JP7047647B2 - Continuous casting method for thin slabs - Google Patents

Description

The present invention relates to a thin slab continuous casting method, and more particularly to a thin slab continuous casting method for efficiently producing a small lot material on the premise of a process in which a thin slab continuous casting process and hot rolling are directly connected.

A thin slab continuous casting method for casting a thin slab (thin slab) having a slab thickness of 150 mm or less and further 40 to 100 mm is known. The cast thin slab is inserted into a tunnel furnace, heated, and then rolled by a small-scale rolling mill with 4 to 7 steps, or a CSP (Compact strip production) method, or a continuous casting machine and a hot rolling process. Various processes such as the ESP (Endless strip production) method, which is a directly connected complete continuous hot rolling method, have been proposed and are in operation. As the continuous casting mold used for thin slab casting, a method using a funnel-shaped mold and a method using a rectangular parallel mold are adopted. In continuous casting of thin slabs, it is necessary to secure productivity by high-speed casting, and industrially, high-speed casting of 5 to 6 m / min and a maximum of 10 m / min is possible (Non-Patent Document 1). reference). The present invention is directed to a continuous casting machine such as the ESP method and thin slab continuous casting in which the hot rolling process is completely continuous.

The continuous casting method of steel is usually poured from a ladle into a tundish and from a tundish into a mold via a dipping nozzle. At this time, it is general that the molten steel continues to have the same composition. On the other hand, in recent years, as the applications of steel have diversified, the lot size and the type of molten steel required for the steelmaking process have also diversified. Conventionally, when manufacturing different types of molten steel, a method of melting up to the ladle is adopted, and even in continuous casting, the component change during casting is performed for each ladle. Therefore, when melting small capacity molten steel, that is, small lot material, the process is adjusted so that there is as little surplus material as possible, but if adjustment is not possible and the order for one ladle is not completed, take it. One cup of ladle is cast in advance and cast, and the portion other than the one that corresponds to the order is stocked as surplus material, which has become a factor that tightens the warehouse capacity.

In addition, since the productivity of the continuous casting equipment is not improved only by continuous casting of the same steel type, "continuous casting of different steel types" is performed in which continuous casting is performed even between steel types having different components. The molten steel composition is changed for each ladle, and with the molten steel of the preceding charge remaining in the tundish, pouring of the molten steel of the next charge with a different composition is started. Therefore, the pre-charge component and the post-charge component are mixed in the tundish. Further, since the molten steel injected into the mold from the tundish is replaced as a discharge flow to a deep range of the unsolidified molten steel, a pre-charged and post-charged component mixing portion exists in the cast slab over a long range. This seam is scrapped because the steel material is not constant and the quality cannot be guaranteed. When changing the components of the ladle unit and replacing the tundish at the same time, insert an iron plate into the unsolidified molten steel in the mold during casting to prevent mixing of the components in the unsolidified molten steel in the mold. (Non-Patent Document 2). In this method, it is necessary to significantly reduce the casting speed when inserting the iron plate.

In the conventional continuous casting process, in order to reduce the generation of scrap at the seams of continuous casting of different steel types, in order to make the mixing area as small as possible when performing continuous casting of different steel types, the casting speed is significantly reduced or preceded. Efforts have been made to adjust the process so that the charge and the subsequent charge produce products with similar concentrations.

Among the prior arts, many methods of adjusting the components in the tundish (tandish aloing) have been reported as a method of manufacturing a small lot material.
In Patent Document 1, the inside of the tundish is divided into an upstream refining chamber and a downstream casting chamber, a closing device is provided in the partition portion, and the partition portion is closed at the time of component change during casting to enter the refining chamber. A method of casting a molten steel whose composition has been adjusted is disclosed in a mold in which an alloy is added to adjust the composition and a dummy bar is inserted in advance and preparation for casting is completed. Since a dummy bar is inserted when the composition is changed, continuous casting is interrupted at that point.
Patent Document 2 discloses a method of producing small lot steel pieces by adjusting the composition of molten steel in a tundish or molten steel discharged into a dedicated tundish and then pouring it into a mold for ingot formation. It does not correspond to the method of changing the composition while continuing continuous casting.

Japanese Unexamined Patent Publication No. 9-239501 Japanese Unexamined Patent Publication No. 2001-162352

5th Edition Steel Handbook Volume 1 Pig Iron / Steelmaking pp. 454-456 3rd Edition Steel Handbook II Ironmaking / Steelmaking Page 644 E.Takeuchi, M.Zeze, H.Tanaka, H.Harada and S.Mizoguchi: Ironmaking and Steelmaking, 24 (1997), 257. M.Zeze, H.Harada and E.Takeuchi: ISIJ-International, 39 (1999), pp.563.

On the other hand, in the thin slab continuous process targeted by the present invention, since it is a continuous process from casting to hot rolling and winding, a decrease in casting speed directly leads to a decrease in productivity and causes variations in the material of the steel sheet. In addition, since the thin slab continuous process is a process suitable for manufacturing thin steel sheets that cannot be manufactured by conventional processes, all steel grades that require thinning are targeted, and a wide range of steel grades (building materials to high) can be achieved with one mill. It is required to manufacture high-strength steel).

Therefore, there is a possibility of manufacturing steel grades that have a large concentration difference between the pre-charge and the post-charge compared to the conventional process. Therefore, it is not realistic, and it is considered desirable to adjust the components within the tundish (tandish aloing). However, the tundish allowing method described in Patent Document 1 and Patent Document 2 is a technique premised on batch casting that is not oriented toward a continuous process as in this case, and cannot be applied to a continuous process.

The present invention is premised on a continuous thin slab process, and an object of the present invention is to provide a continuous casting method of thin slabs capable of producing small lot materials with high efficiency and high yield while continuing continuous casting.

That is, the gist of the present invention is as follows.
[1] A thin slab continuous casting method in which molten steel contained in a ladle is transferred to a tundish and molten steel is injected into a continuous casting mold from a dipping nozzle provided at the bottom of the tundish.
A tundish weir is installed between the molten steel transfer part from the ladle and the immersion nozzle, and the tundish weir has a molten steel passage part, and the ladle molten steel separated by the tundish weir in the tundish. It has a heating means for heating the molten steel in the tundish on the downstream region side, with the transfer side as the upstream region and the opposite side as the downstream region.
A DC magnetic field generator that applies a DC magnetic field in the thickness direction over the entire width of the mold is arranged, and the upper part of the strand sandwiching the DC magnetic field band formed by the DC magnetic field generator is the upper molten steel pool and the lower part is the lower molten steel pool. Then, the molten steel is supplied from the immersion nozzle to the upper molten steel pool.
During continuous casting, by starting continuous addition of a predetermined element or its alloy to the molten steel in the tundish on the downstream region side or by changing the addition amount, the slab cast first and the slab cast later. Shards with different composition compositions were cast between the two, and later cast at the molten steel density ρ (kg / m ³ ) calculated by the following equation (1) from the composition and the molten steel temperature T (° C.) in the tundish. A method for continuous casting of thin slabs, wherein the molten steel density corresponding to the slab is lower than the molten steel density corresponding to the previously cast slab, and the molten steel density difference is 30 kg / m ³ or more.
ρ = 7000-8.0 (T-1823) -80.0 [C] -83.0 [Si] -21.2 [Mn] -67.1 [P] -84.0 [S] -113.0 [Al] -29.0 [Ti] +5.4 [Nb] ] -14.6 [Cr] +4.8 [Ni] +3.6 [Cu] +23.0 [Mo] -46.3 [V] (1)
In the above equation (1), the element symbol means the content (mass%) of each element.
[2] Plasma heating is used as a means for heating the molten steel in the tundish on the downstream region side, and the predetermined element or its alloy is continuously added while gas is blown from the bottom of the tundish in the downstream region. The method for continuously casting a thin slab according to [1].

INDUSTRIAL APPLICABILITY In the continuous casting of thin slabs, according to the present invention, it is possible to produce small lot materials with high efficiency and high yield while continuing continuous casting, and to reduce the component mixing region of the component changing portion.

It is a partial figure which shows the continuous casting state of the thin slab of this invention, and shows the state before the addition of the component in a tundish. It is a partial figure which shows the continuous casting state of the thin slab of this invention, and shows the situation immediately after the start of addition of the component in a tundish. It is a partial figure which shows the continuous casting state of the thin slab of this invention, and shows the state after the start of addition of the component in a tundish. It is a figure which shows the shape of a Tandish weir. It is a figure which shows the relationship between the density difference of molten steel of the front component and the back component, and the length of a component mixing part.

The present invention will be described with reference to FIGS. 1 to 5.
The present invention is intended for a continuous casting method of a thin slab in which molten steel contained in a ladle 1 is transferred to a tundish 2 and molten steel is injected into a continuous casting mold 4 from a dipping nozzle 3 provided at the bottom of the tundish 2. do. Continuous casting of thin slabs means continuous casting in which the thickness of the cast slab is 150 mm or less.

When trying to change the composition of the slab to be cast during continuous casting, the most commonly used method is to change the composition of molten steel in units of ladle. In this method, the same composition is continued for the amount of molten steel in one ladle. On the other hand, in the present invention, while the casting of molten steel in the same ladle is being continued, the molten steel component is changed by continuously adding a predetermined element or an alloy thereof to the molten steel in the tundish. As a result, the composition is changed in units of molten steel that is smaller than the amount of molten steel in the ladle, and small lot materials can be efficiently manufactured in a continuous process in continuous casting of different steel types.

It is necessary to shorten the length of the component mixing part before the component change (pre-component) and after the component change (post-component) as much as possible in the seam slab of different steel grade continuous casting. There are two places where the pre-component and the post-component are mixed, in the tundish and in the unsolidified molten steel in the mold. Hereinafter, a method for reducing component mixing at each location will be described.

In the present invention, as a measure for reducing the component mixing region in the tundish 2, a tundish weir 5 is installed between the molten steel transfer portion 21 from the ladle 1 and the immersion nozzle 3 (see FIG. 1). The tundish weir 5 has a molten steel passage portion 6. The ladle molten steel transfer portion 21 side divided by the tundish weir 5 in the tundish 2 is referred to as an upstream region 22, and the opposite side thereof is referred to as a downstream region 23. The molten steel transferred from the ladle 1 in the upstream region 22 passes through the molten steel passing portion 6 of the tundish weir 5, moves to the downstream region 23, and is further injected into the mold 4 via the immersion nozzle 3. .. Since the tundish weir 5 is provided, the molten steel in the tundish becomes a one-way flow from the upstream region 22 through the molten steel passing portion 6 toward the downstream region 23, and the molten steel moves from the downstream region 23 to the upstream region 22. Can be prevented.

In the present invention, a predetermined element or an alloy thereof is continuously added to the molten steel in the tundish in the downstream region 23 in the tundish. The added component is only mixed in the molten steel in the downstream region 23 and does not spread to the molten steel in the upstream region 22. As a result, the length of the mixed portion of the front component and the rear component can be reduced. As the molten steel capacity of the downstream region 23 is reduced, the mixing portion of the front component and the rear component in the tundish can be reduced. The addition of the element or alloy to the downstream region 23 can be performed by continuously adding a predetermined element or alloy to the downstream region 23 with a wire or the like by the component adding device 8.

Next, in order to reduce the mixed region of the pre-component and the post-component in the unsolidified molten steel in the mold, the DC magnetic field generator 9 is installed and the molten steel density difference between the pre-component and the post-component is formed. The method was applied. Hereinafter, they will be described in order.

The immersion nozzle 3 is provided with a molten steel discharge hole, and the molten steel in the tundish is discharged into the unsolidified molten steel in the mold via the discharge hole of the immersion nozzle. As shown in FIG. 1, it is common practice to provide discharge holes at two positions facing each other of the immersion nozzle to form discharge flows 27 flowing on both sides in the width direction of the slab. The discharge flow 27 discharged from the immersion nozzle 3 collides with the short-side solidification shell and is divided into an ascending flow 28 and a descending flow 29. In normal continuous casting, the downflow 29 reaches a deep position in the unsolidified molten steel pool. Therefore, immediately after the injection of the post-component is started, the molten steel of the post-component is carried to the downstream side of the unsolidified molten steel pool by the downward flow 29, and as a result, the length of the slab in which the pre-component and the post-component are mixed is increased. ..

In the present invention, a DC magnetic field generator 9 that applies a DC magnetic field in the thickness direction over the entire width in the mold width direction is arranged, and the upper part of the strand sandwiching the DC magnetic field band 26 formed by the DC magnetic field generator 9 is the upper molten steel pool 24. The lower part is the lower molten steel pool 25, and the molten steel is supplied from the immersion nozzle 3 to the upper molten steel pool 24. In the DC magnetic field band 26, a DC magnetic field is applied in which the magnetic field lines are directed in the thickness direction of the slab, and the magnetic flux density is made substantially uniform in the mold width direction. By forming such a DC magnetic field band 26, an electromagnetic brake is applied to the molten steel that is about to pass through the DC magnetic field band 26. Even if the discharge flow from the immersion nozzle 3 forms a downward flow 29, the flow is blocked in the DC magnetic field band 26 and is limited to the flow in the upper molten steel pool as shown in FIG. As a result, it is possible to prevent the back component molten steel from being carried to a deep position due to the downward flow caused by the discharge flow of the immersion nozzle 3. Since the molten steel is continuously supplied to the upper molten steel pool 24, the molten steel in the upper molten steel pool 24 passes through the DC magnetic field band 26 as a steady downward flow 33 and moves to the lower molten steel pool 25. The flow velocity of the steady downward flow 33 when passing through the DC magnetic field band 26 is usually a flow velocity close to the casting speed. Here, the DC magnetic field band 26 is in the same range as the core height of the DC magnetic field generator 9. The reason is that a DC magnetic field with a uniform magnetic flux density is applied within this range. If the magnetic flux density of the DC magnetic field band 26 is 0.3 T (tesla) or more, the downward flow 29 can be sufficiently suppressed. This point is also described in Non-Patent Document 3.
The higher the upper limit of the magnetic flux density is, the more preferable it is, but about 1.0 T is the upper limit for forming a DC magnetic field regardless of the superconducting magnet. A magnetic field having an appropriate magnetic flux density may be applied within the range of 0.3T to 1T depending on the casting conditions.

FIG. 1 shows a situation before starting component addition by the component addition device 8 (including before changing the component addition amount). The molten steel component is a pre-component in all of the upstream region 22, the downstream region 23, the upper molten steel pool 24 of the unsolidified molten steel, and the lower molten steel pool 25 in the tundish (pre-component region 31). FIG. 2 shows the situation immediately after the component addition by the component addition device 8 is started (including immediately after the component addition amount is changed). The region where the molten steel component is the rear component (post-component region 32) is dot-hatched. The molten steel in the downstream region 23 in the tundish has changed to a posterior component, and the molten steel in the upper molten steel pool 24 in the mold has also changed to a posterior component. Since the DC magnetic field band 26 is formed, the molten steel of the rear component is not mixed in the lower molten steel pool 25 below the DC magnetic field band 26.

FIG. 3 shows a situation after a predetermined time has elapsed from the start of component addition (change of component addition amount) by the component addition device 8. Due to the continuation of molten steel supply from the tundish 2 into the mold 4, the boundary between the front component region 31 and the rear component region 32 (component boundary portion 30) extends to the lower molten steel pool 25 below the DC magnetic field band 26. It is moving (descending). The unsolidified molten steel part surrounded by the solidified shell is in a state where molten steel flows including natural convection. In the state of FIG. 3, since the DC magnetic field band 26 is not arranged at the component boundary portion 30, if there is a flow of molten steel, the lower side (front component region 31) and the upper side (rear component region 32) of the component boundary portion 30 are present. ) May occur. As a result, the mixed region of the pre-component and the post-component is increased.

In the present invention, the molten steel density corresponding to the slab (post-component) cast later is lower than the molten steel density corresponding to the slab (pre-component) cast earlier, and the molten steel density difference is 30 kg / m ³ or more. It is characterized by being. Since the density of the molten steel of the rear component located above the component boundary 30 is lower than that of the molten steel of the front component located below the component boundary 30, the density difference causes the upper and lower parts of the component boundary 30 to be sandwiched between them. Mixing of molten steel between can be prevented. The molten steel density can be calculated from the component composition and the molten steel temperature T (° C.) in the tundish using the molten steel density ρ (kg / m ³ ) calculated by the following equation (1) (see Non-Patent Document 4). ).
ρ = 7000-8.0 (T-1823) -80.0 [C] -83.0 [Si] -21.2 [Mn] -67.1 [P] -84.0 [S] -113.0 [Al] -29.0 [Ti] +5.4 [Nb] ] -14.6 [Cr] +4.8 [Ni] +3.6 [Cu] +23.0 [Mo] -46.3 [V] (1)
In the above equation (1), the element symbol means the content (mass%) of each element. The component range to which the above formula (1) can be applied is as shown in Table 1.

The effect of the molten steel density difference and the relationship with the electromagnetic brake were investigated by a laboratory test, and when the molten steel density difference between the preceding charge and the succeeding charge was -30 kg / m ³ or more, the effect of reducing the mixing range was more remarkable. This was clarified by a test using a test continuous casting machine. The above formula (1) was used as the above formula for estimating the molten steel density.

In the test, a test continuous casting machine with a ladle molten steel amount of 8 tons was used. The tundish capacity is 5 tons, and the capacity of the downstream region 23 is 0.89 tons using the tundish weir shown in FIGS. 1 and 4 (A). An experiment was carried out at a mold cross-sectional size of 100 (thickness) mm × 800 (width) mm and a casting speed of 5.0 m / min. The DC magnetic field generator 9 was provided 600 mm below the meniscus 34, and the electromagnetic brake was operated. The electromagnetic brake was set under the condition that a DC magnetic field of 0.5 T was applied uniformly in the width direction.

As a component system, low carbon steel of 0.05% C-0.1% Si-0.1% Mn was used as a base composition (pre-component) and supplied from the ladle 1 to the upstream region 22 in the tundish 2. When the amount of residual molten steel in the ladle reached 5 tons, carbon was started to be added to the downstream region 23 in the tundish. The carbon addition was carried out by continuously supplying a wire coated with carbon with iron. By adjusting the wire addition rate, the amount of increase in carbon concentration was changed to make the back component, and the density of the back component was changed to -10, -30, -50 kg / m ³ with respect to the density of the front component. A test was conducted. In addition, the test was conducted under the condition that molybdenum was added and the molten steel density was changed by +10 kg / m ³ .

Immediately after the start of wire addition, it takes time for the molten steel component to rise from the pre-component to the post-component. The smaller the capacity of the downstream region 23 in the tundish and the smaller the capacity of the upper molten steel pool 24 in the mold, the shorter the time required for the change of the molten steel component. In this laboratory test, the tundish arranging time (time for the molten steel component to reach the target component from the start of wire addition) in each component system was constant at about 20 seconds regardless of the test conditions.

FIG. 5 shows the results of comparing the effects of the density difference between the front component and the rear component on the component mixing length of the different steel grade seam in the laboratory test. CN on the vertical axis is the concentration standardized by Eq. (2) using the result of analyzing the concentration of the changed components (carbon, _molybdenum ). C is the analysis result, and C ₁ and C ₂ are the molten steel concentration (pre-component) of the preceding charge and the molten steel concentration (post-component) of the subsequent charge, respectively. The measurement position is 50 mm at the center of the thickness at the center of the width of the slab.
CN = (C _- C ₁ ) / (C ₂ -C ₁ ) (2)

FIG. 5 shows the change in the mixing length under each density difference condition. A region having a normalized _CN value of 0.1 to 0.9 is defined as a mixed region and is shown by hatching in FIG. By reducing the post-component density to -30 kg / m ³ or more for the molten steel density difference (Δρ), a significant reduction in the mixed region can be confirmed, and it is clear that the component mixed length changes significantly depending on the density difference between the front and rear molten steel. bottom.

In the present invention, in the downstream region 23 partitioned by the tundish weir 5, a heating means 7 for heating the molten steel in the tundish is provided together with the component adding device 8. The heating means 7 can be selected from plasma heating, induction heating and the like. It is preferable to install the plasma heating device 11 on the molten steel. By heating the upper part of the molten steel by plasma heating and adding a desired additive element (alloy) as a wire to the heated region, the concentration of the molten steel in the downstream region 23 partitioned by the tundish weir 5 can be changed. It is possible. Thin slab casting has a smaller tundish capacity than ordinary continuous casting equipment. As a result, simply adding a wire to the molten steel will lower the temperature of the molten steel in the tundish, which may cause undissolved residue of the added alloying elements and adversely affect productivity such as nozzle blockage. There is. Therefore, it was decided to provide the heating means 7. It is preferable to install the plasma heating device 11 as the heating means 7. Further, by performing plasma heating, it is possible to install a plurality of wire addition devices as needed to manufacture a desired steel material in a small lot.

In examining the effect of the present invention, it is necessary to consider the natural convection in the mold. Natural convection in the mold is generated by the density difference as the driving force. In addition to the above-mentioned difference in the concentration of contained components, the difference in temperature also has an effect as a factor that causes the difference in density. Since the molten steel temperature is lowered by adding the component in the downstream region of the tundish, the temperature of the molten steel above the component boundary is lowered and the molten steel density is increased, which induces natural convection. , Can cause component mixing above and below the component boundary. On the other hand, by adding plasma heating, the temperature rise due to plasma heating and the temperature decrease due to wire addition are offset, and a large temperature change does not occur. Therefore, the density change due to the temperature difference is small, which is preferable.

As for the plasma heating device 11 as the heating means 7, as shown in FIG. 1, the molten steel surface of the downstream region 23, preferably the wire to be added, is plasma-heated to promote the melting of the additive. In FIG. 1, the torch 12 is used as a cathode, and an iron plate is embedded in a tundish refractory as an anode electrode 13, and the surface thereof is arranged so as to be in contact with molten steel. The torch 12 is connected to the anode electrode 13 and a DC power supply (not shown in FIG. 1) via a gripping device. The output of the DC power supply may be used as a power supply for a normal device that heats plasma, and may be about 2 MW. Here, plasma heating is used as the heating means 7 to promote melting of a metal having a melting point higher than the molten steel temperature by being able to heat near the surface of the molten metal to which an element is added and by using the radiant heat of plasma heating. Because it can be done. Further, although an example of a single torch is shown in FIG. 1, a twin torch system in which two torches are provided side by side may be used.

In the molten steel heated by plasma (wire addition position), the molten steel temperature is locally high, and the high temperature molten steel tends to stay in the upper part. Therefore, in order to efficiently mix the added alloying elements, it is preferable to blow gas from the gas blowing means 10 at the bottom of the tundish as shown in FIG. By promoting the stirring of the molten steel by blowing gas, the composition and temperature become uniform efficiently. Ar gas is preferable as the gas type. The amount of gas blown may be adjusted according to the addition concentration, and may be appropriately selected in the range of 1 NL / min to 10 NL / min.

It is possible to manufacture small lot materials by manufacturing molten steel with some components changed in one tundish using the above equipment and method. In particular, in the manufacturing method according to the present invention, the connecting portion (mixed region) before reaching the desired component is small, and the molten steel component can be changed by changing the alloy wire. At the same time, it is possible to produce small lot materials with high efficiency without the need for major equipment changes.

The shape of the tundish weir 5 will be described. In FIG. 4, the dot hatched portion is the weir-existing portion of the molten steel immersion portion of the tundish weir 5, and the blank portion other than the dot hatched portion indicates the molten steel passing portion 6. As a method of providing the molten steel passing portion 6, it is preferable to use a combination of the upper weir 5a and the lower weir 5b as shown in FIG. 4 (A). Further, the upper weir 5a and the lower weir 5b of the tundish weir 5 are preferably arranged in the same vertical cross section as shown in FIG. 1, but are arranged in different vertical cross sections as shown in FIG. 4 (E). It may be arranged. Further, the tundish weir 5 is formed of one plate, and a part of the tundish weir 5 is opened as shown in FIG. 4 (B) to form a molten steel passing portion 6, as shown in FIG. 4 (C). The tundish weir 5 is used as the lower weir 5b only, the molten steel passage portion 6 is formed between the upper end of the tundish weir 5 and the molten metal surface 35, and the tundish weir 5 is used as the upper weir 5a only as shown in FIG. 4D. The lower part of the dish weir 5 may be the molten steel passing portion 6. In any shape, the larger the size of the molten steel passing portion 6, the easier it is for molten steel to pass through, but the more easily component mixing occurs, and the smaller the size of the molten steel passing portion 6, the more difficult it is for molten steel to pass through. Since it is likely to occur, a suitable size of the molten steel passing portion 6 can be appropriately determined. As shown in FIG. 4C, a relief hole 14 may be provided near the lower end of the tundish weir in order to prevent the molten steel from remaining on the upstream region 22 side when the continuous casting is completed.

A casting test was conducted to verify the effect of the present invention. In this test, the same laboratory casting equipment as in the preliminary test was used. The molten steel capacity of the ladle 1 is 8 tons. The capacity of the tundish 2 is 5 tons, the capacity of the downstream region 23 is 0.89 tons using the tundish weir 5 shown in FIGS. 1 and 4 (A). The tundish weir 5 was installed at a position 230 mm from the immersion nozzle 3. The molten steel in the downstream region 23 in the tundish is heated by plasma heating, and Ar gas is blown into it. The component of the subsequent charge is adjusted by adding a wire. The casting temperature was the liquidus temperature of the initial molten steel composition + 50 ° C., and an electric power of 600 kW was applied to control the temperature of the downstream region 23 in the tundish to be constant by plasma heating. A 13 mm alloy was added as a wire to the heated region, and a test without plasma heating was also performed as a comparative condition. The upper limit of the wire feeding speed was 350 m / min depending on the amount of addition, and a plurality of wire feeding devices were used as needed to add a plurality of wires.

The casting test was performed with a slab size of 100 mm in thickness × 800 mm in width and a casting speed of 5.0 m / min. The DC magnetic field generator 9 was arranged 600 mm below the meniscus 34 to operate the electromagnetic brake. The electromagnetic brake was set to 0.5T under all conditions.

In Table 2, the analysis results of the molten steel component (pre-component) of the preceding charge are shown in the upper row, and the results of the succeeding charge (post-component) are shown in the lower row. In Table 1, it is shown that elements other than “-” and Fe which are not described are not positively added.

As an evaluation method for both the examples of the present invention and the comparative examples, the yield improvement rate was arranged. As a method for measuring the component of the slab, as in the above-mentioned evaluation method, the starting point is the position where the wire is added, and the sample for component analysis is taken from the center of the thickness (50 mm from the surface layer) every 0.2 m in the casting direction. Was collected, and the length of the mixed region (A) up to the desired concentration of the added element was measured. On the other hand, as a conventional method, two ladles are prepared, and the molten steel in the preceding ladles is adjusted to the molten steel component of the preceding charge, and the molten steel in the following ladles is adjusted to the molten steel component of the following charge. When the residual molten steel in the tundish became 2 tons, the molten steel of the subsequent charge was injected into the tundish, and the different steel grades were continuously cast. Then, the mixed region length (B) in this case was evaluated. Then, using the mixed region length (A) and the mixed region length (B), the improvement rate was calculated by setting (BA) / B × 100 (%).

As a comprehensive evaluation, the comprehensive evaluation was marked as ◯ under the conditions that the yield improvement effect was 30% or more and the operation was stable. For conditions where the operation was not stabilized due to nozzle blockage, etc., the evaluation was marked as x even if the yield was improved.

In Examples 1 to 4 of the present invention, even in continuous casting of different steel grades, it was possible to manufacture without slowing down the casting speed, the area of scrap was small, and the effect of improving the yield was confirmed. In addition, stable operation was possible without nozzle blockage.

In the comparative examples in Table 2, items outside the scope of the present invention are underlined. In Comparative Example 1, although the improvement effect due to the difference in molten steel density was slightly observed, the effect of reducing the mixing region was not sufficient because the electromagnetic brake was not used. Further, in Comparative Examples 2 and 3, the difference in the density of the molten steel was not sufficient, the mixing suppression of the molten steel in the strand was not sufficient, and the desired reduction of the mixing region was not achieved. In Comparative Example 4, the molten steel temperature dropped due to the addition of the alloy wire without using the heating means, and the nozzle was finally blocked during the test. Therefore, the evaluation was evaluated as x from the viewpoint of stable operation.

1 Ladle 2 Tandish 3 Immersion nozzle 4 Mold 5 Tandish weir 5a Upper weir 5b Lower weir 6 Molten steel passage 7 Heating means 8 Component addition device 9 DC magnetic field generator 10 Gas blowing means 11 Plasma heating device 12 Torch 13 Anode electrode 14 Relief hole 21 Molten steel transfer part 22 Upstream region 23 Downstream region 24 Upper molten steel pool 25 Lower molten steel pool 26 DC magnetic field band 27 Discharge flow 28 Upflow 29 Downflow 30 Component boundary 31 Front component region 32 Post component region 33 Steady Downflow 34 Meniscus 35 Hot water surface

Claims (2)

It is a continuous casting method of thin slabs in which molten steel contained in a ladle is transferred to a tundish and molten steel is injected into a continuous casting mold from a dipping nozzle provided at the bottom of the tundish.
A tundish weir is installed between the molten steel transfer part from the ladle and the immersion nozzle, and the tundish weir has a molten steel passage part, and the ladle molten steel separated by the tundish weir in the tundish. It has a heating means for heating the molten steel in the tundish on the downstream region side, with the transfer side as the upstream region and the opposite side as the downstream region.
A DC magnetic field generator that applies a DC magnetic field in the thickness direction over the entire width of the mold is arranged, and the upper part of the strand sandwiching the DC magnetic field band formed by the DC magnetic field generator is the upper molten steel pool and the lower part is the lower molten steel pool. Then, the molten steel is supplied from the immersion nozzle to the upper molten steel pool.
During continuous casting, by starting continuous addition of a predetermined element or its alloy to the molten steel in the tundish on the downstream region side or by changing the addition amount, the slab cast first and the slab cast later. Shards with different composition compositions were cast between the two, and later cast at the molten steel density ρ (kg / m ³ ) calculated by the following equation (1) from the composition and the molten steel temperature T (° C.) in the tundish. A method for continuous casting of thin slabs, wherein the molten steel density corresponding to the slab is lower than the molten steel density corresponding to the previously cast slab, and the molten steel density difference is 30 kg / m ³ or more.
ρ = 7000-8.0 (T-1823) -80.0 [C] -83.0 [Si] -21.2 [Mn] -67.1 [P] -84.0 [S] -113.0 [Al] -29.0 [Ti] +5.4 [Nb] ] -14.6 [Cr] +4.8 [Ni] +3.6 [Cu] +23.0 [Mo] -46.3 [V] (1)
In the above equation (1), the element symbol means the content (mass%) of each element. It is characterized in that plasma heating is used as a means for heating the molten steel in the tundish on the downstream region side, and the predetermined element or its alloy is continuously added while gas is blown from the bottom of the tundish in the downstream region. The method for continuously casting a thin slab according to claim 1.

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