Ferritic stainless steel has improved resistance to corrugation, and this steelmaking method

Abstract

The invention relates to a ferritic stainless steel capable of resisting ripple and a method for producing such steel. This ferritic stainless steel capable of resisting ripple comprises, in percentage by mass (% by mass), 0.001% to 0.3% C, 0.01% to 1.0% Si, 0.1% to 3.0% Mn, 10% to 15% Cr, 0.001% to 0.3% N, equal to or less 0.03% P, equal to or less 1.0% Ni, equal to or less 1.0% Cu, equal to or less 1.0% Al, equal to or less 0.003% Mo, equal to or less 1.0% Ti, remainder of which is Fe and other unavoidable impurities, and has a value of γS calculated according to the following expression (1) equal to or greater than 6: γS = 900C-30Si+12Mn+23Ni-17Cr-12Mo+12Cu-49Ti-52Al+950N+178 [Expression (1)] (where C, Si, Mn, Ni, Cr, Mo, Cu, Ti, Al, and N are the content (% by mass) of each element).

Description

Ferritic stainless steel with improved ductility and its manufacturing method

[0001] The present invention relates to ferritic stainless steel and a method for manufacturing the same, and more particularly, to ferritic stainless steel with improved tensile strength and a method for manufacturing the same.

[0002] Stainless steels are generally classified by chemical composition or metallurgical structure. Based on metallurgical structure, stainless steels are categorized into austenitic (300 series), ferritic (400 series), martensitic, and biphasic series.

[0003] Among these, ferritic stainless steels are more price-competitive than austenitic stainless steels due to their low content of expensive alloying elements. Ferritic stainless steels possess excellent surface gloss, drawability, and oxidation resistance, making them widely used in kitchenware, building exteriors, home appliances, and electronic components. In particular, ferritic stainless steels require a high-quality surface gloss for exterior applications.

[0004] However, ferritic stainless steels have a problem with ridged defects, which occur in stripes parallel to the rolling direction during forming processes such as deep drawing. These ridged defects negatively affect the product's appearance, and severe ridged defects require additional polishing after forming, increasing manufacturing costs.

[0005] The cause of leasing has not yet been clearly identified, but it is generally known as follows. Columnar grains formed during slab casting mainly have a {001} / / ND texture, which does not recrystallize well even after hot rolling, and thus remains as a band structure or colony structure.

[0006] In particular, most ferritic stainless steels do not undergo phase transformation from casting to cold rolling annealing, making it more difficult to remove the {001} / / ND texture. Therefore, the {001} / / ND texture remains as long colony structures in the rolling direction even after the final cold rolling. The remaining colony structures exhibit relatively low plastic anisotropy (R value) compared to structures with other surrounding textures. This difference in plastic anisotropy causes a plastic imbalance between the two structures during forming processing, which causes ridging defects in ferritic stainless steels.

[0007] To address leasing defects, various manufacturing methods have been proposed, including hot rolling at extremely low temperatures, continuous rolling, and cold repressing. However, these methods are difficult to apply in the field and increase manufacturing costs, thereby reducing product productivity.

[0008] The present invention relates to ferritic stainless steel and a method for manufacturing the same, and aims to provide a ferritic stainless steel with improved quenching properties and a method for manufacturing the same.

[0009] A ferritic stainless steel having improved tensile strength according to one embodiment of the present invention comprises, in wt%, C: 0.001 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.1 to 3.0%, Cr: 10 to 15%, N: 0.001 to 0.3%, P: 0.03% or less, Ni: 1.0% or less, Cu: 1.0% or less, Al: 1.0% or less, Mo: 0.003% or less, Ti: 1.0% or less, the remainder being Fe and other unavoidable impurities.

[0010] γ expressed by the following equation (1) S It is 6 or more.

[0011] Equation (1): γ S = 900C-30Si+12Mn+23Ni-17Cr-12Mo+12Cu-49Ti-52Al+950N+178

[0012] (Here, C, Si, Mn, Ni, Cr, Mo, Cu, Ti, Al, and N represent the content (weight%) of each element.)

[0013] Additionally, according to one embodiment of the present invention, the ferritic stainless steel can satisfy a ferrite crystal grain size of 15 ㎛ or less.

[0014] In addition, according to one embodiment of the present invention, the ferritic stainless steel can satisfy a ridging height (Wt) of 10 µm or less measured after 15% tension at a thickness of 1.0 mm or less.

[0015] According to another embodiment of the present invention, a method for manufacturing a ferritic stainless steel having improved quenching resistance includes, in wt%, C: 0.001 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.1 to 3.0%, Cr: 10 to 15%, N: 0.001 to 0.3%, P: 0.03% or less, Ni: 1.0% or less, Cu: 1.0% or less, Al: 1.0% or less, Mo: 0.003% or less, Ti: 1.0% or less, the remainder being Fe and other unavoidable impurities, and γ expressed by the following formula (1): S A step of reheating a slab to 1050 to 1250°C, wherein the slab is 6 or more; a step of hot-rolling the reheated slab; and a step of cold-rolling and cold-annealing the hot-rolled material; Including, in the reheating step, γ defined as the austenite weight % at temperature T Wt (T) is controlled to satisfy the following equation (2).

[0016] Equation (1): γ S = 900C-30Si+12Mn+23Ni-17Cr-12Mo+12Cu-49Ti-52Al+950N+178

[0017] (Here, C, Si, Mn, Ni, Cr, Mo, Cu, Ti, Al, and N represent the content (weight%) of each element.)

[0018] Equation (2): γ Wt(1200℃) ≥ 19%

[0019] In addition, according to one embodiment of the present invention, in the reheating step, the following equation (3) can be satisfied.

[0020] Equation (3): γ S *γ Wt (1200℃) ≥ 114

[0021] Additionally, according to one embodiment of the present invention, the hot rolling may include a step of finishing rolling at a temperature of 700 to 950°C.

[0022] Additionally, according to one embodiment of the present invention, after the hot rolling, a step of hot rolling annealing at 600 to 900°C may be further included.

[0023] The present invention can provide a ferritic stainless steel with improved tensile strength and a manufacturing method thereof by optimizing not only the alloy composition and the composition relationship but also the reheating and hot rolling conditions and having a uniform surface quality.

[0024] Figure 1a shows γ of Example 3 using JMatPro. Wt This is a state diagram including (1200℃).

[0025] Figure 1b shows γ of Example 10 using JMatPro. Wt This is a state diagram including (1200℃).

[0026] Figure 1c shows γ of comparative example 1 using JMatPro. Wt This is a state diagram including (1200℃).

[0027] Figure 1d shows γ of comparative example 6 using JMatPro. Wt This is a state diagram including (1200℃).

[0028] Figure 2a is the hot-rolled microstructure of Example 3 investigated using an IQ (Image Quality) map.

[0029] Figure 2b is the hot-rolled microstructure of Example 10 investigated using an IQ (Image Quality) map.

[0030] Figure 2c is the hot-rolled microstructure of Comparative Example 1 investigated using an IQ (Image Quality) map.

[0031] Figure 2d is the hot-rolled microstructure of Comparative Example 6 investigated using an IQ (Image Quality) map.

[0032] Figure 3a is the hot-rolled microstructure of Example 3 investigated using an IPF (Inverse Pole Figure) map.

[0033] Figure 3b is the hot-rolled microstructure of Example 10 investigated using an IPF (Inverse Pole Figure) map.

[0034] Figure 3c is the hot-rolled microstructure of Comparative Example 1 investigated using an IPF (Inverse Pole Figure) map.

[0035] Figure 3d is the hot-rolled microstructure of Comparative Example 6 investigated using an IPF (Inverse Pole Figure) map.

[0036] Figure 4a is a photograph showing the surface microstructure after cold rolling annealing of Example 3.

[0037] Figure 4b is a photograph showing the surface microstructure after cold rolling annealing of Example 10.

[0038] Figure 4c is a photograph showing the surface microstructure after cold rolling annealing of Comparative Example 1.

[0039] Figure 4d is a photograph showing the surface microstructure after cold rolling annealing of Comparative Example 6.

[0040] A ferritic stainless steel having improved tensile strength according to one embodiment of the present invention comprises, in wt%, C: 0.001 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.1 to 3.0%, Cr: 10 to 15%, N: 0.001 to 0.3%, P: 0.03% or less, Ni: 1.0% or less, Cu: 1.0% or less, Al: 1.0% or less, Mo: 0.003% or less, Ti: 1.0% or less, the remainder being Fe and other unavoidable impurities.

[0041] γ expressed by the following equation (1) S It is 6 or more.

[0042] Equation (1): γ S = 900C-30Si+12Mn+23Ni-17Cr-12Mo+12Cu-49Ti-52Al+950N+178

[0043] (Here, C, Si, Mn, Ni, Cr, Mo, Cu, Ti, Al, and N represent the content (weight%) of each element.)

[0044] Preferred embodiments of the present invention are described below. However, the embodiments of the present invention may be modified in various ways, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more fully explain the present invention to those of ordinary skill in the art.

[0045] The terminology used in this application is solely for the purpose of describing specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Additionally, it should be noted that terms such as "comprise" or "have" used in this 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 preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0046] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, specific terms should not be interpreted in an overly idealistic or formal sense. For example, singular expressions herein include plural expressions unless the context clearly indicates otherwise.

[0047] In addition, the terms "about", "substantially", etc. in this specification are used in the sense of or close to the numerical value when manufacturing and material tolerances inherent to the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly using the disclosure contents in which exact or absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0048] The ferritic stainless steel with improved quenching properties according to the present invention contains, in wt%, C: 0.001 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.1 to 3.0%, Cr: 10 to 15%, N: 0.001 to 0.3%, P: 0.03% or less, Ni: 1.0% or less, Cu: 1.0% or less, Al: 1.0% or less, Mo: 0.003% or less, Ti: 1.0% or less, the remainder including Fe and other unavoidable impurities.

[0049] Below, the reasons for limiting the composition range of each alloy element are described.

[0050] The carbon (C) content is 0.001 to 0.3%.

[0051] C, as an interstitial solid-solution strengthening element, improves the strength of ferritic stainless steels. At levels below 0.001%, carbide formation is reduced, preventing sufficient strength from being achieved. However, excessive C content degrades the ductility, toughness, and corrosion resistance of the steel, so the upper limit is set at 0.3%.

[0052] The silicon (Si) content is 0.01 to 1.0%.

[0053] Silicon (Si) is an alloying element essential for deoxidation of molten steel during steelmaking. It enhances strength and corrosion resistance, while stabilizing the ferrite phase. In the present invention, silicon (Si) may be added in amounts of 0.01% or more. However, excessive Si content may result in reduced ductility and formability, and therefore the upper limit is intended to be limited to 1.0%.

[0054] The manganese (Mn) content is 0.1 to 3.0%.

[0055] Manganese (Mn) is an austenite-stabilizing element that promotes grain refinement by inducing austenite nucleation during hot rolling. However, excessive Mn content reduces corrosion resistance, generates manganese fumes during welding, and causes MnS phase precipitation, thereby reducing elongation. Therefore, the present invention aims to control the Mn content to 0.1 to 3.0%.

[0056] The chromium (Cr) content is 10 to 15%.

[0057] Cr (chromium) is an element that enhances corrosion resistance by forming a passive film in Martian environments, and is added at levels of 10% or more. However, excessive Cr content can lead to sticking defects due to the formation of dense oxide scale during hot rolling, increasing manufacturing costs. Therefore, the present invention aims to limit the upper limit of Cr content to 15%.

[0058] The nitrogen (N) content is 0.001 to 0.3%.

[0059] Nitrogen, like carbon, is an interstitial, solid-solution strengthening element that not only improves the strength of ferritic stainless steels but also promotes recrystallization by precipitating an austenite phase during hot rolling. However, excessive content can reduce the ductility of the steel. Therefore, in the present invention, the content of Ni is controlled to 0.001 to 0.3%.

[0060] The phosphorus (P) content is less than 0.03%.

[0061] Phosphorus (P) is an impurity that is inevitably contained in steel, and is a major cause of intergranular corrosion during pickling or of impairing hot workability. Therefore, it is desirable to control its content as low as possible. Therefore, in the present invention, the P content is controlled to 0.03% or less.

[0062] The nickel (Ni) content is less than 1.0%.

[0063] While nickel improves corrosion resistance, adding large amounts increases impurities in the material, which reduces elongation. Furthermore, nickel is a representative austenite-stabilizing element, but it is expensive, increasing manufacturing costs. Therefore, the nickel content in the present invention is controlled to 1.0% or less.

[0064] The copper (Cu) content is less than 1.0%.

[0065] Copper (Cu) is an effective element for improving corrosion resistance, workability, and etching properties. However, adding large amounts of Cu can lead to reduced workability. Therefore, the Cu content in the present invention is controlled to 1.0% or less.

[0066] The content of aluminum (Al) is less than 1.0%.

[0067] Aluminum (Al) is a ferrite-phase stabilizing element and acts as a powerful deoxidizer, reducing the oxygen content in molten steel. However, excessive Al content can reduce room-temperature ductility, increase non-metallic inclusions, and cause sliver defects in cold-rolled strip, while also degrading weldability. Therefore, the present invention controls the Al content to 1.0% or less.

[0068] The content of molybdenum (Mo) is less than 0.003%.

[0069] Molybdenum (Mo) is an effective element for improving the corrosion resistance of stainless steel. However, Mo is an expensive element, leading to increased raw material costs and reducing workability when added in large amounts. Therefore, the present invention controls the Mo content to 0.003% or less.

[0070] The content of titanium (Ti) is less than 1.0%.

[0071] Ti is an effective element for reducing the amount of dissolved carbon and nitrogen in steel and ensuring corrosion resistance by preferentially combining with interstitial elements such as carbon (C) and nitrogen (N) to form precipitates (carbonitrides). However, if its content is excessive, there are problems such as reduced austenite stability, difficulty in obtaining fine grains, reduced toughness, and increased titanium inclusions causing surface defects. Therefore, in the present invention, the titanium content is controlled to 1.0% or less.

[0072] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0073] In addition, the ferritic stainless steel according to one embodiment of the present invention has γ expressed by the following formula (1) S It is 6 or more.

[0074] Equation (1): γ S = 900C-30Si+12Mn+23Ni-17Cr-12Mo+12Cu-49Ti-52Al+950N+178

[0075] (Here, C, Si, Mn, Ni, Cr, Mo, Cu, Ti, Al, and N represent the content (weight%) of each element.)

[0076] γ S (Austenite (gamma-phase) stability) is an index of austenite phase stability corresponding to the maximum amount of austenite at high temperatures. In the present invention, in order to secure austenite phase stability and induce austenite phase transformation during hot rolling, γ S We wanted to limit the value to 6 or more. In addition, γ S When the value is less than 6, the hot-rolled band structure of the ferritic stainless steel is not removed and remains, resulting in severe occurrence of ridging defects.

[0077] The present invention optimizes the alloy composition of ferritic stainless steel to induce austenite phase transformation during hot rolling. Accordingly, the ferritic stainless steel according to one embodiment of the present invention can secure fine single-phase ferrite grains without band or colony structures. The size of the single-phase ferrite grains can be 15 μm or less.

[0078] Next, a method for manufacturing a ferritic stainless steel with improved corrosion resistance according to another embodiment of the present invention will be described.

[0079] A method for manufacturing a ferritic stainless steel according to one embodiment of the present invention comprises, in wt%, C: 0.001 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.1 to 3.0%, Cr: 10 to 15%, N: 0.001 to 0.3%, P: 0.03% or less, Ni: 1.0% or less, Cu: 1.0% or less, Al: 1.0% or less, Mo: 0.003% or less, Ti: 1.0% or less, the remainder being Fe and other unavoidable impurities, and γ expressed by the following formula (1): S A slab having a hardness of 6 or higher is reheated to 1050 to 1250°C, the reheated slab is hot-rolled, the hot-rolled material is cold-rolled and cold-annealed, and in the reheating step, γ, defined as the austenite weight% at temperature T, Wt (T) is controlled to satisfy the following equation (2).

[0080] Equation (1): γ S = 900C-30Si+12Mn+23Ni-17Cr-12Mo+12Cu-49Ti-52Al+950N+178

[0081] (Here, C, Si, Mn, Ni, Cr, Mo, Cu, Ti, Al, and N represent the content (weight%) of each element.)

[0082] Equation (2): γ Wt (1200℃) ≥ 19%

[0083] The reason for limiting the composition range of each alloy element is as described above.

[0084] γ Wt (T)(Austenite(gamma-phase) Weight at temperature T) is the weight percent of austenite at temperature T in the reheating step. γ SEven if the value of 6 or higher is satisfied, the stability of the austenite phase decreases if the reheating temperature is high. If the stability of the austenite phase decreases, the austenite phase transformation does not occur sufficiently during hot rolling, and a hot-rolled band structure remains on the surface of the ferritic stainless steel.

[0085] As a result of examining various control conditions using JmatPro, it was found that when the austenite weight percentage was controlled to 19% or more at a reheating temperature of 1200℃, fine ferrite crystal grains could be secured on the surface of ferritic stainless steel after hot rolling.

[0086] In addition, according to one embodiment of the present invention, in the reheating step, the following equation (3) can be satisfied.

[0087] Equation (3): γ S *γ Wt (1200℃) ≥ 114

[0088] When the value of equation (3), which satisfies equations (1) and (2) and represents the product of equations (1) and (2), is 114 or more, the leasing height can be suppressed to 10 μm or less as intended in the present invention.

[0089] Next, to secure the desired final thickness during hot rolling, finishing rolling (finish rolling) can be performed at 700 to 950°C.

[0090] Maintaining the finishing rolling temperature below 700℃ can cause sticking defects on the slab surface during hot rolling. Furthermore, hot rolling must be performed at a temperature above 700℃ to roll the slab to the appropriate thickness.

[0091] On the other hand, if the temperature of the finishing rolling exceeds 900°C, relatively large ferrite crystal grains are formed. Therefore, in the present invention, the temperature of the finishing rolling is controlled to 900°C or lower so that fine ferrite crystal grains can be formed after hot rolling.

[0092] Hot-rolled steel is surface-pickled for cold rolling. Hot-rolled annealing can be omitted. However, if excessively fine ferrite grains are formed or elongation is reduced due to residual dislocations, hot-rolled steel may be subjected to hot-rolled annealing.

[0093] Accordingly, according to one embodiment of the present invention, a step of hot rolling annealing may be further included after the hot rolling. Hot rolling annealing is preferably performed at 600 to 900°C to prevent the austenite phase from being regenerated and to remove stress formed during hot rolling.

[0094] In this way, by optimizing the alloy composition and composition relationship as well as the reheating and hot rolling processes, the surface properties of ferritic stainless steel can be secured by deriving fine crystal grains of the ferrite single phase.

[0095] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0096] {Example}

[0097] For the various alloy composition ranges shown in Table 1 below, slabs were manufactured by continuous casting and reheated at 1,050 to 1,200°C. Next, the reheated slabs were finish-rolled at a temperature of 700 to 950°C.

[0098] Classification Alloy Composition CSiMnPCrNiCuMoAlTiNbN Example 10.0110.240.480.02411.20.78000.020.1800.01 Example 20.0070.230.50.023110.790.1900.0170.1700.01 Example 30.0070.150.90.0210.600.3000.1500.008 Example 40.0110.210.0210.800.31000.2200.01 Example 50.0110.1510.0210.600.31000.1500.01 Example 60.0070.10.90.0210.500.3000.13500.008Example 70.0110.110.0210.500.31000.13500.01Example 80.0050.10.820.0210.500.21000.13500.004Example 90.0050.150.820.0210.600.21000.1500.004Example 100.0070.20.90.0214.100.400000.057Comparative Example 10.0060.550.30.02211.200000.200.008Comparative Example 20.0060.520.150.02411.10.8000.0260.1900.0072Comparative Example 30.0050.20.820.0210.800.21000.2200.004Comparative Example 40.0070.20.90.0210.800.3000.2200.008Comparative Example 50.0070.850.30.021500000.130.40.006Comparative Example 60.0070.24.60.0214.102000.200.008Comparative Example 70.0070.120.0214.102.500.2000.008

[0099] The hot-rolled material was pickled, cold-rolled to a thickness of 1.0 t or less, and then cold-rolled and annealed at 700 to 900°C to prevent the austenite phase from being regenerated. After that, the cold-rolled and annealed material was pulled by 15% in the rolling direction and the height of the ridging curve was measured with a surface roughness tester. Table 2 below shows the γ of the examples and comparative examples. S, γ Wt (1200℃), γ S *γ Wt(1200℃) and the value indicating the leasing height (㎛).

[0100] Distinction γ S γ Wt (1200℃)(%)γ S *γ Wt (1200℃) Rising height (㎛) Example 1 13.6 33 45 0.16.3 Example 2 17.1 43 73 6.8 4.4 Example 3 14.3 144 1.8 6.2 Example 4 12.7 22 28 0.38.6 Example 5 21.1 42 88 4.9 4.8 Example 6 18.2 39 70 9.25.2 Example 7 25.0 51 12 75 34.1 Example 8 10.5 26 274.28.1 Example 96.6 19 125.6 9.3 Example 108.4 23 19 2.05 5.6 Comparative Example 1-22.1 0.0 22.9 Comparative Example 2-4.58-36.220.5Comparative example 3-1.72-3.421.7Comparative example 45.91165.117.6Comparative example 5-93.300.023.5Comparative example 615.618280.813.8Comparative example 7-7.224-172.812.4

[0101] Looking at Table 2, Examples 1 to 10 are γ S is 6 or more, and γ Wt (1200℃) is 19% or more, and γ S *γ Wt (1200℃) satisfies 114 or more. Accordingly, Examples 1 to 10 were obtained with good surface quality with a ridging height of 10㎛ or less.

[0102] On the other hand, Comparative Example 1 is γ S is less than 6 at -22.1, and γ Wt (1200℃) is less than 19%, γ S *γ Wt (1200℃) is less than 114 and 0. Accordingly, in Comparative Example 1, a leasing defect with a height of 22.9㎛ occurred.

[0103] In addition, Comparative Examples 2 to 5 are γ S is less than 6, and γ S *γ Wt (1200℃) is less than 19%, and γ S *γWt (1200℃) is less than 114. Accordingly, in Comparative Examples 2 to 5, a leasing defect larger than 10㎛ occurred.

[0104] Comparative example 6 is γ S 15.6 satisfies 6 or more presented in the present invention, and γ S *γ Wt (1200℃) has a value of 280.8, which is 114 or higher. However, Comparative Example 6 has γ Wt (1200℃) had a value of 18% less than 19%, resulting in a 13.8㎛ leasing defect larger than 10㎛.

[0105] Comparative example 7 is γ Wt (1200℃) is 24%, which is more than 19%. However, Comparative Example 7 is γ S is less than 6 at -7.2, and γ S *γ Wt (1200℃) is -172.8, which is less than 114. Accordingly, in the comparison, 7 had a leasing defect of 12.4㎛, which is larger than 10㎛.

[0106] Through the disclosed embodiments and comparative examples, γ presented by the present invention S , γ Wt (1200℃) and γ S *γ Wt It was found that when the range of (1200℃) was satisfied, a leasing defect of 10㎛ or less occurred.

[0107] Table 3 below shows the values ​​obtained by measuring whether the band structure was observed after hot rolling, whether the band structure was observed after cold rolling annealing, and the ferrite crystal grain size for Example 3, Example 10, Comparative Example 1, and Comparative Example 6.

[0108] Whether to observe the band structure after hot rolling Whether to observe the band structure after cold rolling annealing After cold rolling annealing Ferrite grain size Remarks Example 3XX10.8㎛Fig. 2a and Fig. 3a Example 10XX11.2㎛Fig. 2b and Fig. 3b Comparative example 1OX35.1㎛Fig. 2c and Fig. 3c Comparative example 6OX18.9㎛Fig. 2d and Fig. 3d

[0109] Looking at Table 3 and Figures 2a to 2d, no band structure was observed in Examples 3 and 10, and fine ferrite crystal grains were evenly distributed. On the other hand, a band structure remaining after hot rolling was observed in Comparative Examples 1 and 6.

[0110] Looking at Table 3 and Figures 4a to 4d, the ferrite grain size of Example 3 is 10.8 µm, and the ferrite grain size of Example 10 is 11.2 µm. On the other hand, the ferrite grain size of Comparative Example 1 is 35.1 µm, and the ferrite grain size of Comparative Example 6 is 18.9 µm.

[0111] After cold rolling annealing, no band structure was observed on the surface of Examples 3 and 10 and Comparative Examples 1 and 6. However, in the case of Comparative Examples 1 and 6, it was confirmed that the size of the ferrite crystal grains was 15 ㎛ or larger, which was coarser than that of the examples.

[0112] According to the disclosed embodiment, by optimizing the alloy composition and composition relationship as well as the reheating and hot rolling conditions, it is possible to secure a uniform surface quality of ferritic stainless steel by suppressing the ridging height to 10 ㎛ or less without developing a band structure or colony structure on the surface.

[0113] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and spirit of the claims set forth below.

[0114] According to one embodiment of the present invention, by optimizing not only the alloy composition and composition relationship, but also the reheating and hot rolling conditions, a ferritic stainless steel with improved tensile strength and uniform surface quality can be provided. Therefore, the steel can be used in various industrial fields.

Claims

1. Contains, by weight%, C: 0.001 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.1 to 3.0%, Cr: 10 to 15%, N: 0.001 to 0.3%, P: 0.03% or less, Ni: 1.0% or less, Cu: 1.0% or less, Al: 1.0% or less, Mo: 0.003% or less, Ti: 1.0% or less, the remainder including Fe and other unavoidable impurities, γ expressed by the following equation (1) S Ferritic stainless steel with improved hardness and corrosion resistance of 6 or higher. Equation (1): γ S = 900C - 30Si + 12Mn + 23Ni - 17Cr - 12Mo + 12Cu - 49Ti - 52Al + 950N + 178 (Here, C, Si, Mn, Ni, Cr, Mo, Cu, Ti, Al, and N represent the content (weight%) of each element.) 2. In claim 1, Ferritic stainless steel with improved hardenability and a ferrite grain size of 15㎛ or less.

3. In claim 1, A ferritic stainless steel with improved tensile strength, having a tensile height (Wt) of 10 ㎛ or less measured after 15% elongation at a thickness of 1.0 mm or less.

4. In weight%, C: 0.001 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.1 to 3.0%, Cr: 10 to 15%, N: 0.001 to 0.3%, P: 0.03% or less, Ni: 1.0% or less, Cu: 1.0% or less, Al: 1.0% or less, Mo: 0.003% or less, Ti: 1.0% or less, the remainder including Fe and other inevitable impurities, and γ expressed by the following formula (1) S A step of reheating the slab to 1050 to 1250℃, with a temperature of 6 or higher; A step of hot rolling the above reheated slab; and A step of cold rolling the above hot-rolled material and cold-rolling annealing; In the reheating step, γ is defined as the weight percent of austenite at temperature T. Wt A method for manufacturing a ferritic stainless steel with improved ductility, wherein (T) is controlled to satisfy the following equation (2). Formula (1): γ S = 900C - 30Si + 12Mn + 23Ni - 17Cr - 12Mo + 12Cu - 49Ti - 52Al + 950N + 178 (Here, C, Si, Mn, Ni, Cr, Mo, Cu, Ti, Al, and N represent the content (weight%) of each element.) Equation (2): γ Wt (1200℃) ≥ 19% 5. In claim 4, A method for manufacturing a ferritic stainless steel with improved ductility, satisfying the following formula (3), in the above reheating step. Formula (3): γ S *γ Wt (1200 °C) ≥ 114 6. In claim 4, A method for manufacturing a ferritic stainless steel with improved tensile strength, wherein the hot rolling includes a step of finishing rolling at a temperature of 700 to 950°C.

7. In claim 4, A method for manufacturing ferritic stainless steel with improved tensile strength, further comprising a step of hot rolling annealing at 600 to 900°C after the above hot rolling.

Images