JPS63242452A - Method for casting by light rolling reduction - Google Patents

Abstract

PURPOSE:To reduce segregation of impurity at center part of a continuously cast slab by executing light rolling reduction to the cast slab containing non- solidified molten metal while adjusting the rolling reduction rate by many rolls and also adjusting castling speed by cooling speed for the cast slab. CONSTITUTION:The cast slab 30 drawn from a mold in continuous casting apparatus has non-solidified molten metal part 34 in the inner part of solidified shell 32 and the non-solidified part eliminates at part of crater end 36. During this section, the cast slab 30 passes through the light rolling reduction device 10 composing of segments 12, 14 and is bent from vertical direction to horizontal direction by many light reducing rolls 16 while cooling by cooling water from many spray nozzles 20 and transferred. In this case, data of the surface temp., etc., of the cast slab 30 at the segment 12 are inputted to a central processing unit (CPU) 40 and through this unit, by a hydraulic control device 42 for the light reducing rolls 16 at the segment 14, a cooling water control device 44 for the nozzles 20 and a drawing speed control device 46 for the cast slab 30 from the mold, rolling reduction force, cooling water rate and drawing speed from the mold are adjusted. By this method, the continuously cast slab having a little center segregation is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a light reduction casting method for preventing center segregation of slabs in continuous casting.

[Prior art] Normally, in continuous casting, when molten steel is cooled by a water-cooled mold, a thin solidified shell is formed at a portion corresponding to the outer periphery of the slab, and the slab that has passed through the mold has unsolidified inside. While holding the molten steel, it is guided by a group of support guide rolls and pulled out by pinch rolls. During the slab drawing process, spray water is injected onto the slab to promote solidification inside the slab, and when the thickness of the solidified shell grows beyond the thickness that can withstand deformation, the slab is pulled out at a predetermined curvature. While bending the slab 90 degrees, apply a light reduction blade to the solidified slab using a light reduction device. The slab is completely solidified in this light reduction zone, and then
At the end position (straightening point) of the light reduction band, the rolls of the straightening device correct the bends in the slab, and the straight slab is cut into a predetermined length by a cutting machine.

In the final solidification part of the slab (crater end), carbon (C),
Component elements such as sulfur (S), manganese (M n ), and phosphorus (P) are concentrated in the unsolidified molten steel.

Since this concentrated molten steel has a low melting point, it does not precipitate when the molten steel is stationary in the solidified shell, but when the concentrated molten steel flows in the solidified shell, it precipitates, resulting in so-called central segregation.

Normally, solidification shrinkage of molten steel causes the molten steel to be sucked and flow toward the bottom of the slab (in the direction of slab withdrawal), so a light reduction blade commensurate with the amount of solidification shrinkage is added to the unsolidified slab to prevent center segregation. There is.

On the other hand, when the casting speed becomes relatively high, the static pressure of the molten steel increases, and so-called bulging occurs, in which the solidified shell partially expands in areas where the reduction amount is insufficient. When bulging occurs, the expanded solidified shell is compressed by roll reduction, causing molten steel to flow toward the top of the slab (in the direction opposite to the direction in which the slab is pulled out), and concentrated molten steel precipitates at the crater end, forming a wide center. Segregation occurs. In particular, when the pitch interval of the rolls is large and when there is deflection or wear on the reduction roll,
Bulging is likely to occur. For this reason, one layer of small-diameter rolls is usually arranged in the light rolling zone to reduce the pitch between the rolls, thereby preventing the occurrence of bulging and reducing center segregation.

In the conventional light reduction casting technology, center segregation of the slab cannot be sufficiently reduced only by reduction using the light reduction device. For this reason, a rotating magnetic field or a moving magnetic field is applied to the crater end using an electromagnetic stirring device (EMS), and the concentrated molten steel at the crater end is stirred by electromagnetic induction to prevent center segregation.

[Problems to be solved by the invention] However, in the conventional light reduction casting technology, concentrated components such as [C] and [P] concentrated at the crater end are simply diffused within a certain area by electromagnetic stirring. Although it is possible to lower the peak value of the concentration of the segregated components, it is not possible to eliminate the segregation itself. For this reason, a segregation zone with an average concentration level is generated within a certain area at the center of the slab. Small island-like segregation (semi-macro segregation) exists in this segregation zone, and various interstitial points as shown below are generated.

In recent years, demands from customers regarding the quality of steel materials have become more sophisticated and diversified, and it is necessary to further reduce the segregation of impurity elements and non-metallic inclusions that inevitably exist in manufactured steel materials. desired.

In other words, in vibrator materials for oil and natural gas transportation, hydrogen-induced cracking (HIC) occurs along the central segregation zone due to the action of sour gas containing hydrogen sulfide.
There is a strong desire to prevent I (IC) by improving materials. It is known that HIC tends to occur in areas where the P concentration is high in the central segregation zone, and generally when the peak concentration of P is about 0. If it exceeds 0.04%, the crack occurrence rate due to HIC increases.In the conventional light reduction casting technology, so-called macro segregation occurs in the center of the slab, where the P concentration reaches about 10 times that of the sound part. When producing HICgil-resistant materials, the P concentration in the ladle molten steel is reduced to a level of 50 ppm or less, and the solidified slab is soaked at approximately 1300°C to diffuse the segregated P and reduce the maximum P concentration. I'm trying to lower the value.

Figure 9 shows the HIC of semi-macro segregated particles for various steel materials with a central segregation zone, with the horizontal axis representing the grain size of centrally segregated particles and the vertical axis representing the number of segregated particles present per slab length loom+a. It is a graph diagram in which the influence of ☆ was investigated. In the figure, black circles indicate those in which cracks occurred due to HIC, and white circles indicate those in which no cracks occurred. As shown in this figure, for HIC generation, the particle size is approximately 0.5
It is known that large segregated particles of mm or more (semi-macro segregation) are particularly harmful, and that reducing this semi-macro segregation is effective in preventing the occurrence of HIC. That is, in order to improve the HIC resistance of steel materials, it is necessary to reduce both the above-mentioned macro segregation and semi-macro segregation.

On the other hand, with respect to steel sheet materials for offshore structures, customers primarily desire improvements in the welding characteristics of the steel materials. In other words, if there is a central segregation zone in the steel material,
It is necessary to preheat the steel material during welding to prevent weld cracking, and after welding, the toughness of the weld heat-affected zone (HAZ) deteriorates and lamellar tear (step-like appearance that occurs along the lamination of the steel plates) occurs in the welded joint. cracks) occur. Therefore, from the viewpoint of improving the reliability of structures, it is strongly desired to reduce the central segregation zone of steel materials.

Furthermore, since segregation has a significant effect on the workability of deep-drawn steel such as beer cans, the development of non-segregating materials is strongly desired.

By the way, although the amount of variation in the light reduction roll and segment during operation is kept to about 0.10 mm or less, the mechanical precision of each part of the equipment decreases due to long-term operation, and the amount of light reduction varies, making it difficult to achieve the specified light reduction. There is a problem in that it becomes difficult to add the blade to the slab, and center segregation tends to occur.

This invention was made in view of such circumstances, and
The purpose of the present invention is to provide a light reduction casting method capable of suppressing the flow of molten metal in an unsolidified slab due to variations in the amount of light reduction and reducing the absolute amount of impurities, etc. segregated in the center of the slab. .

[Means for Solving the Problems] The light reduction casting method according to the present invention is a light reduction casting method in which a continuously cast unsolidified slab is completely solidified while being lightly reduced by a large number of pairs of rolls. While determining the appropriate strain rate that minimizes the flow of molten metal inside the slab, we also measure the roll reaction forces that the multiple pairs of rolls receive from the slab, and determine the distortion of the slab corresponding to each roll reaction force. The method is characterized in that the respective speeds are ascertained, and the amount of roll reduction of each roll is adjusted according to these strain rates, so that the strain rate of each part of the slab matches the appropriate strain rate. In this case, it is preferable to adjust the roll reduction amount, respectively, and also adjust the casting speed and slab cooling rate so that the strain rate of each part of the slab matches the appropriate strain rate.

[Operation] In the light reduction casting method according to the present invention, an appropriate strain rate at which the flow of molten metal inside the slab is minimized is determined from the casting conditions. On the other hand, the roll reaction force that the roll receives from each part of the slab during solidification is measured, and the actual strain rate of the slab corresponding to each roll reaction force is determined. Then, the amount of roll reduction is adjusted according to these strain rates. If the strain rate of the slab corresponding to the roll reaction force becomes larger than the appropriate strain rate due to bulging, reduce the roll reduction amount,
Prevents the flow of molten metal by suppressing excessive squeezing. If the former becomes smaller than the latter, the amount of roll reduction is increased, a force is applied to the molten metal to push back the molten metal sucked by solidification contraction, and the flow of the molten metal inside the slab is prevented.

In addition, by adjusting the roll reduction amount as well as the casting speed and slab cooling rate, conditions such as solidification rate can be stabilized more quickly, making it possible to match the strain rate of each part of the slab to the appropriate strain rate. becomes easier.

[Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First, the basic concept of center segregation reduction will be explained.

FIG. 4 is a schematic diagram showing the generation mechanism of center segregation at the final stage of solidification. In the figure, the shaded area indicates the solid phase, and the area other than the shaded area indicates the liquid phase. In addition, the symbol g in the figure indicates the limit solid phase ratio at which residual molten steel can flow, the symbol LD indicates the thickness of the solid-liquid coexistence region, and the symbol LL indicates the thickness of the liquid phase region (100% liquid phase region). . In the figure, assuming that central segregation is formed by the flowing and mixing of concentrated molten steel existing in the region from the thickness position where the solid fraction is zero to the thickness position where the solid fraction is gl. , the component concentration ratio CL/C of the molten steel at the final stage of solidification is calculated by the following equation (1).

...(1) However, each symbol in the above formula (1) is as follows.

α-LL/LD (ratio of liquid phase region to solid-liquid coexistence region at the final stage of solidification) LL, LD: Thickness of liquid phase region at the final stage of solidification, thickness of solid-liquid coexistence region g: Limit at which residual molten steel can flow The solid phase ratio Ko;
The equilibrium distribution coefficient for each component such as [C] and [P] in Figure 5 shows the solid-liquid coexistence at the final stage of solidification, with the component concentration ratio CL/Co of [C] determined by the above equation (1) taken as the vertical axis. This study examines the relationship between molten steel flow and segregation degree at the final stage of solidification, with α, the ratio of the liquid phase region to the region, as the horizontal axis. In the figure, the numbers written on each curve are the respective solid fraction g
shows. Note that the component concentration ratio CL/Co was calculated by setting the equilibrium distribution coefficient KO to 0.141. As is clear from this figure, as the flow limit solid fraction g of concentrated molten steel increases, the component concentration CL/C0 increases, and as the ratio α of the completely liquid phase region decreases, the component concentration increases. The impact on the conversion rate will be greater. That is, when the flow of concentrated molten steel becomes active, the component concentration rate increases and the degree of segregation increases. Therefore, in order to reduce center segregation, it is necessary to suppress the flow of concentrated molten steel.

As a means of suppressing the flow of this concentrated molten steel, firstly, the pitch between the rolls is narrowed to suppress the occurrence of bulging between the rolls of the slab, and secondly, the concentrated molten steel is moved toward the bottom side by solidification shrinkage. In order to prevent the slab from being sucked toward the downstream side of slab drawing, an appropriate amount of light pressure lower blade corresponding to the amount of solidification shrinkage may be applied to the slab. In addition, according to the results of actual casting tests conducted under various conditions, the effect of roll reduction of the light reduction zone is maximized when the crater end is positioned approximately 3/4 from the starting position of the light reduction zone. It turned out to be.

By the way, the inventors found that the thickness of the unsolidified region was about 30 mm.
As a result of investigating the flow condition of concentrated molten steel at the final stage of solidification by driving studs containing tracer (F e S) into slabs in the middle of solidification and varying the amount of light reduction, the following findings were obtained. did it.

Figure 6 shows the amount of light reduction (the amount of roll spacing reduced per 1 m of slab drawing length) on the horizontal axis, and the flow length of molten steel on the vertical axis, and the amount of light reduction and molten steel based on the above survey results. It is a graph diagram showing the relationship with flow length. As is clear from this figure, the flow length of molten steel decreases as the light reduction amount increases, and it is confirmed that the flow length of molten steel becomes the minimum at a light reduction amount of approximately 1.2 mm/+. Ta.

FIG. 1 is a schematic side view of a light reduction device used in a light reduction casting method according to an embodiment of the present invention, FIG. 2 is a perspective view showing a group of light reduction rolls, and FIG. 3 is a light reduction device. FIG. 2 is a schematic diagram showing a part of the device. A CPU (Central Processing Unit) 40 is provided in a control room of the vertical bending continuous casting machine to control the continuous casting operation. CPU40
The system includes a storage unit that stores input data, a calculation unit that reads data from the storage unit at any time and performs calculations, and a command unit that sends command signals to various control devices 42, 44, and 46 based on the calculation results of the calculation unit. have.

A mold (not shown) is provided in the upper part of the continuous casting machine, and the unsolidified slab 30, which becomes a slab with a predetermined cross-sectional shape, is pulled out from the mold by pinch rolls (not shown). Some support guide rolls (not shown) are arranged below the mold so as to surround the slab, thereby forming a vertical pull-out section. A plurality of pairs of bending rolls 24 are provided below the vertical drawing section, and the bending rolls 24 bend the slab 30 during solidification so that the direction of drawing the slab is changed to horizontal. Further, a light reduction device 10 is provided below the bending rolls 24, and is configured to apply a predetermined amount of reduction to the slab. The light reduction device 10 has two segments 12.14, each segment 12.14 having eight pairs of light reduction rolls 16. A group of spray nozzles 20 are arranged along the width of the slab 30, and each row of spray nozzles is arranged between the light reduction rolls 16, respectively. A group of spray nozzles is a cooling water control device 44 with a water volume adjustment function.
The spray pattern for each zone is controlled by the

As shown in FIG. 2, the light reduction roll 16 has a length of 3
It is divided or divided into six parts. The light reduction roll 16 has a diameter of about 210 +++ m from the conventional approximately 37,511 IIIl.
In addition, the pitch between the rolls has been changed from the conventional approximately 420+++n to approximately 235ma+, and the number of rolls in each segment has been changed from the conventional 5 pairs to 8 pairs. In addition, the length of the vertical drawing part in this continuous casting machine is about 4 m, the radius of curvature of the bent part of the slab is about 8 m, and the height difference from the meniscus in the mold to the light reduction device 10 is about 10.4 to 14 m. ．．
1 m As shown in Fig. 3, these light rolling rolls 16 are each held by a roll chock 17, and a rod 18 of a rolling cylinder 19 is connected to each roll chock 17, and the rod 18 pushes the roll chock 17. and roll 16
is pressed against the slab 30. The output side of a hydraulic control device 42 is connected to each reduction cylinder 19 by a hydraulic circuit. This hydraulic control device 42 has a pressure oil supply source (not shown). Furthermore, the operating circuit of the solenoid valve provided at each pressure oil supply port of this hydraulic control device 42 is C.
It is connected to a command section of the PU 40, and oil at a predetermined pressure is supplied to each reduction cylinder 19 from the hydraulic control device 42 based on a command signal from the CPU 40. on the other hand,
A roll reaction force measuring device 39 equipped with a load cell is installed on each roll 1.
6 is provided in each bearing, and the reaction force that the roll 16 receives from the slab 30 is measured.

The transmission side of each measuring device 39 is connected to the input side of the storage section of the CPU 40, so that the detected reaction force of each measuring device 39 is converted into a signal and transmitted to the CPU 40. Also, C
The calculation section of the PU 40 calculates the bottom direction generated in the molten steel 34 due to solidification shrinkage based on various input data depending on the casting conditions (for example, degree of superheating of the molten metal in the tundish, cooling rate of the slab, rate of slab withdrawal, slab size, etc.). By calculating the flow force a to the slab 30, it is possible to determine the appropriate strain rate to be applied to the slab 30 (the slab strain rate that blocks the flow force a of the molten steel). On the other hand, when roll reaction forces (reaction forces that the roll 16 receives from the slab 30) c and d are generated due to bulging, the reaction forces c and d measured by each roll reaction force measuring device 39 are input to the CPU 40. , based on this, the appropriate strain rate (slab strain rate that prevents the flow force of molten steel)
A command signal is sent from the CPU 40 to the hydraulic control device 42 so that the pressure is applied to the slab 30.

Next, the case of producing slabs by the method of this invention will be specifically explained with reference to FIGS. 1 to 3. The cast steel type is Nb, high tensile strength m for V-type in-pipe (API
-65), and the width of the slab is approximately 1950+a
It is m. Molten steel whose composition has been adjusted by RH degassing treatment and ladle refining treatment is cast into a mold from a tundish. At this time, the temperature of the molten steel in the tanditshu is approximately 1552
℃, and the molten steel is in a superheated state of about 33°C (the solidification temperature of this steel type is about 1519°C). When molten steel is injected into the mold, a solidified shell 32 is formed in contact with the mold wall. The unsolidified slab 30 is pulled out at a casting speed of about 0.75 m/min, and the crater end 36 is positioned approximately at the center of the segment 14. In addition, the cooling water control device 44 adjusts the flow rate control valve of the cooling water supply source to control the amount of cooling water supplied to each spray nozzle, and the specific water amount is 0.54. f'hag (slab 1
Cooling water is injected onto the slab 30 through each spray nozzle in a spray pattern of (amount of spray water per kg).

On the other hand, data detected by sensors in various parts of the continuous casting machine (surface temperature of each part of the slab, degree of superheating of molten steel in the tundish, etc.) is manually input to the CPU 40, and this input data and casting conditions (thickness of the slab, slab width, slab drawing speed, cooling rate,
The calculation unit of the CPU 40 calculates the flow force a of the molten steel caused by solidification contraction using a multivariable mathematical model consisting of a roll pitch, roll pitch, etc. Next, an appropriate strain rate ε0 of the solidified shell 32 is determined by calculation in order to generate a force in the top direction necessary for canceling the flow force a of the molten steel 34 in the final stage of solidification by roll reduction.

FIG. 7 is a graph showing the relationship between strain rate and roll reaction force, with the horizontal axis representing the strain rate of the slab and the vertical axis representing the roll reaction force that the roll receives from the slab. As is clear from this figure, the strain rate and roll reaction force are in a proportional relationship with each other, and if the roll reaction force is known, the corresponding strain rate can be known. For example, when the roll reaction force value detected by the roll reaction force measuring device 39 is 1 Oton-f, the strain rate is 0.9o+a/min.

For example, if bulging occurs and roll reaction forces c and d act on each roll 16, the reaction forces c and d measured by each measuring device 39 are converted into signals and sent to the CPU 40.

In the CPU 40, the strain rate ε1.corresponds to the reaction force C1d. ε
2 and the appropriate strain rate ε. The difference between the two is sent to the hydraulic control device 42 as a voltage difference command signal. When these signals are input, current flows through the operating circuit of each electromagnetic valve, the opening degree of the electromagnetic valve is adjusted, and pressure oil is supplied into each cylinder 19 through each pressure oil supply port. Then, each rod 18 retreats into the cylinder 19, and the pressing force of the roll 16 is relaxed. The rod 18 is driven until the voltage difference signal input from the CPU 40 to the hydraulic control device 42 becomes zero,
It stops when it reaches zero. As a result, the strain rate of the slab becomes the appropriate strain rate ε. Therefore, the flow force in the top direction generated in the molten steel 34 due to bulging is suppressed.

Further, the voltage difference signal described above is sent from the CPU 40 to the cooling water control device 44 and the extraction control device 46, and based on these signals, the amount of cooling water in the nozzle 20 and the drive speed of the pinch roll are adjusted, respectively. Control slab cooling rate and drawing rate. This modifies the solidification rate of the slab 30, making it easier to achieve light reduction casting with an appropriate strain rate ε0.

In Figure 7, the particle size distribution of semi-macro segregated particles was investigated by plotting the particle size of the segregated particles present in the segregation zone on the horizontal axis and the number of segregated particles per 100 mm of slab pulling length on the vertical axis. It is a graph diagram. In the figure, black circles represent semi-macro segregated particles present in the central segregation zone of slabs manufactured by the embodiment of the present invention, and white circles represent semi-macro segregation particles present in the central segregation zone of slabs manufactured by the conventional method. show.

As is clear from this figure, according to the embodiment of the present invention, the number of semi-macro segregated particles with a particle size of 0.51 or more that is harmful to HIC cracking can be significantly reduced compared to the conventional method, and center segregation can be reduced. I was able to achieve this effect.

A sample was taken from the slab manufactured according to the above example (without soaking treatment), and a corrosion test was conducted in a hydrogen sulfide atmosphere (
As a result of investigating the HIC occurrence rate on 24 cross sections using the NACE test, no cracks with a length of 0.01 mm or more were observed.

In the above embodiment, since the diameter of the light reduction roll 16 and the pitch between the rolls are smaller than those of the conventional example, the occurrence of bulging can be effectively prevented.

Furthermore, since the length of the light reduction roll 16 is divided into three or six parts, the driving force of the hydraulic system is reliably transmitted to each divided roll without bending the roll, and roll reduction control can be made more precise. The flow limit solid fraction g of molten steel was reduced from the conventional 0.70 to 0.15.

In addition, although the above example shows the case of manufacturing Nb, V-based high-strength steel, it is not limited to this, and can also be applied to the case of manufacturing other metal materials that exhibit hot work brittleness in a temperature range of about 800°C or less. This invention method can be used.

Furthermore, although the above embodiments have been described with reference to the case of manufacturing slabs, the method of the present invention is not limited to this, and can also be used in the case of manufacturing blooms, round billets, and the like.

[Effects of the Invention] According to the present invention, since the strain rate of the unsolidified slab due to rolling can be controlled, it is possible to reduce the center segregation of the slab in the steady section, and also to It is also possible to reduce center segregation in unsteady parts such as continuous casting joints. Therefore, the quality of the entire slab can be significantly improved.

[Brief explanation of the drawing]

FIG. 1 is a schematic side view of a light reduction device in which a light reduction casting method according to an embodiment of the present invention is used, FIG. 2 is a perspective view showing a group of light reduction rolls, and FIG. 3 is a light reduction device. A schematic diagram showing a part of the apparatus, FIG. 4 is a schematic diagram showing the solidification mechanism of concentrated molten steel, and FIG. 5 is a schematic diagram showing the relationship between the flow of molten steel and the degree of segregation.
Fig. 6 is a graph showing the relationship between light reduction and flow length of molten steel, Fig. 7 is a graph showing the relationship between roll reaction force and strain rate, and Fig. 8 is a graph showing the effects of this invention. , No. 9
The figure is a graph showing the influence of semi-macro segregation on hydrogen-induced cracking (HIC). 10; light reduction device, 16; light reduction roll, 19; reduction cylinder, 30; slab, 32; solidified shell, 34; molten steel, 36
; Crater end, 39; Roll reaction force measuring device, 40;
CPU, 42; Hydraulic control device, 44; Cooling water control device, 46; Drawing control device Patent attorney Patent attorney Takehiko Suzue 24 Figure 5 @ Rolling amount (m hard) Figure 6136 Construction degree C57 Figure: 8 Figure 9

Claims (2)

[Claims] (1) In the light reduction casting method, in which continuously cast unsolidified slabs are completely solidified while being lightly rolled down by multiple pairs of rolls, an appropriate strain rate is determined based on the casting conditions to minimize the flow of molten metal inside the slab. At the same time, the roll reaction force that the multiple pairs of rolls receive from the slab is measured, and the strain rate of the slab corresponding to each roll reaction force is determined, and the roll reduction of each roll is adjusted according to these strain rates. A light reduction casting method characterized by adjusting the amount and matching the strain rate of each part of the slab to an appropriate strain rate. (2) The strain rate of each part of the slab is made to match an appropriate strain rate by adjusting the roll reduction amount of each of the rolls, and also adjusting the casting speed and cooling rate of the slab. Light reduction casting method described in .