JP7477052B2 - Continuously cast slab and its manufacturing method - Google Patents

Description

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

In recent years, in the field of automobiles, in order to achieve both thinner car bodies and collision safety, high strength steels have been further strengthened and, to achieve this, higher alloys have been used. However, the use of higher alloys has significantly reduced the toughness of the slabs.

As the toughness of slabs decreases due to the use of high alloys, cracks during cooling of slabs, so-called "placement cracks," have become more frequent. If 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, the cracks may open during hot rolling of the slab, causing the hot-rolled steel sheet to break. Alternatively, if the cracks in the slab are small, they may appear as surface defects such as scabs and slivers on the steel sheet after hot rolling, cold rolling, annealing, or plating. Usually, cracks on the slab surface are removed with a grinder. However, the toughness of the slab decreases due to the use of high alloys, and the cracks in the slab may progress due to the stress of the grinder, making it impossible to completely remove the cracks in the slab. On the other hand, if the cracks in the slab are small, they 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 cracked part of a high-strength steel slab that had fractured due to a placement crack. As is clear from Figure 1, the fracture surface of the cracked part of the slab appeared to be an intergranular fracture surface along the prior austenite grain boundary. Figure 2 shows a microstructure photograph of the cross section of the cracked part of the slab. The depth of the slab crack was mainly about 20 mm from the surface layer of the slab. The slab crack propagated near the prior austenite grain boundary, and grain boundary ferrite was present at the tip of the cracked part of the slab. 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. Precipitates and ferrite are more likely to form at the grain boundaries than within the grains. Precipitates at the grain boundaries reduce the grain boundary strength and cause the toughness of the slab to decrease. If the prior austenite grains are coarse, the proportion of the grain boundaries decreases and the precipitate density increases, making the grain boundaries even more embrittled. In addition, when intergranular ferrite occurs, a difference in strength occurs between the prior austenite grains and the pearlite and bainite within the grains, so stress is concentrated in the grain boundary ferrite, which has low strength, and even a lower stress can cause the slab to crack. Again, if the prior austenite grains are coarse, thin, linearly elongated intergranular ferrite precipitates, making it impossible to stop the slab crack from propagating, and the damage caused by the slab crack increases. On the other hand, when the slab is cooled, stress is generated due to the difference in thermal contraction and transformation expansion between the surface and the interior of the slab. If this stress is large, the slab will crack when cooled to room temperature. Because recent high-alloy, high-strength steels have low slab toughness, deep cracks that occur in the slabs in this way are difficult to remove by manual maintenance such as with a grinder, and this has been a problem that significantly reduces the slab yield.

From this perspective, a method has been proposed to suppress the occurrence of stabilization cracks in a slab of high-tensile steel. For example, Patent Document 1 proposes a method of suppressing the bainite/martensite transformation by slowly cooling the 700-500°C temperature range in which austenite transforms into ferrite, thereby reducing the stress caused by the transformation expansion. In other words, Patent Document 1 discloses a method capable of suppressing the occurrence of stabilization cracks even in steel types in which stabilization cracks are likely to occur in high-tensile steel. Specifically, the cooling method for a slab of high-tensile steel disclosed in Patent Document 1 is a method of suppressing the occurrence of stabilization cracks by controlling the cooling rate of the slab in accordance with the length of the internal cracks that have occurred in the high-tensile steel, based on the knowledge that the internal stress of high-tensile steel depends on its cooling rate.

In addition, Patent Document 2 proposes a method of reducing temperature differences and stresses during transformation by starting slow cooling immediately after casting of the slab, and further slow cooling to 700-500°C at a temperature of 700°C or higher for 10 hours or more. That is, Patent Document 2 discloses a cooling method for slabs for high-strength steel plates that does not cause slab cracks during cooling of the slab, or quality defects such as scabbing during hot rolling, even for slabs containing Si. Specifically, the cooling method for slabs for high-strength steel plates disclosed in Patent Document 2 is one in which the average cooling rate at 500-700°C of a continuously cast slab of high-strength hot-rolled steel plate with limited contents of chemical components such as C, Si, and Mn is 20°C/hr or less.

JP 2020-139209 A JP 2019-167560 A

However, the above-mentioned conventional techniques have the following problems. The method of cooling a slab of high-tensile steel 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. However, in recent high-alloyed high-strength steels, the toughness of the slab is low, so the state of the prior austenite grain boundaries through which the cracks propagate is also very important. The method described in Patent Document 1 does not control the prior austenite grain size or grain boundary ferrite, so that even if a slab with an increased carbon content is manufactured using the cooling method of a slab of high-tensile steel described in Patent Document 1, the occurrence of cracks in the slab cannot be sufficiently suppressed.

Furthermore, the cooling method for slabs for high-strength steel plates described in Patent Document 2 is based on the knowledge that the cause of slab cracking is the thermal stress generated due to the addition of Si to steel and temperature unevenness in the slab, and focuses on reducing thermal stress to suppress cracking in the slab. However, the cooling method for slabs for high-strength steel plates described in Patent Document 2 does not impose any restrictions on the microstructure of the slab. Therefore, even if a slab is manufactured 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 cracking in the slab.
Furthermore, as a result of intensive research by the present inventors, it was found that the toughness of slabs containing large amounts of C, Si, and Mn according to conventional technology is quite low, it is impossible to completely suppress static cracks, and problems such as holes occurring during rolling occur.

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

The inventors have conducted intensive research to achieve the above object. As a result, they analyzed the fracture morphology of slab cracks and found that the fracture surface is at least one of the intergranular fracture surface along the prior austenite grain boundary and the intragranular fracture surface (cleavage fracture surface) crossing the prior austenite grain boundary. Furthermore, the inventors conducted detailed research and clarified that the slab cracks cannot be suppressed only by controlling the cooling rate and reducing the stress by reducing the temperature unevenness, but are greatly influenced by the morphology of the microstructure. Specifically, they found that by controlling the average prior austenite grain size and microstructure in the continuously cast slab and improving its toughness, it is possible to suppress the slab cracks during the cooling process of the continuously cast slab and prevent the occurrence of hole trouble during rolling, and thus came up with the present invention.

In other words, the continuous cast slab of the present invention, which advantageously solves the above problems, is a continuous cast slab for high-strength steel, characterized in that the average prior austenite grain size at a position 10 mm from the surface of the continuous cast slab is 0.5 mm or more and 2.0 mm or less, and the microstructure has a total area ratio of ferrite and area ratio of pearlite of 90% or more, and the area ratio of ferrite is less than 5% or 10% or more.

In addition, it is considered that a more preferable solution for the continuous cast slab according to the present invention is that it contains, by mass%, (a) C: 0.10% or more and 1.00% or less, Si: 0.10% or more and 2.50% or less, and Mn: 0.40% or more and 5.00% or less.

Furthermore, the manufacturing method of the continuous cast slab according to the present invention is a manufacturing method of the continuous cast slab for high strength steel in which the cracks caused by cooling in the slab are suppressed,
A continuous casting slab having the composition described in (a) is
A first cooling step in which the cooling temperature of the continuous casting slab at a position 10 mm from the surface of the continuous casting slab in the width direction center of the continuous casting slab is 1200 ° C. or more and 1450 ° C. or less, and the residence time of the continuous casting slab is 130 s or less;
A second cooling step in which the continuous casting slab is cooled under cooling conditions in which the average cooling rate at the surface temperature at the center of the width direction of the slab is 20 ° C./hr or less when the surface temperature is 700 ° C. or more and 850 ° C. or less;
and a third cooling step in which the continuous cast slab is cooled under cooling conditions in which the surface temperature at the width center of the slab is 500°C or higher and 700°C or lower at an average cooling rate of 10°C/hr or lower.

According to the present invention, even with the composition system of continuous cast slabs for high strength steel, it is possible to provide a continuous cast slab that does not suffer from 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 high-strength steel continuous casting slab fractured by a plate crack. 1 is a cross-sectional micrograph of the cracked portion. 1 is an enlarged photograph of a continuously cast slab produced in an example (Test No. D-9) of a continuously cast slab according to an embodiment of the present invention, taken with an optical microscope.

Below, the embodiments of the present invention are specifically described. Note that the drawings are schematic and may differ from the actual ones. Furthermore, the following embodiments are intended to exemplify devices and methods for embodying the technical ideas of the present invention, and are not intended to specify the configurations as described below. In other words, the technical ideas 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 the first embodiment will be described. The continuous cast slab according to this embodiment is a continuous cast slab for high strength steel, characterized in that (i) the average prior austenite grain size at a position 10 mm from the surface layer of the continuous cast slab is 0.5 mm or more and 2.0 mm or less, (ii) the microstructure has a total area ratio of ferrite and area ratio of pearlite of 90% or more, and (iii) the area ratio of the ferrite is less than 5% or 10% or more. That is, according to the invention according to this embodiment, by having at least the above characteristics (i) to (iii), even in the case of a continuous cast slab for high strength steel in recent years, in which the toughness of the continuous cast slab is very low, cracks during slab placement during the cooling process are not generated, and hole formation troubles during rolling can be prevented, and a continuous cast slab for high strength steel with a good yield can be provided.

First, we will explain the suitable range of the microstructure of a continuous cast slab and the reasons for its limitation. In the following explanation, "%" indicating the composition ratio of the microstructure means "area %" unless otherwise specified. Furthermore, observation of the microstructure of a continuous cast slab was performed at room temperature.

As mentioned above, when the fracture morphology of the fracture surface of the cracked portion of the high-strength steel continuous casting slab that broke due to the crack was observed, it was found that most of the cracks had progressed to about 20 mm below the surface of the slab, and that the cracks had progressed to the prior austenite grain boundaries, taking the form of "intergranular fracture". In other words, the causes of the cracks due to the grain boundary fracture in the high-strength steel continuous casting slab are the coarse prior austenite grain size and the presence of the grain boundary ferrite structure, which is a factor in embrittling the grain boundaries. In addition to the fact that the toughness of the original continuous casting slab is very low in high-alloy high-strength steel plates, if these embrittling factors are present, the cracks cannot be suppressed even if the slab is cooled slowly to reduce the stress. Therefore, in this embodiment, as the necessary conditions for the high-strength steel continuous casting slab that does not generate the cracks during the cooling process, we focused on these two phenomena consisting of (i) the average prior austenite grain size at a predetermined position from the surface of the continuous casting slab, and (ii) to (iii) the microstructure of the continuous casting slab.

<(i) Average Prior Austenite Grain Size>
The continuously cast slab for high strength steel according to the present embodiment is a continuously cast slab for high strength steel in which the occurrence of cooling-induced cracks is suppressed, and is characterized in that (i) the average prior austenite grain size at a position 10 mm from the surface of the continuously cast slab is 0.5 mm or more and 2.0 mm or less. Here, the average prior austenite grain size is a factor that determines the unit of fracture of the slab. Grain boundaries are characterized in that solute components tend to concentrate, and therefore precipitates tend to concentrate at the grain boundaries. That is, the larger the average prior austenite grain size, the smaller the grain boundary ratio per unit volume, and therefore the precipitate density increases, and as a result, the toughness of the continuously cast slab decreases. Here, the average prior austenite grain size refers to the average value of multiple prior austenite grain sizes calculated from the prior austenite grain sizes measured in multiple fields of view.

In conventional continuously cast slabs, the average prior austenite grain size is very large, at several mm in size. This significantly reduces the toughness of the continuously cast slab. In conventional low alloy steels, the average prior austenite grain size does not pose a problem because the toughness of the original continuously cast slab is also high, but in high alloy high strength steels, the average prior austenite grain size can be a very serious problem. Therefore, in the continuously cast slab according to this embodiment, the average prior austenite grain size at a position 10 mm from the surface layer of the continuously cast slab is set to 2.0 mm or less. If the average prior austenite grain size is 2.0 mm or less, precipitates concentrated at the prior austenite grain boundaries can be dispersed, and the toughness of the continuously cast slab is not reduced, which is preferable.
On the other hand, the lower limit of the average prior austenite grain size is not strictly limited, but in order to make the average prior austenite grain size finer than 0.5 mm, for example, strong cooling is required in the early stage of solidification. In that case, there is a risk of non-uniform solidification breakout occurring. For this reason, the lower limit of the average prior austenite grain size is preferably 0.5 mm or more. The lower limit of the average prior austenite grain size is preferably 0.8 mm or more, and more preferably 1.0 mm or more.

The average prior austenite grain size is defined as the grain size of crystal grains constituting the prior austenite structure at a position 10 mm from the surface layer of the continuously cast slab. The reason why the position 10 mm from the surface layer of the continuously cast slab is specified when setting the average prior austenite grain size is that since most of the cracks in the slab progress to about 20 mm below the surface layer of the slab, the position 10 mm from the surface layer of the continuously cast slab is considered to be a necessary position for suppressing the cracks in the slab.
On the other hand, the region less than 5 mm from the surface layer of the continuous cast slab is directly quenched by the mold or water spray directly below the mold. Therefore, the continuously cast slab has a fine γ grain structure, and the toughness of the slab is high, so it is unlikely that the starting point of the temporary crack occurs in this region. Therefore, the region less than 5 mm from the surface layer of the continuous cast slab can be excluded from the position where the control of the slab structure is required. Therefore, the position where the control of the continuous cast slab structure is required is a position 10 mm from a depth in the slab thickness direction, and may be, for example, a depth of 5 to 20 mm from the surface layer of the continuous cast slab, based on a position 10 mm from the surface layer of the continuous cast slab.

In the continuous cast slab according to the present embodiment, the factor that determines the average prior austenite grain size is the temperature at which the continuous cast slab is cooled. The temperature at which the continuous cast slab is cooled is particularly in the range of 1450°C or less and 1200°C or more, and the residence time has an effect. Furthermore, the longer the residence time of the continuous cast slab, the larger the average prior austenite grain size becomes. That is, in order for the continuous cast slab according to the present embodiment to satisfy the condition (i) that the average prior austenite grain size at a position 10 mm from the surface layer of the continuous cast slab is 0.5 mm or more and 2.0 mm or less, it is important to control the residence time of the continuous cast slab at 1450°C or less and 1200°C or more. Specifically, it is preferable to set the residence time of the continuous cast slab at 1450°C or less and 1200°C or more at a position 10 mm in the thickness direction from the surface layer of the slab to 130 s or less.
If the residence time of the continuously cast slab at 1,450°C or less and 1,200°C or more is 130 s or less, the average prior austenite grain size can be set to 2.0 mm or less. By controlling the average prior austenite grain size to a small size, precipitates and grain boundary ferrite can be dispersed, the toughness of the slab can be improved, and cracking of the slab can be suppressed, which is preferable.
From this viewpoint, the residence time of the continuously cast slab is preferably set to 120 seconds or less, more preferably set to 110 seconds or less, and further preferably set to 100 seconds or less.
Although there is no particular lower limit to the residence time of the continuously cast slab, if the residence time is too short, there is an increased risk of breakout during continuous casting due to non-uniform solidification, so the residence time is set to 40 seconds or more.
That is, if the residence time of the continuously cast slab at 1450° C. or lower and 1200° C. or higher is less than 40 seconds, there is a risk of cracks due to non-uniform solidification leading to breakout, so it is preferable to set it to 40 seconds or more. From this viewpoint, the residence time of the continuously cast slab at 1450° C. or lower and 1200° C. or higher is more preferably 60 seconds or more, and even more preferably 70 seconds or more.

The residence time of a continuously cast slab can be controlled by adjusting the cooling conditions in the early stages of slab casting. For example, in the continuous casting of steel, molten steel with adjusted composition is first poured into a water-cooled copper mold to form an initial solidified shell. Then, withdrawal begins, and after it leaves the water-cooled copper mold, it is cooled with a water spray. The surface temperature of the slab in the above range is greatly affected by the cooling inside the mold and immediately below the mold, so it can be controlled, for example, by improving the thermal conductivity of the mold powder used for lubrication inside the mold, or by increasing the flow rate of the water spray directly below the mold.

By controlling these cooling conditions, the average prior austenite grain size 10 mm below the surface of the continuous cast slab can be controlled. It is difficult to actually measure the cooling temperature of a continuous cast slab. For this reason, the temperature history at a position 10 mm into the slab thickness direction from the surface of the continuous cast slab can be calculated by heat transfer analysis, representing the region from a depth of 5 mm into the slab thickness direction to a depth of 20 mm into the surface of the continuous cast slab, and the cooling temperature of the continuous cast slab can be estimated. In order to ensure that the residence time in the above temperature range is the longest inside the continuous cast slab, the heat transfer analysis position can be set to the center of the width of the continuous cast slab.

<(ii) to (iii) Microstructure of Continuously Cast Slab>
The continuous cast slab according to the present embodiment is characterized in that (ii) the microstructure has a total area ratio of ferrite and area ratio of pearlite of 90% or more, and (iii) the area ratio of ferrite is less than 5% or 10% or more. That is, in addition to the average prior austenite grain size at a position 10 mm from the surface layer of the continuous cast slab being 2.0 mm or less, the ratio of internal structures such as bainite and ferrite is also a factor that determines the unit of fracture, and it is known that the toughness of the slab is improved at an appropriate ratio. Therefore, the inventors have found that the toughness of the slab is improved by controlling the cooling rate and (ii) the microstructure has a total area ratio of ferrite and area ratio of pearlite of 90% or more, and (iii) the area ratio of ferrite is less than 5% or 10% or more. The area ratio of ferrite and the area ratio of pearlite can be calculated based on the observation result of the microstructure of the continuous cast slab using an observation means such as an optical microscope. Then, the ferrite and pearlite contained in the microstructure of the continuous cast slab can be identified using an observation means such as an optical microscope.

Based on the identification result of the microstructure of the continuously cast slab, the area _S of the microstructure of the continuously cast slab and the total area S _{(ferrite + pearlite)} of the _ferrite area S and _{the pearlite} area S are calculated. Then, the ratio of the _total area S _{(ferrite + pearlite)} of the _ferrite area S and the _pearlite area S to the area S of the microstructure of the continuously cast slab is defined as an area ratio (%) and calculated.

The continuous cast slab according to this embodiment is characterized in that (ii) the microstructure has a total area ratio of ferrite and pearlite of 90% or more. That is, in the continuous cast slab according to this embodiment, if the area ratio (%), which is the ratio of the total area S _{(ferrite + pearlite)} of the ferrite area S _ferrite and the pearlite area S _pearlite to the area S _total of the microstructure of the continuous cast slab, is 90% or more, the thermal stress and transformation stress due to the bainite-martensite transformation during the slow cooling of the slab can be reduced, and the generated stress is also dispersed to the ferrite-pearlite present in a sufficient amount in the microstructure, improving the toughness of the continuous cast slab, which is preferable. On the other hand, if the area ratio is less than 90%, the toughness of the continuous cast slab decreases, which is not preferable.

Furthermore, the continuous casting slab according to this embodiment is characterized in that (iii) the area ratio of ferrite is less than 5% or 10% or more. That is, in the continuous casting slab according to this embodiment, when the area ratio of ferrite is 5% or more and less than 10%, thin ferrite is present at the grain boundaries, and stress is concentrated in the soft ferrite portion, which is not preferable because cracks grow. If the area ratio of ferrite is less than 5%, it is preferable because even if cracks grow, they stop immediately, and when the area ratio of ferrite is 10% or more, it is preferable because stress is less likely to concentrate in the ferrite portion and cracks do not grow.

Here, grain boundary ferrite is a factor that determines grain boundary strength. When grain boundary ferrite occurs, it reduces the toughness of the continuously cast slab. In addition, since ferrite has lower strength than austenite, pearlite, and bainite, there is a problem that when stress is applied, stress is likely to concentrate in the grain boundary ferrite. Based on this perspective, the inventors conducted extensive research and discovered that the toughness of the continuously cast slab in this embodiment can be greatly improved by suppressing the generation of grain boundary ferrite even in a pearlite-based structure, in terms of the microstructural type possessed by the continuously cast slab in this embodiment.

Ferrite contains iron with a maximum of 0.02% carbon by mass, and is a structure close to pure iron. Ferrite is a ferromagnetic material from room temperature to 780°C, and is the softest and most ductile of all steel structures. Pearlite is a structure obtained when austenite is cooled slowly. Pearlite is formed by alternating layers of ferrite and cementite.

The precipitation of grain boundary ferrite is greatly influenced by the cooling rate in the ferrite transformation region. If the cooling rate is slower than the critical rate, ferrite precipitation occurs, so it is necessary to control the cooling rate to 850°C or less and 700°C or less. If the cooling rate in the ferrite transformation region is slower than the critical rate but sufficient precipitation time cannot be secured, ferrite will preferentially precipitate at the grain boundaries where precipitation is likely to occur. For this reason, the stress during the pearlite and bainite-martensite transformation that occurs after this will concentrate on the soft ferrite parts, which is not suitable because it will cause cracks when the slab is placed. As a countermeasure, it is possible to grow the ferrite precipitated from the grain boundaries and turn it into polygonal ferrite by reducing the cooling rate in the ferrite transformation region and securing sufficient ferrite precipitation time. In the case of polygonal ferrite, excessive stress concentration is suppressed, and the toughness of the slab can be improved.

In addition, in order to suppress cracking during slab placement, it is important not only to suppress embrittlement of the prior austenite grain boundaries, but also to reduce the stress during transformation. Therefore, by controlling the cooling rate in the pearlite transformation region (700°C or less and 500°C or more), the microstructure of the continuously cast slab can be controlled.

The cooling of the continuously cast slab after it leaves the continuous casting machine can be controlled by changing conditions such as the slab temperature at the exit of the continuous casting machine, the time it takes to stack multiple slabs, the number of slabs to be stacked, whether or not to use a thermal insulation cover, and water toughening treatment. The cooling rate can be measured using a thermocouple. For example, the measurement can be made by placing a thermocouple in the center of the top surface of the slab's wide side (long side) after it comes out of the continuous casting machine.

As described above, according to the invention of the first embodiment, even in recent continuous cast slabs for high strength steel, which have very low toughness, cracks during cooling are not generated, and problems such as holes during rolling can be prevented, making it possible to obtain continuous cast slabs for high strength steel with good yield.

[Second embodiment]
A continuous cast slab according to the second embodiment will be described. The continuous cast slab according to this embodiment contains, in mass %, C: 0.10% or more and 1.00% or less, Si: 0.10% or more and 2.50% or less, and Mn: 0.40 % or more and 5.00% or less in the continuous cast slab according to the above embodiment.
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>
In the continuous cast slab according to the present embodiment, the reasons for limiting each chemical component contained in the continuous cast slab will be described. The content of each chemical component contained in the continuous cast slab is in mass %. The reason for limiting the C content in the continuous cast slab to 0.10% or more and 1.00% or less is as follows. C contained in the continuous cast slab for high strength steel is an element necessary for increasing the strength of a high strength steel plate made from the continuous cast slab. If the C content is less than 0.10%, the strength required for a high strength steel plate 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 plate become insufficient, which is not preferable.

Therefore, from this viewpoint, in the continuous cast slab according to this embodiment, it is preferable that the C content in the continuous cast slab be 0.10% or more and 1.00% or less, more preferably 0.12% or more and 0.40% 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 content of Si contained in the continuous cast slab for high strength steel is set to 0.10% or more and 2.50% or less is as follows. The Si contained in the continuous cast slab is an element necessary for securing retained austenite in the steel plate in the annealing process of the high strength steel plate made from the continuous cast slab. In addition, the Si contained in the continuous cast slab is an essential additive element because it contributes to increasing the strength of the high strength steel plate by solid solution strengthening. If the Si content is less than 0.10%, the strength required for the high strength steel plate 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 is generated on the hot-rolled sheet before it is processed into high-strength steel plate. As a result, the appearance and pickling property of the high-strength steel plate deteriorate, so the upper limit of the Si content is 2.50%.

Therefore, from this viewpoint, in the continuous casting slab according to this embodiment, it is preferable that the Si content contained in the continuous casting slab is 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 content of Mn contained in the continuous cast slab is set to 0.40% or more and 5.00% or less is as follows. Mn contained in the continuous 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 continuous cast slab. If the content of Mn is less than 0.40%, the high strength steel plate cannot be sufficiently strengthened, so the lower limit of the content of Mn is 0.40%. On the other hand, if the content of Mn exceeds 5.00%, the degree of sufficient strengthening of the high strength steel plate is saturated and the manufacturing cost of the high strength steel plate increases, which is not preferable from the viewpoint of economy.

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

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

P segregates at prior austenite grain boundaries and embrittles the grain boundaries, which can cause slab placement cracks. Therefore, the P content is preferably 0.100% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel plate, it is preferable to set it to 0.001% or more. Therefore, the P content is preferably 0.100% or less. Preferably, it is 0.001% or more. More preferably, it is 0.070% or less.

S exists as sulfides 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 is an element that affects the percentage of retained austenite in the slab, as it suppresses the formation of carbides during slab cooling and promotes the formation of retained austenite. It is also preferable to add 0.005% or more for deoxidation. If the Al content exceeds 0.100%, there is a risk of causing slab embrittlement. Therefore, it is preferable that the Al content be 0.100% or less. More preferably, it is 0.010% or more. Even more preferably, it is 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 particular 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 continuous cast slab according to this embodiment, for use as a high strength steel plate, may further contain, in addition to the above-mentioned component composition, at least one element selected from 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, Co: 1.00% or less, and B: 0.0100% or less, either alone or in combination of two or more thereof.

If the Ti, Nb and V contents are each 0.200% or less, large amounts of coarse precipitates and inclusions are not generated, and the toughness of the slab is not reduced. Therefore, it is preferable that the Ti, Nb and V contents are each 0.200% or less. Although there is no particular lower limit for the Ti, Nb and V contents, the Ti, Nb and V contents are more preferably 0.001% or more, since they increase the strength of the steel plate by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing of the continuously cast slab. Therefore, when Ti, Nb and V are contained, their contents are each 0.200% or less. More preferably, they are 0.001% or more. Even more preferably, they are 0.100% or less.

If the content of Ta and W is 0.10% or less, large amounts of coarse precipitates and inclusions are not generated, and the toughness of the slab is not reduced. Therefore, it is preferable that the content of Ta and W is 0.10% or less. Although there is no particular lower limit for the content of Ta and W, it is more preferable that the content of Ta and W is 0.01% or more, since the strength of the steel plate is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of the continuously cast slab. Therefore, when Ta and W are contained, the content of each is 0.10% or less. More preferably, it is 0.01% or more. Even more preferably, it is 0.08% or less.

The continuous casting slab according to this embodiment may contain at least one selected from Cr, Mo, Ni and Cu as necessary within the scope of the present invention. Cr, Mo, Ni and Cu bring about the effect of increasing the strength of the steel plate through structure control in hot rolling of the continuous casting slab. This effect becomes remarkable by adding 0.01% or more of one or more of Cr, Mo, Ni and Cu, respectively, so it is preferable to add 0.01% or more. If the amount of each element exceeds the upper limit of each element, the weldability, hot workability, etc. of the steel plate deteriorates, so the upper limit of the amount of each element of Cr, Mo, Ni and Cu is 1.00%. Therefore, when the continuous casting slab contains Cr, Mo, Ni and Cu, the content of each of them is 1.00% or less. Preferably, it is 0.01% or more. More preferably, it is 0.80% or less.

B may be added because it controls the structural transformation during hot rolling and annealing of the continuous cast slab, and affects the strength through structural strengthening. If B is 0.0100% or less, it does not affect the toughness of the slab. Therefore, it is preferable that the B content be 0.0100% or less. There is no particular lower limit for the B content, but since B is an element that segregates to the austenite grain boundaries during hot rolling and annealing of the continuous cast slab and improves the hardenability, it is more preferable that the B content be 0.0003% or more. Therefore, if B is contained, its content is 0.0100% or less. More preferably, it is 0.0003% or more. Even more preferably, it is 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 decrease. Therefore, it is preferable that the Co content be 1.00% or less. There is no particular lower limit for the Co content, but since Co is an element that improves hardenability, 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.

If Cu is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not decrease. Therefore, it is preferable that the Cu content be 1.00% or less. There is no particular lower limit for the Cu content, but since Cu 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 particular 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, coarse precipitates and inclusions do not increase, and the toughness of the slab does not decrease. Therefore, it is preferable that the Sb content be 0.200% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that suppresses decarburization and enables the strength adjustment 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 will not increase and the toughness of the slab will not decrease. Therefore, it is preferable that the content of Ca, Mg and REM is 0.0100% or less. Although there is no particular lower limit for the content of Ca, Mg and REM, it is more preferable that the content of Ca, Mg and REM is 0.0005% or more, since these elements make the shape of nitrides and sulfides spherical and improve the toughness of the slab. Therefore, when Ca, Mg and REM are contained, the content of each is 0.0100% or less. More preferably, it is 0.0005% or more. Even more preferably, it is 0.0050% or less.

If Zr and Te are each 0.100% or less, coarse precipitates and inclusions do not increase and the toughness of the slab is not reduced. Therefore, it is preferable that the content of each of Zr and Te is 0.100% or less. Although there is no particular lower limit for the content of each of Zr and Te, since Zr and Te are elements that make the shape of nitrides and sulfides spherical and improve the toughness of the slab, it is more preferable that the content of Zr and Te is 0.001% or more. Therefore, when Zr and Te are contained, their contents are each 0.100% or less. More preferably, they are 0.001% or more. Even more preferably, they are 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 decrease. Therefore, it is preferable that the Hf content be 0.10% or less. There is no particular lower limit for the Hf content, but since Hf is an element that makes the shape of nitrides and sulfides spherical 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 decrease. Therefore, it is preferable that the Bi content be 0.200% or less. There is no particular lower limit for the Bi content, but since 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 effect of the present invention will not be impaired, and therefore they are included as unavoidable impurities.

As described above, according to the invention relating to the second embodiment, it is possible to obtain the strength required for high-strength steel, and further to obtain a continuous cast slab having excellent weldability, workability and appearance of high-strength steel.

[Third embodiment]
A method for producing a continuous cast slab according to the third embodiment will be described. 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 in which slab placement cracks caused by cooling are suppressed, and includes a first cooling step in which a continuous cast slab having the component composition of the continuous cast slab described in the above embodiment is cooled under cooling conditions in which the cooling temperature of the continuous cast slab at the center in the width direction of the continuous cast slab and at a position 10 mm from the surface layer of the continuous cast slab is 1200°C or more and 1450°C or less, and the residence time of the continuous cast slab is 130 seconds or less;
A second cooling step in which the continuous casting slab is cooled under cooling conditions in which the average cooling rate at the surface temperature at the center of the width direction of the slab is 20 ° C./hr or less when the surface temperature is 700 ° C. or more and 850 ° C. or less;
and a third cooling step in which the continuous cast slab is cooled under cooling conditions in which the average cooling rate at a surface temperature of 500° C. or higher and 700° C. or lower at a center in the width direction of the slab is 10° C./hr or lower.

Here, the lower limit of the average cooling rate in the second and third cooling steps is not particularly specified, but the average cooling rates of 700°C to 850°C and 500°C to 700°C when multiple slabs are stacked and a heat-insulating cover is used are at least 2°C/hr and 1°C/hr, respectively. In the second and third cooling steps, cooling at an average cooling rate slower than these average cooling rates requires, for example, placing the slab in a heating furnace and applying heat, which is not preferable from the standpoint of economy because it requires equipment. Therefore, in the second cooling step, the lower limit of the average cooling rate of 700°C to 850°C and less is set to 2°C/hr, and in the third cooling step, the lower limit of the average cooling rate of 500°C to 700°C and less is set to 1°C/hr.

In the manufacturing method of the slab for high strength steel plate according to the present embodiment, re-shipping may occur depending on various 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, the handling time required for re-shipping (at most 1 to 2 hours) does not lead to the occurrence of cracks. Therefore, in the present invention, the average cooling rate is specified instead of the maximum cooling rate.
Hereinafter, each step included in the method for producing a continuous cast slab according to this embodiment will be described.

(First cooling step)
The method for producing a continuous cast slab according to the present embodiment is a method for producing a continuous cast slab for high strength steel in which slab placement cracks caused by cooling are suppressed, and the continuous cast slab having the component composition of the continuous cast slab described in the above embodiment is
The method includes a first cooling step in which the cooling temperature of the continuously cast slab at the width center of the continuously cast slab and at a position 10 mm from the surface of the continuously cast slab is 1,200°C or higher and 1,450°C or lower, and the residence time of the continuously cast slab is 130 s or shorter.

The first cooling step is a step for controlling the average prior austenite grain size in the continuously cast slab according to the embodiment to 2.0 mm or less at a predetermined position. By controlling the average prior austenite grain size to 2.0 mm or less, the density of precipitates precipitated at the prior austenite grain boundaries can be reduced, and the precipitation of harmful grain boundary ferrite can be suppressed, thereby improving the toughness of the slab.
In the method for producing a continuous cast slab according to this embodiment, the factor that determines the average prior austenite grain size is the temperature at which the slab is cooled. In the first cooling step, the temperature at which the continuously cast slab is cooled is in the range of 1450°C or lower and 1200°C or higher. In this way, the method for producing a continuous cast slab according to this embodiment focuses on the cooling temperature of the continuously cast slab in the range of 1450°C or lower and 1200°C or higher, which is the factor that determines the average prior austenite grain size, and controls that temperature.

Furthermore, in the first cooling step, the residence time of the continuously cast slab in the above temperature range for cooling the continuously cast slab is 130 seconds or less. If the residence time of the continuously cast slab at the above temperature is 130 seconds or less, the average prior austenite grain size can be reduced to 2.0 mm or less, which is preferable because it is possible to suppress cracking of the slab during placement. Note that there is no particular lower limit for the residence time of the continuously cast slab at 1200°C or more and 1450°C or less, but if the residence time is too short, the risk of breakout during continuous casting due to non-uniform solidification increases, so it is preferable to set it to 40 seconds or more, more preferably 60 seconds or more, and even more preferably 70 seconds or more.

(Second cooling step)
Next, the manufacturing method of the continuous cast slab according to this embodiment includes a second cooling step in which the continuous cast slab is cooled under cooling conditions in which the average cooling rate is 20° C./hr or less when the surface temperature at the center in the width direction of the continuous cast slab is 700° C. or more and 850° C. or less. The second cooling step is a step for suppressing precipitation of grain boundary ferrite contained in the microstructure of the continuously cast slab according to the above embodiment and setting the area ratio of ferrite to less than 5% or 10% or more.

In the second cooling step, the temperature at which the continuous cast slab is further cooled is 700°C or higher and 850°C or lower. In this way, the method for producing a continuous cast slab according to this embodiment controls the temperature by focusing on the cooling rate in the temperature range in the ferrite transformation region where the precipitation of ferrite can be controlled.

In the second cooling step, in the above-mentioned temperature range for cooling the continuously cast slab, the average cooling rate of the continuously cast slab is 20° C./hr or less. If the average cooling rate exceeds 20° C./hr, thin ferrite precipitation occurs only at the prior austenite grain boundaries, embrittling the grain boundaries, which is not suitable. 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 secured, grain boundary ferrite can be grown into polygonal ferrite, and stress concentration on the grain boundary ferrite can be suppressed, which is preferable.
Although there is no strict lower limit to the average cooling rate, since an energy source necessary for controlling the cooling rate is separately required, the average cooling rate is preferably 2° C./hr or more, and more preferably, the average cooling rate is 5° C./hr or more and 18° C./hr or less.

(Third cooling step)
Furthermore, the manufacturing method of the continuous cast slab according to this embodiment includes a third cooling process in which the surface temperature of the continuous cast slab at the widthwise center is cooled under cooling conditions such that the average cooling rate at a temperature of 500°C or higher and 700°C or lower is 10°C/hr or lower.
The third cooling step is a step for making the microstructure of the continuously cast slab according to the above embodiment mainly composed of pearlite and for reducing internal stress. Specifically, the third cooling step is a step for making the area ratio (%), which is the ratio of the total area S _{(ferrite + pearlite)} of the ferrite area S _ferrite and the pearlite area S _pearlite to the area S _total of the microstructure of the continuously cast slab, 90% or more.

In the third cooling step, the temperature at which the continuous cast slab is further cooled is 500°C or higher and 700°C or lower. In this way, the method for producing a continuous cast slab according to this embodiment focuses on the cooling rate in the temperature range in the pearlite transformation region and controls the temperature.

In the third cooling step, within the above-mentioned temperature range for cooling the continuously cast slab, the average cooling rate of the continuously cast slab is 10° C./hr or less. If the average cooling rate of the continuously cast slab exceeds 10° C./hr, bainite-martensite precipitates in the microstructure mainly composed of pearlite, which generates large stress, and is therefore unsuitable. Bainite-martensite has a lower transformation temperature than pearlite, and the transformation stress is applied to the pearlite portion where transformation has already been completed, which is a factor promoting cracking.
From this viewpoint, it is preferable that the average cooling rate of the continuously cast slab is 10° C./hr or less, since this suppresses bainite transformation and forms a structure mainly composed of pearlite, thereby reducing internal stress.
Although there is no strict lower limit to the average cooling rate, since a separate energy source is required to control the cooling rate, the cooling rate is preferably 1° C./hr or more, and more preferably 5° C./hr or more.

In this way, the manufacturing method for continuous cast slabs according to this embodiment employs a three-stage cooling process for the cooling of the continuous cast slab, thereby precisely controlling the average prior austenite grain size and the microstructure of the continuous cast slab, thereby suppressing slab placement cracks that occur due to cooling and making it possible to provide a continuous cast slab for high strength steel that can prevent problems such as hole formation during rolling.

As described above, according to the manufacturing method for continuous cast slabs in the third embodiment, even with the composition system of a continuous cast slab for high strength steel, 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 generate cracks during the cooling process and 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-mentioned embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the technical scope of the present invention. In addition, systems or devices that combine the separate features included in each embodiment in any way are also included in the technical scope of the present invention.

The effects of the present invention will be specifically described below based on examples, but the present invention is not limited to these examples. That is, in order to confirm the effects of the present invention, the inventors produced continuous cast slabs using each steel type as a raw material in comparative examples (Test Nos. A-1 to A-4, Test Nos. B-1 to B-8, Test Nos. C-1 to C-3) and inventive examples (Test Nos. D-1 to D-24). Table 1 shows steel types A to I, which are the raw materials for the continuously cast slabs used in the comparative examples (Test Nos. A-1 to A-4, Test Nos. B-1 to B-8, Test Nos. C-1 to C-3) and inventive examples (Test Nos. D-1 to D-24).

Here, the cooling conditions for the continuous cast slab were a three-stage cooling process consisting of (I) a residence time [s] of 1,200°C or more and 1,450°C or less, (II) an average cooling rate [°C/hr] of 700°C or more and 850°C or less, and (III) an average cooling rate [°C/hr] of 500°C or more and 700°C or less, and cooling was performed by appropriately changing the conditions for each of these stages.

Tables 2 to 4 show the continuous casting slab cooling conditions (I) to (III), the microstructure of the obtained continuous casting slab, and the evaluation of slab placement cracks.

The average prior austenite grain size was measured, the area ratios of ferrite and pearlite were calculated, and the placement cracks of the continuously cast slabs produced in the comparative examples and the invention examples were evaluated 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 out from the width center position of the cooled slab, and the slab thickness cross section parallel to the slab width direction was the observation surface. Next, the observation surface was 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, 5 fields of view were observed at a magnification of 10 times at a position 10 mm below the slab surface layer to obtain a microstructure image of the continuous cast slab. The obtained microstructure image was cut by a cutting method in accordance with JIS G 0551:2020 to obtain the prior austenite grain size obtained by the 5-field observation, and the average value of these was calculated as the average prior austenite grain size.

<Method for measuring ferrite area ratio>
The measurement method of the ferrite area ratio is the same as the measurement method of the average prior austenite grain size, and the observation surface of the slab is prepared. Next, the observation surface is mirror-polished using diamond paste, then finished polished using colloidal silica, and further etched with 3 vol. % nital to reveal the structure. Under the condition of an acceleration voltage of 15 kV, 10 fields of view are observed at a position 10 mm below the surface layer of the slab at a magnification of 50 times using a SEM (Scanning Electron Microscope), and the microstructure image of the obtained continuous cast slab is calculated for the area ratio of ferrite for 10 fields of view using PHOTOSHOP (registered trademark) of Adobe, and the area ratio of ferrite is calculated by averaging these values. Note that ferrite has a larger grain size than other structures (pearlite, bainite, tempered martensite, quenched martensite, and retained austenite), and has a smooth surface and dark contrast, so it can be easily distinguished at a magnification of 50 times.

<Method for measuring area ratio of pearlite>
The method for measuring the area ratio of the pearlite structure is the same as the above-mentioned method for measuring ferrite, in which the structure is revealed on the observation surface of the slab. Under the condition of an acceleration voltage of 15 kV, 10 fields of view are observed at a magnification of 10,000 times at a position 10 mm below the surface layer of the slab using an SEM, with ferrite removed from the field of view, and the microstructure image of the obtained continuous cast slab is calculated for the area ratio of pearlite and the area ratio of bainite for 10 fields of view using Adobe's PHOTOSHOP (registered trademark), and these values are averaged, and calculated so that the total is 100% together with the area ratio of ferrite measured by the above-mentioned method, and the area ratio of each structure is obtained. Pearlite is a eutectoid crystal of ferrite and cementite, and when observed by the above-mentioned scanning electron microscope, the flaky layers of both have a pearl-like luster.

The microstructure of the continuous cast slab according to the present invention is mainly pearlite and has no grain boundary ferrite. However, it is difficult to strictly classify grain boundary ferrite and polygonal ferrite. Therefore, the inventors have thoroughly investigated slabs in which slab placement cracks have occurred, and have found that when the area ratio of ferrite is greater than 5% and less than 10%, a large amount of ferrite is present at the grain boundaries. In other words, the structure has an area ratio of ferrite + pearlite of 90% or more, and an area ratio of ferrite of less than 5% or 10% or more.

<Slab placement crack evaluation>
The evaluation method for slab placement cracks was based on the penetrant test 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 observed to check for placement cracks on the surface of the slab.
In addition, if there is a crack that is 50 mm or more 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 placement cracks were set as follows.
・Cracks in slab placement 〇: No cracks of 50mm or more in length on the slab surface ・Cracks in slab placement ×: Cracks of 50mm or more in length on the slab surface

<Comparative Examples (Test Nos. A-1 to A-4)>
The microstructure of the slabs that the continuously cast slabs produced in Test Nos. A-1 to A-4 satisfied was defined as Condition A. Condition A is a condition for an example in which the average prior austenite grain size at a position 10 mm below the surface layer of the slab was larger than 2.0 mm. In these cases, the toughness of the prior austenite grain boundaries was reduced due to an increase in the precipitate density of the prior austenite grain boundaries, and therefore, even if the conditions for slow cooling of the slab after it left the continuous slab casting machine were varied, it was not possible to suppress the slab from being placed on the ground.

<Comparative Examples (Test Nos. B-1 to B-8)>
The microstructural structure of the slabs that the continuous cast slabs manufactured in Test Nos. B-1 to B-8 satisfied is defined as Condition B. Condition B is a condition in which the average prior austenite grain size at a position 10 mm below the slab surface is 2.0 mm or less, but grain boundary ferrite precipitates at the prior austenite grain boundaries. In these cases, the toughness of the prior austenite grain boundaries is reduced by the grain boundary ferrite, so that the slab placement cracks could not be suppressed even if the cooling rate was changed in various ways thereafter to change the composition of the microstructure.

<Comparative Examples (Test Nos. C-1 to C-3)>
The microstructure of the slabs that is satisfied by the continuous cast slabs manufactured in Test Nos. C-1 to C-3 is defined as Condition C. Condition C is a condition in which the average prior austenite grain size is 2.0 mm or less and the precipitation of grain boundary ferrite is suppressed, but 10% or more of bainite is precipitated, so that it is an example of a condition in which slab cracking could not be suppressed. Bainite transformation occurs at a lower temperature than pearlite transformation, so the density difference with austenite is large and the transformation stress is also large, which is thought to be why slab cracking could not be suppressed.

<Examples of the invention (Test Nos. D-1 to D-24)>
The microstructure of the slabs satisfied by the continuously cast slabs produced in Test Nos. D-1 to D-24 is defined as Condition D. Condition D is a condition of the present invention, and the continuously cast slabs produced in the present invention had an average prior austenite grain size of 2.0 mm or less, and the microstructure had almost no grain boundary ferrite and little bainite. That is, in addition to improving toughness by controlling the prior austenite grain boundaries to be small and dispersing precipitates, toughness was improved by suppressing the precipitation of grain boundary ferrite, and internal stress was reduced by suppressing the precipitation of bainite, so that no cracks occurred during slab placement after cooling.

According to Tables 2 to 4, it was found that (i) the average prior austenite grain size at a position 10 mm from the surface of the slab is 0.5 mm or more and 2.0 mm or less, (ii) the microstructure of the continuously cast slab has an area ratio of ferrite and pearlite in total of 90% or more, and (iii) the area ratio of the ferrite is less than 5% or 10% or more, thereby making it possible to suppress slab placement cracking during cooling of the slab.

That is, the continuous cast slab of the present invention has an average prior austenite grain size at a position 10 mm from the slab surface of 0.5 mm or more and 2.0 mm or less, and the microstructure has an area ratio of ferrite and pearlite in total of 90% or more, and the area ratio of ferrite is less than 5% or 10% or more, so that a slab for high alloy high strength steel can be provided that is free of slab cracks after casting, and it is possible to prevent problems such as hole formation during rolling. That is, according to the invention example and the comparative example, it was found that cracks during cooling of the slab can be suppressed by making the average prior austenite grain size at a position 10 mm from the slab surface of 0.5 mm or more and 2.0 mm or less, and the microstructure has an area ratio of ferrite and pearlite in total of 90% or more, and (iii) the area ratio of the ferrite is less than 5% or 10% or more.

FIG. 3 is an enlarged photograph of a continuous cast slab produced in an invention example (Test No. D-9) of a continuous cast slab, observed by an optical microscope. Based on the enlarged photograph of the continuous cast slab observed by an optical microscope shown in FIG. 3, the metal structure contained in the continuous cast slab was identified, and the ratio of the total area S _{(ferrite + pearlite)} of the ferrite area S _ferrite and the pearlite area S _pearlite to the area S _total of the microstructure of the continuous cast slab was calculated as the area ratio (%). As a result, it was found that the continuous cast slab of the present invention has an average prior austenite grain size at a position 10 mm from the slab surface layer of 0.5 mm or more and 2.0 mm or less, and the microstructure has an area ratio of ferrite and area ratio of pearlite of 90% or more in total. Furthermore, it was found that the continuous cast slab of the present invention satisfies the area ratio of ferrite of less than 5% or 10% or more.

To obtain such a microstructure of a continuously cast slab, it is preferable to adopt a three-stage cooling process in which, for example, the temperature at a position 10 mm below the surface of the slab is cooled to 1,450°C or less and the residence time is 130 seconds or less when the temperature is 1,200°C or more, and then the cooling rate at the surface at the center of the slab width is 20°C/hr or less when the surface temperature is 850°C or less and 700°C or more, and further the average cooling rate at the surface at the center of the slab width is 10°C/hr or less when the surface temperature is 700°C or less and 500°C or more.

In addition, if the continuous cast slab is strongly cooled in the temperature range of 1450°C or less and 1200°C or more, there is a risk of the slab surface cracking due to non-uniform solidification. In such a case, for example, when the temperature at the center of the slab width and 10 mm below the surface layer of the slab is 900°C or more and 1200°C or less, the slab surface is quenched to below the BS point, and then the cooling is stopped and the slab is reheated to above the AC3 point, thereby making it possible to refine the prior austenite grain size by utilizing the austenite reverse transformation. Then, the slab is cooled so that the cooling rate at the center surface of the slab width is 850°C or less and 700°C or more is 20°C/hr or less, and then the average cooling rate at the center surface of the slab width is 700°C or less and 500°C or more is 10°C/hr or less, thereby obtaining a continuous cast slab having such a microstructure. In addition, the manufacturing method of the continuous cast slab having the microstructure of the above-mentioned continuous cast slab is not limited to this.

The continuous cast slab of the present invention has an average prior austenite grain size at a position 10 mm from the slab surface of 0.5 mm or more and 2.0 mm or less, and the microstructure has a total area ratio of ferrite and pearlite of 90% or more, with the area ratio of ferrite being less than 5% or 10% or more. Therefore, a slab for high-strength steel that is free from slab cracking after casting can be provided, and problems such as hole formation during rolling can be prevented, making it industrially useful.

Claims (2)

A continuous casting slab for high strength steel, comprising:
In mass percent,
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;
Furthermore, optionally, at least one element selected from 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% or less, 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 is contained alone or in combination of two or more kinds, with the balance being Fe and inevitable impurities;
The average prior austenite grain size at a position 10 mm from the surface of the continuous casting slab is 0.5 mm or more and 2.0 mm or less, and
The microstructure is pearlite and a remaining unavoidable structure, or the pearlite, one or both selected from ferrite and bainite, and the remaining unavoidable structure ,
The sum of the area ratio of the ferrite and the area ratio of the pearlite is 90% or more, and the area ratio of the ferrite is less than 5% or 10% or more,
A continuously cast slab, characterized in that the sum of the area ratio of the ferrite, the area ratio of the pearlite, and the area ratio of the bainite is 100%. A method for producing a high-strength steel continuous cast slab in which slab placement cracks caused by cooling are suppressed, comprising the steps of:
A first cooling step of cooling the continuous casting slab under cooling conditions in which the surface temperature of the continuous casting slab at a position 10 mm from the surface of the continuous casting slab is in a temperature range of 1200° C. to 1450° C. and the residence time is 130 s or less;
A second cooling step of cooling the continuous casting slab under cooling conditions in which the average cooling rate at the width direction center of the continuous casting slab and the surface temperature of the surface layer of the continuous casting slab is 20 ° C. / hr or less at 700 ° C. or more and 850 ° C. or less;
The method for producing a continuous cast slab as described in claim 1 , further comprising: a third cooling step in which the continuous cast slab is cooled at a center in the width direction thereof and under cooling conditions in which the average cooling rate at a surface temperature of the surface layer of the continuous cast slab is 10°C/hr or less at a temperature of 500°C or more and 700°C or less.

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