Title of invention: Method for manufacturing thin-walled slabs
Technical field
[0001]
　In the present invention, molten steel is supplied to a molten steel pool portion formed by a pair of rotating cooling rolls and a pair of side weirs via a dipping nozzle, and a solidified shell is formed and grown on the peripheral surface of the cooling rolls to form a thin wall. The present invention relates to a method for producing a thin-walled slab for producing a slab.
　The present application claims priority based on Japanese Patent Application No. 2018-188404 filed in Japan on October 3, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　As a method for producing a thin-walled slab of steel, for example, as shown in Patent Documents 1 to 3, a pair of cooling rolls having a water-cooled structure inside and rotating in opposite directions are provided, and a pair of rotating cooling rolls and a plurality of cooling rolls are provided. The molten steel is supplied to the molten steel pool portion formed by the refractory wall of the above, a solidified shell is formed and grown on the peripheral surface of the cooling roll, and the solidified shells formed on the outer peripheral surfaces of the pair of cooling rolls are rolled kiss points. There is provided a double-roll type continuous casting apparatus that is crimped with a steel to produce a thin-walled slab having a predetermined thickness.
[0003]
　In the thin-walled slabs produced by using this double-roll type continuous casting apparatus, the molten steel is rapidly cooled at the time of solidification, so that columnar crystals are formed from the surface layers on both sides toward the 1/2 thick portion. Further, due to the pressing force of the cooling roll, the growth tip of the solidified shell is broken into solidified pieces, which stay in the molten steel pool part, and the solidified pieces are taken in between the solidified shells formed of columnar crystals. An equiaxed crystal zone may be formed in the / 2 thick part.
Prior art literature
Patent documents
[0004]
Patent Document 1: Japanese Patent Application
Laid-Open No. 60-184450 Patent Document 2: Japanese Patent Application Laid-Open No. 05-237603
Patent Document 3: Japanese Patent Application Laid-Open No. 10-180423
Outline of the invention
Problems to be solved by the invention
[0005]
　Here, the equiaxed crystal ratio is defined as the ratio of the equiaxed crystal zone thickness to the total thickness of the slab. It is important to have an appropriate equiaxed crystal ratio in order to obtain thin-walled slabs with few central defects and sound internal quality. When this equiaxed crystal ratio is 0%, that is, when it is composed of only columnar crystals, porosity and central segregation tend to occur in the 1/2 thick portion.
　The equiaxed crystal ratio is known to be related to the molten steel temperature. Table 1 shows an example of the relationship between the superheat degree ΔT of the molten steel in the tundish and the equiaxed crystal ratio in the low carbon steel.
[0006]
[table 1]

[0007]
　As shown in Table 1, in order to increase the equiaxed crystal ratio, it is effective to lower the molten steel temperature in the tundish. Further, if the molten steel temperature fluctuates during casting, the equiaxed crystal ratio in the thin-walled slab also changes, and the cast structure is not stable in the longitudinal direction of the thin-walled slab, which may cause defects.
　In the above-mentioned double-roll type continuous casting apparatus, for example, in the initial stage of casting, heat is taken up by a refractory such as a tundish, and the molten steel temperature drops. The molten steel temperature rises at steady state, but tends to fall again at the end of casting.
[0008]
　Strictly speaking, the equiaxed crystal ratio is affected by the temperature of the molten steel pool section described above, but since it is generally difficult to measure the temperature of the molten steel pool section, it is controlled by the molten steel temperature in the tundish. Since the amount of temperature decrease due to the supply of molten steel from the tundish to the molten steel pool is almost constant depending on the individual continuous casting equipment, the relationship between the molten steel temperature in the tundish and the equiaxed crystal ratio is shown in Table 1. It is required as shown.
[0009]
　Here, when the set temperature of the molten steel in the tundish is lowered in order to improve the equiaxed crystal ratio, the molten steel temperature becomes too low at the initial stage of casting and the final stage of casting, and the immersion nozzle is blocked. There was a risk of trouble. In addition, there is a risk that a hot band or the like may be generated due to the large growth of the bare metal and its entrainment in the thin-walled slab.
　On the other hand, when the set temperature of the molten steel is raised in order to suppress the occurrence of casting troubles at the initial stage of casting and the final stage of casting, the molten steel temperature becomes too high in the steady state and the equiaxed crystal ratio becomes low. There was a risk that it would end up.
[0010]
　It is also conceivable to arrange a heating means or the like in order to keep the molten steel temperature in the tundish within a certain range. However, when trying to adjust the molten steel temperature in the tundish by a heating means, a time lag occurs in the temperature adjustment, and it is difficult to control the molten steel temperature accurately.
[0011]
　The present invention has been made in view of the above-mentioned situation, and controls the molten steel temperature of the molten steel pool portion within a certain range by controlling the molten steel temperature in the tundish within a certain range, and controls the molten steel temperature in the molten steel pool portion within a certain range in the longitudinal direction. It is an object of the present invention to provide a method for producing a thin-walled slab capable of producing a thin-walled slab having a stable equiaxed crystal ratio.
Means to solve problems
[0012]
　In order to solve the above problems, in the method for producing a thin-walled slab according to the present invention, molten steel stored in a tundish is placed in a molten steel pool portion formed by a pair of rotating cooling rolls and a pair of side weirs. A method for producing a thin-walled slab, which is supplied through a dipping nozzle to form and grow a solidified shell on the peripheral surface of the cooling roll to produce a thin-walled slab. From the initial stage of casting to the final stage of casting, a Si additive is added to adjust the Si concentration of the molten steel within a certain range, and to control the temperature of the molten steel in the tundish within a certain range. It is a feature.
[0013]
　According to the method for producing a thin-walled slab having this configuration, the Si concentration of the molten steel is kept constant by adding a Si additive containing Si, which generates heat when dissolved in Fe, to the molten steel in the tundish. Since the temperature of the molten steel in the tundish is controlled within a certain range while being adjusted within the range, the temperature of the molten steel can be controlled accurately and the equiaxed crystal ratio in the longitudinal direction is increased. A stable thin-walled slab can be produced.
　Further, even in the early stage of casting or the final stage of casting when the molten steel temperature becomes low, the molten steel temperature can be raised by adding the Si additive, the occurrence of troubles such as clogging of the immersion nozzle can be suppressed, and stable casting can be performed. It can be performed. Therefore, the molten steel temperature in the steady state can be set low, and a thin-walled slab having a target equiaxed crystal ratio can be produced.
[0014]
　Here, in the method for producing a thin-walled slab of the present invention, a plurality of Si-containing materials having different Si contents are prepared, and one or a plurality of the Si-containing materials are mixed with the molten steel in the tundish. It is preferable to adjust the ratio and add it as the Si additive.
　In this case, the Si concentration of the molten steel can be adjusted within a certain range relatively easily by adjusting the blending ratio of a plurality of Si-containing materials having different Si contents and adding them to the molten steel in the tundish. At the same time, it is possible to control the temperature of the molten steel in the tundish within a certain range, which makes it possible to control the temperature of the molten steel in the molten steel pool portion within a certain range.
[0015]
　Here, in the method for producing a thin-walled slab of the present invention, the addition rate of the Si additive is adjusted according to the compounding ratio of the single or a plurality of Si-containing materials, so that the Si concentration of the molten steel is constant. Can be within range.
[0016]
　Here, in the method for producing a thin-walled slab of the present invention, the Si additive may be added to the molten steel in the tundish after being heated to a temperature exceeding room temperature. In this case, it is efficient. It is possible to raise the molten steel temperature.
Effect of the invention
[0017]
　As described above, according to the present invention, the molten steel temperature in the molten steel pool portion is controlled within a certain range by controlling the molten steel temperature in the tundish within a certain range, and the equiaxed crystal ratio in the longitudinal direction is increased. It is possible to provide a method for producing a thin-walled slab capable of producing a stable thin-walled slab.
A brief description of the drawing
[0018]
FIG. 1 is an explanatory view showing an example of a twin-roll type continuous casting apparatus that implements a method for producing a thin-walled slab according to an embodiment of the present invention.
FIG. 2 is an explanatory diagram showing the result of adjusting the blending ratio of a plurality of Si-containing materials in one embodiment of the present invention.
Mode for carrying out the invention
[0019]
　Hereinafter, a method for producing a thin-walled slab according to an embodiment of the present invention will be described with reference to the attached drawings. The present invention is not limited to the following embodiments. In the following embodiments and examples, the Si-containing material and the Si additive are used at room temperature or at 25 ° C.
　The thin-walled slab 1 produced in the present embodiment is, for example, a Si-containing steel containing Si in a range of 0.5% by mass or more and 8.0% by mass or less.
　Further, in the present embodiment, the width of the thin-walled slab 1 to be manufactured is within the range of 200 mm or more and 1800 mm or less, and the thickness is within the range of 0.8 mm or more and 5 mm or less.
[0020]
　Next, the twin-roll type continuous casting apparatus 10 according to the present embodiment will be described.
　The twin-roll continuous casting apparatus 10 shown in FIG. 1 includes a pair of cooling rolls 11 and 11, pinch rolls 12 and 12 and 13 and 13 for supporting the thin-walled slab 1, and a pair of cooling rolls 11 and 11. A tundish 18 for holding the molten steel 3 supplied to the molten steel pool portion 16 composed of the side weir 15 arranged at the end in the width direction, the pair of cooling rolls 11, 11 and the side weir 15, and the tundish 18. A dipping nozzle 20 for supplying the molten steel 3 stored in the tundish 18 to the molten steel pool portion 16 is provided.
[0021]
　In the twin roll type continuous casting apparatus 10, the molten steel 3 is supplied from the tundish 18 to the molten steel pool portion 16 via the immersion nozzle 20. In the molten steel pool portion 16, the solidified shells 5 and 5 grow on the peripheral surfaces of the cooling rolls 11 and 11 as the molten steel 3 comes into contact with and cooled by the rotating cooling rolls 11 and 11. Then, the solidified shells 5 and 5 formed on the pair of cooling rolls 11 and 11 are pressure-bonded to each other at the roll kiss point, whereby the thin-walled slab 1 having a predetermined thickness is cast.
[0022]
　When casting is performed by the above-mentioned double-roll type continuous casting apparatus 10, the temperature of the refractory material constituting the tundish 18 and the like is low at the initial stage of casting, so that the heat of the molten steel 3 is transferred to the refractory material side, and the molten steel The temperature tends to decrease. Further, even at the final stage of casting, the molten steel temperature tends to decrease as the time changes. That is, usually, when casting is performed using the above-mentioned twin roll type continuous casting apparatus 10, the molten steel temperature is low at the initial stage of casting, high at the steady state, and low at the final stage of casting.
[0023]
　In the above-mentioned twin roll type continuous casting apparatus 10, the molten steel 3 in the tundish 18 is supplied to the molten steel pool portion 16 by using the immersion nozzle 20, but when the molten steel temperature drops, the immersion nozzle 20 uses the immersion nozzle 20. Occlusion occurs, making it difficult to perform casting in a stable manner.
　Further, in the above-mentioned twin roll type continuous casting apparatus 10, since the side weir 15 is always in sliding contact with the cooling roll 11, heat is removed from the cooling roll 11 and cooled. For this reason, bare metal tends to be generated on the surface of the side weir 15. When the molten steel temperature drops in the early stage of casting or the final stage of casting, the metal is more likely to be generated, and if the metal is caught in the thin-walled slab 1 in a state of being greatly grown, a hot band or the like is generated. It becomes impossible to perform stable casting.
　Further, when the molten steel temperature fluctuates, the equiaxed crystal ratio changes, and the structure becomes unstable in the longitudinal direction.
[0024]
　Therefore, in the method for producing a thin-walled slab according to the present embodiment, a Si additive is added to the molten steel 3 in the tundish 18, and the heat generated when Si dissolves in Fe is used to utilize the molten steel. Control the temperature.
　At this time, it is necessary to adjust the amount of the Si additive added so that the Si concentration of the molten steel 3 is within the target range.
　That is, in the method for producing a thin-walled slab according to the present embodiment, a Si additive is added to the molten steel 3 in the tundish 18 to adjust the Si concentration of the molten steel 3 within a certain range. The molten steel temperature in the tundish 18 is controlled within a certain range, and thus the molten steel temperature of the molten steel pool portion 16 is controlled within a constant range.
[0025]
　Specifically, the Si concentration of the molten steel 3 supplied into the tundish 18 is set to be lower than the Si concentration of the product target. In this embodiment, it is preferable that the difference between the Si concentration of the molten steel 3 supplied into the tundish 18 and the Si concentration of the product target is within the range of 0.5% by mass or more and 1.0% by mass or less.
　If the difference between the Si concentration of the molten steel 3 supplied in the tundish 18 and the Si concentration of the product target is less than 0.5% by mass, the addition amount is small and the temperature rise amount is too small. This is because you cannot actually enjoy it. Further, if the difference between the Si concentration of the molten steel 3 supplied into the tundish 18 and the Si concentration of the product target exceeds 1.0% by mass, the addition amount is too large and the adjustment accuracy of the concentration and temperature is lowered. This is because there is a risk of doing so.
[0026]
　The amount of Si additive added is defined by the following formula. The symbols of equations (1) to (4) and (4a) are defined as follows.
　m (i): Formulation of Si-containing material i in Si additive (mass%)
　C (i): Si content of Si-containing material i (mass%)
　Q m : Molten steel throughput (kg / min)
　Q s : Addition rate of Si additive (kg / min)
　ΔC: Insufficient Si concentration difference (mass%) with respect to target Si concentration
　ΔTj: Insufficient temperature difference with respect to target molten steel temperature (° C.)
　ΔQ (i): Si content Molten steel standard addition ratio (mass%) required to increase Si concentration by 1% by mass by adding material i alone
　ΔT (i): When increasing Si concentration by 1% by mass by adding Si-containing material i alone Molten steel temperature increase (° C.)
　ΔT'(i, Tpi): Molten steel temperature increase (° C.) when the Si concentration is increased by 1% by mass by adding the Si-containing material i heated to the temperature Tpi alone.
[0027]
[Number 1]

[0028]
[Number 2]

[0029]
[Number 3]

[0030]
[Number 4]

[0031]
[Number 5]

[0032]
　Here, the above-mentioned equation (3) is an equation for adjusting the Si concentration in the molten steel 3, and the equation (4) is an equation for controlling the molten steel temperature. Equation (4a) is an equation in which ΔT in equation (4) is replaced with ΔT ′ (i, Tpi), and is an equation for controlling the molten steel temperature when a heated Si additive is used.
　Further, in the present embodiment, as the Si additive material, a plurality of types of Si-containing materials having different Si contents are used, and the subscript i corresponds to each Si-containing material.
　Further, since the difference from the target molten steel temperature changes depending on the casting time, the subscript j corresponds to the casting time.
[0033]
　Si of the molten steel 3 by adding a Si additive to the molten steel 3 in the tundish 18 so as to satisfy the above equations (3) and (4) (or the equation (4a)), respectively. It is possible to adjust the concentration within a certain range and control the molten steel temperature in the tundish 18 within a certain range, thereby controlling the molten steel temperature of the molten steel pool portion 16 within a certain range.
　When the Si additive is added to the molten steel 3 in the tundish 18, the addition rate Q s of the Si additive is adjusted according to the blending ratio of the plurality of Si-containing materials i . Specifically, the molten steel throughput Q m (kg / min), the Si concentration difference ΔC (mass%) insufficient with respect to the target Si concentration of the molten steel 3, and the Si additive material so as to satisfy the above-mentioned equation (3). The compounding m (i) (mass%) of the Si-containing material i in the mixture and the addition ratio ΔQ (i) (mass%) of the molten steel standard required for increasing the Si concentration by 1% by mass by adding the Si-containing material i alone. ), The addition rate Q s (kg / min) of the Si additive is adjusted. As a result, even if the Si additive to be added is composed of a plurality of Si-containing materials having different compounding m (i) (mass%) of the Si-containing material i in the Si additive, the compounding ratio of the Si-containing material is cast. The Si concentration of the molten steel 3 can be kept within a certain range even if it differs depending on the time.
　When adding the Si additive to the molten steel 3 in the tundish 18, the Si additive may be added after being heated to a temperature exceeding room temperature. At this time, for each type of Si-containing material, how much temperature the molten steel temperature per addition amount rises when heated to what degree, that is, the Si concentration is increased by 1% by mass by adding a unit amount of the heated Si-containing material i alone. It is advisable to set in advance the amount of molten steel temperature rise (° C.) when raising the temperature by experiments or computer simulations.
[0034]
　Here, as the Si-containing material constituting the Si additive, it is preferable to use metallic Si or ferrosilicon. The ferrosilicon may be one specified in Japanese Industrial Standard JIS2302-1998, or may be one specified in international standard ISO5445-1980.
　As the metallic Si, it is preferable to use one having a purity of 95% by mass or more.
　Further, in ferrosilicon, if the Si content is less than 40% by mass, there is no effect of raising the temperature of the molten steel 3, so it is preferable to use a ferrosilicon having a Si content of 40% by mass or more.
　Ferrosilicon having a Si content of less than 40% by mass can be used when lowering the molten steel temperature or increasing the Si concentration without changing the molten steel temperature.
[0035]
　In this embodiment, for example, metal Si, ferrosilicon No. 2 and ferrosilicon No. 3 are used as the Si-containing material.
　In the case of metallic Si (purity 99% by mass), when it is added alone to increase the molten steel Si concentration by 1% by mass, the addition ratio is 1.00% by mass, and the amount of increase in molten steel temperature is + 31 ° C.
　In Ferrosilicon No. 2 (Si content 75% by mass), when added alone to increase the molten steel Si concentration by 1% by mass, the addition ratio is 1.33% by mass, and the amount of molten steel temperature increase is + 19 ° C. Become. In addition, Ferrosilicon No. 2 is Si: 75 to 80% by mass, C: 0.2% by mass or less, P: 0.05% by mass or less, S: 0.02% by mass or less in Japanese Industrial Standard JIS2302-1998. It is a ferrosilicon defined by the chemical composition of.
　In Ferrosilicon No. 3 (Si content 40% by mass), when added alone to increase the molten steel Si concentration by 1% by mass, the addition ratio is 2.50% by mass, and the amount of molten steel temperature increase is + 3 ° C. Become. In addition, Ferrosilicon No. 3 is Si: 40 to 45% by mass, C: 0.2% by mass or less, P: 0.05% by mass or less, S: 0.02% by mass or less in Japanese Industrial Standard JIS2302-1998. It is a ferrosilicon defined by the chemical composition of.
[0036]
　As described above, the amount of increase in molten steel temperature (° C.) when the Si concentration increases by 1% by mass changes depending on the Si content in the Si-containing material. Therefore, by adjusting the blending ratio of these in the Si additive material, the molten steel 3 It is possible to adjust the Si concentration and the molten steel temperature.
　FIG. 2 shows an example of the relationship between the compounding ratio and the amount of molten steel temperature increase (° C.) when the Si concentration increases by 1% by mass. It is confirmed that the molten steel temperature can be arbitrarily controlled by selecting the compounding ratio of a plurality of Si-containing materials.
[0037]
　It is desirable that the molten steel temperature is continuously measured by a thermocouple arranged on the wall surface of the tundish 18. Alternatively, a consumable temperature measuring probe may be inserted from above the tundish 18 to measure the temperature intermittently.
　Further, if the casting conditions such as the capacity of the tundish 18 and the molten steel throughput are constant, the temperature change of the molten steel when no Si additive is added is obtained in advance, and the temperature is measured by the temperature measuring probe only at the initial stage of casting. May be good.
[0038]
　According to the method for producing a thin-walled slab according to the present embodiment having the above-described configuration, the Si additive is added to the molten steel 3 in the tundish 18 from the initial stage of casting to the final stage of casting. Since the Si concentration of the molten steel 3 is adjusted within a certain range and the temperature of the molten steel 3 in the tundish 18 is controlled within a certain range, the molten steel temperature can be controlled accurately and equiaxed in the longitudinal direction. It is possible to produce thin-walled slabs having a stable crystallinity.
　Further, the molten steel temperature can be set low to promote the formation of equiaxed crystals, and the molten steel temperature can be raised by adding the Si additive even in the early stage of casting or the final stage of casting when the molten steel temperature is low. Therefore, it is possible to suppress the occurrence of troubles such as blockage of the immersion nozzle and generation of a hot band due to the entrainment of the bare metal, and stable casting can be performed.
[0039]
　Further, in a preferred embodiment of the present embodiment, a plurality of Si-containing materials having different Si contents are prepared, and the blending ratio of the molten steel 3 in the tundish 18 alone or a plurality of these Si-containing materials is adjusted. Since it is added as a Si additive, it is relatively easy to adjust the Si concentration of the molten steel 3 within a certain range and control the temperature of the molten steel 3 of the molten steel pool portion 16 within a certain range. It will be possible.
[0040]
　Further, in a preferred embodiment of the present embodiment, the addition rate of the Si additive may be adjusted according to the blending ratio of the plurality of Si-containing materials. In this case, the Si concentration of the molten steel 3 is within a certain range. Can be done.
[0041]
　Further, in a preferred embodiment of the present embodiment, when a Si-containing material such as ferrosilicon is added to the molten steel 3 in the tundish 18 as a Si additive, the Si additive is heated to a temperature exceeding room temperature. Since it is added after that, it is possible to raise the molten steel temperature efficiently.
[0042]
　Although the method for producing a thin-walled slab according to the embodiment of the present invention has been specifically described above, the present invention is not limited to this, and can be appropriately changed without departing from the technical idea of ​​the invention. is there.
　For example, in the description of the present embodiment, as shown in FIG. 1, a twin-roll type continuous casting apparatus in which pinch rolls are arranged has been described as an example, but the arrangement of these rolls and the like is not limited and is appropriately designed. You may change it.
[0043]
　Further, although it has been described that metallic Si, ferrosilicon No. 2 and ferrosilicon No. 3 are used as the Si-containing material, other Si-containing materials may be used.
Example
[0044]
　The results of experiments carried out in order to confirm the effects of the present invention will be described below.
　Using the twin-roll type continuous casting apparatus having the configuration shown in FIG. 1, a thin-walled slab made of carbon steel having a composition in which the target value of Si concentration was 0.80% by mass was cast.
　The size of the thin-walled slab was 2 mm in thickness and 800 mm in width. The casting amount was 10 tons, the casting speed was 50 m / min, and the casting time was 18 minutes.
[0045]
In the
　example of the present invention, the Si concentration in the molten steel supplied into the tundish was set to 0.10% by mass, and the Si concentration difference ΔC insufficient with respect to the target Si concentration was set to 0.70% by mass. ..
　Then, using the three types of Si-containing materials shown in Table 2, a Si additive was added to the molten steel in the tundish as shown in Table 3. Then, the molten steel temperature was adjusted so that the degree of superheat ΔT in the tundish was within an appropriate range (30 to 50 ° C.).
　Then, the structure of the obtained thin-walled slab was observed, and the equiaxed crystal ratio was measured. The measured equiaxed crystal ratios are also shown in Table 3.
[0046]
　Here, regarding the compounding of the Si-containing materials in Table 3, the material selection and the calculation method of the compounding amount will be described below.
　First, the maximum value of the temperature rise is determined by the Si concentration difference ΔC that is insufficient with respect to the target Si concentration. The maximum temperature increase amount ΔT maxi for each Si-containing material is given by the following equation from Table 2.
　　　　ΔT max1 = 31 × ΔC
　　　　ΔT max2 = 19 × ΔC
　　　　ΔT max3 = 3 × ΔC
[0047]
　Therefore, the Si-containing material may be selected according to the insufficient temperature difference ΔTj.
　That is, when the formula: 19 × ΔC ≦ ΔTj ≦ 31 × ΔC is satisfied, ferrosilicon No. 2 (Si content 75% by mass) and metallic Si are used. Further, when the formula: 3 × ΔC ≦ ΔTj ≦ 19 × ΔC is satisfied, ferrosilicon No. 2 (Si content 75% by mass) and ferrosilicon No. 3 (Si content 40% by mass) are used.
　For example, when ΔC = 0.70% by mass and the insufficient temperature difference ΔTj is 10 ° C.,
　　　　3 × 0.70 <10 <19 × 0.70
, so select Ferrosilicon No. 2 and Ferrosilicon No. 3. Just do it.
[0048]
　The required supply amount of each Si-containing material is calculated from the equations (1) to (4).
　From equation (3), the following equation (5) is calculated.
[0049]
[

　Equation 6] From equation (4), the following equation (6) is calculated.
[0050]
[Number 7]

[0051]
　Here, from the equation (1), since m (2) + m (3) = 100, m (2) ≈70 and m (3) ≈30 can be obtained. Substituting these into Eq. (5), the right-hand side becomes 1.18.
　Further, when the molten steel throughput Qm = 584 kg / min, Qs = 6.8 kg / min is obtained.
[0052]
[Table 2]

[0053]
[Table 3]

[0054]
　In the example of the present invention, the molten steel temperature is controlled within a certain range from the initial stage of casting to the final stage of casting by the addition of the Si additive, and the equiaxed crystal ratio is stable within the range of 0.05 to 0.20. Was.
　In addition, there was no blockage of the immersion nozzle or generation of a hot band due to the entrainment of the bare metal, and stable casting could be performed.
[0055]
In
　Comparative Example 1, the Si concentration in the molten steel supplied into the tundish is set to 0.80% by mass, which is a target value, and the degree of superheat in the tundish during steady operation (5 minutes after the start of casting) The molten steel temperature was set so that ΔT was 40 ° C.
[0056]
　In Comparative Example 1, when molten steel was supplied into the tundish at the initial stage of casting, heat was taken from the refractory of the tundish, and the molten steel temperature was lowered by about 20 to 40 ° C. as compared with the steady state. As a result, bullion was generated, and surface flaws and hot bands were generated. Further, even at the final stage of casting, the molten steel temperature was lowered by about 20 to 40 ° C. as compared with the steady state, bare metal was generated, and surface flaws and hot bands were generated.
[0057]
In
　Comparative Example 2, the Si concentration in the molten steel supplied into the tundish is set to 0.80% by mass, which is a target value, and the degree of superheat in the tundish in the steady state (5 minutes after the start of casting). The molten steel temperature was set so that ΔT was 60 ° C.
[0058]
　In Comparative Example 2, the occurrence of casting troubles could be suppressed at the initial stage of casting and the final stage of casting. However, the degree of superheat ΔT in the steady state was high, and equiaxed crystals were not sufficiently generated. Therefore, it was not possible to obtain a thin-walled slab in the range of the target equiaxed crystal ratio of 0.05 to 0.2, and a central defect such as porosity occurred in the 1/2 thick portion.
[0059]
　From the above results, according to the present invention, it is possible to adjust the Si concentration of the molten steel within a certain range and control the temperature of the molten steel in the molten steel pool portion within a certain range, and it is possible to control the temperature of the molten steel in the longitudinal direction equiaxed. It was confirmed that thin-walled slabs with stable crystallinity could be produced. In addition, the occurrence of casting troubles could be suppressed, and stable casting could be performed.
Industrial applicability
[0060]
　According to the present invention, a thin-walled slab in which the molten steel temperature in the molten steel pool portion is controlled within a certain range by controlling the molten steel temperature in the tundish within a certain range and the equiaxed crystal ratio is stable in the longitudinal direction. Can be applied to a method for producing thin-walled slabs capable of producing.
Code description
[0061]
1 Thin-walled slab
3 Molten steel
5 Solidification shell
10 Double roll type continuous casting equipment
11 Cooling roll
15 Side weir
16 Molten steel pool part
18 Tundish
The scope of the claims
[Claim 1]
　The molten steel stored in the tundish is supplied to the molten steel pool portion formed by the pair of rotating cooling rolls and the pair of side weirs via the immersion nozzle, and a solidified shell is formed on the peripheral surface of the cooling rolls. It is a method for producing a thin-walled slab that is grown to produce a thin-walled slab. A
　Si additive is added to the molten steel in the tundish from the initial stage of casting to the final stage of casting, and Si of the molten steel. A method for producing a thin-walled slab in which the concentration is adjusted within a certain range and the temperature of the molten steel in the tundish is controlled within a certain range.
[Claim 2]
　1 The method for producing a thin-walled slab described in 1.
[Claim 3]
　The method for producing a thin-walled slab according to claim 2, wherein the addition rate of the Si additive is adjusted according to the blending ratio of the plurality of Si-containing materials.
[Claim 4]
　The method for producing a thin-walled slab according to any one of claims 1 to 3, wherein the Si additive is added to the molten steel in the tundish after being heated to a temperature exceeding room temperature.