JP2623606B2 - Manufacturing method of ferritic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for stably producing a Ti-added ferritic stainless steel having excellent roping resistance and good surface properties.

<Conventional technology and its problems> Ferritic stainless steel represented by SUS430 has excellent workability and corrosion resistance and is relatively inexpensive. It is a widely used steel grade.

However, ferritic stainless steel has a rough surface called "roping" along the rolling direction when cold-working such as deep drawing or bending is applied to a steel plate manufactured by rolling a continuous cast slab. It has been pointed out that the problem frequently occurs, and this problem is extremely serious because the appearance of the product is deteriorated and the commercial value is impaired.

By the way, the cause of the roving is that the coarse columnar crystal structure generated during continuous casting is not sufficiently destroyed even in the rolling step, and furthermore, the texture remains, and the coarse grains and the texture are formed during hot rolling. It is difficult to recrystallize, and the structure is not refined, and therefore, anisotropy and non-uniformity appear in plastic deformation. " In the case of a steel type such as SUS430 steel, it separates into a two-phase structure of ferrite (α) and austenite (γ) during hot rolling. If α in the cast structure is coarse, it is separated from this. It has been considered that these two phases formed naturally become coarse and cause the above-mentioned inconvenience.

In order to reduce the occurrence of surface defects such as roping, it is considered most effective to refine the crystal grains of steel. Conventionally, "warm rolling" and "rolling with different diameters" have been used. However, it is difficult to achieve a sufficiently satisfactory recrystallization, and it is necessary to add a cold rolling and an annealing step. We did not want to evacuate the process that involved a significant cost increase.

On the other hand, increasing the number of equiaxed zones distributed in the cross-sectional direction of the continuous cast slab, which is a rolled material, leads to the refinement of the casting structure, but this was also confirmed to be effective in improving roping resistance. In order to suppress the roping phenomenon by equiaxing the solidified structure of the slab,
Attempts such as "electromagnetic stirring" and "a method of adding alloy elements such as Ti" have been made, but all of these methods have the following problems and have not been satisfactory.

That is, the low-temperature casting method is a casting method in which the casting temperature is made as close as possible to the solidification temperature, so that nozzle clogging easily occurs during operation, and in the worst case, casting becomes impossible,
Since the viscosity of the molten cast metal is high, the occurrence of slab surface flaws due to "entrainment of continuous casting flux (CC powder)" or "insufficient floating separation of non-metallic inclusions" is noticeable. For this reason, it is difficult to adopt the system on a mass production basis.

In addition, electromagnetic stirring is an effective method for equiaxed crystallization of the solidified structure, but the stable equiaxed crystallization rate is only about 60% or less of the cross section of the slab. It was below the lower limit of the equiaxed crystallization ratio of the cast slab from which a steel sheet excellent in quality could be obtained, ie, “equiaxed crystallization ratio: 70%”.

On the other hand, by adding Ti to ferritic stainless steel, it is easy to make the solidified structure equiaxed, but even in this case, in order to achieve “equiaxed crystallization ratio: 70% or more”, it is necessary to use low-temperature casting. Since the combined use is indispensable, the above-mentioned “problem pointed out by the low-temperature casting method” cannot be escaped. In addition, TiN, which is a reaction product of the added Ti and N present in the molten steel, can be used alone or in addition to the nozzle. The problem of being trapped on the slab surface as a reaction product with refractories and CC powder and causing surface flaws could not be ignored.

<Means for Solving the Problems> From the above-described viewpoints, the present inventors have found that there is no concern about occurrence of roping, which is a problem during cold working such as deep drawing, and that the ferritic stainless steel has good surface properties. Experiments from various angles to provide a means to stably mass-produce
In the course of the research, `` In order to mass-produce ferritic stainless steel with excellent roping resistance without introducing extraordinary equipment and difficult conditions in actual work, it is necessary to add Ti to equiaxed The most practical method is to increase the conversion rate, and by adjusting the amount of Ti added, the N content that easily reacts with Ti, and the casting temperature, ferritic stainless steel with favorable properties can be stabilized without taking special measures. There is a possibility that it is possible to obtain it. "

Therefore, based on the above-mentioned feeling, the present inventors first considered, from the aspect of the component composition, the equiaxial crystal ratio of the cast structure that determines the roving resistance of the ferritic stainless steel and the surface flaw occurrence condition which is a major determining requirement of the quality evaluation. As a result of continuing research focusing on the examination of "Ti content and N content in molten steel", which is thought to have an effect on the above, the following findings were obtained.

(A) For ferritic stainless steel,
With the increase of Ti content and N content, "Equiaxed crystallization ratio"
And "frequency of occurrence of surface flaws" also increased, indicating an inconvenient trend, but in a comparatively narrow range, "a region of 70% or more preferable equiaxed crystallization rate for roving resistance" and "surface flaw generation rate" Is an allowable range "
Clearly there are Ti and N content ranges.

That is, Fig. 1 shows the relationship between the Ti content and the N content in steel and the equiaxed crystallization ratio of a continuous cast slab in a ferritic stainless steel containing 17% Cr (hereinafter, the component ratio is shown by weight%). The graph shows that the equiaxed crystallization ratio increases with the increase of the Ti content and the N content in the molten steel. When the Ti content and the N content exceed the curves in FIG. This shows that the crystallization ratio can be obtained stably. On the other hand, FIG. 2 is a “graph showing the relationship between the Ti content and the N content in steel and the surface flaw of the continuous cast slab” in the same ferrite stainless steel containing 17% Cr.
The occurrence of surface flaws also increases with the increase of Ti content and N content in molten steel, and unless the Ti content and N content are suppressed to values below the curve in FIG. 2, surface flaws are suppressed to an acceptable range. Indicates that it cannot be cut. Attention is drawn to FIG. FIG. 3 shows the results shown in FIG. 1 and the results shown in FIG. 2 collectively in the same figure.
“Equiaxed crystallization ratio: Ti and N that can be obtained stably at 70% or more
It is clear that there is a region (hatched region in FIG. 3) in which the “content” and “the Ti and N contents that can stably suppress the surface flaws within an acceptable range” are present. Is shown.

(B) However, the curve shown in FIG. 1 is obtained from the equation log [% N] = − 19755 / (T + 273) + 7.78 + 0.07 [% Ti] − when the solidification temperature is T (° C.). log [% Ti] +0.045 [% Cr] ……………………………………………………………………………………………………………… (1) On the other hand, the curve shown in FIG. 2 indicates that “17% Cr” calculated by the above equation (1) when the casting temperature when the superheat (ΔT) is 60 ° C. is T (° C.). Of Ti-N at casting temperature (1560 ° C) in ferrite stainless steel
Good equilibrium curve ".

FIG. 4 shows the graph shown in FIG. 3 at a casting temperature (1560 ° C.) at a solidification temperature (1500 ° C.) and a superheat degree (ΔT) of 60 ° C. in a ferrite stainless steel containing 17% Cr.
The Ti-N equilibrium curves are also shown, and the above fact can be clearly confirmed from FIG.

(D) Therefore, looking at the above-mentioned metallurgical drawing of FIG. 4 from the viewpoint of phenomena, the ferrite stainless steel containing 17% Cr has a higher solidification temperature than the Ti-N equilibrium curve at the solidification temperature of 1500 ° C. When the Ti and N contents of the steel are large, equiaxed crystals are easily formed, while the superheating degree (ΔT) is 60 ° C. and the casting temperature (1560
C)), the lower the Ti and N content in the steel than the Ti-N equilibrium curve at 1,500 ° C
In the region of Ti and N content surrounded by the Ti-N equilibrium curve at 1560 ° C, the drawing process is described as “Equiaxed crystallization ratio is 70% or more” and “Almost no surface defects” It shows that a high-quality continuous cast slab suitable for use in such applications can be obtained.

As described above, the equiaxed crystallization ratio in a region where the Ti and N contents in the steel are higher than the Ti-N equilibrium curve at a solidification temperature of 1500 ° C. in 17% Cr-containing ferritic stainless steel is considered to be large. This is because TiN precipitates before the start of solidification of the molten steel, and this precipitates as a nucleus to contribute to the formation of equiaxed crystals.In addition, the Ti-N equilibrium curve at a casting temperature of 1560 ° C. of
The reason that the surface flaws are less likely to occur when the content of Ti and N is reduced is presumed to be that TiN, which contributes to the surface flaws, is not precipitated in the molten steel at the time of casting. Therefore, in the Ti and N content region surrounded by the Ti-N equilibrium curve at the solidification temperature of 1500 ° C and the Ti-N equilibrium curve at the casting temperature of 1560 ° C, TiN is precipitated at the casting temperature. It can be seen that this is a region where a synergistic effect between the effect of removing the surface flaw generation factor due to the absence of the alloy and the effect of increasing the equiaxed crystallization ratio due to the precipitation of TiN before the solidification temperature can be sufficiently expected.

(E) Further, if the above findings are extended to Ti-added ferritic stainless steels in general, the equiaxed crystallization ratio of continuous cast slabs: 70% or more (sufficient roping resistance can be secured). Therefore, it is necessary to precipitate TiN at a solidification temperature or higher determined by the composition of the chemical components. On the other hand, in order to sufficiently suppress the occurrence of surface defects, it is necessary not to precipitate TiN at the casting temperature. In order to obtain a Ti-added ferritic stainless steel excellent in both roping resistance and surface flaw suppression effect, TiN which is a reaction product of Ti and N in the molten steel is determined by various components in the molten steel. It is an important requirement that precipitation be performed at a temperature higher than the solidification temperature (T ₁ ), but not at a temperature higher than the casting temperature (T ₂ ).

The present invention has been made on the basis of the above-described findings. "In the continuous casting of a Ti-added ferritic stainless steel containing 9 to 30% of Cr, the solidification temperature T ₁ (° C.) ), Casting temperature T ₂ (° C), and Ti in steel
Temperature T ₃ (° C) determined based on the amount of N, the amount of Cr and the amount of Cr
Is adjusted so as to satisfy the expression T ₁ ≦ T ₃ <T ₂ ≦ T ₁ +100,
It is characterized by being able to stably mass-produce a Ti-added ferritic stainless steel having excellent roping resistance and hardly any surface flaws.

It should be noted that the present invention is intended for ferrite stainless steel having a Cr content of 9 to 30%, because when the Cr content is less than 9%, the corrosion resistance of the stainless steel is not exhibited, while the content exceeds 30%. This is because the processability of adding Cr deteriorates and the industrial value of applying the method of the present invention is lost.

The solidification temperature T ₁ (° C.) of the target ferritic stainless steel is substantially determined by its chemical composition, and the equation T ₁ (° C.) = 1536− (100.3 [% C] −22.41 [% C] ² +13. 55 [% Si] -0.64 [% Si] ² +5.82 [% Mn] +0.3 [% Mn] ² + 4.2 [% Cu] +3.0 [% Mo] +4.18 [% Ni] +0. Calculated as 01 [% Ni] ² + 1.59 [% Cr]-0.007 [% Cr] ² ). Further, TiN precipitation temperature T ₃ of the Ti added ferritic stainless steel of interest, wherein log [% N] = - 19755 / (T 3 +273) +7.78 +0.07 [% Ti] -log [% Ti] Calculated as +0.045 [% Cr].

If the solidification temperature T ₁ , the casting temperature T _2, and the TiN precipitation temperature T ₃ do not satisfy the condition of [T ₁ ≦ T ₃ <T ₂ ], respectively, a sufficiently satisfactory roping resistance and surface properties can be obtained. Cannot be manufactured stably.

Furthermore, in actual operation, in order to secure breakout resistance and obtain a sound slab, there is an upper limit of the degree of superheat (ΔT). In view of this point, the applicable casting temperature is [solidification temperature + 100 ° C]. ] Casting temperature T ₂ (° C)
The upper limit was regulated under the condition of T ₂ ≦ T ₁ +100 using the solidification temperature T ₁ (° C.). The lower limit of the casting temperature is theoretically better if it is higher than the solidification temperature. However, in actual operation, if the superheat degree (ΔT) is less than 30 ° C., the pouring nozzle is likely to be clogged. For this reason, it is preferable that the casting temperature T ₂ (° C.) be set to [T ₁ +30] (° C.) or more.

Next, the present invention will be described more specifically with reference to examples and comparative examples.

<Example> First, smelted Ti added ferritic stainless steel such component compositions shown in Table 1 in a conventional manner, 50 mm thickness while changing the pouring temperature T ₂ by the laboratory-scale continuous casting apparatus A slab was cast.

Then, for each of the obtained slabs, After examining the conditions and the equiaxed crystallization ratio, this was heated to 1200 ° C. and then hot-rolled to obtain a hot-rolled steel sheet having a thickness of 3 mm. continue,
These hot-rolled steel sheets were annealed under conditions of (soaking at 830 ° C for 16 hours and then furnace-cooled) and then cold-rolled to a thickness of 0.5 mm,
Further, annealing was performed under the condition of [overheating at 830 ° C. for 1 minute and air cooling] to obtain a cold rolled sheet product.

Then, a JIS No. 5 tensile test piece was collected from the obtained cold rolled sheet product, and after applying a 20% tensile strength, the roping resistance was evaluated.

The results thus obtained are also shown in Table 1.

As is clear from the results shown in Table 1, the ferritic stainless steels manufactured under the conditions specified in the present invention show satisfactory results in both roping resistance and surface flaw state. , (Solidification temperature T ₁ ≦ Ti
N precipitation temperature T ₃ ] Test numbers 12 to 16,2 that do not satisfy the condition
In 0 to 21, 25 and 27, the evaluation of roping resistance was unsatisfactory of B to C. On the other hand, test numbers 17 to 19 which did not satisfy the condition of [TiN deposition temperature T ₃ <casting temperature T ₂ ] , 22〜24,
On 26 and 28, the occurrence of surface flaws exceeding the allowable limit was observed.

<Summary of Effects> As described above, according to the present invention, it has excellent roping resistance and good surface properties, and exhibits sufficiently satisfactory performance as a building material, a kitchen product, an electric product, or a car product material. Ferritic stainless steel can be safely produced without taking special facilities or measures that would increase the cost, but an extremely useful effect in industry can be brought about.

[Brief description of the drawings]

FIG. 1 is a graph showing “relationship between Ti content and N content in steel and equiaxed crystallization ratio of continuous cast slab” in 17% Cr-containing ferritic stainless steel. Fig. 2 shows the relationship between the Ti content and N content in steel and the surface flaw of continuous cast slab in ferritic stainless steel containing 17% Cr.
A graph showing. FIG. 3 is a graph in which the curve shown in FIG. 1 and the curve shown in FIG. 2 are collectively shown in the same figure. FIG. 4 shows the graph shown in FIG. 3 at a casting temperature (1560 ° C.) at a solidification temperature (1500 ° C.) and a superheat degree (ΔT) of 60 ° C. in a ferrite stainless steel containing 17% Cr. Ti
-N equilibrium curves are also shown.

Continuation of the front page (72) Inventor Tadahito Sudo 2-12-1 Minatomachi, Joetsu City, Niigata Prefecture Inside Naoetsu Research Laboratory, Japan Stainless Steel Co., Ltd. (56) References JP-A-49-41227 (JP, A) 119622 (JP, A) JP-A-57-127554 (JP, A)

Claims (1)

(57) [Claims] In a continuous casting of a Ti-added ferritic stainless steel containing 9 to 30% of Cr, a solidification temperature T ₁ (° C.) and a casting temperature T ₂ (° C.) are determined based on the composition of the steel. And a TiN precipitation temperature T ₃ determined based on the amounts of Ti, N and Cr in the steel.
(° C.) and a method for producing a Ti-added ferritic stainless steel, characterized by satisfying the following expression: T ₁ ≦ T ₃ <T ₂ ≦ T ₁ +100.