JPS6235617B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the thickness of an equiaxed crystal portion occupying the inside of a slab.

Continuous casting technology, which directly manufactures thick steel plates from molten steel, is an excellent method that meets the demands for resource and energy conservation, and is hoped to become more widespread.However, one of the problems with continuous casting technology is that unavoidable contamination occurs in the molten steel. This is the central segregation of impurity elements that occurs in The degree of center segregation is related to the cast structure, and the higher the proportion of the latter among the columnar crystals and equiaxed crystals that make up the cast structure, the more C, N,
The degree of central segregation of impurity elements such as S, P, and B is reduced. FIG. 1 is a diagram schematically showing the casting structure of a cast slab, where 2 indicates a continuously cast slab and d indicates its thickness. Naturally, the slab 2 is cooled and solidified from the front and back sides, so columnar crystals 2a and 2b extending in the thickness direction develop on the front and back sides, and small-sized equiaxed crystals 2c are formed in the center where the slab 2 finally solidifies. d ₁ indicates the thickness of columnar crystals, and d ₂ indicates the thickness of equiaxed crystals. Since the impurity elements are dispersed within the equiaxed crystals, the degree of center segregation decreases as the equiaxed crystals occur over a wider area. Figure 2 shows the center segregation degree of the slab, C _nax /
The relationship between C ₀ and the proportion (%) of equiaxed crystals is shown for each impurity. The proportion of equiaxed crystals varies greatly depending on the steel type even if the slab thickness is the same.

Possible factors that control the cast structure include the temperature at which molten steel is poured into the continuous caster, the water injection ratio during solidification, the strength of electromagnetic stirring, and the speed at which the slab is pulled out. Alternatively, since a method for online discrimination has not been established, no concrete proposal has been made regarding how to control the above factors in order to obtain an ideal slab.

In the past, a part of the cooled slab was cut,
The old-fashioned method of identifying the cast structure through the processes of grinding, polishing, corrosion, and microscopic observation is used, and information about the cast structure cannot be immediately reflected in the manufacturing process. Discrimination required a huge amount of time and effort. Also, since much of it depends on experience, there is a drawback that the judgment is not very objective.

Therefore, the inventor of the present invention has developed a method for producing cast structures using ultrasonic waves.
We developed a discrimination method and proposed it earlier (patent application 1972-
41850, 52-41851, etc.). This method produces equiaxed crystals by simply emitting ultrasonic waves to the slab and receiving them.
The thickness of each columnar crystal can be determined, and the effective grain size can also be determined. However, this method uses a contact method in which an ultrasonic transmitter is brought into contact with the slab to inject ultrasonic waves into the slab, and reception is also carried out by placing an ultrasonic detector in contact with the slab. It is difficult to apply to high temperature objects.

By the way, ultrasonic waves can also be generated using a pulsed laser. In other words, Q-switching a ruby laser, a YAG laser, etc. generates a powerful pulsed laser beam with an extremely narrow pulse width, and when this is projected onto the sample surface, the material on the surface layer of the sample instantly evaporates and scatters. The reaction force generates pulsed elastic waves (ultrasonic waves) on the sample surface. According to this method, ultrasonic waves can be generated in the sample without contact. Further, by using the optical homodyne method or the electromagnetic induction method, it is possible to detect ultrasonic waves without contact. By using such a non-contact ultrasonic wave generation and reception method, it is possible to perform ultrasonic measurements on moving or hot slabs.

Focusing on this point, the present invention detects the thickness of equiaxed crystals in continuously cast slabs in a non-contact, online manner.
This is then returned to continuous casting control to obtain a slab with a desired structure. Next, this will be explained in detail with reference to the drawings.

Referring again to FIG. 1, if we now assume that an ultrasonic wave is incident from a point P on one side of the slab 2 and detected on the other side Q, the propagation velocity of the ultrasonic wave in the columnar crystals 2a and 2b is v. ₁ , the propagation velocity in the equiaxed crystal 2c is _v2 , and the time from incidence to detection of the ultrasonic wave is t, the following equation holds true.

t=d ₁ /v ₁ +d ₂ /v ₂ ......(1) Here, d ₁ is the thickness of the columnar crystals 2a and 2b, and d ₂ is the thickness of the equiaxed crystal 2c, so the slab is the sum of these. It is equal to the thickness d of 2. That is, d=d ₁ + d ₂ ......(2) Therefore, t=d-d ₂ /v ₁ +d ₂ /v ₂ =d/v ₁ +(11
/v ₂ -1/v ₁ ) d ₂ ......(3) In equation (3), if t is obtained by measurement and d, v ₁ and v ₂ are known, then the thickness d of the equiaxed crystal is Find ₂ . Also, from this, the thickness d ₁ of the columnar crystal can be found from equation (2),
It is also possible to know the ratio d ₂ /d ₁ between equiaxed crystals and columnar crystals. As explained in the specification of the patent application, the propagation velocities v ₁ and v ₂ in columnar crystals and equiaxed crystals are determined by theoretical calculations and are expressed by the following formulas.

<100> For priority direction <110> In case of priority direction Here, C ₁₁ , C ₁₂ , and C ₄₄ are elastic constants. Non-contact transmission and reception of ultrasonic waves can be performed as follows. That is, referring to FIG. 3, in this figure, reference numeral 1 denotes a pulse laser that instantaneously generates a large output by Q-switching, and irradiates one surface 2a of a sample 2, such as a steel material, with pulse laser light _L10 . Laser light sources that can output pulsed laser by Q-switching include ruby laser, glass laser, and
There are YAG lasers, etc. The pulsed laser (also called giant laser) generated by these laser light sources has a high output of, for example, 100MW and a pulse width of 20nS.
This is a short-time process that instantly evaporates and scatters the sample surface layer (apply a protective film such as paint if you do not want to damage it). As mentioned above, this reaction generates an elastic wave within the sample, which propagates at a longitudinal wave propagation speed v within the sample having a thickness of d, and from the laser beam irradiation.
After time t=d/v, it reaches the other surface 2b of the sample and causes a minute displacement (indicated by a dotted line). Laser light L ₁₀
A part (microscopic part) L ₁₁ is connected to a photodetector 4 such as a photocell or photomultiplier tube by a beam splitter 3.
It is detected (photoelectrically converted) here. The output S ₁ of the photodetector 4 gives information on the time when the sample surface 2 a was irradiated with the laser beam L ₁₀ , but this is delayed by a timer 5 for a certain period of time. Then, the output S ₂ of timer 5
The measurement laser 6 is started using the signal as a trigger signal, and is caused to temporarily oscillate or Q-switch.
This trigger signal S ₂ is also used for timing the oscilloscope 11 or memory 12. The measurement laser 6 includes Q lasers such as ruby and YAG lasers.
A switchable laser source (lower output than the laser source 1) is used. Output laser beam of laser 6
L ₂₀ is split into two by beam splitter 7 , and a portion of L ₂₁ reaches mirror 8 , is reflected, returns to beam splitter 7 , passes through this, and enters photodetector 9 . This laser light _L21 becomes a reference signal light to be described later. On the other hand, the remainder _L22 of the laser beam _L20 split by the beam splitter 7 is reflected by the other surface 2b of the sample 2,
The reflected laser light returns to the beam splitter 7,
The light is reflected here and enters the photodetector 9. The photodetector 9 is thus given two inputs L ₂₁ and L ₂₂ and performs root mean square detection of these to obtain an amplitude value corresponding to the phase difference between the two (this has information on the displacement amount of the other surface 2b of the sample). Outputs the signal _S3 . This signal S ₃ is fed to the amplifier 10
After being amplified by , the signal is displayed on an oscilloscope 11 and/or stored in a memory 12 .

Here, the angular frequency of the measurement laser beam _L20 is ω,
If the amplitude of the reference signal L ₂₁ is Ar, the amplitude of the signal light L ₂₂ reflected by the sample surface 2b is As, and the displacement of the sample surface 2b is Z, then the reference signal light L ₂₁ is Ar cosωt, and the signal light L ₂₂
has a phase difference with respect to this, so As cos(ωt
+4π/λZsinωlt), and therefore, the intensity I of the output _S3 of the photodetector 9 subjected to root mean square detection is expressed by the following equation.

I=<{Ar cosωt+As cos (ωt+4π/λZsinωlt)} ² >=<Ar ² cos ² ωt+As ² cos ² (ωt+4π/λZsinω
lt) +2ArAs cosωt cos(ωt+4π/λZsinωlt)>=<1/2(Ar ² +As ² )+1/2{Ar ² cos2ωt+As ² cos2(
ω
t +4π/λZsinωlt)}+ArAs cos(2ωt+4π/λZsinωlt)+ArAs cos(4π/λZsinωlt)＞=1/2(
Ar ² +As ² ) +ArAs cos(4π/λZsinωlt) ………(7) In the above formula, λ is the wavelength of the laser beam L ₂₀ , ωl
is the effective angular frequency when the half-width of the pulsed laser beam _L10 is τl, and there is the following relationship between ωl and τl.

ωl2π/τl (8) However, sinωlt holds true between 0t≓2τl, and is 0 when t>2τl. In equation (7), the first and second terms on the right side are DC components, and the third term is AC component. If this alternating current component is Ia, since Ia is information including displacement Z, vibration displacement Z can be received by appropriately extracting this. By the way, this Ia can be expanded into a series of Betzel functions as shown in the following equation.

Ia=ArAs cos(4πz/λsinωlt)=ArAs{J ₀ (4πz/λ)+2J ₂ (4πz/λ)cos2ωlt+2J ₄ (4πz/λ)cos4ωlt+...} ......(9) In equation (3), Z is smaller than λ and 4πz/
If λ<1, then J ₀ (4πz/λ)1 J ₂ (4πz/λ)2π ² (z/λ) ² J _2o (4πz/λ)0 (n>2). Therefore, IaArAs{1+4π ² (z/λ) ² cosωlt} … …(10) 〓〓〓〓
The displacement Z can be expressed as the square root of the amplitude of the AC signal having the angular velocity ωl.

This root-mean-square detection is based on the optical homodyne method, but the difference from the conventional method is that the phase difference 4πz/λsinωlt due to amplitude displacement only exists for a time corresponding to the pulse width of the pulsed laser beam _L10 . .

In this way, the displacement Z can be detected using the optical homodyne method.
Therefore, non-contact reception of ultrasonic waves becomes possible, but the problem here is the intensity and duration of the signal lights L ₂₁ and L ₂₂ . As mentioned above, the duration of the pulsed laser L ₁₀ is extremely short, and therefore the generation time of the displacement Z and the measurable time are also extremely short. It is necessary to provide sufficient signals, ie, photons, to the photodetector 9 in such a short period of time to ensure reliable detection. In addition, the displacement Z to be detected is extremely small, and in high-temperature environments, fluctuations in the measurement light can also cause false signals, so it is important to provide some kind of filter function to prevent such things from being detected. It is.
Therefore, this device measures only a minute period centered around the time when the pulsed ultrasonic waves generated in the sample 2 by the pulsed laser _L10 reach the sample surface 2b and cause mechanical displacement on that surface. Do it like this.
The timer 5 is provided for this purpose, and its delay time is set depending on the thickness d of the sample 2 and the ultrasonic propagation speed. That is, as mentioned above, laser light
When L ₁₀ is irradiated onto the sample surface 2a at time t = 0, the ultrasonic pulse (elastic wave) generated on that surface propagates within the sample at a speed v and reaches the sample surface 2b at time t = t, and the Since the delay time of the timer 5 is set to a minute width centered on d/v, the delay time of the timer 5 is set to a minute width centered on d/v. To give a specific example, pulse laser 1 has a pulse width of
If we irradiate the sample surface 2a with a pulsed laser beam L ₁₀ of 20n sec and an output of 100MW at time t=0, the output of the photodetector 4 will be applied almost simultaneously (even with a delay of 0.1n).
sec) signal S ₁ appears. Here, assuming that the thickness d of the sample 2 is mm, the propagation speed of the ultrasonic wave (longitudinal wave) is about 6 mm/1 μsec, so a mechanical displacement occurs on the sample surface 2b after about 5 μsec. The measurement laser beam L ₂₀ captures this displacement, and for this purpose, it is preferable to set the delay time of the timer 5 to 4 μS and the duration of the measurement laser 6 to 2 μS. Note that the time required to Q-switch the laser 6 to generate a pulsed laser is about several tens of nanoseconds, and this can also be ignored.

In order to provide sufficient photons to the photodetector 9, the output (continuous output) of the measurement laser light source 6 may be increased, but it is effective to Q-switch the light source 6 to output a pulsed laser. If this Q switching is performed at the above-mentioned timing, there is an advantage that unnecessary noise is not picked up. In this way, this device can raise the detection sensitivity to a sufficient level, and also eliminates noise caused by mechanical vibrations occurring in the measurement sample and noise caused by light fluctuations that occur especially when measuring in hot, high-temperature environments. can be effectively removed.

As explained above, according to the present invention, the equiaxed crystallinity of continuously cast slabs can be measured on-line, which is extremely effective. Of course, the present invention is not limited to measuring the equiaxed crystallinity of slabs during continuous casting, but can also be used to measure the equiaxed crystallinity of general slabs or ingots.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the structure of a slab, FIG. 2 is a graph showing equiaxed crystallinity and degree of segregation, and FIG. 3 is an explanatory diagram showing an example of the present invention. In the drawing, 2 is a slab, 2a and 2b are columnar crystal parts,
2c is an equiaxed crystal part, 1 is a pulsed laser source, 6-1
1 is an ultrasonic detector. 〓〓〓〓

Claims (1)

[Claims] 1 A laser device for measuring only a minute period of time centered on the time when the other side of the slab is displaced by the ultrasonic pulse by irradiating high-power pulsed laser light onto one side of the slab to generate an ultrasonic pulse. Run the
The measurement laser device detects the displacement of the other surface caused by the ultrasonic wave that propagates within the slab and reaches the other surface, measures the time from generation of the ultrasonic wave to detection, and calculates the time and time. A method for measuring the equiaxed crystal ratio of a slab, characterized in that the thickness of the equiaxed crystal portion is determined from the thickness of the slab and the ultrasonic propagation velocity of each portion of the equiaxed crystal and columnar crystal.