JPH0349979B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an Fe-Ni-based low thermal expansion amber type alloy that has excellent rust resistance. In recent years, demand for LNG (liquefied natural gas) has been increasing as part of the diversification of energy sources due to the increasingly serious rise in crude oil prices. Along with stainless steel, Fe--Ni based so-called amber type alloys, which have a low coefficient of thermal expansion, are used in large quantities as materials for membrane tanks of LNG marine transportation ships and land storage low-temperature containers. However, this Fe-Ni-based amber type alloy has low thermal expansion when compared to stainless steel, so it is remarkable that it shows sufficient resistance to concentrated loads due to thermal expansion and contraction when designing and constructing membrane tanks. While it has advantages, it also has the disadvantage of inherently lacking corrosion resistance. When the tank is used after completion, it is purged with nitrogen and stores LNG at extremely low temperatures (-162°C), so there is no problem, but corrosion during construction, especially the formation of rust, is a problem. . The problem is serious because both LNG and land tanks are generally built near the coast, in an environment where corrosion is likely to occur.
There are plans to install large-scale air conditioning equipment during construction, and there are high hopes for this alloy's improved corrosion and rust resistance. This invention meets the above-mentioned demands, and includes:
Ni: 30-45%, C: 0.04% or less, Si:
Besides 0.05~0.25%, Mn: 0.10~0.40%, Co:
Contains 0.01~1.50% or Mo: 0.05~1.50
% or Co: 0.01~1.50% and
For those containing Mo: 0.50-1.50%, Ca: 0.005-0.100%, Cr: 0.50-3.00 if necessary
%, Cu: 0.50~3.00%, Ti: 0.01~0.50%, Zr:
It is an Fe-Ni-based low thermal expansion amber type alloy with excellent rust resistance, containing one or more of 0.01 to 0.50% (excluding Cu), and the remainder consisting of Fe and unavoidable impurities. However, in the case where only Co is included as an essential main component, the selectively added components include Ca alone or two components of Ca and Cu. In other words, for alloys that do not contain Mo, Cr, Ti,
Zr shall not be contained. Fe-Ni-based amber-type alloys do not contain Cr, so it is essentially impossible to expect them to have the same high corrosion resistance as stainless steel. however,
For LNG-related applications, as mentioned above, it is important to suppress the occurrence of surface rust on the material during construction work at the seaside. It is hoped that the rust resistance (or weather resistance) will be improved. Therefore, the inventors thoroughly investigated the effects of various component elements on the rust resistance of this alloy through salt spray tests and atmospheric exposure tests, and found that the above additive elements were added to the Fe-Ni basic component system. It has been found that rust resistance can be improved by adding it. On duty
The gist of the present invention is the finding that adding a small amount of Co is effective. In other words, by appropriately selecting the amount of addition and performing compound addition with other additive elements, we can avoid deterioration of the coefficient of thermal expansion and at the same time reduce the risk of damage such as weld cracking and stress corrosion cracking. It has been clearly shown that the intended purpose can be achieved without the risk of causing problems. Figure 1 shows Fe-36%Ni amber type alloy (C=
0.03%, Si=0.2%, Mn=0.3%, Ni=36.0%)
The results of a salt spray test (JIS Z2371) examining rust resistance when various types of Co or Mo were added are shown.
The figure shows the change in the area incidence of surface rust over the course of the test time. Conventional amber-type alloys without Co or Mo added have a rapid increase in rusting rate in a very short period of time, with the rusting rate reaching 100 in less than 2 hours.
%, that is, the entire surface becomes rusted. In contrast, Co
Alternatively, when Mo is added, the rusting rate and maximum rusting rate decrease sequentially depending on the amount, and a considerable improvement in rust resistance is already seen with Co = 0.2% or Mo = 0.5%, and with Co = 1.0%. Or 18− by adding Mo=1.5%
High rust resistance comparable to that of No. 8 stainless steel (indicated by a dotted line in the figure) was demonstrated. Even if the rust resistance is improved, it will be meaningless if the low expansion property, which is a fundamental characteristic of this alloy, is impaired. This point is illustrated in FIG. That is, the rusting rate and -180°C when the above Fe-36%Ni amber type alloy was subjected to a salt spray test for 4 hours depending on the amount of Co or Mo added.
It also shows the change in the average coefficient of linear expansion at room temperature (20°C). This shows that by adding a small amount of Co or Mo, the rusting rate suddenly decreases to 0.5
With addition of %, it becomes about 1/5 of the case without additive. on the other hand,
The coefficient of thermal expansion increases only slightly up to about 1.5% Co or Mo, but the increasing tendency becomes significant above that. Therefore, in this respect, the addition of Co or Mo naturally has a limit. When Co, which has this effect, is used as the main additive element and one or more of Ca, Cr, Cu, Ti, and Zr are added as a secondary element, the thermal expansion increases as in the case of adding Co alone. It has been found that it is possible to further improve the rust resistance without causing much damage. Moreover, in this case, the addition of expensive Co can be kept to a minimum through secondary addition. Furthermore, instead of Co above, Mo or Co and Mo
In each case, Ca, Cr, Cu,
It has also been found that substantially the same effect as the above result can be achieved when one or more of Ti and Zr is added. In this case, it can be said that it is quite advantageous in terms of cost because it does not contain expensive Co or its amount can be reduced. As mentioned above, amber-type Fe is created by adding elements.
-The mechanism for improving the rust resistance of Ni alloys is currently not clear;
Considering that the contribution of Co to corrosion resistance and rust resistance is still unknown, it is presumed that it will be very difficult to solve the problem. It forms complex hydroxides similar to atoms, and this suppresses the formation of rust. Even if the surface is attacked for some reason and a new surface is generated, Co compounds and Mo compounds are immediately formed, slowing down the growth of rust. It is thought to be attached to a belt. Also, Co or Mo or Co
It is thought that by adding trace amounts of Ca, Mo, Cr, Cu, Ti, Zr, etc. to Co and Mo, the rust resistance of the surface film made of the above Co compound is enhanced. Next, the reasons for limiting the components of the alloy according to the present invention will be described. Ni: 30-45%; Ni is an element that controls the thermal expansion coefficient of the Fe-Ni alloy, and exhibits minimum thermal expansion at around 36% at low temperatures and around 42% at high temperatures. If it is less than 30% or more than 45%, the coefficient of thermal expansion will increase significantly and the toughness will also deteriorate.
It should be 30-45%. C: 0.04% or less; If C exceeds 0.04%, the coefficient of thermal expansion increases and not only does it impair the low thermal expansion characteristic of Fe-Ni alloys, but also carbides precipitate and impair hot workability. Since the toughness after welding deteriorates, its content is limited to 0.04% or less. Si: 0.05-0.25%; Si is added during alloy refining.
0.05% or more is required as a deoxidizing agent, but 0.25%
If it exists in excess of
Limited to 0.05-0.25%. Mn: 0.10-0.40%; Mn is also necessary as a deoxidizing agent during alloy refining at 0.10% or more, but even if it exists in excess of 0.40%, the deoxidizing effect will not change and it will be disadvantageous in terms of cost. , limited to 0.10-0.40%. Co: 0.01-1.50%; Co is an element that characterizes the present invention, and it is necessary to add at least 0.01% to impart excellent rust resistance to the present alloy. but,
If the content exceeds 1.50%, the coefficient of thermal expansion will increase and the cost will increase.
%. Mo: 0.05-1.50%; Mo is also an element that characterizes the present invention, and imparts excellent rust resistance to the present alloy. The amount added should be at least 0.05
% is necessary, but if it exceeds 1.50%, the ductility of the alloy will decrease and the cost will increase, so the
Limited to 1.50%. Ca: 0.005 to 0.100%; Ca is an element that is a secondary feature of the present invention, and its coexistence with Co or Mo gives the present alloy even greater rust resistance.
The amount added should be at least 0.005%, but if it exceeds 0.100%, the hot workability of the alloy will be significantly impaired, so it is limited to 0.005 to 0.100%. Cr, Cu: 0.50 to 3.00% each; Cu has the effect of further improving rust resistance by coexisting with Co or Mo, and Cr is particularly effective in the present invention by coexisting with Mo. Improves rust resistance. The amount added must be at least 0.50%, and if it exceeds 3.00%, the coefficient of thermal expansion will increase, so the upper limit was set at 3.00%. Ti, Zr: 0.01 to 0.50% each of Ti and Zr; both include Co and Mo, especially when Mo is contained alone (not containing Co) in the present invention.
By coexisting with Mo, it has the effect of further improving rust resistance, and the amount added should be at least 0.01%.On the other hand, if it exceeds 0.50%, hot workability will be impaired, so the upper limit should be set. It was set as 0.50%. According to the present invention, particularly excellent rust resistance is exhibited when all of the above conditions are satisfied. This point will be specifically explained in the embodiments of the present invention shown below. Examples Table 1 shows examples (No. 1 to 21) of this invention alloy.
is a reference example (No. 22) that deviates from the composition range of the present invention.
~25) and conventional amber type alloy (No.26,
The chemical composition, rust resistance, coefficient of thermal expansion, and hot workability are shown in comparison with the comparative example 27).

【table】

[Table] Judgment based on surface crack observation results when rolled.
In Table 1, No. 1 to 2 alloys are Co with Ca and Cu added as additive components, No. 3 is Mo alone, and No. 4 to 9 is a composite of Mo with additive components. Additive alloys, No. 10 to 11 are composite additive alloys of Co and Mo, and alloys No. 12 to 20 are Co and Mo with various additive components: Ca, Cr, Cu, Ti, and r. In the examples in which Ni was added, compared to conventional examples No. 25 and 26, the rust resistance was much better, and the coefficient of thermal expansion and hot workability were also good. However, Fe-36%Ni system Nos. 21 to 24, which are reference examples that deviate from the composition range of this invention, show no major difference from the example steel in terms of rust resistance, but they do have excessive Co content. Added No. 21 had a high coefficient of thermal expansion, and Mo was also added in excess.
No. 22 not only has a large coefficient of thermal expansion but also poor hot workability. In addition, No. 23 with excessive Cu added, Ca,
No. 24 with excessive addition of Zr also has poor hot workability. As described above, the alloy of the present invention has significantly improved rust resistance while maintaining low thermal expansion and other basic properties as an amber-type alloy.
It can be applied to membrane tank materials for transportation and storage of liquefied natural gas, and other uses.

[Brief explanation of drawings]

FIGS. 1a and 1b are graphs showing changes in the rusting rate over spraying time in a salt spray test when the amounts of Co and Mo added were varied, respectively.
Figures 2a and b are graphs showing changes in the rusting rate (spray time: 4 hours) and average linear expansion coefficient from -180°C to room temperature (20°C) in a salt spray test with respect to the amounts of Co and Mo added. be.

Claims (1)

[Claims] 1 Ni: 35-37%, C: ≦0.04%, Si: 0.05-37%
0.25%, Mn: 0.10~0.40%, Co: 0.01~1.50% and Ca: 0.005~0.100%, with the balance consisting of Fe and inevitable impurities.
Ni-based low thermal expansion amber type alloy. 2 Ni: 35~37%, C: ≦0.04%, Si: 0.05~
0.25%, Mn: 0.10~0.40%, Co: 0.01~1.50%,
Fe-Ni low thermal expansion amber type alloy with excellent rust resistance, containing Ca: 0.005 to 0.100% and Cu: 0.50 to 3.00%, with the balance consisting of Fe and inevitable impurities. 3 Ni: 35~37%, C: ≦0.04%, Si: 0.05~
0.25%, Mn: 0.10~0.40% and Mo: 0.05~
Fe-Ni based low thermal expansion amber type alloy with excellent rust resistance, containing 1.50% and the remainder consisting of Fe and unavoidable impurities. 4 Ni: 35-37%, C: ≦0.04%, Si: 0.05-
0.25%, Mn: 0.10~0.40% and Mo: 0.05~
Contains 1.50%, additionally Ca: 0.005~0.10%, Cr: 0.50~3.00%, Cu:
0.50~3.00%, Ti: 0.01~0.50%, Zr: 0.01~0.50
%, and the remainder is
Fe-Ni low thermal expansion amber type alloy with excellent rust resistance consisting of Fe and unavoidable impurities. 5 Ni: 35~37%, C: ≦0.04%, Si: 0.05~
0.25%, Mn: 0.10~0.40% and Co: 0.01~
Contains 1.50% and Mo: 0.05~1.50%, the balance is
Fe-Ni low thermal expansion amber type alloy with excellent rust resistance consisting of Fe and unavoidable impurities. 6 Ni: 35~37%, C: ≦0.04%, Si: 0.05~
0.25%, Mn: 0.10~0.40% and Co: 0.01~
Contains 1.50% and Mo: 0.05~1.50%, and
Ca: 0.005~0.100%, Cr: 0.50~3.00%, Cu:
0.50~3.00%, Ti: 0.01~0.50%, Zr: 0.01~0.50
%, and the remainder is
Fe-Ni low thermal expansion amber type alloy with excellent rust resistance consisting of Fe and unavoidable impurities.