Continuously Cast Slab and Method for Producing the Same

The present invention relates to a continuously cast slab that does not cause cracking during cooling, and a method for producing the same. More specifically, the present invention relates to a continuously cast slab for high-strength steel (high tensile steel) that can effectively prevent the occurrence of thermal cracking therein and does not cause problems such as the formation of holes during rolling, and a method for producing the same.

In recent years, the automotive industry has been developing high-strength steels with higher strength and higher alloying levels in order to further reduce the thickness of car bodies and improve crash safety. Increasing the level of alloying has resulted in a significant reduction in the toughness of a slab.

As the toughness of a slab decreases with an increase in the alloying level, cracking in the slab during cooling, known as thermal cracking, in other words, season cracking, has occurred more frequently. Such thermal cracking may cause the slab to fracture while being conveyed, preventing the slab from being hot rolled. Even if the slab does not fracture, the cracks in the slab may open during hot rolling, causing the resulting hot-rolled steel sheet to fracture. Meanwhile, small cracks in a slab may appear as surface defects, such as scabs or slivers, on the resulting steel sheet after hot rolling, cold rolling, annealing, or plating. Typically, cracks in the surface of a slab are removed with a grinder. However, in a case where the toughness of the slab has decreased with an increase in the amount of alloy added and the cracks in the slab develop due to the stress applied by the grinder, it may be impossible to remove the cracks in the slab completely. Furthermore, small cracks in the slab may be overlooked and appear as surface defects on the resulting steel sheet after hot rolling, cold rolling, annealing, or plating. For the above reasons, it is necessary to suppress cracking in slabs.

is a micrograph of a fracture surface of a cracked portion in a slab for high-strength steel, which has fractured due to thermal cracking, shot with a scanning electron microscope (SEM). As is obvious from, the fracture surface of the cracked portion in the slab exhibits an intergranular fracture surface along a prior austenite grain boundary.is a micrograph of a cross-section of the cracked portion in the slab. It is found that the depth of the crack in the slab from the surface is mostly about 20 mm. It is also found that the crack in the slab has propagated around the prior austenite grain boundary, and grain-boundary ferrite is present at the tip end of the cracked portion in the slab. Further, pearlite or pearlite and bainite is/are observed in prior austenite grains.

An intergranular fracture occurs when prior austenite grains are coarse and their grain boundaries are embrittled. Precipitates and ferrite are more likely to be formed at grain boundaries than within grains. Precipitates at grain boundaries are a factor that reduces the grain boundary strength and also reduces the toughness of the slab. When the prior austenite grains are coarse, the ratio of their grain boundaries is low, and the density of precipitates at the grain boundaries is correspondingly high, so that the grain boundaries are further embrittled. When grain-boundary ferrite is formed, there is a difference in strength between the grain-boundary ferrite and the pearlite and bainite in the grains, causing stress concentration at the grain-boundary ferrite portion with lower strength. This can lead to cracks in the slab even with lower stress. In such a case, when the prior austenite grains are coarse, grain-boundary ferrite that is linearly thin and elongated is precipitated, making it difficult to avoid the propagation of the cracks in the slab. This can lead to increased damage due to the cracks in the slab. Meanwhile, when the slab is cooled, stress is caused due to the difference in thermal shrinkage or in transformation expansion between the surface and the inside of the slab. When the stress is high, cracks are caused in the slab while the slab is cooled to room temperature. Since the toughness of a slab for high-alloy, high-strength steel produced in recent years is low, it has been difficult to remove deep cracks that have occurred in the slab in the above manner by using some measures such as a grinder. This has been a problem that greatly reduced the yield of the slab.

From such a viewpoint, a method for suppressing the occurrence of thermal cracking in a slab for high-tensile strength steel has been proposed. For example, Patent Literature 1 proposes a method for suppressing bainite/martensitic transformation by slowly cooling at 700 to 500° C., which corresponds to the temperature range in which the transformation from austenite to ferrite occurs, thereby reducing the stress generated due to the transformation expansion. That is, Patent Literature 1 discloses a method capable of suppressing the occurrence of thermal cracking even in high tensile strength steel with a grade that is likely to cause thermal cracking. Specifically, a method for cooling a slab for high tensile strength steel disclosed in Patent Literature 1 is a method for suppressing the occurrence of thermal cracking by controlling the cooling rate for the slab in accordance with the length of an internal crack that has occurred in high tensile strength steel based on the finding that internal stress in the high tensile strength steel depends on its cooling rate.

Patent Literature 2 proposes a method for reducing a temperature difference and reducing stress due to transformation by starting slow cooling of a slab immediately after the slab is cast, then slowly cooling the slab at a temperature of 700° C. or higher for 10 hours or longer and further from 700 to 500° C. That is, Patent Literature 2 discloses a method for cooling a slab for a high-strength steel sheet that prevents both cracking while the slab is being cooled and defects in quality such as scabs while the slab is being hot-rolled, even if the slab contains Si. Specifically, the cooling method for a slab for a high-strength steel sheet disclosed in Patent Literature 2 includes setting the average cooling rate for a continuously cast slab, which has limited contents of chemical components, such as C, Si, and Mn, for a high-strength hot-rolled steel sheet to 20° C./hr or less in the temperature range of 500 to 700° C.

However, the above conventional technologies have the following problems. The method described in Patent Literature 1 of cooling a slab for high tensile strength steel after casting the slab involves controlling the cooling rate for the slab so as to reduce the internal stress to be generated in the slab by focusing only on the temperature range of 700° C. to 500° C. after the slab is cast and cooled. However, since the toughness of a slab for high-strength steel with a higher amount of alloy added produced in recent years is low, the condition of prior austenite grain boundaries around which thermal cracking propagates is also quite important. However, the method described in Patent Literature 1 does not involve controlling the prior austenite grain size or grain-boundary ferrite. Thus, even if a slab with an increased carbon content is produced by the cooling method for a slab for high tensile strength steel described in Patent Literature 1, it is not possible to sufficiently suppress the occurrence of thermal cracking in the slab.

The method described in Patent Literature 2 for cooling a slab for a high-strength steel sheet is based on the finding that cracking in the slab is caused due to thermal stress, which has been caused by the addition of Si to the steel and by the temperature variation in the slab, and suppresses the occurrence of cracking in the slab by focusing on reducing the thermal stress. However, the method described in Patent Literature 2 for cooling a slab for a high-strength steel sheet does not involve limiting the microstructure of the slab. Therefore, even if a slab is produced by the cooling method described in Patent Literature 2 for a slab for a high-strength steel sheet, it is not possible to sufficiently suppress the occurrence of thermal cracking in the slab.

Further, as a result of intensive studies, the inventors have found that the toughness of a slab produced by conventional technologies to have high C, Si, and Mn contents is significantly low, making it impossible to completely suppress the occurrence of thermal cracking in such a slab, causing a problem such as formation of holes during rolling.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a continuously cast slab that does not cause thermal cracking during cooling of the slab, nor does it cause problems such as the formation of holes during rolling, even when the toughness of the continuously cast slab is low, and a method for producing the same.

The inventors conducted extensive studies in order to achieve the above object. As a result, by analyzing the fracture morphology of slab cracking, the inventors found that its fracture surface includes at least one type selected from an intergranular fracture surface along a prior austenite grain boundary and an intragranular fracture surface (cleavage fracture surface) across a prior austenite grain boundary. Through various detailed studies, the inventors further found that it is impossible to suppress the occurrence of thermal cracking in a slab solely by reducing the stress achieved by controlling the cooling rate and reducing the temperature variation and that the morphology of the microstructure of the slab has a great influence on the occurrence of thermal cracking. Specifically, the inventors found that it is possible to suppress the occurrence of thermal cracking in a continuously cast slab during cooling thereof and to avoid problems such as the formation of holes during rolling, by controlling the average prior austenite grain size and microstructure of the continuously cast slab to increase the toughness of the slab, and thus arrived at the present invention.

That is, a continuously cast slab according to the present invention which advantageously solves the above problems is a continuously cast slab for high-strength steel, characterized in that an average prior austenite grain size at a position 10 mm from a surface layer of the continuously cast slab is in the range of 0.5 mm to 2.0 mm; a total of an area ratio of ferrite and an area ratio of pearlite in the microstructure of the slab is 90% or more; and the area ratio of ferrite is less than 5% or 10% or more.

It is considered that the continuously cast slab according to the present invention may include, in mass %, (a) C: in a range of 0.10% to 1.00%, Si: in a range of 0.10% to 2.50%, and Mn: in a range of 0.40% to 5.00%, as a preferable solution means.

A method for producing a continuously cast slab according to the present invention is a method for producing a continuously cast slab for high-strength steel, the continuously cast slab preventing thermal cracking due to cooling, the method including subjecting the continuously cast slab having the ingredient composition described in (a) to the following:

The present invention can provide a continuously cast slab that causes neither thermal cracking during cooling nor problems such as the formation of holes during rolling, even when the slab has an ingredient composition of a continuously cast slab for high-strength steel.

Hereinafter, embodiments of the present invention will be specifically described. Note that the drawings are only schematic, and thus may differ from the actual ones. In addition, the following embodiments only illustrate examples of an apparatus and a method for embodying the technical idea of the present invention. Thus, the configuration of the present invention is not limited thereto. That is, the technical idea of the present invention can be changed in various ways within the technical scope recited in the claims.

A continuously cast slab according to a first embodiment will be described. The continuously cast slab according to this embodiment is a continuously cast slab for high-strength steel and has the following features: (i) an average prior austenite grain size at a position 10 mm from the surface layer of the continuously cast slab is in the range of 0.5 mm to 2.0 mm, (ii) a total of an area ratio of ferrite and an area ratio of pearlite in a microstructure of the slab is 90% or more, and (iii) the area ratio of ferrite is less than 5% or 10% or more. That is, by satisfying at least the above features (i) to (iii), the invention according to this embodiment can provide a high-yield continuously cast slab for high-strength steel that neither causes thermal cracking during cooling nor problems such as the formation of holes during rolling, even when the slab is a continuously cast slab for high-strength steel of recent years that has extremely low toughness.

First, an appropriate range of the microstructure of the continuously cast slab and the reasons for limiting such a range will be described. In the following description, the symbol “%” representing a constitutional ratio in the microstructure means “area %” unless otherwise stated. It is assumed that the microstructure of the continuously cast slab has been observed at room temperature.

As described above, as a result of observing the fracture morphology of a fracture surface of a cracked portion in a continuously cast slab for high-strength steel that fractured due to thermal cracking, it is found that many of the cracks develop to a position about 20 mm below the surface layer of the slab, and exhibit the morphology of an “intergranular fracture” such that a crack develops in a prior austenite grain boundary. That is, in a continuously cast slab for high-strength steel, thermal cracking due to the fracture of a grain boundary is caused by the coarse prior austenite grain size, and a ferrite structure at the grain boundary which is a factor for embrittlement of the grain boundary. A high-alloy, high-strength steel sheet is made from a continuously cast slab with extremely low toughness, and furthermore, if the continuously cast slab has such an embrittlement factor, it is impossible to suppress the occurrence of thermal cracking in the slab even if the slab is slowly cooled to reduce stress. Thus, this embodiment focuses on the following two including (i) an average prior austenite grain size at a predetermined position from the surface layer of the continuously cast slab, and (ii) to (iii) the microstructure of the continuously cast slab, as the necessary conditions for a continuously cast slab for high-strength steel that does not cause thermal cracking during cooling.

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 thermal cracking due to cooling is prevented, and has the feature (i) that an average prior austenite grain size at a position 10 mm from the surface layer of the continuously cast slab is in the range of 0.5 mm to 2.0 mm. The average prior austenite grain size is a factor that determines the fracture unit of the slab. A grain boundary has a feature such that precipitates tend to concentrate thereon because the solute component tends to condense thereon. This means that the larger the average prior austenite grain size, the smaller the ratio of grain boundaries per unit volume. The density of precipitates thus increases, reducing the toughness of the continuously cast slab. Herein, the average prior austenite grain size refers to a value obtained by averaging the values of a plurality of prior austenite grain sizes calculated from the prior austenite grain sizes measured for a plurality of visual fields.

In a conventional continuously cast slab, the average prior austenite grain size is as large as several millimeters. This significantly reduces the toughness of the continuously cast slab. Since a conventional low-alloy steel is made from a high-toughness continuously cast slab, the average prior austenite grain size has never been a concern. On the other hand, for high-alloy, high-strength steel, the average prior austenite grain size can be a major concern. Thus, 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. When the average prior austenite grain size is 2.0 mm or less, precipitates that are concentrated on prior austenite grain boundaries can be dispersed, which is preferable because the toughness of the continuously cast slab is not reduced.

Meanwhile, the lower limit of the average prior austenite grain size is not strictly defined. However, to achieve a fine average prior austenite grain size of less than 0.5 mm, it is necessary, for example, to strongly cool the slab in the initial stage of solidification, which may cause breakout due to uneven solidification. Therefore, the lower limit of the average prior austenite grain size is preferably 0.5 mm or more. Note that 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 determined by using the size of the grains forming the prior austenite structure at a position 10 mm from the surface layer of the continuously cast slab. The reason for setting the position 10 mm from the surface layer of the continuously cast slab in determining the average prior austenite grain size is that the position 10 mm from the surface layer of the continuously cast slab is considered to be the position necessary to suppress the occurrence of thermal cracking in the slab, since most of the thermal cracking in the slab develops to a position about 20 mm below the surface layer of the slab.

Meanwhile, a region less than 5 mm from the surface layer of the continuously cast slab is rapidly cooled either directly by a casting mold or by a water spray disposed directly below the casting mold. The rapid cooling results in a smaller y grain size and increased toughness in the region of the continuously cast slab. Consequently, this region is less likely to become the starting point for thermal cracking. Therefore, such a region located less than 5 mm from the surface layer of the continuously cast slab can be excluded from structure control. This means that the position where the structure of the continuously cast slab needs to be controlled is a position 10 mm deep in the thickness direction of the slab, and may be, for example, a position 5 to 20 mm deep from the surface layer of the continuously cast slab, based on the position 10 mm from the surface layer of the continuously cast slab.

In the continuously cast slab according to this embodiment, the temperature for cooling the continuously cast slab is a factor that determines the average prior austenite grain size. The temperature for cooling the continuously cast slab is particularly in the range of 1450° C. to 1200° C. and the retention time in such a temperature range has an influence. The longer the retention time of the continuously cast slab in the temperature range, the coarser the average prior austenite grain size. That is, in order for the continuously cast slab according to this embodiment to satisfy the condition (i) that an average prior austenite grain size at a position 10 mm from the surface layer of the continuously cast slab is in the range of 0.5 mm to 2.0 mm, it is essential to control the retention time in the temperature range of 1450° C. to 1200° C. of the continuously cast slab. Specifically, it is preferable to control the retention time within 130 seconds in the temperature range of 1450° C. to 1200° C. at a position 10 mm deep from the surface layer of the slab in the thickness direction of the continuously cast slab.

When the retention time in the temperature range of 1450° C. to 1200° C. of the continuously cast slab is within 130 seconds, it is possible to achieve the average prior austenite grain size of 2.0 mm or less. Controlling the average prior austenite grain size to be small in such a manner can disperse precipitates and grain-boundary ferrite, increasing the toughness of the slab and suppressing the occurrence of thermal cracking in the slab, which is preferable.

Furthermore, from such a viewpoint, the retention time of the continuously cast slab is preferably within 120 seconds, more preferably within 110 seconds, and further preferably within 100 seconds.

It should be noted that the lower limit of the retention time of the continuously cast slab is not defined to a specific value. However, if the retention time is too short, there is a higher risk of breakout due to uneven solidification during continuous casting. Thus, the retention time should be 40 seconds or more.

That is, if the retention time of the continuously cast slab in the temperature range of 1450° C. to 1200° C. is less than 40 seconds, cracking may occur due to uneven solidification, resulting in a risk of breakout. Thus, the retention time is preferably set to 40 seconds or more. From such a viewpoint, the retention time of the continuously cast slab in the temperature range of 1450° C. to 1200° C. is more preferably 60 seconds or more, and further preferably 70 seconds or more.

The retention time of the continuously cast slab can be controlled by adjusting the cooling conditions in the initial stage of the slab casting. For example, in the continuous casting of steel, molten steel with an adjusted ingredient composition is first poured into a water-cooled copper casting mold to form an initial solidified shell. The solidified shell is then removed from the water-cooled copper casting mold and cooled with a water spray. Since the temperature of the slab surface in the above-described range is significantly influenced by cooling performed within the casting mold or immediately below the casting mold, the temperature may be controlled by, for example, increasing the thermal conductivity of mold flux used for lubricating the inside of the casting mold, or by increasing the flow rate of a water spray disposed directly below the casting mold.

By controlling such cooling conditions, the average prior austenite grain size at a position 10 mm below the surface layer of the continuously cast slab can be controlled. The cooling temperature of the continuously cast slab cannot be directly measured, but it can be estimated, for example, by calculating a temperature history at a position 10 mm below the surface layer in the thickness direction of the slab, representing a region from 5 mm to 20 mm below the surface layer in the thickness direction of the continuously cast slab by heat-transfer analysis. To maximize the retention time within the temperature range in the interior of the continuously cast slab, the position for heat-transfer analysis can be set at the center of the wide face of the slab.

<(ii) to (iii) Microstructure of Continuously Cast Slab>

The continuously cast slab according to this embodiment has features such that (ii) a total of an area ratio of ferrite and an area ratio of pearlite in a microstructure of the slab is 90% or more, and (iii) the area ratio of ferrite is less than 5% or 10% or more. That is, in addition to the feature that the average prior austenite grain size at a position 10 mm from the surface layer of the continuously cast slab is 2.0 mm or less, the ratio of internal structures such as ferrite and pearlite is also a factor that determines the unit of fracture, and it is known that controlling such a ratio within appropriate range can increase the toughness of the slab. Thus, the inventors have found that it is possible to increase the toughness of the slab by controlling the cooling rate so as to satisfy the condition (ii) that a total of an area ratio of ferrite and an area ratio of pearlite in a microstructure of the slab is 90% or more, and the condition (iii) that the area ratio of ferrite is less than 5% or 10% or more. Note that the area ratio of ferrite and the area ratio of pearlite can be calculated based on the results of observing the microstructure of the continuously cast slab using an observation means such as an optical microscope. In addition, ferrite and pearlite contained in the microstructure of the continuously cast slab can be identified using an observation means such as an optical microscope.

From the results of identifying the microstructure of the continuously cast slab, the area Sof the microstructure of the continuously cast slab and the area S, which is the sum of the area Sof ferrite and the area Sof pearlite, are calculated. Then, the ratio of the area S, which is the sum of the area Sof ferrite and the area Sof pearlite, to the area Sof the microstructure of the continuously cast slab is calculated as the area ratio (%).

The continuously cast slab according to this embodiment has a feature such that (ii) a total of an area ratio of ferrite and an area ratio of pearlite in a microstructure of the slab is 90% or more. That is, when the continuously cast slab according to this embodiment has a feature such that (ii) the area ratio (%), which is the ratio of the area S, which is the sum of the area Sof ferrite and the area Sof pearlite, to the area Sof the microstructure of the continuously cast slab is 90% or more, it is possible to reduce thermal stress and transformation stress to be applied to the slab due to bainite/martensitic transformation while the slab is slowly cooled, and to allow such generated stress to be dispersed in ferrite and pearlite existing in large amounts within the microstructure, and thus to increase the toughness of the continuously cast slab, which is preferable. Meanwhile, if the area ratio is less than 90%, the toughness of the continuously cast slab decreases, which is unfavorable.

Further, the continuously cast slab according to this embodiment has a feature such that (iii) the area ratio of ferrite is less than 5% or 10% or more. That is, in the continuously cast slab according to this embodiment, when the area ratio of ferrite is 5% or more but less than 10%, the slab is in such a state that thin ferrite is present at grain boundaries and stress is concentrated on the soft ferrite portions, resulting in the development of cracks, which is unfavorable. As long as the area ratio of ferrite is less than 5%, even if cracks have started to develop, the development stops soon, which is preferable. Meanwhile, when the area ratio of ferrite is 10% or more, the stress is unlikely to be concentrated in the ferrite portions, and cracks do not develop, which is preferable.

Herein, grain-boundary ferrite is a factor that determines the strength of grain boundaries. When grain-boundary ferrite is formed, the toughness of the continuously cast slab is reduced. Further, since the strength of ferrite is lower than that of austenite, pearlite, and bainite, the application of stress may cause a problem in that the stress is likely to be concentrated on the grain-boundary ferrite. The inventors have conducted various studies based on such perspectives and have found that even when the microstructure of the continuously cast slab according to this embodiment is a structure of mainly pearlite, it is possible to significantly increase the toughness of the continuously cast slab by suppressing the formation of grain-boundary ferrite.

Note that ferrite contains a maximum carbon content of 0.02 mass % and thus is a structure close to pure iron. Ferrite is a ferromagnetic material from room temperature to 780° C., and is the softest of all structures of steel, with excellent ductility. Pearlite is a structure obtained when austenite is slowly cooled. Pearlite includes ferrite layers and cementite layers and is formed with such layers alternately arranged.

The precipitation of grain-boundary ferrite is largely influenced by the cooling rate in the ferrite transformation range. If the cooling rate is lower than the critical rate, the precipitation of ferrite occurs, so that the cooling rate needs to be controlled in the temperature range of 850° C. to 700° C. If the cooling rate in the ferrite transformation range is lower than the critical rate but a sufficient precipitation time cannot be secured, ferrite is preferentially precipitated at grain boundaries where precipitation is likely to occur. Therefore, the stress applied by subsequent pearlite transformation or bainite/martensitic transformation is concentrated on the soft ferrite portions, resulting in thermal cracking in the slab, which is unfavorable. As a countermeasure, it is possible to grow the ferrite precipitated at the grain boundaries into polygonal ferrite by reducing the cooling rate in the ferrite transformation range and thus securing a sufficient time for the precipitation of ferrite. Forming polygonal ferrite in such a manner can suppress excessive stress concentration, and thus can increase the toughness of the slab.

To suppress the occurrence of thermal cracking in the slab, it is important not only to suppress the embrittlement of prior austenite grain boundaries but also to reduce the stress during transformation. Thus, controlling the cooling rate in the pearlite transformation range (the temperature range of 700° C. to 500° C.) in various manners can also control the microstructure of the continuously cast slab.

Note that cooling that is performed after the continuously cast slab is removed from the continuous casting machine can be controlled by changing conditions, such as the temperature of the slab at the exit side of the continuous casting machine, the time taken to stack a plurality of slabs, the number of slabs to be stacked, the presence or absence of a heat-retention cover, and a water-toughening process, for example. The cooling rate can be measured by a thermocouple. For example, the cooling rate can be measured by disposing a thermocouple at the central portion of the upper surface of a wider face (a longer side) of the slab after the slab is removed from the continuous casting machine.

As described above, the invention according to the first embodiment can obtain a high-yield continuously cast slab for high-strength steel that prevents both thermal cracking of the slab during a cooling process and the occurrence of problems such as the formation of holes during a rolling process, even if such a slab is a continuously cast slab for high-strength steel of recent years that has extremely low toughness.

A continuously cast slab according to a second embodiment will be described. The continuously cast slab according to this embodiment corresponds to the continuously cast slab according to the above embodiment that contains, in mass %, C: in the range of 0.10% to 1.00%, Si: in the range of 0.10% to 2.50%, and Mn: in the range of 1.50% to 5.00%.

Note that in the following description, the symbol “%” representing the content of a constituent element of steel means “mass %” unless otherwise indicated.

The reasons for limiting each chemical composition contained in the continuously cast slab according to this embodiment will be described. Note that the content of each chemical ingredient contained in the continuously cast slab is expressed in mass %. The reasons for setting the C content in the continuously cast slab in the range of 0.10% to 1.00% are as follows. C contained in a continuously cast slab for high-strength steel is the element necessary to increase the strength of a high-strength steel sheet to be formed using the continuously cast slab as a raw material. If the C content is less than 0.10%, the strength required for the high-strength steel sheet cannot be obtained. Therefore, the lower limit of the C content is 0.10%. Meanwhile, if the C content exceeds 1.00%, sufficient weldability or workability of the high-strength steel sheet cannot be obtained, which is unfavorable.

From such a viewpoint, the C content in the continuously cast slab according to this embodiment preferably falls within the range of 0.10% to 1.00%, more preferably within the range of 0.12% to 0.40%, and particularly more preferably within the range of 0.15% to 0.40%.

Next, the reasons for setting the Si content in the continuously cast slab for high-strength steel in the range of 0.10% to 2.50% are as follows. Si contained in the continuously cast slab is the element necessary to obtain the residual austenite in the steel sheet in an annealing step for a high-strength steel sheet produced using the continuously cast slab as a raw material. Further, Si contained in the continuously cast slab is the essential additive element as it contributes to increasing the strength of the high-strength steel sheet by solid-solution strengthening. When the Si content is less than 0.10%, the strength required for the high-strength steel sheet cannot be achieved. Therefore, the lower limit of the Si content is 0.10%.

Meanwhile, when the Si content exceeds 2.50%, the effect of achieving the strength required for the high-strength steel sheet is saturated, and also heavy scale is formed on a hot-rolled sheet that has not yet been processed into a high-strength steel sheet. This deteriorates the appearance and pickling properties of the high-strength steel sheet. Therefore, the upper limit of the Si content is 2.50%.