JP3215296B2 - Method of manufacturing steel material for welded structures with excellent toughness of weld heat affected zone - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel material for a welded structure having excellent toughness of a weld heat affected zone (hereinafter referred to as HAZ) used for ships, marine structures, middle and high rise buildings, and the like. />.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for material properties of welding steel used for large structures used in ships, marine structures, middle and high-rise buildings, and the like, as well as the toughness of the steel itself. Demands on the toughness of the HAZ are also increasing.

Further, when constructing such a structure, it is desired to apply a large heat input welding method typified by a flux-steel backing welding method, an electrogas arc welding method, etc. in order to promote the efficiency of welding. Have been.

[0004] Conventionally, the demand for toughness has been limited to the part to which small-to-medium heat input welding is applied.
For example, Japanese Patent Publication No. 4-14179 and Japanese Patent Laid-Open No.
It was sufficient to control the state of formation of island-like martensite, which controls toughness by regulating the components as disclosed in JP-A-6135. However, in recent years, the application of large heat input welding has been promoted, and in that case, controlling only the island-like martensite is not sufficient.

[0005] In response to this, HAZ of steel at the time of large heat input welding
There have been many proposals focusing on toughness.

[0006] For example, as disclosed in Japanese Patent Publication No. 55-26164, there is a method of reducing the austenite grains of HAZ and improving the toughness by securing fine Ti nitride in steel. Also, Japanese Patent Application Laid-Open No.
JP-A-64614 proposes a method of utilizing a composite precipitate of Ti nitride and MnS as a transformation nucleus of ferrite to improve the toughness of HAZ.

However, Ti nitride almost completely forms a solid solution in the vicinity of a boundary (referred to as a weld bond portion) with a weld metal having a maximum temperature exceeding 1400 ° C. of the HAZ, so that the effect of suppressing the deterioration of toughness is reduced. And it is difficult to achieve the recent demands for severe steel properties.

As a method for improving the toughness in the vicinity of the weld bond, steel containing a Ti oxide is used in various fields such as thick plates and section steels. For example, in the field of thick plates, Japanese Patent Application Laid-Open Nos. 61-79745 and 62-103344.
As exemplified in the publication, steel containing a Ti oxide is very effective in improving the toughness of a large heat input weld, and its application to high tensile strength steel is promising. The principle is that fine ferrite is generated using precipitates such as Ti oxides, Ti nitrides, and MnS as nuclei. As a result, generation of coarse ferrite harmful to toughness is suppressed, and deterioration of toughness can be prevented. . However, the number of such Ti oxides dispersed in steel cannot be so large. The cause is coarsening and aggregation of Ti oxides. If the number of Ti nitrides is to be increased, coarse Ti
Oxides, so-called inclusions, increase. This 5 μm
The inclusions described above are harmful as starting points for structural destruction, and cause a decrease in toughness. Therefore, additional H
In order to achieve an improvement in AZ toughness, it is necessary to utilize an oxide which is less likely to be coarsened and aggregated and which is more finely dispersed than a Ti oxide.

As a method for dispersing such a Ti oxide in steel, there are many methods of adding Ti to molten steel substantially not containing a strong deoxidizing element such as Al. However, it is difficult to control the number and the degree of dispersion of Ti oxides in steel simply by adding Ti to molten steel. Further, the number and the degree of dispersion of precipitates such as TiN and MnS are controlled. It is also difficult. As a result, in steel in which Ti oxide is dispersed only by Ti deoxidation, for example, Ti
Problems such as insufficient number of oxides and variation in toughness in the thickness direction of the thick plate are observed.

Further, in the method disclosed in Japanese Patent Application Laid-Open No. 61-79745, the upper limit of the amount of Al is limited to a very small amount of 0.007% in order to easily form a Ti oxide. . If the amount of Al in the steel is small,
There may be a case where the toughness of the base material is reduced due to a cause such as an insufficient amount of N precipitates. Further, when a steel sheet having a small amount of Al is welded using a commonly used welding material, the toughness of the weld metal may be reduced.

As exemplified in JP-A-4-9448, a method of adding Al to a tundish or a mold after adding Ti has been devised. However, this method is a method for effectively producing AlN,
This is not a method for dispersing oxides and precipitates such as TiN and MnS in steel. In addition, the addition interval between Ti and Al is long, such as when Al is added in a tundish.
The feature is that casting is performed immediately after the addition, in order to minimize the reduction of Ti oxide with Al.
Therefore, the effect of Al on oxide formation cannot be obtained.

Japanese Patent Application Laid-Open No. 3-53044 also proposes a method of adding Al after adding Ti, but this method specifies that the amount of Si before the addition of Ti is made 0.05% or less. are doing. When the amount of Si is small as described above, the adjustment of the dissolved oxygen concentration is unstable, and the dissolved oxygen concentration becomes too high. As a result, coarsening of the oxide occurs. There is a problem that large inclusions are easily generated.

[0013]

Problems to be Solved by the Invention Japanese Patent Application Laid-Open No. 62-1033
No. 44, No. 44, etc., the HAZ characteristics can be further improved, so that they do not become coarse as in the case of Ti oxides, and thus do not become a starting point of destruction, and furthermore, precipitates such as Ti nitride, MnS, etc. Excellent HAZ by forming austenite grains and forming fine ferrite as core sites
Welded structure with stable dispersion of oxide capable of realizing toughness
It is an object of the present invention to provide a method for manufacturing steel materials for use .

[0014]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a semiconductor device having a Si concentration of more than 0.05%,
Molten steel adjusted to have an oxygen concentration of 20 to 80 ppm
In the T, the final content becomes 0.005 to 0.020%
After adding i and deoxidizing, the final content is 0.005 to
0.020% Al was added, and then the final component was
Insufficient Si and other alloys are added to
% By weight, C: more than 0.09 to 0.18% Si: ≤ 0.50% Mn: 0.40 to 2.0% P: ≤ 0.02% S: 0.0010 to 0.010 % Al: 0.005 to 0.020 % Ti: 0.005 to 0.020% N: 0.0020 to 0.0060% , with the balance being Fe and inevitable impurities
And the particle diameter is 0.01 to 1.0 μm and the number of particles is 5
× 10 ³ -1 × 10 ⁵ / mm ² , Ti composition ratio is 5% or less
Above, the total amount of Ti and Al when the Al composition ratio is 95% or less
Is 94% or more of the composite oxide-containing steel,
Excellent heat-affected zone toughness
The first method is a method of manufacturing a steel material for a contact structure.
More than 0.05%, dissolved oxygen concentration is 20-80ppm
The final content of the molten steel adjusted to become 0.00
After adding 5 to 0.020% of Ti and deoxidizing,
Add Al with final content of 0.005 to 0.020%
And then insufficient Si for the final component, and
Other alloys are added, and the component composition is : by weight%, C: more than 0.09 to 0.18% Si: ≤ 0.50% Mn: 0.40 to 2.0% P: ≤ 0.02% S: 0 0.0010 to 0.010% Al: 0.005 to 0.020% Ti: 0.005 to 0.020% N: 0.0020 to 0.0060% as a basic component, and Cu: ≦ 1.0% Ni: ≤ 1.5% Nb: ≤ 0.030% V: ≤ 0.1% Cr: ≤ 0.6% Mo: ≤ 0.6% B: One or two of 0.0002 to 0.0020% Contains more than seeds and the balance is Fe
Casting molten steel consisting of impurities
1.0 μm, the number of particles is 5 × 10 ³ to 1 × 10 ⁵ / m
m ² , Ti composition ratio is 5% or more, Al composition ratio is 95% or less
And a combined oxidation in which the total amount of Ti and Al is 94% or more.
And then rolling the steel
A method of manufacturing a steel material for welded structures with excellent toughness in the heat-affected zone
This is the second means.

[0015]

Hereinafter, the present invention will be described in detail. The inventors of the present invention have considered that metallographic factors for improving the HAZ toughness include (1) austenite grain refinement in a region heated to less than 1400 ° C., and (2) grain size in a region heated to 1400 ° C. or more in the vicinity of a weld bond. It was investigated to achieve the formation of ferrite inside by using an oxide at the same time.

Regarding the above item (1), it is necessary to suppress the growth of austenite grains at a high temperature in order to reduce the size of austenite. As a means for this, a method of pinning austenite grain boundaries with precipitates and stopping the movement of the grain boundaries can be considered. As one of the precipitates having such an action, it is generally considered that Ti nitride is effective. It is a well-known fact that the larger the number of precipitates, the smaller the austenite crystal grain size. Therefore, in order to refine austenite, it is effective to precipitate a large number of Ti nitrides. From such a viewpoint, the present inventors have proposed that Ti
When the nitride was observed in detail, it was found that Ti nitride precipitated as oxide nucleation sites was frequently present. Such an oxide has a Ti composition ratio of 5%
As described above, when the Al composition ratio is 95% or less, the total of Ti and Al
The amount of the composite oxide was 94% or more, and the particle size was 0.01 to 0.1 μm. Since the composite oxide of Ti and Al has good N matching with Ti nitride,
Nitride can preferentially nucleate with oxides as sites. That is, a particle diameter of 0.01 to 0.1 μm
By the presence of a composite oxide of Ti and Al in steel,
The number of sites where Ti nitrides precipitate is increased as compared to the case where no composite oxide of Ti and Al is present, and the number of Ti nitrides deposited is increased. As a result, austenite grains pinned by a large number of Ti nitrides can be refined.

Regarding the above item (2), the present inventors observed the structure of intragranular ferrite generated in austenite grains and investigated the particles contained in the intragranular ferrite. As a result, 0.1 to 1.
It has been found that a composite oxide of Ti and Al having a size of 0 μm and a composite of Ti nitride + MnS deposited on the composite oxide work effectively. The oxide is stable even when heated to a high temperature, and exists stably in steel without change even at 1400 ° C. or higher. In addition, Ti nitride +
In the subsequent cooling process, MnS precipitates a composite oxide of Ti and Al as nucleation sites, so that intragranular ferrite can be generated in the vicinity of the weld bond.

From the above findings, it is necessary to reduce the size of austenite grains in a region heated to less than 1400 ° C. and to generate intragranular ferrite in a region heated to 1400 ° C. or more near a weld bond portion. Diameter 0.1-1.
It is necessary that a composite oxide of 0 μm Ti and Al is present in the steel. According to the findings of the present inventors, when the particle size is less than 0.01 μm, the effect as a Ti nitride precipitation nucleus is weak, and when the particle size exceeds 1.0 μm, there is a possibility that the oxide may serve as a starting point of fracture. And the HAZ toughness may decrease.

Next, the number of composite oxides of Ti and Al will be described. If the number of oxides is too small, sufficient nuclei for formation of Ti nitride and intragranular ferrite at the time of welding cannot be obtained, so it is necessary to allow oxides of 5 × 10 ³ / mm ² or more to be present. As the number of oxides increases, T
i The number of nitrides and intragranular ferrites increases and the HAZ toughness improves, but the presence of an excess of oxide exceeding 1 × 10 ⁵ / mm ² leads to a decrease in the toughness of the HAZ portion and the base material. , The upper limit of the number of oxides is 1 × 10 ⁵ / m
It must be m ^2.

The size and number of the oxide are measured in the following manner. An extract replica is prepared from a steel sheet as a base material, and the size and the number of the oxide are measured by observing at least 20 visual fields at a magnification of 10000 and an observation area of 1000 μm ² or more with an electron microscope. At this time, an extracted replica collected from any part from the surface part to the center part of the steel sheet may be used.

Hereinafter, the production method of the present invention will be described in detail. First, the present inventors attempted deoxidation experiments in various orders using various deoxidizing elements to effectively and uniformly disperse a large number of precipitates such as Ti-Al composite oxides and TiN and MnS. Was. As a result, before performing the deoxidizing treatment, the amount of Si, which is an element having a lower deoxidizing power than Ti, is adjusted so that the dissolved oxygen concentration that balances with the Si amount is 20 to 80 pp.
m in molten steel adjusted to a final content of 0.005 to 0.5.
The most common method is to add Al having a final content of 0.005 to 0.020% immediately after adding 020% of Ti and deoxidizing, and then most often to add Ti-Al composite oxide and TiN,
Precipitates such as MnS were uniformly and finely dispersed, and when the obtained steel material was subjected to large heat input welding, a result was obtained in which the toughness of the HAZ portion was extremely excellent in welded structural steel. That is, the present inventors have found the findings described in the following (3), (4), and (5).

(3) The amount of dissolved oxygen greatly affects the behavior of oxide formation. In order to produce a large number of oxides, an appropriate dissolved oxygen concentration exists, and the value is 20 to 80 ppm. In order to adjust the dissolved oxygen concentration, the amount of Si having a lower deoxidizing power than Ti is adjusted.

(4) If an appropriate amount of Al is added after Ti deoxidation, the number of Ti oxides increases, and the number of precipitated TiN and MnS also increases.

(5) The number of oxides increases as the time interval from the deoxidation of Ti to the addition of Al becomes shorter.

The results of a detailed study of the above three items will be described below.

As for the above item (3), the dissolved oxygen concentration before the introduction of Ti was investigated.
If it is less than pm, the required amount of Ti-based oxide to secure the HAZ toughness is not formed, while if the dissolved oxygen concentration exceeds 80 ppm, the generated oxide becomes coarse and the HAZ toughness is reduced. .

At this time, the dissolved oxygen concentration needs to be adjusted by an equilibrium reaction with Si. There are other methods of adjusting the dissolved oxygen concentration, such as blowing acid, etc., for example, even if the dissolved oxygen concentration is adjusted with blowing acid, the dissolved oxygen concentration changes to an equilibrium value immediately after that, and when the Ti is added, It was found that the dissolved oxygen concentration of could not be adjusted accurately. Therefore, accurate adjustment of the dissolved oxygen concentration at the time of introducing Ti must use an equilibrium reaction that can be stably realized in molten steel. At this time, the Si concentration must be higher than 0.05%. This is because if the Si concentration is 0.05% or less, the dissolved oxygen concentration in equilibrium with Si exceeds 80 ppm, which causes the above-described oxide to be coarse.

As to the above item (4), the effect of Al added after deoxidation of Ti was examined. As a result, it was clarified that Ti oxide was partially reduced and refined by adding Al. The increase in the number of Ti oxides was caused by the decrease in dissolved oxygen concentration due to the addition of Al.
This is probably because the growth of the i-oxide was suppressed, and the oxide was miniaturized and became difficult to float. As a result of conducting an experiment to clarify the optimum range of Al, if the Al content is less than 0.005%, the reduction of the Ti oxide and the reduction of the dissolved oxygen amount are not sufficient, and the Ti oxide becomes coarse. Will emerge.
In addition, when the content exceeds 0.020%, it is clear that the Ti oxide is completely reduced and the number of Ti oxides is reduced. The reason for the increase in TiN is that TiN is generated using fine Ti oxide as a nucleus, and the number of precipitates increases as compared with the case where no Ti oxide exists.

With respect to the above item (5), a molten steel sample after Ti deoxidation was appropriately collected, and the behavior of oxide formation was examined. As shown in FIG. 1, as shown in FIG. It was found that the material grew, aggregated, coarsened, and floated. Therefore, it is effective to introduce Al immediately after mixing Ti into the molten steel uniformly after the introduction of Ti in order to obtain a large amount of oxides. Therefore, Al must be added and added in a deoxidation step in a secondary refining facility such as RH in which Ti is added.
However, when Ti deoxidation is not performed in the secondary refining equipment, for example, when Ti deoxidation is performed at the time of tapping from a converter, Al addition is also performed immediately thereafter. Even if Al is not introduced immediately after Ti deoxidation, the amount of reduction of Ti oxide is not so large as long as it is within 5 minutes, so that it is preferable to be within 5 minutes.
In the claims and the detailed description of the invention, "after adding Ti and deoxidizing or after deoxidizing Ti" means after the charged Ti is uniformly mixed in the molten steel.

When deoxidizing molten steel, a composite oxide of Ti and Al is formed by adding Ti and Al after deoxidizing with an element having a lower deoxidizing power than Ti, such as Si and Mn. I do. This is called a primary oxide. Further, during casting and solidification, a composite oxide of Ti and Al is generated as the temperature of the molten steel decreases. This is called a secondary oxide. In the present invention, either a primary oxide or a secondary oxide may be used.

From the above, in order to control the composition, number and size of oxides to predetermined conditions, a deoxidizing method in the steel making process is important. As a suitable deoxidizing method, after the steel is turned out of the converter, the amount of Si before the deoxidizing treatment is increased to more than 0.05%, and then the molten steel is adjusted to have a dissolved oxygen concentration of 20 to 80 ppm. , RH, etc., in the secondary refining process, Ti is added so that the final content becomes a predetermined component value, and deoxidation is performed. Similarly, in the secondary process such as RH, the final content becomes a predetermined component value%. Al is added, and then Si and other elements that are insufficient for the final component are added to adjust the final component.

In addition, the effect of the oxide has no effect on the process of producing steel materials, such as controlled rolling, a combination of controlled rolling and tempering, and a combination of quenching and tempering as it is in normal rolling. I do not receive.

Next, the reasons for limiting the range of the basic components of the present invention will be described.

C is an effective component for improving the strength of steel, with the lower limit being more than 0.09%, and the excessive addition exceeding 0.18% significantly reduces the weldability and HAZ toughness of the steel material. The upper limit was set to 0.18%.

Although Si is a component necessary for securing the strength of the base material and performing preliminary deoxidation, the upper limit is set to 0.5% in order to prevent the toughness from being reduced by the hardening of the HAZ.

Mn is 0.4% as a component for ensuring the strength and toughness of the base material and for forming transformation nuclei of intragranular ferrite.
Although the above addition is necessary, the upper limit is set to 2.0% within an allowable range such as the toughness and cracking property of the welded portion.

The lower the content of P is, the more desirable it is. However, in order to reduce this industrially, a large cost is required. Therefore, the upper limit is set to 0.020%.

S is 0.00 as an element for forming MnS.
Although 1% is necessary, the upper limit is set to 0.005% within an allowable range of toughness, cracking property and the like of the welded portion.

Al limits the lower limit to 0.03 in order to increase the number of oxides and control the decrease in toughness of the weld metal.
05%. Further, when Al is present in a large amount, all oxides become alumina, and a composite oxide mainly composed of Al-Ti-Mg is not generated.
%.

Ti is an Al—Ti—Mg composite oxide, Ti
0.005% or more is added to form a nitride.
However, when the amount of solute Ti increases, the HAZ toughness decreases. Therefore, the upper limit is set to 0.020%.

N is a very important element for the precipitation of Ti nitride. If the content is less than 0.002%, the amount of Ti nitride deposited is insufficient, and a sufficient amount of ferrite structure cannot be obtained.
Since the increase in the solute N causes a decrease in the HAZ toughness, the upper limit is made 0.006.

Although Cu is effective for improving the strength of the steel material, if it exceeds 1.0%, the HAZ toughness is reduced. Therefore, the upper limit is set to 1.0%.

Although Ni is effective for improving the strength and toughness of the steel material, the upper limit is 1.5% because an increase in the amount of Ni increases the production cost.

Nb is an element effective for improving the strength and toughness of the base material by improving the hardenability, but in the HAZ portion, excessive addition significantly reduces the toughness, so that 0.03% is added. The upper limit was set.

Since V, Cr, and Mo have the same effect as Nb, they are 0.1%, 0.6%,
The upper limit was 0.6%.

B is a grain boundary ferrite harmful to HAZ toughness,
0.0002% or more from the suppression of the growth of the ferrite side plate and the fixation of the solute N in the HAZ by the precipitation of BN.
002% or less.

[0047]

EXAMPLES 40 kg steel and 5 with the chemical composition shown in Table 1
A prototype of 0 kg steel was produced. 1 to 13 are steels of the present invention, 14 to 18
Is a comparative steel. The prototype steel is melted from a converter and deoxidized at RH during vacuum degassing. Before introducing Ti, the dissolved oxygen of molten steel is adjusted with Si, and then Ti and Ai are added in order to perform deoxidation, cast into a 280 mm thick slab by continuous casting, and then subjected to heat rolling to obtain a 32 mm thick steel plate. Manufactured as The obtained steel plate was subjected to l-pass flux copper backing welding (FCB welding). Heat input is 105kj / cm
²

Table 2 shows the deoxidation method, the particle size of the oxide, and the number of particles. Table 3 shows the heat treatment of the steel sheet, the properties of the base material, and the toughness of the HAZ. The Charpy value for evaluating the HAZ toughness is an average value obtained by performing nine tests on a portion 5 mm from the fusion line at a HAZ of 5 mm.

As is clear from Table 3, it is understood that the steels of the present invention of 1 to 10 have excellent HAZ toughness as compared with the comparative steels. That is, when the particle diameter is 0.01 to 1.0 μm and T
Ti and A having an i composition ratio of 5% or more and an Al composition ratio of 95% or less
1 and the number of particles of the complex acid compound is in the range of 5 × 10 ³ to 1 × 10 ⁵ particles / mm ² , and the toughness at −20 ° C. is extremely excellent.

On the other hand, all of Comparative Examples 11 to 20 exhibited low toughness of less than 50 j at -20 ° C in the Charpy test. These causes are as follows: 11, 12, and 13 because the dissolved oxygen amount adjusted by Si did not reach the predetermined amount of the present invention, and 14 was because the dissolved oxygen amount adjusted by Si exceeded the predetermined amount. 16 is because the Al amount was below the predetermined amount, and 16 was because the Al amount exceeded the predetermined amount.
17 and 18 are because the order of addition of Ti and Al was opposite to that of the present invention, and 19 and 20 were that the addition interval between Ti and Al was longer than the predetermined time specified in the present invention.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

[Table 3]

[0054]

According to the present invention, a steel plate which is used at a low temperature and satisfies the strict toughness requirement for destruction of ships, marine structures, middle and high rise buildings, etc. is provided. The contribution to society is very large in terms of structural safety.

[Brief description of the drawings]

FIG. 1 is a graph obtained by investigating the elapsed time after Ti deoxidation and the transition of the solid number of Ti oxide, and shows that the number of oxides decreases 5 minutes after Ti deoxidation.

──────────────────────────────────────────────────続 き Continuing on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/58 C22C 38/58 (72) Inventor Masaaki Nagahara 1 Nishinosu, Oita, Oita City Nippon Steel Corporation Oita Works (56) References JP-A-7-268540 (JP, A) JP-A-8-53734 (JP, A) JP-A-6-293937 (JP, A) JP-A-4-103742 (JP, A) 61-117245 (JP, A) JP 6-21321 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21C 7/06 B22D 11/00-11/12 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. The method according to claim 1, wherein the Si concentration is more than 0.05% and the dissolved acid
In molten steel adjusted to an elemental concentration of 20 to 80 ppm
In addition, Ti with a final content of 0.005 to 0.020%
, And after deoxidation, the final content is 0.005 to 0.5.
020% Al, and then add
Insufficient Si and other alloys are added,
In weight percent, C: more than 0.09 to 0.18% Si: ≦ 0.50% Mn: 0.40 to 2.0% P: ≦ 0.02% S: 0.0010 to 0.010% Al: 0.005 to 0.020% Ti: 0.005 to 0.020% N: 0.0020 to 0.0060% Steel with the balance being Fe and unavoidable impurities
And the particle diameter is 0.01 to 1.0 μm and the number of particles is 5
× 10 ^Three ~ 1 × 10 ^Five Pieces / mm ^Two , Ti composition ratio is 5% or less
Above, the total amount of Ti and Al when the Al composition ratio is 95% or less
Is 94% or more of the composite oxide-containing steel,
Excellent heat-affected zone toughness
Manufacturing method of steel for contact structure. 2. The method according to claim 1, wherein the Si concentration is more than 0.05% and the dissolved acid is
In molten steel adjusted to an elemental concentration of 20 to 80 ppm
In addition, Ti with a final content of 0.005 to 0.020%
, And after deoxidation, the final content is 0.005 to 0.5.
020% Al, and then add
Insufficient Si and other alloys are added,
In weight percent, C: more than 0.09 to 0.18% Si: ≦ 0.50% Mn: 0.40 to 2.0% P: ≦ 0.02% S: 0.0010 to 0.010% Al: 0.005 to 0.020% Ti: 0.005 to 0.020% N: 0.0020 to 0.0060% As a basic component, and Cu: ≦ 1.0% Ni: ≦ 1.5% Nb: ≦ 0.030% V: ≦ 0.1% Cr: ≦ 0.6% Mo: ≦ 0.6% B: 0.0002 to 0.0020% One or more of the following, with the balance being Fe and
Casting molten steel containing impurities
1.0 μm, number of particles is 5 × 10 ^Three ~ 1 × 10 ^Five Pieces / m
m ^Two , Ti composition ratio is 5% or more, Al composition ratio is 95% or less
And a combined oxidation in which the total amount of Ti and Al is 94% or more.
And then rolling the steel
A method for producing a steel material for a welded structure having excellent toughness of a heat-affected zone.