Ferritic stainless steel plate

Abstract

To provide a ferritic stainless steel plate with small production load and high age hardening capability.SOLUTION: A ferritic stainless steel plate has a chemical composition containing, by mass%, C: 0.002-0.03%, Si: 0.1-1.0%, Mn: 1.0% or less, P: 0.04% or less, S: 0.030% or less, Cr: 17.0-19.5%, Mo: 0.10-0.30%, Nb: 0.05-0.2%, Ti: 0.6% or less, Cu: 0.80-1.5%, N: 0.002-0.03%, an optional element, with the balance being Fe and impurities, satisfying [8×(C+N)≤Ti+Nb] and [1.6≤(8×Mo+10×Nb) / Cu≤5.0]. Aging the ferritic stainless steel plate at 600°C for one hour and then air-cooling it results in a Vickers hardness increase ΔHV of 50 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet. [Background technology]

[0002] Compared to austenitic stainless steel, ferritic stainless steel contains less Ni, which is classified as a rare element and is expensive, and therefore can reduce costs by using less alloying. For this reason, ferritic stainless steel is being widely used in kitchens, home appliances, and automobile exhaust components, where austenitic stainless steel has traditionally been used primarily.

[0003] On the other hand, ferritic stainless steels have lower work hardening than austenitic stainless steels, making them difficult to strengthen by temper rolling.Furthermore, because ferritic stainless steels have fast diffusion at high temperatures and low high-temperature strength, they can be difficult to replace austenitic stainless steels in applications requiring strength from room temperature to high temperatures.

[0004] In light of this, ferritic stainless steels containing Cu are disclosed in Patent Documents 1 and 2. The ferritic stainless steels disclosed in Patent Documents 1 and 2 have improved properties by utilizing precipitation strengthening of Cu. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-117985 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-248620 Summary of the Invention [Problem to be solved by the invention]

[0006] Meanwhile, ferritic stainless steels are also required to have a lower recrystallization temperature, which affects heat treatment, and to reduce the manufacturing load during annealing and pickling. If the recrystallization temperature is high, annealing must be performed at a high temperature to complete the recrystallization. This accelerates oxidation, resulting in the formation of a thick oxide scale, which increases the manufacturing load during the pickling process to remove the oxide scale.

[0007] The ferritic stainless steel disclosed in Patent Document 1 is intended for use upstream of exhaust system components exposed to high-temperature exhaust gases emitted from the engine. Therefore, it emphasizes high-temperature properties and contains a large amount of expensive Nb. It also has a high recrystallization temperature, estimated to be over 900°C. Furthermore, if high strength is achieved in hot-rolled sheet annealing, which is performed before cold-rolled sheet annealing, the manufacturing load during subsequent processing such as rolling also increases. Therefore, the ferritic stainless steel disclosed in Patent Document 1 has room for improvement in terms of the manufacturing load resulting from the recrystallization temperature.

[0008] The ferritic stainless steel disclosed in Patent Document 2 has a reduced Nb content and is precipitation strengthened by fine Cu precipitates, but depending on the usage environment, for example, the precipitation of Cu precipitates may become difficult to occur.

[0009] In ferritic stainless steel containing Cu, when the precipitation strengthening mechanism is fully functional, the hardness after heat treatment, known as aging treatment, is greater than before. In other words, the age hardenability is increased. However, the ferritic stainless steel disclosed in Patent Document 2 has room for further improvement in terms of age hardenability.

[0010] In view of the above, an object of the present invention is to provide a ferritic stainless steel sheet that requires a small manufacturing load and has high age hardenability. [Means for solving the problem]

[0011] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following ferritic stainless steel sheet.

[0012] (1) Chemical composition, in mass%, C: 0.002~0.03%, Si: 0.1 to 1.0%, Mn: 1.0% or less, P: 0.04% or less, S: 0.030% or less, Cr: 17.0~19.5%, Mo: 0.10 to 0.30% Nb: 0.05 to 0.2%, Ti: 0.6% or less, Cu: 0.80-1.5% N: 0.002~0.03%, Ni: 0-0.6% V: 0~0.5%, W: 0-0.5%, Co: 0-0.5%, Zr: 0-0.5% Al: 0-1.0%, Sn: 0 to 0.5% B: 0~0.005%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, REM: 0~0.01%, The balance is Fe and impurities. The following formulas (i) and (ii) are satisfied: A ferritic stainless steel sheet that has an increase in Vickers hardness ΔHV of 50 or more when aged at 600°C for 1 hour and then air-cooled. 8 × (C + N) ≦ Ti + Nb (i) 1.6≦(8×Mo+10×Nb) / Cu≦5.0 (ii) However, each element symbol in the above formula represents the content (mass%) of each element contained in the ferritic stainless steel, and if the element is not contained, it is set to zero.

[0013] (2) The chemical composition is in mass%: Ni: 0.01 to 0.6%, V: 0.01 to 0.5%, W: 0.05 to 0.5%, Co: 0.01 to 0.5%, Zr: 0.01 to 0.5%, Al: 0.01 to 1.0%, and Sn: 0.01 to 0.5% The ferritic stainless steel sheet according to (1) above, containing one or more selected from the following:

[0014] (3) The chemical composition is in mass%: B: 0.0002~0.005%, Ca: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, and REM: 0.0002~0.01%, The ferritic stainless steel sheet according to (1) or (2) above, containing one or more selected from the following: [Effects of the Invention]

[0015] According to the present invention, a ferritic stainless steel sheet having a small manufacturing load and high age hardenability can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present inventors have conducted research on ferritic stainless steel sheets and have obtained the following findings (a) to (c).

[0017] (a) To reduce the manufacturing load and raw material costs, it is necessary to minimize the content of elements such as Nb and Mo. This is because Nb and Mo are expensive elements and increase the recrystallization temperature. On the other hand, Nb and Mo are elements that solid-solution strengthen ferritic stainless steels and improve their strength. Nb also suppresses the precipitation of Cr carbonitrides and suppresses sensitization caused by Cr carbonitrides, while Mo has the effect of improving corrosion resistance. Therefore, while reducing Nb, adding Ti ensures sensitization resistance. At the same time, adjusting the above-mentioned Cu and Mo to the appropriate ranges improves age hardenability and achieves high strength.

[0018] (b) The above-mentioned strengthening by Cu is a precipitation strengthening process that causes fine Cu precipitates. Such precipitation strengthening by Cu precipitates occurs when Cu is dissolved in the matrix and then subjected to appropriate aging treatment to form Cu precipitates. On the other hand, Cu precipitates that precipitate before aging treatment simply coarsen in subsequent processes and do not contribute significantly to strengthening. Therefore, to achieve even higher strength, it is necessary to improve the age hardenability so that the hardness increases before and after aging treatment. One possible cause of the decrease in age hardenability is the formation of Cu precipitates before aging treatment, for example, during manufacturing.

[0019] Therefore, the present inventors investigated whether it is possible to suppress the formation of Cu precipitates during manufacturing. They found that adding small amounts of Nb and Mo within a range that does not raise the recrystallization temperature is effective. Although the mechanism behind this is unclear, it is thought that when Cu precipitates are formed, the dissolved Nb and Mo retard the diffusion of Cu in the matrix, narrowing the temperature range in which Cu precipitates can be formed.

[0020] (c) As a result, in a ferritic stainless steel containing Cu and having reduced Nb and Mo contents, it is possible to achieve strength improvement greater than that achieved by solid solution strengthening of Nb and Mo through precipitation strengthening of Cu precipitates.

[0021] An embodiment of the present invention has been made based on the above findings. Each requirement of the ferritic stainless steel sheet of this embodiment will be described in detail below.

[0022] 1.Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0023] C: 0.002 to 0.03% C (carbon) reduces workability and corrosion resistance, so it is preferable to reduce it. Therefore, the C content is set to 0.03% or less. The C content is preferably set to 0.02% or less, and more preferably set to 0.015% or less. However, excessive reduction of C increases refining costs. Therefore, the C content is set to 0.002% or more. The C content is preferably set to 0.003% or more.

[0024] Si: 0.1 to 1.0% Silicon (Si) is an element that is useful as a deoxidizer and also improves oxidation resistance. Therefore, the Si content is set to 0.1% or more. The Si content is preferably set to 0.2% or more. However, excessive Si content reduces ductility at room temperature and workability. Therefore, the Si content is set to 1.0% or less. The Si content is preferably set to 0.6% or less.

[0025] Mn: 1.0% or less If Mn (manganese) is contained in excess, it not only makes it easier for an austenite phase to form at high temperatures but also reduces workability. For this reason, the Mn content is set to 1.0% or less. On the other hand, excessive reduction of Mn increases raw material costs. For this reason, the Mn content is preferably set to 0.1% or more. The Mn content is preferably set in the range of 0.2 to 0.8%.

[0026] P:0.04% or less P (phosphorus) is an impurity element contained in steel and reduces toughness and workability. For this reason, the P content is set to 0.04% or less. The P content is preferably set to 0.03% or less. It is preferable to reduce P as much as possible, but excessive reduction of P increases refining costs. For this reason, the P content is preferably set to 0.01% or more.

[0027] S: 0.030% or less S (sulfur) reduces elongation and has an adverse effect on workability. S also reduces corrosion resistance. For this reason, the S content is set to 0.030% or less. The S content is preferably set to 0.010% or less, and more preferably set to 0.005% or less. It is preferable to reduce S as much as possible, but excessive reduction of S increases refining costs. For this reason, the S content is preferably set to 0.0003% or more.

[0028] Cr: 17.0~19.5% Cr (chromium) is an element effective in improving the corrosion resistance and oxidation resistance that are characteristic of stainless steel. If the Cr content is less than 17.0%, the Cr fraction in the passive film will be insufficient, and corrosion resistance will not be obtained. For this reason, the Cr content is set to 17.0% or more. However, if excessive Cr is added, the steel will undergo solid solution strengthening at room temperature, resulting in hardening and reduced ductility. As a result, workability will decrease. In particular, if the Cr content exceeds 19.5%, the above-mentioned adverse effects will become more pronounced, so the Cr content is set to 19.5% or less. The Cr content is preferably in the range of 17.5 to 19.0%.

[0029] Mo: 0.10 to 0.30% Mo (molybdenum) has the effect of improving high-temperature strength, oxidation resistance, and corrosion resistance. Furthermore, in the ferritic stainless steel sheet of this embodiment, Mo also has the effect of suppressing the formation of Cu precipitates within grains during cooling after recrystallization, thereby bringing Cu into a solid solution state. Therefore, the Mo content is set to 0.10% or more. However, Mo is an expensive element, and excessive inclusion increases alloy costs. Furthermore, Mo increases the recrystallization temperature, thereby increasing the load during manufacturing. Therefore, the Mo content is set to 0.30% or less.

[0030] Nb: 0.05 to 0.2% Niobium (Nb) has the effect of suppressing Cr carbides and improving corrosion resistance. In particular, in the ferritic stainless steel sheet of this embodiment, the combined addition of Nb and Ti suppresses the formation of Cr carbonitrides and improves corrosion resistance. In this case, Ti mainly becomes nitride, and Ti alone is not enough to sufficiently fix C, so it is necessary to add a certain amount of Nb.

[0031] In addition, like Mo, Nb has the effect of suppressing the formation of Cu precipitates at grain boundaries during cooling after recrystallization, and keeping Cu in a solid solution state. Therefore, the Nb content is set to 0.05% or more. However, Nb is an expensive element, and excessive Nb content increases alloy costs. It also increases the recrystallization temperature, increasing the load during manufacturing. Furthermore, it forms a Laves phase containing Nb, which reduces toughness and workability. Therefore, the Nb content is set to 0.2% or less. The Nb content is preferably set to a range of 0.06 to 0.18%.

[0032] Ti: 0.6% or less In the ferritic stainless steel sheet of this embodiment, the Nb content is reduced. Therefore, Ti (titanium) is an important element for fixing C and N. For this reason, the Ti content must satisfy the following formula (i). By satisfying formula (i), C and N are fixed, the occurrence of sensitization is suppressed, and corrosion resistance, particularly intergranular corrosion resistance of welds, is improved. 8 × (C + N) ≦ Ti + Nb (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the ferritic stainless steel, and if the element is not contained, it is set to zero.

[0033] However, excessive Ti content reduces toughness and induces surface defects. Therefore, the Ti content is set to 0.6% or less. Depending on the C and N contents, the Ti content is preferably set in the range of 0.15 to 0.4%, and more preferably in the range of 0.2 to 0.3%.

[0034] Cu: 0.80 to 1.5% Cu (copper) has the effect of improving high-temperature strength by forming fine Cu precipitates. In the ferritic stainless steel sheet of this embodiment, it is important that the sheet is in a solid solution state when cooled after recrystallization, and that fine Cu precipitates are formed by the subsequent aging heat treatment. In light of this, the Cu content is set to 0.80% or more. The Cu content is preferably set to 1.0% or more, and more preferably set to 1.1% or more. However, excessive Cu content will not form a solid solution during annealing, and will form coarse Cu precipitates during cooling. Therefore, the Cu content is set to 1.5% or less. The Cu content is preferably set to 1.4% or less.

[0035] In the ferritic stainless steel sheet of this embodiment, the contents of Mo, Nb, and Cu, which contribute to the formation of Cu precipitates, are controlled in order to cause fine Cu precipitates. Specifically, the contents of Mo, Nb, and Cu must satisfy the following formula (ii):

[0036] 1.6≦(8×Mo+10×Nb) / Cu≦5.0 (ii) However, each element symbol in the above formula represents the content (mass%) of each element contained in the ferritic stainless steel, and if the element is not contained, it is set to zero.

[0037] If the value in equation (ii) is less than 1.6, the amounts of Mo and Nb will be insufficient relative to the amount of Cu, making it difficult to refine the Cu precipitates. As a result, it will be difficult to improve the age hardenability. In other words, the increase in Vickers hardness ΔHV after aging at 600°C for 1 hour and air cooling, as described below, will tend to be less than 50. For this reason, the value in equation (ii) is set to 1.6 or more. The value in equation (ii) is preferably set to 1.7 or more, and more preferably to 1.8 or more.

[0038] On the other hand, if the value in equation (ii) exceeds 5.0, the recrystallization temperature becomes excessively high, reducing manufacturability. Furthermore, it becomes difficult to obtain a recrystallized structure, and the remaining unrecrystallized structure makes it difficult to age-harden even after heat treatment. This reduces the age-hardenability. Therefore, the value in equation (ii) is set to 5.0 or less. It is preferable that the value in equation (ii) is set to 4.5 or less.

[0039] N: 0.002 to 0.03% N (nitrogen) is an impurity contained in steel and deteriorates workability and corrosion resistance. For this reason, the N content is set to 0.03% or less. It is preferable to reduce N as much as possible, but excessive reduction increases refining costs. For this reason, the N content is set to 0.002% or more. The N content is preferably set in the range of 0.003 to 0.02%, and more preferably in the range of 0.003 to 0.015%.

[0040] In addition to the above elements, one or more elements selected from Ni, V, W, Co, Zr, Al, and Sn may be contained within the ranges shown below. The reasons for limiting the content of each element are explained below.

[0041] Ni: 0 to 0.6% Ni (nickel) is an element that improves the toughness and workability of ferritic stainless steel. Therefore, it may be added as needed. However, because Ni is a strong austenite phase-forming element, excessive Ni content makes it easier for austenite phase to form at high temperatures, reducing high-temperature strength. In addition, because Ni is an expensive element, it increases alloy costs. For this reason, the Ni content is set to 0.6% or less. The Ni content is preferably set to 0.5% or less, and more preferably set to 0.3% or less. On the other hand, to obtain the above effects, the Ni content is preferably set to 0.01% or more.

[0042] V: 0 to 0.5% Like Nb and Ti, V (vanadium) is a carbonitride-forming element, and by finely precipitating these carbonitrides, it has the effect of improving high-temperature strength. Therefore, it may be contained as needed. However, excessive V content reduces manufacturability. Therefore, the V content is set to 0.5% or less. The V content is preferably set to 0.4% or less, and more preferably set to 0.3% or less. On the other hand, in order to obtain the above effect, the V content is preferably set to 0.01% or more.

[0043] W: 0 to 0.5% W (tungsten) has the effect of increasing high-temperature strength. Therefore, it may be added as needed. However, excessive W content promotes the formation of intermetallic compounds, reducing the toughness and workability of the steel. Therefore, the W content is set to 0.5% or less. The W content is preferably set to 0.4% or less, and more preferably set to 0.3% or less. On the other hand, to obtain the above effect, the W content is preferably set to 0.05% or more, and more preferably set to 0.1% or more.

[0044] Co: 0 to 0.5% Co (cobalt) has the effect of improving high-temperature strength and lowering the thermal expansion coefficient. Therefore, it may be contained as needed. However, if Co is contained in excess, the steel will harden due to solid solution strengthening, which will reduce workability. Furthermore, Co is an expensive element, which increases alloy costs. Therefore, the Co content is set to 0.5% or less. The Co content is preferably set to 0.4% or less, and more preferably set to 0.3% or less. On the other hand, to obtain the above effects, the Co content is preferably set to 0.01% or more.

[0045] Zr: 0 to 0.5% Zr (zirconium) has the effect of improving oxidation resistance. Therefore, it may be added as needed. However, excessive Zr content generates intermetallic compounds, which reduces the toughness and workability of the steel. Therefore, the Zr content is set to 0.5% or less. The Zr content is preferably set to 0.4% or less, and more preferably set to 0.3% or less. On the other hand, to obtain the above effect, the Zr content is preferably set to 0.01% or more.

[0046] Al: 0 to 1.0% Al (aluminum) is used as a deoxidizer and also has the effect of improving oxidation resistance. Therefore, it may be added as needed. However, excessive Al content hardens the steel through solid solution strengthening, reducing the toughness and workability of the steel. Therefore, the Al content is set to 1.0% or less. The Al content is preferably set to 0.6% or less, and more preferably set to 0.2% or less. On the other hand, to obtain the above effects, the Al content is preferably set to 0.01% or more.

[0047] Sn: 0 to 0.5% Sn (tin) has the effect of improving corrosion resistance without significantly degrading mechanical properties at room temperature. Therefore, it may be contained as needed. However, excessive Sn content significantly reduces manufacturability. Therefore, the Sn content is set to 0.5% or less. The Sn content is preferably set to 0.3% or less, and more preferably set to 0.2% or less. On the other hand, to obtain the above effect, the Sn content is preferably set to 0.01% or more.

[0048] In addition to the above elements, one or more elements selected from B, Ca, Mg, and REM may be contained within the ranges shown below. The reasons for limiting the content of each element will be explained below.

[0049] B: 0 to 0.005% B (boron) has the effect of improving workability, particularly secondary workability. Therefore, it may be contained as needed. However, excessive B content reduces weldability and toughness. Therefore, the B content is set to 0.005% or less. The B content is preferably set to 0.003% or less, and more preferably set to 0.0015% or less. On the other hand, in order to obtain the above effect, the B content is preferably set to 0.0002% or more.

[0050] Ca: 0 to 0.01% Ca (calcium) has the effect of preventing nozzle clogging, which is likely to occur during continuous casting. Therefore, it may be added as needed. However, excessive Ca content makes surface defects more likely to occur. Therefore, the Ca content is set to 0.01% or less. The Ca content is preferably set to 0.005% or less, and more preferably set to 0.003% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably set to 0.0002% or more.

[0051] Mg: 0 to 0.01% Magnesium (Mg) has the effect of increasing the equiaxed crystal ratio of the slab and improving toughness and workability. Therefore, it may be added as needed. However, excessive Mg content reduces the toughness of the steel and also deteriorates the surface properties. Therefore, the Mg content is set to 0.01% or less. The Mg content is preferably set to 0.005% or less, and more preferably set to 0.003% or less. On the other hand, to obtain the above effects, the Mg content is preferably set to 0.0002% or more.

[0052] REM: 0 to 0.01% REM (rare earth elements) have the effect of improving oxidation resistance. Therefore, they may be added as needed. However, excessive REM content reduces weldability and toughness. Therefore, the REM content is set to 0.01% or less. The REM content is preferably set to 0.008% or less, and more preferably set to 0.005% or less. On the other hand, in order to obtain the above effect, the REM content is preferably set to 0.0002% or more.

[0053] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

[0054] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of ferritic stainless steel due to various factors in raw materials such as ore and scrap, and in the production process, and are acceptable within a range that does not adversely affect this embodiment.

[0055] 2.Age hardening ability The ferritic stainless steel sheet of this embodiment has the above-mentioned composition, which enhances the age hardenability. In particular, it is preferable that the Cu contained therein remains in a solid solution state, and the formation of Cu precipitates is minimized. This results in fine Cu precipitates during aging treatment, improving strength. Here, Cu precipitates refer to precipitates containing Cu, such as bcc-Cu, 9R, and ε-Cu Cu particles.

[0056] Here, "Cu is in a solid solution state" means that no Cu precipitates are present during cooling after annealing for recrystallization. However, because Cu precipitates initially have the same bcc structure as the matrix, it is difficult to determine the solid solution state and precipitation state of fine Cu precipitates by structural observation or the like.

[0057] Therefore, the ferritic stainless steel sheet of this embodiment is evaluated for the state of Cu solid solution by the increase in Vickers hardness ΔHV after aging at 600°C for 1 hour and air-cooling. Specifically, the increase in Vickers hardness ΔHV under the above aging and cooling conditions is set to 50 or more. If the increase in Vickers hardness ΔHV under the above aging and cooling conditions is less than 50, Cu precipitates are formed during the manufacturing process, specifically during cooling during annealing. This leads to the formation of precipitates during subsequent aging treatment, and the precipitates become coarse, making it difficult to achieve high strength through age hardening. For this reason, the increase in Vickers hardness ΔHV under the above aging and cooling conditions is set to 50 or more, preferably 55 or more. The upper limit of ΔHV is not particularly limited, but is typically around 80.

[0058] The ΔHV is measured using the following procedure. Specifically, two samples of each steel, measuring 20 mmL x 30 mmW x 2 mmT, are cut out from the cold-rolled annealed sheet, designated Sample A and Sample B. Sample A is aged at 600°C for one hour and then air-cooled. The L cross sections of the aged Sample A and Sample B, which is a cold-rolled annealed sheet, are measured at seven points with a 1 mm pitch using a Vickers hardness tester with a test force of 1.0 kgf, and the average value of five points, excluding the two maximum and minimum values, is calculated. ΔHV is calculated by subtracting the average hardness value of Sample B from the average hardness value of Sample A.

[0059] 3. Manufacturing method The ferritic stainless steel sheet of this embodiment can be stably produced, for example, by the following production method.

[0060] A stainless steel having the above-described chemical composition is melted to produce a steel billet (slab). The obtained slab is hot-rolled to form a hot-rolled sheet. The heating temperature of the slab during hot rolling is not particularly limited, but is usually in the range of 1150 to 1250°C. The total reduction rate during hot rolling is also not particularly limited. The obtained hot-rolled sheet may be annealed and pickled as necessary. The annealing conditions and pickling conditions are not particularly limited. The process may be carried out according to a conventional method as appropriate.

[0061] Next, the hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet. The conditions for cold-rolling are not particularly limited, but for example, the rolling reduction during cold rolling is preferably in the range of 40 to 80%. The obtained cold-rolled sheet is annealed to obtain a cold-rolled annealed sheet. The annealing temperature during annealing is preferably in the range of 850 to 920°C.

[0062] If the annealing temperature is less than 850°C, a sufficient recrystallized structure cannot be obtained. Furthermore, Cu cannot be sufficiently solid-dissolved in the matrix phase. Therefore, the annealing temperature is set to 850°C or higher, and more preferably 860°C or higher. On the other hand, if the annealing temperature exceeds 920°C, this is undesirable from the viewpoint of manufacturability. In particular, a thick scale is formed, which increases the load of the subsequent pickling treatment. Therefore, the annealing temperature is set to 920°C or lower, and more preferably 900°C or lower. The annealing time is not particularly limited, but is usually set to a range of 0.5 to 3 minutes from the viewpoint of manufacturability.

[0063] In the annealing, the average cooling rate in the temperature range of 800 to 400°C, where Cu precipitates are likely to form, is preferably 5°C / s or more.

[0064] After annealing, the sheet is cooled and then pickled to obtain a cold-rolled annealed ferritic stainless steel sheet. The pickling conditions are not particularly limited and may be any conventional method.

[0065] The ferritic stainless steel sheet according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples. [Example]

[0066] Steels having the chemical compositions shown in Table 1 were produced by vacuum melting and cast into 200 mm thick molds. They were then heated at 1230°C for 2 hours and hot-rolled to obtain 5 mm thick hot-rolled sheets. The resulting hot-rolled sheets were then pickled and cold-rolled to a thickness of 2.0 mm to obtain cold-rolled sheets. The cold-rolled sheets were then soaked at 900°C for 2 minutes and then pickled to obtain cold-rolled annealed sheets. In the above examples, the average cooling rate in the temperature range of 800 to 400°C was adjusted to be 5°C / s.

[0067] [Table 1]

[0068] The cold-rolled and annealed sheets were further aged at 600°C for 1 hour and air-cooled to prepare samples, and ΔHV was calculated. Specifically, two samples of each steel, measuring 20 mmL x 30 mmW x 2 mmT, were cut from the cold-rolled and annealed sheets. These were designated Sample A and Sample B. Sample A was aged at 600°C for 1 hour and then air-cooled. The hardness of the L cross section of the aged Sample A and the cold-rolled and annealed Sample B was measured at seven points with a 1 mm pitch using a Vickers hardness tester with a test force of 1.0 kgf. The average value of the five points, excluding the maximum and minimum values, was calculated. ΔHV was calculated by subtracting the average hardness value of Sample B from the average hardness value of Sample A.

[0069] Sample B was evaluated for recrystallization at 900°C for 2 minutes. Specifically, the L-section of Sample B was electropolished, and then the area ratio of regions where the grain average misorientation (GAM) relative to the surrounding measurement points was 1° or less was calculated using EBSD. If this area ratio was 95% or more, it was determined that recrystallization had occurred, and this was recorded as "recrystallized" in Table 2. On the other hand, if the area ratio was less than 95%, it was determined that recrystallization had not been completed, and this was recorded as "not yet" in Table 2. The results are summarized in Table 2 below.

[0070] [Table 2]

[0071] Nos. 1 to 12, which satisfy the requirements for the ferritic stainless steel sheet of this embodiment, achieved complete recrystallization when annealed at 900°C for 2 minutes, and the manufacturing loads of annealing, pickling, etc. were small. They also had large ΔHV values ​​and high age hardenability. On the other hand, Nos. 13 to 19, which do not satisfy the requirements for the ferritic stainless steel sheet of this embodiment, achieved at least one of the following: incomplete recrystallization when annealed at 900°C for 2 minutes, or low age hardenability.

[0072] In No. 13 or 19, the Mo content or Nb content was outside the range of the requirements of this embodiment, and furthermore, formula (ii) was not satisfied, so Cu precipitation could not be suppressed during annealing and cooling, and ΔHV decreased. In Nos. 14 to 17 and 18, at least one of the Nb content, Mo content and Cu content did not satisfy the requirements of this embodiment, and furthermore, formula (ii) was not satisfied, so recrystallization was not completed and ΔHV also decreased. In No. 17, the Cu content was low, so formula (ii) was not satisfied, and ΔHV also decreased.

Claims

1. The chemical composition, in mass%, is C: 0.002-0.03%, Si: 0.1-1.0%, Mn: 1.0% or less, P: 0.04% or less, S: 0.030% or less, Cr: 17.0-19.5%, Mo: 0.10-0.30%, Nb: 0.05-0.2%, Ti: 0.6% or less, Cu: 0.80 to 1.5%, N: 0.002-0.03%, Ni: 0 to 0.6%, V: 0 to 0.5%, W: 0 to 0.5%, Co: 0 to 0.5%, Zr: 0 to 0.5%, Al: 0-1.0%, Sn: 0 to 0.5%, B: 0 to 0.005%, Ca: 0-0.01%, Mg: 0 to 0.01%, REM: 0-0.01%, The balance is Fe and impurities. The following formulas (i) and (ii) are satisfied: A ferritic stainless steel sheet having an increase in Vickers hardness ΔHV of 50 or more when aged at 600°C for 1 hour and air-cooled. 8×(C+N)≦Ti+Nb...(i) 1.6≦(8×Mo+10×Nb) / Cu≦5.0...(ii) In the above formula, each element symbol represents the content (mass%) of each element contained in the ferritic stainless steel, and if the element is not contained, it is set to zero.

2. The chemical composition is, in mass %, Ni: 0.01-0.6%, V: 0.01-0.5%, W: 0.05-0.5%, Co: 0.01-0.5%, Zr: 0.01 to 0.5%, Al: 0.01 to 1.0%, and Sn: 0.01-0.5%, The ferritic stainless steel sheet according to claim 1, comprising one or more selected from the following:

3. The chemical composition is, in mass %, B: 0.0002 to 0.005%, Ca: 0.0002-0.01%, Mg: 0.0002 to 0.01%, and REM: 0.0002-0.01%, The ferritic stainless steel sheet according to claim 1 or 2, comprising one or more selected from the following:

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