JP2015062918A - Continuous casting method of steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce an overall segregation level in the casting piece width direction of central segregation of a continuous casting piece, and to also reduce a variation in the casting piece width direction of a segregation degree.SOLUTION: There is provided a continuous casting method of steel for manufacturing a continuous casting piece, by drafting a casting piece long side surface at a draft speed of 0.3-1.0 mm/min in a light draft belt 14 after bulging the long side surface of a casting piece 10 in a bulging total amount of 3-10 mm in a continuous casting machine. The casting piece is cast by adding mold powder having basicity (mass% CaO)/(mass% SiO) of 0.95 or more and 1.05 or less and the crystallization temperature of 1050°C or higher and 1150°C or lower in a casting mold, and thereby, a variation quantity of a casting piece thickness in the casting direction of the casting piece introduced into the light draft belt, is controlled in 0.10 mm or less.

Description

  TECHNICAL FIELD The present invention relates to a component segregation occurring at the thickness center portion of a continuously cast slab, that is, a steel continuous casting method that suppresses center segregation.

  In the solidification process of steel, solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn) are concentrated on the unsolidified liquid phase side by redistribution during solidification. This is the microsegregation formed between dendrite trees. Due to solidification shrinkage and heat shrinkage of the slab being cast by the continuous casting machine, bulging of the solidified shell generated between the rolls of the continuous casting machine, a void is formed in the center of the slab thickness and negative pressure is generated. Then, the molten steel is absorbed in this part, but since there is not a sufficient amount of molten steel in the unsolidified layer at the end of solidification, the molten steel concentrated by the microsegregation flows and accumulates in the center of the slab. Solidify. In the segregation spot formed in this way, the concentration of the solute element is much higher than the initial concentration of the molten steel. This is generally called macrosegregation, and is called central segregation because of its existence site.

  Quality of transportation line pipe materials such as crude oil and natural gas deteriorates due to central segregation. When manganese sulfide or niobium carbide is generated in the central segregation part, hydrogen that has penetrated into the steel due to corrosion reaction diffuses and accumulates around the manganese sulfide or niobium carbide in the steel, and cracking occurs due to the internal pressure. . Furthermore, since the center segregation part is hard, a crack propagates. This crack is called hydrogen induced cracking (also referred to as “HIC”) and is the main cause of deterioration of the quality of line pipe materials used in sour gas environments.

  In order to cope with this, many countermeasures for reducing or detoxifying the center segregation of the slab from the continuous casting process to the rolling process have been proposed.

  For example, in Patent Document 1 and Patent Document 2, in a continuous casting machine, a slab at the end of solidification having an unsolidified layer is reduced by a slab support roll so as to correspond to the sum of the solidification shrinkage and the heat shrinkage. A method of casting while gradually reducing the pressure is proposed.

  As in Patent Document 1 and Patent Document 2, the technique of gradually reducing the slab being cast with a reduction amount corresponding to the sum of the solidification shrinkage amount and the heat shrinkage amount in a continuous casting machine is “light reduction” or This is called “light reduction”. This light reduction technology uses a plurality of pairs of rolls aligned in the casting direction to gradually reduce the slab by reducing the volume of the unsolidified layer with a reduction amount commensurate with the sum of the solidification shrinkage and heat shrinkage. This is a technique for preventing the formation of voids or negative pressure portions in the center portion of the slab and at the same time preventing the flow of the concentrated molten steel formed between the dendrite trees, thereby reducing the center segregation of the slab.

  In recent years, the segment type continuous casting machine composed of segments with a plurality of pairs of rolls is the mainstream of recent continuous casting machines. In the case of a segment type continuous casting machine, a reduction roll that performs light reduction. A group (referred to as a “light pressure belt”) is also composed of a plurality of segments. In the light rolling belt composed of segments, the predetermined roll amount is given to the slab by adjusting the opening of the opposing rolls on the entry side and the exit side of the segment so that the entry side is larger than the exit side. It is comprised so that.

  By the way, it is known that there is a close relationship between the shape in the slab width direction of the solidification completion position of the slab and the center segregation, and Patent Document 3 detects the solidification completion position in the slab width direction, A method has been proposed in which the molten steel flow in the mold is adjusted or the width of the secondary cooling is adjusted so that the difference between the shortest part and the longest part of the detected solidification completion position is within the standard. . In this technology, when the solidification completion position is different in the slab width direction, the reduction amount in the light reduction zone is different in each position in the slab width direction, and in the position where the solidification completion position extends downstream in the casting direction, the reduction amount This is for the purpose of preventing a decrease in the amount of slag and the inability to obtain a sufficient center segregation improvement effect.

  In addition, it is known that bulging between slab rolls also affects central segregation. In Patent Document 4, bulging between rolls of a slab in a light pressure zone is calculated by unsteady heat transfer solidification calculation. A continuous casting method has been proposed in which the rolling speed applied to the slab is changed in accordance with the calculated inter-roll bulging.

JP-A-8-132203 JP-A-8-192256 JP 2006-198664 A JP 2012-45552 A

  As described above, in order to improve the center segregation of the slab, measures have been taken for the rolling speed during light rolling, the shape of the solidification completion position in the slab width direction, and bulging between rolls. However, the quality requirement level for the nearest continuous cast slab is further increased, and the variation of the segregation degree in the slab width direction is also a problem. In particular, in a segregation-rigid steel material such as a line pipe material, it is difficult to use as a line pipe material if there is a part with poor segregation even at one place in the width direction at the slab stage.

  From this point of view, if the prior art is verified, the prior art has the following problems.

  That is, in Patent Document 1 and Patent Document 2, the segregation degree in the slab width direction decreases as a whole by light reduction, but when the solidification completion position differs in the slab width direction, the segregation improvement effect is not sufficient. Absent. This is because the part where the solidification completion position extends to the downstream side in the casting direction compared to other positions in the slab width direction is a part where solidification has already been completed, and resistance to light reduction is difficult to be applied. In some cases, the above-described hydrogen-induced cracking may occur.

  In Patent Document 3, shape control of the solidification completion position in the slab width direction is adopted as a segregation reduction measure, but the relationship between the shape of the solidification completion position in the slab width direction and the distribution of segregation in the slab width direction. However, even if the target of segregation degree is set, it is not clear how the object can be achieved by specifically controlling the shape of the solidification completion position in the slab width direction. Patent Document 3 states that segregation is sufficiently reduced if the difference in casting direction length between the shortest solidification completion position and the longest solidification completion position is controlled to 2 m or less. The level may not be supported.

  Patent Document 4 employs a method of changing the rolling speed applied to the slab according to the bulging between rolls calculated by unsteady heat transfer solidification calculation. Then, the bulging of the slab is already unsteady bulging that does not return to its original shape due to plastic deformation. Therefore, the entire slab is pushed in at the portion that comes into contact with the roll, and the entire slab swells between the rolls. Since this phenomenon occurs regardless of the rolling speed, even if the rolling speed is increased or decreased, no substantial improvement is achieved. That is, in order to improve the center segregation of the slab, it is necessary to reduce unsteady bulging itself.

  The present invention has been made in view of the above circumstances, and its object is to reduce the overall segregation level of the center segregation in the slab width direction and also reduce the variation of the segregation degree in the slab width direction. It is to provide a continuous casting method for steel.

The gist of the present invention for solving the above problems is as follows.
[1] The roll opening degree of a plurality of pairs of slab support rolls is gradually increased toward the downstream side in the casting direction to bulge the long side surface of the slab with a bulging total amount of 3 to 10 mm, and then a plurality of pairs of slabs In the light pressure zone where the roll opening of the support roll is gradually reduced toward the downstream side in the casting direction, until the solid phase ratio at the center of the slab thickness is at least 0.3 to 0.7. In the continuous casting method of steel for producing a continuous cast slab by reducing the long side surface of the slab at a reduction speed of 0.3 to 1.0 mm / min, the basicity ((mass% CaO) / (mass% SiO ₂ )) Is 0.95 or higher and 1.05 or lower and a crystallization temperature is 1050 ° C. or higher and 1150 ° C. or lower is added into the mold to cast a slab, and the following formula (1) The value calculated in step 5 is used to change the slab thickness in the casting direction. When defined as a dynamic amount, the steel continuous casting method is characterized in that the amount of fluctuation of the slab introduced into the light pressure lower zone is controlled to 0.10 mm or less.

However, in the formula (1), A is the fluctuation amount (mm) of the slab thickness in the casting direction, and Zi is the slab surface from the roll pass line of the slab support roll, which is measured at intervals of 100 mm or less in the casting direction. The i-th measured value (mm) of the distance up to n is an arbitrary integer of 2 or more.
[2] Injection of cooling water per unit area and unit time calculated at intervals of 10 mm or less in the slab width direction from the arrangement of each spray nozzle in the secondary cooling zone and the injection amount of cooling water from each spray nozzle When the value obtained by averaging the amount in the casting direction is defined as the cooling water amount density distribution in the slab width direction in the secondary cooling zone, at least the standard deviation σ of the cooling water amount density distribution from immediately below the mold to a position 10 m below in the casting direction A ratio (σ _SP / Vc) of _SP (L / m ² / s) and slab drawing speed Vc (m / min) is 2.5 or less, and the steel according to [1] above Continuous casting method.
[3] Surface temperature or axial temperature distribution at intervals of 100 mm or less in the slab width direction calculated by heat transfer solidification calculation, or surface temperature at intervals of 100 mm or less in the slab width direction measured online Calculate the solidification completion position at each position in the slab width direction based on the temperature distribution of the axis temperature, determine the distribution in the slab width direction of the solidification completion position from the calculated solidification completion position, and complete the solidification from the obtained distribution When the average value and standard deviation of the position in the slab width direction are calculated, and the value obtained by dividing the calculated average value by the calculated standard deviation is defined as the coefficient of variation in the slab width direction of the solidification completion position, the slab The method of continuous casting of steel according to the above [2], wherein the coefficient of variation is set to 0.03 or less.
[4] The continuous casting method for steel according to [3] above, wherein the coefficient of variation is 0.03 or less for a slab at the entrance of the lower pressure zone.

According to the present invention, the actual rolling speed applied to the slab is controlled in the range of 0.3 to 1.0 mm / min, and the thickness variation amount in the casting direction of the slab introduced into the light rolling belt. Is controlled to be 0.10 mm or less, it is realized to reduce the average value of the segregation degree due to center segregation in the slab width direction. Further, when the ratio (σ _SP / Vc) between the standard deviation (σ _SP ) of the cooling water density distribution and the slab drawing speed (Vc) is controlled to 2.5 or less, the slab of the segregation degree due to center segregation The variation in the width direction can be reduced, and the degree of segregation at all locations in the slab width direction can be reduced.

It is the side schematic diagram of the slab continuous casting machine used when implementing the present invention. It is a figure which shows the example of the profile of the roll opening degree of the slab support roll in this invention. It is a figure which shows typically the method of measuring the fluctuation | variation amount of slab thickness in the casting direction of a slab using a water column ultrasonic sensor. It is a figure which shows the result of having investigated the relationship between the variation | change_quantity of the slab thickness in the casting direction of slab, and the average value of the slab width direction of Mn segregation degree. It is a figure which shows the result of having investigated the relationship between the crystallization temperature of mold powder, and the variation | change_quantity of slab thickness in the casting direction of a slab. It is a figure which shows the example of the result of investigating the relationship between the shape in the slab width direction of the solidification completion position, and the distribution in the slab width direction of Mn segregation degree. It is a figure which shows the relationship between ratio ((sigma) _SP / Vc) and the variation coefficient in the slab width direction of the solidification completion position.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic side view of a vertical bending slab continuous casting machine used in carrying out the present invention.

  As shown in FIG. 1, molten steel 9 is injected into the slab continuous casting machine 1, the molten steel 9 is cooled and solidified, and a mold for forming an outer shell shape of a slab 10 having a rectangular cross section. 5 is installed. A tundish 2 for relaying and supplying molten steel 9 supplied from a ladle (not shown) to the mold 5 is installed at a predetermined position above the mold 5. A sliding nozzle 3 for adjusting the flow rate of the molten steel 9 is installed on the bottom of the tundish 2, and an immersion nozzle 4 is installed on the lower surface of the sliding nozzle 3.

  On the other hand, a plurality of pairs of slab support rolls 6 including a support roll, a guide roll, and a pinch roll are arranged below the mold 5. A spray nozzle (not shown) such as a water spray nozzle or an air mist spray nozzle is disposed in the gap between the slab support rolls 6 adjacent to each other in the casting direction, and the range from directly below the mold to the slab support roll 6 at the end of the machine. In addition, a secondary cooling zone is configured. The slab 10 is cooled by the secondary cooling water sprayed from the spray nozzle in the secondary cooling zone while being drawn.

  A plurality of conveying rolls 7 for conveying the cast slab 10 are installed on the downstream side in the casting direction of the final slab support roll 6 in the casting direction. A slab cutting machine 8 for cutting a slab 10 a having a predetermined length from the slab 10 to be cast is disposed above the transport roll 7.

  Light pressure lower belts 14 are installed on the upstream side and the downstream side in the casting direction across the solidification completion position 13 of the slab 10. The light pressure lower belt 14 is set so that the interval between the slab support rolls facing each other across the slab 10 (this interval is referred to as “roll opening”) is gradually narrowed toward the downstream side in the casting direction. It is composed of a plurality of pairs of cast slab support rolls that are subjected to a rolling gradient. In the light reduction belt 14, it is possible to perform light reduction on the slab 10 in the entire region or a partially selected region. A spray nozzle for cooling the slab 10 is also disposed between the slab support rolls of the light pressure lower belt 14. The slab support roll 6 disposed in the light reduction belt 14 is also referred to as a “reduction roll”.

  In the slab continuous casting machine 1 shown in FIG. 1, the light reduction belt 14 is configured by connecting three segments each including three pairs of reduction rolls in the casting direction. In FIG. 1, the light pressure lower belt 14 is composed of three segments, but it may be one, two, or even four or more. Moreover, in FIG. 1, although the slab support roll 6 arrange | positioned at one segment is 3 pairs, it is not necessary to set it as 3 pairs, and as long as it is 2 pairs or more, it does not matter. Although not shown, the slab support roll 6 other than the light pressure lower belt also has a segment structure.

  Normally, the rolling gradient in the light pressure lower zone 14 is indicated by the narrowing amount of the roll opening per 1 m in the casting direction, that is, “mm / m”. Therefore, the rolling speed of the slab 10 in the light pressure lower zone 14 ( mm / min) is obtained by multiplying the rolling gradient (mm / m) by the slab drawing speed (m / min).

  The slab support roll 6 disposed between the lower end of the mold 5 and the liquidus crater end position of the slab 10 constitutes an intentional bulging band 15. In the intentional bulging zone 15, each slab support roll is formed so that the roll opening gradually increases every roll or every several rolls toward the downstream side in the casting direction until the amount of expansion of the roll opening reaches a predetermined value. 6 is set. The slab support roll 6 installed on the downstream side of the intentional bulging zone 15 is narrowed to such an extent that the roll opening degree corresponds to a constant value or the amount of shrinkage accompanying the temperature drop of the slab 10, and then the downstream side is lightly reduced. It is connected to the belt 14.

  In FIG. 2, the example of the profile of the roll opening degree of the slab support roll in this invention is shown. As shown in FIG. 2, the intentional bulging band 15 intentionally bulges the long side surface of the slab by molten steel static pressure to increase the thickness of the central part of the long side surface of the slab (region b). On the downstream side after passing through the roll, the roll opening is narrowed to a certain value or the amount of shrinkage accompanying the temperature drop of the slab 10 (region c), and then the long side surface of the slab is rolled down by the light reduction belt 14. This is a profile (region d). “A” and “e” in the figure are regions where the roll opening is narrowed to the extent that it corresponds to the amount of shrinkage accompanying the temperature drop of the slab 10. “A ′” in the figure is an example of the roll opening degree of the conventional method in which the roll opening degree is narrowed to an extent corresponding to the amount of shrinkage accompanying the temperature drop of the slab 10.

  In the intentional bulging zone 15, by gradually increasing the roll opening degree of the slab support roll 6 toward the downstream side in the casting direction, the long side surface except for the vicinity of the short side of the slab 10 is made of the molten steel by the unsolidified layer. It is intentionally bulged by the pressure following the slab support roll 6. The vicinity of the short side of the long side surface of the slab is held and restrained by the short side surface of the slab that has been solidified, so that the thickness at the time when intentional bulging is started is maintained. Only the bulged part of the long side surface of the slab is brought into contact with the slab support roll 6 by intentional bulging. Moreover, in the light reduction zone 14, by controlling the total reduction amount to be equal to or less than the total bulging amount, only the bulging portion of the long side surface of the slab is reduced, and the light reduction can be efficiently performed. The total amount of reduction is the amount of reduction of the slab 10 from the start of reduction to the end of reduction in the light reduction zone 14, and the total amount of bulging is the end of intentional bulging in the intentional bulging zone 15 to the end of intentional bulging. The amount of bulging up to.

  In the slab continuous casting machine 1 having this configuration, the molten steel 9 injected from the tundish 2 through the immersion nozzle 4 into the mold 5 is cooled by the mold 5 to form a solidified shell 11. The slab 10 having the solidified shell 11 as an outer shell and having an unsolidified layer 12 inside is continuously pulled out below the mold 5 while being supported by a slab support roll 6 provided below the mold 5. The slab 10 is cooled by the secondary cooling water in the secondary cooling zone while passing through the slab support roll 6, and the thickness of the solidified shell 11 is increased. And the slab 10 increases the thickness of the part except the short side edge part of the long side surface of the slab in the intentional bulging band 15, and the slab 10 is in the solidification completion position 13 while being lightly reduced in the light pressure lower band 14. Complete solidification until. The slab 10 after completion of solidification is cut by the slab cutting machine 8 to become a slab 10a. Mold powder (not shown) that functions as a heat insulating material, a lubricant, an antioxidant, or the like is added into the mold.

  In the present invention, the reason why the intentional bulging band 15 is disposed between the lower end of the mold 5 and the liquid phase crater end position of the slab 10 is as follows. That is, on the upstream side in the casting direction from the liquid phase crater end position of the slab 10, the center part of the slab thickness is the unsolidified layer 12 (liquid phase), and the solidified shell 11 of the slab 10 has a high temperature, Deformation resistance is small and can be easily bulged. In addition, when the slab 10 is intentionally bulged, the center segregation is worsened if the slab 10 is bulged at a time when the unsolidified layer 12 present in the slab 10 is small. However, when bulging is performed upstream of the liquid phase crater end position of the slab 10 in the casting direction, the molten steel having an initial concentration in which the solute elements are not concentrated is abundant in the slab at this point. And this molten steel flows easily. Even if this molten steel flows, segregation does not occur. Therefore, bulging at this point does not cause central segregation.

In addition, the liquidus of the slab 10 is a solidification start temperature determined by the chemical component of the slab 10, and can be obtained from, for example, the following equation (2).
TL = 1536- (78 × [% C] + 7.6 × [% Si] + 4.9 × [% Mn] + 34.4 × [% P] + 38 × [% S] + 4.7 × [% Cu] + 3.1 × [ % Ni] + 1.3 × [% Cr] + 3.6 × [% Al])… (2)
However, in the formula (2), TL is the liquidus temperature (° C.), [% C] is the carbon concentration (mass%) of the molten steel, [% Si] is the silicon concentration (mass%) of the molten steel, and [% Mn]. Is the manganese concentration (mass%) of the molten steel, [% P] is the phosphorus concentration (mass%) of the molten steel, [% S] is the sulfur concentration (mass%) of the molten steel, and [% Cu] is the copper concentration (mass%) of the molten steel. ), [% Ni] is the nickel concentration (mass%) of the molten steel, [% Cr] is the chromium concentration (mass%) of the molten steel, and [% Al] is the aluminum concentration (mass%) of the molten steel.

  The liquidus crater end position of the slab 10 can be obtained by comparing the temperature gradient inside the slab obtained by heat transfer solidification calculation with the liquidus temperature determined by the equation (2).

  Since the intentional bulging band 15 does not require a special mechanism and is configured only by adjusting the roll opening, as long as it is within the range from the lower end of the mold 5 to the liquidus crater end position of the slab 10, It can be installed at any position.

  In the present invention, the load applied to the segment (also referred to as a “lightly reduced segment”) constituting the lightly pressed belt 14 is the size of the slab 10, the rolling gradient in the lightly pressed belt 14, and the unsolidified slab 10 to be lightly pressed. It is determined by the ratio of the layer 12. In order to prevent molten steel flow at the end of solidification, which causes center segregation, it is necessary to apply a light reduction of an amount commensurate with the amount of solidification shrinkage and heat shrinkage. If the size of one piece is large, the load applied to the lightly pressed segment increases.

  When the load applied to the light reduction segment increases, the roll opening degree in the light reduction segment increases. Therefore, even if the slab size and the rolling gradient setting are the same, the load applied to the light rolling segment varies according to the shape of the solidification completion position 13 in the slab width direction, and the roll opening degree also varies depending on the load. Since it fluctuates, the reduction speed actually applied to the slab 10 also fluctuates from the set value. In addition, an increase in the load on the lightly pressed segment leads to a shortened life of the lightly pressed segment.

  For the purpose of efficiently lightly reducing the slab 10, the inventors first changed the bulging total amount in the intentional bulging zone 15, the slab drawing speed, and the setting reduction gradient in the light reduction zone 14. Under the condition, a test for casting a slab 10 (slab slab) having a width of 2100 mm and a thickness of 250 mm is performed, and the influence of the total amount of bulging in the intentional bulging zone 15 and the reduction speed in the light reduction zone 14 on the slab quality is investigated. A test was conducted. In that case, the total amount of bulging in the intentional bulging zone 15 was made larger than the total amount of reduction in the light reduction zone 14 so as not to reduce the short side of the slab 10 that had been solidified during light reduction. Specifically, the total amount of bulging in the intentional bulging zone 15 was changed within a range of 2 to 13 mm. Further, during casting, the roll opening displacement was measured by a non-contact sensor in the lightly reduced segment where the solidification completion position 13 which is the most downstream in the casting direction was previously obtained by heat transfer solidification calculation.

  As a result, when the total amount of bulging is less than 3 mm, the short side of the completely solidified slab 10 is crushed when the light squeezing zone 14 is squeezed, the load on the light squeezing segment becomes excessive, Could hardly do. On the other hand, when the total amount of bulging exceeded 10 mm, an internal crack occurred in the slab 10. Therefore, it was found that the total amount of bulging in the intentional bulging zone 15 needs to be set to 3 to 10 mm.

  In the present invention, when the slab 10 reaches the uppermost stream position of the light pressure lower belt 14, the portion where the long side surface of the slab 10 does not contact the long side surface of the slab 10 and the reduction roll is cast. It is defined that the short side of the slab 10 is not squeezed by the light reduction belt 14 when 50 mm or more exists in the half width direction. In addition, in the intentional bulging zone 15, the amount of expansion of the roll opening per roll is preferably 1.5 mm or less. This is to prevent the occurrence of cracks at the boundary position between the bulging part and the non-bulging part on the long side surface of the slab.

  Moreover, as a result of investigating the relationship between the reduction speed actually applied to the slab 10 and segregation of the slab 10, V segregation occurs when the reduction speed in the light reduction belt 14 is less than 0.3 mm / min, When the rolling speed exceeded 1.0 mm / min, reverse V-shaped segregation occurred. Therefore, it was found that the rolling speed in the light rolling belt 14 needs to be controlled to 0.3 to 1.0 mm / min. The rolling speed actually applied to the slab 10 was determined as the product of the rolling gradient calculated from the displacement of the roll opening in the light rolling segment measured by a non-contact sensor and the slab drawing speed. Is.

  As described above, the segregation of the slab 10 can be improved by optimizing the total amount of bulging in the intentional bulging zone 15 and the reduction speed in the light reduction zone 14, but this is the only required segregation level in recent years. Can not respond.

Therefore, the present inventors further reduced the slab drawing speed at 1.1 m / min, the total bulging amount in the intentional bulging zone 15 to 5.0 mm, and light reduction in continuous casting of the slab 10 having a width of 2100 mm and a thickness of 250 mm. A test for changing the physical property value of the mold powder to be used and a test for changing the amount of cooling water in the secondary cooling zone are performed under the condition that the rolling speed in the belt 14 is constant at 0.70 mm / min. A test was conducted to investigate the effect of the cooling water amount in the next cooling zone on the segregation of slabs. As physical properties of the mold powder, crystallization temperature and basicity ((mass% CaO) / (mass% SiO ₂ )) were used as indices.

  During this test casting, the roll opening of the light rolling segment was measured with a non-contact sensor, the rolling gradient was determined from the measured roll opening, and the rolling speed actually applied to the slab 10 was monitored. . As a result, it was confirmed that the rolling speed was in the range of 0.3 to 1.0 mm / min, which is the appropriate range described above. Further, during casting, longitudinal wave ultrasonic waves are transmitted through the slab 10, the temperature at the center of the slab thickness is obtained from the propagation time of the longitudinal wave ultrasonic wave, and the obtained temperature is illuminated with the center temperature of the slab by heat transfer solidification calculation. By matching, the solidification completion position 13 of the slab 10 was estimated. As a result, it was confirmed that the solidification completion position 13 of the slab 10 was present in the light-down segment on the most downstream side of the light-down zone 14.

  Further, at the time of this test casting, the thickness fluctuation amount in the casting direction of the slab 10 was measured by a water column ultrasonic sensor installed in the gap of the slab support roll 6 immediately before the light pressure lower belt 14. The water column ultrasonic sensor is a device that measures the distance between the slab surface and the water column ultrasonic sensor by ultrasonic waves passing through the water column.

  Here, the fluctuation amount of the slab thickness in the casting direction of the slab 10 is defined by the following equation (1).

  However, in Formula (1), A is the amount of fluctuation (mm) of the slab thickness in the casting direction, and Zi is the slab surface from the roll pass line of the slab support roll measured at intervals of 100 mm or less in the casting direction. The i-th measured value (mm) of the distance up to n is an arbitrary integer of 2 or more.

  FIG. 3 schematically shows a method of measuring the fluctuation amount of the slab thickness in the casting direction of the slab 10 using a water column ultrasonic sensor. As shown in FIG. 3A, the distance from the reference position to the tip position of the water column ultrasonic sensor 16 is x, the distance from the reference position to the center of the slab support roll 6 on the upper surface side is y, and the slab support roll When the radius of 6 is d, the reference distance (Lo) is uniquely determined as “Lo = y + d−x”. That is, the reference distance (Lo) is a distance to the surface of the virtual slab 10. During casting, as shown in FIG. 3B, the distance (Li) is measured at an interval of 100 mm or less in the casting direction by the water column ultrasonic sensor 16, and the measured distance (Li) and the reference distance (Lo) The absolute value of the difference (Zi = Lo-Li) is measured as the amount of thickness variation in the casting direction of the slab 10 with respect to the roll pass line. Two broken lines in FIG. 3 are the roll path lines on the upper surface side and the lower surface side.

  FIG. 3 is a schematic diagram for measuring the fluctuation amount of the slab thickness on the upper surface side of the slab 10, and the lower surface side of the slab 10 also measures the fluctuation amount of the slab thickness in the casting direction by the same method as described above. . However, in the slab continuous casting machine 1 of the vertical bending type, due to the cooling water remaining on the surface of the slab, the amount of fluctuation of the slab thickness in the casting direction tends to increase on the upper surface side of the slab. In this case, there is no problem even if the fluctuation amount of the slab thickness is measured only on the upper surface side of the slab 10.

For the slab 10a obtained by test casting, a test piece at the center of the slab thickness is cut out at the entire width of the slab, and the Mn concentration of the slab is quantitatively analyzed by EPMA (Electron Probe Micro Analyzer), and Mn segregation at each position is performed. The degree of Mn segregation in the slab width direction was measured. The value of the Mn segregation degree was averaged in the slab width direction, and this average value was defined as “the average value of the Mn segregation degree in the slab width direction”. Here, Mn segregation ratio scans the EPMA in a direction perpendicular to the center of the slab thickness, the highest value is the Mn concentration at that time (Mn _i), the samples taken from the molten steel corresponding to the slab It is defined by Mn analysis (Mn _o) and the ratio of the (Mn _i / Mn _o).

  In FIG. 4, the result of having investigated the relationship between the variation | change_quantity of the slab thickness in the casting direction of the slab 10, and the average value of the slab width direction of Mn segregation degree is shown. As is clear from FIG. 4, it was found that the average value of the Mn segregation degree in the slab width direction becomes smaller as the fluctuation amount of the slab thickness in the casting direction is smaller.

  The present inventors have empirically confirmed that in order to reduce hydrogen-induced cracking, the average value of the Mn segregation degree in the slab width direction needs to be 1.025 or less. From the data shown in FIG. 4, in order to prevent hydrogen-induced cracking, the amount of fluctuation in the slab thickness in the casting direction of the slab 10 immediately before the light pressure lower zone, that is, the casting of the slab 10 introduced into the light pressure lower zone 14 It was confirmed that it was necessary to control the fluctuation amount of the slab thickness in the direction to 0.10 mm or less.

  One of the causes for changing the slab thickness in the casting direction of the slab 10 is bulging between rolls of the slab 10. Inter-roll bulging of the slab 10 is a phenomenon in which the slab 10 having an unsolidified layer 12 therein is bulged between adjacent slab support rolls in the casting direction by a molten steel static pressure. However, in the case of bulging between rolls, the slab 10 is returned to a predetermined thickness by the slab support roll 6 installed adjacent to the downstream side. There is no gradual increase.

  As a means for reducing bulging between rolls, a method of increasing the specific water amount in the secondary cooling zone and increasing the thickness of the solidified shell 11 is known. However, when the specific water amount in the secondary cooling zone is increased, cooling unevenness occurs in the slab width direction, and the solidification completion position 13 varies in the slab width direction. Furthermore, there is a concern that excessive secondary cooling water causes variations in heat shrinkage in the casting direction, and conversely increases the amount of fluctuation of the slab thickness in the casting direction of the slab 10. That is, from the viewpoint of reducing the center segregation of the slab 10, the method of increasing the specific water amount is not preferable. The specific water amount means the amount of secondary cooling water required to cool a 1 kg slab in the entire secondary cooling zone of the slab continuous casting machine 1.

  Therefore, the relationship between the crystallization temperature of the mold powder and the amount of fluctuation of the slab thickness in the casting direction of the slab 10 was investigated. In FIG. 5, the result of having investigated the relationship between the crystallization temperature of mold powder and the fluctuation | variation amount of slab thickness in the casting direction of the slab 10 is shown. The basicity of the mold powder shown in FIG. 5 is all 1.00. The crystallization temperature of the mold powder was adjusted by adjusting the components in the mold powder.

  As shown in FIG. 5, when the crystallization temperature of the mold powder to be used is 1050 ° C. or higher and 1150 ° C. or lower, it is confirmed that the fluctuation amount of the slab thickness in the casting direction of the slab 10 is reduced to 0.10 mm or less. did it. This is because if the crystallization temperature of the mold powder becomes too high, the heat removal from the mold 5 becomes too small, the thickness of the solidified shell 11 after the mold exit becomes thin, and bulging between rolls increases. If the crystallization temperature of the mold powder becomes too low, the amount of heat removed from the mold 5 becomes too large, the thickness of the solidified shell 11 becomes non-uniform in the casting direction, and it is considered that bulging between rolls is promoted.

  Furthermore, when the basicity of the mold powder is less than 0.95, and when it exceeds 1.05, the releasability of the mold powder from the cast piece 10 is deteriorated, and secondary cooling is inhibited. It was found that the growth of the solidified shell 11 was delayed, and as a result, bulging between rolls of the slab 10 increased. Therefore, it was found that the basicity of the mold powder needs to be adjusted to 0.95 or more and 1.05 or less.

  That is, the actual amount of reduction applied to the slab 10 is controlled within a range of 0.3 to 1.0 mm / min, and the amount of variation in the thickness of the slab 10 in the casting direction at the time of introduction into the light reduction belt 14. It was found that the average value of the degree of segregation in the slab width direction can be reduced by controlling the thickness to 0.10 mm or less.

  However, in steel materials that are strict against segregation, even if the average value of the segregation degree in the slab width direction is good, the slab cannot be used if there is a part with poor segregation even at one location in the slab width direction. There is a case. Therefore, it is necessary to reduce the average value of the segregation degree in the slab width direction and at the same time reduce the variation in the segregation degree in the slab width direction.

  The inventors have continued further research and examination, and as a result, found that there is a very strong correlation between the variation in the slab width direction of the solidification completion position 13 and the variation of the segregation degree in the slab width direction. It was.

  In FIG. 6, the example of the result of having investigated the relationship between the shape in the slab width direction of the solidification completion position 13 and the distribution in the slab width direction of Mn segregation degree is shown. The slab 10 showing this example is given a reduction speed of 0.3 to 1.0 mm / min, and the amount of variation in the thickness of the slab 10 in the casting direction when it is introduced into the light reduction belt 14. Is controlled to be 0.10 mm or less.

  As shown in FIG. 6, it was confirmed that the degree of segregation of Mn deteriorated at the portion where the solidification completion position 13 extended to the downstream side in the casting direction. As described above, the slab is controlled to have a reduction rate within a proper range and a slab thickness fluctuation amount of 0.10 mm or less when introduced into the light reduction belt 14. The average value in the slab width direction was 1.023, which was less than the average value (1.025) of the Mn segregation degree at which hydrogen-induced cracking was reduced.

  However, there is a difference of about 2.8 m in the casting direction between the shortest solidification completion position 13 and the longest solidification completion position 13, and the variation in the slab width direction of the solidification completion position 13 is large. The segregation degree also changed, and the Mn segregation degree at the position where the solidification completion position 13 was the most downstream side was worse than 1.05. The reason for this is that in the portion where the solidification completion position 13 extends downstream, the other portions of the slab 10 that have already solidified become resistance, the reduction amount decreases, and each slab width position macroscopically. This is because the concentrated molten steel discharged from the solidified portion on the upstream side moves to the downstream side.

  In a portion where the degree of segregation of Mn exceeds 1.05, hydrogen-induced cracking may occur. As described above, in a steel material that is strict against segregation, if there is a portion where segregation has deteriorated even at one location in the slab width direction, it may not be used for that purpose. In order to prevent this, it is necessary to reduce the variation of the segregation degree in the slab width direction.

  In investigating the relationship between the variation of the segregation degree in the slab width direction and the variation in the slab width direction of the solidification completion position 13, the variation defined below is used as an index of the variation in the slab width direction of the solidification completion position 13. A coefficient was adopted.

  That is, the temperature distribution of the slab surface temperature or axial temperature at intervals of 100 mm or less in the slab width direction calculated by heat transfer solidification calculation, or casting at intervals of 100 mm or less in the slab width direction measured online. The solidification completion position 13 at each position in the slab width direction is calculated based on the temperature distribution of the surface temperature or axial temperature of the piece, and the distribution in the slab width direction from the calculated solidification completion position 13 to the solidification completion position 13 is calculated. The average value and standard deviation in the slab width direction of the solidification completion position 13 are calculated from the obtained distribution, and the value obtained by dividing the calculated average value by the calculated standard deviation is calculated in the slab width direction of the solidification completion position 13. Was defined as the coefficient of variation.

  The reason for adopting this coefficient of variation as an index of variation in the slab width direction of the solidification completion position 13 is that if the standard deviation is used as an index, if the average value of the solidification completion position 13 in the slab width direction is large, the variation is Even if is small, the value of the standard deviation becomes large and is not suitable as an index. Therefore, the coefficient of variation obtained by dividing the average value by the standard deviation is adopted as the index of variation. However, even if the variation index in the slab width direction of the solidification completion position 13 is an index other than the above coefficient of variation, there is no problem as long as it is an index that shows accurate variation.

  The coefficient of variation is intended to uniformly perform light reduction in the light pressure lower belt 14 in the width direction of the slab. Therefore, as a matter of course, the solidification completion position 13 to be considered is after the light pressure lower belt 14 It does not include the part that has been solidified by the time it enters the light pressure zone 14.

  As a result, the reduction speed actually applied to the slab 10 was controlled to 0.3 to 1.0 mm / min, and the fluctuation amount of the slab thickness at the position immediately before the light reduction belt was controlled to 0.10 mm or less. In this case, if the variation coefficient in the slab width direction of the solidification completion position 13 obtained at the entrance of the light pressure lower belt 14 is 0.03 or less, the variation in the segregation degree in the slab width direction is reduced, and all castings are performed. It was found that hydrogen-induced cracking did not occur at the half-width position.

  Hereinafter, means for controlling the coefficient of variation of the solidification completion position 13 in the slab width direction to 0.03 or less will be described.

The inventors changed the slab drawing speed and the amount of secondary cooling water to change the standard deviation of the cooling water density density distribution in the slab width direction in the secondary cooling zone (σ _SP , unit = L / m ² / s). And the ratio (σ _SP / Vc, unit = (L × min) / (m ³ × s)) to the slab drawing speed (Vc, unit = m / min). At that time, the solidification completion position 13 in the width direction of the slab was measured by transmitting ultrasonic waves to the slab 10 and measuring the propagation time. FIG. 7 shows the relationship between the ratio (σ _SP / Vc) at that time and the coefficient of variation of the solidification completion position 13 in the slab width direction. Here, the cooling water density density distribution in the slab width direction in the secondary cooling zone is 10 mm or less in the slab width direction from the arrangement of each spray nozzle and the injection amount of cooling water from each spray nozzle in the secondary cooling zone. It is the value which averaged the injection quantity of the cooling water per unit area and unit time calculated for every space | interval in a casting direction.

As shown in FIG. 7, in order to control the coefficient of variation of the solidification completion position 13 in the slab width direction to 0.03 or less, the ratio (σ _SP / Vc) needs to be 2.5 or less. Recognize. To reduce the ratio (σ _SP / Vc), it is necessary to reduce the standard deviation (σ _SP ) or increase the slab extraction speed (Vc), but increase the slab extraction speed (Vc) more than necessary. Then, since the solidification completion position 13 does not enter the light pressure lower belt 14, it is necessary to determine the amount of secondary cooling water and the slab drawing speed (Vc) on the assumption that the solidification completion position 13 enters the light pressure lower belt 14. In determining the amount of water for secondary cooling so that the position on the most downstream side in the casting direction in the solidification completion position 13 in the slab width direction enters the light pressure lower zone 14, the ratio (σ _SP / Vc) is set to 2. If a slab drawing speed (Vc) that can be set to 5 or less is employed, the maximum productivity is achieved while maintaining the internal quality of the slab 10.

  In the above description, the case where the solidification completion position 13 is controlled so as to fall within the range of the light pressure lower belt 14 and continuous casting is taken as an example. In the present invention, the thickness of the slab 10 at the outlet of the light pressure lower belt 14. There is no problem if the solid phase ratio in the center is 0.7 or more. On the other hand, it is necessary to control the solid phase ratio at the center of the thickness of the slab 10 at the entrance of the light pressure lower zone 14 to 0.3 or less. That is, in the light reduction zone 14, it is necessary to apply a reduction force to the slab 10 from the time when the solid phase ratio at the center of the slab thickness is at least 0.3 to the time of 0.7.

  This is because even if the reduction starts after the solid phase ratio at the center of the slab thickness exceeds 0.3, the flow of the concentrated molten steel may occur before that, causing central segregation. The effect of light pressure cannot be obtained sufficiently. In addition, the flow of molten steel may occur when the solid phase ratio at the center of the slab thickness is less than 0.7, and if the reduction is stopped in a state where the solid phase ratio is less than 0.7, the concentration will increase. The flow of molten steel occurs, which causes central segregation, and the effect of light reduction cannot be obtained sufficiently.

  The solid phase ratio at the center portion of the slab thickness can be obtained by heat transfer solidification calculation as in the case of obtaining the liquidus crater end position. Here, the solid phase ratio is defined as the solid phase ratio = 0 before the start of solidification and the solid phase ratio = 1.0 when the solidification is completed. Therefore, the position where the solid phase ratio at the center of the slab thickness is 1.0 is the solidification completion position 13 (solid line crater end position), and the liquidus crater end position is the solid phase at the center of the slab thickness. This corresponds to the most downstream position where the rate is zero.

  In the present invention, the total reduction amount in the light reduction belt 14 is adjusted to be equal to or smaller than the total bulging amount in the intentional bulging belt 15. Further, the solid phase ratio at the center portion of the slab thickness when entering the light pressure lower zone 14 is 0.3 or less, and the solid phase ratio at the center portion of the slab thickness when leaving the lower pressure zone 14 is 0. Adjust one or two or more of the amount of secondary cooling water, the width of the secondary cooling, and the slab drawing speed so as to be 7 or more. Control of the solid phase rate at the center of the slab thickness can be easily performed by obtaining in advance the solid phase rate at the center of the slab thickness under various casting conditions using heat transfer solidification calculation or the like. . Here, “secondary cooling width cutting” is to stop spraying of cooling water to both ends of the long side surface of the slab. By performing the width cutting of the secondary cooling, the secondary cooling is weakened, and generally, the solidification completion position 13 is extended downstream in the casting direction.

As described above, according to the present invention, the actual reduction speed applied to the slab 10 is controlled within the range of 0.3 to 1.0 mm / min, and the casting introduced into the light reduction belt 14 is performed. Since the thickness fluctuation amount in the casting direction of the piece 10 is controlled to 0.10 mm or less, it is realized to reduce the average value of the segregation degree due to center segregation in the slab width direction. Further, when the ratio (σ _SP / Vc) of the standard deviation (σ _SP ) of the cooling water density distribution to the slab drawing speed (Vc) is controlled to 2.5 or less, the slab width at the solidification completion position 13 The coefficient of variation in the direction is controlled to 0.03 or less, which makes it possible to reduce the variation in the segregation degree due to center segregation in the slab width direction, and to reduce the segregation degree at all locations in the slab width direction. Reduction is realized.

  A test for continuously casting low carbon aluminum killed steel with a slab continuous casting machine shown in FIG. 1 was conducted. As for the size of the slab, the thickness was 250 mm, the width was 2100 mm, and the slab drawing speed was 1.1 m / min. The light pressure lower zone of the continuous casting machine includes a seventh segment 19 to 21 m from the molten steel surface in the mold, an eighth segment 21 to 23 m from the molten steel surface in the mold, and a ninth segment 23 to 25 m from the molten steel surface in the mold. The 10th segment of 25 to 27 m from the molten steel surface in the mold. That is, the range of 8 to 19 m from the molten steel surface in the mold is a light pressure lowering zone. In addition, the solid phase ratio at the center of the slab thickness at the entrance of the light pressure lower zone is 0.1 to 0.2 in all tests, and the solid phase ratio at the center of the slab at the outlet of the lower pressure zone is 0. It was 9 or more.

  During casting, the displacement of the roll opening in the lightly pressed segment with the solidification completion position that is the most downstream in the slab width direction obtained in advance by heat transfer solidification calculation is measured by a non-contact sensor, and the slab is calculated from the result. The actual rolling speed applied to the was obtained. Further, a water column ultrasonic sensor was installed in the frame on the entrance side of the seventh segment, and the thickness variation amount in the casting direction of the slab introduced into the light pressure lower zone was measured. Furthermore, longitudinal ultrasonic waves are transmitted at each width position of the slab, the temperature at the center of the slab thickness is obtained from the propagation time, and the slab at the solidification completion position is referred to by the calculation result by heat transfer solidification calculation. The distribution in the width direction was obtained.

  In the test, the total bulging amount in the intentional bulging zone, the reduction gradient in the light reduction zone, the mold powder to be used, and the secondary cooling water density density distribution from the position immediately below the mold to the position 10 m below were changed.

  After casting, the cross section of the test piece taken from the slab (corresponding to the longitudinal section of the slab) was corroded with picric acid, and the presence or absence of V segregation or reverse V segregation and the presence of internal cracks were investigated. Moreover, in the test piece extract | collected from the slab, the segregation of Mn of slab thickness center part was analyzed by EPMA, and the Mn segregation degree of each position of a slab width direction was investigated. Furthermore, a hydrogen resistance-induced cracking test (HIC test) was performed on the test specimens collected from each position in the slab width direction.

  Table 1 shows the total amount of bulging for each test casting, the actual reduction speed, the physical properties of the mold powder used, the thickness variation in the casting direction of the slab, the coefficient of variation in the slab width direction at the solidification completion position, and the test piece Corrosion results, average value of Mn segregation degree in the slab width direction, hydrogen-induced crack resistance test results, and pass / fail judgments are shown.

  The HIC result shown in Table 1 is the ratio (CAR) of the area where hydrogen-induced cracking occurred, and the representative value was the highest CAR among the test pieces taken from the full width of the slab. Then, if there was a test piece having a CAR of 2% or more even at one place in the slab width direction, the test was rejected.

In Examples 1 to 13 of the present invention, the bulging total amount, the rolling speed, and the physical properties of the mold powder are within the range of the present invention, and the ratio (σ _SP / Vc) and the coefficient of variation in the slab width direction of the solidification completion position However, V segregation and reverse V segregation were not observed, and in the HIC test, the CAR was 1% or less in the full width of the slab, and all passed.

In Invention Examples 14 and 15, the ratio (σ _SP / Vc) and the coefficient of variation in the slab width direction of the solidification completion position were outside the preferred range of the present invention, but the total amount of bulging, the rolling speed, the physical properties of the mold powder However, V segregation and reverse V segregation were not observed. In the HIC test, the CAR was less than 2% in the full width of the slab, and all passed.

  On the other hand, in Comparative Example 1, since the total amount of bulging was small, the load on the segment was excessive, the roll opening was enlarged, and the reduction speed actually applied to the slab was reduced. As a result, V segregation was observed, and in the HIC test, hydrogen-induced cracking occurred everywhere in the slab width direction, and the CAR reached 9.50% at the maximum.

  In Comparative Example 2, since the set rolling gradient was too large, the rolling speed actually applied to the slab became excessive, and reverse V segregation was observed. In the HIC test, hydrogen-induced cracks occurred everywhere in the slab width direction, and the CAR reached 7.50% at the maximum.

  In Comparative Examples 3 to 6, the rolling speed actually applied to the slab was in the optimum range, but the physical property value of the mold powder was not the optimum value, and the thickness variation amount in the casting direction of the slab was 0. It exceeded 10 mm. As a result, the center segregation deteriorated, and in the HIC test, hydrogen-induced cracks occurred at several locations in the width direction of the slab, and the CAR reached 4 to 7% at the maximum, failing in all test castings.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Casting piece support roll 7 Conveying roll 8 Cast piece cutting machine 9 Molten steel 10 Cast piece 11 Solidified shell 12 Unsolidified layer 13 Solidification completion position 14 Light pressure lower belt 15 Intentional bulging zone 16 Water column ultrasonic sensor

Claims (4)

The roll opening of the plurality of pairs of slab support rolls is gradually increased toward the downstream side in the casting direction so that the long side surface of the slab is bulged with a total amount of bulging of 3 to 10 mm. In the light pressure lower zone where the roll opening is gradually decreased toward the downstream side in the casting direction, the solid phase ratio at the center of the slab thickness is at least 0.3 to 0.7. In the continuous casting method of steel for producing a continuous cast slab by reducing the long side surface of the slab at a reduction speed of 3 to 1.0 mm / min,
A mold powder having a basicity ((mass% CaO) / (mass% SiO ₂ )) of 0.95 to 1.05 and a crystallization temperature of 1050 ° C. to 1150 ° C. is added to the mold. By casting the slab, when the value calculated by the following equation (1) is defined as the variation amount of the slab thickness in the casting direction, the variation amount of the slab introduced into the lightly pressed belt is set to 0. A method for continuously casting steel, characterized by being controlled to 10 mm or less.

However, in the formula (1), A is the fluctuation amount (mm) of the slab thickness in the casting direction, and Zi is the slab surface from the roll pass line of the slab support roll, which is measured at intervals of 100 mm or less in the casting direction. The i-th measured value (mm) of the distance up to n is an arbitrary integer of 2 or more. Casting unit area and cooling water injection amount per unit time calculated at intervals of 10 mm or less in the slab width direction from the arrangement of each spray nozzle and the injection amount of cooling water from each spray nozzle in the secondary cooling zone When the value averaged in the direction is defined as the cooling water density distribution in the slab width direction in the secondary cooling zone,
The ratio (σ _SP / Vc) of the standard deviation σ _SP (L / m ² / s) of the cooling water density distribution at least from the position immediately below the mold to the position below 10 m in the casting direction and the slab drawing speed Vc (m / min) The continuous casting method for steel according to claim 1, wherein the value is 2.5 or less. Surface temperature or axis temperature distribution at intervals of 100 mm or less in the slab width direction calculated by heat transfer solidification calculation, or surface temperature or axis temperature at intervals of 100 mm or less in the slab width direction measured online The solidification completion position at each position in the slab width direction is calculated based on the temperature distribution of the slab, the distribution in the slab width direction of the solidification completion position is obtained from the calculated solidification completion position, and the casting at the solidification completion position is obtained from the obtained distribution. When calculating the average value and standard deviation in the single width direction and dividing the calculated average value by the calculated standard deviation as the coefficient of variation in the slab width direction of the solidification completion position,
3. The steel continuous casting method according to claim 2, wherein the coefficient of variation of the slab is 0.03 or less.   The continuous casting method for steel according to claim 3, wherein the coefficient of variation is 0.03 or less for a slab at a light pressure lower belt entrance.

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