JP2019044255A - Ferritic stainless steel sheet - Google Patents

Abstract

The object is to provide a ferritic stainless steel sheet which has sufficient corrosion resistance and is excellent in bending formability and surface quality of a bent portion.
According to the present invention, a predetermined component composition is obtained, and the relationship of the following formula (1) is satisfied with respect to the total amount A (volume ppm) of MnS and MnO-SiO ₂ in steel.
A ≦ 800-900 × [% Si] (1)
Here, [% Si] is the Si content [mass%] of the above component composition.
【Selection chart】 None

Description

  The present invention relates to a ferritic stainless steel sheet, and more particularly to a ferritic stainless steel sheet having sufficient corrosion resistance and excellent bending formability and surface quality of a bent portion.

  Japan Industrial Standard: SUS 430 (16 to 18 mass% Cr) specified in JIS G 4305 is inexpensive and excellent in corrosion resistance among ferritic stainless steels, so construction materials, transportation equipment, home appliances, kitchen equipment and automobile parts etc. It is used in various applications.

The steel plate applied to these uses is required to be able to be processed into a predetermined shape by press forming or the like.
Here, press forming is roughly classified into four types of forming methods such as extrusion forming, deep drawing forming, stretch flange forming and bending forming. Among them, bending is widely adopted because the degree of freedom in shape is relatively high, and development of a ferritic stainless steel sheet excellent in bending formability is strongly demanded.

As a ferritic stainless steel, for example, in Patent Document 1,
"Mass%, C: 0.02 to 0.06%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.005% or less, Ti: 0.005% or less, Cr: 11-30% or less, Ni: 0.7% or less is contained, and N is contained so as to satisfy 0.06 ≦ (C + N) ≦ 0.12 and 1 ≦ N / C in relation to the C content, and further V is Containing 1.5 × 10 ^-3 ≦ (V × N) ≦ 1.5 × 10 ^-2 in relation to the N content, the balance being composed of Fe and unavoidable impurities, excellent in formability Ferritic stainless steel sheet. "
Is disclosed.

Patent Document 2 also includes
"In a ferritic stainless steel containing 15 to 20 wt% of Cr, V is contained in a range of 0.005 to 1.0 wt%, and at most 0.005 wt% of Al, 0.001 to 0.007 wt% of O % in the range of, and in the component of oxide inclusions in the steel, _Al 2 _{O 3} and _Cr 2 _{O 3, respectively,} Al ₂ O 3: 5wt% or _less, Cr ₂ O 3: _10~50wt% range A ferritic stainless steel excellent in surface properties and press formability, characterized in that
Is disclosed.

Furthermore, in Patent Document 3,
"A ferritic stainless steel sheet containing, by mass%, C: 0.01 to 0.03%, Mn: 0.5 to 1.0%, Cr: 15 to 20%, Al: 0.01% or less B, characterized in that Cr carbide is dispersed in ferrite and the abundance ratio of metal elements of Fe and Cr in the Cr carbide is Fe / Cr: 0.05 to 0.15 in mass% ratio. Ferritic stainless steel sheet with excellent workability. "
Is disclosed.

In addition, Patent Document 4
“In mass%, C: 0.2% or less, Si: 0.15 to 0.32%, Mn: 0.4% or less, Cr: 10 to 25%, Al: 0.005% or less, and the Mn and Si are Mn / Si: 1.25 A ferritic stainless steel excellent in bending workability and surface properties, characterized in that the above components are contained and the balance is made of Fe and unavoidable impurities. "
Is disclosed.

Patent No. 3584881 Patent No. 3608383 gazette Patent No. 5050565 gazette Japanese Patent Application Laid-Open No. 2002-80941

  However, based on the techniques of Patent Documents 1 and 2, a cold-rolled steel plate is manufactured with a thick plate thickness of 1.0 mm or more, and is used as a material to perform bending, specifically, 90 ° bending 90 ° bending, which is a relatively mild bending process, by bending into a box shape that has a part and a hemmed part that has undergone 180 ° contact bending in a state where tensile strain associated with processing has been introduced Cracks may occur in parts.

  In addition, based on the technology of Patent Document 3, when a cold-rolled steel plate is manufactured with a thick plate thickness, particularly 1.0 mm or more, and this is used as a raw material for the same bending as described above, Although cracking does not occur, cracking may occur in the hemmed portion and it may not be able to be formed into a predetermined shape.

  In this respect, according to the technique of Patent Document 4, even if the plate thickness is increased to about 2.0 mm, the occurrence of cracks can be prevented in both the 90 ° bending portion and the hemming portion. However, even in this case, surface irregularities called ridging may be generated at the bending ridges of the hemmed portion to damage the surface quality, and a problem is left at this point.

  As described above, in the techniques of Patent Documents 1 to 4, it is difficult to say that the bending formability and the surface quality of the bending portion are compatible with each other. Therefore, the bending formability and the surface quality of the bending portion may be compatible with each other At present, there is a demand for the development of possible ferritic stainless steel sheets, in particular, ferritic stainless steel sheets capable of achieving both bending formability and surface quality of bent portions even if the plate thickness is increased.

The present invention has been developed in view of the above-mentioned present situation, and an object of the present invention is to provide a ferritic stainless steel sheet which has sufficient corrosion resistance and is excellent in bending formability and surface quality of a bending portion.
Here, “sufficient corrosion resistance” means salt water spray cycle test (35 ° C., 5 mass% NaCl, spray time: 2 hours) → drying (60 ° C., relative humidity 40) according to the salt spray cycle test defined in JIS H 8502. %, Retention time: 4 hours → wet (50 ° C., relative humidity 9595%, retention time: 2 hours) as one cycle for 8 cycles of rust area ratio on the steel sheet surface (rust generation on steel sheet surface) The area / total surface area of the surface of the test steel plate) × 100 (%)) means that 25% or less.
Further, “excellent in bending formability” means that a crack occurs when a steel sheet is subjected to 180 ° close contact bending (hereinafter also referred to as “stretch bend”) after applying 20% tensile strain with the longitudinal direction perpendicular to the rolling as the longitudinal direction. It means not to do.
Furthermore, “excellent in the surface quality of the bent portion” means that the undulation height measured in parallel and perpendicular to the ridgeline at the bending ridge portion after the above-mentioned stretch bending is measured in accordance with JIS B 0601 (2001). It means that all are 5.0 micrometers or less.

Now, the inventors have conducted various studies in order to solve the above-mentioned problems.
First, the inventors bending-formed using various ferritic stainless steel plates as raw materials, and investigated in detail the cause of the occurrence of ridging occurring at the bending ridge portion of the above-mentioned hemmed portion.
As a result, the ridging generated at the bending ridge portion of the hemmed portion described above is mainly caused by cold rolling annealing of ferrite phase colonies (crystal grain groups having similar crystal orientation) generated during casting and / or hot rolling. It turned out that it arises to survive.

Therefore, the inventors considered that various component compositions were considered to be able to effectively prevent the occurrence of ridging if the above-mentioned colonies could be effectively destroyed.
As a result, from the viewpoint of preventing the occurrence of ridging, it has come to be considered that it is effective to reduce the Si content and to increase the Mn content.
That is, since Si is a ferrite forming element, the amount of austenite phase formed during hot rolling decreases with an increase in the Si content. In addition, Si also has the effect of promoting recovery of the ferrite phase during hot rolling. Therefore, when the amount of Si is too large, the rolling process strain introduced by the hot rolling is promoted to be eliminated by recovery, and the strain which becomes a recrystallization site at the time of the hot-rolled sheet annealing in the next step is reduced. As a result, it is thought that the colony destruction effect by recrystallization of the ferrite phase at the time of hot-rolled sheet annealing is reduced.
On the other hand, Mn is an austenite-forming element, and increases the amount of austenite phase formed during hot rolling. Since the austenite phase is mainly generated from grain boundaries of the ferrite phase, it is believed that the greater the amount of austenite phase generated, the more fracture of the colony is promoted during hot rolling.
Therefore, the inventors thought that it is possible to effectively prevent the occurrence of ridging by reducing the Si content and increasing the Mn content.

  However, when a stainless steel plate prepared to have the above-described component composition was subjected to 180 ° close contact bending in a state in which tensile strain caused by processing was introduced, cracking occurred in the bent portion.

The inventors investigated the cause of this crack in detail. By bending, a void is generated at the interface between the ferrite phase and inclusions such as MnS and MnO-SiO ₂ , and the void is the origin of the crack. It turned out that it has occurred.
Note that the MnO-SiO _2, a mixture of MnO and SiO _2.

Therefore, the inventors examined in detail the relationship between the inclusions as the starting point of the crack and the component composition.
as a result,
The occurrence of cracking is strongly correlated with the Si content of the component composition and the total amount of MnS and MnO-SiO _{2 in} the steel,
By reducing the total amount of MnS and MnO-SiO _{2 to} a predetermined amount or less in relation to the Si content of the component composition, occurrence of cracking is effectively prevented, and bending formability and surface quality of the bending portion It will be possible to achieve both
The findings of the
Further, from the viewpoint of reducing the amount of inclusions described above, in particular MnO-SiO ₂ , secondary refining is performed by a vacuum oxygen decarburization (VOD) method, and the slag basicity at that time (VOD) It is extremely important to carry out bubbling treatment under predetermined conditions during adjustment of CaO / SiO ₂ ) and further during secondary refining, and while performing such treatment, the Mn content is reduced by reducing the amount of S. It was also found that even if the component composition is such that the P content is 0.45% or more, the generation amount of inclusions such as MnS and MnO-SiO ₂ can be significantly reduced.
The present invention has been completed after further investigation based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. In mass%,
C: 0.015 to 0.050%,
Si: 0.05 to 0.40%,
Mn: 0.45 to 1.00%,
P: 0.040% or less,
S: 0.008% or less,
Cr: 15.5 to 18.0%,
Al: 0.001 to 0.010% and N: 0.010 to 0.080%
As well as having a component composition comprising the balance of Fe and unavoidable impurities,
A ferritic stainless steel sheet in which the total amount A (volume ppm) of inclusions MnS and MnO-SiO ₂ in steel satisfies the relationship of the following formula (1).
A ≦ 800-900 × [% Si] (1)
Here, [% Si] is the Si content [mass%] of the above component composition.

2. Said component composition is, in mass%, further Ca: 0.0002 to 0.0020%
The ferritic stainless steel sheet as described in 1 above, which contains

ADVANTAGE OF THE INVENTION According to this invention, while having sufficient corrosion resistance, the ferritic stainless steel plate which is excellent in bending formability and the surface quality of a bending part is obtained.
Further, in the ferritic stainless steel sheet of the present invention, excellent bending formability and surface quality of the bent portion can be obtained even when the steel sheet is thickened. It is particularly advantageous to apply to the commercial kitchen used.

Hereinafter, the present invention will be specifically described.
First, the component composition of the ferritic stainless steel sheet of the present invention will be described. In addition, although the unit in a component composition is all "mass%", unless otherwise indicated, it only shows by "%" unless it refuses.

C: 0.015 to 0.050%
C is an element effective in promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of ridging. From the viewpoint of obtaining such an effect, the C content is made 0.015% or more. However, if the C content exceeds 0.050%, the steel is excessively hardened and the ductility is reduced. In addition, voids may be generated from the interface between the carbide and the ferrite phase, which may lead to cracking during bending. Therefore, the C content is in the range of 0.015 to 0.050%. The lower limit of the C content is preferably 0.025%. The upper limit of the C content is preferably 0.045%.

Si: 0.05 to 0.40%
Si is an element which acts as a deoxidizer at the time of steel melting. From the viewpoint of obtaining such an effect, the Si content is made 0.05% or more. However, if the Si content exceeds 0.40%, the steel becomes excessively hard, and cracking originating from the ferrite phase occurs during bending. Therefore, the desired bend formability can not be obtained. Further, since Si is a ferrite-forming element, the amount of austenite phase formed during hot rolling decreases with an increase in the Si content. Furthermore, Si also has the effect of promoting recovery of the ferrite phase during hot rolling. Therefore, when the amount of Si is too large, the rolling process strain introduced by hot rolling is promoted to be eliminated by the recovery, thereby reducing the strain which becomes a recrystallization site at the time of the hot-rolled sheet annealing in the next process. Resulting in. As a result, the colony destruction effect due to recrystallization of the ferrite phase at the time of hot-rolled sheet annealing is reduced, and ridging occurs in the bending portion to deteriorate the surface quality.
Therefore, the Si content is in the range of 0.05 to 0.40%. The lower limit of the Si content is preferably 0.10%, more preferably 0.20%. The upper limit of the Si content is preferably 0.35%, more preferably 0.30%.

Mn: 0.45 to 1.00%
Mn, like C, is an element effective in promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of ridging. From the viewpoint of obtaining such an effect, the Mn content is set to 0.45% or more. If the Mn content is less than 0.45%, the amount of austenite phase formed during hot rolling decreases. Therefore, the destructive effect of the colony is insufficient, ridging occurs in the bending portion, and the surface quality is impaired. On the other hand, if the Mn content exceeds 1.00%, the steel hardens and the ductility decreases. In addition, the corrosion resistance may also decrease.
Therefore, the Mn content is in the range of 0.45 to 1.00%. The lower limit of the Mn content is preferably 0.50%, more preferably 0.55%. The upper limit of the Mn content is preferably 0.90%, more preferably 0.80%.

P: 0.040% or less P is an element that promotes intergranular fracture due to intergranular segregation. Therefore, the lower the P content, the better, and the upper limit is made 0.040%. Preferably it is 0.030% or less. More preferably, it is 0.010% or less.

S: 0.008% or less S combines with Mn to form MnS. MnS induces the occurrence of cracking and reduces the bending formability. Also, MnS may reduce the corrosion resistance. Therefore, the S content is made 0.008% or less. Preferably it is 0.006% or less.

Cr: 15.5 to 18.0%
Cr is an element having the effect of forming a passive film on the surface of the steel sheet to improve the corrosion resistance. From the viewpoint of obtaining such an effect, the Cr content is made 15.5% or more. However, when the Cr content exceeds 18.0%, the amount of austenite phase formed during hot rolling decreases, which easily causes the occurrence of ridging.
Therefore, the Cr content is in the range of 15.5 to 18.0%. The lower limit of the Cr content is preferably 16.0%. The upper limit of the Cr content is preferably 17.0%, more preferably 16.5%.

Al: 0.001 to 0.010%
Al, like Si, is an element that acts as a deoxidizer. From the viewpoint of obtaining such an effect, the Al content is made 0.001% or more. However, when the Al content exceeds 0.010%, the amount of inclusions, Al ₂ O ₃ , is increased, and the surface quality tends to be degraded.
Therefore, the Al content is in the range of 0.001 to 0.010%. The upper limit of the Al content is preferably 0.007%, more preferably 0.005%.

N: 0.010 to 0.080%
N, like C and Mn, is an element effective in promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of ridging. From the viewpoint of obtaining such an effect, the N content is made 0.010% or more. However, when the N content exceeds 0.080%, the ductility is significantly reduced. In addition, the deposition of Cr nitride may be promoted, and the corrosion resistance may be lowered due to the sensitization.
Therefore, the N content is in the range of 0.010 to 0.080%. The lower limit of the N content is preferably 0.030%, more preferably 0.040%.

As mentioned above, although basic components were explained, in addition to the above-mentioned basic components, Ca: 0.0002 to 0.0020% may be further contained.
Ca: 0.0002 to 0.0020%
Ca is an effective component to prevent the clogging of the nozzle due to the crystallization of inclusions that are likely to occur during continuous casting. In addition, when Ca is contained, a large amount of CaS which remains spherical after rolling is generated in place of MnS which is greatly expanded by rolling, which contributes to the improvement of bending formability. From the viewpoint of obtaining such an effect, the Ca content is preferably 0.0002% or more. However, as the Ca content increases, CaO, which is an inclusion in the steel, also increases. When CaO excessively increases, there is a risk that cracking originating from CaO may occur during bending. Therefore, the content of Ca is preferably 0.0020% or less.
Therefore, when it contains Ca, the content is taken as 0.0002 to 0.0020% of range. The lower limit of the Ca content is more preferably 0.0005%. The upper limit of the Ca content is more preferably 0.0015%, still more preferably 0.0010%.

  The components other than the above are Fe and unavoidable impurities.

Next, inclusions in the steel will be described.
The total amount A (volume ppm) of MnS and MnO-SiO ₂ : 800-900 × [% Si] or less ([% Si] is the Si content (mass%) of the component composition of the steel)
As described above, when the steel sheet is subjected to bending, voids are generated at the interface between the ferrite phase and inclusions such as MnS and MnO-SiO ₂ . And if the total amount of these inclusions increases, these voids will connect and it will become easy to produce a crack. In particular, in a steel plate having a high Si content, the steel plate becomes hard due to a decrease in plastic deformability of the steel and the stress necessary for bending increases, so inclusions such as ferrite phase and MnS or MnO-SiO ₂ As a result, a large stress concentration occurs in the void at the interface between the two, thereby promoting the generation of a crack and, as a result, the bending formability is reduced.
Therefore, in order to obtain the desired bending formability, it is extremely important to adjust the total amount A (volume ppm) of MnS and MnO-SiO ₂ in relation to the Si content of the component composition, thus MnS And the total amount A (volume ppm) of MnO-SiO ₂ is 800-900 × [% Si] or less (that is, satisfy A ≦ 800-900 × [% Si] (1)) It is necessary. The total amount A (volume ppm) of MnS and MnO-SiO ₂ is preferably 700-900 × [% Si] or less, more preferably 650-900 × [% Si] or less.
[% Si] is the Si content (% by mass) of the component composition.

Further, from the viewpoint of improving bending formability, it is preferable to set the amount of MnO—SiO ₂ to 400 ppm by volume or less after satisfying the above equation (1).
The amount of MnS is not particularly limited as long as the above equation (1) is satisfied, but the amount is preferably 400 ppm by volume or less.

Here, the amount of MnS (volume ppm) present as inclusions in the steel is determined as follows.
That is, electrolysis using 10 mass% acetylacetone 1 mass% tetramethyl ammonium chloride methanol solution (AA solution) is performed to the manufactured stainless steel plate. Then, the extraction residue is collected using a filter with a mesh size of 0.2 μm, and the amount of S is quantitatively analyzed by ICP (high frequency inductively coupled plasma) emission analysis using the collected extraction residue. The mass fraction of S obtained here is converted to a molar fraction using the atomic weight of S (= 32), and it is assumed that MnS (atomic weight 87) is generated by the molar fraction of S, the MnS Determine the mole fraction. Then, the mole fraction of MnS is converted to a mass fraction, and further, from the mass fraction of MnS, the density of MnS (5.226 g / cm ³ ) and the density of SUS 430 (7.7 g / cm ³ ) are used. And the volume fraction of MnS (volume ppm).

Further, the amount of MnO-SiO ₂ present as inclusions in the steel is determined as follows.
That is, the produced stainless steel sheet is subjected to a dissolution treatment by a bromine-methanol method. Next, the extraction residue is collected using a filter with a mesh size of 0.2 μm, and the amount of Mn and the amount of Si are quantitatively analyzed by ICP emission analysis using the collected extraction residue. The mass fractions of Mn and Si obtained here are converted to mole fractions using atomic weights of Mn and Si (Mn: 55, Si: 28), respectively, and MnO (atomic weight: 71) Assuming that SiO ₂ (atomic weight: 60) is formed by the mass fraction of Si, the mole fractions of MnO and SiO ₂ are respectively determined. Then, the mole fractions of MnO and SiO ₂ are converted to mass fractions, respectively. Further, the mass fraction of SiO _2, the following formula: 4.4-0.019 by × [mass fraction of SiO _2] (mass%), to calculate the density of the _{MnO-SiO 2 (g / cm} 3) Mass fraction of MnO-SiO ₂ (= [mass fraction of MnO] + [SiO ₂ mass fraction] using the calculated density of MnO-SiO _{2 and} the density of SUS 430 (7.7 g / cm ³ ) ]) and converted to volume fraction of MnO-SiO ₂ (volume ppm).
Then, by summing the volume fraction of MnS which is calculated as described above and MnO-SiO ₂ volume fraction, the total amount A of MnS and MnO-SiO ₂ is obtained.

The reason why the total amount of MnS and MnO-SiO ₂ is defined not by the mass ratio but by the volume ratio is as follows.
That is, most of the cracks are caused by the connection of the ferrite phase which is the matrix phase and the voids generated at the interface of MnS or MnO-SiO ₂ . As the distance between MnS and MnO-SiO ₂ decreases, the connection of voids is promoted. Here, the volume ratio is higher in correlation with the distance between MnS and MnO—SiO ₂ than the mass ratio. Therefore, the total amount of MnS and MnO-SiO ₂ is defined not by the mass ratio but by the volume ratio.
Also, in the above equation (1), the Si content of the component composition is used in mass% units to define the range of the total amount A (volume ppm) of MnS and MnO—SiO ₂ , but Is due to the following reasons.
That is, as described above, the plastic deformability of the ferrite phase, which is the matrix phase, changes in accordance with the Si content (% by mass), so the plastic deformability of this steel represents the influence on the bend formability. It is considered appropriate to use the Si content (% by mass), and in fact, the Si content (% by mass) is used to adjust the total amount A (volume ppm) of MnS and MnO-SiO ₂ This is because the desired bending formability can be obtained.

In addition, the structure of the ferritic stainless steel plate of the present invention has a structure mainly composed of a ferrite phase, specifically, has a ferrite phase of 90% or more in volume ratio to the whole structure, and the remaining structure other than the ferrite phase is a structure It becomes the organization which becomes 10% or less with the volume ratio to the whole. Furthermore, it may be a ferrite single phase. The residual structure mainly includes a martensite phase, and does not include the volume ratio of precipitates and inclusions.
Here, the volume fraction of the ferrite phase is prepared by preparing a test piece for cross-sectional observation from a stainless steel plate and performing an etching process with a picric acid saturated hydrochloric acid solution, and observing with an optical microscope at a magnification of 100 × After the martensitic phase and the ferrite phase are distinguished from the structure shape and the etching strength, the volume fraction of the ferrite phase is determined by image processing, and the average value is calculated.

  The thickness of the ferritic stainless steel plate of the present invention is not particularly limited, but is effective when the thickness is 0.8 mm or more. In particular, it is effective when 1.0 mm or more, more particularly 1.2 mm or more, more particularly 1.5 mm or more, even more particularly 2.0 mm or more. The upper limit of the plate thickness is about 5.0 mm.

Next, the suitable manufacturing method of the ferritic stainless steel plate of this invention is demonstrated.
First, molten steel having the above-mentioned component composition is subjected to primary smelting by a known method using a converter, electric furnace, vacuum melting furnace or the like, and secondary refining by vacuum oxygen decarburizing treatment (hereinafter also referred to as VOD method). It is melted and then made into a steel material (slab) by a continuous casting method.

Here, from the viewpoint of reducing inclusions in the steel, particularly MnO-SiO ₂ and satisfying the above equation (1), secondary refining is performed by the VOD method, and slag basicity (CaO / SiO _{2 is also obtained.} It is important to adjust and maintain b) to 1.5 or more, and to perform bubbling treatment under predetermined conditions during secondary refining.
That is, since the VOD method performs the treatment under vacuum, the amount of oxygen in the molten steel is effectively reduced compared to the argon-oxygen decarburization method (hereinafter also referred to as the AOD method), and MnO-SiO ₂ at the time of casting It is possible to reduce the amount of
Further, by adjusting and maintaining the slag basicity (CaO / SiO ₂ ) to at least 1.5, more preferably at least 1.6, in particular, the deoxidation reaction by Si can be promoted, and as a result, The amount of oxygen in the molten steel is reduced to reduce the amount of MnO-SiO ₂ formed during casting.

  However, in the VOD method, although decarburization and component adjustment are performed under vacuum, pressure is restored to atmospheric pressure, but since molten steel boils violently under vacuum, slag is suspended in molten steel after pressure recovery to atmospheric pressure. It is in a state of Therefore, if casting is performed in this state, a large amount of slag and inclusions generated by secondary refining remain in the molten steel, and the amount of inclusions in the final product increases, and the desired bend formability Can not be obtained.

To avoid this, after the pressure restored to atmospheric pressure, a predetermined condition, specifically, flow: 0.1~0.6Nm ³ / min ( "Nm ^3" means standard conditions (25 ° C. Gas, 1 In addition, it is necessary to perform bubbling treatment of preferably 0.3 to 0.5 Nm ³ / minute) and treatment time: 5 minutes or more (preferably 10 minutes or more).
That is, by performing the above-described bubbling treatment, inclusions generated during secondary refining and suspended in the molten steel are floated to the upper surface of the molten steel utilizing the adsorption effect on bubbles and the specific gravity difference with the molten steel. It becomes possible to separate and remove effectively as slag in the continuous casting tundish which is the next process.
Thereby, inclusions present in the final product plate, in particular, MnO—SiO ₂ can be reduced, and cracking during bending can be prevented.
In addition, it is preferable to use inert gas, such as Ar, as bubbling gas in order to prevent the change of the molten steel component during bubbling processing, and using the mixed gas of Ar and nitrogen in order to prevent denitrification in bubbling processing Is more preferred.

Next, the obtained steel material is heated at 1100 to 1250 ° C. for 1 to 24 hours, or a high temperature slab is directly heated, and then the steel material is subjected to hot rolling to form a hot rolled steel sheet. Subsequently, hot-rolled sheet annealing and pickling are given to the obtained hot-rolled steel plate at a temperature of 800 to 900 ° C., if necessary. Then, the obtained hot rolled steel sheet is subjected to cold rolling and cold rolled sheet annealing to form a cold rolled steel sheet, and further, if necessary, to pickling to obtain a final product sheet.
Here, the rolling reduction of cold rolling is preferably 50% or more from the viewpoint of extensibility, bendability, press formability, and shape correction.
In the case of No. 2B finish which is surface finish specified by JIS G 0203, cold-rolled sheet annealing is performed at a temperature of 800 to 950 ° C. from the viewpoint of obtaining good mechanical properties and pickability. In view of workability, it is more preferable to set the temperature to the austenite transformation point or less. Furthermore, bright annealing (BA annealing) may be performed to obtain more gloss.
Furthermore, cold rolling and cold rolled sheet annealing may be repeated twice or more, respectively.

  In addition, what is necessary is just to follow a usual method about manufacturing conditions other than the above. Further, from the viewpoint of further improving the surface properties, grinding or polishing may be further performed.

Example 1
Molten steel (150 tons) of the component components (the balance is Fe and unavoidable impurities) shown in Table 1 is melted by primary smelting with a converter and secondary refining with VOD method under the conditions shown in Table 2 did. Also, in secondary refining, after adjustment of slag basicity and components is completed, the pressure is restored to atmospheric pressure, and then a gas (flow rate: 0.4 Nm ³ / min) in which Ar and nitrogen are mixed at a volume ratio of 3: 1 The bubbling treatment was carried out using the conditions shown in Table 2.
Subsequently, the molten steel was made into a steel slab having a width of 1000 mm and a thickness of 200 mm by a continuous casting method, and the obtained steel slab was heated at 1150 ° C. for 1 hour. Thereafter, hot rolling consisting of rough rolling and finish rolling is applied, and wound at 700 ° C. to obtain a hot-rolled steel plate having a plate thickness of 5.0 mm.
The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing at an annealing temperature of 830 ° C. and a holding time of 8 hours by box annealing, and then the surface was subjected to shot blasting and descaling by pickling.
Then, the above hot rolled steel sheet is subjected to cold rolling and cold rolled sheet annealing (annealing temperature: 830 ° C.) in a continuous annealing furnace to obtain a cold rolled steel sheet having a thickness of 2.0 mm, further, The cold-rolled steel plate was subjected to descaling treatment by pickling to obtain a ferritic stainless steel plate to be a final product plate.

The amount of MnS and the amount of MnO-SiO ₂ present as inclusions in the steel, and the total amount thereof were determined for the thus obtained ferritic stainless steel sheet by the above-described method. The results are shown in Table 2.
Moreover, when the steel plate structure was identified by the method mentioned above, the ferrite phase was 90% or more by the volume ratio with respect to the whole structure also about any steel plate.
Furthermore, (1) evaluation of bending formability, (2) evaluation of bending part surface quality, and (3) evaluation of corrosion resistance were performed by the following methods. Table 2 shows the results of these evaluations.

(1) Evaluation of Bending Formability From a ferritic stainless steel plate to be a final product plate, a test piece of 30 mm × 200 mm was taken, with the direction perpendicular to rolling being the longitudinal direction. After applying a tensile strain of 20% to the collected test piece, 180 ° contact bending was applied. Then, the presence or absence of the crack in the bending ridge line part of a test piece was confirmed visually, and the case where it could be bend-formed favorably without a crack in a ridgeline part was pass ((circle)), and the case where a crack generate | occur | produced was evaluated as rejection (x).

(2) Evaluation of the surface quality of the bending portion According to JIS B 0601 (2001), the undulation height in the bending ridge portion of the test piece after the 180 ° close contact bending in (1) is parallel to the ridge and Measured in the perpendicular direction.
And, if the waviness height of the bending ridge portion is 5.0 μm or less in any of the direction parallel to and perpendicular to the ridgeline (○), the waviness height of the bending ridge portion is either parallel or perpendicular direction The case where it became more than 5.0 micrometers by one side was evaluated as rejection (x).
However, when a crack is recognized in (1), the column of evaluation of the surface quality of the bent portion in Table 2 is "-".

(3) Evaluation of Corrosion Resistance From a ferritic stainless steel plate to be a final product plate, a test piece of 60 × 100 mm was taken with the rolling direction as the length, and the surface was polished and finished with # 600 emery paper. Thereafter, the end face of the test piece was sealed and subjected to a salt spray cycle test specified in JIS H8502.
Here, the salt spray cycle test is salt spray (35 ° C., 5% by mass NaCl, spray time: 2 hours) → drying (60 ° C., relative humidity 40%, holding time: 4 hours) → wetting (50 ° C., relative 8 cycles were performed, with humidity サ イ ク ル 95%, retention time: 2 hours) as one cycle.
The surface of the test piece after the salt spray cycle test is photographed, the rusted area of the surface of the test piece is measured by image analysis, and the rusted area ratio (( The rust area / total area of the surface of the test piece) × 100 (%)) was calculated.
And the case where calculated rust area ratio was 25% or less was evaluated as pass ((circle)), and the case where it was over 25% as rejection (x).

  In each of the invention examples, excellent bend formability and bent surface quality were obtained, and the corrosion resistance was also excellent.

On the other hand, No. 1 which is a comparative example. In 3, 7 and 11, because the bubbling time is not sufficient, MnO-SiO ₂ formed at the time of secondary refining remains in molten steel without being sufficiently separated and removed as slag, and MnS in the final product plate And the total amount of MnO-SiO ₂ exceeded the appropriate range, and as a result, the desired bend formability was not obtained.
Moreover, No. 1 which is a comparative example. At 4, 8 and 12, since the slag basicity (CaO / SiO ₂ ) in secondary refining is low, deoxidation at the time of secondary refining is insufficient and the amount of oxygen in the molten steel rises, causing a large amount of casting MnO-SiO ₂ was formed, and the total amount of MnS and MnO-SiO _{2 in} the final product plate exceeded the appropriate range, and as a result, the desired bend formability was not obtained.
Furthermore, No. 1 which is a comparative example. In 5, 9 and 13, the bubbling time is not sufficient, and the slag basicity (CaO / SiO ₂ ) in secondary refining is also low, so the total amount of MnS and MnO-SiO _{2 in} the final product plate is also As a result, the desired bending formability could not be obtained.

Example 2
Primary component of molten steel (150 ton) of the components shown in Table 3 (the balance is Fe and unavoidable impurities), and primary smelting with a converter, and VOD adjusted and maintained to a slag basicity (CaO / SiO ₂ ) of 1.7. It was melted by secondary refining according to the method. Also, in secondary refining, after adjustment of slag basicity and components is completed, the pressure is restored to atmospheric pressure, and then a gas (flow rate: 0.4 Nm ³ / min) in which Ar and nitrogen are mixed at a volume ratio of 3: 1 The bubbling process (processing time: 10 minutes) was implemented using.
Subsequently, the molten steel was made into a steel slab having a width of 1000 mm and a thickness of 200 mm by a continuous casting method, and the obtained steel slab was heated at 1150 ° C. for 1 hour. Thereafter, hot rolling consisting of rough rolling and finish rolling is applied, and wound at 700 ° C. to obtain a hot-rolled steel plate having a plate thickness of 5.0 mm. (However, for No. 37 in Table 4, after forming a hot-rolled steel plate having a thickness of 6.0 mm, it was set as a cold-rolled steel plate having a thickness of 4.0 mm by a method described later.)
The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing at an annealing temperature of 830 ° C. and a holding time of 8 hours by box annealing, and then the surface was subjected to shot blasting and descaling by pickling.
Then, the above hot rolled steel sheet is subjected to cold rolling and cold rolled sheet annealing (annealing temperature: 830 ° C.) in a continuous annealing furnace to obtain a cold rolled steel sheet having the thickness shown in Table 4 The cold-rolled steel plate was subjected to descaling treatment by pickling to obtain a ferritic stainless steel plate to be a final product plate.

The amount of MnS and the amount of MnO-SiO ₂ present as inclusions in the steel, and the total amount thereof were determined for the thus obtained ferritic stainless steel sheet by the above-described method. The results are shown in Table 4.
Moreover, when the steel plate structure was identified by the method mentioned above, the ferrite phase was 90% or more by the volume ratio with respect to the whole structure also about any steel plate.
Furthermore, (1) evaluation of bending formability, (2) evaluation of bending surface quality, and (3) evaluation of corrosion resistance were performed in the same manner as in Example 1. The evaluation results are shown in Table 4.

  In each of the invention examples, excellent bend formability and bent surface quality were obtained, and the corrosion resistance was also excellent.

On the other hand, No. 1 which is a comparative example. In No. 38, since the Si content exceeded the appropriate amount, the plastic deformation ability of the ferrite phase was significantly reduced, and the desired bend formability could not be obtained.
Moreover, No. 1 which is a comparative example. In No. 39, since the Mn content exceeds the appropriate amount, desired corrosion resistance can not be obtained.
Furthermore, No. 1 which is a comparative example. In No. 40, since the S content exceeds the appropriate amount, a large amount of MnS is generated, and the total amount of MnS and MnO-SiO _{2 in} the final product plate exceeds the appropriate range, and as a result, the desired bending Moldability was not obtained. In addition, since a large amount of MnS was formed, the desired corrosion resistance was not obtained.
In addition, No. 1 which is a comparative example. In No. 41, since the Mn content is less than the appropriate amount, ridging occurs in the bending ridge portion, and a good bent surface quality can not be obtained.

  The ferritic stainless steel plate of the present invention is used in applications where high bending formability and surface quality of the bent portion, and corrosion resistance are required, for example, cookware in which hemming is applied to the end face of the steel plate and work using caulking It is particularly advantageous to apply to a kitchen or the like.

Claims (2)

In mass%,
C: 0.015 to 0.050%,
Si: 0.05 to 0.40%,
Mn: 0.45 to 1.00%,
P: 0.040% or less,
S: 0.008% or less,
Cr: 15.5 to 18.0%,
Al: 0.001 to 0.010% and N: 0.010 to 0.080%
As well as having a component composition comprising the balance of Fe and unavoidable impurities,
A ferritic stainless steel sheet in which the total amount A (volume ppm) of inclusions MnS and MnO-SiO ₂ in steel satisfies the relationship of the following formula (1).
A ≦ 800-900 × [% Si] (1)
Here, [% Si] is the Si content [mass%] of the above component composition. Said component composition is, in mass%, further Ca: 0.0002 to 0.0020%
The ferritic stainless steel sheet according to claim 1, which contains