Welded joint, method of designing a welded joint, method of manufacturing a welded joint, and hull structure

Abstract

Inhibition of fracture of a welded joint between steel plates due to strain concentration in a heat-affected zone of welding. A butt welded joint formed using a steel plate, wherein the aforementioned steel plate satisfies a standard following a unified standard (Unified Requirement W11 Rev. 9 2017) of the International Association of Classification Societies (IACS), and has a total elongation of 1.40 times or more of a value of the total elongation prescribed in the aforementioned unified standard, the thickness of the aforementioned butt welded joint is set to t (mm), the width of a heat-affected zone of welding is set to Lh (mm), the hardness of the aforementioned heat-affected zone of welding is set to Hh, the hardness of a base material portion is set to Hb, and the hardness of a weld metal portion is set to Hw, in a case where Hh / Hb is less than 0.97, the following formulas (1) to (4) are satisfied, and in a case where Hh / Hb is 0.97 or more, the following formulas (3) to (4) are satisfied.Lh≤(0.034t+0.510) / (1-Hh / Hb) 0.9 ···(1)Hh / Hb≥0.70···(2)Hw / Hb≥1.0···(3)6≤t≤40···(4).

Description

TECHNICAL FIELD

[0001] The present application relates to a welded joint, a design method for a welded joint, a manufacturing method for a welded joint, and a ship structure provided with the welded joint. BACKGROUND

[0002] In recent years, marine pollution caused by marine accidents such as collision of ships, grounding, and the like has become a social problem. For example, even a ship such as a bulk carrier, a coal carrier, or the like, which does not excessively pollute the sea with cargo, sometimes pollutes the sea with fuel oil leakage. In addition, if oil as cargo leaks from a ship such as a tanker, marine pollution becomes more significant. Therefore, it is necessary to suppress a breach of a ship's hull due to collision, grounding, or the like.

[0003] Therefore, Patent Literature 1 proposes a ship structure excellent in collision resistance. The collision resistance here means a property of being able to suppress a breach of a ship's hull even if the ship collides with another ship at a prescribed speed, for example. The ship structure has a ship's hull structure that uses a high ductility steel plate classified as 32, 36, or 40 in strength, which satisfies a standard following a unified standard (Unified Requirement W11 Rev.8 2014) of the International Association of Classification Societies (IACS), a required specification of a total elongation of 1.4 times or more the value of the total elongation prescribed by the unified standard of IACS, and is confirmed to satisfy the above specification, at a part of an outer plate or an inner plate of a ship's side or at all of the outer plate or the inner plate. In this case, by using the above high ductility steel plate in the ship structure, it is possible to suppress a breach of the outer plate, the inner plate, or the like of the ship's hull.

[0004] In addition, Patent Literature 2 proposes a welded joint that suppresses a fracture at a welded metal portion. Patent Literature 2 shows a relational expression with a parameter of a groove angle, a tensile strength of a welded metal, a tensile strength of a base material, or the like in a full penetration welding with a groove having a V shape or the like, and a welded joint proposed thereby satisfies the relational expression. In this case, even if the strength of the welded metal material is lower than that of the base material, it is possible to realize a strong and tough welded joint by avoiding concentration of deformation at the welded metal portion having lower toughness than the base material.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent No. 5893231

[0008] Patent Literature 2: Japanese Patent No. 6319027 SUMMARY

[0009] Problem to be solved by the invention

[0010] However, in a case where the strength of the weld metal material in the welded joint is lower than the strength of the base material, that is, a so-called mismatch case, or a case where a weld heat affected zone (HAZ) is softened, if a tensile stress acts on the welded joint, plastic strain is concentrated in the soft portion. Therefore, it is assumed that the welded joint is broken when the elongation of the member of the welded joint is small.

[0011] In particular, in a case where the microstructure of a steel sheet is controlled using a thermo mechanical control process (TMCP) to increase the strength and elongation characteristics of the steel sheet, the HAZ reaches a temperature higher than the austenitizing temperature due to weld heat. As a result, the microstructure of the base material is not retained in the HAZ, and the strength is sometimes reduced compared to the base material. In this way, the HAZ is easily softened, and the strength and elongation characteristics of the steel sheet can not be fully exhibited for the welded joint.

[0012] The present inventors and others have conducted intensive research, and as a result, it has been found that, as conditions for preventing the HAZ in the welded joint from being broken, the softening rate of the HAZ and the width of the HAZ have an effect, as described later.

[0013] Therefore, the present application was completed in view of the above problems, and aims to suppress the breakage of a welded joint between steel sheets due to strain concentration at a weld heat affected zone.

[0014] Solution for solving the problem

[0015] To solve the above problems, according to one aspect of the present application, a welded joint is provided, which is a butt welded joint formed using a steel sheet, wherein the aforementioned steel sheet satisfies a standard following a unified standard (Unified Requirement W11 Rev. 9 2017) of the International Association of Marine Consultants (IACS), and has a total elongation of 1.40 times or more the value of the total elongation specified by the aforementioned unified standard, the plate thickness of the aforementioned butt welded joint is set to t (mm), the width of the weld heat affected zone is set to Lh (mm), the hardness of the aforementioned weld heat affected zone is set to Hh, the hardness of the base material portion is set to Hb, and the hardness of the weld metal portion is set to Hw, and in a case where Hh / Hb is less than 0.97, the following formulas (1) to (4) are satisfied, and in a case where Hh / Hb is 0.97 or more, the following formulas (3) to (4) are satisfied.

[0016] In the aforementioned welded joint, the aforementioned steel sheet can be classified as 32, 36, or 40 by the strength classification specified by the aforementioned unified standard.

[0017] The total elongation value in the tensile test using a flat joint test piece having a distance between punctuations of 200 mm and a width of 40 mm can be 1.4 times or more the total elongation value of the base material portion prescribed by the aforementioned unified standard.

[0018] In addition, in order to solve the above problem, according to another aspect of the present application, there is provided a design method of a welded joint, which is a design method of a butt welded joint formed using a steel sheet, the design method having: a steel sheet selection step of selecting, as the steel sheet, a steel sheet that satisfies a standard following a unified standard (Unified Requirement W11 Rev.9 2017) of the International Association of Marine Consultants (IACS) and has a total elongation that is 1.40 times or more the total elongation value prescribed by the aforementioned unified standard; and a welding condition setting step of setting welding conditions of the butt welding in such a manner that, when a plate thickness of the aforementioned butt welded joint is set to t (mm), a width of a weld heat-affected zone is set to Lh (mm), a hardness of the aforementioned weld heat-affected zone is set to Hh, a hardness of a base material portion is set to Hb, and a hardness of a weld metal portion is set to Hw, the following equations (1) to (4) are satisfied in a case where Hh / Hb is less than 0.97, and the following equations (3) to (4) are satisfied in a case where Hh / Hb is 0.97 or more.

[0019] In addition, in order to solve the above problem, according to another aspect of the present application, there is provided a design method of a welded joint, which is a design method of a butt welded joint formed using a steel sheet, the design method having: a steel sheet selection step of selecting, as the steel sheet, a steel sheet that satisfies a standard following a unified standard (Unified Requirement W11 Rev.9 2017) of the International Association of Marine Consultants (IACS) and has a total elongation that is 1.40 times or more the total elongation value prescribed by the aforementioned unified standard; and a welding condition setting step of setting welding conditions of the butt welding in such a manner that, when a plate thickness of the aforementioned butt welded joint is set to t (mm), a width of a weld heat-affected zone is set to Lh (mm), a hardness of the aforementioned weld heat-affected zone is set to Hh, a hardness of a base material portion is set to Hb, and a hardness of a weld metal portion is set to Hw, the following equations (1) to (4) are satisfied in a case where Hh / Hb is less than 0.97, and the following equations (3) to (4) are satisfied in a case where Hh / Hb is 0.97 or more.

[0020] In addition, in order to solve the above problem, according to another aspect of the present application, there is provided a hull structure in which a portion of a butt welded joint of an outer plate of a ship side or a ship bottom or all of the butt welded joint, or a portion of a butt welded joint of an inner plate of the ship side or the ship bottom or all of the butt welded joint is the aforementioned welded joint.

[0021] In addition, in order to solve the above problem, according to another aspect of the present application, there is provided a hull structure in which a portion of a butt welded joint of an outer plate of a ship side or a ship bottom or all of the butt welded joint, or a portion of a butt welded joint of an inner plate of the ship side or the ship bottom or all of the butt welded joint is the aforementioned welded joint.

[0022] Lh≤(0.034t+0.510) / (1-Hh / Hb) 0.9 (1)

[0023] Hh / Hb≥0.70 (2)

[0024] Hw / Hb≥1.0 (3)

[0025] 6≤t≤40 (4)

[0026] Effects of the invention

[0027] As explained above, according to the present application, it is possible to provide a welded joint excellent in ductility, which can suppress the fracture of a welded joint between steel plates due to strain concentration at a weld heat affected zone. In addition, by using this welded joint in a portion of a butt welded joint of an outer plate of a ship side or a ship bottom or all of the butt welded joint, it is possible to suppress the fracture of the welded joint due to, for example, a collision of a ship or grounding. As a result, the contribution to industry becomes extremely significant. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is a view for explaining a member of a hull structure.

[0029] Figure 2 is a view in which a ship side and a ship bottom of a hull structure in Figure 1 are enlarged.

[0030] Figure 3 is a view showing a joint tensile test piece used in FEM analysis.

[0031] Figure 4 is a view showing an example of a model used in FEM analysis.

[0032] Figure 5 is a graph showing a comparison between a limit HAZ width derived by an estimation formula and a limit HAZ width derived by FEM analysis.

[0033] Figure 6 This is a diagram showing the state of the joint tensile test piece after the joint tensile test.

[0034] Figure 7 This is a diagram showing the cross-sectional hardness distribution of an EGW joint made of high-ductility steel plate.

[0035] Figure 8 This is a diagram showing the cross-sectional hardness distribution of an existing steel EGW joint. Detailed Implementation

[0036] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that in this specification and the accompanying drawings, constituent elements that have substantially the same functional configuration are omitted from repeated description by using the same reference numerals.

[0037] <Ship Structure>

[0038] First, as an example of ship hull structure (hull structure), the double-hull structure of an oil tank will be explained. For example... Figure 1 and Figure 2 As shown, the main components constituting the side section 10 of the double-hull structure are the outer plating 11 and the inner plating 12, the anti-bending members 13 and 14 attached to the outer plating 11 and the inner plating 12, the transverse beams 15, and the longitudinal beams 16. Additionally, the main components constituting the bottom section 20 are the outer plating 21 and the inner plating 22, the anti-bending members 23 and 24 attached to the outer plating 21 and the inner plating 22, the transverse beams 25, and the longitudinal beams 26. The double-hull structure also includes an upper deck 30 and a bilge 31.

[0039] <Steel Plate>

[0040] In the hull structure of this embodiment, the main components such as the outer and inner plates are made of high-ductility steel plates. These high-ductility steel plates, such as those disclosed in Patent Document 1, meet the standards of the Unified Requirement W11 Rev. 9 2017 of the International Association of Classification Societies (IACS) and have a total elongation of at least 1.40 times the value specified in the IACS unified standard. In this case, the collision resistance of the ship can be significantly improved, and breaches in the hull, such as the outer and inner plates, can be suppressed. It should be noted that the higher the multiple of the total elongation relative to the value specified in the IACS unified standard, the better. While no specific upper limit is specified, approximately 2.20 times is generally considered the upper limit.

[0041] Specifically, as described in Patent Literature 1, in the case of assuming a collision accident of a large crude oil tanker (VLCC: Very Large Crude oil Carrier) as a large ship, a steel sheet having various total elongation is applied to the outer plate, inner plate, and the like of the ship side portion of the hull structure, and analysis is performed based on the finite element method (FEM), and in the case of using a high ductility steel sheet having a total elongation of 1.40 times or more of the value of the total elongation prescribed by the unified standard of IACS, the energy absorption amount can be increased compared to the case of using a conventional steel, and the occurrence of a breach in the hull, such as the outer plate, inner plate, and the like, can be suppressed. In addition, in the case of using the above high ductility steel sheet for the outer plate, inner plate, and the like of the ship bottom portion, the energy absorption amount can also be increased, and the occurrence of a breach in the hull, such as the outer plate, inner plate, and the like, can be suppressed.

[0042] Note that the value of the total elongation prescribed by the unified standard (Unified Requirement W11 Rev.9 2017) is shown in Table 1. In Table 1, the value of the minimum elongation that the hull material used should satisfy is prescribed according to the plate thickness and the grade (Grade). In the unified standard, the letters (A, B, D, E, and F) in the grade indicate the difference in the test temperature required by the Charpy impact test, and the numbers (32, 36, and 40) indicate the classification of the strength. The high ductility steel sheet has an elongation exceeding the standard value of the total elongation shown in Table 1, and satisfies the unified standard. By using a high ductility steel sheet having the above strength classification, the effects of the present application described below become particularly remarkable.

[0043] [Table 1]

[0044] Table 1

[0045]

[0046] Test piece: GL = 200 mm, W = 25 mm

[0047] As described above, a high ductility steel sheet is used in the hull structure of the present embodiment. In the following description, when referred to as a "high ductility steel sheet", it refers to a steel sheet that satisfies the standards following the unified standard of IACS, and has a total elongation of 1.40 times or more of the value of the total elongation prescribed by the unified standard of IACS.

[0048] Note that the high ductility steel sheet is not limited in composition and manufacturing conditions as long as the above conditions are satisfied. For example, as the high ductility steel sheet, a steel sheet having a composition containing, by mass %, C: 0.02 to 0.18%, Si: 0.01 to 0.50%, Mn: 0.9 to 1.6%, Al: 0.001 to 0.100%, N: 0.02% or less, P: 0.02% or less, and S: 0.01% or less, and the balance of Fe and impurities can be used.

[0049] In addition, depending on the required properties such as increasing the strength of the base material portion and increasing the toughness of the welded joint, a steel sheet having a composition different from the above can also be used as the high ductility steel sheet. For example, in the above composition, one or two or more kinds selected from the group consisting of Ni: 0.8% or less, Cr: 0.2% or less, Mo: 0.08% or less, Cu: 0.35% or less, W: 1.0% or less, Co: 1.0% or less, V: 0.1% or less, Nb: 0.05% or less, Ti: 0.02% or less, Zr: 0.05% or less, Ta: 0.05% or less, Hf: 0.005% or less, REM (Rare Earth Metal): 0.005% or less, Y: 0.005% or less, Ca: 0.01% or less, Mg: 0.01% or less, Te: 0.01% or less, Se: 0.005% or less, B: 0.005% or less, and Sn: 0.3% or less can be substituted for a part of Fe.

[0050] In addition, in addition to the high ductility steel sheet classified by strength as described above, for example, a mild steel satisfying the standards following the Unified Requirement W11 Rev.92017 of the International Association of Classification Societies (IACS) and having a total elongation of 1.40 times or more of the value of the total elongation prescribed by the Unified Requirement of IACS can be used.

[0051] < Welded Joint >

[0052] Next, a butt welded joint (hereinafter referred to as "welded joint") formed using a high ductility steel sheet in the ship structure of the present embodiment will be described. As a welding method of the welded joint, for example, a manual arc welding (SMAW), a carbon dioxide gas (CO2) arc welding, an electric arc welding (EGW), a submerged arc welding (SAW), or the like can be used.

[0053] As described above, in the present embodiment, by using a high ductility steel sheet in the ship structure, it is possible to suppress a breach of a hull such as an outer plate, an inner plate, and the like. On the other hand, in the related art, it is not clear whether or not the effect of suppressing a fracture at the welded joint is obtained. For example, at the time of a collision of a ship, grounding, or the like, there is a concern that a fracture occurs along a weld of the welded joint.

[0054] The present inventors have conducted intensive studies in order to eliminate the above concerns, and as a result, have conceived the possibility that the site of fracture would change due to the difference in hardness between the HAZ and the base material portion in the welded joint.

[0055] Accordingly, the present inventors have conducted further verification of the insight conceived in the above studies, and have conducted simulation of a joint tensile test using the finite element method (FEM). As a result, the conditions for a welded joint that does not cause the HAZ to fracture (specifically, the conditions for the softening rate of the HAZ (hereinafter referred to as "HAZ softening rate") and the width of the HAZ (hereinafter referred to as "HAZ width")) were found as described below.

[0056] Figure 3 is a view showing a joint tensile test piece (flat joint test piece) used in the FEM analysis. Figure 3 (a) of FIG. 1 shows a side view, Figure 3 (b) of FIG. 1 shows a plan view. The outer shape of the joint tensile test piece is based on JIS 1A tensile test piece. The joint tensile test piece was modeled such that the weld metal portion WM and the weld heat-affected zone HAZ are located at the center portion in the length direction of the joint tensile test piece. That is, in the modeled joint tensile test piece, the weld metal portion WM is located at the center, the weld heat-affected zone HAZ is located outside the weld metal portion WM, and the base material portion BM is located further outside the weld heat-affected zone HAZ.

[0057] In this model, the length of the joint tensile test piece is 580 mm, the length of the parallel portion is 220 mm, and the distance GL between the marks is 200 mm. The width of the grip portion of the joint tensile test piece is 60 mm, and the width of the parallel portion is 40 mm. In the base material portion BM, the radius of curvature R at the position where the width changes between 60 mm and 40 mm is 25 mm. Note that the width of the parallel portion is not limited to 40 mm, and can be set to 25 mm, for example.

[0058] In addition, the plate thickness t of the joint tensile test piece was set to four cases of 6 mm, 12 mm, 24 mm, and 36 mm. The width of the weld metal portion WM was fixed at 20 mm, and the width Lh of the weld heat-affected zone HAZ was changed at 1 mm intervals in the range of 1 to 15 mm. The range of the HAZ width Lh of 1 to 15 mm is a range assumed for the weld heat-affected zone HAZ under usual welding conditions. The softening rate of the weld heat-affected zone HAZ with respect to the base material portion BM was set to four cases of 5%, 10%, 20%, and 30%.

[0059] An example of the model used in the FEM analysis (right half of the joint tensile test piece) is shown in Figure 4 . Figure 4An example of the model when the plate thickness t of the joint tensile test piece is 12 mm and the width Lh of the weld heat-affected zone HAZ is 5 mm is shown in FIG. 1.

[0060] In the FEM analysis, the true stress-true strain relationship of the material was approximated in accordance with the Swift rule of the following equation (6). In the following equation (6), σ t is the true stress, ε t is the true strain, σ y , α, n are material properties. In addition, the parameters of the Swift rule used in the FEM analysis were set based on data obtained in various experiments performed by the present inventors and others and as shown in Table 2.

[0061] σ t = σ y (1 + ε t / α) n ... (6)

[0062] [Table 2]

[0063] Table 2

[0064] Site σy α n BM 401 0.0159 0.178 WM 551 0.0005 0.071 HAZ (softening rate 5%) 381 0.0159 0.178 HAZ (softening rate 10%) 361 0.0159 0.178 HAZ (softening rate 20%) 321 0.0159 0.178 HAZ (softening rate 30%) 281 0.0159 0.178

[0065] Under the conditions of the plate thickness t (4 cases) and the softening rate of the weld heat-affected zone HAZ (4 cases) of the joint tensile test piece described above, the width Lh of the weld heat-affected zone HAZ was changed in the range of 1 to 15 mm at an interval of 1 mm, and FEM analysis was performed, and the limit of the width of the weld heat-affected zone HAZ at which the weld heat-affected zone HAZ did not break (hereinafter referred to as "limit HAZ width") was found. The results are shown in Table 3.

[0066] [Table 3]

[0067] Table 3

[0068]

[0069] As shown in Table 3, it was found that in the case where the plate thickness t of the joint tensile test piece is small and the softening rate of the weld heat-affected zone HAZ is large, there is a tendency for the limit HAZ width to be small, and the weld heat-affected zone HAZ is easily broken when the width Lh of the weld heat-affected zone HAZ is small. On the other hand, it was found that in the case where the plate thickness t of the joint tensile test piece is large and the softening rate of the weld heat-affected zone HAZ is small, there is a tendency for the limit HAZ width to be large, and the weld heat-affected zone HAZ is not easily broken even when the width Lh of the weld heat-affected zone HAZ is large.

[0070] Next, the present inventors derived an estimation formula of the limit HAZ width based on the FEM analysis results shown in Table 3. It is assumed that when the HAZ softening rate approaches 0% (i.e., Hh / Hb approaches 1), the limit HAZ width will be infinite. Based on this assumption, the limit HAZ width Lh shown in the following equation (7) is defined. LIM An estimation formula of the limit HAZ width Lh is derived.

[0071] Lh LIM = a / (1 - Hh / Hb) b ···(7)

[0072] Using the FEM analysis results of Table 3, the coefficients a and b were derived for each plate thickness t of the joint tensile test piece by the least square method. As a result, it was found that a can be regarded as varying depending on the plate thickness t, while b does not depend on the plate thickness t, and the following equations (8) and (9) were derived.

[0073] a = 0.034t + 0.510 ··· (8)

[0074] b = 0.9 ··· (9)

[0075] According to the above, based on the above equations (7) to (9), an estimation formula of the limit HAZ width shown in the following equation (10) was derived. In the following equation (10), Lh LIM is the limit HAZ width (mm), t is the plate thickness (mm) of the joint tensile test piece (welded joint), Hh is the hardness of the HAZ, and Hb is the hardness of the base material portion.

[0076] Lh LIM = (0.034t + 0.510) / (1 - Hh / Hb) 0.9 ···(10)

[0077] Further, as shown in the following equation (11), if the HAZ width Lh is the limit HAZ width Lh LIM , then the HAZ does not break. Thus, according to equations (10) and (11), a condition that the welded joint should satisfy in the present application, i.e., the following equation (12), was derived. In addition, as described above, since the HAZ softening rate is a value exceeding 0%, Hh / Hb is a value less than 1.00 as shown in the following equation (13).

[0078] Lh ≤ Lh LIM ···(11)

[0079] Lh ≤ (0.034t + 0.510) / (1 - Hh / Hb) 0.9 ···(12)

[0080] Hh / Hb < 1.00 ··· (13)

[0081] Note that the welding joint is set to satisfy the conditions shown in the following equations (14) to (16). As described above, in the FEM analysis, the HAZ softening rate is set to four cases of 5%, 10%, 20%, and 30%. Further, according to equation (12), in the case where the plate thickness is 6 mm, since Hh / Hb is 0.97 or more and the limit HAZ softening width Lh LIM more than 15 mm, equation (14) is derived. Further, since the hardness of the weld metal portion is the same as or more than that of the base material portion, and is so-called overmatching which is assumed based on a general welding joint, equation (15) is derived. Moreover, although the plate thickness t of the joint tensile test piece is set to four cases of 6 mm, 12 mm, 24 mm, and 36 mm in the FEM analysis as described above, it is confirmed by the present inventors and the like that the plate thickness t of 40 mm also satisfies equation (10) as described later. Based on this result, equation (16) is derived.

[0082] 0.97 > Hh / Hb > 0.70... (14)

[0083] Hw / Hb > 1.0... (15)

[0084] 6 < t < 40... (16)

[0085] As described above, the conditions that the welding joint should satisfy in the present application can be divided into different cases according to the value of Hh / Hb, and equations (1) to (4) are satisfied when Hh / Hb is less than 0.97, and equations (3) to (4) are satisfied when Hh / Hb is 0.97 or more. In other words, according to the value of Hh / Hb, if the welding joint satisfies the conditions as described above, the tensile fracture due to the strain concentration of the HAZ (soft portion) can be suppressed, and a welding joint excellent in ductility can be provided. In equations (1) to (4), Hh is the hardness of the HAZ, Hb is the hardness of the base material portion, t is the plate thickness (mm) of the welding joint, Lh is the HAZ width (mm), and Hw is the hardness of the weld metal portion.

[0086] Lh < (0.034t + 0.510) / (1 - Hh / Hb) 0.9 ... (1)

[0087] Hh / Hb > 0.70... (2)

[0088] Hw / Hb > 1.0... (3)

[0089] 6 < t < 40... (4)

[0090] Note that in the present embodiment, the Vickers hardness distribution at the 1 / 4-thickness position and the 3 / 4-thickness position of the cross section of the base material and the welded joint was measured at an interval of 1 mm in accordance with JIS Z2244:2009. At this time, a test piece of the base material portion not affected by heat and a test piece of the welded joint were prepared respectively. In the hardness measurement of the welded portion, a sample was collected in such a manner that the weld was located at the center in a direction perpendicular to the weld extension direction of the welded joint, and was used as the measurement cross section. In addition, in the Vickers hardness distribution measurement, the load was set to 10 kg. The average value of the hardness of the base material portion calculated using the results obtained by the measurement was taken as Hb, the minimum value of the hardness of the HAZ was taken as Hh, and the minimum value of the hardness of the weld metal portion was taken as Hw. In addition, based on the measurement results of the hardness as described above, the width of the HAZ softened portion at the 1 / 4-thickness position and the 3 / 4-thickness position was calculated, and the average value thereof was taken as Lh.

[0091] Note that for the average value of the hardness of the base material portion Hb, the minimum value of the hardness of the HAZ Hh, the minimum value of the hardness of the weld metal portion Hw, and the HAZ width Lh, the definitions and the more specific measurement methods are as described below.

[0092] That is, for the base material portion, 10 points were measured at an interval of 1 mm at the 1 / 4-thickness position and the 3 / 4-thickness position respectively, and a total of 20 points were measured, and the average value of the 20 measurement values obtained was taken as the hardness Hb of the base material portion. In the measurement of the welded joint, after polishing, the cross section of the welded joint was etched with nitric acid alcohol, and thus the weld metal and the HAZ were visualized. Then, at the 1 / 4-thickness position and the 3 / 4-thickness position, starting from the boundary line (fusion line) between the weld metal and the HAZ, the hardness distribution was measured at an interval of 1 mm on the base material side until the base material portion was reached, and the minimum value of the measurement results was taken as the hardness Hh of the HAZ. The region in which the hardness measurement result was 97% or less of the hardness Hb of the base material portion was defined as the HAZ softened region, and the distance from the fusion line to the base material side end portion of the HAZ softened region was calculated at the 1 / 4-thickness position and the 3 / 4-thickness position respectively, and the maximum value of the distances obtained was taken as the HAZ width Lh. In addition, for the weld metal portion, measurement was performed at an interval of 1 mm at the 1 / 4-thickness position and the 3 / 4-thickness position, and the minimum value of the measurement values obtained was taken as the hardness Hw of the weld metal portion.

[0093] Furthermore, the welded joint appropriately satisfies the conditions (1) to (4) above based on the Hh / Hb value. Thus, when a flat joint test piece with a spacing of 200 mm between the puncture points and a width of 40 mm is prepared and subjected to a tensile test, the total elongation value in the tensile test reaches more than 1.40 times the total elongation value of the base material as specified in the aforementioned unified standard. The higher the ratio of the total elongation to the total elongation value of the base material as specified in the aforementioned unified standard, the better. There is no specific upper limit, but in practice, it is around 2.20 times.

[0094] <Verification>

[0095] Among them, the above-mentioned limiting HAZ width Lh LIM The estimated formula is the above formula (10) and is verified. Figure 5 The limiting HAZ width is derived from the estimation formula of equation (10). Figure 5 The horizontal axis) and the limiting HAZ width obtained in the FEM analysis ( Figure 5 A chart comparing the vertical axis (of the two axes). (Refer to...) Figure 5 It can be seen that, for the four cases where the plate thickness t of the welded joint is 6mm, 12mm, 24mm, and 36mm, the estimation results using equation (10) are in good agreement with the FEM analysis results. It should be noted that even when the plate thickness t of the welded joint is 40mm, the results for… Figure 5 The extrapolation of the graphs using equation (10) shows good agreement with the FEM analysis results. Therefore, it can be concluded that the conditions that the welded joint should meet, namely the above equations (1) to (4), are appropriate.

[0096] <Design Methods for Welded Joints>

[0097] Next, the design method for welding joints when manufacturing such welding joints will be explained.

[0098] The welding joint design method of this embodiment is a design method for butt welded joints formed using steel plates. This design method includes: a steel plate selection step, in which a steel plate is selected as the blank for the welding joint; and a welding condition setting step, in which the welding conditions for butt welding are set.

[0099] The steel plate selection step is to select a steel plate as the blank for the welded joint, which meets the standards of the Unified Requirement W11 Rev. 9 2017 of the International Association of Classification Societies (IACS) and has a total elongation of more than 1.40 times the value of the total elongation specified in the Unified Requirement W11 Rev. 9 2017.

[0100] Further, the welding condition setting step is a step of setting the welding condition for the butt welding in such a manner that, when a plate thickness of the welded joint is set as t (mm), a width of the heat-affected zone is set as Lh (mm), a hardness of the heat-affected zone is set as Hh, a hardness of the base material portion is set as Hb, and a hardness of the weld metal portion is set as Hw, the above-described equations (1) to (4) are satisfied.

[0101] In the welding condition setting step, various simulation methods represented by FEM can be implemented, and the welding condition satisfying the above-described equations (1) to (4) can be virtually obtained. Further, using the steel sheet selected in the above-described steel sheet selection step, the butt welding and the verification after the welding can be actually performed while changing the welding condition, and the welding condition satisfying the above-described equations (1) to (4) can be experimentally obtained.

[0102] Through the above-described welding condition setting step, a detailed design drawing for manufacturing the required welded joint including the welding condition can be obtained.

[0103] <Manufacturing method of welded joint>

[0104] Next, the manufacturing method of the above-described welded joint will be described.

[0105] In the manufacturing method of the welded joint of the present embodiment, the steel sheet selected according to the above-described design method of the welded joint is subjected to the butt welding according to the set welding condition, thereby manufacturing the above-described welded joint. That is, it can be said that the manufacturing method of the welded joint of the present embodiment has the above-described steel sheet selection step and welding condition setting step, and a welding step of welding the selected steel sheet according to the set welding condition.

[0106] As the welding method used, for example, SMAW (Shielded Metal Arc Welding), CO2 arc welding, EGW (Electrogas Welding), SAW (Submerged Arc Welding), and the like can be mentioned.

[0107] Further, in the welding, by adopting, for example, the following conditions, the welded joint satisfying the above-described equations (1) to (4) can be reliably manufactured, and thus is particularly preferable.

[0108] That is, in the case of a large welding line energy, there is a tendency for the HAZ to become large, and therefore attention is required. For example, in electric arc welding (EGW), submerged arc welding (SAW), in the case of a welding line energy exceeding 50 kJ / cm, it becomes particularly important to select a steel sheet that is difficult to soften the HAZ. On the other hand, in shielded metal arc welding (SMAW), carbon dioxide gas (C02) arc welding, if the welding line energy is made to be 20 kJ / cm or less, the HAZ becomes small, and therefore, even if the HAZ softening characteristics of the steel sheet are not considered, it is possible to manufacture a welded joint that satisfies the above equations (1) to (4).

[0109] <Applications of the welded joint to a ship structure>

[0110] In a ship structure, a welded joint (hereinafter referred to as "the above-mentioned welded joint") that satisfies the above equations (1) to (4) as above is used for a portion of a butt welded joint of an outer plate of a ship side or a ship bottom, or all portions of the butt welded joint. Also, in a ship structure, the above-mentioned welded joint is used for a portion of a butt welded joint of an inner plate of a ship side or a ship bottom, or all portions of the butt welded joint.

[0111] In particular, the above-mentioned welded joint is used for a portion of a butt welded joint of an outer plate or an inner plate of a ship side or a ship bottom that requires suppression of a break. Among them, the portion that requires suppression of a break is a portion of a ship side or a ship bottom that is likely to be impacted when a ship collides or runs aground, and depends on the type of the ship.

[0112] For example, in a bulk carrier, a portion where there is no ballast tank and the hold is one outer plate (i.e., a portion where there is no inner plate) can be determined as a portion that requires suppression of a break, and the above-mentioned welded joint is used for the welded joint at this portion. Alternatively, a portion where an outer plate that is a part of a fuel tank can be determined as a portion that requires suppression of a break, and the above-mentioned welded joint is used for the welded joint at this portion.

[0113] Also, for example, in a liquid cargo ship, a portion of an outer plate that is opposite to an inner plate where a storage tank that stores a product oil (crude oil in the case of a crude oil liquid cargo ship) can be determined as a portion that requires suppression of a break, and the above-mentioned welded joint is used for the welded joint at this portion.

[0114] Also, for example, in a spherical tank type LNG ship, a portion of a ship side outer plate that is closest to a spherical tank that stores LNG can be determined as a portion that requires suppression of a break. In this case, since the tank is spherical, the portion does not need to be a portion that covers the entire tank in plan view and side view, and only a portion that is closest to the tank. Also, the above-mentioned welded joint can be used for the welded joint at the specific portion. If necessary, a portion of a ship side outer plate that is closest to the spherical tank can also be determined as a portion that requires suppression of a break.

[0115] The above method is a method of determining the portion where the breach is required to be suppressed based on the design drawing of the ship. The energy absorption analysis of each member can also be performed using FEM, and the portion where the breach is required to be suppressed can be determined.

[0116] Note that, in the ship structure, the above-described welding joint can be used for a portion of or all of the butt welding joint of any one of the anti-flexing members, cross beams, and longitudinal beams of the ship side or the ship bottom. In addition, in the ship structure, the above-described welding joint can be used for a portion of or all of the butt welding joint of any one of the upper deck and the bilge.

[0117] In addition, the above-described welding joint can be used not only for large ships but also for small ships, and is particularly effective when applied to large ships. Furthermore, the above-described welding joint can be used for any one of a ship having a double hull structure (double hull) and a ship having a single hull structure (single hull). Note that, in the case of the single hull structure, the outer plate can be regarded as the inner plate (and conversely, the inner plate can be regarded as the outer plate).

[0118] Note that, in the ship structure of the above embodiment, the high ductility steel plate having a total elongation of 1.40 times or more the value of the total elongation prescribed by the unified standard of IACS is used. However, in terms of quality management of the high ductility steel plate, as a realistic manufacturing target of the above-described high ductility steel plate, it is preferable to make it 1.50 times or more the value of the total elongation prescribed by the unified standard of IACS.

[0119] Example

[0120] Next, an example and a comparative example are shown, and the welding joint of the present embodiment is specifically described.

[0121] First, the inventors et al. performed joint tensile tests on the high ductility steel plates and the existing steel shown in Table 4 below, and conducted verification. As the existing steel, YP36 steel (yield stress 36 kgf / mm 2 Note that, in Table 4 below, the yield stress (YP), the tensile strength (TS), and the total elongation (EL) are also described. In addition, the item of "multiple" in Table 4 below indicates the multiple with respect to the value of the total elongation prescribed by the unified standard of IACS.

[0122] [Table 4]

[0123] Table 4

[0124]

[0125] *: times of the value of the total elongation prescribed by the uniform standard of IACS

[0126] Next, two of each of the welded joints between the high ductility steel sheets shown in Table 4 above and the welded joints between the existing steels were produced by EGW, CO2 arc welding or SAW, and joint tensile tests were performed on each of the welded joints by using No. 1A tensile test based on JIS Z 2241:2011 (using a flat joint test piece with a distance between the pin points of 200 mm and a width of 40 mm).

[0127] At this time, the welding conditions for each of the welding methods were set as shown in Table 5 below.

[0128] In addition, Test No. 7 in Table 5 below used a mismatched welding material. In addition, Test No. 8 in Table 5 below was made to have a larger line energy than Test No. 4.

[0129] In addition, the specific values of each of the parameters described in Formulas (1) to (4) were measured for the welded joints before the joint tensile tests by the methods described earlier, and the results were as follows, respectively.

[0130] The results of the joint tensile tests above are shown in Table 5 below. Table 5 shows the results of the measurement of the tensile strength (TS) and the total elongation (EL) of the joint tensile test pieces of each of the steels and each of the welding methods, and the fracture position. Among them, the item of "times" in Table 5 below indicates the times of the value of the total elongation of the base material portion prescribed by the uniform standard of IACS. Further, Figure 6 The state of the joint tensile test pieces after the tests is shown for Test Nos. 1 and 5.

[0131] As shown in Table 5 and Figure 6 It was confirmed that the welded joints between the high ductility steel sheets used in the present tests were fractured at the base material portion, and the ductility (elongation) was not reduced. On the other hand, it was confirmed that the welded joints between the existing steels were fractured at the HAZ, and the ductility was greatly reduced.

[0132] [Table 5]

[0133]

[0134] The present inventors and others conducted a study on the results that the welded joint of the EGW of the high ductility steel sheet (hereinafter referred to as "EGW joint") was fractured at the base material portion, and the EGW joint of the existing steel was fractured at the HAZ in the joint tensile test described above.

[0135] Figure 7 The cross-sectional hardness distribution of the EGW joint of the high ductility steel sheet is shown, Figure 8 The cross-sectional hardness distribution of the EGW joint of the existing steel is shown. Figure 7(b) of FIG. 8, and Figure 8 In (b) of FIG. 8, the horizontal axis indicates the distance from the center of the EGW joint, and the vertical axis indicates the Vickers hardness (Hv). Also, in (b) of FIG. 8, the range corresponding to the HAZ width Lh is indicated by the hatched area. Figure 7 (b) of FIG. 8, and Figure 8 In the welded joint shown in (b) of FIG. 8, the portion at a distance of ±10 mm from the center is the weld metal portion, the outer side of the weld metal portion (the portion at a distance of ±10 mm to ±20 mm from the center) is the HAZ, and further, the outer side of the HAZ (the portion at a distance further than ±20 mm from the center) is the base metal portion. Also, in (b) of FIG. 8, the range corresponding to the HAZ width Lh is indicated by the hatched area. Figure 7 (b) of FIG. 8, and Figure 8 Two graphs are shown in (b) of FIG. 8. "t / 4" is the position corresponding to a depth of 1 / 4 of the plate thickness t from the surface, and "3t / 4" is the position corresponding to a depth of 3 / 4 of the plate thickness t from the surface. Figure 7 (a) of FIG. 8, and Figure 8 In the EGW joint shown in (a) of FIG. 8, "3t / 4" is the position corresponding to a depth of 3 / 4 of the plate thickness t from the surface. Figure 7 (a) of FIG. 8, and Figure 8 In the EGW joint shown in (a) of FIG. 8, "3t / 4" is the position corresponding to a depth of 3 / 4 of the plate thickness t from the surface.

[0136] As shown in (b) of FIG. 8, the hardness of the HAZ of the welded joint of the high ductility steel plate is approximately the same as the hardness of the base metal portion. On the other hand, as shown in (b) of FIG. 8, the hardness of the HAZ of the welded joint of the existing steel is lower than the hardness of the base metal portion, and the HAZ is softened in a wide range. Note that, in (b) of FIG. 8, the range corresponding to the HAZ width Lh is indicated by the hatched area. Figure 7 (a) of FIG. 8, and Figure 8 As shown in (b) of FIG. 8, the hardness of the HAZ of the welded joint of the high ductility steel plate is approximately the same as the hardness of the base metal portion. On the other hand, as shown in (b) of FIG. 8, the hardness of the HAZ of the welded joint of the existing steel is lower than the hardness of the base metal portion, and the HAZ is softened in a wide range. Note that, in (b) of FIG. 8, the range corresponding to the HAZ width Lh is indicated by the hatched area. Figure 8 In (b) of FIG. 8, the range corresponding to the HAZ width Lh is indicated by the hatched area.

[0137] Thus, it was confirmed that, if the HAZ of the welded joint is softened in a wide range, the HAZ is broken, and the ductility (elongation) is greatly reduced. In other words, even if the welded joint of the high ductility steel plate is made by large heat input welding, the HAZ is not easily softened, and thus the base metal portion is broken, and the ductility is not reduced. On the other hand, the welded joint of the existing steel is made by large heat input welding, and thus the HAZ is softened in a wide range, and the HAZ is broken, and the ductility is greatly reduced.

[0138] The preferred embodiments of the present application have been described in detail with reference to the accompanying drawings, but the present application is not limited to the above examples. Various modifications or changes can be made by those skilled in the art who possess the ordinary knowledge in the technical field to which the present application pertains, within the scope of the technical idea recited in the claims, and these naturally should be understood as belonging to the technical scope of the present application.

[0139] Industrial applicability

[0140] The present application can be used for a ship in which the ship body structure is required to have excellent collision resistance.

[0141] Explanation of reference signs

[0142] 10 ship side

[0143] 11 outer plate

[0144] 12 inner plate

[0145] 13 anti-flexing member attached to outer plate

[0146] 14 anti-flexing member attached to inner plate

[0147] 15 cross beam

[0148] 16 longitudinal beam

[0149] 20 ship bottom

[0150] 21 outer plate

[0151] 22 inner plate

[0152] 23 anti-flexing member attached to outer plate

[0153] 24 anti-flexing member attached to inner plate

[0154] 25 cross beam

[0155] 26 longitudinal beam

[0156] 30 upper deck

[0157] 31 bow

Claims

1. A welded joint which is a butt welded joint formed using a steel plate, the steel plate satisfies a standard following Unified Requirement Wl l Rev. 9 2017 of International Association of Marine Offices (IACS), and has a total elongation which is 1.40 times or more of a value of the total elongation prescribed by the standard, when a plate thickness of the butt welded joint is set to t, a width of a weld heat affected zone is set to Lh, a hardness of the weld heat affected zone is set to Hh, a hardness of a base material portion is set to Hb, and a hardness of a weld metal portion is set to Hw, in a case where Hh / Hb is less than 0.97, the following formulas (1) to (4) are satisfied, in a case where Hh / Hb is 0.97 or more, the following formulas (3) to (4) are satisfied, Lh≤ (0.034t + 0.510) / (1 - Hh / Hb) 0.9 ・・・(1) Hh / Hb ≥ 0.70 (2) Hw / Hb ≥ 1.0 (3) 6≤t≤40 ・・・（4）； wherein, The t and Lh are in mm.

2. The welded joint of claim 1, wherein, The steel plate is classified as 32, 36, or 40 by the strength prescribed by the standard.

3. The welded joint of claim 1 or 2, wherein, A value of the total elongation in a tensile test using a flat joint test piece having a distance between marks of 200 mm and a width of 40 mm is 1.40 times or more of a value of the total elongation of the base material portion prescribed by the standard.

4. A design method of a welded joint which is a design method of a butt welded joint formed using a steel plate, the design method has the following steps: a steel plate selection step of selecting, as the steel plate, a steel plate which satisfies a standard following Unified Requirement Wl l Rev. 9 2017 of International Association of Marine Offices (IACS), and has a total elongation which is 1.40 times or more of a value of the total elongation prescribed by the standard; and a welding condition setting step of setting a welding condition of a butt welding in such a manner that, when a plate thickness of the butt welded joint is set to t, a width of a weld heat affected zone is set to Lh, a hardness of the weld heat affected zone is set to Hh, a hardness of a base material portion is set to Hb, and a hardness of a weld metal portion is set to Hw, in a case where Hh / Hb is less than 0.97, the following formulas (1) to (4) are satisfied, and in a case where Hh / Hb is 0.97 or more, the following formulas (3) to (4) are satisfied, Lh≤ (0.034t + 0.510) / (1 - Hh / Hb) 0.9 ・・・(1) Hh / Hb ≥ 0.70 (2) Hw / Hb ≥ 1.0 (3) 6≤t≤40 ・・・（4）； wherein, The t and Lh are in mm.

5. A manufacturing method of a welded joint which is a manufacturing method of a butt welded joint formed using a steel plate, the manufacturing method has the following steps: a steel plate selection step of selecting, as the steel plate, a steel plate which satisfies a standard following Unified Requirement Wl l Rev. 9 2017 of International Association of Marine Offices (IACS), and has a total elongation which is 1.40 times or more of a value of the total elongation prescribed by the standard; and a welding condition setting step of setting a welding condition for a butt joint in such a manner that, when a plate thickness of the butt joint is set as t, a width of a heat-affected zone is set as Lh, a hardness of the heat-affected zone is set as Hh, a hardness of a base material portion is set as Hb, and a hardness of a weld metal portion is set as Hw, the following expressions (1) to (4) are satisfied in a case where Hh / Hb is less than 0.97, and the following expressions (3) to (4) are satisfied in a case where Hh / Hb is 0.97 or more; and a welding step of welding the steel sheet selected in the steel sheet selection step under the welding condition set in the welding condition setting step, Lh≤ (0.034t + 0.510) / (1 - Hh / Hb) 0.9 ・・・(1) Hh / Hb ≥ 0.70... (2) Hw / Hb ≥ 1.0... (3) 6≤t≤40 ・・・（4）； wherein The units of the t and Lh are mm.

6. A hull structure wherein, A part of a butt joint of an outer plate of a ship side or a ship bottom or all parts of the butt joint, or a part of a butt joint of an inner plate of a ship side or a ship bottom or all parts of the butt joint is the butt joint according to any one of claims 1 to 3.

7. A hull structure wherein, A part of a butt joint of an outer plate or an inner plate of a ship side or a ship bottom in which a break is required to be suppressed is the butt joint according to any one of claims 1 to 3.

Images