JP3448542B2 - Ferritic stainless steel sheet excellent in formability and ridging properties and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a ferritic stainless steel sheet having excellent formability and ridging characteristics, and a method for producing the same.

[0002]

2. Description of the Related Art Ferritic stainless steel has excellent corrosion resistance and is used in many applications. However, SUS30, which is a representative steel type of austenitic stainless steel,
There was a problem that the moldability was inferior to that of No. 4, and surface irregularities called ridging occurred during molding.

In order to increase the r value, which is an index of formability,
A method of reducing C and N and reducing solid solution C and N by adding Ti and Nb is used as in JP-A-61-261460. As described above, although the ferritic stainless steel is highly purified and the C and N fixing elements are added, the formability is improved, but on the contrary, the ridging characteristic is sometimes deteriorated.

The cause of ridging is believed to be the coarse grains of the cast structure. Ferritic stainless steel is δ → γ
→ Since there is no α perfect transformation, it is considered that the coarse particles that originally existed remain in the product sheet as a crystal grain group of similar orientations (hereinafter, referred to as colonies), resulting in ridging. Reducing this colony improves ridging.

ECP (Electron Channeling Pattern) method and E method are available for measuring the size of colonies.
BSP (Electron Back Scattering Pattern) method is used. “Iron and Steel” 76-9 (1990), P.1520,
The relationship between the distribution of colonies measured by ECP and the ridging characteristics is described. In addition, “CAMP-ISIJ” 11 (1
998), P.590, a model for predicting ridging occurrence from the colony size measured by EBSP is presented.

As a method for improving ridging (which is considered to be a colony refining method), equiaxed crystallization of a cast structure or hot rolling,
The manufacturing conditions such as annealing have been optimized. As a method for making the solidified structure equiaxed, Japanese Patent Application Laid-Open No. 50-123294 is known.
As typified by the publication, there are electromagnetic stirring during casting, inoculation of solidification nuclei, lowering of casting temperature, and the like. Also, Japanese Patent Publication No. 61-196, which is a method for optimizing hot rolling conditions and annealing conditions.
No. 88 and Japanese Patent Publication No. 57-38655 are known. Recently, as disclosed in JP-A-9-263900 and JP-A-10-330887, methods for defining hot rolling conditions to limit the width of a product plate and the colony size in the plate thickness direction have become known. ing.

[0007] However, the above method does not stably obtain good ridging characteristics, and the manufacturability is greatly reduced, such as the occurrence of scratches due to large reduction (40% or more reduction) in hot rolling. There was a problem. Further, even if the colony size in the plate width and plate thickness directions was limited, ridging could be visually observed in some cases.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a ferritic stainless steel sheet having good formability and stable ridging characteristics without a significant decrease in productivity, and a method for producing the same. Is.

[0009]

The inventors made several kinds of ferritic stainless steel thin plates in a laboratory and investigated the relationship between the colony structure of the steel plates and the ridging characteristics. The thickness of the product plate was 0.4 to 1.5 mm. E for crystal orientation measurement
BSP was used. The definition of {hkl} colony is {hk
Two or more grains having a crystallographic orientation within ± 15 ° from the l} orientation were set to be adjacent to each other. {Hkl} indicates an azimuth perpendicular to the plate surface. The ridging was visually judged from the shape of the test piece after pulling 15% in the L direction.

(1) The colonies mainly consist of {100},
There are {111} and {110}. (2) The smaller the colony size (width, thickness, length), the better the ridging characteristics. The size of the above colonies is 2000 μm or less in the rolling direction and 500 μm in the width direction of the plate.
In the following, the case is 300 μm or less in the plate thickness direction. What is important here is that even if the colony size is reduced only in the plate width direction and the plate thickness direction, ridging is visually recognized when the colony size is expanded in the rolling direction. The ridging was not visually observed when the length of the colony size in the rolling direction was 2000 μm or less.

Further, the influence of the components and the production conditions on the colony size was investigated, and the following findings were obtained. (3) When the appropriate amounts of Ti and Mg were added to the ferritic stainless steel, the colony size became small and the ridging was sometimes very good. Many of the inclusions at this time were found to have a form in which Mg-based inclusions were covered with TiN. (4) The colony size greatly changed depending on the hot rolling conditions, and the colony size was reduced when rolling was performed at 900 ° C to 1200 ° C with a reduction rate of 15% or more 10 times or more. (5) When the above (3) and (4) are combined,
The colony size as shown in (2) is obtained.

The present invention is based on the above findings,
The summary is as follows. (1) Mass%, C: 0.0005 to 0.03%, Si: 0.01 to 1%, Mn: 0.01 to 1%, P: less than 0.04%, S: 0.0001 to 0.01%, Cr: 10 to 25%, Ti: 0.01 to 0.8%, Al: 0.005 to 0.1%, N: 0.0005 to 0.03%, Mg: 0.0005 containing 0.01%, the balance being Fe and unavoidable impurities, the density of the inclusions 3 / mm ² or more in the form of maximum size covers the Mg-based inclusions 0.05~5μm with TiN The size of the largest colony among the {100}, {110}, and {111} colonies is 2000 μm or less in the rolling direction, 500 μm or less in the width direction, and 300 μm or less in the plate thickness direction. Fe-based ferrite with excellent formability and ridging characteristics Nresu steel plate.

(2) In mass%, B: 0.0005 to 0.005%, Nb: 0.05 to 0.5%, V: 0.05 to 0.5%, Ta: 0.05 to 0 0.5%, W: 0.05 to 0.5%, Hf: 0.05 to 0.5%, Zr: 0.05 to 0.5%, and one or more of them may be further contained. A ferritic stainless steel sheet having excellent formability and ridging characteristics described in (1) above. (3) One or more of Mo: 0.1 to 2%, Ni: 0.1 to 2%, and Cu: 0.1 to 2% in mass% are further contained. A ferritic stainless steel sheet having excellent formability and ridging characteristics according to (1) or (2) above. (4) In mass%, Y: 0.0002 to 0.005%, La: 0.0002 to 0.005%, Ce: 0.0002 to 0.005%, and further contains one or more kinds. The ferritic stainless steel sheet having excellent formability and ridging characteristics according to any one of (1) to (3) above.

(5) The above (1) to (10) characterized by further containing Ca: 0.0002 to 0.005% in mass%.
A ferritic stainless steel sheet having excellent formability and ridging characteristics according to any one of (4). (6) One or two kinds of Sb: 0.0002 to 0.005% and Sn: 0.001 to 0.1% in mass% are further contained, and the above (1) to (). A ferritic stainless steel sheet having excellent formability and ridging characteristics according to any one of 5).
(7) Ferrite excellent in formability and ridging property according to any one of (1) to (6), further containing Ag: 0.0005 to 0.3% in mass%. Series stainless steel plate. (8) When manufacturing the ferritic stainless steel sheet according to any one of (1) to (7), rolling at a rolling reduction of 15% or more at 900 to 1200 ° C. is performed 10 times or more in a hot rolling step. A method for producing a ferritic stainless steel sheet having excellent formability and ridging characteristics.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. In addition,% in the following description means mass%. C, N: If a large amount of C, N is added, the strength increases and the moldability decreases, so the upper limit was made 0.03%. On the other hand, the lower limit is 0.0005% that can be manufactured without a significant increase in cost in the refining stage. When used for severe processing applications, both C and N are 0.0005-0.0
It is preferably set to 15%.

Si: Si is necessary as a deoxidizing element, but if added in a large amount, the formability is lowered. Therefore, the upper limit was set to 1%. The lower limit is 0.01 to obtain a deoxidizing effect.
%.

Mn: Mn increases the strength by adding a large amount and lowers the formability, so the upper limit was made 1%. The lower limit is 0.01% from the viewpoint of cost.

From the viewpoint of moldability, P: P is preferably low, and it must be less than 0.04%. The lower the value, the better the ductility, so it is preferably 0.02% or less. The lower limit is preferably about 0.005% from the viewpoint of raw material cost.

S: When S is added in a large amount, corrosion resistance and workability are deteriorated, so the upper limit was made 0.01%. The lower limit is 0.0001% because it can be extremely lowered by recent desulfurization technology.
And

Cr: If Cr is less than 10%, the corrosion resistance as stainless steel is insufficient, and if it exceeds 25%, not only is it difficult to form due to an increase in strength, but also the toughness may decrease, so that 10 The range was -25%. Further, if the amount of Cr increases, the workability decreases, so 10-19% is preferable for processing applications.

Ti: Ti is an important element that not only fixes C and N to enhance the formability but also crystallizes TiN in combination with Mg to improve the ridging property which is the subject of the present invention. In the present invention, the effect of improving the moldability is 0.01% or more, and this is set as the lower limit. Further, if the amount of Ti is too large, the strength increases and the ductility decreases, so the upper limit was made 0.8%. From the viewpoint of ductility and manufacturability, the most preferable range is 0.08 to 0.3%.

Al: Al is an element that fixes N and the like to improve workability, but addition of a large amount thereof causes deterioration of formability and increase of cost, so the upper limit was made 0.1%. On the other hand, since lowering the amount of Al makes it difficult to deoxidize, the lower limit was made 0.005%.

Mg: Mg is an important element that forms inclusions and improves the ridging property which is the subject of the present invention.
The lower limit for exerting the effect is 0.0005%, and this was made the lower limit. Even if added in a large amount, the effect is saturated, so 0.01% was made the upper limit.

B, Nb, V, Ta, W, Hf, Zr: These are elements that improve the moldability, and if necessary, one or more of the above may be added in combination. B:
0.0005%, Nb, V, Ta, W, Hf, Zr:
The effect is exhibited by adding 0.05% or more. But,
Even if B is 0.005% and Nb, V, Ta, W, Hf, and Zr are more than 0.5%, the effect is saturated.

Mo, Ni, Cu: These are elements that improve corrosion resistance, and Mo, N are used in applications where corrosion resistance is a problem.
One or more of i and Cu are added in combination. The effect is exhibited by adding 0.1% or more of each. However, considering workability, the upper limit is 2%
Is.

Y, La, Ce: These improve the fluidity of molten steel and improve cleanliness. The effect is demonstrated 0.0
The lower limit was 002%. Further, even if added over 0.005%, the effect is saturated, so this was made the upper limit. Ca: Ca is used as a deoxidizing and desulfurizing material. The lower limit at which the effect is exhibited is 0.0002%. 0.005
On the other hand, if the content exceeds 0%, the moldability and the corrosion resistance are deteriorated, so the upper limit was made 0.0050%.

Sb, Sn: These elements have an effect of improving ridging because they easily form a deformation zone during rolling.
Since Sb: 0.0002% and Sn: 0.001% or more exert the effect, the lower limit is set. But Sb: 0.0
Addition of more than 05% and Sn: 0.1% has an adverse effect on formability such as strength increase, so the upper limit was made this.

Ag: Ag is an element that improves antibacterial properties. The lower limit was set to 0.0005% at which the effect is exhibited. However, if over 0.3% is added, the formability is deteriorated, so this was made the upper limit.

Further, according to the present invention, the maximum diameter of the Mg-based inclusions is required to be 0.05 to 5 μm. 0.05μ
If it is smaller than m, it does not act as a crystallization nucleus of TiN, and the size of the colony that causes ridging cannot be reduced. 5μ
If it exceeds m, manufacturing defects (scratches) increase.

Further, a form in which TiN exists so as to cover the Mg-based inclusions is required. This indicates that, when observed in an arbitrary cross section, TiN is present so as to surround the entire surface of the Mg-based inclusion as shown in FIG. The distribution density of inclusions in such a form needs to be 3 pieces / mm ² or more. If it is less than this, the colony size cannot be reduced. In order to further improve the ridging property, the number is preferably 30 pieces / mm ² or more. The upper limit of the density is preferably 10,000 pieces / mm ² within a range in which the strength is not significantly increased. The size of these inclusions may be investigated by making an extraction replica or a thin film of the inclusions on an arbitrary cross section of the steel ingot and the steel plate, and inspecting with an electron microscope. The distribution is preferably examined by using an electron microscope or EPMA as described above.

{100} and {11 which cause ridging
The largest of the 0} and {111} colonies has a size of 2000 μm or less in the rolling direction and 5 in the plate width direction.
It is necessary to set the thickness to 00 μm or less and to 300 μm or less in the plate thickness direction. This allows almost no ridging. In order to obtain this colony size, both inclusion control and hot rolling condition control described later are required. The EBSP method may be used to investigate the colony size.

Mg is an element also used as a deoxidizer.
If it remains in molten steel for a long time, it will become coarse and float.
Therefore, the size and distribution of Mg inclusions are
In order to make it exist by casting, cast within 120 min after adding Mg.
It needs to be built. Finally M after adding other component elements
The method of adding g is preferred. Mg-based inclusions are Mg
The inclusions containing are shown. Oxides (MgO, MgAl ₂
O_FourEtc.) and sulfide (MgS), etc.
Yes. The peak of Mg is observed when analyzed by EDS
Refers to existing things.

In the above-mentioned method for producing a ferritic stainless steel, a steel ingot is usually hot rolled, annealed, cold rolled or annealed. However, even if the annealing of the hot rolled sheet is omitted, the annealing is performed during the cold rolling. Alternatively, hot rolling may be omitted. However, it is necessary to perform rolling at a rolling reduction of 15% or more at 900 ° C. to 1200 ° C. 10 times or more in the hot rolling process. When the rolling temperature is lower than 900 ° C. or higher than 1200 ° C., the rolling reduction is lower than 15%, and the number of times is less than 10 times, the colony size does not decrease. Preferably, the rolling reduction is 25% or more in order to promote recrystallization during hot rolling. However, 40%
The above large reduction is not necessary. This method does not significantly reduce manufacturability as compared with ordinary hot rolling.

The cause of obtaining a ferritic stainless steel excellent in formability and ridging property according to the present invention is considered as follows. A large number of Mg inclusions are present in the molten steel,
TiN is crystallized using this as a nucleus. Since this TiN serves as a solidification nucleus when the ferrite phase solidifies, the solidification structure becomes a fine equiaxed crystal. That is, the crystal grains considered as a unit of colony become fine, and the orientation is randomized by reducing the columnar crystals growing to {100}. When hot rolling is started with this structure as a starting point, in addition to the fact that there are many crystal grain boundaries that become recrystallization nuclei because of the fine structure and there are few {100}, which is the orientation that makes hot recrystallization difficult, Easily recrystallized during hot rolling. Therefore, it is considered that the structure is finely divided by recrystallization, that is, the colony that is the cause of ridging is subdivided only by repeating the light reduction during hot rolling.

[0035]

EXAMPLES Examples of the present invention will be shown below. [Example 1] The ferritic stainless steels shown in Table 1 were melted, hot-rolled, then cold-rolled, annealed, or the like to form 0.4 to 1.0 mm.
The steel plate of The hot rolling was performed under the condition of rolling at a rolling reduction of 15% or more in the range of 900 ° C to 1200 ° C 10 times or more. The crystal orientation distribution of the L and C cross sections of the steel sheet was investigated by EBSP, and the colony size was obtained. Further, inclusions were examined from the Z-section structure near the center of the plate thickness by electron microscopy and EPMA.

Tensile test pieces were sampled from the steel sheet and subjected to r value measurement test (15% tensile, average in L, D and C directions (rL + 2rD).
+ RC) / 4)) and a 15% tensile test for ridging measurement. After the tension, the ridging characteristics were evaluated visually and by the ridging height (the maximum difference between the peak and the valley of the surface roughness in the C direction).
The visual rank is A: no ridging is observed, B:
Some ridging is visually recognized, and C: Ridging is clearly visible. Ridging height is A:
5 μm or less, B: 15 μm or less, C: 30 μm or less,
D: It is more than 30 μm.

The results of various evaluations are shown in Table 2. The steel of the present invention is superior to the comparative steel in formability and ridging characteristics. In particular, ridging has been greatly improved so that it cannot be visually observed.

[0038]

[Table 1]

[0039]

[Table 2]

Example 2 With respect to C and G of ferritic stainless steel shown in Table 1, hot rolled sheets were prepared by changing hot rolling conditions by several levels. This hot-rolled sheet was annealed, cold-rolled, etc. to prepare a steel sheet of 0.6 mm. The crystal orientation distribution of the L and C cross sections of the steel sheet was investigated by EBSP, and the colony size was obtained. In addition, the electron microscope and EP
Inclusions were investigated by MA. Tensile test pieces were taken from the steel sheet and subjected to a r-value measurement test and a 15% tensile test for ridging measurement. After the tension, the ridging characteristics were evaluated visually and by the ridging height (the maximum difference between the peak and the valley of the surface roughness in the C direction). Table 3 shows the results of various evaluations. The method of the present invention is superior in moldability and ridging property to the comparative method.

[0041]

[Table 3]

[0042]

According to the present invention, by limiting the components, inclusions and colony size, it is possible to provide a ferritic stainless steel sheet which is excellent in formability and ridging characteristics without significantly reducing the manufacturability.

[Brief description of drawings]

FIG. 1 is a schematic view showing the form of inclusions in the present invention.

Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (8)

(57) [Claims] 1. Mass%, C: 0.0005 to 0.03%, Si: 0.01 to 1%, Mn: 0.01 to 1%, P: less than 0.04%, S: 0.0. 0001 to 0.01%, Cr: 10 to 25%, Ti: 0.01 to 0.8%, Al: 0.005 to 0.1%, N: 0.0005 to 0.03%, Mg: 0 0.0005 to 0.01% of Mg-based inclusions having a maximum diameter of 0.05 to 5 μm and covered with TiN are present in the steel at a density of 3 / mm ² or more. , {100}, {11
The largest of the 0} and {111} colonies has a size of 2000 μm or less in the rolling direction and 500 in the width direction.
A ferritic stainless steel sheet having excellent formability and ridging characteristics, characterized in that the thickness is 300 μm or less in the plate thickness direction. 2. In mass%, B: 0.0005 to 0.005%, Nb: 0.05 to 0.5%, V: 0.05 to 0.5%, Ta: 0.05 to 0. 5%, W: 0.05 to 0.5%, Hf: 0.05 to 0.5%, Zr: 0.05 to 0.5%, or a combination of two or more thereof. A ferritic stainless steel sheet having excellent formability and ridging characteristics according to claim 1. 3. Mass%, Mo: 0.1 to 2%, Ni: 0.1 to 2%, Cu: 0.1 to 2%, and further contains one or more kinds. A ferritic stainless steel sheet excellent in formability and ridging property according to claim 1 or 2. 4. In mass%, one or more of Y: 0.0002 to 0.005%, La: 0.0002 to 0.005%, Ce: 0.0002 to 0.005%, The ferritic stainless steel sheet excellent in formability and ridging property according to any one of claims 1 to 3, further containing. 5. A ferrite system excellent in moldability and ridging property according to claim 1, further comprising Ca: 0.0002 to 0.005% in mass%. Stainless steel plate. 6. In mass%, Sb: 0.0002 to 0.005%, Sn: 0.001 to 0.1%, one or two kinds are further contained. 5. A ferritic stainless steel sheet having excellent formability and ridging characteristics according to any one of 5 above. 7. A ferrite system excellent in formability and ridging property according to claim 1, further comprising Ag: 0.0005 to 0.3% in mass%. Stainless steel plate. 8. When manufacturing the ferritic stainless steel sheet according to any one of claims 1 to 7, rolling in a hot rolling step at 900 to 1200 ° C. with a reduction rate of 15% or more is performed 10 times or more. And a method for producing a ferritic stainless steel sheet having excellent formability and ridging characteristics.