JP2014074649A - Method for observing surface layer part of steel material and method for manufacturing steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for observing a surface layer part of a steel material, which can accurately grasp three-dimensional distribution of a second phase in the surface layer part of the steel material, and to provide a steel material manufacturing method capable of suitably correcting manufacturing conditions based on observation results.SOLUTION: When observing the three-dimensional distribution of the second phase in the surface layer part of the steel material, processing for determining an observation surface in the surface layer part and then treating the observation surface by a focused ion beam so that the treated surface becomes parallel with the observation surface and processing for observing the treated observation surface by a scanning electron microscope are repeated and respective observation results are integrated to grasp the three-dimensional distribution of the second phase on the surface layer part.

Description

  The present invention relates to a method for observing a surface layer portion of a steel material and a method for producing a steel material, and in particular, to observe a surface layer portion of a steel material capable of accurately grasping the three-dimensional distribution of second phases such as precipitates in the surface layer portion of the steel material. The present invention relates to a method and a method for manufacturing a steel material that can appropriately correct manufacturing conditions based on the observation result.

  In general, the mechanical properties of a material are governed by many factors such as the size of a structural unit, crystal orientation, defects in the structure, and the dispersion state of the second phase in the matrix. For example, the strength of the material is inversely proportional to the 1/2 power of the crystal grain size constituting the structure. Moreover, a large precipitation strengthening amount can be obtained by appropriately controlling the particle size and amount of the precipitate finely dispersed in the matrix.

  For this reason, conventionally, the strength of the material has been improved by controlling the structure and precipitates. For example, Patent Document 1 describes a technique for manufacturing a steel material having a desired material by quantitatively controlling the size and amount of precipitates to be used.

  By the way, in order to optimally control the material structure, not only the state of the structure in the stationary part but also the state of the second phase such as precipitates in the unsteady part, for example, near the surface of the steel material treated at high temperature. It is necessary to grasp the oxidation state and the distribution state of inclusions in the weld heat affected zone. In particular, the former is important when the steel material is a hot-dip galvanized steel plate (hereinafter also referred to simply as “plated steel plate”, and in the present invention, a hot-dip galvanized steel plate is also included as a steel material). That is, with the recent increase in the strength of steel sheets for automobiles, a high concentration of Si has been added to the base steel sheet, but during the oxidation and reduction treatment of the steel sheet before plating treatment, oxidation of Si or Si-Mn series is performed. Precipitates such as deposits are dispersed at the interface between the base steel plate and the plating layer or within the plating layer, adversely affecting the alloying process itself of the plating layer and the mechanical properties of the plating layer. In some cases, an unplated portion (including a portion where the alloy phase in the plating layer is not sufficiently formed and a portion where the thickness of the plating layer is thinner than the surroundings) is formed.

  In order to solve such problems, it is necessary to determine the oxidation conditions and reduction conditions of the base steel sheet that suppresses the decrease in plating properties. To that end, the precipitates in the region near the interface between the base steel sheet and the plating layer are required. It is not enough to grasp the size, form and quantity in two dimensions, and it is essential to grasp how they are distributed in three dimensions.

JP 2010-126791 A

  However, the technique described in Patent Document 1 is based on the premise that the size and form of the precipitate to be used and the distribution of the precipitate in the matrix are uniform, like the surface layer portion of the plated steel sheet. It is impossible to grasp the three-dimensional distribution of precipitates in the unsteady part.

Thus, there is no method that can accurately grasp the three-dimensional distribution of the second phase in the surface layer portion of the steel material, and establishment of such a method has been demanded.
Therefore, an object of the present invention is to appropriately correct the manufacturing conditions based on the observation method of the surface layer portion of the steel material, which can accurately grasp the three-dimensional distribution of the second phase in the surface layer portion of the steel material, and this observation result. An object of the present invention is to provide a method for manufacturing a steel material.

  The inventors diligently studied how to solve the above problems. For this purpose, the inventors have come up with a combination of processing using a focused ion beam (FIB) and surface observation using a scanning electron microscope (SEM). In other words, since the steel material sample can be processed on the nanoscale by FIB, the cross section can be made to appear at a very fine pitch in the target region, and each cross section made to appear at the fine pitch can be made. It is found that the distribution of the second phase such as precipitates dispersed on the nanoscale in SEM can be observed by SEM, so that the three-dimensional distribution of the second phase in the surface layer of the steel material can be grasped by integrating the obtained observation results. The present invention has been completed.

That is, the gist of the present invention is as follows.
(1) In observing the three-dimensional distribution of the second phase in the surface layer portion of the steel material, after determining the observation surface in the surface layer portion, the surface processed by the focused ion beam is parallel to the observation surface. The process of processing the observation surface and the process of observing the observation surface after processing by a scanning electron microscope are repeatedly performed, and each observation result is integrated to grasp the three-dimensional distribution of the second phase in the surface layer portion. A method for observing the surface layer of steel.

(2) The method according to (1), wherein the steel material is a hot-dip galvanized steel sheet, and the second phase is an oxide.

(3) After manufacturing a steel material under predetermined manufacturing conditions, and then grasping the three-dimensional distribution of the second phase in the surface layer portion of the manufactured steel material by the method described in (1) or (2), The method for manufacturing a steel material, wherein the predetermined manufacturing condition of the steel material is corrected based on the grasped three-dimensional distribution of the second phase.

According to the method for observing the surface layer portion of the steel material of the present invention, it is possible to accurately grasp the three-dimensional distribution of the second phase in the surface layer portion of the steel material.
Moreover, according to the manufacturing method of the steel materials of this invention, since the three-dimensional distribution of the 2nd phase in steel materials can be grasped | ascertained, steel materials can be manufactured by correcting manufacturing conditions appropriately.

It is a flowchart of the observation method of the surface layer part of the steel materials which concern on this invention. It is a flowchart of the manufacturing method of the steel materials which concern on this invention. It is a SEM image of the depth direction cross section of the surface layer part of a hot-dip galvanized steel plate. It is a part of several SEM image of the depth direction cross section of the surface layer part of a hot dip galvanized steel plate. It is a figure which shows the three-dimensional distribution of the oxide in the surface layer part of a hot dip galvanized steel plate.

(Method for observing the surface layer of steel)
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a flowchart of a method for observing a surface layer portion of a steel material according to the present invention. First, in step S1, an observation surface in the surface layer portion of the steel material is determined. The steel material to be used in the present invention is particularly a galvanized steel sheet, and grasps the three-dimensional distribution of precipitates such as oxides dispersed in the surface layer portion thereof, that is, in the vicinity of the interface between the base steel sheet and the plating layer. As described above, the unplated portion of the surface layer portion of the plated steel sheet is caused by the Si or Si—Mn-based oxide dispersed in the interface between the base steel sheet and the plated layer or in the plated layer. Therefore, for example, when an oxide dispersed in the vicinity of the interface between the plating layer and the steel plate in the plated steel plate is to be observed and the three-dimensional distribution is examined, first, the position of the interface between the plating layer and the steel plate is visually or optical microscope To determine an observation surface of an appropriate size including the interface between the plating layer and the steel plate. In the present invention, the observation surface can be arbitrarily determined, for example, a surface perpendicular to the plate thickness direction of the plated steel plate, or a depth direction cross section of the plated steel plate (that is, the plate thickness direction). (Cross section). Usually, this observation plane is a plane parallel to the thickness direction, but it may be a cross section that is inclined with respect to the thickness direction. Although this observation surface can be revealed by FIB or can be revealed by appropriate cutting and polishing treatment, it is preferable to perform processing by FIB in consideration of processing accuracy and cleanliness. By observing the observation surface in the surface layer portion of the steel material thus revealed by SEM, the three-dimensional distribution of the second phase such as oxide in the surface layer portion of the steel material is grasped.

  In addition, in the subsequent step S5, since the processing width of the observation surface that can be performed by FIB is several tens of μm or less, it can be processed in advance to some extent by general cutting and polishing processing. In this case, the surface of an appropriate size including the observation portion is subjected to surface protection by vapor deposition or the like, and the observation surface is revealed by polishing. By this pre-process, the processing required when performing continuous processing and observation using FIB and SEM in step S5 described later can be largely omitted, and a wider area can be observed in the same processing time. it can.

  In the vicinity of the observation surface thus determined, an appropriate mark is given by a pen, a Vickers indentation, an inscription, or the like so that the observation surface can be confirmed later by SEM.

  Next, in step S2, SEM observation conditions are determined. The SEM observation conditions are specifically the acceleration voltage of electrons, the amount of current, the type of detector used, and the like. The observation conditions determined here are based on the premise that the observation surface processed by the FIB is observed in a later step. That is, in general SEM observation, the mirror-polished surface is etched with a suitable corrosive liquid, and the structure is specified based on the minute irregularities formed on the surface. In many cases, the second phase such as an oxide cannot be preserved. Therefore, in the present invention, the conditions of SEM observation generally performed cannot always be applied as they are.

  On the other hand, since the observation surface in the present invention is a surface processed by FIB, it is very smooth, and it is difficult to specify the structure of the second phase from the unevenness of the surface by the above general method. However, secondary electrons used for SEM observation are superposed by (1) atomic number information and (3) crystal orientation information in addition to (1) unevenness information on the surface of the observation region. Therefore, in the present invention, the presence of the second phase such as an oxide on the observation surface is specified using these combined effects.

Furthermore, since the atomic number information and the crystal orientation information described above can be emphasized by using the backscattered electron image, the secondary electrons and backscattered electrons can be used properly according to the second phase to be observed. Phase identification can be performed more easily.
At that time, secondary electrons are observed using a secondary electron detector, and reflected electrons sensitive to crystal orientation and average atomic number are observed using a reflected electron detector. In general, the backscattered electron detector is often inserted between a sample and a lens, and may not be compatible with sequential processing by FIB, and is used according to the situation.

  In addition, about the base steel plate of the surface layer part of a plated steel plate, and the oxide in a plating layer, it can fully specify only by a secondary electron, without using a reflected electron. In this way, even if the surface is a very smooth observation surface processed by FIB, the structure of the second phase can be specified by making the observation conditions appropriate.

  In SEM observation, the depth of information from the observation surface that can be obtained varies depending on the acceleration voltage of the electrons used. That is, when the accelerating voltage is about 15 to 30 kV with emphasis on the resolution of the SEM, incident electrons penetrate deeply below the observation surface, so the ratio of information on the outermost surface to be observed decreases. On the other hand, when the acceleration voltage is as low as 1 kV or less, the information on the outermost surface is emphasized, but the resolution tends to be lowered. In addition, since the accelerating voltage suitable for observation differs depending on the target sample, in the subsequent step S3, the FIB processing step determined from the size of the second phase to be observed is also taken into consideration and dispersed in the surface layer portion. Set an appropriate acceleration voltage to obtain a three-dimensional distribution of the second phase.

  For example, if the object to be observed is an oxide and the size thereof is about several hundreds of nanometers, it is considered sufficient that the processing interval of FIB is about several tens of nanometers. However, under this condition, the acceleration voltage exceeds 10 kV. Therefore, the observation includes the structure information of the next processed surface. Such a situation is particularly noticeable in high-energy backscattered electron images. In general, in the case of steel materials, the acceleration voltage is set to less than 10 kV, preferably several kV, in order to achieve both submicron horizontal resolution and about 10 nm depth resolution. It is desirable to observe the obtained secondary electron image or reflected electron image. When the observation object is an oxide in the surface layer portion of the plated steel sheet, the size is about several tens to several hundreds of nm, and therefore the acceleration voltage is preferably set to about 5 kV.

  Subsequently, in step S3, processing conditions by FIB are determined. Specifically, the processing step in the direction perpendicular to the observation surface by FIB is selected from the approximate size of the second phase to be observed. This processing step regulates how many thickness units the observation surface is processed, but even if it is set to a small value, it is necessary to specify the structure on the processing surface from the relationship with the acceleration voltage described above. Not only cannot be performed correctly, but also the processing time and data processing time greatly increase, leading to a decrease in the throughput of the tissue evaluation. As a result of the inventors' investigation using various materials, it is possible to obtain a plurality of depth direction sectional views from a plurality of depth direction views without reducing the throughput of the tissue evaluation by reducing the average size of the observation target of interest to about 1/10. It was found that the two-dimensional three-dimensional distribution can be fully understood. For example, when the observation target is an oxide in the surface layer portion of a hot-dip galvanized steel sheet, the size is about several tens to several hundreds of nm, and therefore, the processing step is preferably 3 nm to 100 nm.

  Thereafter, in step S4, it is evaluated whether or not a second phase structure such as an oxide in the surface layer portion of the plated steel sheet can be specified under the conditions of SEM and FIB set in steps S2 and S3. Specifically, this evaluation is performed by performing observation of the observation surface by SEM and processing of the observation surface by FIB at least once, preferably a plurality of times, usually 2 to 3 times. Evaluate whether the phase can be identified. As a result, when it is difficult to recognize the second phase or when discontinuity is recognized in the image, return to step S2 to review the observation conditions of the SEM, or review the processing conditions of the FIB in step S3. I do.

Subsequently, in step S5, the processing of processing the observation surface with FIB and the processing of observing the observation surface after processing with SEM are performed under the processing conditions of FIB and the observation conditions of SEM thus set, Save the image. Here, the processing by FIB is performed so that the surface after processing is parallel to the immediately preceding observation surface (previous processing surface). These processes are repeated a predetermined number of times. For example, the processing step by FIB is set to 100 nm, and processing by FIB and observation by SEM are repeated 100 times. Thereby, an area of about 10 μm ³ is observed, and the three-dimensional distribution of the second phase in this area can be obtained.

  Next, in step S6, it is determined whether or not the process of step S5 has been repeated a predetermined number of times. If not, the process returns to step S5, and if it has been repeated, the process proceeds to step S7.

  Subsequently, in step S7, the observation results obtained in step S5 are integrated to grasp the three-dimensional distribution of the second phase in the surface layer portion. At this stage, a predetermined number of images of the observation surface are obtained. For example, when the processing step is 100 nm and the number of repetitions is 100, 100 images are obtained. Here, these images are integrated to obtain a three-dimensional distribution of the second phase in the surface layer portion. This can be performed using appropriate software (for example, Amira 5 manufactured by Visage Imaging), and the two-dimensional distribution of the second phase structure on each observation surface can be visualized three-dimensionally. At that time, if necessary, correction of misalignment between images and adjustment of contrast can be performed automatically or manually.

  Obtaining quantitative information, such as the average size and volume fraction of the second phase tissue, and the distance between any second phases, from the three-dimensional distribution of the second phase in the observation region, finally obtained in this way And can be appropriately reflected in the manufacturing conditions of the manufacturing method of the steel material.

  Thus, according to the present invention, the three-dimensional distribution of the second phase in the surface layer portion of the steel material can be accurately grasped.

(Method for manufacturing steel materials)
By the method for observing the surface layer portion of the steel material of the present invention as described above, the three-dimensional distribution of the second phase in the surface layer portion of the steel material manufactured under predetermined manufacturing conditions is grasped, and the grasped second-phase three-dimensional distribution is obtained. Based on this, the steel material which has a desired property can be manufactured by correct | amending the manufacturing conditions of steel materials appropriately.

  In FIG. 2, the flowchart of the manufacturing method of the steel materials of this invention is shown. First, in step S11, after manufacturing a steel material under a predetermined manufacturing condition, the three-dimensional distribution of the second phase in the surface layer part is grasped by the method for observing the surface layer part of the steel material of the present invention described above. Specifically, in steps S12 to S18, the same processing as steps S1 to S7 shown in FIG. 1 is performed. Thereafter, in step S19, the predetermined manufacturing condition in step S11 is corrected based on the three-dimensional distribution of the second phase in the surface layer portion obtained in steps S12 to S18.

  There are various effects of the structure of the second phase in steel on the material properties. As described above, grasping the three-dimensional distribution of the second phase is important in setting conditions in the oxidation and reduction treatment of the base steel plate, particularly when the steel material is a plated steel plate. In other words, in the plating layer, when the three-dimensional distribution of oxides near the interface with the steel sheet is observed, and when film-like oxides that adversely affect the plating properties are distributed, the oxygen potential during the oxidation strengthening treatment is reduced. The production conditions can be optimized by increasing the internal oxidation and further suppressing the oxidation of Si on the steel surface. Further, when there is a concern about the adverse effect on the mechanical characteristics due to too much internal oxide, the oxygen potential during the oxidation strengthening process may be reduced.

  As specific judgment criteria, the volume ratio (%) of the internal oxide in the plating layer (alloy layer) in the observed region and the aspect ratio of each oxide are obtained. The frequency of occurrence of oxides ((number of film-like oxides / number of total oxides) × 100 (%)) can be used as an index.

As a result of intensive studies by the inventors, in a normal galvanized steel sheet, the volume ratio of the internal oxide in the vicinity of the interface between the steel and the plated layer, preferably in the plated layer portion in the range of 5 to 10 μm from the interface, is 2.8%. In the following, it was found that when the occurrence frequency of the film-like oxide is 18% or less, non-plating or the like does not occur and a plated steel sheet having a sufficient thickness and having high mechanical properties can be obtained.
However, these indicators are for galvanized steel sheets. When the type of plating is different, or the observation object is not a plated steel sheet, we want to evaluate the weld strength from the state of precipitates in the welded heat affected zone of the thick steel plate welded part. In such a case, a preliminary experiment is appropriately performed to determine what range is appropriate for the three-dimensional distribution state of the oxide, and the present invention can be applied.

The technical significance of the indicators in the above-described plated steel sheet will be described below.
In the case of steel plate plating, it is usual to perform oxidation strengthening treatment of the steel plate before plating the steel plate, and thereafter anneal in a reducing atmosphere to reduce the iron on the surface to facilitate the plating. During this oxidation strengthening treatment, oxides of Si and Mn are formed. If the oxygen potential is not sufficiently high, Si moves to the steel plate surface and is oxidized on the steel plate surface before Si is oxidized. As a result, the oxide forms a film on the surface of the steel plate and covers the surface of the steel plate, and remains on the surface of the steel plate even after reduction annealing, thereby degrading the plateability. Here, the deterioration of the plating property means not only that the plating does not easily adhere, but also that the alloying is difficult to proceed in the subsequent alloying treatment, and a sufficient thickness of the plating layer cannot be obtained.

  On the other hand, if the oxygen potential is too high during the oxidation strengthening treatment, the amount of Si that becomes oxide increases, and the film on the steel sheet surface is oxidized faster than moving to the steel sheet surface. Although no oxide is produced, a large amount of oxide is produced near the surface of the steel sheet, and the volume ratio of the oxide is increased. Since Fe on the surface of the steel sheet is alloyed by the alloying treatment in the plating process to be applied thereafter, a large amount of oxide existing near the surface of the steel sheet is taken into the plating layer, and when the plated steel sheet is processed, Facilitates plating damage.

  On the other hand, by grasping the three-dimensional distribution of the oxide according to the present invention, the amount and distribution form of the oxide that is the second phase can be grasped. In addition, by observing the state of the manufactured plated steel sheet, better plating conditions can be set.

  That is, when the plating layer in the vicinity of the interface between the steel sheet and the plating layer of the plated steel sheet is observed and there are many film-like oxides in the vicinity of the steel interface, the oxygen potential at the time of oxidation strengthening is increased to promote the formation of internal oxides. Moreover, the temperature at the time of the reductive annealing before plating is lowered, and the oxidation of Si on the steel surface is suppressed. On the other hand, when the volume ratio of the internal oxide in the plating layer is high, the oxygen potential at the time of oxidative strengthening may be adjusted in the direction of decreasing.

  At this time, in order to increase the oxygen potential, for example, the oxygen partial pressure at the time of strengthening the oxidation is increased.

  As described above, the manufacturing conditions can be optimized only after the above-described method of the present invention provides the three-dimensional distribution of the second phase. In this way, it is possible to produce a plated steel sheet that has no unplated portion, a sufficient thickness of the plated layer, and mechanical characteristics.

(Steel No. 1-8)
Hot-dip galvanized steel sheets were manufactured according to various manufacturing methods. That is, first, as the base steel plate of the plated steel plate, the thickness 1 having the components shown in Table 1 and the remainder consisting of Fe and unavoidable impurities and having a main composition of 1.5% Si-2.0% Mn. A 0.0 mm high-strength steel sheet was prepared. A plated steel sheet was manufactured by changing the heat treatment conditions before the hot dip galvanizing process for the base steel sheet. Specifically, the plated steel sheet was manufactured by performing heat treatment under eight conditions shown in Table 2 in which the oxygen partial pressure in the combustion gas during the oxygen strengthening treatment and the reduction annealing temperature were changed (Steel No. 1). ~ 8).

  Steel No. manufactured in this way. The structure of the plating layer formed on the steel plate of 1-8 plated steel plates was evaluated. Therefore, as a sample for evaluation, a sample obtained by sampling a 10 mm square from the surface to a depth part of a thickness of 1/4 at the center in the plate width direction of each plated steel sheet was used, and this sample was introduced into the FIB apparatus. A cross section parallel to the thickness direction of the steel plate was produced by a FIB rough machining beam at an arbitrary location, and the observation surface was determined.

  Next, SEM observation conditions were determined, and secondary electron image observation was performed under conditions of an acceleration voltage of 5 kV and a magnification of 5000 times. The obtained image is shown in FIG. As is apparent from this figure, a particulate or linear characteristic contrast was observed in a wide region extending from the vicinity of the interface between the base steel plate and the plating layer to the inside of the plating layer. When these fine structures were evaluated by X-ray analysis, they were found to be Si or an oxide containing Si and Mn. The minimum width of the linear shape of the oxide was about 100 nm. Thus, since it was confirmed that the fine structure of the oxide that is the second phase could be sufficiently specified by a normal secondary electron image, SEM observation was performed at an acceleration voltage of 5 kV, and processing by FIB was performed at a processing step of 50 nm. went. These SEM observations and FIB processing are repeated 80 times in the cross-sectional direction, especially for the area of height: about 8 μm, width: about 10 μm, depth: about 4 μm, including the area near the interface between the underlying steel plate and the plating layer. And a total of 80 images were acquired. Some of these images are shown in FIG.

The image obtained in this manner was three-dimensionally drawn with a dedicated image processing software (Amira 5 manufactured by Visage Imaging) for the oxide distribution in the region. For each sample, the three-dimensional dispersion state and volume ratio of these oxides in a certain volume were determined.
Specifically, the volume fraction (%) of the internal oxide in the plating layer in the observed region was determined. In addition, it was examined whether the oxide distribution was almost uniformly distributed over the entire plating layer, or whether the oxide distribution was particularly large in a portion of 1 μm from the steel interface. Further, the aspect ratio of each oxide was determined, and those having an aspect ratio of 5 or more were used as film oxides, and the occurrence frequency ((number of film oxides / total number of oxides) × 100 (%)) was examined.
In addition, when the volume ratio of the internal oxide in the plating layer in the observed region exceeds 2.8%, “bending workability is poor”, and when the frequency of the film-like oxide exceeds 18%, “plating is difficult”. As predicted the actual evaluation.

Furthermore, as an actual evaluation of the plated steel sheet, the plating at the ridge line when 45 ° bending is performed according to JIS Z 2248 as the alloy layer thickness after alloying treatment as the ease of plating and the workability The number of damage was measured. The alloy layer thickness was measured at 20 arbitrary locations by SEM observation of the cross section, and was averaged. A case where the plating layer thickness was 6 μm or more and the number of plating damages during 45 ° bending was less than 10 was determined as a suitable steel plate. The obtained results are shown in Table 2. As an example, steel No. 3 shows the three-dimensional distribution of the oxide in No. 3. In this figure, the colored portion inside the rectangular parallelepiped frame is an oxide. Further, the upper surface of the rectangular parallelepiped corresponds to the upper surface (surface) of the plated layer of the plated steel sheet, but when performing the FIB processing, a protective vapor deposition layer is formed on the surface of the plated layer.

Steel No. In 1 and 2, it was predicted that the occurrence of film-like oxide was high and plating was difficult to occur, but the thickness of the alloy layer was actually too thin and judged to be incompatible.
On the other hand, Steel No. In 7 and 8, the internal oxide was uniformly distributed, but its volume fraction was high and bending workability was predicted to be poor, but in fact, the number of cracks after bending was large and was judged to be incompatible. .
Steel No. In 3-6, the occurrence frequency of the film-like oxide and the state of the internal oxide were within the appropriate ranges, and it was predicted that the ease of plating and bending workability were good, but the actual evaluation results were also It was good.
Furthermore, steel no. Steel No. 1 has a higher oxygen potential during oxidation strengthening than that of 1 and 2 and a lower temperature during reduction annealing. In Nos. 3-6, Steel No. The occurrence frequency of film-like oxide was not so high as in 1 and 2, and the ease of plating (alloy layer thickness) was also good. Steel No. 3-6 are steel No.3. The oxygen potential during oxidative strengthening is lower than that of steels 7 and 8, and steel no. The volume ratio of the internal oxide was not as high as 7 and 8, and the bending properties were also good.
From the above, it is possible to predict the actual characteristics of the plated steel sheet by examining the three-dimensional distribution state of the oxide in the plated layer of the manufactured plated steel sheet, and film oxidation under certain manufacturing conditions When the frequency of occurrence is high, the oxygen potential can be increased and the temperature during reduction annealing can be decreased to find an appropriate condition. When the volume fraction of the internal oxide is high, the oxygen potential during oxidation strengthening can be found. It was found that the proper condition can be found by lowering the value of.

(Comparative example)
For each of the plated steel plates (steel Nos. 1, 3 and 7) produced in the above examples, one SEM observation sample was prepared in the same manner as in the above examples, and each sample was particularly important. SEM observation was performed on a region having a height of about 8 μm and a width of about 10 μm, including the vicinity of the interface between the base steel plate and the plating layer. However, SEM observation is performed once for one sample. In a two-dimensional image obtained by SEM observation, an oxide having an area ratio of 5 or more and an aspect ratio of 5 or more is used as a film-like oxide, and its occurrence frequency ( (Number of film-like oxides / number of total oxides) × 100 (%)) was evaluated. The obtained results are shown in Table 3.
As shown in Table 3, there is no correlation between the area ratio of the internal oxide and the occurrence frequency of the film-like oxide and the actual evaluation of the plated steel sheet, and there is no correlation with the manufacturing conditions. As described above, it has been found that it is difficult to predict the evaluation of the actual plated steel sheet and to optimize the conditions from the two-dimensional observation.

Claims (3)

  In observing the three-dimensional distribution of the second phase in the surface layer portion of the steel material, after observing the observation surface in the surface layer portion, the observation surface is adjusted so that the surface processed by the focused ion beam is parallel to the observation surface. A process of processing and a process of observing the processed observation surface with a scanning electron microscope are repeatedly performed, and each observation result is integrated to grasp the three-dimensional distribution of the second phase in the surface layer portion. The observation method of the surface layer part of steel materials.   The method according to claim 1, wherein the steel material is a hot-dip galvanized steel sheet, and the second phase is an oxide.   The steel material is manufactured under predetermined manufacturing conditions, and then the second phase obtained after grasping the three-dimensional distribution of the second phase in the surface layer portion of the steel material produced by the method according to claim 1 or 2. A method for manufacturing a steel material, wherein the predetermined manufacturing conditions for the steel material are corrected on the basis of the three-dimensional distribution.