JPH0957413A - Method for preventing cracking and breakout of cast slab in continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the cracking and the breakout of a cast slab by executing the change of an operational condition when a depression depth is larger than a reference value or satisfies a specific relational formula. SOLUTION: A profile of the thickness surface of the cast slab M is continuously detected with a laser beam range finder 4 arranged facing to the thickness surface of the cast slab M at a position befind a mold. The max. value in a prescribed casting length range in an average value deviation obtd. by subtracting the average value of oscillation recessed parts from the average value of projecting parts in each oscillation mark developing period is made to be the depression depth DPmax . At the time of satisfying at least one of the following conditions (1) and (2), the change of the operational condition is executed. (1) When the depression depth is larger than the reference value, (2) when the depression depth in the length range in a casting direction and an estimated shell thickness D derived from the operational condition satisfy the following formula, Uniformity = (D-α×DPmax )/D <= reference value. Wherein, α is an index for recess in the cast slab and is mainly decided from the kind of steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing breakable breakout of a slab in continuous casting.

[0002]

2. Description of the Related Art During continuous casting, when a stress exceeding the shell strength is applied to a portion where the shell thickness is partially thinned due to uneven solidification of the slab, cracks and breakability breakout are caused. Occurs.

When such a situation occurs, energy loss is required because the subsequent steps cannot be taken care of, and the yield is reduced. Further, when a breakout occurs, the equipment is destroyed, which results in a long time. There is a problem of being forced to stop operations.

Therefore, some proposals have hitherto been made as a method or means for preventing the breakable breakout of the cast slab. For example, (a) JP-A-56-954
In JP 61, the mold temperature detecting elements are arranged in the width direction of the continuous casting mold at close intervals, and the surface temperature of the slab and the solidification shell are generated by the mold temperature detected by these mold temperature detecting elements. A method of predicting the occurrence of breakage or breakout of a wire is disclosed.

The applicant of the present invention is (a) JP-A 1-21
In Japanese Patent No. 0160, the mold temperature of the continuous casting mold and the surface temperature of the slab directly below the mold are measured at a plurality of points in the width direction with time, and vertical cracks or vertical cracks are obtained by using these temperature measurement values. To predict the occurrence of sexual breakout,
Furthermore, in (c) Japanese Patent Application Laid-Open No. 4-162949,
Measure the surface shape and surface temperature of the slab directly below the casting mold for continuous casting to determine their distribution, and improve the method for predicting the occurrence of vertical crack defects or vertical crack breakout based on these results. Each addition is disclosed.

In addition, a method combining these methods is also performed.

[0007]

However, each of the above-mentioned conventional techniques has the following problems. (A) In the case of the method disclosed in Japanese Patent Laid-Open No. 56-95461, many temperature measuring points and temperature measuring elements are required to find a local heat removal defect. Therefore, maintenance of the temperature measuring point and its element is very troublesome.

(A) In the case of the technique disclosed in Japanese Patent Laid-Open No. 1-210160, the temperature of the slab is measured just below the mold. Therefore, the measurement accuracy was not high due to the effects of spray water and steam under the mold and the powder film on the mold surface.

Further, (c) in the case of the technique disclosed in Japanese Patent Laid-Open No. 4-162949, since the shape of the slab is also measured, the average displacement amount (bulging) in a wide range can be known. Therefore, in the relationship between the thinness of the shell and the static pressure of molten steel, it is effective for predicting the occurrence of cracks, but even unevenness on the surface of the minute slab that is deeply correlated with the uneven solidification of the slab Since it is not known, it was difficult to predict the occurrence of cracks accompanying it.

Therefore, in the above-mentioned conventional method, although it is possible to detect and predict the occurrence of cracks and breakable breakouts to some extent, the uneven solidification state of the slab, which causes cracks and breakouts, can be detected. Since the operating conditions cannot be optimized by directly and quantitatively, the prediction accuracy of cracks and breakouts was not satisfactory.

On the other hand, in recent years, the mold has been oscillated (vertical vibration), and the surface quality typified by defects and crack defects involving powder is associated with the oscillation mark accompanying this. A method for evaluation is disclosed in JP-A-6-304725.

According to this method, during continuous casting, a signal of an oscillation mark is continuously detected in the casting direction by using a laser distance meter at at least one point in the width direction of the slab, and this signal is subjected to frequency analysis. In the obtained power spectrum, the surface quality of the slab is evaluated by comparing the power spectrum value at the frequency of the oscillation cycle of the mold with the power spectrum value at another frequency.

Further, in the abnormality detection, the power spectrum pattern is compared with the power spectrum pattern in a steady state prepared in advance. When comparing the power spectrum patterns, it is also disclosed that the power spectrum patterns, such as the molten metal level, the pinch roll rotation speed, and the oscillation cycle, are compared.

In this method, attention is paid to the point that the surface defect is associated with the oscillation mark, and in particular to the point that the average value, maximum value, minimum value, distribution, etc. of the depth of the oscillation mark are obtained.

The main object of the present invention is to evaluate the surface quality based on whether or not the uniformity of the pitch of the oscillation marks is secured, and the average value of the depths of the oscillation marks, the maximum Values, minimum values, distributions, etc. are used as criteria for determining whether surface maintenance is required or not after that, and conventionally, the uniformity of the pitch of the oscillation marks was evaluated off-line. In contrast, it is now possible to evaluate online.

With this method, however, it is not possible to judge whether or not there is a risk that a breakable breakout will occur in the slab itself, even if the slab or equipment is abnormal.

Even if the depth of the oscillation mark is deep, if the thickness of the shell at the solidification stage is large, a breakable breakout will not occur and there is little risk of reaching it. Immediately changing the operating conditions based on the above results in unnecessarily confusing other control systems and is not appropriate.

Further, in the case where the oscillation mark is detected on the surface of the slab in the width direction as in the above method, the oscillation mark portion is crushed by the pinch rolls, and no distinct feature appears. As in the present invention, it is not suitable at all when the characteristics of the oscillation mark portion are intensively extracted to judge the breakability breakout.

Therefore, it is an object of the present invention to clearly grasp the oscillation mark portion and intensively extract the characteristics of the oscillation mark portion to appropriately judge the breakability breakout. .

[0020]

The present invention, which has solved the above-mentioned problems, has the following constitution. That is, in order to prevent the breakable breakout of the slab in continuous casting, at the position after the mold, the profile of the thickness surface of the slab is continuously measured by the laser distance meter installed facing the thickness surface of the slab. The oscillation mark generation period in the above profile is detected from the casting speed and the oscillation period, and the following (1) and (2)
A method for preventing breakability of a slab in continuous casting, characterized in that when at least one of the conditions is met, the breakability of the slab is likely to occur. (1) The depletion depth DPmax is the maximum value within the predetermined casting length range in the average value deviation obtained by subtracting the average value of the oscillation concave portions from the average value of the convex portions for each oscillation mark generation cycle. When the depletion depth DPmax is larger than the reference value, (2) the depletion depth DP within a certain casting-direction length range.
When max and the estimated shell thickness D derived from the operating conditions in the casting length range satisfy the following Q expression: Uniformity = (D−α × DPmax) / D ≦ reference value (Q) Here, α is a cast dent index, which is mainly determined by the steel type.

In the present invention, breakable breakout is prevented. For this purpose, it is necessary to accurately and quantitatively grasp the uneven solidification of the slab.

Therefore, in the present invention, the laser distance meter is installed at an appropriate position after the mold so as to face the thickness surface of the slab. With this laser range finder, for example, a (surface) profile on a predetermined line spaced from the ridge of the slab to the center line side of the thickness surface by 10 to 50 mm, more preferably 10 to 30 mm, is continuously detected.

As described above, in the case of detecting the oscillation mark on the widthwise surface of the slab, the oscillation mark portion is crushed by the pinch roll, and no distinct feature appears. Therefore, in the present invention, the pinch is used. The target is the thick surface of the slab, which is not affected by the roll and has little scale adhesion. Therefore, the surface profile of the slab can be extracted accurately.

From the casting speed and oscillation period,
The oscillation mark generation period in the profile can be obtained. From the profile itself,
It is not impossible to specify the oscillation mark portion, but it is not a good idea because it requires a great deal of time and labor for the specific calculation process and may cause an error.

A specific example of changing the operating conditions on the assumption that cracking breakout of the slab may occur will be described in detail below.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of a continuous casting apparatus according to the present invention, and FIG. 2 is a schematic diagram of a state in which a (surface) profile of a thickness surface of a slab M is being measured. Molten steel poured from the tundish is solidified by a mold 1 accompanied by an oscillating operation, is pulled out by the pinch rolls 3, 3 ... Group while being guided by a roll 2, and is cut into a slab of a predetermined size. In the front, the laser range finder 4 is installed while staring at the thickness surface of the cast slab M.

Laser projection light L from the laser rangefinder 4
Is 10 to 3 from the ridge of the slab to the center line side of the thickness surface.
The target is projected on a predetermined line separated in the range of 0 mm, and the profile on the line is continuously detected based on the reflected light.

In this case, since the laser rangefinder 4 is installed in the immediate vicinity of the slab M, it is easily affected by the heat from the slab M. Therefore, in this embodiment, as a measure against radiant heat, the heat insulating plate 5 is provided between the laser range finder 4 and the cast slab M.

When the oscillation mold 1 is used, dents called oscillation marks appear on the surface of the slab M at a cycle determined by the casting speed and the oscillation cycle.

Inhomogeneous solidification in the solidification process of the slab M drawn from the mold 1 is likely to occur along the depression valley where the oscillation mark is deeply grown. Therefore, by extracting the oscillation mark portion and quantifying the depression troughs that have grown deeply, it is possible to grasp the non-uniform solidification and its extent, and thus to prevent the occurrence of breakability breakage of the slab. It can be determined whether there is a fear.

Therefore, the signal from the laser range finder 4 is given to the slab surface profile feature quantity extraction computer 6, and the computer 6 is also informed of steel type, casting speed, oscillation period, amplitude, molten steel temperature ΔT, cooling pattern, etc. It gives casting information and operation information in real time.

The laser range finder used is
In order to accurately grasp the behavior of the oscillation mark,
For example, it is desirable to use a laser beam having a spot diameter of about 1 mm, an accuracy of about 0.2 mm, and a wavelength different from the self-luminous emission of the slab, and having a filter that transmits only that wavelength range. .

The measurement cycle must be determined according to the casting speed and the opening width of the oscillation mark, but is set to 20 msec, for example, and each oscillation generation cycle is calculated from the casting speed and the oscillation mark cycle ( (Casting speed / oscillation cycle), and the feature amount is extracted for each section.

When the solidification of the slab M is substantially uniform, the profile has a uniform cycle and depth (usually about 1 mm) of the oscillation marks as shown in FIG. However, when non-uniform coagulation occurs, as shown in FIG. 5, the period and the depth of the oscillation mark change, and further, the depletion portion where the oscillation mark grows deep is seen.

As described above, since it is found that nonuniform coagulation is likely to occur in this depletion portion, the depth of the depletion portion (depletion depth DPmax) is used as an index of nonuniform coagulation.

A specific example of the method of preventing cracking breakout of the present invention will be described with reference to the flow chart shown in FIG. 3 and the explanatory view of FIG.

First, the laser range finder 4 takes in the surface profile signal of the slab M and the casting information (operating information) into the slab surface profile characteristic amount extraction computer 6.

Then, the oscillation mark generation period is calculated from the casting speed and the oscillation period. Within this oscillation mark generation cycle, the measured value obtained for each measurement cycle is calculated, for example, with 3 points being the maximum value, and from the smaller value being calculated, for example, 3 points being the minimum value. Next, the position in the casting direction is calculated. Then, the deviation (average value deviation) between the maximum value and the minimum value in each oscillation mark generation cycle is obtained, and this is taken as the oscillation mark depth. For the average.

Thereafter, the maximum value of the oscillation mark depth is obtained as the depletion depth (DPmax) for each casting length range, for example, 500 mm, from the calculated correlation with the casting direction position, and the oscillation is performed. Also determine the average and the variance of the mark depth.

Further, the homogeneity expressed by the following equations (1) and (2) is determined.

[0041]

[Equation 1]

[0042]

[Equation 2]

Thus, when at least one of the following conditions (1) and (2) is satisfied, the operating conditions are changed because there is a risk of breakage breakout of the slab.

(1) For each oscillation mark generation cycle, the maximum value within a predetermined casting length range in the average value deviation obtained by subtracting the average value of the oscillation concave portions from the average value of the convex portions is depleted. Depth (DPmax), and when this depletion depth (DPmax) is larger than a reference value, (2) Derived from the depletion depth DPmax in a certain casting direction length range and the operating conditions in the casting length range. When the estimated shell thickness D satisfies the following expression Q: Uniformity = (D−α × DPmax) / D ≦ reference value (Q) where α is a cast dent index, It is the ratio of solidification from the surface and remelting from the inside of the slab, which is mainly determined by the steel type.

The operating conditions can be changed by the operator upon receiving an alarm or by being automated. The operating conditions are not changed when the conditions of the expressions (1) and (2) are not satisfied. The factors of the operating conditions to be changed are mainly the casting speed, the short piece taper amount, the cooling pattern, and the oscillation cycle.

The depletion depth (DPmax) and the amount of depression of the shell (index of non-uniform solidification) show a positive correlation as shown in FIG. 7, and conversely the depletion depth (DPmax).
It turns out that it is effective to capture.

[0047]

As is apparent from the above description, according to the present invention, the oscillation mark portion is clearly captured, the characteristics of the oscillation mark portion are concentratedly extracted, and the breakability breakout is accurately performed. You can judge.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a continuous casting apparatus according to the present invention.

FIG. 2 is a conceptual diagram of a measurement state of a surface profile of a slab.

FIG. 3 is a flowchart showing an example of a crack prevention method according to the present invention.

FIG. 4 is a graph showing a relationship between a casting direction position and a slab surface profile in a normal case.

FIG. 5 is a graph showing the relationship between the casting direction position and the slab surface profile when depletion occurs due to uneven solidification.

FIG. 6 is a flowchart for explaining the arithmetic processing method of the present invention.

FIG. 7 is a graph showing the relationship between depletion depth and shell dent amount.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Mold, 2 ... Roll, 3 ... Pinch roll, 4 ... Laser distance meter, 5 ... Heat insulating plate, 6 ... Slab surface profile feature amount extraction calculator, M ... Slab.

Claims (1)

[Claims] 1. In order to prevent breakage breakout of a slab in continuous casting, a profile of the thickness surface of the slab is measured by a laser distance meter installed facing the thickness surface of the slab at a position after the mold. Continuously detected, the oscillation mark generation period in the profile is obtained from the casting speed and the oscillation period, and when at least one of the following (1) and (2) is satisfied, the slab cracking property is A method for preventing breakable breakout of a slab in continuous casting, characterized in that operating conditions are changed due to the possibility of breakout. (1) The depletion depth DPmax is the maximum value within the predetermined casting length range in the average value deviation obtained by subtracting the average value of the oscillation concave portions from the average value of the convex portions for each oscillation mark generation cycle. When the depletion depth DPmax is larger than a reference value, (2) the depletion depth DPmax in a certain casting direction length range and an estimated shell thickness D derived from operating conditions in the casting length range. When the following Q expression is satisfied: Uniformity = (D−α × DPmax) / D ≦ reference value (Q) where α is a cast dent index and is mainly determined by the steel type.