KR20230153865A - Austenitic stainless steel - Google Patents

Abstract

The austenitic stainless steel according to an example of the present invention has, in weight percent, C: 0.04 to 0.07%, Si: 0.3 to 0.6%, Mn: 0.5 to 1.5%, Cu: 0.1 to 0.4%, Mo: 0.05 to 0.2. %, Ni: 8.0 to 9.0%, Cr: 18.0 to 19.0%, N: 0.02 to 0.05%, including the remaining Fe and inevitable impurities, the thickness is 0.4 to 2.0 mm, and the average grain size at the center of the thickness (d ) value is 3 or more and 10㎛ or less, the martensite fraction after cold rolling is more than 60%, the fraction with Misorientation Angle of 15° or more after cold rolling annealing is more than 95%, the pitting potential value is more than 250mV, and when stretched at 30% The surface roughness (Ra) is 0.50㎛ or less, the surface roughness (Ra) is 0.36㎛ or less when stretched at 20%, the surface roughness (Ra) is 0.25㎛ or less when stretched at 10%, and the age crack limit during drawing processing. If the ratio is more than 2.0, the average earring height is less than 2.2mm, bending cracks may not occur after a 180° bending test.

Description

Austenitic stainless steel {AUSTENITIC STAINLESS STEEL}

The present invention relates to austenitic stainless steel with improved hardness, high strength-high ductility, drawability, bendability and surface properties.

In general, austenitic stainless steel is used for various purposes such as transportation parts and architectural parts due to its excellent formability, work hardening ability, and weldability. However, 304 series stainless steel or 301 series stainless steel has a yield strength of only 200 to 350 MPa, so its application to structures is limited. Therefore, in order to obtain higher yield strength in general-purpose 300 series stainless steel, a temper rolling process is a common method. However, the method of using the temper rolling process has the problem of increased cost and extremely low elongation of the material.

Patent Document 0001 describes a method of manufacturing 300 series stainless steel with small bending even after half-etching by temper rolling a cold-rolled annealed material for a laser metal mask for photo-etching processing and then performing two SR (Stress Relief) heat treatments. there is. However, Patent Document 0001 does not contain technical details on structural parts with a thickness of 0.4 to 2.0 mm as a manufacturing technology for controlling etching properties and bending after etching, and Md30(℃)=497-462*([C] The ASP (Austenitic Stability Parameter) value calculated as +[N])-9.2*[Si]-8.1*[Mn]-13.7*[Cr]-20*[Ni]+[Cu]-18.7*[Mo] Since it has a range of 30 to 50, there is a problem that during forming such as tensile testing, strain-induced martensite transformation occurs at too fast a rate, lowering the elongation rate.

Patent Document 0002 proposed a method of performing long-term heat treatment at 600 to 700°C for more than 48 hours in order to produce an average grain size of 10㎛ or less. However, the method proposed in Patent Document 0002 has problems in that productivity is low and manufacturing costs increase when implemented in an actual production line.

International Publication Patent No. 2016-043125 A1 (Publication Date: March 14, 2016) Japanese Patent Publication No. 2020-050940 A (Publication date: April 2, 2020)

The purpose of the present invention to solve the above-mentioned problems is to provide an ultra-fine manufacturing technology that realizes hardness, high strength-high ductility, drawability, bendability, and sound surface properties of the bending area, so that there are no surface cracks in the bent area and the surface The goal is to provide austenitic stainless steel with excellent roughness.

The austenitic stainless steel according to an embodiment of the present invention has, in weight percent, C: 0.04 to 0.07%, Si: 0.3 to 0.6%, Mn: 0.5 to 1.5%, Cu: 0.1 to 0.4%, Mo: 0.05 to 0.05%. Contains 0.2%, Ni: 8.0-9.0%, Cr: 18.0-19.0%, N: 0.02-0.05%, including the remaining Fe and inevitable impurities;

The Σ value expressed in equation (1) below may be 180 or more and 240 or less.

Equation (1): Σ = 105 + 146d ^-1/2 + 7.36SPM_El + 102[C] + 154[N] + 51.8[Si] + 1.4[Mo] - 17.7[Cu]

In the formula (1), [C], [N], [Si], [Mo], and [Cu] mean the weight percent of each element, d means the average grain size (㎛), and SPM_El is It refers to the difference in elongation (%) before and after Skin Pass Milling.

In addition, the austenitic stainless steel according to the present invention may have an Ω value of 2500 or more, expressed by equation (2) below.

Equation (2): Ω = YS*El - 500*([Ni]+[Cr]+[Cu]+[Mn])

In the formula (2), [Ni], [Cr], [Cu], and [Mn] mean the weight percent of each element, YS means the yield strength (MPa), and El means the elongation (%).

In addition, the austenitic stainless steel according to the present invention may have an Austenitic Stability Parameter (ASP) value of -5 to 12, expressed by equation (3) below.

Equation (3): ASP value = 551 - 462*([C]+[N]) - 9.2*[Si] - 8.1*[Mn] - 13.7*[Cr] - 29*([Ni]+[Cu] ) - 18.5*[Mo]

In the formula (3), [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], and [Mo] mean the weight percent of each element.

In addition, the austenitic stainless steel according to the present invention may have a Π value of 6000 or more, expressed by equation (4) below.

Equation (4): Π = 100*FCRR(Final Cold Rolling Ratio, %)+ 100*ASP(Austenitic Stability Parameter) + CAT(Cold Annealing Temperature, ℃)

In the above equation (4), FCRR means the reduction ratio during cold rolling before final cold rolling annealing, ASP means the value in equation (3), and CAT means a value defined as the temperature of the final cold rolling annealing steel material.

In addition, when the total thickness of the austenitic stainless steel according to the present invention is t, the average grain size (d) value at the center of the thickness of the TD (Transverse Direction) plane may be 3 or more and 10 ㎛ or less.

Additionally, the thickness t may be 0.4 to 2.0 mm.

In addition, the martensite fraction (%) of the cold rolled material of the austenitic stainless steel according to the present invention was determined by analyzing the thickness center of the TD (Transverse Direction) surface using an Electron Back Scatter Diffraction (EBSD) analyzer. This may be more than 60%.

In addition, the austenitic stainless steel according to the present invention has a misorientation angle of 15° or more after cold rolling annealing when the thickness center of the TD (Transverse Direction) surface is analyzed with an Electron Back Scatter Diffraction (EBSD) analyzer ( %) may be 95% or more.

Additionally, the austenitic stainless steel according to the present invention may have a pitting potential value of 250 mV or more in a 3.5% NaCl solution at 30°C.

In addition, the austenitic stainless steel according to the present invention has a surface roughness (Ra) of 0.50 ㎛ or less when stretched at 30%, a surface roughness (Ra) of 0.36 ㎛ or less when stretched at 20%, and a surface roughness (Ra) of 0.36 ㎛ or less when stretched at 10%. The roughness (Ra) may be 0.25㎛ or less.

Additionally, the austenitic stainless steel according to the present invention may have an age cracking limit drawing ratio of 2.0 or more.

In addition, the austenitic stainless steel according to the present invention may have an average earring height of 2.2 mm or less during drawing processing.

Additionally, in the austenitic stainless steel according to the present invention, surface cracks may not occur in the bent area after a 180° bending test.

According to one embodiment of the present invention, ultra-fine grain 300 series stainless steel with excellent hardness can be provided.

According to one embodiment of the present invention, it is possible to provide ultra-fine grain 300 series stainless steel that simultaneously satisfies high strength and high ductility.

According to one embodiment of the present invention, it is possible to provide austenitic stainless steel with no surface cracks in the bent area and excellent surface roughness through an ultra-fine grain manufacturing technology that realizes bending formability and sound surface properties of the bent portion.

Figure 1 is a diagram showing Σ and Ω values of examples and comparative examples.
Figure 2 is a photograph showing the microstructure of an example in which the Π value satisfies 6000 or more.
Figure 3 is a photograph showing an unannealed band in the microstructure of Comparative Example 1 with a Π value of less than 6000.
Figure 4 shows the martensite (red color) fraction (%) when the phase fraction of the thickness center of the TD (Transverse Direction) surface of the material after cold rolling of the example was analyzed using an electron backscatter diffraction pattern analyzer (EBSD). ) is a photo with 91.5% and 73.5%.
Figure 5 shows the martensite (red color) fraction (%) when the phase fraction of the thickness center of the TD (Transverse Direction) surface of the material after cold rolling of the comparative example was analyzed with an electron backscatter diffraction pattern analyzer (EBSD). ) is a photo with 48.5%.
Figure 6 shows the fraction (%) of Misorientation Angle of 15° or more when the thickness center of the TD (Transverse Direction) surface of the material after cold rolling annealing of the example was analyzed with an Electron Back Scatter Diffraction (EBSD) pattern analyzer is 96. This is a photo with %, 96.5%.
Figure 7 shows that when the thickness center of the TD (Transverse Direction) surface of the material after cold rolling annealing of the comparative example was analyzed with an electron backscatter diffraction pattern analyzer (EBSD), the fraction (%) with a misorientation angle of 15° or more was 92.9. It's a % photo. Misorientation Angle less than 15° corresponds to red color.
Figure 8 is a photograph showing the average grain size at the center of the thickness of the TD (Transverse Direction) surface of the 0.4 to 2.0 mm steel manufactured in the example.
Figure 9 is a photograph showing the average grain size at the center of the thickness of the TD (Transverse Direction) surface of the 0.4 to 2.0 mm steel manufactured in Comparative Example.
Figure 10 is a photograph showing the surface roughness Ra (㎛) value according to the tensile strain rate of Example and Comparative Example 2.
Figure 11 is a photograph showing the surface roughness Ra (㎛) shape according to the tensile strain rate of Examples and Comparative Examples.
Figure 12 is a photograph showing the earring height after drawing processing of Examples and Comparative Examples.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. Additionally, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

The terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression, unless the context clearly requires it to be singular. In addition, terms such as “comprise” or “comprise” used in the present application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to indicate other features. It should be noted that it is not used to preliminarily rule out the existence of any elements, steps, functions, components, or combinations thereof.

Meanwhile, unless otherwise defined, all terms used in this specification should be viewed as having the same meaning as generally understood by those skilled in the art to which the present invention pertains. Therefore, unless clearly defined in this specification, specific terms should not be interpreted in an overly idealistic or formal sense. For example, in this specification, singular expressions include plural expressions unless the context clearly dictates otherwise.

In addition, in this specification, "about", "substantially", etc. are used in the meaning of or close to that value when manufacturing and material tolerances inherent in the stated meaning are presented, and are used accurately to aid understanding of the present invention. It is used to prevent unscrupulous infringers from unfairly exploiting disclosures where absolute figures are mentioned.

The austenitic stainless steel according to an example of the present invention has, in weight percent, C: 0.04 to 0.07%, Si: 0.3 to 0.6%, Mn: 0.5 to 1.5%, Cu: 0.1 to 0.4%, Mo: 0.05 to 0.2. %, Ni: 8.0-9.0%, Cr: 18.0-19.0%, N: 0.02-0.05%, and may include the remaining Fe and inevitable impurities.

Below, the reason for limiting the alloy composition will be explained in detail.

The content of C (carbon) may be 0.04% to 0.07%.

C is an austenite phase stabilizing element. Considering this, C can be added in an amount of 0.04% or more. However, if the C content is excessive, chromium carbide may be formed during low-temperature annealing, which may reduce intergranular corrosion resistance. Considering this, the upper limit of C content can be limited to 0.07%.

The content of Si (silicon) may be 0.3 to 0.6%.

Si is a component added as a deoxidizer in the steelmaking stage, and when it goes through the bright annealing process, it has the effect of improving the corrosion resistance of steel by forming Si oxide in the passivation film. Considering this, Si may be added in an amount of 0.3% or more. However, if the Si content is excessive, a problem may occur that reduces the ductility of the steel. Considering this, the upper limit of Si content may be limited to 0.6%.

The content of Mn (manganese) may be 0.5 to 1.5%.

Mn is an austenite phase stabilizing element. Considering this, Mn may be added in an amount of 0.5% or more. However, if the Mn content is excessive, a problem of lowering corrosion resistance may occur. Considering this, the upper limit of Mn content may be limited to 1.5%.

The content of Ni (nickel) may be 8.0 to 9.0%.

Ni is an austenite phase stabilizing element and has the effect of softening steel materials. Considering this, Ni may be added in an amount of 8.0% or more. However, if the Ni content is excessive, the problem of increased cost may occur. Considering this, the upper limit of Ni content can be limited to 9.0%.

The content of Cr (chromium) may be 18.0 to 19.0%.

Cr is a major element for improving the corrosion resistance of stainless steel. Considering this, Cr can be added in an amount of 18.0% or more. However, if the Cr content is excessive, the steel material becomes hard and problems may occur in suppressing strain-induced martensite transformation during cold rolling. Considering this, the upper limit of Cr content can be limited to 19.0%.

The content of Cu (copper) may be 0.1 to 0.4%.

Cu is an austenite phase stabilizing element. Considering this, Cu may be added in an amount of 0.1% or more. However, if the Cu content is excessive, the corrosion resistance of the steel decreases and the problem of increased costs may occur. Considering this, the upper limit of Cu content may be limited to 0.4%.

The content of N (nitrogen) may be 0.02 to 0.05%.

N is an austenite phase stabilizing element and improves the strength of steel. Considering this, N may be added in an amount of 0.02% or more. However, if the N content is excessive, the steel may become hard and hot workability may deteriorate. Considering this, the upper limit of N content may be limited to 0.05%.

The content of Mo (molybdenum) may be 0.05 to 0.2%.

Mo has the effect of improving corrosion resistance and processability. Considering this, Mo may be added in an amount of 0.05% or more. However, if the Mo content is excessive, the problem of increased cost may occur. Considering this, the upper limit of Mo content can be limited to 0.2%.

The remaining ingredient is iron (Fe). However, in the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, so this cannot be ruled out. Since these impurities are known to anyone skilled in the normal manufacturing process, all of them are not specifically mentioned in this specification.

In addition to controlling the alloy composition ratio, the austenitic stainless steel according to an example of the present invention can be determined as follows.

In an example of the present invention, the Σ value expressed by equation (1) below may be 180 or more and 240 or less.

Equation (1): Σ = 105 + 146d ^-1/2 + 7.36SPM_El + 102[C] + 154[N] + 51.8[Si] + 1.4[Mo] - 17.7[Cu]

Σ is an indicator representing the hardness of the material, and the larger the value, the greater the hardness.

In the above formula (1), [C], [N], [Si], [Mo], and [Cu] mean the weight percent of each element, d is the average grain size (㎛), and SPM_El is Skin Pass Milling It refers to the difference in elongation (%) before and after.

SPM_El according to an example of the present invention may be 0.01 to 1.2.

Skin pass milling can be done to improve the gloss of steel and ensure the shape of the coil.

In an example of the present invention, the Ω value expressed by equation (2) below may be 2500 or more.

Equation (2): Ω = YS*El- 500*([Ni]+[Cr]+[Cu]+[Mn])

Ω is an indicator representing high strength and high ductility, and the larger the value, the greater the product of strength and ductility.

In the formula (2), [Ni], [Cr], [Cu], and [Mn] mean the weight percent of each element, YS means the yield strength (MPa), and El means the elongation (%).

YS and El values refer to the values obtained after performing a tensile test on a JIS13B tensile test specimen at room temperature with a crosshead in the range of 10 mm/min to 20 mm/min.

In an example of the present invention, the Austenitic Stability Parameter (ASP) value represented by equation (3) below may be -5 to 12.

Equation (3): ASP value = 551 - 462*([C]+[N]) - 9.2*[Si] - 8.1*[Mn] - 13.7*[Cr] - 29*([Ni]+[Cu] )-18.5*[Mo]

In the formula (3), [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], and [Mo] mean the weight percent of each element.

If the ASP value exceeds -5 to 12, excessive transformation of the material occurs during the tensile test and the elongation desired in the present invention is not satisfied.

In an example of the present invention, the Π value expressed in equation (4) below may be 6000 or more.

Equation (4): Π = 100*FCRR(Final Cold Rolling Ratio, %)+ 100*ASP(Austenitic Stability Parameter) + CAT(Cold Annealing Temperature, ℃)

Π is an indicator representing the completeness of recrystallization, and the larger the value, the greater the degree of recrystallization.

In the above equation (4), FCRR means the reduction ratio during cold rolling before final cold rolling annealing, ASP means the value in equation (3), and CAT means a value defined as the temperature of the final cold rolling annealing steel material.

The average grain size d according to an example of the present invention may be 3 or more and 10 μm.

The average grain size refers to the average grain size at the center of the thickness of the TD (Transverse Direction) plane when the total thickness of the steel is t, and the average refers to the average value of the values measured at five random locations, and the center means 1/4t to 3/4t when the total thickness of the steel is t.

t according to an example of the present invention may be 0.4 to 2.0 mm. In the case of kitchen and building material parts, materials having a thickness of 0.4 to 2.0 mm are widely used, and according to the present invention, stainless steel with excellent hardness in the corresponding thickness range and high strength and high ductility can be provided.

An example of the present invention is that when the thickness center of the TD (Transverse Direction) surface is analyzed with an Electron Back Scatter Diffraction (EBSD) analyzer, the martensite fraction (%) of the cold rolled material is 60% or more, After cold rolling annealing, the fraction (%) with a misorientation angle of 15° or more may be more than 95%.

If the misorientation angle is 15° or more, it means that recrystallization has occurred after cold rolling annealing. If the misorientation angle is less than 15°, the difference in azimuth angle is small and recrystallization has not occurred.

The average grain size (d) of the present invention, the martensite fraction (%) of the cold rolled material, and the fraction (%) with a Misorientation Angle of 15° or more after cold rolling annealing were determined using an Electron Backscattering Diffraction Pattern Analyzer (Electron Backscattering Pattern Analyzer) whose model name is e-Flash FS. The orientation of the center of the thickness was analyzed and measured using Scatter Diffraction (EBSD).

The pitting potential is the critical potential at which corrosion occurs in the form of holes in a passivated metal material. The austenitic stainless steel according to an embodiment of the present invention is immersed in a NaCl solution and pitting occurs by applying a potential. The result of measuring the pitting potential may be more than 250mV. Here, the temperature of the NaCl solution may be 30°C and the concentration may be 3.5%.

The austenitic stainless steel according to an example of the present invention has a surface roughness (Ra) of 0.50 ㎛ or less when stretched at 30%, a surface roughness (Ra) of 0.36 ㎛ or less when stretched at 20%, and a surface roughness (Ra) of 0.36 ㎛ or less when stretched at 10%. Surface roughness (Ra) may be 0.25㎛.

When drawing according to an example of the present invention, the punch size is Φ50 mm and the disc size is Φ100 mm, the age crack limit drawing ratio may be 2.0 or more. Additionally, the average earring height may be 2.2 mm or less.

Age crack limit drawing ratio is the limit drawing ratio at which age cracks do not occur, and refers to the ratio of the maximum diameter of the material and the punch diameter during drawing processing.

Earring height average refers to the average of the sum of the maximum height (H) of the earring from the floor after drawing processing minus the sum of the minimum height (h) of the earring.

After a 180° bending test according to an example of the present invention, surface cracks may not occur in the bent area.

Hereinafter, a method for manufacturing austenitic stainless steel according to an example of the present invention will be described in detail.

The method for manufacturing austenitic stainless steel according to an example of the present invention is by weight percentage, C: 0.04-0.07%, Si: 0.3-0.6%, Mn: 0.5-1.5%, Cu: 0.1-0.4%, Mo: 0.05 Preparing a slab containing ~0.2%, Ni: 8.0~9.0%, Cr: 18.0~19.0%, N: 0.02~0.05% and the remaining Fe and inevitable impurities;

Hot rolling the slab and hot rolling annealing the slab to produce a hot rolled steel sheet;

It may be a method of manufacturing austenitic stainless steel including the step of cold rolling the hot rolled steel sheet and performing cold rolling annealing to produce a cold rolled steel sheet.

The reason for limiting the composition range of each alloy element is as described above, and the manufacturing steps will be described in more detail below.

The hot rolling annealing can be performed at 1000 to 1150°C.

If the hot rolling annealing temperature is less than 1000°C, the residual martensite fraction may be high and the elongation may be poor. On the other hand, if the hot rolling annealing temperature exceeds 1150°C, the strength may decrease due to grain coarsening. Considering this, the hot rolling annealing is performed at 1000 to 1150°C.

The reduction rate of the cold rolling may be 60% or more.

If the reduction ratio is less than 60%, the TRIP transformation amount is too low, so the martensite fraction of the cold rolled material decreases and the retained austenite phase fraction increases. As the amount of processed-induced martensite decreases, the rate of reverse transformation into the austenite phase due to subsequent low-temperature annealing decreases, and the fraction of the residual austenite phase that has not been transformed into martensite is high, making it difficult to secure ultra-fine grains.

The cold rolling annealing can be performed at 800 to 950°C.

If the cold annealing temperature is less than 800°C, recrystallization is not sufficient and the elongation rate is lowered. On the other hand, when the cold annealing temperature exceeds 950°C, the particles become coarse and it is difficult to form ultra-fine particles of 3 to 10㎛ or less, so surface cracks occur at the bent area of austenitic stainless steel and the surface roughness decreases. Problems with deterioration may occur. Considering this, the cold rolling annealing is performed at 800 to 950°C.

Additionally, the manufacturing method may be an austenitic stainless steel manufacturing method including the step of skin pass milling the cold rolled steel sheet.

The SPM_El is set to 0.01 to 1.2 as the difference in elongation (%) before and after Skin Pass Milling.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for illustrating the implementation of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of rights of the present invention is determined by matters stated in the patent claims and matters reasonably inferred therefrom.

{Example}

After hot rolling the slabs with the components shown in Table 1 below, hot rolling annealing was performed at 1000 to 1150°C, and cold rolling was performed at room temperature with a reduction ratio of 60% or more. Then, cold rolled annealed in the range of 800 to 950°C to produce a cold rolled annealed material.

Table 2 below shows the values for equations (1) to (4) according to the component values in Table 1, SPM_El (Skin Pass Mill Elongation, %), FCRR (Final Cold Rolling Ratio, %) values, and CAT (Cold Annealing Temperature, ℃) value and YS(MPa)*El(%) value.

The YS and El values refer to the values obtained after performing a tensile test on a JIS13B tensile test specimen at room temperature with a crosshead in the range of 10 mm/min to 20 mm/min.

The cold-rolled annealed material prepared above was manufactured into specimens with a thickness of 0.4 to 2.0 mm. Afterwards, for the specimen, the average grain size (d) at the center of the thickness, pitting potential (mV), presence or absence of surface cracks in the bent area after the 180° bending test (curvature R value of the bending part is the same as the material thickness), After measuring the surface roughness at each tensile strain rate in the uniaxial tensile test, it is shown in Table 3 below.

The average grain size (d) at the center of the thickness refers to the average grain size (d) at the center of the thickness of the TD (Transverse Direction) surface of a specimen with a thickness of 0.4 to 2.0 mm.

The average grain size (d) of the present invention was measured by analyzing the orientation of the center of the thickness using an Electron Back Scatter Diffraction (EBSD) analyzer with the model name e-Flash FS.

Pitting potential refers to the value measured by immersing in a NaCl solution at a temperature of 30°C and a concentration of 3.5%, where pitting occurs.

The tensile strain value refers to the value when a uniaxial tensile test was performed on a JIS 13B tensile specimen at a crosshead speed of 10 to 20 mm/min.

The occurrence of bending cracks is determined by conducting a 180° bending test to ensure that the curvature R value of the bending part is equal to the material thickness and bending once.

Table 4 below shows the average earring height as an example of the present invention.

Earring height average refers to the average of the sum of the maximum height (H) of the earring from the floor after drawing processing minus the sum of the minimum height (h) of the earring. H means the maximum height, h means the minimum height, and Earring1 to Earring4 are randomly assigned to each earring.

The LDR value and earring height average mean the age crack limit drawing ratio and earring height average during drawing processing using punch size of Φ50 mm and disc size of Φ100 mm.

Referring to Tables 1 and 2 above, in Examples 1 to 6, the Σ value of equation (1) satisfies the range of -180 to 240, and the Ω value of equation (2) satisfied the range of 2500 or more, showing excellent performance. It is possible to achieve hardness, high strength and high ductility.

Referring to Tables 1 to 3, in Examples 1 to 6, the ASP value of equation (3) all satisfies the range of -5 to 12, and the Π value of equation (4) satisfies the range of 6000 or more, so that the 180° No surface cracks occur after the bending test.

Referring to Table 3 above, in Examples 1 to 6, the material thickness was in the range of 0.4 to 2.0 mm and the average grain size (d) value at the center of the material thickness was within the range of 3 to 10 ㎛, and after the uniaxial tensile test, At 30%, the surface roughness (Ra) when stretched is 0.50㎛ or less, at 20%, the surface roughness (Ra) when stretched is 0.36㎛ or less, and at 10%, the surface roughness (Ra) when stretched is 0.25㎛ or less. Additionally, the official potential value satisfies the range of 250mV or more.

On the other hand, Comparative Examples 2 to 4 had Σ values less than 180 and did not satisfy the hardness of the present invention. This can be confirmed in Table 2.

In addition, Comparative Examples 2 to 7 have Ω values less than 2500, so they do not simultaneously satisfy the high strength and high ductility of the present invention. This can be confirmed in Table 2.

Additionally, Comparative Examples 3 to 8 exceeded the ASP values limited by the present invention. Comparative Examples 5 to 8 with ASP values exceeding 12 showed low elongation due to too fast transformation speed. Comparative Examples 2 to 4 with ASP values of less than -5 failed to secure ultra-fine grains due to the high residual austenite phase fraction. This can be confirmed in Table 2 and Table 3.

In addition, Comparative Example 1 had an unannealed band because the Π value was less than 6000, and bending cracks occurred in steel materials including this after a 180° bending test. This can be confirmed in Table 2, Table 3, and Figure 3. In contrast, Figure 2 shows the microstructure of the example, and it can be seen that no bands are visible and no bending cracks occur after the 180° bending test.

In addition, Comparative Examples 6 to 8 had a low Ni content and excessive addition of Mn resulted in low pitting potential values, causing problems with corrosion resistance.

In addition, Comparative Example 2 had a d value of 25.2, and the average grain size was coarse compared to the range of the present invention, so the surface roughness value according to tensile strain was large. This can be confirmed in Table 3 and Figure 6.

Figure 1 shows the ranges of Σ values and Ω values of Examples 1 to 6 and Comparative Examples 1 to 8. Comparative Examples 2 to 8 do not satisfy both the Σ value and the Ω value. Hardness and high strength-high ductility cannot be satisfied at the same time.

Figures 2 and 3 are photographs of the microstructure of the center of the thickness taken through EBSD. Comparative example (FIG. 3) shows the presence of an unannealed band in the microstructure. In contrast, the example (Figure 2) shows that there are no unannealed bands in the microstructure. By comparing this, it can be confirmed that the austenitic stainless steel according to an example of the present invention has ultra-fine grains without band-shaped unannealed portions.

Figures 4 and 5 show the martensite (red color) indicates the fraction (%). This can be compared to the martensite fraction of 60% or more in the example (FIG. 4) and less than 60% in the comparative example (FIG. 5).

Figures 6 and 7 show the fraction with a misorientation angle of 15° or more when the thickness center of the TD (Transverse Direction) surface of the material was analyzed with an electron backscatter diffraction pattern analyzer (EBSD) after cold rolling annealing of the examples and comparative examples. It represents (%). In the Example (FIG. 6), the fraction (%) of Misorientation Angle of 15° or more is more than 95%, which can be compared to less than 95% in the Comparative Example (FIG. 7). Misorientation Angle less than 15° corresponds to red color.

Figures 8 and 9 are photographs showing the average grain size at the center of the thickness of the TD (Transverse Direction) surface as examples and comparative examples of manufactured 0.4 to 2.0 mm steel materials. It can be seen that the average grain size of the comparative example (FIG. 9) is coarser than that of the example (FIG. 8).

Figure 10 is a photograph showing the surface roughness Ra (㎛) value after performing a uniaxial tensile test at each strain rate on Comparative Example 2, in which the average grain size at the center of the surface of the manufactured steel was coarser than that of the Example, at a crosshead speed of 10 to 20 mm/min. . When the average grain size exceeds the range of the present invention, it can be confirmed that the surface roughness Ra value at each strain rate does not satisfy the range of the present invention.

Figure 11 is a photograph showing the surface roughness Ra (㎛) shape after performing a uniaxial tensile test at each strain rate for Examples and Comparative Examples at a crosshead speed of 10 to 20 mm/min. The x-axis and y-axis are in mm units, and the height unit is ㎛. In the comparative example, the height increases as strain is applied, so it can be seen that the surface roughness is high. In contrast, it can be confirmed that the surface roughness of the Example is lower than that of the Comparative Example even if modifications are made.

Figure 12 is a photograph showing the earring height after drawing processing (punch size Φ50 mm, disc size Φ100 mm). Earring height average refers to the average of the sum of the maximum height (H) of the earring from the floor after drawing processing minus the sum of the minimum height (h) of the earring. It can be seen that the average earring height value of the Example is smaller than that of the Comparative Example.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person skilled in the art will recognize the present invention within the scope and spirit of the following claims. You will understand that various changes and modifications are possible.

Claims (13)

In weight percent, C: 0.04~0.07%, Si: 0.3~0.6%, Mn: 0.5~1.5%, Cu: 0.1~0.4%, Mo: 0.05~0.2%, Ni: 8.0~9.0%, Cr: 18.0~ 19.0%, N: 0.02~0.05%, including the remaining Fe and inevitable impurities;
Austenitic stainless steel with a Σ value of 180 or more and 240 or less, as expressed in equation (1) below.
Equation (1): Σ = 105 + 146d ^-1/2 + 7.36SPM_El + 102[C] + 154[N] + 51.8[Si] + 1.4[Mo] - 17.7[Cu]
(In the above equation (1), [C], [N], [Si], [Mo], and [Cu] mean the weight percent of each element, d is the average grain size (㎛), and SPM_El is Skin Pass refers to the difference in elongation (%) before and after milling). In claim 1,
Austenitic stainless steel with an Ω value of 2500 or more, expressed in equation (2) below.
Equation (2): Ω = YS*El- 500*([Ni]+[Cr]+[Cu]+[Mn])
(In the above equation (2), [Ni], [Cr], [Cu], and [Mn] mean the weight% of each element, YS means the yield strength (MPa), and El means the elongation (%) ). In claim 1,
Austenitic stainless steel with an ASP (Austenitic Stability Parameter) value of -5 to 12, expressed in equation (3) below.
Equation (3): ASP value = 551 - 462*([C]+[N]) - 9.2*[Si] - 8.1*[Mn] - 13.7*[Cr] - 29*([Ni]+[Cu] )-18.5*[Mo]
(In the above formula (3), [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], and [Mo] mean the weight percent of each element). In claim 1,
Austenitic stainless steel with a Π value of 6000 or more, expressed in equation (4) below.
Equation (4): Π = 100*FCRR(Final Cold Rolling Ratio, %)+ 100*ASP(Austenitic Stability Parameter) + CAT(Cold Annealing Temperature, ℃)
(In the above equation (4), FCRR means the reduction ratio during cold rolling before final cold rolling annealing, ASP means the value in equation (3), and CAT means the value defined as the temperature of the final cold rolling annealing steel material. ). In claim 1,
When the total thickness of the steel is t, an austenitic stainless steel whose average grain size (d) at the center of the thickness of the TD (Transverse Direction) plane is 3 to 10㎛. In claim 1,
The t is an austenitic stainless steel of 0.4 to 2.0 mm. In claim 1,
Austenitic stainless steel with a martensite fraction (%) of 60% or more at the center of the thickness of the TD (Transverse Direction) surface after cold rolling. In claim 1,
Austenitic stainless steel with a misorientation angle of 15° or more at the center of the thickness of the TD (Transverse Direction) surface after cold rolling annealing is more than 95%. In claim 1,
Austenitic stainless steel with a pitting potential value of 250 mV or more in a 3.5% NaCl solution at 30°C. In claim 1,
An austenitic type with a surface roughness (Ra) of 0.50㎛ or less when stretched at 30%, a surface roughness (Ra) of 0.36㎛ or less when stretched at 20%, and a surface roughness (Ra) of 0.25㎛ or less when stretched at 10%. stainless steel. In claim 1,
An austenitic stainless steel having an age crack limit drawing ratio of 2.0 or more. In claim 1,
Austenitic stainless steel with an average earring height of 2.2 mm or less during drawing processing of the above stainless steel.
(The earring height average refers to the average of the sum of the maximum heights (H) of the earrings from the floor after drawing processing minus the sum of the minimum heights (h) of the earrings). In claim 1,
The stainless steel is an austenitic stainless steel in which surface cracks do not occur in the bent area after a 180° bending test.