JP2008260045A - Solidification delay suppressing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preventing any considerable solidification delay to cause the B.O. (breakout) immediately below a mold while the final object is to prevent the B.O. immediately below the mold. <P>SOLUTION: When a medium carbon steel having the C-content C [wt.%] of 0.08-0.20 is continuously cast by using a mold having the width W [mm] of ≤1,800 at the casting rate Vc [m/min] of ≥1.5, the casting rate Vc [m/min] is reduced assuming that considerable solidification delay occurs if formula (1) is satisfied, where ΔH [mm] is the difference of the molten steel level between at the center in the width direction of the mold and at a corner of the mold in the width direction, and ΔT [°C] is the degree of superheat of the molten steel. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for suppressing solidification delay, in other words, a method for preventing significant solidification delay.

  As this type of technology, Patent Document 1 states that “adjusting the heat flux from the molten steel side to the cooling water side in the mold cooling plate in accordance with the amount of fluctuation in the molten metal level in the mold during casting. Thus, the technique of preventing the occurrence of vertical splitting on the surface of the slab is disclosed (paragraph 0021). As a method of “adjusting the heat flux”, the point of adjusting the casting speed is described (paragraph numbers 0024 and 0026).

  Patent Document 2 describes the relationship between the local fluctuation of the molten metal level and the slab quality (first page, second column, lines 17 to 20), and the fluctuation of the molten metal level by lowering the casting speed by a certain amount. It is pointed out that this can be improved quickly and reliably (page 4, column 3, lines 1-5 from the bottom).

  Patent Document 3 states that “two vortex-type level meters are disposed between the short sides of the immersion nozzle and both ends of the immersion nozzle, and the deviation of each level value measured by the level meter is obtained to determine the surface of the molten steel. “Detecting the bumps and controlling the casting speed to be low so as to suppress the bumps” (page 2, first column, line 48 to second column, second line) There is no point reaching the deep part ”(page 3, column 1, lines 2 to 3).

JP 2001-179413 A Japanese Unexamined Patent Publication No. 1-284471 Japanese Patent Publication No. 7-61529

  As in Patent Documents 1 to 3 above, paying attention to the level of the molten metal in the mold, there are many techniques for improving the slab quality by reducing the casting speed in a predetermined case, all of which are significant. Can be evaluated.

  By the way, as one of the causes of deterioration of the slab quality, there is a breakout that occurs immediately below the mold (hereinafter, simply referred to as “BO immediately below the mold”), and is negative as part of investigating the cause of the BO directly below the mold. As a result of observing the segregation line, it was found that the so-called solidification delay was remarkable in the corner where the BO directly under the mold occurred compared to the other corners. From this, when the solidification delay generated in the mold (that is, in the middle of the mold height direction) is not recovered until it reaches the lower end of the mold, the surface of the solidified shell corresponding to the location where the solidification delay has occurred It can be inferred that stress concentration occurs in the hot part of the steel, so-called vertical splitting, and BO directly under the mold has occurred.

  Regarding the B.O. immediately below the mold caused by this solidification delay, the above Patent Documents 1 to 3 do not describe or suggest any B.O.

  The present invention has been made in view of such various points, and its main purpose is to prevent the BO directly under the mold, and a method for preventing a significant solidification delay that causes the BO directly under the mold. Is to provide.

Means and effects for solving the problems

  Here, the degree of solidification delay Cg [%] is defined as an index of the degree of solidification delay. Please refer to FIG. FIG. 1 is an explanatory diagram of the degree of solidification delay Cg [%]. This degree of solidification delay Cg [%] can be conceived at each corner of the slab based on the negative segregation line visible on the cut surface obtained by cutting the slab perpendicularly to the casting direction. The degree Cg [%] is obtained based on the following formula (3). However, in the following formula (3), A [mm] is the distance between the negative segregation line and the wide surface at a point 5 [cm] away from the narrow surface, and B [mm] is the negative segregation line on the wide surface most. The distance between the negative segregation line and the wide surface at the approaching point.

  Reference is now made to FIG. FIG. 2 is a graph based on the results of the relationship between the degree of solidification delay Cg [%] and the B.O. occurrence frequency [%] directly under the mold. That is, when continuous casting was performed using a mold having a C content C [wt%] of molten steel of 0.08 to 0.20, a mold width W [mm] of 1800 or less, and a casting speed Vc [m / min] of 1.5 or more. For each charge, measure the solidification delay Cg [%] of all corners of the slab 3 times in total, and the highest of the 12 (= 3 × 4) solidification delay Cg [%] obtained The degree of solidification delay Cg [%] was divided into frequencies every 5 [%] on the horizontal axis. Here, “40” on the horizontal axis means “40 to 45”. Then, the above continuous casting was repeated so that the number of samples (number of data, number of charges divided by frequency) became 10 or more for each frequency. When the above sample counts are satisfied for all frequencies, for each frequency, (number of charges classified into the frequency) is used as the denominator (of the number of charges classified into the frequency, the number of charges where the BO directly below the template occurred ) As a molecule is indicated on the vertical axis as “BO occurrence frequency directly under the template [%]”. According to this figure, it can be seen that if the solidification delay degree Cg [%] is operated so as to be less than 40, the occurrence of B.O. directly under the mold can be prevented. In this sense, the main object of the present invention described above is that “the ultimate goal is to prevent the BO directly under the mold, causing the BO directly under the mold”, and the degree of solidification delay Cg [%] is 40 or more. In other words, “to provide a method for preventing clotting delay”. Hereinafter, the “significant solidification delay” in this specification means “a solidification delay having a solidification delay degree Cg [%] of 40 or more”.

<First aspect of the present invention>
The inventors of the present application have obtained the following knowledge regarding the mechanism of the occurrence of coagulation delay after intensive studies. That is, (i) the corner portion of the solidified shell contacts the wide surface and the narrow surface of the mold at the same time and is strongly removed from the heat, so that the solidification shrinkage is remarkable compared with other parts, and the solidified shell and the mold at the corner portion. An air gap is formed between the wide surface and the narrow surface, the heat removal ability is reduced, and solidification is inhibited. (ii) The corner portion of the solidified shell is supplied with heat compared to other parts by a so-called reversal flow generated upward in the vertical direction when the molten steel discharge flow discharged from the immersion nozzle collides with the narrow surface of the mold. Is marked and coagulation is inhibited.

  Based on the above findings, after further intensive research, a medium carbon steel with a C content C [wt%] of 0.08 to 0.20, a mold with a mold width W [mm] of 1800 or less, and a casting speed Vc It has been found that a significant solidification delay can be prevented by the following method when continuously casting at [m / min] of 1.5 or more. That is, if the molten metal surface level difference between the mold width direction center and the mold width direction corner is ΔH [mm], the molten steel superheat degree is ΔT [° C], and the following equation (1) is satisfied, a significant solidification delay occurs. As a result, the casting speed Vc [m / min] is decreased.

  The point of the said invention is as follows. That is, the reversal flow mentioned in (ii) above appears as a molten metal surface level difference ΔH [mm], and the magnitude of the molten metal surface level difference ΔH [mm] is closely related to the solidification delay. The heat supply pointed out in) is considered to be closely related to the molten steel superheat degree ΔT [° C]. Excluding operations that use a mold with a mold width W [mm] exceeding 1800 and operations that make the casting speed Vc [m / min] less than 1.5 does not cause significant solidification delay in the first place. Because.

<Second aspect of the present invention>
And after additional earnest research by the inventors of the present application, (i) the above-described method for preventing the significant coagulation delay includes a slight overdetection, and (ii) the C content C [wt%] is set to 0.08. In continuous casting of medium carbon steel with a mold width W [mm] of 1800 or less and casting speed Vc [m / min] of 1.5 or more, a remarkable solidification delay is as follows. It has been found that the method can prevent it without overdetection. That is, the difference in level between the mold width direction center and the mold width direction corner is ΔH [mm], the molten steel superheat degree is ΔT [° C.], the heat flux at the mold width direction center is Qo [MW / m ² ], If the heat flux at the corner in the mold width direction is Qc [MW / m ² ] and the following equation (2) is satisfied, the casting speed Vc [m / min] is reduced assuming that a significant solidification delay occurs.

  The point of the said invention is as follows. In other words, the heat removal ability for the solidified shell is found to be more easily deteriorated when it is made through the narrow surface of the mold than when it is made through the wide surface of the mold. That's right.

  Another factor that degrades the quality of the slab is a breakout that occurs when the molten steel comes into direct contact with the inner wall of the mold without using mold powder in the mold (hereinafter simply referred to as “restraint BO”). For example). This constraining BO can be easily detected or predicted by observing the output behavior of the thermocouple embedded in the mold, and various useful applications have already been filed for this. I want to be helpful.

  Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 3 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 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 [mm] as the shortest distance between the roll surfaces of the pair of rolls 3a and 3a is configured to be adjustable by an appropriate means.

  Further, upstream of the casting path Q, 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 are appropriately used. Provided. 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 [mm] of the above-described roll pairs 3, 3... Is generally set so as to gradually become narrower as it proceeds to the downstream side of the casting path Q.

  With the above configuration, when starting continuous casting of a slab slab, a dummy bar (not shown) is inserted into the casting path Q in advance before pouring molten steel into the mold 1 and the immersion nozzle 2 is used. Then, the molten steel starts to be poured into the mold 1 at a predetermined flow rate, and the dummy bar is pulled out downstream at a predetermined speed. The dummy bar is collected by an appropriate means when a predetermined meniscus distance is reached. Thus, the slab slab 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 230-280.
-The mold height H [mm] is 800-900.
-Casting speed Vc [m / min] shall be 1.0-2.0.
-The molten steel superheat degree ΔT [° C.] is 0-40.
・ The specific water amount Wt [L / kgSteel] should be 1-3.
-Electromagnetic stirring intensity M-EMS [gauss] in a mold shall be 300-800.
-Molten steel composition is based on an agreement between the parties. 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 will be described later.
The specific water amount Wt [L / kgSteel] means the volume of cooling water used for 1 kg of steel.
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.

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, it is as follows.
-Steel type: Medium carbon steel with C content C [wt%] of 0.08 to 0.20-Mold width W [mm]: 1800 or less.
・ Casting speed Vc [m / min]: 1.5 or more.
-And if the following formula (1) is satisfied, the casting speed Vc [m / min] will be lowered assuming that a significant solidification delay occurs. In the following formula (1), the variable ΔH [mm] means “the difference in the molten metal surface level between the mold width direction center and the mold width direction corner”, and the variable ΔT [° C.] represents the “molten steel superheat degree” as described above. means.

  Next, a method for measuring the level difference ΔH [mm] and the degree of superheat ΔT [° C.] will be described. Please refer to FIG. 4 is a view taken in the direction of arrow A in FIG.

<Depth level difference ΔH [mm]>
As described above, the molten metal surface level difference ΔH [mm] means “the molten metal surface level difference between the mold width direction center and the mold width direction corner”, and “mould width direction center” is an area indicated by reference numeral 10 in the figure. An arbitrary point is selected from the molten metal surfaces existing in the mold, and a “mold width direction corner” is an arbitrary point among the molten metal surfaces existing in the region indicated by reference numeral 11 in the drawing. The region 10 is within 100 [mm] in the mold width direction from the end face of the immersion nozzle 2 in the mold width direction, and within the range of D / 10 [mm] from the center toward both wide surfaces in the mold thickness direction. The region 11 is within 100 [mm] from the narrow surface toward the immersion nozzle 2 in the mold width direction, and within a range of D / 10 [mm] from the center toward both wide surfaces in the mold thickness direction. For example, an eddy current level meter is used for the measurement of the molten metal level. The above-mentioned “molten surface level difference ΔH [mm]” is an absolute value of the difference between the molten metal surface levels measured in these regions 10 and 11. Note that this molten metal surface level difference ΔH [mm] may be measured only on the surface of one narrow surface as viewed from the immersion nozzle 2, or as viewed from the immersion nozzle 2 as shown in FIG. The hot water level on both narrow surfaces may be measured, and a large hot water surface level difference ΔH [mm] among the two obtained hot water surface level differences ΔH [mm] may be employed.

<Molten steel superheat degree ΔT [° C]>
The molten steel superheat degree ΔT [° C.] is “an index of the temperature of the molten steel poured into the mold” and is measured as follows.
(1) The “measurement time” is a predetermined time interval (for example, every minute). In order to correspond to the above-mentioned solidification delay Cg [%], the cross section of the slab for measuring the solidification delay Cg [%], and the molten steel superheat degree ΔT [° C.] of the molten steel corresponding to the cross section Be careful not to lose the correspondence.
(2) “Measurement points” are as follows. That is, the “horizontal position” is the axis of the immersion nozzle 2 provided on the bottom surface of the tundish, and the “vertical position” is a depth of 200 [mm] based on 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 at a point of a depth of 200 [mm], a configuration in which the consumable thermocouple is attached to the tip of an appropriately prepared rod is suitable.
(4) The temperature of the molten steel measured according to the above “measurement time”, “measurement point”, and “measuring instrument” is compared with the liquidus temperature that is uniquely determined by the molten steel composition of the molten steel. The above-mentioned molten steel superheat degree ΔT [° C.] is obtained as the remainder obtained by subtracting the latter from the former and further subtracting 25.

  Hereinafter, a test for confirming the technical effect of the solidification delay suppressing method according to the present embodiment will be described. Each numerical range described above is reasonably supported by the following confirmation test.

  The test conditions and test results of each confirmation test are shown in Table 1 and Table 2 below, and FIG. FIG. 6 is a scatter diagram showing the test results of each confirmation test. In this scatter diagram, the values obtained by substituting the molten metal surface level difference ΔH [mm] and the molten steel superheat degree ΔT [° C.] into the function f (ΔH, ΔT) are plotted on the horizontal axis, and these molten metal surface level differences ΔH [ mm] and the solidification delay Cg [%] measured for the slab part corresponding to the molten steel (or solidified shell) as the measurement target of the molten steel superheat degree ΔT [° C] (however, the four solidifications obtained by one cut surface) The relationship between the maximum degree of coagulation delay Cg [%] among the delay degrees Cg [%] is shown. It should be noted that the hot water surface on both narrow surfaces as viewed from the immersion nozzle 2 is the object of measurement, and the larger hot water surface level difference ΔH [mm] of the two obtained hot water surface level differences ΔH [mm] is adopted. . According to Tables 1 and 2 and FIG. 6 below, it can be seen that a significant solidification delay occurs only when the above formula (1) is satisfied. From this, if the above equation (1) is satisfied, it can be considered that in most cases a significant solidification delay occurs. This remarkable solidification delay disappears before reaching the lower end of the mold 1, in other words, a sufficient solidification shell is formed at least until the lower end of the mold 1 is reached. In order to prevent this, the casting speed Vc [m / min] is decreased, and it is good to gain time until the solidified shell that has been regarded as having undergone a significant solidification delay passes the lower end of the mold 1.

  As described above, in the present embodiment, a medium carbon steel having a C content C [wt%] of 0.08 to 0.20, a mold having a mold width W [mm] of 1800 or less, and a casting speed Vc [m / In continuous casting with a min] of 1.5 or more, significant solidification delay can be prevented by the following method. That is, if the molten metal surface level difference between the mold width direction center and the mold width direction corner is ΔH [mm], the molten steel superheat degree is ΔT [° C], and the following equation (1) is satisfied, a significant solidification delay occurs. As a result, the casting speed Vc [m / min] is decreased.

  In addition to the above description, we investigated how much the casting speed Vc [m / min] could actually be reduced to prevent a significant solidification delay, and the results are shown in Table 3 and FIG. When carrying out the above method, please fully refer to the following description.

  First, Table 3 will be briefly described. See Test No. 137 as an example. The continuous casting in this test is performed according to the conditions shown in Table 3 (C content C [wt%] and mold width W [mm], casting speed Vc [m / min]). When (1) is satisfied, the casting speed Vc [m / min] is immediately decelerated according to the deceleration width ΔVc [m / min] in Table 3, and at the time when the above expression (1) is satisfied. The degree of solidification delay Cg [%] was measured for the slab corresponding to the solidified shell in the mold. Table 3 shows the maximum solidification delay Cg [%] among the four solidification delays Cg [%] obtained at this time. Note that since the reduction of the casting speed Vc [m / min] is directly related to the decrease in productivity, the casting speed Vc [m / min] is set at the time when a predetermined time (for example, 300 [sec]) has elapsed from the time of the above deceleration. The original casting speed Vc [m / min] before deceleration was restored.

  Next, FIG. 7 will be briefly described. FIG. 7 is a graph showing the relationship between the reduction rate ΔVc [m / min] of the casting speed Vc [m / min] and the degree of improvement of the solidification delay Cg [%]. This FIG. 7 is created based on the test results described in Table 3. That is, the test Nos. 101 to 137 shown in Table 3 are divided into frequencies on the horizontal axis according to the deceleration width ΔVc [m / min], and for each deceleration width ΔVc [m / min] (the frequency is divided). The frequency of occurrence of significant clotting delays, with the number of tests in the denominator as the numerator (the number of tests in which the degree of clotting lag Cg [%] listed in Table 3 is 40 or more). Is plotted on the vertical axis. As can be seen from this figure, when the deceleration width ΔVc [m / min] is 0.3 or more, even when a significant solidification delay occurs, the generation can be surely prevented. Therefore, in order to completely prevent a significant solidification delay, the casting speed Vc [m / min] should be reduced by at least 0.3 [m / min] or more when the above formula (1) is satisfied. It will be understood. In consideration of productivity, the actual reduction width ΔVc [m / min] of the casting speed Vc [m / min] is appropriately within the range of 0.0 to 0.3 after sufficiently studying with reference to FIG. I want to be selected. However, even if the deceleration width ΔVc [m / min] is set to 0.4 or more, there will be no particular problem other than a decrease in productivity.

  As described above, the first embodiment of the present invention has been described. However, as shown in FIGS. 6 and 7 described above, the above equation (1) is generated even though no significant solidification delay occurs. It will be understood that some over-detection is included. That is, even when the above formula (1) is satisfied, there are not a few cases where the degree of solidification delay Cg [%] is less than 40. In this regard, although the solidification delay suppressing method according to the first embodiment of the present invention is a sufficiently useful technique as compared with the conventional technique, there is some room for improvement.

  Next, a second embodiment of the present invention will be described. The solidification delay suppressing method according to the present embodiment is an excellent improvement over the solidification delay suppressing method according to the first embodiment. The following description will focus on the differences of the present embodiment from the first embodiment.

In the first embodiment, when the above formula (1) is satisfied, the casting speed Vc [m / min] is reduced assuming that a significant solidification delay occurs, thereby preventing a significant solidification delay. In contrast, in the present embodiment, if the following formula (2) is satisfied, it is assumed that a significant solidification delay occurs, and the casting speed Vc [m / min] is reduced. In the following formula (2), the variable Qo [MW / m ² ] means “heat flux at the center in the mold width direction”, and the variable Qc [MW / m ² ] means “heat flux at the corner in the mold width direction”.

Next, a method for measuring the heat flux Qo [MW / m ² ] and the heat flux Qc [MW / m ² ] will be outlined.

<Heat flux Qo [MW / m ² ] and Heat flux Qc [MW / m ² ]>
The heat flux Qo [MW / m ² ] means “the heat flux at the center of the mold width direction” as described above, and more specifically, “any point of the molten metal surface existing in the region 10 shown in FIG. Passes through a wide surface that is further on the center side in the mold width direction than the level meter installed to measure the surface level in the mold, and within the range of 20 to 40 [mm] from the meniscus toward the casting direction. and heat flux heat flux becomes maximum when measured with [MW / m ^2] a casting direction to 10 [mm] pitch [MW / m ^2] "to. This heat flux measurement may be realized by various known methods. That is, for example, (i) a method in which a plurality of heat flux meters are embedded in the mold wide surface and the output of the heat flux meter is monitored, and (ii) a plurality of thermocouples are embedded in the mold wide surface, the output of this thermocouple, A method for obtaining heat flux by numerical calculation comprehensively considering the temperature of the cooling water flowing through the mold and other various heat transfer coefficients, (iii) A pair of thermocouples are embedded in the mold wide surface in multiple pairs, (I) to (iii) of the method for obtaining the heat flux based on the temperature pair of the mold 1 measured by each pair of thermocouples. Of course, other known computational or analytical methods can also be applied, and the selection thereof will be appropriately selected according to the operating conditions and the equipment environment. On the other hand, the heat flux Qc [MW / m ² ] means “the heat flux at the corner in the mold width direction” as described above, and “at any point of the molten metal surface existing in the region 11 shown in FIG. It passes through a wide surface that is further on the corner side in the mold width direction than the level meter installed to measure the molten metal level, and within the range of 20 to 40 mm from the meniscus toward the casting direction. and heat flux heat flux becomes maximum when measured at 10 [mm] pitch the [MW / m ^2] in the casting direction [MW / m ^2]. " The measurement of the heat flux may be realized by various known methods as described above.

  Hereinafter, a test for confirming the technical effect of the solidification delay suppressing method according to the present embodiment will be described. Each numerical range described above is reasonably supported by the following confirmation test.

First, a method for measuring the heat flux Qo [MW / m ² ] and the heat flux Qc [MW / m ² ] employed in each confirmation test will be described below.
-Mold copper plate thickness dd (see Fig. 8): 20-40 [mm]
-Mold height: 900 [mm]
-Thermocouple type: K thermocouple (+: Chromel,-: Alumel)
-Thermocouple embedding position (mold width direction): 10-50 [mm] pitch across the entire mold width of the wide surface-Thermocouple embedding position (mold height direction) and number: 20-40 [mm] from the meniscus 10 in the casting direction at a pitch of 10 [mm] Depth of embedment of thermocouple: 8 [mm] from the surface of the mold copper plate
・ “Heat flux at the center of mold width direction”: Measured by one of the thermocouple rows on the mold center side from the position of the level meter in the mold width direction, and the maximum heat flux among the 10 heat fluxes Refers to the indicated heat flux.
・ "Corner heat flux" above: Measured in one of the thermocouple rows on the short side of the mold from the position of the level meter in the mold width direction, showing the maximum heat flux among the 10 heat fluxes Refers to heat flux.
-Heat flux calculation method: A plurality of thermocouples are embedded in the direction perpendicular to the casting direction over the entire width of the mold, and the temperature of the inner wall of the mold is measured using these thermocouples. Thermocouple temperature data is taken into the computer every second. The shape of the slit is taken into account from thermocouple temperature data, mold cooling water temperature, mold copper plate thickness, copper thermal conductivity, interfacial heat transfer coefficient between the mold back and cooling water flowing through the slit (calculated from the flow rate of water passing through the slit). Then, using a computer, the heat flux is calculated by a two-dimensional difference method. Details of the difference method are shown below. The shape of the mold copper plate is a repeating shape as shown in FIG. Since it is a repetitive shape, only the region indicated by the thick line in FIG. Heat is input from the surface side of the mold copper plate (q1), and heat is extracted from the back side of the mold copper plate. A copper plate is mesh-divided at a pitch of 1 [mm], an element is created, and the balance of heat quantity passing through the center of the element is calculated as shown in FIG. Given a heat flux as the initial condition of the calculation, compare the measured thermocouple temperature T (i, j) with the calculated temperature T (i, j) of the element corresponding to the thermocouple position until the difference is 1% or less Perform convergence calculation. The value of q1 when converged is taken as the heat flux value. The heat transfer coefficient h1 between the mold back surface and the cooling water and the heat transfer coefficient h2 between the mold back surface and the SUS jacket were the same. The heat transfer coefficient h1 between the mold back surface and the cooling water was obtained from the flow rate of the cooling water flowing through the slit.
In FIG. 8, h1 indicates the heat transfer coefficient of the mold cooling water hole, and q1 indicates the heat flux between the slab and the mold. The copper thermal conductivity λ is 0.849 [cal / cm / sec / deg], and the heat transfer coefficient h1 between the mold back and the cooling water is 0.369 [cal / cm / sec / deg] (Hitachi Shipbuilding Technical Report, Vol. 34) No. 2 (1973)). The mold cooling water temperature Tw is a measured value of the mold outlet water temperature.
・ Calculation formula Boundary conditions (the mold copper plate interface on the slit side) are defined by the heat transfer coefficient as shown in the following formula (4). The boundary condition (cast copper plate interface on the slab side) is defined by the heat flux as shown in the following formula (5). The inside of the copper plate is as shown in the following formula (6). Then, the formulas (4) to (6) are set for each element, and the balance calculation of the temperature T (I, J) is performed. In each equation, Δx is the depth (paper surface thickness) in FIG. 8 (for example, 1 mm).

The test conditions and test results of each confirmation test are shown in Tables 1 and 2 and FIG. FIG. 10 is a scatter diagram showing the test results of each confirmation test. This scatter diagram shows a function g (ΔH, ΔT, Qo, Qc), a surface level difference ΔH [mm], a superheat degree ΔT [° C], a heat flux Qo [MW / m ² ], a heat flux Qc [MW / m ² ] is the value obtained by substituting the horizontal axis for these level differences ΔH [mm], superheat degree ΔT [° C], heat flux Qo [MW / m ² ], heat flux Qc [MW / m ² ] Solidification delay Cg [%] measured for the slab part corresponding to the molten steel (or solidified shell) as the measurement target (however, four solidification delay Cg [%] obtained at one cut surface) The maximum coagulation delay degree Cg [%]) is shown. According to Tables 1 and 2 and FIG. 10, when the above formula (2) is not satisfied, no significant solidification delay occurs. On the other hand, when the above formula (2) is satisfied, a significant solidification delay always occurs. You can see that From this, it can be determined whether or not a significant solidification delay occurs based on whether or not the above equation (2) is satisfied. This remarkable solidification delay disappears by the time it reaches the lower end of the mold 1, in other words, a sufficient solidification shell is formed at least until the lower end of the mold 1 occurs. In order to prevent this, the casting speed Vc [m / min] is decreased, and it is good to gain time until the solidified shell that has been regarded as having undergone a significant solidification delay passes the lower end of the mold 1.

  It should be noted that when the above formula (2) is satisfied and the casting speed Vc [m / min] is not reduced, the degree of solidification delay is always significant. . That is, it is possible to eliminate the overdetection that causes a significant solidification delay even though no significant solidification delay occurs (compare FIGS. 6 and 10).

As described above, in the present embodiment, a medium carbon steel having a C content C [wt%] of 0.08 to 0.20, a mold having a mold width W [mm] of 1800 or less, and a casting speed Vc [m / In continuous casting with a min] of 1.5 or more, significant solidification delay can be prevented without overdetection by the following method. That is, the difference in level between the mold width direction center and the mold width direction corner is ΔH [mm], the molten steel superheat degree is ΔT [° C.], the heat flux at the mold width direction center is Qo [MW / m ² ], If the heat flux at the corner in the mold width direction is Qc [MW / m ² ] and the following equation (2) is satisfied, the casting speed Vc [m / min] is reduced assuming that a significant solidification delay occurs.

  In the form added to the above description, it is investigated how much the casting speed Vc [m / min] is actually reduced to prevent a significant solidification delay, and the results are shown in Table 4 and FIG. When carrying out the above method, please fully refer to the following description.

  Table 4 above is similar to Table 3 above, and FIG. 11 is similar to FIG. 7 above. As can be seen from this figure, when the deceleration width ΔVc [m / min] is 0.3 or more, even when a significant solidification delay occurs, the generation can be surely prevented. Therefore, in order to completely prevent a significant solidification delay, the casting speed Vc [m / min] should be reduced by at least 0.3 [m / min] or more when the above formula (2) is satisfied. It will be understood. In consideration of productivity, the actual reduction width ΔVc [m / min] of the casting speed Vc [m / min] is appropriately within the range of 0.0 to 0.3 after sufficiently studying with reference to FIG. I want to be selected. However, even if the deceleration width ΔVc [m / min] is set to 0.4 or more, there will be no particular problem other than a decrease in productivity.

  The preferred embodiments of the present invention have been described above, but each of the above embodiments can be implemented as follows.

That is, an eddy current level meter for measuring a molten metal surface level difference ΔH [mm], a thermocouple for measuring a molten steel superheat degree ΔT [° C.], a heat flux Qo [MW / m ² ] and a heat flux Qc [ MW / m ² ] is connected to a general-purpose PC via an appropriate A / D converter, etc., and the output results from these measuring instruments are sent to the general-purpose PC at predetermined time intervals. The obtained result is continuously displayed on a display device connected to the general-purpose PC, or the general-purpose PC determines whether the above formula (1) or (2) is satisfied, and the determination result Based on the above, the general-purpose PC may be configured to appropriately change the casting speed Vc [m / min].

  The following are reference materials.

<Electromagnetic stirring intensity in mold M-EMS [gauss]>
Definition: A measure of the strength of the magnetic field that is applied to stir the molten steel in the mold 1.
(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.

Explanation of solidification delay Cg [%] A graph based on the results of the relationship between the degree of solidification delay Cg [%] and B.O. occurrence frequency [%] directly under the mold Schematic diagram of continuous casting machine A view in the direction of arrow A in FIG. Similar to FIG. Scatter chart showing test results of each confirmation test A graph showing the relationship between the reduction speed ΔVc [m / min] of the casting speed Vc [m / min] and the degree of improvement of the solidification delay degree Cg [%]. Partial sectional view of the mold in plan view Explanatory diagram of heat balance A figure similar to FIG. A figure similar to FIG.

Explanation of symbols

1 Mold
2 Immersion nozzle
100 continuous casting machine
Cg solidification delay

Claims (2)

When continuously casting medium carbon steel with a C content C [wt%] of 0.08 to 0.20 and a mold with a mold width W [mm] of 1800 or less and a casting speed Vc [m / min] of 1.5 or more. ,
Assuming that the difference in molten metal level between the mold width direction center and the mold width direction corner is ΔH [mm], the molten steel superheat degree is ΔT [° C], and if the following formula (1) is satisfied, a significant solidification delay occurs: Decrease casting speed Vc [m / min],
Coagulation delay suppression method characterized by the above

When continuously casting medium carbon steel with a C content C [wt%] of 0.08 to 0.20 and a mold with a mold width W [mm] of 1800 or less and a casting speed Vc [m / min] of 1.5 or more. ,
The difference in molten metal level between the mold width direction center and the mold width direction corner is ΔH [mm], the molten steel superheat degree is ΔT [° C], the heat flux at the mold width direction center is Qo [MW / m ² ], and the mold width The heat flux at the direction corner is Qc [MW / m ² ], and if the following formula (2) is satisfied, the solidification delay is generated and the casting speed Vc [m / min] is reduced.
Coagulation delay suppression method characterized by the above

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