JPS63278654A - Light rolling reduction casting method - Google Patents

Abstract

PURPOSE:To make reflection of tested result on internal quality in a cast slab to operation and to reduce center segregation by measuring the cast slab thickness after the strand cast slab is completely solidified, adjusting rolling reduction rate of rolls in case the measured value excesses the permissible range and controlling strain speed in the cast slab at the end time of the solidification. CONSTITUTION:When the strand cast slab 30 is completely solidified, the solidified cast slab 30 is straightened by a straightening roll 26 and cut to the prescribed length with a cutter 37. Next, the thickness of the cut cast slab is measured at each position thereof with measuring instrument 39 and disversing degree of the center segregation is found with an arithmetic unit 40. The permissible range of the prescribed thickness, assuming that the degree of the center segregation is minor, is beforehand grapsed, and in case the measured thickness of the cast slab excesses the permissible range of the prescribed thickness, the rolling reduction rate with the light rolling reduction rolls 16 applied in the cast slab 30 at the end time of solidification, is adjusted. By this method, the strain speed for the cast slab 30 at the end time of the solidification is controlled and the flow of molten steel in the inner part of the cast slab 30 is prevented.

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 bent to a predetermined curvature. While bending the slab by about 90 inches, a light reduction blade is applied to the slab in the middle of solidification 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 (Mn), 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. This segregation zone contains small island-like segregation (semi-macro segregation), which causes various problems as shown below.

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 pipe 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 HIC by improving materials. It is known that HIC is likely 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 0.04% or more, the cracking occurrence rate due to HIC increases. With conventional light reduction casting technology, so-called macro-segregation occurs in the center of the slab, where the P concentration is about 10 times that of the sound part. The concentration is reduced to a level of 50 ppm or less, and the solidified slab is soaked at about 1300° C. to diffuse the segregated P and reduce the maximum value of the P concentration.

Figure 8 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 100+ am of slab length. It is a graph diagram that investigated the influence on 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
Large-sized segregated particles (semi-macro segregation) larger than (2) are particularly harmful, and it is known that reducing this semi-macro segregation is effective in preventing the occurrence of HIC. In other words, the HI resistance of the steel material
In order to improve the C characteristics, 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 segregation-free materials is strongly desired.

Incidentally, in order to discover defective material with significant center segregation, the cross section is usually investigated after cutting the slab. A sulfur print, EPMA, or X-ray microanalyzer is used to investigate the internal quality of the slab, but these methods require a long time to obtain results, so the results cannot be quickly applied to continuous casting operations. There is a problem in that it is not possible to improve the quality of slabs by reflecting these factors.

This invention was made in view of such circumstances, and
The purpose is to provide a light reduction casting method that can quickly reflect the internal quality investigation results of a slab after complete solidification in operations and reduce the absolute amount of impurities segregated in the center of the slab. shall be.

Means for Solving Problem E] The light reduction casting method according to the present invention is a light reduction casting method in which continuously cast unsolidified strand slab is completely solidified while being lightly reduced by a large number of pairs of rolls. After the strand slab is completely solidified, the thickness of the strand slab is measured, and if the measured thickness exceeds a predetermined thickness tolerance range, the reduction amount of the rolls is adjusted to adjust the strain rate of the strand slab at the final stage of solidification. It is characterized by control.

[Operation] In the light reduction casting method according to the present invention, when the strand slab is completely solidified, the thickness of each part of the completely solidified slab is measured and the variation thereof is investigated.

In addition, a predetermined thickness tolerance range in which the degree of central segregation of the slab is estimated to be light should be known in advance, and if the measured thickness of the slab exceeds the predetermined thickness tolerance range, the strands at the final stage of solidification should be Adjust the amount of roll reduction applied to the slab. This results in
The strain rate of the strand slab at the final stage of solidification is controlled, and the flow of molten metal inside the slab is prevented.

[Embodiments] Hereinafter, embodiments of the present invention will be specifically described 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/Co of the molten steel at the final stage of solidification is calculated by the following equation (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 end of solidification) LL + LD; Thickness of liquid phase region at end of solidification, thickness of solid-liquid coexistence region g; Limit to which residual molten steel can flow The solid phase ratio Ko;
The equilibrium partition 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 cL/Co of [C] determined by equation (1) above taken on 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/Co increases, and as the ratio α of the completely liquid phase region decreases, the component concentration increases. The impact 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+n
As a result of investigating the flow condition of concentrated molten steel at the final stage of solidification by driving studs containing a tracer (FeS) into a slab in the middle of solidification and varying the amount of light reduction, the following findings were obtained. I was able to do that.

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, it was confirmed that the flow length of molten steel decreased as the light reduction amount increased, and that the flow length of molten steel became the minimum when the light reduction amount was about 1.211 ml.

Fig. 1 is a schematic diagram showing continuous casting equipment 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 part of the light reduction equipment. FIG. A strand slab 30 is pulled out from a mold of a vertical bending type continuous casting machine by pinch rolls to produce a slab having a predetermined cross-sectional shape. The strand slab 30 pulled out from the mold has a solidified shell 3 on its outer periphery.
2 is formed and holds an unsolidified molten metal 34 inside. Some support guide rolls are arranged below the mold so as to surround the slab, and a vertical pull-out part is formed. A plurality of pairs of bending rolls 24 are provided below the vertical drawing section.
is provided, and the strand slab 30 which is in the middle of solidification is bent by a bending roll 24 at a predetermined curvature so that the direction of drawing the slab is changed to horizontal. Further, below the bending roll 24, a part of the light rolling roll 1 of the light rolling device 10 is provided.
6 is provided to form a light reduction zone, and the strand slab 3
A predetermined reduction blade is added to 0. Some spray nozzles (not shown) are installed in the light reduction zone.
The spray nozzles in each row are arranged along the width of the light reduction rolls 16, respectively, and are arranged in the gaps between the light reduction rolls 16. The spray nozzle is connected to a cooling water supply having an adjustable water f: so that the cooling rate of the strand slab 30 can be controlled by increasing or decreasing the spray pattern in each zone. A crater end 36 of the strand slab 30 is located about 3/4 of the way from the upstream side of the light reduction zone, so that the strand slab 30 is completely solidified within the light reduction zone. Further, a plurality of pairs of straightening rolls 26 are arranged downstream of the group of light reduction rolls 16, so that the bends in the slab 30 can be straightened. Further, a cutting machine 37 is disposed on the downstream side of the straightening roll 26 to cut the straightened strand slab 30 into a straight slab into cut slabs 31 of a predetermined length. The cutting machine 37 is mounted on a movable cart so that it can move together with the slab 30 during cutting. A roller table (
(not shown) is provided so that the cut slab 31 is conveyed toward the slab yard on a roller table. A thickness measuring device 39 is provided in the slab yard, and its input side is connected to a probe 38 that generates ultrasonic waves. This probe 38 can mutually convert electric signals and ultrasonic waves, and generates reflected waves (bottom waves) from the cut slab 31.
is converted into an electrical signal and sent to the thickness measuring device 39. Further, the probe 38 is housed in a casing (not shown) and is provided so as to be movable along the width center of the cut slab 31 together with the casing in the longitudinal direction of the slab.

The output side of the thickness measuring device 39 is connected to the input side of the arithmetic unit 40. In the thickness measuring device 39, a voltage range corresponding to a predetermined allowable thickness range is set, and when a signal exceeding this set voltage range is input, a signal is sent to the arithmetic device 40. When a signal falling within the set voltage signal is input, the signal is not sent to the arithmetic unit 40. Further, the strength of the signal sent from the thickness measuring device 39 to the arithmetic unit 40 is proportional to the voltage difference between the upper limit value or lower limit value of the aforementioned set voltage range and the measurement signal. When the arithmetic unit 40 receives the signal from the thickness measuring device 39, it ranks its strength and counts the number of signals, and performs calculations based on these data using a multivariable formula model. It has become. A hydraulic control device 42 is connected to the arithmetic device 40 so that data can be input and output to and from each other. That is, a signal of the hydraulic pressure of the reduction cylinder 19 is input from the hydraulic control device 42 to the arithmetic device 40, while a hydraulic control signal obtained from this hydraulic pressure data and the thickness measurement data is input from the arithmetic device 40 to the hydraulic control device. 42. The hydraulic control device 42 has a pressure oil supply source (not shown), and upon receiving a hydraulic control signal from the arithmetic device 42, adjusts the solenoid valve of the pressure oil supply port based on this signal.

As shown in FIG. 2, the light reduction roll 16 has a length of 3
It is divided or divided into six parts. The diameter of the light reduction roll 16 has been increased from the conventional approximately 3751 to approximately 210+aa+, and the pitch between the rolls has been increased from the conventional approximately 420a+a+ to approximately 235.
1. The number of rolls in each segment has been increased from 5 pairs to 8.
Each pair has been changed. In addition, the length of the vertical drawing part in this continuous casting machine is approximately 4 m, the radius of curvature of the bent part of the slab is approximately 8 m s, and the height difference from the meniscus in the mold to the light reduction device 10 is approximately 1004 to 14.1 m. It is.

As shown in FIG. 3, the light reduction rolls 16 are supported by roll chocks 17, and a rod 18 of a reduction cylinder 19 is connected to each roll chock 17. When the roll chock 17 is pushed with the rod 18, the roll 16 is moved into a strand. It is pressed against the slab 30. A pressure oil supply port of a hydraulic control device 42 is connected to each reduction cylinder 19 through a hydraulic circuit. Furthermore, the input side of this hydraulic control device 42 is connected to the output side of the calculation device 40, and the hydraulic control device 4 is controlled based on the command signal from the calculation device 40.
2 at a predetermined pressure is supplied to each reduction cylinder 19.

Next, the case of producing slabs by the method of this invention will be specifically explained using FIGS. 1 to 3. The casting steel type is Nb, V-based high tensile strength steel for in-pipe (API X-
65), and the width of the cut slab 31 is approximately 1
950111, whose thickness is approximately 220+am. R.H.
Molten steel whose composition has been adjusted through degassing and ladle refining is cast from a tundish into a mold. At this time, the temperature of the molten steel in the tundish is about 1552°C, 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, it forms a solidified shell 3 in contact with the mold wall.
2 is formed. The unsolidified strand slab 30 is pulled out at a casting speed of about 0.75 m/min, and the crater end 3
6 is located approximately at the center of the segment 14 of the light pressure zone.

Pressure oil at a predetermined pressure is supplied from the hydraulic control device 42 to the reduction cylinder 19, and a reduction amount of approximately 1.2 ma/a+ is applied to the slab 30 by the light reduction roll 16. In addition, the flow control valve of the cooling water supply source was adjusted to control the amount of cooling water supplied to each spray nozzle, and each spray was sprayed in a spray pattern with a specific water amount of 0.54 g (spray water ff1 per 1 kg of slab). Cooling water is injected onto the strand slab 30 through a nozzle.

Then, the completely solidified strand slab 30 is straightened by a straightening roll 26, and then cut into a predetermined length by a cutter 37. The cut slab 31 is conveyed to the slab yard by a roller table, the probe 38 is moved along the width center of the cut slab 31, and the thickness of the slab 31 before being bloomed is measured. Measurement is performed using a device 39. For example, when the allowable thickness is set in the range of 218sn+ to 222cm, the measured thickness of one cut slab 31 is 22cm.
Assume that five signals corresponding to 2■ to 225■ are counted. Then, these measurement signals are sent from the thickness measuring device 39 to the calculation device 40, and based on the measurement signals, the degree of variation in center segregation is calculated using a predetermined mathematical model in the calculation device 40, and this degree of variation and the previous roll are calculated. The amount of correction of the amount of roll reduction is derived from the amount of reduction of 16. Then, a signal corresponding to this correction amount is sent to the hydraulic control device 42, and the solenoid valve of the pressure oil supply port is adjusted to control the amount of pressure oil supplied to the reduction cylinder 19. As a result, the amount of reduction of the roll 16 is adjusted, and an appropriate amount of strain is applied to the slab 30 in the final stage of solidification located near the crater end 36.

That is, as shown in FIG. 3, while casting is being performed while applying the reduction blade Po to the slab 30 from the roll 16, a flow force a is generated in the molten steel 34 near the crater end 36 in the bottom direction. If so, in accordance with the control signal from the arithmetic unit 40, the hydraulic control device 42 supplies each reduction cylinder 19 with pressurized oil whose amount has been increased by the correction amount from the previous amount of pressurized oil. Then, the rod 18 is pushed out and the roll 16 is pressed against the slab 30 more forcefully than before, and the rolling blade P is pressed against the slab 30.
A rolling blade P1 larger than o. P2 acts to increase the strain rate inside the slab 30. As a result, the strain rate of the solidified shell 32 increases, and a flow force in the top direction is generated in the molten steel 34 at the final stage of solidification, which cancels out the flow force a in the bottom direction, and the flow of the molten steel 34 is substantially prevented. , solidification precipitation of concentrated molten steel is prevented.

In Figure 7, the particle size distribution of semi-macro segregated particles is investigated by taking the particle size of the segregated particles existing in the segregation zone on the horizontal axis and the number of segregated particles per 100 cm of slab pulling length on the vertical axis. FIG. 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,
Particle size 0.0 that is harmful to HIC cracking. The number of semi-macro segregated particles of Sra mark or higher could be significantly reduced compared to the conventional method, and the effect of reducing central segregation could be achieved.

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 incidence on 24 cross sections by NACE test, no cracks with a length of 0.01 a+m or more were observed.

Furthermore, in the embodiment described above, since the diameter of the light reduction roll 16 and the pitch between the rolls are smaller than those of the prior art, 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.

Further, in the above embodiment, a thickness DJ meter using ultrasonic waves was used, but a laser or the like may also be used for thickness measurement. On the other hand, in the above embodiment, the thickness measuring device was provided in the post-process of the cutting machine, but the thickness measuring device could also be provided in the pre-process of the cutting machine.

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, the internal quality investigation results of the slab can be quickly reflected in the continuous casting operation using light reduction without using sulfur print, EPMA, or X-ray microanalyzer, etc. Center segregation of slabs can be effectively reduced.

[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.
Figure 6 is a graph showing the relationship between light reduction and flow length of molten steel, Figure 7 is a graph showing the effects of this invention, and Figure 8 is the effect of semi-macro segregation on hydrogen-induced cracking (HIC). FIG. 10; light reduction device, 16; light reduction roll, 19; reduction cylinder, 30; strand slab, 31; cut slab, 32
; Solidified shell, 34; Molten steel, 36; Crater end, 37;
Cutting machine, 38; Probe, 39; Thickness n1 regulator, 40; Arithmetic device, 42; Hydraulic control device. Applicant's agent Patent attorney Takehiko Suzue No. 2121 Figure 3 Figure 4 α Figure 5 Figure 6

Claims (1)

[Claims] In a light reduction casting method in which a continuously cast unsolidified strand slab is completely solidified while being lightly rolled down by a large number of pairs of rolls, the thickness of the strand slab is measured after the strand slab is completely solidified, and the measured thickness is 1. A light reduction casting method, characterized in that when the thickness exceeds a predetermined thickness tolerance range, the reduction amount of the rolls is adjusted to control the strain rate of the strand slab at the final stage of solidification.