Austenitic stainless steel with improved yield ratio and method for producing same

Abstract

To provide an austenitic stainless steel having an improved yield ratio. [Solution] The austenitic stainless steel of the present invention contains, by weight, C: 0.1% or less (excluding 0), N: 0.2% or less (excluding 0), Si: 1.5 to 2.5%, Mn: 6.0 to 10.0%, Cr: 15.0 to 17.0%, Ni: 0.3% or less (excluding 0), Cu: 2.0 to 3.0%, with the remainder consisting of Fe and unavoidable impurities, and is characterized by satisfying the following formulas (1) and (2): Formula (1): 3.2≦5.53+1.4Ni-0.16Cr+17.1(C+N)+0.722Mn+1.4Cu-5.59Si≦7 Formula (2): 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≦110 Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.

Description

[Technical field]

[0001] The present invention relates to an austenitic stainless steel and a method for producing the same, and more particularly to an austenitic stainless steel that can ensure a yield ratio even when final annealing is performed at a temperature of 1,050°C or higher. [Background technology]

[0002] Recent environmental regulations have led to demands for lighter and stronger structural steel suitable for automobiles, trains, etc., to improve energy efficiency, as well as improved stability, crashworthiness, and durability of structural materials to comply with safety regulations for passenger safety. At the same time, the production style of structural materials has changed from the past small-item mass production system to a high-item small-lot production system to meet consumer needs and the trends of the times. Stainless steel provides an alternative to environmental regulations and energy efficiency issues by ensuring strength and formability, and is also suitable for a high-mix low-volume production system because it does not require additional equipment investment to improve corrosion resistance. In particular, austenitic stainless steel has excellent elongation, so there is no problem when making complex shapes, and it has a beautiful appearance, so it can be applied to fields that require forming.

[0003] However, austenitic stainless steels generally have problems in that they are inferior in yield strength and yield ratio to structural carbon steels. In addition, austenitic stainless steels have a relatively low yield ratio because of their low yield strength and high tensile strength due to martensitic transformation. A low yield ratio not only deteriorates the impact properties and durability of structural stainless steel and shortens the lifespan of dies used in manufacturing, but also causes plastic non-uniformity. Therefore, there has been a demand for the development of stainless steel that can secure the same yield strength and high yield ratio as carbon steel.

[0004] On the other hand, in the case of austenitic stainless steel, the alloy components that make up the steel are more expensive than those of general structural carbon steels. In particular, the high price of Ni contained in austenitic stainless steel poses problems in terms of price competitiveness, and extreme fluctuations in material prices not only make the supply and demand of raw materials unstable, but also make it difficult to ensure stability in supply prices, which has limited its use as structural components for automobiles and other products. Therefore, there is a need to develop an austenitic stainless steel that can be used for structural components of automobiles and the like by reducing the content of Ni, an expensive alloying element, while maintaining the yield strength and elongation while improving the yield ratio.

[0005] As a method for solving such problems, the invention of Patent Document 1 has been proposed. In Patent Document 1, an Al-Si plated steel sheet is heated to 850°C or higher and then hot press formed to form the material structure as martensite, but since the aluminum plated layer is present on the surface of the steel sheet, the steel sheet does not oxidize during heating. Not only can ultra-high strength products of 1,000 MPa or more be easily obtained during hot press forming using aluminum plated steel sheet, but formed products with very excellent dimensional precision can also be secured, so this method has attracted attention as a part forming method that is very effective in reducing the weight and improving the rigidity of automobiles. However, recently, hot press forming using aluminum-plated steel sheets has been raising several problems during the forming process and the subsequent joining / welding process between other components. Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide an austenitic stainless steel having an improved yield ratio while ensuring yield strength and elongation. [Means for solving the problem]

[0007] The austenitic stainless steel with an improved yield ratio of the present invention contains, by weight%, C: 0.1% or less (excluding 0), N: 0.2% or less (excluding 0), Si: 1.5 to 2.5%, Mn: 6.0 to 10.0%, Cr: 15.0 to 17.0%, Ni: 0.3% or less (excluding 0), Cu: 2.0 to 3.0%, with the remainder being Fe and unavoidable impurities, and is characterized in that it satisfies the following formulas (1) and (2). Formula (1): 3.2≦5.53+1.4Ni-0.16Cr+17.1(C+N)+0.722Mn+1.4Cu-5.59Si≦7 Formula (2): 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≦110 Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.

[0008] Moreover, according to one embodiment of the present invention, the following formula (3) can be satisfied. Formula (3): [4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1*Mn]+0.16*[((Cr+1.5Si+18) / (Ni+0.52Cu+30(C+N)+0.5Mn+36)+0.262)*161-161]≧17 Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.

[0009] According to one embodiment of the present invention, the yield ratio may be greater than or equal to 0.6. According to one embodiment of the present invention, the yield strength may be greater than or equal to 600 MPa. According to one embodiment of the present invention, the stretch ratio is preferably 35% or more.

[0010] A method for producing an austenitic stainless steel having an improved yield ratio according to another embodiment of the present invention includes, by weight percent, C: 0.1% or less (excluding 0), N: 0.2% or less (excluding 0), Si: 1.5 to 2.5%, Mn: 6.0 to 10.0%, Cr: 15.0 to 17.0%, Ni: 0.3% or less (excluding 0), Cu: 2.0 to 3.0%, and the remainder being Fe and unavoidable impurities, and is characterized in that it includes the steps of producing a slab that satisfies the following formulas (1) and (2), hot rolling the slab, hot-rolling annealing the hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet, and cold-rolling annealing the cold-rolled steel sheet at 1,050°C or more. Formula (1): 3.2≦5.53+1.4Ni-0.16Cr+17.1(C+N)+0.722Mn+1.4Cu-5.59Si≦7 Formula (2): 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≦110 Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.

[0011] Moreover, according to one embodiment of the present invention, the slab can satisfy the following formula (3). Formula (3): [4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1*Mn]+0.16*[((Cr+1.5Si+18) / (Ni+0.52Cu+30(C+N)+0.5Mn+36)+0.262)*161-161]≧17 Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.

[0012] According to an embodiment of the present invention, the cold rolling annealing is preferably performed for 10 seconds to 10 minutes. According to one embodiment of the present invention, the hot rolling can be performed at 1,100 to 1,300°C. According to an embodiment of the present invention, the hot rolling annealing is preferably performed at 1,000 to 1,100° C. for 10 seconds to 10 minutes. Effect of the Invention

[0013] According to the embodiments of the present invention, it is possible to provide a low-cost austenitic stainless steel having a sufficient elongation rate and yield strength, and an improved yield ratio. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a graph illustrating the relationship between the value of formula (1) and the value of formula (2) of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] An austenitic stainless steel with an improved yield ratio according to one embodiment of the present invention contains, by weight percent, C: 0.1% or less (excluding 0), N: 0.2% or less (excluding 0), Si: 1.5 to 2.5%, Mn: 6.0 to 10.0%, Cr: 15.0 to 17.0%, Ni: 0.3% or less (excluding 0), Cu: 2.0 to 3.0%, and the remainder being Fe and unavoidable impurities, and satisfies the following formulas (1) and (2). Formula (1): 3.2≦5.53+1.4Ni-0.16Cr+17.1(C+N)+0.722Mn+1.4Cu-5.59Si≦7 Formula (2): 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≦110 Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.

[0016] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are presented to fully convey the concept of the present invention to those skilled in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. In the drawings, parts that are not relevant to the description may be omitted in order to clarify the present invention, and the size of components may be somewhat exaggerated in order to facilitate understanding. Throughout the specification, when a part "comprises" a certain element, this means that it may further include other elements, not excluding other elements, unless specifically stated to the contrary. The singular expression includes the plural expression unless the context clearly indicates otherwise. Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0017] An austenitic stainless steel with an improved yield ratio according to one aspect of the present invention contains, by weight, C: 0.1% or less (excluding 0), N: 0.2% or less (excluding 0), Si: 1.5 to 2.5%, Mn: 6.0 to 10.0%, Cr: 15.0 to 17.0%, Ni: 0.3% or less (excluding 0), Cu: 2.0 to 3.0%, and the remainder consisting of Fe and unavoidable impurities. The reasons for limiting the numerical values ​​of the metal-containing component contents in the examples of the present invention will be described below. In the following, the unit is weight percent unless otherwise specified.

[0018] The C content is 0.1% or less (excluding 0). Carbon (C) is an effective element for stabilizing the austenite phase, and is added to ensure the yield strength of austenitic stainless steels. However, if its content is excessive, it not only reduces cold workability due to its solid solution strengthening effect, but also induces the precipitation of Cr carbides at grain boundaries, which may adversely affect ductility, toughness, corrosion resistance, etc., so the upper limit is set to 0.1%.

[0019] The N content is 0.2% or less (excluding 0). Nitrogen (N) is a strong austenite stabilizing element and is effective in improving the corrosion resistance and yield strength of austenitic stainless steels. However, if its content is excessive, it may deteriorate the cold workability due to the solid solution strengthening effect, so the upper limit is set to 0.2%.

[0020] The Si content is 1.5~2.5%. Silicon (Si) plays a role as a deoxidizer during the steelmaking process and is an effective element for improving corrosion resistance, and it is preferable to add 1.5% or more. However, as an element effective in stabilizing the ferrite phase, when added in excess, Si not only promotes the formation of delta (δ) ferrite in the cast slab, reducing hot workability, but also has the risk of reducing the ductility / toughness of the steel material due to the solid solution strengthening effect, so the upper limit is limited to 2.5%.

[0021] The Mn content is 6.0 to 10.0%. Manganese (Mn) is an element added in place of nickel (Ni) in the present invention to stabilize the austenite phase, and is preferably added in an amount of 6.0% or more in order to suppress the formation of processing-induced martensite and improve cold rolling properties. However, if the content is excessive, there is a risk of excessive formation of S-based inclusions (MnS) that may reduce the ductility, toughness, and corrosion resistance of the austenitic stainless steel, and Mn fumes may be generated during the steelmaking process, which may pose a manufacturing hazard, so the upper limit is limited to 10.0%.

[0022] The Cr content is 15.0-17.0%. Chromium (Cr) is a ferrite stabilizing element, but it is also effective in suppressing the formation of martensite phase, and is a basic element that ensures the corrosion resistance required of stainless steel, so it is recommended to add 15% or more. However, if the content is excessive, the manufacturing cost increases and there is a risk of forming delta (δ) ferrite in the slab, resulting in a decrease in hot workability, so the upper limit is limited to 17.0%.

[0023] The Ni content is 0.3% or less (excluding 0). Nickel (Ni) is a strong austenite phase stabilizing element and is essential for ensuring good hot and cold workability. However, Ni is an expensive element, and adding a large amount of Ni increases the cost of raw materials. Therefore, taking into consideration both the cost and efficiency of the steel, the upper limit is set at 0.3%.

[0024] The Cu content is 2.0-3.0%. Copper (Cu) is an element added in place of nickel (Ni) in the present invention to stabilize the austenite phase, and it is recommended to add 2.0% or more to improve corrosion resistance in a reducing environment. However, if the content is excessive, not only will the material cost increase, but there will also be problems with liquefaction and low-temperature brittleness. Therefore, taking into consideration the cost, efficiency, and material properties of the steel material, the upper limit is limited to 3.0%.

[0025] The strength-enhanced austenitic stainless steel according to an embodiment of the present invention may further include at least one of P: 0.035% or less and S: 0.01% or less.

[0026] The P content is 0.035% or less. Phosphorus (P) is an impurity that is inevitably contained in steel and is an element that is the main cause of grain boundary corrosion or the impairment of hot workability, so it is preferable to control the P content as low as possible. In the present invention, the upper limit of the P content is controlled to 0.035% or less.

[0027] The S content is 0.01% or less. Sulfur (S) is an impurity that is inevitably contained in steel and is an element that is the main cause of impairing hot workability by segregating at grain boundaries, so it is preferable to control the S content as low as possible. In the present invention, the upper limit of the S content is controlled to 0.01% or less.

[0028] The remaining component of the present invention is iron (Fe). However, in normal manufacturing processes, unintended impurities may be inevitably mixed in from raw materials or the surrounding environment, and it is not possible to exclude them. Since any technician in normal manufacturing processes would know about these impurities, the contents of all of them will not be specifically mentioned in this specification.

[0029] In recent years, not only the weight reduction of structural steel materials but also their stability has become a major issue. As a result, steel materials used in automobile parts, various structural parts, and in environments where loads are applied are required to have not only excellent strength but also a high yield ratio. The yield ratio is the value obtained by dividing the yield strength by the tensile strength, and is an important physical property value for structural steel in terms of manufacturing and use. Austenitic stainless steels generally have the characteristic of having an extremely low yield ratio. When the yield ratio is low, there are restrictions on using it as a structural member, such as the need to change the shape of the part.

[0030] The main physical property required for structural members to support actual loads is yield strength. If the load exceeds the yield strength of a structural member, the structural member will be distorted, which will cause uneven stress and, in extreme cases, may even destroy the structural member. In other words, a high yield strength is an essential element for structural member materials to ensure the stability of the structural material and the reliability of users. On the other hand, as the tensile strength increases, a large amount of energy must be input to deform the material, which shortens the life of the manufacturing equipment. Therefore, when considering the industrial aspect as well as the stable load support of structural members, it is important to improve the yield ratio. In addition, in order to ensure the price competitiveness of osmotic stainless steels, the content of expensive osmotic stabilizing elements such as Ni must be reduced, and it is necessary to predict the amounts of Mn, N, and Cu to be added that can compensate for this.

[0031] However, when Ni is reduced and Mn, N, Cu, etc. are added to ensure price competitiveness, there is a problem that work hardening occurs suddenly, lowering the yield ratio. When the yield ratio of austenitic stainless steel is low, there is a problem that the life of the forming tool and frame is shortened due to a sudden increase in strength caused by deformation during product manufacturing. In order to solve this problem, in the present invention, Si, N, etc. are added and the chemical relationship between Mn, Ni, and N is adjusted to control the deformation behavior, and the following formula (1) has been derived to improve the yield ratio of austenitic stainless steel. Formula (1): 5.53+1.4Ni-0.16Cr+17.1(C+N)+0.722Mn+1.4Cu-5.59Si Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.

[0032] In the austenitic stainless steel with an improved yield ratio according to one embodiment of the present invention, the value represented by the formula (1) satisfies the range of 3.2 or more and 7 or less. The inventors have confirmed that the lower the value of formula (1), the more difficult it becomes for cross-slip of the austenite phase to occur due to external stress. Specifically, when the value of formula (1) is less than 3.2, the austenitic stainless steel only exhibits planar slip behavior in response to deformation, and the accumulation of potential due to external stress progresses, resulting in plastic nonuniformity and high work hardening. This causes a problem of reduced elongation and yield ratio of the austenitic stainless steel, so the lower limit of the value of formula (1) is limited to 3.2. On the other hand, if the value of formula (1) is too high, cross slip occurs frequently, and there is a problem that the occurrence of plastic nonuniformity increases, where stress is concentrated in the weak parts of the steel. The higher the strength of the steel, the greater the impact of such brittleness and plastic nonuniformity, but there is a problem that the elongation rate of the steel cannot be secured, so the upper limit of formula (1) is limited to 7. In addition, in the present invention, the following formula (2) was derived in consideration of the phase transformation that occurs due to deformation of austenitic stainless steel. Formula (2): 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu) Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.

[0033] In the austenitic stainless steel with an improved yield ratio according to one embodiment of the present invention, the value represented by the formula (2) satisfies the range of 110 or less. The inventors have confirmed that the higher the value of formula (2), the easier it is for the austenite phase to transform into martensite by external stress. Specifically, when the value of formula (2) exceeds 110, the austenitic stainless steel exhibits rapid deformation-induced martensite transformation behavior due to external deformation, resulting in plastic non-uniformity. This causes a problem in that the elongation and yield ratio of the austenitic stainless steel are reduced, so the upper limit of the value of formula (2) is limited to 110.

[0034] In addition, in the present invention, in order to ensure the yield strength of austenitic stainless steel, the following formula (3) was derived taking into consideration the effect of the stress field on the yield strength of the steel material, and further, the following formula (4) was derived, which shows the residual ferrite content of austenitic stainless steel. Formula (3):4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1*Mn Formula (4):((Cr+1.5Si+18) / (Ni+0.52Cu+30(C+N)+0.5Mn+36)+0.262)*161-161 Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element. The higher the value of equation (3), the greater the interstitial stress field due to the difference in size of atoms between elements in the alloy, and the greater the limit value for withstanding plastic deformation against external stress. Equation (4) indicates the stability of the ferrite phase at high temperatures, and the higher the value of equation (4), the greater the amount of ferrite formed at high temperatures, which in turn increases the fraction of ferrite remaining at room temperature, thereby improving the yield strength of austenitic stainless steel.

[0035] In the present invention, in order to ensure the yield strength of austenitic stainless steel, the effect of the stress field on the yield strength and the ferrite fraction are simultaneously taken into consideration, the relationship between the above formulas (3) and (4) is determined, and the following formula (5) is derived. Formula (5): [4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1*Mn]+0.16*[((Cr+1.5Si+18) / (Ni+0.52Cu+30(C+N)+0.5Mn+36)+0.262)*161-161] Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element. In the above formula (5), 0.16 is a weighting factor that takes into account the greater effect of the stress field on the yield strength, and is a constant experimentally derived from currently commercialized materials and materials under development.

[0036] In the austenitic stainless steel with an improved yield ratio according to one embodiment of the present invention, the value represented by the formula (5) satisfies the range of not less than 17. If the value of the formula (5) is less than 17, there is a problem in that the yield strength of the austenitic stainless steel cannot be ensured to be 600 MPa or more. The austenitic stainless steel according to the present invention, which satisfies the above-mentioned composition ranges and relational expressions of alloying elements, can ensure not only a yield ratio (yield strength / tensile strength) of 0.6 or more and a yield strength of 600 MPa or more, but also an elongation rate of 35% or more. As described above, even though it is an austenitic stainless steel, it is possible to derive high yield strength and yield ratio. This not only makes it easy to form austenitic stainless steel and fabricate structural members, but also ensures the stability of the fabricated structural members and user trust. Next, a method for producing an austenitic stainless steel having improved strength according to another aspect of the present invention will be described.

[0037] A method for producing an austenitic stainless steel having an improved yield ratio according to one embodiment of the present invention includes the steps of producing a slab containing, by weight%, C: 0.1% or less (excluding 0), N: 0.2% or less (excluding 0), Si: 1.5 to 2.5%, Mn: 6.0 to 10.0%, Cr: 15.0 to 17.0%, Ni: 0.3% or less (excluding 0), Cu: 2.0 to 3.0%, and the remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2); hot rolling the slab; hot-rolling annealing the hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet; and cold-rolling annealing the cold-rolled steel sheet at 1,050°C or more. The reasons for limiting the numerical values ​​of the alloying element contents are as explained above. A stainless steel containing the above composition can be produced into a slab by continuous casting or ingot casting, and then subjected to a series of hot rolling and hot rolling annealing, followed by cold rolling and cold rolling annealing to form a final product.

[0038] Skin pass rolling has been used as a method for improving the strength of austenitic stainless steel. Skin pass rolling is a method that uses the phenomenon in which high work hardening occurs due to the transformation of the austenite phase into work-induced martensite during cold deformation, or the accumulation of electric potential in the steel. However, austenitic stainless steel that has been subjected to skin pass rolling has the disadvantages of a rapid drop in elongation, making subsequent processing difficult, and generating surface defects. In addition, in order to facilitate temper rolling, an alloy composition system that is easy to accumulate potential and undergo phase transformation is generally used. However, this leads to a problem that the work hardening is high and the yield ratio is low, inducing non-uniform plasticity in the steel material.

[0039] Meanwhile, in the past, austenitic stainless steel was annealed at a low temperature of 1,000℃ or less to improve its yield strength. Low-temperature annealing is a method that uses the energy stored in the material during cold rolling without completing recrystallization. However, austenitic stainless steel that has been annealed at such a low temperature has the disadvantages of having uneven material distribution, which makes it difficult to ensure sufficient pickling effect in the subsequent pickling process, and also results in an unattractive surface shape.

[0040] The present invention is an attempt to ensure the yield ratio of austenitic stainless steel even when cold rolling and annealing are performed at high temperatures of 1,050° C. or higher, as a method for solving the above-mentioned shortcomings of temper rolling and low-temperature annealing. For example, the slab is preferably hot rolled at a normal rolling temperature of 1,100 to 1,300° C., and the hot rolled steel sheet is preferably hot rolled and annealed at a temperature range of 1,000 to 1,100° C. In this case, the hot roll annealing is preferably performed for 10 seconds to 10 minutes. The hot rolled steel sheet can then be cold rolled to produce a thin sheet.

[0041] In the present invention, an attempt is made to secure a yield strength of 600 MPa or more, a yield ratio of 0.6 or more, and an elongation rate of 35% or more by performing cold rolling annealing heat treatment at a relatively high temperature of 1,050°C or more after cold rolling. The cold rolling annealing can be performed at a temperature of 1,050° C. or higher. Moreover, the cold rolling annealing according to one embodiment of the present invention is preferably performed at a temperature of 1,050° C. or higher for 10 seconds to 10 minutes. In this way, when the alloy components and the component relationships are controlled, it is possible to obtain a final cold-rolled annealed material through general cold rolling and cold-rolled annealing without performing additional temper rolling or low-temperature annealing, and thus ensure price competitiveness. The austenitic stainless steel having improved strength according to the present invention may be used, for example, in general products for forming, and may be manufactured and used as products such as slabs, blooms, billets, coils, strips, plates, sheets, bars, rods, wires, shape steels, pipes, tubes, and the like.

[0042] The present invention will now be described in more detail with reference to the following examples. For the various alloy composition ranges shown in Table 1 below, slabs were manufactured through ingot melting, heated at 1,250°C for 2 hours, hot rolled, and then hot roll annealed at 1,100°C for 90 seconds. Then, cold rolling was performed with a rolling reduction of 70%, and cold rolling annealing was performed at 1,100°C after cold rolling. The alloy composition (weight %) and the values ​​of formula (1), formula (2), formula (3), formula (4) and formula (5) for each experimental steel type are shown in Table 1 below.

[0043] [Table 1]

[0044] The cold-rolled material having the above composition was subjected to cold-rolling annealing at 1,100°C for 10 seconds, and the elongation, yield strength, tensile strength, and yield ratio of the cold-rolled annealed material were measured. Specifically, the room temperature tensile test was performed according to the ASTM standard, and the yield strength (Yield Strength, MPa), tensile strength (Tensile Strength, MPa), elongation (Elongation, %), and yield ratio (Yield Ratio) measured thereby are shown in Table 2 below.

[0045] [Table 2]

[0046] Fig. 1 is a graph for explaining the relationship between the value of formula (1) and the value of formula (2) of the present invention. As shown in Fig. 1, the ranges of formula (1) and formula (2) are satisfied, but the comparative example corresponds to Comparative Example 8 in which the value of formula (5) does not reach 17. As shown in Table 2, in the case of Examples 1 to 3, which satisfy the alloy composition proposed by the present invention and the ranges of the values ​​of formula (1), formula (2), and formula (5), it was confirmed that not only is it possible to ensure a yield strength of 600 MPa or more and a yield ratio of 0.6 or more, but also that an excellent elongation of 35% or more can be ensured. In addition, it is possible to reduce the content of Ni, which is an expensive austenite stabilizing element, and ensure the price competitiveness of austenitic stainless steel.

[0047] Comparative Examples 1 and 2 are commercially produced standard austenitic stainless steels. However, because of the alloy composition range proposed in the present invention, particularly 7% or more of Ni was added, not only were price competitiveness not ensured, but the value of formula (5) did not reach 17, and the target yield strength of 600 MPa or more could not be ensured. It can be seen that Comparative Example 3 does not satisfy all of the ranges of formulas (1), (2), and (5) proposed in the present invention, and a low yield strength and a low yield ratio due to rapid work hardening were derived.

[0048] In Comparative Example 4, the value of formula (1) is 2.87, which does not reach 3.2; the value of formula (2) is 110 or less, so no sudden martensitic transformation occurs during deformation; and the value of formula (5) is 17 or more, so that very good yield strength can be ensured. However, the value of formula (1) is low, so that accumulation of electric potential due to external stress progresses, which leads to a rapid increase in tensile strength, and a yield ratio of 0.6 or more cannot be ensured. In Comparative Example 5, the value of formula (1) is 8.99, which is greater than 7, and it can be seen that significant plastic non-uniformity occurs and the elongation rate is very low. In Comparative Examples 6 and 7, the values ​​of formula (2) were 113.0 and 165.4, respectively, which exceeded 110, and martensite transformation due to deformation occurred rapidly, causing a rapid increase in tensile strength, and failing to ensure a yield ratio of 0.6 or more. In particular, Comparative Example 6 belongs to the alloy composition proposed by the present invention and satisfies the ranges of formulas (1) and (5), but does not satisfy formula (2), resulting in a rapid increase in tensile strength and a low yield ratio of 0.28.

[0049] Comparative Example 8 is a steel type belonging to the alloy composition proposed by the present invention, which satisfies the ranges of formulas (1) and (2). By controlling the work hardening due to deformation, a yield ratio of 0.6 or more could be ensured. However, the value of formula (5) did not reach 17, and the target yield strength of 600 MPa or more could not be ensured. Thus, according to the disclosed embodiments, by controlling the alloy components and the relational expressions, it is possible to produce austenitic stainless steel having a yield ratio of 0.6 or more, a yield strength of 600 MPa or more, and an elongation rate of 35% or more.

[0050] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and a person having ordinary skill in the art will understand that various changes and modifications are possible without departing from the concept and scope of the claims set forth below. [Industrial Applicability]

[0051] The austenitic stainless steel according to the present invention can ensure the yield strength and elongation while improving the yield ratio, and therefore can be applied to structural members of automobiles and the like.

Claims

1. In weight percent, it contains C: 0.1% or less (excluding 0), N: 0.2% or less (excluding 0), Si: 1.5 to 2.5%, Mn: 6.0 to 10.0%, Cr: 15.0 to 17.0%, Ni: 0.3% or less (excluding 0), Cu: 2.0 to 3.0%, and the remainder is Fe and unavoidable impurities. An austenitic stainless steel characterized by satisfying the following formulas (1) and (2) and having a yield ratio of 0.6 or more. Formula (1): 3.2≦5.53+1.4Ni-0.16Cr+17.1(C+N)+0.722Mn+1.4Cu-5.59Si≦7 Formula (2): 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≦110 (Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.)

2. 2. The austenitic stainless steel according to claim 1, characterized in that the following formula (3) is satisfied: Formula (3): [4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1*Mn]+0.16*[((Cr+1.5Si+18) / (Ni+0.52Cu+30(C+N)+0.5Mn+36)+0.262)*161-161]≧17 (Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.)

3. 2. The austenitic stainless steel according to claim 1, characterized in that the yield strength is 600 MPa or more.

4. 2. The austenitic stainless steel according to claim 1, characterized in that the elongation is 35% or more.

5. A step of producing a slab containing, by weight percent, C: 0.1% or less (excluding 0), N: 0.2% or less (excluding 0), Si: 1.5 to 2.5%, Mn: 6.0 to 10.0%, Cr: 15.0 to 17.0%, Ni: 0.3% or less (excluding 0), Cu: 2.0 to 3.0%, with the remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2); hot rolling the slab at 1,100 to 1,300°C; hot-rolling annealing the hot-rolled steel sheet at 1,000 to 1,100° C. for 10 seconds to 10 minutes; cold rolling the hot rolled steel sheet; and subjecting the cold-rolled steel sheet to cold-rolling annealing at 1,050°C or higher, wherein the yield ratio is 0.6 or higher. Formula (1): 3.2≦5.53+1.4Ni-0.16Cr+17.1(C+N)+0.722Mn+1.4Cu-5.59Si≦7 Formula (2): 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29 (Ni+Cu)≦110 (Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.)

6. The method for producing an austenitic stainless steel according to claim 5, characterized in that the slab satisfies the following formula (3): Formula (3): [4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1*Mn]+0.16*[((Cr+1.5Si+18) / (Ni+0.52Cu+30(C+N)+0.5Mn+36)+0.262)*161-161]≧17 (Here, C, N, Si, Mn, Cr, Ni, and Cu mean the content (wt%) of each element.)

7. The method for producing austenitic stainless steel according to claim 5, wherein the cold rolling annealing is performed for 10 seconds to 10 minutes.

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