TWI580496B - Continuous Casting of Steel - Google Patents

Description

Continuous casting method of steel

The present invention relates to a continuous casting method capable of suppressing segregation of components occurring at a central portion of a thickness of a continuous casting slab, that is, a steel capable of suppressing center segregation.

In the continuous casting of steel, in the final process of solidification, as the solidification shrinks, the unsolidified molten steel (referred to as "unsolidified layer") is attracted, thereby causing the unsolidified molten steel to flow toward the drawing direction of the cast piece. In this unsolidified layer, there are solute elements such as carbon (C), strontium (P), sulfur (S), and manganese (Mn) concentrated in the interior, and this concentrated molten steel is in the center of the cast piece. When it solidifies after flowing, the so-called center segregation phenomenon occurs. The main reason for the flow of the concentrated molten steel at the end of solidification, in addition to the above-mentioned solidification shrinkage, other reasons may be mentioned, for example, the bulging of the slab between the rolls due to the static pressure of the molten steel, the slab The roller positioning of the backup rolls is not aligned.

This central segregation will lead to deterioration of the quality of steel products, especially thick steel plates. For example, in the pipelines for oil transportation or natural gas transportation, due to the action of sour gas, the central segregation will be used as the starting point. A crack induced by hydrogen. In addition, the same problem occurs in marine structures, storage tanks, petroleum oil tanks, and the like. Moreover, in recent years, there are many situations in which the use environment of steel is required to be used in an extremely harsh environment at a lower temperature or a more corrosive environment. Therefore, the importance of reducing the center segregation of the cast piece is increasing. high.

Therefore, many technical solutions have been proposed, which are measures for reducing the center segregation of the cast piece or detoxifying it from the continuous casting process to the rolling process. In these technical solutions, it has been known that a continuous casting slab having an unsolidified layer inside and a "coagulation method at the end of solidification" which is rolled in a continuous casting machine is particularly effective in improving center segregation. . The term "light-pressing method at the end of solidification" as used herein means that a plurality of rolling rolls are disposed in the vicinity of the solidification end position of the slab, and these nip rolls are used for the slabs in continuous casting. The rolling speed of the degree of solidification shrinkage is gradually suppressed, and a method of suppressing the center segregation of the cast piece by suppressing the occurrence of voids in the center portion of the cast piece and suppressing the flow of the concentrated molten steel is suppressed.

In order to effectively utilize the coagulation method at the end of the solidification to prevent the occurrence of center segregation, the time point at which the slab is started and ended during the final clotting period during the final solidification period, and the enthalpy during that period are appropriately set. The amount of pressure is the most important, and technical solutions for various setting methods have been proposed.

For example, the technical solution disclosed in Patent Document 1 is a continuous casting method in which a continuous solidification portion of a continuous casting slab is applied, and a squeezing pressure is applied to the slab, which is: in the section where the squeezing pressure is applied, for the slab It The amount of rolling per unit time is set according to the surface temperature of the slab at the start of rolling and the thickness of the unsolidified layer of the slab at the rolling position.

In the technical solutions disclosed in Patent Document 2 and Patent Document 3, when the solid phase ratio of the center portion of the thickness of the medium-sized cast piece is changed to a temperature of 0.1 to 0.3, the solid phase ratio of the center portion of the thickness becomes a flow limit boundary. In the field from the time point of the temperature of the phase ratio, a continuous casting method in which continuous casting is performed by rolling a plurality of rolls while being pressed is characterized in that the solid phase ratio at the center portion of the thickness of the cast piece becomes higher. On the downstream side of the large casting direction, the rolling speed of the cast piece is set to be larger.

Further, the technical solution disclosed in Patent Document 4 is a continuous casting method for continuously casting a steel while applying a helium pressure to a cast piece in casting, which is characterized in that a cross section perpendicular to the longitudinal direction of the cast piece is formed. The information on the shape and the information on the shape of the unsolidified portion in the section are used to set or adjust the rolling conditions.

[Previous Technical Literature] [Patent Document]

Patent Document 1: Japanese Patent Laid-Open No. Hei 8-132203

Patent Document 2: Japanese Patent Laid-Open No. Hei 3-90263

Patent Document 3: Japanese Patent Laid-Open No. 3-90259

Patent Document 4: Japanese Laid-Open Patent Publication No. 2003-71552

However, the present inventors have obtained an empirical premise that in the continuous casting of the steel slab casting method, the method of applying the squeezing method at the end of solidification, if the thickness of the slab to be cast is different, it should be started. The time point of pressing and the time point at which the light rolling should be finished are not affected by the thickness of the cast piece and do not change, but in the range in which the helium pressure is applied to the cast piece (referred to as "light tapping belt") The optimum rolling speed will vary depending on the thickness of the cast piece.

The thickness of the steel slab is determined by the thickness of the steel product after rolling, and the rolling reduction (casting thickness/steel thickness) required for rolling to achieve the specifications of such a steel product. Therefore, if a new steel product specification is set, the thickness of the cast piece should be set according to the specifications. If the slabs of the set thickness are not casts previously cast using the coagulation method at the end of the coagulation process, it is necessary to re-set the 辗 which is most suitable for the thickness of the slab during the light squeezing. Pressure speed. Therefore, when performing the casting experiment using an actual machine that has set the rolling slope of a plurality of types of tapping belts, it is necessary to determine the optimum rolling slope every time, which takes a lot of time and expense. The problem is. In other words, the technical subject is to achieve: how to easily find: the optimum rolling slope in response to the light pressure of the thickness of the steel slab.

The term "squeezing slope" as used herein means that the interval between the rollers of the opposite rollers (referred to as "roller opening degree") is set such that the roller opening is narrowed toward the downstream side in the casting direction. The state is usually expressed by the amount of narrowing (mm/m) of the opening of the roller per 1 meter. will The value obtained by multiplying the rolling slope (mm/m) by the casting speed (m/min) is the rolling speed (mm/min).

Therefore, the inventors of the present invention have verified the availability of the aforementioned prior art documents from the viewpoint of the above-mentioned problems to be solved.

Patent Document 1 focuses on the thickness of the unsolidified layer of the cast piece as an index for effectively performing the light rolling pressure. According to the description of Patent Document 1, it is based on the idea that the more the rolling is performed on the downstream side of the casting, that is, the more the rolling is performed in the state where the thickness of the unsolidified layer of the cast piece is smaller, The ratio of the amount of rolling pressure set by the rolling roll to the solid-liquid interface of the cast piece (hereinafter referred to as "rolling efficiency") becomes smaller. However, according to the experience of the present inventors, the center segregation is more conspicuous in the field of the center portion of the slab having an unsolidified layer thickness of about 10 mm or less. According to the relationship between the thickness D of the unsolidified layer and the rolling speed required per unit time as shown in Fig. 1 of Patent Document 1, when the thickness of the unsolidified layer is 10 mm and 0 mm, the difference in the required rolling speed is as high as 10 %Degree. Further, in the [Examples] of Patent Document 1, only the test results of the thickness (250 mm) of one type of slab are described, and it is not known whether the optimum rolling conditions described in Patent Document 1 are different for the thickness of the slab. The situation is also valid.

In Patent Documents 2 and 3, the size of the cast piece provided for the test is three types, and the thickness x width thereof is 300 mm × 500 mm, 162 mm × 162 mm, and 380 mm × 560 mm, respectively, but both are with a medium cast (bloom) cast piece. The light squeezing cast related castings. Medium-sized cast In the case, the flatness ratio (width/thickness) of the cross section orthogonal to the drawing direction of the cast piece is smaller than the flat ratio of the steel blank cast piece, so the rolling efficiency at the end of solidification is less than that of the steel blank casting. sheet. Therefore, the setting of the amount of rolling is set to be closer to the end of solidification, and is about 2 to 3 times larger than the example of the steel slab of Patent Document 1. This rolling condition cannot be directly applied to the light rolling of the steel slab.

Further, in Patent Documents 1 to 3, the rolling inclination of the squeezing belt is changed along the drawing direction of the casting, so that the setting of the opening degree of the slab supporting roller is complicated, in order to be on the actual production machine. To achieve, the construction of the equipment has to be more complicated.

Patent Document 4 is directed to a medium-sized slab, and is configured to use a cross-sectional shape perpendicular to the longitudinal direction of the slab, that is, a width and a thickness of the slab to set a light rolling condition. However, the flatness ratio of the slab is used as a reference value, and the light squeezing condition is set according to the amount of change in the flat ratio of the unsolidified portion of the slab relative to the reference value, and the thickness value of the slab is not used. Set the rolling conditions. This is because in the case of a medium-sized slab, the shape of the uncured layer of the slab is different depending on the cooling ratio at the upper and lower surfaces of the slab in the continuous casting machine or the cooling ratio at the left and right surfaces of the slab. In the case where the direction is flat in the left-right direction and the case where the direction is flat in the up-and-down direction, Patent Document 4 is capable of performing the most appropriate tapping pressure regardless of any of the two cases. Its purpose.

In the case of the steel slab cast of the present inventors as a technical subject, the long side of the cast piece is much larger than the short side of the cast piece, and the flat side of the unsolidified layer The direction does not change, and it is always flat in the left and right direction of the cast piece. Therefore, the patent document 4 of the technical problem of the inventors of the present invention has little availability.

Therefore, any of the technical solutions of Patent Documents 1 to 4 cannot effectively solve the technical problems of the present inventors, and therefore it is necessary to develop a completely new technical solution.

The present invention has been developed in view of the above circumstances, and an object thereof is to provide a condition that can be set in accordance with the thickness of a steel slab, thereby preventing segregation of the center of the slab due to insufficient squeezing pressure. The occurrence of steel, and a continuous casting method for preventing the occurrence of cracks in the slab due to excessive rolling.

The gist of the technical content of the present invention for solving the above problems is as follows.

[1] A continuous casting method for steel, wherein the cast piece has a thickness of 160 to 350 mm, a width of 1600 to 2400 mm, and a flat ratio (width/thickness) of 4 to 15 from the cast piece. When the temperature reaches: the temperature at which the solid phase ratio at the center portion of the thickness of the slab becomes 0.1, the temperature of the slab reaches: the time at which the solid phase ratio at the center portion of the slab thickness becomes the temperature at the flow limit solid phase rate In the field of the present invention, a continuous casting method for continuously casting steel is provided by a plurality of slab-supporting roller pairs which are capable of applying a squeezing pressure to a slab, and is subjected to rolling.

The thickness of the cast piece of the cast object, the rolling slope of the above-mentioned tapping belt, and the drawing speed of the cast piece are in accordance with the following formula (1) and the relationship of the following formula (2): 0.3 / (V × α) <Z<1.5/(V×α). . . Number (1)

α=β×(D/Do)+γ. . . Number (2)

However, in the formula (1) and the formula (2), V is the drawing speed (m/min) of the cast piece, α is the thickness coefficient (no dimension), and Z is the rolling slope (mm). /m), D is the thickness (mm) when the cast piece of the casting object is directly under the mold, and Do is the thickness (mm, Do = 187 mm) when the reference cast piece is directly under the mold, and β and γ are The coefficient determined according to the width W (mm) of the cast piece of the cast object is different depending on the width range W of the following cast piece, and has different values: when 1600 ≦ W ≦ 1800, β = -0.61, γ =1.54

When 1800<W≦2000, β=-0.60, γ=1.57

When 2000<W≦2200, β=-0.58, γ=1.58

When 2200<W≦2400, β=-0.53, γ=1.54

[2] The continuous casting method of steel according to the above [1], wherein the thickness of the cast piece to be cast and the total rolling amount of the cast piece are in accordance with the following formula (3): Rt < (D) /Do)×(10/α). . . Equation (3)

However, Rt in the formula (3) is the total amount of rolling (mm) of the cast piece, D is the thickness (mm) when the cast piece of the casting object is directly under the mold, and Do is the reference cast piece. The thickness (mm, Do = 187mm) when the mold is directly under the mold, α is the thickness coefficient (no time yuan).

According to the present invention, in order to reduce the center segregation of the steel slab, a continuous casting of the steel slab is performed for the slab in continuous casting by applying a rolling amount corresponding to the amount of solidification shrinkage in the squeezing belt. In the case of the above formula (1) and the formula (2), the rolling condition is set such that the thickness of the cast piece of the casting object, the rolling slope of the tapping belt, and the drawing speed of the cast piece are in accordance with the relationship of the above formula (1) and the formula (2). Within the scope. In this way, even when the thickness of the slab is different, it is not necessary to perform a real machine experiment composed of a plurality of different levels, which takes a lot of time and cost, and can easily find the most suitable rolling condition. It is very quick to respond to the requirements for the manufacture of steel products of various specifications, which can bring about industrially beneficial effects.

1‧‧‧Steel embryo continuous casting machine

2‧‧‧feed trough

3‧‧‧Sliding nozzle

4‧‧‧Immersion nozzle

5‧‧‧ mould

6‧‧‧ Casting roll

7‧‧‧Handling rollers

8‧‧‧ Casting cutting machine

9‧‧‧Fused steel

10‧‧‧ cast

11‧‧‧ solidified shell

12‧‧‧Unsolidified layer

13‧‧‧End of solidification

14‧‧‧Light pressure belt

15‧‧‧辗压段

16‧‧‧ frame

17‧‧‧ Connecting rod

18‧‧‧ disc spring

19‧‧‧ worm drive

20‧‧‧Motor

21‧‧‧ Roller bearing housing

Fig. 1 is a schematic side view showing a steel continuous casting machine used in the practice of the present invention.

Fig. 2 is a schematic view showing an example of a rolling section of a tapping belt which constitutes a continuous casting machine for a steel preform, and is a schematic view when viewed from the side of the continuous casting machine.

Fig. 3 is a schematic view showing a rolling section shown in Fig. 2 when viewed from the casting direction of the cast piece, that is, a schematic view of a cross section orthogonal to the casting direction.

The invention will be specifically described below with reference to the accompanying drawings. Fig. 1 is a schematic side view showing a steel continuous casting machine used in the practice of the present invention.

As shown in Fig. 1, in the steel continuous casting machine 1, a mold 5 for inserting the molten steel 9 therein to solidify to form the outer shape of the cast piece 10 is provided. At a predetermined position above the mold 5, a molten steel 9 supplied from a ladle (not shown) is supplied to the feed tank 2 of the mold 5, and is provided at the bottom of the feed tank 2 A slide nozzle 3 for adjusting the flow rate of the molten steel 9 is provided, and an impregnating nozzle 4 is provided below the slide nozzle 3.

On the other hand, below the mold 5, a plurality of pairs of slab-supporting rolls 6 composed of a supporting roller, a guiding roller, and a pinch roller are provided. The gap between the adjacent slab-supporting rolls 6 in the casting direction is a secondary cooling belt constituting a spray nozzle (not shown) in which a water mist nozzle or an air droplet nozzle is disposed, and is used for secondary cooling. Cooling water (also referred to as "secondary cooling water") sprayed by the spray nozzle of the belt, and the cast piece 10 is cooled while being pulled. Further, a plurality of transport rollers 7 for transporting the cast slab 10 after casting are provided on the downstream side of the final slab support roll 6 in the casting direction, and are disposed above the transport roller 7 for use. The caster cutting machine 8 of the cast piece 10a of a predetermined length is cut out from the cast piece 10 after casting.

In the upstream side and the downstream side of the casting direction, which are separated from the solidification end position 13 of the cast piece 10, it is provided by a plurality of pairs of castings. The squeezing belt 14 formed by the sheet supporting roller group, and the plurality of slab supporting roller groups are intervals between the slab supporting rollers that face each other with the slab 10 interposed therebetween (this interval is referred to as " The roller opening degree is set to be sequentially narrowed toward the downstream side in the casting direction, that is, the plurality of pairs of the slab supporting roller groups have been set to the rolling inclination (the state of the roller opening degree is set to: Narrowing toward the downstream side of the casting direction). The tapping belt 14 is capable of utilizing the entire field or selected portions of the field to perform a light press on the cast piece 10. Further, a spray nozzle for cooling the cast piece 10 is disposed between the slab support rolls of the tapping press belt 14. Here, the slab support roll 6 disposed on the squeezing belt 14 is also referred to as a squeezing roll.

Generally, the rolling inclination means: the reduction amount of the roller opening degree per metre in the casting direction, in other words, expressed by "mm/m", so the slab 10 at the squeezing belt 14 is The rolling speed (mm/min) can be obtained by multiplying this rolling inclination (mm/m) by the casting speed (m/min).

In the steel continuous casting machine 1 shown in Fig. 1, the tapping press belt 14 is formed by joining three sets of rolling sections in the casting direction, and the rolling section is supported by three pairs of cast pieces. Roller 6 constitutes a set of rolling sections. However, in the present invention, the tapping belt 14 is not necessarily constituted by three sets of rolling sections, and is used to constitute the rolling section of the tapping belt 14, which may be a single group or two groups, or even So four or more groups are fine. Further, in the steel continuous casting machine 1 shown in Fig. 1, each of the rolling sections is constituted by three pairs of slab supporting rolls 6, but a slab supporting roll for constituting a set of rolling sections. The number of 6 can be as long as it is two pairs or more. To.

Figs. 2 and 3 show an example of a rolling section constituting the tapping belt 14. The example shown in Fig. 2 and Fig. 3 is an example in which five pairs of slab support rolls 6 as rolling rolls are disposed in one rolling section 15, and Fig. 2 is viewed from the side of the continuous casting machine. In the schematic view of the time, FIG. 3 is a schematic view when viewed from the casting direction of the cast piece, that is, a schematic view of a cross section orthogonal to the casting direction.

As shown in FIG. 2 and FIG. 3, the rolling section 15 is constituted by the roller bearing housing 21 holding one of the five pairs of slab supporting rollers 6 against the frame body 16 and the frame body 16'. Further, a total of four connecting rods 17 (on both sides on the upstream side and on both sides on the downstream side) that penetrate the frame body 16 and the frame body 16' are disposed. By driving the worm driving device 19 provided on the connecting rod 17 by the motor 20, the interval between the frame 16 and the frame 16' can be adjusted, that is, the rolling slope at the rolling section 15 can be adjusted. . In this case, the roller opening degrees of the five pairs of slab-supporting rolls 6 disposed in the squeezing section 15 are adjusted together.

The worm drive unit 19 is constructed such that, during casting, the worm drive unit 19 is self-locking against the static pressure of the molten steel of the slab 10 having an unsolidified layer to counter the bulging force of the cast piece 10, The adjustment of the rolling slope can be performed under the condition that the slab 10 is not present, that is, the load from the slab 10 does not act on the slab support roll 6 provided in the squeezing section 15. The amount of movement of the casing 16' achieved by the worm driving device 19 is measured and controlled in accordance with the number of rotations of the worm driving device 19, so that the rolling slope of the rolling section 15 can be known.

Further, on the connecting rod 17, a disc spring 18 is provided between the frame body 16' and the worm driving device 19. The disc spring 18 is not constituted by a single disc spring, but is formed by stacking a plurality of disc springs (the more the disc springs overlap, the higher the rigidity). The disc spring 18 is formed such that if no load load exceeding a predetermined load acts on the disc spring 18, it does not shrink but exhibits a certain thickness when there is a predetermined load load. Shrinkage begins to occur, and after a given load is applied, it contracts in proportion to the load.

For example, when the cast piece 10 is in the range of the rolling section 15, and the solidification is completed, the rolling section 15 is subjected to an excessive load by rolling the cast piece 10 after the solidification is completed. When such an excessive load is applied, the disc spring 18 will contract, so that the frame 16' will be opened, that is, the roller opening will be enlarged, and thus, the rolling section 15 will be Will not bear too much load. Further, the frame 16 on the lower side is formed to be fixed to the base of the continuous casting machine and does not move during casting.

Although not shown, the slab support roll 6 disposed other than the slab support roll of the squeezing belt 14 is also configured to form a squeezing section.

Since the squeezing belt 14 shown in Fig. 1 has such a squeezing section structure, the roller opening degrees of the three pairs of slab-supporting rolls 6 disposed in the respective squeezing sections are also adjusted together. In this case, the amount of movement of the upper frame (corresponding to the frame 16') achieved by the worm drive device is Since the measurement and control are performed based on the number of rotations of the worm drive device, the rolling slope of each of the rolling sections can be known.

In the steel slab continuous casting machine 1 of such a configuration, the molten steel 9 injected into the mold 5 from the feed tank 2 via the immersion nozzle 4 is cooled in the mold 5 to form the solidified casing 11 and becomes internal. The cast piece 10 having the unsolidified layer 12 is continuously pulled toward the lower side of the mold 5 while being supported by the cast piece backup roll 6 provided below the mold 5. The slab 10 receives the cooling of the secondary cooling water of the secondary cooling zone while passing through the slab support roller 6, so that the thickness of the solidified casing 11 is increased, and the side of the tamping belt 14 is pressed. The solidification inside the cast piece is finished at the solidification end position 13. The cast piece 10 after the solidification is completed is cut by the caster cutting machine 8 to become the cast piece 10a.

In the present invention, at the time of the light embossing tape 14, at least from the time point when the solid phase ratio of the central portion of the slab thickness becomes 0.1 temperature, the solid phase ratio at the center portion of the slab thickness becomes the flow boundary solid phase ratio. The cast piece 10 is rolled up at the time point of the temperature. Since the flow boundary solid phase ratio is considered to be 0.7 to 0.8, the cast piece 10 is rolled until the solid phase ratio at the center portion of the cast piece thickness becomes 0.7 to 0.8. Therefore, as long as the rolling is continued until the solid phase ratio at the center portion of the thickness of the cast piece becomes 0.8 or more, there is no problem. Since the solid phase ratio at the center portion of the thickness of the cast piece exceeds the flow limit solid phase ratio, the unsolidified layer 12 does not move, so that it is meaningless even if the light pressure is applied. Of course, although the effect of the light pressure is not obtained, the light pressure can be performed even after the flow rate is exceeded. On the other hand, even at the center of the thickness of the slab, the solid phase ratio exceeds After 0.1, the light rolling pressure was started. Before this, there was a possibility that the concentrated molten steel flowed, and thus center segregation occurred, and the effect of sufficiently reducing the center segregation could not be obtained. Therefore, when the solid phase ratio at the center portion of the thickness of the cast piece becomes 0.1, the tapping is started.

The solid phase ratio at the center of the thickness of the slab can be obtained by calculation of the heat transfer solidification of the secondary element. The solid phase ratio referred to herein is defined as the solid phase ratio = 0 at the start of solidification, and is defined as the solid phase ratio = 1.0 at the end of solidification, and the solid phase ratio at the center portion of the slab thickness becomes 1.0. It is equivalent to the solidification end position 13.

At the end of solidification of the molten steel 9, the cast piece 10 is lightly pressed at a predetermined rolling speed, whereby the center segregation of the cast piece 10 can be reduced, which is widely known. However, when the kneading is performed, the deformation of the solidified casing 11 due to the rolling is sometimes caused that the amount of rolling transmitted to the solidification interface of the cast piece 10 is smaller than the amount of rolling applied to the surface of the cast piece. Therefore, there is a case where the rolling speed cannot be controlled as originally set. Here, the ratio of the amount of rolling transmitted to the solidification interface of the cast piece 10 relative to the amount of rolling applied to the surface of the cast piece (the amount of rolling transmitted to the solidified interface / the amount of rolling applied to the surface of the cast piece) It is called: rolling efficiency.

The main factor affecting the magnitude of this rolling efficiency, particularly the thickness of the solidified casing 11, is large, and the greater the thickness of the solidified casing 11, the smaller the rolling efficiency. In other words, the tapping pressure applied to the cast piece 10 is performed at the end of solidification, and the thinner the cast piece 10 having a larger outer shape, the thickness of the solidified casing 11 when the tapping is performed. The greater the degree becomes, the less the rolling efficiency becomes when the light pressure is applied. The outer thickness of the cast piece 10 is determined by the thickness along the short side of the mold in the cavity (the inner space of the mold) at the exit of the mold.

The present inventors have focused on the case where the width of the cast piece is kept constant at 2100 mm, but the cast piece 10 whose thickness of the cast piece is changed in the range of 160 to 350 mm is continuously cast, based on the desire to know: Which kind of slab thickness can be reduced by using the best (suitable) rolling conditions to reduce the center segregation. First, use the casting experiment of the actual casting machine to find: for a 200mm thick slab 10 The optimum (suitable) range of the rolling slope at the tapping belt 14 during continuous casting. As a result, it was found that the optimum (suitable) rolling slope in the case where the thickness of the cast piece 10 was 200 mm was within the range of the following formula (4).

0.3/V<Z<1.5/V. . . Equation (4)

However, in the formula (4), V is the casting speed (m/min) of the cast piece, and Z is the rolling slope (mm/m).

Next, in order to introduce the correction amount for the influence of the thickness of the cast piece 10 which affects the rolling efficiency, the formula (4) is carried out in the thickness of the cast piece of 160 to 350 mm. Numerical simulation calculations relating to deformation of the cast piece 10 at the time of rolling. Then, from the result of the simulation calculation, the relationship between the thickness of the cast piece 10 and the rolling efficiency is obtained, and is derived according to the following formula (5): a thickness coefficient α (no dimension), which is a primary element of the thickness of the cast piece. Approximate formula.

α=-0.58×(D/Do)+1.58. . . Equation (5)

However, in the formula (5), D is the thickness (mm) when the cast piece of the casting object is directly under the mold, and Do is the thickness (mm) of the reference cast piece just below the mold.

The value of the thickness coefficient α is such that the larger the thickness D of the slab is, the smaller the value becomes. This shows the fact that the rolling efficiency becomes smaller as the thickness D of the cast piece becomes larger. Further, the thickness Do of the reference cast piece position immediately below the mold, the thickness of the cast piece when the thickness coefficient α shown by the coefficient formula (5) becomes 1, and the case of the steel blank cast piece having a width of 2100 mm, the Do system 187mm.

Because the thickness of the cast piece 10 of the cast object is the reference thickness The difference of 187 mm is based on the change in the thickness of the cast piece, and the rolling efficiency is changed by the ratio shown by the formula (5). In the present invention, the amount of change in the rolling efficiency which is changed in accordance with the change in the thickness of the cast piece is compensated by adjusting the rolling slope of the tapping belt 14. Specifically, if the rolling efficiency is small, the rolling inclination is increased. Conversely, if the rolling efficiency is increased, the rolling inclination is reduced, and the variation of the rolling efficiency is compensated by this method. . In other words, the thickness coefficient α shown in the formula (5) is introduced into the formula (4), and the following formula (1) is obtained as: the drawing speed of the cast piece, the thickness coefficient α, and the rolling slope. Relationship.

0.3/(V×α)<Z<1.5/(V×α). . . Number (1)

In the case of a cast piece width of 2100mm, the cast piece thickness is 160~ In the case where the 350 mm cast piece 10 is continuously cast, the equations (1) and (5) obtained in the above manner are used, whereby the increase or decrease in the thickness of the cast piece can be prevented. The change in the rolling efficiency can prevent center segregation and pores in the cast piece 10, and can prevent the occurrence of inverted V-shaped segregation or internal cracks in the cast piece 10 due to excessive rolling.

However, the thickness coefficient α of the formula (5) is the value of the slab 10 when the slab width is kept constant at 2100 mm, and on the other hand, the width of the slab 10 cast in the steel slab continuous casting machine 1 is In a wide range of 1600 ~ 2400mm. Therefore, the thickness coefficient α is obtained for the entire range of the cast piece in which the thickness of the cast piece 10 is 160 to 350 mm, the width is 1600 to 2400 mm, and the flat ratio (width/thickness) is 4 to 15.

The main body of the rolling resistance when the tapping belt 14 is lightly pressed is the portion where the short side of the cast piece has been solidified. If the absolute value of the length of the slab in the width direction of this portion is the same as the thickness of the slab 10, the rolling resistance tends to be almost equal to the width of the slab 10 irrespective of the width. Further, if there is still a range in which the unsolidified layer 12 exists inside, since there is still the unsolidified layer 12, the rolling resistance is small, and the both ends of the short side of the cast piece have solidified. Compared with the parts, it can be said to be small enough to ignore.

In other words, for example, in the case of a slab having a width of 1600 mm, the ratio of the portion on the short side of the slab that has solidified to the width of the slab is larger than the case where the width is 2100 mm. As a result, the rolling resistance of a cast piece having a width of 1600 mm is greater than the rolling resistance of a cast piece having a width of 2100 mm. Therefore, if the slab having a width of 1600 mm and the slab having a width of 2100 mm have the same slanting slope set by the squeezing belt 14, the slab having a width of 1600 mm has a reaction effect of the rolling resistance. The force will be higher than the set stress of the disc spring 18, and the roller opening will be enlarged, which may occur: the actual rolling slope becomes smaller than the originally set compression slope.

Therefore, for the case where the slab width is 1700 mm, 1900 mm, or 2300 mm, the same numerical simulation calculation as that performed on the slab having a width of 2100 mm is performed to obtain the thickness coefficient α. The thickness coefficient α is expressed by the following formula (2) in which β and γ are the coefficients of the slab width W (mm) to be cast.

α=β×(D/Do)+γ. . . Number (2)

According to the results of the numerical simulation calculation, the coefficient β and the coefficient γ in the equation (2) are the following values in accordance with the width W (mm) of the cast piece to be cast.

When 1600≦W≦1800, β=-0.61, γ=1.54

When 1800<W≦2000, β=-0.60, γ=1.57

When 2200<W≦2400, β=-0.53, γ=1.54

Here, when 2000 < W ≦ 2200, as shown in the formula (5), β = -0.58 and γ = 1.58.

In addition, the thickness Do of the reference cast piece in the formula (2) is directly below the mold, and the steel blank cast in the range of 1600 to 2400 mm, regardless of the width of the steel blank cast piece In the same manner as in the case of a steel slab having a width of 2,100 mm, it is set to 187 mm.

However, although the light rolling pressure has an effect of preventing the flow of the concentrated molten steel in the final solidified portion, sometimes the cast piece 10 is deformed due to rolling, and internal cracks occur at the solidification interface. It is known that such internal cracks occur as long as the cumulative value of the deformation at the solidification interface reaches a certain value or more.

Therefore, the inventors of the present invention conducted an investigation by using a real machine test on the relationship between the total amount of pressure applied to the cast piece 10 and the presence or absence of internal cracks due to the light pressure. As a result, it is confirmed that the total amount of the slab 10 and the thickness of the slab to be cast are preferably in accordance with the following equation (3).

Rt<(D/Do)×(10/α). . . Equation (3)

However, Rt in the formula (3) is the total amount of rolling (mm) of the cast piece.

That is, in the present invention, it is necessary to reduce the thickness of the cast piece 10 of the casting object, the rolling slope of the tapping belt 14, and the drawing speed of the cast piece so as to fall within the above formula (1) and the formula (2). In the range of the relationship, continuous casting is performed by setting the rolling conditions, and in the case of continuous casting, it is more preferable to increase the total rolling amount of the cast piece 10 and the thickness of the cast piece to be cast. The degree is set within a range that satisfies the relationship of the above formula (3).

Further, in various casting conditions of the continuous casting operation, the thickness of the solidified casing 11 and the solid phase ratio of the central portion of the thickness of the cast piece are determined in advance using the secondary element heat transfer solidification calculation, and is the entry into the tapping belt 14 At the time point, the solid phase ratio at the center portion of the thickness of the slab tends to be 0.1 or less, and the solid phase ratio at the center portion of the thickness of the slab at the time of leaving the squeezing belt 14 tends to be higher than the flow limit solid phase ratio. The way to adjust the amount of secondary cooling water or the drawing speed of the cast piece.

As described above, according to the present invention, the thickness of the cast piece 10 of the casting object, the rolling slope of the tapping belt 14, and the drawing speed of the cast piece fall within the above formula (1) and the formula ( Since the rolling conditions are set in a manner within the relationship of 2), even if the thickness of the cast piece 10 is different, the optimum (suitable) rolling conditions can be easily obtained, and the corresponding conditions can be quickly and varied. Specifications for the manufacture of steel products.

[Examples]

Hereinafter, the present invention will be described in more detail based on examples.

The continuous casting machine used in the test was the same as the continuous casting machine 1 shown in Fig. 1. Casting of low carbon aluminum killed steel is carried out using this continuous casting machine. Table 1 shows the casting conditions at the thickness of three cast pieces of 200 mm, 250 mm, and 300 mm in the continuous casting method of the embodiment of the present invention, and the degree of center segregation and the presence or absence of pores in the cast piece after casting. The result of the investigation of the presence or absence of internal cracks. In addition, it is also shown in Table 1 that it falls within the scope of the present invention at various slab thicknesses. Other conditions, casting conditions and investigation results of the test conducted as a comparative example. The width of the cast piece was 2100 mm in all tests.

The center segregation degree of the cast piece used in the evaluation of the test was measured in accordance with the following method. That is, in the cross section orthogonal to the drawing direction of the slab, the carbon concentration is analyzed at equal intervals along the thickness direction of the slab, and the maximum value of the analysis value in the thickness direction is regarded as C _{max .} The carbon concentration analyzed by the molten steel taken out from the feed tank during casting is regarded as C ₀ , and C _max /C _{0 is} taken as the central segregation degree. Therefore, the closer the center segregation degree is to 1.0, it is that it is a good cast piece with less central segregation. In the present invention, the cast piece having a center segregation degree of 1.10 or more is judged to be inferior in the degree of center segregation.

The pores and the internal cracks of the slab are microscopically observed in the vicinity of the center portion of the thickness of the slab in a cross section orthogonal to the drawing direction of the slab to determine the presence or absence of pores and internal cracks.

The drawing speed of the slabs of the slabs of different thicknesses is in the range of the solid-phase ratio at the center of the thickness of the slab at a solid-phase ratio from 0.1 to the end of the flow-bounding solid-phase ratio. The setting is performed, and test numbers 1 to 3, test numbers 6 to 8, and test numbers 11 to 13 are set, and the rolling slope is set in such a manner as to satisfy the above equations (1) and (2). . In addition, in Test Nos. 4, 9, and 14 which were carried out as comparative examples, the rolling slope was set to be the best (more suitable for the rolling inclination defined by the equations (1) and (2). The upper limit of the range. Further, in Test Nos. 5, 10, and 15, the rolling slope was set to be lower than the lower limit of the optimum (suitable) range of the rolling slope defined by the formulas (1) and (2). Further, in Test Nos. 4 and 9, the rolling slope was set such that the total rolling amount exceeded the upper limit of the formula (3).

It can be seen from the center segregation degree shown in Table 1 that the test numbers 1 to 3, the test numbers 6 to 8, and the test numbers 11 to 13 which fall within the scope of the present invention have a central segregation degree of less than 1.10, which is good. . Moreover, no pores and internal cracks were observed in the cast piece.

Test No. 4, which was carried out as a comparative example, although the optimum rolling slope obtained by the equations (1) and (2) was 0.2 to 1.1 mm/m, but the inclination was too large. , is 1.5mm/m, so The center segregation exceeded 1.10. Moreover, the total amount of rolling is too large, and internal cracks occur in the cast piece. Similarly, in Test Nos. 9 and 14, the rolling inclination was too large, and the center segregation degree was high, and it was confirmed that some of the inverted V-shaped segregation occurred.

Further, in Test No. 15, the optimum rolling slope obtained by the equations (1) and (2) was 0.6 to 3.1 mm/m, but the rolling slope was set to 0.5 mm/m. Therefore, the inclination of the rolling is insufficient, and the center segregation degree exceeds 1.10, and it is also observed that there are pores inside the cast piece. Similarly, in Test Nos. 5 and 10, the rolling slope was too small, and the degree of center segregation was poor.

1‧‧‧Steel embryo continuous casting machine

2‧‧‧feed trough

3‧‧‧Sliding nozzle

4‧‧‧Immersion nozzle

5‧‧‧ mould

6‧‧‧ Casting roll

7‧‧‧Handling rollers

8‧‧‧ Casting cutting machine

9‧‧‧Fused steel

10‧‧‧ cast

10a‧‧‧ cast

11‧‧‧ solidified shell

12‧‧‧Unsolidified layer

13‧‧‧End of solidification

14‧‧‧Light pressure belt

Claims (2)

A continuous casting method for steel, wherein the slab has a thickness of 160 to 350 mm and a width of 1600 to 2400 mm, and a slab having a flatness ratio (width/thickness) of 4 to 15 reaches the temperature of the slab: When the solid phase ratio at the center portion of the slab thickness becomes 0.1, the temperature of the slab reaches the time point when the solid phase ratio at the center portion of the slab thickness becomes the temperature at the flow limit solid phase rate. A continuous casting method for continuously casting steel by applying a plurality of slab-supporting roller pairs to which a squeezing pressure is applied to a slab, and applying a squeezing pressure, characterized in that the cast object is cast The thickness, the rolling slope of the aforementioned tapping belt, and the drawing speed of the slab are in accordance with the following formula (1) and the relationship of the following formula (2): 0.3 / (V × α) < Z < 1.5 / (V×α). . . Equation (1) α = β × (D / Do) + γ. . . In the formula (1) and the formula (2), V is the drawing speed (m/min) of the cast piece, α is the thickness coefficient (no dimension), and Z is 辗. The degree of pressure gradient (mm/m), D is the thickness (mm) when the cast piece of the casting object is directly below the mold, and Do is the thickness (mm, Do = 187 mm) when the reference cast piece is directly below the mold. , β and γ are coefficients which are determined according to the width W (mm) of the cast piece of the casting object, and which have different values depending on the following range of the width W of the cast piece: when 1600 ≦ W ≦ 1800, β =-0.61, γ=1.54 When 1800<W≦2000, β=-0.60, γ=1.57 When 2000<W≦2200, β=-0.58, γ=1.58 When 2200 < W ≦ 2400, β = -0.53, γ = 1.54. The continuous casting method of the steel according to the first aspect of the invention, wherein the thickness of the cast piece of the casting object and the total rolling amount of the cast piece are in accordance with the relationship of the following formula (3): Rt<(D/ Do) × (10 / α). . . In the formula (3), Rt in the formula (3) is the total rolling amount (mm) of the cast piece, and D is the thickness (mm) indicating that the cast piece of the casting object is directly under the mold, and Do It is the thickness (mm, Do = 187 mm) when the reference cast piece is directly under the mold, and α is the thickness coefficient (no dimension).