JPH0551701A - Oxide disperion strengthened ferritic heat resisting steel plate - Google Patents

Abstract

PURPOSE:To provide an oxide dispersion-strengthened ferritic heat resisting steel plate excellent in high temp. creep strength, ductility, and thermal fatigue characteristics and used particularly for high temp. and high pressure boiler, etc. CONSTITUTION:The steel plate is an oxide dispersion-strengthened ferritic heat resisting steel plate characterized by having a composition consisting of, by weight, <=0.08% C, 0.02-2.0% Si, <=2.0% Mn, 9.0-30.0% Cr, 0.1-2.0% Ti, 0.05-0.3% Ni, 0.1-2.0% Mo, <=0.1% Al, <=0.05% P, <=0.05% S, <=0.08% N, one or >=2 elements among Nb, V, Ta, Zr, Hf, B, and Y, and the balance Fe with inevitable impurities and also having a structure where a pretreated powder prepared by mixing the grains of an oxide of >=1500 deg.C melting point with a metal powder is added to a molten steel and the grains of 0.001-0.5mum grain size of the above oxide are incorporated by 0.25-3.0% by volume.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide dispersion strengthened ferritic heat resistant steel sheet which is excellent in high temperature creep strength, ductility and thermal fatigue properties and is particularly used for high temperature and high pressure boilers and the like.

[0002]

2. Description of the Related Art Conventionally, sufficient consideration has been given to the safety of boilers and pressure vessel equipment, especially the safety at the time of water pressure test, and a certain high temperature creep strength for steel for pressure vessels,
Toughness is required. The demand is naturally made also on the welded portion constituting the container. Therefore, as a document relating to a steel sheet for a pressure vessel up to now, for example, as a high-strength ferritic heat-resistant steel excellent in toughness and weldability, Cr-Mo system added with V, Nb, Ni, Co is disclosed in Japanese Patent Laid-Open No.
There is a publication of -238244. In the high Cr steel, the balance between the B and N contents is optimized, and the Si content is reduced, so that the high Cr ferrite heat resistance, toughness and weldability are simultaneously provided. Further, as a similar component system, JP-A-1-8256 is known. This is also similar to the above, in the high Cr steel, the high temperature strength is extremely excellent and the toughness, especially the creep strength is improved by optimizing the product of the Si content and the B and N content, and further the Al content. To obtain excellent steel.

Further, as a dispersion-strengthened alloy in which fine oxide particles that are stable even at high temperature are dispersed in a metal matrix, for example, steel matrix is used, as disclosed in JP-A-63-50448 and JP-A-63-50448. As disclosed in Japanese Patent No. 57-36343, there is MA957 manufactured by Inco Alois Co., Ltd. by a mechanical alloying method. MA957 is Cr-
It is characterized in that an oxide such as Y ₂ O ₃ is dispersed in Ti-Mo steel. As an improved type of MA957, there is a steel characterized by dispersion of TiO ₂ and Y ₂ O ₃ by adding W, as disclosed in JP-A-1-272746. Further, there is JP-A-63-186853 for the purpose of steel having high-temperature strength and toughness and having little swelling when irradiated with neutrons. This is an oxide dispersion strengthened high chromium steel in which Y ₂ O ₃ and particles having a particle diameter of 1 μm or less are dispersed in high Cr steel.

[0004]

However, in order to further increase the high temperature creep strength, the present invention is an oxide dispersion strengthening type in which oxide particles are uniformly dispersed in the molten steel process in addition to the conventionally known steel. It is a ferritic heat-resistant steel plate that is a thick plate, is inexpensive, and has excellent extremely high temperature creep strength. Therefore, neither of the above-mentioned Japanese Patent Laid-Open Nos. 63-238244 and 1-8256 discloses the technical idea of strengthening the high temperature creep by the oxide dispersion strengthening as in the present invention. Further, as a dispersion-strengthened alloy in which fine oxide particles that are stable even at high temperature are dispersed in a metal matrix, JP-A-63-50448, JP-A-57-36343, and JP-A-1-253 / 1982.
In the publication No. 72746, all of them are oxide dispersion type as a sintered body and, unlike steel by uniform dispersion in molten steel, during mechanical alloying during production of oxide dispersion strengthened steel. Since N is mixed in from the atmosphere, this N combines with Cr and Ti in the steel to form coarse (Cr, Ti) N. However, Cr is preferably present as a simple substance in the steel from the viewpoint of solid solution strengthening and high temperature oxidation resistance. Also, Ti
Is an element that promotes uniform and fine dispersion of the oxide, and therefore it is preferable that it is present in combination with the oxide. Further, since the nitride is coarse, the ductility is also reduced, which is a problem. Further, in Japanese Patent Laid-Open No. 63-186853, there is a common point that it is an oxide dispersion strengthened high Cr steel, but this publication produces induced radioactivity in the environment of high density neutron irradiation. For the purpose of not impairing, the addition of elements such as Mo, Ni, Nb is restricted to make it an essential element for increasing the high temperature strength at W, and as described above, it is an oxide dispersion type as a sintered body. It is a difficult problem to manufacture a large number of thick plates as a large steel material.

[0005]

From this point of view, the present invention provides an oxide-dispersion-strengthened ferrite system excellent in high-temperature creep strength, ductility, and thermal fatigue properties, particularly as a thick plate for a high-temperature high-pressure boiler. It is intended to provide heat resistant steel plates. The gist of the invention is (1)% by weight, C: 0.08% or less Si: 0.02-2.0% Mn: 2.0% or less Cr: 9.0-30.0% Ti: 0.1 to 2.0% Ni: 0.05 to 0.3% Mo: 0.1 to 2.0% Al: 0.1% or less P: 0.05% or less S: 0.05% Below N: 0.08% or less, consisting of balance Fe and unavoidable impurities, and melting point 15
A pretreatment powder obtained by mixing oxide particles and metal powder at a temperature of 00 ° C. or higher is added to molten steel, and the oxide having a particle size of 0.001 to 0.5 μm is added at 0% by volume. .25 ~
An oxide dispersion strengthened ferritic heat-resistant steel sheet characterized by containing 3.0%.

(2) C: 0.08% or less Si: 0.02 to 2.0% Mn: 2.0% or less Cr: 9.0 to 30.0% Ti: 0.1% by weight 2.0% Ni: 0.05 to 0.3% Mo: 0.1 to 2.0% P: 0.05% or less S: 0.05% or less Al: 0.1% or less N: 0.08 % Or less as a basic component, and Nb: 0.005 to 0.5% V: 0.01 to 1.0% Ta: 0.01 to 1.0% Zr: 0.005 to 0.5% Hf : 0.01 to 1.0% B: 0.0002 to 0.01% Y: 0.02 to 1.0% One or more types are contained, and the balance consists of Fe and inevitable impurities. And, an oxide having a melting point of 1500 ° C. or more, and a pretreatment powder obtained by mixing the oxide particles and a metal powder is added to molten steel to obtain a particle size of 0.001 to 1 of the oxide.
It is an oxide dispersion strengthened ferritic heat resistant steel sheet characterized by containing 0.25 to 3.0% by volume of 0.5 μm.

The present invention will be described in detail below. In the present invention, the greatest feature is that the pretreated powder obtained by kneading the oxide particles and the metal powder is added to the molten steel. Therefore, first, to increase the specific gravity of the oxide fine particles to facilitate addition to the molten steel and to achieve uniform dispersion in the molten steel, the iron powder is used to attach the oxide fine particles around the iron powder. It is intended to increase the overall specific gravity. That is, the oxide fine particles in the form of being attached around the iron powder are mechanically alloyed, that is, the so-called iron powder and oxide fine particles are treated with high energy.
Mixing in a mill (attritor), mechanical compression, breaking,
The bonding is repeated to form a powder in which fine oxide particles are uniformly dispersed in the matrix. Therefore, it is necessary to perform a pretreatment in which the particle size of the iron powder, which is a metal powder, is regulated to 1 to 1000 μm, and the iron powder is mechanically attached to the surface of the iron powder. The reason for regulating the particle size of the iron powder is that the lower limit of the average particle size of the iron powder is 1 μm or more because the oxide particles are mechanically attached to the surface of the iron powder and uniformly dispersed because of the relationship with the oxide particles. is necessary. Regarding the upper limit, if the size is too large, the oxide fine particles and the iron powder are likely to be non-uniformly mixed.
It is desirable that the thickness is 00 μm or less. Further, the oxide fine particles were set to 0.001 to 0.5 μm. The reason is that the creep strength cannot be improved by dispersion strengthening. Further, the upper limit is set to 0.5 μm or less because cracks easily occur during processing.

The reason for directly adding the oxide fine particles to the molten steel is to continuously mass-produce large products such as thick plates for high temperature and high pressure boilers. Further, the reason for adding as a mixed powder, without adding oxide fine particles alone,
This is because oxide fine particles agglomerate to form a powder, and since the volume increases and the specific gravity is smaller than that of molten steel, when added alone, they float and do not yield in molten steel. The aggregated oxide fine particles were separated into individual oxide fine particles and mixed with powder to reduce the volume. The reason for selecting iron powder is that it does not separate and float when added to molten steel. It should be noted that the present invention is not limited to iron powder only, and Cu powder or the like may be used as long as it achieves this effect, and there is no particular limitation.

Thus, the average particle size is 0.001 to 0.5 μ
This is because the oxide fine particles of m are mechanically attached to the surface of the iron powder to be uniformly dispersed, and the weight ratio of the iron powder as the metal powder to the oxide fine particles is 100: 0.5 to 50. The reason for this is that the oxide volume in the steel sheet is contained in the range of 0.25% to 3.0%. Therefore, the surface area of the iron powder for pretreatment is increased so that the oxide fine particles easily adhere to the iron powder. .. Further, the melting point of the oxide particles is 1500 ° C. or higher, and for example, A
Oxides such as l ₂ O ₃ , Y ₂ O ₃ and Sm ₂ O ₃ are used. The reason why the high temperature oxide particles having a melting point of 1500 ° C. or higher are used is that they are present in an oxide state without being melted by the molten steel even if they are added to the molten steel. As the addition to the molten steel, when the mixed powder is added to the molten steel in the ladle, the molten steel temperature is high, and the oxide particles may be dissolved.Therefore, the place where the mixed powder is added is less likely to dissolve the oxide particles, Moreover, the iron powder is sufficiently dissolved in the tundish and / or the mold.
Cold molten steel is desirable.

[0010]

Next, in order to achieve the desired characteristics of the present invention, it is necessary to limit the amount of each constituent element to an appropriate range as described below. C: 0.08% or less C is an effective component for improving the strength, and is an element that contributes to the improvement of creep rupture strength due to the precipitation of carbides. C was made 0.08% or less in order to produce coarse carbide which is harmful to the alloy. Si: 0.02 to 2.0% When Si is less than 0.02%, sufficient strength and wettability between molten steel and oxide cannot be improved to help disperse the oxide. On the other hand, if it exceeds 2.0%, the toughness decreases, the weldability is adversely affected, and the function of assisting the dispersion of the oxide in the molten steel also decreases. Mn: 2.0% or less Mn improves the creep strength by solid solution strengthening, but if it exceeds 2.0%, its effect disappears. Cr: 9.0 to 30.0% Cr is added to improve high temperature oxidation resistance, high temperature long-term strength, and stabilize ferrite. If it is less than 9.0%, precipitation strengthening by Cr carbide and Solid solution strengthening cannot be expected, high-temperature long-term strength declines, and 9.0%
If it is less than 1, the oxidation resistance at high temperature is lowered. Further, if it exceeds 30%, cracking occurs and the weldability is significantly impaired. Therefore, the Cr content is 9.0 to 30.0%.

Ti: 0.1 to 2.0% Ti improves the wettability of molten steel and oxides and helps the dispersion of oxides, and at the same time, it is a high Cr containing Nb and V according to the second invention.
In steel, the high temperature creep rupture strength is remarkably increased by further adding Ti together with B. 2.
If it exceeds 0%, cracking occurs, so the Ti content is 0.1-0.1%.
2.0%. Ni: 0.05 to 0.3% Ni is added to improve ductility at high temperature, but 0.0
If it is less than 5%, it has no effect, and if it exceeds 0.3%, it is not preferable for hot working. Further, if it exceeds 0.3%, the effect of improving high temperature ductility is not further observed, and Ni is an expensive element. Therefore, the upper limit is set to 0.3%. Mo: 0.1 to 2.0% Mo is an indispensable element for heat-resistant steel because it significantly enhances the high temperature creep strength by solid solution strengthening. In addition to solidifying and strengthening in steel, carbide is precipitated to improve creep strength, but if it is less than 0.1%, this effect does not occur and improvement in high temperature strength is not sufficiently exhibited. On the other hand, if it exceeds 2.0%, the amount of ferrite increases to lower the high temperature strength and cracks occur, so the upper limit was made 2.0%.

Al: 0.1% or less Although Al is effective as a deoxidizing agent, especially when it exceeds 0.1%, the high temperature strength decreases, so the upper limit was made 0.1% or less. P: 0.05% or less P is limited to 0.05% or less because if it exceeds 0.05, grain boundary embrittlement occurs. S: 0.05% or less It is important to suppress hydrogen attack and intergranular embrittlement by suppressing S to be low. Therefore, the S content is 0.05% or less. N: 0.08% or less N is more than 0.08%, and if it exceeds 0.08%, a coarse nitride harmful to the creep is formed, so the content was made 0.08% or less.

Further, the reason why the content range of the constituent elements of the strength improving element group of the second invention is limited to the proper range will be described. Nb: 0.005-0.5% Nb improves the creep strength from both sides of precipitation strengthening and solid solution strengthening, but if it is less than 0.005%, its effect is insufficient. On the other hand, if it exceeds 0.5%, the carbides are remarkably coarsened, the creep rupture strength is lowered, and the ductility is lowered. Therefore, the Nb content is set to 0.005 to 0.5%. V, Ta, Hf: 0.01 to 1.0%, Zr: 0.00
5 to 0.5% Further, regarding V, Ta, Zr, and Hf, the creep strength is improved from both sides of precipitation strengthening and solid solution strengthening similarly to Nb, but V, Ta, and Hf are 0. Less than 01%, Z
If r is less than 0.005%, the effect is insufficient.
Further, in the case of V, Ta, and Hf, if it exceeds 1.0%, and in the case of Zr, if it exceeds 0.5%, the carbides become significantly coarse,
Since the creep rupture strength is lowered and the ductility is lowered, the respective amounts of V, Ta and Hf are set to 1.0% or less and Zr is set to 0.5% or less. B: 0.0002 to 0.01% B is a high Cr steel containing appropriate amounts of Nb and V, and by adding B together with an appropriate amount of Ti, grain boundaries are strengthened and high temperature creep strength is enhanced. Is significantly increased. However, addition over 0.01% causes intergranular cracking, so it must be 0.01% or less. Y: 0.02-1.0% Y forms a solid solution to improve creep strength and prevent high temperature oxidation, but if it is less than 0.02%, the effect is insufficient. .. Further, if it exceeds 1.0%, the ductility is lowered, so the content was made 1.0% or less.

[0014]

【Example】

Example 1 (Method A) A mixed powder was produced by putting iron powder and Y ₂ O ₃ powder in a container together with a steel ball (SUJ-2) and mixing them with an attritor. Average particle size of iron powder at that time 150-200
μm, Y ₂ O ₃ powder average particle size 0.05 μm, weight ratio 1
Mix at a ratio of 0: 1 to obtain a pretreated powder. On the other hand, 30 seconds after Al deoxidation of molten steel whose alloy composition was adjusted in a high-frequency melting furnace with a capacity of 300 kg, molten steel: pretreated powder = 10
The pretreated powder is thrust into the molten steel with a phosphorizer at a weight ratio of 0: 4, and after 2 minutes, the steel is tapped into the mold. Table 1 shows comparative steels such as the steel of the present invention at that time and the oxide fine particles not pretreated alone were added or the composition of the steel was deviated. 1100 ° C hot rolling of steel ingots with these chemical components
A steel plate having a thickness of 8 mm was annealed at 1200 ° C. for 5 hours. Then, in the creep test, the creep rupture strength at 700 ° C. for 10,000 hours was examined. The results are shown in Table 1. As can be seen from Table 1, the invention steels (1) to (7) have high creep strength, while the comparison steels (8) to (7) are high.
The creep strength of (13) is low, and the comparative steel (8) does not yield because the oxide fine particles are added as it is. Is low, the comparative steel (11) has a large amount of Si and Mn and T
Oxide particles were aggregated due to lack of i. Comparative steel (12) is C
There was a large amount of r and Ni, and cracking occurred. Further, the comparative steel (13) lacked strength due to lack of Mo.

Example 2 (Method B) As the mixed powder, copper powder and Al ₂ O ₃ powder were used as steel balls (SUJ-2).
It was manufactured by putting it in a container together with and mixing with an attritor. Average particle size of iron powder at that time 150-200
μm, Al ₂ O ₃ powder average particle size 0.05 μm, weight ratio 1
Mix at a ratio of 0: 1 to obtain a pretreated powder. On the other hand, 30 seconds after Al deoxidation of molten steel whose alloy composition was adjusted in a high-frequency melting furnace with a capacity of 300 kg, molten steel: pretreated powder = 10
A pretreatment powder is injected into the molten steel flow during casting at a weight ratio of 0:10 by argon gas, and then the steel is tapped into a mold. The steel of the present invention at that time was the same as that of Example 1, and the execution No. (4)
It shows in (5). The result is the same as that of the first embodiment.

[0016]

[Table 1A]

[0017]

[Table 1B]

[0018]

As described above, the oxide-dispersion-strengthened ferritic heat-resistant steel excellent in high-temperature creep strength according to the present invention has a higher rupture temperature than the conventional ferritic heat-resistant steel at high temperature, especially creep rupture. Steel with significantly improved strength, excellent ductility, thermal fatigue properties, and good workability, such as high-temperature and high-pressure boiler materials used in high-temperature and high-pressure environments, heat-resistant steel for chemical plants, etc. However, it can be manufactured at a low cost, and thus has an extremely great industrial effect.

Claims (2)

[Claims] 1. By weight%, C: 0.08% or less Si: 0.02 to 2.0% Mn: 2.0% or less Cr: 9.0 to 30.0% Ti: 0.1 to 2 0.0% Ni: 0.05 to 0.3% Mo: 0.1 to 2.0% Al: 0.1% or less P: 0.05% or less S: 0.05% or less N: 0.08% The balance below consists of Fe and unavoidable impurities, and has a melting point of 15
A pretreatment powder obtained by mixing oxide particles and metal powder at a temperature of 00 ° C. or higher is added to molten steel, and the oxide having a particle size of 0.001 to 0.5 μm is added at 0% by volume. .25 ~
An oxide dispersion strengthened ferritic heat-resistant steel sheet characterized by containing 3.0%. 2. C .: 0.08% or less Si: 0.02 to 2.0% Mn: 2.0% or less Cr: 9.0 to 30.0% Ti: 0.1 to 2% by weight 0.0% Ni: 0.05 to 0.3% Mo: 0.1 to 2.0% P: 0.05% or less S: 0.05% or less Al: 0.1% or less N: 0.08% The following are the basic components, and further Nb: 0.005-0.5% V: 0.01-1.0% Ta: 0.01-1.0% Zr: 0.005-0.5% Hf: 0.01 to 1.0% B: 0.0002 to 0.01% Y: 0.02 to 1.0%, one or more of them are contained, and the balance is Fe and inevitable impurities, and A melting point of 1500 ° C. or higher, and a pretreatment powder obtained by mixing the oxide particles and metal powder is added to molten steel to obtain a particle diameter of 0.001 to 0.001 of the oxide.
An oxide dispersion strengthened ferritic heat-resistant steel sheet characterized by containing 0.25 to 3.0% by volume of 0.5 μm.