JP2001138019A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method of a low alloy steel cast slab, with which the development of transverse fracture and transverse crack developed at the straightening of the cast slab even in the case of changing the casting speed or the cast slab size. SOLUTION: When the cast slab having 1.8-10.5 ratio of the width to the thickness is cast, during the section from the starting of cooling just after the outlet of a mold to at least 1.5 m in the casting direction, the cast slab is cooled under condition, in which a value of the water spraying ratio Q (L/kg of steel) of secondary cooling defined with the following formula (A) becomes 0.4-0.75, and the surface temperature of the cast slab is made to not above the Ar3 transformation point and thereafter, returned back to >=850 deg.C to straighten the bending thereof. Q=W/(H×D×Vc×ρ)...(A). Wherein, W: the cooling water quantity (L/min) of the secondary cooling, H: the width (m) of the cast slab, D: the thickness (m) of the cast slab, Vc: the casting speed (m/min) and ρ: the density (kg/m3) of the molten steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method of steel capable of obtaining a low alloy steel slab of good surface quality.

[0002]

2. Description of the Related Art In recent years, low-alloy steels containing alloy elements such as Nb, V, Ni, and Cu have been widely used in thick plate products and the like due to demands on mechanical properties. When these low alloy steels are cast using a curved or vertical bending type continuous casting machine, cracks called horizontal cracks and horizontal cracks are likely to occur on the slab surface. When the slab is straightened, the stress acting on the slab surface exceeds the limit stress inherent in these low alloy steels, so that these cracks occur on the slab surface.

[0003] The reason why lateral cracks and the like are likely to occur in a low alloy steel slab containing alloying elements such as Nb, V, Ni and Cu will be further described as follows. The hot ductility of the cast slabs of these low alloy steels is close to the temperature of the A ₃ transformation point at which the solidified structure of the cast slab is transformed from the γ phase to the α phase, ie, 60 ° C.
In the temperature range of 0 to 850 ° C., the temperature significantly decreases. Furthermore, in these low-alloy steel slabs, AlN, NbC, and the like are likely to precipitate at the γ grain boundary in the secondary cooling process after being drawn from the mold. Γ grain boundaries on which AlN, NbC, etc. are precipitated are liable to crack when stress is applied. Therefore, 600-850 ° C
When the slabs of these low alloy steels are corrected in the above temperature range, lateral cracks and lateral cracks are likely to occur on the surface of the slab due to the brittleness temperature range and precipitates at the γ grain boundary.

Therefore, the slab surface temperature at the time of straightening is set to 600
As a low temperature side or a high temperature side of a temperature range in which hot ductility is reduced to 850 ° C. (hereinafter referred to as an embrittlement temperature range), avoid the embrittlement temperature range and prevent the occurrence of lateral cracks and the like on the slab surface. The method has been adopted.

In Japanese Patent Publication No. 58-3790, when a steel having an embrittlement temperature range of 820 to 950 ° C. is cast, the slab is strongly cooled by secondary cooling to reduce the surface temperature of the slab to 650.
To 700 ° C, and then the surface temperature of the slab at the straightening position is 7
A method has been proposed in which the embrittlement temperature range is set to a low temperature side at a temperature of 00 to 800 ° C. to prevent lateral cracks and the like.

However, in this method, Nb, V, Ni,
In the case of low alloy steel containing alloy elements such as Cu, in order to temporarily lower the slab surface temperature from the embrittlement temperature range,
When the slab is strongly cooled, a thick oxide film is easily generated on the slab surface. The thickness of these oxide films tends to be non-uniform depending on the position of the slab surface, and the slab surface of the thick portion of the oxide film is less likely to be cooled during secondary cooling. Therefore,
The surface temperature of the slab tends to be uneven. For this reason, the temperature of the slab surface may partially become the embrittlement temperature range, and horizontal crazing may occur on the slab surface during straightening.

On the other hand, in Japanese Patent Application Laid-Open No. 9-47854,
After withdrawal of the slab from the mold, the cast slab surface temperature within 1 minute to less than A ₃ transformation point causes then recuperation is, how to cast slab surface temperature 850 ° C. or higher in the straightening points is proposed.

However, according to this method, when the slab size is large or the casting speed is high, the slab surface temperature is reduced to the A ₃ transformation point or lower within one minute, and then the slab surface temperature is set to 850 ° C. or higher. Even if the slab is corrected by reheating to a temperature, lateral cracks or the like may occur on the slab surface.

Further, in Japanese Patent Application Laid-Open No. 9-253814, similarly to the above-mentioned Japanese Patent Application Laid-Open No. 9-47854, the slab is strongly cooled below the outlet of the mold, and the surface temperature of the slab is temporarily reduced to the brittle temperature range. There has been proposed a method of lowering the temperature to a lower temperature side and then recovering the temperature to a higher temperature side than the embrittlement temperature range to prevent the occurrence of lateral cracks and the like when the slab is corrected. By reducing the surface temperature of the slab once and reheating it, the microstructure of the slab is changed to ferrite and pearlite structure in which the γ grain boundaries are obscured, thereby preventing the occurrence of lateral cracks and the like. Is the way.

However, in this method, since the secondary cooling condition is defined by the average water density (liter / cm ² · min) per unit time per unit area of the slab surface, the casting speed and the slab When the size changes, proper cooling may not be performed in some cases. In particular, when the casting speed is high or the size of the slab is large, the amount of cooling water is insufficient, so that the γ grain boundary does not become unclear and the microstructure of the slab may not be properly controlled. Therefore, horizontal cracks and the like are likely to occur on the surface of the slab.

[0011]

DISCLOSURE OF THE INVENTION The present invention relates to cracks such as lateral cracks and lateral cracks that occur when straightening a slab, even if the casting speed or slab size changes (hereinafter referred to as slab surface cracks). It is an object of the present invention to provide a continuous casting method of a low alloy steel slab which can stably prevent the above.

[0012]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a method for casting a slab having a width to thickness ratio of 1.8 to 10.5.
Cooling is started immediately after the mold outlet, and during a period of at least 1.5 m in the casting direction, the value of the specific water amount Q (liter / kg-steel) of the secondary cooling defined by the following equation (A) is 0.1.
By cooling the cast piece under the condition that the 4 to 0.75, the billet surface temperature not more than A ₃ transformation point, then, to correct the bending in a state in which the cast slab surface temperature by recuperation 850 ° C. or higher In the continuous casting method of steel.

Q = W / (H × D × Vc × ρ) (A) where W: cooling water amount for secondary cooling of the slab (liter / liter)
Min) H: slab width (m) D: slab thickness (m) Vc: casting speed (m / min) ρ: density of molten steel (kg / m ³ ) density of molten steel in the above formula (A) 7000 as the value of ρ
(Kg / m ³ ).

The present inventors have solved the above-mentioned problems of the present invention by the following measures.

In the method of the present invention, the specific water amount Q (liter / liter) defined by the above-mentioned equation (A) is used instead of the average water density (liter / cm ² · minute) per unit area of the slab surface.
The slab is secondarily cooled with the value of (kg-steel) as an appropriate condition. Therefore, even if the casting speed or slab size changes,
The secondary cooling condition of the slab can be appropriately maintained.

Cooling is started immediately after the mold outlet, and the specific water amount Q (liter / kg-
The slab is cooled under the condition that the value of (steel) is 0.4 to 0.75, the slab surface temperature is set to the A ₃ transformation point or lower, and then reheated, and the slab surface temperature is set to 850 ° C. or higher. Straighten the slab.

As described above, the slab is secondarily cooled under appropriate conditions immediately after the mold exit, and the slab surface temperature is set to the A ₃ transformation point or lower, so that the slab microstructure and the γ grain boundary are not affected. A clarified ferrite and pearlite structure can be obtained. In addition, the growth of γ grains can be suppressed. The γ grain boundaries of small γ grains with unclear γ grain boundaries are less likely to cause slab surface cracks during slab correction.

The microstructure of a slab composed of ferrite and a pearlite structure in which the γ grain boundaries are obscured means that when the slab is cooled from a high temperature side to a lower temperature side from the A ₃ transformation point, the γ grain It means a microstructure in a state formed in a grain boundary. Since ferrite is generated in a granular form at the γ grain boundary, the γ grain boundary becomes unclear. Such a microstructure is
As described above, it is obtained by strongly cooling the slab under appropriate conditions immediately after the exit of the mold. Also, since the slab is strongly cooled immediately after the mold exit, there is no time for the γ grains to grow large, and small γ grains can be obtained.

Conversely, when the slab immediately after the exit of the mold is weakly secondary cooled, the γ grain boundary becomes clear. The microstructure in which the γ grain boundary is clear is a microstructure in which ferrite is formed in a film shape at the γ grain boundary.

Further, since the slab surface temperature at the time of straightening the slab is set to 850 ° C. or higher, which is higher than the embrittlement temperature range, a low alloy containing alloy elements such as Nb, V, Ni, and Cu is used. Even in the case of steel, the occurrence of surface slab cracks can be prevented.
The embrittlement temperature of low alloy steel is 600 to 850 ° C as described above.
Temperature range.

The surface temperature of the slab is, for example, a surface temperature that can be measured by a radiation thermometer.
It means the temperature from the surface of the slab to just below the skin. Also,
The surface temperature of the slab can also be obtained by calculation by solidification heat transfer analysis. That is, if conditions such as the type of steel, the size of the slab, the casting speed, and the secondary cooling condition of the slab are determined, the surface temperature of the slab according to the distance from the molten steel meniscus can be calculated. Further, by appropriately selecting the surface heat transfer coefficient, the surface temperature of the slab obtained by this calculation can be made to match well with the actually measured surface temperature.

[0022]

FIG. 1 is a view schematically showing an example of a continuous casting apparatus when the method of the present invention is carried out. The slab 2 immediately after being pulled out of the mold 1 has a guide roll pair 3
, Sprayed with water by a mist spray nozzle 4 or the like, and secondary cooled. Thereafter, the slab is pulled out by the pinch roll 5, and the slab is bent by the pinch roll 6 at the position of the correction point.

The slab to which the present invention is directed is a slab having a width to thickness ratio in the range of 1.8 to 10.5.

In the case of bloom or billet slab having a width to thickness ratio of less than 1.8 or a slab slab having a large aspect ratio having a width to thickness ratio of more than 10.5, the slab surface has a width Since it is difficult for cracks and lateral cracks to occur, it is excluded from the target.

In the method of the present invention, the cooling of the slab is started immediately after the mold exit, and the specific water amount of the secondary cooling of the slab is adjusted to an appropriate range within at least 1.5 m in the casting direction. I do.

There are many unsolidified portions in the center of the thickness of the slab between immediately after the mold exit and at least 1.5 m in the casting direction. By cooling such a slab under the conditions of a ratio water proper secondary cooling, the surface temperature of the slab is already solidified solidified shell can be lowered than once A ₃ transformation point. Since the secondary cooling of the slab is terminated at a position at least 1.5 m from the mold outlet in the casting direction, the slab surface temperature can be returned to 850 ° C. or higher thereafter. As described above, by such secondary cooling, the γ grain boundaries are unclear and small γ grains can be obtained.

It is desirable that the end position of the secondary cooling of the slab is between 0.7 and 1.3 m in the casting direction from the mold outlet.

In order to complete the secondary cooling of the slab in the area of less than 0.7 m, the amount of cooling water per mist spray nozzle increases, the mist spray nozzle and the piping for cooling water increase, Costs will be higher. In the case of secondary cooling to a region exceeding 1.3 m, the number of arranged air mist nozzles increases, and the equipment cost increases.

In the method of the present invention, the specific water amount Q (liter / kg-steel) of the secondary cooling defined by the above equation (A) is 0.1.
Casting is performed under the conditions of 4 to 0.75.

The specific water quantity Q (liter / kg-steel) is 0.4
In the secondary cooling of the slab becomes insufficient, once it is difficult to cool to below A ₃ transformation point and the surface temperature of the slab below. Once can not reduce the cast slab surface temperature to below the A ₃ transformation point, the microstructure of the slab can not obscure the γ grain boundary, and since it is impossible to obtain a small γ grains, the cast slab surface cracks More likely to occur. Further, when the specific water amount exceeds 0.75, the slab is supercooled, so that it becomes difficult to recover the temperature to 850 ° C. or more at the time of reheating. Therefore, when the slab is straightened, the surface temperature of the slab becomes the embrittlement temperature range, and the slab surface cracks easily occur.

FIG. 2 shows the specific water amount Q (liter / kg-steel) of the secondary cooling of the slab which affects the slab surface cracking and the casting speed Vc.
It is a figure which shows the relationship with (m / min).

Using a vertical bending type continuous casting machine having a vertical part length of 3.0 m, a medium carbon low content having a C content of 0.10 to 0.15 mass% and an Nb content of 0.015 mass%. Alloy steel was cast. Cooling was started immediately after the mold exit, and the slab was secondarily cooled to 1.5 m in the casting direction. At this time, the test was performed while changing the specific water amount Q and the casting speed Vc of the secondary cooling of the slab. . In addition, the secondary cooling is not performed for the slab located downstream from the region where the secondary cooling is performed.

The curve indicated by reference numeral (a) in FIG. 2 shows a slab having a rectangular cross section, a thickness of 300 mm, a width of 650 mm, and a width to thickness ratio of 2.2. 5 is a curve showing the results of a test in which the cooling water amount was cooled under the condition of 500 liters / minute and the casting speed was changed. Similarly, the curve shown by the symbol (b) is a thickness of 200 mm and a width of 1000 m.
m, the slab having a width-to-thickness ratio of 5.0 was cooled under the condition that the cooling water volume at the time of the secondary cooling was 1500 liters / minute, and the result of the test in which the casting speed was changed was represented by a code ( The curve shown in c) has a thickness of 230 mm and a width of 2300 m
m, the slab having a width-to-thickness ratio of 10.0 was cooled under the condition that the cooling water amount at the time of secondary cooling was 2300 liters / minute, and the results of tests in which the casting speed was changed were shown by curves respectively. is there.

From FIG. 2, even if at least the casting speed or the slab size changes, cooling is started immediately after the mold exit,
By setting the specific water amount Q of the secondary cooling of the slab to 1.5 m in the casting direction to 0.4 to 0.75 (liter / kg-steel), it is possible to prevent the occurrence of a slab surface crack. I understand.

[0035]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the apparatus configuration shown in FIG.
Using a vertical bending type continuous casting machine of 0 m, the test was performed while changing the conditions of the slab size, the casting speed, and the specific water amount Q of the secondary cooling. The region for secondary cooling of the slab was cooled immediately after exiting the mold within the range defined by the method of the present invention, and was set to a region up to 0.8 m in the casting direction. The slab at a position downstream of the secondary cooling area was not subjected to the secondary cooling of the slab. In Table 1,
The chemical composition of the medium carbon low alloy steel used is shown. Nb, Cu,
It is a low alloy steel containing Ni. The A ₃ transformation point of this steel is 86
5 ° C., and the embrittlement temperature range is 710 to 840 ° C.

[0036]

[Table 1]

The slab size is a size within the range of the ratio of the width to the thickness specified by the method of the present invention.
Tested by type. A slab having a thickness of 300 mm, a width of 650 mm, and a width to thickness ratio of 2.2, a thickness of 200 mm,
There were three types of slabs, a slab having a width of 1000 mm and a width to thickness of 5.0, and a slab having a thickness of 230 mm, a width of 2300 mm and a width to thickness of 10.0.

In each test, three heats were cast continuously, and the surface temperature of the slab immediately after the secondary cooling and the surface temperature of the slab at the straightening position were measured by a radiation thermometer during the casting. In each test, the length in the casting direction from each heat was 1 mm.
A slab sample of m is collected, and the surface of the slab sample is scarfed to remove oxides on the surface layer of the slab so that cracks on the slab surface can be easily observed. , And the state of occurrence of surface slab cracks was visually observed. Evaluation of degree of occurrence of slab surface cracking A
Means that no slab is generated, and evaluation B is that a slab surface crack is generated to the extent that the slab surface requires maintenance before hot rolling using the slab as a raw material. Surface cracks have occurred.

Further, a sample for observation with an optical microscope was cut out from the slab sample, polished and subjected to 5% nital corrosion, and the state of formation of ferrite at the γ grain boundary was observed with an optical microscope. Table 2 shows each test condition and each test result.

[0040]

[Table 2]

Test No. of the present invention example 1 to No. In 5,
Within the range of the condition of the ratio of the width to the thickness specified by the method of the present invention, the range of the condition of the specific water amount Q of the secondary cooling of the slab specified by the method of the present invention for three types of slabs Tested within.

In these test nos. 1 to No. In No. 5, the surface temperature of the slab immediately after the secondary cooling was temporarily set to 720 to 800 ° C. below the A ₃ transformation point by the secondary cooling of the slab. Thereafter, the steel was reheated, and the slab was straightened at a slab surface temperature of 880 ° C. to 950 ° C., which is a high temperature side of the brittle temperature range of the steel. Therefore, the microstructure of the surface layer of the slab, ferrite is generated in a granular manner at the γ grain boundary, and the γ grain boundary is unclear,
The evaluation of the slab surface crack generation was A, and no slab surface cracks such as lateral cracks were generated.

Test No. of Comparative Example 6-No. 8, within the range of the condition of the ratio of the width to the thickness specified by the method of the present invention, for the slabs of three kinds of sizes, the specific water amount Q of the secondary cooling of the slab specified by the method of the present invention. The test was performed with the conditions removed.

Test No. In No. 6, the specific water amount Q in the secondary cooling of the slab was set to 1.05 liter / kg-steel, which exceeded the upper limit of the condition specified by the method of the present invention. The evaluation of the occurrence of slab surface cracking was C, and significant slab surface cracking occurred. In the microstructure of the surface layer of the slab, ferrite was generated in a granular form at the γ grain boundary, and the γ grain boundary was unclear. However, because the slab is supercooled, once the surface temperature of the slab to obtain the following 670 ° C. A ₃ transformation point, could not be recuperation to a temperature above the A ₃ transformation point. for that reason,
The surface temperature of the slab at the time of straightening is the brittle temperature range of this steel.
The temperature was 20 ° C., and when the slab was corrected, significant slab surface cracks occurred.

Test No. 7 and No. 7 In No. 8, the specific water amount Q of the secondary cooling of the slab is set to 0.35 or 0.29 liter / kg which is less than the lower limit of the condition specified by the method of the present invention.
-Steel. In each case, the evaluation of the occurrence of slab surface cracking was B, indicating that slab surface cracking required maintenance was generated.
The surface temperature of the cast slab at the time of straightening is 890 to 970 ° C., and although the brittle temperature range of the steel is avoided on the high temperature side, the secondary cooling of the cast slab is insufficient and the surface temperature of the cast slab is reduced. once it could not be cooled to below a ₃ transformation point. Therefore, in the microstructure of the surface layer of the slab, ferrite was formed in a film shape at the γ grain boundary, and the γ grain boundary became clear. Therefore, when the slab is corrected, the slab surface cracks are easily generated.

[0046]

According to the method of the present invention, even if the casting speed or the slab size changes, the occurrence of lateral cracks or lateral cracks on the slab surface can be prevented, and the casting of low alloy steel with good surface quality can be prevented. You can get a piece.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating an example of a continuous casting apparatus when a method of the present invention is performed.

FIG. 2 is a diagram showing a relationship between a specific water amount Q of secondary cooling of a slab and a casting speed Vc, which affect a slab surface crack.

[Explanation of symbols]

1: mold 2: cast slab 3: guide roll pair 4: mist spray nozzle 5: pinch roll 6: pinch roll at the position of the correction point

Claims (1)

[Claims] (1) When casting a slab having a width to thickness ratio of 1.8 to 10.5, cooling is started immediately after the mold exit and the following (A) ) The secondary cooling specific water volume Q (liter /
(kg-steel), the slab is cooled under the condition that the value of the slab is 0.4 to 0.75, the slab surface temperature is set to the A ₃ transformation point or lower, and then the slab is reheated to set the slab surface temperature to 850 ° C. A continuous casting method for steel, wherein the bending is corrected in the above state. Q = W / (H × D × Vc × ρ) (A) where W: cooling water amount for secondary cooling of the slab (liter / liter)
Min) H: slab width (m) D: slab thickness (m) Vc: casting speed (m / min) ρ: density of molten steel (kg / m ³ )