Ferritic stainless steel with improved workability and ridging resistance and method for producing the same

Abstract

A ferritic stainless steel with improved ridging and a method for manufacturing the same are provided. [Solution] A ferritic stainless steel with improved workability and ridging resistance according to one embodiment of the present invention contains, in weight percent, carbon (C): 0.0005 to 0.02%, nitrogen (N): 0.01 to 0.2%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 0.01 to 1.0%, phosphorus (P): 0.001 to 0.05%, chromium (Cr): 13.0 to 20.0%, titanium (Ti): 0.05 to 0.2%, with the remainder being iron (Fe) and unavoidable impurities, and satisfies the following formula (1), and the grain size can satisfy the range of 20 to 25 μm. Formula (1): 2*[Ti] / [N] ≦ 2.3 In the above formula (1), [Ti] and [N] represent the weight percent content of each element.

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel having improved workability and ridging resistance, and a method for producing the same. [Background technology]

[0002] Ferritic stainless steel has excellent corrosion resistance despite the addition of small amounts of expensive alloying elements, making it more cost-competitive than austenitic stainless steel. Ferritic stainless steel is used in a variety of applications, including building materials, transportation equipment, home appliances, and kitchen appliances.

[0003] Ferritic stainless steel cold-rolled products have the problem of striped ridging defects occurring during forming processes such as deep drawing. Not only does this ridging defect mar the product's appearance, but if the ridging is severe, an additional polishing process is required after forming, which increases the manufacturing cost.

[0004] To solve the ridging defect, various manufacturing methods have been proposed in the past, such as ultra-low temperature hot rolling, differential speed rolling, and cold rolling repressing, etc. However, the previously proposed manufacturing methods have problems such as being difficult to apply on-site, increasing manufacturing costs, and reducing product productivity. Summary of the Invention [Problem to be solved by the invention]

[0005] The object of the present invention is to provide a ferritic stainless steel having improved workability and ridging by optimizing the steel composition and manufacturing process to control the grain size of the final cold-rolled and annealed material in order to improve the workability of cold-rolled products, and a manufacturing method thereof.

[0006] However, the problems to be solved by this specification are not limited to the above problems, and other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0007] A ferritic stainless steel with improved workability and ridging resistance according to one embodiment of the present invention contains, by weight, 0.0005 to 0.02% carbon (C), 0.01 to 0.2% nitrogen (N), 0.01 to 1.0% silicon (Si), 0.01 to 1.0% manganese (Mn), 0.001 to 0.05% phosphorus (P), 13.0 to 20.0% chromium (Cr), 0.05 to 0.2% titanium (Ti), with the remainder being iron (Fe) and unavoidable impurities, and satisfies the following formula (1), and the grain size can satisfy the range of 20 to 25 μm.

[0008] Formula (1): 2*[Ti] / [N] ≦ 2.3

[0009] In the above formula (1), [Ti] and [N] represent the weight percent content of each element.

[0010] The ferritic stainless steel having improved workability and ridging resistance according to one embodiment may have a ridging height of 10 μm or less measured after being stretched by 15%.

[0011] The ferritic stainless steel having improved workability and ridging resistance according to one embodiment may have an elongation of 32% or more.

[0012] A method for producing a ferritic stainless steel having improved workability and ridging resistance according to one aspect of the present invention includes the steps of producing a slab consisting of, by weight, 0.0005 to 0.02% carbon (C), 0.01 to 0.2% nitrogen (N), 0.01 to 1.0% silicon (Si), 0.01 to 1.0% manganese (Mn), 0.001 to 0.05% phosphorus (P), 13.0 to 20.0% chromium (Cr), 0.05 to 0.2% titanium (Ti), and the remainder being iron (Fe) and unavoidable impurities, and satisfying the following formula (1): reheating the slab; hot-rolling the reheated slab followed by hot-rolling annealing; and cold-rolling the hot-rolled and annealed hot-rolled material at 800 to 850°C.

[0013] Formula (1): 2*[Ti] / [N] ≦ 2.3

[0014] In the above formula (1), [Ti] and [N] represent the weight percent content of each element.

[0015] In the method for producing a ferritic stainless steel having improved workability and ridging resistance according to one embodiment, the reheating may be performed at a temperature of 1,000 to 1,300°C.

[0016] In one embodiment of the method for producing a ferritic stainless steel having improved workability and ridging resistance, the stainless steel can satisfy the grain size range of 20 to 25 μm.

[0017] In one embodiment of a method for producing a ferritic stainless steel having improved workability and ridging resistance, the stainless steel may have a ridging height of 10 μm or less measured after being stretched by 15%.

[0018] In the method for producing a ferritic stainless steel having improved workability and ridging resistance according to one embodiment, the stainless steel may have an elongation of 32% or more. [Effects of the Invention]

[0019] The present invention provides a ferritic stainless steel with improved workability and ridge resistance, and a manufacturing method thereof, by optimizing the steel composition and manufacturing process to control the grain size of the final cold-rolled and annealed material in order to improve the workability of the cold-rolled product. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a graph showing ridging height (solid line) and elongation (dotted line) depending on the annealing temperature during production of stainless steel according to one embodiment. [Figure 2a] 1 is a photograph showing the grain size of stainless steel produced at a cold rolling annealing temperature of 750°C. [Figure 2b] 1 is a photograph showing the grain size of stainless steel produced at a cold rolling annealing temperature of 800°C. [Figure 2c] 1 is a photograph showing the grain size of stainless steel produced at a cold rolling annealing temperature of 850°C. [Figure 2d] 1 is a photograph showing the grain size of stainless steel produced at a cold rolling annealing temperature of 900°C. [Figure 2e] 1 is a photograph showing the grain size of stainless steel produced at a cold rolling annealing temperature of 950°C. DETAILED DESCRIPTION OF THE INVENTION

[0021] According to one embodiment, the ferritic stainless steel with improved workability and ridging resistance is composed of, in weight percent, carbon (C): 0.0005 to 0.02%, nitrogen (N): 0.01 to 0.2%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 0.01 to 1.0%, phosphorus (P): 0.001 to 0.05%, chromium (Cr): 13.0 to 20.0%, titanium (Ti): 0.05 to 0.2%, with the remainder being iron (Fe) and unavoidable impurities, and satisfies the following formula (1), and the grain size can satisfy the range of 20 to 25 μm.

[0022] Formula (1): 2*[Ti] / [N] ≦ 2.3

[0023] In the above formula (1), [Ti] and [N] represent the weight percent content of each element.

[0024] The following describes preferred embodiments of the present invention. However, the embodiments of the present invention can be modified into various other forms, 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 completely explain the present invention to those with average knowledge in the art.

[0025] The terms used in this application are merely used to describe specific examples. Thus, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Note that the terms "comprise" or "include" 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 intended to preclude the presence of other features, steps, functions, components, or combinations thereof.

[0026] On the other hand, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Therefore, unless clearly defined herein, specific terms should not be interpreted in an overly ideal or formal sense. For example, singular expressions in this specification include plural expressions unless there is a clear exception in the context.

[0027] Furthermore, in this specification, the terms "about," "substantially," and the like are used to mean from or near a numerical value when manufacturing and material tolerances inherent in the stated meaning are given, and are used to prevent unscrupulous infringers from unfairly taking advantage of disclosures in which precise or absolute numerical values ​​are stated to aid in the understanding of the present invention.

[0028] A ferritic stainless steel with improved workability and ridging resistance according to one embodiment of the present invention is composed, by weight, of carbon (C): 0.0005 to 0.02%, nitrogen (N): 0.01 to 0.2%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 0.01 to 1.0%, phosphorus (P): 0.001 to 0.05%, chromium (Cr): 13.0 to 20.0%, titanium (Ti): 0.05 to 0.2%, and the remainder being iron (Fe) and unavoidable impurities.

[0029] The reasons for limiting the range of each alloy element will be explained below. Unless otherwise specified, the units are % by weight.

[0030] The carbon (C) content may be 0.0005 to 0.02 wt%, preferably 0.01 to 0.02 wt%.

[0031] Carbon (C) is an element that significantly affects the strength of steel. Even if the carbon content is less than 0.0005 wt%, the refining cost of producing high-purity products can be high. However, if the carbon content exceeds 0.02 wt%, corrosion resistance and formability can be reduced.

[0032] The nitrogen (N) content may be 0.01 to 0.2 wt%, preferably 0.04 to 0.2 wt%.

[0033] Nitrogen exists in an interstitial form as a nitride-forming element, and excessive nitrogen content can lead to a decrease in impact toughness and formability. Taking this into consideration, the upper limit of the nitrogen content is set at 0.2 wt%. However, if the nitrogen (N) content is too low, TiN crystallization may be reduced, resulting in a low equiaxed crystal fraction of the slab.

[0034] The content of silicon (Si) may be 0.01 to 1.0 wt%, preferably 0.05 to 0.60 wt%.

[0035] Silicon can be added to deoxidize molten steel during steelmaking and is an effective element for stabilizing ferrite. If the silicon content is less than 0.01 wt%, the refining cost becomes high. However, if the silicon content exceeds 1.0 wt%, the number of impurities increases and formability decreases.

[0036] The manganese (Mn) content may be 0.01 to 1.0 wt%, preferably 0.20 to 0.95 wt%.

[0037] Mn is an element that is effective in improving corrosion resistance. If the amount of manganese (Mn) is less than 0.01 wt%, the refining cost becomes high, while if it exceeds 1.0 wt%, the amount of impurities increases, resulting in a decrease in formability.

[0038] The phosphorus (P) content may be 0.001 to 0.05 wt%, preferably 0.001 to 0.020 wt%.

[0039] If the amount of phosphorus (P) is less than 0.001 wt%, the refining cost will be high, but if the amount of phosphorus exceeds 0.05 wt%, the amount of impurities will increase and the formability will decrease.

[0040] The chromium (Cr) content may be 13.0 to 20.0 wt%, preferably 13.5 to 17.5 wt%.

[0041] Cr is an effective element for ensuring the corrosion resistance of steel. If the chromium content is less than 13.0 wt%, corrosion resistance will be impaired. However, if the chromium content exceeds 20.0 wt%, formability will be impaired.

[0042] The content of titanium (Ti) may be 0.05 to 0.2 wt%, preferably 0.05 to 0.17 wt%.

[0043] Ti is an element that can preferentially combine with interstitial elements such as C and N to form precipitates. If the amount of titanium (Ti) is less than 0.05 wt%, it may be difficult to form Ti-based inclusions during manufacturing. If the titanium content is excessive, the Ti component may react with oxygen, causing surface defects that turn yellow.

[0044] The remaining component of the ferritic stainless steel according to the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may inevitably be mixed in from the raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in normal manufacturing processes, not all of the details thereof will be specifically mentioned in this specification.

[0045] Moreover, the ferritic stainless steel according to one embodiment can satisfy the following formula (1).

[0046] Formula (1): 2*[Ti] / [N] ≦ 2.3

[0047] In the above formula (1), [Ti] and [N] represent the weight percent content of each element.

[0048] In the present invention, in order to improve the ridging resistance of stainless steel, a fine cast structure is obtained by forming TiN precipitates by adjusting the contents of Ti and N. When the value of 2*[Ti] / [N] in equation (1), calculated based on the alloy components, exceeds 2.3, there is not enough N to bond with Ti, and the effect of refining the cast structure cannot be achieved.

[0049] The value of 2*[Ti] / [N] in the above formula (1) may be specifically 0.5 to 2.3, more specifically 1.0 to 2.3, and even more specifically 1.7 to 2.3. Within the above range, the ferritic stainless steel according to an embodiment of the present invention can have a further improved effect of controlling the microstructure and can have better workability and ridging resistance.

[0050] The ferritic stainless steel according to one embodiment may have a ridging height of 10 μm or less, measured after being stretched 15% in a direction perpendicular to the rolling direction. The ferritic stainless steel according to one embodiment may also have an elongation of 32% or more, preferably an elongation of a cold-rolled annealed material having a thickness of about 0.4 to 0.6 mm of 32% or more. The higher the elongation, the more improved the workability.

[0051] For the workability of cold-rolled stainless steel products used in home appliances, both ridging resistance and elongation must be satisfied. Because the steel must be highly elongated during forming, a minimum elongation of 32% is required. However, to minimize the occurrence of streaks after processing and achieve the desired gloss, it is preferable to limit the ridging height to approximately 10 μm or less, preferably 8.5 μm or less.

[0052] The ferritic stainless steel according to one embodiment can satisfy the grain size range of 20 to 25 μm, preferably 20 to 23 μm.

[0053] If the average crystal grain size of the stainless steel is limited to 20 to 25 μm, the desired elongation rate and ridging height can be simultaneously satisfied by refining the cast structure.

[0054] Next, a method for producing a ferritic stainless steel having improved ridging resistance according to another embodiment of the present invention will be described.

[0055] A method for producing a ferritic stainless steel having improved workability and ridging resistance according to one aspect of the present invention includes the steps of producing a slab consisting of, by weight, 0.0005 to 0.02% carbon (C), 0.01 to 0.2% nitrogen (N), 0.01 to 1.0% silicon (Si), 0.01 to 1.0% manganese (Mn), 0.001 to 0.05% phosphorus (P), 13.0 to 20.0% chromium (Cr), 0.05 to 0.2% titanium (Ti), and the remainder being iron (Fe) and unavoidable impurities, and satisfying the following formula (1): reheating the slab; hot-rolling the reheated slab followed by hot-rolling annealing; and cold-rolling the hot-rolled material followed by cold-rolling annealing.

[0056] Formula (1): 2*[Ti] / [N] ≦ 2.3

[0057] In the above formula (1), [Ti] and [N] represent the weight percent content of each element.

[0058] The reasons for limiting the ranges of the numerical values ​​of the components of each alloy composition and formula (1) are as described above, and each manufacturing step will be described in more detail below.

[0059] First, a slab satisfying the above alloy composition is produced, and then the slab can undergo a series of steps including reheating, hot rolling, hot rolling annealing, cold rolling, and cold rolling annealing.

[0060] First, the slab is heated to a temperature of 1,000 to 1,300° C. in a hot rolling furnace, and then hot rolled to produce a hot-rolled steel sheet.

[0061] If the heating temperature is too low, it may be difficult to redissolve the coarse precipitates formed during slab production. Taking this into consideration, the heating temperature can be set to 1,000°C or higher. However, if the heating temperature is too high, the internal crystal grains may become too coarse, which may cause severe surface oxidation and lead to surface defects. Taking this into consideration, the upper limit of the heating temperature can be limited to 1,300°C.

[0062] In the above hot rolling, the finish rolling can be carried out at 700 to 900°C.

[0063] If the finish rolling temperature is less than 700°C, sticking may occur on the surface of the slab during hot rolling. However, if the finish rolling temperature exceeds 900°C, coarse ferrite grains may form, which may reduce ridging resistance.

[0064] The hot-rolled material can be subjected to hot rolling annealing to recrystallize the cast structure, which can be carried out at a temperature of 700 to 900°C.

[0065] If the hot rolling annealing temperature is low, the stress formed during hot rolling may not be sufficiently removed, and workability may be reduced. However, if the hot rolling annealing temperature is too high, the strength may be reduced due to coarsening of the crystal grains, and the ridging resistance may be reduced.

[0066] The hot-rolled and annealed hot-rolled material can be cold-rolled and then cold-annealed to produce a cold-rolled steel sheet. In this case, the cold-rolling and annealing can be performed at a temperature of 800 to 850°C.

[0067] By satisfying the cold rolling annealing temperature, the grain size can be controlled to 20 to 25 μm. If the cold rolling annealing temperature exceeds 850°C, the grain size becomes coarse, resulting in the growth of a {001} / / ND crystal orientation structure, which reduces ridging resistance. The growth of a {001} / / ND crystal orientation structure increases plastic anisotropy with the matrix, not only reducing ridging resistance but also potentially resulting in poor surface roughness. However, if the cold rolling annealing temperature is less than 800°C, recrystallization does not occur, potentially resulting in poor elongation and ridging resistance.

[0068] As described above, by optimizing the alloying elements and elemental relationships as well as the reheating, hot rolling annealing, and cold rolling annealing processes, it is possible to realize grain refinement in the cast structure and ensure the ridging resistance and elongation rate of ferritic stainless steel.

[0069] The stainless steel produced according to one embodiment can have a grain size in the range of 20 to 25 μm. By satisfying the grain size range, it is possible to obtain a stainless steel with excellent workability and desired workability and ridging resistance.

[0070] The stainless steel produced according to one embodiment may have a ridging height of 10 μm or less, measured after being stretched 15% in a direction perpendicular to the rolling direction. By satisfying the above range of ridging height, it is possible to obtain stainless steel with a desired glossiness and with little occurrence of streaks after processing.

[0071] The stainless steel manufactured according to one embodiment may have an elongation ratio of 32% or more. By satisfying this range of elongation ratio, the steel material can be stretched well during forming, resulting in a stainless steel with excellent formability.

[0072] The present invention will be described in more detail below with reference to the following embodiments. However, the following embodiments are intended to more specifically describe the present invention, and the scope of the present invention is not limited to the following embodiments.

[0073] [Example] Slabs were produced in a vacuum induction melting furnace for various alloy composition ranges shown in Table 1 below. The produced slabs were reheated in a heating furnace at 1,100°C, then hot-rolled to produce hot-rolled steel sheets, which were then air-cooled. The air-cooled hot-rolled steel sheets were subjected to hot-rolling annealing at 850°C, cold-rolled to a thickness of 0.5 mm, and then cold-rolled annealing at the temperatures shown in Table 2 below to produce cold-rolled steel sheet test pieces.

[0074] Moreover, the values ​​of the following formula (1) are calculated and shown in Table 1 below.

[0075] Formula (1): 2*[Ti] / [N]

[0076] In the above formula (1), [Ti] and [N] represent the weight percent content of each element.

[0077] [Table 1]

[0078] Table 2 below shows the average grain size in the cast structure for each cold-rolling annealing temperature of steel having the alloy composition shown in Table 1, as well as the ridging height and elongation measured after 15% stretching. Figure 1 also shows the correlation between the ridging height and elongation for each cold-rolling annealing temperature of steel having the alloy composition of Steel 3. The grain size was measured by photographing the cast structure using an optical microscope (OM). Figures 2a to 2e are photographs of the grain size of stainless steels obtained by cold-rolling annealing steel having the alloy composition of Steel 3 at temperatures of 750°C, 800°C, 850°C, 900°C, and 950°C, respectively. The ridging height was measured by stretching the specimen by 15% in a direction perpendicular to the rolling direction of the specimen, and then measuring the ridging bending height using a surface roughness tester.

[0079] The elongation ratio was calculated by dividing the amount of elongation until breakage occurred when the stainless steel cold-rolled product was uniaxially stretched by the initial length.

[0080] [Table 2]

[0081] Referring to Tables 1 and 2, Steels 3, 5, 6, and 9 satisfy the alloy composition, element range, and formula (1) proposed by the present invention. Furthermore, when these satisfy the cold rolling annealing temperature, the grain size in the cast structure is refined to 20-25 μm (see FIGS. 2b and 2c), and the ridging height is 10 μm or less, while the elongation is 32% or more. In other words, it can be seen that the inventive examples, which combine the alloy composition, element range, formula (1), and cold rolling annealing temperature, exhibit excellent workability and ridging resistance. However, the comparative examples, which satisfy the alloy composition, element range, and formula (1) proposed by the present invention but do not satisfy the cold rolling annealing temperature, fail to satisfy one or more of the physical properties of grain size, ridging height, or elongation.

[0082] Specifically, referring to FIGS. 1 and 2a, when the cold rolling annealing temperature was less than 800° C., no recrystallization was formed, the ridging height exceeded 10 μm, and the elongation ratio was significantly reduced.

[0083] 1, 2d, and 2e, when the cold rolling annealing temperature exceeded 850°C, the elongation improved, but the crystal grains in the cast structure became coarse, resulting in a ridging height exceeding 10 μm. In other words, it can be seen that as the cold rolling annealing temperature increases, the elongation improves, but the cast structure becomes coarser, which tends to increase the ridging height.

[0084] In addition, in Steels 1, 2, 4, 7, and 8, the value of formula (1) exceeded 2.3, and even though the cold rolling annealing temperature was 800 to 850°C, the content of N that can bond with Ti was low, and refinement of equiaxed crystal grains could not be realized. This confirmed that the ridging resistance was also inferior.

[0085] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it will be understood by those skilled in the art that various changes and modifications can be made without departing from the concept and scope of the claims set forth below. [Industrial Applicability]

[0086] According to the present invention, in order to improve the workability of cold-rolled products, the steel composition and manufacturing process are optimized to control the grain size of the final cold-rolled annealed material, thereby providing a ferritic stainless steel with improved workability and ridging, and a manufacturing method thereof, which is therefore recognized as having industrial applicability.

Claims

1. In weight percent, carbon (C): 0.0005 to 0.02%, nitrogen (N): 0.01 to 0.2%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 0.01 to 1.0%, phosphorus (P): 0.001 to 0.05%, chromium (Cr): 13.0 to 20.0%, titanium (Ti): 0.05 to 0.2%, and the remainder being iron (Fe) and unavoidable impurities, The following formula (1) is satisfied: A ferritic stainless steel having improved workability and ridging resistance, characterized in that the grain size satisfies the range of 20 to 25 μm. Formula (1): 2*[Ti] / [N] ≦ 2.3 In the above formula (1), [Ti] and [N] represent the weight percent contents of each element.

2. 2. The ferritic stainless steel having improved workability and ridging resistance according to claim 1, wherein the ridging height measured after 15% tension is 10 μm or less.

3. 2. The ferritic stainless steel having improved workability and ridging resistance according to claim 1, wherein the elongation is 32% or more.

4. A step of producing a slab consisting of, in weight percent, carbon (C): 0.0005 to 0.02%, nitrogen (N): 0.01 to 0.2%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 0.01 to 1.0%, phosphorus (P): 0.001 to 0.05%, chromium (Cr): 13.0 to 20.0%, titanium (Ti): 0.05 to 0.2%, the remainder being iron (Fe) and unavoidable impurities, and satisfying the following formula (1): reheating the slab; hot rolling and then hot annealing the reheated slab; and cold rolling the hot-rolled material after hot-rolling and annealing, and then cold-rolling and annealing at 800 to 850°C; 1. A method for producing a ferritic stainless steel having improved workability and ridging resistance, comprising: Formula (1): 2*[Ti] / [N] ≦ 2.3 In the above formula (1), [Ti] and [N] represent the weight percent contents of each element.

5. 5. The method of claim 4, wherein the reheating step is performed at a temperature of 1,000 to 1,300°C.

6. 5. The method for producing a ferritic stainless steel having improved workability and ridging resistance according to claim 4, wherein the stainless steel has a crystal grain size in the range of 20 to 25 μm.

7. 5. The method for producing a ferritic stainless steel having improved workability and ridging resistance according to claim 4, wherein the stainless steel has a ridging height of 10 μm or less as measured after being stretched by 15%.

8. 5. The method for producing a ferritic stainless steel having improved workability and ridging resistance according to claim 4, wherein the stainless steel has an elongation rate of 32% or more.

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