JPS5852456B2 - Continuous casting method for undeoxidized steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing so-called undeoxidized steel, which is equivalent to rimmed steel or semi-killed steel, by continuous casting.

Attempts have been made to produce steel equivalent to rimmed steel or semi-killed steel by continuous casting for a long time, but it has not yet been put to practical use due to problems with operability and quality, especially surface bubble defects.

The current equivalents of these steel types are excessive AA
, Si deoxidized products are generally cast by subjecting them to continuous casting, but the use of alloys required for these deoxidizations reduces the advantages of continuous casting.

As an improvement measure, attempts have been made to reduce the amount of deoxidizers used, such as A# and Si, but as the amount of deoxidizers used decreases, the free oxygen concentration in molten steel increases and reaches a certain level. This oxygen reacts with the carbon in the molten steel to produce CO gas, which together with some H2tN2 forms pinhole defects on the slab surface.

On the other hand, there are several known examples in which electromagnetic stirring is used to continuously cast high-oxygen molten steel to remove air bubbles forming at the solidification interface.

These ideas are based on the idea that air bubbles generated at the solidification interface are peeled off and floated as quickly as possible by strong stirring.

The molten steel flow rate required for such physical bubble removal is approximately 1.0
m/sec or more, and this value is, for example, disclosed in JP-A-51-
This value agrees well with the value disclosed in Japanese Patent No. 2621.

However, when considering current continuous casting operations, mold powder is indispensable for lubricating the inside of the mold, preventing the temperature of the molten steel from dropping, preventing re-oxidation, adsorbing inclusions in the molten steel, etc. If there is an intensely stirred molten steel flow of 1 m/sec or more in the mold, there is an extremely high risk that this mold powder will be drawn in during positive solidification, resulting in quality defects.

Furthermore, even if it does not reach the point of entrainment, the powder will flow unevenly and the powder will not be able to perform its original function.

An example of a report on lowering the flow rate of molten steel by electromagnetic stirring is, for example, Japanese Patent Application Laid-Open No. 1983-68915, which discloses that an electromagnetic stirrer is installed below the mold perpendicular to the casting direction to reduce the flow rate of molten steel by 0.2 to 0. .5 m/sec or more.

However, although the lower limit value is lower than the above-mentioned 1.0 m/SeC, the underlying idea is to aim for a value as high as possible.

Moreover, since the molten steel flow to be obtained is a convection or circulation flow perpendicular to the molten metal surface in the mold, even though the lower limit of the molten steel flow has been lowered to, for example, 0.5 m/sec, it is not practical in actual work. However, there are still concerns about non-uniform distribution of powder on the meniscus, non-uniform powder flow, and even entrainment.

In order to eliminate the above-mentioned drawbacks, the applicant filed a patent application in 1973.
No. 3-99972 invention was provided.

According to this, the molten steel flows at a gentle flow rate of 0.1 to 0.5 m/sec in order to give a flow in the long side direction of the mold to the surface of the molten metal in the mold, and to suppress the generation of bubbles themselves that later become gas bubbles at the initial stage of casting. Can be done.

As described above, according to the invention of Japanese Patent Application No. 53-99972, an optimum flow rate has been found which prevents the generation of bubbles and does not interfere with casting operations.

However, subsequent experiments and research results by the present inventors revealed that undeoxidized steel could be continuously cast more stably if other conditions were set in addition to the above flow rate.

That is, when, for example, a circulating flow pattern perpendicular to the surface of the molten steel in the mold is adopted as an example of obtaining the stirring flow at the above-mentioned flow rate, a disadvantage arises.

Figure 1 schematically shows the results of a water model experiment with the above-mentioned stirring pattern.As shown in the experimental results, when the circulating flow is perpendicular to the molten steel surface 1 in the mold, this molten steel flow 2 When the molten steel reaches the molten metal surface, it is not possible to uniformly flow the molten steel at the desired flow rate over the entire molten metal surface 1.

In other words, the molten steel flow at the desired flow rate cannot necessarily be applied to the molten metal surface on the three short sides of the mold, and a stagnation portion of the flow occurs in the portion shown in FIG. 1A.

Furthermore, the molten steel flow that has reached the molten metal surface collides with the immersion nozzle 4, and the flow velocity is reduced, resulting in another 6" stagnation section on the opposite side of the mold on the short side 3 (section B in FIG. 1).

When the molten steel flow is convection, 6 stagnation areas also occur at the short sides of both molds or at the center.

This phenomenon occurs when casting with the above stirring pattern.
This is evidenced by the fact that the higher the oxygen content, the more surface bubbles appear on both short sides.

Therefore, even if the molten steel flows at a rate of 0.1 to 0.5 m/sec at the center of the mold in the long side direction of the hot water surface 1 according to the stirring pattern shown in Fig. 0 to 0.1 m/sec, and for this reason, surface bubbles tend to remain in this part, and depending on the oxygen concentration in the molten steel, the degree of bubbles may deteriorate significantly, causing defects such as surface scratches on the rolled product. It will give rise to

The above-mentioned stagnation area can be minimized by increasing the flow velocity of the molten steel, but in this case, the molten steel flow velocity at the center of the molten metal surface in the longitudinal direction of the mold should be set to a considerable value, for example, 1 m/Se.
C or higher, resulting in uneven distribution of the powder 5 due to uneven portions C of the powder 5 and areas where no powder 5 is present, uneven powder flow, and further, bad phenomena such as powder entrainment.

As described above, in the prior invention, the optimum flow velocity and the position at which it is applied are stabilized to enable continuous casting of unoxidized steel, but the present invention stably provides the flow velocity throughout the entire body. This explains how to give it.

The present invention is an improvement on the prior invention described above, and has developed a method of providing a stirring flow that enables stable continuous casting of undeoxidized steel using an electromagnetic stirring device.

In other words, by applying a flow rate of molten steel to the molten steel before and after the solidification start point so as to spread it to every corner of the cross section of the slab mold so as not to interfere with the casting operation, such a slow flow of molten steel as possible can be achieved. This prevents the generation of gas bubbles, which will be described later, throughout the entire mold including the short sides of the mold, thereby preventing powder entrainment, non-uniform distribution of powder on the surface of the mold, and non-uniform flow of powder. This makes it possible to continuously cast unoxidized steel without causing quality or operational problems.

The present invention will be explained in detail below.

The present inventors first investigated the formation mechanism of slab surface defects due to (CO) gas or (CO+CO2) gas generated when deoxidation is insufficient, as in the case of continuous casting of rimmed and semi-killed steel.

In other words, the process in which bubbles are generated at the solidification interface and a so-called pinhole is formed can be considered to be divided into bubble nucleation and bubble growth.

According to experimental results, once a bubble is generated, a gas partial pressure of Pco: 1 (aim) within the bubble is sufficient for the bubble to grow, but Pco: 2 to 3 is sufficient for nucleation at the solidification interface.
(ATM) was required.

This fact means that although nucleation itself is difficult to occur, once a nucleation occurs, bubbles grow easily.

The bubble-like formation that causes the pinholes mentioned above is caused by the concentration of carbon and oxygen in the molten steel, and as shown in Figure 2, this is likely to occur at the solidification interface where the constituent elements in the molten steel are concentrated. When the concentration exceeds a certain level, that is, C, for example Cx in FIG. 2, bubble nuclei are generated.

This indicates that bubble nuclei are generated which grow even later from the point of onset of solidification and give rise to pinhole defects.

Therefore, when a flow of more than a certain value is applied to the solidification interface, the concentration distribution of the elements involved in the formation of bubbles will be as shown by the dotted line at point 1 in Figure 2, and the value at the interface will be lower than Cx. In the case of C/, it is possible to prevent bubble nucleation.

In FIG. 2, Cl indicates the element concentration in the liquid phase, and C5 indicates the element concentration in the solid phase.

The present invention is based on the above research results. 1) The nucleation of bubbles occurs less than the growth of the bubbles, and the C1 is higher than a predetermined value.
2) the fact that the bubble nuclei that generate surface bubbles are already generated from the start of solidification; and 3) powder entrainment and non-uniform distribution in molten steel convection or circulation flow perpendicular to the mold surface. Furthermore, we focused on the fact that 4) Stagnant areas of the molten steel flow occur on both short sides of the mold, and pinholes are likely to occur. (to every corner of the hot water surface)
It is designed to provide a uniform flow of molten steel in the horizontal plane of the cross section of the mold at a speed that prevents the formation of bubble nuclei.

By applying such a molten steel flow to the molten metal surface in the mold, powder entrainment, average distribution, and pinholes on both short sides of the slab can be prevented. This effectively enables continuous casting of non-deoxidized steel.

That is, in the conventional example disclosed in Japanese Patent Application Laid-Open No. 51-2621, the idea is to physically float the bubbles themselves that are generated after the bubble nuclei are generated, so the flow velocity of the molten steel inside the mold is reduced to 1. Intense stirring of ~3m/sec or more is required, and
In the molten steel flow pattern that provides convection or circulation flow perpendicular to the mold surface as shown in Publication No. 915, stagnation occurs on the short sides of the mold inner surface, and the flow velocity required to prevent the generation of bubble nuclei occurs. As a result, the flow rate of molten steel in the mold becomes non-uniform, and the concentration of C and O at the solidification interface on the short side of the mold exceeds the value Cx required for nucleation, resulting in shortening of the slab. Surface bubbles remain on the edges.

In addition, there are also problems such as powder entrainment and non-uniform distribution.

In addition, in the previous application, although the flow rate itself was extremely low and effective, the method of providing molten steel flow was not clear, so a uniform molten steel flow rate could not be obtained, and stagnation occurred on the short sides of the mold, resulting in stable bubble formation. The occurrence could not be prevented.

In contrast to these examples, the present invention uses a mold 6 as shown in FIG.
The idea is to give the molten steel rotating flow 7 to the inner melt surface 1 itself, which is horizontal to its cross section and at a flow velocity sufficient to prevent the generation of bubble nuclei, and to spread the molten steel flow to every corner of the mold 6. , there is no stagnation part in the flow of molten steel at the molten metal surface 1, and an overall uniform molten steel flow γ is obtained.

This eliminates the need for an intense molten steel flow as in the past, and completely prevents the generation of surface bubbles with only the minimum amount of molten steel flow required to prevent the generation of bubble nuclei. Coupled with the fact that the flow pattern is a rotating flow γ horizontal to the cross section of the mold, the problems of mold powder entrainment, non-uniform flow, and even non-uniform distribution can be solved at the same time.

Thus, according to the present invention, it is possible to realize industrial production of unoxidized steel equivalent to rimmed or semi-killed steel.

Based on the present invention, the flow velocity of the rotational flow applied to the mold surface in the horizontal direction with respect to its cross section is as follows.

First, the undeoxidized steel that is the object of the present invention is almost the same as rimmed or semi-killed steel in the ingot method, and rimmed steel is
C<0.10%, Si<0.10%.

Mn<0.30%, P<0.025%, S<0.0
Add a deoxidizing agent of O~2.0KGI/l-5tee1 to 25 pieces of molten steel to reduce free oxygen in the molten steel to 50~400 ppm.
For semi-killed steel, C=0.10 to 0.
30%, Si=0.02~0.20%, Mn=0
．． 20-1600%, P<0.025%, S<0.0
25%.

5olAl=trs Free oxygen is 50 to 1100 pp.

Note that in rimmed steel, a boiling phenomenon occurs, making the casting operation itself difficult, so a lower upper limit is preferable.

The optimal flow rate of the surface rotation flow when continuously casting the above-mentioned undeoxidized steel was quantified through experiments.

In other words, the overall flow velocity of molten steel at the surface of the mold is 0.1 m/
sec or more, the desired effect of not generating surface bubbles even on the short sides of the slab was stably achieved.

The upper limit is regulated to ensure operability.

In the prior invention, since the flow is applied to the molten metal surface in only one direction, the flow velocity is highest at the center of the mold in the thickness direction, causing turbulence on the molten metal surface and uneven powder at the injection nozzle position. Therefore, it was necessary to keep the upper limit to a relatively low value of 0.5 m/sec.

On the other hand, with the horizontal rotating flow in the present invention, the desired flow velocity is only obtained at the periphery of the mold (solidification interface within the mold), and almost no flow occurs in the center, resulting in a stable molten metal surface. Therefore, the upper limit value could be raised higher than the 0.5 m/SeC of the previous application.

However, if the flow rate of this rotational flow is too fast, the powder will not be able to lubricate the mold surface and the slab surface, and one of the original functions of the powder will not be achieved.

The critical point at which this lubrication begins to become unbalanced is 1.0 m.
/sec, which was quantified experimentally.

If the lubricating action of the powder is unbalanced, the solidified thickness will also be unbalanced, leading to surface defects due to stress differences and, in the worst case, breakout.

As described above, the optimum flow velocity range of the rotational flow given to the molten steel in the mold according to the present invention is 0.1 to 1.0 m/sec.

Considering stability, the more preferable range is 0.1 to 0.5 m.
/sec.

FIG. 8 shows the relationship between the rotational flow velocity (m/5ec) of molten steel, pinhole determination, and powder entrainment occurrence rate.

In Figure 8, pinhole judgments A and B are passed, C and D.
is demoted and E is scrapped.

From this figure, it can be seen that according to the present invention, the rotational flow rate of molten steel is 0.1~
It can be seen that by limiting the speed to 1.0 m/sec, a sound slab without pinholes can be obtained without involving powder.

Next, as a specific means for applying a rotating flow of molten steel to the molten steel at the solidification interface in the mold according to the present invention, an electromagnetic stirring device is optimal in consideration of economic efficiency and stability.

The rotational flow rate is controlled by changing the current applied to an electromagnetic stirring device, for example, an electromagnetic stirring coil (linear motor).

Of course, the current applied will vary depending on the thickness of the copper plate in the mold, the electrical conductivity of the copper plate, etc., but the electrical conductivity of the copper plate is 4.2.
X 10” U/cm, when the copper plate thickness is 10 mm, the current is 40 to 400 A, and the rotational flow rate is 0.1 to 1.0 m/se.
Corresponds to c.

Note that the current and rotational flow speed have a linear function relationship.

Thus, it is necessary to provide an electromagnetic stirring device in the mold itself in order to apply a rotational flow to the molten metal surface in the mold.

In this case, the mold for steel casting usually uses a copper mold,
It is preferable to use a low frequency power source as the power source.

However, if the electromagnetic stirring device 9 is provided on only one long side 8 of the slab mold 6 as shown in FIG. 4, the molten steel will flow at the desired flow rate only on the side where the device 9 is provided, depending on the casting thickness. does not occur and rotational flow cannot be obtained.

If the output is increased, the effect is also exerted on the opposite side, and a rotational flow itself can be obtained, but in this rotational flow, the installation side is strong and the opposite side is weak, so that the solidification interface is uniformly distributed over the entire solidification interface. .1 to 1.0 m/sec cannot be satisfied, and the intended purpose cannot be achieved.

Furthermore, increasing the output itself requires installation of the electromagnetic stirring device 9 in the mold 6, which results in an increase in the size of the device, which is disadvantageous in terms of equipment.

However, as shown in FIG.
An electromagnetic stirring device 9 is provided at each of the molten metal, and thrust forces 10 and 10' in opposite directions are applied to each of the two, so that the output of the device 9 can be, for example, 0.1 to 1.0 m/min, without increasing the output of the device 9.
sec, which is effective for removing bubble nuclei, is uniformly provided, and this is a distant relative of the original purpose of the present invention.

When applying rotational flow to the molten metal surface in the mold with the above arrangement, it is insufficient to apply rotational flow only to the molten metal surface at a flow rate of 0.1 to 1.0 m/sec.

That is, in the actual process, since a considerable amount of scale-off is likely to occur during reheating and rolling of the cast slab, it is necessary to position the generated bubbles inside by an amount corresponding to this amount.

Therefore, in carrying out the present invention, the hot water level should be 0.1 to 1.
It is necessary not only to provide a rotational flow of 0 m/sec, but also to provide molten steel flow at the solidification interface until a solidification thickness equivalent to or greater than the above-mentioned scale-off amount is obtained.

However, in reality, this scale-off amount is about 1 to 2 m/m, and the corresponding solidification thickness position is 20 m/m below the molten metal surface.
Since the electromagnetic stirring device is installed at a position of about 20 m/m, care must be taken to ensure that a flow velocity of 0.1 m/sec or more is applied even at a position 20 m/m below the molten metal surface.

FIG. 6 shows the arrangement of the electromagnetic stirring device 9 in the height direction.

In this case, naturally, the flow velocity at the surface of the hot water is 0.1 to 1.0 m/sec.
Adjust the output of the electromagnetic stirring device so that

(The peak of the flow rate is the installation point.) The present invention performs continuous casting of undeoxidized steel as described above.

In the practice of the present invention, bubbles may occur inside the slab, but this is because a healthy surface layer is formed in excess of the scale-off amount by carrying out the present invention, so there is no oxidation and it is crimped in the subsequent process. However, it is not a defect and does not cause any practical problems.

It is well known to install an electromagnetic stirring device in a mold, but all of these methods involve the incorporation of powder into a slab mold and the prevention of uneven distribution for the purpose of casting undeoxidized steel such as rimmed and semi-killed steel. The rotational flow is as gentle as possible from the surface of the hot water.
This does not imply the idea of the present invention that the thickness is given within a range until a desired thickness is formed.

When installing the above-mentioned electromagnetic stirring device, if the stirring** flow interferes with the pouring jet 7', it may cause turbulence in the rotating flow of the hot water surface, which may cause powder entrainment, uneven distribution, and even stagnation. Therefore, it is preferable to install the injection nozzle 4 above the ejection hole 11 position as shown in FIG.

Further, in carrying out the present invention, it is preferable that the gap between the molten steel injection nozzle 4 and the long side 8 of the mold be approximately 20 m/m or more on one side, since this will not impede the flow of the molten steel.

Furthermore, the mold itself may be improved as shown in FIGS. 7a and 7b so as to obtain a rotating flow of molten steel at a desired flow rate with as little output as possible.

Although the cross-sectional shape of the slab obtained with this mold is special, it poses no problem during rolling and can be made into a final product using conventional processes.

Next, examples of the present invention and comparative examples will be shown.

The present invention was carried out on rimmed steel (A1, 2) and semi-killed steel (Arashi 3t4) shown in the table below.

The casting conditions are as follows.

The processing amount is 100T in both cases.

Casting dimensions: 250% (thickness) x 2100% (width) Casting speed: 0.7 m/min Electromagnetic stirring device installation position and thrust direction: 20 below the surface of the mold (1 piece on each long side in m, installed so that the direction of each thrust is opposite. Thickness of solidified slab at the installation position of the electromagnetic stirring device... 3 pieces. Output of the electromagnetic stirring device... Adjustment so that the flow rate of the horizontal rotation flow on the surface of the mold is within the range of 0.1 to 0.5 rn/SeC Injection nozzle... 100% outer diameter is used in the center of the mold Results Example 1 ~4 In all cases, a horizontally rotating flow was obtained within the mold, and a healthy slab without surface defects (pinholes) could be obtained without involving powder on the surface of the molten metal.

When we investigated the cross-sectional condition of the slabs obtained in the above examples, we found that in the rimmed steel slabs shown in 1 and 2, air bubbles were located within 23% from the surface, and in the semi-killed steel slabs shown in 3 and 4. There was no generation of bubbles.

The slabs obtained in Examples 1 to 4 were then reheated, hot rolled, and cold rolled to produce final products according to conventional methods, but no surface defects were observed in the final products in any case.

Comparative Example 1 Molten steel having the same composition as Examples 1 to 4 was cast so that the horizontal rotational flow at the surface of the mold was 0.1 m/SeC or less.

As a result, large bubbles were already generated in the surface layer of the slabs obtained from the molten steel of compositions 1 and 2, and the slabs could not be passed to subsequent steps.

In addition, the slabs obtained from the molten steel of compositions 3 and 4 had many pinholes on their surfaces, and when they were made into final products, the final products had many surface defects, resulting in a significant decrease in yield.

Comparative Example 2 Molten steel having the same composition as Examples 1 to 4 was cast so that the horizontal rotational flow at the surface of the mold was 1.0 m/sec or more.

As a result, fragmentary vertical cracks occurred on the surface of both the slabs obtained from the molten steel with compositions 1 and 2 and the slabs obtained from the molten steel with compositions 3 and 4, and regardless of their size, they were scarfed and the subsequent Although the product was passed through a conventional process, surface defects appeared on the surface of the final product, resulting in a decrease in yield.

This is thought to be caused by scarfing of a slab with large vertical cracks during the slab stage, which caused secondary scratches.

The above-mentioned vertical cracks are thought to be due to the fact that in this example, the rotational flow was strong and the powder tended to collect in the center, which prevented smooth lubrication between the slab and the mold.

Comparative Example 3 (Previous Application Example) Molten steel having the same composition as Examples 1 to 4 was stirred using an electromagnetic stirring device (below the mold) with a stirring pattern as shown in Fig. 1 so that the average flow velocity of the molten steel in the mold was 0.5 m/sec. Casting was performed by adjusting the output of the guide roll (which has a thrust perpendicular to the casting direction).

As a result, no pinholes were observed in samples 3 and 4, but in the slabs obtained from the molten steel of compositions 1 and 2, pinholes were observed on both short sides of the slab. Surface flaws occur on both ends of the product.
Yield decreased.

Therefore, when the flow velocity was increased to 1.0 m/sec, 1
Although pinholes did not occur in the ~4 samples, defects did occur in fragments due to powder entrainment.

Although this was removed by scarfing, some of it affected the final product, resulting in a decrease in yield.

The following powders were used in the above examples and comparative examples.

CaO/5i02=1.0 A1203=10% Na+=3.5% +=2.5% F-=4% C=4.5% Viscosity at 1511°C, 2.3 poise Melting point 11500C Comparative Example 1 In this case, the surface bubbles were generated because the generation of gas bubble nuclei could not be prevented due to insufficient flow velocity of the semi-rotational flow of water in the mold.

In Comparative Example 2, because the flow rate was too fast, the lubricating effect of the powder could not be obtained, and vertical cracks occurred.

In Comparative Example 3, the stirring pattern was a circulating flow perpendicular to the mold surface, so pinholes occurred at both ends of the slab, which would become stagnation areas in the case of high oxygen levels.
Also, since the flow velocity was increased, powder was drawn in.

As described above, the present invention is based on the analysis of the mechanism of gas bubble generation during casting of non-deoxidized steel.The present invention is based on the analysis of the mechanism of gas bubble generation during casting of non-deoxidized steel. A horizontal rotational flow is applied to the inner steel of the mold by a stirring device, which affects every corner of the mold and prevents the generation of gas bubble nuclei at the entire solidification interface, which affects the lubricating effect of the powder. It is possible to prevent the generation of gas bubbles over the entire surface of the slab without causing powder to be mixed in as with conventional stirring patterns. This made continuous casting of acid steel possible.

As is clear from the above Examples and Comparative Examples, the present invention contributes to the industrial implementation of unoxidized steel equivalent to rimmed and semi-killed steel.

[Brief explanation of the drawing]

Figure 1 is a model diagram showing the conventional stirring pattern, Figure 2 is a diagram showing the concentration status of component elements, Figure 3 is a model diagram of the horizontal rotational flow applied to the mold surface according to the present invention, and Figure 4 is a diagram showing the concentration status of component elements. Figure 5 shows a flow pattern obtained when an electromagnetic stirrer is installed on only one long side of a slab mold. Figure 6 shows the installation position of the electromagnetic stirring device in the mold height direction.
Figures 7a and 7b are diagrams showing cross-sectional examples of molds suitable for implementing the present invention, and Figure 8 is a diagram showing the molten steel rotational flow velocity (m/5ec) and pinhole determination. FIG. 3 is a diagram showing the relationship with the powder entrainment incidence rate. 1: Molten metal surface, 2: Molten steel flow, 3: Mold short side, 4: Injection nozzle, 5: Powder, 6: Mold, 7: Horizontal rotating flow, 8. : Long side of mold, 9: Electromagnetic stirring device, 10°10'...
Thrust direction, 11: Ejection hole.

Claims (1)

[Claims] 1 An electromagnetic stirrer installed on both long sides of the slab mold applies thrust to the molten steel in the mold in contact with both long sides in different directions along the length of the long sides to achieve a flow rate of 0.1 to 1.0 m/sec. A continuous casting method for undeoxidized steel characterized by forming a horizontal rotating flow of molten steel over the entire pro-solidification interface of the mold.