JP7743935B2 - Continuously cast slab and its manufacturing method - Google Patents

Description

The present invention relates to a continuously cast slab that prevents cracking during cooling and a method for manufacturing the same. More specifically, the present invention relates to a continuously cast slab for high-strength steel (hi-tensile), which is effective in preventing cracking during slab placement and does not cause the problem of holes during rolling, and a method for manufacturing the same.

In recent years, in the automotive industry, in order to achieve both thinner car bodies and collision safety, high-strength steel has been further strengthened and, to that end, has been increasingly alloyed. However, the use of high-strength steel with high alloying significantly reduces the toughness of the slab.

As the toughness of slabs decreases due to the use of high alloys, cracks occurring during slab cooling, so-called slab placement cracks, have become more frequent. If slab placement cracks occur, the slab may break during transportation, making it impossible to use the slab for hot rolling. Even if the slab does not break, cracks in the slab may open during hot rolling, causing the hot-rolled steel sheet to break.
Alternatively, when a slab having small cracks is subjected to hot rolling, cold rolling, annealing, or plating treatment, the small cracks in the slab appear as surface defects such as scabs and slivers on the steel sheet produced by these treatments.
Cracks on the surface of a slab are usually removed by a grinder. However, in a highly alloyed slab, the toughness of the slab decreases due to the high alloying, and the cracks in the slab propagate due to the stress of the grinder, so that the cracks in the slab may not be completely removed.
On the other hand, small cracks in the slab may be overlooked and appear as surface defects on the steel sheet after hot rolling, cold rolling, annealing, or plating. For these reasons, it is necessary to suppress cracks in the slab.

Figure 1 is an enlarged photograph taken with a scanning electron microscope (SEM) of the fracture surface of a crack in a high-strength steel slab that fractured due to slab cracking. As is clear from Figure 1, the fracture surface of the slab crack appeared to be an intergranular fracture along the prior austenite grain boundary. Figure 2 shows a microstructure photograph of the cross section of the slab crack. The depth of the slab crack was mainly approximately 20 mm from the surface of the slab. The slab crack propagated near the prior austenite grain boundary, and grain boundary ferrite was present at the tip of the slab crack. Pearlite, or pearlite and bainite, was also observed within the prior austenite grains.

Intergranular fracture occurs when the prior austenite grains are coarse and the grain boundaries are embrittled. When intergranular ferrite forms, there is a difference in strength between the intragranular pearlite and bainite, causing stress concentration in the weak intergranular ferrite, which can develop into cracks in the slab even at lower stress levels.
On the other hand, when a slab is cooled, stress is generated due to differences in thermal contraction and transformation expansion between the surface and interior of the slab. If this stress is large, slab cracking occurs when the slab is cooled to room temperature. Recent high-alloy, high-strength steels have high hardenability, and the prior austenite grain size in the slab is coarse. Therefore, conventional slow cooling processes cannot prevent the precipitation of low-temperature transformation phases (bainite, martensite, etc.).
In addition, since the toughness of the slab is low, deep cracks that occur in the slab are difficult to remove by manual maintenance such as using a grinder, which has been a problem that significantly reduces the yield of the slab.

From this perspective, methods have been proposed to prevent the occurrence of thermal cracking in high-tensile steel slabs. For example, Patent Document 1 proposes a method of suppressing the bainite-martensite transformation by slow cooling from 700 to 500°C, the temperature range in which austenite transforms to ferrite, thereby reducing the stress generated by the transformation expansion. In other words, Patent Document 1 discloses a method that can prevent the occurrence of thermal cracking in slabs, even in high-tensile steel types that are prone to thermal cracking. Specifically, the cooling method for high-tensile steel slabs disclosed in Patent Document 1 is a method of preventing the occurrence of thermal cracking by controlling the slab cooling rate in accordance with the length of internal cracks that have occurred in the high-tensile steel, based on the finding that the internal stress of high-tensile steel depends on the cooling rate.

Furthermore, Patent Document 2 proposes a method of reducing the temperature difference and stress in the slab that occurs during transformation by starting slow cooling of the slab immediately after casting, and further slow cooling the slab at a temperature of 700°C or higher for 10 hours or more at a temperature of 700 to 500°C.
That is, Patent Document 2 discloses a cooling method for slabs for high-strength steel plates that not only prevents slab cracking during cooling of the slab but also prevents quality defects such as scabs during hot rolling, even for slabs containing Si. Specifically, the cooling method for slabs for high-strength steel plates disclosed in Patent Document 2 sets the average cooling rate of a continuously cast slab of high-strength hot-rolled steel plate, which has limited contents of chemical components such as C, Si, and Mn, at 500 to 700°C, to 20°C/hr or less.

Japanese Patent Application Laid-Open No. 2020-139209 Japanese Patent Application Laid-Open No. 2019-167560

However, the above-mentioned prior art has the following problems. The method of cooling a high-tensile steel slab after casting described in Patent Document 1 focuses only on the temperature range from 700°C to 500°C when the slab is cooled after casting, and controls the internal stress generated in the slab to be small. For this reason, even if a slab with a higher carbon content is manufactured using the cooling method of a high-tensile steel slab described in Patent Document 1, the occurrence of slab placement cracks cannot be sufficiently suppressed.

Furthermore, the cooling method for slabs for high-strength steel plates described in Patent Document 2 is based on the finding that slab cracking is caused by the addition of Si to steel and thermal stress generated due to temperature variations within the slab, and focuses on reducing thermal stress to suppress slab cracking. However, the cooling method for slabs for high-strength steel plates described in Patent Document 2 does not limit the microstructure of the slab. Therefore, even if a slab is produced using the cooling method for slabs for high-strength steel plates described in Patent Document 2, it is not possible to sufficiently suppress the occurrence of slab placement cracking.
Furthermore, as a result of intensive research by the present inventors, it was found that slabs containing large amounts of C, Si, and Mn according to conventional technology have a significantly low toughness, making it impossible to completely suppress cracking during slab placement, and causing problems such as holes during rolling.

The present invention was made in consideration of these circumstances, and aims to provide a continuously cast slab and a manufacturing method thereof that does not cause slab placement cracks during cooling, even in the case of continuously cast slabs with low toughness, and that do not cause hole formation problems during rolling.

The inventors conducted extensive research to achieve the above-mentioned object. As a result, they analyzed the fracture morphology of slab cracks and found that the fracture surface contained at least one of the following fracture surfaces: an intergranular fracture along the prior austenite grain boundary, and an intragranular fracture (cleavage fracture) that crosses the prior austenite grain boundary. Furthermore, the microstructure at the position where the slab crack propagated was mainly composed of ferrite and pearlite, while the microstructure further inside was mainly composed of bainite.
Here, the term "bainite-based structure" refers to a microstructure that contains at least one selected from bainite, tempered martensite, quenched martensite, and retained austenite. In other words, the term "bainite-based structure" refers to a microstructure that mainly contains bainite and may also contain at least one unavoidable structure selected from quenched martensite, tempered martensite, and retained austenite. The bainite-based structure is a low-temperature transformation phase that undergoes phase transformation at a lower temperature than the ferrite and pearlite contained in the microstructure.
Normally, an object is cooled from the surface, so the precipitation of low-temperature transformed phases does not occur inside the object. However, because the prior austenite grain size in a slab is coarse, the prior austenite grain size significantly affects the transformation time of the phases that make up the microstructure. In other words, when the prior austenite grain size inside the slab is larger than that on the surface of the slab, the structure that precipitates in the slab and the timing of the precipitation of the structure differ, even if the slab is cooled in the same way.
From this, it was discovered that, after the transformation of the surface layer of the slab is completed, the low-temperature transformation phase inside the slab undergoes transformation expansion during thermal contraction, generating tensile stress on the surface of the surface layer of the slab, leading to slab cracking. Furthermore, after extensive research, the inventors discovered that by controlling the microstructure of the continuously cast slab and reducing the stress during transformation in the low-temperature transformation phase inside the slab, it is possible to suppress slab placement cracking during the cooling process of the continuously cast slab and prevent the occurrence of hole formation problems during rolling, and thus arrived at the present invention.

That is, the continuously cast slab according to the present invention, which advantageously solves the above problems, has the following features:
(a) A continuous cast slab for high strength steel, comprising:
In mass%,
C: 0.10% or more and 1.00% or less,
Si: 0.10% or more and 2.50% or less,
Mn: 0.40% or more and 5.00% or less;
Optionally, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less;
Further, optionally, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.20 at least one element selected from the group consisting of Cr: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination of two or more elements, with the balance being Fe and inevitable impurities;
When the average prior austenite grain size at a position 10 mm below the surface of a continuously cast slab is defined _{as d10} and the average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as _d20 , the average prior austenite grain size ratio ( _d20 / _d10 ) is 1.0 or more and 4.0 or less,
The microstructure consists of ferrite, pearlite, and a low-temperature transformation phase,
the low-temperature transformation phase includes at least one selected from bainite, tempered martensite, quenched martensite, and retained austenite;
The microstructure at a position 10 mm below the surface of the continuously cast slab has an area ratio of the ferrite and an area ratio of the pearlite totaling 80% or more, and the area ratio of the ferrite, the area ratio of the pearlite, and the area ratio of the low-temperature transformation phase totaling 100%; and
The microstructure at a position 20 mm below the surface of the continuous casting slab is characterized in that the sum of the area ratio of the ferrite and the area ratio of the pearlite is 60% or more, and the sum of the area ratio of the ferrite, the area ratio of the pearlite, and the area ratio of the low-temperature transformation phase is 100%.

Furthermore, the method for producing a continuous cast slab according to the present invention is a method for producing a continuous cast slab for high strength steel, comprising the steps of:
A continuously cast slab having the composition described in (a)
The total heat dissipation amount Q _MD in the continuous casting mold satisfies the following relational expression (1),
A first cooling step in which the temperature _T20 of the continuously cast slab at the center in the width direction of the continuously cast slab and at a position 20 mm from the surface of the continuously cast slab is cooled under cooling conditions in which the temperature T20 is in the temperature range of 1200 ° C. or more and 1450 ° C. or less and the residence time is 230 s or less;
a second cooling step in which the continuous cast slab is cooled at a center in the width direction thereof and at a surface temperature _T0 of the surface layer of the continuous cast slab of 700°C or higher and 850°C or lower at an average cooling rate of 20°C/hr or lower;
and a third cooling step of cooling the continuously cast slab at a center in the width direction of the continuously cast slab under cooling conditions in which the average cooling rate at a surface temperature _T0 of the surface layer of the continuously cast slab is 10°C/hr or less at a surface temperature T0 of 500°C or more and 700°C or less.
In the above relational expression (1), Q _MD represents the total heat removal amount in the continuous mold [MW/m ² ], L _MD represents the effective mold length [m], and Vc represents the casting speed [m/min].

According to the present invention, even with the chemical composition of continuous cast slabs for high-strength steel, it is possible to provide a continuously cast slab that does not undergo cracking during the cooling process and does not cause problems with holes during rolling.

1 is a photograph taken by a scanning electron microscope (SEM) of a fracture surface of a cracked portion of a continuously cast slab for high strength steel that has fractured due to a temporary crack. 1 is a cross-sectional photograph of the cracked portion. This is an enlarged photograph of an optical microscope of a continuously cast slab produced in an example (Test No. D-2) of a continuously cast slab according to an embodiment of the present invention, showing the microstructure at a position 10 mm below the surface of the continuously cast slab. This is an enlarged photograph of an optical microscope of a continuously cast slab produced in an example (Test No. D-2) of a continuously cast slab according to an embodiment of the present invention, showing the microstructure at a position 20 mm below the surface of the continuously cast slab.

The following describes in detail the embodiments of the present invention. Note that the drawings are schematic and may differ from the actual product. Furthermore, the following embodiments exemplify devices and methods that embody the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[First embodiment]
A continuous cast slab according to a first embodiment will be described. The continuous cast slab according to this embodiment is a continuous cast slab for high strength steel,
In mass%,
C: 0.10% or more and 1.00% or less,
Si: 0.10% or more and 2.50% or less,
Mn: 0.40% or more and 5.00% or less;
Optionally, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less;
Further, optionally, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.20 at least one element selected from the group consisting of Cr: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination of two or more elements, with the balance being Fe and inevitable impurities;
(i) When the average prior austenite grain size at a position 10 mm below the surface of a continuously cast slab is defined as _d10 and the average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as _d20 , the average prior austenite grain size ratio ( _d20 / _d10 ) is 1.0 or more and 4.0 or less,
(ii) the microstructure consists of ferrite, pearlite, and a low-temperature transformation phase;
the low-temperature transformation phase includes at least one selected from bainite, tempered martensite, quenched martensite, and retained austenite;
The microstructure at a position 10 mm below the surface of the continuously cast slab has an area ratio of the ferrite and an area ratio of the pearlite totaling 80% or more, and the area ratio of the ferrite, the area ratio of the pearlite, and the area ratio of the low-temperature transformation phase totaling 100%; and
The microstructure at a position 20 mm below the surface of the continuous casting slab is characterized in that the sum of the area ratio of the ferrite and the area ratio of the pearlite is 60% or more, and the sum of the area ratio of the ferrite, the area ratio of the pearlite, and the area ratio of the low-temperature transformation phase is 100%.
That is, according to the invention of this embodiment, by limiting the component composition contained in the continuously cast slab and providing it with at least the above-mentioned properties (i) to (ii), even in the case of recent continuously cast slabs for high-strength steel, which have very low toughness, cracking during slab placement does not occur during the cooling process, and problems such as perforation during rolling of the continuously cast slab can be prevented, making it possible to provide a continuously cast slab for high-strength steel with a high yield.

First, we will explain the suitable range of the microstructure of the continuously cast slab according to this embodiment and the reasons for limiting it. In the following explanation, "%" indicating the composition ratio of the microstructure means "area %" unless otherwise specified. Furthermore, observation of the microstructure of the continuously cast slab was performed at room temperature.

As mentioned above, when the fracture morphology of the fracture surface of the cracked portion of a high-strength continuous cast slab for steel that had fractured due to slab placement cracking was observed, it was found that many of the slab placement cracks had progressed to a depth of approximately 20 mm below the surface of the slab, and that they had taken the form of "intergranular fracture" in which the crack had progressed to the prior austenite grain boundary, and that the microstructure of the cracked portion was mainly composed of ferrite and pearlite, while the microstructure inside the crack (towards the center of the slab in the thickness direction) was mainly composed of bainite.
In other words, in continuously cast slabs for high-strength steel, the causes of slab cracking due to fracture of grain boundaries are thought to be a coarse prior austenite grain size, differences in the microstructure between the slab surface and the interior, and the precipitation of low-temperature transformation phases within the slab. When the prior austenite grain size is coarse, embrittlement of the grain boundaries due to grain boundary segregation and precipitation of grain boundary ferrite is likely to occur, which is a cause of slab cracking. Furthermore, when the microstructure of the slab surface differs from that of the interior, and low-temperature transformation phases are precipitated within the slab, the transformation expansion of the low-temperature transformation phases is a cause of slab cracking.
Therefore, the invention according to this embodiment limits the component composition contained in the continuously cast slab as a necessary condition for a continuously cast slab for high-strength steel that does not generate slab placement cracks during the cooling process, and focuses on two phenomena: (i) the average prior austenite grain size ratio, which is calculated from the average prior austenite grain size at each of several predetermined positions set from the surface layer of the continuously cast slab, and (ii) the microstructure of the continuously cast slab.

<(i) Average Prior Austenite Grain Size Ratio>
The continuously cast slab for high strength steel according to this embodiment is characterized in that (i) when the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab is defined as _d10 and the average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as _d20 , the average prior austenite grain size ratio ( _d20 / _d10 ) is 1.0 or more and 4.0 or less. Here, the average prior austenite grain size refers to the average value of a plurality of prior austenite grain sizes measured in a plurality of fields of view.

In conventional continuously cast slabs, the average prior austenite grain size is very large, at several mm. This significantly reduces the toughness of the continuously cast slab. In addition, the average prior austenite grain size significantly affects the transformation behavior of the microstructure of the continuously cast slab. The larger the prior austenite grain size, the longer the transformation start time. As a result, even if the continuously cast slab is slowly cooled, low-temperature transformation phases are more likely to precipitate in the microstructure. Furthermore, the greater the difference in the average prior austenite grain size, the more likely it is that a difference in the microstructure will occur between the surface and the interior of the slab.
From this technical viewpoint, the continuously cast slab according to this embodiment has an average prior austenite grain size ratio ( _d20 /d10) set to 1.0 or more and 4.0 or less, where d10 is the average prior austenite grain size at a position ₁₀ mm below the surface of the continuously cast slab and _d20 is the average prior austenite grain size at a position ₂₀ mm below the surface of the continuously cast slab.
Thus, the upper limit of the average prior austenite grain size ratio ( _d20 / _d10 ) is preferably 4.0 or less, because if the average prior austenite grain size ratio ( _d20 / _d10 ) is 4.0 or less, the difference in microstructure between the slab surface layer and the slab interior can be reduced.
On the other hand, the lower limit of the average prior austenite grain size ratio ( _d20 / _d10 ) is not strictly limited. However, special cooling control is required to make the average prior austenite grain size inside the slab smaller than the average prior austenite grain size in the slab surface layer, and this special cooling control requires capital investment. For this reason, the average prior austenite grain size ratio ( _d20 / _d10 ) is preferably 1.0 or more. The lower limit of the average prior austenite grain size ratio ( _d20 / _d10 ) is more preferably 1.2 or more, and even more preferably 1.5 or more.

Here, the reason why the positions 10 mm below the surface of the continuously cast slab and 20 mm below the surface of the continuously cast slab are specified when setting the average prior austenite grain size is that, since most slab placement cracks progress to about 20 mm below the surface of the slab, it is considered that the positions 20 mm below the surface of the continuously cast slab and the position 10 mm below the surface of the continuously cast slab, which is located halfway between the surface of the continuously cast slab and the position 20 mm below the surface of the continuously cast slab, are necessary positions to suppress slab placement cracks.
On the other hand, the region of the continuously cast slab less than 5 mm below the surface is directly quenched by a water spray in the mold or just below the mold.
The region of less than 5 mm below the surface of the continuously cast slab has a fine structure in which the grains constituting the continuously cast slab are of γ grain size, and the continuously cast slab has high toughness.
Therefore, it is unlikely that the origin of the slab placement cracks occurs in an area less than 5 mm below the surface of the continuously cast slab.
From this technical viewpoint, in the continuously cast slab according to this embodiment, the region less than 5 mm from the surface layer of the continuously cast slab can be excluded from the position where control of the microstructure of the continuously cast slab is required.
Therefore, the positions where control of the continuously cast slab structure is required are the first position, which is located inside the slab and is 20 mm from the depth in the slab thickness direction, and the second position, which is located on the surface of the slab and is 10 mm from the depth in the slab thickness direction.
That is, as a first position below the surface layer of the continuously cast slab, the average prior austenite grain size may be set, for example, at a position 20 mm from the surface layer of the continuously cast slab inside the slab in the depth direction from the surface layer of the continuously cast slab to 18 to 22 mm and 15 to 25 mm.
Furthermore, as a second position below the surface layer of the continuously cast slab, a position 10 mm from the surface layer of the continuously cast slab inside the slab may be used as a reference, and the average prior austenite grain size may be set, for example, at a depth of 8 to 12 mm or 5 to 15 mm from the surface layer of the continuously cast slab.

In the continuously cast slab according to this embodiment, the factor that determines the average prior austenite grain size is the temperature at which the continuously cast slab is cooled. Austenite grains grow at a high rate, particularly in the temperature range of 1200°C to 1450°C. Therefore, the temperature at which the continuously cast slab is cooled is particularly in the range of 1200°C to 1450°C, and the cooling rate and residence time of the continuously cast slab in this temperature range affect the temperature.
That is, in the temperature range of 1200°C or higher and 1450°C or lower, the slower the cooling rate of the continuously cast slab or the longer the residence time of the continuously cast slab, the coarser the average prior austenite grain size becomes. The prior austenite grain size of a continuously cast slab is fine on the surface side of the slab and coarser toward the interior of the slab. To reduce the difference between the average prior austenite grain size on the surface side of the slab and the average prior austenite grain size inside the slab, it is necessary to coarsen the average prior austenite grain size on the surface side of the slab and refine the average prior austenite grain size inside the slab.

That is, in order for the continuously cast slab according to this embodiment to satisfy the condition (i) that the average prior austenite grain size ratio ( _d20 / _d10 ) is _1.0 or more and 4.0 or less, where _d10 is the average prior austenite grain size 10 mm below the surface of the continuously cast slab and d20 is the average prior austenite grain size 20 mm below the surface of the continuously cast slab, it is important to produce the continuously cast slab by controlling the cooling of the continuously cast slab so that the temperatures _T10 and _T20 of the continuously cast slab at the center in the width direction of the continuously cast slab and at positions 10 mm and 20 mm from the surface of the continuously cast slab are in the temperature range of 1200°C or more and 1450°C or less. Here, _T10 and _T20 are the temperatures at positions 10 mm and 20 mm below the surface of the continuously cast slab, respectively.
In order to control the cooling rate in the temperature range of 1200°C or more and 1450°C or less where the temperature _T10 at a position 10 mm from the surface of the continuously cast slab is 1200°C or more and 1450°C or less, it is necessary to take into account the total amount of heat dissipation within the continuous casting mold in the manufacturing process of the continuously cast slab according to this embodiment.
In other words, the temperature _T10 at a position 10 mm from the surface of the continuously cast slab falls within the temperature range of 1200°C or higher and 1450°C or lower when the continuously cast slab passes through the continuous casting mold. Therefore, taking into account the relationship between the total heat removal amount in the continuous casting mold per unit area QMD [MW/ ^m2 ], the effective mold length LMD [m], and the casting speed Vc [m/min], the continuously cast slab must be cooled so that the total heat removal amount in the continuous casting mold satisfies the following relationship (1).
In the above relational expression (1), Q _MD represents the total heat removal amount in the continuous casting mold [MW/m ² ], L _MD represents the effective length of the mold [m], and Vc represents the casting speed [m/min].

The total heat removal amount _QMD in the continuous casting mold is calculated by calculating the total heat removal amount from the flow rate of cooling water to the continuous casting mold and the temperature difference between the inlet and outlet of the continuous casting mold, and dividing the calculated total heat removal amount by the contact area between the mold copper plate that constitutes the continuous casting mold and the cast piece.
The effective mold length _LMD is the length of the mold in which molten steel poured from the tundish can solidify. The effective mold length _LMD depends on the shape of the continuous casting mold, but is preferably 0.7 to 0.9 m, for example.
The casting speed Vc is the speed at which molten steel is poured into the continuous casting mold, and is preferably, for example, 0.8 to 2.0 m/min.

Here, the relationship between the total heat removal amount Q _MD [MW/m ² ] in the continuous casting mold, the effective mold length L _MD [m], and the casting speed Vc [m/min] is defined by the above-mentioned relational expression (1), and the value calculated by relational expression (1) is preferably 0.6 to 1.5. If the value calculated by relational expression (1) is 1.5 or less, the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab can be coarsened, and the difference with the average prior austenite grain size inside the slab can be reduced, which is preferable.
On the other hand, if the value calculated by the relational expression (1) is 0.6 or more, it is preferable because the shell thickness of the continuously cast slab at the outlet side of the continuous casting mold can be ensured and the average prior austenite grain size of the continuously cast slab can be coarsened without the risk of breakout.

Furthermore, in order to control the residence time during which the temperature _T20 at a position 20 mm from the surface of the continuously cast slab falls within the range of 1200°C or more and 1450°C or less, it is necessary to control the cooling by spray water cooling in the manufacturing process of the continuously cast slab according to this embodiment.
That is, the temperature _T20 at a position 20 mm from the surface of the continuously cast slab falls within the range of 1200°C or higher and 1450°C or lower when the continuously cast slab passes through the secondary cooling zone directly below the continuous casting mold. Therefore, it is preferable to control the amount of cooling water in the secondary cooling zone and set the residence time during which the temperature _T20 at a position 20 mm from the surface of the continuously cast slab falls within the range of 1200°C or higher and 1450°C or lower to 230 seconds or shorter.
If the total heat removal amount QMD in the continuous casting mold satisfies the above relational expression (1) and the residence time in which the temperature _T20 at a position 20 mm from the surface of the continuously cast slab falls within the temperature range of 1200°C or higher and 1450°C or lower is 230 s or shorter, the average prior austenite grain size ratio ( _{d20/ d10} ), which is the ratio of the average prior austenite grain size d10 at a position 10 mm from the surface of the continuously cast slab to the average prior austenite grain size d20 at a position ₂₀ mm below the surface _of the slab, can be set to 4.0 or less, which is preferable because it can suppress slab placement cracking.

From this viewpoint, the residence time of the continuously cast slab is preferably 220 seconds or less, more preferably 210 seconds or less, and even more preferably 200 seconds or less. The lower limit of the residence time of the continuously cast slab is not particularly limited, but is set to 60 seconds or more because too short a residence time increases the risk of breakout during continuous casting due to non-uniform solidification.
In other words, if the residence time of the continuously cast slab is less than 60 seconds, cracks will occur in the continuously cast slab due to uneven solidification of the initial solidified shell, which may lead to breakout, so it is preferable to set the residence time of the continuously cast slab to 60 seconds or more.
From this viewpoint, the residence time of the continuously cast slab in the temperature range of 1200°C or more and 1450°C or less at a position ₂₀ mm from the surface of the continuously cast slab is more preferably 80 seconds or more, and even more preferably 90 seconds or more.

The cooling rate and residence time of the continuously cast slab in the temperature range of 1200°C to 1450°C _(T10 , _T20) at positions 10 mm and 20 mm from the surface of the continuously cast slab can be controlled by adjusting the cooling conditions in the initial stage of slab casting. For example, in the continuous casting of steel, molten steel with an adjusted composition is first poured into a water-cooled copper mold to form an initial solidified shell. Then, the continuously cast slab begins to be withdrawn from the water-cooled copper mold, and after the continuously cast slab leaves the water-cooled copper mold, the continuously cast slab is cooled with a water spray.
The temperature _T10 at a position 10 mm below the surface of the continuously cast slab is greatly affected by the cooling inside the continuous casting mold. Therefore, for example, the thermal conductivity of the mold powder used to lubricate the inside of the continuous casting mold may be reduced, or the amount of cooling water for the continuous casting mold may be reduced.
On the other hand, the temperature _T20 at a position 20 mm below the surface of the continuously cast slab is greatly affected by the cooling just below the continuous casting mold, and therefore, this temperature can be controlled, for example, by increasing the flow rate of the water spray just below the continuous casting mold. When the spray just below the continuous casting mold is a two-fluid spray of water and air, this temperature can also be controlled by increasing the water flow rate and the air flow rate.

By controlling these cooling conditions, it is possible to control the average prior austenite grain size at positions 10 mm and 20 mm from the surface of the continuously cast slab.
However, it is difficult to actually measure the temperature inside the continuously cast slab. Therefore, the temperature history at positions 10 mm and 20 mm below the surface of the continuously cast slab can be calculated by heat transfer analysis, and the temperatures _T10 and _T20 of the continuously cast slab can be estimated. In order to ensure that the residence time in the above temperature range is the longest inside the continuously cast slab, the heat transfer analysis position can be set at the center of the slab width.

<(ii) Microstructure of Continuously Cast Slabs>
The continuously cast slab according to this embodiment is characterized in that (ii) the microstructure at a position 10 mm below the surface of the slab has a total area ratio of ferrite and pearlite of 80% or more, and the microstructure at a position 20 mm below the surface of the slab has a total area ratio of ferrite and pearlite of 60% or more. That is, (i) in addition to the average prior austenite grain size ratio ( _d20/d10) , which is the ratio of the average prior austenite grain size d10 at a position 10 mm below the surface of the continuously cast slab to the average prior austenite grain size _d20 at a position _{20 mm below} the surface of the continuously cast slab, being 4.0 or less, the ratio of the internal structures that make up the microstructure of the continuously cast slab, such as bainite and ferrite, is also a factor that determines the unit of fracture of the slab, and it is known that the stress applied to the slab changes depending on this ratio.
In particular, when a low-temperature transformation phase, which is an unavoidable structure that mainly contains bainite and also includes quenched martensite, tempered martensite, and retained austenite, occurs in the microstructure inside a slab, the amount of expansion during transformation of the low-temperature transformation phase is large, and in addition, the ferrite and pearlite formed around the low-temperature transformation phase, including bainite, etc., have already completed their transformation and subsequently undergo thermal contraction, resulting in large stress concentrations there.
Therefore, the inventors discovered that by controlling the cooling rate and (ii) ensuring that the microstructure at a position 10 mm below the surface of the continuously cast slab has a total area ratio of ferrite and pearlite of 80% or more, and that the microstructure at a position 20 mm below the surface of the continuously cast slab has a total area ratio of ferrite and pearlite of 60% or more, the transformation stress applied to the surface of the slab can be reduced.
Here, the low-temperature transformation phase, which is a microstructure other than ferrite and pearlite, is mainly composed of bainite, but is not limited to bainite alone, and may also include quenched martensite, tempered martensite, and retained austenite in addition to bainite.
The area ratio of ferrite and the area ratio of pearlite can be calculated based on the results of observation of the microstructure of the continuously cast slab using an observation means such as an optical microscope, an electron microscope, etc. Then, the ferrite and pearlite contained in the microstructure of the continuously cast slab can be distinguished using an observation means such as an optical microscope or an electron microscope.

Based on the results of identifying the microstructure of the continuously cast slab, the _area S of the microstructure of the continuously cast slab and the area S ( _{ferrite + pearlite) obtained by summing the area S of ferrite and the area S of pearlite are} calculated. Then, the ratio of the area S _{(ferrite + pearlite)} obtained by _summing the area S of _ferrite and the area S of _pearlite to the area S of the microstructure of the continuously cast slab is defined as an area ratio (%) and is calculated.
The microstructure of a continuously cast slab is composed of ferrite, pearlite, and a low-temperature transformation phase, and the sum of the area ratio (%) of ferrite, the area ratio (%) of pearlite, and the area ratio (%) of the low-temperature transformation phase is 100%. Therefore, the area ratio (%), which is the ratio of the area S to _X of the low-temperature transformation phase, is calculated by subtracting the total area ratio of the area ratio (%) of ferrite and the area ratio (%) of pearlite from 100%.

The continuously cast slab of this embodiment is characterized in that (ii) the microstructure at a position 10 mm below the surface of the continuously cast slab has a total area ratio of ferrite and area ratio of pearlite of 80% or more, and the total area ratio of the ferrite, area ratio of pearlite, and area ratio of the low-temperature transformation phase is 100%, and the microstructure at a position 20 mm below the surface of the continuously cast slab has a total area ratio of ferrite and area ratio of pearlite of 60% or more, and the total area ratio of the ferrite, area ratio of pearlite, and area ratio of the low-temperature transformation phase is 100%.
That is, in the continuously cast slab according to this embodiment, (ii) if the area ratio (%), which is the ratio of _{the total} area S10 _{(ferrite + pearlite) of the ferrite area S10ferrite and the pearlite area S10pearlite to the area S10total of the microstructure at a position 10 mm below the surface of the continuously cast slab, is 80% or more, and the area ratio (%), which is the ratio of the total area S20(ferrite + pearlite) of the ferrite area S20ferrite} and the _pearlite area S20pearlite to the area S20total of the microstructure at a position 20 mm below the surface of the continuously cast slab, is 60% or more, this is preferable because it can alleviate the thermal stress and transformation stress that occur in the surface layer of the slab due to slow cooling of the slab and suppress slab placement cracking of the continuously cast slab.
Furthermore, when the area ratio of ferrite is 5% or more but less than 10%, a small amount of grain boundary ferrite precipitates, which reduces the grain boundary strength and worsens the degree of cracking in the slab. Therefore, for both S10 _ferrite and S20 _ferrite , it is preferable that the area ratio of these ferrites is less than 5% or 10% or more.
On the other hand, if the microstructure at a position 10 mm below the surface of a continuously cast slab has a total area ratio of ferrite and pearlite of less than 80%, the stress during cooling and transformation of the low-temperature transformation phase cannot be absorbed, and cracks will occur during slab placement, which is not desirable.
Furthermore, if the microstructure at a position 20 mm below the surface of a continuously cast slab is such that the sum of the area ratio of ferrite and the area ratio of pearlite is less than 60%, the transformation expansion of the low-temperature transformation phase will cause excessive tensile stress in the surface of the slab where transformation has already been completed, which is undesirable because it will cause slab placement cracks in the continuously cast slab.
The microstructure at a position 10 mm below the surface of a continuously cast slab consists of ferrite, pearlite, and a low-temperature transformation phase, and the sum of the area ratio of the ferrite area (S10 _ferrite ), the area ratio of the pearlite area (S10 _pearlite ), and the area ratio of the low-temperature transformation phase (S10 _X) is 100%. Similarly, the microstructure at a position 20 mm below the surface of a continuously cast slab consists of ferrite, pearlite, and a low-temperature transformation phase, and the sum of the area ratio of the ferrite area (S20 _ferrite) , the area ratio of the pearlite area (S20 _pearlite) , and the area ratio of the low-temperature transformation phase (S20 _X) is 100%.

Ferrite contains iron with a maximum of 0.02% carbon by mass, and is a structure close to pure iron. Ferrite is the softest and most ductile of all steel structures. Pearlite is a structure obtained when austenite is slowly cooled. Pearlite consists of ferrite layers and cementite layers, and is formed by these layers being arranged alternately.

To control slab cracking, it is important not only to reduce thermal and transformation stresses by cooling the slab, but also to control the microstructure of the continuously cast slab. By controlling the cooling rate below 850°C and above 700°C, which is the ferrite transformation region, it is possible to control the precipitation of ferrite within the microstructure. Specifically, by reducing the cooling rate of the continuously cast slab in the ferrite transformation region, the amount of ferrite precipitation can be increased, and the ferrite precipitated from grain boundaries, which causes stress concentration problems, can be rendered harmless.

In addition, to prevent slab cracking during placement, it is important not only to suppress embrittlement of prior austenite grain boundaries but also to reduce stress during transformation of low-temperature transformation phases. In addition to controlling the average prior austenite grain size at multiple predetermined locations from the surface of the continuously cast slab, various controls for the cooling rate in the pearlite transformation region (700°C or lower, 500°C or higher) can be used to control the microstructure of the continuously cast slab and reduce stress during transformation of low-temperature transformation phases. Specifically, reducing the cooling rate in the pearlite transformation region suppresses the precipitation of low-temperature transformation phases, primarily bainite, and prevents excessive transformation expansion of the low-temperature transformation phases from affecting the ferrite and pearlite phases, which have already completed their transformation. This can prevent slab cracking during placement in continuously cast slabs.

The cooling of the continuously cast slab after it leaves the continuous casting machine can be controlled by changing the slab temperature at the outlet of the continuous casting machine, the time it takes to stack multiple slabs, the number of slabs to be stacked, whether or not a heat-insulating cover is used, water-tight treatment, and other conditions.
The cooling rate of the continuously cast slab after it leaves the continuous casting machine can be measured using a thermocouple. For example, a thermocouple is installed in the center of the wide surface (long side) of the surface layer of the continuously cast slab after it leaves the continuous casting machine, and the surface temperature _T0 of the surface layer of the continuously cast slab is measured, thereby allowing the cooling rate to be calculated.

As explained above, according to the invention of this embodiment, even in the case of recent continuous cast slabs for high-strength steel, which have very low toughness, cracking of the slab during cooling is prevented, and problems such as holes during rolling can be prevented, making it possible to obtain continuous cast slabs for high-strength steel with a high yield.

Furthermore, the continuously cast slab according to this embodiment contains, by mass%, C: 0.10% to 1.00%, Si: 0.10% to 2.50%, and Mn: 0.40% to 5.00%.
In the following description, "%" representing the content of a component element in steel means "mass %" unless otherwise specified.

<C: 0.10% or more and 1.00% or less>
The reasons for limiting the chemical components contained in the continuously cast slab according to this embodiment are explained below. The content of each chemical component contained in the continuously cast slab is expressed in mass %. The reason for limiting the C content in the continuously cast slab to 0.10% or more and 1.00% or less is as follows. C contained in the continuously cast slab for high-strength steel is an element necessary for increasing the strength of high-strength steel plates made from the continuously cast slab. If the C content is less than 0.10%, the strength required for high-strength steel plates cannot be obtained, so the lower limit of the C content is 0.10%. On the other hand, if the C content exceeds 1.00%, the weldability and workability of the high-strength steel plates become insufficient, which is undesirable.

Therefore, from this perspective, in the continuously cast slab according to this embodiment, it is preferable that the C content contained in the continuously cast slab be 0.10% or more and 1.00% or less, more preferably 0.12% or more and 0.45% or less, and particularly preferably 0.15% or more and 0.40% or less.

<Si: 0.10% or more and 2.50% or less>
Next, the reason why the Si content in continuously cast slabs for high-strength steel is set to 0.10% or more and 2.50% or less is as follows. Si contained in continuously cast slabs is an element necessary for ensuring retained austenite in the steel sheet during the annealing process of high-strength steel sheet made from continuously cast slabs. In addition, Si contained in continuously cast slabs is an essential additive element because it contributes to increasing the strength of high-strength steel sheet through solid solution strengthening. If the Si content is less than 0.10%, the strength required for high-strength steel sheet cannot be obtained, so the lower limit of the Si content is 0.10%.

On the other hand, if the Si content exceeds 2.50%, the effect of obtaining the strength required for high-strength steel plate saturates, and strong scale forms on the hot-rolled sheet before it is processed into high-strength steel plate. As a result, the appearance and pickling properties of the high-strength steel plate deteriorate, so the upper limit for the Si content is 2.50%.

Therefore, from this perspective, in the continuously cast slab according to this embodiment, it is preferable that the Si content contained in the continuously cast slab be 0.10% or more and 2.50% or less, more preferably 0.50% or more and 2.00% or less, and even more preferably 1.00% or more and 1.80% or less.

<Mn: 0.40% or more and 5.00% or less>
Furthermore, the reason why the Mn content in the continuously cast slab is set to 0.40% or more and 5.00% or less is as follows. Mn contained in the continuously cast slab is an element necessary for further increasing the strength of the high-strength steel plate. Specifically, Mn is an element added to control the strength of the high-strength steel plate through transformation control in the hot rolling process of the continuously cast slab. If the Mn content is less than 0.40%, the high-strength steel plate cannot be sufficiently strengthened, so the lower limit of the Mn content is 0.40%. On the other hand, if the Mn content exceeds 5.00%, the degree of sufficient strengthening of the high-strength steel plate saturates and the manufacturing cost of the high-strength steel plate increases, which is undesirable from an economic standpoint.

Therefore, from this perspective, in the continuously cast slab according to this embodiment, it is preferable that the Mn content contained in the continuously cast slab be 0.40% or more and 5.00% or less, more preferably 1.20% or more and 4.50% or less, and even more preferably 1.40% or more and 4.00% or less.

The continuously cast slab according to this embodiment has the above-mentioned chemical composition, with the remainder consisting of Fe and unavoidable impurities, and has an average prior austenite grain size and microstructure of appropriate composition. To that extent, taking other properties into consideration, it may contain up to 0.100% P, up to 0.0200% S, up to 0.100% Al, up to 0.0100% N, and up to 0.0100% O. Examples of unavoidable impurities include Zn, Pb, and As. A total content of up to 0.100% of these unavoidable impurities is permitted.

P segregates at prior austenite grain boundaries, embrittling the grain boundaries and potentially causing slab cracking. Therefore, the P content is preferably 0.100% or less. While there is no specific lower limit for the P content, it is preferable that it be 0.001% or more, as P is a solid solution strengthening element and can increase the strength of steel sheet. Therefore, the P content is preferably 0.100% or less. Preferably, it is 0.001% or more. Even more preferably, it is 0.070% or less.

Sulfur exists as sulfide and is an element that causes slab embrittlement. Therefore, it is preferable that the S content be 0.0200% or less. There is no particular lower limit for the S content, but due to production technology constraints, it is preferable that it be 0.0001% or more. Therefore, the S content is preferably 0.0200% or less. Preferably, it is 0.0001% or more. More preferably, it is 0.0050% or less.

Al suppresses the formation of carbides during slab cooling and promotes the formation of retained austenite, and is therefore an element that affects the fraction of retained austenite in the slab. It is also preferable to add 0.005% or more for deoxidation. If the Al content exceeds 0.100%, there is a risk of slab embrittlement. Therefore, the Al content is preferably 0.100% or less. It is even more preferable to set it to 0.010% or more. It is even more preferable to set it to 0.080% or less.

N exists as a nitride and is an element that causes embrittlement of the slab. Therefore, it is preferable that the N content be 0.0100% or less. There is no specific lower limit for the N content, but due to production technology constraints, it is preferable that the N content be 0.0001% or more. Therefore, the N content is preferably 0.0100% or less. Preferably, it is 0.0001% or more. More preferably, it is 0.0050% or less.

O exists as an oxide and is an element that causes embrittlement of the slab. Therefore, it is preferable that the O content be 0.0100% or less. There is no particular lower limit for the O content, but due to production technology constraints, it is preferable that the O content be 0.0001% or more. Therefore, the O content is preferably 0.0100% or less. Preferably, it is 0.0001% or more. More preferably, it is 0.0050% or less.

The continuously cast slab according to this embodiment is for use in high-strength steel plates and, in addition to the above-mentioned chemical composition, further contains Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, B: 0.0100% or less, Co: 1.00 %, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less may be contained alone or in combination of two or more thereof.

If the content of Ti, Nb, and V is 0.200% or less, large amounts of coarse precipitates and inclusions are not formed in the slab, and the toughness of the slab is not reduced. Therefore, the content of Ti, Nb, and V is preferably 0.200% or less.
Although there are no particular lower limits for the contents of Ti, Nb, and V, the contents of Ti, Nb, and V are each preferably 0.001% or more because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of continuously cast slabs. Therefore, when Ti, Nb, and V are contained, their contents are each preferably 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

Ta and W, if contained at 0.10% or less each, do not form large amounts of coarse precipitates or inclusions, and do not reduce the toughness of the slab. Therefore, it is preferable that the Ta and W contents be 0.10% or less each. While there are no specific lower limits for the Ta and W contents, they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of continuously cast slabs. Therefore, if Ta and W are contained, their contents should be 0.10% or less each. More preferably, they should be 0.01% or more. Even more preferably, they should be 0.08% or less.

The continuously cast slab according to this embodiment may contain at least one element selected from Cr, Mo, Ni, and Cu, as needed, provided that the objectives of the present invention are not impaired. Cr, Mo, Ni, and Cu enhance the strength of the steel sheet through structural control during hot rolling of the continuously cast slab. This effect is enhanced by adding 0.01% or more of one or more of Cr, Mo, Ni, and Cu, respectively, and therefore adding 0.01% or more is preferable. Since exceeding the upper limit of each element degrades the weldability, hot workability, and other properties of the steel sheet, the upper limit of each of Cr, Mo, Ni, and Cu is set to 1.00%. Therefore, when the continuously cast slab contains Cr, Mo, Ni, and Cu, the content of each element should be 1.00% or less. Preferably, the content should be 0.01% or more. More preferably, the content should be 1.00% or less.

B may be added because it controls the structural transformation of continuously cast slabs during hot rolling and annealing, thereby affecting strength through structural strengthening. B does not affect slab toughness if its content is 0.0100% or less. Therefore, it is preferable that the B content be 0.0100% or less. While there is no specific lower limit for the B content, it is more preferable that the B content be 0.0003% or more because B is an element that segregates to austenite grain boundaries during hot rolling and annealing of continuously cast slabs and improves hardenability. Therefore, if B is contained, its content should be 0.0100% or less. It is more preferable that it be 0.0003% or more. It is even more preferable that it be 0.0080% or less.

If Co is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not be reduced. Therefore, it is preferable that the Co content be 1.00% or less. There is no specific lower limit for the Co content, but because Co is an element that improves the hardenability of the slab, it is more preferable that the Co content be 0.001% or more. Therefore, if Co is contained, its content should be 1.00% or less. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.80% or less.

Cu content of 1.00% or less does not increase coarse precipitates or inclusions, and does not reduce the toughness of the slab. Therefore, it is preferable that the Cu content be 1.00% or less. There is no specific lower limit for the Cu content, but because it is an element that improves hardenability, it is more preferable that the Cu content be 0.01% or more. Therefore, if Cu is contained, its content should be 1.00% or less. More preferably, it should be 0.01% or more. Even more preferably, it should be 0.80% or less.

Sn does not affect the toughness of the slab if it is 0.200% or less. Therefore, it is preferable that the Sn content be 0.200% or less. There is no specific lower limit for the Sn content, but since Sn is an element that improves hardenability, it is more preferable that the Sn content be 0.001% or more. Therefore, if Sn is contained, its content should be 0.200% or less. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.100% or less.

If Sb is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not be reduced. Therefore, it is preferable that the Sb content be 0.200% or less. There is no specific lower limit for the Sb content, but since Sb is an element that suppresses decarburization and makes it possible to adjust the strength of the steel plate, it is more preferable that the Sb content be 0.001% or more. Therefore, if Sb is contained, its content should be 0.200% or less. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.100% or less.

If the content of Ca, Mg, and REM is 0.0100% or less, the amount of coarse precipitates and inclusions does not increase and the toughness of the slab does not decrease. Therefore, the content of each of Ca, Mg, and REM is preferably 0.0100% or less.
Although the lower limits of the contents of Ca, Mg, and REM are not particularly specified, since Ca, Mg, and REM are elements that make the shape of nitrides and sulfides spheroidal and improve the toughness of the slab, it is more preferable that the contents of these elements are each 0.0005% or more.
Therefore, when Ca, Mg and REM are contained, their contents are each set to 0.0100% or less, more preferably 0.0005% or more, and even more preferably 0.0050% or less.

Zr and Te, if each is 0.100% or less, do not increase coarse precipitates or inclusions in the slab, and do not reduce the toughness of the slab. Therefore, it is preferable that the Zr and Te contents be 0.100% or less. While there are no specific lower limits for the Zr and Te contents, since Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of the slab, it is more preferable that the Zr and Te contents be 0.001% or more. Therefore, if Zr and Te are contained, their contents should be 0.100% or less, more preferably 0.001% or more, and even more preferably 0.080% or less.

If Hf is 0.10% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not be reduced. Therefore, it is preferable that the Hf content be 0.10% or less. There is no specific lower limit for the Hf content, but since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel plate, it is more preferable that the Hf content be 0.01% or more. Therefore, if Hf is contained, its content should be 0.10% or less. More preferably, it should be 0.01% or more. Even more preferably, it should be 0.08% or less.

If Bi is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not be reduced. Therefore, it is preferable that the Bi content be 0.200% or less. There is no specific lower limit for the Bi content, but because Bi is an element that reduces segregation, it is more preferable that the Bi content be 0.001% or more. Therefore, if Bi is contained, its content should be 0.200% or less. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.100% or less.

In addition, with regard to the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi, if the content of each is less than the preferred lower limit value, the effects of the present invention will not be impaired, and therefore they are considered to be included as unavoidable impurities.

As described above, the invention relating to the first embodiment makes it possible to obtain the strength required for high-strength steel, and furthermore, to obtain continuous cast slabs that have excellent weldability, workability, and appearance of high-strength steel.

[Second embodiment]
A method for producing a continuously cast slab according to a second embodiment will be described. The method for producing a continuously cast slab according to this embodiment is a method for producing a continuously cast slab for high strength steel, and comprises the steps of:
A first cooling step in which the total heat dissipation amount Q _MD in the continuous casting mold satisfies the following relational expression (1), and the temperature T ₂₀ of the continuously cast slab at the center in the width direction of the continuously cast slab and 20 mm from the surface of the continuously cast slab is in the temperature range of 1200 ° C. or more and 1450 ° C. or less, and the residence time is 230 s or less;
a second cooling step in which the continuous cast slab is cooled at a center in the width direction thereof and at a surface temperature _T0 of the surface layer of the continuous cast slab of 700°C or higher and 850°C or lower at an average cooling rate of 20°C/hr or lower;
and a third cooling step of cooling the continuously cast slab at a center in the width direction thereof under cooling conditions in which the average cooling rate at a surface temperature T0 of the surface layer of the continuously cast slab is 10°C/hr or less at a surface temperature T0 of 500°C _or more and 700°C or less.
In the above relational expression (1), Q _MD represents the total heat removal amount in the continuous casting mold [MW/m ² ], L _MD represents the effective length of the mold [m], and Vc represents the casting speed [m/min].

In the manufacturing method for slabs for high-strength steel plates according to this embodiment, re-shipping may occur depending on the conditions of the manufacturing process. When re-shipping occurs, the cooling rate of the slab may temporarily exceed the specified cooling rate. However, since the time required for transformation is very slow, at 10 hours or more, handling times equivalent to the time required for re-shipping (at most 1 to 2 hours) do not lead to the occurrence of cracks. Therefore, in the manufacturing method for slabs for high-strength steel plates according to this embodiment, the cooling rate when cooling the continuously cast slab is specified as an average cooling rate rather than a maximum cooling rate.
Hereinafter, each step included in the method for producing a continuously cast slab according to this embodiment will be described.

(First cooling process)
The method for producing a continuous cast slab according to this embodiment is a method for producing a continuous cast slab for high strength steel, which comprises:
The method includes a first cooling step in which the total heat removal amount Q _MD in the continuous casting mold satisfies the following relational expression (1), and the temperature T ₂₀ of the continuously cast slab, which is at the center in the width direction of the continuously cast slab and 20 mm from the surface of the continuously cast slab, is cooled under cooling conditions in which the residence time in the temperature range of 1200°C or higher and 1450°C or lower is 230 seconds or shorter.
In the above relational expression (1), Q _MD represents the total heat removal amount in the continuous casting mold [MW/m ² ], L _MD represents the effective length of the mold [m], and Vc represents the casting speed [m/min].

The first cooling step is a step for controlling the average prior austenite grain size ratio ( _{d20/ d10} ), which is the ratio of the average prior austenite grain size d10 at a position 10 mm below the surface of the continuously cast slab to the average prior austenite grain size _d20 at a position ₂₀ mm below the surface of the continuously cast slab according to the above embodiment, to 4.0 or less. In the method for producing a continuously cast slab according to this embodiment, the factor that determines the average prior austenite grain size is the temperature at which the continuously cast slab is cooled. In the first cooling step, the temperature at which the continuously cast slab is cooled is in the range of 1200°C or higher and 1450°C or lower. In this way, the method for producing a continuously cast slab according to this embodiment focuses on the temperature range of 1200°C or higher and 1450°C or lower of the continuously cast slab, which is a factor that determines the average prior austenite grain size, and controls that temperature.
In the first cooling step, since it is difficult to actually measure the temperature at which the continuously cast slab is cooled in the temperature range of 1200°C to 1450°C, the temperature history at a position 20 mm from the surface of the continuously cast slab was calculated by heat transfer analysis. The analysis position was the central part of the slab width, where the residence time in the above temperature range is the longest within the slab.

In order to reduce the average prior austenite grain size ratio ( _d20 / _d10) , which is the ratio of the average prior austenite grain size _d10 at a position 10 mm below the surface of the continuously cast slab to the average prior austenite grain size _d20 at a position 20 mm below the surface of the continuously cast slab, it is necessary to increase the average prior austenite grain size _d10 at a position 10 mm below the surface of the continuously cast slab and decrease the average prior austenite grain size d20 at a position ₂₀ mm below the surface of the continuously cast slab.
In other words, it is necessary to slow down the cooling at a position 10 mm below the surface of the continuously cast slab, and to speed up the cooling at a position 20 mm below the surface of the continuously cast slab.

Furthermore, in the first cooling step, the total heat removal amount Q _MD in the continuous casting mold must satisfy the following relational expression (1) in terms of the relationship between the effective mold length L _MD [m] and the casting speed Vc [m/min]: In the above temperature range for cooling the continuously cast slab at a position 20 mm from the surface, the residence time of the continuously cast slab is 230 seconds or less.
Furthermore, if the total heat removal quantity Q _MD in the continuous casting mold satisfies the following relational expression (1) in relation to the effective mold length L _MD [m] and the casting speed Vc [m/min], and the residence time of the continuously cast slab at the above temperature is 230 seconds or less, the average prior austenite grain size ratio (d ₂₀ /d ₁₀ ) can be made 4.0 or less, and slab placement cracking can be suppressed, which is preferable.
Although there is no particular lower limit for the residence time of the continuously cast slab in the temperature range of 1200°C to 1450°C, if the residence time is too short, the risk of breakout during continuous casting due to non-uniform solidification increases, so the residence time is preferably 60 seconds or more, more preferably 80 seconds or more, and even more preferably 90 seconds or more.
In the above relational expression (1), Q _MD represents the total heat removal amount in the continuous casting mold [MW/m ² ], L _MD represents the effective length of the mold [m], and Vc represents the casting speed [m/min].

(Second cooling process)
Next, the method for producing a continuously cast slab according to this embodiment includes a second cooling step in which the continuously cast slab is cooled at a center in the width direction thereof and at an average cooling rate of 20° C./hr or less when the surface temperature _T0 of the surface layer of the continuously cast slab is 700° C. or more and 850° C. or less. The second cooling step is a step for increasing the area ratio of ferrite contained in the microstructure of the continuously cast slab according to the above embodiment.

In the second cooling process, the temperature at which the continuously cast slab is further cooled is 700°C or higher and 850°C or lower. In this way, the method for producing a continuously cast slab according to this embodiment controls the cooling rate within the temperature range in the ferrite transformation region, which can increase the amount of ferrite precipitation. In the second cooling process, the cooling rate was measured using a thermocouple. A thermocouple was placed in the center of the top surface of the slab's wide side (long side) after the slab emerged from the continuous casting machine, and the cooling rate was calculated from the measured temperature. The cooling rate in the third cooling process, described below, can also be measured in a similar manner.

In the second cooling step, the average cooling rate of the continuously cast slab within the above-mentioned temperature range is 20° C./hr or less. If the average cooling rate of the continuously cast slab is 20° C./hr or less, the residence time of the continuously cast slab in the ferrite transformation temperature range can be sufficiently ensured, the area ratio of ferrite can be increased, the precipitation of grain boundary ferrite can be suppressed, and the toughness of the slab can be improved, which is preferable.
Although there is no strict lower limit to the average cooling rate, since a separate energy source is required for control, the average cooling rate is preferably 2°C/hr or more, and more preferably 5°C/hr or more and 18°C/hr or less.

(Third cooling process)
Furthermore, the method for producing a continuously cast slab according to this embodiment includes a third cooling step in which the continuously cast slab is cooled at a center in the width direction thereof under cooling conditions in which the average cooling rate at a surface temperature _T0 of the surface layer of the continuously cast slab is 10°C/hr or less at a surface temperature T0 of 500°C or more and 700°C or less.
The third cooling step is a step for changing the microstructure of the continuously cast slab according to the embodiment to a structure mainly composed of pearlite, and for reducing the internal stress of the microstructure.
In other words, this is a process for suppressing the precipitation of low-temperature transformation phases in the microstructure of the continuously cast slab, and preventing the stress caused by the transformation expansion from being applied to the ferrite and pearlite regions where transformation has already been completed.
Specifically, the third cooling step is a step for setting an area ratio (%), which is the ratio of _{the total} area S10 _{(ferrite + pearlite)} of the ferrite area _S10ferrite and the _pearlite area S10pearlite to the area S10total of the microstructure at a position 10 mm below the surface of the continuously cast slab, to 80% or more, and a step for setting an area ratio (%), which is the ratio of _{the total} area S20( _{ferrite + pearlite)} of the ferrite area S20ferrite and the _pearlite area S20pearlite to the area S20total of the microstructure at a position 20 mm below the surface of the continuously cast slab, to 60% or more.

In the third cooling process, the temperature to which the continuously cast slab is further cooled is 500°C or higher and 700°C or lower. In this way, the method for manufacturing a continuously cast slab according to this embodiment controls the temperature of the continuously cast slab by focusing on the cooling rate in the temperature range in the pearlite transformation region.

In the third cooling step, the average cooling rate of the continuously cast slab within the above-mentioned temperature range is 10°C/hr or less. If the average cooling rate of the continuously cast slab is 10°C/hr or less while controlling the aforementioned average prior austenite grain size ratio ( _d20 / _d10 ) to 4.0 or less, the transformation of low-temperature transformation phases including bainite can be suppressed, and the microstructure of the continuously cast slab can be made mainly of pearlite, thereby reducing the internal stress, which is preferable.
Although there is no strict lower limit to the average cooling rate when cooling a continuously cast slab, since a separate energy source is required for control, the rate is preferably 2°C/hr or more, and more preferably 5°C/hr or more.

In this way, the method for manufacturing continuous cast slabs according to this embodiment employs a three-stage cooling process for continuously cast slabs, and by precisely controlling the average prior austenite grain size ratio and the microstructure of the continuously cast slab, it is possible to provide a continuously cast slab for high-strength steel that suppresses slab placement cracks that occur during cooling and also prevents problems such as perforation during rolling.

As explained above, according to the second embodiment of the method for manufacturing a continuous cast slab, even if the composition of a continuous cast slab for high-strength steel is used, by dividing the cooling process into three stages and precisely controlling each cooling process, it is possible to provide a continuous cast slab for high-strength steel that does not develop cracks during the cooling process and that can prevent problems such as holes during rolling.

[Other embodiments]
Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Various modifications can be made to the configuration and details of the present invention that are understandable to those skilled in the art within the technical scope of the present invention. Furthermore, systems or devices that combine the separate features included in each embodiment in any manner are also included in the technical scope of the present invention.

The effects of the present invention will be specifically explained below based on examples, but the present invention is not limited to these examples. Specifically, to confirm the effects of the present invention, the inventors produced continuously cast slabs using various steel types as raw materials in comparative examples (Test Nos. A-1 to A-4, Test Nos. B-1 to B-4, Test Nos. C-1 to C-4) and inventive examples (Test Nos. D-1 to D-24). Table 1 shows the compositions of the steels used as raw materials for the continuously cast slabs used in comparative examples (Test Nos. A-1 to A-6, Test Nos. B-1 to B-4, Test Nos. C-1 to C-4) and inventive examples (Test Nos. D-1 to D-24).

Here, the cooling conditions for the continuously cast slab were (I) the continuous casting mold total heat removal function [-], (II) a residence time [s] at 1450 to 1200°C, (III) an average cooling rate [°C/hr] at 850 to 700°C, and (IV) a three-stage cooling process consisting of an average cooling rate [°C/hr] at 700 to 500°C, and the conditions for each stage were appropriately changed to perform cooling. Tables 2 to 4 show the continuously cast slab cooling conditions (I) to (IV), the microstructure of the obtained continuously cast slab, and an evaluation of slab placement cracks.
The measurement of the average prior austenite grain size, the calculation of the ferrite and pearlite area ratios, and the evaluation of slab placement cracks in the continuously cast slabs produced in the comparative examples and inventive examples were carried out as follows.

<Measurement of average prior austenite grain size>
Here, the method for measuring the average prior austenite grain size is as follows. A sample was cut from the width center of the cooled slab, and the slab thickness cross section parallel to the slab width direction was used as the observation surface. The observation surface was then mirror-polished using diamond paste, then finish-polished using colloidal silica, and further etched with 3 vol. % nital to reveal the structure on the observation surface. Using an optical microscope, five fields of view were observed at 10 mm and 20 mm below the slab surface at 10x magnification to obtain structural images. The obtained structural images were used to determine the average prior austenite grain size using a cutting method in accordance with JIS G 0551:2020.

<Method for measuring ferrite area ratio>
The ferrite area ratio was measured in the same manner as in the measurement of the average prior austenite grain size. The observation surface was then mirror-polished using diamond paste, then polished to a finish using colloidal silica, and then etched with 3 vol.% nital to reveal the structure. Using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV, 10 fields of view were observed at 50x magnification at positions 10 mm and 20 mm below the surface of the slab. The obtained structure images were then analyzed using Adobe's PHOTOSHOP (registered trademark), and the ferrite area ratio was calculated for each of the 10 fields of view. The average of these values was used to determine the ferrite area ratio.
Ferrite has a larger grain size than pearlite and other low-temperature transformation phases including bainite (bainite, tempered martensite, quenched martensite, and retained austenite), and has a smooth surface with dark contrast, so it can be easily distinguished at 50x magnification.
It is difficult to strictly classify grain boundary ferrite and intragranular ferrite. Therefore, the inventors have conducted extensive research on slabs in which cracking occurred during slab placement and have found that when the area ratio of ferrite is greater than 5% and less than 10%, a large amount of harmful ferrite is present at the grain boundaries. In other words, a structure in which the area ratio of ferrite is 5% or more and less than 10% is determined to be grain boundary ferrite.

<Method for measuring the area ratio of pearlite>
The area ratio of the pearlite structure was measured in the same manner as in the above-mentioned ferrite measurement method, by revealing the structure on the observation surface of the slab. Using an SEM at an acceleration voltage of 15 kV, 10 fields of view were observed at a magnification of 10,000 times at positions 10 mm and 20 mm below the surface of the slab, with the ferrite removed from the field of view. The obtained structure images were then used with Adobe PHOTOSHOP (registered trademark) to calculate the area ratio of pearlite for each of the 10 fields of view, and the average of these values was determined as the area ratio of pearlite.
Pearlite is a eutectoid crystal of ferrite and cementite, and when observed with the above-mentioned scanning electron microscope, it is a structure in which thin flake layers of both crystals exhibit a pearl-like luster.

The microstructure of the continuously cast slab according to the present invention is composed of ferrite, pearlite, and a low-temperature transformation phase. Therefore, the area ratio of the area S _X of the low-temperature transformation phase constituting the microstructure can be calculated by subtracting the sum of the area ratios of ferrite and pearlite from 100%, taking into consideration that the sum of the area ratios of ferrite, pearlite, and low-temperature transformation phase is 100%.

<Evaluation of cracks in slab placement>
The evaluation method for slab placement cracks was based on the penetrant testing specified in JIS Z 2343:2017, and the presence or absence of cracks on the wide and narrow surfaces of the slab was evaluated. After applying the developer, the appearance of the penetrant was visually checked to see if any slab placement cracks had occurred on the surface.
In addition, if there is a crack that is 50 mm or longer in length, there is a high risk of the slab breaking during slab handling or in the heating furnace, and it is also highly likely to lead to hole formation problems during rolling. Therefore, the evaluation criteria for slab cracks when placed are as follows.
・Slab placement cracks 〇: No cracks of 50 mm or more in length on the slab surface. ・Slab placement cracks △: No cracks of 50 mm or more in length on the slab surface, but defects appeared after rolling. ・Slab placement cracks ×: Cracks of 50 mm or more in length on the slab surface.

<Comparative Examples (Test Nos. A-1 to A-4)>
The slab microstructures satisfied by the continuously cast slabs produced in Test Nos. A-1 to A-4 are designated Condition A. Condition A represents an example in which the ratio of the average prior austenite grain size at a position 10 mm below the slab surface to the average prior austenite grain size at a position 20 mm below the surface was greater than 4.0. In these cases, even if various conditions for slow cooling of the slab after it left the continuous slab casting machine were used, differences in the prior austenite grain size resulted in a difference in transformation rate, and the transformation timing at positions 10 mm and 20 mm below the slab surface differed significantly. This resulted in the concentration of transformation stress in the slab surface, making it impossible to suppress slab placement cracking.

<Comparative Examples (Test Nos. B-1 to B-4)>
The slab microstructures satisfied by the continuously cast slabs produced in Tests B-1 to B-4 are designated Condition B. Condition B represents an example in which the ratio of the average prior austenite grain size 10 mm below the slab surface to the average prior austenite grain size 20 mm below the surface is 4.0 or less, but the amount of ferrite precipitation 10 mm below the slab surface is small, resulting in the precipitation of grain boundary ferrite. In these cases, although the cooling rate between 700 and 500°C is small and the transformation stress generated is also small, embrittlement of the grain boundaries due to grain boundary ferrite occurs, and slab rest cracking cannot be suppressed.

<Comparative Examples (Test Nos. C-1 to C-4)>
The slab microstructures satisfied by the continuously cast slabs produced in Test Nos. C-1 to C-4 are designated as Condition C. Condition C is an example of a condition in which the ratio of the average prior austenite grain size at a position 10 mm below the slab surface to the average prior austenite grain size at a position 20 mm below the surface is 4.0 or less, but low-temperature transformed phases including bainite and the like precipitated at the position 20 mm below the slab surface, making it impossible to suppress slab cracking. In particular, because bainite transformation occurs at a lower temperature than pearlite transformation, the density difference with austenite is large and the transformation stress is also large. Therefore, it is believed that this stress is concentrated in the ferrite and pearlite portions where transformation has already been completed, making it impossible to suppress slab cracking.

<Examples of the Invention (Test Nos. D-1 to D-24)>
The slab microstructures satisfied by the continuously cast slabs produced in Test Nos. D-1 to D-24 are designated Condition D. Condition D is a condition for the present invention examples, in which the average prior austenite grain size ratio is 4.0 or less, and the microstructure at a position 10 mm below the slab surface has a ferrite area ratio and pearlite area ratio of 80% or more in total, and the microstructure at a position 20 mm below the slab surface has a ferrite area ratio and pearlite area ratio of 60% or more in total. The continuously cast slabs produced in Test Nos. D-1 to D-24 also did not experience slab placement cracking after cooling.

Figure 3 is an enlarged optical microscope photograph of a continuously cast slab produced in an example of the present invention (Test No. D-2) of a continuously cast slab according to the present invention. The enlarged photograph in Figure 3A shows the microstructure at a position 10 mm below the surface of the continuously cast slab. The enlarged photograph in Figure 3B shows the microstructure at a position 20 mm below the surface of the continuously cast slab.

As is clear from Figure 3, the continuously cast slab produced in the present invention example of a continuously cast slab (Test No. D-2) had controlled average prior austenite grain sizes at a position 10 mm below the slab surface and at a position 20 mm below the slab surface, and was found to have a microstructure corresponding to the area ratio of ferrite at each position below the slab surface.

3, it was found that slab placement cracking during slab cooling can be suppressed by (i) setting the average prior austenite grain size ratio ( _d20 / _d10 ) to 1.0 or more and _4.0 or less, where _d10 is the average prior austenite grain size 10 mm below the surface of the continuously cast slab and d20 is the average prior austenite grain size 20 mm below the surface of the continuously cast slab, and (ii) setting the microstructure at a position 10 mm below the surface of the continuously cast slab such that the sum of the area fractions of ferrite and pearlite is 80% or more and the microstructure at a position 20 mm below the surface of the continuously cast slab such that the sum of the area fractions of ferrite and pearlite is 60% or more. Note that the sum of the area fractions of ferrite, pearlite, and low-temperature transformation phases at positions 10 mm below the surface and 20 mm below the surface of the continuously cast slab is 100%.

That is, in the continuously cast slab of the present invention, when the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab is _d10 and the average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is _d20 , the average prior austenite grain size ratio ( _d20 / _d10 ) is 1.0 or more and 4.0 or less, and the microstructure at a position 10 mm below the surface of the continuously cast slab has a total area ratio of ferrite and pearlite of 80% or more, and the microstructure at a position 20 mm below the surface of the continuously cast slab has a total area ratio of ferrite and pearlite of 60% or more. Therefore, a slab for high-alloy, high-strength steel that is free from slab cracking after casting can be provided, and problems such as perforation during rolling can also be prevented.

In the continuously cast slab of the present invention, when the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab _{is d10} and the average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is d20, the average prior austenite grain size ratio ( _d20 / _d10 ) is 1.0 or more and 4.0 or less, and the microstructure at a position 10 mm below the surface of the continuously cast slab has a total area ratio of ferrite and pearlite of 80% or more, and the microstructure at a position 20 mm below the surface of the continuously cast slab has a total area ratio of ferrite and pearlite of 60% or more.Therefore, a slab for high-strength steel that is free from slab cracking after casting can be provided, and problems such as perforation during rolling can be prevented, making it industrially useful.

Claims (2)

A continuous casting slab for high strength steel,
In mass%,
C: 0.10% or more and 1.00% or less,
Si: 0.10% or more and 2.50% or less,
Mn: 0.40% or more and 5.00% or less;
Optionally, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less;
Further, optionally, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.20 at least one element selected from the group consisting of Cr: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination of two or more elements, with the balance being Fe and inevitable impurities;
When the average prior austenite grain size at a position 10 mm below the surface of a continuously cast slab is defined _{as d10} and the average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as _d20 , the average prior austenite grain size ratio ( _d20 / _d10 ) is 1.0 or more and 4.0 or less,
The microstructure consists of ferrite, pearlite, and low-temperature transformation phases (excluding pearlite), or
the microstructure is composed of the ferrite, the pearlite, the low-temperature transformation phase (excluding pearlite) and retained austenite,
The low-temperature transformation phase (excluding pearlite) is composed of one or more of bainite, tempered martensite, and quenched martensite ,
The microstructure at a position 10 mm below the surface of the continuously cast slab has an area ratio of the ferrite and an area ratio of the pearlite totaling 80% or more, and the area ratio of the ferrite, the area ratio of the pearlite, and the area ratio of the low-temperature transformation phase totaling 100%; and
A continuously cast slab characterized in that the microstructure at a position 20 mm below the surface of the continuously cast slab has a total area ratio of the ferrite and the pearlite of 60% or more, and the total area ratio of the ferrite, the pearlite, and the low-temperature transformation phase is 100%. A method for producing a continuous casting slab for high strength steel, comprising:
A continuously cast slab having the component composition according to claim 1,
The total heat dissipation amount Q _MD in the continuous casting mold satisfies the following relational expression (1),
A first cooling step in which the temperature _T20 of the continuously cast slab at the center in the width direction of the continuously cast slab and at a position 20 mm from the surface of the continuously cast slab is cooled under cooling conditions in which the temperature T20 is in the temperature range of 1200 ° C. or more and 1450 ° C. or less and the residence time is 230 s or less;
a second cooling step of cooling the continuously cast slab at a center in the width direction thereof and under cooling conditions in which the average cooling rate at a surface temperature T0 of the surface layer of the continuously cast slab is 20°C/hr or less at a temperature of 700°C _or more and 850°C or less;
and a third cooling step of cooling the continuously cast slab at a center in the width direction of the continuously cast slab and under cooling conditions in which the average cooling rate at a surface temperature _T0 of the surface layer of the continuously cast slab is 10°C/hr or less at a temperature of 500°C or more and 700°C or less.
In the above relational expression (1), Q _MD represents the total heat removal amount in the continuous casting mold [MW/m ² ], L _MD represents the effective length of the mold [m], and Vc represents the casting speed [m/min].