JP7260335B2 - Cooling method for slabs of high-strength steel - Google Patents

Description

The present invention relates to a method for cooling a slab of high-strength steel after casting.

In recent years, the concentrations of carbon (C), silicon (Si), and manganese (Mn) in high-strength steel have been increased for the purpose of increasing strength. For the same purpose, chromium (Cr), titanium (Ti), boron (B), etc. may be added. However, due to the addition of these additives, cracks (hereinafter referred to as "placement cracks") are likely to occur when the cast slab is cooled after being cast.

Patent Literature 1 describes a method for suppressing placement cracks in continuous cast slabs of high-strength steel. In the method of Patent Document 1, in high-strength steel, as soon as the continuously cast piece reaches 150 ° C. or less, brittleness increases rapidly. controls the cooling rate from 500°C to 150°C.

JP 2007-83274 A

In order to further increase the strength of high-strength steel, it is conceivable to increase the amount of carbon (C), silicon ( Si) and manganese (Mn) added. However, as the amount of these elements added increases, the frequency of occurrence of placement cracks increases.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method capable of suppressing the occurrence of placement cracks even in high-strength steels that are susceptible to placement cracks.

The inventors' studies have revealed the following.
When the slab is cooled, a large internal stress is generated in the longitudinal direction of the slab. It was found that the main stress causing placement cracks was this longitudinal stress. Furthermore, it was found that the placement cracks originate from internal cracks that occur in the direction orthogonal to this stress.
As a result of further research by the present inventors, it was found that the degree of internal stress at which the placement crack occurs varies depending on the length of the internal crack. From this, it is considered that by controlling the internal stress according to the length of the internal crack, the occurrence of the placement crack can be suppressed. Since it was found that the internal stress depends on the cooling rate, it was found that the internal stress can be controlled by controlling the cooling rate according to the internal crack length.

The present invention is made based on the above findings. Specifically, in the method of cooling a high-strength steel slab of the present invention, the carbon (C) content is 0.16 mass% or more and 0.35 mass% or less, and the silicon (Si) content is 1.0 mass% or more. 5 mass% or less, internal crack length measured after casting in a high-strength steel slab with a manganese (Mn) content of 1.2 mass% or more and 4.0 mass% or less, or the internal after casting obtained based on the casting conditions A method of cooling a high-strength steel slab with a crack length of 50 mm or less, cutting the slab after the slab passes through the final roll of the continuous casting machine, and cutting the slab after passing the final roll of the continuous casting machine. Stack 3 or more cut slabs within 50 minutes.
Here, for at least one slab excluding the top and bottom slabs stacked three or more slabs, the total area of one wide surface and the other wide surface of the slab, the one wide surface is placed on it Three or more cut slabs are stacked so that the ratio of the sum of the area in contact with the slab and the area in which the other wide surface contacts with the slab placed below is 60% or more.
Maintain the state of stacking 3 or more cut slabs for 8 hours or more.

According to the present invention, when a high-strength steel slab is cooled to room temperature (approximately 20° C.) after being cast, it is possible to suppress the occurrence of placement cracks. In addition, the slab can be cooled to room temperature without the occurrence of standing cracks, even in the case of high-strength steel that is prone to cracking.

1 is a perspective view of a cut high-strength steel slab; FIG. It is a figure which shows the cut surface of II of FIG. It is a figure which shows the result of a fracture-toughness test. It is a figure which shows the relationship between internal stress and internal crack length. FIG. 4 is a diagram showing the relationship among internal stress, length of internal cracks, axial cooling rate of the slab, and occurrence of placement cracks. It is a figure which shows the state which piled up the slab. FIG. 4 is a diagram showing the relationship among internal stress, length of internal cracks, axial cooling rate of the slab, and occurrence of placement cracks. It is a figure explaining an example of an overlap rate. It is a figure which shows the relationship between an internal stress and an overlap rate. FIG. 4 is a diagram showing an example of the relationship between the core temperature of the slab and time.

Preferred embodiments of the present invention are described below.

The high-strength steel targeted by the method for cooling a slab of high-strength steel according to the present embodiment will be described. The high-strength steel targeted by this embodiment has the following composition.
Carbon (C) content: 0.16 mass% or more and 0.35 mass% or less Silicon (Si) content: 1.0 mass% or more and 2.5 mass% or less Manganese (Mn) content: 1.2 mass% or more and 4.0 mass% The high-strength steel targeted by the present embodiment has a tensile strength of 780 MPa or more due to the addition of the above elements. The size of the slab is, for example, 230 mm or more and 270 mm or less in thickness and 800 mm or more and 1,400 mm or less in width.

The upper limit of the alloy content including carbon of the high-strength steel targeted by the present embodiment is higher than the upper limit of the alloy content of the conventional high-strength steel. Therefore, the high-strength steel of this embodiment includes steel having a higher strength than the conventional one. However, when the carbon content is high, the frequency of cracking during cooling is high. Therefore, the high-strength steel of the present embodiment includes steel more likely to cause placement cracks than conventional steel.

After casting a slab of high tensile steel as described above, the slab is cut to length. A cut slab X is shown in FIG. The width direction, thickness direction and longitudinal direction shown in FIG. 1 are orthogonal to each other. In this embodiment, the cut slab X is cooled to room temperature without being heated.

The present inventors have studied the cause of the occurrence of placement cracks when the slab X is cooled. As a result, we found the following.
Internal stress is generated in the slab X when the slab X is cooled. When the magnitude of the internal stress was compared in the width direction, thickness direction and longitudinal direction of the slab X, it was found that the stress in the longitudinal direction was the largest. It was found that this longitudinal stress is the main stress that causes placement cracks.

FIG. 2 shows an example of a cut plane along the longitudinal direction of the slab X. As shown in FIG. The cut plane shown in FIG. 2 is the cut plane of II in FIG. As shown in FIG. 2, the slab X may have internal cracks. Internal cracks occur in a direction orthogonal to the longitudinal direction of the slab. This direction was perpendicular to the longitudinal direction in which the direction of stress was greatest. It was found that the placement cracks are caused by internal stress, starting from internal cracks that occur in the direction orthogonal to the stress in the longitudinal direction.

The internal stress generated when the slab X is cooled is generated as a result of the action of both thermal stress due to thermal expansion/contraction and transformation stress due to transformation expansion/contraction. Thermal stress is caused by the difference between the surface temperature and the internal temperature of the slab X and can be controlled by the cooling rate.

In conventional cooling methods for high-strength steel, the occurrence of placement cracks is suppressed by controlling the cooling rate of continuous cast slabs. However, in the high-strength steel of this embodiment, since the upper limit of the carbon content is higher than in the past, it was found that the occurrence of placement cracks could not be suppressed only by controlling the cooling rate of the continuously cast slab. For this reason, in the present embodiment, not only the thermal stress that can be controlled by the cooling rate but also the transformation stress is focused on. In addition, since the fracture surface of the slab holding crack is a brittle fracture surface and is a fracture phenomenon, attention was paid to the size of the crack starting point for the occurrence of the holding crack. As a result, we found the following.

The transformation stress varies depending on the difference in the timing of transformation occurrence between the surface side and the inside side of the slab, and the strength (structure) of the material after transformation. When the structure of the slab during cooling transforms from an austenite structure to a ferrite structure, a pearlite structure, or a structure in which both of these exist, the transformation stress is small. However, when the slab structure changes to a bainite structure or a martensite structure, the transformation stress increases.

In the high-strength steel of this embodiment, it was found that transformation stress may be reduced when transformation (transformation to ferrite structure or pearlite structure) occurs when the slab is at 500° C. or higher and 700° C. or lower. That is, it is important to control the cooling rate within this temperature range. On the other hand, when the temperature is higher than the above temperature range, no transformation stress occurs because the slab structure does not transform. Further, if the transformation occurs in a lower temperature range than the above temperature range, a bainite structure or a martensite structure is generated and the strength of the material increases, resulting in an increase in transformation stress.

As the transformation stress increases, the internal stress, which is the sum of the thermal stress and the transformation stress, increases, and the internal stress remaining in the slab when cooled to room temperature increases. Therefore, by causing transformation to occur in the range of 500° C. or higher and 700° C. or lower, the transformation stress is suppressed and the generated internal stress is reduced. It is important to reduce the internal (residual) stress of

The greater the internal stress generated during cooling, the greater the (residual) stress inside the slab when cooled to room temperature, and the easier it is for cracks to occur. If not, cracks are likely to occur. On the other hand, the cooling rate in the range below 500° C. and in the range above 700° C. also contributes to changes in thermal stress, but the contribution is small and does not directly cause placement cracks.

From the above knowledge, in the high-strength steel of the present embodiment, when the slab is cooled after casting, the slab is cooled from 700 ° C. to 500 ° C. (when 500 ° C. or higher and 700 ° C. or lower). We thought that the occurrence of placement cracks could be suppressed by controlling the generated internal stress to be small.

As mentioned above, a placement crack is a brittle fracture phenomenon that occurs when the internal (residual) stress acting on the crack origin exceeds the fracture toughness of the material, and the stress leading to fracture depends on the size of the crack origin. do. Therefore, the relationship between the occurrence of placement cracks, internal cracks, and internal stress was investigated.

The fracture toughness test was performed under the following conditions, considering the fracture in the fracture toughness test as the placement crack of the slab.

Slabs of the steel grades shown in Table 1 were cast. The steel grades A to C shown in Table 1 are steel grades of the high-strength steel of this embodiment. After casting, a sample was taken from the slab and subjected to a fracture toughness test according to JIS G0564 Metallic Materials-Plane Strain Fracture Toughness Test Method. The sample fracture, fracture origin (notch length) and fracture stress in the fracture toughness test can be considered as slab placement cracks, internal cracks (internal crack length) and internal stress, respectively.

FIG. 3 shows the results of the fracture toughness test. The vertical axis in FIG. 3 is the breaking stress at room temperature, and the horizontal axis in FIG. 3 is the breaking starting point (notch length). The curve shown in FIG. 3 shows the fracture toughness characteristics. It was found that the slab failed above the curve, but not below the curve.

Replacing the fracture, fracture starting point (notch length), and fracture stress shown in Fig. 3 with the slab's standing crack, internal crack (internal crack length), and internal (residual) stress at room temperature, respectively, is shown in Fig. 4. The figure shown was obtained. The vertical axis of FIG. 4 is the internal stress δ, and the horizontal axis of FIG. 4 is the internal crack length L. The curve shown in FIG. 4 can be considered as the maximum internal stress at which placement cracks do not occur. The upper range of the curve is the "range where placement cracks occur", and the lower range of the curve is the "range where placement cracks do not occur".

From FIG. 4, it was found that the range of internal (residual) stress .delta. As the internal crack length L becomes longer, the internal stress δ at which the placement crack does not occur becomes smaller. However, it was found that even if the internal crack length L is long, if the internal stress δ is within the "range in which no cracking occurs", the cracking does not occur. In other words, it was found that even if the internal crack length L is long, if the internal stress δ is reduced, the occurrence of the placement crack can be suppressed. On the other hand, when the length L of the internal crack is short, the range of the internal stress δ within which the crack does not occur is wide. Therefore, when the internal crack length L is short, even if the internal stress δ is large, the occurrence of placement cracks can be suppressed.

Based on the above, it is considered that the occurrence of standing cracks can be suppressed by controlling the internal stress δ so that the internal stress δ falls within the “range in which no standing cracks occur” according to the length L of the internal cracks.

Internal stress is generated as a result of the action of thermal stress and transformation stress, as described above. Thermal stress and transformation stress can be controlled by the cooling rate of the slab. Therefore, the relationship between cooling rate and internal stress was investigated.

<Cooling rate>
The cooling rate of the slab was calculated using CASTEM (heat transfer solidification program). Here, the cooling conditions (casting speed , specific water content) and the cooling conditions after casting, the heat transfer coefficients of the slab surface and the mist, the air, and the rolls were given as functions of temperature as boundary conditions. In addition, the surface temperature of the slab immediately after passing the final roll of the continuous casting machine and the surface temperature of the slab during cooling after casting were measured, and the surface temperature of the slab was adjusted so that the surface temperature history obtained by calculation matched the measured value. The transmission coefficient was corrected. The final roll of the continuous casting machine is the roll located at the longest casting distance from the mold in the group of rolls extending in the casting direction from directly below the mold. In addition, in the analysis of internal stress described later, the internal stress was maximum at the center of the slab in the width direction, the center in the thickness direction, and the center in the longitudinal direction. The cooling rate at the center in the direction and the center in the longitudinal direction was adopted. Hereinafter, the cooling rate at the center in the width direction, the center in the thickness direction, and the center in the longitudinal direction of the slab may be referred to as the "axial cooling rate of the slab" or simply the "axial cooling rate."

<Internal stress>
The internal stress was determined by numerical analysis using general-purpose software ABAQUS. At this time, by giving the temperature distribution of the slab calculated by the above-mentioned CASTEM and sequentially giving the physical property values (stress-strain relationship, linear expansion coefficient) according to the temperature and cooling rate, the stress distribution in the slab was calculated. From this analysis, it was found that the internal stress is greatest in the longitudinal direction among the slab width direction, thickness direction, and longitudinal direction. In addition, it was found that the internal stress in the width direction, thickness direction and longitudinal direction of the slab becomes maximum at the width direction center, thickness direction center and longitudinal direction center, respectively.

From the cooling rate CR and the internal (residual) stress δ at room temperature, it was found that the internal stress δ depends on the cooling rate CR. From this, it was found that the internal stress δ can be controlled by controlling the cooling rate CR.

The results shown in FIG. 5 were obtained from the relationship between the cooling rate CR and the internal stress δ, and the relationship between the internal stress δ, the internal crack length L, and the occurrence of placement cracks shown in FIG. FIG. 5 shows the relationship between the internal stress δ, the length L of internal cracks, the cooling rate CR, and the presence or absence of occurrence of placement cracks.

The curve shown in FIG. 5 is also a curve indicating the maximum internal (residual) stress δ at room temperature at which placement cracks do not occur, and is also a curve indicating the maximum cooling rate CR at which placement cracks do not occur. The upper range of the curve is the "range where placement cracks occur", and the lower range of the curve is the "range where placement cracks do not occur". Depending on the internal crack length L, the range of cooling rate CR in which placement cracks do not occur differs. From FIG. 5, it can be seen that the occurrence of placement cracks can be suppressed by controlling the cooling rate CR so that the cooling rate CR falls within the “range in which placement cracks do not occur” according to the internal crack length L. Conceivable.

In order to confirm the above findings, the following experiments were conducted.
Slabs were cast at the casting speed and specific water content shown in Table 3. At the start of drawing the slab, the slab was drawn out at a rate of 0.2 m/min. for about 100 seconds. After that, the speed was gradually increased to the target casting speed at an acceleration of 0.12 m/ ^min.2 . After casting, the slabs were cut to length and prepared for the slab cooling conditions shown in Table 3 at room temperature. Specifically, the slab was cooled as shown in Table 3 within 50 minutes after the slab passed the final roll of the continuous casting machine. After that, the slab was cooled under the slab cooling conditions shown in Table 3. The slab cooling state shown in Table 3 is a cooling state in which the axial temperature of the slab is from 700°C to 500°C. After that, the slab was allowed to cool to room temperature, and then it was placed on the slab to confirm whether or not cracks had occurred (whether or not there was slab breakage).

The conditions shown in Table 3 are described below.
Steel types A to C shown in Table 3 are steel types A to C shown in Tables 1 and 2.
The "slab cooling conditions" shown in Table 3 are explained below.
・No.1 "water cooling"
The slabs were placed on mambos (ties) and spray watered the slabs from above and below for 12 hours.
・No.2 and No.5 "Single air cooling"
One slab was placed on a mambo (sleeper) and left at room temperature for 96 hours.
・No.3 "Stacking 8 sheets (top layer)"
As shown in Figure 6, 8 slabs were stacked and left at room temperature for 96 hours. Hereinafter, stacking a plurality of slabs may be referred to as "stacking". Stacking was performed so that the centers of the width direction and the length direction of the eight slabs were arranged in the same vertical line. Among eight slabs, it was confirmed whether cracks occurred in the uppermost slab.
・No.4 "Stacking 8 sheets (2nd sheet from top)"
As in No. 3, 8 slabs were stacked and left at room temperature for 96 hours. It was confirmed whether the second slab from the top of the eight slabs was placed and cracked.

The "core cooling rate" shown in Table 3 is the core cooling rate of the slab when cooling in the cooling state shown in Table 3. "Axial cooling rate" is a value calculated using the heat transfer solidification program CASTEM.

"Internal crack length L" shown in Table 3 is the length measured by the following method.
After casting, the slabs cut to length were cut longitudinally. A sample was taken from the cut surface and an acid was applied to the sample. The length of the portion corroded by acid was measured. The length in the thickness direction was defined as the internal crack length L. When a plurality of corroded portions existed, the length of the longest corroded portion was taken as the internal crack length L. Since it is difficult to distinguish a portion with large micro-segregation between dendrites and an internal crack, the lower limit of detection in this method is 10 mm. Therefore, all cracks with a length of less than 10 mm were treated as 10 mm.

The results are shown in Table 3 and FIG. FIG. 7 plots the results shown in Table 3 on FIG.

From Table 3, No. 1 to 4 did not cause cracking while placing, but No. 5 did. Nos. 1 to 4 in which placement cracks did not occur were in the "range where placement cracks do not occur" as shown in FIG. On the other hand, No. 5 in which placement cracks occurred was in the "range where placement cracks occur". From this, it was confirmed that the lower range of the curve in FIG. 7 is "the range in which placement cracks do not occur" and the upper range of the curve in FIG. 7 is the "range in which placement cracks occur".

From the above, when the slab is cut after casting and cooled, the temperature at the center of the slab is from 700 ° C. to 500 ° C. (within the range of 500 ° C. or higher and 700 ° C. or lower), depending on the internal crack length L , the occurrence of placement cracks can be suppressed by controlling the cooling rate CR so that the cooling rate CR falls within the "range in which placement cracks do not occur" shown in FIG.

As described above, transformation stress does not occur until the temperature of the core of the slab reaches 700° C., so the internal stress generated in the slab is small. Therefore, placement cracks are less likely to occur without controlling the axial center cooling rate CR of the slab. Therefore, for example, the slab may be left standing (air cooled) until the core temperature of the slab reaches 700°C. When one slab is left (air-cooled), the slab core cooling rate CR is 2.5° C./min or less. Therefore, in a range where the slab core temperature exceeds 700° C., the slab core cooling rate CR should be controlled as follows.
CR≤2.5°C/min.

Moreover, even when the temperature of the core of the slab falls below 500°C, if the cooling rate from 700°C to 500°C is controlled, the transformation will not occur because the transformation has already been completed, or the transformation will not occur at 500°C or less. It is assumed that the length of the internal crack does not occur even if the internal stress increases due to the occurrence of stress. Therefore, placement cracks are less likely to occur without controlling the axial center cooling rate CR of the slab. Therefore, it is not necessary to control the cooling rate after the core temperature of the slab falls below 500°C. For example, even after the core temperature of the slab falls below 500°C, the cooling state may be continued up to 500°C. Also, the cooling state up to 500° C. may be stopped.

The "slab core cooling rate CR" can be calculated, for example, by heat transfer calculation from the surface temperature history of the slab (for example, the cooling rate of the slab surface). Further, if the slab casting conditions, post-casting cooling conditions, etc. are determined, the "slab axial center cooling rate CR" can be calculated according to those conditions. For example, a value obtained by (temperature difference between temperature A and temperature B)/(time from temperature A to temperature B) may be used as the “slab core cooling rate CR”. The "slab core cooling rate CR when the slab core temperature is from 700°C to 500°C" (the slab core cooling rate CR when the slab core temperature is 500°C or higher and 700°C or lower) is, for example, After casting, it is possible to control for each operation by the packing style when the slab is cooled from 700 ° C. to 500 ° C. (for example, "slab cooling state" shown in Table 3), the time until the packing style, etc. be.

The "shaft center temperature of the slab" was calculated using CASTEM (heat transfer solidification program), but it is obtained by general heat transfer solidification calculation, for example, by calculation using a commercially available code. The "axial temperature of the slab" is the temperature at the center in the width direction, the center in the thickness direction, and the center in the longitudinal direction of the slab, and is the temperature at the highest temperature position in the slab. In this embodiment, an attempt is made to control the stress generated inside the slab, and it is the cooling rate that controls the internal stress. Since the internal stress is greatest at the center of the slab, the "cooling rate of the center of the slab" is adopted as the cooling rate of the slab. Therefore, the "slab core temperature" is used as the slab temperature.

The “internal crack length L” may be, for example, a length estimated based on the slab casting conditions (casting speed, specific water content, slab size, usage of roll stand, etc.). The "internal crack length L" may be, for example, an actual value measured on the cut surface of the slab after casting. When multiple internal cracks occur, the longest internal crack length is defined as "internal crack length L".
Also, for example, if there is a length estimated as the "internal crack length L", when casting the slab under the same casting conditions as the casting conditions used for estimating the length, the "internal crack length L" of the slab The crack length L" may be an already existing inferred length. For example, when there is a measured value of "internal crack length L", when a slab is cast under the same casting conditions as those under which the measured value was obtained, the "internal crack length L" of the slab is value.

Based on the above findings, a specific cooling method was examined.

When the high-strength steel of this embodiment is cast under normal casting conditions, the internal crack length L is assumed to be about 50 mm at maximum. Therefore, in the present embodiment, a cooling method capable of suppressing placement cracks when the internal crack length L is 50 mm or less was investigated.

An experiment was conducted in which the cooling conditions were changed when the slab was cooled from 700°C to 500°C. By changing the cooling conditions, the axial cooling rate of the slab is changed. Tables 3 and 4 show the casting conditions and cooling conditions of the slabs.

The cooling conditions shown in Table 5 are described below. In this experiment, the centers of the stacked slabs in the width direction and the longitudinal direction were arranged on the same vertical line. Hereinafter, the slab to be checked for occurrence of placement cracks is referred to as a "target slab".

If the target slab is a slab excluding the topmost and bottommost slabs among the stacked slabs, the "time to stacking" is the process of stacking the slabs from the time the slab passes the final roll. , is the time until another slab is piled on top of the target slab.
When the target slab is the topmost slab among multiple stacked slabs, the "time to stacking" is the time from when the slab passes the final roll to when the target slab is stacked. is the time until is accumulated.
For example, if 3 slabs are stacked in 3 layers and the target slab is the middle slab, the "time until stacking" is the time from the time the slab passes the final roll until the top slab is placed on top of the middle slab. (until the upper and lower surfaces of the middle slab come into contact with other slabs). In addition, when multiple slabs are stacked and the target slab is the top slab, the "time until stacking" is calculated from the time the slab passes the final roll until the target slab (top slab) This is the time until the top slab is placed on the top surface of the top slab (until the bottom surface of the top slab comes into contact with another slab).

A "contact surface" is the surface where the target slab contacts another slab. "Top" is the upper broad surface of the subject slab, and "Bottom" is the lower broad surface of the subject slab. The wide faces of the slab are, as shown in FIG. 1, the two faces with the largest area among the four faces parallel to the longitudinal direction of the slab. In No. 6 and No. 7, the "contact surface" is "upper and lower", so the upper wide surface and the lower wide surface of the target slabs of No. 6 and No. 7 are in contact with other slabs. In No. 8, the "contact surface" is "bottom", so in No. 8, only the lower broad surface of the target slab is in contact with other slabs. The No. 8 target slab is the uppermost slab among the stacked slabs.

If the target slab is a slab excluding the top and bottom slabs among multiple stacked slabs, the "stacking maintenance time" is the state in which the top and bottom surfaces of the target slab are in contact with other slabs. It's time to be In other words, if the target slab is a slab excluding the top and bottom slabs among the stacked slabs, the "stacking maintenance time" is the time from when another slab is stacked on top of the target slab. (From when the upper and lower surfaces of the target slab come into contact with other slabs) to when the slabs stacked on the target slab are removed (when the upper surface of the target slab stops contacting with other slabs) until).
When the target slab is the uppermost slab among a plurality of stacked slabs, the "stacking maintenance time" is the time during which the bottom surface of the target slab is kept in contact with other slabs. . In other words, if the target slab is the topmost slab among multiple stacked slabs, the “stacking maintenance time” is from when the target slab is stacked on top of another slab ( It is the time from when the bottom surface comes into contact with other slabs) to when the target slab is removed (until the bottom surface of the target slab stops contacting with other slabs).
For example, when three slabs are stacked and the target slab is the middle slab (the second slab), the "stacking maintenance time" is set to This is the time until the slab is removed. Also, when multiple slabs are stacked and the target slab is the top slab, the "stacking maintenance time" is the time from when the target slab (top slab) is placed on top of another slab (target It is the time from when the bottom surface of the slab comes into contact with another slab) to when the target slab is removed.

The “overlap ratio” is the “total area of the upper wide surface (one wide surface) and the lower wide surface (the other wide surface) of the target slab”, and the “upper wide surface (one wide surface) is placed on it”. It is the ratio of the sum of the area in contact with the slab and the area in which the lower wide surface (the other wide surface) contacts the slab placed below it.
The area of the upper wide surface (one wide surface) and the area of the lower wide surface (the other wide surface) of a single slab may be treated in the same way. For example, when the widthwise and longitudinal centers of a plurality of stacked slabs are arranged on the same vertical line, the area of the upper wide surface of the target slab is S ₀₁ and the area of the lower wide surface of the target slab is S ₀₂ . , the area of the lower broad surface of the slab placed above the target slab is _S1 , and the area of the upper broad surface of the slab placed above the target slab is _S2 . can be done.

FIG. 8 shows an example of stacking three slabs. The three tiered slabs are stacked such that the centers of the slabs in the width direction and the longitudinal direction are positioned on the same vertical line. The overlapping rate when the middle slab of the three slabs is the target slab will be described.
(Case 1)
When both the upper slab and the lower slab are stacked with slabs having a wider surface area than the middle slab (target slab), S ₁ >S ₀₁ and S ₂ >S ₀₂ . In this case, the entire upper wide surface of the middle slab is in contact with the upper slab, and the entire lower wide surface of the middle slab is in contact with the lower slab. The overlap ratio for case 1 is represented by the following equation.
Overlap ratio = 100 × (S ₀₁ + S ₀₂ )/(S ₀₁ + S ₀₂ )
= 100
(Case 2)
When the upper slab has a wider surface area than the middle slab and the lower slab has a wider surface area than the middle slab, S ₁ ≦S ₀₁ and S ₂ ≧S ₀₂ . In this case, part of the upper wide surface of the middle slab is in contact with the entire lower wide surface of the upper slab. The entire lower wide surface of the middle slab is in contact with the lower slab. In this case, the overlap rate of case 2 is represented by the following equation.
Overlap ratio=100×(S ₁ +S ₀₂ )/(S ₀₁ +S ₀₂ )
When the upper slab has a wider surface area than the middle slab and the lower slab has a wider surface area than the middle slab, S ₁ ≧S ₀₁ and S ₂ ≦S ₀₂ are satisfied. In this case, the overlap ratio is 100×(S ₀₁ +S ₂ )/(S ₀₁ +S ₀₂ ).
(Case 3)
When both the upper slab and the lower slab are stacked with slabs having a wider surface area than the middle slab, S ₁ <S ₀₁ and S ₂ <S ₀₂ . In this case, part of the upper wide surface of the middle slab is in contact with the entire lower wide surface of the upper slab. A portion of the lower wide surface of the middle slab is in contact with the entire upper wide surface of the lower slab. In this case, the overlap rate of case 3 is represented by the following equation.
Overlap ratio=100×(S ₁ +S ₂ )/(S ₀₁ +S ₀₂ )

The "internal crack length" was estimated based on the casting conditions.
The “internal stress” was calculated by numerical analysis using general-purpose software ABAQUS.

"Presence or absence of placement cracks" was judged by the following method.
The internal crack length L of Nos. 6 to 8 is 50 mm or less. From FIG. 7, when the internal crack length L is 50 mm or less, the internal stress capable of suppressing the occurrence of standing cracks is 177 MPa or less. From this, when the internal crack length L is 50 mm or less and the internal stress is 177 MPa or less, the placement crack does not occur.
Therefore, when the internal stress was 177 MPa or less, it was determined that the occurrence of placement cracks could be suppressed, and is indicated by "◯" in Table 5. On the other hand, when the internal stress exceeded 177 MPa, it was determined that the occurrence of placement cracks could not be suppressed, and indicated as "x" in Table 5.

The results are shown in FIG. The vertical axis of FIG. 9 is the internal stress, and the horizontal axis of FIG. 9 is the overlap ratio. FIG. 9 shows approximate curves of Nos. 6-8. In Nos. 6 to 8, as described above, the internal crack length L is 50 mm or less, so the internal stress capable of suppressing the occurrence of standing cracks is 177 MPa or less. Therefore, in FIG. 9, it is considered that the occurrence of placement cracks can be suppressed when the internal stress is in the range of 177 MPa or less.

From FIG. 9, in the approximation curves of Nos. 6 to 8, the internal stress is less than 177 MPa when the overlap rate is 60%. From this, it is considered that the boundary between the range in which the occurrence of placement cracks can be suppressed and the range in which the occurrence of placement cracks cannot be suppressed is when the overlap rate is 60%, and when the overlap rate is 60% or more, the occurrence of placement cracks can be suppressed. .

In order to confirm the above findings, slabs were cast under the conditions shown in Table 4 and Table 6 below, and then cooled. In No. 9 to No. 11 shown in Table 6, the slabs were stacked, but in No. 12, one slab was left until it reached room temperature (air cooling).

"Presence or absence of placement cracks" shown in Table 6 was judged by the following method.
<No.9 to No.12>
After the slab was cast, it was cooled to room temperature and then placed on the slab to check for cracks. In No. 9 to No. 11, no placement cracks occurred, which is indicated by "○" in Table 6. In No. 12, since placement cracks occurred, "x" is shown in Table 6.

From Table 6, the following was found.
In Nos. 9 to 11 with an overlap rate of 60% or more, the occurrence of placement cracks could be suppressed, but in No. 12 with an overlap rate of less than 60%, the occurrence of placement cracks could not be suppressed. From this, it was confirmed that when the overlap rate is 60% or more, the occurrence of placement cracks can be suppressed.

From the above, when the slabs are cooled, the slabs are stacked so that the overlapping ratio of the target slabs is 60% or more when the slab core temperature is from 700°C to 500°C. Thereby, generation|occurrence|production of a placement crack can be suppressed.

Moreover, for the following reasons, the slabs are kept stacked such that the overlap rate of the target slabs is 60% or more for 8 hours or more.
The higher the overlapping ratio of the target slabs, the slower the cooling rate. When the overlap rate is the highest at 100%, the cooling rate is the slowest. In Table 3, No. 4 is an experiment with an overlap rate of 100%. The cooling rate at this time was 0.44° C./min. At this cooling rate, it takes (700-500)/0.44=455 minutes for the core temperature of the slab to go from 700°C to 500°C. Since it takes 455 minutes at the slowest cooling rate, the core temperature of the slab drops from 700° C. to 500° C. in less than 455 minutes when cooling at a faster cooling rate.

The cooling rate when stacked with an overlap rate of 60% or more is equal to or faster than the cooling rate when the overlap rate is 100%. Therefore, cooling by stacking for 455 minutes or longer reduces the core temperature of the slab from 700°C to 500°C.

From the above, the state in which the slabs are stacked so that the overlap ratio of the target slabs is 60% or more is maintained for 8 hours or more. This reduces the core temperature of the slab from 700°C to 500°C. After that, the slabs may remain stacked or may be unstacked.

Here, "the state in which the slabs are stacked so that the overlap rate of the target slab is 60% or more" means that the slabs to be stacked are stacked so that the overlap rate of at least the target slab is 60% or more. It is stacked. "The state in which the slabs are stacked so that the overlap rate of the target slab is 60% or more" means that if the overlap rate of the target slab is 60% or more, not all slabs need to be stacked. All slabs may be stacked. Also, "the state in which the slabs are stacked so that the overlap rate of the target slab is 60% or more" may mean that the slabs are being stacked as long as the overlap rate of the target slab is 60% or more. The number of target slabs may be one, or two or more. For example, if 4 slabs are stacked in 4 layers and the slab on the 3rd layer from the top is the target slab, when the slab on the 2nd layer from the top is stacked, the slab on the 3rd layer (target slab ) becomes 60% or more. In this case, "the state in which the slabs are stacked so that the overlap ratio of the target slab is 60% or more" means that if the second slab from the top is stacked, the top slab is not stacked. or the top slab may be stacked.

The timing of starting stacking is not particularly limited as long as it is after the slabs are cut. However, the reason why the slabs are stacked so that the target slab overlap ratio is 60% or more is within 50 minutes after the slab has passed the final roll of the continuous casting machine for the following reasons. do.

In order to prevent placement cracks from occurring, when the slab core temperature reaches 700° C., the target slab must be cooled in a state where the overlapping ratio is 60% or more. Therefore, it is preferable that the slabs are stacked so that the overlapping ratio of the target slabs is 60% or more before the core temperature of the slabs reaches 700°C.

When a slab is cast under normal casting conditions, the shaft core temperature of the slab on the final roll of the continuous casting machine exceeds 800°C. For example, when the casting speed is as slow as 1.0 m/min. is 827° C. The lower the casting speed and the higher the specific water content, the easier it is for the shaft core temperature to drop. Since the core temperature exceeds 800° C. even under conditions of slow casting speed and high specific water content, the core temperature reliably exceeds 800° C. under conditions of high casting speed and low specific water content. Therefore, when a slab is cast under normal casting conditions, the axial temperature of the slab in the final roll of the continuous casting machine exceeds 800°C.

When the slab is left standing (air cooling), the axial cooling rate of the slab is 1.92° C./min. In this case, it takes (800-700)/1.92≈52 minutes for the slab core temperature to rise from 800°C to 700°C. After this time, the core temperature of the slab may drop below 700°C. Therefore, the slabs are stacked so that the overlapping ratio of the target slabs is 60% or more within 50 minutes after the slabs pass the final roll of the continuous casting machine.
As a result, after the core temperature of the slab reaches 700° C., the target slab is cooled while the overlapping ratio of the target slab is 60% or more.

As described above, when the length of the internal crack in the high-strength steel slab of the present embodiment is 50 mm or less, the slab is cooled to room temperature by the following method.
After the slabs pass through the final roll of the continuous casting machine, the slabs are cut and stacked such that the target slab overlap ratio is 60% or more. Within 50 minutes after the slab has passed the final roll of the continuous casting machine, the overlap ratio of the target slab should be 60% or more. Thereafter, the state in which the target slab overlap rate is 60% or more is maintained for 8 hours or more. In other words, the state in which the slabs are stacked such that the overlap ratio of the target slabs is 60% or more is maintained for 8 hours or more.
"Overlap ratio" is the ratio of the "total area of one broad surface and the other wide surface of the slab" to the "upper It is the ratio of the sum of the area where the broad side (one wide side) contacts the slab placed above it and the area where the lower wide side (the other wide side) contacts the slab below it.

When the slab is cooled by the above-described method, the slab core cooling rate CR is within the "range in which placement cracks do not occur" shown in FIG. 6 from 700°C to 500°C. Thus, the core cooling rate CR can be controlled. As a result, it is possible to suppress the occurrence of placement cracks in the high-strength steel slab of the present embodiment.

FIG. 10 shows an example of "relationship between slab core temperature and time" when the cooling method described above is carried out. "A" in FIG. 10 is when the slab passes the last roll of the continuous caster. At "A" in FIG. 10, the core temperature of the slab exceeds 800.degree. After "A", the slabs are stacked, and at "B" within 50 minutes from "A", the overlapping rate of the target slab becomes 60% or more. In "B", the core temperature of the slab exceeds 700°C. At "B", all slabs may or may not be completely stacked. For 8 hours or more from "B", the state where the overlap rate of the target slab is 60% or more is maintained. In other words, the slabs are kept stacked such that the target slab overlap rate is 60% or more for 8 hours or more from "B". Thereby, the core temperature of the slab falls below 500°C. After this, the slabs may be kept stacked or unstacked.

In FIG. 10, at "A", the core temperature of the slab exceeds 800.degree. Also, in FIG. 10, the slab core temperature exceeds 700° C. at "B", but the slab core temperature may be 700° C. at "B".

Although the embodiments of the present invention have been described above with reference to the drawings, it should be considered that the specific configuration is not limited to these embodiments. The scope of the present invention is indicated by the scope of the claims rather than the above description, and includes all modifications within the scope and meaning equivalent to the scope of the claims.

X slab

Claims (1)

A slab of high-strength steel having a carbon content of 0.16 mass% or more and 0.35 mass% or less, a silicon content of 1.0 mass% or more and 2.5 mass% or less, and a manganese content of 1.2 mass% or more and 4.0 mass% or less. A cooling method for a high-strength steel slab having an internal crack length of 50 mm or less after casting determined based on the internal crack length measured after casting or the casting conditions in
Cutting the slab after the slab has passed through the final roll of the continuous casting machine,
Stacking three or more cut slabs within 50 minutes after the slab has passed the final roll of the continuous casting machine,
Here, for at least one slab excluding the top and bottom slabs stacked three or more slabs, the total area of one wide surface and the other wide surface of the slab, the one wide surface is placed on it Stack three or more cut slabs so that the total ratio of the area in contact with the slab and the area in which the other wide surface contacts the slab placed below is 60% or more,
A method for cooling high-strength steel slabs, characterized by maintaining a state in which three or more cut slabs are piled up for eight hours or more.

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