JPH0763827B2 - Continuous casting method for steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to carbon steel, low alloy steel, stainless steel, and other iron-based alloys (these are referred to as “ (Collectively referred to as "steel").

(Conventional technology) The continuous casting method has many advantages such as efficiency of casting work, energy saving, and improvement of yield, and is one of the modern steel production technologies indispensable. The continuous casting method of steel has been continuously improved since its origin, and is almost finished, but it still has some unsolved problems. One of them is a fine crack generated inside the slab.

In continuous casting, the molten metal supplied into the mold is cooled by the mold to form a solidified shell, which is gradually cooled by water spray while being supported by the support rolls to gradually solidify and finally complete. A method of cutting the solidified slab into a predetermined length is adopted. The slab is pulled out by sandwiching the slab with opposing support rolls called pinch rolls, rotating the roll while applying pressure to the slab, and pulling it downward. At this time, there is an unsolidified portion inside the slab, bending of the slab during continuous casting, straightening, bulging, misalignment, and tearing of the solidification interface due to tensile strain due to thermal stress, etc. It is believed that internal cracking occurs in the.

Therefore, various techniques for preventing the occurrence of internal cracks have been proposed as follows, except for the individual factors that cause the tensile strain as described above. It should be noted that since the tensile strain is all that matters in the present invention, the tensile strain is simply referred to as “strain” in this specification.

JP, 59-197365, A "Bill guide straightening device" This is a device in which rolls are arranged so that the amount of change in curvature of the straightening profile for each roll increases toward the latter stage of the straightening zone. As a result, the strain (correction strain) generated in the correction region is dispersed and internal cracks can be prevented.

Japanese Patent Application Laid-Open No. 59-118254 "Bending / rectifying crack prevention device in continuous machine" Upstream of bending portion or straightening portion of curved casting machine and / or
Alternatively, it is a device in which a drive mechanism and a control mechanism are provided in the solidified shell support mechanism on the downstream side where tensile strain acts, respectively. As a result, compressive strain is applied to the solidified shell on the side where the tensile strain is working, so that the tensile strain is relaxed and solidified shell cracking can be prevented.

Iron and Steel, 73 (1987), S909 "Development of online internal crack diagnosis and prevention system" This is to diagnose cracks by determining the amount of strain for each roll and comparing it with the critical strain of cracks. Is the way. As a crack prevention method, a compressive strain is applied to the solidified shell of a slab corresponding to the roll position where the crack generation is diagnosed to reduce the strain and at the same time reduce the casting speed.

Materials and Processes, vol.1 (1988), P161 "Mold removal heat and secondary cooling conditions during high-speed casting" This technology, as the casting speed is increased, the bulging strain of each part in the longitudinal direction of the slab depends on Grows up to
At the time of high-speed casting of 2.0 m / min, there is a part that exceeds the critical strain for cracking, and based on the knowledge that internal cracking occurs at that part, it is to strengthen the cooling in the vicinity and prevent internal cracking. is there. However, there is no proportional relationship between the strength of the secondary cooling and the solidification rate, and the effect of improving the solidification rate is small even if the strong cooling is performed to a certain degree or more. There is a limit to this method in that it lowers and causes surface cracking. Therefore, the following technique has been proposed focusing on the fact that the bulging strain greatly changes depending on the support interval of the support rolls.

Japanese Patent Laid-Open No. 50-62128 "Method of supporting and guiding slab in secondary cooling zone of continuous casting apparatus" As an alternative to the slab support roll, a fixed supporter having a surface with a constant interval parallel to the surface of the slab and casting By providing a plurality of rollers in the gap with one surface, the support interval of the slab is reduced and bulging is suppressed.

Now, while the unsolidified portion exists inside the slab during continuous casting, cracks occur at the solidification interface even when the pressure of the support roll is inappropriate, for example, when the slab is overpressurized. In particular, in high-speed casting, which is a recent trend, the range of the unsolidified portion is widened, so that the risk region where such cracking occurs is widened, and the demand for proper pressure adjustment of the support rolls is becoming more severe. Also, when the casting speed changes at the start and end of casting, or when there is a temporary change in casting speed due to breakout prediction, unbalanced heat is generated in the circumferential direction of the roll and the roll bends. Even if the pressure of the roll is appropriate, there is a risk of over-compression of the slab and internal cracking.

The following methods have been proposed as measures against these problems.

JP-B-62-39065 "Roll bending correction method for continuous casting machine" JP-A-63-303668 "Continuous slab reduction control device" The various internal crack prevention measures proposed so far have been mentioned. However, each has the following drawbacks.

The above-mentioned is a corrective strain, is a bending strain and a corrective strain, is mainly focusing on misalignment and strain, and is a bulging strain, respectively, and shows a preventive measure against internal cracks caused by these strains. Also asks for distortion between rolls or for each roll and discusses the magnitude.

The method does not suggest how long the rollers or split rolls should be arranged at what pitch. In order to reduce the bulging strain, these aim to shorten the diameter or length of the rollers or rolls as much as possible, and to shorten the distance between the rollers or rolls. However, if the distance between the rollers and rolls is shortened, the influence of the misalignment strain history increases as the number of times the slab passes through the rollers and rolls increases, and the misalignment strain itself for each roller or roll also increases. Therefore, the effect of preventing internal cracks may not be obtained depending on the arrangement method of the rollers or rolls.

Is a method of externally cooling the roll to prevent bending of the roll, but since the cooling water is also applied to the slab, the temperature of the slab becomes uneven and the surface quality deteriorates. Further, not only elastic deformation due to thermal stress but also plastic deformation due to roll pressure occurs after long-term use, so roll bending cannot be eliminated only by cooling the roll. Further, since the pinch roll pressing pressure is usually constant regardless of the fluctuation of the casting speed, when roll bending occurs, overpressure and insufficient pressure on the slab are repeated.

Is a method of supporting the solidified shell of the slab with a surface instead of supporting the slab by the roll as described above. However, this is a method for preventing the center segregation by reducing the surface pressure only at the final solidification position, and is not a support method for preventing a wide range of misalignment.

(Problems to be Solved by the Invention) Although various preventive measures against internal cracks in continuously cast slabs have been proposed as described above, there are still many problems in trying to increase the casting speed. As the casting speed increases, various distortions are applied to the slab in a state where the solidified thickness is thin and the temperature is high, and internal cracking is likely to occur. Further, in steel, cracking susceptibility changes depending on the contents of components such as C, S, and P, and therefore, the upper limit of the casting speed is often restricted by the contents of these components.

In order to further increase the productivity in continuous casting, a technique capable of performing high speed casting without causing internal cracks is required.

Despite various efforts so far, the only reason why internal cracks cannot be eliminated is that conventional measures against internal cracks are only examining the magnitude of strain that is received between rolls or between rolls, and the effect of strain history is It has not been taken into consideration, and it is considered that the measures are limited to local measures. That is, there is a possibility that the slab comes out of the mold and undergoes various deformations until the central portion is completely solidified, and the deformation received earlier has an influence later. In this case, internal cracks cannot be completely eliminated by local countermeasures at the time (location) when cracks occur.

In continuous casting, especially in the straightening zone, the frequency of occurrence of internal cracks is highest, and despite the measures (for example, the above examples) being taken based on various straightening theories, a reliable solution has been found. It has not been. As described above, the measures taken so far focus only on the magnitude of the strain received by one straightening roll, and it is not possible to adjust the placement of straightening points and the straightening amount in consideration of the influence of the strain history. The reason is that it is not considered and only local measures are taken.

The purpose of the present invention is to comprehensively examine the factors that cause internal cracking of continuously cast slabs, determine casting conditions after determining the limits of internal cracking occurrence, and especially to revise the method for straightening continuous slabs A continuous casting method for steel that produces a slab without internal cracks with maximum production efficiency by devising a method of supporting the slab at the straightening point that greatly affects the accumulation of strain and the subsequent horizontal portion until complete solidification. To provide.

(Means and Actions for Solving the Problems) The inventors of the present invention comprehensively studied the factors causing the internal cracks in the continuous cast slab, and obtained the following findings.

It may not be possible to predict the occurrence of internal cracks by the conventional way of thinking that the distortion at each position of the slab is taken individually and the magnitude of the distortion is discussed.

The influence of the casting speed cannot be quantitatively grasped by the conventional method that determines the crack occurrence based on the strain amount at each position without considering the strain accumulation.

It is assumed that the strain that each part of the slab receives while being in the temperature range between the tensile strength appearance temperature (ZST) and the ductility appearance temperature (ZDT) during casting will accumulate as it is without being relaxed each time the strain is received. Internal cracks occur when the total amount of strains exceeds the critical strain of the steel to be cast, and cracks do not occur if the total strain is less than that.

The invention of this application was made based on the above findings, and the basic invention is that "each part of the slab is at least between tensile strength appearance temperature (ZST) and ductility appearance temperature (ZDT) during casting. A continuous casting method for steel, characterized in that the total amount of strain received while in the temperature range does not exceed the critical strain of the type of steel to be cast ”.

First, the basic invention will be described.

It is known that each part of the slab during continuous casting first undergoes a tensile strength appearance temperature (ZST) and a ductility appearance temperature (ZDT) from the liquid state to the solid-liquid coexisting state and then to complete solidification and cooling. ing. These ZST and ZDT are values determined by the steel type, and the values are, for example, Arch. Eisen-httenwesen54 (1983) .357.
Can be measured by a known method. Also, ZS
It is also known that the values of T and ZDT correspond well with the solid fraction, and ZST almost coincides with the solid fraction of 0.8 and ZDT almost coincides with the solid fraction of 0.99 (“Iron and Steel” '87. -S896).

FIG. 1 is a diagram schematically showing a solidification process of a slab in continuous casting. In the figure, the molten steel injected into the mold M is cooled as it is drawn downward, and the solidified layer (shell) S is gradually thickened. Between the solidification layer S and the liquid layer (L) in the central portion, a solid-liquid coexisting layer (S +
L) Now, assuming that the A ₁ point is the solid phase point of 0.8 at a fixed position P ₁ in the longitudinal direction (drawing direction) of the slab, the tensile strength starts to appear from here. That is, the temperature at point A ₁ is ZST.
At the same position P ₁ , if the B ₁ point is the solid fraction 0.99,
Ductility appears from here. The temperature at this point is ZDT.

If you follow the previous A ₁ point in the drawing direction of the slab, Z ₂
And ductility will appear. ZST is the maximum temperature at which stress starts to generate strain, and ZDT is the temperature below which cracking does not occur due to ductility even if strain occurs. Each part of the slab is subjected to various strains as described above during the course of ZST to ZDT due to bending, bulging, straightening, thermal stress, etc. during continuous casting.

The present inventors, strain received while each part of the slab is in the temperature range from ZST to ZDT, do not relax each time the strain is received,
Assuming that it is accumulated as it is, the total amount of accumulated distortion Exceeds a certain limit value (limit strain ε _c ), cracking occurs,
It was confirmed by repeating various experiments that cracks did not occur if the thickness was less than that.

FIG. 2 is one of the experiments and explains the method for measuring the critical strain for cracking (for details, see our paper, Materials and Processes, Vol. 1 (1988), P. 1229). ).

The internal cracking limit strain is determined by applying various deformations to a small ingot cast so that the cooling conditions are almost the same as those of the continuous cast slab, and examining the magnitude of the strain and the occurrence of cracks. I asked. That is, as shown in FIG. 2, uniaxial tensile deformation was applied to an ingot whose central portion was unsolidified to generate strain. The amount of deformation, the number of deformations, and the time interval between deformations were variously changed, and these were combined to give deformation. In addition, a thermocouple for temperature measurement was set inside the ingot, and the strain amount applied between ZST and ZDT of the total strain amount was obtained while confirming the temperature.

By changing the cooling condition within the range that can be considered for continuous casting slab, the cooling rate at the temperature measurement point is changed so that each point is ZST to Z.
The time during DT was changed.

FIG. 3 shows the results of examining the presence or absence of internal cracking by taking the cooling condition on the horizontal axis and the total amount of strain generated on the vertical axis. The strain amount is the sum of the generated strain amounts (Σε) and the sum of the strain amounts added between ZST and ZDT. Are shown separately.

As shown in Figure 3, However, if it exceeds about 1.6%, internal cracking occurs and this becomes the critical strain. Further, it can be seen that this critical strain does not change depending on the cooling conditions. On the other hand, the added total strain Σε
The crack generation limit value judged by the above is about 1.6% above.
Greater than. This is because the strain applied below ZDT is also counted, and the strain is absorbed by the ductility of the solidified shell, which is absorbed and does not cause cracking. Further, the crack generation limit value due to the total strain amount Σε changes depending on the cooling conditions, and increases as the cooling rate increases.

Is a combination of the various strains described above, but cracks occur when the total strain amount exceeds the limit value regardless of the type of deformation.
The main strains applied to the slab in continuous casting are bending strain, corrective strain, bulging strain, misalignment strain and thermal strain. Of these strains, each part of the slab is
The sum of the strains applied between ZST and ZDT is Σε
_{Let r} , Σε _u , Σε _b , Σε _m and Σε _t be the sum of these, that is, Is as follows.

Σε _r + Σε _u + Σε _b + Σε _m + Σε _t = E This E is a certain limit value ε _c (about 1.6% in the previous FIG. 3)
If it exceeds, internal cracking will occur. Therefore, it is necessary to perform the casting while maintaining the condition of E ≦ ε _c at least until the slab is completely solidified. When there is a temperature range in which the ductility is extremely small even in the temperature range of ZDT or less depending on the steel type, it is preferable to expand to the temperature range in which the ductility is sufficiently increased and suppress the total strain during that time to the critical strain or less.

The critical strain for cracking (ε _c ) is almost constant depending on the type of steel to be cast. The value is particularly affected by the contents of components such as C, S, and P, and the higher the content of these components, the more likely cracking occurs. Table 1 is an example of the limiting strain obtained by performing the above experiment with different steel types and determining the steel types. Regarding the critical strain itself of such cracking, for example, “Mechanical Behavior in Continuous Casting”, April 1985, Iron and Steel Institute of Japan,
It is known from the materials of the Japan Iron and Steel Joint Research Group, P.3 to 7, and it is used for the method of suppressing the crack generation by suppressing the strain amount due to the same deformation below this value. Although the data of the critical strain obtained from such a conventional single deformation can also be used in the method of the present invention, the data of these critical strains are ZST to ZDT.
The range of the distance is not considered, and it is considered that the strain applied outside the ZST-ZDT is included depending on the cooling conditions of the test material. Therefore, when using these data, the cooling conditions of the test material must be considered.

The amount of various strains applied to the slab in the casting process is, by heat transfer analysis, the temperature of the slab at each position of the continuous casting machine is determined, the temperature dependence of the mechanical properties of the steel to be cast, the thermal elasto-plasticity,
It can be obtained by finite element method stress analysis (FEM) considering creep and the like.

As mentioned above, the basic invention of this application is that each part of the slab is ZS.
The gist is to perform the casting under the condition that the total amount of strain received while in the temperature range between T and ZDT does not exceed the above-mentioned critical strain. Hereinafter, a specific method for implementing this will be described. The method is roughly divided into the following in principle.

A method of increasing the critical strain (ε _c ) of cast steel.

Since the critical strain of steel is affected by its composition, the content of the components (C, P, C) that lower the critical strain is reduced. This method is also effective in reducing the distance between the following ZST and ZDT, but the required C content of steel has been specified, and there is a limit in refining technology in reducing P and S. However, it is not a very realistic method.

 How to make E small.

This method is further subdivided as follows.

-1 A method to reduce the distance between ZST and ZDT.

As mentioned above, it is the accumulated strain between ZST and ZDT that causes internal cracks, so if the interval is shortened, the accumulated strain (E)
Becomes smaller. Specifically, measures such as lowering the casting speed, strengthening cooling, and reducing the thickness of the slab are conceivable.However, lowering the casting speed lowers productivity, so it is not adopted. It is difficult and it is not realistic to change the thickness of the slab only to prevent internal cracking. After all, what is practical is enhanced cooling.

-2 A method of bringing Σε _r , Σε _u , Σε _b , Σε _m , and Σε _t out of the range of ZST to ZDT when there is a particularly large one.

For example, if the straightening points to which straightening strain is applied are dispersed and each part of the slab passes through at least one of the straightening points and the temperature is outside the range of ZST to ZDT, Σε _u becomes smaller. As a result, E can be suppressed to ε _c or less. This method can be performed by changing the design of the continuous casting machine, but the objective can be achieved even if ZDT appears early by the cooling enhancement described above.

-3 A method to reduce various distortions applied between ZST and ZDT.

This corresponds to the above Σε _r , Σε _u , Σε _b , Σε _m , Σε
_This is a method in which _t is made as small as possible and E is made small. For example, in order to reduce Σε _b , there is a method of making the roll arrangement dense or increasing the rigidity of the continuous casting machine to reduce bulging. Other distortions can be reduced by a method known per se.

A method of positively applying compressive strain in the casting direction of the slab to offset the tensile strain that causes internal cracking is also effective.

The method of the present invention, which is carried out in these various specific methods, predicts the occurrence of internal cracks during a casting operation and gives an alarm,
It can also be used to prevent cracking by changing the casting conditions.

That is, in the method as described above, the strain accumulation range and the total strain amount in the range are obtained, and when the total strain amount reaches 95% of the internal crack occurrence limit strain value of the cast steel type, the strain is Generates an internal crack occurrence prediction alarm that includes the occurrence location and strain amount. Based on this alarm, the amount of cooling water in the entire strain accumulation range is increased so that the total amount of strain decreases to, for example, 90% or less of the crack generation limit strain. If the cooling water supply capacity reaches the limit of the continuous casting machine, the casting speed may be reduced.

In a conventional roll support type continuous casting machine, a peak of accumulated strain appears particularly from the straightening portion to the horizontal portion.
Therefore, the bulging amount in this region is actually measured, the bulging strain (ε _b ) is calculated from the measured value, and the secondary cooling is strengthened, for example, so that the total accumulated strain does not exceed the limit strain.
Operating methods such as controlling the casting speed are also effective in reducing internal cracking.

Hereinafter, preferred embodiments of the present invention will be described.

(I) "The straightening rolls are arranged so that each part of the slab is at a temperature higher than the tensile strength appearance temperature (ZST) or lower than the ductility appearance temperature (ZDT) when passing through at least one straightening point. A continuous casting method for steel, characterized in that this method is characterized in that the amount of accumulated strain is kept below the critical strain by disposing the straightening points that have the greatest effect on strain accumulation in a distributed manner. Internal cracks can be very effectively prevented.

(Ii) “The total amount of strain that the slab receives while in the temperature range between ZST and ZDT during casting exceeds the critical strain of the steel to be cast by adjusting the rotation speed of the support roll in the slab area. , A continuous casting method of steel in which a compressive strain in the casting direction is added to a cast piece in the region to cast the metal '', for example, a cumulative strain gives a pushing force to a roll group on the upstream side in a region in which the internal crack limit strain exceeds, By applying a braking force to the group of rolls on the downstream side, compressive strain is applied to the slab in that region, and the accumulated strain amount is suppressed below the critical strain.

(Iii) "Support rolls in which continuously cast slabs having an unsolidified phase inside are divided into a plurality of pieces in the width direction of the slabs and arranged in a zigzag pattern (hereinafter referred to as zigzag array division support rolls)
In the length W, the slab width end portion including the slab support length W in the slab width direction by the split support rolls and the interval L between the split support rolls that follow each other in the casting direction are included in the length W. It was determined that W / 5 ≤ 1.5 for split support rolls and W / L ≤ 2.0 for split support rolls that do not include it, and the interval L and zigzag arrangement interval of the split support rolls described above appear in tensile strength appearance temperature (ZST) and ductility. Continuous casting method for steel, characterized in that the total amount of strain received by the slab during the temperature range between the temperatures (ZDT) is determined so as not to exceed the critical strain of the steel type to be cast. The feature is that the zigzag array division support rolls are used, and the arrangement section, the slab support length, and the roll interval are appropriately determined to suppress the accumulated strain amount of bulging and misalignment to the critical strain or less.

(Iv) "Continuous casting method for steel that supports the horizontal portion from the straightening portion of the slab to complete solidification by the surface that moves in synchronization with the slab" This method is a risk area for internal cracking, that is, , It is characterized by supporting the horizontal part from the straightening part until the slab is completely solidified by the surface that moves in synchronization with the slab,
This suppresses the accumulated strain to be lower than the limit strain and effectively prevents the occurrence of internal cracks. When carrying out this method, it is desirable to provide a displacement gauge between the support surfaces and operate while controlling the distance between the surfaces, that is, the thickness of the slab to be constant.

Hereinafter, the present invention will be specifically described with reference to preferred embodiments by examples.

(Example 1) FIG. 4 shows C: 0.15%, P: 0.02%, S: 270 mn thick and 180 mm wide.
When the 0.01% carbon steel slab is cast at a casting speed of 1.6 m / min, the amount of strain at the solidification interface (at the solid phase ratio of 0.8 which is ZST) for each roll is shown as the distance from the meniscus. Is. The maximum amount of strain is as small as 0.3%, which does not exceed the critical strain (1.6%) for cracking of this steel type.

FIG. 5 shows the positions of ZST and ZDT at this time by the distance from the meniscus. About 22m from the meniscus in the figure
At the position (point B), the ZDT is about 80 mm from the surface of the slab, and the ZST at this point is about 14 m from the meniscus.
Position (point A). Therefore, the distortion between A and B is accumulated at the position of point B. With this same idea,
FIG. 6 shows that the accumulated strain at each position from the meniscus was obtained. The accumulated strain is highest immediately after leaving the straightening zone, and exceeds the crack initiation critical strain just before the exit side of the straightening zone.

When the cross-section of the slab after casting was sulfa printed,
The presence of internal cracks was confirmed at a position of 80 to 90 mm from the epidermis. From FIG. 5, it can be seen that this position corresponds to the vicinity of the correction area output side, and cracks have occurred in the portion exceeding the critical strain. On the other hand, no crack has occurred in the portion that does not exceed the critical strain.

The chain line in FIG. 6 shows the accumulated strain amount when the bulging strain is reduced by strongly cooling the upstream side of the straightening region. When the total accumulated amount of strain was reduced below the critical strain by such strong cooling, no internal cracks were found.

Figure 4 above shows the amount of strain at each position when strain accumulation is not considered, and is the maximum at a position of about 3.5 m from the meniscus (position where bending strain is applied), but it is still small at about 0.33%. It is a value. These strain amounts are limited to the critical strain (1.6
%), It means that cracking does not occur according to the conventional wisdom, but in reality cracking occurs. In other words, it is impossible to predict the occurrence of cracks in this case by the conventional method of individually taking up the strain at each position of the slab.

(Example 2) FIG. 7 shows the same steel type as in Example 1 at a casting speed of 2.3 m / mi.
Z when cast by changing to high and low levels of n and 1.0 m / min
It shows the positions of ST and ZDT. FIG. 8 shows the above-mentioned No. 6
It is the sum of strain amounts between ZST and ZDT obtained in the same manner as in the figure. The width between ZST and ZDT increases as the casting speed increases, and the range of strain accumulation also increases accordingly. As a result, the strain accumulation amount increases as the casting speed increases, as shown in FIG. When the casting speed was 2.3 m / min, the accumulated strain amount greatly exceeded the critical strain and internal cracking occurred. On the other hand, when the casting speed was 1.0 m / min, internal cracking did not occur.

Although not shown, the amount of strain at each position where strain accumulation is not considered is only increased by about 20% as the casting speed is increased from 1.0 m / min to 2.3 m / min. However, when considering the accumulation of strain, the total amount of strain increases significantly due to the expansion of the strain accumulation range (increasing the width between ZST and ZDT) due to the increase in casting speed. Thus, without considering the accumulation of strain, the conventional method of determining the occurrence of cracks based on the amount of strain at each position, the difference between the method of the present invention is clear, in the conventional method, It is almost impossible to quantitatively understand the relationship between the casting speed and the occurrence of internal cracks.

(Embodiment 3) FIG. 9 shows that, with respect to No. 1, No. 2 and No. 4 steel types shown in Table 1 above, slabs of the same size were cast under the same cooling conditions and various casting speeds were used. This is the result of investigating the occurrence of internal cracking by changing. The horizontal axis represents the casting speed, and the vertical axis represents the maximum value of the accumulated strain amount obtained in the same manner as in FIGS. 6 and 8 to show the presence or absence of internal cracks for each steel type. It can be seen that internal cracks occurred in each steel type when the critical strain values shown in Table 1 were exceeded. Even if the steel types differ in this way, the respective critical strains must be known and the
If ZST and ZDT are calculated and the strain accumulation between them is calculated, continuous casting can be performed under the condition that internal cracking does not occur.

(Embodiment 4) This embodiment is an example in which correction distortion occupying a large portion of accumulated total distortion is focused and the distortion is reduced as much as possible. Specifically, the straightening points are dispersed, in other words, the straightening zone is expanded, and when each part of the slab is at a temperature higher than the tensile strength appearance temperature (ZST) or at a temperature lower than the ductility appearance temperature (ZDT). At least one of the correction points is passed when in zone or both. Straightening of each part of the slab is ZST to Z
If it is done during DT, the correction strain will be extremely accumulated, so at least a part of the correction is performed outside the range of ZST to ZDT to suppress the accumulated strain as small as possible. As a concrete means therefor, it is possible to adopt a means of strengthening the cooling of the slab and narrowing the temperature range between ZST and ZDT. However, in recent years, by directly connecting continuous casting and hot rolling,
A technique for rolling continuously cast slabs without heating or with only a minimum amount of auxiliary heating is prevalent, and for that purpose, it is desirable to raise the temperature of the slabs leaving the continuous casting machine. Therefore,
In carrying out this method, at least a part of the straightening of the bend is performed by the design change of the continuous casting machine rather than the cooling strengthening.
It should be done outside the ZST-ZDT range.

The steel grade, slab size, and casting speed used in this example are the same as those in Example 1. First, for comparison, the test results will be described in which straightening points were cast by arranging them in the conventional manner without considering the strain accumulation range (temperature range between ZST and ZDT).

FIG. 10 shows the solidification interface strain when four correction points are arranged at positions of 18 m, 19.2 m, 20.4 m, and 21.6 m from the meniscus.
(FIGS. 10 and 11 are substantially the same as FIGS. 4 and 5 above, but are reproduced for convenience of explanation.) FIG.
It shows the type and amount of strain generated at the solidification interface of each part of the cast slab (indicated by the distance from the meniscus). As shown in the figure, the thermal stress strain and the bulging strain are generated over the entire section, but the bending strain is generated at the curved portion of the slab and the correction strain is generated at the bending correction region. No matter what point you take, the amount of distortion is maximum
About 0.33%, the cracking limit strain of this steel type is 1.6%
Has not exceeded. However, in reality, internal cracks occur,
The cause is that the accumulated amount of strain in the temperature range between ZST and ZDT
More than 1.6%.

FIG. 11 is a diagram showing the positions of ZST and ZDT at this time. As shown, 80mm (A-A line) and 90mm (B-
Between lines B), the entire correction area (position about 18 to 22 m from the meniscus) is between ZST and ZDT, and as a result,
All correction strains will be accumulated. FIG. 12 is a diagram showing the accumulated distortion amount. As shown in the figure, the critical strain of this steel type exceeds 1.6% in the straightening zone, which causes internal cracking.

Next, according to the present invention, the arrangement area of the straightening roll is expanded,
The test results for allowing straightening to be performed even when each part of the slab is at a temperature higher than ZST or lower than ZDT will be described.

As shown in FIG. 13 (the same type as FIG. 10 described above), seven correction points were dispersed and arranged in a range of 13 m to 25 m from the meniscus. Placement points are 13m, 15m, 17m, 19m, 21 from Meniscus
It is located at m, 23m and 25m. By distributing the correction points to more points, the correction strain of one correction point becomes about 1/2 of the original correction strain. As shown in Figure 14, the positions of ZST and ZDT are
Although it is the same as in Fig. 11, the range of the correction area is wider than the strain accumulation range of ZST to ZDT at any place because the arrangement of the correction points is different. For example, at a position 80 mm from the surface of the ingot (A
In the (A line), the latter half of the correction point is lower than ZDT, and in the position of 90 mm (BB line), the front part of the correction point is higher than ZST. Therefore, at least a part of the correction strain will not be accumulated. As a result, as shown in FIG. 15, the total accumulated strain amount was below the critical strain and no internal cracking occurred.

In the example of a narrow straightening zone shown in FIG. 10 (comparative example), internal cracking occurs in the straightening zone at a casting speed of 1.6 m / min, whereas in this embodiment the casting speed should be increased to 1.8 m / min. It was possible to improve productivity.

As described above, the strain accumulation range (width between ZST and ZDT) increases as the casting speed increases, and the strain accumulation range increases accordingly. Therefore, as a way of arranging the correction points, keep in mind the limit of the casting speed due to restrictions from other parts of the continuous casting machine (for example, restrictions due to the solidification phenomenon in the mold), and internal cracks are generated within this casting speed. It is better not to put it out.

FIG. 16 is a diagram showing the effect of the ratio of the strain accumulation range and the width of the straightening zone on the maximum speed (maximum casting speed) at which casting is possible without internal cracking.

That is, a test for dispersing straightening points was performed using a slab of the same steel type and the same size as the previous slab, and the limit casting speed at which internal cracking occurred was determined. As an index Was used. The denominator of the above equation is the maximum value of the strain accumulation range (ZST to ZDT) when operating at the maximum casting speed possible with the continuous casting machine. FIG. 16 shows the test results in which the horizontal axis represents the value of Ra and the vertical axis represents the critical casting rate at which internal cracking occurs. The critical speed begins to increase when Ra exceeds 1.0, and the internal crack initiation critical casting speed increases as the straightening area becomes wider than the strain accumulation area (Ra increases).

(Example 5) In this example, a compressive strain is applied to a crack generation region of a cast slab to reduce accumulated strain. Specifically, the accumulated strain received during the temperature range between ZST and ZDT during casting is calculated by heat transfer analysis and stress analysis, and the accumulated strain exceeds the critical strain of the steel type to be cast. , That is,
Determine the cracking area. Then, in this region, compressive strain is applied in the longitudinal direction (casting direction) of the slab.

As a specific means for applying compressive strain to the crack generation region of the cast slab, as shown in FIG. 17, for example, the driving rolls (R ₀ , R ₁ , R ₂ , ... R _i-in front of the crack generation region ₁ ), the drive rolls in the crack generation region (R _i , R _{i + 1} , R _{i + 2} , ... R _{i + n-1} ) and the drive rolls after the crack generation region (R _{i + n} , R _{i + n + 1} , R _{i + n + 2} , ・ ・ ・ R
_end ) rotation speed is V ₀ , V ₁ , V ₂ , ・ ・ ・ V _i-1 , V _i , V
_{i + 1} , V _{i + 2} , ・ ・ ・ V _{i + n-1} and V _{i + n} , V _{i + n + 1} , V _{i + n + 2} , ・
..Assuming V _end , V ₀ = V ₁ = V ₂ ... = V _i-1 > V _i ≧ V _{i + 1}
≧ V _{i + 2}・ ・ ・ ≧ V _{i + n-1} ＞ V _{i + n} ＝ V _{i + n + 1} ＝ V _{i + n + 2}・ ・ ・ ＝
V _end (however, V ₀ , V ₁ , V _2, ..., V _i-1 are rotation speeds of the rolls synchronized with the casting speed) may be adjusted to pull out the slab. That is, by adjusting the roll rotation _{speed, R i, R i + 1} , R i + 2, acts compressive strain between ··· R _{i + n-1} of the roll and the roll portion, compression on the entire crack region You can add distortion.

Depending on the occurrence of internal cracks on the top and bottom sides of the slab, the roll speed on one side (for example, the ground side) can be adjusted while keeping the roll speed at the same speed as the casting speed. It may be performed only for the roll (for example, the top side). Further, the rotation speeds of both the top and bottom rolls may be adjusted as described above.

Hereinafter, this embodiment will be described in detail. The steel type, slab size, and casting speed used here are the same as in Example 1.

FIG. 18 shows the amount of accumulated strain when compressive strain is applied in the strain accumulating range (hereinafter referred to as compression casting) and when no compressive strain is applied (hereinafter referred to as comparative example).

In the comparative example in which compression casting was not carried out, points A and B in the figure
Between the points, the accumulated strain exceeded the internal cracking limit strain, and internal cracking occurred in the slab slab.

In compression casting, the range of compressive strain was set between A _{1 and} B _{2 in} Fig. 18 considering the strain accumulation range. That is, it is point A that exceeds the critical strain before compression casting, and this accumulated strain is A ₁ − in the figure.
The strain applied between A ₂ is involved. Further, the limit strain is exceeded up to point B, but the accumulated strain at point B involves the strain added between B _{1 and} B ₂ in the figure. Therefore, a compressive strain was applied to the slab in the crack generation region by applying a braking force to the roll group after B ₂ and a pushing force to the roll group before A ₁ . As a result,
As shown by the alternate long and short dash line (...) in Fig. 18, the peak value of accumulated strain was below the critical strain and internal cracking of the slab did not completely occur.

As described above, internal cracks could be prevented by giving a compressive strain (0.45% compressive strain in this case) sufficient to alleviate the amount exceeding the critical strain in consideration of the strain accumulation range.

FIG. 19 shows the steel types as described above, the compression casting of the present invention by varying the casting speed in the range of 1.0 to 3.0 m / min with the slab size (the accumulated strain amount is calculated for each casting speed, and the critical strain and Is calculated, and a compressive strain equal to or more than the difference is given), and the charge ratio of internal cracking in the slab is investigated. As a comparative example, an example in which the compression casting shown by the solid line in FIG. 18 was not carried out, and as a conventional example, focusing only on the correction area, the maximum amount of the correction strain ( 0.1-0.2
%) Is added.

As shown in FIG. 19, in the comparative example in which compression casting was not carried out, internal cracking began to occur at each charge when the casting speed was 1.5 m / min or more. Further, in the conventional example, the charge ratio of internal crack generation is lower than that in the comparative example, but it increases as the casting speed increases. This is because the strain accumulation range increases as the casting speed increases, and it cannot be dealt with by a local method.

On the other hand, in the examples of the present invention, no internal cracks were generated even at a high casting speed of 3 m / min, and the effect of preventing internal cracks by the method of the present invention is clear.

(Example 6) This example is an example in which staggered array division support rolls are used to reduce bulging strain and minimize accumulated strain during casting in the temperature range between ZST and ZDT.

FIG. 20 is a diagram showing an example of a roll arrangement of zigzag arrangement division support rolls. The support rolls (R-a, R-b) that suppress the long side surface of the slab S are in the length direction (the slab width direction).
It is divided into multiple parts and arranged in a staggered pattern. Therefore, for example, compared with the case where the support rolls are not arranged in a staggered manner, the bulging strain (ε _b ) of the row of divided rolls R-a is reduced by the deformation suppressing effect of the adjacent row of divided rolls Rb. However, the relationship between W / L and ε _{b in} the case of zigzag arrangement rolls differs depending on whether the rolls support the slab width edge or not.
Internal cracks cannot be prevented unless is set.

Fig. 20 shows the relationship between the W / L of the zigzag array division support rolls and the maximum strain (calculated value) in the casting direction directly below each roll.
Figure 21 shows. The maximum strain is shown as a value normalized by ε _b of a conventional normal roll (W / L ≧ 4.1). In the roll supporting the slab including the slab width end (roll R-a in FIG. 20, indicated by ● in FIG. 21), W ₁ /L=≦1.5, the slab width end is Width center roll not included (roll R-b in FIG. 20, roll 21
(Indicated by a circle in the figure), if the length (W) in the width direction of the slab is determined so that W ₂ /L≦2.0, ε _b is 90 in the case of the conventional roll arrangement (not in the zigzag arrangement). % Or less.

Next, it is necessary to determine the roll interval L. In this case,
As already mentioned, the strain history must be taken into account. The shorter L is, the smaller the bulging strain (ε _b ) is, but the number of rolls that pass while each part of the slab is in the ZST to ZDT range increases, and the accumulated amount of misalignment strain (ε _m ) increases. Will increase. Moreover, as L becomes shorter, ε _{m for} each roll also becomes larger. We have ε
It was confirmed that _b is proportional to L ⁵ and ε _m is proportional to L ^-2 .

The absolute value of ε _b varies depending on the solidification shell thickness, molten steel static pressure, slab temperature, casting speed, W / L in addition to L, and ε _m is the solidification shell thickness, misalignment amount δ ([mm]:> 0), but in most cases in a normal curved continuous casting machine, ε _b = 0.03 to 0.3% and ε _m = 0.1δ to 0.4δ%. Here, the roll alignment is managed so that δ approaches 0, but it is usually about 0.5 mm due to the gap of the roll bearing part, the deflection of the segment frame fixing the roll, the wear of the roll and the thermal deformation during casting. The existence of δ is inevitable. For this reason, it is not possible to unduly shorten the roll interval, which leads to an increase in ε _m .

In this example, the solidification shell thickness in the vicinity of the straightening part of the curved continuous casting machine was 92 mm, the static pressure of molten steel was 0.095 kg / mm ² , and the casting surface temperature was 900 ° C. at a casting speed of 2.0 m / min and L = 100. ~ 400mm,
Ε _b when W = 1000 mm and ε _m when δ = 0.25 mm were calculated. Since the correction strain and the thermal stress strain hardly change depending on the roll interval, only ε _b and ε _m were examined.

FIG. 22 shows changes in ε _b and ε _m when the roll interval (L) is changed. Since ε _b is proportional to L ⁵ , it should change like the dotted line a in the figure, but due to the effect that W / L increases as L decreases, the change in ε _b actually changes from the solid line b. Become. On the other hand, ε _m is the solid line c. As shown in the figure, ε _b can be reduced by making L physically as small as possible, but on the contrary, ε _m increases, so it is difficult to determine the optimum roll interval. In the present invention, the optimum value of L is determined as follows in consideration of the accumulated distortion of ε _b and ε _m .

FIG. 23 is the same as FIG. 5 described above and shows the positions of ZST and ZDT. The number of times each part of the slab passes through the support roll while passing through the range of ZST to ZDT can be read from the figure. The point 92 mm from the surface of the slab passed the ZST at the point A about 20.5 m from the meniscus, and the point B passed the ZDT at the point B at about 28 m. Consider the history of ε _b and ε _m received by the rolls in the range of 7.5 m in the meantime. The bulging strain (ε _b ) is generated every time the slab passes through the rolls in the section A-B, but the misalignment strain (ε _m ) is rarely generated in each roll. It often occurs in. Therefore,
If ε _m is generated for every two rolls, and the strain history is considered under the conditions shown in FIG. 21, the sum Σε _b + Σε _m of the bulging accumulated strain and the misalignment accumulated strain in this case can be obtained.

FIG. 24 shows the relationship between the roll interval (L) and the bulging and misalignment accumulated strain.

When L = 400 mm, Σε _b + Σε _m is slightly less than 1.6%,
It can be seen that the value decreases as L decreases, reaches a minimum at L = 320 mm, and then increases, and becomes almost the same value as L = 400 mm at L = 265 mm. In fact in addition to the thermal stress-strain Shigumaipushiron _T is about 0.05% of Shigumaipushiron _b and Shigumaipushiron _m, since corrective strain Shigumaipushiron _u is present about 0.2%, the total accumulated distortion _{E = Σε b + Σε m +} Σε T + Σε u
Is less than 1.6%, Σε _b + Σε _m is 1.35% or less,
That is, L = 310 mm to 340 mm, and otherwise the occurrence of internal cracks cannot be prevented. If the strain history is not taken into consideration, the absolute value of the strain is very small compared to the limit strain as shown in FIG. 22 and is optimal as in the method of the present invention. Difficult to find a good roll interval.

Next, specific test conditions and the results will be described.

〔Test condition〕

Target steel type… C: 0.15%, P: 0.02%, S: 0.013% carbon steel slab (slab) Slab size… 270 mm thickness, 180 mm width Casting speed… 1.5 to 4.3 m / min Continuous casting machine… Vertical bending bending type (Machine length 43 m) Straightening section: Distance from meniscus 18 m to 22 m When casting with a conventional support roll, the distance from meniscus is 20 to 28 m at a casting speed (Vc) of 1.6 m / min or more. Internal cracking occurred in the range (roll interval L: 400 mm).

Therefore, according to the present invention, considering the strain history between ZST and ZDT, the distance from the meniscus of this continuous casting machine is 15 to 28 m.
The zigzag array division support roll was applied to the section. One of them is the roll arrangement shown in FIG. 25 (this is called type I), and the roll interval is shorter than the conventional one. the other one is,
With the roll arrangement shown above and shown in FIG. 20 (this is referred to as Type II), the roll spacing is similar to the conventional one.

Table 2 shows the relationship between the dimensions of the zigzag array support rolls and the internal cracking limit casting speed (Vcmax.m / min).

When the support length W by the roll, the roll interval L and the zigzag arrangement range are within the scope of the present invention, as shown in Table 2, the limit Vc at which internal cracking does not occur, and in the case of type I
From 1.5m / min (comparative example) to 2.2m / min (example), in the case of type II 1.6m / min (comparative example, from 2.0m / m)
It could be improved to in (Example). Comparative example
W ₁ / L and W ₂ / L, Comparative example W ₂ / L, Comparative example W ₁ / L is out of the scope of the present invention, W ₁ to prevent internal cracking
/L≦1.5 and W ₂ /L≦2.0 are desirable. In the comparative example, the installation range of the zigzag array roll was set equal to the internal crack generation range when the conventional roll was used, but the effect of preventing the internal crack by the zigzag array split support roll was not obtained. In the example, the effect of preventing internal cracking is obtained, and it is clear that it is necessary to determine the installation range of the zigzag array roll in consideration of the strain history between ZST and ZDT.

The zigzag arrangement division support roll of the present invention was applied to a vertical bending curve continuous casting machine (machine length 15 m) having a slab thickness of 100 mm and a maximum slab width of 1000 mm. [C] = 0.12%, [P] = 0.018%,
When a steel grade of [S] = 0.014% was cast using a conventional support roll, internal cracking occurred in the range of 8 to 10 m from the meniscus when Vc = 3.6 m / min or more (roll interval L in this section was 245 mm. ). In consideration of the strain history so that the generated strain falls below the critical strain ε _c , the range of 5 to 10 m from the meniscus is shown in Table 2 as an example.
II staggered roll array. In this case, the internal crack generation limit casting speed Vcmax. Could be increased to 4.3 m / min.

(Embodiment 7) This is an example in which a slab that moves in synchronization with the slab in a danger zone of internal cracking is supported to reduce accumulated strain due to bulging and misalignment.

As described above, in the case of a normal curved continuous casting machine, a peak of accumulated strain appears within the range where the slab of the horizontal portion from the straightening portion is completely solidified, and internal cracking occurs when this exceeds the critical strain. .

FIG. 26 shows accumulated strain in the slab when continuous casting is performed by a conventional roll-support type curved continuous casting machine. As shown in the figure, among the accumulated internal strains, the bulging strain (ε _b ) and the misalignment strain (ε _b )
_m ) accounts for a large proportion. Internal cracking occurs in the horizontal portion from the straightening portion where the total strain exceeds the critical strain (ε _c ) to the complete solidification. In a curved continuous casting machine, generation of a straightening portion (ε _u ) and thermal stress strain (ε _t ) is inevitable. Therefore, in order to make the total accumulated distortion smaller than the critical distortion, it is necessary to devise to reduce the bulging distortion and the misalignment distortion. However, as long as the straightening part and the horizontal part are supported by the rolls that are in line contact with the slab, bulging strain occurs, and the misalignment strain due to bending due to thermal deformation of the rolls becomes a considerably large value. It is extremely difficult to keep the total accumulated strain added with the stress strain to a value smaller than the critical strain.

FIG. 27 is a schematic cross-sectional view showing an example of a method for supporting a slab of straightening and horizontal parts of a slab according to the present invention. In this example, the correction section A and the horizontal section B are independent support members 1 respectively.
And 2 (both are composed of a pair of upper and lower slabs S). The supports 1 and 2 are in the shape of a caterpillar in which the segments 3 are connected to each other, and move in synchronism with the moving speed of the slab while supporting the upper and lower surfaces of the slab in surface contact.

The segment 3 constituting the caterpillar is pressed against the surface 6 of the slab S by a mechanism including a hydraulic cylinder 4 and a small-diameter roll 5, as schematically shown in FIG. The individual segments 3 are always pressed by at least one small-diameter roll 5. The pressing force may be such a pressure as to prevent the entire contact surface of the caterpillar with the cast slab from being bent. The movement of the caterpillar (rotation in the direction shown by the arrow in FIG. 27) can be performed according to the moving speed (casting speed) of the slab by a method known per se. The correction part and the support of the horizontal part may be integrated. However, as shown in FIG. 27, it is easier to design and manufacture them by separating them.
In this case, by providing the roll support 7 between both supports,
It is desirable to suppress bulging in this part. Also,
The support (and its segments) must be cooled appropriately, such as by water spray.

The support by the support as described above is a portion where the accumulated strain exceeds the critical strain as shown in FIG. 26, that is, from the straightening portion A shown in FIG. 26 to the point C where the slab of the horizontal portion B is completely solidified. Is enough. The other part (for example, the upstream side of the caterpillar) may be supported by a roll as in the conventional case, but it is more effective if the zigzag array division support roll described above is used in combination.

As shown in FIG. 27, displacement gauges 8 are attached to each of the support bodies at three positions, ie, the inlet side, the center, and the outlet side, and the distance between the upper and lower segments, that is, the distance between both sides of the caterpillar is measured. It is desirable to control the pressing force of the small-diameter roll 5 that suppresses each segment so that the thickness of the slab is always constant.

Hereinafter, this example will be specifically described based on the test results.

A vertical bending type continuous casting machine (2-strand type) having a bending radius of 10 m casts a slab having a thickness of 200 mm and a width of 1800 mm at a casting speed of 3.0 m / min. The steel type is carbon steel shown in Table 3.

The surface support method of the present invention equipped with a displacement meter as shown in FIG. 27 was used for No. 1 strand, and the conventional roll support method was used for No. 2 strand for comparison.

FIG. 29 is an internal crack generation code determined by a sulfur print of a slab cross section.

As shown in FIG. 29, internal cracking is remarkably reduced in the surface support method of the present invention as compared with the conventional roll support method. In the measurement result of the surface distance by the displacement meter, in the case of the method of the present invention, it is within the range of ± 0.1 mm, and the alignment is almost normal, whereas the measurement result of the roll distance in the conventional method is about 2 mm. There was a periodic fluctuation of. This is because roll bending occurs, and it is presumed that internal bending frequently occurred because the cast piece was over-pressurized by this bending.

FIG. 30 is a diagram showing an internal strain accumulation state of a cast piece when the method of the present invention is used. From the straightening part to the horizontal part, there is virtually no bulging strain and misalignment strain, and the total accumulated strain is much smaller than the critical strain. This is a major factor in reducing the internal cracking shown in Fig. 29.

(Effects of the Invention) The present invention is based on new findings obtained as a result of comprehensively investigating the cause of internal cracks in continuously cast slabs, ZST and Z
It provides an epoch-making method of controlling accumulated distortion between DTs.

According to the continuous casting method of the present invention, it is possible to accurately determine the limit of casting without generating internal cracks,
It is possible to manufacture a slab of good quality without internal cracks at the maximum casting speed according to the type of steel to be cast. Specifically, a casting method in which straightening points are dispersed outside the strain accumulation range, a compression casting method in which a accumulated strain inside the slab exceeds the critical strain of internal cracking by applying a compressive strain, and a slab support in the region is provided. The present invention can be implemented by a method of supporting in a zigzag array division support roll or a surface that moves in synchronization with the slab.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a continuously cast slab explaining ZST and ZDT, FIG. 2 is an explanatory diagram of an experimental method for obtaining a crack initiation critical strain, and FIG. 3 is a total amount of strain applied to the slab. It is a figure which shows the relationship between a crack generation. 4 to 6 relate to Example 1, and FIG. 4 is a diagram showing the results of obtaining the strain amount for each roll position (distance from the meniscus) of the continuous casting machine, and FIG. The figure which shows the position of ZST and ZDT in the casting condition of an example, FIG. 6 is a figure which shows the change of the accumulated strain in the same Example,
Is. 7 and 8 relate to the second embodiment, and FIG. 7 shows the ZST in the embodiment with two kinds of casting speeds.
And FIG. 8 is a diagram showing the positions of ZDT, and FIG. 8 is a diagram showing changes in accumulated distortion. FIG. 9 relates to Example 3 and is a diagram showing the relationship between accumulated strain and occurrence of internal cracks when three types of steel are continuously cast. 10 to 16 relate to Example 4, FIG. 10 is a diagram showing strain at solidification interface when the correction points are not dispersed (conventional method), and FIG. 11 is the same correction points. ZST to Z when not distributing
Fig. 12 shows the relationship between DT, position, and correction area. Fig. 12 shows the accumulated strain amount when the correction points are not dispersed. Fig. 13 is the coagulation when the correction points are dispersed. Figure 14 shows the strain generated at the interface, and Figure 14 shows ZST to ZDT when the correction points were also dispersed.
Fig. 15 shows the relationship between the position and the correction area, Fig. 15 shows the accumulated strain amount when the correction points are similarly dispersed, and Fig. 16 shows the maximum value of the strain accumulation range (ZST to ZDT). It is a figure which shows the relationship between the value (Ra) which divided the width of the straightening area | region with, and the limit casting speed which can be cast without generating an internal crack. 17 to 19 relate to Example 5, FIG. 17 is a schematic diagram showing an example of a compression casting method, and FIG. 18 shows a case of not applying compression strain and a case of applying compression strain. The relationship between the position of ZST ~ ZDT and the amount of accumulated strain and the amount of accumulated strain, Fig. 19 shows the internal cracking of the slab and the casting speed when no compressive strain was applied and when compressive strain was applied. It is a figure which shows a relationship. 20 to 25 are diagrams relating to Example 6, FIG. 20 is a schematic view showing an example of a zigzag array dividing support roll, and FIG. 21 is a zigzag array dividing support roll having a slab width end portion. Bulging strain (ε _b ) and W / L with and without
FIG. 22 shows the relationship between the bulging strain (ε _b ) and the misalignment strain (ε _m ) when using a conventional support roll, and the roll interval L. FIG. FIG. 24 is a diagram showing the positions of ZST to ZDT in the casting conditions of the embodiment, and FIG. 24 is a bulging accumulated strain (Σε _b ) and a misalignment accumulated strain (Σ) when a conventional support roll is used.
FIG. 25 is a diagram showing the relationship between the sum of ε _m ) and the roll interval L. FIG. 25 is a schematic diagram showing another example of the zigzag array division support roll. 26 to 30 are diagrams relating to Example 7, FIG. 26 is a diagram showing accumulated strain inside a slab in a conventional continuous casting method, and FIG. 27 is a diagram showing an apparatus for carrying out the method of the present invention. A schematic view showing an example, FIG. 28 is a partially enlarged view of the apparatus of FIG. 27, FIG. 29 is a view showing the internal cracking occurrence state in Examples and Comparative Examples, and FIG. 30 is in the method of the present invention. It is a figure which shows the accumulation | storage strain inside a slab.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Keiji Nakajima 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Kazuo Okamura, 4-chome Kitahama, Chuo-ku, Osaka City, Osaka Prefecture No. 33 Sumitomo Metal Industries Co., Ltd. (72) Inventor Kei Kanazawa 4-5-3 Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture Sumitomo Metal Industries Co., Ltd. (72) Inventor, Koji Kajiwara 4 Kitahama, Chuo-ku, Osaka-shi, Osaka 5-33, Sumitomo Metal Industries, Ltd.

Claims (5)

[Claims] 1. The critical strain of the cast steel type is the sum of the strain amounts that each part of the slab receives during casting at least in the temperature range between the tensile strength appearance temperature (ZST) and the ductility appearance temperature (ZDT). A continuous casting method for steel, which is characterized in that the casting is performed under the condition not exceeding 2. A straightening point by a straightening roll so that each part of the slab is at a temperature higher than the tensile strength appearance temperature (ZST) or a lower temperature than the ductility appearance temperature (ZDT) when passing through at least one straightening point. A continuous casting method for steel, which is characterized by arranging. 3. A slab area in which the total amount of strain received by the slab exceeds the critical strain of the type of steel to be cast during the temperature range between the tensile strength appearance temperature (ZST) and the ductility appearance temperature (ZDT) during casting. Adjusting the rotation speed of the support roll of the casting, adding a compressive strain in the casting direction to the slab in the region to cast, the sum of the strain amounts is characterized by not exceeding the critical strain. Continuous casting method for steel. 4. A continuously cast slab having an unsolidified phase inside,
Supporting with support rolls that are divided into a plurality of pieces in the width direction of the slab and arranged in a staggered manner, and the slab support length W in the width direction of the slab by the above-mentioned split support rolls and the split support rolls that follow each other in the casting direction And the gap L of the split support roll including the width end of the slab within the length W is set to W / L ≦ 1.5, and for the split support roll not including W / L ≦ 2.0. Of tensile strength appearance temperature (ZST) and ductility appearance temperature (ZDT)
A method for continuous casting of steel, characterized in that the total amount of strain received by the slab during the temperature range between is determined so as not to exceed the critical strain of the type of steel cast. 5. A horizontal portion from the straightening portion of the slab to complete solidification is supported by a surface that moves in synchronization with the slab,
At least the tensile strength appearance temperature (ZS
A continuous casting method for steel, characterized in that the total amount of strain received while in the temperature range between T) and the ductility appearance temperature (ZDT) does not exceed the critical strain of the steel type to be cast.