JP4363321B2 - Welded joint with excellent fatigue characteristics - Google Patents

Description

The present invention relates to a welded joint having excellent fatigue strength used in the fields of shipbuilding, construction structures, bridges, and the like, and particularly to a welded joint having a tensile strength of 50 to 60 kgf / mm ² class.

  In recent years, in order to reduce the maintenance cost of structures, there is an increasing demand for extending the life cycle and reducing the maintenance cost. For example, bridges tend to be maintained and used for a period exceeding 100 years.

In order to meet the above requirements, it is necessary to realize a longer life than ever in terms of materials. In other words, not only the structural material itself has a very long life but also the development of a welded joint with a very long life is required. The ultra-long life here means the fatigue life in use at 2 × 10 ⁶ times or more. The normal life means a fatigue life that is assumed to be used less than 2 × 10 ⁶ times.

In the case of an ultra-long life of 2 × 10 ⁶ times or more, as a design consideration, the fatigue strength value determined by the fatigue test of 2 × 10 ⁶ times is set as a limit, and the safety limit is set for that limit value. The load of the amplitude exceeding is not applied.

However, as a practical matter, fatigue strength value determined by the fatigue test of about 2 × 10 ⁶ times, a major problem of whether treated as defines the Usage fatigue life as is 2 × 10 ⁶ or more times Remaining.

  Patent Document 1 (Patent No. 3469443) is a technique aimed at improving the fatigue strength of a steel plate welded part, and specifies the structure of the weld heat affected zone where fatigue cracks occur. ing. Specifically, the area ratio is specified as 15 to 80% because it is effective to increase the ferrite area ratio to suppress the generation and propagation of fatigue cracks in the weld heat affected zone.

  In the invention disclosed in Patent Document 2 (Japanese Patent No. 35267702), the base metal structure is a bainite-based structure including martensite with respect to the high-tensile welded steel sheet, and the welded HAZ structure is 60. By making it a welded joint made of bainite exceeding%, a high-strength welded structural steel sheet with excellent fatigue strength of the welded joint has been realized.

  However, in order to make the base material structure martensite, it is necessary to add a considerable amount of alloy elements, which causes a problem in terms of alloy costs. Furthermore, when a large amount of alloy element is added, the weldability is deteriorated, and when martensite is generated in the base material, the workability is also deteriorated.

  None of the above-mentioned patent documents describes or suggests fatigue properties in the long-life region, which is a problem of the present invention.

Japanese Patent No. 3469443 Japanese Patent No. 35267702

  The technique disclosed in Patent Document 1 stipulates that the ferrite area ratio is increased with respect to the structure of the weld heat affected zone, so that the strength is extremely limited. Therefore, the strength range of the steel material in which the fatigue strength improvement effect is exhibited is restricted.

  Further, as described above, in the invention of Patent Document 2, it is necessary to add a considerable amount of alloy elements, which causes a problem in terms of alloy costs. Furthermore, if a large amount of alloy element is added, the weldability is deteriorated, and if martensite is generated in the base material, there is a problem in workability. Furthermore, none of these inventions are intended for use in a long-life range.

  An object of the present invention is to provide a welded joint with improved fatigue characteristics in a long service life region. The present invention is based on the invention of the steel material excellent in fatigue crack growth characteristics of Japanese Patent Application No. 2003-175156 previously proposed by the present inventors, and is an invention directed to the welded joint.

  The present inventors limited the hardness value in the weld heat-affected zone with respect to the steel material excellent in fatigue crack growth characteristics with limited metal structure and chemical composition, and the weld heat-affected zone. By limiting the ratio of the hardness of the base metal or weld metal to the lower hardness value, as a synergistic effect, it was found that the fatigue characteristics in the long-life region were remarkably improved, and the present invention was completed. It was.

  The gist of the present invention resides in the following welded joint. Hereinafter, “%” regarding the component content means “% by mass”.

(1) It has the following chemical composition, the metal structure is mainly composed of ferrite and bainite, the area ratio of pearlite is 10% or less, and the half width of the X-ray diffraction intensity from the (110) plane is 0.13 Is a welded joint using a steel material that satisfies the following formulas (a) and (b), the hardness value in the weld heat affected zone is Hv 300 or less, and A welded joint characterized in that the hardness value in the heat affected zone is 80% or more of the lower hardness of the base metal or the weld metal.

6 ≦ 20C + 5Si + 10Mn ≦ 30 (a)
0.01 ≦ C / Mn ≦ 0.10 (b)
However, the element symbols in the formulas (a) and (b) represent the content (% by mass) of each element.

The chemical composition of the above steel contains C: 0.01-0.10%, Si: 0.03-0.60%, Mn: 0.5-2.0%, sol.Al: over 0.005% to 0.10%, N: 0.0005-0.0080% The balance consists of Fe and impurities . This steel, B: 0.0003~0.0030% may contain. However, when B is included, Mn is set to 0.3 to 2.0%.

  The steel may further contain at least one component selected from at least one of the following first group to third group.

Group 1: Nb: 0.08% or less, Ti: 0.03% or less and V: 0.08% or less Group 2: Cu: less than 0.7%, Ni: less than 3.0%, Cr: less than 1.0% and Mo: less than 0.8% Group: Ca: 0.007% or less, Mg: 0.007% or less, Ce: 0.007% or less, Y: 0.5% or less, and Nd: 0.5% or less.

  When the first group component is contained, the following formula (c) needs to be satisfied instead of the formula (b). Moreover, when it contains the 2nd group component, it replaces with the said (b) Formula, and it is necessary to satisfy | fill following (d) Formula.

0.01 ≦ C / (Mn + 20Nb + 10Ti + 5V) ≦ 0.10 (c)
0.01 ≦ C / {Mn + (1/10) Cu + (1/2) Ni + (1/4) Cr + Mo + 20Nb + 10Ti + 5V} ≦ 0.10
... (d)
However, the element symbols in the formulas (c) and (d) represent the content (% by mass) of each element.

  The present inventors first observed in detail macroscopically the state of crack initiation / propagation in a fatigue test piece of a welded joint. In this process, a specimen with a weld length of 400 mm was manufactured and a fatigue test was conducted. The following knowledge was obtained.

  (1) In the normal fatigue life region, most fatigue cracks occur from a number of locations in the weld heat affected zone, several of which grow together and reach the final fracture.

(2) On the other hand, in the long-life region, there are very few fatigue cracks at one or two places, and the fatigue cracks generated at the few places grow slowly and repeat 2 × 10 ⁶ times or more. After loading, it breaks.
In other words, as described in the “Background Art” column, as a conventional fatigue design, the fatigue strength value determined by a fatigue test of about 2 × 10 ⁶ times has been used as a limit. When observed, a welded joint that breaks more than 2 × 10 ⁶ times has the characteristics described above.

In the long life range of 2 × 10 ⁶ or more repeated loads, the stress applied is suppressed to a relatively low level, so the fatigue crack occurrence point is suppressed to about 1 or 2 per 400 mm weld length. And the occurrence and growth of the fatigue crack restricts the hardness value in the weld heat affected zone with respect to the steel material excellent in fatigue crack propagation characteristics with limited structure and components, and the weld heat affected zone. By limiting the ratio between the hardness value and the hardness value of the base metal or weld metal, whichever has the lower hardness, it was found that the synergistic effect of these was suppressed, and the fatigue characteristics in the long life range were greatly improved. did.

  Limiting the hardness value in the weld heat affected zone with respect to the weld zone, and limiting the ratio between the hardness value in the weld heat affected zone and the hardness value of the lower hardness of the base metal or weld metal, This has the effect of reducing the stress concentration in the welded joint.

Limiting the structure and composition of the steel material affects the structure of the heat-affected zone produced by welding and affects the hardness suppression of the heat-affected zone. When the heat input, such as manual welding or CO ₂ welding, is small, the structure of the weld heat-affected zone of the base metal is considered to be affected by the base metal structure. It is thought that it will remain in the part.

  In the present invention, the reason for limiting the metal structure and chemical composition of the steel is as follows.

1. Metal structure The structure of steel (base material) constituting the welded joint of the present invention is mainly composed of ferrite and bainite in order to obtain a high-strength joint. The bainite includes structures such as upper bainite, lower bainite, acicular ferrite, and granular bainite.

  Here, “mainly” means that the composition ratios of these structures in the steel structure are 90% or more in terms of area ratio in total. The remaining structure is not particularly limited, and may be a normally observed structure such as pearlite or pseudo-pearlite structure.

2. Half-width of X-ray diffraction The half-width is a value indicating the distribution width of the portion where the diffraction intensity is ½ of the peak intensity in the distribution of the X-ray diffraction intensity by the diffraction angle. A half-width is smaller as a braid produced at high temperature and having a lower dislocation density. A structure with a larger half width has a higher dislocation density and better fatigue crack propagation resistance.

  The crystal plane on which X-ray diffraction is performed is the (110) plane because it is most commonly used. In the present invention, in order to obtain good fatigue crack growth resistance, the half-value width of the diffraction intensity at the (110) plane is 0.13 degrees or more. In the case of steel of 490 MPa class, it is desirable that the angle is 0.13 to 0.24 degrees from the viewpoint of balance of strength and the like.

  FIG. 1 is a schematic diagram illustrating a method for analyzing a half-value width in X-ray diffraction. FIGS. 1A and 1B are graphs showing diffraction intensities at the (110) plane, respectively. As shown in FIG. 1, the half width is the angle of the distribution width at the half of the highest intensity value at the peak of the diffraction intensity. When the peak is divided into two, it takes 1/2 the value of the higher peak.

The above half width is measured with the value of Kα ₁ when the peaks of Kα ₁ and Kα ₂ appear independently in the diffraction pattern, and with the total width when the values of Kα ₁ and Kα ₂ appear overlapping. . In addition, the measurement of the said half width shall be performed in the surface parallel to the rolling surface in the site | part which entered 1 mm inside from the steel material surface in the thickness direction.

  And the limitation of the half-value width in the X-ray diffraction of such a base material structure means a dense structure having a high dislocation density, and is rapidly heated by limiting the area ratio of pearlite to 10% or less. In the welding heat-affected zone, a microstructure with a high C is excluded.

As is well known, pearlite is a layered structure of ferrite and Fe ₃ C (cementite), and is locally understood as a structure having a C content of about 0.8% from the Fe—C phase diagram. In such a structure, C may not be sufficiently uniformed by welding heat, and a micro-hardened structure or an island martensite structure may be generated. Producing such a structure is not preferable for improving the fatigue characteristics of the welded joint in the long life range which is the object of the present invention.

3. (a), (b), (c) and (d)
When the value of “20C + 5Si + 10Mn” in the formula (a) is less than 6, the ratio of bainite in the ferrite + bainite structure is not sufficient, and an appropriate half-value width can be obtained even if a steel plate is manufactured under the manufacturing conditions described later. It cannot be obtained, and good fatigue crack growth resistance cannot be obtained. If the value exceeds 30, if the strength is to be 490 MPa class, the ferrite structure in the ferrite + bainite structure must be increased, and in this case as well, good fatigue crack growth resistance cannot be obtained.

  “C / Mn” in the formula (b), “C / (Mn + 20Nb + 10Ti + 5V)” in the formula (c) and “C / {Mn + (1/10) Cu + (1/2) Ni + (1/4) in the formula (d) When the value of “) Cr + Mo + 20Nb + 10Ti + 5V}” is less than 0.01, the hardness of the bainite structure becomes insufficient and good fatigue crack growth resistance cannot be obtained. Conversely, when it exceeds 0.10, the cooling rate dependency of the progress of transformation becomes large, and it is not easy to obtain uniform fatigue crack growth resistance in the entire steel sheet.

4). Chemical composition of steel The effects of the components of steel and the reasons for limiting the content are as follows.

C: 0.01-0.10%
C is an element effective for increasing the strength of the steel, and is contained in an amount of 0.01% or more in order to obtain the strength of the steel. However, if the content exceeds 0.10%, the strength becomes too high and the toughness deteriorates. To avoid this, the content is made 0.10% or less. More desirable is 0.03 to 0.07%. In addition, since C content has big influence on the hardness of a welding heat affected zone, such restriction | limiting of C content of steel is important for realization of the welded joint of this invention.

Si: 0.03-0.60%
Si is an element effective for deoxidation of steel, and in order to obtain the effect, 0.03% or more is contained. However, if it exceeds 0.60%, formation of the MA structure is promoted. The MA structure is a kind of island martensite formed in the bainite structure, and is an MA transformation product containing residual austenite. It is known that the MA structure has very high hardness and easily deteriorates toughness. Therefore, the Si content is set to 0.60% or less in order to avoid inertia deterioration. More desirable is 0.3 to 0.5%.

Mn: 0.5-2.0% or 0.3-2.0%
Mn is an element effective for improving the hardenability, and is contained in an amount of 0.5% or more in order to improve strength and fatigue crack growth resistance. On the other hand, if it exceeds 2.0%, the toughness deteriorates, so the upper limit of the Mn content is 2.0%. However, when it contains B as mentioned later, it is good also as 0.3 to 2.0%.

sol.Al: more than 0.005% to 0.10%
Al is an element necessary for deoxidation together with Si, and in order to obtain the effect, sol.Al exceeding 0.005% is contained. On the other hand, if the sol.Al content exceeds 0.10%, the MA ratio (existence ratio of the MA structure) increases and the fertility deteriorates. In order to avoid this, the sol.Al content is 0.10% or less.

N: 0.0005-0.0080%
N combines with Al and Ti to form precipitates, and contributes to the refinement of austenite grains and improves toughness. In order to acquire this effect, N is contained 0.0005% or more. However, if the N content exceeds 0.0080%, the MA ratio increases and the toughness deteriorates. In order to avoid this, the upper limit of the N content is 0.0080%.

B: 0.0003-0.0030%
B is not an essential component. However, B has an effect of remarkably increasing hardenability, and is effective in improving strength and fatigue crack growth resistance. Therefore, it is added when these effects are desired. In order to obtain the above effect, it is effective to contain 0.0003% or more. However, if the content of B exceeds 0.0030%, the inertia deteriorates, so the upper limit is desirably 0.0030%.

  In addition to the above-described components, one or more components selected from at least one group from the first group to the third group may be contained. Hereinafter, components belonging to these groups will be described.

(1) First group components: Nb, Ti, V
Nb: 0.08% or less
Nb is not an essential component, but has an effect of improving toughness through a fine graining action. Moreover, since hardenability is increased, it is effective in improving strength and suppressing fatigue crack growth. Therefore, you may make it contain in order to acquire these effects. In that case, it is desirable to contain 0.005% or more. On the other hand, if its content exceeds 0.08%, toughness deteriorates, so 0.08% is made the upper limit. More preferred is 0.06% or less.

Ti: 0.03% or less
Ti is not an essential component, but Ti is effective in improving strength and suppressing fatigue crack growth, and may be contained in order to obtain these effects. In order to acquire the said effect, it is desirable to make it contain 0.005% or more. On the other hand, if it exceeds 0.03%, the toughness deteriorates, so the upper limit is preferably 0.03%.

V: 0.08% or less V is not an essential component, but is effective in improving strength and suppressing fatigue crack growth. In particular, the tendency to improve becomes significant for thick materials. Therefore, it may be contained in order to obtain these effects. When contained, it is desirable to contain 0.005% or more in order to obtain the above effect. On the other hand, if it exceeds 0.08%, the toughness deteriorates, so the upper limit is preferably 0.08%.

(2) Second group component: Cu, Ni, Cr, Mo
Cu: Less than 0.7%
Although Cu is not an essential component, it has the effect of increasing the strength of steel, so it may be contained for that purpose. In order to obtain the effect, the content is preferably 0.1% or more, and more preferably 0.3% or more. However, if the content is 0.7% or more, the toughness of the steel deteriorates, so even if it is contained, the upper limit is made less than 0.7%. More desirable is less than 0.5%.

Ni: 3.0% or less
Ni is not an essential component, but has the effect of increasing the strength of the steel and is also effective in suppressing fatigue crack growth. Therefore, it may be contained in order to obtain these effects. In order to obtain the effect, the content is preferably 0.2% or more. However, if the content exceeds 3.0%, high strength sufficient to meet the cost increase and fatigue crack growth suppressing effect are not seen, so even if it is contained, the upper limit is made 3.0%.

Cr: less than 1.0%
Cr is not an essential component, but has the effect of increasing the strength of the steel and is also effective in suppressing fatigue crack growth. Therefore, it may be contained in order to obtain these effects. In that case, the content is preferably 0.1% or more, and more preferably 0.3% or more. However, if Cr is excessively contained, toughness deteriorates, so even if it is included, the content is made less than 1.0%.

Mo: 0.8% or less
Mo is not essential. However, Mo is an element effective in improving hardenability and improving strength. However, if the Mo content exceeds 0.8%, not only the toughness is deteriorated but also the cost is increased, so the upper limit of the content is 0.8%. When Mo is added, its content is preferably 0.1% or more, and more preferably 0.2% or more.

Group 3 components: Ca, Mg, Ce, Y, Nd
Ca: 0.007% or less
Ca contributes to toughness improvement through refinement of the structure. In order to obtain the effect, the content is preferably 0.0015% or more. However, if its content exceeds 0.007%, the amount of Ca inclusions becomes excessive and the toughness deteriorates. Therefore, when adding Ca, the content needs to be 0.007% or less. A more desirable content is 0.0020 to 0.0030%.

Mg: 0.007% or less
Mg also contributes to toughness improvement through microstructure refinement. In order to obtain the effect, the content is preferably 0.0005% or more. However, if it exceeds 0.007%, the amount of Mg inclusions becomes excessive, resulting in toughness deterioration similar to Ca. Therefore, when adding Mg, the content needs to be 0.007% or less. A more desirable content is 0.0010 to 0.0030%.

Ce: 0.007% or less
Ce contributes to the improvement of toughness through microstructure refinement. In order to obtain the effect, the content is preferably 0.0005% or more. However, if the Ce content exceeds 0.007%, the amount of Ce inclusions becomes excessive and the toughness deteriorates. Therefore, when Ce is added, its content needs to be 0.007% or less. A more desirable content is 0.0008 to 0.0030%.

Y: 0.5% or less Y contributes to toughness improvement through microstructure refinement. In order to obtain the effect, the content of 0.01% or more is desirable. However, if its content exceeds 0.5%, the amount of Y inclusions becomes excessive, and the toughness deteriorates. Therefore, when Y is used, its content needs to be 0.5% or less. A more desirable content is 0.02 to 0.05% or less.

Nd: 0.5% or less
Nd contributes to toughness improvement through refinement of the structure. In order to acquire the effect, it is desirable to make it contain 0.01% or more. However, if the Nd content exceeds 0.5%, the amount of Nd inclusions becomes excessive and the toughness deteriorates. Therefore, when Nd is added, its content needs to be 0.5% or less. A more desirable content is 0.02 to 0.05%.

5. Method for Measuring Hardness of Heat Affected Zone FIG. 2 shows a method for measuring the hardness of the heat affected zone. In the figure, 1 is a main plate, 2 is a gusset, 3 is a weld metal, and 4 is a weld heat affected zone. The hardness of the heat-affected zone is a straight line (a-a line) perpendicular to the steel sheet surface, passing through a point 1mm into the weld metal side parallel to the steel sheet surface from the weld toe stop (position of the broken line in the figure). ) Above, Vickers hardness shall be measured in the plate thickness direction to a region exceeding the weld heat affected zone with a pressing load of 9.8 N and a measurement interval of 0.5 mm.

  In addition, since it is often difficult to carry out such a measurement for a welded joint of an actual structure, welding under the same conditions may be performed on another steel material of the same material, and the welded portion may be measured.

6). Manufacturing Method A means for manufacturing a steel material constituting a welded joint having excellent fatigue crack growth resistance according to the present invention is manufactured using a known hot rolling facility or a known hot rolling facility and a known heat treatment facility. can do. The manufacturing conditions are preferably those described below.

  A cast slab having the above chemical composition is heated to 1000 ° C. to 1250 ° C. and then hot-rolled. Then, when cooling this, in the cooling step, accelerated cooling is performed so that the average cooling rate between 650 ° C. and 400 ° C. is 5 ° C./s or more, preferably 5 to 25 ° C./s. Stop at a temperature below ℃. Thereafter, the cooling is finished so that the recuperated temperature range becomes 70 ° C. or less. Here, the recuperation temperature range means the difference between the temperature reached when cooling is stopped and the temperature when the surface temperature rises and stabilizes due to the heat inside the steel plate after cooling stops.

  When the heating temperature of the casting slab is less than 1000 ° C., the ferrite rate increases and the crack growth rate increases. If it exceeds 1250 ° C, the structure becomes coarse and the toughness deteriorates. When the average cooling rate between 650 ° C. and 400 ° C. in the cooling process is less than 5 ° C./s, the ferrite rate increases and the crack growth rate increases. However, it is preferably 25 ° C./s or less. When the recuperated temperature range from the accelerated cooling stop to the end of cooling exceeds 70 ° C, the dislocation density decreases and the crack growth rate increases. When the accelerated cooling stop temperature is higher than 400 ° C., the ferrite rate increases and the crack growth rate increases. However, the preferred stop temperature is 350 ° C. or higher.

  In order to reduce the recuperation temperature range, it is necessary to reduce the temperature difference between the steel sheet surface layer and the center part during cooling, and to end the phase transformation of at least the surface layer part at the end of cooling. In order to reduce the temperature difference between the steel sheet surface layer and the central portion, it is preferable to increase the cooling rate at the subsequent stage from the previous stage of the cooling zone. Further, in order to complete the phase transformation of the surface layer portion when the accelerated cooling is stopped, it is necessary to set the stop temperature of the accelerated cooling to 400 ° C. or lower.

7). Hardness of weld
(1) Hardness value in weld heat affected zone is Hv 300 or less:
That the hardness value in the weld heat affected zone exceeds Hv 300 means that the weld heat affected zone is excessively hardened because of a lot of martensite. The martensitic transformation is a phenomenon accompanied by rapid expansion, which causes local residual stress, which is not preferable for improving the fatigue characteristics of the welded joint in the long-life region which is the object of the present invention. In addition, martensite is brittle, and after fatigue cracks occur, the life of the martensite is shortened, so the formation of martensite should be avoided. Therefore, it is necessary to limit it to Hv 300 or less. Hv is a symbol indicating Vickers hardness.

  Needless to say, the hardness of the heat affected zone is determined by the combination of the base metal and welding. However, in the steel of the present invention, the composition of the steel is limited to relatively low carbon, so the welding conditions are appropriate. This condition can be fully achieved by choosing

(2) The hardness value in the weld heat affected zone is 80% or more of the lower hardness value of either the base metal or the weld metal:
It is not preferable that the weld heat affected zone softens and its hardness value is too low. When the weld heat-affected zone is significantly softened and becomes less than 80% of the hardness value of the lower one of the base metal and the weld metal, strain is concentrated on the softened portion, and in the long-life region intended by the present invention. It is not preferable for improving the fatigue characteristics of welded joints. Therefore, 80% was set as the lower limit. This value can be secured, for example, by appropriately selecting welding conditions or by selecting a steel material component in accordance with the welding method.

  Slabs were produced by melting steels having chemical compositions shown in Tables 1 and 2 in a converter, and each slab was hot-rolled to an appropriate thickness. Table 3 shows the manufacturing conditions of the steel sheet.

  A part of the steel plate prepared as described above was cut out, the metal structure as a base material was observed, and the half width and hardness of the (110) plane were measured. In addition, steel sheets were welded under the same conditions as those for the bridge weld structure model described later, and the hardness of the weld heat affected zone and weld metal was measured. This hardness measurement is a simulation of the hardness measurement of the weld heat affected zone and the weld metal of the weld structure model.

  Furthermore, a welded structure model of the bridge was prepared using the same steel plate and subjected to a long life fatigue test. The target part is the joint between the main girder and cross girder of the bridge.

  FIG. 3 shows the shape and dimensions of the structural model specimen. (A) of the same figure is a side view, (b) is a longitudinal cross-sectional view of a center part. The specimen was an I-shaped cross-sectional welded girder consisting of an upper flange 11, a web 12 and a lower flange 13, and an out-of-plane gusset 14 was provided under a stiffener 15 at the center of the span. The gusset 14 was welded with a K-shaped groove, and the weld 17 was turned from the top and bottom and the end under the conditions shown in Table 4. In order to prevent buckling, stiffeners 16 were applied to the three-point bending fulcrums at the lower ends of the specimen.

The above structural model specimen was repeatedly subjected to a three-point bending load and 10 ⁷ times fatigue strength was derived. When a three-point bending load is applied, a fatigue crack 18 is generated from the toe of the gusset. If a fatigue crack occurs at one location, the toe close to the stiffener 15 (point A in FIG. 3). If a fatigue crack occurs at two locations, start from both toes (points A and B in FIG. 3). Fatigue cracks occur. Note that the distance from the stiffener 15 is doubled from the point A at the point B. After the fatigue test, the hardness distribution was measured at the toe where no fatigue crack occurred.

Table 5 shows together with the test conditions, the metal structure of the base metal, the hardness, and the fatigue test results. Long life fatigue strength evaluated in 10 ⁷ times fatigue strength of the welded joint of the present invention from the table that is significantly higher apparent.

  According to the present invention, it is possible to obtain a super-long-life welded joint having excellent fatigue strength. This welded joint is used in the fields of shipbuilding, construction structures, bridges, etc., and greatly contributes to the long-term use and maintenance of these structures.

It is a schematic diagram explaining the analysis method of the half value width in X-ray diffraction. It is a figure which shows the measuring method of the hardness of a welding heat affected zone. It is a figure of the welding structure model for fatigue tests, (a) is a side view, (b) is a longitudinal cross-sectional view of the center part.

Explanation of symbols

1: Main plate, 2: Gusset, 3: Weld metal, 4: Weld heat affected zone, 11: Upper flange (main girder flange), 12: Web (main girder web), 13: Lower flange (main girder flange) ), 14: Gusset (flange of cross beam), 15, 16: Stiffener (web of cross beam), 17: Turned weld, 18: Fatigue crack

Claims (5)

In mass%, C: 0.01-0.10%, Si: 0.03-0.60%, Mn: 0.5-2.0%, sol.Al: over 0.005% to 0.10%, N: 0.0005-0.0080%, the balance being Fe and impurities, the metal structure is mainly composed of ferrite and bainite, the area ratio of pearlite is 10% or less, and the half width of the X-ray diffraction intensity from the (110) plane is 0.13 degrees or more, Further, a welded joint using a steel material whose chemical composition satisfies the following formulas (a) and (b), the hardness value in the weld heat affected zone is Hv 300 or less, and the hardness in the weld heat affected zone A welded joint characterized in that the value is 80% or more of the lower hardness of the base metal or weld metal.
6 ≦ 20C + 5Si + 10Mn ≦ 30 (a)
0.01 ≦ C / Mn ≦ 0.10 (b)
However, the element symbols in the formulas (a) and (b) represent the content (% by mass) of each element. In mass%, C: 0.01 to 0.10%, Si: 0.03 to 0.60%, Mn: 0.3 to 2.0%, sol.Al: over 0.005% to 0.10%, N: 0.0005 to 0.0080%, B: 0.0003 to 0.0030 The balance is composed of Fe and impurities, the metal structure is mainly composed of ferrite and bainite, the area ratio of pearlite is 10% or less, and the half-value width of the X-ray diffraction intensity from the (110) plane Is a welded joint using a steel material in which the chemical composition of the steel satisfies the following formulas (a) and (b), the hardness value in the weld heat affected zone is Hv 300 or less, Furthermore, a welded joint characterized in that the hardness value in the weld heat affected zone is 80% or more of the lower hardness of the base metal or the weld metal.
6 ≦ 20C + 5Si + 10Mn ≦ 30 (a)
0.01 ≦ C / Mn ≦ 0.10 (b)
However, the element symbols in the formulas (a) and (b) represent the content (% by mass) of each element. In addition to the above-mentioned components, the steel further contains at least one member selected from the group consisting of Nb: 0.08% or less, Ti: 0.03% or less, and V: 0.08% or less, and the following formula (a): The weld joint according to claim 1 or 2 , wherein the expression (c) is satisfied.
6 ≦ 20C + 5Si + 10Mn ≦ 30 (a)
0.01 ≦ C / (Mn + 20Nb + 10Ti + 5V) ≦ 0.10 (c)
However, the element symbols in the formulas (a) and (c) represent the content (% by mass) of each element. In addition to the above-mentioned components, the steel further contains at least one member selected from the group consisting of Cu: less than 0.7%, Ni: less than 3.0%, Cr: less than 1.0% and Mo: less than 0.8%, following equation (a) and welded joint according to any one of claims 1 to 3, characterized by satisfying the equation (d).
6 ≦ 20C + 5Si + 10Mn ≦ 30 (a)
0.01 ≦ C / {Mn + (1/10) Cu + (1/2) Ni + (1/4) Cr + Mo + 20Nb + 10Ti + 5V} ≦ 0.10 (d)
However, the element symbols in the formulas (a) and (d) represent the content (% by mass) of each element. In addition to the above-mentioned components, the steel further comprises, in mass%, Ca: 0.007% or less, Mg: 0.007% or less, Ce: 0.007% or less, Y: 0.5% or less, and Nd: 0.5% or less. containing more species, welded joint according to any one of claims 1 to 4, characterized by satisfying the formula (a) and equation (d) below.
6 ≦ 20C + 5Si + 10Mn ≦ 30 (a)
0.01 ≦ C / {Mn + (1/10) Cu + (1/2) Ni + (1/4) Cr + Mo + 20Nb + 10Ti + 5V} ≦ 0.10 (d)
However, the element symbols in the formulas (a) and (d) represent the content (% by mass) of each element.

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