High-strength welded joint with improved heat input and toughness

Abstract

An object of the present invention is to provide a high-strength welded joint having improved large heat input toughness by controlling the microstructure. [Solution] The high-strength welded joint with improved high heat input toughness of the present invention has, by weight%, C: 0.03% or more and 0.045% or less, Mn: 1.45% or more and 1.55% or less, Si: 0.3% or more and 0.4% or less, Al: 0.02% or more and 0.03% or less, Ni: 1.55% or more and 1.70% or less, Cr: 0.05% or more and 0.07% or less, Cu: 0.05% or more and 0.08% or less, M It is characterized by the following: O: 0.14% or more and 0.2% or less, Ti: 0.06% or more and 0.07% or less, Nb: 0.004% or more and 0.005% or less, O: 0.035% or more and 0.05% or less, N: 40 ppm or more and 180 ppm or less, P: more than 0 ppm and 80 ppm or less, S: more than 0 ppm and 90 ppm or less, with the remainder being iron (Fe) and other unavoidable impurities.

Description

Technical Field

[0001] The present invention relates to a high-strength welded joint with improved large heat input toughness, and more particularly, to a high-strength welded joint with improved large heat input toughness by controlling the microstructure.

Background Art

[0002] In recent years, as the Arctic sea ice area has rapidly decreased due to the rising temperature caused by global warming, interest in the opening of the Arctic shipping route has been increasing. In order to operate the Arctic shipping route, it is essential to build icebreakers that can break sea ice and开拓 the route.

[0003] The steel used for the hull of an icebreaker must have excellent impact toughness at low temperatures and requires high strength to protect the hull from the impact during icebreaking. Therefore, the steel and welded joints for icebreakers must be composed of a fine high-strength low-temperature transformation phase structure, and for this purpose, a large amount of alloying elements are added.

[0004] Recently, shipbuilding companies have been continuously demanding steel and welded joints that can apply electrogas welding in order to improve the productivity during the construction of icebreakers. However, when a large amount of alloying elements are added, there may be a problem that the toughness of the welded joint drops sharply due to the formation of a bainite single phase in the welded joint and the heat affected zone. Therefore, in fact, there are few inventions for high-strength steel and welded joints for large heat input welding that can apply electrogas welding.

Summary of the Invention

Problems to be Solved by the Invention

[0005] <​​​​

[0006] The high-strength welded joint section of the present invention, which exhibits improved high heat input toughness, has the following composition in weight percent: C: 0.03% to 0.045%, Mn: 1.45% to 1.55%, Si: 0.3% to 0.4%, Al: 0.02% to 0.03%, Ni: 1.55% to 1.70%, Cr: 0.05% to 0.07%, Cu: 0.05% to 0.08%, Mo: 0.14% to 0.2%, Ti: 0.06% to 0. It contains 0.7% or less, Nb: 0.004% to 0.005%, O: 0.035% to 0.05%, N: 40 ppm to 180 ppm, P: over 0 ppm and up to 80 ppm, S: over 0 ppm and up to 90 ppm, with the remainder consisting of iron (Fe) and other unavoidable impurities, and is characterized by containing, in terms of area fraction, needle-shaped ferrite of 85% or more, grain boundary ferrite of 14% or less, and upper bainite of 0.1% or less.

[0007] Furthermore, the high-strength welded joint with improved high heat input toughness according to the present invention has a carbon equivalent (Ceq) of 0.45% or more and 0.50% or less, represented by the following formula (1).

[0008] Formula (1): [C]+[Mn] / 6+(Cr+Mo) / 5+(Ni+Cu) / 15

[0009] In formula (1) above, C, Mn, Cr, Mo, Ni, and Cu represent the content (by weight) of each component.

[0010] Furthermore, the high-strength welded joint portion with improved high heat input toughness according to the present invention has an MA (island martensite-austenite) phase content of 0.5% or less.

[0011] Furthermore, the high-strength welded joint with improved high heat input toughness according to the present invention has a yield strength of 500 MPa or more.

[0012] Furthermore, the high-strength welded joint section with improved high heat input toughness according to the present invention has a tensile strength of 610 to 770 MPa.

[0013] Furthermore, the high-strength welded joint with improved heat input toughness according to the present invention has an elongation ratio of 21% or more.

[0014] Furthermore, the high-strength welded joint with improved high heat input toughness according to the present invention has an impact toughness of 50 J or more at -20°C.

[0015] Furthermore, the manufacturing method for a high-strength welded joint with improved high heat input toughness according to the present invention is as follows (by weight%): C: 0.03% to 0.045%, Mn: 1.45% to 1.55%, Si: 0.3% to 0.4%, Al: 0.02% to 0.03%, Ni: 1.55% to 1.70%, Cr: 0.05% to 0.07%, Cu: 0.05% to 0.08%, Mo: 0.14% to 0.2%, Ti: 0.06% or more. The method is characterized by including the step of electrogas welding a steel material containing 0.07% or less of the following: Nb: 0.004% to 0.005%, O: 0.035% to 0.05%, N: 40 ppm to 180 ppm, P: greater than 0 ppm and 80 ppm or less, S: greater than 0 ppm and 90 ppm or less, with the remainder being iron (Fe) and other unavoidable impurities, and having a carbon equivalent (Ceq) of 0.45% to 0.50% represented by the following formula (1).

[0016] Formula (1): [C]+[Mn] / 6+(Cr+Mo) / 5+(Ni+Cu) / 15

[0017] In formula (1) above, C, Mn, Cr, Mo, Ni, and Cu represent the content (by weight) of each component.

[0018] Furthermore, in the method for manufacturing a high-strength welded joint with improved high heat input toughness according to the present invention, the electrogas welding is performed with a heat input of 160 kJ / cm or more. [Effects of the Invention]

[0019] According to the present invention, by maximizing the acicular ferrite fraction and minimizing the grain boundary ferrite and upper bainite fractions to refine the grain size, it is possible to provide a high-strength welded joint with improved high heat input toughness that simultaneously achieves low-temperature impact toughness and high strength, as well as a method for manufacturing the same. [Modes for carrying out the invention]

[0020] The high-strength welded joint with improved high heat input toughness according to one embodiment of the present invention has the following composition in weight percent: C: 0.03% to 0.045%, Mn: 1.45% to 1.55%, Si: 0.3% to 0.4%, Al: 0.02% to 0.03%, Ni: 1.55% to 1.70%, Cr: 0.05% to 0.07%, Cu: 0.05% to 0.08%, Mo: 0.14% to 0.2%, Ti: 0.0 It contains 6% to 0.07% of the following: Nb: 0.004% to 0.005%, O: 0.035% to 0.05%, N: 40 ppm to 180 ppm, P: over 0 ppm and up to 80 ppm, S: over 0 ppm and up to 90 ppm, with the remainder consisting of iron (Fe) and other unavoidable impurities, and may also contain, by area fraction, acicular ferrite of 85% or more, grain boundary ferrite of 14% or less, and upper bainite of 0.1% or less.

[0021] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the idea of ​​the present invention to those who have ordinary skill in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. In order to clarify the present invention, the drawings may omit illustrations of parts not relevant to the description and may slightly exaggerate the sizes of components to aid understanding.

[0022] When a specification as a whole states that a part "includes" a certain component, this means, unless otherwise stated, that it may include other components rather than excluding them.

[0023] A singular expression includes plural forms unless there is a clear exception in the context.

[0024] The following explains the reasons for the numerical limitations on the alloy component content in the embodiments of the present invention. Unless otherwise specified, the unit is weight percent.

[0025] In a high-strength welded joint with improved large heat input toughness according to an embodiment of the present invention, in terms of weight%, C: 0.03% or more and 0.045% or less, Mn: 1.45% or more and 1.55% or less, Si: 0.3% or more and 0.4% or less, Al: 0.02% or more and 0.03% or less, Ni: 1.55% or more and 1.70% or less, Cr: 0.05% or more and 0.07% or less, Cu: 0.05% or more and 0.08% or less, Mo: 0.14% or more and 0.2% or less, Ti: 0.06% or more and 0.07% or less, Nb: 0.004% or more and 0.005% or less, O: 0.035% or more and 0.05% or less, N: 40 ppm or more and 180 ppm or less, P: more than 0 ppm and 80 ppm or less, S: more than 0 ppm and 90 ppm or less, and the balance is preferably composed of iron (Fe) and other inevitable impurities.

[0026] The content of C (carbon) may be 0.03% or more and 0.045% or less.

[0027] C is an element effective for increasing the material strength by solid solution strengthening. Considering this, C may be added at 0.03% or more. However, when the content of C is excessive, the hardening ability may be excessively improved and the upper bainite fraction may increase. Also, when the content of C is excessive, a large amount of island-shaped martensite phase may be generated and the toughness may decrease. Considering this, the upper limit of the C content may be limited to 0.045%. Preferably, the content of C may be 0.032% or more and 0.043% or less.

[0028] The content of Mn (manganese) may be 1.45% or more and 1.55% or less.

[0029] Mn is an effective element for improving strength through solid solution strengthening and for enhancing hardening ability so that low-temperature transformation phases are generated. Considering this, Mn may be added in amounts of 1.45% or more. However, if the Mn content is excessive, the excessive increase in hardening ability may promote the formation of upper bainite and island martensite in the weld heat-affected zone and base material structure. Therefore, excessive Mn content can significantly reduce impact toughness. Considering this, the upper limit of the Mn content may be limited to 1.55%. Preferably, the Mn content may be between 1.49% and 1.55%.

[0030] The silicon (Si) content may be between 0.3% and 0.4%.

[0031] Si is an essential element for deoxidation work by precipitating dissolved oxygen in steelmaking and welded joints into a slag-like form. Considering this, Si may be added in amounts of 0.3% or more. However, if the Si content is excessive, coarse oxides may be formed, or a large amount of nonmetallic inclusions or coarse island-like martensite may be formed in the microstructure. Considering this, the upper limit of the Si content may be limited to 0.4%. Preferably, the Si content may be between 0.34% and 0.4%.

[0032] The aluminum (Al) content may be between 0.02% and 0.03%.

[0033] Al, like Si, is an effective element for deoxidation. Considering this, Al may be added in amounts of 0.02% or more. However, excessive Al content can lead to the formation of coarse oxides or coarse island-like martensite. Considering this, the upper limit of Al content may be limited to 0.03%. Preferably, the Al content may be between 0.022% and 0.025%.

[0034] The nickel (Ni) content may be between 1.55% and 1.70%.

[0035] Ni is an important element for improving strength by enhancing low-temperature impact toughness. Considering this, Ni may be added in amounts of 1.55% or more. However, if the Ni content is excessive, the toughness may decrease due to the formation of upper bainite caused by an excessive increase in hardening ability. Furthermore, if the Ni content is excessive, the manufacturing cost may increase. Considering this, the upper limit of the Ni content may be restricted to 1.70%. Preferably, the Ni content may be between 1.57% and 1.69%.

[0036] The chromium (Cr) content may be between 0.05% and 0.07%.

[0037] Cr is an effective element for ensuring strength by improving hardenability. Considering this, Cr may be added in amounts of 0.05% or more. However, if the Cr content is excessive, the toughness may decrease due to the formation of upper bainite caused by an excessive increase in hardening ability. Furthermore, if the Cr content is excessive, the manufacturing cost may increase. Considering this, the upper limit of the Cr content may be limited to 0.07%. Preferably, the Cr content may be between 0.052% and 0.061%.

[0038] The copper (Cu) content may be between 0.05% and 0.08%.

[0039] Cu is an effective element for improving strength and ensuring low-temperature toughness. Considering this, Cu may be added in amounts of 0.05% or more. However, excessive Cu content can increase manufacturing costs. Considering this, the upper limit of Cu content may be restricted to 0.08%. Preferably, the Cu content may be between 0.057% and 0.071%.

[0040] The molybdenum (Mo) content may be between 0.14% and 0.2%.

[0041] Mo is an effective element for ensuring strength by improving hardenability. Considering this, Mo may be added in amounts of 0.14% or more. However, if the Mo content is excessive, the toughness may decrease due to the formation of upper bainite caused by an excessive increase in hardening ability. Furthermore, if the Mo content is excessive, the manufacturing cost may increase. Considering this, the upper limit of the Mo content may be limited to 0.2%. Preferably, the Mo content may be between 0.146% and 0.196%.

[0042] The titanium (Ti) content may be between 0.06% and 0.07%.

[0043] Ti is an effective element for promoting the nucleation of intragranular acicular ferrites through the crystallization of Ti oxide (TiO2) at welded joints. Furthermore, Ti is an effective element for refining crystal grains through the precipitation of carbonitrides. Considering this, Ti may be added in amounts of 0.06% or more. However, excessive Ti content can reduce low-temperature impact toughness by causing excessive oxide formation. Considering this, the upper limit of the Ti content may be limited to 0.07%. Preferably, the Ti content may be between 0.06% and 0.063%.

[0044] The niobium (Nb) content may be between 0.004% and 0.005%.

[0045] Nb is an effective element for improving strength by bonding with C and N to form Nb-based precipitates. Considering this, Nb may be added in amounts of 0.004% or more. However, if the Nb content is excessive, a large amount of island-like martensite may form in the welded joint, reducing toughness. Considering this, the upper limit of the Nb content may be limited to 0.005%.

[0046] The oxygen (O) content may be between 0.035% and 0.05%.

[0047] O is an element that reacts with Ti in the molten metal to form Ti oxide. Ti oxide can play an effective role in refining the crystal grains by promoting the nucleation of intragranular acicular ferrite in the weld. Considering this, O may be added in amounts of 0.035% or more. However, if the O content is excessive, it will worsen the impact toughness of the weld due to the unnecessary formation of coarse Ti oxide and other oxides. Considering this, the upper limit of the O content may be limited to 0.05%.

[0048] The nitrogen (N) content may be between 40 ppm and 180 ppm.

[0049] N is an element that refines the grain size by preventing the growth of prior austenite grains through the precipitation of TiN. Considering this, N may be added at a concentration of 40 ppm or more. However, if the N content is excessive, free nitrogen is generated, reducing toughness and potentially causing cracks in the slab due to AlN precipitation. Considering this, the upper limit of the N content may be limited to 180 ppm. Preferably, the N content may be between 90 ppm and 170 ppm.

[0050] The phosphorus (P) level may be between 0 ppm and 80 ppm.

[0051] P is an element that causes brittleness at grain boundaries. Therefore, to improve resistance to brittle crack propagation, the upper limit of the P content may be limited to 80 ppm.

[0052] Sulfur (S) may be present in concentrations above 0 ppm and below 90 ppm.

[0053] S is an element that causes brittleness by forming coarse inclusions. Therefore, to improve propagation resistance, the upper limit of S content may be limited to 90 ppm.

[0054] The remaining component of this invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be introduced from the raw materials or the surrounding environment, and therefore cannot be eliminated. Since these impurities are known to any technician in the normal manufacturing process, not all of them are specifically mentioned herein.

[0055] A high-strength welded joint with improved high heat input toughness according to one embodiment of the present invention may have a carbon equivalent (Ceq) represented by the following formula (1) of 0.45% or more and 0.50% or less.

[0056] Formula (1): [C]+[Mn] / 6+(Cr+Mo) / 5+(Ni+Cu) / 15

[0057] In formula (1), C, Mn, Cr, Mo, Ni, and Cu represent the content (by weight) of each component.

[0058] If the carbon equivalent (Ceq) is less than 0.45%, the grain boundary ferrite phase may increase, leading to a deterioration in low-temperature impact toughness. However, if the carbon equivalent (Ceq) exceeds 0.50%, the upper bainite phase may increase, resulting in a decrease in low-temperature impact toughness.

[0059] The microstructure of a high-strength welded joint with improved high heat input toughness according to one embodiment of the present invention may contain 85% or more acicular ferrite, 14% or less grain boundary ferrite, and 0.1% or less upper bainite by area fraction.

[0060] To simultaneously ensure the strength and toughness of high-heat-input welded joints, microstructure refinement is essential. Therefore, it is necessary to minimize the grain boundary ferrite phase and upper bainite phase, which reduce low-temperature impact toughness, and maximize the acicular ferrite phase.

[0061] The acicular ferrite phase can be maximized by increasing the amount of titanium oxide produced. Furthermore, the grain boundary ferrite phase and the upper bainite phase can be minimized by appropriately controlling the carbon equivalent (Ceq) in welding according to an example of the present invention. In other words, by controlling the alloy composition and manufacturing method to ensure a fine microstructure, both low-temperature impact toughness and high strength can be achieved simultaneously.

[0062] Furthermore, the MA phase reduces low-temperature impact toughness and therefore needs to be minimized. Accordingly, in one embodiment of the present invention, the high-strength welded joint with improved high heat input toughness may contain 0.5% or less of the MA (island martensite-austenite) phase.

[0063] The MA phase can be minimized by reducing the amount of alloying elements that promote the formation of the MA phase, such as C, Si, and Nb.

[0064] In one embodiment of the present invention, a high-strength welded joint with improved high heat input toughness may have a yield strength of 500 MPa or more and a tensile strength of 610 to 770 MPa by controlling the microstructure. Furthermore, in one embodiment of the present invention, a high-strength welded joint with improved high heat input toughness may have an elongation ratio of 21% or more and a -20°C impact toughness of 50 J or more.

[0065] Next, a method for manufacturing a high-strength welded joint with improved high heat input toughness according to another aspect of the present invention will be described.

[0066] A method for manufacturing a high-strength welded joint with improved high heat input toughness according to one embodiment of the present invention uses the following composition in weight percent: C: 0.03% to 0.045%, Mn: 1.45% to 1.55%, Si: 0.3% to 0.4%, Al: 0.02% to 0.03%, Ni: 1.55% to 1.70%, Cr: 0.05% to 0.07%, Cu: 0.05% to 0.08%, Mo: 0.14% to 0.2%, Ti: 0.06% The process may include a step of electrogas welding a steel material containing % or more and 0.07% or less, Nb: 0.004% or more and 0.005% or less, O: 0.035% or more and 0.05% or less, N: 40 ppm or more and 180 ppm or less, P: more than 0 ppm and 80 ppm or less, S: more than 0 ppm and 90 ppm or less, with the remainder being iron (Fe) and other unavoidable impurities, and having a carbon equivalent (Ceq) of 0.45% or more and 0.50% or less represented by the following formula (1).

[0067] Formula (1): [C]+[Mn] / 6+(Cr+Mo) / 5+(Ni+Cu) / 15

[0068] In formula (1), C, Mn, Cr, Mo, Ni, and Cu represent the content (by weight) of each component.

[0069] The reasons for the component ranges of each alloy composition and the numerical limitations in formula (1) are as described above, and the manufacturing method will be explained in more detail below.

[0070] A method for manufacturing a high-strength welded joint with improved high heat input toughness according to one embodiment of the present invention may include a step of electrogas welding in order to improve the productivity of the welded joint.

[0071] Electrogas welding may be performed with a heat input of 160 kJ / cm or more.

[0072] Generally, when a large amount of alloying elements are added, the toughness of the weld decreases as the heat input increases due to the formation of a single bainite phase in the weld joint and heat-affected zone. However, in this invention, by controlling the microstructure, it is possible to realize a weld joint with excellent toughness even at high heat inputs of 160 kJ / cm or more.

[0073] The present invention will be described in more detail below through examples. However, such examples are provided to illustrate the implementation of the present invention and do not limit the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0074] {Example} Welded joints were manufactured by electrogas welding with a heat input of 160 kJ / cm on steel materials having various alloy composition ranges as shown in Table 1 below.

[0075] [Table 1]

[0076] Table 2 below shows the carbon equivalent, acicular ferrite area fraction, grain boundary ferrite area fraction, upper bainite area fraction, and MA area fraction.

[0077] The carbon equivalent was calculated using the following formula (1).

[0078] Formula (1): [C]+[Mn] / 6+(Cr+Mo) / 5+(Ni+Cu) / 15

[0079] In formula (1), C, Mn, Cr, Mo, Ni, and Cu represent the content (by weight) of each component.

[0080] The area fractions of acicular ferrite, grain boundary ferrite, upper bainite, and MA were measured according to ASTM E562-11 after imaging with an optical microscope.

[0081] [Table 2]

[0082] Table 3 below shows the yield strength, tensile strength, elongation, and -20°C impact toughness.

[0083] Yield strength, tensile strength, and elongation were measured at room temperature using a Zwick / Roell ZWICK Z250 tensile testing machine.

[0084] -20°C impact toughness was measured at a low temperature of -20°C using an impact testing machine from Zwick Roell.

[0085] [Table 3]

[0086] As shown in Tables 2 and 3, Examples 1 and 2 satisfied the alloy composition, component range, formula (1), and microstructure fraction presented in the present invention. Therefore, Examples 1 and 2 satisfied a yield strength of 500 MPa or more, a tensile strength of 610 to 770 MPa, an elongation of 21% or more, and an impact toughness of 50 J or more at -20°C. In other words, Examples 1 and 2 simultaneously embodied low-temperature impact toughness and high strength.

[0087] However, Comparative Examples 1 and 2 had low Ni and Ti content and relatively low carbon equivalent. Therefore, Comparative Examples 1 and 2 had a high grain boundary ferrite fraction and could not satisfy the yield strength and tensile strength presented in the present invention.

[0088] Furthermore, Comparative Example 3 had low Ni and Ti content and inferior yield strength. Also, because Comparative Example 3 had a high carbon equivalent, it had poor impact toughness due to a high upper bainite fraction.

[0089] Furthermore, Comparative Examples 4 and 5 had high C, Mn, and / or Mo content, and high carbon equivalent. Consequently, Comparative Examples 4 and 5 had high upper bainite fraction and were inferior in elongation and -20°C impact toughness. [Industrial applicability]

[0090] According to the present invention, by controlling the microstructure and refining the grain size, it is possible to provide a high-strength welded joint with improved high heat input toughness that simultaneously achieves low-temperature impact toughness and high strength, as well as a method for manufacturing the same. Therefore, it can be used in various industrial fields such as various home appliances.

Claims

1. In weight percent, it contains C: 0.03% to 0.045%, Mn: 1.45% to 1.55%, Si: 0.3% to 0.4%, Al: 0.02% to 0.03%, Ni: 1.55% to 1.70%, Cr: 0.05% to 0.07%, Cu: 0.05% to 0.08%, Mo: 0.14% to 0.2%, Ti: 0.06% to 0.07%, Nb: 0.004% to 0.005%, O: 0.035% to 0.05%, N: 40 ppm to 180 ppm, P: over 0 ppm and up to 80 ppm, S: over 0 ppm and up to 90 ppm, with the remainder being iron (Fe) and other unavoidable impurities. In terms of area fraction, it contains 85% or more acicular ferrite, 14% or less grain boundary ferrite, and 0.1% or less upper bainite. A high-strength welded joint with improved high heat input toughness, characterized in that the carbon equivalent (Ceq) represented by the following formula (1) is 0.45% or more and 0.50% or less. Formula (1): [C]+[Mn] / 6+(Cr+Mo) / 5+(Ni+Cu) / 15 (In formula (1) above, C, Mn, Cr, Mo, Ni, and Cu represent the content (by weight %) of each component.)

2. The high-strength welded joint with improved high heat input toughness according to claim 1, characterized in that the yield strength is 500 MPa or more.

3. The high-strength welded joint portion with improved high heat input toughness according to claim 1, characterized in that the tensile strength is 610 to 770 MPa.

4. The high-strength welded joint with improved heat input toughness according to claim 1, characterized in that the elongation rate is 21% or more.

5. The high-strength welded joint portion with improved high heat input toughness according to claim 1, characterized in that the impact toughness at -20°C is 50 J or more.

6. In weight percent, C: 0.03% to 0.045%, Mn: 1.45% to 1.55%, Si: 0.3% to 0.4%, Al: 0.02% to 0.03%, Ni: 1.55% to 1.70%, Cr: 0.05% to 0.07%, Cu: 0.05% to 0.08%, Mo: 0.14% to 0.2%, Ti: 0.06% to 0.07%, Nb: 0.004% to 0.005%, O A method for manufacturing a high-strength welded joint with improved high heat input toughness, characterized by including the step of electrogas welding a steel material that contains 0.035% to 0.05%, N: 40 ppm to 180 ppm, P: more than 0 ppm and up to 80 ppm, S: more than 0 ppm and up to 90 ppm, with the remainder being iron (Fe) and other unavoidable impurities, and having a carbon equivalent (Ceq) of 0.45% to 0.50% represented by the following formula (1). Formula (1): [C]+[Mn] / 6+(Cr+Mo) / 5+(Ni+Cu) / 15 (In formula (1) above, C, Mn, Cr, Mo, Ni, and Cu represent the content (by weight %) of each component.)

7. The method for manufacturing a high-strength welded joint with improved heat toughness, as described in claim 6, characterized in that the electrogas welding is performed with a heat input of 160 kJ / cm or more.

Images