JPS58100661A - High ni alloy with superior weldability and corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a high Ni alloy with superior weldability and corrosion resistance by restricting the N content of a high-Ni iron alloy showing characteristics of ''Invar '' as well as the P, S and O contents to an extremely low level. CONSTITUTION:This high Ni alloy consists of 30-45% Ni, <=0.4% Mn, <=0.25% Si, <=0.05% C, 0.2-0.4% Cr, (Cr-0.1)%<=Co<=Cr%, <=0.005% P, <=0.001% S, 0.003% O, <=0.002% N and the balance essentially Fe. When this high Ni alloy is used as the material of the lining of an LNG storage house which is welded under very severe welding conditions, in order to perfectly prevent the occurrence of weld defects during welding, it is necessary to regulate not only the P, S and O contents but also the N content to said ranges. When the amounts of the elements exceed said ranges, the occurrence of cracks in the weld metal and at the zone affected by heating can not be prevented. The elements are required to satisfy said conditions at the same time.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an iron alloy containing 30 to 45% Nl exhibiting Invar characteristics, and provides a high N1 alloy with excellent weldability and corrosion resistance.

Iron alloys containing 30 to 45 inches of Nl are
Because it has a small coefficient of thermal expansion in a temperature range of 6°C and excellent low-temperature toughness, it is an alloy material whose uses are expanding as containers for storing low-temperature liquids and various functional materials.

However, since such high N1 alloys are austenitic single-phase alloys, they also have poor thermal conductivity, and when welding, the melted solidified areas and heat-exposed areas tend to become extremely brittle, resulting in weld cracking and other problems. This was a big problem. Therefore, as a result of various studies on these matters, the present inventors found that
The reason why the heat-affected zone in the melted and solidified part is prone to embrittlement is that trace impurity elements p, s, o and KN particles contained in the high Ni alloy segregate at the solidified tendrite interface during welding, or due to multiple welding thermal cycles. It was clearly shown that this was caused by the segregation and precipitation at the austenite grain boundaries.

It has been known that Si, P, S, and O promote hot cracking susceptibility during welding.
It is said that it is desirable to set it to 0.4 inches or less, Po, 015 inches or less, and SO, 01% or less.

Furthermore, C& and Mg are added to remove harmful substances such as S and θ.

The method of adding L&*C@ etc. has been found to be effective in reducing high temperature cracking susceptibility. However, the technology to control the amount of these elements added is difficult, adding large amounts may actually promote cracking susceptibility, impair the toughness of the base metal, and addition of small amounts may result in no desulfurization or deoxidizing effect. For these reasons, it is not necessarily an appropriate method to prevent cracking.

The present invention relates to a high-Ni alloy that has significantly reduced susceptibility to weld cracking by suppressing N in addition to p, s, and o to extremely low contents. No cracks occur even when the steel is rolled, and it has extremely excellent workability even in the subsequent hot rolling process.

In the present invention, N130 to 45% and Mn 0.41 or less.

sio, zs- or less, C0,05- or less* Cr 0.
2 to 0.4%, (Cr-0,1)-≦Co≦Cr%, Po, 005- or less, SO, 001% or less, 80.
003-below.

It is a high Ni alloy with NO, 0.002% or less, and the balance is substantially F.

The reason for limiting each component element will be described below.

It has a small coefficient of thermal expansion in the low temperature range from 200°C to -196°C (hereinafter referred to as Invar properties), and has excellent low-temperature toughness and S
The reason for limiting the main elements Ni, Mn, 81, C, Co, and KCr constituting the matrix in order to have excellent contact and corrosion resistance is as follows.

The reason why N1 is set to 30 to 45 inches is that outside this range, the coefficient of thermal expansion becomes large and it becomes difficult to exhibit invar characteristics.

When Mn is contained in excess of 0.4 mm, the low temperature toughness and Invar properties deteriorate. Further, the reason why Sl is limited to 0.25% or less is that if it is contained more than that, the susceptibility to cracking during welding increases.

If C exceeds 0.05-, the corrosion resistance will deteriorate, and the invar properties and low-temperature toughness will also decrease.
,05% or less.

The F alloy containing N130 to N45, which is the object of the present invention, is susceptible to rusting in the atmosphere, which poses a problem during construction and use. However, this rusting property also has a Cr content of 0.2 to 0.
Adding 4 tsp improves dramatically. The effect of adding Cr is that when surface KCr becomes concentrated and segregated during bright annealing, it forms a stable surface film.

As the amount of Co added increases, it causes deterioration of mechanical properties at low temperatures and increases product cost, but it is necessary to add an appropriate amount to compensate for the deterioration of invar properties due to the addition of Cr. Found out. It has also been found that in order to maintain the maximum effect of maintaining Invar characteristics and eliminate the adverse effects of Cr addition, it is sufficient to use Coi in an amount that is approximately the same as the Cr content or approximately 0.1 inch lower than the Cr content. Therefore, if the Cr content is expressed as Cr, (Cr-0,1)-≦Co≦Cr %
becomes.

Low thermal expansion coefficient in low temperature range, low temperature toughness.

The desirable addition amount range of the main elements constituting the matrix for excellent corrosion resistance 1 strength and weldability is Nl 30
~451, Mn 0.1~0.3 fs, 81
0.1~0.2S, C0.02~0.05%, CrO,
2-0.4%-(Cr-0,1)S≦Co≦Cr%
It is.

Next, describe the upper limits for s, o, p, and N.

Conventionally, for s, o, and p, from the viewpoint of preventing weld cracking,
For example, it has been reported that cracking can be prevented by regulating 80.005% or less, 00.025% or less, and P (8 + P) 0.020% or less, and in some cases, Ca +
It has been reported that one or more types of La r CI T Y are added.

In addition, from the viewpoint of preventing bubbles generated during solidification of molten steel, N should be regulated to 0.004- or less or N should be 0.004- or less.
TI when contained in an amount of 4 to 0.03%.

It has been reported that it is effective to add Zr or the like in an amount of 0.02 to 0.1. However, no knowledge has been obtained regarding whether N is harmful to weld cracking or K.

The present inventors have determined that when using a high-density Ni alloy for the lining of an LNG storage tank where welding conditions are extremely severe, we added P to o, ooss to J6 to eliminate welding defects during welding.
Hereinafter, it has been found that it is necessary to restrict SlO, 0O to 11 or less, 0 to 0.003 or less, and N to 0.002 or less.

If the content of these elements exceeds the above-mentioned range, it will not be possible to prevent the occurrence of cracks in the heat-affected zone, which is located in the weld metal part.

Furthermore, these elements will eventually need to satisfy the above conditions at the same time; for example, even if s, o, and p are regulated, N
In the absence of regulations, even if no defects such as cracks are observed in the first weld, cracks may occur in the first weld that has undergone the welding heat cycle during subsequent welding, such as when repairing a crossed welder. I end up.

Next, the present invention will be explained in detail by way of examples.

Example 1 High N1 alloys having the components A to F shown in Table 81 were made into slabs by continuous casting, hot rolled, hot rolled plate annealed, cold rolled and bright annealed to make sheet products.

These expansion coefficients (-180°C to 0°C) were measured and the laminated specimens were immersed in tap water to observe rusting behavior. The results are shown in Table 2, and the judgments in the table are those that are excellent in both thermal expansion coefficient and rust resistance (marked with ○) and those that are inferior (marked with ○).
(marked with an x).

Table 2 Thermal expansion coefficient of test steel and initial test results Example 2 A steel of each chemical composition of A1-40 as shown in Tables 3 and 4 was made into m-thickness. It was processed into a hot rolled sheet and subjected to solution heat treatment (1030°C, held in the weather for 30 minutes, then water cooled). From these steel plates, the length is 320cm, the medium is 40cm,
A test plate with a thickness of 5 cm was prepared and subjected to the Paris strain test (Va
High-temperature cracking susceptibility during welding was evaluated by restraint T@sts (forced bending strain cracking test during welding). 1st
The figure shows the sample shape of the Paristrain, (b) shows a conceptual diagram during test welding on a testing machine, and (c) shows the shape of the test plate after the test.

In the figure, 1 is the test board, 2 is the planned test SM position, and TIG from 3 companies.
@@ ) - Chi, 4 is a bend with a curvature of 8! Lock, 5 indicates weld cracking.

Normally, the Paris strain test examines the occurrence of cracks by applying forced bending surface strain during one weld bead formation.
Here, in order to evaluate the hot cracking resistance during reverse welding and multi-layer welding such as repair welding, we first performed the first bead welding without applying forced bending surface strain, and then (Old bead) Partially overlap IfC and perform second bead welding (new bead).
．． Forced surface distortion of 5 inches was applied. A conceptual diagram of the test plate shape and crack occurrence after such a double bead Paris strain test is shown in Figure 2. 1 In the figure, 6 is the first bead (old bead), 7 is the second bead (
New Peteco, 5 shows weld cracking.

In order to give a forced bending surface strain of 2.5 seconds, K is a bending professional since the test plate thickness is 5.0 -, and the curvature R of R is 100.
■ was used. Further, the welding of the second bead was performed several minutes after the welding of @1 bead. The state of surface cracks that occurred near the point where surface strain was applied was examined using a binocular stereomicroscope with a magnification of 30 times, and the total crack length was calculated.

Cracks occur in the base metal heat-affected zone of the first bead (old bead), second bead (new bead), and the first and second beads, but in the IA skeleton test material, cracking in the base metal heat-affected zone is very small. Furthermore, there was no significant difference in the occurrence status between the test materials. Therefore, the crack K between the first bead and the second bead
Table 5 shows the results of observation and measurement. The test was repeated three times using the same steel type, and the respective crack lengths and their average values are recorded in the table.

M1 to M181 in Table 3 are comparative steels, of which 4
1~44.45~49.410~&12゜413~A1
5 and I&16~hired 18Fi P, 8, respectively.
Six amounts of N, 81, and O are each larger than in the steel of the present invention. In this double-peed Paris strain test, cracking was observed in both cases. Only the N amount is out of the range of the steel of the present invention, and the tail cliff is 10 to 412.0.
For the beads 16 to 118 where only the amount is off, cracking does not occur in the second bead, but only in the first bead (old bead). In addition, in 413 to 15, which are out of the range of the steels of the present invention by an amount of 61, reverse K and cracking did not occur in the first bead but only in the second bead (new bead). Steels with P content and S content outside the range of the steel according to the present invention are classified as @1. Second
Cracks occurred in all beads K. Regarding the occurrence of cracks, the cracks in the second bead increase to the same extent as the amount of KP and 8P increase, whereas the cracks in the first bead increase in size with S
It reacts very sensitively to an increase in the amount of large P.
It is rather insensitive to changes in the amount.7゜In general, cracks in the 1st and 2nd beads are often encountered in austenitic stainless steels. 1) When molten metal solidifies, p, s, st, etc. segregate at grain boundaries. This is solidification cracking caused by K. On the other hand, the cracks observed in the tIc1 bead appear to be caused by K, which occurs when the once solidified weld metal undergoes a welding thermal cycle again, causing grain boundary liquefaction and grain boundary embrittlement. In particular, cracks due to nitridation due to N and OK and grain boundary embrittlement due to oxidation are expected to occur at a considerably low temperature range (approximately 550° C.). In fact, cracks in the first bead occurred in a dispersed range from the vicinity of the strain application point to the welding star) @Ic).

ga table, drop 4 table 419-440 F! Although there are steels according to the present invention, no cracking was observed in any of the steel types, regardless of the first bead or the second bead. 419~mu25. A2
6~Mu37.438~Mu40 are each C*Nl e
The amount of Co was varied within the range of the steel of the present invention, but in the Paris strain test, as shown in Table 5, the amount of Co was 1.
No cracks were observed in any of the second beads.

In addition, the austenitic stainless steel 8U shown in Table 4
The results of the double-peed Paris strain test for 8304, 316, and 3108 are also listed in Table 5, and the first bead cracking was not observed at all, while the second bead cracking occurred severely in 5U8316Ki and 5US3108. . With an S content of 8304, no cracking was observed in any of the beads. These results are in very good agreement with the results obtained in actual welding of austenitic stainless steel.

Based on the above, the steel of the present invention is welded in multiple layers.

Austenitic stainless steel SU8, including repair welding
It was revealed that it has extremely excellent hot cracking resistance and susceptibility to the same level as No. 304.

[Brief explanation of the drawing]

Figure 1 shows a conceptual diagram of the burr strain test, in which (1) shows the shape of the test piece, and (b) shows the test welding being carried out on the testing machine. ) indicates the shape of the test plate after the test. FIG. 2 is an explanatory diagram of the double bead Paris strain test. Figure 7 (b) (C)

Claims (1)

[Claims] Ni 30-45%, No 0.4% or less, 8
10.25% or less, co, oss or less, CrO,2
~0.4% and (Or -0,1)$≦co≦Cr
% 6J), Po, 005S or less, 80.001% or less, Oo, 003% or less, NO, 002- or less, and the balance is substantially 1cIP@ High N1 with excellent weldability and corrosion resistance
alloy.