JPH0533012A - Method for working steel reinforced by dispersion with small plane anisotropy - Google Patents

Abstract

PURPOSE:To reduce the plane anisotropy of ferritic steel reinforced by oxide dispersion by heat-treating and working the steel under specified conditions. CONSTITUTION:Ferritic steel contg. dispersed fine oxide particles such as sintered steel obtd. by extrusion-compacting particles obtd. by mechanical alloying is subjected to recrystallization heat treatment at 1,250-1,400 deg.C and worked at >=50% working rate at <=800 deg.C. Works of ferritic steel reinforced by oxide dispersion and having small plane anisotropy can easily be obtd. These works are useful as parts for apparatus used at high temp. such as a heating furnace, a boiler, an internal-combustion engine, a turbine, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for working an oxide dispersion strengthened steel having a small in-plane anisotropy. Such steel is promising as a material for equipment used at high temperatures such as for heating furnaces, boilers, internal combustion engines, and turbines.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for materials that can withstand high temperatures and have corrosion resistance. One of the promising alloys as a material that meets these requirements is a dispersion strengthening alloy. This dispersion strengthening alloy, especially an oxide dispersion strengthening alloy, is a sintered material in which fine inert particles are uniformly dispersed in a matrix, and can exhibit useful strength up to a temperature close to the melting point of the matrix alloy. The most common method for producing dispersion strengthened alloys is by powder metallurgy, in which mechanical alloying is performed first, and then the obtained alloyed powder is extruded into a sintered material. is there.

As a mechanical alloying method, a method is generally used in which metal powder and hard fine particles (oxide, carbide, nitride, etc.) are strongly pulverized and mixed in a high energy ball mill. Such a process is described in
No. 37631. The dispersion-strengthened alloy powder produced by the mechanical alloying method in this way is vacuum-encapsulated in a steel capsule and sintered, but in producing a steel sheet, extrusion and rolling steps are indispensable. However, in general, when a metallic material undergoes large strains, the material develops a deformed texture. In this case, the grains of the material are oriented such that the particular crystallographic orientations are aligned parallel to the processing direction.

While such texture can be mitigated by subsequent processing and heat treatment, the material rarely recovers a random crystallographic orientation. In particular, oxide dispersion strengthened alloys have a very high recrystallization temperature (sometimes 1300 ° C or higher), and it is very difficult to control the texture by ordinary heat treatment. Since the crystal orientation affects the directionality of the physical properties of the material, structural materials that have biaxial stress at high temperature and have such a structure have strength in the direction perpendicular to the processing direction, especially creep rupture strength. However, the problem is that the strength is significantly inferior to that expected from the processing direction (this is called the anisotropy of creep rupture strength), and an oxide dispersion strengthened alloy without this anisotropy was developed. There is a need to.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
No. 83406 discloses an extrusion method of a dispersion-strengthened metal extruded article that does not substantially contain a texture by specifying the shape of the die, but as a product as it is extruded in terms of dimensional accuracy, surface properties, etc. It is considered that there is a small possibility that the anisotropy of creep rupture strength will be increased due to the development of texture again in the subsequent steps such as rolling. on the other hand,
Japanese Examined Patent Publication No. 59-1779 shows a rolling / forging method for extruded oxide dispersion strengthened alloys, and this method is also hot rolling (forging) at a temperature of 840 to 1150 ° C as processing conditions.
It only shows that, and is about Ni-based and Co-based alloy containing special oxide fine particles, there is no mention of ferrite steel is a body-centered cubic metal,
Nor does it mention the anisotropy of creep rupture strength.

Regarding such anisotropy, Japanese Patent Publication No. 58-3604
As described in Japanese Patent Publication No. 3, this is a method of hot rolling an extruded material at a temperature of not less than the recrystallization temperature and not more than 1120 ° C at a predetermined strain rate. It is hard to say that the method can be easily carried out, for example, that all the steps from extrusion, rolling and complete secondary recrystallization heat treatment are limited in fine processing conditions, and in some cases, cross rolling is required. Japanese Unexamined Patent Publication No. 54-146206 discloses a heat treatment method for eliminating anisotropy for Ni-based and Co-based alloys, but creep rupture strength by recrystallization in a direction perpendicular to the working direction. It has been shown that the anisotropy of can be reduced. However, this is not practical because hot rolling is followed by recrystallization, and such unidirectional recrystallization requires a special device.

On the other hand, a method for producing an ultra-thin steel sheet having a small in-plane anisotropy is disclosed in Japanese Patent Laid-Open No. 63-310924, which uses fine carbides during recrystallization after cold rolling. It controls the texture. This is commonly referred to as deep drawing steel and is not intended for high temperature strength. Further, the oxide dispersion strengthened alloy targeted by the present invention has a very high recrystallization temperature (sometimes 1300 ° C. or higher). It is very possible to control the texture by the usual heat treatment as described above. difficult. As described above, the in-plane anisotropy has been a problem in the oxide-dispersed strong steel, but the present situation is that there is no practically advantageous method for solving it due to complicated treatment steps. An object of the present invention is to provide a simple processing method for an oxide dispersion strengthened steel having a very small anisotropy, particularly an in-plane anisotropy of creep rupture strength.

[0008]

In order to achieve the above object, the present inventor has conducted extensive studies on the influence of texture on the anisotropy of high temperature strength of dispersion strengthened alloys. Figure 1
MA95, an oxide dispersion strengthened ferritic steel from INCO
7 Steel (nominal composition: Fe-14Cr-1Ti-0.3Mo-0.25Y ₂ O ₃ ) a,
Processed materials b and c, a: Cross-rolled material rolled at a working rate of 70% in one direction and then at a working rate of 50% in the right direction, b: Extruded material (extrusion ratio 5), c: Extruded material (extrusion ratio 15)
For 650 ℃ × 1000h creep rupture strength in the longitudinal direction (L) and its orthogonal direction (T), respectively, and the degree of integration on the stress axis in the <110> direction, that is, the <110> X-ray integrated strength (I / I
_This is shown in a graph by investigating the relationship with ₀ ).

As shown by the broken line in the figure, it was found that the strength of the oxide dispersion strengthened ferritic steel increases when the <110> orientation is integrated on the stress axis. But its right angle
If the degree of integration in the <110> orientation of (T) is small, the anisotropy of creep rupture strength becomes remarkable. For example, in the case of the extruded material (c) in Fig. 1, the integrated strength in the L direction is 17 and the creep rupture strength is 31 kg.
Whereas shows f / mm ² or more, the integrated intensity of the T direction is 0.3, the creep rupture strength 11 kgf / mm ² only about without intensity difference is also about 20 kgf / mm ^2, the ratio (T / L) is 35
%.

Here, "accumulate" means when the X-ray integrated intensity of a certain crystal orientation is measured along the stress axis of the test material.
It means that (I) has an axial density (I / I ₀ ) that is more than 5 times that (I ₀ ) obtained from a disordered sample such as a powder sample. The present inventor, based on the above findings, focusing on the correlation between the anisotropy of creep rupture strength and the deformation texture,
We proposed a processing method to reduce the anisotropy of creep rupture strength that reduces the disadvantageous texture by performing spinning processing (Japanese Patent Application No. 1-284294), and also in the perpendicular direction.
We proposed a processing method that develops <110> orientation and reduces the anisotropy of creep rupture strength (Japanese Patent Application No. 2-37687).

The present inventor further continued research and development. After carrying out recrystallization heat treatment at 1250 to 1400 ° C., 800 ° C.
It was found that the in-plane anisotropy of creep rupture strength becomes extremely small by performing processing with a processing rate of 50% or more below,
The present invention has been completed. Here, in the broadest sense, the present invention is
Oxide dispersion with extremely small in-plane anisotropy of creep rupture strength is obtained by recrystallizing the extruded sintered material at high temperature to make the texture isotropic, and then performing processing such as rolling to recover strength. This is a method for manufacturing a reinforced steel.

More specifically, in the present invention, ferritic steel containing dispersed oxide fine particles is subjected to recrystallization heat treatment at 1250 to 1400 ° C. and then processed at 800 ° C. or less at a processing rate of 50% or more. This is a method for processing an oxide dispersion strengthened steel having a small in-plane anisotropy. The time of holding at the recrystallization heat treatment temperature is not particularly limited as long as the texture is isotropic, but is preferably 10 to 40 minutes. The "ferrite steel containing oxide dispersed therein" is, for example, a sintered steel obtained by extruding particles obtained by a mechanical alloying method.

[0013]

Next, the reason why each processing condition is limited as described above in the present invention will be explained together with its function and effect. In the present invention, the target steel is ferritic steel containing oxides dispersed therein, but is not particularly limited. Commercially available products include MA956 and MA957 steels (both are INCO's product names), and other products such as "NUCLEAR TECHNOLOGY" Vol. 70, AU
DT2906 steel, DT2203Y05 steel, etc. in which Ti ₂ O ₃ or Y ₂ O ₃ described in Table 1 on page 216 of G. 1985 are dispersed are included.

The recrystallization heat treatment temperature is limited to 1250 to 1400 ° C. This is for making the texture found in the extruded material isotropic, and if the temperature is less than 1250 ° C., a sufficient recrystallization structure cannot be obtained. The organization does not become isotropic. If it exceeds 1400 ° C,
The dispersed particles become coarse, and the strength cannot be recovered by subsequent processing such as rolling. Therefore, the heat treatment temperature range should be 12
It was set to 50 to 1400 ° C. FIG. 2 shows the MA957 as in the case of FIG.
It is a graph which shows the relationship between the recrystallization heat treatment temperature at the time of performing the recrystallization heat treatment which heats the steel for 30 minutes at each indicated temperature, and then cools it in the furnace, the average particle diameter of dispersed particles, and the <110> X-ray integrated intensity. As can be seen from the graph, it can be seen that sufficient isotropicity is observed in the temperature range of 1250 to 1400 ° C.

According to the present invention, the strength is recovered by the work hardening of the rolling after the recrystallization heat treatment, but the temperature is 800 ° C.
If it exceeds the limit, or if the degree of processing is less than 50%, a sufficient effect cannot be obtained. Therefore, processing conditions such as rolling are set to 80.
The processing rate is limited to 50% or more at 0 ° C or less. If the rolling temperature is too low, the working resistance increases, so it is preferable to set the rolling temperature to 300 ° C or higher. The working such as rolling may be carried out only once or may be carried out plural times without interposing the softening heat treatment between the working repetitions.

The "working method" referred to in the present invention is, first of all, rolling, but in addition, as for the forging and drawing, the "working method" referred to in the present invention is the "working method" as long as the strength recovery effect by work hardening is observed. Included. Further, the rolling mill used for rolling is not particularly limited like the roll rolling machine for sheet rolling, but in addition, considering the case of pipe rolling, for example, a Pilger mill, a plug mill or a mandrel mill is exemplified. It The "working ratio (reduction ratio)" is the area reduction ratio defined by the following formula.

{(AA ₀ ) / A} × 100 (%) A: Cross-sectional area before processing (rolling) A ₀ : Cross-sectional area after processing (rolling) Oxide dispersion strengthened ferrite thus obtained The steel can then be used as it is without any further processing, with no substantial in-plane anisotropy, for example 650
℃ × 10 ³ h Used as a high temperature member with a creep rupture strength of 20 kgf / mm ² .

[0018]

EXAMPLES Examples of the present invention will be shown below. In this example, IN
25 mmt x 100 mmw x 500 of oxide dispersion strengthened ferritic steel (trade name "MA957 steel") that is an extruded material marketed by CO
Extruded material with dimensions of mml was used. Table 1 shows the chemical composition of this test steel. The unit is% by weight.

[0019]

[Table 1]

The extruded material thus prepared was subjected to recrystallization heat treatment and subsequent hot rolling under the conditions shown in Table 2. All recrystallization heat treatments were carried out at the indicated temperature in a vacuum furnace at 30
After heating for a minute, the furnace was cooled. The rolling direction was the L direction, which was the same as the extrusion direction of the extruded material, and the T direction was the direction perpendicular thereto. After that, 1100 ℃ × for all conditions
A solution treatment for 10 minutes was performed and the test material was used for the test. In the test, <110> X-ray integrated intensity in each direction and 650 ℃ ×
1000 h creep rupture strength was measured. The results are shown in Table 2. Further, the anisotropy of creep rupture strength (σ _T / σ _L ) and the amount of change in strength in the L direction (σ _L / σ _O ) are also shown.

Conditions A and B are the examples of the present invention, conditions C to F are comparative examples, condition G is the extruded material, and condition H is the extruded material that has been subjected only to recrystallization heat treatment. The condition H can be said to be a conventional example. As is clear from the results shown in Table 2, according to the present invention, the anisotropy of creep rupture strength (σ _T / σ _L ) and the amount of change in strength in the L direction (σ
It is understood that both _L / σ _O ) are 80% or more, and the anisotropy is reduced without impairing the high temperature strength.

[0022]

[Table 2]

[0023]

According to the present invention, a processed product of oxide dispersion strengthened ferritic steel having a small in-plane anisotropy can be easily obtained, and the processed steel product is a heating furnace, a boiler, an internal combustion engine,
It is useful as a component for equipment used at high temperatures, such as for turbines.

[Brief description of drawings]

[Figure 1] 650 ° C for materials processed under various conditions
It is a graph which shows the result of having investigated the 1000h creep rupture strength and the degree of integration on the stress axis of two directions.

FIG. 2 is a graph showing changes in <110> X-ray integrated intensity and average particle diameter of dispersed particles depending on a recrystallization heat treatment temperature.

Claims (1)

Claims: 1. A ferritic steel containing oxide fine particles dispersed therein is subjected to recrystallization heat treatment at 1250 to 1400 ° C, and then 800 ° C.
Characterized by processing at a processing rate of 50% or more,
Processing method of oxide dispersion strengthened steel with small in-plane anisotropy.