KR102517499B1 - Ferritic stainless steel sheet and manufacturing method thereof - Google Patents

Abstract

In addition to the predetermined component composition, the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 μm or more is 3.0 μm or more, and the minimum value of the maximum logarithmic strain of the forming limit based on the forming limit curve is 0.20 or more.

Description

Ferritic stainless steel sheet and manufacturing method thereof

[0001] The present invention relates to a ferritic stainless steel sheet having sufficient corrosion resistance and excellent in formability, particularly in sheet formability, and a method for producing the same.

SUS430 (16-18 mass% Cr) ferritic stainless steel sheet is economical and has excellent corrosion resistance, so it is applied to various applications such as building materials, transportation equipment, home appliances, kitchen equipment, and automobile parts. The scope of application has been further expanded in recent years.

Steel sheets applied to these applications require not only corrosion resistance but also sufficient formability to be processed into a predetermined shape by press forming or the like.

As such a ferritic stainless steel sheet, for example, in Patent Document 1,

"By 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 to 30% or less, Ni: 0.7% or less, and N is contained so that 0.06 ≤ (C + N) ≤ 0.12 and 1 ≤ N / C are satisfied in relation to the C content, and further V , a ferritic stainless steel sheet with excellent formability, characterized in that it contains so as to satisfy 1.5 × 10 ^-3 ≤ (V × N) ≤ 1.5 × 10 ^-2 in relation to the N content, and the balance consists of Fe and unavoidable impurities. .”

This is disclosed.

Also, in Patent Literature 2,

"By 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 of Cr carbonitride It is contained so that the volume fraction v is 0.09% or less, and further contains Si: 0.4% or less, P: 0.05% or less, S: 0.010% or less, and has a composition consisting of Fe and unavoidable impurities, and further contains ferrite grains. A ferritic stainless cold-rolled steel sheet with excellent press formability, characterized by having an average grain size of 10 μm or more and a single ferrite structure in which 50 or less Cr carbonitrides are dispersed per ferrite grain.”

This is disclosed.

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

By the way, press forming is roughly divided into four types of forming modes such as stretch forming, deep drawing forming, stretch flange forming and bending forming.

In recent years, members whose molding mode in press molding is mainly stretch molding, for example, exterior members such as exterior hoods of annular louvers used for exhaust ducts and ventilation openings, and design by embossing Alternatively, the application of ferritic stainless steel to interior panel members and the like with improved functionality is progressing. For this reason, development of a ferritic stainless steel sheet having excellent pour formability that can be processed into such a member shape has been desired.

However, it could not be said that the ferritic stainless steel sheets disclosed in Patent Literatures 1 and 2 had sufficient pour formability.

So, the inventors first, 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: 0.01% 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 remainder being Fe and unavoidable impurities, elongation at break of 28% or more, average r value of 0.75 or more, and FLD (molding limit line) Fig.) ferritic stainless steel sheet having a minimum value of the maximum logarithmic strain of the forming limit based on 0.15 or more.”

has been developed.

As a result, compared to the ferritic stainless steel sheets disclosed in Patent Literatures 1 and 2, a ferritic stainless steel sheet with significantly improved pour formability was obtained.

However, when attempting to mold the ferritic stainless steel sheet of Patent Document 3 into a member requiring particularly high pour formability, such as an exhaust duct, more cracks may occur, and therefore, further improvement in pour formability is required. What exists is the status quo.

The present invention was developed in view of the above-described current situation, and an object of the present invention is to provide a ferritic stainless steel sheet having sufficient corrosion resistance and excellent pour formability together with an advantageous manufacturing method.

Here, "sufficient corrosion resistance" refers to the salt spray cycle test specified in JIS H 8502, salt spray (35 ° C, 5 mass% NaCl, spraying time: 2 hours) → drying (60 ° C, 40% relative humidity, holding time) : 4 hours) → Wetting (50 ° C., relative humidity ≥ 95%, holding time: 2 hours) as one cycle, and rust generation area ratio on the steel sheet surface when 8 cycles were performed (rust generation area on the steel sheet surface/ This means that the total area of the steel sheet surface) x 100 (%)) is 25% or less.

In addition, "excellent pour formability" means that the minimum value of the maximum logarithmic strain of the forming limit determined based on the Forming Limit Diagram (hereinafter also referred to as FLD) measured in accordance with ISO12004-2: 2008 is 0.20 or more. means that

Then, inventors repeated various examinations in order to solve the above-mentioned subject.

First, the inventors prepared various types of ferritic stainless steel sheets having different component compositions and manufacturing methods, and using these steel sheets, press working tests were performed on members including portions that are equal biaxially extended and unequal biaxially extended. was carried out.

In general, it is considered that the higher the elongation, the higher the pour formability, but in this press working test, cracks may occur even in a steel sheet having a high elongation at break. It was found that the size of the kidney alone was not determined.

Therefore, the inventors separately prepared a steel sheet with cracks in the previous test, and using the steel sheet, again conducted a press working test under the same conditions, and press-fitted the upper mold with cracks in the previous test. Press working was stopped immediately before the end position (bottom dead center + 2 mm), a test piece was taken from the steel sheet, and the metal structure was observed in detail. Specifically, after stopping the above press working, the steel sheet is extracted from the mold, and then, after the end face of the steel sheet is mirror-polished, corrosion treatment is performed with a saturated picric acid-5% hydrochloric acid aqueous solution. Thus, a test piece for metal structure observation was produced, and the test piece was observed at a magnification of 500 times with a scanning electron microscope (secondary electron image).

As a result, in all of the steel sheets where cracks occurred in the previous test, a large number of voids were already formed at the interface between the Cr-based carbonitride and the ferrite matrix phase in the middle of press working, and some of the voids were connected to nearby voids. It was confirmed that it was growing into a micro crack.

On the other hand, in the metal structure after press working of the steel sheet, which was able to be pressed without cracking in the previous test, voids were formed at the interface between the Cr-based carbonitride and the ferrite matrix phase, but the occurrence of microcracks due to the connection of the voids Not confirmed.

In view of the above, the inventors believe that the superiority or inferiority of the pour formability is greatly influenced by the metal structure of the steel sheet, and press working of both the steel sheet with cracks in the previous press working test and the steel sheet that could be formed without cracking. The previous metal structure was examined and a detailed comparison between the two was performed.

As a result, although both metal structures were ferrite structures in which Cr-based carbonitrides were dispersed, it was found that the distance between Cr-based carbonitrides tended to be relatively short in the steel sheet in which cracks occurred in the previous press working test.

Then, the inventors paid attention to the relationship between the casting formability and the distance between Cr-based carbonitrides, and repeated experiments and examinations. As a result, it was confirmed that there is a correlation between the extrusion formability and the average distance between Cr-based carbonitrides having a certain size or more. In particular, by setting the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 μm or more to 3.0 μm or more, excellent injection moldability was obtained. Specifically, excellent pour 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 was found that a member requiring particularly high pour formability, such as an exhaust duct, can be press formed without cracking.

Here, the inventors consider the reason why excellent pour formability is obtained by increasing the average distance between Cr-based carbonitrides of 0.05 μm or more in equivalent circle diameter as follows.

That is, when processing a steel sheet, voids are generated at the interface between the ferrite parent phase and the Cr-based carbonitride in the metal structure as the amount of strain increases. These voids increase and grow with an increase in deformation amount and/or increase in stress concentration, become cracks by connecting with other adjacent voids, and finally lead to fracture of the steel sheet.

In this way, voids grow by receiving stress concentration along with an increase in deformation amount, and grow into microcracks by connecting with nearby voids. In particular, in deformation under multiaxial stress in which stress acts in two or three dimensions, the growth of cracks is further encouraged by increasing the degree of triaxial stress. In the case of deformation under such multiaxial stress, voids tend to grow compared to deformation under uniaxial stress (deformation under uniaxial stress is used for evaluation of elongation and is represented by a tensile test). Therefore, it is considered that the fracture limit of the material is lowered (in other words, it becomes easier to fracture) compared to under uniaxial stress.

Stretch forming is usually deformation under multiaxial stress, and since omnidirectional void connection in the steel sheet tends to occur, fracture is more likely to occur than deformation under uniaxial stress.

For this reason, even in a steel sheet that exhibits high elongation at break in deformation under uniaxial stress such as in a tensile test, if the average distance between Cr-based carbonitrides of a certain size or more is short, void connection occurs in deformation under multiaxial stress, and void connection occurs. The occurrence of micro cracks caused by and their progress is encouraged.

On the other hand, if the average distance of Cr-based carbonitrides having a certain size or more is sufficiently long, even in the case of carrying out injection molding in which deformation under multiaxial stress is performed, it is difficult to cause void connection, and for this reason, microscopic microstructures resulting from void connection The occurrence of cracks and their propagation are suppressed.

For this reason, the inventors believe that the casting formability is greatly improved by lengthening the average distance between Cr-based carbonitrides of 0.05 μm or more in equivalent circle diameter.

In addition, as a result of further examination by the inventors, in order to set the average distance between Cr-based carbonitrides of 0.05 μm or more in equivalent circle diameter to 3.0 μm or more, hot-rolled sheet annealing is performed by holding in a predetermined temperature range for a predetermined time or longer, , The metal structure after hot-rolled sheet annealing is once made into a ferrite single-phase structure in which Cr-based carbonitride is precipitated, and then, in cold-rolled sheet annealing after cold rolling,

(1) Slowing the heating rate from 500 ° C. to a heating temperature to simultaneously promote aggregation and coarsening of Cr-based carbonitrides and solid solution of Cr-based carbonitrides in the ferrite phase (in addition, Cr-based carbonitrides The solid solution in the ferrite phase is a phenomenon in which Cr-based carbonitride is decomposed into Cr, carbon and nitrogen in an atomic unit, and each element is contained in the ferrite phase),

(2) Properly controlling the heating temperature and holding time to further promote the solid solution of Cr-based carbonitride in the ferrite phase, and

(3) Increasing the cooling rate from the heating temperature to 500°C to suppress re-precipitation of the dissolved Cr-based carbonitride

This is important, and it was found that the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 μm or more can be set to 3.0 μm or more by simultaneously satisfying all of these conditions.

The present invention was completed after further examination based on the above findings.

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

1. In mass %,

C: 0.025 to 0.050%,

Si: 0.10 to 0.40%,

Mn: 0.45 to 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%

While having a component composition in which the balance consists of Fe and unavoidable impurities,

The average distance between Cr-based carbonitrides having an equivalent circle diameter 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. A steel material having the component composition described in 1 above is hot-rolled to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is subjected to hot-rolled sheet annealing at a heating temperature of 800 to 900°C and a holding time of 1 hour or more, followed by cold rolling and cold rolling steel sheet, and then, 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 annealing, the average heating rate at 500 ° C. to heating temperature is 20 ° C. / s or less, and the average cooling rate at heating temperature to 500 ° C. is 10 ° C. / s or more, ferrite A method for manufacturing a stainless steel sheet.

ADVANTAGE OF THE INVENTION According to this invention, while having sufficient corrosion resistance, the ferritic stainless steel sheet excellent in pour formability is obtained.

In addition, the use of the ferritic stainless steel sheet of the present invention is very advantageous industrially because it is possible to manufacture by press forming a member requiring particularly high pour formability, such as an exhaust duct.

1 is a photograph of the metal structure of No. 1 of Example.
Fig. 2 is a photograph of the metal structure of No. 12 of Example.

Hereinafter, the present invention will be described in detail.

First, the component composition of the ferritic stainless steel sheet of the present invention will be described. In addition, all units in the component composition are "mass %", but hereinafter, unless otherwise specified, it is simply expressed as "%".

C: 0.025 to 0.050%

C is an element effective in promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of ridging. From the viewpoint of obtaining such an effect, the C content is made 0.025% or more.

However, when the C content exceeds 0.050%, the precipitation amount of Cr-based carbonitrides increases excessively during hot rolling and annealing of the hot-rolled sheet, making it difficult to lengthen the average distance between Cr-based carbonitrides. Therefore, at the time of injection molding, it is not possible to prevent cracks caused by the connection of voids and breakage due to propagation, so that desired injection molding properties cannot be obtained. Also, the steel is hardened excessively and the ductility is lowered.

Therefore, C content is made into the range of 0.025 to 0.050%. The lower limit of the C content is preferably 0.030%, more preferably 0.035%. Moreover, the upper limit of C content is preferably 0.045%.

Si: 0.10 to 0.40%

Si is an element that acts as a deoxidizer during steel melting. From the viewpoint of obtaining such an effect, the Si content is made 0.10% or more.

However, when the Si content exceeds 0.40%, the steel hardens excessively and the rolling load during hot rolling increases. Moreover, the ductility of the steel sheet obtained after cold-rolled sheet annealing falls.

Therefore, Si content is made into the range of 0.10 to 0.40%. The lower limit of the Si content is preferably 0.20%. The upper limit of Si content is preferably 0.30%.

Mn: 0.45 to 1.00%

Mn, like C, is an element effective in accelerating the formation of an austenite phase and suppressing the occurrence of leasing. From the viewpoint of obtaining such an effect, the Mn content is made 0.45% or more.

However, when the Mn content exceeds 1.00%, the steel hardens excessively and the rolling load during hot rolling increases. Moreover, the ductility of the steel sheet obtained after cold-rolled sheet annealing falls.

Therefore, Mn content is made into 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 encourages grain boundary destruction by grain boundary segregation. Therefore, the one with less P content is preferable, and makes an upper limit into 0.04 %. Preferably it is 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 desorption of P causes an increase in cost. Therefore, it is preferable to make the lower limit of P content into 0.005 %.

S: 0.010% or less

S is an element that exists in steel as sulfide-based inclusions such as MnS and deteriorates ductility, corrosion resistance, and the like, and especially when the S content exceeds 0.010%, the adverse effect occurs remarkably. Therefore, the one where S content is as low as possible is preferable, and the upper limit of S content is made into 0.010%. Preferably it is 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 desorption of 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 an effect of improving corrosion resistance by forming a passivation film on the surface of a steel sheet. From the viewpoint of obtaining such an effect, the Cr content is made 16.0% or more.

However, when the Cr content exceeds 18.0%, the amount of formation of the austenite phase during hot rolling decreases, and there is a possibility that the resistance to hardening may deteriorate.

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, like Si, is an element that acts as a deoxidizer. 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, which tends to cause deterioration in surface properties.

Therefore, Al content is made into 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%

N, like C and Mn, is an element effective in promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of ridging. From the viewpoint of obtaining such an effect, the N content is made 0.025% or more.

However, when the N content exceeds 0.060%, the ductility of the steel sheet obtained after cold-rolled sheet annealing is significantly reduced. In addition, the precipitation amount of Cr-based carbonitrides during hot rolling and hot-rolled sheet annealing becomes excessively large, making it difficult to lengthen the average distance between Cr-based carbonitrides. For this reason, in the case of performing pour molding, it is not possible to prevent breakage due to generation and propagation of cracks resulting from the connection of voids, and desired pour moldability cannot be obtained.

Therefore, N content is made into 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 formation of an austenite phase during hot rolling, and has an effect of improving hardening resistance. Moreover, Ni is an element effective also for the improvement of corrosion resistance. From the viewpoint of obtaining such an effect, the Ni content is made 0.05% or more. However, when the Ni content exceeds 0.60%, the steel hardens excessively and formability deteriorates. Therefore, Ni content is made into 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%.

In addition, 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 ferritic stainless steel sheet of the present invention is a structure mainly composed of a ferrite phase, specifically, has a ferrite phase of 90% or more in volume ratio with respect to the entire structure, and the remaining structure other than the ferrite phase is relative to the entire structure It becomes a structure whose volume ratio is 10% or less. Alternatively, single-phase ferrite may be used. In addition, as a residual structure|tissue, a martensite phase is mainly mentioned, and the volume ratio of a precipitate and an inclusion shall not be included.

Here, the volume ratio of the ferrite phase is determined by making a test piece for cross-section observation from a stainless steel plate, performing an etching treatment with a saturated picric acid-5% by mass aqueous hydrochloric acid solution after mirror polishing, and then arbitrarily at the position of 1/4 of the plate thickness. 10 fields of view are observed with an optical microscope at a magnification of 100, and after distinguishing martensite phase and ferrite phase from the shape of the metal structure, the volume ratio of the ferrite phase is obtained by image processing, and the average value is calculated. .

In the metal structure of the ferritic stainless steel sheet of the present invention, as described above, it is important to set the average distance between Cr-based carbonitrides of 0.05 μm or more in equivalent circle diameter precipitated in the steel to 3.0 μm or more.

Here, the equivalent circle diameter is

Image processing is performed on a digital photograph (magnification: 500 times) in which Cr-based carbonitride appears on the metal structure of the test piece for cross-sectional observation described above, and the area of the Cr-based carbonitride is measured,

From the area of the measured Cr-based carbonitride, a circle diameter calculated on the assumption that the shape of the Cr-based carbonitride is a perfect circle (= {(4 × [area of the measured Cr-based carbonitride])/π} ^0.5 )

means

Average distance between Cr-based carbonitrides of 0.05 μm or more in equivalent circle diameter: 3.0 μm or more

When the average distance of Cr-based carbonitrides with an equivalent circle diameter of 0.05 μm or more is increased to 3.0 μm or more, even in the case of carrying out injection molding that deforms under multiaxial stress, it is difficult to form a connection between voids, and as a result, voids Generation of microcracks resulting from connection and their propagation are suppressed.

For this reason, the average distance of the Cr-based carbonitride having an equivalent circle diameter of 0.05 μm or more is 3.0 μm or more. Preferably it is 4.0 micrometers or more. The upper limit is not particularly limited, and is usually about 6.0 µm.

In addition, Cr-based carbonitrides with an equivalent circle diameter of less than 0.05 μm are not targeted, since very fine Cr-based carbonitrides with an equivalent circle diameter of less than 0.05 μm have a small area in contact with the parent ferrite phase, This is because, even if plastic deformation is applied, voids are hardly generated at the interface between the ferrite phase and the Cr-based carbonitride, and therefore, the influence on the formability, particularly the pour formability, is almost negligible.

In addition, Cr-based carbonitride here is a general term for Cr carbide and Cr nitride. Examples of the Cr carbide include Cr ₂₃ C ₆ , and examples of the Cr nitride include Cr ₂ N . Incidentally, those in which a part of Cr in Cr carbides and Cr nitrides is substituted with elements such as Fe and Mn are included in the Cr-based carbonitrides herein.

In addition, for Cr-based carbonitrides with an equivalent circle diameter of 0.05 μm or more, the voids generated with the increase in the amount of deformation are mainly generated at the interface between the ferrite parent phase and the Cr-based carbonitride with an equivalent circle diameter of 0.05 μm or more. This is because the distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 μm or more particularly affects the connection of voids and, consequently, the extrusion formability.

In addition, the size of the Cr-based carbonitride is usually about 0.5 μm in equivalent circle diameter.

In addition, the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 µm or more was measured as follows.

That is, the rolled parallel cross section of the steel sheet is etched with a picric acid saturated hydrochloric acid solution after mirror polishing to make a metal structure appear, and a piece of the metal structure at a position of 1/4 the sheet thickness is photographed with an optical microscope at a magnification of 500 times. .

In addition, it can be confirmed that the precipitate grasped in the metallographic photograph is a Cr-based carbonitride by performing component analysis of the precipitate by energy dispersive X-ray spectroscopy under a scanning electron microscope. Specifically, the peak of Cr in the element spectrum obtained from the precipitate by energy dispersive X-ray spectroscopy is higher than the peak of Cr in the element spectrum obtained from the parent phase by the same method, and each element of the precipitate In the quantitative analysis value of each element calculated from the spectral intensity ratio of , when the main components of the precipitate are Cr, Fe, C and N, the precipitate can be determined to be a Cr-based carbonitride.

Next, in the obtained metallographic photograph, any Cr-based carbonitride having an equivalent circle diameter of 0.05 μm or more (hereinafter also referred to as a reference carbonitride) is selected, and the distance from the reference carbonitride is decreased in order of decreasing equivalent circle diameter. Ten Cr-based carbonitrides (also referred to as target carbonitrides) of 0.05 µm or more are selected, and the distance between the reference carbonitride and each target carbonitride (distance between centers) is measured on a metal structure photograph.

This measurement is carried out 20 times with the reference carbonitride arbitrarily changed, and the average distance between the Cr-based carbonitrides is obtained by arithmetic averaging the distances between all measured reference carbonitrides and the target carbonitride.

In addition, the above measurement is not limited to a single ferrite grain, and may span grain boundaries. In addition, in order to perform a representative measurement, each measurement point is sufficiently far from each other so that the reference carbonitride and target carbonitride selected in the previous measurement do not become the reference carbonitride or target carbonitride in other measurements. choose

In this way, the ferritic stainless steel sheet of the present invention has the above-described component composition, and by setting the average distance between Cr-based carbonitrides of 0.05 μm or more in equivalent circle diameter to 3.0 μm or more, based on the forming limit diagram (FLD) When the minimum value of the maximum logarithmic strain of the molding limit determined by the above is set to 0.20 or more, preferably 0.23 or more, excellent pour formability can be obtained.

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

Next, the manufacturing method of the ferritic stainless steel sheet of this invention is demonstrated.

In the ferritic stainless steel sheet of the present invention, a steel material having the above component composition is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is subjected to hot-rolled sheet annealing at a heating temperature of 800 to 900°C and a holding time of 1 hour or more. , 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 annealing, 500 It can manufacture by making the average heating rate in °C - heating temperature 20 °C/s or less, and also making the average cooling rate in heating temperature - 500 °C 10 °C/s or more.

That is, the ferritic stainless steel sheet according to one embodiment of the present invention is a ferritic stainless steel cold rolled annealed steel sheet.

First, molten steel having the above component composition is melted by a known method such as a converter, an electric furnace, or a vacuum melting furnace, and used as a steel raw material (slab) by a continuous casting method or an ingot-breaking method.

Next, after heating the obtained steel material, preferably at 1100 to 1250°C for 1 to 24 hours, or directly heating a high-temperature slab, the steel material is hot-rolled to obtain a hot-rolled steel sheet. In addition, about hot rolling conditions, it is good to follow a conventional method.

Next, the obtained hot-rolled steel sheet is subjected to hot-rolled sheet annealing under the following conditions.

<Heating temperature of hot-rolled sheet annealing: 800 to 900 ° C., holding time: 1 hour or more>

When the coiling temperature during hot rolling is high, the metal structure of the hot-rolled steel sheet is a metal structure in which a ferrite phase and a ferrite phase formed by decomposition of an austenite phase formed at a high temperature are layered, and a coiling temperature during hot rolling is low. In this case, a metal structure in which a ferrite phase and a martensite phase formed by transformation of an austenite phase formed at high temperature are laminated in layers are formed.

In addition, when the coiling temperature is high, Cr-based carbonitrides precipitated along with decomposition of the austenite phase are unevenly distributed in the vicinity of the ferrite phase generated by decomposition of the austenite phase, and the distribution of the Cr-based carbonitride in the entire metal structure is inhomogeneous

In addition, the coiling temperature is not particularly limited, but when the coiling temperature is set to 450°C to 500°C, a significant decrease in toughness of the hot-rolled steel sheet due to embrittlement at 475°C may occur. Therefore, it is preferable to make the coiling temperature into more than 500 degreeC or less than 450 degreeC. Further, from the viewpoint of obtaining a predetermined metal structure more easily after hot-rolled sheet annealing, it is advantageous to sufficiently precipitate Cr-based carbonitride after hot rolling and before hot-rolled sheet annealing. For this reason, the coiling temperature is more preferably set to 600° C. or higher, where decomposition into Cr-based carbonitride and ferrite phase of the austenite phase is more promoted.

By subjecting a hot-rolled steel sheet having such a metal structure to hot-rolled sheet annealing held at a temperature range of 800 to 900 ° C. for 1 hour or more, recrystallization and precipitation of Cr-based carbonitrides occur in the metal structure, and the obtained after annealing the hot-rolled sheet In the steel sheet, a metal structure in which Cr-based carbonitride is sufficiently and uniformly dispersed in a single phase ferrite structure is obtained.

Here, when the heating temperature of the hot-rolled sheet annealing is set to less than 800°C, aggregation and coarsening of Cr-based carbonitride and solid solution to the ferrite phase become insufficient, and a predetermined metal structure cannot be obtained. In addition, recrystallization becomes insufficient, and a layered structure formed during hot rolling remains, particularly in the central portion of the sheet thickness. Therefore, after annealing the cold-rolled sheet, a non-uniform metallographic structure having prominent full-length grains is generated in the central portion of the sheet thickness, and there is a possibility that the scratch resistance may be lowered.

On the other hand, when the hot-rolled sheet annealing temperature exceeds 900°C, the austenite phase is regenerated during maintenance of the hot-rolled sheet annealing, and the Cr-based carbonitride precipitated in the hot rolling step dissolves into the austenite phase. For this reason, Cr-based carbonitrides cannot be sufficiently precipitated in the metal structure of the steel sheet obtained after annealing the hot-rolled sheet. Also, during cooling of the annealing of the hot-rolled sheet, a decomposition reaction of the austenite phase into a ferrite phase and Cr-based carbonitride occurs. As a result, the metal structure after the annealing of the hot-rolled sheet becomes a ferrite phase generated by decomposition of the ferrite phase and the austenite phase, in short, a mixed structure of the ferrite phase in which a large amount of Cr-based carbonitride is distributed around it, and Cr-based carbonitride The distribution of becomes non-uniform. For this reason, even if cold-rolled sheet annealing is performed under predetermined conditions in a subsequent step, a region in which the average distance between Cr-based carbonitrides is not sufficient is locally generated, and the predetermined pour formability is not obtained.

Therefore, the heating temperature in hot-rolled sheet annealing is in the range of 800 to 900°C. Preferably it is the range of 800-860 degreeC.

In addition, when the holding time in hot-rolled sheet annealing is less than 1 hour, precipitation of Cr-based carbonitrides becomes insufficient, and even if cold-rolled sheet annealing is performed under predetermined conditions in a subsequent step, the average between Cr-based carbonitrides The distance cannot be made sufficiently long, and the predetermined ejection moldability cannot be obtained either.

Therefore, the holding time in hot-rolled sheet annealing is 1 hour or more. Preferably it is 3 hours or more, More preferably, it is 5 hours or more. In addition, there is no particular limitation on the upper limit of the holding time, but it is preferably 24 hours or less from the viewpoint of productivity.

Next, the steel sheet (hot-rolled annealed steel sheet) obtained after the hot-rolled sheet annealing is pickled as necessary, and cold-rolled to obtain a cold-rolled steel sheet. Cold rolling is preferably performed at a reduction ratio of 50% or more from the viewpoints of elongation, bendability and shape correction. Further, within a range that satisfies the cold-rolled sheet annealing conditions described later, the cold-rolled sheet annealing may be repeated twice or more. Further, in order to improve the surface properties, grinding or polishing may be applied to the steel sheet obtained after hot-rolled sheet annealing.

Cold-rolled sheet annealing is given to the cold-rolled steel sheet obtained in this way under the following conditions.

<Average heating rate at 500°C to heating temperature of cold-rolled sheet annealing: 20°C/s or less>

Cold-rolled sheet annealing is a process for recrystallizing the rolled structure formed by cold rolling and making the average distance between Cr-based carbonitrides sufficiently long. To do so, the average heating rate at a heating temperature of 500 ° C. It is important to set it below ℃/s.

That is, when the average heating rate at a heating temperature of 500 ° C. is lowered, the driving force for recrystallization becomes smaller, so the temperature at which recrystallization starts becomes higher, and dislocations or shear bands introduced by cold rolling are maintained up to a higher temperature.

In addition, in a high temperature range close to the heating temperature, aggregation and coarsening of Cr-based carbonitrides generated during annealing of the hot-rolled sheet (each Cr-based carbonitride increases while the volume ratio remains almost constant, and the number density of Cr-based carbonitrides increases) shrinking phenomenon) and solid solution to the ferrite phase occurs. This aggregation and coarsening is rate-limited to the diffusion of Cr, which is a major constituent element of Cr-based carbonitrides. As described above, when the dislocation or shear zone is maintained up to a high temperature, high-speed diffusion of Cr occurs through the dislocation or shear zone, and aggregation and coarsening of Cr-based carbonitrides are promoted.

In addition, the reason that the solid solution of Cr-based carbonitride occurs in a high temperature range close to the heating temperature is due to the increase in the upper limit (solid solution limit) of C and N that can be dissolved in the ferrite phase. The average distance between Cr-based carbonitrides increases due to the effect of accelerating the aggregation and coarsening of these Cr-based carbonitrides and the solid solution to the ferrite phase. That is, by controlling the heating rate slowly, specifically, by controlling the average heating rate at a heating temperature of 500°C 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 at a heating temperature of 500°C to 20°C/s 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, resulting in cold rolling. Processed structures such as introduced dislocations or shear zones are replaced with recrystallized grains in the low temperature region. As a result, the effect of accelerating the aggregation and coarsening of Cr-based carbonitrides is insufficient, and the average distance between Cr-based carbonitrides in the steel sheet obtained after cold-rolled sheet annealing is shortened, so desired pour formability cannot be obtained.

Therefore, the average heating rate at a heating temperature of 500°C to 20°C/s is set to 20°C/s or less. Preferably it is 15 degrees C/s or less, More preferably, it is 12 degrees C/s or less. In addition, the lower limit of the average heating rate is not particularly limited, but if the heating rate is too slow, productivity will decrease, so it is preferably set to 1°C/s or more.

In addition, the control of the heating rate can be controlled by setting the furnace temperature or the sheet passing speed of the continuous annealing line, for example, in the case of the continuous annealing method.

The reason why the controlled temperature range is 500°C or higher is that recovery and recrystallization do not occur in a temperature range lower than 500°C.

<Heating temperature of cold-rolled sheet annealing: 800 to 900 ° C., holding time: 5 to 300 seconds>

The upper limit (limit of solid solution) of C and N that can be dissolved in the ferrite phase increases as the temperature increases. In cold-rolled sheet annealing, by setting the heating temperature: 800 to 900 ° C. and the holding time: 5 to 300 seconds, some of the Cr-based carbonitrides generated during hot-rolled sheet annealing are dissolved in the ferrite phase, and the number density of Cr-based carbonitrides is reduced. By doing so, the average distance between Cr-based carbonitrides can be increased.

For this reason, the heating temperature in cold-rolled sheet annealing is 800-900 degreeC, and holding time is 5-300 second. Preferably, the heating temperature in cold-rolled sheet annealing is 800 to 860°C, and the holding time is 15 seconds to 180 seconds.

In addition, the retention time here is the retention time in the temperature range of +/-10 degreeC of heating temperature.

Here, when the heating temperature is less than 800°C, the solid solution dissolution of C and N in the ferrite phase is not sufficiently high, the amount of Cr-based carbonitride dissolved in the ferrite phase is reduced, and the distance between the Cr-based carbonitrides becomes short. . As a result, the desired ejection moldability cannot be obtained. In addition, non-recrystallized grains remain and ductility is greatly reduced.

On the other hand, when the heating temperature exceeds 900°C, an austenite phase is generated during holding, and the austenite phase is transformed into a martensite phase during subsequent cooling to significantly harden the steel sheet. In addition, the metal structure of the final product sheet becomes a two-phase structure of ferrite phase and martensite phase, the plastic deformability is remarkably lowered, and the desired ejection moldability is not obtained.

In addition, if the holding time is less than 5 seconds, the solid solution of the Cr-based carbonitride in the ferrite phase during the holding is incomplete, the distance between the Cr-based carbonitrides is shortened, and the desired pour formability is not obtained. Also, since non-recrystallized grains remain, ductility is greatly reduced.

On the other hand, when the holding time exceeds 300 seconds, the crystal grains are markedly coarsened and the glossiness of the steel sheet is lowered, 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 cooling after the holding described above, re-precipitation of Cr-based carbonitride occurs. That is, during maintenance of cold-rolled sheet annealing, Cr-based carbonitride is dissolved in the ferrite matrix phase. As a result, dissolved C and N in the ferrite phase increase, supersaturate the ferrite phase during cooling, and re-precipitate as Cr-based carbonitrides.

For this reason, from the viewpoint of lengthening the average distance of Cr-based carbonitrides, re-precipitation of Cr-based carbonitrides is suppressed by increasing the cooling rate in the temperature range of 500 ° C. or higher, which is the precipitation temperature range of Cr-based carbonitrides, in particular, It becomes important to maintain a metal structure with a sufficiently long average distance between Cr-based carbonitrides formed before cooling.

Here, when the average cooling rate is less than 10 ° C./s, re-precipitation of solid solution C and N in the ferrite phase generated during maintenance of cold-rolled sheet annealing as Cr-based carbonitrides cannot be sufficiently suppressed, so The average distance becomes short, and the desired ejection moldability is not obtained

Therefore, the average cooling rate at the heating temperature of cold-rolled sheet annealing to 500°C is 10°C/s or more. Preferably it is 15 degrees C/s or more, More preferably, it is 20 degrees C/s or more. The upper limit of the average cooling rate is not particularly limited, but the average cooling rate is preferably 200°C/s or less, since deformation may occur in the steel sheet when cooling is performed rapidly.

In addition, there is no particular limitation on the cooling method, and gas jet cooling, mist cooling, roll cooling, or the like can be used. Moreover, about cold-rolled sheet annealing, in order to obtain higher gloss, you may perform BA annealing (bright annealing).

Then, after the cold-rolled sheet annealing described above, pickling is performed as necessary to manufacture the above ferritic stainless steel sheet.

Example

Example 1

Components shown in Table 1 Molten steel (the remainder being Fe and unavoidable impurities) was melted by refining using a converter with a capacity: 150 ton and a vacuum oxygen decarburization (VOD) method, and then continuously cast to a width: 1000 mm and thickness: It was set as a 200 mm slab.

After the slab was heated at 1200°C for 1 hour, hot rolling was performed by 7 passes of rough rolling using a reverse rolling mill with 3 stands and 7 passes with a one-way rolling mill with 7 stands. Rolling was performed and coiling treatment was performed at about 750°C to obtain a hot-rolled steel sheet having a sheet thickness of about 5.0 mm.

Next, after subjecting these hot-rolled steel sheets to hot-rolled sheet annealing using the box annealing method under the conditions shown in Table 2, the surface was subjected to shot blasting treatment and descaling by pickling.

After cold-rolling the steel sheet obtained in this way to sheet thickness: 1.0 mm, cold-rolled sheet annealing was performed under the conditions shown in Table 2. In addition, cooling after holding was performed by gas jet cooling or mist cooling. In addition, after cooling, descaling treatment by pickling was performed.

Here, the retention time in Table 2 is the retention time in a temperature range of heating temperature ±10°C.

In addition, the average heating rate described in Table 2 is the average heating rate until reaching 500 degreeC - heating temperature. In addition, the average cooling rate described in Table 2 is the average cooling rate until reaching a heating temperature - 500 degreeC.

For the steel sheet thus obtained, identification of the metal structure, measurement of the volume fraction of ferrite, and measurement of the average distance between Cr-based carbonitrides with an equivalent circle diameter of 0.05 μm or more were performed.

Here, identification of the metal structure and measurement of the volume fraction of ferrite were carried out by the methods described above. That is, a test piece for cross-sectional observation is prepared from the obtained steel sheet, subjected to an etching treatment with a picric acid saturated hydrochloric acid solution, and then observed with an optical microscope at a magnification of 100 times for 10 fields of view at a position of 1/4 of the sheet thickness, After distinguishing the martensite phase from the ferrite phase from the shape of the metal structure, the volume ratio of the ferrite phase was determined in each visual field by image processing, and the average value was taken as the volume ratio of the ferrite phase. In addition, the volume ratio of precipitates and inclusions is excluded.

In addition, the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 μm or more was measured by the method described above.

These results are similarly shown in Table 2. Also, for reference, FIGS. 1 and 2 show metal structure photographs for No. 1 and No. 12 in Table 2 used for measuring the average distance between Cr-based carbonitrides.

In addition, evaluation of (1) pour-out moldability and (2) evaluation of corrosion resistance were performed by the following methods. The evaluation results are listed together in Table 2.

(1) Evaluation of extrusion moldability

A forming test based on ISO12004-2: 2008 was conducted with the rolling parallel direction, the rolling 45° direction, and the rolling perpendicular direction as the maximum logarithmic strain directions, respectively, of the obtained steel sheet, and a forming limit diagram (FLD) was created.

Specifically, a scribed circle with a diameter of 5 mm was marked on the surface of the steel sheet so that the distance between the marks was 1 mm, and the forming test under various conditions, that is,

・For forming limit under uniaxial stress, tensile test using JIS No. 5 tensile test piece,

· Regarding the molding limit in the plane deformation state, a bulging test using a test piece in which a bead was provided on the circumference after shearing at 130 mm in width and length,

· Regarding the forming limit under equibiaxial stress, a bulging test using a test piece blanked in a circular shape,

· Regarding the forming limit under unequal biaxial stress, a bulging test using test pieces blanked into ellipsoids with various ellipticities

were carried out, respectively, and the specimens before and after each test were photographed, respectively. Subsequently, 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 the photograph, and the strain imparted by each test was measured to create a forming limit diagram (FLD).

The minimum value of the maximum logarithmic strain of the molding limit was obtained from the obtained molding limit curve (FLD), and the case where the minimum value of the maximum logarithmic strain was 0.20 or more was passed (○), and the case of less than 0.20 was evaluated as rejected (×).

(2) Evaluation of corrosion resistance

From the obtained steel sheet, a test piece of 60 × 100 mm was taken, and the surface was polished with #600 emery paper. After that, the end face of the test piece was sealed and subjected to a salt spray cycle test specified in JIS H 8502.

Here, the salt spray cycle test is salt spray (35 ° C., 5 mass% NaCl, spraying time: 2 hours) → drying (60 ° C., 40% relative humidity, holding time: 4 hours) → wet (50 ° C., relative humidity ≥ 95%, retention time: 2 hours) was used as one cycle, and 8 cycles were performed.

The surface of the test piece after the salt spray cycle test was photographed, and 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 / test piece) Total surface area) × 100 (%)) was calculated.

Then, a case where the calculated rust generation area ratio was 10% or less was evaluated as pass (◎, particularly excellent), a case where more than 10% and 25% or less was regarded as pass (○), and a case where more than 25% was evaluated as disqualified (×).

In all of the examples of the invention, excellent pour moldability and excellent corrosion resistance were obtained.

In particular, in No.6 (Steel A6) containing 0.58% of Ni and No.8 (Steel A8) containing 17.8% of Cr, the rust area ratio in the salt spray cycle test was 10% or less (◎) As a result, more excellent corrosion resistance was obtained.

On the other hand, in Comparative Examples No. 12 (Steel B1) and No. 13 (Steel B2), although the production conditions were appropriate, the C content and N content exceeded the respective appropriate ranges, so the precipitation amount of Cr-based carbonitride was excessive. As a result, it was not possible to sufficiently secure an average distance between Cr-based carbonitrides, and the desired pour formability was not obtained.

In No. 14 (Steel B3), since the Si content exceeded the appropriate range, the steel sheet became hard, the plastic deformability decreased, and the desired pour formability was not obtained.

In No. 15 and No. 16, since the average heating rate in cold-rolled sheet annealing exceeded the appropriate range, a sufficient average distance between Cr-based carbonitrides could not be secured, and the desired pour formability was not obtained.

In No. 17 and No. 18, since the heating temperature of cold-rolled sheet annealing exceeds the appropriate range, an austenite phase is generated during maintenance of cold-rolled sheet annealing, and during cooling after holding, the austenite phase is transformed into martensite phase, , the steel sheet was significantly hardened. In addition, since the metal structure of the final product sheet became a two-phase structure composed of a ferrite phase and a martensite phase, the plastic deformability of the steel sheet was remarkably reduced. For this reason, the desired ejection moldability was not obtained.

In No.19 and No.20, since the heating temperature of cold-rolled sheet annealing is below the appropriate range, the average distance between Cr-based carbonitrides cannot be sufficiently secured, and the metal structure in which non-recrystallized grains remain, Desired ejection moldability was not obtained.

In No.21 and No.22, since the holding time of cold-rolled sheet annealing is less than the appropriate range, the average distance between Cr-based carbonitrides cannot be sufficiently secured, and the metal structure in which non-recrystallized grains remain, Desired ejection moldability was not obtained.

In No.23 and No.24, since the cooling rate of cold-rolled sheet annealing is below the appropriate range, Cr-based carbonitrides are re-precipitated in a large amount and finely during the cooling, so that the average distance between Cr-based carbonitrides can be sufficiently secured. Therefore, the desired ejection moldability could not be obtained.

In No. 28, since the heating temperature of hot-rolled sheet annealing is less than the appropriate range, aggregation and coarsening of Cr-based carbonitrides and solid solution to the ferrite phase in hot-rolled sheet annealing become insufficient, and the distribution of Cr-based carbonitrides of non-uniformity occurred. As a result, a region where the average distance between Cr-based carbonitrides is insufficient was locally formed, and desired extrusion moldability was not obtained.

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 non-uniform distribution of Cr-based carbonitrides in the metal structure after the hot-rolled sheet annealing occurred. As a result, a region where the average distance between Cr-based carbonitrides is insufficient was locally formed, and desired extrusion moldability was not obtained.

In No. 30, since the holding time of the hot-rolled sheet annealing is less than the appropriate range, the aggregation and coarsening of the Cr-based carbonitride in the annealing of the hot-rolled sheet and the solid solution to the ferrite phase become insufficient, and the distribution of the Cr-based carbonitride of non-uniformity occurred. As a result, a region where the average distance between Cr-based carbonitrides is insufficient was locally formed, and desired extrusion moldability was not obtained.

industrial applicability

The ferritic stainless steel sheet of the present invention is particularly advantageous for application requiring high pour formability during press forming, for example, exterior members, kitchen utensils, and tableware.

Claims (2)

in mass %,
C: 0.030 to 0.050%,
Si: 0.10 to 0.40%,
Mn: 0.45 to 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: 0.030 to 0.060% and
Ni: 0.05 to 0.60%
While having a component composition in which the balance consists of Fe and unavoidable impurities,
The average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 μm or more is 3.0 μm or more,
The minimum value of the maximum logarithmic strain of the forming limit based on the forming limit diagram is 0.20 or more,
In the salt spray cycle test specified in JIS H 8502, salt spray (35 ° C., 5 mass% NaCl, spraying time: 2 hours) → drying (60 ° C., 40% relative humidity, holding time: 4 hours) → wet (50 °C, relative humidity ≥ 95%, holding time: 2 hours) as one cycle, rust generation area ratio on the steel sheet surface when 8 cycles were performed ((rust generation area on the steel sheet surface/total area on the steel sheet surface) × A ferritic stainless steel sheet in which 100 (%)) is 25% or less. A method for producing the ferritic stainless steel sheet of claim 1,
A steel material having the component composition according to claim 1 is hot-rolled to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is subjected to hot-rolled sheet annealing at a heating temperature of 800 to 900°C and a holding time of 1 hour or more, and then cold-rolled to obtain a cold-rolled steel sheet. Then, 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 annealing, the average heating rate at 500 ° C. to heating temperature is 20 ° C. / s or less, and the average cooling rate at heating temperature to 500 ° C. is 10 ° C. / s or more, ferrite A method for manufacturing a stainless steel sheet.

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