High-strength austenitic stainless steel and method for producing same

Abstract

The high-strength austenitic stainless steel according to one embodiment may contain 0.05% to 0.1% (inclusive) of C, 0.1% to 1.0% (inclusive) of Si, 1.0% to 5.0% (inclusive) of Mn, less than 0.05% of P, less than 0.03% of S, 14.0% to 18.0% (inclusive) of Cr, 1.0% to 5.0% (inclusive) of Ni, 0.1% to 2.0% (inclusive) of Cu, 0.1% to 0.2% (inclusive) of N, and a remainder of Fe and unavoidable impurities, and may have a diameter deviation between crystal grains in the thickness direction of 2 or less.

Description

Technical Field

[0001] The present invention relates to a high-strength austenitic stainless steel and a method for manufacturing the same. Background Art

[0002] In recent years, due to the rising price and increasing volatility of Ni, the demand for low-Ni austenitic stainless steels has been continuously increasing. However, there are problems such as a yield strength of 250 MPa or less and poor yield strength.

[0003] In addition, in order to reduce the content of Ni, which is a high-cost element, attempts have been made to replace Ni with austenite stabilizing elements such as Mn and N. However, there is a problem that the corrosion resistance deteriorates due to the formation of MnS.

[0004] In order to increase the strength of austenitic stainless steels, methods such as work hardening by temper rolling or adding a large amount of interstitial elements such as C and N are used. However, due to the poor elongation of the tempered material, the usability is reduced. The addition of high C reduces the weldability, and the addition of high N reduces the hot workability.

[0005] In Patent Document 0001, an austenitic fine-grained stainless steel having excellent strength and ductility is disclosed. However, Patent Document 0001 does not disclose the difference in fine grain formation due to the difference in austenite phase stability according to the composition and the change in hot workability according to the fine grain formation.

[0006] [Prior Art Documents]

[0007] (Patent Document 1) Published Patent Gazette No. 10-2007-0067905 (Publication Date: June 29, 2007) Summary of the Invention

[0008] (I) Technical Problem to be Solved

[0009] An object of the invention disclosed to solve the above problems is to provide a low-Ni austenitic stainless steel and a method for manufacturing the same, which have a high yield strength, excellent hot workability, and corrosion resistance through composition control and grain refinement.

[0010] (II) Technical Solution

[0011] A high-strength austenitic stainless steel according to an embodiment may include: 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 balance being Fe and inevitable impurities, and the diameter deviation between grains in the thickness direction may be 2 or less.

[0012] In addition, the value of the following formula (1) may be less than 1.0.

[0013] Formula (1): 0.18Si + 0.45C + 4.4N

[0014] In the formula (1), Si, C, and N represent the contents (weight %) of the respective elements.

[0015] In addition, the value of the following formula (2) may be 3.6 or less.

[0016] Formula (2): 0.02Si + 0.21Mn + 0.09Cr + 0.15Ni + 2.73(C + N)

[0017] In the formula (2), Si, Mn, Cr, Ni, C, and N represent the contents (weight %) of the respective elements.

[0018] In addition, the value of the following formula (3) may be 0.35 or more.

[0019] Formula (3): -0.04Si + 0.05Mn - 0.01Cr + 0.05Ni + 1.18(C + N)

[0020] In the formula (3), Si, Mn, Cr, Ni, C, and N represent the contents (weight %) of the respective elements.

[0021] In addition, the average grain diameter at the center of the thickness may be 10 μm or less.

[0022] In addition, the yield strength may be 600 MPa or more.

[0023] In addition, the elongation may be 35% or more.

[0024] In addition, the pitting potential may be 200 mV or more.

[0025] In addition, the thickness may be 0.5 - 2.0 mm.

[0026] The manufacturing method of a high-strength austenitic stainless steel according to an embodiment may include the following steps: manufacturing a slab, the slab including: 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 balance being Fe and unavoidable impurities; hot rolling the slab and then reheating it to manufacture a hot-rolled material; and cold rolling the hot-rolled steel sheet and then performing cold rolling annealing at 850 - 900 °C to manufacture a cold-rolled material.

[0027] The value of the following formula (1) of the slab may be less than 1.0.

[0028] Formula (1): 0.18Si + 0.45C + 4.4N

[0029] In the formula (1), Si, C, and N represent the contents (wt%) of the respective elements.

[0030] In addition, the value of the following formula (2) of the slab may be 3.6 or less.

[0031] Formula (2): 0.02Si + 0.21Mn + 0.09Cr + 0.15Ni + 2.73(C + N)

[0032] In the formula (2), Si, Mn, Cr, Ni, C, and N represent the contents (wt%) of the respective elements.

[0033] In addition, the value of the following formula (3) of the slab may be 0.35 or more.

[0034] Formula (3): -0.04Si + 0.05Mn - 0.01Cr + 0.05Ni + 1.18(C + N)

[0035] In the formula (3), Si, Mn, Cr, Ni, C, and N represent the contents (wt%) of the respective elements.

[0036] The reheating may be performed at 1000 - 1150 °C.

[0037] The cold rolling may be performed at a thickness reduction rate of 60% or more.

[0038] (III) Beneficial effects

[0039] According to an embodiment of the disclosed invention, a low-Ni austenitic stainless steel and a method for manufacturing the same can be provided, which have excellent hot workability and corrosion resistance by achieving high yield strength while controlling phase stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a diagram showing the microstructure of a high-strength austenitic stainless steel according to an example of the disclosed invention taken by Electron Backscatter Diffraction (EBSD). DETAILED DESCRIPTION

[0041] Embodiments of the present invention will be described in detail below with reference to the drawings. The following embodiments are proposed to fully convey the idea of the present invention to those skilled in the art to which the present invention pertains. The present invention is not limited to the embodiments presented in this specification, but can also be embodied in other forms. To make the present invention clear, illustrations of parts irrelevant to the description may be omitted in the drawings, and the dimensions of the components may be slightly exaggerated to aid understanding.

[0042] Throughout the specification, when a certain part "includes" or "comprises" a certain component, unless otherwise specifically stated to the contrary, it means that other components can be further included or comprised, rather than excluding other components.

[0043] Unless there is a clear exception in the context, singular expressions include plural expressions.

[0044] Below, the reasons for numerically limiting the alloy component contents in the embodiments of the present invention will be described. Below, unless otherwise specifically stated, the unit is wt%.

[0045] The high-strength austenitic stainless steel according to an embodiment may include: 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 balance being Fe and inevitable impurities.

[0046] The content of carbon (C) may be 0.05% or more and 0.1% or less.

[0047] C is an element that is very effective and inexpensive for stabilizing austenite. C is an interstitial element and is an element that contributes to increasing strength through solid-solution strengthening effects. Considering this, C can be added in an amount of more than 0.05%. However, when the content of C is excessive, after welding, due to the precipitation of carbides such as Cr 23 C6 at grain boundaries, sensitization occurs, and thus ductility, toughness, corrosion resistance, etc. may deteriorate. Considering this, the upper limit of the content of C can be restricted to 0.1%. Preferably, the content of C can be 0.058% or more and 0.096% or less.

[0048] The content of silicon (Si) can be 0.1% or more and 1.0% or less.

[0049] Si acts as a deoxidizer in the steelmaking process and is an effective element for improving corrosion resistance. Considering this, Si can be added in an amount of 0.1% or more. However, when the content of Si is too high, a δ-ferrite phase may be formed through the peritectic reaction during casting, and thus the hot workability may be reduced. Considering this, the upper limit of the content of Si can be restricted to 1.0%. Preferably, the content of Si can be 0.34% or more and 0.97% or less.

[0050] The content of manganese (Mn) can be 1.0% or more and 5.0% or less.

[0051] Mn is an inexpensive element for stabilizing the austenite phase. In addition, Mn is an effective element for increasing low-temperature impact toughness by suppressing thermally induced and strain-induced martensitic transformations. Considering this, Mn can be added in an amount of 1.0% or more. However, when the content of Mn is too high, as the inclusions (MnS) increase, the corrosion resistance and hot workability of the steel may be reduced. Considering this, the upper limit of the content of Mn can be restricted to 5.0%. Preferably, the content of Mn can be 1.02% or more and 3.79% or less.

[0052] The content of phosphorus (P) can be 0% to less than 0.05% or more than 0% and less than 0.05%.

[0053] P is an impurity inevitably contained in steel and is an element that reduces corrosion resistance and hot workability. Considering this, the content of P can be 0% or more than 0% and less than 0.05%. Preferably, the content of P can be 0.02% or less. In this case, while minimizing the influence of the content of P on physical properties, the effect of reducing process costs can be further improved.

[0054] The content of sulfur (S) can be 0% to less than 0.03% or more than 0% and less than 0.03%.

[0055] Like P, S is an impurity inevitably contained in steel and is an element that reduces corrosion resistance and hot workability. Considering this, the content of S can be 0% or more than 0% and less than 0.03%. Preferably, the content of S can be 0.002% or less. In this case, while minimizing the impact of the S content on physical properties, the effect of reducing process costs can be further improved.

[0056] The content of chromium (Cr) can be 14.0% or more and 18.0% or less.

[0057] Cr is an essential element to ensure corrosion resistance and phase stability. Considering this, Cr can be added in an amount of 14.0% or more. However, when the content of Cr is too high, the δ-ferrite phase is formed by the peritectic reaction, so the hot workability may be reduced. Considering this, the upper limit of the Cr content can be limited to 18.0%.

[0058] The content of nickel (Ni) can be 1.0% or more and 5.0% or less.

[0059] Ni is a powerful element for stabilizing the austenite phase. In addition, Ni inhibits thermally induced and processing-induced martensitic transformation, thus effectively preventing the reduction of toughness at extremely low temperatures. In addition, when Ni is added, the hot workability and cold workability can be improved. Considering this, Ni can be added in an amount of 1.0% or more. However, when the content of Ni is too high, the refinement of grains may be reduced. In addition, when the content of Ni is too high, it may lead to an increase in raw material costs. Considering this, the upper limit of the Ni content can be limited to 5.0%. Preferably, the content of Ni can be 2.61% or more and 4.45% or less.

[0060] The content of copper (Cu) can be 0.1% or more and 2.0% or less.

[0061] Cu is an effective element for stabilizing the austenite phase. In addition, Cu is an effective element for inhibiting thermally induced and processing-induced martensitic transformation. Considering this, Cu can be added in an amount of 0.1% or more. However, when the content of Cu is too high, due to Cu solidification segregation, the hot workability may be reduced. Considering this, the upper limit of the Cu content can be limited to 2.0%. Preferably, the content of Cu can be 1.19% or more and 1.88% or less.

[0062] The content of nitrogen (N) can be 0.1% or more and 0.2% or less.

[0063] N is an element that is very effective and inexpensive for stabilizing the austenite phase. In addition, N is an effective element for increasing strength and improving corrosion resistance through solid solution strengthening. Considering this, N can be added in an amount of more than 0.1%. However, when the content of N is too high, the hot workability may deteriorate. Considering this, the upper limit of the content of N can be limited to 0.2%. Preferably, the content of N can be more than 0.173% and less than 0.189%.

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

[0065] Generally, austenitic stainless steels can have a developed mechanically-induced martensite phase (mechanically-induced ε, α`-martensite) during cold rolling. The development tendency of the mechanically-induced phase tends to change according to the stability of the austenite phase. Austenitic stainless steels with low phase stability have developed ε-martensite bands at the initial stage of deformation, and as the strain increases, α`-martensite can be generated from the intersections within the bands.

[0066] In order to reduce the high-cost Ni and improve cost competitiveness, it is necessary to control the phase stability by using other austenite phase stabilizing elements other than Ni. For this purpose, it is necessary to control the free energy change (ΔG γ-α ) value of the austenite-to-martensite phase transformation at room temperature.

[0067] In addition, when the skin-passed material is subjected to cold rolling annealing heat treatment, a reverse transformation (reversion transformation) from the mechanically-induced martensite phase to the austenite phase can occur. The reverse transformation process can be roughly divided into diffusional reversion and shear reversion. The reverse transformation process can proceed according to the free energy change (ΔG α-γ ) from the martensite phase to the austenite phase in the annealing step. Generally, the martensite shear reverse transformation requires a larger free energy change (ΔG α-γ ) than the diffusional reversion.

[0068] Therefore, in the disclosed invention, in order to suppress the reduction in hot workability caused by the reduction of Ni, it is necessary to control the alloy composition. In addition, by controlling the free energy change (ΔG γ-α ) at room temperature during cold rolling, the transformation from austenite to martensite is promoted. In addition, by controlling the free energy change (ΔG α-γ) It aims to induce the reverse phase transformation of martensite during cold rolling annealing to easily recrystallize into austenite.

[0069] In addition, in the disclosed invention, it aims to ensure excellent strength and corrosion resistance by controlling the grain diameter and uniformity.

[0070] The value of the following formula (1) for a high-strength austenitic stainless steel according to one embodiment can be less than 1.0, and preferably can be 0.48 or more and less than 1.0.

[0071] Formula (1): 0.18Si + 0.45C + 4.4N

[0072] In the formula (1), Si, C, and N represent the contents (wt%) of the respective elements.

[0073] Formula (1) represents the Hot Rolling Index (HRI) as an index of hot workability.

[0074] If the value of formula (1) is controlled to be less than 1.0, the hot workability is improved and slab edge cracks may not occur. In addition, if the value of formula (1) is controlled to be 0.48 or more, the effect of grain refinement can be improved. Specifically, the value of formula (1) can be 0.49 or more and less than 1.0, more specifically 0.76 or more and less than 1.0, and more specifically 0.86 or more and less than 1.0. Within the above range, the hot workability of the high-strength austenitic stainless steel according to one embodiment of the present invention is improved, slab edge cracks do not occur, and the balance of composition control and grain refinement is further improved, thereby enabling more excellent hot workability and corrosion resistance.

[0075] In the disclosed invention, it aims to evaluate the phase stability of the austenite phase and the martensite phase using the thermodynamic database of the Thermo-Calc. TCFE 6.0 thermodynamic analysis program. Thereby, the content of alloying elements and the free energy changes of the austenite phase and the ferrite phase according to temperature changes are calculated to obtain a phase stability index.

[0076] The value of the following formula (2) for a high-strength austenitic stainless steel according to one embodiment can be 3.6 or less, and preferably can be 2.0 or more and 3.6 or less.

[0077] Formula (2): 0.02Si + 0.21Mn + 0.09Cr + 0.15Ni + 2.73(C + N)

[0078] In the formula (2), Si, Mn, Cr, Ni, C, and N represent the contents (wt%) of the respective elements.

[0079] Equation (2) is an austenite phase stability index, representing the Austenite Stability Index (ASI).

[0080] When the value of Equation (2) is controlled below 3.6, the free energy change (ΔG γ-α ) for the transformation from the austenite phase to the martensite phase can be -2.1 kJ / mol or less. Therefore, when the value of Equation (2) is controlled below 3.6, it is easy to transform into martensite, which can compensate for the austenite phase stability caused by the decrease in Ni content and is beneficial to fine grain size. In addition, when the value of Equation (2) is controlled below 3.6, the retained austenite structure remaining in the form of banded structure during cold rolling is minimized, thereby enabling uniform grains. When the value of Equation (2) is controlled above 2.0, the hot workability and the effect of grain refinement can be improved. Specifically, the value of Equation (2) can be 2.6 or more and 3.6 or less, more specifically 2.9 or more and 3.6 or less, and more specifically 3.1 or more and 3.6 or less. Within the above range, the free energy change (ΔG γ-α ) for the transformation from the austenite phase to the martensite phase of the high-strength austenitic stainless steel according to an embodiment of the present invention is -2.1 kJ / mol or less, and the balance between composition control and grain refinement is further improved, thereby enabling more excellent hot workability and corrosion resistance.

[0081] According to an embodiment, the value of the following Equation (3) for the high-strength austenitic stainless steel can be 0.35 or more, preferably 0.35 or more and 0.71 or less.

[0082] Equation (3): -0.04Si + 0.05Mn - 0.01Cr + 0.05Ni + 1.18(C + N)

[0083] In the said Equation (3), Si, Mn, Cr, Ni, C, and N represent the contents (weight %) of the respective elements.

[0084] Equation (3) is a recrystallization index of the austenite phase, representing the Austenite Recrystallization Index (ARI).

[0085] When the value of Equation (3) is 0.35 or more, the free energy change (ΔG α-γ) may be -0.44 kJ / mol or less. Therefore, when the value of formula (3) is 0.35 or more, recrystallization by diffusion reverse phase transformation during cold rolling annealing may be advantageous. In addition, when the value of formula (3) is 0.35 or more, by minimizing the residual martensite band structure, defects generated during forming can be suppressed, thereby 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, and 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 more specifically 0.44 or more and less than 0.6. Within the above range, the free energy change (ΔG α-γ ) from the martensite phase to the austenite phase of the high-strength austenitic stainless steel according to one embodiment of the present invention is -0.44 kJ / mol or less, further improving the balance between composition control and grain refinement, thereby achieving more excellent hot workability and corrosion resistance.

[0086] The free energy change (ΔG γ-α ) for the austenite-to-martensite phase transformation of the high-strength austenitic stainless steel according to one embodiment at 25 °C can be -2.1 kJ / mol or less, specifically -2.19 kJ / mol or less, and more specifically -2.19 kJ / mol or less. In addition, the lower limit of the free energy change (ΔG γ-α ) from the austenite phase to the martensite phase at 25 °C can be, for example, -5.0 kJ / mol or more, -3.0 kJ / mol or more. Within the above range, the high-strength austenitic stainless steel according to one embodiment can compensate for the austenite phase stability caused by the decrease in the Ni content and is beneficial to fine grain formation, thereby achieving higher yield strength and excellent hot workability and corrosion resistance.

[0087] The free energy change (ΔG α-γ ) from the martensite phase to the austenite phase of the high-strength austenitic stainless steel according to one embodiment at 850 °C can be -0.44 kJ / mol or less, specifically -0.5 kJ / mol or less, and more specifically -0.55 kJ / mol or less. In addition, the lower limit of the free energy change (ΔG α-γ ) from the martensite phase to the austenite phase at 850 °C can be, for example, -1.9 kJ / mol or more, -0.9 kJ / mol or more. Within the above range, the high-strength austenitic stainless steel according to one embodiment is more favorable for recrystallization due to diffusion reverse phase transformation during cold rolling annealing, is beneficial to minimizing the residual martensite band structure, thereby achieving higher yield strength and excellent hot workability and corrosion resistance.

[0088] By controlling the above alloying components, formulas (1), (2), (3), and the following manufacturing method, the diameter deviation between grains in the thickness direction of the high-strength austenitic stainless steel according to one embodiment can be 2 or less, and the average grain diameter at the thickness center can be 10 μm or less. In this case, by improving the balance between composition control and grain refinement, a low-Ni austenitic stainless steel with high yield strength, excellent hot workability, and corrosion resistance can be provided.

[0089] In addition, the yield strength of the high-strength austenitic stainless steel according to one embodiment can be 600 MPa or more, the elongation can be 35% or more, and the pitting potential can be 200 mV or more.

[0090] In addition, the high-strength austenitic stainless steel according to one embodiment can achieve sufficient strength and corrosion resistance, so the thickness can be 0.5 - 2.0 mm.

[0091] Next, according to another aspect of the disclosed invention, a manufacturing method of the high-strength austenitic stainless steel will be described.

[0092] The manufacturing method of the high-strength austenitic stainless steel according to one embodiment may include the following steps: manufacturing 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, the balance of Fe and unavoidable impurities; reheating the slab after hot rolling to manufacture a hot-rolled material; and cold rolling the hot-rolled material and then performing cold rolling annealing at 850 - 900 °C to manufacture a cold-rolled material.

[0093] The value of the following formula (1) of the slab can be less than 1.0, and preferably can be 0.48 or more and less than 1.0.

[0094] Formula (1): 0.18Si + 0.45C + 4.4N

[0095] In the formula (1), Si, C, and N represent the contents (weight %) of the respective elements.

[0096] In addition, the value of the following formula (2) of the slab can be 3.6 or less, and preferably can be 2.0 or more and 3.6 or less.

[0097] Formula (2): 0.02Si + 0.21Mn + 0.09Cr + 0.15Ni + 2.73(C + N)

[0098] In the formula (2), Si, Mn, Cr, Ni, C, and N represent the contents (wt%) of the respective elements.

[0099] In addition, the value of the following formula (3) for the slab may be 0.35 or more, preferably 0.35 or more and 0.71 or less.

[0100] Formula (3): -0.04Si + 0.05Mn - 0.01Cr + 0.05Ni + 1.18(C + N)

[0101] In the formula (3), Si, Mn, Cr, Ni, C, and N represent the contents (wt%) of the respective elements.

[0102] The compositional ranges of the above alloy compositions and the reasons for the numerical limitations of formulas (1), (2), and (3) are as described above. The following manufacturing steps will be described in more detail.

[0103] A slab that satisfies the alloy composition, formulas (1), (2), and (3) can be manufactured, and then it can undergo a series of hot rolling, reheating, cold rolling, and cold rolling annealing processes.

[0104] First, the slab can be heated at 1200 - 1350 °C and then hot rolled, and then reheated to 1000 - 1150 °C to manufacture a hot rolled material.

[0105] By reheating to 1000 - 1150 °C, the coarse precipitates generated during the manufacture of the hot rolled material are redissolved, and the internal grains can be controlled to an appropriate size.

[0106] Next, the hot rolled material can be cold rolled with a thickness reduction rate of 60% or more.

[0107] By cold pressing with a thickness reduction rate of 60% or more, most of the structure is transformed into martensite, compensating for the austenite phase stability, and at the same time, grain refinement can be achieved.

[0108] Cold rolling annealing can be carried out at 850 - 900 °C after cold rolling to manufacture a cold rolled material.

[0109] By carrying out cold rolling annealing at 850 - 900 °C, the reverse phase transformation recrystallization from martensite to austenite can be easily carried out. Preferably, the cold rolling annealing can be carried out at 850 - 890 °C.

[0110] Hereinafter, the present invention will be described in more detail by way of examples. However, the descriptions of these examples are only for illustrating the implementation of the present invention, and the present invention is not limited by these examples. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

[0111] {Example}

[0112] For the various alloy composition ranges shown in Table 1 below, slabs were manufactured in a vacuum induction melting furnace. The obtained slabs were heated at 1250 °C for 2 hours and then hot-rolled to a thickness of 3.0 mm to manufacture hot-rolled materials. The hot-rolled materials were reheated at 1100 °C for 10 minutes and then water-cooled to manufacture hot-rolled materials. The hot-rolled materials were cold-rolled with a thickness reduction rate of 70% to manufacture cold-rolled materials with a thickness of 0.9 mm. The cold-rolled materials were cold-rolled annealed at 850 °C to manufacture cold-rolled materials.

[0113] [Table 1]

[0114]

[0115] In Table 2 below, the values of formula (1), formula (2), formula (3), the calculated value of the thermodynamic free energy change ΔG γ-α (25 °C) and the calculated value of ΔG α-γ (850 °C) are shown. The value of formula (1) is calculated and shown by the following formula (1). Formula (1): 0.18Si + 0.45C + 4.4N

[0116] In the formula (1), Si, C, and N represent the contents (wt%) of the respective elements.

[0117] The value of formula (2) is calculated and shown by the following formula (2).

[0118] Formula (2): 0.02Si + 0.21Mn + 0.09Cr + 0.15Ni + 2.73(C + N)

[0119] In the formula (2), Si, Mn, Cr, Ni, C, and N represent the contents (wt%) of the respective elements.

[0120] The value of formula (3) is calculated and shown by the following formula (3).

[0121] Formula (3): -0.04Si + 0.05Mn - 0.01Cr + 0.05Ni + 1.18(C + N)

[0122] In the formula (3), Si, Mn, Cr, Ni, C, and N represent the contents (wt%) of the respective elements.

[0123] For the thermodynamic free energy change ΔGγ-α (25 °C) Calculated values and ΔG α-γ (850 °C) Calculated values. Using the Thermo-Calc. TCFE 6.0 thermodynamic database, the free energy changes of the austenite phase and ferrite phase according to the alloy element content and temperature change are calculated and shown.

[0124] [Table 2]

[0125]

[0126] In Table 3 below, the average grain diameter, the diameter deviation between grains, whether cracks occur in the hot-rolled material, whether recrystallization occurs in the cold-rolled material, the yield strength, the elongation, and the pitting potential are shown. The average grain diameter is measured by taking a photograph of the thickness center of the cold-rolled material using a scanning electron microscope (SEM). In addition, in the disclosed invention, the average refers to the average value of the values measured at any 5 positions. Furthermore, when the thickness is referred to as t, the thickness center refers to the position from 1 / 4t to 3 / 4t.

[0127] For the diameter deviation between grains, the thickness center of the cold-rolled material is photographed with a scanning electron microscope (SEM), and the diameter deviation between grains in the thickness direction is calculated to show the diameter deviation between grains.

[0128] In addition, in the disclosed invention, the deviation is calculated as the standard deviation by a conventional method.

[0129] For whether cracks occur in the hot-rolled material, it is judged according to whether edge cracks occur in the hot-rolled material. For whether cracks occur in the hot-rolled material, when no edge cracks occur, it is indicated as "good", and when edge cracks occur, it is indicated as "poor".

[0130] In addition, in the disclosed invention, when cracks of 3 mm or more appear in the width direction, it is considered that edge cracks occur.

[0131] For whether recrystallization occurs in the cold-rolled material, the thickness center of the cold-rolled material is photographed with a scanning electron microscope (SEM), and it is judged based on the area fraction of the retained martensite band structure. For whether recrystallization occurs in the cold-rolled material, when the area fraction of the retained martensite band structure is less than 3%, it is indicated as "good", and when the area fraction of the retained martensite band structure is 3% or more, it is indicated as "poor".

[0132] For the yield strength and elongation, a JIS13B tensile test piece is tested at room temperature with a tensile speed of 20 mm per minute using a tensile testing machine of Zwick Roell company to measure the yield strength and elongation.

[0133] The pitting potential is measured using a potentiostat device according to the KSD0238 standard. At this time, when the stainless steel is immersed in an NaCl solution and a voltage of 20 mV / min is applied, the potential (pitting potential) value when the measured current reaches 100 μA is shown. Among them, the temperature of the NaCl solution is set to 30 °C and the concentration is set to 3.5%. In addition, the higher the pitting potential value, the more excellent the corrosion resistance.

[0134] [Table 3]

[0135]

[0136] Referring to Table 2 and Table 3, Examples 1 to 5 satisfy the alloy composition, the values of Formulas (1) to (3), and the manufacturing method proposed by the disclosed invention. Therefore, the diameter deviation between grains in the thickness direction of Examples 1 to 5 satisfies 2 or less, the average grain diameter at the thickness center satisfies 10 μm or less, and both the hot-rolled material and the cold-rolled material are in good condition in terms of whether cracks are generated and whether recrystallization occurs. In addition, the yield strength of Examples 1 to 5 satisfies 600 MPa or more, the elongation rate satisfies 35% or more, and the pitting potential satisfies 200 mV or more. However, the value of Formula (2) in Comparative Example 1, Comparative Example 2, and Comparative Example 5 cannot satisfy 3.6 or less, and the ΔG γ-α (25 °C) value cannot satisfy -2.1 kJ / mol or less. Therefore, the diameter deviation between grains in the thickness direction of Comparative Example 1, Comparative Example 2, and Comparative Example 5 cannot satisfy 2 or less, and the average grain diameter at the thickness center cannot satisfy 10 μm or less, and the yield strength cannot satisfy 600 MPa or more. That is, in Comparative Example 1, Comparative Example 2, and Comparative Example 5, since sufficient processing-induced martensitic transformation cannot be achieved, retained austenite structure remains in the form of a coarse structure, so the strength deteriorates.

[0137] The value of Formula (1) in Comparative Example 3 to Comparative Example 5 cannot satisfy less than 1.0. Therefore, the evaluation result of whether cracks are generated in the hot-rolled materials of Comparative Example 3 to Comparative Example 5 is poor, so the hot workability is reduced.

[0138] The value of Formula (3) in Comparative Example 4 cannot satisfy 0.35 or more. Therefore, in Comparative Example 4, although the austenite phase stability is low, a large amount of retained martensite structure remains, and the evaluation result of whether the cold-rolled material recrystallizes is poor. Therefore, Comparative Example 4 has a high yield strength, but cannot achieve an elongation rate of 35% or more. In addition, there are a large number of non-recrystallized structures in Comparative Example 4, so defects are caused during forming processing, which may lead to a reduction in quality.

[0139] Comparative Example 2 and Comparative Example 5 contain too much Mn. Therefore, the pitting potential of Comparative Example 2 and Comparative Example 5 cannot satisfy 200 mV or more. Therefore, the corrosion resistance of Comparative Example 2 and Comparative Example 5 is poor.

[0140] Figure 1 It is a figure of the microstructure of a high-strength austenitic stainless steel of an example of the disclosed invention taken by electron backscatter diffraction (EBSD).

[0141] Referring to Figure 1 , according to an example of the disclosed invention, it can be confirmed that high yield strength and corrosion resistance are achieved by realizing grain refinement.

[0142] According to an embodiment of the disclosed invention, grain refinement is achieved by controlling phase stability, thereby providing a low-cost austenitic stainless steel that achieves high yield strength and corrosion resistance and has improved manufacturability, and a manufacturing method thereof.

Claims

1. A high-strength austenitic stainless steel, comprising: 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 balance being Fe and unavoidable impurities, The diameter deviation between grains in the thickness direction is 2 or less.

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 formula (1), Si, C, and N represent the contents of the respective elements in weight %.

3. 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 formula (2), Si, Mn, Cr, Ni, C, and N represent the contents of the respective elements in weight %.

4. 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 formula (3), Si, Mn, Cr, Ni, C, and N represent the contents of the respective elements in weight %.

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

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

8. The high-strength austenitic stainless steel according to claim 1, wherein, The pitting potential of the high-strength austenitic stainless steel is 200 mV or more.

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

10. A method for manufacturing a high-strength austenitic stainless steel, comprising the following steps: Manufacturing a slab, the slab comprising: 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 balance being Fe and unavoidable impurities; Hot-rolling the slab and then reheating it to manufacture a hot-rolled material; And Cold-rolling the hot-rolled steel sheet and then performing cold-rolling annealing at 850 - 900 °C to manufacture a cold-rolled material.

11. The manufacturing method of the high-strength austenitic stainless steel according to claim 10, wherein, The value of the following formula (1) for the slab is less than 1.0, Formula (1): 0.18Si + 0.45C + 4.4N In the formula (1), Si, C, and N represent the contents of the respective elements in weight %.

12. The manufacturing method of the high-strength austenitic stainless steel according to claim 10, wherein, The value of the following formula (2) for the slab is 3.6 or less, Formula (2): 0.02Si + 0.21Mn + 0.09Cr + 0.15Ni + 2.73(C + N) In the formula (2), Si, Mn, Cr, Ni, C, and N represent the contents of the respective elements, with the unit being wt%.

13. The manufacturing method of the high-strength austenitic stainless steel according to claim 10, wherein, The value of the following formula (3) for the slab is 0.35 or more, Formula (3): -0.04Si + 0.05Mn - 0.01Cr + 0.05Ni + 1.18(C + N) In the formula (3), Si, Mn, Cr, Ni, C, and N represent the contents of the respective elements, with the unit being wt%.

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

15. The manufacturing method of the 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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