KR20130126715A - Thick steel sheet having superior fatigue resistance properties in sheet thickness direction, method for producing same, and fillet welded joint using said thick steel sheet - Google Patents

Abstract

Provided are a thick steel sheet excellent in fatigue resistance in a plate thickness direction suitable for welded steel structures such as pressure vessels, a method for producing the same, and a fillet welded joint using the thick steel sheet. Specifically, the X-ray intensity ratio of the (110) plane parallel to the plate surface is at least 2.0 in a range from a position of 2 mm to a 3/10 position of the plate thickness from at least both sides or one side of the rolled surface of the steel sheet in the plate thickness direction. The X-ray intensity ratio of the (100) plane parallel to a plate surface is 160 MPa or more, and the average value of plate | board thickness direction compressive residual stress is 1.1 or less, and includes C, Si, Mn, and further Ti After having a composition comprising one or two of Nb, if necessary, one or two or more of Cu, Ni, Cr, Mo, V, W, Zr, B, Al, and the balance Fe and unavoidable impurities It is a fillet welded joint using a steel plate, its manufacturing method, and a thick steel plate.

Description

THICK STEEL SHEET HAVING SUPERIOR FATIGUE RESISTANCE PROPERTIES IN SHEET THICKNESS DIRECTION, METHOD FOR PRODUCING SAME, AND FILLET WELDED JOINT USING SAID THICK STEEL SHEET}

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to plate thickness endothelium suitable for welded steel structures, such as ships, marine structures, bridges, constructions, pressure vessels, and the like. The present invention relates to a steel plate excellent in fatigue resistance and a manufacturing method thereof, and a fillet welded joint using the thick steel plate.

Steel sheets used in welded steel structures such as ships, offshore structures, bridges, buildings, pressure vessels, etc., of course, are excellent in mechanical properties such as strength and toughness or weldability, Also for a steady cyclic load at the time of operation, or an unsteady cyclic load resulting from vibrations such as wind and earthquake. It is required to have a characteristic that can ensure the structural safety of the structure. Especially in recent years, it is strongly requested | required that the steel plate is excellent in fatigue resistance.

In a welded steel structure, a large number of stress concentration portions exist in the weld end portion, but the stress tends to concentrate on the weld end portion, and the residual stress in tension also acts. Fatigue cracks are likely to occur from the weld toe, and the weld toe is often a source of fatigue cracks.

In order to prevent the occurrence of such fatigue cracks, measures such as improvement of the shape of the end portion and introduction of compressive residual stress are known. However, since many weld edges exist in a welded steel structure, implementing measures to prevent the occurrence of fatigue cracks for each weld edge requires enormous effort and time, and increases the number of construction times, It leads to higher construction costs.

Therefore, it is thought that the fatigue resistance of the steel plate itself to be used is improved, and the fatigue resistance of a welded steel structure can be improved instead of the measure which prevents generation of such a fatigue crack. By improving the fatigue resistance of the steel sheet itself, the growth of fatigue cracks is suppressed and the fatigue life of the welded steel structure can be extended.

With respect to such a request, for example, in Patent Document 1, a microstructure in which a stripe-shaped second phase extending in the steel sheet rolling direction is interspersed at an area ratio of 5 to 50% in the mother phase is present. to have, a has a hardness (hardness) H _V is high, crack propagation properties in the endothelial than 30% than the hardness H _V of the parent phase (fatigue crack propagation properties) is good on the second steel plate is proposed.

The technique described in Patent Literature 1 disperses the second phase having a high hardness in the mother phase, and when the fatigue crack reaches the hard second phase, the propagation of the crack is significantly delayed. It is said that it is preferable to make the aspect ratio of a 2nd phase into four or more by improving a characteristic. It is described that when such a steel sheet is used for a large structure in which fatigue cracks are generated and propagated from the surface, high fatigue crack propagation preventing properties can be given to the large structure without requiring special consideration.

Among the weld joints, fillet welded joints such as box arc welds, cruciform arc welds, cover plate welds, stud welds, etc. It is known that the fatigue strength is the lowest, and the improvement of the fatigue strength in fillet welded joints of heavy gauge steel, especially for recent large container vessels, is urgent issue. It is considered). In the case of a fillet welded joint, fatigue cracks generated from the weld edges are advanced in the plate thickness direction, and therefore, it is effective to use a steel sheet excellent in fatigue resistance in the plate thickness direction because it improves the fatigue resistance as a joint.

Further, Patent Document 2 contains, 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, and S: 0.02% or less, and the balance Fe and It is composed of unavoidable impurities, and has a (diffracted intensity ratio) of 2.0 to 15.0 in the plate thickness direction measured by X-ray, and is also a recovery ferrite grain or recrystallized ferrite grain ( A thick steel sheet having a low fatigue crack growth rate in the plate thickness direction, in which the area ratio of recrystallized ferrite grains is 15 to 40%, is described.

Japanese Patent Application Laid-Open No. 7-90478 Japanese Patent Application Laid-open No. Hei 8-199286

However, in the technique described in Patent Literature 1, in order to lower the fatigue crack propagation speed and retard the propagation of fatigue cracks remarkably, it is necessary to increase the hardness of the second phase relative to the mother phase and to disperse it in large quantities. A problem arises in that ductility and toughness decrease. Although the fall of the ductility and toughness of a steel plate can be prevented by containing a large amount of alloying elements, the problem of causing high material cost cannot be avoided.

In addition, in the technique described in Patent Literature 2, the (200) diffraction intensity ratio in the plate thickness direction is set to 2.0 or more, that is, the (100) plane develops textures provided parallel to the plate surface, and fatigue cracking occurs. Various slip systems are activated at the fatigue crack tip to generate interference between dislocations, and the propagation of cracks is suppressed to reduce the propagation rate of fatigue cracks in the plate thickness direction. have. However, the (100) plane is a cleavage plane, and the thick steel plate provided with the (100) plane parallel to the plate surface has left a problem that the toughness in the plate thickness direction deteriorates.

Moreover, although the technique of patent documents 1 and 2 reduces the fatigue crack propagation rate, the fatigue life of total does not remarkably increase.

As described above, the thick steel sheet excellent in fatigue resistance characteristics described in Patent Literatures 1 and 2 has room to be improved in terms of cost and performance for welded structures. There is no known welding method for improving furnace characteristics.

An object of the present invention is to advantageously solve such problems of the prior art and to provide a thick steel sheet excellent in fatigue resistance in the plate thickness direction and suitable for use in welded steel structures, and a method of manufacturing the same.

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

MEANS TO SOLVE THE PROBLEM In order to improve a fatigue characteristic, this inventor repeated earnest research, and acquired the following knowledge, in order to improve a fatigue characteristic, without accompanying the fall of toughness of a plate thickness direction.

(1) In order to improve the fatigue properties, the (110) plane is parallel to the plate surface in a range from a position of 2 mm to a 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel sheet in the plate thickness direction. It is effective to set it as the organization ((110) aggregate organization which developed).

(2) In order to suppress the fall of toughness in the plate thickness direction, it is effective to make it the structure which suppressed the development of the (100) plane parallel to a plate surface in the said range.

(3) In order to improve a fatigue characteristic, without accompanying the fall of toughness of a plate | board thickness direction, it is effective to introduce | transduce a plate | board thickness direction residual stress and to make the average value as small as possible (to make it the compression side).

 (4) The aggregated structure having the characteristics of (1) and (2) is characterized in that hot rolling is carried out from both sides or one side of the rolled surface of the steel sheet to a position of 2 mm in the plate thickness direction to 3/10 position of the plate thickness. In the temperature range where the range becomes the two-phase temperature range, rolling is performed in which the reduction ratio of the average of one pass is less than 3.5% so as to be 50% or more in the cumulative reduction ratio, and the sheet thickness direction residual stress is accumulated in the reduction ratio. It is introduce | transduced by adjustment of the cooling rate of the accelerated cooling after 2-phase reverse rolling or hot rolling of 50% or more of rate.

In addition, (5) limiting the welding heat input and the number of laminations during the production of the fillet welded joint is effective for improving the fatigue strength of the fillet welded portion.

In addition, this invention uses the steel plate thickness: 50 mm or more, and uses the three-point bend fatigue specimen of the dimension shape shown in FIG. 1 with "excellent fatigue resistance property." Then, a fatigue test was conducted under the condition that the stress ratio (= minimum load / maximum load) became 0.1, and the fatigue life in the plate thickness direction was obtained to obtain a stress range of 340 MPa. The fatigue life is assumed to be 2 million times or more.

In addition, the present invention is directed to a fillet welded joint of a thick steel sheet having a plate thickness of 50 mm or more. If the plate thickness is less than 50 mm, the decrease in the fatigue strength due to the thickness effect is not so significant, and it is also based on various fatigue design curves based on many fatigue test databases in the past. In this case, fatigue resistant safety is ensured even without using the present invention. "Excellent fatigue resistance" means a fatigue test under the condition that the stress ratio is 0.1 by using a three-point bent fillet welded joint fatigue test piece in which the dimensional shape shown in FIG. 3 is cut out, and the fatigue in the plate thickness direction is performed. It is assumed that the service life is obtained and the fatigue life in the stress range of 340 MPa is 250,000 times or more.

This invention is completed based on the said knowledge, adding further examination. That is, the gist of the present invention is as follows.

(1) The X-ray intensity ratio of the (110) plane parallel to the plate surface is at least 2.0 in a range from at least 2 mm to 3/10 position of the plate thickness in the plate thickness direction from at least both sides or one side of the rolled surface of the steel sheet. A thick steel sheet having an aggregate structure to be formed and having an excellent fatigue resistance in the plate thickness direction, wherein an average value of the compressive residual stress in the plate thickness direction is 160 MPa or more.

(2) A thick steel sheet having excellent fatigue resistance in the plate thickness direction as described in (1), wherein an X-ray intensity ratio of the (100) plane parallel to the plate surface in the aggregate structure is 1.1 or less.

(3) The thick steel sheet contains, in mass%, C: 0.03 to 0.15%, Si: 0.60% or less, Mn: 0.80 to 1.80%, and further Ti: 0.005 to 0.050%, Nb: 0.001 to 0.1%. The thick steel plate which contains the 1 type (s) or 2 types selected from these, and has the composition which consists of remainder Fe and an unavoidable impurity, and is excellent in the fatigue resistance of the plate thickness direction as described in (1) or (2).

(4) In addition to the above composition, in mass%, 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, A thick steel sheet excellent in fatigue resistance in the sheet thickness direction as described in (3), characterized by containing one or two or more selected from Zr: 0.5% or less and B: 0.0050% or less.

(5) The thick steel sheet excellent in fatigue resistance in the plate thickness direction as described in (3) or (4) characterized by the above-mentioned composition being a composition containing Al: 0.1% or less by mass% further.

(6) When the steel material having the composition according to any one of (3) to (5) is heated and hot rolled to form a thick steel sheet, the hot rolling is a temperature equal to or higher than the austenitic partial recrystallization temperature. In the reverse direction, the cumulative reduction ratio: the range corresponding to the first rolling to be 10% or more and the position corresponding to 3/10 of the plate thickness from the position of 2 mm in the plate thickness direction from both sides or one side of the rolled surface of the thick steel sheet is 2 In the temperature range which becomes a phase structure, it has a 2nd rolling whose average reduction ratio of each path | pass is less than 3.5%, and a cumulative reduction ratio: 50% or more, 1 degreeC after completion | finish of hot rolling at 600 degreeC or more by steel plate surface temperature. A method for producing a thick steel sheet having excellent fatigue resistance in a plate thickness direction by performing accelerated cooling at a cooling rate of / s or more and cooling to 400 ° C. or less.

(7) Fillet part of thick steel plate with excellent fatigue resistance in plate thickness direction of 50mm or more of plate thickness is laminated with heat less than 30kJ / cm, 3 layers or less, 6 passes or less A fillet welded joint having excellent fatigue strength, characterized by welding.

(8) The thick steel sheet having a plate thickness of 50 mm or more is parallel to the plate surface in a range from a position of 2 mm to a 3/10 position of the plate thickness in the plate thickness direction from at least both sides or one side of the rolled surface of the steel sheet. The fillet welded joint excellent in the fatigue strength as described in (7) characterized by having the site | part which X-ray intensity ratio of (110) plane becomes 2.0 or more.

(9) The fillet welded joint having excellent fatigue strength according to (8), wherein an X-ray intensity ratio of the (100) plane in which the structure of the thick steel plate having the plate thickness of 50 mm or more is parallel to the plate surface is 1.1 or less.

(10) The fillet welded joint having excellent fatigue strength according to (8) or (9), wherein an average value of the plate thickness direction compressive residual stress of the thick steel sheet having a sheet thickness of 50 mm or more is 160 MPa or more.

According to the present invention, a thick steel sheet having a plate thickness of 50 mm or more having excellent fatigue resistance in the plate thickness direction can be easily and inexpensively manufactured without impairing the ductility and toughness, and exhibits an excellent industrial effect.

According to the present invention, the fatigue properties of the 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 improved by using a thick steel sheet having ductility and toughness as a welding structure. It is possible to show a special industrial effect.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram explaining the dimension shape of the 3-point bending test piece used for a fatigue test.
It is a schematic diagram explaining the generation | occurrence | production state of the slip in the fatigue crack tip which advances in a plate thickness direction cross section.
It is explanatory drawing which shows schematically the dimension shape of the notched three-point bending fillet weld joint fatigue test piece used for a fatigue test.
It is a figure explaining the welding conditions of a fillet weld joint.

(Mode for carrying out the invention)

Hereinafter, the structure, plate | board thickness direction compressive residual stress, preferable component composition, and manufacturing conditions which are prescribed | regulated by this invention are demonstrated.

[group]

The thick steel sheet according to the present invention is an X-ray of (110) plane parallel to the plate surface in a range from at least 2 mm to 3/10 position of the plate thickness in the plate thickness direction from at least both sides or one side of the rolled surface of the steel sheet. The aggregated structure has an intensity ratio of 2.0 or more.

In order to suppress the propagation (propagation) of the fatigue crack (cracked surface is the plate thickness surface) advancing in the plate thickness direction, the (110) plane is parallel to the structure, that is, the plate surface 90 ° tilted from the crack surface (plate thickness surface). In this case, the (110) aggregated tissue (110) is integrated, and the X-ray intensity ratio is 2.0 or more.

It is a schematic diagram explaining the generation | occurrence | production state of the slip in the fatigue crack tip which advances in a plate thickness direction cross section. In general, fatigue cracking causes irreversible slip at the tip of the crack at an inclination of about 45 ° from the crack surface at which the shear stress is maximum due to the action of cyclic stress, which accumulates and progresses. Ganda (slip deformation occurs in the slip system (slip plane slip direction) where the shear stress is the highest in the relationship between the stress field of the crack tip and the crystal orientation, and the crack progresses).

Therefore, when the (110) plane, which is the principal slip plane of the steel plate of the body-centered cubic structure, is tilted 90 ° from the crack plane, the shear stress is maximized. Slip on the surface inclined about 45 ° from the crack surface to be suppressed is suppressed.

If the X-ray intensity ratio of the (110) plane parallel to the plate surface is less than 2.0, the effect of lowering the fatigue crack propagation speed and improving the fatigue characteristic in the plate thickness direction is not sufficiently obtained. In addition, 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 direction. Refers to the ratio of X-ray intensity from the (110) plane that exists parallel to the plate surface. The X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more, compared to the steel plate having a random crystal orientation, and the (110) plane parallel to the plate surface is accumulated at 2.0 times or more, so that the (110) aggregate structure is formed. It means forming.

The thick steel plate according to the present invention is a plate structure in which the X-ray intensity ratio of the (110) plane parallel to the plate surface becomes 2.0 or more, at least from a position of 2 mm in the plate thickness direction from both sides or one side of the rolled surface of the steel sheet. It is provided in the range up to 3/10 position of thickness.

The fatigue crack propagated in the sheet thickness direction is generated from a stress concentration region near the surface of the steel sheet, for example, a weld portion such as a member attached to the surface, but the region, in particular, the portion up to 2 mm from the surface of the steel sheet. In this case, the aggregate structure provided by the welding heat for attachment of the member or the like is lost.

On the other hand, in the fatigue crack which has advanced to the plate thickness center, the crack is large, the stress intensity factor of the crack tip is large, and the fatigue crack growth per cycle of cyclic load is large, (110 The effect of reducing the fatigue crack propagation rate due to the presence of the aggregated structure is hardly obtained.

Therefore, the said aggregate structure is formed in the range from the position of 2 mm to 3/10 position of plate | board thickness at least from the both sides or one side of the rolling surface of a steel plate. However, the effect of the present invention is not impaired even if the entire steel sheet is a (110) aggregate structure, and the thick steel sheet according to the present invention does not interfere with the entire plate thickness direction as the aggregate structure.

In a body-centered cubic (bcc) structural steel sheet, the (100) plane is a cleaved plane, and the presence of the (100) plane parallel to the plate surface lowers the toughness in the plate thickness direction, and when the (100) plane develops parallel to the plate plane, Since the formation of the aggregate structure is inhibited, it is parallel to the plate surface in a range from a position of 2 mm in the plate thickness direction to a position of 3/10 of the plate thickness from at least both sides or one side of the rolled surface of the steel sheet. The X-ray intensity ratio of the plane) is 1.1 or less, preferably reduced as much as possible. In addition, the X-ray intensity ratio of the (100) plane parallel to a plate surface is based on the X-ray intensity from the (100) plane parallel to the plate surface in the steel plate which has a random orientation, and parallel to it The ratio of the X-ray intensity from the (100) plane present. The X-ray intensity ratio of the (100) plane parallel to the plate surface is 1.1 or less, compared to the steel plate having a random orientation, and the integration of the (100) plane parallel to the plate surface is 1.1 times or less, and almost (100) aggregate structure is formed. It means not doing.

[Compressive residual stress in plate thickness direction]

Although the compressive residual stress in the plate thickness direction is effective for suppressing the toughness drop in the plate thickness direction and reducing the fatigue crack propagation speed in the plate thickness direction, since the above-described excellent fatigue resistance characteristics are not obtained at less than 160 MPa, 160 It is set to MPa or more. The average value of the compressive residual stress in the plate thickness direction measures the residual stress in the plate thickness direction (crack propagation direction) at a pitch of 4 mm in the plate thickness direction by X-ray measurement, and the value on the compression side ( It was set as the absolute value of the average value of negative value).

It is preferable to combine the thick steel sheet according to the present invention with strength and toughness (tensile strength TS: 490 MPa or more, absorbed energy at -40 ° C: 200 J or more) for welding steel structures. , Component composition and manufacturing conditions are as follows.

[Component Composition] In the description,% is made by mass%.

C: 0.03-0.15%

C is an element having the effect of increasing the strength of the steel, and in order to ensure the desired high strength, it is preferable to contain 0.03% or more, but when it contains more than 0.15%, the weld heat affected zone (welded heat-affected zone) The toughness of the is lowered. 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 which acts as a deoxidizing agent and has a function of solid solution to increase the strength of steel. In order to acquire such an effect, it is preferable to contain 0.01% or more. On the other hand, containing exceeding 0.60% reduces the toughness of a weld heat affected zone. For this reason, it is preferable to limit Si to 0.60% or less. Moreover, More preferably, it is 0.50% or less.

Mn: 0.80 to 1.80%

Mn is an element having the effect of increasing the strength of the steel, and in order to secure a desired high strength, it is preferable to contain 0.80% or more. However, when Mn is contained in an amount exceeding 1.80%, the base metal toughness may be reduced. For this reason, it is preferable to limit Mn to 0.80 to 1.80% of range. Moreover, More preferably, it is 0.9-1.60%.

1 type or 2 types selected from Ti: 0.005-0.050% and Nb: 0.001-0.1%

Ti and Nb are elements that increase the strength through precipitation strengthening and inhibit the growth of austenite grains during heating and contribute to the refinement of the steel sheet structure. It contains.

Ti forms carbides and nitrides, contributes to the miniaturization of austenite grains during steel sheet production, and suppresses coarsening of crystal grains in the weld heat affected zones, thereby affecting weld heat. Improves the toughness of wealth In order to acquire such an effect, it is preferable to contain 0.005% or more. On the other hand, containing exceeding 0.050% reduces toughness. For this reason, it is preferable to limit Ti to 0.005 to 0.050% of range. Moreover, More preferably, it is 0.005 to 0.02%.

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

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

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: 0.0050% or less Or two or more

Cu, Ni, Cr, Mo, V, W, Zr, and B are elements that improve the strength and toughness of the steel, and contain one or two or more kinds depending on desired characteristics.

Cu mainly contributes to increasing the strength of the steel through precipitation strengthening. In order to acquire such an effect, it is preferable to contain 0.05% or more, but the content exceeding 2.0% becomes excessive precipitation strengthening and toughness falls. For this reason, when it contains, it is preferable to limit Cu to 2.0% or less. Moreover, More preferably, it is 0.35% or less.

Ni increases the strength of the steel and also contributes to the toughness improvement. Moreover, Ni acts effectively in order to prevent the crack at the time of hot rolling by Cu. In order to acquire such an effect, it is preferable to contain 0.05% or more. However, even when it contains a large amount exceeding 2.0%, the effect becomes saturated and the effect suitable for content cannot be expected, and it becomes economically disadvantageous, Ni is an expensive element, and a high content contains high material cost. . For this reason, when it contains, it is preferable to limit Ni to 2.0% or less. Moreover, More preferably, it is 0.1% or more.

Cr increases the amount of pearlite (amont of pearlite), contributing to the increase in strength of the steel. In order to acquire such an effect, it is preferable to contain 0.01% or more, but the content exceeding 0.6% reduces the toughness of a weld part. For this reason, when it contains, it is preferable to limit Cr to 0.6% or less. Moreover, More preferably, it is 0.01 to 0.2%.

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

V contributes to increasing the strength of the steel through solid solution strength and precipitation strengthening. In order to acquire such an effect, it is preferable to contain 0.05% or more, but the content exceeding 0.2% will remarkably reduce base material toughness and weldability. For this reason, it is preferable to limit V to 0.2% or less. Moreover, More preferably, it is 0.05 to 0.1%.

W contributes to the increase in strength of the steel, in particular to the increase in strength at high temperatures. In order to acquire such an effect, it is preferable to contain 0.1% or more, but large amount containing more than 0.5% reduces the toughness of a weld part. In addition, the high content of expensive W results in higher material costs. For this reason, when it contains, it is preferable to limit W to 0.5% or less. More preferably, it is 0.2 to 0.4%.

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

B contributes to increasing the strength of the steel through improvement of the hardenability, precipitates as BN during rolling, and contributes to miniaturization of ferrite grains after rolling. In order to acquire such an effect, it is preferable to contain 0.0010% or more, but the content 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 also contributes to the refinement of crystal grains. In order to obtain such an effect, Al is preferably contained at 0.015% or more, but excessive content exceeding 0.1% leads to a decrease in toughness. For this reason, when it contains, Al was limited to 0.1% or less. Moreover, Preferably it is 0.08% or less.

Remainder other than the above-mentioned component is Fe and an unavoidable impurity, and can accept P: 0.035% or less, S: 0.035% or less, and N: 0.012% or less.

[Manufacturing conditions]

The manufacturing method of steel materials, such as slab, is not specifically limited. The molten steel of the above composition is melted using a commercially available solvent furnace such as a converter furnace, and used as a steel material such as slab by a commercial method such as a continuous casting method. And to a temperature of 900 to 1350 ° C.

If the heating temperature is lower than 900 ° C, the desired hot rolling becomes difficult. On the other hand, at heating temperatures exceeding 1350 ° C., surface oxidation becomes remarkable, and coarsening of crystal grains becomes remarkable. For this reason, it is preferable to limit the heating temperature of steel materials to the temperature of the range of 900-1350 degreeC. More preferably, it is 1150 degrees C or less from a viewpoint of toughness improvement.

Hot rolled steel is applied. Hot rolling has a 1st rolling and a 2nd rolling, and 1st rolling has a temperature range more than austenite partial recrystallization temperature (in the case of the said component composition, the temperature range more than austenite partial recrystallization temperature is 1000-1000 in surface temperature). At a cumulative reduction ratio of 10% or more at 850 ° C. Since the austenite grains recrystallize at least partially, the steel sheet structure can be made fine and uniform. In addition, in order for at least austenite grains to recrystallize at least partially, it is preferable to make cumulative reduction ratio: 10% or more. In the rolling temperature range, the uniformity of the crystal grains cannot be expected in the austenite unrecrystallized temperature range. Moreover, it is preferable to make an upper limit of a cumulative reduction ratio into 30% from a viewpoint of securing the reduction ratio of 2nd rolling.

After said first rolling, the average of each path | pass in the temperature range which the range from the position of 2 mm to the 3/10 position of plate | board thickness from two sides or one side of the rolling surface of a steel plate to a 2/10 position of plate | board thickness becomes a two-phase structure. The rolling reduction is less than 3.5%, cumulative reduction: 50% or more, and rolling finish temperature: 600 ° C or higher.

In 2nd rolling, the average rolling reduction of each path | route introduce | transduces a shear strain in the range from the position of 2 mm to 3/10 position of plate | board thickness from the both sides or one side of the rolling surface of a steel plate, and accumulates When the rolling reduction temperature is set to 50% or more and the rolling end temperature is 600 ° C or more, in order to form an (110) aggregate structure in which the X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more, it is less than 3.5%. .

If the cumulative reduction ratio is less than 50%, the X-ray intensity ratio of the (110) plane parallel to the plate surface cannot be 2.0 or more.

In the case of the composition range, the surface temperature ranges from about 2 mm in the plate thickness direction to 3/10 the plate thickness in the plate thickness direction from both or one side of the rolled surface of the steel sheet in the temperature range of 900 to 600 ° C. Become a phase organization. Rolling end temperature is made into the temperature of the temperature range of 600 degreeC or more as surface temperature.

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. Therefore, the rolling end temperature is set to 600 ° C or higher, preferably 850 to 600 ° C.

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

In hot rolling, a steel sheet having a sheet thickness of 50 mm or more is used. When the sheet thickness is less than 50 mm, at the time of hot rolling, at least, from the both sides or one side of the rolling surface of the steel sheet to the position from 2 mm to 3/10 position of the sheet thickness in the plate thickness direction, (110) It is difficult to introduce a shear strain that is effective in the development. Moreover, when plate | board thickness is less than 50 mm, the fall of steel plate buckling performance is feared by introduction of plate | board thickness direction compressive residual stress. As mentioned above, it is set as the thick steel plate of 50 mm or more of plate | board thickness. In addition, hot rolling may perform rolling in the range which does not impair the effect of these rolling other than 1st rolling and 2nd rolling.

After the second rolling, accelerated cooling is performed at a cooling rate of 1 ° C / s or more and cooled to 400 ° C or less. If the cooling stop temperature exceeds 400 ° C at a cooling rate of less than 1 ° C / s, it is difficult to set the average value of the compressive residual stress in the sheet thickness direction to 160 MPa or more, so that the cooling rate is 1 ° C / s or more and the cooling stop temperature is It is set to 400 degrees C or less. Moreover, More preferably, it cools to 350 degrees C or less at the cooling rate of 5 degrees C / s or more.

In the present invention, the welding heat input (kJ / cm) and the lamination method are defined as welding conditions for the fillet joint of a thick steel sheet excellent in fatigue resistance in the plate thickness direction. Welding heat input (simply referred to as heat input) is 30 kJ / cm or less. Fillet welding with a heat input of more than 30 kJ / cm results in a change in the structure of the steel sheet or the form of internal residual stress due to the heat effect of the welding, which adversely affects the fatigue properties of the steel sheet having excellent fatigue resistance in the plate thickness direction. It is set to 30 kJ / cm or less.

In addition, when the fillet weld joint is manufactured by the lamination exceeding 3 layers and 6 passes, even if the welding heat input is 30 kJ / cm or less, the compressive residual stress of the weld end portion becomes high, and the effect of improving fatigue characteristics is not obtained. Or 6 passes or less. In addition, the welding method is not specifically prescribed. Can be applied to the welding hand (hand welding), MIG welding (metal inert gas welding), welding CO ₂ (carbon dioxide welding) or the like.

Example 1

Hot rolling was performed to the steel raw material of the composition shown in Table 1 on the conditions shown in Table 2, and it was set as the thick steel plate of 50-80 mm of plate | board thickness. These thick steel sheets were subjected to structure observation, tensile test, toughness test, and fatigue crack propagation test.

(1) microstructure observation

From the 1/4 position (representation of the range of 2 mm-3/10 position of plate thickness from surface to plate thickness direction) of the obtained thick steel plate, the test piece for magnitude | size observation parallel to a plate surface (size: thickness 1.5mm x Width 25mm x length 30mm) was extract | collected, and the X-ray-diffraction intensity of the (110) plane and (100) plane parallel to a board surface was calculated | required by the X-ray diffraction method. The ratio between the obtained X-ray intensity and the X-ray diffraction intensities of the (110) plane and the (100) plane of the random tissue standard sample is the X-ray intensity ratio of the (110) plane parallel to the plate surface, respectively. It was set as the X-ray intensity ratio of the (100) plane parallel to a plate surface.

(2) residual stress measurement

From the obtained thick steel sheet, a test piece for measuring residual stress by X-ray (size: sheet thickness (as steel sheet intact thickness) × 12.5 mm × 300 mm [plate thickness direction dimension × rolling right angle direction dimension × rolling direction dimension]) was taken After electrolytic polishing was performed on the measurement surface [surface of 12.5 mm x 300 mm] [rolling right angle dimension x rolling direction dimension], the plate thickness direction residual stress was measured by X-ray at a pitch of 4 mm in the plate thickness direction. . Among the measured residual stresses, the value on the compression side (negative side) was averaged, and the absolute value was made the average value of the compression residual stress in the plate thickness direction.

(3) tensile test

From the obtained thick steel plate, JIS No. 4 tensile test piece (parallel part diameter: 14 mm) was extract | collected so that the tension direction might become a direction perpendicular to the rolling direction of a steel plate based on the specification of JISZ2201 (1998). The sampling position of the test piece was made into 1/4 position of plate | board thickness (representation of the range of 2 mm-3/10 position of plate | board thickness direction from a surface). The tensile test was carried out in accordance with JIS Z 2241 (1998), and YS: yield strength σ _YS or 0.2% yield strength sigma _0.2 , TS: tensile strength σ _TS and elongation El were determined to evaluate tensile properties at static tension.

(4) toughness test

From the obtained thick steel plate, in accordance with the provisions of JIS Z 2242 (2005), the V-notch test piece was sampled so that a longitudinal direction might become parallel to a rolling direction, the absorption energy in -40 degreeC was calculated | required, and toughness was evaluated. In addition, the V-notch test piece was extract | collected from the 1/4 position of plate | board thickness (represented in the range of 2/10-3/10 position of plate | board thickness from a surface direction from the surface).

(5) fatigue test

From the obtained thick steel sheet, a test piece for fatigue testing (size: sheet thickness (steel plate intact as it is)) × 12.5 mm x 300 to 350 mm so that the propagation direction of the fatigue crack becomes the sheet thickness direction (plate thickness direction dimension X rolling right angle direction dimension X rolling) Direction dimension]). The test piece is the three-point bending fatigue test piece which cut out the dimension shape shown in FIG. 1 mentioned above, and when plate | board thickness is 50-65 mm in order to make the bending span at the time of a fatigue test four times as plate thickness. When the dimension of the rolling direction was 300 mm and the plate | board thickness was 80 mm, the dimension of the rolling direction was 350 mm. In the fatigue test, a fatigue test was conducted under conditions in which 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 plate thickness direction were obtained.

The obtained fatigue life of 2 million times or more was evaluated as " excellent fatigue resistance in plate thickness direction " as ○, and the other cases as x. In addition, the cutout (notch) of a test piece is a machined notch of width 0.1mm.

Examples of the present invention (Nos. 4, 7, 9, 11, 14, 17) are all 1/4 positions of the plate thickness (representation of the range of 2 mm to 3/10 positions of the plate thickness from the surface) The X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more, the average value of the plate thickness direction compressive residual stress is 160 MPa or more, and the X-ray intensity ratio of the (100) plane parallel to the plate surface is 1.1 or less. There is no fall in toughness in the plate thickness direction, and a thick steel sheet excellent in fatigue resistance in the plate thickness direction is obtained.

On the other hand, Comparative Examples (No. 1, 2, 3, 5, 6, 8, 10, 12, 13, 15, 16) outside the scope of the present invention have an X-ray intensity ratio of (110) plane parallel to the plate surface. The average value of the compressive residual stress of less than 2.0 or of the plate thickness direction is less than 160 MPa, and the fatigue resistance of the plate thickness direction is inferior.

[Example 2]

The fillet weld joint was produced using the thick steel plate 1 excellent in the fatigue characteristic of the plate thickness direction of 50-80 mm of plate thickness which shows a chemical component in Table 3, and manufacturing conditions and a characteristic in Table 4, and shows a shape in FIG. The three-point bending fatigue test was implemented using the notched three-point bending fillet weld joint fatigue test piece. The test method for confirming the structure, mechanical properties, and plate | board thickness direction fatigue characteristics of the thick steel plate 1 was performed similarly to Example 1.

After the characteristic was confirmed by the test mentioned above, the fillet weld joint was produced on the conditions shown in FIG. 4 using the steel plate 1, and the fatigue test was done. As a fatigue test piece, using the three-point bending fillet welded joint fatigue test piece which cut out the dimension shape shown in FIG. 3, it implemented on the conditions which a stress range is 340 Mpa and stress ratio R (= minimum load / maximum load) will be 0.1. Saved fatigue life. Table 5 shows the results obtained with the thick steel plate 1.

In the thick steel plate 1, all the examples of the present invention (test Nos. 3, 4 and 6) were obtained under the strict conditions of the stress range of 340 MPa, and a fillet welded joint having a fatigue life of 250,000 times or more and excellent fatigue resistance was obtained. It became. On the other hand, comparative examples (test Nos. 1 and 2) outside the range of welding conditions specified in the present invention (30 kJ / cm of heat input, lamination conditions of 3 layers and 6 passes or less) and fatigue life in the plate thickness direction are inferior. The comparative example (test No. 5) using the thick steel plate does not ensure fatigue resistance.

Claims (10)

Aggregation in which the X-ray intensity ratio of the (110) plane parallel to the plate surface is at least 2.0 in a range from a position of 2 mm to a 3/10 position of the plate thickness at least from both sides or one side of the rolled surface of the steel sheet in the plate thickness direction. The thick steel plate which has a structure and whose average value of plate | board thickness direction compressive residual stress is 160 Mpa or more. The method of claim 1,
The thick steel sheet in the said aggregate structure whose X-ray intensity ratio of the (100) plane parallel to a plate surface is 1.1 or less. 3. The method according to claim 1 or 2,
The thick steel sheet contains, in mass%, C: 0.03 to 0.15%, Si: 0.60% or less, Mn: 0.80 to 1.80%, and further selected from Ti: 0.005 to 0.050% and Nb: 0.001 to 0.1%. The thick steel plate which contains the seed | species or 2 types, and has a composition which consists of remainder Fe and an unavoidable impurity. The method of claim 3,
In addition to the above composition, in mass%, 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: A thick steel sheet having a composition containing one or two or more selected from 0.0050% or less. The method according to claim 3 or 4,
In addition to the above-mentioned composition, a thick steel sheet having a composition containing Al: 0.1% or less by mass%. The steel sheet having the composition according to any one of claims 3 to 5 is subjected to hot rolling to form a thick steel sheet, wherein the hot rolling is a cumulative reduction ratio at a temperature range equal to or higher than the austenite partial recrystallization temperature: 10% The temperature range where the range corresponding to the 2/10 position of a plate thickness from the position of 2 mm in a plate thickness direction from the both sides or one side of the rolling surface of the said thick steel plate, and the above-mentioned 1st rolling and the said steel plate becomes a two-phase structure In the pass, the average reduction ratio of each pass is less than 3.5% and has a cumulative reduction ratio: 50% or more, and has a cooling rate of 1 ° C / s or more after completion of hot rolling at 600 ° C or higher at the steel plate surface temperature. The manufacturing method of the thick steel plate which accelerates cooling and cools to 400 degrees C or less. A fillet welded joint in which a fillet portion of a thick steel sheet having excellent fatigue resistance in a plate thickness direction of 50 mm or more is welded in a stack having a heat input of 30 kJ / cm or less, 3 layers or less, and 6 passes or less. The method of claim 7, wherein
The thick steel sheet having a sheet thickness of 50 mm or more is parallel to the plate surface in a range from a position of 2 mm to a 3/10 position of the plate thickness at least from both sides or one side of the rolled surface of the steel sheet in the plate thickness direction. A fillet welded joint having a portion where the surface X-ray intensity ratio is 2.0 or more. 9. The method of claim 8,
A fillet welded joint in which the X-ray intensity ratio of the (100) plane in which the structure of the thick steel plate having the plate thickness of 50 mm or more is parallel to the plate surface is 1.1 or less. 10. The method according to claim 8 or 9,
The fillet welded joint in which the average value of the plate thickness direction compressive residual stress of the thick steel plate of 50 mm or more of said plate | board thickness is 160 Mpa or more.

Images