JP2703162B2 - Thick steel plate for welded structure excellent in toughness of electron beam weld and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel plate for a welded structure,
More particularly, the present invention relates to a high-strength steel plate excellent in impact characteristics of an electron beam weld and having a tensile strength exceeding 590 MPa, and a method for producing the same.

[0002]

2. Description of the Related Art In addition to high tensile strength, steel sheets used in pressure vessels and reaction vessels of nuclear power plants and chemical plants are required to have excellent low-temperature toughness and good weldability. In addition, these steel sheets are often thick materials with a thickness exceeding 50 mm, and recently, for example, ultra-thick steel sheets with a thickness of as much as 200 mm due to improvements in reaction efficiency and higher output of reactors. The use of has been considered. Conventionally, as a steel material suitable for this kind of application, a steel material such as A533 system of ASTM standard or SQV system of JIS standard, which is mainly composed of Mn-Ni-Mo system, has been used.

On the other hand, from the viewpoint of recent shortage of welders and improvement of welding work efficiency, electron beam welding is a new welding method capable of welding thick steel plates in one pass and simultaneously improving both welding efficiency and quality. (Hereinafter simply referred to as EBW)
Has been developed, and its practical use is rapidly progressing. However, when EBW is applied to the above thick steel plate, EBW is applied to important structures because the low-temperature impact characteristics of the weld metal are low even when heat treatment is performed after welding, not to mention welding. I couldn't.

To solve the above problem, Japanese Patent Application Laid-Open
In JP 61-246345 A, the toughness of the EB weld metal is improved by coexisting TiN and V carbide.
According to experiments by the inventors, when heat treatment was performed after EB welding, variation in toughness was observed, and it was difficult to say that the effect was sufficient.

JP-A-64-15321 discloses that the toughness of an EB weld metal can be ensured by coexisting Ti ₂ O ₃ and Nb in an Al-free component system. However, the steel material targeted by this technique originally has a low strength level, and the toughness of the EB weld metal varies during the heat treatment after the EB welding, so that its effect is hardly sufficient. In addition, it is difficult to secure the toughness of the base material, especially the stability of the NRL drop weight characteristics.

Furthermore, Japanese Patent Publication No. 61-3376 proposes a method for improving the toughness by heating an EB-welded extra-thick steel sheet again to an austenitizing temperature of 880-920 ° C. and then cooling it. However, heating a large structure after welding to a high temperature is not only difficult in terms of reality and economy, but also changes dimensions and shapes and is not practical.

Further, Japanese Patent Application Laid-Open No. 2-77557 discloses that
It is disclosed that the toughness of an electron beam weld is improved by reducing the contents of P and N in steel. However, in a steel sheet manufactured from a slab by a continuous casting method, which is advantageous in cost, P segregates at the center of the sheet thickness, and EB
Due to the re-melting and solidification by W, segregation further progresses in the final solidified portion of the EBW portion, so that it is difficult to obtain a weld having excellent toughness.

In the case of continuous casting, in addition to the solidification shrinkage of the solidification front end, the center segregation causing such an adverse effect is caused by C, P, Cr at the solidification front end due to the vacuum suction force of the void generated by bulging of the solidified shell. Etc. are formed by sucking the concentrated molten steel component. As a preventive measure, for example, electromagnetic stirring in a secondary cooling zone has been attempted, but it has not been possible to reduce semi-micro segregation. In addition, the so-called in-line reduction method in which a large reduction is performed using a pair of rolls at the end of solidification
60th year (1974) No. 7 pp. 875-884 was attempted, but if the reduction in the slab area with a large unsolidified layer was insufficient, cracks occurred at the solidification interface, and conversely the reduction was reduced. If it is too large, there is a problem that a strong negative segregation occurs at the center in the thickness direction of the slab.

In this regard, Japanese Patent Application Laid-Open No. 49-121738 discloses a method in which a light reduction is performed by a pair of rolls near the solidification front end of a slab, and the amount of solidification shrinkage of the portion is compensated by reduction. JP 52-54625 A proposes a method of using a forging die to greatly reduce the pressure near the solidification completion point of a slab. However, in the case of light rolling with rolls, solidification shrinkage and bulging that occur between roll pitches cannot be sufficiently prevented even if rolling down several mm by multiple pairs of rolls, and the rolling position is appropriate. Otherwise, there is a problem that the center segregation worsens. On the other hand, when a forging die is used to greatly reduce the vicinity of the solidification completion point of a slab, the solidification interface is less likely to crack than in the case of a large reduction by a roll such as an in-line reduction method, and negative segregation and even semi-macro segregation occur. Although it has been clarified that the slab can be improved drastically, cracking occurs at the solidification interface if the reduction in the slab area where the unsolidified layer is large is insufficient, and conversely if the reduction is too large, the slab There is a disadvantage that a strong negative segregation occurs in the center, and furthermore, even if the unsolidified thickness is reduced, the effect cannot be obtained.
The reality is that we are searching for optimal rolling conditions.

[0010]

As described above, EB
The prior art aimed at improving the low-temperature toughness of the weld metal has had some problems in terms of economy, dimensional accuracy, characteristic dispersion and strength, and furthermore, center segregation and the like. SUMMARY OF THE INVENTION An object of the present invention is to develop a steel plate for a welding structure capable of solving all of the problems of the above prior arts. We propose a thick steel plate for welded structures that has excellent low-temperature toughness and a tensile strength of 590 MPa or more, together with its advantageous production method.

[0011]

Means for Solving the Problems The inventors first studied the effects of various elements on the toughness of the EB weld in order to achieve the above-mentioned object. As a result, when the C content was relatively high, it was found that good low-temperature toughness could not be stably secured in a component system in which Ti was added and Nb and V were added in combination. . The reason is that Ti, Nb and V dissolved and dissolved during EB welding precipitate as carbides in the subsequent heat treatment (heat treatment after welding) and fragile island martensite is formed in the EB weld metal part. Is mentioned.

Therefore, the present inventors have attempted to develop a new component system which can suppress the formation of island-like martensite and secure the base material toughness based on a component system in which Ti, Nb, and V are not added. As a result of intensive experiments and studies, it was found that excellent low-temperature toughness and tensile strength can be obtained only by adjusting Si, Al and N and P and S. In addition, the present inventors utilize the forging process in the continuous casting process, so that P
It has also been found that segregation of can be advantageously prevented. The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows. 1. C: 0.07 to 0.20 wt% (hereinafter simply indicated by "%"), Si: 0.05 to 0.20%, Mn: 0.20 to 1.80%, P: 0.008% or less, S: 0.005% or less, solAl: 0.005 to 0.040% , N: 0.0020 to 0.0070%, Ni: 0.05 to 1.50%, Cr: 0.05 to 0.50%, Mo: 0.05 to 1.00%, and O: 0.0040% or less N / Al: 0.11 to 0.40, with the balance being the balance. Is the composition of Fe and unavoidable impurities, and the ratio P / of the P content (P) at the center of the plate thickness to the average P content (P ₀ ) in steel.
A thick steel plate for a welded structure having excellent toughness in an electron beam weld having a P ₀ of 1.3 or less (first invention).

[0014] 2. C: 0.07 to 0.20%, Si: 0.05 to 0.20%, Mn: 0.20 to 1.80%, P: 0.008% or less, S: 0.005% or less, solAl: 0.005 to 0.040%, N: 0.0020 to 0.0070%, Ni: 0.05 1.50%, Cr: 0.05-0.50%, Mo: 0.05-1.00% and O: 0.0040% or less, N / Al: 0.11-0.40, Ca: 0.0005-0.0050%, and REM: 0.0005-0.0050 %,
One or two selected from the group consisting of: Fe and the unavoidable impurities, and the ratio of the P content (P) at the center of the sheet thickness to the average P content (P ₀ ) in steel P /
A thick steel plate for a welded structure having excellent toughness of an electron beam weld having a P ₀ of 1.3 or less (second invention).

[0015] 3. C: 0.07 to 0.20%, Si: 0.05 to 0.20%, Mn: 0.20 to 1.80%, P: 0.008% or less, S: 0.005% or less, solAl: 0.005 to 0.040%, N: 0.0020 to 0.0070%, Ni: 0.05 -1.50%, Cr: 0.05-0.50%, Mo: 0.05-1.00% and O: 0.0040% or less, N / Al: 0.11-0.40, the balance being molten steel with composition of Fe and unavoidable impurities Is continuously cast, and after hot rolling or forging to form a thick steel plate, by performing quenching and tempering, when manufacturing a thick steel plate for a welding structure, in the continuous casting process, the internal molten steel of the continuous cast piece solidifies. In the vicinity of the crater end to be completed,
Forging structure excellent in toughness of electron beam welded part, characterized by performing forging processing in which the ratio P / P ₀ of the P content (P) at the center of the sheet thickness to the content (P ₀ ) is 1.3 or less. A method for manufacturing thick steel plates.

[0016] 4. C: 0.07 to 0.20%, Si: 0.05 to 0.20%, Mn: 0.20 to 1.80%, P: 0.008% or less, S: 0.005% or less, solAl: 0.005 to 0.040%, N: 0.0020 to 0.0070%, Ni: 0.05 1.50%, Cr: 0.05-0.50%, Mo: 0.05-1.00% and O: 0.0040% or less, N / Al: 0.11-0.40, Ca: 0.0005-0.0050%, and REM: 0.0005-0.0050 %,
Containing one or two selected from the other, the remainder is molten steel having a composition of Fe and unavoidable impurities, by continuous casting, after hot rolling or forging into a thick steel plate, by quenching-tempering In manufacturing a thick steel plate for a welded structure, in the continuous casting process, the molten steel in the ladle is placed near the crater end where the molten steel inside the continuous cast piece completes solidification.
Forging structure excellent in toughness of electron beam welded part, characterized by performing forging processing in which the ratio P / P ₀ of the P content (P) at the center of the sheet thickness to the content (P ₀ ) is 1.3 or less. A method for manufacturing thick steel plates.

[0017]

The development of the present invention will be described below.
According to the study of the inventors, the toughness degradation of the EB weld metal is
(1) precipitation embrittlement due to Ti, Nb and V, (2) formation of brittle island-like martensite, (3) P and S segregate between dendrites of EB weld metal, which is generally considered It has been found that (4) when the specific N content exceeds 70 ppm, the toughness is significantly deteriorated. Regarding (2), a new finding was found that the amount of Al content had a significant effect.

Further, this kind of steel material has a toughness of a base material,
In particular, good falling weight characteristics are also required. In general,
As means for securing good dropping weight characteristics with this kind of thick steel plate, grain refinement of the crystal grains is known. Specifically, AlN is precipitated by setting the N content to about 100 ppm or more, and this AlN is deposited. Thereby, the grains are refined. In this case, the balance between the amounts of N and Al is adjusted and added so that N / Al ≧ 0.5. However, in the conventional component design described above, that is, in a high N component system in which N / Al ≧ 0.5, the low temperature toughness of the EB weld metal deteriorates. However, simply reducing the N in terms of improving the toughness of the EB weld metal causes a new problem that high toughness of the base material cannot be secured.

Accordingly, the inventors have conducted intensive studies aiming at achieving both of the above-mentioned conflicting characteristics. As a result, the intended purpose can be advantageously achieved by simply adjusting the general balance of Si, Al, N, P and S as an additional alloying element without requiring a special alloying element. It was an unexpected new finding that could be achieved. That is, P and S are reduced as much as possible, and low Si, particularly preferably 0.15% or less, and N / Al is set to 0.1 as low N.
It has been found that by adjusting to be 11 to 0.40, both good base material toughness and EB weld portion toughness can be achieved.

First, the reasons for limiting the amount of each alloy element to be added will be described. C: 0.07 to 0.20% C requires at least 0.07% to secure the strength, but if it exceeds 0.20%, the low-temperature toughness of the base metal and the EB weld metal deteriorates. did.

Si: 0.05 to 0.20% Si is an element that is effective not only for deoxidation but also for improving the strength, but is an element that greatly affects the toughness of the EB weld.
Large additions are undesirable. At least 0.05% is required for effective use, but if it exceeds 0.20%, not only does the low-temperature toughness of the EB weld metal deteriorate, but also the base metal properties, particularly the dropping weight properties, are not satisfied with the composition steel of the present invention. , 0.05
0.20.20% (preferably 0.05 to less than 0.15%).

Mn: 0.20 to 1.80% Mn requires at least 0.20% to secure desired strength, but if it exceeds 1.80%, the weldability and the low-temperature toughness of the EB weld metal are reduced. The range was 1.80%.

P: 0.008% or less, S: 0.005% or less Since both P and S significantly deteriorate the low-temperature toughness of the EB weld metal, it is desirable to reduce it as much as possible, and to secure good base metal toughness. To make each upper limit P
0.008%, S is up to 0.005%.

Sol Al: 0.005 to 0.040% Al is an element used for deoxidizing steel, and plays an important role together with N in the present invention. That is, low Si, low P,
In low S and low N steels, when the Al content is reduced to less than 0.005%, coarse island martensite is formed in the EB weld metal and the low temperature toughness is significantly deteriorated. Since the effect of grain refinement cannot be expected at all, and the low-temperature toughness of the base material also deteriorates, the lower limit is set to 0.005%. On the other hand, in the low-N steel of the present invention, 0.040% of Al
If a large amount is added beyond that, coarse AlN is formed and the low-temperature toughness of the base material is degraded, so the upper limit was made 0.040%.

N: 0.0020 to 0.0070% N degrades the low-temperature toughness of the EB weld metal, so that it is lower than that of this type of general-purpose steel.
If it is 0.0070% or less, sufficient low-temperature toughness of the EB weld metal can be secured. However, if it is reduced to less than 0.0020%, precipitation of AlN will not occur and the low-temperature toughness of the base material will be impaired, so that 0.0020 to 0.0070% (preferably 0.0020 to 0.00
(Less than 50%).

Ni: 0.05-1.50% Ni is a useful element for improving the strength and toughness of the base metal without impairing the low-temperature toughness of the EB weld metal, and 0.05% or more is required to obtain the target properties. is there. However, even if added in excess of 1.50%, the effect of improving the characteristics is small, and it is an expensive element. Therefore, the content is set in the range of 0.05 to 1.50%.

Cr: 0.05 to 0.50% Cr is a useful element for improving the hardenability and, consequently, the strength of the base metal and the EB weld metal part.
However, if the content exceeds 0.50%, the weldability and the low-temperature toughness of the EB weld metal are degraded.

Mo: 0.05 to 1.00% Mo is an element that improves both the strength and toughness of the base material, and requires 0.05% or more. If it exceeds 1.00%, the weldability and the low-temperature toughness of the EB weld metal are increased. 0.05 ~
The range was 1.00%.

O: 0.0040% or less If O is too much, blowholes are generated in the EB weld metal, so it is necessary to reduce it as much as possible. However, up to 0.0040% is acceptable.

N / Al: 0.11 to 0.40% Although the basic components have been described above, it is not enough to limit the component composition to the above range in the present invention.
It is important to limit the ratio N / Al to a predetermined range. In other words, the present invention regulates the amount of solute N, the amount of solute Al and the precipitation form of AlN, which affect both properties, in order to simultaneously improve both the base metal toughness and the EB weld metal toughness of a thick steel plate. , In addition to controlling, the amount of added N and
The ratio N / Al to the amount of Al was determined to be within the above range.
The reason for this is that if N / Al is less than 0.11, the precipitation amount of AlN is small, and even if it is within the composition range of the present invention, the low-temperature toughness of the base material is significantly deteriorated.
If it exceeds 0.40, the base material toughness is good, but the low-temperature toughness of the EB weld metal is significantly deteriorated.

C: 0.18%, Si: 0.15%, Mn: 1.45%,
P: 0.004%, S: 0.002%, Ni: 0.65%, Cr: 0.13
%, Mo: 0.55% and O: 0.0020% as basic components, Al
And N amount, Al: 0.006 to 0.050%, N: 0.0020 to 0.0120
%, And the relationship between N / Al and the low-temperature toughness (vTrs) of the base metal and the absorbed energy (vE- ₄₀ ) of the EB weld metal were examined when N / Al was variously changed. The results are shown in FIG. As is clear from FIG.
Only when 0.11 to 0.40 is satisfied, both excellent base metal toughness and good EB weld metal toughness can be obtained.

Further, in the present invention, Ca and REM can be added in the following range in addition to the above basic components. Ca: 0.0005% to 0.0050% Ca is a useful element that controls the morphology of sulfide (MnS) and has an effect on improving the low-temperature toughness and anisotropy of the base metal and improving the resistance to hydrogen-induced cracking. If the content is less than 0.0005%, the effect of the addition is poor, while if it exceeds 0.0050%, Ca
Oxysulfides (inclusions) are formed and impair the low-temperature toughness and cleanliness. Therefore, the content is set in the range of 0.0005 to 0.0050%.

REM: 0.0005% to 0.0050% REM, like Ca, effectively contributes to controlling the morphology of sulfide (MnS), improving the low-temperature toughness and anisotropy of the base metal, and improving the resistance to hydrogen-induced cracking. If the content is less than 0.0005%, the effect of the addition is poor. On the other hand, if it exceeds 0.0050%, the low-temperature toughness and cleanliness deteriorate, so the content was made 0.0005 to 0.0050%.

In the steel sheet according to the present invention, when the molten steel whose chemical composition is adjusted as described above is made into a slab by continuous casting, the molten steel in the ladle is placed near the crater end where the internal molten steel of the continuous slab completes solidification. The ratio P / P ₀ of the P content (P) at the center of the plate thickness at the crater end to the P content (P ₀ ) (which is approximately equal to the average P content in steel) is 1.3
Forging can be performed by the following forging process, followed by hot rolling or forging, followed by quenching and tempering by a conventional method or direct quenching and tempering after hot rolling. By the forging described above, segregation of P, which harms the toughness of the electron beam weld, is effectively prevented.

Here, P /
The reason why the P ₀ ratio can be controlled is as follows. That is, in the final stage of solidification of the internal molten steel, since the molten steel in which P has been enriched is present near the crater end, if it is solidified as it is, central segregation will occur, but if forging is performed before solidification, As a result of the P-enriched molten steel being pushed upward,
The P concentration at the center does not increase so much. Therefore, if the forging process is performed in accordance with the concentration of P, the P content at the center of the sheet thickness can be adjusted. In this regard, in the case of a conventional steel sheet in which such control is not performed and the P / P ₀ ratio exceeds 1.3, the center segregation portion further repeats melting and solidification by EBW, thereby reducing the E value.
In the BW final solidification part, a segregation part in which P is further concentrated occurs. As a result, even in a steel in which the content of P in the steel is reduced, there is a locally deteriorated toughness portion in the EBW portion,
It may be a starting point of destruction. Further, it is reduced in the steel in the content of P is economically very disadvantageous in current steelmaking technology, by the P / P ₀ ratio than 1.3, to reduce the economic burden on such steel Can also. Note that P
The rolling reduction in forging, suitable for setting the / P ₀ ratio to 1.3 or less, is about 5% or more and about 15% or less. Also, P
As a means for controlling the / P ₀ ratio to 1.3 or less, not only forging processing but also processing such as diffusion annealing is effective.

[0036]

【Example】

Example 1 Molten steel having a chemical composition shown in Table 1 was continuously cast, forged at the vicinity of the crater end (rolling reduction: 5 to 15%), and then hot-rolled into a 120 mm thick steel sheet. These steel sheets were heated to 900 ° C, air-cooled, and further cooled to 880
After heating and holding at 3 ° C. for 3 hours, the mixture was quenched with water, heated to 645 ° C., and air-cooled. Then, for such a 120mm thick steel plate,
Pass electron beam welding was performed. Then, Table 1 also shows the results of examining the tensile properties and impact properties of the steel sheet and the impact properties of the EB weld metal after performing the post-weld heat treatment at 615 ° C. for 5 hours. Table 1 also shows, for comparison, the results of an investigation on steel sheets obtained according to the conventional method without forging as in the present invention.

[0037]

[Table 1]

As is clear from the table, the steel sheet obtained according to the present invention has a high tensile strength of 590 MPa or more, and has good low-temperature toughness of the base metal and the EB weld metal. On the other hand, when the Al content was too large or too small, the toughness of the base material was mainly deteriorated, and the toughness of the EB weld metal was deteriorated in steels with a large amount of N. Further, the toughness of the EB weld also deteriorated for steels with N / Al outside the proper range. Further, in the steel material obtained according to the conventional method, good low-temperature toughness was not obtained due to the central segregation of P.

Example 2 Molten steel having the chemical composition shown in Table 2 was continuously cast, in which case a forging process (reduction rate: 5 to 15%) was performed in the vicinity of the crater end, and a 100 mm-thick steel sheet was hot-rolled. And These steel sheets were heated at 880 ° C. for 3 hours, quenched with water, heated to 645 ° C., and air-cooled. Then, one-pass electron beam welding was performed on the steel sheet having a thickness of 100 mm. Then, Table 2 also shows the results of examining the tensile properties and impact properties of the steel sheet and the impact properties of the EB weld metal after the post-weld heat treatment at 615 ° C. for 5 hours. Table 2 also shows, for comparison, the results of a survey on steel sheets obtained according to the conventional method without forging as in the present invention.

[0040]

[Table 2]

As is clear from the table, the steel sheet obtained according to the present invention had a high tensile strength of 590 MPa or more, and also had good low-temperature toughness of the base metal and the EB weld metal. On the other hand, if the content of Al or N is too large, EB
The low-temperature toughness of the weld metal deteriorated, and when the P content exceeded the allowable upper limit, the low-temperature toughness of the base metal and the EB weld metal also deteriorated. Further, in the steel material obtained according to the conventional method, the low-temperature toughness of the EB weld metal, in particular, was significantly deteriorated due to the central segregation of P.

Example 3 A steel slab having the composition shown in Table 3 was subjected to a diffusion annealing treatment, and then hot-rolled to obtain a steel sheet having a thickness of 100 mm. These steel sheets were heated and maintained at 880 ° C. for 3 hours, then water-quenched, and then air-cooled to 645 ° C. Then, the sheet thickness 1
One pass electron beam welding was performed on a 00 mm steel plate. Then, Table 3 also shows the results of examining the tensile properties and impact properties of the steel sheet and the impact properties of the EB weld metal after the post-weld heat treatment at 615 ° C. for 5 hours. As is clear from the table, the steel sheet obtained according to the present invention has a tensile strength of 5%.
It had a high strength of 90 MPa or more, and the low-temperature toughness of the base metal and the EB weld metal was also good.

[0043]

[Table 3]

[0044]

As described above, the Mn-Ni-Mo steel according to the present invention has a high strength of at least 580 MPa and a good tensile strength as a steel for a thick-walled welding structure used in a pressure vessel or a reaction vessel of a nuclear power plant or a chemical plant. It has low-temperature toughness and ensures sufficient toughness of the joint even when electron beam welding is applied.
Accordingly, remarkable improvement in welding efficiency and high reliability as a structure can be obtained, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of N / Al on the low-temperature toughness (vTrs) of a base metal and the absorbed energy (vE- ₄₀ ) of an EB weld metal.

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location // B23K 103: 04 (56) References JP-A-2-77562 (JP, A) JP-A 2-77561 (JP, A) JP-A-3-254340 (JP, A) Japan Society for the Promotion of Science, Steelmaking 19th Committee, “Iron and Alloy Elements (2)” (Showa 41-3-25) Seibundo Shinkosha p. 135 Steelmaking and Alloying Elements (1), 19th Committee of Steelmaking, Japan Society for the Promotion of Science (Showa 41-2-28) Seibundo Shinkosha p. 230−231

Claims (4)

(57) [Claims] 1. C: 0.07 to 0.20 wt%, Si: 0.05 to 0.20 wt%, Mn: 0.20 to 1.80 wt%, P: 0.008 wt% or less, S: 0.005 wt% or less, solAl: 0.005 to 0.040 wt% , N: 0.0020 to 0.0070 wt%, Ni: 0.05 to 1.50 wt%, Cr: 0.05 to 0.50 wt%, Mo: 0.05 to 1.00 wt% and O: 0.0040 wt% or less, N / Al: 0.11 to 0.40 And the balance has the composition of Fe and unavoidable impurities, and the ratio of the P content (P) of the central portion of the sheet thickness to the average P content (P ₀ ) in the steel, P /
A thick steel plate for a welded structure excellent in toughness of an electron beam weld, wherein P ₀ is 1.3 or less. 2. C: 0.07 to 0.20 wt%, Si: 0.05 to 0.20 wt%, Mn: 0.20 to 1.80 wt%, P: 0.008 wt% or less, S: 0.005 wt% or less, solAl: 0.005 to 0.040 wt% N: 0.0020 to 0.0070 wt%, Ni: 0.05 to 1.50 wt%, Cr: 0.05 to 0.50 wt%, Mo: 0.05 to 1.00 wt% and O: 0.0040 wt% or less, N / Al: 0.11 to 0.40 And Ca: 0.0005-0.0050wt% and REM: 0.0005-0.0050wt%
%, One or two selected from the following, with the balance being Fe
An electron beam having a composition of unavoidable impurities and a ratio P / P ₀ of the P content (P) at the center of the plate thickness to the average P content (P ₀ ) in the steel of 1.3 or less. Thick steel plate for welded structures with excellent toughness in welds. 3. C: 0.07 to 0.20 wt%, Si: 0.05 to 0.20 wt%, Mn: 0.20 to 1.80 wt%, P: 0.008 wt% or less, S: 0.005 wt% or less, solAl: 0.005 to 0.040 wt% , N: 0.0020 to 0.0070 wt%, Ni: 0.05 to 1.50 wt%, Cr: 0.05 to 0.50 wt%, Mo: 0.05 to 1.00 wt% and O: 0.0040 wt% or less, N / Al: 0.11 to 0.40 , The remainder is molten steel that has a composition of Fe and unavoidable impurities, continuously cast, after hot rolling or forging into a thick steel plate, by quenching-tempering, to produce a thick steel plate for welding structure In the continuous casting process, the ratio of the P content (P) at the center of the plate thickness to the P content (P ₀ ) of the molten steel in the ladle with respect to the vicinity of the crater end where the internal molten steel of the continuous cast piece completes solidification P / P ₀ of 1.3 or less and comprising forged machining of welding structural steel plate superior in toughness of the electron beam weld, wherein the subjecting Production method. 4. C: 0.07 to 0.20 wt%, Si: 0.05 to 0.20 wt%, Mn: 0.20 to 1.80 wt%, P: 0.008 wt% or less, S: 0.005 wt% or less, solAl: 0.005 to 0.040 wt% N: 0.0020 to 0.0070 wt%, Ni: 0.05 to 1.50 wt%, Cr: 0.05 to 0.50 wt%, Mo: 0.05 to 1.00 wt% and O: 0.0040 wt% or less, N / Al: 0.11 to 0.40 And Ca: 0.0005-0.0050wt% and REM: 0.0005-0.0050wt%
%, One or two selected from the following, with the balance being Fe
In the continuous casting process, when manufacturing molten steel having a composition of unavoidable impurities, continuously casting, forming a thick steel plate by hot rolling or forging, and then performing quenching and tempering to manufacture a thick steel plate for a welded structure, The ratio P / P ₀ of the P content (P) at the center of the plate thickness to the P content (P ₀ ) of the molten steel in the ladle is 1.3 or less, in the vicinity of the crater end where the internal molten steel of the continuous cast iron completes solidification A method for producing a thick steel plate for a welded structure having excellent toughness in an electron beam welded part, wherein forging is performed.