JPH1017997A - High strength invar alloy excellent in hot workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength invar alloy withstanding mass-production and having excellent hot workability without deteriorating its thermal expansibility even in a low temp. region from a room temp. to a liq. nitrogen temp. SOLUTION: This invar alloy has a chemical compsn. contg., by weight, 0.015 to 0.10% C, <=0.35% Si, <=1.0% Mn, <=0.015% P, <=0.0010% S, <=0.3% Cr, 35 to 37% Ni, 0 to 0.5% Mo, 0 to 0.05% V, <=0.01% Al, 0.15 to <1.0% Nb, <=0.003% Ti and <=0.005% N or, in which the content of S is regulated to <=0.002% and that of Ti to 0.05 to 0.2%. The alloy furthermore contains 0.0005 to 0.005% B according to necessary, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an invar alloy (invar alloy) having high strength, excellent hot workability, low production cost, and low thermal expansion coefficient at room temperature or lower.
It is about.

[0002]

2. Description of the Related Art Fe-Ni based low thermal expansion invar alloys containing 34-45% (hereinafter, "%" representing a component ratio is referred to as "% by weight") of Ni have a temperature range from extremely low temperature to about 300 ° C. Because it shows a low coefficient of thermal expansion over a relatively wide temperature range,
In the past, taking advantage of this feature, it has been frequently used in tanks for liquefied natural gas transport vessels, shadow masks for large-sized televisions and cathode ray tubes for still images, IC lead frames, glass sealing materials, and the like. .

[0003] It should be noted that the characteristics more strongly required for the above-mentioned invar alloy vary depending on the "temperature range to be used" and the "degree of thermal expansion coefficient required for the alloy member". In the case of so-called “36Ni alloy”,
In particular, the "thermal expansion characteristic" is the most important characteristic because it was developed to utilize a small coefficient of thermal expansion. It is known that reducing the content of alloying elements such as C, Si, and Mn is effective in reducing the "coefficient of thermal expansion near room temperature" of Invar alloys. An extremely low coefficient of thermal expansion of the order of 1.2 × 10 ⁻⁶ / ° C. can be achieved by reducing as much as possible.

However, such high-purity invar alloys are often used in a form having a wall thickness of less than 2 mm, and therefore, it is desired that the high-purity invar alloy has sufficient strength to make up for it. The strength at room temperature is about 20 to 40% lower than that of "9% Ni steel" or "SUS304 steel" used as the same low temperature material. Therefore, when an attempt is made to use the steel sheet as a thick steel sheet, in this case, there is a problem that the crystal grain size tends to increase in the manufacturing process, and the strength is further reduced.

However, it is known that by adding an alloy element, it is possible to increase the strength of the invar alloy, and it is effective to add, for example, Co, Nb, C, Cr or the like as an alloy element. Other known techniques for increasing the strength include a "method of refining crystal grains by adjusting the final annealing temperature" and a "method of strengthening by cold working". It can be said that "addition" is a universal method capable of increasing the strength of not only the base material but also the welded portion.

However, the addition of alloying elements to increase the strength often lowers the hot workability of the Invar alloy. That is, FIG. 1 shows that S: less than 0.001%, P: less than 0.005%,
N: less than 0.002%, Al: less than 0.003%, the contents of S, P, N and Al are each regulated, and Nb is added in various proportions as a strengthening element. 0.03% C-0.2% Si-0.3% Mn- 36% Ni
The results obtained by evaluating the hot ductility of the -Nb-based invar alloy by the "reduced value (cross-sectional shrinkage) at the time of a tensile test at a strain rate (相当 1 × s ^-1 ) corresponding to hot rolling". FIG. 1 also shows that the addition of a strengthening element such as Nb to the Invar alloy significantly reduces the hot ductility.

[0007] Originally, invar alloys cannot be said to have good hot workability. Particularly, when a continuous cast slab is used, the hot workability is significantly reduced. Nevertheless, cracking and surface flaws often occur during hot rolling, and are low in productivity and production yield. In addition, since a large amount of expensive Ni is contained as a component element, a reduction in the production yield has resulted in a significant increase in cost. Therefore, increasing the strength of an invar alloy by adding an alloying element such as Nb or C, which lowers hot workability, means that even when the ingot method is used, ear cracks, surface flaws, etc. in the hot working step. However, in the case of considering mass production of hot working flaws and considering mass production of materials, it cannot be said that such means can be adopted as it is.

It is known that it is effective to add an appropriate amount of B to improve the hot workability of an Invar alloy. For example, Japanese Patent Publication No. 64-8669 and Japanese Patent Publication No.
No. 8692 reports that "the addition of B improves the hot workability of an invar alloy (invar alloy)". Japanese Patent Application Laid-Open No. 5-171357 discloses that "the hot workability of an Nb-containing invar alloy is improved by adding a small amount of B".

However, although the addition of B is certainly an effective means for improving the hot workability of an Invar alloy, it is necessary to finely adjust its content in order to secure a desired effect. If the adjustment is not made, there is a problem that boride is formed, and conversely, the hot workability is significantly adversely affected. In particular, in the case of an Nb-containing invar alloy, as pointed out in the above-mentioned JP-A-5-171357, unless the amount of B added is strictly controlled in accordance with the Nb content, a remarkable decrease in hot workability is caused. There was a danger, and it could not be said to be suitable as a means for mass-producing alloy materials.

[0010] However, Japanese Patent Publication No. Sho 64-8 mentions that the addition of B improves the hot workability of an invar alloy.
No. 696, Japanese Patent Publication No. Hei 1-8692 or Japanese Patent Application Laid-Open No. Hei 5-171357 disclose any of the techniques disclosed in
Used in the "temperature range higher than room temperature" up to about 400 ° C, such as lead frames, power transmission core wires, precision machine parts, etc., in which relatively small heat of less than 10 ^-6 / ° C is used. It is intended only for high-strength invar alloys for applications that do not necessarily need to be mass-produced, such as for ultra-thin sheets and wires that maintain the expansion coefficient. However, in order to produce raw materials for low-temperature storage tanks such as tanks for transporting liquefied natural gas, "establishing a mass production system" is an inevitable requirement. Nevertheless, "a low-cost practical invar alloy with high strength while maintaining excellent thermal expansion characteristics of 1.5 × 10 ^-6 / ° C or less in the temperature range below room temperature" Has not yet established a mass production system.

In view of the above, an object of the present invention is to provide an excellent hot workability capable of withstanding mass production without impairing low thermal expansion characteristics even in a low temperature range from room temperature to liquid nitrogen temperature. The purpose of the present invention is to establish a means for stably providing a high-strength invar alloy having the following features.

[0012]

Means for Solving the Problems The present inventor has made intensive studies to achieve the above object, and as a result, has obtained the following findings. a) To improve the strength of Invar alloy, C and Nb are used as alloying elements.
Is a very effective and stable means to add
In this case, there is a concern that the addition of Nb significantly reduces the hot workability of the alloy as described above. However, according to the study of the present inventors, the decrease in hot workability when Nb is added is caused not only by Nb itself,
Ti which is inevitably contained in the Nb raw material is also a major cause of the deterioration of hot workability, and this trace amount of Ti and S which is also contained in the alloy as an impurity in the hot working temperature range. It has been clarified that sulfide precipitates at the crystal grain boundaries and promotes deterioration of hot workability. Therefore, based on this fact, `` Even if the strength of an invar alloy is increased by adding Nb, it is necessary to appropriately adjust the Nb content and minimize the content of the impurity elements S and Ti. For example, the hot workability sufficient for mass production can be ensured without having a particular adverse effect on the thermal expansion characteristics. "

B) On the other hand, even when Ti is positively added and contained in a large amount in the invar alloy, the sulfide crystal grain boundary precipitation during hot rolling does not occur, so that the Nb content is appropriately adjusted. If this is the case, even if the S content is somewhat high, hot workability sufficient for mass production can be ensured. FIG. 2 is a graph showing the examination results of the Ti content and the hot ductility (reduced value) of the 36% Ni-0.3Nb-0.001% S-Ti-based invar alloy, which are shown in FIG. The results show that the Ti content of the Nb-added invar alloy is as low as possible (0.0005%),
Conversely, when it is contained in a certain amount (0.12%), the tendency of the hot ductility is improved.

C) Further, if the Nb content is properly adjusted, the addition of B as needed can be an effective means of improving hot workability without being disadvantageous for mass production.

The present invention has been made based on the above findings and the like. "The chemical composition of an invar alloy is as follows: C: 0.015 to 0.10%, Si: 0.35% or less, Mn: 1.0% or less, P: 0.015% %, S: 0.0010% or less, Cr: 0.3% or less, Ni: 35 to 37%, Mo: 0 to 0.5%, V: 0 to 0.05%, Al: 0.01% or less, Nb: 0.15% to less than 1.0% , Ti: 0.003% or less, N: 0.005% or less, or C: 0.015 to 0.10%, Si: 0.35% or less, Mn: 1.0% or less, P: 0.015% or less, S: 0.002% or less, Cr : 0.3% or less, Ni: 35 to 37%, Mo: 0.5% or less, V: 0.05% or less, Al: 0.01% or less, Nb: 0.15% to less than 1.0%, Ti: 0.05 to 0.2%, N: 0.005% By containing B and 0.0005 to 0.005% as necessary, and adjusting the balance so that the balance consists of Fe and unavoidable impurities, excellent thermal expansion at room temperature or lower can be achieved. Properties, that was allowed combines high strength and good hot workability "
It has great features.

[0016]

As described above, the present invention relates to an invar alloy which has been strengthened by the combined addition of C and Nb, and has a hot working strength capable of withstanding mass production of a material without impairing the thermal expansion characteristics from room temperature to liquid nitrogen temperature. The chemical composition of the alloy is devised so that the workability is imparted. The reason why the chemical composition of the alloy is limited as described above in the present invention together with the action of each element will be described in detail below. C: C is added because it is effective for increasing the strength of the alloy. However, if the content is less than 0.015%, the desired strength of the alloy cannot be secured. Since the addition impairs the excellent thermal expansion characteristics (low thermal expansion coefficient) of the alloy, the C content was determined to be 0.015 to 0.10%.

Si and Mn: Si and Mn are basic elements necessary as a deoxidizing agent in the production of an alloy, but if both are contained in excess, the thermal expansion characteristics of the alloy are impaired. Therefore, the contents of Si and Mn are determined to be 0.35% or less and 1.0% or less, respectively.

P: P is an impurity element which remarkably enhances the susceptibility of the alloy to solidification cracking and welding cracking. If the content of P is 0.015% or less, a mass production system can be maintained. Therefore, the upper limit of the P content is set to 0.015%.

Cr: If the Cr content exceeds 0.3%, the adverse effect on the thermal expansion coefficient of the alloy becomes remarkable. Therefore, the Cr content was determined to be 0.3% or less.

Ni: Ni is the most important element that controls the thermal expansion characteristics of the alloy, and its thermal expansion coefficient can be minimized by adjusting its content to 35 to 37%. Therefore, Ni
The content was determined to be 35 to 37%.

Mo and V: Since Mo and V are both elements that deteriorate the thermal expansion characteristics of the alloy, the addition of these elements must be avoided. At least 0.5 for Mo content
% Or less, and the V content must be suppressed to 0.05% or less to ensure desired thermal expansion characteristics.

Al: Al is an element that degrades the hot workability of the alloy, and its content must be regulated from the viewpoint of hot workability. In particular, when the Al content exceeds 0.01%, the adverse effect on hot workability becomes remarkable, so the content is set to 0.01% or less.

Nb: Nb is an important element necessary for improving the strength of the alloy, and the desired strength cannot be ensured unless the content is 0.15% or more. On the other hand, in the alloy of the present invention, when the Nb content is 1.0% or more, the thermal expansion characteristic tends to be deteriorated. Therefore, it must be strictly avoided to contain 1.0% or more Nb. Therefore, the Nb content is determined to be 0.15% or more and less than 1.0%.

S and Ti: Both S and Ti are unavoidable impurities that are mixed in when the alloy of the present invention is melted (Ti is unavoidably mixed particularly with the Nb raw material). It is an important element that affects the hot workability of an alloy. That is, S and Ti in the alloy precipitate at the grain boundaries as sulfides in the hot working temperature range, and significantly degrade hot workability. However, S
Reduce the Ti content to 0.0010% or less (preferably 0.0005% or less) and reduce the Ti content to 0.003% or less (preferably 0.0
When the content is regulated to 01% or less, the hot workability sufficient for mass production can be secured without significantly adversely affecting the thermal expansion characteristics in combination with the effect of adjusting the Nb content.

On the other hand, in the alloy of the present invention, Ti
Even in the case of containing a large amount as described above, sulphide crystal grain boundary precipitation during hot rolling does not occur, so that good hot workability can be ensured. When Ti is positively contained at 0.05% or more, the allowable upper limit of the S content is increased to 0.002%. However, excessive addition of Ti degrades the thermal expansion characteristics, so even in this case it is necessary to limit the Ti content to 0.2% or less.

Therefore, regarding the contents of S and Ti,
"Ti content is 0.003% or less and S content is 0.00
10% or less "or" Ti content is 0.05 to 0.2% and S content is 0.002% or less ".

N: N is an unavoidable impurity element that deteriorates the hot workability of the alloy. In the alloy of the present invention, if its content exceeds 0.005%, the tendency of the hot workability to deteriorate becomes remarkable. It has been determined that the N content should be reduced to 0.005% or less.

B: B has the effect of improving the hot workability of the alloy, and the Nb content is adjusted to a range of 0.15% or more and less than 1.0%. Is effective. However, if the B content is less than 0.0005%, the desired effect cannot be obtained by the above-mentioned action. On the other hand, if the B content exceeds 0.005%, the thermal expansion coefficient increases significantly and the properties of the invar alloy deteriorate. Therefore, the B content was determined to be 0.0005 to 0.005%.

When melting the alloy of the present invention, Co or Cu
Etc. are often mixed as unavoidable impurity elements,
Even if these Co and Cu are contained as impurities, there is no particular problem as long as their contents are in the range of 0.1% or less.

Hereinafter, the present invention will be described with reference to examples.

EXAMPLE First, an invar alloy having the chemical composition shown in Table 1 was melted and cast into an ingot by a 25 kg vacuum induction furnace, and then hot forged, hot rolled and "900 ° C. × 15 minutes Heat treatment was performed to obtain a 12 mm thick plate.

[0031]

[Table 1]

Next, for the evaluation of hot ductility, a smooth round bar test piece of “diameter 10 mm × length 130 mm” was sampled from each hot-rolled sheet, and these were heated at “1200 ° C. × 5 minutes”. Strain rate 1 in the temperature range of 1100 to 700 ° C during cooling of
The specimen was broken by a tensile test at × s ^−1, and the drawing value (= cross-sectional shrinkage) was measured.

For the evaluation of the thermal expansion characteristics, square rod test pieces of “thickness 2 mm × width 5 mm × length 50 mm” were sampled from each of the hot-rolled sheets and subjected to linear expansion at 20 ° C. to -196 ° C. Was measured. Then, the average coefficient of thermal expansion was calculated based on the measured values at -180 ° C to 20 ° C.

Further, for the evaluation of mechanical properties, a tensile test piece having a parallel part of “diameter 6 mm × length 40 mm” was taken from each of the hot-rolled sheets, and was subjected to a 0.2% yield strength (YS) and a tensile strength. (T
S) was measured. Table 2 shows the measurement results (1000 ° C, 9
(Hot ductility at 00 ° C and 800 ° C, 0.2% yield strength and tensile strength at room temperature, average thermal expansion coefficient from 20 ° C to -180 ° C).

[0035]

[Table 2]

From the results shown in Table 2, it can be seen that the comparative alloy (comparative example) whose chemical composition does not satisfy the conditions specified in the present invention has inferior hot ductility and an aperture value of 60 at high temperature.
% Or the strength at room temperature is not enough
While the 0.2% yield strength has a problem of being less than 250 MPa, the alloy of the present invention (example of the present invention) has a high-temperature drawing value of more than 60% at any temperature and the 0.2% yield strength. It can be seen that the material exceeds 300 MPa and has excellent hot workability and strength properties. In addition, it was confirmed that there was no occurrence of edge cracks from the hot-rolled sheet according to the alloy of the present invention. Furthermore, the average thermal expansion coefficient of each of the alloys of the present invention is 1.5
It is less than × 10 ^-6 , and it is clear that the alloy of the present invention is an invar alloy having high strength, good hot workability and excellent thermal expansion characteristics.

[0037]

As described above, according to the present invention, it is possible to provide a high-strength invar alloy having excellent hot workability, and it is possible to provide a high-strength invar alloy in the production of sheet materials. Since the retention is greatly improved, industrially useful effects such as a low-temperature storage tank material or the like for which high strength is desired can be provided relatively inexpensively.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the Nb content of a 0.03% C-0.2% Si-0.3% Mn-36% Ni-Nb-based invar alloy and hot ductility (high-temperature drawing value).

FIG. 2 is a graph showing the relationship between Ti content and hot ductility (high-temperature drawing value) for a 36% Ni-0.3Nb-0.001% S-Ti-based invar alloy.

Claims (4)

[Claims] C. 0.015 to 0.10%, Si: 0.35% or less, Mn: 1.0% or less, P: 0.015% or less, S: 0.0010% or less, Cr: 0.3% or less, Ni: 35 to 37%, Mo: 0 to 0.5%, V: 0 to 0.05%, Al: 0.01% or less, Nb: 0.15% to less than 1.0%, Ti: 0.003% or less, N: 0.005% or less, with the balance Fe And high-strength invar with excellent hot workability characterized by being composed of unavoidable impurities.
alloy. 2. The composition according to claim 1, further comprising B
Characterized by containing 0.0005 to 0.005% by weight,
High strength invar alloy with excellent hot workability. 3. The weight ratio of C: 0.015 to 0.10%, Si: 0.35% or less, Mn: 1.0% or less, P: 0.015% or less, S: 0.002% or less, Cr: 0.3% or less, Ni: 35 to 37%, Mo: 0.5% or less, V: 0.05% or less, Al: 0.01% or less, Nb: 0.15% or more and less than 1.0%, Ti: 0.003% or less, N: 0.005% or less, with the balance Fe and unavoidable Strength, excellent in hot workability, characterized by being composed of chemical impurities
alloy. 4. A compound according to claim 3, further comprising B
Characterized by containing 0.0005 to 0.005% by weight,
High strength invar alloy with excellent hot workability.