JPS5934225B2 - Fe-Ni low thermal expansion amber type alloy with excellent welding hot cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an Fe--Ni low thermal expansion amber type alloy that has excellent weld hot cracking resistance.

In recent years, demand for LNG (liquefied natural gas) has been increasing as part of the diversification of energy sources due to the soaring price of crude oil.

Along with stainless steel, a so-called amber type alloy based on Fe--Ni, which has a low coefficient of thermal expansion, is used in large quantities as a material for membrane tanks of LNG marine transportation ships and land storage low-temperature containers.

The low thermal expansion amber type alloy used for the above membrane tank has sufficient strength, ductility, and toughness at cryogenic LNG temperatures (approximately -162°C), but a major drawback is that it is susceptible to so-called hot cracking during welding assembly construction. It becomes.

This high-temperature cracking is particularly likely to occur during repair welding of a portion of the tank corner, so it was necessary to perform multiple rework and perform multi-layer welding.

However, even if such inefficient and troublesome construction is carried out, it is important to take all possible precautions (not only will it not be possible to completely prevent tank leaks, but also the possibility of cracking caused by fluctuating loads due to temperature changes). There was a concern that this could become a starting point and cause macroscopic fatigue failure.

Therefore, there has been a desire for an Fe-Ni-based amber-type alloy that has improved weld hot cracking resistance without impairing the physical properties (low thermal expansion) and mechanical properties of this type of alloy. be.

An object of the present invention is to provide such a new amber-type alloy while also considering manufacturability and economy.

The Fe-Ni-based amber type alloy of the present invention is as follows.

Ni: 30-45%, C: ≦0.04%, Si: 0.0
5-0.25%, Mn: 0.10-0.40%,
A Fe-Ni alloy containing 60.020% of the total of P and S, with the remainder consisting of Fe, and 0:≦0.00
60, cleanliness: ≦0.05, and additional elements Ta: 01005-0.500%, Hf: 0
．． 001-0.100%, Mo: 0.10-2.
00%.

A low thermal expansion amber type alloy containing one or more of W=□ and 10 to 2.00% and having excellent welding hot cracking resistance.

In accordance with JIS GO 555, cleanliness is determined by placing a glass plate with 20 vertical and horizontal grid lines in the eyepiece of a microscope and repeatedly inspecting the surface to be examined at random. This is a test method in which the number of point centers is counted, and in this case, the standard number of fields of view to be measured is 60, and the standard magnification of the microscope is 400 times. The area percentage occupied by inclusions is calculated using the following formula based on the number n of lattice point centers occupied by objects, and the cleanliness d of the steel is determined.

The present inventors have found that the weld hot cracking that has become a problem in the use of amber-type alloys is related to the grain boundary segregation of P, S, and 0 (cleanliness) based on examples of failures occurring in faplicators. Since it was possible to estimate from the experimental results, the prerequisite was to reduce 9 (cleanliness) below a certain level, and P and S
Various additive elements were explored to fix the grain boundaries and strengthen the grain boundaries.

First, the welding hot cracking test conditions conducted by the present inventors in order to explore such additive elements will be described.

TIG welding is performed under the following conditions to provide a first bead in the central short direction of the sample between sample sizes 811B x 60 x 350.

That is, welding voltage: 12v1, welding current: 200A, welding speed: 100rtatt/min, shielding gas flow rate: 1513/win, W) - 2.4m, extension approximately 2cm.

Next, TIG welding is performed in the center longitudinal direction under the same welding conditions, and when the torch reaches the center of the width of the first bead, an additional strain of 2.0% is applied by applying rapid bending deformation. Set the bead.

As a result, it is possible to evaluate both types of weld hot cracking that occurs in alloys, namely solidification cracking and reheating cracking.

In other words, solidification cracking occurs on the second bead, and reheating cracking occurs on the first bead.

In general, austenitic high alloys are prone to high-temperature cracking during welding, but this is because the material undergoes high-temperature embrittlement during the solidification-cooling process or reheating-cooling process, resulting in a significant decrease in ductility. The chemical cause is the presence of impurity elements P and S.

From this point of view, as a countermeasure to prevent welding hot cracking of austenitic high alloys (reducing the amounts of P and S as much as possible, or fixing S mainly in the structure and preventing its adverse effects, sulfide formation is recommended). T 1 s with a strong tendency
N b , V t Z r s Ca eMg, R,
Attempts have been made since ancient times to add such substances.

In this regard, it is possible to naturally add these elements to prevent hot cracking for Fe-Ni-based amber type alloys, but
According to the results of detailed experiments and studies by the present inventors, there is almost no effect even if the above-mentioned additive elements are used under the condition that P+S≦0.020%, which is contained in normal steelmaking and refining processes (
, 0≦0.0060, cleanliness≦0.05, Ta=0.005-0.500%, Hf=0.
001~0.100%, Mo=0.10~2.0
They found that a significant effect can only be obtained by adding one or more of the following: 0%, W = 0.10 to 2.00.

Here, even if the conditions of O≦0.0060% and cleanliness≦0.05 are satisfied, the above-mentioned Ti and Nb.

The effect A of adding V, Zr, Ca, Mg, R, M, etc. is still poor, and when adding one or more of Ta, Hf, W, and Mo, Ti and Nb.

There is no problem with the combined addition of V, R, etc. (However, even if they are added in combination, no effect can be expected, so
There is no need to add them in combination), Zr, Ca
, and Mg must be avoided since it will destroy the remarkable effects of Setsuka.

Some of the effects of the above additive elements are shown in the figures.

Here, FIG. 1 shows the case where the conditions of the present invention are satisfied, 0≦0.
．． 0060%, cleanliness≦0.05゜(P1S≦0.02
0%), Ta, Hf.

The effect of each added element Mo and W is compared with the case without added element, and hot cracking is caused by solidification cracking on the second bead and
The figure also shows the reheat cracks on the bead.

From this, the effect of the added elements is clearly recognized.

FIG. 2 shows 0, a case where the cleanliness value is not regulated within the regulated range of the present invention, and the results without the added element are even worse than those in FIG. 1, and the effect of the added element is also small.

Figure 3 shows the case where the O1 cleanliness value is not regulated and Ca is added, Ta and Hf.

In the case without the addition of Mo or W, there is no significant difference in crackability compared to that shown in FIG. 2 (in other words, this shows that the effect of adding Ca is small), which is a feature of the present invention.

The effect of adding Hf, Mo, and W is also small.

From the above results, it is known that in order to improve weld hot cracking of an amber type alloy, it is necessary to satisfy the alloy conditions of the present invention described above.

The reason for this result is as follows.

The contribution of non-metallic inclusions cannot be ignored in the reduction in ductility at the phase boundary at high temperatures that leads to hot cracking in the alloy of the present invention, and as can be seen in Figures 1 and 2, it is possible to reduce the amount of inclusions to a certain extent. It is possible to improve the high temperature cracking resistance of

And 0. The cleanliness is lowered below a certain value to reduce nonmetallic inclusions, and Ta, Hf.

When Mo, W, etc. are added, the decrease in high temperature ductility is suppressed due to the solid solution strengthening of the phase boundaries and the effect of fixing S (and P), thereby preventing hot cracking from occurring.

Therefore, 0. If the cleanliness is not regulated below a certain value, the above low nonmetallic inclusion condition will not be met, and Zr,
When Ca, Mg, etc. coexist, the strong affinity of these elements with O acts and the cleanliness inevitably deteriorates, so the low nonmetallic inclusion condition is not met, so the hot cracking resistance improves. Not done.

Ti and Nb have traditionally been considered as S-fixing elements.

VsZrxCasMg-RByl is
9. Regardless of the cleanliness value, this alloy has the ability to strengthen phase boundaries due to solid solution strengthening and S fixation effects compared to Ta and Hf.

Since it is smaller than Mo and W, the effect of improving hot cracking resistance is small.

Next, the reasons for limiting the composition range and inclusion conditions of this invention alloy will be described.

Ni: 30-45%pN1 is an element that controls the thermal expansion coefficient of the Fe-Ni alloy, and exhibits a thermal expansion limit near 36 coefficients at low temperatures and near 42% at high temperatures.

If the coefficient is less than 30% or exceeds 45, the coefficient of thermal expansion will become significantly large, and the toughness will also deteriorate.
It shall be 45%.

C≦0.04%; If it exceeds 0.04%, the coefficient of thermal expansion will increase and not only will the low thermal expansion, which is a characteristic of Fe-Ni alloy, be impaired, but also carbides will precipitate. Since hot workability and toughness after welding deteriorate, its content should be reduced to 0.
．． Limited to 0.4% or less.

Si: 0.05-0.25%; Si is required as a deoxidizing agent in alloy refining in an amount of 0.05% or more, but 0.2%
If it exists in excess of 5%, hot workability will be impaired, so 0.
0.05% to 0.25%.

Mn: 0.10-0.40%; Mn is also required as a deoxidizing agent during alloy refining in an amount of 0.10% or more, but 0.10% to 0.40%;
．． Even if the amount exceeds 40%, the deoxidizing effect remains the same and the cost is disadvantageous, so the amount is limited to 0.10 to 0.40.

P and s: p10s≦0.020%: The sum of P and S is 0.
If it exists in excess of 0.020%, it will be difficult to obtain stability during welding work (as work efficiency will be significantly impaired,
The upper limit is limited to 0.020%.

0: 0.0060% or less, and cleanliness: 0.05 or less;
Reduces the occurrence of welding hot cracking, Ta, Hf.

As a prerequisite for bringing out the effects of addition of Mo, W, etc., it is necessary to reduce the amount of various non-metallic inclusions such as oxides and sulfides.
060, the cleanliness level must be 0.05.

Ta: 0.005-0.500%; 0≦0.006
It is an element that is effective in improving weld hot cracking when added under the conditions of coefficient 0 and cleanliness ≦0.05, and constitutes a part of the characteristics of the present invention.

If the content is less than 0.005%, it is difficult to expect an effect;
If the content exceeds 500%, hot workability will be inhibited,
This will increase the cost (therefore, it is limited to 0.005~Q, 500 units).

Hf: 0.001 to 0.100, an element that is effective in improving weld hot cracking when added under the conditions of 0≦0.0060 and cleanliness≦0.05, which is also a feature of this invention. It becomes a part of it.

Less than 0.001% is unlikely to be effective; 0.100%
If the content exceeds 0.0%, the ductility of the material will decrease and the cost will increase, so it is limited to 0.00% to 0.100%.

MO and W2 each 0.10 to 2.00%; these components also characterize this invention, and 0≦0.0060
%, and when added under the condition of cleanliness≦0.05, the weld hot cracking property is improved.

M. In both cases, this effect is not exhibited when less than 0.10% of M and W is added, and when added in excess of 2.00%,
In the case of o, hot workability deteriorates, and in the case of W, low-temperature toughness is impaired, so each content is limited to 0.1θ to 2.00%.

The amber-type alloy of the present invention exhibits excellent weld hot cracking resistance when all of the above-mentioned composition and nonmetallic inclusion conditions are satisfied, and this point is specifically explained in the examples shown in the table below. As explained below.

Examples The table below compares the present alloy with reference examples that slightly deviate from the conditions of the present invention and comparative examples of conventional amber-type alloys, and compares their chemical composition, weld cracking resistance, and hot-temperature This shows the workability.

Each alloy is made by refining regular electric furnace molten steel in an extra-furnace vacuum refining furnace, using zircon bricks to lower the oxygen content and cleanliness of the steel, and creating ingots by non-oxidizing injection. .

Thereafter, blooming, hot rolling, and cold rolling were carefully performed, followed by bright annealing to produce the final product.

Note that the weld cracking property was determined by the above-mentioned test method.

In the table, A1 to A8 are Fe-36%Ni based example steels according to the present invention, which have significantly improved weld hot cracking resistance compared to conventional comparative steels of this type.

In particular, reheat cracking, which is the most problematic, has been reduced.

Reducing the number of cracks through such forced testing (even if the number of cracks is not O) brings about a significant improvement effect in actual work.

Moreover, the coefficient of thermal expansion of the example exhibits a value similar to that of the comparative steel, and the low thermal expansion property as an amber-type alloy is not impaired in the slightest.

However, Fe-36%Ni system A9-12 listed as a reference example which is out of the range of the composition and nonmetallic inclusion conditions of this invention
(Y9 is O1, Bian10 is cleanliness, boiled 11 is T a, A
Although the weldability of the weldability Q-transformed comparative steels with l 2 and Mo deviated from each other is slightly improved, it is far inferior to the invention steel.

The weldability of All is as good as that of invented steel, but T
Hot workability became poor because a was added in excess.

As described above, the alloy of the present invention has significantly improved weld hot cracking resistance without impairing the low thermal expansion as an amber-type alloy and retains other basic properties, and is therefore preferable. It can be applied as a membrane tank material for transporting and storing liquefied natural gas8, but it can also be applied to components such as superconducting magnet support members, cameras and electrical components, and (liquid) hydrogen utilization technology.

[Brief explanation of the drawing]

Figure 1 shows the effect of added elements on weld cracking in the alloy of the present invention, which satisfies the O and cleanliness conditions of the present invention.
Fig. 2 is a graph showing the comparison steel with no additive elements, and Fig. 2 shows the influence of the same additive elements on weld cracking in cases where the cleanliness conditions of the present invention are not satisfied. FIG. 3 is a graph shown in comparison with comparative steel. It is a graph showing the influence of the added element on weldability in a case where the O and cleanliness conditions of the present invention are not satisfied and Ca is added, in comparison with a comparative steel without the added element.

Claims (1)

[Claims] iNi2 30-45%, C: ≦0.04%,
Si: 0.05-0.25%, Mn: 0.
In a Fe-Ni alloy containing 10 to 0.40%, P and S2≦0.020%, and the balance consisting of Fe, further O2≦0
．． 0060, adjust the cleanliness to 2≦0.05, and add T a : 0.005 to 0.500 as an added element.
%, Hf20,001~0.100%, Mo: 0
．． 10-2.00%. W: A low thermal expansion amber-type alloy with excellent weld hot cracking resistance, characterized by containing one or more of 0.10 to 2.00%.