WO2023106204A1

WO2023106204A1 - Ferritic stainless steel and method for manufacturing same

Info

Publication number: WO2023106204A1
Application number: PCT/JP2022/044388
Authority: WO
Inventors: 佳幸藤村; 尊仁濱田; 純一濱田
Original assignee: 日鉄ステンレス株式会社
Priority date: 2021-12-09
Filing date: 2022-12-01
Publication date: 2023-06-15
Also published as: JPWO2023106204A1

Abstract

A ferritic stainless steel according to one aspect of the present invention contains at most 0.030 mass% of C, 0.01-1.5 mass% of Si, 0.01-1.00 mass% of Mn, at most 0.050 mass% of P, at most 0.005 mass% of S, 15.0-25.0 mass% of Cr, 2.0-4.0 mass% of Al, at most 1.00 mass% of Ni, 0.01-0.70 mass% of Nb, at most 0.030 mass% of N, 0.0003-0.01 mass% of B, and 0.01-0.20 mass% of REM, with the balance consisting of Fe and unavoidable impurities, wherein a dislocation density ρ derived by means of the Williamson and Hall method is at least 0.91×10¹⁴ [m⁻²].

Description

Ferritic stainless steel and its manufacturing method

The present invention relates to ferritic stainless steel and its manufacturing method.

Ferritic stainless steel, which is used for catalyst carriers for purifying exhaust gases (including electrically heated types) mounted on automobiles and motorcycles (including electrically heated types), stove combustion cylinders, and combustion gas exhaust systems in plants, has high oxidation resistance at high temperatures. high temperature oxidation) is required.

Patent Document 1 discloses a high-Al-containing ferritic stainless steel with further improved high-temperature oxidation resistance. The high Al content ferritic stainless steel disclosed in Patent Document 1 contains 15-25% Cr and 4.5-6.0% Al. Furthermore, by suppressing the amounts of Mn and Si added, Mn and Si are reduced, and by containing Mo as an essential element, the high-temperature oxidation resistance of the high-Al-containing ferritic stainless steel is improved.

Japanese Patent No. 3351836

However, in the above-described conventional technology, the excessive amounts of Al and Mo added may reduce toughness and adversely affect manufacturability.

An object of one aspect of the present invention is to realize a ferritic stainless steel that is excellent in high-temperature oxidation resistance and toughness.

In order to solve the above problems, the ferritic stainless steel according to one aspect of the present invention has, in mass %, C: 0.030% or less, Si: 0.01 to 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% Below, Nb: 0.01 to 0.70%, N: 0.030% or less, B: 0.0003 to 0.01%, REM: 0.01 to 0.20%, the balance being Fe and It consists of unavoidable impurities, has a dislocation density ρ derived using the Williamson and Hall method of 0.91×10 ¹⁴ [m ⁻² ] or more, and is examined using a scanning electron microscope in a plane perpendicular to the rolling direction. Carbide having a Nb concentration of 5 wt % or more and a particle diameter of 0.1 μm or more as measured by energy dispersive X-ray analysis when the cut cross section is randomly observed at three locations in the range of 30 μm × 30 μm each. The number is 2 or more and 15 or less on average.

In addition, in the method for producing ferritic stainless steel according to one aspect of the present invention, 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 balance being Fe and inevitable impurities A method for producing ferritic stainless steel, comprising: annealing a steel strip after hot rolling; , after the final annealing step, a cold rolling step of rolling until the dislocation density ρ derived using the Williamson and Hall method is 0.91×10 ¹⁴ [m ⁻² ] or more.

According to one aspect of the present invention, ferritic stainless steel with excellent high-temperature oxidation resistance and toughness can be realized.

FIG. 3 is a partially enlarged schematic diagram of a cross section of an alumina layer formed by heating an exemplary ferritic stainless steel according to an embodiment at 1050° C. for 50 hours, cut in the thickness direction. FIG. 4 is a partially enlarged schematic diagram of an alumina layer of a comparative example;

[Embodiment]
An embodiment of the present invention will be described in detail below. As used herein, the term "stainless steel" means a stainless steel material whose specific shape is not limited. Examples of this stainless steel material include steel plates, steel pipes, bar steels, and the like. In this specification, "%", which is the unit of content of each component element, means "% by mass" unless otherwise specified. In addition, in the present application, "A to B" indicates that A or more and B or less.

(Component composition of ferritic stainless steel)
First, essential elements constituting the ferritic stainless steel in this embodiment will be described.

The ferritic stainless steel according to one embodiment of the present invention has a steel composition in mass % of 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 component composition has a reduced Al content compared to conventional high-Al-containing ferritic stainless steels. When the ferritic stainless steel according to one embodiment of the present invention has the above chemical composition, it is possible to obtain a ferritic stainless steel having excellent toughness.

The significance of the content of each element contained in the ferritic stainless steel according to one embodiment of the present invention will be described below. In addition, the ferritic stainless steel is composed of iron (Fe) or a small amount of unavoidable impurities (inevitable impurities) other than the components shown below.

<C: Carbon>
C is an essential element in the ferritic stainless steel according to one embodiment of the present invention. On the other hand, C tends to cause abnormal oxidation as the content increases. Also, excessive C content deteriorates the toughness of slabs and hot coils, making it difficult to process them into plate materials by hot working. Therefore, in one aspect of the present invention, the upper limit of the C content is set to 0.030%. If the C content is 0.020% or less, the possibility of abnormal oxidation can be further reduced and workability can be improved. Considering the above reasons, the more preferable content of C is 0.002 to 0.015%.

<Si: silicon>
Si is an element effective in improving oxidation resistance, and is an essential element in the ferritic stainless steel according to one embodiment of the present invention. On the other hand, an excessive Si content may reduce toughness and workability. Therefore, in one aspect of the present invention, the Si content is 0.01 to 1.50%. By setting the Si content to 0.01 to 1.0%, preferably 0.01 to 0.50%, the effect as a deoxidizing agent and workability are further improved.

<Mn: Manganese>
Mn is an essential element in the ferritic stainless steel according to one embodiment of the present invention. On the other hand, an excessive Mn content may destabilize the ferrite phase and lower the high-temperature oxidation resistance. Therefore, in one aspect of the present invention, the content of Mn 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 occurrence of corrosion starting points is further reduced.

<P: Phosphorus>
P is an essential element in the ferritic stainless steel according to one embodiment of the present invention. On the other hand, an excessive P content may deteriorate the oxidation resistance and toughness of the hot-rolled sheet. Therefore, in one aspect of the present invention, the P content is specified to be 0.050% or less. By setting the P content to 0.04% or less, deterioration of workability can be further reduced. Considering the above reasons, the preferable content of P is 0.005 to 0.03%.

<S: Sulfur>
S is an essential element in the ferritic stainless steel according to one embodiment of the present invention. On the other hand, an excessive S content may adversely affect the formation of the Al ₂ O ₃ film in the ferritic stainless steel, degrading the oxidation resistance. Therefore, in one aspect of the present invention, the S content is specified to be 0.005% or less. Considering the above reasons, the more preferable S content is 0.0001 to 0.002%.

<Cr: Chromium>
Cr is a basic alloying element necessary to improve the high-temperature oxidation resistance of ferritic stainless steel. By containing a predetermined amount or more of Cr, an oxide film is formed on the surface of the stainless steel, and oxidation of the stainless steel is suppressed. On the other hand, an excessive Cr content lowers toughness and deteriorates manufacturability. Therefore, in one aspect of the present invention, the Cr content is specified to be 15.0 to 25.0%. By setting the Cr content to 16.0 to 22.0%, more preferably 17.0 to 20.0%, the oxidation suppressing effect and manufacturability can be further improved.

<Al: aluminum>
Al is a basic alloying element necessary to improve the high-temperature oxidation resistance of ferritic stainless steel. By containing a predetermined amount or more of Al, an oxide film of Al ₂ O ₃ is formed on the surface of the stainless steel, and oxidation of the stainless steel is suppressed. Further, when REM or Y is added, the oxide film becomes denser and the adhesion to the base steel is improved, thereby suppressing the occurrence of abnormal oxidation. On the other hand, an 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 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%, high-temperature oxidation resistance and manufacturability can be further improved.

<Ni: Nickel>
Ni is an element that improves the corrosion resistance of ferritic stainless steel, and is an essential element in the ferritic stainless steel according to one embodiment of the present invention. On the other hand, an excessive Ni content destabilizes the ferrite phase and increases the material cost. Therefore, in one aspect of the present invention, the Ni content is specified to be 1.00% or less. By setting the Ni content to 0.50% or less, it is possible to further suppress destabilization of the ferrite phase and an increase in production cost due to excessive Ni content. Considering the above reasons, the more preferable Ni content is 0.02 to 0.30%.

<N: Nitrogen>
N is an essential element in the ferritic stainless steel according to one embodiment of the present invention. On the other hand, if it is excessively contained, it may combine with Al in the steel to form AlN and become a starting point for accelerated oxidation. Therefore, in one aspect of the present invention, the content of N is specified to be 0.030% or less. By setting the N content to 0.025% or less, the possibility of hardening can be further reduced. Considering the above reasons, the more preferable content of N is 0.003 to 0.020%.

<Nb (Niobium), B (Boron), REM (Rare Earth Element)>
Nb is an element added to ensure high-temperature strength. Furthermore, Nb has the effect of promoting the formation of the Al ₂ O ₃ film. In addition, it suppresses recrystallization of stainless steel and refines crystal grains, thereby widening the grain boundary area. On the other hand, an excessive Nb content may deteriorate the toughness of the hot-rolled sheet.

B is an element that improves the secondary workability and oxidation resistance of molded products manufactured using ferritic stainless steel. On the other hand, if B is contained excessively, the compound of B becomes an inclusion (impurity).

REM (rare earth metals) refers to lanthanoid elements (elements with atomic numbers of 57 to 71, such as La, Ce, Pr, Nd, and Sm). REM is an element that improves high-temperature oxidation resistance. The Al oxide film is stabilized by containing a predetermined amount or more of REM. Also, by improving the adhesion between the base material and the oxide, the oxidation resistance is improved. On the other hand, an excessive REM content causes surface defects during hot rolling, resulting in lower manufacturability.

For the above reason, in one aspect of the present invention, the Nb content is specified 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 workability can be further reduced. The upper limit of the Nb content is even more preferably 0.20% or 0.15%. Also, the content of B is defined as 0.0003 to 0.01%. By setting the content of B to 0.0003 to 0.005%, the existence of inclusions can be further reduced and the secondary workability can be improved. Also, the content of REM is defined as 0.01 to 0.20%. The content of REM is preferably 0.02-0.15%, more preferably 0.04-0.10%.

(other ingredients)
The ferritic stainless steel according to one aspect of the present invention further contains at least one element selected from Zr, V, Cu, Mo, W, Hf, Sn, Ta, Ti, Mg, and Ca as elements other than those described above. You may

<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, 0.50% or less of Zr may be contained. Considering the reduction of hardening, etc., the Zr content is more preferably 0.01 to 0.40%.

<V: Vanadium>
V is an element that improves workability and weld zone toughness. On the other hand, excessive addition of V may deteriorate the toughness of the hot-rolled sheet. One aspect of the present invention may contain 0.50% or less of V. Considering the reduction of hardening, etc., the V content is more preferably 0.02 to 0.35%.

<Cu: Copper>
Cu is an element that improves the corrosion resistance of ferritic stainless steel. On the other hand, an excessive Cu content may lead to deterioration in oxidation resistance and hot workability. Therefore, in one embodiment of the present invention, 1.0% or less of Cu may be contained. Considering material costs and the like, the Cu content is more preferably 0.01 to 0.85%.

<Mo: molybdenum>
Mo is an element that improves corrosion resistance. On the other hand, if Mo is contained excessively, the steel becomes hard, the toughness is lowered, and the material cost is increased. Therefore, in one aspect of the present invention, 2.0% or less of Mo may be contained. Considering workability, material cost, etc., the Mo content is more preferably 0.01 to 1.0%.

<W: Tungsten>
W is an element added to ensure high-temperature strength. On the other hand, an excessive W content deteriorates the toughness of the hot-rolled sheet and increases the material cost. Therefore, in one aspect of the present invention, 2.0% or less of W may be contained. Considering material costs and the like, the content of W is more preferably 0.01 to 1.0%.

<Hf: Hafnium>
Hf is an element that improves oxidation resistance. On the other hand, an excessive Hf content lowers the toughness of the hot-rolled sheet and increases the material cost. Therefore, in one embodiment of the present invention, 0.50% or less Hf may be contained. Considering toughness and material cost, the Hf content is more preferably 0.001 to 0.20%.

<Sn: Tin>
Sn (tin) is an element that improves the corrosion resistance of ferritic stainless steel. On the other hand, if Sn is excessively contained, the workability is lowered and the material cost is increased. Therefore, in one aspect of the present invention, 0.50% or less of Sn may be contained. Considering processability, cost, etc., the Sn content is more preferably 0.005 to 0.20%.

<Ta: Tantalum>
Ta is an element that improves the cleanliness and oxidation resistance of steel. On the other hand, an excessive Ta content lowers the toughness and increases the material cost. Therefore, in one aspect of the present invention, 0.5% or less of Ta may be contained. Considering toughness and material cost, the Ta content is preferably 0.40% or less. Considering the above reasons, the more preferable Ta content is 0.001 to 0.30%.

<Ti: Titanium>
Ti reacts with C and/or N to turn ferritic stainless steel into a ferritic single layer at 900-1000°C. On the other hand, if Ti is contained excessively, TiO ₂ is generated in the Al oxide, which may deteriorate the oxidation life. Therefore, in one aspect of the present invention, 0.20% or less of Ti may be contained. Considering workability and the like, the Ti content is more preferably 0.005 to 0.10%.

<Mg: Magnesium>
Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizing agent. On the other hand, an excessive Mg content lowers the toughness of the steel and lowers the manufacturability. Therefore, in one aspect of the present invention, 0.015% or less of Mg may be contained. Considering the above reasons, the preferred content of Mg is 0.0002 to 0.0080%.

<Ca: Calcium>
Ca is an element that improves hot workability. On the other hand, if Ca is excessively contained, the toughness of the steel is lowered and the manufacturability is lowered. Therefore, in one aspect of the present invention, 0.015% or less of Ca may be contained. Considering the above reasons, the preferable content of Ca is 0.0001 to 0.012%.

The ferritic stainless steel according to the present embodiment may satisfy 100×[C]/[Nb]≦35, where [C] is mass % of C and [Nb] is mass % of Nb. As a result, Nb-based carbides are generated during hot rolling or annealing, which increases the amount of accumulated strain in the final cold rolling, making it possible to obtain the desired dislocation density.

(dislocation density)
The ferritic stainless steel according to the present embodiment has a dislocation density ρ of 0.91×10 ¹⁴ [m ⁻² ] or higher as determined by the Williamson and Hall method using X-ray diffraction. X-ray diffraction is measured from the surface in this embodiment.

Dislocation density is a value that indicates the amount of dislocations in a crystal, and is the number of coordination lines [m ⁻² ] penetrating a unit area of a crystal cross section or the length of dislocation lines present in a unit volume of a crystal. is indicated by the sum of [m/m ⁻³ ]. Since the ferritic stainless steel according to the present embodiment has a dislocation density ρ of 0.91×10 ¹⁴ [m ⁻² ] or more, Al and Cr diffuse rapidly, and an alumina layer can be formed quickly. . Therefore, oxidation resistance can be improved.

In this embodiment, the dislocation density ρ[m ⁻² ] is derived using the Williamson and Hall method. More specifically, for example, it is derived as follows. That is, an X-ray diffractometer using a Co tube as an X-ray source measured α(110) 52.2°, α(211) 99.3°, α(229) A diffraction intensity curve is measured for each diffraction peak (2θ) at 123.3°. The diffraction peak (2θ) in the obtained diffraction intensity curve is separated into a peak due to the Kα ₁ line and a peak due to the Kα ₂ line. Regarding the diffraction peaks due to the separated Kα ₁ line, the peak top method is used to specify the diffraction angle 2θ, and the angle between half the peak intensity is calculated as the half width. The true half-value width β can be calculated using the following formula (1) using the half-value width β _m of the steel material after cold rolling and the half-value width β ₀ of the steel material after final annealing. .

β ² = β _m ² - β ₀ ² (1)
The true half-value width β calculated by the above formula ( ₁ ) is obtained by the following formula ( ₂ ).

β = β ₁ + β ₂ (2)
The FWHM broadening β ₁ due to the crystallite size (D) is represented by the following formula (3), and the FWHM broadening β ₂ due to the strain (ε) is represented by the following formula (4). It is known.

β ₁ =0.9λ/(D cos θ) (3)
β ₂ =2ε tan θ (4)
Here, λ in Equation (3) is the wavelength of X-rays.

The following formula (5) is obtained by rearranging the above formula (2) using the above formula (3) and formula (4).

βcos θ/λ=(0.9/D)+(2εsin θ/λ) (5)
As shown in the above formula (5), the strain ε can be calculated from the slope of the graph created by plotting β cos θ/λ against sin θ/λ.

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

ρ=(14.4×ε ² )/b ² (3)
(Nb carbide)
For the ferritic stainless steel according to the present embodiment, a scanning electron microscope (SEM) is used to observe a cross section cut along a plane perpendicular to the rolling direction at three random locations within a range of 30 μm × 30 μm. The average number of Nb carbides having a Nb concentration of 5 wt % or more and a particle size of 0.1 μm or more measured by EDS analysis is 2 or more and 15 or less. When the average value is 2 or more, strain is likely to be accumulated in the structure during cold rolling. Further, when the average value is 15 or less, the toughness of the stainless steel is less likely to decrease. The particle size of the carbide is calculated from the size of particles in an image taken with a scanning electron microscope. Specifically, the particle diameter of the carbide is defined as the average width between the width with the largest distance and the width with the smallest distance in the carbide.

(alumina layer)
The ferritic stainless steel according to the present disclosure can be suitably applied to applications requiring oxidation resistance at high temperatures. Therefore, use conditions mean high temperature conditions. Below, the alumina layer 10 formed when the ferritic stainless steel according to the present disclosure is heated at 1050° C. for 50 hours will be described.

As a result of intensive research, the present inventors have found that when Nb, Cr, and REM are contained as essential elements in ferritic stainless steel at concentrations within appropriate ranges, columnar alumina layers are formed under conditions of use. It was found that the crystallization was improved. This is believed 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 this embodiment, the total concentration of Nb oxides, Cr oxides and REM oxides present in grain boundaries is 3.5 wt % or more. Since this suppresses the inward diffusion of oxygen, the alumina layer 10 has excellent oxidation resistance. That is, the ferritic stainless steel according to this embodiment has excellent oxidation resistance under high temperature conditions.

In addition, the present inventors have found that the columnar crystallization is also improved by containing B as an essential element at a concentration within an appropriate range.

The ferritic stainless steel according to the present embodiment has C: 0.030% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.00%, and P: 0.050% by mass. % 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: 0.0003-0.01%, and REM: 0.01-0.20%.

The alumina layer 10 formed by heating the ferritic stainless steel containing the above components at 1050° C. for 50 hours has the following characteristics. That is, in a cross section of the alumina layer 10 cut in the thickness direction, the total grain boundary length included in any region having an area of 2.25 μm ² is 5.5 μm or less.

FIG. 1 is a partially enlarged schematic cross-sectional view of an alumina layer 10 formed by heating an exemplary ferritic stainless steel according to the present embodiment at 1050° C. for 50 hours, cut in the thickness direction. A region with an arbitrary area of 2.25 μm ² as shown in FIG. The grain boundary length included in the region having an area of 2.25 μm ² is the sum of the lengths of all the grain boundaries GB existing within the region having an area of 2.25 μm ² . In the example shown in FIG. 1, the grain boundary length included in the region of 2.25 μm ² is 5.5 μm or less.

FIG. 2 is a partially enlarged schematic view of a comparative alumina layer 20 in which the grain boundary length contained in 2.25 μm ² is longer than 5.5 μm. As shown in FIG. 2, the comparative alumina layer 20 in which the grain boundary length included in 2.25 μm ² is longer than 5.5 μm has a higher proportion of equiaxed grains than the example shown in FIG.

Here, the columnar crystal means a structure in which crystal grains grown elongated in the thickness direction of the alumina layer are arranged. The equiaxed crystal means a polycrystalline structure in which the shape and orientation of crystal grains constituting the equiaxed crystal are isotropic.

As is clear from a comparison between FIG. 1 and FIG. 2, the alumina layer with a high ratio of columnar crystals (FIG. 1) has a higher per unit area than the alumina layer with a high ratio of equiaxed crystals (FIG. 2). It can be seen that the length of the grain boundary GB becomes shorter.

The grain boundary length included in an arbitrary 2.25 μm ² in the cross section of the alumina layer 10 according to this embodiment cut in the thickness direction is 5.5 μm or less. In other words, the alumina layer 10 has a high columnar crystal ratio. Since equiaxed crystals have a higher grain boundary density than columnar crystals, grain boundary diffusion paths for oxygen increase. Therefore, equiaxed crystals have a shorter oxidation life than columnar crystals. Therefore, the ferritic stainless steel according to the present embodiment has a high columnar crystal ratio, and thus has excellent oxidation resistance under high-temperature conditions.

(Production method)
First, an example of the manufacturing process of ferritic stainless steel in this embodiment will be schematically described. The manufacturing process of ferritic stainless steel in this embodiment includes a pretreatment process, a hot rolling process, an annealing process, a pickling process, and a cold rolling process.

In the pretreatment process, first, a vacuum or argon atmosphere melting furnace is used to melt steel with a composition adjusted to fall within the scope of the present invention, and this steel is cast to produce a slab. Thereafter, slab pieces for hot rolling are cut from the slab. Then, the slab piece is heated to a temperature range of 1100° C. to 1300° C. in the atmosphere. The time to heat and hold the slab pieces is not limited. In addition, when performing a pretreatment process industrially, the said casting may be continuous casting.

The hot rolling process is a process of hot rolling the slab (steel ingot) obtained in the pretreatment process to produce a hot rolled steel strip with a predetermined thickness.

The annealing process is a process for softening the steel strip by heating the hot rolled steel strip obtained in the hot rolling process to, for example, 900 to 1050°C. In the annealing step, the steel strip after annealing is cooled from the annealing temperature to 400° C. for 30 seconds or longer. As a result, Nb carbide can be precipitated inside the structure (that is, grain boundaries and grain interiors).

The pickling process is a process of washing off scale adhering to the surface of the annealed steel strip obtained by the annealing process using a pickling solution such as hydrochloric acid or a mixed solution of nitric acid and hydrofluoric acid.

The cold rolling process is a process of further thinning the annealed steel strip from which the scale has been removed in the first pickling process. The rolling reduction in the cold rolling process is 65% or more, preferably 75% or more. By setting the rolling reduction in the cold rolling step to 65% or more, the strain in the steel can be increased. More specifically, by setting the rolling reduction in the cold rolling step to 65% or more, the dislocation density ρ derived by the Williamson and Hall method using X-ray diffraction is 0.91×10 ¹⁴ [m ⁻² ]. That's it. In other words, in order to produce a cold-rolled sheet in which the dislocation density ρ derived by the Williamson and Hall method using X-ray diffraction is 0.91×10 ¹⁴ [m ⁻² ] or more, rolling in the cold rolling step The ratio should be 65% or more, preferably 75% or more.

A series of steps from the annealing step to the cold rolling step may be performed multiple times. When the series of steps is single, the annealing step is called the final annealing step. When the series of steps is performed multiple times, the final annealing step is called the final annealing step, and the other annealing steps are called intermediate annealing steps.

Moreover, in the manufacturing method according to the present embodiment, the rolling reduction in the cold rolling step after the final annealing step is 65% or more. In other words, the cold rolling step after the final annealing step is cold rolling until the dislocation density ρ derived by the Williamson and Hall method using X-ray diffraction reaches 0.91 × 10 ¹⁴ [m ^-2 ] or more. This is the rolling process.

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 this embodiment is a cold rolled steel strip after the cold rolling process. Since the ferritic stainless steel is a cold-rolled steel strip, strain remains accumulated in the steel, accelerating the diffusion of Al and Cr. Therefore, an alumina layer can be formed early under high-temperature conditions, and high high-temperature oxidation resistance can be realized. Moreover, since there is no need to perform final annealing after cold rolling, manufacturing costs can be reduced.

(summary)
The ferritic stainless steel according to aspect 1 of the present disclosure has, 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: 15.0-25.0%, Al: 2.0-4.0%, Ni: 1.00% or less, Nb: 0.01-0 .70%, N: 0.030% or less, B: 0.0003 to 0.01%, REM: 0.01 to 0.20%, the balance being Fe and unavoidable impurities, X-ray diffraction The dislocation density ρ derived by the Williamson and Hall method using is 0.91×10 ¹⁴ [m ⁻² ] or more.

According to the above configuration, since the Al content is 4.0% or less, the toughness is excellent. In addition, since the dislocation density ρ is 0.91×10 ¹⁴ [m ⁻² ] or more, an alumina layer can be formed early, so that a ferritic stainless steel having excellent oxidation resistance at high temperatures can be realized. be able to.

The ferritic stainless steel according to aspect 2 of the present disclosure forms an alumina layer mainly composed of alumina when heated at 1050 ° C. for 50 hours in the above aspect 1, and the alumina layer is formed in the thickness direction of the alumina layer. The total length of grain boundaries included in an arbitrary region having an area of 2.25 μm ² may be 5.5 μm or less in a cross section when cut into two.

In the ferritic stainless steel according to Aspect 3 of the present disclosure, in Aspect 2 above, the total concentration of Nb oxides, Cr oxides, and REM oxides present in grain boundaries in the alumina layer is 3.5 wt% or more. may be

The ferritic stainless steel according to Aspect 4 of the present disclosure is, in any one of Aspects 1 to 3, Zr: 0.50% or less, V: 0.50% or less, and Cu: 1.0% or less in terms of % by mass. , 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 Ca: 0.015% or less, and Ca: 0.015% or less.

In the ferritic stainless steel according to aspect 5 of the present disclosure, in any one of aspects 1 to 4, when [C] is the mass% of C and [Nb] is the mass% of Nb, 100 × [C] /[Nb]≦35 may be satisfied.

In the method for producing ferritic stainless steel according to aspect 6 of the present disclosure, 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: 15.0-25.0%, Al: 2.0-4.0%, Ni: 1.00% or less, Nb: 0.00%. 01 to 0.70%, N: 0.030% or less, B: 0.0003 to 0.01%, REM: 0.01 to 0.20%, the balance consisting of Fe and unavoidable impurities, A method for producing ferritic stainless steel, comprising: annealing a steel strip after hot rolling; cooling the steel strip after annealing from the annealing temperature to 400° C. for 30 seconds or longer; and final annealing. After the step, a cold rolling step of rolling until the dislocation density ρ derived using the Williamson and Hall method is 0.91×10 ¹⁴ [m ⁻² ] or more.

According to the above configuration, ferritic stainless steel with excellent toughness and high-temperature oxidation resistance can be realized.

In the method for manufacturing ferritic stainless steel according to Aspect 7 of the present disclosure, in Aspect 6 above, the rolling reduction in the cold rolling step may be 65% or more.

In the method for producing ferritic stainless steel according to aspect 8 of the present disclosure, in aspect 6 or 7, the ferritic stainless steel obtained through the cold rolling step is measured in the rolling direction using a scanning electron microscope. When a cross section cut along a vertical plane is randomly observed at three locations in the range of 30 μm × 30 μm each, the Nb concentration measured by energy dispersive X-ray analysis is 5 wt% or more and the particle diameter is 0.1 μm or more. The average number of carbides of 2 or more and 15 or less may be present.

A method for producing ferritic stainless steel according to aspect 9 of the present disclosure is, in any one of aspects 6 to 8, wherein the ferritic stainless steel obtained through the cold rolling step is heated at 1050° C. for 50 hours. In this case, an alumina layer is formed, and the alumina layer has an area of ^2.25 μm in a cross section when the alumina layer is cut in the thickness direction. It may be below.

A method for producing ferritic stainless steel according to Aspect 10 of the present disclosure is the method for producing ferritic stainless steel according to Aspect 9 above, wherein the total concentration of Nb oxide, Cr oxide and REM oxide present in grain boundaries in the alumina layer is 3.0%. It may be 5 wt % or more.

A method for producing ferritic stainless steel according to aspect 11 of the present disclosure is the method according to any one of aspects 6 to 10, wherein the ferritic stainless steel contains, by 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.

A method for producing a ferritic stainless steel according to Aspect 12 of the present disclosure is, in any one of Aspects 6 to 11, wherein the ferritic stainless steel contains [C] as % by mass of C, and [Nb] as % by mass of Nb. , 100×[C]/[Nb]≦35 may be satisfied.

In the method for manufacturing ferritic stainless steel according to Aspect 13 of the present disclosure, in any one of Aspects 6 to 12 above, the heating temperature in the final annealing step may be 900°C to 1050°C. With this configuration, it is possible to obtain an annealed steel strip suitable for realizing a ferritic stainless steel excellent in toughness and oxidation resistance at high temperatures.

〔Example〕
EXAMPLES In order to evaluate the physical properties of the ferritic stainless steel of the present invention, ferritic stainless steels were produced from the ingredients shown in Table 1 below as invention example steels and comparative example steels. In Table 1, the steel type No. 1 to 16 are ferritic stainless steels as examples of the present invention produced within the scope of the present invention. Also, in Table 1, the steel type No. Nos. 17 to 27 are ferritic stainless steels as comparative examples produced under conditions outside the scope of the present invention.

　In order to manufacture the steel materials of the steel grades shown in Table 1, first, the steel having the components shown in Table 1 was vacuum melted to produce a 30 kg slab. After heating the slab at 1230° C. for 2 hours, it was subjected to hot rolling to prepare a hot-rolled sheet having a thickness of 3 mm. The obtained hot-rolled sheet was annealed between 900 and 1050° C. to prepare a hot-rolled and annealed sheet. The obtained hot-rolled and annealed sheet was cold-rolled and annealed twice, and then finally cold-rolled to produce a cold-rolled sheet with a thickness of 50 μm. Table 2 shows the cooling time from the annealing temperature to 400°C in the annealing process.

The cold rolling up to the second time was carried out at a rolling reduction of 60 to 85% for both the inventive examples and the comparative examples, and the annealing after cold rolling was carried out in the temperature range of 900 to 1050°C. The rolling reduction in the final cold rolling is described in the column of "Final rolling reduction" in Table 2. As shown in Table 2, the rolling reduction in the final cold rolling of the inventive examples is 65% or more. On the other hand, the rolling reduction in the final cold rolling of the comparative example is less than 65%. In addition, the manufacturing method described in this embodiment is an example, and the manufacturing method is not limited.

Table 1 shows the composition of the components contained in each steel type in mass%. The balance other than the components shown in Table 1 is Fe or a small amount of unavoidable impurities (unavoidable impurities). The underlines in Table 1 indicate that the range of each component contained in each stainless steel according to the comparative examples of the present invention is outside the scope of the present invention.

(Measurement of dislocation density)
Below, the measurement of the dislocation density ρ performed on the cold-rolled sheets of the steel types of the present invention and the comparative steel types shown in Table 1 will be described. The dislocation density ρ was measured according to the method described in the section (dislocation density) in the embodiment. Table 2 shows the measurement results of the dislocation density ρ. Steel type No. which is an example of the present invention. 1 to 16 all had a dislocation density ρ of 0.91×10 ¹⁴ [m ⁻² ] or more. On the other hand, comparative example steel No. All of Nos. 17 to 27 had a dislocation density ρ of less than 0.91×10 ¹⁴ [m ⁻² ]. From these results, it was demonstrated that the dislocation density ρ was 0.91×10 ¹⁴ [m ⁻² ] or more when the rolling reduction in the final cold rolling was 65% or more. On the other hand, it was demonstrated that when the rolling reduction in the final cold rolling is less than 65%, the dislocation density ρ is less than 0.91×10 ¹⁴ [m ⁻² ].

(Nb carbide)
The number of Nb carbides present in the structure of the cold-rolled steel sheets of the invention example steels and comparative example steels shown in Table 1 was investigated. The investigation was conducted as follows. First, the cold-rolled sheet was cut along a plane perpendicular to the rolling direction. Next, using a scanning electron microscope, the cut surface is observed at random three places in the range of 30 μm × 30 μm each, and the Nb concentration measured by energy dispersive X-ray analysis is 5 wt% or more and the particle diameter is 0. .The average number of carbides of 1 μm or more was calculated. The calculated average value of Nb carbides is shown in Table 2 as "average number of Nb carbides". As shown in Table 2, the steel type No. 1, which is an example of the present invention. 1 to 16, the average number of carbides was in the range of 2 to 15.

(Measurement of grain boundary length)
First, the cold-rolled sheets of the steel grades of the invention examples and the steel grades of the comparative examples shown in Table 1 were heated at 1050° C. for 50 hours. After heating, each steel plate was observed by STEM from the cross section. The STEM observation was performed using an HD-2700 manufactured by Hitachi High-Tech at a voltage of 200 V and an observation magnification of 30,000. For the measurement of the alumina grain boundary length, a range of 1.5 μm×1.5 μm was randomly selected from the central portion of the alumina film, and the total grain boundary length within the range was determined. The measured length was the average value of three randomly selected locations.

In the determination of the grain boundary length in Table 2, “◯ (good)” indicates a grain boundary length of 5.5 μm or less within the range of 2.25 μm ² , and “× (poor)” indicates 2.25 μm ^2. The grain boundary length within the range of is greater than 5.5 μm.

(Element concentration in grain boundary of alumina layer)
The concentrations of Nb, Cr and REM elements in the alumina grain boundaries of the inventive examples and comparative examples shown in Table 1 are described below.

First, the cold-rolled sheets of the steel grades of the invention examples and the steel grades of the comparative examples shown in Table 1 were heated at 1050°C for 50 hours. After heating, each steel sheet was observed from the cross section, and the concentration of each element in the grain boundary was measured by STEM-EDX. The STEM image observation was carried out by spot analysis of the central part of the grain boundary using an HD-2700 manufactured by Hitachi High-Tech Corporation at a voltage of 200 V and an observation magnification of 4,000,000 times. For EDX (energy dispersive X-ray analysis), an energy dispersive X-ray spectrometer EDAX Octane T Ultra W manufactured by Ametech was used. Analysis time was 300 seconds.

Table 2 shows the sum of the element concentrations of Nb, Cr, Ce, La, and Nd. This value is the total concentration of Nb oxide, Cr oxide and REM oxide in the grain boundary.

　In the determination of the concentrated element concentration in Table 2, when the total concentration of Nb oxide, Cr oxide and REM oxide in the grain boundary was 3.5 wt% or more, it was indicated as "Good (good)". On the other hand, when it was less than 3.5 wt%, it was described as "x (defective)" in Table 2.

(High-temperature oxidation resistance evaluation test)
In the following, high-temperature oxidation resistance evaluation tests conducted on the invention examples and comparative examples shown in Tables 1 and 2 will be described. First, for each of the steel grades shown in Table 1, three test pieces of 20 mm width and 25 mm length were taken from the cold-rolled sheet having a thickness of 50 μm described in the production of the steel material. The test piece was subjected to an air atmosphere at 1050° C. for 50 hours, and the average weight gain by oxidation of the three pieces was measured. This high-temperature oxidation resistance property evaluation test was conducted in the atmosphere using an EREMA electric furnace. The results are shown in Table 3 below. In the determination of high-temperature oxidation resistance in Table 3, "○ (good)" indicates that the average oxidation weight gain was 1 mg/cm ² or less, and "× (poor)" indicates that it exceeded 1 mg/cm ^{2 .} ing.

(Toughness evaluation test)
Toughness evaluation tests performed on the inventive examples and comparative examples shown in Table 1 will be described below. First, a test piece used in this evaluation test was produced based on the V-notch test piece of the JIS standard (JIS Z 2242 (2018)). The plate thickness was adjusted by surface-cutting the 3 mm-thick hot-rolled plate described in the production of the steel material to a plate thickness of 2.5 mm. A test piece was taken from the steel plate so that the longitudinal direction of the test piece was parallel to the rolling direction. Also, a notch was made in the test piece so as to be perpendicular to the rolling direction.

This evaluation test was conducted based on the JIS standard (JIS Z 2242 (2018)). This evaluation test was carried out at room temperature (23° C.±2° C.) for 5 pieces of each steel type to determine the Charpy impact value (absorbed energy). In this evaluation test, an IC-30B type Charpy impact tester manufactured by Tokyo Shoki Seisakusho Co., Ltd. was used. The results are shown in Table 2 below. In Table 2, "◯ (good)" indicates a Charpy impact value of 20 J/cm ² or more, and "× (poor)" indicates a Charpy impact value of less than 20 J/cm ² .

As shown in Table 2, invention example steel No. 1 to 16 all met the above criteria for high temperature oxidation resistance and toughness. Comparative Example Steel No. Nos. 17 to 27 did not meet the above criteria in either or both high temperature oxidation resistance and toughness.

That is, it was demonstrated that the ferritic stainless steel within the scope of the present invention is excellent in high-temperature oxidation resistance and toughness.

Comparative example steel No. The reason why No. 17 to No. 27 do not show better results than the invention example steels will be explained.

　Comparative example steel No. In No. 17, the grain boundary length satisfies the standard, but because the B content is low, the concentrations of Nb, Cr and REM in the grain boundary do not satisfy the standard. As a result, excellent results in high-temperature oxidation resistance were not obtained.

　Comparative example steel No. In No. 18, the content of REM was less than 0.01%, so the concentrations of Nb, Cr and REM in the grain boundaries did not meet the criteria. Comparative example steel type No. In No. 18, since the Ti content was more than 0.20%, equiaxed crystallization was likely to occur and high-temperature oxidation resistance was not excellent.

　Comparative example steel No. In No. 19, the content of Nb was less than 0.01%, the concentrations of Nb, Cr and REM in the grain boundaries did not meet the criteria, and the result was not excellent in high-temperature oxidation resistance.

　Comparative example steel No. In No. 20, since the Zr content was more than 0.50%, Zr was likely to segregate at the alumina grain boundaries, promoting the equiaxed crystallization of crystals, and did not exhibit excellent results in high-temperature oxidation resistance.

Comparative Example Steel No. No. 21 has a Si content of more than 1.5%, and shows excellent results in high-temperature oxidation resistance due to the influence of Si-based oxides such as SiO ₂ . On the other hand, comparative example steel No. No. 21 had a Si content of more than 1.5% and did not show good results for toughness.

Comparative Example Steel No. In No. 22, the Al content is lower than 2.0%, and an oxide film of Al ₂ O ₃ is difficult to form. As a result, the oxygen partial pressure becomes high, and equiaxed crystals are likely to occur, so good results were not obtained in terms of high-temperature oxidation resistance.

　Comparative example steel No. In No. 23, since the Ti content was higher than 0.20%, equiaxed crystallization was likely to occur, and good results in high-temperature oxidation resistance were not obtained.

　Comparative example steel No. In No. 24, the Nb content was higher than 0.70%, and equiaxed crystallization was likely to occur, and therefore, the high-temperature oxidation resistance did not show good results.

　Comparative example steel No. No. 25 did not show good results for toughness because the Al content was higher than 4.0%.

　Comparative example steel No. In No. 26, the Cr content was higher than 25.0%, Cr was concentrated at the alumina grain boundary, and equiaxed crystals were likely to be formed, and did not exhibit good results in terms of high-temperature oxidation resistance.

Comparative Example Steel No. No. 27 did not show good results in terms of toughness due to the formation of oxides such as Y ₂ O ₃ or CeO ₂ due to the REM content higher than 0.20%.

10 alumina layer

Claims

% by 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 balance being Fe and unavoidable impurities, and the dislocation density ρ derived using the Williamson and Hall method is 0.00. 91×10 14 [m −2 ] or more,
Using a scanning electron microscope, when a cross section cut along a plane perpendicular to the rolling direction is randomly observed at three locations in the range of 30 μm × 30 μm each, the Nb concentration measured by energy dispersive X-ray analysis is 5 wt. % or more and the average number of carbides having a particle size of 0.1 μm or more is 2 or more and 15 or less.
Forming an alumina layer mainly composed of alumina when heated at 1050 ° C. for 50 hours,
2. The alumina layer has a total grain boundary length of 5.5 μm or less in an arbitrary region having an area of 2.25 μm 2 in a cross section when the alumina layer is cut in the thickness direction. A ferritic stainless steel as described.
The ferritic stainless steel according to claim 2, wherein the alumina layer has a total concentration of 3.5 wt% or more of Nb oxide, Cr oxide and REM oxide existing in grain boundaries.
% by 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% Further contains one or more of 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 The ferritic stainless steel according to any one of claims 1 to 3.
When [C] is mass % of C and [Nb] is mass % of Nb,
4. The ferritic stainless steel according to claim 1, which satisfies 100*[C]/[Nb]≦35.
% by 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 balance being Fe and inevitable impurities, a method for producing ferritic stainless steel,
An annealing step in which the 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 a cold rolling step of rolling until the dislocation density ρ derived using the Williamson and Hall method reaches 0.91×10 14 [m −2 ] or more after the final annealing step. Production method.
The method for producing ferritic stainless steel according to claim 6, wherein the rolling reduction in the cold rolling step is 65% or more.
The ferritic stainless steel obtained through the cold-rolling step was observed with a scanning electron microscope at random three locations in the range of 30 μm × 30 μm in cross section cut along a plane perpendicular to the rolling direction. Claim 6 or 7, wherein the average number of carbides having a Nb concentration of 5 wt% or more and a particle diameter of 0.1 µm or more as measured by energy dispersive X-ray analysis is 2 or more and 15 or less. A method for producing ferritic stainless steel according to 1.
The ferritic stainless steel obtained through the cold rolling step forms an alumina layer when heated at 1050° C. for 50 hours,
6 or 8. The method for producing ferritic stainless steel according to 7.
The method for producing ferritic stainless steel according to claim 9, wherein the alumina layer has a total concentration of 3.5 wt% or more of Nb oxides, Cr oxides and REM oxides present in grain boundaries.
The ferritic stainless steel is, 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 The method for producing ferritic stainless steel according to claim 6 or 7, further comprising one or more of them.
In the ferritic stainless steel, when [C] is mass % of C and [Nb] is mass % of Nb,
8. The method for producing ferritic stainless steel according to claim 6, wherein 100×[C]/[Nb]≦35 is satisfied.