JPH0637699B2 - Manufacturing method of A-l-Mg base alloy plate for welded structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Field of the Invention The present invention relates to Al-used in various large-scale welded structural materials and the like.
Regarding a method for manufacturing a thick plate made of an Mg-based alloy, in particular, when the welding with a high heat input is performed, the weldability is improved, that is, the weld joint performance is improved and the micro-cracking of the base metal during welding is prevented. The present invention relates to a method for manufacturing a thick plate having a thickness of 20 mm or more.

2. Description of the Related Art A typical Al-Mg-based alloy, JIS 5083 series alloy, is a non-heat treatment type high-strength material. Therefore, with the progress of Al welding technology in recent years, LNG (liquefied natural gas) It has been widely used for large-scale welded structures such as tanks for land storage and tankers, and as a plate material used for these large-scale welded structures, a plate with a thickness of 20 mm or more is required. It is like this. When welding such thick plates with a thickness of 20 mm or more, it is desirable to reduce the number of welding layers from the viewpoint of efficiency of welding work, and in this case, the welding heat input per unit plate thickness must be large. I don't get it.

By the way, the welding heat input per unit plate thickness is 18,000 J / cm / cm
When an Al-Mg based alloy slab is welded with a large heat input exceeding 50 ° C, in the Al-Mg based alloy slab obtained by a conventional general manufacturing method, the eutectic of the heat-affected zone of the base metal Due to the fact that some of the components and grain boundaries are melted by the heat at the time of welding and thermal stress at the time of welding is applied to the melted part,
Microcracks may occur parallel to the rolled surface, and if such microcracks occur, there is a risk of deteriorating the safety and other qualities of the welded structure. Further, if the welding heat input is extremely large, a decrease in the strength of the heat-affected zone cannot be ignored, and there is a problem that the joint part with a stress where stress tends to concentrate particularly in the heat-affected zone easily breaks from the heat-affected zone. .

Conventionally, Al-Mg represented by JIS 5083 series alloy
As a method for preventing the generation of microcracks in the heat-affected zone during welding in the production of a thick base alloy material, JP-B-55-34860 or JP-B-60-5641 is known.
As shown in No. 7, homogenization treatment of an alloy ingot containing a predetermined component element is performed at 500 to 540 ° C, and hot rolling is performed so that the end temperature is 400 ° C or more. A with a grain length of 15 mm or less in the rolling direction of the macrostructure
A method for obtaining an 1-Mg-based alloy plate has been proposed. However, even with these methods, it is practically difficult to surely prevent the occurrence of microcracks in the heat-affected zone of the base material during large heat input welding and to sufficiently suppress the softening of the heat-affected zone.

Problems to be Solved by the Invention As described above, with a thick-walled material made of an Al-Mg-based alloy and having a thickness of 20 mm or more, microcracks are easily generated in the heat-affected zone during high heat input welding, and the heat-affected zone is softened. Therefore, the problem of strength reduction in the joint member with extra reinforcement cannot be ignored, and the method already proposed for preventing the occurrence of microcracks cannot reliably and sufficiently prevent these problems. The fact is that safety measures have not been taken yet.

This invention has been made in the background of the above circumstances, even in the large heat input welding in the thick material of the plate thickness 20mm or more, it is possible to reliably prevent the generation of microcracks in the heat affected zone and soften the heat affected zone. It is an object of the present invention to provide a method for producing an Al-Mg-based alloy slab that is sufficiently suppressed and thereby sufficiently secures the safety of a welded structure. Micro cracks generated in the heat-affected zone due to large heat input welding are
As described above, a large amount of heat input melts a part of the eutectic component and grain boundaries of the base material and opens due to welding thermal stress. Therefore, in order to prevent microcracking, the eutectic grains are refined in order to disperse the thermal stress applied to the grain boundaries, and at the same time the amount of eutectic compound is reduced and the eutectic compound is finely and uniformly dispersed. It is considered effective to reduce impurities other than the essential alloying elements as much as possible in order to improve the high temperature strength of the boundary. In addition, in order to minimize the softening of the heat affected zone during high heat input welding, it is considered necessary to strictly control the chemical composition and the soaking / hot rolling official conditions.

From this point of view, the present inventors have reexamined the chemical composition and hot rolling conditions in the Al-Mg based alloy, and as a result, by comprehensively combining the following means, the high heat input welding The inventors have found that it is possible to prevent the occurrence of microcracks in the heat-affected zone and suppress the softening of the heat-affected zone, and have completed the present invention.

That is, first, in order to refine the base material crystal grains, which is one of the countermeasures for the occurrence of microcracks in the heat-affected zone during high heat input welding, the structure in the ingot stage was refined and the refinement was performed. To obtain a fine fibrous structure by leaving the ingot structure as it is in the subsequent soaking / hot rolling process, that is, in the condition that recrystallization or crystal grain coarsening does not easily occur in the soaking / hot rolling process. Is effective. And as a concrete means for that,
First, for refining the ingot structure, it is effective to add Ti or Ti-B as a refining material. Furthermore, in order to suppress recrystallization and grain coarsening during the soaking / hot rolling process, the precipitation of insoluble compounds containing transition metals is refined during the soaking / hot rolling process to prevent the recrystallization of the structure. It is effective to have it, and for that purpose soaking (homogenization treatment)
It is performed in the low temperature range of less than 500 ℃, and hot rolling is 500 ℃.
It has been found that it is effective to carry out in a low temperature range below.

Further, as another measure for preventing micro-cracking in the heat-affected zone during high heat input welding, there is a decrease in the amount of precipitation of the eutectic compound and fine / uniform dispersion of the eutectic compound as described above. In order to control the eutectic compound in this way without deteriorating the performance of the alloy, the content of the impurity components Fe and Si should be kept lower than the level of the conventional normal Al-mg base alloy. It was found that it is effective to suppress Fe and Si to less than 0.10%, respectively.

Further, in order to minimize the softening of the heat-affected zone during high heat input welding, it is necessary to make the precipitate of the insoluble compound containing the transition metal fine during the soaking / hot rolling process as described above. Conditions: homogenization treatment, hot rolling 500 ℃ each
It was found that it is effective to carry out in the low temperature range below.

Therefore, the method of the first invention of the present application is Mg 4.0 to 5.5,
Mn 0.40 to 1.0%, Cr 0.05 to 0.35%, Ti 0.005 to
In producing an Al-Mg-based alloy thick plate for welded structure having a plate thickness of 20 mm or more containing 0.2% and the balance being Al and unavoidable impurities, the contents of Fe and Si as impurity components in the alloy ingot Are regulated to less than 0.10% respectively, and the ingot is homogenized at 460 ° C or higher but lower than 500 ° C, and then hot-rolled at a temperature in the range of 350 ° C or higher but lower than 500 ° C. It is a thing.

In the method of the second invention of the present application, in addition to the component elements specified in the first invention, 0.01 to 0.1% of B is added, and
As in the first invention, each of e and Si is suppressed to less than 0.10%, and the alloy ingot is subjected to homogenization treatment and hot rolling under the same conditions as in the first invention.

Operation First, the reasons for limiting the components of the alloy used in the method for producing an Al—Mg based alloy plate for welded structure of the present invention will be explained.

Mg: Mg is an essential component for obtaining high strength by solid solution strengthening in a 5083 type Al-Mg based alloy which is a non-heat treatment type alloy. If Mg is less than 4.0%, strength is not sufficient as a large structural material. On the other hand, if it exceeds 5.5%, hot working becomes difficult. Therefore, Mg is set within the range of 4.0 to 5.5%.

Mn: Mn is also an essential element in the 5083-based Al-Mg-based alloy targeted by the present invention, and improves strength and Fe.
Although it is effective in preventing the corrosion resistance from being deteriorated, if less than 0.40%, sufficient strength cannot be obtained. On the other hand, if it exceeds 1.0%, coarsening of the compound becomes remarkable and microcracking may occur. Therefore, Mn is set within the range of 0.40 to 1.0%.

Cr: Cr, like Mn, is effective in improving strength and preventing lowering of corrosion resistance, but if it is less than 0.05%, these effects cannot be expected, while if it exceeds 0.35%, ductility is impaired.
Within the range of up to 0.35%.

Ti: Ti contributes to the refinement of the ingot structure and the refinement and dispersion of the crystallized compound, but if it is less than 0.005%, its effect cannot be expected, while if it exceeds 0.2%, it causes an increase in cost and castability. Since it causes a decrease, it was set within the range of 0.005 to 0.2%.

B: By adding B together with Ti, the refinement of the ingot crystal grains becomes more remarkable, and it is particularly effective for the refinement of the ingot crystal grains in which the Fe and Si contents are kept low. Therefore, B is added in the second invention. However, B
However, if less than 0.001%, the effect is small, while B is 0.1.
%, The toughness deteriorates. Therefore, in the second invention, B
The amount added was 0.001 to 0.1%.

Fe, Si: Fe and Si are Mg ₂ Si and Al-Fe-M, respectively.
An n-type crystallization compound is formed, and these eutectic-type compounds cause microcracking by melting before the matrix in the heat affected zone during high heat input welding. Therefore, in the present invention, it was decided to limit each to less than 0.10%. It should be noted that limiting the Fe content to less than 0.10% is not only effective in preventing microcracks in the heat-affected zone during high heat input welding as described above, but also sends out Mn precipitation during homogenization treatment. Therefore, it is also effective in suppressing softening of the weld heat affected zone.

Next, the process conditions in the manufacturing method of the Al—Mg based alloy thick plate for welded structure of the present invention will be described.

First, an Al-Mg based alloy ingot is cast by a conventional method so as to have the above-mentioned composition, and then homogenized.
Perform at temperatures above ℃ and below 500 ℃ If the homogenization temperature is 500 ° C or higher, the precipitation of the transition metal-based insoluble compound becomes coarse, and it is easy to cause crystal grain coarsening and softening due to heat in the high temperature heat cycle during high heat input welding. It becomes difficult to secure the strength of the welded joint, and it becomes difficult to make the microstructure of the structure after hot rolling due to the coarsening of the insoluble compound, and it is possible to sufficiently prevent the occurrence of microcracks in the heat affected zone during high heat input welding. Disappear. On the other hand, if the temperature is lower than 460 ° C, the homogenizing effect cannot be sufficiently obtained. Therefore, the ingot homogenization process is 4
It must be performed at a temperature of 60 ℃ or more and less than 500 ℃. The holding time for homogenization treatment is preferably 5 to 30 hours. Five
If the time is shorter than the time, the effect of homogenization cannot be sufficiently obtained, and even if the treatment is performed for a long time exceeding 30 hours, no further effect can be expected.

As described above, the homogenization treatment is carried out in a relatively low temperature range of less than 500 ° C. and 460 ° C. or higher, so that precipitation of the insoluble compound becomes fine. And then hot rolling below 500 ℃, 3
By carrying out in the temperature range of 50 ° C. or higher, the fine precipitation of the insoluble compound is further continued and promoted to bring about the development of a fine substructure, and the crystal grain shape becomes a uniform fine fibrous structure. By forming a uniform fine fibrous structure in this way, the thermal stress applied to the grain boundaries is dispersed even at the time of high heat input welding, and it is not shaving, but the Fe and Si contents are suppressed to a small amount as described above. As a result, since the amount of eutectic compound is also reduced, the chance of melting the eutectic compound during high heat input welding is reduced, and the high temperature strength of the grain boundary itself is increased due to the reduction of Fe and Sih impurities. Therefore, they act synergistically to effectively prevent the generation of microcracks in the heat-affected zone during high heat input welding. In addition, since the precipitation of insoluble compounds is fine as described above, there is little risk of crystal grain coarsening and eventually softening of the heat-affected zone in the heat cycle during high heat input welding, and the strength of the welded joint is sufficient. Can be secured.

In addition, performing hot rolling at a temperature of less than 500 ° C. and 350 ° C. or higher is synonymous with setting the hot rolling start temperature to less than 500 ° C. and the hot rolling end temperature to 350 ° C. or more.
When hot rolling is performed at a high temperature of 500 ° C or higher, the above insoluble compounds coarsely precipitate, which in turn leads to coarsening of crystal grains, causing microcracks in the heat-affected zone during high heat input welding. The effect of preventing softening and the effect of preventing softening are not sufficiently obtained. On the other hand, when the temperature is lower than 350 ° C, hot rolling becomes difficult. Therefore, hot rolling is performed in the temperature range of lower than 500 ° C and higher than 350 ° C.

After hot rolling, it is usual to carry out finish annealing for finishing to a soft material (O material) at 350 ° C to 400 ° C for 2 to 5 hours, if necessary.

Example For each of the sample materials Nos. 1 to 3 having the composition shown in Table 1, an ingot of actual production scale having a thickness of 500 mm and a width of 1400 mm was cast and homogenized under the conditions shown in Table 1. Then, hot rolling was performed to produce a large plate having a thickness of 50 mm and a width of 2620 mm, and further finish annealing was performed at 350 ° C. for 5 hours to obtain a soft material.

Welding test pieces having a width of 350 mm and a length of 1000 mm were cut out from each of these large plates of soft material, and downward facing double-sided single-pass butt welding was performed by high-current MIG welding. The welding heat input condition is the second
As shown in the table, the current on the F side was changed and set to three levels: "large heat input", "medium heat input", and "small heat input". Table 2 shows each welding condition. As a filler metal, A518
3-WY 4.8 mmφ was used, and the groove shape was X groove.

For each of the obtained welded materials, a full thickness tensile test is performed in each state of the extra welded material and the extra welded material to check the mechanical properties of the welded joint, and a side bending test is performed to determine the microstructure of the weld heat affected zone. The presence or absence of cracking was examined. The results are shown in Table 3. In Table 3, "Presence or absence of micro-cracks in side bending test" is taken at a bending radius R / t = 3 for each of 10 sliced slices (thickness 9 mm) of the welded portion at right angles to the welding direction. After 180 ° bending, generation of microcracks observable with the naked eye in the heat affected zone of welding was examined, and the number of microcracks generated was expressed as a molecule.

As is clear from Table 3, in the test material No. 1 in which the material having the composition within the composition range of the present invention was processed under the homogenization treatment condition and the hot rolling condition of the present invention, the welding current was large. It was found that even under the heat input welding conditions, the occurrence of microcracks in the heat-affected zone was not observed, and the decrease in tensile strength of the welded joint with extra reinforcement was small.

EFFECTS OF THE INVENTION According to the method for manufacturing an Al—Mg based alloy plate for welded structure of the present invention, the contents of Fe and Si, which are impurity components in the alloy ingot, are suppressed to less than 0.10% and homogenized. Welded under a high heat input welding condition for thick wall materials of 20 mm or more by performing the treatment in the temperature range of less than 500 ° C and 460 ° C or more and hot rolling in the temperature range of less than 500 ° C and 350 ° C or more. Even in the case, it is possible to surely prevent the occurrence of microcracks in the heat-affected zone, and also to suppress the softening of the heat-affected zone to the minimum and sufficiently secure its strength even in the welded joint member with a reinforcement, Therefore, by using the Al-Mg based alloy thick plate obtained by the method of the present invention, the safety of the thick-walled welded structure to which the large heat input welding is applied can be remarkably improved as compared with the conventional one.

Claims (2)

[Claims] 1. Mg 4.0 to 5.5 (% by weight, the same applies hereinafter), M
n 0.40 to 1.0%, Cr 0.05 to 0.35%, Ti 0.005
In producing an Al-Mg-based alloy thick plate for welded structures having a plate thickness of 20 mm or more, containing 0.2% to the balance of Al and unavoidable impurities, Fe and Si as impurity components in the alloy ingot are produced.
Content of 0.10% or less, and the ingot is homogenized at 460 ℃ to less than 500 ℃.
A method for producing an Al-Mg-based alloy thick plate for a welded structure, which comprises performing hot rolling at a temperature in the range of ℃ to less than 500 ℃. 2. Mg 4.0 to 5.5%, Mn 0.40 to 0.1%,
Cr 0.05 to 0.35%, Ti 0.005 to 0.2%, B 0.001
In producing an Al-Mg-based alloy thick plate for welded structures having a plate thickness of 20 mm or more containing 0.1% to 0.1% and the balance being Al and unavoidable impurities, Fe and Si as impurity components in the alloy ingot are produced.
Content of 0.10% or less, and the ingot is homogenized at 460 ℃ to less than 500 ℃.
A method for producing an Al-Mg-based alloy thick plate for a welded structure, which comprises performing hot rolling at a temperature in the range of ℃ to less than 500 ℃.