JP6881666B2 - Manufacturing method of ferritic stainless steel sheet - Google Patents

Description

The present invention relates to a ferritic stainless steel sheet having sufficient corrosion resistance and excellent moldability, particularly overhang moldability, and a method for producing the same.

Since SUS430 (16-18 mass% Cr) -based ferritic stainless steel sheets are economical and have excellent corrosion resistance, they are applied to various applications such as building materials, transportation equipment, home appliances, kitchen equipment and automobile parts. , Its scope of application has expanded further in recent years.

Steel sheets applied to these applications are required to have not only corrosion resistance but also sufficient formability that can be processed into a predetermined shape by press molding or the like.
As such a ferritic stainless steel sheet, for example, Patent Document 1
"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: It contains 11 to 30% or less, Ni: 0.7% or less, and contains N so as to satisfy 0.06 ≦ (C + N) ≦ 0.12 and 1 ≦ N / C in relation to the C content, and further contains V. contained so as to satisfy ^{1.5 × 10 -3 ≦ (V ×} N) ≦ 1.5 × 10 -2 in relation to the N content, excellent in formability, characterized in that the balance Fe and unavoidable impurities Ferritic stainless steel sheet. "
Is disclosed.

Further, in Patent Document 2,
"In mass%, C: 0.010 to 0.045%, N: 0.01 to 0.05%, Mn: 1% or less, Cr: 13 to 20%, Al: 0.01% or less, and C and N are Cr carbonitrides. It contains so that the volume ratio v is 0.09% or less, Si: 0.4% or less, P: 0.05% or less, S: 0.010% or less, has a composition consisting of the balance Fe and unavoidable impurities, and further ferrite. A ferritic stainless cold-rolled steel sheet having excellent press formability, characterized in that the average crystal grain size of the grains is 10 μm or more and that Cr carbonitride has a ferrite single structure in which 50 or less are dispersed per ferrite grain. "
Is disclosed.

Japanese Patent No. 3584881 Japanese Patent No. 4682806 Japanese Patent No. 5884211

By the way, press forming is roughly classified into four types of forming modes such as overhang forming, deep drawing forming, stretch flange forming and bending forming.
In recent years, members whose molding mode in press molding is mainly overhang molding, for example, exterior members such as exhaust ducts and outdoor hoods of round louvers used for ventilation openings, and design or functionality by embossing. Ferritic stainless steel is being applied to interior panel members, etc. Therefore, it is desired to develop a ferritic stainless steel sheet having excellent overhang formability that can be processed into such a member shape.

However, it cannot be said that the ferritic stainless steel sheets disclosed in Patent Documents 1 and 2 have sufficient overhang formability.

Therefore, the inventors first described in Patent Document 3.
"In mass%, C: 0.005 to 0.025%, Si: 0.02 to 0.50%, Mn: 0.55 to 1.00%, P: 0.04% or less, S: 0. Contains 01% or less, Al: 0.001 to 0.10%, Cr: 15.5 to 18.0%, Ni: 0.1 to 1.0%, N: 0.005 to 0.025%. The balance consists of Fe and unavoidable impurities, the elongation at break is 28% or more, the average r value is 0.75 or more, and the minimum value of the maximum logarithmic strain of the molding limit based on FLD (ferritic stainless steel) is 0. Ferritic stainless steel plate of 15 or more. "
Was developed.
As a result, a ferritic stainless steel sheet having significantly improved overhang formability can be obtained as compared with the ferritic stainless steel sheets disclosed in Patent Documents 1 and 2.

However, when an attempt is made to form a ferritic stainless steel sheet of Patent Document 3 into a member such as an exhaust duct that requires particularly high overhang formability, cracks may still occur, and therefore, overhang formability is further improved. Is currently required.

The present invention has been developed in view of the above situation, and an object of the present invention is to provide a ferritic stainless steel sheet having sufficient corrosion resistance and excellent overhang formability, together with an advantageous manufacturing method thereof.

Here, "sufficient corrosion resistance" means that the salt spray cycle test specified in JIS H8502 is performed by salt spray (35 ° C, 5% by mass NaCl, spray time: 2 hours) → drying (60 ° C, relative humidity 40). %, Retention time: 4 hours) → Wetness (50 ° C, relative humidity ≥ 95%, Retention time: 2 hours) as one cycle for 8 cycles, the rust area ratio on the steel sheet surface (rust on the steel sheet surface) Area / total area of steel sheet surface) × 100 (%)) means that it is 25% or less.
Further, "excellent overhang formability" means a molding limit determined based on a molding limit diagram (Forming Limit Di ■ gram, hereinafter also referred to as FLD) measured in accordance with ISO12004-2: 2008. It means that the minimum value of the maximum logarithmic strain is 0.20 or more.

Now, the inventors have repeated various studies in order to solve the above-mentioned problems.
First, the inventors prepare various ferritic stainless steel sheets having different composition and manufacturing method, and use these steel sheets to form a member including a portion having equal biaxial overhang and unequal biaxial overhang. A press working test was conducted.
Generally, it is considered that the one with high elongation is superior in overhang formability, but in this press working test, even a steel sheet with high breaking elongation may crack, and from this test result, overhang molding is performed. It was found that the superiority or inferiority of sex is not necessarily determined only by the magnitude of elongation at break.

Therefore, the inventors separately prepared a steel plate in which the crack occurred in the previous test, and again performed a press working test using the steel plate under the same conditions, and used the upper mold in which the crack occurred in the previous test. The press working was stopped immediately before the pushing end position (bottom dead center + 2 mm), a test piece was collected from the steel sheet, and the metal structure was observed in detail. Specifically, after the above-mentioned press working is stopped, the steel sheet is extracted from the mold, and then the cross section of the steel sheet is mirror-polished and then corroded with a saturated picric acid-5% by mass hydrochloric acid aqueous solution to carry out a metallographic structure. An observation test piece was prepared, and the test piece was observed with a scanning electron microscope (secondary electron image) at a magnification of 500 times.
As a result, in all the steel sheets that were cracked in the previous test, a large amount of voids were already formed at the interface between the Cr-based carbonitride and the ferrite matrix in the middle of the press working, and some voids were generated. , It was confirmed that it was connected to nearby voids and grew into microcracks.

On the other hand, in the metal structure of the steel sheet that was press-processed without cracking in the previous test, voids were formed at the interface between the Cr-based carbonitride and the ferrite matrix, but the voids were connected to each other. No microcracks were observed.

From the above, the inventors consider that the superiority or inferiority of the overhang formability is greatly influenced by the metal structure of the steel sheet, and both the steel sheet with cracks and the steel sheet that can be formed without cracks in the previous press working test. The metallographic structure before press working was investigated, and a detailed comparison was made between the two.
As a result, both of the metal structures were ferrite structures in which Cr-based carbonitrides were dispersed, but in the steel sheet in which cracks occurred in the previous press working test, the distance between Cr-based carbonitrides tended to be relatively short. It was found that it is in.

Therefore, the inventors have repeated experiments and studies focusing on the relationship between the overhang formability and the distance between Cr-based carbonitrides. As a result, it was found that there is a correlation between the overhang formability and the average distance between Cr-based carbonitrides having a certain size or more. In particular, excellent overhang formability was obtained by setting the average distance between Cr-based carbonitrides having a diameter equivalent to a circle of 0.05 μm or more to 3.0 μm or more. Specifically, excellent overhang formability was obtained in which the minimum value of the maximum logarithmic strain of the molding limit determined based on the molding limit diagram (FLD) was 0.20 or more. As a result, it has been found that members such as exhaust ducts, which require particularly high overhang formability, can be press-molded without cracking.

Here, the inventors consider the reason why excellent overhang formability can be obtained by increasing the average distance between Cr-based carbonitrides having a diameter equivalent to a circle of 0.05 μm or more as follows.
That is, when processing a steel sheet, voids are generated at the interface between the ferrite matrix and the Cr-based carbonitride in the metal structure as the amount of strain increases. This void increases and grows as the amount of strain increases and / or the stress concentration increases, and when it is connected to other adjacent voids, it becomes a crack, which eventually leads to the fracture of the steel sheet.
In this way, the void grows by receiving stress concentration as the strain amount increases, and grows into a microcrack by connecting with a nearby void. In particular, in deformation under multiaxial stress in which stress acts two-dimensionally or three-dimensionally, the growth of cracks is further promoted by increasing the degree of triaxial stress. In the case of such deformation under multiaxial stress, the void grows as compared with the deformation under uniaxial stress (deformation under uniaxial stress is used for evaluation of elongation and is represented by a tensile test). It's easy to do. Therefore, it is considered that the fracture limit of the material is lower than that under uniaxial stress (that is, it is easily fractured).
Overhang molding is usually deformation under multiaxial stress, and omnidirectional void connection is likely to occur in the steel sheet, so that fracture is more likely to occur than deformation under uniaxial stress.
Therefore, even if the steel sheet shows high breaking elongation in deformation under uniaxial stress such as a tensile test, if the average distance between Cr-based carbonitrides having a certain size or more is short, it will be under multiaxial stress. The deformation causes void connection, which promotes the generation and growth of microcracks due to the void connection.
On the other hand, if the average distance of a Cr-based carbonitride having a size of a certain size or more is sufficiently long, voids are unlikely to be connected even in the case of overhang molding in which deformation occurs under multiaxial stress. In addition, the generation and growth of microcracks due to the connection of voids are suppressed.
For this reason, the inventors believe that the overhang formability is significantly improved by increasing the average distance between Cr-based carbonitrides having a diameter equivalent to a circle of 0.05 μm or more.

Further studies by the inventors revealed that in order for the average distance between Cr-based carbonitides having a diameter equivalent to a circle of 0.05 μm or more to be 3.0 μm or more, a certain period of time or more is required in a predetermined temperature range. The hot-rolled plate is annealed to be retained, and the metal structure after the hot-rolled plate is annealed to be a ferrite single-phase structure in which Cr-based carbonitrides are once precipitated, and then in the cold-rolled plate annealing after cold rolling. ,
(1) By slowing down the heating rate from 500 ° C. to the heating temperature, the aggregation and coarsening of Cr-based carbonitoxide and the solid solution of Cr-based carbonitride into the ferrite phase are promoted at the same time. Solid solution of Cr-based carbonitoxide into the ferrite phase is a phenomenon in which Cr-based carbonitride is decomposed into Cr, carbon and nitrogen in atomic units, and each element is contained in the ferrite phase).
(2) Appropriately controlling the heating temperature and holding time to further promote the solid solution of Cr-based carbonitride into the ferrite phase, and
(3) Accelerate the cooling rate from the heating temperature to 500 ° C. to suppress the reprecipitation of the solid-dissolved Cr-based carbonitride.
Is important, and by satisfying all of these conditions at the same time, it is possible to set the average distance between Cr-based carbonitrides having a diameter equivalent to a circle of 0.05 μm or more to 3.0 μm or more. Got
The present invention has been completed after further studies based on the above findings.

That is, the gist structure of the present invention is as follows.
1. 1. By mass%
C: 0.025 to 0.050%,
Si: 0.10 to 0.40%,
Mn: 0.45-1.00%,
P: 0.04% or less,
S: 0.010% or less,
Cr: 16.0 to 18.0%,
Al: 0.001 to 0.010%,
N: 0.025 to 0.060% and Ni: 0.05 to 0.60%
Has a component composition in which the balance is composed of Fe and unavoidable impurities, and
The average distance between Cr-based carbonitrides with a diameter equivalent to a circle of 0.05 μm or more is 3.0 μm or more.
A ferritic stainless steel sheet in which the minimum value of the maximum logarithmic strain of the forming limit based on the forming limit diagram is 0.20 or more.

2. The steel material having the component composition described in 1 was hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet was annealed with a hot-rolled sheet at a heating temperature of 800 to 900 ° C. and a holding time of 1 hour or more. After that, it is cold-rolled to obtain a cold-rolled steel sheet, and then the cold-rolled steel sheet is annealed with a heating temperature of 800 to 900 ° C. and a holding time of 5 to 300 seconds.
Manufacture of ferritic stainless steel sheets in which the average heating rate from 500 ° C. to heating temperature is 20 ° C./s or less and the average cooling rate from heating temperature to 500 ° C. is 10 ° C./s or more in the above-mentioned cold rolled sheet annealing. Method.

According to the present invention, a ferritic stainless steel sheet having sufficient corrosion resistance and excellent overhang formability can be obtained.
Further, if the ferritic stainless steel sheet of the present invention is used, a member such as an exhaust duct that requires particularly high overhang formability can be manufactured by press molding, which is extremely advantageous in industry.

Example No. It is a metal structure photograph of 1. Example No. 12 metal structure photographs.

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. The unit in the component composition is "mass%", but hereinafter, unless otherwise specified, it is simply indicated by "%".

C: 0.025 to 0.050%
C is an element effective for promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of rigging. From the viewpoint of obtaining such an effect, the C content is set to 0.025% or more.
However, when the C content exceeds 0.050%, the amount of Cr-based carbonitrides precipitated during hot rolling and hot-rolled sheet annealing becomes excessively large, and the average distance between Cr-based carbonitrides becomes long. Becomes difficult. Therefore, at the time of overhang molding, it is not possible to prevent the occurrence and breakage of cracks due to the connection of voids, and the desired overhang moldability cannot be obtained. In addition, the steel becomes excessively hard and the ductility decreases.
Therefore, the C content is set in the range of 0.025 to 0.050%. The lower limit of the C content is preferably 0.030%, more preferably 0.035%. The upper limit of the C content is preferably 0.045%.

Si: 0.10 to 0.40%
Si is an element that acts as an antacid during melting of steel. From the viewpoint of obtaining such an effect, the Si content is set to 0.10% or more.
However, when the Si content exceeds 0.40%, the steel becomes excessively hard and the rolling load during hot rolling increases. In addition, the ductility of the steel sheet obtained after annealing the cold-rolled sheet is reduced.
Therefore, the Si content is in the range of 0.10 to 0.40%. The lower limit of the Si content is preferably 0.20%. The upper limit of the Si content is preferably 0.30%.

Mn: 0.45-1.00%
Like C, Mn is an element effective in promoting the formation of an austenite phase and suppressing the occurrence of rigging. From the viewpoint of obtaining such an effect, the Mn content is set to 0.45% or more.
However, if the Mn content exceeds 1.00%, the steel becomes excessively hard and the rolling load during hot rolling increases. In addition, the ductility of the steel sheet obtained after annealing the cold-rolled sheet is reduced.
Therefore, the Mn content is set in the range of 0.45 to 1.00%. The lower limit of the Mn content is preferably 0.60%. The upper limit of the Mn content is preferably 0.75%, more preferably 0.70%.

P: 0.04% or less P is an element that promotes grain boundary fracture due to grain boundary segregation. Therefore, it is desirable that the P content is low, and the upper limit is 0.04%. It is preferably 0.03% or less. More preferably, it is 0.01% or less. The lower limit of the P content is not particularly limited, but excessive de-P causes an increase in cost. Therefore, the lower limit of the P content is preferably 0.005%.

S: 0.010% or less S is an element that exists in steel as a sulfide-based inclusion such as MnS and lowers ductility, corrosion resistance, etc., especially when the S content exceeds 0.010%. In addition, the adverse effect is remarkable. Therefore, it is desirable that the S content is as low as possible, and the upper limit of the S content is 0.010%. It is preferably 0.007% or less. More preferably, it is 0.005% or less. The lower limit of the S content is not particularly limited, but excessive de-S causes an increase in cost. Therefore, the lower limit of the S content is preferably 0.001%.

Cr: 16.0 to 18.0%
Cr is an element having the effect of forming a passivation film on the surface of a steel sheet to improve corrosion resistance. From the viewpoint of obtaining such an effect, the Cr content is set to 16.0% or more.
However, if the Cr content exceeds 18.0%, the amount of austenite phase formed during hot rolling may decrease and the rigging resistance may decrease.
Therefore, the Cr content is in the range of 16.0 to 18.0%. The upper limit of the Cr content is preferably 17.0%, more preferably 16.5%.

Al: 0.001 to 0.010%
Al is an element that acts as an antacid, like Si. From the viewpoint of obtaining such an effect, the Al content is set to 0.001% or more.
However, when the Al content exceeds 0.010% _{, Al-based inclusions such as Al 2} O ₃ increase, and the surface texture tends to deteriorate.
Therefore, the Al content is in the range of 0.001 to 0.010%. The lower limit of the Al content is preferably 0.002%. The upper limit of the Al content is preferably 0.007%, more preferably 0.005%.

N: 0.025 to 0.060%
Like C and Mn, N is an element effective in promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of rigging. From the viewpoint of obtaining such an effect, the N content is set to 0.025% or more.
However, when the N content exceeds 0.060%, the ductility of the steel sheet obtained after annealing the cold-rolled sheet is significantly reduced. Further, the amount of Cr-based carbonitrides precipitated during hot rolling and hot-rolled sheet annealing becomes excessively large, and it becomes difficult to increase the average distance between Cr-based carbonitrides. Therefore, when the overhang molding is performed, it is not possible to prevent the occurrence and breakage due to the expansion of cracks due to the connection of the voids, and the desired overhang moldability cannot be obtained.
Therefore, the N content is in the range of 0.025 to 0.060%. The lower limit of the N content is preferably 0.030%, more preferably 0.040%. The upper limit of the N content is preferably 0.055%, more preferably 0.050%.

Ni: 0.05 to 0.60%
Ni is an element that promotes the formation of an austenite phase, increases the amount of austenite phase formed during hot rolling, and has the effect of improving rigging resistance. Ni is also an element effective for improving corrosion resistance. From the viewpoint of obtaining such an effect, the Ni content is set to 0.05% or more. However, when the Ni content exceeds 0.60%, the steel becomes excessively hard and the formability deteriorates. Therefore, the Ni content is in the range of 0.05 to 0.60%. The lower limit of the Ni content is preferably 0.10%. The upper limit of the Ni content is preferably 0.50%, more preferably 0.30%.

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

Next, the metal structure of the ferritic stainless steel sheet of the present invention will be described.
The metal structure of the ferrite-based stainless steel plate of the present invention has a structure mainly composed of a ferrite phase, specifically, a ferrite phase having a volume ratio of 90% or more with respect to the entire structure, and the remaining structure other than the ferrite phase is the entire structure. The structure has a volume ratio of 10% or less. Further, it may be a ferrite single phase. The residual structure mainly includes the martensite phase, and does not include the volume fraction of precipitates and inclusions.
Here, the volume fraction of the ferrite phase is determined by preparing a test piece for cross-sectional observation from a stainless steel plate, mirror-polishing it, and then etching it with a saturated aqueous solution of picrinic acid-5 mass% hydrochloric acid, and then at a plate thickness of 1/4. After observing an arbitrary 10 fields with an optical microscope at a magnification of 100 times and distinguishing between the martensite phase and the ferrite phase from the morphology of the metal structure, the volume fraction of the ferrite phase is obtained by image processing and the average value is calculated. Ask for it.

In the metal structure of the ferritic stainless steel sheet of the present invention, as described above, the average distance between Cr-based carbonitides having a diameter equivalent to a circle deposited in the steel of 0.05 μm or more can be set to 3.0 μm or more. It is essential.
Here, the diameter equivalent to a circle is
The area of the Cr-based carbonitride was measured by performing image processing on a digital photograph (magnification of 500 times) in which the Cr-based carbonitride that appeared in the metal structure of the test piece for cross-sectional observation was taken.
From the measured area of Cr-based carbonitride, the circle diameter calculated based on the assumption that the shape of the Cr-based carbonitride is a perfect circle (= {(4 × [measured Cr-based carbonitride] Area of object]) / π} ^0.5 ),
Means.

Average distance between Cr-based carbonitrides with a circle-equivalent diameter of 0.05 μm or more: 3.0 μm or more When the average distance between Cr-based carbonitrides with a circle-equivalent diameter of 0.05 μm or more is increased to 3.0 μm or more, many Even in the case of overhang molding that deforms under axial stress, it is difficult for voids to be connected to each other, and as a result, the generation and growth of microcracks due to the connection of voids are suppressed.
Therefore, the average distance of Cr-based carbonitrides having a diameter equivalent to a circle of 0.05 μm or more is set to 3.0 μm or more. It is preferably 4.0 μm or more. The upper limit is not particularly limited, and is usually about 6.0 μm.
The reason why Cr-based carbonitrides with a circle-equivalent diameter of less than 0.05 μm are not targeted is that extremely fine Cr-based carbonitrides with a circle-equivalent diameter of less than 0.05 μm are in contact with the ferrite phase, which is the matrix phase. Since the area is small, even if plastic deformation is applied by press working or the like, voids are hardly generated at the interface between the ferrite phase and the Cr-based carbonitride, and thus the effect on moldability, especially overhang formability. This is because can be almost ignored.

Further, the Cr-based carbonitride referred to here is a general term for Cr carbides and Cr nitrides. Examples of the Cr carbide include Cr ₂₃ C ₆ , and examples of the Cr nitride include Cr ₂ N. Further, the Cr carbide and Cr nitride in which a part of Cr is replaced with an element such as Fe and Mn are also included in the Cr-based carbonitride.

In addition, the Cr-based carbonitrides with a circle-equivalent diameter of 0.05 μm or more were targeted because the voids generated as the strain amount increased were mainly ferrite matrix and Cr-based with a circle-equivalent diameter of 0.05 μm or more. This is because the distance between Cr-based carbonitrides having a diameter equivalent to a circle of 0.05 μm or more, which is formed at the interface of the carbonitride, particularly affects the connection of voids and, by extension, the overhang formability.
The size of the Cr-based carbonitride is usually about 0.5 μm in diameter equivalent to a circle.

The average distance between Cr-based carbonitrides having a diameter equivalent to a circle of 0.05 μm or more is measured as follows.
That is, the rolled parallel cross section of the steel sheet is mirror-polished and then etched with a saturated hydrochloric acid solution of picrinate to reveal the metal structure, and the metal structure at the plate thickness of 1/4 is photographed with an optical microscope at a magnification of 500 times. ..
It should be confirmed that the precipitate captured in the metallographic photograph is Cr-based carbonitride by analyzing the components of the precipitate by energy dispersive X-ray spectroscopy under a scanning electron microscope. Can be done. Specifically, the peak of Cr in the element spectrum obtained from the precipitate by energy dispersion type X-ray spectroscopy is higher than the peak of Cr in the element spectrum obtained from the parent phase by the same method, and the precipitate In the quantitative analysis value of each element calculated from the spectral intensity ratio of each element, when the main components of the precipitate are Cr, Fe, C and N, the precipitate is determined to be a Cr-based carbonitride. be able to.
Then, in the obtained metallographic photograph, any Cr-based carbonitride having a diameter equivalent to a circle of 0.05 μm or more (hereinafter, also referred to as a reference carbonitride) is selected, and the distances from the standard carbonitride are in ascending order. , Select 10 Cr-based carbonitrides (also referred to as target carbonitrides) having a diameter equivalent to a circle of 0.05 μm or more, and determine the distance (distance between centers) between the reference carbonitride and each target carbonitride. Measure on the metallographic photograph.
This measurement is performed 20 times with the reference carbonitride changed arbitrarily, and the average distance between the Cr-based carbonitrides is calculated by arithmetically averaging the distances between all the measured reference carbonitrides and the target carbonitrides. Ask.
The above measurement is not limited to single ferrite grains, and may straddle grain boundaries. In addition, in order to make a representative measurement, each measurement point is made so that the reference carbonitride and the target carbonitride selected in the previous measurement do not become the reference carbonitride or the target carbonitride in another measurement. Selects locations that are sufficiently distant from each other.

As described above, the ferritic stainless steel sheet of the present invention is formed by using the above-mentioned composition and setting the average distance between Cr-based carbonitides having a diameter equivalent to a circle of 0.05 μm or more to 3.0 μm or more. Excellent overhang formability can be obtained by setting the minimum value of the maximum logarithmic strain of the molding limit determined based on the limit diagram (FLD) to 0.20 or more, preferably 0.23 or more.

The thickness of the ferritic stainless steel sheet according to the embodiment of the present invention is not particularly limited, but is, for example, 0.8 to 2.0 mm.

Next, the method for manufacturing the ferritic stainless steel sheet of the present invention will be described.
The ferrite-based stainless steel plate of the present invention is obtained by hot-rolling a steel material having the above-mentioned composition to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is heated at a heating temperature of 800 to 900 ° C. and a holding time of 1 hour or more. After the rolled-rolled plate is annealed, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, and the cold-rolled steel sheet is subjected to cold-rolled sheet annealing at a heating temperature of 800 to 900 ° C. and a holding time of 5 to 300 seconds. In the cold-rolled sheet baking, the average heating rate from 500 ° C. to heating temperature is 20 ° C./s or less, and the average cooling rate from heating temperature to 500 ° C. is 10 ° C./s or more. can do.
That is, the ferritic stainless steel sheet according to the embodiment of the present invention is a ferritic stainless cold-rolled annealed steel sheet.

First, molten steel having the above-mentioned composition is melted by a known method such as a converter, an electric furnace, a vacuum melting furnace, etc., and a steel material (slab) is obtained by a continuous casting method or an ingot-packing method.
Then, the obtained steel material is preferably 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 hot-rolled to be a hot-rolled steel sheet. And. The hot rolling conditions may be in accordance with a conventional method.
Then, the obtained hot-rolled steel sheet is annealed by hot-rolled sheet under the following conditions.

<Heating temperature for hot-rolled sheet annealing: 800-900 ° C, holding time: 1 hour or more>
The metal structure of a hot-rolled steel sheet is a metal structure in which a ferrite phase and a ferrite phase generated by decomposition of an austenite phase generated at a high temperature are laminated in layers when the winding temperature during hot rolling is high. When the take-up temperature during hot rolling is low, the ferrite phase and the martensite phase generated by transformation of the austenite phase generated at high temperature are laminated in layers to form a metal structure.
In the vicinity of the ferrite phase formed by the decomposition of the austenite phase when the winding temperature is high, Cr-based carbonitrides precipitated by the decomposition of the austenite phase are unevenly distributed in the entire metal structure. The distribution of Cr-based carbonitride is non-uniform.
The winding temperature is not particularly limited, but when the winding temperature is 450 ° C. to 500 ° C., the toughness of the hot-rolled steel sheet may be significantly reduced due to embrittlement at 475 ° C. Therefore, the winding temperature is preferably more than 500 ° C or less than 450 ° C. Further, from the viewpoint of more easily obtaining a predetermined metal structure after hot rolling plate annealing, Cr-based carbonitride was sufficiently precipitated after hot rolling and before hot rolling plate annealing. Is more advantageous. Therefore, the winding temperature is more preferably 600 ° C. or higher, which further promotes the decomposition of the austenite phase into Cr-based carbonitride and ferrite phases.
By performing hot-rolled sheet annealing in which the hot-rolled steel sheet having such a metal structure is held in a temperature range of 800 to 900 ° C. for 1 hour or more, recrystallization and precipitation of Cr-based carbonitride occur in the metal structure. In the steel sheet obtained after hot-rolled sheet annealing, a metal structure in which Cr-based carbonitrides are sufficiently and uniformly dispersed in the ferrite single-phase structure can be obtained.

Here, when the heating temperature of the hot-rolled sheet annealing is set to less than 800 ° C., the aggregation and coarsening of Cr-based carbonitride and the solid solution to the ferrite phase become insufficient, and a predetermined metal structure cannot be obtained. .. In addition, the layered structure formed during hot rolling due to insufficient recrystallization remains particularly in the central portion of the plate thickness. Therefore, after the cold-rolled plate is annealed, a non-uniform metal structure having remarkably expanded grains may be formed in the central portion of the plate thickness, and the rigging resistance may be lowered.
On the other hand, when the hot-rolled plate annealing temperature exceeds 900 ° C., the austenite phase is regenerated during the holding of the hot-rolled plate annealing, and the Cr-based carbonitride precipitated in the hot rolling step is solid-solved in the austenite phase. Therefore, Cr-based carbonitrides cannot be sufficiently precipitated in the metal structure of the steel sheet obtained after hot-rolled sheet annealing. Further, during the cooling of the hot-rolled sheet annealing, a decomposition reaction between the ferrite phase and the Cr-based carbonitride occurs in the austenite phase. As a result, the metallographic structure after annealing the hot-rolled plate is a mixture of the ferrite phase and the ferrite phase formed by the decomposition of the austenite phase, that is, the ferrite phase in which a large amount of Cr-based carbonitride is distributed around the ferrite phase. It becomes a structure and the distribution of Cr-based carbonitride becomes non-uniform. Therefore, even if the cold-rolled sheet is annealed under predetermined conditions in the subsequent step, a region where the average distance between Cr-based carbonitrides is not sufficient is locally generated, and a predetermined overhang formability can be obtained. I can't.
Therefore, the heating temperature in the hot-rolled sheet annealing is in the range of 800 to 900 ° C. It is preferably in the range of 800 to 860 ° C.

Further, when the holding time in the hot-rolled plate annealing is less than 1 hour, the precipitation of Cr-based carbonitride becomes insufficient, and even if the cold-rolled plate annealing is performed under predetermined conditions in the subsequent steps, the Cr-based carbonitride is obtained. The average distance between the carbonitrides cannot be made sufficiently long, and the predetermined overhang formability cannot be obtained.
Therefore, the holding time in hot-rolled sheet annealing is set to 1 hour or more. It is preferably 3 hours or more, more preferably 5 hours or more. The upper limit of the holding time is not particularly limited, but is preferably 24 hours or less from the viewpoint of productivity.

Then, the steel sheet (hot-rolled annealed steel sheet) obtained after the hot-rolled sheet is annealed, if necessary, pickled and cold-rolled to obtain a cold-rolled steel sheet. Cold rolling is preferably performed at a rolling reduction of 50% or more from the viewpoint of extensibility, bendability and shape correction. Further, the cold-rolled-cold-rolled plate annealing may be repeated twice or more as long as the cold-rolled plate annealing conditions described later are satisfied. Further, in order to improve the surface texture, the steel sheet obtained after annealing the hot-rolled sheet may be ground or polished.
The cold-rolled steel sheet thus obtained is annealed by cold-rolled sheet under the following conditions.

<Average heating rate of cold rolled sheet annealing from 500 ° C to heating temperature: 20 ° C / s or less>
Cold-rolled sheet annealing is a process for recrystallizing a rolled structure formed by cold rolling and sufficiently lengthening the average distance between Cr-based carbonitrides. For that purpose, heating is performed from 500 ° C. It is important that the average heating rate at temperature is 20 ° C./s or less.
That is, when the average heating rate from 500 ° C. to the heating temperature is slowed down, the driving force for recrystallization becomes smaller, so that the temperature at which recrystallization starts becomes higher, and the dislocations or shear bands introduced by cold rolling become higher. Is maintained until.
Further, in a high temperature region close to the heating temperature, the Cr-based carbonitrides produced during the annealing of the hot-rolled plate are aggregated and coarsened (the individual Cr-based carbonitrides become large while the volume ratio remains almost constant, and the Cr-based carbonitrides become large. The phenomenon that the number density of objects decreases) and solid solution to the ferrite phase occur. This aggregation / coarsening is rate-determined by the diffusion of Cr, which is a major constituent element of Cr-based carbonitrides. When the dislocation or shear band is maintained at a high temperature as described above, high-speed diffusion of Cr through the dislocation or shear band occurs, and aggregation and coarsening of Cr-based carbonitride are promoted.
Further, the solid solution of Cr-based carbonitride occurs in a high temperature range close to the heating temperature because the upper limits (solid solution limit) of C and N that can be solid-solved in the ferrite phase increase. The average distance between Cr-based carbonitrides becomes longer due to the effect of promoting aggregation and coarsening of these Cr-based carbonitrides and the solid solution into the ferrite phase. That is, by slowing the heating rate, specifically, controlling the average heating rate from 500 ° C. to the heating temperature to 20 ° C./s or less, it is possible to increase the average distance between Cr-based carbonitrides. ..
On the other hand, when the average heating rate from 500 ° C. to the heating temperature exceeds 20 ° C./s, the driving force for recrystallization of the ferrite phase becomes excessively large, and recrystallization of the ferrite phase occurs from a relatively low temperature region in the heating process. , Processed structures such as dislocations or shear zones introduced by cold rolling are replaced with recrystallized grains in the low temperature region. As a result, the effect of promoting aggregation and coarsening of Cr-based carbonitrides becomes insufficient, and the average distance between Cr-based carbonitrides in the steel sheet obtained after annealing the cold-rolled sheet becomes short, so that the desired overhang formability is obtained. Cannot be obtained.
Therefore, the average heating rate from 500 ° C. to the heating temperature is set to 20 ° C./s or less. It is preferably 15 ° C./s or less, more preferably 12 ° C./s or less. The lower limit of the average heating rate is not particularly limited, but if the heating rate is excessively slowed down, the productivity decreases, so that the average heating rate is preferably 1 ° C./s or more.
The heating rate can be controlled by, for example, in the case of the continuous annealing method, the furnace temperature is set, the plate passing speed of the continuous annealing line, or the like.
Further, the temperature range to be controlled is set to 500 ° C. or higher because recovery and recrystallization do not occur in a temperature range of less than 500 ° C.

<Heating temperature for cold rolled sheet annealing: 800-900 ° C, holding time: 5-300 seconds>
The upper limit (solid solution limit) of C and N that can be solid-solved in the ferrite phase increases as the temperature rises. In cold-rolled sheet annealing, the heating temperature is 800 to 900 ° C. and the holding time is 5 to 300 seconds, so that a part of the Cr-based carbonitride produced during hot-rolled sheet annealing is dissolved in the ferrite phase to solidify Cr. The number density of the carbon nitrides can be reduced and the average distance between the Cr carbonitrides can be increased.
Therefore, the heating temperature in the cold rolled sheet annealing is 800 to 900 ° C., and the holding time is 5 to 300 seconds. Preferably, the heating temperature in the cold rolled sheet annealing is 800 to 860 ° C., and the holding time is 15 seconds to 180 seconds.
The holding time referred to here is a residence time in a temperature range of a heating temperature of ± 10 ° C.

Here, when the heating temperature is less than 800 ° C., the solid solution limits of C and N in the ferrite phase do not become sufficiently large, and the amount of Cr-based carbonitrides solid-solved in the ferrite phase decreases, resulting in Cr-based carbonitride. The distance between objects becomes shorter. Therefore, the desired overhang formability cannot be obtained. In addition, unrecrystallized grains remain and the ductility is greatly reduced.
On the other hand, when the heating temperature exceeds 900 ° C., an austenite phase is formed during holding, and the austenite phase is transformed into a martensite phase in the subsequent cooling to remarkably harden the steel sheet. Further, the metal structure of the final product plate becomes a two-phase structure of a ferrite phase and a martensite phase, the plastic deformability is remarkably lowered, and the desired overhang formability cannot be obtained.

Further, when the holding time is less than 5 seconds, the solid solution of the Cr-based carbonitride during holding into the ferrite phase becomes incomplete, the distance between the Cr-based carbonitrides becomes short, and the desired overhang formability is obtained. Cannot be obtained. Further, the ductility is greatly reduced because unrecrystallized grains remain.
On the other hand, if the holding time exceeds 300 seconds, the crystal grains become remarkably coarse and the glossiness of the steel sheet decreases, which is not preferable from the viewpoint of surface quality.

<Average cooling rate at heating temperature of cold rolled sheet annealing to 500 ° C: 10 ° C / s or more>
In the above cooling after holding, reprecipitation of Cr-based carbonitride occurs. That is, the Cr-based carbonitride dissolves in the ferrite matrix during the cold-rolled sheet annealing. As a result, the solid solution C and N in the ferrite phase increase, become supersaturated with respect to the ferrite phase during cooling, and reprecipitate as Cr-based carbonitride.
Therefore, from the viewpoint of increasing the average distance of the Cr-based carbonitride, the Cr-based carbonitride is particularly produced by increasing the cooling rate in the temperature range of 500 ° C. or higher, which is the precipitation temperature range of the Cr-based carbonitride. It is important to suppress the reprecipitation of the metal structure and maintain a metal structure in which the average distance between the Cr-based carbonitrides formed before cooling is sufficiently long.
Here, when the average cooling rate is less than 10 ° C./s, the solid solution C and N in the ferrite phase generated during the holding of the cold rolled sheet annealing are sufficiently suppressed from being reprecipitated as Cr-based carbonitridants. Therefore, the average distance between Cr-based carbonitides is shortened and the desired overhang formability cannot be obtained. Therefore, the average cooling rate of cold rolled sheet annealing at a heating temperature of to 500 ° C. is 10 ° C./s or more. And. It is preferably 15 ° C./s or higher, more preferably 20 ° C./s or higher. Although the upper limit of the average cooling rate is not particularly limited, the average cooling rate is preferably 200 ° C./s or less because the steel sheet may be distorted when cooling is performed rapidly.

The cooling method is not particularly limited, and gas jet cooling, mist cooling, roll cooling, and the like can be used. Further, as for the cold rolled sheet annealing, BA annealing (bright annealing) may be performed in order to obtain higher gloss.
Then, after the cold-rolled sheet is annealed, the above-mentioned ferritic stainless steel sheet is manufactured by pickling if necessary.

Example 1
The molten steel of the components shown in Table 1 (the balance is Fe and unavoidable impurities) is melted by refining using a converter with a capacity of 150 tons and the vacuum oxygen decarburization (VOD) method, and then by continuous casting. A slab having a width of 1000 mm and a thickness of 200 mm was used.
After heating the slab at 1200 ° C. for 1 hour, 7-pass rough rolling using a reverse rolling mill consisting of a 3-stage stand and a unidirectional rolling mill consisting of a 7-stage stand were used for hot rolling. It was subjected to finish rolling consisting of 7 passes and wound at about 750 ° C. to obtain a hot-rolled steel sheet having a plate thickness of about 5.0 mm.
Then, these hot-rolled steel sheets were annealed by hot-rolled sheet using the box annealing method under the conditions shown in Table 2, and then the surface was descaled by shot blasting and pickling.
The steel sheet thus obtained was cold-rolled to a sheet thickness of 1.0 mm, and then cold-rolled sheet was annealed under the conditions shown in Table 2. The cooling after holding was performed by gas jet cooling or mist cooling. In addition, after cooling was completed, descale treatment by pickling was performed.
Here, the holding time in Table 2 is the residence time in the temperature range of the heating temperature ± 10 ° C.
The average heating rate shown in Table 2 is the average heating rate from 500 ° C. to reaching the heating temperature. The average cooling rate shown in Table 2 is the average cooling rate until the heating temperature reaches 500 ° C.

With respect to the steel sheet thus obtained, the metallographic structure was identified, the volume fraction of ferrite was measured, and the average distance between Cr-based carbonitrides having a diameter equivalent to a circle of 0.05 μm or more was measured.
Here, the identification of the metallographic structure and the measurement of the volume fraction of ferrite were carried out by the method described above. That is, a test piece for cross-sectional observation is prepared from the obtained steel plate, subjected to etching treatment with a saturated hydrochloric acid solution of picrinate, and then observed with an optical microscope at a magnification of 100 times for 10 fields at a plate thickness of 1/4. After distinguishing between the martensite phase and the ferrite phase from the morphology of the metal structure, the volume ratio of the ferrite phase was determined in each field by image processing, and the average value was taken as the volume ratio of the ferrite phase. The volume fractions of precipitates and inclusions are excluded.
Further, the average distance between Cr-based carbonitrides having a diameter equivalent to a circle of 0.05 μm or more was also measured by the method described above.
These results are also shown in Table 2. In addition, for reference, No. 1 in Table 2 is shown in FIGS. 1 and 2. 1 and No. A photograph of the metallographic structure used for measuring the average distance between Cr-based carbonitrides is shown for No. 12.

In addition, (1) overhang formability and (2) corrosion resistance were evaluated by the following methods. The evaluation results are also shown in Table 2.

(1) Evaluation of overhang formability A molding test in accordance with ISO12004-2: 2008 was performed with the rolling parallel direction, rolling 45 ° direction, and rolling perpendicular direction of the obtained steel sheet as the maximum logarithmic strain directions, respectively, and a molding limit diagram was performed. (FLD) was created.
Specifically, a scribed circle having a diameter of 5 mm is marked on the surface of the steel sheet so that the distance between the scores is 1 mm, and a forming test under various conditions, that is, that is,
-For the molding limit under uniaxial stress, a tensile test using JIS No. 5 tensile test piece,
-For the molding limit in the plane strain state, a bulging test using a test piece with a bead on the circumference after shearing to a 130 mm square.
・ For the molding limit under isobiaxial stress, a bulging test using a test piece blanked in a perfect circle,
-For the molding limit under unequal biaxial stress, a bulging test using test pieces blanked in an elliptical shape with various ellipticity.
Was performed, and the test pieces before and after each test were photographed. Then, the amount of change in the shape of the scribed circle of the test piece before and after each test was quantitatively measured by image processing of photographs, the strain applied by each test was measured, and a molding limit diagram (FLD) was created. ..
The minimum value of the maximum logarithmic strain of the molding limit is obtained from the obtained molding limit diagram (FLD), and the case where the minimum value of the maximum logarithmic strain is 0.20 or more is passed (○), and the case where it is less than 0.20 is not accepted. It was evaluated as a pass (x).

(2) Evaluation of Corrosion Resistance From the obtained steel sheet, a test piece having a size of 60 × 100 mm was collected, and the surface was polished and finished with # 600 emery paper. Then, the end face of the test piece was sealed and subjected to a salt spray cycle test specified in JIS H8502.
Here, in the salt spray cycle test, salt spray (35 ° C, 5% by mass NaCl, spray time: 2 hours) → dry (60 ° C, relative humidity 40%, holding time: 4 hours) → wet (50 ° C, relative). Eight cycles were performed with humidity ≥95% and holding time (holding time: 2 hours) as one cycle.
The surface of the test piece after the salt spray cycle test was photographed, the rusted area on the surface of the test piece was measured by image analysis, and the rusted area ratio ((Rust on the surface of the test piece) was calculated from the ratio to the total area of the test piece surface. Rust area / total area of the surface of the test piece) × 100 (%)) was calculated.
Then, the case where the calculated rusted area ratio was 10% or less was evaluated as a pass (⊚, particularly excellent), the case of more than 10% and 25% or less was evaluated as a pass (◯), and the case of more than 25% was evaluated as a fail (x).

In each of the examples of the invention, excellent overhang moldability and excellent corrosion resistance were obtained.
In particular, No. 1 containing 0.58% of Ni. No. 6 (steel A6) and No. 1 containing 17.8% of Cr. In No. 8 (Steel A8), the rusted area ratio in the salt spray cycle test was 10% or less (⊚), and more excellent corrosion resistance was obtained.

On the other hand, No. 12 (Steel B1) and No. Although the production conditions of 13 (steel B2) are appropriate, the C content and the N content each exceed the appropriate ranges, so that the amount of Cr-based carbonitrides precipitated becomes excessive and between Cr-based carbonitrides. A sufficient average distance could not be secured, and the desired overhang formability could not be obtained.
No. Since the Si content of 14 (steel B3) exceeds the appropriate range, the steel sheet is hardened and the plastic deformability is lowered, so that the desired overhang formability cannot be obtained.
No. 15 and No. In No. 16, since the average heating rate of cold-rolled sheet annealing exceeded the appropriate range, the average distance between Cr-based carbonitrides could not be sufficiently secured, and the desired overhang formability could not be obtained.
No. 17 and No. In No. 18, since the heating temperature of the cold-rolled sheet annealing exceeds the appropriate range, an austenite phase is generated during the holding of the cold-rolled sheet annealing, and the austenite phase is transformed into the martensite phase in the cooling after the holding, and the steel sheet is remarkably formed. Hardened. In addition, since the metal structure of the final product plate has a two-phase structure consisting of a ferrite phase and a martensite phase, the plastic deformability of the steel sheet is significantly reduced. Therefore, the desired overhang moldability could not be obtained.
No. 19 and No. In No. 20, since the heating temperature of cold-rolled sheet annealing is below the appropriate range, a sufficient average distance between Cr-based carbonitrides cannot be secured, and unrecrystallized grains remain in the metal structure. , The desired overhang moldability could not be obtained.
No. 21 and No. In No. 22, since the holding time for cold-rolled sheet annealing was less than the appropriate range, a sufficient average distance between Cr-based carbonitrides could not be secured, and unrecrystallized grains remained in the metal structure. , The desired overhang moldability could not be obtained.
No. 23 and No. In No. 24, since the cooling rate of the cold rolled sheet annealing is lower than the appropriate range, a large amount and finely reprecipitated Cr-based carbonitride during the cooling ensures a sufficient average distance between Cr-based carbonitrides. It was not possible to obtain the desired overhang formability.
No. In No. 28, since the heating temperature of the hot-rolled sheet annealing is below the appropriate range, the aggregation and coarsening of the Cr-based carbonitride in the hot-rolled sheet annealing and the solid solution to the ferrite phase become insufficient, and the Cr-based carbonitride is insufficient. Non-uniform distribution of objects occurred. Therefore, a region where the average distance between Cr-based carbonitrides is not sufficient is locally formed, and the desired overhang formability cannot be obtained.
No. In No. 29, since the heating temperature of the hot-rolled sheet annealing exceeds the appropriate range, the austenite phase is regenerated in the hot-rolled sheet annealing, and as a result, the distribution of Cr-based carbonitrides is poor in the metal structure after the hot-rolled sheet annealing. Uniformity has occurred. Therefore, a region where the average distance between Cr-based carbonitrides is not sufficient is locally formed, and the desired overhang formability cannot be obtained.
No. In No. 30, since the holding time of the hot-rolled plate annealing is less than the appropriate range, the aggregation and coarsening of the Cr-based carbonitride in the hot-rolled plate annealing and the solid solution to the ferrite phase become insufficient, and the Cr-based carbonitride is insufficient. Non-uniform distribution of objects occurred. Therefore, a region where the average distance between Cr-based carbonitrides is not sufficient is locally formed, and the desired overhang formability cannot be obtained.

The ferritic stainless steel sheet of the present invention is particularly advantageous when applied to applications that require high overhang formability during press molding, for example, exterior members, kitchen utensils, and tableware.

Claims (1)

The average distance between Cr-based ferritics with a diameter equivalent to a circle of 0.05 μm or more is 3.0 μm or more, and the minimum value of the maximum logarithmic strain of the molding limit based on the molding limit diagram is 0.20 or more. , A method for manufacturing ferritic stainless steel sheets,
By mass%
C: 0.025 to 0.050%,
Si: 0.10 to 0.40%,
Mn: 0.45-1.00%,
P: 0.04% or less,
S: 0.010% or less,
Cr: 16.0 to 18.0%,
Al: 0.001 to 0.010%,
N: 0.025 to 0.060% and
Ni: 0.05 to 0.60%
A steel material having a component composition containing Fe and unavoidable impurities is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet has a heating temperature of 800 to 900 ° C. and a holding time of 1 hour or more. After hot-rolling the hot-rolled sheet, it is cold-rolled to obtain a cold-rolled steel sheet, and then the cold-rolled steel sheet is annealed with a heating temperature of 800 to 900 ° C. and a holding time of 5 to 300 seconds. ,
Manufacture of ferritic stainless steel sheets in which the average heating rate from 500 ° C. to heating temperature is 20 ° C./s or less and the average cooling rate from heating temperature to 500 ° C. is 10 ° C./s or more in the above-mentioned cold rolled sheet annealing. Method.

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