JP5050565B2 - Ferritic stainless steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a ferritic stainless steel plate used for kitchens and utensils, and particularly to a ferritic stainless steel plate excellent in bending workability and a method for producing the same.

  Ferritic stainless steel sheets have a good balance of corrosion resistance and price, and are widely used in building exteriors, kitchen appliances, chemical plants, and the like. A ferritic stainless steel sheet containing 0.01% by mass or more of C is sensitized by forming a Cr-deficient layer by Cr carbonitride precipitation in a welding heat cycle. For this reason, the end portion of the press-formed product may be subjected to severe bending such as hem. However, bending of a ferritic stainless steel sheet causes bending cracks, so that a severe bending process cannot be performed particularly with a thin material having a thickness of about 1.5 mm or less, and the shape of the pressed product is restricted.

  Although this bending workability is not intended directly, for example, Patent Document 1 (Japanese Patent Laid-Open No. 54-112320) defines the contents of Cr, Si, Mn, and contains C, N. A method of improving stretch flangeability by adding Zr according to the amount to form Zr carbonitride is disclosed.

  Patent Document 2 (Japanese Patent Laid-Open No. 10-99952) discloses a method for producing a ferritic stainless steel in which an austenite forming element is added to an unsolidified portion during casting to improve ductility.

  Patent Document 3 (Japanese Patent Laid-Open No. 2000-282186) discloses a ferritic stainless steel sheet excellent in ductility that defines the C / N ratio and the amount of austenite forming elements.

Patent Document 4 (Japanese Patent Application Laid-Open No. 2001-207244) discloses a ferritic stainless steel sheet having an excellent ductility in which the aspect ratio of a coarse colony structure is 5 or less.
JP 54-112320 A Japanese Patent Laid-Open No. 10-99952 JP 2000-282186 A JP 2001-207244 A

  However, the method disclosed in Patent Document 1 has a problem that the bending workability is not improved even if the stretch flangeability is excellent.

  In addition, the method disclosed in Patent Document 2 has a problem that a difference in structure occurs between the surface layer and the inside, and cracks occur at the boundary between the structures during bending.

  In addition, the method disclosed in Patent Document 3 has a problem that bending cracks cannot be avoided although elongation is improved.

  Further, the method disclosed in Patent Document 4 has a problem that bending cracks cannot be avoided even if ductility is improved.

  As described above, the above-described prior art has a problem that even if improvement in ductility can be achieved, it is not suitable for severe bending.

  The present invention has been made to solve the above problems, and an object thereof is to provide a ferritic stainless steel sheet excellent in bending workability and a method for producing the same.

  The present inventors have conducted a detailed study on the relationship between the bending workability of a ferritic stainless steel sheet and the structure of the steel sheet. As a result, in the conventional ferritic stainless steel sheet, it was clarified that Cr carbides were arranged in a row, and cracks progressed along the rows of Cr carbides to cause bending cracks.

  This row of Cr carbides does not affect the elongation value of a normal tensile test. However, in bending, the outer side of the bending is deformed and the inner side is contracted, so that the deformation direction is reversed in the plate thickness direction. For this reason, it was found that shearing easily occurs above and below the thickness direction of Cr carbides arranged in parallel to the plate surface, and Cr carbides are the starting point of shear fracture. In the normal tensile test, the entire plate is pulled in the same direction, so that it is not affected by the form of the row of Cr carbides.

When the cause of the formation of this row of Cr carbide was investigated, when box-annealing a hot-rolled sheet having a structure expanded in the rolling direction, a piece of Cr carbide precipitated along the expanded ferrite grain boundary. It has been found that when this is pulverized by cold rolling, a row of Cr carbide precipitates in the cold-rolled steel sheet. In order to prevent the Cr carbides from forming a line, it is only necessary to prevent the Cr carbides from precipitating in the form of flakes at the ferrite grain boundaries expanded in the rolling direction during box annealing. Then, the present inventors replace part of Cr in Cr carbide (mainly Cr ₂₃ C ₆ or Cr carbide having the same crystal structure) with Fe, whereby the flaky Cr carbide is spheroidized. I found out. Furthermore, it has been found that the Fe substitution rate in Cr carbide can be controlled by adding an appropriate amount instead of reducing Mn, which has been considered to increase strength by solid solution strengthening and deteriorate ductility. .

  The reason for this is not yet clear, but it is considered that Mn interacts with C and affects the precipitation rate of Cr carbide, and as a result, Fe is taken into Cr carbide.

The present invention has been completed based on the above findings and has been completed and has the following characteristics.
[1] Ferritic stainless steel sheet containing, in mass%, C: 0.01 to 0.03%, Mn: 0.5 to 1.0%, Cr: 15 to 20%, Al: 0.01% or less There,
While Cr carbide is dispersed in the ferrite, the abundance ratio of Fe and Cr metal elements in the Cr carbide is a mass% ratio,
Fe / Cr: 0.05 to 0.15
A ferritic stainless steel sheet excellent in bending workability, characterized by being
[2] In the above [1], the ferritic stainless steel sheet is further, in mass%, Si: 0.25% or less, P: 0.040% or less, S: 0.01% or less, N: 0.05 % Ferritic stainless steel sheet, characterized by comprising
[3] slab, heated above 1000 ° C., finish rolling temperature 900 ° C. or higher, after hot rolling at a coiling temperature 800 ° C. or higher, above, characterized in that the annealing hot rolled sheet at 850 ° C. or less [ The manufacturing method of the ferritic stainless steel plate excellent in the bending workability as described in 1] or [2] .

  ADVANTAGE OF THE INVENTION According to this invention, the ferritic stainless steel plate excellent in bending workability and its manufacturing method are provided.

  Hereinafter, an example of the best mode for carrying out the present invention will be described.

  The reason for limiting the composition of the ferritic stainless steel sheet of the present invention will be described below. In the following description, “%” represents mass%. Moreover, this invention is used suitably for the thin steel plate whose plate | board thickness is about 0.1-0.15 mm.

[C: 0.01 to 0.03%]
C forms a solid solution in steel, stabilizes the austenite phase during hot rolling, and combines with Cr to form Cr carbide (mainly Cr ₂₃ C ₆ or Cr carbide having the same crystal structure). To do. When C is less than 0.01%, even during hot rolling, the structure becomes a ferrite single structure, C concentration in austenite and coarse precipitation of Cr carbide are suppressed, Cr carbide is precipitated in ferrite, Hardens and decreases ductility.

  On the other hand, when C exceeds 0.03%, the amount of Cr carbide increases and ductility itself decreases. For this reason, content of C was made into 0.01 to 0.03%.

[Mn: 0.5 to 1.0%]
Since Mn solidifies and strengthens steel, it has been considered preferable to reduce Mn. In the present invention, an optimum amount of Mn is added to control the morphology of Cr carbide. By adding 0.5% or more of Mn, a part of Cr in Cr ₂₃ C ₆ can be easily replaced with Fe. On the other hand, if it is added over 1.0%, a decrease in ductility due to solid solution strengthening of Mn cannot be ignored. Moreover, the corrosiveness is deteriorated. For this reason, the Mn content is set to 0.5 to 1.0%.

Here, Cr ₂₃ C ₆ or a state of Cr carbide in which a part of Cr element in Cr carbide having the same crystal structure is substituted with Fe element is expressed as Cr _23-i Fe _i C _6. . Note that i represents an integer of 1 to 22. Further, Cr _23-i Fe _i C ₆ lacking a part of Fe or Cr at the lattice point is included as long as the crystal structure is the same.

[Cr: 15-20%]
Cr is an element that forms Cr oxide on the steel surface and contributes to the improvement of corrosion resistance. Since Mn is actively added, the corrosion resistance as stainless steel cannot be maintained with a Cr addition amount of less than 15%. On the other hand, if added over 20%, the ductility drop due to the solid solution strengthening of Cr is remarkable. For this reason, the content of Cr is set to 15 to 20%.

[Al: 0.01% or less]
Al acts as a deoxidizing agent and also bonds with N in steel and precipitates as AlN. Since AlN is finely precipitated at the ferrite grain boundaries in the hot rolled sheet, it inhibits equiaxed hot-rolled sheet annealing of the expanded hot-rolled sheet structure, and flaky precipitation of Cr carbide on the ferrite grain boundaries Promote. For this reason, in order to reduce the precipitation amount of AlN, the Al content is set to 0.01% or less, preferably 0.005% or less, and more preferably 0.003% or less.

  In the present invention, in addition to the above components, Si: 0.20% or less, P: 0.040% or less, S: 0.01% or less, N: 0.05% or less are preferably contained. . Hereinafter, the reasons for limiting the composition will be described.

[Si: 0.25% or less]
Si is an element that acts as a deoxidizer, and is an element that significantly strengthens steel by solid solution. In the present invention for reducing Al, addition of 0.05% or more is preferable for the purpose of deoxidation with Si. On the other hand, since the C content is lowered, it is preferable to keep the austenite fraction during hot rolling by setting the Si content to 0.25% or less. Accordingly, the Si content is preferably 0.25% or less, and more preferably 0.05% or more and 0.25% or less.

[P: 0.040% or less]
P strengthens the steel by solid solution. Moreover, it segregates at the ferrite grain boundaries and embrittles the steel. For this reason, the P content is preferably 0.040% or less, more preferably 0.030% or less, and still more preferably 0.020% or less.

[S: 0.01% or less]
S forms sulfides in the steel. MnS precipitates in steel containing Mn. Since this MnS precipitates in the form of flakes at the ferrite grain boundaries, ductility is deteriorated, particularly bending workability is deteriorated, so S is preferably reduced as much as possible. For this reason, the S content is preferably 0.01% or less, and more preferably 0.005% or less.

[N: 0.05% or less]
N stabilizes austenite during hot rolling and forms Cr nitride (Cr ₂ N). Even if the Cr carbide and Cr nitride have the same content, the volume of the precipitate is greatly different, and the volume of Cr nitride is smaller. Therefore, the N content is preferably 0.05% or less, and preferably 0.01% or more for stabilizing austenite.

Other than the above components are Fe and inevitable impurities. The impurities include Ni: 0.5% or less, Cu: 0.1% or less, Mo: 0.1% or less, V: 0.06% or less, Nb: 0.03% or less, Ti: 0.03 % Or less, Ca: 0.01% or less, Mg: 0.01% or less, and B: 0.001% or less are acceptable, but the smaller the better.

[Fe / Cr: 0.05 to 0.15]
The abundance ratio of Fe / Cr in the Cr carbide Cr _23-i Fe _i C ₆ is important in the present invention. If the abundance ratio of Fe and Cr metal elements (Fe / Cr) in Cr _23-i Fe _i C ₆ is less than 0.05 by mass%, that is, if the amount of Fe in Cr carbide is too low, Cr Since the carbide precipitates in a piece shape, a row of Cr carbides is generated in the cold-rolled sheet. On the other hand, if the Fe / Cr exceeds 0.15, Cr carbides are refined, and the steel is hardened and reduced in ductility. For this reason, the abundance ratio of Fe and Cr metal elements in Cr _23-i Fe _i C ₆ is set to 0.05 to 0.15 in mass% ratio.

As described above, the ferritic stainless steel sheet of the present invention has a ferrite single composition comprising a ferrite phase, Cr carbide, Cr nitride, and a composite thereof. However, precipitates due to inevitable impurities, such as TiS, Ti ₄ C ₂ S ₂ , NbC, and VN, are acceptable if the content is inevitable impurity level. When a bainite phase, martensite phase, or austenite phase other than ferrite is mixed, not only the steel becomes hard and high ductility cannot be obtained, but also bending workability is deteriorated. For this reason, in the present invention, a single ferrite composition is preferable.

In the ferritic stainless steel sheet of the present invention, the Cr carbide Cr _23-i Fe _i C ₆ is dispersed in ferrite. Here, the dispersed state is not in a row in the rolling direction but is randomly dispersed. The column shape is a plate thickness section parallel to the rolling direction and having a length of 5 mm in the rolling direction, and five or more Cr carbides are arranged in the rolling direction. Although not particularly limited, ductility tends to be deteriorated when there are 10 or more groups of rows having a row-to-row spacing of 20 μm or less in the thickness direction. For this reason, the group of rows is preferably 9 or less in the thickness direction.

  Hereinafter, an example of the manufacturing method of the ferritic stainless steel sheet of the present invention will be described.

  The stainless steel sheet of the present invention comprises a hot rolling process, a hot rolled sheet annealing process for subjecting the ferritic stainless steel material to hot rolling sheet annealing and pickling treatment, a cold rolling process, and a cold rolled sheet annealing process. Manufactured.

  After the molten stainless steel having the above composition is melted by a known melting method, a slab (also referred to as “slab”) is obtained by a known casting method, preferably a continuous casting method. In the present invention, the melting method and the casting method are not particularly limited, and any of ordinary methods can be applied.

  In the hot rolling step, the slab is heated to 1000 ° C. or higher. When the heating temperature is less than 1000 ° C., the rolling load is increased, the surface roughness is remarkably increased, and the surface quality is deteriorated. This becomes a starting point of the bending crack, and the bending crack is likely to occur.

  The stainless slab heated to 1000 ° C. or higher is hot-rolled at a finish rolling temperature of 900 ° C. or higher, wound at a winding temperature of 800 ° C. or higher, and the hot rolling step is completed. By rolling at 900 ° C. or higher, it is possible to prevent the ferrite grains in the hot rolled sheet from spreading significantly. Further, by setting the coiling temperature to 800 ° C. or higher, the equiaxed ferrite grains in the hot rolled sheet are promoted. When the finish rolling temperature is less than 900 ° C. or the winding temperature is less than 800 ° C., the ferrite grains in the hot rolled sheet remain stretched, so that Cr carbide tends to precipitate in the hot rolled sheet in the form of a piece, Since this is cracked by cold rolling and it becomes easy to form a row of Cr carbides arranged in the rolling direction, bending workability deteriorates.

  Moreover, in this invention, after the said hot rolling completion | finish, hot-rolled sheet annealing is performed at 850 degrees C or less. When this hot-rolled sheet annealing temperature exceeds 850 ° C., the growth along the ferrite grain boundary of Cr carbide becomes remarkable, resulting in a flake shape. This is also pulverized by cold rolling to form row-like Cr carbide in the cold-rolled annealed plate, and the bending workability is deteriorated. Then, a hot-rolled sheet annealing process is complete | finished by performing a pickling process by a normal method.

  After the hot-rolled sheet annealing step, the ferritic stainless steel sheet according to the present invention is manufactured through a cold rolling process and a cold-rolled sheet annealing process by a normal method.

Stainless steel having the composition shown in Table 1 was melted to form a slab. Next, these slabs were heated to 1200 ° C. and hot-rolled under the conditions shown in Table 2. The obtained hot-rolled sheet was box-annealed at the temperature shown in Table 2. Subsequently, mixed acid: pickling with _{(HF 3mass% + HNO 3)} , to prepare a hot rolled sheet having a thickness of 4 mm.

  This hot-rolled sheet was cold-rolled at a rolling rate of 80%. Then, it annealed by 830 degreeC for 30 second soaking, and produced the cold rolled sheet. The obtained cold-rolled sheet was subjected to tensile test, bending test and observation of precipitates by SEM and TEM.

  Hereinafter, the conditions for the tensile test, the bending test, and the observation of precipitates are shown.

[Tensile test]
A tensile test piece specified in JIS No. 13B is collected so that the rolling direction and the tensile direction are parallel, and a tensile test is performed in accordance with the specification of JIS Z2241, yield stress YS (MPa), tensile strength TS (MPa ) And ductility EL (%) were measured.

[Bending test]
A rectangular plate having a width of 30 mm and a length of 200 mm was cut out so that the width direction was parallel to the rolling direction, and was bent by 180 ° with a bending radius t / 2. The presence or absence of cracks in the bent portion was visually determined, and those having no cracks were evaluated as “◯”, and those having cracks were evaluated as “×”.

[Precipitation observation]
(A) SEM (scanning electron microscope) observation The central part of the plate thickness section parallel to the rolling direction was polished and corroded by aqua regia. The cross section was observed by SEM, and the presence or absence of the precipitation of Cr carbide was confirmed. The observation range is 5 mm long in the rolling direction. Even if precipitation in a row is recognized locally, bending cracks do not occur. Therefore, a row having a remarkable row shape or a row having 10 or more rows in the thickness direction at the same longitudinal position is referred to as “row shape”. "Deposition" and the others were evaluated as "random".

(B) TEM (Transmission Electron Microscope) Observation Cr carbide was collected by a replica method, and the abundance ratio (mass% ratio) of Cr and Fe in the Cr carbide was measured by EDX (energy dispersive X-ray detector).

  Table 3 shows the results of the precipitate observation, the tensile test, and the bending test. As shown in Table 3, the inventive examples showed better results in the bending test than the comparative examples, and it was confirmed that the bending workability was excellent.

Claims (3)

A ferritic stainless steel sheet containing, in mass%, C: 0.01 to 0.03%, Mn: 0.5 to 1.0%, Cr: 15 to 20%, Al: 0.01% or less,
While Cr carbide is dispersed in the ferrite, the abundance ratio of Fe and Cr metal elements in the Cr carbide is a mass% ratio,
Fe / Cr: 0.05 to 0.15
A ferritic stainless steel sheet excellent in bending workability, characterized by being   The ferritic stainless steel sheet further includes, by mass%, Si: 0.25% or less, P: 0.040% or less, S: 0.01% or less, and N: 0.05% or less. The ferritic stainless steel sheet according to claim 1. The slab, heated to above 1000 ° C., finish rolling temperature 900 ° C. or higher, after hot rolling at a coiling temperature 800 ° C. or higher, according to claim 1 or 2, characterized in that annealing hot rolled sheet at 850 ° C. or less The manufacturing method of the ferritic stainless steel plate excellent in the bending workability of description .