Method for evaluating brittle crack arrestability of steel plate

Abstract

Provided is a method for evaluating brittle crack arrestability of a steel plate by using a large test piece, in which a notch is disposed on one edge in a central portion, in which an embrittled region having a predetermined length L is formed at a tip of the notch or formed so as to include the tip, and in which a fusion zone spaced from the embrittled region is disposed. The fusion zone is formed on one side or both sides of the embrittled region at a distance d from the embrittled region, where d is determined in relation to a thickness t of the steel plate, and a length of the fusion zone is determined by adding ΔL1 (0.3L to −0.3L) to a length L of the embrittled region and by subtracting ΔL2 (0 to 0.4L) from the length L.

Claims

1. A method for evaluating brittle crack arrestability of a steel plate, the method comprising using a large test piece, cooling the large test piece to a predetermined temperature, applying a predetermined stress to the cooled large test piece, forming a crack propagating in the stressed large test piece, and evaluating brittle crack arrestability of the steel plate, wherein the large test piece is a test piece having a width of 500 mm or more and a length of 500 mm or more taken from the steel plate, a notch, at which a brittle crack is generated, is disposed on one edge in a central portion of the one edge in a direction in which the predetermined stress is to be applied to the large test piece, an embrittled region having a predetermined length L extending in a propagation direction of the brittle crack is formed by performing fusion welding before application of the predetermined stress to the large test piece at a tip of the notch or so as to include the tip, and a fusion zone spaced from the embrittled region is formed before application of the predetermined stress to the large test piece in at least one location in the test piece after the embrittled region is formed, the fusion zone being formed at a position satisfying expression (1) below and having lengths satisfying expressions (2) and (3) below:
1.0t≤d≤7.0t  (1), where d: distance (mm) between a center of the embrittled region and a center of the fusion zone and t: thickness (mm) of the test piece (steel plate),
−0.3L≤ΔL1≤0.3L  (2),
0≤ΔL2≤0.4L  (3), where L: length (mm) of the embrittled region, ΔL1: distance (mm) in the propagation direction of the brittle crack between a front edge of the embrittled region and a front edge of the fusion zone (where negative value denotes a position on a non-evaluation region side from the front edge of the embrittled region and a positive value denotes a position on an evaluation region side from the front edge of the embrittled region), and ΔL2: distance (mm) in the propagation direction of the brittle crack between an edge of the test piece and an edge of the fusion zone.

2. The method for evaluating brittle crack arrestability of a steel plate according to claim 1, wherein the step of forming the crack propagating in the stressed large test piece comprises forming a crack using a double tensile process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The FIGURE is a schematic diagram of a large test piece used in accordance with aspects of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(2) One aspect of the present invention is a method for evaluating brittle crack arrestability of a steel plate. In accordance with aspects of the present invention, the brittle crack arrestability of a steel plate is evaluated by using a large test piece, by cooling the large test piece to a predetermined temperature, by applying a predetermined stress to the large test piece, and by forming a crack propagating in the stressed test piece.

(3) In accordance with the FIGURE, the large test piece used in accordance with aspects of the present invention will be described.

(4) The FIGURE is a schematic diagram of a large test piece used in accordance with aspects of the present invention. As illustrated in the FIGURE, in accordance with aspects of the present invention, a large test piece 1 (hereinafter, also referred to as a “test piece”) which is taken from a steel plate (having a thickness of t mm), that is, an evaluation object, and which has a width of 500 mm or more and a length of 500 mm or more is used.

(5) In the case of the large test piece used, a notch 2, in which a brittle crack is generated, is disposed on one edge in the central portion in the stress-application direction of the large test piece. Although there is no particular limitation on the shape of the disposed notch 2, the shape is set to be one with which it is possible to form a brittle crack by applying striking energy via, for example, a wedge. For example, it is preferable that the shape be a V-shape, and a saw-cut notch may further be formed at the notch tip. Moreover, in the case of the large test piece 1 used in accordance with aspects of the present invention, an embrittled region 3 having a predetermined length L (and a width of about 5 mm) extending in the propagation direction of the brittle crack is formed at the tip of the notch 2 or formed so as to include the tip. It is preferable that such an embrittled region 3 be a narrow region extending in the propagation direction of the crack formed by performing fusion welding by using an electron beam welding method. Examples of fusion welding other than that performed by using an electron beam welding method include that performed by using a laser beam welding method or an arc welding method. Here, it is preferable that the width of the embrittled region 3 be 1 mm to 10 mm.

(6) In the case where the embrittled region 3 is formed, a tensile residual stress in the welding line direction due to fusion welding is generated in the test piece 1 and a tensile stress in a direction perpendicular to the direction of the residual stress is applied when a test is performed, and the embrittled region is thus in a biaxial stress state. Therefore, a brittle crack, which has been generated in the notch 2, may deflect from the predetermined embrittled region 3. Here, the tensile residual stress in the welding line direction in the embrittled region 3 generates a compressive residual stress in the welding line direction in a base metal outside the embrittled region 3 and stress balance is thus maintained across the whole test piece 1.

(7) Therefore, in accordance with aspects of the present invention, by forming a fusion zone 4 in the base metal region, in which the compressive residual stress in the welding line direction has been generated, to remove the compressive residual stress, the tensile residual stress in the embrittled region 3 is removed. With this, the brittle crack, which has been generated in the notch 2, is prevented from deflecting from the predetermined embrittled region 3.

(8) The range of the compressive residual stress field, which is formed outside the embrittled region 3, depends on the length L of the embrittled region 3 and the thickness t of the test piece 1. Therefore, in accordance with aspects of the present invention, the fusion zone 4 is formed so that expression (1) below and expressions (2) and (3) below are satisfied.
1.0t≤d≤7.0t  (1)

(9) (where d: distance (mm) between the center of the embrittled region and the center of the fusion zone and t: thickness (mm) of the test piece (steel plate))
−0.3L≤ΔL1≤0.3L  (2)
0≤ΔL2≤0.4L  (3)

(10) (where L: length (mm) of the embrittled region, ΔL1: distance (mm) in the propagation direction of the brittle crack between the front edge of the embrittled region and the front edge of the fusion zone (where negative value denotes a position on the non-evaluation region side from the front edge of the embrittled region 3 and a positive value denotes a position on the evaluation region side from the front edge of the embrittled region 3), and ΔL2: distance (mm) in the propagation direction of the brittle crack between the edge of the test piece and the edge of the fusion zone).

(11) Here, as illustrated in the FIGURE, the term “non-evaluation region” denotes a region in which an artificially generated brittle crack propagates through the embrittled region 3, and the term “evaluation region” denotes a region in which the embrittled region 3 does not exist and which is made only of the material which is the evaluation object. Here, the non-evaluation region in the FIGURE is a region which is located above the “evaluation region” in the FIGURE and in which the embrittled region 3 exists.

(12) In expression (1) above, in the case where d (d: distance (mm) between the center of the embrittled region 3 and the center of the fusion zone 4) is less than 1.0t (t: thickness (mm) of the test piece (steel plate) 1), the residual stress generated in the base metal region due to the embrittled region 3 is not compressive but tensile, and therefore there is no effect of a compressive residual stress even when the embrittled region 3 is formed, and rather an additional tensile residual stress is generated in the embrittled region 3 due to the fusion zone 4, which results in an increased tendency for FPD to occur. On the other hand, in the case where d is more than 7.0t, since the compressive residual stress generated in the base metal region due to the embrittled region 3 fades away almost completely, there is no effect of removing the residual stress.

(13) In expression (2) above, in the case where ΔL1 (ΔL1: distance (mm) in the propagation direction of the brittle crack between the front edge of the embrittled region 3 and the front edge of the fusion zone 4) is less than −0.3L (L: length (mm) of the embrittled region 3), the effect, which is caused by the fusion zone 4, of removing the residual stress does not reach the whole embrittled region 3, and FPD thus occurs. On the other hand, in the case where ΔL1 is more than 0.3L, since a large portion of the residual stress generated by the fusion zone 4 is distributed in the evaluation region, it is not possible to perform appropriate evaluation.

(14) In expression (3) above, in the case where ΔL2 (ΔL2: distance (mm) in the propagation direction of the brittle crack between the edge of the test piece 1 and the edge of the fusion zone 4) is less than 0, it denotes a position outside the test piece 1, and it is thus physically impossible to form a fusion zone. On the other hand, in the case where ΔL2 is more than 0.4L, since the effect, which is caused by the fusion zone 4, of removing the residual stress does not reach the whole embrittled region 3, FPD occurs.

(15) In the case where at least one of the expressions (1) through (3) described above is not satisfied by the formed fusion zone 4, it is not possible to remove the compressive residual stress, and therefore it is not possible to expect the predetermined effect according to aspects of the present invention to be realized.

(16) In accordance with aspects of the present invention, as illustrated in the FIGURE, the fusion zone 4 is formed by preferably using an electron beam welding method so that expression (1), expression (2), and expression (3) described above are satisfied. A fusion zone 4 is formed in at least one location, and the fusion zone 4 is formed after the embrittled region 3 is formed so that the fusion zone 4 is spaced from the embrittled region 3 and arranged on one side or both sides of the embrittled region 3. In the case where the fusion zones 4 are arranged on both sides of the embrittled region 3 (the right-hand side and the left-hand side of the embrittled region 3), it is not necessary that the fusion zones 4 be arranged symmetrically with respect to the embrittled region 3. Here, the fusion zone 4 is preferably formed so that the expressions 1.0t≤d≤2.0t, −0.2L≤ΔL1≤0.2L, and ΔL2=0 are satisfied from the viewpoint of maximally realizing the effects.

(17) In addition, although it is preferable that the fusion zone 4 be arranged parallel to the embrittled region 3 from the viewpoint of preparation of the test piece 1, it is needless to say that the arrangement is not limited to such an example as long as the compressive residual stress in the base metal is removed or decreased. Here, it is acceptable that the fusion zone 4 have an inclination of 60° or less, preferably 30° or less, or more preferably 10° or less with respect to the center line of the embrittled region 3. By setting the inclination of the fusion zone 4 to be 60° or less, it is possible to realize the effect, which is caused by the fusion zone 4, of removing the residual stress across the whole embrittled region 3, and it is thus possible to realize the effects according to aspects of the present invention to a higher degree.

(18) In accordance with aspects of the present invention, although there is no particular limitation on the width of the fusion zone 4 as long as it is possible to perform fusion across the whole plate thickness, it is preferable that the width be, for example, 0.5 mm to 30 mm, because this makes it possible to perform fusion welding across the whole thickness of a steel plate in a practical operation.

(19) The method for evaluating brittle crack arrestability of a steel plate according to aspects of the present invention may be used regardless of the strength of the steel plate, and the evaluation may be performed on, for example, a practical steel plate having a yield strength of 400 MPa to 800 MPa.

(20) The large test piece according to aspects of the present invention may be used regardless of a method for generating an artificial crack, and such a method may be a method used in a double tensile test instead of a crack generation method utilizing impact used in a CAT. Even in such a case, it is possible to realize the effect of preventing FPD as in the case of a CAT.

(21) Hereafter, aspects of the present invention will be further described in accordance with examples.

Examples

(22) High-strength steel plates having a thickness of 19 mm and 80 mm and a strength of 470 MPa grade (having a yield strength of 470 MPa or more) or 600 MPa grade (having a yield strength of 600 MPa or more) were prepared. A large test piece 1 (having the thickness t, a width of 500 mm, and a length of 500 mm), as illustrated in the FIGURE, was taken from each of the steel plates so that the length direction of the large test piece 1 was the rolling direction.

(23) A V-notch 2 was formed on one edge in the central portion in the length direction of the large test piece 1 taken, and an embrittled region 3 including the V-notch and having a length L extending in the propagation direction of the crack was formed by performing fusion welding by using an electron beam welding method. Here, the width of the embrittled region 3 was about 5 mm in the case of a thickness of 19 mm and about 10 mm in the case of a thickness of 80 mm. In this case, the stress-application direction was the length direction of the large test piece 1.

(24) Subsequently, fusion zones 4 were formed in the large test piece 1 so that the fusion zones 4 were spaced from the embrittled region 3 and arranged parallel to the embrittled region 3. In this case, the fusion zones 4 having a length represented by length differences ΔL1 and ΔL2 with respect to the length L of the embrittled region 3 were arranged on both sides of the embrittled region 3 at a distance of d (mm) from the embrittled region 3. Here, “d (mm)” denotes the center-to-center distance between the embrittled region 3 and the fusion zone 4. In the case of test piece Nos. 1 through 10, fusion zones 4 identically shaped were arranged on both sides of the embrittled region 3. In the case of test piece No. 11, only one fusion zone 4 was arranged on one side of the embrittled region 3. In the case of test piece No. 12, only one fusion zone 4 having an inclination of 60° with respect to the center line of the embrittled region 3 was arranged on one side of the embrittled region 3. Here, “d (mm)” in the case of the fusion zone inclined denotes the shortest distance between the embrittled region 3 and the fusion zone 4. In the case of test piece No. 13, fusion zones 4 were arranged on both sides of the embrittled region 3 so that the fusion zones 4 had sizes different from each other. The combinations of d, ΔL1, and ΔL2 of the fusion zones 4 formed are given in Table 1.

(25) As indicated in Table 1, large test pieces 1 having fusion zones 4 having various lengths and positions were used. A CAT was performed such that the large test piece 1 was cooled to a predetermined temperature (−70° C. in the case of a thickness of 19 mm and −10° C. in the case of a thickness of 80 mm), a predetermined striking energy was applied to the notch 2 after having applied a predetermined stress to the large test piece 1, and a brittle crack was generated, allowed to propagate, and arrested. In this CAT, the applied stress was 324 MPa in the case of a thickness of 19 mm and 307 MPa in the case of a thickness of 80 mm.

(26) After having performed the CAT, the propagation state of the brittle crack was observed, and investigation into whether FPD (Fracture Pass Deviation) of the brittle crack occurred was conducted by performing visual observation on the appearance of the large test piece 1. Here, a case where the artificially generated brittle crack propagated outside the embrittled region 3 was judged as a case where FPD occurred, and a case where the artificially generated brittle crack propagated inside the embrittled region 3 to reach the evaluation region was judged as a case where no FPD occurred.

(27) The obtained results are given in Table 1.

(28) TABLE-US-00001 TABLE 1 Steel Plate Fusion Zone-forming Condition**** Strength Embrittled Level Region Conforming/ Conforming/ Conforming/ Brittle Crack (Yield Length L of Nonconforming Nonconforming Nonconforming Arrest Test Test Strength) Thickness Embrittled d to Expression ΔL1 to Expression ΔL2 to Expression Occurrence No. (MPa) t (mm) Region (mm) (mm) (1)* (mm) (2)** (mm) (3)*** of FPD Note 1 600 19 50 5.0t ∘ 0.3L ∘ 0.1L ∘ No Example 5.0t ∘ 0.3L ∘ 0.1L ∘ 2 600 19 100 7.0t ∘ −0.3L ∘ 0.3L ∘ No Example 7.0t ∘ −0.3L ∘ 0.3L ∘ 3 600 19 150 2.0t ∘ 0 ∘ 0 ∘ No Example 2.0t ∘ 0 ∘ 0 ∘ 4 600 19 150 1.0t ∘ 0 ∘ 0 ∘ No Example 1.0t ∘ 0 ∘ 0 ∘ 5 470 80 150 3.0t ∘ 0.2L ∘ 0.2L ∘ No Example 3.0t ∘ 0.2L ∘ 0.2L ∘ 6 470 80 150 1.5t ∘ 0 ∘ 0 ∘ No Example 1.5t ∘ 0 ∘ 0 ∘ 7 600 19 150 0.2t x 0 ∘ 0 ∘ Yes Comparative 0.2t x 0 ∘ 0 ∘ Example 8 600 19 150 10.0t x 0.4L x 0 ∘ Yes Comparative 10.0t x 0.4L x 0 ∘ Example 9 600 19 150 2.0t ∘ 0.6L x 0.3L ∘ Yes Comparative 2.0t ∘ 0.6L x 0.3L ∘ Example 10 600 19 150 10.0t x −0.3L ∘ 0.5L x Yes Comparative 10.0t x −0.3L ∘ 0.5L x Example 11 600 19 150 2.5t ∘ −0.1L ∘ 0.3L ∘ No Example — — — — — — 12 600 19 150 3.0t ∘ −0.15L ∘ 0.35L ∘ No Example — — — — — — 13 600 19 150 1.0t ∘ 0 ∘ 0.2L ∘ No Example 6.0t ∘ −0.2L ∘ 0 ∘ *1.0t ≤ d ≤ 7.0t . . . (1) **−0.3L ≤ ΔL1 ≤ 0.3L . . . (2) ***0 ≤ ΔL2 ≤ 0.4L . . . (3) ****Refer to the figure (Each of the cases with conditions given only in one row is a case where a fusion zone was formed only on one side.)

(29) In the case of all the examples of the present invention (test Nos. 1 through 6 and test Nos. 11 through 13), the formed fusion zones satisfied expressions (1) through (3), and it was possible to prevent FPD. As a result, effective CAT results were obtained. On the other hand, in the case of the comparative examples, which were out of the range of the present invention, the formed fusion zones did not satisfy at least one of expressions (1) through (3), and FPD occurred. As a result, no effective CAT result was obtained.

(30) In the case of test No. 7, expression (1) (condition regarding d) was not satisfied while expression (2) (condition regarding ΔL1) and expression (3) (condition regarding ΔL2) were satisfied, which was out of the range of the present invention, and FPD occurred. In the case of test No. 8, neither expression (1) (condition regarding d) nor expression (2) (condition regarding ΔL1) was satisfied while expression (3) (condition regarding ΔL2) was satisfied, which was out of the range of the present invention, and FPD occurred. In the case of test No. 9, expression (2) (condition regarding ΔL1) was not satisfied while expression (1) (condition regarding d) and expression (3) (condition regarding ΔL2) were satisfied, which was out of the range of the present invention, and FPD occurred. In the case of test No. 10, neither expression (1) (condition regarding d) nor expression (3) (condition regarding ΔL2) was satisfied while expression (2) (condition regarding ΔL1) was satisfied, which was out of the range of the present invention, and FPD occurred.

REFERENCE SIGNS LIST

(31) 1 large test piece 2 notch 3 embrittled region 4 fusion zone