JP7477051B2 - Continuously cast slab and its manufacturing method - Google Patents

Description

The present invention relates to a continuously cast slab that is prevented from cracking during cooling and a method for manufacturing the same, and more particularly to a continuously cast slab for high strength steel (hi-tensile steel) that is effective in preventing cracking during cooling 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 higher alloys have been used for that purpose. However, the higher alloys have significantly reduced the toughness of the slabs.

As the toughness of the slab decreases due to the use of high alloying, cracks during cooling of the slab, so-called "placement cracks", have become more frequent. If a placement crack occurs, the slab may break during transportation, and the slab may not be able to be used 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 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 alloying, 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 of the fracture surface of a cracked portion of a high-strength steel slab fractured by a temporary crack, taken with a scanning electron microscope (SEM). As is clear from Figure 1, the fracture surface of the cracked portion of the slab exhibited the appearance of a grain boundary fracture surface along the prior austenite grain boundary. Figure 2 shows a microstructure photograph of the cross section of the cracked portion 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 portion of the slab. In addition, pearlite or pearlite and bainite were 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 intergranular 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 extension of the slab crack, 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, slab cracks will occur when the slab is 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 using a grinder, and this has been a problem that significantly reduces the slab yield.

From this viewpoint, a method for suppressing the occurrence of lattice cracks in a slab of high tensile steel has been proposed. For example, Patent Document 1 proposes a method for suppressing the bainite/martensite transformation by slow cooling from 700 to 500°C, which is a temperature range in which austenite transforms into ferrite, and reducing the stress caused by the transformation expansion. That is, Patent Document 1 discloses a method capable of suppressing the occurrence of lattice cracks even in a steel type in which lattice 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 for suppressing the occurrence of lattice cracks by controlling the cooling rate of the slab in accordance with the length of an internal crack that has occurred in the high tensile steel based on the knowledge that the internal stress of high tensile steel depends on the cooling rate.

In addition, Patent Document 2 proposes a method of reducing the temperature difference and stress during transformation by starting slow cooling immediately after casting of the slab, and further slowly cooling the temperature to 700 to 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 a slab for a high-strength steel plate, which does not cause slab cracks during cooling of the slab, even if the slab contains Si, and does not cause quality defects such as scabs during hot rolling. Specifically, the cooling method for a slab for a high-strength steel plate disclosed in Patent Document 2 is a method of cooling a slab for a high-strength hot-rolled steel plate, in which the content of chemical components such as C, Si, and Mn is limited, at 500 to 700 ° C., the average cooling rate is 20 ° C./hr or less.

JP 2020-139209 A JP 2019-167560 A

However, the 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 when the temperature of the slab reaches 700°C to 500°C when cooling after casting the slab, and controls the internal stress generated in the slab to be small. However, since the toughness of the slab is low in recent high-alloyed high-strength steels, the state of the prior austenite grain boundaries through which the staging 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 staging cracks in the slab cannot be sufficiently suppressed.

Furthermore, the cooling method for slabs for high-strength steel plates described in Patent Document 2 suppresses cracking of the slab by focusing on reducing thermal stress, based on the knowledge that the cause of slab cracking is thermal stress generated due to the addition of Si to steel and temperature unevenness 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 extensive research, the inventors have found that the toughness of slabs containing large amounts of C, Si, and Mn as produced by conventional techniques is quite low, and that it is impossible to completely suppress temporary cracking by only slow cooling aimed at reducing thermal stress.

The present invention has been made in consideration of the above circumstances, and aims to provide a continuous cast slab and a manufacturing method thereof in which no cracking occurs in the slab during cooling, even in the case of a continuously cast slab with low toughness.

The inventors have conducted extensive research to achieve the above object. As a result, they have analyzed the fracture morphology of slab cracks and found that the fracture surface includes at least one of a grain boundary fracture surface along the prior austenite grain boundary and an intragranular fracture surface (cleavage fracture surface) crossing the prior austenite grain boundary. Furthermore, the inventors have conducted detailed research and found 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 affected by the morphology of the microstructure. Specifically, they have found that the slab cracks during the cooling process of the continuously cast slab can be suppressed by controlling the average prior austenite grain size and microstructure of the continuously cast slab and improving its toughness, and have come up with the present invention.

In other words, the continuous cast slab of the present invention, which advantageously solves the above-mentioned 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 continuously cast slab is 0.5 mm or more and 2.0 mm or less, and the microstructure has a total area ratio of bainite and area ratio of ferrite of 90% or more, and the area ratio of ferrite is 0% or more or 3% or more.

In addition, it is considered that a more preferable solution for the continuous cast slab according to the present invention is to contain, by mass%, (a) C: 0.10% or more and 0.40% or less, Si: 0.10% or more and 2.50% or less, and Mn: 1.00% 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 of the continuous casting slab at the center in the width direction is 700° C. or more and 850° C. or less is 25° C./hr or more and 40° C./hr or less, or 50° C./hr or more;
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 15°C/hr or higher.

According to the present invention, it is possible to provide a continuously cast slab that does not generate cracks during the cooling process, even if the slab has a composition system for a continuously cast slab of high strength steel.

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-2) of a continuously cast slab according to an embodiment of the present invention, taken with an optical microscope.

Hereinafter, the embodiments of the present invention will be described in detail. Note that the drawings are schematic and may differ from the actual ones. Furthermore, the following embodiments are merely examples of devices and methods for embodying the technical idea of the present invention, and are not intended to limit the configuration to the following. In other words, the technical idea 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 total area ratio of bainite and ferrite in the microstructure is 90% or more, and (iii) the area ratio of the ferrite is 0% or 3% 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 recent continuous cast slabs for high strength steel in which the toughness of the continuous cast slab is very low, it is possible to provide a continuous cast slab for high strength steel with a good yield without causing slab placement cracks during the cooling process.

First, the suitable range of the microstructure of the continuous cast slab and the reason for limiting it will be explained. In the following explanation, "%" indicating the composition ratio of the microstructure means "area %" unless otherwise specified. In addition, the observation of the microstructure of the continuous cast slab was performed at room temperature.

As described above, when the fracture morphology of the fracture surface of the cracked portion of the high-strength steel continuous casting slab fractured by the placement crack was observed, it was found that most of the cracks had progressed to about 20 mm below the slab surface, and had taken the form of "intergranular fracture" in which the cracks had progressed to the prior austenite grain boundaries. In other words, the causes of placement cracks due to the fracture of the grain boundaries in the high-strength steel continuous casting slab are the coarse prior austenite grain size and the presence of grain boundary ferrite structure, which is a factor in embrittling the grain boundaries. Therefore, in this embodiment, as the necessary conditions for the high-strength steel continuous casting slab that does not generate the placement crack during the cooling process, attention was paid to these two phenomena consisting of (i) the average prior austenite grain size at a predetermined position from the surface layer 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 this 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. In other words, the larger the average prior austenite grain size, the lower the toughness of the continuously cast slab. Here, the average prior austenite grain size refers to an 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, it is preferable because it does not reduce the toughness of the continuously cast slab.
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. The lower limit of the average prior austenite grain size is more preferably 0.8 mm, and further preferably 1.0 mm.

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 γ grain size of the continuous cast slab is fine, 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 microstructure of the continuous cast slab is required is a position 10 mm from a depth in the slab thickness direction, and for example, may be 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 affects the temperature. Furthermore, the longer the residence time of the continuous cast slab in that temperature range, 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, if the residence time of the continuous cast slab at a position 10 mm in the thickness direction from the surface layer of the continuous cast slab at 1450°C or less and 1200°C or more is 130 s or less, the average prior austenite grain size can be made 2.0 mm or less, which is preferable because the average prior austenite grain size can be controlled to be small to suppress the placement cracking of the slab.
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 less and 1200° C. or more is 40 s or more, the risk of breakout can be reduced, the average prior austenite grain size can be set to 2.0 mm or less, and cracking of the slab during placement can be suppressed, which is preferable. The residence time of the continuously cast slab is more preferably 60 s or more, and further preferably 70 s or more.

The residence time 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 adjusted composition is first poured into a water-cooled copper mold to generate an initial solidified shell. Then, the drawing starts, and after it leaves the water-cooled copper mold, it is cooled by water spray. The surface temperature of the slab in the above range is greatly affected by the cooling in the mold and immediately below the mold, so it can be controlled by, for example, improving the thermal conductivity of the mold powder for lubrication in the mold or increasing the flow rate of the water spray immediately below the mold.

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

<(ii) to (iii) Regarding the microstructure of continuously cast slabs>
The continuous cast slab according to the present embodiment is characterized in that (ii) the microstructure has an area ratio of bainite and an area ratio of ferrite of 90% or more in total, and (iii) the area ratio of ferrite is 0% or 3% or more. That is, in addition to the average prior austenite grain size at a position 10 mm from the surface layer of the continuously cast slab being 2.0 mm or less, the ratio of the internal structure 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 the microstructure has an area ratio of bainite and ferrite of 90% or more in total, and the area ratio of ferrite is 0% or 3% or more. The area ratio of bainite and the area ratio of ferrite can be calculated based on the observation result of the microstructure of the continuously cast slab using an observation means such as an optical microscope. Then, the bainite and ferrite contained in the microstructure of the continuously 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, the area S of _bainite , the area S _{of ferrite} , and the total area S of the _bainite and _ferrite , S _{(bainite + ferrite)} , are calculated. Then, the ratio of the area S of _bainite to the _area S of the microstructure of the continuously cast slab, the ratio of the area S of _ferrite to the _area S of the microstructure of the continuously cast slab, and the _ratio of the area S of _bainite to the area S of the microstructure of the continuously cast slab _{(bainite + ferrite ) are} defined and calculated as the area ratio of bainite, the area ratio of ferrite, and the total area ratio (%) of bainite and ferrite, respectively.

The continuously cast slab according to this embodiment is characterized in that (ii) the microstructure has an area ratio of bainite and ferrite of 90% or more in total. That is, in the continuously cast slab according to this embodiment, (ii) if the area ratio ( _% ), which is the ratio of the area S _{(bainite + ferrite )} obtained by summing the area S of _bainite and the area S of _ferrite to the area S of the microstructure of the continuously cast slab, is 90% or more, the toughness of the continuously cast slab can be improved, which is preferable. On the other hand, if the area ratio is less than 90%, the toughness of the continuously cast slab decreases, which is not preferable.

Furthermore, the continuous cast slab according to this embodiment is characterized in that (iii) the area ratio of ferrite is 0% or less or 3% or more. That is, in the continuously cast slab according to this embodiment, in the microstructure mainly composed of bainite, the ferrite area ratio, which is the ratio of the area S _ferrite of the ferrite to the area S _total of the microstructure of the continuously cast slab, is 0% or 3% or more. If the area ratio of ferrite is 0%, it is preferable because cracks due to stress concentration on the soft ferrite do not occur, and if the area ratio of ferrite is 3% or more, it is preferable because the proportion of the ferrite part is sufficiently secured and cracks due to stress concentration in the ferrite part do not progress. On the other hand, if the area ratio of ferrite is more than 0% and less than 3%, it is not preferable because thin ferrite exists at the grain boundary, and stress is concentrated in the small ferrite part and cracks progress.

Here, grain boundary ferrite is a factor that determines grain boundary strength. When grain boundary ferrite is generated, it reduces the toughness of the continuously cast slab. In addition, since ferrite has a lower strength than austenite, pearlite, and bainite, there is a problem that stress is likely to concentrate on grain boundary ferrite when stress is applied. Based on this viewpoint, the inventors conducted extensive research and found that, regarding the microstructural type possessed by the continuously cast slab according to the present embodiment, even in a structure mainly composed of bainite, the toughness of the continuously cast slab can be significantly improved by suppressing the generation of grain boundary ferrite or ensuring a sufficient thickness of ferrite.

Ferrite contains iron with a maximum of 0.02% by mass of carbon, 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 slowly cooled. Pearlite is formed by alternating ferrite layers and cementite layers.

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 the cooling rates below 850°C and above 700°C and below 700°C and above 500°C are controlled to a certain level or higher to suppress the precipitation of grain boundary ferrite and pearlite in each temperature range. Specifically, by increasing the cooling rate in the ferrite-pearlite transformation region, it is possible to suppress the precipitation and make the microstructure of the continuously cast slab mainly bainite, thereby improving the toughness of the slab.

Bainite is a type of transformed structure formed from austenite in carbon steel or alloy steel, and is superior in ductility and impact resistance, toughness, and durability to structures obtained by normal quenching and tempering. Bainite is a black needle-like crystal with mechanical properties intermediate between martensite and fine pearlite. Martensite is formed when austenite structures are rapidly cooled, and is a hard and brittle structure.

The cooling of the continuously cast slab after it leaves the continuous casting machine can be controlled by changing the slab temperature at the exit of the continuous casting machine, the time until multiple slabs are stacked, the number of slabs to be stacked, the water toughening treatment conditions, etc. The cooling rate can be measured using a thermocouple. For example, the measurement can be performed by installing a thermocouple in the center of the top surface of the wide surface (long side) of the slab after it leaves the continuous casting machine.

Continuously cast slabs containing a large amount of C, Si, and Mn have extremely low toughness. Therefore, by controlling only (i) the average prior austenite grain size at a position 10 mm from the surface of the continuously cast slab to be 0.5 mm or more and 2.0 mm or less, (ii) the total area ratio of bainite and ferrite in the microstructure is 90% or more, or (iii) the area ratio of ferrite is 0% or 3% or more, it is not possible to ensure sufficient slab toughness to prevent slab placement cracks, and slab placement cracks will occur. Therefore, it is important that the continuously cast slab for high strength steel according to this embodiment simultaneously satisfies the requirements of (i) the average prior austenite grain size and (ii) to (iii) the microstructure.

As described above, according to the invention of the first embodiment, even in the case of recent high-strength steel slabs in which the toughness of continuous cast slabs is very low, no slab placement cracks occur during the cooling process, and a high-strength steel continuous cast slab can be obtained 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% to 0.40%, Si: 0.10% to 2.50%, and Mn: 1.00% 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 0.40% or less>
The reason for limiting the composition of each chemical component contained in the continuously cast slab according to the present embodiment will be described. The content of each chemical component contained in the continuously cast slab is expressed in mass %. The content of C contained in the continuously cast slab is set to 0.10% or more and 0.40% or less.
The reason for setting the C content at 0.10% or less is as follows. C contained in a 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 0.40%, it is not preferable because the microstructure mainly composed of bainite and ferrite as described above cannot be obtained within the range of the cooling rate of the continuous cast slab.

From this viewpoint, therefore, in the continuous cast slab according to this embodiment, the C content of the continuous cast slab is preferably 0.10% or more and 0.40% or less, more preferably 0.12% or more and 0.35% or less, and even more preferably 0.15% or more and 0.30% 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 the high-strength steel plate is saturated, and strong scale is generated on the hot-rolled sheet before being processed into the high-strength steel plate, which results in deterioration of the appearance and pickling property of the high-strength steel plate, so the upper limit of the Si content is 2.50%.

Therefore, from this viewpoint, in the continuous casting slab according to this embodiment, the Si content contained in the continuous casting slab is preferably 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: 1.00% or more and 5.00% or less>
Furthermore, the reason why the content of Mn contained in the continuous cast slab is set to 1.00% 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 1.00%, the high strength steel plate cannot be sufficiently strengthened, so the lower limit of the content of Mn is 1.00%. 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 casting slab according to this embodiment, the Mn content contained in the continuous casting slab is preferably 1.00% or more and 5.00% or less, more preferably 1.50% or more and 4.50% or less, and even more preferably 1.80% or more and 4.00% or less.

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

P segregates at prior austenite grain boundaries and embrittles the grain boundaries, which may cause slab placement cracks. Therefore, the P content is preferably 0.100% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable that the P content be 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 sulfide and is an element that causes slab embrittlement. Therefore, the S content is preferably 0.0200% or less. Although the lower limit of the S content is not particularly specified, it is preferably 0.0001% or more due to constraints on production technology. 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 suppresses the formation of carbides during slab cooling and promotes the formation of retained austenite, and thus 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 causing slab embrittlement. Therefore, the Al content is preferably 0.100% or less, more preferably 0.010% or more, and even more preferably 0.080% or less.

N exists as a nitride and is an element that causes embrittlement of the slab. Therefore, the N content is preferably 0.0100% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 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, the O content is preferably 0.0100% or less. Although the lower limit of the O content is not particularly specified, the O content is preferably 0.0001% or more due to constraints on production technology. 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 contents of Ti, Nb and V 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, the contents of Ti, Nb and V are preferably 0.200% or less. Although the lower limits of the contents of Ti, Nb and V are not particularly specified, the contents of Ti, Nb and V are more preferably 0.001% or more, since the strength of the steel sheet is increased 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, the contents of each are 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, the content of Ta and W is preferably 0.10% or less. Although the lower limit of the content of Ta and W is not particularly specified, the content of Ta and W is more preferably 0.01% or more, since the strength of the steel sheet 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 cast slab according to this embodiment may contain at least one selected from Cr, Mo, Ni and Cu as necessary within a range that does not impair the object 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 cast 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 cast 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 continuously 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, the content of B is preferably 0.0100% or less. Although the lower limit of the content of B is not particularly specified, since B is an element that segregates to the austenite grain boundaries during hot rolling and annealing of the continuously cast slab and improves the hardenability, the content of B is more preferably 0.0003% or more. Therefore, when 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, coarse precipitates and inclusions do not increase, and the toughness of the slab is not reduced. Therefore, the Co content is preferably 1.00% or less. Although the lower limit of the Co content is not particularly specified, since Co is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, when Co is contained, the content is 1.00% or less. More preferably, it is 0.001% or more. Further preferably, it is 0.80% or less.

If Cu is 1.00% or less, coarse precipitates and inclusions do not increase, and the toughness of the slab is not reduced. Therefore, the Cu content is preferably 1.00% or less. Although the lower limit of the Cu content is not particularly specified, since Cu is an element that improves hardenability, the Cu content is more preferably 0.01% or more. Therefore, when Cu is contained, its content is 1.00% or less. More preferably, it is 0.01% or more. Even more preferably, it is 0.80% or less.

Sn does not affect the toughness of the slab if it is 0.200% or less. Therefore, the Sn content is preferably 0.200% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability, the Sn content is more preferably 0.001% or more. Therefore, when Sn is contained, its content is 0.200% or less. More preferably, it is 0.001% or more. Further preferably, it is 0.100% or less.

If Sb is 0.200% or less, coarse precipitates and inclusions do not increase, and the toughness of the slab is not reduced. Therefore, the Sb content is preferably 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, when Sb is contained, its content is 0.200% or less. More preferably, it is 0.001% or more. Even more preferably, it is 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, the content of each of Ca, Mg and REM is preferably 0.0100% or less. Although the lower limit of the content of each of Ca, Mg and REM is not particularly specified, since these elements make the shape of nitrides and sulfides spherical and improve the toughness of the slab, the content of each of Ca, Mg and REM is more preferably 0.0005% or more. Therefore, when Ca, Mg and REM are contained, the content of each of them 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, the contents of Zr and Te are preferably 0.100% or less. Although the lower limits of the contents of Zr and Te are not particularly specified, 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 contents of Zr and Te are each 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, the Hf content is preferably 0.10% or less. Although the lower limit of the Hf content is not particularly specified, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel plate, the Hf content is more preferably 0.01% or more. Therefore, when Hf is contained, its content is 0.10% or less. More preferably, it is 0.01% or more. Even more preferably, it is 0.08% or less.

If Bi is 0.200% or less, coarse precipitates and inclusions do not increase, and the toughness of the slab is not reduced. Therefore, the Bi content is preferably 0.200% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, when Bi is contained, the content is 0.200% or less. More preferably, it is 0.001% or more. Even more preferably, it is 0.100% or less.

In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi is less than the preferable lower limit value, the effect of the present invention is not impaired, and therefore, these elements 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 a high-strength steel plate, and further, it is possible to obtain a continuous cast slab having excellent weldability, workability, and appearance of the high-strength steel plate.

[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 of the continuous casting slab at the center in the width direction is 700° C. or more and 850° C. or less is 25° C./hr or more and 40° C./hr or less, or 50° C./hr or more;
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 width direction center of the slab is 15° C./hr or higher.

Here, the upper limit of the average cooling rate in the second cooling step and the third cooling step is not particularly specified, but the average cooling rates of 700°C to 850°C and 500°C to 700°C when one slab is allowed to cool in the air are maximum 120°C/hr and 70°C/hr, respectively. In the second cooling step and the third cooling step, cooling at an average cooling rate faster than these average cooling rates requires, for example, water spraying or air blowing on the slab, which requires equipment and is not preferable from the viewpoint of economy. Therefore, in the second cooling step, the upper limit of the average cooling rate of 700°C to 850°C is set to 120°C/hr, and in the third cooling step, the upper limit of the average cooling rate of 500°C to 700°C is set to 70°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, if the handling time is about the same as re-shipping (1 to 2 hours at most), the occurrence of cracks due to placement will not occur. Therefore, in the present invention, the average cooling rate is specified instead of the maximum cooling rate. Each step included in the manufacturing method of the continuous casting slab according to the present embodiment will be described below.

(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 contained in the continuously cast slab according to the above embodiment to 2.0 mm or less at a predetermined position. In the method for producing a continuously cast slab according to this embodiment, a 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 less and 1200°C or more. In this way, the method for producing a continuously cast slab according to this embodiment focuses on the cooling temperature of the continuously cast slab in the range of 1450°C or less and 1200°C or more, which is a 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 made 2.0 mm or less, and cracking of the slab can be suppressed, which is preferable. Note that, although 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, if the residence time is too short, the risk of breakout during continuous casting due to non-uniform solidification increases, so it is preferably 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 25° C./hr to 40° C./hr, or 50° C./hr or more when the surface temperature at the center in the width direction of the continuous cast slab is 700° C. to 850° C. The second cooling step is a step for controlling the precipitation of ferrite contained in the microstructure of the continuously cast slab according to the above embodiment, and making the microstructure a structure mainly composed of bainite.

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

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 25°C/hr or more and 40°C/hr or less, or 50°C/hr or more. If the average cooling rate of the continuously cast slab is less than 25°C/hr, after ferrite is generated, residual austenite is generated in which solutes such as carbon exceeding the solid solubility limit of the ferrite are concentrated, and then a large amount of pearlite precipitates from the residual austenite, which is not preferable. Also, if the average cooling rate of the continuously cast slab is more than 40°C/hr and less than 50°C/hr, thin ferrite precipitates only at the prior austenite grain boundaries depending on the steel composition and the average prior austenite grain size, which is not preferable because it embrittles the grain boundaries.
From this viewpoint, it is preferable that the average cooling rate of the continuous cast slab is 25°C/hr or more and 40°C/hr or less, or 50°C/hr or more, since this optimizes the amount of ferrite precipitation, suppresses the precipitation of pearlite, and suppresses cracking of the slab during placement.
Although the upper limit of the average cooling rate is not particularly specified, the average cooling rate of 700°C or more and 850°C or less when one continuous cast slab is allowed to cool in the air is a maximum of 120°C/hr, and cooling at an average cooling rate faster than this requires, for example, water sprinkling or air blowing onto the continuous cast slab, which is not preferable from the viewpoint of economy because it requires equipment. Therefore, the upper limit of the average cooling rate of 700°C or more and 850°C or less should be set to 120°C/hr.

(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 center in the width direction is cooled under cooling conditions in which the average cooling rate is 15°C/hr or more when the surface temperature is 500°C or more and 700°C or less.
The third cooling step is a step for further suppressing the precipitation of pearlite contained in the microstructure of the continuously cast slab according to the embodiment, and making the microstructure a structure mainly composed of bainite. Specifically, the third cooling step is a step for setting the area ratio (%), which is the ratio of the total area S _{(bainite +} ferrite ₎ of the bainite area S _bainite and the ferrite area S _ferrite to the area S _total of the microstructure of the continuously cast slab, to 90% or more.

In the third cooling step, the temperature to which the continuously cast slab is further cooled is 500° C. to 700° C. In this manner, the method for producing a continuously cast slab according to the present embodiment controls the cooling temperature by focusing on the cooling rate in the temperature range in the pearlite transformation region where the precipitation of pearlite can be suppressed.

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 15° C./hr or more. If the average cooling rate of the continuously cast slab is less than 15° C./hr, pearlite precipitates in a structure mainly composed of bainite. Pearlite has a higher transformation temperature than bainite, and transformation stress is applied to the pearlite portion that precipitated earlier. In addition, pearlite simply has lower strength than bainite, so that strain is concentrated in pearlite due to the difference in strength between pearlite and bainite, which is a factor promoting cracking.
From this viewpoint, it is preferable that the average cooling rate of the continuously cast slab is 15° C./hr or more, since this can avoid a large amount of pearlite precipitation and suppress cracking of the slab when placed.
Although the upper limit of the average cooling rate is not particularly specified, the average cooling rate of 500°C to 700°C in the case of cooling one continuous cast slab in the air is a maximum of 70°C/hr, and cooling at an average cooling rate faster than this requires, for example, water sprinkling or air blowing onto the slab, which is not preferable from the viewpoint of economy because it requires equipment. Therefore, the upper limit of the average cooling rate of 500°C to 700°C should be set at 70°C/hr.

In this way, the method for manufacturing a continuous cast slab according to this embodiment employs a three-stage cooling process as the cooling process for the continuous cast slab, thereby precisely controlling the average prior austenite grain size and the microstructure of the continuous cast slab, thereby making it possible to provide a continuous cast slab for high-strength steel in which slab placement cracks caused by cooling are suppressed.

As described above, according to the manufacturing method of the continuous cast slab of the third embodiment, even if the slab has a component system 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 placement cracks during the cooling process.

[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.

Hereinafter, the effects of the present invention will be specifically described 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-20). Table 1 shows the steel types A to F that are the raw materials of 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-20).

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 cooling conditions (I) to (III) of the continuously cast slabs, the microstructures of the obtained continuously cast slabs, and the evaluation of cracks during slab placement. In Tables 2 to 4, F, P, and B in the microstructure column represent ferrite, pearlite, and bainite, respectively.

In the continuously cast slabs produced in the comparative examples and the invention examples, the average prior austenite grain size was measured, the area ratios of bainite, pearlite and ferrite were calculated, and the continuous cast slabs were evaluated for cracking in the following manner.

<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 slab after cooling, 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, five fields of view were observed at a magnification of 10 times at a position 10 mm from the slab surface layer to obtain a microstructure image of the continuous casting 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 five-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 from the slab surface layer 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 ferrite area ratio for 10 fields of view using Adobe's PHOTOSHOP (registered trademark), and the values are averaged to obtain the ferrite area ratio. 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 and bainite>
The area ratio of the pearlite and bainite structures is measured in the same manner as in the above-mentioned ferrite measurement method, by revealing the structure 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 from the surface layer of the slab, with ferrite removed from the field of view, using an SEM. The area ratios of pearlite and bainite are calculated for the 10 fields of view using the obtained structure images with PHOTOSHOP (registered trademark) of Adobe, 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 ratios of each structure are obtained. Bainite is a structure of the recesses, and pearlite is a structure of the recesses that contains lamellar carbides.

A comprehensive evaluation of the microstructure was performed. The evaluation criteria are as follows:
Microstructure evaluation: ◯ ... The total area ratio (%) of bainite and ferrite, which is the ratio of S _{(bainite + ferrite)} to S _total , is 90% or more, and the area ratio (%) of ferrite, which is the ratio of S _{( ferrite)} to S _total , is 0% or 3% or more. Microstructure evaluation: × ... The total area ratio (%) of bainite and ferrite, which is the ratio of S _{(bainite + ferrite)} to S _total , is less than 90%, or the area ratio (%) of S (bainite + ferrite) to S _total is less than 90%.
The area ratio of _ferrite (%) is 0 to less than 3%

<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 and defects on the surface of the slab.
In addition, since slab cracks and defects with a length of 10 mm or less do not cause holes, openings, or breaks during heating in the hot rolling process, the evaluation criteria for slab placement cracks were set as follows.
・Cracks in slab placement: ◯ ... No cracks or scratches longer than 10 mm were found on the surface of the slab by visual inspection. ・Cracks in slab placement: × ... Cracks or scratches longer than 10 mm were found on the surface of the slab by visual inspection.

<Comparative Examples (Test Nos. A-1 to A-5)>
The microstructure of the slabs satisfied by the continuously cast slabs produced in Test Nos. A-1 to A-5 is 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 from the surface layer of the slab in the thickness direction is larger than 2.0 mm. In these cases, the toughness of the prior austenite grain boundaries is 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 leaves the continuous slab casting machine are varied, it is not possible to suppress the cracking of the slab during placement.

<Comparative Examples (Test Nos. B-1 to B-8)>
The microstructure of the slabs satisfied by the continuous cast slabs produced in Test Nos. B-1 to B-8 is defined as Condition B. Condition B is a condition for an example in which the average prior austenite grain size at a position 10 mm from the slab surface is 2.0 mm or less, but pearlite is precipitated at 10% or more, and cracking during slab placement cannot be suppressed.

<Comparative Examples (Test Nos. C-1 to C-3)>
The microstructural structure of the slabs that is satisfied by the continuously cast slabs produced in Test Nos. C-1 to C-3 is defined as Condition C. Condition C is an example of a condition in which the average prior austenite grain size at a position 10 mm from the slab surface is 2.0 mm or less, the area ratio of bainite and ferrite is 90% or more, but the area ratio of ferrite is 1% or 2%, thin ferrite precipitates at the grain boundaries, and slab placement cracking cannot be suppressed.

<Examples (Test Nos. D-1 to D-20)>
The microstructural structure of the slabs satisfied by the continuously cast slabs produced in Test Nos. D-1 to D-20 is defined as Condition D. Condition D is a condition of the present invention example, and the continuously cast slabs produced in the present invention example had an average prior austenite grain size of 0.5 mm to 2.0 mm at 10 mm from the slab surface, and the microstructure of the continuously cast slab was a bainite single phase or a bainite + ferrite structure. In other words, by improving the toughness of the slab from the viewpoint of refining the prior austenite grain size and optimizing the microstructure, no slab placement cracks were generated after the slab was cooled.

According to Tables 2 to 4, it was found that slab placement cracking during cooling of the slab can be suppressed by setting the average prior austenite grain size at a position 10 mm from the surface of the slab in the slab thickness direction to 2.0 mm or less, and by setting the microstructure of a continuously cast slab to have a total area ratio of bainite and ferrite of 90% or more and an area ratio of ferrite of 0% or 3% or more.

Fig. 3 is an enlarged photograph of a continuous cast slab produced in an invention example (Test No. D-2) 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 _{(bainite + ferrite)} of the area S of _bainite and the area S of _ferrite to the area S 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 invention has an average prior austenite grain size of 0.5 mm or more and 2.0 mm or less at a position 10 mm from the slab surface in the slab thickness direction, and the microstructure has an area ratio of bainite and ferrite of 90% or more in total, and the area ratio of ferrite is 0% or 3% or more.

In this way, the continuous cast slab of the present invention has an average prior austenite grain size of 0.5 mm or more and 2.0 mm or less at a position 10 mm from the slab surface, and the microstructure has an area ratio of bainite and ferrite of 90% or more in total and an area ratio of ferrite of 0% or more or 3% or more, so that a slab for high alloy high strength steel can be provided that is free from slab placement cracks after casting, and it becomes possible to prevent problems such as hole formation during rolling. That is, according to the examples and comparative examples, it was found that slab placement cracks during cooling of the slab can be suppressed by setting the average prior austenite grain size of 0.5 mm or more and 2.0 mm or less at a position 10 mm from the slab surface in the slab thickness direction, and setting the microstructure to an area ratio of bainite and ferrite of 90% or more in total and an area ratio of ferrite of 0% or more or 3% or more.

In order to obtain such a microstructure of a slab for high strength steel plate, it is preferable to adopt a three-stage cooling in which, for example, the slab surface temperature is cooled for a residence time of 130 s or less from 1200° C. to 1450° C., and then the slab is cooled so that the cooling rate at the slab width center surface where the temperature is 850° C. to 700° C. is 25° C./hr to 40° C./hr, or 50° C./hr or more, and further the average cooling rate at the slab width center surface where the temperature is 700° C. to 500° C. is 15° C./hr or more. Note that the manufacturing method of a continuous cast slab having a microstructure of a slab for high strength steel plate 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 bainite and ferrite of 90% or more, and the area ratio of ferrite is 0% or 3% or more. Therefore, a slab for high strength steel that is free from cracks during slab placement 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 0.40% or less,
Si: 0.10% or more and 2.50% or less,
Mn: 1.00% 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, B: 0.0100% or less, Co: 1.00% or less, Cu: 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 a bainite and a remaining unavoidable structure, or the bainite, and either one or both selected from ferrite and pearlite, and the remaining unavoidable structure ,
a sum of an area ratio of the bainite and an area ratio of the ferrite is 90% or more, and an area ratio of the ferrite is 0% or 3% or more;
A continuously cast slab, characterized in that the sum of the area ratio of bainite, the area ratio of ferrite, and the area ratio of pearlite 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 in which the continuous casting slab is cooled 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 700° C. or higher and 850° C. or lower is 25° C./hr or higher and 40° C./hr or lower, or 50° C./hr or higher;
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 continuous cast slab is 15°C/hr or more at a temperature of 500°C or more and 700°C or less.

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