Austenitic stainless steel and method for producing austenitic stainless steel

Abstract

The present invention achieves an austenitic stainless steel having high productivity while reducing processing load during production and increasing the strength of the final product. The austenitic stainless steel contains, in mass%, 0.005-0.03% of C, 0.1-2.0% of Si, 0.3-2.5% of Mn, 0.04% or less of P, 0.015% or less of S, 3.0-6.0% of Ni, 16.0-18.5% of Cr, 1.5-4.0% of Cu, and 0.08-0.25% of N, with the remainder comprising Fe and inevitable impurities, includes 20 vol% or more of an austenite phase, and a Cu-rich phase having a number density of 1.0×103·μm-3 or more and a major axis of 30 nm or less, with the remainder comprising deformation-induced martensite phases and inevitable formation phases, and has an Md30 value of 0.0-80.0.

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel and a method for producing an austenitic stainless steel. [Background technology]

[0002] Metastable austenitic stainless steels, such as SUS301, are known as austenitic stainless steels used in applications requiring corrosion resistance and strength. These austenitic stainless steels are used, for example, as materials for spring products such as cylinder head gaskets for automobile engines and structural components such as automotive battery frame materials.

[0003] Such stainless steels are generally strengthened by increasing the reduction ratio of cold rolling, etc., and therefore tend to have a large processing load during the manufacturing process, such as rolling. In order to reduce such load, for example, Patent Document 1 proposes a method for manufacturing a spring material having a martensite phase in which precipitates made of a Cu-rich phase are dispersed, in which a spring steel sheet exhibiting a multi-phase structure without precipitation of a Cu-rich phase is subjected to aging treatment. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2008-195976 Summary of the Invention [Problem to be solved by the invention]

[0005] Precipitation of a Cu-rich phase is effective in increasing the strength of stainless steel. Therefore, according to the method described in Patent Document 1, by subjecting a spring steel sheet to aging treatment to precipitate a Cu-rich phase, it is possible to reduce the processing load in the manufacturing process of the spring steel sheet while achieving high strength in the final product, the spring material. However, the need for an aging treatment poses a problem in the productivity of the spring material.

[0006] An object of one aspect of the present invention is to realize an austenitic stainless steel that achieves both a reduction in the processing load during manufacturing and an increase in the strength of the final product, and that is highly productive.

[0007] Furthermore, because the precipitation temperature of the Cu-rich phase and that of Cr carbide are relatively close, we noticed that in order to reduce the decrease in corrosion resistance due to the precipitation of Cr carbide, it is preferable to utilize N without excessively increasing the amount of C. Lowering the amount of C in austenitic stainless steel is also preferable for reducing the desired processing load. [Means for solving the problem]

[0008] In order to solve the above problems, an austenitic stainless steel according to one aspect of the present invention contains, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and has 20% or more by volume of austenite phase and a number density of 1.0×10 3 pieces μm -3 The above-mentioned Cu-rich phases with a long diameter of 30 nm or less are included, and the remainder is a deformation-induced martensite phase and an unavoidably formed phase, and Md 30 is between 0.0 and 80.0 inclusive: Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1) In the element symbols in the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

[0009] In order to solve the above problems, one aspect of the present invention provides a method for producing an austenitic stainless steel containing, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and Md 30 A method for producing an austenitic stainless steel having a value of 0.0 or more and 80.0 or less, the method comprising a finish annealing step of performing finish annealing at a temperature of 750°C or more and 980°C or less, wherein when the maximum temperature reached in the finish annealing step is 850°C or more, the heating time at 850°C or more is set to 30 seconds or less, and in the finish annealing step, the average cooling rate from 700°C to 500°C after the finish annealing is set to 1°C / second or more: Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1) In the element symbols in the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added. [Effects of the Invention]

[0010] According to one aspect of the present invention, it is possible to realize an austenitic stainless steel that is highly productive and achieves both a reduction in the processing load during manufacturing and an increase in the strength of the final product. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 shows an EBSD grain boundary map and TEM imaging image of an austenitic stainless steel according to an embodiment. [Figure 2] FIG. 1 is a diagram showing the relationship between 0.2% yield strength (YS 18%) and reference strength (HV 60%) of austenitic stainless steels according to an example and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0012] An austenitic stainless steel according to one embodiment of the present invention will be described in detail below. The following description is provided to provide a better understanding of the gist of the invention, and is not intended to limit the scope of the invention unless otherwise specified.

[0013] [Organizational structure] An austenitic stainless steel according to one embodiment of the present invention is a stainless steel containing 20% ​​or more by volume of an austenite phase. Hereinafter, "austenitic stainless steel" refers to an austenitic stainless steel according to one embodiment of the present invention unless otherwise specified. The austenitic stainless steel may be, for example, a steel plate or a steel strip.

[0014] Austenitic stainless steel contains a stress-induced martensite phase, which is a result of a portion of the austenite phase being transformed by the stress-induced transformation plasticity (TRIP) phenomenon. From the viewpoint of increasing strength, the proportion of the stress-induced martensite phase in the austenitic stainless steel is preferably 5% by volume or more, more preferably 10% by volume or more, even more preferably 15% by volume or more, and even more preferably 20% by volume or more. Furthermore, the proportion of the stress-induced martensite phase in the austenitic stainless steel is preferably less than 80% by volume, and more preferably 75% by volume or less. The proportion of the austenite phase in the austenitic stainless steel may decrease as the proportion of the stress-induced martensite phase increases, as long as it is 20% by volume or more.

[0015] Austenitic stainless steel further contains a Cu-rich phase. The Cu-rich phase is a phase containing 60 atomic % or more of Cu (copper), such as an ε-Cu phase. Austenitic stainless steel has a number density of at least 1.0 × 10 3 pieces μm -3The Cu-rich phase contains a Cu-rich phase having a major axis of 30 nm or less. The major axis refers to the longest diameter of the Cu-rich phase precipitated in particulate form. Note that the austenitic stainless steel may contain a Cu-rich phase having a major axis of more than 30 nm. The Cu-rich phase may be dispersed in the austenite phase, in the deformation-induced martensite phase, or in the unavoidably formed phase described below.

[0016] The Cu-rich phase may be identified by structural observation using a transmission electron microscope (TEM). For example, a TEM sample containing an arbitrary cross section of austenitic stainless steel is prepared, and a predetermined region of the cross section is observed using a TEM to count the number of Cu-rich phases within that region whose cross section has a major axis of 30 nm or less. Furthermore, the number density per volume can be calculated by calculating the volume based on the thickness of the TEM sample used for the number count and the area of ​​the region where the number count was performed. The thickness of the TEM sample may be, for example, an actual measured value of the thickness of the TEM sample, or an estimated value based on the method used to prepare the TEM sample. Methods for preparing TEM samples include, but are not limited to, electrolytic polishing.

[0017] The finer the precipitated Cu-rich phase and the greater the amount present, the higher the strength of austenitic stainless steel. Cu-rich phases of the above-mentioned amount and size are effective in increasing the strength of austenitic stainless steel. During the manufacturing process of austenitic stainless steel, such as during cold rolling before finish annealing, the Cu-rich phase is not precipitated, thereby keeping the strength low and reducing the processing load. Then, by precipitating the Cu-rich phase in the finish annealing process, high strength is achieved in the manufactured austenitic stainless steel. Manufacturing processes such as the finish annealing process will be described later.

[0018] Furthermore, austenitic stainless steels may contain unavoidably formed phases other than the austenite phase, the strain-induced martensite phase, and the Cu-rich phase. The unavoidably formed phases are not particularly limited, but examples include a δ-ferrite phase and a phase containing carbides, nitrides, and / or oxides. Examples of the phase containing carbides, nitrides, and / or oxides include a phase containing carbides and / or nitrides of Cr, Ti, and / or Nb, and a phase containing oxides of Si, Ti, Al, Mg, and / or Ca.

[0019] The austenitic stainless steel preferably has an average crystal grain size of 10.0 μm or less. The strength of austenitic stainless steel improves as the crystal grains become finer. Generally, when the strength of austenitic stainless steel is improved, the ductility decreases. However, by making the crystal grains finer, it is possible to achieve both improved strength and improved ductility in austenitic stainless steel.

[0020] The average grain size may be measured using EBSD (Electron Backscattering Diffraction). For example, for any cross section of an austenitic stainless steel, the grain sizes in multiple fields of view may be calculated using EBSD, and the average value of the grain sizes calculated in the multiple fields of view may be used as the average grain size. The average grain size may also be measured using a method other than EBSD. For example, a method other than EBSD may include a method such as that specified in JIS G0551, in which grain boundaries are revealed by nitric acid electrolysis and then measured by an intercept method.

[0021] [Component composition] Austenitic stainless steel contains, by mass%, C: 0.005% to 0.03%, Si: 0.1% to 2.0%, Mn: 0.3% to less than 2.5%, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% to less than 6.0%, Cr: 16.0% to 18.5%, Cu: 1.5% to 3.8%, and N: 0.08% to 0.25%. The remainder of the austenitic stainless steel may consist of Fe (iron) and unavoidable impurities. The significance of the content of each element in austenitic stainless steel will be explained below.

[0022] (C) Carbon (C) is an austenite-forming element that facilitates the formation of the austenite phase, has a high solid-solution strengthening effect, and is also an effective element for obtaining strength. Austenitic stainless steel contains 0.005% by mass or more and 0.03% by mass or less of C. If the C content is 0.005% by mass or more, sufficient solid-solution strengthening effect is exerted and austenitic stainless steel with good strength can be obtained.

[0023] Excessive addition of C causes precipitation of Cr carbides during annealing at relatively low temperatures, leading to a decrease in the corrosion resistance of the austenitic stainless steel, particularly in welds, so the C content is set to 0.03 mass% or less. If the C content is 0.03 mass% or less, an austenitic stainless steel with good corrosion resistance can be obtained, even in welds.

[0024] (Si) Silicon (Si) is an element that is effective as a deoxidizer and also has a solid-solution strengthening effect. Austenitic stainless steel contains 0.1% by mass or more and 2.0% by mass or less of Si, and preferably 0.2% by mass or more and 1.0% by mass or less of Si. If the Si content is 0.1% by mass or more, the austenitic stainless steel effectively exhibits the deoxidizing effect and solid-solution strengthening effect. If the Si content is 0.2% by mass or more, it is more preferable.

[0025] In addition, Si is a ferrite-forming element that facilitates the formation of the ferrite phase. The δ-ferrite phase causes edge breaks or splitting during hot rolling. From the viewpoint of reducing the formation of the δ-ferrite phase, the Si content is set to 2.0 mass% or less, and preferably 1.0 mass% or less.

[0026] (Mn) Manganese (Mn) is an austenite-forming element and is effective in maintaining the austenite phase. Furthermore, Mn promotes the precipitation of a Cu-rich phase. Austenitic stainless steel contains 0.3 to 2.5 mass% Mn, preferably 0.5 to 2.0 mass% Mn. A Mn content of 0.3 mass% or more facilitates the precipitation of a Cu-rich phase, and a Mn content of 0.5 mass% or more is more preferable. Excessive addition of Mn reduces the hot workability of austenitic stainless steel. Therefore, the Mn content should be 2.5 mass% or less, preferably 2.0 mass% or less.

[0027] (P) P (phosphorus) is an element that is mixed in as an unavoidable impurity, and the lower the P content, the better. From the viewpoint of manufacturability, austenitic stainless steel may contain 0.04 mass% or less of P. If the P content is 0.04 mass% or less, adverse effects on material properties such as ductility can be reduced in austenitic stainless steel.

[0028] (S) S (sulfur) is an element that is mixed in as an unavoidable impurity, and the lower the S content, the better. From the viewpoint of manufacturability, austenitic stainless steel may contain 0.015 mass% or less of S. If the S content is 0.015 mass% or less, adverse effects on material properties such as ductility can be reduced in austenitic stainless steel.

[0029] (Ni) Ni (nickel) is an austenite-forming element and is also effective in maintaining the austenite phase. Austenitic stainless steel contains 3.0% by mass or more and less than 6.0% by mass of Ni, preferably 3.5% by mass or more and less than 5.5% by mass of Ni, and more preferably 4.0% by mass or more and less than 5.0% by mass of Ni. If the Ni content is 3.0% by mass or more, the formation and maintenance of the austenite phase is favorable. It is more preferable that the Ni content is 4.5% by mass or more.

[0030] On the other hand, Ni is an expensive element, and if added in excess, it stabilizes the austenite phase, reducing the amount of strain-induced martensite phase formed. Therefore, the Ni content is set to less than 6.0 mass%, preferably 5.5 mass% or less, and more preferably less than 5.0 mass%.

[0031] (Cr) Cr (chromium) is an element effective in ensuring the corrosion resistance of austenitic stainless steel. Austenitic stainless steel contains 16.0 mass% to 18.5 mass% Cr, and preferably 16.5 mass% to 18.0 mass% Cr. If the Cr content is 16.0 mass% or more, the corrosion resistance of the austenitic stainless steel can be well ensured. It is more preferable if the Cr content is 16.5 mass% or more.

[0032] On the other hand, since Cr, like Si, is also a ferrite forming element, excessive addition of Cr results in excessive formation of the delta ferrite phase, so the Cr content is set to 18.5 mass% or less, and preferably 18.0 mass% or less.

[0033] (Cu) Cu is an austenite-forming element and is also effective in maintaining the austenite phase. It is also effective in increasing the strength of austenitic stainless steels by precipitating a Cu-rich phase. Cu is also an element that effectively refines grains. This is thought to be because the Cu-rich phase inhibits grain growth. In addition, Cu reduces the work hardening of the austenite phase in its solid solution state, which reduces the rolling load in the manufacturing process of austenitic stainless steels.

[0034] Austenitic stainless steel contains 1.5% by mass or more and 4.0% by mass or less of Cu, preferably 2.0% by mass or more and 3.5% by mass or less of Cu, and more preferably more than 2.0% by mass and 3.5% by mass or less of Cu. If the Cu content is 1.5% by mass or more, the generation and maintenance of the austenite phase is favorable, and the precipitation of the Cu-rich phase is favorable. If the Cu content is 2.0% by mass or more, it is more favorable, and if it exceeds 2.0% by mass, it is even more favorable.

[0035] On the other hand, if excessive Cu is added, a CuMn phase will form in the center of the slab during solidification, which will reduce the hot workability of the slab. Therefore, the Cu content should be 4.0 mass% or less, and preferably 3.5 mass% or less.

[0036] (N) N (nitrogen) is an austenite-forming element and also has the effect of solid solution strengthening and improving corrosion resistance. Since the C content of austenitic stainless steel is set to 0.03 mass% or less to ensure corrosion resistance of welds, the N content is set to 0.08 mass% or more, preferably 0.10 mass% or more, more preferably 0.11 mass% or more, and even more preferably 0.12 mass% or more. This N content is effective in ensuring the strength and corrosion resistance required of austenitic stainless steel.

[0037] Furthermore, excessive addition of N increases the rolling load of the austenitic stainless steel, so the N content is set to 0.25 mass % or less, and preferably 0.20 mass % or less.

[0038] (Other elements) In addition to the above elements, the austenitic stainless steel may further contain, by mass%, one or more selected from Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, B: 0.0001% or more and 0.01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb: 0.5% or less, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less, and REM (rare earth metals): 0.2% or less.

[0039] (Mo, W, V) Mo (molybdenum), W (tungsten), and V (vanadium) are elements effective in improving corrosion resistance. However, because Mo, W, and V are ferrite-forming elements and expensive elements, excessive addition is undesirable. Therefore, it is preferable that austenitic stainless steel contain one or more elements selected from 1.0 mass% or less of Mo, 1.0 mass% or less of W, and 0.5 mass% or less of V.

[0040] (B) B (boron) is an element that improves hot workability and is effective in reducing edge breaks and lamination during hot rolling. Austenitic stainless steel preferably contains 0.0001% by mass or more and 0.01% by mass or less of B. A B content of 0.0001% by mass or more is effective in improving hot workability and reducing edge breaks and lamination during hot rolling. However, excessive addition of B to austenitic stainless steel containing Cr leads to a decrease in corrosion resistance due to the precipitation of CrB. Therefore, the B content is preferably 0.01% by mass or less.

[0041] (Co) Co (cobalt) is an element effective in ensuring the corrosion resistance of austenitic stainless steel. It also contributes to reducing the coarsening of the Cu-rich phase and maintaining it fine. To achieve this effect, it is preferable to contain 0.10 mass% or more of Co. However, since Co is an expensive element, from the viewpoint of cost reduction, it is preferable to limit the Co content to 0.8 mass% or less.

[0042] (Sn) Sn (tin) is an element that is effective in ensuring the corrosion resistance of austenitic stainless steel. However, since excessive addition of Sn leads to a decrease in the hot workability of austenitic stainless steel, the Sn content is preferably 0.1 mass% or less.

[0043] (Al, Ca, Mg, Ti) Al (aluminum), Ca (calcium), Mg (magnesium), and Ti (titanium) are all elements that have a deoxidizing effect. Austenitic stainless steel preferably contains one or more deoxidizers selected from 0.3 mass% or less of Al, 0.03 mass% or less of Ca, 0.03 mass% or less of Mg, and 0.5 mass% or less of Ti.

[0044] (Nb) Niobium (Nb) is an element that is effective in reducing sensitization of austenitic stainless steel. It is also effective in making the structure finer and more uniform. Austenitic stainless steel preferably contains 0.5 mass % or less of Nb.

[0045] (Sb, Zr, Ta, Hf, REM) Sb (antimony), Zr (zirconium), Ta (tantalum), Hf (hafnium), and REMs (rare earth metals) are all elements that improve hot workability and are also effective in improving oxidation resistance. Austenitic stainless steel preferably contains one or more elements selected from 0.5 mass% or less of Sb, 0.5 mass% or less of Zr, 0.03 mass% or less of Ta, 0.03 mass% or less of Hf, and 0.2 mass% or less of REMs.

[0046] [Md 30 value of Austenitic stainless steel has Md as shown in the following formula (1). 30 The value is 0.0 or more and 80.0 or less, and preferably 20.0 or more and 70.0 or less.

[0047] Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1) In the element symbols in the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

[0048] In austenitic stainless steel, Md 30 The value of Md indicates the temperature (℃) at which 50% of the structure of austenitic stainless steel transforms into martensite when a 30% tensile strain is applied to austenitic single-phase austenitic stainless steel. 30 The value of Md can be used as an index of the stability of the austenite phase. 30 The value of can also be used as an index that influences the susceptibility of the TRIP phenomenon to occur in austenitic stainless steel.

[0049] Md of austenitic stainless steel according to one embodiment of the present invention 30 The value of Md is preferably 0.0 or more and 80.0 or less. 30 The larger the value of , the more likely the transformation from the austenite phase to the strain-induced martensite phase occurs, and the more likely it is that high strength can be obtained by applying slight cold-rolling strain, while also ensuring excellent ductility. Furthermore, when austenitic stainless steel is formed, parts that have been subjected to processing strain, such as bent parts, tend to achieve even higher strength due to the TRIP phenomenon.

[0050] In addition, in the manufacturing process of austenitic stainless steel, the presence of the strain-induced martensite phase in the rolled material before the final annealing is effective in refining the crystal grains by the final annealing. 30 This is particularly noticeable when the value of Md is 0.0 or greater. 30 If the value exceeds 80.0, the TRIP phenomenon is likely to occur excessively, making it difficult to stabilize the properties of the austenitic stainless steel.

[0051] Therefore, Md, which is an index of the stability of the austenite phase, 30 When the value of is 0.0 or more and 80.0 or less, austenitic stainless steel having high strength and good ductility can be stably produced.

[0052] In addition, the conventionally known Md 30 In the component regression equation, the same value is generally used for the coefficients of Ni and Cu. 30 In the component regression equation, the coefficient for Cu is set smaller than the coefficient for Ni. 30 Many of the elemental regression equations are based on the results of non-Ni-reduced austenitic stainless steels. In contrast, in the Ni-reduced elements of the present invention, it has been found that the effect of Cu on the stabilization of the austenite phase is clearly smaller than that of conventional knowledge. This is a new finding obtained as a result of intensive research by the present inventors, and based on this finding, 30 This makes it easier to adjust the Cu content, and increases the flexibility in manufacturing austenitic stainless steel.

[0053] [Manufacturing method] A method for producing an austenitic stainless steel according to one embodiment of the present invention comprises producing an austenitic stainless steel containing, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and having Md 30 The method for producing austenitic stainless steel has a value of 0.0 or more and 80.0 or less. The method for producing austenitic stainless steel also includes a finish annealing step.

[0054] The method for producing austenitic stainless steel may include general austenitic stainless steel production processes other than the finish annealing step. An example of a method for producing austenitic stainless steel according to one embodiment of the present invention will be described below, but the present invention is not limited to this.

[0055] In a method for producing austenitic stainless steel according to one embodiment of the present invention, for example, a slab is produced by continuously casting molten steel with adjusted composition. The slab produced by continuous casting is then heated to 1100°C or higher and 1300°C or lower, and then hot-rolled to produce a hot-rolled steel strip. After hot-rolling, the precipitation rate of the Cu-rich phase from the less-strained austenite phase is slow. Therefore, the finishing temperature and coiling temperature of the hot-rolled steel strip after hot-rolling may be the same as those in general methods for producing austenitic stainless steel. From the viewpoint of minimizing the precipitation of the Cu-rich phase before finish annealing, the coiling temperature of the hot-rolled steel strip after hot-rolling is preferably 850°C or lower, and more preferably 650°C or lower.

[0056] The hot-rolled steel strip may be pickled. The hot-rolled steel strip may be annealed before pickling, or may be pickled without annealing. When the hot-rolled steel strip is annealed before pickling, the annealing temperature is preferably in the range of 900°C to 1150°C, and more preferably in the range of 980°C to 1150°C in order to completely dissolve Cu, but is not limited to the above range. Then, the hot-rolled steel strip after pickling is cold-rolled until it has a predetermined thickness to obtain a cold-rolled steel strip.

[0057] In the manufacturing method of austenitic stainless steel, recrystallization and precipitation of a Cu-rich phase proceed simultaneously in the finish annealing process after the cold rolling process. Because the Cu-rich phase is particularly likely to precipitate from the strain-induced martensite phase, the cold rolling process is preferably performed at a rolling reduction ratio and rolling temperature such that the strain-induced martensite phase accounts for 20% or more by volume of the entire cold-rolled steel strip. By performing such a cold rolling process, the Cu-rich phase can be effectively precipitated in the steel strip in the subsequent finish annealing process.

[0058] In addition, austenitic stainless steel is Md 30 Adjust the value of Md to be between 0.0 and 80.0. 30 In an austenitic stainless steel having a value of , the amount of Cu-rich phase specified in one embodiment of the present invention will precipitate regardless of the amount of strain-induced martensite phase in the cold-rolled steel strip. However, if necessary, increasing the rolling ratio in the cold-rolling step, controlling the temperature in the cold-rolling step to a low value, etc., is more effective for precipitation of the Cu-rich phase.

[0059] In order to achieve a strain-induced martensite phase content of 20% by volume or more in the cold-rolled steel strip, the rolling ratio in the cold-rolling step is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more. The temperature in the cold-rolling step is preferably 90°C or less, and more preferably 60°C or less.

[0060] (finish annealing process) The cold-rolled steel strip is then subjected to finish annealing. The finish annealing process is carried out under conditions that promote the precipitation of Cu-rich phases. The Cu-rich phase is effective in increasing the strength of austenitic stainless steel. Therefore, the strength of the hot-rolled steel strip and cold-rolled steel strip before the precipitation of the Cu-rich phase is low, which reduces the rolling load in the cold rolling process. Furthermore, the precipitation of the Cu-rich phase during the finish annealing process results in high strength in the austenitic stainless steel after finish annealing.

[0061] The precipitation of the Cu-rich phase is also effective in refining the recrystallized grains of the austenite phase, so the average grain size can be controlled to 10.0 μm or less by utilizing the precipitation of the Cu-rich phase.

[0062] As described above, the method for producing an austenitic stainless steel according to one embodiment of the present invention can achieve both a reduction in the processing load during production and high strength in the final product at a high level. Furthermore, since an additional step of aging treatment is not required for precipitation of the Cu-rich phase, as in the conventional method, the productivity of the austenitic stainless steel is also good.

[0063] The temperature of the finish annealing step is set to 750°C or higher and 980°C or lower, preferably 800°C or higher and 925°C or lower, so that the Cu-rich phase effectively precipitates in the austenitic stainless steel. If the finish annealing temperature is lower than 750°C, the recrystallization of the structure will be insufficient. If the finish annealing temperature exceeds 980°C, the Cu-rich phase will dissolve, and the amount of Cu-rich phase remaining after the finish annealing will be insufficient.

[0064] Furthermore, the Cu-rich phase precipitated from the deformation-induced martensite phase is particularly likely to dissolve into the austenite phase when held at a temperature of 850°C or higher for a long period of time during finish annealing. Therefore, when the maximum temperature reached in the finish annealing step is 850°C or higher, it is preferable to shorten the heating time at 850°C or higher. Specifically, when the maximum temperature reached in the finish annealing step is 850°C or higher, the heating time at 850°C or higher is set to 30 seconds or less, and preferably 15 seconds or less. When the time during which the temperature reaches 850°C or higher in the finish annealing step is divided into multiple steps, the "heating time at 850°C or higher" refers to the total time of the multiple steps.

[0065] Since austenitic stainless steels have a C content of 0.03 mass% or less, precipitation of Cr carbides during cooling is unlikely to occur. Therefore, the cooling rate after finish annealing may be the same as that used in general stainless steel manufacturing methods. From the viewpoint of productivity, a faster cooling rate is preferable, but a relatively slow average cooling rate of 1°C / sec or more from 700°C to 500°C may also be acceptable, and from the viewpoint of productivity, a rate of 5°C / sec or more is preferable. Furthermore, from the viewpoint of the flatness of the steel sheet, a cooling rate of less than 75°C / sec is preferable, and 50°C / sec or less is more preferable.

[0066] In the cold rolling process, intermediate annealing and intermediate rolling may be performed as necessary. Furthermore, temper rolling may be performed as necessary to further increase the strength of the steel strip after final annealing. When prioritizing reduction of rolling load, the temperature of intermediate annealing is preferably 980°C or higher and 1150°C or lower to avoid precipitation of Cu-rich phases. In order to aim for high strength by repeating precipitation treatments, the temperature of intermediate annealing is preferably the same as that of final annealing. The temperature of intermediate annealing is not limited to the above-mentioned range.

[0067] [Strength evaluation] An austenitic stainless steel according to one embodiment of the present invention has a lower strength during the manufacturing process to reduce the rolling load, and yet achieves high strength after manufacturing. Such properties of austenitic stainless steel can be expressed, for example, by the relationship between 0.2% yield strength (YS 18%, MPa) and reference strength (HV 60%).

[0068] 0.2% proof stress (YS18%) is an index of the strength of austenitic stainless steel. It indicates the 0.2% proof stress when austenitic stainless steel is subjected to further temper rolling to an elongation of 18% after finish annealing. 0.2% proof stress can be evaluated using a method based on JIS Z2241.

[0069] The reference strength (HV60%) is an index that hypothetically indicates the strength of an austenitic stainless steel before the precipitation of a Cu-rich phase in the final annealing process. The reference strength (HV60%) indicates the Vickers hardness when the austenitic stainless steel has the same composition but is produced by a partially modified production method from that of one embodiment of the present invention, in which hot rolling is followed by annealing at 1050°C and cold rolling at a rolling reduction of 60%. In other words, the reference strength (HV60%) does not indicate the strength of the austenitic stainless steel according to one embodiment of the present invention, but may be the strength of a steel strip produced for evaluation, for example. The Vickers hardness can be measured based on the Vickers hardness test method in accordance with JIS Z2244.

[0070] The inventors have discovered that for austenitic stainless steels that achieve both reduced processing loads during manufacturing and high strength in the final product, the relationship between 0.2% yield strength (YS 18%) and reference strength (HV 60%) satisfies the following formula (2):

[0071] YS18%≧3.75HV60%-575 (2) According to a manufacturing method according to one embodiment of the present invention, it is possible to manufacture an austenitic stainless steel that satisfies the above formula (2) and achieves both a reduction in the processing load during manufacturing and high strength in the final product.

[0072] [Suitable uses] Austenitic stainless steel has extremely high strength and corrosion resistance. Therefore, austenitic stainless steel is suitable as a material for spring products that require high strength and corrosion resistance, such as cylinder head gaskets, spiral springs, springs for electronic components, electric railcar components, on-board battery frame materials, structural materials, and metal packing. In particular, austenitic stainless steel has excellent corrosion resistance (weldability) at welded joints, even when welded. Therefore, the austenitic stainless steel according to one embodiment of the present invention can be suitably used even in applications where a relatively large number of welded structures are used, such as electric railcar components or on-board battery frame materials manufactured for welding.

[0073] 〔summary〕 The austenitic stainless steel according to a first aspect of the present invention contains, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and has 20% or more by volume of austenite phase and a number density of 1.0×10 3 pieces μm -3 The above-mentioned Cu-rich phases with a long diameter of 30 nm or less are included, and the remainder is a deformation-induced martensite phase and an unavoidably formed phase, and Md 30 is between 0.0 and 80.0 inclusive: Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1) In the element symbols in the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

[0074] The austenitic stainless steel according to aspect 2 of the present invention, in accordance with aspect 1 above, may further contain, by mass%, one or more selected from Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, B: 0.0001% or more but 0.01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb: 0.5% or less, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less, and REM (rare earth metals): 0.2% or less.

[0075] The austenitic stainless steel according to a third aspect of the present invention may be the same as that of the first or second aspect, and may have an average grain size of 10.0 μm or less.

[0076] A method for producing an austenitic stainless steel according to a fourth aspect of the present invention comprises a steel sheet containing, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and having Md 30 A method for producing an austenitic stainless steel having a value of 0.0 or more and 80.0 or less, the method comprising a finish annealing step of performing finish annealing at a temperature of 750°C or more and 980°C or less, wherein when the maximum temperature reached in the finish annealing step is 850°C or more, the heating time at 850°C or more is set to 30 seconds or less, and in the finish annealing step, the average cooling rate from 700°C to 500°C after the finish annealing is set to 1°C / second or more: Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1) In the element symbols in the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

[0077] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Example]

[0078] The results of evaluation of the austenitic stainless steels according to the inventive examples and comparative examples of the present invention will be described below.

[0079] [Evaluation conditions] <Component composition> The composition (mass%) and Md of the austenitic stainless steels according to the examples of the present invention (invention steels A1 to A15) and the austenitic stainless steels according to the comparative examples (comparison steels B1 to B5) 30 The values ​​of Md are shown in Table 1 below. 30 The values ​​were calculated using the above formula (1). In Table 1 below, underlined values ​​indicate values ​​outside the range specified in the present invention. The same applies to Table 2 below.

[0080] [Table 1]

[0081] <Manufacturing method> The austenitic stainless steels according to the examples and comparative examples of the present invention were produced by the following methods. Austenitic stainless steels having the chemical compositions shown in Table 1 were melted and then subjected to processes from hot rolling to finish annealing by a production method according to an example of the present invention (Invention Examples C1 to C8) or a production method according to a comparative example (Comparative Examples D1 and D2), to obtain cold-rolled and annealed materials. The conditions for each production method are shown in Table 2 below.

[0082] [Table 2]

[0083] In the final annealing step, when the final annealing temperature was 850°C or higher, the time for which the temperature was maintained at 850°C or higher was adjusted as shown in Table 2. In Example C3, the heating was adjusted so that the temperature began to decrease when the final annealing temperature reached 850°C, but for convenience, the time for which the temperature was maintained at 850°C or higher is recorded as "1 second" in Table 2.

[0084] <Evaluation method> The austenitic stainless steels according to the examples of the present invention and the comparative examples were evaluated for various indices as shown below.

[0085] (Number density of Cu-rich phase) TEM samples were prepared using electropolishing from the cold-rolled and annealed materials produced under each condition. Three 400 nm x 400 nm areas were observed on the TEM sample, parallel to the rolling direction of the cold-rolled and annealed material. Cu-rich phases were identified from the contrast in the TEM images, and the number of Cu-rich phases was counted. The thickness of the TEM sample was assumed to be 150 nm, and the number density per unit volume was calculated. As the Cu-rich phases coarsened, they began to be observed as distinct shapes rather than as contrast. Cu-rich phases with a major axis exceeding 30 nm were excluded from the measurement.

[0086] (crystal grain size) The average grain size was evaluated using the EBSD method. The cross section of the cold-rolled and annealed material produced under each condition, parallel to the rolling direction and perpendicular to the rolling surface, was mechanically polished and then electrolytically polished. Then, EBSD analysis was performed at a magnification of 2000x over a 40 μm x 40 μm area on the cross section at a step interval of 0.2 μm. Regarding the misorientation in the orientation relationship that satisfies the Σ3 coincidence grain boundary, annealing twins with a misorientation of 1° or less were excluded, and boundaries with a misorientation of 2° or more were considered as grain boundaries, and the area of ​​each grain was calculated as S (μm 2 ), and the diameter of a circle having the same area as the crystal grain was defined as D (μm), and the crystal grain size was calculated using the following formula (3). This was performed for five visual fields, and the average of the crystal grain sizes obtained in the five visual fields was calculated as the average crystal grain size.

[0087] Grain size={Σ(D×S)} / 40×40 (3) (amount of martensite phase) The amount of martensite phase (volume %) was measured as is for sheet thicknesses of 1.5 mm or more, and for sheets less than 1.5 mm, the cold-rolled or temper-rolled material was layered so that the total thickness was 1.5 mm or more. These materials were measured using a ferrite scope (Fischer FMP30, electromagnetic induction method), and the measured value was divided by 0.7475 to determine the amount of martensite phase. The amount of austenite phase (volume %) was calculated by subtracting the amount of martensite phase from the entire austenitic stainless steel matrix, which was set at 100 volume %. The amounts of Cu-rich phases and unavoidably formed phases in austenitic stainless steel can be calculated as external quantities because their small proportions make accurate measurement difficult.

[0088] (Tensile properties) As an index of tensile properties, the 0.2% yield strength (YS18%) was evaluated when temper rolling was performed to achieve an elongation of 18%. JIS No. 13B test pieces were prepared and measured in a tensile test in accordance with JIS Z2241. The 0.2% yield strength (YS18%) was measured at a crosshead speed of 3 mm / min.

[0089] (strength) In each example and comparative example, the manufacturing conditions were partially changed, and the hot-rolled steel strip was annealed at 1050°C, and the rolling ratio was set to 60%, followed by cold rolling. The Vickers hardness of the 60% rolled material was measured as a reference strength (HV60%). The Vickers hardness was measured by conducting a Vickers hardness test (JIS Z2244) on the surface of the 60% rolled material using a Vickers hardness tester. The load during the Vickers hardness test was 10 kg.

[0090] (Corrosion resistance of welded joints) TIG fillet welding was performed on 1.5mm thick cold-rolled annealed material under Ar gas sealing conditions with an electrode diameter of 1.6mm, a welding speed of 70cm / min, and a welding current of 90A. The evaluation surface was a 10mm x 10mm area including the weld, and after #600 polishing to remove any film influence, the corrosion resistance of the evaluation surface was evaluated using the electrochemical reactivation rate as an index.

[0091] The reactivation rate was measured in accordance with JIS G0580. Specifically, the specimen was polarized in a 0.5 mol / L sulfuric acid and 0.01 mol / L potassium thiocyanate aqueous solution at a liquid temperature of 30°C from the spontaneous potential to 0.3 V (vs. SCE) at a sweep rate of 100 mV / min (hereinafter referred to as the "forward run"). After reaching 0.3 V (vs. SCE), the potential was swept in the opposite direction to the forward run, and after reactivation of the hot-rolled specimen, the sweep was terminated at a potential where the anodic current again became zero (hereinafter referred to as the "return run").

[0092] The ratio (ir / ia) of the maximum current density ia in the forward path to the maximum current density ir in the return path was calculated as the reactivation rate. Because this evaluation method is strict as a sensitization assessment method for evaluating corrosion resistance, a reactivation rate of, for example, 1.5% is considered to be no problem in practical environments. However, considering that the cold-rolled annealed material according to one embodiment of the present invention may have fine crystal grains, making it difficult to evaluate corrosion resistance, a reactivation rate of 1% or less can be said to have favorable corrosion resistance. Therefore, with regard to the corrosion resistance of the weld, a reactivation rate of 1% or less was evaluated as "O" (good), and a reactivation rate of more than 1% was evaluated as "x" (poor).

[0093] 〔result〕 For invention steel A2, the amount of Cu-rich phase precipitation and the grain size of the cold-rolled and annealed material obtained under each of the conditions shown in Table 2 are shown in Table 3. Table 3 also shows the 0.2% yield strength (YS 18%) under each condition, and the amount of martensite phase after cold rolling (before finish annealing) and after temper rolling to give an elongation of 18% after finish annealing.

[0094] In Table 3 below, the underlined values ​​indicate that the amount of precipitated Cu-rich phase is outside the range specified in the present invention.

[0095] [Table 3]

[0096] For the invention steel A2, the cold-rolled and annealed materials produced under the conditions of invention examples C1 to C8 had a Cu-rich phase precipitation amount within the range specified by the present invention and exhibited a fine average grain size of 10.0 μm or less. On the other hand, the cold-rolled and annealed materials produced under the conditions of comparative examples D1 and D2 did not show any Cu-rich phase precipitation.

[0097] For the cold-rolled and annealed steel of Invention Steel A2 produced under the conditions of Invention Example C2, an EBSD grain boundary map is shown on the left side of Fig. 1, and a TEM image is shown on the right side of Fig. 1. As shown in the TEM image on the right side of Fig. 1, precipitation of a Cu-rich phase (shown as "Cu" in Fig. 1) was observed in the austenitic stainless steel according to one embodiment of the present invention.

[0098] Furthermore, since the reference strength (HV60%) of invention steel A2 was 445, the 0.2% yield strength (YS18%) is preferably 1094 MPa or more based on the above formula (2). The cold-rolled and annealed materials of invention steel A2 produced under the conditions of invention examples C1 to C8 all had a 0.2% yield strength (YS18%) of 1094 MPa or more. On the other hand, the cold-rolled and annealed materials produced under the conditions of comparative examples D1 and D2 all had a 0.2% yield strength (YS18%) lower than 1094 MPa. As such, it was demonstrated that, because a Cu-rich phase does not precipitate under the conditions of comparative examples D1 and D2, it is difficult to obtain an austenitic stainless steel that has a good balance between workability before final annealing and high strength after final annealing.

[0099] Next, the amount of Cu-rich phase precipitated and the grain size after final annealing of cold-rolled and annealed materials produced from invention steels A1 to A15 or comparative steels B1 to B5 under the production conditions shown in invention example C2 are shown in Table 4. Table 4 also shows the 0.2% yield strength (YS 18%), reference strength (HV 60%), and corrosion resistance of welds under each of these conditions.

[0100] In Table 4 below, the underlined parts indicate that the amount of Cu-rich phase precipitation is outside the range specified in the present invention, the 0.2% yield strength (YS18%) is a value that does not satisfy the above formula (2), or the corrosion resistance of the weld is poor.

[0101] [Table 4]

[0102] The cold-rolled and annealed steels A1 to A15 of the invention had a Cu-rich phase precipitation amount within the range specified in the present invention, and exhibited a fine average grain size of 10.0 μm or less.Furthermore, all of them exhibited good values ​​for 0.2% proof stress (YS 18%) that satisfied the above formula (2).

[0103] On the other hand, the cold-rolled and annealed material of comparative steel B1 had poor corrosion resistance in the welded zone. The cold-rolled and annealed materials of comparative steels B2 to B5 did not satisfy the above formula (2) in terms of 0.2% yield strength (YS 18%), and it was not possible to obtain austenitic stainless steels with a good balance between workability before final annealing and high strength after final annealing.

[0104] Figure 2 shows a plot of the relationship between 0.2% proof stress (YS 18%) and reference strength (HV 60%) under each condition in Table 4. In Figure 2, an example of the present invention is indicated by a white circle, and a comparative example is indicated by a black arrowhead. In the graph shown in Figure 2, the plotted value is located to the upper left, indicating a better balance between workability before finish annealing and high strength after finish annealing.

[0105] The above results are all for cold-rolled and annealed materials obtained under the condition of a cooling rate of 25°C / sec after final annealing. Here, cold-rolled and annealed materials were produced using invention steels A1, A2, and A5 under the conditions of invention example C2 shown in Table 2, with the cooling rate from 700°C to 500°C after final annealing varied within the range of 0.3 to 100°C / sec. The amount of Cu-rich phase precipitated in the obtained cold-rolled and annealed materials is shown in Table 5 below.

[0106] [Table 5]

[0107] When the cooling rate was 5°C / s or more, there was no change in the amount of Cu precipitates in the cold-rolled annealed material. It can be said that if the cooling rate was sufficiently fast, coarsening of the Cu-rich phase during cooling and the accompanying disappearance of the Cu-rich phase did not occur. When the cooling rate was 2°C / s, the amount of Cu precipitates decreased slightly. This is thought to be due to coarsening of the Cu-rich phase during cooling and the accompanying disappearance of the Cu-rich phase. This is a phenomenon generally known as Ostwald ripening. When the cooling rate was less than 1°C / s, coarsening of the Cu-rich phase during cooling and the accompanying disappearance of the Cu-rich phase progressed further, and the amount of precipitation reached 1.0 × 10 3 pieces μm -3 It became less than.

[0108] As shown in Table 4 and Figure 2, it was shown that the cold-rolled annealed material produced by the production method according to one embodiment of the present invention using an austenitic stainless steel having a composition according to one embodiment of the present invention achieves both a reduction in the processing load during production and high strength in the product. Furthermore, it was shown that such cold-rolled annealed material also has excellent corrosion resistance in the welded parts, making it suitable for applications where frequent welding is performed.

Claims

1. The steel sheet contains, in mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 2.0% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, 20% by volume or more of austenite phase and a number density of 1.0 × 10 ３ pieces・μm －３ The Cu-rich phase has a major axis of 30 nm or less, and the remainder is a deformation-induced martensite phase and an unavoidably formed phase, Md shown in the following formula (1) ３０ Austenitic stainless steel having a value of 0.0 or more and 80.0 or less: Md ３０ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1) In the element symbols of the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

2. 2. The austenitic stainless steel according to claim 1, further containing, in mass%, one or more selected from Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, B: 0.0001% or more and 0.01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb: 0.5% or less, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less, and REM (rare earth metals): 0.2% or less.

3. 3. The austenitic stainless steel according to claim 1, wherein the average grain size is 10.0 μm or less.

4. The alloy contains, by mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 2.0% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, with the balance being Fe and unavoidable impurities, and has a composition of Md represented by the following formula (1): ３０ A method for producing an austenitic stainless steel in which the value of is 0.0 or more and 80.0 or less, A final annealing step of performing final annealing at a temperature of 750°C or higher and 980°C or lower, When the maximum temperature reached in the finish annealing step is 850 ° C or higher, the heating time at 850 ° C or higher is set to 30 seconds or less, A method for producing an austenitic stainless steel, wherein in the finish annealing step, the average cooling rate from 700°C to 500°C after the finish annealing is 1°C / second or more: Md ３０ =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1) In the element symbols of the formula (1), the content (mass %) of each element contained in the austenitic stainless steel is substituted, and 0 is substituted for elements that are not added.

Images