JP5096088B2 - Welded joints with excellent toughness and fatigue cracking suppression properties - Google Patents

Description

  The present invention relates to a welded joint obtained by welding high-yield ratio high-tensile steel plates used for various applications such as civil engineering, architecture, bridges, offshore structures, pipes, ships, storage, and construction machinery, and particularly high tensile strength. The present invention relates to a welded joint that suppresses the occurrence of fatigue cracks in a weld heat-affected zone (hereinafter sometimes referred to as “HAZ”) of a steel sheet to ensure a good fatigue life and is excellent in HAZ toughness. .

  There are many structural materials that are applied to the above-mentioned various applications, and many stresses are repeatedly applied. Therefore, in order to ensure the safety of structural materials, the steel sheets used as raw materials must have good fatigue characteristics. Is extremely important in design.

  Researches to improve the fatigue properties of steel materials have been widely conducted so far, and the research can be broadly divided into (1) suppression of fatigue cracks at stress-concentrated parts and (2) progress of cracks once generated. It is classified into two technologies called suppression. However, actual welded structures such as offshore structures and shipbuilding are regularly inspected and repaired when fatigue cracks are confirmed. For these reasons, there is a strong demand for a technique for improving the characteristic (1).

  In the case of a welded structure constructed with a welded joint, extremely small defects (microcracks) introduced during welding are the starting points of fatigue cracks. Will not occur or progress. However, when the microcrack becomes larger than a certain size, a crack (that is, fatigue crack) that develops due to repeated load is generated and progresses.

  For these reasons, it is an important matter to suppress the occurrence of fatigue cracks in the HAZ, which is the boundary between the weld metal and the base material (steel plate), such as a weld toe in fillet welding. Further, in such HAZ, since toughness is likely to deteriorate, it is also important to ensure good toughness together with the occurrence of fatigue cracks.

Various techniques for generating fatigue cracks and securing toughness in HAZ have been proposed so far. For example, in Patent Document 1, the metal structure of a steel sheet is a composite structure of ferrite and bainite, and the (100) plane is used. 490-590 N / mm ² class (50-60 kgf / mm ² class) welded joints by controlling the half-value width of X-ray diffraction intensity from the X-ray diffraction intensity to 0.13 degrees or more and controlling the hardness of HAZ and weld metal It has been proposed to improve the weld fatigue characteristics in the long life region.

  This technique is intended to improve the resistance to fatigue crack growth after the occurrence of a crack, and particularly improves the effect of improving the fatigue life in a long time. However, in an actual welded structure, repairs are made at the stage where fatigue cracks are generated by periodic inspection, and it is difficult to say that a practically effective effect is exhibited.

  Further, in Patent Document 2, by making the Si content of the steel sheet 0.3 to 0.6%, a large amount of island-like martensite is generated between the laths of the upper bainite, and the Cu content is set. A technique for improving the fatigue strength and preventing a remarkable decrease in toughness due to a bainite substrate has been proposed by strengthening the ferrite substrate by solid solution strengthening by adjusting the content of Al to 0.5 to 1.2%.

  In this technique, a large amount of island-like martensite is generated to suppress the occurrence of cracks. However, island-like martensite greatly reduces the HAZ toughness, so that the HAZ toughness is required. In fields such as severe offshore structures and shipbuilding, it becomes difficult to satisfy toughness requirements.

  On the other hand, in Patent Document 3, the structure of the welded joint is made of soft ferrite, and the Si content of the steel sheet is set to about 0.6 to 2%, thereby causing the stacking fault energy to be reduced and reducing the cross slip. A technique has been proposed in which localization of deformation at the time of deformation is suppressed to increase reversibility of plastic deformation. However, inclusion of 0.5% or more of Si in the steel sheet reduces the HAZ toughness of the welded joint, and satisfies the toughness requirement in the fields of offshore structures and shipbuilding where HAZ toughness is severely demanded. It becomes difficult to let them.

  Further, in Patent Document 4, the carbon equivalent Ceq is controlled to 0.4 to 0.8%, and the HAZ structure contains 60% or more of martensite, and the difference in strength between the martensite lath and lath interface is described. There has been proposed a technique for suppressing the occurrence of HAZ fatigue cracks by reducing the size.

However, martensite is effective in improving fatigue strength, but is a brittle structure. Therefore, in marine structures and shipbuilding where HAZ toughness is severely demanded, it still does not satisfy toughness requirements. It becomes difficult.
JP 2006-169602 A JP-A-6-235044 Japanese Patent No. 2911725 JP-A-8-209296

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is fatigue starting from microcracks at the boundary between the weld metal and the base material (steel plate), which has been difficult to suppress so far. An object of the present invention is to provide a welded joint capable of suppressing the occurrence of cracks and improving the HAZ toughness as much as possible.

  The welded joint of the present invention that has achieved the above object is a welded joint obtained by welding high-tensile steel plates, and the structure of the weld heat-affected zone of the high-tensile steel plate has an average grain size of prior austenite. When a region surrounded by a large-angle grain boundary having a bainite fraction of 90% by area or more and an orientation difference between two crystals of 15 ° or more is used as a crystal grain, the average of the crystal grains is 200 μm or less. It has a gist in that the equivalent circle diameter is 9 μm or less. The “average equivalent circle diameter” is an average value of diameters (equivalent circle diameters) when crystal grains surrounded by large-angle grain boundaries having an orientation difference of 15 ° or more are converted into circles of the same area. is there.

  The steel type of the high-strength steel sheet used in the welded joint of the present invention is not particularly limited as long as it is a high-strength steel sheet. For example, C: 0.01 to 0.06% (meaning mass%, the same applies hereinafter), Si: 0.01-0.5%, Mn: 0.5-2.0%, Al: 0.005-0.10%, Nb: 0.005-0.05%, Ti: 0.008- 0.03%, B: 0.001 to 0.005% and N: 0.0030 to 0.0080%, Cr: 0.3 to 1.0%, Cu: 0.1 to 1 0.0%, Ni: 0.1 to 1.0%, Mo: 0.03 to 0.5% and V: 0.02 to 0.05% selected from the group consisting of one or more And the balance of Ti content [Ti] and N content [N] satisfy the following formula (1), (2) A value defined by the formula is preferably one which satisfies the following 0.15%.

[Ti] / [N] ≦ 3.4 (1)
A value = [C] + [Si] / 19 + [Mn] / 30 + [Cu] / 49 + [Ni] / 51 + [Cr] / 33 + [Mo] / 25 + [V] / 13 (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are C, Si, Mn, Cu, Ni, Cr, Mo and The V content (% by mass) is shown.

  Moreover, as a high-tensile steel plate used in the present invention, in addition to the above preferable chemical component composition, P in the inevitable impurities is 0.025% or less (not including 0%), S is 0.02% or less (0%) It is also preferable to contain and suppress Ca: 0.005% or less (not including 0%) and / or rare earth elements: 0.005% or less (not including 0%). Or according to the kind of component to contain, the characteristic (characteristic which reflected the characteristic of the steel plate) of a welded joint is further improved.

  In the welded joint of the present invention, the HAZ of the high-strength steel plate exhibits excellent toughness (low temperature toughness) having an average Charpy absorbed energy at −40 ° C. of 50 J or more.

  In the welded joint of the present invention, as HAZ of the steel sheet constituting the welded joint, it has a structure mainly composed of bainite (90% or more in area ratio), appropriately defines each crystal orientation relationship of bainite, and By appropriately defining the size of austenite, it is possible to realize a welded joint that is excellent in suppressing the occurrence of fatigue and that can secure good HAZ toughness. Such welded joints can be used in civil engineering, architecture, bridges, offshore structures, pipes, ships, It is useful for application to various welded structures such as storage and construction machinery.

  In order to solve the above problems, the present inventors have studied from various angles. As a result, the following knowledge was obtained. That is, in order to suppress the occurrence of fatigue cracks starting from microcracks at the boundary between the weld metal and the base metal (steel plate), which has conventionally been difficult to suppress, in the base metal constituting the weld joint, By using a single structure of homogeneous bainite, it becomes possible to control ΔKth (described later) representing the lower limit characteristic of fatigue crack growth to a high value, and it is favorable by refining the prior austenite grain size. The inventors have found that the HAZ toughness can be secured, and that the above-mentioned object can be achieved brilliantly, thereby completing the present invention. Next, the effect of each requirement prescribed | regulated by this invention is demonstrated.

  In a single phase structure mainly composed of a bainite phase, it is considered that the grain boundary serves as a resistance to fatigue crack initiation. However, at a small-angle grain boundary (small tilt boundary) where the orientation difference between both ends forming a grain boundary is small (for example, a small tilt boundary), the grain boundary energy is small and the effect is small, so the orientation difference is 15 ° or more. It is necessary to target a large-angle grain boundary (large tilt boundary). That is, a crystal grain surrounded by a large-angle grain boundary having an orientation difference of 15 ° or more and having an average diameter (equivalent circle diameter) of 9 μm or less when converted to a circle of the same area. As a result, a welded joint excellent in the effect of suppressing the occurrence of fatigue cracks could be realized. This crystal grain is preferably 8 μm or less, more preferably 7 μm or less. The “orientation difference” is also referred to as “shift angle” or “inclination angle”, and may be hereinafter referred to as “crystal orientation difference”. In order to measure such a crystal orientation difference, an EBSP method (Electron Backscattering Pattern Method) may be employed.

  Further, as a homogeneous structure, the bainite fraction of the steel sheet constituting the welded joint needs to be 90 area% or more. By satisfying these requirements, it is excellent in suppressing the occurrence of fatigue cracks. This bainite fraction is preferably 95 area% or more, more preferably 97 area% or more. Incidentally, the bainite structure in the present invention is a structure excluding polygonal ferrite and martensite, and various kinds of materials generated in an intermediate speed range such as bainitic ferrite, acicular ferrite, and Widmanstatten ferrite. Includes organizations.

  On the other hand, in order to improve the HAZ toughness, the average grain size of prior austenite (old γ) in the steel sheet constituting the welded joint needs to be 200 μm or less. By satisfying these requirements, good toughness in the HAZ of the welded joint can be ensured. The average particle diameter of the prior austenite is preferably 190 μm or less, and more preferably 175 μm or less. The prior austenite means an austenite structure in which HAZ undergoes austenite transformation by heat input during welding and thereafter grows in the process of being maintained at a high temperature. Moreover, although this prior austenite is transformed again by cooling after welding and becomes a predetermined HAZ structure, its particle size can be measured.

  In the welded joint of the present invention, by satisfying the above-mentioned requirements, it is possible to suppress the occurrence of fatigue cracks starting from microcracks at the boundary between the weld metal and the base material (steel plate) and to ensure good toughness in HAZ. The steel type of the high-tensile steel plate used in the welded joint of the present invention is not particularly limited as long as it is a high-strength steel plate. Is preferably satisfied. The reasons for setting the ranges of these components are as follows.

[C: 0.01 to 0.06%]
C is an element necessary for ensuring the strength of the steel sheet. In order to ensure the strength economically, it is preferable to contain a large amount, but if it is contained excessively, island-shaped martensite (MA) rapidly increases in the HAZ of the welded joint, which is harmful to the toughness of the HAZ. is there. On the other hand, if the C content exceeds 0.06%, the HAZ structure contains a large amount of bainite structure having low toughness and large structural units, and the toughness and fatigue generation characteristics are greatly reduced. Therefore, the upper limit of the C content is preferably 0.06% or less. On the other hand, when the extremely low C is reduced to less than 0.01%, pro-eutectoid ferrite is more likely to be generated than the prior austenite grain boundaries in the HAZ of the weld zone, and the fatigue generation characteristics are deteriorated. Therefore, the C content is preferably 0.01% or more. In addition, a more preferable lower limit of the C content is 0.015%, and a preferable upper limit is 0.055%.

[Si: 0.01 to 0.5%]
Si is an effective element for deoxidation and ensuring strength. However, when it is excessively contained, a large amount of island martensite phase (MA phase) is precipitated on the steel material (base material) to deteriorate toughness. For these reasons, the upper limit is preferably set to 0.5%. In addition, the upper limit with more preferable Si content is 0.35%.

[Mn: 0.5 to 2.0%]
Mn is an effective element for securing the strength and toughness of the steel sheet, and in order to exert such effects, it is preferable to contain Mn in an amount of 0.5% or more. However, if Mn is contained excessively, the toughness of the base material is deteriorated, so the upper limit is made 2.0%. A more preferable lower limit of the Mn content is 0.9%, and a more preferable upper limit is 1.7%.

[Al: 0.005 to 0.10%]
Al is an effective element as a deoxidizing agent, and if it is less than 0.005%, such an effect is not exhibited. However, if it is contained excessively, a large amount of Al oxide or nitride is generated and the toughness of the welded joint is deteriorated. For these reasons, the upper limit is preferably 0.10%. In addition, the more preferable minimum of Al content is 0.015%, and a more preferable upper limit is 0.07%.

[Nb: 0.005 to 0.05%]
Nb is an effective element for ensuring a homogeneous bainite structure while suppressing the transformation of polygonal ferrite with the C content reduced. In order to exhibit these effects, it is preferable to contain Nb 0.005% or more. However, if the Nb content becomes excessive and exceeds 0.05%, it becomes difficult to ensure the HAZ toughness of the welded joint. A more preferable lower limit of the Nb content is 0.01% (more preferably 0.015%), and a more preferable upper limit is 0.03%.

[Ti: 0.008 to 0.03%]
Ti uniformly disperses nitride (TiN), thereby suppressing coarsening of prior austenite (former γ) during welding heat input, improving HAZ toughness and suppressing proeutectoid ferrite formation, and reducing HAZ microstructure It is an effective element for promoting homogenization. In order to exert such effects, the Ti content needs to be 0.008% or more. However, if Ti is contained excessively, coarse inclusions are precipitated and the HAZ toughness is deteriorated on the contrary, so the upper limit is made 0.03%. A more preferable lower limit of the Ti content is 0.01%, and a more preferable upper limit is 0.02%.

[B: 0.001 to 0.005%]
B is an effective element for suppressing pro-eutectoid ferrite and promoting homogeneous bainite transformation of the HAZ structure. In order to exert such an effect, B is preferably contained in an amount of 0.001% or more. However, when B is excessively contained, not only the effect is saturated, but also inclusions (B nitride) in the HAZ structure increase and the HAZ toughness decreases, so the upper limit of the B content is 0.005. % Is required. A more preferable lower limit of the B content is 0.0015%, and a more preferable upper limit is 0.003%.

[N: 0.003 to 0.008%]
In order to ensure high toughness in the HAZ of the welded joint, it is effective to finely precipitate TiN in the prior austenite grains and prevent coarsening of the prior austenite grains. In order to exert such effects, the N content is preferably set to 0.003% or more. However, if the N content becomes excessive and exceeds 0.008%, coarse TiN precipitates and becomes the starting point of fracture. A more preferable lower limit of the N content is 0.004%, and a more preferable upper limit is 0.0065%.

From the viewpoint of generating TiN, it is preferable that the relationship of the following formula (1) is satisfied in the relationship between the Ti content and the N content.
[Ti] / [N] ≦ 3.4 (1)

[Cr: 0.3-1.0%, Cu: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.03-0.5% and V: 0.02 1 type or 2 types or more selected from the group consisting of ˜0.05%]
These elements exhibit an effect of making the texture unit fine by suppressing transformation and lowering the bainite transformation start temperature Bs. In addition, bainite transforms with a KS relationship (Kurdjimov-Sachs relationship) at the time of transformation, but by transforming at a low temperature, a fine block consisting of a single variant (so-called sibling) is generated. It becomes like this. In order to exhibit such an effect, it is preferable to make it contain more than each above-mentioned minimum, but since it will impair weldability when it contains abundantly, it is good to make it below an upper limit.

The steel plate constituting the welded joint in the present invention preferably satisfies the chemical composition as described above, but it is also important that the A value defined by the following formula (2) is 0.15% or less. It is a necessary requirement. That is, in order to produce bainite, it is necessary to contain elements for improving hardenability such as C, Si, Mn, Cr, Mo and V, but for the production of fine bainite, the content of these elements is It is preferable to reduce as much as possible.
A value = [C] + [Si] / 19 + [Mn] / 30 + [Cu] / 49 + [Ni] / 51 + [Cr] / 33 + [Mo] / 25 + [V] / 13 (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are C, Si, Mn, Cu, Ni, Cr, Mo and The V content (% by mass) is shown.

  The inventors of the present invention conducted various reproducible thermal cycle tests using various steel sheets having the chemical composition shown in Table 1 below, and conducted fatigue crack propagation tests using CT test pieces from the test pieces. The coefficient was obtained (the steel plate manufacturing method and fatigue crack propagation test method are the same as those in Examples described later).

The relationship between ΔKth (lower limit stress intensity factor range: see Examples described later) and each component calculated at this time is as shown in the following formulas (3) to (10). By arranging these relationships, The above equation (2) was obtained (for the meanings of [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] the same).
In (ΔKth) = 2.522-15.413 [C] (3)
In (ΔKth) = 2.343-0.829 [Si] (4)
In (ΔKth) = 2.003−0.507 [Mn] (5)
In (ΔKth) = 1.766-0.312 [Cu] (6)
In (ΔKth) = 1.877−0.305 [Ni] (7)
In (ΔKth) = 1.531-0.461 [Cr] (8)
In (ΔKth) = 1.964-0.621 [Mo] (9)
In (ΔKth) = 2.381-1.159 [V] (10)

  The preferable chemical composition of the high-strength steel sheet used in the present invention is as described above, and the balance is composed of iron and inevitable impurities (for example, P, S, O, etc.). However, it is preferable to suppress to P: 0.025% or less (not including 0%) and S: 0.02% or less (not including 0%) from the following viewpoints.

[P: 0.025% or less (not including 0%) and S: 0.02% or less (not including 0%)]
P is an impurity that segregates in crystal grains and adversely affects ductility and toughness, so it is preferable that it be as small as possible, but 0.025% or less (more preferably) considering that it is inevitably mixed into the steel material. Is preferably 0.020% or less). Further, S is an impurity that reacts with alloy elements in the steel material to form various inclusions and adversely affects the ductility and toughness of the steel material. Therefore, it is preferably as small as possible, but inevitably mixed in. In consideration of this, it is good to suppress to 0.02% or less (more preferably 0.015% or less).

  Further, in the steel sheet used in the present invention, in addition to the above components, if necessary, Ca: 0.005% or less (not including 0%) and / or rare earth elements: 0.005% or less (not including 0%) It is also effective to contain these elements, and the inclusion of these elements further improves the properties of the steel sheet (that is, the properties of the welded joint).

[Ca: 0.005% or less (not including 0%) and / or rare earth element: 0.005% or less (not including 0%)]
Ca and rare earth elements (REM) are effective elements for reducing the inclusion shape anisotropy and improving the HAZ toughness. Such an effect increases as the content thereof increases, but if it is excessively contained, inclusions become coarse and the HAZ toughness decreases. For these reasons, when these are contained, the content is preferably 0.005% or less (total of one or two types). In addition, the preferable minimum for exhibiting the said effect effectively is all 0.0005%.

  By using the steel plate as described above to form a welded joint by welding, the HAZ of the high-tensile steel plate in the welded joint has an average Charpy absorbed energy at −40 ° C. of 50 J or more, as shown in Examples below. Excellent toughness (low temperature toughness) will be exhibited. The welding method at this time is not particularly limited, and a carbon dioxide arc welding method, a submerged arc welding method, an electrogas arc welding method, and other methods can be applied, and any welding method is applied. In addition, it is possible to realize a welded joint having excellent fatigue crack generation suppression characteristics and toughness.

In the welded joint of the present invention, the HAZ structure at the stage of forming the welded joint is composed mainly of bainite, but HAZ has a temperature of Ac ₃ or higher (usually 1300 to 1) due to heat input during welding. 1450 ° C.) and complete austenite transformation, the characteristics of the base material are determined without being affected by the metal structure of the base material. Therefore, in order to satisfy good characteristics in the welded joint, it is preferable to satisfy the chemical component composition as described above.

  For this reason, the high-tensile steel plate used in the present invention does not impose any restrictions on the manufacturing method.

For example, as an example of specific production conditions, after heating in the temperature range of 950 to 1250 ° C. and finishing rolling in the temperature range of Ar ₃ transformation point to 900 ° C., the air cooling or cooling rate is 5 ° C./second or more. The method of manufacturing by water cooling which becomes becomes. In addition, after heating in the temperature range of 950 to 1250 ° C. and finishing the rolling in the temperature range of Ar ₃ transformation point to 900 ° C., after performing online or offline quenching treatment, tempering in the temperature range of 500 to 700 ° C. The method of processing and manufacturing is also mentioned.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
Steels (experiment Nos. 1 to 22) having chemical composition shown in Table 2 below were melted using a 150 kg vacuum melting furnace to produce ingots. Table 2 also shows the A value [value of equation (2)] and the [Ti] / [N] ratio [value on the left side of equation (1)]. In addition, experiment No. in Table 2 Nos. 1 to 11 satisfy the preferred chemical component composition of the present invention. Nos. 12 to 22 are those in which any of the preferable requirements (chemical component composition, [Ti] / [N], A value) of the present invention is not satisfied.

  The ingot was reheated to 1200 ° C. (± 50 ° C.) and hot forged to obtain a slab (slab thickness: 135 mm, slab width: 180 mm). This slab is reheated to a heating temperature of 1100 ° C (± 30 ° C), hot-rolled to a hot end temperature of 900 ° C (± 20 ° C) using a small rolling mill, and then air-cooled. Thus, a test steel plate was produced. In addition, since the rolling conditions at this time are reheated to 1200 ° C. or higher at the time of welding, only the chemical components have an influence without affecting the HAZ.

  Using each of the test steel plates, butt welding was performed under the welding conditions (I to III) under the conditions shown in Table 3 below, and weld joints were produced. From this welded joint, a fatigue crack growth specimen and a Charpy impact specimen were collected as described below and subjected to the following characteristic evaluation. In collecting the test pieces, the HAZ when the welding conditions I and II were performed was set to the sampling site on the vertical side of the lathe groove (optional in the case of welding conditions III).

[Fatigue crack growth specimen collection and fatigue crack growth rate test]
(1) From the welded joint, cutting to 12.5 mm is performed so that t / 4 (t is the plate thickness) of the steel plate is the center, surface microetching is performed, and the welding heat effect confirmed by microetching A compact test piece (CT test piece) was sampled so that the center of the part (HAZ) was in the crack propagation direction. The shape of the CT test piece obtained at this time is shown in FIG.
(2) Using the CT test piece, ΔKth (lower limit stress intensity factor range) was measured by conducting a fatigue crack growth test in accordance with ASTM E647. Note that ΔKth is the value of ΔK (N / mm ^1.5 ) when the fatigue crack when the ΔK is lowered by the gradual reduction method is measured and the drop is 1 × 10 ⁻⁷ (mm / cycle) or less. Value (general definition). Other test conditions at this time are as follows.

Test method: Electrohydraulic servo type ± 10 ton fatigue tester is used. Crack length is measured by a computer-controlled compliance method using a load gradual reduction method K value reduction method (the load is automatically reduced as the crack progresses). ΔKth is measured by the method) Test environment: room temperature, atmospheric control method: load control stress ratio: R = 0.1
Test speed: 600-1200 cpm

At this time, in order to remove the influence of welding residual stress on ΔKth, the following stress removal annealing was performed. When the value measured in this way was ⁵ N / mm ^1.5 or more, it was evaluated that the fatigue crack generation suppressing property was excellent.
(Annealing conditions)
Heating temperature: 630 ° C
Holding time: 13 hours Heating rate: 30 ° C / hour (± 5 ° C)
Cooling rate: 30 ° C / hour (± 5 ° C) [However, 200 ° C or less is air-cooled]

[Collecting Charpy Impact Test Piece and Charpy Impact Test (HAZ Toughness)]
Cross-sectional micro-etching was performed on the welded joint, and three test pieces with notches centered at a position 3 mm from the bond portion at the t / 4 (t is the plate thickness) portion of the steel plate were collected. The shape of the test piece at this time was a JIS Z 2201 No. 4 v-notch test piece. Using this test piece, a Charpy impact test was performed in accordance with JIS Z 2242 to determine an average absorbed energy vE- ₄₀ at -40 ° C. At this time, the average absorbed energy vE _-40 (average value of three) was 50 J or more.

  Further, from each of the obtained welded joints, the bainite fraction in HAZ, the prior austenite grain size, the grain size of the crystal grains surrounded by the large angle grain boundary (large angle grain size: average equivalent circle diameter), and the like were measured by the following methods. . These results are shown in Table 4 below together with the fatigue test results and the HAZ toughness test results.

[Bainite fraction (area ratio)]
A test piece was taken from a position corresponding to t / 4 (t is the plate thickness) of the steel sheet, the cross section in the rolling direction was mirror-polished, and this was etched with a 2% nitric acid-ethanol solution (a nital solution). Observation was performed at 400 times using an optical microscope, and the bainite fraction (area%) in the steel structure was measured by image analysis. At this time, all lath structures other than ferrite (including polygonal ferrite and pseudopolygonal ferrite) and island martensite were regarded as bainite.

[Measurement of prior austenite grain size]
A test piece was taken from a position corresponding to t / 4 (t is the plate thickness) of the steel plate, the cross section in the rolling direction was mirror-polished, and this was etched with picric acid, and then 100 times using an optical microscope in 10 fields of view. Observation was performed, and the prior austenite (old γ) particle size in the structure was measured by image analysis.

[Large-angle grain boundary diameter (average equivalent circle diameter)]
Measurement was performed by FE-SEM-EBSP (electron backscattering diffraction image method using an electron emission scanning electron microscope) at a position corresponding to a cross section parallel to the rolling direction of the steel sheet. Specifically, an EBSP apparatus (trade name: “OIM”) manufactured by Tex SEM Laboratories is used in combination with EF-SEM, and a grain having a tilt angle (crystal orientation difference) of 15 ° or more is used as a crystal grain boundary. The diameter was measured. The measurement conditions at this time were a measurement area: 200 μm, a measurement step: 0.5 μm interval, and measurement points having a confidence index (Confidence Index) indicating reliability of the measurement direction smaller than 0.1 were excluded from the analysis target. . Thus, the average value of the crystal grain diameter calculated | required was computed, and it was set as the average crystal grain diameter (large angle particle diameter) in this invention. Incidentally, those having a crystal grain size of 2.0 μm or less were judged as measurement noise and excluded from the target of calculating the average value of the crystal grain size.

  These results are shown together with fatigue test results and HAZ toughness test results in Table 4 below, but satisfy the requirements of the present invention (Experiment Nos. 1 to 11). It turns out that it is excellent in. On the other hand, if any of the requirements defined in the present invention deviates (Experiment Nos. 12 to 22), it can be seen that any of the characteristics is deteriorated.

Based on the results of Table 4 above, FIG. 2 shows the effect of the bainite fraction on the large-angle particle size and ΔKth, FIG. 3 shows the relationship between the prior γ particle size and the HAZ toughness (vE _-40 ), and the A value and the large-angle particle size. These relationships are shown in FIG. As is clear from these results, (1) By controlling the bainite fraction to an appropriate range, the large-angle particle size and ΔKth are in an appropriate range, and fatigue crack generation suppression characteristics are improved. 2) It can be seen that good HAZ toughness is exhibited by appropriately controlling the prior γ grain size, and (3) controlling the A value is effective in reducing the large-angle grain size.

It is explanatory drawing which shows the shape of a fatigue crack growth test piece. It is a graph which shows the influence which a bainite fraction has on large angle particle size and (DELTA) Kth. It is a graph which shows the relationship between an old gamma particle size and HAZ toughness (vE- ₄₀ ). It is a graph which shows the relationship between A value and a large angle particle size.

Claims (4)

A welded joint in which high-tensile steel plates are joined by welding, the high-tensile steel plate being C: 0.01 to 0.06% (meaning of mass%, the same shall apply hereinafter), Si: 0.01 to 0.5% , Mn: 0.5 to 2.0%, Al: 0.005 to 0.10%, Nb: 0.005 to 0.05%, Ti: 0.008 to 0.03%, B: 0.001 -0.005% and N: 0.003-0.008%, Cr: 0.3-1.0%, Cu: 0.1-1.0%, Ni: 0.1 1.0%, containing one or more selected from the group consisting of Mo: 0.03-0.5% and V: 0.02-0.05%, the balance being inevitable impurities, and Ti The content [Ti] of N and the content [N] of N satisfy the relationship of the following formula (1), and the A value defined by the following formula (2) is 0.15% or less. Is intended to satisfy the organization of weld heat affected zone of high-tensile steel plate, with an average particle size of prior austenite is 200μm or less, a fraction of bainite of 90 area% or more, and the orientation difference between two crystal Is a welded joint excellent in toughness and fatigue crack generation suppression characteristics, wherein when the region surrounded by a large-angle grain boundary of 15 ° or more is a crystal grain, the average equivalent circle diameter of the crystal grain is 9 μm or less .
[Ti] / [N] ≦ 3.4 (1)
A value = [C] + [Si] / 19 + [Mn] / 30 + [Cu] / 49 + [Ni] / 51 + [Cr] / 33 + [Mo] / 25 + [V] / 13 (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] are C, Si, Mn, Cu, Ni, Cr, Mo and The V content (% by mass) is shown. The high tensile steel sheet, 0.025% or less of P in unavoidable impurities (not including 0%), in claim 1 is obtained by respectively suppressed to 0.02% or less of S (0% exclusive) The weld joint described. The high tensile steel sheet, Ca: 0.005% or less (not including 0%) and / or rare earth elements: 0.005% or less (not including 0%) in claim 1 or 2 are those which contain The weld joint described. The weld joint according to any one of claims 1 to 3 , wherein the heat-affected zone of the high-tensile steel plate has an average Charpy absorbed energy at -40 ° C of 50 J or more.

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