JPH1099952A - Method for producing ferritic stainless steel sheet with excellent ductility and ridging properties - Google Patents

Abstract

(57) [Problem] To provide a method for producing a ferritic stainless steel sheet having excellent ductility and ridging properties. [Achievement means] Cr: 10% by weight by a continuous casting method
% To 23% or less in a method for producing a ferritic stainless steel sheet, wherein the base molten steel is Cr: 10% by weight%.
A ferritic stainless steel containing at least 23% or less and having a value of γ potential represented by the formula (1) of 23% or less, and then an element that increases the γ potential in the central unsolidified portion in the mold during continuous casting. A method for producing a ferritic stainless steel sheet having excellent ductility and ridging properties, characterized by adding particles contained therein and increasing the γ potential of the central part of the slab by 5% or more from the surface part. γp = 189 + 470 × [% N] + 420 × [% C] + 23 × [% Ni] + 7 × [% Mn] + 9 × [% Cu] -11.5 × [% Cr] -11.5 × [% Si] −52 × [% acid-soluble Al] −12 × [% Mo] (1)

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic stainless steel sheet represented by SUS430 steel. It has been recognized that ferritic stainless steel sheets such as SUS430 steel cause fine irregularities on the surface when cold-rolled, and impair the surface smoothness. Also, when press working or tensile working is applied to a thin steel plate,
It has been recognized that wrinkles of unevenness parallel to the rolling direction occur, which also impairs the appearance of the surface. These irregularities are generally called roping, ridging, and living (hereinafter collectively referred to as ridging), and are considered to be fatal defects peculiar to ferritic stainless steel sheets not found in austenitic stainless steel sheets such as SUS430 steel. ing.
The present invention relates to a method for producing a ferritic stainless steel sheet in which such ridging is extremely light and excellent in ductility.

[0002]

2. Description of the Related Art Ferritic stainless steel sheets are widely used for kitchen appliances and other household goods, building materials, home appliances, and automobile parts, because they can easily realize high smoothness by clear color tone and cold rolling. . However, cold rolling of ferritic stainless steel sheet causes RMS
It has been recognized that unevenness of 1 μm or less is generated and that the smoothness is impaired. In addition, it has been recognized that when press forming or tensile processing is applied, wrinkles of unevenness are generated parallel to the rolling direction, and in extreme cases, they are deformed into a corrugated sheet shape. These not only impair the color tone and the smoothness, making it difficult to carry out a polishing treatment to recover the color tone and the smoothness, but also may affect functional problems such as lowering the adhesion depending on parts.

Although the cause of ridging is not always clear, it is generally considered as follows. That is, one coarse crystal grain at the time of casting is subjected to working heat treatment such as hot rolling or annealing to be recrystallized and refined. However, since most of the refined grains have almost the same crystal orientation, it is necessary to regard them as forming pseudo-coarse grains corresponding to the original coarse grains in terms of texture. Is done. Therefore, the deformation behavior is the same as that of a super-coarse-grained thin steel sheet, and the difference in the deformation behavior of each pseudo-coarse-grain appears as unevenness on the surface.

In the conventional ridging countermeasures, concrete measures have been generally implemented based on the following three types of ideas based on this estimation mechanism. Of course, in many cases, the effects have been increased by superimposing each of the components, instead of implementing them alone. 1) Refinement of solidified crystal grains that are the source of pseudo coarse grains 2) Randomization of texture of pseudo coarse grains 3) Decomposition of pseudo coarse grains

[0005] The idea of 1) is related to the miniaturization of pseudo-coarse grains. For example, as described in Japanese Patent Application Laid-Open No. 50-123294, there is an electromagnetic method aiming at equiaxed columnar crystals. Stirring, rapid solidification due to the introduction of solidified crystal nuclei aiming at miniaturization of solidified crystal grains and lowering of casting temperature have been implemented as specific measures.

According to the idea of 2), recrystallization is promoted in the manufacturing process (Japanese Patent Application Laid-Open No. 57-70234), and hot rolling temperature (heating, finish winding temperature, etc.) is set to repeat a plurality of times. Rate, annealing temperature, etc., and the addition of an intermediate annealing process during cold rolling to increase the number of cold rolling recrystallizations,
Use of intragranular precipitates and γ phase aimed at applying widening deformation in addition to elongation deformation at the time of hot rolling and optimization of hot rolling lubrication may be mentioned.

In 3), a component change intended to introduce a transformation (Japanese Patent Laid-Open No. 6-81036) and a special heat process are proposed.

[0008] However, the method 1) has an advantage that it can be dealt with only at the time of casting and there is little interference with the subsequent steps, but if this idea is promoted, casting obstacles such as nozzle clogging and breakout may occur. There were drawbacks, such as easy occurrence.

The method 2) is a method which can be carried out after the rolling step, and therefore has an advantage that it can be easily adapted to the condition of the slab, but the design of the manufacturing process is complicated because many steps affect each other in a complicated manner. Is an extremely difficult method. For example, if you try to increase the number of recrystallizations, you have to increase the number of steps, if you try to promote recrystallization in one step, the recrystallization in other steps becomes insufficient, the thickness of the slab and the final product Since the optimum process differs depending on the thickness, there is a degree of freedom in the process design, and a situation arises in which the process must be changed for each slab. In addition, recrystallization alone is not enough for texture randomization, and requires the use of precipitates and contrivance of a rolling method.
It has many drawbacks, such as the potential for hidden cost increase factors, such as the inability to swing materials.

[0010] The method 3) is extremely effective in destroying the cast structure because it utilizes transformation as in the case of carbon steel. However, the method is limited to components and heat processes, and is most commonly used for ferritic stainless steel sheets. It cannot be applied to the SUS430 steel used for steel, and is a solution applicable only to special steel types. In addition, since the transformation phase is generally hard, deterioration of mechanical properties such as ductility and workability cannot be avoided.

[0011]

On the other hand, the present inventors increased the γ potential in the central portion of the sheet thickness where the effect of improving the ridging property in hot rolling was small, and introduced shearing strain near the γ phase. Invented a ferritic stainless steel with improved ductility and ridging property by lowering the γ potential in the surface layer having a large ridging property improving effect in hot rolling.

However, providing a component difference between the central part of the plate thickness and the surface layer is expensive and not practical.
For example, it is possible to achieve this by using a clad steel sheet by the rolling method or the casting method.However, in the rolling method, large costs are inevitable for assembling the slab and refining the interface. Special equipment such as work or interface refining treatment or double nozzles is required, and large costs are inevitable. For this reason, the ferritic stainless steel improved in ductility and ridging property invented by the present inventors was only able to respond to special applications and requirements.

[0013] The object of the present invention, in view of such a situation,
It is an object of the present invention to provide a method of manufacturing a ferritic stainless steel sheet which has a simple operation process and requires no special equipment, and as a result, has low manufacturing costs and improved ductility and ridging properties.

[0014]

As a method of providing a component difference between the central portion of the plate thickness and the surface layer portion, there is a method of cladding in a solid phase or a liquid phase as described above. However, in the case of the steel according to the present invention, since the component difference between the central part of the plate thickness and the surface layer is a slight difference in the γ potential, the difference in the concentration of elements for that purpose does not necessarily have to be large. In order to provide a slight concentration difference, it is not always necessary to adopt a method by cladding established as a conventional technique. For example, it is possible to reduce the γ potential of the surface layer by decarburization after plate rolling or diffusion and infiltration treatment of Al or the like.

However, in order to ensure ductility and to increase the γ potential in the central portion, a process requiring diffusion in a solid phase not only takes an extremely long time but also causes a concentration gradient. Inability to build appropriate materials.

Therefore, the present inventors have recalled that the γ-potential increasing element is injected only into the central portion by utilizing the diffusion including the stirring in the liquid phase. That is, the material containing the γ-potential increasing element is wrapped around the solidified portion of the slab surface layer corresponding to the thickness of the product plate surface layer that keeps the γ-potential low during continuous casting. Remembering to inject the particles. In the liquid phase, the substance containing the γ-potential increasing element dissolves,
Immediately, it diffuses into the entire liquid phase, that is, the central layer of the slab. However, there is only diffusion in the solid phase portion in accordance with the diffusion rate in the solid phase. Naturally, diffusion in the solid phase is by far the slowest compared to diffusion in the liquid phase. Therefore, the element that increases the γ potential does not actually diffuse into the solid phase portion of the surface layer, and solidification is completed with a low γ potential. become.

According to this method, it is possible to provide a difference in concentration between the surface layer and the inner layer of the slab without requiring special equipment or steps. The thickness of the surface layer that does not increase the γ potential can be controlled by changing the type and thickness of the coating material of the particles to be melted by the molten steel.

Based on the above findings, the inventors of the present invention increased the γ potential at the center of the slab by introducing the above-mentioned particles into the mold at the time of casting. Invented a method for producing a ferritic stainless steel sheet having excellent ductility and ridging properties in the presence of a large amount of.

That is, the present invention relates to a process including continuous casting, hot rolling, cold rolling and annealing, wherein Cr:
In a method of manufacturing a ferritic stainless steel sheet containing 10% or more and 23% or less, a mother molten steel is made of Cr:
Ferritic stainless steel containing 10% or more and 23% or less and having a γ potential value represented by the following formula (1) of 23% or less, and then increasing the γ potential in the central unsolidified portion in the mold during continuous casting. The particles containing the alloy to be alloyed are charged, and the charged particles are melted and solid-dissolved by the latent heat of the unsolidified portion, so that the value of the γ potential of the inner layer of the slab represented by the formula (1) is 5% of the value of the mother molten steel. This is a method for producing a ferritic stainless steel sheet excellent in ductility and ridging characteristic, characterized by increasing the above. γp = 189 + 470 × [% N] + 420 × [% C] + 23 × [% Ni] + 7 × [% Mn] + 9 × [% Cu] -11.5 × [% Cr] -11.5 × [% Si] −52 × [% acid-soluble Al] −12 × [% Mo] (1)

In the above method of the present invention, the thickness d of the surface layer of the slab made of the base molten steel component and the total thickness D of the slab are 0.1 ≦
It has a relationship of d / D ≦ 0.3, and the value of the γ potential of the slab inner layer portion, which increases the value of the γ potential from that of the base molten steel, should be 28% or more and 60% or less. It is preferable as a method for producing a ferritic stainless steel sheet.

In the method of the present invention, all components of the base molten steel can be applied as long as they are contained in ferritic stainless steel. Often required
Ferritic stainless steel containing up to 08% C,
Specifically, C: 0.08% or less in weight%, Si: 0.0
5% or more and 1.0% or less, Mn: 0.05% or more and 1.0% or less
Hereafter, Cr: 10% or more and 23% or less, acid-soluble Al: 0.
It is effective to apply the present invention to steel containing 001% or more and 0.2% or less and N: 0.06% or less, with the balance being Fe and unavoidable impurities. For the purpose of improving corrosion resistance, Ni: 2% or less, Mo: 3% or less, C:
u: It can be applied to a ferritic stainless steel to which one or two or more of 1% or less are added.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the present invention will be described below. First, the content of Cr in the base molten steel of ferritic stainless steel to which the method of the present invention is applied is set to 10% or more and 23% or less. If it is less than 10%, the basic corrosion resistance of the stainless steel sheet will be insufficient, while if it exceeds 23%, the ductility and workability will be reduced, and it will not be used for machining applications where ridging is a problem.

In the base molten steel, the value of the γ potential shown by the following equation (1) is set to 23% or less. The lower the value of the γ potential is, the lower the value is, the more effective it is in improving ductility and workability, and since there is no particular problem with other quality characteristics,
There is no particular lower limit. However, if it exceeds 23%, the ductility decreases when the γ potential at the center is increased, so that the upper limit is 23%. γp = 189 + 470 × [% N] + 420 × [% C] + 23 × [% Ni] + 7 × [% Mn] + 9 × [% Cu] -11.5 × [% Cr] -11.5 × [% Si] −52 × [% acid-soluble Al] −12 × [% Mo] (1) Other preferable components of the ferritic stainless steel to which the method of the present invention is applied will be described. First, if C exceeds 0.08% on average, ductile workability decreases,
0.08% is defined as the following.

If the content of Si is less than 0.05%, there is a risk that the deoxidization becomes insufficient and a large amount of nonmetallic inclusions remain.
Since it is possible to reliably perform deoxidation by other methods, there is no problem if it is less than 0.05% in that case, but since the cost to reduce to that level is required,
0.05% or more. On the other hand, if it exceeds 1.0%, not only the hardness becomes hard and the workability of the final product deteriorates, but also the hot workability also deteriorates, so the content is limited to 1.0% or less.

If Mn is less than 0.05%, S, which is an inevitable impurity, is insufficiently fixed and causes surface flaws, so Mn is made 0.05% or more. However, if it exceeds 1.0%, it is difficult to secure a low γ potential in the surface layer, and the γ potential lowering element (Cr, Si, Al, M
Since a large amount of o) is required and the ductility is greatly reduced, the content is set to 1.0% or less.

If the acid-soluble Al content is less than 0.001%, the deoxidation becomes insufficient, and a large amount of nonmetallic inclusions remain to deteriorate the workability and corrosion resistance. However, if it exceeds 0.2%, the ductility deteriorates, so that
% Or less.

If N exceeds 0.06%, it becomes difficult to secure a low γ potential in the surface layer portion, and it is necessary to add a large amount of a γ potential lowering element (Cr, Si, Al, Mo). Not only does this cause a large decrease in ductility workability, but also the precipitation of Cr nitride increases to deteriorate the workability and corrosion resistance, so that the content is made 0.05% or more.

On the basis of the above components, Ni, Mo and Cu may be selectively added, if necessary.
Ni has an effect of improving corrosion resistance, and it is effective to add Ni to the surface portion in consideration of the fact that corrosion resistance mainly exerts an effect on the surface side. However, if a large amount is added, the γ-potential of the surface layer portion is increased, and a large amount of a γ-potential lowering element (Cr, Si, Al, Mo) is required, resulting in a large decrease in ductile workability.
2.0% or less.

Mo, on the contrary, is an element that lowers the γ potential, but when added in a large amount, becomes hard and deteriorates ductility, so that the content is made 3% or less.

Cu is an element that raises the γ potential like Ni, but must be added to the surface part in view of the fact that corrosion resistance mainly exerts an effect on the surface side. Therefore, if a large amount is added, the γ potential of the surface layer portion is increased, and the γ potential lowering element (C
(r, Si, Al, Mo) is required to be added in a large amount, resulting in a large decrease in ductile workability.

After the ferritic stainless steel comprising the above components is melted, particles containing an alloy for increasing the γ potential are introduced into the central unsolidified portion in the mold during casting of the slab by continuous casting. Then, the input particles are melted and solid-dissolved by the latent heat of the unsolidified portion to increase the γ potential of the inner layer portion of the slab. As a method of introducing the particles into a central portion in the mold, a method of introducing molten steel from inside a stopper used for injecting molten steel from a dandish into a mold via a nozzle is generally adopted.

With respect to the particles for increasing the γ potential at the center of the slab, the core material at the center of the particles is γ
It is necessary that the substance contains an element that increases the potential. Such elements include Cr, Ni, M
Examples thereof include n, C, N, and Cu. It is preferable to use, as the core material, a powder of an alloy containing these elements (a compound formed of a compound such as a carbide or a nitride may be used). On the other hand, the outside of the particles is required to be melted by the latent heat inside the mold.
It is preferably a steel foil made of austenitic stainless steel such as 04.

The width of increasing the γ-potential at the center of the slab from the base molten steel due to the solid solution of the particles is 5
If it is less than 5%, the ridging improvement effect is not recognized, so that the content is set to 5% or more.

The rise width of the γ potential of the slab center layer is 5
%, The ridging improvement effect is recognized, but in order to make the ridging improvement effect sufficient, it is desirable that the value of the γ potential at the center of the slab be 28% or more. On the other hand, the higher the value is, the better the ridging property is, but if it exceeds 60%, the ridging property is correspondingly improved, but the ductility and workability can be secured even if a base molten steel having extremely low γ potential is used. Therefore, the value of the γ potential at the center of the slab is desirably 60% or less.

As described above, the slab continuously cast by the method of the present invention is made of mother molten steel, and the surface layer of the slab forming the surface of the slab and the center of the slab having a γ potential increased more than that of the mother molten steel. It consists of a part. Here, assuming that the thickness of the surface layer of the slab on one side from the slab surface is d and the total thickness of the slab is D, the relationship of 0.1 ≦ d / D ≦ 0.3 may be satisfied. . When d / D <0.1, the function as the surface layer cannot be secured, and when d / D> 0.3, the ridging improvement effect is not recognized even if the γ potential at the center of the slab is increased.

In the steel obtained by the method of the present invention, texture randomization occurs in the surface layer and the central portion by different mechanisms, and ridging is improved. In the surface layer, even if the composition is not particularly controlled and the structure is not controlled, the texture is sufficiently randomized by the shear deformation at the time of hot rolling, and the ridging property is improved. On the other hand, since shear deformation due to rolling cannot be expected at the center, a relatively large amount of hard γ phase is present in α phase during rolling, and microscopic shear deformation is applied by utilizing the difference in hardness. Thus, randomization in a collective manner is performed, and the ridging property is improved. At this time, in order to cause a large amount of the γ phase to be present in the central portion, the components are adjusted in a direction that impairs the ductility. Therefore, in order to ensure the ductility and workability of the steel sheet as a whole, it is desirable that the surface layer portion is relatively softened so as to retain large ductility.

As described above, by providing a component difference in the thickness direction of the steel sheet, a ferritic stainless steel sheet having excellent ductility and ridging properties can be manufactured.

[0038]

EXAMPLES In continuous casting of molten ferritic stainless steel having the chemical composition shown in Table 1, particles of SUS304 steel foil filled with various substances that increase the γ potential were passed through a casting stopper. It was put into the center of the mold to produce a slab 190 mm thick. This slab is hot-rolled to 4 mm by a usual method, and hot-rolled sheet annealing (H
OT-AP) or descaling without performing hot-rolled sheet annealing, cold-rolled to 0.5 mm,
A final annealing at 925 ° C. × 5 min.

[0039]

[Table 1]

Thereafter, as an evaluation test, ridging property evaluation and ductility evaluation by a tensile test were performed. The ridging property was determined by cutting out a JIS-5 test piece from the final cold-rolled annealed sheet, pulling it in the rolling direction by 20%, and evaluating the height of the striped undulation generated in the direction perpendicular to the rolling direction. Table 2 shows the situation of the component difference of the final cold-rolled sheet (component composition of the surface layer and the central layer, γ potential (γp)) and the values of ridging property and ductility of the cold-rolled annealed sheet.

[0041]

[Table 2]

Each of the cold-rolled steel sheets produced by the method of the present invention had a ridging height of less than 20 μm and a ductility in the rolling direction of more than 30%. However, no. When the amount of increase in the γ potential of the central layer was less than 5% as in Comparative Example 6, the effect of improving ridging was small. In addition, No. The steel sheet having no component difference in the thickness direction shown as a comparative example in FIG. 7 had a high ductility but a poor ridging property of 20 μm or more, and the ductility and the ridging property were not compatible.

[0043]

According to the method of the present invention, particles containing an element for increasing the γ potential are introduced into the central unsolidified portion in the mold during the continuous casting of ferritic stainless steel, and the γ in the central portion of the slab is cast. Since the potential is made higher than the surface layer portion, it has become possible to manufacture a ferritic stainless steel sheet having both ridging properties and ductile workability. Since the steel sheet according to the method of the present invention has good ridging properties and ductile workability, it is used in applications where austenitic stainless steel containing a large amount of Ni had to be used although it was conventionally unnecessary from corrosion resistance. Also, it became possible to apply the inexpensive ferritic stainless steel according to the present invention. Therefore, the present invention also has a secondary effect such as the preservation of Ni for which concern about depletion remains.

Claims (4)

[Claims] In a method for producing a ferritic stainless steel sheet containing Cr: 10% or more and 23% or less by weight in a process including continuous casting, hot rolling, cold rolling and annealing, the mother molten steel is subjected to weight %: Cr: A ferritic stainless steel containing 10% or more and 23% or less and having a γ potential value of 23% or less represented by the following formula (1), and then a central unsolidified portion in the mold during continuous casting: Particles containing an element that increases the γ potential are charged, and the charged particles are melted and solid-dissolved by the latent heat of the unsolidified portion, whereby the value of the γ potential of the inner layer portion of the slab, expressed by the formula (1), is set to the value of the mother molten steel. A method for producing a ferritic stainless steel sheet having excellent ductility and ridging properties, characterized by increasing the value by at least 5%. γp = 189 + 470 × [% N] + 420 × [% C] + 23 × [% Ni] + 7 × [% Mn] + 9 × [% Cu] -11.5 × [% Cr] -11.5 × [% Si] −52 × [% acid-soluble Al] −12 × [% Mo] (1) 2. A thickness d of a surface layer portion of a slab made of a base molten steel component.
And the total thickness D of the slab have a relationship of 0.1 ≦ d / D ≦ 0.3, and the value of the γ potential of the inner layer portion of the slab which increases the value of the γ potential from the base molten steel is 28% or more and 60% or more. % Or less, the method for producing a ferritic stainless steel sheet having excellent ductility and ridging property according to claim 1. 3. The mother molten steel is, in terms of% by weight, C: 0.08% or less, Si: 0.05% to 1.0%, Mn: 0.05% to 1.0%, Cr: 10 % To 23%, Acid-soluble Al: 0.001% to 0.2%, N: 0.06% or less, the balance being Fe and inevitable impurities. 2. The method for producing a ferritic stainless steel sheet having excellent ductility and ridging property according to 2. 4. The method according to claim 1, wherein the base molten steel is, by weight%, further Ni: 2%.
4. The method for producing a ferritic stainless steel sheet having excellent ductility and ridging property according to claim 3, wherein one or more of Mo: 3% or less and Cu: 1% or less are contained.