JP3502842B2 - 600MPa class steel with excellent low YR characteristics and super high heat input weld joint toughness - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 600 MPa class steel which is suitable for use in large-scale building structures such as middle- and high-rise buildings and bridges and has excellent low YR characteristics and super-high heat input welded joint toughness.

[0002]

2. Description of the Related Art In recent years, demands for material characteristics of steel materials for welding used in large-scale building structures such as middle and high-rise buildings and bridges have become more severe. Further, in constructing such a structure, in order to promote efficiency of welding, application of large heat input welding method represented by flux-copper backing welding method, electrogas welding method, electroslag welding method, etc. Is desired, and HAZ as well as the toughness of steel itself
The demands on the toughness of steel are increasing. In addition, since large building structures require earthquake resistance, the low YR of steel itself
There is a need for 600 MPa grade steel with (YP / TS) properties and high tensile strength.

In order to achieve a tensile strength of 590 MPa or more under microstructural conditions as a 600 MPa class steel, it is particularly necessary to increase C in the steel component system and to add Cu / Ni and Nb. If the adjustment is performed, there occurs a problem that the super large heat input welded joint toughness decreases.

[0004] There have been many proposals that have focused on the HAZ toughness of steel during high heat input welding.

[0005] 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 a fine Ti nitride in the steel. In addition, JP-A-3-2
Japanese Patent No. 64614 proposes a method of utilizing the composite precipitate of Ti nitride and MnS as a transformation nucleus of ferrite to improve the toughness of HAZ.

However, 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 whose maximum temperature exceeds 1400 ° C., so that the effect of improving the toughness is deteriorated. However, it is difficult to meet the recent demands for strict steel material properties.

As a method for improving the toughness in the vicinity of the weld bond, steel containing Ti oxide is used in various fields such as thick plate and shaped steel. For example, in the field of thick plates, JP-A-61-79745 and JP-A-62-103344.
As exemplified in the publication, steel containing a Ti oxide is very effective in improving the toughness of a large heat input welded portion, and its application to high-strength steel is promising. The principle is that Ti oxide, which is stable even at the melting point of steel, is used as a site, Ti nitride, MnS, etc. are precipitated during the temperature decrease after welding, and fine ferrite is generated using them as sites, resulting in toughness. Generation of harmful coarse ferrite is suppressed, and deterioration of toughness can be prevented. However, the number of such Ti oxides dispersed in steel cannot be increased so much. The cause is coarsening or aggregation of Ti oxides, and 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 serve as a starting point of structural destruction, which is harmful and causes a decrease in toughness.
Therefore, in order to further improve the HAZ toughness, it is necessary to utilize an oxide that is less prone to coarsening and agglomeration and finely dispersed than the Ti oxide.

Further, as a method of dispersing such a Ti oxide in steel, there is often a method of adding Ti to molten steel which does not substantially contain a strong deoxidizing element such as Al. 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. Is also difficult. As a result, in the steel in which the Ti oxide is dispersed only by Ti deoxidation, for example, Ti
Problems such as insufficient number of oxides and variation in toughness of the thick plate in the plate thickness direction 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 produce Ti oxide. . When the amount of Al in steel is small, Al
The toughness of the base material may decrease due to a lack of N precipitate amount or the like. 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.

With respect to such a problem, Japanese Patent Laid-Open No. 6-29
Japanese Patent No. 3937 proposes a technique of utilizing the Ti-Al composite oxide generated by adding Al immediately after the addition of Ti. With this technology, large heat input welding HAZ
Although it is possible to significantly improve toughness, recently, in the construction industry, etc., it is required to weld a large structural material for construction, 500 kJ / cm or more, and a large one is more than 1000 kJ / cm. Welding heat input is increasing, and HAZ toughness is even higher.
0 MPa grade steel is required. At this time, it is particularly necessary to improve the toughness in the vicinity of the welded fusion portion. In addition, large-scale structural materials will also be required to have earthquake resistance.
It is also necessary to make a steel material having a low YR characteristic of not more than%.

[0011]

The present invention has a low YR characteristic (YR ≦ YR) that achieves excellent HAZ toughness even during welding with an extremely large heat input of 500 kJ / cm or more as described above.
(80%) 600 MPa class steel.

[0012]

The present inventor has conducted research to provide a 600 MPa class steel having low YR characteristics, and as a result, YR≤
In order to achieve 80%, it was found that the microstructure needs to have two phases of ferrite and bainite and a ferrite fraction of 25 to 75%. Then, in order to achieve TS ≧ 590 MPa under such a microstructure condition, C is increased as a component of steel, and Cu, Ni, N are added.
Although the addition of b is necessary, these elements reduce the super high heat input welded joint toughness. The present invention has been completed by finding that the reduction of the toughness of the ultra-high heat input welded joint can be prevented by the oxide dispersion. The gist of the invention is as follows.

(1) In mass%, C: 0.1 to 0.2%, Si: 0.05 to 0.25%, Mn: 0.4 to 2.0%, P: 0.02% or less , S: 0.02% or less, Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Al: 0.005 to 0.04%, Ti: 0.005 to 0. Steel containing 03%, Nb: 0.005 to 0.05%, Ca: 0.0005 to 0.003%, and the balance being Fe and inevitable impurities, and having a circle equivalent diameter of 0 in the steel. 0.005-2.0μ
The number density of the oxide particles of m is 100 to 100 per unit area.
3000 pieces / mm ² contained, the composition of the oxide particles thereof contains at least Ca, Al, and O, and the elements other than O are contained in a mass ratio of Ca: 5% or more, Al: 5% or more, and A 600 MPa class steel excellent in low YR characteristics and super large heat input welded joint toughness, characterized in that the microstructure of the steel is two phases of ferrite and bainite and the ferrite fraction is 25% to 75%.

(2) The composition of the oxide particles contains at least Ca, Al, O, and S, and the elements other than O are mass ratio, Ca: 5% or more, Al: 5% or more, S: 1% A 600 MPa grade steel containing the above, and the balance consisting of other unavoidable impurities, which is excellent in low YR characteristics and super large heat input welded joint toughness according to the above (1).

(3) C: 0.1 to 0.2%, Si: 0.05 to 0.25%, Mn: 0.4 to 2.0%, P: 0.02% or less in mass% , S: 0.02% or less, Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Al: 0.005 to 0.04%, Ti: 0.005 to 0. 03%, Nb: 0.005 to 0.05%, Ca: 0.0005 to 0.003%, Mg: 0.002% or less, the balance being steel consisting of Fe and unavoidable impurities, and Equivalent circle diameter in steel 0.005-2.0μ
The number density of the oxide particles of m is 100 to 100 per unit area.
3000 pieces / mm ² contained, the composition of the oxide particles includes at least Ca, Al, Mg and O, and the elements except O are mass ratio, Ca: 5% or more, Al: 5% or more, Mg: Excellent in low YR characteristics and super large heat input welded joint toughness, characterized by containing 1% or more, and having a steel microstructure with two phases of ferrite and bainite and a ferrite fraction of 25% to 75%. 600 MPa grade steel.

(4) The composition of the oxide particles contains at least Ca, Al, Mg, O, and S, and the elements except O are mass ratio, Ca: 5% or more, Al: 5% or more, Mg: 1 MPa or more, S: 1% or more, and the balance consisting of other unavoidable impurities, 600 MPa class steel excellent in low YR characteristics and super large heat input welded joint toughness according to the above (3).

(5) It is characterized by containing one or more of V: 0.1% or less, Cr: 0.6% or less, and Mo: 0.6% or less in mass%. A 600 MPa grade steel excellent in the low YR characteristics and the super large heat input welded joint toughness according to any one of the above (1) to (4).

(6) Low YR characteristics and super-high heat input welding as set forth in any one of (1) to (5) above, characterized by containing B: 0.0005 to 0.003% in mass%. 600 MPa grade steel with excellent joint toughness.

(7) The oxide particles have a circle equivalent diameter of 0.
1 to 2.0 μm, characterized in that (1) to
A 600 MPa class steel excellent in low YR characteristics and super large heat input welded joint toughness according to any one of (6).

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The present inventors have studied, as a metallographic factor for improving the HAZ toughness, achieving 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 austenite grains at high temperature. The most effective method is to pin the austenite grain boundaries with dispersed particles and stop the movement of the grain boundaries. As one of the dispersed particles having such a function, it has been conventionally considered that Ti nitride and oxide are effective. However, as described above, the Ti nitride has a large proportion of solid solution at a high temperature of 1400 ° C. or higher, so that the pinning effect decreases. On the other hand, it is necessary to utilize oxides that are stable at high temperatures as pinning particles.

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 particle 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 various studies so as to increase the volume fraction of the oxide and to obtain an appropriate particle size.

One of the means for increasing the volume fraction of oxide is to increase the amount of oxygen. However, increasing the amount of oxygen causes a large amount of coarse oxide which is 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 a strong deoxidizing element, which has a small solubility product with oxygen. However, Al alone is not sufficient to sufficiently utilize oxygen, and a deoxidizing element stronger than Al is necessary, and it is important to utilize Ca that is generally used in the deoxidizing step of steel. Since Ca has a small solubility product with oxygen, it is possible to generate a larger amount of oxide than Al for the same amount of oxygen. As a result of an experiment using Ca as a deoxidizing element, the composition of the oxide particles produced in the steel contains 5% or more of Ca and 5% or more of Al, and thus the volume fraction of the oxide, that is, the oxidation. We have found that it is possible to increase the quantity. Based on this result, the composition of the oxide particles contained in the steel should be at least Ca, A
The mass ratio of the elements including l and O and excluding O was 5% or more for Ca and 5% or more for Al.

Using Mg together with Ca is also effective in producing a large number of oxides. Mg is Ca
Although not so effective, it is a deoxidizing element stronger than Al and has a small solubility product with oxygen. Therefore, Mg to C
By combining with a and using it for deoxidation, the number of oxides can be further increased. The inventors conducted an experiment using Ca as a deoxidizing element, and as a result, as a composition of oxide particles generated in steel, Ca was 5% or more and Al was 5% or more,
It has been found that inclusion of 1% or more of Mg makes it possible to further increase the volume fraction of oxide, that is, the amount of oxide. Based on this result, the composition of the oxide particles contained in the steel was determined to include at least Ca, Al, Mg, and O.
The mass ratio of elements excluding 5% or more was 5% or more for Ca, 5% or more for Al, and 1% or more for Mg.

Furthermore, the present inventors have found that the precipitation of sulfides such as CaS and MgS around oxides causes
They have found that it is possible to further increase the volume fraction by combining oxides and sulfides. Based on this result, the composition of the particles contained in the steel should be at least Ca, A
An element including 1, O, and S, except O, has a Ca content of 5
% Or more, Al is 5% or more, S is 1% or more, or at least Ca, Al, Mg, O, and S are included, and elements excluding O are 5% or more Ca and 5% or more Al in mass ratio. , Mg
Of 1% or more and S of 1% or more.

Next, the size of the oxide particles 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 diameter of one particle is larger. However, when the volume ratio of the particles is constant, the size of one oxide particle is large. It was thought that the smaller the value, the larger the number of particles and the greater the pinning effect. However, if the value is too small, the ratio of the particles existing at the grain boundary becomes smaller, so that the effect is considered to be 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 changed, it was found that the grain size of 0.005 to 2.0 μm was found for pinning. He found that the effect was great. Further, it has been found that the pinning force for stopping the movement of the austenite grain boundaries is stronger as the size of the dispersed particles is larger, and the particle size of 0.1 to 2.0 μm is particularly preferable among the particle diameters of 0.005 to 2.0 μm. We came to discover that it was effective. When it is less than 0.1 μm, the pinning effect gradually decreases, and when it is less than 0.005 μm, the pinning effect is hardly exhibited. Also,
Oxide particles larger than 2.0 μm have a pinning effect, but may be the starting point of brittle fracture, and are unsuitable for the properties of steel materials. From this result, the required particle size was set to 0.
005-2.0 μm, especially 0.1-2.0 μm
m.

Next, the number of pinning particles required for HAZ toughness was examined.

The larger the number of oxide particles, the finer the organizational unit.
The higher the number of particles, the higher the HAZ toughness, but
HAZ toughness required for steel materials is used and used
It varies depending on the welding method. Especially the required characteristics are strict
Large heat input welding is performed with high strength steel for shipbuilding
HAZ toughness required, for example, test temperature-
The absorption energy of 50 J or more should be satisfied at 40 ° C.
To achieve this, as shown in FIG.
The number of oxide particles of 2.0 μm is 100 / mm²More required
It was discovered that However, as the number of particles increases,
However, the effect of improving the toughness becomes smaller and the number of particles is increased more than necessary.
Increasing the number produces coarse particles that are detrimental to toughness.
Considering that the possibility is high, the upper limit of the number of particles is 30.
00 / mm ²Is appropriate.

The size and number of the oxide particles are measured, for example, in the following manner. An extraction replica is made from a steel plate that is a base material, and the replica is made with an electron microscope at 10000.
The size and the number of the oxides are measured by observing at least 20 fields of view and an observation area of 1000 μm ² or more. To measure the size, for example, the equivalent circle diameter is obtained based on a photograph of particles. At this time, the extracted replica collected from any portion from the surface layer portion to the central portion of the steel sheet may be used. Further, if the particles can be properly observed, the observation magnification may be lowered.

As a process for producing a steel material, the effect of the oxide is not affected even if it is normally rolled, controlled rolling, a combination of this and controlled cooling and tempering, and a combination of quenching and tempering.

Further, it will be described based on FIG. 2 that the HAZ toughness is improved by dispersing the oxide particles.

FIG. 2 is a diagram showing the relationship between P _HAZ and HAZ toughness. The solid line shows the oxide particles of 0.005 to 2.0 μm in number density per unit area of 100 to 3000 / mm.
The average value of the present invention steel containing ² and the dotted line shows the average value of the comparative steel containing oxide particles in a number density of less than 100 particles / mm ² . Note that P _HAZ is represented by the following formula. P _HAZ = 340% C + 42% Si + 13% Mn + 68
0% P + 1250% S + 130% Cr + 220% Mo +
220% V + 720% Nb-260% Al + 5600
(% N-% Ti14 / 48) + 3470% B

As shown in FIG. 2, the components were adjusted to achieve TS ≧ 590 MPa (increasing C, Cu, Ni,
For steels (with Nb added), the higher the P _HAZ , the lower the HAZ toughness. For example, the HAZ toughness satisfying the absorbed energy of 50 J or more at the test temperature of -40 ° C is a value of P _HAZ .
Even if it is achieved with the steel of the present invention in the range of to b, it cannot be achieved with the comparative steel. That is, it can be seen that the HAZ toughness is significantly improved by dispersing the oxide particles.

Next, YR characteristics and tensile strength (TS) of steel
I will describe.

As a property of steel, Y which is the object of the present invention.
In order to achieve R (YP / TS) ≦ 80% and TS ≧ 590 MPa, the cooling rate after the hot rolling is finished is 4 to 7.5 ° C.
/ Sec, preferably 5 to 6.5 [deg.] C./sec, it is necessary that the microstructure of the steel has a ferrite fraction of 25 to 75% in the two phases of ferrite and bainite.

FIG. 3A shows the relationship between the ferrite fraction and YR, and FIG. 3B shows the ferrite fraction and TS.
It is a figure which shows the relationship with.

In the steel of the present invention, as shown in FIG. 3 (a), YR decreases with an increase in the ferrite (α) fraction. Therefore, in order to achieve YR of 80% or less, the microstructure should be ferrite and bainite. And a ferrite fraction of 25% or more. Further, as shown in FIG. 3B, TS (MPa) also decreases as the ferrite (α) fraction increases, so TS590M
In order to achieve Pa or more, it is necessary to set the ferrite fraction to 75% or less. Therefore, in the present invention, the ferrite fraction is limited to 25 to 75% as the microstructure of steel.

Further, since the oxide serves as a ferrite nucleation site for ferrite transformation, a large amount of oxide is dispersed, so that ferrite is easily generated even if the cooling rate is increased. Increasing the cooling rate increases the strength of the bainite phase, resulting in low YR. The steel of the present invention is also advantageous in this respect, and HA
It is possible to achieve both Z toughness and low YR.

The ferrite fraction is an area ratio. The measurement is performed by corroding a steel material with nital, observing with an optical microscope, and taking a photograph to measure a mainly white phase fraction that can be distinguished from ferrite. For the fraction measurement, a point counting method is suitable in which a lattice line is drawn on the photograph and the number of lattices contained in the ferrite phase is counted. Further, a photographic magnification of 200 to 400 is suitable.

Further, the range of basic components of the present invention will be described.

C is an effective component for improving the strength of steel, and as is clear from the graph showing the relationship between YR and the amount of C in FIG. 4, when C is less than 0.1%, the ferrite fraction is 25% or more. Cannot be achieved, and YR: 80% or less cannot be achieved, so the lower limit is made 0.1%, and excessive addition significantly lowers the weldability and HAZ toughness of the steel material, so the upper limit is 0%. It was set to 0.2%.

Si is a component necessary for securing the strength of the base material, deoxidizing, etc., but the upper limit was set to 0.25% in order to prevent deterioration of toughness due to hardening of the HAZ.

Mn must be added in an amount of 0.4% or more as an effective component for securing the strength and toughness of the base metal, but the upper limit is 2.0% within the allowable range of the toughness and crackability of the welded portion. And

The smaller the content of P is, the more preferable 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 preferable it is. However, it takes a great amount of cost to reduce this industrially, so 0.02% was made the upper limit.

Cu is effective for improving the strength of the steel material and is required to be 0.1% or more, but if it exceeds 1.0%, the HAZ toughness is lowered, so 1.0% is made the upper limit. .

Ni is effective for improving the strength of the steel material and is required to be 0.1% or more. However, since an increase in the amount of Ni increases the manufacturing cost, 1.0% was made the upper limit.

Al is an important deoxidizing element, and the lower limit is set to 0.005%. Also, when Al is present in a large amount,
Since the surface quality of the slab deteriorates, the upper limit was made 0.04%.

Ti is added in an amount of 0.005% or more in order to combine with N to form Ti nitride. However, solid solution Ti
As the amount increases, the HAZ toughness decreases, so 0.03%
Was set as the upper limit.

Nb is an element effective for improving the strength of steel by improving the hardenability, and 0.0
It is necessary to be 05% or more, but in the HAZ part, an excessive addition significantly reduces the toughness, so 0.05% was made the upper limit.

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, 0.0
The upper limit was 03%.

Mg is an element that increases the number of oxides when used in combination with Ca for deoxidation. However,
Since excessive addition produces coarse inclusions, Mg is less than 0.1%.
Although it was 002% or less, preferably 0.0001 to
It is 0.002%.

V, Cr and Mo have the effect of improving the strength and toughness of steel similarly to Nb, but excessive addition in the HAZ part significantly reduces toughness, so 0.1% and 0.6% respectively. , 0.6% was made the upper limit.

B is an element that improves the hardenability and strength of steel, but if it is less than 0.0005%, a sufficient effect cannot be obtained, while if it exceeds 0.003%, the toughness decreases. Therefore, B was made 0.0005 to 0.003%.

[0056]

Example A 590 MPa class steel was trial-produced with the chemical composition shown in Table 1. 1 to 8 are steels of the present invention, and 9 to 20 are comparative steels. The trial steel is melted in a converter and deoxidized during vacuum degassing at RH. After being cast into a 280 mm thick slab by continuous casting, it was subjected to hot rolling and water cooling to produce a steel plate having a thickness of 50 mm. The obtained steel sheet was welded with 1-pass electroslag using a general-purpose welding material. Heat input is about 90
It is 0 kJ / cm.

Table 2 shows the composition of oxides, the number of particles, the conditions for hot rolling and water cooling of steel sheets, the base material characteristics, the ferrite fraction, and the HAZ toughness. The Charpy value for toughness evaluation is an average value of 6 tests performed at a test temperature of 0 ° C. at the fusion line portion.

[0058]

[Table 1]

[0059]

[Table 2]

As is clear from Table 2, the inventive steels 1 to 8 have excellent HAZ toughness with absorbed energy of 50 J or more and excellent low Y of 80% or less as compared with the comparative steel.
It can be seen that it has R characteristics. That is, the particle size is 0.
The number of particles of the oxide having a chemical composition within the range of the present invention is 005 to 2.0 μm, and is in the range of 100 to 3000 particles / mm ² , and the HAZ toughness at 0 ° C. and the low YR characteristics are extremely excellent.

On the other hand, in Comparative Examples 9 to 20, HAZ toughness in the Charpy test shows low toughness of less than 40 J, YR exceeds 80%, and strength (TS) is 590 M.
It did not reach Pa. Nos. 9 to 16 had inferior HAZ toughness because the oxide composition or the number of oxides was out of the range of the present invention. In Nos. 17 and 18, the ferrite fraction was out of the range of the present invention, so that the YR exceeded 80%. In Nos. 19 and 20, the ferrite fraction was out of the range of the present invention, so that the strength was insufficient.

[0062]

INDUSTRIAL APPLICABILITY The present invention can provide a steel sheet having a weld joint toughness of YR ≦ 80% and TS ≧ 590 MPa, which is excellent even for large heat input welding of 500 kJ / cm or more and super large heat input welding. The effect brought to the industrial field where manufacturing of structures and the like by welding is required is extremely large, and the contribution to society is also very large because of the safety of the structures.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a relationship between P _HAZ and HAZ toughness.

3A is a diagram showing a relationship between a ferrite fraction and YR, and FIG. 3B is a diagram showing a relationship between a ferrite fraction and TS.

FIG. 4 is a diagram showing a relationship between YR and C amount.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yukihiro Chiba No. 1 Nishinoshu, Oita-shi, Oita Steel Co., Ltd. (56) Reference JP-A-11-293382 (JP, A) Kaihei 10-183295 (JP, A) JP 5-163516 (JP, A) JP 8-100234 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38 / 00-38/60 C21C 7/00-7/10

Claims (7)

(57) [Claims] 1. In mass%, C: 0.1 to 0.2%, Si: 0.05 to 0.25%, Mn: 0.4 to 2.0%, P: 0.02% or less, S: 0.02% or less, Cu: 0.1-1.0%, Ni: 0.1-1.0%, Al: 0.005-0.04%, Ti: 0.005-0.03 %, Nb: 0.005 to 0.05%, Ca: 0.0005 to 0.003%, the balance being Fe and unavoidable impurities, and the steel having a circle equivalent diameter of 0. 005-2.0μ
The number density of the oxide particles of m is 100 to 100 per unit area.
3000 pieces / mm ² contained, the composition of the oxide particles thereof contains at least Ca, Al, and O, and the elements other than O are contained in a mass ratio of Ca: 5% or more, Al: 5% or more, and A 600 MPa class steel excellent in low YR characteristics and super large heat input welded joint toughness, characterized in that the microstructure of the steel is two phases of ferrite and bainite and the ferrite fraction is 25% to 75%. 2. The composition of the oxide particles is at least C
A mass ratio of elements including a, Al, O, and S, excluding O: Ca: 5% or more, Al: 5% or more, S: 1% or more, and the balance being other unavoidable impurities. A 600 MPa class steel excellent in low YR characteristics and ultra-high heat input welded joint toughness according to claim 1. 3. In mass%, C: 0.1 to 0.2%, Si: 0.05 to 0.25%, Mn: 0.4 to 2.0%, P: 0.02% or less, S: 0.02% or less, Cu: 0.1-1.0%, Ni: 0.1-1.0%, Al: 0.005-0.04%, Ti: 0.005-0.03 %, Nb: 0.005 to 0.05%, Ca: 0.0005 to 0.003%, Mg: 0.002% or less, and the balance being Fe and unavoidable impurities. 0.005-2.0μ in equivalent circle diameter
The number density of the oxide particles of m is 100 to 100 per unit area.
3000 pieces / mm ² contained, the composition of the oxide particles includes at least Ca, Al, Mg and O, and the elements except O are mass ratio, Ca: 5% or more, Al: 5% or more, Mg: Excellent in low YR characteristics and super large heat input welded joint toughness, characterized by containing 1% or more, and having a steel microstructure with two phases of ferrite and bainite and a ferrite fraction of 25% to 75%. 600 MPa grade steel. 4. The composition of the oxide particles is at least C.
A mass ratio of elements including a, Al, Mg, O, and S, excluding O: Ca: 5% or more, Al: 5% or more, Mg: 1% or more, S: 1% or more, and the balance Is composed of other unavoidable impurities, and the 600 MPa class steel excellent in low YR characteristics and super large heat input welded joint toughness according to claim 3. 5. In mass%, V: 0.1% or less, Cr: 0.6% or less, Mo: 0.6% or less, and one or more kinds are contained. A 600 MPa grade steel excellent in the low YR characteristics and the super large heat input welded joint toughness according to any one of claims 1 to 4. 6. Low YR characteristics and super large heat input welded joint toughness according to any one of claims 1 to 5, characterized in that B: 0.0005 to 0.003% by mass% is contained. Excellent 600 MPa class steel. 7. The oxide particles have a circle equivalent diameter of 0.1 to 0.1.
It is 2.0 micrometers, Claims 1-6 characterized by the above-mentioned.
600 MPa class steel excellent in low YR characteristics and super large heat input welded joint toughness according to any one of 1.