JP2010230666A - Method for determining brittle crack propagation inhibiting performance of high-strength steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for determining brittle crack propagation inhibiting performance of high-strength steel plates which greatly improves miniature testing method and its evaluation method, and simply/easily estimates arrest performance of any high-strength steel plate with thickness of 50 mm or more without conducting any large test such as ESSO test or double tensile test to inspect performance of high-strength steel plates. <P>SOLUTION: A complex miniature test includes a process to conduct a drop weight test using surface miniature specimen, and a process to conduct a miniature test for measuring brittle fracture surface ratio or absorbed energy by using an internal miniature specimen. In this complex miniature test, the miniature test is conducted on the surface miniature specimen and the internal miniature specimen with a different way, respectively. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention provides a simple and rational method for stopping brittle crack propagation of high-strength thick steel plates used in the construction of structures that require prevention of large-scale damage or damage caused by the propagation of brittle cracks. The present invention relates to a method for estimating and inspecting the performance of a high-strength thick steel plate.
The present application includes Japanese Patent Application No. 2009-050983 filed in Japan on March 4, 2009, Japanese Patent Application No. 2009-051005 filed in Japan on March 4, 2009, and March 4, 2009. The priority is claimed on the basis of Japanese Patent Application No. 2009-050991 filed in Japan, the contents of which are incorporated herein by reference.

  Unlike tankers, container ships and bulk carriers, which are welded structures, have few partition walls in the hold and a large opening at the top of the ship. On the contrary, the tanker is finely partitioned by the oil tank, and the inner wall and upper deck also have a structure that bears the hull strength.

  On the other hand, container ships have a hull structure with a large opening at the top to improve loading capacity and cargo handling efficiency. For this reason, in a container ship, in order to ensure the strength of a hull structure, it is especially necessary to use a high-strength steel plate as a hull outer plate.

In recent years, container ships have become larger, and large container ships of 6000 to 20000 TEU (Twenty Fee Equivalent Unit) have been built and planned. Accordingly, the steel plate for the outer hull plate, with thickened, and high strength, with a thickness 50 to 100 mm, the yield strength 390 N / mm ² grade, 470N / mm ² class of thick steel plate (steel plate ) Has come to be used.

  The TEU is an index indicating the loading capacity of the container ship, and is displayed as the number of containers when converted into a container having a length of 20 feet.

  As described above, in a thick steel plate used for ships and the like, brittle crack propagation stopping performance (hereinafter sometimes referred to as “arrest performance”) is a very important characteristic in evaluating the safety of the thick steel plate.

  In order to improve the arrest performance, various component compositions, steel materials with manufacturing processes, various large welded structures, and the like have been developed and manufactured. In order to quantitatively evaluate the arrest performance of these newly developed steel materials, the ESSO test (Brittle Crack Propagation Stop Test: the ability to artificially generate a brittle crack in the specimen and stop the brittle crack) Evaluated tests) and large-scale tests such as double tensile tests. For example, in the case of the ESSO test, a large test piece having dimensions of about 500 mm × 500 mm × plate thickness is produced, and a V-notch is formed at the end of the test piece. A temperature gradient is applied to the test piece, and an impact load is applied to the V notch through a wedge to artificially generate a brittle crack. A Kca value (fracture toughness value) is calculated based on the stress applied to the specimen, the temperature at the position where the propagation of the brittle crack is stopped, and the length of the crack.

The test is performed by changing the temperature gradient condition and the load load condition, the relationship between the crack stop temperature and the K value is obtained, and the arrest performance at an arbitrary temperature is evaluated by the Kca value. Further, as an index of arrest performance, a limit temperature (minimum temperature) at which a predetermined Kca can be secured is called a target Kca limit temperature and expressed as TKca. Specifically, for example, the target Kca limit temperature when the target Kca value is 6000 N / mm ^1.5 is expressed as TKca6000.
When a steel plate is used for a steel structure such as a container ship, a design temperature or a minimum use temperature of the steel structure is determined. If TKca6000 measured or estimated about a predetermined steel plate is the temperature below the design temperature, it will evaluate that said steel plate can ensure sufficient arrest performance in this design temperature.

  However, in order to actually measure the Kca value, a large test piece and a large test machine are required, and a lot of work and time are required until a test result is obtained. In particular, a large-scale testing machine capable of applying a tensile load of 1000 tons or more is required for a large-scale test of a thick steel plate having a thickness of 50 mm or more. Therefore, in conventional performance inspection or quality control of steel plates, it is necessary to obtain in advance a correlation between the arrest performance of the entire thick steel plate and a simple small test result using a small test piece taken from the thick steel plate. (See, for example, Patent Document 1 and Non-Patent Documents 1 to 3). Then, based on the preliminary examination results, based on the required value of the small test calculated from the required arrest performance, a small test is performed on a steel plate manufactured at a steelworks, etc. Performance inspection or quality control of steel sheets has been performed by a method for determining whether or not a required value is satisfied.

Non-Patent Document 1 describes the correlation between the arrest performance of a low-temperature steel having a thickness of about 16 mm and a small test. Non-Patent Document 2 describes a simple evaluation method for arrest performance of a steel sheet having a multi-layer structure having a special ultrafine grain structure in the steel sheet surface layer. In Non-Patent Document 3, the arrest performance is simply evaluated from the fracture surface transition temperature obtained in the Charpy impact test of the central part of the plate thickness, 1/4 and 2 mm below the surface, using an extremely thick high strength 790 N / mm grade ² steel plate. A method is described.

JP 2007-302993 A

Steel Society Presentation Summary, CAMP-ISIJ Vol. 4 (1991) -918, “Correlation between Kca and small arrest test method and factors controlling arrest performance” (Examination of arrest performance of steel sheet (5)) Western Shipbuilding Association Bulletin No. 106 (August 2003), P275-280, “Easy Evaluation Method for Arrest Performance of Surface Superfine-Grained Steel Sheet (Part 1) —Establishment of Arrest Performance Estimation Formula” Summary of National Welding Society Conference Lecture, No. 49 (issued on August 25, 1991), P108-109, “Effects of plate thickness and plate thickness direction toughness distribution on the arrestability of extra-thick HT790”

In order to ensure the safety of large container ships, the plate thickness used is 50mm to 100mm, and the yield strength is 390N / mm grade ² , 470N / mm grade ² thick steel plate (thick steel plate) is 4000N at -10 ° C. / from ^{mm 1.5} 6000N ^{/ mm 1.5} about the arrest performance it has been found to be very important. To meet this arrestability request, but steel sheet having a high arrest performance have been developed, it is equipped with all the arrest performance from 4000 N ^{/ mm 1.5} of about 6000 N ^{/ mm 1.5} relative to steel sheet It was not easy. For this reason, it has come to be required to confirm the arrest performance by a steel sheet shipping test at a steelworks. However, as described above, it is difficult to perform a large-scale test such as an ESSO test on all steel plates. In addition, there is a large variation in the correlation between the arrest performance in the large test and the small test result, and taking into account the variation, it is necessary to have extremely strict requirements for small test that cannot be stably manufactured at the steelworks. There wasn't. For this reason, there is a need for a method for simply evaluating the arrest performance with high accuracy using a small test for thick steel plates having a thickness of 50 mm to 100 mm and a yield strength of 390 N / mm grade ² , 470 N / mm grade ^2. I came.
Therefore, the present inventors have developed a method for obtaining the correlation between the arrest performance of the entire thick steel plate and a simple small test result using a small test piece taken from the thick steel plate with much higher accuracy than before. investigated.

  The inventors first manufactured high-strength thick steel plates with a plurality of compositions and production methods, and determined the Kca value of the high-strength thick steel plates by the ESSO test using the methods disclosed in Non-Patent Documents 1 and 2. Moreover, a small test piece was sampled from the above-mentioned thick steel plate, and various small tests (V-notch Charpy impact test, drop weight test, etc.) were performed on the small test piece to determine the characteristic value of the small test piece. Then, the correspondence relationship between the Kca value, which is the result of the large test obtained, and the characteristic value of the small test piece was investigated. As described above, when the correlation between the characteristic value obtained in the small test and the arrest performance is related with the existing method for obtaining the correlation related to the steel plate having a thickness of about 20 mm, the correlation with sufficient accuracy cannot be obtained. There was found.

  Therefore, the present invention greatly improves the small test method and its evaluation method, and is a simple technique for the arrest performance of a high-strength thick steel plate having a thickness of 50 mm or more without performing a large test such as an ESSO test or a double tensile test. One object is to provide a method for inspecting the performance of a high-strength thick steel plate.

  The present inventors diligently studied why the conventional method relating the result of the small test and the arrest performance cannot be applied to the correlation between the arrest performance of the high-strength thick steel plate having a thickness of 50 mm or more and the result of the small test. In particular, we examined methods applicable to a wide range of steel grades and manufacturing methods. As a result, we have obtained the knowledge that the propagation behavior of a brittle crack differs depending on the position in the thickness direction where the brittle crack occurs, and this difference in propagation behavior greatly affects the arrest performance of the entire steel sheet. .

  Based on the above findings, the present inventors investigated in detail the brittle crack propagation behavior in a high-strength thick steel plate having a thickness of 50 mm or more. As a result, multiple small specimens are sampled along the thickness direction so that the difference in crack propagation behavior in the thickness direction is reflected in the evaluation of arrest performance, and the optimum method is selected according to the sampling position. It was found that by performing a small test with, and appropriately combining the test results, the combined result can be obtained with high accuracy with the Kca value obtained in the large test.

  Further, as a result of further detailed investigation, in order that the difference in crack propagation behavior in the plate thickness direction is reflected in the evaluation of arrest performance, at least the steel plate surface layer and the steel plate central portion (plate thickness 1/2) ), And the results of the composite small test conducted by a method suitable for each sampling position have a very good correlation with the K value (fracture toughness value) obtained in the large test. Found.

  Moreover, as a result of investigating the characteristic evaluation method for small test pieces on the steel sheet surface layer, which is an important factor for estimating the arrest performance of high-strength thick steel plates, we found that the optimum method is drop weight test. Moreover, about the small test piece extract | collected from the inside of a steel plate, the drop weight test was not preferable, and it discovered that the small test which measures a brittle fracture surface rate or absorbed energy was optimal. That is, it is desirable to use different small test methods for the small test piece on the surface layer and the small test piece inside.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) A method for discriminating brittle crack propagation stopping performance of a high-strength thick steel plate according to one aspect of the present invention includes a step of performing a large-scale test and a composite small-size test using standard steel; A step of calculating a correlation model between a result of the large test and a result of the composite small test; a step of performing the composite small test using sample steel; and the correlation of the result of the composite small test using the sample steel Substituting into a model and estimating the brittle crack propagation stopping performance of the sample steel, and the composite small test includes: (a) collecting a surface layer small test piece including a steel plate surface layer portion; ) A step of collecting internal small test pieces from one or two or more internal regions not including the steel plate surface layer portion; (c) a step of performing a drop weight test using the surface layer small test pieces; and (d). Using the internal small test piece A step performing a small test that measures sexual fracture rate or absorption energy; wherein said composite compact test is made small test differently respectively the internal small test piece with the surface layer small specimens.

(2) In the method of (1) above, Y is the NDT temperature that is the result of the drop test of the surface small test piece, and X1 is the fracture surface transition temperature that is the result of the small test using the internal small test piece. Alternatively, if the absorption energy transition temperature, a, b, d are coefficients, and the target Kca limit temperature is TKca, the correlation model may be a · Y + b · X1 + d = TKca.

(3) In the method of (2) above, in the step of estimating the brittle crack propagation stopping performance of the sample steel, Y ′ and X1 ′, which are the results of the composite compact test using the sample steel, are used as the correlation model. TKca ′, which is an estimated value of the target Kca limit temperature of the sample steel, by substituting into TKca; and the TKca ′ is compared with TKca, which is the actually measured target Kca limit temperature of the standard steel. And a step of determining that the brittle crack propagation stopping performance of the sample steel is good when ≦ TKca.

(4) In the method of (1), Y is the NDT temperature that is the result of the drop test of the surface small test piece, and X1 is a small test using the internal small test piece taken from the first internal region. Fracture surface transition temperature, X2 is a fracture surface transition temperature, a, b, c, d, which is a result of a small test using the small internal test piece taken from the second internal region, and a target Kca If the limit temperature is TKca, the correlation model may be a · Y + b · X1 + c · X2 + d = TKca.

(5) In the method of (4), in the step of estimating the brittle crack propagation stopping performance of the sample steel, Y ′, X1 ′, and X2 that are the results of the composite small test using the sample steel Calculating TKca ′, which is an estimated value of the target Kca limit temperature of the sample steel by substituting 'in the correlation model; and TKca ′, which is the measured target Kca limit temperature of the standard steel, And, when TKca ′ ≦ TKca, the step of determining that the brittle crack propagation stopping performance of the sample steel is good may be further included.

(6) In the method according to any one of (1) to (5), the small internal test piece is a chevron notch Charpy impact test piece, a V-notch Charpy impact test piece, a sharp notch Charpy impact test piece, or a press notch. It may be any one of a Charpy impact test piece, a pre-cracked Charpy impact test piece, a three-surface sharp notch Charpy impact test piece, and a U-notch Charpy impact test piece.

(7) In the method according to any one of (1) to (5), even if the internal region does not include a plate thickness center portion and includes a position within 5 mm from the plate thickness center portion. Good.

(8) In the method according to any one of (1) to (5), the internal region may be a region including a position having a thickness of 1/4.

(9) In the method according to any one of (1) to (5), a yield strength of the high-strength thick steel plate may be 240 to 600 N / mm ² .

(10) In the method according to any one of (1) to (5), a thickness of the surface small test piece and the internal small test piece may be 10 to 25 mm.

(11) In the method according to any one of (1) to (5), the high-strength thick steel plate may be a steel plate for a large hull.

(12) In the method according to any one of (1) to (5) above, a plate thickness of the high-strength thick steel plate may be 50 mm or more.

(13) In the method according to any one of (1) to (5), in the drop weight test, a crack initiation weld is formed on a surface corresponding to a surface layer side of the sample steel among surfaces of the surface layer small test piece. And a notch may be provided along the thickness direction of the sample steel in the surface of the internal small test piece.

  According to the present invention, when evaluating the arrest performance of a high-strength thick steel plate used for a large-sized welded structure, a large-sized test piece such as a test piece for an ESSO test for each steel piece of each lot manufactured, It is not necessary to use a large-scale destructive testing machine of tons or more Obtain a correlation between the large test result and the small test result by conducting a large test once in advance on the standard sample steel and performing a small test on a small test piece taken from the same standard sample steel. Can do. Thereafter, a small specimen is collected and a small test is performed on the test steels of a plurality of production lots. By applying the correlation obtained in advance to these small test results, it is possible to easily estimate the arrest performance of each test steel that is usually obtained only in a large test. In addition, a drop weight test is used for small test pieces taken from the surface layer of the test steel. On the other hand, a test for measuring a brittle fracture surface ratio or absorbed energy is used for a test piece taken from the inside of the test steel. This improves the accuracy of the arrest performance estimation and can eliminate the large-scale test for each lot. As a result, it is possible to quickly and accurately evaluate whether a high-strength thick steel plate of each lot can prevent a fatal large-scale damage or damage that would destroy a large welded structure by a simple method. . Therefore, the method of the present invention can be effectively applied to quality control at the time of production of a high-strength thick steel plate used for a large welded structure, for example.

It is a figure which shows typically the propagation behavior of the brittle crack which propagates in the steel plate. The propagation behavior of a brittle crack in a thick steel plate having a plate thickness T5 of 50 mm or more is shown. It is a figure which shows typically the propagation behavior of the brittle crack which propagates in the steel plate. The propagation behavior of a brittle crack in a steel plate having a thickness T2 of about 20 mm is shown. It is a figure which shows the position which extract | collects a small test piece from a thick steel plate. It is a figure which shows the position which extract | collects a small test piece from a thick steel plate. It is a figure which shows the position and direction which extract | collect a test piece from a thick steel plate. It is this schematic diagram which shows arrangement | positioning of the testing machine of a drop weight test, and a small test piece. It is this schematic diagram which shows the determination method of the test result of a drop weight test. It is a figure which shows the shape of a small test piece, and shows a falling weight test piece. The numerical value in a figure shows a dimension (unit: mm). It is a figure which shows the shape of a small test piece, and is a schematic side view and front view showing a V-notch Charpy impact test piece. It is a figure which shows the shape of a small test piece, and shows a 1-side sharp notch Charpy impact test piece. It is a figure which shows the shape of a small test piece, and shows a 3 surface sharp notch Charpy impact test piece. The numerical value in a figure shows a dimension (unit: mm). It is a figure which shows the shape of a small test piece, and shows the shape of a chevron notch Charpy impact test piece. 6A to 6E show comparative examples, and are diagrams showing a correlation example between a composite small test using only the V-notch Charpy impact test and the arrest performance measured in the large test. Test pieces taken from various positions were used for the small test. FIG. 6A is a correlation diagram in the case of using either the small test piece 10a or the small test piece 11 shown in FIG. 2B. FIG. 6B is a correlation diagram when the small test piece 8a shown in FIG. 2B is used. FIG. 6C is a correlation diagram when the small test piece 9a shown in FIG. 2B is used. FIG. 6D is a correlation diagram in the case where both the small test pieces 10a and 9a shown in FIG. 2B are used and the obtained small test results are weighted averaged. FIG. 6E is a correlation diagram in the case where both the small test pieces 10a and 8a shown in FIG. 2B are used and the obtained small test results are weighted averaged. 7A to 7E show comparative examples, and are diagrams showing a correlation example between a composite small test using only a chevron notch Charpy impact test and arrest performance measured in a large test. Test pieces taken from various positions were used for the small test. FIG. 7A is a correlation diagram when one of the small test piece 10a and the small test piece 11 shown in FIG. 2B is used. FIG. 7B is a correlation diagram when the small test piece 8a shown in FIG. 2B is used. FIG. 7C is a correlation diagram when the small test piece 9a shown in FIG. 2B is used. FIG. 7D is a correlation diagram in the case where both the small test pieces 10a and 9a shown in FIG. 2B are used and the obtained small test results are weighted averaged. FIG. 7E is a correlation diagram in the case where both the small test pieces 10a and 8a shown in FIG. 2B are used and the obtained small test results are weighted averaged. It is a figure which shows the correlation example of the composite small test result using the method concerning the 1st Embodiment of this invention, and the arrest performance measured by the large sized test. A drop weight test was conducted for the surface small test, a chevron notch Charpy impact test was conducted for the internal small test, and these results were used by weighted averaging. FIG. 8B is a diagram illustrating a correlation example between the composite small test result using the method according to the comparative example and the arrest performance measured in the large test. A drop weight test was conducted as a small surface test, and a drop weight test was conducted as an internal small test, and these results were used by weighted averaging. It is a figure which shows the correlation example of the composite small test result using the method concerning the 2nd Embodiment of this invention, and the arrest performance measured by the large sized test. Internal small test pieces were collected from two locations. A drop weight test was performed as a surface small test, and a chevron notch Charpy impact test was performed as an internal small test, and these three results were used as a weighted average. It is a figure which shows the correlation example of the composite small test result using the method concerning the 1st Embodiment of this invention, and the arrest performance measured by the large sized test. The difference from FIG. 8A is that a V-notch Charpy impact test was performed instead of the chevron notch Charpy impact test in the small internal test. FIGS. 9A and 9B are flowcharts showing a coefficient determination process for estimating the arrest performance of the sample steel from the result of the small test. FIG. 9A shows a process in the case of two-point measurement. FIG. 9B shows a determination process of coefficients added in the case of three-point measurement. It is a schematic diagram of the fracture surface of the drop weight test of surface superfine-grained steel. It is a schematic diagram of the fracture surface of the drop weight test of general steel.

Hereinafter, a method for determining brittle crack propagation stopping performance of a high-strength thick steel plate according to a first embodiment of the present invention will be described with reference to the drawings.
In this specification, the small test refers to a test performed by cutting out a part of the test material prepared as described above, collecting a small test piece, and using the small test piece. As will be described in detail later, various small test pieces may be collected from one place in the test material or from a plurality of places. The small test can also be called a partial test.
In the present embodiment, when estimating the brittle crack propagation stopping performance of a high-strength thick steel plate based on the result of a small test, the small test piece used for the small test was divided in the thickness direction of the high-strength thick steel plate. It is characterized by sampling from one area on the surface and at least one area inside the plurality of areas.

FIG. 1A is a schematic cross-sectional view along the thickness direction of the thick steel plate 7, and schematically shows the propagation behavior of a brittle crack propagating through the steel plate in the direction of the arrow. The vertical direction of the paper is the thickness direction of the thick steel plate 7, and the upper and lower ends indicate the front and back steel plate surfaces. The crack is generated at the end of the steel plate at the left end of the page and propagates in the direction of the arrow.
FIG. 1A shows the propagation behavior of a brittle crack in a thick steel plate 7 having a plate thickness T5 of 50 mm or more, and FIG. 1B shows the propagation behavior of a brittle crack in a steel plate having a plate thickness T2 of about 20 mm.

  First, the difference between the propagation behavior of a brittle crack in a steel plate having a thickness of 50 mm or more and the propagation behavior of a brittle crack in a steel plate having a thickness of about 20 mm will be described.

  So far, the correlation between the brittle crack propagation characteristics and the results of the small test has been studied for a steel sheet having a thickness of about 20 mm. As a result, the propagation mechanism of the brittle crack is considered as follows.

  As shown in FIG. 1B, in the steel plate 6 having a plate thickness of about 20 mm, the crack tip d of the brittle crack propagation surface Zd (formed in the plate thickness direction of the steel plate 6 and reaching the surface) moves in the direction of the arrow. And brittle cracks propagate. Since the brittle crack propagation surface Zd is a continuous surface along the plate thickness direction of the steel plate 6, the result of the small test is not greatly influenced by the sampling position of the small test piece (steel plate surface layer portion, steel plate central portion). This value is a constant value, and this value represents the brittle crack propagation characteristics of a steel sheet having a thickness of about 20 mm.

  However, in a thick steel plate having a thickness of 50 mm or more, a plurality of brittle crack propagation surfaces (Za, Zb, Zc in the figure) are formed in the thickness direction of the thick steel plate 7 as shown in FIG. 1A. Each of the brittle crack propagation surfaces (Za, Zb, Zc) is a continuous surface along the thickness direction of the thick steel plate 7. On the other hand, the brittle crack propagation surfaces exist discontinuously along the thickness direction of the thick steel plate 7. That is, generally, the brittle crack propagation surfaces (Za, Zb, Zc) are included on separate planes that are substantially parallel to the plate thickness direction and do not overlap each other. Then, the crack tips a, b, c move independently in the direction of the arrow, and the brittle crack proceeds.

  That is, as shown in the right diagram of FIG. 1A, there may be a step (see the position surrounded by the dotted line) between the brittle crack propagation surfaces (Za, Zb, Zc). Therefore, when a small test piece is collected from a thick steel plate having a thickness of 50 mm or more and a small test is performed, the test results are greatly dispersed due to the influence of the collection position of the small test piece. After all, the test results of small specimens arbitrarily collected from thick steel plates with a thickness of 50 mm or more are not sufficient to represent the brittle crack propagation characteristics of thick steel plates with a thickness of 50 mm or more by a single test alone. is there.

  Thus, the brittle crack propagation behavior in a thick steel plate having a thickness of 50 mm or more is greatly influenced by the difference in brittle crack propagation characteristics at each position in the thickness direction, and is not uniform in the thickness direction.

  Here, the reason why the crack propagation behavior in the central portion of the plate thickness in the thick steel plate having a thickness of 50 mm or more and the crack propagation behavior in the vicinity of the steel plate surface layer portion will be described.

  The cause of the difference in brittle crack propagation behavior in the thickness direction of thick steel plates with a thickness of 50 mm or more is related to the fact that the crack tip is in a plane strain state inside the plate thickness and in the plane stress state near the surface. To do. That is, since the crack tip is in a plane strain state within the plate thickness of the thick steel plate, the size of the plastic zone formed at the crack tip is larger than that of the crack tip existing near the plate thickness surface. As a result, the resistance to propagation of cracks is reduced, and brittle cracks are likely to progress.

  On the other hand, in the vicinity of the surface of the thick steel plate (surface layer), the crack tip is in a plane stress state, so the dimension of the plastic zone formed at the crack tip is the plastic zone of the crack tip existing inside the plate thickness. As a result, the brittle crack is harder to propagate than inside the plate thickness. For this reason, it is well known that a shear lip is formed in the vicinity of the surface layer portion and contributes greatly to the improvement of brittle crack propagation stopping performance. Thus, a phenomenon completely different from the inside of the plate thickness occurs in the surface layer portion of the steel plate. As a result of examining the optimum test method in the surface layer portion, the present inventors have found that a drop weight test that can directly evaluate the propagation of a brittle crack on the surface is the best method.

  In this way, in a thick steel plate having a thickness of 50 mm or more, the stress-strain field at the crack tip, which is the driving force for propagating a brittle crack, and the spread of the plastic zone at the crack tip differ in the thickness direction. The brittle crack propagation behavior differs in the plate thickness direction.

  Furthermore, in the case of a thick steel plate having a thickness of 50 mm or more, the temperature history in the thickness direction differs during production, and the strain acting on the inside of the steel plate during rolling may also differ in the thickness direction. For this reason, the crystal grain size and texture of the steel sheet structure often differ greatly in the thickness direction.

  Therefore, when a small test piece is collected from a thick steel plate with a thickness of 50 mm or more at any position and a small test is performed, the test results are greatly dispersed due to the influence of the sampling position of the small test piece. It does not represent the brittle crack propagation characteristics of the entire thick steel plate having a thickness of 50 mm or more.

  In the present embodiment, based on the fact that the brittle crack propagation behavior of a thick steel plate having a thickness of 50 mm or more is different in the plate thickness direction, the small test piece to be subjected to a small test is divided in the plate thickness direction of the high strength thick steel plate. Collect from multiple areas.

  In the following description, a test piece collected so as to include at least the surface of the thick steel plate or a position up to about 2 mm below the surface is referred to as a small surface test piece. In this embodiment, in addition to the surface layer small test pieces, the test pieces are collected from at least one place, more preferably two or more positions from the inside in the thickness direction of the thick steel plate. In the following description, such a test piece that does not include the surface portion of the thick steel plate and is sampled from the inside of 10 mm or more below the surface in the thickness direction of the thick steel plate is referred to as an internal small test piece.

  When sampling an internal small test piece, it may be preferable that the sampling position avoids the thickness center part (center segregation part) of a thick steel plate. As shown in FIG. 2A, if a small test piece 8 is sampled from the thickness center portion of the thick steel plate 7, it is usually considered that the test result shows a brittle crack propagation behavior at the thickness center portion. However, steel sheets manufactured by a continuous casting process often have an alloy element enrichment zone called a center segregation part at the center of the plate thickness, and when the center segregation is remarkable, this center segregation part is brittle. It is thought that it greatly affects the propagation of cracks.

  Therefore, the present inventors experimentally examined the influence of the central segregation part on the brittle crack propagation behavior. As a result, the direct influence of local central segregation on brittle crack propagation behavior was found to be small.

  On the other hand, as a result of another experiment, it has been found that the center segregation part is a cause of significantly reducing the brittle crack initiation characteristics. The small test result is a result of superimposing brittle crack initiation characteristics and brittle crack propagation characteristics. Therefore, even if the brittle crack propagation performance is the same, if the central segregation part greatly affects the brittle crack initiation characteristics, there will be a large difference in the results of the small test, and the problem that the brittle crack propagation characteristics cannot be estimated accurately arises. .

  Therefore, the inventors of the present invention, when center segregation is remarkable in the thick steel plate, as shown in FIG. 2A, the small test piece is formed in a region (9) where the brittle crack propagation surface does not include the center segregation portion. It was found that it is preferable to collect. If center segregation is not noticeable inside the thick steel plate, a small test piece including the center portion of the plate thickness may be collected.

  When collecting a small test piece that does not include the plate thickness center portion in the vicinity of the plate thickness center portion of the thick steel plate, the region is 0.1 mm or more away from the plate thickness center portion (more preferably 1 mm or more away). It is preferable to collect a region including a position within 5 mm from the center of the plate thickness. The test results for small test specimens collected so as to include the position within 5 mm from the center of the plate thickness, not including the center of the plate thickness of the thick steel plate, the influence of the center segregation portion is excluded. The brittle crack propagation characteristics of the part are accurately shown.

  As shown in FIG. 2A, when a small test piece 10 is collected from the vicinity of the steel plate surface (surface layer portion) where the crack tip is in a plane stress state and the brittle crack is difficult to propagate, the test piece including the steel plate surface (surface layer small size) It is most preferable to collect a test piece). After the steel plate is manufactured, if there are layers with significantly different characteristics compared to the inside of the steel plate, such as scales and decarburized parts, remove these layers by cutting them as thin as possible before collecting the specimen. May be. Even when the surface is cut, if it is cut greatly beyond 2 mm from the steel sheet surface, there is a possibility that a test piece representative of the crack propagation characteristics on the steel sheet surface cannot be obtained. Even when the steel sheet surface is cut for reasons such as avoiding the effects of scale, decarburized parts, etc., it is necessary to keep the steel sheet surface within 2 mm.

  Moreover, you may extract | collect the small test piece containing the depth position of plate | board thickness 1/4 from the surface of a thick steel plate. The region including the position of the plate thickness ¼ of the thick steel plate is a region that can represent the brittle crack propagation behavior of the region including the brittle crack propagation surfaces Za and Zc shown in FIG. This is a preferred region.

  FIG. 2B shows how the small test piece is collected. The small test piece 9a is a small test piece including a position within 5 mm from the center of the plate thickness. The small test piece 10a is a small test piece including a position 2 mm below the surface of the steel plate. The small test piece 11 is a mode in which a small test piece including a position of a thickness ¼ of a thick steel plate is collected. The small test piece 8a is a small test piece including a center portion of the plate thickness. The small test piece 12 includes a position 2 mm below the steel plate surface and the steel plate surface.

  And in an internal small-sized test, in order to avoid the influence of a shear lip, it is necessary to extract | collect 10 mm or more away from the steel plate surface. As an internal small test piece, a test piece including a plate thickness center portion, a test piece collected so as to include a position within 5 mm from the plate thickness center portion, and a position of a plate thickness 1/4 of a thick steel plate are included. The collected small test pieces can be used.

  The dimension of the internal small test piece along the thickness direction of the thick steel plate is preferably set to 10 to 20 mm, roughly corresponding to the width of the brittle crack propagation surface. In the case of a thick steel plate having a plate thickness of 50 mm or more, since the brittle crack propagation behavior differs in the plate thickness direction, it is more preferable to collect a plurality of small test pieces along the plate thickness direction.

  Thus, by collecting a small test piece and performing a small test, it is possible to obtain a test result corresponding to a brittle crack propagation behavior that differs in the plate thickness direction. Based on this test result, the arrest performance of a thick steel plate having a thickness of 50 mm or more can be estimated. This estimation method will be described later.

A small test using a small specimen (internal small specimen) taken from an area other than the surface layer of a cracked steel plate is a specific test if it is a small test that measures the brittle fracture surface rate or absorbed energy. It is not limited. In the case of such a test, the behavior when a crack progresses inside a small test piece can be sufficiently evaluated.
For example, various Charpy type impact test specimens, specifically, V-notch Charpy impact test specimens, sharp notch Charpy impact test specimens, press notch Charpy impact test specimens, pre-crack Charpy impact test specimens, and three-face sharp notch Charpy impact test specimens Chevron notch Charpy impact test pieces and U-notch Charpy impact test pieces can be used. In any test piece, the absorbed energy during the impact test can be measured. Moreover, the ductile fracture surface ratio or brittle fracture surface ratio of a fracture surface can be measured. There is usually a positive correlation between the ductile fracture surface rate and the absorbed energy in small tests of certain steel materials. In the measurement of the brittle fracture surface ratio and the absorbed energy, the results of stress acting on the entire cross section of the specimen including the inside of the specimen as well as the surface part of the small specimen are integrated and accumulated. Is fully reflected in the test results. For this reason, as an evaluation method of the internal small test piece, by measuring the brittle fracture surface ratio (or ductile fracture surface ratio) or absorbed energy, it is possible to evaluate the brittle crack transmission characteristics inside the thick steel plate with high accuracy. it is conceivable that. Since the ductile fracture surface ratio and the brittle fracture surface ratio are 100%, the ductile fracture surface ratio may be used instead of the brittle fracture surface ratio.
On the other hand, when the drop weight test is used as the internal small test, an embrittled weld bead (crack initiation weld) is disposed on the surface arbitrarily formed when the internal small test piece is formed. For this reason, a test result directly dependent on the crack propagation characteristics on the surface of an arbitrary test piece, which is different from the surface originally present in the thick steel material, is obtained. In this case, the brittle crack propagation characteristics of the original thick steel material cannot always be accurately evaluated.

  The V-notch Charpy impact test and drop weight test may be performed using a test method based on the ASTM standard or JIS standard. For chevron notch Charpy impact test and sharp notch Charpy impact test, the notch shape is devised so that brittle cracks are likely to occur, and the shape of the specimen is adjusted so that the contribution of brittle crack propagation characteristics can be extracted greatly. Is desirable. A preferred test piece shape will be described in Examples.

  5A to 5E show preferred small test piece shapes. 5A is a drop weight test piece, FIG. 5B is a V-notch Charpy impact test piece, FIG. 5C is a (one surface) sharp notch Charpy impact test piece, FIG. 5D is a three-surface sharp notch Charpy impact test piece, and FIG. 5E is a chevron notch Charpy test piece. Representative shapes of impact test pieces are shown respectively.

  When the drop weight test is used as the surface layer small test according to the method of the present embodiment, the brittle crack propagation characteristics of the outermost surface portion of the thick steel plate can be directly evaluated. For this reason, the arrest performance of a thick steel plate can be evaluated with high accuracy as compared with the case where the surface layer small test is tested by other methods such as a V-notch Charpy impact test.

  Further, since the V-notch Charpy impact test is a condition in which a brittle crack is relatively unlikely to occur, the brittle crack generation characteristics may be dominant in the test result. On the other hand, if a small test piece that easily generates a brittle crack is used in a small test, the brittle crack propagation characteristic may be greatly reflected in the small test result.

  Therefore, it is more desirable to adopt a test method using a small test piece that is easy to generate a brittle crack. For example, a chevron notch Charpy impact test using a test piece in which a notch portion is processed into a slit shape and the notch shape is formed in a chevron shape is more desirable.

  About the dimension of the chevron notch Charpy impact test piece (see FIG. 5E), the dimension taken in the plate thickness direction is about 10 mm, and within the conditions of this embodiment, the area of the brittle crack propagation region is increased. This is desirable in view of the correlation with a large test for testing the brittle crack propagation stopping performance of a thick steel plate having a thickness of 50 mm or more. In FIG. 5E, the angle formed by the notch valley portion indicated by R1 is less than 60 degrees, and the curvature radius of the bottom of the notch valley bottom portion is preferably 0.1 to 0.2 mm. The notch extends in the width direction of the test piece and is processed into a shape that bends in a mountain shape at the front view point (view point along the longitudinal direction of the test piece). The apex portion of the chevron (R2 in FIG. 5E) becomes a crack initiation point during the test. The radius of curvature of the notch bend at the apex portion R2 is preferably 0.1 to 0.2 mm. The height of the R2 portion from the bottom of the test piece is desirably 13 mm.

In the drop weight test, as shown in FIG. 5A, a weld (weld bead, 101b in FIG. 3) is formed on the surface of the test piece, and a notch (crack initiation weld, slit processing) is formed in the weld. The drop weight test piece 101 is collected so as to include a surface layer portion of a thick steel plate as shown in FIG. That is, one surface 101a of the drop weight test piece 101 is collected so as to correspond to the surface of the thick steel plate. As schematically shown in FIG. 3, the weld bead 101b is formed on the surface 101a.

Using the obtained test piece 101, an NRL (Naval Research Laboratory) drop test specified in E208-06 of ASTM (Standards of American Society for Testing and Materials) is conducted. A welding bead 101b having a length of about 64 mm is attached to one surface 101a of the test piece 101 in the longitudinal direction with a welding material in accordance with the above-mentioned regulations (FIG. 5A). This bead acts as a crack start weld. Further, a slit having a width of 1.5 mm or less is formed in the weld bead 101b.
Next, as shown in FIG. 4A, the drop test piece 101 is installed on the test piece installation base 200 b of the drop test machine 200. At this time, it is installed so that the surface 101a on which the weld bead 101b is formed faces downward. In the drop weight test, a weight 200 a having a specified shape and weight falls on the test piece 101. If the toughness of the steel material of the test piece 101 is low, a brittle crack generated from the notch of the weld bead propagates into the test piece 101 depending on conditions such as the test temperature.
When a crack starting from the notch propagates on the surface 101a of the test piece in the width direction of the test piece 101 and proceeds to the end thereof (state in FIG. 4B), the test result is determined to be Break (with crack propagation). . When a crack does not reach the end in the width direction, the test result is determined as No Break (no crack propagation). The above test operation was repeated with two test pieces while changing the test piece temperature in increments of 5 ° C., and the temperature of the two test pieces was 5 ° C. lower than the lowest temperature at which no break was obtained. Set to NDT temperature.
In some cases, the crack penetrates the test piece and proceeds to the surface opposite to the weld bead installation surface 101a. However, in the drop weight test employed in this embodiment, the presence or absence of this penetration is not included in the evaluation.

As an example of using the NDT temperature in the drop weight test for the simple evaluation of the arrest toughness value Kca, there is an example used for the fracture evaluation of a special steel plate called a super-fine grain steel (Non-Patent Document 2). In the super-fine-grain steel, a layer having a substantially uniform ultra-fine grain structure that is difficult to brittle fracture exists in the super-fine grain steel. In the above-mentioned document, a drop weight test is used for the evaluation of super-fine grain steel. However, in this document, test conditions, correlation equations, etc. are used so that the main evaluation criterion is whether or not the brittle cracks generated in the drop weight test penetrate the surface ultrafine-grained steel and reach the back surface. Optimized. For this reason, the usage of the entire test, interpretation of the results, correlation equations, and the like are greatly different from the general operation method of the drop weight test of the ASTM standard. That is, with this prior art, although a special correlation formula dedicated to the evaluation of the characteristics of the superfine particle layer can be obtained, it is difficult to use test conditions such as the correlation formula for general steel materials. Moreover, in the test of the said literature, the propagation of the crack of the aspect along a bead installation surface does not necessarily have a decisive influence directly on a test result. This is because in super-fine-grain steel, the crack propagates vertically through the super-fine grain part, and then the crack propagates inside (under) the super-fine grain part. This is because it occurs. FIG. 10A shows the state of the fracture surface in the drop test of the super-fine grain steel, and FIG. 10B shows the state of the fracture surface in the drop test of the general steel material. In the drop weight test of the super-fine grain steel, the brittle crack penetrates the super-fine grained surface layer portion, and when the area P of FIG. If the crack does not penetrate to the region P, the operation is stopped. In other words, if the crack reaches the boundary of the substantial ultrafine grain region, it is determined as Go (propagation). That is, in the drop weight test of the superfine grain steel, it is equivalent to evaluating only the superficial part composed of the superfine grain structure. On the other hand, in a drop test of a general steel material, a brittle crack propagates in both the plate thickness direction and the specimen width direction (arrow in FIG. 10B), and is judged as Break (with crack propagation) by penetration in the width direction. . As described above, the drop test of the general steel material is greatly different from the drop test of the super-fine grain steel in that the propagation characteristic of the brittle crack parallel to the plate surface in the vicinity of the plate surface is evaluated. For this reason, the test method of the said literature is set to the conditions applicable only to surface ultrafine-grained steel, and cannot be applied to the test of general steel.
In the above document, the inside of the plate thickness is also evaluated by the drop weight test. The main mechanism by which the inside of the plate thickness contributes to arrestability is the energy of brittle crack propagation resistance, not the energy of shear lip formation. For this reason, it is considered that evaluating the internal test piece by the drop weight test causes an evaluation error due to the deviation of the steel properties generated for each production batch and production method of the test steel. In the steel sheet, since the contribution to the arrest characteristics of the super-fine grain region is too large, such an error does not increase and is put into practical use. For this reason, the said evaluation formula cannot be applied to general steel materials.
On the other hand, regarding the evaluation of the plate thickness surface layer of a general steel material, it is important to use different optimum methods separately for the outermost surface evaluation and the internal evaluation. The inventors of the present invention have found that the NDT temperature of the drop weight test specified in the ASTM standard is suitable for the evaluation of the outermost surface in thick steel plates having a wide range of processing conditions and compositions. In the drop weight test specified in this embodiment, whether or not a crack has reached the back layer of the test piece is not included in the evaluation. This drop weight test is a test method in which the contribution on the surface layer side is the largest and the contribution on the back surface side is small, and is suitable for evaluating the shear lip effect of the steel material surface layer.

  The present embodiment relates to a performance inspection method related to the arrest performance of a thick steel plate. Therefore, all the methods for guaranteeing performance by estimating the arrest performance, which is the overall performance in the plate thickness direction, in accordance with the above-described method may be included in the scope of the technical idea of the present embodiment. The scope of the present invention is defined by the appended claims.

  This embodiment is based on the basic idea of estimating the arrest performance of a thick steel plate based on the result of a small test, and the characteristics based on the component composition appear in the test result. For this reason, the method of this embodiment is applicable with respect to the thick steel plate of a wide component composition. The thick steel plate used in the present embodiment may be manufactured from a structural steel for welding having a known component composition. That is, the thick steel plate used in the present embodiment may be a thick steel plate having a known component composition.

  In addition, as structural steel for welding, for example, in mass%, C: 0.02 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, Al : 0.001 to 0.20%, N: 0.02% or less, P: 0.01% or less, S: 0.01% or less as basic components, and improvement of base material strength and joint toughness is required. 1 type or 2 types or more of Ni, Cr, Mo, Cu, W, Co, V, Nb, Ti, Zr, Ta, Hf, REM, Y, Ca, Mg, Te, Se, B You may use the thick steel plate containing this.

  Next, examples of the present embodiment will be described. The conditions of the examples are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the scope of application of the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  Table 1 shows the composition of the steel plates used in this example. Table 2 shows main rolling conditions, cooling conditions, heat treatment conditions, sheet thickness, yield strength YS, and the like at the time of manufacturing each steel sheet. From these thick steel plates having a thickness of 70 mm, small test pieces were sampled in the sampling mode shown in FIG. 2B. Table 3 shows the types of small tests and sampling positions.

  The result of the V-notch Charpy impact test is represented by a temperature: vTrs indicating a 50% brittle fracture surface ratio called a ductile brittle fracture surface transition temperature. The result of the chevron notch Charpy impact test is shown by a transition temperature at which 70 J of absorbed energy can be secured. The result of the sharp notch Charpy impact test is shown as a transition temperature at which an absorbed energy of 40 J can be secured.

  Next, the correlation between the small test result and the arrest performance Kca measured in the large test performed on each steel material will be described. The small test was performed using the method according to the present embodiment and the method according to the comparative example.

6A to 6E show the results according to the comparative example, and are diagrams showing a correlation example between the small test using only the V-notch Charpy impact test and the arrest performance measured in the large test. Test pieces taken from various positions were used for the small test.
The V-notch Charpy impact test piece (test piece thickness: 10 mm) is a standard test piece defined in JIS (see FIG. 5B).
FIG. 6A is a correlation diagram in the case of using either the small test piece 10a or the small test piece 11 shown in FIG. 2B.
FIG. 6B is a correlation diagram when the small test piece 8a shown in FIG. 2B is used.
FIG. 6C is a correlation diagram when the small test piece 9a shown in FIG. 2B is used.
FIG. 6D is a correlation diagram in the case where both the small test pieces 10a and 9a shown in FIG. 2B are used and the obtained small test results are weighted averaged.
FIG. 6E is a correlation diagram in the case where both the small test pieces 10a and 8a shown in FIG. 2B are used and the obtained small test results are weighted averaged.
The weighted average was obtained by multiplying the surface test result by 0.4 and the internal test result by multiplying by 0.6. The procedure for obtaining this weighting coefficient will be described later.

  In the correlation examples shown in FIGS. 6A, 6B, and 6C, the correlation that varies greatly is slightly improved in FIG. 6D. Further, in FIG. 6E using the test piece 8a affected by the center segregation, the variation in correlation is larger than that in FIG. 6D. However, it is more preferable to further improve the correlation of FIG. 6D in order to guarantee the arrest characteristic by using only the small test for the shipping test.

7A to 7E, a correlation example between the result of a small test obtained by collecting only chevron notch Charpy impact test pieces as small test pieces from 10 types of thick steel plates with a thickness of 70 mm and the arrest performance measured in the large test. Indicates.
About a chevron notch Charpy impact test piece (test piece thickness: 10 mm), FIG. 5E shows a specific shape and dimensions (unit: mm).
Except for the difference in the shape of the small test piece and the small test method, the sampling position of the small test piece and the arrangement method of the results are the same as in the case of FIGS.

  As a result of adopting the chevron notch Charpy impact test, the contribution of brittle crack initiation characteristics is reduced, so the correlation shown in FIG. 7D is slightly improved compared to the correlation shown in FIG. 6D.

  FIG. 7E shows the correlation when the test piece 8a shown in FIG. 2B is used. Due to the use of the small test piece 8a including the center segregation, even the chevron notch Charpy impact test piece has a lower degree of correlation than the correlation shown in FIG. 7D.

  Although the detailed results are not described, the V-notch Charpy impact test piece is used because the contribution of brittle crack generation characteristics is suppressed to a small degree even in the sharp notch Charpy impact test piece as in the case of the chevron notch Charpy impact test piece. The correlation tends to be better than that.

  For the one-side sharp notch Charpy impact test piece (test piece thickness: 10 mm), FIG. 5C shows specific shapes and dimensions (unit: mm). A three-surface sharp notch Charpy impact test piece (FIG. 5D) in which sharp notches are formed on three surfaces of the test material can also be used.

Correlation between the test results of the small test pieces of the comparative examples shown in FIGS. 6A to 6E and FIGS. 7A to 7E and the temperature at which the Kca value measured in the large test is 6000 N / mm ^1.5 : TKca 6000 (° C.). From the above, the Kca value of the thick steel plate was estimated. Table 3 shows the estimation results. Table 3 also shows Tkca6000 actually measured in the large test.

In Table 3, VNC is a V-notch Charpy impact test piece, CNC is a chevron notch Charpy impact test piece, and NDT is a falling weight test piece defined by ASTM. FIG. 5A shows specific shapes and dimensions (unit: mm).
In Comparative Example VNC, the V-notch Charpy impact test was used for both the surface layer small test and the internal small test.
In Comparative Example CNC, the chevron notch Charpy impact test was used for both the surface layer small test and the internal small test.
In Comparative Example NDT, the drop weight test was used for both the surface small test and the internal small test.
In Comparative Example 8a, the V-notch Charpy impact test was used for both the surface layer small test and the internal small test. In another example, the test piece for the internal small test is taken from the position 9a, whereas in the comparative example 8a, the test piece for the internal small test is taken from the position 8a.
In the inventive example, a drop weight test was used for the surface small test, and a chevron notch Charpy impact test piece was used for the internal small test.

In the comparative examples VNC and CNC, the difference between the small test and the large test varies greatly depending on the steel type, and there are steel types for which an estimated value with sufficient accuracy cannot be obtained.
Although the comparative example 8a uses the small test result of the 8a sampling position including the center segregation part, the estimation error is large. In the comparative example NDT, the evaluation method related to the super-fine grain steel in the non-patent document 2 was applied. However, in general steel materials, it is clear that the evaluation method using the drop weight test on the central test piece has low accuracy. became. In the invention example, the difference between the value estimated from the small test and the value measured in the large test is smaller than that of the comparative example, and the result of the large test is estimated industrially with high accuracy by the result of the small test. I understand that

  In invention example VNC, the correlation was obtained using a drop test on the surface and a V-notch Charpy impact test inside the plate thickness. Although the accuracy is slightly lower than that of the above-described invention example, it is understood that the accuracy is better than that of the comparative example and estimation is possible.

  In the embodiment of the invention, the error is relatively small, but since the error is strongly influenced by the number of data, the error tends to increase when the number of data is larger than the embodiment included in this patent. The error in this embodiment is data when N = 10, and the error becomes even larger when N> 10. That is, when N is several tens to several hundreds as an actual steel material evaluation test, it is expected that the error will be larger than that of the present example. Even in such a case, the present invention provides a simple determination method that is sufficiently useful and accurate.

  FIG. 8A is a diagram illustrating a correlation example between the composite small test result using the method according to the first embodiment of the present invention and the arrest performance measured in the large test. A drop weight test was conducted on the surface small test piece, and a chevron notch Charpy impact test was conducted on the internal small test piece, and these results were used by weighted averaging. A very high correlation appears with respect to the comparative example.

  FIG. 8B is a diagram illustrating a correlation example between the composite small test result using the method according to the comparative example NDT and the arrest performance measured in the large test.

  In Steel 3, Steel 5, Steel 7, and Steel 10, the estimation accuracy error was 4 ° C. or higher even in the method of the inventive example according to the first embodiment of the present invention. For this reason, a small test was added, and estimation by a small test at three positions was attempted. Table 4 shows Example 2 of the invention. In Example 2, the error is further reduced, and it can be seen that the result of the large test can be estimated with high accuracy from the small test result.

  FIG. 8C is a diagram illustrating a correlation example between the composite small test result using the method according to the second embodiment of the present invention and the arrest performance measured in the large test. Here, the surface small test pieces were collected from the position 12 shown in FIG. 2B, and the internal small test pieces were taken from two positions 9a and 11 shown in FIG. 2B. A drop weight test (NDT, small test 1) is performed on the surface small test piece, a chevron notch Charpy impact test (CNC, small test 2, 3) is performed on the two internal small test pieces, and these three small test results are weighted. Used on average. The highest correlation was obtained during this test.

  FIG. 8D is a diagram showing a correlation example between the composite small test result using the method according to the first embodiment of the present invention and the arrest performance measured in the large test, as in FIG. 8A. A drop weight test was performed on the surface small test piece, and a V-notch Charpy impact test was performed on the internal small test piece, and these results were used by weighted averaging. The difference between the test of FIG. 8A and the test of FIG. 8D is whether the internal small test piece is a chevron notch Charpy impact test piece (FIG. 8A) or a V-notch Charpy impact test piece (FIG. 8D). Also in the result of FIG. 8D, a sufficiently high correlation appears with respect to the comparative example.

第２の実施形態にかかる発明例２の方法では、鋼３、鋼５、鋼７、鋼１０においても、実測Ｔｋｃａ６０００を高精度で推定することができている。

In the method of Invention Example 2 according to the second embodiment, the measured Tkca 6000 can be estimated with high accuracy even in the steel 3, the steel 5, the steel 7, and the steel 10.

   9A and 9B show a flowchart of a method for calculating the coefficient of the estimation formula. First, a linear inclination, that is, a ratio between a and b is obtained from the pass / fail criterion, and Y + aX / b is compared with TKca using the obtained ratio to determine a correlation equation. At this time, if the accuracy is insufficient, a small test at the third position is used to change a: b, and the above procedure is performed to minimize the error.

In the first and second embodiments described above, in the invention example, the difference between the value estimated from the small test and the value measured in the large test is smaller than that of the comparative example. The result of the test can be estimated industrially with high accuracy.
That is, according to the present invention, it can be seen that the arrest performance Kca estimated based on the results of the small test and the arrest performance K obtained in the actual large test coincide with each other with high accuracy over a wide range of steel types and construction methods.

  According to the method of the present invention, a large test apparatus is required as in the ESSO test, and it is possible to omit a large cost test. That is, a sample steel material is selected from the product steel materials of each batch, a small test is performed using a small test piece cut out from the sample steel material, and the arrest performance of the actual product can be estimated with the same high accuracy as the large test. By using the method of the present invention, it is possible to provide quality assurance at the product steel level of each batch based on a small test instead of quality assurance at the component / process level.

Za, Zb, Zc, Zd Brittle crack propagation surface a, b, c, d Crack tip 7 Thick steel plate 8, 8a, 9, 9a, 10, 10a, 11, 12 Small test piece 101 Drop weight test piece 101a One side Surface 101b weld bead 102 small test piece 103 small test piece N notch 200 drop weight tester 200b test piece installation base 200a weight

Claims (13)

A method to determine brittle crack propagation stopping performance of high strength thick steel plates:
A process of conducting a large test and a combined small test using standard steel;
Calculating a correlation model between the result of the large test using the standard steel and the result of the composite small test;
Performing the composite compact test using sample steel;
Substituting the result of the composite compact test using the sample steel into the correlation model to estimate the brittle crack propagation stopping performance of the sample steel;
Including
The combined small test is:
(A) a step of collecting a surface layer small test piece including a steel sheet surface layer portion;
(B) collecting a small internal test piece from one or two or more internal regions not including the steel sheet surface layer portion;
(C) performing a drop weight test using the surface small test piece;
(D) performing a small test for measuring a brittle fracture surface ratio or absorbed energy using the internal small test piece;
Including
In the composite small test, the surface small test piece and the internal small test piece are each subjected to a small test by different methods.
A method for judging brittle crack propagation stopping performance of a high-strength thick steel plate. Y is the NDT temperature that is the result of the drop test of the surface small test piece,
Fracture surface transition temperature or absorbed energy transition temperature, which is the result of a small test using X1 as the internal small test piece,
a, b, d are coefficients,
If the target Kca limit temperature is TKca,
The correlation model is
a · Y + b · X1 + d = TKca
The method for determining brittle crack propagation stopping performance of a high-strength thick steel plate according to claim 1, wherein: In the step of estimating the brittle crack propagation stopping performance of the sample steel, the target Kca of the sample steel is substituted by substituting Y ′ and X1 ′, which are the results of the composite small test using the sample steel, into the correlation model. Calculating TKca ′ which is an estimated value of the limit temperature;
Comparing TKca 'with TKca, which is an actual target Kca limit temperature measured for the standard steel;
TKca '≦ TKca
A step of determining that the brittle crack propagation stopping performance of the sample steel is good;
The method for determining brittle crack propagation stopping performance of a high-strength thick steel plate according to claim 2, further comprising: Y is the NDT temperature that is the result of the drop test of the surface small test piece,
Fracture surface transition temperature, which is the result of a small test using the internal small test piece collected from the first internal region, X1,
Fracture surface transition temperature, which is the result of a small test using the small internal test piece taken from the second internal region, X2,
a, b, c, d are coefficients,
If the target Kca limit temperature is TKca,
The correlation model is
a · Y + b · X1 + c · X2 + d = TKca
The method for determining brittle crack propagation stopping performance of a high-strength thick steel plate according to claim 1, wherein: In the step of estimating the brittle crack propagation stopping performance of the sample steel, Y ′, X1 ′, and X2 ′, which are the results of the composite small test using the sample steel, are substituted into the correlation model. Calculating TKca ′, which is an estimated value of the target Kca limit temperature of the sample steel;
Comparing TKca 'with TKca, which is an actual target Kca limit temperature measured for the standard steel;
TKca '≦ TKca
A step of determining that the brittle crack propagation stopping performance of the sample steel is good;
The method for determining brittle crack propagation stopping performance of a high-strength thick steel plate according to claim 4, further comprising:   The internal small test piece is a chevron notch Charpy impact test piece, a V-notch Charpy impact test piece, a sharp notch Charpy impact test piece, a press notch Charpy impact test piece, a pre-crack Charpy impact test piece, a three-face sharp notch Charpy impact test piece. The determination method of the brittle crack propagation stop performance of the high-strength thick steel plate according to any one of claims 1 to 5, wherein the determination method is a U-notch Charpy impact test piece.   The high-strength thickness according to any one of claims 1 to 5, wherein the internal region is a region that does not include a plate thickness center portion and includes a position within 5 mm from the plate thickness center portion. A method for judging brittle crack propagation stopping performance of steel sheets.   The method for determining brittle crack propagation stopping performance of a high-strength thick steel plate according to any one of claims 1 to 5, wherein the internal region is a region including a position having a thickness of 1/4. . Yield strength of the high strength thick steel plate, characterized in that it is a 240~600N / mm ^2, determined in the brittle crack arrest performance of high-strength steel plate according to any one of claims 1 to 5 Method.   The brittle crack propagation stop of the high-strength thick steel plate according to any one of claims 1 to 5, wherein the thickness of the surface small test piece and the internal small test piece is 10 to 25 mm. Judgment method of performance.   The said high-strength thick steel plate is a steel plate for large hulls, The determination method of the brittle crack propagation stop performance of the high-strength thick steel plate as described in any one of Claims 1-5 characterized by the above-mentioned.   The method for judging the brittle crack propagation stopping performance of a high-strength thick steel plate according to any one of claims 1 to 5, wherein the thickness of the high-strength thick steel plate is 50 mm or more. In the drop weight test, among the surfaces of the surface layer small test piece, a crack start weld is provided on the surface corresponding to the surface layer side of the sample steel,
Of the surface of the internal small test piece, a notch is provided along the thickness direction of the sample steel.
The determination method of the brittle crack propagation stop performance of the high-strength thick steel plate as described in any one of Claims 1-5 characterized by the above-mentioned.

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