JP5906868B2 - Thick steel plate with excellent fatigue resistance in the thickness direction and method for producing the same - Google Patents

Description

  The present invention relates to a steel plate excellent in fatigue resistance in the thickness direction suitable for welded steel structures such as ships, offshore structures, bridges, buildings, and pressure vessels, and a method for producing the same.

  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.

In response to such a request, for example, in Patent Document 1, the striped second phase extending in the rolling direction of the steel sheet has a microscopic structure in which the matrix phase is scattered at an area ratio of 5 to 50%. hardness H _V phase 30% or 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.

  Moreover, in patent document 2, it is the mass%, C: 0.015-0.20%, Si: 0.05-2.0%, Mn: 0.1-2.0%, P: 0.05 %, 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 Alternatively, a thick steel plate having a low fatigue crack propagation rate in the thickness direction in which the area ratio of recrystallized ferrite grains is 15 to 40% is described.

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

  However, 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 can be increased compared to the parent phase and dispersed in a large amount. Necessary, there arises a problem that the ductility and toughness of the steel sheet are significantly lowered. Although reduction of the ductility and toughness of the steel sheet may be prevented by the inclusion of a large amount of alloy elements, the problem of incurring an increase in material costs is inevitable.

  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 is not significantly increased.

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

In order to improve the fatigue characteristics without accompanying a decrease in toughness in the thickness direction, the present inventors have made extensive studies focusing on the texture and obtained the following knowledge.
(1) In order to improve the fatigue characteristics, the (110) plane is parallel to the plate surface in the range from the position of 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. It is effective to use an organization that has been developed (sometimes referred to as (110) texture).
(2) In order to suppress a reduction in toughness in the plate thickness direction, it is effective to have a structure in which the development of the (100) plane is suppressed in parallel with the plate surface in the above range.
(3) In order to improve fatigue properties without accompanying toughness reduction in the thickness direction, it is effective to introduce residual stress in the thickness direction and make the average value as small as possible (to the compression side). is there.

  (4) The texture having the characteristics (1) and (2) described above is for hot rolling from a position 2 mm in the thickness direction from both sides or one side of the rolling surface of the steel plate to a 3/10 position of the thickness. In the temperature range where the range is a two-phase temperature range, rolling is performed so that the average reduction rate of one pass is less than 3.5%, so that the cumulative reduction rate is 50% or more. Residual stress is introduced by adjusting the cooling rate of accelerated cooling after two-phase rolling or hot rolling with a cumulative rolling reduction of 50% or more.

  The present invention is intended for a steel sheet having a thickness of 50 mm or more, and “excellent in fatigue resistance” means that a stress ratio is measured using a three-point bending fatigue test piece having a dimension and shape shown in FIG. The fatigue test is carried out under the condition of 0.1, the fatigue life in the thickness direction is obtained, and the fatigue life in the stress range of 340 MPa is 2 million 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) The X-ray intensity ratio of the (110) plane parallel to the plate surface is 2 at least in the range from the position 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. A thick steel plate having excellent texture resistance in the plate thickness direction, having a texture of 0.0 or more and having an average value of compressive residual stress in the plate thickness direction of 160 MPa or more.
(2) In the texture, the X-ray intensity ratio of the (100) plane parallel to the plate surface is 1.1 or less, and excellent in fatigue resistance in the plate thickness direction according to (1) Thick steel plate.
(3) The said thick steel plate contains C: 0.03-0.15%, Si: 0.60% or less, Mn: 0.80-1.80% by mass%, and also Ti: 0.005 -0.050%, Nb: containing one or two selected from 0.001 to 0.1%, and having a composition comprising the balance Fe and inevitable impurities (1) Or the thick steel plate excellent in the fatigue resistance of the thickness direction as described in (2).
(4) In addition to the above composition, Cu: 2.0% or less; Ni: 2.0% or less; Cr: 0.6% or less; Mo: 0.6% or less; 2 or less, W: 0.5% or less, Zr: 0.5% or less, and B: 0.0050% or less, containing one or more selected from (3) A steel plate with excellent fatigue resistance in the thickness direction as described in 1.
(5) In addition to the above composition, the composition further contains Al: 0.1% or less by mass%, and the fatigue resistance property in the thickness direction according to (3) or (4) Excellent thick steel plate.
(6) When the steel material having the composition according to any one of (3) to (5) is heated and hot-rolled to obtain a thick steel plate, the hot-rolling is performed at an austenite partial recrystallization temperature or higher. In the temperature range, the rolling reduction ratio is 10% or more, and the range corresponding to the position from 2 mm to 3/10 position of the sheet thickness in the sheet thickness direction from both sides or one side of the rolled surface of the thick steel sheet. In the temperature range in which a two-phase structure is formed, the average rolling reduction of each pass is less than 3.5%, and the cumulative rolling reduction is 50% or more. A method for producing a thick steel plate having excellent fatigue resistance in the thickness direction, characterized by performing accelerated cooling at a cooling rate of 1 ° C./s or more after completion of cold rolling and cooling to 400 ° C. or less.

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

The schematic diagram explaining the dimension shape of the 3 point | piece bending test piece used for a fatigue test. The schematic diagram explaining the generation | occurrence | production state of the slip in the fatigue crack tip which progresses in a sheet thickness direction cross section.

Hereinafter, the structure, the thickness direction compressive residual stress, the preferred component composition, and the production conditions defined in the present invention will be described.
[Organization]
The steel plate according to the present invention has an X-ray intensity of (110) plane parallel to the plate surface at least in the range from 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. It has a texture with a ratio of 2.0 or more.

  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.

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

  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 effect of the present invention is not impaired even if the entire steel sheet is made into a (110) texture, and the thick steel sheet according to the present invention does not prevent the entire texture in the 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.

The preferred component composition and production conditions for combining the steel plate according to the present invention with strength and toughness for welded steel structures (tensile strength TS: 490 MPa or more, absorbed energy at −40 ° C .: 200 J or more) are as follows: It is as follows.
[Component 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: 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% and Nb: 0.001 to 0.1% Ti and Nb increase the strength through precipitation strengthening and at the time of heating Is an element that suppresses the growth of the austenite grains and contributes to the refinement of the steel sheet structure. In the present invention, it contains 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, and N: 0.012% or less 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, and cooling is performed to 400 ° C. or less. The 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 compressive residual stress in the plate thickness direction to 160 MPa or more. More preferably, it is cooled to 350 ° C. or lower at a cooling rate of 5 ° C./s or higher.

  The steel material having the composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a thick steel plate having a thickness of 50 to 80 mm. These thick steel plates were subjected to a structure observation, a tensile test, a toughness test, and a fatigue crack propagation test. Table 2 shows these test results together with the hot rolling conditions.

(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 of the obtained X-ray intensity and the X-ray diffraction intensity of the (110) plane and (100) plane of the random test piece is respectively compared with the X-ray intensity ratio of the (110) plane parallel to the plate plane, the plate plane The X-ray intensity ratio of the (100) plane parallel to.
(2) Residual stress measurement From the obtained thick steel plate, a test piece for X-ray residual stress measurement (size: plate thickness (as the original thickness of the steel plate) x 12.5 mm x 300 mm [plate thickness direction dimension x rolling perpendicular direction dimension x The dimension in the rolling direction]) was collected and subjected to electropolishing on the measurement surface [surface of 12.5 mm × 300 mm] [dimension in the perpendicular direction of rolling × dimension in the rolling direction], and then the plate thickness was measured by X-rays at a pitch of 4 mm in the plate thickness direction. Directional residual stress 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) Tensile test JIS No. 4 tensile specimen (diameter of parallel part) was obtained from the obtained thick steel plate in accordance with the provisions of JIS Z 2201 (1998) so that the tensile direction was perpendicular to the rolling direction of the steel plate. : 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 is performed in accordance with JIS Z 2241 (1998). YS: Yield strength σ _YS or 0.2% yield strength σ _0.2 , TS: Tensile strength σ _TS , elongation El, static tensile The tensile properties at the time were evaluated.

(4) Toughness test V-notch test specimens were collected from the obtained thick steel sheets in accordance with the provisions of JIS Z 2242 (2005) so that the longitudinal direction was 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 (representing a range of 2 mm to 3/10 position of the plate thickness in the plate thickness direction from the surface).

(5) Fatigue test From the obtained thick steel plate, a fatigue test specimen (size: plate thickness (the original steel plate thickness)) x 12.5 mm x 300 to be such that the propagation direction of fatigue cracks is the plate thickness direction. 350 mm [size in the plate thickness direction × size in the direction perpendicular to the rolling × size in the rolling direction]) was collected. The test piece is a notched three-point bending fatigue test piece having the dimensions shown in FIG. 1 and the bending span during the fatigue test is 4 times the plate thickness. Therefore, when the plate thickness is 50 to 65 mm, rolling is performed. When the direction dimension was 300 mm and the plate thickness was 80 mm, the rolling direction dimension 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.

  The case where the obtained fatigue life was 2 million times or more was evaluated as “Excellent in fatigue resistance in the thickness direction”, and the other cases were evaluated as ×. In addition, the notch (notch) of a test piece is a machining notch with a width of 0.1 mm.

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

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

Claims (4)

In mass%, C: 0.03 to 0.15%, Si: 0.60% or less, Mn: 0.80 to 1.80%, Al: 0.1% or less, and Ti: 0.005 -0.050%, Nb: containing one or two selected from 0.001-0.1%, having a composition consisting of the balance Fe and inevitable impurities,
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 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. A thick steel plate having excellent fatigue resistance in the plate thickness direction, having a texture that becomes a mean value and having an average value of compressive residual stress in the plate thickness direction of 160 MPa or more.   2. The thick steel plate having excellent fatigue resistance in the thickness direction according to claim 1, wherein an X-ray intensity ratio of a (100) plane parallel to the plate surface in the texture is 1.1 or less. In addition to the above-mentioned composition, further, by 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: claim 1, characterized in that a composition containing one or more selected from among 0.0050% or less Or a thick steel plate having excellent fatigue resistance in the thickness direction according to 2 ; It is a manufacturing method of the thick steel plate as described in any one of Claims 1-3 , and when the steel raw material is heated and hot-rolled to make a thick steel plate, the said hot rolling is more than an austenite partial recrystallization temperature. In the temperature range, the rolling reduction ratio is 10% or more, and the range corresponding to the position from 2 mm to 3/10 position of the sheet thickness in the sheet thickness direction from both sides or one side of the rolled surface of the thick steel sheet. In the temperature range in which a two-phase structure is formed, the average rolling reduction of each pass is less than 3.5%, and the cumulative rolling reduction is 50% or more. A method for producing a thick steel plate having excellent fatigue resistance in the thickness direction, characterized by performing accelerated cooling at a cooling rate of 1 ° C./s or more after completion of cold rolling and cooling to 400 ° C. or less.

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