JP3256118B2 - Ultra-high heat input welding High-strength steel for welding with excellent heat-affected zone toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel for heat-affected zone (hereinafter referred to as HAZ) toughness in ultra-high heat input welding such as electroslag welding applied to assembling box columns of a high-rise building. It is about.
Particularly, when the heat input is 500 kJ / cm or more, for example, 150 kJ / cm.
It has excellent HAZ toughness even at about 0 kJ / cm.

[0002]

2. Description of the Related Art With the recent increase in the height of building structures, steel columns have become larger and the thickness of steel materials used for the columns has also increased. When assembling such large steel columns by welding,
It is necessary to perform welding with high efficiency, and electroslag welding, which can weld extremely thick steel plates in one pass, has been widely applied. Typical heat input range is 500-1
In such a super large heat input welding, unlike a large heat input welding such as a submerged arc welding (heat input is 100 to 200 kJ / cm) used in shipbuilding or the like, the heat history received by the HAZ is 1350. The high-temperature residence time at a temperature of not less than ℃ was extremely long, and the coarsening of austenite grains was extremely remarkable, and it was difficult to secure the toughness of HAZ. It is an urgent task to secure the reliability of the building structure in response to the recent large earthquake, and it is extremely important to achieve such improvement in the toughness of the super-large heat input welding HAZ.

[0003] Conventionally, there are many knowledge and techniques for improving the HAZ toughness of large heat input welding, as described below. However, as described above, the heat histories of the HAZ greatly differ between ultra-high heat input welding and large heat input welding. Therefore, the technology for improving the HAZ toughness of the high heat input welding cannot be simply applied to the target field of the present invention.

[0004] The conventional high heat input welding HAZ toughness improvement is largely based on two basic technologies. One is a technique for preventing austenite grain coarsening using the pinning effect of particles in steel, and the other is an effective grain refinement technique using ferrite transformation in austenite grains.

Iron and steel, No. 11, 1975 (1975)
On page 68, the effect of suppressing austenite grain growth on various types of nitrides and carbides in steel was examined. In the case of Ti-added steel, fine particles of TiN were formed in the steel.
A technique for effectively suppressing austenite grain growth in Z is disclosed.

[0006] Japanese Patent Application Laid-Open No. 60-184663 discloses A
1 to 0.04 to 0.10%, Ti to 0.002 to 0.0
2%, and 0.003 to rare earth element (REM)
Heat input of 150 kJ / c in steel containing 0.05%
A technique for improving the high heat input welding HAZ toughness of m is disclosed. This is because the REM has an action of forming sulfur / oxide and preventing the HAZ portion from coarsening during large heat input welding.

Japanese Patent Application Laid-Open No. 60-245768 discloses that the particle size is 0.1 to 3.0 μm and the number of particles is 5 × 10 ³ to 1 ×.
10 ⁷ / mm ³ of Ti oxide, or Ti oxide and T
In steels containing any of the composites with i-nitrides, these particles act as ferrite transformation nuclei in a large heat input welding HAZ with a heat input of 100 kJ / cm, thereby miniaturizing the HAZ structure and improving HAZ toughness. Techniques that can be improved are disclosed.

[0008] Japanese Patent Application No. 2-254118 discloses Ti
In a steel containing a proper amount of S and S, intragranular ferrite is formed in the large heat input welding HAZ structure by using a composite precipitate of TiN and MnS as a nucleus, and the HAZ structure is refined.
A technique that can improve toughness is disclosed.

Japanese Patent Application Laid-Open No. 61-253344 discloses A
1 is 0.005 to 0.08%, and B is 0.0003 to 0.
Steel containing at least 0050% and at least 0.03% of at least one of Ti, Ca and REM is subjected to a cooling process starting from unmelted REM / Ca oxide / sulfide or TiN in the large heat input welding HAZ. A technique is disclosed in which a high heat input HAZ toughness is improved by forming BN from the ferrite and forming ferrite from the BN.

[0010] Iron and steel, No. 11, 1975
The technique disclosed on page 68 is intended to suppress austenite grain growth by using nitrides such as TiN, and is effective in large heat input welding. In heat input welding, since the residence time at 1350 ° C. or more is extremely long, most of TiN forms a solid solution and loses the effect of grain growth. Therefore, this technique cannot be applied to the toughness of the ultra-high heat input welding HAZ intended by the present invention.

The technique disclosed in Japanese Patent Application Laid-Open No. 60-184661 is a composite of a sulfide and an oxide of REM (hereinafter referred to as sulfuric acid).
Oxide) is used to prevent coarsening of the HAZ during large heat input welding. Sulfur / oxide has a higher stability at a high temperature of 1350 ° C. or higher than nitride, so that the effect of suppressing grain growth is maintained. However, it is difficult to finely disperse sulfur oxides. Due to the low sulfur and oxide density, even if the pinning effect of individual particles is maintained, there is a limit to reducing the austenite particle size of the ultra-high heat input welded HAZ, and it is impossible to improve toughness by itself. Can not.

The technique described in Japanese Patent Application Laid-Open No. 60-245768 discloses a method in which a HAZ structure is formed by particles of either Ti oxide or a composite of Ti oxide and Ti nitride acting as ferrite transformation nuclei. HAZ
It improves toughness, and its effect is maintained even in ultra-high heat input welding in consideration of the high-temperature stability of Ti oxide. However, the crystal orientation of ferrite generated from the intragranular transformation nucleus is not completely random, and is affected by the crystal orientation of the parent phase austenite. Therefore, in the case of ultra-high heat input welding HAZ, when austenite grains become coarse, there is a limit to making the HAZ structure finer only by intragranular transformation.

The technique disclosed in Japanese Patent Application Laid-Open No. 2-254118 is to transform ferrite from a composite precipitate obtained by depositing MnS on TiN, and has a residence time of 1350 ° C. or more as in large heat input welding. Is relatively short, but in ultra-high heat input welding, the residence time at 1350 ° C. or more is long, and during this time, TiN dissolves, so the ferrite transformation nuclei disappear and the effect is exhibited. Can not.

The technique disclosed in Japanese Patent Application Laid-Open No. Sho 61-253344 discloses an oxide / sulfide of REM / Ca or Ti
BN is formed on N and ferrite is formed from the BN to refine the HAZ structure. Similar effects can be expected in ultra-high heat input welding. However, it is difficult to increase the number of oxides and sulfides of REM / Ca, and TiN cannot form a ferrite nucleus because it forms a solid solution. There is a limit to the improvement in toughness.

[0015]

SUMMARY OF THE INVENTION The present invention relates to a welding head having excellent HAZ toughness in ultra-high heat input welding of 500 kJ / cm or more, such as electroslag welding, which is applied to assemble box columns of a high-rise building. It is to provide a tension steel.

[0016]

In order to improve the toughness of the ultra-high heat input welding HAZ, it is essential that the HAZ structure be refined. It has been concluded that this is possible only by promoting the ferrite transformation of and making the effective crystal grains fine by synergistic action of these two.

The gist of the present invention is as follows. (1) Mg-containing oxide having an A particle diameter of 0.01 to 0.20 μm is used in an amount of 40,000 to 100,000 per square mm.
A super-large heat input welding heat, characterized in that the steel contains 20 to 400 composites per square mm of a Ti-containing oxide and MnS having a particle diameter of 0.20 to 5.0 μm. High strength steel for welding with excellent toughness in the affected zone. (2) 0.04 ≦ C ≦ 0.20,0.0 in A weight%
2 ≦ Si ≦ 0.15,0.6 ≦ Mn ≦ 2.0, P ≦
0.02, 0.0020 ≦ S ≦ 0.0060, Al ≦
0.005, 0.005 ≦ Ti ≦ 0.025,0.00
20 ≦ N ≦ 0.0080, 0.0002 ≦ Mg ≦ 0.
030, 0.0005 ≦ O ≦ 0.0080, and the balance of Fe and inevitable impurities is excellent in the toughness of the heat-affected zone of the ultra-high heat input welding described in (1) above. High strength steel for welding. (3) A In addition to the steel of the above (2), a base metal strength increasing element group, 05 ≦ Cu ≦ 1.5, 05 ≦ Ni ≦ 2.0,
02 ≦ Cr ≦ 1.0, 02 ≦ Mo ≦ 1.0, 005 ≦ N
b ≦ 0.05,005 ≦ V ≦ 0.10,0004 ≦
High strength steel for welding excellent in toughness of a heat affected zone of a very large heat input weld, characterized in that one or two or more of B ≦ 0.0040 are contained.

Here, the number of Mg-containing oxides is, for example, 1
After holding at 400 ° C. for 20 seconds, an extraction replica is collected from the quenched sample and measured with a transmission electron microscope. Also, Ti
The number of composites composed of the contained oxide and MnS is, for example, 1
After holding at 400 ° C. for 20 seconds, a sample cooled at 800 ° C. to 500 ° C. for 800 seconds is measured by a CMA analyzer.

The present inventors have conducted detailed investigations and studies on the relationship between the structure and toughness of the ultra-high heat input welding HAZ. As a result, the present inventors have applied the conventional method of controlling the structure or improving the toughness of the large heat input welding HAZ. However, it has been found that the improvement of the HAZ toughness of the ultra-high heat input welding is limited, and the toughness can be improved only by combining a plurality of microstructure control techniques. As described above, in order to improve the HAZ toughness, it is essential to refine the structure of the HAZ, that is, to refine the effective crystal grains.
The refinement of effective crystal grains includes refinement of austenite grains and fragmentation and refinement of the structure within austenite grains by utilizing intragranular ferrite transformation. In ultra-high heat input welding, the residence time at a high temperature of 1350 ° C or more is extremely long, so it is not possible to obtain sufficient structure refinement by using these two structure refinement techniques alone. We have come to the conclusion that it is possible.

First, it is effective to use the pinning effect of the particles in the steel to refine the austenite grains, but Ti is considered to be the most thermally stable among nitrides.
Even if N is heated to 1350 ° C. or more for a long time, it melts and loses the pinning effect, so that it cannot be applied to ultra-high heat input welding. Therefore, it is essential to use oxide particles that are stable at high temperatures. However, prior art REM or Ca
For oxides (including acids and sulfides), ultra-high heat input welding HAZ
It is impossible to finely disperse these oxides in steel to an extent sufficient to suppress austenite grain coarsening. The present inventors have compared various oxides. 50 kg
A steel sheet was manufactured from a steel ingot manufactured by performing a complex deoxidation with Ti and other deoxidizing elements using a vacuum melting furnace described above, and the distribution of particles was examined by an extraction replica method. Here, the super large heat input H
In AZ, to eliminate the number of nitrides that are invalid for pinning,
The measurement was performed on a sample that was quenched after holding at 1400 ° C. for 20 seconds. Thus, only particles effective for pinning can be counted. According to this measurement method, 0.01 to 0.20 μm
The number of m-type oxide particles was counted. The result is shown in FIG. Deoxidized molten steel with Mg in combination with Ti
It has been found that generation of an oxide containing g is most effective for fine dispersion of particles, and can effectively prevent coarsening of austenite grains in the ultra-high heat input welding HAZ. It is produced when Ti and Mg are combined and deoxidized.
The oxide having a size of 20 μm or less is an oxide mainly composed of Mg. It is difficult to generate a fine oxide effective for pinning by single deoxidation of Ti, and it is easy to cause agglomeration and coarsening of Mg oxide even by single deoxidation of Mg to obtain sufficient fine dispersion. It is not possible.

As conventionally known, assuming that the volume fraction of particles is f and the average diameter is r, the pinned austenite particle diameter and the average distance between particles are proportional to f / r. In order to pin the austenite grains, a large amount of fine particles need to be dispersed. Particles effective for pinning have a particle size of 0.01 to 0.20 μm, particularly particles having a particle size of 0.1 μm or less. The average distance between particles is related to the number of particles observed per unit area of steel. Measurement surface 1 in measurement by the extraction replica method based on the above conditions
At least 40,000 particles per square mm are required. If the number is less than this, the super large heat input welding HAZ
The effect of suppressing austenite grain coarsening becomes weaker. However, when a large amount of particles are present in the steel, the formation of dimples at the time of plastic deformation is promoted, which causes a significant decrease in the ductility of the steel. For this reason, it is necessary to set an upper limit on the number of particles. From the relationship between the ductility of steel (elongation / drawing in a tensile test and the upper shelf absorbed energy in a Charpy impact test) and the number of particles, the upper limit is set to 100 per square mm of the measurement surface. 2,000.

In the present invention, in addition to suppressing austenite grain growth, it is necessary to refine effective crystal grains by intragranular ferrite transformation. Technology for transforming intragranular ferrite from TiN or composite particles containing TiN (see, for example,
No. 254118), as described above, in ultra-high heat input welding, TiN dissolves, so that ferrite transformation nuclei disappear,
Invalid. Therefore, it is essential to use oxide-based ferrite transformation nuclei that are stable at high temperatures. At this time, it must be possible to simultaneously generate Mg oxide effective for pinning austenite grains. As a result of conducting various studies on oxides satisfying such conditions, it was concluded that Ti oxide was the most suitable. JP 6
It is widely known that composite particles mainly composed of Ti oxide have a high action as a ferrite transformation nucleus, as seen in Japanese Patent Application No. 0-245768. In the present invention, Ti
It has been clarified that, in the combination of and the other deoxidizing element (referred to as X), the X oxide coexists in the Ti oxide, and the ferrite transformation ability greatly changes. This is because the precipitation of MnS on the Ti oxide is a necessary condition for the ferrite transformation, and the coexistence of the X oxide with the Ti oxide greatly changes the MnS precipitation ability. Using a 50 kg vacuum melting furnace, the present inventors varied X in the complex deoxidation system of Ti and X, and measured the frequency of MnS precipitation on the Ti oxide. The maximum heating temperature is 1400 as a heat cycle equivalent to ultra-high heat input welding HAZ
C., the holding time was 20 seconds, and the cooling time from 800 to 500 ° C. was 800 seconds.
An A analyzer (a device that performs EPMA analysis two-dimensionally and measures the two-dimensional distribution of elements) is analyzed.
The number of Ti oxide-containing oxides having a size of 2 μm or more and the number of Ti-containing oxides in which MnS was precipitated were measured. Here, particles in which Ti and O were simultaneously detected were determined as Ti-containing oxides, and particles in which Mn and S were simultaneously detected were determined as MnS. The results are shown in FIG. The number of Ti-containing oxides is increased in the case of complex deoxidation of Ti and X as compared with the case of single deoxidation of Ti, and among them, it can be considered that they have the ability as intragranular ferrite transformation nuclei. When the number of Ti-containing oxides on which MnS is precipitated is compared, the difference depending on the type of X becomes clear. In other words, it has been clarified that the composite of Ti and Mg produces the largest amount of the composite of Ti-containing oxide and MnS and the largest amount of the intragranular ferrite transformation nucleus. Corresponding to FIG. 2, the intragranular ferrite area ratio in the former austenite grains of the heat cycle material is also the highest in the composite deoxidizing system of Ti and Mg. In addition, even in the case of particles which promote the intragranular ferrite transformation due to the crystal composition, if the particle diameter is less than 0.2 μm, the probability of forming intragranular ferrite is sharply reduced. Therefore, the intragranular ferrite transformation nucleus needs to be 0.20 μm or more. vice versa,
Particles having a size exceeding 5.0 μm tend to be a starting point of brittle fracture and increase the variation in toughness. Therefore, the particle diameter of the transformation nucleus needs to be 5.0 μm or less. There must be at least 20 transformation nuclei of such a size per square mm of the steel inspection surface. If the number is less than 20, it is insufficient to secure ultra-high heat input welding HAZ toughness. Conversely, if the number exceeds 400, ductility decreases, so the upper limit was set to 400.

From the above results, it can be seen that in a steel deoxidized by combining Ti and Mg, the grain growth of austenite in the ultra-high heat input welding HAZ is suppressed and, at the same time, the intragranular ferrite transformation is promoted, so that the HAZ is effectively used. It became clear that crystal grains can be refined.

The reasons for limiting the components are shown below.

C is an element capable of increasing the strength of the base material.
If it is less than 0.04%, the base material strength cannot be secured, so 0.04% was made the lower limit. Conversely, if a large amount of C is contained,
To increase cementite, which is the starting point of brittle fracture,
Decreases the toughness of base metal and HAZ. If it exceeds 0.20%, the toughness is significantly reduced.

Si is an element effective for increasing the strength of the base material.
If less than 0.02%, this effect cannot be obtained, so the lower limit is set to 0.02%. Conversely, if the content exceeds 0.15%, H
Island martensite is generated in the AZ structure, and improvement in toughness cannot be obtained even if the effective crystal grain size is reduced. Therefore, the upper limit is set to 0.15%.

Mn is an element effective for increasing the strength of the base material. If the content is less than 0.6%, this effect cannot be obtained, so the lower limit is set to 0.6%. Conversely, if the content exceeds 2.0%, the toughness is significantly reduced. Therefore, the upper limit is set to 2.0%.

P is an element that causes grain boundary embrittlement and is harmful to toughness. If the content exceeds 0.02%, the toughness is significantly reduced, so the upper limit is 0.02%.

In the present invention, S represents MnS on Ti-containing oxide.
Is an essential element for precipitating and producing intragranular ferrite. If it is less than 0.0020%, MnS precipitation is insufficient and intragranular ferrite is not generated. Therefore, the lower limit is set to 0.
0020%. Conversely, when the content exceeds 0.0060%, MnS precipitates not only on the Ti oxide but also in the steel, generates elongated inclusions, and the ductility in the sheet thickness direction is significantly reduced.
Therefore, the upper limit was made 0.0060%.

Since Al suppresses formation of Ti oxide,
In the present invention, lower is better. If the content exceeds 0.005%, the content of the Ti-containing oxide is significantly reduced.
Was set as the upper limit.

Ti is an essential element for forming oxides and providing intragranular ferrite transformation nuclei. If the content is less than 0.005%, the number of Ti-containing oxides becomes insufficient.
% Was defined as the lower limit. Conversely, if the content exceeds 0.025%, a coarse Ti-containing oxide serving as a fracture starting point is generated to cause a decrease in toughness, and a large amount of TiC harmful to toughness is precipitated. Therefore, the upper limit is set to 0.025%.

N combines with the residual Ti present as an oxide to precipitate TiN. TiN is H closest to the weld fusion line
In AZ, the effect of suppressing austenite grain coarsening is lost, but in HAZ which is far from the fusion line and has a low heating temperature, TiN
Is effective in suppressing grain growth because it does not dissolve. HA
In order to improve the toughness over the entire range of Z, it is essential to use TiN, and therefore, it is necessary to contain N. If N is less than 0.0020%, TiN precipitation becomes insufficient, so the lower limit was made 0.0020%.
Conversely, if the content exceeds 0.0080%, the solid solution in the ferrite ground causes a decrease in toughness. Therefore, the upper limit is 0.008
0%.

Mg forms the Mg-containing oxide finely without lowering the ferrite transformation ability of the Ti-containing oxide,
Works to suppress austenite grain coarsening. If less than 0.0002%, this effect cannot be obtained, so the lower limit is 0.0002%.
%. On the other hand, when the content exceeds 0.0030%, the Mg concentration in the Ti-containing oxide increases, and the transgranular ferrite transformation ability decreases, and the Mg-containing oxide in the steel increases, and the ductility decreases. Therefore, the upper limit is 0.003
0%.

O is an essential element for forming oxides of Mg and Ti. If it is less than 0.0005%, the number of both oxides will not be sufficient, so 0.0005% is made the lower limit. Conversely, if the content exceeds 0.0080%, the amount of oxides becomes too large, resulting in a decrease in ductility. Therefore, the upper limit is 0.00
80%.

Further, the limited range of the selected element effective for increasing the base material strength was determined for the following reasons.

Cu is an element effective for increasing the base material strength.
In particular, a remarkable increase in strength can be obtained by precipitating a fine Cu phase by aging heat treatment. If less than 0.05%, no increase in strength can be obtained, so 0.05% was set as the lower limit.
Conversely, if the content exceeds 1.5%, embrittlement becomes remarkable, so the upper limit was set to 1.5%.

Ni has the effect of increasing the base material strength by increasing the hardenability, and further improves the toughness.
If the content is less than 0.05%, these effects cannot be obtained, so the lower limit is set to 0.05%. Conversely, if the content exceeds 2.0%, the quenchability becomes too high, so that a HAZ hardened structure is easily generated, and the HAZ toughness is reduced. Therefore, the upper limit is set to 2.
0%.

Cr has the effect of increasing the strength of the base material. 0.
If it is less than 02%, this effect cannot be obtained, so the lower limit is set to 0.
02%. Conversely, if the content exceeds 1.0%, a hardened structure is formed in the HAZ, and the HAZ toughness is reduced. Therefore, the upper limit was set to 1.0%.

Mo is effective in increasing the strength of the base material. 0.
If it is less than 02%, this effect cannot be obtained, so the lower limit is set to 0.
02%. Conversely, if the content exceeds 1.0%, a hardened structure is formed in the HAZ, and the HAZ toughness is reduced. Therefore, the upper limit was set to 1.0%.

Nb is an element effective for increasing the strength and refining the base material. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, 0.
If the content exceeds 0.05%, precipitation of carbonitrides in the HAZ becomes remarkable, and the reduction in HAZ toughness becomes remarkable. Therefore, the upper limit is set to 0.05%.

V is an element effective for increasing the strength and refining the base material. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, 0.1
When the content exceeds 0%, precipitation of carbonitrides in the HAZ becomes remarkable, and HAZ toughness is significantly reduced. Therefore, the upper limit is set to 0.10%.

B exerts a particularly remarkable increase in strength when subjected to controlled cooling and quenching heat treatment. Further, it has an effect of precipitating intragranular ferrite transformation by precipitating B nitride or carbon boride on the Ti-containing oxide of the ultra-high heat input welding HAZ. If the content is less than 0.0004%, the above effect cannot be obtained, so the lower limit was made 0.0004%. vice versa,
When the content exceeds 0.0040%, coarse nitrides and carbide borides are precipitated and serve as starting points of fracture, so that toughness is reduced. Therefore, the upper limit is set to 0.0040%.

[0043]

BEST MODE FOR CARRYING OUT THE INVENTION

(Examples) Examples of the present invention will be described below. Steel was melted by a converter, and a slab having a thickness of 240 mm was manufactured by continuous casting. Table 1-1 and Table 1-2 show the chemical components of the steel material. Since the HAZ toughness also depends greatly on the carbon equivalent,
In order to confirm the effects of the present invention, steels were prepared by melting only Al, N, Ti, Mg, and O with substantially the same chemical components and comparing them.

Table 2 shows the manufacturing method and thickness of the steel sheet, and the mechanical properties of the base material. As shown in the table, steel sheets were manufactured by a controlled rolling / controlled cooling method, a quenching / tempering method, and a direct quenching / tempering method. The plate thickness was 40 to 100 mm.

A weld specimen was prepared by electroslag welding shown in FIG. Welding current is 380A, voltage is 4
6V, the speed was 1.14 cm / min. Heat input is 920k
J / cm. As shown in FIG. 3, a Charpy impact test specimen was collected so that the weld fusion line and the position 3 mm from the weld fusion line coincided with the notch position. For comparison, submerged arc welding with a heat input of 150 kJ / cm was also performed. A Charpy impact test specimen was collected so as to be at the same notch position as in electroslag welding. The impact test was performed at 0 ° C., and the results of evaluating the toughness by the average value of two repetitions are shown in Table 3. Further, the microstructure of the HAZ immediately near the fusion line of the electroslag weld was observed, and the γ grain size and the intragranular ferrite fraction were measured. Further, the number of Mg-containing oxides and the number of composites of Ti-containing oxide and MnS were measured according to the above-described method. Table 3 shows the results. FIG. 4 shows the HAZ toughness of the submerged arc welding, and FIG. 5 shows the HAZ toughness of the electroslag welding.

As is apparent from Table 3, the invention steel has a large number of Mg-containing oxides and has a very large heat input (electroslag).
The γ grain size of the welded HAZ is small, and at the same time, the number of composites of Ti-containing oxide and MnS is large, and the HAZ has a high intragranular ferrite fraction. As a result, the toughness of the ultra-high heat input welding HAZ is high. In the case of large heat input welding, the HAZ of the invention steel is higher than that of the comparative steel
Although the toughness is high, the difference is greater in the ultra-high heat input welding HAZ. This is because, in ultra-high heat input welding, remarkable improvement in toughness can be obtained with a steel that simultaneously achieves the suppression of gamma grain growth and the intragranular ferrite transformation as in the present invention.

The comparative steels Nos. 4 and 13 have a large number of composites of Ti-containing oxide and MnS, but have a small number of Mg-containing oxides. The improvement in HAZ toughness by heat welding is insufficient. The comparative steels of Nos. 11 and 23 have Al higher than the range of the present invention and Ti
Since the number of composites of the contained oxide and MnS is small, the improvement of the HAZ toughness of the ultra-high heat input welding is insufficient.

[0048]

[Table 1-1]

[0049]

[Table 1-2]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

As described above, in the steel of the present invention, Mg
Heat input welding HAZ with a heat input of 500 kJ / cm or more by a composite mainly composed of a titanium-containing oxide and a titanium-containing oxide
The effective crystal grains of HAZ are refined by the synergistic action of the suppression of γ grains and the promotion of intragranular ferrite transformation, and the HAZ toughness can be remarkably improved. By applying the present invention to a building structure to which ultra-high heat input welding is applied, it is possible to manufacture a highly reliable welded structure. Therefore, the present invention is extremely effective industrially.

[Brief description of the drawings]

FIG. 1 is a diagram showing a change in the number of fine oxides effective for suppressing γ grain growth depending on the type of deoxidizing element.

FIG. 2 is a diagram showing a change in the number of composites of Ti-containing oxide and MnS effective for intragranular ferrite transformation depending on the type of deoxidizing element.

FIG. 3 is a diagram showing electroslag welding conditions.

FIG. 4 is a diagram in which large heat input (submerged arc) welding HAZ toughness is plotted against Pcm.

FIG. 5 is a diagram in which ultra-high heat input (electroslag) welding HAZ toughness is plotted against Pcm.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. Mg having a particle diameter of 0.01 to 0.20 μm.
The content of oxides is 40,000 to 100,
1 mm 2 of a composite comprising MnS and a Ti-containing oxide having a particle size of 0.20 to 5.0 μm and containing
A high-strength steel for welding with excellent toughness in a heat-affected zone of ultra-high heat input welding, characterized in that the steel contains 20 to 400 steels per steel. 2. In% by weight, 0.04 ≦ C ≦ 0.20,
0.02 ≦ Si ≦ 0.15, 0.6 ≦ Mn ≦ 2.0,
P ≦ 0.02,0.0020 ≦ S ≦ 0.0060, A
l ≦ 0.005, 0.005 ≦ Ti ≦ 0.025,0.
0020 ≦ N ≦ 0.0080, 0.0002 ≦ Mg
≦ 0.0030, 0.0005 ≦ O ≦ 0.0080,
The high tensile strength steel for welding excellent in toughness of the ultra-high heat input welding heat-affected zone according to claim 1, characterized in that it is a steel containing Fe and unavoidable impurities. 3. The steel according to claim 2, further comprising, in% by weight, a base metal strength increasing element group: 0.05 ≦ Cu ≦ 1.5, 0.05 ≦ N
i ≦ 2.0, 0.02 ≦ Cr ≦ 1.0, 0.02 ≦ Mo
≦ 1.0, 0.005 ≦ Nb ≦ 0.05, 0.005 ≦
The welding having excellent toughness of the heat-affected zone of the ultra-high heat input welding according to claim 2, wherein one or two or more of V ≦ 0.10, 0.0004 ≦ B ≦ 0.0040 are contained. For high tensile steel.