JP2727887B2 - Horizontal continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a horizontal continuous casting method for blooms or billets, and more particularly, to a method for horizontally casting blooms or billets for hot extrusion of carbon steel, low alloy steel, stainless steel, high alloy steel, superalloy and the like. The present invention relates to a horizontal continuous casting method capable of suppressing the existence range and size of cavities and porosity generated at the center of a slab during continuous casting.

[0002]

2. Description of the Related Art Generally, in a hot extrusion pipe making method such as the Ugine-Sejournet method, the center portion is drilled at the time of pipe making. Removed, no quality problem. However, if the existing diameter of the cavity is larger than the diameter of the perforation, if the pipe is made as it is, it will cause flaws on the inner surface of the pipe,
This leads to poor quality of the tube. On the other hand, if the porosity is removed by increasing the diameter of the hole at the time of drilling in order to prevent the occurrence of a flaw on the inner surface of the pipe, the yield loss is caused by the amount of the hole and the economic efficiency is deteriorated.

[0003] Horizontal continuous casting equipment is lower in height than vertical and curved continuous casting equipment, and does not require a large slab support mechanism.
There are also advantages such as easy maintenance and inspection. For this reason, practical use has been attempted for small lots, various kinds of stainless steel, etc., for which continuous casting has been delayed. Furthermore, since continuous continuous casting does not require bending or straightening at high temperatures, continuous casting by this method has been further promoted in recent years for high alloy steels and Ni-base superalloys that are highly sensitive to hot cracking. I am trying to do.

However, in horizontal continuous casting, since the height of the equipment is low as described above, the static pressure of molten steel near the final solidification position of the slab becomes small, so that a sink cavity due to solidification shrinkage is formed at the center of the slab. It is easy to occur and the cavity tends to remain in the center. The cavity at the center tends to be generated as the sectional area or thickness of the slab or the casting speed increases, and the existing diameter of the cavity tends to increase. In addition, such as stainless steel, high alloy steel, and Ni-based superalloy, which have a lower melting point and a wider solid-liquid coexisting phase temperature range than general steel, are more likely to generate cavities.

In order to solve the above problems, Japanese Patent Application Laid-Open
Japanese Patent Application Laid-Open No. 57-75258 proposes a method in which at least two stages of a linear electromagnetic stirrer are installed and a center porosity is prevented by moving equiaxed crystal pieces to a crater end side.

Japanese Patent Application Laid-Open No. Sho 59-133957 discloses that at least two rotating magnetic field type electromagnetic stirrers are provided at a fixed interval determined from a slab drawing speed and a liquid core value at the rear end of the first stage electromagnetic stirrer. A method has been proposed in which the unsolidified molten metal is agitated to prevent sedimentation of equiaxed crystal pieces and improve the microcavity at the center of the slab. All of these methods are intended to disperse equiaxed crystal nuclei or equiaxed crystal pieces in an unsolidified molten metal by electromagnetic stirring. However, with these prior arts, prevention of center porosity and cavity was not sufficient.

[0007]

In order to solve the problems of the prior art, the present inventor has previously filed Japanese Patent Application No. Hei 3-23.
No. 2660 proposed a method of performing three-stage electromagnetic stirring to suppress porosity and cavity formation at the center of the slab.
However, it was found that the effect was insufficient when the casting speed was increased to improve productivity. Accordingly, a general object of the present invention is to develop a method that enables high-speed continuous casting without center porosity and cavities.

[0008] Accordingly, the present inventor has filed a separate application to
The second-stage electromagnetic stirring is performed at a position corresponding to the solidification start position in the mold, and the second-stage electromagnetic stirring is performed between the mold outlet and a position at which the solid fraction at the center of the slab does not exceed 0, and A horizontal continuous casting method was proposed in which the slab surface was strongly cooled from the time when the solid fraction at the center of the slab did not exceed 0 until the solid fraction at the center of the slab reached 1.0.

However, it has been found that this method has the following problems. (1) In horizontal continuous casting, slab size, casting speed,
The solidification start position at the center of the slab changes greatly depending on the secondary cooling conditions. (2) Among the factors affecting the solidification start position at the center of the slab, the casting speed usually fluctuates even during casting. In particular, in the early stage of casting, it is general to set a steady speed while gradually increasing the speed from a low casting speed. (3) It is most effective to start cooling from the point where the slab center just started to solidify.

[0010] That is, a specific object of the present invention is to form the center porosity and the cavity so that the center of the slab always starts to cool strongly from the position where solidification has started from the initial casting to the steady speed. The aim is to develop a method that enables continuous casting that has been eliminated.

[0011]

The present inventor has conducted various studies and experiments in order to solve the above-mentioned problems, and as a result, has found that the following means can solve the problems. (1) As an indirect method for determining the position where the slab center starts to solidify, the surface temperature of the slab during casting is always detected. (2) Provide a cooling device that can move in the longitudinal direction of the slab, so that cooling can be started at the point when the slab center just started to solidify, according to the change in the solidification start position of the slab center. Move.

Here, the gist of the present invention is as follows.
A method for producing a billet or bloom by horizontal continuous casting, wherein at least two stages of a rotating magnetic field type electromagnetic stirrer are arranged in series, and the first stage of electromagnetic stirring is performed at a position corresponding to a solidification start position in a mold. The second-stage electromagnetic stirring is performed between the exit of the mold and the position where the solid fraction at the center of the slab does not exceed 0, and the solid fraction at the center of the slab is downstream of these electromagnetic stirring devices. During the period from the point when the temperature exceeds 0 to the time when the solid phase ratio at the center of the slab becomes 1.0 , the solidification shrinkage is compensated for on the slab surface.
Strong cooling that can apply compensating compressive force to the slab surface
When performing the strong cooling, the cooling device capable of moving in the longitudinal direction of the slab while detecting the surface temperature and the casting speed of the slab, the position where the solid fraction of the slab center exceeds 0 This is a horizontal continuous casting method characterized by starting cooling from the beginning.

In the present invention, the term "solid phase ratio" refers to the volume ratio of the solid phase to the total volume in a certain region of the melt which is a solid-liquid coexisting phase. There is a one-to-one correspondence between the solid phase ratio and the temperature. The solid phase ratio is 0 above the liquidus temperature, and 1 below the solidus temperature. The distribution of the solid fraction can be calculated by actually measuring the temperature distribution in the slab or by calculating the heat transfer.

[0014]

Next, the operation of the present invention will be described more specifically with reference to the accompanying drawings. FIG. 1 shows an example of an apparatus for carrying out the present invention, and FIG. 2 shows a configuration of a cooling device moving system. In the figure, the molten metal once stored in the tundish 4
10 is cooled through the mold 5 and the secondary cooling zone 6,
Is grown and turned into a slab 8, which is drawn in a horizontal direction (right-hand direction in the drawing) by a pinch roll (not shown) through a mobile cooling device 3, which is a terminal cooling device, for example, a mobile spray cooling device. . The cooling device 3 is mounted on a rail and is configured to be movable.

According to the invention, a diameter of 265 mm × length
An example of casting stainless steel at a casting speed of 1.2 m / min using a 300 mm mold will be described below. According to the present invention, the electromagnetic stirrer 1 in the first stage mold, the electromagnetic stirrer 2 in the second stage, and the mobile cooling device 3 are provided at predetermined positions. The specifications of the electromagnetic stirrer are almost the same as those shown in Japanese Patent Application No. 3-232660,
For example, as shown in Table 1 below.

Spray cooling device 3 which is a mobile cooling device
The entire spray is mounted on a gantry, and the gantry moves on a rail laid in parallel with the axis of the slab in the longitudinal direction of the slab. The surface temperature of the slab is measured by the radiation thermometer 11. The surface temperature may be measured at at least one location, and may be provided at two or more locations to improve the accuracy of the system. Reference numeral 12 is a roller contact type casting speed detecting device.

The reason why the positions of the first and second stages of electromagnetic stirring are defined in the present invention as described above is as follows. First, the first-stage electromagnetic stirrer 1 has an effective electromagnetic stirring length of, for example, 200 mm, and a mold so that the molten metal 10 in the mold 5 at the position where the initial solidified shell 9 starts to be formed can be sufficiently stirred. 5 is arranged near the outer periphery.

As described above, the reason why the electromagnetic stirring is performed at the position corresponding to the solidification start position in the mold 5 is that the molten metal 10 is agitated on the front surface of the solidified shell 9 which solidifies at the time when the cooling rate is the fastest. By this, many nuclei of fine equiaxed crystals are melted
This is because they can be dispersed and released within the same. In addition,
In this example, the intensity of the first-stage electromagnetic stirring is obtained by applying a rotating magnetic field having a center magnetic flux density of 1200 gauss and a magnetic field rotation speed of 3 Hz. Negative segregation occurs at the solidification interface if
This is undesirable because it impairs the uniformity of the slab.

Next, the electromagnetic stirring device 2 of the second stage has an effective electromagnetic stirring length of, for example, 300 mm.
In the position of, the solid phase fraction (fs) at the center of the slab is 0, and there is a molten metal in the center of the slab only in the liquid phase. The reason why the electromagnetic stirring position of the second stage is set between the exit of the mold and the position where the solid phase ratio at the center of the slab does not exceed 0 is that the superheat of the unsolidified molten metal is reduced by stirring during this period. Uniform to prevent the growth of the equiaxed crystals dispersed and released in the melt 10 to increase the grain size, and to generate new equiaxed nuclei on the front surface of the solidified shell 9 to produce fine equiaxed crystals. This is because the number of crystals can be increased and sedimentation of equiaxed crystals can be prevented.

On the other hand, when the solid fraction at the center of the slab 8 exceeds 0, the superheat of the molten metal 10 cannot be used and the flow resistance rapidly increases, so that the above effects cannot be expected. . According to the present invention, strong cooling is performed when the solid phase ratio at the center of the slab 8 is between 0 and 1. At this time, the surface temperature and the casting speed of the slab 8 are detected, so that the center of the slab 8 is detected. The position where the solid fraction of the part is 0 is determined.

FIG. 2 shows an outline of a system for determining a solidification start position at the center of a slab. As shown in FIG.
The slab surface temperature and casting speed data are input online to the computer of the system, and the slab temperature distribution is calculated hourly by heat transfer calculation. Data necessary for heat transfer calculation, such as slab size, secondary cooling conditions, and physical properties of the cast steel type, are input to this computer in advance. In order to improve the accuracy of the heat transfer calculation, the surface temperature of the slab is constantly measured, and parameters on the system are determined so as to match the calculation result.

From the heat transfer calculation result, the temperature distribution of the slab is found, and the temperature of the center of the slab is estimated based on the temperature distribution. This is performed for each unit time, and the solidification position of the center is estimated each time. Then, information on such a solidification start position of the slab is output to the spray movement control device, and the slab is moved.
In the process of increasing the casting speed from the start of casting to the steady state, the solidification position fluctuates, but after achieving the steady state,
Solidification of the center of the slab starts at a substantially constant position.

As described above, the fluctuation of the casting speed during casting,
In accordance with changes in slab size, secondary cooling conditions, and steel type, the position where the slab center starts to solidify is detected, and the terminal intensive cooling of the slab surface is started just after the slab center starts to solidify. It starts.

The reason for cooling the surface of the slab in the final stage of solidification is that the surface is contracted by cooling the surface, and as a result, a compressive force is applied to the inside of the slab, causing cavities or porosity. The amount of coagulation shrinkage can be compensated, and the occurrence thereof can be suppressed. While the last stage of electromagnetic stirring is effective in preventing bridging due to the merging of equiaxed crystals, the method according to the present invention suppresses the formation of cavities or porosity itself by more positively applying compressive force. What you want to do. Therefore, at this time, the cooling capacity, that is, "strong cooling" in the present invention can be said to be a cooling capacity capable of applying such a compressive force, and specifically, for example, about 0.5 to 2 ° C./sec is desirable.

From the point where the center of the slab just started to solidify, the reasons for starting cooling are as follows: (1) If cooling is delayed,
The effects described above cannot be expected. (2) If the cooling starts earlier, the slab surface will undergo thermal contraction before the slab center starts to solidify, and the slab shrinks after the slab center starts to solidify. This is because the margin is consumed first.

[0026]

EXAMPLE Next, according to the present invention, a casting test was performed using the apparatus and system shown in FIGS. 1 and 2, and the results are shown in comparison with those of a comparative example. The casting conditions were such that a mold having a diameter of 265 mm and a length of 300 mm was used, and the average casting speed was varied between 0.8 and 1.6 m / min. 1st stage, 2nd stage
The arrangement and specifications of the electromagnetic stirring device at the stage were as shown in Table 1.

As shown in FIG. 2, the temperature distribution of the slab is continuously calculated by a heat transfer calculation by a computer, and thereby the solidification starting point at the center of the slab is determined. In this example, cooling was started immediately after the start of solidification at the center of the slab, and strong cooling was continued until solidification was completed. In the early stage of casting, during the period of raising the casting speed to the predetermined speed, the solidification start position was fluctuating, so the mobile cooling device was moved each time, but in the steady state, the solidification start point was almost the same, Strong cooling could be performed in a substantially fixed state.

The amount of cooling water at this time was changed depending on the average casting speed and the slab diameter, and the specific water amount was 0.25 l / kg-steel.
The movement amount of the spray cooling device in this example is 265 mm in diameter during the period from the start of casting to the transition to the steady state.
8m, 1.6m when casting a slab at a casting speed of 0.8m / min
18m in case of / min, and 0.8m / min for 300mm diameter slab
It was 12 m when cast at, and 23 m when 1.6 m / min.

On the other hand, the amount of movement of the cooling device accompanying the fluctuation of the casting speed in the steady state is 0.5 m when casting a 265 mm diameter slab at 0.8 m / min, 1 m at 1.6 m / min, and 0.8 when casting 300 mm slab at 0.8 m / min
m, 1.5 m in the case of 1.6 m / min.

As a comparative example, a fixed spray cooling device was used in which the casting conditions were exactly the same as above and the cooling start position was set in advance by the slab diameter and the average casting speed.

FIGS. 3 and 4 show a diameter of 265 mm,
The center porosity or the existing diameter of the cavity for each casting speed in the case of 300 mm is shown by the average value and the variation in the slab longitudinal direction.

As shown in FIGS. 3 and 4, according to the present invention, the average value of the center porosity or the existing diameter of the cavity is reduced, and the variation is also reduced, and the effect is clear.

[0033]

[Table 1]

[0034]

As described in detail above, according to the present invention, the formation of center porosity or cavity, which is a major problem in horizontal continuous casting slabs, can be performed without being affected by changes or fluctuations in casting conditions. It is possible to stably suppress.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing an example of an apparatus for carrying out the present invention.

FIG. 2 is a configuration diagram showing an example of a cooling device moving system used in the present invention.

FIG. 3 is a graph showing the effect of the present invention in an example.

FIG. 4 is a graph showing the effect of the present invention in the embodiment.

[Explanation of symbols]

1: First-stage electromagnetic stirrer 2: Second-stage electromagnetic stirrer 3: Mobile cooling device 4: Tundish 5: Mold 6: Secondary cooling zone 8: Cast piece 9: Solidified shell 10: Molten metal

Claims (1)

(57) [Claims] 1. A method for producing a billet or bloom by horizontal continuous casting, wherein at least two stages of a rotating magnetic field type electromagnetic stirrer are arranged in series, and the first stage of electromagnetic stirring is performed at a solidification start position in a mold. The electromagnetic stirring of the second stage is performed between the exit of the mold and a position where the solid phase ratio at the center of the slab does not exceed 0, and the center of the slab is downstream of these electromagnetic stirring devices. From the time when the solid phase ratio exceeds 0 to the time when the solid phase ratio at the center of the slab becomes 1.0 ,
Strong cooling that can apply compressive force to compensate for solidification shrinkage
When performing cooling on the slab surface and performing strong cooling,
A cooling device capable of moving in the longitudinal direction of the slab while detecting the surface temperature and casting speed of the slab, and starting cooling from a position where the solid fraction of the slab central portion exceeds 0. Continuous casting method.