JPH0512066B2 - - Google Patents

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. [Prior Art] In recent years, it has become extremely important to uniformize the solidification state inside the slab in continuous casting operations as the trend toward higher quality products has been achieved. In particular, it has long been known that center cracks and center segregation that occur in the center of slabs have a significant effect on the quality of rolled products, and various methods have been devised to predict these 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. In addition, as shown in Japanese Patent Application Laid-Open No. 59-156557, the solidification profile is calculated from the casting conditions, and from the solidification profile, the non-flow area of the 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. [Problems to be Solved by the Invention] In the above-mentioned Japanese Patent Application Laid-Open No. 57-114850, the measuring device may be damaged because the surface displacement of the slab is directly measured, and it is difficult to accurately measure the surface displacement of the slab. There was a problem in that measurement was not possible in some cases, and it was difficult to continuously and accurately estimate the occurrence of center cracks over a long period of time. In addition, in JP-A No. 59-156557, various operational fluctuations during casting,
For example, the profile of the unsolidified tip changes in a complicated manner due to changes in casting speed, molten steel temperature, secondary cooling water volume, etc., and the calculated profile of the unsolidified tip may not match the actual profile. As a result, there was a problem in that an error occurred in calculating the suction index of the concentrated molten steel, and it was not possible 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 solidification position, calculate the time difference t _f1 until complete solidification based on the measurement time from the measurement value detected by the measuring device at the set period ΔT,
When the time difference t _f2 at the time of the next measurement satisfies the following formula (1), it is determined that the unsolidified molten steel has been trapped, and the amount of unsolidified molten steel trapped is calculated using the following formula (2), and the sealed molten steel is determined to have occurred. This is a method for estimating the occurrence of a center crack in continuous casting, characterized in that the occurrence of a center crack is predicted when the amount of filling exceeds an allowable limit amount preset from the correlation between the amount of filling and the center crack. t _f1 > t _f2 ……(1) Q=n×Vc×ΔT×{2K(t _fi +t _fi+o )} ^0.5 ……(2) However, Q: Confinement amount of unsolidified molten steel n: Confinement Number of consecutive judgments f _fi : Time difference until complete solidification at measurement time f _fi+1 : ΔT× from the measurement time when the above t _fi was obtained when the unsolidified molten steel containment judgment occurred n times in a row Time difference Vc until complete solidification after n: Casting speed ΔT: Measurement period [Function] 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
The molten steel 50 is once injected into the mold 4 through the nozzle 3. 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 guide rolls 6 disposed after the mold 4, and is continuously blown 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 a specified size 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 cutter is used to cut the slab into specified dimensions, 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. It is generally believed that this center crack occurs when the tip (hereinafter referred to as crater end) of unsolidified molten steel moves significantly in the casting direction. The following may be the cause of the change in the crater end in the casting direction. When the thickness of the slab changes locally due to the slab being rolled down by horizontal rolls, and indirectly the position of the crater end changes, or when the casting speed suddenly changes, Molten steel flow occurs at
When confinement of molten steel occurs (same effect as the crater end position fluctuating), or because the slab is straightened horizontally in curved continuous casting equipment, the straightening distortion that occurs due to straightening is reduced. For this purpose, there is a casting method that applies compressive force to the slab, and due to periodic fluctuations in this compressive force, the thickness of the solidified shell becomes partially uneven, resulting in the above-mentioned horizontal part. 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 based on casting conditions such as casting thickness, secondary cooling conditions, casting steel type, and casting speed. Measuring device 1 capable of measuring the thickness of a solidified shell or determining the average temperature in the thickness direction of a slab using electromagnetic ultrasonic waves
1 (hereinafter referred to as a complete coagulation position detection device) was installed. First, to know the location of the crater end,
The method for determining the time difference between the time of complete coagulation and the measurement time is shown 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 equation (3). t _f = (D ² /4 - D × d _M + d _M ) / 2 / K ... (3) where, t _f : Time of complete solidification d _M : Thickness of solidified shell D : Thickness of slab K : Accelerated solidification coefficient Further, the time difference when complete solidification occurs in front of the device 11 can be determined by the following equation (4). t _f =C _R (θ _S −θ _SU −(θ _AVE −θ _SU )/KK ...(4) where, C _R : Center cooling rate θ _SU : Slab surface temperature θ _S : Solidus temperature KK: Constant θ _AVE : Average temperature in the slab thickness direction Here, if the time difference between the measurement time and the time at which complete solidification occurs at the measurement time is t _f1 , and the time difference after the measurement period ΔT from the measurement time is t _f2 , then The state of the crater end can be expressed as in equations (1) and (5) to (7) below. t _f1 > t _f2 ...(1) t _f1 +ΔT=t _f2 ...(5) t _f1 +ΔT＜t _f2 ...(6) t _f1 <t _f2 <t _f1 +ΔT ...(7) In the case of the above formula (5), the position of the crater end is stable without fluctuation, and in the case of the above formula (6) In the case of formula (7), the crater end is shortened in the direction of the mold, and in the case of formula (1), the crater end is extremely shortened in the mold direction, and the crater end is shortened in the mold direction. containment of molten steel (hereinafter referred to as
This means that containment (also known as containment) has occurred. As described above, it is necessary to determine which of the four conditions the crater end is in. Then, only when formula (1) is satisfied, t _f1 > t _f2 ,
It is important to determine that containment has occurred.
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 = n × Vc × ΔT × {2K (t _fi + t _{fi + o} )} ^0.5 ... (2) However, Q: Entrapment amount of unsolidified molten steel n: Number of times confinement judgment occurred continuously f _fi : Measurement Time difference until complete solidification at time f _{fi +1} : Time difference Vc from the measurement time when the above-mentioned t _fi was obtained when the unsolidified molten steel is judged to be contained n times in a row until complete solidification after ΔT×n : Casting speed ΔT : Measurement period Next, the present inventors calculated the time difference until complete solidification at each measurement time using equations (2) to (4) described above, and determined that confinement had occurred. Once completed, 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. We investigated the correlation. The casting conditions at that time are shown in Table 1 below.

〔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] 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 was set to 40 as described above, and casting was performed under the casting conditions shown in Table 2. 5, which will be described later in Table 3.
The presence or absence of containment judgment for each slab in the figure, the amount of containment, and the presence or absence of center crack prediction are shown.

〔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 to prevent the center cracks from occurring by taking appropriate measures.
Furthermore, operational troubles can be avoided, and 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... Tundishyu, 3... Nozzle, 4... Mold, 5... Solidified shell, 6... Support roll, 7... Drawing roll, 8... Secondary cooling zone,
9... Cutter, 10... Process computer, 11... Complete solidification position detection device, 12... Defect occurrence prediction device, 13... Monitor.

Claims (1)

[Scope of Claims] 1. When estimating the complete solidification position of a slab from the measured value by a shell thickness measuring device or a slab cross-sectional temperature measuring device installed at the end of the machine, the measuring device detects at a set period ΔT. From the measured value, calculate the time difference t _f1 until complete solidification based on the measurement time, and
When the time difference t _f2 at the next measurement satisfies the following formula (1), it is determined that the unsolidified molten steel has been trapped, and the amount of unsolidified molten steel trapped is calculated from the following formula (2), and It is possible to predict the occurrence of a center crack when the amount of containment exceeds the allowable limit amount, which is a containment amount of 40, which corresponds to a defect occurrence index of 50, which is set in advance from the correlation between the amount of containment and the center crack. A method for estimating the occurrence of center cracks in continuous casting. t _f1 > t _f2 ……(1) Q=n×Vc×ΔT×{2K(t _fi +t _fi+o )} ^0.5 ……(2) However, t _f =C _R (θ _S −θ _SU −( θ _AVE −θ _SU )/KK ...(4) where, C _R : Center cooling rate θ _SU : Slab surface temperature θ _S : Solidus temperature KK : Constant θ _AVE : Average temperature in the slab thickness direction Here , the time difference between the time at which complete solidification occurs and the measurement time is t _f1 , and the time difference after the measurement period ΔT from the measurement time is t _f2 Q: Confinement amount of unsolidified molten steel n: Confinement judgment is Number of consecutive occurrences t _fi : Time difference until complete solidification at measurement time t _fi+o : ΔT×n after the measurement time at which f _fi was obtained when the unsolidified molten steel is judged to have been contained n times in a row Time difference until complete solidification Vc: Casting speed ΔT: Measurement period