JPH10183295A - Steel material excellent in toughness at heat-affected zone in large heat input weld, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel material improved in toughness in HAZ to a greater extent by specifying the composition of a steel and incorporating a compound oxide which contains Ca, Ti, and Al and in which grain size and the number of grains are respectively specified. SOLUTION: A molten steel, having a composition consisting of, by weight, 0.03-0.18% C, <=0.50% Si, 0.40-2.0% Mn, <=0.02% P, 0.0010-0.010 S, 0.005-0.020% Al, 0.005-0.020% Ti, 0.0005-0.0030% Ca, 0.0020-0.0060% N, and the balance Fe, is cast and then rolled, by which a steel containing a compound oxide which contains Ca and Ti and/or Al and in which grain size and the number of grains are regulated, respectively, to 0.01-1.0μm and (5×10<3> to 1×10<5> ) pieces/mm<2> is obtained. After Ti of 0.005-0.20%, by weight, of final content is added to the molten steel in which Si concentration and dissolved oxygen concentration are regulated, respectively, to 0.05-0.20% and 20-80ppm, Al of 0.005-0.020% final content is added and further Ca of 0.0005-0.0030% final content is added, and then, Si in an amount compensating the shortage base on the final composition and other alloys are added, by which the steel material can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding heat affected zone (hereinafter referred to as H) used for ships, offshore structures, middle and high rise buildings and the like.
AZ) and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the demand for material properties of welding steel materials used for large structures used in ships, marine structures, middle and high-rise buildings and the like has been increasing strictly. The requirements for 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-copper 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 portion 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 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 austenite grains of HAZ and improving toughness by securing fine Ti nitride in steel. Also, Japanese Patent Application Laid-Open No.
JP-A-64614 proposes a method for improving the toughness of HAZ by utilizing a composite precipitate of Ti nitride and MnS as a transformation nucleus of ferrite.

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, steels containing Ti oxide are 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 Ti oxide is very effective in improving the toughness of a large heat input weld, and its application to high-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 oxides 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. 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] In order to solve such a problem, a part of the present inventors has disclosed a technique in which a Ti-Al composite oxide generated by adding Al immediately after Ti addition in Japanese Patent Application Laid-Open No. 6-293937 is used. Has been devised. With this technology,
It is possible to greatly improve the HAZ toughness of large heat input welding, but recently, in shipbuilding and construction industries, 200
With further increase in welding heat input of kJ / cm or more, steel materials having even higher HAZ toughness are required. At this time, it is necessary to improve the toughness particularly in the vicinity of the weld fusion part.

[0014]

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
An object of the present invention is to stably disperse an oxide capable of realizing toughness, and to further improve the toughness of a weld fusion part in particular.

[0015]

According to the present invention, in order to solve the above-mentioned problems, C: 0.03 to 0.18% Si: .ltoreq.0.50% Mn: 0.40 to 2% by weight 0.0% P: ≦ 0.02% S: 0.0010 to 0.010% Al: 0.005 to 0.020% Ti: 0.005 to 0.020% Ca: 0.0005 to 0.0030% N: 0.0020 to 0.0060%, the balance being Fe and unavoidable impurities, and having a particle diameter of 0.01 to 1.0 μm and a particle number of 5 × 10 ^3.
１1 × 10 ⁵ / mm ² , containing Ca, Ti and Al
The first means is a steel material for a welded structure excellent in toughness of a weld heat-affected zone, characterized by containing a composite oxide containing at least one of the following: C: 0.03 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 -0.020% Ca: 0.0005-0.0030% N: 0.0020-0.0060% as a basic component, and further, Cu: ≦ 1.0% Ni: ≦ 1.5% Nb: ≦ 0. 030% V: ≦ 0.1% Cr: ≦ 0.6% Mo: ≦ 0.6% B: One or more of 0.0002 to 0.0020%, the balance being Fe and unavoidable impurities Consisting of, and having a particle size of 0.01 to 1.0 μm
m, the number of particles is from 5 × 10 ³ to 1 × 10 ⁵ / mm ² , containing Ca, and further containing a composite oxide containing at least one of Ti and Al. Is used as the second means, and in producing the steel material of the first and second means, the Si concentration is 0.05 to 0.20% and the dissolved oxygen concentration is 20%. ~ 8
After adding Ti having a final content of 0.005 to 0.020% to molten steel adjusted to be 0 ppm and deoxidizing, A having a final content of 0.005 to 0.020% is added.
1 and a final content of 0.0005-0.0
030% Ca is added, and then Si and other alloys are added to the final component, which are insufficient, and the component composition is% by weight. C: 0.03-0.18% 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% Ca : 0.0005 to 0.0030% N: 0.0020 to 0.0060%, and the balance is made of molten steel consisting of Fe and unavoidable impurities and then rolled after casting. The method for producing a steel material for a welded structure is defined as a third means.
i concentration is 0.05 to 0.20% and dissolved oxygen concentration is 20
After adding Ti having a final content of 0.005 to 0.020% into molten steel adjusted to be ~ 80 ppm and deoxidizing, the final content becomes 0.005 to 0.020%. Al is added and the final content is 0.0005 to
0.0030% of Ca is added, and then Si and other alloys, which are insufficient with respect to the final component, are added, and the composition of the components is% by weight. C: 0.03-0.18% 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 % Ca: 0.0005 to 0.0030% N: 0.0020 to 0.0060% as a basic component, and further Cu: ≦ 1.0% Ni: ≦ 1.5% Nb: ≦ 0.030% V: ≦ 0.1% Cr: ≦ 0.6% Mo: ≦ 0.6% B: One or more of 0.0002 to 0.0020%, and the balance is molten steel composed of Fe and unavoidable impurities. Method of producing steel for welded structures with excellent toughness in the heat affected zone, characterized by rolling after casting It is referred to as the fourth means.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The inventors of the present invention have considered that the 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 studied that the formation of internal ferrite was achieved simultaneously using oxides.

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 oxides include Ca,
Furthermore, there is an oxide containing one or more of Ti and Al (hereinafter, Ti-Al-Ca oxide), and it has been found that the particle diameter is 0.01 to 0.1 µm.

That is, when Ti-Al-Ca oxide having a particle diameter of 0.01 to 0.1 μm is present in steel, sites where Ti nitride precipitates are compared with the case where these oxides are not present. And the number of precipitated Ti nitrides increases. 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 of a Ti—Al—Ca oxide having a size of 0 μm and a Ti nitride + MnS deposited thereon works 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. Also, Ti nitride + M
Since nS precipitates a Ti—Al—Ca oxide as a nucleation site in a subsequent cooling process, intragranular ferrite can be generated in the vicinity of the weld bond.

One of the oxides having the effect of promoting the formation of intragranular ferrite is a Ti-Al composite oxide, but Ti-Al-Ca oxide is more ferrite-forming than Ti-Al oxide. It has been found that when the ability is high and the same number of oxides are present, the number of intragranular ferrites increases when the Ti-Al-Ca oxide is present.

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 0.01 ~
It is necessary that 1.0 μm of the Ti—Al—Ca composite oxide 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 Ti-Al-Ca oxides will be described. If the number of oxides is too small, sufficient T
Since i-nitrides and intragranular ferrite formation nuclei cannot be obtained, it is necessary to have oxides of 5 × 10 ³ / mm ² or more. As the number of oxides increases, Ti
The number of nitrides and intragranular ferrites increases and the HAZ toughness improves, but the presence of an excess oxide exceeding 1 × 10 ⁵ / mm ² causes 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 / mm ²
Must.

The size and number of the oxide are measured as follows. 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 conducted deoxidation experiments in various orders using various deoxidizing elements to effectively and finely disperse a large number of precipitates such as Ti-Al-Ca oxides and TiN and MnS. Tried. 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, and the dissolved oxygen concentration that balances with the amount of Si is adjusted to 20 to 80 pp.
m in molten steel adjusted to a final content of 0.005 to 0.5.
Immediately after adding 020% Ti and deoxidizing, Al having a final content of 0.005 to 0.020% is added, and thereafter, a final content of 0.0005 to 0.020% is added.
030% Ca is most frequently added to Ti-A
When l-Ca oxide and precipitates such as TiN and MnS are uniformly and finely dispersed, and the obtained steel material is subjected to high heat input welding, HA
A result was obtained in which the toughness of the Z portion was a very excellent welded structural steel. Compared to the case where Al was added immediately after Ti deoxidation,
Further, the effect of increasing the number of oxides is increased by adding Ca.

That is, the present inventors have the following (3):
The findings described in (4) and (5) were found.

(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) When an appropriate amount of Al is added after deoxidation of Ti, the number of Ti oxides increases, and when Ca is further added, Ca is taken into the oxide to form a Ti-Al-Ca oxide.

(5) By adding Al after Ti deoxidation and then further adding Ca, the number of oxides present in the steel is increased as compared to the case where only Al is added after Ti deoxidation.

Hereinafter, the results of the above three items will be described in detail.

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 HAZ toughness is not formed. On the other hand, 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, when the Si concentration is 0.05% or less, the dissolved oxygen concentration which equilibrates with Si exceeds 80 ppm, so that the above-mentioned oxide is coarsened.

As for the above items (4) and (5), the effect of Al and Ca added after Ti deoxidation was examined. As a result, it was found that Ti oxide was partially reduced and refined by adding Al. It became. Also, Ti-A
It is considered that the reason why the number of 1-oxides increased was that, because the concentration of dissolved oxygen was reduced by the addition of Al, the growth of Ti-Al oxides was suppressed, the Ti-Al oxides were miniaturized, and it became difficult to float. As a result of conducting experiments to clarify the optimum range of Al, if the Al content is less than 0.005%, reduction of Ti oxide and reduction of dissolved oxygen amount are not sufficient, and Ti
Oxide coarsens and floats. In addition, 0.020%
It is clear that when the ratio exceeds 1, the Ti oxide is completely reduced, and the number of Ti oxides is reduced.

Next, by further adding Ca, which has a stronger deoxidizing power than Ti and Al, the oxides already generated are partially reduced to Ti-Al-Ca oxides. Further, the dissolved oxygen concentration further decreases, and Ti-Al-
The growth of Ca oxide is further suppressed, and the oxide can be dispersed while keeping the fineness. At this time, excessive addition of Ca is not appropriate because it promotes the formation of CaS and hinders the subsequent precipitation of MnS.

Further, a sample of molten steel after Ti deoxidation was appropriately collected, and the formation behavior of the oxide was investigated. As shown in FIG. It has been clarified that the particles are aggregated, coarsened, and float. 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
It must be added and added in a deoxidation step in a secondary refining facility such as RH where Ti is added. However, when Ti deoxidation is not performed in the secondary refining facility, for example, when Ti deoxidation is performed during converter tapping, 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. The addition of Ca is the same as the addition of Al, and it is desirable to add Ca within a short time after the addition of Al.

The Ti-Al-Ca oxide is generated when deoxidizing molten steel. This is called a primary oxide. Furthermore, during casting and solidification, the temperature of molten steel decreases and Ti-Al-C
a oxide is formed. 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 deoxidation method, after the steel output from the converter, the amount of Si before the deoxidization treatment is increased to more than 0.05%, and 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%. After adding Al and further adding Ca, an insufficient amount of Si and other elements with respect to the final component is added to adjust the final component.

The effect of the oxide is not affected by the process of producing steel material, which is usually performed as-rolled, controlled-rolling, a combination of controlled rolling and tempering, and a combination of quenching and tempering. .

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 0.03%, and an excessive addition exceeding 0.18% significantly reduces the weldability and HAZ toughness of the steel material. Was set to 0.18%.

Si is a component necessary for securing the strength of the base material, preliminary deoxidation, and the like, but the upper limit is set to 0.5% in order to prevent a decrease in toughness due to hardening of the HAZ.

Mn is 0.4% as a component for ensuring the strength and toughness of the base material and for generating 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 smaller the content of P is, the more desirable it is. However, in order to reduce this industrially, a great deal of cost is required. Therefore, the upper limit is set to 0.020%.

S is an element that forms MnS at 0.00
Although 1% is necessary, the upper limit is set to 0.005% within an allowable range such as toughness and cracking of the welded portion.

Al has a lower limit of 0.0 to increase the number of oxides and to suppress a decrease in the toughness of the weld metal.
05%. In addition, when Al is present in a large amount, all oxides become alumina and Ti-Al-Ca oxide is not generated, so the upper limit was made 0.020%.

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

Ca must be added in an amount of 0.0005% or more in order to form a Ti—Al—Ca oxide. However, excessive addition hinders the precipitation of MnS and consequently reduces the intragranular ferrite structure, so that 0.0030
% As the upper limit.

N is a very important element for the precipitation of Ti nitride, and if less than 0.002%, the amount of Ti nitride deposited is insufficient, and a sufficient amount of ferrite structure cannot be obtained.
The upper limit of 0.006 is set because an increase in solid solution N causes a decrease in HAZ toughness.

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

V, Cr, and Mo also have the same effect as Nb, so that 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.

[0053]

EXAMPLE A 50 kg steel was prototyped with the chemical composition shown in Table 1. 1 to 10 are steels of the present invention and 11 to 22 are comparative steels. 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
After that, Ti and Al were added in order, and deoxidation was performed. After casting into a 280 mm thick slab by continuous casting, it was heated and rolled to produce a 45 mm thick steel sheet. The obtained steel plate was subjected to one-pass SEGARC welding. The heat input is about 200 kJ / cm ² .

Table 2 shows the deoxidation method, the particle diameter of the oxide, and the number of particles. Table 3 shows rolling conditions, base metal properties, and HAZ toughness of the steel sheet. The Charpy value for evaluating the HAZ toughness is an average value obtained by performing nine tests at a position 1 mm from the fusion line at a HAZ of 1 mm.

As is evident from Table 3, 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
The number of particles of i- and Al-Ca oxide is 5 × 10 ³ to 1 × 1
0 is in the ⁵ / mm ² range, HAZ toughness -20 ° C. is extremely excellent.

On the other hand, all of Comparative Examples 11 to 22 exhibited low toughness of less than 40 J at -20 ° C. in the Charpy test. The causes are 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. Since the amount was less than the predetermined amount, 16 is 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.
21 is because the Ca amount exceeded the predetermined amount, and 22 was because the Ca amount was below the predetermined amount.

[0057]

[Table 1]

[0058]

[Table 2]

[0059]

[Table 3]

[0060]

According to the present invention, a steel sheet which satisfies the strict toughness requirements for destruction of ships, marine structures, middle and high-rise buildings, etc. is provided, and the effect brought to this kind of industrial field is extremely large. The contribution to society is very large in terms of safety.

Claims (4)

[Claims] C: 0.03 to 0.18% Si: ≦ 0.50% Mn: 0.40 to 2.0% P: ≦ 0.02% S: 0.0010 to 0% by weight% 0.010% Al: 0.005 to 0.020% Ti: 0.005 to 0.020% Ca: 0.0005 to 0.0030% N: 0.0020 to 0.0060%, the balance being Fe And an unavoidable impurity having a particle diameter of 0.01 to 1.0 μm and a particle number of 5 × 10 ^3.
１1 × 10 ⁵ / mm ² , containing Ca, Ti and Al
A welded structural steel having excellent toughness in a heat affected zone of a weld, comprising a composite oxide containing at least one of the following: 2. In weight%, C: 0.03-0.18% Si: ≦ 0.50% Mn: 0.40-2.0% P: ≦ 0.02% S: 0.0010-0 0.010% Al: 0.005 to 0.020% Ti: 0.005 to 0.020% Ca: 0.0005 to 0.0030% N: 0.0020 to 0.0060% : ≦ 1.0% Ni: ≦ 1.5% Nb: ≦ 0.030% V: ≦ 0.1% Cr: ≦ 0.6% Mo: ≦ 0.6% B: 0.0002 to 0.0020 % Or more, the balance being Fe and unavoidable impurities, and having a particle size of 0.01 to 1.0 μm.
m, the number of particles is from 5 × 10 ³ to 1 × 10 ⁵ / mm ² , containing Ca, and further containing a composite oxide containing at least one of Ti and Al. Excellent steel material for welded structures. 3. A T steel having a final content of 0.005 to 0.020% in molten steel adjusted to have a Si concentration of 0.05 to 0.20% and a dissolved oxygen concentration of 20 to 80 ppm.
After adding i and deoxidizing, the final content is 0.005 to
0.020% of Al is added, Ca is added to a final content of 0.0005 to 0.0030%, and then Si and other alloys are added, which are insufficient for the final components. Ingredient composition is% by weight, C: 0.03 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% Ca: 0.0005 to 0.0030% N: 0.0020 to 0.0060%, the balance being Fe and inevitable A method for producing a steel material for a welded structure having excellent toughness in a heat affected zone of a weld, characterized in that molten steel containing impurities is cast and then rolled. 4. A T steel having a final content of 0.005 to 0.020% in molten steel adjusted to have a Si concentration of 0.05 to 0.20% and a dissolved oxygen concentration of 20 to 80 ppm.
After adding i and deoxidizing, the final content is 0.005 to
0.020% of Al is added, Ca is added to a final content of 0.0005 to 0.0030%, and then Si and other alloys are added, which are insufficient for the final components. Ingredient composition is% by weight, C: 0.03 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% Ca: 0.0005 to 0.0030% N: 0.0020 to 0.0060% as a basic component, and further Cu: ≦ 1.0% Ni: ≦ 1.5% Nb: ≦ 0.030% V: ≦ 0.1% Cr: ≦ 0.6% Mo: ≦ 0.6% B: 0.0002 to 0.0020% Roll molten steel containing one or more types, the balance being Fe and inevitable impurities Method for producing a weld heat-affected zone toughness having excellent welding structural steel, characterized in that.