JP5435837B2 - Welded joint of high-tensile thick steel plate - Google Patents

Description

  The present invention relates to a welded joint of a high-tensile steel plate obtained by welding a high-tensile steel plate having a tensile strength of 490 MPa or more and a plate thickness of 35 mm or more.

  In recent years, with the increase in size of welded structures such as ships and offshore structures, the strength and thickness of steel used has been increased. In addition, from the pursuit of safety, demands for low temperature toughness have been tightened along with demands for thick and high strength.

  The stricter low-temperature toughness is reflected in the low temperature of the test temperature in Charpy impact test and the increase in the required absorbed energy, but the demand for fracture toughness value by large-scale fracture test at full thickness closer to the actual structure is also required. It has increased. In other words, with regard to the fracture toughness (Kc value) of brittle fracture, not only the base metal but also welded joints are required to be guaranteed in the deep notch test and CTOD test at full thickness, and once the brittle crack has occurred. As a minimum, it is required to guarantee a Kca value as a brittle crack propagation stop fracture toughness value representing the stopping ability of a base material.

  Conventionally, regarding the brittle fracture characteristics of joints, attention has been paid to the toughness of a heat affected zone (HAZ) where the toughness is most likely to deteriorate in a welded joint. In order to improve the toughness of this part, HAZ Improvement of metallographic toughness has been achieved with a focus on the refinement of the structure. And as its evaluation means, Charpy impact test using a small test piece collected from a certain position of the steel sheet is common, metallographic technique to improve the result of this Charpy impact test, It is also applied to improve the brittle fracture characteristics of the entire steel sheet.

  However, in the base material, the structure, strength characteristics, and toughness of the steel sheet change in the thickness direction, and in HAZ, the structure has a more complicated structure and distribution of mechanical properties. For this reason, the fracture toughness value at the total thickness evaluated by the Charpy impact property is determined as an overall characteristic of this complex structure and distribution of mechanical properties. In particular, the thicker the base material, the greater the difference between the fracture toughness value measured at full thickness and a small test such as the Charpy impact test. Therefore, there are cases where the Kc value and the Kca value cannot always be improved directly by only improving the toughness at a specific position based on the Charpy impact characteristics. In addition, in order to reliably improve the fracture toughness value of the base steel sheet and the fracture toughness of the joint as a whole, it is necessary to increase the toughness at all positions of the steel sheet and joint. is there. For this reason, it is necessary to add a large amount of an expensive alloy element or to make full use of the TMCP technique, which causes problems associated with an increase in manufacturing cost and deterioration of weldability.

Thus, conventionally, a technique for improving the fracture toughness value of the entire steel sheet by controlling the toughness distribution has been proposed (see, for example, Patent Document 1). In the structural steel sheet for welding described in Patent Document 1, in order to achieve both Kca characteristics and NDT characteristics without adding an expensive element such as Ni, after rough rolling, only the surface layer portion is cooled and Ar ₁ point. After the following, by rolling while recuperating by sensible heat inside the plate thickness, a range of at least 0.1 mm or more from the surface layer is composed of ferrite grains having an average equivalent circle diameter of 3 μm or less, and the same ferrite grains The colony of the texture having a crystal orientation is composed of a structure having an aspect ratio (major axis / minor axis) of 4 or more.

JP-A-5-148542

  However, the conventional techniques described above have the following problems. The technique described in Patent Document 1 is a technique for increasing the Kca value of the steel sheet as a whole by forming an ultrafine grain structure with extremely good toughness on the surface layer of the steel sheet. Is not considered. Furthermore, conventionally, in addition to improving the Charpy impact properties by adjusting the chemical composition and structure control at each position, in terms of comprehensive control of the structure and mechanical property distribution in the plate thickness direction of the steel sheet or in the entire joint, In addition to the Kca value of the base material, a technique for improving the fracture toughness value (Kc value) of occurrence of brittle fracture of the base material and the joint has not been developed.

  The present invention has been made in view of the above-described problems, and in a joint in which a thick high-tensile steel plate having a thickness of 35 mm or more is welded, the fracture toughness value (Kc value) of brittle fracture occurrence and brittle fracture crack propagation. It aims at providing the welded joint of the high-tensile steel plate with favorable both stop fracture toughness values (Kca value).

The welded joint of the high strength thick steel plate according to the present invention is a welded joint of a high strength thick steel plate obtained by welding a high strength thick steel plate having a tensile strength of 490 MPa to 724 MPa and a plate thickness of 35 mm to 70 mm . It consists of the said steel plate and the welding part containing the welding heat affected zone of this steel plate, and the said steel plate is the mass%, C: 0.01-0.3%, Si: 0.01-2%, Mn: 0.00. 1 to 3%, Al: 0.003 to 0.1%, N: 0.001 to 0.01%, P: 0.02% or less and S: 0.01% or less, The balance consists of Fe and unavoidable impurities, and the steel plate and the weld heat affected zone have a 2 mm V notch Charpy impact property at a position of ¼ of the plate thickness and a fracture surface transition temperature of −20 ° C. or less, Yield stress YPS in the range from the surface of the sheet to 1/6 of the plate thickness The ratio of the yield stress YPC ranging from 1/4 position in the plate thickness to the thickness center (YPS / YPC) is equal to or more than 1.3.

In this welded joint, the steel sheet further contains, by mass%, Ti: 0.005 to 0.03% and Ca: 0.0005 to 0.003%, and the steel sheet and the welding heat effect. In the structure of the part, 100 to 3000 particles / mm ² of oxide particles having an average equivalent circle diameter of 0.005 to 2 μm may be contained per unit area.

  Moreover, the said steel plate can also contain Mg: 0.0001-0.002% by the mass% further.

  Furthermore, S content in the said steel plate is good also as 0.002-0.01% by mass%, for example.

Furthermore, the steel sheet further contains, by mass%, C u: 0.05~1.5%, Cr: 0.05~2%, Mo: 0.05~2%, W: 0.05~2 %, V: 0.01-0.2%, Nb: 0.003-0.1%, Ta: 0.01-0.2% and Zr: 0.005-0.1% 1 type selected from the group consisting of Y: 0.001 to 0.01% and Ce: 0.005 to 0.1% may be contained. Or you may contain 2 or more types of elements.

  According to the present invention, in a welded joint of a thick high-tensile steel plate having a tensile strength of 490 MPa or more and a plate thickness of 35 mm or more, the composition of the steel plate that serves as a base material is optimized, and the plate in the steel plate and the weld heat affected zone. The fracture toughness value (Kc value) and brittle fracture crack propagation due to the occurrence of brittle fracture because the ratio of the low-temperature toughness at the 1/4 position of the thickness and the ratio of the yield stress inside the steel sheet surface layer in the base metal is optimized. Both the stop fracture toughness value (Kca value) can be improved, and the industrial effect is extremely large.

  Hereinafter, the best mode for carrying out the present invention will be described in detail. The present inventor has studied in detail the factors governing the fracture toughness value of welded joints, and as is known from the past, in a relatively thin welded joint, the Charpy impact characteristics at ¼ position of the plate thickness. There is a correlation between the total thickness of brittle fracture occurrence fracture toughness value (Kc value) or brittle fracture crack propagation stop fracture toughness value (Kca value), but a thick welded joint with a plate thickness of 35 mm or more In the above, it was confirmed that the Kc value and / or Kca value of the total thickness of the welded joint may be greatly different even if the Charpy impact characteristics at the 1/4 position of the plate thickness are equivalent. Moreover, as a result of investigating this cause from the composition, structure, mechanical properties, etc. of the steel material, the Kc value and the fracture toughness value of the Kca value in the total thickness of the welded joint are particularly the base material in addition to the Charpy impact characteristics. It was found that the yield stress (YP) distribution in the plate thickness direction of the steel plate has a great influence.

  Based on the above knowledge, the present invention is a thick steel plate having a thickness of 35 mm or more, which is a base material of a welded joint, and low-temperature toughness at 1/4 position of the thickness in the weld heat affected zone, and the base material (steel plate) By optimizing the ratio between the yield stress of the surface layer and the internal yield stress, the fracture toughness value of the weld joint full thickness Kc value and Kca value is improved, and the requirements are as follows. Street.

That is, the high-tensile thick steel plate welded joint of the present invention (hereinafter simply referred to as a welded joint) is a joint obtained by welding a high-tensile thick steel plate having a thickness of 35 mm or more, and a steel plate as a base material, It is comprised by the welding part containing the welding heat affected zone of this steel plate. And this steel plate is the mass%, C: 0.01-0.3%, Si: 0.01-2%, Mn: 0.1-3%, P: 0.02% or less, S: 0 .01% or less, Al: 0.003 to 0.1%, N: 0.001 to 0.01%, with the balance being Fe and inevitable impurities. The steel plate and its weld heat-affected zone have a 2 mm V notch Charpy impact property at a fracture surface transition temperature of −20 ° C. or less at a quarter of the plate thickness, and 1/6 of the plate thickness from the surface of the steel plate. The ratio (YP _S / YP _C ) of the yield stress YP _{S in} the range up to the position of γ and the yield stress YP _{C in} the range from the position of ¼ the plate thickness to the center of the plate thickness satisfies the following formula (1) To do.

FIG. 1 shows the ratio (YP _S / YP _C ) between the yield stress YP _S of the surface layer portion of the steel sheet and the yield stress YP _C inside the steel sheet on the horizontal axis, and the Kc value of the joint on the vertical axis. FIG. 2 is a graph showing the relationship between the Kc value of the joint at the fracture surface transition temperature (vTrs) and YP _S / YP _C, and FIG. 2 is a conceptual diagram showing the difference between the Charpy impact test and the large fracture test. The Kc value shown in FIG. 1 is measured by a deep notch test in which a notch is made in the entire thickness as shown in FIG. 2 at the boundary (fusion line: FL) between the weld metal of the weld joint and the weld heat affected zone. Kc value at -20 ° C. The ratio of the fracture surface transition temperature (vTrs) and the yield stress YP _S of the surface layer portion of the steel sheet to the yield stress YP _C inside the steel sheet (YP _S / YP _C ) is centered on the t / 4 position of the plate thickness. It is a value measured by a 2 mm V notch Charpy impact test in which notches are made only in the range.

Further, the yield stress YP _S of the surface layer portion of the steel plate is the yield stress in the range from the surface of the steel plate, which is the base material portion of the welded joint, to the 1/6 position of the plate thickness, and the yield stress YP _C inside the steel plate is The yield stress in the range from the 1/4 position of the thickness to the center of the plate thickness is shown. These yield stresses YP _S and YP _C are measured using a plate-like tensile test piece having a thickness from the surface of the steel plate to the 1/6 position of the plate thickness and from the 1/4 position of the plate thickness to the center of the plate thickness. It is preferable to use it, but when the steel plate is thick and the thickness of each test piece exceeds the capacity of the tensile tester, each plate is divided into a plurality of plate or round bar tensile test pieces. After collecting and testing using these specimens, the yield stress at each split position may be averaged.

  Furthermore, the welded joint used in the above-described measurement is subjected to rolling conditions or after rolling when high-tensile steel having various chemical compositions having a carbon equivalent of about 0.3 to 0.5% by mass is hot-rolled. By changing the cooling and heat treatment conditions, the strength distribution in the plate thickness direction is changed to produce thick steel plates with a plate thickness of 50 mm and a tensile strength TS of 490 to 600 MPa, and these thick steel plates are one pass large by a simple elegance welding method. It was produced by heat input welding.

As shown in FIG. 1, even when the Charpy impact characteristics (fracture surface transition temperature: vTrs) at the 1/4 position of the plate thickness measured by the Charpy test are similar, the total plate thickness measured by the deep notch test is The fracture toughness (Kc value) of the occurrence of brittle fracture in thickness maintenance is affected by the yield stress ratio (YP _S / YP _C ) between the surface layer portion and the inside of the steel sheet. In particular, when the ratio (YP _S / YP _C ) between the yield stress YP _S of the surface layer portion of the steel sheet defined by the above formula (1) and the yield stress YP _C inside the steel sheet exceeds 1.3, the joint The Kc value of is greatly reduced. This is because when the welded joint receives a load, the yield stress YP _S of the surface layer portion of the base steel plate is higher than the yield stress YP _C inside the steel plate, and the above-described yield stress ratio (YP _S / YP _C ). When the condition exceeds 1.3, the deformation of the steel plate is constrained, and as a result, the local stress at the central portion of the plate thickness increases more than usual. For these reasons, even if the low temperature toughness at the 1/4 position of the plate thickness obtained by a small test such as the Charpy impact test is good, the Kc value may be greatly reduced.

  The inventor of the present invention is particularly prominent when the base material is a thick plate having a thickness of 35 mm or more, regardless of the chemical composition and microstructure, and regardless of the difference between the joint and the base material. Furthermore, not only the fracture toughness value (Kc value) of the occurrence of brittle fracture required by the deep notch test and the CTOD test, but also the propagation stop fracture toughness value (Kca value) of the brittle crack measured by the double tensile test and the ESSO test. The same was confirmed.

On the other hand, as shown in FIG. 1, the ratio (YP _S / YP _C ) between the yield stress YP _S of the surface layer portion of the steel sheet defined by the above formula (1) and the yield stress YP _C inside the steel sheet is 1.3. Even under the following conditions, the fracture toughness value (Kc value) of the occurrence of brittle fracture is lowered due to a decrease in Charpy impact characteristics (vTrs) at a 1/4 position of the plate thickness. For this reason, in order to secure the target fracture toughness value (Kc value) for the occurrence of brittle fracture, it is necessary to improve the Charpy impact characteristics (vTrs) at the 1/4 position of the plate thickness.

According to FIG. 1, the Charpy impact property (vTrs) at a 1/4 position of the plate thickness in the welded joint satisfies −20 ° C. or less, and the surface layer portion of the steel plate defined by the above formula (1) (of the steel plate) The ratio of the yield stress YP _{S in} the range from the surface to the 1/6 position of the plate thickness) and the yield stress YP _{C in} the interior of the steel plate (the range from the 1/4 position of the plate thickness to the center of the plate thickness) (YP _S / YP _C ) is a condition satisfying 1.3 or less, the fracture toughness value (Kc) of occurrence of brittle fracture in the entire thickness of the welded joint is 3920 N / mm ^1.5 (400 kgf / mm ^1.5 ) or more. can do.

Similarly, the inventor of the present invention has obtained the above-described yield stress ratio (YP _S / YP _C ) for the welded joint obtained by welding a thick steel plate having a thickness of 50 mm, and the steel plate (base material) of the welded joint measured by the ESSO test. We also investigated the relationship between the propagation stop fracture toughness value (Kca value) of brittle cracks and the Charpy impact property (vTrs) at 1/4 position of the plate thickness, and the Charpy impact property at 1/4 position of the plate thickness. When (vTrs) satisfies −20 ° C. or less and the yield stress ratio (YP _S / YP _C ) satisfies 1.3 or less, the propagation of a brittle crack in the entire thickness of the steel plate (base material) It has been confirmed that the temperature at which the stop fracture toughness value (Kca value) is 3920 N / mm ^1.5 (400 kgf / mm ^1.5 ) is −40 ° C. or less, and the propagation stop fracture toughness is also improved.

From the above examination results, in the present invention, the fracture toughness value (Kc) of the occurrence of brittle fracture in the entire thickness of the welded joint and the propagation stop fracture toughness value (Kca value) of the brittle crack of the steel plate in the welded joint are sufficiently improved. Therefore, for each of the steel plate (base material) and the weld heat-affected zone in the welded joint, the 2 mmV notch Charpy impact property at the 1/4 position of the plate thickness satisfies a fracture surface transition temperature of −20 ° C. or less, and The ratio of the yield stress YP _{S in} the range from the surface of the steel plate to 1/6 position of the plate thickness and the yield stress YP _{C in} the range from the 1/4 position of the plate thickness to the center of the plate thickness (YP _S / YP _C ) Satisfies the relationship of the above formula (1).

The lower limit of the yield stress ratio (YP _S / YP _C ) described above is not particularly limited from the viewpoint of improving the fracture toughness value (Kc) of occurrence of brittle fracture, and excessive softening of the surface layer portion of the steel sheet. Thus, the yield stress ratio (YP _S / YP _C ) can be reduced within a range where the joint strength does not fall below the required value.

  Moreover, the fracture toughness value (Kc value) of brittle fracture occurrence, the brittle fracture crack propagation stop fracture toughness value (Kca value), and the Charpy impact property of a welded joint can be measured by the following methods. The fracture toughness value (Kc value) of the occurrence of brittle fracture can be determined, for example, by a deep notch test. Specifically, as shown in FIG. 2, a test piece having a width of about 400 mm is prepared from the butt joint, and the steel plate is machined by machining with a small tip diameter at the boundary between the weld metal and the weld heat affected zone of the base steel plate. Notches covering the entire thickness (hereinafter also referred to as notches) are provided on both sides of the end of the test piece or in the center in the width direction, and a load is applied in a direction perpendicular to the notches. Determine the fracture toughness value (Kc value) of the occurrence of brittle fracture.

  The brittle fracture crack propagation stop fracture toughness value (Kca value) can be determined, for example, by an ESSO test. Specifically, for a full-thickness test piece with the same specimen width as the deep notch test described above, a notch is introduced to the end piece side of the test piece, a certain preload is applied, and the test is started from the notch part. A brittle fracture is generated by, for example, driving a wedge into the notch portion after providing a temperature gradient that increases as it moves away from the notch over the opposite side in the one-width direction. Then, a stress intensity factor at the time of brittle fracture stop, that is, a brittle fracture crack propagation stop fracture toughness value (Kca value) is obtained from the position and load at which this brittle fracture has stopped. Thus, as described above, the fracture toughness value (Kc value) of brittle fracture occurrence and the brittle fracture crack propagation stop fracture toughness value (Kca value) are measured for the full thickness test piece of the welded joint, Usually, a large test apparatus is used.

  On the other hand, the Charpy impact test can use a simple small test apparatus as compared with the deep notch test and the ESSO test, and the thickness of the test piece is typically about 10 mm. For this reason, in a steel plate with a large plate thickness, it does not represent the toughness of the entire thickness, and, for example, the steel material by the absorbed energy and the fracture surface transition temperature (vTrs) at a specific temperature at the 1/4 position of the plate thickness. It is used for relative evaluation of toughness between each other and simple quality control. The Charpy test piece has a crack propagation length of 8 mm at the maximum, and the notch is a mechanical notch with a not so small tip radius, so the Charpy impact characteristic represents the characteristic of occurrence of brittle fracture. Conceivable.

In the present invention, for each of the steel plate (base material) and the weld heat affected zone in the above-described welded joint, both the 2 mm V notch Charpy impact characteristics at the 1/4 position of the plate thickness satisfy a fracture surface transition temperature of −20 ° C. or less. The ratio of the yield stress YP _{S in} the range from the surface of the steel plate to 1/6 position of the plate thickness and the yield stress YP _{C in} the range from 1/4 position of the plate thickness to the center of the plate thickness (YP _S / It is necessary to define the chemical composition of the base steel sheet of the welded joint so that YP _C ) satisfies the relationship of the above formula (1).

  Next, the reason for limiting the component composition of the base steel sheet in the welded joint of the present invention will be described. In the welded joint of the present invention, the portion excluding the weld metal in the welded portion, that is, the weld heat affected zone is not mixed with components due to melting of the welding material, so the component composition is the base material steel plate shown below. It becomes the same thing as the component composition. In the following description, “%” means “mass%” unless otherwise specified.

C: 0.01 to 0.3%
C is contained as an effective component for improving the strength of steel. However, if the C content is less than 0.01%, it is difficult to ensure the base metal strength necessary for the welded joint. On the other hand, if the C content exceeds 0.3%, the toughness of the base metal deteriorates significantly, so that the fracture toughness value (Kc value) of brittle fracture occurrence and the brittle fracture crack propagation stop fracture toughness value ( Both Kca value) deteriorates. On the other hand, if the C content exceeds 0.3%, the weld crack resistance also decreases, which is not preferable. Therefore, the C content is in the range of 0.01 to 0.3%.

Si: 0.01-2%
Si is an element effective as a deoxidizing element in steel and for securing the strength of the base material. However, when the Si content is less than 0.01%, a clear effect cannot be obtained with respect to these. On the other hand, when the Si content exceeds 2%, a coarse oxide is formed in the steel, resulting in deterioration of the ductility and toughness of the base material. Therefore, the Si content is set to 0.01 to 2%.

Mn: 0.1 to 3%
Mn is an element necessary for ensuring the strength and toughness of the base material, and also combines with S to form a sulfide, and further compounded with an oxide or precipitated around the oxide, In particular, it has an effect on refinement of heated austenite grains by pinning of the heat affected zone.
However, such an effect cannot be obtained when the Mn content in the base steel sheet is less than 0.1%. On the other hand, when Mn is excessively contained, specifically, when the Mn content exceeds 3%, the toughness of the base metal, the toughness of the welded part, and further the weld cracking property due to the formation of the hard phase and the grain boundary embrittlement. Etc. deteriorate. Therefore, the Mn content is 0.1 to 3%.

Al: 0.003-0.1%
In addition to acting as deoxidation in steel, Al forms an Al oxide and is an effective element for refining the heated austenite grain size by pinning in the heat affected zone. However, when the Al content is less than 0.003%, the effect of increasing the toughness cannot be obtained by reducing the heated austenite grain size. On the other hand, when the Al content is excessive, the Al content is specifically 0. If it exceeds 0.1%, it becomes difficult to finely disperse the Al oxide effective for refining the heated austenite by pinning in the weld heat affected zone, and on the contrary, a coarse oxide is formed and the toughness deteriorates. For this reason, Al content shall be 0.003%-0.1%.

N: 0.001 to 0.01%
N adversely affects the ductility and toughness of the base metal when in a solid solution state in steel. For this reason, in order to ensure the ductility and toughness of the base material, it is desirable to reduce the N content as much as possible. Specifically, when the N content exceeds 0.01%, the adverse effect on ductility and toughness due to an increase in solid solution N becomes significant. On the other hand, it is industrially impossible to completely remove N in steel, and it is not preferable to reduce N more than necessary because it places an excessive load on the manufacturing process. In addition, N combines with Al and Ti described later to form a nitride, and effectively works for refining and precipitation strengthening of austenite grains in the base steel plate and its weld heat affected zone, so its content is very small. If present, it is effective for improving mechanical properties such as strength and toughness of the base steel sheet. However, such an effect cannot be obtained when the N content is less than 0.001%. Therefore, the N content is within a range where the influence on ductility and toughness can be tolerated and can be industrially controlled, and the load on the manufacturing process can be allowed, that is, 0.001 to 0.01%.

P: 0.02% or less P is an impurity element and is harmful to the mechanical properties of the base steel sheet. Therefore, it is preferable to reduce it as much as possible. In particular, when the P content exceeds 0.02%, the mechanical properties deteriorate. Therefore, in the welded joint of the present invention, the P content in the base steel plate is restricted to an amount that is practically acceptable for adverse effects, that is, 0.02% or less.

S: 0.01% or less S is also an impurity element that adversely affects the ductility and toughness of the base steel sheet, as in the case of P described above, and if it contains S in an amount exceeding 0.01%, Toughness decreases. Therefore, the S content is restricted to 0.01% or less. In addition, S is bonded to Mn and the like, and by forming these sulfides around the oxide particles, it suppresses the coarsening of the heated austenite grain size by pinning particularly in the weld heat affected zone of the weld joint, Has the effect of increasing toughness. In order to exert this effect, the S content is preferably 0.002% or more. Furthermore, in order to further improve the ductility and toughness of the base steel sheet, the S content is preferably 0.005% or less.

  The above is the reason for limiting the basic component composition of the base steel plate of the welded joint of the present invention. In the welded joint of the present invention, in particular, to improve the low temperature toughness in the base steel plate and its weld heat affected zone. In addition to the above elements, Ti and Ca can also be contained in the base material.

Ti: 0.005 to 0.03%
Ti is an element having a deoxidizing action like Al described above. In addition, Ti has a lower affinity with O than Al, so there are few oxides remaining in the steel compared to Al, but it has the effect of finely dispersing Al oxide, so it can be added together with Al. Especially, in the heat affected zone, there is an effect of promoting the refinement of the heated austenite grain size by pinning and increasing the toughness. Furthermore, it combines with N to form Ti nitride, and has the effect of improving mechanical properties such as strength and toughness of the base steel sheet. However, when the Ti content is less than 0.005%, these effects cannot be sufficiently exhibited. On the other hand, if the Ti content exceeds 0.03%, coarse TiN and oxides are formed, and the toughness may be reduced. For this reason, in the welded joint of this invention, when making a base material steel plate contain Ti, the content shall be 0.005-0.03%.

Ca: 0.0005 to 0.003%
Ca has a stronger deoxidizing effect than Al and Ti, and when added together with Al, it has the effect of finely dispersing oxides in the heat affected zone to promote refinement of heated austenite and further enhancing toughness. . However, this effect is not exhibited when the Ca content is less than 0.0005%. On the other hand, if the Ca content exceeds 0.003%, coarse sulfides and oxides are formed, so that the toughness may be lowered. For this reason, in the welded joint of this invention, when making a base material steel plate contain Ca, the content shall be 0.0005 to 0.003%.

  In the welded joint of the present invention, Mg may be contained in the base material in addition to the above elements.

Mg: 0.0001 to 0.002%
Mg has a stronger deoxidizing action than Al and Ti described above, and when added together with Al, it has the effect of promoting fine oxide dispersion in the weld heat affected zone and increasing toughness. However, this effect cannot be obtained when the Mg content is less than 0.0001%. On the other hand, when the Mg content exceeds 0.002%, there is a concern that the oxide becomes coarse and the toughness is lowered. Therefore, when making a base material steel plate contain Mg, the content shall be 0.0001 to 0.002%.

  In the welded joint of the present invention, Ni, Cu, Cr, Mo, W, V, in addition to the above components, as necessary, for adjusting the strength and toughness of the base steel sheet in the welded joint. One or more elements selected from the group consisting of Nb, Ta, Zr and B can also be contained.

Cu: 0.05 to 1.5%
Cu is an element having an effect of improving both the strength and toughness of a base steel plate substantially similar to Ni. However, when the Cu content is less than 0.05%, the effect is not exhibited. On the other hand, if the Cu content exceeds 1.5%, problems arise in hot workability and HAZ toughness. For this reason, in the welded joint of this invention, when adding Cu to a base material steel plate, the content shall be 0.05 to 1.5%.

Cr: 0.05-2%
Cr is an element effective for improving the hardenability and improving the strength by solid solution strengthening. However, when the Cr content in the base steel sheet is less than 0.05%, the effect cannot be obtained. On the other hand, when Cr is excessively contained, specifically, when the Cr content exceeds 2%, the toughness of the base steel plate and the HAZ is adversely affected by the increase in hardness and the formation of coarse precipitates. Therefore, when adding Cu to a base material steel plate, the content is made 0.05 to 2%.

Mo: 0.05-2%
Mo also has the same effect as Cr described above, and is an element effective for improving the strength. However, when the Mo content is less than 0.05%, the effect cannot be obtained, and when the Mo content exceeds 2%, other characteristics are adversely affected. Therefore, when adding Mo to a base material steel plate, the content is limited to a range in which the addition effect can be exhibited and the other characteristics are not adversely affected, that is, 0.05 to 2%.

W: 0.05-2%
W is an element effective for improving the strength by the same effect as Cr and Mo described above. However, if the W content is less than 0.05%, the effect cannot be obtained, and if the W content exceeds 2%, other properties are adversely affected. Therefore, W is added to the base steel sheet. In that case, the content is limited to a range in which the effect of addition can be exhibited and the other characteristics are not adversely affected, that is, 0.05 to 2%.

V: 0.01 to 0.2%
V is an element that contributes to high strength mainly by precipitation strengthening. However, when the V content is less than 0.01%, the effect is not exhibited. On the other hand, when V is excessively contained, specifically, when the V content exceeds 0.2%, the toughness is extremely deteriorated due to precipitation embrittlement. Therefore, in the welded joint of this invention, when adding V to a base material steel plate, the content is limited to the range of 0.01 to 0.2%.

Nb: 0.003 to 0.1%
Nb is an element that contributes to high strength even in a small amount by transformation strengthening and precipitation strengthening. Nb is also effective in improving the toughness of the base steel sheet because it significantly affects the processing and recrystallization behavior of austenite. However, when the Nb content is less than 0.003%, these effects are not exhibited. On the other hand, when Nb is contained excessively, specifically, when the Nb content exceeds 0.1%, the toughness is extremely deteriorated. Therefore, in the welded joint of the present invention, when Nb is added to the base steel plate, its content is limited to a range of 0.003 to 0.1%.

Ta: 0.01 to 0.2%
Ta also has the same effect as Nb described above, and contributes to the improvement of strength and toughness by adding an appropriate amount. However, if the Ta content is less than 0.01%, the effect is not clearly produced, and if the Ta content exceeds 0.2%, the toughness deterioration due to coarse precipitates becomes remarkable. . Therefore, when adding Ta to a base material steel plate, the content is made 0.01 to 0.2%.

Zr: 0.005 to 0.1%
Zr is also an element effective for improving the strength, but when the content is less than 0.005%, the effect is not exhibited. On the other hand, if Zr is contained excessively, specifically, if the Zr content exceeds 0.1%, coarse precipitates are formed, which adversely affects toughness. Therefore, when adding Zr to a base material steel plate, the content is made 0.005 to 0.1%.

  Furthermore, in the welded joint of the present invention, Y and / or Ce can be contained in addition to the above-described components as necessary for the purpose of improving the ductility of the base steel plate and improving the joint toughness.

Y: 0.001 to 0.01%, Ce: 0.005 to 0.1%
Y and Ce are both elements that effectively refine the oxide and effectively improve the ductility and toughness of the base steel plate and the weld heat affected zone. However, when the Y content is less than 0.001% or the Ce content is less than 0.005%, the effect is not exhibited. On the other hand, when these elements are contained excessively, specifically, when the Y content exceeds 0.01% or the Ce content exceeds 0.1%, sulfides and oxides become coarse, It causes deterioration of ductility and toughness. Therefore, when adding Y and / or Ce to the base steel sheet, the Y content is set to 0.001 to 0.01% and the Ce content is set to 0.005 to 0.1%, respectively.

  The above is the reason for limitation regarding the component composition of the base steel sheet in the welded joint of the present invention. In addition, the remainder in the base material steel plate of the welded joint of the present invention is Fe and inevitable impurities.

  Among the above-mentioned base material components, Al, Ti, Ca and Mg, as described above, have the effect of refining the heated austenite grains by pinning in the base steel plate and its welding heat affected zone. In the welded joint, in order to improve these effects more stably, the preferable range of the component composition and number density in the oxide finely dispersed in the base steel plate and the weld heat affected zone of the welded joint is further specified. .

  Al, Ti, Ca, and Mg oxides are particularly effective in improving toughness by promoting the refinement of austenite grain size by pinning, particularly in welding heat-affected zones that are exposed to high temperatures during welding. In addition, these oxides also reduce the hardenability of the steel due to the refinement of the heated austenite grain size even in the base material, so that the strength on the steel sheet surface side is not excessively increased in the quenching and thermomechanical treatment of the steel sheet, As a result, it is possible to contribute to the improvement of the fracture toughness of the base metal by reducing the ratio of the yield stress between the steel sheet surface layer and the inside.

  Conventionally, it has been known that dispersed particles present in steel pin the movement of austenite grain boundaries at high temperatures to suppress grain growth and promote refining of reheated austenite grains. Conventionally, Ti nitride is known as a dispersed particle having such an action. However, although Ti nitride is effective in the hot rolling process, the proportion of solid solution increases at a high temperature of 1400 ° C. or higher during welding, and the pinning effect is reduced. Therefore, the effect of refinement of reheated austenite grains is reduced. Is not enough.

  Therefore, in the welded joint of the present invention, oxides of Al, Ti, Ca, and Mg that can be stably present as pinning particles without dissolving in the heat affected zone exposed to high temperatures during welding are utilized. .

  On the other hand, Mn, Ca and Mg in the steel material combine with S to form sulfides, and are compounded with the above-mentioned oxides or precipitated around these oxides, thereby reducing the size of dispersed particles. There is an effect of enlarging and further promoting the refinement of the reheated austenite grains by the pinning described above.

  In general, the pinning effect of the grain boundaries by the dispersed particles increases as the cumulative volume ratio of the dispersed particles in the steel increases and as the size of the dispersed particles per particle increases. However, the increase in the cumulative volume fraction of the dispersed particles in the steel is limited by the content of the constituent elements of the dispersed particles. Further, the increase in the size of the dispersed particles per particle is limited because the cumulative volume ratio of the dispersed particles is lowered and coarse inclusions are harmful to the material.

  When the O content and the S content, which are constituent elements of the dispersed particles, increase, the dispersed particles composed of oxides or oxides and sulfides are not preferable because coarse inclusions harmful to the material increase. Therefore, in the welded joint of the present invention, since the cumulative volume fraction of dispersed particles consisting of oxide or oxide and sulfide is increased without increasing the O content, the solubility product with O is small. That is, Al, which is a strong deoxidizing element, is utilized, Ca, which is a stronger deoxidizing element than Al, and further Mg if necessary.

  Ti has a smaller deoxidizing power than Al, Ca, and Mg, but by adding Ti together with Al, Ca, and Mg, oxides of Al, Ca, and Mg are finely dispersed and oxidized or oxidized. This has the effect of increasing the cumulative volume fraction of dispersed particles composed of a material and a sulfide.

  According to the results of the dissolution experiment in which the present inventor combined Al and one or more deoxidizing elements such as Ti, Ca and Mg, the dispersed particles composed of oxides or oxides and sulfides. In order to increase the cumulative volume fraction and further promote refining of reheated austenite grains by pinning, O in the oxide particles was removed as a composition of dispersed particles of oxide generated in steel. When (1) Al, Ti, and Ca are added in a ratio (mass%) with respect to the mass of all component elements, the contents of Ca and Al are 5% or more, respectively. (2) Al, Ti, and Ca When Mg and Mg are added in combination, the Ca and Al contents are preferably 5% or more and the Mg content is preferably 1% or more. In addition, Ca and Mg described above are elements having a smaller solubility product with S than Mn, which is a sulfide-forming element, and are sulfides (MnS) that are complexed with oxides or precipitate around oxides. , CaS, MgS), there is an effect of increasing the cumulative volume fraction of dispersed particles composed of oxides and sulfides without increasing the S content.

  In addition, according to the results of dissolution experiments in which the present inventor combined Al and one or more deoxidizing elements such as Ti, Ca, and Mg, accumulation of dispersed particles composed of oxides and sulfides. In order to increase the volume ratio and further promote the refining of reheated austenite grains by pinning, the composition of dispersed particles composed of oxides and sulfides formed in the steel is changed to O in the oxide particles. (3) When Al, Ti, and Ca are added together, the content of Ca and Al is set to 5% or more and the S content is 1 (4) When Al and Ti, Ca and Mg are added in combination, the Ca and Al contents should be 5% or more and the Mg and S contents should be 1% or more. preferable.

  Next, limitation of the preferable range of the average equivalent circle diameter of the oxide particles present in the structure of the steel plate and the weld heat affected zone in the weld joint of the present invention, and the number per unit area (hereinafter also referred to as number density). The reason will be explained. In the case where the oxide contains S, in the case where the oxide and the sulfide are complexed, in any case where the sulfide is precipitated around the oxide with the oxide as a nucleus, It has the same effect for suppressing the growth of austenite. For this reason, in the following description of particle size and number density, oxide dispersed particles and oxide and sulfide dispersed particles are treated equally unless otherwise specified.

Average equivalent circle diameter: 0.005 to 2 μm
As described above, the pinning effect of the grain boundary of the weld heat affected zone is more effective as the cumulative volume fraction of dispersed particles in the steel increases and as the size of the dispersed particles per particle increases. growing. According to the results of a detailed investigation of the austenite particle size when heated to a high temperature using a welded joint specimen in which the size of the dispersed particles composed of oxide or oxide and sulfide was changed, When the average equivalent circle diameter is less than 0.005 μm, the pinning effect per dispersed particle is reduced, and the proportion of dispersed particles existing at the grain boundary is reduced, so that the refined effect of the heated austenite grains is stabilized. It can no longer be obtained. On the other hand, if the average equivalent circle diameter of the dispersed particles exceeds 2 μm, the cumulative volume fraction of the dispersed particles in the steel decreases, and similarly, the effect of refining the heated austenite grains cannot be stably obtained. For these reasons, in order to stably pin the austenite particle size at the heating temperature and holding time corresponding to large heat input welding to super large heat input welding, the size of the dispersed particles is set to the average equivalent circle diameter. The thickness is preferably 0.005 to 2 μm.

Number per unit area: 100 to 3000 / mm ²
As described above, the pinning effect of the grain boundary of the weld heat affected zone is more effective as the cumulative volume fraction of dispersed particles in the steel increases and as the size of the dispersed particles per particle increases. growing. The austenite grains when the present inventor uses a welded joint test piece in which the number density is changed under the condition that the size of dispersed particles composed of oxide or oxide and sulfide is constant, and is heated to a high temperature. As a result of examining the diameter in detail, the pinning effect is reduced when the number density of the dispersed particles is less than 100 particles / mm ² even if the size of the dispersed particles satisfies the above-mentioned range of the average equivalent circle diameter. It has been found that the stable effect of the refinement of the heated austenite grains cannot be obtained. According to the study of the present inventor, the austenite grain size of the weld heat affected zone within 2 mm from the fusion line (FL: boundary between the molten metal and the weld heat affected zone) is reliably 200 μm or less. In order to make the particles finer, it is preferable that the number of particles having a particle diameter of 0.005 to 2 μm is 100 particles / mm ² or more.

On the other hand, as the number density of the dispersed particles increases, the structural unit by pinning becomes finer, and the heated austenite particles in the weld heat affected zone become finer, but when the number density of the dispersed particles exceeds 3000 / mm ² , The effect is saturated and on the contrary increases the possibility of producing coarse particles which are detrimental to ductility and toughness. For this reason, in the welded joint of the present invention, it is preferable that the upper limit of the number density of dispersed particles is 3000 / mm ² .

In addition, the measurement of the magnitude | size and number density (number per unit area) of the dispersed particle which consists of an oxide or an oxide and sulfide mentioned above can be performed in the following ways, for example. First, an extraction replica is produced from a steel plate as a base material of a welded joint and its heat-affected zone, and the extracted replica is observed with an electron microscope at a magnification of 10,000 times for 20 fields of view and an observation area of 1000 μm ² or more. Thus, the size and number of the oxides are measured. At this time, an extraction replica collected from any part may be used as long as it is between the surface layer part and the center part of the steel plate. If the particles can be observed properly, there is no problem even if the observation magnification is lowered.

  Next, a preferred embodiment for producing a high-tensile steel plate having a plate thickness of 35 mm or more used as a base material in the welded joint of the present invention will be described. In the welded joint of the present invention, if it is a steel plate that satisfies the above-mentioned requirements, it is not necessary to specifically limit the manufacturing method, but preferable methods and conditions for manufacturing a steel plate that satisfies the above-mentioned requirements are as follows. Shown in However, the method for satisfying the metallographic and mechanical requirements in the welded joint of the present invention is not limited to the method shown below.

  Examples of the steel sheet manufacturing method that can satisfy the above-described requirements of the present invention include the following methods (a) to (f).

(A) C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.1 to 3%, Al: 0.003 to 0.1%, N: 0.001 to 0 0.01% and P: 0.02% or less and S: 0.01% or less, and further Ti: 0.005-0.03% and Ca: 0.0005 as necessary. -0.003%, Mg: 0.0001-0.002% , Cu: 0.05-1.5%, Cr: 0.05-2%, Mo: 0.05-2%, W: 0 0.05 to 2%, V: 0.01 to 0.2%, Nb: 0.003 to 0.1%, Ta: 0.01 to 0.2%, and Zr: 0.005 to 0.1% one or more elements selected from the group consisting containing a slab of the balance being Fe and unavoidable impurities, was heated to AC ₃ transformation point to 1300 ° C., and hot-rolled The average austenite grain size before transformation on which the 50μm or less, an average cooling rate of 3 to 100 ° C. / sec, to accelerated cooling from Ar ₃ transformation point or more temperature to a temperature range of 600 ° C. to 200 DEG ° C..

(B) A steel slab having the same steel composition as that of (a) described above is heated to an AC ₃ transformation point to 1300 ° C., and then hot-rolled in an austenite recrystallization region or a partial recrystallization region to obtain average austenite grains. From this state, hot rolling with a cumulative rolling reduction of 30 to 90% is performed in the austenite non-recrystallized region. Thereafter, the cooling rate is set to ₃ to 100 ° C./second, and the cooling is accelerated from the Ar ₃ transformation point to the temperature range of 600 to 200 ° C.

  (C) In the method of (a) described above, in addition to hot rolling in the austenite region, the cumulative rolling reduction is 10 to 60 in the ferrite / austenite two-phase region where the ferrite fraction at the start of rolling is 10 to 30%. % Rolling.

  (D) In the method of (b) described above, in addition to hot rolling in the austenite region, the cumulative rolling reduction is 10 to 60 in the ferrite / austenite two-phase region where the ferrite fraction at the start of rolling is 10 to 30%. % Rolling.

(E) After performing the above-described hot rolling and accelerated cooling of (a) to (d), tempering is performed in the temperature range of 450 ° C. to AC ₁ transformation point as necessary.

  The methods (a) and (c) described above are preferable production conditions when the austenite state is recrystallized or not recrystallized. The methods (b) and (d) described above are This is a preferable production condition when the state of austenite is more limited.

In addition, when tempering is performed, a method of controlling the surface yield stress by induction heating is also effective. At that time, the heating temperature for tempering by induction heating is preferably in the range of 500 ° C. to AC ₁ transformation point in terms of surface temperature. Tempering by induction heating can be applied to the steel plate produced by the above-described methods (a) to (d) as necessary, but the hot rolling conditions are as shown in (a) to (d). However, the present invention can be applied to a steel plate manufactured by the method (f) shown below.

(F) A steel slab having the same steel composition as (a) described above is heated to AC ₃ transformation point to 1300 ° C. and hot-rolled, and then an average cooling rate of 3 to 100 ° C./second is set to 400 ° C. or less. Cool to acceleration.

When performing normal furnace heating and tempering as necessary, the above-described hot rolling and accelerated cooling processes (a) to (d) are assumed. On the other hand, in the case of induction heating tempering, the surface layer portion is more softened by tempering than the inside, which is advantageous in reducing the material difference between the surface layer portion and the inside. For this reason, a steel plate may be produced by the above-described methods (a) to (d), or a method in which the austenite state is not particularly defined, that is, the steel slab is subjected to AC ₃ transformation point as in (f) above. After heating to ˜1300 ° C. and hot rolling, a method of accelerating cooling to 400 ° C. or less may be applied at an average cooling rate of 3 to 100 ° C./second.

  The above is the detailed description about the requirements of the steel plate in the welded joint of the present invention. The base metal steel sheet of the welded joint of the present invention can suppress the coarsening of the weld heat-affected zone of the steel sheet due to the welding heat and secure the required toughness even if the welded joint is produced by high heat input welding. Therefore, the welding method and conditions for producing a welded joint are not limited. That is, when the welded joint of the present invention is manufactured, the base steel plate is not limited to multi-layer welding by MAG welding or the like, but also by one-pass large heat input welding such as elegance welding and electroslag welding. The effect is not impaired.

As described above in detail, in the welded joint of the present invention, the low temperature toughness at the 1/4 position of the thickness of the base steel plate and the weld heat affected zone of the thick welded joint having a thickness of 35 mm or more is optimized. In addition, since the ratio (YP _S / YP _C ) between the yield stress YP _S of the surface layer portion of the base steel sheet and the internal yield stress YP _C is optimized, the fracture toughness of the Kc value and Kca value of the weld joint full thickness The value can be improved.

  Hereinafter, the effects of the present invention will be described in detail by way of examples of the present invention. In this example, steel pieces in the form of ingots and slabs having the compositions shown in Table 1 below were processed and heat-treated under the conditions shown in Table 2 below to produce thick steel plates having a thickness of 50 to 80 mm. In Table 1 below, No. 1-No. No. 10 steel slab has a steel composition within the scope of the present invention. 11-No. The steel bill of 15 has a steel composition outside the scope of the present invention. And the underline in following Table 1 shows that it is outside the scope of the present invention. Further, the balance in the steel composition shown in Table 1 below is Fe and inevitable impurities. Furthermore, No. 2 in Table 2 below. A1-No. The steel sheet of A13 is an example that satisfies all the requirements of the present invention. B1-No. The steel plate of B8 is a comparative example in which any requirement is out of the scope of the present invention. Furthermore, Table 1 below also shows the heating transformation point when each steel slab is heated at a rate of temperature increase of 2.5 ° C./min.

  Further, the composition of oxide particles contained in the structure of each steel plate and the number of particles having a particle size of 0.005 to 2 μm were examined. The composition, size, and number of oxide particles contained in the structure of each steel plate are the same as those of the steel slab, and are not affected by subsequent processing conditions such as rolling and heat treatment. That is, steel plates using the same steel slab have the same composition, size and number of oxide particles contained in the structure. For this reason, these measurements were made by taking test pieces from 10 or more positions of each steel piece ¼ position and the central part of the thickness, and observing the extracted replicas made from the test pieces with an electron microscope. It was done by doing. The results are shown in Table 3 below. In addition, the oxide composition shown in Table 3 below is a ratio (% by mass) to the mass of all component elements excluding O.

  Next, evaluation of the mechanical properties of each steel sheet of Examples and Comparative Examples produced by the above-described method, evaluation of Charpy impact characteristics of welded joints obtained by welding these steel sheets, and a deep notch test were performed. The characteristics of each steel plate (base material) were examined for the surface layer portion and internal tensile properties, Charpy impact properties, and brittle crack propagation stopping properties (Kca) by temperature gradient type ESSO test at the full thickness.

  The tensile test was performed at room temperature using a plate-like tensile test specimen collected so that the longitudinal direction of the test specimen was parallel to the rolling direction. And, the tensile property of the surface layer part is set so that the thickness of the test piece is from the steel sheet surface to 1/6 of the plate thickness, and the internal tensile characteristic is ¼ to the central part of the plate thickness. It measured with the test piece extract | collected so that it might become.

  The Charpy impact characteristics were evaluated by the fracture surface transition temperature (vTrs). At that time, the Charpy impact test piece is the same as the tensile test piece in the direction in which the longitudinal direction of the test piece is parallel to the rolling direction, and the center of the test piece is positioned 6 mm from the surface of the steel plate for the surface layer portion. Was collected so that the center of the test piece was at the position of 1/4 of the thickness of the steel sheet.

Furthermore, the brittle crack propagation stop characteristic is based on the temperature at which the Kca value becomes 3920 N / mm ^1.5 (400 kgf / mm ^1.5 ) based on a plurality of test results obtained by changing the temperature gradient condition and the load stress. Tkca400).

  On the other hand, the welded joint was produced by one-pass welding with a heat input of about 30 to 45 kJ / mm using a simple electrogas welding machine with the steel plate thickness still being maintained. In the Charpy impact test, a test piece having a notch aligned with the Fusion Line at a position where the center of the test piece is 1/4 of the steel plate thickness was used, and the evaluation was performed based on the fracture surface transition temperature (vTrs). Moreover, Kc value calculated | required by the deep notch test was used for the fracture toughness value of a welded joint. Further, the deep notch test was a plate thickness through center notch type test piece, in which a notch was introduced into the fusion line and the test was performed at -20 ° C.

The above results are summarized in Table 4 below. In addition, the austenite particle size shown in the following Table 4 is an average austenite particle size at the time when the recrystallization region to partial recrystallization region rolling is completed. The upper limit temperature for non-recrystallization is the upper limit temperature at which 100% non-recrystallized grains are formed. The recrystallization rate is the recrystallization rate immediately before the start of accelerated cooling, and is a value estimated based on the results of a small simulation test. is there. Furthermore, the cumulative rolling reduction is cumulative rolling reduction of rolling in the temperature range of the pre-recrystallization upper limit temperature to Ar ₃ transformation point, Ar ₃ transformation point after the non-recrystallization region rolling, a measured value of air cooling. Furthermore, the two-phase zone rolling is rolling in a temperature range below the Ar ₃ transformation point, and the ferrite fraction is a measured value obtained by an experiment in which intermediate quenching was performed before the start of the two-phase zone rolling. Furthermore, the tempering method is tempering in a heat treatment furnace in the case of furnace heating, and tempering in high-frequency induction heating in the case of induction heating. In the furnace heating, the holding time at the heating temperature was set to 10 to 120 minutes, and in the induction heating, the holding was not performed, and the cooling was performed by air cooling. Furthermore, the tempering temperature is the furnace temperature in the case of furnace heating, and is the actual measured value of the surface in the case of high frequency induction heating.

  As shown in Tables 2-4 above, Example No. A1-No. Since the steel sheet of A13 satisfies all the requirements of the present invention, both the base steel sheet and the welded joint have good toughness by the Charpy impact test, and the large fracture toughness value performed at the full thickness of the steel sheet is also a good value. there were.

  In contrast, Comparative Example No. B1-No. Since the steel sheet of B8 does not satisfy any of the requirements of the present invention, as apparent from the mechanical properties shown in Table 4, the above-mentioned Example No. A1-No. Compared to the steel sheet of A13, both Charpy impact properties and fracture toughness values were inferior, or even when Charpy impact properties were good, the fracture toughness values in large-scale fracture tests were inferior.

Specifically, no. In the steel plate of B1, since the C content exceeds the range of the present invention, both the base steel plate and HAZ have significantly deteriorated the toughness value by the Charpy impact test, and as a result, the fracture toughness value was extremely low. . No. B2-No. Each of the steel plates of B5 has a Si, Mn, P or S content exceeding the range of the present invention, and therefore a good fracture toughness value is obtained for the same reason as when the C content is excessive. There wasn't. Furthermore, no. B6-No. Since the steel composition of B8 has a steel composition within the scope of the present invention, both the base steel plate and HAZ have sufficiently high toughness. However, since the steel plate manufacturing method is not appropriate, the yield stress of the steel plate surface layer portion is Is excessive compared to the internal yield stress. For this reason, one of the requirements of the present invention that the ratio of the yield stress of the surface layer portion to the internal yield stress (YP _S / YP _C ) is 1.3 or less is not satisfied. The brittle crack propagation stopping property (Kca value) of the base steel plate and the brittle fracture occurrence property (Kc value) of the joint were greatly inferior to those of the steel plates of the examples having the same steel composition.

  From the above results, the welded joint of the present invention is not only toughness by Charpy impact test, but also toughness by large-scale fracture test that is not easy to obtain good characteristics, measured at full thickness, in the base steel plate and welded joint. The value was also confirmed to be very good.

The horizontal axis represents the ratio of the yield stress YP _{S in} the surface layer of the steel sheet to the yield stress YP _{C in the} steel sheet (YP _S / YP _C ), and the vertical axis represents the Kc value of the joint. is a graph showing the relationship between the Kc value of the joint and _YP S / YP _C in temperature (vTrs). It is a conceptual diagram which shows the difference between a Charpy impact test and a large-scale fracture test.

Claims (6)

In a welded joint of a high strength thick steel plate obtained by welding a high strength thick steel plate having a tensile strength of 490 MPa to 724 MPa and a plate thickness of 35 mm to 70 mm ,
It consists of the steel plate and a welded portion including the weld heat affected zone of this steel plate,
The steel plate
% By mass
C: 0.01 to 0.3%
Si: 0.01-2%
Mn: 0.1 to 3%
Al: 0.003 to 0.1%,
N: containing 0.001 to 0.01%,
P: 0.02% or less and S: 0.01% or less,
The balance consists of Fe and inevitable impurities,
The steel plate and the weld heat affected zone have a 2 mm V notch Charpy impact property at a position of 1/4 of the plate thickness, which is −20 ° C. or less at the fracture surface transition temperature,
The ratio of the yield stress YPS in the range from the surface of the steel plate to 1/6 of the plate thickness and the yield stress YPC in the range from the 1/4 position of the plate thickness to the center of the plate thickness (YPS / YPC) Is a welded joint of high-tensile thick steel plate, characterized by being 1.3 or less. The steel sheet further contains, by mass%, Ti: 0.005 to 0.03% and Ca: 0.0005 to 0.003%,
And in the structure of the steel plate and the weld heat affected zone, oxide particles having an average equivalent circle diameter of 0.005 to 2 μm are included in a number per unit area of 100 to 3000 / mm ^2. The welded joint of the high-tensile thick steel plate according to claim 1.   The high strength steel plate welded joint according to claim 2, wherein the steel sheet further contains Mg: 0.0001 to 0.002% by mass.   The S content in the steel plate is 0.002 to 0.01% by mass%, and the welded joint for high-tensile thick steel plate according to any one of claims 1 to 3. The steel sheet further contains, by mass%, C u: 0.05~1.5%, Cr: 0.05~2%, Mo: 0.05~2%, W: 0.05~2%, V 1 selected from the group consisting of: 0.01-0.2%, Nb: 0.003-0.1%, Ta: 0.01-0.2% and Zr: 0.005-0.1% The welded joint for high-strength thick steel plates according to any one of claims 1 to 4, comprising seeds or two or more elements.   The steel sheet further contains one or more elements selected from the group consisting of Y: 0.001 to 0.01% and Ce: 0.005 to 0.1% by mass%. The welded joint for high-tensile thick steel plates according to any one of claims 1 to 5.

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