JP2003136200A - Method of manufacturing cleans steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a clean steel employing a method of detecting inclusion, which enables 'in-situ measurement' in continuous casting. SOLUTION: The method comprises the step of determining the range of a particle size permitted to inclusion and each of thresholds of the size and the number of the inclusions if necessary depending on types and purposes of steel, the step of determining an orifis diameter used for a probe on the basis of the threshold of the particle size permitted to the inclusions, the step of dipping an inclusion sensor in a ladle, a tundish or a molten steel in a casting mold to detect the data for calculating the particle diameter or the number of inclusions, the step of calculating the particle diameter of inclusion and the number of inclusions, and the step of immediately taking in-situ countermeasures, e.g. alarming to order inclusion reducing countermeasures in refining-continuous casting steps or changing a destination of obtained products if either the obtained diameter of inclusion or the number of inclusions exceeds the respective threshold.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing clean steel, and more particularly to a method for producing clean steel in which casting is performed while monitoring inclusions in molten steel during continuous casting.

[0002]

2. Description of the Related Art Conventionally, for the measurement of inclusions in steel, a small piece is cut out from a cast piece or a product after solidification, and it is observed with a microscope. There is a method of extracting non-metallic inclusions and analyzing their particle size and number. All of these methods require a period of several days to one week or more for analysis after casting, which is not sufficient for quality determination of a slab before hot rolling.

Therefore, the relationship between the operating conditions and the slab cleanliness is usually empirically obtained, and when the operating conditions exceed a certain range, the slabs are regarded as an abnormal product and applied to scrap or another purpose. The measures to do are used. In addition, for steel grades that have a high product degradation rate due to inclusions such as DI cans and deep drawing steel for automobiles, at the beginning of casting, the joint between ladle and ladle (continuous joint), the end of casting We are taking measures such as manufacturing on the premise that the slab will be scrapped or diverted to another purpose in advance.

Further, in the case of a high pressure resistant material such as a fuel injection pump material for a diesel engine, inclusions are generally evaluated / judged by observing the obtained product under a microscope. It goes without saying that it requires a lot of effort and time. Moreover, there is no guarantee that inclusions that appear infrequently can be reliably measured.

[0005]

[Problems to be Solved by the Invention] Meanwhile, with the increasing functionality of steel, the need for clean steel is becoming more and more stringent, and at the same time, there is also a demand for cost reduction. Here, strictness for clean steel means that the cleanliness of steel can be accurately grasped with information such as other chemical composition, molten steel temperature, level of molten metal in the mold, and detection of ladle slag.
On the other hand, in terms of cost, on the other hand, in order to improve the yield, it is required to perform "in-situ measurement" and feed back the result to the operating condition in real time.

In particular, in recent years, in so-called near net shape casting such as steelmaking-rolling direct coupling operation, thin slab casting, and strip casting, the processing speed from steelmaking to products is remarkably high, and therefore inclusions are also out of specification. If a cast piece is made to flow downstream, it will cause a large loss economically. Therefore, a technique for immediately determining the quality of a cast piece (product) is indispensable.

The object of the present invention is to continuously cast clean steel, for example, near net shape casting.
It is an object of the present invention to provide a highly reliable method for producing clean steel that employs a method of detecting inclusions that enables “in-situ measurement”.

[0008]

[Means for Solving the Problems] Various investigations have been carried out in order to achieve such a problem, and the present inventors have conceived of adopting a so-called ESZ method for detecting inclusions, and investigated various problems in its application. The present invention has been completed by finding out a solution to the problem and finding that an unexpected effect can be obtained.

That is, the present inventors previously developed continuous type and one-shot type inclusion sensors for molten steel by the ESZ method. Patent Nos. 02594681 and 02591253
issue. However, these patents did not disclose the specific quality judgment method described above.
Moreover, no method for producing clean steel was disclosed.

In subsequent research, by applying this ESZ method to the refining stage or continuous casting stage of steel,
The present inventor can make a quality judgment on the spot while manufacturing clean steel from a completely new viewpoint, and according to it, a highly reliable clean steel can be reliably manufactured at low cost. Was found.

That is, the ESZ method is theoretically an excellent method capable of accurately measuring the number of inclusions. However, for example, as a practical technique for near net shape casting, it can be used on the spot. It was unclear whether necessary and sufficient data could be obtained. Therefore, as a result of further research by the present inventor, thresholds are set for the size and the number of inclusions, and in consideration of those thresholds, especially the threshold of the size of inclusions, The present invention has been completed, knowing that such an object can be easily realized by determining the diameter of the orifice provided in the probe constituting the object sensor.

That is, when the present inventor repeated a series of experiments, the measurement sometimes became impossible. The orifice was clogged with inclusions. When the correlation between the size of the inclusions and the diameter of the orifice at that time was examined, it was found that there was a certain correlation between the diameter of the orifice and the minimum value of the diameter of the inclusion that clogs the orifice. Therefore, if the present inventors know such a correlation in advance and define the orifice diameter to a predetermined value accordingly, it can be used for the determination of coarse inclusions due to the clogging of the orifice. it can.

By the way, conventionally, the number N _A of inclusions per unit area obtained by microscopic observation is simply converted into the number N _V of inclusions per unit volume by the following equation. That ^{_{N V = N A 3/2 ················· (}} 1) for example, 100 mm ² is usually applied in extreme statistical method
One inclusion counted in the field of view is 4 per kg
This corresponds to a count of 054 inclusions. However, such a measurement under a microscope lacks reliability, particularly when determining the influence of coarse inclusions that rarely appear. In other words, since relatively large inclusions do not occur evenly,
It may not be detected. Moreover, such measurement under a microscope is performed ex post facto, and is not a mere confirmation area.

Here, regarding the count number of inclusions by the ESZ method, particularly with respect to coarse inclusions, if any such coarse inclusions are detected, the product is immediately changed in the direction and cleaned. It is possible to secure reliability as steel. Therefore, as described above, if the critical diameter of coarse inclusions is defined by the orifice diameter, the use of a probe with a predetermined orifice diameter will immediately cause a quality non-standard to be detected when the blockage is experienced. I can judge.

Furthermore, when the accurate distribution and size (number) of inclusions are measured by the ESZ method, it is judged whether or not it is outside the target range, and if it is out of the target range, Immediately taking the countermeasures enables the stable production of clean steel containing only inclusions within the target range. In that case, if the tundish is used for measurement and it is found that the inclusions are out of distribution and coarse inclusions are present, for example, the tundish heater applied current (force) is increased. Immediately take steps such as lowering the casting speed and extending the refining time outside the furnace. Then, even when the amount of inclusions is large, for example, immediately increase the power applied to the tundish heater, decrease the casting speed, strengthen the degassing seal between the ladle and the tundish, strengthen the slag reforming outside the furnace, etc. To take. Of course, the location where such measurement data was obtained or the molten steel portion may be recorded and rejected as a product. Since the inclusion control as described above has a very high responsiveness, it is particularly effective as a countermeasure for “in-situ measurement” as in the present invention.

That is, the present invention is as follows. (1) In continuous casting of molten steel, the inclusions contained in the molten steel passing through the orifice provided in the probe that constitutes the inclusion sensor immersed in the molten steel are based on the potential difference pulse between the facing surfaces of the orifice. In the method for producing clean steel in which the cleanliness of molten steel is “in-situ measured” by the counting ESZ method, the threshold value of the maximum diameter of inclusions that can be allowed is determined according to the steel type and application of molten steel. Stage; based on a threshold for the maximum diameter of the inclusions,
Determining the orifice diameter used for the probe; immersing the inclusion sensor in molten steel in a ladle, tundish, or mold; and sensing that the orifice is blocked by inclusions above the threshold , A warning steel that orders measures to reduce inclusions in the stages of refining to continuous casting, whether the product obtained is redirected, or other measures immediately taken on the spot; Manufacturing method.

(2) During continuous casting of molten steel, a potential difference pulse is generated between the facing surfaces of the molten steel, the inclusions contained in the molten steel passing through the orifice provided in the probe constituting the inclusion sensor immersed in the molten steel. In the method for producing a clean steel in which the cleanliness of the molten steel is “in-situ measured” by the ESZ method of counting based on the above, in the range of the grain size allowed for inclusions according to the steel type and the application of the molten steel, Further, if necessary, determining each threshold value with the number of inclusions in the range; determining the orifice diameter used for the probe based on the threshold value of the range of the allowable particle size of the inclusions. Immersing the inclusion sensor in molten steel in a ladle, a tundish, or a mold to detect data for calculating the diameter of inclusions or the diameter of inclusions and the number of inclusions; detection The step of calculating the diameter of the inclusions and the number of the inclusions if necessary from the data obtained; and when calculating the diameter of the obtained inclusions or the diameter of the inclusions, either the diameter of the inclusions or the number of the inclusions is When the threshold value is exceeded, a warning is issued to order measures to reduce inclusions in the stages of refining to continuous casting, the product is redirected to the product obtained, etc. , A method for producing clean steel.

(3) When performing continuous measurement using a continuous measurement type inclusion sensor, the above data is detected and sampled by setting the immersion depth from the molten metal surface of the orifice to 300 mm or more. (2) The method for producing the clean steel described above.

(4) When intermittent measurement is performed using a one-shot type inclusion sensor, the above-mentioned data is detected and collected by setting the immersion depth from the molten metal surface of the orifice to 300 mm or more. ) Or (2) The method for producing clean steel described in (2) above.

(5) The method for producing clean steel according to the above (2), wherein the threshold value of the number of inclusions is 50000 or less per kg of molten steel determined by the following equation for molten steel in the tundish or trough.

[0021]

[Equation 2]

S ≦ 5 × 10 ⁴ P / kg-steel D: Orifice diameter x: Inclusion particle size

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is directed to ESZ (Electric Sensing).
In-situ measurement of inclusions in molten steel by the Zone) method,
In the process of continuously casting clean steel while monitoring the cleanliness of molten steel, clean steel that ensures quality that does not exceed the respective predetermined thresholds of coarse inclusion diameter and / or number of inclusions A manufacturing method is provided, and its specific content will be described in detail below.

FIG. 1 is an explanatory view of the principle of the inclusion detection method by the ESZ method. In the figure, when non-conductive fine particles, that is, inclusion particles, sequentially pass through the orifice in the order of 1 → 2 → 3 → 4 → 5, a kind of pulse current is generated between the electrodes provided on the facing surface of the orifice. With this you know the passage of inclusions,
The magnitude is calculated from the height of the potential (ΔV).

FIG. 2 illustrates a system for measuring the size and amount of inclusion particles passing through an orifice provided in a probe when the probe is immersed in molten steel based on such a principle.

That is, Δ measured as described above
V is output by a PHA (pulse height analyzer) through a preamplifier and a logamplifier. An arithmetic processing system is interposed between the preamplifier and the log amplifier to form a noise separation circuit.

Here, according to the present invention, a small tundish used in a ladle, a tundish, a trough, that is, a strip casting in a continuous casting, or in a mold, is continuously or intermittently interposed at an appropriate interval. "In-situ measurement" of the concentration and particle size of inclusions in molten steel with an object sensor. The orifice diameter of the probe to be applied is selected from the range of 100 μm to 2000 μm in consideration of the threshold of the diameter of coarse inclusions determined by the specifications of clean steel.

The particle size to be excluded of non-metallic inclusions in the ladle, the tundish, and the mold is X μm.
When the above is satisfied, the diameter of the orifice is set to 2.5 X,
When the orifice is clogged during measurement in molten steel, the rejection of the molten steel is determined, or a warning is issued to instruct inclusion reduction measures in the refining to tundish stages.

According to the research conducted by the present inventor,
Inclusions with a diameter of 40% or more do not completely ride on the streamline when being sucked into the probe from the orifice and adhere to the inner surface of the orifice. Further inclusions grow here,
There is a problem of clogging the orifice. Therefore, in the present invention, an orifice diameter of 2.5 times or more of the minimum diameter of a single inclusion that is harmful to the product quality of the steel type is used.

On the other hand, when an orifice having a diameter 2.5 times larger than the allowable minimum diameter of a single harmful inclusion is used, inclusions having an equivalent diameter larger than the minimum diameter of the single harmful inclusion are trapped in the orifice. And the orifice is clogged.

As a result, it becomes impossible to suck molten steel into the probe thereafter, and the baseline of the current value becomes unstable. This is clearly discriminated on the oscilloscope of the inclusion sensor system or a system capable of observing the time course of the potential difference pulse corresponding to this, and it is instantly judged that the molten steel does not satisfy the required final product quality. can do.

In the broadest sense of the present invention, in continuous casting of molten steel, inclusions contained in the molten steel passing through the orifice provided in the probe immersed in the molten steel are included in the surfaces of the facing surfaces of the orifice. It is a method for producing clean steel, in which the cleanliness of molten steel is cast while "in-situ measurement" is performed by the ESZ method, which counts based on a potential difference pulse between the two.

According to this specific embodiment, the method for producing clean steel according to the present invention has the following steps. (i) Determining the respective threshold values of the range of the particle size of the inclusions that are allowed and, if necessary, the number of the inclusions in that range, depending on the steel type and application of the molten steel: The maximum allowable diameter of inclusions is determined in advance or according to the standard, and this is used as the threshold value. This is because, as will be described later, when inclusions of such a diameter are measured, it cannot be applied to the intended use.
It needs to be fairly strict. For example, the maximum allowable diameter
It is determined as 120 μm.

(Ii) Determining the orifice diameter used for the probe based on the threshold value for the maximum diameter of the inclusions: As described above, the diameter of the acceptable inclusions, that is, the threshold value of the maximum diameter is Once determined, for example, the orifice diameter with which it can be measured is also determined as described above.

For example, when the diameter of the allowable inclusion is X or less, the diameter of the orifice is set to 2.5 X or less, so that the inclusion of the X diameter is present in the molten steel, the orifice is clogged. It can be judged that it is out of the standard.

(Iii) Immersing the inclusion sensor in the molten steel in a ladle, a tundish, or a mold: At this stage, the inclusion sensor is immersed in the molten steel. The immersion depth is 300 mm or less on the molten steel surface. The place of detection of the inclusions at this time is not particularly limited and may be any of the ladle, the tundish and the mold.

(Iv) When it is detected that the orifice is blocked by inclusions exceeding the threshold, refining-
Immediately take measures on the spot, such as issuing a warning ordering measures to reduce inclusions at the stage of continuous casting, or changing the destination of the obtained products.

In the case of the present invention, when such a blockage is observed, a large number of coarse inclusions are stochastically present per 1 kg of molten steel, and it is necessary to immediately take the above measures. .

According to still another aspect of the present invention, the method for producing clean steel according to the present invention has the following steps. (i) Determining the respective thresholds of the allowable grain size range of inclusions and, if necessary, the number of inclusions in that range, depending on the steel type and application: Alternatively, the allowable particle size range of inclusions and the number of allowed inclusions are determined in advance according to the standard, and the threshold value is used.

Specifically, for example, the allowable particle size range is 20 to
It is 120 μm, and the allowable number of inclusions is 50,000 / kg within the range of the inclusion diameter. (ii) Determining the orifice diameter used for the inclusion sensor probe to be used, based on the threshold value of the allowable particle size range of the inclusions: When the threshold value is determined, the orifice diameter with which it can be measured is also determined, as described above. From the practical point of view, an orifice having a diameter close to the determined value of the orifice diameter is used.

For example, when the maximum diameter in the allowable particle diameter range is X, the size and number of inclusions having a diameter of X or less in molten steel can be easily measured by setting the orifice diameter to 2.5 X or less. You can do it.

(Iii) The step of immersing the sensor probe in molten steel in a ladle, a tundish, or a mold to detect inclusion diameter and, if necessary, data for calculating the number of such inclusions: inclusions The place of detection is not particularly limited and may be any of a ladle, a tundish, and a mold, but considering that it is possible to take measures to reduce inclusions, it is preferable to detect with a tundish. This is because, if an abnormality is detected such that each threshold value is exceeded, for example, inclusion floating processing can be performed on the spot, and processing operation in the ladle can be changed. In the mold, the molten steel in the mold cannot be changed any more, and when an abnormality is detected, it is possible to barely perform such a treatment operation on the molten steel in the tundish by feeding back.

(Iv) Step of calculating inclusion diameter and number of inclusions from the detected data: This may be performed by the already known ESZ method. In the case of a continuous probe, it may be determined continuously by measuring during casting.

When performing intermittent measurement using a one-shot type probe which is inexpensive in cost, the continuous casting steady part and unsteady part (continuous start part, continuous stop part, continuous connection part) of each heat are connected. (Part) can be specified in advance, and the measured value at the representative part can be regarded as the representative value of each cast piece.

(V) Steps for taking measures: When any of the obtained inclusion diameters or the inclusion diameters and the number of inclusions exceeds the above threshold value, the steps of refining to continuous casting, for example, converter refining In this case, it is necessary to immediately take measures on the spot, such as issuing a warning ordering measures to reduce inclusions in the converter-tundish stage, or changing the destination of the obtained product.

In the case of converter refining, the following means (i) to (iii) can be considered as measures for reducing inclusions at this time. (i) In the converter, a. Prevention of blowdown, b. Reduction of slag outflow from tapping steel, c. Deoxidation strengthening during tapping, etc. (ii) In furnace refining a. Extension of treatment time (inclusions) (Floatation, separation promotion), b. Slag reforming, c. Increase in stirring power, etc. (iii) For tundish, a. Strengthening Ar seal, b.
Preventing slag from mixing between ladle and tundish, and c. Strengthening Ar bubbling to promote floating of inclusions.

On the other hand, when the measurement is performed using the one-shot type sensor probe, the continuous casting steady part of each heat,
Unsteady parts (continuous casting start part, continuous casting stop part, continuous casting joint part) can be specified in advance, and the measured value at the representative part can be regarded as the representative value of each slab.

As described above, the orifice diameter is defined by the maximum allowable diameter of a single harmful inclusion required for each product quality. Here, the case where the high-purity steel used for the high pressure resistant material is manufactured according to the present invention will be described as follows.

In high pressure resistant materials, fatigue strength is usually an important item when evaluating products. Generally, the fatigue limit evaluation formula for inclusions is expressed as follows. ¹⁾・ Inclusions on the surface σ _w = 1.43 (H _V +120) × (√area) ^-1/6・ ・ ・ ・ ・ ・ (2) ・^{Inclusions on the} surface σ _w = 1.41 (H _V +120) ) × (√area) ^-1/6・ ・ ・ ・ ・ ・ (3) ・ Internal inclusions distant from the surface σ _w = 1.56 (H _V +120) × (√area) ^-1/6・ ・・ ・ ・ ・ ・ ・ (4) Note 1) Takanori Murakami: “Fatigue of Metals: Effects of Small Defects and Inclusions” (1993), p.90, Yokendo According to the present invention, the required quality of the product is required. If the σ _w is given, the minimum diameter √area of the inclusion that is harmful to the product alone can be obtained from the above equations (2), (3), and (4).

If the probe is clogged with an orifice having a diameter of 2.5 times or more of this minimum diameter √area, this molten steel or cast part (corresponding slab) cannot be applied to the product. Is instantly known. Furthermore, the particle size distribution of inclusions in the molten steel can be measured using an orifice having a diameter of 2.5 times or more of the minimum diameter √ area. From the maximum diameter of inclusions detected at that time,
It is also possible to estimate the quality or σ _w when the molten steel is used as a product. Then, it becomes possible to judge the product allocation of the molten steel.

If it is attempted to perform the above with a speculum, it takes time, but according to the present invention using an inclusion sensor, it is possible to make an instant determination. Next, a determination method in the case of a product type in which the maximum particle size of inclusions is a problem but the total number is also a problem will be described.

As described above, the maximum diameter of inclusions that can be stably measured is 40% of the orifice diameter. Therefore, the upper limit of the measurement range is 40% of the orifice diameter. Normally, as mentioned above, an orifice diameter of 2.5 times or more the minimum diameter of a single inclusion that is harmful to the product quality of the steel is used, but if an orifice in the vicinity of this diameter is used, not only the minimum diameter of a harmful single inclusion, The quality can be judged from the number of inclusions.

That is, the maximum particle size of inclusions is also a problem, but the total number is also useful for the determination in the case of a product type. According to the research conducted by the present inventor,
When a probe with an orifice diameter of (2.5 ± 0.1) times is used, the basic characteristics as clean steel are satisfied by Δ of 10 μV or more.
This is the case where the number of inclusions having a diameter in the range of the minimum diameter of the harmful single inclusion from the inclusion diameter giving V ₂ is 5 × 10 ⁴ or less per 1 kg of molten steel.

That is, in the case of a general system, the preferred embodiment of the present invention can be easily performed in a tundish with respect to particles in the inclusion size range of, for example, the lower limit of the orifice diameter of 6 to 10% to the upper limit of 40%. It is a method for producing clean steel, characterized in that the molten steel in the trough is cast while confirming that the number S of inclusions is in the range of S ≦ 5 × 10 ⁴ pieces / kg-steel.

In this case, as described above, the orifice diameter
Do not detect (measure) a diameter of 40% or more. That is, it goes without saying that it does not exist. In FIG. 1, fine particles 1
Change in electrical resistance value when the gas passes through the orifice 2 ΔR
Are spherical particles, assuming a cylindrical orifice, is given by ^{_{ΔR = (4ρ e d 3)}} / (πD 4) ········ (5). Where ρ _e is the resistivity of the fluid, d is the particle diameter, and D is the diameter of the orifice. Actually, the equation (5) requires the correction coefficient F (d / D) represented by the following equation (6).

F (d / D) = {1-0.8 (d / D) ³ } ^-1 (6) After all, ΔR is ΔR = (4ρ _e d ³ ) / (πD ⁴ ) × {1-0.8 (d / D) ³ } ⁻¹ ... (7) Stable measurement without clogging (d /
Since D) is considered to be almost 0.3 or less, F (d / D
) Can be set to 1. Thus, [Delta] R is ^{_{ΔR = (4ρ e d 3)}} / (πD 4) ··········· (8) On the other hand, as an example of the basic measuring system, there is a circuit set forth in FIG.

The potential difference pulse ΔV ₂ when the non-conductive spherical particles pass through the orifice is represented by the following equation. ΔV ₂ = V _E × R ₃ × ΔR / {(R ₂ + R ₃ + ΔR) (R ₂ + R ₃ )} (9) where ΔR is sufficiently smaller than (R ₂ + R ₃ ). Therefore, after all, ΔV ₂ ≈V _E × R ₃ × ΔR / (R ₂ + R ₃ ) ² (10)

Alternatively, in equation (10), (R ₂ + R ₃ ) = V _E
Substituting / I, ΔV ₂ = R ₃ × I × ΔR / (R ₂ + R ₃ ) ... (11) Alternatively, ΔV ₂ = I ² × R ₃ × ΔR / V _E ... (12) where I is the measured current value. Equation (8) is transformed into Equation (11)
When the potential difference pulse ΔV ₂ is calculated by substituting into, the following equation (13) is obtained.

[0059] _{ΔV 2 = R 3 × I ×} (4ρ e d 3) / {(πD 4) (R 2 + R 3)} ·· (13) where the noise level is measured in the usual 10μV below, this In this case, the detection limit d _min is ΔV ₂ = 10 μV, and d _min = 18 μm.

This is to detect and count a signal whose ΔV ₂ in the equation (12) is at least 10 μV or more as being due to inclusions. Generally, when trying to use this system in a steel mill, it is affected by noise. Therefore, various signal processings are performed that are used as conditions for identifying a filter, a waveform inclination, and the like from noise. However, although noise of high intensity can be discriminated to some extent by the above measures, it is difficult to discriminate noise of low intensity or a signal of a small potential difference pulse due to a very small inclusion itself from the baseline.

According to the research conducted by the present inventor, it is difficult to reduce the influence of noise to 10 μV or less in a mass-produced steelmaking factory. Therefore, in this case, the range in which the particle size distribution of inclusions can be accurately measured can be said to be a signal having an intensity of ΔV ₂ > 10 μV. In this case, for example, the orifice diameter is 300 μ
In the case of m, it is possible to measure inclusions having a diameter of 6% or more.

That is, the dynamic range of the inclusion sensor when using this orifice can be said to be 6% to 40% of the orifice diameter. In this effective dynamic range, the cleanliness of the slab can be instantly determined by setting a threshold value for each measured value based on the correspondence between the product quality and the measured data of the inclusion sensor.

That is, the orifice diameter is first set to 2.5, which is the minimum diameter of harmful inclusions alone as a product quality to be measured.
More than double. When using the 300 μm orifice in the system of FIG. 3, ΔV ₂ > 10 μV is d> 18 μ.
Therefore, the detection range of inclusion diameter is 18 μm to 120 μm at maximum. Thresholds are determined for the particle size distribution or concentration (number / kg) of inclusions in this range and used for quality judgment.

That is, if there is a harmful inclusion alone, it can be determined by clogging the orifice, and the total number (inclusion concentration) and particle size distribution can also be determined from the output of PHA. Thereby, when the respective threshold values are exceeded, the cast slab or / and molten steel is rejected,
The usage can be changed.

Further, it becomes possible to reflect these data in process control (extending refining time, strengthening stirring, strengthening tundish induction heating, lowering casting speed, etc.). According to the research by the present inventor, the orifice diameter is 100 μm.
When it is less than the above value, the orifice is clogged in a normal commercial production process, and stable measurement cannot be performed. This clogging is caused by the inclusion itself to be measured or by the molten steel being cooled by the probe body and solidifying in the orifice. In addition, it is expensive in terms of cost to form a hole with a diameter of less than 100 μm in a probe normally made of quartz or the like.

On the other hand, when the orifice diameter exceeds 2000 μm, when molten steel enters the probe through this orifice, a large flow occurs in the probe due to the high inflow velocity, and the baseline of the current value is not stable. , I can't measure accurately.

Further, in general, the detectable range of inclusions is an inclusion having a diameter such that ΔV ₂ > 10 μV in the above formula (12), and at this time, the minimum inclusion detection limit by a probe having an orifice diameter of 2000 μm, for example. Is 226 μm, which is not practical when measuring clean steel, which has the problem of inclusions with a smaller diameter.

Based on the above findings, the present inventor has set the diameter of the probe orifice that can be put to practical use in the measurement around the continuous casting machine to 100 μm or more and 2000 μm or less.

[0069]

EXAMPLES Example 1 In this example, the system shown in FIG. 1 was used, and measurement was carried out using a continuous measurement type and one-shot type sensor probe equipped with an orifice having a diameter of 300 μm. The measurement was performed on the molten steel in the tundish of the Bloom continuous casting machine. The sensor probe was immersed in molten steel, and the molten steel at 1550 ° C. was sucked into the inside through an orifice for measurement.

In this example, the maximum allowable diameter of coarse inclusions is 100
μm, and the threshold value for the number of inclusions was 50,000 / kg. The immersion depth of the sensor is 400 m below the surface of the orifice.
The measurement was performed as m.

It can be seen that the potential difference pulse detected during the measurement of about 1 kg of molten steel has a stable baseline and a smooth waveform. FIG. 3 shows an example of the measurement system adopted in this example.

The input ΔV _in to the log amplifier is ΔV _in = G × ΔV ₂ (14) where the gain of the preamplifier is G, and the output ΔV _{out of the} log amplifier is ΔV _out = 10/3 × {log ₁₀ (ΔV _in ) +2} ··· (15) This PHA divides the output range of 8V into 512 channels, and if the channel number is Ch, Ch = (512/8) × ΔV _out ... (16) Here, ΔV _out is the output of the log amp expressed by the equation (15). The gain G was 100.

There is the maximum diameter output near 250Ch, and I = 20
When A, V _E = 6.4 V and R ₃ = 0.125 Ω, ρ _e = 1.4
X10 ^-6 Ωm, the above equations (8), (12), (14), (15),
From (16), the diameter of this maximum inclusion is estimated to be 95 μm. Since the maximum diameter of 95 μm is sufficiently smaller than 40% of the orifice diameter, it was possible to measure the particle size distribution and the number of inclusions without causing the orifice clogging.

Further, during continuous casting of this molten steel, ΔV ₂
Inclusions of 10 μV or more, that is, measurement using the orifice. If the number of inclusions with a diameter of 18 μm or more exceeds 50,000 pieces / kg, change the destination, and if less than 50,000 pieces / kg, make a regular destination. I assigned it to.

In the case of this steel type, the total diameter together with the maximum diameter of the inclusions becomes a problem, and empirically, when the maximum diameter is 120 μm or less and the number of inclusions having the maximum diameter of 18 μm or more is 50,000 / kg or less, These were used as threshold values because they did not substantially affect the characteristics. Specifically, many of the slabs at the joint part of the continuous casting (the part where the ladle is replaced) exceed this threshold value.

The threshold value of the minimum diameter of the permissible harmful single inclusions is empirically obtained depending on the steel type, the destination and the application, and further, the threshold value of the number of inclusions per unit steel sample. Is defined by equation (17).

[0077]

[Equation 3]

S ≦ 5 × 10 ⁴ P / kg-steel D: Orifice diameter x: Inclusion particle size Here, FIG. 4 is a graph showing the contents of the formula (17), which is proportional to the orifice diameter. Inclusion diameter (x) and fatigue characteristics
Relationship with (N): A graph showing N = F (x).
The value integrated from 0.06D to 0.4D is S, that is, the number of inclusions.

In any case, according to the present invention, an appropriate orifice diameter is selected for each steel type, destination and application, and the maximum diameter of inclusions and / or the threshold value of the number of inclusions per unit steel sample It has become possible to judge the quality of molten steel "on the spot" and assign it to the application.

Here, the following examples can be given as typical examples of the steel composition to which the present invention is applied. C: 1.2% or less, Si: 3.0% or less, Mn: 1.15% or less,
Elements such as P: 0.025% or less, S: 0.2% or less, Al: 1.0% or less, and Cr and Mo may be appropriately contained if necessary.

Example 2 Using the system shown in FIG. 3, measurement was performed using a sensor probe equipped with an orifice having a diameter of 100 μm.

In this example, the maximum allowable diameter of coarse inclusions is 40 μm.
m was set as a threshold value for inclusion diameter, and the threshold value for the number of inclusions was set at 50000 pieces / kg. The measurement was performed on the molten steel in the tundish of the Bloom continuous casting machine. 15
The measurement was performed by sucking molten steel at 45 ° C. About to suck molten steel
After 10 seconds, the orifice was clogged and the measurement was stopped.

The clogging of the orifice was judged by the disturbance of the baseline on the oscilloscope. When the sensor probe was pulled up from the molten steel, it was observed that the tip of the probe was actually blocked. The molten steel superheat degree at this time is 33
It was ℃. When the signal 10 seconds before the start of the measurement was analyzed, inclusions having a diameter of 40 μm or more were found to be scattered according to equations (8) and (12).

From this, it is immediately judged that this molten steel contains inclusions having a diameter of 40 μm or more, and it is shown that the orifice clogging becomes a condition for judging the existence of harmful inclusions by itself.

When a sample taken from a billet obtained by rolling a bloom obtained by casting this molten steel was microscopically observed, inclusions having a diameter of 40 μm or more were observed.
The maximum allowable diameter of inclusions of this material was 40 μm, so the product was not assigned to its original use.

The microscopic observation of inclusions at the billet stage was obtained 7 days after the completion of casting. As described above, in the conventional method, it took seven days to determine inclusions, but by applying the present invention, it becomes possible to determine inclusions at the molten steel stage.

Example 3 The measurement was carried out using a sensor probe equipped with an orifice having a diameter of 200 μm. The measurement was performed on the molten steel in the tundish of the Bloom continuous casting machine.

The measurement was performed by sucking molten steel at 1552 ° C. In this example, the maximum allowable diameter of coarse inclusions is set to 25 μm, which is used as the threshold value for inclusion diameter, and the number of inclusions in the range of 10 μm to 25 μm is set to 50000 pieces / kg, which is used as the threshold value. .

As a result of the measurement, the maximum diameter range of the inclusions measured was in the range of 25 μm to 30 μm, and the number of inclusion particles in this particle size range was estimated to be about 300 particles per kg of molten steel.

The steel type is used for the purpose of being repeatedly fatigued under a high pressure condition, and empirically based on the maximum allowable diameter of inclusions measured by the present invention and the fatigue strength required for the product, the formula (2) The maximum diameter of allowable inclusions obtained from (3) and (4) was 25 μm. The number of inclusions in the range of 10 to 30 μm was 5.5 × 10 ⁴ pieces / kg. Based on the results of this measurement, the slab cast from the molten steel was judged immediately after casting and could not be used for the original purpose.

Based on the measurement results, immediately after that,
As a result of extending the stirring and holding time in the secondary refining process and strengthening the slag modification in the ladle, the same measurement was carried out in another casting trial immediately after that, and as a result, inclusions in the molten steel in the continuous casting tundish were The maximum diameter was reduced from 15 to 20 μm, and the number of inclusions in the range of 10 to 20 μm was 2.5.
× 10 ⁴ pieces / kg. It became possible to allocate the slab to the original purpose. There were no abnormalities on the product.

It is possible to carry out these process improvement procedures within tens of minutes to several hours, and the method of waiting for the results of microscopic observation of conventional products to judge and improve the time is from 1 week to 10 days after casting. Compared to the
It is now possible to fully meet the customer's demand for delivery date.

[0093]

According to the present invention, it is possible to manufacture a clean steel whose inclusion distribution can be specified, the reliability of the quality is remarkably improved, and such a high-quality clean steel can be stably manufactured. It has great practical significance.

[Brief description of drawings]

FIG. 1 is an explanatory view of the principle of inclusion detection.

FIG. 2 is a conceptual diagram of a basic measurement system.

FIG. 3 is an explanatory diagram of a system for carrying out the present invention.

FIG. 4 is a graph illustrating a relationship with N-X in an example.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B22D 46/00 B22D 46/00

Claims (5)

[Claims] 1. In continuous casting of molten steel, inclusions contained in the molten steel passing through an orifice provided in a probe constituting an inclusion sensor immersed in the molten steel are converted into a potential difference pulse between facing surfaces of the orifice. In the method for producing clean steel, in which the cleanliness of molten steel is “measured in-situ” by the ESZ method that is counted based on the Determining; determining the orifice diameter used for the probe based on the threshold for the maximum diameter of the inclusions; immersing the inclusion sensor in molten steel in a ladle, tundish, or mold;
And when it is detected that the orifice is blocked by inclusions exceeding the threshold value, a warning is issued to order measures to reduce inclusions in the stages of refining to continuous casting, or the obtained product is changed. A method for producing clean steel, which comprises the step of immediately taking measures such as 2. In continuous casting of molten steel, inclusions contained in the molten steel passing through an orifice provided in a probe forming an inclusion sensor immersed in the molten steel are converted into a potential difference pulse between facing surfaces of the orifice. A method for producing a clean steel in which the cleanliness of molten steel is "in-situ measured" by an ESZ method that counts based on a method for producing a clean steel. If necessary, determining each threshold value with the number of inclusions in that range; determining the orifice diameter used for the probe based on the threshold value of the range of the allowable particle size of the inclusions; The step of immersing the inclusion sensor in molten steel in a ladle, a tundish, or a mold to detect data for calculating the diameter of inclusions or the diameter of inclusions and the number of inclusions; detection The step of calculating the diameter of inclusions and the number of inclusions if necessary from the data obtained; and the diameter of the obtained inclusions, or when calculating the number of inclusions, either the diameter of inclusions or the number of inclusions When the threshold value is exceeded, a warning is issued to order measures to reduce inclusions in the stages of refining to continuous casting, the product is redirected to the product obtained, or the like, and measures are taken immediately on the spot; Method for producing clean steel. 3. When performing continuous measurement using a continuous measurement type inclusion sensor, the above-mentioned data is detected and collected with an immersion depth of 300 mm or more from the molten metal surface of the orifice,
The method for producing clean steel according to claim 1 or 2. 4. When the intermittent measurement is performed using a one-shot type inclusion sensor, the data is detected and collected by setting the immersion depth from the molten metal surface of the orifice to 300 mm or more. 2. The method for producing clean steel according to 2. 5. The method for producing clean steel according to claim 2, wherein the threshold value of the number of inclusions is 50000 or less per 1 kg of molten steel determined by the following formula for molten steel in the tundish or trough. [Equation 1] S ≦ 5 × 10 ⁴ P / kg-steel D: Orifice diameter x: Inclusion particle size