DE69829012T2 - Ferritic, heat-resistant steel and method of manufacture - Google Patents

Description

AREA OF INVENTION

The The present invention relates to a ferritic heat-resistant steel and a method for its production: more particularly, it relates to a ferritic heat-resistant steel, the for Materials for Apparatus is suitable under high temperature and high pressure conditions used, such as steam boilers, apparatus in the chemical industry, etc., and it relates to a method to its Production. In particular, it relates to a ferritic heat-resistant steel, the excellent oxidation resistance at high temperatures has, in particular, a steam oxidation resistance, not even at high temperatures greater than 630 ° C worse and that has a high creep resistance with that of ordinary Steel is comparable, and it concerns a method to its Production.

BACKGROUND THE INVENTION

in the general is heat-resistant steel for use for high temperature heat resistant and pressure-resistant Parts of steam boilers, nuclear-powered apparatus and other apparatus needed in the chemical industry, high-temperature strength, toughness, High temperature erosion resistance, oxidation resistance, etc. to have. For such austenitic stainless steel such as JIS-SUS321H, JIS-SUS347H etc., low alloy steel such as JIS-STBA24 (2-1 / 4Cr-1Mo steel), etc., and 9- to 12-Cr high-ferrite steel such as JIS-STBA26 (9Cr-1Mo steel).

In front In all, ferritic high-Cr steel was widely used in the state of the art Technique used because it has several advantages. Especially he has a higher one Strength and a higher Erosion resistance as low alloy steel at temperatures between 500 and 650 ° C lie, and it is cheaper than austenitic stainless steel. Furthermore, high Cr ferrite steel has a good thermal fatigue resistance, because its thermal conductivity is high and its thermal expansion is small while it is hardly a detachment caused by scale and stress erosion breakage.

On the other side are in current thermal electric Power plants Steam boilers at high temperature and high pressure conditions for the Purpose to improve the thermal efficiency therein. to Time will be steam boilers in such facilities under supercritical Pressure conditions at 538 ° C and 246 atmospheres operated, but in the future they are under ultra-supercritical Pressure conditions at 650 ° C and 350 atmospheres operated. In this given situation it is demanded that steel for steam boiler an extremely high performance has, and conventional high-Cr ferrite steel could meet this requirement at high Oxidation resistance and long-term creep resistance, in particular No longer meet steam oxidation resistance. When the steam oxidation resistance of steam boilers is bad, oxide films form on the inside surface of steel pipes of steam boilers, through which high temperature steam passes. After this they have grown to a certain thickness, the oxide films dissolve due to the thermal stress caused by the temperature changes in steam boilers can be caused when operated for example Steam boilers are stopped, causing the pipes to become clogged. Therefore, the prevention of steam oxidation of steel pipes, in particular the prevention of detachments Of oxide films an important topic.

austenitic stainless steel is known as a material that is capable of to meet the above noted requirements. However, it is austenitic stainless steel expensive, and its use in commercial facilities is from economic establish limited. additionally Its thermal stress is large, due to the temperature changes when stopping the operation or is caused due to austenitic stainless steel a big one has thermal expansion coefficient. For these reasons is the use of austenitic stainless steel in plants due the difficulties in the design and operation of the facilities problematic who use it. In the face of this, it is desirable the efficiency of ferritic steel, which has a smaller thermal Has expansion coefficients and is cheaper.

To meet the requirements, various types of ferritic heat-resistant steel have recently been proposed. For example, in Japanese Patent Application Publication (JP-A) Hei-3-097832, there has been proposed a Cu-containing high-Cr heat-resistant steel whose W content is higher than that of conventional steel. Cu is added to improve its high temperature oxidation resistance. JP-A-Hei-4-371551 and Hei-4-371552 have proposed a heat-resistant high-Cr steel. In this, the ratio of Mo / W is optimized, and Co and B are both (there to), increasing the high-temperature strength and the toughness of the steel. Although its high-temperature creep strength is increased because a large amount of W is added, such steel types are still problematic because the reduction of their toughness is unavoidable. This is because W such as Mo and Cr is a ferrite-forming element and therefore forms d-ferrite when added in such a large amount, lowering the toughness of the W-containing steel.

Around to solve this problem, It is most effective to have a single martensitic phase in steel to build. For this purpose, it has been proposed, for example, in JP-A-Hei-5-263195, etc. to lessen the amount of Cr added to steel, and Hei-5-311342, Hei-5-311343, Hei-5-311344, Hei-5-3111345, Hei-5-311-346, etc. has been proposed, a large amount of austenite-forming To add elements such as Ni, Cu, Co and the corresponding to steel. This is done to the tenacity of the steel through the proposed techniques.

However, the former steel proposed in JP-A-Hei-5-263196 could not have an intact scale structure since Mo enters the scale consisting essentially of Cr. Therefore, it has a bad steam oxidation resistance. In order to solve this problem, another proposal has been proposed in JP-A-Hei-8-85847 wherein no Mo or only a small amount of Mo to W-containing steel is added. In the proposed steel, W is an essential element that has been added to reinforce it. However, since it contains a large amount of Ni and Cu, like the steel disclosed in JP-A-Hei-5-311342, this steel is still insufficient in that the structure is changed by oxides consisting essentially of Cr ₂ Os exist and that its steam oxidation resistance is poor.

On the other hand, the high Cr ferrite steel disclosed in JP-A-5-311342 and elsewhere has a low A ₁ transformation point and a low A ₃ transformation point since it contains a large amount of Ni, Cu, etc. contains. As a result, the temper softening resistance of the steel is poor, and in addition, carbides and nitrides in the steel rapidly aggregate to form large coarse grains therein. Therefore, the long-term creep strength of the steel is low. Moreover, Ni, Cu and other elements added to the steel change the scale layer formed to cause it to have a brittle structure, as in the heat resistant steel disclosed in JP-A-Hei-5-263196, wherein the steam oxidation resistance of the steel is worsened.

As mentioned above, there is no known satisfactory ferritic heat-resistant steel, the sufficient oxidation resistance and steam oxidation resistance for use in ultra-supercritical Pressure conditions at high temperatures and high pressures has.

SUMMARY THE INVENTION

The The present invention has been made in consideration of the above-mentioned present situation. and its subject matter is the provision of a ferritic steel, free of the disadvantages of conventional ferritic steel is. In particular, the subject of the invention is the provision of ferritic steel, of which the steam oxidation resistance even higher at high temperatures not 630 ° C is lowered, and the excellent long-term creep resistance Has. To the above mentioned To solve problems, As in claim 1, the invention provides a ferritic heat-resistant steel ready, which has a steam oxidation resistance, wherein the ferritic heat resistant steel based on the weight of 0.06 to 0.18% C, from 0 to 1.0% Si, from 0.05 to 1.5% Mn, not more than 0.030% P, not more than 0.05% S, from 8.0 to 13.0% Cr, from 0 to 4.0% W, from 0 to 2.0% Mo with the proviso that W + 2Mo ≤ 4.0% is from 0.02 to 0.14% Nb, from 0.10 to 0.50% V, from 0 to 0.10% N, from 0 to 0.01% B, not more than 0.010% O and from 0 to 0.050% dissolved Al, at least one of Pd and Pt in an amount of 0.3 to 5.0% Pd and from 0.3 to 5.0% Pt and in a ratio of 0.3% ≤ Pd + Pt ≤ 5.0% and the remainder comprises Fe and unavoidable impurities.

SHORT DESCRIPTION THE DRAWINGS

1 Figure 3 is a cross-sectional view of a steel sample according to the invention, showing graphically the relational structure of the oxide grains formed therein and the scale formed on the steel base.

2 (A) Fig. 12 is a cross-sectional view of a conventional steel sample in which the scale formed peels off due to the voids formed therein; 2 B) is a cross-sectional view of a steel sample according to the invention, in which due to the oxide particles formed therein is prevented that the scale formed peels off.

In these is 1 an outer scale layer, 2 is an inner scale layer, 3 is a steel base, 4 is an oxide particle and 5 is a gap.

DESCRIPTION THE PREFERRED EMBODIMENTS

The The present invention is characterized by the above-mentioned characteristics characterized. The problems with steel, which has a bad oxidation resistance have, are that the oxide film on the inner surfaces of Steel pipes is formed, peels off and settle in the pipes to clog them, and that the detached one Oxid fildm distributed in steel pipes and eroded the apparatus, in a later one Zone is arranged. From this point of view, the present became Invention, and its subject matter is as indicated above homogeneous formation of ultrafine oxide particles of a size of not more than 1 μm in and / or around the interface between that on the surface a steel base formed oxide film and the steel base directly under the oxide film, wherein the adhesion between increase the oxide film and steel base.

It is known to be adding a big one Amount of Cr or Si to steel is effective, causing the steel has a high Cr or Si content in order to reduce the high temperature oxidation resistance of To improve steel. However, high-Cr steel is problematic in that that d-ferrite is formed in it, which lowers the toughness of the steel. Therefore becomes an austenite stabilizing element such as Ni, Co, Added Cu or equivalent to conventional high Cr steel. however is adding of the element disadvantageous, since a stable oxide film difficult on To form the steel that contains the element, which leads to the oxidation resistance of the steel is reduced. On the other The side of high-Si steel is also inadequate in that the one formed on it Film peels off easily, although its erosion is slowed down.

• (1) Wenn feine Oxidpartikel in der und/oder um die Grenzfläche zwischen einer Metallbasis und dem darauf gebildeten Oxidfilm vorhanden sind, insbesondere in der Region direkt unter dem Film, könnten sie lückenfüllende Punkte im Film sein und desweiteren als eine Barriere für das Wachstum von Lücken wirken, die sich in der Grenzfläche bilden. Überdies wird das Adhäsionsvermögen zwischen dem Film und der Basis durch den Brückeneffekt der Partikel erhöht, wobei der Film am Ablösen gehindert wird.
• (2) Wenn zu große Oxidpartikel in der Region direkt unter dem Film gebildet werden, können sie jedoch nicht mehr der Ablösung des Films widerstehen. Daher besteht kein signifikanter Unterschied zwischen der Anwesenheit von derartigen großen Oxidpartikeln und deren Abwesenheit.

In this given situation, we, the present inventors, have studied the structure of oxide films formed on various steel samples as well as the structure of the film / steel interface in these samples and that of film / steel interface structures, and the following findings receive. Based on these findings, we have completed the present invention.

• (1) When fine oxide particles are present in and / or around the interface between a metal base and the oxide film formed thereon, especially in the region directly under the film, they may be gap-filling points in the film and further as a barrier to the growth of Gaps appear that form in the interface. Moreover, the adhesiveness between the film and the base is increased by the bridging effect of the particles, whereby the film is prevented from peeling off.
• (2) When too large oxide particles are formed in the region directly under the film, however, they can no longer resist peeling of the film. Therefore, there is no significant difference between the presence of such large oxide particles and their absence.

Based Based on these results, the invention provides a ferritic heat-resistant steel ready, which has both a good oxidation resistance and a has good creep resistance even at high temperatures of 600 ° C or higher.

The The nature of the invention is described in more detail below.

<Oxide precipitates>

Of the essential reason for the replacement of the oxide film is thermal stress caused by the temperature change caused in steel. The thermal stress should with the growth of the oxide film on steel increases (i.e., with the increase the thickness of the film). When the thermal stress is the adhesiveness (adhesion strength) between the film and the underlying steel base, dissolves the film from the steel base. Therefore, increasing the adhesiveness of the film is on the steel base effective to peeling to prevent the film.

The Adhesiveness of the Films are generally formed by densifying the oxide film itself increased by Create conditions in which pores or gaps in the interface between The film and the steel base are difficult to form. In contrast however, in the present invention, fine particles are used for this purpose in the interface formed between the oxide film and the steel base, so they as a barrier to the Continuation of the film release in the film / base interface act, preventing the film from swelling.

The effect of the invention for preventing the detachment of the scale could interpre as follows to be trained:
In the invention, oxides are formed by internal oxidation in steel, while the steel is to have a Zunderstruktur consisting of an outer scale layer (of Fe oxides) ( 1 ) and an inner scale layer (made of Fe-Cr oxides) ( 2 ), as it sits on the surface of the steel base ( 3 ) as in

1 formed in the fine oxide particles ( 4 ) exist around the scale / base interface.

Ordinary steel is thought to aggregate the pores existing in the scale in the interface between the scale and the steel base to fill gaps ( 5 ) as in 2 (A) and such gaps ( 5 ) are connected to each other so that they cause a detachment of the scale. If, however, as in 2 B) fine oxide particles ( 4 ) around the interface between the scale layers ( 1 ) ( 2 ) and the steel base ( 3 ), especially in the region directly under the tinder ( 2 ), they could be gap-filling points and even as a barrier to 5 ) Act. Furthermore, the particles could continue to act as a mechanical bond between the scale and the steel base, preventing the scale from swelling or peeling.

existing Oxide particles in the interface and / or around the interface between the oxide film and the steel base, which is a size of not more than 1 μm have, but preferably not more than 0.5 microns, prevent the film from peeling and are effective to achieve the intended purpose. But when big particles in the interface exist, which have a size of 3 microns or more they are not for the intended purpose effective, but actually support the replacement of the film.

<Steel composition>

• (1) Cr: In general, the oxide film formed on ferritic heat-resistant steel is composed of an outer layer consisting essentially of Fe oxides and an inner layer consisting essentially of Cr oxides or Fe-Cr Oxides exists. The stabilization of the intact Cr ₂ O ₃ film without its detachment is effective for improving the oxidation resistance of the steel. From this point of view, Cr is an essential alloying element according to the invention. With regard to the amount added, Cr must be added to the steel in an amount not smaller than 8.0% to form an intact oxide film. However, when the amount of Cr added is larger than 13.0%, much Cr promotes the formation of d-ferrite, greatly deteriorating the properties of the steel including its toughness. For these reasons, the Cr content of the steel according to the invention is preferably between 8.0 and 13.0%. The other elements are added to steel as in the prior art to cause the steel to have the necessary performance such as creep resistance and toughness. For their quantity, therefore, reference is made to the normal knowledge known in the art.
• (2) C: C is an element constituting carbides of various types, MC [as in the case of carbonitrides M (C, N) in which M indicates an alloying element and the same shall apply below], M ₇ C ₃ , M ₆ C, M ₂₃ C ₆ , and this has a great influence on the properties of steel. In particular, fine carbide particles of VC, NbC and the like are precipitated in steel while the steel is used, and they contribute to the increase of the long-term creep strength of steel. In order to effectively precipitate such fine carbide particles to strengthen the steel, the amount of C contained in the steel may not be less than 0.06%. However, when greater than 0.18%, so much C forms coarse and large carbide aggregates during early stages of use, undesirably lowering the long-term creep strength of steel. For these reasons, the C content of steel is suitably defined to be between 0.06 and 0.18%.
• (3) Si: Si is an element used for deoxidizing a molten steel and to improve the high-temperature steam oxidation resistance of steel is effective. However, too much Si lowers the toughness of Stole. Therefore, in the prior art, the Si content generally becomes steel defined to be between 0.01 and 1.0%. Accordingly, also in the invention, the upper limit of the Si content is 1.0 %.
• (4) Mn: Mn is an element added to steel for the purpose of deoxidizing and desulfurization of molten steel, and this is effective, the short-term creep strength of steel under high load to increase. In order to achieve its effect, Mn must be added in an amount which is not less than 0.05%. On the other hand, it is known that if greater than 1.6%, so much Mn's toughness reduced by steel. For these reasons, it is appropriate that the amount of Mn that added is between 0.05 and 1.5%.
• (5) Mo, W: Mo is effective for solution hardening of steel. It also stabilizes M ₂₃ C ₆ and increases the high-temperature strength of steel. However, if its amount is more than 2%, Mo supports the Formation of d-ferrite while assisting the precipitation and aggregation of M ₆ C and Laves phases, giving coarse and large particles. Therefore, the upper limit is defined to be 2%. Like Mo, W is also suitable for solution hardening of steel. Furthermore, it contributes to the precipitation of fine particles of M ₂₃ C ₆ while preventing aggregating of carbides to give coarse and large particles. Due to such effects, W greatly increases the high-temperature and long-term creep strength of steel. However, if it is larger than 4%, so much W often forms d-ferrite and coarse Laves phases, thereby lowering the toughness of steel. Therefore, it is appropriate that the upper limit of W is 4%. When Mo and W are both added to steel, it is appropriate that the total amount of W + 2 Mo is up to 4%.
• (6) V: V is an element that fine carbide, nitride and carbonitrite particles forms, which contribute to the increase in creep resistance of steel. In order to achieve its effect, V has to be added to steel in an amount which is not less than 0.10%. But even in one Amount greater than 0.50% is added, is so much V no longer effective, since the effect of V is saturated, if the amount is up to 0.50%. Therefore, it is appropriate that the V content is between 0.10 and 0.50%.
• (7) Nb: Nb precipitated in steel in the form of its carbides, nitrides and carbonitrides, where it increases the high temperature strength of steel. Furthermore, it works, the To make microstructure of steel fine, taking it toughness increased by steel. Therefore, it is said that the lower limit of Nb in steel is 0.02% is. When Nb is added in an amount of 0.15% or more, However, it is believed that it is not completely in the matrix of steel can penetrate to a solid solution at normalization temperatures, and it could therefore its effect for strengthening the steel does not develop sufficiently. Accordingly, it is appropriate that the Nb content be between 0.02 and 0.14%.
• (8) N: N is an element that forms nitrides and carbonitrides while increasing the creep resistance of steel. If the N content more than 0.1%, In general, however, the nitrides formed grow to be coarse and larger size Particles, which actually greatly reduces the toughness of steel. Therefore, amounts the upper limit of the N content is preferably 0.1%.
• (9) It is known that B is effective for strengthening the intergranular strength of steel and finely dispersing M ₂₃ C ₆ carbides in steel, and that it contributes to the increase of high-temperature strength of steel, and it is for improving the quenchability effective from steel. It is also known to form so much B of more than 0.01% coarse and large B-containing precipitates, thereby embrittling the steel. Therefore, it is appropriate that the upper limit of B is 0.01%.
• (1) solved Al: Al added to steel essentially acts as a deoxidizer for molten steel. In steel the added one exists Al in the form of its oxides and in any other form. In analyzes the latter becomes HCl-soluble Al (solved Al, sol. Al). As far as steel is concerned by any other elements, the added be deoxidized, will be solved Al not specifically needed. If it's in a lot bigger than Added 0.050 wt .-% As much Al lowers the creep resistance of steel. The salary at solved Aluminum is made of steel suitably from 0 to 0.050 wt .-%.
• (11) P and S: P and S are both inevitable impurities in steel. These elements have a certain negative influence on the hot workability of steel, the toughness of welded steel parts etc. Therefore, their content is preferably as small as possible. Specific P should not be greater than 0.030 % By weight and S not greater than 0.05 wt .-% be.
• (12) O: O is also an inevitable impurity in steel. If it is local in steel in the form of coarse and large oxide particles exists, the particles have a certain negative influence on toughness and other properties of steel. To ensure the toughness of steel, is it desirable that the O content of steel is minimized as much as possible. If the O content is not greater than 0.010 wt.%, is his influence on the tenacity of steel satisfactorily small. Therefore, the O content should not greater than Be 0.010%.

As mentioned above, the object of the present invention is the formation of fine Oxide particles that are a size of not more than 1 μm directly under the film formed on the steel, thereby causing the bridge effect The oxide particles are prevented from the film peeling off. It is therefore, of course, that the constituents which form the steel according to the invention, not as always limited to those are specifically referred to here as far as the steel the subject invention achieved.

Further, the ferritic heat-resistant steel of the present invention characterized by the above-mentioned specific objects has been completed on the basis of the following findings resulting from the detailed study data including the present inventors regarding the relationship between the properties of the steel have made high long-term creep resistance and its high steam oxidation resistance and chemical constituents that protect the steel and the metal form the metallic structure (microstructure) of the steel.

<Long-term creep>

Of the Ferritic according to the invention heat resistant steel can with any ordinary Equipment and with methods generally used in the art will be produced.

stole is melted, for example, in an oven, such as a electric furnace, a converter or the like, and deoxidizer and alloying elements are added to the steel composition to control. If a strict modulation of the steel composition is specifically required The molten steel can be subjected to a vacuum treatment before adding the be subjected to alloying elements.

The Molten steel that has been specifically modulated to give it a predetermined has chemical composition, then is in plates, billets or Ingots in a continuous casting process or a process poured for plate making, and then in tubes, sheets, etc. be formed. For example, if made of seamless steel tubes become, become clubs extruded and forged to steel tubes. For the production of steel sheets plates are hot rolled into hot rolled sheets. The resulting hot-rolled sheets can cold rolled in cold rolled sheets. When the hot working of followed by cold working such as cold rolling, it is desirable that the hot worked sheets are tempered and washed with acids before they become ordinary Be subjected to cold working.

The optionally produced steel tubes and sheets, a heat treatment such as tempering or similar to be made to have predetermined characteristics to have.

The The invention will be explained in more detail below with reference to the following Examples are described, but not as limiting on the scope the invention are meant.

example 1

Various Steel types, each containing the chemical Composition were in a vacuum high frequency induction furnace made of a capacity for 10 kg of steel has.

Table 1

each Molten steel was poured into ingots with a diameter of 70 mm, which then varied at a temperature varying between 1250 ° C and 1000 ° C, were hot forged in sheets, which have a square area of 45 mm × 45 mm and one length of 400 mm had. These were then heated at a temperature between 1100 ° C and 900 ° C varied, cold-rolled in sheets, which has a square area of 15 mm × 15 mm.

The inventive samples Nos. 1 to 6 in Table 1 were thereafter kept at 1100 ° C for 1 hour and then normalized by air cooling or were kept at 800 ° C for one hour and then tempered by air cooling.

On on the other hand, Comparative Samples 1 and 2 were in Table 1 of an ordinary post-heat treatment subjected. Briefly, these were at 950 ° C for one hour held and then by air cooling normalized or were at 750 ° C held and then by air cooling started. Comparative Samples 1 and 2 had a chemical composition from ASTM-A213-T91 or DIN-X20CrMoWV121.

Test pieces were drawn as samples from these eight samples and on the high temperature creep resistance and tested the steam oxidation resistance.

[Steam oxidation resistance]

The test conditions are mentioned below. Test piece: Diameter 8.0 mm measuring length 40 mm Test temperature: (1) 650 ° C, (2) 700 ° C stress: (1) 140 MPa, (2) 120 MPa Measurement: Time before the break

The Data obtained in these tests are shown in Table 2.

Table 2

in the Case of Samples 1-6 if the thickness of the scale layer formed is less than 36 μm (625 ° C × 1000 h), less than 48 gm (650 ° C × 1000 h) and less than 57 gm (700 ° C x 1000 h). It was found that each steel of Samples 1-6 had superior steam oxidation resistance at the high temperature of over 630 ° C and has is extremely stable.

Of course it is the invention is not limited to the ones illustrated here embodiments limited. In terms of its details, the invention, any changes and modifications which do not exceed their scope.

As has been described in detail above, the invention provides a ferritic heat-resistant steel ready which has excellent steam oxidation resistance and creep resistance characteristics. The creep resistance of the steel according to the invention is at least comparable with or greater than those of ordinary steel. The steel according to the invention can for high temperature heat resistant and pressure-resistant Parts are used in various industrial fields widely used, for example for parts steam boilers, nuclear-powered apparatus and other apparatus the chemical industry. The steel can be used, for example, for pipes, Sheets for Pressure vessel, Turbines etc. are used.

Claims (1)

Ferritic heat-resistant steel with a steam oxidation resistance, based on the weight from 0.06 to 0.18% C, from 0 to 1.0% Si, from 0.05 to 1.5% Mn more than 0.030% P, not more than 0.05% S, from 8.0 to 13.0% Cr, from 0 to 4.0% W, from 0 to 2.0% Mo, with the proviso that W + 2Mo ≤ 4.0%, from 0.02 to 0.14% Nb, from 0.10 to 0.50% V, from 0 to 0.10% N, from 0 to 0.01% B, not more than 0.010% O and from 0 to 0.050% dissolved Al, at least one of Pd and Pt in an amount of 0.3 to 5.0% Pd and from 0.3 to 5.0% Pt and in a ratio of 0.3% ≤ Pd + Pt ≤ 5.0% and as balance Fe and unavoidable impurities.