JPS6139144B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a continuous casting method for stainless steel slabs that prevents internal cracking of continuously cast stainless steel slabs. As is well known, internal cracks often occur in slabs during continuous casting of stainless steel slabs.
This internal cracking occurs before the slab is completely solidified, and therefore, at the time of cracking, the concentrated molten steel at the final stage of dendrite solidification enters the area and forms large chain-shaped sulfides. This impairs the bendability of the product and poses a quality problem. The present invention relates to a continuous casting method for stainless steel slabs that prevents the occurrence of internal cracks in continuously cast slabs that cause product defects. In the secondary cooling zone after
/Kg- steel or more, the length of the secondary cooling zone is extended to a length that allows cooling water to be injected below the fully solidified position of the slab, and this secondary cooling zone is divided into multiple zones (preferably The total amount of secondary cooling water is divided into 4 zones) and the total amount of secondary cooling water is distributed in the ratio of water injection amount in the upper zone (upper 2 zones): water injection amount in the lower zone (lower 2 zones) = 6 to 7: 3 to 4. be. As a result, it is possible to suppress the expansion of the solidified slab shell due to the static pressure of the remaining molten steel, and also to prevent the generation of thermal stress within the solidified shell, thereby preventing the occurrence of internal cracks. In addition, the secondary cooling zone should be positioned below the completely solidified position of the slab.
It is uneconomical to extend it further than 2m, and the amount of water injected per unit length of slab should be constant for each zone (if subdivided, for each subdivided zone). However, the length of the upper zone is preferably 1/3 to 2/5 of the total length of the secondary cooling zone. Hereinafter, the principle and embodiments of this method will be explained in detail based on test results. A slab cast into a mold by the continuous casting method is cooled by the mold and secondary cooling zone, solidifying from the surface and moving downward, but some unsolidified molten steel still exists. In this range, the static pressure of the molten steel will be applied to the solidified shell of the slab. Therefore, if the solidified shell is too weak to withstand the static pressure of the molten steel, the solidified shell will expand between the rolls. The swollen slab is then subjected to a reduction when it comes into contact with the rolls, and at this time tensile stress is generated inside the solidified shell. However, the solidified shell,
In particular, since the temperature near the inside is extremely high near the solidus line, the tensile strength is extremely low, and therefore, cracks occur due to the tensile stress caused by rolling reduction. This is one mechanism by which internal cracks occur. In addition, if the distribution of cooling water in the secondary cooling zone is inappropriate, when the slab moves from one zone to the zone below it, or from the secondary cooling zone to the natural cooling zone, , reheating occurs on the surface side of the slab. However, if unsolidified molten steel still remains inside the slab at the time (position) where this reheating occurs, tensile stress is generated inside the slab shell. On the other hand, since the inner side of this solidified shell is at high temperature and has extremely low tensile strength as described above, internal cracks will occur in this case. This is the second mechanism for the occurrence of internal cracks. Figure 1 shows the results of an investigation of the stress distribution in the longitudinal direction of the slab in the thickness direction of the solidified shell during reheating of the surface temperature of the slab, which provided a clue as to the second mechanism.
However, in Figure 1, a represents the temperature distribution and stress distribution in the solidified shell thickness direction at the start of reheating, and b represents the 60°
These are shown after reheating at °C, where T _SL is the solidus temperature position and SC is the slab center. In view of the first and second internal crack generation mechanisms, measures to prevent their occurrence include (1) eliminating any bulge between the rolls of the solidified slab shell;
and (2) it is effective and important to prevent reheating from occurring on the surface of the slab before it is completely solidified. As a result of intensive study on a method of injecting cooling water into the secondary cooling zone that satisfies these conditions, the inventors determined that the amount of cooling water injected should be 1.0/Kg-steel or more, and the length of the secondary cooling zone should be If the cooling zone is extended below the complete solidification point of the piece, and the secondary cooling zone is divided into 4 zones, and the distribution ratio of water injection amount between the upper 2 zones and the lower 2 zones is controlled at 6 to 7 to 3 to 4. For example, it has been found that conditions (1) and (2) above are satisfied, thereby completely preventing the occurrence of internal cracks during continuous casting of stainless steel slabs. Figure 2 shows one of the results of investigating the effect of secondary cooling water distribution on the transition of SUS430 steel slab surface temperature.
This is an example, and the amount of secondary cooling water is 1.0/
Kg - steel, and divide the secondary cooling zone into four zones 1, 2, 3, and 4 at the distance shown on the vertical axis in Figure 2 (M is molded, AC is natural cooling zone). band),
The distribution of cooling water to the upper (1+2) zone and the lower (3+4) zone is shown in A, B, and Table 1 below.
It shows the change in the surface temperature of the cast slab when it is made as shown in C. In addition, in the example of Fig. 2, the upper zone (1+2)
The length of is approximately 1/3 of the total secondary cooling zone length,
The length of the lower zone (3+4) is the total secondary cooling zone length minus the upper two zones (that is, approximately 2/3 of the total secondary cooling zone length), and this lower zone (actually the zone 4) shows an example in which a complete solidification position exists, but the division ratio of the length of this upper zone and lower zone has a length of 2/5 or less of the total length of the secondary cooling zone. an upper zone;
It is sufficient to divide the secondary cooling zone into a lower zone having a length equal to the total length of the secondary cooling zone minus the upper zone and including the complete solidification position within that length. The present invention is suitably implemented by dividing the secondary cooling water and then distributing the total amount of secondary cooling water in the ratio of upper zone water injection amount: lower zone water injection amount = 6 to 7: 3 to 4. I can do it.

[Table] According to FIG. 2, in the case of cooling water distribution condition A according to the present invention, the surface temperature of the slab is 2.
When it moves to the next cooling zone, it rapidly drops to around 900℃, and after that, it maintains a relatively uniform temperature between 870 and 900℃ until the slab moves to the natural cooling zone, AC. No reheating of the slab surface temperature was observed. A sample was taken from a SUS430 steel slab that was continuously cast under Condition A, and the occurrence of internal cracks was investigated, but no internal cracks were observed. On the other hand, in condition B, the lower part (3+4)
Since the mixing ratio of cooling water to the zone is slightly lower than in condition A, when the slab moves from zone 2 to zone 3, reheating occurs with a maximum surface temperature of about 85°C. When samples were taken from the SUS430 steel slab cast under Condition B and the occurrence of internal cracks was investigated, quite severe internal cracks were observed. . The time of occurrence was calculated from the position of the starting point of this internal crack, and it was found to be the point at which the slab moved 30 cm downward from the start of recuperation (the point at which the amount of recuperation reached approximately 40°C). Condition C is a short secondary cooling zone (conventional example) in which the slab leaves the secondary cooling zone before it is completely solidified (there is no fourth zone in Fig. 2), and a lower zone (no fourth zone in Fig. 2). The distribution of cooling water to 3 zones) is also low. In the case of condition C, as shown in Fig. 2, after leaving the secondary cooling zone (after leaving the third zone), the surface temperature of the slab recuperates by approximately 140°C. When samples were taken from the SUS430 steel slab cast under Condition C and investigated for the occurrence of internal cracks, it was observed that internal cracks occurred frequently. Also, condition B
Similarly, after calculating the timing of internal cracking based on the location of the starting point of the crack, it was determined that it was the time when the internal crack was pulled out by 20 cm. A comparison of Conditions A and B above shows that it is important to prevent internal cracking from occurring on the surface of the slab, and a comparison of Conditions A and C shows that the length of the secondary cooling zone is It is clear that it is necessary to extend the length of the casting to such a length that the cooling water is injected below the point of complete solidification of the slab. It is very effective to distribute the cooling water in the secondary cooling zone so that recuperation does not occur, or even if it does occur, it is kept to a low level (approximately 30℃ or less) that does not cause internal cracks. It is clear that this distribution ratio is sufficiently satisfactory if, as in condition A, the amount of water injected into the upper two zones: the amount of water injected into the lower two zones = 65:35. This distribution ratio may be slightly changed as shown in the examples below, and is 60 to 70:30 to 40, that is, 6 to
The objective of the invention is achieved with a distribution ratio of 7:3-4. In order to appropriately distribute the amount of cooling water injected in this way, it is necessary to divide the secondary cooling zone into a number of zones.From the viewpoint of equipment maintenance, it is preferable to divide the secondary cooling zone into four zones, and the upper two It is convenient for actual operation to set the distribution ratio of the water injection amount between the zone and the lower two zones as described above. Figure 3 shows the influence of the amount of secondary cooling water on the occurrence of internal cracks. From the results shown in Figure 3, 1.0
/Kg - If the amount of cooling water is higher than steel, it can be cast without internal cracking even if the casting speed is increased, but if the amount of cooling water is less than this, the tendency for cracking to occur becomes more pronounced as the amount is smaller. I understand. Therefore, it is an important requirement for the present invention to increase the amount of secondary cooling water injected to 1.0/Kg-steel or more in order to prevent the occurrence of internal cracks. It will be possible to produce. In addition,
Internal cracks are more likely to occur at low water injection amounts, less than 1.0/Kg-steel, because the surface temperature of the slab increases,
This is thought to be because the strength of the solidified shell deteriorates and bulges tend to occur between the rolls. Below, some examples of the present invention and comparative examples are summarized in Table 2. In addition, in these Examples and Comparative Examples,
The length of the upper zone (Zone 1+Zone 2) was about 1/3 of the total secondary cooling zone length, as in the example of FIG.

[Table] In Table 2, Comparative Examples 1 and 2 are conventional examples in which the slab exits the secondary cooling zone before completely solidifying, and the distribution of secondary cooling water is also outside the scope of the present invention. Comparative Examples 3 and 4 are examples in which the length of the secondary cooling zone and the distribution of secondary cooling water are within the range of the present invention, but the amount of water injected is less than 1.0/Kg-steel. Comparative Examples 5 and 6 are examples in which the length of the secondary cooling zone and the amount of water injection are within the range of the present invention, but the distribution of the amount of water injection is outside the scope of the invention. In Examples 7 and 8, the length of the secondary cooling zone was extended to the point below where the cooling water was injected past the completely solidified position of the slab, and the amount of water injected was also 1.0/Kg-steel or more, and the upper zone The distribution of the amount of water injected into the lower zone and the lower zone is also in the range of 6-7:3-4, which is the implementation result according to the present invention. From the results in Table 2, it is clear that even if one of the water injection conditions of the present invention is missing, the occurrence of internal cracks cannot be suppressed.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the temperature distribution in the thickness direction of the solidified shell of a continuous slab and the internal stress generated thereby, and Figure 2 shows the influence of the secondary cooling water distribution conditions on the transition of the slab surface temperature. FIG. 3 is a relational diagram showing the influence of the amount of secondary cooling water injected on the occurrence of internal cracks.

Claims (1)

[Claims] 1. During continuous casting of stainless steel slabs, in the secondary cooling zone after the slab passes through the mold,
The length of this secondary cooling zone is extended to a length that allows cooling water to be injected within 2m below the fully solidified position of the slab, and the amount of secondary cooling water injected is reduced to 1.0%.
Kg steel or more, and this secondary cooling zone is 1/3 to 2/5 of the total length.
The area is divided into an upper zone having the same length and a lower zone having the remaining length and including the completely solidified position within that length, and the amount of water injected per unit length of the slab is constant for each zone. At the same time, the total amount of secondary cooling water is calculated as follows: Upper zone water injection amount: Lower zone water injection amount =
A continuous casting method for stainless steel slabs, characterized in that the ratio is 6-7:3-4. 2. A patent claim in which the upper zone is further subdivided into two zones, the lower zone is further subdivided into two zones, and the amount of water injected per unit length of slab is constant for each of these subdivided zones. A continuous casting method for stainless steel slabs according to item 1.