JPH11264055A - Oxdide-dispersed low thermal expansion alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low thermal expansion alloy excellent in hot workability, cold workability, ductility and toughness, particularly impact characteristics at -196 deg.C. SOLUTION: (1) This alloy contains, by weight, 30 to 45% Ni, <=0.03% C, <=0.2% Si, 0.1 to 0.6% Mn, 0.001 to 0.008% O, 0.003% sol.Al and 0 to 10% Co, also satisfying 0.0002 to 0.003% insol.Al or contg. >=two kinds among insol.Al, Ca and Mg so as to satisfy the inequality of 0.0002<=insol.Al+Ca+2.Mg <=0.006, in which among inevitable impurity elements, the content of P is regulated to <=0.01%, that of S to <=0.005% and that of N to <=0.006% respectively, and the balance substantial Fe. (2) The alloy contains oxides by 0.005 to 0.10% by cleanliness. (3) The average grains size is regulated to <=100 μm. (4) The average grain size is regulated to <=100 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low thermal expansion alloy having excellent hot workability, cold workability and the like.

[0002]

2. Description of the Related Art It is widely known that an Fe-Ni alloy having a specific chemical composition has a very small linear expansion coefficient as an Invar effect. Typical examples thereof include Fe-Ni alloys such as Fe-36% Ni and Fe-42% Ni. These parts utilize their low coefficient of thermal expansion to expand and contract due to temperature changes, such as liquid nitrogen temperature (-
It is beginning to be used for storage of liquefied natural gas cooled to 196 ° C) and its peripheral equipment. However, these low-thermal-expansion alloys have extremely poor hot workability, causing cracks or flaws on the end faces and surfaces of the hot rolling top and tail, resulting in a significant decrease in yield. This is because the impurity elements P, S, O, etc. segregate at the grain boundaries during casting, and phosphides,
It is said that sulfides and oxides are generated.

[0003] This has a bad effect on hot workability.
S, O, C, N, Mn, Al, limit the amount respectively,
Alloys that improve hot workability by adding Ti when C and N are contained in excess are proposed (Japanese Patent Publication No.
-42141). Further, while reducing the total of P and S to 0.020% or less, one of Ti, Nb, V, Al, Ta, Zr, La, Ce and Y is contained 0.005% to 0.10% and Ca and Mg are contained. An alloy capable of improving hot workability by containing at least one of 0.002 to 0.03% has been proposed (Japanese Patent Publication No. 57-15656). Also, P + 3 ・ S is 0.009
wt% or less, O (oxygen) 0.0030% or less, cleanliness 0.0
There is also a proposal for an alloy that prevents high-temperature embrittlement by setting the content to 5% or less (Japanese Patent Publication No. 57-35260). However, it is impossible to improve the hot workability to a level that does not cause any problem only by the limitation of the impurity elements P, S, O, etc. for the reasons described below. Further, the addition of Ti, Ca, etc. generates nitrides and sulfides, and although the adverse effects of impurities are reduced, these nitrides and sulfides deteriorate the ductility and toughness, especially the impact value at -196 ° C.

[0004] Therefore, to solve these problems, B
An alloy has been proposed in which the content of 0.001 to 0.03% is contained to suppress segregation of impurity elements at grain boundaries and improve hot workability (Japanese Patent Publication No. 64-8696). However, the addition of B precipitates BN, hardens the matrix with the BN and solid solution B, and generates flaws and the like during cold working.

[0005] Therefore, a low thermal expansion alloy having excellent performance in all of hot workability, cold workability, ductility and toughness, especially impact properties at -196 ° C, has not been known so far.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a low thermal expansion alloy which is excellent in hot workability, cold workability, ductility and toughness, especially impact characteristics at -196 ° C.

[0007]

The present inventors applied Auger electron spectroscopy or the like to the grain boundary fracture surface of the above-mentioned low thermal expansion alloy which had been subjected to grain boundary destruction in a high vacuum. Analysis about segregation was performed. As a result, the cause of the deterioration of hot workability is that the impurity elements S, P, and O segregate at the grain boundaries during solidification, lowering the bonding strength of the grain boundaries, and sulfides generated at this time.
It was confirmed that low melting point compounds such as phosphides and oxides promote grain boundary destruction. However, it was also confirmed that deterioration of hot workability could not be prevented even if S, P, and O were reduced as much as possible using current steelmaking technology. this is,
This is because the crystal grains are coarsened by heating before hot working, so that the susceptibility of the grain boundaries to cracking is further increased. Even if S, P, O
Even if these impurities are segregated even at a small amount in the grain boundaries where cracking sensitivity is increased, grain boundary cracks occur during hot working or the like.

Therefore, the focus was changed to insol.Al, Ca and Mg, which have a crystal grain refining action rather than the reduction of impurity elements.
The effect of slag on hot workability was investigated. As a result, if insol.Al alone or two of these three elements are controlled and contained in a certain narrow range, the crystal grain size becomes fine, and hot workability, cold workability, ductility and The effect of clearly improving the toughness, especially the impact property at -196 ° C, could be recognized.

However, the effect of miniaturization is that high-melting oxides formed in a certain range of insol.Al or an alloy containing two or more of insol.Al, Ca and Mg in a certain range are used. Limited. When the content in the alloy does not fall within the range of the formula even if it contains two or more of Al, Ca and Mg, in the case of a composite oxide not containing these elements, and in the case of a low melting point coarse oxide such as Si and Mn When a large amount of is contained, a fine-grained structure cannot be obtained. On the contrary, the cold workability, ductility and toughness are deteriorated due to these composite oxides and coarse oxides.

The present invention relates to the range of insol.Al which produces oxides exhibiting the above-mentioned grain refining effect, and the range of insol.Al, Ca, Mg
It was completed after a detailed investigation of the range and trial production at a large-scale smelting facility at the site. The gist lies in the following low thermal expansion alloys.

(1) Ni: 30 to 45%, C: 0.03% or less, Si: 0.2% or less, Mn: 0.1 to 0.6%, insol.Al: 0.0002 by weight%
0.003%, sol.Al: 0.003% or less, O (oxygen): 0.001 to 0.00
8% and Co: 0 to 10%, P and S among inevitable impurities
And N are respectively P: 0.01% or less, S: 0.005% or less and
N: An alloy containing 0.006% or less, with the balance being substantially Fe.

(2) Ni: 30 to 45%, C: 0.03% or less, Si: 0.2% or less, Mn: 0.1 to 0.6%, O: 0.001 to 0.008% by weight
%, Sol.Al: 0.003% or less, Co: 0 to 10%, and two or more of insol.Al, Ca and Mg so as to satisfy the following formula, and among the unavoidable impurity elements, P and S , N are each P: 0.01% or less, S: 0.005% or less, N: 0.006% or less, and the balance is substantially Fe, and the low thermal expansion alloy is characterized by being Fe.

0.0002 ≦ insol.Al + Ca + 2 · Mg ≦ 0.006 Here, the symbol of the element indicates the weight% of the element.

(3) The alloy according to (1) or (2), which contains 0.005 to 0.10% or less of an oxide in cleanliness.

(4) The alloy according to (1) or (2), wherein the average crystal grain size is 100 μm or less.

(5) The average crystal grain size is 100 μm or less;
The alloy according to (1) or (2), which contains an oxide in an amount of 0.005 to 0.10% or less in cleanliness.

The above (1), (2) or (3) is for an alloy after final hot working, an alloy after heat treatment (annealing), an alloy after cold working, and an annealing after cold working. Target alloys. On the other hand, the alloys of (4) and (5) above include alloys after final processing, alloys that have been subjected to heat treatment (annealing), and alloys that have been cold-worked and annealed.

"The balance is substantially Fe" means that among the unavoidable impurities, P, S and N are included in the above-mentioned range, and the other unavoidable impurities include Fe and the balance is Fe.

The cleanliness of the oxide is measured by the following method. As in the present invention, containing a certain amount of O and Al, S: 0.005
% Or less and Mn: 0.1 to 0.6%, sulfides such as MnS are found in the visual field of an optical microscope in view of their morphology and color to form Al.
It is clearly distinguished from oxides and the like. Therefore, it is possible to easily measure the cleanliness of only the oxide in accordance with JIS G 0555 while avoiding sulfides. Average grain size is JIS G
Measure according to 0551.

[0020]

Next, the reason why the present invention is limited as described above will be described. In the following description, “%” of the alloy element indicates “% by weight”.

Ni: 30 to 45% Ni is a main element constituting the low thermal expansion alloy. In order to obtain a sufficiently low coefficient of linear expansion, it is necessary to contain 30 to 45%, and even if it is less than 30% or exceeds 45%, a sufficiently low coefficient of linear expansion cannot be obtained. A desirable lower limit is 32%, and a more desirable lower limit is 34%. Also, the desirable upper limit is 43%
It is.

C: 0.03% or less C is an element for stabilizing the austenite phase which is the main phase of the base material. However, if contained excessively, both ductility and toughness are reduced, so the content is made 0.03% or less. A more desirable upper limit is 0.02%.

Si: 0.2% or less Si is added for the purpose of deoxidation during steelmaking. However, when added in a larger amount as the content exceeds 0.2% and the content is higher, a coarse Si-based oxide having a low melting point is generated, which causes deterioration in hot workability. Further, since a composite oxide is generated with Mn and the like, and the cold workability is deteriorated, the upper limit of the Si content is 0.2%. A desirable upper limit is 0.15%, more preferably 0.1%.
%. Although there is no particular need to set a lower limit, an extremely low value causes a remarkable increase in the refining cost.

Mn: 0.1-0.6% Mn is added for the purpose of deoxidation at the time of steel making like Si. However, when added in such a large amount that the Mn content exceeds 0.6%, Si
In the same manner as described above, a coarse oxide having a low melting point is generated, and hot workability, toughness, and ductility are deteriorated. The sulfide generated at this time deteriorates corrosion resistance. For this reason, Mn is 0.1-0.6%
And A more desirable range is 0.12 to 0.55%, and even more desirably 0.1 to 0.5%.

O: 0.001 to 0.008% O (oxygen) is conventionally considered as an unavoidable impurity, and efforts have been continuously made to reduce it. However,
O combines with Al to generate an Al-based oxide, and contributes to improvement of hot workability through refinement of austenite crystal grains. For that purpose, a content of 0.001% or more is required. However, if the concentration is too high, the composite oxide, Si, Mn
Oxide which is not effective for equal miniaturization is generated, but rather deteriorates hot workability and cold workability. Also, increasing the volume fraction of the oxide reduces ductility and impact value at -196 ° C. Therefore, O is set to 0.008% or less. A more desirable upper limit is 0.006%.

Sol.Al: 0.003% or less Forming Al and nitride dissolved in the matrix
The total of Al, that is, sol.Al, is dispersed as AlN during hot working or cooling after hot working, causing dispersion hardening, significantly deteriorating hot workability, and lowering toughness and ductility. Therefore, the content is set to 0.003% or less. Although the lower limit is not particularly limited, it is preferable that sol.Al is contained in an amount of 0.0005% or more in order to sufficiently perform deoxidation.

Co: 0 to 10% Co may not be contained. However, Co is a main element for stabilizing the austenite phase which is a matrix of the base material, and has an effect of lowering the linear expansion coefficient like Ni. Therefore, it may be contained in a range of 10% or less. But 10%
Exceeding this causes a decrease in ductility and impact value at -196 ° C, and a significant increase in alloy cost. A desirable upper limit is
8%, and a more desirable upper limit is 5%.

Next, the oxide will be described.

Insol.Al: 0.0002-0.003% Al is a strong deoxidizing element and is added as a deoxidizing agent during steel making. Combined with O (oxygen) to form oxide insol.Al, refine the crystal grains, suppress the coarsening of crystal grains during hot working, and improve hot workability. Insol.Al means the content of Al that forms an oxide, and is a value determined as Al that does not dissolve in acid in the state analysis of Al. Insol.Al necessary to express the effect of this oxide is
It is 0.0002% or more, and more preferably 0.0005% or more. However, if the insol.Al exceeds 0.003%, the Al oxide coarsens and the effect is diminished, and the volume ratio of the Al oxide increases, leading to a decrease in toughness at -196 ° C.
003%. More preferably, it is set to 0.002% or less.

As described above, the oxide can exert a sufficient grain refining effect only by an aluminum oxide containing Al as a main component, but as will be described below, insol. Even when two or more kinds are included in the following ranges, a fine graining effect can be stably obtained.

Insol. Al, Ca and Mg: containing two or more kinds,
And the range satisfying the formula (0.0002 ≤ insol.Al + Ca + 2 ・ Mg ≤
0.006 ···) Al is a strong deoxidizing element and is added as a deoxidizing agent.
Al combines with O (oxygen) to form an oxide, refines the crystal grains and suppresses the coarsening of the crystal grains during hot working,
It has the effect of improving hot workability. Where insol.Al
The term refers to the content of Al present as an oxide in the alloy. That is, it refers to the Al content obtained by subtracting the total of solid solution Al and Al as nitride from all Al.

Ca and Mg are also strong deoxidizing elements like Al, and almost all exist as oxides in the alloy. These oxides refine crystal grains and contribute to improvement in hot workability and cold workability. “Insol.Al + Ca + 2 · Mg” is adopted as an index for judging the effects of these elements comprehensively. Again, each element symbol is the weight percent of that element
Is displayed. Therefore, the value of this index is% by weight, but "% by weight" is omitted. From now on, this index will be referred to as “refining index”. The grain refining effect has the largest Mg among the above elements, and improves the hot workability with a very small content. Therefore, the above-mentioned grain refining index is weighted twice as much as other elements.

In order for the alloy of the present invention to be sufficiently fine-grained and to have all of the excellent hot workability, cold workability, ductility and toughness, the fine-graining index is set to 0.0002 or more. More preferably, it is set to 0.0005 or more. On the other hand, when the grain refining index exceeds 0.006, the oxide becomes coarse and the cleanliness (volume ratio) of the oxide increases, the grain refining effect decreases, the ductility and
Since the impact property at -196 ° C is deteriorated, the upper limit is made 0.006.

Of the unavoidable impurities, P, S and N must be limited to the following ranges.

P: 0.01% or less P is an unavoidable impurity. However, in order to secure hot workability and reduce the susceptibility to hot cracking during welding of a product, 0.01% or less.
The following is assumed. Desirably, it is 0.008% or less. Although there is no particular lower limit, an extremely reduced amount leads to an increase in cost. Therefore, the lower limit is preferably set to 0.0005% or more. More preferably, the content is 0.0008% or more.

S: 0.005% or less Like P, it is an inevitable impurity. Since the hot workability and the hot cracking susceptibility during welding and the reheat cracking susceptibility during multi-layer welding are extremely increased, the content must be 0.005% or less. Further, it is desirable to set the content to 0.004% or less. Although there is no particular lower limit, an extremely reduced amount leads to an increase in cost. Therefore, the lower limit is preferably set to 0.0002% or more. More preferably, it is set to 0.0005% or more.

N: 0.006% or less N is an unavoidable impurity element. However, if it is contained excessively, it combines with solute Al during hot working to form a nitride, hardens the matrix, and improves hot workability. Reduces cold workability, toughness and ductility. Therefore, the upper limit is made 0.006%. Further, extremely lowering causes an increase in refining-casting cost. Therefore, a lower limit is not particularly set, but is preferably 0.0005% or more.

Oxide cleanliness: 0.005 to 0.1% O (oxygen) and insol.Al content within the above range, as long as appropriate melting and casting are carried out, the target hot working of the present invention Workability, cold workability, ductility and impact properties at -196 ° C can be ensured. However, if the molten metal is not sufficiently agitated at the time of smelting, the oxide such as Si is not sufficiently floated and contains a large amount of oxide such as Si.
The impact value at -196 ° C decreases. O (oxygen), insol.Al, Si
When Mn and Mn are within the above-defined range of the present invention, when the cleanliness of the oxide exceeds 0.1%, ductility and toughness decrease, so that the content is set to 0.1% or less. More preferably, it is set to 0.08% or less. On the other hand, when the cleanliness of the oxide is less than 0.005%, the austenite crystal grains are not sufficiently refined and the hot workability is not improved to a satisfactory level, so the content is made 0.005% or more. More preferably, the content is 0.008% or more.

Average crystal grain size: 100 μm or less When a product alloy is hot or cold formed, crystal grains may be coarsened and cracked by a heat cycle during heating. In addition, if the crystal grains of the product are coarse, cracks are likely to occur due to cold forming. In order to prevent such a problem, if the average crystal grain size in the product state is 100 μm or less, the above O (oxygen) or insol.Al
Even if heating is performed for hot working due to the effect of (1), the crystal grains do not significantly increase in size and cracks do not occur. In addition, the cold workability, and further, the ductility and toughness were also improved in the average crystal grain size.
When the thickness is 0 μm or less, it is greatly improved.

In order to reduce the average crystal grain size to 100 μm or less, the heating temperature in the final hot working should be set as long as the content of O (oxygen) and insol.Al is within the scope of the invention defined as the present invention.
1300 ° C or less, the in-furnace time of the heating is 2h or less per 10mm in the thickness of the maximum thickness part of the alloy, and the hot working finishing temperature is
1050 ° C or lower, workability of about 3 to 35, annealing temperature of 7
It can be realized by setting the annealing furnace time to about 100 to about 1050 ° C. and about 0.5 to about 9 hours. Here, the degree of processing is defined by (thickness before height increase) / (finish thickness) of the final processing. Further, the annealing treatment after the cold working is also the same as the annealing condition after the final hot working, so that the
It can be 0 μm or less. The average crystal grain is preferably as fine as 100 μm or less, and the lower limit is not particularly defined,
The minimum value obtained in completing the present invention is 35 to 40 μ
m.

[0041]

EXAMPLES The effects of the present invention will now be described with reference to examples.

[Test on Invention (1)] First, the test on the above invention (1) will be described.

150 kg of 10 kinds of alloys were melted under vacuum,
100mm thickness, 200mm width, heated to 150-1250 ℃
Forged to a length of 00 mm.

Table 1 shows the chemical compositions of these alloys.
After forging, at the time of final hot working, the alloy is heated to each temperature of 900 ~ 1250 ℃, at a finishing temperature of 900 ℃ 9.5mm thickness, 300m
Hot rolling was performed to a width of m and a length of 2000 mm. Processing degree is about 10.5
(100 / 9.5).

[0045]

[Table 1]

The hot workability was judged as acceptable or unacceptable when no scratches or cracks appeared on the rolling top, tail end or surface during hot forging and final hot rolling, and the others were rejected. Thereafter, the steel sheet was annealed at 850 ° C. × 0.5 h, and then pickled. After pickling, the steel sheet was cut to a thickness of 9.5 mm, a width of 300 mm, and a length of 500 mm, and from one of them, a tensile test piece (JIS Z22015 No. 5 test piece) and a Charpy impact test piece (JIS Z2202 No. 4 test piece) A 5 mm piece of Sapsei) was sampled and tested at room temperature and at -196 ° C, respectively, to evaluate the performance of the steel sheet. Pass / fail judgment is that ductility is 20 by tensile test
% Or more, and those satisfying both the Charpy impact value of 100 J / cm ² were judged as acceptable. In addition, a gold phase specimen was collected,
Examination of the cleanliness of the oxide and measurement of the average crystal grain size were performed by a microscope using an optical microscope. Investigation of oxide cleanliness is JIS G
The test was carried out at an observation field number of 60 and an observation magnification of 400 times in accordance with “Microscopic observation test method for non-metallic inclusions in steel” described in 0555. At this time, the measurement was performed while avoiding sulfides as described above.
Thereafter, the steel sheet was cold rolled to 2.0 mm, and the cold workability was evaluated. In the pass / fail judgment, as in the case of hot rolling, a case where no defect such as a flaw was generated on the surface was passed, and the other cases were rejected. Further, after that, the steel sheet was annealed under the condition of 850 ° C. × 0.5 h, then pickled, a No. 5 tensile test piece described in JIS Z2201 was sampled, and a tensile test was performed. Then, the crystal grain size and the oxide cleanliness were measured. Further, a bending test piece (JIS Z 22043) was collected and subjected to a 180 ° bending test. For pass / fail judgment, perform a penetrant inspection test of the bent part,
The test was performed with or without indication.

Table 2 shows the results of the above tests. Note that the cleanliness of the oxides in Table 2 is different after the final hot rolling annealing and after the cold working annealing, but the difference is not remarkable and both are close to each other and can be considered as a range of the measurement error.

[0048]

[Table 2]

In test number NB1, which is a comparative example, insol.
Is contained in a large amount of 0.0036%, the oxide was coarsened, the effect of refining the crystal grains was not sufficiently obtained, and cracks occurred at the edges during the final hot rolling. In addition, although sufficient toughness and ductility were obtained after hot rolling, fine cracks were generated by bending due to insufficient ductility after cold rolling due to coarse oxides. NB2, which is also a comparative example, is sol.A
Since l was excessive, that is, 0.0042%, AlN was precipitated, and a number of cracks occurred at the edge and surface during hot working. Furthermore, the toughness and ductility deteriorated due to the influence of AlN precipitation, and cracks were found at the edges even during cold working. NB3 of Comparative Example
Is N in excess of 0.008%, as in the case of NB2, Al
N was precipitated, and hot workability, cold workability, toughness, ductility, and bending performance were poor. Further, Comparative Example NB4 is O (oxygen)
Is an excess of 0.010%, so that the oxide was a complex oxide such as Si and Mn, the effect of miniaturization was not obtained, and many edge cracks occurred during hot working. Furthermore, the cleanliness of the oxide is 0.
Since the content was as large as 115%, the toughness was poor. Also,
Many surface scratches occurred during cold working, and the cold workability was poor. Furthermore, ductility was deteriorated, and fine cracks were generated by a bending test. In Comparative Example NB5, since Si is out of the range defined by the present invention, the oxide is a low-melting-point oxide mainly composed of Si, and the effect of miniaturization is not obtained, and satisfactory hot workability and cold workability are obtained. I couldn't. Further, the cleanliness of the oxide was increased, the low-temperature toughness and ductility were poor, and the bending performance was poor. Further, in Comparative Example NB6, since insol.Al and sol.Al were respectively excessive, Al oxide was coarsened and AlN was precipitated, and numerous cracks were generated during hot forging, and cracks were generated even in hot rolling. It progressed, and subsequent test piece processing and the like were impossible.

On the other hand, it has been found that the alloy of the present invention has excellent hot workability, cold workability and toughness, and can achieve the target performance of the present invention only within the defined range of the present invention. .

[Test for Invention (2)] Twelve kinds of alloys were melted in a vacuum of 150 kg and the steel ingot was heated to 1150-1250 ° C. for 100 hours.
Forged to t × 200w × 300lmm.

Table 3 shows the chemical compositions of these alloys.

[0053]

[Table 3]

The steel obtained by forging was subjected to hot rolling, annealing and pickling in the same manner as in the test for the invention (1), and subjected to the same test as above. Thereafter, cold rolling, annealing and pickling were performed in the same manner as above, and the same test was performed.

Table 4 is a list showing the results of the above test.

[0056]

[Table 4]

Test No. MB1, which is a comparative example, has a grain refining index of 0.0066, which exceeds the upper limit of the definition range of the present invention of 0.006, so that the oxide becomes coarse and the crystal diameter after hot rolling is 103%.
It was coarsened to μm. For this reason, cracks occurred at the rolling top and tail end faces during hot rolling.

Also, the ductility and the impact value at -196 ° C. deteriorated. Further, fine cracks were generated by bending. Comparative Example MB
Similarly, in the case of No. 2, the grain refining index was 0.0065, and since it exceeded 0.006, the hot workability, ductility, toughness, and bendability were poor. In Comparative Example MB3, O (oxygen) is 0.011% and is excessive, so that a composite oxide such as Si or Mn is generated, and the effect of miniaturization by an oxide such as Al or Ca is not obtained. Many edge cracks occurred. Furthermore, the toughness deteriorated due to the high oxide cleanliness of 0.140%. In addition, many surface flaws were generated at the time of cold working, cracks were generated in a bending test of the product, and the result that cold workability was poor was obtained. Comparative example
MB4 has an excess of N of 0.009%, so AlN
Were precipitated, the hot workability was deteriorated, and the matrix was hardened by the AlN, so that many flaws were generated on the surface during cold rolling. The toughness, ductility and bending workability were also poor. In Comparative Example MB5, Si and Mn were out of the defined range of the present invention, so that a low-melting oxide containing Si and Mn as main components was generated,
The effect of miniaturization was not obtained, and satisfactory hot workability and cold workability were not obtained. In addition, the cleanliness of the oxides increased and the toughness also deteriorated. For Comparative Example MB6, the refinement index is 0.0072 and excessive, and sol.Al is 0.0038% out of the defined range of the present invention, the effect of the refinement is not obtained,
Due to the high-density AlN precipitation, a large number of cracks occurred during hot forging, and the cracks developed during hot rolling, so that hot rolling could not be continued.

On the other hand, MA1 to MA9 using alloys within the definition range of the present invention are excellent in all of hot workability, cold workability, ductility and toughness, especially in all impact values at -196 ° C. It became clear that there was.

[0060]

Industrial Applicability According to the present invention, a large amount of industrial materials having low thermal expansion can be supplied. In particular, the transportation and storage of liquefied natural gas, whose demand is continuously increasing as clean energy, and new peripheral equipment for the same. It is expected to contribute to technology development and its use.

Claims (5)

[Claims] (1) Ni: 30 to 45%, C: 0.03% or less, S
i: 0.2% or less, Mn: 0.1 to 0.6%, insol.Al: 0.0002 to 0.00
3%, sol.Al: 0.003% or less, O (oxygen): 0.001-0.008% and Co: 0-10%, P, S and unavoidable impurities
N is P: 0.01% or less, S: 0.005% or less, and N: 0.
A low thermal expansion alloy characterized by being at most 006% and the balance being substantially Fe. 2. Ni: 30 to 45% by weight, C: 0.03% or less, S
i: 0.2% or less, Mn: 0.1 to 0.6%, sol.Al: 0.003% or less, O
(Oxygen): 0.001 to 0.008% and Co: 0 to 10%, and insol.Al, Ca and Mg are included so as to satisfy the following formula, and P, S and inevitable impurity elements are included. N
However, P: 0.01% or less, S: 0.005% or less and N: 0.0
A low thermal expansion alloy characterized by being at most 06%, with the balance being substantially Fe. 0.0002 ≦ insol.Al + Ca + 2 · Mg ≦ 0.006 Here, the element symbol indicates the weight% of the element. 3. The low thermal expansion alloy according to claim 1, wherein the low-thermal-expansion alloy contains 0.005 to 0.10% or less in terms of cleanliness. 4. The low thermal expansion alloy according to claim 1, wherein the average crystal grain size is 100 μm or less. 5. The low thermal expansion alloy according to claim 1, having an average crystal grain size of 100 μm or less and containing an oxide of 0.005 to 0.10% or less in cleanliness.