6 or less: Formula (2): 0.02Si+0.21Mn+0.09Cr+0.15Ni+2.73(C+N) (In the above formula (2), Si, Mn, Cr, Ni, C, and N represent the content (wt%) of each element.)

4. 2. The high-strength austenitic stainless steel according to claim 1, wherein the value of the following formula (3) is 0.35 or more: Formula (3): -0.04Si+0.05Mn-0.01Cr+0.05Ni+1.18(C+N) (In the above formula (3), Si, Mn, Cr, Ni, C, and N represent the content (wt%) of each element.)

5. 2. The high-strength austenitic stainless steel according to claim 1, wherein the average grain diameter at the center of the thickness is 10 μm or less.

6. 2. The high-strength austenitic stainless steel according to claim 1, wherein the yield strength is 600 MPa or more.

7. 2. The high-strength austenitic stainless steel according to claim 1, wherein the elongation is 35% or more.

8. 2. The high-strength austenitic stainless steel according to claim 1, characterized in that it has a pitting potential of 200 mV or more.

9. 2. The high-strength austenitic stainless steel according to claim 1, wherein the thickness is 0.5 to 2.0 mm.

10. a step of producing a slab containing C: 0.05% or more and 0.1% or less, Si: 0.1% or more and 1.0% or less, Mn: 1.0% or more and 5.0% or less, P: less than 0.05%, S: less than 0.03%, Cr: 14.0% or more and 18.0% or less, Ni: 1.0% or more and 5.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.1% or more and 0.2% or less, with the remainder being Fe and unavoidable impurities; hot rolling the slab and then reheating it to produce a hot-rolled material; and and cold-rolling the hot-rolled material and then cold-rolling and annealing the hot-rolled material at 850 to 900°C to produce a cold-rolled material.

11. The slab is 11. The method for producing a high-strength austenitic stainless steel according to claim 10, wherein the value of the following formula (1) is less than 1.0: Formula (1): 0.18Si+0.45C+4.4N (In the above formula (1), Si, C, and N represent the content (wt%) of each element.)

12. The slab is 11. The method for producing a high-strength austenitic stainless steel according to claim 10, wherein the value of the following formula (2) is 3.6 or less: Formula (2): 0.02Si+0.21Mn+0.09Cr+0.15Ni+2.73(C+N) (In the above formula (2), Si, Mn, Cr, Ni, C, and N represent the content (wt%) of each element.)

13. The slab is 11. The method for producing a high-strength austenitic stainless steel according to claim 10, wherein the value of the following formula (3) is 0.35 or more: Formula (3): -0.04Si+0.05Mn-0.01Cr+0.05Ni+1.18(C+N) (In the above formula (3), Si, Mn, Cr, Ni, C, and N represent the content (wt%) of each element.)

14. 11. The method for producing high-strength austenitic stainless steel according to claim 10, wherein the reheating is carried out at a temperature of 1000 to 1150°C.

15. 11. The method for producing a high-strength austenitic stainless steel according to claim 10, wherein the cold rolling is carried out at a thickness reduction rate of 60% or more.

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