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(11) | EP 4 800 145 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | AUSTENITIC STAINLESS STEEL WITH EXCELLENT FORMABILITY |
| (57) The present disclosure relates to an austenitic stainless steel having excellent
formability, the austenitic stainless steel comprising, in percent by weight (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%, and the balance of Fe and other unavoidable impurities,
and satisfying Formula (1) below. wherein C, N, Mn, Cr, Ni, and Mo mean the weight% of each element |
[Technical Field]
[0001] The present disclosure relates to an austenitic stainless steel having excellent formability.
[Background Art]
[0002] The use of stainless steel as a material for home appliances and kitchenware is recently increasing. In particular, the expansion of products requiring deep drawing to emphasize aesthetic elements necessitates a material having excellent formability.
[0003] Generally, an austenitic stainless steel, having high strength and excellent elongation, is used for products of various shapes. Particularly, the occurrence of work hardening during forming is known to provide good formability during deep drawing.
[0004] However, an excessive strength increase during forming demands changes in forming conditions. For processes requiring prolonged uniform deformation, such as deep drawing, significant changes in forming equipment loads occur. Accordingly, a continuous increase in forming load causes an increase in equipment burden, thereby making stable forming difficult. Moreover, for steel grades with high work-hardening capacity, where significant strength changes occur during processing, a higher likelihood of defects during processing exists.
[0005] Moreover, for forming operations having a strain of 20% or more, such as deep drawing, significant work hardening occurs. Anisotropy resulting from differences in work-hardening capability among rolling directions causes the deformation characteristics to vary according to the sheet direction during forming. Consequently, achieving uniform forming by direction presents a difficulty.
[0006] Therefore, the development of an austenitic stainless steel is required, the stainless steel having applicability as a deep drawing material through minimization of a strength increase due to work hardening during forming and reduction of anisotropy according to rolling direction.
[Disclosure]
[Technical Problem]
[0007] For solving the above-described problem, the present disclosure aims to provide a highly formable austenitic stainless steel, enabling stable forming even during deep drawing without excessive strength increase, by increasing an initial strength prior to forming the stainless steel and minimizing a strength increase due to work-hardening during forming.
[0008] Also, the present disclosure aims to provide an austenitic stainless steel having excellent formability, enabling uniform forming by reducing anisotropy according to the rolling direction.
[0009] The present disclosure also aims to provide an austenitic stainless steel with excellent formability, said steel offering a reduction in strength increase during deep drawing, thereby preventing equipment failure caused by increased load on forming equipment due to strength changes, minimizing equipment load fluctuations, enabling forming operations under stable conditions, and being effective for equipment maintenance.
[Technical Solution]
[0010] An austenitic stainless steel having excellent formability, comprising, in percent by weight (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%, and the balance of Fe and other unavoidable impurities, wherein Formula (1) below is satisfied, and a work-hardening exponent (n) in a strain range of 20 to 30% according to Formula (2) below is 0.5 or less.
(wherein C, N, Mn, Cr, Ni, and Mo mean the weight% of each element)
(wherein σ is stress, K is a strength coefficient, ε is strain, and n is a work-hardening exponent).
[0011] Also, an example stainless steel of the present disclosure has a deviation of the work-hardening exponent (n) according to the Formula (2) in the strain range of 20 to 30% among a rolling direction, a 0° direction, a 45° direction, and a 90° direction that is 0.02 or less.
[0012] Additionally, an austenitic stainless steel according to an example of the present disclosure may further comprise P: more than 0% and 0.0500% or less or S: more than 0% and 0.0060% or less.
[0013] Further, the stainless steel of an example of the present disclosure may have a thickness of 0.1 to 3.0 mm and an average grain size of 10 µm or less.
[0014] Furthermore, the stainless steel according to an example of the present disclosure may have an austenite phase of 95% or more in terms of area fraction.
[0015] Additionally, the stainless steel according to an example of the present disclosure may have a yield ratio (yield strength/tensile strength) at room temperature of 0.43 or more.
[0016] Also, the stainless steel according to an example of the present disclosure may have, after deep drawing with a disk diameter of 105 mm and a punch diameter of 50 mm, an average earing height difference according to Formula (3) below of 3.0 or less.
(wherein H means a value of a part where an earing height is high, and h means a value of a part where an earing height is low)
[0017] Also, an Hv hardness value of the austenitic stainless steel of an example of the present disclosure may exceed 180.
[0018] Further, in an example of the present disclosure, a yield strength of the stainless steel at room temperature may be 320 MPa or more.
[0019] A method for manufacturing an austenitic stainless steel having excellent formability, in an example of the present disclosure, the method comprising: hot rolling a slab comprising, in percent by weight (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%, and the balance of Fe and other unavoidable impurities, satisfying Formula (1) below, and having a work-hardening exponent (n) in a strain range of 20 to 30% according to Formula (2) below of 0.5 or less; hot-rolled annealing after the hot rolling; final cold rolling after the hot-rolled annealing; and final annealing after the cold rolling.
[0021] Furthermore, the hot-rolled annealing according to an example of the present disclosure can be performed at a temperature of 1000 to 1150°C.
[0022] Also, according to an example of the present disclosure, the final cold rolling can be performed such that a thickness reduction rate may be 40% or more at room temperature.
[0023] Further, the final annealing according to an example of the present disclosure can be performed at a temperature of 700 to 950°C.
[0024] The austenitic stainless steel according to an example of the present disclosure may have a deviation of the work-hardening exponent (n) according to the Formula (2) in the strain range of 20 to 30% among a rolling direction, a 0° direction, a 45° direction, and a 90° direction is 0.02 or less.
[0025] The stainless steel according to an example of the present disclosure can further comprise P: more than 0% and 0.0500% or less or S: more than 0% and 0.0060% or less.
[0026] Additionally, the stainless steel according to an example of the present disclosure may have a thickness of 0.1 to 3.0 mm and an average grain size of 10 µm or less.
[0027] Furthermore, the stainless steel according to an example of the present disclosure may have an austenite phase of 95% or more in terms of area fraction.
[0028] Also, the austenitic stainless steel of the present disclosure according to an example may have a yield ratio (yield strength/tensile strength) at room temperature of 0.43 or more.
[0029] Furthermore, the said stainless steel according to an example of the present disclosure, after deep drawing with a disk diameter of 105 mm and a punch diameter of 50 mm, may have an average earing height difference according to the said Formula (3) of 3.0 or less.
[0030] Furthermore, the stainless steel according to an example of the present disclosure may have an Hv hardness value exceeding 180.
[0031] Additionally, in an example of the present disclosure, the stainless steel may have a yield strength at room temperature of 320 MPa or more.
[Advantageous Effects]
[0032] The highly formable austenitic stainless steel of the present disclosure provides an increase in initial strength before forming and a minimization of strength increase due to work-hardening during forming, enabling stable forming without excessive strength increase even during deep drawing. A reduction in anisotropy according to the rolling direction enables uniform forming.
[0033] Furthermore, a reduction of strength increase during deep drawing enables the prevention of equipment failure from an increased load on forming equipment due to strength changes. A minimization of equipment load fluctuations facilitates the performance of forming operations under stable conditions, thereby proving effective for equipment maintenance.
[Description of Drawings]
[0034] FIG. 1 is a drawing experiment specimen for measuring an average earing height difference according to an example of the present disclosure.
[Modes of the Invention]
[0035] Hereinafter, examples of the present disclosure are described in detail with reference to the accompanying drawings. The following examples are provided to a person of ordinary skill in the art to which the present disclosure pertains to sufficiently convey the spirit of the present disclosure. The present disclosure is not limited to the examples presented herein and may be embodied in other forms. The drawings omit illustrations of irrelevant parts for clarity of the present disclosure, and for ease of understanding, may exaggerate the size of components.
[0036] Moreover, a statement of a part comprising a component does not exclude other components but means the further inclusion of other components, absent a specific contrary statement.
[Technical Problem]
[0038] The present disclosure controls the content of major constituent elements and regulates final cold rolling and final annealing conditions for improving formability. This control refines grains and secures sufficient initial strength, enabling the derivation of conditions capable of minimizing strength increase during forming. Particularly, the present disclosure derives conditions for reducing the work-hardening exponent in deep drawing conditions of 20% or more, thereby mitigating strength increase even with extensive deformation. Furthermore, a reduction in anisotropy according to the rolling direction resolves the problem of formability degradation arising from differing deformation characteristics based on the sheet direction.
[0039] The austenitic stainless steel of the present disclosure comprises C, Si, Mn, Ni, Cr, Mo, Cu, N, Fe, and other unavoidable impurities.
[0040] Hereinafter, the reasons for limiting the component composition of the said steel are described in detail. The following component compositions all refer to percent by weight (wt%) unless otherwise specified.
[0042] C is an austenite phase stabilizing element, added for suppressing martensite formation during deformation and for securing strength. The C content is 0.005% or more due to the effect of stabilizing the austenite phase with a greater addition amount. However, a C content exceeding 0.100% forms chromium carbides during low-temperature annealing, potentially degrading intergranular corrosion resistance. The limitation of its upper bound to 0.100% is desirable. More preferably, the C content is 0.005% to 0.080%. Most preferably, the C content is 0.010% to 0.030%. In such a case, the excellent intergranular corrosion resistance and austenite phase stabilization effects further improve hardness and yield strength while further improving the work-hardening exponent.
[0044] Si is a component added as a deoxidizer in the steelmaking stage, and an element securing the strength and corrosion resistance of austenitic stainless steel. A certain amount of Si addition forms silicon oxide (Si oxide) on a passive film in cases of undergoing bright annealing, improving the corrosion resistance of the steel. However, an Si content greater than 1.00% can reduce the ductility of the steel, such that limiting the upper limit to 1.00% is desirable. More preferably, the Si content is included from 0.10% to 0.90%, and most preferably, the Si content is included from 0.20% to 0.50%. In such a case, the effect of improving the strength and corrosion resistance of the austenite phase is excellent, and the ductility of the steel is also excellent, such that formability can be further improved.
[0046] Manganese is an austenitic phase stabilizing element having an effect of suppressing a strength increase generated during forming. Mn is added at 0.10% or more, owing to its increased effect of stabilizing the austenitic phase with a greater addition amount. However, an Mn content exceeding 2.00% can lower corrosion resistance and surface gloss, thus, limiting the upper limit to 2.00% is desirable. More preferably, the Mn content is 0.30% to 1.90%, and most preferably, 0.60% to 1.50%. In such a case, excellent corrosion resistance and surface gloss are attained, and the austenitic phase stabilizing effect is excellent, leading to further improvements in hardness and yield strength while further improving formability.
[0048] Ni is the most potent austenite phase stabilizing element, with an increasing content thereof stabilizing the austenite phase and softening the material. For suppressing work hardening caused by the occurrence of deformation-induced martensite, 6.00% or more can be added. However, an excessive addition of expensive Ni causes a cost increase. In consideration of the cost and efficiency of the steel material, limiting the upper limit thereof to 12.00% is preferable. More preferably, the Ni content is 7.00% to 10.50%, and most preferably, the Ni content is 8.00% to 9.50%. Such a content range enables an improvement in hardness and yield strength and an improvement in the work-hardening exponent, and is further preferable in terms of cost and efficiency.
[0050] Cr is an essential element for improving stainless steel corrosion resistance. A Cr content of 16.00% or more is added for securing atmospheric corrosion resistance. However, a Cr content exceeding 20.00% hardens the material and suppresses deformation-induced martensite during cold rolling. Limiting the upper limit to 20.00% is preferred. More preferably, the Cr content is 16.10% to 19.40%. Most preferably, the Cr content is 17.00% to 19.00%. In this case, the corrosion resistance and the martensite suppression effect are excellent, thereby ensuring stable strength and further improving formability.
[0052] Mo is a useful element for improving corrosion resistance. However, large additions of Mo, as an expensive constituent element, cause a cost increase. Furthermore, the precipitation of intermetallic compounds such as sigma (σ) phase causes problems of deteriorating mechanical properties and corrosion resistance. Therefore, the upper limit is desirably limited to 0.20% or less. More preferably, the Mo content is 0.01% to 0.18%. Most preferably, the Mo content is 0.05% to 0.15%. Such a case provides excellent effects of cost reduction and improvement in mechanical properties and corrosion resistance.
[0054] Cu is a useful element for the stabilization of an austenite phase, capable of use instead of expensive Ni. However, an excessive Cu content forms a low-melting-point ε-Cu precipitate phase, degrades hot workability, and is capable of degrading surface quality. Therefore, limiting the upper limit to 0.50% is desirable. More preferably, the Cu content is included at 0.01% to 0.48%, and most preferably, at 0.10% to 0.40%. In such a case, excellent hot workability and an excellent austenite phase stabilization effect improve hardness and yield strength, and surface quality is capable of improvement.
[0056] N is an austenitic phase stabilizing element. The effect of N in stabilizing the austenitic phase and improving material strength permits its addition in an amount of 0.010% or more, with a greater addition amount contributing to austenitic phase stabilization. However, N content greater than 0.100% can cause material hardening and hot workability degradation. A limitation of its upper content to 0.100% is desirable. More preferably, the N content is 0.011% to 0.091%. Most preferably, the N content is 0.030% to 0.080%. In such a case, the excellent hot workability and austenitic phase stabilizing effect lead to improved hardness and yield strength, alongside enhanced surface quality.
[0058] P is an unavoidably contained impurity in the steel, and is an element that may causes grain boundary corrosion or hinders hot formability. The upper limit of the P content is limited to 0.0500%. More preferably, the P content is 0.0200% to 0.0450%, and most preferably, the P content is 0.0300% to 0.0400%. In such a case, prevention of grain boundary corrosion and hot formability can be further improved.
[0060] S constitutes an unavoidable impurity within steel. S segregation at grain boundaries functions as a main cause of hindering hot workability. The upper limit of the S content is accordingly limited to 0.0060 wt%. More preferably, the S content is 0.0020 wt% to 0.0055 wt%. Most preferably, the S content is 0.0030 wt% to 0.0045 wt%. Such content ranges provide excellent hot workability.
[0061] The balance is Fe. However, during conventional manufacturing processes, unintended impurities are inevitably mixed in from raw materials or the surrounding environment, making their exclusion impossible. An ordinary person skilled in the art of conventional manufacturing processes knows these impurities; therefore, the present disclosure does not specifically mention all details.
[0062] Work-hardening of austenitic stainless steel generally results from transformation of an unstable austenite phase at room temperature into a martensite phase by stress resulting from plastic deformation.
[0063] Ongoing deformation results in continuous phase transformation. Such phase transformation causes an increase in strength for the austenitic stainless steel until material fracture. The minimization of strength increase during forming to secure formability requires the suppression of martensite phase transformation.
[0064] In the present disclosure, considering the deformation-induced phase transformation of the austenitic stainless steel, Formula (1) below was derived.
[0065] Specifically, the present disclosure increases the stabilization degree of the austenite phase by controlling the value of Formula (1) among the alloy components to be 0 or more. Consequently, phase transformation to a martensite phase is suppressed, and work hardening of the austenitic stainless steel can be suppressed.
wherein C, N, Mn, Cr, Ni, and Mo mean the weight% of each element
[0066] An austenitic stainless steel having excellent formability according to an example of the present disclosure satisfies a range where the value represented by Formula (1) is 0 or more. A value of Formula (1) less than 0 causes the aforementioned austenitic stainless steel of the alloy component system to exhibit rapid deformation-induced martensite transformation behavior or to generate plastic inhomogeneity due to twin formation. Accordingly, a problem of decreased strength, formability, and particularly deep drawability of the austenitic stainless steel arises, thereby limiting the lower limit of Formula (1) to 0. That is, the value represented 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, stabilization of the austenitic stainless steel is possible, whereby the strength, formability, and deep drawability of the steel can be further improved. The upper limit of Formula (1) is not particularly limited; for example, it may be 116 or less, 100 or less, 50 or less, or 20 or less. Within the said range, the effect of improving deep drawability can be further enhanced.
[0067] Meanwhile, an austenitic stainless steel exhibits a decrease in the work-hardening exponent after reaching a maximum work-hardening exponent point (hereinafter, referred to as the 'maximum work-hardening exponent point'), despite the progression of deformation. That is, after the maximum work-hardening exponent point, strength gradually increases with the continuous decrease of the work-hardening exponent value.
[0068] Accordingly, the present disclosure focuses on the necessity of securing a certain amount or more of deformation without an excessive strength increase for improving the formability of an austenitic stainless steel. The arrangement of the work-hardening exponent's maximum point in a relatively low strain range of 20 to 30% secures sufficient strength in an early stage. Furthermore, the securing of a certain amount or more of deformation from the work-hardening exponent's maximum point causes a continuous decrease in the value of the work-hardening exponent and a gradual increase in strength, thereby minimizing the strength increase during forming.
[0069] In other words, the exhibition of a maximum work-hardening exponent in a strain range of 20 to 30% and the securing of continuous deformation without an excessive strength increase enables the securing of the strength of the austenitic stainless steel while improving the formability thereof and significantly reducing fracture occurrence.
[0070] Therefore, an austenitic stainless steel having excellent formability according to an example of the present disclosure can exhibit a work-hardening exponent (n) in a strain range of 20 to 30% according to Formula (2) below of 0.5 or less.
(wherein σ is stress, K is a strength coefficient, ε is strain, and n is a work-hardening exponent).
[0071] The work-hardening exponent n in the said Formula (2) corresponds to the slope of a graph. A larger slope signifies severe strength increase of the material during plastic deformation. Improvement in the formability of the present disclosure's austenitic stainless steel requires the securing of continuous deformation without an excessive strength increase.
[0072] The present disclosure's austenitic stainless steel has a work-hardening exponent (n) of 0.5 or less in a strain range of 20 to 30%. A work-hardening exponent exceeding 0.5 causes an excessive strength increase, reducing formability and causing rapid fracture. Accordingly, a reduction in the yield ratio of the austenitic stainless steel occurs, making a limitation of the upper limit of the work-hardening exponent to 0.5 desirable. More desirably, the work-hardening exponent is 0.48 or less, and most desirably, 0.45 or less. The satisfaction of this condition secures continuous deformation without excessive strength increase, further improving formability. The lower limit of Formula (2) is not particularly limited; for example, it can be 0.01 or more, 0.1 or more, 0.2 or more, 0.3 or more, or 0.4 or more. Within the aforementioned range, the effect of securing continuous deformation can be further improved.
[0073] Specifically, the present disclosure, through the satisfaction of Formula (1) as 0 or more, suppresses phase transformation to a martensite phase and suppresses work hardening of the austenitic stainless steel, thereby minimizing strength increase due to work hardening during forming. Furthermore, the satisfaction of the work-hardening exponent (n) in a strain range of 20 to 30% according to Formula (2) below as 0.5 or less increases initial strength before forming and minimizes strength increase due to work hardening during forming, enabling stable forming even during deep drawing without excessive strength increase.
[0074] The austenitic stainless steel having excellent formability according to an example of the present disclosure further has a deviation of the work-hardening exponent (n) according to Formula (2) in the strain range of 20 to 30% among a rolling direction, a 0° direction, a 45° direction, and a 90° direction of 0.02 or less.
[0075] The 0° direction means the average value of the work-hardening exponent measured at a position forming a 0° angle with the rolling direction, the 45° direction means the average value of the work-hardening exponent measured at a position forming a 45° angle with the rolling direction, and the 90° direction means the average value of the work-hardening exponent measured at a position forming a 90° angle with the rolling direction.
[0076] For forming operations such as deep drawing, work hardening significantly occurs during forming. The strength increase becomes significant due to work hardening. Accordingly, a difference in strength occurs according to a difference in work-hardening ability by rolling direction. Such differences in strength by direction cause the deformation characteristics to vary according to the direction of the sheet during forming, and thus uniform forming may be difficult.
[0077] Therefore, in the present disclosure, it is preferable that a deviation among work-hardening exponents in a rolling direction, a 0° direction, a 45° direction, and a 90° direction in a 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, anisotropy is reduced, and forming defects depending on the direction of the sheet material can be reduced, thereby enabling uniform forming.
[0078] As described above, the austenitic stainless steel of the present disclosure, which satisfies the alloy element composition range and the relational formula, the work-hardening exponent in the strain range of 20 to 30%, and the deviation among the work-hardening exponents in the rolling direction, the 0° direction, the 45° direction, and the 90° direction, may satisfy an austenite phase of 95% or more in terms of area fraction, a thickness of the steel material of 0.1 to 3.0 mm, and an average grain size of 10 µm or less.
[0079] When the average grain size exceeds 10 µm, an excessive strength increase due to work hardening may occur, thereby degrading formability. Therefore, it is preferable to limit the upper limit thereof to 10 µm. More preferably, the average grain size is 9 µm or less, and most preferably 8 µm or less, whereby formability can be further improved.
[0080] In addition, the thickness of the steel material may be limited to 0.1 to 3.0 mm, preferably 0.15 to 0.30 mm, for forming without cracks or fracture.
[0081] In addition, the austenitic stainless steel according to an example of the present disclosure may have a yield ratio(yield strength/tensile strength) at room temperature of 0.43 or more.
[0082] The yield ratio is a ratio of yield strength to tensile strength, and is a value representing an increase in strength from a yield point, at which deformation starts during forming, to a maximum strength. A large yield ratio means that variation in strength is small.
[0083] When the yield ratio at room temperature is less than 0.43, a large change in strength due to work hardening may occur during forming, thereby reducing formability. In particular, during deep drawing, an equipment load may occur due to an increase in load of forming equipment caused by a change in strength. Therefore, the yield ratio at room temperature is preferably 0.43 or more, 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, an advantageous effect can be achieved in attaining desired strength and formability while reducing equipment load during forming.
[0084] In addition, the austenitic stainless steel according to an example of the present disclosure may have an Hv hardness value exceeding 180, preferably exceeding 185, and may satisfy a yield strength at room temperature of 320 MPa or more, preferably 330 MPa or more, thereby realizing excellent strength while reducing overload of equipment 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, the balance between high strength and high formability can be improved, equipment load during processing can be reduced, and the deviation of the work-hardening exponent can be reduced.
[0085] In addition, the austenitic stainless steel according to an example of the present disclosure may secure an average earing height difference according to Formula (3) below of 3.0 or less after deep drawing with a disk diameter of 105 mm and a punch diameter of 50 mm. Accordingly, formability can be improved, and uniform forming without cracks or fracture can be achieved. Preferably, the average earing 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 can also be achieved.
wherein H means a value of a portion having a high earing height, and h means a value of a portion having a low earing height.
[0086] A method for manufacturing the austenitic stainless steel having excellent formability of the present disclosure as described above will be described below.
[0087] A method for manufacturing an austenitic stainless steel according to an example of the present disclosure may include: hot rolling a slab comprising, in percent by weight (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%, and the balance of Fe and other unavoidable impurities, satisfying Formula (1), and satisfying a work-hardening exponent (n) of 0.5 or less in a strain range of 20 to 30% according to Formula (2); performing hot-rolled annealing after the hot rolling; performing final cold rolling after the hot-rolled annealing; and performing final annealing after the final cold rolling.
[0088] The temperature during the hot-rolled annealing after the hot rolling greatly influences residual stress relief and microstructure. Therefore, the hot-rolled annealing is preferably performed at a temperature of 1000 to 1150°C.
[0089] A hot-rolled annealing temperature less than 1000°C prevents sufficient recrystallization and annealing, thereby potentially causing a decrease in ductility. A hot-rolled annealing temperature exceeding 1150°C can result in extremely coarse grain growth. Therefore, the hot-rolled annealing temperature is preferably limited to 1000 to 1150°C.
[0090] Furthermore, as described above, satisfaction of the alloy element composition and the relational formula, satisfaction of the work-hardening exponent in the strain range of 20 to 30%, and satisfaction of the work-hardening exponent deviation among the rolling direction, the 0° direction, the 45° direction, and the 90° direction, combined with control of the final cold rolling and final annealing conditions, enables the average grain size of the austenitic stainless steel to satisfy 10 µm or less.
[0091] The final cold rolling is performed after the hot-rolled annealing at room temperature such that a thickness reduction rate is 40% or more. The number of passes for the final cold rolling may be one pass or multiple passes, and performance through two or more repeated rollings is particularly preferable. Upon performance of the final cold rolling such that a thickness reduction rate is 40% or more, execution is possible without limitation on a reduction rate per pass and a number of passes.
[0092] Specifically, the present disclosure performs cold rolling such that a thickness reduction rate is 40% or more to increase the amount of carbonitride precipitates, suppress grain growth, and satisfy an average grain size of 10 µm or less. A thickness reduction rate less than 40% during the cold rolling prevents obtaining fine grains, thereby hindering the improvement of formability.
[0093] Additionally, the final annealing is performed at a temperature of 700 to 950°C, a temperature lower than the preceding hot-rolled annealing process, for removing distortion caused by cold rolling and for precipitating fine carbonitrides to refine grains. The final annealing temperature exceeding the said range may result in the formation of coarse carbonitrides and a non-uniform structure. Therefore, the final annealing temperature is limited to 700 to 950°C.
[0094] The austenitic stainless steel of the present disclosure manufactured by such a method has an austenite phase of 90% or more in terms of area fraction, and an average grain size of 10 µm or less.
example
[0096] Using steel having the composition of Table 1 below, a slab having a thickness of 200 mm was manufactured through a continuous casting process. After heating for 2 hours at 1250°C, hot rolling was performed to a thickness of 4 to 8 mm, and after the hot rolling, hot-rolled annealing was performed at 1150°C. Then, final cold rolling was performed such that a thickness reduction rate was 40% or more at room temperature, and final annealing was performed at 700 to 950°C to manufacture a coil.
[0098] In Table 2 below, the Formula (1) value is derived by substituting into Formula (1) below, the n value is the work-hardening exponent (n) value in the strain range of 20 to 30% according to Formula (2) below, and the yield ratio means the ratio of the yield strength to the tensile strength.
(wherein C, N, Mn, Cr, Ni, and Mo mean the weight% of each element)
(wherein σ is stress, K is a strength coefficient, ε is strain, and n is a work-hardening exponent).
[Table 1]
| Class ification | C | Si | Mn | P | S | Cr | Ni | Mo | Cu | N |
| Comparative Examp-le 1 | 0. 0 1 2 | .0 6 3 | 0 . 5 1 | 0. 02 33 | 0. 00 29 | 1 8. 0 1 | 8. 2 9 | 0 .0 8 | 0 . 2 6 | 0. 0 8 1 |
| Comparative Exam-ple 2 | 0. 0 8 3 | 0 .2 0 | 1 .1 0 | 0. 03 29 | 0. 00 25 | 1 7. 9 6 | 7. 0 6 | 0 .0 8 | 0 . 4 4 | 0. 0 6 2 |
| Comparative Exam-ple 3 | 0. 0 6 1 | 0 .4 0 | 1 .0 8 | 0. 03 36 | 0. 00 31 | 1 8. 1 7 | 8. 0 0 | 0 .0 7 | 0 .1 6 | 0. 0 3 6 |
| Com parati ve Exa m-ple 4 | 0. 0 3 3 | 0 9 0 | 1 6 2 | 0. 02 83 | 0. 00 42 | 1 9. 4 7 | 6. 2 9 | 0 1 7 | 0 . 3 9 | 0. 0 9 3 |
| Com parati ve Exa m-ple 5 | 0. 0 5 4 | 0 4 2 | 1 9 3 | 0. 04 01 | 0. 00 47 | 1 8. 6 2 | 7. 3 3 | 0 1 2 | 0 . 3 1 | 0. 0 5 7 |
| Com parati ve Exa m-ple 6 | 0. 0 5 6 | 0 3 3 | 1 0 5 | 0. 03 10 | 0. 00 43 | 1 8. 1 6 | 8. 0 8 | 0 1 9 | 0 1 9 | 0. 0 3 1 |
| Comparative Exam-ple 7 | 0. 0 1 6 | 0 .2 6 | 0 . 6 2 | 0. 03 20 | 0. 00 33 | 1 6. 3 2 | 1 0. 2 1 | 0 . 0 8 | 0 .1 1 | 0. 0 1 8 |
| Comparative Exam-ple 8 | 0. 0 3 7 | 0 .4 0 | 0 . 8 2 | 0. 03 51 | 0. 00 21 | 1 6. 7 1 | 9. 3 3 | 0 .2 0 | 0 . 2 1 | 0. 0 2 8 |
| Comparative Exam-ple 9 | 0. 0 5 4 | 0 .4 1 | 1 .0 4 | 0. 03 24 | 0. 00 27 | 1 8. 1 3 | 8. 0 6 | 0 .1 1 | 0 . 2 5 | 0. 0 3 4 |
| Exam-ple 1 | 0. 0 5 1 | 0 .3 4 | 1 .1 1 | 0. 02 87 | 0. 00 21 | 1 6. 3 4 | 9. 1 2 | 0 .0 6 | 0 .1 7 | 0. 0 1 9 |
| Exam-ple 2 | 0. 0 4 0 | 0 . 9 1 | 0 . 6 3 | 0. 04 12 | 0. 00 32 | 1 9. 4 1 | 7. 4 2 | 0 .1 6 | 0 . 4 2 | 0. 0 2 9 |
| Exam-ple 3 | 0. 0 5 4 | 0 .4 3 | 1 .0 3 | 0. 03 06 | 0. 00 25 | 1 8. 2 4 | 7. 9 1 | 0 .0 9 | 0 . 2 4 | 0. 0 3 2 |
| Exam-ple 4 | 0. 0 1 3 | 0 .2 2 | 0 . 3 8 | 0. 02 62 | 0. 00 26 | 1 6. 1 2 | 1 0. 3 3 | 0 . 0 4 | 0 .1 1 | 0. 0 1 1 |
| Exam-ple 5 | 0. 0 2 1 | 0 .3 6 | 1 .3 4 | 0. 04 09 | 0. 00 20 | 1 7. 0 1 | 8. 3 0 | 0 . 0 8 | 0 . 3 6 | 0. 0 6 5 |
| Exam-ple 6 | 0. 0 5 4 | 0 .4 0 | 1 .0 5 | 0. 03 52 | 0. 00 25 | 1 8. 3 2 | 7. 6 0 | 0 .1 3 | 0 . 3 7 | 0. 0 3 9 |
| Exam-ple 7 | 0. 0 1 3 | 0 .5 2 | 0 . 4 9 | 0. 02 21 | 0. 00 28 | 1 8. 0 8 | 8. 1 0 | 0 .0 8 | 0 . 2 1 | 0. 0 8 1 |
| Exam-ple 8 | 0. 0 8 3 | 0 .2 1 | 1 .0 8 | 0. 03 30 | 0. 00 31 | 1 7. 8 8 | 7. 0 1 | 0 .0 8 | 0 . 4 7 | 0. 0 6 2 |
| Exam-ple 9 | 0. 0 5 3 | 0 .4 0 | 1 .0 1 | 0. 03 12 | 0. 00 29 | 1 8. 1 1 | 8. 0 7 | 0 .0 8 | 0 .1 7 | 0. 0 3 6 |
| Exam-ple 10 | 0. 0 3 2 | 0 .9 0 | 1 .6 1 | 0. 03 19 | 0. 00 45 | 1 9. 3 3 | 6. 3 1 | 0 .1 8 | 0 . 4 1 | 0. 0 9 1 |
| Exam-ple 11 | 0. 0 5 4 | 0 .4 3 | 1 .9 4 | 0. 03 29 | 0. 00 30 | 1 8. 5 9 | 7. 2 8 | 0 .1 1 | 0 . 2 9 | 0. 0 4 1 |
| Exam-ple 12 | 0. 0 3 3 | 0 .4 2 | 0 .7 6 | 0. 03 19 | 0. 00 53 | 1 8. 1 4 | 8. 1 0 | 0 .1 4 | 0 . 2 8 | 0. 0 3 9 |
[Table 2]
| Classification | Formula (1) value | n-23value | Average Grain Size (µm) | Yield Ratio at RT |
| Comparative Example 1 | -1.611 | 0.560 | 23.0 | 0.42 |
| Comparative Example 2 | -0.054 | 0.557 | 25.0 | 0.42 |
| Comparative Example 3 | -2.554 | 0.554 | 26.0 | 0.42 |
| Comparative Example 4 | -0.408 | 0.521 | 24.3 | 0.42 |
| Comparative Example 5 | -7.89 | 0.519 | 20.0 | 0.42 |
| Comparative Example 6 | -5.062 | 0.520 | 18.0 | 0.42 |
| Comparative Example 7 | -0.19 | 0.578 | 19.0 | 0.42 |
| Comparative Example 8 | -5.314 | 0.592 | 18.0 | 0.41 |
| Comparative Example 9 | -0.803 | 0.579 | 18.0 | 0.41 |
| Example 1 | 5.512 | 0.478 | 6.8 | 0.47 |
| Example 2 | 7.776 | 0.478 | 7.1 | 0.47 |
| Example 3 | 2.825 | 0.478 | 6.9 | 0.47 |
| Example 4 | 7.438 | 0.443 | 7.8 | 0.47 |
| Example 5 | 6.644 | 0.443 | 7.7 | 0.47 |
| Example 6 | 4.749 | 0.443 | 8.0 | 0.47 |
| Example 7 | 2.039 | 0.447 | 8.9 | 0.45 |
| Example 8 | 2.348 | 0.449 | 8.6 | 0.45 |
| Example 9 | 0.028 | 0.449 | 8.8 | 0.45 |
| Example 10 | 1.808 | 0.441 | 7.8 | 0.47 |
| Example 11 | 1.034 | 0.442 | 8.4 | 0.46 |
| Example 12 | 6.085 | 0.443 | 8.2 | 0.46 |
[0099] As shown in the said Table 2, for Examples 1 to 12 according to the present disclosure, the value derived by substituting into Formula (1) is 0 or more, and the work-hardening exponent in the strain range of 20 to 30% according to the said Formula (2) satisfies 0.5 or less. Confirmation of the austenite phase grain size being 10 µm or less and the yield ratio value satisfying 0.43 or more was observed. In contrast, for Comparative Examples 1 to 9, where the content of each component is included in the alloy composition presented in the present disclosure, but the value derived by substituting into Formula (1) is less than 0, confirmation of the work-hardening exponent in the strain range of 20 to 30% exceeding 0.5, the grain size also exceeding 10 µm, and the yield ratio also appearing as less than 0.43 was observed.
[0100] Through these results, satisfaction of an alloy composition according to the present disclosure with a value derived by substitution into Formula (1), a work-hardening exponent in a strain range of 20 to 30% according to Formula (2) of 0.5 or less, an average grain size of 10 µm or less, and a yield ratio of 0.43 or more, thereby minimizes strength increase due to work hardening during forming, enabling stable forming without excessive strength increase even during deep drawing, and preventing equipment failure due to increased load on forming equipment from strength variation. This allows prediction of effectiveness in equipment maintenance under stable conditions.
[0101] Additionally, for each steel sheet of Table 1, the work-hardening exponent (n) value and the deviation (maximum work-hardening exponent value - minimum work-hardening exponent value) were each measured for the rolling direction, the 0° direction, the 45° direction, and the 90° direction in the strain range of 20 to 30% according to Formula (2). A large deviation of the work-hardening exponent for each direction causes strength variation to appear differently by direction during forming, meaning difficulty in achieving uniform forming.
[0102] Also, the earing height difference was measured using drawing test specimens manufactured by deep drawing with a disk diameter of 105 mm and a punch diameter of 50 mm, and Hv hardness and yield strength were measured. The results are shown in Table 3 below.
[0103] Measurement of the earing height difference was performed according to Formula (3) below. As illustrated in FIG. 1, a high earing height portion of the specimen is designated H, and a low earing height portion is designated h. The four high earing height portions are arbitrarily designated H1, H2, H3, and H4, respectively, and the four low earing height portions are arbitrarily designated h1, h2, h3, and h4, respectively.
[0104] Formula (3): {(H1+H2+H3+H4)-(h1+h2+h3+h4)} / 4 (wherein H means a value of a part where an earing height is high, and h means a value of a part where an earing height is low).
(wherein H means a value of a part where an earing height is high, and h means a value of a part where an earing height is low).
[TABLE 3]
| Classification | n-value by Rolling Direction | n-value Deviation | Ear Height Difference | Hv Hardness | Yield Strength (MPa) | ||
| 0° | 45 ° | 90 ° | |||||
| Comparative Example 1 | 0.537 | 0.552 | 0.560 | 0.023 | 3.07 | 173 | 312.8 |
| Compa rative Examp le 2 | 0. 53 1 | 0. 54 1 | 0. 55 7 | 0.02 6 | 3.48 | 179 | 312. 7 |
| Compa rative Examp le 3 | 0. 53 4 | 0. 54 7 | 0. 55 4 | 0.02 0 | 3.51 | 176 | 313. 6 |
| Compa rative Examp le 4 | 0. 49 5 | 0. 50 0 | 0. 52 1 | 0.02 6 | 3.06 | 173 | 301. 2 |
| Comparative Example 5 | 0. 49 6 | 0. 50 3 | 0. 51 9 | 0.02 3 | 3.18 | 180 | 301. 9 |
| ComparativeExample 6 | 0. 49 8 | 0. 50 3 | 0. 52 0 | 0.02 2 | 3.36 | 179 | 301. 2 |
| ComparativeExample 7 | 0. 55 6 | 0. 57 1 | 0. 57 8 | 0.02 2 | 3.50 | 180 | 302. 0 |
| ComparativeExample 8 | 0. 56 8 | 0. 57 3 | 0. 59 2 | 0.02 4 | 3.27 | 176 | 302. 5 |
| ComparativeExample 9 | 0. 55 4 | 0. 56 5 | 0. 57 9 | 0.02 5 | 3.21 | 180 | 302. 8 |
| Example 1 | 0. 46 0 | 0. 46 8 | 0. 47 8 | 0.01 8 | 2.61 | 184 | 328. 1 |
| Example 2 | 0. 45 9 | 0. 46 2 | 0. 47 8 | 0.01 9 | 2.57 | 193 | 327. 4 |
| Examp le 3 | 0. 46 1 | 0. 46 6 | 0. 47 8 | 0.01 7 | 2.03 | 181 | 326. 9 |
| Examp le 4 | 0. 42 5 | 0. 42 7 | 0. 44 3 | 0.01 8 | 2.74 | 193 | 343. 4 |
| Examp le 5 | 0. 42 4 | 0. 42 8 | 0. 44 3 | 0.01 9 | 2.50 | 184 | 345. 2 |
| Examp le 6 | 0. 42 5 | 0. 42 6 | 0. 44 3 | 0.01 8 | 2.65 | 189 | 343. 3 |
| Examp le 7 | 0. 43 3 | 0. 43 5 | 0. 44 7 | 0.01 4 | 2.14 | 181 | 324. 4 |
| Examp le 8 | 0. 43 5 | 0. 43 1 | 0. 44 9 | 0.01 8 | 2.62 | 185 | 326. 9 |
| Examp le 9 | 0. 43 4 | 0. 43 2 | 0. 44 9 | 0.01 7 | 2.53 | 187 | 324. 4 |
| Examp le 10 | 0. 42 8 | 0. 42 6 | 0. 44 1 | 0.01 5 | 2.71 | 184 | 341. 5 |
| Example 11 | 0.424 | 0.426 | 0.442 | 0.018 | 2.28 | 189 | 337.2 |
| Example 12 | 0.430 | 0.426 | 0.443 | 0.017 | 2.51 | 185 | 334.4 |
[0105] As shown in Table 3, for examples 1 to 12 according to the present disclosure, the value derived by substitution into Formula (1) is 0 or more, and satisfaction of a work-hardening exponent in a strain range of 20 to 30% according to Formula (2) of 0.5 or less resulted in a deviation of the work-hardening exponent of 0.02 or less among a rolling direction, a 0° direction, a 45° direction, and a 90° direction, from which a conclusion was drawn that uniform formability is possible due to less variation in strength in each rolling direction during forming. Also, the average earing height difference according to Formula (3) is 3.0 or less, the Hv hardness value exceeds 180, and the yield strength also satisfies 320 MPa or more, which was confirmed. In contrast, for comparative examples 1 to 9, where the content of each component is included in the alloy composition presented in the present disclosure, but the value derived by substitution into Formula (1) is less than 0, the deviation of the work-hardening exponent among a rolling direction, a 0° direction, a 45° direction, and a 90° direction exceeded 0.02, the average earing height difference also exceeded 3.0, the Hv hardness value was less than 180, and the yield strength was also less than 320 MPa, which was confirmed.
[0106] Through these results, the alloy composition according to the present disclosure, satisfying the value derived by substituting into Formula (1), and with the work-hardening exponent in the strain range of 20 to 30% according to Formula (2) being 0.5 or less, demonstrates satisfaction of a deviation of the work-hardening exponent of 0.02 or less among the rolling direction, the 0° direction, the 45° direction, and the 90° direction in the strain range of 20 to 30%, an average earing height difference of 3.0 or less, an Hv hardness value exceeding 180, and a yield strength of 320 MPa or more. Such satisfaction minimizes strength increase during forming, reduces anisotropy according to the rolling direction, and enables uniform forming. Furthermore, the prevention of equipment failure due to an increase in load of forming equipment caused by strength changes ensures effectiveness in equipment maintenance under stable conditions.
[0107] The foregoing disclosure describes exemplary embodiments of the present disclosure. The present disclosure is not limited thereto. A person having ordinary skill in the art will understand the possibility of various changes and modifications without departing from the concept and scope of the claims described below.
1. An austenitic stainless steel having excellent formability, comprising, in percent
by weight (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%, and the balance of Fe and other unavoidable impurities,
wherein Formula (1) below is satisfied, and
a work-hardening exponent (n) in a strain range of 20 to 30% according to Formula
(2) below is 0.5 or less:
(wherein C, N, Mn, Cr, Ni, and Mo mean the weight% of each element)
(wherein σ is stress, K is a strength coefficient, ε is strain, and n is a work-hardening exponent).
2. The austenitic stainless steel of claim 1,
wherein a deviation of the work-hardening exponent (n) according to the Formula (2) in the strain range of 20 to 30% among a rolling direction, a 0° direction, a 45° direction, and a 90° direction is 0.02 or less.
wherein a deviation of the work-hardening exponent (n) according to the Formula (2) in the strain range of 20 to 30% among a rolling direction, a 0° direction, a 45° direction, and a 90° direction is 0.02 or less.
3. The austenitic stainless steel of claim 1,
further comprising P: more than 0% and 0.0500% or less or S: more than 0% and 0.0060% or less.
further comprising P: more than 0% and 0.0500% or less or S: more than 0% and 0.0060% or less.
4. The austenitic stainless steel of claim 1,
wherein a thickness is 0.1 to 3.0 mm, and an average grain size is 10 µm or less.
wherein a thickness is 0.1 to 3.0 mm, and an average grain size is 10 µm or less.
5. The austenitic stainless steel of claim 1,
wherein an austenite phase is 95% or more in terms of area fraction.
wherein an austenite phase is 95% or more in terms of area fraction.
6. The austenitic stainless steel of claim 1,
wherein a yield ratio (yield strength/tensile strength) at room temperature is 0.43 or more.
wherein a yield ratio (yield strength/tensile strength) at room temperature is 0.43 or more.
7. The austenitic stainless steel of claim 1,
wherein, after deep drawing with a disk diameter of 105 mm and a punch diameter of
50 mm, an average earing height difference according to Formula (3) below is 3.0 or
less:
(wherein H means a value of a part where an earing height is high, and h means a value of a part where an earing height is low).
8. The austenitic stainless steel of claim 1,
wherein an Hv hardness value exceeds 180.
wherein an Hv hardness value exceeds 180.
9. The austenitic stainless steel of claim 1,
wherein a yield strength at room temperature is 320 MPa or more.
wherein a yield strength at room temperature is 320 MPa or more.
10. A method for manufacturing an austenitic stainless steel having excellent formability,
the method comprising:
hot rolling a slab comprising, in percent by weight (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%, and the balance of Fe and other unavoidable impurities, satisfying Formula (1) below, and having a work-hardening exponent (n) in a strain range of 20 to 30% according to Formula (2) below of 0.5 or less;
hot-rolled annealing after the hot rolling;
final cold rolling after the hot-rolled annealing; and
final annealing after the cold rolling;
(wherein C, N, Mn, Cr, Ni, and Mo mean the weight% of each element)
(wherein σ is stress, K is a strength coefficient, ε is strain, and n is a work-hardening
exponent).
11. The method of claim 10,
wherein the hot rolling is performed at a temperature of 1150 to 1280°C.
wherein the hot rolling is performed at a temperature of 1150 to 1280°C.
12. The method of claim 10,
wherein the hot-rolled annealing is performed at a temperature of 1000 to 1150°C.
wherein the hot-rolled annealing is performed at a temperature of 1000 to 1150°C.
13. The method of claim 10,
wherein the final cold rolling is performed such that a thickness reduction rate is 40% or more at room temperature.
wherein the final cold rolling is performed such that a thickness reduction rate is 40% or more at room temperature.
14. The method of claim 10,
wherein the final annealing is performed at a temperature of 700 to 950°C.
wherein the final annealing is performed at a temperature of 700 to 950°C.
15. The method of claim 10,
wherein a deviation of the work-hardening exponent (n) according to the Formula (2) in the strain range of 20 to 30% among a rolling direction, a 0° direction, a 45° direction, and a 90° direction is 0.02 or less.
wherein a deviation of the work-hardening exponent (n) according to the Formula (2) in the strain range of 20 to 30% among a rolling direction, a 0° direction, a 45° direction, and a 90° direction is 0.02 or less.
16. The method of claim 10,
wherein the stainless steel further comprises P: more than 0% and 0.05% or less or S: more than 0% and 0.006% or less.
wherein the stainless steel further comprises P: more than 0% and 0.05% or less or S: more than 0% and 0.006% or less.
17. The method of claim 10,
wherein the stainless steel has a thickness of 0.1 to 3.0 mm and an average grain size of 10 µm or less.
wherein the stainless steel has a thickness of 0.1 to 3.0 mm and an average grain size of 10 µm or less.
18. The method of claim 10,
wherein the stainless steel has an austenite phase of 95% or more in terms of area fraction.
wherein the stainless steel has an austenite phase of 95% or more in terms of area fraction.
19. The method of claim 10,
wherein the stainless steel has a yield ratio (yield strength/tensile strength) at room temperature of 0.43 or more.
wherein the stainless steel has a yield ratio (yield strength/tensile strength) at room temperature of 0.43 or more.
20. The method of claim 10,
wherein, after deep drawing with a disk diameter of 105 mm and a punch diameter of 50 mm, an average earing height difference according to Formula (3) below is 3.0 or less:
(wherein H means a value of a part where an earing height is high, and h means a value of a part where an earing height is low).
wherein, after deep drawing with a disk diameter of 105 mm and a punch diameter of 50 mm, an average earing height difference according to Formula (3) below is 3.0 or less:
(wherein H means a value of a part where an earing height is high, and h means a value of a part where an earing height is low).