JP2002371338A - Steel with excellent toughness in laser welds - Google Patents

Abstract

(57) [Summary] The present invention has a tensile strength of 400 MPa to 490.
An object of the present invention is to provide a steel excellent in the toughness of a laser welded portion, which is used for all structures requiring toughness, mainly in the MPa class. SOLUTION: In mass%, C: 0.01 to 0.1%, S
i: 0.05 to 0.5%, Mn: 0.1 to 1.5%,
P: 0.01% or less, S: 0.005% or less, Al:
0.005 to 0.06%, N: 0.001 to 0.01%
And, if necessary, Ti, Ca, Mg, C
u, Ni, Cr, Mo, W, V, Nb, Ta, Zr,
In a steel welded joint containing B, Y, and Ce and having a carbon equivalent represented by the formula (1) of 0.3% or less and a balance of Fe and unavoidable impurities, the heated austenite particle size of the weld heat affected zone is A steel excellent in toughness of a laser welded portion having a ferrite fraction of 100% or less and a ferrite fraction of 40% or more. Ceq. = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (V% + Mo% + Cr%) / 5 (1)

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a tensile strength of 400
The present invention relates to steel excellent in the toughness of a laser welded portion, which is provided for all structures requiring toughness, mainly in the MPa to 490 MPa class. For example, it can be used for welding structures such as building structures, marine structures, ships, bridges, and line pipes. Although the form of the steel is not particularly limited, the steel is used as a structural member, and is useful for a steel plate, particularly a thick plate, a steel pipe material, or a shaped steel that requires low-temperature toughness.

[0002]

2. Description of the Related Art Recently, laser welding, which can perform high-speed welding and is relatively easy to automate, is being applied to welding of a thick steel plate. However, conventional welded structural steel that guarantees the toughness of the welded portion is designed with components assuming that it is welded by the existing welding method centered on arc welding, and when welded by laser welding No consideration is given to toughness. Recently, steels with excellent laser weldability such as the soundness of laser welds and the cracking properties of HAZ have been actively developed, but steels that can guarantee the toughness of the welds have hardly been studied. Is the actual situation.

[0003] Recently, in Japanese Patent Application Laid-Open No. Hei 10-94890, by adjusting the ideal quenching critical diameter (DI value) of steel in accordance with laser welding conditions, the welded joint structure is made to have a lower bainite-based structure. A laser welding method for improving laser weld joint toughness is shown.
From the viewpoint of improving toughness, it is preferable to mainly use lower bainite as shown in this publication, but since the chemical composition range of the lower bainite structure is relatively narrow, and it is necessary to adjust according to welding conditions, It is complicated and there is a problem that the applicable range is limited.

[0004]

When compared at the same thickness, laser welding generally has a narrow groove width and a small amount of weld metal, so that the welding energy is lower than that of conventional arc welding. As a result, the cooling rate of the heat affected zone exposed to a high temperature in the cooling stage is high, and the HAZ has a hardened structure with a lower transformation point than in arc welding. In general, unlike arc welding, which is multi-layer welding, since it is a single-layer welding, it is not possible to expect the effect of refining the structure once formed by reheating the subsequent welding pass or tempering. Therefore, the HAZ structure formed by laser welding is
Unless the chemical composition is strictly adjusted according to the welding conditions as in JP-A-10-94890, coarse bainite or martensite or a mixed structure thereof is often formed. This is the same in the fusion zone.

[0005] Coarse bainite, especially upper bainite, must be avoided because of the large effective grain size and significant toughness degradation due to island martensite. As a means for improving the HAZ toughness in general without using the upper bainite structure without depending on the laser welding conditions, it is conceivable to use a martensite single phase or a fine-grain ferrite-based structure.

In the case of a single-phase martensite structure, generally, the toughness of martensite itself formed in the HAZ is also low, and in order to improve the toughness, means such as fine structure, low C, and addition of Ni are used. Need to be taken. In addition, in order to suppress the formation of upper bainite and obtain a martensite single phase structure, it is necessary to add an alloy element at a certain level or more, so that a high strength steel is inevitably obtained. Therefore, it is difficult to employ a means for forming a martensitic single-phase structure in a welded structural steel having a low alloy content and a tensile strength of mainly 400 to 490 MPa.

In view of the above, according to the present invention, the tensile strength is 4
For welding structural steels requiring low-temperature toughness, mainly in the class of 00 to 490 MPa, HAZ is generally used without depending on laser welding conditions, based on the premise that the laser welded structure is mainly composed of fine grain ferrite. It was an object to provide means for increasing toughness. It should be noted that the present invention is not limited by the strength level of the base material, and the effects of the present invention can be obtained regardless of the base material strength level within the component range of the present invention.

[0008]

Means for Solving the Problems The present inventors differ from the means disclosed in Japanese Patent Application Laid-Open No. Hei 10-94890 by adjusting the chemical composition in accordance with the welding conditions to form a lower bainite-based structure. On the premise that the structure of the welded joint was a ferrite-based structure, a means for improving the toughness of the ferrite-based structure was studied. As a result, as a heat history at the time of welding, the cooling rate is large, characterized by being a one-layer welding without a subsequent pass, in order to improve the toughness of the HAZ in laser welding, the generation of upper bainite is suppressed, In order to reduce the amount of the hard second phase which becomes the embrittlement phase as much as possible, it is necessary to reduce C and limit the carbon equivalent. It has been found that miniaturization is important.

Incidentally, in order to stably refine the heated austenite grains of the HAZ in laser welding, 13
Even when exposed to a high temperature of about 50 ° C. or higher, it is preferable that strong pinning particles capable of suppressing grain growth of austenite are dispersed at high density in the steel. It is effective to disperse thermally stable particles at a high density. The present invention has been invented based on new knowledge on the HAZ toughness controlling factors formed by the laser welding and development of means for achieving the same, and the requirements are as follows.

(1) In mass%, C: 0.01 to 0.1
%, Si: 0.05 to 0.5%, Mn: 0.1 to 1.5
%, P: 0.01% or less, S: 0.005% or less, A
l: 0.005 to 0.06%, N: 0.001 to 0.0
1% and a carbon equivalent (Ce) represented by the formula (1)
q. ) Is 0.3% or less, and when the steel consisting of the balance Fe and inevitable impurities is laser-welded, the heat-affected zone in the weld heat-affected zone has a heated austenite particle size of 100 μm or less;
And the ratio of ferrite in the structure of the heat affected zone is 40
% Steel with excellent toughness in laser welds, characterized in that it is at least 80%. Ceq. = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (V% + Mo% + Cr%) / 5 (1)

(2) Ti: 0.005 to 5% by mass
0.03%, Ca: 0.0005 to 0.003%, and oxide particles having a circle equivalent diameter of 0.005 to 2 μm in steel per unit area of 100 to 3000.
/ Mm ² , and the composition of the oxide is at least C
a, Al, O, and the element excluding O is mass%,
a) containing at least 5% and at least 5% of Al, wherein the total of Ca and Al is at least 50%.
A steel with excellent toughness in laser welds as described in section.

(3) The composition of the oxide particles contains at least Ca, Al, O, and S, and the element excluding O is mass%.
Wherein Ca: 5% or more, Al: 5% or more, S: 1% or more, wherein the total of Ca, Al and S is 50% or more. Steel with excellent toughness in laser welds.

(4) Ti: 0.005 to 5% by mass
0.03%, Ca: 0.0005 to 0.003%, M
g: contains 0.0001 to 0.002%, and contains 100 to 3000 particles / mm ² of oxide particles having an equivalent circle diameter of 0.005 to 2 μm per unit area in steel. The composition of the oxide is at least Ca, Al, Mg,
Element containing O and excluding O is in mass%, containing 5% or more of Ca, 5% or more of Al, and 1% or more of Mg, and the total of Ca, Al, and Mg is 50% or more. The steel excellent in toughness of a laser weld according to the above item (1), characterized in that:

(5) The composition of the oxide particles contains at least Ca, Al, Mg, O, and S, and the element excluding O is represented by mass%. Ca: 5% or more, Al: 5% or more, Mg: 1
%, S: 1% or more, Ca, Al, Mg, and S
The steel having excellent toughness of the laser weld as described in the above item (4), wherein the total of the steels is 50% or more.

(6) The laser-welded part according to any one of the above (1) to (5), wherein the ratio of ferrite in the melted part after laser welding is 40% or more. Steel with excellent toughness.

(7) Cu: 0.05-1.
5%, Ni: 0.05-3%, Cr: 0.05-1%,
Mo: 0.05-1%, W: 0.1-2%, V: 0.0
1 to 0.2%, Nb: 0.003 to 0.05%, Ta:
0.01-0.2%, Zr: 0.005-0.1%,
B: 0.0002 to 0.005%, one or more of the above-mentioned (1) to (6), wherein the steel having excellent toughness of the laser welded part. .

(8) Y: 0.001 to 0.
(1) to (7), wherein one or two of Ce, 0.005 to 0.1%, are contained.
A steel excellent in the toughness of a laser weld according to any one of the above items.

[0018]

Embodiments of the present invention will be described below in detail. In the present invention, it is essential to optimize the chemical composition at the same time as limiting the tissue requirements. Therefore, first, the reasons for limiting the chemical composition and its effects are described, and then, as an example of a means for satisfying the structure requirements, as an example of a means for satisfying the structure requirements, pinning into steel for suppressing the growth of heated austenite grains is described. Describe the requirements for particle dispersion.

C can be reduced to reduce the carbon equivalent so as to have a ferrite-based structure, and to reduce the agglomerates of island-like martensite and cementite, which become the embrittlement phase, and to reduce the HAZ toughness. Although preferred, it is also an essential element for ensuring the strength of steel. If it is less than 0.01%, it is difficult to secure the strength required for structural steel, so the lower limit is set to 0.
01%. On the other hand, assuming that the austenite grain size of the HAZ limited in the present invention is 100 μm or less, the upper limit in which the deterioration of toughness is allowable is set to 0.1% in the present invention based on experimental results.

Next, Si is an element effective as a deoxidizing element and for securing the strength of the base material. In order to make these effects clear, it is necessary to add 0.05% or more. On the other hand, an excessive content exceeding 0.5% forms a coarse oxide and causes deterioration of ductility and toughness. Therefore, the range of Si is set to 0.05 to 0.5%.

Mn is an element necessary for ensuring the strength and toughness of the base material, and must be contained at least 0.1% or more.
Since Mn tends to suppress the formation of ferrite and increase the proportion of coarse upper bainite, and to promote the formation of island martensite, the HAZ structure as in the present invention is mainly composed of fine-grain ferrite to improve toughness. If you plan to 1.
It is necessary to suppress it to 5% or less.

P is an impurity element and is harmful to the ductility and toughness of steel. Although it is preferable to reduce the amount as much as possible, in the present invention, the upper limit is set to 0.01% as an amount in which practically adverse effects can be tolerated.

S is an impurity element which has an adverse effect on the ductility characteristics, and is preferably reduced to 0.005% or less.
However, when it is contained in the oxide particles and exerts an effect on the refinement of the heated austenite grain size in the weld heat affected zone, it is added positively. In that case, 0.002-0.005%
Is preferred.

Al is an element which is effective for deoxidation, refining of the heated γ particle diameter of the base material, and the like.
It is necessary to contain at least 05%. On the other hand, if it is contained in excess of 0.06%, it has an adverse effect on the dispersion of fine oxides effective for refining the austenite by heating in the heat affected zone and forms a coarse oxide to deteriorate ductility. , 0.005% to 0.06%.

N is unfavorable in the present invention because it adversely affects ductility and toughness in a solid solution state.
In combination with Ti and Ti, etc., it works effectively for refinement of austenite grains and precipitation strengthening. Therefore, a small amount is effective for improving mechanical properties. Further, it is impossible to industrially completely remove N in steel, and it is not preferable to reduce N more than necessary because an excessive load is applied to a manufacturing process. Therefore, the lower limit is set to 0.001% as a range in which adverse effects on ductility and toughness can be tolerated, and industrially controllable, and a load on the manufacturing process can be tolerated. If it is contained excessively, solid solution N increases, which may adversely affect ductility and toughness. Therefore, the upper limit is set to 0.01% as an allowable range.

Ti is required to be properly added when the heat-affected zone (HAZ) of the laser welding heat-reduced austenite is refined by pinning with an oxide. Although the details will be described later, 0.005% or more is required to exhibit the effect. On the other hand, if the content exceeds 0.03%, coarse TiN or oxide may be formed.
Is limited to 0.005 to 0.03%.

Like Ti, Ca also has a fine dispersion of oxides of H.
When AZ is used for heat austenite miniaturization, it is an essential element. 0.0005% or more is required to exhibit the effect of reducing the heated austenite particle size. On the other hand, if the content exceeds 0.003%, coarse sulfides and oxides may be formed.
005 to 0.003%.

Mg is also effective in finely dispersing the oxide, and is added as necessary. If added, 0.00
01 to 0.002%, which is 0.000%
If the content is less than 1%, the effect is not clear, and if it exceeds 0.002%, the oxide may be coarse.

The above are the reasons for limiting the essential or important elements and impurity elements in the present invention. In the present invention, Cu, Ni, Cr, Mo, W, V, Nb, Ta, Z
One or more of r and B can be contained.

Cu is effective in enhancing hardenability and, in the case of performing tempering, increasing the strength by precipitation strengthening. In addition, it is an element that does not degrade toughness despite the increase in strength.
It is effective for improving the toughness balance. In order to exhibit the effect, addition of 0.05% or more is necessary, and addition of more than 1.5% causes a problem in hot workability and HAZ toughness. Limit to 1.5% range.

Ni enhances the hardenability and the strength. Further, it is the only element capable of improving toughness regardless of the structure by solid solution toughening. In order to exhibit the effect, addition of 0.05% or more is necessary. The toughness is improved as the Ni content is increased. However, the excessive addition of more than 3% saturates the effect but increases the hardenability, and the proportion of ferrite in the HAZ structure satisfies the present invention. Becomes difficult.
Therefore, in the present invention, the Ni content is set to 0.05 to 3%.

Cr is an element effective for improving strength by improving hardenability and solid solution strengthening.
5% or more is necessary, but excessive addition of Cr adversely affects the toughness of the base metal and HAZ through increase in hardness, formation of coarse precipitates, etc., so the upper limit is limited to 1% as an allowable range. I do.

Mo is also an element effective for increasing the strength by the same effect as Cr, but is limited to 0.05 to 1% as long as it can exert the effect and does not adversely affect other characteristics.

W is an element effective for increasing the strength by the same effect as Cr and Mo, but is limited to 0.1 to 2% as long as it can exhibit the effect and does not adversely affect other characteristics.

V mainly contributes to strengthening by precipitation strengthening. In order to exhibit the effect, 0.01% or more is necessary. However, if added in excess of 0.2%, the ductility and toughness will be extremely deteriorated. Therefore, in the present invention, the content is limited to the range of 0.01 to 0.2%.

Nb contributes to high strength in a trace amount by transformation strengthening and precipitation strengthening. Further, since it greatly affects the processing and recrystallization behavior of γ, it is also effective in improving the base material toughness. In order to exert the effect, 0.003% or more is necessary. However, if added in excess of 0.05%, the ductility and toughness are extremely deteriorated.
It is limited to the range of 0.003 to 0.05%.

Ta also has the same effect as Nb, and when added in an appropriate amount, contributes to improvement in strength and toughness.
If it is less than 0.2%, the effect will not be clearly produced, and if it is added in excess of 0.2%, the toughness deterioration due to coarse precipitates will be remarkable, so the range is 0.01 to 0.2%.

Zr is also an effective element for improving the strength, but is required to be 0.005% or more in order to exhibit the effect. On the other hand, if added in excess of 0.1%, coarse precipitates are formed and the toughness is adversely affected, so the upper limit is made 0.1%.

B is an element which enhances hardenability in a trace amount,
It is an effective element for increasing strength. B is hardened by segregating at the γ grain boundary in a solid solution state to enhance the hardenability. Therefore, even a very small amount is effective. This is not preferable because the effect of improving the properties is likely to be insufficient or the effect tends to vary. On the other hand, if added in excess of 0.005%, coarse precipitates are often formed during the production of steel slabs or during the reheating stage, so that the effect of improving hardenability becomes insufficient,
The risk of ductility deterioration and toughness deterioration due to cracks and precipitates in the billet also increases. Therefore, in the present invention,
The range of B is set to 0.0002 to 0.005%.

Further, in the present invention, the ductility is improved,
For the purpose of improving joint toughness, etc., one or two of Y and Ce can be contained as necessary. Both Y and Ce make the oxide finer to make the base material, HAZ ductility and HA
Effectively works to improve Z toughness. The lower limit contents for exhibiting the effect are as follows: Y is 0.001%, Ce is 0.005%.
It is. On the other hand, if it is contained excessively, sulfides and oxides are coarsened and ductility and toughness are deteriorated. Therefore, the upper limits are set to 0.01% for Y and 0.1% for Ce, respectively.

In the present invention, the carbon equivalent (Ceq.) Represented by the formula (1) is further limited to 0.3% or less. Ceq. = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (V% + Mo% + Cr%) / 5 (1) Ceq. Is limited to 0.3% or less, as described in the reason for limitation described below, even when the austenite grain size of the laser welded HAZ is reduced to 100 μm or less, the HAZ structure stably changes the ferrite ratio. This is because a ferrite-based structure of 40% or more is formed.

The above is the reason for limiting the chemical composition in the present invention.

In order to attain the good HAZ toughness of the laser-welded portion, which is the object of the present invention, the structure of the HAZ needs to be as follows on the premise that the above chemical composition is limited. That is, heated austenite particle size: 100 μm or less Ferrite fraction: 40% or more

The heated austenite particle size of 100 μm
The following is to guarantee the fineness of the transformed structure. That is, when the HAZ structure in laser welding, which is the object of the present invention, is a ferrite-based structure, it is the ferrite grain size that has a dominant effect on toughness,
In a heat history in which the cooling rate is relatively high, such as laser welding, if the heated austenite particle size is 100 μm or less, the average ferrite particle size will surely be about 20 μm or less, and the appearance of coarse grain boundary ferrite will also be suppressed. Na H
AZ toughness can be ensured.

The reason why the ferrite fraction is set to 40% or more is that the detrimental effect on the toughness of the upper bainite is suppressed to an acceptable level when the ferrite fraction is 40% or more. As a result, under the condition of 50% or more of ferrite, the influence of the chemical composition and the grain size of the heated austenite on the toughness can be reduced. The same applies to the fusion zone.

The above are the requirements regarding the chemical composition and the structure for improving the HAZ toughness of the laser welded joint in the present invention. As long as the organizational requirements and chemical composition requirements are satisfied, there is no limitation on the means of achieving them. For example, as means for refining the austenite grain size, as in the present invention described below,
In addition to the method of using oxides as pinning particles, a method of including a large amount of a nitride-forming element such as Ti or N may be considered.

However, it is not always sufficient as means for reducing the prior austenite grain size of the HAZ without deterioration of the material.

On the other hand, in the present invention, as a specific means for stably reducing the prior austenite grain size of HAZ to 200 μm or less without deterioration of the material, fine dispersion of pinning particles mainly composed of thermally stable oxides is used. Law.

In order to make the heated austenite grains of the HAZ finer, it is necessary to suppress the growth of austenite grains at a high temperature. The most effective method for this is a method in which the austenite grain boundaries are pinned by the dispersed particles to suppress the movement of the grain boundaries.

As one of the dispersed particles having such an action, Ti nitride has conventionally been considered to be effective. However, at a high temperature of 1400 ° C. or higher, the proportion of solid solution of Ti nitride increases, so that the pinning effect decreases.

On the other hand, it is effective to utilize oxides and sulfides that are stable at high temperatures as pinning particles. Further, the pinning effect of the crystal grain boundary by the dispersed particles is larger as the volume ratio of the dispersed particles is larger and the diameter of one particle is larger.

However, since the volume ratio of the dispersed particles has an upper limit depending on the concentration of the element constituting the particles contained in the steel,
Assuming that the volume ratio is constant, a smaller particle size is more effective for pinning.

One of the means for increasing the volume fraction of oxides and sulfides is to increase the amount of O and S. However, the increase of the amount of O and S reduces coarse inclusions harmful to the material. This is not an effective means because it also causes a large number of data to be generated. O
In order to increase the volume fraction of oxides and sulfides without increasing the amount and the amount of S, it is effective to use an element having a small solubility product with O and S.

Al has a small solubility product with O, that is, Al is generally used as a strong deoxidizing element. However, Al alone is not sufficient to sufficiently utilize O, and further requires a stronger deoxidizing element than Al. It is important to utilize Ca and, if necessary, Mg.

Mn is an element that easily forms sulfides. However, Mn alone is not enough to utilize S, and as in the oxides described above, it is important to utilize Ca and Mg, which have a small solubility product with S, that is, elements that generate stable sulfides. is there.

From the results of dissolution experiments using various deoxidizing elements such as Ca and Mg, the composition of the oxide particles generated in the steel was such that the element excluding O was in a mass ratio of 5% or more, excluding O. 5% or more of Al, preferably 40% in total
If it is included, or if it contains
% Or more, it was found that the volume fraction of the oxide, that is, the amount of the oxide can be increased.

Further, the fact that sulfides such as CaS and MgS are precipitated around the oxides enables the volume fraction to be further increased by combining the oxides and the sulfides. I found it. In that case, the composition when the oxide and sulfide are considered as one particle together,
In the case where Mg is not contained, the element except O is represented by mass ratio,
It is necessary that a and Al are contained in 5% or more and S is contained in 1% or more. When the particles contain Mg, the elements excluding O are in a mass ratio of 5% or more of Ca and Al, and 1% of Mg and S.
It needs to be included.

In addition, when S is contained in the oxide, when the oxide and the sulfide are complexed, and when the sulfide is precipitated around the oxide as the nucleus, the austenitic oxide is used. Has the same effect in suppressing the growth of. In the following, oxide or pinning particles
Unless otherwise specified, the above particles are included.

Next, the particle size effective for pinning the heated austenite grains of the HAZ will be described.

The pinning effect of the crystal grain boundary by the dispersed particles is larger as the volume ratio of the dispersed particles is larger and the diameter of one particle is larger. However, when the volume ratio of the particles is constant, the smaller the size of one particle is, Although the number of particles increases and the pinning effect increases, the effect decreases when the particle size is too small, because the proportion of the particles present at the grain boundaries decreases.

A detailed examination of the austenite grain size when heated to a high temperature using specimens having variously changed grain sizes showed that the austenite grain size was stable at a temperature and holding time corresponding to laser welding. It has been found that the effective size of the stable particles having the above-mentioned composition is 0.005 to 2 μm for pinning. Oxide particles smaller than 0.005 μm were hardly observed. From these results, the required particle diameter was set to 0.005 to 2 μm.

Next, the number of necessary pinning particles was examined. The greater the number of particles, the finer the texture unit,
Therefore, the HAZ structure becomes finer as the number of particles increases. HAZ in laser welding, object of the present invention
In order to ensure that the austenite particle size is 100 μm or less, 100 particles / mm ² or more having a particle size of 0.005 to 2 μm are required. However, as the number of particles increases, the pinning force increases.However, the effect is saturated, and increasing the number of particles more than necessary may generate coarse particles that are harmful to ductility and toughness. Will also increase. Considering that there is a limit in the current industrial technology, in the present invention, the upper limit of the number of particles is 3000 particles / mm.
^{And 2} .

The size and number of the oxide are measured, for example, in the following manner. An extraction 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 10,000 visual fields at a magnification of 10,000 with an electron microscope at an observation area of 1000 μm ² or more. 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.

In the present invention, the HAZ toughness of a laser welded joint is improved by optimizing the chemical composition and reducing the austenite particle size by heating with an oxide. The HAZ in the vicinity of the fusion line is exposed to a high temperature, and the matrix structure is almost completely eliminated. Therefore, in the present invention, a process for producing a steel material does not matter. As it is, hot rolling can be performed as it is by controlled rolling, controlled cooling, a combination of these processes and tempering, and further, reheating treatment such as normalizing, quenching and tempering.

The present invention aims to provide a high-strength steel having excellent laser welded joint toughness. However, the heat history applied to the steel sheet during welding can be considered to be similar to that of laser welding, that is, after welding, If the cooling rate is relatively high, specifically, if the cooling rate from 800 ° C. to 500 ° C. is 20 s or less and, in principle, one-pass welding does not receive heat from a subsequent welding pass, the welding method may be varied. The same effect is exhibited.

[0066]

Embodiments of the present invention will be described below. Tables 1 and 2 show the chemical compositions of the test steels used in this example. Steel number A1
A10 is the steel of the present invention, and B1 to B5 are comparative steels. Prototype steel is smelted by continuous casting or vacuum melting, cast into slabs or ingots, and then heated and rolled to a thickness of 12 m.
m. All the steel sheets were subjected to reheating quenching and tempering (QT). Quenching and tempering conditions were adjusted according to the chemical composition.

[0067]

[Table 1]

[0068]

[Table 2]

Tables 1 and 2 also show the composition of oxide particles in steel and the measurement results of the number of particles having a particle diameter of 0.005 to 2 μm. Note that, as described above, oxide particles mean that S
Particles containing oxides and sulfides,
It contains all particles in which oxides are nuclei and sulfides are precipitated around the oxides.

In laser welding, laser-cut or saw-cut I-shaped end faces were butt-welded without a root gap. Using a 10 kW CO ₂ laser welding machine, laser welding was performed at a welding speed of about 1.5 m / min.

Table 3 shows the results of an investigation of the tensile properties of the base metal, the HAZ after welding, and the morphology of the molten zone, and the respective properties. As the HAZ structure form, the heated austenite grain size and the ferrite fraction are shown, and as the molten part structure form, the ferrite fraction is shown. The heated austenite grain size and the ferrite fraction were measured from optical microstructure photographs. The heated austenite grain size indicates the maximum diameter in the HAZ within 1 mm on the HAZ side from the fusion line. HA from the fusion line
Magnification 200 photographed on the Z side 1 mm and near the center of the fusion zone
Magnified photographs are shown as an average of 5 visual fields.

As toughness, the tensile properties of the base material and the HAZ
And the 2 mm V notch Charpy impact characteristics of the fusion zone were investigated. The tensile properties of the base material were measured by sampling a round bar tensile test piece from the center of the sheet thickness in a direction perpendicular to the rolling direction. 2mm
The V-notch Charpy test piece has a notch position 1 mm on the HAZ side from the fusion line and the center of the fusion zone.
The test piece was a standard test piece having a thickness of 10 mm. The tensile test was performed at room temperature, the Charpy test was performed at various temperatures, and the fracture surface transition temperature (vTrs) and the absorbed energy at -40 ° C (vE-40) were determined.

[0073]

[Table 3]

In Tables 1 and 2, steel material numbers A1 to A10
Are manufactured according to the present invention and have steel material numbers B1 to B
No. 9 does not satisfy any of the requirements of the present invention. As is clear from the mechanical properties in Table 3, the steel materials Nos. A1 to A10 according to the present invention satisfy the requirements of the present invention in the microstructure of the HAZ, and all have good weld toughness (−4).
Absorbed energy and fracture surface transition temperature (vTr
It is clear that s)) has been achieved.

On the other hand, as can be seen from Table 3, the comparative examples of the steel materials Nos. B1 to B5 are significantly inferior in the weld toughness of the laser welded joint as compared with the present invention.

The reason why the HAZ toughness is inferior to the comparative example will be described below.

The steel material number B1 has a HAZ
Although the structure satisfies the present invention, the adverse effect of C on the toughness is remarkable, and the HAZ toughness is much lower than that of the present invention.

In steel material No. B2, since the carbon equivalent is excessive as compared with the present invention, the ferrite fraction in the HAZ and the molten portion is too small, and therefore, the bainite structure other than the ferrite phase becomes dominant in toughness, and The toughness is significantly inferior to the present invention.

In steel material No. B3, since the amount of Mn is excessive as compared with the present invention, the carbon equivalent is within the range of the present invention, but the ratio of the ferrite structure in the HAZ and the molten portion is not sufficiently ensured. As a result, weld toughness is significantly inferior to the present invention.

The steel materials Nos. B4 and B5 each had a heated austenite grain size of HAZ that was large and did not satisfy the present invention, and the HAZ toughness was significantly deteriorated.

From the above examples, it can be seen that the steel of the present invention is a relatively low alloy steel capable of making the HAZ structure in laser welding mainly ferrite, and has extremely excellent weld toughness in laser welding. It is clear.

[0082]

According to the present invention, the tensile strength is 400 to 49.
It is possible to improve the HAZ toughness of laser welded joints of relatively low alloy steel centering on 0 MPa class,
The industrial effects are very significant.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hiroyuki Shirahata 1 Nishinosu, Oita-shi, Oita Prefecture F-term in Nippon Steel Corporation Oita Works (reference) 4E068 DB01

Claims (8)

[Claims] C .: 0.01 to 0.1% by mass, S:
i: 0.05 to 0.5%, Mn: 0.1 to 1.5%,
P: 0.01% or less, S: 0.005% or less, Al:
0.005 to 0.06%, N: 0.001 to 0.01%
And the carbon equivalent (Ceq.) Represented by the formula (1)
Is 0.3% or less, and the ratio of ferrite in the weld heat-affected zone structure in the heat-affected zone of the welded heat-affected zone is 100 μm or less in the heat-affected zone when the steel consisting of the balance Fe and unavoidable impurities is laser-welded Steel having excellent toughness in a laser welded part, wherein the steel has a ratio of not less than 40%. Ceq. = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (V% + Mo% + Cr%) / 5 (1) 2. Ti: 0.005 to 0.03 by mass%
%, Ca: 0.0005 to 0.003%, and oxide particles having a circle equivalent diameter of 0.005 to 2 μm in steel per unit area of 100 to 3000 particles / m 2.
m ² , and the composition of the oxide is at least Ca, A
Element containing l and O, excluding O is mass%, and Ca: 5%
The steel having excellent toughness of a laser weld according to claim 1, wherein the steel contains 5% or more of Al, and the total of Ca and Al is 50% or more. 3. The composition of the oxide particles having at least C
elements containing a, Al, O, S and excluding O are mass%,
The laser welded part according to claim 2, wherein Ca: 5% or more, Al: 5% or more, S: 1% or more, and the total of Ca, Al, and S is 50% or more. Steel with excellent toughness. 4. Ti: 0.005 to 0.03 by mass%
%, Ca: 0.0005 to 0.003%, Mg: 0.0
001-0.002%, and 100-3000 / mm ² of oxide particles having a circle equivalent diameter of 0.005-2 μm per unit area in steel. Contains at least Ca, Al, Mg, O,
The element excluding O is mass%, Ca: 5% or more, Al: 5
%, And Mg: 1% or more, and the total of Ca, Al and Mg is 50% or more, The steel excellent in toughness of a laser welded part according to claim 1. 5. The composition of the oxide particles having at least C
a, containing Al, Mg, O, and S, and excluding O in mass%, containing Ca: 5% or more, Al: 5% or more, Mg: 1% or more, S: 1% or more; The steel with excellent toughness of a laser weld according to claim 4, wherein the total of Al, Mg and S is 50% or more. 6. The steel excellent in toughness of a laser-welded portion according to claim 1, wherein the ratio of ferrite in the molten portion when laser-welded is 40% or more. . 7. In mass%, Cu: 0.05-1.5%,
Ni: 0.05-3%, Cr: 0.05-1%, Mo:
0.05-1%, W: 0.1-2%, V: 0.01-
0.2%, Nb: 0.003-0.05%, Ta: 0.
01-0.2%, Zr: 0.005-0.1%, B:
The steel having excellent toughness of a laser weld according to any one of claims 1 to 6, comprising one or more of 0.0002 to 0.005%. 8. Y: 0.001 to 0.01 by mass%
%, Ce: 0.005 to 0.1%, one or two of them
The steel excellent in toughness of a laser weld according to any one of claims 1 to 7, wherein the steel contains a seed.