Austenitic stainless steel with excellent formability

Abstract

The present invention relates to an austenitic stainless steel having excellent formability and, more specifically, to an austenitic stainless steel that contains, in wt%, 0.005-0.100% of C, 0.10-1.00% of Si, 0.10-2.00% of Mn, 6.00-12.00% of Ni, 16.00-20.00% of Cr, more than 0% and not more than 0.20% of Mo, more than 0% and not more than 0.50% of Cu, and 0.010-0.100% of N, with the remainder comprising Fe and inevitable impurities, and satisfies Expression (1) below. Expression (1): 454-440×(C+N)-8×Mn-12×Cr-23×Ni-45×Mo ≥ 0 (where, C, N, Mn, Cr, Ni, and Mo each represent the wt% of the corresponding element.)

Description

Austenitic stainless steel with excellent formability

[0001] The present invention relates to an austenitic stainless steel having excellent formability.

[0002] Stainless steel is increasingly being used as a material for home appliances and kitchenware. In particular, the number of products requiring deep processing to emphasize aesthetic elements is increasing, necessitating materials with superior processability.

[0003] Austenitic stainless steels are generally used in a variety of shapes due to their high strength and excellent elongation. In particular, they are known to exhibit work hardening during forming, making them ideal for deep forming.

[0004] However, excessive strength increases during forming require changes in forming conditions. Furthermore, in processes requiring consistent deformation over extended periods, such as deep drawing, this leads to significant changes in the load on the forming equipment. Consequently, the continuous increase in forming load leads to increased equipment load, making stable forming difficult. Furthermore, steel grades with high work-hardenability, which exhibit significant strength changes during forming, are more likely to experience defects during processing.

[0005] In addition, in the case of forming with a strain rate of 20% or more, such as deep forming, work hardening occurs significantly, and due to anisotropy caused by differences in work hardening ability in each rolling direction, the deformation characteristics differ depending on the direction of the sheet material during forming, and accordingly, there is a problem that uniform forming in each direction is difficult.

[0006] Therefore, there is a need for the development of an austenitic stainless steel that can be applied as a deep-processing material by minimizing the increase in strength due to work hardening during forming and reducing anisotropy in each rolling direction.

[0007] In order to solve the above-described problem, the present invention aims to provide a highly formable austenitic stainless steel that increases the initial strength before forming the stainless steel and minimizes the increase in strength due to work hardening during forming, thereby enabling stable forming without excessive increase in strength even during deep forming.

[0008] In addition, the present invention aims to provide a highly formable austenitic stainless steel capable of uniform forming by reducing anisotropy according to the rolling direction.

[0009] In addition, the present invention aims to provide a highly formable austenitic stainless steel that can prevent equipment failure due to increased load on forming equipment due to changes in strength by reducing the increase in strength during deep processing, and can perform forming work under stable conditions by minimizing changes in equipment load, thereby being effective in equipment maintenance.

[0010] In order to achieve the above object, an austenitic stainless steel having excellent formability according to one embodiment of the present invention contains, in wt%, C: 0.005% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 2.00%, Ni: 6.00% to 12.00%, Cr: 16.00% to 20.00%, Mo: more than 0% and 0.20% or less, Cu: more than 0% and 0.50% or less, N: 0.010% to 0.100%, the remainder Fe and other unavoidable impurities, and satisfies the following equation (1), and the work-hardening exponent (n) in the strain range of 20 to 30% according to the following equation (2) satisfies 0.5 or less.

[0011] Formula (1): 454-440×(C+N)-8×Mn-12×Cr-23×Ni-45×Mo ≥ 0

[0012] (Here, C, N, Mn, Cr, Ni, Mo represent the weight% of each element)

[0013] Equation (2): logσ = logK + nlogε

[0014] (Here, σ represents stress, K represents strength factor, ε represents strain, and n represents work hardening exponent)

[0015] In addition, the stainless steel according to one embodiment of the present invention can satisfy a deviation of the work-hardening exponent (n) in the rolling direction and the 0° direction, 45° direction, and 90° direction in the strain range of 20 to 30% according to the above formula (2) of 0.02 or less.

[0016] Additionally, the stainless steel according to one embodiment of the present invention may additionally include P: more than 0% and 0.0500% or less or S: more than 0% and 0.0060% or less.

[0017] In addition, the stainless steel according to one embodiment of the present invention may have a thickness of 0.1 to 3.0 mm and an average grain size of 10 μm or less.

[0018] Additionally, the stainless steel according to one embodiment of the present invention may have an austenite phase with an area fraction of 95% or more.

[0019] Additionally, the stainless steel according to one embodiment of the present invention may have a yield ratio (yield strength / tensile strength) at room temperature of 0.43 or more.

[0020] In addition, the stainless steel according to one embodiment of the present invention may have an average ear height difference of 3.0 or less according to the following formula (3) after deep drawing at a disk diameter of 105 mm and a punch diameter of 50 mm.

[0021] Equation (3): {(H1+H2+H3+H4)-(h1+h2+h3+h4)} / 4

[0022] (Here, H represents the value of the high ear height part, and h represents the value of the low ear height part)

[0023] Additionally, the stainless steel according to one embodiment of the present invention may have an Hv hardness value exceeding 180.

[0024] Additionally, the stainless steel according to one embodiment of the present invention may have a yield strength of 320 MPa or more at room temperature.

[0025] In addition, a method for manufacturing an austenitic stainless steel having excellent formability according to an embodiment of the present invention comprises the steps of hot-rolling a slab containing, in wt%, C: 0.005% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 2.00%, Ni: 6.00% to 12.00%, Cr: 16.00% to 20.00%, Mo: more than 0% and 0.20% or less, Cu: more than 0% and 0.50% or less, N: 0.010% to 0.100%, the remainder Fe and other unavoidable impurities, satisfying the above formula (1), and having a work-hardening exponent (n) of 0.5 or less in a strain range of 20 to 30% according to the above formula (2); It includes a step of hot rolling annealing after the hot rolling; a step of final cold rolling after the hot rolling annealing; and a step of final annealing after the cold rolling.

[0026] Additionally, the hot rolling according to one embodiment of the present invention can be performed at a temperature of 1150 to 1280°C.

[0027] Additionally, the hot rolling annealing according to one embodiment of the present invention can be performed at a temperature of 1000 to 1150°C.

[0028] In addition, the final cold rolling according to one embodiment of the present invention can be performed at room temperature so that the thickness reduction rate is 40% or more.

[0029] Additionally, the final annealing according to one embodiment of the present invention can be performed at a temperature of 700 to 950°C.

[0030] In addition, the stainless steel according to one embodiment of the present invention can satisfy a deviation of the work-hardening exponent (n) in the rolling direction and the 0° direction, 45° direction, and 90° direction in the strain range of 20 to 30% according to the above formula (2) of 0.02 or less.

[0031] In addition, the stainless steel according to one embodiment of the present invention may additionally include P: more than 0% and less than or equal to 0.0500% or S: more than 0% and less than or equal to 0.0060%.

[0032] In addition, the stainless steel according to one embodiment of the present invention may have a thickness of 0.1 to 3.0 mm and an average grain size of 10 μm or less.

[0033] Additionally, the stainless steel according to one embodiment of the present invention may have an austenite phase with an area fraction of 95% or more.

[0034] In addition, the stainless steel according to one embodiment of the present invention may have a yield ratio (yield strength / tensile strength) at room temperature of 0.43 or more.

[0035] In addition, the stainless steel according to one embodiment of the present invention may have an average ear height difference of 3.0 or less according to the above formula (3) after deep drawing at a disk diameter of 105 mm and a punch diameter of 50 mm.

[0036] Additionally, the stainless steel according to one embodiment of the present invention may have an Hv hardness value exceeding 180.

[0037] Additionally, the stainless steel according to one embodiment of the present invention may have a yield strength of 320 MPa or more at room temperature.

[0038] The high-formability austenitic stainless steel of the present invention has the effect of increasing the initial strength before forming and minimizing the increase in strength due to work hardening during forming, thereby enabling stable forming without excessive increase in strength even during deep working, and reducing anisotropy according to the rolling direction, thereby enabling uniform forming.

[0039] Furthermore, by reducing the increase in strength during deep processing, equipment failure due to increased load on forming equipment due to changes in strength can be prevented, and by minimizing equipment load fluctuations, forming work can be performed under stable conditions, which has an effective effect on equipment maintenance.

[0040] Fig. 1 is a drawing test specimen for measuring the average ear height difference according to one embodiment of the present invention.

[0041] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the present invention, the drawings may omit portions irrelevant to the description, and the sizes of components may be slightly exaggerated to facilitate understanding.

[0042] Additionally, when a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise specifically stated.

[0043] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0044] The present invention aims to derive conditions that can minimize strength increase during forming by controlling the content of major component elements to improve formability, controlling the final cold rolling and final annealing conditions to refine grains, and securing sufficient initial strength. In particular, the present invention derives conditions that can reduce strength increase even when the amount of processing is large by reducing the work hardening index under deep drawing conditions of 20% or more, and can solve the problem of reduced formability due to different deformation characteristics depending on the direction of the sheet material by reducing the anisotropy according to the rolling direction.

[0045] The austenitic stainless steel of the present invention contains C, Si, Mn, Ni, Cr, Mo, Cu, N, Fe and other unavoidable impurities.

[0046] Below, the reasons for limiting the composition of the above lecture are explained in detail. Unless otherwise specified, the composition of the above lecture lecture refers to weight %.

[0047] The carbon (C) content can be 0.005% to 0.100%.

[0048] C is an austenite phase stabilizing element added to suppress martensite formation during deformation and secure strength. Since C has a greater effect of stabilizing the austenite phase as the amount added increases, it can be added in amounts of 0.005% or more. However, if the C content exceeds 0.100%, chromium carbide may be formed during low-temperature annealing, thereby reducing intergranular corrosion resistance. Therefore, it is preferable to limit the upper limit to 0.100%. More preferably, the C content is included in an amount of 0.005% to 0.080%, and most preferably, it is included in an amount of 0.010% to 0.030%. In this case, the intergranular corrosion resistance and the austenite phase stabilizing effect are excellent, so that the hardness and yield strength can be further improved, and the work hardening exponent can be further improved.

[0049] The silicon (Si) content may be 0.10% to 1.00%.

[0050] Silicon (Si) is a component added as a deoxidizer during the steelmaking process, and is an element that ensures the strength and corrosion resistance of austenitic stainless steel. When a certain amount of Si is added, it has the effect of improving the corrosion resistance of steel by forming silicon oxide in the passive film when bright annealing is performed. However, if the Si content exceeds 1.00%, it may reduce the ductility of the steel, so it is preferable to limit the upper limit to 1.00%. More preferably, the Si content is included in an amount of 0.10% to 0.90%, and most preferably, it is included in an amount of 0.20% to 0.50%. In this case, the effect of improving the strength and corrosion resistance of the austenitic phase is excellent, and the ductility of the steel is also excellent, so that the formability can be further improved.

[0051] The manganese (Mn) content may be 0.10% to 2.00%.

[0052] Manganese is an austenite phase stabilizing element that suppresses the increase in strength that occurs during forming and processing. Since Mn has a greater effect of stabilizing the austenite phase as the amount added increases, it can be added in amounts of 0.10% or more. However, if the Mn content exceeds 2.00%, corrosion resistance and surface gloss may deteriorate, so it is preferable to limit the upper limit to 2.00%. More preferably, the Mn content is included in an amount of 0.30% to 1.90%, and most preferably, it is included in an amount of 0.60% to 1.50%. In this case, corrosion resistance and surface gloss are excellent, and the austenite phase stabilizing effect is excellent, so that hardness and yield strength can be further improved, and formability can be further improved.

[0053] The nickel (Ni) content may be 6.00% to 12.00%.

[0054] Ni is the strongest austenite phase stabilizing element. As its content increases, the austenite phase becomes more stabilized, softening the material. To suppress work hardening caused by the formation of deformation-induced martensite, it can be added in amounts exceeding 6.00%. However, excessive addition of expensive Ni causes an increase in cost. Therefore, considering the cost and efficiency of the steel, it is preferable to limit the upper limit to 12.00%. More preferably, the Ni content is included in an amount of 7.00% to 10.50%, and most preferably, it is included in an amount of 8.00% to 9.50%. In this case, the hardness and yield strength can be improved, and the work hardening exponent can be improved, which is more preferable in terms of cost and efficiency.

[0055] The chromium (Cr) content may be 16.00% to 20.00%.

[0056] Chromium (Cr) is an essential element for improving the corrosion resistance of stainless steel, and can be added in amounts exceeding 16.00% to ensure corrosion resistance in atmospheric environments. However, if the Cr content exceeds 20.00%, the material may be hardened and deformation-induced martensite may be suppressed during cold rolling, so it is preferable to limit the upper limit to 20.00%. More preferably, the Cr content is included in an amount of 16.10% to 19.40%, and most preferably, it is included in an amount of 17.00% to 19.00%. In this case, the corrosion resistance and martensite suppression effects are excellent, so that stable strength can be secured and formability can be further improved.

[0057] The content of molybdenum (Mo) may be greater than 0% and less than or equal to 0.20%.

[0058] Mo is a useful element for improving corrosion resistance, but it is an expensive component, so adding it in large quantities increases costs. Furthermore, it precipitates intermetallic compounds such as the sigma (σ) phase, which reduces mechanical properties and corrosion resistance. Therefore, it is desirable to limit the upper limit to 0.20% or less. More preferably, the Mo content is comprised between 0.01% and 0.18%, and most preferably between 0.05% and 0.15%. In this case, the effects of cost reduction and improvement in mechanical properties and corrosion resistance are excellent.

[0059] The content of copper (Cu) may be greater than 0% and less than or equal to 0.50%.

[0060] Cu is a useful element for stabilizing the austenite phase and can be used as a substitute for expensive nickel. However, if the content of Cu is excessive, it can form a low-melting-point ε-Cu precipitate phase, which can lower hot workability and thus surface quality. Therefore, it is preferable to limit the upper limit to 0.50%. More preferably, the content of Cu is included in the range of 0.01% to 0.48%, and most preferably, it is included in the range of 0.10% to 0.04%. In this case, the hot workability and the austenite phase stabilization effect are excellent, so that hardness and yield strength can be improved while improving surface quality.

[0061] The nitrogen (N) content can be 0.010% to 0.100%.

[0062] N is an austenite phase stabilizing element. As the amount added increases, the effect of stabilizing the austenite phase increases. Therefore, it can be added in an amount of 0.010% or more to stabilize the austenite phase and improve the strength of the material. However, if the N content exceeds 0.100%, it may harden the material and reduce hot workability, so it is preferable to limit the upper limit to 0.100%. More preferably, the N content is included in an amount of 0.011% to 0.091%, and most preferably, it is included in an amount of 0.030% to 0.080%. In this case, the hot workability and the austenite phase stabilizing effect are excellent, so that hardness and yield strength are improved, and surface quality can be improved.

[0063] The content of phosphorus (P) may be greater than 0% and less than or equal to 0.0500%.

[0064] Phosphorus (P) is an impurity that is inevitably contained in steel and is a major cause of intergranular corrosion and deterioration of hot workability. Therefore, its upper limit may be limited to 0.0500%. More preferably, the P content is comprised between 0.0200% and 0.4500%, and most preferably between 0.0300% and 0.0400%. In such cases, intergranular corrosion prevention and hot workability can be further improved.

[0065] The sulfur (S) content may be greater than 0% and less than or equal to 0.0060%.

[0066] S is an impurity that is inevitably contained in steel, and is a major element that hinders hot workability by segregating at grain boundaries. Therefore, its upper limit can be limited to 0.0060%. More preferably, the S content is included in an amount of 0.0020% to 0.0055%, and most preferably, it is included in an amount of 0.0030% to 0.0045%. In such cases, the hot workability effect is excellent.

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

[0068] In general, work hardening of austenitic stainless steels occurs when the austenite phase, which is unstable at room temperature, transforms into the martensite phase due to stress resulting from plastic deformation.

[0069] As deformation continues, continuous phase transformation occurs, and this phase transformation increases the strength of the austenitic stainless steel until the material breaks. In order to minimize the increase in strength during forming and secure formability, it is necessary to suppress the transformation to martensite.

[0070] In the present invention, the following equation (1) was derived by considering the phase transformation occurring due to deformation of austenitic stainless steel.

[0071] Specifically, in the present invention, the stability of the austenite phase can be increased by controlling the value of the following formula (1) among the alloy components to be 0 or greater. Accordingly, phase transformation into the martensite phase can be suppressed, and work hardening of the austenitic stainless steel can be suppressed.

[0072] Formula (1): 454-440×(C+N)-8×Mn-12×Cr-23×Ni-45×Mo ≥ 0

[0073] Here, C, N, Mn, Cr, Ni, and Mo represent the weight percent of each element.

[0074] According to one embodiment of the present invention, an austenitic stainless steel having excellent formability satisfies a range in which the value expressed by the above formula (1) is 0 or more. When the value of formula (1) is lower than 0, the austenitic stainless steel of the above-mentioned alloy composition system may exhibit a rapid strain-induced martensitic transformation behavior due to external deformation, or plastic non-uniformity due to twin formation may occur. Accordingly, there is a problem in that the strength and formability, especially the deep workability, of the austenitic stainless steel are reduced, and therefore the lower limit of formula (1) may be limited to 0. That is, the value expressed by formula (1) may be 0 or more, preferably 2 or more, more preferably 4.7 or more, and most preferably 5.5 or more. In this case, the austenitic stainless steel can be stabilized, so that the strength, formability, and deep workability of the steel can be further improved. The upper limit of equation (1) is not particularly limited, but may be, for example, 116 or less, 100 or less, 50 or less, or 20 or less. Within the above range, the effect of improving deep workability can be further enhanced.

[0075] Meanwhile, in the case of austenitic stainless steels, the strain hardening exponent decreases even as deformation progresses after the point where the strain hardening exponent reaches its maximum (hereinafter referred to as the "maximum strain hardening exponent"). In other words, after the maximum strain hardening exponent, the strain hardening exponent value continues to decrease, resulting in a gradual increase in strength.

[0076] Accordingly, in the present invention, in order to improve the formability of austenitic stainless steel, it is necessary to secure a certain amount of deformation without excessive strength increase, and the maximum point of the work hardening exponent is placed in a relatively low strain range of 20 to 30% so that sufficient strength can be secured initially, and a certain amount of deformation is secured from the maximum point of the work hardening exponent so that the strength increases gradually while the work hardening exponent value continuously decreases, thereby minimizing the increase in strength during forming.

[0077] In other words, if the maximum strain hardening exponent can be achieved in the strain range of 20 to 30% and continuous deformation can be secured without excessive strength increase, the strength of austenitic stainless steel can be secured while improving formability and significantly reducing the occurrence of fracture.

[0078] Therefore, the austenitic stainless steel having excellent formability according to one embodiment of the present invention can satisfy a work-hardening exponent (n) of 0.5 or less in a strain range of 20 to 30% according to the following equation (2).

[0079] Equation (2): logσ = log K + nlogε

[0080] Here, σ represents stress, K represents strength factor, ε represents strain, and n represents strain hardening exponent.

[0081] In the above equation (2), the work hardening exponent n corresponds to the slope of the graph. A greater slope indicates a greater increase in the strength of the material during plastic deformation. In order to improve the formability of the austenitic stainless steel of the present invention, continuous deformation must be ensured without excessive increase in strength.

[0082] Accordingly, the austenitic stainless steel of the present invention can satisfy a work-hardening exponent (n) of 0.5 or less in the strain range of 20 to 30%. When the work-hardening exponent exceeds 0.5, an excessive increase in strength occurs, which reduces formability and causes a problem of rapid fracture. Accordingly, there is a problem of a decrease in the yield ratio of the austenitic stainless steel, so it is preferable to limit the upper limit of the work-hardening exponent to 0.5. More preferably, the work-hardening exponent is 0.48 or less, and most preferably, it is 0.45 or less. In this case, continuous deformation can be secured without an excessive increase in strength, thereby further improving formability. The lower limit of Equation (2) is not particularly limited, but may be, for example, 0.01 or more, 0.1 or more, 0.2 or more, 0.3 or more, or 0.4 or more. Within the above range, the effect of securing continuous deformation can be further improved.

[0083] That is, the present invention satisfies the value of the above formula (1) as 0 or more, thereby suppressing phase transformation into a martensite phase and suppressing work hardening of austenitic stainless steel, thereby minimizing the increase in strength due to work hardening during forming, and furthermore, the work-hardening exponent (n) in the 20-30% strain range according to the following formula (2) satisfies 0.5 or less, thereby increasing the initial strength before forming and minimizing the increase in strength due to work hardening during forming, thereby enabling stable forming without excessive increase in strength even during deep forming.

[0084] In addition, an austenitic stainless steel having excellent formability according to one embodiment of the present invention can satisfy a deviation of work-hardening exponent (n) in the rolling direction and the 0° direction, 45° direction, and 90° direction in the strain range of 20 to 30% according to the above formula (2) of 0.02 or less.

[0085] The 0° direction in the rolling direction means the average value of the strain hardening index measured at a position forming a 0° angle with the rolling direction, the 45° direction in the rolling direction means the average value of the strain hardening index measured at a position forming a 45° angle with the rolling direction, and the 90° direction in the rolling direction means the average value of the strain hardening index measured at a position forming a 90° angle with the rolling direction.

[0086] In forming processes such as deep drawing, significant work hardening occurs during forming, leading to significant strength increases. Consequently, differences in work hardening capacity across the rolling direction lead to strength differences. These directional strength differences lead to variations in sheet deformation characteristics depending on the direction of the forming process, making uniform forming difficult.

[0087] Therefore, the present invention is preferably such that the deviation of the work hardening exponent in the rolling direction and the 0° direction, 45° direction, and 90° direction in the strain range of 20 to 30% is 0.02 or less, more preferably 0.019 or less, and most preferably 0.017 or less. In this case, the anisotropy is small, so that forming defects depending on the direction of the sheet material can be reduced, thereby enabling uniform forming.

[0088] The austenitic stainless steel of the present invention, which satisfies the alloy element composition range and relationship as described above, and the strain hardening index in the strain range of 20 to 30%, and the strain hardening index deviations in the rolling direction and the 0° direction, 45° direction, and 90° direction, can satisfy an area fraction of 95% or more of an austenite phase, a steel thickness of 0.1 to 3.0 mm, and an average grain size of 10 μm or less.

[0089] If the average grain size exceeds 10 µm, excessive strength increase due to work hardening may occur, which may reduce formability. Therefore, it is preferable to limit the upper limit to 10 µm. More preferably, the average grain size is 9 µm or less, and most preferably, 8 µm or less can further improve formability.

[0090] Additionally, the thickness of the above steel material can be limited to 0.1 to 3.0 mm, preferably 0.15 to 0.30 mm, for forming without cracks or fractures.

[0091] Additionally, the austenitic stainless steel according to one embodiment of the present invention may have a yield ratio (yield strength / tensile strength) at room temperature of 0.43 or more.

[0092] The yield ratio is the ratio of the yield strength to the tensile strength, and is a value that indicates the increase in strength from the yield point where deformation begins during forming to the maximum strength. A large yield ratio means that there is less fluctuation in strength.

[0093] When the yield ratio at room temperature is less than 0.43, a significant change in strength due to work hardening during molding may occur, which may reduce formability. In particular, an increase in the load of the molding equipment due to the change in strength during deep drawing may cause equipment load. Therefore, the yield ratio at room temperature is preferably 0.43, and more preferably 0.45 or more. The upper limit of the yield ratio is not limited, but may be, for example, 1 or less or 0.8 or less. Within the above range, a favorable effect of achieving the desired strength and formability while reducing the equipment load during molding can be realized.

[0094] In addition, the austenitic stainless steel according to one embodiment of the present invention has an Hv hardness value of more than 180, preferably more than 185, and a yield strength at room temperature of 320 MPa or more, preferably 330 MPa or more, so that it can implement excellent strength while reducing equipment overload during forming. In addition, the upper limit of the Hv hardness value is not limited, but may be, for example, 300 or less or 250 or less. The upper limit of the yield strength is not limited, but may be 600 MPa or less, 500 MPa or less, or 480 MPa or less. Within the above range, it is possible to improve the balance between high strength and high formability, while reducing equipment load during processing and reducing the variation of the work hardening exponent.

[0095] In addition, the austenitic stainless steel according to one embodiment of the present invention can secure an average ear height difference of 3.0 or less according to the following formula (3) after deep drawing with a disk diameter of 105 mm and a punch diameter of 50 mm, thereby improving formability and enabling uniform forming without cracks or fractures. Preferably, the average ear height difference is 2.7 or less, and more preferably, 2.5 or less. In this case, excellent formability can be secured, and uniform forming is also possible.

[0096] Equation (3): {(H1+H2+H3+H4)-(h1+h2+h3+h4)} / 4

[0097] Here, H represents the value of the high ear height part, and h represents the value of the low ear height part.

[0098] The method for manufacturing the austenitic stainless steel having excellent formability according to the present invention as described above is described as follows.

[0099] According to one embodiment of the present invention, a method for manufacturing an austenitic stainless steel comprises the steps of: hot rolling a slab containing, in wt%, C: 0.005% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 2.00%, Ni: 6.00% to 12.00%, Cr: 16.00% to 20.00%, Mo: more than 0% and 0.20% or less, Cu: more than 0% and 0.50% or less, N: 0.010% to 0.100%, the remainder Fe and other unavoidable impurities, satisfying the above formula (1), and a work-hardening exponent (n) of 0.5 or less in a strain range of 20 to 30% according to the above formula (2); hot rolling annealing after the hot rolling; It may include a step of final cold rolling after the above hot rolling annealing; and a step of final annealing after the above cold rolling.

[0100] The temperature during hot annealing following the above hot rolling process significantly affects the release of residual stress and the microstructure. Therefore, it is recommended that the above hot annealing be performed at a temperature of 1000 to 1150°C.

[0101] If the hot rolling annealing temperature is lower than 1000℃, sufficient recrystallization and annealing may not occur, resulting in a decrease in ductility. If it exceeds 1150℃, the grains may become extremely coarsened. Therefore, it is desirable to limit the hot rolling annealing temperature to 1000 to 1150℃.

[0102] In addition, in the present invention, the average grain size of the austenitic stainless steel can be satisfied to be 10㎛ or less by controlling the final cold rolling and final annealing conditions while satisfying the alloy element composition and relationship as described above, satisfying the strain hardening index in the strain range of 20 to 30%, and the strain hardening index deviation in the rolling direction and the 0° direction, 45° direction, and 90° direction.

[0103] The final cold rolling is performed at room temperature after annealing to ensure a thickness reduction of at least 40%. Cold rolling may be performed in one or multiple passes, and is particularly recommended to be performed through repeated rolling at least twice. As long as the final cold rolling is performed to ensure a thickness reduction of at least 40%, there are no restrictions on the reduction ratio per pass or the number of passes.

[0104] That is, in the present invention, cold rolling is performed so that the thickness reduction rate is 40% or more so that the average grain size can be satisfied to 10㎛ or less by increasing the amount of carbonitride precipitation and suppressing grain growth. If the thickness reduction rate during cold rolling is less than 40%, fine grains cannot be obtained, which may hinder the improvement of formability.

[0105] In addition, the final annealing is performed at a temperature of 700 to 950°C, which is lower than the preceding hot rolling annealing process, in order to remove distortion caused by cold rolling and to refine grains by precipitating fine carbonitrides. If the final annealing temperature exceeds the above range, coarse carbonitrides may be generated, resulting in an uneven structure. Therefore, the final annealing temperature may be limited to 700 to 950°C.

[0106] The austenitic stainless steel of the present invention manufactured by this method may have an area fraction of 90% or more of an austenite phase and an average grain size of 10 ㎛ or less.

[0107] Hereinafter, the present invention will be described in more detail through preferred embodiments.

[0108] Example

[0109] Using steel having the composition shown in Table 1 below, a 200 mm thick slab was manufactured through a continuous casting process, heated at 1250°C for 2 hours, hot rolled to a thickness of 4 to 8 mm, and hot annealed at 1150°C after hot rolling. Next, final cold rolling was performed so that the thickness reduction rate was 40% or more at room temperature, and final annealing was performed at 700 to 950°C to manufacture a coil.

[0110] The units in Table 1 below are weight%.

[0111] In Table 2 below, the value of Equation (1) is a value derived by substituting it into Equation (1), the value of n is a work-hardening exponent (n) value in the strain range of 20 to 30% according to Equation (2), and the yield ratio means the ratio of the yield strength to the tensile strength.

[0112] Formula (1): 454-440×(C+N)-8×Mn-12×Cr-23×Ni-45×Mo ≥ 0

[0113] (Here, C, N, Mn, Cr, Ni, Mo represent the weight% of each element)

[0114] Equation (2): logσ = logK + nlogε

[0115] (Here, σ represents stress, K represents strength factor, ε represents strain, and n represents work hardening exponent)

[0116] Classification CSiMnPSCrNiMoCuNComparative Example 10.0120.630.510.02330.002918.018.290.080.260.081Comparative Example 20.0830.201.100.03290.002517.967.060.080.440.062Comparative Example 3 0.0610.401.080.03360.003118.178.000.070.160.036Comparative Example 40.0330.901.620.02830.004219.476.290.170.390.093Comparative Example 50.0540.421.930.04010.00 4718.627.330.120.310.057Comparative Example60.0560.331.050.03100.004318.168.080.190.190.031Comparative Example70.0160.260.620.03200.003316.3210.210.080.110. 018Comparative Example 80.0370.400.820.03510.002116.719.330.200.210.028Comparative Example 90.0540.411.040.03240.002718.138.060.110.250.034Example 10.0510.341.110.0 2870.002116.349.120.060.170.019Example 20.0400.910.630.04120.003219.417.420.160.420.029Example 30.0540.431.030.03060.002518.247.910.09 0.240.032Example 40.0130.220.380.02620.002616.1210.330.040.110.011Example 50.0210.361.340.04090.002017.018.300.080.360.065Example 60.0540.40 1.050.03520.002518.327.600.130.370.039Example 70.0130.520.490.02210.002818.088.100.080.210.081Example 80.0830.211.080.03300.003117.887. 010.080.470.062Example 90.0530.401.010.03120.002918.118.070.080.170.036Example 100.0320.901.610.03190.004519.336.310.180.410.091Example 110.0540.431.940.03290.003018.597.280.110.290.041Example 120.0330.420.760.03190.005318.148.100.140.280.039.

[0117] Classification formula (1) Value n Value Average grain size (㎛) Room temperature Yield ratioComparative example 1 - 1.6110.56023.00.42Comparative example 2 - 0.0540.55725.00.42Comparative example 3 - 2.5540.55426.00.42Comparative example 4 - 0.4080.52124.30.42Comparative example 5 - 7.890.51920.00.42Comparative example 6 - 5.0620.52018.00.42Comparative example 7 - 0.190.57819.00.42Comparative example 8 - 5.3140.59218.00.41Comparative example 9 - 0.8030.57918.00.41Example 15.5120.4786.80.47Example Example 27.7760.4787.10.47 Example 32.8250.4786.90.47 Example 47.4380.4437.80.47 Example 56.6440.4437.70.47 Example 64.7490.4438.00.47 Example 72.0390.4478.90.45 Example 82.3480.4498.60.45 Example 90.0280.4498.80.45 Example 101.8080.4417.80.47 Example 111.0340.4428.40.46 Example 126.0850.4438.20.46

[0118] As shown in Table 2 above, in the case of Examples 1 to 12 according to the present invention, the value derived by substituting into Equation (1) is 0 or more, and the work hardening index in the strain range of 20 to 30% according to Equation (2) satisfies 0.5 or less, and therefore, it was confirmed that the grain size of the austenite phase was 10㎛ or less, and the yield ratio value was 0.43 or more. On the other hand, in the case of Comparative Examples 1 to 9, in which the content of each component was included in the alloy composition presented in the present invention but the value derived by substituting into Equation (1) was less than 0, it was confirmed that the work hardening index in the strain range of 20 to 30% exceeded 0.5, the grain size also exceeded 10㎛, and the yield ratio was also less than 0.43.

[0119] Through these results, when the alloy composition according to the present invention is satisfied with the value derived by substituting into the above formula (1), the work hardening exponent in the strain range of 20 to 30% according to the above formula (2) is satisfied as 5.0 or less, the crystal grain size is 10㎛ or less, and the yield ratio is also 0.43 or more, thereby minimizing the increase in strength due to work hardening during forming, enabling stable forming without excessive strength increase even during deep forming, and preventing equipment failure due to increased load on the forming equipment due to strength change, so that it can be predicted to be effective for equipment maintenance under stable conditions.

[0120] In addition, for each steel plate in Table 1, the strain hardening exponent n value and deviation (maximum strain hardening exponent - minimum strain hardening exponent) in the rolling direction and the 0° direction, 45° direction, and 90° direction in the strain range of 20 to 30% according to the above equation (2) were measured. If the deviation of the strain hardening exponent by direction is large, it means that the strength fluctuations in each direction during forming are different, making it difficult to achieve uniform forming.

[0121] In addition, the ear height difference was measured using a drawing test specimen manufactured by deep drawing to have a disk diameter of 105 mm and a punch diameter of 50 mm, and the hardness Hv and yield strength were measured, and the results are shown in Table 3 below.

[0122] The difference in ear height was measured according to the following formula (3), with the high ear height part of the specimen as H and the low ear height part as h, as shown in Fig. 1, and the four high ear height parts were arbitrarily designated as H1, H2, H3, and H4, and the four low ear height parts were arbitrarily designated as h1, h2, h3, and h4, respectively.

[0123] Equation (3): {(H1+H2+H3+H4)-(h1+h2+h3+h4)} / 4

[0124] (Here, H represents the value of the high ear height part, and h represents the value of the low ear height part)

[0125] n value by rolling directionStrain hardening indexDeviationEar height differenceHvHardnessYield strength(MPa)0°45°90°Comparative example 10.5370.5520.5600.0233.07173312.8Comparative example 20.5310.5410.5570.0263.48179312.7Comparative example 30.5340.5470.5540.0203.51176313.6Comparative example 40.4950.5000.5210.0263.06173301.2Comparative example 50.4960.5030.5190.0233.18180301.9Comparative example 60.4980.5030.5200.0223.36179301.2 Comparative Example 70.5560.5710.5780.0223.50180302.0 Comparative Example 80.5680.5730.5920.0243.27176302.5 Comparative Example 90.5540.5650.5790.0253.21180302.8 Exemplary Example 10.4600.4680.4780.0182.61184328.1 Exemplary Example 20.4590.4620.4780.0192.57193327.4 Exemplary Example 30.4610.4660.4780.0172.03181326.9 Example 40.4250.4270.4430.0182.74193343.4 Example 50.4240.4280.4430.0192.50184345.2 Example 60.4250.4260.4430.0182.65189343.3 Example 70.4330.4350.4470.0142.14181324.4 Example 80.4350.4310.4490.0182.62185326.9 Example 90.4340.4320.4490.0172.53187324.4 Example 100.4280.4260.4410.0152.71184341.5 Example 110.4240.4260.4420.0182.28189337.2 Example 120.4300.4260.4430.0172.51185334.4

[0126] As shown in Table 3 above, in the case of Examples 1 to 12 according to the present invention, the value derived by substituting into Equation (1) was 0 or more, and the strain hardening index in the 20 to 30% strain range according to Equation (2) was 0.5 or less, so that the deviation of the strain hardening index in the rolling direction and the 0° direction, 45° direction, and 90° direction was 0.02 or less, and from this, it was found that the strength fluctuation in each rolling direction during forming was small, so that uniform forming would be possible. In addition, it was confirmed that the average ear height difference according to the above formula (3) was 3.0 or less, the hardness Hv exceeded 180, and the yield strength also satisfied 320 MPa or more. On the other hand, in the case of comparative examples 1 to 9 in which the content of each component was included in the alloy composition suggested in the present invention but the value derived by substituting it into formula (1) was less than 0, it was confirmed that the deviation of the work hardening index in the rolling direction and the 0° direction, 45° direction, and 90° direction exceeded 0.02, and the average ear height difference also exceeded 3.0, the hardness Hv was less than 180, and the yield strength was less than 320 MPa.

[0127] Through these results, it was predicted that when the alloy composition according to the present invention satisfies the value derived by substituting into the above formula (1), and the strain hardening exponent in the 20 to 30% strain range according to the above formula (2) is 0.5 or less, the deviation of the strain hardening exponent in the rolling direction and the 0° direction, 45° direction, and 90° direction in the 20 to 30% strain range satisfies 0.02 or less, the average ear height difference is 3.0 or less, the hardness Hv exceeds 180, and the yield strength also satisfies 320 MPa or more, thereby minimizing the increase in strength during forming, reducing anisotropy according to the rolling direction, enabling uniform forming, and preventing equipment failure due to an increase in the load of the forming equipment according to the change in strength, and thus being effective for maintenance of equipment under stable conditions.

[0128] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and spirit of the claims set forth below.

Claims

1. Containing, in wt%, C: 0.005% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 2.00%, Ni: 6.00% to 12.00%, Cr: 16.00% to 20.00%, Mo: more than 0% and 0.20% or less, Cu: more than 0% and 0.50% or less, N: 0.010% to 0.100%, the remainder being Fe and other unavoidable impurities, Satisfies the following equation (1), An austenitic stainless steel having excellent formability, wherein the work-hardening exponent (n) in the strain range of 20 to 30% according to the following equation (2) is 0.5 or less. Formula (1): 454-440×(C+N)-8×Mn-12×Cr-23×Ni-45×Mo ≥ 0 (Here, C, N, Mn, Cr, Ni, Mo represent the weight% of each element) Equation (2): logσ = logK + nlogε (Here, σ represents stress, K represents strength factor, ε represents strain, and n represents strain hardening exponent.) 2. In paragraph 1, The above stainless steel is an austenitic stainless steel that satisfies the deviation of the work-hardening exponent (n) in the rolling direction and the 0° direction, 45° direction, and 90° direction in the strain range of 20 to 30% according to the above formula (2) of 0.02 or less.

3. In paragraph 1, The above stainless steel is an austenitic stainless steel additionally containing P: more than 0% and not more than 0.0500% or S: more than 0% and not more than 0.0060%.

4. In paragraph 1, The above stainless steel is an austenitic stainless steel with a thickness of 0.1 to 3.0 mm and an average grain size of 10 μm or less.

5. In paragraph 1, The above stainless steel is an austenitic stainless steel having an area fraction of 95% or more of the austenite phase.

6. In paragraph 1, The above stainless steel is an austenitic stainless steel with a yield ratio (yield strength / tensile strength) of 0.43 or more at room temperature.

7. In paragraph 1, The above stainless steel is an austenitic stainless steel having an average ear height difference of 3.0 or less according to the following formula (3) after deep drawing with a disk diameter of 105 mm and a punch diameter of 50 mm: Equation (3): {(H1+H2+H3+H4)-(h1+h2+h3+h4)} / 4 (Here, H represents the value of the high ear height part, and h represents the value of the low ear height part) 8. In paragraph 1, The above stainless steel is an austenitic stainless steel with a Hv hardness value exceeding 180.

9. In paragraph 1, The above stainless steel is an austenitic stainless steel having a yield strength of 320 MPa or more at room temperature. A step of hot-rolling a slab containing 10. wt% of C: 0.005% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 2.00%, Ni: 6.00% to 12.00%, Cr: 16.00% to 20.00%, Mo: more than 0% and 0.20% or less, Cu: more than 0% and 0.50% or less, N: 0.010% to 0.100%, the remainder being Fe and other unavoidable impurities, and satisfying the following formula (1), and a work-hardening exponent (n) in a strain range of 20 to 30% according to the following formula (2) satisfying 0.5 or less; A step of hot annealing after the above hot rolling; The step of final cold rolling after the above hot rolling annealing; and The final annealing step after the above cold rolling; A method for manufacturing an austenitic stainless steel having excellent formability, the method comprising: Formula (1): 454-440×(C+N)-8×Mn-12×Cr-23×Ni-45×Mo ≥0 (Here, C, N, Mn, Cr, Ni, Mo represent the weight% of each element) Equation (2): logσ = logK + nlogε (Here, σ represents stress, K represents strength factor, ε represents strain, and n represents strain hardening exponent.) 11. In paragraph 10, The above hot rolling is a method for manufacturing austenitic stainless steel, performed at a temperature of 1150 to 1280°C.

12. In paragraph 10, The above hot rolling annealing is a method for manufacturing austenitic stainless steel, performed at a temperature of 1000 to 1150°C.

13. In paragraph 10, A method for manufacturing austenitic stainless steel, wherein the final cold rolling is performed at room temperature so that the thickness reduction rate is 40% or more.

14. In paragraph 10, A method for manufacturing austenitic stainless steel, wherein the final annealing is performed at a temperature of 700 to 950°C.

15. In paragraph 10, The above stainless steel is a method for manufacturing an austenitic stainless steel, wherein the deviation of the work-hardening exponent (n) in the rolling direction and the 0° direction, 45° direction, and 90° direction in the strain range of 20 to 30% according to the above formula (2) is 0.02 or less.

16. In paragraph 10, The above stainless steel is a method for manufacturing an austenitic stainless steel, wherein the stainless steel additionally contains P: more than 0% and not more than 0.05% or S: more than 0% and not more than 0.006%.

17. In paragraph 10, The above stainless steel is a method for manufacturing austenitic stainless steel having a thickness of 0.1 to 3.0 mm and an average grain size of 10 μm or less.

18. In paragraph 10, The above stainless steel is a method for manufacturing austenitic stainless steel having an area fraction of 95% or more of an austenite phase.

19. In paragraph 10, The above stainless steel is a method for manufacturing austenitic stainless steel having a yield ratio (yield strength / tensile strength) of 0.43 or more at room temperature.

20. In paragraph 10, The above stainless steel is deep-drawn at a disk diameter of 105 mm and a punch diameter of 50 mm, and then a method for manufacturing an austenitic stainless steel having an average ear height difference of 3.0 or less according to the following formula (3): Equation (3): {(H1+H2+H3+H4)-(h1+h2+h3+h4)} / 4 (Here, H represents the value of the high ear height part, and h represents the value of the low ear height part)

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