JP2003313629A - Steel product superior in toughness of weld heat- affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel product having superior toughness in a HAZ (heat- affected zone). <P>SOLUTION: The steel product superior in toughness of a weld heat-affected zone comprises, by mass%, 0.03-0.18% C, 0.50% or less Si, 0.40-2.0% Mn, 0.02% or less P, 0.02% or less S, 0.6-4.0% Ni, 0.005-0.10% Nb, 0.005-0.070% Al, 0.005-0.030% Ti, 0.0005-0.0050% Ca, 0.0005-0.0070% N, 0.0005-0.0030% B, the balance Fe with unavoidable impurities, ENI which satisfies ENI≥0 in a chemical equivalent expression of ENI=(%Ni)-18(%C)-36(%Nb)+1, and EN which satisfies 0≤EN≤0.002 in the chemical equivalent expression of EN =(%N)-0.292(%Ti)-1.292(%B); and further particles in the number of 100-3,000 pieces/mm<SP>2</SP>, which have circle-equivalent particle sizes of 0.005-2.0 μm, and such a composition as to contain at least Ca, Al, O, and comprise 3% or more Ca and 1% or more Al on average mass% of elements except O, and the balance the other deoxidized elements and/or unavoidable impurities. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded structural steel material having excellent toughness in a weld heat affected zone (hereinafter referred to as HAZ) used in ships, offshore structures, middle and high-rise buildings, bridges and the like.

[0002]

2. Description of the Related Art In recent years, ships, offshore structures, middle and high-rise buildings,
The demands on the material characteristics of steel for welding used in large-scale structures such as bridges are becoming more and more severe. Further, when constructing such a structure, in order to promote efficiency of welding, application of a large heat input welding method represented by a flux-backing welding method, an electrogas welding method, an electroslag welding method, or the like. The demand for HAZ toughness is increasing, as is the toughness of steel itself.

There have been many proposals that have focused on the HAZ toughness of steel materials during high heat input welding. For example, Japanese Examined Japanese Patent Publication 55-2
As disclosed in Japanese Patent No. 6164 and the like, there is a method of reducing the austenite grains of the HAZ and improving the toughness by securing a fine Ti nitride in the steel.

Further, Japanese Patent Laid-Open No. 3-264614 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. Further, JP-A-4-143246 proposes a method of utilizing a composite precipitate of Ti nitride and BN as a precipitation nucleus of grain boundary ferrite to improve the HAZ toughness.

However, since the Ti nitride almost forms a solid solution in the vicinity of the boundary (referred to as a weld bond portion) between the HAZ and the weld metal having the highest ultimate temperature exceeding 1400 ° C., the effect of improving the toughness is deteriorated. However, it is difficult to achieve the severe requirements for HAZ toughness in recent years and the HAZ toughness in ultra-high heat input welding.

As a method of improving the toughness in the vicinity of the welded bond, the inclusion of Ti oxide in steel is used in various fields such as thick plate and shaped steel. For example,
In the field of thick plates, as illustrated in JP-A-61-79745 and JP-A-62-103344, T
The inclusion of i oxide is very effective in improving the toughness of the high heat input welded portion, and the application of Ti oxide to high strength steel is promising.

This principle is based on the fact that T which is stable even at the melting point of steel.
With the i-oxide as the site, T
The i-nitride, MnS, etc. are precipitated, and fine ferrite is generated using them as sites. As a result, the generation of coarse ferrite harmful to the toughness is suppressed, and the deterioration of the toughness can be prevented.

However, such a Ti oxide is
It is not possible to increase the number of particles dispersed in steel.
The cause is coarsening and aggregation of Ti oxide,
If an attempt is made to increase the number of Ti oxides, coarse Ti oxides of 5 μm or more, so-called inclusions will increase.

The inclusions having a size of 5 μm or more are harmful as a starting point of structural destruction and cause a decrease in toughness.
Therefore, in order to further improve the HAZ toughness, it is necessary to utilize an oxide that is less likely to coarsen or aggregate and is more finely dispersed than the Ti oxide.

As a method of dispersing such Ti oxide in steel, a method of adding Ti to molten steel that does not substantially contain a strong deoxidizing element such as Al is often used. However, it is difficult to control the number and degree of dispersion of Ti oxide in the steel by simply adding Ti to the molten steel, and further control the number and degree of dispersion of precipitates such as TiN and MnS. It is also difficult to do.

As a result, in the steel in which the Ti oxide is dispersed only by Ti deoxidation, for example, the number of Ti oxides is not sufficient, or the toughness of the thick plate varies in the plate thickness direction. Points are recognized.

Further, in the method disclosed in Japanese Patent 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 Ti oxide. However, when the amount of Al in the steel material is small, A
The toughness of the base material may decrease due to a lack of 1N precipitates. Further, when a steel plate having a small amount of Al is welded by using a commonly used welding material, the toughness of the weld metal may decrease.

To solve such a problem, Japanese Patent Laid-Open No. 6-29
Japanese Patent No. 3937 proposes a technique of adding Al immediately after the addition of Ti and utilizing the Ti-Al composite oxide generated by this addition. With this technology, it is possible to significantly improve the high heat input welding HAZ toughness,
Most recently, 200 kJ / c in the shipbuilding and construction industries
Welding heat input is being further increased by 1000 kJ / cm or more for large m or more HAZ.
A steel material having toughness is required. At this time, it is particularly necessary to improve the toughness in the vicinity of the weld fusion portion.

Further, the steel materials used are required to be thicker and have higher strength. For example, in the shipping industry, the hull is becoming larger due to the expansion of logistics, and thicker and stronger steel materials are required from the viewpoint of construction and transportation efficiency. Further, in the fields of architecture, bridges, marine structures, etc., thick and high strength steel is required to secure a wider space as the size of buildings increases.

In general, the strength of a steel material is increased by increasing the number of additional elements and increasing the carbon equivalent (hereinafter referred to as Ceq). However, since the weldability is significantly impaired, the strength improvement without increasing Ceq is achieved. desired. The most promising among them is the improvement of strength by Nb.

However, increasing the amount of Nb hardens the grain boundary ferrite in the HAZ structure and increases the embrittlement phase such as retained austenite, resulting in a large decrease in the HAZ toughness. HAZ is enough
It cannot obtain toughness.

Therefore, a steel material having high HAZ toughness is required even when Nb is added in order to reduce the Ceq and increase the thickness and strength.

[0018]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In order to dramatically improve the HAZ characteristic, coarsening of austenite grains when heated to a high temperature for a long time as compared with the above-mentioned conventional methods described in JP-A-44, JP-A-6-293937 and the like. Even if Nb is added to further suppress the heat and increase the strength, excellent HA
The challenge is to achieve Z toughness.

[0019]

Means for Solving the Problems As a result of intensive research to solve the above problems, the present inventor has found that the equivalent formula ENI = (%
Ni) -18 (% C) -36 (% Nb) +1, and
EN = (% N) -0.292 (% Ti) -1.292
In (% B), ENI and EN are respectively contained in a predetermined range, and the particle diameter of the dispersed oxide particles,
It was found that the toughness of the weld heat affected zone is remarkably improved when the composition and the number of dispersed particles are each within a predetermined range.

The present invention was made based on the above findings, and the summary thereof is as follows.

(1)% by mass, C: 0.03 to 0.18% Si: ≤ 0.50% Mn: 0.40 to 2.0% P: ≤ 0.02% S: ≤ 0.02 % Ni: 0.6 to 4.0% Nb: 0.005 to 0.10% Al: 0.005 to 0.070% Ti: 0.005 to 0.030% Ca: 0.0005 to 0.0050 % N: 0.0005 to 0.0070% B: 0.0005 to 0.0030%, the balance Fe and inevitable impurities, ENI = (% Ni) -18 (% C) -36 (% Nb) +
In the equivalent formula of 1, ENI ≧ 0 is satisfied, and EN = (% N) −0.292 (% Ti) −1.292.
In the equivalent formula of (% B), 0 ≦ EN ≦ 0.002 is satisfied, the equivalent circle diameter is 0.005 to 2.0 μm, and the composition contains at least Ca, Al, and O.
Average mass% of elements excluding O, particles containing Ca: 3% or more and Al: 1% or more, and the balance consisting of other deoxidizing elements and / or unavoidable impurities, the number of particles is 100 to 3000 / mm.
A steel material with excellent toughness in the weld heat affected zone, which is characterized by containing ² .

(2) In mass%, C: 0.03 to 0.18% Si: ≤ 0.50% Mn: 0.40 to 2.0% P: ≤ 0.02% S: ≤ 0.02 % Ni: 0.6 to 4.0% Nb: 0.005 to 0.10% Al: 0.005 to 0.070% Ti: 0.005 to 0.030% Ca: 0.0005 to 0.0050 % N: 0.0005 to 0.0070% B: 0.0005 to 0.0030% as a basic component, and further Cu: ≤ 1.0% V: ≤ 0.1% Cr: ≤ 0.6% Mo: ≦ 0.6% Mg: ≦ 0.0050% REM: ≦ 0.100% One or more kinds are contained, and the balance is Fe and inevitable impurities. ENI = (% Ni) -18 (% C ) -36 (% Nb) +
In the equivalent formula of 1, ENI ≧ 0 is satisfied, and EN = (% N) −0.292 (% Ti) −1.292.
In the equivalent formula of (% B), 0 ≦ EN ≦ 0.002 is satisfied, the equivalent circle diameter is 0.005 to 2.0 μm, and the composition contains at least Ca, Al, and O.
Average mass% of elements excluding O, particles containing Ca: 3% or more and Al: 1% or more, and the balance consisting of other deoxidizing elements and / or unavoidable impurities, the number of particles is 100 to 3000 / mm.
A steel material with excellent toughness in the weld heat affected zone, which is characterized by containing ² .

(3) In the steel material having excellent weld heat affected zone toughness according to (1) or (2), the equivalent circle particle diameter is 0.1 to 2.0 μm and the composition is at least C.
a, Al, O in average mass% of elements excluding O, Ca: 3% or more, containing Al: 1% or more, the balance is particles of other deoxidizing element and / or unavoidable impurities, Number of particles 100-3000 / mm
A steel material with excellent toughness in the weld heat affected zone, which is characterized by containing ² .

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. As a metallographic factor for improving the HAZ toughness, the present inventors have studied to achieve reheat austenite grain refinement in the HAZ region heated to 1400 ° C. or higher by using an oxide.

In order to refine the reheated austenite grains, it is necessary to suppress the growth of the austenite grains at high temperature. As the most effective method as that means,
Pinning the austenite grain boundaries with dispersed particles,
A method of stopping the movement of the grain boundaries can be considered. And, as one of the dispersed particles having such an action, it has been conventionally considered that Ti nitride is effective.

However, Ti nitride is 1400 ° C.
Since the proportion of solid solution increases at the above high temperatures, the pinning effect decreases as the welding heat input increases, as described above. Therefore, it is necessary to utilize an oxide stable as a pinning particle at a higher temperature than Ti nitride.

Further, the pinning effect of the crystal grain boundaries by the dispersed particles becomes larger as the volume ratio of the dispersed particles becomes larger and the diameter of each particle becomes larger. However, since the volume ratio of dispersed particles has an upper limit depending on the concentration of the elements that make up the particles contained in the steel, if the volume ratio is assumed to be constant, a smaller particle size is more effective for pinning. .

From such a viewpoint, the present inventors have made it possible to increase the volume fraction of the oxide and to obtain an appropriate particle size.
Various studies were conducted.

As one of means for increasing the volume fraction of oxide, there is a method of increasing the oxygen amount. However, since increasing the oxygen amount causes a large number of coarse inclusions harmful to the material. , Not an effective means.

Therefore, the present inventors examined the utilization of an element having a small solubility product with oxygen in order to maximize the utilization of oxygen. Al is generally used as an element having a small solubility product with oxygen, that is, a strong deoxidizing element. However, Al alone is not sufficient to make sufficient use of oxygen, and a deoxidizing element stronger than Al is necessary.

As a result of various studies, the present inventor has concluded that it is effective to utilize Ca, which has the strongest deoxidizing power in molten steel. Then, as a result of various experiments mainly including Ca as a deoxidizing element, as a composition of oxide particles generated in steel, Ca is contained by 3% or more and Al by 1% or more,
It was found that it is possible to increase the volume fraction of oxide, that is, the amount of oxide.

Based on these results, the composition of the particles contained in the steel is such that at least Ca, Al and O are contained, and the elements excluding O are mass% and Ca: 3% or more and Al: 1% or more. did.

At this time, Mg or / and RE having a deoxidizing power between Al and Ca are used as the remaining oxide constituent elements.
Even if M is included, the effect of the present invention is effective, and deoxidizing elements weaker than Al such as Si, Mn, and Ti inevitably mixed, and / or S inevitably bound to Ca or the like. It was confirmed that the effect of the present invention is not affected even if the impurity element of 1) is included.

Next, the particle size effective for pinning will be described. The pinning effect of the crystal grain boundaries by the dispersed particles is larger as the volume ratio of the dispersed particles is larger and the particle size of one particle is larger, but when the volume ratio of the particles is constant, the size of one particle is smaller. However, the number of particles increases and the pinning effect increases.

However, if the size of the particles is too small, the ratio of the particles existing at the grain boundaries becomes small, and it is considered that the effect is reduced. As a result of detailed examination of the austenite grain size when heated to a high temperature using test pieces having various grain sizes, it was found that the pinning was 0.005 to 2.0 μm. Is effective, and among them, 0.1-2.
It has been found that particles having a particle size of 0 μm are particularly effective. In addition, almost no oxide particles smaller than 0.005 μm were observed.

From this result, the required particle size was 0.005.
To 2.0 μm, and in particular, 0.1 to 2.0 μm
And

Next, the number of pinning particles required for HAZ toughness was examined. The larger the number of particles, the finer the structural unit. Therefore, as shown in FIG. 1, the larger the number of particles, the higher the HAZ toughness.

The HAZ toughness required for steel material varies in a complicated manner depending on its application, the welding method used, etc.
In order to satisfy the HAZ toughness required for high heat input welding in high-strength steel for shipbuilding, which is considered to have particularly severe required characteristics, the number of particles must be at least 100 particles / mm ² or more. I found out that.

On the other hand, when the number of particles exceeds 3000 particles / mm ² , the particle spacing becomes small, which is effective for refining the heated austenite grains, but since the spacing between fractures originating from inclusions becomes small, Charpy It was found to be rather harmful to the toughness represented by shock absorption energy.

Therefore, the effective and required number of particles is
It was set to 100 to 3000 pieces / mm ² .

The size and number of the above oxide particles are measured, for example, in the following manner. An extraction replica is made from the steel plate that is the base material, and it is made to 1000 by an electron microscope.
The size and the number of the oxide particles are measured by observing 20 fields of view at 0 times and an observation area of 1000 μm ² or more. At this time, if the oxide particles can be properly observed, the observation magnification may be lowered.

Oxide particles are produced when deoxidizing molten steel. This is called a primary oxide. Furthermore, during casting and solidification, Ti-Al-Ca oxide is generated as the molten steel temperature decreases. This is called a secondary oxide. In the present invention,
Either a primary oxide or a secondary oxide may be used.

As a process for producing a steel material, the effect of the oxide particles can be obtained even if normal rolling, controlled rolling, a combination of this and controlled cooling / tempering, a combination of quenching / tempering, etc. are adopted. Is not affected.

On the other hand, by dispersing the oxide particles in the steel in this way, the reheated austenite particles of the HAZ are extremely effectively fined by the pinning by the oxide particles, and the HAZ toughness is also improved accordingly. At the same time, as the austenite grains become finer, the grain boundary area increases, and the ferrite forming ability from the grain boundaries also increases. Especially, in the extremely strict toughness requirement, especially at the corners of the grain boundaries (grain boundary triple points).
It has been found that a new problem is that the relatively coarse ferrite generated at the starting point acts as a starting point to control the improvement of toughness.

In other words, if the relatively coarse ferrite generated at the grain boundaries and the triple points of the grain boundaries can be suppressed and improved, the effect of refining the HAZ structure can be superposed and the toughness can be greatly increased. It is possible to improve.

The problem of the relatively coarse ferrite formed at the grain boundaries and the triple points of the grain boundaries is that HA of high heat input welding.
It was discovered for the first time by making the Z structure finer than before by pinning with oxide particles.

The inventor of the present invention further studied in order to dramatically improve the effect of improving the toughness by making the HAZ structure finer. As a result, when a large number of fine oxide particles are dispersed to refine the reheated austenite grains, in order to suppress the growth of ferrite at the grain boundaries and the grain boundary triple points in the process of forming the HAZ structure, B It has been found that the addition of is extremely effective.

Further, as a result of detailed investigation of the mechanism of the effect of addition of B, the balance between B and N is important, and in terms of atomic ratio with B, more than the equivalent amount of solute N remains in the stage of ferrite formation. It has been clarified that the effect of adding B is enhanced, and the toughness of the fine-grained HAZ is significantly improved and stabilized.

In the Ti-added steel, the affinity between Ti and N is extremely large. As a result of considering N consumed by Ti, the HAZ toughness is EN = (% N) as shown in FIG.
-0.292 (% Ti) -1.292 (% B) can be well organized by the equivalent formula, and if the equivalent value is in the range of "0 to 0.002", the addition effect is the largest and the toughness is significantly large. It was found to improve.

When the equivalent value is less than 0, the effect of B is not recognized, while when it exceeds 0.002, the ferrite is refined, but the HAZ toughness is greatly reduced by the excess N.

Under the above conditions, particularly in the Si-Mn steel, the HAZ toughness is remarkably improved.
Further, when strengthening the steel material, sufficient H is obtained under the above conditions.
A new problem arose that the AZ toughness could not be obtained.

When increasing the strength of steel, it is necessary to suppress Ceq to a low level from the viewpoint of weldability as described above. Therefore, Nb, which is a strength element that does not affect Ceq, must be added. This is because the addition or increase of Nb, Nb greatly reduces the HAZ toughness.

Therefore, the present inventor further diligently studied the conditions under which high HAZ toughness can be obtained with Nb-added steel. First,
As a result of detailed investigation on the cause of the decrease in HAZ toughness, H
The AZ toughness decreases, but the grain boundary ferrite in the HAZ structure is N
It has been found that it is caused by the fact that it is hardened by the addition or increase of the amount of b and becomes a starting point of brittle fracture.

Then, attention was paid to the characteristics of the hardened grain boundary ferrite which is apt to be the origin of brittle fracture, and as a result of various investigations, as a result of adding Ni of 0.6% or more to the steel, the hardened grain boundary ferrite was found. The critical fracture stress of ferrite is improved,
It has been found that the fracture starting point is less likely to occur and the reduction in HAZ toughness is suppressed.

This is different from the conventional toughening of the matrix by Ni, which improves the critical fracture stress of hardened ferrite by adding Ni, thereby increasing resistance to fracture and improving HAZ toughness. This is the first time that the present inventors have found out by adding 0.6% or more of Ni to Nb-added steel.

Further, since the hardness of the grain boundary ferrite increases with the addition amount of Nb, unless the addition amount of Ni is also increased with the addition amount of Nb, the critical fracture required for maintaining high HAZ toughness is obtained. It has been found that it cannot have stress.

The condition is to increase Ni by 1 or more with respect to the Nb increase amount 36, and below that, the grain boundary ferrite becomes a fracture starting point, and the HAZ toughness decreases.

Further, when the grain boundary ferrite is less likely to become the starting point of brittle fracture, then the embrittlement phase is more likely to become the starting point of fracture, and the HAZ toughness is easily affected by the C content. Further, it was also found that by adding an appropriate amount of Ni, the embrittled phase is finely dispersed and suppresses the reduction in HAZ toughness.

Then, it was found out that in order to subdivide the embrittlement phase by adding Ni, 1 or more Ni is necessary for the amount of C increase 18.

Under the above conditions, the amounts of Ni, C, Nb and H
As a result of the relationship with AZ toughness, as shown in FIG.
Each amount of Ni, C, Nb is equivalent formula: ENI = (% Ni)
ENI at -18 (% C) -36 (% Nb) +1
It has been found that when ≧ 0 is satisfied, the reduction in HAZ toughness is suppressed and high HAZ toughness is obtained.

That is, if this equivalent value is less than 0,
The effect of Ni is not sufficient, and the embrittlement phase is coarsened by C,
And / or the grain boundary ferrite hardened by Nb easily becomes a starting point of brittle fracture, so that the HAZ toughness decreases.

Next, the reasons for limiting the range of the basic components of the present invention will be described. In addition,% means mass%.

C has a lower limit of 0.03% as an effective component for improving the strength of steel. On the other hand, an excessive addition of C significantly lowers the weldability and HAZ toughness of the steel material, so the upper limit is 0.18. %.

Si is a component necessary for securing the strength of the base material, deoxidizing, etc., but the upper limit was made 0.50% in order to prevent deterioration of toughness due to hardening of the HAZ.

Mn needs to be added in an amount of 0.40% or more as an effective component for ensuring the strength and toughness of the base metal, but the upper limit is 2.0 in the allowable range of the toughness and crackability of the welded portion.
%.

The smaller the content of P is, the more desirable it is, but in order to reduce this industrially, it takes a lot of cost, so 0.02% was made the upper limit.

The smaller the content of S is, the more desirable it is, but in order to reduce this industrially, a great deal of cost is required, so 0.02% was made the upper limit.

Ni is effective for improving the strength and toughness of the base material of the steel, and in particular, HAZ by Nb and C.
Although it is effective in suppressing deterioration of toughness, addition of less than 0.6% is not sufficient for suppressing deterioration of HAZ toughness.
Since the addition of more than 4.0% increases the manufacturing cost,
The range of Ni was 0.6% or more and 4.0% or less.

Nb is an effective element for improving the strength of the base material by improving the hardenability, but at a low Ceq, addition of less than 0.005% cannot sufficiently increase the strength. Further, excessive addition exceeding 0.10% also significantly reduces the toughness of the base material, so the addition range was made 0.005 to 0.10%.

Al is an important deoxidizing element, and the lower limit value is 0.005%. On the other hand, if Al is present in a large amount,
Since the surface quality of the slab deteriorates, the upper limit was made 0.070%.

Ti is added in an amount of 0.005% or more in order to combine with N to form a Ti nitride. However, as the amount of solid solution Ti increases, the HAZ toughness decreases, so 0.0
The upper limit was 30%.

Ca is required to be added in an amount of 0.0005% or more in order to form a Ca-based oxide. However, since excessive addition produces coarse inclusions,
The upper limit was 0050%.

N precipitates as TiN, so that HAZ
The lower limit was made 0.0005% because it is effective in improving toughness. However, if the amount of solute N increases, the HAZ toughness decreases, so 0.0070% was made the upper limit.

B is an element effective in suppressing the growth of ferrite generated in the heated austenite grain boundaries in the coexistence with N, and is added at least 0.0005%. However, if added in a large amount, the toughness of the steel material deteriorates, so the upper limit was made 0.0030%.

Cu is effective for improving the strength of the steel material, but if it exceeds 1.0%, it lowers the HAZ toughness, so 1.0% was made the upper limit.

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

Both Mg and REM have the deoxidizing power next to Ca in molten steel and have the function of assisting the formation of fine oxides by Ca. However, if added in excess, the cost will be higher than that of Ca. Is large and coarse inclusions are formed to hinder the toughness of the steel plate and HAZ, so the upper limits were made 0.0050% and 0.100%, respectively.

[0078]

(Example) (Example) With steel having the chemical composition shown in Table 1, 5
0-60 kg steel was trial-produced. Steel types 1 to 8 are steels of the present invention, and steel types 9 to 16 are comparative steels. Prototype steel is melted in a converter,
Deoxidation was performed when vacuum degassing was performed by the RH method. T
Before adding i, adjust the dissolved oxygen of molten steel with Si, then
i, Al, and Ca were sequentially added to perform deoxidation, and continuous casting was performed to cast a slab having a thickness of 280 mm. Then, this slab was heated and rolled to manufacture a steel plate having a plate thickness of 70 mm. The obtained steel sheets were welded by 1-pass electrogas welding. The heat input is about 410 kJ / cm ² .

[0079]

[Table 1]

[0080]

[Table 2]

Table 2 shows the average composition of oxide particles, the number of particles having a particle size of 0.005 to 2.0 μm and the number of particles having a particle size of 0.1 to 2.0 μm measured by an electron microscope, and ENI = (% N
i) -18 (% C) -36 (% Nb) +1 value, EN =
(% N) -0.292 (% Ti) -1.292 (% B)
Value, the average grain size of the austenite grains of the HAZ structure, the maximum ferrite size (width) at the austenite grain boundary or the grain boundary triple point, and the toughness of the HAZ measured by the cutting method in a field of view of an optical microscope photograph of 100 times. Show.

The Charpy test for evaluating the HAZ toughness was conducted at -40 ° C. Nine pieces of the test were conducted at a site of HAZ 1 mm from the bond, and the average value was taken.

As is clear from Table 2, the steels of the present invention of steel types 1 to 8 have excellent HAZ toughness as compared with the comparative steels. That is, the particle size is 0.005-2.0μ
m, the number of oxide particles containing Ca and Al in a predetermined composition is in the range of 100 to 3000 particles / mm ² , so that the austenite grain size of the HAZ structure is smaller than that of the comparative steel, and , B also reduces the ferrite at the austenite grain boundaries or at the grain boundary triple points.

As a result, the Charpy absorbed energy value at −40 ° C. greatly exceeds the average of 50 J generally required from the viewpoint of the fracture mechanics of steel structures, and the steel of the present invention is extremely excellent in HAZ toughness. It is clear that

Steel types 1, 2, 4, 5, 7 and 8
Is 100 particles / m for particles having a particle diameter of 0.1 to 2.0 μm
m ² or more, the austenite grain size is relatively small, and the Charpy absorbed energy is relatively high as compared with the steel types 3 and 6.

On the other hand, all of the comparative steel types 9 to 16 showed a low toughness of less than 50 J at the Charpy test of -40 ° C.

These causes are that, in steel types 9 to 12, the chemical composition is out of the range of the present invention, the oxide does not have the predetermined composition of the present invention, and / or the number of oxide particles is predetermined. This is because the number of oxides was less than the number, and in Steel Type 13, the oxide composition and number were within the scope of the present invention, but the ENI equivalent and the EN equivalent were outside the scope of the present invention, and in Steel Type 14, the oxides were Composition, number, E
The N equivalent is within the range of the present invention, but the ENI equivalent is out of the range of the present invention. In Steel Type 15, the oxide composition, the number, and the ENI equivalent are within the range of the present invention.
This is because the N equivalent is outside the range of the present invention. Also,
Steel type 16 had a higher oxygen content in the steel than the other steels and a larger number of oxide particles than the predetermined number, and thus had a lower toughness than the steel of the present invention.

[0088]

INDUSTRIAL APPLICABILITY The present invention provides a steel sheet that satisfies severe toughness requirements for fracture of ships, offshore structures, middle- and high-rise buildings, bridges, etc., and has an extremely large effect on the industrial field of this type. In terms of the safety of structures, it makes a great contribution to society.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the number of oxide particles and HAZ toughness.

FIG. 2 shows EN = (% N) -0.292 (% Ti) -1.
It is a figure which shows the relationship between 292 (% B) and HAZ toughness.

FIG. 3 ENI = (% Ni) -18 (% Ti) -36
It is a figure which shows the relationship between (% B) +1 and HAZ toughness.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masanori Minagawa             No. 1 Nishinosu, Oita City, Oita Prefecture Made in New Japan             Oita Steel Works, Ltd. (72) Inventor Toshinaga Hasegawa             No. 1 Nishinosu, Oita City, Oita Prefecture Made in New Japan             Oita Steel Works, Ltd. (72) Inventor Koji Ishida             No. 1 Nishinosu, Oita City, Oita Prefecture Made in New Japan             Oita Steel Works, Ltd.

Claims (3)

[Claims] 1. In mass%, C: 0.03 to 0.18% Si: ≤ 0.50% Mn: 0.40 to 2.0% P: ≤ 0.02% S: ≤ 0.02% Ni: 0.6 to 4.0% Nb: 0.005 to 0.10% Al: 0.005 to 0.070% Ti: 0.005 to 0.030% Ca: 0.0005 to 0.0050% N: 0.0005 to 0.0070% B: 0.0005 to 0.0030%, the balance Fe and inevitable impurities, ENI = (% Ni) -18 (% C) -36 (% Nb ) +
In the equivalent formula of 1, ENI ≧ 0 is satisfied, and EN = (% N) −0.292 (% Ti) −1.292.
In the equivalent formula of (% B), 0 ≦ EN ≦ 0.002 is satisfied, the equivalent circle diameter is 0.005 to 2.0 μm, and the composition contains at least Ca, Al, and O.
Average mass% of elements excluding O, particles containing Ca: 3% or more and Al: 1% or more, and the balance consisting of other deoxidizing elements and / or unavoidable impurities, the number of particles is 100 to 3000 / mm.
A steel material with excellent toughness in the weld heat affected zone, which is characterized by containing ² . 2. In mass%, C: 0.03 to 0.18% Si: ≤ 0.50% Mn: 0.40 to 2.0% P: ≤ 0.02% S: ≤ 0.02% Ni: 0.6 to 4.0% Nb: 0.005 to 0.10% Al: 0.005 to 0.070% Ti: 0.005 to 0.030% Ca: 0.0005 to 0.0050% N: 0.0005 to 0.0070% B: 0.0005 to 0.0030% as a basic component, and further Cu: ≤ 1.0% V: ≤ 0.1% Cr: ≤ 0.6% Mo: ≦ 0.6% Mg: ≦ 0.0050% REM: ≦ 0.100% One or more kinds are contained, and the balance is Fe and inevitable impurities. ENI = (% Ni) -18 (% C ) -36 (% Nb) +
In the equivalent formula of 1, ENI ≧ 0 is satisfied, and EN = (% N) −0.292 (% Ti) −1.292.
In the equivalent formula of (% B), 0 ≦ EN ≦ 0.002 is satisfied, the equivalent circle diameter is 0.005 to 2.0 μm, and the composition contains at least Ca, Al, and O.
Average mass% of elements excluding O, particles containing Ca: 3% or more and Al: 1% or more, and the balance consisting of other deoxidizing elements and / or unavoidable impurities, the number of particles is 100 to 3000 / mm.
A steel material with excellent toughness in the weld heat affected zone, which is characterized by containing ² . 3. The steel material having excellent weld heat affected zone toughness according to claim 1 or 2, having a circle-equivalent particle diameter of 0.1 to 0.1.
The composition is 2.0 μm, and the composition is at least Ca and A.
The average mass% of elements including 1, O and excluding O, Ca:
Particles containing 3% or more Al: 1% or more, with the balance being other deoxidizing elements and / or unavoidable impurities, the number of particles is 100 to 3000 / mm.
A steel material with excellent toughness in the weld heat affected zone, which is characterized by containing ² .