JP5884150B2 - Method for manufacturing fillet welded joint - Google Patents

Description

  The present invention relates to a fillet welded joint, and particularly to a joint made of an extremely thick steel plate having a thickness of 50 mm or more and having excellent fatigue characteristics.

  Steel plates used for welded steel structures such as ships, offshore structures, bridges, buildings, pressure vessels, etc. are of course excellent in mechanical properties such as strength and toughness and weldability. It is required to have a characteristic that can ensure the structural safety of the structure even with respect to a steady cyclic load and a non-steady cyclic load caused by vibrations such as wind and earthquake. Particularly in recent years, steel sheets are strongly required to have excellent fatigue resistance.

  In welded steel structures, there are many stress concentration parts at the weld toe, etc., but stress tends to concentrate at the weld toe part, and tensile residual stress also acts, so repeated loads were applied. In some cases, fatigue cracks are likely to occur from the weld toe, and the weld toe is often the source of fatigue cracks.

  In order to prevent the occurrence of such fatigue cracks, measures such as improvement of the shape of the toe portion and introduction of compressive residual stress are known. However, since there are a large number of weld toes in a welded steel structure, it takes a lot of labor and time to implement the above-described measures for preventing the occurrence of fatigue cracks for each weld toe. , Increase in the number of construction steps and increase in construction cost.

  Therefore, it has been considered to improve the fatigue resistance characteristics of the welded steel structure by improving the fatigue resistance characteristics of the steel sheet used in place of measures for preventing the occurrence of such fatigue cracks. By improving the fatigue resistance of the steel plate itself, the growth of fatigue cracks is suppressed and the fatigue life of the welded steel structure can be extended.

As a steel plate excellent in fatigue resistance characteristics, for example, in Patent Document 1, a striped second phase extending in the steel plate rolling direction has a microscopic structure scattered in an area ratio of 5 to 50% in the matrix phase, hardness H _V of the second phase is 30% or more higher than the hardness H _V of the mother phase, excellent steel fatigue crack growth properties have been proposed.

  The technique described in Patent Document 1 disperses a high-hardness second phase in the matrix phase, and when a fatigue crack reaches the vicinity of the hard second phase, the propagation of cracks is significantly delayed, and the steel plate It is intended to improve fatigue crack propagation resistance, and the aspect ratio of the second phase is preferably 4 or more. It is described that if such a steel plate is used for a large structure in which fatigue cracks are generated from the surface and propagated, no special consideration is required and a high fatigue crack propagation preventing property can be imparted to the large structure. ing.

Among welded joints, fillet welded joints such as corner welding, cross welding, cover plate welding, stud welding, etc. are known to have the lowest fatigue strength, and are especially applicable to recent large container ships. Improvement of fatigue strength in fillet welded joints of thick steel plates is an urgent issue.
In the case of fillet welded joints, fatigue cracks generated from the weld toes propagate in the plate thickness direction, so using a steel plate with excellent fatigue resistance in the plate thickness direction improves the fatigue resistance of the joint. It is effective for.

  In Patent Document 2, in mass%, C: 0.015 to 0.20%, Si: 0.05 to 2.0%, Mn: 0.1 to 2.0%, P: 0.05% or less , S: 0.02% or less, the balance being Fe and inevitable impurities, the (200) diffraction intensity ratio in the plate thickness direction measured by X-ray is 2.0 to 15.0, and recovery or re- A thick steel sheet having a low fatigue crack propagation rate in the thickness direction, in which the area ratio of crystal ferrite grains is 15 to 40%, is described.

JP-A-7-90478 JP-A-8-199286

  In the technique described in Patent Document 1, in order to reduce the fatigue crack propagation rate and significantly delay the propagation of fatigue cracks, the hardness of the second phase is increased compared to the parent phase, and a large amount of hard second phase is added. Need to be distributed. For this reason, the problem that the ductility of a steel plate and the toughness fall remarkably arises. In some cases, a decrease in the ductility and toughness of the steel sheet can be prevented by containing a large amount of alloy elements. However, the inclusion of a large amount of alloy elements inevitably raises the material cost.

  Further, in the technique described in Patent Document 2, the (200) diffraction intensity ratio in the plate thickness direction is set to 2.0 or more, that is, a texture in which the (100) plane is aligned parallel to the plate surface is developed, and fatigue Various slip systems are activated at the crack tip, causing interference between dislocations, suppressing crack propagation and reducing the fatigue crack propagation rate in the plate thickness direction. However, the (100) plane is a cleavage plane, and the thick steel plate having the (100) plane parallel to the plate surface has a problem that the toughness in the plate thickness direction deteriorates. Furthermore, in the techniques described in Patent Documents 1 and 2, the fatigue crack propagation speed is reduced, but the total fatigue life including the fatigue crack generation life is not significantly increased.

  As described above, the thick steel plates with excellent fatigue resistance described in Patent Documents 1 and 2 have room to be improved in terms of cost and performance for use in welded structures, while manufacturing fillet welded joints. However, a welding method for improving fatigue resistance as a joint is not disclosed.

  Accordingly, an object of the present invention is to provide a fillet welded joint having excellent fatigue resistance, which is a fillet joint using a thick steel plate having excellent fatigue resistance in the thickness direction.

  In order to achieve the above object, the present inventors have determined that the influence of the texture and compressive residual stress in the thickness direction of the thick steel plate on the fatigue resistance property of the steel plate and the welding conditions of the fillet welded joint are the fatigue resistance properties of the joint. The following findings were obtained through extensive research on the effects on the environment.

  In order to improve the fatigue resistance in the plate thickness direction of the thick steel plate, (1) at least a range from a position of 2 mm to 3/10 position of the plate thickness in the plate thickness direction from both sides or one side of the rolled surface of the steel plate. Suppose that the (110) plane is developed parallel to the plate surface ((110) texture).

  (2) In the above (1), the structure further suppresses the development of the (100) plane parallel to the plate surface. (3) Introduce compressive residual stress in the plate thickness direction, and make the average value as small as possible (on the compression side).

  (4) It is also effective to set the compressive residual stress in the direction perpendicular to the sheet thickness direction of the steel sheet to 100 MPa or more in the range from both sides or one side of the rolled surface of the steel sheet to 4 mm in the sheet thickness direction.

  Moreover, (5) Limiting the welding heat input and the number of laminations when producing fillet welded joints is effective in improving the fatigue strength of fillet welds.

  The present invention is directed to a fillet welded joint of thick steel plates having a thickness of 50 mm or more. If the thickness is less than 50 mm, the decrease in fatigue strength due to the thickness effect is not so significant, and if it conforms to various fatigue design curves based on many past fatigue test databases, the fatigue resistance safety can be achieved without using the present invention. Sex is ensured. “Excellent fatigue resistance” means that a fatigue test was conducted under the condition that the stress ratio was 0.1 using a notched three-point bend fillet welded joint fatigue test piece with the dimensions shown in FIG. The fatigue life in the plate thickness direction is obtained, and the fatigue life in the stress range of 340 MPa is assumed to be 250,000 times or more.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) Fatigue characterized by welding a fillet portion of a thick steel plate having excellent fatigue resistance in the thickness direction of a thickness of 50 mm or more, with a heat input of 30 kJ / cm or less and a lamination of 3 layers and 6 passes or less. Fillet welded joint with excellent strength.
(2) The steel plate having a thickness of 50 mm or more is parallel to the plate surface at least in the range from 2 mm position to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate in the plate thickness direction ( 110) The fillet welded joint having excellent fatigue strength according to (1), wherein the X-ray intensity ratio of the surface is 2.0 or more.
(3) The fatigue according to (2), wherein the structure of the thick steel plate having a thickness of 50 mm or more is such that the X-ray intensity ratio of the (100) plane parallel to the plate surface is 1.1 or less. Fillet welded joint with excellent strength.
(4) The fillet weld having excellent fatigue strength according to (2) or (3), wherein an average value of compressive residual stress in the thickness direction of the thick steel plate having a thickness of 50 mm or more is 160 MPa or more. The joint (5) is characterized in that the compressive residual stress in the direction perpendicular to the plate thickness direction is 100 MPa or more in a range from both sides or one side of the rolled surface of the thick steel plate having a plate thickness of 50 mm or more to 4 mm in the plate thickness direction. A fillet welded joint having excellent fatigue strength according to (1).

  According to the present invention, the fatigue characteristics of a fillet welded portion of a thick steel plate having a thickness of 50 mm or more in which fatigue strength is a particular problem can be easily and inexpensively obtained using a thick steel plate having ductility and toughness as a welded structure. It can be greatly improved and has an industrially significant effect.

Explanatory drawing which shows typically the dimension shape of the 3 point bending fillet welded joint fatigue test piece with a notch used for a fatigue test. Explanatory drawing which shows typically the generation | occurrence | production state of the slip in the fatigue crack front-end | tip in the plate | board thickness direction cross section of the thick steel plate applied to a fillet welded joint. Explanatory drawing which shows typically the dimension shape of the 3 point | piece bending fatigue test piece with a notch used for a fatigue test. The figure explaining the welding conditions of a fillet welded joint.

  In the present invention, welding heat input (kJ / cm) and a lamination method are defined as welding conditions for a fillet joint of a thick steel plate having excellent fatigue resistance in the thickness direction. The welding heat input (sometimes referred to as heat input) is 30 kJ / cm or less. When fillet welding is performed at a heat input exceeding 30 kJ / cm, the structure of the steel sheet or the form of internal residual stress changes due to the thermal effect of the welding, which adversely affects the fatigue characteristics of the steel sheet with excellent fatigue resistance in the thickness direction. Therefore, it is set to 30 kJ / cm or less.

Moreover, even if the welding heat input is 30 kJ / cm or less, if a fillet welded joint is produced with a laminate exceeding three layers and six passes, the compressive residual stress at the weld toe portion becomes high, and the effect of improving fatigue characteristics cannot be obtained. The lamination is 3 layers or 6 passes or less. The welding method is not specified. Manual welding, MIG welding, CO ₂ welding, etc. can be applied.

Thick steel plate 1 (having a specific structure and compressive residual stress in the plate thickness direction) or thick steel plate 2 (perpendicular to the plate thickness direction) described below as a steel plate having excellent fatigue resistance in the plate thickness direction of 50 mm or more With a compressive residual stress in the direction) is preferred. First, the structure of the thick steel plate 1 and the compressive residual stress in the plate thickness direction will be described.
[Organization]
The X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more at least in the range from the position of 2 mm in the plate thickness direction from both sides or one side of the rolled surface of the steel plate to 3/10 position of the plate thickness. It has a texture.

  In order to suppress the progress (propagation) of fatigue cracks that propagate in the plate thickness direction (the crack surface is the plate thickness surface), a structure in which the (110) surface is inclined by 90 ° from the crack surface (plate thickness surface), that is, a plate A structure ((110) texture) in which the (110) planes are accumulated parallel to the plane is set, and the X-ray intensity ratio is set to 2.0 or more.

  FIG. 2 is a schematic diagram for explaining the occurrence of slip at the tip of a progressing fatigue crack in the cross section in the thickness direction. Generally, fatigue cracks cause irreversible slip at the crack tip at a crack angle of about 45 ° from the crack surface where the shear stress is maximum due to the action of repeated stress, which accumulates and propagates (crack tip). In the slip system (slip direction in the slip plane) where the shear stress is the highest due to the relationship between the stress field and the crystal orientation, slip deformation occurs and the crack progresses.

  Therefore, when the (110) plane, which is the main sliding surface of the body-centered cubic (bcc) structural steel plate, is tilted by 90 ° from the crack plane, the shear stress becomes maximum. For example, slipping on a surface inclined by about 45 ° from the crack surface is suppressed.

  Further, if the X-ray intensity ratio of the (110) plane parallel to the plate surface is less than 2.0, the effect of reducing the fatigue crack propagation rate and improving the fatigue characteristics in the plate thickness direction cannot be obtained sufficiently. 0 or more. The X-ray intensity ratio of the (110) plane parallel to the plate surface is based on the X-ray intensity from the (110) plane parallel to the plate surface in the steel plate having a random orientation, The ratio of the X-ray intensity from the (110) plane existing in parallel. The X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more means that the (110) plane parallel to the plate surface is 2.0 times or more compared to a steel plate having a random crystal orientation. It means that it is highly accumulated and forms a (110) texture.

  Fatigue cracks propagating in the plate thickness direction are generated from stress-concentrated portions in the vicinity of the steel plate surface, for example, welds such as members attached to the surface. The texture imparted by the welding heat for attaching the metal will disappear.

  On the other hand, the fatigue crack that has propagated to the center of the plate thickness is large, the stress intensity factor at the crack tip is large, the amount of fatigue crack growth per cycle is large, and the presence of (110) texture The effect of reducing the fatigue crack propagation rate due to is hardly obtained.

  Therefore, the texture is formed in a range from a position of 2 mm in the sheet thickness direction from at least one side or one side of the rolled surface of the steel sheet to a 3/10 position of the sheet thickness. However, the entire steel plate may be a (110) texture, and this does not prevent the entire thickness direction from being the texture.

  In the body-centered cubic (bcc) structure steel plate, the (100) plane is a cleavage plane, and the presence of the (100) plane parallel to the plate surface reduces the toughness in the plate thickness direction, and the (100) plane becomes the plate surface. When it develops in parallel, the formation of (110) texture is inhibited, so at least in the range from 2 mm in the thickness direction to the 3/10 position of the plate thickness from both sides or one side of the rolled surface of the plate. The X-ray intensity ratio of the parallel (100) plane is 1.1 or less, preferably reduced as much as possible. The X-ray intensity ratio of the (100) plane parallel to the plate surface is based on the X-ray intensity from the (100) plane parallel to the plate surface in a steel plate having a random orientation, The ratio of X-ray intensity from (100) planes existing in parallel. The X-ray intensity ratio of the (100) plane parallel to the plate surface is 1.1 or less means that the accumulation of the (100) plane parallel to the plate surface is 1.1 times or less compared to the steel plate having a random orientation. It means that (100) texture is hardly formed.

[Thickness direction compressive residual stress]
The compressive residual stress in the plate thickness direction is effective for suppressing toughness reduction in the plate thickness direction and reducing the fatigue crack propagation rate in the plate thickness direction, but if it is less than 160 MPa, the excellent fatigue resistance characteristics described above cannot be obtained. 160 MPa or more. The average value of compressive residual stress in the plate thickness direction is measured by X-ray measurement of the residual stress in the plate thickness direction (crack propagation direction) at a pitch of 4 mm in the plate thickness direction. The absolute value was taken.

  Next, the compressive residual stress in the direction perpendicular to the plate thickness direction of the thick steel plate 2 will be described. When improving fatigue resistance in the plate thickness direction, it is also effective to set the compressive residual stress in the direction perpendicular to the plate thickness direction to 100 MPa or more in the range from both sides or one side of the rolled surface of the thick steel plate to 4 mm in the plate thickness direction. It is.

  In the production of a welded structure, tack welding or scratching on the steel plate surface is inevitable, and the compressive residual stress is impaired at the very surface of the steel plate. It is set to 4 mm in the plate | board thickness direction from the both sides or one side of a rolling surface.

  On the other hand, if the range of the compressive residual stress extends beyond 4 mm from the surface to the inside of the plate thickness, the compressive residual stress in the vicinity of the surface portion where fatigue cracks occur due to the balance of internal stress decreases, so The range is 4 mm from both sides or one side in the plate thickness direction.

  The compressive residual stress in the direction perpendicular to the plate thickness direction within the above range is 100 MPa or more. In order to suppress the propagation of fatigue cracks, it is effective to apply a compressive stress in a direction perpendicular to the crack surface (crack propagation surface). Since the present invention targets cracks propagating in the plate thickness direction, the compression direction of the compressive residual stress is set to a direction perpendicular to the plate thickness direction.

  If the compressive residual stress is less than 100 MPa, the fatigue crack propagation rate is reduced, but a remarkable effect cannot be obtained so as to lead to an improvement in fatigue life. In addition, More preferably, it is 150 MPa or more. The compressive residual stress in the direction perpendicular to the plate thickness direction in the steel plate exceeding the range of 4 mm in the plate thickness direction from both sides or one side of the rolled surface of the steel plate is not particularly specified, but is usually in the range of up to 4 mm in the plate thickness direction. Smaller size.

Strength and toughness for welded steel structures on steel plates 1 and 2 having a thickness of 50 mm or more, excellent in fatigue resistance characteristics in the thickness direction as described above (tensile strength TS: 490 MPa or more, absorbed energy at −40 ° C .: In the present invention, the component composition and production conditions for combining 200 J or more) are not particularly defined, but preferred component compositions and production conditions are as follows.
In the case of the thick steel plate 1 [component composition] In the explanation,% is mass%.

C: 0.03-0.15%
C is an element having an effect of increasing the strength of steel, and in order to ensure a desired high strength, it is preferably contained in an amount of 0.03% or more. Heat-affected zone toughness decreases. For this reason, it is preferable to limit C to 0.03 to 0.15% of range.

Si: 0.60% or less Si is an element that acts as a deoxidizer and has a function of increasing the strength of steel by solid solution. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, the content exceeding 0.60% lowers the weld heat affected zone toughness. For this reason, it is preferable to limit Si to 0.60% or less. In addition, More preferably, it is 0.50% or less.

Mn: 0.80 to 1.80%
Mn is an element that has the effect of increasing the strength of steel. In order to ensure the desired high strength, Mn is preferably contained in an amount of 0.80% or more, but if contained in excess of 1.80%, There is concern about a reduction in material toughness. For this reason, Mn is preferably limited to a range of 0.80 to 1.80%. In addition, More preferably, it is 0.9 to 1.60%.

One or two selected from Ti: 0.005 to 0.050%, Nb: 0.001 to 0.1%
Ti and Nb are elements that increase the strength through precipitation strengthening and suppress the growth of austenite grains during heating and contribute to the refinement of the steel sheet structure. In the present invention, Ti and Nb contain one or two kinds.

  Ti forms carbides and nitrides, contributes to the refinement of austenite grains during steel plate production, suppresses crystal grain coarsening in the weld heat affected zone, and improves the weld heat affected zone toughness. In order to acquire such an effect, it is preferable to contain 0.005% or more. On the other hand, the content exceeding 0.050% reduces toughness. For this reason, Ti is preferably limited to a range of 0.005 to 0.050%. In addition, More preferably, it is 0.005 to 0.02%.

  Nb, like Ti, increases the strength through precipitation strengthening, further refines the structure, suppresses recrystallization of austenite, and promotes the effect of rolling to form the desired structure. Have. In order to acquire such an effect, it is preferable to contain 0.001% or more, but when it exceeds 0.1%, the structure tends to become needle-like and the toughness tends to decrease. For this reason, Nb is preferably limited to a range of 0.001 to 0.1%. In addition, More preferably, it is 0.02 to 0.05%.

  Furthermore, when improving a characteristic, in addition to the said basic component, 1 type (s) or 2 or more types of Cu, Ni, Cr, Mo, V, W, Zr, B, and Al can be contained.

Cu: 2.0% or less, Ni: 2.0% or less, Cr: 0.6% or less, Mo: 0.6% or less, V: 0.2% or less, W: 0.5% or less, Zr: 0.5% or less, B: One or more of 0.0050% or less Cu, Ni, Cr, Mo, V, W, Zr, B are elements that improve the strength and toughness of steel, and are desired It contains 1 type or 2 types or more depending on the characteristics.

  Cu contributes to the strength increase of steel mainly through precipitation strengthening. In order to obtain such an effect, it is desirable to contain 0.05% or more. However, if it exceeds 2.0%, precipitation strengthening is excessive and toughness decreases. For this reason, when it contains, it is preferable to limit Cu to 2.0% or less. In addition, More preferably, it is 0.35% or less.

  Ni increases the strength of the steel and contributes to improved toughness. Ni acts effectively to prevent cracking during hot rolling with Cu. In order to acquire such an effect, it is desirable to contain 0.05% or more. However, even if it is contained in a large amount exceeding 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected and it is economically disadvantageous, and Ni is an expensive element. Invite the soaring. For this reason, when it contains, it is preferable to limit Ni to 2.0% or less. In addition, More preferably, it is 0.1% or more.

  Cr increases the amount of pearlite and contributes to an increase in steel strength. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.6% reduces the toughness of a welded part. For this reason, when it contains, it is preferable to limit Cr to 0.6% or less. In addition, More preferably, it is 0.01 to 0.2%.

  Mo contributes to an increase in steel strength. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.6% reduces the toughness of a welded part. For this reason, when it contains, it is preferable to limit Mo to 0.6% or less. In addition, More preferably, it is 0.01 to 0.08%.

  V contributes to increasing the strength of steel through solid solution strengthening and precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.05% or more, but inclusion exceeding 0.2% significantly reduces the base metal toughness and weldability. For this reason, it is preferable to limit V to 0.2% or less. In addition, More preferably, it is 0.05 to 0.1%.

  W contributes to an increase in the strength of steel, particularly at a high temperature. In order to acquire such an effect, it is desirable to contain 0.1% or more, but if it contains more than 0.5%, the toughness of the welded portion is lowered. In addition, a large amount of expensive W causes a material cost to rise. For this reason, when contained, W is preferably limited to 0.5% or less. In addition, More preferably, it is 0.2 to 0.4%.

  Zr contributes to increasing the strength of steel and improves the resistance to plating cracking in the galvanized material. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.5% lowers the toughness of the weld. For this reason, when it contains, it is preferable to limit to 0.5% or less. In addition, More preferably, it is 0.01 to 0.1%.

  B contributes to an increase in the strength of the steel through improvement of hardenability and also precipitates as BN during rolling and contributes to refinement of ferrite grains after rolling. In order to acquire such an effect, it is desirable to contain 0.0010% or more, but inclusion exceeding 0.0050% deteriorates toughness. For this reason, when it contains, it is preferable to limit B to 0.0050% or less. More preferably, it is 0.0010 to 0.0035%.

Al: 0.1% or less Al acts as a deoxidizer and contributes to the refinement of crystal grains, and in order to obtain such an effect, it is desirable to contain 0.015% or more, An excessive content exceeding 0.1% leads to a decrease in toughness. For this reason, when it contained, Al was limited to 0.1% or less. In addition, Preferably it is 0.08% or less.

  The balance other than the above components is Fe and inevitable impurities, and P: 0.035% or less, S: 0.035% or less, N: 0.012% or less, etc. are acceptable.

[Production conditions]
The manufacturing method of steel materials, such as a slab, is not specifically limited. The molten steel having the above composition is melted using a conventional melting furnace such as a converter, and is made into a steel material such as a slab by a conventional method such as a continuous casting method, and heated to a temperature of 900 to 1350 ° C.

  If heating temperature is less than 900 degreeC, desired hot rolling will become difficult. On the other hand, when the heating temperature exceeds 1350 ° C., the surface oxidation becomes remarkable and the coarsening of the crystal grains becomes remarkable. For this reason, it is preferable to limit the heating temperature of a steel raw material to the temperature of the range of 900-1350 degreeC. More preferably, it is 1150 ° C. or less from the viewpoint of improving toughness.

  The heated steel material is hot rolled. The hot rolling includes the first rolling and the second rolling, and the first rolling is a temperature range above the austenite partial recrystallization temperature (in the case of the above component composition, the temperature range above the austenite partial recrystallization temperature is The cumulative rolling reduction is 10% or more at a surface temperature of 1000 to 850 ° C. Since the austenite grains are at least partially recrystallized, the steel sheet structure can be made fine and uniform. In order to at least partially recrystallize the austenite grains, it is preferable that the cumulative rolling reduction is 10% or more. When the rolling temperature range is the austenite non-recrystallization temperature range, it becomes impossible to expect uniform crystal grains. The upper limit of the cumulative rolling reduction is preferably 30% from the viewpoint of securing the rolling reduction of the second rolling.

  After the first rolling described above, the range from the position of 2 mm in the plate thickness direction to the 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate is the temperature range where the two-phase structure is formed, and the average of each pass Second rolling is performed at a rolling reduction of less than 3.5%, a cumulative rolling reduction of 50% or more, and a rolling end temperature of 600 ° C. or more.

In the second rolling, the average reduction ratio of each pass is determined by introducing shear strain in the range from the position of 2 mm in the sheet thickness direction to the position of 3/10 of the sheet thickness from both sides or one side of the rolled surface of the sheet. 2. When the rate is 50% or more and the rolling end temperature is 600 ° C. or more, an (110) texture whose X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more is formed. Less than 5%.
If the cumulative rolling reduction is less than 50%, the X-ray intensity ratio of the (110) plane parallel to the plate surface cannot be made 2.0 or more.

  In the case of the above composition range, the range from the position of 2 mm in the sheet thickness direction to the position of 3/10 of the sheet thickness from both sides or one side of the rolled surface of the steel sheet in the temperature range of 900 to 600 ° C. is substantially two-phase. Become an organization. The rolling end temperature is a surface temperature of 600 ° C. or higher.

  If the rolling end temperature is less than 600 ° C. at the surface temperature, excessive work strain is introduced into the ferrite and the toughness is lowered, so that it is 600 ° C. or higher, preferably 850 to 600 ° C.

  The thick steel plate produced by the above manufacturing method has at least (100) plane X-rays parallel to the plate surface in a range from a position 2 mm in the plate thickness direction to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate. The strength ratio is 1.1 or less, and toughness deterioration in the thickness direction is suppressed.

  In hot rolling, a steel plate having a thickness of 50 mm or more is used. When the plate thickness is less than 50 mm, at the time of hot rolling, the development of (110) texture occurs at least in the range from 2 mm position to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate in the plate thickness direction. It is difficult to introduce an effective shear strain in the case. Furthermore, if the plate thickness is less than 50 mm, there is a concern that the steel plate buckling performance may be lowered due to the introduction of the plate thickness direction compressive residual stress. From the above, a thick steel plate having a thickness of 50 mm or more is obtained. In addition to the first rolling and the second rolling, the hot rolling may be performed as long as the effects of the rolling are not impaired.

  After the second rolling, accelerated cooling is performed at a cooling rate of 1 ° C./s or more. If the cooling rate is less than 1 ° C./s, it is difficult to set the average value of the compressive residual stress in the plate thickness direction to 160 MPa or more. More preferably, it is cooled to 400 ° C. or lower at a cooling rate of 5 ° C./s or higher.

  In hot rolling, a steel plate having a thickness of 50 mm or more is used. In addition to the reasons described above, if the plate thickness is less than 50 mm, at the time of hot rolling, at least in the range from the position of 2 mm in the plate thickness direction from both sides or one side of the rolled surface of the steel plate to the 3/10 position of the plate thickness. , (110) It becomes difficult to introduce shear strain effective for the development of texture. Furthermore, if the plate thickness is less than 50 mm, there is a concern that the steel plate buckling performance may be lowered due to the introduction of the plate thickness direction compressive residual stress. In the hot rolling, in addition to the first rolling and the second rolling, the rolling may be performed as long as the effects of the rolling are not impaired.

  After the second rolling, accelerated cooling is performed at a cooling rate of 1 ° C./s or more. If the cooling rate is less than 1 ° C./s, it is difficult to set the average value of the compressive residual stress in the plate thickness direction to 160 MPa or more. More preferably, it is cooled to 400 ° C. or lower at a cooling rate of 5 ° C./s or higher.

In the case of the thick steel plate 2 [Ingredient composition] In the description,% is mass%.

C: 0.03-0.15%
C is an element having an effect of increasing the strength of steel, and in order to ensure a desired high strength, it is preferably contained in an amount of 0.03% or more. Heat-affected zone toughness decreases. For this reason, it is preferable to limit C to 0.03 to 0.15% of range.

Si: 1.0% or less Si is an element that acts as a deoxidizer and has a function of increasing the strength of steel by solid solution. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, the content exceeding 1.0% lowers the weld heat affected zone toughness. For this reason, it is preferable to limit Si to 1.0% or less. In addition, More preferably, it is 0.50% or less.

Mn: 1.0-2.0%
Mn is an element that has the effect of increasing the strength of steel. In order to ensure a desired high strength, Mn is preferably contained in an amount of 1.0% or more. There is concern about a reduction in material toughness. For this reason, it is preferable to limit Mn to the range of 1.0 to 2.0%. In addition, More preferably, it is 0.9 to 1.60%.

One or two of Ti: 0.005 to 0.05% and Nb: 0.001 to 0.05%
Ti and Nb are elements that increase the strength through precipitation strengthening and suppress the growth of austenite grains during heating and contribute to the refinement of the steel sheet structure. In the present invention, Ti and Nb contain one or two kinds.

  Ti forms carbides and nitrides, contributes to the refinement of austenite grains during steel plate production, suppresses crystal grain coarsening in the weld heat affected zone, and improves the weld heat affected zone toughness. In order to acquire such an effect, it is preferable to contain 0.005% or more. On the other hand, the content exceeding 0.05% reduces toughness. For this reason, it is preferable to limit Ti to 0.005 to 0.05% of range. In addition, More preferably, it is 0.005 to 0.02%.

  Nb, like Ti, increases the strength through precipitation strengthening, further refines the structure, suppresses recrystallization of austenite, and promotes the effect of rolling to form the desired structure. Have. In order to acquire such an effect, it is preferable to contain 0.001% or more. However, if it exceeds 0.05%, the structure tends to become needle-like and the toughness tends to decrease. For this reason, it is preferable to limit Nb to 0.001 to 0.05% of range. In addition, More preferably, it is 0.02 to 0.05%.

Al: 0.1% or less Al is an element that acts as a deoxidizer and contributes to refinement of crystal grains, and can be contained as necessary. In order to acquire such an effect, it is desirable to contain 0.015% or more, However, Excess content exceeding 0.1% will lead to the fall of toughness. For this reason, when it contained, Al was limited to 0.1% or less. In addition, Preferably it is 0.08% or less.

N: 0.0035 to 0.0075%
N is an element necessary for securing the necessary amount of TiN. If it is less than 0.0035%, a sufficient amount of TiN cannot be obtained, and if it exceeds 0.0075%, it is solidified in the region where TiN is dissolved by the welding heat cycle. The amount of dissolved N increases, and in any case, the toughness of the welded portion is significantly reduced.

  Furthermore, when improving a characteristic, in addition to the said basic component, 1 type (s) or 2 or more types of Cu, Ni, Cr, Mo, V, W, Zr, B, and Ca can be contained.

Cu: 0.01-0.5%, Ni: 2.0% or less, Cr: 0.01-0.5%, Mo: 0.01-0.5%, V: 0.001-0.1 %, W: 0.5% or less, Zr: 0.5% or less, Ca: 0.0005 to 0.0030%, B: 0.0005 to 0.0020% Cu, Ni, Cr, Mo, V, W, Zr, and B are elements that improve the strength and toughness of the steel, and contain one or more kinds depending on the desired properties.

  Cu contributes to the strength increase of steel mainly through precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.01% or more, but when it exceeds 0.5%, precipitation strengthening is excessive and toughness is lowered. For this reason, when it contains, it is preferable to limit Cu to 0.5% or less. In addition, More preferably, it is 0.35% or less.

  Ni acts effectively to prevent cracking during hot rolling with Cu. In order to acquire such an effect, it is desirable to contain 0.05% or more. However, even if it is contained in a large amount exceeding 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected and it is economically disadvantageous, and Ni is an expensive element. Invite the soaring. For this reason, when it contains, it is preferable to limit Ni to 2.0% or less. In addition, More preferably, it is 0.1% or more.

  Cr increases the amount of pearlite and contributes to an increase in steel strength. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.5% reduces the toughness of a welded part. For this reason, when contained, Cr is preferably limited to 0.5% or less. In addition, More preferably, it is 0.01 to 0.2%.

  Mo contributes to an increase in steel strength. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.5% reduces the toughness of a welded part. For this reason, when it contains, it is preferable to limit Mo to 0.5% or less. In addition, More preferably, it is 0.01 to 0.08%.

  V contributes to increasing the strength of steel through solid solution strengthening and precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.001% or more, but the content exceeding 0.1% remarkably lowers the base metal toughness and weldability. For this reason, it is preferable to limit V to 0.1% or less. In addition, More preferably, it is 0.05 to 0.1%.

  W contributes to an increase in the strength of steel, particularly at a high temperature. In order to acquire such an effect, it is desirable to contain 0.1% or more, but if it contains more than 0.5%, the toughness of the welded portion is lowered. In addition, a large amount of expensive W causes a material cost to rise. For this reason, when contained, W is preferably limited to 0.5% or less. In addition, More preferably, it is 0.2 to 0.4%.

  Zr contributes to increasing the strength of steel and improves the resistance to plating cracking in the galvanized material. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.5% lowers the toughness of the weld. For this reason, when it contains, it is preferable to limit to 0.5% or less. In addition, More preferably, it is 0.01 to 0.1%.

  B contributes to an increase in the strength of the steel through improvement of hardenability and also precipitates as BN during rolling and contributes to refinement of ferrite grains after rolling. In order to acquire such an effect, it is desirable to contain 0.0005% or more, but inclusion exceeding 0.0020% deteriorates toughness. For this reason, when it contains, it is preferable to limit B to 0.0020% or less. In addition, More preferably, it is 0.001 to 0.003%.

Ca: 0.0005% to 0.0030%
Ca is an element having an effect of improving toughness by fixing S. In order to exert such an effect, it is necessary to contain at least 0.0005%, but even if it exceeds 0.0030%, the effect is saturated, so 0.0005% to 0.0030% And

The balance other than the above components is Fe and inevitable impurities, and P: 0.035% or less, S: 0.035% or less, etc. are acceptable.
[Production conditions]
The manufacturing method of steel materials, such as a slab, is not specifically limited. The molten steel having the above composition is melted using a conventional melting furnace such as a converter, and is made into a steel material such as a slab by a conventional method such as a continuous casting method, and heated to a temperature of 1000 to 1250 ° C.

  If heating temperature is less than 1000 degreeC, desired hot rolling will become difficult. On the other hand, at a heating temperature exceeding 1250 ° C., surface oxidation becomes significant and crystal grain coarsening becomes significant. For this reason, it is preferable to limit the heating temperature of a steel raw material to the temperature of the range of 1000-1250 degreeC. In addition, more preferably, it is 1200 degrees C or less from a viewpoint of a toughness improvement.

  The heated steel material is hot rolled. Hot rolling is performed at a temperature of (Ar3 point +50) ° C. or higher with a cumulative reduction of 30% or more, and in combination with the cooling conditions described later, 4 mm from both sides or one side of the rolled surface of the steel plate in the thickness direction. In the range up to this point, compressive residual stress in the direction perpendicular to the thickness direction of 100 MPa or more is introduced. Ar3 point is calculated | required by Ar3 (degreeC) = 910-273 * C-74 * Mn-57 * Ni-16 * Cr-9 * Mo-5 * Cu (each element is content (mass%)), for example. Is possible.

  In hot rolling, a steel plate having a thickness of 50 mm or more is used. Compressive residual stress improves fatigue properties, but reduces buckling performance, and the reduction is more noticeable for steel sheets with thinner plate thickness, and if the plate thickness is less than 50 mm, there is a concern that the buckling performance of the steel plate itself may decrease. The thickness is 50 mm or more.

  In addition, this invention does not restrict | limit rolling outside the prescribed | regulated temperature range, It is possible to perform rough rolling etc. implemented at the high temperature after slab heating.

  After the completion of rolling, it is cooled to 350 ° C. or lower at a cooling rate of 3 ° C./s or higher. If either the cooling rate or the cooling stop temperature deviates from the above, a compressive residual stress of 100 MPa or more perpendicular to the plate thickness direction is obtained in the range from both sides or one side of the steel plate to 4 mm in the plate thickness direction. I can't. More preferably, it cools to 300 degrees C or less with the cooling rate of 5 degrees C / s or more.

  Table 1 shows chemical components, Table 2 shows manufacturing conditions and characteristics, thick steel plate 1 having excellent thickness in the thickness direction of 50 to 80 mm, Table 4 shows chemical components, and Table 5 shows manufacturing conditions and characteristics. A fillet welded joint was prepared using a thick steel plate 2 having excellent fatigue properties in the thickness direction of 55 to 70 mm, and three points were used using a notched three-point bend fillet welded joint fatigue test piece whose shape is shown in FIG. A bending fatigue test was performed. Test methods for confirming the structure, mechanical properties and thickness direction fatigue properties of the thick steel plates 1 and 2 were as follows (1) to (6).

(1) Microstructure observation From the 1/4 position of the thickness of the obtained thick steel plate (representative of the range of 2 mm to 3/10 position of the plate thickness from the surface to the thickness direction), the structure observation test parallel to the plate surface A piece (size: thickness 1.5 mm × width 25 mm × length 30 mm) was sampled, and X-ray diffraction intensities of the (110) plane and (100) plane parallel to the plate surface were determined by X-ray diffraction. . The ratio between the obtained X-ray diffraction intensity and the X-ray diffraction intensity of the (110) plane and (100) plane of the random test piece is determined by comparing the X-ray intensity ratio of the (110) plane parallel to the plate plane and the plate, respectively. The X-ray intensity ratio of the (100) plane parallel to the plane was used.
(2) Measurement of compressive residual stress in the plate thickness direction From the obtained thick steel plate, a test piece for measuring X-ray residual stress (size: plate thickness (the original thickness of the steel plate)) x 12.5 mm x 300 mm [plate thickness direction dimension x rolling Vertical dimension × rolling direction dimension]), and after the electrolytic polishing is performed on the measurement surface [surface of dimension 12.5 mm × 350 mm], the compressive residual stress in the plate thickness direction is measured by X-rays at a pitch of 4 mm in the plate thickness direction. It was measured. Among the measured residual stresses, the values on the compression side (minus side) were averaged, and the absolute value was defined as the average value of the compressive residual stresses in the thickness direction.

(3) Measurement of compressive residual stress in the direction perpendicular to the plate thickness direction From the obtained thick steel plate, a test piece for measuring X-ray residual stress (size: plate thickness (the original thickness of the steel plate) x 12.5 mm x 300 mm [plate thickness Direction dimension × rolling vertical direction × rolling direction]), and after electropolishing the measurement surface [surface of dimension 12.5 mm × 350 mm], perpendicular to the plate thickness direction by X-rays at 4 mm pitch in the plate thickness direction Residual stress in the direction was measured. The number of lines measured at a 4 mm pitch in the thickness direction was 5 lines. From the distribution of residual stress in the thickness direction obtained by averaging five points of the measured residual stress for each thickness position, obtain the residual stress (negative value) at a position 4 mm from the front surface / back surface, The absolute value was defined as compressive residual stress.

(4) Tensile test JIS No. 4 tensile specimen (diameter of parallel part) was obtained from the obtained thick steel sheet in accordance with the provisions of JIS Z 2201 (1998) so that the tensile direction was perpendicular to the rolling direction of the steel sheet. : 14 mm). The sampling position of the test piece was a 1/4 position of the plate thickness (representative of a range of 2 mm to 3/10 position of the plate thickness from the surface to the plate thickness direction). The tensile test was performed according to JIS Z 2241 (1998), and YS: yield strength or 0.2% proof stress, TS: tensile strength, elongation EL were determined, and the tensile properties during static tension were evaluated.

(5) Toughness test V-notch specimens were sampled from the resulting thick steel plate in accordance with the provisions of JIS Z 2242 (2005) so that the longitudinal direction is parallel to the rolling direction, and absorption at −40 ° C. Energy was determined and toughness was evaluated. The V-notch test piece was taken from a 1/4 position of the plate thickness (representative of a range from 2 mm to 3/10 position of the plate thickness in the plate thickness direction from the surface).

(6) Sheet thickness direction fatigue property confirmation test From the obtained thick steel plate, a fatigue test specimen (size: plate thickness (the original thickness of the steel plate) x 12) so that the propagation direction of fatigue cracks is the plate thickness direction. .5 mm × 300 to 350 mm [plate thickness direction size × rolling vertical direction size × rolling direction size]). The test piece is a notched three-point bending fatigue test piece having the dimensions shown in FIG. 3, and the bending span during the fatigue test is 4 times the plate thickness. Therefore, when the plate thickness is 50 to 65 mm, the rolling direction When the dimension was 300 mm and the plate thickness was 80 mm, the dimension in the rolling direction was 350 mm. In the fatigue test, a fatigue test was performed under the conditions that the stress range was 340 MPa and the stress ratio R (= minimum load / maximum load) was 0.1, and the fatigue characteristics (fatigue life) in the thickness direction were obtained.

  Using the thick steel plates 1 and 2 whose characteristics were confirmed by the test described above, fillet welded joints were produced under the conditions shown in FIG. 4, and a fatigue test was performed. As a fatigue test piece, a notched three-point bending fillet welded joint fatigue test piece having the dimensions shown in FIG. 1 is used, and the stress range is 340 MPa and the stress ratio R (= minimum load / maximum load) is 0.1. The fatigue life was obtained. The results obtained with the thick steel plate 1 are shown in Table 3, and the results obtained with the thick steel plate 2 are shown in Table 6.

  In the thick steel plates 1 and 2, the present invention examples (in the case of the thick steel plate 1, tests No. 3, 4, 6 and in the case of the thick steel plate 2, the test Nos. 2, 7, 8, 10) all have a stress range of 340 MPa. It was confirmed that a fillet welded joint having excellent fatigue resistance with a fatigue life of 250,000 times or more can be obtained under severe conditions. On the other hand, a comparative example (in the case of the thick steel plate 1, test Nos. 1 and 2 and the thick steel plate 2) out of the range of the welding conditions specified in the present invention (heat input 30 kJ / cm or less, 3 layers and 6 passes or less). In this case, test Nos. 4 and 5) and a comparative example using a thick steel plate with inferior fatigue life in the thickness direction (in the case of thick steel plate 1, in test No. 5 and in the case of thick steel plate 2, test No. 1, 3) , 6, 9) have not secured fatigue resistance.

Claims (2)

An average value of compressive residual stress in the plate thickness direction is 160 MPa or more, and a fillet portion of the thick steel plate having excellent fatigue resistance in the plate thickness direction of 50 mm or more is applied to a heat input of 30 kJ / cm or less, three layers and six passes. Welding with the following lamination,
The steel plate having a thickness of 50 mm or more is at least a (110) plane parallel to the plate surface in a range from a position of 2 mm to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate in the plate thickness direction. The manufacturing method of the fillet welded joint excellent in fatigue strength characterized by having the site | part from which X-ray intensity ratio of becomes 2.0 or more . The steel plate having a thickness of 50 mm or more is 2 mm in the plate thickness direction from both sides or one side of the rolled surface of the steel plate.
2. The fatigue strength according to claim 1 , wherein the X-ray intensity ratio of the (100) plane parallel to the plate surface is 1.1 or less in a range from the position of 3 to the 3/10 position of the plate thickness. Manufacturing method of excellent fillet welded joints.

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