阅读说明：本技术 铁素体系不锈钢板的制造方法 (Method for producing ferritic stainless steel sheet ) 是由 吉野正崇 持田哲男 于 2018-09-05 设计创作，主要内容包括：本发明提供具有充分的耐腐蚀性并且弯曲成形性和弯曲部的表面品质优良的铁素体系不锈钢板的制造方法。本发明的制造方法,包括：将钢水通过一次精炼、以及利用利用真空氧脱碳处理法的二次精炼进行熔炼,接着,通过连续铸造法制成钢原材的工序；对所述钢原材实施热轧,制成热轧钢板的工序；和对所述热轧钢板实施冷轧和冷轧板退火,制成冷轧钢板的工序,在所述二次精炼中,将炉渣碱度(CaO/SiO-2)设定为1.5以上,并且,在恢复至大气压后,进行流量：0.1～0.6Nm～3/分钟、处理时间：5分钟以上的鼓泡处理。(The present invention provides a method for producing a ferritic stainless steel sheet having sufficient corrosion resistance and excellent bending formability and surface quality of a bent portion. The manufacturing method of the present invention includes: a step in which molten steel is subjected to primary refining and secondary refining by a vacuum oxygen decarburization treatment, and then is made into a steel material by a continuous casting method; a step of hot rolling the steel material to produce a hot-rolled steel sheet; and a step of cold-rolling and cold-sheet annealing the hot-rolled steel sheet to produce a cold-rolled steel sheet, wherein in the secondary refining, slag basicity (CaO/SiO) is adjusted 2 ) Set to 1.5 or more, and, after returning to atmospheric pressure, perform the flow rate: 0.1 to 0.6Nm 3 Per minute, treatment time: bubbling treatment for 5 minutes or more.)

1. A method for manufacturing a ferritic stainless steel sheet, comprising:

will have a composition containing, in mass%, C: 0.015 to 0.050%, Si: 0.05 to 0.40%, Mn: 0.45-1.00%, P: 0.040% or less, S: 0.008% or less, Cr: 15.5 to 18.0%, Al: 0.001-0.010% and N: a step in which 0.010 to 0.080% of molten steel, the balance of which is composed of Fe and unavoidable impurities, is subjected to primary refining and secondary refining by a vacuum oxygen decarburization method, and then the molten steel is produced into a steel material by a continuous casting method;

a step of hot rolling the steel material to produce a hot-rolled steel sheet; and

a step of cold rolling and cold sheet annealing the hot-rolled steel sheet to produce a cold-rolled steel sheet,

in the secondary refining, slag basicity (CaO/SiO)₂) Set to 1.5 or more, and, after returning to atmospheric pressure, perform the flow rate: 0.1 to 0.6Nm³Per minute, treatment time: bubbling treatment for 5 minutes or more.

2. The method for producing a ferritic stainless steel sheet according to claim 1, wherein the composition further contains, in mass%, Ca: 0.0002 to 0.0020%.

3. The method for producing a ferritic stainless steel sheet according to claim 1, wherein the slag basicity is 1.6 or more.

4. The method for producing a ferritic stainless steel sheet according to claim 2, wherein the slag basicity is 1.6 or more.

5. The method for producing a ferritic stainless steel sheet according to any one of claims 1 to 4, wherein the flow rate in the bubbling treatment is 0.3 to 0.5Nm³The treatment time is 10 minutes or more per minute.

6. The method for producing a ferritic stainless steel sheet according to any one of claims 1 to 4, wherein a bubbling gas in the bubbling treatment is a mixed gas of Ar and nitrogen.

7. The method for producing a ferritic stainless steel sheet according to claim 5, wherein a bubbling gas in the bubbling treatment is a mixed gas of Ar and nitrogen.

8. A method for manufacturing a ferritic stainless steel sheet, comprising:

will have a composition containing, in mass%, C: 0.015 to 0.050%, Si: 0.05 to 0.40%, Mn: 0.45-1.00%, P: 0.040% or less, S: 0.008% or less, Cr: 15.5 to 18.0%, Al: 0.001-0.010% and N: a step in which 0.010 to 0.080% of molten steel, the balance of which is composed of Fe and unavoidable impurities, is subjected to primary refining and secondary refining by a vacuum oxygen decarburization method, and then the molten steel is produced into a steel material by a continuous casting method;

a step of hot rolling the steel material to produce a hot-rolled steel sheet; and

a step of cold rolling and cold sheet annealing the hot-rolled steel sheet to produce a cold-rolled steel sheet,

in the secondary refining, slag basicity (CaO/SiO)₂) Set to 1.5 or more, and, after returning to atmospheric pressure, perform the flow rate: 0.1 to 0.6Nm³Per minute, treatment time: a bubbling treatment for 5 minutes or more,

MnS and MnO-SiO as inclusions in steel in the ferritic stainless steel sheet₂The total amount A satisfies the following formula (1), wherein the unit of the total amount A is volume ppm,

201≤A≤800-900×[％Si]…(1)

here, [% Si ] is the Si content in the composition of the above-described ferritic stainless steel sheet, and the unit is mass%.

9. The method for producing a ferritic stainless steel sheet according to claim 8, wherein the composition further contains, in mass%, Ca: 0.0002 to 0.0020%.

10. The method for producing a ferritic stainless steel sheet according to claim 8, wherein the slag basicity is 1.6 or more.

11. The method for producing a ferritic stainless steel sheet according to claim 9, wherein the slag basicity is 1.6 or more.

12. The method for producing a ferritic stainless steel sheet according to any one of claims 8 to 11, wherein the flow rate in the bubbling treatment is 0.3 to 0.5Nm³The treatment time is 10 minutes or more per minute.

13. The method for producing a ferritic stainless steel sheet according to any one of claims 8 to 11, wherein a bubbling gas in the bubbling treatment is a mixed gas of Ar and nitrogen.

14. The method for producing a ferritic stainless steel sheet according to claim 12, wherein a bubbling gas in the bubbling treatment is a mixed gas of Ar and nitrogen.

Technical Field

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

Background

Japanese industrial standards: SUS430(16 to 18 mass% Cr) specified in JIS G4305 is also inexpensive and excellent in corrosion resistance in ferritic stainless steel, and therefore is used in various applications such as building materials, transportation equipment, home electric appliances, kitchen equipment, and automobile parts.

Steel sheets used for these applications are 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 forming methods, i.e., bulging forming, deep drawing forming, stretch flange forming, and bending forming. Among them, bending is often used because of a relatively high degree of freedom of shape, and development of a ferritic stainless steel sheet having excellent bending formability is strongly required.

As a ferritic stainless steel, for example, patent document 1 discloses "a ferritic stainless steel excellent in formabilityA stainless steel sheet, characterized by containing, in 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, and N is contained so as to satisfy 0.06 ≦ (C + N). ltoreq.0.12 and 1 ≦ N/C in relation to the C content, and so as to satisfy 1.5X 10 in relation to the N content^-3≤(V×N)≤1.5×10^-2The embodiment (1) contains V, and the balance is Fe and inevitable impurities. ".

Patent document 2 discloses "a ferritic stainless steel excellent in surface properties and press formability, characterized by containing Cr: 15 to 20 wt% of ferritic stainless steel, V is contained in the range of 0.005 to 1.0 wt%, Al is 0.005 wt% or less, O is 0.001 to 0.007 wt%, and Al is contained in the oxide inclusion component in the steel₂O₃And Cr₂O₃Are respectively set to Al₂O₃: 5% by weight or less of Cr₂O₃: 10 to 50 wt%. ".

Further, patent document 3 discloses "a ferritic stainless steel sheet having excellent bending workability, which contains, in mass%, C: 0.01-0.03%, Mn: 0.5 to 1.0%, Cr: 15-20%, Al: 0.01% or less of a ferritic stainless steel sheet characterized in that Cr carbide is dispersed in ferrite, and the ratio of Fe to Cr metal elements in the Cr carbide is Fe/Cr: 0.05 to 0.15. ".

Patent document 4 discloses "a ferritic stainless steel excellent in bending workability and surface properties, characterized by containing, in mass%, C: 0.2% or less, Si: 0.15 to 0.32%, Mn: 0.4% or less, Cr: 10-25%, Al: 0.005% or less, and the above Mn and Si are such that Mn/Si: 1.25 or more, and the balance being Fe and unavoidable impurities. ".

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 3584881

Patent document 2: japanese patent No. 3608383

Patent document 3: japanese patent No. 5050565

Patent document 4: japanese laid-open patent publication No. 2002-80941

Disclosure of Invention

Problems to be solved by the invention

However, according to the techniques of patent documents 1 and 2, when a cold-rolled steel sheet is manufactured with a large thickness, particularly 1.0mm or more, and the steel sheet is bent as a raw material, specifically, when the steel sheet is bent into a box shape having a 90 ° bending portion and a hemming portion subjected to 180 ° close bending in a state where tensile strain is introduced by working, breakage may occur in the 90 ° bending portion of relatively mild bending.

Here, the hemming process refers to a process of folding back a key for bending the end surface portion of the steel plate at 180 °, or a process of folding back a key for bending the steel plate at 180 ° while sandwiching different steel plate members, and connecting the members.

Further, according to the technique of patent document 3, when a cold-rolled steel sheet is manufactured by increasing the thickness, particularly by setting the thickness to 1.0mm or more, and bending the steel sheet as a material in the same manner as described above, although no crack occurs in the 90 ° bent portion, a crack occurs in the hemming portion, and the steel sheet may not be formed into a predetermined shape.

In this regard, in the technique of patent document 4, even if the plate thickness is increased to about 2.0mm, the occurrence of cracking can be prevented in both the 90 ° bend processed portion and the hemming processed portion. However, in this case, surface irregularities called wrinkles may occur in the curved ridge line portion of the hemming portion, and the surface quality may be deteriorated. Therefore, in applications where surface aesthetics are required, when surface irregularities (specifically, more than 5.0 μm in terms of undulation height) due to significant wrinkling are generated, the surface irregularities need to be removed by polishing after processing, and a problem remains that the manufacturing process and manufacturing cost increase.

It is apparent that the techniques of patent documents 1 to 4 cannot be said that both the bending formability and the surface quality of the bent portion are compatible. Therefore, development of a ferritic stainless steel sheet that can achieve both bending formability and surface quality of a bent portion, particularly a ferritic stainless steel sheet that can achieve both bending formability and surface quality of a bent portion even when the sheet thickness is increased, is currently desired.

The present invention has been made in view of the above-described situation, and an object thereof is to provide a ferritic stainless steel sheet having sufficient corrosion resistance and excellent bending formability and surface quality of a bent portion.

Here, "sufficient corrosion resistance" means that the rust area ratio ((rust area on the steel sheet surface/total area on the steel sheet surface) × 100 (%)) of the steel sheet surface is 25% or less when 8 cycles are performed with one cycle of brine spray (35 ℃, 5 mass% NaCl, spray time: 2 hours) → dry (60 ℃, relative humidity 40%, hold time: 4 hours) → wet (50 ℃, relative humidity ≥ 95%, hold time: 2 hours) in the brine spray cycle test prescribed in JIS H8502.

The phrase "excellent bending formability" means that cracking does not occur when a steel sheet is subjected to 180 ° tight bending (hereinafter, also referred to as stretch bending) after a tensile strain of 20% is applied to the steel sheet in the direction perpendicular to the rolling direction as the longitudinal direction.

The phrase "the surface quality of the bent portion is excellent" means that the undulation height measured in the direction parallel to the ridge line and in the direction perpendicular to the ridge line of the bent ridge line portion after the stretch bending is 5.0 μm or less as measured in accordance with JIS B0601 (2001).

Means for solving the problems

The present inventors have made various studies to solve the above problems.

First, the present inventors conducted bending forming using various ferritic stainless steel sheets as a material, and examined in detail the cause of the occurrence of wrinkles generated in the bent edge line portion of the above-described hemmed portion.

As a result, it was found that the wrinkles generated at the curved ridge line portion of the above-described hemmed portion were mainly generated due to the fact that crystal grains of a ferrite phase (crystal grain group having a similar crystal orientation) generated at the time of casting and/or hot rolling remained after the cold rolling annealing.

Therefore, the present inventors have considered that if the above-described crystal grains can be effectively broken, generation of wrinkles may be effectively prevented, and thus have studied various composition compositions.

As a result, it is thought that it is effective to reduce the Si content and increase the Mn content from the viewpoint of preventing generation of wrinkles.

That is, since Si is a ferrite-forming element, the amount of austenite phase formed during hot rolling decreases as the Si content increases. Si also has an effect of promoting recovery of a ferrite phase during hot rolling. Therefore, if Si is excessive, the phenomenon of recovery cancellation of the rolling strain induced by hot rolling is promoted, and the strain serving as a recrystallization site is reduced in the next step of annealing the hot-rolled sheet. As a result, it is considered that the effect of breaking the crystal grains by recrystallization of the ferrite phase at the time of annealing of the hot-rolled sheet is reduced.

On the other hand, Mn is an austenite forming element, and increases the amount of austenite phase formed during hot rolling. The austenite phase is mainly generated from the grain boundaries of the ferrite phase, and therefore, it is considered that the destruction of the crystal grains at the time of hot rolling is promoted as the generation amount of the austenite phase is increased.

Therefore, the present inventors thought that generation of wrinkles could be prevented effectively, perhaps, by decreasing the Si content and increasing the Mn content.

However, the stainless steel sheet adjusted to the above-described composition was subjected to 180 ° close bending in a state where tensile strain accompanying working was introduced, and as a result, cracking occurred in the bent portion.

The present inventors examined the cause of the cracking in detail and found that the ferrite phase and MnS, MnO-SiO phase are formed by bending₂The interface of such inclusions generates voids, which serve as starting points and are broken.

Note that MnO-SiO₂Is MnO and SiO₂A mixture of (a).

Therefore, the present inventors have studied in detail the relationship between the above-described inclusions, which become the starting points of fracture, and the composition of the components.

As a result, the following findings were obtained: occurrence of cracking and Si content of composition and MnS and MnO-SiO in steel₂Has a strong correlation with the sum of MnS and MnO-SiO₂The total amount of (a) is reduced to a predetermined amount or less in relation to the Si content of the component composition, so that the occurrence of cracks can be effectively prevented, and both bending formability and surface quality of a bent portion can be achieved.

In addition, it has been found that: from reduction of the above-mentioned inclusions, in particular MnO-SiO₂From the viewpoint of the amount, the secondary refining and the slag basicity (mass% of CaO in the slag/SiO in the slag) at this time are performed by a Vacuum Oxygen Decarburization (VOD) method₂Hereinafter also referred to as CaO/SiO₂) The adjustment of (3) and the bubbling treatment under predetermined conditions in the secondary refining are extremely important, and by performing such a treatment and reducing the S content, MnS and MnO-SiO can be greatly reduced even with a component composition having an Mn content of 0.45% or more₂The amount of such inclusions formed.

The present invention has been completed based on the above findings.

That is, the gist of the present invention is as follows.

1. A ferritic stainless steel sheet having a composition containing, in mass%, C: 0.015 to 0.050%, Si: 0.05 to 0.40%, Mn: 0.45-1.00%, P: 0.040% or less, S: 0.008% or less, Cr: 15.5 to 18.0%, Al: 0.001-0.010% and N: 0.010-0.080%, and the balance of Fe and inevitable impurities, and

MnS and MnO-SiO as inclusions in steel₂The total amount A (volume ppm) of (A) satisfies the relationship of the following formula (1).

A≤800-900×[％Si]…(1)

Here, [% Si ] is the Si content [ mass% ] of the above-described composition.

2. The ferritic stainless steel sheet according to claim 1, wherein the composition further contains, in mass%, Ca: 0.0002 to 0.0020%.

Effects of the invention

According to the present invention, a ferritic stainless steel sheet having sufficient corrosion resistance and excellent bending formability and surface quality of a bent portion can be obtained.

In addition, the ferritic stainless steel sheet of the present invention is particularly advantageously applied to a cooking device in which edge curling is performed on an end surface portion of the steel sheet or a service kitchen using clinch joining because it can obtain excellent bending formability and surface quality of a bent portion even when the steel sheet is thickened.

Detailed Description

The present invention will be specifically described below.

First, the composition of the ferritic stainless steel sheet of the present invention will be described. The units in the component compositions are all expressed as "% by mass", and hereinafter, unless otherwise specified, they are merely expressed as "%".

C：0.015～0.050％

C is an element effective for promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of wrinkles. From the viewpoint of obtaining such effects, the C content is set to 0.015% or more. However, if the C content exceeds 0.050%, the steel is excessively hardened and ductility is reduced. Further, voids are generated from the interface between the carbide and the ferrite phase, and there is a possibility that cracking may be induced during bending. Therefore, the C content is set to be in the range of 0.015 to 0.050%. The lower limit of the C content is preferably 0.025%. In addition, the upper limit of the C content is preferably 0.045%.

Si：0.05～0.40％

Si is an element that acts as a deoxidizer during steel melting. From the viewpoint of obtaining such effects, the Si content is set to 0.05% or more. However, if the Si content exceeds 0.40%, the steel is excessively hardened, and cracking occurs from the ferrite phase at the time of bending. Therefore, desired bending formability is not obtained. Since Si is a ferrite-forming element, the amount of austenite phase formed during hot rolling decreases as the Si content increases. In addition, Si has an effect of promoting recovery of a ferrite phase during hot rolling. Therefore, if Si is excessive, the phenomenon of recovery cancellation of the rolling strain induced by hot rolling is promoted, and thus the strain that becomes a recrystallization site at the time of hot-rolled sheet annealing in the next step is reduced. As a result, the crystal grain destruction effect by the recrystallization of the ferrite phase at the time of annealing of the hot-rolled sheet is reduced, and wrinkles are generated in the bent formed portion, thereby deteriorating the surface quality.

Therefore, the Si content is set to be 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：0.45～1.00％

Like C, Mn is an element effective for promoting the formation of an austenite phase during hot rolling and suppressing the formation of wrinkles. From the viewpoint of obtaining such effects, the Mn content is set to 0.45% or more. When the Mn content is less than 0.45%, the amount of austenite phase formed during hot rolling decreases. Therefore, the effect of breaking the crystal grains is insufficient, and wrinkles are generated in the bent portion, which deteriorates the surface quality. On the other hand, if the Mn content exceeds 1.00%, the steel is hardened and the ductility is reduced. Further, the corrosion resistance may also decrease.

Therefore, the Mn content is set to be 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%, and still more preferably 0.70%.

P: less than 0.040%

P is an element that promotes grain boundary fracture caused by grain boundary segregation. Therefore, the lower the P content, the more preferable the upper limit is set to 0.040%. Preferably 0.035% or less. More preferably 0.030% or less.

S: less than 0.008%

S combines with Mn to form MnS. MnS induces cracking to degrade bending formability. In addition, MnS may also reduce corrosion resistance. Therefore, the S content is set to 0.008% or less. Preferably 0.006% or less.

Cr：15.5～18.0％

Cr is an element having an effect of improving corrosion resistance by forming a passive film on the surface of the steel sheet. From the viewpoint of obtaining such effects, the Cr content is set to 15.5% or more. However, if the Cr content exceeds 18.0%, the amount of austenite phase formed during hot rolling is reduced, and wrinkling is likely to occur.

Therefore, the Cr content is set to be 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：0.001～0.010％

Like Si, Al is an element that functions as a deoxidizer. From the viewpoint of obtaining such effects, the Al content is set to 0.001% or more. However, if the Al content exceeds 0.010%, Al as an inclusion is contained₂O₃The amount of (2) increases, and the surface quality tends to be lowered.

Therefore, the Al content is set to be 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：0.010～0.080％

N is an element effective for promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of wrinkles, similarly to C and Mn. From the viewpoint of obtaining such effects, the N content is set to 0.010% or more. However, when the N content exceeds 0.080%, ductility is greatly reduced. In addition, precipitation of Cr nitride may be promoted, which may cause a decrease in corrosion resistance due to sensitization.

Therefore, the N content is set to be in the range of 0.010 to 0.080%. The lower limit of the N content is preferably 0.030%, more preferably 0.040%. The upper limit of the N content is preferably 0.070%, more preferably 0.060%, and still more preferably 0.050%.

While the basic components have been described above, the basic components may further contain Ca: 0.0002 to 0.0020%.

Ca：0.0002～0.0020％

Ca is an effective component for preventing clogging of the nozzle due to crystallization of inclusions which are likely to occur during continuous casting. In addition, when Ca is contained, CaS maintaining a spherical shape after rolling is produced in a large amount instead of MnS greatly elongated by rolling, and therefore, this contributes to improvement of bending formability. From the viewpoint of obtaining such effects, the Ca content is preferably set to 0.0002% or more. However, with an increase in the Ca content, CaO, which is an inclusion in steel, also increases. If CaO is excessively increased, cracking may occur from CaO during bending. Therefore, the content of Ca is preferably set to 0.0020% or less.

Therefore, when Ca is contained, the content is set to be in the range of 0.0002 to 0.0020%. The lower limit of the Ca content is more preferably 0.0005%. The upper limit of the Ca content is more preferably 0.0015%, and still more preferably 0.0010%.

The other components are Fe and inevitable impurities.

Next, inclusions in steel will be described.

MnS and MnO-SiO₂Total amount of (a) (volume ppm): 800 × [% Si-]Hereinafter ([% Si)]Si content (mass%) of the composition of steel)

As described above, when the steel sheet is bent, the ferrite phase is MnS and MnO-SiO₂The interface of such inclusions generates voids. When the total amount of these inclusions is increased, these voids are connected and easily broken. In particular, in a steel sheet containing a large amount of Si, the steel sheet is hardened due to the reduction of the plastic deformability of the steel, and the stress required for bending deformation increases, so that MnS and MnO-SiO phases are present in the ferrite phase₂Such a large stress concentration occurs in the voids at the interface of the inclusions, and the occurrence of cracks is promoted, resulting in a decrease in bending formability.

Therefore, in order to obtain desired bending formability, MnS and MnO-SiO are used₂Since it is extremely important to adjust the total amount A (volume ppm) of (A) in relation to the Si content of the component composition, it is necessary to adjust MnS and MnO-SiO₂The total amount A (volume ppm) of (A) is set to 800-]The following (i.e., satisfying A. ltoreq. 800-]… (1). MnS and MnO-SiO₂The total amount A of (A), (B)Volume ppm) preferably 700-]More preferably 650 × [% Si]The following.

The content (% by mass) of Si in the composition is [% Si ].

From the viewpoint of improving the bending formability, MnO-SiO is preferably added to satisfy the above formula (1)₂The amount of (B) is set to 400 ppm by volume or less.

The amount of MnS is not particularly limited as long as it satisfies the above formula (1), and is preferably set to 400 ppm by volume or less.

Here, the MnS amount (volume ppm) in the form of inclusions in the steel was determined as follows.

That is, the produced stainless steel sheet was subjected to electrolysis using a 10 mass% acetylacetone-1 mass% tetramethylammonium chloride-methanol solution (AA solution). Subsequently, the extraction residue was collected by using a filter having a mesh size of 0.2 μm, and the amount of S was quantitatively analyzed by ICP (high frequency inductively coupled plasma) emission spectrometry using the collected extraction residue. The mass percentage of S obtained here was converted into a molar percentage using the atomic weight of S (═ 32), and the molar percentage of MnS was determined assuming that MnS (atomic weight 87) in an amount corresponding to the molar percentage of S was produced. Then, the molar percentage of MnS was converted to a mass percentage, and the density of MnS (5.226 g/cm) was used from the mass percentage of MnS³) And density of SUS430 (7.7 g/cm)³) Converted to volume percentage of MnS (volume ppm).

In addition, MnO-SiO present in the form of inclusions in the steel₂The amount was determined as follows.

That is, the produced stainless steel sheet was subjected to a dissolution treatment by the bromine-methanol method. Subsequently, the extraction residue was collected using a filter having a mesh size of 0.2 μm, and the amount of Mn and the amount of Si were quantitatively analyzed by ICP emission spectroscopy using the collected extraction residue. The mass percentages of Mn and Si obtained here were converted to molar percentages using the atomic weights of Mn and Si (Mn: 55, Si: 28), respectively, assuming that MnO (atomic weight: 71) was produced in an amount corresponding to the mass percentage of Mn and SiO was produced in an amount corresponding to the mass percentage of Si₂(atomic weight: 60)Separately obtain MnO and SiO₂Mole percent of (c). Then, MnO and SiO₂The molar percentages of (a) and (b) are each converted to mass percentages. Further, it is made of SiO₂The mass percentage of (a) is represented by the following formula: 4.4-0.019 x [ SiO ]₂Mass percent of](mass%) calculation of MnO-SiO₂Density (g/cm)³) Using the calculated MnO-SiO₂Density of SUS430 (7.7 g/cm)³) MnO-SiO₂(ii) mass percentage of [ MnO [% by mass)]+[SiO₂Mass percent of]) Converted to MnO-SiO₂Volume percentage (volume ppm).

Then, the volume fraction of MnS calculated in the above manner is compared with MnO-SiO₂Are added together, thereby obtaining MnS and MnO-SiO₂The total amount A of (a).

Note that the volume ratio is used instead of the mass ratio for MnS and MnO-SiO₂The total amount of (b) is defined for the following reasons.

That is, most of the cracks are due to the ferrite phase as the matrix phase and MnS, MnO-SiO₂The void generated at the interface of (a) and (b) is connected. MnS, MnO-SiO₂The closer the distance therebetween, the more the connection of the voids is promoted. Here, the volume ratio is compared with the mass ratio of MnS, MnO-SiO₂The correlation of the distance between them is higher. Thus, MnS and MnO-SiO₂The total amount of (a) is specified by a volume ratio rather than a mass ratio.

In the above formula (1), the Si content of the component composition is used in mass%, and MnS and MnO-SiO are used₂The range of the total amount A (volume ppm) of (B) is defined for the following reasons.

That is, as described above, since the plastic deformability of the steel, which is the ferrite phase of the matrix phase, changes depending on the Si content (mass%), it is considered that the effect of the plastic deformability of the steel on the bending formability is suitably expressed by using the Si content (mass%), and that MnS and MnO — SiO are actually affected by using the Si content (mass%)₂The total amount A (volume ppm) of (A) is adjusted to obtain the desired bending formability.

The structure of the ferritic stainless steel sheet of the present invention is a structure mainly composed of a ferrite phase, specifically, a structure having a ferrite phase at a volume ratio of 90% or more with respect to the entire structure, and a remaining structure other than the ferrite phase at a volume ratio of 10% or less with respect to the entire structure. Further, a ferrite single phase may be used. The balance structure is mainly a martensite phase, and the volume ratio of precipitates and inclusions is not included.

Here, the volume fraction of the ferrite phase is determined by the following method: a test piece for cross-section observation was prepared from a stainless steel plate, subjected to etching treatment with picric acid saturated hydrochloric acid solution, observed with an optical microscope at a magnification of 100 times in 10 visual fields, and then the martensite phase and the ferrite phase were distinguished from each other by the structure shape and the etching strength, and the volume fraction of the ferrite phase was obtained by image processing to calculate the average value thereof.

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

Next, a preferred method for producing the ferritic stainless steel sheet of the present invention will be described.

First, molten steel having the above-described composition is melted by primary refining by a known method such as a converter, an electric furnace, or a vacuum melting furnace, and secondary refining by a vacuum oxygen decarburization method (hereinafter, also referred to as a VOD method), and then, a steel material (billet) is produced by a continuous casting method.

Here, the inclusion in the steel, in particular MnO-SiO, is reduced₂From the viewpoint of satisfying the above expression (1), the secondary refining is performed by the VOD method and the slag basicity (CaO/SiO)₂) It is important to adjust and maintain the pressure to 1.5 or more and perform the bubbling treatment under predetermined conditions in the secondary refining. Need toThe bubbling treatment is a treatment of: inert gas such as Ar (in some cases, nitrogen gas) is blown from the bottom surface of the refining vessel to stir the molten steel, and the slag floats on the molten steel bath surface by being adsorbed on the foam of the inert gas generated by the blowing of the inert gas.

That is, in the VOD method, the treatment is performed under vacuum, so that the amount of oxygen in the molten steel is effectively reduced as compared with the argon-oxygen decarburization method (hereinafter, also referred to as AOD method), and MnO-SiO at the time of casting can be reduced₂The amount of production of (c).

In addition, in particular, by reducing the slag basicity (CaO/SiO)₂) Adjustment and maintenance to 1.5 or more, preferably 1.6 or more, promotes the deoxidation reaction by Si, and as a result, the amount of oxygen in the molten steel can be reduced, and MnO-SiO during casting can be reduced₂The amount of production of (c).

However, in the VOD method, decarburization and component adjustment are performed under vacuum and then the pressure is returned to atmospheric pressure, and since molten steel is vigorously boiled under vacuum, slag is suspended in the molten steel after the pressure is returned to atmospheric pressure. Therefore, when casting is performed in this state, a large amount of slag and inclusions generated by secondary refining remain in molten steel, and the amount of inclusions in the final product increases, so that desired bending formability cannot be obtained.

In order to avoid the above phenomenon, after returning to atmospheric pressure, predetermined conditions, specifically, flow rates: 0.1 to 0.6Nm³In terms of/minute (` Nm `)³"refers to the volume of a gas in the standard state (25 ℃ C., 1 atm). In addition, it is preferably 0.3 to 0.5Nm³Per minute), treatment time: bubbling treatment for 5 minutes or longer (preferably 10 minutes or longer).

That is, by performing the bubbling treatment, the inclusions suspended in the molten steel and generated during the secondary refining are floated on the upper surface of the molten steel by effectively utilizing the adsorption effect to the bubbles and the difference in specific gravity with the molten steel, and can be effectively separated and removed as slag in the tundish of the continuous casting which is the next step.

Thus, inclusions, in particular MnO-SiO, present in the final product sheet₂The occurrence of cracks during bending can be prevented.

In order to prevent the change in the molten steel composition during the bubbling process, an inert gas such as Ar is preferably used as the bubbling gas, and a mixed gas of Ar and nitrogen is more preferably used to prevent the denitrification during the bubbling process.

Then, the obtained steel material is heated at 1100 to 1250 ℃ for 1 to 24 hours or a high-temperature billet is directly heated, and then the steel material is hot-rolled to produce a hot-rolled steel sheet. Then, the hot-rolled steel sheet obtained is subjected to hot-rolled sheet annealing and pickling at a temperature of 800 to 900 ℃ as required. Subsequently, the obtained hot-rolled steel sheet is subjected to cold rolling and cold sheet annealing to obtain a cold-rolled steel sheet, which is further subjected to acid pickling as necessary to obtain a final product sheet.

Here, the reduction ratio of the cold rolling is preferably set to 50% or more from the viewpoint of elongation, bendability, press formability, and shape correction.

In addition, in the case of the No.2B finish as the surface finish specified in JIS G0203, the cold-rolled sheet annealing is preferably performed at a temperature of 800 to 950 ℃ from the viewpoint of obtaining good mechanical properties and pickling properties, and is more preferably set at a temperature of not more than the austenite transformation point from the viewpoint of workability. Further, bright annealing (BA annealing) may be performed to further improve the gloss.

Further, the cold rolling and the annealing of the cold-rolled sheet may be repeated two or more times, respectively.

Production conditions other than those described above may be used according to a conventional method. In addition, from the viewpoint of further improving the surface properties, grinding, polishing, and the like may be further performed.

[ examples ]

Example 1

Molten steel (150 tons) having the composition shown in table 1 (balance Fe and inevitable impurities) was subjected to primary refining in a converter and secondary refining by a VOD method under the conditions shown in table 2, respectively, to perform melting. In addition, in the secondary refining, the slag basicity is completed andafter the adjustment of the components, the pressure was returned to atmospheric pressure, and then, Ar and nitrogen were used in a volume ratio of 3: 1 mixed gas (flow rate: 0.4 Nm)³Per minute), the bubbling treatment was performed under the conditions shown in table 2.

Next, the molten steel was cast into a slab having a width of 1000mm and a thickness of 200mm by a continuous casting method, and the obtained slab was heated at 1150 ℃ for 1 hour. Then, hot rolling consisting of rough rolling and finish rolling was performed, and the resultant was coiled at 700 ℃ to obtain a hot-rolled steel sheet having a thickness of 5.0 mm.

The obtained hot rolled steel sheet was subjected to an annealing temperature by a box annealing method: 830 ℃, retention time: the hot-rolled sheet was annealed for 8 hours, and then shot blasting and descaling by acid pickling were performed on the surface. The pickling was carried out by immersing the plate in a 250g/l sulfuric acid aqueous solution at 80 ℃ for 40 seconds and then in a mixed aqueous solution of 20g/l hydrofluoric acid and 120g/l nitric acid at 60 ℃ for 10 seconds.

Next, the hot-rolled steel sheet was subjected to cold rolling and cold sheet annealing (annealing temperature: 830 ℃) using a continuous annealing furnace to obtain a cold-rolled steel sheet having a thickness of 2.0mm, and this cold-rolled steel sheet was subjected to descaling treatment by pickling to obtain a ferritic stainless steel sheet as a final product sheet. The acid wash was carried out at a current density of 40C/dm in 180g/l aqueous sodium sulfate solution set at 80 deg.C²Electrolysis was carried out for 60 seconds.

The thus obtained ferritic stainless steel sheet was subjected to the above-described method to determine the MnS content and MnO-SiO content as inclusions in the steel₂Amounts, and their total amounts. The results are also shown in Table 2.

As a result of identifying the steel sheet structure by the above-described method, the volume fraction of the ferrite phase with respect to the entire structure was 90% or more for any of the steel sheets.

Further, (1) the bending formability was evaluated, (2) the surface quality of the bent portion was evaluated, and (3) the corrosion resistance was evaluated by the following methods. The results of these evaluations are also shown in table 2.

(1) Evaluation of bending formability

A test piece of 30 mm. times.200 mm was cut out from a ferritic stainless steel sheet to be a final product sheet, with the rolling direction being the longitudinal direction. After applying a tensile strain of 20% to the cut test piece, the piece was subjected to tight bending at 180 °. Then, the test piece was visually checked for the presence of cracking at the bent edge portion, and the test piece was evaluated as acceptable when the test piece was bent well without cracking at the edge portion (o), and as unacceptable when cracking occurred at the edge portion (x).

(2) Evaluation of surface quality of bent portion

The undulation height of the bent ridge portion in the test piece after 180 ° intimate bending in (1) above was measured in the direction parallel to the ridge line and in the direction perpendicular to the ridge line in accordance with JIS B0601 (2001).

Then, the case where the undulation height of the curved ridge portion was 5.0 μm or less in both the direction parallel to the ridge line and the direction perpendicular to the ridge line was evaluated as pass (o), and the case where the undulation height of the curved ridge portion was more than 5.0 μm in either the direction parallel to the ridge line or the direction perpendicular to the ridge line was evaluated as fail (x).

In the case where cracking was observed in (1), the evaluation column of the surface quality of the bent portion in table 2 is represented as "-".

(3) Evaluation of Corrosion resistance

A60X 100mm test piece was cut out from a ferritic stainless steel plate to be a final product plate with the rolling direction as the longitudinal direction, and the surface was polished with a #600 sandpaper. Then, the end faces of the test pieces were sealed and subjected to a salt water spray cycle test as defined in JIS H8502.

In the brine spray cycle test, 8 cycles were carried out with brine spray (35 ℃, 5 mass% NaCl, spray time: 2 hours) → dry (60 ℃, relative humidity 40%, hold time: 4 hours) → wet (50 ℃, relative humidity ≥ 95%, hold time: 2 hours) as one cycle.

The surface of the test piece after the brine spray cycle test was photographed, the rust area on the surface of the test piece was measured by image analysis, and the rust area ratio ((rust area on the surface of the test piece/total area on the surface of the test piece) × 100 (%) was calculated from the ratio to the total area on the surface of the test piece.

Then, the calculated rust area rate was evaluated as "pass" when it was 25% or less, and as "fail" when it was more than 25%.

The invention examples all obtained excellent bending formability and surface quality of the bent portion, and also had excellent corrosion resistance.

On the other hand, in comparative examples Nos. 3, 7 and 11, since the bubbling time was insufficient, MnO-SiO generated at the time of secondary refining was insufficient₂MnS and MnO-SiO in the final product sheet remain in the molten steel without being sufficiently separated and removed in the form of slag₂If the total amount of (A) exceeds the appropriate range, the desired bending formability is not obtained.

In comparative examples Nos. 4, 8 and 12, the basicity of slag (CaO/SiO) in secondary refining was determined₂) Therefore, deoxidation at the time of secondary refining becomes insufficient, the amount of oxygen in molten steel increases, and a large amount of MnO-SiO is generated at the time of casting₂MnS and MnO-SiO in the final product sheet₂If the total amount of (A) exceeds the appropriate range, the desired bending formability is not obtained.

In addition, in Nos. 5, 9 and 13 as comparative examples, the bubbling time was insufficient, and the slag basicity (CaO/SiO) in secondary refining was insufficient₂) Also low, and therefore, MnS and MnO-SiO in the final product sheet₂Still exceeds the appropriate range, and as a result, the desired bending formability is not obtained.

Example 2

Molten steel (150) having a composition shown in Table 3 (the balance being Fe and unavoidable impurities)Ton) by primary refining using a converter and reducing the slag basicity (CaO/SiO)₂) The secondary refining by the VOD method was adjusted to 1.7 and maintained for melting. In the secondary refining, after the adjustment of the slag basicity and the components is completed, the pressure is returned to the atmospheric pressure, and then, Ar and nitrogen are used in a volume ratio of 3: 1 mixed gas (flow rate: 0.4 Nm)³Per minute), a bubbling treatment was performed (treatment time: 10 minutes).

Next, the molten steel was cast into a slab having a width of 1000mm and a thickness of 200mm by a continuous casting method, and the obtained slab was heated at 1150 ℃ for 1 hour. Then, hot rolling consisting of rough rolling and finish rolling was performed, and the resultant was coiled at 700 ℃ to obtain a hot-rolled steel sheet having a thickness of 5.0 mm. (however, in No.37 of Table 4, a hot-rolled steel sheet having a thickness of 6.0mm was produced, and then a cold-rolled steel sheet having a thickness of 4.0mm was produced by the method described later.)

The obtained hot rolled steel sheet was subjected to an annealing temperature by a box annealing method: 830 ℃, retention time: after 8 hours of hot-rolled sheet annealing, shot blasting and descaling by acid pickling were performed on the surface. The pickling was carried out by immersing the plate in a 250g/l sulfuric acid aqueous solution at 80 ℃ for 40 seconds and then in a mixed aqueous solution of 20g/l hydrofluoric acid and 120g/l nitric acid at 60 ℃ for 10 seconds.

Subsequently, the hot-rolled steel sheet was subjected to cold rolling and cold sheet annealing (annealing temperature: 830 ℃) using a continuous annealing furnace to produce cold-rolled steel sheets having thicknesses in Table 4, and the cold-rolled steel sheets were subjected to descaling treatment by pickling to obtain ferritic stainless steel sheets as final product sheets. The acid wash was carried out at a current density of 40C/dm in 180g/l aqueous sodium sulfate solution set at 80 deg.C²Electrolysis was carried out for 60 seconds.

The thus obtained ferritic stainless steel sheet was subjected to the above-described method to determine the MnS content and MnO-SiO content as inclusions in the steel₂Amounts, and their total amounts. The results are also shown in Table 4.

As a result of identifying the steel sheet structure by the above-described method, the volume fraction of the ferrite phase with respect to the entire structure was 90% or more for any of the steel sheets.

Further, (1) the bending formability was evaluated, (2) the surface quality of the bent portion was evaluated, and (3) the corrosion resistance was evaluated by the same method as in example 1. The results of these evaluations are shown in table 4.

The invention examples all obtained excellent bending formability and surface quality of the bent portion, and also had excellent corrosion resistance.

On the other hand, in comparative example No.38, since the Si content exceeded an appropriate amount, the plastic deformability of the ferrite phase was greatly reduced, and the desired bending formability was not obtained.

In addition, in comparative example No.39, the Mn content exceeded the appropriate amount, and therefore the desired corrosion resistance was not obtained.

In addition, in No.40 as a comparative example, the S content exceeded the appropriate amount, and therefore, MnS was generated in a large amount, and MnS and MnO-SiO in the final product sheet₂If the total amount of (A) exceeds the appropriate range, the desired bending formability is not obtained. In addition, since MnS is produced in a large amount, desired corrosion resistance is not obtained.

In addition, in No.41 as a comparative example, since the Mn content was less than an appropriate amount, wrinkles occurred in the curved ridge line portion, and good surface quality of the curved portion was not obtained.

Industrial applicability

The ferritic stainless steel sheet of the present invention is particularly advantageously used in applications where high bending formability and surface quality of a bent portion, and further corrosion resistance are required, for example, in cooking utensils where edge curling is performed on end faces of the steel sheet, or in service kitchens where caulking is used.