Patent Grant
7037464
The present invention relates to an oxide dispersion strengthened (ODS) martensitic steel excellent in high-temperature strength and a method of manufacturing this steel.
The oxide dispersion strengthened martensitic steel of the present invention can be advantageously used as a fuel cladding tube material of a fast breeder reactor, a first wall material of a nuclear fusion reactor, a material for thermal power generation, etc. in which excellent high-temperature strength and creep strength are required.
Although austenitic stainless steels have hitherto been used in the component members of nuclear reactors, especially fast reactors which are required to have excellent high-temperature strength and resistance to neutron irradiation, they have limitations on irradiation resistance such as swelling resistance. On the other hand, martensitic stainless steels have the disadvantage of low high-temperature strength although they are excellent in irradiation resistance.
Therefore, oxide dispersion strengthened martensitic steels have been developed as materials that combine irradiation resistance and high-temperature strength and there have been proposed techniques for improving high-temperature strength by adding Ti to oxide dispersion strengthened martensitic steels, thereby finely dispersing oxide particles.
For example, Japanese Patent Laid-Open No. 5-18897/1993 discloses a tempered oxide dispersion strengthened martensitic steel which comprises, as expressed by % by weight, 0.05 to 0.25% C, not more than 0.1% Si, not more than 0.1% Mn, 8 to 12% Cr (12% being excluded), 0.1 to 4.0% in total of Mo+W, not more than 0.01% O (O in Y2O3 and TiO2 being excluded) with the balance being Fe and unavoidable impurities, and in which complex oxide particles comprising Y2O3 and TiO2 having an average particle diameter of not more than 1000 Å are homogeneously dispersed in the matrix in an amount of 0.1 to 1.0% in total of Y2O3+TiO2 and in the range of 0.5 to 2.0 of the molecular ratio TiO2/Y2O3.
However, even when oxide dispersion strengthened martensitic steels are produced by adjusting the total amount of Y2O3 and TiO2 and the ratio of these oxides and besides the total amount of Mo and W as disclosed in the Japanese Patent Laid-Open No. 5-18997/1993, there are cases where oxide particles are not finely dispersed in a homogeneous manner and it follows that in such cases the expected effect on an improvement in high-temperature strength cannot be achieved.
An object of the present invention is, therefore, to provide an oxide dispersion strengthened martensitic steel in which oxide particles are finely and homogeneously dispersed at a high density is positively obtained, with the result that excellent high-temperature strength is obtained, and to provide a method of manufacturing this steel.
Paying attention to the fact that an excess oxygen content Ex.O (a value obtained by subtracting an oxygen content in Y2O3 from an oxygen content in steel) in an oxide dispersion strengthened martensitic steel has a close relation to high-temperature strength, the present inventors have found that high-temperature strength can be positively improved by adjusting the level of the excess oxygen content in steel within a predetermined range, thus having accomplished the present invention.
According to the present invention, there is provided an oxide dispersion strengthened martensitic steel excellent in high-temperature strength which comprises, as expressed by % by weight, 0.05 to 0.25% C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y2O3 with the balance being Fe and unavoidable impurities and in which Y2O3 particles are dispersed in the steel, characterized in that the oxide particles are finely dispersed and highly densified by adjusting the Ti content within the range of 0.1 to 1.0% so that an excess oxygen content Ex.O in the steel satisfies [0.22×Ti (% by weight)<Ex.O (% by weight)<0.46×Ti (% by weight)].
Incidentally, in the following descriptions of this specification, “%” denotes “% by weight” unless otherwise noted.
In the present invention, by adjusting the Ti content in steel within the range of 0.1 to 1.0% so that the excess oxygen content Ex.O in steel becomes a predetermined range, it becomes possible to finely disperse oxide particles in steel and increase the density of them at a high level, with the result that it becomes possible to improve the high-temperature short-time strength and high-temperature long-time strength of the steel.
The steel of the invention described above can be manufactured by subjecting either element powders or alloy powders and a Y2O3 powder to mechanical alloying treatment in an Ar atmosphere. In this manufacturing process, by reducing the amount of oxygen which is included in the steel, it is also possible to keep the excess oxygen content in the resulting steel in a predetermined range.
Accordingly, the present invention provides a method of manufacturing an oxide dispersion strengthened martensitic steel excellent in high-temperature strength, the method comprising subjecting either element powders or alloy powders and a Y2O3 powder to mechanical alloying treatment in an Ar atmosphere to manufacture an oxide dispersion strengthened martensitic steel which comprises 0.05 to 0.25% C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y2O3 with the balance being Fe and unavoidable impurities and in which Y2O3 particles are dispersed in the steel, characterized in that an Ar gas having a purity of not less than 99.9999% is used as the Ar atmosphere so that an excess oxygen content Ex.O in the steel satisfies [0.22×Ti (% by weight)<Ex.O (% by weight)<0.46×Ti (% by weight)].
The present invention further provides a method of manufacturing an oxide dispersion strengthened martensitic steel excellent in high-temperature strength, the method comprising subjecting either element powders or alloy powders and a Y2O3 powder to mechanical alloying treatment in an Ar atmosphere to manufacture an oxide dispersion strengthened martensitic steel which comprises 0.05 to 0.25% C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y2O3 with the balance being Fe and unavoidable impurities and in which Y2O3 particles are dispersed in the steel, characterized in that a stirring energy during the mechanical alloying treatment decreases to suppress oxygen contamination during stirring so that an excess oxygen content Ex.O in the steel satisfies [0.22×Ti (% by weight)<Ex.O (% by weight)<0.46×Ti (% by weight)].
The present invention further provides a method of manufacturing an oxide dispersion strengthened martensitic steel excellent in high-temperature strength, the method comprising subjecting either element powders or alloy powders and a Y2O3 powder to mechanical alloying treatment in an Ar atmosphere to manufacture an oxide dispersion strengthened martensitic steel which comprises 0.05 to 0.25% C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y2O3 with the balance being Fe and unavoidable impurities and in which Y2O3 particles are dispersed in the steel, characterized in that a metal Y powder or a Fe2Y powder is used in place of the Y2O3 powder so that an excess oxygen content Ex.O in the steel satisfies [0.22×Ti (% by weight)<Ex.O (% by weight)<0.46×Ti (% by weight)].
The chemical composition of the oxide dispersion strengthened martensitic steel of the present invention and the reasons for the limitation of its components will be described below.
Cr (chromium) is an element important for ensuring corrosion resistance, and if the Cr content is less than 8.0%, the worsening of corrosion resistance becomes remarkable. If the Cr content exceeds 12.0%, a decrease in toughness and ductility is feared. For this reason, the Cr content should be 8.0 to 12.0%.
When the Cr content is 8.0 to 12.0%, it is necessary that C (carbon) be contained in an amount of not less than 0.05% in order to make the structure a stable martensite structure. This martensite structure is obtained by conducting heat treatment including normalizing at 1000 to 1150° C.+tempering at 700 to 800° C. The higher the C content, the amount of precipitated carbides (M23C6, M6C, etc.) and high-temperature strength increases. However, workability deteriorates if C is contained in an amount exceeding 0.25%. For this reason, the C content should be 0.05 to 0.25%.
W (tungsten) is an important element which dissolves into an alloy in a solid solution state to improve high-temperature strength, and is added in an amount of not less than 0.1%. A high W content improves creep rupture strength due to the solid solution strengthening, the strengthening by carbide (M23C6, M6C, etc.) precipitation and the strengthening by intermetallic compound precipitation. However, if the W content exceeds 4.0%, the amount of δ-ferrite increases and contrarily strength decreases. For this reason, the W content should be 0.1 to 4.0%.
Ti (titanium) plays an important role in the dispersion strengthening of Y2O3 and forms the complex oxide Y2Ti2O7 or Y2TiO5 by reacting with Y2O3, thereby functioning to finely disperse oxide particles. This action tends to reach a level of saturation when the Ti content exceeds 1.0%, and the finely dispersing action is small when the Ti content is less than 0.1%. For this reason, the Ti content should be 0.1 to 1.0%.
Y2O3 is an important additive which improves high-temperature strength due to dispersion strengthening. When the Y2O3 content is less than 0.1%, the effect of dispersion strengthening is small and strength is low. On the other hand, when Y2O3 is contained in an amount exceeding 0.5%, hardening occurs remarkably and a problem arises in workability. For this reason, the Y2O3 content should be 0.1 to 0.5%.
A method described below may be used as a general manufacturing method of the oxide dispersion strengthened martensitic steel of the present invention. The above-described components as either element powders or alloy powders and a Y2O3 powder are mixed so as to obtain a target composition. The resulting powder mixture is subjected to mechanical alloying treatment which comprises charging the powder mixture into a high-energy attritor and stirring the powder mixture in an Ar atmosphere. Thereafter, the resulting alloyed powder is filled in a capsule made of a mild steel. The capsule is then degassed and sealed, and hot extrusion is carried out after heating it to 1150° C. to thereby solidify the alloyed powder.
In this manufacturing process, an Ar gas having a purity of 99.99% is usually used as the atmosphere gas during the mechanical alloying treatment. However, even when such a high-purity Ar gas is used, it is impossible to avoid the oxygen contamination into steel, though slight in quantity. In the present invention, by using a high purity Ar gas of not less than 99.9999%, it is possible to reduce the oxygen contamination into steel, with the result that it is possible to adjust the excess oxygen content in the resulting steel within a predetermined range.
Furthermore, in carrying out the mechanical alloying treatment by charging the raw material powder mixture into the high-energy attritor and stirring the powder mixture, by decreasing the stirring energy in the attritor and suppressing the amount of entrapped oxygen during the stirring, it is also possible to reduce the excess oxygen content in steel and to adjust the excess oxygen content in the resulting steel within a predetermined range. As specific means of decreasing the stirring energy, it is considered to lower the rotary speed of an agitator of the attritor, to shorten the length of a pin attached to the agitator, and the like.
Moreover, in the step of mixing either element powders or alloy powders and a Y2O3 powder to prepare a target composition, a metal Y powder or an Fe2Y powder is used as a raw material powder in place of the Y2O3 powder. By using such a metal Y powder or an Fe2Y powder, the Y metal reacts with the oxygen which is contaminated during the manufacturing process such as the mechanical alloying treatment or with the oxygen from mixed unstable oxides (Fe2O3 etc.), to thereby form thermodynamically stable dispersed Y2O3 particles. As a result, it is possible to effectively adjust the excess oxygen content in steel to a predetermined range. Incidentally, the excess oxygen content in steel in this case is calculated on the assumption that the whole amount of the added metal Y becomes Y2O3.
Table 1 collectively shows the target compositions of test materials of oxide dispersion strengthened martensitic steel, features of the compositions, and manufacturing conditions.
In each test material, either element powders or alloy powders and a Y2O3 powder were blended to obtain a target composition, charged into a high-energy attritor and thereafter subjected to mechanical alloying treatment by stirring in an Ar atmosphere. The number of revolutions of the attritor was about 220 revolutions per minute (rpm) and the stirring time was about 48 hours. The resulting alloyed powder was filled in a capsule made of a mild steel, degassed at a high temperature in a vacuum, and then subjected to hot extrusion at about 1150 to 1200° C. in an extrusion ratio of 7 to 8:1, to thereby obtain a hot extruded rod-shaped material.
In each of the test materials shown in Table 1, not only a Y2O3 powder but also Ti was added to try to finely disperse and highly densify dispersed oxide particles by the formation of complex oxides of Ti and Y. The test materials MM11, MM13, T14 and E5 have a basic composition. T3 is a test material in which the excess oxygen content was intentionally increased by adding an unstable oxide (Fe2O3) to the basic composition of MM13 and T14. T4 is a test material in which the amount of added Ti was increased by adding higher amount of Ti powder to the basic composition of M13 and T14. T5 is a test material in which the excess oxygen content was increased by adding an unstable oxide (Fe2O3) and the amount of added Ti was also increased.
“Stirring energy” in the manufacturing conditions (mechanical alloying treatment conditions) of Table 1 shows the difference in the length of the pin attached to the agitator of the attritor which stirs the raw material powders during the mechanical alloying treatment. “Stirring energy: Large” means the use of the pin having a normal length, and “Stirring energy: Small” means the use of the pin having a length shorter than normal. That is, even when the number of revolutions of the agitator is the same, the stirring energy is smaller in the case of the shorter pin than in the case of the pin having a normal length and hence the amount of entrapped oxygen is reduced during the stirring. For only MM11 in Table 1, an agitator which has the shorter pin and in which the stirring energy is small was used. In all other test materials, an agitator which has the pin of normal length and in which the stirring energy is large was used. For the Ar atmosphere, a super high purity Ar gas having a purity of 99.9999% was used in only E5 in Table 1 and a high purity Ar gas having a purity of 99.99% was used in all other test materials.
Table 2 collectively shows the results of chemical analysis of each test material which was prepared as described above.
<Creep Rupture Test>
Among the hot extruded rod-shaped materials obtained above, T14, T3, T4, T5 and E5 were subjected to final heat treatment involving normalizing (1050° C.×1 hr, air cooling)+tempering (800° C.×1 hr, air cooling) and finished as rod-shaped materials. MM11 and MM13 were first formed in tubular shape and then subjected to final heat treatment involving normalizing (1050° C.×1 hr, air cooling)+tempering (800° C.×1 hr, air cooling). The tube making process was carried out by the first cold rolling+heat treatment for softening →the second cold rolling+heat treatment for softening→the third cold rolling+heat treatment for softening→the fourth cold rolling+final heat treatment.
For rod-shaped test pieces (T14, T3, T4, T5, E5) and tubular test pieces (MM11, MM13) thus obtained, a creep rupture test at 700° C. was conducted. The results of the test are shown in the graph shown in
Incidentally, the arrow in the graph shown in
<Tensile Strength Test>
For the test materials MM13, MM11 and T5, a tensile strength test was conducted at test temperatures of 700° C. and 800° C. The results of the test are shown in the graphs shown in
As is understood from the graphs shown in
<Microscopic Observation>
For each of the test materials prepared by subjecting the hot extruded rod-shaped materials obtained above to heat treatment for normalizing (1050° C.×1 hr), an observation by a transmission electron microscope (TEM) was carried out. The results of the microscopic observation are shown in
In
<Ti Content and Excess Oxygen Content>
For each of the test materials, the relationship between the Ti content and the excess oxygen content (Ex.O) shown in the results of chemical analysis in Table 2 are illustrated in the graph shown in
Incidentally, in the graph shown
<Adjustment of Ti Content>
A comparison between the test material MM13 of basic composition (Ti content: 0.21%, excess oxygen content 0.137>0.46×Ti) and the test material T4 in which the Ti content was increased (Ti content: 0.46%, excess oxygen content 0.107<0.46×Ti) reveals that T4 shows dispersed Y2O3 particles which are more finely dispersed and more increased in density at a higher level and has higher creep rupture strength.
In the test material T3 (Ti content: 0.21%, excess oxygen content 0.147>0.46×Ti) in which the excess oxygen content was intentionally increased by adding Fe2O3 to the test material MM13 of the basic composition, dispersed Y2O3 particles are more coarsened than the test material MM13 of the basic composition and creep rupture strength also decreases. However, by adding a further increased amount of Ti to the test material T3 in which the excess oxygen content was increased, it is possible to make the excess oxygen content less than 0.46×Ti % as seen in the test material T5 (Ti content: 0.46%, excess oxygen content 0.167<0.46×Ti), to more finely disperse and more highly densify dispersed Y2O3 particles at a higher level than T3, and to improve the creep rupture strength.
From these facts, it is understood that in the oxide dispersion strengthened martensitic steel in which the Ti content in steel is adjusted within the range of 0.1 to 0.5% so that the excess oxygen content becomes less than 0.46×Ti, Y2O3 particles are finely dispersed and highly densified and the high-temperature strength of this steel is excellent.
<Purity of Ar Gas>
Even in the test material E5 (excess oxygen content 0.084<0.46×Ti) having the same composition as the test material MM13 of the basic composition (excess oxygen content 0.137>0.46×Ti), by changing the purity of Ar gas used in the Ar atmosphere during mechanical alloying treatment from a high purity of 99.99% to a super high purity of 99.9999%, it is possible to reduce the oxygen contamination during the stirring in the attritor and hence the excess oxygen content in steel can be held to less than 0.46×Ti %.
From this fact, it is understood that by using a super high purity Ar gas of not less than 99.9999% as the Ar atmosphere during mechanical alloying treatment, it is possible to obtain an oxide dispersion strengthened martensitic steel in which Y2O3 particles are finely dispersed and highly densified and which is excellent in high-temperature strength.
<Adjustment of Stirring Energy During Mechanical Alloying Treatment>
A comparison between the test material MM13 of the basic composition (excess oxygen content 0.137>0.46×Ti) and the test material MM11 of the same composition (excess oxygen content 0.07<0.46×Ti) reveals that in the test material MM11 which was obtained by reducing stirring energy during mechanical alloying treatment by use of a pin attached to the agitator in the attritor having a length shorter than normal length, it is possible to hold the excess oxygen content to less than 0.46×Ti %.
In the test material MM11, Y2O3 particles can be finely dispersed and highly densified in comparison with the test material MM13 and creep rupture strength and tensile temperature strength can be improved.
From this fact, it is understood that by reducing the stirring energy during mechanical alloying treatment to limit the amount of entrapped oxygen during stirring, it is possible to obtain an oxide dispersion strengthened martensitic steel in which Y2O3 particles are finely dispersed and highly densified and which is excellent in high-temperature strength.
<Use of Metal Y Powder in Place of Y2O3 Powder>
Table 3 collectively shows the target compositions and the target excess oxygen contents of the test materials. Incidentally, E5 and T3 in Table 3 are the same as the test materials in Table 1.
E5 and E7 are standard materials of the basic composition to which a Y2O3 powder is added and the target excess oxygen content is 0.08%. Y1, Y2 and Y3 are materials to which a metal Y powder is added in place of a Y2O3 powder. That is, in Y1, a metal Y powder is added without the addition of an unstable oxide (Fe2O3) and the target excess oxygen content is 0%. In Y2 and Y3, a Fe2O3 powder, along with a metal Y powder, is added in an amount of 0.15% and 0.29%, respectively, and the target excess oxygen content is 0.05% and 0.09%, respectively. In T3, the excess oxygen content is increased by adding Fe2O3 powder to the basic composition of E5 and E7.
The test materials Y1, Y2, Y3 and E7 were all produced as hot extruded rod-shaped materials by the same manufacturing method and under the same manufacturing conditions as with MM13 described above, and heating and cooling in furnace (1050° C.×1 hr→600° C. (30° C./hr)) or normalizing (1050° C.×1 hr·air cooling)+tempering (780° C.×1 hr·air cooling) was carried out as final heat treatment.
The results of chemical analysis of each test material are collectively shown in Table 4.
From
As described above, Ti forms complex oxides by reacting with a Y2O3 powder, thereby functioning to finely disperse oxide particles. This action tends to reach a level of saturation when the Ti content exceeds 1.0%, and becomes small when the Ti content is less than 0.1%. From this fact, when the amount of added Ti is in the range of 0.1% to 1.0%, by controlling the excess oxygen content within the range of [0.22×Ti (% by weight)<Ex.0 (% by weight)<0.464×Ti (% by weight)], namely, within the diagonally shaded range in the graph of
As is apparent from the above descriptions, according to the present invention, by paying attention to the excess oxygen content in steel, it is possible to positively obtain a structure in which oxide particles are finely dispersed and highly densified by adjusting the Ti content or by reducing the amount of oxygen contamination during the manufacturing process so that the excess oxygen content becomes within a predetermined range. As a result, it is possible to provide an oxide dispersion strengthened martensitic steel excellent in high-temperature strength.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002-231780 | Aug 2002 | JP | national |
| 2003-276554 | Jul 2003 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP03/10081 | 8/7/2003 | WO | 00 | 7/23/2004 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2004/015154 | 2/19/2004 | WO | A |
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| 0 949 346 | Oct 1999 | EP |
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| Number | Date | Country | |
|---|---|---|---|
| 20050084405 A1 | Apr 2005 | US |
