JP2020139209A - Cooling method of slab of high tensile steel - Google Patents

Abstract

To suppress occurrence of cracks in a slab of a high tensile steel.SOLUTION: When cutting and cooling after a slab of a high tensile steel having a carbon content of 0.16 mass% or larger and 0.35 mass% or smaller, a silicon content of 1.0 mass% or larger and 2.5 mass% or smaller, and a Mn content of 1.2 mass% or larger and 4.0 mass% or smaller is cast, when a length of an inner crack generated on the slab after casting is 50 mm or smaller, within 50 minutes after the slab passing a final roll of a continuous casting machine, three or more sheets of the cut slabs are stacked. A state where three or more sheets of slabs are stacked is maintained for 8 hours or longer. About at least one sheet of slab excepting the uppermost stage and the lowermost stage in the three sheets of stacked slabs, a ratio of a sum total of an area where one broad surface contacts with a slab placed thereabove and an area where other broad surface contacts with the slab arranged therebelow, to a total area of the one broad surface and the other broad surface of the slab is set to be 60% or larger.SELECTED DRAWING: Figure 9

Description

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

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

Patent Document 1 describes a method of suppressing a crack in a continuous slab of high-strength steel. In the method of Patent Document 1, in high-strength steel, brittleness increases sharply as soon as the continuous slab becomes 150 ° C. or lower, so that the temperature range on the higher temperature side than 150 ° C., specifically Controls the cooling rate from 500 ° C to 150 ° C.

JP-A-2007-83274

In order to further increase the strength of the 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 cracking occurs more frequently.

An object of the present invention is to provide a method capable of suppressing the occurrence of laying cracks even in a steel type in which laying cracks are likely to occur in high-strength steel.

From the research of the present inventors, the following was found.
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 that causes the cracking is this longitudinal stress. Furthermore, it was found that the place cracks occur from the internal cracks generated in the direction orthogonal to this stress.
As a result of further research, the present inventors have found that the degree of internal stress at which a placement crack occurs differs depending on the length of the internal crack. From this, it is considered that the occurrence of cracks can be suppressed by controlling the internal stress according to the length of the internal cracks. 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 based on the above findings. Specifically, the method for cooling a slab of high-strength steel of the present invention has a carbon (C) content of 0.16 mass% or more and 0.35 mass% or less, and a silicon (Si) content of 1.0 mass% or more. Length of internal cracks generated in the slab after casting when cutting and cooling a slab of high-strength steel having a manganese (Mn) content of 1.2 mass% or more and 4.0 mass% or less of 5 mass% or less. When the value is 50 mm or less, the slab is cut after passing through the final roll of the continuous casting machine, and three or more pieces are cut within 50 minutes after the slab passes through the final roll of the continuous casting machine. Stack the slabs.
Here, for at least one slab excluding the top and bottom slabs in which three or more slabs are stacked, the one wide surface is arranged on the one wide surface and the other wide surface of the slab. Three or more cut slabs are stacked so that the total ratio of the area in contact with the slab and the area in contact with the slab on which the other wide surface is arranged is 60% or more.
Maintain the state in which three or more cut slabs are stacked for 8 hours or more.

According to the present invention, when a slab of high-strength steel is cast and then cooled to room temperature (about 20 ° C.), it is possible to suppress the occurrence of cracks. In addition, even in high-strength steels that are prone to cracking, the slab can be cooled to room temperature without cracking.

It is a perspective view of the slab of a cut high-strength steel. It is a figure which shows the cut surface of II of FIG. It is a figure which shows the result of the fracture toughness test. It is a figure which shows the relationship between the internal stress and the internal crack length. It is a figure which shows the relationship between the internal stress, the internal crack length, the axial cooling rate of a slab, and the presence or absence of the occurrence of a crack. It is a figure which shows the state which slabs are piled up. It is a figure which shows the relationship between the internal stress, the internal crack length, the axial cooling rate of a slab, and the presence or absence of the occurrence of a crack. It is a figure explaining an example of the overlap rate. It is a figure which shows the relationship between the internal stress and the overlap rate. It is a figure which shows an example of the relationship between the axial temperature of a slab and time.

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

The high-strength steel targeted by the method for cooling the slab of the 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% or less Hereinafter, 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, a size of 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 carbon-containing alloy content 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 the present embodiment includes steel having higher strength than the conventional one. However, when the carbon content is high, cracking occurs frequently when cooled. Therefore, the high-strength steel of the present embodiment includes steel that is more likely to be cracked than before.

After casting the above-mentioned high-strength steel slab, the slab is cut to a predetermined length. FIG. 1 shows the cut slab X. The width direction, the thickness direction, and the longitudinal direction shown in FIG. 1 are orthogonal to each other. In this embodiment, the cut slab X is cooled to room temperature without heating.

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

FIG. 2 shows an example of a cut surface along the longitudinal direction of the slab X. The cut surface shown in FIG. 2 is the cut surface of II in FIG. As shown in FIG. 2, internal cracks may occur in the slab X. Internal cracks occur in the direction orthogonal to the longitudinal direction of the slab. This direction was orthogonal to the longitudinal direction, where the direction of stress was greatest. It was found that the laying cracks are generated by the internal stress starting from the internal cracks generated 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 the thermal stress due to thermal expansion / contraction and the transformation stress due to transformation expansion / contraction. The 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 the conventional cooling method for high-strength steel, the occurrence of cracks is suppressed by controlling the cooling rate of the continuous slab. However, in the high-strength steel of the present embodiment, since the upper limit of the carbon content is higher than before, it has been found that the occurrence of cracks may not be suppressed only by controlling the cooling rate of the continuous slab. Therefore, in this embodiment, attention is paid not only to the thermal stress that can be controlled by the cooling rate but also to the transformation stress. In addition, since the fracture surface of the slab placement crack is a brittle fracture surface and is a fracture phenomenon, we focused on the size of the starting point of the crack for the occurrence of the placement crack. As a result, the following was found.

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

It has been found that in the high-strength steel of the present embodiment, the transformation stress may be reduced if transformation (transformation to a ferrite structure or a pearlite structure) occurs when the slab is 500 ° C. or higher and 700 ° C. or lower. That is, it is important to control the cooling rate in this temperature range. On the other hand, in a temperature range higher than the above temperature range, the slab structure does not transform, so that transformation stress does not occur. Further, when the transformation is generated in a temperature range lower than the above temperature range, a bainite structure or a martensite structure is generated, and the strength of the material is increased, so that the transformation stress is increased.

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 increases when cooled to room temperature. Therefore, by generating transformation 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. As a result, when the slab is cooled to room temperature, cracks are generated at room temperature. It is important to reduce the internal (residual) stress of the room temperature.

The greater the internal stress generated during cooling, the greater the (residual) stress inside the slab when cooled to room temperature, and the more likely it is that cracks will occur. Therefore, appropriate transformation is generated in the range of 500 ° C or higher and 700 ° C or lower. If not, cracks are likely to occur. On the other hand, the cooling rate in the range of less than 500 ° C. and the range of more than 700 ° C. also contributes to the change in thermal stress, but the degree of contribution is small and does not directly cause cracking.

From the above findings, in the high-strength steel of the present embodiment, when the slab is cast and then cooled, the slab becomes a slab from the temperature of 700 ° C to 500 ° C (when 500 ° C or more and 700 ° C or less). It was thought that the occurrence of cracks could be suppressed by controlling the internal stress to be generated to be small.

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

Fracture by the fracture toughness test was considered to be a crack in the slab, and the fracture toughness test was conducted under the following conditions.

The slabs of the steel types shown in Table 1 were cast. The steel grades A to C shown in Table 1 are the steel grades of the high-strength steel of the present embodiment. After casting, a sample was taken from the slab and a fracture toughness test was carried out according to the metal material-plane strain fracture toughness test method of JISG0564. The fracture of the sample in the fracture toughness test, the starting point of fracture (cutout length) and the fracture stress can be considered as the slab's placement crack, internal crack (internal crack length) and internal stress, respectively.

FIG. 3 shows the results of the fracture toughness test. The vertical axis of FIG. 3 is the fracture stress at room temperature, and the horizontal axis of FIG. 3 is the starting point of fracture (cutout length). The curve shown in FIG. 3 shows the characteristics of fracture toughness. It was found that the slab breaks on the upper side of the curve, but not on the lower side of the curve.

When the fracture, the starting point of fracture (cutout length) and the fracture stress shown in FIG. 3 are replaced with the slab's placement crack, internal crack (internal crack length) and internal (residual) stress at room temperature, respectively, FIG. 4 shows. The figure shown is 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 no cracking occurs. The upper range of the curve is the "range where cracks occur", and the lower range of the curve is the "range where cracks do not occur".

From FIG. 4, it was found that the range of the internal (residual) stress δ at room temperature at which no placing crack occurs differs depending on the internal crack length L. 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 the placement crack does not occur”, the placement crack does not occur. In other words, it was found that even if the internal crack length L is long, the occurrence of cracks can be suppressed by reducing the internal stress δ. On the other hand, when the internal crack length L is short, the range of the internal stress δ at which the placement crack does not occur is wide. Therefore, when the internal crack length L is short, the occurrence of cracks can be suppressed even if the internal stress δ is large.

From the above, it is considered that the occurrence of the placement crack can be suppressed by controlling the internal stress δ so that the internal stress δ is in the “range in which the placement crack does not occur” according to the internal crack length L.

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

<Cooling rate>
The cooling rate of the slab was calculated using CASTEM (heat transfer coagulation program). Here, based on the physical property values (specific heat, solidification latent heat, thermal conductivity, density, etc.) collected by various tests of the slab samples of the components shown in Table 1 above, the cooling conditions (casting speed) of the continuous casting machine shown in Table 2 below , Specific water volume) and the cooling conditions after casting, and the heat transfer coefficients of the slab surface, mist, air, and roll were given as a function of temperature as boundary conditions. In addition, the slab surface temperature immediately after passing through the final roll of the continuous casting machine and the slab surface temperature during cooling after casting are measured, and the heat of the slab surface is adjusted so that the surface temperature history obtained by calculation matches the measured value. The transmission coefficient was corrected. The final roll of the continuous casting machine is a roll located at the position where the casting distance is the longest from the mold among the roll group connected from directly under the mold in the casting direction. Further, in the analysis of the internal stress described later, since the internal stress was maximized at the positions of the center in the width direction, the center in the thickness direction, and the center in the longitudinal direction of the slab, the cooling rate was the center in the width direction and the thickness of the slab. The cooling rate at the center of the direction and the center of the longitudinal direction was adopted. In the following, 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 ABAQUS, a general-purpose software. At this time, the stress distribution in the slab is given 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 the cooling rate. Was calculated. From this analysis, it was found that the internal stress in the longitudinal direction is the largest among the width direction, the thickness direction, and the longitudinal direction of the slab. It was also found that the internal stresses in the width direction, the thickness direction, and the longitudinal direction of the slab were maximum in the center in the width direction, the center in the thickness direction, and the center in the longitudinal direction, 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 and the internal crack length L shown in FIG. 4 and the presence or absence of the occurrence of cracks. FIG. 5 shows the relationship between the internal stress δ, the internal crack length L, the cooling rate CR, and the presence or absence of the occurrence of cracks.

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

The following experiments were conducted to confirm the above findings.
The slab was cast at the casting speed and specific water volume shown in Table 3. At the start of drawing out the slab, the slab was pulled out at 0.2 m / min. For about 100 seconds. Then, 0.12 m / min. Was sequentially accelerated up to the target casting speed at ^second acceleration. After casting, the slab was cut to a predetermined length and prepared to be in the slab cooling state shown in Table 3 at room temperature. Specifically, the slab was cooled as shown in Table 3 within 50 minutes after the slab passed through the final roll of the continuous casting machine. Then, the slab was cooled in the slab cooling state shown in Table 3. The slab cooling state shown in Table 3 is a cooling state in which the axial temperature of the slab ranges from 700 ° C. to 500 ° C. Then, after cooling the slab to room temperature, it was placed on the slab and checked whether cracks were generated (whether or not the slab was broken).

The conditions shown in Table 3 will be described below.
The steel types A to C shown in Table 3 are the steel types A to C shown in Tables 1 and 2.
The "slab cooling state" shown in Table 3 will be described below.
・ No.1 "water cooling"
The slab was placed on a mambo (sleepers) and sprayed onto the slab 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 "8-sheet stacking (top)"
As shown in FIG. 6, eight slabs were stacked and left at room temperature for 96 hours. In the following, stacking a plurality of slabs may be referred to as "stacking". The stacking was such that the centers of the eight slabs in the width and longitudinal directions were arranged in the same vertical line. It was confirmed whether or not a crack occurred in the uppermost slab out of the eight slabs.
・ No. 4 "8-sheet stacking (second from the top)"
As in No. 3, eight slabs were stacked and left at room temperature for 96 hours. Of the eight slabs, the second slab from the top was placed and it was confirmed whether cracks had occurred.

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

The “internal crack length L” shown in Table 3 is a length measured by the following method.
After casting, slabs cut to a predetermined length were cut along the longitudinal direction. A sample was taken from the cut surface and acid was applied to the sample. The length of the part corroded by the acid was measured. The length in the thickness direction was defined as the internal crack length L. When there were a plurality of corroded portions, the one having the longest corroded portion was defined as the internal crack length L. Since it is difficult to distinguish between a portion having a large microsegregation and an internal crack between dendrite trees, the lower limit of detection by 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 is a plot of the results shown in Table 3 in FIG.

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

From the above, when the slab is cast, cut, and cooled, the temperature of the axis of the slab ranges from 700 ° C to 500 ° C (in the range of 500 ° C or more and 700 ° C or less) according to the internal crack length L. By controlling the cooling rate CR so that the cooling rate CR is within the “range in which the cracking does not occur” shown in FIG. 7, the occurrence of the cracking can be suppressed.

As described above, the transformation stress does not occur until the axial temperature of the slab reaches 700 ° C., so that the internal stress generated in the slab is small. Therefore, even if the axial cooling rate CR of the slab is not controlled, cracking is unlikely to occur. Therefore, for example, the slab may be left to stand (air-cooled) until the axial temperature of the slab reaches 700 ° C. When one slab is left to stand (air-cooled), the axial cooling rate CR of the slab is 2.5 ° C./min. Or less. Therefore, in the range where the axial temperature of the slab exceeds 700 ° C., the axial cooling rate CR of the slab may be controlled as follows.
CR ≤ 2.5 ° C / min.

Further, even when the axial temperature of the slab falls below 500 ° C, if the cooling rate from 700 ° C to 500 ° C is controlled, the transformation has already been completed and the transformation does not occur, or the transformation occurs at 500 ° C or less. It is assumed that the length of the internal crack does not cause the crack even if the internal stress increases due to the occurrence of. Therefore, even if the axial cooling rate CR of the slab is not controlled, cracking is unlikely to occur. Therefore, it is not necessary to control the cooling rate after the axial temperature of the slab falls below 500 ° C. For example, even after the axial temperature of the slab falls below 500 ° C., the cooling state up to 500 ° C. may be continued. Further, the cooling state up to 500 ° C. may be stopped.

The "axial cooling rate CR of the slab" 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, the "axial cooling rate CR of the slab" can be calculated according to the casting conditions of the slab, the cooling conditions after casting, and the like, for example. For example, as the "axial cooling rate CR of the slab", a value obtained by (temperature difference between temperature A and temperature B) / (time from temperature A to temperature B) may be used. "Slab axis cooling rate CR from slab axis temperature from 700 ° C to 500 ° C" (slab axis cooling rate CR when slab axis temperature is 500 ° C or more and 700 ° C or less) is, for example, It can be controlled for each operation according to the packing style when the slab is cooled from 700 ° C to 500 ° C after casting (for example, the “slab cooling state” shown in Table 3), the time required for the packing shape, and the like. is there.

The "axial temperature of the slab" was calculated using CASTEM (heat transfer coagulation program), but can be calculated by general heat transfer coagulation calculation, for example, by 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, the stress generated inside the slab is controlled, and the internal stress is controlled by the cooling rate. Since the maximum internal stress is in the slab axis, the "slab axis cooling rate" is adopted as the slab cooling rate. Therefore, "the axial temperature of the slab" is adopted as the temperature of the slab.

The "internal crack length L" may be, for example, a length estimated based on the casting conditions of the slab (casting speed, specific water amount, slab size, usage status of the roll stand, etc.). The “internal crack length L” may be, for example, an actually measured value measured on the cut surface of the slab after casting. When a plurality of internal cracks occur, the longest internal crack length is defined as "internal crack length L".
Further, for example, when a slab has a length estimated as "internal crack length L", when the slab is cast under the same casting conditions as the casting conditions used for estimating the length, the "inside" of the slab is used. The crack length L may be an estimated length that already exists. For example, if there is an actually measured value of "internal crack length L", when casting a slab under the same casting conditions as the casting condition for which the measured value was obtained, the "internal crack length L" of the slab is measured as described above. It may be a value.

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

When the high-strength steel of the present 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 cracking when the internal crack length L is 50 mm or less has been 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 changes. Tables 3 and 4 show the casting conditions and cooling conditions of the slab.

The cooling conditions shown in Table 5 will be described below. In this experiment, the centers of the stacked slabs in the width and longitudinal directions were arranged in the same vertical line. In the following, a slab for confirming the presence or absence of cracking will be referred to as a “target slab”.

If the target slab is a slab other than the top and bottom slabs of the multiple stacked slabs, the "time to stack" is the process of stacking the slabs from the time the slab passes the final roll. In, it is the time until another slab is stacked on the target slab.
If the target slab is the top slab of the multiple stacked slabs, the "time to stack" is the target slab in the process of stacking the slabs from the time the slab passes the final roll. It is the time until is piled up.
For example, when three slabs are stacked in three stages and the target slab is a middle slab, the "time until stacking" is such that the top slab is placed on the upper surface of the middle slab from the time the slab passes the final roll. It is the time until it is closed (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 the target slab (top slab) below it from the time the slab passes the final roll. It is the time until it is placed on the upper surface of the slab (until the lower surface of the uppermost slab comes into contact with another slab).

The "contact surface" is the surface on which the target slab is in contact with another slab. "Upper" is the upper wide surface of the target slab, and "lower" is the lower wide surface of the target slab. As shown in FIG. 1, the wide surface of the slab is two of the four surfaces parallel to the longitudinal direction of the slab, which have a large area. In No. 6 and No. 7, since the "contact surface" is "up and down", 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, since the "contact surface" is "lower", in No. 8, only the wide surface below the target slab is in contact with the other slab. The target slab of No. 8 is the uppermost slab among the stacked slabs.

When the target slab is a slab other than the top and bottom slabs among the plurality of stacked slabs, the "stacking maintenance time" is maintained so that the upper and lower surfaces of the target slab are in contact with other slabs. It's time to spend. In other words, if the target slab is a slab other than the top and bottom of the stacked slabs, the "stacking maintenance time" is from the time when another slab is stacked on the top surface of the target slab. (From when the upper and lower surfaces of the target slab are in contact with other slabs) until when the slabs stacked on the target slab are removed (when the upper surface of the target slab is no longer in contact with other slabs) Until) time.
When the target slab is the uppermost slab of the plurality of stacked slabs, the "stacked maintenance time" is the time during which the lower surface of the target slab is maintained in contact with other slabs. .. In other words, if the target slab is the top slab of the stacked slabs, the "stacking maintenance time" will be from the time the target slab is stacked on top of the other slabs (of the target slab). It is the time from when the lower surface comes into contact with other slabs) to when the target slab is removed (until when the lower surface of the target slab no longer contacts other slabs).
For example, when three slabs are stacked and the target slab is the middle slab (second slab), the "stacking maintenance time" is the highest tier from the time when the three slabs are stacked. It is the time until the slab is removed. In addition, when a plurality of slabs are stacked and the target slab is the top slab, the "stacking maintenance time" is set from the time when the target slab (top slab) is placed on the upper surface of another slab (target). It is the time from when the lower surface of the slab comes into contact with another slab) to when the target slab is removed.

For the "overlap ratio", the "upper wide surface (one wide surface)" was arranged on the "total area of the upper wide surface (one wide surface) and the lower wide surface (the other wide surface) of the target slab". It is the ratio of the area in contact with the slab and the area where the lower wide surface (the other wide surface) is in contact with 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 one slab may be treated in the same manner. For example, when the centers in the width direction and the longitudinal direction of a plurality of stacked slabs are arranged in 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. , When the area of the lower wide surface of the slab placed on the target slab is S ₁ and the area of the upper wide surface of the slab placed on the target slab is S ₂ , the overlap ratio is calculated from these areas. Can be done.

FIG. 8 shows an example in which three stages of slabs are stacked. The three-tiered slabs are stacked so that the centers of the slabs in the width direction and the longitudinal direction are located in the same vertical direction. The overlap rate when the middle slab is used as the target slab among the three slabs will be described.
(Case 1)
When both the upper slab and the lower slab are stacked with slabs having a wider surface area larger 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 rate of Case 1 is expressed by the following equation.
Overlap rate = 100 x (S ₀₁ + S ₀₂ ) / (S ₀₁ + S ₀₂ )
= 100
(Case 2)
When the upper slab is a slab having a wide surface area equal to or less than the middle slab and the lower slab is stacked with a slab having a wide surface area equal to or larger than the middle slab, S ₁ ≤ S ₀₁ and S ₂ ≥ S ₀₂ . In this case, a 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 expressed by the following equation.
Overlap rate = 100 x (S ₁ + S ₀₂ ) / (S ₀₁ + S ₀₂ )
When the upper slab is a slab having a wider area than the middle slab and the lower slab is a slab having a wider area less than the middle slab, S ₁ ≧ S ₀₁ and S ₂ ≦ S ₀₂ . 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 smaller than the middle slab, S ₁ <S ₀₁ and S ₂ <S ₀₂ . In this case, a part of the upper wide surface of the middle slab is in contact with the entire lower wide surface of the upper slab. A part 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 expressed by the following equation.
Overlap rate = 100 x (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 ABAQUS, a general-purpose software.

"Presence or absence of cracking" was determined 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 placing 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 is 177 MPa or less, it is judged that the occurrence of cracks can be suppressed, and “◯” is shown in Table 5. On the other hand, when the internal stress exceeds 177 MPa, it is judged that the occurrence of cracks cannot be suppressed, and “x” is shown 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 to 8. In Nos. 6 to 8, as described above, since the internal crack length L is 50 mm or less, the internal stress capable of suppressing the occurrence of placing cracks is 177 MPa or less. Therefore, in FIG. 9, it is considered that the occurrence of cracks can be suppressed in the range where the internal stress is 177 MPa or less.

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

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

The “presence or absence of cracking” shown in Table 6 was determined by the following method.
<No.9 to No.12>
After casting the slab, it was cooled to room temperature and then placed on the slab to check if cracks had occurred. In No. 9 to No. 11, since no cracking occurred, “◯” is shown in Table 6. In No. 12, “x” is shown in Table 6 because a crack was generated.

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

From the above, when cooling the slabs, the slabs are stacked so that the overlap rate of the target slabs is 60% or more from 700 ° C. to 500 ° C. at the axial temperature of the slabs. As a result, the occurrence of cracks can be suppressed.

Further, for the following reasons, the state in which the slabs are stacked so that the overlap rate of the target slabs is 60% or more is maintained for 8 hours or more.
The higher the overlap rate of the target slab, the slower the cooling rate. When the overlap rate is 100%, which is the highest, the cooling rate is the slowest. In Table 3, No. 4 is an experiment in which the overlap rate is 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 axial temperature of the slab to reach from 700 ° C to 500 ° C. Since it takes 455 minutes at the slowest cooling rate, the axial temperature of the slab drops from 700 ° C. to 500 ° C. in less than 455 minutes when cooled at a faster cooling rate.

The cooling rate when stacked so that the overlap rate is 60% or more is equal to or faster than the cooling rate when the overlap rate is 100%. Therefore, the axial temperature of the slab is lowered from 700 ° C. to 500 ° C. by cooling by stacking for 455 minutes or more.

From the above, the state in which the slabs are stacked so that the overlap rate of the target slabs is 60% or more is maintained for 8 hours or more. As a result, the axial temperature of the slab is lowered from 700 ° C. to 500 ° C. After that, the slabs may be left stacked or the slabs may be stopped.

Here, "a state in which slabs are stacked so that the overlap rate of the target slabs is 60% or more" means that the slabs are stacked so that the overlap rate of the target slabs is at least 60% or more among the stacked slabs. It is in a stacked state. "A state in which slabs are stacked so that the overlap rate of the target slabs is 60% or more" means that if the overlap rate of the target slabs is 60% or more, not all slabs need to be stacked. All slabs may be stacked. Further, "a state in which slabs are stacked so that the overlapping rate of the target slabs is 60% or more" may be in the middle of stacking the slabs as long as the overlapping rate of the target slabs is 60% or more. The number of target slabs may be one or two or more. For example, when four slabs are stacked in four stages and the third slab from the top is the target slab, when the second slab from the top is stacked, the third slab (target slab) ) Overlap rate is 60% or more. In this case, "a state in which slabs are stacked so that the overlap rate of the target slabs is 60% or more" means that if the second slab from the top is stacked, the top slab is not stacked. It may be piled up with the top slabs.

The timing at which stacking is started is not particularly limited as long as it is after cutting the slab. However, "the state where the slabs are stacked so that the overlap rate of the target slabs is 60% or more" is set within 50 minutes after the slabs have passed the final roll of the continuous casting machine for the following reasons. To do.

In order to prevent cracking, when the axial temperature of the slab reaches 700 ° C., cooling must be performed in a state where the overlap rate of the target slab is 60% or more. Therefore, before the axial temperature of the slabs reaches 700 ° C., it is preferable that the slabs are stacked so that the overlap ratio of the target slabs is 60% or more.

When the 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. For example, when casting is performed under conditions where the casting speed is as slow as 1.0 m / min. And the specific water content is as high as 4.0 L / kg-steel, the axial temperature of the slab in the final roll of the continuous casting machine is calculated by heat transfer solidification. 827 ° C. Since the core temperature tends to decrease as the casting speed is slower and the specific water content is larger, it is considered that the shaft core temperature is further higher under the conditions where the casting speed is faster and the specific water content is smaller than the above conditions. Since the axial core temperature exceeds 800 ° C. even under the condition that the casting speed is slow and the specific water content is large, the axial core temperature surely exceeds 800 ° C. under the condition that the casting speed is high and the specific water content is small. Therefore, when the 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 unattended (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 axial temperature of the slab to reach from 800 ° C. to 700 ° C. After this time, the axial temperature of the slab may drop below 700 ° C. Therefore, within 50 minutes after the slab has passed through the final roll of the continuous casting machine, the slabs are stacked so that the overlap rate of the target slabs is 60% or more.
As a result, the target slab is cooled with the overlap rate of the target slab being 60% or more from the time when the axial temperature of the slab reaches 700 ° C.

From the above, when the length of the internal crack of 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 slab has passed through the final roll of the continuous casting machine, the slab is cut and the slabs are stacked so that the overlap ratio of the target slab is 60% or more. Within 50 minutes after the slab has passed the final roll of the continuous casting machine, the overlap rate of the target slab should be 60% or more. After that, the state where the overlap rate of the target slab is 60% or more is maintained for 8 hours or more. In other words, the state in which the slabs are stacked so that the overlap rate of the target slabs is 60% or more is maintained for 8 hours or more.
The "overlap ratio" is the "upper side" with respect to the "total area of one wide surface and the other wide surface" of at least one slab excluding the top and bottom slabs in a plurality of stacked slabs. It is the ratio of the total area where the wide surface (one wide surface) contacts the slab placed above it and the area where the lower wide surface (the other wide face) contacts the slab placed below it.

When the slab is cooled by the method described above, the axial cooling rate CR of the slab exists in the "range where cracks do not occur" shown in FIG. 6 from 700 ° C. to 500 ° C. As described above, the axial cooling rate CR can be controlled. As a result, it is possible to prevent the slab of the high-strength steel of the present embodiment from being placed and cracked.

An example of "relationship between the axial temperature of the slab and time" when the above-mentioned cooling method is carried out is shown in FIG. “A” in FIG. 10 is when the slab has passed the final roll of the continuous casting machine. In "A" of FIG. 10, the axial temperature of the slab is higher than 800 ° C. After "A", slabs are stacked, and at "B" within 50 minutes from "A", the overlap rate of the target slab becomes 60% or more. In "B", the axial temperature of the slab exceeds 700 ° C. In "B", the stacking of all slabs may be completed, and the stacking of all slabs may not be completed. 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 state in which the slabs are stacked so that the overlap rate of the target slabs is 60% or more is maintained for 8 hours or more from "B". As a result, the axial temperature of the slab falls below 500 ° C. After this, the slabs may be maintained in a stacked state, or the stacking may be stopped.

In FIG. 10, in "A", the axial temperature of the slab is higher than 800 ° C., but in "A", the axial temperature of the slab may be 800 ° C. Further, in FIG. 10, in "B", the axial temperature of the slab is higher than 700 ° C., but in "B", the axial temperature of the slab may be 700 ° C.

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 shown by the scope of claims rather than the above description, and includes all modifications within the meaning and scope equivalent to the scope of claims.

X slab

Claims (1)

High-strength steel slab with carbon content of 0.16 mass% or more and 0.35 mass% or less, silicon content of 1.0 mass% or more and 2.5 mass% or less, and manganese content of 1.2 mass% or more and 4.0 mass% or less. After casting, when cutting and cooling,
When the length of internal cracks generated in the slab after casting is 50 mm or less
After the slab has passed the final roll of the continuous casting machine, the slab is cut and
Within 50 minutes after the slab has passed the final roll of the continuous casting machine, stack three or more cut slabs.
Here, for at least one slab excluding the top and bottom slabs in which three or more slabs are stacked, the one wide surface is arranged on the one wide surface and the other wide surface of the slab. Three or more cut slabs are stacked so that the total ratio of the area in contact with the slab and the area in contact with the slab on which the other wide surface is arranged is 60% or more.
A method for cooling a slab of high-strength steel, which comprises maintaining a state in which three or more cut slabs are stacked for 8 hours or more.