JP7102988B2 - Manufacturing method of ferritic stainless steel sheet, clad material and ferritic stainless steel sheet - Google Patents

Description

The present invention relates to a ferritic stainless steel sheet, a clad material, and a method for producing a ferritic stainless steel sheet.

Stainless steel plates and clad materials made of stainless steel plates are used for equipment that supports IH (induction heating: high-frequency induction heating) cooking, such as pots, frying pans, and inner pots of rice cookers. In these applications, in addition to properties such as strength and moldability, excellent designability may be required in order to appeal to consumers. The designability depends on the times and consumer needs. For example, by coarsening the metal crystal grains so that they can be visually observed, it is possible to impart an unprecedented design property as the crystal grain structure pattern. Conceivable. Further, it is expected that the appearance of the crystal grain structure pattern on the surface makes the flaws formed on the surface of the stainless steel sheet less noticeable.

However, for example, in a ferritic stainless steel sheet, a high temperature and long time heat treatment is required to coarsen the ferrite crystal grains to a level that can be visually confirmed. Generally, when a ferrite stainless steel sheet is heat-treated at a high temperature for a long time using an atmospheric furnace used for mass production heat treatment, a thick scale is generated on the surface of the material. The scale formed on the surface of the stainless steel sheet is difficult to remove by pickling, and there is a problem that the formation of thick scale leads to a large reduction in plate thickness.

In addition, metal materials used as equipment compatible with IH cooking are required to have high relative magnetic permeability in order to improve heating efficiency by electromagnetic induction in addition to moldability and corrosion resistance that can be molded into pots and the like. ..

Furthermore, stainless steel, which enables visual confirmation of metal crystal grains, has design properties such as building interior or exterior materials, electronic device housings, tableware, and fine arts, in addition to equipment that supports IH cooking. There is a demand for applications that require.

In Patent Document 1 (Japanese Unexamined Patent Publication No. 2002-129292), in mass%, C + N: 0.02% or less, Si: 0.3% or less, Mn: 0.5% or less, Cr: 11 to 35%, It contains P: 0.05% or less, S: 0.010% or less, Al: 0.02% or less, has a steel composition consisting of the balance Fe and unavoidable impurities, and has a surface roughness of 0.5 μm in Ry. The following ferritic stainless cold-rolled steel sheets for jar pot containers, which are excellent in corrosion resistance and water stain resistance, are described.

Patent Document 2 (Japanese Unexamined Patent Publication No. 2013-249519) describes, in terms of mass%, C: 0.06% or less, N: 0.06% or less, Si + Al: 0.6% or more and 2% or less, Cr: 13%. Described are ferritic stainless steels for cooking utensils used in IH cookers, which contain 20% or less, Mn: 2.0% or less, and have a component composition in which the balance is Fe and unavoidable impurities.

Japanese Unexamined Patent Publication No. 2002-129292 Japanese Unexamined Patent Publication No. 2013-249519

However, none of the ferritic stainless steels described in Patent Documents 1 or 2 has coarsened crystal grains, and Patent Documents 1 and 2 describe a production method for coarsening crystal grains. Is not listed.

The present invention has been made in view of the above circumstances, and a ferritic stainless steel plate and a clad material which are provided with crystal grains having a size that can be visually confirmed, have excellent design, and can be applied to an IH cooker or the like. The challenge is to provide. Another object of the present invention is to provide a method for producing a ferritic stainless steel sheet capable of forming crystal grains having a size that can be visually confirmed without performing heat treatment at a high temperature for a long time.

In order to solve the above problems, the present invention adopts the following configuration.
[1] By mass%
C: 0.030% or less,
N: 0.030% or less,
Si: 0.20% or more and 0.80% or less,
Cr: 15% or more and 20% or less,
Mn: 0.80% or less,
Ni: 1.0% or less,
Cu: Contains 0.80% or less,
Further, any one or more of Nb, V, and Ti are contained in a total of 0.10% or more and 0.80% or less.
The balance has a component composition consisting of Fe and impurities,
A ferritic stainless steel sheet having an average crystal grain size of 100 μm or more.
[2] The ferritic stainless steel sheet according to [1], wherein the relative magnetic permeability at a frequency of 25 kHz is 90 or more.
[3] The ferrite-based stainless steel sheet according to [1] or [2], which is used as a member of equipment compatible with high-frequency induction cooking.
[4] A clad material characterized in that the ferrite-based stainless steel plate according to any one of [1] to [3] is joined to an aluminum plate.
[5] The first step of subjecting the stainless steel sheet material having the component composition according to [1] to an intermediate heat treatment at 900 ° C. or higher and 1100 ° C. or lower for 1.0 minute or longer.
The second step of performing cold rolling for finishing with a plate thickness reduction rate of 5% or more and 10% or less, and
The ferrite-based stainless steel according to any one of [1] to [3] , which comprises sequentially performing a third step of performing a finish heat treatment at 980 ° C. or higher and 1150 ° C. or lower for 1.0 minute or longer. Method of manufacturing steel sheet.

According to the present invention, it is possible to provide a ferritic stainless steel sheet and a clad material which are provided with crystal grains having a size that can be visually confirmed, have excellent design, and can be applied to an IH cooker or the like. Further, according to the present invention, it is possible to provide a method for producing a ferritic stainless steel sheet capable of forming crystal grains having a size that can be visually confirmed without performing heat treatment at a high temperature for a long time.
In addition, the ferrite-based stainless steel plate and clad material of the present invention have design characteristics such as pots, frying pans, inner pots of rice cookers and other devices compatible with IH cooking, housings for electronic devices, tableware, fine arts, building materials, etc. It can be suitably used in a required field.

In order to obtain a ferritic stainless steel sheet that has crystal grains of a size that can be visually confirmed, has excellent design, and can be applied to IH cookers, etc., the present inventors have discussed the chemical composition and manufacturing method of the material. As a result of detailed examination, the following findings were obtained, and the present invention was completed.

(A) By using a chemical component in which the ferrite phase, which diffuses quickly, exists stably up to a high temperature, the crystal grain growth during heat treatment is fast, and a coarse crystal grain structure can be easily obtained.
(B) By performing cold rolling with a plate thickness reduction rate of 5% or more and 10% or less before the heat treatment at a high temperature, the crystal grain growth during the heat treatment is remarkably promoted, and the crystal grains are coarse even in a short heat treatment. To become.
(C) By coarsening the crystal grains, an aesthetic appearance is created and the flaws become inconspicuous.
(D) The relative magnetic permeability is improved by coarsening the crystal grains.

Hereinafter, a method for producing a ferrite-based stainless steel sheet, a clad material, and a ferrite-based stainless steel sheet according to an embodiment of the present invention will be described.

The ferritic stainless steel plate of the present embodiment has C: 0.030% or less, N: 0.030% or less, Si: 0.20% or more and 0.80% or less, Cr: 15% or more and 20% in mass%. Hereinafter, Mn: 0.80% or less, Ni: 1.0% or less, Cu: 0.80% or less are contained, and any one or more of Nb, V, and Ti are added in total to 0. A ferritic stainless steel sheet containing 10% or more and 0.80% or less, having a component composition in which the balance is composed of Fe and impurities, and having an average crystal grain size of 100 μm or more.
Further, the ferrite-based stainless steel sheet of the present embodiment preferably has a relative magnetic permeability of 90 or more at a frequency of 25 kHz.
Further, the ferrite-based stainless steel sheet of the present embodiment is preferably used as a member of an apparatus compatible with high-frequency induction cooking.
Furthermore, the clad material of the present embodiment is a clad material formed by joining the ferritic stainless steel plate of the present embodiment and an aluminum plate.

Hereinafter, the component composition of the ferritic stainless steel sheet of the present embodiment will be described. In the description of the component composition, "%" means mass%. The component composition of the ferritic stainless steel sheet of the present embodiment is a chemical component in which a fast-diffusing ferrite phase is stably present up to a high temperature.

C: 0.030% or less C is an element that stabilizes the austenite phase when it is dissolved in the matrix phase. Therefore, if the amount of C is large, the austenite phase is formed at high temperature and the crystal grain growth is slowed down. It becomes difficult to obtain coarse crystal grains. Further, if the amount of C is large, the steel becomes hard and the formability is lowered. Therefore, the amount of C is preferably as small as possible, and the upper limit is 0.030% or less. Desirably, it is 0.020% or less. The amount of C is preferably as small as possible, but reducing the amount of C increases the cost, so the lower limit should be 0.001% or more.

N: 0.030% or less N, like C, is an element that stabilizes the austenite phase when dissolved in the parent phase. When the amount of N is large, an austenite phase is formed at a high temperature, the crystal grain growth is slowed down, and it becomes difficult to obtain coarse crystal grains. Therefore, the amount of N is set to 0.030% or less. Desirably, it is 0.020% or less. It is preferable that the amount of N is as small as possible, but since reducing the amount of N increases the cost, the lower limit should be 0.001% or more.

Si: 0.20% or more and 0.80% or less Si is contained in 0.20% or more in order to improve the protective property of the oxide film. On the other hand, if the amount of Si is too large, the hot workability is remarkably deteriorated, ear cracks occur, and the maintenance cost increases. Therefore, the upper limit is set to 0.80% or less. Preferably, it is 0.25% or more and 0.70% or less.

Cr: 15% or more, 20% or less Cr is an essential element from the viewpoint of ensuring corrosion resistance as stainless steel, and is 15% or more from the viewpoint of ensuring sufficient corrosion resistance. On the other hand, if the amount of Cr is too large, a coarse embrittled phase is formed during annealing, so the amount is set to 20% or less. Preferably, it is 16% or more and 18% or less.

Mn: 0.80% or less Mn is mixed from raw material scrap and the like. In order to significantly reduce the amount of Mn, it is necessary to reduce the use of scrap, which leads to an increase in cost. On the other hand, if the amount of Mn is too large, the hot workability is deteriorated and the corrosion resistance of the material is deteriorated. Therefore, the amount of Mn is set to 0.80% or less. It is preferable that the amount of Mn is as small as possible, but since reducing the amount of Mn increases the cost, the lower limit of Mn is 0.20% or more, more preferably 0.10% or more, which is acceptable.

Ni: 1.0% or less Ni is a strong austenite stabilizing element and promotes the transformation from the ferrite phase to the austenite phase at high temperatures. As a result, the crystal grain growth is slowed down, and it becomes difficult to obtain coarse crystal grains. Therefore, the amount of Ni is set to 1.0% or less. Desirably, it is 0.3% or less. The smaller the amount of Ni, the better, and the lower limit thereof is 0.001% or more.

Cu: 0.80% or less Cu is mixed from raw material scrap. In order to significantly reduce the amount of Cu, it is necessary to reduce the use of scrap, which leads to an increase in cost. On the other hand, Cu is a strong austenite stabilizing element like Ni, and promotes the transformation from the ferrite phase to the austenite phase at high temperature. As a result, the growth of crystal grains is slowed down, which makes it difficult to obtain coarse crystal grains. Therefore, the upper limit of the amount of Cu is 0.80% or less. Desirably, it is 0.70% or less. It is preferable that the amount of Cu is as small as possible, but since reducing the amount of Cu increases the cost, the lower limit of Cu is 0.25% or more, 0.20% or more, or 0.02% or more, which is acceptable.

One or more of Nb, V, and Ti in total: 0.10% or more and 0.80% or less Nb, V, and Ti are all elements that form a compound with C and N. When C and N become compounds, the solid solution C and N are reduced, and the austenite stability of the matrix is lowered. As a result, the ferrite phase is maintained up to a high temperature and the grain growth is promoted. This effect can be obtained by containing 0.10% or more in total of any one or more of Nb, V, and Ti. On the other hand, if the content of Nb, V, and Ti is too high, coarse carbides and nitrides are formed during melting, which promotes ear cracking during hot rolling and cold rolling, or remains in the product as inclusions and is molded. Deteriorates sex. Therefore, the total content of Nb, V, and Ti is set to 0.80% or less.

The rest excluding the above elements are Fe and impurities. Impurities are elements that are inevitably mixed in from the steel raw material and / or in the steelmaking process, and are permissible elements as long as they do not impair the characteristics of the ferritic stainless steel sheet of the present embodiment.

The average crystal grain size is 100 μm or more. In order to produce a crystal grain pattern at a level that can be visually confirmed, it is desirable that the average crystal grain size is large. In addition, the coarsening of crystal grains also has the effect of increasing the relative magnetic permeability. In order to stabilize the relative magnetic permeability to 90 or more, it is necessary to make the average crystal grain size 100 μm or more. Therefore, the average crystal grain size is defined as 100 μm or more. It is preferably 150 μm or more. Since the stainless steel sheet of the present embodiment is a ferrite-based stainless steel sheet, the average crystal grain size means the average crystal grain size of ferrite. The upper limit is not particularly specified, but when the size of the crystal grains is the same level as that of a steel plate or a product manufactured by using a steel plate, the crystal grain pattern is meaningless, so it is desirable that the size is 10 mm or less.

Next, a method for measuring the average crystal grain size of the ferritic stainless steel sheet will be described.
When the width of the ferritic stainless steel sheet is w, (1/10) w, (1/4) w, (1/2) w, (3/4) w and (9/10) from one end in the width direction. Samples are collected so that the cross section perpendicular to the rolling direction (C cross section) is the observation surface at the five points w. The area of the entire observation surface is 5 mm ² , and the area of one observation surface is 1 mm ² . The dimension of each observation surface in the plate thickness direction is the plate thickness t (mm). Therefore, the length of each observation surface in the plate width direction is 1 / t (mm). After mirror polishing the observation surface, electrolytic etching is performed with 10% oxalic acid, and ferrite crystal grains are observed with an optical microscope or a scanning electron microscope, and the number of crystal grains is counted. For crystal grains that partially protrude from the observation surface when the contour lines in the plate width direction and plate thickness direction of the observation surface cross over the crystal grains, the number of crystal grains is counted as 0.5. The total number of ferrites measured on the five observation surfaces is divided by the area of all observation surfaces to obtain the area per crystal grain. Then, the diameter equivalent to the circle of the crystal grains is obtained from the area per crystal grain, and this is used as the average crystal grain size.

Next, a method for manufacturing the ferritic stainless steel sheet of the present embodiment will be described.
The method for producing a ferrite-based stainless steel plate of the present embodiment includes a first step of subjecting a stainless steel plate material having the above-mentioned component composition to an intermediate heat treatment at 900 ° C. or higher and 1100 ° C. or lower for 1.0 minute or longer, and a plate thickness. The second step of performing the finish cold rolling with a reduction rate of 5% or more and 10% or less and the third step of performing the finish heat treatment at 980 ° C. or higher and 1150 ° C. or lower for 1.0 minute or longer are sequentially performed. By heating the stainless steel sheet material in the first step, the structure of the stainless steel is completely restored and recrystallized, in the second step, it is reduced to give strain, and in the third step, it is heated to crystallize. Grow the grain. By applying strain in the second step, the holding time of the heat treatment required for grain growth in the third step can be significantly shortened.

As the stainless steel plate material, any of a hot-rolled steel plate, a cold-rolled steel plate, and a cast plate having the above-mentioned composition can be used. The hot-rolled steel sheet may be a hot-rolled steel sheet obtained by hot-rolling a slab and cooling it, or may be a steel sheet obtained by hot-rolling a slab, cooling it, and then hot-rolling the sheet. Pickling may be performed before annealing the hot-rolled plate. The cold-rolled steel sheet may be a hot-rolled sheet that has been cold-rolled one or more times, or may be a sheet that has been repeatedly cold-rolled and intermediate-annealed a plurality of times. The hot-rolled steel sheet may be pickled before cold rolling. Further, as the cast plate, a plate cast from molten steel into a plate shape can be mentioned.

In the first step, the stainless steel sheet material is subjected to an intermediate heat treatment at 900 ° C. or higher and 1100 ° C. or lower for 1.0 minute or longer. By the first step, the structure of the stainless steel sheet material is completely restored and recrystallized. If the heat treatment temperature is less than 900 ° C. or the holding time is less than 1.0 minute, the recovery and recrystallization of the steel structure will be insufficient, and the grain growth of the crystal grains will not proceed sufficiently. Further, if the heat treatment temperature exceeds 1100 ° C., the steel sheet becomes too soft and the operation becomes difficult. Therefore, the heat treatment conditions are such that the heat treatment temperature is 900 ° C. or higher and 1100 ° C. or lower, and the holding time is 1.0 minute or longer. The upper limit of the holding time is not particularly limited, but if the holding time is long, the productivity is lowered and the scale may be thickened. Therefore, 20 minutes or less is preferable, and 10 minutes or less is more preferable. It is good, more preferably 3.0 minutes or less. When various heat treatments are performed to control the material during the production of the stainless steel sheet, the intermediate heat treatment in the first step of the present embodiment is called an intermediate heat treatment because it is a heat treatment performed before the finishing heat treatment.

In the present embodiment, it is preferable to perform pickling between the first step and the second step to remove the scale generated in the first step. If the surface condition of the stainless steel sheet material after the first step is good, this pickling may be omitted.

In the second step, the stainless steel sheet material is subjected to finish cold rolling with a plate thickness reduction rate of 5% or more and 10% or less. By giving strain to the stainless steel sheet material in the second step, the grain growth of crystal grains in the next third step is promoted, and the heat treatment time in the third step is shortened. If the plate thickness reduction rate is less than 5%, the stainless steel sheet material cannot be sufficiently strained and the grain growth of crystal grains cannot be promoted. Further, when the plate thickness reduction rate exceeds 10%, excessive strain is applied to the stainless steel plate material to generate new crystal grain nuclei, and in the third step, new crystal grains are generated to form the crystal grains. It is not preferable because the number density increases and the average crystal grain size decreases. Therefore, the plate thickness reduction rate is set to 5 to 10%. The plate thickness reduction rate is the plate thickness reduction rate before and after the second step. The plate thickness reduction rate when the cold rolling pass is performed a plurality of times in the second step is 100 × (when the plate thickness t1 before the first cold rolling and the plate thickness t2 after the last cold rolling are taken. It becomes t1-t2) / t1. The temperature of the stainless steel sheet material in the second step is allowed to be in the range of room temperature to 300 ° C. or lower. If the temperature of the steel sheet exceeds 300 ° C., it becomes difficult to give sufficient strain necessary for grain growth, which is not preferable.

When the stainless steel sheet is manufactured, cold rolling is performed to adjust the plate thickness. However, since the finish cold rolling in the second step of the present embodiment is the final cold rolling in the present invention, the finish cold rolling is performed. It is called inter-rolling. However, this does not mean that cold rolling is not performed at all after the finish cold rolling, and the skin pass is performed after the third step as long as the quality and characteristics of the ferritic stainless steel sheet of the present embodiment are not affected. It is permissible to perform rolling under light pressure such as.

In the third step, the stainless steel sheet material after cold rolling is subjected to a finish heat treatment at 980 ° C. or higher and 1150 ° C. or lower for 1.0 minute or longer. In the third step, the crystal grains grow, and crystal grains having an average crystal grain of 100 μm or more can be obtained. If the heat treatment temperature is less than 980 ° C. or the holding time is less than 1.0 minute, the grain growth of the crystal grains does not proceed sufficiently. Further, if the heat treatment temperature exceeds 1150 ° C., the steel sheet becomes too soft and the operation becomes difficult. Therefore, the heat treatment conditions are such that the heat treatment temperature is 980 ° C. or higher and 1150 ° C. or lower, and the holding time is 1.0 minute or longer. The upper limit of the holding time is not particularly limited, but if the holding time is long, the productivity is lowered and the scale may be thickened. Therefore, 20 minutes or less is preferable, and 10 minutes or less is more preferable. It is good, more preferably 3.0 minutes or less.

The cooling after the third step is not particularly limited, and may be cooled in the heating furnace used in the third step, may be naturally cooled or air-cooled outside the heating furnace, and may be water-cooled. You may. The cooling rate is the average cooling rate from the holding temperature of the third step to 200 ° C., and is preferably in the range of 0.03 ° C./sec to 100 ° C./sec, but even if it deviates from this range, the material is not affected.

The finish heat treatment in the third step is referred to as a finish heat treatment because it is the final heat treatment in the present invention. However, this does not mean that the heat treatment is not performed at all after the finish heat treatment, and that the heat treatment is performed after the third step within a range that does not affect the quality and characteristics of the ferritic stainless steel sheet of the present embodiment. Permissible.

By sequentially performing the above steps, the ferritic stainless steel sheet of the present embodiment can be obtained.
The ferrite-based stainless steel sheet of the present embodiment has crystal grains having a size that can be visually confirmed, and has excellent designability. Further, since the relative magnetic permeability at a frequency of 25 kHz is 90 or more, it can be applied to an IH cooker or the like.

The ferritic stainless steel plate of the present embodiment may be used as a clad material together with the aluminum plate. The aluminum plate may be a pure aluminum plate or an aluminum alloy plate. Regardless of the method for forming the clad, a commonly performed method, for example, lap rolling, explosive welding, diffusion welding, or the like may be used.

Further, the ferritic stainless steel plate or clad material of the present embodiment may be molded into a pot, a frying pan, an inner pot of a rice cooker, or the like used for equipment compatible with IH cooking. Further, the use of the ferrite-based stainless steel sheet of the present embodiment is not limited to IH cooking, and can be applied to applications requiring design, for example, housings for electronic devices such as personal computers and home appliances. It may be used for housings of products, tableware, works of art, and the like. Further, the ferrite-based stainless steel sheet of the present embodiment may be used as a building material, and can be used, for example, as an exterior material or an interior material of a building.

A method for measuring relative magnetic permeability will be described. When the width of the ferritic stainless steel sheet is w, (1/10) w, (1/4) w, (1/2) w, (3/4) w and (9/10) from one end in the width direction. At the five points w, the stainless steel plate is cut into strips having a width of 5 mm and a length of 50 mm by electric discharge processing, and these are used as measurement samples. The longitudinal direction of the measurement sample is the plate thickness direction. The relative magnetic permeability of the measurement sample is measured by the impedance method. The measurement solenoid length is 42 mm, the number of windings is 50 turns, the coil diameter is 10.25 mm, the measurement magnetic field is 11.05 A / m, the measurement direction is the long side direction, and the measurement frequency is 25 kHz. The relative magnetic permeability of the clad material is measured only with a stainless steel plate after the aluminum plate is peeled from the clad material. The average value of the five locations is defined as the relative magnetic permeability.

Next, the present invention will be described in more detail by way of examples.
As a material, a stainless steel ingot having the chemical composition shown in Table 1 was used. Steels A to D in Table 1 are stainless steel ingots satisfying the provisions of the present invention, and steels E to L are comparative stainless steel ingots not invented. Steel I is a martensitic stainless steel, and steel J is an austenitic stainless steel. The ingots of steels A to L were hot-rolled to obtain a hot-rolled plate having a plate thickness of 4 mm. Except for some hot-rolled sheets, further, intermediate heat treatment at 1050 ° C. for 3 minutes, cold rolling with a plate thickness reduction rate of 63%, intermediate heat treatment at 1000 ° C. for 3 minutes, cold rolling with a plate thickness reduction rate of 67%. To obtain a cold-rolled plate having a plate thickness of 0.5 mm. The hot-rolled and cold-rolled plates were subjected to intermediate heat treatment (first step), finish cold rolling (second step), and finish heat treatment (third step) under the conditions shown in Table 2. The heat treatment was carried out in an atmospheric furnace, and the scale generated by the heat treatment was removed by immersion in fluorinated nitric acid. The cooling after the third step was performed in the range of 0.03 ° C./sec to 100 ° C./sec at an average cooling rate from the holding temperature to 200 ° C. Some stainless steel sheets were further joined and rolled with a pure aluminum plate to form a clad material. In this way, the stainless steel plates of Test Examples 1 to 25 shown in Table 2 were obtained.

The evaluation method for evaluating the average crystal grain size, designability (aesthetic appearance, conspicuous flaws), and relative magnetic permeability of the obtained stainless steel sheet will be described below.

When the width of the average crystal grain size stainless steel sheet is w, (1/10) w, (1/4) w, (1/2) w, (3/4) w and (9 /) from one end in the width direction. 10) Samples were taken at 5 points w so that the cross section perpendicular to the rolling direction (C cross section) was the observation surface. The area of the entire observation surface was 5 mm ² , and the area of one observation surface was 1 mm ² . The dimension of each observation surface in the plate thickness direction was the plate thickness t (mm). The length of each observation surface in the plate width direction was 1 / t (mm). After mirror polishing the observation surface, electrolytic etching was performed with 10% oxalic acid, and ferrite crystal grains were observed with an optical microscope or a scanning electron microscope, and the number of crystal grains was counted. The number of crystal grains that partially protruded from the observation surface when the contour lines in the plate width direction and the plate thickness direction of the observation surface crossed over the crystal grains was counted as 0.5. The total number of ferrites measured on the five observation surfaces was divided by the area of all observation surfaces to obtain the area per crystal grain. Then, the diameter equivalent to the circle of the crystal grains was obtained from the area per crystal grain, and this was used as the average crystal grain size.

Design (aesthetic, conspicuous flaws)
The design was evaluated from the two viewpoints of the aesthetic appearance and the conspicuousness of the flaws. As for the aesthetics, 20 evaluators visually observed the appearance of the stainless steel sheet, which was immersed in a mixed acid of 3% hydrofluoric acid and 7% nitric acid for 10 minutes to reveal crystal grains, and felt a different aesthetic appearance. The percentage of those who answered was x, 50% or more and less than 80% was Δ, 80% or more and less than 90% was ○, and 90% or more was ⊚. ○ The above was passed. The degree of conspicuousness of the flaw depends on whether the flaw can be identified by 20 evaluators using a sample (50 mm square) in which a scratch flaw with a length of 10 mm is attached to the surface of a steel plate with the tip of a wire having a diameter of 0.5 mm. evaluated. The percentage of people who could identify defects within 10 seconds was marked as x for less than 50%, Δ for 50% or more and less than 80%, ○ for 80% or more and less than 90%, and ⊚ for 90% or more. ○ The above was passed.

When the width of the specific magnetic permeability stainless steel sheet is w, (1/10) w, (1/4) w, (1/2) w, (3/4) w and (9/10) from one end in the width direction. ) At 5 points w, the stainless steel plate was cut into strips having a width of 5 mm and a length of 50 mm by electric discharge processing, and these were used as measurement samples. The longitudinal direction of the measurement sample was defined as the plate thickness direction. The relative magnetic permeability of the measurement sample was measured by the impedance method. The solenoid length for measurement was 42 mm, the number of windings was 50 turns, the coil diameter was 10.25 mm, the measurement magnetic field was 11.05 A / m, the measurement direction was the long side direction, and the measurement frequency was 25 kHz. The relative magnetic permeability of the clad material was measured only with a stainless steel plate after the aluminum plate was peeled from the clad material. The average value of the five locations was taken as the relative magnetic permeability. A relative magnetic permeability of 90 or more was accepted.

The stainless steel sheets shown in Test Examples 1 to 9 in Table 2 are examples of inventions that satisfy the composition and production conditions of the steel sheet specified in the present invention. Further, in Test Example 10, the stainless steel plate of Test Example 1 is clad-rolled together with a pure aluminum plate to obtain a clad material. On the other hand, the stainless steel sheets shown in Test Examples 11 to 25 are comparative examples that do not satisfy either the composition of the steel sheet or the production conditions specified in the present invention.

As shown in Table 2, it can be seen that Test Examples 1 to 10 (Invention Examples) have an average crystal grain size of 100 μm or more and a relative magnetic permeability of 90 or more. Further, when the average crystal grain size is 100 μm or more, it can be seen that 80% or more of the people recognize the unprecedented aesthetic appearance and judge that the flaws are inconspicuous. Further, when the average crystal grain size is 150 μm or more, it can be seen that 90% or more of the people recognize the aesthetic appearance and judge that the flaws are inconspicuous. Further, although Invention Example 3 is made of a hot-rolled plate, it satisfies a predetermined crystal grain size, aesthetic appearance, flaw conspicuity, and relative magnetic permeability by going through the steps specified in the present invention.

A comparative example will be described below.
In Test Example 11, although the component composition of the material satisfies the provisions of the present invention, the heat treatment temperature in the first step is low, the holding time is short, the heat treatment is insufficient, and the average crystal grain size of the final product is small.
In Test Examples 12 and 13, the component composition of the material satisfies the provisions of the present invention, but the average crystal grain size is small. This is because the finish cold rolling before the finish heat treatment is not performed, or the plate thickness reduction rate is smaller than the specification of the present invention.

In Test Examples 14 and 15, the component composition of the material satisfies the provisions of the present invention, but the average crystal grain size is small. This is because the plate thickness reduction rate in the finish cold rolling before the finish heat treatment is larger than the specification of the present invention.

From the results of Invention Examples 1 and 6 and Comparative Examples 12 to 15, it can be seen that coarse crystal grains can be obtained by performing the finish cold rolling at the plate thickness reduction rate specified in the present invention before the finish heat treatment.

In Test Example 16, the composition of the material satisfies the provisions of the present invention, but the average crystal grain size is small. This is because the holding temperature during the finish heat treatment is lower than the specification of the present invention.
In Test Example 17, the composition of the material satisfies the provisions of the present invention, but the average crystal grain size is small. This is because the holding time during the finish heat treatment is shorter than the specification of the present invention.

In Test Example 18, the total amount of Ti, Nb, and V in the component composition of the material is small, does not satisfy the range of the present invention, and the average crystal grain size is small. This is because the austenite phase is formed during the finish heat treatment and the crystal grain growth rate is slowed down.
In Test Example 19, the amount of C in the component composition of the material is excessive, does not satisfy the range of the present invention, and the average crystal grain size is small. This is because the amount of C that contributes to the stabilization of austenite is large, the austenite phase is formed during the finish heat treatment, and the crystal grain growth rate is slowed down.

In Test Example 20, the amount of N in the component composition of the material is excessive, does not satisfy the range of the present invention, and the average crystal grain size is small. This is because the amount of N that contributes to the stabilization of austenite is large, the austenite phase is formed during the finish heat treatment, and the crystal grain growth rate is slow.
In Test Example 21, the total amount of Ti, Nb, and V in the component composition of the material became excessive, which did not satisfy the scope of the present invention, and a large number of ear cracks occurred during hot rolling, and rolling was not possible.

In Test Example 22, the material was martensitic stainless steel, the chemical composition did not meet the scope of the present invention, and the average crystal grain size was reduced.
In Test Example 23, the material is austenitic stainless steel, the chemical composition does not meet the scope of the present invention, and the average crystal grain size is small. Moreover, since it is an austenitic stainless steel, its relative magnetic permeability is very small.

In Test Example 24, the amount of Ni in the chemical components of the material is excessive, does not satisfy the range of the present invention, and the average crystal grain size is small. This is because the amount of Ni that contributes to the stabilization of austenite is large, the austenite phase is formed during the finish heat treatment, and the crystal grain growth rate is slowed down.
In Test Example 25, the amount of Cu among the chemical components of the material is excessive, does not satisfy the range of the present invention, and the average crystal grain size is small. This is because the amount of Cu that contributes to the stabilization of austenite is large, the austenite phase is formed during the finish heat treatment, and the crystal grain growth rate is slowed down.

Claims (5)

By mass%
C: 0.030% or less,
N: 0.030% or less,
Si: 0.20% or more and 0.80% or less,
Cr: 15% or more and 20% or less,
Mn: 0.80% or less,
Ni: 1.0% or less,
Cu: Contains 0.80% or less,
Further, any one or more of Nb, V, and Ti are contained in a total of 0.10% or more and 0.80% or less.
The balance has a component composition consisting of Fe and impurities,
A ferritic stainless steel sheet having an average crystal grain size of 100 μm or more. The ferrite-based stainless steel sheet according to claim 1, wherein the relative magnetic permeability at a frequency of 25 kHz is 90 or more. The ferrite-based stainless steel sheet according to claim 1 or 2, wherein it is used as a member of an apparatus compatible with high-frequency induction cooking. A clad material obtained by joining the ferritic stainless steel plate according to any one of claims 1 to 3 to an aluminum plate. The first step of subjecting the stainless steel sheet material having the component composition according to claim 1 to an intermediate heat treatment at 900 ° C. or higher and 1100 ° C. or lower for 1.0 minute or longer.
The second step of performing cold rolling for finishing with a plate thickness reduction rate of 5% or more and 10% or less, and
The ferritic stainless steel according to any one of claims 1 to 3, wherein a third step of performing a finish heat treatment at 980 ° C. or higher and 1150 ° C. or lower for 1.0 minute or longer is sequentially performed. Method of manufacturing steel sheet.