JP4394996B2 - Welded joints with excellent brittle fracture resistance - Google Patents

Description

  The present invention relates to a weld metal that has greatly improved the brittle fracture resistance of welded joints, which are the highest sites of fracture in welded structures such as building structures and large shipbuilding structures such as large ships. .

  In a welded structure such as a building structure and a large shipbuilding structure such as a large ship, a portion having the highest possibility of fracture occurrence is a welded joint. The reason for this is that there is a possibility that a weld defect may occur during welding of the welded joint part, and there is a high possibility that a stress concentration part that is the starting point of fracture exists, and the structure of the steel sheet becomes coarse due to the influence of welding heat. Fracture toughness value used as an index for the occurrence of brittle fracture: Kc (set according to the standards of the Japan Maritime Association) is lowered.

  Therefore, in order to prevent the deformation and strain from concentrating on the welded joint, it is essential to make the weld metal stronger and harder than the base metal that will be welded. Therefore, when selecting the weld metal, the joint design is always over-matched compared to the strength of the base material.

  As a method for evaluating the fracture toughness value in a welded joint, a deep notch test in which a test piece width is 400 mm and a notch with a length of 240 mm is machined in the center of the test piece at a position assumed to be the weakest part of the welded joint. So far, many tests have been conducted centering on steel plates for hull structures with a plate thickness of 50 mm or less, and the necessary performance of steel plates for hulls has been discussed based on these data. For example, Patent Document 1 discloses a steel plate (TMCP) having excellent brittle fracture characteristics and fatigue characteristics as a hull steel plate in consideration of the fracture toughness value of a weld.

Until now, large tankers and container ships of 6000 TEU or less have used TMCP steel sheets with a thickness of about 50 mm. However, there is a growing need for construction of large container ships of 6000 TEU or more, and steel plates having a plate thickness of 60 mm or more are also practically used as hull steel plates. Currently, the upper limit of the strength of structural steel plates for ship hulls in practical use is the 390 MPa level in terms of yield strength. If the size of the container ship is further increased, the plate thickness will increase and the number of man-hours for welding will increase. There are industrial problems such as a significant increase in construction costs and an increase in the weight of the container ship itself.
JP-A-6-88161

  As described above, the size of a welded structure such as a container ship has been increased, and a high-tensile steel thick steel sheet having a plate thickness exceeding 50 mm and a high design stress is desired for container ships exceeding 6000 TEU.

  The present inventors can obtain a large heat input welded joint having a thickness of 50 mm or more by a deep notch test which is a large fracture test even if it shows a good result in a V notch Charpy impact test which is a small test. Fracture toughness value: It was newly found that Kc (set according to the standards of the Japan Maritime Society) does not necessarily show a good value.

  An object of the present invention is to provide a welded joint having a high strength steel having a yield strength of 460 MPa and a sufficiently high fracture toughness of the welded joint in a steel plate for ship hull having a thickness of about 70 mm.

The present inventors are the basic in producing a welded joint, “in order to prevent deformation and strain from concentrating on the welded joint, the strength and hardness of the weld metal is made higher than the base metal, We have developed a new joint design technology that can ensure the fracture toughness value of welded joints while following the joint design that is overmatched when compared to the base metal strength when selecting so-called weld metal. That is, in a high heat input welded joint, the hardness of the weld heat affected zone (HAZ) region is controlled to be 85% or less of the hardness of the base metal, so that the welded joint has notches and defects and breaks. By preventing the local stress from occurring in the HAZ region having the lowest toughness, the fracture toughness value despite the fact that the yield strength is 460 MPa class and the plate thickness is about 50 mm or more and about 70 mm is a thick steel plate: The present invention has been completed as a technique for realizing a welded joint having a high Kc. The gist of the present invention is as follows.
(1) In a butt weld joint forming a welded structure, the width of the region where the hardness of the weld heat affected zone is 85% or less of the hardness of the base material is 10% or more of the plate thickness. that excellent welded joint resistant to brittle fracture characteristics.
(2) The brittle fracture resistance according to (1) above, wherein the width of the region where the hardness of the weld heat affected zone is 85% or less of the hardness of the base material is 30% or less of the plate thickness A welded joint with excellent generation characteristics.
(3), wherein the weld heat affected zone which is in contact with the welding fusion line, the austenite grain size (HAZ) is 200μm or less, excellent in brittle fracture properties described in the above (1) or (2) Welded joints.
(4) welding metal is characterized in that at least 110% of the hardness of the base material, the above-mentioned (1) to (3) welded joint having excellent brittle fracture characteristics of any one of .
(5) plate thickness 50mm or more, wherein the yield strength is not less than 430 MPa, the (1) to (4) any one welded joint having excellent brittle fracture properties described in Section.

  INDUSTRIAL APPLICABILITY The present invention can provide a welded joint that is a high-strength steel having a yield strength of 460 MPa and a sufficiently high fracture toughness of the welded joint in a steel plate for ship hull having a thickness of about 70 mm.

  First, the reason for ensuring the brittle fracture resistance in the weld heat affected zone (HAZ) softened region in the present invention will be described.

The design of conventional welded joints is basically to increase the strength and hardness of the weld metal compared to the base metal in order to prevent deformation and distortion from concentrating on the welded joint. It was selected to be overmatched compared to the material strength. Therefore, a steel sheet having a yield strength of 460 MPa was prototyped, a weld joint was manufactured using a welding material selected so that the weld metal was overmatched, and a deep notch test was performed . As a result, although the V-notch Charpy test result of the welded joint showed a sufficient value of 90 J or more at the test temperature of -20 ° C, and the fracture surface transition temperature showed a very good value of -20 ° C. The Kc value was as extremely low as 2000 N / mm 1.5 or less, and a new finding was obtained that deviated greatly from the correlation between the Charpy test result and the deep notch test result used conventionally.

Therefore, as a result of detailed investigation of the fracture occurrence point in the deep notch test, the fracture occurrence position is the boundary between the weld metal and the HAZ, and the microstructure of the fractured part is the fracture observed in the Charpy specimen Ascertaining that the microstructure is the same as the microstructure of the generating part, we predicted that the distribution of local stress, which is the driving force of fracture, was significantly different between the deep notch test and the Charpy test. Next, as a result of conducting a three-dimensional finite element method analysis in order to obtain the local stress distribution of both, when the plate thickness exceeds 50 mm and becomes about 70 mm, the degree of restraint in the plate thickness direction increases remarkably, and the weld metal It was found that the local stress significantly increases at the boundary between the weld metal and HAZ when the strength of the steel is higher than that of the base metal. It has also been found that this local stress can be reduced as the HAZ area softened by welding increases. Based on these analyses, the degree of softening of the HAZ part and the size of the region were variously changed, and the fracture toughness value: Kc was measured. As shown in the black circles in FIG. It has been found that if a region of 85% or less of the hardness can be ensured by 10% or more of the plate thickness, the fracture toughness value can be prevented from decreasing due to an increase in local stress.

In the present invention, the reason why the prior austenite grain size of the HAZ part is regulated to 200 μm or less will be described. Fracture toughness value of welded joint: As described above, it is important not to increase the local stress at FL (weld fusion line) which is the most fragile part of the welded joint in order to secure Kc. At the same time, it is important to improve the microscopic brittle fracture resistance in the vicinity of the FL. As a result of investigating and examining the mechanism of brittle fracture occurring in the vicinity of FL, proeutectoid ferrite generated around old austenite, upper bainite and ferrite side plates generated in lath form inside old austenite, etc. became the starting point of fracture. It has also been found that the brittle fracture resistance is improved by controlling the prior austenite grain size to be small (see FIG. 2) .

  Furthermore, the fact that the hardness of the weld metal is specified to be 110% or more of the base material hardness in the present invention will be described. In the present invention, even if the hardness of the weld metal is higher than that of the base metal, the hardness of the HAZ part adjacent to the FL part is suppressed to be low, so that the local stress in the FL part is not extremely increased. Therefore, by making the hardness of the weld metal higher than the hardness of the base metal, it is possible to prevent deformation and strain from concentrating on the weld metal even when the weld joint is subjected to a stress exceeding the yield strength. In other words, if these conditions (conditions for securing a region where the hardness of the HAZ portion is 85% or less of the base material hardness of 10% or more of the plate thickness) are not satisfied, distortion and deformation concentration on the weld metal are prevented. If the hardness of the weld metal is made to be greater than the hardness of the base metal, the fracture toughness of the joint will be significantly reduced. However, if the above-mentioned conditions of the present invention are satisfied, the fracture toughness of the joint will be secured while ensuring the weld toughness. The hardness of the metal can be increased to prevent the concentration of distortion and deformation.

  The steel plate of the present invention, in particular, the hull outer plate can be manufactured from a well-known welded structural steel. For example, in mass%, C: 0.02 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, Al: 0.001 to 0.20%, N: 0.02% or less, P: 0.01% or less, S: 0.01% or less steel as a basic component, required for purposes such as increasing the strength of the base metal and improving joint toughness. 1 selected from, for example, Ni, Cr, Mo, Cu, W, Co, V, Nb, Ti, Zr, Ta, Hf, REM, Y, Ca, Mg, Te, Se, B It is a steel for welded structures having a known component containing seeds or two or more kinds. In addition, as a welding material, for example, in mass%, C: 0.01 to 0.06%, Si: 0.20 to 1.20%, Mn: 1.0 to 2.50%, Cu: 0.00. 10 to 0.80%, Ni: 0.50 to 3.5%, Ti: 0.01 to 0.06%, B: 0.0030 to 0.0090%, and if necessary, Cr: 0.20 %, Nb: 0.02% or less, Mo: 0.5% or less, and welding is preferably performed using a welding material composed of the remaining Fe and inevitable impurities. In addition, as a welding method, it is only necessary to achieve the weld metal composition and metal structure defined in the present invention while controlling conditions such as heat input and cooling rate during welding, and temperature between welding passes. There are no special restrictions.

Plate thickness: 20 to 100 mm thick steel plate (YP47 , YP56 ) is prepared, heat input: 20-530 kilojoule / cm, two-electrode VEGA welding (VEGA-II), normal electrogas welding (EG), latent Arc welding (SAW) and carbon dioxide welding (CO ₂ ) were performed.

The characteristics of the welded joint and the results of the joint performance test are shown in Table 1. In addition, the evaluation method of the result obtained by the present Example is as follows.
Hardness: Hv (BM) is an average value of the hardness in the thickness direction of the base material measured by an indentation at a load of 10 kg. Hv (WM) is a hardness value measured by an indentation of 10 kg at the center of the plate thickness of the weld metal. Hv (HAZ) measures the hardness from the FL to the base metal in the direction of the base metal at a quarter of the plate thickness with an indentation at 10 mm load at 0.5 mm pitch, and the hardness is most softened by the heat effect. It shows the parts of the Hv to have.
-HAZ width: It is the width of the HAZ area | region which shows the hardness of 85% or less of the hardness of a base material.
・ HAZ old γ grain size: Old austenite grain (equivalent circle diameter) at the HAZ part in contact with the weld line
· Kc: the value marked in> a value obtained at -20 ° C. the test temperature in a deep notch test described above, sufficiently deformed notch of the test piece, which after ductile crack was confirmed No Kc value could be measured because of the test piece width of 400 mm.

  In addition, as a welded joint performance, the result of obtaining the fracture surface transition temperature by changing the test temperature with a test piece taken so that the melt line (FL), which is the weakest part of the joint, is at the center of the thickness of the test piece It is. Further, the joint strength is a result of a joint tensile test using an NKU No. 1 test piece, and shows the strength at which the fracture occurred.

  As shown in Table 1, No. of the present invention example. 1 to 11 satisfied the conditions defined in the present invention, and both Kc value and joint strength showed sufficient values. In addition, No. of the present invention example. In Nos. 6 and 7, the value of Hv (WM) / Hv (BM) is 1 or less, so the joint tensile strength is slightly smaller than that of other inventions. Furthermore, No. of this invention example. 10 and 11 have a HAZ width / thickness greater than 0.3, so the joint tensile strength is smaller than that of the other invention examples. However, the fracture toughness value Kc of the joint is sufficient in any of the invention examples. It can be seen that In addition, No. of the present invention example. Nos. 12 and 13 had a value of 200 μm or more only in the prior austenite grain size of the HAZ part, and thus the Kc value was lower than that of the other invention examples, but it was found that the Kc value was higher than that of the comparative example.

  In contrast, No. of the comparative example. Nos. 14 and 15 have Hv (HAZ) / Hv (BM) not less than a predetermined value. Nos. 16 and 17 have a HAZ width / plate thickness ratio equal to or less than a predetermined value. 18, 19, and 20 are Hv (HAZ) / Hv (BM) values, and the ratio of HAZ width / thickness is not more than a predetermined value. Therefore, the fracture toughness value Kc may not show a sufficient value. I understand.

  According to the present invention, in a high heat input welded joint having a high strength and a large plate thickness, it is difficult for brittle cracks to occur even if welding defects exist or fatigue cracks are generated and grown, and the welded structure. It is possible to prevent a fatal damage such as destroying it.

It is a figure which shows the influence of the ratio of the width | variety of a HAZ softening part, and board thickness which acts on Kc value. (The HAZ softened portion is the width of the HAZ region showing a hardness of 85% or less of the hardness of the base material.) It is a figure explaining the relationship between the prior-austenite particle size of a HAZ part, and Kc value.

Claims (5)

In butt welded joint to form a welded structure, you wherein the width of the region hardness of the welding heat affected zone is less than or equal to 85% of the hardness of the base material is the plate thickness of more than 10%, resistance to A welded joint with excellent brittle fracture characteristics.   The excellent resistance to occurrence of brittle fracture according to claim 1, wherein the width of the region where the hardness of the weld heat affected zone is 85% or less of the hardness of the base material is 30% or less of the plate thickness. Welded joints. Wherein the weld heat affected zone which is in contact with the welding fusion line, the austenite grain size (HAZ) is 200μm or less, welded joint having excellent brittle fracture characteristics according to claim 1 or 2. Wherein the weld metal is not less than 110% of the hardness of the base material, weld joint having excellent brittle fracture characteristics according to any one of claims 1 to 3. Thickness 50mm or more, a welded joint which yield strength is equal to or not less than 430 MPa, which is excellent in brittle fracture characteristics according to any one of claims 1 to 4.

Images