JP2008261036A - Method for cooling continuously cast bloom - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively prevent formation of surface cracks in continuously cast bloom when the bloom is rolled. <P>SOLUTION: The method is one comprising cutting a continuously cast bloom 1 to a specified length and cooling it outside the continuous casting machine. In this method, the bloom 1 is cooled from a temperature above the Ar<SB>3</SB>transformation temperature under conditions in which the cooling strength Cs defined by the formula: Cs=Hw×T falls within the range of from 500 to 2,500 (W*min/m<SP>2</SP>*K), wherein Hw is the heat transfer rate (W/m<SP>2</SP>*K) in the state where the surface temperature of the bloom 1 is 1,000 K; and T is the time (min) necessary for the bloom 1 to pass through the cooling zone (heat transfer rate Hw). Thus, it becomes possible to increase high-temperature ductility, to prevent crack formation during a heat expansion process, and to prevent bloom surface cracking otherwise occurring during bloom rolling. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for cooling a continuously cast bloom slab.

  The bloom slab is obtained by injecting molten steel from a ladle into a mold through a tundish, forming a solidified shell in the mold, and then pulling out the solidified slab from the lower part of the mold while placing it on the lower part of the mold. Manufactured by further cooling in a cooling spray zone.

Furthermore, various cooling methods have been proposed in order to prevent slab surface cracks during reheating of the slab when the bloom slab is continuously cast.
In particular, for the purpose of refining the surface structure of the bloom slab, it is called tertiary cooling, and the bloom slab is cooled outside the continuous casting machine.

For example, in Patent Document 1, the bloom slab after cutting to a predetermined length is subjected to a bloom cooler outside the continuous casting machine, and the water density on the upper surface of the bloom slab is set to 5 × 10 ⁻ from the temperature range immediately above the Ar ₃ transformation point. A method of cooling by changing the water density ratio of the side surface and the lower surface to the upper surface at ⁴ to 4 × 10 ⁻³ m ³ / sm ² is disclosed. According to this method, it is described that generation of surface flaws over the entire surface that occurs during cooling can be prevented.
Japanese Patent Laid-Open No. 10-1719

Patent Document 2 discloses a cooling method in which the moving speed of the bloom slab is 3 to 10 m / min when cooling from a temperature range immediately above the Ar ₃ transformation point using a bloom cooler outside the continuous casting machine. ing. According to this method, it is described that surface defects are reduced.
JP 2005-40837 A

The tertiary cooling described in Patent Document 1 and Patent Document 2 is that the bloom slab is simply cooled from the temperature range immediately above the Ar ₃ transformation point, and the structure is refined by recuperation and reheating (granular grain refinement). This is effective for preventing and suppressing crack growth.

  However, a thermal stress crack may occur due to a temperature difference between the surface layer and the inside during heating, or a portion that has been quenched and martensitic transformed may break due to expansion during heating. In addition, there is an influence by the cast steel type.

  As described above, the tertiary cooling disclosed in Patent Literature 1 and Patent Literature 2 cannot effectively prevent the slab surface layer cracking that occurs during the ingot rolling.

The problem to be solved by the present invention is that the bloom cast slab is simply cooled from the temperature range immediately above the Ar ₃ transformation point, and in the conventional tertiary cooling aimed at refining the structure by reheating and re-heating of the lump, It is a point that the slab surface layer crack which generate | occur | produces sometimes cannot be prevented effectively.

  In the tertiary cooling of the continuously cast bloom slab, the inventors made the surface layer of the bloom slab have a solidified structure of an unclear γ grain boundary described in JP-A No. 2002-307149, and the high temperature ductility It was thought that the conventional problem could be solved by forming a certain structure.

That is, the cooling method of the bloom slab of the present invention,
A method of cooling outside a continuous casting machine after cutting a continuous cast bloom slab to a predetermined length,
When the surface temperature of the bloom slab is 1000K, the heat transfer coefficient (W / m ² · K) is Hw, and the time (min) for the bloom slab to pass through the cooling zone (heat transfer coefficient Hw) is T. ,
From the temperature at which the surface temperature of the bloom slab exceeds the Ar ₃ transformation point,
The most important feature is that the cooling is performed under the condition that the cooling strength Cs defined by Cs = Hw × T is in the range of 500 to 2500 (W · min / m ² · K).

  According to the method for cooling a bloom slab according to the present invention, a solidified structure of an unclear γ grain boundary is present in the range of several mm of the surface part of the bloom slab, thereby forming a high-temperature ductile structure. it can. Therefore, it is possible to effectively prevent slab surface layer cracking during rolling by suppressing reheat cracking in the slab reheating process and suppressing crack progress by refining the slab surface layer structure by reheating the slab.

In the bloom slab cooling method of the present invention, the cooling is started from the temperature at which the surface temperature of the bloom slab exceeds the Ar ₃ transformation point in order not to form a ferrite structure that is the origin of surface defects. .

In the method for cooling a bloom slab according to the present invention, the cooling is performed under the condition that the cooling strength Cs defined by Cs = Hw × T is in the range of 500 to 2500 (W · min / m ² · K). For the following reasons.

The upper and lower limit values of the cooling strength Cs correspond to the thickness of the γ grain boundary unclear structure that is the object of the present invention, and are meaningless unless the tertiary cooling in the present invention is performed and the target structure is obtained.
Therefore, there exists a range of the cooling strength Cs as the optimum cooling condition.

According to the investigation by the inventors, when the cooling strength Cs is less than 500, sufficient tertiary cooling cannot be performed. Therefore, after passing through the Ar ₃ transformation point, the time until the reheating is shortened, and the structure reforming is performed. It becomes insufficient.

On the other hand, when the cooling strength Cs exceeds 2500, the recuperation after passing through the Ar ₃ transformation point is insufficient, so that it becomes a rapidly cooled structure, and in the recuperation + reheating process described in Patent Documents 1 and 2. Therefore, the target γ grain boundary unclear structure cannot be obtained.

In the present invention, the inventors have found that an unclear structure of γ grain boundaries is formed without necessarily reheating to a temperature not lower than the Ar ₃ transformation point.

The method described in JP-A No. 2002-307149 starts secondary cooling directly under the mold to temporarily lower the slab surface temperature from the Ar ₃ transformation point, and immediately after that, a temperature higher than the Ar ₃ transformation point. In this method, cooling and double heat time are specified. According to this method, it is possible to form a structure in which granular ferrite and pearlite are mixed in the γ grain boundary, and to obtain a solidified structure of an unclear γ grain boundary. It is described that it can be prevented.

  According to the present invention, by tertiary cooling, a mixed structure of granular ferrite and pearlite can be formed at the γ grain boundary, and an unclear γ grain boundary solidified structure can be obtained. Cracks can be prevented, and slab surface layer cracks that occur during block rolling can be prevented.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for carrying out the present invention will be described with reference to the accompanying drawings along with the progress from the idea of the present invention to the solution of a problem.
In order to embody the idea of the present invention, the inventors made a test apparatus shown in FIG. 1 and investigated the slab surface layer structure modification and cooling conditions.

  In FIG. 1, 1 is a bloom slab, 2 is a drive device for simulating the moving speed of the bloom slab 1, and 3 is cooling water on only one side of the long side and the short side of the bloom slab 1. It is a cooling spray for spraying.

The experiment was performed according to the following procedure under the implementation conditions shown in Table 1 below.
First, an ingot corresponding to the cross section of a bloom cast slab was continuously cast using molten steel whose components and temperature were adjusted so as to be the test steel types shown in Table 1 below.

Thereafter, after die cutting and transporting to the test apparatus shown in FIG. 1, the time during which the bloom cast slab 1 passes the spray cooling zone from the temperature above the Ar ₃ transformation point on only one side of the long side and the short side. Cooling water was sprayed from the cooling spray 3 for T only and cooled. After cooling, after cooling water injection was stopped and reheated (see FIG. 2), the sample was cut out and the structure was observed (including a reheating test).

FIGS. 3 and 4 show the observation results of the surface structure of the cast slab that has been exposed by etching with nital (HNO ₃ : C ₂ H ₅ OH = 10%: 90%). Table 2 below shows the cooling conditions during the experiment when the solidified structure shown in FIGS. 3 and 4 was obtained.

  The heat transfer coefficient Hw during cooling shown in Table 2 was obtained based on the temperature transition of the bloom slab measured by an embedded thermocouple set at a position of 25 mm of the surface of the bloom slab. That is, using a cooling curve as shown in FIG. 2 obtained by evaluating the amount of heat removed by cooling from the temperature transition, the time (min) T during which the bloom slab 1 passes through the spray cooling zone (heat transfer coefficient Hw) and It was obtained by the structure investigation (appearance of γ grain boundary unclear area).

After cooling the bloom slab so that it does not fall below the Ar ₃ transformation point when passing through the secondary cooling zone, the cooling strength Cs defined by Cs = Hw × T is 1855.29 (W · min / When cooled under the condition of m ² · K) (invention example), a target γ grain boundary unclear structure appeared in the range of the surface layer of 4 mm as shown in FIG.

On the other hand, when cooling is performed under the condition that the cooling strength Cs is 2600.01 (W · min / m ² · K) (comparative example), as shown in FIG. A clear structure does not appear, and the grain boundary of the slab surface layer is clear.

FIG. 5 is a diagram in which the relationship between the cooling strength Cs defined by Cs = Hw × T and the structure modification thickness (gamma grain boundary unclear structure) is arranged. As shown in FIG. 5, the cooling condition of the third cooling starting from the temperature exceeding the Ar ₃ transformation point, that is, the cooling strength Cs is appropriate to be in the range of 500 to 2500 (W · min / m ² · K). It became clear that a γ grain boundary unclear structure appeared in the surface layer of the bloom cast slab regardless of the presence or absence of Nb.

The method for cooling a bloom slab of the present invention is based on the above investigation results,
A method of spray-cooling outside a continuous casting machine after cutting a continuous cast bloom slab to a predetermined length,
When the surface temperature of the bloom slab is 1000K, the heat transfer coefficient (W / m ² · K) is Hw, and the time (min) for the bloom slab to pass through the spray cooling zone (heat transfer coefficient Hw) is T In addition,
From the temperature at which the surface temperature of the bloom slab exceeds the Ar ₃ transformation point,
Cooling is performed under conditions where the cooling strength Cs defined by Cs = Hw × T is in the range of 500 to 2500 (W · min / m ² · K).

  In the method of the present invention, the control of the cooling strength Cs in the tertiary cooling is not simply determined by the amount of cooling water, it is important how the bloom slab is cooled, and the amount of cooling water (cooling) sprayed on the bloom slab is important. This is done by controlling the water and air) and the feed rate of the transfer table.

  That is, the heat transfer coefficient Hw in the cooling zone is estimated from the above-described transition of the thermocouple temperature, and the product is arranged by the product of the time T when the bloom slab surface passes through the cooling zone. Therefore, it is possible to grasp the optimum condition of the steel and to stably modify the slab surface layer structure.

  In order to confirm the effect of the method of the present invention, the inventors used the relationship between the surface structure modification of the bloom slab and the cooling strength Cs, and in the actual machine operation, in the slab surface defect initial pass rate, The effect of quality improvement was confirmed.

  Table 3 below shows actual machine test conditions, Table 4 below shows cooling conditions, and FIG. 6 shows a comparison of the initial test pass rates showing the effects of the present invention.

  As shown in FIG. 6, the initial test pass rate when the method of the present invention is implemented is 71.5%, the initial test pass rate when the method of the present invention is not performed is 57.3%, and the initial test pass rate is 14%. Improved by 2%. The improvement rate was improved by about 31%, and the effect of the present invention was proved.

  The present invention is not limited to the above example, and it goes without saying that the embodiments may be changed as appropriate within the scope of the technical idea described in the claims.

  For example, in the above example, the heat transfer coefficient Hw of the spray uses the value calculated based on the thermocouple depth, material properties, etc., by inputting the temperature measured by the thermocouple. It can also obtain | require by following (1) Formula and (2) Formula (Special Report No. 29, forced cooling of steel materials, Japan Iron and Steel Institute, S53.11.10 issue, p58).

Hw = 10 ^1.399・ Tw ^-0.1358・ W ^0.6293・ V ^0.2734 … (1)
V = 10 ^3.25・ R ^0.35・ P ^0.62・ S ^0.52・ H ^−0.4 (2)
However, V: Flow velocity (m / s), R: Water volume / Air volume, P: Air pressure (kgf / cm ² ), H: Nozzle height (cm), S: Slit width (cm), Tw: Heat transfer surface Temperature (° C), W: Water density (L / min · m ² )

  The results of calculating the heat transfer coefficient Hw using the above formulas (1) and (2) are shown in the experimental values at the time of tertiary cooling shown in FIG. 7 and at the time of cooling by mist spray shown in FIG. It can be seen that a heat transfer coefficient substantially equal to the value can be obtained.

  In the present invention, since the relationship with the cooling strength Cs is particularly important, the cooling water sprayed from the cooling zone may be sprayed in any spraying manner such as mist spray or high pressure spray.

  The present invention can be applied not only to the medium carbon steel bloom cast as shown in the examples, but also to continuous casting of low carbon steel bloom cast and high carbon steel bloom cast.

It is a figure explaining the test device of the method of the present invention. It is the figure which showed the cooling pattern used for experiment. It is the figure which showed the observation result of slab surface layer structure, and is a case where the unclear structure of the target (gamma) grain boundary is obtained. It is the figure which showed the observation result of slab surface layer structure, and is a case where the unclear structure of the target (gamma) grain boundary was not obtained. It is the figure which arranged the relationship between the cooling intensity | strength Cs defined by Cs = HwxT, and structure | tissue modification thickness (gamma grain boundary unclear structure). It is a figure which shows the comparison of the initial examination pass rate which shows the effect of this invention. It is a figure which shows the result of having calculated the heat transfer rate Hw at the time of tertiary cooling. It is the figure which showed the heat transfer rate h by mist spray.

Explanation of symbols

1 Bloom slab 3 Cooling spray

Claims (1)

A method of cooling outside a continuous casting machine after cutting a continuous cast bloom slab to a predetermined length,
From the temperature at which the surface temperature of the bloom slab exceeds the Ar ₃ transformation point,
A cooling method for a bloom slab, wherein the cooling strength Cs defined by the following formula is cooled in a range of 500 to 2500 (W · min / m ² · K).
Cs = Hw × T
Here, Hw: heat transfer coefficient when the surface temperature of the bloom slab is 1000K
(W / m ²・ K)
T: Time for the bloom slab to pass through the cooling zone (heat transfer coefficient Hw) (min)