JP6165701B2 - Depth prediction method for cracks caused by delayed fracture at the cut end of steel plate. - Google Patents

Description

  The present invention relates to a method for predicting the depth of a crack generated by delayed fracture at a cut end of a steel plate.

  In recent years, in order to achieve both weight reduction and collision safety of automobiles, higher strength is required for automobile steel sheets used for frame parts. However, the adoption of high-strength steel plates as automotive steel plates is a concern for delayed fracture, which is an obstacle to high strength.

  Here, delayed fracture is a phenomenon in which steel breaks brittlely after a certain period of time in a state where a static load is applied, and is considered to be caused by hydrogen that has entered the steel. It has also been reported that delayed fracture is promoted when plastic strain is introduced into the steel sheet.

  In particular, since large plastic strain is introduced into the cut end of the thin steel plate due to shearing, the delayed fracture resistance is inferior to that of the unstrained portion that is not affected by shearing. It is said that delayed fracture is likely to occur from the end. If a delayed fracture occurs at the cut end in an actual use environment and grows into a large crack, the strength of the member may be degraded, leading to a serious accident. Therefore, establishment of the method of estimating the depth of the crack which generate | occur | produces by the delayed fracture in the cutting edge part of the steel plate in an actual use environment is desired.

  As a test for evaluating delayed fracture characteristics in an actual environment, there is an atmospheric exposure test in which exposure is performed in an atmospheric environment and whether or not delayed fracture occurs and the time until the occurrence is used as an index (Non-Patent Document 1). However, the atmospheric exposure test has a very long test period of several months to several years, and is not suitable as an industrial characterization test.

  In addition, there is an immersion test in an acidic solution as a known accelerated test method. For example, in hydrochloric acid immersion as described in Non-Patent Document 2, the corrosion rate may be significantly higher than the actual environment. Results may differ from test results at.

Junichiro Kinugasa, 4 others, “Evaluation of delayed fracture resistance of high-strength steel sheets (1470 MPa class steel sheet, atmospheric exposure 48 months evaluation results)”, Materials and Processes, Japan Iron and Steel Institute, 2009, Vol. 22, No. 1, p. 596 Yuki Taji, 4 others, “Method for evaluating hydrogen embrittlement resistance of high strength thin steel sheet”, Iron and Steel, Japan Iron and Steel Institute, 2009, Vol. 95, No. 12, p. 887-894

  Accordingly, an object of the present invention is to provide a method capable of accurately predicting the depth of a crack caused by delayed fracture at a cut end of a steel plate in a short time.

The method for predicting the depth of a crack caused by delayed fracture at the steel sheet cutting end according to the first invention of the present invention is as follows.
For a steel sheet having a structure in which the tensile strength is 1000 MPa or more and the total area ratio of martensite and / or bainite is 80% or more, the depth of cracks caused by delayed fracture at the cut end of the steel sheet is predicted. A method,
With respect to the thickness cross section perpendicular to the cut end surface of the steel sheet, by performing EBSP measurement at the unstrained part, the unstrained part crystal orientation data acquisition step for obtaining crystal orientation data at the unstrained part,
A cutting edge crystal orientation data acquisition step for obtaining crystal orientation data at the cutting edge by performing EBSP measurement at the cutting edge with respect to the plate thickness cross section,
From the crystal orientation data in the unstrained part, a non-strained part KAM value calculating step for calculating an average KAM (Kernel Average Misoration) value K0 in the unstrained part,
From the crystal orientation data at the cut end, a cut end KAM value calculating step for calculating an average KAM value K1 in a predetermined measurement region for each fixed distance from the cut end face toward the facing end face direction;
The average KAM value K0 in the unstrained part is compared with the average KAM value K1 in the predetermined measurement area at the fixed distance at the cut end, and K1 is more than K0 [measurement interval of crystal orientation data (unit: μm). ) × 2] out of the predetermined measurement region that satisfies the condition that it is greater than or equal to °, a crack depth determination step for determining the distance from the cut end surface to the farthest predetermined measurement region as the crack depth;
It is characterized by having.

The method for predicting the depth of cracks caused by delayed fracture at the steel sheet cutting edge according to the second invention of the present invention is as follows.
In the first invention,
In the strain-free crystal orientation data acquisition step,
In the unstrained portion, an average KAM value K0 is obtained by arithmetically averaging a plurality of KAM values calculated from crystal orientation data measured at each measurement point in a measurement region of 100 μm ² or more, and
In the cutting end KAM value calculating step,
In using the crystal orientation data obtained from the predetermined measurement region having a width of 0.5 to 2 μm in the opposite end face direction at the constant distance of 5 to 75 μm from the cut end face at the cut end, The predetermined measurement area is divided into a plurality of measurement sections having a thickness of 20 to 300 μm in the thickness direction, and the KAM values at the respective measurement points in the respective measurement sections of the plurality of measurement sections are arithmetically averaged to obtain the respective measurement sections. The average KAM value is obtained, and the largest of the average KAM values in the respective measurement sections is set as the average KAM value K1 in the predetermined measurement region.

  According to the present invention, by using the orientation difference (KAM value) calculated from the crystal orientation data measured at each of the cut end portion and the unstrained portion of the steel sheet, by evaluating the amount of plastic strain introduced at the time of cutting, It has become possible to provide a method for accurately predicting the depth of a crack generated by delayed fracture at the cut end of a steel plate in a short time.

It is a flowchart for demonstrating the structure of the prediction method which concerns on embodiment of this invention. It is a graph which shows typically the relation between the distance from a cut end face, and a local average KAM value. It is a front view which shows the measurement site | part by EBSP of the steel plate which has a cutting | disconnection edge part in 1st Embodiment. It is a front view which shows the measurement site | part by EBSP of the steel plate which has a cutting | disconnection edge part in 2nd Embodiment.

  Hereinafter, the present invention will be described in more detail with reference to the drawings.

[Applicable steel plate of the present invention]
The target steel plate to which the prediction method according to the present invention is applied is a “steel plate having a structure in which the tensile strength is 1000 MPa or more and the total area ratio of martensite and / or bainite is 80% or more”.

  Delayed fracture is a phenomenon that occurs mainly in high-strength steel. For steel sheets with a tensile strength of less than 1000 MPa, the crack depth estimated by the prediction method according to the present invention and exposure under atmospheric conditions The crack depth obtained as a result of the test may vary.

  Moreover, the structure of the steel sheet having a tensile strength of 1000 MPa or more is mainly composed of hard martensite or bainite, but the structure corresponds to the structure when the ratio of the second phase other than the main phase becomes too large. Due to the very micro plastic strain distribution, the crack depth estimated (predicted) by the prediction method according to the present invention may differ from the crack depth obtained as a result of the exposure test in the atmospheric environment. is there.

  Therefore, the steel sheet to which the prediction method according to the present invention is applied is “a steel sheet having a structure in which the tensile strength is 1000 MPa or more and the total area ratio of martensite and / or bainite is 80% or more”.

  In addition, although it does not specifically limit about the plate | board thickness of the said application target steel plate, It is especially preferable to apply the prediction method which concerns on this invention to the thin steel plate of plate | board thickness less than 3 mm.

[Main Configuration of the Present Invention]
As shown in FIG. 1, the prediction method according to the present invention includes a strain-free portion crystal orientation data acquisition step (step S1) for acquiring crystal orientation data in an unstrained portion by EBSP measurement, and a crystal at a cut end by EBSP measurement. Cutting end crystal orientation data acquiring step (step S2) for acquiring orientation data, unstrained portion KAM value calculating step (step S3) for calculating average KAM value K0 from crystal orientation data in the unstrained portion, and cutting end Cutting end KAM value calculating step (step S4) for calculating an average KAM value K1 for each fixed distance from the cutting end, and K1 ≧ K0 + [crystal orientation data measurement interval (unit: μm) × 2 And a crack depth determining step (step S5) for determining the maximum distance from the cut end surface satisfying the angle as the crack depth as a main component. That.

  Here, either the unstrained part crystal orientation acquisition step (step S1) or the cut end crystal orientation acquisition step (step S2) may be performed first. In addition, either the undistorted portion KAM value calculating step (step S3) or the cut end KAM value calculating step (step S4) may be performed first. Hereinafter, each process is demonstrated in detail based on embodiment.

[First Embodiment]
(1) Unstrained part crystal orientation data acquisition step This step obtains crystal orientation data in the unstrained part by performing EBSP measurement at the unstrained part on the thickness cross section orthogonal to the cut end face of the steel sheet. It is a process.

  The cut end of the steel sheet has a plastic strain distribution. That is, as schematically shown in FIG. 2, the plastic strain amount (local average KAM value) increases as the distance from the cut end increases, and decreases as the distance from the cut end increases. In this invention, since it is necessary to evaluate this plastic strain distribution quantitatively, EBSP measurement is performed with respect to the plate | board thickness cross section orthogonal to the cutting end surface of a steel plate.

  Here, EBSP is a Kikuchi pattern (also referred to as “Kikuchi Line”) obtained from reflected electrons generated when an electron beam is incident on the surface of a test piece, and this Kikuchi pattern is analyzed. Thus, the crystal orientation at the electron beam incident position can be determined. The Kikuchi pattern (Kikuchi line) is a pattern that appears behind the electron diffraction pattern as a pair of black and white parallel lines or strips or arrays when the electron beam hitting the crystal is scattered and diffracted. Point to.

  The determination of crystal orientation by EBSP is to identify the steel structure in the thickness direction, which is the same in ordinary microscopic observations but with different crystal orientation differences, by (1) color difference, (2) TEM (transmission) Type electron microscope (Transmission Electron Microscope) makes it possible to measure bulk samples, (3) No need to make a thin film sample for observation, (4) Significantly shortens measurement and analysis time There are advantages such as being possible.

  The measurement of the amount of crystal grain distortion by EBSP is preferably performed by structural evaluation using an FE-SEM (Field Emission-Scanning Electron Microscope) equipped with an EBSP detector. The crystal orientation data on the surface of the test piece can be analyzed by scanning the surface of the test piece two-dimensionally with the FE-SEM and measuring the crystal orientation at every predetermined pitch. In addition, as a FE-SEM provided with the EBSP detector, for example, “JEOL field emission scanning electron microscope JSM-6500F” can be used.

As the measurement area B1 by EBSP in the unstrained portion B, as shown in FIGS. 3 and 4, for example, at a position separated from the cut end face 11 by 10 mm or more, ¼ of the thickness t of the steel plate (test piece) 1 is obtained. A constant region centered at the depth position (t / 4) is defined as a measurement region B1, and the area of the measurement region B1 is preferably 100 μm ² or more.

Here, the “position away from the cut end face 11 by 10 mm or more” is for measurement at a position where the influence of the plastic strain introduced into the cut end A can be ignored. In addition, “1/4 depth position of the thickness t” means that the average component composition and structure avoiding the surface portion and the center of the plate thickness where segregation of components or a special structure may exist. This is because the measurement is performed at the middle portion having a thickness. Further, “the area of the constant region B1 is 100 μm ² or more” is to obtain representative crystal orientation data in the unstrained portion B by measuring at a large number of measurement points in a sufficiently wide region. Because. A more preferable area of the constant region B is 400 μm ² or more, and a particularly preferable area of the constant region B is 1000 μm ² or more.

  The crystal orientation data measuring means will be specifically described below.

  First, a steel plate having a cut end face is cut in the thickness direction orthogonal to the cut end face, and polished to make the cut face a measurement surface, but electrolytic polishing may be performed to prevent the influence of the structure change due to polishing. preferable.

  Next, a test piece is set in the lens barrel of the FE-SEM, the measurement site is irradiated with an electron beam, and EBSP is projected on the screen. This is photographed with a high-sensitivity camera and captured as image data into a computer. Then, EBSP image analysis is performed, and the orientation of the recognized crystal system is determined by comparison with a simulation pattern using a known crystal system (FCC: face-centered cubic lattice, BCC: body-centered cubic lattice). As software used for analysis, for example, “OIM (Orientation Imaging Microscopy) Analysis 6 × 64 manufactured by EDAX-TSL” can be used. In addition, it is preferable to analyze by excluding data of CI ≦ 0.1, which has extremely low reliability in determining the azimuth angle including grain boundaries.

(2) Cutting Edge Crystal Orientation Data Acquisition Step Next, as the measurement region A1 by EBSP at the cutting edge A, for example, as shown in FIG. A range from (for example, 40 μm) × the cut end surface 11 to a predetermined distance in the opposite end surface direction (not shown) (for example, 0 to 300 μm) may be selected as the measurement region A1 (see Invention Examples 1 and 3 in Examples described later). This makes it possible to accurately estimate (predict) the crack depth with a short time measurement without carrying out an atmospheric exposure test.

  As the means for measuring crystal orientation data, the same means as described in the strain-free crystal orientation data acquisition step may be used.

(3) Unstrained part KAM value calculation step From the crystal orientation data in the unstrained part B, an average KAM (Kernel Average Misoration) value K0 in the unstrained part B is calculated. The average KAM value K0 in the unstrained portion B can be obtained by arithmetically averaging the KAM values at all measurement points in the measurement region B1.

  As evaluation of orientation difference of crystal grains by EBSP, local orientation difference can be quantitatively evaluated by using KAM (Kernel Average Misorientation). KAM indicates a strain distribution based on a local azimuth change, and is represented by an average value of azimuth differences between six adjacent pixels at each measurement point. A minute angle change between pixels can be considered to correspond to a dislocation density determined by the amount of plastic strain introduced into the region. Therefore, it is possible to evaluate the plastic strain distribution based on the local orientation change using the KAM value.

  Here, if there is an orientation difference exceeding 5 ° between adjacent measurement points, the orientation difference is considered as a subgrain boundary or cell wall and excluded from the calculation of KAM. Therefore, the analysis was performed considering only the orientation difference of 5 ° or less as the orientation fluctuation in the crystal grains.

(4) Cutting end KAM value calculating step Next, in the same manner as the unstrained portion KAM value calculating step, from the crystal orientation data measured in the measurement region A1 in the cutting end A, the direction from the cutting end surface 11 to the facing end surface An average KAM value K1 is calculated for each predetermined measurement region A11 having a constant width (for example, 1 μm) toward the front.

(5) Crack depth determination step The average KAM value K0 in the unstrained part B and the average KAM value K1 for each predetermined measurement region A11 in the cut end A are compared, and K1 is determined from K0 [measurement interval of crystal orientation data. Of the measurement region A1 that satisfies the condition of t1 (unit: μm) × 2] ° or more, the distance from the cut end surface 11 to the farthest predetermined measurement region A11 is determined as the crack depth.

  As described above, the present inventors have found that the dominant factor of delayed fracture at the cutting edge is the amount of plastic strain, and calculated from crystal orientation data obtained using EBSP as an index for evaluating the amount of plastic strain. The inventors have come up with a method that uses KAM values. For various steel sheets, the average KAM value at the cutting edge is reduced by comparing the crack depth predicted using the prediction method according to the present invention with the crack depth actually measured by the atmospheric exposure test. It was found that delayed fracture occurs in a measurement region that satisfies the condition of [measurement interval of crystal orientation data (unit: μm) × 2] ° or more than the average KAM value of the strained portion. And based on this knowledge, it discovered that a crack depth could be estimated by the distance of the measurement area | region farthest from the cutting end surface in the measurement area | region which satisfy | fills the said conditions, and a cutting end surface.

  The reason why the difference from the average KAM value of the unstrained part depends on the measurement interval of the crystal orientation data is that the absolute value of the KAM value itself changes depending on the measurement interval of the crystal orientation data. In the EBSP measurement in the present invention, it is known that the absolute value of the KAM value is proportional to the measurement interval of crystal orientation data. As shown in FIG. 2, when the measurement interval is 0.1 μm, the measurement area whose average KAM value with respect to the unstrained part is 0.2 ° or more, from the cut end face to the farthest measurement area. It was found that the distance can be estimated as the crack depth. Therefore, as described above, the threshold value of the difference from the average KAM value of the unstrained portion is defined as [measurement interval of crystal orientation data (unit: μm) × 2] °.

[Second Embodiment]
This embodiment is different from the first embodiment in (2) cutting edge crystal orientation data acquisition step and (4) cutting edge KAM value calculation step. The above (1) unstrained portion crystal orientation data acquisition step, (3) unstrained portion KAM value calculation step, and (5) crack depth determination step are the same as those in the first embodiment. Description is omitted.

(2) Cutting end crystal orientation data acquisition step As shown in FIG. 4, the measurement region A11 by EBSP at the cutting end A is from the cutting end surface 11 to a predetermined distance (for example, 300 μm) in the opposite end surface direction (not shown). It is more preferable to set a predetermined measurement area A11 having a thickness t × 0.5 to 2 μm width d in the opposite end face direction at a constant distance L of 5 to 75 μm from the cut end face 11 (invention examples in the examples described later) 2 and 4). As a result, the crack depth can be predicted with high accuracy in a shorter time.

  Here, the constant distance L is set to 5 to 75 μm. If the constant distance L is made too small, the measurement points at the cut end A become too many and the measurement time becomes excessive, while the constant distance L is increased. This is because if it is too large, the crack depth prediction accuracy deteriorates. A more preferable constant distance L is 20 to 70 μm, and a particularly preferable constant distance L is 50 μm.

  The width d is set to 0.5 to 2 μm because if the width d is too small, the crack depth prediction accuracy deteriorates. On the other hand, if the width d is too large, the measurement point at the cut end A is measured. This is because the measurement time becomes excessive due to the excessive amount of. A more preferable width d is 0.7 to 1.5 μm, and a particularly preferable width d is 1.0 μm.

  The measurement interval Δt of crystal orientation data is preferably 0.02 to 0.5 μm. This is because the block interval of martensite and bainite is about 1 μm, so it is difficult to evaluate the orientation difference in the block at a measurement interval exceeding 0.5 μm, while the measurement interval is less than 0.02 μm. This is because it cannot be a practical method because it takes too much time. A more preferable measurement interval Δt is 0.05 to 0.2 μm, and a particularly preferable measurement interval Δt is 0.1 μm.

(4) Cutting edge portion KAM value calculation step In the cutting edge portion A, a predetermined measurement region A11 having a thickness t × a facing edge surface width d at every predetermined distance L from the cutting edge surface 11 toward the facing edge surface (not shown). From the crystal orientation data measured in step 1, the average KAM value K1 in the predetermined measurement region A11 is calculated for each fixed distance L from the cut end surface 11 toward the facing end surface.

  In calculating the average KAM value K1 in the predetermined measurement area A11, as shown in FIG. 4, the predetermined measurement area A11 is divided into a plurality of sections A2i (i = 1 to n; n) having a thickness t1 of 20 to 300 μm. Is the number of divisions), and the KAM value at each measurement point in each of the plurality of sections A2i (i = 1 to n) is arithmetically averaged to obtain the average KAM value K1i (i = 1 to n), and the largest one of the average KAM values K1i (i = 1 to n) in the respective sections A2i (i = 1 to n) is set as the average KAM value K1 in the predetermined measurement area A11. More preferably.

  The amount of plastic strain introduced at the cutting end by shear cutting etc. has a distribution in the plate thickness direction, but the maximum position in the plate thickness direction varies depending on the material of the steel plate, cutting conditions, etc. Correspondingly, the generation position in the plate thickness direction of the crack generated at the cut end due to delayed fracture also changes variously. For this reason, as described above, the average KAM value that is the maximum in the thickness direction for each fixed distance from the cut end surface is used as a representative value, so that the place where cracks are most likely to occur in the thickness section is two-dimensional. Therefore, the prediction accuracy of crack depth can be further improved.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Invention Example]
(Use steel plate)
In this example, two types of steel plates having the tensile strength and steel structure shown in Table 1 below were used. In addition, these steel plates are manufactured by a conventionally well-known method, and the plate | board thickness is 1.0 mm in both.

(Measurement method of area ratio of each phase)
The area ratio of each phase described in Table 1 below was measured as follows. That is, each steel plate as a test material was mirror-polished, its surface was corroded with a 3% nital solution to reveal a metal structure, and then a thickness of 1/4 part was measured using a scanning electron microscope (SEM). A region of approximately 40 μm × 30 μm was observed at a magnification of 2000 times, and among regions observed in black, those containing carbides observed in white inside were defined as bainite, and regions that appeared gray were defined as martensite. In addition, although carbide may exist inside bainite and martensite, the carbide is considered to be a part of the structure (martensite or bainite) containing this, and the area ratio of martensite and bainite is obtained. It was.

(Production of steel plate with cut end face)
The plate-shaped steel plate having a thickness of 1.0 mm was subjected to shear cutting to produce a steel plate having a cut end face. In addition, it cut | disconnected in the magnitude | size of 30 mmWx50mmLx1.0mmt so that a cut end surface might be set to 30 mm width.

(EBSP measurement)
In order to observe the plate thickness cross section perpendicular to the cut end face of the 30 mmW × 50 mmL × 1.0 mmt steel plate, it is cut to 15 mmW × 50 mmL × 1.0 mmt by electric discharge machining and electrolysis is performed on the electric discharge machining cut surface. Polishing was performed, and EBSP measurement was performed at a measurement interval of 0.1 μm. For the EBSP measurement, a “field emission scanning electron microscope JSM-6500F manufactured by JEOL Ltd.” was used.

  And as an unstrained part, EBSP measurement was performed in a 40 μm × 40 μm region centered on a position of a thickness of ¼ at a position 15 mm away from the cut end face in the direction of the facing end face.

  Further, as the cut end portion, in Invention Examples 1 and 3, EBSP measurement was performed in a region having a plate thickness direction of 40 μm centered on the plate thickness center position and a distance of 0 to 300 μm from the cut end surface.

  On the other hand, in the inventive examples 2 and 4 as the cut end portion, an EBSP measurement was performed on a region of plate thickness × 1 μm width at a distance of 50 μm from the cut end surface in a range from the cut end surface to 300 μm.

(Calculation of KAM value)
As analysis software, “EDM-TSL OIM 6 × 64” was used to calculate the KAM value at each measurement point. The analysis was performed excluding data of CI ≦ 0.1, which has extremely low reliability in determining the azimuth angle including grain boundaries. Here, when there was an orientation difference exceeding 5 ° between adjacent measurement points, the orientation difference was considered as a subgrain boundary or a cell wall, and was excluded from the calculation of KAM. Therefore, the analysis was performed considering only the orientation difference of 5 ° or less as the orientation fluctuation in the crystal grains.

  The average KAM value K0 of the unstrained part was obtained by averaging the KAM values of the entire 40 μm × 40 μm measurement visual field.

  At the cutting end, in Invention Examples 1 and 3, the KAM value at each measurement point is arithmetically averaged for each measurement region having a constant width of 1 μm and a plate thickness direction of 40 μm from the cutting end surface, so that the constant distance from the cutting end surface is 1 μm Average KAM value K1 was obtained (corresponding to the first embodiment).

  On the other hand, in Examples 2 and 4, one measurement region (plate thickness × 1 μm width) is divided every 40 μm in the plate thickness direction, and an average KAM value is obtained for each of the divided plural sections. The largest average KAM value is calculated as a representative value of the average KAM value of the entire measurement area, and an average KAM value K1 is obtained for each fixed distance of 50 μm from the cut end surface (corresponding to the second embodiment). ).

(Estimation of crack depth)
Among those that satisfy the condition that the average KAM value K1 for each fixed distance at the cut end is larger than the average KAM value K0 at the unstrained portion by [measurement interval (unit: μm) × 2] ° or more, the maximum KAM value from the cut end face. The distance to the far measurement area was the crack depth.

[Reference example]
As a reference example that serves as a reference for evaluating the applicability of the present invention, using the same two types of steel sheets as in the above invention examples, the steel sheet having a cut end produced by the same method and conditions as in the above invention examples, An air exposure test was conducted. The atmospheric exposure test was performed under the condition of leaving in the atmosphere for 48 months by a method according to JIS Z 2381. After carrying out the atmospheric exposure test, the cut surface cut into the same dimensions as the above invention example by electric discharge machining was mirror-polished, the crack was observed using an optical microscope, and the crack depth was measured.

[Comparative Example]
Moreover, the hydrochloric acid immersion test was implemented with respect to the steel plate which has the cut end part produced by the same method and conditions as the said invention example using the same two types of steel plate as the said invention example as a comparative example. The hydrochloric acid immersion test was performed under the condition of immersing in hydrochloric acid having a pH of 1.0 and a liquid temperature of 25 ° C. for 48 hours. After the hydrochloric acid immersion test, as in the above reference example, the cut surface cut into the same dimensions as the above invention example by electrical discharge machining was mirror-polished and the crack was observed using an optical microscope, and the crack depth was determined. It was measured.

〔Measurement result〕
The measurement results are shown in Table 2 below. In the table, “difference from crack depth in air exposure test” means crack depth Lc ₀ measured in air exposure test and crack depth predicted or measured in each test for the same steel type. This means the absolute value | Lc ₀ −Lc | of the difference from the thickness Lc. In addition, when | Lc0−Lc | is less than 50 μm, it is considered that the crack depth prediction accuracy is a particularly excellent evaluation method, and when | Lc ₀ −Lc | is 50 μm or more and less than 100 μm, As an evaluation method with excellent crack depth prediction accuracy, ○, and when | Lc ₀ -Lc | is 100 μm or more, each is indicated by × as an evaluation method with poor crack depth prediction accuracy, ◎ and ○ were accepted.

  As is clear from the table, the crack depth measured in the hydrochloric acid immersion test, which is one of the conventional accelerated tests, is largely separated from the crack depth measured in the atmospheric exposure test. It was confirmed that the prediction accuracy of the crack depth was greatly improved by using the prediction method according to the present invention.

1 ... Steel plate (test piece)
DESCRIPTION OF SYMBOLS 11 ... Cutting end surface A ... Cutting end A1 ... Measurement area A11 ... Predetermined measurement area B ... Unstrained part B1 ... Measurement area d ... Width L ... Constant distance t ... Plate thickness

Claims (2)

For a steel sheet having a structure in which the tensile strength is 1000 MPa or more and the total area ratio of martensite and / or bainite is 80% or more, the depth of cracks caused by delayed fracture at the cut end of the steel sheet is predicted. A method,
With respect to the thickness cross section perpendicular to the cut end surface of the steel sheet, by performing EBSP measurement at the unstrained part, the unstrained part crystal orientation data acquisition step for obtaining crystal orientation data at the unstrained part,
A cutting edge crystal orientation data acquisition step for obtaining crystal orientation data at the cutting edge by performing EBSP measurement at the cutting edge with respect to the plate thickness cross section,
From the crystal orientation data in the unstrained part, a non-strained part KAM value calculating step for calculating an average KAM (Kernel Average Misoration) value K0 in the unstrained part,
From the crystal orientation data at the cut end, a cut end KAM value calculating step for calculating an average KAM value K1 in a predetermined measurement region for each fixed distance from the cut end face toward the facing end face direction;
The average KAM value K0 in the unstrained part is compared with the average KAM value K1 in the predetermined measurement area at the fixed distance at the cut end, and K1 is more than K0 [measurement interval of crystal orientation data (unit: μm). ) × 2] out of the predetermined measurement region that satisfies the condition that it is greater than or equal to °, a crack depth determination step for determining the distance from the cut end surface to the farthest predetermined measurement region as the crack depth;
It is characterized by having
A method for predicting the depth of cracks caused by delayed fracture at the cut end of a steel plate. In the strain-free crystal orientation data acquisition step,
In the unstrained portion, an average KAM value K0 is obtained by arithmetically averaging a plurality of KAM values calculated from crystal orientation data measured at each measurement point in a measurement region of 100 μm ² or more, and
In the cutting end KAM value calculating step,
In using the crystal orientation data obtained from the predetermined measurement region having a width of 0.5 to 2 μm in the opposite end face direction at the constant distance of 5 to 75 μm from the cut end face at the cut end, The predetermined measurement area is divided into a plurality of measurement sections having a thickness of 20 to 300 μm in the thickness direction, and the KAM values at the respective measurement points in the respective measurement sections of the plurality of measurement sections are arithmetically averaged to obtain the respective measurement sections. The average KAM value at the respective measurement intervals is obtained, and the maximum one of the average KAM values in the respective measurement sections is set as the average KAM value K1 in the predetermined measurement region.
The depth prediction method of the crack which generate | occur | produces by the delayed fracture in the steel plate cutting edge part of Claim 1.

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