JP3298519B2 - Steel sheet free of hydrogen defects and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate having no hydrogen defects therein and manufactured by rolling a steel slab by a continuous casting method, and a method of manufacturing the same.

[0002]

2. Description of the Related Art When a slab is cast by a continuous casting method and rolled into a thick steel plate, defects detected by ultrasonic flaw detection considered to be based on hydrogen may be found. Here, this is referred to as a hydrogen defect.In the process of cooling after rolling, hydrogen dissolved in the steel becomes gas at inclusions and component segregation parts, and the pressure causes cracks inside the steel. This is considered to be caused and detected as a defect.
Therefore, it is necessary to take measures such as reducing hydrogen in steel as much as possible and improving the resistance of steel to cracking caused by hydrogen.

[0003] As a method for reducing the hydrogen concentration in steel, degassing of molten steel, heat retention of cast slabs after casting, and heat insulation of steel sheets after rolling are also performed. In the case of continuous casting, most of the molten steel is degassed for the purpose of removing impurities and adjusting the components. For this purpose, the RH degassing method is mainly used. Hydrogen in molten steel can be sufficiently reduced by lengthening the time of this degassing process,
As the processing time increases, the processing cost increases, and the time is limited by the cycle of continuous casting, so that there is a limit. More sufficient reduction in hydrogen concentration is achieved by keeping the slabs immediately after pouring at intervals and covering them with a cover, keeping them warm, and extending the high temperature time to diffuse hydrogen in the slabs to the surface and eliminate them. It can be done by doing. in this case,
Thick slabs require a few days or more to stand because the diffusion of hydrogen takes time, and the temperature of the slabs decreases.

[0004] In recent years, from the viewpoint of efficient use of energy and shortening of the manufacturing process, a hot charge or a hot cast slab immediately after casting is immediately adjusted to a required temperature without cooling and rolled to a product size. Many direct rolling manufacturing methods have been adopted. In this case, the hydrogen remaining in the steel during the casting process is rolled into a thick steel plate shape without being sufficiently removed, so in the cooling process after rolling, if hydrogen is not released and lowered while the temperature is high,
The danger of hydrogen deficiency increases. For this reason, it is necessary to stack the rolled steel sheets and gradually cool them down, or to heat them to keep them warm, and there are problems in the process, such as the place for slow cooling, heating and heat keeping equipment, and the increase in the required time due to them. Inevitable.

As means for improving the resistance of steel to cracking caused by hydrogen, it is conceivable to reduce inclusions serving as starting points of hydrogen cracking, suppress the formation thereof, and suppress local hardening such as center segregation. Cluster-like inclusions such as MnS and oxides are present in the form of a band in the rolled steel, along which hydrogen dissolved in the steel becomes gas and causes hydrogen cracking. These inclusions are likely to occur in non-homogeneous portions of the steel, such as the center segregation, such as components, structure, and hardness, and hydrogen defects of the steel sheet mainly occur in the center segregation of the steel.

The cause of the center segregation peculiar to continuous casting is that the molten steel whose components have been enriched due to solidification moves and accumulates locally, that is, the residual molten steel between the dendrites in the final stage of solidification causes solidification shrinkage of the molten steel and bulging of the solidified shell. By
This is for moving to the solidification completion point of the final solidification part and solidifying there. As measures to prevent this, a method of minimizing the occurrence of bulging, a method of applying reduction at the solidification completion point, and the like have been developed.More recently, after bulging has been actively generated during continuous casting, the reduction is reduced. In addition, a method has been proposed in which the bulging is pushed back.

Conventionally, the countermeasures against center segregation of continuous cast materials are to crack the central part of the plate thickness at the time of welding of a thick steel plate or the like, for example, when the steel plate is used for line pipes and tanks in an acidic environment (sour environment) containing H ₂ S. Hydrogen-induced cracking (H
Various studies have been made on IC). This HIC is a crack that occurs during use of a steel sheet having a very low level of hydrogen in the steel after shipment from the factory. The HIC is caused by the corrosion of the surface or the like and the hydrogen that has entered the steel. The amount of hydrogen outside the site is negligible. In contrast,
The target here is a defect detected by ultrasonic flaw detection found after rolling a steel sheet, the defect generation position is concentrated at the center segregation position, and it is common in that hydrogen is involved However, the amount of hydrogen in the steel at the time of the occurrence of the defect is much larger. Therefore, it is necessary to take measures from a viewpoint different from the conventional center segregation.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to produce a slab by continuous casting and roll it into a thick steel plate. The present invention provides a steel sheet that does not require a dehydrogenation treatment, and a method for producing the same.In particular, a direct charge method in which a slab is put into a heating furnace without cooling the slab and the temperature is adjusted and then rolled, or the slab is directly used as it is. An object of the present invention is to prevent hydrogen-based defects in a steel sheet manufactured by a direct rolling method of rolling.

[0009]

Means for Solving the Problems The present inventor has proposed a measure for suppressing the occurrence of a defect which is generated in a thick plate and is detected by ultrasonic flaw detection which is considered to be based on hydrogen, that is, a hydrogen defect. Various studies were conducted. Defects detected by this ultrasonic flaw detection hardly occur when the slab is cooled once after continuous casting, then reheated and rolled, and the direct charge method in which the slab is heated immediately after casting and rolled is common. This occurs particularly when cooling is performed at a relatively high speed such as cooling after the completion of rolling. Therefore, since it is presumed that hydrogen is involved, the amount of hydrogen in the steel sheet and the state of the occurrence of defects were investigated, and it was found that the frequency of the occurrence of defects increased rapidly when the concentration exceeded about 2 ppm. This hydrogen, which could not be sufficiently reduced by the degassing treatment of the molten steel, was presumed to have been brought to the steel sheet as it was, and it was thought that it could be eliminated if the degassing treatment was performed reliably. However, in ordinary molten steel degassing, it can be easily reduced to about 1 to 2 ppm, but if it is surely reduced to less than 1 ppm, especially if it is tried to lower it to less than 1 ppm, degassing treatment The time must be greatly increased, and the process management and the cost increase due to the increase in the processing time become problems. From a different point of view, this means that even if it contains about 1 to 2 ppm hydrogen,
If the generation of hydrogen defects can be sufficiently suppressed, the burden on the degassing process can be reduced, and the processing time should be rationalized.

[0010] When the location of the hydrogen defect was investigated, almost all of them were found to be located at the center of the plate thickness t, that is, 1/2.
It was found that it was found near the position of t. It was also apparent that the place of occurrence was a so-called center segregation part where the Mn segregation degree of high Mn concentration, that is, the concentration ratio to ladle analysis value greatly exceeded 1. Therefore, if the portion having a high Mn concentration disappeared, it was thought that the hydrogen concentration of the steel sheet at which cracking began to occur, that is, the critical hydrogen concentration at which cracking occurred, could be increased. Thus, a slab was produced by continuous casting in which the degree of Mn segregation at the center of the sheet thickness did not exceed 1.0, and the occurrence of hydrogen defects was examined. However, the effect was not always clear.

According to various examination results on HIC,
It is said that inclusions such as MnS are the starting points of crack generation. Since inclusions must be a starting point of crack generation even in this hydrogen defect, the relationship between the Mn content and the S content and the hydrogen defect generation was further examined. As a result, M
Even when the degree of n segregation does not exceed 1.0, the product of the amount of Mn and the amount of S (hereinafter, the content represented by weight% is expressed as [Mn] or [S], and the product is [Mn] × [S ] Exceeds 0.005), hydrogen defects appear, and if [Mn] × [S] is 0.005 or less and the Mn segregation degree is 1.0 or less, about 1-2 ppm of hydrogen is contained. Even so, it became clear that no hydrogen defects were generated.

[0012] Hydrogen defects occur during the cooling process after the completion of hot rolling, when the cooling rate is relatively high, and when the temperature decreases without diffusion of hydrogen to the steel surface to sufficiently reduce the concentration in the steel, It is presumed that this becomes a nucleus around inclusions such as MnS and hydrogen gasifies to locally increase the pressure, causing cracks to occur in hard and brittle portions where Mn and the like are segregated. When the content of Mn and S in steel is high,
That is, when [Mn] × [S] is large, MnS is formed during the process of solidification of the molten steel, becomes an inclusion, is elongated by hot rolling, and becomes a core of gasification of hydrogen.

On the other hand, when [Mn] × [S] is small, it is in a solid solution state at the stage of solidification, and gradually becomes finer as the temperature decreases. [M
The fact that n] × [S] is 0.005 or less is presumed to be a concentration at which this MnS does not become a large inclusion but becomes a fine precipitate and is dispersed in the steel. Since it does not become a large inclusion, it does not become a nucleus for gasification of hydrogen and generation of cracks is suppressed. It is considered that hydrogen-induced defects cannot be suppressed only by eliminating center segregation and reducing the degree of Mn segregation to 1.0 or less.

As a method for reducing the center segregation, there is a method in which a reduction is applied immediately before the solidification of the slab to complete the molten steel in the central part of the thickness, in which the components are concentrated. As a method to further increase the amount of molten steel to be pushed back, positively perform bulging to increase the thickness of the slab in the process of developing a surface solidification layer during casting, reduce the thickness by rolling down near the solidification end point, A technique has been developed in which the reduction is stopped when the total thickness of the solidified layer reaches the length of the short side of the original mold. In this way, by pushing back the component-concentrated molten steel in the central portion, the degree of Mn segregation in the central portion of the slab thickness or in the vicinity of 1/2 thickness of the slab can be reduced to 1.0 or less.
However, the development of the solidified layer is not always uniform,
In particular, the development of the solidified layer is slower at the center in the width direction than at the edge, and when the slab is pressed down to the length of the short side of the mold initially, the concentrated steel is removed from the portion close to the edge. ,
It may remain at the center of the width. Once formed, the solidified layer is difficult to deform, so it becomes uneven in the width direction,
Although segregation is reduced, a part where the degree of segregation of Mn exceeds 1.0 occurs. As the concentration of Mn increases,
If the amount of S is the same, [Mn] × [S] is increased in that portion, MnS inclusions are generated, and a hydrogen defect appears.

Therefore, a casting method in which a region where the degree of segregation of Mn exceeds 1.0 does not appear in the center of the plate thickness over the widest possible range of the plate width was studied. As a result, when bulging is performed after the solidified layer has developed to some extent, and then the reduction is performed at a stage earlier than the completion of the solidification and under the condition of applying the large reduction in the shortest possible time, the degree of segregation of Mn is reduced.
It was found that an area of 1.0 or less was reliably obtained, and that the area was obtained over a range close to the entire width in the plate width direction. The reason for such a result is not always clear, but by delaying the timing of the start of bulging, the unsolidified region will be sufficiently widened in the width direction,
Non-uniformity in the width direction is reduced, and the amount of movement of the component-enriched molten steel is increased by short-time large pressure, and the degree of segregation is reliably 1.
It is thought that it will be reduced to 0 or less. However, due to the increase in the area where the segregation degree is 1.0 near the center of the sheet thickness, a part where the segregation degree slightly exceeds 1.0 may occur in parts other than the center part, and in the later casting compared to the slab at the beginning of casting. The average Mn content of the pieces tended to increase somewhat, but it was confirmed that each of them was within the range of variation.

Based on the above-described examination results, the conditions and the limits of the range were further confirmed, and the present invention was completed. The gist of the present invention is as follows.

(1) Assuming that the thickness of the steel sheet is t, a region having a thickness of 0.20 t or less in which the degree of segregation of Mn is 1.0 or less is 1% of the sheet thickness.
The content of Mn and S in the range of ± 0.2t centered on the / 2t position and represented by wt% of the steel composition is Mn (%) × S (%) ≦ 0.005. Steel plate.

Here, the range of ± 0.2t is defined as
With the center of the plate thickness set to 0, the upper and lower parts are portions up to 0.2t.

(2) Continuous casting of slabs in which bulging is caused by unsolidified slabs in the center during casting, and then reduced to the short side length of the original mold using a reduction roll provided in a casting line. In the method, bulging is caused in the range of 0.5 to 0.7 of the center solid phase ratio so that the maximum thickness of the slab is 110 to 150% of the length of the short side of the mold. ~
In the range of 0.8, a reduction of at least 10% of the length of the short side of the mold per pair of reduction rolls is applied to obtain a slab that becomes a rolled material of a thick steel plate. Steel plate manufacturing method.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS When the segregation degree of Mn exceeds 1.0,
At that location, inclusions of MnS are likely to be present and harden, so that hydrogen defects are likely to occur. Normally, slabs produced by continuous casting show a tendency of segregation at the center, and there is a portion near the center in the thickness direction where the degree of segregation with a high Mn concentration exceeds 1.0. And exists in a position near the center of the thickness of the steel sheet in a state of being spread in the width and length directions parallel to the sheet surface. In order to suppress the hydrogen defect, there is no portion having a high Mn concentration, but it is necessary that the Mn concentration in this portion is equal to or lower than the upper and lower portions. However, Mn is an element that plays an important role in the strength of steel by coexisting with C,
An increase in the portion of the Mn concentration having a low segregation degree of 1.0 or less is likely to make it impossible to secure the strength of the steel sheet.
The thickness of this region shall be within 0.2t. The thickness of the region having a segregation degree of 1.0 or less may be 0t as long as there is substantially no portion where the segregation degree exceeds 1.0 near the center of the thickness of the steel sheet. In addition, the position of the region where the segregation degree is 1.0 or less may not always coincide with the center position of the plate thickness due to the structure of the casting equipment, but since the center position of the plate thickness is relatively easily affected by hydrogen, The region where the segregation degree is 1.0 or less needs to be within a range of ± 0.2 t centered on the position of 1/2 t of the plate thickness, that is, substantially in the center of the plate thickness.

FIG. 1 shows the results obtained by examining the degree of segregation of Mn at the center of the sheet thickness, the hydrogen concentration of the steel sheet, and the presence or absence of hydrogen defects in the steel sheet. In this case, the slab is left to cool after hot rolling by hot charging, thereby making it easier to cause hydrogen defects. As can be seen from FIG. 1, when the segregation degree of Mn exceeds 1.0, a hydrogen defect occurs in any case where the hydrogen concentration is high. When the segregation degree is 1.0 or less, a hydrogen concentration is in the range of 1 to 2 ppm, and even if the hydrogen concentration exceeds 2 ppm, ones having no hydrogen defect appear. Next, when the relationship between the hydrogen concentration and the hydrogen defect with respect to [Mn] × [S] was examined for those having a Mn segregation degree of 1.0 or less, the results shown in FIG. 2 were obtained. From this, the Mn segregation degree is set to 1.0 or less, and 2pp
To suppress hydrogen defects at a hydrogen concentration of about m
It can be seen that n] × [S] should be set to 0.005 or less.

That is, according to the present invention, the region having a segregation degree of Mn of 1.0 or less and a thickness of 0.20 t or less extending parallel to the plate surface is within a range of ± 0.2 t centered on a half position of the plate thickness t. Mn (%) × S
(%) ≦ 0.005. This steel plate, in particular, without cooling the slab after casting, in the manufacturing method of rolling immediately or rolling into a heating furnace for rolling,
Generation of hydrogen defects can be suppressed. Note that Mn
A region having a segregation degree of 1.0 or less and a thickness of 0.20 t or less does not need to exist over the entire width of the sheet, and if it is 80% or more of the sheet width, a steel sheet without hydrogen defects can be obtained.

The steel sheet of the present invention has an Mn content and an S content of [M
n] × [S] ≦ 0.005,
The composition is not particularly limited, but when a steel sheet is actually produced, the chemical composition preferably has the following range.

C is an important element that determines the strength of steel, and it must be contained in an amount of 0.05% or more in ordinary applications. However, if it is too much, the weldability, which is essential for a thick plate, is deteriorated. However, the content is preferably 0.2% or less.

Mn is an element essential for suppressing hot embrittlement by S, and is an element that improves the strength by being contained together with C. Therefore, Mn is contained for these purposes.
The range is preferably 0.2 to 2.5%, but if it is less than 0.2%, the effect is not obtained, and if it exceeds 2.5%, the toughness of the steel sheet is deteriorated.

Si is contained for the purpose of deoxidizing molten steel. For that purpose, the addition of 0.05% or more is necessary, but if it is increased, the surface properties of the steel sheet are deteriorated. Therefore, the upper limit content is set to 0.60%.

P is one of the unavoidable impurities of steel and degrades the toughness of steel, so the smaller the content, the better. The range in which the adverse effect is not remarkable is desirably at most 0.20% or less.

S is one of the unavoidable impurities of steel and degrades the toughness and other properties of the steel. The presence of Mn reduces its adverse effects, but increasing it increases MnS inclusions, so 0.01%
The following is assumed. [Mn] × [S] is 0.
005% or less.

Al is contained for deoxidizing molten steel. It is also effective in reducing the grain size of steel. To get this effect,
At least 0.001% or more must be contained. However, even if a large amount is added, the effect is saturated and only the cost is increased, so it is preferable to set the content to 0.20% or less.

Cu, Ni, Cr and Mo are elements that improve the strength of the steel by improving the structure by heat treatment, and do not need to be added, but may be contained up to 1.0% as necessary. If the content exceeds 1.0%, the toughness is degraded, and the effect is saturated, resulting in cost increase. Note that Cu has an effect of improving corrosion resistance in addition to improving strength.

Nb, V and Ti have the effect of refining the crystal grains and strengthening the precipitation with a small amount of addition, and need not be added. However, if such an effect is desired, 0.10% or less is required. It is contained in the range. If the content exceeds 0.10%, the toughness of the steel may be deteriorated.

A portion where the degree of segregation of Mn is 1.0 or less is formed at the center of the thickness of the slab.
In order to manufacture a steel sheet having a region with a thickness of not more than t and not having a high Mn concentration, when producing a slab as a material of a thick plate by a continuous casting method, bulging is actively generated, and then Use the method of rolling down. In this case, when the solid fraction in the center of the slab is in the range of 0.5 to 0.7, bulging occurs, and the maximum thickness of the slab is 110 to 150% of the short side length of the mold. Thereafter, when the solid fraction of the slab central portion becomes 0.7 to 0.8, the slab is lowered to return the slab to the same thickness as the short side length of the original mold. The amount of reduction per hit shall be 10% or more of the short side length of the mold.

When the bulging is caused, the solid phase ratio in the central portion of the slab is set in the range of 0.5 to 0.7 because the solidified layer is developed to some extent and then the interval between the molten steel portions between the solidified layers is increased. The reason for this is that a region having a segregation degree of 1.0 or less is as uniform as possible in the width direction of the plate and the range in the width direction is as large as possible. When the solid fraction in the central portion is less than 0.5, the thickness of the region where the degree of segregation is 1.0 or less becomes uneven, and there is a possibility that a portion exceeding 1.0 may appear. The solid fraction in the center is
If bulging occurs after exceeding 0.7, bulging does not sufficiently occur, and the degree of segregation exceeds 1.0. Therefore, bulging occurs when the solid fraction at the center is in the range of 0.5 to 0.7. The bulging thickness is 11
If it is less than 0%, the Mn segregation degree cannot be reduced to 1.0 or less. In this case, it is preferable that the bulging is large. However, if the bulging exceeds 150%, there is a risk of side breakout, and more rolling force is required to reduce the pressure to the original thickness. Therefore, the short side length of the mold
110% to 150%.

After the bulging occurs, when the solid fraction in the center becomes 0.7 to 0.8, the slab is reduced to return the cast slab to the same thickness as the short side length of the original mold. This is because if it is less than 0.7, the Mn segregation degree cannot be reduced to 1.0 or less, and if it exceeds 0.8, the solidified layer becomes too thick, and it cannot be reduced to the short side length of the original mold. The reason why the amount of reduction per pair of reduction rolls is set to 10% or more of the length of the short side of the mold during the reduction is to cause the reduction rapidly and to push back the molten steel with the concentrated components as effectively as possible. If the amount of reduction per pair of reduction rolls is less than this, the formation of a region below the Mn segregation degree of 1.0 becomes insufficient.

[0035]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A continuous casting apparatus provided with a rolling roll in which guide rolls arranged in a drawing direction from immediately below a mold are variable in a thickness direction of a slab and can reduce the thickness of a slab near a solidification completion point. , A steel slab having the composition shown in Table 1 was cast. The short side of the mold is 235 mm and the long side is 2260 mm. In the casting, the thickness of the mold was first set, a solidified layer was formed, bulging was caused, the solidified layer was further developed, and then a reduction was applied by a pair of rolls to the short side thickness of the original mold. Table 2 shows the solid phase ratio at the start of bulging, the bulging thickness, the solid phase ratio at the start of bulging reduction, and the like. The obtained slab was immediately put into a heating furnace, soaked at 1200 ° C., taken out of the furnace, subjected to hot rolling, finished at 40 ° C. at about 900 ° C., and allowed to cool.

[0036]

[Table 1]

[0037]

[Table 2]

Each steel number was examined for a steel plate having a length of 10 m, which was collected from a position substantially corresponding to the center of the charge. The segregation degree of Mn and the hydrogen concentration are 1 in the width direction.
At the / 4 position, analysis was performed at three locations: the head, center, and tail of the steel plate to be inspected, and the results were averaged. Ultrasonic flaw detection was performed on the entire surface of the steel sheet to be inspected, and when even one hydrogen defect was found, it was determined that there was a defect. Table 2 also shows these results.

As is clear from the examination results of the chemical composition in Table 1 and the steel sheet in Table 2, the test numbers in which [Mn] × [S] is 0.005 or less and the degree of Mn segregation at the center of the sheet thickness is 1 or less. No hydrogen defects were found in any of the steel sheets 1, 2, and 5. In contrast, Test Nos. 3 and 4 indicate [M
n] × [S] exceeds 0.005, the hydrogen concentration of the steel sheet is lower than that of the steel sheets of other test numbers, and hydrogen defects are generated even though the Mn segregation degree is 1 or less. In Test Nos. 6 to 10, the conditions of bulging or bulging pressure deviated from the scope of the present invention, and the Mn segregation degree at the center of the plate thickness exceeded 1 in all cases. For this reason, [Mn] ×
Although [S] is 0.005 or less, hydrogen deficiency has occurred.

[0040]

The steel plate according to the present invention is a steel plate in which hydrogen defects are unlikely to occur during production. When a slab is manufactured by continuous casting and rolled into a thick steel plate, the slab or the rolled steel plate is used. The steel sheet immediately after the steel sheet does not necessarily require dehydrogenation treatment such as slow cooling and heat retention. In recent years, in order to save energy and simplify the process, in recent years, the direct charging method, in which a slab after continuous casting is put into a heating furnace without cooling and the temperature is adjusted and then rolled, or the direct rolling method in which the slab is immediately rolled as it is, without cooling, has been adopted. Although it is increasingly used, it is effective in suppressing hydrogen-induced defects in the steel sheet, which are likely to occur in such cases.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the degree of Mn segregation of a steel sheet and the hydrogen concentration of the steel sheet, and the presence or absence of hydrogen defects in the steel sheet.

FIG. 2 is a diagram showing the relationship between the product of the Mn concentration (%) and the S concentration (%) of steel, the hydrogen concentration of the steel sheet, and the presence or absence of hydrogen defects in the steel sheet.

Continuation of front page (56) References JP-A-9-314298 (JP, A) JP-A-9-57410 (JP, A) JP-A-4-305350 (JP, A) JP-A-10-244349 (JP) JP-A-11-179489 (JP, A) JP-A-11-61328 (JP, A) JP-A-9-253703 (JP, A) JP-A-6-179910 (JP, A) 63-20142 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/16 B22D 11/128 350 B22D 11/20 C22C 38/00 301

Claims (2)

(57) [Claims] When the thickness of a steel sheet is represented by t, a region having a thickness of 0.20 t or less in which the degree of segregation of Mn is 1.0 or less is ｔ t of the sheet thickness.
A thick steel plate which is present within a range of ± 0.2 t centered on the position, and wherein the content of Mn and S expressed by weight% of the steel composition is Mn (%) × S (%) ≦ 0.005. . 2. A continuous casting method for casting slabs in which bulging is caused in a slab that is not solidified in the center during casting, and then reduced to the short side length of the original mold by using a reduction roll provided in a casting line. In, the bulging occurs in the center solid phase ratio in the range of 0.5 to 0.7, the maximum thickness of the slab is set to 11 of the short side length of the mold.
0 to 150%, and then, when the solid phase ratio in the center of the slab is in the range of 0.7 to 0.8, the length of the short side of the mold is 10 per pair of reduction rolls.
The method for producing a thick steel plate according to claim 1, wherein a slab that is a rolling material of the thick steel plate is formed by applying a reduction of at least%.