JP2001049396A - Oxide dispersion strengthened steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxide dispersion strengthened steel capable of recrystallizing at <=1300 deg.C even in the case where steel powder of >0.15 wt.% oxygen content, such as low-cost SUS410L steel powder prepared by water atomization, is used and having excellent creep characteristics even under multi-axial stress. SOLUTION: The steel is an alloy steel having a composition consisting of, by weight, 8 to 18% Cr, 0.1 to 1.5% of (1/2.W+Mo) (where Mo and W are 0.1 to 1.5% and 0-2.8% W respectively), 1.5 to 2.5% Ti, 0.12 to 0.32% Y, 0.18 to 0.40% O, and the balance Fe with inevitable impurities and also having a structure in which 0.43 to 1.0 wt.% oxides are dispersed in a parent phase and recrystallized structure is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an oxide dispersion strengthened steel having excellent creep strength even under multiaxial stress.

[0002]

2. Description of the Related Art Conventionally, a method using mechanical alloying has been known as a method for producing an oxide dispersion strengthened steel having excellent high-temperature strength. In this method, F
Raw material powder of steel containing e, Cr, etc. and ThO ₂ , Y ₂ O ₃
Powder or a metal powder that oxidizes to form an oxide such as ThO ₂ , Y ₂ O _3, and in some cases, Ti
The mixture of the powders is passed through a high-energy ball mill such as an attritor or a vibration mill to apply processing such as large plastic deformation, pulverization, mixing and recombination. This process is called mechanical alloying. The powder subjected to mechanical alloying (hereinafter referred to as mechanical alloying powder)
Vacuum sealed in mild steel capsules and hot isostatic pressing
It is formed into a bulk body by a known hot forming technique such as HIP) or hot extrusion. The oxides are finely and uniformly dispersed in the steel in this series of steps.

[0003] Oxide dispersion strengthened steel produced through mechanical alloying exhibits excellent uniaxial creep strength.
However, when used in a high-temperature plant, it is often necessary to consider use in a multiaxial stress environment.However, the creep strength of a conventional oxide dispersion strengthened steel in a multiaxial stress environment is uniaxial. There is a problem that it is weaker than the creep strength and breaks in a short time. Specifically, in the normal creep rupture test, V-shaped or U-shaped
The notch creep rupture test, in which a constant load is applied to the circular cross section at the bottom of the notch to check the rupture time, can realize a multiaxial stress environment. Oxide-strengthened steel breaks in a short time. In order to prevent the fracture under such multiaxial stress, recrystallization of the oxide dispersion strengthened steel has been studied. The recrystallization temperature changes depending on the composition of the matrix alloy steel, etc.
Since recrystallization at a high temperature causes coarsening of finely dispersed oxides and deteriorates creep characteristics, it is desirable to recrystallize at a temperature as low as possible.

Japanese Patent Application Laid-Open No. Hei 8-225891 discloses an oxide dispersion-strengthened steel in which the internal pressure creep corresponding to a multiaxial stress is improved by recrystallizing the oxide dispersion-strengthened steel at a temperature of 1300 ° C. or less, and a method thereof. Manufacturing methods have been proposed. According to the publication, Cr: 7 to 18 by weight%.
%, (1 / 2W + Mo): 0.1-3%, Ti: 0.1
0 to 1.0%, the balance being Fe and inevitable impurities
Yttria ferritic metal matrix in which mainly the -Cr in (Y ₂ O ₃₎ oxide dispersion strengthened ferritic steel obtained by dispersing, yttria and excess oxygen content (Excess O)
But 0.10% by weight <Y ₂ O ₃ ≦ 0.30% by weight;
03% by weight ≦ Excess O ≦ 0.15% by weight, Excess O
It is shown that the range of ≦ 0.25-0.5 × ₃ % by weight of Y ₂ O is necessary to impart a recrystallized structure at 1300 ° C. or less. Here, the excess oxygen amount means the total oxygen amount (Total O) in the oxide dispersion strengthened steel from Y ₂ O
Oxygen (O i) bonded to yttrium (Y) as ₃
n Y ₂ O ₃ ) ([Excess O] =
[Total O]-[O in Y ₂ O ₃ ]). The excess oxygen amount is determined to reflect oxygen contained in raw materials other than oxides and oxygen mixed in during the process. In practice, for example, SUS410L steel powder containing 12% by weight of Cr contains oxygen. In particular, powder of SUS410L steel, which is suitable for mass production and produced by the low-cost water atomization method,
Oxygen is contained in about 0.2% by weight, and it is very difficult to reduce oxygen to 0.15% by weight or less. On the other hand, powder of SUS410L steel by the high-cost gas atomizing method
The oxygen content is low and can be easily reduced to 0.1% by weight or less.

[0005]

The above-mentioned Japanese Patent Application Laid-Open No. Hei 8-225
According to the method of JP-A-891, the amount of excess oxygen must be 0.15% by weight or less in order to recrystallize the oxide dispersion strengthened steel. For this purpose, gas atomized powder must be used as the raw steel powder. I can't get it. As a result, the material cost of the oxide dispersion strengthened steel becomes high. The present invention relates to the use of steel powders containing more than 0.15% by weight of oxygen, such as low cost water atomized SUS410L steel,
An object of the present invention is to provide an oxide dispersion-strengthened steel that can be recrystallized at 1300 ° C. or less and has excellent creep characteristics even under multiaxial stress.

[0006]

SUMMARY OF THE INVENTION The present invention provides a Cr:
8 to 18%, (1/2 · W + Mo): 0.1 to 1.5%
(However, Mo: 0.1 to 1.5%, W: 0 to 2.8
%), Ti: 1.5 to 2.5%, Y: 0.12 to 0.3
2%, O: 0.18 to 0.40%, with the balance being Fe and unavoidable impurities.
An oxide dispersion-strengthened steel comprising 43 to 1.0% by weight of an oxide dispersed therein and having a recrystallized structure.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A feature of the present invention is that
In the prior art such as Japanese Patent No. 225891, recrystallization is realized at 1300 ° C. or lower by reducing the amount of excess oxygen in the assigned parent phase. In the present invention, Ti powder is used as a Ti powder during mechanical alloying. By increasing the amount of added Ti from 1% by weight or less in the prior art to 1.5 to 2.5% by weight, a steel powder raw material containing 0.15 to 0.32% by weight of oxygen is used. However, an object of the present invention is to provide an oxide dispersion-strengthened steel which can be recrystallized at 1300 ° C. or lower and exhibits excellent creep characteristics under multiaxial stress.

The oxide dispersion strengthened steel according to the present invention (hereinafter referred to as O
DS steel: sometimes referred to as Oxide Dispersion Strengthened Steel) is as follows. Cr depends on the use time of the steel material, but is based on the excellent creep property of the ODS steel according to the present invention.
When used at a high temperature of not less than ° C., it is necessary to be at least 8% by weight in order to impart sufficient oxidation resistance. However, there is a part depending on the use temperature history. However, if Cr is added excessively, it becomes brittle due to long-time use. Therefore, the addition of Cr is set to 18% by weight or less.

Mo must be added in an amount of 0.1% by weight or more in order to act as a solid solution strengthening element. However, Mo is embrittled by excessive addition, so that the maximum amount is 1.5% by weight. W
Is a solid solution strengthening element similar to Mo, and exhibits an excellent solid solution strengthening effect when added in combination with Mo. However, if added excessively, it becomes brittle, so the maximum amount of W added is 2.8% by weight, and the content of (1 / 2.W + Mo) is 0.1 to 1.
5% by weight. Particularly in the region where W is 1.5 to 2.8% by weight and (1 / 2W + Mo) is 1 to 1.5% by weight, the composite addition of W and Mo has a significant effect as compared with the addition of Mo alone. Produces.

The OSD steel according to the present invention contains Mn, Si, Ni, and C as unavoidable impurities. These elements are mixed in the process of producing the steel powder and during the mechanical alloying. Mn and Si are added as deoxidizers in the process of producing the steel powder, and the excessive presence adversely affects the toughness at room temperature and the strength at high temperature.
i is preferably 1.0% by weight or less. Ni is mixed in the steel powder production process or during mechanical alloying and adversely affects the creep strength. Therefore, it is preferable that Ni be 0.2% by weight or less. C is included as an unavoidable impurity in the process of producing the alloy steel powder as a raw material and in the production process of mechanical alloying and the like, and its excessive presence has an adverse effect on the fine dispersion of oxides, so C is set to 0.04% by weight or less.

[0011] Ti is mainly added at the time of mechanical alloying, and is considered to be dissolved in a steel powder raw material in the process of mechanical alloying. Then, when the mechanically alloyed powder produced in the mechanical alloying process is hot-extruded or hot-formed by HIP to produce an oxide-dispersed steel as a bulk body, the oxide in the mechanically alloyed powder is combined with oxygen. It forms TiO ₂ , and similarly combines with oxygen and Y in the mechanical alloying powder to form Y ₂ Ti ₂ O ₇ . Y ₂ Ti ₂ O ₇ is finely dispersed in the steel, and exhibits excellent creep strength of the oxide dispersion strengthened steel. O (oxygen) content in steel powder raw material is 0.15
When the amount of the Y ₂ O ₃ powder to be added at the time of mechanical alloying is 0.15 to 0.40% by weight and the recrystallized structure is obtained at 1300 ° C. or less, Ti
Is required to be 1.5% by weight or more. However, if it exists in excess, it becomes brittle, so that the maximum content is 2.5% by weight.

Y is Y ₂ O during mechanical alloying.
_It is added as ₃ and becomes a solid solution in the raw material of steel powder in the process of mechanical alloying. The mechanical alloying powder produced in the mechanical alloying process is hot-extruded or H
When producing oxide-dispersed steel by hot forming with IP, the oxide Y is combined with oxygen and Ti in the mechanical alloying powder.
₂ Ti ₂ O ₇ is formed. This oxide is finely dispersed in the steel, and exhibits excellent creep strength of the oxide dispersion strengthened steel. In order to exhibit the effect, the addition amount of Y is 0.1
It must be at least 2% by weight. However, if it is added in excess, it cannot be recrystallized below 1300 ° C.
It needs to be 0.32% by weight or less.

O in the oxide dispersion strengthened steel is mixed with O in the low-cost steel powder raw material by the water atomization method, O in the Y ₂ O ₃ powder added at the time of mechanical alloying, and mixed during the production process. O. O in the oxide dispersion steel is
Considering the thermodynamic equilibrium state, it is considered that most of the oxides precipitate as oxides, and the amount of O dissolved in the mother phase is extremely small. Even if the mixing of oxygen during the manufacturing process is minimized,
Oxygen in the oxide-dispersed steel is obtained by combining oxygen in the steel powder raw material with Y ₂ O
₃ It is not less than the sum of the amount of oxygen in the powder and 0.1
8% by weight or more. If O is excessive, that is, if the oxide is excessive, recrystallization cannot be performed. Therefore, O is set to 0.40% by weight or less.

The oxide dispersion-strengthened steel having the above-mentioned component composition is usually produced by mechanical alloying followed by hot working. The crystal grains of the oxide dispersion strengthened steel formed in this process are fine, and the average crystal grain size is 1 to
It is several μm or less. In addition, oxide Y ₂ Ti ₂ O
The average particle size of the _seven particles is about several to several tens of nm. This oxide dispersion strengthened steel exhibits excellent creep properties under uniaxial stress, but has extremely poor creep properties under multiaxial stress such as notch creep. On the other hand, the oxide dispersion-strengthened steel is cold-worked by rolling or the like, and further subjected to a heat treatment at 1300 ° C. or less to form a recrystallized structure, which is excellent even in a multiaxial stress environment such as notch creep. It shows creep characteristics. Various recrystallization conditions can be considered depending on the creep characteristics, strength characteristics, ductility, etc. imparted to the oxide-dispersed steel. When recrystallized at a high temperature, the finely dispersed oxide becomes coarse, resulting in poor creep characteristics. For this reason, a heat treatment of 1300 ° C. or less is required as the recrystallization temperature, and 1200 ° C. or less is preferable.

[0015]

The present invention will be described more specifically with reference to the following examples. (Example) A steel powder having a composition shown in Table 1 and a predetermined amount of Y ₂ O ₃ powder and Ti powder shown in Table 1 were added and mixed together with steel balls into a stainless steel pot for a vibration mill. Vacuum the inside of the pot with a rotary pump, and purify 9
9.9995% of Ar gas was introduced. Further, the inside of the pot is again evacuated with a rotary pump, and the same A
r gas was introduced. This pot was placed on a vibration mill for 60 hours and subjected to mechanical alloying. The powder produced by the mechanical alloying (hereinafter referred to as mechanical alloying powder) was vacuum-sealed in a mild steel capsule, heated to 1100 ° C., and subjected to hot extrusion to produce an ODS steel.
Table 2 shows the composition of the produced ODS steel. In addition, in addition to the elements shown in Table 2, C (0.0
4% by weight or less), Si (1.0% by weight or less), Mn
(0.3% by weight or less) and Ni (0.2% by weight or less) were confirmed to be present.

Comparative Example A steel powder having a composition shown in Table 3 was prepared by adding a predetermined amount of Y ₂ O ₃ powder, Ti powder and Fe ₂ O shown in Table 3.
₃ Powder was added, and the O powder for comparison was prepared in the same procedure as in the example.
DS steel was produced. Fe ₂ O ₃ powder is not usually added during the production of ODS steel, but was added to investigate the effect of the amount of O in ODS steel. Table 4 shows the composition of the comparative ODS steel.
Shown in Note that the comparative ODS steel produced by the electron probe microanalyzer also contained C as an unavoidable impurity.
(0.04% by weight or less), Si (1.0% by weight or less),
Mn (0.3% by weight or less), Ni (0.2% by weight or less)
Confirmed that there is.

All the Os produced in the above Examples and Comparative Examples
In the DS steel, the crystal extends in the extrusion direction, and the average value of the crystal size in the direction perpendicular to the extrusion direction is 2 μm.
It was below. When cold working at a working ratio of 50 to 60% was performed on these ODS steels and heat treatment was performed at a temperature of 1300 ° C. or lower to examine recrystallization, the steel of the present invention (steel type 1 in Table 2)
In all of (16) to (16), the average value of the crystal size in the direction perpendicular to the extrusion direction became 5 μm or more, and the crystals were recrystallized.
On the other hand, the ODS steel for comparison has an R content of 0.39% by weight of Y.
1 steel, R2 steel containing 0.45% by weight of O and Ti
None of the weight percent R3 steel recrystallized.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

[Table 4]

Steel type 7 was subjected to cold working at a working rate of 40% to
Notched creep rupture tests were performed at 650 ° C. on the steel recrystallized by a heat treatment at 150 ° C. and the non-recrystallized steel 7 while hot extruded. Notch creep rupture test
The notch has a V-shaped opening angle of 60 degrees, the ratio of the diameter of the smooth portion to the diameter of the notch bottom is 1.2, and the curvature of the notch bottom is 0.12 mm.
The applied load is 25 kg / m2 on the area of the circle including the notch bottom.
The load was multiplied by m ² . As a result, the notched creep rupture time of the steel of the present invention was 5,000 hours, while that of the unrecrystallized steel type 7 ruptured in about 100 hours. Therefore, it can be seen that the recrystallized ODS steel has excellent notch creep strength, which is a multiaxial stress environment.

[0023]

The oxide dispersion strengthened steel of the present invention (ODS steel)
Is an ODS with excellent creep properties even under multiaxial stress
This steel is suitable for applications such as exhaust gas turbocharger blades for diesel engines and bolts and nuts for high-temperature plants. Further, the ODS steel of the present invention can be produced by recrystallization treatment at a temperature of 1300 ° C. or less even if a steel powder containing 0.15% by weight or more of oxygen obtained at low cost can be used. Its industrial value is extremely large.

[Procedure amendment]

[Submission date] August 26, 1999 (1999.8.2
6)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0020

[Correction method] Change

[Correction contents]

[0020]

[Table 3]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0021

[Correction method] Change

[Correction contents]

[0021]

[Table 4]

Claims (1)

[Claims] (1) Cr: 8 to 18% by weight%;
W + Mo): 0.1 to 1.5% (Mo: 0.1%)
-1.5%, W: 0-2.8%), Ti: 1.5-2.
5%, Y: 0.12-0.32%, O: 0.18-0.
40%, the balance being an alloy steel composed of Fe and unavoidable impurities, in which 0.43 to 1.0% by weight of an oxide is dispersed in the matrix and which has a recrystallized structure. Characterized by oxide dispersion strengthened steel.