TW202022127A - Ferritic stainless-steel sheet and method for manufacturing same - Google Patents

Abstract

The present invention is configured so as to have a predetermined component composition, and so that the average distance between Cr-based carbonitrides having an equivalent circle diameter of at least 0.05 [mu]m is at least 3.0 [mu]m, and the smallest value of the maximum logarithmic strain at the forming limit based on a forming limit diagram is at least 0.20.

Description

Fertilizer-based stainless steel steel plate and manufacturing method thereof

The present invention relates to a ferrous iron-based stainless steel plate having sufficient corrosion resistance and excellent formability, especially outstanding formability, and a method for manufacturing the same.

Stainless steel (SUS, Steel Use Stainless) 430 (16~18mass% of Cr) series ferrous iron stainless steel plate is economical and excellent in corrosion resistance, so it is suitable for construction materials, transportation equipment, home appliances, kitchen equipment and For various purposes such as automobile parts, its application range has been further expanded in recent years.

For steel plates used for these purposes, not only corrosion resistance is required, but also sufficient formability that can be processed into a predetermined shape by techniques such as press forming. As such a ferrite-based stainless steel sheet, for example, Patent Document 1 discloses "a ferrite-based stainless steel sheet with excellent formability, characterized in that it contains C: 0.02 to 0.06% and Si: 1.0 in terms of mass%. % Or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.005% or less, Ti: 0.005% or less, Cr: 11-30% or less, Ni: 0.7% or less, and more than The relationship between C content satisfies 0.06≦(C+N)≦0.12 and 1≦N/C, and N is contained in the way, and then the relationship with N content satisfies 1.5×10 ^-3 ≦(V×N)≦1.5×10 ^-2 The method contains V, and the remainder is composed of Fe and inevitable impurities.”

In addition, Patent Document 2 discloses "a ferrous iron-based stainless steel cold-rolled steel sheet with excellent press formability, characterized in that it contains C: 0.010 to 0.045%, N: 0.01 to 0.05%, and mass%. Mn: 1% or less, Cr: 13~20%, Al: 0.01% or less, and contains C and N so that the volume ratio v of the Cr carbonitride becomes 0.09% or less, and further contains Si: 0.4% or less, P: 0.05% or less, S: 0.010% or less, and the remainder It is composed of Fe and unavoidable impurities, and has a single ferrite iron with an average crystal grain size of 10μm or more, and 50 or less Cr carbonitrides dispersed per ferrite iron. organization".

[Prior Technical Literature]

[Patent Literature]

Patent Document 1: Japanese Patent No. 3584881

Patent Document 2: Japanese Patent No. 4682806

Patent Document 3: Japanese Patent No. 5884211

However, press forming is roughly divided into four forming modes: protrusion forming, deep drawing forming, extended flange forming, and bending forming. In recent years, the ferrite-based stainless steel is gradually being used in the following components: the forming mode of press forming is mainly for the prominent forming of the components, such as the outdoor cover of the circular transom used in the exhaust pipe or the ventilation opening. Components, and interior panel components that improve design or functionality by embossing. Therefore, it is desired to develop a ferritic-based stainless steel steel sheet having excellent outstanding formability that can be processed into such a member shape.

However, it cannot be said that the ferrous iron-based stainless steel sheets disclosed in Patent Documents 1 and 2 have sufficient protruding formability.

Therefore, the inventors have previously developed in Patent Document 3 "a ferrous iron-based stainless steel steel sheet containing C: 0.005 to 0.025% and Si: 0.02 in mass%. ~0.50%, Mn: 0.55~1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001~0.10%, Cr: 15.5~18.0%, Ni: 0.1~1.0%, N: 0.005~0.025% , And the remainder is composed of Fe and inevitable impurities, the elongation at break is more than 28%, the average r value is more than 0.75, and the minimum value of the maximum logarithmic strain based on the forming limit of FLD (forming limit line diagram) is 0.15 the above". As a result, compared with the ferrous iron-based stainless steel steel plates disclosed in Patent Documents 1 and 2, a ferritic-based stainless steel steel plate with greatly improved outstanding formability can be obtained.

However, if the ferrite-based stainless steel sheet of Patent Document 3 is to be formed into a member with extremely high outstanding formability as required for exhaust pipes, cracks may occur further. Therefore, the current situation requires outstanding formability Further improve.

The present invention was developed in view of the above-mentioned current situation, and its object is to provide a ferrite-based stainless steel sheet with sufficient corrosion resistance and excellent formability, and to provide an advantageous manufacturing method thereof.

Here, "sufficient corrosion resistance" means salt spray (35°C, 5 mass% NaCl, spray time: 2 hours) → drying (60°C, relative humidity 40%, holding time: 4 hours) → moistening (50℃, relative humidity ≧95%, retention time: 2 hours) is 1 cycle. When the salt spray cycle test specified in JIS H 8502 is carried out for 8 cycles, the rust area rate of the steel plate surface (rust on the steel plate surface Area/total area of steel plate surface)×100(%)) is 25% or less. In addition, the so-called "excellent outstanding formability" means that the minimum value of the maximum logarithmic strain of the maximum logarithmic strain of the forming limit determined based on the forming limit diagram (Forming Limit Diagram, also referred to as FLD) measured in accordance with ISO12004-2:2008 is 0.20 or more.

Then, the inventors have repeatedly carried out various researches to solve the above-mentioned problems. Research. First, the inventors prepared various ferrous iron-based stainless steel steel plates with different composition or manufacturing methods, and used these steel plates to perform press processing tests on members including equal biaxial protrusions and unequal biaxial protrusions. Generally speaking, it is considered that the one with higher elongation is superior in overhang formability. However, in this press working test, even steel plates with higher breaking elongation may crack. The test results indicate that the overhang formability The pros and cons may not only be determined by the elongation at break.

Therefore, the inventors prepared a steel plate that had cracks in the previous test. Using this steel plate, the pressure processing test was performed again under the same conditions. The cracks occurred in the previous test and the mold was just before the end of press-fitting ( (Bottom dead center + 2mm) stop press processing, take a test piece from the steel plate, and observe the metal structure in detail. Specifically, after the above-mentioned pressing process is stopped, the steel sheet is pulled out from the mold, and the cross section of the steel sheet is mirror-polished, and then corroded by a saturated picric acid-5 mass% hydrochloric acid aqueous solution to produce a metal structure observation Using the test piece, observe the test piece with a scanning electron microscope (secondary electron image) at a magnification of 500 times. As a result, it was confirmed that the steel plates with cracks in the previous tests were all in the middle of the press working. A large number of pores were formed at the interface between the Cr-based carbonitride and the ferrous iron matrix phase, and a part of the pores were connected with nearby pores to grow Tiny cracks.

In contrast, in the metal structure of the steel sheet that can be press-processed without cracks in the previous test, pores are formed at the interface between the Cr-based carbonitride and the ferrous iron matrix, but the pores cannot be confirmed The generation of tiny cracks caused by the connection between each other.

Based on the above situation, the inventors believe that the pros and cons of outstanding formability are largely affected by the metal structure of the steel sheet, and investigated the addition of the steel sheet that cracked in the previous press working test and the steel sheet that can be formed without cracks. Metal before pressing Organize and compare the two in detail. As a result, it was found that the metal structure of both of them had the ferrous iron structure of Cr-based carbonitrides dispersed, but in the steel plate that cracked in the previous press working test, the distance between the Cr-based carbonitrides tended to be shorter. .

Therefore, the inventors have repeatedly conducted experiments and studies focusing on the relationship between the prominent formability and the distance between the Cr-based carbonitrides. As a result, it was confirmed that the protrusion formability is correlated with the average distance between Cr-based carbonitrides of 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 protruding formability can be obtained. Specifically, it is possible to obtain excellent outstanding formability in which the minimum value of the maximum logarithmic strain of the forming limit determined based on the forming limit diagram (FLD) is 0.20 or more. As a result, it has been found that components such as exhaust pipes requiring extremely high protruding formability can be press-formed without cracks.

Here, the inventors consider the following as the reason why excellent protrusion formability can be obtained by extending the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 μm or more. That is, in the case of processing steel plates, as the amount of strain increases, pores are generated at the interface between the ferrite matrix phase and the Cr-based carbonitride in the metal structure. The pores increase and grow with the increase of the strain and/or the increase of the stress concentration, and are connected with other pores close to them to become cracks, and finally the steel plate is broken. In this way, the pores grow due to stress concentration as the amount of strain increases, and connect with nearby pores to grow into tiny cracks. In particular, in the deformation under multiaxial stress where the stress acts in two or three dimensions, the degree of triaxial stress increases, thereby further promoting the growth of cracks. In the case of deformation under such multiaxial stress, pores are easier to grow compared to deformation under uniaxial stress (deformation under uniaxial stress is used to evaluate elongation, represented by a tensile test). Therefore, it is considered that the failure limit of the material becomes lower than that under uniaxial stress (that is, it breaks easily). Extrusion forming is usually deformation under multiaxial stress. It is easy to produce omnidirectional pores in the steel plate. Therefore, it is easier to fracture than the deformation under uniaxial stress. Therefore, even if it is a steel plate showing a higher elongation at break in the deformation under uniaxial stress such as a tensile test, if the average distance between Cr-based carbonitrides of a certain size or more is short, it will be The deformation under stress also produces the connection of pores, and promotes the generation and development of micro cracks caused by the connection of pores. On the other hand, if the average distance of Cr-based carbonitrides of a certain size or more is sufficiently extended, even in the case of deformation under multiaxial stress, the connection of voids will be difficult to produce, so that the The generation and development of tiny cracks caused by the connection of pores. For this reason, the inventors believe that by extending the average distance between Cr-based carbonitrides having a circle-equivalent diameter of 0.05 μm or more, the protrusion formability is greatly improved.

In addition, the inventors have conducted further studies and found that in order to make the average distance between Cr-based carbonitrides with a circle equivalent diameter of 0.05 μm or more, 3.0 μm or more, annealing of hot-rolled sheets is performed in a predetermined temperature zone for a certain period of time or more. , The metal structure of the hot-rolled sheet after annealing is temporarily used as a single-phase structure of ferrous iron with Cr-based carbonitrides precipitated, and in the cold-rolled sheet annealing after cold rolling, the important thing is: (1) slow down by 500 The heating rate from ℃ to heating temperature promotes the aggregation and coarsening of Cr-based carbonitrides and the solid solution of Cr-based carbonitrides to the ferrous iron phase. (Furthermore, the so-called Cr-based carbonitrides dissolve in the ferrous iron The phase-based Cr-based carbonitrides are decomposed into Cr, carbon and nitrogen in atomic units, and each element is contained in the ferrous iron phase); (2) The heating temperature and holding time are appropriately controlled to further promote the Cr-based carbonitrides Solid dissolve in the ferrous iron phase; and (3) Speed up the cooling rate at the heating temperature to 500°C, and inhibit the re-precipitation of the solid dissolved Cr-based carbonitrides; by satisfying all these conditions at the same time, the equivalent circle diameter can be made The average distance between Cr-based carbonitrides of 0.05 μm or more is 3.0 μm or more. The present invention is based on the above-mentioned knowledge and completed after further research.

That is, the main structure of the present invention is as follows. 1. A ferrous iron-based stainless steel steel plate containing C: 0.025~0.050%, Si: 0.10~0.40%, Mn: 0.45~1.00%, P: 0.04% or less, S: 0.010% or less by mass%, Cr: 16.0~18.0%, Al: 0.001~0.010%, N: 0.025~0.060% and Ni: 0.05~0.60%, and the remainder is composed of Fe and inevitable impurities, and the equivalent circle diameter is 0.05 The average distance between Cr-based carbonitrides of 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 line diagram is 0.20 or more.

2. A method for manufacturing a ferrous iron-based stainless steel sheet, which hot-rolls a steel material having the composition described in 1 above to produce a hot-rolled steel sheet, and heats the hot-rolled steel sheet at a temperature of 800 to 900°C 、After annealing the hot-rolled sheet with a holding time of more than 1 hour, it is cold-rolled to produce a cold-rolled steel sheet. Secondly, the cold-rolled steel sheet is subjected to cold rolling with a heating temperature of 800 to 900°C and a holding time of 5 to 300 seconds Sheet annealing, and in the above-mentioned cold-rolled sheet annealing, the average heating rate from 500°C to the heating temperature is set to 20°C/s or less, and the average cooling rate from the heating temperature to 500°C is set to 10°C/s or more.

According to the present invention, it is possible to obtain a ferrite-based stainless steel steel plate having sufficient corrosion resistance and excellent outstanding formability. In addition, if the ferrous iron-based stainless steel plate of the present invention is used, it is possible to manufacture components such as exhaust pipes that require extremely high formability by press forming technology, which is extremely advantageous industrially.

Figure 1 is a photo of the metal structure of No. 1 of the embodiment.

Figure 2 is a photo of the metal structure of No. 12 of the embodiment.

Hereinafter, the present invention will be described in detail. First, the composition of the ferritic stainless steel sheet of the present invention will be described. In addition, the units in the composition of ingredients are all "mass%". Below, unless otherwise specified, only "%" is used.

C: 0.025~0.050%

C series is an effective element to promote the formation of austenitic iron phase during hot rolling and inhibit the formation of ridges. From the viewpoint of obtaining such an effect, the C content is set to 0.025% or more. However, if the C content exceeds 0.050%, the precipitation amount of Cr-based carbonitrides during hot-rolled and hot-rolled sheet annealing is excessive, making it difficult to extend the average distance between Cr-based carbonitrides. Therefore, when the protrusion is formed, it is impossible to prevent the generation and the progress of the crack caused by the connection of the pores from the fracture, and it is impossible to obtain the required protrusion formability. In addition, the steel becomes excessively hardened 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%. Furthermore, the upper limit of the C content is preferably 0.045%.

Si: 0.10~0.40%

Si is an element that functions as an oxygen scavenger when steel is melted. From the viewpoint of obtaining such an effect, the Si content is set to 0.10% or more. However, if the Si content exceeds 0.40%, the steel becomes excessively hardened and the rolling load during hot rolling increases. In addition, the ductility of the steel sheet obtained after annealing of the cold rolled sheet is reduced. Therefore, the Si content is set 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%

The Mn series is the same as C to promote the formation of austenitic iron phase and inhibit the formation of ridges. The element of effect. 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 hardened and the rolling load during hot rolling increases. In addition, the ductility of the steel sheet obtained after annealing of the cold rolled sheet is reduced. Therefore, the Mn content is set to 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: Below 0.04%

P is an element that promotes the destruction of grain boundaries caused by grain boundary segregation. Therefore, the P content is preferably smaller, and the upper limit is set to 0.04%. Preferably it is 0.03% or less. More preferably, it is 0.01% or less. The lower limit of P content is not particularly limited, but excessive de-P will result in an increase in cost. Therefore, the lower limit of the P content is preferably set to 0.005%.

S: Below 0.010%

S is an element that exists in steel as a sulfide-based intermediary such as MnS and reduces ductility and corrosion resistance. Especially when the S content exceeds 0.010%, its adverse effects are significant. Therefore, the S content is preferably extremely low, and the upper limit of the S content is set to 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 desulfurization will increase the cost. Therefore, the lower limit of the S content is preferably set to 0.001%.

Cr: 16.0~18.0%

Cr is an element that has the effect of forming a passive film on the surface of the 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 austenitic iron phase generated during hot rolling will decrease and the Nylon The risk of reduced ridgeliness. Therefore, the Cr content is set 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: 0.001~0.010%

Al is an element that functions as an oxygen scavenger like Si. From the viewpoint of obtaining such an effect, the Al content is set to 0.001% or more. However, if the Al content exceeds 0.010%, Al-based intermediaries such as Al ₂ O ₃ increase, which tends to cause a decrease in surface properties. Therefore, the Al content is set 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: 0.025~0.060%

Like C and Mn, N is an effective element to promote the formation of austenitic iron phase during hot rolling and inhibit the formation of ridges. From the viewpoint of obtaining such an effect, the N content is set to 0.025% or more. However, if the N content exceeds 0.060%, the ductility of the steel sheet obtained after the cold-rolled sheet annealing is greatly reduced. In addition, the precipitation amount of Cr-based carbonitrides during hot-rolled and hot-rolled sheet annealing is too large, and it is difficult to extend the average distance between Cr-based carbonitrides. Therefore, in the case of protruding molding, the generation and progress of cracks caused by the connection of pores cannot be prevented, and the required protruding formability cannot be obtained. Therefore, the N content is set to 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: 0.05~0.60%

Ni series can promote the formation of austenitic iron phase and increase the austenitic iron phase during hot rolling The amount of production, the element that improves the effect of ridge resistance. In addition, Ni is an element that is also effective in improving corrosion resistance. From the viewpoint of obtaining such an effect, the Ni content is set to 0.05% or more. However, if the Ni content exceeds 0.60%, the steel becomes excessively hardened and formability decreases. Therefore, the Ni content is set to 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%.

Furthermore, the components other than the above are Fe and inevitable impurities.

Next, the metal structure of the ferrous iron-based stainless steel sheet of the present invention will be described. The metallic structure of the ferrous iron-based stainless steel steel plate of the present invention is a structure mainly composed of ferrous iron phase, specifically, a ferrous iron phase having a volume ratio of 90% or more relative to the entire structure, and is defertilized The remaining part of the structure other than the iron grain phase is a structure whose volume ratio relative to the whole structure is 10% or less. Furthermore, it may be a single phase of ferrous iron. Furthermore, as the remaining part of the structure, the Asada scattered iron phase is mainly cited, excluding the volume ratio of precipitates and intermediaries. Here, the volume ratio of the ferrous iron phase is determined by the following method: a test piece for cross-sectional observation is made from a stainless steel steel plate, mirror-polished, and then etched with a saturated picric acid-5 mass% hydrochloric acid aqueous solution. Arbitrary 10 viewing angles at 1/4 position are observed with an optical microscope at a magnification of 100 times, and the Asada iron phase and the fat iron phase are distinguished according to the shape of the metal structure, and then the fat iron phase is obtained by image processing Volume rate, calculate the average value.

In addition, in the metallic structure of the ferrous iron-based stainless steel sheet of the present invention, as described above, it is important that the average distance between Cr-based carbonitrides with an equivalent circle diameter of 0.05 μm or more precipitated in the steel is 3.0 μm the above. Here, the “circle equivalent diameter” means that a digital photograph (magnification of 500 times) obtained by photographing the Cr-based carbonitrides present in the metal structure of the cross-sectional observation test piece is subjected to image processing to measure the Cr-based carbonitride The area of the compound is based on the measured area of the Cr-based carbonitride and the diameter of the circle calculated based on the assumption that the shape of the Cr-based carbonitride is a true circle (={(4×[measured Cr-based carbonitride The area of the substance])/π} ^0.5 ).

The average distance between Cr-based carbonitrides with an equivalent circle diameter of 0.05μm or more: 3.0μm or more

If the average distance of Cr-based carbonitrides with a circle-equivalent diameter of 0.05μm or more is extended to 3.0μm or more, even in the case of deformation under multiaxial stress, it is difficult to connect the voids to each other. , Inhibit the generation and progress of tiny cracks caused by the connection of pores. Therefore, the average distance of Cr-based carbonitrides with an equivalent circle diameter of 0.05 μm or more is set to 3.0 μm or more. Preferably it is 4.0 μm or more. In addition, the upper limit is not particularly limited, but is usually about 6.0 μm. Furthermore, Cr-based carbonitrides with a circle-equivalent diameter of less than 0.05μm are not targeted because the area where extremely fine Cr-based carbonitrides with a circle-equivalent diameter of less than 0.05μm are in contact with the ferrous iron as the parent phase Since it is small, even if plastic deformation by press working or the like is applied, there are almost no voids at the interface between the ferrite phase and the Cr-based carbonitride, so the impact on the formability, especially the outstanding formability, can be ignored. influences.

In addition, the so-called Cr-based carbonitride-based Cr carbide and Cr nitride are collectively referred to herein. Examples of Cr carbides include Cr ₂₃ C ₆ , and examples of Cr nitrides include Cr ₂ N. In addition, Cr carbides and Cr nitrides obtained by replacing a part of Cr with elements such as Fe or Mn are also included in the so-called Cr-based carbonitrides herein.

In addition, the Cr-based carbonitrides with an equivalent circle diameter of 0.05 μm or more are used as the target. The pores generated with the increase of the strain are mainly generated in the ferrite matrix and the Cr-based alloy with a circle equivalent diameter of 0.05 μm or more. The interface of carbonitrides, the distance between the Cr-based carbonitrides with an equivalent circle diameter of 0.05μm or more to connect the pores Even highlight formability has a special impact. Furthermore, the size of Cr-based carbonitrides is usually about 0.5 μm in circle equivalent diameter.

In addition, the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 μm or more is measured as follows. That is, after mirror-polishing the rolled parallel section of the steel sheet, the metal structure appears by etching with a picric acid-saturated hydrochloric acid solution, and a piece of metal structure at 1/4 of the plate thickness is taken with an optical microscope with a magnification of 500 times. Furthermore, the precipitates captured in the metal structure photograph are Cr-based carbonitrides, which can be confirmed by the following method: the composition analysis of the precipitates is performed 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 according to the precipitate In the quantitative analysis value of each element calculated from the spectral intensity ratio of each element of the substance, when the main components of the precipitate are Cr, Fe, C, and N, it can be judged that the precipitate is a Cr-based carbonitride. Secondly, in the obtained metal structure photos, select any Cr-based carbonitrides (hereinafter, also referred to as reference carbonitrides) with an equivalent circle diameter of 0.05μm or more, so that the distance from the reference carbonitrides is as close as Select 10 Cr-based carbonitrides (also called target carbonitrides) with an equivalent circle diameter of 0.05μm or more in the order of farthest, and measure the distance between the reference carbonitride and each target carbonitride (between centers) on the metal structure photo Distance). The reference carbonitrides are arbitrarily changed, the measurement is performed 20 times, and the distances between all the measured reference carbonitrides and the target carbonitrides are arithmetic averaged to obtain the average distance between the Cr-based carbonitrides. Furthermore, the above measurement is not limited to a single ferrite grain, and may also cross grain boundaries. In addition, in order to perform a representative measurement, each measurement location is selected 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. The part that is far away from each other Bit.

In this way, the ferrous iron-based stainless steel steel plate of the present invention contains the above-mentioned composition, and the average distance between Cr-based carbonitrides with an equivalent circle diameter of 0.05 μm or more is 3.0 μm or more, which will be based on the forming limit line diagram (FLD ) The minimum value of the maximum logarithmic strain of the determined forming limit is set to 0.20 or more, preferably 0.23 or more, so that excellent protrusion formability can be obtained.

In addition, the thickness of the ferrous iron-based stainless steel steel plate in one embodiment of the present invention is not particularly limited, and is, for example, 0.8 to 2.0 mm.

Next, the manufacturing method of the ferrite-based stainless steel sheet of the present invention will be described. The ferrous iron-based stainless steel sheet of the present invention can be manufactured by hot-rolling a steel material having the above composition to form a hot-rolled steel sheet, and heating the hot-rolled steel sheet at a temperature of 800 to 900°C and a holding time After annealing the hot-rolled sheet for more than 1 hour, the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet, and the cold-rolled steel sheet is subjected to cold rolling with a heating temperature of 800 to 900°C and a holding time of 5 to 300 seconds For sheet annealing, in the above-mentioned cold-rolled sheet annealing, the average heating rate from 500°C to the heating temperature is set to 20°C/s or less, and the average cooling rate from the heating temperature to 500°C is set to 10°C/s or more. That is, the ferrous iron-based stainless steel sheet of one embodiment of the present invention is a ferrous iron-based stainless steel cold-rolled and annealed steel sheet.

First, molten steel composed of the above-mentioned components is melted by a known method such as a converter, an electric furnace, and a vacuum melting furnace, and a steel material (slab) is made by a continuous casting method or a block-block method. Secondly, the obtained steel material is preferably heated at 1100 to 1250°C for 1 to 24 hours, or after the high-temperature steel billet is directly heated, the steel material is hot-rolled to produce a hot-rolled steel sheet. In addition, the hot rolling conditions may follow a conventional method. Next, the obtained hot-rolled steel sheet is annealed under the following conditions.

<Heat temperature for annealing of hot rolled sheet: 800~900℃, holding time: more than 1 hour>

The metal structure of the hot-rolled steel sheet, when the coiling temperature during hot rolling is high, is a layered layer with ferrous iron phase, and fertilizer particles generated by decomposing the austenitic iron phase generated at high temperature The metallic structure of the iron phase, when the coiling temperature during hot rolling is low, is a layered layered ferrous iron phase, which is formed by phase change with the austenitic iron phase generated at high temperature. Phase metal structure. Furthermore, when the coiling temperature is relatively high, near the fat iron phase generated by the decomposition of the austenitic iron phase, the Cr-based carbonitrides precipitated with the decomposition of the austenitic iron phase Uneven distribution, uneven distribution of Cr-based carbonitrides in the whole metal structure. In addition, the coiling temperature is not particularly limited. When the coiling temperature is set to 450°C to 500°C, the toughness of the hot-rolled steel sheet due to embrittlement at 475°C may be significantly reduced. Therefore, the coiling temperature is preferably more than 500°C or less than 450°C. Furthermore, from the viewpoint of obtaining a predetermined metal structure more easily after the hot-rolled sheet is annealed, it is advantageous that the Cr-based carbonitrides are fully analyzed in the stage after the hot-rolled and before the hot-rolled sheet is annealed. Therefore, the coiling temperature is more preferably set to 600° C. or higher that further promotes the decomposition of the austenitic iron phase into Cr-based carbonitrides and ferrous iron phases. By annealing the hot-rolled steel sheet with such a metal structure at a temperature range of 800 to 900°C for more than 1 hour, recrystallization and precipitation of Cr-based carbonitrides are generated in the metal structure. In the steel sheet obtained after the rolled sheet is annealed, a metallic structure in which the Cr-based carbonitrides are sufficiently and uniformly dispersed in the single-phase structure of the ferrous iron is obtained.

Here, when the heating temperature of the hot-rolled sheet annealing is less than 800°C, the aggregation and coarsening of the Cr-based carbonitrides and the solid solution to the ferrous iron phase are insufficient, and the predetermined metal structure cannot be obtained. In addition, the recrystallization is insufficient, and the layered structure formed during hot rolling particularly remains in the center of the plate thickness. Therefore, the thickness of the central part of the cold-rolled sheet will be produced after annealing. Produces a non-uniform metal structure with markedly stretched grains, and may reduce the ridge resistance. On the other hand, if the annealing temperature of the hot-rolled sheet exceeds 900°C, the austenitic iron phase will be formed again when the hot-rolled sheet is kept annealed, and the Cr-based carbonitrides precipitated in the hot rolling step will dissolve in the austenitic iron phase. . Therefore, in the metal structure of the steel sheet obtained after the hot-rolled sheet is annealed, the Cr-based carbonitride cannot be charged out. In addition, during the cooling of the hot-rolled sheet annealing, the austenitic iron phase is decomposed into a ferrous iron phase and a Cr-based carbonitride reaction. As a result, the metal structure of the hot-rolled sheet after annealing is a fat iron phase and a mixture of the fat iron phase generated by the decomposition of the austenitic iron phase, that is, the fat iron phase with a large amount of Cr-based carbonitrides distributed around it. Crystal structure, the distribution of Cr-based carbonitrides is uneven. Therefore, even if the cold-rolled sheet is annealed under the predetermined conditions in the subsequent steps, a region where the average distance between the Cr-based carbonitrides is insufficient is locally formed, and the predetermined outstanding formability cannot be obtained. Therefore, the heating temperature for annealing the hot-rolled sheet is set in the range of 800 to 900°C. It is preferably in the range of 800 to 860°C.

In addition, when the holding time of the hot-rolled sheet annealing is less than 1 hour, the precipitation of Cr-based carbonitrides is insufficient, and even if the cold-rolled sheet is annealed under predetermined conditions in the subsequent steps, the Cr-based carbonitrides cannot be fully extended. The average distance between carbonitrides still cannot achieve the established outstanding formability. Therefore, the holding time for annealing the hot rolled sheet is set to 1 hour or more. It is preferably 3 hours or more, more preferably 5 hours or more. In addition, the upper limit of the retention time is not particularly limited, but from the viewpoint of productivity, it is preferably 24 hours or less.

Next, the steel sheet (hot-rolled and annealed steel sheet) obtained after the hot-rolled sheet is annealed as necessary is pickled and cold-rolled to produce a cold-rolled steel sheet. From the viewpoints of elongation, bendability, and shape correction, cold rolling is preferably performed at a reduction ratio of 50% or more. In addition, the cold-rolled-cold-rolled sheet can be withdrawn within the range of satisfying the cold-rolled sheet annealing conditions described later Repeat the fire 2 more times. Furthermore, in order to improve the surface properties, the steel sheet obtained after annealing of the hot-rolled sheet may be ground or polished. The cold-rolled steel sheet thus obtained was subjected to cold-rolled sheet annealing under the following conditions.

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

Cold-rolled sheet annealing is a step used to recrystallize the rolled structure formed by cold rolling and to sufficiently extend the average distance between Cr-based carbonitrides. Therefore, it is important to set the average temperature from 500°C to the heating temperature The heating rate is set to 20°C/s or less. That is, if it slows down by 500. The average heating rate from ℃ to the heating temperature reduces the driving force for recrystallization, so the temperature at which recrystallization starts becomes higher, and the differential or shear band introduced by cold rolling is maintained to a higher temperature. In addition, in a high-temperature region close to the heating temperature, agglomeration and coarsening of the Cr-based carbonitrides produced during annealing of the hot-rolled sheet (each Cr-based carbonitride with a substantially constant volume ratio becomes larger, and the Cr-based carbonitride The phenomenon that the number density of the compound decreases) and the solid solution to the ferrous iron phase. This aggregation and coarsening restrict the diffusion rate of Cr, which is the main constituent element of Cr-based carbonitrides. As described above, if the differential or shear zone is maintained to a high temperature, high-speed diffusion of Cr through the differential or shear zone occurs, which promotes the aggregation and coarsening of Cr-based carbonitrides. Furthermore, the solid solution of Cr-based carbonitrides generated in a high temperature region close to the heating temperature is increased by the upper limit (solid solution limit) of C and N that are solid-soluble in the ferrite phase. The average distance between the Cr-based carbonitrides becomes longer due to the effect of promoting aggregation and coarsening of the Cr-based carbonitrides and the solid solution to the ferrite phase. That is, by slowing down the heating rate, specifically, by controlling the average heating rate from 500° C. to the heating temperature to 20° C./s or less, the average distance between the Cr-based carbonitrides can be extended. On the other hand, if the average heating rate from 500°C to the heating temperature exceeds 20°C/s, the driving force for recrystallization of the ferrous iron phase is too large, resulting in a lower temperature region in the heating process. The recrystallization of the iron phase of the raw fertilizer grains is replaced with recrystallized grains in the low temperature region by the processed structure such as the differential row or shear band introduced by cold rolling. As a result, the effect of promoting agglomeration and coarsening of Cr-based carbonitrides is insufficient, and the average distance between the Cr-based carbonitrides of the steel sheet obtained after annealing of the cold-rolled sheet becomes shorter, so that the desired outstanding formability 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. In addition, 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 it is preferably set to 1°C/s or more. Furthermore, the heating rate can be controlled by the setting of the furnace temperature or the plate passing speed of the continuous annealing line, for example in the case of the continuous annealing method. Also, setting the controlled temperature range to 500°C or higher is because no recovery or recrystallization occurs in the temperature range below 500°C.

<Heating temperature for cold-rolled sheet annealing: 800~900℃, holding time: 5~300 seconds>

The higher the temperature, the higher the upper limit (solid solution limit) of C and N that can be dissolved in the ferrite phase. In the cold-rolled sheet annealing, by setting the heating temperature to 800~900℃ and the holding time to 5~300 seconds, part of the Cr-based carbonitrides generated during the annealing of the hot-rolled sheet can be dissolved in the ferrous iron Phase, reduce the number density of Cr-based carbonitrides and extend the average distance between Cr-based carbonitrides. Therefore, the heating temperature of cold-rolled sheet annealing is set to 800~900°C, and the holding time is set to 5~300 seconds. Preferably, the heating temperature for annealing the cold rolled sheet is 800 to 860° C., and the holding time is 15 seconds to 180 seconds. Furthermore, the so-called holding time here is the residence time in the temperature range of the heating temperature ±10°C.

Here, if the heating temperature is less than 800°C, the solid solution limit of C and N in the ferrous iron phase is not fully expanded, the amount of Cr-based carbonitrides dissolved in the ferrous iron phase decreases, and the Cr-based carbon nitrogen The distance between objects becomes shorter. Therefore, it is impossible to obtain the required prominence Formability. In addition, unrecrystallized grains remained, and ductility was greatly reduced. On the other hand, if the heating temperature exceeds 900° C., an austenitic iron phase is generated during the holding, and during subsequent cooling, the austenitic iron phase is transformed into an astigmatite scattered iron phase, and the steel sheet is significantly hardened. In addition, the metal structure of the final product plate is a two-phase structure of a fat iron phase and an Asada scattered iron phase, and the plastic deformation ability is significantly reduced, and the required outstanding formability cannot be obtained.

In addition, if the holding time is less than 5 seconds, the Cr-based carbonitrides during the holding are not completely dissolved in the ferrite phase and the distance between the Cr-based carbonitrides becomes short, and the desired protrusion formability cannot be obtained. Furthermore, since unrecrystallized grains remain, the ductility is greatly reduced. On the other hand, if the holding time exceeds 300 seconds, the crystal grains are remarkably coarsened and the gloss of the steel sheet decreases, which is not good from the viewpoint of surface quality.

<Average cooling rate from heating temperature to 500℃ for annealing cold rolled sheet: 10℃/s or more>

During the cooling after the above-mentioned holding, re-precipitation of Cr-based carbonitrides occurs. That is, when the cold-rolled sheet is kept annealed, the Cr-based carbonitride is dissolved in the ferrite matrix. As a result, the solid solution C and N in the ferrite phase increase, and the ferrite phase becomes supersaturated during cooling and re-precipitates as Cr-based carbonitrides. Therefore, from the viewpoint of extending the average distance of Cr-based carbonitrides, it is particularly important to increase the cooling rate in the temperature range above 500°C, which is the precipitation temperature range of Cr-based carbonitrides, to suppress Cr-based carbonitrides The re-precipitation of the compounds maintains a metal structure with a very long average distance between the Cr-based carbonitrides formed before cooling. Here, when the average cooling rate is less than 10°C/s, it is not possible to sufficiently suppress the re-precipitation of solid solution C and N in the ferrous iron phase generated during annealing of the cold-rolled sheet as Cr-based carbonitrides. Therefore, the average distance between the Cr-based carbonitrides becomes shorter, and the required outstanding formability cannot be obtained. Therefore, the heating temperature of cold-rolled sheet annealing is to The average cooling rate at 500°C is set to 10°C/s or more. It is preferably 15°C/s or higher, and more preferably 20°C/s or higher. In addition, the upper limit of the average cooling rate is not particularly limited. However, in the case of rapid cooling, since the steel sheet may be strained, the average cooling rate is preferably 200°C/s or less.

In addition, the cooling method is not particularly limited, and gas jet cooling, spray cooling, roll cooling, or the like can be used. In addition, for cold-rolled sheet annealing, BA annealing (bright annealing) can be performed in order to require higher gloss. In addition, after the cold-rolled sheet is annealed, pickling is performed as necessary to manufacture the ferrous iron-based stainless steel sheet.

[Example]

[Example 1]

The molten steel with the composition shown in Table 1 (the remaining part is Fe and unavoidable impurities) is prepared by a converter with a capacity of 150 ton and a refining method using vacuum oxygen decarburization (VOD). Secondly, by Continuous casting produces billets with a width of 1000mm and a thickness of 200mm. After heating the billet at 1200°C for 1 hour, as hot rolling, 7-pass rough rolling using a reverse rolling mill composed of 3 stages and unidirectional rolling using 7-stage supports The machine consists of 7-way finish rolling and coiling at about 750°C to produce hot-rolled steel plates with a thickness of about 5.0mm. Next, the hot-rolled steel sheets were subjected to hot-rolled sheet annealing using the bell annealing method under the conditions described in Table 2, and then shot blasting and pickling were performed on the surface to remove rust. The steel sheet thus obtained was cold-rolled to a thickness of 1.0 mm, and then cold-rolled sheet annealing was performed under the conditions described in Table 2. Furthermore, cooling after holding is performed by gas jet cooling or spray cooling. In addition, after the cooling is completed, rust removal treatment is performed by pickling. Here, the holding time in Table 2 is the residence time in the temperature range of the heating temperature ±10°C. Furthermore, the average heating described in Table 2 The speed is the average heating speed from 500°C to the heating temperature. In addition, the average cooling rate described in Table 2 is the average cooling rate since the heating temperature reaches 500°C.

The steel plate thus obtained was subjected to the identification of the metal structure, the measurement of the volume ratio of the ferrite, and the measurement of the average distance between the Cr-based carbonitrides with an equivalent circle diameter of 0.05 μm or more. Here, the identification of the metal structure and the determination of the volume ratio of the ferrite are carried out by the aforementioned method. That is, a test piece for cross-sectional observation was prepared from the obtained steel plate, and after etching treatment with a picric acid saturated hydrochloric acid solution, 10 viewing angles at a position of 1/4 of the plate thickness were observed by an optical microscope at a magnification of 100 times. The morphology of the metal structure distinguishes the Asada scattered iron phase and the fat iron phase. The volume fraction of the fat iron phase is obtained from various viewing angles by image processing, and the average value is used as the volume fraction of the fat iron phase. Furthermore, the volume rate of precipitates and intermediaries is excluded. In addition, the measurement of the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 μm or more was also performed by the aforementioned method. These results are also shown in Table 2. In addition, for reference, the metal structure photographs used to measure the average distance between Cr-based carbonitrides of No. 1 and No. 12 of Table 2 are shown in FIGS. 1 and 2.

In addition, (1) evaluation of protrusion formability and (2) evaluation of corrosion resistance were performed by the following methods. Record the evaluation results in Table 2.

(1) Evaluation of outstanding formability

The rolling parallel direction, the rolling 45° direction, and the rolling right angle direction of the obtained steel sheet were respectively set as the maximum logarithmic strain direction, and the forming test according to ISO12004-2:2008 was performed to produce the forming limit line diagram (FLD). Specifically, marking a 5mm diameter scribed circle on the surface of the steel plate with a gauge length of 1mm, and performing forming tests under various conditions, namely, (a) For the forming limit under uniaxial stress, use Tensile test on JIS No. 5 tensile test piece; (b) For the formed pole under plane strain For the bulging test, use a test piece cut to 130mm square and then form a bead on the circumference; (c) For the forming limit under equal biaxial stress, use a blanking test to form a perfect circle The bulging test is carried out on the sheet; (d) For the forming limit under unequal biaxial stress, the test sheet punched into an ellipse with various ellipticities is used for the bulging test; photographs are taken of the test sheets before and after each test. Next, the shape change of the circular grid of the test piece before and after each test was quantitatively measured by the image processing of the photograph, and the strain imparted by each test was measured to create a forming limit line diagram (FLD). Calculate the minimum value of the maximum logarithmic strain of the forming limit based on the obtained forming limit line diagram (FLD). If the minimum value of the maximum logarithmic strain is 0.20 or more, it is evaluated as pass (○), and if it is less than 0.20, it is evaluated as unacceptable (× ).

(2) Evaluation of corrosion resistance

A 60×100mm test piece is obtained from the obtained steel plate, and the surface is polished with #600 emery paper. After that, the end surface of the test piece was sealed for the salt spray cycle test specified in JIS H 8502. Here, the salt spray cycle test is based on salt spray (35°C, 5 mass% NaCl, spray time: 2 hours) → drying (60°C, relative humidity 40%, retention time: 4 hours) → humidification (50°C, Relative humidity ≧95%, holding time: 2 hours) is 1 cycle, and 8 cycles are performed. Take a photo of the surface of the test piece after the salt spray cycle test, measure the rust area on the surface of the test piece by image analysis, and calculate the rust area rate based on the ratio to the total area of the test piece surface Rust area/total area of test piece surface)×100(%)). In addition, when the calculated rust area rate is 10% or less, it is evaluated as pass (◎, particularly excellent), when it exceeds 10% and 25% or less, it is evaluated as pass (○), and when it exceeds 25%, it is evaluated as unqualified (×) .

In the inventive examples, excellent outstanding formability and excellent corrosion resistance can be obtained. Especially, in No. 6 (steel A6) containing 0.58% Ni and No. 8 (steel A8) containing 17.8% Cr, the rust area rate in the salt spray cycle test is less than 10% (◎) , Can obtain more excellent corrosion resistance.

On the other hand, No. 12 (steel B1) and No. 13 (steel B2), which are comparative examples, have appropriate manufacturing conditions, but the C content and N content are higher than the appropriate ranges, so the precipitation of Cr-based carbonitrides is excessive However, the average distance between the Cr-based carbonitrides cannot be sufficiently ensured, and the required outstanding formability cannot be obtained. In No. 14 (steel B3), since the Si content is higher than the appropriate range, the steel plate is hardened and the plastic deformability is reduced, and the required protruding formability cannot be obtained. No. 15 and No. 16 have an average heating rate higher than the appropriate range for cold-rolled sheet annealing, so the average distance between the Cr-based carbonitrides cannot be sufficiently ensured, and the required outstanding formability cannot be obtained. For No.17 and No.18, since the heating temperature of cold-rolled sheet annealing is higher than the appropriate range, the austenitic iron phase is generated when the cold-rolled sheet is annealed, and the austenitic iron phase is transformed into matian during cooling after holding Scattered iron phase, the steel plate is significantly hardened. In addition, the metal structure of the final product plate is a two-phase structure composed of a fat iron phase and an Asada scattered iron phase, so the plastic deformation ability of the steel plate is significantly reduced. Therefore, the required protruding formability cannot be obtained. No. 19 and No. 20 have a cold-rolled sheet annealing heating temperature lower than the appropriate range, so the average distance between the Cr-based carbonitrides cannot be sufficiently ensured, and the metallic structure with unrecrystallized grains remains and cannot be obtained. The required outstanding formability. No. 21 and No. 22 have a cold-rolled sheet annealing holding time below the appropriate range, so the average distance between the Cr-based carbonitrides cannot be sufficiently ensured, and the metallic structure with unrecrystallized grains remains, and cannot be obtained. The required outstanding formability. In No.23 and No.24, since the cooling rate of the cold-rolled sheet annealing is lower than the appropriate range, the Cr-based carbonitrides are re-precipitated in large amounts and finely during the cooling, and the Cr-based carbonitrides cannot be sufficiently ensured. The average distance cannot obtain the required outstanding formability. No. 28 Since the heating temperature of the hot-rolled sheet annealing is lower than the appropriate range, the agglomeration, coarsening and solid solution of the Cr-based carbonitrides in the hot-rolled sheet annealing to the ferrous iron phase are insufficient, resulting in the generation of Cr-based carbon and nitrogen Uneven distribution of compounds. Therefore, a region where the average distance between the Cr-based carbonitrides is insufficient is formed locally, and the desired protrusion formability cannot be obtained. No. 29 Since the heating temperature of the hot-rolled sheet annealing is higher than the appropriate range, the austenitic iron phase is regenerated during the hot-rolled sheet annealing. As a result, Cr-based carbonitrides are generated in the metal structure of the hot-rolled sheet after annealing. The distribution is uneven. Therefore, a region where the average distance between the Cr-based carbonitrides is insufficient is formed locally, and the desired protrusion formability cannot be obtained. No. 30 Since the retention time of hot-rolled sheet annealing is less than the appropriate range, the agglomeration, coarsening and solid solution of Cr-based carbonitrides in the hot-rolled sheet annealing and the solid solution into the ferrous iron phase are insufficient, resulting in Cr-based carbon and nitrogen The distribution of substances is uneven. Therefore, a region where the average distance between the Cr-based carbonitrides is insufficient is formed locally, and the desired protrusion formability cannot be obtained.

(Industry availability)

The ferrous iron-based stainless steel plate of the present invention is particularly advantageous for applications that require high outstanding formability during press forming, such as outdoor components, kitchen appliances, and tableware.

Claims (2)

A kind of ferrous iron-based stainless steel steel plate, which has, by mass%, contains C: 0.025~0.050%, Si: 0.10~0.40%, Mn: 0.45~1.00%, P: 0.04% or less, S: 0.010% or less, Cr: 16.0~18.0%, Al: 0.001~0.010%, N: 0.025~0.060% and Ni: 0.05~0.60%, And the remainder is composed of Fe and unavoidable impurities, The average distance between Cr-based carbonitrides with an equivalent circle diameter of 0.05μm or more is 3.0μm or more, and The minimum value of the maximum logarithmic strain of the forming limit based on the forming limit line diagram is 0.20 or more. A method for manufacturing a ferrite-based stainless steel plate, which hot-rolls a steel material having the composition of claim 1 to produce a hot-rolled steel plate, and applies a heating temperature of 800 to 900°C and a holding time of the hot-rolled steel plate After the hot-rolled sheet is annealed for more than 1 hour, it is cold-rolled to produce a cold-rolled steel sheet. Next, 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, and In the above cold-rolled sheet annealing, the average heating rate from 500°C to the heating temperature is set to 20°C/s or less, and the average cooling rate from the heating temperature to 500°C is set to 10°C/s or more.

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