KR20130125822A - Thick steel sheet having superior fatigue resistance properties in direction of sheet thickness, 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, a method for producing the same, and a fillet welded joint using the thick steel sheet, suitable for welded steel structures such as pressure vessels. Specifically, the compressive residual stress at right angles to the plate thickness direction is 100 MPa or more in a range of up to 4 mm in the plate thickness direction from both or one side of the rolled surface of the steel sheet, preferably in mass%, C: 0.03% to 0.15%, Si: 1.0% or less, Mn: 1.0% to 2.0%, further including Ti: 0.005% to 0.05%, Nb: 0.001% to 0.05%, one or two kinds, and Al: 0.1% or less. And a thick steel sheet having a composition containing at least one of Cu, Ni, Cr, Mo, V, W, Zr, Ca, and B, and a method for producing the same, and a fillet weld joint using the thick steel sheet.

Description

THICK STEEL SHEET HAVING SUPERIOR FATIGUE RESISTANCE PROPERTIES IN DIRECTION OF SHEET THICKNESS, 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 an increase in construction cost.

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 welded edges are advanced in the plate thickness direction, and therefore, it is effective to use a steel sheet having excellent fatigue resistance in the plate thickness direction to improve 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.

Patent Literature 3 discloses a cooling process in which the steel sheet is tempered at a temperature equal to or lower than Ac1 point after performing an on-line heat treatment such as quenching and normalizing, or an on-line heat treatment such as direct quenching or accelerated cooling. By performing forced cooling to set the maximum value of the temperature difference between the steel sheet surface and the steel sheet sheet thickness center in the range of 200 ° C. or more, compressive residual stress is imparted to the steel sheet surface to obtain a steel sheet having excellent fatigue strength.

Japanese Patent Application Laid-Open No. 7-90478 Japanese Patent Application Laid-open No. Hei 8-199286 Japanese Patent Laid-Open No. 6-100947

However, in the technique described in Patent Literature 1, in order to lower the fatigue crack propagation rate and retard the propagation of the fatigue crack remarkably, the hardness of the second phase is higher than that of the mother phase, and the hard second phase is further. It is necessary to disperse a large amount. For this reason, the problem that the fall of the ductility and toughness of a steel plate becomes remarkable arises. 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 that containing a large amount of alloying elements raises a 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 speed | rate, there existed a fundamental problem that the fatigue life of the total including fatigue crack generation life 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. The welding method for improving furnace characteristics is not clear.

Moreover, in the technique of patent document 3, in order to give a compressive residual stress to the steel plate surface, tempering process is essential and it has left the problem that it is impossible to provide a steel plate as it is with high productivity.

It is an object of the present invention 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 having a suitable strength and toughness for 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 The present inventors earnestly researched in consideration of the internal residual stress of a steel plate, in order to improve a fatigue characteristic as it is, with a productive rolling, without accompanying the fall of the ductility and toughness of a steel plate, and the fall of the toughness of a sheet thickness direction, The following findings were obtained.

(1) The fatigue property in the plate thickness direction is improved by setting the compressive residual stress perpendicular to the plate thickness direction to be 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 plate thickness direction. do.

(2) The steel sheet provided with the compressive residual stress is subjected to hot rolling with a cumulative reduction ratio of 30% or more with the temperature of the plate thickness center portion at (Ar 3 point + 50) ° C or higher, and thereafter at a cooling rate of 3 ° C / s or more. If it cools to 350 degrees C or less, it can be manufactured as it is by rolling (without tempering treatment).

In addition, (3) this invention applies to the steel plate of 50 mm or more of plate | board thickness, and is "excellent in fatigue-proof property", and stress ratio is 0.1 using the three-point bending fatigue test piece of the dimensional shape shown in FIG. A fatigue test is performed under such conditions to determine the fatigue life in the plate thickness direction, and the fatigue life in the stress range of 340 MPa is 2 million times or more.

(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 or one side of the rolled surface of the steel sheet to 4 mm in the sheet thickness direction.

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, 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 fatigue strength due to the plate thickness effect is not so remarkable, and, based on various fatigue design curves based on many fatigue test databases in the past, fatigue resistance is not required even if the present invention is not used. Secured. "Excellent fatigue resistance" means a fatigue test under the condition that the stress ratio is 0.1 using a three-point bent fillet welded joint fatigue test piece in which the dimensional shape shown in FIG. 1 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.

The present invention has been completed by further examining the obtained knowledge, that is, the gist of the present invention is as follows.

(1) The compressive residual stress at right angles to the plate thickness direction is 100 MPa or more in the range from both sides or one side of the rolled surface of the steel sheet to the plate thickness direction, wherein the fatigue resistance in the plate thickness direction is Excellent thick steel plate.

(2) The thick steel sheet contains, in mass%, C: 0.03 to 0.15%, Si: 1.0% or less, Mn: 1.0 to 2.0%, and further Ti: 0.005 to 0.05% and Nb: 0.001 to 0.05%. The thick steel plate as described in (1) containing 1 or 2 types of, N: 0.0035 to 0.0075%, and having a composition which consists of remainder Fe and an unavoidable impurity.

(3) Further, in mass%, Cu: 0.01 to 0.5%, Ni: 2.0% or less, Cr: 0.01 to 0.5%, Mo: 0.01 to 0.5%, V: 0.001 to 0.1%, W: 0.5% or less, Zr : 0.5% or less, Ca: 0.0005 to 0.0030%, B: 0.0005 to 0.0020% The steel plate as described in (2) characterized by the composition containing 1 type, or 2 or more types.

(4) Moreover, the thick steel sheet as described in (2) or (3) characterized by the composition which contains Al: 0.1% or less by mass%.

(5) After heating the steel material which has a chemical component in any one of (2)-(4) at the temperature of 1000-1250 degreeC, it accumulates in the temperature range where a plate thickness center part becomes (Ar3 point +50) degreeC or more. A hot-rolling sheet having an excellent fatigue resistance in the plate thickness direction, characterized by performing hot rolling with a reduction ratio of 30% or more and then cooling to 350 ° C or lower at a cooling rate of 3 ° C / s or more.

(6) Fillet welding excellent in fatigue strength characterized by welding a fillet portion of a thick steel sheet having excellent fatigue resistance in a plate thickness direction of 50 mm or more in a laminate having a heat input of 30 kJ / cm or less and three layers of six passes or less. Joint.

(7) The compressive residual stress in the direction perpendicular to the plate thickness direction is 100 MPa or more in the range from the both sides or the 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. Fillet welded joint with excellent fatigue strength as described in 6).

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 explanatory drawing which shows schematically the dimensional shape of the 3-point bending test piece used for a fatigue test.
It is explanatory drawing which shows schematically the dimensional shape of the notched three-point bending fillet weld joint fatigue test piece used for a fatigue test.
FIG. 3 is an explanatory diagram schematically showing a state of occurrence of slip at the leading edge of fatigue cracking in the plate thickness direction cross section of the thick steel sheet applied to the fillet weld joint. FIG.
It is a figure explaining the welding conditions of a fillet weld joint.

(Mode for carrying out the invention)

Hereinafter, the compressive residual stress, preferable component composition, and manufacturing conditions of the steel plate prescribed | regulated by this invention are demonstrated.

[Compressive residual stress of steel sheet]

The thick steel sheet according to the present invention has a compressive residual stress of 100 MPa or more that is perpendicular to the plate thickness direction in a range of up to 4 mm in the plate thickness direction from both or one side of the rolled surface of the steel sheet.

In the production of a welded structure, temporary adhesion welding to a steel plate surface part or a dent cannot be avoided, and the compressive residual stress is damaged at the very front and back surfaces of the steel plate. The range which exists is made into 4 mm in the plate | board thickness direction from the both sides or one side of the rolling surface of a steel plate.

On the other hand, if the range of the compressive residual stress exceeds 4 mm from the surface to the inside of the plate thickness, the compressive residual stress in the vicinity of the surface portion where the fatigue crack occurs due to the balance of the internal stress decreases, so that the rolling surface of the steel sheet It is set as the range to 4 mm in a plate thickness direction from both sides or one side.

The compressive residual stress in the direction perpendicular to the plate thickness direction within the above range is 100 MPa or more. To suppress the propagation of fatigue cracks, it is effective to apply compressive stress in a direction perpendicular to the crack surface. In the present invention, since the crack propagates in the plate thickness direction, the compression direction of the compressive residual stress is a direction perpendicular to the plate thickness direction.

If the compressive residual stress is less than 100 MPa, the fatigue crack propagation speed is reduced, but since the remarkable effect is not obtained so as to lead to the improvement of the fatigue life, the compressive stress is set to 100 MPa or more. Moreover, More preferably, it is 150 Mpa or more. The compressive residual stress in the direction perpendicular to the plate thickness direction in the plate thickness direction exceeding the range up to 4 mm in the plate thickness direction from both sides or one side of the rolled surface of the steel sheet is not particularly specified, but is usually 4 mm in the plate thickness direction. It will be smaller in size.

Combine the strength and toughness (tensile strength TS: 490 Mpa or more, absorption energy at -40 degreeC as -40 degreeC as Charpy impact value of 1/4 plate | board thickness collection) to the thick steel plate which concerns on this invention. Preferred preferred component compositions and production 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 secure a 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 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 having a function of acting as a deoxidizer and increasing the strength of steel by solid solution. In order to acquire such an effect, it is preferable to contain 0.01% or more. On the other hand, containing exceeding 1.0% reduces the toughness of weld heat affected part. For this reason, it is preferable to limit Si to 1.0% or less. Moreover, More preferably, it is 0.50% or less.

Mn: 1.0 to 2.0%

Mn is an element having the effect of increasing the strength of the steel, and in order to secure the desired high strength, it is preferable to contain 1.0% or more. For this reason, it is preferable to limit Mn to 1.0 to 2.0% of range. Moreover, More preferably, it is 0.9-1.60%.

Ti: 0.005 to 0.05%, Nb: 0.001 to 0.05%, one or two

Ti and Nb are elements which increase the strength through precipitation strengthening, suppress the growth of austenite grains during heating, and contribute to miniaturization of the steel sheet structure. In the present invention, one or two is contained.

Ti forms carbides and nitrides, contributes to miniaturization of austenite grains during steel sheet production, suppresses grain coarsening of the weld heat affected zone, and improves 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, containing exceeding 0.05% reduces toughness. For this reason, it is preferable to limit Ti to 0.005 to 0.05% 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 0.05% tends to needle-like structure and fall toughness. For this reason, it is preferable to limit Nb to 0.001 to 0.05% of range. More preferably, it is 0.02 to 0.05%.

Al: 0.1% or less

Al is an element which acts as a deoxidizer and contributes to the refinement of crystal grains, and may be contained as necessary. In order to acquire such an effect, it is preferable to contain 0.015% or more, but excess content exceeding 0.1% leads to the fall of toughness. For this reason, when it contains, Al was limited to 0.1% or less. Moreover, Preferably it is 0.08% or less.

N: 0.0035% to 0.0075%

N is an element necessary to secure the required amount of TiN, and if less than 0.0035%, a sufficient amount of TiN is not obtained, and if it exceeds 0.0075%, the amount of solid solution N increases in the region where TiN is dissolved by welding thermal cycle. Since the toughness of the weld is also significantly reduced, the welding is made 0.0075% or less.

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, Ca in addition to the said basic component.

Cu: 0.01 to 0.5%, Ni: 2.0% or less, Cr: 0.01 to 0.5%, Mo: 0.01 to 0.5%, V: 0.001 to 0.1%, W: 0.5% or less, Zr: 0.5% or less, Ca: 0.0005 to 0.0030%, B: 0.0005% to 0.0020% of one kind or two or more kinds

Cu, Ni, Cr, Mo, V, W, Zr, and B are elements which 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 0.01% or more, but the content exceeding 0.5% becomes precipitation strengthening excessively, and toughness falls. For this reason, when it contains, it is preferable to limit Cu to 0.5% 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.

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 0.1% or more. However, even if 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, and Ni is an expensive element, and a large amount of content raises a material cost. . For this reason, when it contains, it is preferable to limit Ni to 2.0% or less. Moreover, More preferably, it is 0.05% or more.

Cr increases the amount of pearlite and contributes to increasing the strength of the steel. In order to acquire such an effect, it is preferable to contain 0.01% or more, but the content exceeding 0.5% reduces the toughness of a weld part. For this reason, when it contains, it is preferable to limit Cr to 0.5% 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 0.5% reduces the toughness of a weld part. For this reason, when it contains, it is preferable to limit Mo to 0.5% or less. Moreover, More preferably, it is 0.01 to 0.08%.

V contributes to increasing the strength of the steel through strengthening employment and strengthening precipitation. In order to acquire such an effect, it is preferable to contain 0.001% or more, but the content exceeding 0.1% will remarkably reduce base material toughness and weldability. For this reason, it is preferable to limit V to 0.1% 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 leads to an increase in the material cost. 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 0.0005% or more, but the content exceeding 0.0020% deteriorates toughness. For this reason, when it contains, it is preferable to limit B to 0.0020% or less. More preferably, it is 0.001 to 0.003%.

Ca: 0.0005% to 0.0030%

Ca is an element having the toughness improvement effect by the fixation of S. In order to exhibit such an effect, it is necessary to contain at least 0.0005%, but since the effect is saturated even if it contains exceeding 0.0030%, it is made into 0.0005%-0.0030%.

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

[Manufacturing conditions]

The manufacturing method of steel materials, such as slabs, is not specifically limited. The molten steel of the said composition is melted using commercially available solvent furnaces, such as a converter, and is heated by the temperature of 1000-1250 degreeC as steel materials, such as slabs, by commercial methods, such as a continuous casting method. do.

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

Hot rolled steel is applied. Hot rolling rolls 30% or more of a cumulative reduction ratio in the temperature range of (Ar3 point +50) degreeC or more, and it combines with the cooling conditions mentioned later and is 4 mm in both directions from the both sides or one side of the rolling surface of a steel plate. In the range of, a compressive residual stress in a direction perpendicular to the plate thickness direction of 100 MPa or more is introduced. Ar3 point is calculated | required by Ar3 (degreeC) = 910-273xC-74xMn-57xNi-16xCr-9xMo-5xCu (each element is content (mass%)), for example. It is possible.

In hot rolling, a steel sheet having a sheet thickness of 50 mm or more is used. Although the compressive residual stress improves the fatigue characteristics, the buckling performance is lowered, and the lowering of the buckling performance is remarkable as the sheet thickness is thinner, and when the sheet thickness is less than 50 mm, the buckling performance of the steel sheet itself may be deteriorated. The thickness is 50 mm or more.

In addition, this invention does not restrict rolling outside a prescribed temperature range, It is possible to perform the rough rolling etc. which are performed at the high temperature after slab heating.

After the end of rolling, it cools to 350 degrees C or less at the cooling rate of 3 degrees C / s or more. If either of the cooling rate and the cooling stop temperature deviates from the above provisions, the compressive residual stress of 100 MPa or more perpendicular to the plate thickness direction in the range of up to 4 mm in the plate thickness direction from both or one side of the rolled surface of the steel sheet. This is not obtained. More preferably, it cools to 300 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. When fillet welding with a heat input of more than 30 kJ / cm, due to the heat effect of the welding, the structure of the steel sheet or the form of internal residual stress changes, which adversely affects the fatigue properties of the steel sheet excellent in 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 lamination exceeding 3 layers or 6 passes even if the welding heat input is 30 kJ / cm or less, the compressive residual stress of the weld end is increased, and the effect of improving fatigue characteristics is not obtained. Or less than 6 passes. 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 on 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 55-70 mm of plate | board thickness. Residual stress measurements, tensile tests, toughness tests, and fatigue tests were performed on these thick steel sheets. The test method was as follows.

(1) residual stress measurement

From the obtained thick steel plate, a test piece for measuring residual stress by X-ray (size: plate thickness (steel plate original thickness intact) × 12.5 mm × 300 mm [plate thickness direction dimension x rolling right angle direction dimension x rolling direction dimension]) is collected. After performing electrolytic polishing on the measurement surface [surface of 12.5 mm x 300 mm], the residual stress in the direction perpendicular to the plate thickness direction was measured by X-ray at a pitch of 4 mm in the plate thickness direction. The number of lines measured by 4 mm pitch in the plate | board thickness direction was 5 lines. The residual stress (negative value) at the position of 4 mm from the surface / rear surface is obtained from the plate thickness direction distribution chart of the residual stress obtained by averaging the measured residual stresses of the five lines by five points for each sheet thickness position. And the absolute value were made into compressive residual stress.

(2) 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. The tensile test was carried out in accordance with JIS Z 2241 (1998), and YS: yield strength or 0.2% yield strength TS: tensile strength and stretched EL were determined to evaluate tensile properties at static tension.

(3) 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.

(4) fatigue test

From the obtained thick steel sheet, the test piece for fatigue test (size: sheet thickness (steel plate intact as it is)) x 12.5 mm x 300 to 350 mm so that the propagation direction of the fatigue crack becomes the sheet thickness direction. Rolling direction dimension]) was taken out. A test piece is a three-point bending fatigue test piece which cut out the dimension shape shown in FIG. 1, 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 of plate | board thickness, it rolls. When the direction dimension was 300 mm and the plate thickness was 80 mm, the rolling direction dimension was 350 mm. The fatigue test was carried out under the condition 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 plate thickness direction were obtained.

The obtained results are shown in Table 2. Examples of the present invention (No. 2, 4, 5, 7, 8, 10, 13, 15, 17) all have a compressive residual stress at a position of 4 mm from the surface / rear (from the surface / rear to 4 mm). At the lowest position), the compressive residual stress in the direction perpendicular to the plate thickness direction is 100 MPa or more, and there is no drop in toughness, and a thick steel sheet excellent in fatigue resistance in the plate thickness direction is obtained.

On the other hand, in Comparative Examples (No. 1, 3, 6, 9, 11, 12, 14, 16), the compressive residual stress in the direction perpendicular to the plate thickness direction was less than 100 MPa, and the fatigue resistance in the plate thickness direction was observed. This is inferior. In Comparative Example 11, since the amount of C was 0.23 mass% in the component composition of steel and exceeds the upper limit of the preferable content of this invention, compressive residual stress is less than 100 Mpa, and is inferior in fatigue resistance in a plate thickness direction.

[Example 2]

The fillet weld joint was produced using the thick steel plate 2 excellent in the fatigue characteristic of the plate thickness direction of 55-70 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 2 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 2, and the fatigue test was done. As a fatigue test piece, using the three-point bent fillet welded joint fatigue test piece which cut out the dimension shape shown in FIG. 2, it implemented on the conditions which 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 2.

In the thick steel plate 2, all of the examples of the present invention (test Nos. 2, 7, 8, and 10) were obtained under a strict condition of a stress range of 340 MPa, and a fillet welded joint having excellent fatigue resistance with a fatigue life of 250,000 times or more was obtained. It was confirmed. On the other hand, the comparative examples (test Nos. 4 and 5) which deviate from the range of welding conditions prescribed | regulated by this invention (30 kJ / cm of heat input, lamination conditions of 3 layers or less and 6 passes or less), and fatigue life of a plate thickness direction Comparative examples (test Nos. 1, 3, 6, and 9) using inferior thick steel sheets do not secure fatigue resistance.

Claims (7)

The thick steel plate whose compressive residual stress which becomes perpendicular to a plate thickness direction is 100 Mpa or more in the range from the both sides or one side of the rolled surface of a steel plate to a plate thickness direction to 4 mm. The method of claim 1,
The thick steel sheet contains, in mass%, C: 0.03 to 0.15%, Si: 1.0% or less, Mn: 1.0 to 2.0%, and further, Ti: 0.005 to 0.05% and Nb: 0.001 to 0.05%. Or 2, N: 0.0035% to 0.0075%, and a thick steel sheet having a composition comprising remainder Fe and unavoidable impurities. 3. The method of claim 2,
Furthermore, in mass%, 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% A thick steel sheet having a composition containing one or two or more. The method according to claim 2 or 3,
Furthermore, the thick steel sheet which sets it as the composition containing Al: 0.1% or less by mass%. Cumulative reduction ratio in the temperature range where the plate thickness center part becomes (Ar3 point + 50) degreeC after heating the steel raw material which has the chemical component of any one of Claims 2-4 to the temperature of 1000-1250 degreeC. The manufacturing method of the thick steel plate which performs 30% or more of hot rolling, and cools to 350 degrees C or less at the cooling rate of 3 degrees C / s or more after that. 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 according to claim 6,
A fillet welded joint having a compressive residual stress in a direction perpendicular to the plate thickness direction of 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 a plate thickness direction at least 100 MPa.

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