JP3058079B2 - Steel continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting steel, and more particularly to a method for continuously casting steel in which surface cracks such as lateral cracks and lateral cracks in slabs are prevented.

[0002]

2. Description of the Related Art In recent years, the necessity of improving the orthogonality of continuous cast slabs has been increasing from the viewpoint of reducing the production cost of steel products. There is a problem of lateral cracking or surface cracking called lateral cracking.

Recently, Nb, V,
The production of low-alloy steel containing various alloying elements such as Ni and Cu is increasing, but with the addition of these alloying elements, the frequency of surface cracks in continuous cast slabs has increased, leading to lower production costs. In response to the demand for reduction, the achievement rate has been at a standstill.

[0004] These surface cracks cause the surface temperature of the slab to be close to the γ → α transformation temperature (about 750 to 850 ° C) at which the hot ductility decreases during the secondary cooling in continuous casting. It is known that this occurs due to mechanical stress such as slab correction.

[0005] Therefore, a method of avoiding the slab surface temperature in the slab bending portion or the slab straightening portion to a lower or higher temperature side than the above-mentioned region where the hot ductility is reduced (hereinafter referred to as embrittlement temperature region) is usually employed. Has been adopted. However, it is difficult to eliminate surface cracks only by avoiding the brittle temperature range of the slab temperature, and several techniques relating to the slab cooling history focusing on the slab surface layer structure have been disclosed.

For example, Japanese Patent Publication No. 58-3790 discloses that the upper surface of a secondary cooling zone is forcibly cooled to reduce the surface temperature of a slab to 650 ° C.
A method is disclosed in which a γ → α transformation is carried out by cooling to 750 ° C., followed by a gradual recuperation to avoid a slab surface temperature in a slab correction section below the brittle temperature range.

Further, Japanese Patent Application Laid-Open No. 58-224054,
Japanese Patent No. 224055 discloses a heat history limited to both corners of a slab, but the slab surface temperature is 750 to 750 to just below the mold.
After cooling to 900 ° C to improve the surface layer structure (there is a description of refining γ grains), cool the slab surface at the slab bending section and slab correction section to 800 ° C or higher. Is disclosed.

[0008] As described above, several methods for preventing slab surface cracks have been proposed so far, and although they are effective for ordinary steel containing no alloying element, Ni, Cu, V, Nb
However, the casting of low alloy steels containing the same still has the above-mentioned drawbacks, and no effective solution to the slab surface crack has yet been found.

[0009]

SUMMARY OF THE INVENTION The object of the present invention is to provide not only ordinary steel but also low alloy steel containing Ni, Cu, V, Nb, etc., which have a high surface cracking susceptibility which has been increasing in recent years. An object of the present invention is to provide a continuous casting method capable of effectively avoiding a slab surface crack even in continuous casting.

[0010]

Means for Solving the Problems By the way, as described above,
In low alloy steel containing Ni, Cu, Nb, V, etc., the embrittlement temperature range shifts to the low temperature side, the cooling characteristics change due to scale change due to alloying element addition, etc., in the bent part, straightening part The temperature has to be avoided on the high temperature side. For example, when a small amount of Ni is contained, a layer called a sub-scale is likely to adhere to the slab surface in some places, and uneven cooling is likely to occur in a portion where a sub-scale is present and in a portion where no sub-scale is present. In the method of avoiding the slab surface temperature at a lower temperature side than the embrittlement temperature range in the bending portion and the straightening portion, this uneven cooling is promoted, and the method disclosed in the above-mentioned Japanese Patent Publication No. 58-3790 is disclosed. Has proven difficult to apply to low alloy steels.

Further, Japanese Patent Application Laid-Open Nos. 58-224054 and 58-24054
In the method disclosed in Japanese Patent No. 224055, in the case of the low-alloy steel as described above, the viewpoint of improving the slab surface layer structure not only at the slab corner but also at the entire width of the slab is necessary. When a reproduction test was performed based on the specification of the above publication, the slab surface temperature at the bent portion and the straightening portion was set to 850 ° C. or more to prevent surface cracking. It has been found that it cannot be realized unless the cooling conditions are devised.

Therefore, the present inventor focused on improving the surface structure of the slab due to the cooling history and avoiding the slab surface temperature in the bent portion and the straightened portion on the high temperature side of the embrittlement temperature range. Such a study was conducted.

FIG. 1 is a graph showing the high-temperature ductility of a Ni-containing steel having a high frequency of occurrence of slab surface cracks in an actual production line together with the steel composition. "%" On the graph in the figure
The amount indicates the Ni content. In this example, the embrittlement temperature range moves to a lower temperature side than that of ordinary steel by containing several percent of alloys such as Ni. (The embrittlement temperature range of ordinary steel is 750 to 850 ° C, It is 600-850 ° C for Ni-containing steel). Furthermore, it has already been known that the cooling characteristics change due to the change in the properties of the oxide scale on the slab surface layer, and that the cooling tends to be uneven at a temperature lower than the embrittlement temperature range.

Therefore, when such low alloy steel is continuously cast, the temperature of the slab is higher than the embrittlement temperature range, that is, 850 ° C., in the bent portion and the straightening portion where the slab is subjected to a large stress.
It is necessary to avoid this.

However, based on the actual results of casting low-alloy steels, it was impossible to eliminate surface cracks by simply setting the slab surface temperature higher than the brittle temperature range. . Even if the slab surface temperature at the bent portion and the straightening portion is avoided to be higher than the embrittlement temperature range, the surface layer structure of the slab having surface cracks is almost always shown in FIG. 2 (a). Thus, the prior austenite grain boundaries had a clear structure with high crack susceptibility, and the cracks occurred along the prior austenite grain boundaries. Based on such findings, FIG.
It is presumed that the unclear surface structure of the prior austenite grain boundary as shown in (b) is effective in dulling the surface cracking susceptibility.

On the other hand, the slab surface layer structure is closely related to the slab heat history, and if an appropriate heat history is selected, the crack susceptibility of the slab surface layer structure may be reduced. Therefore, the correlation between the heat history of the slab and the surface structure of the slab was investigated by a 200 kg scale molten steel experiment. The molten steel composition was 0.7% Ni steel. The experiment is
After casting molten steel in a 400 × 400 × 200 mm static mold, the cast piece was taken out of the mold before complete solidification, and the cast piece was cooled by mist spraying. The temperature history of the surface of the slab was previously measured by using a thermocouple or a radiation thermometer that had been previously cast, and the surface structure of the slab was investigated.

FIG. 3 shows the correlation between various cooling conditions and the surface structure of the slab. Here, the cooling conditions at that time were changed according to the time t during which secondary cooling was performed after the slab was released from the mold, and the temperature Tc at which the surface of the slab dropped due to the secondary cooling. The surface layer structure of the slab was obtained when the old γ grain boundary was clear in the ferrite pearlite structure, when the old γ grain boundary was unclear in the ferrite pearlite structure, and when the secondary cooling was not strong enough to recover the heat. The bainite structure was classified into three types.

As can be seen from FIG. 3, the structure having high crack susceptibility has a secondary cooling time of 2 minutes regardless of the cooling time when Tc is equal to or more than the Ar ₃ point of steel and even when Tc is equal to or less than the Ar ₃ point. Occurs when longer. Although the structure of No. 1 is considered to have low cracking susceptibility, the cooling time is within 2 minutes and the Tc
Is formed when the temperature is below 600 ° C. and below the Ar ₃ point (γ → γ + α transformation temperature). The structure was formed when Tc was below 600 ° C. However, in the case of such a structure, the slab surface temperature is too low, so that the slab temperature is likely to be uneven, which is not preferable. The most preferred of the structures is a structure in which the γ grain boundaries are unclear. In order to obtain such a structure, for example, cooling may be performed at a water volume density of 0.025 to 0.090 L / cm ² / min immediately below the mold, which corresponds to a cooling water volume which is twice or more that of ordinary cooling.

Based on the above findings, the correlation between the cooling history, the slab surface layer structure, and the state of occurrence of slab surface cracks was investigated using an actual continuous casting apparatus. FIG. 4 shows the thermal history of the continuously cast slab while comparing the cooling pattern according to the present invention with the cooling pattern of the conventional method. From the heat history, the minimum temperature Tm, the bending part temperature Tb (in the case of a vertical bending type continuous casting machine), and the correction part temperature Tu, which are reached by cooling from immediately below the mold, are obtained to obtain the surface structure of the slab and the slab surface. The relationship between cracks was determined. The cast slab that has exited the mold is immediately subjected to secondary cooling, and then is drawn out of the continuous casting machine through a bending section and a straightening section. Cooling curves A to E in the figure
Represents a cooling curve in a case described later.

FIG. 5 shows the relationship between the surface temperature of each slab of Tm and Tb, the surface structure of the slab, and the surface cracks generated on the slab ground side.
The relationship between Tm, Tu, the surface structure of the slab, and the surface cracks generated on the top side of the slab is shown below. In each case, the molten steel composition is
It was 0.7% Ni-0.01% Nb steel. 5 and 6, the occurrence of the surface layer structure of the slab is almost the same as the result of the basic test in FIG. 3, and the structure in which the γ grain boundaries predicted to have low cracking susceptibility in the basic test are unclear is directly below the mold. 600 ° C ≦ Tm within 2 minutes at
Generated when <Ar ₃ points are reached.

However, when the strong cooling immediately below the mold was less than 0.5 minutes, the surface temperature of the slab did not become lower than the Ar ₃ point, and a structure in which the γ grain boundaries were unclear was not formed. This is because a cooling capacity of lowering the slab surface temperature to the Ar ₃ point or less cannot be secured by cooling for less than 0.5 minutes.

The ground side slab surface cracks were eliminated when the old γ grain boundary had an unclear structure (Tm <Ar ₃ points) and Tb ≧ 850 ° C. Even if the former γ grain boundary is unclear, there is no surface cracking when Tb <850 ° C.
Even at ≧ 850 ° C., there was no surface cracking in the structure with clear old γ grain boundaries (see FIG. 5). In Similarly old γ grain boundaries indistinct organization for top-side surface cracks (Tm <Ar ₃ point), Tu
It disappeared when ≧ 850 ° C. (see FIG. 6).

As can be understood from the results shown in FIGS. 5 and 6, both the improvement of the surface layer structure to the former unclear γ grain boundary structure and the avoidance of the high temperature side of the slab surface temperature in the bent portion and the straightening portion are compatible. By doing so, surface cracks can be eliminated. However, even if the surface layer structure is improved to such an unclear structure, if the surface temperature of the slab at the bent portion and the straightening portion falls within the brittle temperature range, surface cracking occurs. This is because both the improvement of the surface layer structure and the avoidance on the high-temperature side are thermally contradictory actions, so that it is difficult to achieve both when overlapping with various restrictions in an actual continuous casting apparatus.

The restrictions in such an actual continuous casting apparatus include, for example: a) A vertical bending type continuous casting machine for slabs generally has a vertical portion of 2-3 m and a mold length of 0.7-0.7 m. Considering that it is 0.9 m, the temperature of the bending part and the temperature of the straightening part ≧
In order to achieve 850 ° C., it is necessary to cool the slab temperature to less than ₃ points of Ar in a very short range immediately below the mold. B) The curved continuous casting machine for slabs is required to float inclusions. From the viewpoint, low-speed casting operation is generally performed, and in this case, too, it is necessary to cool the slab temperature to less than the Ar ₃ point in a very short range in order to achieve the straightening section temperature ≥850 ° C. And the like.

Therefore, the present inventor further studied a method for reliably obtaining a heat history for eliminating the surface crack even under such various practical constraints. In the primary cooling and the secondary cooling using the mold, the secondary cooling has a higher cooling capacity. Further, the recuperation ability of the slab is higher in the early stage of casting having a large heat capacity. From these facts, in order to secure sufficient recuperation ability of the slab by strong cooling directly below the mold, it is effective to start strong cooling early in the region where the solidified shell thickness is small and finish strong cooling early. Was reached.

FIG. 7 shows a condition of 600 ° C. ≦ Tm <Ar ₃ points of a molten steel similar to that of FIGS. Of the solidified shell thickness and bend to start secondary cooling
The relationship of the surface temperature Tb at the vertical portion (3 m in this example) is shown. In this case, however, the cooling time from the time when the mold is released to the time when the temperature reaches Tm is 1 minute. In a preferred embodiment, the shell thickness at the mold outlet may be adjusted by adjusting the cooling capacity of the mold, specifically, by changing the amount of cooling water.

By setting the thickness of the solidified shell for starting the secondary cooling to 15 mm or less in this way, recuperation capability is ensured (early secondary cooling starts, early strong cooling ends), and the formation and bending of the γ grain obscured structure It is easy to achieve compatibility of the unit temperature Tb ≧ 850 ° C.

However, in this case, if the solidified shell thickness is less than 10 mm, operation troubles such as breakouts occur frequently, which is not preferable. The above knowledge can be similarly applied to the straightening temperature of the vertical bending type continuous casting machine and the bending type continuous casting machine.

Here, the heat history of the present invention is shown as a cooling curve A in FIG. It is desirable that this condition be satisfied at all circumferential positions on the surface of the slab. The shell thickness for starting the secondary cooling, Tm, the time from leaving the mold to reaching Tm, etc. are determined by the type of the continuous caster, the profile of the continuous caster (bend position, straightening position), casting conditions (Vc, (Steel temperature).

Here, the present invention is as follows. (1) A continuous casting method characterized by cooling a slab under the following conditions when producing a slab of steel using a vertical bending type continuous casting machine.

When the thickness of the solidified shell is 10 mm or more and 15 mm or less, the primary cooling by the mold is finished, and the secondary cooling is started. The surface temperature of the entire slab is once lowered to at least 600 ° C. and at most Ar ₃ points within 2 minutes after leaving the mold. Secondary cooling is performed so that both the slab surface temperature in the bent portion and the slab surface temperature in the straightening portion are 850 ° C. or more.

(2) A continuous casting method characterized by cooling a slab under the following conditions when producing a steel slab using a curved continuous caster.

When the thickness of the solidified shell is 10 mm or more and 15 mm or less, the primary cooling by the mold is finished, and the secondary cooling is started. The surface temperature of the entire slab is once lowered to at least 600 ° C. and at most Ar ₃ points within 2 minutes after leaving the mold. Secondary cooling is performed so that the slab surface temperature in the straightening section becomes 850 ° C. or higher.

According to the present invention, the surface temperature of the slab in the bent portion and the straightening portion is controlled to at least 850 ° C., but the upper limit of the surface temperature of the slab in the bent portion and the straightening portion is limited by the heat transfer. There are restrictions. Therefore, in the preferred embodiment of the present invention, the upper limit temperature is 1300 ° C. in the bent portion and 1200 ° C. in the straightening portion.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described together with their functions. The present invention is not limited to ordinary steel, Nb,
It is intended for continuous casting of low alloy steel containing various alloying elements such as V, Ni, Cu, and the like, and the mechanism for preventing lateral cracking and lateral cracking of a slab according to the present invention is as follows.

FIG. 4 is a graph showing the cooling pattern of the continuous cast slab according to the present invention, that is, the heat history of the slab surface in comparison with the cooling pattern of the conventional method. Here, the cooling curve A
In the present invention, B is a cooling pattern and a cooling curve C for gradually lowering the surface temperature of the slab without strong cooling directly below the mold to avoid the temperature of the bent portion and the straightened portion on the high temperature side of the embrittlement temperature range. Is a cooling pattern in which strong cooling is performed immediately below the mold in the same manner as in the present invention, and then re-heated. If secondary cooling is started from a place where the shell thickness is 15 mm or more, the cooling curve D indicates strong cooling immediately below the mold. The cooling pattern and cooling curve E for avoiding the slab surface temperature to the lower temperature side of the embrittlement temperature range in the bent part and straightening part
Is a cooling pattern in which cooling is performed more strongly than in the case of D to transform the surface structure into bainite.

According to the present invention, as shown by the cooling curve A in FIG. 4, secondary cooling start position is started from the position of the shell thickness 10-15 mm, also, 600 ° C. or higher within 2 minutes Ar ₃ Cool below the point. As a result, the surface layer structure of the slab becomes a ferrite pearlite structure with low susceptibility to cracking and indistinct γ grain boundaries. The strong cooling immediately below the mold starts early and ends early, so the slab has sufficient recuperation ability, and it is possible to avoid the high temperature side of the embrittlement zone in the bent part and straightening part (850 ° C or more) . As a result, it is possible to achieve both the reduction of the crack sensitivity of the surface structure of the mold and the avoidance of the high temperature side of the embrittlement zone in the bent portion and the straightening portion, and it is possible to make almost no slab surface cracks.

Each of the cooling patterns B to E in FIG. 4 has the following disadvantages, and the superiority of the present invention is apparent. Although the cooling curve B avoids the higher side than the embrittlement temperature range in the bent portion and the straightened portion, the surface layer structure of the slab becomes a clear structure of γ grain boundaries having high crack sensitivity, and the surface crack does not decrease. .

The cooling curve C shows that the surface layer structure of the slab becomes indistinct in the γ grain boundary due to the strong cooling immediately below the mold, but the secondary cooling start time is late, and the bending section and the straightening section start from the brittle temperature range. Is not avoided, and surface cracking does not decrease.

The cooling curve D shows that the surface structure of the slab becomes an indistinct structure of the γ grain boundary due to the strong cooling immediately below the mold. As described above, in a low alloy steel, the embrittlement temperature range often shifts to a lower temperature side, which is not always advantageous. Cooling curve E
Temperature tends to be uneven due to extremely strong cooling, and cracks occur due to thermal stress.

The present invention is directed to low alloy steels, especially Ni, V, Cu, Nb.
When applied to the continuous casting of low alloy steel having a total amount of at least one alloy element selected from the group consisting of 2.0% or less,
Surface cracks are prevented more effectively, and the excellent effect is exhibited.

[0042]

EXAMPLE In this example, continuous casting of a slab-shaped slab was performed using a vertical bending type and a bending type continuous casting machine. Table 1 shows the specifications of the continuous casting machine and the casting conditions.

The secondary cooling conditions were variously changed, and the correlation between the temperature history, the surface structure, and the surface cracks on the slab surface was investigated. The temperature history was measured from below the mold at a position 100 mm from the corner where the frequency of surface cracking was high, using a bite type thermocouple.

Table 2 shows the chemical components of the cast steel types. In order to make it easy to understand the evaluation of the correlation between the temperature history, the surface structure, and the state of occurrence of surface cracks, a steel type with a high crack occurrence frequency was selected as the cast steel type.

[0045]

[Table 1]

[0046]

[Table 2]

Table 3 summarizes the heat history, the surface structure of the slab, and the state of occurrence of surface cracks in the examples of the present invention. Table 4 summarizes the heat history, the surface structure of the slab, and the state of occurrence of surface cracks in the comparative example and the conventional example. Was.

In Comparative Examples 1-2 and 20-21, the vertical cooling type (hereinafter referred to as VB type) and the curved type (hereinafter referred to as S type) continuous casters each have a secondary cooling start shell thickness defined by the present invention. This is a smaller example. In this case, since the solidified shell strength was insufficient, breakout occurred at the exit of the mold.

Comparative Examples 3 to 4 and 22 to 23 are VB type, respectively.
This is an example in which the secondary cooling start shell thickness is larger than that of the range specified in the present invention in the S type. In this case, the surface layer of the slab is γ
Despite the microstructure where the susceptibility of the grain boundaries to cracking was low, the secondary cooling was started too late, and the recuperation ability of the slab could not be secured. The brittle temperature range could not be avoided on the high temperature side in the straightening section. As a result, a corner crack having a depth of about 5 mm was generated on the slab top side.

Comparative Examples 5 to 7 were VB molds in which the cooling time immediately below the mold was too short to reduce the surface temperature of the slab to the Ar ₃ point or less as specified in the present invention. It is. In this case, the amount of water for temporarily cooling the slab surface temperature to the Ar ₃ point or less could not be secured, and a structure having a clear susceptibility to cracking at the γ grain boundary was formed in the surface layer. As a result, a slight crack of 2-3 mm in depth was found within 100 mm from the corners on both sides of the slab (hereinafter, near the corners).
The location of the lateral crack near the corner corresponds to the location where dents occur on the slab surface due to the difference in cooling between the short side and the long side. Could not be prevented.

Comparative Examples 24 to 26 are examples in which the slab surface temperature could not be reduced to the range specified in the present invention because the S type was too short to cool strongly under the mold.
In this case as well, a reduced lateral crack having a depth of about 3 mm occurred near the corner due to the structure having high crack susceptibility.

In Comparative Examples 8 to 10, in the VB type, the strong cooling time immediately below the mold was within the range specified in the present invention. However, due to insufficient cooling, the minimum temperature after the strong cooling immediately below the mold did not fall below the Ar ₃ point. This is an example. In this case, Comparative Examples 5 to 7
With the same mechanism as above, slight lateral cracks occurred near the corners on both sides of the slab.

In Comparative Examples 27 to 29, in S type, the strong cooling time immediately below the mold was within the range specified in the present invention. However, due to insufficient cooling, if the minimum temperature immediately after the strong cooling immediately below the mold was lower than the Ar ₃ point. This is an example that did not exist. In this case, Comparative Examples 24-26
With the same mechanism as above, slight lateral cracking occurred near the corner on the slab top side.

Comparative Examples 11 to 13 and 30 to 32 were VB
The minimum temperature after strong cooling directly below the mold for molds and S molds is 600
This is an example in which the temperature is lower than ℃. In either case, VB
A bainite structure was formed on the surface layer regardless of the mold type and the S type, and lateral cracks with a depth of about 20 mm considered to be caused by thermal stress occurred on the entire top and bottom.

Comparative Examples 14 to 16 are examples in which the minimum temperature Tm immediately below the mold in the VB type was within an appropriate range, but the temperature of the bent portion Tb was 850 ° C. or less due to improper selection of the amount of water during reheating. is there. In this case, an unclear structure of the γ grain boundary was formed in the surface layer, but the embrittlement temperature range could not be avoided at the bent portion, and a lateral crack having a depth of about 10 mm occurred almost entirely on the slab side.

Comparative Examples 17 to 19 and 33 to 35 were VB
In this example, the correction section temperature Tu was 850 ° C. or less due to improper selection of the amount of water at the time of slab reheating in the mold and S mold. In this case, an unclear structure of the γ grain boundary was formed in the surface layer, but the embrittlement temperature region could not be avoided in the straightening portion, and a lateral crack having a depth of about 10 mm occurred almost entirely on the slab top side.

In the conventional example 1, the embrittlement temperature range is set to the high temperature side in the bent portion and the straightening portion without vigorously cooling immediately below the mold in the VB mold.
(850 ° C or higher). In this case, despite avoiding the embrittlement temperature range on the high temperature side, the surface layer of the slab had a structure with a clear susceptibility to cracking at the γ grain boundary. Lateral cracks were scattered.

In prior art example 2, after the S-type was cooled strongly just below the mold, it was attempted to avoid the embrittlement temperature range on the low-temperature side at the bending portion and the straightening portion (the method disclosed in Japanese Patent Publication No. 58-3790). Reproduction test). However, lateral cracks with a depth of about 30 mm occurred frequently on the entire surface of the slab. From this, it was estimated that the embrittlement temperature range of the steel types in Table 2 spread considerably to the lower temperature side.

Incidentally, Japanese Patent Application Laid-Open Nos. 58-224054 and 58-22
The cooling conditions corresponding to Japanese Patent No. 4055 are Comparative Examples 3, 4, 15, and 1.
8, 22, 23, and 34, but as described above, at least one condition out of the range of the present invention was present, so that surface cracks occurred.

On the other hand, in Examples 1 to 36 of the present invention, the secondary cooling start shell thickness, the strong cooling time immediately below the mold, and the strong Since the minimum temperature, bending part temperature and straightening part temperature at the time of cooling were appropriate, the reduction of crack sensitivity of the surface layer structure of the slab and the avoidance of the high temperature side of the brittle temperature range in the bent part and straightening part were compatible, and the surface cracking Slabs of good quality without blemishes were obtained.

[0061]

[Table 3]

[0062]

[Table 4]

[0063]

According to the present invention, it is possible to reduce the crack sensitivity of the surface structure of the slab (structure with unclear γ grain boundaries), and to bend and straighten a vertical bending type continuous caster and a straightening part of a curved type continuous caster. And avoidance of the high temperature side of the embrittlement temperature range, and the surface cracks of the continuous cast slab were effectively eliminated. As a result, the cast slab was formed into a North Scarf, the surface was unmaintained, and the straightness ratio of the continuous cast slab was improved, thereby greatly contributing to a reduction in manufacturing cost.

[Brief description of the drawings]

FIG. 1 is a graph showing the hot ductility of a Ni-containing steel together with its steel composition.

FIG. 2 is a conceptual diagram of the surface layer structure of a slab, and FIG. 2 (a) shows a structure in which old austenite grain boundaries are clearer than the surface layer and has higher crack susceptibility, and FIG. It is a schematic diagram which respectively shows the structure | tissue with low susceptibility to cracking.

FIG. 3 is a cooling condition (a time T during which secondary cooling is performed after a slab comes out of a mold, and a temperature T at which a slab surface portion is reduced by the secondary cooling).
It is the graph which calculated | required the relationship between c) and the slab surface layer structure by the experiment of a lab scale.

FIG. 4 is a graph comparing a cooling pattern of a slab according to the present invention with a cooling pattern of a slab according to a conventional method.

FIG. 5: Temperature Tm of cooling by strong cooling just below the mold
FIG. 4 is a diagram showing a relationship between a bending portion temperature Tb, a slab surface layer structure, and a surface crack generated on a slab ground side.

FIG. 6 is a diagram showing a relationship between Tm, a correction portion temperature Tu, a slab surface layer structure, and a surface crack generated on a slab top side.

FIG. 7 is a graph showing a relationship between a solidified shell thickness at which secondary cooling is started and a slab surface temperature Tb at a bent portion.

Continuation of front page (56) References JP-A-2-37941 (JP, A) JP-A-58-224056 (JP, A) JP-A-58-224054 (JP, A) JP-A-61-195761 (JP) JP-A-55-14173 (JP, A) JP-A-53-106335 (JP, A) JP-A-4-305338 (JP, A) JP-A-7-90504 (JP, A) 58-224055 (JP, A) JP-A-7-178526 (JP, A) JP-A-9-47854 (JP, A) JP-A-8-33964 (JP, A) JP-A-8-10919 (JP, A) A) JP-A-8-10920 (JP, A) JP-A-6-87054 (JP, A) JP-A-63-112058 (JP, A) JP-A-7-308743 (JP, A) JP-A-63 JP-A-63559 (JP, A) JP-A-61-193758 (JP, A) JP-A-61-189851 (JP, A) JP-A-61-9952 (JP, A) JP-A-60-56453 (JP, A) JP-A-57-187150 (JP, A) JP-A-3-193253 (JP, A) JP-A-57-121866 (JP, A) JP-A-57-11957 (JP, A) JP-A-61-189851 (JP, A) JP-A-61-195742 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11 / 124 B22D 11/00 B22D 11/22

Claims (2)

(57) [Claims] 1. A continuous casting method characterized by cooling a slab under the following conditions when a steel slab is manufactured by a vertical bending type continuous casting machine. When the thickness of the solidified shell is 10 mm or more and 15 mm or less, the primary cooling by the mold is finished, and the secondary cooling is started. The surface temperature of the entire slab is once lowered to at least 600 ° C. and at most Ar ₃ points within 2 minutes after leaving the mold. Secondary cooling is performed so that both the slab surface temperature in the bent portion and the slab surface temperature in the straightening portion are 850 ° C. or more. 2. A continuous casting method characterized by cooling a slab under the following conditions when producing a steel slab using a curved continuous caster. When the thickness of the solidified shell is 10 mm or more and 15 mm or less, the primary cooling by the mold is finished, and the secondary cooling is started. The surface temperature of the entire slab is once lowered to at least 600 ° C. and at most Ar ₃ points within 2 minutes after leaving the mold. Secondary cooling is performed so that the slab surface temperature in the straightening section becomes 850 ° C. or higher.