JP4612585B2 - Method for evaluating deformation structure of ferritic steel sheet - Google Patents

Description

  The present invention relates to a method for quantitatively evaluating a deformed structure formed inside a ferritic steel sheet with deformation by a microstructural analysis technique utilizing a scanning electron microscope.

Ferritic steel sheets are widely used not only for automobiles but also for home appliances and building materials. The workability of this ferritic steel sheet is a very important product index in designing products and structures manufactured using the steel sheet.
In order to form a ferritic steel sheet, it is necessary to apply a certain stress or more to cause plastic deformation. With this plastic deformation, dislocations are formed in the ferrite grains, and strain is introduced. In the early stage of plastic deformation, the dislocations are widely distributed within the grains. However, when the amount of deformation increases, the dislocations multiply and the dislocations are entangled and accumulated. Along with the reaction between the dislocations, a region in which the dislocations are accumulated and a region in which the dislocations hardly exist are formed in the ferrite grains, and this is generally understood as a dislocation cell structure (for example, see Non-Patent Document 1). ). The formation of this dislocation cell structure is considered to be one of the major dominating factors related to the macro plastic deformation behavior of steel sheets, and it is known that the dislocation cell structure varies greatly depending not only on the deformation mode and deformation amount but also on the crystal orientation of the ferrite grains. ing. For this reason, in examining the workability of the ferrite steel sheet, it has been important to elucidate the formation behavior of the dislocation cell structure formed in the ferrite grains with deformation.

  Until now, microscopic techniques such as an optical microscope and an electron microscope have been utilized to evaluate the deformation structure of a ferritic steel sheet. The optical microscope can observe the deformation band or the slip band formed on the steel sheet surface and inside with deformation, but the observation of the dislocation cell structure in the ferrite grain is important for studying the plastic deformation behavior. Measurement of crystal orientation is impossible. On the other hand, with a TEM (transmission electron microscope), which is one of electron microscopes, it is possible to observe a dislocation cell structure that could not be observed with an optical microscope. Furthermore, the crystal orientation of arbitrary ferrite grains can be measured by electron diffraction. However, in this evaluation method using TEM, it is necessary to produce a thin film sample as an observation sample, and it takes a long time to produce it. Furthermore, when the crystal orientation is evaluated from the electron diffraction pattern obtained from the ferrite grains, there is a problem that the analysis takes a long time. Furthermore, in order to efficiently obtain information on the dislocation cell structure and crystal orientation from a large number of ferrite grains required in the evaluation of the deformation structure of a ferritic steel sheet, automatic measurement is difficult with the existing TEM method. It took a lot of time and time to collect data. As described above, the conventional method using an optical microscope or a TEM is not able to quickly collect data from a large number of ferrite grains, which is necessary for evaluating the deformation structure of a ferritic steel sheet. It was a problem.

Concept of material strength (2004, issued by Agne Technology Center), page 240

In order to study the plastic deformation behavior of deformed ferritic steel sheets, it is necessary to develop a deformation structure evaluation method that quickly and abundantly measures information on the dislocation cell structure and crystal orientation for each ferrite grain forming the steel sheet. It was.
An object of the present invention is to provide a method for measuring a deformation structure of a deformed ferritic steel sheet rapidly and in large quantities based on the current state of the prior art.

The research group including the present inventor has conducted extensive research on the evaluation method of the deformation structure of the deformed ferritic steel sheet. As a result of reflection electron images using SEM (scanning electron microscope), the microstructure of the deformed ferritic steel sheet By optimizing the observation conditions, the dislocation cell structure is observed in the ferrite grains, and a binarized image representing the form of the dislocation cell structure can be extracted from the reflected electron image by image processing. I understood. In addition, the dislocation cell structure for each ferrite grain is formed by superimposing a binarized image showing the form of the extracted dislocation cell structure on the orientation map of the same observation field obtained by the EBSP (electron backscatter diffraction) method. It was found that the state can be visualized together with the crystal orientation.
That is, in the same SEM, it is possible to evaluate the form of the dislocation cell structure for each ferrite grain and its crystal orientation, and invented a method for more quickly evaluating the deformation structure of a ferrite steel sheet.

The present invention is configured based on the above-mentioned knowledge, and the main points thereof are as follows.
(1) In the evaluation of the microstructure of the deformed ferritic steel sheet, a binary image showing the form of the dislocation cell structure formed in the ferrite grain is obtained from the reflected electron image inside the steel sheet obtained using SEM. Furthermore, by superimposing a binarized image showing the form of the extracted dislocation cell structure on the orientation map of the same observation field obtained by the EBSP method, the dislocation cell structure of each ferrite grain is extracted. A method for evaluating a deformation structure of a ferritic steel sheet, characterized by visualizing a formation state together with a crystal orientation.
(2) The deformation structure of the ferritic steel sheet according to the above (1), wherein the plastic deformation behavior for each ferrite grain is continuously evaluated by changing the deformation amount applied to the ferritic steel sheet and repeating the measurement. Evaluation methods.

  According to the present invention, it is possible to quickly evaluate the plastic deformation characteristics of a deformed ferritic steel sheet, which has been considered difficult so far, from the viewpoint of the formation behavior of a dislocation cell structure. This greatly contributes as an evaluation method in technology development for improving the workability of steel. Specifically, it becomes possible to evaluate the deformation structure in the ferritic steel sheet after forming and quantitatively measure the amount of strain in the local region. As a result, when a problem of forming failure occurs in the ferritic steel sheet, the cause of the defect can be specified based on the amount of strain in the local region, and the defect rate in press forming of the ferritic steel sheet can be reduced. Further, by feeding back information on the plastic deformation state of each ferrite grain obtained by the above evaluation method to a numerical simulation such as a finite element method, the prediction accuracy can be greatly improved. Due to the significant improvement in prediction accuracy of this finite element method, the press formability of the ferritic steel sheet can be remarkably improved.

The contents of the present invention will be described in detail below.
First, the concept of a dislocation cell structure formed in a ferrite crystal grain with plastic deformation of a ferritic steel sheet will be described. FIG. 1 schematically shows a typical dislocation cell structure observed inside a ferritic steel sheet due to deformation of the ferritic steel sheet. In FIG. 1, the straight line portion shown in the grain boundary 2 of the ferrite crystal 1 is a dislocation cell wall 3, which is a region where dislocations are deposited with a high density due to plastic deformation of the ferrite steel sheet and exist in a wall shape. This dislocation cell wall 3 often exists along one of the {110} plane, {112} plane, or {123} plane that is the crystal plane of the ferrite crystal 1. The crystal plane along which the dislocation cell wall 3 extends can be determined by analyzing an electron diffraction pattern obtained by a transmission electron microscope. Dislocation cells are formed in ferrite crystal grains due to plastic deformation of ferritic steel sheets. The relationship between the direction of stress applied to each ferrite crystal grain and the crystal orientation of the grain, and further activity in ferrite crystal 1 It is known that the dislocation cell structure formed in the ferrite crystal 1 differs in relation to the sliding system.
However, the detailed mechanism is not clear, and future research is awaited.

Next, using a backscattered electron detector mounted on a scanning electron microscope (hereinafter referred to as SEM: Scanning Electron Microscope), a method for observing the dislocation cell structure formed in the ferrite grains with the deformation of the ferrite steel sheet will be explained. To do.
First, the SEM apparatus configuration used for observing the morphology of the dislocation cell structure and analyzing the orientation of ferrite grains will be described.
FIG. 2 is a block diagram of the SEM used in the present invention, which is roughly divided into three parts. One is the SEM body 5, an electron gun 6 for generating an electron image, a focusing lens 8 for focusing the electron beam 7, a deflection coil 9 for scanning the electron beam 7 on the surface of the sample piece 12, Objective lens 10, backscattered electron detector 15 for detecting backscattered electrons generated from the surface of sample piece 12, and EBSP detector for detecting electron backscatter diffraction pattern (hereinafter referred to as EBSP: Electron Back Scatter Diffraction Pattern) from sample piece 12. Consists of 16.
The electron gun 6 irradiates the sample piece 12 with the electron beam 7, observes the morphology of the dislocation cell structure by the reflected electron image from the surface of the sample piece 12, and the crystal orientation of the ferrite grains by EBSP of the sample piece 12 In order to perform the analysis stably, it is desirable to be a thermal Schottky type field emission type electron gun.

  The second is a backscattered electron detector system 20 that captures and displays backscattered electron image data from the surface of the sample piece 12 detected by the backscattered electron detector 15, and further performs image processing. The backscattered electron detector system 20 includes a backscattered electron image control unit 21 for capturing and storing the backscattered electron image data, a backscattered electron image display unit 22 for displaying the backscattered electron image, and backscattered electron image data. Is binarized, and an image processing unit 23 for extracting the form of the dislocation cell structure as a binarized image from the reflected electron image is constituted. In order to detect the reflected electron image with high sensitivity, it is necessary to have a system in which a bias voltage can be applied to the sample piece 12 (the bias voltage is set and applied before the sample piece is irradiated with the electron beam). is there.

The third is a crystal orientation analysis system 17 by the EBSP method. This system includes an EBP control unit 18 for capturing and storing the EBP image data, and an EBP image display unit 19 for displaying the EBP image, and a commercially available system can be used.
The EBSP crystal orientation analysis system and the backscattered electron detector system are connected by a communication cable so that image data can be transmitted and received. The crystal orientation map obtained from the former system And a binarized image showing the form of the dislocation cell structure obtained from the latter system can be processed.

Next, a specific procedure for visualizing the morphology of the dislocation cell structure for each ferrite grain together with the crystal orientation according to the present invention will be described. FIG. 3 illustrates the procedure.
First, a method for producing an observation sample will be described.
First, a sample piece having a size of about 10 mm × 10 mm is prepared from the deformed ferritic steel sheet. The sample piece is mechanically polished using emery paper and then mirror-finished by buffing. Further, the altered layer on the surface of the sample piece formed during the mechanical polishing and buffing is removed by electrolytic polishing. Next, using a hardness tester such as a Vickers hardness tester, an indentation is applied to the sample surface as a mark for specifying the measurement range. As a result, it is possible to observe the dislocation cell structure by the reflected electron image and to measure the orientation of the ferrite grains by the electron backscatter diffraction method in the same region of the sample piece.

  The sample piece prepared in the above procedure is fixed on a support stand dedicated to the SEM, inserted into the sample chamber of the SEM, an electron beam is generated, and the sample piece is irradiated. Next, using the driving mechanism of the sample stage, the indentation given as a mark on the sample surface is found while observing the SEM image, and the measurement range is specified. Thereafter, a backscattered electron detector mounted on the SEM is inserted, and the dislocation cell structure is observed with a backscattered electron image. Specifically, the acceleration voltage is set to 15 to 20 kV, and the working distance (distance between the objective lens and the observation sample) is set to 3 mm or less. Further, the bias voltage applied to the observation sample is set to 0.5 to 1V.

  Under these observation conditions, the dislocation cell walls constituting the dislocation cell structure in the ferrite grains can be clearly observed as black linear contrast. In this way, after image observation of the dislocation cell structure of each ferrite grain, the reflected electron image is collected as image data and stored in a PC dedicated to SEM control. The gradation per pixel in the image data when stored may be 256 gradations. The observation magnification of the dislocation cell structure based on the reflected electron image is preferably about several thousand times to 20,000 times considering the grain size of the ferrite grains and the size of the dislocation cells.

Next, a method for extracting the form of the dislocation cell structure from the reflected electron image obtained by observation by image processing will be described. Commercially available image processing software may be used.
First, a reflected electron image obtained through observation is read out on image processing software and binarized. As a binarization method, there are a method of performing binarization processing by setting one threshold value and a method of binarization method by setting two threshold values. In order to extract the form of the dislocation cell structure from the backscattered electron image, it is necessary to appropriately extract an image that emphasizes the dislocation cell wall observed as a linear black contrast in the dislocation cell structure. For this reason, it is desirable to use a binarization method capable of setting two threshold values that can be binarized with high accuracy for an image to be processed. Furthermore, the image processing software to be used displays the reflected electron image before the binarization process and the reflected electron image after the binarization process on the monitor so as to overlap each other to display the binarized image from the reflected electron image. It is desirable to have a function to properly extract. Note that the threshold value for binarization processing for the reflected electron image may be set once because the range of change in image contrast is constant if the shooting conditions of the reflected electron image are constant. It is possible to binarize the backscattered electron image in a short time.

  Next, a method for measuring crystal orientation with respect to ferrite grains by the EBSP method will be described. The EBSP (Electron Backscattering Diffraction) method is a method of determining an orientation of a measurement point by analyzing an electron backscattering diffraction pattern obtained at each measurement point as needed while scanning an electron beam on an observation sample. According to this method, it is possible to visualize the distribution of crystal orientation of ferrite grains existing on the sample surface as a color map image, and it is possible to perform a fully automatic analysis of crystal orientation in real time. By this EBSP (electron backscattering diffraction) method, the crystal orientation is measured for ferrite grains present in the measurement region where the dislocation cell structure is observed by the reflected electron image.

Specifically, the observed sample is tilted so that the sample observed from the reflected electron image forms an angle of 60 ° to 80 ° with respect to the detector for EBSP measurement using a driving mechanism of the sample stage.
Next, an indentation provided as a mark on the sample surface is found while observing the SEM image. Thereafter, the measurement range for collecting the crystal orientation map is set, and the measurement time per point and the measurement interval are determined. The measurement time per measurement point depends on the processing capacity of the PC of the system used for the measurement, but it may be about 0.05 sec, and 0.03 sec or less is desirable for efficient measurement. The measurement interval depends on the size of the crystal grain of the ferrite steel sheet to be measured. If the size is about several tens of micrometers, it may be about 0.5 to 1 μm.

  By superimposing a binary image showing the form of the dislocation cell structure extracted from the above-described backscattered electron image by image processing on the orientation map of the ferrite grains obtained by the above measurement procedure, The plastic deformation state is visualized together with the crystal orientation.

By the evaluation method of the present invention, a clear dislocation cell structure needs to be observed as a desirable deformation structure when visualizing the plastic deformation state of ferrite grains. In a ferrite steel sheet with a deformation amount of 10% or more, a deformation structure develops and a dislocation cell structure is observed. However, if the deformation amount is less than 10%, only dislocations randomly distributed in the ferrite crystal grains are observed. A dislocation cell structure having a clear direction cannot be observed. For this reason, in order to accurately evaluate the plastic deformation state, it is desirable that the deformation amount of the ferritic steel sheet to be evaluated is 10% or more.
Moreover, the plastic deformation state can be evaluated not only for ferrite single-phase steel sheets but also for steel sheets containing precipitates and inclusions in ferrite crystal grains by the evaluation method invented this time.

  The average distance between dislocation cells in the dislocation cell structure in the ferrite grains shows a good correlation with the amount of strain. In the present invention, since it is possible to statistically analyze the crystal orientation of arbitrary ferrite grains and the average cell spacing of dislocation cell structures formed in the grains, the amount of strain in the local region can be evaluated more precisely. be able to. In addition, we will propose a texture that further improves the workability of ferritic steel sheets by evaluating the workability of each ferrite grain from the relationship between the crystal orientation of the ferrite grains and the amount of strain obtained from the average spacing of the dislocation cells. Is also possible.

From a ferritic steel plate (0.002C-0.003N-0.04Ti: mass%, strength: 270 MPa, plate thickness: 1.2 mm), a tensile test piece of a JIS No. 13 B type rolling method was prepared. Furthermore, a tensile test with a deformation amount of 30% was performed on these test pieces using a tensile test piece. The nominal equivalent strain is 10 ⁻³ / sec. Next, the sample piece for observing a deformation | transformation structure | tissue was produced from the tension test piece.
First, a plate-like sample piece of size L: 10 mm × W: 10 mm was cut out from the center of a tensile test piece that had undergone plastic deformation in the tensile direction by a tensile test. The cut micro sample piece was mechanically polished using emery paper and further mirror finished by buffing. Subsequently, the altered layer formed on the sample surface by mechanical polishing was removed by an electropolishing method using a 5% perchloric acid-95% acetic acid solution. Further, in order to specify a measurement range of 10 μm width × 25 μm length using a Vickers hardness tester, a square pyramid-shaped indentation of 50 μm in size was given to the four corners.

  The sample piece prepared by the above procedure was fixed on a dedicated support base and inserted into the SEM sample chamber. After setting the acceleration voltage to 15 kV, an electron beam was generated and irradiated on the sample piece. Next, using the sample stage drive mechanism, an indentation applied to the sample surface as a mark was found while observing the SEM image, and the measurement range was specified. Thereafter, a backscattered electron detector mounted on the SEM was inserted, the working distance was set to 3 mm, the voltage applied to the observation sample was set to 1 V, and the dislocation cell structure in the ferrite grains was observed. The magnification at the time of observation was 6000 times. After observation, the backscattered electron images were collected as image data with 256 gradations per pixel, and these images were stored in a PC dedicated to SEM control.

  Next, the form of the dislocation cell structure was extracted from the reflected electron image obtained by observation by image processing. The image processing software used is Win ROOF Professional Ver. 5.0 made by Mitani Corporation. The reflected electron image obtained through the above observation was read on image processing software, and binarization processing was performed by setting two threshold values and binarizing. Furthermore, a binary image indicating the form of the dislocation cell structure is displayed by superimposing and displaying the reflected electron image before binarization processing and the reflected electron image after binarization processing on a monitor, and performing binarization processing appropriately. The chemical image was extracted.

  Next, the crystal orientation of ferrite grains was measured by the EBSP method, and the orientation distribution was visualized as a color map image. Specifically, the observation sample was tilted so as to form an angle of 70 ° with respect to the detector for EBSP measurement using a driving mechanism of the sample stage. Next, an indentation applied to the sample surface was found while observing the SEM image. Thereafter, a measurement range was set, and a crystal orientation map was collected with a measurement time per measurement point of 0.05 sec and a measurement interval of 0.5 μm step.

By superimposing a binary image showing the form of the dislocation cell structure extracted from the reflected electron image by image processing on the orientation map (FIG. 5) of the ferrite particles obtained by the above measurement procedure, The result of visualizing the morphology of each dislocation cell structure together with the crystal orientation is shown in FIG.
From the above results, the binarized image indicating the form of the dislocation cell structure formed in the ferrite grain by image processing is extracted from the backscattered electron image inside the steel sheet obtained by using the evaluation method of the present invention, that is, SEM. In addition, the evaluation method of superimposing a binarized image showing the form of the extracted dislocation cell structure on the crystal orientation map of the same field obtained by the EBSP method is for accurately evaluating the plastic deformation state of the ferritic steel sheet. It is clear that this is an effective method.

  As described above, the present invention greatly contributes as an evaluation method in technological development for improving the workability of ferritic steel sheets. Specifically, according to the present invention, it is possible to identify the cause of defects in poorly formed ferrite steel sheets, and to reduce the defect rate in press forming of ferritic steel sheets. Therefore, the present invention has high applicability in steel sheet manufacturing technology.

It is a figure which shows typically the dislocation cell structure observed with a ferrite steel plate. It is a figure which shows the structure of an apparatus. It is a figure which shows the specific procedure of an evaluation method. It is the binarized image which shows the form of the dislocation cell structure extracted from the backscattered electron image. It is a figure which shows the orientation map obtained by the EBSP method. It is the figure which superimposed the binarized image which shows the form of a dislocation cell structure on the orientation map obtained by EBSP method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ferrite crystal 2 Grain boundary 3 Dislocation cell wall 4 Dislocation cell space | interval 5 Scanning electron microscope main body 6 Electron gun 7 Electron beam 8 Focusing lens 9 Deflection coil 10 Objective lens 11 Scan 12 Sample piece 13 Reflected electron 15 Reflected electron detector 16 For EBSP Detector 17 EBSP analysis system 18 EBSP display unit 19 EBSP control unit 20 Backscattered electron detector system 21 Backscattered electron control unit 22 Backscattered electron image display unit 23 Image processing

Claims (2)

  In the evaluation of the microstructure of deformed ferritic steel sheet, the binarized image showing the form of dislocation cell structure formed in the ferrite grain is image-processed from the reflected electron image inside the steel sheet obtained using a scanning electron microscope. And by superimposing a binarized image showing the form of the extracted dislocation cell structure on the crystal orientation map of the same observation field obtained by electron backscatter diffraction method, dislocation cell structure for each ferrite grain A method for evaluating the deformation structure of a ferritic steel sheet, characterized by visualizing the formation state of the steel together with the crystal orientation.   2. The method for evaluating a deformation structure of a ferritic steel sheet according to claim 1, wherein the plastic deformation behavior for each ferrite grain is continuously evaluated by changing the deformation amount applied to the ferritic steel sheet and repeating the measurement. .