Ferritic stainless steel sheet

Abstract

To provide a ferritic stainless steel sheet excellent in ridging resistance.SOLUTION: A ferritic stainless steel having a predetermined composition and satisfying formula (1) is provided. In the ferritic stainless steel, a composite inclusion having an oxide and a major axis of 1.0 μm or more is defined as composite inclusion A, and among the composite inclusion A, a composite inclusion in which the oxide satisfies formulas (2) to (4) in mass percentage is defined as composite inclusion B, the number ratio of the number of composite inclusion A to the number of composite inclusion B satisfies formula (5), the number density of composite inclusion C, having a major axis of 2.0 to 15.0 μm among the composite inclusion B, is 2.0 to 20 pieces / mm2, and the number density of sulfide-oxide D having a circle-equivalent diameter of 5.0 μm or more and containing S of 5% or more among the composite inclusion A is 0.50 pieces / mm2 or less. 70≥(0.50×[%C]+0.20×[%N]+0.65×[%P]+3.0×[%S])×1000+6×[%Si]+20×[%Ti] +30×[%Nb]≥20 Formula (1) Al2O3 / MgO≤4 Formula (2)CaO+CaS≤20% Formula (3) Al2O3+MgO≥75% Formula (4) Number of composite inclusion B / Number of composite inclusion A≥0.70 Formula (5).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet, and more particularly to a ferritic stainless steel sheet having excellent ridging resistance. [Background technology]

[0002] Ferritic stainless steel has high corrosion resistance and heat resistance, and because it contains less Ni than austenitic stainless steel and is therefore less expensive, it is widely used in building materials, commercial kitchens, automotive exhaust system parts, etc. However, after processing, the steel sheet surface is prone to irregularities (ridging), which not only requires a grinding process to maintain the surface appearance, but can also cause cracks if the irregularities are significant.

[0003] To reduce ridging, it is effective to make steel sheet with less anisotropy by refining the crystal grains. However, ferritic stainless steel does not produce an austenite phase at high temperatures, so grain refinement by transformation cannot be utilized during manufacturing, and the effects of the coarse cast structure tend to remain. Therefore, it is necessary to reduce the columnar grain diameter during casting and increase the equiaxed crystal ratio.

[0004] For example, a well-known method is to actively utilize inclusions as nuclei for the formation of δ-Fe, the primary crystal during solidification, to increase the equiaxed crystal ratio. Mg-Al oxides such as spinel (MgO·Al2O3) and TiN are known to have crystal lattice constants close to those of δ-Fe, and spinel is also considered advantageous for the formation of δ-Fe because it promotes the formation of TiN.

[0005] Although the inclusion of Ti is advantageous from the viewpoint of rust resistance in that it generates Ti(C,S), which is resistant to rust, there are concerns that it may cause nozzle clogging during casting, reduce productivity due to reduced toughness of the hot-rolled sheet caused by coarse TiN, and even if TiN is generated finely, it may cluster and cause surface defects in the steel sheet.

[0006] Patent Document 1 is characterized by containing 4(C+N) to 0.40% of Ti and setting the Mg / Al mass ratio in inclusions to 0.55 or more to refine the solidification structure, and also by setting V×N to 0.0005 to 0.0015 to promote recrystallization during hot rolling.

[0007] Patent Document 2 discloses that even when the Ti and N contents are small, the number of Mg-Al oxides dispersed in molten steel is 20 to 1000 particles / mm 2 It has been shown that fine equiaxed grains can be generated during casting by controlling the grain size.

[0008] In Patent Document 3, in order to solve the problem that the solidification structure is not refined when the Mg-containing oxide contains Ca, the Mg-containing oxide with an Mg / Ca ratio of 0.5 or more is set to 3 particles / mm 2 The presence of more than one

[0009] Patent Document 4 suggests that it is possible to promote the generation of equiaxed crystals using Mg-Al-based oxides or TiN containing Mg-Al-based oxides by controlling the composition, constituent ratio, and number density of oxides and by lowering the solidus temperature to ensure a solidification temperature range. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-285717 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-194505 [Patent Document 3] Japanese Patent Application Laid-Open No. 2001-288542 [Patent Document 4] International Publication No. 2019 / 189858 Summary of the Invention [Problem to be solved by the invention]

[0011] However, there are problems that cannot be solved by the above techniques. In the technology of Patent Document 1, the Ti content is relatively high at 0.2% or more, making it difficult to stably suppress nozzle clogging caused by TiN. In addition, only the Mg / Al ratio in the Mg-Al-based inclusions is controlled, which may not necessarily lead to a reduction in ridging.

[0012] The technology of Patent Document 2 requires a reduced CaO concentration to obtain the effect of promoting the formation of δ-Fe by Mg-Al inclusions. This is because, if the CaO concentration is high, the effect is not exhibited, and CaO cannot become the nucleus of δ-Fe or TiN, and refinement of equiaxed grains may not be achieved.

[0013] The technology of Patent Document 3 does not contribute to refining the solidification structure when the Mg / Al ratio is low (high Al2O3 corundum is formed) even if the oxide has a Mg / Ca ratio of 0.5 or more, and may not be able to reduce ridging.

[0014] Although the technology of Patent Document 4 may produce a steel sheet with excellent ridging resistance, it is difficult to stably reduce surface defects caused by TiN, and the grinding step of the product may not be omitted.

[0015] The present invention addresses the problem of improving ridging resistance by refining the cast structure using mainly Mg-Al oxides, while reducing sulfides that act as rust initiation sites, thereby ensuring corrosion resistance in ferritic stainless steel that utilizes Nb to reduce the C and N content in the steel, and aims to consistently provide ferritic stainless steel sheets with excellent ridging resistance. [Means for solving the problem]

[0016] The present inventors conducted a detailed investigation into the factors that affect the surface properties and corrosion resistance of Nb-containing ferritic stainless steel sheets manufactured by various methods, and clarified the relationship between the state of complex inclusions, the composition and constituent ratio of oxides contained in the complex inclusions, and the ridging properties.

[0017] In this specification, the term "composite inclusions" refers to so-called inclusions. For example, when a nitride surrounds an oxide, the size of the inclusion refers to the size of the inclusion including the nitride. Furthermore, sulfides are often formed around or inside oxides, making it difficult to distinguish between them. Therefore, when sulfur is contained in an amount of 5% or more, it is referred to as an oxysulfide.

[0018] That is, the composition of the composite inclusions must satisfy the following requirements: the ratio of Al2O3 to MgO (Al2O3 / MgO) must be 4 or less, CaO + CaS must be 20% or less, and the sum of Al2O3 and MgO must be 75% or more; and the number of composite inclusions with a major axis of 2.0 μm or more must be 2.0 pieces / mm 2 It has been found that, for inclusions present at a density of 1 μm or more and with a major axis of 1 μm or more, by making the ratio of the number of inclusions that satisfy the above-mentioned composition of composite inclusions to the number of inclusions that do not satisfy the above-mentioned composition 0.70 or more, ridging resistance is improved.

[0019] In addition, the number density of oxysulfides with a circle equivalent diameter of 5.0 μm or more and containing 5% or more of S is 0.50 particles / mm 2 It has been found that corrosion resistance can be improved by the following.

[0020] The present invention was made based on the above findings, and the gist of the present invention is as follows.

[0021] [1] In mass%, C: 0.001 to 0.05%, Si: 1.0% or less, Mn: 0.02 to 1.0%, P: 0.005~0.040%, S: 0.005% or less, Cr: 11-21%, Al: 0.01 to 0.2%, Ti: 0.015% or less, Nb: 0.1 to 0.6%, O: 0.0005 to 0.005%, N: 0.001 to 0.05%, Ca: 0.0015% or less, Mg: 0.0003% to 0.0030%, B: 0~0.002%, Mo: 0-2.5% Ni: 0 to 2.0% Cu: 0-2.0% Co: 0-1.0% Sn: 0 to 0.5% W: 0-1.0%, Ta: 0 to 0.10%, Sb: 0 to 0.30% the balance being Fe and impurities, Formula (1) is satisfied, and It contains oxide-containing composite inclusions A with a major axis of 1.0 μm or more, When composite inclusions B are composite inclusions A that satisfy formulas (2) to (4), the ratio of the number of composite inclusions A to the number of composite inclusions B satisfies formula (5), and the number density of composite inclusions C among composite inclusions B, which have a major axis of 2.0 to 15.0 μm, is 2.0 to 20 pieces / mm 2 and Among the composite inclusions A, the number density of oxysulfides D having a circle equivalent diameter of 5.0 μm or more and containing 5% or more of S is 0.50 pieces / mm 2 A ferritic stainless steel sheet characterized by the following: 70≧(0.50×[%C]+0.20×[%N]+0.65×[%P]+3.0×[%S])×1000+6×[%Si]+20×[%Ti]+30×[%Nb]≧20 ··· Equation (1) Al2O3 / MgO≦4 ··· Formula (2) CaO+CaS≦20% ··· Formula (3) Al2O3+MgO≧75% ··· Formula (4) Number of composite inclusions B / Number of composite inclusions A ≥ 0.70 Equation (5) However, [%C] and the like in formula (1) indicate the content (mass%) of each element in the steel sheet, and Al2O3, MgO, CaO, and CaS in formulas (2) to (4) indicate the mass% of each element in the complex inclusion A. [2] Furthermore, in mass%, B: 0.0001~0.002%, Mo: 0.1 to 2.5% Ni: 0.1 to 2.0% Cu: 0.1 to 2.0% Co: 0.05 to 1.0%, Sn: 0.01 to 0.5% W: 0.05 to 1.0%, Ta: 0.001 to 0.10%, The ferritic stainless steel sheet according to [1], characterized in that it contains one or more of Sb: 0.01 to 0.30%. [Effects of the Invention]

[0022] According to the present invention, it is possible to stably provide steel sheets made of ferritic stainless steel that are excellent in ridging resistance while also ensuring corrosion resistance. DETAILED DESCRIPTION OF THE INVENTION

[0023] An embodiment of the present invention (hereinafter simply referred to as the present invention) will be described below. Unless otherwise specified, "%" for components indicates mass% in the steel. When no lower limit is specified or when the lower limit is 0%, this includes the case where no component is contained (0%).

[0024] As mentioned above, a method for finely graining equiaxed crystals in cast slabs is known to disperse Mg-Al inclusions and TiN. Although the inclusion of Ti is advantageous in that it generates Ti(C,S), which is resistant to rust, the clustered TiN can cause surface defects, making it impossible to omit the grinding process.

[0025] In ordinary Nb-containing ferritic stainless steel sheets, carbonitrides do not form during solidification, and Nb itself lowers the solidus temperature, lengthening the solidification time and promoting the formation of δ-Fe during solidification. On the other hand, the growth of inclusions is also promoted, so further investigation was conducted to maximize the use of Mg-Al inclusions to improve the equiaxed crystal ratio, and to minimize the formation of CaS, which forms at high temperatures from casting to hot rolling, to improve corrosion resistance.

[0026] In Nb-containing ferritic stainless steels, the relationship between ridging resistance and corrosion resistance was clarified, along with the concentration of O and S, which indicate various deoxidizing elements and the state of deoxidation, the area ratio of CaS, the state of existence of complex inclusions, and the composition and composition ratio of oxides contained in the complex inclusions.

[0027] <About steel composition> The range of components that can be suitably used is shown below.

[0028] C: 0.001 to 0.05% C reduces corrosion resistance by forming Cr carbides, so it is set to 0.05% or less, preferably 0.03% or less, or 0.01% or less. On the other hand, C is set to 0.001% or more to lower the solidus temperature and facilitate the formation of equiaxed crystals, but an excessive decrease increases the refining load, so it is preferably set to 0.002% or more.

[0029] Si: 1.0% or less Excessive Si content reduces workability, so the Si content is set to 1.0% or less. To ensure toughness, the Si content is preferably 0.5% or less, or 0.3% or less. Although there is no lower limit, the Si content is preferably 0.01% or more to efficiently reduce and recover Cr oxides produced during stainless steel decarburization. To ensure oxidation resistance, the Si content is more preferably 0.05% or more.

[0030] Mn: 0.02 to 1.0% Like Si, excessive Mn content reduces workability, so it is set to 1.0% or less. To improve corrosion resistance by forming fine MnS, the content is preferably 0.5% or less, or 0.3% or less. On the other hand, excessive Mn content prevents the formation of (Mn,Cr)S (sulfides of Mn or Cr, or their composite sulfides), and promotes the formation of CaS, reducing corrosion resistance, so the content is set to 0.02% or more. Because Mn is an element that contributes to deoxidation, the content is preferably 0.03% or more, or 0.05% or more.

[0031] P: 0.005 to 0.040% P significantly reduces corrosion resistance and toughness, so its content is set to 0.040% or less. To ensure hot workability, it is preferably set to 0.030% or less. On the other hand, it lowers the solidus temperature and facilitates the formation of equiaxed crystals, so it is set to 0.005% or more. P is an element contained as an impurity in main raw materials such as ferrochromium, and excessive reduction in its content leads to increased costs, so it is preferably set to 0.010% or more.

[0032] S: 0.005% or less S is harmful to stainless steel because it generates sulfides and reduces corrosion resistance. The lower the upper limit of its content, the better, and the upper limit is set to 0.005%, preferably 0.004% or less. Although there is no lower limit, it is preferably set to 0.0003% or more in order to lower the solidus temperature and facilitate the formation of equiaxed crystals. An excessive decrease in S content increases the load during refining, leading to increased costs, so it is more preferably 0.001% or more.

[0033] Cr: 11~21% Cr is an important element that provides corrosion resistance to stainless steel, and its content should be 11% or more. Furthermore, Cr, together with Mn, fixes S and suppresses CaS, so its content should preferably be 13% or more. On the other hand, to prevent the formation of coarse carbides containing Cr, its content should be 21% or less, and preferably 19% or less.

[0034] Al: 0.01% to 0.2% Al is an element for deoxidation and desulfurization, and is set to 0.01% or more. To stably desulfurize and reduce sulfides that affect corrosion resistance, 0.02% or more, 0.03% or more, or 0.035% or more is preferred. However, since excessive content reduces workability, it should be set to 0.20% or less, preferably 0.15% or less.

[0035] Ti: 0.015% or less Ti is an element that causes surface defects in products due to TiN clusters, so its content is set to 0.015% or less. It is preferable to set it to 0.012% or less in order to minimize TiN and promote the formation of equiaxed crystals by lowering the solidus temperature of N. While there is no lower limit, if Ti itself is contained in molten steel, it lowers the solidus temperature, so its content is preferably set to 0.001% or more. Ti is an element that forms fine carbonitrides, thereby suppressing the deterioration of surface properties, sensitization, and a decrease in corrosion resistance due to the precipitation of chromium mononitrides, so its content is more preferably set to 0.002% or more.

[0036] Nb: 0.1 to 0.6% Nb is an element that lowers the solidus temperature, facilitates the formation of equiaxed crystals, and suppresses the deterioration of corrosion resistance by refining carbonitrides, so it is advisable to set the Nb content to 0.1% or more, preferably 0.15% or more. However, since excessive Nb content increases the recrystallization temperature of the steel sheet, resulting in a decrease in workability, the Nb content is set to 0.6% or less, preferably 0.54% or less.

[0037] O: 0.0005 to 0.005% The O content is set to 0.0005% or more to obtain the effect of promoting the formation of δ-Fe by Mg-Al inclusions. To form many equiaxed grains and refine the structure, the O content is preferably 0.0010% or more. However, if the O content is excessive, lower oxides of Mn, Cr, etc. other than Mg-Al inclusions are formed, which makes it impossible to promote the formation of δ-Fe. Therefore, the O content is set to 0.005% or less, preferably 0.0045% or less.

[0038] N: 0.001 to 0.05% N has the same effect as C, and reduces corrosion resistance by forming Cr nitrides, so its content should be 0.05% or less, preferably 0.01% or less. On the other hand, N lowers the solidus temperature and facilitates the formation of equiaxed crystals, so its content should be 0.001% or more. Excessive lowering of N increases the refining load, so its content should preferably be 0.002% or more.

[0039] Ca: 0.0015% or less Ca is an element effective for deoxidation and desulfurization, but when it is formed as an oxide, it inhibits the formation of δ-Fe by Mg-Al inclusions, and if contained in excess, it forms CaS, so the content is set to 0.0015% or less, preferably 0.0010% or less. Although there is no specified lower limit, an excessive decrease increases the refining load, so the content is preferably 0.0001% or more, and more preferably 0.0002% or more.

[0040] Mg: 0.0003 to 0.0030% Mg is an element effective for deoxidation and desulfurization, and promotes the formation of δ-Fe by forming Mg-Al inclusions, and further generates more equiaxed crystals to refine the structure. Therefore, the content should be 0.0003% or more, preferably 0.0006% or more. On the other hand, excessive Mg content forms sulfides that inhibit the formation of equiaxed crystals due to a lowering of the solidus of S, so the content should be 0.0030% or less, preferably 0.0027% or less.

[0041] Furthermore, one or more of B, Mo, Ni, Cu, Co, Sn, W, Ta, and Sb may be contained. Although these elements do not necessarily have to be contained, further effects can be obtained by containing them. These elements will be explained below.

[0042] B: 0.002% or less Since excessive B content leads to a decrease in hot workability, it should be 0.002% or less, preferably 0.0014% or less. On the other hand, B contributes to improving workability, so it may be contained in an amount of 0.0001% or more, preferably 0.0004% or more.

[0043] Mo: 2.5% or less Mo should be 2.5% or less, preferably 1.5% or less, because high Cr forms a brittle sigma phase, which deteriorates manufacturability. On the other hand, Mo not only has the effect of improving the corrosion resistance of sensitized parts, but also has the effect of finely dispersing precipitates by dissolving Mo in carbonitrides, so the content is preferably 0.1% or more, or 0.2% or more.

[0044] Ni: 2.0% or less Since a large amount of Ni may reduce formability, increase alloy costs, and hinder manufacturability, it is recommended that the Ni content be 2.0% or less, and preferably 1.5% or less. On the other hand, Ni is an element that is effective in improving the corrosion resistance of stainless steel by suppressing the progression of localized corrosion, so the Ni content is preferably 0.1% or 0.5% or more.

[0045] Cu:2.0% or less Since excessive Cu content degrades manufacturability by reducing hot workability, etc., it is recommended that the Cu content be 2.0% or less, and preferably 1.5% or less. On the other hand, Cu is an element that is effective in improving the corrosion resistance of stainless steel by suppressing the progression of localized corrosion, so it is recommended that the Cu content be 0.1% or more, or 0.5% or more.

[0046] Co: 1.0% or less Since excessive Co content leads to a decrease in workability, the upper limit should be set to 1.0%, preferably 0.5% or less. On the other hand, Co has the effect of increasing the strength of the steel sheet, so the Co content should preferably be 0.05% or more, or 0.1% or more.

[0047] Sn: 0.5% or less Since excessive Sn content leads to a decrease in workability, it is recommended that the content be 0.5% or less, and preferably 0.1% or less. On the other hand, Sn is an element that is effective in increasing the corrosion resistance of stainless steel, so the content should preferably be 0.01% or more, or 0.02% or more.

[0048] W: 1.0% or less Since excessive W content reduces toughness, the upper limit should be set at 1.0%. On the other hand, W has the effect of improving corrosion resistance, especially pitting corrosion resistance, so the content should preferably be 0.05% or more.

[0049] Ta: 0.10% or less Excessive Ta content reduces room temperature ductility and toughness, so it is recommended that the Ta content be 0.10% or less. On the other hand, Ta is an effective element for deoxidation and desulfurization, so the Ta content is preferably 0.001% or more.

[0050] Sb: 0.30% or less Since excessive Sb content reduces manufacturability, the content should be 0.30% or less, whereas Sb has the effect of improving corrosion resistance, so the content should preferably be 0.01% or more.

[0051] The balance of the above steel components is Fe and unavoidable impurities. Here, the unavoidable impurities refer to components that are mixed in during industrial steel production due to various factors in the production process, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention.

[0052] [70≧(0.50×[%C]+0.20×[%N]+0.65×[%P]+3.0×[%S])×1000+6×[%Si]+20×[%Ti]+30×[%Nb]≧20] It was discovered that when the components in steel satisfy the condition of the following formula (1), δ-Fe formation occurs easily and the formed δ-Fe is difficult to redissolve. Furthermore, when the condition of formula (1) is satisfied, the coarsening of oxysulfides during solidification can be suppressed, and the occurrence of rust and surface defects can be suppressed. Therefore, it is possible to ensure excellent surface cleanliness in ferritic stainless steel sheets. 70≧(0.50×[%C]+0.20×[%N]+0.65×[%P]+3.0×[%S])×1000+6×[%Si]+20×[%Ti]+30×[%Nb]≧20 ··· Equation (1) However, [%C], [%Si], [%P], [%S], [%Ti], [%Nb], and [%N] indicate the mass percentage of each element in the steel, and if the element is not contained, enter 0.

[0053] <About compound inclusions> In this specification, composite inclusions containing oxides and having a major axis of 1.0 μm or more are referred to as composite inclusions A, and composite inclusions A whose oxides satisfy the following formulas (2) to (4) are referred to as composite inclusions B. Note that Al2O3, MgO, CaO, and CaS in formulas (2) to (4) represent the mass percentages of each of them in the composite inclusion.

[0054] <Composition of compound inclusions> [Al2O3 / MgO≦4.0] When Al2O3 / MgO = 4.0, it corresponds to a nearly pure spinel composition. Al2O3-MgO inclusions with compositions ranging from pure spinel to pure MgO are effective in promoting the formation of δ-Fe. The closer the composition is to pure MgO, the better the ability to form δ-Fe, so Al2O3 / MgO should be 4.0 or less (equation (2)). Desirably, Al2O3 / MgO is 3.0 or less, 2.0 or less, or 1.0 or less. Furthermore, under conditions where TiN forms, TiN is more likely to form if the composition is within the above range. Al2O3 / MgO≦4.0 ··· Formula (2)

[0055] [CaO+CaS≦20%] If the sum of the CaO and CaS concentrations in the composite inclusions is high, the melting point of the steel will be lowered and the inclusions will not solidify at the temperature at which δ-Fe solidifies. Alternatively, CaO and CaS have poor lattice matching with δ-Fe and TiN, so they do not act as nuclei for δ-Fe or TiN, preventing refinement of the solidification structure. Since a lower sum of the CaO and CaS concentrations promotes the formation of δ-Fe and TiN, the CaO + CaS content should be 20% or less (Equation (3)). Preferably, the CaO + CaS content should be 15% or less, and even more preferably, 10% or less. CaO+CaS≦20% ··· Formula (3)

[0056] [Al2O3+MgO≧75%] Al2O3 and MgO have good lattice matching with δ-Fe and TiN. In addition, if there are a lot of oxides other than CaO, Al2O3, and MgO, the melting point will be low or the crystal structure will change, so the sum of Al2O3 and MgO should be 75% or more (formula (4)), and preferably 80% or more, or 85% or more. Al2O3+MgO≧75% ··· Formula (4)

[0057] [Number of compound inclusions B / Number of compound inclusions A ≥ 0.70] Among composite inclusions A containing oxides and having a major axis of 1.0 μm or more, composite inclusions containing oxides that do not satisfy the conditions of formulas (2) to (4) inhibit composite inclusions B containing oxides that satisfy the conditions of formulas (2) to (4) from acting as nuclei for δ-Fe and TiN. In particular, if the ratio of the number of composite inclusions B to the number of composite inclusions A is small, composite inclusions B are less likely to act as nuclei for δ-Fe and TiN. Therefore, the ratio of the number of composite inclusions B to the number of composite inclusions A should be 0.70 (70%) or more (formula (5)), preferably 0.75 or more, or 0.80 or more. Number of composite inclusions B / Number of composite inclusions A ≥ 0.70 Equation (5)

[0058] [Number density of composite inclusions C with a major axis of 2.0 to 15.0 μm among composite inclusions B: 2.0 to 20.0 pieces / mm 2 ] Among the complex inclusions B, those with a maximum diameter of 2.0 μm or more are particularly likely to become solidification nuclei of δ-Fe. However, if the size exceeds 15.0 μm, it can cause surface defects, so the size is set to 15.0 μm or less. It is preferably 10.5 μm or less, and more preferably 5.0 μm or less. Note that the complex inclusions B here are particles in the steel containing oxides that satisfy the conditions of formulas (2) to (4), and may be in the form of carbonitrides such as Nb(C,N) or Ti(C,N) surrounding the oxides.

[0059] Among the composite inclusions B, composite inclusions C with major diameters of 2.0 to 15.0 μm were selected at a rate of 2.0 pieces / mm 2Dispersing them in the steel effectively acts as solidification nuclei, increasing the equiaxed crystal ratio and improving ridging resistance. On the other hand, the Al2O3-MgO oxides contained in the complex inclusions C, which have a major axis of 2.0 to 15.0 μm, have a high melting point and are hard, and if present in large quantities, they are likely to cause surface defects and cracks. Therefore, the number density of the complex inclusions C is set to 20 pieces / mm 2 or less, preferably 19 / mm 2 Below, 15 pieces / mm 2 or less, or 13 pieces / mm 2 The following would be appropriate.

[0060] (The number density of oxysulfides D, which have a circle equivalent diameter of 5.0 μm or more and contain 5% or more of S, among the composite inclusions A: 0.50 pieces / mm 2 below) If the S content is 5% or more, the S cannot be completely dissolved in the oxysulfides, and sulfides are formed around or inside them, which are prone to becoming rust initiation points. Furthermore, when the relationship between inclusions before the corrosion resistance test and the rusted areas after the test was investigated, it was found that there was a high probability that rust would originate from oxysulfides with a circle equivalent diameter of 5.0 μm or more, so oxysulfides with a circle equivalent diameter of 5.0 μm or more were targeted. Furthermore, since the presence of many coarse oxysulfides with a circle equivalent diameter of 5.0 μm or more increases the number of rust initiation points, the number density of these coarse oxysulfides was set at 0.50 particles / mm 2 The number of particles per mm should be less than 0.40, preferably 0.40 particles per mm 2 The following would be appropriate.

[0061] [Method for measuring inclusions] The following describes the method for measuring inclusions. A cross section of a steel sheet is observed, and at least 100 inclusions containing oxides and with a major axis of 1.0 μm or larger are randomly selected. This is used as a population. The inclusions in the population are analyzed using SEM-EDS to identify the size, type, and number of inclusions. The observed area is also recorded. For steel sheets, the above procedure is performed by observing a cross section perpendicular to the rolling direction. In the case of steel sheets, the inclusions observed at the time of observation are those that have been deformed by rolling, etc., and therefore, evaluation is often impossible based on the major axis in a cross section parallel to the rolling direction. On the other hand, since there is almost no deformation in the sheet width direction, the major axis of inclusions observed in a perpendicular cross section is thought to be approximately the same as the diameter of the inclusions at the time of solidification. Therefore, for steel sheets, a cross section perpendicular to the rolling direction is observed.

[0062] <Manufacturing method> The steel sheet of the present invention is manufactured by a method for manufacturing a normal ferritic stainless steel sheet. For example, in the case of a cold-rolled steel sheet, the hot-rolled steel sheet is manufactured through the steps of melting and casting, hot-rolling, annealing and pickling of the hot-rolled sheet, and then cold-rolling, annealing and pickling of the cold-rolled sheet.

[0063] When producing the above steel, deoxidation treatment with Al is performed in the early stages of secondary refining, reducing the O concentration in the molten steel to 0.006% or less. Subsequently, strong deoxidizing elements that easily form oxides, such as Si, Ti, Al, Ca, and Mg, are controlled by adding alloys and reducing the refining slag. This makes it possible to stably increase the amount and ratio of complex inclusions that satisfy the formula (4), Al2O3 + MgO ≥ 75%.

[0064] In particular, magnesium can be added in the form of metallic magnesium or alloys such as Ni-Mg. It can also be added indirectly by adding magnesium oxide (MgO) to the refining slag and reducing the magnesium from the slag to the molten steel. In this case, a high MgO activity in the slag is desirable. While this cannot be uniquely determined due to the relationship with other components, a ratio of approximately 0.7 based on pure solid MgO is generally desirable. This allows for a stable increase in the amount and ratio of complex inclusions that satisfy the relationship between Al2O3 / MgO ≤ 4 (as shown in equation (2)) and CaO ≤ 20% (as shown in equation (3)). Because it is difficult to measure the MgO activity during operation, it can be calculated by measuring the slag composition and comparing it with a thermodynamic data set or using general-purpose thermodynamic calculation software.

[0065] By performing the above-mentioned deoxidation treatment and controlling the activity of strong deoxidizing elements such as Si, Ti, Al, Ca, and Mg, and the activity of MgO in the slag, and by satisfying the conditions of formulas (2) to (4) and adjusting the composition to a predetermined level, it is possible to satisfy the relationship of the number of composite inclusions B / the number of composite inclusions A ≥ 0.70 shown in formula (5).

[0066] Furthermore, after the secondary refining, in order to ensure time for the inclusions to float, the number density of composite inclusion B is kept at 2.0 to 20.0 pieces / mm by leaving it for 3 minutes or more. 2 The number density of oxysulfides with a circle equivalent diameter of 5.0 μm or more is set to 0.50 pieces / mm 2 The temperature can be controlled as follows. After casting, steel sheets can be manufactured under known manufacturing conditions. For example, in the case of cold-rolled steel sheets, the slab is heated to 1100 to 1300°C, and then rough-rolled and finish-rolled to a thickness of 2 to 8 mm. The sheet is then pickled to remove surface scale, resulting in a hot-rolled steel sheet. The hot-rolled sheet is then annealed, cold-rolled, and cold-rolled again to form a product. [Example]

[0067] The effects of the present invention will be explained below using examples, but the present invention is not limited to the conditions used in the following examples.

[0068] In this example, steel with the chemical composition shown in Table 1 was melted and continuously cast into a 200 mm thick slab. Deoxidation treatment with Al was performed in the early stages of secondary refining, reducing the O content in the molten steel to 0.0060% or less. Subsequently, strong deoxidizing elements that readily form oxides, such as Si, Ti, Al, Ca, and Mg, were controlled. This resulted in a steel that satisfied formula (4). Furthermore, the amount of MgO added to the refining slag was calculated in advance so that the MgO activity of the refining slag would be 1.0 based on pure solid MgO. This resulted in a steel that satisfied formulas (2) and (3). By controlling the deoxidation treatment, strong deoxidizing elements, and MgO in the slag, and adjusting the composition to a predetermined level, formula (5) was satisfied.

[0069] After the secondary refining, the number density of the complex inclusions was controlled by allowing the slab to stand for 3 minutes or more. The slab was heated to 1150°C to 1250°C for 2 hours and then hot-rolled to form a hot-rolled steel sheet with a thickness of 5 mm. Then, hot-rolled sheet annealing, cold rolling, and cold-rolled sheet annealing were performed to produce a 0.6 mm thick stainless steel sheet. In the hot-rolled sheet annealing and cold-rolled sheet annealing, the recrystallization temperature T was measured and then the annealing temperature was determined. The hot-rolled sheet annealing temperature was T+10 (°C), and the cold-rolled sheet annealing temperature was T (°C). The annealing time (holding time) in the hot-rolled sheet annealing and cold-rolled sheet annealing was 30 seconds, and intermediate annealing was omitted in this example. The cold-rolling reduction was 85%.

[0070] <Evaluation of complex inclusions> The composition of the composite inclusions was determined by randomly selecting 100 composite inclusions A containing oxides and having a major axis of 1.0 μm or more from a cross section of the cold-rolled sheet perpendicular to the rolling direction, and measuring the composition of the composite inclusions by SEM-EDS to determine the number of composite inclusions B and the number of oxysulfides D. The area observed along with the major axis of the composite inclusions was recorded, and the number density of composite inclusions C and the number density of oxysulfides D were calculated.

[0071] <Evaluation of steel sheet surface properties> [Riding] To measure ridging height, No. 5 tensile test pieces conforming to JIS Z2241 were taken and subjected to a 15% tensile strain in the rolling direction. After tensioning, an unevenness profile was obtained for the center of the parallel part of the test piece using a two-dimensional contact surface roughness measuring device. From the unevenness profile, the maximum value of the length in the plate thickness direction between the apexes of adjacent concave and convex portions (height of the concave and convex portions) was defined as the ridging height. A ridging height of less than 10 μm was judged to be good (◯), and a ridging height of 10 μm or more was judged to be poor (×).

[0072] [Corrosion resistance] Corrosion resistance was tested by spraying salt water. The NaCl concentration was 5%, the test temperature was 50°C, and other test specimen shapes and test conditions conformed to JIS Z 2371. Corrosion resistance was evaluated as poor (×) if red rust was observed on the appearance after 48 tests, and good (◯) if no red rust was observed.

[0073] [Surface defects] The quality of the surface defects was evaluated by visually inspecting the entire length of the obtained cold-rolled sheet at intervals of 1 m. If the area where surface defects with a length of 10 mm or more were found accounted for less than 5% of the total area, it was marked as ○, and if it accounted for more than 5%, it was marked as ×.

[0074] As shown in Table 2, the steel components and the composition, amount and number ratio of the complex inclusions in the samples B1 to B23 satisfied the present invention, ensuring corrosion resistance and ridging resistance, and exhibiting good results with no surface defects.

[0075] The specimen b1 had insufficient corrosion resistance due to a high C concentration and coarse Cr carbides. In addition, the value of formula (1) was high, and the number density of oxysulfides D was also high.

[0076] In the specimen b2, the C concentration was low, the solidus temperature was high, and the number density of the complex inclusions B that act as nuclei for the equiaxed crystals was low, resulting in significant ridging.

[0077] In the specimen b3, the Si concentration was high, which resulted in poor workability, and the number of composite inclusions A containing Si increased, while the number of composite inclusions B, which act as nuclei for equiaxed crystals, was small, resulting in significant ridging.

[0078] In the specimen b4, the Mn concentration was low, (Mn,Cr)S was not formed, and CaS was easily formed, so the number density of oxysulfide D was high and the corrosion resistance was insufficient.

[0079] In the specimen b5, the Mn concentration was high, which resulted in poor workability, and the number of composite inclusions A containing Mn was large, resulting in a small number of composite inclusions B, which act as nuclei for equiaxed crystals, resulting in significant ridging. In addition, the tendency for (Mn,Cr)S to form resulted in a high density of oxysulfides D, resulting in insufficient corrosion resistance.

[0080] In the specimen b6, the P concentration was high, and together with the solidification segregation of P, the solidus temperature was low, so the number density of oxysulfides D was also high and the corrosion resistance was insufficient.

[0081] The specimen b7 had a low P concentration, a high solidus temperature, and a low density of complex inclusions B, which act as nuclei for equiaxed crystals. As a result, significant ridging occurred.

[0082] In the specimen b8, the S concentration was high and (Mn,Cr)S was easily formed, resulting in a high density of oxysulfides D and insufficient corrosion resistance.

[0083] In the specimen b9, the Cr concentration was low, (Mn,Cr)S was not formed, and CaS was easily formed, so the number density of oxysulfide D was high and the corrosion resistance was insufficient.

[0084] The specimen b10 had a high Cr concentration and coarse Cr carbides, resulting in insufficient corrosion resistance. Furthermore, the number of complex inclusions A containing Cr was high, while the number of complex inclusions B, which act as nuclei for equiaxed crystals, was low. This resulted in significant ridging.

[0085] The specimen b11 had a high Al concentration, and many of the composite inclusions A contained Al, while the number of composite inclusions B, which act as nuclei for equiaxed crystals, was small. As a result, significant ridging occurred. In addition, a large amount of alumina was generated, making surface defects more likely to occur.

[0086] The specimen b12 had a low Al concentration and a high S concentration, and therefore had a high density of oxysulfides D, resulting in insufficient corrosion resistance.

[0087] Reference symbol b13 had a high Ti concentration, and a large amount of TiN was generated, which made it prone to surface defects.

[0088] In the specimen b14, the Nb concentration was low, the solidus temperature was high, and the number density of the complex inclusions B that act as nuclei for the equiaxed crystals was low. As a result, significant ridging occurred.

[0089] The specimen b15 had a high Nb concentration, a relatively high value of formula (1), and a relatively high number density of oxysulfides D.

[0090] In the specimen b16, the O concentration was high, and the number of composite inclusions A containing Mn and Cr was high, while the number of composite inclusions B, which act as nuclei for equiaxed crystals, was low, resulting in significant ridging. Furthermore, the quantity of inclusions was also high, making surface defects more likely to occur.

[0091] In the specimen b17, the O concentration was low and the number density of complex inclusions C, which act as nuclei for equiaxed crystals, was not satisfied, resulting in significant ridging.

[0092] In the specimen b18, the N concentration was low, the solidus temperature was high, and the number density of the complex inclusions B that act as nuclei for the equiaxed crystals was not satisfied, resulting in significant ridging.

[0093] The specimen b19 had insufficient corrosion resistance due to a high N concentration and coarse Cr nitrides. In addition, the formula (1) was high, and the number density of oxysulfides D was also high.

[0094] In the specimen b20, the Ca concentration was high, and many of the composite inclusions A contained Ca, while there were few composite inclusions B, which act as nuclei for equiaxed crystals, resulting in significant ridging.

[0095] The specimen b21 had a high Mg concentration, a high number density of oxysulfides D, and insufficient corrosion resistance.

[0096] In the specimen b22, the Mg concentration was low, the number of composite inclusions A containing Mg was small, and the number of composite inclusions B that serve as nuclei for equiaxed crystals was small, resulting in significant ridging.

[0097] In the specimen b23, the value of formula (1) was high, which promoted the coarsening of oxysulfide D, and therefore the corrosion resistance was insufficient.

[0098] In the case of symbol b24, the value of formula (1) was low, and δ-Fe generation was difficult, so large ridging occurred.

[0099] [Table 1]

[0100] [Table 2] [Industrial Applicability]

[0101] The ferritic stainless steel according to the present invention can be used in a variety of industrial products, including vehicles and home appliances, and is particularly suitable for industrial products with a high level of design.

Claims

1. In mass%, C: 0.001-0.05%, Si: 1.0% or less, Mn: 0.02 to 1.0%, P: 0.005-0.040%, S: 0.005% or less, Cr: 11-21%, Al: 0.01-0.2%, Ti: 0.015% or less, Nb: 0.1 to 0.6%, O: 0.0005-0.005%, N: 0.001-0.05%, Ca: 0.0015% or less, Mg: 0.0003% to 0.0030%, B: 0 to 0.002%, Mo: 0 to 2.5%, Ni: 0-2.0%, Cu: 0-2.0%, Co: 0 to 1.0%, Sn: 0 to 0.5%, W: 0 to 1.0%, Ta: 0-0.10%, Sb: 0 to 0.30%; the balance being Fe and impurities; Formula (1) is satisfied, and Composite inclusions A containing oxides and having a major axis of 1.0 μm or more are included, When composite inclusions B are composite inclusions A that satisfy formulas (2) to (4), the ratio of the number of composite inclusions A to the number of composite inclusions B satisfies formula (5), and the number density of composite inclusions C among composite inclusions B, which have a major axis of 2.0 to 15.0 μm, is 2.0 to 20 pieces / mm ２ and Among the composite inclusions A, the number density of oxysulfides D having a circle equivalent diameter of 5.0 μm or more and containing 5% or more of S is 0.50 pieces / mm ２ A ferritic stainless steel sheet characterized by the following: 70≧(0.50×[%C]+0.20×[%N]+0.65×[%P]+3.0×[%S])×1000+6×[%Si]+20×[%Ti]+30×[%Nb]≧20 ... Formula (1) Al ２ O ３ / MgO≦4... Formula (2) CaO+CaS≦20%... Formula (3) Al ２ O ３ +MgO≧75%... Formula (4) Number of composite inclusions B / Number of composite inclusions A≧0.70 Equation (5) In the formula (1), [% C] and the like indicate the content (mass%) of each element in the steel sheet, and Al in the formulas (2) to (4) ２ O ３ , MgO, CaO, and CaS indicate the mass % of each in the complex inclusion A.

2. Furthermore, in mass%, B: 0.0001-0.002%, Mo: 0.1 to 2.5%, Ni: 0.1-2.0%, Cu: 0.1-2.0%, Co: 0.05-1.0%, Sn: 0.01-0.5%, W: 0.05-1.0%, Ta: 0.001 to 0.10%, Sb: 0.01~0.30% 2. The ferritic stainless steel sheet according to claim 1, further comprising one or more of the following:

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