JP2903927B2 - 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 of casting a slab continuously using a vertical bending type continuous casting machine without causing surface cracks (grain boundary cracks) in a continuous cast piece at 1.6 m / min or more. The present invention relates to a continuous casting method devised so that continuous casting can be performed at a high speed.

[0002]

2. Description of the Related Art As a cooling method in a secondary cooling zone when performing slab continuous casting using a vertical bending type continuous casting machine,
(1) A method of cooling while uniquely controlling the amount of secondary cooling water according to the casting speed of molten steel, and (2) Obtaining the required amount of water for each part of the continuous cast piece to be drawn and continuously casting to the target position. There is known a method of adjusting the amount of cooling water in each cooling zone in the longitudinal direction of the continuous cast piece to be drawn so that the piece can be completely solidified.

However, the above method (1) is a method of uniquely setting a cooling water amount (hereinafter, sometimes referred to as a specific water amount) with respect to a continuous cast piece weight passing through a secondary cooling zone in accordance with a casting speed. It is a cooling control method with the main purpose of stable operation, and it is suitable for surface cracks (grain boundary cracks) which often become a problem in the continuous casting slab bending part and straightening part (bend return part) of vertical bending type continuous casting. Is not taken into account.

On the other hand, in the method (2), the total amount of cooling water with respect to the casting speed obtained in the same manner as in the method (1) is distributed to each zone in the longitudinal direction of the secondary cooling zone. It is also intended to improve the surface cracks and internal cracks of the continuous cast slab, but in the case of slab continuous casting, the flatness ratio of the cast slab is large, so the surface temperature of the corner in the width direction of the continuous cast slab is Since the temperature is lower than the surface temperature, it is difficult to reliably prevent surface cracks only by adjusting the cooling water amount in the longitudinal direction of the continuous cast slab.

Surface cracks, which are particularly problematic in vertical bending continuous casting, generally tend to become more pronounced as the casting speed is increased. This tendency is the same when the above-mentioned cooling control (2) is performed. Therefore, even when the method (2) is adopted, the casting speed is actually suppressed to about 1.5 m / min or less so as not to cause the surface crack of the slab cast.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object the purpose of performing slab continuous casting using a vertical bending type continuous casting machine.
Even if a casting speed of 6 m / min or more is adopted, it is an object of the present invention to establish a technology capable of performing continuous casting smoothly and with good productivity without causing surface cracks in the continuous slab cast.

[0007]

The structure of the continuous casting method according to the present invention, which can solve the above-mentioned problems, uses a vertical bending type continuous casting apparatus and has a steel casting speed of 1.6 m / min or more. In performing the slab continuous casting, the surface temperature of the continuous cast slab when the continuous cast slab passes through the first bending portion is set to a temperature exceeding the embrittlement temperature range of the steel material, and the continuous cast slab is placed downstream thereof. The point is that the surface temperature of the continuous cast piece when passing through the straightening portion on the side is set to a temperature lower than the embrittlement temperature range. However, in the above description, the embrittlement temperature range means a temperature range in which the reduction value in a high-temperature tensile test of a steel material as described later is 50% or less of the reduction value in the tensile test.

[0008]

In order to establish a method that can prevent surface cracking even when a higher casting speed is employed in the vertical bending type continuous casting than in the conventional example, the present inventors firstly developed the mechanism of surface cracking. Was studied. As a result, the following facts were confirmed.

Considering the situation during solidification of the continuous cast slab, P and S are segregated between dendrites in the solidified shell of the cast slab, and the dendrite trees are in a molten state to easily cause internal cracks.
In addition, a phenomenon in which nitride segregates at a crystal grain boundary after solidification and causes grain boundary fracture (hereinafter, referred to as high-temperature embrittlement) occurs. A temperature region in which such a phenomenon occurs (referred to as an embrittlement temperature region) is as follows. A specific range is shown depending on the composition of the steel.

For example, the drawing value (RA) determined by a high-temperature tensile test of steel A having the composition shown in Table 1 below.
Shows a low value in the temperature range of 700 to 900 ° C. as shown in FIG. 1, and in this temperature range, the tensile strength decreases and the embrittlement phenomenon in which the ductility decreases also occurs. FIG. 2 similarly shows the results of a high-temperature tensile test of steel B, and the range of 700 to 1000 ° C. is the embrittlement temperature range.

[0011]

[Table 1]

FIG. 3 shows the result of examining the relationship between the nitrogen content in steel and the embrittlement temperature range. As the nitrogen content increases, the embrittlement temperature range tends to expand. Can be confirmed. Also, according to the observation of the fracture surface of the test piece used in this experiment, precipitation of nitride at the grain boundary was confirmed,
It is judged that embrittlement was caused by the occurrence of stress concentration starting from the fine nitride precipitated at the grain boundary.

In a vertical bending type continuous casting machine, if the surface temperature of the slab is within the embrittlement temperature range when the slab passes through the bent portion and the straightening portion (bent back portion), the slab surface layer portion It is considered that the segregation of the nitride causes grain boundaries to be broken by the bending stress due to the segregation of the nitride, which easily leads to surface cracking.

The present invention has been completed as a result of further research based on the above-mentioned findings, and has been described in the following embodiment (FIG. 5).
As described in detail in 6), the surface temperature of the slab when passing through the bending portion of the vertical bending continuous caster is controlled to a temperature exceeding the embrittlement temperature range inherent in the slab, and the downstream side thereof is controlled. The cooling conditions are controlled so that the slab surface temperature when passing through the straightening section located at a temperature lower than the embrittlement temperature region, whereby the slab surface temperature is in the embrittlement temperature region. By preventing bending stress from acting on the slab, surface cracks caused by high-temperature embrittlement at the time of continuous casting slab drawing are prevented.

By the way, in the conventional slab cooling method in which cooling water is sprayed uniformly in the width direction of the continuously cast slab which is continuously drawn, particularly in the case of a slab continuous slab having a large aspect ratio, the slab continuous slab is not slid in the width direction. The cooling rate becomes uneven (the vicinity of the corner of the slab becomes easier to cool than the center: see FIG. 4A). Therefore, it is difficult to control the surface temperature of the slab when passing through the bending portion and the straightening portion of the vertical bending type continuous casting machine so as to be outside the embrittlement temperature region in the entire width direction, and therefore, the center of the slab is Even if the temperature of the part is higher than the embrittlement temperature range, surface cracks may occur near the corner of the slab, and even if the surface temperature of the corner is lower than the embrittlement temperature range, May enter the embrittlement temperature range and cause surface cracks at the center.

Therefore, in order to avoid such a problem, FIG.
As shown in (B), the cooling capacity of the continuous slab slab can be changed in the width direction of the slab, and the cooling capacity on both sides of the slab continuous slab can be set lower than the center in the width direction of the slab. Thereby, the temperature in the width direction of the continuous slab slab becomes substantially uniform, and surface cracks can be prevented in the entire area of the slab continuous slab in the width direction.

The method of adjusting the cooling capacity in the width direction of the slab is not particularly limited, but the most common method is to supply the slab continuous slab 1 to the central portion and both side portions thereof as schematically shown in FIG. The cooling water supply pipes 2a and 2b are provided separately so that the supply water amount (or water temperature) from each can be controlled separately, and the supply water amount (or water temperature) is set so that the cooling capacity at the center is higher than at both sides. There is a control method. In this case, if the cooling water supply pipe is further subdivided in the slab width direction and a gradient is provided so that the cooling capacity gradually decreases toward both sides, the surface temperature in the width direction is made more uniform. Is preferred.

[0018]

EXAMPLES Hereinafter, the structure and operation and effect of the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and can conform to the spirit of the preceding and following examples. Of course, the present invention can be carried out while being changed within the scope, and all of them are included in the technical scope of the present invention.

Example 1 Steel B was melted in a converter using the composition shown in Table 1 as a target value, and the steel B, the composition of which was adjusted by a vacuum degassing apparatus, was supplied to a vertical bending type continuous casting machine. 230m section size per minute
A slab having a thickness of m and a width of 1800 mm was continuously cast. At this time, the amount of cooling water sprayed in a range of 300 mm from both side edges of the slab slab is controlled to be smaller than the amount of cooling water sprayed to the center, so that the surface temperature in the entire width direction is controlled to be substantially uniform. Continuous casting was performed while adjusting the amount of cooling water in the slab feed direction such that the relationship between the position of the slab in the drawing direction and the surface temperature was as shown in FIG. That is, in the figure, 1 is a slab slab, 3 is a mold, A is a vertical portion, B is a bent portion, C is an arc portion, D is a straightening portion (bent-back portion), and E is a horizontal portion. As described above, the surface temperature of the bent portion B exceeds the embrittlement temperature range (700 to 900 ° C.) of the steel A,
The surface temperature of the straightening portion D was adjusted to be lower than the brittle temperature range of the steel A. As a result, no surface cracks occurred in the obtained slab slab.

On the other hand, for comparison, continuous casting was performed while adjusting the amount of cooling water so that the surface temperature of each slab of the bent portion and the straightening portion was as shown in the comparative example of Table 2, and the slab taken off was obtained. Surface cracks were observed in 30% of the slabs.

[0021]

[Table 2]

Example 2 As a steel material, steel B shown in Table 1 (brittle temperature range: 650 to 1)
Slab continuous casting was performed in the same manner as in Example 1 except that the casting speed was set at 2.0 m / min and the cooling conditions were as shown in Table 3. No surface cracks were observed in the continuous cast slab. For comparison, continuous casting was performed while adjusting the amount of cooling water so that the surface temperature was as shown in the comparative example in Table 3. As a result, 50% of the surface cracks occurred in the slab slab to be taken. I was

[0023]

[Table 3]

[0024]

The present invention is constituted as described above, and even when slab continuous casting is performed at a speed of 1.6 m / min or more using a vertical bending type continuous casting machine, the surface of the bent portion and the straightening portion can be obtained. By controlling the temperature in the continuous slab slab drawing direction such that the temperature is out of the embrittlement temperature range determined according to the type of steel used, it is possible to eliminate the surface cracks of the slab slab.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of a high-temperature tensile test of steel A used in an experiment.

FIG. 2 is a graph showing the results of a high-temperature tensile test of steel B used in the experiment.

FIG. 3 is a graph showing the relationship between the temperature and the aperture value when the N content in steel is changed.

FIG. 4 is a diagram showing the relationship between the cooling capacity in the slab slab slab width direction and the slab surface temperature.

FIG. 5 is an explanatory view showing a preferred cooling method employed in the present invention.

FIG. 6 is a graph showing the relationship between the position of the slab in the drawing direction and the surface temperature during continuous slab casting employed in the example.

FIG. 7 is a graph showing the relationship between the position in the drawing direction of the slab and the surface temperature during continuous slab casting employed in the example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Slab slab 2a, 2b Cooling water supply pipe 3 Mold A Vertical part B Bending part C Circular part D Straightening part E Horizontal part

────────────────────────────────────────────────── (5) References JP-A-53-106335 (JP, A) JP-A-56-109149 (JP, A) JP-A-63-154250 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) B22D 11/124 B22D 11/16 B22D 11/22

Claims (2)

(57) [Claims] 1. Using a vertical bending type continuous casting apparatus,
In performing slab continuous casting of steel at a casting speed of 6 m / min or more, the surface temperature of the continuous cast piece when the continuous cast piece passes through the first bent portion is set to a temperature exceeding the brittle temperature range of the steel material. And a surface temperature of the continuous cast piece when the continuous cast piece passes through a straightening portion on the downstream side thereof is set to a temperature lower than the embrittlement temperature range. However, the embrittlement temperature range means a temperature range in which the drawn value in a high-temperature tensile test of a steel material is 50% or less of the drawn value in the tensile test. 2. The continuous casting method according to claim 1, wherein the cooling capacity on both side edges in the width direction of the slab slab to be drawn is made smaller than that in the center portion, and the surface temperature in the width direction is made uniform.