JPH08155658A - Welding method for low alloy high tensile steel - Google Patents

Abstract

PURPOSE: To provide a welding method of the low alloy high tensile steel by the high energy density beam to improve the toughness of a weld zone. CONSTITUTION: In welding the low alloy high tensile steel by irradiation of the high energy density beam, the welding is achieved using the steel containing 0.003-0.06% Ti and 0.001-0.015% sol.Al by shielding the part to be welded by the atmosphere consisting of oxygen whose partial pressure is 10Pa-40kPa and the balance the inert gas. When the steel to be welded is a steel plate, the inert gas containing no oxygen may be used on the beam irradiating side, and the inert gas containing oxygen may be used on the opposite side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to high energy density beam welding in structures, machine parts, structural steel pipes, steel pipes for piping etc. Particularly, the present invention relates to a welding method capable of obtaining an excellent weld portion.

[0002]

2. Description of the Related Art As a demand for steel materials, there is an increase in strength for the purpose of reducing the weight of structures and the pursuit of economic efficiency therefor, and the environment in which they are used is becoming increasingly severe in recent years. For example, the development of energy such as oil and gas has begun to take place in the Arctic region and in extremely cold regions near it, and steel materials used for offshore structures and line pipes also have excellent toughness at low temperatures. Things are becoming required.

In the case of a steel material to be used by welding, various studies have been made to make the performance of the welded portion as close as possible to the performance of the base material, but the most severely required performance is toughness. One of the factors that largely controls this toughness is the cleanliness and structure of the weld metal.

Due to the progress of steelmaking technology in recent years, S in steel,
Impurity components that cause inclusions such as O, N, and P were significantly reduced, and the cleanliness of the low alloy steel was significantly improved.
On the other hand, a welding method that uses a high energy density beam, that is, an electron beam, a laser beam, or an ion beam has also been developed, and by irradiating these beams to the butt portion, the heat input can be reduced and fusion welding becomes possible. It was Since these beams can be irradiated in vacuum or in an inert gas, it is possible to prevent the molten metal from being contaminated from the atmosphere and maintain the same high cleanliness as the base material.

However, cleaning steel generally tends to cause coarsening of crystal grains at high temperatures. During welding, the temperature of the solidification from melting and the subsequent cooling is high for a short time, but the weld metal has a coarse-grained structure. Since coarse-grained steel has poor toughness, the toughness of the welded portion is not good in this state. Post-heat treatment after welding is a method of refining the structure, but it is not productive if the post-heat treatment is performed by heating with another heat source again while adopting the welding method with small heat input and small heat-affected zone. Then
In the case of a pressed material, the strength after heat treatment may not be secured in some cases.

As a method of refining the structure of the weld metal and improving the toughness, a method of containing Ti in the weld metal has been known in the submerged arc welding (for example, Journal of Welding Society, Vol. 44 (1975)). No. 10, p. 815) In this case, fine TiN precipitates are said to be effective for the refinement of the structure.

In the welding of high energy density beams, Japanese Patent Publication No. Sho 56-50793 discloses that a layer containing an oxide is previously formed on a portion of a steel material to be remelted during welding by means such as arc welding. , A method of refining the structure of weld metal is shown. However, this method requires additional steps in advance, which complicates the manufacturing process.

In Japanese Patent Publication No. 28474/1992, an oxide of Ti is uniformly dispersed in steel in advance, and a high energy density beam such as an electron beam or a laser beam is applied in a vacuum during welding. It has been proposed that by irradiating the steel in a highly clean environment such as in an inert gas atmosphere or in an inert gas atmosphere, fine precipitates are uniformly dispersed in the weld metal to refine the microstructure and improve the low temperature toughness. .

This method seems to easily obtain a weld having excellent performance when applying high energy density beam welding, but first, there is a problem in the production of steel. In order to obtain a sound slab with few defects in continuous casting, it is generally necessary to sufficiently deoxidize, and if deoxidation is insufficient, surface defects and internal defects due to inclusions tend to increase. Al is usually used for deoxidation, and if the deoxidation is sufficiently performed, acid-soluble Al (sol.Al) is present in the steel.
However, since the oxide of Ti is easily reduced to sol.Al, in order to make it remain in the steel, the addition of Al is limited, and on top of that, severe manufacturing conditions for reducing defects on the surface and inside are required. Need management.

Secondly, the oxygen level in the steel tends to increase due to the dispersion of the Ti oxide. If the oxygen level is high, coarse oxides are likely to be formed due to the aggregation and aggregation of oxides during solidification of molten steel, and the toughness of steel is likely to be deteriorated. That is, even if the toughness of the welded portion is excellent, there is a risk of a steel sheet having much poorer toughness of the base material.

[0011]

DISCLOSURE OF THE INVENTION The present invention greatly improves the toughness of a welded portion when welding a high-strength low-alloy steel with good cleanliness and high toughness by irradiation with a high energy density beam such as a laser. The purpose of the present invention is to provide a welding method that can be performed.

[0012]

The present inventor has taken advantage of the advantage of the high energy density beam welding method that the heat-affected zone is small for high-strength, low-alloy steel with good cleanliness, and is excellent without post-heat treatment. A method for improving the toughness of the weld metal was studied to obtain a welded joint. As a result, the following things became clear.

(A) In high energy density beam welding, if an appropriate amount of oxygen is mixed with the inert gas used for shielding the weld, the toughness of the weld is greatly improved.

(B) In this case, the toughness is further improved by controlling the Al and Ti contents appropriately.

(C) When the shield gas was mixed with oxygen and welded, the amount of oxygen increased when the weld metal was examined in detail, and fine Al oxide and Ti oxide were dispersed. It is considered that this refines the structure of the weld metal, and as a result, the toughness of the weld metal is improved.

(D) The gases covering the welded portions need not all contain the same amount of oxygen. For example, even if the inert gas on the side that irradiates the beam does not contain oxygen and oxygen is mixed into the gas on the back side, the amount of oxygen in the weld metal increases and the same effect can be obtained.

Particularly in the case of a laser, a plasma is generated by beam irradiation, which obstructs the transmission of the beam and makes the welding unstable, but the presence of oxygen facilitates the generation of this plasma. Therefore, in the case of butt welding of steel sheets, if an inert gas that does not contain oxygen is used on the side that irradiates the beam and a gas that contains oxygen is used on the side that does not irradiate, stable welding can be performed while supplying oxygen to the weld metal. It will be possible.

Since the electron beam and the ion beam can be generated and accelerated only in a vacuum, the member to be welded is usually placed in a vacuum container connected to the beam source.
Welding is performed by irradiating a beam in a state of being evacuated to a vacuum. Therefore, when using an electron beam or an ion beam, it is usually difficult to control the atmospheric gas component during welding.

On the other hand, while keeping the beam generating portion and its accelerating portion in vacuum, the beam is guided to another container under reduced pressure through a hole having a small diameter, and the member placed there is welded. Welding is possible while controlling the atmosphere inside. In this case, it is necessary to evacuate the beam generating and accelerating parts with a vacuum pump having a sufficient evacuation capacity, and constantly replenish the gas lost from the small diameter hole in the container in which welding is performed. By using such a differential evacuation mechanism, the welding atmosphere control in high energy beam welding can be applied to electron beams and ion beams. It was found that the same effect can be obtained by controlling the oxygen partial pressure in the welding atmosphere by such a method.

The present invention was made based on the above findings, and the gist of the present invention is "Ti: 0.003 to
When welding low alloy high strength steel containing 0.06% and sol.Al: 0.001 to 0.015% by irradiation with a high energy density beam, the welded part contains oxygen with a partial pressure of 10 Pa to 40 kPa and the rest is an inert gas. In the welding method performed while shielding in the atmosphere, when applying a laser beam, which is one of the high energy density beams, to the low alloy high strength steel plate containing the above components, "the beam irradiating side is an inert gas Partial pressure is 10 Pa on the side that is shielded and not irradiated.
This is a welding method in which the atmosphere is controlled to contain an oxygen of up to 40 kPa and the balance consists of an inert gas.

The welding method of the present invention uses Ti: 0.003 to 0.06.
% And sol.Al: 0.001 to 0.015% in a low alloy high strength steel, and there are no particular restrictions on other steel components. However, the welding method by which a weld having excellent toughness can be obtained should be applied to steel having high strength and high toughness as a matter of course.

Therefore, a desirable steel composition other than Ti and Al is C: 0.02 to 0.30% by weight, S is S by weight.
i: 0.01 to 0.80%, Mn: 0.10 to 2.0%, P: 0.030%
Below, S: 0.030% or less, N: 0.0100% or less, O: 0.00
10 to 0.0070% as a basic component, Ni: 5.0% or less, C
r: 1.0% or less, Mo: 0.5% or less, Nb: 0.15% or less, V: 0.15% or less, Cu: 1.50% or less, B: 0.0030%
Below, Ca: 0.007% or less, and rare earth element: 0.02%
Below, the balance is steel consisting of Fe and inevitable impurities.

[0023]

The chemical composition of the high-strength steel to which the present invention is applied, its desirable range, and the reasons for limiting the shield atmosphere conditions during welding will be described below.

(1) Ti Ti, in addition to increasing the strength of steel by precipitation strengthening,
It is also added to fix N and prevent deterioration of toughness. In the present invention, in particular, the weld metal is made to have an action of forming an oxide and refining the weld metal structure. Therefore, it is necessary to contain 0.003% or more. However, if the content exceeds 0.06%, the toughness of the base material deteriorates, so this value is made the upper limit.

(2) sol.Al Al is added for the purpose of deoxidizing steel, and is usually contained in the form of sol.Al in an amount of about 0.02%. However, in the present invention, the purpose is to oxidize almost all sol.Al in the weld metal portion.
When the amount of Al increases, the oxide of Al increases in the weld metal, the cleanliness decreases, and the toughness deteriorates. For this reason, in the present invention, sol.Al in the steel plate must be 0.015% or less.

On the other hand, when the steel is deoxidized and the sol.Al remains, oxidizing inclusions such as Ti oxides that deteriorate the toughness of the steel are substantially not included. For this, sol.A
Since it is sufficient to contain 0.001% as l, this value is the lower limit.

(3) The C content is controlled for the purpose of ensuring the strength of the steel, but if it is less than 0.02%, the required strength cannot be obtained. On the other hand, if it exceeds 0.30%, not only are weld cracks liable to occur, but also hardenability is excessively increased, resulting in a decrease in toughness of both the base material and the welded portion.
Therefore, the C content range is preferably 0.02% to 0.30%.

(4) Si Si is added to deoxidize the steel and secure the strength. However, if the content is too large, the toughness of both the base material and the welded part deteriorates, so 0.80% is made the upper limit. In addition, in the heat affected zone of welding and the like, the content may be reduced in order to suppress the formation of abnormal structure, but it is difficult to reduce the content to less than 0.01% by a normal production method. In addition, there is a risk of increasing oxides if reduced to below 0.01%. For this reason, the preferable Si content range is 0.01 to 0.80%.

(5) Mn Mn is a component contained to secure the strength of the steel material,
Determine the content while balancing with other ingredients for securing strength. However, if the content is less than 0.10%, not only is there no effect in securing strength, but there is a deterioration in toughness, hot work cracking, etc.
The effect of suppressing the adverse effect due to is lost. Further, if the content exceeds 2.00%, the toughness of both the base material and the joint is deteriorated.
Therefore, the desirable range is 0.10 to 2.00%.

(6) P P is an impurity, and the lower the content, the better. 0.030%
If it is contained in excess of 1.0, weld cracking will occur, so it is preferable to keep this value or less.

(7) S S is an impurity and becomes MnS-based inclusions due to the presence of Mn. However, toughness, HIC (hydrogen-induced cracking) resistance, and SS resistance are high.
Since the C (stress corrosion cracking) property is deteriorated, the lower the content, the better. When it exceeds 0.020%, the effect becomes remarkable, so less than this value is desirable.

(8) N N is an unavoidable impurity by nature, and generally, the lower the toughness, the better.

However, when Ti is present, TiN is formed, and TiN suppresses the coarsening of the structure of the weld metal and the weld heat affected zone, and sometimes becomes a welded joint having excellent toughness. However, if the content of N exceeds 0.010%, the toughness deteriorates due to the increase of coarse TiN and solid solution N.
0.010% or less is preferable.

(9) O (oxygen) O is mostly Al _{2 in} fully deoxidized steel.
It is in the form of O ₃ -based oxidation inclusions. If such a large amount of inclusions is contained, the toughness is deteriorated. As a limit where no noticeable effect appears,
0.007% or less is preferable. Also, if it is extremely lowered, the weld metal may not be refined, so at least 0.001
It is desirable that the content be at least%.

(10) Cr, Mo, Cu, Ni, Nb and V It is not necessary to add these elements, but if they are added in appropriate amounts as required and combined with hot rolling conditions, toughness is impaired. Without increasing the strength of the steel. To obtain the effect, 0.01% Cr and M, respectively
0.01% for o, 0.01% for Cu, 0.01% for Ni, 0.00 for Nb
It is desirable to contain one or more of 2% and 0.002% of V.

If any element is added excessively,
Since the strength of steel is excessively increased and the toughness is impaired, the upper limit of each is 1.0% for Cr, 0.5% for Mo, 1.5% for Cu,
It is preferable that Ni is 5.0%, Nb is 0.15%, and V is 0.15%.

(11) Ca and rare earth elements These elements may not be added, but since they have the effect of changing the morphology of the sulfide-based inclusions in the steel, they exhibit toughness and HIC resistance.
Added as needed for the purpose of improving the properties and SSC resistance.

In order to bring out the effect of the addition, Ca is 0.
It is desirable to contain 002% or more, and 0.001% or more for rare earth elements. However, on the other hand, since these elements are inclusion forming elements, if added excessively, the cleanliness is deteriorated and the toughness is deteriorated.
It is preferably 007% or less, and 0.02% or less for rare earth elements.

(12) BB It is not necessary to add B, but if it is added, a small amount of γ → α
It has the effect of delaying transformation and improves the structure and toughness. To obtain the effect of addition, the content is preferably 0.0001% or more, and the effect is saturated even if a large amount is added, so the upper limit is preferably 0.003%.

(13) Atmospheric gas composition Al and Ti are oxidized during welding, and fine oxides of these are dispersed to refine the structure of the weld metal. Contains oxygen. The content is 10 Pa to 40 kPa in terms of oxygen partial pressure.
This is because if the pressure is less than 10 Pa, the effect of refining the weld metal cannot be obtained, and if it exceeds 40 kPa, severe oxidation occurs, defects such as bubbles occur in the weld metal, and the mechanical properties of the joint deteriorate.

If the steel to be welded is a steel plate, the oxygen partial pressure is higher on the upper surface, that is, on the side where the beam is irradiated and on the opposite side.
As long as it is 10 Pa to 40 kPa, the oxygen content of the atmospheric gas does not necessarily have to be the same. The oxygen partial pressure of either one may be 10 Pa to 40 kPa.

Therefore, the same effect can be obtained by using an inert gas containing no oxygen on the side irradiated with the beam and using a gas having an oxygen partial pressure of 10 Pa to 40 kPa on the side not irradiated. Particularly when a laser is used, the generation of plasma increases and interferes with the transmission of the beam, so there is a problem that the penetration depth does not increase even if the output is increased. Since oxygen easily generates plasma, stable welding can be performed by using an inert gas containing no oxygen on the side that irradiates the beam.

At the time of welding, it is desirable that the abutting portion is installed so as to be substantially horizontal, and the beam is directed from the upper side to the lower side in the same vertical direction as gravity. This is to stably hold the molten metal pond formed by the beam irradiation, but if the molten metal can be stabilized by some method, this positional relationship can be changed. Good.

In the present invention, the product is basically as-welded, but if necessary, heat treatment may be performed after welding, in which case the toughness of the joint can be further improved. .

[0045]

EXAMPLE Steels having the chemical composition shown in Table 1 were melted and hot rolled to manufacture steel sheets. Yield point of the obtained steel sheet (Y
S), tensile strength (TS) and impact test fracture surface transition temperature (vTs) by 2 mmV notch Charpy test piece are shown in Table 2.

The inclusions present in the steel sheet were investigated by a scanning electron microscope, and the contained elements were investigated by an attached energy dispersive EPMA. The results are also shown in Table 2.

[0047]

[Table 1]

[0048]

[Table 2]

Using these steel plates, butt welding of the I-shaped groove with a laser beam and an electron beam was performed. At that time, the beam was irradiated while shielding the welded portion with an inert gas (He gas). In order to confirm the effect of the present invention in the inert gas, various amounts of oxygen are mixed, and when oxygen having the same partial pressure is mixed in the upper and lower surfaces of the steel plate, or the gas irradiating side does not contain oxygen. , When the gas whose oxygen partial pressure was controlled was supplied to the opposite side, and the like were compared.

The electron beam irradiation was compared and examined in a vacuum and a case where oxygen was mixed in an inert gas by the differential evacuation method.

In order to evaluate the toughness of the weld metal, a 2 mm V notch Charpy impact test piece was cut out from the welded portion and the change in absorbed energy depending on the test temperature was investigated. Board thickness is 10
Below 5 mm, a sub-size test piece with a width of 5 mm was used. In this case, it is preferable that the absorbed energy is high even at a low temperature.

The above welding conditions and welding results are shown in Table 3 together.

[0053]

[Table 3]

As can be seen in Table 1, Table 2 and Table 3,
It can be seen that excellent toughness of the welded portion can be obtained when steel containing Ti and Al in the content range defined in the present invention is used and the oxygen partial pressure of the atmosphere during welding is controlled within the range of the present invention. Despite the fact that Steel 13 is within the scope of the present invention, it has a slightly poor toughness because the N content is too high.

The present invention is based on the discovery that during butt welding with a high energy density beam, fine oxides produced by reaction with the atmosphere during welding refine the structure of the weld metal and improve its toughness. . At that time, the oxide of Ti existing in the steel in advance may have the same effect. Steel 17 does not have sol.Al and has higher oxygen than other steels, and the oxide of Ti is present in the steel, and the welded part has relatively good toughness in welding in an atmosphere with almost no oxygen. Indicated. However, the toughness of the base metal is
Was inferior compared to the fully deoxidized steel sheet. Further, in Steel 15 as well as Steel 17, Ti oxide exists,
Welding in a vacuum containing substantially no oxygen does not improve the toughness of the welded portion, and the toughness of the base material is also poor.

Further, the steel 14 contains a large amount of sol.Al, is sufficiently deoxidized, and has a good base material toughness. However, when applying the welding method of the present invention, the amount of sol.Al is too large and the formation of an oxide that refines the structure during welding becomes insufficient,
As shown in Table 3, the toughness of the weld metal was somewhat insufficient. Steel 16 also has a high sol.Al content, so as shown in Table 3, even if an attempt is made to increase the oxygen partial pressure during welding to promote oxidation to obtain a finer structure, porosity will be generated if it is excessively oxidized. The integrity of the weld is lost.

To reduce S and control the morphology of sulfides, Ca
Although REM and REM are added, in a slightly loose deoxidized state where an oxide of Ti exists, Ca and REM become oxides rather than combining with S, and the effect of addition is lost. The method of the present invention is also effective when adding such Ca or REM.

Steel 9 is an example of such a Ca addition. Further, since Steel 9 has high Mn and contains Ni and V, in high energy density beam welding in a normal high purity atmosphere, the weld metal is likely to become coarse upper bainite and it is difficult to obtain good toughness, According to the method of the present invention, the weld metal is refined and the toughness is improved.

As can be seen from the above examples, the steel having the components within the scope of the present invention can greatly improve the toughness of the weld metal by the welding method of the present invention.

[0060]

As described above, according to the method of the present invention,
In welding low alloy high strength steel with high energy density beams such as steel structures and line pipes, select steel components,
By combining this with management of the welding atmosphere,
It is possible to significantly improve the performance of the weld metal part, particularly the toughness, and since there is no problem in the performance of the base steel sheet and its production, a very beneficial effect in industry is obtained.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C22C 38/14 // B23K 9/16 J 8315-4E

Claims (2)

[Claims] 1. When welding a low alloy high strength steel containing Ti: 0.003 to 0.06% and sol.Al: 0.001 to 0.015% by weight by irradiation with a high energy density beam,
A method of welding low alloy high strength steel, characterized in that the welded part is shielded in an atmosphere containing a partial pressure of 10 Pa to 40 kPa of oxygen and the balance consisting of an inert gas. 2. When welding a low alloy high-strength steel sheet containing Ti: 0.003 to 0.06% and sol.Al: 0.001 to 0.015% in a weight ratio by a laser beam, the steel sheet is irradiated with the laser beam. Welding of low-alloy high-strength steel sheets, characterized in that the side is shielded with an inert gas, and the non-irradiating side is controlled and welded in an atmosphere containing oxygen with a partial pressure of 10 Pa to 40 kPa and the balance consisting of an inert gas. Method.