JPH10195531A - Production of invar alloy excellent in strength and toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide as method for producing Fe-Ni invar alloy excellent in strength and toughness without checking its physical properties (low thermal expansion characteristics) and producibility. SOLUTION: In a method for producing an Fe-Ni series invar alloy contg., by weight, 30 to 45% Ni and 0.001 to 0.04% C, the alloy is heated at 900 to 1150 deg.C and is subjected to hot rolling at a cumulative draft of >=5% at <=TR deg.C expressed by the formula of TR( deg.C)=2,500×C%+750.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to LNG transport,
Fe with excellent strength and toughness suitable for storage container structural materials, etc.
The present invention relates to a method for producing a Ni-based invar alloy.

[0002]

2. Description of the Related Art In recent years, with the diversification of energy sources, natural gas has been in the spotlight and its demand is increasing worldwide. When transporting and storing natural gas,
It is carried out as liquefied natural gas (LNG) at a very low temperature of -160 ° C. The LNG marine transport ship and cryogenic container for land storage each have several types of structures, but with the recent trend of large capacity, LNG ships have changed from an independent tank type to a membrane type, We are moving from a metal shell with a double shell structure to a semi-underground membrane tank.

[0003] In order to reduce thermal expansion and contraction caused by the rise and fall of the liquid level of LNG, and to improve weldability by simplifying the design of a welded part, Fe has a low coefficient of thermal expansion. -A Ni-based invar alloy is used.

However, since this amber alloy has an austenitic structure, the low strength and low proof stress peculiar to austenitic steel are major drawbacks. In the case of an amber alloy, the tensile rupture stress level at the time of complete solution treatment has a maximum of 450 to 500 MPa and a proof stress of only 250 to 350 MPa. Considering that the strength properties important for ensuring the structural reliability and weight reduction of the membrane material can be obtained by solid solution hardening and precipitation strengthening by adding a small amount of additional elements to the base material, the addition of impurity elements is There is a problem that the amber characteristic (low thermal expansion characteristic) which is also called basic performance of the alloy is impaired, the addition of a trace element itself leads to an increase in the amount of inclusions in the parent phase, and the toughness is reduced. As the same austenitic steel, for austenitic stainless steel, Japanese Patent Application Laid-Open No. 60-26619 discloses a method for achieving high tensile strength by grain reinforcement as a measure for increasing strength. Discloses a method in which a reduction is applied in a non-recrystallized region to increase the tensile strength by strengthening the dislocation. Further, JP-A-63-1868
In Japanese Patent Publication No. 22, a stable high strength is ensured by rolling while avoiding the partial recrystallization temperature range. Such an attempt has not been made on an invar alloy, and a method for producing an Fe—Ni-based invar alloy having excellent strength properties without greatly impairing physical properties (low thermal expansion properties) and toughness has been desired. Things.

[0005]

However, it is known that the grain size dependence of strength when austenitic steel is refined is as small as 2/3 or less of that of low alloy steel, and that the effect is small. Further, the amber alloy has low oxidation resistance at high temperatures, cannot be heated at high temperatures, and has a partial recrystallization temperature of 700 to
Since the temperature is as wide as about 950 ° C., it is difficult to apply a method based on a technique such as Japanese Patent Application Laid-Open No. 63-186822 for stainless steel.

An object of the present invention is to provide a method for producing an Fe—Ni-based invar alloy having excellent strength and toughness without impairing physical properties (low thermal expansion properties) and manufacturability.

[0007]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) The alloy of the present invention has a Ni content of 30 to 45% by weight.
And a method for producing an Fe—Ni-based invar alloy containing 0.001% to 0.04% of C:
The invar alloy having excellent strength and toughness characterized in that it is heated to 1150 ° C. and hot rolled at a cumulative reduction rate of 5% or more at a temperature of not more than T _R ° C represented by the following formula (1). It is a manufacturing method. T _R (° C.) = 2,500 × C% + 750 (1)

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor has proposed an Fe-Ni-based Invar alloy having excellent strength and toughness without impairing physical properties (low thermal expansion properties) and manufacturability. Intensive research was conducted on the correlation between the rolling end temperature and the strength improvement.

As a result, first, in slab heating from the viewpoint of high-temperature oxidation resistance, surface oxidation is within an allowable range unless the temperature exceeds 1150 ° C. When heated at a temperature higher than this, grain boundary oxidation remarkably progresses. In addition, it was found that a healthy plate was not obtained due to flaws or cracks after rolling, and that even in a mild case, it took a great deal of time and effort to repair the surface.

[0010] Further, the complete recrystallization temperature and the partial recrystallization initiation temperature after hot working of an invar alloy are closely correlated with the C content, and the recrystallization temperature and the hardness after a compression test simulating hot working are re-evaluated. As a result of the judgment of the crystal structure, it became clear that the quantitative relationship between the complete recrystallization temperature, the partial recrystallization temperature, and the C amount can be grasped.

FIG. 1 shows the relationship between the rolling end temperature and the hardness.
Now, C content, can correlation equations of the rolling end temperature and strength improvement, the temperature T _R which is determined by _{T R (℃) = 2,500 ×} C% + 750 is, expresses the partial recrystallization starting temperature You can see. In other words, rolling at below this temperature
In processing, it was found that strength improvement due to residual strain was achieved.

On the other hand, as long as the C content is small and the amount of carbon that cannot be completely dissolved in the mother phase is almost zero, even in the partial recrystallization temperature range (coarse grain + fine grain mixed grain range), The undissolved carbides do not inhibit the growth of crystal grains, and in particular, the recovery of dislocations proceeds almost the same as the complete recrystallization temperature range,
It could be judged from the results of the hardness test and the like.

[0013] The present inventor has concluded from the results that although the amber alloy has the same austenitic structure, unlike the austenitic stainless steel in which the precipitation and retention of carbides play a dominant role in the recrystallization behavior, the strength improvement is not achieved. It is large in the non-recrystallization temperature range, but small in the partial recrystallization temperature range, meaning that it can be treated as a recrystallization temperature range, allowing high strength while allowing hot working in a wide range. They have newly found that they can be secured stably.

Based on the above findings, the present inventor has controlled the alloy composition and the slab heating temperature, and has been active in a range not lower than the partial recrystallization temperature defined by the amount of C (that is, within the partial recrystallization temperature range). By using the partial recrystallization temperature range for processing, it is possible to process even products in which hot working extends to a relatively low temperature range (that is, thin materials can be easily manufactured because of the large number of rolling passes), and The present inventors have found a method for producing an invar alloy excellent in strength and toughness according to the present invention, which can stably improve strength, and completed the present invention.

That is, the present invention limits the alloy composition and the manufacturing conditions to the following ranges, thereby preventing the physical properties (low thermal expansion properties) and the manufacturability without impairing the strength and toughness of the Fe-Ni-based alloy. Amber alloy can be obtained.

The reasons for adding the components, the reasons for limiting the components, and the reasons for limiting the production conditions of the present invention are described below. (1) Component composition range Ni: 30 to 45% In the present invention, a low thermal expansion Fe—Ni-based invar alloy is targeted, and Ni is an element that controls the thermal expansion coefficient of the present alloy. 30-45% Ni-Fe bal. The required low thermal expansion characteristics can be obtained in the alloy range of 1. Ni is 30
If the content is less than 45% or more than 45%, the required low thermal expansion characteristics cannot be obtained, so the range of Ni is 30 to 45%. C: 0.001 to 0.04% C is an additive element for improving the strength in the present alloy, and its content greatly affects the recrystallization temperature.
However, since the processing temperature can be set according to the amount of C added, the addition range is defined from the viewpoint of strength. That is, if C is less than 0.001%, the strength properties are inferior, and if C is less than 0.001%,
Decreasing the amount is not common in ordinary refining, while exceeding 0.04% impairs the low thermal expansion characteristics,
The amount of addition is 0.001 to 0.04%.

Incidentally, in the present alloy, Si, Mn, P, S, N, O,
Even if Co is mixed, if necessary, addition of 1% or less of Cr, Mo, Cu, Nb effective for strength and corrosion resistance, and Hf, Ta, Al effective for improving hot workability, etc. , T
Addition of 0.1% or less of i, Zr, and B, Ca, M
Even if 0.01% or less of g is added, the effect of the present invention is not impaired and the content is acceptable.

By adjusting to the above component composition range, it is possible to obtain an Fe—Ni-based Invar alloy excellent in strength and toughness without impairing physical properties (low thermal expansion properties) and manufacturability. .

An alloy having such characteristics can be manufactured by the following manufacturing method. (2) Alloy plate manufacturing process After melting the alloy adjusted to the above component composition range in a converter, it is made into a cast slab, and the alloy is heated to 900 to 1150 ° C.

Slab heating from the viewpoint of high-temperature oxidation resistance is as follows.
If the temperature does not exceed 1150 ° C., oxidation of the surface is within an allowable range. If heated at a temperature higher than this temperature, grain boundary oxidation remarkably progresses, resulting in flaws or cracks after rolling, failing to obtain a sound plate, Even in the case of hard work and time to clean the surface,
Since the cost is required, the upper limit of the slab heating temperature is 1150 ° C. On the other hand, if the heating temperature is too low,
The lower limit is 900 ° C. because the carbide precipitation element in the slab does not sufficiently form a solid solution and the balance between strength and toughness deteriorates.

Next, for the heated alloy, the following (1)
Hot rolling is performed at a temperature equal to or lower than T _R ° C represented by the equation and a cumulative draft of 5% or more. T _R (° C.) = 2,500 × C% + 750 (1) Regarding the rolling method, the temperature T _R defined by the equation (1)
When rolling is performed at (° C.) or lower, a strength improving effect corresponding to the cumulative rolling reduction is obtained. On the other hand, even when rolling is performed at a temperature higher than T _R (° C.), there is almost no strength improving effect. Also,
If the cumulative rolling reduction is less than 5%, the effect on strength improvement is small. FIG. 2 shows the relationship between the cumulative draft, the tensile strength, and the proof stress when rolling is performed at T _R (° C.) or less. As is clear from the figure, when the cumulative rolling reduction is 5% or more, the tensile strength is 450N.
/ Mm ² or more, and good strength characteristics of proof strength 280 N / mm ² or more. Therefore, under pressure below T _R (° C.)
And the cumulative rolling reduction is 5% or more. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0022]

EXAMPLES Tables 1 and 2 show the chemical composition of the alloy of the present invention and the comparative alloy, the critical recovery temperature T _R (° C.), the cumulative rolling reduction below T _R (° C.), and other manufacturing conditions, respectively. Tables 3 and 4 show the occurrence of surface flaws after rolling of the respective inventive alloys and comparative alloys (the presence or absence of grain boundary cracks was visually observed), and the results of tensile tests and Charpy impact tests performed on these alloys. Is shown. Table 3 also shows the results of a tensile test and a Charpy impact test after annealing at a high temperature for comparison. Each alloy was vacuum-melted in an experimental furnace, and the obtained ingot was hot-rolled to prepare a test material.

As shown in Table 1, the alloy No. 1-A to N
o. 7-D is an alloy of the present invention, which was manufactured under the manufacturing conditions of the present invention, in which the slab was heated to 1150 ° C. or less and the cumulative draft was 5% or more at a recovery critical temperature T _R (° C.) or less.

As shown in Table 3, the alloy of the present invention has a good surface condition after rolling, and has a tensile strength of about 460 N / mm ^2.
As described above, good strength characteristics with a proof strength of about 300 N / mm ² or more are exhibited. Compared to the tensile properties after annealing, shown in the right column of the same table, the good strength properties in the rolled state are still clear. On the other hand, the toughness shows a higher value in the annealed state, but it is a sufficient value of 200 J / cm ² or more even as-rolled.

On the other hand, as shown in Table 4, comparative alloy No.
As seen in 3-N, 3-P, 4-N, 4-P, 5-N and 6-N, in the rolled material heated by slab at a temperature exceeding 1150 ° C., after rolling in various component systems, The surface condition showed the appearance of grain boundary cracking. Observation of the structure revealed that oxidation had occurred at the grain boundaries of amber, and cracks during rolling caused by high-temperature heating were recognized. In addition, the comparative alloy No. 1-L, 1-M, 2-L, 2-M, 5-R, 5
-L, 5-M, in 6-M is, T for cumulative reduction rate at _R following is finished with missing or T _R, equal to or greater than about than the tensile strength of the present invention alloy in average 50 N / mm ^{2. The} yield strength is inferior to the average by about 100 N / mm ² . Comparative alloy No. In 8-A, since the content of C is small, the improvement of the strength characteristics is inferior to that of the alloy having a higher C content. Comparative alloy No. In 9-B, since the content of C is large, the average thermal expansion coefficient from low temperature to around room temperature is 2.3 × 10 ⁻⁶ / K, deteriorating the amber characteristic itself (average thermal expansion of amber of the present invention). Coefficient: about 1.6 × 10 ⁻⁶ / K).

As exemplified above, in order to obtain good surface properties after rolling and high strength properties and toughness intended in the present invention, the amount of C is set to an appropriate range as specified in the present invention. Above, the slab heating temperature is set to 1150 ° C or less, and T
It is understood that it is necessary to produce at a cumulative draft of 5% or more at _R (° C.) or less.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

[0031]

According to the present invention, the physical properties (low thermal expansion properties),
A high-strength Fe—Ni-based invar alloy having excellent mechanical properties can be provided without impairing manufacturability. INDUSTRIAL APPLICABILITY The alloy of the present invention can be used for members requiring strength and toughness, such as LNG transportation and structural materials for storage containers, and has industrially useful effects.

[Brief description of the drawings]

FIG. 1 is a view showing a relationship between a C content affecting a hardness of an alloy according to an embodiment of the present invention and a rolling end temperature.

FIG. 2 is a diagram showing the relationship between the cumulative draft and the tensile strength and proof stress at a temperature equal to or lower than T _{R according} to the embodiment of the present invention.

Claims (1)

[Claims] (1) Ni: 30 to 45% by weight and C:
A method of producing a Fe-Ni-based amber alloy containing 0.001 to .04%, heating the alloy to 900 to 1150 ° C., at T _R ° C. below the temperature shown by the following equation (1), A method for producing an invar alloy excellent in strength and toughness, wherein hot rolling is performed at a cumulative rolling reduction of 5% or more. T _R (° C.) = 2,500 × C% + 750 (1)