JP7355285B1 - Continuous steel casting method - Google Patents

Abstract

We propose a continuous casting method for steel that can effectively reduce center segregation that occurs in slabs. Along the slab drawing direction in the continuous casting machine, from the starting point where the average value of the solid fraction along the thickness direction at the final solidification part in the slab width direction is 0.8 or less, When the first section is the range up to the end point where the average value of the solid fraction along the thickness direction in the solidified part is larger than the solid fraction at the starting point and 1.0 or less, within this first section A continuous casting method for steel, characterized in that the slab is cooled at a water density per slab surface area of 50 to 2000 L/(m2×min) or less.

Description

The present invention relates to a continuous casting method for steel, and particularly proposes a continuous casting method for steel that is effective in reducing center segregation occurring in slabs.

Generally, during the solidification process of steel, solute elements such as carbon, phosphorus, sulfur, and manganese are concentrated in the unsolidified liquid phase by redistribution during solidification, and as a result, micro-segregation occurs between dendrite trees. is known to be formed.

In addition, when casting steel using a continuous casting machine, the continuously cast slab (hereinafter also simply referred to as "slab") that is solidifying is affected by solidification shrinkage, heat shrinkage, and the formation of solidified shells that occur between the rolls of the continuous casting machine. Due to bulging, etc., voids may be formed in the center of the thickness of the slab, or negative pressure may occur. Therefore, molten steel is drawn into the center of the thickness of the slab. However, since there is not a sufficient amount of molten steel in the unsolidified layer at the final stage of solidification, the molten steel between the dendrite trees, where the solute elements are concentrated, is attracted to the center of the thickness and solidifies there. . In the thus formed segregation spot at the center of the thickness of the slab, the concentration of solute elements is much higher than the initial concentration of molten steel. This phenomenon is generally called "macro segregation" and is also called "central segregation" due to its location.

The center segregation of the slab significantly reduces the quality of steel materials, for example, the quality of line pipe materials for transportation of crude oil, natural gas, etc. Such quality deterioration of steel materials is caused by, for example, hydrogen penetrating into the steel due to a corrosion reaction, which diffuses and accumulates around manganese sulfide (MnS) and niobium carbide (NbC) generated in the central segregation area, resulting in an increase in the internal pressure. This is caused by the occurrence of cracks caused by Moreover, since such a central segregation part is hardened by a high concentration of solute elements, the crack further propagates and expands to the periphery. This cracking is called hydrogen induced cracking (HIC). Therefore, reducing center segregation at the center of the thickness of a slab is extremely important in improving the quality of steel products.

Conventionally, many techniques have been proposed for reducing the center segregation of slabs and rendering them harmless during the process from the continuous casting process to the rolling process. For example, in Patent Document 1 and Patent Document 2, in a continuous casting machine, a slab at the final stage of solidification having an unsolidified layer is reduced by a slab support roll to an extent equivalent to the sum of the amount of solidification shrinkage and the amount of heat shrinkage. A technique has been proposed in which the material is cast while being gradually reduced. This technique is called the light reduction method. In this light reduction method, when the slab is pulled out using multiple pairs of slab support rolls lined up in the casting direction, the slab is gradually rolled down by a reduction amount commensurate with the sum of the amount of solidification shrinkage and the amount of heat shrinkage. By reducing the volume of the unsolidified layer, the formation of voids and negative pressure areas at the center of the slab is prevented. By adopting such a mechanism (light reduction method), the concentrated molten steel between the dendrite trees is prevented from being sucked from the dendrite trees to the center of the thickness of the slab, and the center segregation that occurs within the slab is prevented. It reduces it.

Furthermore, it is known that there is a close relationship between the morphology of the dendrite structure at the center of the thickness and center segregation. For example, Patent Document 3 discloses that by setting the specific water amount at a specific position in the casting direction of the secondary cooling zone of a continuous casting machine to 0.5 L/kg-steel or more, the solidification structure is refined and the equiaxed crystallization is achieved. Techniques have been proposed to promote this and reduce center segregation. Furthermore, Patent Document 4 proposes a technique for reducing center segregation by appropriately adjusting rolling conditions and cooling conditions to make the primary dendrite arm spacing at the center of slab thickness 1.6 mm or less. There is.

On the other hand, although this technology aims to prevent surface cracks in slabs, Patent Document 5 proposes a technique for heating the surface of slabs to increase the temperature as a method for controlling the temperature of slabs in a continuous casting machine. are doing. The technique proposed in Patent Document 5 prevents surface cracking during straightening of the slab by heating the surface layer of the slab at an average rate of 30° C./min or more within the straightening zone of the continuous casting machine.

Japanese Patent Application Publication No. 08-132203 Japanese Patent Application Publication No. 08-192256 Japanese Patent Application Publication No. 08-224650 JP2016-28827A Japanese Patent Application Publication No. 2008-100249

The inventions described in Patent Document 1 and Patent Document 2 state that center segregation can be reduced by employing a light reduction method. However, these techniques are not sufficient as methods for reducing center segregation to the level of quality required for steel pipes such as line pipes in recent years.

Furthermore, in the inventions described in Patent Document 3 and Patent Document 4, in addition to adopting the light reduction method, the secondary cooling conditions are adjusted to refine the solidified structure and reduce center segregation. However, the level of segregation reduction required for steel pipes such as line pipe materials is increasing year by year, and it is still insufficient to reduce the degree of segregation to the level that will be required in the future. Further, in order to further reduce segregation, it is possible to continuously cast steel under optimal light reduction conditions, but the methods of Patent Documents 3 and 4 do not allow center segregation to be reduced beyond the current level. Have difficulty.

In addition, since the installation space in the continuous casting machine is limited, the slab heating device of Patent Document 5 can be used as a local heating method, but it cannot control the temperature of the entire slab to be uniform. .

The present invention is a method developed in view of the above-mentioned problems faced by the conventional technology, and its purpose is to develop a new steel that can effectively reduce the center segregation that occurs in slabs. The objective is to propose a continuous casting method.

Therefore, the inventors conducted intensive studies to solve the above-mentioned problems faced by the prior art. As a result, they discovered that in the cooling process of slabs in continuous steel casting, if the slabs are cooled in specific sections with a preferred water flow density, center segregation within the slabs can be significantly reduced, and the present invention was developed. I ended up doing it.

The present invention has been made based on the above findings, and the gist and structure thereof are as follows.
Along the slab drawing direction in the continuous casting machine, from the starting point where the average value of the solid fraction along the thickness direction at the final solidified part in the slab width direction is 0.8 or less, When the first section is defined as the range up to the end point where the average value of the solid phase percentage along the thickness direction in the solidified part is larger than the solid phase percentage at the starting point and 1.0 or less, within this first section A continuous casting method for steel, characterized in that the slab is cooled at a water density per slab surface area of 50 L/(m ² × min) or more and 2000 L/(m ² × min) or less. .

It is also a more preferable aspect of the present invention to employ the following embodiments.
(1) When a second section is defined as a section located downstream of the first section in which the average value of the solid fraction in the width direction at the end point is 1.0 or less, Cooling the slab within the range of water flow density per surface area of 50 L/(m ² × min) to 300 L/(m ² × min);
(2) At least from the end of the first section to the passage through the second section, the slab is cast while maintaining a nucleate boiling state where the surface temperature is 200°C or less, and the thickness of the slab at the final stage of solidification is The temperature gradient in the vicinity should be 1.7 to 3.5 K/mm,
(3) The first section and the second section are installed within an area limited to a horizontal segment or a vertical segment within the continuous casting machine;
(4) applying a light reduction in the first section and the second section, and rolling down the long side surface of the slab at a reduction rate of 0.3 to 2.0 mm/min;
(5) By increasing the roll opening degree of the plurality of pairs of slab support rolls in stages toward the downstream side in the casting direction, the long sides of the slab are bulged with a total bulging amount of 2 to 20 mm, and then The opening degree of the slab support rolls is gradually reduced toward the downstream side in the casting direction to lightly reduce the slab.

By adopting the continuous casting method for steel according to the present invention having the above-mentioned main structure, it becomes possible to effectively cool the continuous slab, which is expected to occur in the continuously cast slab. This makes it possible to reduce center segregation and internal cracks.

FIG. 1 is a schematic diagram showing an example of a continuous casting machine that is effective for carrying out the continuous steel casting method according to the present invention. It is a figure explaining the final solidification position in the slab width direction. FIG. 3 is a cross-sectional view of the slab cut along a plane perpendicular to the slab drawing direction. 2 is a graph showing the relationship between the temperature gradient near the center of slab thickness and the number of segregated grains in Experiment 1. 3 is a graph showing the relationship between the water flow density and the temperature gradient near the center of slab thickness in Experiment 2. 3 is a graph showing the relationship between water volume density and temperature drop time in Experiment 3. 3 is a graph showing the relationship between the average value of the solid fraction at the start of strong cooling and the temperature gradient near the center of slab thickness in Experiment 4. It is a graph showing the influence of reduction speed during strong cooling and light reduction on the degree of segregation.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. However, the scope of the present invention is not limited to the illustrated example.

FIG. 1 is a schematic diagram showing an example of a continuous casting machine that is effective for carrying out the continuous steel casting method according to the present invention. The continuous casting machine 11 illustrated in FIG. 1 is an example of a vertical bending type continuous casting machine. In addition, in the present invention, a curved type or completely vertical type continuous casting machine can also be used instead of the vertical bending type.

The continuous casting machine 11 shown in FIG. 1 includes a mold 13, a tundish 14, a plurality of pairs of slab support rolls 16, a plurality of spray nozzles 17, and the like. As shown in this figure, the slab 18 is pulled out in the slab drawing direction D1. In addition, in this figure, the tundish 14 side along the slab drawing direction D1 will be referred to as the upstream side, and the side from which the slab 18 is pulled out will be referred to as the downstream side in the following description.

The tundish 14 is disposed above the mold 13 and is used to supply molten steel 12 to the mold 13. Molten steel 12 supplied from a ladle (not shown) is stored in this tundish 14. To this end, a sliding nozzle (not shown) for adjusting the flow rate of the molten steel 12 is installed at the bottom of the tundish 14, and an immersion nozzle 15 is installed at the bottom of this sliding nozzle.

A continuous casting mold 13 is installed below a tundish 14 , and molten steel 12 is injected into the mold 13 from the tundish 14 through an immersion nozzle 15 . The injected molten steel 12 is cooled (primary cooling) in the mold 13, thereby forming the outer shell shape of the slab 18.

As shown in FIG. 1, a plurality of pairs of slab support rolls 16 are arranged so as to support the slab 18 from both sides along the slab drawing direction D1. The plurality of pairs of slab support rolls 16 are composed of a plurality of pairs of support rolls, for example, a pair of support rolls, a pair of guide rolls, and a pair of pinch rolls. Further, as shown in this figure, a plurality of pairs of slab support rolls 16 are assembled to form one segment 20.

The plurality of spray nozzles 17 are arranged between adjacent slab support rolls 16 along the slab drawing direction D1. These spray nozzles 17 are nozzles for secondary cooling of the slab 18 by spraying cooling water toward the slab 18 . As the spray nozzle 17, a nozzle such as a water spray nozzle (one-fluid spray nozzle) or an air mist spray nozzle (two-fluid spray nozzle) can be used.

The slab 18 is cooled by cooling water (secondary cooling water) sprayed from the plurality of spray nozzles 17 while being pulled out along the slab drawing direction D1. In addition, in FIG. 1, the unsolidified portion 18a of molten steel in the slab 18 is shown with diagonal lines. Further, in FIG. 1, the solidification completion position where the unsolidified portion 18a is eliminated and solidification is completed is indicated by the reference numeral 18b.

The downstream portion of the continuous casting machine 11 is a light rolling zone 19 that lightly rolls down the slab 18. This light reduction band 19 is provided with a plurality of segments 20a and 20b each of which is constituted by a plurality of pairs of slab support rolls 16. The plurality of slab support rolls 16 of this light reduction band 19 are arranged so that the roll interval in the thickness direction of the slab 18 of each roll pair gradually narrows toward the slab drawing direction D1. , the slab 18 passing through the light rolling zone 19 is lightly rolled down. Note that the reference numeral 22 in FIG. 1 indicates a lower correction position provided within the area of the light compression zone 19.

In the downstream part of the continuous casting machine 11 described above, there is also a horizontal band region A1 that transports the slab 18 in the horizontal direction. In addition, in FIG. 1, among the segments constituted by the slab support roll 16, the segment existing in the horizontal band area A1 is shown as 20a, and the segment located upstream of the horizontal band area A1 is shown as 20b. ing.

A plurality of conveyor rolls 21 for conveying the completely solidified slab 18 are provided downstream of the horizontal band area A1. A slab cutting machine (not shown) that cuts the slab 18 into a predetermined length is disposed above the conveyance roll 21 .

In the present invention, in the section along the slab drawing direction D1 of the continuous casting machine 11, the solid phase rate along the thickness direction in the final solidified part in the width direction has the lowest solid phase rate in the width direction of the slab. From the starting point where the average value of is within the range of 0.8 or less, particularly 0.1 or more and 0.8 or less, The section up to the end point in which the average value of the solid phase ratio is larger than the average value of the solid phase ratio at the starting point and within the range of 1.0 or less is defined as the first section.

Here, the solid phase ratio is an index representing the progress of solidification, and the solid phase ratio is expressed in the range of 0 to 1.0, with solid phase ratio = 0 (zero) representing non-solidification, A solid phase ratio of 1.0 represents complete solidification.

In the present invention, a section following the first section is referred to as a second section, and this second section refers to a state where the surface temperature state (nucleate boiling state) cooled in the first section has a central solid fraction of 1.0. This is the interval that is complemented until .

In the continuous steel casting method according to the present invention, the slab is cooled by water spray sprayed from a water spray nozzle in the first section, and at this time, the water density per slab surface area is set to 50 L/(m ² × min) or more and 2000 L/(m ² × min) or less. By performing such cooling, the temperature gradient at the center of the thickness of the slab is greatly increased, the solidification structure at the center of the thickness of the slab is refined, and center segregation can be efficiently reduced. Here, in the first section, the slab is cooled with cooling water, with the water density per slab surface area being within the range of 50 L/(m ² × min) to 2000 L/(m ² × min). This is called "strong cooling."

Hereinafter, the thickness direction of the final solidified portion in the width direction where the solid phase ratio is lowest in the width direction of the slab will be explained using FIGS. 2 and 3.

FIG. 2 is a diagram where C1 is the position of the final solidified part in the width direction where the solid phase ratio is lowest in the width direction of the slab. FIG. 2 shows a plan view of the slab 18 when the upper and lower surfaces of the slab 18 are supported by slab support rolls 16. In FIG. 2, the direction "rear←→front" corresponds to the slab drawing direction D1, and the direction "right←→left" corresponds to the width direction D2 of the slab 18. The position C1 of the final solidified portion in the width direction where the solid fraction is lowest in the width direction of the slab is located along the slab drawing direction D1 at a position near the center of the slab 18 in the width direction.

FIG. 3 is a cross-sectional view of the slab 18 cut along a plane perpendicular to the slab drawing direction D1. In FIG. 3, the direction “left←→right” corresponds to the width direction D2 of the slab 18, and the direction “top←→bottom” corresponds to the thickness direction D3 of the slab 18. The position C2 in the thickness direction of the final solidified part in the width direction with the lowest solid phase ratio in the width direction of the slab is a position parallel to the thickness direction D3 at C1 in the cross section of the slab 18, and is shown in FIG. Indicated by a broken line.

In the following explanation, the heat transfer coefficient for cooling from the slab surface by water spray is calculated using a regression equation, and the physical property values for other steels are calculated using the physical property values corresponding to each temperature from the data book. For temperature, we used a value obtained by performing a proportional calculation using data at temperatures before and after that temperature.

In addition, in the following explanation, the heat transfer coefficient on the slab surface due to water spray is, for example, Publication 2 (Masashi Mitsuka, Tetsu to Hagane, Vol. 91, 2005, p. 685-693, Japan Iron and Steel Institute) The values described in Publication 3 (Toshio Teshima et al., Tetsu to Hagane, Vol. 74, 1988, p. 1282-1289, Japan Iron and Steel Institute) were used.

In the present invention, under the above-mentioned premise, it was decided to obtain the cross-sectional temperature distribution of the slab by performing an unsteady heat transfer solidification analysis.

Furthermore, the solidus ratio at a certain position arbitrarily selected in the thickness direction of the slab cross section is calculated using the temperature at the arbitrarily selected position, the solidus temperature of molten steel, and the liquidus temperature of molten steel. The temperature at the arbitrarily selected position was determined using the above-mentioned cross-sectional temperature distribution of the slab. Also, when the temperature at that location is below the solidus temperature of molten steel, the solidus fraction is 1.0, and when the temperature at that location is above the liquidus temperature of molten steel, the solidus fraction is 0. be. Furthermore, when the temperature at that position is higher than the solidus temperature of molten steel and lower than the liquidus temperature of molten steel, the solidus fraction is greater than 0 and smaller than 1.0. It is determined by the temperature at that location.

Then, from the solid fraction at each position in the slab thickness direction calculated in this way, the average value of the solid fraction along the thickness direction was determined.

As described above, in the continuous casting method for steel according to the present invention, in the first section, the water density per slab surface area is set to 50 L/(m ² ×min) or more and 2000 L/(m ² ×min) or less. within the range of In order to more efficiently obtain the effect of reducing segregation, the water density per slab surface area in this first section is more preferably 300 L/(m ² ×min) or more. Furthermore, according to the findings of the inventors, within this first section, when the water density per slab surface area is 2000L/(m ² ×min) and when it is 1000L/(m ² ×min), It was found that there were no significant differences in the temperature gradient at the final stage of solidification or the number of segregated grains. In addition, if the water density is reduced, the required amount of water can be reduced and the cost can be reduced, so it is desirable to set it to 1000 L/(m ² ×min) or less.

Regarding cooling in this first section, the effect of the present invention can be obtained if the slab is cooled with the water volume density that is compatible with the present invention. From the viewpoint of effectively obtaining the above effect, it is preferable that the difference in the solid fraction average value between the starting point and the ending point is 0.2 or more, and more preferably 0.4 or more.

Next, it is preferable that the starting point of the first section is located either in a horizontal band that horizontally conveys the slab within the continuous casting machine, or in a curved band located upstream of the horizontal band. For example, it is preferable that the first section is within a region A1 of a horizontal band that horizontally conveys slabs in a continuous casting machine. The reason for this is that strong cooling within the horizontal band region allows uniform cooling and suppresses the influence of thermal stress, making it more difficult for internal cracks to occur in the slab.

On the other hand, even if the starting point of the first section is a curved zone, the effects of the present invention can be obtained, but there is a possibility that problems such as temperature drop and correction becoming impossible or cracking due to surface stress may occur. However, even if the starting point of the first section is located within the curved band, it is within the allowable range of the present invention.

Next, regarding the cooling in the second section, the flow rate is set to a flow rate that can maintain the surface temperature of the slab at 200°C or less at a water density per slab surface area that is smaller than the water density per slab surface area in the first section. . The reason for this is that cooling at the same water density as in the first section would result in expensive equipment. However, by doing so, it is possible to suppress equipment investment costs and obtain the same effect as when strong cooling is performed only in the first section. Further, it is possible to obtain the effect of suppressing rapid recuperation and preventing internal cracking of the slab due to reheating.

In addition, from the viewpoint of obtaining the above effect, in this second section, the water density per slab surface area is set within the range of 50 L/(m ² × min) or more and 300 L/(m ² × min) or less. Preferably, the slab is cooled by spraying.

The surface temperature of the slab refers to the temperature at the center of the width of the outermost surface of the slab in the cross-sectional temperature distribution of the slab determined by the above-mentioned unsteady heat transfer solidification analysis. Although this calculated value is used for the surface temperature in the present invention, the surface temperature of the slab can also be actually measured. When actually measuring the surface temperature, the temperature of the outermost surface of the slab is measured as the surface temperature using, for example, a radiation thermometer or a thermocouple.

Furthermore, for materials with severe center segregation, it is preferable to lightly reduce the long sides of the slab at a reduction rate of 0.3 to 2.0 mm/min in the first section and the second section. The reason for this is that the suction of concentrated molten steel due to solidification shrinkage can be suppressed. In the present invention, it is also possible to further reduce center segregation by combining strong cooling and light reduction.

In addition, the roll opening degrees of the multiple pairs of slab support rolls were changed to a range that does not include the straightening point (starting after the upper straightening and ending before the lower straightening, and narrowing down the opening degree as much as possible by keeping the opening degree flat at the straightening point. (not) stepwise toward the downstream side, bulging the long sides of the slab with a total bulging amount of 2 to 20 mm, and then changing the roll opening of the multiple pairs of slab supporting rolls toward the downstream side in the casting direction. Further improvement in efficiency can be expected by reducing the amount in stages and lightly reducing the amount to a total amount that is less than the short side heat shrinkage.

Below, an experiment was conducted regarding the requirements for reducing center segregation, that is, the implementation conditions for reducing center segregation.

In this experiment, a vertical bending type continuous casting machine shown in FIG. 1 was used to cast medium carbon aluminum killed steel. The machine length of the continuous casting machine is 49 m, the thickness of the slab is 250 mm, the width of the slab is 2100 mm, and air mist spray is used for secondary cooling except for the first and second sections, and the range of secondary cooling is The distance was from just below the mold to the exit of the continuous casting machine. The chemical component concentration of medium carbon aluminum killed steel is 0.20% by mass of carbon (C), 0.25% by mass of silicon (Si), 1.1% by mass of manganese (Mn), and 0.0% by mass of phosphorus (P). 01% by mass and 0.002% by mass of sulfur (S) were used.

In addition, in this experiment, the temperature gradient near the thickness center at the solidification completion position and the final stage of solidification of the slab is defined as follows. In addition, the number of segregated grains, internal crack length, and roughness of the slab were measured as follows, and used to evaluate the degree of segregation, internal cracks, and roughness, respectively. Here, "zaku" refers to voids that remain in the closed spaces between dendrites during solidification and appear as voids in the slab due to volumetric contraction of molten steel with concentrated segregated components during solidification.

<Position of solidification completion>
The solidification completion position of the slab was calculated by unsteady heat transfer solidification analysis.

<Temperature gradient near the center of slab thickness at the final stage of solidification>
The temperature gradient near the thickness center of the slab at the final stage of solidification was calculated using the unsteady heat transfer solidification analysis described above.

In this analysis, the average temperature of an area within a range of 1 mm in the thickness direction and 10 mm in the width direction from the center position of the slab was calculated in the cross section of the slab 1 m upstream in the slab drawing direction D1 from the solidification completion position. Next, in the cross section of the slab 1 m upstream in the slab drawing direction D1 from the solidification completion position, with a position 10 mm in the thickness direction from the center position of the slab, ±1 mm in the thickness direction and 10 mm in the width direction The average temperature in the area within the range was calculated, and the difference between these two average temperatures was divided by 10 mm, which was taken as the temperature gradient (K/mm) near the center of the thickness of the slab at the final stage of solidification.

<Number of segregated grains>
The number of segregated grains was measured by the following method and used for evaluation of segregation.

In the cross section of the slab perpendicular to the slab drawing direction D1, it has a width of 15 mm, includes a central segregation part in the center, and has a triple point on one side from the center of the width (solidified shells on the short side and long side have grown). A slab sample with a length up to the point where the specimen met was taken. The cross section of the collected slab sample perpendicular to the slab drawing direction D1 is polished, and the surface is corroded with a saturated aqueous solution of picric acid to reveal a segregation zone, and the slab thickness is ±7 from the center of the segregation zone. The range of .5 mm was defined as the central segregation area. A slab sample in the segregation zone near the center of thickness (near the solidified area) was divided into small pieces in the width direction of the slab, and then divided into slabs with an electron beam diameter of 100 μm using an Electron Probe Micro Analyzer (EPMA). The manganese (Mn) concentration of the sample was analyzed over the entire surface. Then, the distribution of manganese (Mn) segregation degree was determined, and those in which regions with Mn segregation degree of 1.33 or more were connected were defined as one segregated grain. The number of segregated grains was counted, and the number of segregated grains divided by the length of the sample in the width direction of the slab was defined as the number of segregated grains (pieces/mm). Here, the Mn segregation degree is the Mn concentration in the segregated portion divided by the Mn concentration at a position 10 mm away from the center of thickness.

<Internal crack length of slab>
The length of internal cracks in the slab was measured by the following method and used for evaluation of internal cracks.

In the slab after casting, a cross section of the slab perpendicular to the slab drawing direction D1 was observed, and the length of the internal crack along the thickness direction of the slab was measured. Among the lengths of the internal cracks, the maximum length within the observed cross section was defined as the internal crack length (mm). If no internal cracks were confirmed, the internal crack length was set to 0.

<Zaku Judgment>
After milling a cross section perpendicular to the slab drawing direction D1 of the completely solidified slab and etching it with hydrochloric acid, a macro print was taken, and the scratches were confirmed by a visual sensory test. Those in which no cracks were observed were judged as good (◯), and those in which slight bumps were observed that did not affect the quality of the product were judged to be fair (△).

The inventors conducted the following experiments and investigated conditions for reducing center segregation.
[Experiment 1]
In this experiment, the temperature gradient near the center of the thickness of the slab at the final stage of solidification of the slab and the number of segregated grains were calculated or measured using the method described above, and the relationship between these was considered. A graph plotting the results is shown in FIG. From the results shown in FIG. 4, it was found that increasing the temperature gradient (K/mm) near the center of slab thickness at the end of solidification tends to reduce the number of segregated grains and reduce center segregation. The temperature gradient near the center of slab thickness at the final stage of solidification is 1.7 to 3.5, more preferably 2.0 to 3.5. The reason why center segregation can be reduced is considered to be that by increasing the temperature gradient, the solidified structure at the center of the thickness of the slab could be made finer.

[Experiment 2]
In this experiment, when secondary cooling a slab using a continuous casting machine, slabs were manufactured by changing the conditions of the water amount density per slab surface area in water spray, and the The relationship with the temperature gradient near the center of thickness at the final stage of solidification of the piece was investigated. We also investigated the optimal water flow density range to achieve a temperature gradient at the center of the slab thickness that can reduce center segregation. A graph plotting the results is shown in FIG.

From the results shown in FIG. 5, it was found that when the water density per slab surface area was 50 L/(m ² ×min) or more, the temperature gradient at the center of the slab thickness became significantly large. That is, based on the results of Experiment 1, it can be seen that center segregation can be significantly reduced by cooling with a water flow density per slab surface area of 50 L/(m ² ×min) or more. Further, even when the water density per slab surface area was made larger than 1000 L/(m ² ×min), the temperature gradient at the center of the slab thickness did not become large.

Therefore, although there are some differences between the first section and the second section, in order to efficiently increase the temperature gradient at the center of the slab thickness, the water density per slab surface area is 2000 L/(m ² × min). Hereinafter, it is preferably 1000 L/(m ² ×min) or less. Moreover, the lower limit is 50 L/(m ² ×min) or more, more preferably 300 L/(m ² ×min) or more.

[Experiment 3]
In this experiment, the effect of slab cooling was investigated. According to the findings of the inventors, the effect of cooling the slab is greatly influenced by the surface temperature of the slab. This is thought to be because the boiling form of the cooling water changes depending on the surface temperature of the slab. If the surface temperature of the slab is sufficiently lowered, the form of boiling at the surface layer will be nucleate boiling, and stable cooling can be achieved. Therefore, when performing secondary cooling of slabs using a continuous casting machine, we changed the water spray density per slab surface area to reduce the surface temperature of the slabs from 800°C to 300°C. We calculated the time it took for the temperature to drop (temperature drop time) and investigated the effect of water volume density on the temperature drop time. A graph plotting these results is shown in FIG.

From the results shown in Figure 6, when the water density per slab surface area is around 50 L/(m ² × min), the temperature drop time for the slab surface temperature to drop from 800°C to 300°C is 200 seconds. Therefore, it was found that the water density per slab surface area is preferably 50 L/(m ² ×min) or more. Furthermore, when the water density per slab surface area was greater than 2000 L/(m ² ×min), there was no significant change in the falling time.

Therefore, from the viewpoint of efficient cooling, it was found that it is desirable that the water density per slab surface area, particularly in the first section, be 2000 L/(m ² ×min) or less.

[Experiment 4]
Next, the inventors also investigated the starting position of strong cooling that can effectively increase the temperature gradient at the center of the thickness of the slab. Using a continuous casting machine, the slab is cooled by changing the conditions of the average solid fraction along the thickness direction of the slab at the start of intense cooling, and the average solid fraction at the start of intense cooling is The relationship between this value and the temperature gradient near the center of thickness of the slab at the final stage of solidification was investigated. The thickness of the slab was 250 mm, the water density per slab surface area during strong cooling was 300 L/(m ² ×min), and strong cooling was continued until the slab was completely solidified. FIG. 7 shows a graph plotting the results of the relationship between the average solid fraction at the start of strong cooling and the temperature gradient near the center of thickness at the final stage of solidification of the slab.

From the results shown in FIG. 7, it was found that the smaller the average value of the solid fraction at the start of strong cooling, the larger the temperature gradient at the center of the slab tends to be. However, the temperature gradient when the solid fraction average value at the start of strong cooling is 0.26 is not significantly different from the temperature gradient when the solid phase fraction average value is 0.43 at the start of strong cooling, and The value was 0.8, and the temperature gradient at the center of the slab was 1.7 or more. Therefore, in order for the effects of the present invention to be fully exhibited, and to make the strong cooling equipment more compact and increase equipment investment and operational efficiency, the average value of the solid fraction at the start of strong cooling must be 0.4 or more. I found out that it's good to have. Further, when the average value of solid fraction at the start of strong cooling was larger than 0.9, the temperature gradient did not become large.

Examples carried out to verify the effects of the present invention will be described below.
Medium carbon aluminum killed steel was cast using a vertical bending type continuous casting machine shown in FIG. The machine length of the continuous casting machine is 49 m, the thickness of the slab is 250 mm, the width of the slab is 2100 mm, and air mist spray is used for secondary cooling except for the first and second sections, and the range of secondary cooling is The distance was from just below the mold to the exit of the continuous casting machine. The chemical component concentration of medium carbon aluminum killed steel is 0.20% by mass of carbon (C), 0.25% by mass of silicon (Si), 1.1% by mass of manganese (Mn), and 0.0% by mass of phosphorus (P). 01% by mass and 0.002% by mass of sulfur (S) were used.

[Example 1]
In this example, the solid phase rate at the starting point in the "first section", the solid phase rate at the end point, the water density per slab surface area (L/(m ² × min)), the first section outlet surface Temperature (°C), solid phase ratio at the start point in the "second section", solid phase rate at the end point, water density per slab surface area (L/(m ² × min)), maximum surface temperature in the second section (°C) was investigated under the conditions shown in Table 1.

Regarding the results, the temperature gradient (K/mm) at the final stage of solidification, light reduction conditions (reduction speed: 0.3 to 2.0 m/min), and total bulging amount (mm) were investigated, and these are summarized in Table 1. . At the same time, the number of segregated grains (pieces/mm) was examined as an evaluation of the degree of segregation, and the internal crack length (mm) was examined as an evaluation of internal cracks, and these are summarized in Table 1. In addition, in these tables, those under conditions that are compatible with the present invention are shown as invention examples, and those under conditions that are not compatible are shown as comparative examples. In the table below, the degree of segregation is evaluated when the number of segregated grains is 1.3 particles/mm or less, which is considered good and marked with "◎", and when the number of segregated particles is more than 1.3 particles/mm and less than 3.5 particles/mm, it is considered acceptable. , and 3.5 pieces/mm or more were judged as unacceptable and evaluated as "×". For evaluation of internal cracks, if the length of the internal crack is 1.0 mm or less, it is considered good and marked with "◎", if it is more than 1.0 mm and less than 10 mm, it is marked as "○", and if it is 10 mm or more, it is marked as "x". It was evaluated by

No. 1 to 7 and No. All of Nos. 10 to 14 are examples in which operations were carried out within the range of conditions required for the operation in the first section that are compatible with the present invention. As a result, both the segregation degree and internal crack evaluation were good. On the other hand, No. shown as a comparative example. No. 8 is an example in which the water volume density in the first section is off. 9 is an example in which the solid phase ratio at the starting point is off. No. 1 with low water density. In No. 8, the nucleate boiling state could not be maintained and the evaluation of the number of segregated grains and the evaluation of internal cracks were also poor. In addition, No. 1 whose solid phase ratio was off. In No. 9, the temperature gradient at the center of the slab became small and the evaluation of the number of segregated grains was poor.

[Example 2]
In this example, the conditions required for the second section treatment in the present invention (water density per slab surface area (L/(m ² × min)) and the second section maximum surface temperature (°C) were investigated. In this example, the conditions required for the second section were studied based on the conditions required for the first section of [Example 1] above.The conditions and results are summarized in Table 2. Each evaluation was performed in the same manner as in [Example 1].As is clear from Table 2, Nos. 20 to 26 satisfied the conditions of the present invention, while Nos. 27 and 28 were evaluated in this second section. Therefore, although Nos. 27 and 28 satisfy the conditions of the present invention required in the first section, evaluation of segregation degree and internal Both crack evaluations were slightly inferior to Nos. 20 to 26.

[Example 3]
In this example, the temperature gradient (K/mm) at the center of the thickness of the slab at the end of solidification, the reduction rate (m/min) as a light reduction condition, and the total amount of bulging (mm) are determined in the first section and the second section described above. The conditions required for this are studied on the assumption that the conditions are compatible with the present invention. The conditions and results are summarized in Table 3. Each evaluation was performed in the same manner as in [Example 1]. As is clear from Table 3, No. Nos. 30 to 37 are all cases that are suitable as invention examples, and the evaluations of segregation degree and internal cracking were generally good. However, No. Regarding Nos. 38 and 39, the temperature gradient (K/mm) near the center of slab thickness at the end of solidification is small due to insufficient water density in the second section, and the maximum surface temperature in the second section is also high (>200°C). became. Therefore, No. 38 and 39 are No. It was found that the same effect as 30-37 could not be obtained.

From the results of these examples, the present invention assumes that the water density per slab surface area in at least the first section is within a predetermined range, and the water density per slab surface area in the second section is adjusted as necessary. It was confirmed that continuous casting slabs can be effectively cooled by controlling the It has also been found that it is more desirable to consider the temperature gradient near the center of slab thickness at the final stage of solidification and the conditions of light reduction in the first and second sections.

[Example 4]
In this example, the cooling conditions required for the first section described above were set to a stronger cooling range, the cooling conditions required for the second section were set to 180 L/(m ² ×min), and the total amount of bulging was kept constant. The influence of rolling speed (m/min) was also investigated as a light rolling condition. The conditions and results are summarized in Table 4, and FIG. 8 shows the relationship between the reduction rate and the degree of segregation during light reduction. Each evaluation was performed in the same manner as in [Example 1]. In addition, for the evaluation of blemishes, those in which no blemishes were observed were evaluated as good (◯), and those in which slight burrs were observed that did not affect the quality of the product were judged to be fair (△). As is clear from Table 4, No. Cases 41 to 45 and 48 to 52 are all suitable as invention examples, and the evaluations of segregation degree and roughness judgment were generally good. However, No. Regarding Nos. 40 and 47, the number of segregated grains was lower than the number of segregated grains because the reduction speed was too low under the light reduction condition. The numbers increased from 41 to 45 and 48 to 52, and slight zaku was observed. On the other hand, No. Regarding No. 46 and No. 53, the number of segregated grains was lower than that of No. 1 because the reduction speed was too high under the light reduction condition. It was more than 41-45 and 48-52. Note that no internal cracking occurred under the conditions shown in Table 4. From this result, as shown in Figure 8, by setting the cooling condition in the first section to a stronger cooling range and setting the rolling speed to 0.3 to 2.0 m/min as a light rolling condition, the degree of segregation can be evaluated. It was found that slabs with excellent internal cracking and roughness evaluation can be produced.

The reason for this is thought to be as follows. By applying ultra-strong cooling of the slab in the unsolidified region, the temperature gradient increases and dendrite refinement can be achieved. In order to further improve center segregation, it is preferable to combine light reduction. However, if the light reduction is too weak, that is, if the reduction rate is too low, the flow accompanying solidification shrinkage cannot be suppressed, resulting in positive segregation. In addition, solidification and shrinkage may cause cracks. On the other hand, if the light reduction is too strong, that is, if the reduction rate is too fast, the reduction will be excessive and the concentrated molten steel will flow backwards, causing inverted V segregation and worsening the segregation.

The unit of volume "L" herein is 10 ⁻³ m ³ . “x to y” representing a range of numerical values represents a value greater than or equal to x and less than or equal to y, and includes boundary values.

11 Continuous casting machine 12 Molten steel 13 Mold 14 Tundish 15 Immersion nozzle 16 Slab support roll 17 Spray nozzle 18 Slab 18a Unsolidified portion in slab 18b Solidification completion position 19 Light reduction zone 20 Segment 20a Segment 20b Segment 21 Conveyance roll

Claims (5)

A starting point where the average value of the solid fraction along the thickness direction at the final solidified part in the width direction, where the solid fraction is the lowest in the width direction of the slab, is 0.8 or less along the slab drawing direction in the continuous casting machine. Therefore, the average value of the solid fraction along the thickness direction at the final solidified part in the width direction where the solid fraction is lowest in the width direction of the slab is larger than the solid fraction at the starting point and 1.0 or less. The range up to a certain end point is defined as a first section, and the section located downstream of the first section in which the average solid phase ratio along the thickness direction at the end point is 1.0 or less is defined as a second section. and when,
The slab is cooled at a water density per slab surface area in the first section within a range of 50 to 2000 L/(m ² × min),
Optionally, a continuous casting method for steel, in which the slab is cooled at a water flow density per slab surface area in the second section within a range of 50 to 300 L/(m ² ×min). At least from the end of the first section to the passage through the second section, the slab is cast while maintaining a nucleate boiling state where the surface temperature is 200°C or less, and the slab thickness near the center at the final stage of solidification is maintained. The continuous casting method for steel according to claim 1, wherein the temperature gradient is 1.7 to 3.5 K/mm. The continuous casting method for steel according to claim 1 or 2, wherein the first section and the second section are installed in a region limited to a horizontal segment or a vertical segment in a continuous casting machine. The continuous steel according to claim 1 or 2, wherein in the first section and the second section, a light reduction is applied and the long side surface of the slab is reduced at a reduction rate of 0.3 to 2.0 mm/min. Casting method. By increasing the roll opening degree of the plurality of pairs of slab support rolls in stages toward the downstream side in the casting direction, the long sides of the slab are bulged with a total bulging amount of 2 to 20 mm, and then the plurality of pairs of slab support rolls are bulged. The continuous steel casting method according to claim 1 or 2, wherein the roll opening degree of the rolls is gradually reduced toward the downstream side in the casting direction to lightly reduce the slab.

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