Method for calculating thickness of grain boundary oxide layer, method for determining plating quality, method for manufacturing plated steel sheet, and thickness calculation device

Abstract

To provide a film thickness calculation method for a grain boundary oxide layer that can calculate a film thickness of a grain boundary oxide layer remaining on a steel plate after acid cleaning processing with high precision.SOLUTION: A film thickness calculation method for a grain boundary oxide layer remaining on a steel plate after acid cleaning processing according to one embodiment of the present invention comprises: an intensity measuring process of measuring fluorescent X-ray intensity to elements included in the steel plate and constituting oxide in the grain boundary oxide layer; and a film thickness calculating process of calculating the film thickness of the grain boundary oxide layer based upon correlation between previously extracted fluorescent X-ray intensity and the film thickness of the grain boundary oxide layer and the fluorescent X-ray intensity measured in the intensity measuring process.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for calculating the thickness of an intergranular oxide layer, a method for determining platability, a method for producing a plated steel sheet, and a thickness calculation device. [Background technology]

[0002] Surface-treated steel sheets imparted with rust-resistance, particularly hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets, which have excellent rust-resistance, are known as high-strength steel sheets used for automobile bodies, etc. Hot-dip galvanized steel sheets are generally produced by using a strip-shaped steel sheet obtained by hot-rolling and cold-rolling a slab as a base steel sheet, subjecting this base steel sheet to recrystallization annealing in an annealing furnace under a reducing atmosphere, and then performing hot-dip galvanizing.

[0003] In such high-strength steel sheets, the addition of Si, Mn, etc. is effective in increasing the strength. However, when the steel sheet contains Si or Mn, oxidation proceeds even in a reducing atmosphere containing reducing hydrogen gas in which iron does not oxidize, and oxides of Si or Mn are formed on the steel sheet surface. These oxides reduce the wettability of the molten zinc and the steel sheet during the plating process, so when a base steel sheet containing Si, Mn, etc. is used, the galvanizability is likely to decrease.

[0004] A manufacturing method using an oxidation-reduction process that uses an annealing furnace having an oxidation zone and a reduction zone has been put to practical use as a method for improving the galvanizability of base steel sheet containing added elements such as Si and Mn. In this manufacturing method, an iron oxide film is formed on the surface of the steel sheet, and this oxide film is reduced in a reducing atmosphere containing hydrogen, after which a plating process is performed.

[0005] It is known that the formation of oxide films in the oxidation-reduction process is influenced by the grain boundary oxide layer. The grain boundary oxide layer is a layer formed when elements dissolved in Fe move to the grain boundary and combine with oxygen diffusing along the grain boundary to form an oxide. The thickness of this grain boundary oxide layer can be adjusted by the pickling conditions performed before the oxidation-reduction process.

[0006] For example, if the grain boundary oxide layer is thin, i.e., if the amount of solute elements is large, Si and Mn, which are less noble than Fe, are selectively oxidized during oxidation-reduction annealing, which may inhibit the formation of the oxide film and cause non-plating or uneven alloying. On the other hand, if the grain boundary oxide layer is thick, there is a risk that alloying will proceed excessively, making it difficult to ensure platability. Thus, the thickness of the grain boundary oxide layer is an important factor in ensuring platability.

[0007] As an in-line method for determining the amount of the grain boundary oxide layer, which is thought to be correlated with the thickness of the grain boundary oxide layer, a method for determining the amount of oxygen per unit area of ​​the steel sheet, i.e., the oxygen weight, by Compton scattering using X-rays has been proposed (see JP 2001-272360 A). In this method, the oxygen weight is uniquely determined from the actually measured scattering intensity, utilizing the fact that the Compton scattering intensity increases in response to the increase in the amount of oxygen due to the oxidation of the surface layer. Then, under the premise that this amount of oxygen is proportional to the amount of the grain boundary oxide layer, the oxygen amount is used as an index of the amount of the grain boundary oxide layer. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2001-272360 A Summary of the Invention [Problem to be solved by the invention]

[0009] In the conventional method, the amount of oxygen is assumed to be proportional to the amount of the grain boundary oxide layer. In order for this assumption to be valid, it is necessary that the composition of the oxide contained in the grain boundary oxide layer does not change. Here, as described above, the thickness of the grain boundary oxide layer before the oxide film is formed in the oxidation-reduction method is adjusted by the pickling conditions. Specifically, by pickling, a part of the thick grain boundary oxide layer is dissolved and thinned to a desired thickness so that it remains. At this time, since a specific oxide is selectively dissolved in the acid, the composition of the oxide contained in the grain boundary oxide layer changes before and after pickling, and depending on the amount of the grain boundary oxide layer dissolved by pickling.

[0010] Therefore, in the conventional method for determining the thickness of the grain boundary oxide layer remaining on the steel sheet after pickling treatment, the assumption that the amount of oxygen is proportional to the amount of the grain boundary oxide layer does not hold true, so it is difficult to determine the thickness of the grain boundary oxide layer with high accuracy. Therefore, it is difficult to accurately evaluate the platability of the steel sheet using the method using the amount of oxygen.

[0011] The present invention has been made in light of the above-mentioned circumstances, and has an object to provide a method and device for calculating the thickness of a grain boundary oxide layer that can calculate with high accuracy the thickness of a grain boundary oxide layer remaining on a steel sheet after pickling treatment, a method for determining platability of a steel sheet with high accuracy from the grain boundary oxide layer remaining on a steel sheet after pickling treatment, and a method for manufacturing a plated steel sheet using the above-mentioned method for determining platability. [Means for solving the problem]

[0012] In the steel sheet before pickling, a relatively thick intergranular oxide layer is formed on the surface of the steel sheet base. At this stage, the concentration of the elements contained in the steel sheet and constituting the oxide in the intergranular oxide layer is uniform in the steel sheet base and in the intergranular oxide layer. When the steel sheet is pickled, the oxide of the above element is selectively dissolved, so that the concentration of the element in the intergranular oxide layer decreases. When the fluorescent X-ray intensity of the above elements in the steel sheet is measured, the intensity is determined by the above elements near the surface, so that the fluorescent X-ray intensity is lower than that when the steel sheet base is measured. The present inventors further focused on the fact that when the pickling treatment is continued and the thickness of the remaining intergranular oxide layer approaches a desired value, the intergranular oxide layer becomes sufficiently thin, so that the intensity from the above elements contained in the steel sheet base is superimposed on the fluorescent X-ray intensity. That is, the inventors found that in this region, as the grain boundary oxide layer becomes thinner, the ratio of fluorescent X-rays superimposed from the above elements contained in the steel sheet base increases, and the measured fluorescent X-ray intensity increases. The inventors also found that the fluorescent X-ray intensity has a high correlation with the thickness of the grain boundary oxide layer, and completed the present invention.

[0013] That is, a method for calculating the thickness of a grain boundary oxide layer according to one embodiment of the present invention is a method for calculating the thickness of a grain boundary oxide layer remaining on a steel sheet after pickling treatment, and includes an intensity measuring step of measuring fluorescent X-ray intensity for elements contained in the steel sheet and constituting oxides in the grain boundary oxide layer, and a thickness calculating step of calculating the thickness of the grain boundary oxide layer based on a correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer that has been extracted in advance, and the fluorescent X-ray intensity measured in the intensity measuring step.

[0014] In the method for calculating the thickness of the intergranular oxide layer, the fluorescent X-ray intensity of elements contained in the steel sheet and constituting the oxide in the intergranular oxide layer is measured. Since the fluorescent X-ray intensity has a high correlation with the thickness of the intergranular oxide layer remaining on the steel sheet after pickling, the thickness of the intergranular oxide layer can be calculated with high accuracy based on the fluorescent X-ray intensity.

[0015] A plurality of correlations are extracted in advance according to the manufacturing conditions of the steel plate, and in the film thickness calculation step, one correlation is selected from the plurality of correlations according to the manufacturing conditions of the steel plate. For example, fluorescent X-ray intensity depends on Mn concentration. When the manufacturing conditions of the steel plate differ in this way, the correlations may differ. Therefore, by making it possible to select an appropriate correlation according to the manufacturing conditions of the steel plate, the calculation accuracy of the film thickness can be further improved.

[0016] The method further includes a distance measurement step of measuring a measurement distance between the measurement position of the fluorescent X-ray intensity measured in the intensity measurement step and the steel sheet, and an intensity correction step of correcting the fluorescent X-ray intensity measured in the intensity measurement step based on the measurement distance, and the film thickness calculation step may use the fluorescent X-ray intensity corrected in the intensity correction step instead of the fluorescent X-ray intensity measured in the intensity measurement step. The fluorescent X-ray intensity is affected by the measurement distance. Therefore, by normalizing the fluorescent X-ray intensity based on the measurement distance, it is possible to prevent a decrease in the film thickness calculation accuracy.

[0017] The acid used in the pickling treatment may be hydrochloric acid, and the element may be Mn. By using hydrochloric acid as the acid in the pickling treatment and Mn as the element in this manner, the measurement sensitivity of the fluorescent X-ray intensity is improved, and the calculation accuracy of the film thickness can be further improved.

[0018] A platability determination method according to another aspect of the present invention is a method for determining the platability of a steel sheet from a grain boundary oxide layer remaining on the steel sheet after pickling treatment, and includes a calculation step of calculating a thickness of the grain boundary oxide layer, and a determination step of determining the platability of the steel sheet using the thickness of the grain boundary oxide layer calculated in the calculation step as a determination index, and the calculation step uses the method for calculating the thickness of the grain boundary oxide layer of the present invention.

[0019] In the method for determining platability, the thickness of the grain boundary oxide layer can be calculated with high accuracy using the method for calculating the thickness of the grain boundary oxide layer of the present invention. Therefore, the platability of the steel sheet can be determined with high accuracy based on the calculated thickness.

[0020] A platability determination method according to yet another embodiment of the present invention is a method for determining the platability of a steel sheet from a grain boundary oxide layer remaining on the steel sheet after pickling treatment, and includes an intensity measurement step of measuring fluorescent X-ray intensity for elements contained in the steel sheet and constituting oxides in the grain boundary oxide layer, and a determination step of directly determining the platability of the steel sheet based on the fluorescent X-ray intensity measured in the intensity measurement step.

[0021] In the method for determining platability, the fluorescent X-ray intensity of elements contained in the steel sheet and constituting the oxide in the grain boundary oxide layer is measured. Since the fluorescent X-ray intensity has a high correlation with the thickness of the grain boundary oxide layer remaining on the steel sheet after pickling treatment, the platability of the steel sheet can be calculated directly with high accuracy based on the fluorescent X-ray intensity.

[0022] A method for producing a plated steel sheet according to another embodiment of the present invention includes a hot rolling process in which a steel sheet serving as a base material is hot rolled, a pickling process in which oxide scale on the surface of the steel sheet after the hot rolling process is removed with an acid, and a platability determination process in which the platability of the steel sheet is determined from the grain boundary oxide layer remaining on the steel sheet after the pickling process, and the platability determination method of the present invention is used in the platability determination process.

[0023] In the manufacturing method of the plated steel sheet, the galvanic property of the steel sheet can be determined with high accuracy by using the galvanic property determination method of the present invention after the pickling process. This allows the manufacturing conditions after the pickling process to be optimized according to the galvanic property of the steel sheet. Therefore, the manufacturing efficiency of the plated steel sheet can be improved by using the manufacturing method of the plated steel sheet.

[0024] A thickness calculation device according to yet another embodiment of the present invention is a thickness calculation device that calculates the thickness of a grain boundary oxide layer remaining on a steel sheet after pickling treatment, and includes an intensity measurement unit that measures fluorescent X-ray intensity for elements contained in the steel sheet and constituting oxides in the grain boundary oxide layer, a distance measurement unit that measures a measurement distance that is the distance between a measurement position of the fluorescent X-ray intensity of the intensity measurement unit and the steel sheet, an intensity correction unit that corrects the fluorescent X-ray intensity measured by the intensity measurement unit based on the measurement distance, and a thickness calculation unit that calculates the thickness of the grain boundary oxide layer based on a correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer that has been extracted in advance and the fluorescent X-ray intensity corrected by the intensity correction unit.

[0025] The thickness calculation device includes an intensity measurement unit that measures the fluorescent X-ray intensity of elements included in the steel sheet and that constitute the oxide in the intergranular oxide layer. The fluorescent X-ray intensity has a high correlation with the thickness of the intergranular oxide layer formed on the steel sheet after pickling treatment, so the thickness calculation unit of the thickness calculation device can accurately calculate the thickness of the intergranular oxide layer based on the fluorescent X-ray intensity. In addition, the thickness calculation device can prevent a decrease in calculation accuracy by normalizing the fluorescent X-ray intensity, which is affected by the measurement distance, by the measurement distance. Therefore, the thickness calculation device can calculate the thickness of the intergranular oxide layer with high accuracy.

[0026] It is preferable that the apparatus further includes a storage unit that stores a plurality of correlations extracted according to the manufacturing conditions of the steel sheet, and the film thickness calculation unit has a function of selecting one correlation from the plurality of correlations according to the manufacturing conditions of the steel sheet. By being able to select an appropriate correlation according to the manufacturing conditions of the steel sheet in this manner, it is possible to further improve the calculation accuracy of the film thickness. Effect of the Invention

[0027] As described above, the intergranular oxide layer thickness calculation method and the intergranular oxide layer thickness calculation device of the present invention can calculate the thickness of the intergranular oxide layer remaining on the steel sheet after pickling with high accuracy. The platability determination method of the present invention can accurately determine the platability of the steel sheet from the intergranular oxide layer remaining on the steel sheet after pickling. In addition, the plated steel sheet manufacturing method of the present invention includes the platability determination method that can accurately determine the platability of the steel sheet from the intergranular oxide layer, and can improve manufacturing efficiency. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a flow diagram showing a film thickness calculation method according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a flow diagram showing a plating quality determination method according to one embodiment of the present invention. [Diagram 3] FIG. 3 is a flow diagram showing a method for producing a plated steel sheet according to one embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram of a film thickness calculation device according to one embodiment of the present invention, a platability determination device in which this film thickness calculation device is used, and a plated steel sheet manufacturing device in which this platability determination device is used. [Diagram 5] FIG. 5 is a flow chart showing a method for extracting the correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer used in the determination step of FIG. [Figure 6] FIG. 6 is a schematic diagram of a film thickness calculation device according to an embodiment different from the film thickness calculation device of FIG. [Figure 7]FIG. 7 is a graph illustrating the correlation between the relative intensity of fluorescent X-rays and the thickness of the grain boundary oxide layer. [Figure 8] FIG. 8 is a flow chart showing a plating quality determination method according to an embodiment different from that shown in FIG. [Figure 9] FIG. 9 is a graph showing the relationship between the pickling treatment time and the mass loss due to the pickling treatment in the examples. [Figure 10] FIG. 10 is a graph showing the correlation between fluorescent X-ray intensity and the thickness of the grain boundary oxide layer in the examples. [Figure 11] FIG. 11 is a graph showing the correlation between the thickness of the grain boundary oxide layer calculated according to the determination method of FIG. 4 in the examples and the actually measured values. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] [First embodiment] Hereinafter, a method for calculating the thickness of an intergranular oxide layer, a method for determining platability, a method for producing a plated steel sheet, and a thickness calculation device according to one embodiment of the present invention will be described.

[0030] The method for calculating the thickness of the grain boundary oxide layer of the present invention is a method for calculating the thickness of the grain boundary oxide layer remaining on a steel sheet after pickling treatment, and includes an intensity measurement step S1, a distance measurement step S2, an intensity correction step S3, and a thickness calculation step S4, as shown in FIG. 1. The method for calculating the thickness of the grain boundary oxide layer is used in a method for determining platability, which is an embodiment of the present invention. Specifically, the method for determining platability of a steel sheet is a method for determining the platability of the steel sheet from the grain boundary oxide layer remaining on the steel sheet after pickling treatment, and includes a calculation step S5 and a determination step S6, as shown in FIG. 2, in which the method for calculating the thickness of the grain boundary oxide layer is used in the calculation step S5. The method for determining platability is used in a method for manufacturing a plated steel sheet, which is an embodiment of the present invention. Specifically, the method for manufacturing a plated steel sheet includes a hot rolling step S11, a pickling step S12, a plateability determination step S13, and a cold rolling step S14, as shown in FIG. 3, in which the method for determining platability is used in the plateability determination step S13.

[0031] The method for calculating the film thickness of the grain boundary oxide layer can be performed by a film thickness calculation device 1 which is itself an embodiment of the present invention as shown in FIG. 4. The film thickness calculation device 1 is used in the plating property determination device 2 shown in FIG. 4, and the plating property determination device 2 is used in the plating steel sheet manufacturing device 100 shown in FIG. 4. The plating steel sheet manufacturing device 100 includes a hot rolling section 110 for hot rolling a steel sheet M as a base material, a pickling section 120 for removing the surface oxide scale of the steel sheet M rolled in the hot rolling section 110 with an acid, and a cold rolling section 130 for cold rolling the steel sheet M from which the oxide scale has been removed in the pickling section 120. The film thickness calculation device 1, and thus the plating property determination device 2 including the same, is disposed on the downstream side of the pickling section 120 and the upstream side of the cold rolling section 130. That is, the steel sheet M measured by the strength measurement section 10 of the film thickness calculation device 1 described later has been hot rolled but not cold rolled.

[0032] The film thickness calculation device 1 is a film thickness calculation device that calculates the film thickness of the grain boundary oxide layer remaining on the steel sheet M after pickling treatment. Specifically, it calculates the film thickness of the grain boundary oxide layer (calculated film thickness 1a) remaining on the steel sheet M after pickling treatment. The film thickness calculation device 1 includes an intensity measurement section 10 that measures the fluorescence X-ray intensity for the elements contained in the steel sheet M and constituting the oxides in the grain boundary oxide layer, a distance measurement section 20 that measures the measurement distance L which is the distance between the measurement position of the fluorescence X-ray intensity of the intensity measurement section 10 and the steel sheet M, an intensity correction section 30 that corrects the fluorescence X-ray intensity measured by the intensity measurement section 10 based on the measurement distance L, and a film thickness calculation section 40 that calculates the film thickness of the grain boundary oxide layer based on the correlation between the fluorescence X-ray intensity extracted in advance and the film thickness of the grain boundary oxide layer and the fluorescence X-ray intensity corrected by the intensity correction section 30.

[0033] In addition to the film thickness calculation device 1, the plating property determination device 2 includes a determination section 50 that determines the plating property of the steel sheet M using the film thickness of the grain boundary oxide layer (calculated film thickness 1a) calculated by the film thickness calculation section 40 as a determination index.

[0034] 〔Method for manufacturing a plated steel sheet〕 Hereinafter, each step of the method for manufacturing the plated steel sheet will be described.

[0035] <Hot rolling process> In the hot rolling step S11, the steel sheet M serving as the base material is hot rolled. The hot rolling step S11 is performed by the hot rolling section 110.

[0036] Specifically, the procedure is as follows: For example, a cast steel material such as a slab is wound into a coil shape and cooled to room temperature to produce a cast coil C1, from which a steel sheet M is fed in a sheet passing direction R and charged into the hot rolling section 110. Note that the cast steel material such as a slab can also be charged directly into the hot rolling section 110.

[0037] The steel sheet M charged in the hot rolling section 110 is soaked and hot rolled. The rolling conditions are not particularly limited. The hot rolled steel sheet M is at a high temperature of, for example, 500°C or higher, and is wound on a reel. The pre-cooling hot rolled coil C2 after winding is cooled (for example, air-cooled) to room temperature.

[0038] <Acid washing process> In the pickling step S12, oxide scale on the surface of the steel sheet M after the hot rolling step S11 is removed by acid (pickling treatment).

[0039] Specifically, the process is as follows. The steel sheet M is fed in the sheet passing direction R from the cooled hot-rolled coil C3 that has reached room temperature, and is charged into the pickling section 120. The pickling section 120 has, for example, a pickling tank in which acid is stored, and the steel sheet M is immersed in the pickling tank to dissolve and remove the oxide film formed on the surface in the hot rolling step S11. At this time, from the viewpoint of plating properties, a grain boundary oxide layer is left on the surface of the steel sheet M. The average thickness of the grain boundary oxide layer to be left is preferably 5 μm or more and 20 μm or less. Here, the "average thickness" refers to the average of thicknesses measured at any 10 points.

[0040] Examples of the acid used in the pickling treatment include hydrochloric acid and sulfuric acid, but hydrochloric acid is preferred. By using hydrochloric acid as the acid in the pickling treatment, the sensitivity of the calculation step S5 of the plating property determination step S13 described later, i.e., the thickness calculation method, is improved, and the accuracy of extraction of the thickness of the grain boundary oxide layer can be further improved.

[0041] <Plating quality evaluation process> In the plateability determination step S13, the plateability of the steel sheet M is determined from the grain boundary oxide layer remaining on the steel sheet M after the pickling step S12. Specifically, the thickness of the grain boundary oxide layer remaining on the steel sheet M after the pickling step S12 is extracted, and the plateability is determined.

[0042] The platability determination step S13 uses the platability determination method shown in Fig. 2. The platability determination method will be described in detail later.

[0043] The film thickness calculation position is preferably a position where tension is applied to the steel sheet M by a bride roll or the like after pickling, and sheet vibration is suppressed. The calculation results are preferably stored for each coil.

[0044] Using the calculated thickness of the grain boundary oxide layer, it is possible to group the steel sheets M according to the thickness of the grain boundary oxide layer. By grouping according to the thickness of the grain boundary oxide layer in this way, it is possible to manufacture plated steel sheets with reduced original sheet variation, for example. In addition, since it is possible to predict in advance which steel sheets M may have plating defects due to the original sheet, measures such as discarding the steel sheets M can be taken as necessary. Furthermore, since it is possible to separate the causes of plating defects from those of the original sheet and the manufacturing conditions, it is easy to optimize the manufacturing conditions.

[0045] This platability determination step S13 is preferably performed continuously along the longitudinal direction (sheet passing direction R) of the steel sheet M. By performing the platability determination step S13 continuously along the longitudinal direction of the steel sheet M in this manner, when a region where the thickness of the grain boundary oxide layer is partially thin occurs, it becomes possible to cut the corresponding portion and use the steel sheet.

[0046] In the manufacturing method of the plated steel sheet, a pickling step S12 is performed before a cold rolling step S14. In a galvanizability determination step S13 after the pickling step S12 and before the cold rolling step S14, the thickness of the grain boundary oxide layer formed on the steel sheet M after the pickling treatment is extracted, and the manufacturing conditions of the next step, the cold rolling step S14 and thereafter, can be optimized according to the thickness of the grain boundary oxide layer. Therefore, by using the manufacturing method of the plated steel sheet, the manufacturing efficiency of the plated steel sheet can be improved.

[0047] <Cold rolling process> In the cold rolling step S14, the steel sheet M after the galvanization determining step S13 is cold rolled. The cold rolling step S14 is performed by the cold rolling section 130.

[0048] Specifically, cold rolling can be performed by sandwiching the steel sheet M between rolling rolls during passing. The rolling rolls of the cold rolling section 130 may be configured to have a plurality of pairs of cold rolling rolls as shown in Fig. 3, or a reverse rolling mill that repeatedly rolls the steel sheet M with one mill may be used.

[0049] The rolled steel sheet M is wound around a reel to form a coil of cold-rolled steel sheet (cold-rolled coil C4).

[0050] The cold-rolled coil C4 is subjected to conventional processes, namely, an oxidation process for oxidizing the surface of the steel material M, a reduction process for reducing the oxide film formed in the oxidation process, and a plating process for plating the surface of the steel material M after the reduction process, to obtain a plated steel sheet.

[0051] [Method of determining plating quality] Next, each step of the platability determination method used in the platability determination step S13 of the above-mentioned method for producing a plated steel sheet will be described.

[0052] <Calculation process> In the calculation step S5, the thickness of the intergranular oxide layer is calculated. As described above, in the calculation step S5, the method for calculating the thickness of the intergranular oxide layer shown in Fig. 1 is used. Hereinafter, each step of the method for calculating the thickness of the intergranular oxide layer will be described as the calculation step S5.

[0053] (Strength measurement process) In the strength measurement step S1, the fluorescent X-ray intensity of elements that are contained in the steel sheet M and that constitute the oxides in the grain boundary oxide layer is measured.

[0054] The intensity measuring step S1 is performed by an intensity measuring unit 10. As the intensity measuring unit 10, a known fluorescent X-ray analyzer can be used.

[0055] The above elements are selected from elements contained in the steel sheet M, whose oxides are selectively removed by pickling. Among them, it is preferable that the above element is Mn, and it is more preferable that the acid used in the above pickling treatment is hydrochloric acid and the above element is Mn. The oxides selectively removed and the ratio of the oxides removed are determined by a combination of the type of pickling solution and the type of element. A combination of hydrochloric acid as the acid used in the above pickling treatment and Mn as the above element results in a high ratio of the oxides removed. In the method for calculating the thickness of an intergranular oxide layer, the higher the ratio of the oxides removed, the higher the measurement sensitivity. Therefore, the above combination can further improve the accuracy of the calculation of the thickness, and thus the accuracy of the determination in the method for determining platability.

[0056] (distance measurement process) In the distance measuring step S2, a measurement distance L, which is the distance between the measurement position of the fluorescent X-ray intensity measured in the intensity measuring step S1 and the steel sheet M, is measured.

[0057] The distance measurement step S2 is performed by the distance measurement unit 20. As the distance measurement unit 20, for example, a known laser displacement meter can be used.

[0058] (Strength correction process) In the intensity correction step S3, the fluorescent X-ray intensity measured in the intensity measurement step S1 is corrected based on the measurement distance L.

[0059] The intensity correction step S3 is performed by the intensity correction unit 30. As the intensity correction unit 30, a known arithmetic device such as a microcontroller or a personal computer can be used.

[0060] If we use the X-ray fluorescence analyzer that extracts the correlation described below, and measure the same distance L 0 Even if the strength measurement step S1 is performed, the actual measurement distance L is the measurement distance L when the correlation is extracted. 0 Since the fluorescent X-ray intensity is affected by the measurement distance L, this error directly leads to an error in the calculated film thickness. In this intensity correction step S3, the measurement distance L is actually measured, and the measurement distance L when the correlation is extracted is 0 The fluorescent X-ray intensity is normalized based on the measurement distance.

[0061] In this way, the measurement distance L 0 However, even if the actually measured measurement distance L is different, by providing a distance measurement process S2 and an intensity correction process S3 and normalizing the fluorescent X-ray intensity by the measured distance, it is possible to prevent a decrease in the determination accuracy.

[0062] For example, since the fluorescent X-ray intensity decreases inversely proportional to the square of the measurement distance L, when the fluorescent X-ray intensity measured in the intensity measurement step S1 is A and the corrected fluorescent X-ray intensity is Ac, correction can be made using the following formula 1. Ac = A × (L / L 0 ) 2 1

[0063] Alternatively, the fluorescent X-ray intensity may be measured in advance within the range of the actually expected measurement distance L, and the relationship between the measurement distance L and the corrected fluorescent X-ray intensity Ac may be stored in a database, for example in the form of a look-up table, and the correction value may be determined based on this database.

[0064] (Thickness calculation process) In the film thickness calculation step S4, the film thickness of the intergranular oxide layer is calculated based on the correlation between the fluorescent X-ray intensity and the film thickness of the intergranular oxide layer that has been extracted in advance and the fluorescent X-ray intensity measured in the intensity measurement step.

[0065] The film thickness calculation step S4 is performed by the film thickness calculation unit 40. A known arithmetic device such as a microcontroller or a personal computer can be used as the film thickness calculation unit 40. The arithmetic device of the intensity correction unit 30 may be shared as the film thickness calculation unit 40.

[0066] Here, a method for extracting the correlation between fluorescent X-ray intensity and the thickness of the grain boundary oxide layer will be described. The correlation can be extracted for a plurality of steel plate samples having grain boundary oxide layers with different thicknesses by an extraction method including an individual X-ray intensity measurement step S21, a break point extraction step S22, an individual thickness calculation step S23, and a correlation equation determination step S24, as shown in FIG.

[0067] (Individual X-ray intensity measurement process) In the individual X-ray intensity measuring step S21, the fluorescent X-ray intensity for the above elements of each steel plate sample is measured.

[0068] This step is preferably performed using the same device as the strength measuring unit 10 used in the strength measuring step S1, and at the same measurement distance L. By thus aligning the measurement conditions with those in the strength measuring step S1, errors can be reduced.

[0069] (Break point extraction process) In the break point extraction step S22, a break point of a line representing the relationship between the pickling treatment time of the steel sheet sample measured in the individual X-ray intensity measurement step S21 and the mass loss per unit area due to the pickling treatment is obtained.

[0070] When the relationship between the pickling time and the mass loss per unit area due to the pickling process is taken for each steel sheet sample, the mass is reduced for a certain period of time after the start of the pickling process due to the removal of the grain boundary oxide layer. When the grain boundary oxide layer is completely removed, the base material of the steel sheet is subsequently removed. Since the grain boundary oxide layer and the base material of the steel sheet have different compositions, the mass loss per unit time that is dissolved and reduced by the pickling process, that is, the mass loss rate, is different. Therefore, the relationship between the pickling time and the mass loss due to the pickling process for each steel sheet sample is a broken line (see, for example, FIG. 9 in the examples). This breaking point means the boundary between the grain boundary oxide layer and the base material of the steel sheet. This breaking point can be obtained, for example, by linear approximation of the relationship between the pickling time and the mass loss due to the pickling process, which is plotted discretely, at one point.

[0071] (Individual film thickness calculation process) In the individual film thickness calculation step S23, the equivalent film thickness of the grain boundary oxide layer of the steel sheet sample is calculated by dividing the mass reduction amount at the break point by the density of the steel sheet.

[0072] The above-mentioned break point represents the point where the intergranular oxide layer has been completely removed, but the steel sheet base material has not yet been removed, so the mass loss per unit area at the break point represents the total amount of the intergranular oxide layer remaining in the steel sheet sample per unit area. Since the density of the intergranular oxide layer is substantially equal to the density of the steel sheet, the above-mentioned mass loss at the break point can be calculated by multiplying the density of the steel sheet, for example, 7.87 g / cm3 of iron. 3 By dividing by this, the thickness of the grain boundary oxide layer of the steel sheet sample can be determined with high accuracy.

[0073] (Correlation formula determination process) In the correlation equation determination step S24, a correlation equation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer is extracted from the fluorescent X-ray intensity and the equivalent thickness of the grain boundary oxide layer of each steel plate sample obtained in the individual X-ray intensity measurement step S21 and the individual thickness calculation step S23.

[0074] As described above, the grain boundary oxide layer is relatively thin, so when the fluorescent X-ray intensity of the above elements of the steel sheet M is measured, the fluorescent X-ray intensity is measured with the intensity from the above elements contained in the steel sheet base superimposed. The thinner the grain boundary oxide layer is, the higher the proportion of the intensity from the above elements contained in the steel sheet base superimposed. Here, since oxides containing the above elements are selectively removed in the grain boundary oxide layer, the concentration of the above elements in the grain boundary oxide layer is lower than the concentration of the above elements contained in the steel sheet base. Therefore, the thinner the grain boundary oxide layer is, the higher the fluorescent X-ray intensity is, so there is a negative correlation between the fluorescent X-ray intensity and the film thickness of the grain boundary oxide layer (see, for example, FIG. 9 in the examples).

[0075] In the correlation equation determination step S24, this correlation is expressed by a correlation equation. For example, a linear equation may be used as the correlation equation. The thickness of the grain boundary oxide layer has a desired value, and the pickling treatment conditions are determined so as to obtain this desired value. Therefore, it is difficult to imagine a large variation, and sufficient accuracy can be obtained by linear approximation. For example, the well-known least squares method can be used as the linear approximation.

[0076] By determining the correlation equation between the thickness of the intergranular oxide layer calculated in the individual thickness calculation step S23 and the fluorescent X-ray intensity of the steel plate sample calculated in the individual X-ray intensity measurement step S21 in this manner, the thickness of the intergranular oxide layer (calculated thickness 1a) can be calculated easily and accurately based on the fluorescent X-ray intensity corrected in the intensity correction step S3.

[0077] <Judgment process> In the judgment step S6, the thickness of the intergranular oxide layer calculated in the calculation step S5 is used as a judgment index to judge the platability of the steel sheet M. Specifically, it is judged whether the thickness of the remaining intergranular oxide layer is a desired value. In the judgment, even if it is not exactly equal to the desired value, it may be judged that the platability is good when it is within a certain range.

[0078] The determination step S6 is performed by the determination unit 50. A known arithmetic device such as a microcontroller or a personal computer can be used as the determination unit 50. The arithmetic device of the intensity correction unit 30 or the film thickness calculation step S4 may be shared as the determination unit 50.

[0079] <Advantages> In the method for calculating the thickness of an intergranular oxide layer, the fluorescent X-ray intensity of elements that are contained in the steel sheet M and that constitute the oxides in the intergranular oxide layer is measured. The fluorescent X-ray intensity has a high correlation with the thickness of the intergranular oxide layer remaining in the steel sheet M after pickling treatment, so the thickness can be calculated with high accuracy based on the fluorescent X-ray intensity.

[0080] In the plating property determination method, the thickness of the grain boundary oxide layer can be calculated with high accuracy using the method for calculating the thickness of the grain boundary oxide layer of the present invention. Therefore, the plating property of the steel sheet M can be determined with high accuracy based on the calculated thickness.

[0081] In the manufacturing method of the plated steel sheet, the platability of the steel sheet M can be determined with high accuracy by using the platability determination method of the present invention after the pickling step S12. This makes it possible to optimize the manufacturing conditions after the pickling step S12 in accordance with the platability of the steel sheet M. Therefore, by using the manufacturing method of the plated steel sheet, the manufacturing efficiency of the plated steel sheet can be improved.

[0082] The thickness calculation device 1 includes an intensity measurement unit 10 that measures the fluorescent X-ray intensity for elements that are included in the steel sheet M and that constitute the oxide in the intergranular oxide layer. The fluorescent X-ray intensity has a high correlation with the thickness of the intergranular oxide layer formed on the steel sheet M after pickling treatment, so the thickness calculation unit 40 of the thickness calculation device 1 can accurately calculate the thickness of the intergranular oxide layer based on the fluorescent X-ray intensity. In addition, the thickness calculation device 1 can prevent a decrease in calculation accuracy by normalizing the fluorescent X-ray intensity, which is affected by the measurement distance L, by the measurement distance L. Therefore, the thickness calculation device 1 can calculate the thickness of the intergranular oxide layer with high accuracy.

[0083] [Second embodiment] Hereinafter, a method and device for calculating the thickness of an intergranular oxide layer according to a second embodiment of the present invention will be described.

[0084] [Film thickness calculation device] A thickness calculation device 3 shown in Fig. 6 is a thickness calculation device that calculates the thickness of an intergranular oxide layer remaining on a steel sheet M after pickling treatment. The thickness calculation device 3 can be used in place of the thickness calculation device 1 shown in Fig. 1.

[0085] The film thickness calculation device 3 includes an intensity measurement unit 10 that measures the fluorescent X-ray intensity for elements contained in the steel sheet M and constituting the oxide in the intergranular oxide layer, a distance measurement unit 20 that measures a measurement distance L that is the distance between the measurement position of the fluorescent X-ray intensity of the intensity measurement unit 10 and the steel sheet M, an intensity correction unit 30 that corrects the fluorescent X-ray intensity measured by the intensity measurement unit 10 based on the measurement distance L, and a film thickness calculation unit 41 that calculates the film thickness of the intergranular oxide layer (calculated film thickness 1a) based on the correlation between the fluorescent X-ray intensity and the film thickness of the intergranular oxide layer that has been extracted in advance and the fluorescent X-ray intensity corrected by the intensity correction unit 30. The film thickness calculation device 3 further includes a storage unit 60.

[0086] The intensity measuring unit 10, the distance measuring unit 20 and the intensity correcting unit 30 can be configured in the same manner as the intensity measuring unit 10, the distance measuring unit 20 and the intensity correcting unit 30 in FIG. 1, so they are denoted by the same reference numerals and detailed description thereof will be omitted.

[0087] (Storage part) The storage unit 60 stores a plurality of correlations extracted according to the manufacturing conditions of the steel sheet M.

[0088] Examples of the manufacturing conditions of the steel sheet M include the acid used in the pickling treatment, and the types and concentrations of elements contained in the steel sheet M. Among them, the Mn concentration is preferable as the manufacturing condition of the steel sheet M. It is known that the fluorescent X-ray intensity depends on the Mn concentration.

[0089] To explain the Mn concentration dependency of fluorescent X-ray intensity in detail, when a single-wavelength X-ray is incident on a steel sheet M having a two-layer structure of a steel sheet base material and a grain boundary oxide layer, assuming that the Mn concentrations in the steel sheet base material and the grain boundary oxide layer are constant, the relative intensity of fluorescent X-rays emitted from Mn in the steel sheet M is expressed by the following formula 2. Here, W 1 is the Mn concentration (mass%) in the grain boundary oxide layer, and W 2 is the Mn concentration (mass%) in the steel sheet substrate, T 1 is the thickness of the grain boundary oxide layer (cm), ρ 1 is the density of the grain boundary oxide layer (g / cm 3 ), A is a constant expressed by the following formula 3. Const. is a constant determined by the measurement element and the X-ray optical system. In addition, in the following formula 3, μ 1 (λ) is the mass absorption coefficient (cm 2 / g), μ 1 (ip) is the mass absorption coefficient (cm 2 / g), Φ is the incident angle of the incident X-ray (rad), and Ψ is the detection angle of the fluorescent X-ray (rad).

[0090]

number

[0091] Since the main component in the grain boundary oxide layer is Fe, if we assume that the mass absorption coefficient in the grain boundary oxide layer does not change, A is considered to be constant even if the Mn concentration varies. In other words, the fluorescent X-ray intensity I is W 1 , W 2 and T 1 It is a function of .

[0092] Assume that the design value of the steel sheet M is a Mn concentration of 2.0 mass%, and that the Mn concentration has changed to 1.8 mass% or 2.2 mass% due to steelmaking variations. Also, assume that the amount of Mn oxide formed in the grain boundary oxide layer is uniformly 0.5 mass%, and that the Mn oxide is completely removed in the pickling process. Then, the Mn concentration W in the grain boundary oxide layer after pickling is 1is 1.3 mass% to 1.7 mass%. Using these values ​​and formula 2 above to calculate the correlation between the relative intensity of the fluorescent X-rays and the thickness of the grain boundary oxide layer, the graph in Figure 7 is obtained. Here, the incident X-rays are Rh Ka rays, the fluorescent X-rays are Mn Ka rays, the incident angle of the incident X-rays is 60°, the detection angle is 90°, and the density of the grain boundary oxide layer is 7.87 g / cm 3 In addition, the fluorescent X-ray intensity at a grain boundary oxide layer thickness of 0 μm when the Mn concentration in the grain boundary oxide layer is 1.5 mass% and the Mn concentration in the steel sheet substrate is 2.0 mass% is normalized as 1.

[0093] 7, it can be seen that when the Mn concentration in the steel sheet base material changes, the thickness of the grain boundary oxide layer can change even if the same fluorescent X-ray intensity is obtained. For this reason, the thickness calculation device 3 prepares a number of correlations extracted according to the manufacturing conditions of the steel sheet M (in this case, the Mn concentration in the steel sheet base material).

[0094] The number of correlations to be stored is not particularly limited, and is appropriately determined depending on the manufacturing conditions of the steel sheet M and the associated differences in correlations.

[0095] (Film thickness calculation section) The film thickness calculation unit 41 has a function of selecting one of the above-mentioned multiple correlations according to the manufacturing conditions of the steel sheet M. By being able to select an appropriate correlation according to the manufacturing conditions of the steel sheet M in this manner, the calculation accuracy of the film thickness can be further improved.

[0096] Except for having the above functions, the film thickness calculation unit 41 can be configured similarly to the film thickness calculation unit 40 shown in FIG. 4, and therefore a detailed description thereof will be omitted.

[0097] [Method of calculating the thickness of grain boundary oxide layer] The method for calculating the thickness of an intergranular oxide layer using the thickness calculation device 3 shown in FIG. 6 is a method for calculating the thickness of an intergranular oxide layer remaining on a steel sheet M after pickling treatment, and includes an intensity measurement step of measuring fluorescent X-ray intensity for elements contained in the steel sheet M and constituting an oxide in the intergranular oxide layer, a distance measurement step of measuring a measurement distance L between the measurement position of the fluorescent X-ray intensity measured in the intensity measurement step and the steel sheet M, an intensity correction step of correcting the fluorescent X-ray intensity measured in the intensity measurement step based on the measurement distance L, and a thickness calculation step of calculating the thickness of the intergranular oxide layer based on a correlation between the fluorescent X-ray intensity and the thickness of the intergranular oxide layer that has been extracted in advance and the fluorescent X-ray intensity measured in the intensity correction step.

[0098] The intensity measuring step, the distance measuring step, and the intensity correcting step are similar to the intensity measuring step S1, the distance measuring step S2, and the intensity correcting step S3 in the first embodiment.

[0099] (Thickness calculation process) In the method for calculating the thickness of the grain boundary oxide layer, a plurality of correlations are extracted in advance according to the manufacturing conditions of the steel sheet M. For example, a case will be described in which a plurality of correlations are extracted in advance according to the Mn concentration of the steel sheet M, but this does not mean that the manufacturing conditions of the steel sheet M are limited to the Mn concentration of the steel sheet M. Note that each correlation can be extracted, for example, by using the method for extracting the correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer described in the first embodiment.

[0100] The above correlations have different Mn concentrations. The Mn concentrations may be determined so as to cover a range that takes manufacturing variations into consideration, centered on the design reference value of the steel sheet M. In the example of the graph in Fig. 7, in addition to the case of a Mn concentration of 2.0 mass%, which is the design reference value, a total of three correlations are prepared, with Mn concentrations of 1.8 mass% and 2.2 mass%, to cover the range of manufacturing variations.

[0101] In the film thickness calculation step, one correlation is selected from the multiple correlations according to the manufacturing conditions of the steel sheet M. When the manufacturing condition is Mn, the Mn concentration of the steel sheet M can be, for example, a measurement result of the base material performed during operation. By using this measurement result, the Mn concentration of each steel sheet M can be known, so that a correlation of the Mn concentration close to the measurement result can be selected.

[0102] Using this selected correlation, the thickness of the grain boundary oxide layer (calculated thickness 1a) can be calculated easily and accurately based on the fluorescent X-ray intensity.

[0103] <Advantages> For example, the fluorescent X-ray intensity depends on the Mn concentration. In this way, when the manufacturing conditions of the steel sheet M are different, the above correlation may be different. Therefore, in the method for calculating the thickness of the intergranular oxide layer, it is possible to select an appropriate correlation according to the manufacturing conditions of the steel sheet M, thereby further improving the calculation accuracy of the thickness. Therefore, even if manufacturing variations occur in the steel sheet M, the platability of the steel sheet M can be accurately determined from, for example, the intergranular oxide layer.

[0104] [Third embodiment] Hereinafter, a plating quality determination method according to a third embodiment of the present invention will be described.

[0105] [Method of determining plating quality] The platability determination method shown in Fig. 8 is a method for determining the platability of a steel sheet M from a grain boundary oxide layer remaining on the steel sheet M after pickling treatment, and includes an intensity measurement step S1 for measuring the fluorescent X-ray intensity for elements contained in the steel sheet M and constituting the oxide in the grain boundary oxide layer, a distance measurement step S2 for measuring a measurement distance L between the measurement position of the fluorescent X-ray intensity measured in the intensity measurement step S1 and the steel sheet M, an intensity correction step S3 for correcting the fluorescent X-ray intensity measured in the intensity measurement step S1 based on the measurement distance L, and a determination step S7 for directly determining the platability of the steel sheet M based on the fluorescent X-ray intensity measured in the intensity correction step S3. The platability determination method can be used in the platability determination step S13 shown in Fig. 3 of the first embodiment instead of the platability determination method shown in Fig. 2.

[0106] The strength measuring step S1, the distance measuring step S2, and the strength correcting step S3 are similar to the strength measuring step S1, the distance measuring step S2, and the strength correcting step S3 in the plating quality determination step of the first embodiment shown in Figure 2, so detailed explanations will be omitted.

[0107] <Judgment process> In the plating quality evaluation process of the first embodiment shown in FIG. 2, the thickness of the grain boundary oxide layer is calculated and used as the evaluation index, whereas in this plating quality evaluation process, the plating quality is directly evaluated from the numerical value of the fluorescent X-ray intensity itself, without using the thickness of the grain boundary oxide layer.

[0108] As mentioned above, there is a correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer, and the platability can be judged if the thickness of the grain boundary oxide layer is determined. In this case, the judgment result of the platability can be uniquely determined if the fluorescent X-ray intensity is known, so the platability can be specified as a function of the fluorescent X-ray intensity. In this case, the thickness of the grain boundary oxide layer is not calculated as a positive value, but the same result as that obtained by calculating the thickness of the grain boundary oxide layer can be obtained.

[0109] <Advantages> In the platability evaluation method, the fluorescent X-ray intensity of elements that are contained in the steel sheet M and that constitute the oxides in the grain boundary oxide layer is measured. Since the fluorescent X-ray intensity has a high correlation with the thickness of the grain boundary oxide layer remaining in the steel sheet M after pickling treatment, the platability of the steel sheet M can be calculated directly with high accuracy based on the fluorescent X-ray intensity.

[0110] [Other embodiments] It should be noted that the present invention is not limited to the above-described embodiment.

[0111] In the above embodiment, the case where the plating quality judgment method includes the intensity correction step has been described, but the intensity correction step is not an essential configuration and can be omitted. In this case, the intensity correction unit of the plating quality judgment device can also be omitted.

[0112] In the plating quality evaluation method not including the intensity correction step, it is preferable to include a distance correction step for mechanically making the measurement distance constant. In this case, the plating quality evaluation device is provided with a distance correction unit for mechanically making the measurement distance constant. The method for mechanically making the measurement distance constant can be appropriately selected, and for example, a method of moving the intensity measurement unit based on the measurement distance measured in the distance measurement step can be mentioned. In addition, in the plating quality evaluation method not including the intensity correction step, the thickness of the grain boundary oxide layer is calculated in the evaluation step based on the correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer extracted in advance and the fluorescent X-ray intensity measured in the intensity measurement step.

[0113] It is also possible to have a configuration in which only the intensity correction step is provided without including the distance measurement step. For example, in the intensity measurement step, the fluorescent X-ray intensity of Fe element in addition to the target element can be obtained, and correction can be made based on the difference in the intensity of this Fe element.

[0114] Furthermore, the present invention also contemplates a platability evaluation method that does not include both the distance measurement process and the intensity correction process. For example, in an environment where the fluctuation of the measurement distance is guaranteed to be small, the thickness of the grain boundary oxide layer can be calculated with high accuracy without correction. In this case, both the distance measurement unit and the intensity correction unit can be omitted in the platability evaluation device.

[0115] In the first embodiment, the correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer is determined from the relationship between the pickling treatment time of the steel sheet sample and the mass loss due to the pickling treatment, but the method of extracting the correlation is not limited to this. For example, the thickness of the grain boundary oxide layer may be determined from the cross section of each steel sheet sample by SEM observation or the like, and the correlation may be specified.

[0116] In the first embodiment, the correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer is expressed by a correlation equation. However, the correlation may be expressed by other methods, such as by a lookup table format.

[0117] In the above embodiment, the thickness calculation device is described as being disposed downstream of the pickling section and upstream of the cold rolling section, but the thickness calculation device may be disposed at another position as long as it is downstream of the pickling section. Similarly, in the manufacturing method of the plated steel sheet, the galvanization determining step is not limited to being before the cold rolling step, so long as it is after the pickling step. The same effect can be achieved by measuring the cold-rolled steel sheet with the strength measuring section of the galvanization determining device.

[0118] In the above first and second embodiments, the case where the thickness of the grain boundary oxide layer is calculated and used as the judgment index is described, and in the above third embodiment, the case where the plating property is judged directly from the numerical value of the fluorescent X-ray intensity itself is described, but the judgment index may be another index. EXAMPLES

[0119] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0120] A steel sheet containing 2.35 mass % of Mn was prepared.

[0121] In order to determine the correlation equation according to the method for extracting the correlation between the fluorescent X-ray intensity and the thickness of the intergranular oxide layer described in the thickness calculation step S4 of the first embodiment, 15 steel plate samples each having remaining intergranular oxide layers with different thicknesses were prepared.

[0122] First, the fluorescent X-ray intensity of each of the steel sheet samples was measured using a Rigaku Simultix-14 fluorescent X-ray analyzer.

[0123] Next, the relationship between the pickling treatment time and the mass loss per unit area due to the pickling treatment for each steel sheet sample was determined. An example is shown in Figure 9. As shown in Figure 9, the relationship between the pickling treatment time and the mass loss due to the pickling treatment for each steel sheet sample was a broken line. The mass loss at the breaking point was calculated based on the density of iron, 7.87 g / cm. 3 The equivalent thickness of the grain boundary oxide layer of each steel sheet sample was calculated by dividing the thickness by 1 / 2.

[0124] A graph plotting the relationship between the fluorescent X-ray intensity and the thickness of the intergranular oxide layer for each of the above steel sheet samples is shown in Figure 10. As can be seen from the graph in Figure 10, there is a negative correlation between the fluorescent X-ray intensity and the thickness of the intergranular oxide layer.

[0125] The relationship between the fluorescent X-ray intensity x (kcps) and the thickness y (μm) of the grain boundary oxide layer was calculated using a linear approximation by the least squares method, and the following formula 4 was obtained. y=-0.0832x+28.216 4

[0126] The correlation between the thickness of the grain boundary oxide layer calculated according to the plating property evaluation method of the present invention using the above correlation formula 4 and the actual measurement value for the 15 steel sheet samples is shown in Figure 11. The actual measurement value was calculated by calculating the mass loss from the break point of the line of the above pickling treatment time and the mass loss due to the pickling treatment for each steel sheet sample, and the density of iron was 7.87 g / cm. 3 This is the equivalent film thickness value calculated by dividing by .

[0127] The results in Figure 11 show that measurements were made with a high degree of accuracy, with a standard deviation of σ = 0.88 μm, in the measurement range of 2 μm to 8 μm. Therefore, it can be said that the galvanic property of steel sheets can be judged by using the thickness of the grain boundary oxide layer calculated based on the fluorescent X-ray intensity as a judgment index. [Industrial Applicability]

[0128] The intergranular oxide layer thickness calculation method and thickness calculation device of the present invention can calculate the thickness of the intergranular oxide layer remaining on the steel sheet after pickling with high accuracy. The platability determination method of the present invention can accurately determine the platability of the steel sheet from the intergranular oxide layer remaining on the steel sheet after pickling. In addition, the plated steel sheet manufacturing method of the present invention includes the platability determination method that can accurately determine the platability of the steel sheet from the intergranular oxide layer, thereby improving manufacturing efficiency. [Explanation of symbols]

[0129] 1, 3 Film thickness calculation device 2. Plating quality assessment device 1a Calculated film thickness 10 Strength measurement section 20 Distance measurement unit 30 Intensity correction section 40, 41 Film thickness calculation section 50 Judgment section 60 Storage section 100. Plated steel sheet manufacturing equipment 110 Hot Rolling Section 120 Pickling section 130 Cold Rolling Section M steel plate C1 Cast coil C2 Uncooled hot rolled coil C3 Cooled hot rolled coil C4 cold rolled coil L Measurement distance R Threading direction

Claims

1. A method for calculating the thickness of a grain boundary oxide layer remaining on a steel sheet after pickling treatment, comprising the steps of: an intensity measuring step of measuring a fluorescent X-ray intensity for an element that is contained in the steel sheet and that constitutes an oxide in the grain boundary oxide layer; a thickness calculation step of calculating the thickness of the grain boundary oxide layer based on a correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer, which has been extracted in advance, and the fluorescent X-ray intensity measured in the intensity measurement step; A method for calculating the thickness of a grain boundary oxide layer comprising:

2. A plurality of correlations are extracted in advance according to manufacturing conditions of the steel plate, 2. The method for calculating a thickness of an intergranular oxide layer according to claim 1, wherein in the thickness calculation step, one correlation is selected from the plurality of correlations depending on manufacturing conditions of the steel sheet.

3. a distance measuring step of measuring a measurement distance between the measurement position of the fluorescent X-ray intensity measured in the intensity measuring step and the steel sheet; an intensity correction step of correcting the fluorescent X-ray intensity measured in the intensity measurement step based on the measurement distance; Further equipped with 3. The method for calculating a thickness of an intergranular oxide layer according to claim 1, wherein in the thickness calculation step, the fluorescent X-ray intensity corrected in the intensity correction step is used instead of the fluorescent X-ray intensity measured in the intensity measurement step.

4. The acid used in the pickling treatment is hydrochloric acid, 3. The method for calculating the thickness of an intergranular oxide layer according to claim 1, wherein the element is Mn.

5. A method for determining the platability of a steel sheet from a grain boundary oxide layer remaining on the steel sheet after pickling treatment, comprising: A calculation step of calculating the thickness of the grain boundary oxide layer; a determination step of determining the galvanizability of the steel sheet using the thickness of the grain boundary oxide layer calculated in the calculation step as a determination index; Equipped with A plating quality judgment method, in which the calculation step uses the method for calculating the thickness of an intergranular oxide layer according to claim 1.

6. A method for determining the platability of a steel sheet from a grain boundary oxide layer remaining on the steel sheet after pickling treatment, comprising: an intensity measuring step of measuring a fluorescent X-ray intensity for an element that is contained in the steel sheet and that constitutes an oxide in the grain boundary oxide layer; a determining step of directly determining the platability of the steel sheet based on the fluorescent X-ray intensity measured in the strength measuring step; A plating quality determination method comprising:

7. a hot rolling process for hot rolling a steel plate serving as a base material; a pickling process for removing oxide scale from the surface of the steel sheet after the hot rolling process using an acid; a platability determination step of determining the platability of the steel sheet from the grain boundary oxide layer remaining on the steel sheet after the pickling step; Equipped with A method for producing a plated steel sheet, in which the plateability determining method according to claim 5 or 6 is used in the plateability determining step.

8. A thickness calculation device for calculating the thickness of an intergranular oxide layer remaining on a steel sheet after pickling treatment, an intensity measuring unit that measures a fluorescent X-ray intensity for an element that is contained in the steel sheet and that constitutes an oxide in the grain boundary oxide layer; a distance measuring unit that measures a measurement distance between a measurement position of the fluorescent X-ray intensity of the intensity measuring unit and the steel sheet; an intensity correction unit that corrects the fluorescent X-ray intensity measured by the intensity measurement unit based on the measurement distance; a thickness calculation unit that calculates the thickness of the grain boundary oxide layer based on a correlation between the fluorescent X-ray intensity and the thickness of the grain boundary oxide layer that has been extracted in advance and the fluorescent X-ray intensity corrected by the intensity correction unit; A film thickness calculation device comprising:

9. A storage unit that stores a plurality of correlations extracted according to the manufacturing conditions of the steel plate, 9. The thickness calculation device according to claim 8, wherein the thickness calculation unit has a function of selecting one of the plurality of correlations in accordance with manufacturing conditions of the steel plate.

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