JP2005305459A - Method for working welded joint comprising weld metal having satisfactory low temperature toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for working a welded joint comprising a weld metal having satisfactory low temperature toughness, after the performance of welding applied in the case an actual welded structure is produced, by forming a welded joint as well, thus suppressing the strain age hardening of a weld metal generated in a welded joint to be subjected to heat treatment, and reducing deterioration in its toughness caused thereby. <P>SOLUTION: In the method for working a welded joint comprising a weld metal having satisfactory low temperature toughness, a weld metal having a composition comprising, by mass, 0.03 to 0.09% C, 0.08 to 0.4% Si, 1.0 to 2.3% Mn, 0.005 to 3.3% Ni, 0.005 to 1.5% Cr and 0.005 to 2% Mo, and in which Pcw value defined by formula (1) satisfies 0.2 to 0.8, and the balance Fe with inevitable components is formed on the weld zone in a welded joint, the welded joint is subjected to heating treatment under the conditions satisfying a heating temperature of 50 to 450°C and a holding time of ≥1 s, and thereafter, the welded joint is formed, and is heat-treated. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for forming and heat-treating a welded joint used particularly in the manufacture of hulls in the field of shipbuilding and the manufacture of surface-treated steel pipes in the field of steel pipes, and more specifically, processing of welded joints having a weld metal with good low-temperature toughness. Regarding the method.

  Generally, when welding a steel material having a thickness of 1 mm or more at the joint portion to produce a welded joint, a method of welding by adding a filler material such as a welding wire to the joint portion is used. In particular, when welding a relatively thick steel material having a plate thickness of 3 to 5 mm or more, the groove portion is subjected to multi-layer welding by an arc welding method or the like using a filler material.

  Also, when welding steel materials with a plate thickness of 10 mm or more, for example, when building large welded structures such as shipbuilding, or when productivity such as steel pipe manufacturing is required, use welding wires from the viewpoint of welding efficiency. In many cases, a high heat input and high efficiency welding method such as the MAG welding used and the multi-electrode submerged arc welding is applied.

  On the other hand, in the case of manufacturing an actual structure, it is often the case that after welding, the obtained welded joint is subjected to forming processing such as bending, compression, and tension and further subjected to processing accompanied by heat treatment. For example, when building a large welded structure such as a shipbuilding, in order to correct the deformation of the welded joint after welding, the joint including the welded part is subjected to forming processing such as tension or bending, and then the residual due to processing strain A method of removing the stress by heat treatment is used. Moreover, at the time of manufacture of a steel pipe, pipe expansion molding is performed to increase the roundness of the steel pipe obtained by welding the seam portion. Furthermore, a coating process may be performed while heating a thermosetting resin or the like for the purpose of improving the corrosion resistance of the steel pipe surface.

  In this way, when a welded joint including a welded part is subjected to heat treatment after forming the welded joint, in the heat treatment, the dislocation caused by the work strain generated in the weld metal of the welded part by the forming process is carbonized. Further, there is a problem that strain aging caused by adhesion of nitrogen and nitrogen easily occurs, the weld metal becomes brittle, and toughness decreases. In particular, such a problem of toughness reduction tends to occur in a welded joint having a weld metal having a relatively high strength.

  Conventionally, various methods for improving the weld joint toughness of the welded joint have been proposed from the viewpoint of the composition of the weld metal. For example, in Patent Document 1, the carbon equivalent (Ceq) of the component composition of the weld metal formed in the weld joint is defined, and further, Ti oxide and B oxide are appropriately dispersed in the weld metal, so A method for improving toughness is disclosed. Patent Document 2 discloses a method for improving the toughness of the weld metal by limiting the C content of the welding material during welding to an extremely low amount in consideration of reheat embrittlement. Furthermore, Patent Document 3 secures the strength and toughness of the weld metal by further defining the chemical composition of the weld metal with the increase in the strength of the base material.

On the other hand, a method for improving weld toughness by heat-treating a welded joint has also been proposed. For example, Patent Document 4 discloses a method in which a welded portion of a joint is once heated to an austenite temperature and then held immediately below the _AC1 transformation temperature.

  However, each of the above prior arts is a method for improving the weld joint toughness when using the welded joint as it is, and is a problem peculiar to a welded joint subjected to processing for further forming and heat-treating the welded joint. It is not a method for suppressing a decrease in toughness due to strain aging of metals.

JP-A 62-40996 JP 62-34694 A JP 2000-199036 A JP 56-77334 A

  The present invention further suppresses the strain age hardening of the weld metal that occurs in the welded joint after forming the welded joint and applying heat treatment after the welding applied to manufacture an actual welded structure. It aims at providing the processing method of the welded joint which has a weld metal with favorable low temperature toughness by reducing a toughness fall.

  In order to suppress the strain age hardening of the weld metal that occurs in a welded joint that forms a welded joint and is subjected to a heat treatment after welding, the present inventors have prescribed the composition of the weld metal in the welded joint. In addition, it was found that heat treatment under a predetermined condition before forming the welded joint is effective as a method for suppressing strain age hardening of the weld metal and reducing toughness reduction. The present invention has been made based on this finding, and the gist of the present invention is as follows.

(1) In a welded joint processing method in which a welded joint produced by welding is molded and heat treated, the welded portion of the welded joint in the welding is mass%, C: 0.03 to 0.09%, Si : 0.08 to 0.4%, Mn: 1.0 to 2.3%, Ni: 0.005 to 3.3%, Cr: 0.005 to 1.5%, Mo: 0.005 to 2 %, And the Pcw value defined by the following formula (1) satisfies 0.2 to 0.8, with the balance forming a weld metal composed of Fe and inevitable components, and heating the welded joint to the heating temperature. A method for processing a welded joint having a weld metal with good low-temperature toughness, wherein the welded joint is molded and heat-treated after being subjected to heat treatment at 50 to 450 ° C and a holding time of 1 second or longer.
Pcw = [C] + 0.09 × [Si] + 0.08 × [Mn] + 0.06 × [Ni] + 0.11 × [Cr] + 0.14 × [Mo] (1)
However, said [C], [Si], [Mn], [Ni], [Cr], [Mo] show content (mass%) of C, Si, Mn, Ni, Cr, and Mo, respectively.

  (2) Tensile strength of said weld metal is 500-1200 MPa, The processing method of the welded joint which has a weld metal with favorable low-temperature toughness as described in (1) characterized by the above-mentioned.

  According to the present invention, after welding construction applied when manufacturing an actual welded structure, further, forming a welded joint, suppressing the strain age hardening of the weld metal that occurs in the welded joint such as heat treatment, By reducing the accompanying decrease in toughness, it is possible to provide a method for processing a welded joint having a weld metal with good low-temperature toughness. As a result, it is possible to improve the quality and reliability of welded parts in the field of manufacturing large welded structures such as shipbuilding and steel pipes subjected to surface treatment, and there is a significant industrial contribution.

  An embodiment of the present invention will be described.

  First, the technical idea of the present invention will be described.

  The inventors of the present invention, after welding construction applied when producing an actual welded structure, further suppresses the strain age hardening of the weld metal that occurs in the welded joint such as forming a welded joint and applying heat treatment, The method of reducing the accompanying toughness reduction was examined.

  The strain age hardening of the weld metal and the accompanying decrease in toughness are caused by the fact that N and C are fixed and hardened around the dislocations caused by the work strain when the welded joint is formed and heat-treated. Conceivable. In particular, the cooling rate of the weld metal formed by welding is very fast, and N and C in the weld metal exist as excessive solute N or solute C, which facilitates the accumulation around the dislocations. It is considered that this is a major cause of the decrease in toughness due to the strain age hardening.

In order to suppress the strain age hardening, it is conceivable to reduce the content of N or C from the viewpoint of the component composition of the weld metal. However, N is usually known as an element that causes embrittlement in a low alloy component based weld metal, and is a component that is limited in the welding material as an unavoidable impurity component.
On the other hand, C is an essential element for improving the hardenability and ensuring strength of the weld metal in the process of melting and solidifying and transforming the molten metal after welding. In particular, in order to form a weld metal having a tensile strength of 500 MPa or more, there is a limit in reducing its content.

  Therefore, the present inventors are indispensable to ensure the strength of the weld metal, but fix and uniformly fix C that is excessively dissolved C in the weld metal, which is a major cause of the decrease in toughness due to strain age hardening. The method of detoxifying by dispersion etc. was examined by various experiments.

  As a result, after forming the welded joint and before forming the welded joint, heat treatment of the welded joint fixes or homogenizes the C present as excessive solid solution C in the weld metal, and lowers the toughness due to strain age hardening. It was found that it can be suppressed.

  FIG. 1 is a diagram showing a welding joint processing step between the prior art and an embodiment of the present invention.

  Conventionally, a processing method in which a welded joint produced by welding is formed as it is and heat-treated has been performed. In addition, as a shaping | molding process, the press forming etc. which were conventionally performed in order to correct the deformation | transformation of a welded joint after welding construction, the pipe expansion for raising roundness after the seam welding of a steel pipe, etc. are mentioned. In addition, as a heat treatment process performed after forming a welded joint, surface treatment such as thermosetting resin and baking coating, which has been conventionally performed to improve the corrosion resistance of steel materials such as steel pipes, and deformation of welded joints are performed. For example, a heat treatment performed to remove residual strain generated during correction is included.

  On the other hand, the embodiment of the present invention is characterized in that a heat treatment process is performed under a predetermined condition described later before a conventional forming process using a welded joint having a weld metal having a predetermined component composition described later. And

  The heat treatment conditions of the present invention and the weld metal formed on the welded joint will be described below.

  First, the reasons for limiting the heat treatment conditions before forming the welded joint in the present invention will be described.

  In the following description, for convenience, the heat treatment before forming the welded joint may be referred to as “primary heat treatment”, and the heat treatment after the formation may be referred to as “secondary heat treatment”.

  The reason for limiting the heating temperature of the primary heat treatment, which is a feature of the present invention, will be described.

  2 and 3 are graphs showing the relationship between the heating temperature during the primary heat treatment of the welded joint and the toughness (Charpy absorbed energy at −30 ° C.) of each weld metal before and after the secondary heat treatment. 4 and 5 are graphs showing the relationship between the heating temperature during the primary heat treatment of the welded joint and the tensile strength of each weld metal before and after the secondary heat treatment. 2 and 4 show the condition that the holding time during the primary heat treatment is 60 seconds, and FIGS. 3 and 5 show that the holding time is 1200 seconds. Moreover, as for the secondary heat processing conditions of FIGS. 2-5, heating temperature is 300 degreeC and the holding time is 60 second in all.

  Welded joints are made by processing a groove with a bevel angle of 45 degrees on a high-strength steel plate having a length of 1000 mm, a width of 150 mm, and a thickness of 12 mm, and a tensile strength of 1000 MPa, forming a V groove with a groove angle of 90 degrees. It was prepared by one-layer three-electrode submerged arc welding. The welding wire used was a commercially available 1000 MPa welding wire, and the flux was also a commercially available melt type. Tables 1 to 3 show the chemical composition of the base metal, the welding conditions, and the mechanical properties of the weld metal.

  The forming process of the welded joint was deformed so as to add a strain of 0.5% in a direction perpendicular to the weld line of the weld metal. Both the primary heat treatment before the molding process and the secondary heat treatment after the molding process were heated using a heat treatment furnace. As shown in FIG. 6, the weld metal Charpy impact test and tensile test specimen of the welded joint are from a 1/2 plate thickness position, the weld metal tensile test specimen 1 is parallel to the welding direction, and the impact test specimen 2 is The samples were taken in the direction perpendicular to the welding direction. Further, the impact test piece was sampled at the notch position of 2 mmV notch (groove) 3 from the center of the weld metal as shown in FIG.

  2 and 3, the effect of the primary heat treatment can be obtained when the heating temperature is less than 50 ° C., regardless of whether the holding time during the primary heat treatment is 60 seconds or 1200 seconds. In addition, the weld metal toughness of the welded joint after the secondary heat treatment is reduced to about 130 J or less by the average value of Charpy absorbed energy at −30 ° C. (about 20 to 25% as a reduction rate with respect to the toughness before the secondary heat treatment) ) In the condition where the heating temperature during the primary heat treatment exceeds 450 ° C., the Charpy absorbed energy at −30 ° C. is already obtained after the primary heating under the condition where the holding time is particularly long (holding time: 1200 seconds, see FIG. 3). Decrease. This is because the toughness decreases due to excessive increase and coarsening of precipitates such as carbides and nitrides in the weld metal as the heating temperature and holding time during the primary heat treatment increase.

  On the other hand, when the heating temperature at the time of the primary heat treatment is 50 ° C. or higher and 450 ° C. or lower, a toughness of about 150 J or more is ensured with an average value of Charpy energy at −30 ° C. against the toughness before the secondary heat treatment. The reduction rate is about 10%) and good toughness is obtained.

  The reason why the primary heat treatment of the present invention can suppress a decrease in toughness of the weld metal after secondary heating is considered as follows.

  In other words, excessive solid solution C or solid solution N in the weld metal is formed as a precipitate of carbide, nitride, etc. (hereinafter referred to as carbide, etc.) by primary heating performed before forming the welded joint of the present invention. The effect of dispersing and homogenizing the solid solution C or solid solution N that is fixed or segregated is obtained. As a result, the accumulation of solute C and solute N to dislocations in the weld metal during secondary heating after forming the welded joint is suppressed, and the reduction in toughness caused by fixing and hardening the dislocation movement is reduced. This is because it can be done.

  On the other hand, from FIGS. 4 and 5, under the condition where the heating temperature during the primary heat treatment is 450 ° C. or less, there is almost no decrease in the tensile strength of the weld metal (less than 5%), and there is no deterioration in strength due to heating. Tensile strength can be secured. However, when the heating temperature exceeds 450 ° C., the tensile strength of the weld metal after primary heating is significantly reduced. In particular, when the holding time is long (holding time: 1200 seconds, see FIG. 5), the tensile strength of the weld metal tends to decrease greatly when the heating temperature exceeds 450 ° C. In order to ensure the tensile strength of the weld metal, it is necessary to form a hardened structure by a hardenable element such as C, Mn, Cr, etc. in the weld metal and to strengthen the solution by solid solution C. The decrease in tensile strength under high temperature conditions where the heating temperature exceeds 450 ° C. and the holding time is long, the decrease in solid solution C contributes to strengthening due to the increase in the precipitation amount of carbide in supersaturated solid solution C, and the coarsening of the precipitate. This is probably because of this.

  Based on the above knowledge, after forming the welded joint in the present invention, the deterioration of toughness due to strain age hardening of the weld metal during secondary heat treatment is suppressed, and good tensile strength and toughness of the weld metal are obtained. Therefore, the heating temperature in the primary heat treatment of the welded joint, that is, the heat treatment performed before forming is set to 50 to 450 ° C. In any case where the heating temperature is less than 50 ° C. or exceeds 450 ° C., the toughness of the weld metal is lowered, which is not preferable. Moreover, when the heating temperature exceeds 450 degreeC, since the tensile strength of a weld metal also falls, it is unpreferable.

  Next, the reason for limiting the holding time in the primary heat treatment will be described.

  7 and 8 are graphs showing the relationship between the holding time during the primary heat treatment of the welded joint and the toughness (Charpy absorbed energy at −30 ° C.) of each weld metal before and after the secondary heat treatment. 9 and 10 are graphs showing the relationship between the holding time during the primary heat treatment of the welded joint and the tensile strength of each weld metal before and after the secondary heat treatment. 7 and 9 are conditions corresponding to the lower limit value of the heating temperature defined by the present invention, in which the heating temperature during the primary heat treatment is 50 ° C. FIGS. 8 and 10 show that the heating temperature is 450 ° C. and corresponds to the upper limit value of the heating temperature defined by the present invention. Also, in all of the secondary heat treatment conditions of FIGS. 8 to 10, the heating temperature is 300 ° C. and the holding time is 60 seconds.

  From FIGS. 7 and 8, the toughness of the weld metal of the welded joint after the secondary heat treatment is the average value of Charpy absorbed energy at −30 ° C. under the condition that the holding time during the primary heat treatment is less than 1 second. It is reduced to 120 J or less (about 20 to 25% in terms of the reduction rate with respect to toughness before the secondary heat treatment). This is because excessive solid solution C, solid solution N, etc. in the weld metal are precipitated during the primary heat treatment, or segregated solid solution C, solid solution N, etc. are dispersed, homogenized, and toughness during secondary heating. This is because a holding time of at least 1 second or more is necessary to sufficiently suppress the decrease. Moreover, since the precipitation amount of carbides, nitrides, and the like is determined by the heating temperature, the precipitation is saturated on the long side of the holding time, but does not significantly affect the action of suppressing the toughness deterioration of the weld metal.

  9 and 1, the tensile strength of the weld metal of the welded joint after the secondary heat treatment is 1 second when the heating temperature during the primary heat treatment is 450 ° C. (see FIG. 10). However, the tensile strength of 900 MPa or more can be maintained at a satisfactory tensile strength, but no reduction in strength is observed.

  Based on the above knowledge, after forming the welded joint in the present invention, it suppresses the decrease in toughness due to strain age hardening of the weld metal when the secondary heat treatment is performed, and provides good weld metal tensile strength and toughness. In order to obtain this, the lower limit of the holding time in the primary heat treatment of the welded joint, that is, the heat treatment performed before the forming process, was set to 1 second. When this holding time is less than 1 second, the toughness of the weld metal is lowered, which is not preferable. Further, the upper limit of the holding time is not particularly required from the viewpoint of mechanical properties such as the strength and toughness of the weld metal, but since the heat treatment efficiency of the welded joint is lowered when the holding time is excessively long, The upper limit is preferably about one hour.

  In the present invention, the tensile strength of the weld metal formed on the weld joint is preferably 500 to 1200 MPa. When the tensile strength is less than 500 MPa, the amount of C added for improving the hardenability and strength of the weld metal is small, so that the solid solution C in the weld metal is originally a problem in the secondary heat treatment. Strain age hardening due to the amount and the like, and the degree of decrease in the toughness of the weld metal due thereto is small. Therefore, in order to obtain the effect of the present invention more, it is desirable to apply to a high strength weld metal having a tensile strength of 500 MPa or more. On the other hand, when the tensile strength of the weld metal exceeds 1200 MPa, the weld metal structure is mainly a martensite structure and the toughness of the weld metal is lowered, which is not preferable.

Next, the reasons for limiting the component composition of the weld metal formed in the weld joint of the present invention will be described.
In the following description, “%” means “mass%” unless otherwise specified.

  In order to obtain good tensile strength and toughness of the weld metal formed on the welded joint in the present invention, it is particularly necessary to define the chemical composition by the following chemical composition and Pcw value.

C: 0.03% or more and 0.09% or less C is an element effective for sufficiently improving the tensile strength of the weld metal, so the lower limit of its content is 0.03%. However, if C is contained excessively, the hardness of the weld metal becomes too high to obtain the necessary toughness, and the risk of cold cracking also increases, so the upper limit of the content is 0.09%. did.

Si: 0.08% or more and 0.4% or less Si is required to contain 0.08% or more from the viewpoint of preventing the blowhole invention by the deoxidation action of the weld metal, but contains Si excessively. In order to reduce the toughness of the weld metal, the upper limit of its content was made 0.40%.

Mn: 1.0% or more, 2.3% or less Since Mn is an important element in improving the hardenability of weld metal and improving tensile strength, the lower limit of its content is set to 1.0%. . On the other hand, when it exceeds 2.3%, the toughness of the weld metal is lowered, so the upper limit of its content is set to 2.3%.

Ni: 0.005% or more and 3.3% or less The amount of Ni is one of the elements necessary for improving the hardenability of the weld metal, improving the tensile strength, and improving the toughness. In order to obtain these effects sufficiently, the lower limit of the content is made 0.005%. However, excessive Ni addition tends to cause hot cracking of the weld metal, so the upper limit of its content was made 3.3%.

Cr: 0.005% or more and 1.5% or less Cr is required to be contained in an amount of 0.005% or more in order to improve the hardenability of the weld metal and improve the tensile strength. However, excessive addition of Cr causes a reduction in the toughness of the weld metal, so the upper limit of its content was made 1.5%.

Mo: 0.005% or more and 2% or less Mo, like Cr, is an element necessary for improving the hardenability of the weld metal and increasing the tensile strength, so it is necessary to contain 0.005% or more. However, excessive addition of Mo causes the tensile strength of the weld metal to become excessively high, leading to a decrease in toughness, so the upper limit of its content was made 2% or less.

Furthermore, in the present invention, in order to ensure good tensile strength and toughness of the weld metal, in addition to the definition of the component content, the component content is 0.2 or more and 0 as defined by the following formula (1). It is necessary to limit so as to satisfy the range of .8 or less. When this Pcw value is less than 0.2, the tensile strength of the weld metal is lowered, which is not preferable. On the other hand, if this Pcw value exceeds 0.8, the strength of the weld metal becomes too high, and sufficient toughness of the weld metal cannot be obtained.
Pcw = [C] + 0.09 × [Si] + 0.08 × [Mn] + 0.06 × [Ni] + 0.11 × [Cr] + 0.14 × [Mo] (1)
However, said [C], [Si], [Mn], [Ni], [Cr], [Mo] show content (mass%) of C, Si, Mn, Ni, Cr, and Mo, respectively.

  The effects of the present invention will be described below based on examples.

After performing CO ₂ welding or submerged arc welding on the steel sheet having the component composition, tensile strength, sheet thickness and the like shown in Table 4 under the joints and welding conditions shown in Table 5, primary heat treatment is performed under the conditions shown in Table 6. Molding and secondary heat treatment were performed. Table 8 shows the component composition of the welding wire used at the time of welding. In the submerged arc welding, tack welding was performed by CO ₂ welding using a 1000 MPa class welding wire in order to prevent burn-off, and then main welding was subsequently performed by submerged arc welding. As the flux for submerged arc welding, a molten flux was used.

  In order to measure the mechanical properties of the weld metal of the obtained welded joint, a test piece was taken from the weld and subjected to a Charpy impact test and a tensile test, respectively. When the welded joint was a plate joint, as shown in FIG. 6, Charpy impact test pieces were collected from the 1/2 plate thickness in the direction perpendicular to the weld line, and weld metal tensile test pieces were taken in the weld line direction. When the welded joint is a UO steel pipe, as shown in FIG. 11, the impact test piece 2 is placed on the outer surface so that the intersection of the fusion line of the inner weld metal of the X groove 4 and the outer weld metal intersects the center line of the impact test piece. A weld metal tensile test piece 1 was collected from the weld. Table 7 shows the Charpy impact value (toughness) and tensile strength of the weld metal at 30 ° C. together with the composition of the weld metal.

Inventive Examples 1, 2 and 3 shown in Tables 6 and 7 are multilayer prime welding of one side of a steel plate by CO ₂ welding, and in Inventive Example 4, two layers on one side of the steel plate are temporarily welded by CO ₂ welding. Submerged arc welding was performed. In invention example 5, the steel pipe seam portion was tack welded by CO2 welding, and then each layer of the inner and outer surfaces was submerged arc welded. Thereafter, each of the obtained welded joints was subjected to primary heat treatment under conditions within the range defined by the present invention, and then subjected to molding and secondary heat treatment. In addition, as for the forming process of each welded joint, in Examples 1-4, the press straightening process of the out-of-plane deformation of the welded joint was performed, and in Inventive Example 5, the pipe expanding process was performed for the purpose of improving the roundness of the seam-welded steel pipe. . Further, the secondary heat treatment of each welded joint is performed by baking the paint at a baking temperature of 250 ° C. in Invention Example 1, baking the paint at a baking temperature of 300 ° C. in Invention Example 2, and baking in the Invention Example 3. A baking coating treatment at a temperature of 350 ° C. was performed in the invention example 4 with a resin coating treatment at a holding temperature of 300 ° C., and in the invention example 5 with a coating treatment at a temperature of 250 ° C. and a holding time of 1 minute. In any case of Invention Examples 1 to 5, since the heating temperature and holding time of the primary heat treatment, the component composition of the weld metal, and the Pcw value are within the range defined by the present invention, the welded joint is subjected to secondary treatment after processing. The Charpy absorbed energy at −30 ° C. after the heat treatment was almost unchanged even after the secondary heating, as shown by the toughness reduction rate immediately after welding after the secondary heating in Table 7, and good toughness was obtained. . Also, the tensile strength of the weld metal was comparable to that of all the weld metal.

  On the other hand, Comparative Examples 1 to 4 are examples in which any of the heating temperature, the holding time, the component composition of the weld metal, and the Pcw value of the primary heat treatment is out of the range defined by the present invention.

  In Comparative Examples 1, 2, 3 and 4, the same welded joints as in Invention Examples 1, 2, 3 and 4 were used, and the processing conditions after welding were changed.

  Comparative Example 1 is the same as Invention Example 1 except that the primary heat treatment is not performed. Since there is no primary heating compared to before the secondary heating, the toughness of the weld metal is reduced by about 20% to 179J on average due to the secondary heating.

  Comparative Example 2 is the same as that of Invention Example 2 except that the holding time of the primary heat treatment of Invention Example is 0.5 seconds.

  Since the holding time of the primary heat treatment of the joint after welding specified by the present invention is short, after the baking correction treatment of the paint at 300 ° C. after the press straightening treatment of the weld joint. The average absorbed energy at −30 ° C. of the weld metal was 169 J, which was about 22% lower than the toughness immediately after welding.

  Comparative Example 3 is the same as Inventive Example 3 except that the condition of the primary addition treatment with Inventive Example 3 is 30 seconds at 30 ° C.

Since the primary heat treatment temperature of the joint after welding specified by the present invention is low, the time is within the range of the present invention of 60 seconds. However, the paint is baked and applied at 300 ° C. after the press straightening treatment of the welded joint. After the treatment, the average value of the absorbed energy at −30 ° C. of the weld metal was 143 J, and the toughness was reduced by about 24% as compared with the toughness of the weld metal before the secondary heating.
Comparative Example 4 is the same as Invention Example 4 except for the primary heat treatment conditions.

In Comparative Example 3, the toughness of the weld metal was lowered because the temperature of the primary heat treatment exceeded the upper limit specified in the present invention. The tensile strength of the weld metal is also lowered by the primary heat treatment at a high temperature exceeding the range of the present invention. In Invention Example 4, it can be understood that the primary heating at a high temperature is the cause because the tensile strength of the weld metal does not decrease so much even at the same secondary heating temperature.
In Comparative Example 5, the base material is the same as that of Invention Example 4, but the primary heat treatment is not performed at all, and the weld metal component composition and the Pcw value are changed by using a welding material having a high strength of 1200 MPa class. Is. In Comparative Example 5, since the contents of C, Ni and Cr in the weld metal deviate from the specified range of the present invention, the toughness of the weld metal is 72 J on average even immediately after welding, which is not suitable for practical use.

  Since the components of the weld metal were C, Ni and Cr higher than the specified range of the present invention, the tensile strength of the weld metal was too high at 1235 MPa even immediately after welding. In addition to not performing the primary heat treatment, this caused the toughness to further decrease after the secondary heating.

  Comparative Example 6 is the same base material as Invention Example 5, but uses a high-strength welding material of 1200 MPa class, and the component composition and Pcw value of the weld metal are different. In Comparative Example 6, since the contents of Mn, Ni, Cr, and Mo and the Pcw value of the weld metal deviate from the specified range of the present invention, the toughness of the weld metal is as low as 54 J on average even immediately after welding, and is suitable for practical use. Absent.

 Since the components of the weld metal are Mn, Ni, Cr, and Mo are higher than the specified range of the present invention, and Pcw is 0.980, which is higher than the specified range of the present invention, the tensile strength of the weld metal is too high at 1350 MPa even immediately after welding. It was. This caused the low toughness after the secondary heating in addition to not performing the primary heat treatment.

  Comparative Example 7 uses the same base material as that of Invention Example 2 and uses a 1200 MPa class high-strength welding material. In Comparative Example 7, the composition of the weld metal is within the specified range of the present invention, but since Pcw is not high, the toughness of the weld metal is as low as 75 J on average even immediately after welding and is not suitable for practical use.

  Even within the range, the weld metal component had a Pcw of 0.814, which is higher than the specified range of the present invention, and thus the tensile strength of the weld metal was too high at 1230 MPa even immediately after welding. This caused the low toughness after the secondary heating in addition to not performing the primary heat treatment.

  In Comparative Example 8, the same base material as that of Invention Example 1 was used, and a low-strength commercial molten material for mild steel was used and CO2 welded. Therefore, Pcw is as low as 0.152, Ni, Cr and Mo are lower than the range of the present invention, and the strength of the weld metal is as low as 470 MPa. Therefore, the toughness of the weld metal is good regardless of the presence or absence of primary heating, and no toughness reduction due to secondary heating is observed.

It is a figure which shows the manufacturing process of the weld joint of a prior art and embodiment of this invention. It is a graph which shows the relationship between the heating temperature at the time of the primary heat processing of a welded joint (holding time: 60 seconds), and the toughness of each weld metal before and after secondary heat processing. It is a graph which shows the relationship between the heating temperature (holding time: 1200 second) at the time of the primary heat processing of a welded joint, and the toughness of each weld metal before and after secondary heat processing. It is a graph which shows the relationship between the heating temperature at the time of the primary heat processing of a welded joint (holding time: 60 seconds), and the tensile strength of each weld metal before and after secondary heat processing. It is a graph which shows the relationship between the heating temperature at the time of the primary heat treatment of a welded joint (holding time: 1200 seconds) and the tensile strength of each weld metal before and after the secondary heat treatment. It is a figure which shows the sampling procedure of the mechanical test piece in the weld joint of V groove | channel. It is the graph which showed the relationship between the retention time (heating temperature: 50 degreeC) at the time of the primary heat processing of a welded joint, and the toughness of each weld metal before and after secondary heat processing. It is the graph which showed the relationship between the retention time (heating temperature: 450 degreeC) at the time of the primary heat processing of a welded joint, and the toughness of each weld metal before and after secondary heat processing. It is a graph which shows the relationship between the retention time (heating temperature 50 degreeC) at the time of the primary heat processing of a welded joint, and the tensile strength of each weld metal before and after secondary heat processing. It is a graph which shows the relationship between the retention time (heating temperature 450 degreeC) at the time of the primary heat processing of a welded joint, and the tensile strength of each weld metal before and after secondary heat processing. It is a figure which shows the sampling procedure of the mechanical test piece in the weld joint of X groove | channel.

Explanation of symbols

1 weld metal tensile test piece 2 impact test piece 3 V groove 4 X groove

Claims (2)

In the method for processing a welded joint in which a welded joint produced by welding is formed and subjected to heat treatment, C: 0.03 to 0.09%, Si: 0.00% in the welded portion of the welded joint in the welding. Contains 08 to 0.4%, Mn: 1.0 to 2.3%, Ni: 0.005 to 3.3%, Cr: 0.005 to 1.5%, Mo: 0.005 to 2% And the Pcw value defined by the following formula (1) satisfies 0.2 to 0.8, the remainder forms a weld metal composed of Fe and inevitable components, and the weld joint is heated to a heating temperature of 50 to 450. A method for processing a welded joint having a weld metal with good low-temperature toughness, wherein the welded joint is molded and heat-treated after heat treatment under the conditions of ° C and a holding time of 1 second or longer.
Pcw = [C] + 0.09 × [Si] + 0.08 × [Mn] + 0.06 × [Ni] + 0.11 × [Cr] + 0.14 × [Mo] (1)
However, said [C], [Si], [Mn], [Ni], [Cr], [Mo] show content (mass%) of C, Si, Mn, Ni, Cr, and Mo, respectively.   2. The method for processing a welded joint having a weld metal with good low temperature toughness according to claim 1, wherein the weld metal has a tensile strength of 500 to 1200 MPa.

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