JPH0949010A - Production of continuous casting ferritic stainless steel slab having high equi-axial crystal ratio - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve ridging resistance property of a ferritic stainless steel by improving an equi-axial crystal ratio of a continuous casting ferritic stainless steel slab. SOLUTION: The continuous casting slab of the ferritic stainless steel is produced in the range of the ordinary casting temp. and casting speed decided with an equipment specification. At this time, Ti is added after deoxidizing the slab with Al at the step of adjusting the components before continuous casting, the continuous casting slab of the ferritic stainless steel having high equi-axial crystal ratio is produced by adjusting Al and Ti contents in the steel so that the ratio of Ti/Al becomes >=8.

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 continuous casting slab of ferrite type stainless steel having a high equiaxed crystal ratio by appropriately adjusting the components in steel.

[0002]

2. Description of the Related Art Ferritic stainless steel slabs manufactured by continuous casting tend to develop columnar crystals from the surface of the slab to the inside during the solidification process, and the equiaxed zone in the center tends to narrow. . In the solidification structure in which columnar crystals have developed, band-shaped coarse ferrite grains due to columnar crystals tend to remain in the final steel sheet product. Therefore, when the steel sheet is pressed, irregularities called ridging occur along the rolling direction. ， There is a problem that the appearance is significantly impaired. Further, if the unevenness of this ridging is large, it must be removed by polishing, and therefore an extra step is required.

Although the problem of ridging of ferrite type stainless steel sheets has been improved in all aspects,
Suppressing the growth of columnar crystals in the continuous casting slab, which is the basic factor, in other words, increasing the equiaxed crystal zone (increasing the equiaxed crystal ratio) is the fundamental solution. .

It is known that the equiaxed crystal ratio of a ferritic stainless steel continuous cast slab is affected by casting conditions.
It is generally said that the equiaxed crystal ratio increases when the casting temperature is low and the casting speed is slow.

In order to increase the equiaxed crystal ratio, attempts are made to destroy the columnar crystals by electromagnetic stirring or ultrasonic vibration during continuous casting, and measures to suppress the growth of columnar crystals by casting at the lowest possible temperature. Has been taken. However, even if such a method is used, it is not always possible to completely suppress the occurrence of ridging, and in this case, the slab extraction temperature and finish rolling temperature during hot rolling are lowered to randomize the metal structure. Alternatively, measures have been taken to uniformly refine the metal structure by twice cold rolling annealing or three cold rolling annealing.

[0006]

As described above, although measures such as control of casting conditions and metallographic control after hot rolling impose a heavy load on the operation, the problem of ridging of ferritic stainless steel is completely eliminated. The reality is that it cannot be solved. Therefore, the present invention has an object to suppress the growth of columnar crystals in a continuously cast slab, which is a basic factor in the occurrence of ridging of the ferrite stainless steel, from the viewpoint of adjusting the composition of the steel.

In particular, Ti-containing ferritic stainless steel tends to cause ridging. Therefore, the main problem is to increase the equiaxed crystal ratio of the continuously cast slab of Ti-containing ferritic stainless steel.

[0008]

According to the present invention, when a continuous casting slab of ferrite type stainless steel is manufactured within a normal casting temperature range and casting speed range determined by equipment specifications, before continuous casting After deoxidizing Al at the component adjustment stage, Ti
Was added, the content of Al and Ti in the steel was changed to Ti
Provided is a method for producing a continuous cast slab of ferrite type stainless steel having a high equiaxed crystal ratio, which is characterized by adjusting the contents of these elements so that the ratio of / Al is 8 or more.

Further, according to the present invention, when a continuous casting slab of a ferrite type stainless steel is manufactured within a range of a normal casting temperature and a range of casting speed determined by equipment specifications, at the stage of component adjustment before continuous casting. After deoxidizing Si, add Ti,
At that time, the Ti content in the steel is adjusted to be in the range of 0.01 to 0.05%, and the following formula (1), namely, the Ti content x (1560 ° C-casting temperature) x 100
A method for producing a continuous casting slab of ferrite type stainless steel having a high equiaxed crystal ratio, characterized in that the casting temperature is adjusted so that the value calculated by the formula (1) is 42 or more.
Here, the casting temperature is the temperature of molten steel in the tundish.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION When Ti is added to molten steel deoxidized by Al during continuous casting slab production of Ti-containing ferrite type stainless steel, the content of Al and Ti in the steel is Ti /
It was found that by adjusting the contents of these elements so that the Al ratio is 8 or more, a slab having an equiaxed crystal ratio of 60% or more can be obtained. In particular, the Al content in steel is 0.
001 to 0.040%, Ti content is 0.10
The Ti / Al ratio is preferably 8 or more in the range of 1.00%. Other components than Al and Ti are not particularly limited, but this effect is particularly remarkable when the C content is in the range of 0.001 to 0.030%.

When a Ti-containing ferrite stainless steel is produced without deoxidizing Al, Si is deoxidized at the stage of adjusting the components before continuous casting, and then Ti is added. The Ti content of was adjusted to be in the range of 0.01 to 0.05%, and the value calculated by the above formula (1) was 4
It was found that the equiaxed crystal ratio of the obtained slab can be increased to 60% or more by adjusting the casting temperature so as to be 2 or more, preferably 80 or more. In this case, the Si content should be 0.75% or less, and the C content should be 0.12%.
% Or less.

In both the former case of deoxidizing Al and the latter case of deoxidizing Si, the suppression of columnar crystals of the continuous casting slab (increased equiaxed crystal), which is the subject of the present invention, causes ridging. The method of the present invention can be applied to any ferrite composition stainless steel, and any Ti / ferrite composition stainless steel composition can be used.

Among these steels, those having a typical composition are given as follows:
% By weight, C: 0.001 to 0.030%, Si: 1.0
0% or less, Mn: 1.00% or less, Cr: 10.50-2
0.00%, Mo: 2.20% or less, Ti: 0.10-1.
Ferrite-based stainless steels including 00%, Al: 0.001 to 0.040%, the balance being unavoidable impurities and Fe. This steel is a ferritic single phase steel with a relatively high Ti content. Hereinafter, this steel will be referred to as "low C, high Ti Al deoxidized steel".

In the low C high Ti Al deoxidized steel ferrite single phase steel, it is preferable that C is low in consideration of corrosion resistance, so it is set to 0.030% or less, and Si increases the strength, but if too much, Since it causes deterioration of workability, it is set to 1.00% or less, and if Mn is too much, it promotes the formation of non-metallic inclusions.
Since it deteriorates the workability, it should be 1.00% or less, and Cr is preferably high from the viewpoint of corrosion resistance, but it is 10.50 to 20.00% in consideration of economic efficiency. Mo improves corrosion resistance, but it is expensive. Therefore, considering economic efficiency, it is set to 2.20% or less, and Ti fixes C and N to improve corrosion resistance.
If it is too large, there is a concern that surface flaws will occur.
It is 1.00%.

In the latter case, the typical composition of components for deoxidizing Si is as follows: C: 0.12% or less by weight, Si: 0.75% or less, Mn: 1.0% or less, Cr:
16.00-18.00%, Ti: 0.01-0.05%,
A ferritic stainless steel composed of the balance unavoidable impurities and Fe can be mentioned. This steel has a relationship in which a trace amount of Ti is added to SUS430. In the case of this steel, since the oxygen concentration in the steel is relatively high when Si is deoxidized, TiO ₂ is generated when the Ti content is large, and there is a concern that surface flaws may occur, so the Ti content is 0.01%. It is set to ~ 0.05. Hereinafter, this steel will be referred to as "low Ti Si deoxidized steel". Even in the case of this low Ti Ti deoxidizing steel, some Al is contained. However, the amount is 0.02% or less unless otherwise specified.

The inventors of the present invention have described the above "low C high Ti Al
We have extensively investigated various factors affecting the equiaxed crystal ratio of continuously cast slabs of Ti-containing ferritic stainless steels such as "deoxidized steel" and "low Ti Si deoxidized steel". I got the knowledge.

[Factors Affecting Equiaxed Crystal Ratio of Low C, High Ti Al Deoxidized Steel] As a test material, C: 0.001 by weight%
~ 0.030%, Si: 1.00% or less, Mn: 1.00
% Or less, Cr: 10.50 to 20.00%, Mo: 2.2
0% or less, Ti: 0.10 to 1.00%, other unavoidable impurities, and ferritic stainless steel consisting of Fe are used to produce continuously cast slabs with varying amounts of Al and Ti. Samples are produced from this slab. The samples were sampled to measure the equiaxed crystal ratio, and the effects of the components on the equiaxed crystal ratio were investigated.

As a result, it was confirmed that the lower the Al content and the higher the Ti content, the higher the equiaxed crystal ratio of the continuously cast slab, that is, the larger the Ti / Al ratio, the higher the equiaxed crystal content.

Further, based on this result, in order to know the influence of other factors on the equiaxed crystal ratio, the equiaxed crystal ratio was changed between the casting temperature, the casting speed, the O content, the N content, the Ti content and the Al content. The following regression equations (2) and (3) were obtained by performing multiple regression analysis with.

Y (equiaxed crystal ratio) = 449.1 + 29.5 × Ti (%)-118.5 × Al (%) + 4.13 × (Ti / Al) −571.0 × N (%)-0 .26 x casting temperature (° C) -36.15 x casting speed (m / min) +0.129 x O (ppm) ··· (2)

Y (equiaxed crystal ratio) = 449.1 + Ti × (29.5 + 4.13 / Al) -118.5 × Al (%) −571.0 × N (%) − 0.26 × casting temperature ( ℃) -36.15 x casting temperature (m / min) +0.129 x O (ppm) ··· (3) However, these regression equations are 29 observations, multiple correlation coefficient R
= 0.908.

From this multiple regression equation, Al, N, casting temperature,
The lower the value, the higher the equiaxed crystal ratio,
It was found that the higher the values of i and O, the higher the equiaxed crystal ratio. However, since the absolute value of the partial correlation coefficient (t value) of O, N, the casting temperature, and the casting speed is small, it does not affect the equiaxed crystal ratio so much.

The analytical results obtained from this multiple regression analysis are summarized in Table 1. Among the factors (X values) in Table 1, the X value 1 (Ti amount) that has a particularly large effect on Y,
The observed value graphs of the X value 2 (Al amount) and the X value 3 (Ti / Al) are shown in FIGS. 1, 2 and 3. In addition,
In these tests, the equiaxed crystal ratio was measured by cutting out the cross-section of the continuous cast slab, polishing the cross-section, etching with aqua regia, and then determining the ratio of the columnar crystal zones of the macrostructure that appeared on the surface. Is. As illustrated in FIG. 4, this ratio is obtained by measuring the width of the equiaxed zone at three points and simply averaging the ratio of the widths using the formula in the figure.

[0024]

[Table 1]

From these results, it is understood that Y (equiaxed crystal ratio) can be satisfactorily organized especially with Ti / Al, and Y increases with the increase of Ti / Al (FIG. 3). Since ridging can be reduced if the equiaxed crystal ratio is 60% or more, it is understood from FIG. 3 that Ti / Al having the equiaxed crystal ratio of 60% or more exists in the vicinity of about 8.

However, as can be seen in FIG. 2, the decrease in Y is remarkable when the amount of Al increases. For example, the amount of Al is 0.05
If it is more than%, Y will drop to 30% or less. However, if the Al amount is 0.04% or less, Y can be secured to be 60% or more. Therefore, if the Al amount is 0.04% or less and Ti / Al is 8 or more, a high equiaxed crystal ratio of 60% or more can be secured.

Next, the result of examining the influence of each factor on the equiaxed crystal ratio of "low Ti Si deoxidized steel" will be described.

[Factors Affecting Equiaxed Crystal Ratio of Low-Ti Si-Deoxidized Steel] As a test material, in% by weight, C: 0.12% or less,
Si: 0.75% or less, Mn: 1.00% or less, Cr: 1
6.00 to 18.00%, other unavoidable impurities and Fe
After deoxidizing silicon during vacuum degassing, using ferritic stainless steel consisting of Ti: 0.01 to 0.0
5 was added and continuous casting was performed. At that time, the casting temperature was also changed. A sample was taken from each slab and the interaction between the amount of Ti and the casting temperature on the equiaxed crystal ratio was investigated.

As a result, it was found that the equiaxed crystal ratio increased as the Ti content increased and the casting temperature decreased. When both are combined, it was found that the equiaxed crystal ratio has a good correlation with the formula of Ti × (1560−casting temperature) × 100.

Also in this example, in order to know the influence of other factors affecting the equiaxed crystal ratio, the equiaxed crystal ratio, casting temperature, casting speed, O
The following regression equations (4), (5) and (6) were obtained by performing a multiple regression analysis among the amounts, the amounts of N, the amounts of Ti and the amounts of Al. In these expressions, ΔT = 1560−casting temperature (° C.) is meant.

Y (equiaxed crystal ratio) = 8.12 + 18.5 × casting speed (m / min) −1868 × Ti (%) − 24.8 × Al (%) + 193 × N (%) − 0.002 × O (ppm) -0.376 × ΔT (° C) + 1.09 × (ΔT × Ti × 100) (4)

Y (equiaxed crystal ratio) = 8.12 + 18.5 × casting speed (m / min) −24.8 × Al (%) + 193 × N (%) − 0.002 × O (ppm) −0 .376 × ΔT (° C.) + Ti × (109 × ΔT-1868) (5) However, when ΔT is 18 or more, (109 × ΔT-18
68) is always positive

Y (equiaxed crystal ratio) = 8.12 + 18.5 × casting speed (m / min) −1868 × Ti (%) − 24.8 × Al (%) + 193 × N (%) − 0.002 × O (ppm) + ΔT × (109 × Ti-0.376) ··· (6) However, since 0.01 <Ti <0.05, (109 ×
Ti-0.376) is always positive.

These regression equations were obtained with 56 observations and a multiple correlation coefficient R = 0.887. Equation (4) is an equation obtained by multiple regression analysis. Equation (5) is a summary of this equation (4) for Ti, and equation (6) is a summary of ΔT.

From the equation (5), (109 × ΔT-1868) becomes positive only when ΔT is 18 or more, and Ti is effective. In other words, unless ΔT is secured to some extent, Ti
It was found that increasing the value had no effect on the equiaxed crystal ratio.

The Ti content is 0.01% <Ti <0.05.
%. Therefore, (109 × Ti in equation (6)
-0.376) is always positive, and it has been proved that the larger ΔT is, the more effective the equiaxed crystal ratio is.

From the results of this multiple regression analysis, the casting temperature, A
It was found that the lower the l, O, the higher the equiaxed crystal ratio, and the higher the Ti, N, and the casting speed, the higher the equiaxed crystal ratio. However, the casting rate, Al, O, and N are partial correlation coefficients (t
The absolute value of (value) is small, and it does not significantly affect the equiaxed crystal ratio.

The analytical results obtained from this multiple regression analysis are summarized in Table 2. Among the factors (X values) in Table 2, X value 1 (casting speed), X value 2 (Ti%), X value 6
The observed value graphs of (ΔT ° C.) and X value (ΔT × Ti × 100) are shown in FIGS. 5, 6, 7 and 8.
The equiaxed crystal ratio was measured according to the method shown in FIG.

[0039]

[Table 2]

From these results, it can be understood that Y (equiaxed crystal ratio) can be arranged especially by the value of (ΔT × Ti × 100) in FIG. 8, and the higher this value is, the better Y is. More specifically, if this value is 42 or more, the probability that the equiaxed crystal ratio is 60% or more is high, and if this value is 80 or more, the equiaxed crystal ratio is 60%.
The above can be secured.

[0041]

[Examples] Two types of ferritic stainless steels shown in Table 3 were melted by a converter-one degassing method, Ti was added at the final component adjusting stage, and the casting temperature and casting speed shown in Table 3 were obtained. The continuous casting slab obtained by continuous casting was measured for equiaxed crystal ratio by the method described in the text. As a result, all the slabs were slabs with a high equiaxed crystal ratio of 90%.

[0042]

[Table 3]

In the composition system of "Al-deoxidizing steel with low C and high Ti" in Table 3, the Ti amount and the Al amount were changed as shown in Table 4 (thus, the Ti / Al ratio was changed), and the same continuation was performed. Casting was performed. Table 4 shows the casting temperature, the casting speed, and the obtained equiaxed crystal ratio at that time. In addition, a photograph of the cross section of the slab where these equiaxed crystal ratios were measured is shown in FIG.
6) and FIG. 10 (1/4 magnification). Of these, the A to F steels are comparative examples that deviate from the ranges specified in the present invention, and the G steel is an example of the present invention.

[0044]

[Table 4]

In the composition system of "Low Ti Si deoxidized steel" in Table 3, the same amount of Ti and casting temperature were changed as shown in Table 5, and the same continuous casting was performed. The obtained equiaxed crystal ratio is shown in Table 5.
It was also described in. In addition, photographs of the cross section of the slab where these equiaxed crystal ratios were measured are shown in FIG. 11 (magnification × 1/6) and FIG.
(Magnification 1/4). Of these, steels (a) to (e) are comparative examples outside the range specified in the present invention, and steel (f) is an example of the present invention.

[0046]

[Table 5]

[0047]

As described above, according to the present invention,
The equiaxed crystal ratio of the continuous cast slab of Ti-containing ferrite stainless steel can be increased by adjusting its composition, and the continuous cast slab with the equiaxed crystal ratio of 60% or more can be stably manufactured. Therefore, the problem of ridging, which is peculiar to Ti-containing ferrite stainless steel cold-rolled steel sheet, can be prevented by the source of ridging. Countermeasures can be avoided, and industrially advantageous and stable production of high-quality steel sheets.

[Brief description of drawings]

FIG. 1 is an observed value graph showing the relationship between the Ti content and the equiaxed crystal ratio in a low C, high Ti Al deoxidized steel.

FIG. 2 is an observed value graph showing the relationship between the Al content and the equiaxed crystal ratio in a low C, high Ti Al deoxidized steel.

FIG. 3 is an observed value graph showing the relationship between the Ti / Al ratio and the equiaxed crystal ratio in a low C, high Ti Al deoxidized steel.

FIG. 4 is a diagram showing a method for measuring an equiaxed crystal ratio.

FIG. 5 is an observed value graph showing the relationship between the casting rate and the equiaxed crystal ratio in low Ti Si deoxidized steel.

FIG. 6 is an observed value graph showing the relationship between the Ti content and the equiaxed crystal ratio in low Ti Si deoxidized steel.

FIG. 7 is an observed value graph showing the relationship between T (° C.) and equiaxed crystal ratio in low Ti Si deoxidized steel.

FIG. 8: (ΔT × Ti × 1 in low Ti Si deoxidized steel)
00) and the equiaxed crystal ratio.

FIG. 9 is a photograph (magnification × 1/6) showing a metal structure of a slab cross section of an Al deoxidized steel with low C and high Ti.

FIG. 10 is a photograph (magnification × 1/4) showing a metal structure of a slab cross section of an Al deoxidized steel with low C and high Ti.

FIG. 11 is a photograph (magnification × 1/6) showing a metal structure of a slab cross section of an Al deoxidized steel with low C and high Ti.

FIG. 12 is a photograph (magnification × 1/4) showing the metallographic structure of the slab cross section of low Ti Si deoxidized steel.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C21C 7/04 C21C 7/04 B 7/06 7/06

Claims (6)

[Claims] 1. When manufacturing a continuous casting slab of ferritic stainless steel within a normal casting temperature range and casting speed range determined by equipment specifications, A is adjusted at the component adjustment stage before continuous casting.
Deoxidizing Ti and adding Ti.
And Ti content is adjusted so that the Ti / Al ratio is 8 or more, the method for producing a continuous cast slab of ferrite type stainless steel with high equiaxed crystal ratio, characterized by adjusting the content of these elements . 2. The manufacturing method according to claim 1, wherein the equiaxed crystal ratio is 60% or more. 3. An Al content in the range of 0.001 to 0.040% and a Ti content in the range of 0.10 to 1.00% Ti /
The manufacturing method according to claim 1, wherein the ratio of Al is 8 or more. 4. The method according to claim 1, wherein the ferritic stainless steel has a C content in the range of 0.001 to 0.030%. . 5. When manufacturing a continuous casting slab of ferrite type stainless steel within a range of normal casting temperature and casting speed determined by equipment specifications, S is added at the component adjustment stage before continuous casting.
i After deoxidizing Ti, add Ti.
The content is adjusted to be in the range of 0.01 to 0.05%, and the value calculated by the following formula (1) Ti content x (1560 ° C-casting temperature) x 100 ··· (1) is A method for producing a continuous casting slab of ferritic stainless steel having a high equiaxed crystal ratio, which is characterized in that the casting temperature is adjusted to 42 or more. 6. The value calculated by the equation (1) is 80 or more, Si
Content is 0.75% or less, and slab equiaxed crystal ratio is 6
The method for increasing the equiaxed crystal ratio according to claim 5, which is 0% or more.