WO2024203317A1 - Ferritic stainless steel sheet - Google Patents

Abstract

Provided are a ferritic stainless-steel sheet that has low proof stress and ridging resistance and reduces in-plane anisotropy while having a high r value, and a method for producing the same. The ferritic stainless steel sheet (1) has a crystal grain size of 15-40 μm inclusive and satisfies the relationships I₁₁₁ ≥ 10.0 and 0.2 ≤ (Ia/Ib) ≤ 4.0 when the larger of the [111]<110> crystal orientation strength Ia and the [111]<112> crystal orientation strength Ib in a cross-section (13) parallel to the rolling surface in the plate thickness center is expressed as I₁₁₁.

Description

Ferritic Stainless Steel Sheet

The present invention relates to ferritic stainless steel sheets.

When ferritic stainless steel sheet is deep drawn, unevenness may appear on the surface of the formed product due to the in-plane anisotropy of the material structure. In addition, ridge-like undulations may appear on the surface of the formed product along the rolling direction of the ferritic stainless steel sheet; this type of surface defect is generally referred to as ridging.

　Technologies to improve the formability of ferritic stainless steel sheets have been studied (see, for example, Patent Documents 1 to 5).

Japanese Patent No. 5505555 Japanese Patent No. 4083669 Japanese Patent No. 2772237 Japanese Patent Application Publication No. 8-311542 Japan Special Publication No. 51-35369

When ferritic stainless steel has high yield strength, the forming load during forming increases and the shape and dimensional accuracy after processing can deteriorate (springback). Coarsening the crystal grains is an effective way to reduce yield strength, but if a texture in a specific orientation develops during the grain growth process, the in-plane anisotropy increases. Furthermore, reducing the in-plane anisotropy tends to reduce the r-value and worsen ridging.

One aspect of the present invention aims to provide a ferritic stainless steel sheet that has a high r-value, reduced in-plane anisotropy, and low yield strength and ridging resistance.

In order to solve the above problems, a ferritic stainless steel sheet according to one embodiment of the present invention contains, by mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, Cr: 10.5-30.0%, N: 0.001-0.030%, and P: 0.005-0.050%, and also contains at least one of Ti: 0.01-0.50% and Nb: 0.01-0.50%, and the S content is 0.0100% or less. and the balance being Fe and unavoidable impurities, the crystal grain size calculated by a cutting method in a cross section parallel to the rolling direction of the ferritic stainless steel plate and perpendicular to the rolling surface is 15 μm or more and 40 μm or less, and in a cross section parallel to the rolling surface in the center part of the plate thickness of the ferritic stainless steel plate, the {111}<110> crystal orientation intensity is Ia and the {111}<112> crystal orientation intensity is Ib, the larger of the values Ia and Ib is referred to as _I111 , and the relationships _I111 ≧10.0 and 0.2≦(Ia/Ib)≦4.0 are satisfied.

According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet that has a high r-value, reduced in-plane anisotropy, and low yield strength and ridging resistance.

FIG. 1 is a schematic diagram showing a cross section of a stainless steel plate according to one embodiment of the present invention. FIG. 1 is a schematic diagram showing a cross section of a stainless steel plate according to one embodiment of the present invention. 1 is a flowchart showing an example of a method for producing a ferritic stainless steel sheet according to one embodiment of the present invention.

The following describes the embodiments of the present invention. Note that the following description is intended to provide a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. In this application, "A to B" indicates that the range is greater than or equal to A and less than or equal to B.

In this specification, the term "ferritic stainless steel" is not limited to the specific shape of steel strip, steel plate, etc., and is used to explain the properties of the material itself. Also, since a "steel plate" can be considered to be a part of a "steel strip," "ferritic stainless steel plate" includes a "ferritic stainless steel strip."

The ferritic stainless steel sheet in one embodiment of the present invention has a chemical composition that is a single phase of ferrite up to the melting point, has reduced carbon and nitrogen content, and contains carbide stabilizing elements (Ti, Nb, etc.). Ferritic stainless steel with such a chemical composition is sometimes called high-purity ferritic stainless steel.

<Overview of the present invention>
In general, the formability (deep drawability) of a ferritic stainless steel sheet can be evaluated by the r-value (Lankford value, plastic working strain ratio). Typically, the deep drawability can be evaluated by the average r-value obtained by averaging multiple r-values obtained by measuring multiple different in-plane directions based on the rolling direction. In addition, the in-plane anisotropy can be evaluated by the in-plane anisotropy index Δr calculated from the multiple r-values.

Ferritic stainless steel sheets (high-purity ferritic stainless steel sheets) that have a chemical composition that does not undergo austenite transformation when heated have texture in their metal structure, and are prone to the presence of colony textures (hereafter referred to as colonies). These colonies are formed by the aggregation of crystal grains with similar crystal orientations. The crystal orientation of the colony differs from the crystal orientation of the texture in the surrounding area of the colony.

　There is a known process that omits hot-rolled sheet annealing in order to reduce the in-plane anisotropy Δr, but in this case the colonies are not sufficiently separated, and ridging worsens. On the other hand, when hot-rolled sheet annealing is performed, ridging improves due to the separation of the colonies, but in-plane anisotropy increases because only certain orientations are preferentially developed. For this reason, it is not easy to improve ridging resistance while maintaining a small Δr.

The inventors have conducted extensive research into technology that reduces in-plane anisotropy while maintaining a high r-value, and also provides low yield strength and resistance to ridging, and have arrived at the present invention. The ferritic stainless steel sheet in one embodiment of the present invention is realized by a manufacturing process approach that differs from conventional approaches. The various properties and manufacturing method of the ferritic stainless steel sheet in one embodiment of the present invention will be described in detail later.

(Regarding the Prior Art)
In order to facilitate understanding of the method for producing a ferritic stainless steel sheet according to one embodiment of the present invention, the techniques described in Patent Documents 1 to 5 will be briefly described below.

　Generally, the manufacturing process for stainless steel sheets includes, in this order, a steelmaking process, a hot rolling process, a hot-rolled sheet annealing process, intermediate processes such as cold rolling, and a final annealing process.

In the technology described in Patent Document 1, the forming workability is improved by refining the crystal grains, and the reduction in the yield strength is not taken into consideration. In the technology described in Patent Documents 2 and 4, the texture is controlled in the hot rolling process, and the hot-rolled sheet annealing process is omitted. Patent Document 2 describes that by increasing the concentration of both the {111}<112> orientation and the {111}<011> orientation, the in-plane anisotropy is reduced while maintaining a high r value, but the objective is not to achieve both low yield strength and ridging resistance. Patent Document 4 describes that by highly reducing the carbon content to 0.005% or less and the nitrogen content to 0.012% or less, the texture can be effectively controlled in the hot rolling process, and the in-plane anisotropy is reduced while maintaining ridging resistance and a high r value. However, in the technology described in Patent Documents 2 and 4, the hot-rolled sheet annealing process is omitted, so ridging may worsen. The technology described in Patent Document 4 takes ridging resistance into consideration, but the improvement in ridging resistance may be limited, and highly reducing the carbon and nitrogen content leads to problems with production costs.

Patent Document 3 describes reducing in-plane anisotropy while retaining ridging resistance and a high r-value by controlling the conditions of the hot rolling process. However, it is not intended to combine low yield strength and low Δr value. When the annealing temperature of the cold-rolled sheet is lowered, the crystal grains become finer and the yield strength increases, while when the annealing temperature of the cold-rolled sheet is increased, the yield strength decreases while Δr increases. In addition, the technology described in Patent Document 5 reduces the carbon and nitrogen content and further fixes carbon and nitrogen by adding Ti, improving the workability of ferritic stainless steel. However, the ridging resistance is unknown, and recrystallization can be suppressed by Ti-based precipitates. Therefore, there is a limit to increasing the r-value.

<Steel plate composition>
First, the chemical composition of the ferritic stainless steel sheet according to one embodiment of the present invention will be described below. In the following description, the ferritic stainless steel sheet according to one embodiment of the present invention will sometimes be abbreviated as "the present stainless steel sheet."

The stainless steel plate may have a chemical composition that contains, by mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, Cr: 10.5-30.0%, N: 0.001-0.030%, and P: 0.005-0.050%, as well as at least one of Ti: 0.01-0.50% and Nb: 0.01-0.50%, with an S content of 0.0100% or less.

The stainless steel plate may have a chemical composition with the remainder being iron (Fe) and unavoidable impurities. Each of the above elements is explained below.

(C: Carbon)
C is an element that forms carbides with Cr and the like, thereby generating interfaces that become the source of dislocations when ferritic stainless steel is deformed. However, if C is added in excess, the intergranular corrosion resistance and workability decrease, and the cost required for refining increases. Therefore, the C content may be 0.001 to 0.030 mass%, 0.001 to 0.020 mass%, or 0.002 to 0.010 mass%.

(Si: Silicon)
Silicon has an effect as a deoxidizer in the smelting stage. However, if excessive silicon is added, the ferritic stainless steel becomes hard and its ductility decreases. Therefore, the silicon content may be 0.01 to 1.00 mass%, 0.02 to 0.70 mass%, or 0.03 to 0.30 mass%.

(Mn: Manganese)
Mn has an effect as a deoxidizer. However, if Mn is added in excess, the amount of MnS produced increases, and the corrosion resistance of the ferritic stainless steel decreases. Therefore, the Mn content may be 0.01 to 1.00 mass%, 0.02 to 0.70 mass%, or 0.03 to 0.30 mass%.

(Cr: Chromium)
Cr is necessary to form a passive film on the surface of the cold-rolled steel sheet to improve corrosion resistance. However, if Cr is added in excess, the ductility of the ferritic stainless steel decreases. Therefore, the Cr content may be 10.5 to 30.0 mass%, or 12.0 to 25.0 mass%.

(N: nitrogen)
N is an element that forms nitrides with Cr and the like, thereby generating interfaces that become the source of dislocations when ferritic stainless steel is deformed. However, if excessive N is added, ductility decreases due to solid solution strengthening. Therefore, the N content may be 0.001 to 0.030 mass%, or 0.005 to 0.025 mass%. In addition, in this stainless steel sheet, the C+N content, which is the total value of the C content and the N content, may be 0.050 mass% or less, or 0.045 mass% or less. If the C+N content is too high, the amount of precipitation of carbonitrides may be excessive. On the other hand, since excessive reduction of the C+N content increases refining costs, the C+N content may be 0.010 mass% or more, or 0.015 mass% or more. The C+N content, in mass %, may be in the range of 0.010≦C+N≦0.050, and may be in the range of 0.015≦C+N≦0.050.

(P: Rin)
If P is contained excessively, the weldability, the toughness of the weld, and the workability may be deteriorated. In addition, P is related to precipitates in the material structure (described later). Therefore, the P content may be 0.005 to 0.050 mass%, 0.005 to 0.040 mass%, or 0.010 to 0.030 mass%.

(Ti and Nb: titanium and niobium)
Ti and Nb combine with C or N to fix C and N as precipitates such as TiC, TiN, NbC, or NbN, and therefore the average r-value and product elongation can be improved by purifying ferritic stainless steel. On the other hand, excessive Ti and Nb content increases raw material costs and may reduce manufacturability due to an increase in recrystallization temperature.

Therefore, in one embodiment of the present invention, the Ti content may be 0.01 to 0.50 mass%, 0.02 to 0.40 mass%, or 0.10 to 0.30 mass%. The Nb content may be 0.01 to 0.50 mass%, 0.02 to 0.40 mass%, or 0.10 to 0.30 mass%. The ferritic stainless steel may contain only either Ti or Nb, or may contain both Ti and Nb.

(S: sulfur)
S is an impurity atom that adversely affects hot workability, corrosion resistance, and oxidation resistance. Therefore, the S content may be 0.0100 mass% or less. Ferritic stainless steel may not contain S, and there is no particular lower limit for the S content. The S content may be 0 (including no addition) to 0.0100 mass%. The S content of "0 (including no addition)" means that S is allowed to be contained as an unavoidable impurity.

(Other ingredients)
The present stainless steel sheet may have a chemical composition further containing, by mass%, one or more elements selected from the group consisting of Mo, Ni, Co, Cu, Al, Ca, Mg, B, V, W, Sn, Sb, Zr, Y, Hf, and rare earth elements.

(Mo: Molybdenum)
Mo is an element effective in improving corrosion resistance. However, if excessive Mo is added, the raw material cost of stainless steel increases. Therefore, when Mo is included in the chemical composition, the Mo content may be 0.05 to 2.00 mass%.

(Ni: Nickel)
Ni is an element effective in improving corrosion resistance. On the other hand, excessive Ni content destabilizes the ferrite phase and increases the raw material cost of the ferritic stainless steel. Therefore, when Ni is included in the chemical composition, the Ni content may be 0.01 to 1.00 mass%. The Ni content may be 0.01 to 0.10 mass%. In this stainless steel, the Ni content may be 0.40 mass% or less, 0.10 mass% or less, or 0 (including no addition) to 0.10 mass%. "No addition" means that Ni is not artificially added during steelmaking. The Ni content of "0 (including no addition)" allows the inclusion of Ni as an unavoidable impurity.

(Co: Cobalt)
Co is an element that is effective in improving corrosion resistance and heat resistance. However, if excessive Co is added, the raw material cost of ferritic stainless steel increases. Therefore, when Co is included in the chemical composition, the Co content may be 0.005 to 0.500 mass%.

(Cu: Copper)
Cu is an element effective for improving corrosion resistance, and therefore, when Cu is included in the chemical composition, the Cu content may be 0.05 to 1.00% by mass.

(Al: Aluminum)
Al is an effective element for deoxidization and can reduce _A2 -based inclusions that adversely affect press workability. However, if Al is added in excess, surface defects increase. Therefore, when Al is included in the chemical composition, the content of Al may be 0.01 to 1.00 mass%.

(Ca: Calcium)
Ca is an element effective for degassing, and therefore, when Ca is included in the chemical composition, the Ca content may be 0.0001 to 0.0050 mass%.

(Mg: Magnesium)
Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizer. On the other hand, if Mg is contained in excess, the toughness of the ferritic stainless steel decreases, and the manufacturability decreases. Therefore, when Mg is contained in the chemical composition, the Mg content may be 0.0001 to 0.0050 mass%.

(B: Boron)
B is an element effective in improving toughness. However, if an excessive amount of B is contained, the effect is saturated. Therefore, when B is contained in the chemical composition, the content of B may be 0.0001 to 0.0025 mass%.

(V: Vanadium)
V is an element effective in improving hardness and strength. However, if an excessive amount of V is added, the raw material cost of the ferritic stainless steel increases. Therefore, when V is included in the chemical composition, the V content may be 0.05 to 0.50 mass%.

(W: Tungsten)
W is an element effective in improving high-temperature strength. However, if excessive W is added, the raw material cost of the ferritic stainless steel increases. Therefore, when W is included in the chemical composition, the W content may be 0.05 to 1.00 mass%.

(Sn: tin)
Sn is an element effective in improving corrosion resistance. However, if excessive Sn is added, hot workability and toughness are reduced. Therefore, when Sn is included in the chemical composition, the Sn content may be 0.005 to 0.500 mass%.

(Sb: Antimony)
Sb is effective in improving workability by promoting the formation of deformation bands during rolling. On the other hand, if an excessive amount of Sb is contained, the effect is saturated and workability is further reduced. Therefore, when Sb is contained in the chemical composition, the Sb content may be 0.005 to 0.500 mass%.

(Zr: zirconium)
Zr is an element effective for denitrification, deoxidation, and desulfurization. However, if Zr is added in excess, the raw material cost of stainless steel increases. Therefore, when Zr is included in the chemical composition, the Zr content may be 0.050 to 0.500 mass%.

(Y: Yttrium)
Y is an element effective in improving hot workability and oxidation resistance. However, these effects are saturated when the content exceeds 0.20%. When Y is included in the chemical composition, the Y content may be 0.001 to 0.100 mass%.

(Hf: Hafnium)
Hf is an element that improves oxidation resistance. On the other hand, if an excessive amount of Hf is contained, the toughness of the steel plate is reduced and the raw material cost of the stainless steel increases. Therefore, when Hf is contained in the chemical composition, the Hf content may be 0.001 to 0.100 mass%.

(REM: rare earth element)
Rare Earth Metals refers to lanthanoid elements (La, Ce, Pr, Nd, Sm, and other elements with atomic numbers 57 to 71). REM is used to improve hot workability and oxidation resistance. However, these effects are saturated when the content exceeds 0.100%. Therefore, when REM is included in the chemical composition, the content of REM may be 0.001 to 0.100 mass%. .

<Steel plate characteristics>
This stainless steel sheet has the above-mentioned chemical composition, and by controlling the manufacturing conditions, a material structure (internal structure) having the following characteristics is formed. Schematically, in the hot-rolled sheet annealing process, the colonies are divided, the metal structure is recrystallized, and low-temperature precipitates are dissolved. These low-temperature precipitates have the effect of suppressing grain growth (so-called "pinning" effect). Then, in the final annealing process, by raising the temperature to a temperature range where precipitates are not formed, recrystallized nuclei of crystal grains with various crystal orientations are formed and the grains grow, so that randomization of the crystal orientation can be promoted. More details will be described later in conjunction with the explanation of the manufacturing method of this stainless steel sheet.

(Grain size of steel sheet)
Fig. 1 is a schematic diagram showing a cross section 12 of a stainless steel sheet 1 according to one embodiment of the present invention. As shown in Fig. 1, the cross section 12 is a cross section parallel to the rolling direction of the stainless steel sheet 1 and perpendicular to the rolled surface 11. The thickness and width of the stainless steel sheet 1 are denoted as t and w, respectively.

In the cross section 12 of the stainless steel plate 1, the crystal grain size (average grain size d, described below) calculated by the cutting method is 15 μm or more and 40 μm or less, and preferably 18 μm or more and 35 μm or less.

The grain size calculated by the intercept method can be measured by the method specified in the JIS standard (JIS G 0551:2020). Specifically, first, a line segment with a total length L parallel to the rolling direction is drawn on the cross section 12, and the number n of grains that this line crosses is measured. Note that grains with the end of the line segment inside are counted as 1/2 grain. The average grain size d can be calculated by the formula d = L/n.

(Crystal orientation)
Fig. 2 is a schematic diagram showing a cross section 13 of a stainless steel sheet 1 according to one embodiment of the present invention. As shown in Fig. 2, the cross section 13 is a cross section of the center of the sheet thickness parallel to the rolled surface 11 of the steel sheet 1. The rolling direction is abbreviated as RD (Rolling Direction), the normal direction to the rolled surface is abbreviated as ND (Normal Direction), and the direction perpendicular to the rolling is abbreviated as TD (Transverse Direction). The cross section 13 is a so-called ND surface.

Pole figure data can be obtained by performing XRD measurement on cross section 13. From the obtained pole figure data, the crystal orientation distribution function (ODF) can be analyzed to measure the intensity for each crystal orientation (crystal orientation intensity). For example, pole figure data can be obtained by performing pole measurements using SmartLab manufactured by Rigaku. SmartLab Studio II can be used as ODF analysis software. As known analysis methods can be used in this way, detailed explanations are omitted, but a general explanation is as follows.

When the (hkl) plane of a certain grain is parallel to the ND plane and the rolling direction is the [uvw] direction, the grain's crystal orientation is expressed as (hkl)[uvw]. The equivalent orientation group is expressed as {hkl}<uvw>.

ODF is a function of three variables (φ1, Φ, φ2) that uniquely specify the crystal orientation of the crystal grains with respect to the material coordinate axis system. φ1, Φ, and φ2 are Euler angles defined by Bunge's method. In the material coordinate axis system, the x, y, and z axes are RD, TD, and ND, respectively. φ1 is the counterclockwise rotation angle around the z axis, Φ is the counterclockwise rotation angle around the x' axis after rotation by φ1, and φ2 is the counterclockwise rotation angle around the z' axis after rotation by Φ.

In the φ2=45° cross section of Euler space, the position where Φ=55° and φ1=30° is the {111}<112> orientation, and the position where Φ=55° and φ1=0° is the {111}<110> orientation.

For the stainless steel plate 1, (200), (110), and (211) pole figures are obtained by performing XRD measurement. The obtained pole figures are used to perform ODF analysis. The {111}<110> crystal orientation intensity obtained by ODF analysis is Ia, and the {111}<112> crystal orientation intensity obtained by ODF analysis is Ib. The values of Ia and Ib can be obtained, for example, by outputting the contour line data (ODF diagram) obtained using SmartLab Studio II as a numerical value. In addition, the calculation method in the ODF analysis can be the WIMV method by Matthies and Vinel, which does not use a continuous function in the analysis. The larger value of Ia and Ib is referred to as I _111. For example, SmartLab can be used as the X-ray diffraction device, and a Mo source can be used as the X-ray source. In this case, the pole figure data used in the ODF analysis is intensity normalized by correction processes such as background correction and randomization in the processing in SmartLab. This normalization process is performed under fixed conditions without setting individual conditions by selecting a normalization check box displayed on the display of the user interface of the X-ray diffraction device. The above crystal orientation intensity can also be expressed as an orientation density or an X-ray random intensity ratio.

The stainless steel plate 1 has _{an I111} of 10.0 or more and satisfies the relationship 0.2≦(Ia/Ib)≦4.0. The higher the _I111 , the larger the r-value can be. When Ia/Ib satisfies the above relationship, the in-plane anisotropy can be reduced.

The stainless steel sheet 1 may have an I ₁₁₁ of 10.0 or more and 30.0 or less, or may have an I 111 of 15.0 or more and 30.0 or less, and may satisfy the relationship of 1.0≦(Ia/Ib)≦3.8. When the I ₁₁₁ exceeds 30.0, it becomes difficult to satisfy the relationship of 0.2≦(Ia/Ib)≦4.0. When the stainless steel sheet 1 is manufactured through the steps of hot rolling, hot-rolled sheet annealing, cold rolling, and final annealing (i.e., when intermediate annealing and additional cold rolling are not performed), Ia≧Ib is often satisfied. Therefore, (Ia/Ib) may be 1 or more. (Ia/Ib) may be 3.8 or less, which can further reduce the in-plane anisotropy.

(Average r value and Δr)
The present stainless steel sheet can have an average r-value (average Lankford value) of 1.4 or more. The average r-value can be calculated by the following formula (1) using the r-value measured at a plastic strain of 14.4% according to the method specified in the JIS standard (JIS Z 2254:2021):
Average r value = (r _L + 2r _D + r _C ) / 4 ... (1).

In formula (1), _rL , _rD, and _rC are r-values measured on JIS No. 13B test pieces taken in directions of 0° (parallel), 45°, and 90° to the rolling direction, respectively. The stainless steel sheet may have an average r-value of 1.4 to 2.1, or 1.9 to 2.1.

Furthermore, the stainless steel sheet of the present invention may have a Δr of -0.50 or more and 0.50 or less. The range of Δr is preferably 0 or more and 0.50 or less, and more preferably 0 or more and 0.48 or less. Δr can be calculated using the above r value according to the following formula (2):
Δr=(r _L −2r _D +r _C )/2 (2).

(Riding height)
The present stainless steel sheet can have a ridging height of 15 μm or less. The ridging height is a value obtained by measuring the waviness height in the width direction (direction perpendicular to the rolling direction of the stainless steel sheet) at the center of the rating interval for a test piece obtained by taking a JIS No. 5 tensile test piece perpendicular to the rolling direction and then subjecting the test piece to a 16% strain using a tensile test method specified by the JIS standard (JIS Z 2241:2011). The waviness height is the average height of the waviness curve element measured by the surface property measurement specified in JIS B 0601:2001, etc. The average height of the waviness curve element measured in this manner is the waviness height (i.e., the ridging height) of the present stainless steel sheet.

(Proof strength)
The present stainless steel sheet can have a yield strength of 320 MPa or less. The yield strength is a value obtained by measuring the 0.2% yield strength of a JIS No. 13 B test piece by a tensile test method specified by the JIS standard (JIS Z 2241:2011). The present stainless steel sheet may have a yield strength of 310 MPa or less, or may have a yield strength of 300 MPa or less.

<Method of manufacturing steel sheet>
Fig. 3 is a flow chart showing an example of a method for producing a ferritic stainless steel sheet according to one embodiment of the present invention. As shown in Fig. 3, the method for producing the present stainless steel sheet (hereinafter sometimes abbreviated as the present production method) includes a hot rolling step, a hot-rolled sheet annealing step, an intermediate step, and a final annealing step, in this order.

The method for producing the slabs to be subjected to the hot rolling process is not particularly limited, but may, for example, include a steelmaking process prior to the hot rolling process. In the steelmaking process, molten steel having the desired composition is poured into a mold and cooled to produce a slab of ferritic stainless steel. The slab is cut to the desired length and used in the hot rolling process. In addition, this manufacturing method may include a post-process after the final annealing process as appropriate.

(Hot rolling process)
The hot rolling process is a process for producing a hot-rolled sheet of a predetermined thickness by rolling (hot rolling) a slab at a high temperature. The hot rolling process can be performed by known equipment and methods. In this manufacturing method, general manufacturing conditions in the hot rolling process can be adopted. For example, the heating temperature (rolling temperature) can be 1150 to 1250°C, and the total reduction rate can be 95 to 99%. The coiling temperature after hot rolling can be 200 to 500°C.

(Hot-rolled sheet annealing process)
In the hot-rolled sheet annealing process, the hot-rolled sheet is heated to a temperature range of 900 to 1000° C. and soaked. The soaking temperature in the hot-rolled sheet annealing process is preferably 910 to 980° C. The soaking time in the hot-rolled sheet annealing process is 20 to 120 s (seconds).

In this manufacturing method, by carrying out the hot-rolled sheet annealing process as described above, the precipitates can be dissolved in the ferritic stainless steel. In this specification, a distinction is made between low-temperature precipitates and high-temperature precipitates.

The low-temperature precipitates are precipitates that precipitate in ferritic stainless steels in the temperature range of 600° C. or more and less than 900° C. Examples of low-temperature precipitates include compounds containing Fe and at least one of Nb, Ti, and P. More specifically, examples of low-temperature precipitates include (i) phosphides, such as FeTiP, FeNbP, and Fe(Ti,Nb)P, and (ii) intermetallic compounds that form a phase called a Laves phase, such as Fe ₂ Nb.

Furthermore, precipitates of compounds with melting points of 900°C or higher are called high-temperature precipitates. Examples of high-temperature precipitates include metal carbides (e.g., TiC and NbC) and metal nitrides (e.g., TiN and NbN).

In the hot-rolled sheet annealing process, the low-temperature precipitates and high-temperature precipitates are dissolved in the ferritic stainless steel, and the hot-rolled structure that causes ridging is disrupted by recrystallization. For this reason, the lower limit of the heating temperature is set to 900°C, and the lower limit of the soaking time is set to 20 seconds. On the other hand, if the recrystallized structure of the hot-rolled sheet becomes coarse, ridging will worsen. For this reason, the upper limit of the heating temperature is set to 1000°C, and the upper limit of the soaking time is set to 120 seconds. This disrupts the colonies in the metal structure.

The cooling rate after soaking in the hot-rolled sheet annealing process can be 5°C/s or more in the temperature range from the soaking temperature to 500°C. This makes it possible to suppress the precipitation of low-temperature precipitates during cooling. There are no particular limitations on the cooling rate in the temperature range below 500°C. The semi-finished product after cooling is called a hot-rolled annealed sheet.

The annealing equipment used in the hot-rolled sheet annealing process is not particularly limited, and known equipment such as a continuous annealing furnace or a batch furnace can be used.

(Intermediate process)
After the hot-rolled sheet annealing step, at least one cold rolling step is typically performed before the final annealing step. The hot-rolled annealed sheet may be subjected to an acid pickling step. The acid pickling step is a step of removing scale adhering to the surface of the hot-rolled annealed sheet using an acid pickling solution such as sulfuric acid, hydrochloric acid, or a mixture of nitric acid and hydrofluoric acid.

The cold rolling process is a process in which the above-mentioned hot-rolled annealed sheet is cold-rolled (for example, at room temperature to 200°C) to obtain a cold-rolled sheet of a specified thickness. The rolling ratio in the cold rolling process is preferably 75% or more, since a higher rolling ratio is more effective in improving the average r-value and formability. The rolling ratio in the cold rolling process may be 90% or less.

The rolling equipment used in the cold rolling process is not particularly limited, and publicly known equipment can be used.

In the intermediate process, the cold rolling process may be performed only once, or may be performed two or more times. When the cold rolling process is performed two or more times, an intermediate annealing process may be performed between the cold rolling processes. The heating temperature in the intermediate annealing process may be, for example, 900 to 1000°C.

<Final annealing process>
The final annealing step is a step of heating the cold-rolled sheet to a temperature range of 900 to 1000° C. and then soaking for 0 to 120 seconds. The heating rate in the final annealing step is 100° C./s or more. The heating rate may be 100 to 2000° C./s, and is preferably 100 to 1500° C./s.

In the final annealing process, the cold-rolled sheet is heated under the above conditions, which makes it difficult for low-temperature precipitates to precipitate during heating, and promotes the growth rate of crystal grains while causing recrystallization in random orientations. By setting the heating rate at 100°C/s or more, the amount of low-temperature precipitates that precipitate during the heating process can be effectively reduced. Furthermore, if the heating time is too short, the amount of heat required for recrystallization may be insufficient, so the heating rate may be set to 2000°C/s or less, or 1500°C/s or less.

The temperature increase under the above conditions in the final annealing process suppresses the precipitation of low-temperature precipitates and promotes the growth of crystal grains in the metal structure. By dissolving the low-temperature precipitates in the hot-rolled sheet annealing process and then increasing the temperature under the above conditions in the final annealing process, it is possible to promote the grain growth of crystal grains with various crystal orientations.

The lower limit of the heating temperature (soaking temperature) in the final annealing process is 900°C, at which point no low-temperature precipitates form and recrystallization is complete. On the other hand, if grain growth becomes too large, the in-plane anisotropy increases. Therefore, the upper limit of the heating temperature is set to 1000°C, and the upper limit of the soaking time is set to 120 seconds. The soaking temperature in the final annealing process is preferably 910 to 980°C, and the soaking time is preferably 5 to 100 seconds.

The final annealing step in this manufacturing method is described in more detail below. First, for comparison, a case where annealing is performed at a general heating rate will be described. When heating is performed at a general heating rate, recrystallization occurs in the order of {111} grains, {211} grains, {311} grains, and {100} grains as the temperature rises. That is, during heating, recrystallization nuclei of {111} grains are generated at a temperature of, for example, about 800°C. Then, recrystallization nuclei of grains in other orientations are generated as the temperature rises. As recrystallization by annealing progresses, crystal grains having a specific direction (for example, {111}<112> orientation) among the {111} grains that were nucleated earlier grow preferentially. Therefore, when annealing is performed at a general heating rate, the anisotropy of the ferritic stainless steel sheet becomes large. In contrast, in the final annealing step in this manufacturing method, by performing heating under the above conditions, recrystallization nuclei are generated not only in {111} grains but also in other orientations at approximately the same timing. This makes it possible to prevent preferential growth in only one direction.

The annealing equipment used in the final annealing process is not particularly limited, and known equipment such as a continuous annealing furnace or a batch furnace can be used.

After the final annealing process, a pickling process may be performed if necessary. Also, if necessary, a post-process may be performed, such as temper rolling and cutting to the desired shape.

〔summary〕
The ferritic stainless steel sheet in the first aspect of the present invention contains, by mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, Cr: 10.5-30.0%, N: 0.001-0.030%, and P: 0.005-0.050%, as well as at least one of Ti: 0.01-0.50% and Nb: 0.01-0.50%, with the S content being 0.0100% or less, and the balance being Fe and unavoidable impurities. A ferritic stainless steel sheet having a pure chemical composition, wherein in a cross section of the ferritic stainless steel sheet parallel to the rolling direction and perpendicular to the rolling surface, the crystal grain size calculated by a cutting method is 15 μm or more and 40 μm or less, and in a cross section parallel to the rolling surface in a center portion of the sheet thickness of the ferritic stainless steel sheet, the {111}<110> crystal orientation intensity is Ia and the {111}<112> crystal orientation intensity is Ib, and the larger of Ia and Ib is referred to as _I111 , and the relationships _I111 ≧10.0 and 0.2≦(Ia/Ib)≦4.0 are satisfied.

The ferritic stainless steel sheet according to aspect 2 of the present invention is the same as that according to aspect 1, wherein the yield strength is 320 MPa or less, the average r-value is 1.4 or more, and the in-plane anisotropy index Δr of the r-value is −0.50 or more and 0.50 or less, where the average r-value=(r _L +2r _D +r _C )/4, Δr=(r _L -2r _D +r _C )/2, r _L : Lankford value in a direction at 0° to the rolling direction, r _D : Lankford value in a direction at 45° to the rolling direction, and r _C : Lankford value in a direction at 90° to the rolling direction.

The ferritic stainless steel sheet in aspect 3 of the present invention is the ferritic stainless steel sheet in aspect 1 or 2, in which a JIS No. 5 tensile test piece is taken from the ferritic stainless steel sheet perpendicular to the rolling direction, and the JIS No. 5 tensile test piece is subjected to a 16% strain to measure the waviness height in the width direction at the center of the rating line, and the ridging height obtained is 15 μm or less.

The ferritic stainless steel sheet in aspect 4 of the present invention is any one of aspects 1 to 3 described above, and contains, by mass%, Mo: 0.05 to 2.00%, Ni: 0.01 to 1.00%, Co: 0.005 to 0.500%, Cu: 0.05 to 1.00%, Al: 0.01 to 1.00%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, B: 0.00 It further contains one or more elements selected from the group consisting of 0.01-0.0025%, V: 0.05-0.50%, W: 0.05-1.00%, Sn: 0.005-0.500%, Sb: 0.005-0.500%, Zr: 0.050-0.500%, Y: 0.001-0.100%, Hf: 0.001-0.100%, and rare earth elements: 0.001-0.100%.

The manufacturing method of the ferritic stainless steel sheet in aspect 5 of the present invention contains, by mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, Cr: 10.5-30.0%, N: 0.001-0.030%, and P: 0.005-0.050%, as well as at least one of Ti: 0.01-0.50% and Nb: 0.01-0.50%, and the S content is 0.0 The process includes a hot-rolled sheet annealing process in which a hot-rolled sheet having a chemical composition in which the content of Zn is 0.1% or less, with the remainder being Fe and unavoidable impurities, is heated to a temperature range of 900 to 1000°C and held at that temperature for 20 to 120 seconds, and a final annealing process in which the cold-rolled sheet obtained by cold-rolling the hot-rolled annealed sheet obtained by the hot-rolled sheet annealing process is heated to a temperature range of 900 to 1000°C at a heating rate of 100°C/s or more, and then held at that temperature for 120 seconds or less or cooled without being held at that temperature.

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

An embodiment of the present invention is described below. Note that the manufacturing method of the ferritic stainless steel sheet described in this embodiment is just one example.

(Slab manufacturing)
Steels having the composition shown in Table 1 below were vacuum melted to produce 30 kg slabs. In Table 1, steel types A to N have chemical compositions within the range of the present invention. Also, in Table 1, steel types P, R, and S have chemical compositions outside the range of the present invention. Table 1 shows the composition of the components contained in each steel type in mass %. The balance other than each component shown in Table 1 is Fe or unavoidable impurities. Also, the underlines in Table 1 indicate that the composition of each component contained in each steel type according to the comparative example is outside the range of the present invention.

The slab was heated at 1200°C for 2 hours and then hot rolled to produce a hot-rolled sheet with a thickness of 3 mm.

Next, the hot-rolled sheet annealing process and the final annealing process were performed under the manufacturing conditions shown in Table 2, and steel sheets No. 1 to 38 having a sheet thickness of 0.6 mm were manufactured. In this example, cold rolling was performed only once as an intermediate process. The rolling ratio of the cold rolling was 80%. Table 2 also shows the results of evaluating various physical properties for each steel sheet. The underlines in Table 2 indicate that the manufacturing conditions and various physical properties of the steel sheet are outside the range of the present invention or outside the preferred range of the present invention. The grain size, crystal orientation strength (I ₁₁₁ , Ia, Ib), yield strength, average r value, Δr, and ridging height were measured or calculated by the same method as in the above-mentioned embodiment. The diagonal lines in the description column for the conditions of the hot-rolled sheet annealing process for steel sheet No. 3 indicate that the hot-rolled sheet annealing process was not performed (omitted).

As shown in Table 2, all of the steel sheets of the present invention examples manufactured by the manufacturing method of ferritic stainless steel sheet according to one embodiment of the present invention described above had the following characteristics: grain size, crystal orientation strength, yield strength, average r value, Δr, and ridging height.

In contrast, the comparative steel sheets did not meet the criteria for at least one of the above characteristics. Comparative steel sheet No. 3 had yield strength, average r value, and Δr within the range of the present invention, but the omission of the hot-rolled sheet annealing process resulted in insufficient colony separation and insufficient ridging resistance. Comparative steel sheet No. 6 also showed insufficient colony separation. Comparative steel sheet No. 8 had small crystal grain size and increased yield strength due to insufficient heating in the hot-rolled sheet annealing process, and therefore had insufficient ridging resistance. Comparative steel sheets No. 10 and 24 had coarse recrystallized structures, which deteriorated ridging resistance.

In comparative steel sheet No. 13, the heating rate in the final annealing process was outside the range of the present invention, and the in-plane anisotropy increased due to the precipitation of low-temperature precipitates and preferential grain growth in a specific direction (e.g., the {111}<112> orientation). In comparative steel sheets No. 16 and 28, the crystal grain size was small and the yield strength increased due to insufficient heating in the final annealing process. On the other hand, in comparative steel sheets No. 19, 21, 30, and 33, the grain growth due to heating in the final annealing process was too large, and the in-plane anisotropy increased.

1 Stainless steel plate 11 Rolled surface 12 Cross section 13 Cross section

Claims (4)

A ferritic stainless steel sheet having a chemical composition containing, by mass%, C: 0.001-0.030%, Si: 0.01-1.00%, Mn: 0.01-1.00%, Cr: 10.5-30.0%, N: 0.001-0.030%, and P: 0.005-0.050%, as well as at least one of Ti: 0.01-0.50% and Nb: 0.01-0.50%, with an S content of 0.0100% or less, and the balance being Fe and unavoidable impurities,
In a cross section of the ferritic stainless steel plate parallel to the rolling direction and perpendicular to the rolling surface, the grain size calculated by a cutting method is 15 μm or more and 40 μm or less,
In a cross section parallel to the rolled surface at the center of the thickness of the ferritic stainless steel plate, the {111}<110> crystal orientation intensity is Ia, the {111}<112> crystal orientation intensity is Ib, and the larger of Ia and Ib is referred to as _I111 ,
A ferritic stainless steel sheet satisfying the relationships I ₁₁₁ ≧10.0 and 0.2≦(Ia/Ib)≦4.0. 2. The ferritic stainless steel sheet according to claim 1, having a yield strength of 320 MPa or less, an average r-value of 1.4 or more, and an in-plane anisotropy index Δr of the r-value of -0.50 or more and 0.50 or less.
(where:
Average r value = (r _L + 2r _D + r _C )/4
Δr=(r _L −2r _D +r _C )/2
r _L : Lankford value in the direction of 0° to the rolling direction, r _D : Lankford value in the direction of 45° to the rolling direction, r _C : Lankford value in the direction of 90° to the rolling direction) The ferritic stainless steel sheet according to claim 1 or 2, in which a JIS No. 5 tensile test piece is taken from the ferritic stainless steel sheet perpendicular to the rolling direction, and the JIS No. 5 tensile test piece is subjected to a 16% strain to measure the waviness height in the width direction at the center of the rating, and the ridging height obtained is 15 μm or less. In mass%, Mo: 0.05-2.00%, Ni: 0.01-1.00%, Co: 0.005-0.500%, Cu: 0.05-1.00%, Al: 0.01-1.00%, Ca: 0.0001-0.0050%, Mg: 0.0001-0.0050%, B: 0.0001-0.0025%, V: 0.05-0.50%, W: 0.05-1.00%, S The ferritic stainless steel sheet according to any one of claims 1 to 3, further comprising one or more selected from the group consisting of n: 0.005 to 0.500%, Sb: 0.005 to 0.500%, Zr: 0.050 to 0.500%, Y: 0.001 to 0.100%, Hf: 0.001 to 0.100%, and rare earth elements: 0.001 to 0.100%.

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