JP2002316283A - Method of manufacturing extra-low-carbon steel welded joint having excellent welded joint toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing an extra-low-carbon steel welded joint having excellent welded joint toughness by laser beam welding. SOLUTION: Steel products having a steel product composition containing 0.005 to 0.25% C, 0.1 to 0.50% Si, <=2.5% Mn, <=0.003% S, <=0.05% Al, 0.005 to 0.08% Ti, 0.0003 to 0.0060% B and <=0.0035% N and a steel product composition containing >=90% extra-low-carbon bainite and <=5% carbide are welded and joined by the laser beam welding by using inert gas containing 1 to 40 vol% oxygen supply gas as shielding gas. A fly wire may be used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 490 MPa tensile strength suitable for use in welded structures such as ships, storage tanks and pipelines.
The present invention relates to a method for welding a steel material for a welding structure having a pressure of 700 MPa or less, and more particularly to improvement of toughness in a laser beam welded joint. The steel material in the present invention includes a steel pipe, a steel plate, and a shaped steel.

[0002]

2. Description of the Related Art Welding using a laser beam is capable of obtaining a high energy density, so that high-speed welding with deep penetration is possible, and is expected as a highly efficient welding method. Laser output has been increasing year by year due to improvements in optical systems and reliability of control devices. For example, the available power of a carbon dioxide laser reaches as much as 50 kW, and it is possible to penetrate and weld several tens of mm thick steel. Recently, a high-output YAG laser is also commercially available, and its use as a laser welding machine is expanding due to the flexibility of beam transmission.

[0003] However, since welding using this laser beam is extremely localized, the amount of heat input per sheet thickness is significantly smaller than that of conventional arc welding.
This is what is called small heat input welding. For this reason, there is a problem that the cooling rate of the welded portion is increased, the weld metal is significantly hardened, and the weld metal toughness is often deteriorated. To solve such a problem, electron beam welding, which has a high cooling rate like laser beam welding, is disclosed in, for example,
Japanese Patent No. 64486 proposes a technique for reducing the hardenability of a weld by increasing the oxygen content of steel and reducing the hardenability of the steel. However, an increase in the oxygen content in the steel may significantly deteriorate the base metal toughness, which is not a practical improvement measure.

Japanese Patent Application Laid-Open No. 282446/1990 discloses a yield strength of 60 kgf / mm ² excellent in weldability in electron beam welding capable of deep penetration welding as in laser beam welding.
(588MPa) or higher high-strength steel has been proposed. JP 2
In the technique described in -282446, the hardenability of the steel material is adjusted so that a lower bainite or a mixed structure of lower bainite and martensite is easily generated, instead of the upper bainite structure that easily causes toughness degradation. It is stated that the toughness of the electron beam weld is dramatically improved. However, in the technique described in Japanese Patent Application Laid-Open No. 2-282446, the target steel is limited to high-strength steel having a tensile strength of 686 MPa or more.

On the other hand, the tensile strength targeted by the present invention is
When high-speed welding such as laser beam welding is applied to general-purpose commercial steel of 490 to 700 MPa, upper martensite containing hard martensite and coarse carbides or island-like martensite is formed in the weld. It is not easy to prevent the occurrence of remarkable hardening of the weld metal and deterioration of toughness.

Japanese Patent Application Laid-Open No. 8-155658 discloses that Ti:
When performing high energy density beam welding of steel containing 0.003 to 0.06% and sol.Al: 0.001 to 0.015%, by mixing an appropriate amount of oxygen with the shielding gas,
A welding method has been proposed in which fine oxides of Al and Ti are dispersed in a weld metal, thereby making the structure of the weld metal finer and improving the toughness of a welded portion.

[0007] However, in the technique described in Japanese Patent Application Laid-Open No. 8-155658, from the viewpoint of securing the strength of the steel material, the target steel material is limited to a low alloy high tensile strength steel having a C content of 0.02 mass% or more. I have. According to the technique described in Japanese Patent Application Laid-Open No. 8-155658, the structure of the weld metal is refined and the toughness is increased, but there is no effect on the toughness of the weld heat affected zone (HAZ). There is.

[0008]

The present invention advantageously solves the above-mentioned problems of the prior art, and has a tensile strength of 490 to 700 MPa.
It is an object of the present invention to propose a method of manufacturing an ultra-low carbon steel welded joint capable of obtaining excellent welded joint toughness in a method of manufacturing a welded joint by welding a steel material by laser beam welding.

[0009]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have developed a welded joint obtained by welding steel materials by laser beam welding having a low welding heat input and a high cooling rate. Various factors affecting toughness were studied diligently. Hitherto, it has been reported that the structure having the best low-temperature toughness is acicular ferrite (for example, the structure and toughness of a weld, Yukihiko Horii: No. 12).
8th Nishiyama Memorial Lecture, etc.).

The present inventors have paid attention to acicular ferrite, and have reached the idea that, even in laser welding, if the weld metal structure can be made mainly of acicular ferrite, it is possible to increase the toughness of the weld metal. We have:
Furthermore, as a result of the research, the steel composition containing C as the material to be welded was reduced to an extremely low carbon region, and further contained B and Ti within an appropriate range.
By using a steel material with a steel structure mainly composed of an ultra-low carbon bainite phase containing only a very small amount of carbide, the toughness of the weld joint is improved by adjusting the shielding gas composition of laser beam welding. Was found to be achieved. Specifically, the shielding gas for laser beam welding is made an inert gas containing an appropriate amount of oxygen supply gas, and oxygen is supplied into the welding metal from the shielding gas, thereby oxidizing Ti and the like in the welding metal to obtain Ti. Precipitates and disperses in the weld metal as a composite oxide, thereby enabling the weld metal structure to have a fine structure mainly composed of an acicular ferrite phase rich in toughness. It has been found that by reducing the amount, deterioration of HAZ toughness can be suppressed.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows (1) to (7). (1) In mass%, C: 0.005 to 0.025%, Si: 0.1 to 0.
5%, Mn: 2.5% or less, S: 0.003% or less, Al: 0.05%
Hereinafter, Ti: 0.005 to 0.08%, B: 0.0003 to 0.0060%,
N: a steel material containing 0.0035% or less, preferably a steel material composition comprising the balance of Fe and unavoidable impurities, and a steel material having a steel structure having an area ratio of 90% or more of ultra-low carbon bainite and 5% or less of carbide, In producing a welded joint by laser beam welding, the shield gas of the laser beam welding is an inert gas containing 1 to 40 vol% oxygen supply gas, and has excellent weld metal toughness. Manufacturing method of ultra low carbon steel welded joint. (2) In (1), in addition to the steel composition, the steel material may further include, by mass%, Cu: 0.05 to 2.00% and Ni: 0.05 to
3.00%, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.08%
Below, V: one or two selected from 0.08% or less
A method for producing an ultra-low carbon steel welded joint comprising at least one kind. (3) In (1) or (2), the steel material may further contain, by mass%, Ca: 0.0005 to 0.0005 in addition to the steel material composition.
A method for producing a welded joint of ultra low carbon steel, comprising one or two selected from 40% and REM: 0.0020 to 0.0080%. (4) The method according to any one of (1) to (3), wherein the welded joint has a weld metal structure mainly composed of acicular ferrite. (5) The method for producing an ultra-low carbon steel welded joint according to any one of (1) to (4), wherein the laser beam welding is laser beam welding using a filler wire. (6) In (5), the filler wire may have a mass%
And C: 0.010 to 0.10%, Mn: 0.6 to 2.5%, Ti: 0.01
0-0.220%, B: 0.0020-0.0200%, with the balance being
A method for producing an ultra-low carbon welded joint having a composition comprising Fe and unavoidable impurities. (7) In (5) or (6), the following equation (1) is used: Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1) (where C, Mn, Cr, Mo, V, Cu, Ni: Ceq defined by the content of each element (% by mass) is 0.15 to 0.35 in mass%.
% Of a weld metal having a low carbon content.

The ultra-low carbon bainite in the present invention means quasi-polygonal α (αq), granular Zw-α
(Α _B ), bainitic ferrite (α ° _B ) and a mixed structure thereof. _{Incidentally, αq, α B, α °} B is "(pole)
Recent Research on Bainite Structure and Transformation Behavior of Low Carbon Steel "(Bainite Research Group, Basic Research Group of the Iron and Steel Institute of Japan, Final Report, published July 30, 1994:
It means the same organization as the organization described in Table 1) on page 8.

Further, the ultra-low carbon bainite according to the present invention,
The area ratio of the acicular ferrite and the carbide means an average value of the area ratio obtained by observation with an optical microscope or a scanning electron microscope.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, an ultra-low carbon steel welded joint is produced by welding and joining a steel having an appropriate steel composition and a steel structure by laser beam welding. First, the reasons for limiting the steel composition of the steel used in the present invention will be described. Hereinafter, mass% in the composition is simply expressed as%.

C: 0.005 to 0.025% C is an important element that governs the microstructure of the base metal of steel and the heat affected zone (HAZ). In the present invention, the base metal, HA
In the equilibrium state of both Z, the generation of cementite (one of carbides) is eliminated, and in the laser beam welded joint, H
In order to suppress the formation of hard martensite which deteriorates the toughness in the AZ structure, C is made extremely low carbon of 0.025% or less. By making C a very low carbon of 0.025% or less and adding an appropriate alloying element, the base metal structure can be made a very low carbon bainite structure without depending on the cooling rate, and a tensile strength of 490 MPa or more can be obtained. A steel material having relatively high strength can be obtained.

Further, by making C an extremely low carbon of 0.025% or less, the HAZ structure can be made of an extremely low carbon bainite having an extremely small amount of carbide even in laser beam welding at a high cooling rate. Further, in the weld metal of the laser beam weld joint, the C content of the steel material is also reduced to 0.025.
% Or less, the formation of hard martensite which degrades toughness is suppressed. Furthermore, the combination of ultra-low carbonization of this steel material and the use of an inert gas containing an oxygen supply gas as the shielding gas in laser beam welding described below mainly enables the structure of the weld metal to be mainly made of tough acicular ferrite. Thus, a weld metal having excellent toughness can be obtained, and a laser beam welded joint having excellent toughness can be obtained.

As the C content decreases, the solid solution C decreases, so that the grain boundary strength decreases, the grain boundary fracture easily occurs, and the toughness of the steel material decreases. Therefore, C is 0.005%
The above content is required. Therefore, in the present invention, C is limited to the range of 0.005 to 0.025%. In addition,
It is preferably at most 0.020%. Si: 0.1 to 0.5% Si
Is an element that acts as a deoxidizing agent and improves the toughness of the steel material. Such an effect becomes remarkable at a content of 0.1% or more. On the other hand, if the content exceeds 0.5%, since Si is a ferrite-forming element, the steel structure tends to change from an extremely low-carbon bainite structure to granular ferrite (polygonal ferrite (αp)), and the strength is reduced and the steel toughness is significantly deteriorated. May be. For this reason, Si is limited to 0.1-0.5%. Preferably, it is at most 0.3%.

Mn: 2.5% or less Mn is an element that greatly affects the continuous cooling transformation behavior of steel in an extremely low carbon region, and the lath (internal structure of bainite) formed is refined and has very small toughness. In order to obtain a bainite structure, the content is preferably 0.5% or more. On the other hand, if the content exceeds 2.5%, a structure having low toughness such as island-like martensite is formed even if the total C content is reduced. Therefore, Mn is limited to 2.5% or less. Preferably, the content is 0.8 to 1.8% from the viewpoint of strength and toughness.

S: 0.003% or less In laser beam welding, no filler metal is usually used, so that the weld metal has almost the same composition as the steel material. For this reason, in order to prevent the occurrence of solidification cracking during welding, it is necessary to make the steel composition a composition free of solidification cracking. S
Is an element that forms sulfides and reduces ductility and resistance to solidification cracking. It is desirable that S in steel materials be reduced as much as possible. In particular, by reducing the carbon to an extremely low level,
Since the solidus temperature approaches and solidification cracks easily occur,
Reduction of S becomes important. If S exceeds 0.003%, welding solidification cracks frequently occur in the final solidified portion of the weld metal during welding. Therefore, in the present invention, S is limited to 0.003% or less. Incidentally, the content is preferably 0.001% or less.

Al: 0.05% or less Al acts as a deoxidizing agent, but when added in a large amount, the concentration of Al ₂ O _{3 in} inclusions increases, and large cluster inclusions are formed. For this reason, Al was limited to 0.05% or less. Ti: 0.005 to 0.08% Ti has a large bonding force with N, and fixes N bonded to B as TiN to promote effective use of B. By the effective use of B, the structure of the steel material can be a structure mainly composed of fine ultra-low carbon bainite, and the strength and toughness of the steel material are increased. In addition, Ti precipitates as TiN, and has the effect of suppressing grain growth at high temperatures and improving toughness. Such an effect is observed at a content of 0.005% or more.

On the other hand, a content exceeding 0.08% not only increases insoluble precipitates, but also forms carbides in the steel material, lowers grain boundary strength and lowers toughness. For this reason, Ti was limited to 0.005 to 0.08%. Note that Ti also acts as a deoxidizing agent. In order to allow Ti to sufficiently act as a deoxidizing agent, it is preferable to contain Ti in an amount of 0.025% or more.

In addition, Ti contributes to increasing the toughness of the weld metal by laser beam welding. By performing laser beam welding in an inert gas shielding gas containing an oxygen supply gas, Ti contained in the steel material forms a complex oxide with other elements having a high affinity for oxygen in the weld metal. .
The composite oxide containing Ti effectively functions as a nucleation site for a fine structure such as acicular ferrite, and makes the weld metal structure a fine structure mainly composed of acicular ferrite. This results in a tough weld metal.

B: 0.0003% to 0.0060% B is an element for improving hardenability, suppresses the formation of soft ferrite and coarse acicular ferrite, and stably obtains a uniform and fine ultra-low carbon bainite phase. This is one of the important elements in the present invention. In order to make the steel microstructure a very low carbon bainite with high toughness, it is necessary to contain B at least 0.0003% or more. B also has the effect of suppressing the generation of grain boundary ferrite in the weld metal of laser beam welding performed in a shielding gas of an inert gas containing an oxygen supply gas, and can prevent the toughness of the weld metal from deteriorating. On the other hand, when the content exceeds 0.0060%, an Fe-B-based precipitate is generated, and thus the effect of increasing toughness cannot be obtained. Therefore, B is
The range is limited to 0.0003 to 0.0060%. Preferably,
0.0010 to 0.0025%.

N: 0.0035% or less N combines with B to form BN, thereby reducing effective B. Effective B
Since the reduction of the amount promotes the formation of ferrite and coarse upper bainite which are likely to be generated from the grain boundary, it is difficult to form a fine ultra-low carbon bainite having a high toughness. N contributes to the stabilization of austenite as solid solution N, promotes the formation of a microscopic hard phase such as island martensite, and deteriorates toughness. Therefore, in the present invention, N is reduced as much as possible. However, an extreme decrease in the N content causes an increase in manufacturing cost. Therefore, in the present invention, the allowable upper limit of N is set to 0.0035%.
And

The steel material used in the present invention preferably further contains the following components in addition to the above components. Cu: 0.05 to 2.00%, Ni: 0.05 to 3.00%, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.08% or less, V: 0.08% or less One or more of Cu, Ni, Cr, Mo, Nb, and V are all elements that increase the strength. If necessary, one or two of these elements may be used.
More than one species can be selected and contained.

Cu has the effect of increasing the strength and regulating the bainite transformation temperature. Steel structure,
In order to obtain a structure mainly composed of ultra-low carbon bainite having a high HAZ structure toughness, the content is preferably 0.05% or more.
On the other hand, when the content exceeds 2.00%, the concentration of C solid solution in the matrix is reduced, and island martensite is easily formed,
Deteriorates toughness. For this reason, Cu is preferably limited to the range of 0.05 to 2.00%.

Ni, like Cu, has the effect of increasing strength and regulating the bainite transformation temperature. The steel material structure and the HAZ structure are preferably contained at 0.05% or more in order to make the structure mainly composed of extremely low carbon bainite having high toughness. On the other hand, when the content exceeds 3.00%, the solid solution concentration of C in the matrix decreases, so that it becomes easy to form island-like martensite and the toughness is deteriorated. Therefore, Ni is 0.05
Preferably, it is limited to the range of ~ 3.00%.

Cr is an element that increases the strength of steel. However, if the content exceeds 1.0%, no further increase in strength is expected, and conversely, the toughness is deteriorated. Therefore, the content of Cr is preferably limited to 1.0% or less. In addition, more preferably 0.1 to
0.8%. Mo is an element that increases the strength of steel, but if it exceeds 1.0%, no further increase in strength can be expected, and conversely, toughness is degraded. For this reason, Mo is preferably limited to 1.0% or less. In addition, more preferably 0.1 to
0.8%.

V is an element which increases the strength by forming a solid solution or forming a carbide or a nitride. When V exceeds 0.08%, the tendency to form ferrite is increased. Therefore, V is preferably set to 0.08% or less. From the viewpoint of improving the strength, the content is more preferably 0.002% or more. Nb facilitates accumulation of austenite processing strain,
Promotes increased strength. In order to obtain such an effect, the content is preferably 0.01% or more. But 0.08%
If it is contained in excess of, carbides are formed, the grain boundary strength of the steel material is reduced, and the toughness is reduced. For this reason, Nb is preferably limited to 0.08% or less. In addition, more preferably, from the viewpoint of improving the strength toughness of the base material and improving the HAZ toughness, 0.01%
0 to 0.050%.

One or two of Ca: 0.0005 to 0.0040% and REM (rare earth element): 0.0020 to 0.0080% Ca and REM both have the effect of suppressing the coarsening of the weld heat affected zone. If necessary, one or two of Ca and REM can be contained. Both Ca and REM have the effect of forming inclusions (precipitates) and suppressing the coarsening of steel at high temperatures. Such an effect, Ca: 0.0005%
As described above, REM is recognized at a content of 0.0020% or more. on the other hand,
If the Ca content exceeds 0.0040% and the REM content exceeds 0.0080%, the inclusions become excessive and become cluster-like inclusions, which serve as starting points for fracture and rather adversely affect toughness. For this reason, Ca is 0.0005 to 0.0040%, and REM is 0.0020 to 0.0080%.
% Is preferable. REM is Sc,
It is preferable to use one or more of Y and elements from La of atomic number 57 to Lu of atomic number 71. Among them, Ce and / or La are preferred from the viewpoint of availability of materials.

The balance other than the above components is Fe and inevitable impurities. P: 0.005% as inevitable impurities
Below, O: 0.0035% or less is acceptable. Next, the reason for limiting the steel structure of the steel used in the present invention will be described.
The steel material used in the present invention has a structure containing, in addition to the above-described composition, an extremely low carbon bainite of 90% or more in area ratio and a carbide of 5% or less in area ratio. In the present invention, the area ratio in the structure means the area ratio occupied by each structure when observing the cross section of the steel material.

By containing ultra-low carbon bainite in an area ratio of 90% or more, a steel material excellent in both strength and toughness can be obtained. If the area ratio of the ultra-low carbon bainite is less than 90%, ferrite is formed and the strength is reduced, or coarse upper bainite and island martensite containing carbides are formed, and the toughness of the base material and the HAZ is deteriorated. Therefore, in the present invention,
90% area ratio of ultra-low carbon bainite in steel structure
Limited to the above.

The steel material of the present invention contains 5% or less of carbide in area ratio. If the amount of carbide exceeds 5%, the toughness of HAZ deteriorates. The toughness of steel and HAZ is also reduced by the hard phase contained in the matrix. The hard phase includes carbides, and the influence of such a hard phase increases as the difference in hardness between the matrix and the hard phase increases. Therefore, in order to obtain a steel material having good toughness, it is necessary to limit the amount of carbide, which is a hard phase. For this reason, the carbide was limited to 5% or less.

The above-mentioned phases other than the ultra-low carbon bainite and carbide, for example, granular and acicular ferrite, upper bainite, lower bainite, martensite, etc. are lower in hardness than carbides, so that If the total amount is less than 10% in area ratio, there is no particularly strong adverse effect on toughness. However, the content of island martensite is preferably set to 6% or less.

In the present invention, an ultra-low carbon steel material having the above-described steel material composition and steel structure is welded and joined by laser beam welding to produce an ultra-low carbon steel welded joint. As a beam source for laser beam welding used in the present invention, a carbon dioxide laser or a YAG laser is preferable, but not limited thereto. Any known beam source can be applied.

In the present invention, the shield gas in laser beam welding is an inert gas containing an oxygen supply gas.
Since the shielding gas contains an oxygen supply gas, oxygen is supplied to the weld metal during welding, Ti and the like supplied from steel are oxidized in the weld metal, and a composite oxide containing Ti is generated, and the weld metal is formed. Dispersed inside. These composite oxides act as nucleation sites for a fine structure such as acicular ferrite to make the structure of the weld metal a fine structure with rich toughness, thereby improving the toughness of the weld metal. Preferably, this fine structure is mainly composed of acicular ferrite, whereby the toughness of the weld metal is further improved.

Here, the structure mainly composed of the acicular ferrite refers to a structure in which the content of the acicular ferrite phase is 60% or more. The remaining structure of the weld metal other than the acicular ferrite phase is bainite or grain boundary ferrite. The content of the oxygen supply gas in the shield gas is 1 to 40 vol%. As the inert gas, He gas or Ar gas is preferable from the viewpoint of easy availability. If the oxygen supply gas content in the shielding gas is less than 1 vol%, remarkable refining of the weld metal structure cannot be obtained, while if it exceeds 40 vol%, severe oxidation occurs, causing defects such as bubbles and welding. Many grain boundary ferrites are generated in the metal, and the toughness of the weld metal deteriorates.
For this reason, the content of the oxygen supply gas in the shielding gas is set to 1 to 40 vol%.

The oxygen supply gas referred to in the present invention is a gas having a function of supplying oxygen to a weld metal during welding. As the oxygen feed gas, but CO ₂ gas is preferably inexpensive in terms of cost, there is no problem even replace some or all of the CO ₂ gas O ₂ gas. Further, in the present invention, in the above-described laser beam welding, laser beam welding may be performed using a filler wire.

When a filler wire is used, welding can be performed while filling the root gap, so that the groove angle can be increased and stable welding can be performed at high speed. Next, the reason for limiting the composition of the filler wire will be described. The filler wire suitable for the laser beam welding described above is, in mass%,
C: 0.010 to 0.10%, Mn: 0.6 to 2.5%, Ti: 0.010 to
It is a wire containing 0.220% and B: 0.0020 to 0.0200%, with the balance being Fe and unavoidable impurities. Hereinafter, the mass% in the composition is simply expressed as%.

C: 0.010% to 0.10% C is an element that increases the hardenability. In laser beam welding, the weld metal is rapidly cooled. Therefore, when the C content in the weld metal increases, martensite and bainite are formed. It tends to harden the weld metal and deteriorate the toughness. For this reason, it is preferable to reduce the C content in the filler wire as much as possible from the viewpoint of increasing the toughness of the weld metal. The steel material used in the production of a welded joint in the present invention is C:
Since it is an extremely low carbon steel material of 0.025% or less and the shielding gas contains an oxygen supply gas, C in the weld metal reacts with oxygen to become CO gas, and the C content in the weld metal decreases. Therefore, if the C content in the filler wire is 0.10% or less, the hardening of the weld metal and a decrease in toughness can be suppressed. However, when the C content in the filler wire is reduced to less than 0.010%, the grain boundary strength of the weld metal decreases, the grain boundary is likely to break, and the toughness of the weld metal decreases. For this reason, C in the filler wire is preferably limited to the range of 0.010 to 0.10%.

Mn: 0.6-2.5% Mn is an element that increases the hardenability, but when an oxygen supply gas is contained in the shield gas for laser beam welding, it is combined with oxygen and discharged from the weld metal as slag. Will be increased. Therefore, in order to ensure the strength of the weld metal, it is preferable that Mn be contained in the filler wire in an amount of 0.6% or more. On the other hand, if the content exceeds 2.5% in the filler wire, the formation of a low toughness structure such as island martensite cannot be avoided even if the C content is reduced. Therefore, it is preferable that Mn in the filler wire is limited to the range of 0.6 to 2.5%. In addition, more preferably, 0.8 to 1.8%
It is.

Ti: 0.010 to 0.220% Ti is an element that contributes to increasing the toughness of the weld metal in laser beam welding. In laser beam welding using an inert gas containing an oxygen supply gas as a shielding gas, Ti combines with oxygen together with an element having a strong affinity for oxygen to form a composite oxide. The Ti-containing composite oxide effectively acts as a nucleation site for a fine structure such as acicular ferrite, and has a function of forming a weld metal into a structure mainly composed of a fine acicular ferrite and a high toughness weld metal. Such an effect is remarkable when the content of Ti in the weld metal is 0.010% or more. On the other hand, when the content exceeds 0.220%, the effect is saturated and unnecessary precipitates are formed in the weld metal. Will increase. For this reason,
Preferably, Ti is limited to the range of 0.010 to 0.220%. It is to be noted that the entire amount of Ti in the filler wire does not always remain in the weld metal as a composite oxide, but may be discharged out of the system as slag.

B: 0.0020 to 0.0200% B is an element that segregates at the crystal grain boundaries and suppresses the generation of grain boundary ferrite, and greatly contributes to increasing the toughness of the weld metal.
In the case of the present invention in which oxygen is supplied from the shielding gas to form a composite oxide containing Ti, grain boundary ferrite is easily generated, and from the viewpoint of increasing the toughness of the weld metal, the addition of B is particularly effective. . The addition of B to the weld metal may be made by diluting only the steel material to be welded. However, in order to suppress the formation of grain boundary ferrite, the steel material must contain more than 0.0030% of B. Required. However, the B content exceeding 0.0200% lowers the toughness of the heat affected zone. Therefore, B is preferably supplied from a filler wire. If the B content in the filler wire is less than 0.0020%, it is difficult to suppress the formation of grain boundary ferrite.
If the content exceeds 0.0200%, B including B
Form compounds to reduce the toughness of the weld metal. Therefore, B in the filler wire is preferably limited to the range of 0.0020 to 0.0200%.

In order to further increase the toughness of the weld metal when a filler wire is used, the composition of the weld metal is expressed by the following equation (1): Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 ... (1) Here, C, Mn, Cr, Mo, V, Cu, Ni: Ceq defined by the content (% by mass) of each element is 0.15 to 0.35% by mass.
Therefore, it is preferable to adjust the composition of the filler wire to be used in accordance with the composition of the steel material to be welded and the welding conditions.

When the carbon equivalent Ceq of the weld metal exceeds 0.35%, in a weld having a high cooling rate such as laser welding, the weld metal is significantly hardened and the toughness is reduced. on the other hand,
If the carbon equivalent Ceq of the weld metal is less than 0.15%, coarse grain boundary ferrite is generated even in welding with a high cooling rate such as laser welding, and the toughness is reduced. For this reason, it is preferable that the carbon equivalent Ceq of the weld metal be in the range of 0.15 to 0.35%.

[0046]

Next, the effects of the present invention will be described in detail based on examples. (Example 1) Molten steel having the composition shown in Table 1 was melted in a converter, and then cast into a slab having a thickness of 310 mm by a continuous casting method. Then, these slabs were heated to 1120 ° C and then 980 ° C.
Hot rolling was performed so that the rolling reduction temperature was 750 to 850 ° C. in the following temperature ranges with a cumulative draft of 50% or more, and after hot rolling, air-cooled to obtain a 12 mm thick steel sheet.

With respect to the obtained steel sheet, a structure examination, a tensile test, and a Charpy impact test were performed to examine the steel material properties.
In the tensile test, a JIS No. 4 test piece was used in accordance with the provisions of JIS Z 2201, and in the Charpy impact test, a JIS No. 4 test piece was used in accordance with the provisions of JIS Z 2202. In the structure examination of the steel material, an optical microscope and a scanning electron microscope (SEM) were observed to determine the amount of carbide and the content ratio of the structure. For tissue, at least 5 and field of view observed with an optical microscope (400-fold), granular ferrite, pearlite, the area ratio of martensite measuring, .alpha.q rest, alpha _B, the area ratio of the ultra low carbon bainite phase of alpha DEG _B The average value of each visual field was determined, and the average value was defined as the content ratio of the extremely low carbon bainite phase. For carbide, after performing two-stage electrolytic etching, SEM photographs (1000 times) were randomly photographed in 5 fields of view, the area ratio of each field was determined, and the average value of each field was taken as the carbide content ratio. .

Further, the obtained steel sheet was subjected to bead-on-plate welding under the laser beam welding conditions shown in Table 2 to produce a welded joint. The laser welding machine used was a carbon dioxide laser welding machine with a maximum output of 50 kW, and was a horizontal penetration welding. In the laser beam welding, a shielding gas having a gas composition shown in Table 3 was used. The ductile-brittle fracture transition temperature (vTrs) of the weld metal was determined using a Charpy impact test specimen taken from the center of the weld metal and the HAZ of the obtained weld joint. It can be said that the lower the vTrs, the better the toughness of the welded joint. The Charpy impact test specimen has a phenomenon in which a crack escapes to the base material side during the Charpy impact test (Fracture
Pass Deviation) to prevent JIS No. 4 test piece
A three-notch specimen into which a side notch having a depth of 0.5 mm was introduced was used.

The structure of the weld metal was measured by observing a specimen subjected to nital etching using an optical microscope, and the content ratio of acicular ferrite was measured. The content ratio of the acicular ferrite was determined by observing at least five visual fields with an optical microscope (400 times), obtaining the area ratio of the acicular ferrite in each visual field, and taking the average value as the content ratio.

Table 3 shows the results.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

[Table 3]

In the example of the present invention, the toughness of the HAZ is excellent such that the fracture surface transition temperature vTrs is -30 ° C. or less.
rs is -50 ° C or less, indicating excellent weld joint toughness. On the other hand, in a comparative example outside the scope of the present invention,
There is no example in which both HAZ and weld metal show good toughness. (Example 2) Molten steel having the composition shown in Table 4 was melted in a converter, and then cast into a slab having a thickness of 310 mm by a continuous casting method. Then, these slabs were heated to 1120 ° C and then 980 ° C.
Hot rolling was performed so that the rolling reduction temperature was 750 to 850 ° C. in the following temperature ranges with a cumulative draft of 50% or more, and after hot rolling, air-cooled to obtain a 12 mm thick steel sheet.

The obtained steel sheet was subjected to a structure examination, a tensile test and a Charpy impact test in the same manner as in Example 1 to examine the steel material properties. In addition, the obtained steel sheet was subjected to laser beam welding under the laser beam welding conditions shown in Table 5 using a filler wire (1.2 mm φ) having the composition shown in Table 6 and an I-shaped butt groove having a root gap of 1.4 mm. Beam welding was performed to produce a welded joint. The feed rate of the filler wire was 8.0 mm / min. The laser welding machine used was a carbon dioxide laser welding machine with a maximum output of 50 kW, and was a horizontal penetration welding. The shielding gas in the laser beam welding was a shielding gas having a gas composition shown in Table 7.

The central portion of the weld metal of the obtained welded joint, HA
Example 1 using a Charpy impact test specimen taken from Z
Similarly, the transition temperature between ductile and brittle fracture surfaces (vTrs) of the weld metal
I asked. It can be said that the lower the vTrs, the better the toughness of the welded joint. The Charpy impact test specimen is
The phenomenon in which cracks escape to the base metal side during the Charpy impact test (Fr
In order to prevent acture pass deviation), a 3 notch test piece having a 0.5 mm deep side notch introduced into a JIS No. 4 test piece was used as in Example 1.

In the same manner as in Example 1, the structure of the weld metal was measured by observing a sample subjected to nital etching using an optical microscope, and the area ratio of acicular ferrite was measured. Table 7 shows the results.

[0058]

[Table 4]

[0059]

[Table 5]

[0060]

[Table 6]

[0061]

[Table 7]

In the example of the present invention, the toughness of the HAZ is excellent such that the fracture surface transition temperature vTrs is -30 ° C. or less, and the vT
rs is -50 ° C or less, indicating excellent weld joint toughness. On the other hand, in a comparative example outside the scope of the present invention,
There is no example in which both HAZ and weld metal show good toughness.

[0063]

As described in detail above, according to the present invention,
Even if laser beam welding is performed on steel materials with a tensile strength of 490 to 700 MPa, welded joints with excellent weld toughness can be manufactured,
High-efficiency welding can be performed stably, and it has a remarkable industrial effect.

Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (Reference) C22C 38/58 C22C 38/58 B23K 103: 04 B23K 103: 04 (72) Inventor Koichi Yasuda Chuo-ku, Chiba-shi, Chiba 1 Kawasakicho F-term in the Technical Research Institute, Kawasaki Steel Corporation (reference) 4E068 BA06 BD00 CJ06 DA00 DB01

Claims (7)

[Claims] C. 0.005 to 0.025%, Si: 0.1 to 0.5%, Mn: 2.5% or less, S: 0.003% or less, Al: 0.05% or less, Ti: 0.005 to 0.08%, B: Laser beam welding of steel composition containing 0.0003-0.0060%, N: 0.0035% or less, and steel composition containing 90% or more of ultra-low carbon bainite and 5% or less of carbide in area ratio by laser beam welding An ultra-low carbon steel having excellent toughness of a welded joint portion, wherein a shielding gas for the laser beam welding is an inert gas containing an oxygen supply gas of 1 to 40 vol% in joining and manufacturing a welded joint. Manufacturing method for welded joints. 2. The steel material further comprises, in mass%, Cu: 0.05 to 2.00%, Ni: 0.05 to 3.00%, in addition to the steel material composition.
Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.08% or less,
The method for producing an ultra-low carbon steel welded joint according to claim 1, wherein one or more selected from V: 0.08% or less are contained. 3. The steel material further comprises, in mass%, Ca: 0.0005 to 0.0040%, REM: 0.0020 to
The method for producing an ultra-low carbon steel welded joint according to claim 1 or 2, comprising one or two selected from 0.0080%. 4. The method according to claim 1, wherein the welded joint has a weld metal structure mainly composed of acicular ferrite. 5. The method for manufacturing an ultra-low carbon steel welded joint according to claim 1, wherein the laser beam welding is laser beam welding using a filler wire. 6. The filler wire contains, by mass%, C: 0.010 to 0.10%, Mn: 0.6 to 2.5%, Ti: 0.010 to 0.220%, B: 0.0020 to 0.0200%, with the balance being Fe and unavoidable. The method for producing an ultra-low carbon welded joint according to claim 5, wherein the joint has a composition composed of impurities. 7. A carbon equivalent Ceq defined by the following formula (1):
The method for producing an ultra-low carbon welded joint according to claim 5 or 6, wherein a weld metal having 0.15 to 0.35% by mass is formed. Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1) where C, Mn, Cr, Mo, V, Cu, Ni: content of each element (% by mass)