Method of analyzing particle-containing liquid, method of maintaining hot-dip galvanizing bath, method of producing hot-dip galvanized steel sheet, and device for analyzing particle-containing liquid

Abstract

This method of analyzing a particle-containing liquid is for analyzing a particle-containing liquid by irradiating the same with a laser and running spectrometry on the light emission that occurs at every laser irradiation, the method comprising: a step for focusing the laser; a step for obtaining the light emission intensity of a first element selected from elements included in the liquid and a light emission intensity of a second element selected from elements contained in the liquid and the particles; a step for normalizing the light emission intensity of the second element with the light emission intensity of the first element to obtain a normalized intensity of the second element; a step for creating a frequency distribution graph of the normalized intensity; a step for identifying liquid-induced peaks and particle-induced peaks in the frequency distribution graph; and a step for quantifying the abundance of the particles on the basis of the ratio of a frequency derived from the particle-induced peaks to the total frequency of the frequency distribution graph.

Description

Method for analyzing liquid containing particles, method for managing hot-dip galvanizing bath, method for manufacturing hot-dip galvanized steel sheet, and apparatus for analyzing liquid containing particles

[0001] The present invention relates to a method for analyzing a liquid containing particles, a method for managing a hot-dip galvanizing bath, a method for manufacturing a hot-dip galvanized steel sheet, and an apparatus for analyzing a liquid containing particles. This application claims priority to Japanese Patent Application No. 2022-202524, filed on December 19, 2022, the contents of which are incorporated herein by reference.

[0002] In a molten metal bath such as a hot-dip galvanizing bath used in the production of plated steel sheets, it is necessary to monitor and control the state of the molten metal for quality control of hot-dip galvanized steel sheets, which are one of the products obtained using the bath, and for control of operating conditions.

[0003] Patent Document 1 proposes laser emission spectroscopy of molten metal as a method for analyzing molten metal during refining, and Patent Document 2 discloses a method and apparatus for applying laser-induced breakdown spectroscopy (LIBS) to the analysis of molten metal materials.

[0004] Furthermore, Patent Document 3 discloses an immersion sensor for analyzing a liquid or melt using LIBS, in which the measurement point is located at the point where the liquid or melt is newly supplied into the sample chamber from an inlet opening.

[0005] Furthermore, Non-Patent Document 1 discloses a method for obtaining information on molten Al only by obtaining the Al / Zn ratio from the spectrum of each LIBS laser shot for molten zinc, creating a frequency distribution chart, and excluding the dross portion.

[0006] Japanese Patent Application Publication No. 2006-300819 Japanese Patent Application Publication No. 2005-530989 Japanese Patent Application Publication No. 2008-96433

[0007] Eric Baril et al, "Novel Method for On-Line Chemical Analysis of Continuous Galvanizing Baths", Galvatech2004 Conference Proceedings, 1095-1104

[0008] Molten metal often contains particles of μm size, which can affect the quality of industrial products manufactured from the molten metal. For example, alumina in molten steel or molten aluminum, and oxides and nitrides in molten titanium, can act as inclusions and cause cracks or defects in products. Furthermore, in the case of galvanized steel sheets, which are manufactured by continuously immersing steel sheets in molten zinc, solid matter containing Zn-Al-Fe alloys, known as dross, can adhere to the steel sheets during plating, resulting in defects and poor appearance. Therefore, managing particles in molten metal is extremely important for improving product quality and yield.

[0009] However, Patent Documents 1 to 3 do not disclose the analysis of particles in liquids such as molten metals, and it is unclear whether the amount of particles present can be analyzed. Furthermore, the technology disclosed in Non-Patent Document 1 is a technology aimed at improving the accuracy of quantitative determination of molten Al, but does not go so far as to separately quantify particles and molten Al.

[0010] Conventionally, to quantify particles in molten metal, a portion of the molten metal is first collected as an analytical sample using a ladle or other tool. The molten metal is then solidified using a mold or other tool. However, during this process, the state of the particles contained in the collected analytical sample may change due to alloying or other factors. To avoid this change in particle state, the molten metal is rapidly solidified by water cooling or using a mold made of a material with high thermal conductivity, such as copper. The solidified analytical sample is then removed from the mold, cut, and polished to obtain an observation surface. This observation surface is then observed using an optical microscope or electron microscope to measure the particle count per volume (concentration), size, and elemental composition. However, this method requires time and effort for cutting, polishing, and observing the sample, and the representativeness of the obtained quantitative values ​​is questionable due to the technique of observing the cross-section of the analytical sample.

[0011] Therefore, the present invention has been made in consideration of the above problems, and aims to provide a method for analyzing a liquid containing particles, a method for managing a hot-dip galvanized bath, and a method for manufacturing a hot-dip galvanized steel sheet, which are capable of easily quantifying the amount of particles present in the liquid.

[0012] The inventors have found that by controlling the laser focusing conditions in LIBS, it is possible to distinguish between the emission spectrum of the liquid and the emission spectrum of particles in the liquid, and to quantify the amount of particles present in the liquid based on the ratio of the frequency attributable to particle-attributed peaks to the total frequency in a frequency distribution graph of normalized intensity obtained by normalizing the emission intensity of the emission spectrum.

[0013] The gist of the present invention, which was completed based on the above findings, is as follows: [1] A method for analyzing a liquid containing particles according to one aspect of the present invention is a method for analyzing a liquid containing particles by irradiating the liquid containing particles with a laser and spectroscopically analyzing the emission generated at each irradiation of the laser, the method comprising: a laser focusing step of focusing the laser; an emission intensity measurement step of obtaining an emission intensity of a first element selected from elements contained in the liquid and an emission intensity of a second element selected from elements contained in the liquid and the particles, the second element having a concentration different from that in the liquid; the normalization step of normalizing the intensity of the second element by the emission intensity of the first element to obtain a normalized intensity of the second element; a frequency distribution graph creation step of creating a frequency distribution graph showing a distribution of frequencies of the normalized intensities; a peak identification step of identifying, in the frequency distribution graph, liquid-attributed peaks that are peaks attributable to the liquid and particle-attributed peaks that are peaks attributable to the particles; and a particle quantification step of calculating the sum of the frequencies attributable to the particle-attributed peaks and quantifying the amount of the particles present based on the ratio of the frequency attributable to the particle-attributed peaks to the total frequency in the frequency distribution graph. [2] The method for analyzing a particle-containing liquid according to the above [1] may include a step of preparing liquids containing particles with different abundances, measuring the abundances of the particles in the liquids by analyzing samples collected from the liquids, and applying the analytical method according to claim 1 to the liquids when collecting the samples from the liquids to calculate a ratio of the sum of the frequencies attributable to the particle-attributed peaks to the total frequency in the frequency distribution graph, thereby creating a calibration curve showing the relationship between the ratio of the sum of the frequencies attributable to the particle-attributed peaks to the total frequency in the frequency distribution graph and the abundances of the particles. [3] In the method for analyzing a particle-containing liquid according to the above [1], the peak identification step may include calculating an approximation curve in the frequency distribution graph to identify the liquid-attributed peak and the particle-attributed peak, and the particle quantification step may include quantifying the frequencies attributable to the particle-attributed peaks from the calculated approximation curve.[4] The method for analyzing a particle-containing liquid according to any one of [1] to [3] above may include an average particle diameter measurement step of measuring the average particle diameter of the particles present in the liquid before the laser focusing step. [5] In the method for analyzing a particle-containing liquid according to any one of [1] to [4] above, in the laser focusing step, the diameter of the laser's focused spot may be set to a value less than 10 times the average particle diameter of the particles. [6] In the method for analyzing a particle-containing liquid according to any one of [1] to [5] above, in the laser focusing step, the diameter of the laser's focused spot may be changed while performing the emission intensity measurement step, and the normalization step, the frequency distribution graph creation step, and the peak identification step may be performed for each focused spot diameter to determine the laser focusing conditions. [7] In the method for analyzing a particle-containing liquid according to any one of [1] to [6] above, the liquid may be molten metal. [8] In the method for analyzing a particle-containing liquid according to any one of [1] to [7] above, the liquid may be a molten zinc alloy in which the molten zinc contains at least aluminum. [9] In the method for analyzing a liquid containing particles according to any one of [1] to [8] above, in the laser focusing step, the diameter of the laser focusing spot may be set to less than 300 μm.

[10] In the method for analyzing a liquid containing particles according to any one of [1] to [9] above, in the laser focusing step, the diameter of the laser focusing spot may be controlled to be 5 μm or more and less than 100 μm.

[0014]

[11] A method for managing a hot-dip galvanizing bath according to another aspect of the present invention comprises irradiating a hot-dip galvanizing bath containing particles with a laser and spectroscopically analyzing the emission generated at each irradiation of the laser, thereby analyzing the hot-dip galvanizing bath and managing the hot-dip galvanizing bath, the method comprising: a laser focusing step of focusing the laser; an emission intensity measurement step of obtaining an emission intensity of a first element selected from elements contained in the hot-dip galvanizing bath and an emission intensity of a second element selected from elements contained in the liquid and the particles, the second element having a concentration in the liquid different from that in the particles; The method includes a normalization step of normalizing the luminescence intensity of a second element by the luminescence intensity of the first element to obtain a normalized intensity of the second element; a frequency distribution graph creation step of creating a frequency distribution graph showing a distribution of frequencies of the normalized intensities; a peak identification step of identifying, in the frequency distribution graph, liquid-attributed peaks that are peaks attributable to the liquid and particle-attributed peaks that are peaks attributable to the particles; and a particle quantification step of calculating the sum of the frequencies attributable to the particle-attributed peaks and quantifying the amount of the particles present based on the ratio of the frequency attributable to the particle-attributed peaks to the total frequency in the frequency distribution graph.

[0015]

[12] A method for producing a hot-dip galvanized steel sheet according to yet another aspect of the present invention includes a step of managing a hot-dip galvanizing bath managed by the method for managing a hot-dip galvanizing bath according to the above

[11] .

[0016]

[13] According to yet another aspect of the present invention, an apparatus for analyzing a liquid containing particles is provided for analyzing the liquid containing particles by irradiating the liquid with a laser and spectroscopically analyzing the emission generated with each irradiation of the laser. The apparatus includes: a laser focusing unit that focuses the laser; an emission intensity measuring unit that obtains an emission intensity of a first element selected from elements contained in the liquid and an emission intensity of a second element selected from elements contained in the liquid and the particles, the second element having a concentration in the liquid different from that in the particles; a normalization unit that normalizes the emission intensity of the second element by the emission intensity of the first element to obtain a normalized intensity of the second element; a frequency distribution graph creation unit that creates a frequency distribution graph representing a distribution of frequencies of the normalized intensities; a peak identification unit that identifies, in the frequency distribution graph, liquid-attributed peaks that are peaks attributable to the liquid and particle-attributed peaks that are peaks attributable to the particles; and a particle quantification unit that calculates the sum of frequencies attributable to the particle-attributed peaks and quantifies the amount of particles present based on a ratio of the frequency attributable to the particle-attributed peaks to the total frequency in the frequency distribution graph.

[0017] As explained above, according to the above aspects of the present invention, it is possible to simply and easily quantify the amount of particles present in a liquid.

[0018] 1 is a diagram showing an example of the configuration of a component analysis system applicable to a method for analyzing particles in a liquid according to one embodiment of the present invention. FIG. 2 is a functional block diagram showing the functional configuration of a signal processing unit. FIG. 3 is a flowchart showing the flow of the method for analyzing particles according to the same embodiment. FIG. 4 is an example of an emission spectrum of an Al-containing molten zinc alloy measured by LIBS. FIG. 5 is another example of an emission spectrum of an Al-containing molten zinc alloy measured by LIBS. FIG. 6 is an example of a frequency distribution graph in the same embodiment, showing an example of a frequency distribution graph of an Al-containing molten zinc alloy including dross. FIG. 7 is an example of peaks identified in the frequency distribution graph in the same embodiment, showing an example of a peak due to dross and a peak due to liquid. FIG. 8 is an example of a graph showing a calibration curve in the same embodiment, showing the calibration curve when the horizontal axis represents the number of dross and the vertical axis represents the ratio of the frequency due to particle-related peaks to the total frequency in the frequency distribution graph. FIG. 9 is an example of a graph showing a calibration curve in the same embodiment, showing the calibration curve when the horizontal axis represents the Al concentration in an Al-containing molten zinc alloy and the vertical axis represents the normalized intensity at the peak top position of the liquid-related peak in the frequency distribution graph. Fig. 1 is a diagram showing an example of a schematic configuration of a hot-dip galvanizing apparatus to which the method for analyzing particles in a liquid according to one embodiment of the present invention can be applied. Fig. 2 is a plan view showing an example of a schematic configuration of a hot-dip galvanizing apparatus to which the method for analyzing particles in a liquid according to the same embodiment can be applied. Fig. 3 is a frequency distribution graph of level 1 in an example. Fig. 4 is a frequency distribution graph of level 2 in an example. Fig. 5 is a frequency distribution graph of level 3 in an example. Fig. 6 is a frequency distribution graph of level 4 in an example. Fig. 7 is a frequency distribution graph of a comparative example.

[0019] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0020] <Particle Analysis System> First, prior to describing a method for analyzing a particle-containing liquid according to one embodiment of the present invention, an example of a particle component analysis system capable of performing the method for analyzing a particle-containing liquid according to this embodiment will be described. FIG. 1 is a diagram showing an example of the configuration of a component analysis system 11 according to this embodiment. As shown in FIG. 1, the component analysis system 11 according to this embodiment has a configuration for performing LIBS, and includes, for example, a laser device 12, a cylindrical probe 13, a transmission cable 14, a support member 17, a fixing member 18, a drive unit 19, and a processing device 20. In the description of the particle analysis device, the liquid will be described as molten metal, which is one form of liquid.

[0021] The laser device 12 includes a laser oscillator (not shown) that emits pulsed laser light (hereinafter also simply referred to as "laser") and an optical system (not shown) that guides the pulsed laser light to a cylindrical probe 13 whose tip is immersed in the molten metal 3 and guides plasma-induced light received via the cylindrical probe 13 to a transmission cable 14. The laser oscillator has a function of generating plasma at the gas-liquid interface between the molten metal 3 and a gas supplied to the molten metal 3 by the cylindrical probe 13 (described later) using the laser it emits. The laser oscillator preferably has a function of emitting pulsed laser light that non-selectively vaporizes components contained in the molten metal 3 and particles contained in the molten metal 3. The laser oscillator, for example, emits pulsed laser light using a Q-switching method or the like, with an output of 10 to 200 mJ / pulse, and emits laser light with wavelengths ranging from the fundamental wave to the third harmonic. For example, the laser oscillator may be an Nd:YAG laser or a diode laser, which are widely used as high-power pulsed lasers. The pulse frequency may be, for example, about 1 to 50 Hz, and the laser pulse width is preferably several nanoseconds to several psec.

[0022] The laser device 12 may also be provided with an adjustment mechanism for adjusting the laser spot diameter and the like.

[0023] The optical system includes an optical system that guides the laser emitted from the laser oscillator to the cylindrical probe 13, and an optical system that guides the plasma-induced light received via the cylindrical probe 13 to the transmission cable 14. These optical systems are composed of optical members such as lenses, mirrors, dichroic mirrors, and collimators. The optical system may include an adjustment mechanism for adjusting the laser spot diameter, etc.

[0024] The cylindrical probe 13 is a cylindrical member that forms an optical path between the molten metal 3 and the laser device 12. One end of the cylindrical probe 13 is connected to the laser device 12, and the other end, which is an open end 131, is disposed so as to be immersed in the molten metal 3. The pulsed laser light emitted from the laser device 12 is guided through the cylindrical probe 13 and condensed by a dichroic mirror or a lens to focus on the molten metal 3 containing particles to be analyzed at the open end 131 side. An inert gas is also supplied to the cylindrical probe 13 from an arbitrary position toward the open end 131. As shown in FIG. 1 , the inert gas is released from the open end 131 into the molten metal 3 and foams, generating bubbles 15. When the laser light guided through the cylindrical probe 13 is irradiated onto the molten metal 3, plasma 16 is generated at the gas-liquid interface between the generated bubbles 15 and the molten metal 3. The particles and elements constituting the molten metal are decomposed to the atomic level in the plasma 16, then excited, and emit light upon relaxation. The inert gas is preferably an inert gas commonly used in plasma emission spectrometry, such as Ar or He. The open end 131 of the cylindrical probe 13 may be perpendicular to the longitudinal direction or may be oblique.

[0025] The transmission cable 14 is a cable for transmitting light that is generated by plasma emission in the molten metal 3 and guided to the laser device 12 via the cylindrical probe 13 to the processing device 20. The transmission cable 14 is realized by a general light-guiding cable such as an optical fiber cable.

[0026] The support member 17 supports the laser device 12 and the cylindrical probe 13. The support member 17 may be, for example, a plate-shaped platform. The support member 17 is driven by a driving means, such as a built-in motor, of the drive unit 19 (described later) so as to form an angle with respect to the horizontal plane. This makes it possible to change the angle of the cylindrical probe 13 with respect to the vertical direction. The support member is not limited to the illustrated embodiment and may be, for example, a member that can hold or suspend the laser device 12 and the cylindrical probe 13.

[0027] The fixed member 18 is a rod-shaped member that is disposed outside the plating tank 4 and whose positional relationship with respect to the plating tank 4 is fixed. A drive unit 19 is attached to the fixed member 18. The drive unit 19 fixes the support member 17 and is configured to be movable along the fixed member 18. The drive unit 19 is moved to any position along the fixed member 18 by a drive means such as a built-in motor. This makes it possible to change the immersion depth of the open end 131 of the cylindrical probe 13 in the plating bath 3. In other words, the support member 17 that supports the laser device 12 and the cylindrical probe 13 can be moved relative to the fixed member 18 using the drive unit 19, thereby controlling the position of the open end 131 of the cylindrical probe 13.

[0028] The processing device 20 has a function of separating and detecting the light transmitted via the transmission cable 14 and processing the obtained signal. The processing device 20 includes a detection unit 21 and a signal processing unit 22.

[0029] The detector 21 disperses the light transmitted via the transmission cable 14 and records the dispersed light as an emission spectrum. The emission spectrum is collected and recorded for each shot (pulse) of the laser. For example, the detector 21 can be realized by a spectrometer and a photoelectric converter. The spectrometer has sufficient wavelength resolution and sensitivity for the emission intensity at the wavelengths of the particles and elements in the molten metal to be measured, and is preferably an Echelle or Czerny-Turner type, for example. The photoelectric converter has an operating speed capable of processing all laser pulses at a predetermined frequency and has sufficient sensitivity for the wavelengths of the constituent elements of the particles and molten metal to be measured. The photoelectric converter may be, for example, an optical sensor such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) equipped with an image intensifier, or a PMT (Photomultiplier Tube). The detection unit 21 according to this embodiment measures signals in a wavelength band including wavelengths corresponding to elements contained in the molten metal 3 and elements contained in the particles, and outputs the signals as measurement data. The measurement data relating to the signals corresponding to each element detected by the detection unit 21 is output to the signal processing unit 22.

[0030] The signal processing unit 22 has a function of processing the acquired measurement data (signals). The signal processing unit 22 calculates the peak intensity of the wavelengths of the particles and elements in the molten metal to be measured from the collected spectra, normalizes the spectrum, calculates the frequency of occurrence, creates a frequency distribution graph, and separates the peaks. Figure 2 is an example of a functional block diagram showing the functional configuration of the signal processing unit 22. As shown in FIG. 2 , the signal processing unit 22 includes: a laser focusing unit 221 that focuses the laser; an emission intensity measuring unit 222 that obtains the emission intensity of a first element selected from elements contained in the molten metal 3 and the emission intensity of a second element selected from elements contained in the molten metal 3 and the particles, and having a concentration in the molten metal 3 different from that in the particles; a normalization unit 223 that normalizes the emission intensity of the second element with the emission intensity of the first element to obtain a normalized intensity of the second element; a frequency distribution graph creation unit 224 that creates a frequency distribution graph showing the distribution of frequencies of the normalized intensities; a peak identification unit 225 that identifies, in the frequency distribution graph, liquid-attributed peaks that are peaks attributable to the molten metal 3 and particle-attributed peaks that are peaks attributable to the particles; and a particle quantification unit 226 that calculates the sum of the frequencies attributable to the particle-attributed peaks and quantifies the amount of particles present based on the ratio of the frequency attributable to the particle-attributed peaks to the total frequency in the frequency distribution graph. The laser focusing unit 221 controls the optical system so that the diameter of the laser focused spot (focused diameter) is determined in advance. Specifically, the focusing device (for example, an optical device also called an optical system or lens system, such as a collimator and a focusing lens) is controlled to achieve the target focused diameter. Note that if appropriate focusing conditions are determined in advance by a preliminary test or the like and the optical system is set to those focusing conditions, control of the focusing conditions as signal processing is not required.

[0031] The signal processing unit 22 is realized by hardware including, for example, an arithmetic unit such as a CPU or a processor, a main storage device such as a ROM (Read Only Memory) or a RAM (Random Access Memory), and an auxiliary storage device such as a hard disk or a flash memory. The signal processing unit 22 may be configured by a single piece of hardware or by multiple pieces of hardware. The signal processing unit 22 may also be realized by an embedded system.

[0032] <Method for analyzing liquid> Next, a method for analyzing a liquid containing particles according to one embodiment of the present invention will be described with reference to Fig. 3. Fig. 3 is a flowchart showing the flow of the method for analyzing a liquid containing particles according to this embodiment. Below, a method for analyzing a liquid containing particles using the component analysis system 11 described above will be described. However, the present invention is not limited to this example.

[0033] The method for analyzing a liquid containing particles according to this embodiment is a method for analyzing a liquid containing particles by irradiating a laser onto the liquid containing particles and spectroscopically analyzing the light emission generated with each laser irradiation. The method for analyzing a liquid containing particles according to this embodiment includes a laser focusing step (step S1) of focusing a laser beam, an emission intensity measurement step (step S2) of obtaining an emission intensity of a first element selected from elements contained in the liquid and an emission intensity of a second element selected from elements contained in the liquid and the particles, the second element being selected from elements whose concentrations in the liquid and the particles differ from each other, a normalization step (step S3) of normalizing the emission intensity of the second element with the emission intensity of the first element to obtain a normalized intensity of the second element, a frequency distribution graph creation step (step S4) of creating a frequency distribution graph representing a distribution of frequencies of the normalized intensities, a particle-attributed peak identification step (step S5) of identifying, in the frequency distribution graph, liquid-attributed peaks that are peaks attributable to the liquid and particle-attributed peaks that are peaks attributable to the particles, and a particle quantification step (step S6) of calculating the sum of frequencies attributable to the particle-attributed peaks and quantifying the amount of the particles present based on the ratio of the frequency attributable to the particle-attributed peaks to the total frequency in the frequency distribution graph. The liquid to be measured is not particularly limited and may be, for example, water containing particles or molten metal, and the molten metal may be a molten zinc alloy in which at least aluminum is contained in molten zinc (Al-containing molten zinc alloy). Note that the first element is different from the second element.

[0034] [Laser Focusing Process (Step S1)] In the laser focusing process, the diameter (focusing diameter) of the laser focus spot is adjusted to an appropriate diameter that allows identification of the liquid-induced peak and the particle-induced peak in the peak identification process described below, and the laser is irradiated onto the liquid containing particles. In this process, it is preferable to adjust the focusing diameter to, for example, less than 10 times the average particle diameter of the particles. This is to prevent signals due to the particles from being buried in the signals of the liquid matrix. In this case, it becomes easier to identify the liquid-induced peak and the particle-induced peak in the peak identification process described below, that is, to separate the liquid-induced peak and the particle-induced peak. For example, if the particles present in the laser focusing spot have a single size and that size, i.e., the diameter of the particle, is 10 times the diameter of the laser focusing spot, the area occupied by the particles in the laser focusing spot will be 1 / 100. In other words, it is expected that the intensity of the emission spectrum from the particles will be 1% of the emission spectrum from the non-particle portion, i.e., the molten metal. On the other hand, in galvannealed hot-dip galvanizing baths, the particles being measured are dross, and their composition has been reported to be 44 to 45 mass% Al, 37 to 38 mass% Fe, and 18 to 19 mass% Zn (ISIJ International, Vol. 61 (2021), No. 3, pp. 937-944). The composition of galvannealed hot-dip galvanizing baths generally contains approximately 0.12 to 0.14 mass% Al and 0.03 to 0.05 mass% Fe. Therefore, the particles contain 314 to 375 times more Al and 740 to 1266 times more Fe than the molten metal. Therefore, if a particle with a diameter (average particle diameter) 1 / 10 of the diameter of the laser focal spot is included, the emission intensity of Al can be calculated to be approximately 3 times higher, and the emission intensity of Fe can be calculated to be approximately 7 to 12 times higher. On the other hand, in the case of lasers commonly used in LIBS, which have an output of several tens of mJ / pulse and a pulse width of several nanoseconds, the stability of the emission intensity is generally several percent to several tens of percent. In the case of molten metal, taking into account subtle fluctuations in the position of the analysis surface, the stability is expected to be 20% or more.Here, the lower limit of quantification in an analysis is the minimum amount or minimum concentration at which a certain analytical method can quantify an analyte, and is generally considered to be 10 times the background variation (standard deviation). If the stability of the emission intensity in the absence of particles is 20%, then 10 times that amount is 200%, i.e., an increase in signal intensity of about twice the original emission intensity does not yield a significant signal for quantitative analysis. Here, if the particle diameter is larger than 1 / 10 of the diameter of the laser focused spot, it can be calculated that a signal intensity of about 3 times (300%) or more can be obtained for Al, and about 7 to 12 times (700 to 1200%) or more can be obtained for Fe. Therefore, since a signal significantly exceeding 200%, which is 10 times the variation in emission intensity, can be obtained, quantitative analysis can be said to be possible. If the particle diameter is smaller than 1 / 10 of the diameter of the laser irradiation spot, the proportion of the emission signal from the particles in the obtained emission signal decreases exponentially, which is insignificant relative to the stability of the emission intensity, and therefore sufficient signal intensity cannot be obtained to distinguish between particles and perform analysis. Therefore, although the laser emission intensity and its variation are greatly affected by the wavelength, output, pulse width, focusing conditions, and state of the object of analysis of the laser beam, for quantitative analysis, it is preferable that the size of the laser's focused spot be less than 10 times the average particle diameter (average particle size). For example, when the average particle size is 10 to 20 μm, the diameter of the laser's focused spot is preferably approximately 50 μm. On the other hand, while techniques such as cutting with an aperture can be used to focus the laser, such techniques can sometimes be difficult to focus while maintaining the photon density. Therefore, in order to perform laser-induced emission analysis while maintaining analytical sensitivity, it is preferable that the diameter of the focused spot be 5 μm or more. In conventional LIBS, the laser's focused diameter is approximately 300 μm to 1 mm, which is significantly larger than the diameter of particles contained in the liquid. Therefore, it is difficult to distinguish between liquid and particles and perform accurate analysis using conventional methods. In this embodiment, from the viewpoint of performing a highly accurate analysis, the focused diameter of the laser is preferably less than 300 μm, and more preferably less than 200 μm, less than 100 μm, less than 80 μm, or less than 60 μm.Reducing the diameter of the laser's focused spot reduces the frequency of particles contained within the spot, but increasing the laser's oscillation frequency or appropriately setting the measurement time can improve analytical accuracy. Regarding the diameter of the laser's focused spot irradiated onto the particle-containing liquid, it is preferable to first determine the diameter at the position of the analysis target (the surface of the liquid containing the particles to be analyzed) using, for example, thermal paper or a laser beam profiler, depending on the laser and focusing device (e.g., an optical device also known as an optical system or lens system, such as a collimator and a focusing lens), and then select or adjust an appropriate optical system according to the average particle size of the particles to be measured. Examples of selecting or adjusting the optical system and lens system include selecting or adjusting the focal length of the aperture or lens, or adjusting the position of the lens's optical axis. If the diameter of the laser's focused spot is significantly larger than the particle size (average particle size), the probability of particles being present directly below the laser increases, but the probability of particles not being present directly below the laser decreases significantly. As a result, a frequency distribution graph with two peaks cannot be obtained in the frequency distribution graph creation process described below. Therefore, it is necessary to reduce the focused spot diameter to increase the probability that particles are not present directly below the laser. On the other hand, if the focused spot diameter is made too small, the probability of particles being present directly below the laser also decreases significantly, which may prevent the generation of a frequency distribution graph with two peaks in the frequency distribution graph creation process described below. Therefore, there is an appropriate range for the laser focused spot diameter. The average particle diameter of particles in a liquid can be determined by measuring the particle size distribution using laser scattering measurement, for example, when the particle-containing liquid is an aqueous solution. In the case of molten metal, the molten metal can be sampled, cut after solidification, and polished. The cut and polished surface can be measured under a microscope, and the average particle diameter can be determined using image processing. In the peak identification process, the appropriate spot diameter (focused diameter) that can identify the liquid-attributed peak and the particle-attributed peak depends not only on the particle diameter (average particle diameter) but also on the difference in chemical composition between the liquid and the particles (the difference in the content of the second element described below) and the variation in particle diameter. For example, if the difference in chemical composition is small, a smaller spot diameter is required.For this reason, even if the spot diameter is mechanically multiplied by a coefficient, it may not be possible to identify (separate) the liquid-induced peak and the particle-induced peak. It is essential to focus the light to a spot diameter that allows the liquid-induced peak and the particle-induced peak to be identified in the peak identification step. If the liquid-induced peak and the particle-induced peak can be identified (separated) in the peak identification step, there is no need to adjust the ratio between the average particle diameter and the focused spot diameter. For example, if, by chance, the liquid-induced peak and the particle-induced peak can be identified (separated) in the peak identification step when focusing light with a certain optical system and lens system in a preliminary test, the focusing conditions of the optical system and lens system obtained by chance may be adopted (without the need to adjust the spot diameter itself).

[0035] In the laser focusing process, it is preferable to focus the laser under multiple focusing conditions in advance, and then perform the emission intensity measurement process, normalization process, frequency distribution graph creation process, and particle-attributed peak identification process (described later) for each focusing condition to determine the optimal focusing conditions that can identify (separate) the liquid-attributed peak and the particle-attributed peak. This allows the focusing conditions to be optimized, i.e., the focused diameter to be more appropriate, making it possible to distinguish between liquid and particles with higher accuracy. For example, it is preferable to control the diameter of the laser focused spot to be 5 μm or more and less than 100 μm.

[0036] [Emission Intensity Measurement Step (Step S2)] In the emission intensity measurement step, the emission intensity of a first element selected from elements contained in the liquid and the second element selected from elements contained in the liquid and particles, and whose concentration in the liquid differs from that in the particles, is obtained. This step is performed using LIBS. Specifically, an emission spectrum is collected for each laser pulse, and from the collected spectrum, the emission intensity of a first element selected from elements contained in the liquid and the second element selected from elements contained in the liquid and particles, and whose concentration in the liquid differs from that in the particles, is calculated. Because the emission spectrum includes not only the emission from each element but also the continuous light generated by the plasma, the peak intensity is calculated using the peak height method or the peak area method after subtracting the baseline (background due to the continuous light from the plasma). The supply of inert gas, laser irradiation, and detection and spectroscopic analysis of plasma emission can be performed using conventional methods.

[0037] The first element may be any element contained in the liquid, but is preferably the element that is most abundant among the elements that make up the liquid, in that concentration change is small and measurement accuracy is improved. As described above, the second element is an element that is contained in the liquid and the particles, and whose concentration in the liquid is different from that in the particles.

[0038] For example, when the liquid is an Al-containing molten zinc alloy, the Al-containing molten zinc alloy may contain an alloy called dross (Fe, ２ Al ５ Zn ｘ) particles. In the case of an Al-containing molten zinc alloy containing dross, for example, the first element is Zn and the second element is Al, and the emission intensities of these elements are calculated. FIG. 4 shows an example of an emission spectrum estimated when a laser is irradiated onto a molten metal portion of an Al-containing molten zinc alloy measured by LIBS. In FIG. 5, the emission intensity of Zn is relatively low and the emission intensity of Al is relatively high compared to the emission spectrum in FIG. 4. Therefore, FIG. 5 shows an example of an emission spectrum when a laser is irradiated onto an Al-containing molten zinc alloy containing a large amount of dross. Since an Al-containing molten zinc alloy is a liquid, it is easily vaporized by the thermal energy applied by the laser, and many atoms are excited and luminescent in the plasma generated by the laser. Therefore, as shown in FIG. 4, when irradiated onto molten metal, the emission intensity of Zn is high. On the other hand, when dross is irradiated with a laser, the dross is solid and has a high boiling point, so the number of atoms vaporized and introduced into the plasma is smaller than that of the molten metal. As a result, when dross is irradiated, the emission intensity of Zn is low, as shown in Figure 5. Furthermore, the ratio of the emission intensity of Al to the emission intensity of Zn is such that the emission intensity of Al is relatively higher in dross than in molten metal. This is because the proportion of Al is relatively higher in dross, which is an alloy of Al, Fe, and Zn, than in molten metal, in which Al is dissolved in a relatively dilute amount.

[0039] The emission spectrum includes emission (atomic beam) due to relaxation from an electronically excited state and emission (ion beam) due to relaxation from an ionized state. As a method for selecting the first element and the second element, it is preferable to select the same type of ion beam or atomic beam, more preferably to select beams having similar energy levels leading to excitation. However, this is not limiting, and beams having similar emission intensities or emission wavelengths may also be selected.

[0040] (Normalization Step (Step S3)) In the normalization step, the luminescence intensity of the second element is normalized by the luminescence intensity of the first element to obtain a normalized intensity of the second element. That is, in the normalization step, the ratio of the luminescence intensity of the second element to the luminescence intensity of the first element (luminescence intensity of the second element / luminescence intensity of the first element) is calculated.

[0041] (Frequency Distribution Graph Creation Process (Step S4)) In the frequency distribution graph creation process, a frequency distribution graph (histogram) is created that represents the distribution of the frequency of occurrence of the normalized intensity of the second element normalized in the normalization process. The emission wavelength and emission intensity of the element reflect the elemental composition of the laser irradiation location. In this process, in order to classify emission due to the liquid alone from emission due to particles in the next particle-attributed peak identification process, a frequency distribution graph is created for the element (second element) contained in the particles, with the normalized intensity on the horizontal axis and the frequency of occurrence of each normalized intensity per pulse on the vertical axis. When quantifying particles, it is preferable to calculate the total frequency due to particle-attributed peaks from an approximation curve, so the width of each bin in the histogram is all the same. The bins on the horizontal axis of the frequency distribution graph and the number of pulses n per analysis are preferably determined based on the size and number of particles in the liquid to be measured and the fluidity of the liquid, so that the frequency distribution graph forms a smooth curve. For example, if the liquid contains particles at a concentration of several tens of ppm, it is preferable to obtain data from several thousand to 10,000 pulses. To obtain higher analytical precision, it is preferable to increase the integration time to collect more data, or to increase the number of bins when creating a histogram.

[0042] 6 is an example of a frequency distribution graph for an Al-containing molten zinc alloy containing dross, which is created by plotting the Al / Zn intensity on the horizontal axis and the frequency of occurrence on the vertical axis. As shown in FIG. 6, for an Al-containing molten zinc alloy containing dross, a bimodal (two-peak) frequency distribution graph is obtained.

[0043] (Peak Identification Step (Step S5)) In the peak identification step, a liquid-related peak, which is a peak attributable to the liquid, and a particle-related peak, which is a peak attributable to the particles, are identified in the frequency distribution graph. When there is overlap in the distributions in the frequency distribution graph, the peaks are separated using a peak deconvolution method. Specifically, it is preferable to separate each peak by fitting using the Gauss-Newton method or the like, more specifically, fitting an approximate curve using a known method such as the Levenberg-Marquardt method or Powell's Dog Leg method. In other words, when there is overlap in the distributions in the frequency distribution graph, it is preferable to separate the liquid-related peak and the particle-related peak by calculating an approximate curve in the frequency distribution graph. When the liquid-related peak and the particle-related peak do not overlap each other in the frequency distribution graph, it is not necessary to calculate an approximate curve. In such a case, the frequency derived from the liquid-induced peak and the frequency derived from the particle-induced peak can be easily determined without separating the liquid-induced peak and the particle-induced peak, and therefore the particle quantification step described below may be performed. In this embodiment, identifying the liquid-induced peak and the particle-induced peak means separating the liquid-induced peak and the particle-induced peak using approximate curve fitting or the like as necessary. However, if the liquid-induced peak and the particle-induced peak do not overlap with each other, identifying the liquid-induced peak and the particle-induced peak means confirming that the liquid-induced peak and the particle-induced peak do not overlap with each other.

[0044] When only the liquid is irradiated with a laser, the emission signal contains information about the elements that make up the liquid. When the laser is irradiated to a portion of the liquid containing particles, the emission signal contains information about the elements that make up the particles in addition to the liquid. Because particles in a liquid are solids with a different composition from the liquid, when a laser is irradiated to a portion of the liquid containing particles, the emission signal contains different elemental information or has a different emission intensity than when only the liquid is irradiated. Therefore, the frequency distribution graph created in the frequency distribution graph creation process contains information about the particles in the liquid. For example, if a common element contained in the particles and the liquid is targeted and the particle composition contains more of that element than the liquid composition, the frequency distribution graph will have a two-peak intensity distribution. Of these, if the content of the second element in the particles is greater than the content of the second element in the liquid, the lower-intensity peak will reflect the liquid, and the higher-intensity peak will reflect the particles. Both peaks have a broad distribution, but whereas the low-intensity peak due to the liquid follows a distribution close to a normal distribution based on analytical variability, the high-intensity peak due to particles often appears broader, including variability due to particle size in addition to analytical variability.

[0045] 7 shows examples of peaks identified in a frequency distribution graph in this embodiment, including a peak of dross and a peak of an Al-containing molten zinc alloy. When the liquid is an Al-containing molten zinc alloy, as described above, the ratio Al / Zn of the emission intensity of Al to the emission intensity of Zn in the dross is relatively larger than the Al / Zn ratio in the molten metal, so it can be determined that the peak located on the low Al / Zn side is a peak caused by molten zinc, and the peak located on the high Al / Zn side is a peak caused by dross.

[0046] (Particle quantification step (step S6)) In the particle quantification step, the sum of the frequencies originating from particle-induced peaks is calculated, and the amount of particles present is quantified based on the ratio of the frequencies originating from the particle-induced peaks to the total frequencies in the frequency distribution graph. When an approximation curve of the frequency distribution graph is calculated in the peak identification step, it is preferable to quantitate the frequencies originating from particle-induced peaks from the calculated approximation curve. In this case, the area of ​​the approximation curve originating from the particle-induced peaks may be regarded as the frequencies originating from the particle-induced peaks, and the sum of the areas of the approximation curves originating from the liquid-induced peaks and the particle-induced peaks may be regarded as the total frequencies in the frequency distribution graph. In other words, the "area of ​​the approximation curve originating from the liquid-induced peaks" / ("area of ​​the approximation curve originating from the liquid-induced peaks" + "area of ​​the approximation curve originating from the particle-induced peaks") may be regarded as the ratio of the frequencies originating from particle-induced peaks to the total frequencies in the frequency distribution graph. If necessary, the amount of particles present may be quantified using a parameter other than the ratio of the frequency of particle-attributed peaks to the total frequency of the frequency distribution graph, as long as the parameter is uniquely determined from the ratio of the frequency of particle-attributed peaks to the total frequency of the frequency distribution graph. For example, instead of the ratio of the frequency of particle-attributed peaks to the total frequency of the frequency distribution graph, the amount of particles present may be quantified using the ratio of the frequency of liquid-attributed peaks to the frequency of particle-attributed peaks, etc. In addition, if the total frequency can be limited to a fixed frequency by fixing the laser pulse frequency, laser pulse width, or LIBS measurement time, including the calibration curve creation process, the amount of particles present may be quantified simply using the frequency of liquid-attributed peaks (without calculating the total frequency). In this embodiment, these modifications are not excluded.

[0047] (Calibration Curve Creation Step (Step S0)) In order to determine the number density or the like of the amount of particles present, for example, it is preferable to include the following calibration curve creation step before the particle quantification step. In the calibration curve creation step, it is preferable to prepare in advance liquids containing particles with different amounts of particles present, measure the amount of particles present in the liquid (e.g., number density) by analyzing samples collected from the liquid, and create in advance a calibration curve showing the relationship between the ratio of the frequency derived from particle-attributed peaks to the total frequency of a frequency distribution graph obtained by applying the above-mentioned analytical method to the liquid when the sample is collected from the liquid, and the amount of particles present. As the standard samples, multiple standard samples are used that differ in the amount of particles and at least one of the average particle size and particle size distribution. When the liquid is molten metal, the quantity of particles in the standard sample is quantified, for example, by observing the cut and polished surface as described above. The components of the molten metal are determined by cutting a portion of the above-mentioned analysis sample with a drill or the like, dissolving the cuttings in acid or the like, and then analyzing them with a high-frequency inductively coupled plasma analyzer, atomic absorption analyzer, or the like. The analytical value may be calculated from the composition and temperature of the molten metal using a phase diagram or the like. Since the analytical value is the sum of the molten metal and particles, the components of the molten metal can be calculated by subtracting the quantitative particle results obtained by observing the cut and polished surface or by calculation using the phase diagram. FIG. 8 is an example of a graph showing a calibration curve in this embodiment, with the horizontal axis representing the number of dross particles and the vertical axis representing the ratio of the frequency attributable to particle-attributed peaks to the total frequency in the frequency distribution graph. Here, the ratio of the frequency attributable to particle-attributed peaks to the total frequency in the frequency distribution graph was calculated based on the curve obtained by curve fitting the frequency distribution graph and the area of ​​the region enclosed by the X-axis of the frequency distribution graph. In other words, the "area of ​​the approximation curve attributable to the liquid-attributed peaks" / ("area of ​​the approximation curve attributable to the liquid-attributed peaks" + "area of ​​the approximation curve attributable to particle-attributed peaks") was considered to be the ratio of the frequency attributable to particle-attributed peaks to the total frequency in the frequency distribution graph. In the calibration curve creation step, if the liquid-attributed peak and the particle-attributed peak do not overlap in the frequency distribution graph, it is not necessary to calculate an approximation curve. If they overlap, it is preferable to calculate an approximation curve.In order to obtain a highly accurate calibration curve, it is preferable to create a calibration curve for different laser transmitters, optical systems for guiding and focusing light (e.g., focusing diameter), and optical systems for guiding the received plasma-induced light to the transmission cable 14.

[0048] Next, when a calibration curve has been created, in the particle quantification step, the amount of particles present in the liquid containing particles as the measurement sample is quantified based on the measurement result of the ratio of the frequency attributable to the particle-attributed peak to the total frequency in the frequency distribution graph based on the calibration curve. For example, the number of dross particles in an Al-containing molten zinc alloy can be quantified by applying the ratio of the frequency attributable to the particle-attributed peak to the total frequency in the measured frequency distribution graph to the calibration curve as shown in Figure 8.

[0049] The calibration curve creation step may be omitted. For example, when using this analysis method to reduce defects and appearance defects in a steel sheet after plating, the calibration curve creation step may be omitted, and the "ratio of the frequency attributable to particle-attributed peaks to the total frequency in the frequency distribution graph" itself may be controlled, and the plating bath may be managed or controlled without calculating the amount of particles (dross) present using the calibration curve created in the calibration curve creation step. More specifically, if the relationship between the defect rate (or appearance defect rate) of the steel sheet after plating and the "ratio of the frequency attributable to particle-attributed peaks to the total frequency in the frequency distribution graph" can be clarified, an appropriate range of the "ratio of the frequency attributable to particle-attributed peaks to the total frequency in the frequency distribution graph" may be predetermined according to the clarified content, and the plating bath operating conditions (such as the plating bath temperature and the chemical composition of the plating bath) may be managed or controlled so that the "ratio of the frequency attributable to particle-attributed peaks to the total frequency in the frequency distribution graph" falls within the predetermined appropriate range.

[0050] Since the area ratio of particles (dross) to the laser irradiation area in each laser pulse varies to a certain extent, the width of the particle-induced peak in the frequency distribution graph is often larger than the width of the liquid-induced peak. As described above, the inventors have found that the ratio of the frequency attributable to the particle-induced peak to the total frequency in the frequency distribution graph shows a good correlation with the particle number density in the liquid.

[0051] (Liquid element concentration quantification step (step S7)) Furthermore, in the present method, when the liquid contains a second element, it is also possible to quantify the second element in the liquid from the frequency distribution graph. FIG. 9 is an example of a graph showing a calibration curve in this embodiment, and is a graph showing a calibration curve when the horizontal axis represents the Al concentration in the liquid (Al-containing molten zinc alloy) and the vertical axis represents the normalized intensity (emission intensity of the second element / emission intensity of the first element) at the peak top position attributable to the liquid in the frequency distribution graph (the center position of the liquid-attributed peak on the horizontal axis in FIG. 7). The vertical axis in FIG. 9 represents the normalized intensity, and the vertical axis is dimensionless. When quantifying Al in an Al-containing molten zinc alloy, for example, as shown in FIG. 9, a correlation (calibration curve) between the normalized intensity at the peak top position of the liquid-attributed peak and Al in the liquid (Al-containing molten zinc alloy) is created in advance. Furthermore, if the normalized intensity at the peak top position of the liquid-attributed peak can be measured by LIBS on the liquid (Al-containing molten zinc alloy) to be measured, the Al concentration in the liquid (Al-containing molten zinc alloy) can be quantified using a pre-created calibration curve. Therefore, by using not only the correlation between the number of particles and the ratio of the frequency attributable to the particle-attributed peak to the total frequency in the frequency distribution graph, but also the correlation between the concentration of the second element in the liquid and the normalized intensity at the peak top position of the liquid-attributed peak in the frequency distribution graph, it is possible to quantify not only the amount of particles present in the liquid containing the particles, which is the sample to be measured, but also the concentration of the second element in the liquid. Therefore, the method for analyzing a liquid containing particles according to this embodiment preferably further includes a liquid element concentration quantification step, in which the concentration of the second element in the liquid is quantified based on the correlation between the concentration of the second element in the liquid and the normalized intensity at the peak top position of the liquid-attributed peak in the frequency distribution graph.

[0052] According to the method for analyzing a liquid containing particles according to this embodiment, by creating a calibration curve, the cutting, polishing, and observation of a sample for each liquid for quantifying particles can be omitted, which saves time and effort. Thus, particle quantification can be easily performed.

[0053] <Hot-dip galvanizing apparatus> The liquid-submerged particle analysis system according to this embodiment can be applied to a hot-dip galvanizing apparatus. An example of a hot-dip galvanizing apparatus will now be described. FIG. 10 is a side view showing a schematic configuration of a hot-dip galvanizing apparatus 1 to which the liquid-submerged particle analysis method according to this embodiment can be applied, and FIG. 11 is a plan view showing a schematic configuration of the hot-dip galvanizing apparatus 1 shown in FIG. 10. Note that, as an example of a molten metal bath, a hot-dip galvanizing bath 3 (hereinafter also simply referred to as a "galvanizing bath") in the hot-dip galvanizing apparatus 1 will be described as a representative example. However, the present invention is not limited thereto and can be applied to any other molten metal bath.

[0054] The hot-dip galvanizing apparatus 1 is an apparatus for continuously depositing molten zinc on the surface of a steel strip 2 by immersing the steel strip 2 in a coating bath 3 filled with molten zinc. The hot-dip galvanizing apparatus 1 includes a coating tank 4, a snout 5, a pair of upper and lower support rolls 7, 7, an inductor 8, a gas wiping device 9, an alloying furnace 10, and a component analysis system 11.

[0055] The coating tank 4 stores a coating bath 3 made of molten zinc. In addition to Zn, the coating bath 3 may contain, for example, approximately 0.12 to 0.15 mass% Al and approximately 0.02 to 0.1 mass% Fe. The temperature of the coating bath 3 is approximately 440 to 480°C. The snout 5 is disposed at an angle so that one end thereof is immersed in the coating bath 3. The submerged roll 6 is disposed at the lowest position inside the coating tank 4. The submerged roll 6 rotates in the direction of the arrow shown in the figure due to contact with and shearing of the steel strip 2.

[0056] The support rolls 7 are arranged inside the coating tank 4 downstream of the submerged rolls 6 in the transport direction of the steel strip 2, and are arranged so as to sandwich the steel strip 2 fed from the submerged rolls 6 on both the left and right sides. The support rolls 7 are rotatably supported by bearings (not shown) (e.g., plain bearings, rolling bearings, etc.). Only one support roll, three or more support rolls may be installed, or no support rolls may be installed.

[0057] The inductor 8 is an example of a heating device that heats the plating bath 3 filled in the plating tank 4. As shown in Fig. 10, a plurality of inductors 8 are provided on the side wall of the plating tank 4, and adjust the temperature of the plating bath 3 to a predetermined bath temperature. Note that the means for heating the plating bath 3 is not limited to the inductor 8, and known techniques can be used.

[0058] The gas wiping device 9 is positioned above the plating tank 4 and has the function of blowing gas (e.g., nitrogen, air) onto the surfaces on both sides of the steel strip 2 to scrape off molten metal adhering to the surface of the steel strip 2 and control the amount of molten metal adhering to the surface.

[0059] The alloying furnace 10 is an example of a heating device that heats the steel strip 2 after gas wiping to a predetermined temperature. The alloying furnace 10 raises the temperature of the steel strip 2 by heating, and promotes alloying of the coating layer of molten metal adhering to the surface of the steel strip 2. Note that a known technology, such as an induction heater, is used as the alloying furnace 10.

[0060] The steel strip 2 that has been annealed in an annealing furnace, which is an upstream process, is immersed in a coating tank 4 filled with a coating bath 3 through a snout 5, passes through a bath roll 6 and support rolls 7, 7, and is pulled up vertically to be transported out of the coating bath 3. The steel strip 2 transported out of the coating bath 3 passes through an alloying furnace 10 after the basis weight of the molten metal adhering to the surface is adjusted by a gas wiping device 9.

[0061] 3, the component analysis system 11 is disposed near a corner of the plating bath 3, but is not limited to the illustrated embodiment and can be disposed at any location in the plating bath 3 and can be used to measure any position in the plating bath 3. The component analysis system 11 is as described above.

[0062] As described above, the method for analyzing a liquid containing particles according to one embodiment of the present invention can be applied to a hot-dip galvanizing apparatus and is suitable for managing a hot-dip galvanizing bath. Therefore, in another aspect, this embodiment provides a method for managing a hot-dip galvanizing bath, which involves irradiating a hot-dip galvanizing bath containing particles with a laser and spectroscopically analyzing the emission generated at each laser irradiation, thereby analyzing the hot-dip galvanizing bath and managing the hot-dip galvanizing bath, the method comprising: a laser focusing step of focusing the laser; an emission intensity measurement step of obtaining an emission intensity of a first element selected from elements contained in the liquid and an emission intensity of a second element selected from elements contained in the liquid and particles, the second element having a concentration different from that in the liquid; and a step of calculating the emission intensity of the second element based on the emission intensity of the first element. the particle-attributed peak identifying step of identifying a liquid-attributed peak that is a peak attributable to the liquid and a particle-attributed peak that is a peak attributable to the particles in the frequency distribution graph, and a particle quantification step of calculating the sum of the frequencies attributable to the particle-attributed peaks and quantifying the amount of the particles based on the ratio of the frequency attributable to the particle-attributed peak to the total frequency in the frequency distribution graph. In managing the hot-dip galvanizing bath, if necessary, liquids containing particles with different amounts of particles may be prepared in advance, and a calibration curve may be created in the calibration curve creating step, and the amount of the particles may be quantified using the calibration curve in the particle quantification step. Furthermore, when managing the hot-dip galvanizing bath, if necessary, the peak identifying step may calculate an approximation curve in a frequency distribution graph to identify the liquid-attributable peak and the particle-attributable peak, and the particle quantifying step may quantify the frequency attributable to the particle-attributable peak from the calculated approximation curve.

[0063] Furthermore, a hot-dip galvanized steel sheet can be suitably produced using the hot-dip galvanizing bath controlled in this manner. Therefore, in another aspect, the present invention also relates to a method for producing a hot-dip galvanized steel sheet, the method including a step of controlling a hot-dip galvanizing bath controlled by the method for controlling a hot-dip galvanizing bath.

[0064] Although the present invention has been described above based on preferred embodiments thereof, the present invention is not limited thereto. The above is merely an example, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention.

[0065] For example, in the laser focusing process, it is preferable to adjust the focused diameter so that it is smaller than 10 times the average particle diameter of the particles. To adjust the focused diameter in this way, an average particle diameter measurement process may be performed before the laser focusing process. Measuring the particle size distribution of particles present in the liquid and measuring the average particle diameter of the particles can facilitate control of the laser focusing conditions. The average particle diameter can be measured using known methods. For example, methods for measuring fine particles in a liquid include light scattering, light diffraction, microscopy (e.g., electron microscope, X-ray microscope), electrostatic methods, sedimentation, inertial methods, and diffusion methods. In the case of liquid metal, the size of the fine particles contained can be determined by collecting a portion of the liquid metal, rapidly solidifying it, cutting and polishing it, and observing it under a microscope. Note that if the lower limit of the average particle diameter of particles present in the liquid is known, the average particle measurement process is unnecessary, and the focused diameter may be set to 10 times or less of the lower limit of the average particle diameter.

[0066] Furthermore, it is preferable that the liquid itself is in flow or that a mechanism for causing the liquid to flow is in place so that the laser does not repeatedly irradiate the same part of the liquid but instead irradiates a new surface of the liquid. The mechanism for causing the liquid to flow can be thermal convection, or, if the liquid is molten metal, forced stirring can be achieved using a metal pump or the like. Another efficient method is to immerse the laser and the probe for transmitting the emitted light in the liquid and then inject an appropriate amount of inert gas into the probe to stir the liquid inserted into the probe and expose a new surface of the liquid for analysis. This method ensures representativeness of the analysis because the laser irradiates a new surface of the liquid at all times.

[0067] Furthermore, the above-described liquid analysis method has been described using an example of a liquid containing one type of particle, specifically, an Al-containing molten zinc alloy containing dross. However, the above-described liquid analysis method can also be applied to liquids containing multiple particles of different compositions. For example, when a liquid contains multiple types of particles of different compositions, a histogram showing a peak for the liquid and multiple peaks representing peaks for each particle of different compositions can be obtained. Furthermore, the above-described method for analyzing liquids containing particles is not limited to molten metals, but can be widely applied to suspended particles in liquids. For example, dissolved carbon dioxide in water and suspended organic matter such as microorganisms and microplastics may be separately quantified by focusing on the carbon element, which is a common element contained in both. Furthermore, suspended matter formed as a solid in liquid oil due to oil degradation, such as oxidation or saponification, may also be separately analyzed.

[0068] The present invention will be described in more detail below with reference to examples. Note that the examples described below are merely examples of the present invention and are not intended to limit the present invention.

[0069] Example 1 A laser emission spectrometer was used to analyze a coating bath used in continuous hot-dip galvanizing using the Sendzimir method. A small amount of Al is added to the molten zinc used as the coating bath, and a small amount of Fe is dissolved into the bath when the steel sheet is passed through it. As a result, an alloy called top dross (Fe) is present near the surface of the coating bath. ２ Al ５ Zn ｘ The particles were suspended with an average particle size of 10 μm to 20 μm and a particle size distribution range of several μm to approximately 100 μm. The amount of dross in the plating bath was also changed by adjusting the components, temperature, holding time, etc.

[0070] A laser was generated using a Quantel Ultra 100 manufactured by Lumibird. The laser oscillation conditions were wavelength: 1064 nm, output: 100 mJ / pulse, and frequency: 20 Hz. The oscillated laser passed through a 1000 mm long cylindrical probe made of SiAlON and was focused by an anti-reflection coated lens so as to be focused on the plating bath surface. The focal length f of the lens was 900 mm, and the focused diameter of the laser was 50 μm.

[0071] The light emitted from the plating bath surface by laser irradiation was passed through a cylindrical probe, offset by 90° from the optical axis of the laser by a dichroic mirror, and then focused by a lens. The light was then transmitted to a spectrometer via a 15 m long, 32-core optical fiber (I-type standard fiber manufactured by Mitsubishi Electric Wire Co., Ltd.). The spectrometer used was a double grating spectrometer (NP250-2) manufactured by SOL Instrument. The detector used was an ICCD camera iStar manufactured by Andor.

[0072] For the spectrum obtained for each laser irradiation pulse, the emission intensity of Al (second element) at a wavelength of 309.27 nm was divided by the emission intensity of Zn (first element) at a wavelength of 303.58 nm, and this value was expressed as Al / Zn. Spectra were collected 1,000 times, and a frequency distribution graph was created with Al / Zn on the horizontal axis and its occurrence frequency on the vertical axis. In the frequency distribution graph, the number of bins was set to 25 for 1,000 data points. Next, approximation based on the Levenberg-Marquardt method was performed, and two peaks, namely a liquid-induced peak and a particle-induced peak, were separated in the frequency distribution graph.

[0073] For each molten zinc alloy having an Al concentration and dross count shown in Table 1, levels 1 to 4, the liquid-attributed peak and particle-attributed peak (dross peak) in the frequency distribution graph were identified using the method described above. Because the Al content in the particles (dross) was greater than the Al content in the liquid and therefore relatively greater than the Al / Zn ratio of the molten metal, the peaks located on the high Al / Zn side were determined to be peaks due to dross. Furthermore, the Al concentration and amount of dross present in the molten zinc alloy were measured using a conventional method. Specifically, approximately 300 g of molten metal from levels 1 to 4 were collected with a ladle. The collected analysis sample was immediately quenched and solidified in a water-cooled copper mold. This analysis sample was removed from the mold and cut into two pieces, one of which was cut with a drill. 1 g of the cuttings was weighed, completely decomposed with nitric acid and hydrochloric acid, and then quantitatively analyzed for Al concentration using an ICP atomic emission spectrometer. The remaining piece was milled to obtain a smooth surface, and then a cross section was prepared using a mirror polishing device. The cross section was subjected to nital etching and then observed using an optical microscope to determine the number of dross particles of 10 μm or more per particle area (particles / cm ２ The cross-sectional observation was carried out on 20 new cross-sections of an area of ​​approximately 2 cm x 2 cm, which were repeatedly milled and mirror-polished. ２ This observation was carried out on the analytical samples collected under the conditions of levels 1 to 4. The average diameter of the dross in levels 1 to 4 was less than 5 μm.

[0074] The upper row of the Reference Value column in Table 1 shows the Al concentration in the molten zinc measured by ICP emission spectroscopy (the Al concentration in Table 1), and the lower row of the Reference Value column shows the number of dross particles measured by cross-sectional observation. The upper row of the Examples column in Table 1 shows the normalized intensity at the peak top position of molten zinc in the frequency distribution graph, and the lower row of the Examples column shows the ratio of the frequency attributable to particle (dross) peaks to the total frequency in the frequency distribution graph. Furthermore, FIG. 12 shows a frequency distribution graph with peaks separated for Level 1, FIG. 13 shows a frequency distribution graph with peaks separated for Level 2, FIG. 14 shows a frequency distribution graph with peaks separated for Level 3, and FIG. 15 shows a frequency distribution graph with peaks separated for Level 4. As shown in FIGS. 12 to 15 , the liquid-attributed peaks and particle-attributed peaks overlapped with each other in Levels 1 to 4, so approximation curve fitting based on the Levenberg-Marquardt method was performed as described above. Then, "area of ​​the approximation curve derived from the liquid-induced peak" / ("area of ​​the approximation curve derived from the liquid-induced peak" + "area of ​​the approximation curve derived from the particle-induced peak") was regarded as the ratio of the frequency derived from the particle-induced peak to the total frequency in the frequency distribution graph.

[0075]

[0076] Here, in FIG. 8, the number of dross particles of 10 μm or more per particle area (particles / cm ) was measured using an optical microscope for levels 1 to 4. ２) and the ratio of the frequency attributable to particle-attributed peaks to the total frequency in the frequency distribution graph. As shown in Figure 8, the number of dross particles is proportional to the ratio of the frequency attributable to particle-attributed peaks to the total frequency in the frequency distribution graph, and therefore it was found that quantitative analysis of dross can be performed using the calibration curve shown in Figure 8. For this Sendzimir-type continuous hot-dip galvanizing bath, the ratio of the frequency attributable to particle (dross) peaks to the total frequency in the frequency distribution graph was measured by the same LIBS as above at multiple different times using the same laser optical system (i.e., the same focusing conditions), and the number of dross particles measured by LIBS was determined using the calibration curve of Figure 8. At the same time, analysis samples were collected in the same manner as above, and the number of dross particles measured by cross-sectional observation was measured. The ratio between the number of dross particles measured by LIBS and the number of dross particles measured by cross-sectional observation was within 1.00 ± 0.10, and the number of dross particles measured by LIBS was in good agreement with the number of dross particles measured by cross-sectional observation.

[0077] 9 shows the relationship between the Al concentration in the liquid (Al-containing molten zinc alloy) and the normalized intensity at the peak top position of the liquid-attributable peak for levels 1 to 4. As shown in Fig. 9, the Al concentration in the liquid (Al-containing molten zinc alloy) is proportional to the normalized intensity at the peak top position of the liquid-attributable peak, and therefore it was found that the Al concentration in molten zinc can be quantitatively analyzed using the calibration curve shown in Fig. 9.

[0078] On the other hand, the conventional measurement methods for Levels 1 to 4, namely, measuring the Al concentration of molten zinc by ICP atomic emission spectrometry and measuring the number of dross particles by cross-sectional observation, required the work of one operator for approximately 10 days. Therefore, when measuring the number of dross particles or the Al concentration of molten zinc containing dross to be measured, the method according to the present invention allows measurement by LIBS using a calibration curve, and it was found that these can be measured simply and in a short time compared to conventional methods.

[0079] [Example 2] Analysis was performed under the conditions of Level 1, but with a different focusing lens and a focusing diameter of 500 μm. A histogram was created using the same signal processing as in the invention example. The results are shown in Figure 15. Although several spectra with high Al / Zn ratios likely due to dross were detected, the histogram was broad. This is thought to be because the laser focusing diameter was much larger than the dross, and even if dross was present directly below the laser, the light emitted from the molten metal was dominant, resulting in an averaged spectrum. Therefore, it was found that by adjusting the focusing diameter within an appropriate range, molten zinc and dross can be distinguished with greater accuracy.

[0080] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0081] REFERENCE SIGNS LIST 1 Hot-dip galvanizing apparatus 2 Steel strip 3 Galvanizing bath (molten metal) 4 Galvanizing tank 5 Snout 6 Submerged roll 7 Support roll 8 Inductor 9 Gas wiping device 10 Alloying furnace 11 Component analysis system 12 Laser device 13 Cylindrical probe 131 Open end 14 Transmission cable 15 Bubbles 16 Plasma 17 Support member 18 Fixing member 19 Driving unit 20 Processing device 21 Detection unit 22 Signal processing unit 221 Laser focusing unit 222 Emission intensity measurement unit 223 Normalization unit 224 Frequency distribution graph creation unit 225 Peak identification unit 226 Particle quantification unit

Claims

1. A method for analyzing a liquid containing particles, which comprises irradiating a liquid containing particles with a laser and spectroscopically analyzing the luminescence generated each time the laser is irradiated, comprising: a laser focusing step of focusing the laser; a luminescence intensity measurement step of obtaining the luminescence intensity of a first element selected from the elements contained in the liquid and the luminescence intensity of a second element selected from the elements contained in the liquid and the particles and having different concentrations in the liquid and in the particles; a normalization step of normalizing the luminescence intensity of the second element by the luminescence intensity of the first element to obtain a normalized intensity of the second element; a frequency distribution graph creation step of creating a frequency distribution graph representing the frequency distribution of the normalized intensity; a peak identification step of identifying, in the frequency distribution graph, a liquid-originated peak that is a peak attributable to the liquid and a particle-originated peak that is a peak attributable to the particles; a particle quantification step of obtaining the total of the frequencies derived from the particle-originated peak and quantifying the abundance of the particles based on the ratio of the frequency derived from the particle-originated peak to the total frequency of the frequency distribution graph; A method for analyzing a liquid containing particles, comprising the above steps.

2. Preparing in advance liquids containing particles with different abundances of the particles, measuring the abundance of the particles in the liquid by analyzing a sample taken from the liquid, and when taking the sample from the liquid, applying the analysis method according to Claim 1 to the liquid to obtain the ratio of the total of the frequencies derived from the particle-originated peak to the total frequency of the frequency distribution graph, including a calibration curve creation step of creating in advance a calibration curve showing the relationship between the ratio of the total of the frequencies derived from the particle-originated peak to the total frequency of the frequency distribution graph and the abundance of the particles, wherein in the particle quantification step, the abundance of the particles is quantified using the calibration curve. The method for analyzing a liquid containing particles according to Claim 1.

3. In the peak identification step, the liquid-originated peak and the particle-originated peak are identified by calculating an approximate curve in the frequency distribution graph, wherein in the particle quantification step, the frequency derived from the particle-originated peak is quantified from the calculated approximate curve. The method for analyzing a liquid containing particles according to Claim 1 or 2.

4. Before the laser focusing step, The method for analyzing a liquid containing particles according to claim 1 or 2, which has an average particle diameter measurement step of measuring the average particle diameter of the particles present in the liquid.

5. The method for analyzing a liquid containing particles according to claim 1 or 2, wherein in the laser focusing step, the diameter of the laser focusing spot is set to a value smaller than 10 times the average particle diameter of the particles.

6. The method for analyzing a liquid containing particles according to claim 1 or 2, wherein in the laser focusing step, the diameter of the laser focusing spot is changed to perform the light emission intensity measurement step, and for each diameter of the focusing spot, the normalization step, the frequency distribution graph creation step, and the peak identification step are performed to determine the laser focusing conditions.

7. The method for analyzing a liquid containing particles according to claim 1 or 2, wherein the liquid is a molten metal.

8. The method for analyzing a liquid containing particles according to claim 1 or 2, wherein the liquid is a molten zinc alloy containing at least aluminum in molten zinc.

9. The method for analyzing a liquid containing particles according to claim 1 or 2, wherein in the laser focusing step, the diameter of the laser focusing spot is set to less than 300 μm.

10. The method for analyzing a liquid containing particles according to claim 1 or 2, wherein in the laser focusing step, the diameter of the laser focusing spot is controlled to be 5 μm or more and less than 100 μm.

11. A method for managing a molten zinc plating bath, which irradiates a laser to a molten zinc plating bath containing particles, analyzes the molten zinc plating bath by spectro-analyzing the light emission generated each time the laser is irradiated, and manages the molten zinc plating bath, comprising: a laser focusing step of focusing the laser; a light emission intensity measurement step of obtaining the light emission intensity of a first element selected from the elements contained in the molten zinc plating bath and the light emission intensity of a second element selected from the elements contained in the liquid and the particles and having different concentrations in the liquid and in the particles; a normalization step of normalizing the light emission intensity of the second element by the light emission intensity of the first element to obtain the normalized intensity of the second element; a frequency distribution graph creation step of creating a frequency distribution graph representing the distribution of the frequencies of the normalized intensities; a peak identification step of identifying a liquid-originated peak that is a peak caused by the liquid and a particle-originated peak that is a peak caused by the particles in the frequency distribution graph. A particle quantification step of obtaining the total frequency derived from the peak caused by the particles and quantifying the abundance of the particles based on the ratio of the frequency derived from the peak caused by the particles to the total frequency of the frequency distribution graph. A method for managing a molten zinc plating bath, including the above.

12. A method for manufacturing a hot-dip galvanized steel sheet, including a step of managing a molten zinc plating bath managed by the method for managing a molten zinc plating bath according to Claim 11.

13. An analyzer for analyzing a liquid containing particles, which irradiates a laser on the liquid containing particles and spectro-analyzes the light emission generated each time the laser is irradiated, comprising: A laser condensing unit that condenses the laser; A light emission intensity measurement unit that obtains the light emission intensity of a first element selected from the elements contained in the liquid and the light emission intensity of a second element selected from the elements contained in the liquid and the particles and having different concentrations in the liquid and in the particles; A normalization unit that normalizes the light emission intensity of the second element by the light emission intensity of the first element to obtain the normalized intensity of the second element; A frequency distribution graph creation unit that creates a frequency distribution graph representing the distribution of the frequencies of the normalized intensities; A peak identification unit that identifies a liquid-originated peak, which is a peak caused by the liquid, and a particle-originated peak, which is a peak caused by the particles, in the frequency distribution graph; A particle quantification unit that obtains the total frequency derived from the particle-originated peak and quantifies the abundance of the particles based on the ratio of the frequency derived from the particle-originated peak to the total frequency of the frequency distribution graph; An analyzer for a liquid containing particles, including the above.