JPH06246411A - Continuous casting method - Google Patents

Abstract

PURPOSE:To eliminate the surface crack of a cast slab by controlling the temp. of the continuously cast slab in the drawing direction so that the surface temps. at a bending part and a straightening part are apart from a temp. range becoming brittleness cided according to the used kind of steel, in the case of executing the continuous casting to the slab at a high by using a vertical bending type continuous casting. CONSTITUTION:At the time of executing the continuous casting to the steel slab at the casting speed of >=1.6m/min by using the vertical bending type continuous casting apparatus, the surface temp. of the cast slab 1 when the cast slab 1 passes through the first bending part B, is set to the temp. exceeding the temp. range becoming the brittleness of the steel material. Further, the surface temp. of the cast slab 1 when the cast slab 1 passes through the straightening part D at the downstream side, is set below the temp. range becoming the brittleness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, when performing continuous slab casting using a vertical bending type continuous casting machine, casts 1.6 m / min or more without causing surface cracks (grain boundary cracks) of the continuous cast pieces. The present invention relates to a continuous casting method devised so that continuous casting can be performed at high speed.

[0002]

2. Description of the Related Art As a cooling method in a secondary cooling zone when performing continuous slab 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 continuously cast piece being drawn, and continuously casting to the target position. A method is known in which the amount of cooling water in each cooling zone in the longitudinal direction of the continuous cast piece to be drawn is adjusted so that the piece can be completely solidified.

However, the above method (1) is a method of uniquely setting the amount of cooling water (hereinafter, sometimes referred to as a specific water amount) with respect to the weight of the continuous cast piece passing through the secondary cooling zone in accordance with the casting speed. Yes, it is a cooling control method mainly for stable operation, and for surface cracks (intergranular cracks) that are often a problem in the bending part and straightening part (bending-back part) of vertical bending type continuous casting. Is not considered.

On the other hand, in the above method (2), the total amount of cooling water with respect to the casting speed obtained in the same manner as in the above method (1) is distributed to each zone in the longitudinal direction of the secondary cooling zone. Although it is intended to improve the surface cracks and internal cracks of the continuous cast slab, in the case of slab continuous casting, the flatness ratio of the slab is large, so the surface temperature of the corner part 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 cracking only by adjusting the amount of cooling water only in the longitudinal direction of the continuous cast piece.

Surface cracks, which are particularly problematic in vertical bending continuous casting, generally tend to become more prominent as the casting speed is increased, and this tendency also applies to the cooling control of the above (2). Therefore, even when the method (2) is adopted, it is the actual situation that the casting speed is suppressed to about 1.5 m / min or less so as not to cause surface cracking of the slab cast piece.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its objects are: 1. When performing continuous slab casting using a vertical bending type continuous casting machine.
Even if a casting speed of 6 m / min or more is adopted, it is intended to establish a technique capable of smoothly performing continuous casting with good productivity without causing surface cracks in a slab continuous cast piece.

[0007]

The structure of the continuous casting method according to the present invention, which has been able to solve the above-mentioned problems, uses a vertical bending type continuous casting apparatus and is capable of casting steel at a casting speed of 1.6 m / min or more. In performing slab continuous casting, the surface temperature of the continuous cast piece when the continuous cast piece passes through the first bending portion is set to a temperature exceeding the embrittlement temperature range of the steel material, and the continuous cast piece is downstream thereof. The gist 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, the embrittlement temperature range means a temperature range in which the drawing value in the high temperature tensile test of the steel material as described later is 50% or less of the drawing value in the tensile test.

[0008]

In order to establish a method capable of preventing surface cracks even when a casting speed higher than that of the conventional example is adopted when performing vertical bending type continuous casting, the inventors of the present invention firstly study the mechanism of surface cracking. Researched. As a result, the following facts were confirmed.

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

For example, the reduction value (RA) obtained by a high temperature tensile test of steel A having the composition shown in Table 1 below.
Shows a low value in a temperature range of 700 to 900 ° C. as shown in FIG. 1, and in this temperature range, an embrittlement phenomenon occurs in which tensile strength is lowered and ductility is also lowered. Similarly, FIG. 2 shows the high-temperature tensile test result of Steel B, and the range of 700 to 1000 ° C. is the embrittlement temperature region.

[0011]

[Table 1]

FIG. 3 shows the results of examining the relationship between the nitrogen content in steel and the embrittlement temperature range. The embrittlement temperature range tends to expand as the nitrogen content increases. Can be confirmed. Further, observation of the fracture surface of the test piece used in this experiment confirmed the precipitation of nitrides at the grain boundaries,
It is considered that the embrittlement phenomenon occurred due to stress concentration starting from the fine nitrides precipitated at the grain boundaries.

In the vertical bending type continuous casting machine, when the surface temperature of the slab is within the embrittlement temperature range when the slab passes through the bending part and the straightening part (bending back part), the surface layer of the slab is Therefore, it is considered that segregation of nitride causes grain boundary fracture due to bending stress in the crystal grain boundary, which easily leads to surface cracking.

The present invention has been completed as a result of further research based on the above findings, and is completed in Examples described later (FIG. 5, FIG.
As described in detail in 6), the surface temperature of the slab when passing through the bending part of the vertical bending continuous casting machine is controlled to a temperature exceeding the embrittlement temperature region specific to the slab, and the downstream side thereof. The slab surface temperature when passing through the straightening section located at is controlled by controlling the cooling conditions so that the slab surface temperature falls below the embrittlement temperature region, whereby the slab surface temperature is kept in the embrittlement temperature region during casting. By preventing bending stress from acting on the piece, surface cracking due to high temperature embrittlement during drawing of the continuous cast piece is prevented.

By the way, in the conventional slab cooling method in which the cooling water is uniformly sprayed in the width direction of the continuously cast slab that is continuously drawn, particularly in the case of a slab continuous slab having a large aspect ratio, the The cooling rate becomes non-uniform (it becomes easier to cool near the corners of the slab than at the center: see FIG. 4 (A)). Therefore, it is difficult to control the surface temperature of the slab when passing through the bending part and the straightening part of the vertical bending type continuous casting machine so as to be out of the embrittlement temperature range in the entire width direction. 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, the surface temperature of the center part However, there is a possibility that surface cracks will occur in the central part when entering the embrittlement temperature range.

Therefore, in order to avoid such a problem, FIG.
As shown in (B), the cooling ability of the slab continuous cast piece can be changed in the width direction of the cast piece, and the cooling ability on both sides of the continuous piece of the slab continuous piece can be set to be low, As a result, the temperature in the width direction of the continuous slab slab becomes substantially uniform, and surface cracks can be prevented in the entire width direction of the continuous slab slab.

The method for adjusting the cooling capacity in the width direction of the cast slab is not particularly limited, but the most common method is to supply the slab continuous cast 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 separately provided so that the amount of supplied water (or water temperature) from each can be controlled separately, and the amount of supplied water (or water temperature) is adjusted so that the cooling capacity of the central part is higher than that of both side parts. The method of controlling is mentioned. In this case, if the cooling water supply pipe is further subdivided in the width direction of the slab and a gradient is given such that the cooling capacity gradually decreases toward both sides, the surface temperature in the width direction becomes more uniform. It is possible to do so, which is preferable.

[0018]

EXAMPLES The constitutions and effects of the present invention will be described in more detail with reference to the following examples, but the present invention is not limited by the following examples and may be adapted to the gist of the preceding and following examples. It is needless to say that the range is changed and carried out, and all of them are included in the technical scope of the present invention.

Example 1 Steel B having the composition shown in Table 1 as a target value was melted in a converter and the composition of the steel B was adjusted by a vacuum degassing apparatus, and the steel B was supplied to a vertical bending type continuous casting machine at a speed of 2.0 m. Section size 230 m / min
A slab of m thickness x 1800 mm width was continuously cast. At this time, by controlling the amount of cooling water sprayed within a range of 300 mm from both side edges of the slab slab to be smaller than the amount of cooling water sprayed to the central portion, it is possible to control the surface temperature in the entire width direction to be substantially uniform. Continuous casting was performed while adjusting the cooling water amount in the slab feed direction so 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 (bending back portion), and E is a horizontal portion, and shown in Table 2. As described above, the surface temperature of the bent portion B exceeds the embrittlement temperature region (700 to 900 ° C.) of the steel A,
The surface temperature of the straightening portion D was adjusted so as to be lower than the embrittlement temperature region of the steel A. As a result, no surface cracking occurred in the obtained slab cast piece.

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 straightened portion was as shown in the comparative example of Table 2, and the slab that was taken Surface cracks were found in 30% of the cast pieces.

[0021]

[Table 2]

Example 2 As a steel material, Steel B in the above Table 1 (brittle temperature range: 650 to 1)
000 ° C.), the casting speed was set to 2.0 m / min, and the cooling conditions were as shown in Table 3, and slab continuous casting was performed in the same manner as in Example 1 above. No surface cracks were observed on the continuous cast piece. For comparison, when continuous casting was performed while adjusting the amount of cooling water so that the surface temperature was as shown in the comparative example of Table 3, the slab slabs to be taken off had surface cracks at a ratio of 50% of the whole. Was there.

[0023]

[Table 3]

[0024]

EFFECTS OF THE INVENTION The present invention is constructed as described above, and even when performing continuous slab casting at a speed of 1.6 m / min or more using a vertical bending type continuous casting machine, the surface of the bending portion and the straightening portion Surface cracking of the slab cast piece can be eliminated by controlling the temperature of the continuous cast piece drawing direction so that the temperature is out of the embrittlement temperature range determined depending on the steel type used.

[Brief description of 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 high temperature tensile test of Steel B used in the experiment.

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

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

FIG. 5 is an explanatory view showing a preferable cooling method adopted in the present invention.

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

[Explanation of symbols]

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

Claims (2)

[Claims] 1. A vertical bending type continuous casting apparatus is used.
When performing continuous slab casting of steel at a casting speed of 6 m / min or more, the surface temperature of the continuous casting piece when the continuous casting piece passes through the first bending portion is set to a temperature exceeding the embrittlement temperature range of the steel material. In addition, the continuous casting method is characterized in that the surface temperature of the continuous cast piece when the continuous cast piece passes through the straightening portion on the downstream side is set to a temperature lower than the embrittlement temperature range. However, the embrittlement temperature range means a temperature range in which the drawing value of the steel material in the high temperature tensile test is 50% or less of the drawing value in the tensile test. 2. The continuous casting method according to claim 1, wherein the cooling ability of both side edges in the width direction of the drawn slab slab is made smaller than that in the central portion, and the surface temperature in the width direction is made uniform.