JPS6058779B2 - High Ni alloy with excellent weldability and corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an iron alloy containing 30 to 45% Ni that exhibits Invar characteristics, and provides a high Ni alloy that has excellent weldability and corrosion resistance.

Iron alloys containing 30 to 45% Ni have a temperature of -1 from 2000C.
Because it has a small coefficient of thermal expansion in a temperature range of 96°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 Ni alloys are austenitic single-phase alloys, they also have poor thermal conductivity, and when welding, the molten and solidified parts and heat-affected zones tend to become extremely brittle, causing weld cracks, etc. It had become a big problem.

Therefore, as a result of various studies on these issues, the inventors of the present invention found that the reason why melted solidified parts and heat-affected zones tend to become brittle is as follows.
The trace impurity elements P, S, and O contained in the high Ni alloy are
It was revealed that this is caused by segregation and precipitation at austenite grain boundaries due to repeated welding thermal cycles or multiple welding thermal cycles. It has been known that Si, P, S, and O promote hot cracking susceptibility during welding, and to prevent cracking, Si should be 0.4% or less, Po, 0.15% or less, and 50.01%.
The following is considered desirable.

Furthermore, in order to remove harmful substances such as 5 and 0, Ca, Mg, and I
It has been found that a method of adding -a, Ce, etc. is also effective for reducing hot cracking susceptibility. However, the technology to control the amount of these elements added is difficult; adding them in large amounts may actually promote cracking susceptibility, impairing the toughness of the base metal, and adding small amounts will not produce any desulfurization or deoxidizing effects. For these reasons, it has not necessarily become an appropriate method to prevent cracking. The present invention relates to a high Ni alloy whose weld cracking susceptibility is significantly reduced by suppressing the contents of N in addition to P, S, and O to extremely low levels.This alloy is manufactured by a continuous casting method. No cracks occur even in the hot rolling process, and the workability is extremely excellent even in the subsequent hot rolling process.

The present invention is characterized by N130-45%, Mn0.4% or less, Si
0.25% or less, Co, 0.5% or less, Cr0.2-0
．． 4%, including (Cr-0.1)%≦CO≦Cr%, P
O. OO5% or less, SO. OOl% or less, 00.003
% or less, NO. It is a high Ni alloy in which the OO is regulated to 2% or less, and the balance is substantially made of Fe.

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

The main elements Nj, Mn, and Si that make up the matrix have a small coefficient of thermal expansion in the low-temperature range from 200°C to -196°C (hereinafter referred to as Invar properties) and have excellent low-temperature toughness, weldability, and corrosion resistance. , C, CO, and Cr are as follows.

The reason why Ni is set to 30 to 45% 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 an amount exceeding 0.4%, low-temperature toughness and Invar properties deteriorate.

Further, the reason why Si 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 is contained in an amount exceeding 0.05%, corrosion resistance will deteriorate, as well as Invar properties and low temperature toughness. O
It was limited to 5% or less.

The Fe alloy containing ~45% Nl3O, 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 is also dramatically improved by adding 0.2 to 0.4% of Cr. The effect of adding Cr is that Cr is concentrated and segregated on the surface during bright annealing, thereby forming 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 the addition of Cr causes deterioration of invar properties. It has been found that an appropriate amount of addition is necessary to compensate for the

In order to maintain the maximum effect of maintaining Invar properties and eliminate the negative effects of Cr addition, the Cr content should be approximately the same as the Cr content or 0.
．． It was found that CO as low as about 1% is sufficient. Therefore C
Display r content as Cr! Then (Cr-0.1)%≦C
O≦Cr%. In order to have a small coefficient of thermal expansion in a low temperature range and excellent low temperature toughness, corrosion resistance, strength, and weldability, the desirable addition amounts of the main elements constituting the matrix include Nl3O to 45%, MnO. l~0.3%, SiO.
1~0.24%, CO. O2~0.05%, CrO. 2
~0.4%, (Cr-0.1)%≦CO≦Cr%. Next, upper limit regulations for S, O, P, and N will be described.

Conventionally, for S, O, and P, from the viewpoint of preventing weld cracking,
For example, S.O. It has been reported that cracking can be prevented by regulations such as OO 5% or less, 00.025% or less, and P (S+P) 0.020% or less, and in some cases, Ca, L
It has been reported that one or more of a, Ce, and Y are added. Regarding N, from the viewpoint of preventing bubbles generated during solidification of molten steel, NO. Regulate OO to 4% or less or N is 0
．． It has been reported that it is effective to add Ti, Zr, etc. in an amount of 0.02 to 0.1% when the content is 0.04 to 0.03%.

However, no knowledge has been obtained that N is harmful to weld cracking. The present inventors have determined that when using real high N1 alloy for the lining of LNG storage, where welding conditions are extremely severe, in order to eliminate any welding defects during welding, P should be 0.005% or less and S should be 0. It has been found that it is necessary to restrict O to 0.001% or less, O 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 weld metal part and heat affected zone.

Furthermore, all of these elements must meet the above conditions at the same time. For example, if S, O, and P are regulated but N is not regulated, even if no defects such as cracks are observed in the initial welding. , during subsequent welding such as cross welding or repair, cracking occurs in the initial weld that has undergone the welding thermal cycle.

Next, the present invention will be explained in detail with reference to Examples.

Example 1 A high Nj alloy of A-F having the components shown in Table 1 was made into a slab by continuous casting, hot rolled and hot rolled plate annealed, then cold rolled and bright annealed to make a sheet product. . 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 evaluation in the table was divided into those that were excellent in both thermal expansion coefficient and rust resistance (marked O) and those that were inferior (marked x). Example 2 No. 2 as shown in Tables 3 and 4. Steel with each chemical composition of 1 to 40 is melted and made into 6.5? A thick hot-rolled plate was processed and subjected to solution heat treatment (1030°C, held in 3-powder atmosphere and then water-cooled).

From these steel plates, the length is 320Tf0n and the width is 40TnIn.
, test plates with 5 thicknesses were created, and a burr strain test (V
The susceptibility to hot cracking during welding was evaluated using a restraint test (forced bending strain cracking test during welding). 1st
The figure shows a conceptual diagram of the burr strain test.
In the figure, a shows the shape of the test plate, b shows a conceptual diagram during test welding while attached to a testing machine, and c shows the shape of the test plate after the test. In the figure, 1 is a test plate, 2 is a planned test welding position, 3 is a TlG welding torch, 4 is a bent block having a curvature R, and 5 is a weld crack.

Normally, the burr strain test examines the occurrence of cracks by applying forced bending surface strain during the formation of a single weld bead.

However, in order to evaluate the hot cracking resistance during repeated welding such as repair welding and multi-layer welding, first bead welding without forced bending surface strain is performed, and then this first bead A second bead weld (new bead) was performed partially overlapping the (old bead), and a forced bending surface strain of 2.5% was applied during the second bead welding. Figure 2 shows a conceptual diagram of the shape of the test plate and the occurrence of cracks after such a double bead burr strain test. In the figure, 6 is the first bead (
7 indicates a second bead (new bead), and 5 indicates a weld crack.

In order to give a forced bending surface strain of 2.5%, the test plate thickness is 5.0 mm, so the curvature R of the bending block is 10.
cyfI1n was used.

Further, the second bead was welded several minutes after the first bead was welded. The state of surface cracks that occurred near the point where surface strain was applied was examined using a three-way binocular stereomicroscope, and the total crack length was calculated. Cracks occur in the first bead (old bead), second bead (new bead), and in the heat-affected zone of the second bead on the base metal side, but in this test material, cracks in the base metal heat-affected zone were extremely rare. Moreover, no significant difference was observed between the test materials in the occurrence status.

Therefore, Table 5 shows the results of observations and measurements regarding the cracks in the first and second beads. The test was repeated on three pieces of the same steel type, and the crack lengths and average values for each were recorded in the table. No. 3 in Table 3. There are comparative steels from No.1 to No.8, among which No.1 to No.1. 4, NO. 5~NO.
9, NO. lO~NO. 12, NO, 13 to NO15 and NO16 to NO18 each have a larger amount of P, S, N, Si and 0 than the steel of the present invention.

In this double bead burr strain test, cracking was observed in both cases. In NOlO to NOl2, where only the N amount is out of the range of the steel of the present invention, and NOl6 to NOl8, where only the O amount is out of range, cracking does not occur in the second bead, but only in the first bead (old bead). . Also,
NO. only the Si amount is out of the range of the steel of the present invention. l3
~NO. 15, on the other hand, cracks do not occur in the first bead but in the second bead.
This occurs only on beads (new beads). In steels with P and S contents out of the range of the steel according to the present invention, cracks occurred in both the first and second beads. Regarding the occurrence of cracks, the cracks in the second bead become larger as the amounts of P and S increase, whereas the cracks in the first bead become very sensitive to increases in the amount of S. It reacts and becomes larger, and is rather insensitive to changes in the amount of P. Generally, cracks in the second bead are often encountered in austenitic stainless steels, and are solidification cracks caused by segregation of P, S, Si, etc. at grain boundaries when molten metal solidifies.

On the other hand, the cracks observed in the first bead appear to be caused by grain boundary liquefaction or grain boundary embrittlement when once solidified weld metal undergoes a welding heat cycle again. In particular, cracks caused by nitridation due to N and O and grain boundary embrittlement due to oxidation are expected to occur up to a considerably low temperature range (approximately 550° C.). In fact, the cracks in the first bead were dispersed over a considerable range from the vicinity of the strain application point toward the welding start side. No. 9 to N in Tables 3 and 4
Although O4O is the copper of the present invention, no cracking was observed in any of the copper types, regardless of the first bead or the second bead.

No. l9~NO. 25, NO. 26~NO. 37,N
O. 38~NO. Although the amounts of C, Nl, and CO in 4O were varied within the range of the steel according to the present invention, in the burr strain test, as shown in Table 5, there was no cracking in either the first or second bead. It was not recognized. In addition, the austenitic stainless steel SUS3O4,3 shown in Table 4
The results of the double bead burr strain test for 16,31OS are also listed in Table 5, and the first bead cracking was not observed at all, while the second bead cracking occurred slightly in SUS316 and quite severely in SUS31OS.

Claims (1)

[Claims] 1 Ni 30-45%, Mn 0.4% or less, Si 0.2
5% or less, C0.05% or less, Cr0.2-0.4%, and (Cr-0.1)%≦Co≦Cr%, and P0.
A high Ni alloy with excellent weldability and corrosion resistance, containing 0.005% or less, S 0.001 or less, O 0.003% or less, N 0.002% or less, and the balance being substantially Fe.