JPH03174962A - Method for continuously casting steel - Google Patents

Abstract

PURPOSE:To prevent the development of inner crack to a continuously cast slab by casting molten metal under condition, which the total strain quantity received in the temp. range of tensile strength appearance temp. and ductility appearance temp. does not exceed the limited strain in casting steel kind at each part in the continuously cast slab during casting at the time of continuously casting the molten steel. CONSTITUTION:At the time of perfectly solidifying unsolidified part at the center while drawing the continuously cast slab S from a mold with plural pinch rolls, the accumulated strain received between the tensile strength appearance temp. (ZST) and the ductile appearance temp. (ZDT) during casting is calculated with heat conducting analysis and stress analysis, and cooling zone in the continuously cast slab, which this value exceeds the limited value in the steel kind, is decided to the crack developing range. In this range, plural pieces of supporting rolls Ra, Rb are set as zigzag-state to the longitudinal direction of the cast slab S, and relation between this interval L and length W supporting the continuously cast slab S with the supporting rolls, is made to W/L <=1.5-2.0, and by applying compressive strain in the range, which the accumulated strain in inner part of the cast slab exceeds the limited value of developing the inner crack, the development of continuously cast slab having deteriorated quality caused by developing the crack in the inner part of continuously cast slab, is prevented.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is capable of manufacturing carbon steel, low-alloy steel, stainless steel, and other iron-based metals (in the present invention, they can This invention relates to a method for continuously casting steel (collectively referred to as "steel").

(Prior Art) Continuous casting has many advantages, such as increased efficiency in casting operations, energy savings, and improved yield, and has become an indispensable part of modern steel production technology. Since its inception, the continuous casting method for steel has been constantly improved and is now nearing completion, but there are still some unresolved problems. One of these is minute cracks that occur inside the slab.

In continuous casting, the molten metal placed in the mold is cooled by the mold to form a solidified shell, then gradually solidified as it is cooled by water spray while being supported by support rolls, and finally solidified. A method is adopted in which a completely solidified slab is cut into predetermined lengths. To draw a slab, the slab is sandwiched between opposing support rolls called vinci rolls, and the rolls are rotated while applying pressure to the slab and pulled out downwards. It is thought that the above-mentioned internal cracks occur when the solidification interface ruptures due to tensile strain caused by bending, bend straightening, bulging, misalignment, thermal stress, etc. of the slab during continuous casting. ing.

Therefore, various techniques have been proposed to prevent the occurrence of internal cracks by eliminating the individual factors that cause the above-mentioned tensile strain, as described below. In addition, since the problem in the present invention is all about tensile strain, tensile strain is simply referred to as "strain" in this specification.

■ JP-A No. 59-197365 “Slab guide straightening device”
This device has rolls arranged so that the amount of change in the curvature of the straightening profile for each roll increases toward the rear of the straightening zone. According to the company, this allows the distortion (amount of correction) generated in the correction area to be dispersed, thereby preventing internal cracks.

■ JP-A No. 59-118254 "Bending/straightening crack prevention device in continuous casting machine"
Alternatively, it is a device in which the solidified shell support mechanism on the downstream side where tensile strain is applied is provided with a drive mechanism and a control mechanism, respectively. This applies compressive strain to the solidified shell on the side where the tensile strain is acting, so the tensile strain is alleviated and cracking of the solidified shell can be prevented.

■ Tetsu to Hagane, 73 (1987), 3909 r Development of online internal crack diagnosis and prevention system J This diagnoses the occurrence of cracks by determining the amount of strain for each roll and comparing it with the critical strain for crack occurrence. As a method to prevent zero cracking, compressive strain is applied to the solidification well of the slab corresponding to the roll position where cracking has been diagnosed, thereby reducing the strain and at the same time reducing the casting speed.

■ Materials and Processes, νo1.1 (198B), P2
S5 r Mold heat extraction and secondary cooling conditions during high-speed casting" This technology gradually increases the bulging strain at various parts in the longitudinal direction of the slab as the casting speed increases.
2. During on/1 inch high-speed casting, there are parts that exceed the critical strain for cracking, and based on the knowledge that internal cracks will occur in those parts, cooling in the vicinity is strengthened to prevent internal cracks. . However, there is no proportional relationship between the intensity of secondary cooling and the solidification rate, and even strong cooling beyond a certain level has little effect on improving the solidification rate. This method has its limitations, such as lowering the surface area and causing cracks on the surface.

Therefore, the following technology has been proposed, focusing on the fact that bulging strain varies greatly depending on the support interval between support rolls.

■ JP-A No. 50-62128 ``Method for supporting and guiding slabs in the secondary cooling zone of continuous casting equipment'' As an alternative to slab support rolls, a fixed supporter with surfaces parallel to the slab surface and spaced apart at regular intervals is used. This method reduces bulging by providing a plurality of rollers in the gap between the slab and the slab surface to reduce the support interval between the slabs.

Now, cracks also occur at the solidification interface if the pressure of the support roll is inappropriate, for example, if the slab is over-pressured while an unsolidified portion exists inside the slab during continuous casting. In particular, in the recent trend of high-speed casting, the range of unsolidified parts is expanded, so the dangerous area where such cracks occur is widened, and the requirements for optimizing the pressure of the support roll are becoming more severe. In addition, when the casting speed changes at the start and end of casting, or when there is a temporary change in casting speed due to predicted breakout, uneven heat is generated in the circumferential direction of the roll, causing the roll to bend. Even if the pressure of the rolls is appropriate, there is a risk that overpressure will occur in the slab and internal cracks will occur.

As a countermeasure to these problems, the following methods have been proposed.

■ Japanese Patent Publication No. 62-39065 "Method for correcting roll gaps in continuous casting machine" ■ Japanese Patent Application Publication No. 63-303668 "Continuous Slab Reduction Control Device" The above are various internal crack prevention measures proposed so far. However, each has the following drawbacks.

The above (■) focuses on corrective distortion, (■) focuses on distortion and correction distortion, (2) mainly focuses on cross-alignment distortion, and (2) and (2) focus on bulging distortion, respectively, and indicate measures to prevent internal cracks caused by these distortions. However, these methods merely determine the distortion between rolls or for each roll and discuss its magnitude.

In method ①, it is not suggested how long the rollers or the pitch of the wart-split rolls should be arranged. The aim is to make it as short as possible and shorten the intervals between rollers and rolls. However, when the spacing between the rollers and rolls is shortened, the number of times the slab passes through the rollers and rolls increases, and the influence of the H history of misalignment distortion increases, and the misalignment distortion itself in each roller and roll increases. Therefore, depending on the rollers or how they are arranged, the effect of preventing internal cracks may not be obtained.

Method (2) is a method in which the rolls are externally cooled to prevent roll spacing, but since the cooling water is also applied to the slab, the problem arises that the temperature of the slab becomes uneven and the surface quality deteriorates. In addition, not only elastic deformation due to thermal stress but also plastic deformation due to roll pressure occurs during long-term use, so it is not possible to eliminate roll gaps only by cooling the rolls.Furthermore, the normal pinch roll pressing pressure is Since it is constant regardless of speed fluctuations, if roll spacing occurs, the slab will repeatedly be under pressure and under pressure.

Method (2) is a method in which the solidified shell of the slab is supported by a surface instead of supporting the slab by rolls as described above. However, this is a method of reducing mold surface pressure only at the final solidification position to prevent center segregation, and is not a support method for preventing misalignment over a wide range.

(Problems to be Solved by the Invention) Various preventive measures have been proposed for internal cracking in continuously cast slabs, as described above, but there are still many problems when trying to increase the casting speed. As the speed increases, the solidification thickness becomes thinner and various strains are applied to the slab at high temperatures, making it more likely that internal cracks will occur.

In addition, in steel, the cracking susceptibility changes depending on the content of components such as c, s, and p, and therefore, the upper limit of the casting speed is often restricted depending on the content of these components.

In order to further increase productivity in continuous casting, a technology that can perform high-speed casting without causing internal cracks is required.

Despite various efforts to date, the reason why internal cracks cannot be eliminated is that conventional countermeasures for internal cracks only consider the magnitude of strain applied to each roll or between rolls; is not taken into account,
It is possible that the measures are only local. In other words, after the slab leaves the mold, the center undergoes various deformations until it is completely solidified, and the deformations received earlier may have an effect later on. In this case, internal cracks cannot be completely eliminated by taking local measures at the point (location) where the cracks occur.

In continuous casting, internal cracks occur most frequently, especially in the straightening zone, and although countermeasures have been taken based on various straightening theories (for example, example ① above), no reliable solution has been found. It has not been.

As mentioned above, the measures taken so far have only focused on the magnitude of strain received by a single straightening roll, and have not adjusted the placement of straightening points or the amount of straightening in consideration of the influence of the history of strain. This is because the problem is not considered and the measures are only localized.

The purpose of the present invention is to comprehensively examine the factors that cause internal cracks in continuous cast slabs, determine the limits of internal crack occurrence, determine casting conditions, and in particular to revise the straightening method for continuous slabs. A continuous steel casting method that manufactures slabs without internal cracks with maximum production efficiency by devising a method of supporting slabs at the straightening point, which greatly affects the accumulation of strain, and at the subsequent horizontal section until complete solidification. Our goal is to provide the following.

(Means for solving the problem) The present inventors 1. We comprehensively investigated the causes of internal cracks in continuous slabs and obtained the following knowledge.

■ The conventional way of thinking, which considers the strain at each position in the slab individually and discusses its magnitude, may not predict the occurrence of internal cracks.

■ The conventional method of determining the occurrence of cracks based on the amount of strain at individual locations without considering the accumulation of strain cannot quantitatively grasp the influence of casting speed.

■ During casting, each part of the slab reaches a tensile strength onset temperature (ZST).
) and the ductility onset temperature (ZDT), each time it is subjected to strain, it does not relax and accumulates as it is, then the total amount of accumulated strain is the critical strain of the type of steel to be cast. Internal cracks will occur when it exceeds the limit, and no cracks will occur if it is below that limit.

The invention of this application was made based on the above-mentioned knowledge, and the basic invention is that each part of the cast slab reaches at least the tensile strength appearance temperature (
Continuous casting method 1 for steel, characterized in that the total amount of strain received during the temperature range between ZST) and ductility appearance temperature (ZDT) is cast under conditions that the total amount of strain does not exceed the critical strain of the steel type to be cast. This is the summary.

First, this basic invention will be explained.

It is known that each part of a slab during continuous casting initially goes from a liquid state to a solid-liquid coexistence state, and then passes through a tensile strength onset temperature (ZST) and a ductility onset temperature (ZDT) during complete solidification and cooling. ing. These ZST and ZDT are values determined by the steel type, and the values are, for example, Arch, Eisenhiit
It can be measured by a known method according to TenweSen 54 (1983), 357. Also, ZST and Z
It is also known that the value of DT corresponds well to the solid phase ratio,
ZST has a solid phase rate of 0.8, and ZDT has a solid phase rate of 0.99.
It almost coincides with the point of
.

FIG. 1 is a diagram schematically showing the 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 gradually becomes thicker.
A solid-liquid coexistence layer (S+
L), now at a certain position P1 in the longitudinal direction (drawing direction) of the slab, if point A is defined as a point with a solid phase ratio of 0.8, tensile strength begins to appear from here. That is, the temperature at point A1 is ZS
It is T.

At the same position P, if the 81st point is a point with a solid phase ratio of 0.99, ductility appears from this point. The temperature at this point is ZD
It is T.

If the previous point A1 is followed in the drawing direction of the slab, ZDT will be reached at the point At, and ductility will appear.

ZST is the maximum temperature at which stress begins to be applied and strain begins to occur.
ZDT is the temperature limit below which cracking does not occur even if strain occurs due to ductility. During continuous casting, each part of the slab is subjected to various strains as described above due to bending, bulging, straightening, thermal stress, etc. while passing from ZST to ZDT.

The present inventors have calculated that the total amount of accumulated strain (Σε and It has been confirmed by repeating various experiments that cracks occur when the limit value 1 (limit strain ε9) is exceeded, and cracks do not occur below this limit.

Figure 2 is one of the experiments and explains the method for measuring the critical strain for cracking (details can be found in the paper by the present inventors, Materials and Processes, Vol. 01988).

1229).

The critical strain for internal cracking can be determined by applying various deformations to a small ingot cast under almost the same cooling conditions as continuous casting slabs, and examining the magnitude of the strain and whether or not cracks occur. I asked for it. That is, a uniaxial tensile deformation was applied to an ingot whose center portion was in an unsolidified state as shown in FIG. 2 to cause strain. The amount of deformation, the number of times of deformation, and the time interval between deformations were varied, and these were combined to give deformation. Further, a thermocouple for temperature measurement was set inside the ingot, and while checking the temperature, the amount of strain applied between ZST and ZDT out of the total amount of strain was determined.

By changing the cooling conditions within the conceivable range for continuously cast slabs, we can change the cooling rate at the temperature measurement points and achieve ZST at each point.
- Vary the time between ZDT.

FIG. 3 shows the results of examining the presence or absence of internal cracks by taking the cooling conditions on the horizontal axis and the total amount of strain produced on the vertical axis. The amount of strain is shown separately as the total amount of strain that occurs (Σε) and the total amount of strain added between ZST-ZDT (Σε)ZST.

As shown in Fig. 3, when Σε exceeds about 1.6% ZST, internal cracks occur, and this becomes the critical strain. It can also be seen that this critical strain does not change depending on the cooling conditions. On the other hand, the crack occurrence limit determined by the total applied strain Σε is larger than the above-mentioned approximately 1.6%, because the strain applied below ZDT is also counted, and that strain is absorbed as elongation due to the ductility of the solidified shell, and does not become a factor in cracking.
The crack occurrence limit value based on the total amount of strain Σε changes depending on the cooling conditions, and increases as the cooling rate increases.

Although Tt is a combination of the various strains mentioned above, ντT Regardless of the type of deformation, cracking occurs when the total amount of strain exceeds a limit value. The main strains that are applied to slabs during continuous casting are bending strain, straightening amount, bulging strain, ξ alignment strain, and thermal strain. Among these respective strains, the sum of the strains applied when each part of the slab is between ZST-207 is Σε1 and Σε, respectively. , Σε1, Σε, and Σε
, then let Σε, ten Σε5+Σε, +Σε, +Σε be E. This E is a certain limit value εC (approximately 1.
6%), internal cracks will occur. Therefore, until the slab reaches complete solidification, at least E≦ε,
It is necessary to perform casting while maintaining the following conditions. In addition,
Depending on the type of steel, if there is a temperature range where the ductility is extremely low even in the temperature range below the ZDT, it is preferable to expand the temperature range to a temperature range where the ductility is sufficiently large and suppress the total content in that range to below the critical strain.

The critical strain for cracking (εC) is almost constant depending on the type of steel to be cast. The value is particularly affected by the content of components such as c, s, and p, and the higher the content of these components, the more likely cracks will occur. This is an example of critical strain. The critical strain for crack generation itself is known, for example, from ``Mechanical Behavior in Continuous Casting,'' April 1985, Iron and Steel Institute of Japan, Steel Basics Joint Research Group Materials, pp. 3-7. It is used as a method to prevent cracking by suppressing the amount of strain caused by deformation to below this value. Although the limit strain data obtained from such conventional one-time deformation can also be used in the method of the present invention, these limit strain data are
The range between T-Zl)7 is not considered, and depending on the cooling conditions of the test material, it is thought that the amount of strain applied outside the range between ZST-207 is also included. Therefore, when using these data, the cooling conditions of the test material must be considered.

(The following is a blank space) Table 1 The amount of various strains applied to the slab during the casting process is determined by determining the temperature of the slab at each position of the continuous casting machine by heat transfer analysis, and the temperature dependence of the mechanical properties of the steel to be cast. It can be determined by finite element method stress analysis (FEM) that takes into consideration factors such as elasticity, thermoelasticity, and creep.

As mentioned above, the basic invention of this application is that each part of the slab is
The gist of this is to perform casting under conditions such that the total amount of strain received while in the temperature range between ST and ZDT does not exceed the above-mentioned critical strain. A specific method for implementing this will be described below. The methods can be broadly classified as follows in principle.

■ A method of increasing the critical strain (ε,) of the steel to be cast.

Since the critical strain of steel is affected by its component composition, the content of components (C, P, S) that lower the critical strain is lowered. This method is also effective for reducing the distance between ZST and ZDT described below, but the required C of the steel
This is not a very practical method because the content is specified and there are limits to refining technology in reducing P and S.

■ How to reduce E.

This method is further subdivided as follows.

■-1 A method of reducing the distance between ZST and ZDT positions.

As mentioned above, it is ZST to ZD that cause internal cracks.
Since this is the accumulated strain between T, if the interval is shortened, the accumulated strain (E) becomes smaller. Specifically, measures such as reducing the casting speed, strengthening cooling, and reducing the thickness of the slab can be considered, but reducing the casting speed reduces productivity, so it is not recommended to adopt such measures. It is difficult and impractical to change the thickness of the slab just to prevent internal cracking; in the end, what is practical is to strengthen the cooling.

(2) If there is a particularly large one among Σε1, Σε1, Σε1, Σε, a method of bringing it out of the range of ZST-ZDT.

For example, if the straightening points that apply the straightening amount are distributed so that when each part of the slab passes at least one of the straightening points, the temperature is outside the range of ZST-ZDT, Σε,
becomes small, and as a result, E can be suppressed to ε or less. This method can be carried out by changing the design of the continuous a casting machine, but the purpose can also be achieved even if the ZDT appears early by the above-mentioned weight reinforcement.

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

This is the above-mentioned Σε1, Σε1, Σε2, Σε1, Σε
For example, in order to reduce Σε, there is a method to reduce bulging by making the roll arrangement denser or by increasing the rigidity of the continuous casting machine. Other distortion reductions can also be carried out by methods known per se.

Another effective method is to actively apply compressive strain to the slab in the casting direction to offset the tensile strain that causes internal cracks.

The method of the present invention, which is implemented using these various specific methods, predicts the occurrence of internal cracks during casting operations and issues a warning;
It can also be used to prevent cracks from occurring by changing casting conditions.

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

In conventional roll support type continuous casting machines, the peak of accumulated strain appears particularly from the straightening section to the horizontal section.

Therefore, the amount of bulging in this region is actually measured, the amount of bulging (ε,) is calculated from that value, and, for example, secondary cooling is strengthened so that the total accumulated strain does not exceed the critical strain.
Operational techniques such as controlling casting speed are also effective in reducing internal cracking.

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

(i) Straightening points by straightening rolls are arranged so that when each part of the slab passes through at least one straightening point, the temperature is higher than the tensile strength onset temperature (ZST) or lower than the ductility onset temperature (ZDT). A continuous steel casting method characterized by:

This method is characterized by suppressing the amount of accumulated strain below the critical strain by distributing the bending correction points that have the greatest effect on strain accumulation, thereby making it possible to extremely effectively prevent internal cracks.

(ii) Adjust the rotation speed of the support rolls in areas where 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 type to be cast. , a continuous steel casting method in which the slab in that area is cast by applying compressive strain in the pouring direction.

In this method, for example, compressive strain is applied to the slab in that area by applying pushing force to the roll group upstream of the area where the accumulated strain exceeds the critical strain for internal cracking and applying braking force to the downstream roll group. It is characterized by suppressing the amount of accumulated strain below a critical strain.

(iii) Supporting the continuously cast slab having an unsolidified phase inside r with support rolls that are divided into a plurality of parts in the width direction of the slab and arranged in a staggered manner (hereinafter referred to as staggered divided support rolls); and the supporting length W of the slab in the width direction of the slab by the split support rolls and the interval between the successive split support rolls in the casting direction, W/L when the width end of the slab is included in the length W. ≦1.5, W if not included
/L≦2.0, and the spacing and staggered arrangement section of the split support rolls are determined based on the tensile strength appearance temperature (
Continuous casting of steel, characterized in that the total amount of strain that the slab undergoes while in the temperature range between ZST) and ductility appearance temperature (ZDT) is determined so that it does not exceed the critical strain of four types of casting. This method uses staggered split support rolls and appropriately determines the arrangement section, slab support length, and roll spacing to keep the amount of accumulated strain due to bulging and misalignment below the critical strain. It is characterized by

(iv) Continuous casting method for steel in which the horizontal part of the slab from the bend straightening part until complete solidification is supported by a surface that moves in synchronization with the slab.'' The feature is that the horizontal part from the straightening section until the slab completely solidifies is supported by a surface that moves in synchronization with the slab.
This suppresses M stacking to a level lower than the critical strain and effectively prevents the occurrence of internal cracks. When implementing this method, it is desirable to provide a displacement meter between the support surfaces and operate while controlling the distance between the surfaces, that is, the thickness of the slab, to be kept constant.

EXAMPLES Hereinafter, the present invention will be specifically explained along with its desirable aspects using examples.

(Example 1) Figure 4 shows 27 (1w+m thickness, 1800+nm width C:
Solidification interface of each roll (solid phase ratio ZST 0.8) is expressed as a distance from the meniscus. The amount of strain is small, about 0.3% at most, and does not exceed the critical strain for cracking (1.6%) for this steel type.

FIG. 5 shows the positions of ZST and ZDT at this time in terms of distance from the meniscus. At 1t (point B), which is about 22m from the meniscus in the figure, ZDT is at a position of about 80 degrees from the surface of the slab, and this point becomes ZST at a position about 14m from the meniscus (point A). Yes, therefore B
This means that distortion between A and B has been accumulated at the position of the point.

Using the same concept, the M stack at each position from the meniscus is calculated as shown in FIG. The accumulated strain is at its highest immediately after exiting the correction area, and exceeds the critical strain for cracking a little before exiting the correction area.

When the cross section of the cast slab was sulfur printed,
The presence of internal cracks was confirmed at a position 80 to 90 mm from the epidermis. From FIG. 5, it can be seen that this position corresponds to the vicinity of the exit side of the correction area, and that cracks have occurred in the portion where the strain exceeds the critical strain. On the other hand, no cracking occurs in areas where the critical strain is not exceeded.

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

Figure 4 above shows the amount of strain at each position without considering the accumulation of strain, and although it is maximum at a position approximately 3.5 m from the meniscus (position where bending strain is applied), it is still approximately 0.33 m. It is a small value of %. Since these amounts of strain are smaller than the critical strain (1.6%) at any position, conventional wisdom would indicate that no cracking occurs, but in reality, cracking does occur.

In other words, the occurrence of cracks in this case cannot be predicted using the conventional method of individually measuring strain at each position of the slab.

(Example 2) Figure 7 shows the same type m as in Example 1 at a casting speed of 2.3 m.
This figure shows the positions of ZST and ZDT when casting was performed at two high and low levels of 1.0+a/l1in and 1.0+a/min.

Figure 8 shows ZST and ZD obtained in the same manner as in Figure 6 above.
This is the total amount of distortion between T. The width between ZST and ZDT increases as the casting speed increases, and the range of strain accumulation increases accordingly. As a result, the amount of accumulated strain increases as the casting speed increases, as shown in FIG. When the casting speed was 2.3 m/min, the amount of accumulated strain greatly exceeded the critical strain and internal cracks occurred. On the other hand, when the casting speed was 1.0 m/win, no internal cracks occurred.

Although not shown, the amount of strain at each position without consideration of strain accumulation increases by only about 20% as the casting speed increases from 1.0 m/win to 2.3 m/ll1n.

However, when considering the accumulation of strain, the total placement amount becomes significantly larger due to the expansion of the strain M product range (the width between ZST and ZDT) due to the increase in casting speed. In this way, 1 of the strain
There is a clear difference between the conventional method of determining the occurrence of cracks based on the amount of strain at each location without considering the product, and the method of the present invention. It is almost impossible to quantitatively understand the relationship between

(Example 3) Figure 9 shows Nα1, No, 2, Nα shown in Table 1 above.
These are the results of examining the occurrence of internal cracks by casting slabs of the same size under the same cooling conditions and varying the casting speed for steel types No. 4. The casting speed is plotted on the horizontal axis, and the maximum value of the M stacking amount obtained in the same manner as in FIGS. 6 and 8 is plotted on the vertical axis, and the shots at which internal cracks occur are shown for each steel type.

It can be seen that internal cracks occur in each steel type when the critical strain values shown in Table 1 are exceeded. Even if the steel types are different, if you understand the critical strain of each steel type, find ZST and ZDT for each steel type, and calculate the accumulation of strain between them, continuous casting can be performed under conditions that do not cause internal cracks. be able to.

(Embodiment 4) This embodiment focuses on corrective distortion that occupies a large portion of the total accumulated position, and is an example of reducing the distortion as much as possible. Specifically, by dispersing the straightening points, in other words, by widening the straightening area, when each part of the slab is in a higher temperature range than the tensile strength onset temperature (ZST) or lower than the ductility onset temperature (ZDT), At least one of the correction points is passed when the object is in the area, or both. All parts of the slab are straightened by Z.
If the correction is performed between ST and ZDT, the accumulation of correction distortion becomes extremely large, so at least part of the correction is performed outside the range of ZST and ZDT to keep the accumulated distortion as small as possible. As a specific means for this purpose, it is also possible to adopt a method of strengthening cooling of the slab to narrow the temperature range between ZST and ZDT. However, in recent years, the technology of directly linking continuous casting and hot rolling to roll continuously cast slabs without heating or with minimal auxiliary heating has become contradictory. It is desirable to raise the temperature of

Therefore, when implementing this method, it is preferable to change the design of the continuous casting machine so that at least a portion of the bending straightening is performed outside the range of ZST - ZDT rather than strengthening the cooling.

The steel type, slab size, and casting speed employed in this example are the same as those in Example 1.

First, for comparison, the correction points are placed in the strain accumulation range (ZST and Z
We will discuss the test results of casting with the conventional arrangement without considering the temperature range between DT and DT.

Adjust the correction point to 18m, 19.2m from the meniscus, 20.
Figure 10 shows the solidification external surface strain when four points are placed at positions of 4 m and 21.6 m. (Figs. 10 and 11 are substantially the same as the previous Figs. 4 and 5, but for convenience of explanation, they will be repeated.) Fig. 1O shows each part of the slab (meniscus). As shown in the figure, thermal stress strain and bulging strain occur over the entire section, but eruption strain occurs due to the curvature of the slab. and the amount of correction occurs in the oily correction area, respectively. No matter which point you take, the maximum amount of strain is about 0.33%, which does not exceed the cracking limit strain of 1.6% for this steel type.
However, in reality, internal cracks occur, and the cause is ZST.
This is because the amount of strain accumulated in the temperature range between ZDT and ZDT exceeds 1.6%.

Figure 11 is a diagram showing the positions of ZST and ZDT at this time.
line) and 90mm (line B-B), the entire correction area (approximately 18-22m from the meniscus) is ZST and ZD.
As a result, the entire amount of correction will be accumulated. FIG. 12 is a diagram showing the amount of accumulated distortion. As shown in the figure, the limit strain of this steel type is 1.6% within the straightening area.
This causes internal cracks to occur.

Next, in accordance with the present invention, the arrangement area of the straightening roll is expanded,
The results of a test in which straightening was performed even when each part of the slab was at a temperature higher than ZST or lower than ZDT will be explained.

As shown in FIG. 13 (the same type of diagram as the above-mentioned FIG. 10), seven correction points were distributed in a range of 13 m to 25 m from the meniscus. The placement point is 13m from the meniscus, 15
The positions are m, 17m, 19m, 21m, 23m, and 25m. By distributing the correction points to a larger number of points, the amount of correction per correction point is reduced to the original amount of approximately 172. 14th
As shown in the figure, the positions of zST and ZDT are the same as in Fig. 11, but the arrangement of the correction points is different, so the range of the correction area is wider than the strain accumulation range of ZST-ZDT at any location. For example, about 80 degrees from the ingot surface! (A
-A line), the rear part of the correction point is lower than ZDT, and at the position 90III11 (B-B line), the front part of the correction point is higher than ZST. Therefore, at least a part of the correction amount will not be multiplied by M. As a result, as shown in FIG. 15, the total amount of accumulated strain was less than the critical strain, and no internal cracks occurred.

In the example (comparative example) with a narrow straightening area shown in Fig. 10, internal cracks occur in the straightening area at a casting speed of 1.6 m/sin, whereas in this example, the casting speed is 1.8+m/win.
As a result, we were able to improve productivity.

As mentioned above, the strain accumulation range (width between ZST and ZDT) increases as the casting speed increases, and the strain accumulation range also widens accordingly. Therefore, when arranging straightening points, keep in mind the limits of the casting speed due to constraints from other sources in the continuous casting machine (for example, constraints due to the solidification phenomenon within the mold), and avoid internal cracks within this casting speed. It is better not to release it.

FIG. 16 is a diagram illustrating the influence of the ratio of the strain accumulation range to the width of the correction area on the maximum casting speed (maximum limit casting speed) that can be cast without internal cracking.

That is, using a slab of the same steel type and size as the previous slab, a test was conducted in which the straightening points were dispersed, and the critical casting speed at which internal cracking occurred was determined. As an index, the strain accumulation range (ZS
The highest value of T-ZDT) was used. The denominator of the above equation is the maximum value of the strain M product range (ZST-ZDT) when the continuous casting machine is operated at the highest possible casting speed. FIG. 16 shows the test results, with the value of Ra taken on the horizontal axis and the critical casting speed for internal cracking on the vertical axis. Ra is 1.0
The critical casting speed begins to increase when exceeding , and as the straightening region is made wider than the strain accumulation range (Ra becomes larger), the critical casting speed for internal crack occurrence increases.

(Example 5) This example attempts to reduce accumulated strain by applying compressive strain to the crack occurrence area of a slab. Specifically, the accumulated strain received while in the temperature range between ZST and ZDTO during casting is calculated by heat transfer analysis and stress analysis, and the area of the slab where the accumulated strain exceeds the critical strain of the steel type to be cast, That is, the crack occurrence area is determined. Then, compressive strain is applied in this region in the longitudinal direction (casting direction) of the slab.

For example, as shown in Fig. 17, as a specific means of applying compressive strain to the crack occurrence area of a cast slab, the driving rolls (Ro, R1, R1, . . . Drive roll of generation area (RlRt-+, Rt.7.・
・・R+-,,-+), and the drive roll after the crack occurrence area (RIIRI RI+*+l+ RI+a+2+・
...Rent) rotation speeds, respectively. +Vl+
V! +...V, -1, ■.

■I m I + V l +□・・・V i+n-1
, and V, □■i, □1. ■i and 70. ...■,,
, d, then Vo = V + = V z .
・= V t −I> V t≧V r + +≧V1
□・ ・ ・≧■i□−+ > V ,,, l== V
l+,, +1-V (+,, *t...=V-,,
a (However, ■., vl.

v2...Vl-1 is the rotational speed of the roll that is synchronized with the casting speed), and the slab may be drawn. That is, by adjusting the roll rotation speed, Ri+
R1411R+*z+... Compressive strain acts between the rolls and at the roll portion of R15L1-1, and compressive strain can be applied to the entire crack occurrence area.

The rotational speed of the rolls is adjusted depending on the occurrence of internal cracks on the top and bottom sides of the slab, while keeping the rolls on one side (for example, the bottom side) at a speed synchronized with the casting speed. You can also target only the roll on the top side (for example, on the top side).
Further, the rotation speeds of both the top and bottom rolls may be adjusted as described above.

This example will be explained in detail below. Note that the steel type, slab size, and casting speed used here were also the same as in Example 1.

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

In the comparative example where compression casting was not performed, points A and B in the figure
The accumulated strain between the points exceeded the critical strain for internal cracking, and internal cracking had occurred in the slab slab.

In compression casting, the range to which compressive strain is applied was set between A and -82 in FIG. 18 in consideration of the strain accumulation range. That is, before compression casting, the critical strain is exceeded at point A, and this accumulated strain is related to the strain added between A and -A in the figure. The limit strain is further exceeded up to point B, but the accumulated strain at point B is
The strain added between I and B2 is involved. Therefore, a braking force was applied to the roll group after 82, and a pushing force was applied to the roll group before AI, thereby applying compressive strain to the slab in the cracking area. As a result, as shown by the dashed line (-) in FIG. 18, the peak value of accumulated strain became less than the critical strain, and internal cracks in the slab completely stopped occurring.

As mentioned above, considering the strain accumulation range, the compressive strain is enough to alleviate the amount exceeding the critical strain (here, 0.45% compressive strain)
Internal cracking could be prevented by giving

FIG. 19 shows the compression casting of the present invention using the steel type and billet size as described above and varying the casting speed in the range of 1.0 to 3.0 m/l 1 inch (the amount of accumulated strain was calculated for each casting speed, The difference from the critical strain was calculated, and a compressive strain greater than that difference was given.)
These are the results of investigating the charge rate at which internal cracks occur in slabs. The comparative example shown by the solid line in Fig. 18 is an example in which compression casting was not carried out, and the conventional example shows only the correction area and the maximum correction strain ( This is an example in which a compressive strain of 0.1% to 0.2% was applied.

As shown in FIG. 19, in the comparative example in which compression casting was not performed, internal cracks began to occur for each charge when the casting speed became 1.5 m/min or more. In addition, in the conventional example, the rate of internal crack occurrence charge was lower than in the comparative example, but it increased as the casting speed increased. This is because as the casting speed increases, the strain accumulation range increases and cannot be addressed by local methods.

On the other hand, in the examples of the present invention, no internal cracks occurred even at a high casting speed of 3 m/win, and the effect of preventing internal cracks by the old anode of the present invention is clear.

(Example 6) This example is an example of using staggered split support rolls to reduce bulging strain and minimize the N build-up experienced while in the temperature range between ZST and ZDT during casting. .

FIG. 20 is a diagram showing an example of the roll arrangement of the staggered split support rolls. The support rolls (R-a, R-b) that hold down the long sides of the slab S are divided into a plurality of pieces in the length direction (s) (f-width direction) and arranged in a staggered manner. Therefore, for example, compared to the case where the support rolls are not arranged in a staggered manner, the bulging strain (ε,) of the row of divided rolls R-a becomes lower due to the deformation suppressing effect of the adjacent row of divided rows JLtR-b. However, the relationship between W/L and ε in the case of colt-aligned rolls differs depending on whether the roll includes and supports the slab width end, so W/L should be adjusted appropriately depending on each roll. Unless set, internal cracking cannot be prevented.

W/L of the staggered split support roll shown in Figure 20
Figure 21 shows the relationship between the maximum strain (calculated value) in the casting direction directly under each roll.
The values are normalized by ε5 of ≧4.1). In the roll that supports the slab including the width end of the slab (roll R-a in Figure 20, indicated by the mark in Figure 21), W, /L
≦1.5, width center roll (20th
For roll R-b in the figure (indicated by a circle in Figure 21), the length in the width direction of the slab (W) is set so that W, /L≦2,0.
By determining ε, it is possible to reduce ε to 90% or less of the conventional (non-divided staggered) roll arrangement.

Next you need to determine the roll spacing, in this case,
As already mentioned, the history of strain must be taken into account; the shorter L is, the smaller the bulging strain (ε,) will be.
The number of rolls passing through each part of the slab increases while it is in the ZST-ZDT range, resulting in misalignment distortion (
The accumulated amount of ε, ) will increase. Moreover, when L is shortened, ε for each roll also increases.
is in LS, ε.

was confirmed to be proportional to Ll.

In addition to L, the absolute value of ε changes depending on the solidified shell thickness, 18fi static pressure, slab salinity, casting speed, and W/L, and ε1 is the solidified shell thickness and misalignment amount δ((mn+):
>O), but in most cases with normal curved continuous casting machines, ε, = 0.03 to 0.3%, ε, -
It falls within the range of 0.1 δ to 0.4 δ%. Here, roll alignment is managed so that δ approaches O, but it is usually around 0.5 mm due to gaps in the roll bearings, deflection of the segment frame that fixes the rolls, wear of the rolls, and thermal deformation during casting. Therefore, it is impossible to shorten the roll interval unnecessarily, which would lead to an increase in ε1.

In this example, the solidified shell I'X92 near the straightening part of a curved continuous caster, the static pressure of molten steel 0.095 kg/m+a'',
Under the condition of slab surface temperature 900°C, casting speed 2
．． 0m/min, L = 100~400I1m, W
= 10(1(ε5 in the lnm case, and δ
ε1 was calculated when −0,25+n+a. Since the straightened area and thermal stress strain hardly change depending on the roll spacing, only ε and ε were studied.

FIG. 22 shows ε when the roll interval (L) is changed,
It shows the change in and ε. Since ε is proportional to L5, it should change as shown by the dotted line a in the figure, but due to the effect of W/L increasing as L decreases, the change in ε actually becomes a solid line. . On the other hand, ε.

becomes a solid line C. As shown in the figure, ε can be reduced by making L as small as physically possible, but on the other hand, ε increases, making it difficult to determine the optimal roll spacing. In this case, the optimum value of L is determined as follows, taking into account the accumulated strain of ε and ε.

Figure 23 is the same figure as the above-mentioned Figure 5, etc., and shows ZST and ZD.
The location of T is indicated. Each part of the slab is ZST~ZDT
The number of times the support roll is passed while passing through the range can be read from the diagram. 92m+m from the slab surface
It is about 20 minutes from the meniscus that the point at
．． It was the IA point at about 5m, and it was about 28 that passed the ZDT.
The mth place is the IZB point. Consider the history of ε5, ε, received by rolls in the range of 7.51I during this period.

Bulging strain (ε,) occurs every time the slab passes through the rolls in this section AB, but misalignment strain (ε,) rarely occurs on each roll, and roll 2
This often occurs in more than one book. Therefore, assuming that ε1 occurs for every two rolls, the strain ff is
Considering the history, it is possible to obtain the sum Σε, 10Σε, of the bulging □ device and misalignment accumulated strain in this case.

FIG. 24 shows the relationship between the roll spacing (L) and the bulging and spacing alignment M devices.

When L = 400mm, Σε, +Σε is 1.6%
Although it is weak, it decreases as L decreases, reaches a minimum between L=320, increases after that, and reaches almost the same value as L=400 questions at L=26511111. Actually Σε.

In addition to
ε1←Σε+Σε, +Σε1 is less than 1.6%, Σε, +Σε0 is 1.35% or less, that is, L=3
The length is in the range of 10 mm to 340 mm, and the occurrence of internal cracks cannot be prevented outside of this range. Note that when the history of strain is not considered, the absolute value of strain is very small compared to the critical strain, as seen from the strain values shown in FIG. It is difficult to find the right roll spacing.

Next, specific test conditions and results will be explained.

(Test conditions) Target steel type...C: 0.15%, p: o, oz%, s:
o, ot3% carbon steel slab (slab) Slab size: 270 mm thick, 1800 mm width Casting speed: 1.5 to 4.3 m/min Continuous casting machine: Vertical bending bay type (machine length 43I11) Correction section section: Distance from the meniscus fi18m to 22I11 range When casting with conventional support rolls, casting speed (Vc) 1.6m/a+in or more, distance from the meniscus range from 20m to 28m (Roll spacing L: 400 m++s) Internal cracking occurred.

Therefore, in accordance with the present invention, a staggered split support roll was applied to a section with a distance of 15 to 28 m from the meniscus of this continuous Vi casting machine, taking into account the strain history between ZST and Zn2. One of them is the roll arrangement shown in FIG. 25 (this is called type I), and the roll interval is shorter than that of the conventional one.

The other type is the roll arrangement shown in FIG. 20 (referred to as type n), in which the roll spacing is the same as that of the conventional roll arrangement.

Table 2 shows the dimensional specifications of the r′0 array split support roll and the critical casting speed (Vcmax, m/m
1n).

When the support length W by the rolls, the roll spacing, and the staggered arrangement range are within the range of the present invention, the limit Vc that does not cause internal cracks as shown in Table 2 is 1.5 m/ min (comparative example) to 2.2o+/w
in (in the example), 1.6 m/w for type H
in (2.Om/min from comparative examples ■, ■, ■
Example 2). Comparative example ■ is W1
/I- and W, /L, comparative example ■ is W, /L, comparative example ■
W, /L is out of the range of the present invention, and W, /L≦1.5 and W, /L≦2.0 are desirable to prevent internal cracking. Comparative example Although the installation range was made equal to the range where internal δ warping occurs when using conventional rolls,
The effect of preventing internal cracking by the staggered split support rolls was not achieved. In the example, the effect of preventing internal cracking was obtained, and it is clear that the installation range of the staggered rolls must be determined in consideration of the strain history between ZST and ZDT.

The staggered split support roll of the present invention was applied to a vertical bending bay type continuous casting machine (machine length 15 m) with a slab ff1oo opening and a maximum slab width of 1000III11. [C] =0.
12%, CP) = 0.018%, (S) = 0.014
When casting % of steel using conventional support rolls, V
When c=3.6 m/ll1 inch or more, internal cracks occurred in a range of 8 to 10 m from the meniscus (the roll spacing in this area was 245 mm). The generated strain is the critical strain ε.

5 to 1 from the meniscus considering the strain history so that it is less than
The range of 0 m was arranged into a staggered roll arrangement of type (2) with the dimensions shown in Example (2) in Table 2 above. In this case, the internal crack generation limit casting speed Vcmax is set to 4.3 m/+in.
I was able to improve my performance until now.

(Embodiment 7) This is an example in which the slab is supported by a surface that moves in synchronization with the slab in the danger area of internal cracking, thereby reducing the accumulated strain due to bulging and misalignment.

As mentioned above, in the case of a normal curved continuous casting machine, the peak of accumulated strain appears within the range from the straightening section to the horizontal section where the slab completely solidifies, and when this exceeds the critical strain, internal cracks occur. .

Figure 26 shows the accumulated strain inside the slab when continuous casting is carried out using a conventional roll support type curved continuous casting machine.As shown in the figure, the bulging strain ( ) and misalignment distortion (ε, )
a) accounts for a large proportion. Internal cracks occur in the horizontal part from the straightened part where the total strain exceeds the critical strain (εC) until complete solidification. In a curved continuous casting machine, the occurrence of straightening portions (εU) and thermal stress strains (εL) is unavoidable. Therefore, in order to make the total M stack smaller than the critical strain, it is necessary to take measures to reduce the bulging strain and the misalignment strain.

However, as long as the straightening section and the horizontal section are supported by rolls that are in line contact with the slab, bulging distortion will occur, and misalignment distortion due to oil leakage caused by thermal deformation of the rolls will also be quite large, and these distortions and It is extremely difficult to keep the total accumulated strain including thermal 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 in a bend straightening section and a horizontal section of a slab according to the present invention. In this example, the straightening section A and the horizontal section B are independent supports 1 and 2, respectively (each consisting of a pair of upper and lower slabs S).
It is supported by The support body 1.2 has a caterpillar shape in which segments 3 are connected, and supports the slab by coming into surface contact with the upper and lower surfaces thereof while moving in synchronization with the moving speed of the slab.

As shown schematically in FIG. 28, the segments 3 constituting the caterpillar are pressed against the surface 6 of the slab S by a mechanism consisting of a hydraulic cylinder 4 and a small diameter roll 5.
The individual segments 3 are always pressed with at least one small-diameter roll 5 . This pressing force may be sufficient to suppress the entire contact surface of the caterpillar with the slab from being bent.Note that the movement of the caterpillar (rotation in the direction shown by the arrow in Fig. 27) This can be carried out in accordance with the moving speed (casting speed) of the slab by a method known per se. The supports for the correction part and the horizontal part may be integrated, but it is easier to design and manufacture them by separating them as shown in FIG. 27. In this case, it is desirable to provide a roll support 7 between both supports to suppress bulging in this area, and the support (and its segments) need to be appropriately cooled by water spray or the like.

The above-mentioned support is applied to the part where the accumulated strain exceeds the critical strain, as shown in Fig. 26, that is, from the straightening part A to the point C where the slab in the horizontal part B completely solidifies, as shown in Fig. 26. is sufficient. Other parts (for example, the upstream side of the caterpillar) may be supported by rolls as in the past, but it will be more effective if the previously described staggered arrangement divided support rolls are used in combination.

As shown in Fig. 27, displacement meters 8 are installed at three locations on each support: the entry side, the center, and the exit 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 presses each segment depending on the value so that the thickness of the slab is always constant.

This example will be specifically described below based on the test results.

A slab with a thickness of 200 mm + and a width of 1800 mm was cast at a casting speed of 3.0 mm by vertical bending type continuous casting i (2 strand type) with an oil refinement radius of 10 m. Casting was performed on/min. The steel type is carbon steel shown in Table 3.

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

FIG. 29 shows the internal crack occurrence code determined by the sulfur print on the cross section of the slab.

As shown in FIG. 29, internal cracks are significantly reduced in the surface support method of the present invention compared to the conventional roll support method. The results of measuring the distance between surfaces using a displacement meter show that the method of the present invention has ±0. It is within the range of lll1m,
While the alignment was almost normal, the results of measuring the roll spacing using the conventional method showed periodic fluctuations of about 2 degrees. This is due to the bending of the rolls, and it is presumed that this bending caused the slab to be over-compressed, resulting in frequent internal cracks.

FIG. 30 is a diagram showing the state of internal strain accumulation in a slab when the method of the present invention is used. Bulging distortion and misalignment distortion are virtually eliminated from the correction part to the horizontal part,
The total accumulated strain is much smaller than the critical strain, which is the second
This is a major factor in reducing the number of internal cracks shown in Figure 9.

(Effect of the invention) The present invention is based on new knowledge obtained as a result of comprehensively investigating the causes of internal cracks in continuously cast slabs.
This provides an innovative method for controlling the accumulated distortion between the ZDT and the ZDT.

According to the continuous casting method of the present invention, it is possible to accurately determine the limit of casting without causing internal cracks.
It becomes possible to produce slabs with good wall quality and no internal cracks at the maximum casting speed depending on the type of steel to be cast. Specifically, we have developed a casting method that disperses straightening points outside the strain accumulation range, a compression casting method that applies compressive strain to areas where the accumulated strain inside the slab exceeds the critical strain for internal cracking, and a casting method that applies compression strain to areas where the accumulated strain inside the slab exceeds the critical strain for internal cracking. Same as staggered split support roll or slab! III. The present invention can be implemented by a method of supporting the moving surface.

[Brief explanation of drawings]

Figure 1 is a schematic cross-sectional view of a continuously cast slab to explain ZST and ZDT. Figure 2 is an explanatory diagram of the experimental method for determining the critical strain for cracking. Figure 3 is the total amount of strain applied to the slab. FIG. Figures 4 to 6 relate to Example 1, and Figure 4 is
Figure 5 is a diagram showing the results of determining the amount of strain for each roll position (distance from the meniscus) of the continuous casting machine. Figure 5 is a diagram showing the positions of ZST and ZDT under the casting conditions of the example. Figure 6 is the same. A diagram showing changes in accumulated strain in Examples,
It is. FIG. 7 and FIG. 8 relate to Example 2, and FIG.
ZST and ZDT in examples at two different pouring speeds
FIG. 8 is a diagram showing the change in accumulated strain. 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. Figures 10 to 16 relate to Example 4, and are related to Example 1.
Figure 0 shows the strain generated at the solidification interface when the dispersion of the straightening points is not measured (conventional method), and Figure 11 shows the ZS when the dispersion of the straightening points is not measured.
Figure 12 is a diagram showing the relationship between the T-ZDT position and the correction area, Figure 12 is a diagram showing the amount of accumulated strain when the correction points are not dispersed, and Figure 13 is the case when the correction points are distributed. Figure 14 shows the strain generated at the solidification interface of ZST when the straightening points are similarly distributed.
- A diagram showing the relationship between the position of ZDT and the correction area, Figure 15 is a diagram showing the amount of accumulated strain when the correction points are similarly distributed, and Figure 16 is a diagram showing the strain accumulation range (ZST-ZDT) This is a diagram showing the relationship between (1! (Ra), which is the width of the straightening area divided by the highest value of Regarding example 5, the first
Figure 7 is a schematic diagram showing an example of a compression casting method, and Figure 18 is a ZST diagram with no compressive strain and with compressive strain.
Figure 19 shows the relationship between the position of ~ZD↑ and the compressive strain application area and the amount of accumulated strain. Figure 19 shows the relationship between internal cracks in slabs and casting speed when compressive strain is not applied and when compressive strain is applied. FIG. FIG. 20 to FIG. 25 are diagrams related to Example 6, and FIG.
The figure is a schematic diagram showing an example of a staggered split support roll. Figure 21 shows the bulging strain (ε,) and W when the staggered split support roll includes and does not include the slab width end.
Figure 22 shows the relationship between bulging strain (ε,) and misalignment strain (ε,) when using a conventional support roll.
FIG. 23 is a diagram showing the positions of ZST to ZDT under the casting conditions of the example, and FIG. 24 is a diagram showing the bulging accumulated strain (Σε) when conventional support rolls are used. , ) and ξ alignment accumulated distortion (
FIG. 25 is a diagram showing the relationship between the sum of Σε, ) and the roll spacing. FIG. 25 is a schematic diagram showing another example of a staggered arrangement of divided support rolls. FIG. 26 to FIG. 30 are diagrams related to Example 7, and FIG.
27 is a schematic diagram showing an example of an apparatus for carrying out the method of the present invention. FIG. 28 is a diagram showing an example of the apparatus shown in FIG. 27. FIG. 29 is an enlarged view showing the occurrence of internal cracks in Examples and Comparative Examples, and FIG. 30 is a view showing the accumulated strain inside the slab in the method of the present invention.

Claims (5)

[Claims] (1) During casting, the total amount of strain that each part of the slab receives while it is in the temperature range at least between the tensile strength onset temperature (ZST) and the ductility onset temperature (ZDT) exceeds the critical strain of the steel type to be cast. A method for continuous casting of steel characterized by casting under conditions such as: (2) Straightening points by straightening rolls are arranged so that when each part of the slab passes through at least one straightening point, the temperature is higher than the tensile strength onset temperature (ZST) or lower than the ductility onset temperature (ZDT). A continuous casting method for steel, characterized by: (3) Support for areas where the total amount of strain that the slab receives while in the temperature range between the tensile strength appearance temperature (ZST) and the ductility appearance temperature (ZDT) during casting exceeds the critical strain of the steel type to be cast. The rotation speed of the rolls is adjusted to apply compressive strain in the casting direction to the slab in that area, and the total amount of strain is
A continuous casting method for steel, characterized in that the above-mentioned critical strain is not exceeded. (4) Supporting a continuously cast slab having an unsolidified phase inside with support rolls that are divided into a plurality of parts in the width direction of the slab and arranged in a staggered manner; and The slab support length W and the interval L between the divided support rolls adjacent in the casting direction are defined as W/L≦1.5 when the width end of the slab is included in the length W, and W/L when it is not included.
The spacing L of the split support rolls and the staggered arrangement section are determined so that L≦2.0, and the tensile strength appearance temperature (Z
A continuous casting method for steel, characterized in that the total amount of strain that a slab undergoes while in the temperature range between ST) and ductility onset temperature (ZDT) is determined so that it does not exceed the critical strain of the steel type to be cast. . (5) The horizontal part of the slab from the bend straightening part until it completely solidifies is supported by a surface that moves in synchronization with the slab, so that each part of the slab reaches at least the tensile strength appearance temperature (ZST) during casting.
A continuous casting method for steel, characterized in that the total amount of strain received during the temperature range between ZDT and ZDT does not exceed the critical strain of the type of steel to be cast.