JP2008260056A - Continuous casting method for slab steel less in central segregation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method for slab steel by which rolling reduction conditions are set not based on a solid phase ratio extremely difficult in precise estimation but based on easily understandable actual casting conditions to securely suppress central segregation with high reproducibility. <P>SOLUTION: A slab steel having a C content: C [wt.%] of 0.08 to 0.55, an Si content: Si [wt.%] of 0.02 to 0.60; and an Mn content: Mn [wt.%] of 0.3 to 1.5 is used as an object. The thickness D [mm] of a mold is allowed to satisfy 230≤D≤250, casting velocity Vc [m/min] is allowed to satisfy 1.50≤Vc≤1.70, and specific water quantity Wt [L/kgSteel] is allowed to satisfy 0.5≤Wt≤1.5. The rolling reduction gradient Ak[mm/m] in the first section Int1 as a section where meniscus distance M [m] is 28 to 37 is controlled to 0.20 to 1.00. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a continuous casting method of slab steel with little center segregation.

  So-called center segregation of slabs cast by a continuous casting machine is a problem from the viewpoint of ensuring the toughness of steel and preventing hydrogen-induced cracking. This central segregation is due to the high concentration of the so-called concentrated molten steel (carbon, manganese, silicon, etc.) remaining between the dendritic crystals (so-called dendrites) with the solidification shrinkage of the molten steel at the end of solidification in continuous casting. It is thought that this is caused by suction flow and accumulation in the center of the slab. As a method for preventing the above-mentioned center segregation, a method is generally known in which the slab is squeezed from the outside to compensate for the solidification shrinkage of the molten steel at the end of solidification and the above-described suction flow of the concentrated molten steel is suppressed. .

  And in order to improve said center segregation suitably, it is important to set the amount of reduction appropriately according to the solidification state of a slab. That is, when the reduction amount is too small, the reduction applied from the outside is not sufficiently transmitted to the center of the slab, and the center segregation is hardly improved. On the other hand, if the amount of reduction is excessive, the above-mentioned concentrated molten steel left between the dendritic trees is squeezed out in the direction opposite to the casting direction, so-called reverse V segregation (the cross section parallel to the narrow surface of the slab). , A pattern (opened in a V-shape toward the downstream side in the casting direction), which can be visually recognized in the vicinity of the center of the slab thickness direction, leads to deterioration of center segregation.

  As this type of technology, for example, Patent Document 1 discloses a reduction method in which a reduction condition such as a reduction interval and a reduction amount is set according to the solid phase ratio inside the slab.

JP 2001-259810 A

  In the rolling method disclosed in the above-mentioned Patent Document 1, since the rolling condition is set according to the solid phase rate inside the slab, as a premise thereof, the solid phase rate inside the slab is sufficiently accurately grasped. There is a need. Since this solid phase ratio is extremely difficult to measure in an actual continuous casting process, it is generally obtained by solidification heat transfer calculation. In order to accurately perform solidification heat transfer calculations in this continuous casting process, at least the physical property data (e.g. solidification latent heat / thermal conductivity / specific heat) of the steel grade and external heat removal conditions (inside the mold) It is necessary to accurately grasp the calculation conditions such as heat removal at the heat / heat transfer coefficient by spray or mist cooling in the secondary cooling zone / heat transfer coefficient by roll cooling). Among the above calculation conditions, the ones that greatly affect the calculation results are as follows: (1) (physical property data) solidification latent heat and (2) (external heat removal conditions) heat transfer coefficient / roll in the secondary cooling zone And a heat transfer coefficient by cooling.

  The former (1) solidification latent heat generally has a value of about 55 to 65 [cal / g], but it is extremely difficult to accurately determine the solidification latent heat of steel containing many elements. The heat transfer coefficient in the secondary cooling zone of the latter (2) is generally estimated on the basis of experimental measurements of temperature changes when steel is cooled at a predetermined spray flow rate. It ’s just that. However, it has been reported that the heat transfer coefficient of spray / mist cooling in the secondary cooling zone is expressed as a complex function in which various parameters are linked (Mitsuka et al .: Iron and Steel, 69 (1983), 262 / Mitsuka: Iron and steel, 91 (2005), see 1). The parameters are, for example, spray flow rate / water droplet size and momentum / air amount and pressure / slab surface temperature. And, even if these parameters are appropriately determined, the present condition is that the heat transfer coefficient varies greatly depending on the measurement conditions. In addition, in the above experiment, (a) the difference in cooling capacity between the upper and lower surfaces of the slab, the change caused by the movement of the slab, (b) the effect of clogging of the immersion nozzle, (c) the accumulation between the guide rolls Naturally, it is impossible to estimate various effects that can occur in actual machines, such as the effects of water, (d) the effects of cooling from low-temperature rolls, and (e) the effects of slab oxidation (scale adhesion thickness).

  As described in (1) and (2) above, as long as the calculation conditions for solidification heat transfer calculation include many uncertain factors, the solid phase ratio inside the slab can be accurately predicted according to the individual steel type / casting conditions. This is extremely difficult at present. As a reference, an example of the calculation result of solidification heat transfer calculation is shown in FIG. This figure is calculated by using the prediction formula described in the above-mentioned Mitsuka et al. Literature and setting the latent heat of solidification to 55 or 65 [cal / g]. In this figure, the solid line is calculated with the latent heat of solidification as 65 [cal / g], and the broken line is calculated with 55 [cal / g]. As can be seen from the figure, in the present situation in which the latent heat of solidification is determined almost subjectively, as a result, a large deviation of, for example, several [m] order has occurred in the relationship between the solid phase ratio and the meniscus distance. It ends up. In addition, it is difficult to think that Mitsuka et al.'S prediction formula described above is suitable for all casting conditions, and depending on which prediction formula is used, there will be a large shift in the relationship between the solid phase ratio and the meniscus distance. Is easily guessed. Therefore, with the rolling method disclosed in Patent Document 1 above, even the solid phase ratio, which is the setting criterion for the rolling conditions, cannot be predicted with high accuracy, so it is unlikely that the center segregation will be really improved.

  The present invention has been made in view of such various points, and its main purpose is not based on a solid phase ratio that is extremely difficult to predict with high accuracy, but on actual casting conditions that can be easily grasped. An object of the present invention is to provide a continuous casting method of slab steel that can reliably suppress center segregation with high reproducibility by setting the rolling conditions.

Means and effects for solving the problems

  The problems to be solved by the present invention are as described above. Next, means for solving the problems and the effects thereof will be described.

  According to the first aspect of the present invention, the C content C [wt%] is 0.08 to 0.55, the Si content Si [wt%] is 0.02 to 0.60, and the Mn content Mn [wt%] is 0.3 to Continuous casting of slab steel with 1.5 is performed by the following method. That is, the mold thickness D [mm] is 230 ≦ D ≦ 250, the casting speed Vc [m / min] is 1.50 ≦ Vc ≦ 1.70, and the specific water amount Wt [L / kgSteel] is 0.5 ≦ Wt ≦ 1.5. The rolling gradient Ak [mm / m] in the first section Int1 as the section in which the meniscus distance M [m] is 28 to 37 is set to 0.20 to 1.00. According to this, based on specific casting conditions such as molten steel composition, mold size, casting speed, etc., the rolling gradient Ak [mm / m], the meniscus distance M [m] to start and stop the rolling, Is specifically required. In other words, the center segregation is well controlled with high reproducibility by setting the rolling conditions based on the actual casting conditions that can be easily grasped, not based on the solid phase ratio, which is extremely difficult to predict with high accuracy. it can. As a result, the UT defect rate of the product can be reduced. These effects are particularly beneficial when manufacturing steel materials for shipbuilding or construction and bridges with a rolling reduction ratio of 10 or less and a final product thickness Df [mm] of 20 or more.

  The continuous casting of the slab steel is preferably performed by the following method. That is, the rolling gradient Ak [mm / m] in the second section Int2 as the section in which the meniscus distance M [m] is 20 to 28 is set to 0.10 to 1.00. According to this, center segregation can be suppressed very well.

  Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic view of a continuous casting machine. First, based on this figure, the structure and operation | movement of the continuous casting machine 100 used for the continuous casting of the slab steel which concerns on this embodiment are demonstrated easily as an example.

  The continuous casting machine 100 smoothly pours molten metal held in a tundish (not shown) into the mold 1 at a predetermined flow rate for cooling the molten steel to form a solidified shell having a predetermined shape. And a plurality of roll pairs 3, 3... Arranged in parallel along the casting path Q from directly below the mold 1. In the present embodiment, the casting path Q is provided in order from the upstream side thereof, a vertical path portion extending in a substantially vertical direction, an arc path portion connected to the vertical path portion and extending in an arc shape, and further provided on the downstream side thereof. And a horizontal path portion extending in the horizontal direction and a correction path portion for smoothly connecting the arc path portion and the horizontal path portion.

  Further, each of the roll pairs 3, 3... Is composed of a pair of rolls 3a, 3a for sandwiching a slab as a casting object with both wide surfaces. The roll gap G [mm] (see also FIG. 2) as the shortest distance between the roll surfaces of the pair of rolls 3a and 3a is configured to be adjustable by appropriate means.

  Further, the vertical path part and the arc path part are provided with cooling sprays 4, 4... Spraying cooling water at a predetermined flow rate on the solidified shell formed in the mold 1 and pulled out from the mold 1. It is provided as appropriate. In general, the mold 1 is referred to as a primary cooling zone, and in this sense, a path portion in which the cooling sprays 4, 4,... Are arranged is referred to as a secondary cooling zone.

  The solidified shell drawn out from the mold 1 and transported along the casting path Q is further cooled and contracted by natural heat dissipation, the cooling sprays 4, 4. Accordingly, the roll gap G [mm] of the above-described roll pairs 3, 3... Is generally set so as to be gradually narrowed as it proceeds to the downstream side of the casting path Q.

  With the above configuration, when starting continuous casting of slab steel, before pouring the molten steel into the mold 1, a dummy bar (not shown) is inserted in the casting path Q in advance and is inserted through the immersion nozzle 2. The molten steel starts to be poured into the mold 1 at a predetermined flow rate, and the dummy bar is drawn downstream at a predetermined speed. The dummy bar is collected by an appropriate means when a predetermined meniscus distance is reached. As a result, the slab steel is continuously cast.

Next, general operating conditions of the continuous casting machine 100 will be briefly introduced.
-The mold width W [mm] is 800-2100.
-Mold thickness D [mm] shall be 220-310.
-Mold height H [mm] shall be 900, for example.
-Casting speed Vc [m / min] shall be 0.8-2.0.
• The molten steel superheat degree ΔT [° C.] is 10 to 45.
-The specific water amount Wt [L / kgSteel] should be 0.4 to 2.0.
-Electromagnetic stirring intensity M-EMS [gauss] in a mold shall be 0-800.
-The rolling gradient Ak [mm / m] is normally set to 0.05 to 0.10 in consideration of thermal shrinkage of the slab.
-Molten steel components are based on standard agreements. Typical components are C, Si, and Mn. To this, Cr, Cu or the like is appropriately added. P and S are adjusted to be as small as possible. Contains other inevitable impurities.

Here, each term is briefly explained.
The mold width W [mm] and the mold thickness D [mm] are considered at the upper end of the mold 1.
The casting speed Vc [m / min] is a slab drawing speed, and is thought of as the peripheral speed of the roll pair 3 arranged in the uppermost stream among the plurality of roll pairs 3.
The molten steel superheat degree ΔT [° C.] is an index of the temperature of the molten steel poured into the mold 1. Details are described at the end of this specification.
The meniscus distance M [m] means a distance [m] that starts with the molten steel surface (meniscus) in the mold 1 and is considered along the casting path Q.
The specific water amount Wt [L / kgSteel] means the volume of cooling water used for 1 kg of steel. This cooling water is injected / sprayed on the slab in the above-mentioned secondary cooling zone, which is conceived with a meniscus distance [m] of 1 to 40.
In-mold electromagnetic stirring strength M-EMS [gauss] is an index of the strength of the magnetic field that is applied to stir the molten steel in the mold 1. Details are described at the end of this specification.
The rolling gradient Ak [mm / m] means the decreasing gradient [mm / m] of the roll gap G [mm] of the roll pairs 3. FIG. 2 is an explanatory diagram of a rolling gradient. As shown in the figure, the rolling gradient Ak (x, x + 1) [mm / m] between the meniscus distance M (x) [m] to the meniscus distance M (x + 1) [m] is the meniscus distance. The roll gap G (x) [mm] of the roll pair 3 (x) arranged at M (x) [m] and the roll pair 3 (x +) arranged at the meniscus distance M (x + 1) [m] When the roll gap G (x + 1) [mm] of 1) is used, it is defined by the following formula.
Ak (x, x + 1) = (G (x) -G (x + 1)) / (M (x + 1) -M (x))

Next, specific operating conditions of the continuous casting machine 100 according to the present embodiment will be described. In the continuous casting according to the present embodiment, the C content C [wt%] is 0.08 to 0.55, the Si content Si [wt%] is 0.02 to 0.60, and the Mn content Mn [wt%] is 0.3 to 1.5. Slab steel (mainly for thick plate products).
(1) The mold thickness D [mm] is 230 ≦ D ≦ 250.
(2) The casting speed Vc [m / min] is 1.50 ≦ Vc ≦ 1.70.
(3) The specific water amount Wt [L / kgSteel] is 0.5 ≦ Wt ≦ 1.5.
(4) The rolling gradient Ak [mm / m] in the first section Int1 as the section in which the meniscus distance M [m] is 28 to 37 is set to 0.20 to 1.00. Since the setting of the rolling gradient Ak [mm / m] is illustrated in FIG. 3, please refer to it appropriately. FIG. 3 is an explanatory diagram for setting the rolling gradient Ak [mm / m] according to the first embodiment of the present invention. That is, in the present embodiment, the rolling gradient Ak [mm / m] is set so as to be within the hatched region shown in the drawing in the first section Int1. At this time, the aspect of the rolling gradient Ak [mm / m] in the first section Int1 is not necessarily linear. Further, the rolling gradient Ak [mm / m] in the section other than the first section Int1 is arbitrary, but it is better to follow general operating conditions.

Among the steel type components targeted for continuous casting of the slab steel according to the present embodiment, as described above, only C, Si, and Mn are specifically specified with numerical values because the central segregation of these three components is determined. , When using slab steel as a steel material for shipbuilding or construction, bridges with a rolling reduction ratio of 10 or less and a final product thickness Df [mm] of 20 or more, the quality (for example, UT defects) is particularly affected. It is because it is supposed to affect. Other components (additive elements) are briefly exemplified below.
・ Cu [wt%]: 0 ~ 0.50
・ Al [wt%] ： 0 ～ 0.08
・ Ni [wt%] ： 0 ～ 1.0
・ Cr [wt%]: 0 to 1.0
・ Mo [wt%] ： 0 ～ 0.60
・ V [wt%]: 0 ~ 0.10
・ Nb [wt%]: 0 ~ 0.05
・ Ti [wt%] ： 0 ～ 0.10
・ B [wt%] ： 0 ～ 0.002
・ Ca [wt%] ： 0 ～ 0.002

Further, for reference, other components (impurity elements) generally contained in the slab steel are also introduced below.
・ P [wt%] ： ≦ 0.03
・ S [wt%] ： ≦ 0.015

  Next, a method for setting the above-described rolling gradient Ak [mm / m] will be described. Please refer to FIG. FIG. 5 is a diagram illustrating a method for setting the rolling gradient.

  Here, a method for setting the rolling gradient Ak [mm / m] when the plurality of roll pairs 3, 3,... Are rotatably supported by a roll stand for each predetermined pair will be described. In this case, it is preferable that the roll alignment of the plurality of rolls 3a and 3a supported by one roll stand is as uniform as possible.

  For convenience of explanation, among the plurality of roll pairs 3, 3... Supported by the roll stand arranged on the upstream side, the most downstream roll pair 3 is referred to as a roll pair 3 (i) and on the downstream side. The number of pairs of roll pairs 3, 3... Supported by the roll stand is n, and the most downstream roll pair 3 is referred to as a roll pair 3 (i + j). That is, (i + j) − (i−1) = n is established.

  Hereinafter, the rolling gradient Ak (i, i + j) [mm / m] in the section where the meniscus distance M [m] is M (i) to M (i + j) will be set.

<STEP1: (1) to (3)>
(1) The roll gap G (i) of the roll pair 3 (i) is measured.
Example: G (i) [mm] = 240
(2) The distance M (i + j) −M (i) [m] between the roll pair 3 (i + j) and the roll pair 3 (i) is calculated. This is not the case if it is known.
Example: (M (i + j) −M (i)) [m] = 1.5
(3) As shown in the following formula, the roll gap G (i + j) to be applied to the roll pair 3 (i + j) is calculated, and at least of the roll stands provided in a pair so as to sandwich the slab The roll gap G (i + j) obtained by calculation is applied to the roll pair 3 (i + j) by moving one of them by an appropriate means.
G (i + j) = G (i) −Ak (i, i + j) × (M (i + j) −M (i))
Example: Ak (i, i + j) [mm / m] = 0.6, G (i + j) [mm] = 240−0.6 × 1.5 = 239.1

<STEP 2: (4) to (5)>
(4) A distance M (i + 1, i + j) [m] between the roll pair 3 (i + j) and the roll pair 3 (i + 1) is obtained. This is not the case if it is known.
Example: (M (i + j) −M (i + 1)) [m] = 1.2
(5) As shown in the following formula, the roll gap G (i + 1) to be applied to the roll pair 3 (i + 1) is calculated, and at least of the roll stands provided in a pair so as to sandwich the slab. The roll gap G (i + 1) obtained by calculation is applied to the roll pair 3 (i + 1) by moving one of them by an appropriate means.
G (i + 1) = G (i + j) + Ak (i, i + j) × (M (i + j) −M (i + 1))
Example: G (i + 1) [mm] = 239.1 + 0.6 × 1.2 = 239.8

  Hereinafter, the test for confirming the technical effect of the continuous casting method of the slab steel which concerns on this embodiment is demonstrated. Each numerical range described above is reasonably supported by the following confirmation test.

  First, the evaluation criteria of the technical effect in each confirmation test will be described. Evaluation of technical effects in each confirmation test is based on the degree of central segregation. The center segregation means the ratio of the maximum value of the specific element content in the final solidified portion to the average content of the specific element in the slab. For example, when focusing on C (carbon) as a specific element, assuming that the average content of C in the slab is Co and the maximum value of C content in the final solidified part is Cmax, the center segregation is Cmax / Co. It is written. This center segregation is measured as follows. In the following, the case of focusing on C as the specific element will be described, but the same measurement can be performed when focusing on other elements.

(1) Cutting See FIG. FIG. 6 is a perspective view of a slab cut perpendicular to the casting direction. As shown in this figure, among the slabs continuous in the casting direction, the part of the slab whose central segregation is to be measured is cut perpendicular to the casting direction.
(2) Sample collection On the cut surface obtained by the above cutting, segregation traces due to solidification of the so-called concentrated molten steel are visually recognized as shown by the broken line in this figure after undergoing an appropriate corrosion process. The The segregation traces include those extending along the wide surface and those extending at a predetermined angle with respect to the wide surface and the narrow surface. Among these, on the segregation trace extending along the wide surface, the slab is perpendicular to the cut surface at a predetermined interval p (p = 10 [mm]) in the slab width direction using a φ5 mm drill blade. Drilling is performed at a predetermined depth dp (dp = 20 [mm]), and a total of 150 to 170 chip samples (appropriately increase / decrease within this range depending on the slab width) are collected.
(3) Sample component investigation The C content C [wt%] of all the chips collected by the above drilling is measured by the combustion infrared absorption method. The C content C [wt%] of the chip sample having the highest C content C [wt%] among all the chip samples collected by the drilling is recorded as Cmax [wt%]. On the other hand, the molten steel corresponding to the part of the slab whose central segregation is to be measured is collected in advance when the molten steel is present in the tundish, and the C content C [wt%] of the molten steel is determined as Co. Record as [wt%]. Then, Cmax [wt%] is divided by Co [wt%] to obtain center segregation Cmax / Co having C as a specific element.
(4) Numerical criteria
(4.1) The confirmation test with Cmax / Co ≦ 1.1 is evaluated as “「 (minimum center segregation) ”.
(4.2) Evaluate the confirmation test with 1.1 <Cmax / Co ≤ 1.2 as "○ (small center segregation)".
(4.3) Evaluate the confirmation test with 1.2 <Cmax / Co as “× (significant center segregation)”.
(5) Other specific elements The center segregation when focusing on Si (silicon) as a specific element is indicated by Simax / Sio in Table 1 (and Table 2) below. The center segregation when focusing on Mn (manganese) as a specific element is indicated by Mnmax / Mno in Table 1 (and Table 2) below. These “numerical criteria” are the same as those for central segregation with C as the specific element. The reason for focusing on the central segregation of the three elements C, Si, and Mn is that they strongly affect the product quality of thick plate products including UT defects.
(6) Additional notes The above “criteria for determining numerical values” has been set for the following reasons. That is, among the rolled products that use slab steel as a base material, defects due to center segregation become a problem. In particular, shipbuilding with a rolling reduction ratio of 10 or less and a final product thickness Df [mm] of 20 or more. Or it is a case where steel materials for construction and bridges are manufactured (this center segregation diffuses and disappears as it is rolled). A typical defect among the defects caused by the center segregation is a so-called UT defect, and this UT defect is detected as a defect echo height by an ultrasonic flaw detection test defined in JIS B0901. If the maximum value of the defect echo height of this UT defect exceeds 5%, the rolled product will be pierced during welding, or corrosion will preferentially progress, causing defects or failures. In order for the maximum value of the defect echo height of this UT defect to be 5% or less, the thickness before rolling of the slab steel to which the above embodiment is applied depends on other operating conditions (related Japanese Patent Application No. 2006-190470, In consideration of the fact that it is slightly smaller than that of Japanese Patent Application No. 2006-190471), the present inventors have found that the above-mentioned center segregation with the specific elements C, Si, and Mn being 1.2 or less, preferably 1.1 or less. Evidence from these other trials. For the reasons described above, the above “numerical criteria” is set. When the above-mentioned center segregation with specific elements C, Si, and Mn is reduced, the steel UT for shipbuilding or construction and bridges with a rolling reduction ratio of 10 or less and a final product thickness Df [mm] of 20 or more. It is considered that the defects are suppressed because the amount of manganese in the slab shaft core is reduced, and as a result, the bainite structure is reduced, thereby preventing hydrogen defects. In addition, if each center segregation with the specific elements C, Si, and Mn being 1.1 or less, the UT defects of the rolled product can be suppressed very well, and steps such as soaking diffusion treatment and breakdown rolling can be omitted. This will result in shortening the manufacturing period and reducing energy consumption. These effects are also recognized for dies that require extremely strict quality.

  Next, test conditions and test results in each confirmation test are shown in Table 1 below. The diameter of each roll 3a, 3a of the roll pair 3, 3,... Provided in the continuous casting machine 100 is φ280 [mm], and the pitch between the rolls 3a, 3a adjacent in the casting direction is 300 [mm].

  As described above, in the first embodiment, the C content C [wt%] is 0.08 to 0.55, the Si content Si [wt%] is 0.02 to 0.60, and the Mn content Mn [wt%] is Continuous casting of slab steel with 0.3 to 1.5 is performed by the following method. That is, the mold thickness D [mm] is 230 ≦ D ≦ 250, the casting speed Vc [m / min] is 1.50 ≦ Vc ≦ 1.70, and the specific water amount Wt [L / kgSteel] is 0.5 ≦ Wt ≦ 1.5. The rolling gradient Ak [mm / m] in the first section Int1 as the section in which the meniscus distance M [m] is 28 to 37 is set to 0.20 to 1.00. According to this, based on specific casting conditions such as molten steel composition, mold size, casting speed, etc., the rolling gradient Ak [mm / m], the meniscus distance M [m] to start and stop the rolling, Is specifically required. In other words, it is not based on the solid phase ratio, which is extremely difficult to predict accurately, but the rolling conditions are set based on the actual casting conditions that can be easily grasped, so that center segregation can be suppressed well with high reproducibility. . As a result, the UT defect rate of the product can be reduced. These effects are particularly beneficial when manufacturing steel materials for shipbuilding or construction and bridges with a rolling reduction ratio of 10 or less and a final product thickness Df [mm] of 20 or more.

  Next, a second embodiment of the present invention will be described. Since the content overlapping with the above description of the first embodiment is omitted as appropriate, refer to the above description of the first embodiment as necessary.

  Specific operating conditions of the continuous casting machine 100 according to this embodiment will be described. That is, in the first embodiment, the rolling gradient Ak [mm / m] in the first section Int1 is set to 0.20 to 1.00. On the other hand, other sections are not special values and are arbitrary, for example, general operating conditions. On the other hand, in this embodiment, in addition to the first embodiment, the rolling gradient Ak [mm / m] in the second section Int2 as the section in which the meniscus distance M [m] is 20 to 28 is set to 0.10 to 1.00. And The setting of the rolling gradient Ak [mm / m] is illustrated in FIG. FIG. 4 is an explanatory diagram for setting the rolling gradient Ak [mm / m] according to the second embodiment of the present invention. That is, in this embodiment, the rolling gradient Ak [mm / m] is set so as to be within the hatched area shown in the drawing in the first section Int1 and the second section Int2. At this time, the aspect of the rolling gradient Ak [mm / m] in the second section Int2 is not necessarily linear. Further, the rolling gradient Ak [mm / m] in the sections other than the first section Int1 and the second section Int2 is arbitrary, but it is better to follow general operating conditions.

  Hereinafter, the test for confirming the technical effect of the continuous casting method of the slab steel which concerns on this embodiment is demonstrated. Each numerical range described above is reasonably supported by the following confirmation test.

  Table 2 below shows test conditions and test results in each confirmation test.

  As described above, in the second embodiment, the C content C [wt%] is 0.08 to 0.55, the Si content Si [wt%] is 0.02 to 0.60, and the Mn content Mn [wt%] is Continuous casting of slab steel with 0.3 to 1.5 is performed by the following method. That is, the rolling gradient Ak [mm / m] in the second section Int2 as the section in which the meniscus distance M [m] is 20 to 28 is set to 0.10 to 1.00. According to this, as can be seen from Table 2, the center segregation can be suppressed very well.

  The following is an attached document.

Molten steel superheat ΔT [℃]
Definition: A measure of the temperature of the molten steel poured into the mold.
(1) “Measurement time” is “the time when the flow of molten steel in the tundish reaches a steady state, more specifically, it is accommodated in a ladle for conveying molten steel from the converter to the tundish. The time when about 1/4 to 1/3 of the molten steel is poured into the tundish is defined as “a time when the molten steel is poured into the tundish”.
(2) “Measurement points” are as follows. That is, the “horizontal position” is the axis of the immersion nozzle provided on the bottom surface of the tundish, and the “vertical position” is 100 mm deep with reference to the molten steel surface held in the tundish.
(3) The “measuring instrument” shall use a consumable thermocouple. As described above, since the consumable thermocouple is immersed in a point having a depth of 100 mm, a configuration in which the consumable thermocouple is attached to the tip of a suitably prepared rod is suitable.
(4) Compare the liquidus temperature determined solely by the molten steel component of the molten steel from the temperature of the molten steel measured according to the above “measurement time”, “measurement point”, and “measuring instrument”. The above-described molten steel superheat degree ΔT [° C.] is obtained as the remainder obtained by removing (subtracting) the latter from the former.
(5) From various viewpoints, the molten steel superheat degree ΔT [° C.] is preferably 10 to 45.

Electromagnetic stirring strength in mold M-EMS [gauss]
Definition: A measure of the strength of a magnetic field applied to stir molten steel in a mold.
(1) The “measurement time” is arbitrary.
(2) “Measurement points” are as follows. That is, the “horizontal position” is (i) the center in the mold width direction, (ii) 15 [mm] from the mold inner wall surface to the center in the mold thickness direction, and (iii) in the mold height direction. Align with the coil center of the electromagnetic coil embedded in the mold.
(3) Use an appropriate gauss meter as the “measuring instrument”.
(4) Measure multiple times according to the above “Measurement time”, “Measurement point”, and “Measurement instrument”. The above-described in-mold electromagnetic stirring intensity M-EMS [gauss] is obtained by averaging the plurality of measured values.
(5) From various viewpoints, the above-mentioned electromagnetic stirring intensity M-EMS [gauss] in the mold is preferably 0 to 1000, and the magnetic field frequency [Hz] ("magnetic field frequency applied to the molten steel in the mold""Means the number of times the current conducted to the electromagnetic coil changes direction per second.) Is preferably 1 to 5, and generally 2 is adopted as the frequency [Hz] of this magnetic field.

Schematic diagram of continuous casting machine Illustration of rolling gradient Explanatory drawing of the setting of rolling-down gradient Ak [mm / m] which concerns on 1st embodiment of this invention Explanatory drawing of the setting of rolling-down gradient Ak [mm / m] which concerns on 2nd embodiment of this invention The figure which illustrates the setting method of a rolling-down gradient Perspective view of a slab cut perpendicular to the casting direction Illustration of difficulty in calculating solid fraction

Explanation of symbols

1 Mold
3 roll pairs
Int1 1st section
Int2 2nd section
G Roll gap

Claims (2)

In the continuous casting method of slab steel in which the C content C [wt%] is 0.08 to 0.55, the Si content Si [wt%] is 0.02 to 0.60, and the Mn content Mn [wt%] is 0.3 to 1.5,
Mold thickness D [mm] is 230 ≦ D ≦ 250,
The casting speed Vc [m / min] is 1.50 ≦ Vc ≦ 1.70,
Specific water amount Wt [L / kgSteel] is 0.5 ≦ Wt ≦ 1.5,
The rolling gradient Ak [mm / m] in the first section Int1 as the section in which the meniscus distance M [m] is 28 to 37 is 0.20 to 1.00.
Continuous casting method for slab steel The rolling gradient Ak [mm / m] in the second section Int2 as the section in which the meniscus distance M [m] is 20 to 28 is set to 0.10 to 1.00.
The continuous casting method of slab steel according to claim 1

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