JP4747954B2 - Continuous casting method of high alloy steel - Google Patents

Description

  The present invention relates to a continuous casting method for high alloy steel having a total content of Ni and Cr of 40% by mass or more, and more specifically, by reducing the depth of oscillation marks generated during continuous casting. The present invention relates to a continuous casting method of high alloy steel that can prevent cracking during forging of a slab made of high alloy steel.

  In continuous casting of cast slabs, the molten steel cast into the mold is rapidly cooled to form a solidified shell, which is continuously pulled out of the mold by a plurality of guide rolls and pinch rolls arranged below the mold. Further cooling with secondary cooling water.

  Thus, when continuously casting a slab, in order to prevent seizure of the solidified shell generated from the molten steel cast into the mold to the inner surface of the mold and reduce friction with the mold, While adding powder as a lubricant, the mold is vibrated in the vertical direction (hereinafter also referred to as “mold vibration”).

  The mold vibration described above is effective in preventing seizure between the solidified shell and the mold, but on the other hand, a recess called an oscillation mark is formed on the surface of the slab due to the vibration of the mold. When the oscillation mark is deep, there is a problem that cracking occurs starting from the oscillation mark in forging performed after continuous casting.

  As a means for solving the surface crack caused by such an oscillation mark, it is effective to reduce the depth of the oscillation mark.

  FIG. 1 is a diagram schematically showing the relationship between the mold vibration speed and the casting speed when the vibration waveform is a sine wave, based on the elapsed time. The broken line portion in FIG. 1 is a time zone in which the mold lowering speed is higher than the casting speed, that is, a time period in which the relative speed of the slab with respect to the mold is opposite to the casting direction, and is referred to as a negative strip time. .

  It is known that as the negative strip time becomes shorter, the depth of the oscillation mark generated on the slab surface due to mold vibration can be reduced. As a means for reducing the depth of the oscillation mark using such characteristics, a method of increasing the frequency of mold vibration is generally used.

  However, when the mold vibration frequency, that is, the vibration frequency is increased, the movement of the actuator may not match the movement of the mold due to insufficient rigidity of the connection portion between the mold and the drive actuator. As described above, it is not easy to vibrate the mold according to the target waveform, and there is a limit to reducing the negative strip time only by increasing the frequency of the mold vibration.

  Furthermore, a high alloy steel having a total content of Ni and Cr of 40% by mass or more has a wider solid-liquid coexistence temperature range, a lower melting point, and an initial solidified shell than stainless steel such as ordinary steel and SUS304. It is characterized by low strength. Therefore, when a high alloy steel with a high content of Ni and Cr is continuously cast under the same casting conditions as ordinary steel and stainless steel, breakout may occur, and the slab surface may be rashed. And serious defects such as vertical cracks may occur. For this reason, various methods have been proposed for the purpose of reducing the oscillation marks on the surface of the slab.

  In Patent Document 1, the amount of powder flowing into the gap between the mold and the solidified shell is controlled by controlling the heat flux of the mold and setting the negative strip time to 0.10 to 0.20 (s). Steel continuous casting technology that can prevent occurrence of vertical cracks and breakout has been disclosed.

  However, as described above, it is difficult to prevent cracks that occur when forging a continuously cast high alloy steel simply by shortening the negative strip time as a condition for mold vibration.

  In Patent Document 2, by avoiding the surface crack at the corner portion of the slab, the convex portion of the oscillation mark generated at the corner portion of the slab is crushed at a position above the bent portion. There has been disclosed a method for producing a continuous cast slab that can go straight from a continuous casting to a rolling process without requiring an auxiliary step such as maintenance.

  However, in the method disclosed in Patent Document 2, it is possible to reduce the depth of the oscillation mark on the long side surface in contact with the roll, but the oscillation mark generated on the short side surface that does not contact the roll. The depth is not reduced, and as a result, cracking occurs during forging, so it is difficult to solve the above problem.

  In Patent Document 3, a mold powder and a liquid lubricant are used in combination as a lubricant between the mold and the slab, and the negative strip time due to the mold oscillation is set to 0.25 (s) or less, whereby the surface of the slab is obtained. The continuous casting method of steel which can reduce the oscillation mark of this and can reduce a slab surface crack and a subepidermal inclusion is disclosed.

  However, high alloy steel has the characteristics that the solid-liquid coexistence temperature range is wide, the melting point is low, and the initial solidified shell strength is low compared to ordinary steel, etc. By simply applying the shortening to continuous casting of high alloy steel, it is difficult to prevent cracking during forging of the slab.

Japanese Patent Application Laid-Open No. 08-257695 JP 2000-197953 A JP 2001-170743 A

  As described above, since there is a limit to reducing the negative strip time only by increasing the mold vibration frequency, the conventional technology that only shortens the negative strip time as a condition of the mold vibration is a continuously cast high alloy. It is difficult to prevent cracks that occur when forging steel.

  The present invention has been made in view of the above-mentioned problems. While maintaining the negative strip time within a predetermined short time range, the mold vibration conditions other than the negative strip time are controlled, and continuous casting is performed. It aims at providing the continuous casting method of the high alloy steel which can prevent the generation of the crack at the time of the forging of the slab made of the high alloy steel by reducing the depth of the oscillation mark generated at the time .

  The present inventors not only shorten the negative strip time as a condition of mold vibration, but also control the mold vibration conditions other than the negative strip time, thereby cracking at the time of forging of a slab made of high alloy steel. The continuous casting method of high alloy steel that can prevent the occurrence of metal was studied.

  As described above, it is known to shorten the negative strip time as a means for shallowing the oscillation mark. To shorten the negative strip time, it is effective to increase the frequency and reduce the stroke. .

  However, the present inventors have experimentally determined that the oscillation mark is maintained by reducing the frequency and stroke and reducing the average lowering speed of the mold while maintaining the negative strip time within a predetermined short time range. The knowledge that the depth of can be reduced was obtained.

  Furthermore, in the mold flux conventionally used as a lubricant, the oscillation mark may suddenly become deep, but by using a higher viscosity mold flux, the oscillation mark suddenly becomes deep. We found that the depth can be reduced on average.

The present invention has been completed based on the above findings, and the gist of the continuous casting method under Symbol high alloy steel.
In a continuous casting method of high alloy steel having a total content of Ni and Cr of 40% by mass or more, the mold is vibrated with a sinusoidal vibration waveform, negative strip time T _N (s), average mold lowering speed V _M (M / min) and casting speed V _C (m / min) satisfy the following formulas (1) to (5), and the viscosity at 1300 ° C. is 0.3 Pa · s or more and 2.0 Pa · s or less. A continuous casting method of high alloy steel, characterized by casting using mold flux .

0.16 ≦ T _N <0.25 (1)
T _N = (60 / (π × f)) × cos ⁻¹ (V _C / (π × f × s)) (2)
V _M ≦ 1.0 (3)
V _M = 2 × f × s (4)
0.2 ≦ V _C <0.8 (5)
Where π: Pi
f: Mold frequency (cpm)
s: Stroke of mold vibration (m )

In the present invention, the “average mold lowering speed V _M ” means an average lowering speed over the entire time that the mold is lowered.

  Moreover, in the continuous casting method of high alloy steel of this invention, it aims at reducing the depth of an oscillation mark to less than 0.3 (mm). The reason for this setting is that it has been empirically confirmed that no crack is generated on the surface of the slab when the depth of the oscillation mark is less than 0.3 (mm).

  According to the continuous casting method of the present invention, even in a high-alloy steel having a high Ni and Cr content, which is liable to cause surface defects such as vertical cracks, it is possible to continuously control the mold vibration conditions within an appropriate range. The depth of the oscillation mark generated at the time of casting can be reduced.

  Furthermore, the effect of reducing the depth of the oscillation mark can be further exhibited by using a high-viscosity mold flux as the lubricant between the mold and the slab.

  As a result, it is possible to prevent cracks caused by the oscillation mark when forging a slab made of high alloy steel.

The high alloy steel continuous casting method of the present invention is a high alloy steel continuous casting method in which the total content of Ni and Cr is 40% by mass or more. Casting is performed such that _TN (s), average mold lowering speed V _M (m / min), and casting speed V _C (m / min) satisfy the following expressions (1) to (5). Yes.

0.16 ≦ T _N <0.25 (1)
T _N = (60 / (π × f)) × cos ⁻¹ (V _C / (π × f × s)) (2)
V _M ≦ 1.0 (3)
V _M = 2 × f × s (4)
0.2 ≦ V _C <0.8 (5)
Here, π is the circularity ratio, f is the mold vibration frequency (cpm), and s is the mold vibration stroke (m).

Furthermore, in the continuous casting method of the present invention, it is desirable to use a mold flux having a viscosity at 1300 ° C. of 0.3 Pa · s to 2.0 Pa · s. Below, the reason and the preferable range which prescribed | regulated the continuous casting method of this invention as mentioned above are demonstrated.
1) Negative strip time T _N
In the continuous casting method of the high alloy steel of the present invention, the casting speed V so that the negative strip time T _N (s) in the mold vibration whose vibration waveform is a sine wave satisfies the following expressions (1) to (2). _C (m / min), mold frequency f (cpm), and mold vibration stroke s (m) are adjusted.

0.16 ≦ T _N <0.25 (1)
T _N = (60 / (π × f)) × cos ⁻¹ (V _C / (π × f × s)) (2)
In order to make the negative strip time shown in the above equation (2) less than 0.16 (s), it is necessary to increase the frequency f of the mold. However, for example, when the casting speed is 0.4 (m / min) and the stroke is 3.1 × 10 ^{−3 to} 5.0 × 10 ⁻³ (m), the negative strip time is less than 0.16 (s). In order to achieve this, the frequency needs to be 160 (cpm) or more. At such a high frequency, the lubricity between the mold and the slab deteriorates. For this reason, in the continuous casting method of the high alloy steel of the present invention, the negative strip time T _N is defined as 0.16 (s) or more.

  When the negative strip time is 0.25 (s) or more, the depth of the oscillation mark may exceed 0.3 (mm). As described above, when the depth of the oscillation mark exceeds 0.3 (mm), the surface of the forged product may be cracked.

Therefore, the negative strip time _TN in the mold vibration in which the vibration waveform is a sine wave needs to be in the range of 0.16 (s) or more and less than 0.25 (s).
2) Average mold lowering speed V _M
In the continuous casting method of the high alloy steel of the present invention, the average mold lowering speed V _M (m / min) in the mold vibration whose vibration waveform is a sine wave satisfies the following formulas (3) to (4): The mold frequency f (cpm) and the mold vibration stroke s (m) are adjusted.

V _M ≦ 1.0 (3)
V _M = 2 × f × s (4)
The present inventors have summarized the relationship between the pressure P _X (Pa) of the initial solidified shell of the mean mold descending speed V _M and the molten steel is subjected to give (6) below.

P _X = 6 × μ × (V _C + V _M ) × Q _X + P ₀ (6)
Where μ: viscosity of mold flux (Pa · s), Q _X : term depending on flow rate of mold flux, P ₀ : atmospheric pressure (Pa).

From the above equation (6), when the average mold descending speed V _M is small, the initial solidified shell in the mold it was confirmed that the pressure P _X received from the melting of the mold flux is reduced.

Further, when the continuous casting of high-alloy steel, (4) the average template lowering speed V _M in the expression of more than 1.0 (m / min), large shearing force acting on the initial solidification shell, oscillation May further increase the depth of the mark.

Therefore, the average mold lowering speed needs to be 1.0 (m / min) or less, preferably 0.9 (m / min) or less.
3) Casting speed V _C
When continuously casting high alloy steel, if the casting speed is 0.8 (m / min) or higher, the solidified shell may be broken because the strength of the initial solidified shell is weak. On the other hand, when the casting speed is less than 0.2 (m / min), the molten steel temperature in the ladle is lowered, and nozzle clogging may occur. Considering the influence of these casting speeds, the casting speed needs to be in the range of 0.2 (m / min) or more and less than 0.8 (m / min).
4) Mold Flux In the continuous casting method of the high alloy steel of the present invention, the reason for using the high viscosity mold flux as the lubricant is that the melt flux film in the mold becomes thick by adding the high viscosity mold flux. This is because the pressure acting on the initial solidified shell decreases.

  When continuously casting high alloy steel, if the viscosity of a mold flux used as a lubricant at 1300 ° C. is less than 0.3 Pa · s, the oscillation mark may suddenly become deep. On the other hand, when the viscosity of the mold flux at 1300 ° C. exceeds 2.0 Pa · s, the lubrication performance between the mold and the slab deteriorates, surface defects such as vertical cracks occur frequently, and further breakout occurs. May occur. For this reason, it is desirable that the viscosity of the mold flux is 0.3 Pa · s or more and 2.0 Pa · s or less. More preferably, it is 0.4 Pa · s or more and 2.0 Pa · s or less.

Further, the mold flux has a property that its viscosity increases by increasing the concentration of contained SiO ₂ . Therefore, by adjusting the concentration of SiO ₂ , the viscosity of the mold flux can be set within a desirable range defined in the present invention.

In order to confirm the effect of the present invention, the following continuous casting test was performed to evaluate the depth of the oscillation mark generated on the surface of the slab.
[Test conditions]
Table 1 shows the chemical composition of the high alloy steel used in this test, and Table 2 shows the physical properties and chemical composition (representative components) of the mold flux.

In this test, as shown in Table 1, high alloy steel having a total content of Ni and Cr of 40% by mass or more was used. Further, as shown in Table 2, the mold flux A has a high viscosity of 0.50 Pa · s. Since the viscosity of the mold flux increases by increasing the concentration of SiO ₂ contained, the viscosity of the mold flux A used in the present invention was also increased by the same method.

  Furthermore, Table 3 shows the specifications of the continuous casting machine used in this test, and Table 4 shows the vibration conditions of the mold vibration.

As shown in Table 4, the vibration condition 1 is such that the negative strip time T _N of the mold vibration is constant within 0.16 (s) or more and less than 0.25 (s), and the average mold lowering speed V _The frequency, stroke, and casting speed V _C were set so that _M was 1.0 (m / min) or less. In addition, the vibration frequency, stroke, and casting speed V _C were set so that the average mold lowering speed V _M in vibration condition 2 and the negative strip time T _N in vibration condition 3 exceeded the ranges specified in the present invention, respectively. The casting speed V _C was 0.4 (m / min) under all vibration conditions.
[Evaluation methods]
Continuous casting was performed under the above conditions, and the depth of the oscillation mark generated on the surface of the obtained slab was measured. From the measured values, the average oscillation mark depth and the maximum oscillation mark depth were calculated. The evaluation of the depth of the oscillation mark is evaluated as ○ when the ratio where the depth of the oscillation mark is 0.3 (mm) or more is 0%, and as Δ when it exceeds 0% and 10% or less. The case where the evaluation exceeded 10% was defined as a cross mark evaluation.
[Test results]
Table 5 shows the test results of test numbers T1 to T5.

  In Invention Example T1 in which the vibration condition satisfies the range defined in the present invention and the mold flux A having high viscosity is used, the maximum oscillation mark depth was not 0.3 (mm) or more. Further, as is clear from the comparative example T4, the present invention example T2 using the low-viscosity mold flux B has the oscillation mark shallower when the vibration condition satisfies the range defined by the present invention.

  On the other hand, Comparative Example T3 uses a high-viscosity mold flux, but the depth of the oscillation mark is improved as compared with other Comparative Examples T4 and T5, but the vibration conditions are within the range specified by the present invention. The maximum oscillation mark depth was 0.3 (mm) or more.

  Further, in Comparative Examples T4 and T5 in which the vibration condition does not satisfy the range defined in the present invention and the low-viscosity mold flux B is used, the maximum oscillation mark depth is 0.5 (mm) or more. The average oscillation mark depth was also deep.

  From the comparison of the maximum oscillation mark depths of Comparative Examples T4 and T5, it was found that the specifications of the continuous casting machine had a small effect on the depth of the oscillation mark.

  According to the continuous casting method of the present invention, it is possible to control the vibration conditions of the mold within an appropriate range even for high alloy steels with high Ni and Cr contents that are prone to surface defects such as rashes and vertical cracks. Therefore, the depth of the oscillation mark generated during continuous casting can be reduced.

  Furthermore, the use of a high-viscosity mold flux as the lubricant between the mold and the slab can further exert the effect of reducing the depth of the oscillation mark.

  As a result, it is possible to prevent cracks caused by the oscillation mark during forging performed after continuous casting.

It is a figure which shows typically the relationship between the vibration speed of a casting_mold | template and casting speed in case a vibration waveform is a sine wave based on elapsed time.

Claims (1)

In the continuous casting method of high alloy steel in which the total content of Ni and Cr is 40% by mass or more,
The mold is vibrated with a sinusoidal vibration waveform, and the negative strip time T _N (s), the average mold lowering speed V _M (m / min), and the casting speed V _C (m / min) are the following (1) to ( 5) satisfies the equation ,
A continuous casting method of high alloy steel, characterized by casting using a mold flux having a viscosity at 1300 ° C. of 0.3 Pa · s to 2.0 Pa · s .
0.16 ≦ T _N <0.25 (1)
T _N = (60 / (π × f)) × cos ⁻¹ (V _C / (π × f × s)) (2)
V _M ≦ 1.0 (3)
V _M = 2 × f × s (4)
0.2 ≦ V _C <0.8 (5)
Where π: Pi
f: Mold frequency (cpm)
s: Stroke of mold vibration (m )

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