JP2002317241A - 600MPa grade steel with excellent weldability and weld joint toughness - Google Patents

Abstract

(57) [Summary] [PROBLEMS] Excellent HAZ even when welding with very large heat input
It has toughness and high hardness of HAZ part, and TS ≧ 59
A 600 MPa grade steel excellent in welding workability satisfying 0 MPa is provided. SOLUTION: A predetermined component satisfies the condition that P _cm is 0.21 or less, and the content particle diameter is 0.005 to 2.0 μm.
m, especially 0.1 to 2.0 μm, at least C
a, Al, and O, but excluding O, by weight, C
a: 5% or more, Al: 5% or more, and the balance is composed of other unavoidable impurities.
Pieces / mm ² , the hardness of the weld heat affected zone is 80% of the base metal
600 excellent in welding workability having the above hardness (Hv)
MPa grade steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a large heat input welded joint toughness suitable for use in a large building structure such as a bridge or a middle and high-rise building, and a high hardness (Hv) of a weld heat affected zone. The present invention relates to a 600 MPa class steel having a strength of YS ≧ 590 MPa and excellent in weldability.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for material properties of welding steel materials used for large building structures such as bridges and middle-rise buildings. Furthermore, when constructing such a structure, in order to promote the efficiency of welding, application of a large heat input welding method typified by flux-copper backing welding method, electrogas welding method, electroslag welding method, etc. Has been desired, and as with the toughness of the steel itself, HAZ
Demands on toughness are also increasing. However, if an attempt is made to improve the joint toughness with a steel component, the weld heat affected zone (H
Since the strength of the AZ part decreases, there is a demand for the development of steel that can satisfy both toughness and strength.

There have been many proposals focusing on the HAZ toughness of steel during large heat input welding.

[0004] 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 improving toughness is reduced. Therefore, it is difficult to achieve the recent strict requirements for steel material 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. This principle is based on the fact that Ti nitrides, MnS, etc., are precipitated while the temperature is lowered after welding, and fine ferrite is generated using the Ti oxides, which are stable even at the melting point of steel, as a site. The formation of harmful coarse ferrite is suppressed, and the deterioration of toughness can be prevented. However, the number of such Ti oxides dispersed in steel cannot be so large. This is due to coarsening and aggregation of Ti oxides. If the number of Ti oxides is to be increased, 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 the destruction of the structure and cause a decrease in toughness.
Therefore, in order to further improve the HAZ 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 Ti oxides in steel, there are many methods in which Ti is added to molten steel substantially containing no 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 or the like, 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.

To solve such a problem, Japanese Patent Laid-Open No. 6-29
No. 3937 and JP-A-6-293937, Ti formed by adding Al immediately after adding Ti
-A technology utilizing an Al composite oxide has been proposed. With this technology, it is possible to greatly improve the large heat input welding HAZ toughness.

[0010] As described above, the HAZ at the time of large heat input welding is used.
Various techniques for improving toughness have been proposed, but recently, in the construction industry, etc., it has been required to weld large-scale bridge structural materials using 600 MPa class steel.
0 kJ / cm or more, 1000 kJ / cm for large ones
Further increase in welding heat input is progressing,
In a MPa grade steel, HAZ toughness is improved and HA
At present, a 600 MPa class steel excellent in welding workability which can satisfy TS ≧ 590 MPa without lowering the strength of HAZ which is a property contradictory to Z toughness has not yet been proposed.

[0011]

SUMMARY OF THE INVENTION The present invention has excellent HAZ toughness and high hardness of the HAZ portion even when welding with a very large heat input equal to or more than the very large heat input as described above.
An object of the present invention is to provide a 600 MPa class steel excellent in welding workability and weld joint toughness satisfying S ≧ 590 MPa.

[0012]

As an index of weldability, the steel component P _cm shown in the following equation (1) is known. P _cm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + B * 5 ≦ 0.21 (1)

It is known that if the P _cm of the steel component is 0.21 or less, good weldability is obtained. To lower the P _cm in order to improve the weldability, the steel components C and Ni in the formula (1) may be reduced, but if these components are reduced, the strength (TS) of the HAZ decreases. Resulting in. Therefore, a large amount of Nb, which is an element unrelated to P _cm , is added and H
Although it is conceivable to improve the hardness of the AZ portion, the addition of a large amount of Nb will decrease the ultra-high heat input welded joint toughness. In other words, the toughness and strength of the HAZ are contradictory properties, and it is difficult to achieve both by simply adjusting the steel composition.

The present inventor has determined that the HAZ strength (TS ≧ 590M
Research was conducted on preventing the reduction of HAZ toughness while maintaining Pa) by adding Nb and V, and it was found that the HAZ toughness could be improved by dispersing an oxide in steel.

Further, it has been found that the hardness (Hv) of the HAZ portion must be 80% or more (160 to 180 Hv) of the hardness of the base material in order to prevent the steel material from breaking at the HAZ portion. For this purpose, it is necessary to add a large amount of Nb and V. However, it has been found that a decrease in the toughness of the ultra-high heat input welded joint due to the addition of Nb and V can be prevented by dispersing an oxide in the steel.

The present invention has been completed based on the above findings, and the gist of the invention is as follows.

(1) In mass%, C: 0.05 to 0.1
5%, Si: 0.1-0.5%, Mn: 1.0-2.0
%, P: 0.02% or less, S: 0.02% or less, Al:
0.005 to 0.04%, Ti: 0.005 to 0.03
%, Nb: 0.01 to 0.06%, V: 0.02 to 0.
0.6%, Ca: 0.0005-0.003%, P _cm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + B * 5 ≦
0.21 with the balance being Fe and unavoidable impurities, and 0.005 to 2.
0 μm oxide particles at a number density per unit area of 10
0 to 3000 particles / mm ^2, and the composition of the oxide particles contains at least Ca, Al and O, and the elements excluding O contain, by mass ratio, Ca: 5% or more and Al: 5% or more, respectively. 600 MPa excellent in welding workability, characterized in that the total of Ca and Al is 50% or more and the hardness of the weld heat affected zone has a hardness (Hv) of 80% or more of the base metal.
Grade steel.

(2) The composition of the oxide particles contains at least Ca, Al, O, and S, and the elements excluding O are represented by mass ratio of Ca: 5% or more, Al: 5% or more, S: 1% Each containing the above, the total of Ca and Al is 50% or more,
The 600 MPa class steel excellent in welding workability according to the above (1), wherein the balance is composed of other unavoidable impurities.

(3) In mass%, C: 0.05-0.1
5%, Si: 0.1-0.5%, Mn: 1.0-2.0
%, P: 0.02% or less, S: 0.02% or less, Al:
0.005 to 0.04%, Ti: 0.005 to 0.03
%, Nb: 0.01 to 0.06%, V: 0.02 to 0.
06%, Ca: 0.0005 to 0.003%, Mg:
0.002% or less, P _cm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + B * 5 ≦
0.21 with the balance being Fe and unavoidable impurities, and 0.005 to 2.
0 μm oxide particles at a number density per unit area of 10
0 to 3000 particles / mm ^2, and the composition of the oxide particles contains at least Ca, Al, Mg, and O, and the elements excluding O are in a mass ratio of Ca: 5% or more, Al: 5% or more, M
g: each containing 1% or more, the total of Ca and Al being 50% or more, and the hardness of the weld heat affected zone being 80% of the base metal
A 600 MPa grade steel excellent in welding workability, having the above hardness (Hv).

(4) The composition of the oxide particles contains at least Ca, Al, Mg, O, and S, and the elements excluding O are represented by mass ratio of Ca: 5% or more, Al: 5% or more, Mg: 1
% Or more and S: 1% or more, and the balance is made of other unavoidable impurities.

(5) In mass%, Cu: 1.0% or less,
Ni: 1.5% or less, Cr: 0.6% or less, Mo: 0.
600MPa grade steel excellent in welding workability according to any one of the above (1) to (4), characterized by containing one or more of 6% or less.

(6) B: 0.0005 to 5% by mass
(1) to (3), which contain 0.003%.
(5) 600 excellent in welding workability according to any of the above items.
MPa grade steel.

(7) The oxide particles have a circle equivalent diameter of 0.1.
(1) to (1) to 2.0 μm.
600 excellent in welding workability according to any one of the above items (6).
MPa grade steel.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The present inventors have found that the hardness (Hv) of HAZ is 8
0% or more and strength is TS ≧ 590 MPa 600
Nb and V were contained as steel components in order to obtain a MPa-grade steel. However, by including Nb and V, H
AZ toughness decreased. Then, the present inventors considered reheating austenite grain refinement in the HAZ region heated to 1400 ° C. or more as a metal structure factor for improving the HAZ toughness.
It has been studied to achieve this using oxides.

In order to reduce the size of the reheated austenite grains, it is necessary to suppress the growth of austenite grains at a high temperature. The most effective method is to pin the austenite grain boundaries with dispersed particles and stop the movement of the grain boundaries. Conventionally, Ti nitrides and oxides have been considered to be effective as one of the dispersed particles having such an effect. However, as described above, since the proportion of solid solution of Ti nitride at a high temperature of 1400 ° C. or higher increases, the pinning effect decreases. On the other hand, it is necessary to utilize oxides stable at high temperatures as pinning particles.

The effect of the dispersed particles on the pinning of the crystal grain boundaries increases as the volume ratio of the dispersed particles increases and as the diameter of one particle increases. However, since the volume fraction of dispersed particles has an upper limit depending on the concentration of the elements constituting the particles contained in the steel, if the volume fraction is assumed to be constant, a smaller particle diameter is more effective for pinning. . From such a viewpoint, the present inventors have conducted various studies to increase the volume fraction of the oxide and to obtain an appropriate particle size.

One way to increase the volume fraction of oxides is to increase the amount of oxygen. However, an increase in the amount of oxygen causes the formation of a large number of coarse oxides that are harmful to the material. It is not an effective means. Therefore, the present inventors have studied the use of an element having a small solubility product with oxygen in order to make maximum use of oxygen. Al is generally used as a material having a small solubility product with oxygen, that is, as a strongly deoxidizing element. However, Al alone is not enough to sufficiently utilize oxygen, and further requires a deoxidizing element stronger than Al. Therefore, it is important to utilize Ca which is widely used in the deoxidizing step of steel. Since Ca has a small solubility product with oxygen, Ca can produce 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 generated in the steel contained 5% or more of Ca and 5% or more of Al. It has been found that the physical quantity can be increased. Based on this result, the composition of the oxide particles contained in the steel was changed to at least Ca, A
The elements containing l and O and excluding O were made to have a mass ratio of Ca of 5% or more and Al of 5% or more.

The use of Mg together with Ca is also effective in producing a large number of oxides. Mg is Ca
Although not as effective as Al, it is a stronger deoxidizing element than Al and has a small solubility product with oxygen. Therefore, Mg is converted to C
By using it for deoxidation in combination with a, 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, Al was 5% or more,
It has been found that the inclusion of 1% or more of Mg makes it possible to further increase the volume fraction of the oxide, that is, the amount of the oxide. Based on this result, the composition of the oxide particles contained in the steel was changed to include at least Ca, Al, Mg, O
The elements excluding are set to 5% or more of Ca, 5% or more of Al, and 1% or more of Mg by mass ratio.

Furthermore, the present inventors have found that sulfides such as CaS and MgS precipitate around the oxide,
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 was changed to at least Ca, A
Elements containing l, O, and S, but excluding O,
% Or more, Al is 5% or more, S is 1% or more, or an element containing at least Ca, Al, Mg, O, and S and excluding O is 5% or more of Ca and 5% or more of Al by mass ratio. , Mg
Was set to 1% or more, and S was set to 1% or more.

Next, the size of the oxide particles effective for pinning will be described.

The effect of the dispersed particles on the pinning of the crystal grain boundaries increases as the volume fraction of the dispersed particles increases and as the diameter of one particle increases, but when the volume ratio of the particles is constant, the size of one oxide particle is reduced. It was considered that the smaller the number, the larger the number of particles and the larger the pinning effect, but the smaller the value, the smaller the proportion of the particles present at the grain boundaries, and thus considered that the effect was reduced. As a result of detailed investigation of the austenite particle size when heated to a high temperature using test pieces with variously changed particle sizes, pinning has a particle size of 0.005 to 2.0 μm. I found that the effect was great. Furthermore, it has been found that the pinning force for stopping the movement of the austenite grain boundary is stronger as the size of the dispersed particles is larger, and among the particle sizes of 0.005 to 2.0 μm, the size of the particles of 0.1 to 2.0 μm is particularly large. They have found that it is effective. When it is smaller than 0.1 μm, the pinning effect gradually decreases, and when it is smaller 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 a starting point of brittle fracture, and are therefore unsuitable in the properties of steel materials. Based on this result, the required particle size was set at 0.
005 to 2.0 μm, among which 0.1 to 2.0 μm
m.

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

The greater the number of oxide particles, the finer the texture unit
And HAZ toughness improves as the number of particles increases.
HAZ toughness required for steel materials is
It depends on the welding method. Particularly required characteristics
Large heat input welding with high strength shipbuilding steel
Required HAZ toughness, such as test temperature-
Satisfies absorption energy of 50 J or more at 40 ° C
First, as shown in FIG.
The number of oxide particles of 2.0 μm is 100 / mm^TwoNeed more
It was found that. However, as the number of particles increases,
However, the effect of improving toughness is reduced, and
Higher numbers produce coarser particles that are detrimental to toughness
Considering that the possibility increases, the upper limit of the number of particles is 30.
00 pieces / mm ^TwoIs appropriate.

The size and the number of the oxide particles are measured, for example, in the following manner. An extract replica is prepared from a steel sheet as a base material, and the size and the number of the oxide are measured by observing at least 20 visual fields at a magnification of 10000 and an observation area of 1000 μm ² or more with an electron microscope. In the measurement of the size, for example, a circle-equivalent diameter is obtained based on a photograph of a particle. 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. If the particles can be properly observed, the observation magnification may be reduced.

The effects of oxides are not affected by the process of producing steel materials, which is usually performed as-rolled, controlled-rolling, a combination thereof with controlled cooling and tempering, or a combination of quenching and tempering.

The fact that the HAZ toughness is improved by dispersing oxide particles in steel containing Nb and V will be described with reference to FIG.

FIG. 2 is a graph showing the relationship between P _HAZ and HAZ toughness.
0 / mm ² Comparative containing steel, and the solid line 0.1
1 shows a steel according to the invention containing about 500 particles / mm ^{2 of} 2.0 μm oxide particles.

As shown in FIG. 2, the HAZ toughness decreases as the added amount of Nb and V increases.
H that satisfies absorption energy of 50 J or more at 40 ° C.
AZ toughness can not be achieved with even the comparative steels have been achieved in the present invention steels in the range of 80 to 140 of P _HAZ. That is, it is understood that the HAZ toughness is greatly improved by dispersing the oxide particles.

Next, the hardness of the HAZ portion will be described.

FIG. 3 is a diagram showing the relationship between the hardness (Hv) and the strength (TS) of the HAZ portion of a 600 MPa class steel material.
A test piece was prepared by welding a 600 MPa class steel material at a heat input of 100 kJ / cm, a tensile test was performed, and the hardness (H
The relationship between v) and strength (TS) was determined.

As shown in FIG. 3, the hardness of the HAZ portion is 16
At 0 Hv, the TS became 590 MPa. Hardness is 1
If it is less than 60 Hv, the TS will be less than 590 MPa, and the desired TS ≧ 590 MPa in the present invention cannot be obtained.
A break occurred in the part.

Therefore, it was found that it was necessary to set the hardness to 80% or more of the hardness (200 Hv) of the base material in order not to break at the HAZ. For this reason, in the present invention, the hardness of the heat affected zone is defined as a hardness (Hv) of 80% or more of the base metal.

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

C is an effective component for improving the strength of steel, with the lower limit being 0.05%, and the excessive addition significantly reduces the weldability and HAZ toughness of the steel material, so the upper limit is 0.15%. %.

Si is required to be 0.1% or more, which is a component necessary for securing the strength of the base material and deoxidizing, but the upper limit is set to 0.5% in order to prevent the toughness from being reduced by the hardening of the HAZ.

Mn needs to be added in an amount of 1.0% or more as an effective component for securing the strength and toughness of the base material, but the upper limit is 2.0% in an allowable range of the toughness and cracking property of the welded portion. And

The smaller the content of P is, the more desirable it is. However, in order to reduce this industrially, a large cost is required. Therefore, the upper limit is set to 0.02%.

The lower the content of S is, the more desirable it is. However, in order to reduce this industrially, a large cost is required. Therefore, the upper limit is set to 0.02%.

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 is set to 0.04%.

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

Nb is an element effective for improving the hardness and strength of the steel by improving the hardenability, and 0.01% for achieving TS ≧ 590 MPa.
% Or more is necessary, but in the HAZ portion, an excessive addition significantly lowers toughness, so the upper limit was made 0.06%.

V, like Nb, is an effective element for improving the hardness and hardness of steel, and is added in an amount of 0.02% or more. However, V also reduces the toughness of the HAZ,
0.06% was made the upper limit.

Since the reduction in the toughness of the HAZ portion can be prevented by adding both Nb and V in combination rather than adding Nb alone, it was decided to add both Nb and V.

Ca must be added in an amount of 0.0005% or more in order to form a Ca-based oxide. However, excessive addition produces coarse inclusions,
The upper limit was 03%.

Mg is an element that increases the number of oxides when used in combination with Ca for deoxidation. However,
Excessive addition causes the formation of coarse inclusions, so that Mg is added in an amount of 0.1%.
002% or less, preferably from 0.0001 to
0.002%.

Although Cu is effective for improving the strength of the steel material, if it exceeds 1.0%, the HAZ toughness is reduced and P _cm is increased. Therefore, the upper limit is set to 1.0%. Preferably, it is 0.1 to 1.0%.

Although Ni is effective for improving the strength and toughness of the steel material, the upper limit is set to 1.0% because an increase in the Ni content increases P _cm and increases the production cost. Preferably, it is 0.1 to 1.0%.

Cr and Mo have the effect of improving the strength and toughness of the steel, but excessive addition of HAZ significantly reduces the toughness.
% As the upper limit.

B is an element which improves the hardenability of the steel and at the same time increases the strength. However, 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 set to 0.0005 to 0.003%.

[0060]

EXAMPLES A 590 MPa class steel was experimentally produced with the chemical components shown in Table 1. 1 to 7 are inventive steels, and 8 to 19 are comparative steels. The prototype steel is melted from a converter and deoxidized at RH during vacuum degassing. After casting into a 280 mm thick slab by continuous casting, it was manufactured as a steel sheet having a thickness of 50 mm through hot rolling and water cooling. The obtained steel sheet was subjected to one-pass electroslag welding using a general-purpose welding material. Heat input is about 900
kJ / cm.

Table 2 shows the composition of the oxide, the number of particles, the hardness of the steel sheet, the hardness of the HAZ portion, and the HAZ toughness. The Charpy value for toughness evaluation is an average value obtained by performing six tests at a test temperature of 0 ° C. at a fusion line site.

As is clear from Table 2, the inventive steels Nos. 1 to 7 have excellent HAZ toughness having an absorption energy of 50 J or more as compared with the comparative steel, and the hardness of the HAZ portion / the hardness of the base material is 80%. As described above, it can be seen that the joint has sufficient joint strength and excellent welding workability. That is, when the particle size is 0.
005-2 μm, the number of oxide particles having a chemical composition within the range of the present invention is 100-3000 particles / mm ² , and both HAZ toughness at 0 ° C. and weldability are extremely excellent.

On the other hand, Comparative Examples 8 to 19 show a low HAZ toughness of less than 40 J in the Charpy test, or the hardness of the HAZ portion / hardness of the base material does not reach 80%.
Nos. 8 to 15 are inferior in HAZ toughness because the composition of the oxide or the number of oxides is out of the range of the present invention. 16
In Nos. To 19, V was not added, so that the HAZ toughness was good, but the hardness of the HAZ portion / hardness of the base material did not reach 80%.

[0064]

[Table 1]

[0065]

[Table 2]

[0066]

According to the present invention, YR ≦ 80%, which has excellent weld joint toughness for large heat input welding and very large heat input welding, T
The steel plate of S ≧ 590 MPa can be provided, and the effect brought to the industrial field where large-scale building structures and the like are required to be manufactured by welding is extremely large, and further, the contribution to society is very large in terms of the safety of the structures.

[Brief description of the drawings]

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

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

FIG. 3 is a hardness (Hv) of a HAZ portion of a 600 MPa class steel material.
FIG. 6 is a diagram showing a relationship between the strength and the strength (TS).

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Tomohiko Hata Oita-shi, Ozai No. 1 Nippon Steel Corporation Oita Works (72) Inventor Chiba Yukihiro Oita-shi Oaza Nishi No. 1 New Japan Inside Oita Steel Works

Claims (7)

[Claims] 1. A mass% of C: 0.05 to 0.15%,
Si: 0.1 to 0.5%, Mn: 1.0 to 2.0%,
P: 0.02% or less, S: 0.02% or less, Al: 0.
005 to 0.04%, Ti: 0.005 to 0.03%,
Nb: 0.01 to 0.06%, V: 0.02 to 0.06
%, Ca: 0.0005 to 0.003%, P _cm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + B * 5 ≦
0.21 with the balance being Fe and unavoidable impurities, and 0.005 to 2.
0 μm oxide particles at a number density per unit area of 10
0 to 3000 particles / mm ^2, and the composition of the oxide particles contains at least Ca, Al and O, and the elements excluding O contain, by mass ratio, Ca: 5% or more and Al: 5% or more, respectively. 600 MPa excellent in welding workability, characterized in that the total of Ca and Al is 50% or more and the hardness of the weld heat affected zone has a hardness (Hv) of 80% or more of the base metal.
Grade steel. 2. The composition of the oxide particles having at least C
elements containing a, Al, O, S, and excluding O, by mass ratio,
2. The composition according to claim 1, further comprising Ca: 5% or more, Al: 5% or more, and S: 1% or more, wherein the total of Ca and Al is 50% or more, and the balance consists of other unavoidable impurities. 600MPa grade steel with excellent weldability. 3. C: 0.05 to 0.15% by mass%,
Si: 0.1 to 0.5%, Mn: 1.0 to 2.0%,
P: 0.02% or less, S: 0.02% or less, Al: 0.
005 to 0.04%, Ti: 0.005 to 0.03%,
Nb: 0.01 to 0.06%, V: 0.02 to 0.06
%, Ca: 0.0005 to 0.003%, Mg: 0.0
02% or less, P _cm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + B * 5 ≦
0.21 with the balance being Fe and unavoidable impurities, and 0.005 to 2.
0 μm oxide particles at a number density per unit area of 10
0 to 3000 particles / mm ^2, and the composition of the oxide particles contains at least Ca, Al, Mg, and O, and the elements excluding O are in a mass ratio of Ca: 5% or more, Al: 5% or more, M
g: each containing 1% or more, the total of Ca and Al being 50% or more, and the hardness of the weld heat affected zone being 80% of the base metal
A 600 MPa grade steel excellent in welding workability, having the above hardness (Hv). 4. The composition of the oxide particles having at least C
a, containing elements of Al, Mg, O, and S, and excluding O, containing, by mass ratio, 5% or more of Ca, 5% or more of Al, 1% or more of Mg, and 1% or more of S; 4. The welding workability according to claim 3, wherein the total of Ca and Al is 50% or more, and the balance is composed of other unavoidable impurities.
00MPa grade steel. 5. A mass% of Cu: 1.0% or less, Ni:
The method according to claim 1, wherein one or more of 1.5% or less, Cr: 0.6% or less, and Mo: 0.6% or less are contained. 600MPa grade steel with excellent weldability. 6. B: 0.0005 to 0.00% by mass%
The 600 MPa class steel excellent in welding workability according to any one of claims 1 to 5, which contains 3%. 7. The oxide particles having a circle equivalent diameter of 0.1 to
The thickness is 2.0 μm.
A 600 MPa class steel excellent in welding workability according to any one of the above.