Ferritic stainless steel hot-rolled sheet and method for manufacturing the same

Abstract

To provide a ferritic stainless hot-rolled steel sheet in which the possibility that the toughness of hot-rolled steel sheet will decrease in Nb-added steel having increased Si content, is reduced.SOLUTION: The ferritic stainless hot-rolled steel sheet (1) contains Si: 1.0 to 3.5% and Nb: 0.1 to 0.4%, etc., and in which the charpy impact value is 20 J / cm2 or more, and in the cross section (12) parallel to the rolling direction and perpendicular to the rolling surface (11), the major axis of the Nb carbide in the region (13) at a distance of 1 / 2 to 1 / 3 of the sheet thickness from the rolling surface is 4.5 μm or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a ferritic stainless hot-rolled steel sheet and a method for manufacturing the same.

Background Art

[0002] In order to improve the toughness of a ferritic stainless hot-rolled steel sheet (hereinafter abbreviated as "hot-rolled steel sheet"), various techniques have been developed. For example, Patent Document 1 discloses a technique for improving the toughness of a hot-rolled steel sheet by immediately cooling with water and winding it after hot rolling. Patent Document 2 discloses a technique for improving the toughness of a hot-rolled steel sheet by adjusting the cooling rate and reduction ratio in the hot rolling process.

[0003] Patent Document 3 discloses a technique for improving the toughness of a Nb-containing hot-rolled steel sheet by water cooling after hot rolling at a hot rolling finishing temperature of 890 °C or higher, winding at a coiling temperature of 400 °C or lower, and then immersing in water. Patent Document 4 discloses a technique for improving the toughness of a hot-rolled steel sheet by adjusting the annealing temperature and the cooling rate after annealing.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, when the Si content in hot-rolled steel sheets is increased to improve oxidation resistance at high temperatures, even when applying the technologies described in Patent Documents 1 to 4, a problem arises in which toughness decreases due to the precipitation of coarse Nb carbides.

[0006] One aspect of the present invention aims to reduce the possibility of a decrease in the toughness of a hot-rolled steel sheet in Nb-added steel with an increased Si content. [Means for solving the problem]

[0007] To solve the above problems, a ferritic stainless steel hot-rolled sheet according to one aspect of the present invention has a composition in which, by mass%, C: 0.001~0.02%, Si: 1.0~3.5%, Mn: 0.2~0.4%, Cr: 15~20%, N: 0.001~0.03%, Nb: 0.1~0.4%, and Al: 0.002~0.1%, with a P content of 0.05% or less, a S content of 0.03% or less, an O content of 0.01% or less, and the remainder being Fe and unavoidable impurities, and has a Charpy impact value of 20 J / cm². 2 In the above configuration, in a cross-section of the ferritic stainless steel hot-rolled sheet parallel to the rolling direction and perpendicular to the rolling surface, the major axis of the Nb carbide in the region where the distance from the rolling surface is 1 / 2 to 1 / 3 of the sheet thickness is 4.5 μm or less. With the above configuration, the possibility of a decrease in the toughness of the hot-rolled steel sheet can be reduced in Nb-added steel with an increased Si content.

[0008] Furthermore, a ferritic stainless steel hot-rolled sheet according to one aspect of the present invention may further contain one or more of the following: Mo: 0.1-2.0%, Ni: 0.1-1.0%, Co: 0.01-0.5%, Cu: 0.1-1.2%, Ca: 0.005% or less, Mg: 0.005% or less, B: 0.005% or less, V: 1.0% or less, W: 1.0% or less, Sn: 0.5% or less, Sb: 0.5% or less, Ga: 0.01% or less, Ta: 0.01% or less, Zr: 0.5% or less, Y: 0.1% or less, Hf: 0.01% or less, and rare earth elements: 0.1% or less. With the above configuration, the possibility of a decrease in the toughness of the hot-rolled steel sheet can be further reduced in Nb-added steel with an increased Si content.

[0009] Furthermore, a method for manufacturing a ferritic stainless hot-rolled steel sheet according to one aspect of the present invention has a composition in which, by mass%, C: 0.001~0.02%, Si: 1.0~3.5%, Mn: 0.2~0.4%, Cr: 15~20%, N: 0.001~0.03%, Nb: 0.1~0.4%, and Al: 0.002~0.1%, with a P content of 0.05% or less, a S content of 0.03% or less, an O content of 0.01% or less, and the remainder being Fe and unavoidable impurities. The method includes: a heating step of heating a ferritic stainless steel slab to a temperature range of 1200°C to 1300°C and maintaining uniform heat in the said temperature range for 30 minutes or more; a rolling step of rolling the ferritic stainless steel slab after the heating step at a hot-rolling finishing temperature of 800°C or higher to form a steel sheet; a first cooling step of water-cooling the steel sheet after the rolling step; a winding step of winding the steel sheet after the first cooling step at a winding temperature of 480°C or lower; and a second cooling step of water-cooling the steel sheet after the winding step. According to the above method, the possibility of a decrease in the toughness of the hot-rolled steel sheet can be reduced in Nb-added steel with an increased Si content.

[0010] Furthermore, in a method for manufacturing a ferritic stainless steel hot-rolled sheet according to one aspect of the present invention, the ferritic stainless steel slab may further contain one or more of the following: Mo: 0.1-2.0%, Ni: 0.1-1.0%, Co: 0.01-0.5%, Cu: 0.1-1.2%, Ca: 0.005% or less, Mg: 0.005% or less, B: 0.005% or less, V: 1.0% or less, W: 1.0% or less, Sn: 0.5% or less, Sb: 0.5% or less, Ga: 0.01% or less, Ta: 0.01% or less, Zr: 0.5% or less, Y: 0.1% or less, Hf: 0.01% or less, and rare earth elements: 0.1% or less. According to the above method, the possibility of a decrease in the toughness of the hot-rolled steel sheet can be further reduced in Nb-added steel with an increased Si content. [Effects of the Invention]

[0011] According to one aspect of the present invention, in Nb-added steel with an increased Si content, the possibility of a decrease in the toughness of the hot-rolled steel sheet can be reduced. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram showing a hot-rolled steel sheet according to one embodiment of the present invention. [Figure 2] This figure shows a typical SEM image of a hot-rolled steel sheet according to one embodiment of the present invention. [Figure 3] This is a flowchart showing a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention. [Modes for carrying out the invention]

[0013] One embodiment of the present invention will be described in detail below. The following description is intended to help you better understand the spirit of the invention and does not limit the present invention unless otherwise specified.

[0014] In this application, unless otherwise specified, "%" as the unit of the content of each component element means "% by mass". Also, in this application, for numerical values X and Y (where X < Y), "X to Y" shall mean "not less than X and not more than Y". Further, in this application, "ferritic stainless steel" is abbreviated as "stainless steel".

[0015] 〔Composition of Hot-rolled Steel Sheet〕 The hot-rolled steel sheet according to one aspect of the present invention contains, in % by mass, C: 0.001 to 0.02%, Si: 1.0 to 3.5%, Mn: 0.2 to 0.4%, Cr: 15 to 20%, N: 0.001 to 0.03%, Nb: 0.1 to 0.4% and Al: 0.002 to 0.1%, and has a composition in which the content of P is 0.05% or less, the content of S is 0.03% or less, and the content of O is 0.01% or less, and the balance is Fe and unavoidable impurities.

[0016] Also, the hot-rolled steel sheet according to one aspect of the present invention may further contain, in % by mass, one or more of Mo: 0.1 to 2.0%, Ni: 0.1 to 1.0%, Co: 0.01 to 0.5%, Cu: 0.1 to 1.2%, Ca: 0.005% or less, Mg: 0.005% or less, B: 0.005% or less, V: 1.0% or less, W: 1.0% or less, Sn: 0.5% or less, Sb: 0.5% or less, Ga: 0.01% or less, Ta: 0.01% or less, Zr: 0.5% or less, Y: 0.1% or less, Hf: 0.01% or less, and rare earth elements: 0.1% or less.

[0017] Hereinafter, the significance of the content of each element contained in the hot-rolled steel sheet according to one aspect of the present invention will be described. The hot-rolled steel sheet is composed of iron (Fe) or a small amount of unavoidably mixed impurities (unavoidable impurities) other than the following components.

[0018] <C: Carbon> C is an essential element in the hot-rolled steel sheet according to one aspect of the present invention. On the other hand, as the content of C increases, abnormal oxidation is likely to occur. Also, if C is contained excessively, the toughness of the slab and the hot-rolled steel sheet is reduced, and it becomes difficult to process the sheet material by hot working. Therefore, in one aspect of the present invention, the content rate of C is 0.001 to 0.02%, preferably 0.008 to 0.018%.

[0019] <Si: Silicon> Si is an essential element in the hot-rolled steel sheet according to one aspect of the present invention. Si is an element effective for improving oxidation resistance at high temperatures and is also effective as a deoxidizer during steelmaking. On the other hand, if Si is contained excessively, there is a possibility of reducing toughness and workability. Therefore, in one aspect of the present invention, the content rate of Si is 1.0 to 3.5%, preferably 2.55 to 2.75%.

[0020] <Mn: Manganese> Mn is an essential element in the hot-rolled steel sheet according to one aspect of the present invention. On the other hand, if Mn is contained excessively, the ferrite phase may be destabilized and the high-temperature oxidation resistance may be reduced. Therefore, in one aspect of the present invention, the content rate of Mn is 0.2 to 0.4%, preferably 0.25 to 0.35%.

[0021] <Cr: Chromium> Cr is an essential element in the hot-rolled steel sheet according to one aspect of the present invention and is a basic alloy element necessary for improving the high-temperature oxidation characteristics of stainless steel. By containing Cr above a predetermined amount, an oxide film is formed on the surface of the stainless steel and the oxidation of the stainless steel is suppressed. On the other hand, if excessive Cr is contained, the toughness is reduced and the manufacturability deteriorates. Therefore, in one aspect of the present invention, the content rate of Cr is 15 to 20%, preferably 17.5 to 18.1%.

[0022] <N: Nitrogen> N is an essential element in hot-rolled steel sheets according to one aspect of the present invention. On the other hand, if present in excess, it can combine with Al in the steel to form AlN, which can become the starting point for accelerated oxidation. Therefore, in one aspect of the present invention, the N content is 0.001 to 0.03%, preferably 0.001 to 0.013%.

[0023] <Nb:ニオブ> Nb is an essential element in hot-rolled steel sheets according to one embodiment of the present invention. Nb is effective in improving oxidation resistance at high temperatures and is an element added to ensure high-temperature strength. Furthermore, Nb improves the toughness of stainless steel by generating carbonitrides. In addition, Nb has the effect of promoting the formation of Al2O3 films. It also suppresses recrystallization of stainless steel and widens the grain boundary area by refining the crystal grains. On the other hand, excessive Nb content may reduce the toughness of the hot-rolled steel sheet. Therefore, in one embodiment of the present invention, the Nb content is 0.1 to 0.4%, preferably 0.25 to 0.35%.

[0024] <Al:アルミニウム> Al is an essential element in the hot-rolled steel sheet according to one aspect of the present invention and is a fundamental alloying element necessary to improve the high-temperature oxidation resistance of stainless steel. By containing Al in amounts greater than a predetermined amount, an Al2O3 oxide film is formed on the surface of the stainless steel, suppressing oxidation of the stainless steel. Furthermore, when REM or Y is added, the oxide film becomes denser and its adhesion to the base steel improves, suppressing the occurrence of abnormal oxidation. On the other hand, excessive Al content deteriorates the toughness of the stainless steel, resulting in poor manufacturability and workability. Therefore, in one aspect of the present invention, the Al content is 0.002 to 0.1%, preferably 0.002 to 0.05%.

[0025] <P:リン> In the hot-rolled steel sheet according to one aspect of the present invention, P is an element whose upper limit of content needs to be controlled. If P is contained excessively, the oxidation resistance and the toughness of the hot-rolled steel sheet may deteriorate. Therefore, in one aspect of the present invention, the content of P is 0 to 0.05%, preferably 0 to 0.03%. The hot-rolled steel sheet according to one aspect of the present invention may not contain P.

[0026] <S: Sulfur> In the hot-rolled steel sheet according to one aspect of the present invention, S is an element whose upper limit of content needs to be controlled. If S is contained excessively, it may have an adverse effect on the formation of the Al2O3 film in stainless steel and deteriorate the oxidation resistance. Therefore, in one aspect of the present invention, the content of S is 0 to 0.03%, preferably 0 to 0.003%. The hot-rolled steel sheet according to one aspect of the present invention may not contain S.

[0027] <O: Oxygen> In the hot-rolled steel sheet according to one aspect of the present invention, O is an element whose upper limit of content needs to be controlled. O generates non-metallic inclusions and reduces the impact value and fatigue life. Therefore, in one aspect of the present invention, the content of O is 0 to 0.01%. The hot-rolled steel sheet according to one aspect of the present invention may not contain O.

[0028] <<Other components>> The hot-rolled steel sheet according to one aspect of the present invention preferably further contains at least one element selected from Mo, Ni, Co, Cu, Ca, Mg, B, V, W, Sn, Sb, Ga, Ta, Zr, Y, Hf and rare earth elements as elements other than those described above. Also, the hot-rolled steel sheet according to one aspect of the present invention may further contain Ti.

[0029] <Mo: Molybdenum> Mo is an element that improves corrosion resistance. On the other hand, if Mo is contained excessively, it will harden, the toughness will decrease, and the material cost will increase. Therefore, in one aspect of the present invention, the content of Mo is preferably 0.05 to 2.0%.

[0030] <Ni: Nickel> Ni is an element that improves the corrosion resistance of stainless steel and is an essential element in the hot-rolled steel sheet according to one aspect of the present invention. On the other hand, when Ni is contained excessively, the ferrite phase becomes unstable and the material cost increases. Therefore, in one aspect of the present invention, the content rate of Ni is preferably 0.1 to 1.0%, and more preferably 0.1 to 0.2%.

[0031] <Co: Cobalt> Co is an element effective for improving corrosion resistance and heat resistance. However, when Co is added excessively, the raw material cost of stainless steel increases. Therefore, in one aspect of the present invention, the content rate of Co is preferably 0.01 to 0.5%.

[0032] <Cu: Copper> Cu is an element that improves the corrosion resistance of stainless steel. On the other hand, when Cu is contained excessively, it may cause a decrease in oxidation resistance and hot working properties, and the material cost increases. Therefore, in one aspect of the present invention, the content rate of Cu is preferably 0.1 to 1.2%.

[0033] <Ca: Calcium> Ca is an element that improves hot working properties. On the other hand, when Ca is contained excessively, the toughness of the steel decreases and the manufacturability decreases. Therefore, in one aspect of the present invention, the content rate of Ca is preferably 0 to 0.005%. The hot-rolled steel sheet according to one aspect of the present invention may not contain Ca.

[0034] <Mg: Magnesium> Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizer. On the other hand, when Mg is contained excessively, the toughness of the steel decreases and the manufacturability decreases. Therefore, in one aspect of the present invention, the content rate of Mg is preferably 0 to 0.005%. The hot-rolled steel sheet according to one aspect of the present invention may not contain Mg.

[0035] <B: Boron> B is an element that improves the secondary workability and oxidation resistance of molded products manufactured using stainless steel. On the other hand, if B is included in excess, B compounds will become inclusions (impurities). Therefore, in one aspect of the present invention, the B content is preferably 0 to 0.005%. The hot-rolled steel sheet according to one aspect of the present invention does not need to contain B.

[0036] <V:バナジウム> V is an element that improves workability and weld toughness. On the other hand, adding too much V may degrade the toughness of hot-rolled steel sheets. Therefore, in one aspect of the present invention, the V content is preferably 0 to 1.0%, and more preferably 0 to 0.15%. Hot-rolled steel sheets according to one aspect of the present invention do not need to contain V.

[0037] <W:タングステン> W is an element added to ensure high-temperature strength. On the other hand, excessive W content degrades the toughness of the hot-rolled steel sheet and increases material costs. Therefore, in one aspect of the present invention, the W content is preferably 0 to 1.0%. The hot-rolled steel sheet according to one aspect of the present invention does not need to contain W.

[0038] <Sn:スズ> Tin (Sn) is an element that improves the corrosion resistance of stainless steel. On the other hand, excessive Sn content reduces workability and increases material costs. Therefore, in one aspect of the present invention, the Sn content is preferably 0 to 0.5%. Hot-rolled steel sheets according to one aspect of the present invention do not need to contain Sn.

[0039] <Sb:アンチモン> Sb is effective in improving workability by promoting the formation of deformation strips during rolling. On the other hand, if it contains an excessive amount of Sb, the effect saturates and workability further decreases. For this reason, in one aspect of the present invention, the Sb content is preferably 0 to 0.5%. The hot-rolled steel sheet according to one aspect of the present invention does not need to contain Sb.

[0040] <Ga:ガリウム> Ga is an element that improves corrosion resistance. On the other hand, excessive inclusion of Ga increases the material cost. In one aspect of the present invention, the content rate of Ga is preferably 0 to 0.01%. The hot-rolled steel sheet according to one aspect of the present invention may not contain Ga.

[0041] <Ta: Tantalum> Ta is an element that improves the cleanliness and oxidation resistance of steel. On the other hand, excessive inclusion of Ta reduces toughness and increases the material cost. Therefore, in one aspect of the present invention, the content rate of Ta is preferably 0 to 0.01%. The hot-rolled steel sheet according to one aspect of the present invention may not contain Ta.

[0042] <Zr: Zirconium> Zr is an element that improves oxidation resistance. On the other hand, excessive addition of Zr may harden the steel and cause a decrease in toughness. Therefore, in one aspect of the present invention, the content rate of Zr is preferably 0 to 0.5%. The hot-rolled steel sheet according to one aspect of the present invention may not contain Zr.

[0043] <Y: Yttrium> Y is an element that reduces the viscosity of molten steel and improves cleanliness. On the other hand, excessive inclusion of Y saturates its effect and further reduces workability. Therefore, in one aspect of the present invention, the content rate of Y is preferably 0 to 0.1%. The hot-rolled steel sheet according to one aspect of the present invention may not contain Y.

[0044] <Hf: Hafnium> Hf is an element that improves oxidation resistance. On the other hand, excessive inclusion of Hf reduces the toughness of the hot-rolled steel sheet and increases the material cost. Therefore, in one aspect of the present invention, the content rate of Hf is preferably 0 to 0.01%. The hot-rolled steel sheet according to one aspect of the present invention may not contain Hf.

[0045] <REM: Rare earth element> Rare earth elements (hereinafter abbreviated as "REM") refer to lanthanide elements (elements with atomic numbers 57 to 71, such as La, Ce, Pr, Nd, and Sm). REM is an element that improves high-temperature oxidation resistance. By including more than a certain amount of REM, the Al oxide film is stabilized. Furthermore, oxidation resistance is improved by improving the adhesion between the base material and the oxide. On the other hand, if REM is included in excess, surface defects occur during hot rolling, and manufacturability decreases. For this reason, in one embodiment of the present invention, the REM content is preferably 0 to 0.1%. Hot-rolled steel sheets according to one embodiment of the present invention do not need to contain REM.

[0046] <Ti:チタン> Ti can react with C and / or N to form a ferritic monolayer in stainless steel at 900-1000°C. On the other hand, excessive Ti content can generate TiO2 in the Al oxide, potentially degrading the oxidation life. Therefore, in one aspect of the present invention, the Ti content is preferably 0-0.02%. The hot-rolled steel sheet according to one aspect of the present invention may not contain Ti.

[0047] [Charpy impact value of ferritic stainless steel] A hot-rolled steel sheet according to one aspect of the present invention has a Charpy impact value of 20 J / cm² at 25°C. 2 This concludes the explanation. Here, the Charpy impact value in this application is defined as the minimum value among the Charpy impact values ​​obtained by measuring three C-direction V-notch Charpy test specimens made from hot-rolled steel sheets using the method specified in the JIS standard (JIS Z2242 (2018)).

[0048] Furthermore, a C-direction V-notch Charpy test specimen can be prepared by taking a specimen from a hot-rolled steel sheet so that the longitudinal direction of the specimen is parallel to the rolling direction, and then making a V-notch perpendicular to the rolling direction.

[0049] [Longest axis of Nb carbide] Figure 1 is a schematic diagram showing a hot-rolled steel sheet 1 according to one embodiment of the present invention. As shown in Figure 1, the cross section 12 is a cross section obtained when the hot-rolled steel sheet 1 is cut in a direction parallel to the rolling direction and perpendicular to the rolling surface 11. Region 13 is a region in the cross section 12 of the hot-rolled steel sheet 1 where the distance from the rolling surface 11 is 1 / 2 to 1 / 3 of the sheet thickness t. In the hot-rolled steel sheet 1, the major axis of Nb carbide (NbC) in region 13 is 4.5 μm or less, and more preferably 4.3 μm or less.

[0050] Figure 2 is a representative SEM image of Nb carbides observed in a cross-section of a hot-rolled steel sheet according to one embodiment of the present invention. In the cross-section 12 of the hot-rolled steel sheet according to one embodiment of the present invention, Nb carbides can be observed as granular material dispersed within the material. As shown in Figure 2, the major axis of the Nb carbide refers to the length of the longest line segment connecting two points at the edge of the image of the particulate Nb carbide. In this application, the major axis of the Nb carbide is defined as the average value of multiple measurements obtained by taking a backscattered electron image of the cross-section 12 at a magnification of 1000x using a scanning electron microscope (SEM), and then measuring the major axis of each NbC present in the specified region 13 in the SEM image.

[0051] [Manufacturing method] Figure 3 is a flowchart showing an example of a method for manufacturing a hot-rolled steel sheet according to one aspect of the present invention. As shown in Figure 3, the method for manufacturing a hot-rolled steel sheet includes a heating step S1, a rolling step S2, a first cooling step S3, a winding step S4, and a second cooling step S5. In this application, the heating step S1, rolling step S2, first cooling step S3, winding step S4, and second cooling step S5 are collectively referred to as the "hot rolling step." Each of the above steps will be described below.

[0052] <Heating process S1> Heating step S1 is a process of heating a ferritic stainless steel slab (hereinafter abbreviated as "slab") having the composition described above in [Component composition of hot-rolled steel sheet] to a temperature range of 1200°C to 1300°C and maintaining uniform heat in this temperature range for 30 minutes or more. The heating equipment used in heating step S1 is not particularly limited and known equipment can be used, for example, a heating furnace can be used.

[0053] The slab used in heating step S1 can be manufactured, for example, by using pig iron or iron scrap as a raw material, removing impurities, adding various components, and heating to obtain molten steel with a desired composition, and then cooling and solidifying the molten steel.

[0054] In this embodiment, the slab has a Si content of 1.0% or more and an Nb content of 0.1% or more. Si and Nb are elements effective in improving oxidation resistance at high temperatures. However, the addition of Si increases the activity of Nb and C, promoting the precipitation of Nb carbides. Therefore, in the case of high-Si-content Nb-added steel, in which the Si content is increased to improve oxidation resistance, when the molten steel is solidified to produce a slab, a problem may arise in which coarse Nb carbides precipitate in areas where Si or Nb solidification segregates. Here, high-Si-content Nb-added steel is, for example, stainless steel in which the Si content is 1.0% to 3.5%, preferably 2.55% to 2.75% or more, and the Nb content is 0.1% to 0.4%, preferably 0.25% to 0.35%. When such coarse Nb carbides remain in the hot-rolled steel sheet, the toughness of the hot-rolled steel sheet decreases. When the toughness of hot-rolled steel sheets decreases, cracks are more likely to occur during subsequent processing, for example, reducing the efficiency of the processing.

[0055] In one aspect of the present invention, in heating step S1, the slab is heated to a temperature range of 1200°C to 1300°C and maintained at this temperature range for 30 minutes or more. This causes the coarse Nb carbides contained in the slab to be dissolved and reduced in size. Therefore, the possibility of a decrease in the toughness of the hot-rolled steel sheet can be reduced.

[0056] Furthermore, in stainless steel with a low Si content (e.g., less than 1.0%), heating the slab above 1200°C can lead to excessive grain growth, resulting in reduced toughness and deformation of the slab due to its own weight. If the slab deforms due to its own weight, uniform rolling may become difficult in the subsequent rolling process. Therefore, in the hot rolling process of such stainless steel, the slab is usually heated below 1200°C.

[0057] On the other hand, a slab according to one aspect of the present invention is less prone to a decrease in toughness and slab sagging even when heated to 1200°C or higher. The reason why a decrease in toughness is less likely to occur is, although not limited to, presumed to be partly due to the fact that the Si content is 1.0% or more and the Nb content is 0.1% or more, which reduces the growth of crystal grains due to the precipitation of Nb nitrides. Furthermore, the reason why deformation of the slab is less likely to occur is presumed to be partly due to the fact that the strength is improved because the Si content is 1.0% or more.

[0058] After the heating process S1 and before the rolling process S2, scale adhering to the surface of the slab may be removed. Scale removal may be performed, for example, by applying high-pressure water to the surface of the slab.

[0059] <Rolling process S2> The rolling process S2 is a process in which the slab after the heating process S1 is rolled at a hot-rolling finish temperature of 800°C or higher to form a steel sheet. In this application, "hot-rolling finish temperature" refers to the temperature of the steel sheet immediately after the final rolling treatment in the rolling process S2. Since the first cooling process S3 and the second cooling process S5 are performed after the rolling process S2, cold rolling is not included in the rolling process S2 of this application.

[0060] The rolling process S2 may include a rough rolling process and a finish rolling process. In this case, in the rough rolling process, the slab after the heating process S1 is rolled to form a steel sheet, and then in the finish rolling process, the steel sheet after the rough rolling process is further rolled.

[0061] The number of rolling operations in rolling process S2 is not particularly limited. Furthermore, the rolling equipment used in rolling process S2 is not particularly limited; known equipment can be used, however, a multi-stage rolling mill is preferred.

[0062] <First cooling process S3> The first cooling step S3 is a step of water-cooling the steel sheet after the rolling step S2. This reduces the precipitation of the Laves phase and reduces the possibility of a decrease in the toughness of the hot-rolled steel sheet. The cooling equipment used in the first cooling step S3 is not particularly limited and known equipment can be used. The temperature of the steel sheet after water-cooling in the first cooling step S3 is preferably, for example, 480°C or lower.

[0063] <Winding process S4> The winding process S4 is a process in which the steel sheet after the first cooling process S3 is wound at a winding temperature of 480°C or lower. The winding equipment used in the winding process S4 is not particularly limited, and known equipment can be used.

[0064] <Second cooling process S5> The second cooling step S5 is a step of water-cooling the steel sheet after the winding step S4. This reduces 475°C brittleness and the possibility of a decrease in the toughness of the hot-rolled steel sheet. The cooling equipment used in the second cooling step S5 is not particularly limited and known equipment can be used. From the viewpoint of simplifying the cooling equipment in the second cooling step S5 and reducing running costs, it is preferable to perform the second cooling step S5 by immersing the steel sheet after the winding step S4 in water. The immersion time can be appropriately selected according to the volume of the steel sheet, and may be, for example, about 1 to 12 hours.

[0065] A hot-rolled steel sheet according to one aspect of the present invention may be further processed by a method comprising at least one of an annealing step, a pickling step, a cold rolling step, and a finishing step. The annealing step, pickling step, cold rolling step, and finishing step can be carried out using known conditions and equipment.

[0066] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0067] [Examples] An embodiment of the present invention will be described below. Note that the method for manufacturing hot-rolled steel sheets described in this embodiment is merely an example and does not limit the method for manufacturing hot-rolled steel sheets according to one aspect of the present invention.

[0068] <Slab manufacturing> To evaluate the physical properties of a hot-rolled steel sheet according to one aspect of the present invention, a 30 kg slab was first produced by vacuum melting steel having the component composition shown in Table 1 below. In Table 1, steel grades A to K are stainless steels produced within the scope of the present invention, serving as examples of the present invention. In addition, steel grades L to N in Table 1 are stainless steels produced under conditions outside the scope of the present invention, serving as comparative examples.

[0069] [Table 1]

[0070] Table 1 shows the composition of the components contained in each steel grade in mass percent. The remainder other than the components shown in Table 1 is Fe or small amounts of unavoidable impurities. Furthermore, the underlined text in Table 1 indicates that the composition of the components contained in each steel grade in the comparative examples falls outside the scope of the present invention.

[0071] <Manufacturing and property evaluation of hot-rolled steel sheets> Next, hot-rolled steel sheets No. 1 to 22 with a thickness of 3.2 mm were manufactured under the manufacturing conditions shown in Table 2. The precipitate diameter and Charpy impact value for each hot-rolled steel sheet were also measured and are shown in Table 2. Underlined text in Table 2 indicates that the manufacturing conditions, precipitate diameter, and Charpy impact value of the hot-rolled steel sheet are outside the scope of the present invention. Furthermore, in the toughness assessment in Table 2, "○ (Good)" indicates a Charpy impact value of 20 J / cm². 2That's all. "× (Defective)" indicates a Charpy impact value of 20 J / cm². 2 This indicates that it is less than [a certain value].

[0072] [Table 2]

[0073] The V-notch test specimens used in the Charpy impact test were prepared by surface-cutting 3mm thick hot-rolled steel plates down to a thickness of 2.5mm. The Charpy impact test was conducted using an IC-30B Charpy impact testing machine manufactured by Tokyo Shoki Seisakusho Co., Ltd.

[0074] As shown in Table 2, hot-rolled steel sheets No. 1, 2, 4, 6, 7, 9, 11, 13, 15, 17, 19, and 20, which are examples of the present invention and manufactured by the method for manufacturing hot-rolled steel sheets according to one aspect of the present invention, were all demonstrated to have excellent toughness.

[0075] In contrast, hot-rolled steel sheets No. 3, 5, 8, 10, 12, 14, 16, 18, and 21-23, which are comparative examples manufactured under conditions outside the scope of the hot-rolled steel sheet manufacturing method according to one aspect of the present invention, were all demonstrated to have poor toughness.

[0076] Furthermore, it has been demonstrated that hot-rolled steel sheets No. 1, 2, 4, 6, 7, 9, 11, 13, 15, 17, 19, and 20, which correspond to the examples of the present invention and satisfy the criteria for the component composition, major axis of Nb carbide, and Charpy impact value of the hot-rolled steel sheet according to one aspect of the present invention, all exhibit excellent toughness.

[0077] In contrast, hot-rolled steel sheets No. 3, 5, 8, 10, 12, 14, 16, 18, and 21-23, which were comparative examples, all failed to meet the criteria in terms of their component composition, the major axis of the Nb carbide, or the Charpy impact value, demonstrating poor toughness. [Industrial applicability]

[0078] The present invention can be used, for example, in exhaust gas path components for automobiles and motorcycles, burner combustion cylinders, chimneys for heating equipment, heating elements for thermoelectric power generation, and reformers, housings, and piping for fuel cells. [Explanation of symbols]

[0079] 1. Ferritic stainless steel hot-rolled sheet 11 Rolling surface 12 Cross-section 13 areas t Plate thickness S1 Heating process S2 Rolling process S3 1st cooling process S4 Winding process S5 2nd cooling process

Claims

1. A ferritic stainless hot-rolled steel sheet having a composition in mass percent containing C: 0.001-0.02%, Si: 1.0-3.5%, Mn: 0.2-0.4%, Cr: 15-20%, N: 0.001-0.03%, Nb: 0.1-0.4%, and Al: 0.002-0.1%, with a P content of 0.05% or less, a S content of 0.03% or less, an O content of 0.01% or less, and the remainder being Fe and unavoidable impurities, Charpy impact value is 20 J / cm ２ That's all. A ferritic stainless steel hot-rolled sheet, wherein in a cross section parallel to the rolling direction and perpendicular to the rolling surface, the major axis of the Nb carbide in the region where the distance from the rolling surface is 1 / 2 to 1 / 3 of the sheet thickness is 4.5 μm or less.

2. The ferritic stainless hot-rolled steel sheet according to claim 1, further comprising one or more of the following elements: Ni: 0.1-0.12%, Cu: 0.1-0.15%, Ca: 0.005% or less, Mg: 0.002% or less, B: 0.003% or less, V: 0.09% or less, Sn: 0.3% or less, Zr: 0.25% or less, and rare earth elements: 0.05% or less.

3. A method for producing a ferritic stainless steel hot-rolled sheet according to claim 1, A heating step is performed in which a ferritic stainless steel slab having a composition containing, by mass%, C: 0.001-0.02%, Si: 1.0-3.5%, Mn: 0.2-0.4%, Cr: 15-20%, N: 0.001-0.03%, Nb: 0.1-0.4%, and Al: 0.002-0.1%, with a P content of 0.05% or less, a S content of 0.03% or less, an O content of 0.01% or less, and the remainder being Fe and unavoidable impurities is heated to a temperature range of 1200°C to 1300°C and maintained uniformly at that temperature range for 30 minutes or more. A rolling process in which the ferritic stainless steel slab after the heating process is rolled at a hot rolling finishing temperature of 800°C or higher to form a steel sheet, A first cooling step involves water-cooling the steel sheet after the rolling process, A winding step in which the steel sheet after the first cooling step is wound at a winding temperature of 480°C or less, A method for manufacturing a ferritic stainless hot-rolled steel sheet, comprising a second cooling step of water-cooling the steel sheet after the winding step.

4. The method for producing a ferritic stainless steel hot-rolled sheet according to claim 3, wherein the ferritic stainless steel slab has a composition further containing one or more of the following: Ni: 0.1 to 0.12%, Cu: 0.1 to 0.15%, Ca: 0.005% or less, Mg: 0.002% or less, B: 0.003% or less, V: 0.09% or less, Sn: 0.3% or less, Zr: 0.25% or less, and rare earth elements: 0.05% or less.

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