Ferritic stainless steel and method of manufacturing the same

Abstract

A ferritic stainless steel according to one embodiment of the present invention contains, in mass%, C: 0.030% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.00%, P: 0.050% or less, S: 0.005% or less, Cr: 15.0 to 25.0%, Al: 2.0 to 4.0%, Ni: 1.00% or less, Nb: 0.01 to 0.70%, N: 0.030% or less, B: 0.0003 to 0.01%, REM: 0.01 to 0.20%, the remainder being Fe and unavoidable impurities, and the dislocation density ρ derived using the Williamson-Hall method is 0.91×1014[m-2] or more.

Description

Technology Field

[0001] The present invention relates to ferritic stainless steel and a method for manufacturing the same. Background Technology

[0002] Ferritic stainless steels used as catalyst carriers for exhaust gas purification (including electric heating types) mounted on automobiles, motorcycles, etc., stove combustion chambers, or combustion gas exhaust devices in plants require high oxidation resistance at high temperatures (high-temperature oxidation resistance).

[0003] Patent document 1 discloses a high-Al content ferritic stainless steel with further improved high-temperature oxidation resistance. The high-Al content ferritic stainless steel disclosed in patent document 1 contains 15 to 25% Cr and 4.5 to 6.0% Al. In addition, by suppressing the amount of Mn and Si added, the Mn and Si content are reduced, and by including Mo as an essential element, the high-temperature oxidation resistance of the high-Al content ferritic stainless steel is improved. Prior art literature

[0004] Patent Document 1: Japanese Patent Publication No. 3351836 The problem to be solved

[0005] However, the conventional technology described above may have a negative impact on manufacturability due to reduced toughness caused by the excessive addition of Al and Mo.

[0006] One embodiment of the present invention aims to realize a ferritic stainless steel with excellent high-temperature oxidation resistance and toughness. means of solving the problem

[0007] To solve the above problem, a ferritic stainless steel according to one embodiment of the present invention contains, in mass%, C: 0.030% or less, Si: 0.01–1.5%, Mn: 0.01–1.00%, P: 0.050% or less, S: 0.005% or less, Cr: 15.0–25.0%, Al: 2.0–4.0%, Ni: 1.00% or less, Nb: 0.01–0.70%, N: 0.030% or less, B: 0.0003–0.01%, REM: 0.01–0.20%, the remainder being Fe and unavoidable impurities, and the dislocation density ρ derived using the Williamson-Hall method is 0.91 × 10⁻⁶ 14 [m -2 ] That is all, and when three randomly selected cross-sections cut in a plane perpendicular to the rolling direction using a scanning electron microscope were observed in a range of 30㎛×30㎛, the number of carbides with an average value of 2 or more and a particle size of 0.1㎛ or more, measured by Energy Dispersive X-ray Spectroscopy (EDS), is 5 wt% or more.

[0008] In addition, a method for manufacturing a ferritic stainless steel according to one embodiment of the present invention is a method for manufacturing a ferritic stainless steel comprising, in mass%, C: 0.030% or less, Si: 0.01–1.5%, Mn: 0.01–1.00%, P: 0.050% or less, S: 0.005% or less, Cr: 15.0–25.0%, Al: 2.0–4.0%, Ni: 1.00% or less, Nb: 0.01–0.70%, N: 0.030% or less, B: 0.0003–0.01%, REM: 0.01–0.20%, wherein the remainder is Fe and unavoidable impurities, the steel strip after hot rolling is annealed, and the steel strip after annealing is from the annealing temperature An annealing process in which the cooling time to 400°C is 30 seconds or more, and after the final annealing process, the dislocation density ρ derived using the Williamson-Hall method is 0.91×10⁻⁶ 14 [m -2 It includes a cold rolling process that rolls until it becomes ] or more. Effects of the invention

[0009] According to one embodiment of the present invention, a ferritic stainless steel with excellent high-temperature oxidation resistance and toughness can be realized. Brief explanation of the drawing

[0010] FIG. 1 is a partially enlarged schematic diagram of a cross-section when an alumina layer formed by heating an exemplary ferritic stainless steel according to an embodiment at 1050°C for 50 hours is cut in the thickness direction. Figure 2 is a partial enlarged schematic diagram of the alumina layer of the comparative example. Specific details for implementing the invention

[0011] [Embodiment]

[0012] Hereinafter, an embodiment of the present invention will be described in detail. In this specification, the term "stainless steel" refers to a stainless steel material whose specific shape is not limited. Examples of stainless steel materials include steel sheets, steel pipes, bar steel, etc. Meanwhile, in this specification, "%", which is the unit of the content of each constituent element, means "mass%" unless specifically stated otherwise. Furthermore, in this application, "A to B" indicates that it is A or greater and B or less.

[0013] (Composition of ferritic stainless steel)

[0014] First, the essential elements constituting the ferritic stainless steel in this embodiment will be described.

[0015] A ferritic stainless steel according to one embodiment of the present invention contains, in mass% as a steel component composition, C: 0.030% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.00%, P: 0.050% or less, S: 0.005% or less, Cr: 15.0 to 25.0%, Al: 2.0 to 4.0%, Ni: 1.00% or less, Nb: 0.01 to 0.70%, N: 0.030% or less, B: 0.0003 to 0.01%, and REM: 0.01 to 0.20%.

[0016] This composition has a reduced Al content compared to conventional high-Al ferritic stainless steel. By having the above composition of the ferritic stainless steel according to one embodiment of the present invention, a ferritic stainless steel with excellent toughness can be obtained.

[0017] Hereinafter, the significance of the content of each element contained in the ferritic stainless steel according to one embodiment of the present invention will be explained. Meanwhile, in addition to the components shown below, the ferritic stainless steel is composed of iron (Fe) or a small amount of unavoidable impurities (unavoidable impurities).

[0018] <C: Carbon>

[0019] C is an essential element in ferritic stainless steel according to one embodiment of the present invention. Meanwhile, as the content of C increases, abnormal oxidation becomes more likely to occur. Furthermore, if C is contained excessively, the toughness of slabs and hot coils deteriorates, making it difficult to process them into plates by hot working. Therefore, in one embodiment of the present invention, the upper limit of the C content is set to 0.030%. If the C content is set to 0.020% or less, the possibility of abnormal oxidation can be further reduced, thereby improving workability. Taking these reasons into account, a more preferable C content is 0.002 to 0.015%.

[0020] <Si: Silicon>

[0021] Si is an element effective for improving oxidation resistance and is an essential element in ferritic stainless steel according to one embodiment of the present invention. On the other hand, excessive Si content may reduce toughness and machinability. Therefore, in one embodiment of the present invention, the Si content is 0.01 to 1.50%. By setting the Si content to 0.01 to 1.0%, more preferably 0.01 to 0.50%, the effect as a deoxidizer and machinability are further improved.

[0022] <Mn: Manganese>

[0023] Mn is an essential element in ferritic stainless steel according to one embodiment of the present invention. On the other hand, if Mn is excessively contained, not only is the ferritic phase destabilized, but there is also a possibility that high-temperature oxidation resistance may be reduced. Therefore, in one embodiment of the present invention, the Mn content is 0.01 to 1.00%. By setting the Mn content to 0.01 to 0.80%, more preferably 0.01 to 0.50%, the possibility of a corrosion initiation point occurring is further reduced.

[0024] <P: In>

[0025] P is an essential element in ferritic stainless steel according to one embodiment of the present invention. On the other hand, if P is contained excessively, there is a possibility that oxidation resistance and hot-rolled sheet toughness may deteriorate. Therefore, in one embodiment of the present invention, the P content is specified to be 0.050% or less. By reducing the P content to 0.04% or less, the deterioration of workability can be further reduced. Taking these reasons into account, a more preferable P content is 0.005 to 0.03%.

[0026] <S: Hwang>

[0027] S is an essential element in ferritic stainless steel according to one embodiment of the present invention. On the other hand, if S is contained excessively, it may adversely affect the formation of an Al2O3 film in ferritic stainless steel and potentially degrade oxidation resistance. Therefore, in one embodiment of the present invention, the S content is specified as 0.005% or less. Taking these reasons into account, a more preferable S content is 0.0001 to 0.002%.

[0028] <Cr: Chrome>

[0029] Cr is a basic alloying element necessary to improve the high-temperature oxidation resistance of ferritic stainless steel. By containing more than a predetermined amount of Cr, an oxide film is formed on the surface of the stainless steel, thereby suppressing the oxidation of the stainless steel. On the other hand, if Cr is contained excessively, toughness decreases and manufacturability deteriorates. Therefore, in one embodiment of the present invention, the Cr content is specified as 15.0 to 25.0%. By setting the Cr content to 16.0 to 22.0%, and more preferably 17.0 to 20.0%, the oxidation suppression effect and manufacturability can be further improved.

[0030] <Al: Aluminum>

[0031] Al is a basic alloying element necessary to improve the high-temperature oxidation resistance of ferritic stainless steel. By containing more than a predetermined amount of Al, an oxide film of Al2O3 is formed on the surface of the stainless steel, thereby suppressing the oxidation of the stainless steel. In addition, when REM or Y is added, the oxide film becomes denser and adhesion to the base steel is improved, thereby suppressing the occurrence of abnormal oxidation. On the other hand, if Al is contained excessively, the toughness of the stainless steel deteriorates, resulting in poor manufacturability and processability. Therefore, in one embodiment of the present invention, the Al content is specified as 2.0 to 4.0%. By setting the Al content to 2.5 to 3.7%, more preferably 2.8 to 3.5%, the high-temperature oxidation resistance and manufacturability can be further improved.

[0032] <Ni: Nickel>

[0033] Ni is an element that improves the corrosion resistance of ferritic stainless steel and is an essential element in ferritic stainless steel according to one embodiment of the present invention. On the other hand, if Ni is contained in excess, not only is the ferrite phase destabilized, but material costs also increase. Therefore, in one embodiment of the present invention, the Ni content is specified to be 1.00% or less. By keeping the Ni content at 0.50% or less, the destabilization of the ferrite phase and the increase in material costs caused by excessive content can be further suppressed. Taking these reasons into account, a more preferable Ni content is 0.02 to 0.30%.

[0034] <N: Nitrogen>

[0035] N is an essential element in ferritic stainless steel according to one embodiment of the present invention. On the other hand, if it is contained in excessive amounts, it combines with Al in the steel to form AlN, which may become a starting point for accelerated oxidation. Therefore, in one embodiment of the present invention, the content of N is specified to be 0.030% or less. By keeping the content of N to 0.025% or less, the possibility of hardening can be further reduced. Taking these reasons into account, a more preferable content of N is 0.003 to 0.020%.

[0036] <Nb (Niobium), B (Boron), REM (Rare Earth Elements)>

[0037] Nb is an element added to ensure high-temperature strength. Additionally, Nb has the effect of promoting the formation of an Al2O3 film. Furthermore, it inhibits the recrystallization of stainless steel and widens the grain boundary area by refining the grain size. On the other hand, excessive Nb content may lead to a deterioration in the toughness of hot-rolled sheets.

[0038] B is an element that improves the secondary processability and oxidation resistance of molded products made using ferritic stainless steel. On the other hand, if B is contained in excessive amounts, B compounds become inclusions (impurities).

[0039] REM (rare earth metals) refers to lanthanoid 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 containing a predetermined amount of REM, the Al oxide film is stabilized. In addition, oxidation resistance is improved by enhancing the adhesion between the base material and the oxide. On the other hand, if REM is contained in excess, surface defects occur during hot rolling, which reduces manufacturability.

[0040] For the reasons stated above, in one embodiment of the present invention, the Nb content is defined as 0.01 to 0.70%. By setting the Nb content to 0.05 to 0.50%, more preferably 0.08 to 0.30%, the possibility of deterioration in processability can be further reduced. The upper limit of the Nb content is more preferably 0.20% or 0.15%. In addition, the B content is defined as 0.0003 to 0.01%. By setting the B content to 0.0003 to 0.005%, the presence of inclusions can be further reduced, thereby improving secondary processability. In addition, the REM content is defined as 0.01 to 0.20%. The REM content is preferably 0.02 to 0.15%, and more preferably 0.04 to 0.10%.

[0041] (Other ingredients)

[0042] A ferritic stainless steel according to one embodiment of the present invention may further contain at least one element among Zr, V, Cu, Mo, W, Hf, Sn, Ta, Ti, Mg, and Ca as an element other than the aforementioned elements.

[0043] <Zr: Zirconium>

[0044] Zr is an element that improves oxidation resistance. On the other hand, excessive addition of Zr may harden the steel, potentially leading to a decrease in toughness. Therefore, in one embodiment of the present invention, Zr may be contained in an amount of 0.50% or less. Considering the reduction of hardening, it is more preferable that the Zr content be 0.01 to 0.40%.

[0045] <V: Vanadium>

[0046] V is an element that improves processability and weld toughness. On the other hand, excessive addition of V may degrade the toughness of the hot-rolled sheet. In one embodiment of the present invention, V may be contained in an amount of 0.50% or less. Considering the reduction of hardening, it is more preferable that the V content be 0.02 to 0.35%.

[0047] <Cu: Dong>

[0048] Cu is an element that improves the corrosion resistance of ferritic stainless steel. On the other hand, an excessive content of Cu may lead to a decrease in oxidation resistance or hot workability. Therefore, in one embodiment of the present invention, Cu may be contained in an amount of 1.0% or less. Considering material costs, it is more preferable that the Cu content be 0.01 to 0.85%.

[0049] <Mo: Molybdenum>

[0050] Mo is an element that improves corrosion resistance. On the other hand, if Mo is contained in excessive amounts, not only is toughness reduced due to hardening, but material costs also increase. Therefore, in one embodiment of the present invention, Mo may be contained in an amount of 2.0% or less. Considering processability, material costs, etc., it is more preferable that the Mo content be 0.01 to 1.0%.

[0051] <W: Tungsten>

[0052] W is an element added to ensure high-temperature strength. On the other hand, excessive W content not only degrades the toughness of the hot-rolled sheet but also increases material costs. Therefore, in one embodiment of the present invention, W may be contained in an amount of 2.0% or less. Considering material costs and the like, it is more preferable that the W content be 0.01 to 1.0%.

[0053] <Hf: Hafnium>

[0054] Hf is an element that improves oxidation resistance. On the other hand, excessive Hf content not only reduces the toughness of the hot-rolled sheet but also increases material costs. Therefore, in one embodiment of the present invention, Hf may be contained in an amount of 0.50% or less. Considering toughness and material costs, it is more preferable that the Hf content be 0.001 to 0.20%.

[0055] <Sn: Comment>

[0056] Sn (tin) is an element that improves the corrosion resistance of ferritic stainless steel. On the other hand, excessive Sn content reduces workability and increases material costs. Therefore, in one embodiment of the present invention, Sn may be contained in an amount of 0.50% or less. Considering workability, cost, etc., it is more preferable that the Sn content be 0.005 to 0.20%.

[0057] <Ta: Tantalum>

[0058] Ta is an element that improves the cleanability and oxidation resistance of steel. On the other hand, excessive Ta content not only lowers toughness but also increases material costs. Therefore, in one embodiment of the present invention, Ta may be contained in an amount of 0.5% or less. Considering toughness and material costs, it is preferable that the Ta content be 0.40% or less. Taking these reasons into account, a more preferable Ta content is 0.001 to 0.30%.

[0059] <Ti: Titanium>

[0060] By reacting with C and / or N, Ti can form a ferritic single layer in ferritic stainless steel at 900 to 1000°C. On the other hand, if Ti is contained in excessive amounts, TiO2 is generated in the oxides of Al, which may degrade the oxidation life. Therefore, in one embodiment of the present invention, Ti may be contained in an amount of 0.20% or less. Considering processability and the like, it is more preferable that the Ti content be 0.005 to 0.10%.

[0061] <Mg: Magnesium>

[0062] Mg acts as a deoxidizer by forming Mg oxide together with Al in molten steel. On the other hand, if Mg is contained in excess, the toughness of the steel decreases, and manufacturability decreases. Therefore, in one embodiment of the present invention, Mg may be contained in an amount of 0.015% or less. Taking these reasons into account, the preferred content of Mg is 0.0002 to 0.0080%.

[0063] <Ca: Calcium>

[0064] Ca is an element that improves hot workability. On the other hand, if Ca is contained excessively, the toughness of the steel decreases, and manufacturability decreases. Therefore, in one embodiment of the present invention, Ca may be contained in an amount of 0.015% or less. Taking these reasons into account, the preferred content of Ca is 0.0001 to 0.012%.

[0065] The ferritic stainless steel according to the present embodiment can satisfy 100×[C] / [Nb]≤35, where [C] is the mass% of C and [Nb] is the mass% of Nb. Accordingly, Nb-based carbides are generated during hot rolling or annealing, thereby increasing the amount of deformation accumulation during final cold rolling and obtaining the desired dislocation density.

[0066] (Potential density)

[0067] The ferritic stainless steel according to the present embodiment has a dislocation density ρ derived by the Williamson-Hall method using X-ray diffraction of 0.91×10⁻⁶ 14 [m -2 That is all. In this embodiment, X-ray diffraction is measured from the surface.

[0068] Dislocation density is a value representing the amount of dislocations within a crystal, defined as the number of coordination lines penetrating a unit area of ​​the crystal's cross-section [m -2 ] or the total length of dislocation lines existing within a unit volume of crystal [m / m -3 It is indicated as ]. The ferritic stainless steel according to the present embodiment has a dislocation density ρ of 0.91×10 14 [m -2 As such, the diffusion of Al and Cr is rapid, allowing for the rapid formation of an alumina layer. Therefore, oxidation resistance can be improved.

[0069] In this embodiment, the potential density ρ[m -2 ] is derived using the Williamson-Hall method. More specifically, for example, it is derived as follows. That is, for a sample subjected to electrolytic polishing treatment, a diffraction intensity curve is measured for each diffraction peak (2θ) at α (110) 52.2°, α (211) 99.3°, and α (229) 123.3° using an X-ray diffraction apparatus utilizing a Co vacuum tube as an X-ray source. The diffraction peak (2θ) in the obtained diffraction intensity curve is separated into a peak due to the Kα1 line and a peak due to the Kα2 line. For the separated diffraction peak due to the Kα1 line, the diffraction angle 2θ is determined using the peak top method, and the angle between the intensities of half the peak intensity is calculated as the half-width. Meanwhile, the true half-width β is the half-width β of the steel after cold rolling. m It can be calculated using the following equation (1) by using the half width β0 of the steel after final annealing.

[0070] β 2 =βm 2 -β0 2 … (1)

[0071] The true half-width β calculated by the above equation (1) is the sum of the half-width diffusion β1 due to crystallite size D and the half-width diffusion β2 due to strain ε, and is expressed as shown in the following equation (2).

[0072] β=β1+β2… (2)

[0073] It is known that the diffusion of half width β1 due to crystallite size D is represented by the following equation (3), and the diffusion of half width β2 due to strain ε is represented by the following equation (4).

[0074] β1=0.9λ / (Dcosθ) … (3)

[0075] β2=2εtanθ … (4)

[0076] Here, λ in Equation (3) is the wavelength of the X-ray.

[0077] By rearranging the above equation (2) using the above equations (3) and (4), the following equation (5) is obtained.

[0078] βcosθ / λ=(0.9 / D)+(2εsinθ / λ) … (5)

[0079] As shown in the equation (5) above, the strain ε can be calculated from the slope of the graph produced by plotting βcosθ / λ against sinθ / λ.

[0080] Then, the dislocation density ρ is calculated using the calculated strain ε, the magnitude b (=0.25 nm) of the dislocation Burgers vector, and the following equation (6).

[0081] ρ=(14.4×ε 2 ) / b 2 … (3)

[0082] (Nb carbide)

[0083] The ferritic stainless steel according to the present embodiment, when a cross-section cut in a plane perpendicular to the rolling direction is observed at three random locations within a range of 30㎛ × 30㎛ using a scanning electron microscope (SEM), has an Nb concentration of 5 wt% or more as measured by EDS analysis, and the number of Nb carbides with a particle size of 0.1㎛ or more is an average of 2 to 15. Because the average value is 2 or more, it is easy to accumulate deformation within the microstructure during cold rolling. In addition, because the average value is 15 or less, the toughness of the stainless steel does not easily deteriorate. The particle size of the carbides is calculated from the particle size in the image captured by the scanning electron microscope. Specifically, the average width of the largest and smallest distances from the carbides is defined as the particle size of the carbides.

[0084] (Alumina layer)

[0085] The ferritic stainless steel according to the present invention can be preferably applied to applications requiring oxidation resistance at high temperatures. Therefore, "under usage conditions" means under high temperature conditions. Below, an alumina layer (10) formed when the ferritic stainless steel according to the present invention is heated at 1050°C for 50 hours will be described.

[0086] The inventors have discovered, through diligent research, that when Nb, Cr, and REM are included as essential elements in a concentration within an appropriate range as components of ferritic stainless steel, the columnar crystallization of the alumina layer formed under usage conditions is improved. This is thought to be due to the concentration of Nb, Cr, and REM at the grain boundaries of the alumina layer. In the alumina layer (10) according to the present embodiment, the total concentration of Nb oxide, Cr oxide, and REM-based oxide present at the grain boundaries is 3.5 wt% or more. Accordingly, since the inward diffusion of oxygen is suppressed, the alumina layer (10) has excellent oxidation resistance. That is, the ferritic stainless steel according to the present embodiment has excellent oxidation resistance under high temperature conditions.

[0087] In addition, the inventors discovered that columnar purification is improved by including B as an essential element at a concentration within an appropriate range.

[0088] The ferritic stainless steel according to the present embodiment contains, in mass%, C: 0.030% or less, Si: 0.01–1.5%, Mn: 0.01–1.00%, P: 0.050% or less, S: 0.005% or less, Cr: 15.0–25.0%, Al: 2.0–4.0%, Ni: 1.00% or less, Nb: 0.01–0.70%, N: 0.030% or less, B: 0.0003–0.01%, and REM: 0.01–0.20%.

[0089] The alumina layer (10) formed by heating a ferritic stainless steel containing the above components at 1050°C for 50 hours has the following characteristics. That is, in the cross-section when the alumina layer (10) is cut in the thickness direction, the area is 2.25 μm 2 The sum of the grain boundary lengths included in any region is 5.5 μm or less.

[0090] FIG. 1 is a partially enlarged schematic diagram of a cross-section when an alumina layer (10) formed by heating an exemplary ferritic stainless steel according to the present embodiment at 1050°C for 50 hours is cut in the thickness direction. An arbitrary area as shown in FIG. 1 is 2.25 μm 2 The region may be, for example, a region of 1.5 μm in width and length centered at the center in the thickness direction of the alumina layer (10). Area 2.25 μm 2 The grain boundary length included in the region refers to an area of ​​2.25 μm 2 It is the sum of the lengths of all grain boundaries (GBs) existing within the region. In the example shown in FIG. 1, 2.25 μm 2 The grain boundary length included in the region is 5.5㎛ or less.

[0091] Fig. 2 shows 2.25 μm 2 This is a partial schematic enlarged view of a comparative example alumina layer (20) in which the grain boundary length included is longer than 5.5 μm. As shown in FIG. 2, 2.25 μm 2 The comparative example alumina layer (20) in which the grain boundary length is longer than 5.5 μm has a higher proportion of equiaxial crystals than the example shown in FIG. 1.

[0092] Here, columnar crystal refers to a structure in which crystal grains that have grown elongated and slender in the thickness direction of the alumina layer are arranged. Isometric crystal refers to a polycrystalline structure in which the shape and orientation of the crystal grains constituting the isometric crystal are isotropic.

[0093] As is evident when comparing Fig. 1 and Fig. 2, the alumina layer with a high proportion of columnar grains (Fig. 1) has a shorter length of grain boundaries (GB) per unit area than the alumina layer with a high proportion of equiaxed grains (Fig. 2).

[0094] Any 2.25 μm in the cross-section when the alumina layer (10) according to the present embodiment is cut in the thickness direction 2The grain boundary length included in is 5.5 μm or less. In other words, the alumina layer (10) has a high proportion of columnar grains. Since equiaxed grains have a higher grain boundary density than columnar grains, the diffusion path of oxygen into the grain boundaries increases. For this reason, equiaxed grains have a shorter oxidation life than columnar grains. Therefore, the ferritic stainless steel according to the present embodiment has excellent oxidation resistance under high temperature conditions by having a high proportion of columnar grains.

[0095] (Manufacturing method)

[0096] First, an example of a manufacturing process for ferritic stainless steel in the present embodiment is described in detail. The manufacturing process for ferritic stainless steel in the present embodiment includes a pretreatment process, a hot rolling process, an annealing process, a pickling process, and a cold rolling process.

[0097] In the pretreatment process, first, steel with a composition adjusted to be within the scope of the present invention is melted using a melting furnace in a vacuum or argon atmosphere, and the steel is cast to produce a slab. Then, a slab piece for hot rolling is cut from the slab. Then, the slab piece is heated in an atmospheric atmosphere to a temperature range of 1100°C to 1300°C. The time for heating and holding the slab piece is not limited. Meanwhile, when the pretreatment process is carried out industrially, the casting may be continuous casting.

[0098] The hot rolling process is a process for manufacturing hot-rolled steel strips of a predetermined thickness by hot-rolling a slab (steel ingot) obtained from a pretreatment process.

[0099] The annealing process is a process that aims to soften the steel strip by heating the hot-rolled steel strip obtained from the hot rolling process to, for example, 900 to 1050°C. In the annealing process, the steel strip after annealing is cooled from the annealing temperature to 400°C for a cooling time of 30 seconds or more. Accordingly, Nb carbides can be precipitated within the structure (i.e., at grain boundaries and within grains).

[0100] The pickling process is a process of washing away scale attached to the surface of the annealed steel strip obtained by the above annealing process using a pickling solution such as hydrochloric acid or a mixture of nitric acid and hydrofluoric acid.

[0101] The cold rolling process is a process of rolling an annealed steel strip, from which scale has been removed in the first pickling process, into a thinner layer. The reduction rate in the cold rolling process is 65% or more, preferably 75% or more. By making the reduction rate in the cold rolling process 65% or more, the deformation within the steel can be increased. More specifically, by making the reduction rate in the cold rolling process 65% or more, the dislocation density ρ derived by the Williamson-Hall method using X-ray diffraction is 0.91 × 10⁻⁶ 14 [m -2 ] becomes greater than. In other words, the dislocation density ρ derived by the Williamson-Hall method using X-ray diffraction is 0.91×10 14 [m -2 In order to produce a cold-rolled sheet that is ] or larger, the rolling reduction rate in the cold rolling process should be 65% or more, preferably 75% or more.

[0102] Meanwhile, the series of processes from the annealing process to the cold rolling process may be performed multiple times. If the above series of processes is performed only once, the annealing process is referred to as the final annealing process. If the above series of processes is performed multiple times, the last annealing process is referred to as the final annealing process, and the other annealing processes are referred to as intermediate annealing processes.

[0103] In addition, in the manufacturing method according to the present embodiment, the reduction rate in the cold rolling process after the final annealing process is 65% or more. In other words, in the cold rolling process after the final annealing process, the dislocation density ρ derived by the Williamson-Hall method using X-ray diffraction is 0.91 × 10⁻⁶ 14 [m -2It is a cold rolling process that rolls until it becomes ] or more.

[0104] The method for manufacturing ferritic stainless steel according to the present embodiment is characterized by not including an annealing process after the cold rolling process. That is, the ferritic stainless steel according to the present embodiment is a cold-rolled steel strip after the cold rolling process. Since the ferritic stainless steel is a cold-rolled steel strip, deformation accumulates within the steel, and the diffusion of Al and Cr accelerates. For this reason, an alumina layer can be formed early under high-temperature conditions, thereby realizing high resistance to high-temperature oxidation. In addition, since there is no need to perform final annealing after cold rolling, material costs can be reduced.

[0105] (organize)

[0106] The ferritic stainless steel according to Embodiment 1 of the present invention contains, in mass%, C: 0.030% or less, Si: 0.01–1.5%, Mn: 0.01–1.00%, P: 0.050% or less, S: 0.005% or less, Cr: 15.0–25.0%, Al: 2.0–4.0%, Ni: 1.00% or less, Nb: 0.01–0.70%, N: 0.030% or less, B: 0.0003–0.01%, and REM: 0.01–0.20%, with the remainder consisting of Fe and unavoidable impurities, and the dislocation density ρ derived by the Williamson-Hall method using X-ray diffraction is 0.91 × 10⁻⁶. 14 [m -2 That is all.

[0107] With this composition, the toughness is excellent because the Al content is 4.0% or less. In addition, the dislocation density ρ is 0.91×10⁻⁶ 14 [m -2 From the above, since an alumina layer can be formed early, it is possible to realize a ferritic stainless steel with excellent oxidation resistance at high temperatures.

[0108] The ferritic stainless steel according to Embodiment 2 of the present invention, in Embodiment 1, forms an alumina layer mainly composed of alumina when heated at 1050°C for 50 hours, and the alumina layer has an area of ​​2.25 μm in the cross-section when the alumina layer is cut in the thickness direction. 2 The sum of the grain boundary lengths included in any region of the crystal may be 5.5 μm or less.

[0109] In the ferritic stainless steel according to Embodiment 3 of the present invention, the total concentration of Nb oxide, Cr oxide, and REM oxide present in the grain boundaries of the alumina layer in Embodiment 2 may be 3.5 wt% or more.

[0110] The ferritic stainless steel according to Embodiment 4 of the present invention may further contain, in any one of Embodiments 1 to 3, one or more of the following in mass%: Zr: 0.50% or less, V: 0.50% or less, Cu: 1.0% or less, Mo: 2.0% or less, W: 2.0% or less, Hf: 0.50% or less, Sn: 0.50% or less, Ta: 0.5% or less, Ti: 0.20% or less, Mg: 0.015% or less, and Ca: 0.015% or less.

[0111] The ferritic stainless steel according to Embodiment 5 of the present invention can satisfy 100×[C] / [Nb]≤35 when [C] is the mass% of C and [Nb] is the mass% of Nb in any one of Embodiments 1 to 4.

[0112] A method for manufacturing a ferritic stainless steel according to Embodiment 6 of the present invention comprises, in mass%, C: 0.030% or less, Si: 0.01–1.5%, Mn: 0.01–1.00%, P: 0.050% or less, S: 0.005% or less, Cr: 15.0–25.0%, Al: 2.0–4.0%, Ni: 1.00% or less, Nb: 0.01–0.70%, N: 0.030% or less, B: 0.0003–0.01%, REM: 0.01–0.20%, wherein the remainder is Fe and unavoidable impurities, the method comprises an annealing process in which a steel strip after hot rolling is annealed, and the cooling time from the annealing temperature of the steel strip after annealing to 400°C is 30 seconds or more, and after the final annealing process, The potential density ρ derived using the Williamson-Hall method is 0.91×10 14 [m -2 It includes a cold rolling process that rolls until it becomes ] or more.

[0113] With this composition, it is possible to realize a ferritic stainless steel with excellent toughness and oxidation resistance at high temperatures.

[0114] In the method for manufacturing ferritic stainless steel according to Embodiment 7 of the present invention, in Embodiment 6, the rolling reduction rate in the cold rolling process may be 65% or more.

[0115] The method for manufacturing a ferritic stainless steel according to Embodiment 8 of the present invention may be such that, in Embodiment 6 or 7, the ferritic stainless steel obtained through the cold rolling process has a composition in which, when a cross-section cut in a plane perpendicular to the rolling direction is observed at three random locations within a range of 30㎛×30㎛ using a scanning electron microscope, the Nb concentration measured by energy-dispersive X-ray analysis is 5 wt% or more, and the number of carbides with a particle size of 0.1㎛ or more is 2 or more and 15 or less as an average value.

[0116] A method for manufacturing ferritic stainless steel according to any one of forms 6 to 8, wherein the ferritic stainless steel obtained through the cold rolling process forms an alumina layer when heated at 1050°C for 50 hours, and the alumina layer has an area of ​​2.25 μm in the cross-section when the alumina layer is cut in the thickness direction. 2 The sum of the grain boundary lengths included in any region of the crystal may be 5.5 μm or less.

[0117] The method for manufacturing ferritic stainless steel according to Embodiment 10 of the present invention, in Embodiment 9, may have a total concentration of Nb oxide, Cr oxide, and REM oxide present in the grain boundaries of the alumina layer of 3.5 wt% or more.

[0118] The method for manufacturing a ferritic stainless steel according to any one of forms 6 to 10 of the present invention may further contain, in mass%, one or more of Zr: 0.50% or less, V: 0.50% or less, Cu: 1.0% or less, Mo: 2.0% or less, W: 2.0% or less, Hf: 0.50% or less, Sn: 0.50% or less, Ta: 0.5% or less, Ti: 0.20% or less, Mg: 0.015% or less, and Ca: 0.015% or less.

[0119] The method for manufacturing a ferritic stainless steel according to Embodiment 12 of the present invention, in any one of Embodiments 6 to 11, can satisfy 100×[C] / [Nb]≤35 when [C] is the mass% of C and [Nb] is the mass% of Nb.

[0120] In any one of forms 6 to 12, the method for manufacturing ferritic stainless steel according to form 13 of the present invention may have a heating temperature of 900°C to 1050°C during the final annealing process. With such a configuration, an annealed steel strip suitable for realizing ferritic stainless steel with excellent toughness and oxidation resistance at high temperatures can be obtained.

[0121] Example

[0122] In order to evaluate the physical properties of the ferritic stainless steel of the present invention, ferritic stainless steel was manufactured using the components shown in Table 1 below as raw materials, as steel grades of the inventive examples and steel grades of comparative examples. In Table 1, steel grades No. 1 to 16 are ferritic stainless steels as inventive examples manufactured within the scope of the present invention. In addition, in Table 1, steel grades No. 17 to 27 are ferritic stainless steels as comparative examples manufactured under conditions outside the scope of the present invention.

[0123] In manufacturing steel of the grade shown in Table 1, first, steel with the composition shown in Table 1 was vacuum melted to produce a 30 kg slab. After heating this slab at 1230°C for 2 hours, hot rolling was performed to produce a hot-rolled plate with a thickness of 3 mm. The obtained hot-rolled plate was annealed at a temperature between 900 and 1050°C to produce a hot-rolled annealed plate. Cold rolling and annealing were each performed twice on the obtained hot-rolled annealed plate, and a final cold rolling was performed further to produce a cold-rolled plate with a thickness of 50 μm. The cooling time from the annealing temperature to 400°C during the annealing process is shown in Table 2.

[0124] Meanwhile, the first two cold rolling steps were performed with a reduction rate of 60–85% for both the example of the present invention and the comparative example, and the annealing after cold rolling was performed under conditions within a temperature range of 900–1050°C. The reduction rate in the final cold rolling step is indicated in the column 'Final Reduction Rate' of Table 2. As shown in Table 2, the reduction rate in the final cold rolling step of the example of the present invention is 65% or higher. On the other hand, the reduction rate in the final cold rolling step of the comparative example is less than 65%. Meanwhile, the manufacturing method described in this example is merely an example and is not limiting.

[0125]

[0126] In Table 1, the composition of the components included in each steel grade is listed in mass%. Meanwhile, the remainder other than the components shown in Table 1 is Fe or a small amount of unavoidable impurities (unavoidable impurities). The underlined parts in Table 1 indicate that the range of each component contained in each stainless steel according to the comparative example of the present invention is outside the range of the present invention.

[0127] (Measurement of potential density)

[0128] Hereinafter, the measurement of dislocation density ρ performed on cold-rolled plates of the steel grades of the inventive examples and comparative examples shown in Table 1 will be described. The dislocation density ρ was measured according to the method described in the aforementioned section (dislocation density). Table 2 shows the measurement results of the dislocation density ρ. For steel grades No. 1 to 16 of the inventive examples, the dislocation density ρ was 0.91 × 10⁻⁶ 14 [m -2 ] was above. Meanwhile, the comparative examples steel grades No. 17 to 27 all had a dislocation density ρ of 0.91×10 14 [m -2 ] was less than. From this result, when the rolling reduction in the final cold rolling was 65% or more, the dislocation density ρ was 0.91×10 14 [m -2It has been demonstrated that it is greater than ]. On the other hand, when the rolling reduction rate in the final cold rolling is less than 65%, the dislocation density ρ is 0.91×10 14 [m -2 It was proven that it becomes less than ].

[0129] (Nb carbide)

[0130] For the cold-rolled sheets of the steel grades of the inventive examples and comparative examples shown in Table 1, the number of Nb carbides present in the microstructure was investigated. The investigation was carried out as follows. First, the cold-rolled sheet was cut in a plane perpendicular to the rolling direction. Next, the cross-section was observed at three random locations within a range of 30 μm × 30 μm using a scanning electron microscope, and the average number of carbides was calculated, in which the Nb concentration measured by energy-dispersive X-ray analysis was 5 wt% or more and the particle size was 0.1 μm or more. The calculated average value of Nb carbides is shown in Table 2 as the 'average number of Nb carbides'. As shown in Table 2, the average number of carbides for steel grades No. 1 to 16 of the inventive examples was in the range of 2 to 15.

[0131] (Measurement of grain boundary length)

[0132] First, the cold-rolled plates of the steel grades of the inventive example and the comparative example shown in Table 1 were heated at 1050°C for 50 hours. After heating, each steel plate was observed via STEM from a cross-section. STEM observation was performed using a Hitachi High-Tech HD-2700 at a voltage of 200V and a magnification of 30,000x. To measure the alumina grain boundary length, a range of 1.5㎛ × 1.5㎛ was randomly selected from the center of the alumina film, and the total length of the grain boundaries within this range was calculated. Meanwhile, the measured length was taken as the average value of three randomly selected locations.

[0133] In the determination of grain boundary length in Table 2, '○ (Good)' is 2.25 µm 2 It indicates that the grain boundary length within the range is 5.5㎛ or less, and '× (Defective)' is 2.25㎛. 2It indicates that the grain boundary length within the range is greater than 5.5㎛.

[0134] (Elemental concentration in the grain boundaries of the alumina layer)

[0135] Below, the concentrations of Nb, Cr, and REM elements in the alumina grain boundaries of the inventive examples and comparative examples shown in Table 1 will be described.

[0136] First, the cold-rolled plates of the steel grades of the inventive example and the comparative example shown in Table 1 were heated at 1050°C for 50 hours. After heating, each steel plate was observed from a cross-section, and the concentration of each element within the grain boundaries was measured using STEM-EDX. STEM observation was performed using a Hitachi High-Tech HD-2700 at a voltage of 200V, an observation magnification of 4 million times, and spot analysis of the center of the grain boundaries. EDX (Energy Dispersive X-ray Analysis) was performed using an AMETEK EDAX Octane T Ultra W energy dispersive X-ray analyzer. The analysis time was set to 300 seconds.

[0137] Table 2 lists the sum of the elemental concentrations of Nb, Cr, Ce, La, and Nd. This value is the sum of the concentrations of Nb oxide, Cr oxide, and REM-based oxide in the grain boundaries.

[0138] In the determination of enriched element concentrations in Table 2, if the total concentration of Nb oxide, Cr oxide, and REM-based oxide in the grain boundaries is 3.5 wt% or higher, it is marked as '○ (Good)'. On the other hand, if it is less than 3.5 wt%, it is marked as '× (Poor)' in Table 2.

[0139] (High-temperature oxidation resistance evaluation test)

[0140] Below, the high-temperature oxidation resistance evaluation test conducted on the examples of the present invention and comparative examples shown in Tables 1 and 2 will be described. First, for each steel grade shown in Table 1, three test specimens with a width of 20 mm and a length of 25 mm were taken from a cold-rolled plate with a thickness of 50 μm, as described in the steel manufacturing section. These test specimens were subjected to an atmospheric temperature of 1050°C for 50 hours, and the average oxidation weight of the three specimens was measured. This high-temperature oxidation resistance evaluation test was conducted in the atmosphere using an EREMA electric furnace. The results are shown in Table 3 below. In the judgment of high-temperature oxidation resistance in Table 3, '○ (Good)' indicates that the average oxidation weight is 1 mg / cm² or less, and '× (Poor)' indicates that it exceeds 1 mg / cm².

[0141] (Personality Assessment Test)

[0142] The toughness evaluation test performed on the examples of the present invention and comparative examples shown in Table 1 is described below. First, the test specimen used in this evaluation test was manufactured based on the V-notch test specimen of the JIS standard (JIS Z 2242 (2018)). The plate thickness was adjusted by surface cutting a hot-rolled plate with a thickness of 3 mm, as described in the section on steel manufacturing, to a thickness of 2.5 mm. The test specimen was taken from the steel plate so that the longitudinal direction of the test specimen was parallel to the rolling direction. In addition, a notch was made in the test specimen so that it was perpendicular to the rolling direction.

[0143] This evaluation test was conducted based on the JIS standard (JIS Z 2242 (2018)). The test was performed at room temperature (23℃±2℃) on five samples of each steel grade, and the Charpy impact value (absorbed energy) was calculated. For this evaluation test, a TOKYO KOKI IC-30B type Charpy impact tester was used. The results are shown in Table 2 below. In Table 2, '○ (Good)' indicates a Charpy impact value of 20 J / cm² 2 It indicates an abnormality, and '× (Defective)' means the Charpy impact value is 20 J / cm² 2It indicates that it is less than

[0144]

[0145] As shown in Table 2, steel grades No. 1 to 16 of the inventive examples all satisfied the above criteria for high-temperature oxidation resistance and toughness. Steel grades No. 17 to 27 of the comparative examples did not satisfy the above criteria for one or both of high-temperature oxidation resistance and toughness.

[0146] In other words, it has been demonstrated that the ferritic stainless steel within the scope of the present invention exhibits excellent high-temperature oxidation resistance and toughness.

[0147] Below, the reason why steel grades No. 17 to 27 of the comparative examples do not show better results than the steel grades of the present invention examples will be explained.

[0148] Although the grain boundary length of Comparative Example Steel No. 17 satisfies the standard, the concentrations of Nb, Cr, and REM in the grain boundaries did not meet the standard due to the low B content. Consequently, it did not exhibit excellent high-temperature oxidation resistance.

[0149] In Comparative Example steel grade No. 18, the concentrations of Nb, Cr, and REM in the grain boundaries did not meet the standards because the REM content was less than 0.01%. In addition, in Comparative Example steel grade No. 18, the Ti content was greater than 0.20%, so it was prone to equiaxed crystallization and did not show excellent high-temperature oxidation resistance.

[0150] Comparative example steel grade No. 19 had a Nb content of less than 0.01%, and the concentrations of Nb, Cr, and REM in the grain boundaries did not meet the standards, and did not show excellent high-temperature oxidation resistance.

[0151] Comparative example steel grade No. 20 did not show excellent high-temperature oxidation resistance because, as the Zr content was greater than 0.50%, Zr tended to segregate at the alumina grain boundaries, promoting equiaxed crystals.

[0152] Comparative Example steel grade No. 21 had a Si content greater than 1.5%, and showed excellent high-temperature oxidation resistance due to the influence of Si-based oxides such as SiO2. On the other hand, Comparative Example steel grade No. 21 had a Si content greater than 1.5%, and did not show good results regarding toughness.

[0153] Comparative example steel grade No. 22 has an Al content lower than 2.0%, making it difficult to form an Al2O3 oxide film. Consequently, the oxygen partial pressure is high, making it easy to equiaxed, and thus it did not show good results regarding high-temperature oxidation resistance.

[0154] Comparative example steel grade No. 23 did not show good results regarding high temperature oxidation resistance because the Ti content was higher than 0.20%, making it prone to equiaxed crystallization.

[0155] Comparative example steel grade No. 24 had a higher Nb content than 0.70%, and since it was prone to equiaxed crystallization, it did not show good results regarding high-temperature oxidation resistance.

[0156] Comparative example steel grade No. 25 did not show good results regarding toughness because the Al content was higher than 4.0%.

[0157] Comparative example steel grade No. 26 had a Cr content higher than 25.0%, which made it easier for Cr to be concentrated at the alumina grain boundaries and form equiaxed grains, and did not show good results regarding high-temperature oxidation resistance.

[0158] Comparative example steel grade No. 27 did not show good results regarding toughness because the REM content was greater than 0.20%, and oxides such as Y2O3 or CeO2 were formed. Explanation of the symbols

[0159] 10: Alumina layer

Claims

Claim 1 In mass%, it contains C: 0.030% or less, Si: 0.01–1.5%, Mn: 0.01–1.00%, P: 0.050% or less, S: 0.005% or less, Cr: 15.0–25.0%, Al: 2.0–4.0%, Ni: 0.02–1.00%, Nb: 0.01–0.70%, N: 0.030% or less, B: 0.0003–0.01%, and REM: 0.01–0.20%, with the remainder consisting of Fe and unavoidable impurities, and the dislocation density ρ derived using the Williamson-Hall method is 0.91×10⁻⁶ 14 [m -2 A ferritic stainless steel that satisfies 100×[C] / [Nb]≤35 when [C] is the mass% of C and [Nb] is the mass% of Nb, and when three randomly selected locations within a range of 30㎛×30㎛ are observed using a scanning electron microscope on cross-sections cut in a plane perpendicular to the rolling direction, the Nb concentration measured by energy-dispersive X-ray analysis is 5 wt% or more, and the number of carbides with a particle size of 0.1㎛ or more is an average of 2 or more and 15 or less. Claim 2 In claim 1, when heated at 1050℃ for 50 hours, an alumina layer mainly composed of alumina is formed, and said alumina layer has an area of ​​2.25 μm in the cross-section when said alumina layer is cut in the thickness direction. 2 Ferritic stainless steel having a total grain boundary length of 5.5 μm or less in any region. Claim 3 A ferritic stainless steel according to claim 2, wherein the total concentration of Nb oxide, Cr oxide, and REM-based oxide present in the grain boundaries of the alumina layer is 3.5 wt% or more. Claim 4 A ferritic stainless steel according to any one of claims 1 to 3, further containing, in mass%, one or more of Zr: 0.50% or less, V: 0.50% or less, Cu: 1.0% or less, Mo: 2.0% or less, W: 2.0% or less, Hf: 0.50% or less, Sn: 0.50% or less, Ta: 0.5% or less, Ti: 0.20% or less, Mg: 0.015% or less, and Ca: 0.015% or less. Claim 5 delete Claim 6 A method for manufacturing a ferritic stainless steel having, in mass%, C: 0.030% or less, Si: 0.01–1.5%, Mn: 0.01–1.00%, P: 0.050% or less, S: 0.005% or less, Cr: 15.0–25.0%, Al: 2.0–4.0%, Ni: 0.02–1.00%, Nb: 0.01–0.70%, N: 0.030% or less, B: 0.0003–0.01%, REM: 0.01–0.20%, with the remainder consisting of Fe and unavoidable impurities, satisfying 100×[C] / [Nb]≤35 where [C] is the mass% of C and [Nb] is the mass% of Nb, wherein the steel strip after hot rolling is annealed at a temperature An annealing process in which the steel strip is heated to 900–1050°C and the cooling time from the annealing temperature to 400°C after heating is 30 seconds or more, and after the final annealing process, the reduction ratio is 65% or more, and the dislocation density ρ derived using the Williamson-Hall method is 0.91×10⁻⁶ 14 [m -2 A method for manufacturing ferritic stainless steel, comprising a cold rolling process of rolling until it becomes ] or larger. Claim 7 delete Claim 8 A method for manufacturing ferritic stainless steel according to claim 6, wherein the ferritic stainless steel obtained through the above cold rolling process has, when a cross-section cut in a plane perpendicular to the rolling direction is observed at three random locations within a range of 30㎛×30㎛ using a scanning electron microscope, the Nb concentration measured by energy-dispersive X-ray analysis is 5 wt% or more, and the number of carbides with a particle size of 0.1㎛ or more exists as an average value of 2 or more and 15 or less. Claim 9 In claim 6, the ferritic stainless steel obtained through the above cold rolling process forms an alumina layer when heated at 1050°C for 50 hours, and the alumina layer has an area of ​​2.25 μm in the cross-section when the alumina layer is cut in the thickness direction. 2 A method for manufacturing ferritic stainless steel in which the total length of grain boundaries included in any region is 5.5 μm or less. Claim 10 A method for manufacturing ferritic stainless steel according to claim 9, wherein the total concentration of Nb oxide, Cr oxide, and REM-based oxide present in the grain boundaries of the alumina layer is 3.5 wt% or more. Claim 11 A method for manufacturing a ferritic stainless steel according to claim 6, wherein the ferritic stainless steel further contains, in mass%, one or more of Zr: 0.50% or less, V: 0.50% or less, Cu: 1.0% or less, Mo: 2.0% or less, W: 2.0% or less, Hf: 0.50% or less, Sn: 0.50% or less, Ta: 0.5% or less, Ti: 0.20% or less, Mg: 0.015% or less, and Ca: 0.015% or less. Claim 12 delete

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