JPH0620629B2 - Continuous casting method for steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention is capable of producing slab surface defects such as melt level fluctuations, breakouts and vertical cracks, which tend to occur during continuous casting, regardless of steel grade.
In addition, it can be effectively prevented without changing the mold, and the casting speed in the "unsteady range of casting (when the casting speed rises up to the steady casting speed in the early stage of casting)" and "steady range" The present invention relates to a method of continuously casting steel, which is greatly increased.

<Prior Art> In recent years, in the production of steel, a continuous casting process using a vertical or curved type continuous casting machine has been indispensable, but such a continuous casting method allows blooms, billets, and slabs to be produced. When manufacturing steel slabs, etc., changes in the molten metal surface during casting, breakout caused by baking of the slab and mold in the mold, thermal stress caused by cooling,
Or it is said that surface flaws occur due to bending stress applied to the slab, or similar surface flaws occur during hot rolling of the obtained slab (particularly remarkable during hot direct rolling or hot charge rolling) Often caused trouble.

Among them, when continuously casting medium carbon steel with a carbon content of 0.09 to 0.20% by weight, the rate of vertical cracking occurring in the slab is high, and if such vertical cracking occurs, a slab care step is required. In particular, hot direct rolling and hot charging are not possible, which has been a major obstacle in promoting labor and energy saving in the steel manufacturing process.

The reason why cracking is likely to occur in medium carbon steel is that steel in this carbon content range has a high temperature at which it becomes an austenite single phase corresponding to the peritectic composition, and therefore austenite grains become coarse during cooling and cracking susceptibility is increased. However, in addition to this, a large amount of shrinkage during solidification due to the peritectic reaction tends to cause a local gap between the mold and the solidified shell, resulting in the formation of a heterogeneous solidified shell. It is also possible to mention the fact that it is easy to cause cracking due to thermal stress.

Therefore, when continuously casting medium carbon steel, in order to prevent vertical cracking and breakout due to uneven solidification, the casting speed must be reduced compared to when casting low carbon steel or high carbon steel. It was

Even in the case of low carbon steel or high carbon steel, the flow of powder between the mold and the solidified shell tends to become insufficient or uneven during the unsteady region or high speed casting. Correspondingly, the thickness becomes insufficient or non-uniform, and breakout and cracking due to thermal stress are likely to occur. Therefore, at least in the unsteady region, the casting speed had to be reduced significantly compared to in the steady region.

Therefore, in the past, even in the case of continuous casting of medium carbon steel or continuous casting of other steel types, inconveniences such as "breakout" and "slab surface cracking" that tend to occur in the unsteady region are reduced. In order to carry out high-efficiency casting with increased casting speed,
The following means have been tried.

(a) Optimize the viscosity and crystallization temperature of the powder to ensure uniform inflow of the powder, (b) Bond a metal with low thermal conductivity to the inner surface of the copper plate that constitutes the mold, or insulate air on the inner surface. To prevent non-uniform solidification by using a so-called "slow cooling mold" in which heat is removed from the molten steel by providing minute grooves to maintain the layer, (c) Molten steel between the immersion nozzle and the long side of the mold (Japanese Patent Application Laid-Open No. 172663/1986), (d) The continuous casting is carried out using a mold in which only a specific portion of the mold facing the dipping nozzle is slowly cooled (Japanese Patent Application No. 62- 1952
81), (e) Continuous casting is performed using a so-called "tapered mold" in which the inner dimension of the mold is reduced from the upper end to the lower end by the amount corresponding to the solidification shrinkage of the slab.

<Problems to be Solved by the Invention> However, according to the means described in the above items (a) and (b), when the casting speed is relatively slow (particularly 1.0 m / min)
The following) has a considerable effect, but the casting speed
It was not possible to completely prevent breakout and vertical cracking at high speed casting of 1.1 m / min or more.

On the other hand, the method shown in the item (c) is aimed at preventing vertical cracking and increasing the casting speed, but in practice, it is very difficult to sufficiently flow the molten steel in the mold. Inevitably, the flow tends to stay only locally, so that it is practically difficult to reliably prevent vertical slab cracking by this means.

Further, although the means described in the above (d) and (e) is effective in preventing vertical cracks during continuous casting of medium carbon steel, it is possible to use the same mold to obtain "carbon content 0.09% When casting "less than low carbon steel" or "high carbon steel with carbon content exceeding 0.20%", operational troubles (breakout, etc.) due to seizure between mold and slab are likely to occur. Problems such as these were recognized.

On the other hand, apart from these, with the aim of performing continuous casting of all steel types in the same mold without depending on the carbon content, during casting of medium carbon steel the amount of mold cooling water is reduced to slow cooling, while for other steel types medium carbon is used. Although a method of increasing the amount of mold cooling water compared to the case of steel has been attempted, it is difficult to sufficiently suppress the amount of solidification shrinkage during medium carbon steel casting to the extent that vertical cracking does not occur even with this method. It was

Further, as is clear from the relationship between the casting speed and the fluctuation amount of the molten metal surface shown in FIG. 9, when the casting speed is increased, the discharge flow velocity from the dipping nozzle is increased to increase the fluctuation of the molten metal surface. Since the frequency of breakouts and surface defects increases, it has been attempted to suppress the fluctuation of the molten metal level using radioisotope (RI) level meters and eddy current level meters, but the results are also satisfactory. It wasn't possible.

As described above, the stabilization means for the continuous casting operation that has been tried so far cannot completely prevent the fluctuation of the molten metal surface, the breakout and the flaws on the surface of the slab, which is insufficient for promoting the high speed casting. It was

Therefore, the object of the present invention is, in particular, breakout and slab surface defects that tend to occur in continuous casting of medium carbon steel, or the molten metal surface that occurs in the unsteady region of continuous casting common to all steel types (initial casting) By providing a continuous casting means capable of reliably preventing breakout and slab surface flaws due to fluctuations without requiring mold replacement, and capable of sufficiently increasing the casting speed regardless of the steady region or unsteady region. is there.

<Means for Solving the Problems> In order to achieve the above-mentioned object, the inventors of the present invention have various viewpoints regarding melt level fluctuation, breakout, and slab surface flaw when a slab is manufactured by a continuous casting machine. As a result of repeated research while studying, the following facts were found.

That is, as schematically shown in FIG. 1, a) The shape casting of the entire continuous casting mold 1 [FIG. 1 (a)] or a part thereof [FIG. 1 (b)] including the side in contact with the molten steel 2 is performed. It is composed of an alloy (the shape memory alloy portion is shown by meshes in the drawing), and Ms point (temperature at which martensitic transformation starts) before using for casting
The shape memory alloy portion is deformed at the following temperature to form a mold taper, and b) a cooling medium 3 circulating in the mold.
By increasing or decreasing the flow rate of (water, liquid nitrogen, liquid oxygen, He gas, etc.), the degree of taper of the mold can be freely changed according to the shape stored in the shape memory alloy part, and c) and the meniscus. A molten steel holding device 5 (hereinafter referred to as "electromagnetic holder") that uses electromagnetic force for the purpose of maintaining the shape of molten steel (including the solidified shell 4) and suppressing fluctuations in the molten metal surface is installed near the mold taper and meniscus. When performing continuous casting of steel while controlling the molten steel shape of
A) Even without requiring mold replacement, in the unsteady region (the region where the casting speed is increased in the initial stage of casting), the mold taper degree is reduced while keeping the same mold, and the electromagnetic holder 5 is provided between the mold 1 and the solidified shell 4. Since it is possible to control such that the molten steel shape is maintained so that the gap between the two becomes large, it becomes easy for the molten powder 6A to flow evenly between the mold 1 and the solidification shell 4 without deficiency, and the fluctuation of the molten metal level can be suppressed. Can be done effectively,
(B) In the steady region, by controlling the mold shape so that the mold taper becomes large during the casting of medium carbon steel, the coarsening of γ grains and the tendency of uneven solidification are effectively prevented. ) Furthermore, even if powder lubrication failure occurs in the steady state, in this case, the mold temperature rises, so the mold shape automatically changes to reduce the mold taper and the powder lubrication failure can be automatically eliminated. Regardless of this, it is possible to prevent breakout and slab surface flaws during casting as much as possible, and it is possible to greatly increase the casting speed.

In the drawings, reference numeral 6B denotes unmelted powder, 7 denotes an immersion nozzle, and 8 denotes slugrim.

The present invention has been completed based on the above findings and the like. "In the continuous casting of steel, the whole of the casting mold for continuous casting or a part of the side in contact with molten steel is formed of a shape memory alloy, and By controlling the taper of the mold by adjusting the flow rate of the flowing cooling medium, and casting while maintaining the electromagnetic force of the molten steel shape including the solidified shell near the meniscus, the casting speed without breakout or slab surface flaws. It is characterized in that it has been made possible to increase significantly.

As the shape memory alloy applied to the continuous casting mold according to the present invention, Cu-Zn system, Cu-Al system, Cu-Mu system, Cu-Zn-
Any shape memory alloy such as Al-based, Cu-Al-Ni-based, Cu-Zn-Ni-based may be adopted, and it is appropriately selected according to the required degree of change of the mold taper.

In addition, the change rate of the mold taper is not limited to the increase / decrease in the flow rate of the cooling medium (water, liquid nitrogen, liquid oxygen, He gas, etc.) and the above-mentioned selection of the chemical composition of the shape memory alloy. It goes without saying that the amount can be adjusted by changing the amount. By dividing the mold into several stages in the casting direction and changing the flow rate of the cooling medium and changing the chemical composition and deformation amount of the shape memory alloy, a so-called "multi-taper" in which the degree of taper varies in the casting direction is realized. Is also easy.

Next, the present invention will be described in more detail with the effects and advantages thereof.

<Operation> As described above, in the past, in the unsteady region of continuous casting (the region where the casting speed is increased), the thickness of the powder melting layer was not sufficient regardless of the steel type, and there was a sufficient gap between the mold and the solidification shell. In addition to the inability to ensure the inflow of powder, the fluctuation of the molten metal surface causes a large amount of breakout or slab surface flaws due to seizure between the mold and the solidified shell or uneven solidification, which significantly increases the casting speed. There was no choice but to decrease.

However, according to the present invention, which uses a mold in which the whole or a part of the side in contact with molten steel is made of a shape memory alloy and has a shape memory, according to the present invention, in this unsteady region, the flow rate of the cooling medium in the mold is reduced and the temperature of the alloy portion in addition to reducing the degree of taper as shown in FIG. 2 (a) by approximating the a _r point (the temperature at which the reverse martensite transformation is terminated),
Since the electromagnetic holder 5 holds the shape of the molten steel 2 and suppresses the fluctuation of the molten metal surface, the molten powder 6A can be made to flow between the mold 1 and the solidification shell 4 in a larger amount and more uniformly than in the conventional case.
The casting speed can be increased while effectively preventing breakout and slab surface defects.

Further, in the steady-state region of continuous casting, conventionally, when medium carbon steel is cast, the γ-single phase temperature is high, so that the γ grains are coarsened and the cracking susceptibility is easily increased. Since uniform solidification occurred, breakouts and vertical cracks frequently occurred. However, in the present invention, as shown in FIG. 2 (b), the cooling medium flow rate in the mold is increased to increase the temperature of the shape memory alloy part by M _s. Since the degree of taper of the mold is increased by setting the temperature below the point, the mold wall can be made to follow a large amount of solidification shrinkage caused by the peritectic reaction, nonuniform solidification is prevented, and the powder inflow is made uniform. At the same time, the increase in the cooling rate of the solidified shell 4 due to the increased degree of contact between the mold 1 and the solidified shell 4 suppresses the coarsening of γ grains. Therefore, breakout and vertical cracking of the slab are effectively prevented, and the casting speed can be increased.

Furthermore, if powder lubrication failure occurs in the steady state, it may cause breakout or slab surface flaws, but if powder lubrication failure occurs, the mold temperature rises, so a shape memory alloy is used to memorize a specific shape. In the mold according to the present invention, the degree of taper of the mold is automatically reduced, the inflow of powder is promoted, and the poor lubrication is eliminated.

Therefore, if the continuous casting method according to the present invention is applied, the molten metal level variation, breakout, and slab surface can be obtained under the above-described action, both in the casting of medium carbon steel and in the unsteady region casting of all steel types. Defects can be reliably prevented without changing the mold due to the steel type, that is, with the same mold, and the casting speed can be greatly increased in both the unsteady region and the steady region.

Next, the present invention will be described with reference to examples.

<Example> FIG. 3 shows a continuous casting machine (curvature radius 1) used in this example.
0m) is a schematic view showing a cast portion, and the mold 1 is made of a shape memory alloy of Cu-18wt% Zn-5wt% Al (Ms point: 29
(0 ° C, A _f point: 330 ° C), and it is a multi-tapered mold that can freely adjust the taper degree of each part by dividing the cooling water flow rate into four regions in the casting direction (No. 1). The symbol Q in FIG. 3 indicates the flow rate of the cooling water, and its value is Q ₁ > Q ₂ > Q ₃ > Q ₄ .

Note that FIG. 4 shows the taper adjustable region (within the shaded area) of the mold.

Now, using the above continuous casting machine, a continuous casting test of three types of low carbon steel, medium carbon steel and high carbon steel shown in Table 1 was conducted (at this time, the cooling water flow rate was appropriately adjusted so as to obtain the optimum mold taper). And the molten steel shape near the meniscus was controlled by the electromagnetic holder 5), each 200 m
A mφ slab was manufactured. However, for comparison, No. 1 strand of the continuous casting machine was cast by applying the conventional method (continuous casting using a normal mold with a taper degree of 1% / m) to obtain No. 2
Casting according to the method of the invention was carried out on the strands.

Then, when the frequency of occurrence of surface flaws in the slab in the unsteady region was investigated by this casting test, it was confirmed that the application of the method of the present invention significantly reduced the surface flaws as shown in FIG.

Further, FIG. 6 is a graph comparing the casting speeds in the continuous casting unsteady region of the application example of the present invention and the conventional example based on the test results. It is clear that the application of the method can significantly increase the casting speed in the unsteady region as compared with the conventional method.

Further, FIG. 7 shows the results of investigation of the frequency of occurrence of vertical cracks in the slab when casting medium carbon steel. The results shown in FIG.
It is clearly shown that application of the method of the present invention significantly reduces vertical cracking of the slab.

FIG. 8 is a graph comparing the “total casting speed of the steady region and the unsteady region” for the application example of the present invention and the conventional example. It is apparent that the application can significantly increase the casting speed as compared with the conventional method regardless of the steel type.

<Effects of the Invention> As described above, according to the present invention, even in the continuous casting of medium carbon steel, the continuous casting unsteady region of all other steel types.
Even in the initial stage of casting, the same casting mold is used without changing the casting mold to effectively prevent melt level fluctuations, breakouts, and slab surface flaws, and to achieve continuous casting speed in all processes throughout the steady and unsteady regions. It is possible to greatly improve the value, and it will bring an extremely useful effect in industry.

[Brief description of drawings]

FIG. 1 is a conceptual diagram illustrating an example of a continuous casting method according to the present invention, and FIGS. 1 (a) and 1 (b) show different examples. FIG. 2 is an explanatory view of the implementation situation of the continuous casting method according to the present invention, and FIGS. 1 (a) and 1 (b) show the casting situation under different conditions. FIG. 3 is an explanatory view of the continuous casting equipment used in the examples according to the present invention. FIG. 4 is a graph showing the taper adjustable region of the continuous casting mold used in the example according to the present invention. FIG. 5 is a graph comparing the occurrence of surface flaws in a slab cast by the method of the present invention and the conventional method. FIG. 6 is a graph comparing the casting speeds in the unsteady region of the method of the present invention and the conventional method. FIG. 7 is a graph comparing the occurrence of vertical cracks in a slab cast by the method of the present invention and the conventional method. FIG. 8 is a graph comparing casting speeds according to the method of the present invention and the conventional method. FIG. 9 is a graph showing the relationship between the casting speed and the fluctuation amount of the molten metal surface in continuous casting. In the drawing, 1 ... Mold, 2 ... Molten steel, 3 ... Cooling medium, 4 ... Solidification shell, 5 ... Electromagnetic holder, 6A ... Powder, 6B ... Unmelted powder, 7 ... Immersion nozzle, 8 ...... Sragrim.

Claims (1)

[Claims] 1. In continuous casting of steel, the entire casting mold or a part on the side in contact with molten steel is made of a shape memory alloy, and the taper of the casting mold is controlled by adjusting the flow rate of a cooling medium flowing in the casting mold. And a continuous casting method for steel, characterized in that the molten steel containing the solidified shell near the meniscus is cast by electromagnetic force while being cast.