JP5652362B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a steel continuous casting method using a mold vibration device, and more particularly to a steel continuous casting method in which vibration conditions are controlled according to the amount of heat removed from molten steel by a mold.

  In continuous casting of steel, mold powder is used as a lubricant to reduce friction between the mold and slab and prevent seizure, and the mold uses a vertical vibration called oscillation. Is granted.

  Examples of the mold vibration device include a mechanical vibration device using an electro-oil actuator and a link as described in Patent Document 1, and a hydraulic vibration device as described in Patent Document 2.

  Conventionally, it is known that the mold vibration conditions (mold vibration stroke and vibration frequency) affect the depth of the oscillation mark formed on the surface of the slab and the inflow amount of the mold powder. In general, it has been recognized that the shorter the mold vibration stroke and the higher the frequency, the lower the consumption of mold powder and the smaller the depth of the oscillation mark.

  Further, the mold powder flows between the mold wall surface and the slab and acts as a lubricant, and also acts as a heat resistance material that reduces the amount of heat removed from the molten steel by the mold wall surface.

  When the amount of heat extracted from the molten steel by the mold becomes excessive at the time of casting a slab, such as when the consumption of mold powder is small, a large temperature gradient is generated in the thickness direction of the solidified shell. It is known that the thickness of the solidified shell becomes non-uniform in order to disperse the internal stress caused by this temperature gradient, and as a result, the solidified shell is deformed and the possibility of surface defects such as vertical cracks in the slab increases. It has been.

  In order to suppress adverse effects such as surface cracks of the slab due to excess or shortage of heat removal by the mold, Patent Document 3 discloses a mold cooling control method in which the flow rate of mold cooling water is substantially constant and the temperature is controlled within a predetermined range. Is disclosed. However, in this method, there is a problem that a large-scale device for detecting the temperature at the time of supplying the mold cooling water and controlling the temperature within a predetermined range is necessary.

  In Patent Document 4, as a technique capable of stably casting a slab having few surface defects such as vertical cracks, a temperature measuring element is embedded in a mold copper plate and a temperature measuring sensor is embedded in the vicinity of the temperature measuring element. Calculate the thermal resistance at the interface between the mold flux film and the mold copper plate from the temperature inside the mold copper plate measured by the element and sensor and the surface temperature of the mold flux film, and control the casting conditions so that this thermal resistance falls within the specified range. Techniques to do this are disclosed. However, in this technology, if a failure occurs in the embedded temperature sensor, it is difficult to repair due to the complicated structure of the device, and it is difficult to control the casting conditions because the thermal resistance cannot be calculated. There is a problem of becoming.

Japanese Patent No. 3026173 Japanese National Patent Publication No. 11-506982 JP-A-8-90188 JP 2002-11558 A

  The present invention has been made in view of the above-mentioned problems, and does not require a large-scale and complicated apparatus, and does not have surface defects such as vertical cracks, and can stably cast a slab having excellent surface properties. It is an object of the present invention to provide a method capable of continuous casting.

  The mechanism of occurrence of vertical cracks on the slab surface during the continuous casting process of steel is as follows. When the amount of heat removed from the molten steel by the mold (hereinafter referred to as “molten heat extraction amount”) is large, the thickness of the initial solidified shell becomes non-uniform, and solidification shrinkage occurs in the thick portion of the solidified shell. As a result, when tensile stress is generated in a thin portion of the solidified shell and buckling occurs in the solidified shell, vertical cracks are generated on the surface of the slab.

  As a result of studying based on this mechanism, the present inventors have monitored the amount of heat extracted from the molten steel by the mold during continuous casting, and appropriately controlled the vibration conditions of the mold in accordance with the change in the amount of heat removed from the molten steel. It was found that it is possible to suppress the occurrence of surface defects such as vertical cracks on the surface of the slab by appropriately controlling the inflow amount of steel and keeping the heat extraction from the molten steel within a predetermined range.

The present invention has been made based on this finding, the gist lies in the continuous casting method of steel shown below SL.

A method of continuously casting steel using a curved continuous casting machine while moving a water-cooled mold up and down with a vibration device, wherein the vibration device is a hydraulic mold vibration system including a hydraulic cylinder, and the bending of the mold The hydraulic cylinder is provided inside and outside the surface, the hydraulic cylinder is supported by a leaf spring and directly connected to the mold, and the vibration stroke of the mold is utilized by using the resonance with the leaf spring to raise and lower the mold. And a direct-acting hydraulic vibration device that can control the vibration frequency independently,
Further, the temperature of the cooling water supplied to the mold and the temperature of the cooling water discharged from the mold are respectively measured, the amount of heat removed from the molten steel by the mold calculated from the difference between these temperatures is obtained, and the amount of heat removed When it is determined that there is a large amount, an operation to increase the vibration stroke of the mold or reduce the vibration frequency is performed, and when it is determined that the amount of heat removal is small, an operation to decrease the vibration stroke of the mold or increase the vibration frequency It was carried out, the continuous casting method of steel, characterized by controlling the inflow of mold powder.

  According to the steel continuous casting method of the present invention, the amount of mold powder inflow during continuous casting and the amount of heat extracted from the molten steel by the mold can be maintained in an appropriate range, so that the surface properties with few surface defects such as vertical cracks are excellent. The slab can be stably cast continuously. Moreover, a slab can be continuously cast without requiring a large-scale and complicated apparatus.

It is a figure which shows the relationship between the amount of molten steel heat extraction by a casting_mold | template, the consumption of mold powder, and the surface property of a steel piece. It is a block diagram of the hydraulic vibration apparatus applicable to the method of this invention.

1. Continuous casting method of steel of the present invention The continuous casting method of steel of the present invention is a method of continuously casting steel while raising and lowering a water-cooled mold by a vibration device, and the temperature of cooling water supplied to the mold And measuring the temperature of the cooling water discharged from the mold, setting the vibration stroke and vibration frequency of the mold according to the amount of heat removed from the molten steel by the mold calculated from the difference between these temperatures, and molding powder This is a method for controlling the amount of inflow.

The amount of heat extracted from the molten steel by the mold during continuous casting can be expressed by the following equation (1).
D = {W × (T _out −T _in ) × C} / (L / Vc) (1)
Here, D: amount of heat removed from molten steel by mold, W: amount of mold cooling water, T _out : temperature of mold cooling water at mold outlet, T _in : temperature of mold cooling water at mold water inlet, C: mold cooling water Specific heat, L: length in the casting direction of the contact portion between the molten steel and the mold wall surface, and Vc: casting speed.

Here, in principle, the casting speed and the amount of mold cooling water during continuous casting are maintained constant. Therefore, the dominant factor for the change in the amount of heat extracted from the molten steel from the mold is the difference between the temperature at the mold outlet of the mold cooling water and the temperature at the water inlet (T _out −T _in ), that is, the mold cooling water in the mold. It is a temperature change. It can be said that when the temperature change of the mold cooling water is large, the amount of heat removal from the molten steel by the mold is large, and when the temperature change is small, the amount of heat removal from the molten steel is small.

  As described above, surface defects such as vertical cracks on the surface of the slab are caused by uneven solidification that occurs when the amount of heat extracted from the molten steel by the mold is large. In the steel continuous casting method of the present invention, even when the amount of heat removal from the molten steel by the mold is large, by increasing the mold powder inflow by increasing the vibration stroke of the mold or by reducing the vibration frequency. It is possible to reduce the amount of heat removal to such an extent that the surface defects of the pieces do not occur.

  In addition, when the amount of mold powder inflow is too large, a deep oscillation mark is formed on the surface of the slab, and surface cracks may occur in the steel slab obtained by rolling the slab. In this case, since the amount of heat extracted from the molten steel by the mold is small and the temperature change of the mold cooling water in the mold is small, the continuous casting method of steel of the present invention reduces the vibration stroke of the mold or increases the vibration frequency. Thus, by reducing the inflow amount of the mold powder, it is possible to make the depth of the oscillation mark formed on the surface of the slab so as not to cause surface cracks in the rolled steel slab.

  In the method of the present invention, the temperature of the mold cooling water can be measured by a temperature sensor provided at or near the water inlet and drain of the mold. As described above, since the temperature sensor can be provided outside the mold, it can be easily repaired or replaced even if it breaks down.

  In the steel continuous casting method of the present invention, the mold vibration conditions (vibration stroke and vibration conditions set in advance for the amount of mold cooling water, the temperature change of the mold cooling water in the mold, and the casting speed are controlled for mold vibration control. Based on the set value of (frequency), it is possible to use a method of automatically changing the vibration conditions of the mold in accordance with the temperature change of the mold cooling water in the mold and the casting speed at each time point.

  The purpose of mold vibration control is to control the inflow amount of mold powder in the mold, so that the cooling strength of the slab in the mold, that is, the amount of heat extracted from the molten steel by the mold represented by the above formula (1) is within an appropriate range. There is to keep on.

  When the temperature change of the mold cooling water is large, the amount of heat extracted from the molten steel by the mold is large, and surface defects such as vertical cracks on the slab surface may occur due to uneven solidification in the initial solidification stage of the slab. is there. In this case, the inflow of mold powder is increased to increase the thermal resistance between the mold wall surface and the molten steel, so that the slab is slowly cooled.

The relationship shown in the following equation (2) is known for mold vibration conditions and mold powder consumption.
Q = k / (T ^1.6 η ^0.5 ) × (A ^0.4 / Vc) × COS ⁻¹ {−1000 Vc / (2π
fA)} (2)
Here, Q: mold powder consumption (kg / m ² ), k: proportional constant, T: crystallization temperature of mold powder (° C.), η: viscosity of mold powder at 1300 ° C. (pois)
e), A: vibration stroke of mold (mm), Vc: casting speed (m / min), f: vibration frequency of mold (cpm: cycle per minute).

  Based on the above equation (2), in order to increase the inflow amount of mold powder, the negative strip rate (1 of the mold vibration) is increased by increasing the vibration stroke or reducing the vibration frequency among the mold vibration conditions. In the cycle, the ratio of the time during which the mold lowering speed exceeds the casting speed may be reduced.

  If the temperature change of the mold cooling water is small, the mold powder may flow in excessively. In this case, since the oscillation mark may become too deep, in order to reduce the consumption of mold powder, the vibration stroke of the mold is reduced and the vibration frequency is increased to increase the negative strip rate.

  In general, the inflow amount of mold powder varies depending on the vibration conditions of the mold, and the inflow amount tends to increase as the vibration stroke increases and the inflow amount decreases as the vibration frequency increases. Therefore, it is important to appropriately control both the vibration stroke and vibration frequency conditions.

3. Threshold value of molten steel heat removal amount There is a relationship shown in FIG. 1 between the molten steel heat removal amount by the mold and the consumption basic unit of the mold powder and the surface property of the steel piece obtained by performing the ingot rolling of the slab.

  FIG. 1 is a diagram showing the relationship between the amount of heat extracted from molten steel by a mold, the amount of mold powder consumed, and the surface properties of steel pieces. In the figure, the horizontal axis is the amount of heat removed from the molten steel by the mold, and the vertical axis is the consumption unit of the mold powder. The consumption unit of mold powder is the consumption of mold powder per ton of cast steel. Moreover, the test number in the figure corresponds to the test number of the Example mentioned later, and the symbol ((circle) and (triangle | delta)) in a figure respond | corresponds to evaluation of the surface property of the steel slab in Table 2 mentioned later. ○ means that the evaluation of the surface property of the steel slab was good, and Δ means that it was acceptable.

  As shown in FIG. 1, in the test numbers 1 and 2 in which the consumption basic unit of the mold powder was appropriate, many steel pieces having good surface properties were obtained. On the other hand, in test number 3 where the consumption unit of mold powder was too small, vertical cracks occurred on the surface of many cast pieces. In Test No. 4 where the consumption unit of mold powder was excessive, the oscillation mark formed on the surface of the slab was deep, and surface cracks occurred in many steel slabs obtained by rolling this slab.

  From the results shown in FIG. 1, when changing the vibration condition of the mold based on the temperature change of the mold cooling water in the steel continuous casting method of the present invention, the amount of heat removal from the molten steel by the mold is 21.5-33.0 kcal. Is preferred.

4). Preferable vibration conditions The vibration stroke of the mold is preferably 2.0 to 4.0 mm in amplitude, and the vibration frequency is preferably 32 to 255 cpm.

  The smaller the vibration stroke and the higher the vibration frequency, the greater the vibration acceleration of the mold and the greater the load on the equipment. For this reason, the lower limit value (2.0 mm) of the vibration stroke and the upper limit value (255 cpm) of the vibration stroke under the preferable conditions described above are values that do not make this load excessive.

  Also, if the vibration stroke is too large, the amount of mold powder inflow will be excessive, the depth of the oscillation mark generated on the surface of the slab will be too deep, and when rolling the steel slab starting from that oscillation mark There is a risk of surface cracking. Therefore, the upper limit of the vibration stroke is set to 4.0 mm as a value that does not cause the surface crack of the steel piece.

  Furthermore, the lower limit (32 cpm) of the vibration frequency is a lower limit of a threshold value at which stable casting is possible at a low casting speed (0.30 m / min or less). When the vibration frequency falls below 32 cpm, the inflow of mold powder decreases, and seizure occurs between the slab and the mold, which may make it difficult to continue continuous casting.

5. Other Casting Conditions The steel continuous casting method of the present invention applies to a wide variety of steel types such as general carbon steel, low alloy steel, and high alloy steel. Therefore, no special regulation is necessary.

  As preferable casting conditions, the cross-sectional size of the slab is 300 to 400 mm in thickness, 400 to 500 mm in width, the casting speed is 0.40 to 0.85 m / min, and the secondary cooling specific water amount is 0.15 to 0.50 L. / Kg-steel.

  Among the above casting conditions, the upper limit and the lower limit of the casting speed are limited by the time required for the molten steel refining treatment process, which is a process that is possible in stable continuous casting and upstream of the continuous casting process.

  The secondary cooling specific water amount varies depending on the steel type and is controlled for the purpose of controlling the final solidification position in the slab, which varies with the casting speed.

6). FIG. 2 is a block diagram of a hydraulic vibration device applicable to the method of the present invention. Two hydraulic cylinders 1 of the hydraulic vibration device are connected to a mold 2 of a continuous casting machine, and are arranged so as to sandwich a slab 4 in which molten steel supplied into the mold 2 from an immersion nozzle 3 is solidified. Vibrate up and down. When applied to a vertical bending type and a curved type continuous casting machine, the hydraulic cylinders 1 are arranged inside and outside the curved part, respectively. The vibration stroke and vibration frequency of each hydraulic cylinder 1 can be controlled independently. The mold 2 is water-cooled and has a water supply port and a water discharge port for mold cooling water.

  This hydraulic vibration device can be applied regardless of the type of continuous casting apparatus. In the continuous casting apparatus of the vertical type and the vertical bending type, the vibration strokes and vibration frequencies of both the hydraulic cylinders 1 are the same and vibrate synchronously. In the curved continuous casting machine, the mold thickness center at the upper limit position and lower limit position of the mold vibration stroke depends on the radius of curvature of the curvature at the mold part and the distance between the inner and outer surfaces of the curved surface. The magnitude of the vibration stroke of the hydraulic cylinder 1 inside and outside the curved portion is controlled so as to be located on a straight line connecting the portion and the center point of the curved radius of the continuous casting machine.

  In the case of performing control to change the vibration stroke and frequency of the mold at any time during the continuous casting process, the vibration device for vibrating the mold is preferably a hydraulic vibration device. According to the hydraulic vibration device, vibration control can be performed with a higher degree of freedom and can be installed at a lower cost than a mechanical mold vibration device using a link. The mechanical mold vibration device has a more complicated structure than the hydraulic vibration device, requires a large amount of equipment, and has a huge installation cost.

  As the hydraulic vibration device, a direct-acting type device in which the mold 2 is supported by a leaf spring (not shown) and the resonance by the leaf spring is used for vibration is preferable. A direct acting hydraulic vibration device using resonance has a simple structure, small inertia, and good response. In addition, the equipment itself is compact, and replacement work can be performed relatively easily.

  In order to confirm the effect of the continuous casting method of the steel of the present invention, the following tests were conducted and the results were evaluated.

1. Test conditions A curved continuous casting machine was used as the continuous casting apparatus, and the apparatus shown in FIG. 2 was used as the mold vibration apparatus. The steel types used were A and B shown in Table 1, and the casting conditions were the conditions shown in Table 2 including the vibration conditions of the mold.

Among the test numbers 1 to 4 shown in Table 2, test numbers 1 and 2 are examples of the present invention in which the vibration conditions of the mold were changed according to the amount of heat extracted from the molten steel by the mold calculated using the equation (1). is there. “Mold cooling water temperature change” shown in the table is the difference between the temperature of the mold cooling water measured by the temperature sensor provided at the mold drain and the temperature measured by the temperature sensor provided at the mold water supply (see above). (T _out −T _in ) _{in the} equation (1).

  Test numbers 3 and 4 are comparative examples, and the vibration conditions of test numbers 1 and 2 are different except that the temperature change of the mold cooling water in the mold is slightly different and the vibration conditions of the mold are not changed. The conditions were the same as before the change. Moreover, the slab manufactured on each condition was carried out by the lump rolling, and was made into the steel slab (billet). 36 pieces of steel were produced for each condition.

  In test number 1, the temperature change of the mold cooling water in the mold was 10.2 ° C, and the amount of heat extracted from the molten steel by the mold was 32.6 kcal, so the vibration stroke of the mold was increased from 3.0 mm to 3.5 mm in amplitude. And the vibration frequency was reduced from 1.63 Hz (97.8 cpm) to 1.08 Hz (64.8 cpm). Thereby, the consumption basic unit of mold powder increased from 0.38 kg / t to 0.43 kg / t. Further, along with the increase in thermal resistance due to the increase in consumption unit of the mold powder, the temperature change of the mold cooling water in the mold became 8.9 ° C., and the amount of heat extracted from the molten steel by the mold decreased to 28.5 kcal.

  In test number 2, the temperature change of the mold cooling water in the mold was 7.2 ° C, and the amount of heat extracted from the molten steel by the mold was as small as 21.4 kcal, so the vibration stroke of the mold was reduced from 3.5 mm to 3.0 mm in amplitude. The vibration frequency was increased from 1.17 Hz (70.2 cpm) to 1.75 Hz (105 cpm). As a result, the consumption unit of mold powder was reduced from 0.52 kg / t to 0.48 kg / t. Further, along with a decrease in thermal resistance due to a decrease in the consumption unit of the mold powder, the temperature change of the mold cooling water in the mold became 8.4 ° C., and the amount of heat extracted from the molten steel by the mold increased to 25 kcal.

2. Test results Evaluation of surface properties was performed using slabs manufactured under the above conditions for surface defects such as vertical cracks, and for steel slabs obtained by split-rolling slabs, steel slab care rate. It was.

  The billet light care rate is the result of surface surface care and is an index of the surface property of the billet. The steel slab obtained by the block rolling was subjected to scoring to take care of defects such as surface cracks, and the surface properties were evaluated for each steel slab by the length of the scoring. Depending on the evaluation results of the surface properties, each steel slab is classified into lightly-cured steel slabs, medium-steel slabs and heavy-steel slabs. The ratio of the number of light billets to the total number of billets was defined as the billet bill percentage. The higher the billet care rate, the greater the number of billets with good surface properties.

  Here, the lightly billeted steel slab refers to a steel slab having a total shave length of less than 2 m on each side as a result of staking on the four sides of the slab. The medium-handed steel slab refers to a steel slab having a surface with a total length of 2 m or more on one surface among the four side surfaces of the steel slab. A major steel billet refers to a steel piece that has two or more of the four side faces of the steel piece with a total cutting length of 2 m or more.

  Table 2 shows the result of the steel bill light care rate together with the casting conditions. The case where the billet care rate was 90% or more was evaluated as ◯ (the surface property of the steel piece was good), and the case where it was less than 90% was evaluated as Δ (the surface property of the steel piece was acceptable). FIG. 1 shows the relationship between the amount of heat extracted from the molten steel by the mold, the amount of mold powder consumed, and the surface properties of the steel pieces.

  As shown in Table 2 and FIG. 1, in Test Nos. 1 and 2 in which the vibration conditions of the mold were changed in accordance with the amount of heat removal from the molten steel and the amount of heat removal from the molten steel was appropriate, the steel bill light care rate was 94% or more. And the evaluation was good.

  On the other hand, in Test Nos. 3 and 4 in which the vibration conditions of the mold were not changed even though the amount of heat removal from the molten steel was excessive or small, the slab light care rate was as low as 81% or less, and the evaluation was △ Met. In test number 3 in which the consumption basic unit of the mold powder was small and the amount of heat removal from the molten steel was excessive, vertical cracks occurred on the surface of many cast pieces. Further, in Test No. 4 where the consumption unit of mold powder was excessive, the oscillation mark formed on the surface of the slab was deep, and surface cracks occurred in many steel slabs obtained by rolling this slab.

  According to the steel continuous casting method of the present invention, the amount of mold powder inflow during continuous casting and the amount of heat extracted from the molten steel by the mold can be maintained in an appropriate range, so that the surface properties with few surface defects such as vertical cracks are excellent. The slab can be stably cast continuously. Moreover, a slab can be continuously cast without requiring a large-scale and complicated apparatus.

1: hydraulic cylinder, 2: mold, 3: immersion nozzle, 4: slab

Claims (1)

A method of continuously casting steel while using a curved continuous casting machine and raising and lowering a water-cooled mold using a vibration device,
The vibration device is a hydraulic mold vibration system including a hydraulic cylinder,
The hydraulic cylinder is provided inside and outside the curved surface of the mold,
The hydraulic cylinder is supported by a leaf spring and directly connected to the mold,
Using a resonance with the leaf spring for the vertical vibration of the mold, a direct acting hydraulic vibration device capable of independently controlling the vibration stroke and vibration frequency of the mold is configured,
Furthermore, the temperature of the cooling water supplied to the mold and the temperature of the cooling water discharged from the mold are respectively measured, and the amount of heat removed from the molten steel by the mold calculated from the difference between these temperatures is obtained,
When determining that the amount of heat removal is large, increase the vibration stroke of the mold or reduce the vibration frequency,
A continuous casting method for steel, characterized in that when it is determined that the amount of heat removal is small, an operation of reducing the vibration stroke of the mold or increasing the vibration frequency is performed to control the inflow amount of mold powder.

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