JP4916746B2 - Evaluation method of strain in local region of formed ferritic steel sheet - Google Patents

Description

  The present invention relates to a method for measuring and evaluating a strain amount in a local region from a deformation structure of a formed ferritic thin steel sheet.

  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.

  The processing state of the ferritic steel sheet after forming is a very important factor in examining the formability of the steel sheet, such as not only whether the formability of the steel sheet is good but also optimization of the material and processing method of the steel sheet used.

  Until now, the amount of strain in the region where the shape of the steel sheet has changed due to plastic deformation associated with forming has been predicted by a finite element method (FEM) which is one of the numerical simulation methods (for example, Non-Patent Document 1, reference).

  The finite element method virtually divides an object that deforms under external force into a number of small areas (elements), and numerically calculates the overall deformation shape, strain distribution and / or stress distribution from the relationship between the displacement and force within each element By calculating the input parameters, it is possible to investigate the plastic processing state of the molded product under various processing conditions.

  A method of displaying the simulation result by the finite element method on a display screen or the like by a computer is widely used, and the analysis result can be visually recognized.

  However, in the simulation technology of plastic working by the finite element method, the distribution state of stress and strain at each part can be predicted macroscopically by considering the shape of the steel sheet after forming, but it covers the details after actual forming. There is no method for evaluating the plastic working state, that is, the distribution state of stress and strain in the local region, which has been a big problem in the development of steel sheets.

  Furthermore, in the current finite element method, since the object is divided into homogeneous elements and the simulation is performed, it has been difficult to examine the workability of the steel sheet in consideration of the crystal orientation of individual ferrite crystal grains such as texture. .

Plastic working (published in 1980 by Doukabo), page 83

  Thus, in examining the plastic working state of the formed ferritic steel sheet, the development of a technique for evaluating the distribution state of stress and strain in a local region has been required. The present invention fundamentally solves this problem and provides a method for evaluating the strain amount in a local region of a ferritic steel sheet after forming.

  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 crystal 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 where dislocations are accumulated and a region where almost no dislocations exist are formed in the ferrite crystal grains, and this is generally understood as a dislocation cell structure.

  The formation of the dislocation cell structure was regarded as one of the major controlling factors related to the macroscopic work hardening behavior of the steel sheet.

  The research group including the present inventor conducted extensive research on the microstructure of the ferrite steel sheet after plastic deformation, and the dislocation cell structure formed in the ferrite crystal grains inside the steel sheet was lined with directional dislocation cells. It was found that a dislocation cell structure of the form was obtained.

  Furthermore, we investigated the distance between dislocation cells and the amount of strain during plastic deformation in these dislocation cell structures, and found that there was a difference between the average cell spacing and the amount of strain in the dislocation cell structure in which dislocation cells having the same direction were arranged. It turns out that there is a good correspondence.

  FIG. 1 is a diagram schematically showing ferrite crystal grains having a dislocation cell structure in which dislocation cells having the same directionality are arranged.

  Also in the steel sheet actually formed and processed, ferrite crystal grains 1 having a dislocation cell structure as shown in FIG. 1 are observed, and the correspondence between the average value of the cell spacing 4 of the dislocation cells 5 and the strain amount is utilized. Thus, it was found that the strain amount of the steel sheet after forming can be measured in a local region. As a result, the present evaluation method was invented.

  The present invention is configured based on the above-mentioned knowledge, and the main points thereof are as follows.

(1) In a portion subjected to plastic deformation of a ferritic steel sheet mainly composed of a formed ferrite phase (or a ferrite single phase), a dislocation cell wall having a deformation amount of 10% or more and having the same directionality Measure the distance between adjacent dislocation cell walls for ferrite crystal grains with a dislocation cell structure in which linear dislocation cells are arranged, and perform the same measurement for ferrite crystal grains in which the same dislocation cell structure is observed. And measuring the average distance of the dislocation cells as an average distance of the dislocation cell structure, and evaluating the strain amount of the portion, and determining the strain in the local region of the formed ferritic steel sheet Quantity evaluation method.
(2) A dislocation cell in which a dislocation cell wall is lined with linear dislocation cells with a deformation amount of 10% or more in a portion subjected to plastic deformation of a formed ferrite single-phase ferritic steel sheet For ferrite crystal grains composed of a structure, the distance between adjacent dislocation cell walls is measured, and for ferrite crystal grains in which the same dislocation cell structure is observed, the same measurement is performed, and the average value is calculated as the dislocation cell structure. A method for evaluating the amount of strain in a local region of a shaped ferritic steel sheet, wherein the amount of strain at the part is evaluated by measuring the average distance between the dislocation cells as the average spacing of the dislocation cells.

( 3 ) The average distance between dislocation cells of a dislocation cell structure in which dislocation cells having the same direction are arranged in the portion subjected to the plastic deformation is measured by an observation technique using an electron microscope. (1) The evaluation method of the distortion amount in the local region of the ferrite steel plate shape | molded as described in (2) .

  According to the present invention, it is possible to measure the amount of strain in a local region of a ferritic thin steel sheet after forming, and the present invention contributes as an evaluation method in technical development for improving the workability of a ferritic steel sheet. However, it is big.

  Specifically, when applied to a poorly formed ferritic steel sheet, the cause of the defect can be identified through measurement of the amount of strain in the local region according to the present invention. Thereby, it becomes possible to reduce the defect rate in the press forming of the ferritic steel sheet.

  Furthermore, by feeding back information on the amount of distortion in the local region obtained by the above evaluation method to a numerical simulation by the finite element method, it is possible to expect a significant improvement in the prediction accuracy.

  The contents of the present invention will be described in detail below. First, a dislocation cell structure formed in a ferrite crystal grain with plastic deformation will be described.

  FIG. 1 schematically shows ferrite grains having a dislocation cell structure in which dislocation cells having the same direction are arranged. In FIG. 1, the straight line portion shown in the grain boundary 2 of the ferrite crystal 1 is the dislocation cell wall 3, which is a region where dislocations are accumulated at a high density by plastic deformation and exist in a wall shape.

  This dislocation cell wall 3 often exists along one crystal plane of {110} plane, {112} plane, or {123} plane which is a crystal plane. The crystal plane along which these dislocation cell walls 3 are aligned can be determined by analyzing an electron diffraction pattern obtained with a transmission electron microscope.

  In the present invention, a region partitioned by a pair of dislocation cell walls is defined as a dislocation cell 5, and a dislocation cell wall 3 that is observed in a straight line as shown in FIG. It is defined as

  Dislocation cells 5 are formed in the ferrite crystal grains as a result of plastic deformation. The relationship between the direction of stress applied to each ferrite crystal grain and the crystal orientation of the grain, and the slip system that is active in the ferrite crystal 1 It is known that the dislocation cell structure formed in the ferrite crystal 1 differs in relation to the above.

  However, the detailed mechanism has not been clarified, and future research is awaited.

  As a result of detailed studies by the present inventors, among various types of dislocation cell structures, as shown in FIG. 1, a dislocation cell in which dislocation cells 5 having the same directionality in which dislocation cell walls 3 are linear is arranged. It has been confirmed that the dislocation cell interval 4 in the ferrite grain 1 having the structure has a good correspondence with the strain amount of the actually formed steel sheet.

  The present invention has been made on the basis of the above knowledge, and among the dislocation cell structures observed in the part of the formed ferritic steel sheet that has undergone plastic deformation, the dislocation cells are arranged with dislocation cells having the same direction. A ferrite grain having a structure is characterized in that the amount of strain at the portion is evaluated by measuring the average distance between dislocation cells.

  When evaluating the strain amount of the formed ferritic steel sheet subjected to plastic deformation, a tensile test of a known test material is performed in advance, and the correspondence between the average distance between the dislocation cells and the strain amount of the steel sheet It is preferable to evaluate the amount of strain formed in the local region after forming an actual steel sheet using this relationship.

  As a form of the dislocation cell structure, a dislocation cell structure in which dislocation cells having two or more different orientations are arranged, or a dislocation cell constituted by dislocation cells having no directionality in which the dislocation cell wall has a curved shape or the like. The cell structure is not preferable in evaluating the amount of strain from the average interval of dislocation cells at the site subjected to plastic deformation.

  Next, a method for evaluating the strain amount in the local region of the steel sheet after forming from the measurement of the cell spacing in the dislocation cell structure in which dislocation cells having the same directionality are arranged will be described.

  First, sample pieces for mechanical tests such as shear deformation and tensile deformation are prepared from the same steel sheet as that used for forming, and the test is performed while systematically changing the strain amount. Next, for the sample piece after the test, the deformation structure inside the steel plate is investigated, and the cell spacing is measured for the dislocation cell structure in which dislocation cells having the same direction are arranged in the observed dislocation cell structure.

  Specifically, the distance between two cell walls adjacent in parallel is measured. In this cell interval measurement, an image processing technique such as a wavelet method is utilized as necessary.

  The same applies to 30 or more ferrite crystal grains in which a dislocation cell structure in which dislocation cell structures in which dislocation cells having the same direction are arranged is observed is measured with respect to one dislocation cell structure at 20 arbitrary positions. And the average value thereof is taken as the average interval of the dislocation cell structure.

  With respect to the dislocation cells in which the obtained dislocation cells are arranged in the same direction, the correspondence relationship between the average distance between the dislocation cells and the strain amount is summarized in a graph.

  Next, in the actually formed ferritic steel sheet, for the part to be evaluated, the deformation structure similar to that of the above-described test piece is investigated, and the dislocation cell structure in which dislocation cells having the same direction are arranged is dislocated. Determine the value of the average cell spacing, and determine the amount of strain in the local region of the molding site from the relationship between the average distance of the dislocation cells and the amount of strain in the transition cell structure in which dislocation cells with the same direction in the previous specimen are arranged. evaluate.

  In order to measure the cell spacing of the dislocation cell structure, an observation technique using a transmission electron microscope capable of observing the dislocation is required.

  Next, an observation method of the dislocation cell structure using this transmission electron microscope will be described.

  In transmission electron microscopy, a thin specimen for observation having a thickness of 0.1 μm or less is placed in a lens barrel of an electron microscope, and the electron beam transmitted through the thin specimen in the irradiated electron beam is combined. This is a technique for projecting and observing a three-dimensional microstructure formed inside a steel plate as a two-dimensional image.

  Next, a method for producing an observation sample for a transmission electron microscope will be described.

  A sample piece after a mechanical test such as shear deformation or a plate-like minute sample piece having an appropriate size is taken out from a site to be evaluated of a formed steel sheet using a precision cutting machine or the like. Next, the cut micro sample piece is mechanically polished with an abrasive paper such as emery paper, and a sample piece having a thickness of about 100 μm is produced from a 1/2 t region that is the center of the steel plate.

  A disk-shaped sample having a diameter of 3 mmφ is prepared from the foil-shaped sample piece using a dedicated punch. Subsequently, a thin film sample for observation with a transmission electron microscope is produced by electrolytic polishing or ion milling.

  When the shape of the formed steel plate is complicated, it may be difficult to cut a plate-shaped minute sample piece from a site to be evaluated by a cutting machine. In that case, a microsampling method using a focused ion beam apparatus is applied.

  That is, a micro sample piece having a size of 100 μm or less is extracted by a micro sampling method, and is fixed to a support stand dedicated for electron microscope observation. Further, a thin piece sample is prepared from the fine sample piece fixed on the support base by a fine processing technique using a focused ion beam, and is used for transmission electron microscope observation.

  Next, a method for observing the dislocation cell structure in the ferrite crystal grains with a transmission electron microscope will be described. The thin piece sample prepared in the above production procedure is fixed to the tip of a sample holder dedicated to a transmission electron microscope, and is loaded into the sample chamber of the main body of the electron microscope. After charging, an electron beam is generated and the thin film sample is irradiated with the electron beam. The dislocation cell structure in the ferrite crystal grains is observed by observing an electron microscope image projected on the fluorescent screen in the observation chamber.

  The region where the dislocation cell structure exists is a region where the crystal structure of the ferrite is microscopically disturbed. For this reason, in the region where the dislocation cell exists, the electron beam traveling straight is scattered, and if only the transmitted wave is imaged using the objective aperture provided in the electron microscope, the dislocation cell is observed as black contrast. Will be.

  The image magnification in the observation of the dislocation cell structure is preferably in the range of 5000 to 20000 times in consideration of the distance between the dislocation cells. After the observation, the observed dislocation cell structure is photographed and stored on a film dedicated to an electron microscope, or stored as a digital image using a CCD camera.

  As a device for observing the dislocation cell structure, in addition to the transmission electron microscope, a scanning transmission electron microscope for observing a microscope image while scanning a thin piece sample with a beam diameter of about several nanometers may be used. Good.

  It is necessary to observe a clear dislocation cell structure as a desirable deformed structure in accurately evaluating the strain amount in the local region. A dislocation cell structure is observed in a ferrite steel sheet having a deformation amount of 10% or more. However, in a steel sheet having a deformation amount of less than 10%, only dislocations randomly distributed in ferrite crystal grains are observed, and clear dislocations are observed. No cell structure is observed.

  For this reason, in order to accurately evaluate the amount of distortion in the local region from the deformed tissue, it is desirable that the amount of deformation of the processing site to be evaluated is 10% or more.

  Further, not only the ferrite single-phase steel sheet but also the steel sheet containing precipitates and inclusions in the ferrite crystal grains, the amount of strain in the local region can be measured by the evaluation method invented this time.

  Hereinafter, an embodiment of a method for evaluating a strain amount in a local region of a ferritic steel sheet after forming according to the present invention will be described.

From a ferritic steel sheet having the mechanical properties shown in Table 1, simple shear test pieces parallel to the rolling direction were prepared. The size is 30 mm long × 20 mm wide × 1.4 mm thick. Further, a simple shear test was performed on these test pieces by using a simple shear tester and setting the amount of deformation so that the strain amount was 0.1 every 0.1 to 0.6. The nominal equivalent strain rate is 10 ⁻³ / sec. FIG. 2 is a stress-strain curve obtained in a simple shear test.

  Next, as shown in FIG. 3, a simple shear test piece 7 (30 mm length × 20 mm width × 1.4 mm thickness) parallel to the rolling direction 6 is sampled in a cross section parallel to the shearing direction 8. It was. FIG. 3 is a schematic diagram showing a sampling situation.

  First, from a central portion of the region 9 subjected to simple shear deformation, a fine sample piece exposing an observation cross section 10 of a steel sheet having a size of length L30 mm × width W1.4 mm × thickness T0.2 mm using a precision cutting machine or the like. 11 was cut out.

  A sample piece having a thickness of about 100 μm was produced from the cut fine sample piece 8 by mechanical polishing using polishing paper such as emery paper. A strip-shaped sample of 3 mm × 1.4 mm was prepared from the foil-shaped sample piece using a dedicated punch. Subsequently, a thin sample for observation with a transmission electron microscope was prepared by an electropolishing method using a 5% perchloric acid-95% acetic acid solution.

  Next, the thin piece sample prepared according to the above procedure was loaded into a transmission electron microscope with an acceleration voltage of 200 kV, and the dislocation cell structure formed in the ferrite crystal grains was observed in the range of 5000 to 20000 times the image magnification. Carried out. In particular, the bright field method using an objective aperture was used to observe the dislocation cell structure with enhanced contrast.

  FIG. 4 is a diagram showing a typical transmission electron micrograph of a dislocation cell structure in which dislocation cells having the same direction observed in ferrite crystal grains are arranged after simple shear deformation with an equivalent strain of 30%. Further, the observed dislocation cell structure was stored in a personal computer as a digital image using a dedicated CCD camera attached to a transmission electron microscope.

  Next, the image data of the obtained dislocation cell structure is displayed on a monitor dedicated to a personal computer, and using a commercially available microscope image analysis software, the dislocation cell structure in which dislocation cells having the same direction are arranged, The cell spacing was measured and the average value was obtained. There are 20 measurement points. The average spacing of the dislocation cells was measured for 30 ferrite crystal grains, and the average value of the whole was used as data for one test piece.

  Based on the obtained data, the relationship between the average interval of dislocation cells and the amount of strain was summarized in a graph. The graph is shown in FIG.

  Next, assuming the actual forming process, a hat-shaped bending process was performed on the same steel sheet as the ferritic steel sheet subjected to the simple shear test. The test piece had a width of 50 mm and a length of 270 mm, and was processed using a die having a punch width of 78 mm, a punch shoulder R5, and a die shoulder R5. FIG. 6 is a schematic diagram showing the shape of a ferritic steel sheet after bending.

  First, an L-shaped sample piece 14 of about 20 mm × 20 mm was collected from the shoulder 13 of the steel plate 12 formed into a hat shape by bending with a cutting machine. After embedding this L-shaped sample piece 14 in a resin, the cross section of the steel sheet was exposed by emery paper and buffing.

  Thereafter, from the cross-section of the shoulder portion 13, at a position corresponding to a depth of 100 μm from the surface of the steel plate and at a position having a thickness of ½ t, using a focused ion beam device with an acceleration voltage of 30 kV equipped with a microsampling system, A sample piece was extracted, and a cross-sectional thin film sample was prepared with a focused ion beam.

  Furthermore, the deformation structure was investigated using the transmission electron microscope, and the average distance between dislocation cells was determined in the dislocation cell structure in which dislocation cells having the same directionality observed were arranged.

  Similar to the investigation on the simple shear specimen, one dislocation cell structure was measured at 20 locations to determine the average cell spacing. Furthermore, this measurement was performed on 30 ferrite crystal grains in which a dislocation cell structure in which dislocation cells having the same directionality are arranged is observed.

  As a result of the measurement, the average distance between dislocation cells was 0.47 μm at a depth of 100 μm from the surface and 0.51 μm at a thickness of 1/2 t.

  From the correspondence relationship between the average distance between dislocation cells determined earlier and the amount of strain, the amount of strain at the shoulder corresponds to a strain amount of 0.43 at a position of 100 μm depth from the surface, It was found that the thickness position corresponds to a strain amount of 0.32, and the strain amount varies depending on the depth from the surface. It has been found that the strain amount at the position of 1/2 t corresponds well with the strain amount predicted by the finite element method.

  As described above, it was found that the strain amount evaluation method using the deformed structure according to the present invention can sufficiently cope with the strain evaluation in the local region of the actually formed ferrite steel sheet.

  As described above, the present invention greatly contributes as an evaluation method in technology 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 stress-strain curve of the test steel obtained by the simple shear test. It is a figure which shows typically the shape of the sample piece for simple shear tests. It is a figure which shows the transmission electron microscope photograph of the dislocation cell structure observed in the ferritic steel plate by which simple shear deformation was carried out. It is a figure which shows the relationship between the distortion amount obtained by the simple shear test, and the average space | interval of a dislocation cell. It is a figure which shows typically the ferrite steel plate which gave the hat-shaped bending process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ferrite crystal 2 Grain boundary 3 Dislocation cell wall 4 Dislocation cell space | interval 5 Dislocation cell 6 Rolling direction 7 Simple shear test piece 8 Shearing direction 9 Area | region which received simple shear deformation 10 Observation cross-section 11 Micro test piece 12 It shape | molded in hat type Steel plate 13 Shoulder 14 L-shaped test piece L Length of micro test piece W Width of micro test piece T Thickness of micro sample piece

Claims (3)

A dislocation cell structure in which a dislocation cell wall in which the deformation amount is 10% or more and the dislocation cell walls having the same direction are arranged in a straight line in a portion subjected to plastic deformation of a formed ferritic steel sheet mainly composed of a ferrite phase. For the ferrite crystal grains, the distance between adjacent dislocation cell walls is measured, and for the ferrite crystal grains in which the same dislocation cell structure is observed, the same measurement is performed, and the average value is calculated for the dislocation cell structure. A method for evaluating a strain amount in a local region of a formed ferritic steel sheet, wherein the strain amount of the part is evaluated by measuring an average interval of the dislocation cells as an average interval. In the part subjected to plastic deformation of the formed ferrite single-phase ferritic steel sheet, the amount of deformation is 10% or more, and the dislocation cell wall having the same direction has a dislocation cell structure in which linear dislocation cells are arranged. For ferrite crystal grains, the distance between adjacent dislocation cell walls is measured, and for ferrite crystal grains in which the same dislocation cell structure is observed, the same measurement is performed, and the average value is calculated as the average distance between the dislocation cell structures. As an evaluation method of the strain amount in the local region of the formed ferritic steel sheet, the strain amount of the part is evaluated by measuring the average interval of the dislocation cells. At sites that received a plastic deformation of the, the average distance between dislocations cell dislocation cell structure arranged dislocation cells having the same direction, according to claim 1 or, characterized in that measured by observation technique using an electron microscope The evaluation method of the distortion amount in the local region of the formed ferritic steel sheet of 2 .