JP5582233B1 - Evaluation method of brittle fracture propagation stop performance of thick steel plate - Google Patents

Abstract

A simple method for evaluating the brittle fracture propagation stopping performance of a thick steel plate by a small test is provided.
As a small test, a Charpy impact test using a deformed press notch Charpy impact test piece in which the sampling position is at the center of the plate thickness and the press notch is introduced in the propagation direction of the brittle crack is obtained by the Charpy impact test. deformation press notch Charpy absorbed energy temperature showing the 20~225J energy transition temperature (℃) (℃): pT based on _E, to evaluate the brittle fracture propagation stop performance (Kca value).
[Selection] Figure 3

Description

  The present invention relates to a method for evaluating brittle fracture propagation stopping performance of thick steel plates used for large structures such as ships, marine structures, low-temperature storage tanks, and construction / civil engineering structures, particularly thick steel plates having a thickness of 50 mm or more.

  For large structures such as ships, offshore structures, low-temperature storage tanks, and construction / civil engineering structures, accidents associated with brittle fractures have a significant impact on the economy and the environment, and are used because safety is always required. Steel materials to be used are required to have toughness and brittle fracture propagation stopping performance at operating temperatures.

  The evaluation of the brittle fracture propagation stop performance is usually performed by a large test represented by an ESSO test or a double tensile test. However, since these tests are large, many days and costs are required to perform the tests, and it is difficult to perform them easily.

  For this reason, a method for predicting brittle fracture propagation stop performance from the fracture surface transition temperature vTrs of the V-notch Charpy test has been established in WES3003-1995, but the prediction accuracy is poor for materials with a plate thickness of more than 50 mm in recent years. It is difficult to evaluate.

  In order to solve this problem, comparatively small and simple evaluation methods such as a Charpy impact test and a drop weight test in which the shape of the test piece is devised have been developed instead of the large test. Regarding the drop weight test, Patent Document 1 discloses a method of producing a test piece with a press notch after applying compressive deformation in the thickness direction of the test piece so that the test proceeds more stably from a brittle fracture surface. Has been proposed.

  For the Charpy impact test, instead of the Charpy impact test piece, as a method of proceeding the test more efficiently from the brittle fracture surface, a saw notch having a depth of 2 mm or less after depositing a weld bead on the portion corresponding to the notch of the Charpy impact test piece It is proposed in Patent Document 2 to use a test piece into which the mark is inserted.

  In Non-Patent Document 1, because of the distribution of toughness in the plate thickness position direction, the Kca value obtained by the ESSO test, which represents brittle fracture propagation stop performance, is strongly influenced by the low toughness region. It is described that the brittle fracture propagation stop performance is evaluated by weighting the toughness value at the plate thickness position to the value obtained by taking the average of the area of the steel plate and further the value at the central portion of the plate thickness.

  In addition, as a simple evaluation method for the brittle fracture propagation stopping performance considering the plate thickness effect, the Kca value is predicted from the result obtained by performing a three-point bending test on the test piece taken from the center portion of the plate thickness and the surface layer portion. A method is proposed in Patent Document 3. Patent Document 4 proposes a technique for evaluating brittle fracture propagation stopping performance using a deformed Charpy impact test piece having a special shape.

Patent Document 5 relates to a technique for evaluating the brittle fracture propagation stopping performance of a thick steel plate having a thickness of 50 mm or more using a press notch Charpy impact test piece, and the center portion and the surface of the thickness of the thick steel plate having a thickness of 50 mm or more. The Charpy impact test was performed using Charpy impact test specimens that were sampled from 1/4 of the plate thickness and introduced with a press notch, and the fracture surface transition temperature vTrs ^* obtained by the Charpy impact test for each specimen was measured ^. Based on this, a method for evaluating the brittle fracture propagation stopping performance of a thick steel sheet characterized by evaluating the brittle fracture propagation stopping performance is described.

  Further, Patent Document 6 discloses a small test piece as a method for obtaining a correlation between the arrest performance of a whole thick steel plate having a thickness of 50 mm or more and a test result using a small test piece collected from the thick steel plate with high accuracy. Collect a plurality of samples along the thickness direction, and perform a small test using the optimum method according to the sampling position, that is, the drop surface test for the steel sheet surface layer, and the method for measuring the brittle fracture surface ratio and absorbed energy inside the steel sheet. It has been proposed to appropriately combine test results and to estimate the Kca value obtained in a large test from the combined results.

JP-A-63-67544 JP-A-62-274258 JP 2008-46106 A JP 2009-47462 A JP 2011-33457 A Japanese Patent No. 4795487

49th Annual Meeting of the Japan Welding Society 108 (1991) Brittle crack arrest design guidelines Japan Maritime Association (2009)

  However, the method of collecting the test pieces in Patent Documents 1 to 3 is not necessarily a simple method because the preparation of the test pieces is complicated, such as the size of the test pieces and processing again after welding. The technique of Patent Document 4 is poor in versatility because the shape of the test piece is special. In addition, the method for estimating a large-scale test result based on the Kca value obtained in consideration of the toughness value at the plate thickness position described in Non-Patent Document 1 shows a certain degree of correlation, but there is variation as a whole. The current situation is that they are large and are not consistent enough to be used instead of a large test.

  Since the techniques of Patent Document 5 and Patent Document 6 require that test pieces are collected from a plurality of positions in the thickness direction of the thick steel plate and the test is performed, the collection of the test pieces is complicated and not necessarily a simple method. Is hard to say.

  Then, an object of this invention is to provide the simple method of evaluating the brittle fracture propagation stop performance of a thick steel plate by a small test.

The present inventors have introduced a press notch, and a deformed Charpy impact test piece having a cross-sectional area in the direction perpendicular to the longitudinal direction exceeding 100 mm ² (in the present invention, also referred to as “deformed press notch Charpy impact test piece or deformed press notch Charpy test piece”). ) Charpy impact test was conducted under various conditions, and the 20 to 225J energy transition temperature (° C) of the absorbed energy by the deformed press-notched Charpy impact specimen taken from the thickness center of the thick steel plate stopped brittle fracture propagation. It was found that it shows the best correlation with the performance value.

Therefore, using the evaluation method of brittle fracture propagation stop performance of thick steel sheet described in Patent Document 5, brittle fracture propagation stop performance by thick ESO test with thick plate thickness of 50 mm or more and Charpy by deformed press notch Charpy impact test piece. Further investigation was made on the correlation between the impact test results, and the following findings were newly obtained. FIG. 4 shows a schematic diagram of a fracture surface of an ESSO test piece of a thick material having a thickness of 50 mm or more used for the study. The brittle crack has different propagation behavior in the cross section in the thickness direction, and the brittle crack length in the central portion of the thickness is shorter than the brittle crack length in the vicinity of the surface portion, and a concave portion in which the central portion of the thickness is recessed is formed. In addition, on the fracture surface of the thick ESSO test piece with a plate thickness of 50 mm or more, as shown in FIG. 5, the central portion of the plate thickness is a convex portion, and the upper and lower regions sandwiching the central portion of the plate thickness form a concave portion. However, this is not the subject of the present invention.
1. Even when the propagation behavior of the brittle crack in the cross section in the plate thickness direction is different (as shown in Fig. 4, the brittle crack in the surface portion propagates longer than the plate thickness central portion), the brittle crack in the plate thickness central portion propagates. If stopped, the dynamic stress intensity factor of the surface portion will decrease mechanically and it will be easy to stop propagation, so the brittle crack propagation stop toughness at the center of the plate thickness represents the brittle crack propagation stop toughness of the entire steel plate. To do.
2. Were taken from mid-thickness of the thick steel plates, fracture form modified press notch Charpy test pieces 100 mm ² beyond the greater than perpendicular to become the cross-sectional area normal specimen with respect to the longitudinal transition to ductile fracture brittle fracture The brittle crack propagation stop is reproduced in the specimen. On the other hand, the deformed press notch Charpy test piece sampled near the surface of the thick steel plate often has a transition from ductile fracture to brittle fracture, and brittle crack propagation stop cannot be reproduced in the specimen.
3. In a deformed press notch Charpy impact test conducted on a test piece taken from the center of the thickness of a thick steel plate, when the cross section of the deformed press notch Charpy test piece is 15 mm square, the absorption energy is 100 J (pT). _In the case of a 13 mm square, the temperature indicating 68 _J (referred to as pT _68J ) is 20 to 225 J energy transition temperature (° C.), showing a good correlation with the brittle fracture propagation stopping performance value of the steel sheet.
4). As shown in FIG. 3, there is a correlation between the Charpy absorbed energy and the brittle fracture surface ratio, and in FIG. 3, both modified press notch Charpy when Charpy absorbed energy is 15 mm square and 100 J, and when 13 mm square is 68 J. The brittle fracture surface ratio of the test piece is 63%. Temperature at which brittle fracture surface ratio of deformed press notch Charpy test piece shows 63% (° C.): 63% fracture surface transition temperature BATT shows good correlation with the value of brittle fracture propagation stop performance as well as press notch Charpy absorbed energy because, instead of the pT _E, can be used to evaluate the brittle fracture propagation stopping performance.

The present invention has been made by further study based on the above-described findings, and the gist thereof is as follows.
(1) An evaluation method for brittle fracture propagation stoppage performance of a thick steel plate in which the brittle fracture propagation stoppage performance of the thick steel plate is estimated from a small test, wherein the sampling position is a press notch at the center of the plate thickness. Is introduced in the direction of propagation of a brittle crack, and the Charpy absorbed energy obtained by the Charpy impact test is 20 to 225 J in the deformation press notch Charpy impact test using a deformed press notch Charpy impact test piece having a rectangular cross-section exceeding 100 mm ^2. temperature: pT based on E _(° C.), the evaluation method of brittle fracture propagation stopping performance of thick steel plate and evaluating brittle fracture propagation stop performance (Kca value).
(2) Based on the above pT _E (° C), the Tk (° C) calculated according to the equation (1) is the target value for the brittle fracture propagation stop performance (Kca value). 2. The brittle fracture propagation stop performance is evaluated by comparing the stop temperature with a set temperature at which a target value of the brittle fracture propagation stop performance (Kca value) is set. The evaluation method of the brittle fracture propagation stop performance of the described thick steel plate.
Tk = a × pT _E + b (1)
However, pT _E: Charpy absorbed energy 20~225J energy transition temperature (℃) a, b: in place of the coefficient _{(3) pT E (℃)} , brittle fracture rate of deformation press notch Charpy impact test piece 50 The method for evaluating brittle fracture propagation stopping performance of a thick steel plate according to (1) or (2), wherein a fracture surface transition temperature BATT (° C.) of 50 to 90% showing 90% fracture surface transition is used.

  According to the present invention, the brittle fracture propagation stopping performance of a thick steel plate having a thickness of 50 mm or more is measured using a test piece having the same size as that of a normal Charpy impact test without performing a large brittle crack propagation test such as an ESSO test. In addition, it is extremely useful industrially because it can be easily and accurately evaluated at one place where the plate thickness is collected.

The figure explaining the collection position of a deformation | transformation press notch Charpy test piece. The figure explaining a deformation | transformation press notch Charpy test piece. The figure which shows the relationship between a deformation | transformation press notch Charpy absorbed energy (J) and a brittle fracture surface rate (%) (when the longitudinal cross-section of a deformation press notch Charpy test piece is 13 mm square and 15 mm square). The schematic diagram of the fracture surface of the ESSO test piece of thick material with a plate thickness of 50 mm or more. The schematic diagram of the other fracture surface of the ESSO test piece of the thick material whose board thickness is 50 mm or more.

  The present invention is intended for a thick steel plate having a thickness of 50 mm or more and a fracture surface of an ESSO test piece having the shape shown in the schematic diagram of FIG. This is an estimation method to be estimated.

Charpy tests in sampling position is the center of plate thickness position, press notch is introduced in the direction of propagation of brittle cracks, Charpy performing longitudinal direction perpendicular cross-sectional area using a modified press notch Charpy test piece 100mm ² ~225mm ² Take the test. The sampling position of the plate thickness center portion means that the press notch center of the deformed press notch Charpy test piece is sampled in accordance with the position of 40% to 60% of the plate thickness of the steel plate. FIG. 1 shows the sampling position.

  A modified Charpy test piece in which a notch by a press is introduced instead of a notch by a notch, and the dimensions of the main body are 50 to 60 mm in the longitudinal direction and (10 to 15) × (10 to 15) mm in the cross section in the longitudinal direction. In this case, the Charpy test result shows a good correlation with the result of the brittle crack propagation performance test.

  The press notch is preferably introduced as follows. Taking into account the direction of the test piece, the material from which the test piece is to be collected is divided and cut, and further, the cut-out part is cut with a blade shape against the rectangular parallelepiped small steel piece obtained by finishing the outer shape. Press fit. The present invention uses a deformed press notch Charpy test piece having a 2 mm V notch having a depth of 2 mm and an angle of 45 degrees as shown in FIG.

  In the Charpy test, in order to eliminate the influence of the brittle fracture occurrence characteristics from the obtained test results, the deformed press notch Charpy test piece after the test was observed, and the test piece in which no fracture occurred from the brittle crack stopped the brittle crack propagation It is considered that the performance has not been evaluated, and is deleted from the test results. The impact test result arranged only with the test piece in which the fracture has occurred from the brittle crack excludes the influence of the brittle fracture occurrence characteristic and reflects only the brittle fracture propagation stop performance.

In the present invention, the deformation absorbing energy obtained by press-notch Charpy test is 20~225J 20~225J energy transition temperature (° C.): to evaluate the brittle fracture propagation stopping performance based on pT _E. The case where the brittle fracture propagation stop performance is evaluated by estimating the temperature at which the brittle fracture propagation stop performance Kca value is 6000 N / mm ^1.5 will be described below.

In the case of a steel plate having a thickness of 75 mm or less, if the Kca value at −10 ° C. is 6000 N / mm ^1.5 or more, the brittle crack stops at −10 ° C. (Non-patent Document 2). The absorbed energy obtained by conducting a Charpy test using a deformed press notch Charpy test piece is 20 to 225 J energy transition temperature: using pT _E (° C.), the temperature at which the Kca value is 6000 N / mm ^1.5 is obtained, The brittle fracture propagation stopping performance is evaluated depending on whether the temperature is higher or lower than −10 ° C.

STEP1:
For a thick steel plate with a thickness of 50 mm or more, a deformed press notch Charpy impact test piece is collected from the center position of the plate thickness, and after introducing the press notch, a Charpy impact test is performed at various test temperatures. . Since the direction of introduction of the press notch should be taken in the direction in which the crack propagates, it is put in the rolling direction or the rolling width direction in combination with the notch direction in the ESSO test.

STEP2
From the result of the Charpy impact test, the temperature at which the absorbed energy is 20 to 225 J is obtained and is set to 20 to 225 J energy transition temperature: pT _E (° C.), and the value is substituted into equation (1) to obtain Tk (° C.). . The Tk (° C.) obtained here shows a very good correlation with the temperature Tk (6000) (° C.) at which the brittle fracture propagation stop performance Kca value is 6000 N / mm ^1.5 , measured by the ESSO test. Evaluation of brittle fracture propagation stop performance is possible at Tk (° C.) calculated as described above.
Tk = a × pT _E + b (1)
However, pT _E : Deformation press notch Charpy absorbed energy of 15 mm square is 100 J, 13 mm square is 68 J temperature, a, b: coefficient Yield strength of steel sheet to be evaluated for brittle fracture propagation stop performance is 360 MPa In the case of the class or higher, a good correlation is obtained in the range of 0.4 <a <1.5 and 0 <b <40.

Equation (1) measures pT _E (° C.) for various test specimens when the deformed press notch Charpy absorbed energy at the center of the plate thickness is 15 mm square, and when it is 13 mm square, it indicates 68 J. An ESSO test is performed on the test specimens common to these test pieces to obtain a temperature: Tk (6000) (° C.), and these measurement results are arranged, and pT _E (° C.) and temperature: Tk (6000) (° C.) This is an empirical formula for obtaining the correlation.

It is known that there is a correlation between press notch Charpy absorbed energy and brittle fracture surface ratio. When the deformed press notch Charpy absorbed energy is 15 mm square and the temperature is 100 J and 13 mm square is 68 J, the fracture surface ratio (in the present invention, it can be recognized that the brittle crack generated from the press notch has stopped due to the characteristics of the steel sheet. , Brittle fracture surface ratio is 63%). When the temperature at which the brittle fracture surface ratio is 63% is defined as 63% BATT (° C.), the temperature indicates 63 J BATT (° C.) and Charpy absorbed energy of 15 mm square and 100 J, and 13 mm square is 68 J. Are substantially the same temperature, and pT _E (° C.) in the formula (1) may be replaced with 63% BATT (50-90% fracture surface transition temperature BATT) (° C.), and the temperature: Tk (6000) (° C.) is good. Correlation is obtained.

STEP3
Temperature: When Tk (6000) (° C.) is lower than −10 ° C., it is determined that the brittle fracture propagation stopping performance is excellent.

The present invention, brittle fracture propagation stop performance (Kca value) and 4000 N ^{/ mm 1.5} or 8000 N ^{/ mm 1.5,} also when a value other than 6000 N ^{/ mm 1.5,} applicable, respectively Evaluation similar to the above can be performed by deriving the correlation equation from the experimental results.

When there are a plurality of thick steel plates of the same steel type to be evaluated for brittle fracture propagation stop performance, the present invention is preliminarily implemented for one of them to evaluate the brittle fracture propagation stop performance of the steel plate. for the other steel sheets seeking absorbed energy variations press notch Charpy test specimens at test temperature pT _E (J), by the following equation, it is possible to determine the brittle crack arrest performance at -10 ° C.. In the case of a 15 mm square deformed press notch Charpy test piece that satisfies the formula (2) at a test temperature pT ₁₀₀ (° C.), it has excellent brittle crack propagation stopping performance.
pE ≧ 100 (J) (2)
pE: Absorption energy (J) of deformed press notch Charpy at test temperature pT _E (° C.)

  For a thick steel plate with a thickness of 50 mm or more, a Charpy impact test specimen material that is not notched from the center of the thickness is collected, and a deformed press notch is introduced into the specimen specimen using a blade die made of hard steel. And subjected to Charpy impact test. The deformed press notch Charpy test pieces were manufactured in 15 mm square and 13 mm square. Table 1 shows the component composition of the thick steel plate, and Table 2 shows the production conditions.

The Charpy impact test was performed at various temperatures, and the deformed press notch Charpy absorbed energy was found to be a temperature: pT ₁₀₀ (° C.) when the 15 mm square was 100 J, and a temperature: pT ₆₈ (° C.) where the 13 mm square was ₆₈ J. In the deformed press notch Charpy impact test, the deformed press notch Charpy impact test piece was observed after the test, and the test piece in which no fracture occurred from the brittle crack was deemed to have not been evaluated for brittle crack propagation stop performance and was excluded. Thus, the average value of five test pieces in which fracture occurred from brittle cracks at each test temperature was taken. Thereafter, the value of pT ₁₀₀ (° C.) or pT ₆₈ (° C.) was substituted into the above-described equation (1), and the respective temperatures Tk * (° C.) of 15 mm square and 13 mm square were obtained. Further, the respective temperatures Tk ** of 15 mm square and 13 mm square obtained from the brittle fracture surface ratio were also obtained by the same method.

On the other hand, an ESSO test is performed as a large brittle crack propagation test on the same thick steel plate together with a deformed press notch Charpy impact test, and the temperature at which the Kca value becomes 6000 N / mm ^1.5 : Tk6000 (° C.) Asked. Table 3 shows Tk (° C.) and Tk 6000 (° C.). The comparative example is the result of prediction based on the ductile brittle fracture surface transition temperature vTrs of the V-notch Charpy specimen used in the conventional prediction.

  In the comparative example, the prediction error is large and the error is 30 ° C. or more. On the other hand, in the present invention, the prediction errors are all very accurate within 10 ° C., and the usefulness of the evaluation method for brittle crack propagation stopping performance according to the present invention was confirmed.

1 Deformation press notch test piece 2 2mmV press notch

Claims (2)

A method for evaluating the brittle fracture propagation stoppage performance of a thick steel plate, estimating the brittle fracture propagation stoppage performance of a thick steel plate from a small test, wherein the sampling position is the center position of the plate thickness and the press notch is a brittle crack 20 to 225 J energy transition temperature of Charpy absorbed energy obtained by the Charpy impact test in a deformed press notch Charpy impact test using a deformed press notch Charpy impact test piece having a rectangular cross-section exceeding 100 mm ^2. _{Based on E} (° C.), Tk (° C.) calculated according to the equation (1) is set as a stop temperature at which the brittle fracture propagation stop performance (Kca value) becomes a target value, and the stop temperature and the brittle fracture propagation stop performance steel plate by comparing the set temperature target value is set for (Kca value), and evaluating brittle fracture propagation stop performance (Kca value) Evaluation method of brittle fracture propagation stop performance.
Tk = a × pT _E + b (1)
However, pT _E : Charpy absorption energy 20 to 225J energy transition temperature (° C.) a and b are coefficients. In place of pT _E (° C.), a fracture surface transition temperature BATT (° C.) of 50 to 90% showing a fracture surface transition with a brittle fracture surface ratio of a deformed press notch Charpy impact test piece of 50 to 90% is used. The evaluation method of the brittle fracture propagation stop performance of the thick steel plate according to claim 1 .

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