JP2022015256A - METHOD FOR MANUFACTURING WELDED JOINT USING Ni STEEL FOR LOW TEMPERATURE - Google Patents

Abstract

To provide a method for manufacturing a welded joint using Ni steel for low temperature capable of manufacturing a high quality welded joint in a state where the Ni steel for low temperature has residual magnetism without performing demagnetization treatment as in a conventional method.SOLUTION: Provided is a method for manufacturing a welded joint using Ni steel for low temperature, which is characterized in that in a state where the Ni steel for low temperature has a residual magnetism of 30 gauss or more, the welded joint is obtained by hot wire laser composite welding for supplying a welding wire subjected to electric heating while being melted by applying a laser beam.SELECTED DRAWING: None

Description

The present invention relates to a method for manufacturing a welded joint using Ni steel for low temperature.

Liquefied natural gas (LNG), which has a liquefaction temperature of -162 ° C, is known as clean energy, and its demand tends to increase more and more due to efforts to tackle environmental problems. In the storage tank for storing such LNG, low-temperature Ni steel to which about 6 to 9% of Ni is added is generally used in order to secure toughness at extremely low temperatures. Similarly, low temperature Ni steel is also used in LNG fuel tanks and ships transporting ethane, ethylene and liquefied petroleum gas (LPG).

In the welding of this low-temperature Ni steel, a Ni-based alloy (high Ni alloy) containing 50% or more of Ni is used as the welding material in order to ensure the strength and toughness of the weld metal constituting the welded joint at extremely low temperatures. It is used, and as a welding method, TIG welding, submerged arc welding (SAW), and shield metal arc welding (SMAW) are generally adopted (see, for example, Patent Document 1). Incidentally, regarding the use of such a welding material, a Ni-based alloy having a high Ni content has an austenite structure, which is stable at an extremely low temperature and is unlikely to cause brittle fracture.

When welding low-temperature Ni steel, low-temperature Ni steel tends to be magnetized, and if the residual magnetism at the welded portion of the base metal forming the welded joint is high, magnetic blowing will occur. Consideration is required. That is, since nickel steel for low temperature has high coercive force and magnetic permeability, residual magnetism is easily formed, and the influence of this residual magnetism disturbs the welding arc and makes welding difficult, and the quality of the weld metal deteriorates. Will be invited. Therefore, for low-temperature Ni steel, it is necessary to carry out demagnetization treatment to remove residual magnetism at the time of shipment of the steel material.

As the demagnetization treatment, a method is generally adopted in which a steel plate is passed through a coil, an alternating current is passed through the coil, and the magnetic field is gradually weakened to reduce residual magnetism (see, for example, Patent Document 1). ). Such a method has been used for a long time and must be shipped after reducing the residual magnetism of the steel material to a certain level (usually to about 30 gauss).

However, a huge amount of steel is used to manufacture a storage tank such as LNG. In particular, in recent years, the size of tanks has increased, and the amount of Ni steel for low temperature used has increased accordingly, so that demagnetization processing alone consumes a great deal of cost and labor. In addition, low-temperature Ni steel may be magnetized not only after being transported to the construction site but also during processing and assembly at the tank manufacturing site, even after the demagnetization treatment is completed. Therefore, for example, during transportation, magnet lifts and the like used for other steel materials cannot be used, and control is performed so that low-temperature Ni steel is not magnetized until the storage tank is manufactured through transportation, processing, and assembly. It is a big burden just by itself.

JP-A-2015-123457 Japanese Unexamined Patent Publication No. 59-184505

As a result of diligent studies on a method for improving the welding of low-temperature Ni steel, which has been burdensome due to the problem of residual magnetism in manufacturing LNG storage tanks, etc., the laser beam. By adopting hot wire / laser composite welding that supplies welding wire that has been energized and heated while irradiating and melting, it is not affected by magnetic blowing even if the low temperature Ni steel has residual magnetism. The present invention has been completed by finding that a welded joint can be manufactured and a high-quality welded joint having excellent reliability can be obtained.

Therefore, an object of the present invention is to provide a low-temperature Ni steel capable of producing a high-quality welded joint in a state where the low-temperature Ni steel has residual magnetism without performing a demagnetization treatment as in the conventional case. It is an object of the present invention to provide the manufacturing method of the welded joint used.

That is, the gist of the present invention is as follows.
(1) A method of manufacturing a welded joint using Ni steel for low temperature.
It is characterized in that a welded joint is obtained by hot wire / laser composite welding in which the low temperature Ni steel has a residual magnetism of 30 gauss or more and supplies a welded wire heated by energization while being melted by irradiating a laser beam. A method for manufacturing a welded joint using Ni steel for low temperature.
(2) The method for manufacturing a welded joint using the low temperature Ni steel according to (1), wherein the low temperature Ni steel has a plate thickness of 4 to 60 mm.
(3) The low temperature Ni steel has a chemical composition of% by mass.
C: 0.03 to 0.10%,
Si: 0.01-0.5%,
Mn: 0.3-1.5%,
Ni: 5-10%,
P: 0.015% or less,
S: 0.003% or less,
Al: 0.005 to 0.08%,
B: 0.001% or less,
Ti: 0.010% or less,
Nb: 0.010% or less,
V: 0.010% or less,
N: 0.010% or less,
O: 0.005% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
Remaining: Fe and impurities,
The method for manufacturing a welded joint using the Ni steel for low temperature according to (1) or (2).

According to the present invention, it becomes possible to manufacture a welded joint of Ni steel for low temperature, which is easily magnetized, without being affected by magnetic blowing or the like without performing demagnetization treatment as in the conventional case. Moreover, it is possible to obtain a high-quality welded joint with excellent reliability.

Hereinafter, the present invention will be described in detail.
First, in the method for manufacturing a welded joint in the present invention, hot wire / laser composite welding is adopted as a welding means to obtain a welded joint by supplying a welded wire heated by energization while irradiating a laser beam to melt the base metal. do. This hot wire / laser composite welding is a welding method that combines a hot wire system that heats and inserts the weld wire to near the melting temperature and a laser heat source. By using the laser heat source together with the hot wire system, the groove accuracy Because the weld metal supplied from the hot wire is heated to just below the melting point, while compensating for the disadvantages of laser welding using a laser heat source, such as a small margin for welding and a large effect of the base metal component on the characteristics of the joint. Sufficient molten metal can be formed even by irradiation with a laser beam having a low energy density.

In such hot wire / laser composite welding, since the base metal is melted by irradiating the laser beam, it is possible to suppress the generation of magnetic blow that causes the welding arc to be deflected by magnetic force, and in particular, demagnetization treatment is performed. Welding of low temperature Ni steel can be performed stably even if it is not performed. That is, even if the low-temperature Ni steel has a residual magnetism of 30 gauss or more, a high-quality welded joint can be stably obtained. However, if the residual magnetism is extremely magnetized to exceed 300 gauss, the soundness of the weld metal may be affected. Therefore, the Ni steel for low temperature is preferably 30 gauss or more and 300 gauss or less. It is better to obtain a welded joint by hot wire / laser composite welding. The residual magnetism of Ni steel for low temperature refers to the value at the welded portion of the base metal (Ni steel for low temperature) forming the welded joint in order to consider the influence during welding. For example, in the case of butt welding, it is the value at the groove formed for butt welding, and in the case of fillet welding, it is the value at the corner of the steel plate to be welded.

Further, since such a feature of hot wire / laser composite welding has an advantageous effect in welding low-temperature Ni steel using a welding material made of a high Ni alloy, the heat input during welding is controlled to some extent, while controlling the heat input during welding. Welded joints can be obtained by reducing the number of welding passes. That is, in a high Ni alloy welding material, it is generally necessary to control the heat input during welding in order to suppress the occurrence of high temperature cracking due to the addition of a large amount of Ni and the deterioration of the toughness of the weld heat affected zone (HAZ). .. On the other hand, if the welding heat input is reduced, the number of weldings (the number of welding passes) for obtaining a predetermined welded joint is inevitably increased, so that the welding efficiency is lowered. On the other hand, in hot wire / laser composite welding, a sufficient molten metal is formed even by irradiation with a laser beam having a low energy density while reducing welding heat input as compared with a high efficiency welding method such as electrogas arc welding. Therefore, one-pass welding is also preferably possible, and it is possible to suppress the occurrence of high-temperature cracking and the decrease in toughness in HAZ while ensuring welding efficiency.

In this hot wire / laser composite welding, a laser beam is irradiated to melt the base metal, and the welding wire is energized with the base metal to form a hot wire, and the energized and heated welding wire is used to melt the base metal. The laser beam may be moved while supplying to the portion to form a molten pool composed of the molten base metal and the weld wire to obtain a welded joint, and the same method as a known method can be used. The type of welded joint is not particularly limited, and examples thereof include butt welding and fillet welding, as well as square joints, lap joints, T-shaped joints, and cross joints. At that time, considering the suppression of welding defects such as fusion defects and high temperature cracks, it is preferable to control the energy density to 300 W / mm ² or less for the welding conditions in the hot wire / laser composite welding. Here, the energy density is a value obtained by dividing the output (kW) in laser welding by the spot area (mm ² ) at the time of welding. The shape of the spot is generally circular, rectangular, or the like, but it can be controlled by the energy density regardless of the shape. Controlling the energy density is effective in suppressing welding deformation and reducing weld joint toughness.

Further, in the present invention, for low temperature use to which about 6 to 9% Ni is added in order to secure toughness at extremely low temperature, for example, a storage tank for storing LNG and LPG, a tanker for transporting these, and the like. A welded joint is obtained by using Ni steel as a base material. As for this Ni steel for low temperature, it is preferable to use one having a predetermined chemical composition. The reason for specifying the chemical composition is as described below. In addition, "%" in these explanations represents "mass%" unless otherwise specified.

(C: 0.03 to 0.10%)
C needs to be contained in an amount of 0.03% or more from the viewpoint of ensuring strength. However, if the content is too large, the toughness will decrease, so the upper limit is set to 0.10%. Preferably, the C content is 0.04% or more and 0.07% or less.

(Si: 0.01-0.5%)
Si has a deoxidizing effect and is an element that improves strength, and a content of 0.01% or more is required. However, if the content is too large, the toughness of the welded joint will decrease, so the upper limit is 0.50%, preferably 0.03 to 0.4%.

(Mn: 0.3-1.5%)
Mn is an element that improves strength and toughness, and needs to be contained in an amount of 0.3% or more. However, if the content is too large, the toughness of the base metal and HAZ deteriorates, so the upper limit is 1.5%. It is preferably 0.4 to 1.2%.

(Ni: 5.0 to 10.0%)
Ni has the effect of improving strength and toughness at the same time, and is indispensable as a base material for manufacturing tanks for storing low-temperature liquids, especially LNG tanks for storing cryogenic LNG at -165 ° C. It is a non-elemental element and requires a content of 5.0% or more. However, even if it is contained in excess of 10.0%, the effect is saturated and the cost is increased. It is preferably 5.5 to 9.5%.

(P: 0.015% or less)
P is an impurity element inevitably contained in steel, and since it is a grain boundary segregation element, it causes grain boundary cracking in HAZ. In order to improve the toughness of the base metal and HAZ, the content of P should be 0.015% or less. It is preferably 0.010% or less. The content of P may be 0, but it is preferably 0.003% or more because excessive reduction causes an increase in cost. It should be noted that P is the same as the impurities as the balance described later in that it is inevitably mixed in the production of steel materials, but the content of P is separately specified from the viewpoint of improving the toughness of HAZ as described above. is doing.

(S: 0.003% or less)
When S is present in a large amount, it produces a precipitate of MnS alone, which is the starting point of welding cracks. Therefore, the content of S needs to be 0.003% or less. It is preferably 0.002% or less. The content of S may be 0, but it is preferably 0.0002% or more because excessive reduction causes an increase in cost. It should be noted that S is the same as impurities as a balance described later in that it is inevitably mixed in the production of steel materials, but S has a separately specified content from the viewpoint of suppressing welding cracks as described above. ing.

(Al: 0.005 to 0.08%)
Al is a deoxidizing element and needs to be contained in an amount of 0.005% or more in order to ensure the cleanliness of steel. However, if the content is too large, coarse Al ₂ O ₃ is generated and the toughness of the welded joint is lowered, so that the upper limit is 0.08%. It is preferably 0.01 to 0.05%.

(B: 0.001% or less)
B has an effect of increasing strength. That is, B segregates at the grain boundaries and has a strength improving effect. However, if the B content exceeds 0.001%, the toughness is impaired. It is preferably 0.0005% or less. The content of B may be 0, but it is preferably 0.0003% or more in order to exhibit the above-mentioned action.

(Ti: 0.010% or less)
Since Ti causes toughness deterioration due to an increase in the starting point of fracture through the formation of carbonitride, it is necessary to suppress it to 0.010% or less. It is preferably 0.005% or less. The content of Ti may be 0, but since Ti forms carbonitride and contributes to microstructural granulation, it should be 0.003% or more in order to exhibit such an effect. good.

(Nb: 0.010% or less)
Since Nb causes deterioration of toughness due to an increase in the starting point of fracture through the formation of carbonitride, it is necessary to suppress it to 0.010% or less. It is preferably 0.005% or less. The content of Nb may be 0, but since Nb forms a carbonitride and contributes to microstructural granulation, it should be 0.003% or more in order to exhibit such an effect. good.

(V: 0.010% or less)
V causes toughness deterioration due to an increase in the starting point of fracture through the formation of carbonitride, and therefore needs to be suppressed to 0.010% or less. It is preferably 0.005% or less. The content of V may be 0, but since V forms carbonitride and contributes to microstructural granulation, it should be 0.005% or more in order to exhibit such an effect. good.

(N: 0.010% or less)
N is an element mixed as an unavoidable impurity and may cause deterioration of toughness, so it is reduced to 0.010% or less. It is preferably 0.006% or less. The content of N may be 0, but it is preferably 0.002% or more because excessive reduction causes an increase in cost. It should be noted that N is the same as impurities as a balance described later in that it is inevitably mixed in the production of steel materials, but N has a separately specified content from the viewpoint of suppressing toughness deterioration as described above. ing.

(O: 0.005% or less)
O must be 0.005% or less because it causes toughness deterioration through the formation of oxides. It is preferably 0.003% or less. The content of O may be 0, but it is preferably 0.001% or more because excessive reduction causes an increase in cost. It should be noted that O is the same as the impurities as the balance described later in that it is inevitably mixed in the production of steel materials, but O is separately defined for its content from the viewpoint of suppressing toughness deterioration as described above. ing.

(Cu: 1.0% or less)
Cu has the effect of increasing the strength. However, if the content exceeds 1.0%, the toughness of HAZ is lowered. It is preferably 0.5% or less. The content of Cu may be 0, but it is preferably 0.02% or more in order to exhibit the above-mentioned action.

(Cr: 1.0% or less)
Cr has the effect of increasing the strength. However, if the content exceeds 1.0%, the toughness of HAZ is lowered. It is preferably 0.8% or less. The Cr content may be 0, but it is preferably 0.02% or more in order to exhibit the above-mentioned action.

(Mo: 1.0% or less)
Mo has the effect of increasing the strength. However, if the content exceeds 1.0%, the toughness of HAZ is lowered. It is preferably 0.5% or less. The Mo content may be 0, but it is preferably 0.02% or more in order to exhibit the above-mentioned action.

The rest of the above components are iron (Fe) and impurities. Here, the "impurity" is a component mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when steel is industrially manufactured, and has an adverse effect on the present invention. It means what is allowed within the range not given.

In the present invention, by adopting hot wire / laser composite welding, a sufficient molten metal can be formed even by irradiation with a laser beam having a low energy density. Therefore, the plate thickness T is 4 to 60 mm, preferably 10. It is desirable to obtain a welded joint for low temperature Ni steel of ~ 50 mm.

Further, the welding material used for obtaining the welded joint may be any material as long as it can be used as a welding wire to be supplied while energizing and heating in hot wire / laser composite welding, and generally, Ni steel for low temperature is preferable. It is preferable to use a high Ni alloy welding material as used in welding. That is, a Ni alloy having a high Ni content has an austenite structure, which is stable at extremely low temperatures and is unlikely to cause brittle fracture. Therefore, for example, it is preferable to use a Ni-based alloy (high Ni alloy) having an austenite structure containing 50% or more, preferably 55 to 75% of Ni in terms of mass ratio. Examples of commercially available products for such high Ni alloy welding materials include NITTETSU FILLER 196 (above, product name manufactured by Nippon Steel Welding & Co., Ltd.), TG-S709S (above, product name manufactured by Kobe Steel), and the like. Can be done.

Next, the present invention will be described based on examples, but the present invention is not limited to these contents.

(Test Example 1)
The steel plates (Ni steel for low temperature) according to Invention Examples 1 to 10 and Comparative Examples 1 to 19 having the chemical components and mechanical properties shown in Table 1 were used as the base material (welded material). Steel sheets were manufactured by quenching tempering (QT) or thermal machining controlled rolling and then tempering (TMCT). Of these, QT heats the slab to 1000 to 1200 ° C, rolls it to a predetermined size, cools it, heats it to 800 to 900 ° C, quenches it, and then heats it to 560 to 600 ° C. And then tempered. The TMCT was heated to 900 to 1200 ° C., then controlled rolling was completed at a temperature of 700 ° C. or higher, cooled, and then heated to 560 to 600 ° C. for tempering. Both QT and TMCT were subjected to an intermediate heat treatment at 605 to 750 ° C. before quenching, if necessary. In the chemical composition of each steel sheet shown in Table 1, the balance of each chemical component is Fe and impurities.

As for the mechanical properties of these steel sheets, regarding the tensile properties of the steel sheet, for steel sheets with a plate thickness (T) of 16 mm or less, JIS Z2241: 2011-5 test pieces were sampled at the total thickness in the direction perpendicular to the rolling direction. For steel sheets with a plate thickness (T) of more than 16 mm, JIS Z2241: 2011-4 test pieces are sampled from the position of 1/4 of the plate thickness (1 / 4t) in the direction perpendicular to the rolling direction and tested at room temperature. The tensile strength (TS) was obtained. Regarding the impact characteristics of steel sheets, 2 mm V notch Charpy test pieces specified in JIS Z2242: 2018 were taken in parallel with the position of 1/4 of the plate thickness (1 / 4t) and the rolling direction in all steel sheets. , -A Charpy impact test was carried out at a temperature of 196 ° C to determine the absorbed energy value (svE-196 ° C).

In this test example 1, two steel plates shown in Table 1 are arranged side by side to form a 20 ° V-shaped (V-shaped) groove, and a welded joint is obtained by butt welding, and a V-shape is formed at that time. Then, the welding start position and any five points are selected from the facing surface (groove surface) of each steel sheet to be the welding execution part, and the residual magnetism is measured with a Gauss meter, and the largest value (maximum value) among these is measured. ) Is shown in Table 1 as the residual magnetism in the welded portion of the steel sheet. For the backing metal at the time of welding, the same steel plate as the base material was used. The type of welded joint is not particularly limited, and examples thereof include groove welding, fillet welding, wire welding, and slot welding.

Further, as the welding material, a nickel (Ni) -based alloy wire (φ1.2 mm) equivalent to YGT9NI-2 specified in JIS Z3332: 2007 is used, and as a welding wire to be energized and heated by hot wire / laser composite welding. Welded joints were manufactured under the welding conditions shown in Table 2. Here, in hot wire laser composite welding (HWL), a laser welder and a welding power supply for hot wire were used. Then, using the steel plate shown in Table 1 and the above-mentioned Ni-based alloy wire, a welded joint having a welding length of 500 mm was produced by a hot wire / laser composite welding method. At that time, the wire heating current was 100 to 400 A for hot wire welding, and the wire supply speed was 2 to 30 m / min. The energy density was controlled by adjusting the welding spot diameter with a welding speed of 1 to 5 m / min. And a laser output of 3 to 20 kW as parameters in laser welding. The shield gas used was Ar (flow rate 30 L / min.).
On the other hand, as a comparison, we also made joints by gas tungsten arc welding (GTAW). As the welding material, the same nickel-based alloy and Ar shield gas as in the hot wire / laser composite welding were used, and the current value was 280 A, the voltage value was 10 V, and the target heat input amount was 18 kJ / cm.

For such Test Example 1, Table 2 shows the combination of the steel sheet and the welding conditions shown in Table 1. Regarding each of these evaluations, first, as the determination of magnetic blowing, x was evaluated when the molten bead was biased or meandering during welding, and ○ was evaluated otherwise. Further, in order to evaluate the impact characteristics of the obtained welded joint, the bond portion (the ratio of the thickness of the weld metal and the HAZ is 1: 1) at the position (1 / 4t) of the plate thickness of the steel plate which is the base material is 1: A 2 mm V notch Charpy test piece specified in JIS Z2242: 2018 was taken with 1) as the notch position, and a Charpy impact test was performed at a temperature of -196 ° C. to obtain an absorbed energy value (joint charpy). Furthermore, the obtained welded joints were examined for welding defects by a radiation transmission test method. The test was conducted according to JIS Z 3106: 2001, and those in which two or more types of defects were confirmed were judged as x, and those in which other defects were judged as ◯.

As can be seen from the above results, according to the present invention, a welded joint is manufactured for low-temperature Ni steel, which is easily magnetized, without being affected by magnetic blowing, etc., without being demagnetized as in the conventional case. It is possible to obtain a high-quality welded joint with excellent reliability.

Claims (3)

It is a method of manufacturing welded joints using Ni steel for low temperature.
It is characterized in that a welded joint is obtained by hot wire / laser composite welding in which the low temperature Ni steel has a residual magnetism of 30 gauss or more and supplies a welded wire heated by energization while being melted by irradiating a laser beam. A method for manufacturing a welded joint using Ni steel for low temperature. The method for manufacturing a welded joint using the low temperature Ni steel according to claim 1, wherein the low temperature Ni steel has a plate thickness of 4 to 60 mm. The low temperature Ni steel has a chemical composition of% by mass.
C: 0.03 to 0.10%,
Si: 0.01-0.5%,
Mn: 0.3-1.5%,
Ni: 5-10%,
P: 0.015% or less,
S: 0.003% or less,
Al: 0.005 to 0.08%,
B: 0.001% or less,
Ti: 0.010% or less,
Nb: 0.010% or less,
V: 0.010% or less,
N: 0.010% or less,
O: 0.005% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
Remaining: Fe and impurities,
The method for manufacturing a welded joint using the low temperature Ni steel according to claim 1 or 2.