JPH0225257A - Method of estimating generation of central crack in continuous casting - Google Patents

Abstract

PURPOSE:To make adequate processing and to prevent the central crack by deciding the generation of the entrapment of an unsolidified molten steel and predicting the generation of the central trap when the entrapment quantity of the unsolidified molten steel exceeds the permissible limit quantity. CONSTITUTION:The generation of the entrapment of the unsolidified molten steel is decided when the time difference tf1 from the measurement time detected at a set period T by a measuring instrument before the complete solidification based on the measured time and the time difference tf2 of the next time measurement satisfies equation I at the time of estimating the complete solidification position of an ingot from the measured value by the shell thickness measuring instrument or ingot section temp. measuring instrument installed to a machine end part. The entrapment quantity of the unsolidified molten steel is calculated by equation II. In equation, Q: the entrapment quantity of the unsolidified molten steel; (n): the number of the times when the decision of the entrapment is continuously generated; tf1: the time difference before the complete solidification at the measurement time, tfi+n: the time difference before the measurement time when the tf1 at the generation of the entrapment decision of the unsolidified molten steel continuously (n) times is obtd. before the complete solidification after TXn; Vc: the casting speed, T: measurement period. The generation of the central crack is decided when the entrapment quantity exceeds the permissible limit quantity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for estimating the occurrence of center cracks in continuously cast slabs in continuous steel casting equipment.

[Conventional technology]

In recent years, it has become extremely important to uniformize the solidification state inside the slab during continuous casting operations as products are becoming more sophisticated. In particular, it has been known for a long time that center cracks and center segregation that occur in the center of the slab have a large impact on the quality of the product after rolling, so various methods have been devised to predict them in advance. For example, the present applicant has invented a method of installing slab surface displacement meters on the front and back surfaces of a slab, measuring the surface displacement of the slab, and estimating the center crack of the slab from the amount of displacement. JP-A-57-114850 was provided.

Furthermore, as shown in Japanese Patent Application Laid-open No. 59-156557, a solidification profile is calculated from the casting conditions, and from the solidification profile, the non-flowing area of unsolidified molten steel at the solidified tip and the amount of bulging in that part are determined, A method of estimating center segregation by calculating the attraction index of concentrated molten steel has also been proposed.

[Problem to be solved by the invention]

In the above-mentioned Japanese Patent Application Laid-Open No. 57-114850, since the surface displacement of the slab is directly measured, the measuring instrument may be damaged and the surface displacement of the slab may not be accurately measured. There has been a problem in that it is difficult to continuously and accurately estimate the occurrence of center cracks over the entire area. In addition, in JP-A-59-156557, various operational fluctuations during casting, such as casting speed, molten steel temperature, secondary As the amount of cooling water changes, the profile of the unsolidified tip will change in a complicated manner, and the calculated profile of the unsolidified tip will not match the actual profile, resulting in the above concentration. There was a problem in that an error occurred in calculating the suction index of the molten steel, making it impossible to accurately estimate the occurrence of center segregation of the slab.

[Means for Solving the Problems] The present invention has been made in view of the above circumstances, and it is possible to determine the thickness of a slab based on the measured value by a shell thickness measuring device installed at the end of the machine or a slab cross-sectional temperature measuring device. When estimating the complete coagulation position, the time difference t0 until complete coagulation based on the measurement time is determined from the measurement value detected by the measuring device at the set cycle ΔT, and the time difference L0 at the next measurement is calculated as follows (1
), it is determined that the entrapment of unsolidified molten steel has occurred, and the enclosed amount of the unsolidified molten steel is calculated according to the following equation (2), and the enclosed amount is determined by comparing the enclosed amount and the center crack in advance. This is a method for estimating the occurrence of a center crack in continuous casting, which is characterized by predicting the occurrence of a center crack when the amount exceeds an allowable limit set based on the correlation with the amount.

tfl>tf!・・・・・・
(1) Q=nXVcXΔT X (2K(tri +
trr−11+ ) ” '・・・・・・(2) However, Q: Entrapment amount of unsolidified molten steel n: Number of consecutive occurrences of entrapment determination trt: Time difference trt until complete solidification at measurement time. 7
: Time difference vc from the measurement time when the jfi was obtained when the containment judgment of unsolidified molten steel occurred 0 times in a row until complete solidification after ΔTXn : Casting speed ΔT : @ Fixed period [effect] Figure 1 shows FIG. 1 is a diagram illustrating a well-known continuous casting facility together with equipment for configuring the method of the present invention.

In Figure 1, from molten steel ladle 1 to tundish 2? 8
Once E50 is injected, molten steel 5 is poured into the mold 4 through the nozzle 3.
0 is injected. The molten steel 50 injected into the mold 4 loses heat from the mold 4 and forms a solidified shell 5 from the side in contact with the mold 4. The solidified shell 5 is supported by a guide roll 6 disposed after the mold 4, and is continuously pulled out by a drawing roll 7. The solidification 5 increases its thickness by spraying cooling water onto its outer surface in the secondary cooling zone 8 after the mold 4. After the completely solidified slab is pulled out from the end of the machine, it is cut into specified dimensions by a cutter 9, and then supplied to the subsequent rolling process.

Now, Fig. 2 schematically shows the situation of center cracks occurring in slabs. As is well known, center cracks are cracks that occur at the center of the thickness of a slab, and cracks occur at certain intervals as shown in FIG.

This interval is generally said to be approximately equal to the circumference of the horizontal roll. As mentioned above, since the slab is cut into specified dimensions with a cutter, a central crack may appear on the cut surface depending on the cutting position. If rolling is carried out with a central crack appearing on the cut surface, the edges will open during rolling, leading to problems that will make rolling impossible. In this way, when a central crack appears on the cut surface, the ends of the slab are generally cut to ensure that there is no central crack on the cut surface, and then rolling is performed.

Furthermore, even if there is no center crack on the cut surface, the quality of the product after rolling may be affected, such as insufficient strength or elongation in the product thickness direction, depending on the degree of cracking.

Generally, this central crack is the tip of unsolidified molten steel (hereinafter referred to as
It is thought that this occurs when the part (called a crater end) fluctuates significantly in the casting direction. The following may be considered as the cause of the fluctuation of the crater end in the casting direction. If the thickness of the slab changes locally due to the slab being rolled down by horizontal rolls, which indirectly changes the position of the crater end, or if the casting speed changes suddenly, the molten steel will leak near the crater end. When flow occurs and molten steel is confined (same effect as the crater end position fluctuates), or when curved continuous casting equipment straightens the slab horizontally, straightening distortion occurs due to straightening. There is a casting method that applies compressive force to the slab with the aim of reducing It is conceivable that the position of the crater end fluctuates in the same way as when a slab is rolled down with a roll.

Now, the inventors of the present invention have determined the position of the crater end 51 shown in FIG. 1 in advance according to casting conditions such as casting thickness, secondary cooling conditions, casting steel type, and casting speed. A measuring device 11 (hereinafter referred to as a complete solidification position detection device) capable of measuring the thickness of the solidified shell using ultrasonic waves or determining the average temperature in the thickness direction of the slab was installed.

First, in order to know the position of the crater end, a method for determining the time difference between the time of complete solidification and the measurement time will be described below. As is well known, when the crater end is behind the completely solidified position detection device 11, that is, when unsolidified molten steel exists at the location of the device 11, the time difference can be determined by the following (3) 2.

t, f = (D"/4-DXdH+dH)/2/'
K...-(3) However, L: Time of complete solidification d, 4: Thickness of solidified shell D: Thickness of slab K: Accelerated solidification coefficient Also, when complete solidification occurs in front of the device 11 The above time difference is as follows (4)
It can be found by 2.

Lr= CR(θ, -θ, U-(θAVE-θs u
) / KK --(4) However, CR: Center cooling rate θ, u: Slab surface temperature θ5
Two solidus temperatures KK: Constant θ^vE: Average temperature in the thickness direction of the slab Here, the time difference between the time of complete solidification and the measurement time is trI, and the measurement period ΔT from the measurement time
Assuming that the subsequent time difference is 111, the state of the crater-end portion can be expressed as in the following equations (1), (5) to (force formula).

Lr, > t tz −'=(1
)tr+1Δ'r=at, ・・・・・・
・(5) tr++ΔT<Ltt ・・
...(6) 1 , , < 1.2 < 1 , , +ΔT
...... (7) In the case of the above equation (5), the position of the crater end is stable without fluctuation, and the above (6)
In the case of formula (7), the crater end extends in the direction of the mold, in the case of formula (7), the crater end shortens in the direction of the mold, and in the case of formula (1), the crater end extends extremely toward the mold. The molten steel is confined at the crater end (
(hereinafter referred to as containment) has occurred. As described above, it is necessary to determine which of the four conditions the crater end is in. And 1. ,
It is important to determine that confinement has occurred only when formula (1), >L, 2, is satisfied.

The containment situation can be schematically described as shown in Figure 3.

The shaded area in FIG. 3 is the area where molten steel is sealed. At this time, the amount of encapsulation (hereinafter referred to as encapsulation amount) Q in the shaded area was determined by the following equation (2).

Q=nXVcXΔT X (2K(tri + ttL
-R) ) 0・'・・・・・・(2) However, Q: Entrapment amount of unsolidified molten steel n: Number of consecutive occurrences of entrapment judgment is also 7,: Time difference until complete solidification at measurement time t□. 7:
Said t2. when the determination of containment of unsolidified molten steel occurs zero times in a row. Time difference vc from the measurement time obtained to complete solidification after ΔTXn: Casting speed ΔT: Measurement period Next, the present inventors calculated the complete solidification at each measurement time using equations (2) to (4) described above. Find the time difference until solidification,
When it is determined that containment has occurred, the amount of containment is calculated, and the center of the slab is examined using a well-known ultrasonic flaw detection device with the slab in question as a cold slab. The correlation between occurrence and the amount of containment was investigated. The casting conditions at that time are shown in Table 1 below.

Table 1 FIG. 4 shows the relationship between the amount of confinement under the casting conditions shown in Table 1 and the defect occurrence index, which means the probability that a center crack will occur in the slab. As is clear from FIG. 4, a good correlation was observed between the amount of encapsulation and the defect occurrence index. When producing slabs, it is naturally desired that the slabs be defect-free, but the inventors of the present invention have found that the defect occurrence index can be determined empirically and does not pose a problem in terms of product quality or continued production. In their experience, it was known that a defect occurrence index exceeding 50 is undesirable in terms of product quality and continued production. When the number of defect occurrences exceeds 50, the amount of containment is 40 or more, and the inventors have determined that this value (amount of containment = 4)
0) is the permissible limit of containment amount (hereinafter referred to as the permissible limit amount)
And so. That is, it is possible to compare the containment amount calculated by the formulas (2) to (4) with the allowable limit amount, and predict the occurrence of a center crack when the allowable limit amount is exceeded.

Details of center crack prediction will be explained using FIG. 1.

The eJ type information to be cast from the process computer IO shown in FIG.
is set to At the same time, the allowable limit amount is set in the defect prediction device 11 based on the steel type information transmitted from the process computer IO to the defect prediction device 12. The complete coagulation position detection device 11 has the above-mentioned (2) to (3).
The time difference until complete solidification at the measurement time is determined from time to time using the formula, and the time difference is transmitted to the defect occurrence prediction device 12. The defect occurrence prediction device 12 determines whether or not entrapment has occurred using the method described above, and when it is determined that entrapment has occurred, calculates the amount of entrapment using equation (2) described above.

When the calculated containment amount exceeds the allowable limit amount, the occurrence of a center crack is predicted, and the occurrence location is displayed on the monitor 13, for example, or the occurrence location is transmitted to the process computer, and the center crack is predicted to occur. Data containing defects is stored as part of the quality information.

It was possible to predict the occurrence of a center crack. In this case, if the slab is subjected to direct rolling with the rolling process, -
Once the slab is cooled, we check the condition of the central crack in the slab, i.e. the extent of the crack, and whether or not there are any cracks on the cut surface.After confirming that there is no problem, we supply it to the rolling process and prevent it from occurring in the rolling process. troubles can be avoided. In addition, the present inventors have found that there are several possible causes of center cracking as described above, but the one that has the most adverse effect is the above-mentioned cause of center cracking. This was due to fluctuations in compressive force.

Therefore, if the occurrence of a center crack can be predicted, a control method for reducing the amount of fluctuation in the compressive force, for example, by reducing the compressive force from time to time, can be used to prevent the center A'l deviation from occurring. You can also.

As described above, the method of the present invention does not directly measure the displacement of the slab or predict the occurrence of center cracks by simple simulation as in the conventional technology, but uses a non-contact method to predict the occurrence of center cracks. Because data is directly measured or calculated, it has excellent durability and extremely high estimation accuracy (eliminating the problems of conventional technology).

[Example] The method for estimating the occurrence of center cracking based on the present invention described above was carried out in a curved continuous casting facility with a machine length of 37 m and a monthly production capacity of 160,000 tons. The complete solidification position detection device 11 shown in FIG. 1 was installed at a position 34 m away from the mold 4. Further, the casting conditions were as shown in Table 2 below.

Table 2 The allowable limit amount may be determined in advance by investigating the correlation between the defect occurrence index of the slab and the amount of encapsulation using the method described above. In this example, the allowable limit amount is 4 as described above.
0, and casting was performed under the casting conditions shown in Table 2.

Table 3 shows the presence or absence of containment judgment for each slab in Fig. 5, which will be described later, as well as its containment star and center crack prediction.

Table 3 FIG. 5 is a diagram showing the occurrence of center cracks at that time.

As shown in FIG. 5, center cracks have occurred in the slabs in which center cracks were predicted to have occurred. As for the Nα2 and N093 slabs shown in Fig. 5, center cracks appeared on the cut surfaces, so both slabs were re-cut at the position indicated by the broken line 21°31 before being fed to the rolling process. As for the other slabs, since the center crack did not appear on the cut surface, the slabs were supplied to the rolling process as they were and subjected to rolling, but no problems occurred. Furthermore, as described above, after the occurrence of center υ1 deviation was predicted, the compressive force applied to the slab was increased from 80 tons to 50 tons, thereby reducing the amount of fluctuation in the compressive force from 25 tons to 7 tons. No center separation was observed in the slabs of No. 015 and above, in which the compressive force was reduced and its fluctuation became smaller, and it was also confirmed that center cracking could be prevented.

[Effect of the invention] According to the present invention, it is possible to predict the occurrence of center cracks that occur in the center of slabs, and by taking appropriate measures, it is possible to prevent the center cracks from occurring and also avoid operational troubles. It is clear that this method is highly effective in stably supplying defect-free slabs to the rolling process.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the configuration of equipment necessary for predicting center cracks according to the present invention in well-known continuous casting equipment, Figure 2 is a diagram schematically showing center cracks, and Figure 3 is a diagram showing the configuration of equipment necessary for predicting center cracks according to the present invention. Fig. 4 is a diagram showing the relationship between the amount of containment and the defect occurrence index of the slab, and Fig. 5 is the center of the actual slab when the method of the present invention is applied. FIG. 3 is a diagram showing a state in which cracks occur. DESCRIPTION OF SYMBOLS 1... Pot, 2... Tundishu, 3... Nozzle, 4... Mold, 5... Solidified shell, 6... Support roll, 7... Drawing roll, 8... Two Next cooling zone, 9... Cutter 10... Process computer 11... Complete solidification position detection device, 12.
...Defect prediction device, 13...Monitor Applicant: Nippon Steel Corporation

Claims (1)

[Claims] 1. When estimating the complete solidification position of a slab from the measurement value by a shell thickness measuring device or a slab cross-section temperature measuring device installed at the end of the machine, the measuring device is used to estimate the complete solidification position of the slab at a set period ΔT. The time difference t_f_1 until complete solidification based on the measurement time is determined from the detected measurement value, and when the time difference t_f_2 at the next measurement satisfies the following formula (1), it is determined that unsolidified molten steel has occurred. At the same time, the enclosed amount of the unsolidified molten steel is calculated using the following formula (2), and when the enclosed amount exceeds the allowable limit amount preset from the correlation between the enclosed amount and the center crack, the center A method for estimating the occurrence of a central crack in continuous casting, which is characterized by predicting the occurrence of a crack. t_f_1>t_f_2...(1) Q=n×Vc×ΔT×{2K(t_f_i+t_f_i
＿＋＿n）｝＾0＾． ^5...(2) However, Q: Amount of unsolidified molten steel sealed n: Number of times that the containment determination occurred consecutively t_f_i: Time difference until complete solidification at measurement time t_f_i_+_n: Unsolidified molten steel sealed n times in a row Δ from the measurement time when the t_f_i was obtained when the error judgment occurred.
Time difference until complete solidification after T×n Vc: Casting speed ΔT: Measurement period