DE69904336T2 - HIGH CHROME, HEAT RESISTANT, FERITIC STEEL - Google Patents

Description

Technical area

The present invention relates to a heat-resistant high Cr ferritic steel, particularly to a high Cr ferritic steel which has excellent long-term creep strength and toughness at high temperatures and is suitable for use, for example, as a heat-resistant steel pipe or heat-resistant steel piping, a steel plate of a pressure vessel or a steel member of a turbine used under high temperature and high pressure conditions in a steam boiler, or in a nuclear power plant and a chemical plant.

State of the art

A heat-resistant steel used under high temperature and high pressure conditions such as those in a steam boiler, a nuclear power plant and a chemical plant is generally required to have excellent properties in terms of high temperature strength, corrosion resistance, oxidation resistance and toughness.

An austenitic stainless steel such as JIS-SUS321H steel (equivalent to AISI-TP321H), JIS-SUS347H steel (equivalent to AISI-TP347H), a low alloy steel such as 2¹/₄Cr-1Mo steel and a high Cr ferritic steel such as 9-12 Cr steel have been used for these applications in the past. Among them, a high Cr ferritic steel is better than a low alloy steel in terms of high temperature strength and corrosion resistance at a temperature of 500ºC to 650ºC. Furthermore, a high Cr ferritic steel has many advantages over an austenitic stainless steel, such as being less expensive, having high thermal conductivity and Due to its low coefficient of thermal expansion, it has excellent thermal fatigue properties and is also not susceptible to stress corrosion cracking.

The steam state in a steam boiler has been changing in the past as the temperature and pressure have become higher and higher to further improve the thermal efficiency in a thermal power plant. An example of such a steam state is a temperature of 538ºC and a pressure of 246 atm, which is called a supercritical state. Moreover, a steam state corresponding to a temperature of 625ºC or higher and a pressure of 300 atm or higher, a so-called ultra-supercritical state, has been studied for future application. Due to these changes in the steam state, the property requirements for a steel pipe or piping used in a steam boiler are becoming higher. A conventional ferritic steel with a high Cr content cannot therefore fully meet the long-term creep strength at a high temperature required at higher steam states.

An austenitic stainless steel has the properties that can satisfy the extreme conditions mentioned above, but it is expensive. Therefore, many studies have been conducted on improving the properties of a high Cr ferritic steel in order to utilize a high Cr ferritic steel that is less expensive compared with an austenitic stainless steel.

The inventors have already developed a high-Cr heat-resistant ferritic steel which is less expensive and has excellent creep resistance and toughness when used for a long period of time under the ultra-supercritical conditions of high temperature and high pressure (Published Japanese Patent Applications 8-85850, 9-71845, 9-71846).

However, further improvement in creep strength is needed to use a high Cr ferritic steel in a thermal steam boiler operating under more extreme conditions than the ultra-supercritical conditions mentioned above.

Disclosure of the invention

The object of this invention is to provide a high Cr heat-resistant ferritic steel which has excellent long-term high-temperature creep strength and toughness and can be used under high-temperature and high-pressure conditions such as 625ºC or higher and 300 atm or more.

The gist of the present invention relates to a high Cr heat-resistant ferritic steel described below.

A high Cr heat resistant ferritic steel having excellent long term creep strength at high temperatures and toughness comprising, in wt%,

C: 0.02 to 0.15% Mn: 0.05 to 1.5%

P: 0.03% or less S: 0.015% or less

Cr: 8 to 13% W: 1.5 to 4%

Co: 2 to 6% V: 0.1 to 0.5%

Ta: 0.01 to 0.15% Nb: 0.01 to 0.15%

Nd: 0.001 to 0.2% N: less than 0.02%

B: 0.0005 to 0.02% Al: 0.001 to 0.05%

Mo: 0 to 1% Si: 0 to 1%

Ca: 0 to 0.02% La: 0 to 0.2%

Ce: 0 to 0.2% Y: 0 to 0.2%

Hf: 0 to 0.2%,

where the proportions of Nd and N satisfy the following condition and the remainder is Fe and impurities

Nd(%) ≤ 5 · N(%) + 0.1(%)

The inventors have extensively studied the effect of various alloying elements on the long-term creep strength and toughness at high temperatures of a heat-resistant Nd-containing high Cr ferritic steel previously developed by the inventors in order to further improve this steel. As a result, the inventors made the following discoveries, which led to the present invention.

(a) Nd is effective in oxygen fixation of a steel by forming Nd oxides, thereby preventing the formation of oxides containing small amounts of Nd and V, which are precipitation strengthening elements, and which contribute to increasing creep strength by precipitating fine carbides. Furthermore, Nd forms carbides such as NdC2. These carbides precipitate finely and stably even when used for a long period of time at a high temperature, and therefore contribute greatly to increasing long-term creep strength at high temperatures. However, Nd has a strong affinity for N (nitrogen) and produces inclusions of coarse NdN in a steel containing large amounts of nitrogen. As a result, in such a steel, the effects of Nd in preventing the formation of oxides of Nd and V and in precipitating fine carbides such as NdC2 to enhance precipitation become insufficient and the effect of Nd in improving creep strength is not fully exerted.

(b) The formation of coarse NdN can be prevented by controlling the amount of N to be less than 0.02% in a high Cr heat-resistant Nd-containing ferritic steel. As a result, fine carbides of Nd and V and fine carbides such as NdC2 precipitate stably even when exposed to high temperatures for a long period of time, thereby the recovery and softening phenomenon of a martensite structure is suppressed even when used for a long period of time under high temperatures, thereby greatly improving the creep strength.

(c) To ensure the toughness of a high Cr ferritic steel containing Nd, the balance of the amounts of Nd and N is important and must satisfy the above-mentioned condition.

Best mode for carrying out the invention

The following explains the reasons why the chemical composition of the heat-resistant steel according to the invention is set (hereinafter, % stands for % by weight).

C: 0.02 to 0.15%

C forms MC-type carbides (M represents an alloying element) such as M₇C₃ and M₂₃C₆-type carbides. These carbides contribute to increasing creep strength and C itself also stabilizes a metallurgical structure by serving as an austenite-stabilizing element. However, if the C content is less than 0.02%, precipitation of carbides is insufficient and the amount of δ-ferrite increases, which cannot achieve satisfactory creep strength and toughness. However, if C is contained in an excess amount of more than 0.15%, carbide coarsening occurs at an early stage of operation, lowering long-term creep strength and deteriorating formability and weldability. Therefore, the upper limit of the C content is 0.15%. Accordingly, the C content is preferably 0.05 to 0.13%.

Mn: 0.05 to 1.5%

Mn is an element that can be used effectively as a deoxidizer and in the fixation of S and also serves as an austenite stabilizing element. To obtain these effects, the Mn content must be 0.05% or more. However, if the Mn content exceeds 1.5%, the toughness of the steel will deteriorate. Therefore, the Mn content is 0.05 to 1.5%, preferably 0.05 to 0.7%.

P: 0.03% or less, S: 0.015% or less

P and S, which are impurities, are detrimental to hot workability, weldability and toughness. Therefore, it is preferable that the P and S content be as low as possible, but a P content of 0.03% or less and an S content of 0.015% or less do not directly affect the properties of the steel of the present invention. Therefore, the upper limits of the P and S content are 0.03% and 0.015%, respectively.

Cr: 8 to 13%

Cr is an essential element for corrosion resistance and oxidation resistance, particularly steam oxidation resistance at high temperatures, in the steel of the present invention. In addition, Cr forms carbides, thereby increasing creep resistance. Cr also forms an impermeable oxide film, thereby improving corrosion resistance and oxidation resistance. To obtain these effects, the Cr content must be 8% or more. However, too high a Cr content promotes the formation of δ-ferrite, which deteriorates the toughness of the steel. Therefore, the upper limit of the Cr content is 13%. The Cr content is preferably 9 to 12%.

W: 1.5 to 4%

W is one of the main elements of the steel according to the invention and serves as a reinforcing element. W precipitates during high temperature operation in a fine and dispersed form in the form of an intermetallic compound such as the Fe₇W₆-like u-phase and the Fe₂W-like Laves phase and thereby contributes to the improvement of the long-term creep strength. Furthermore, W is also partially present in the Cr carbides and prevents the accumulation and grain coarsening of the carbides, whereby W helps to maintain the strength of the steel. However, too much W promotes the formation of δ-ferrite. Therefore, the W content is 1.5 to 4%, preferably 2 to 3.5%.

Co: 2 to 6%

Co is an austenite stabilizing element and is essential to the steel of the present invention, into which W is intentionally added. Co does not contribute to creep strength, and rather increases creep strength. In this respect, Co is different from Ni, which is also an austenite stabilizing element. To achieve these effects, Co must be added in an amount of 2% or more, however, an excessive amount of Co of more than 6% significantly lowers the Ac1 transition temperature of the steel and deteriorates the creep strength of the steel. Therefore, the Co content is preferably 2 to 4%.

V: 0.1 to 0.5%

V is an important element for the steel of the present invention to form fine carbonitrides, whereby V contributes to the improvement of creep strength. To obtain this effect, the V content must be 0.1% or more, but if the V content is more than 0.5%, this effect is saturated. Therefore, the V content is 0.1 to 0.5%, preferably 0.15 to 0.35%.

Ta: 0.01 to 0.15%; Nb: 0.01 to 0.15%

Ta and Nb, as well as V, form fine carbonitrides and contribute to improving creep strength. To obtain this effect, the content of both Ta and Nb must be 0.01% or more, but if each content is more than 0.15%, the effect saturates. Therefore, the content of both Ta and Nb is 0.01 to 0.15%, preferably 0.01 to 0.1%.

Nd: 0.001 to 0.2%

Nd can finely and stably precipitate carbides such as NdC2 even after long-term use under high temperatures, whereby Nd greatly contributes to preventing the recovery and softening of a martensite structure and increasing the creep strength. To obtain these effects, the Nd content must be 0.001% or more, but too high Nd content of more than 0.2% will deteriorate the toughness of the steel. Therefore, the Nd content is 0.001 to 0.2%, preferably 0.005 to 0.15%.

N: less than 0.02%

N, like C, serves as an austenite stabilizing element. However, in the Nd-containing steel, coarse NdN remains in the form of inclusions as the amount of N increases, which affects the improvement of creep strength by Nd and also the toughness of the steel. Therefore, the upper limit of the N content in the steel must be less than 0.02% in order to fully ensure the effect of Nd. When the toughness of the steel is particularly important, the ratio of the amount of Nd and the amount of N is preferably adjusted to satisfy the following condition. The N content is preferably not more than 0.017%.

Nd(%) ≤ 5 · N(%) + 0.1(%)

B: 0.0005 to 0.02%

B can precipitate M₂₃C₆ type carbide in a fine and dispersed form when B is added in a small amount. Consequently, B contributes to improving the long-term creep strength at high temperatures. B is also effective in enhancing the quench hardening properties, especially for a heavy-wall steel product whose cooling rate after heat treatment is low, thereby B plays an important role in ensuring the high temperature resistance. This effect is pronounced when the B content is 0.0005% or more, but a B content of more than 0.02% forms coarse precipitates, thereby reducing the toughness of the steel. Therefore, the B content is 0.0005 to 0.02%, preferably 0.002 to 0.01%.

Al: 0.001 to 0.05%

Al must be contained in an amount of 0.001% or more to serve as a deoxidizer in the molten steel. However, too high Al content of more than 0.05% will reduce the creep strength. Therefore, the Al content is 0.001 to 0.05%, preferably 0.001 to 0.03%.

Si: 0 to 1%

Si is optionally added to the molten steel as a deoxidizer. Si can increase the steam oxidation resistance at high temperatures. However, since too high a Si content of more than 1% deteriorates the toughness of the steel, the Si content is 0 to 1%. When the steam oxidation resistance is particularly important, the Si content is preferably 0.1% or more.

Mon: 0 to 1%

Mo is an optional element that contributes to the improvement of creep strength by acting as a solution strengthening element. However, if the Mo content is more than 1%, it accelerates the precipitation of an intermetallic compound such as the Laves phase. In the steel containing Mo, the intermetallic compound precipitates very coarsely, which therefore does not contribute to the improvement of creep strength and also reduces the toughness of the steel after quenching and tempering. Therefore, the Mo content is 0 to 1%.

Ca: 0 to 0.02% La: 0 to 0.2% Ce: 0 to 0.2%

Y: 0 to 0.2% Hf: 0 to 0.2%

At least one element selected from Ca, La, Ce, Y and Hf is added if necessary. These elements can improve the creep resistance and hot workability of the steel by strengthening the grain boundaries even if the amount added is very small. However, too much of these elements deteriorates the hot workability of the steel. Therefore, the upper limit of the Ca content is 0.02% and the upper limits for La, Ce, Y and Hf are 0.2% respectively.

Examples

The present invention will now be described in detail using various examples, but these examples should not be construed as limiting the invention.

The steels having the chemical compositions shown in Table 1 and Table 2 were melted in a vacuum induction furnace and cast into 50 kg ingots having a diameter of 144 mm. In the tables, steels No. 1 to No. 23 are examples of the steels of the present invention, and steels A to Q are the comparative examples. Table 1

The symbol indicates values that are outside the range defined by the present invention.

Steels Nos. 1 to 12: Examples of the present invention.

Steels No. A to H: Comparison examples Table 2

The symbol indicates values that are outside the range defined by the present invention.

Steels Nos. 13 to 23: Examples of the present invention.

Steels No. A to H: Comparison examples

These steel blocks were hot forged and then hot rolled into steel plates with a thickness of 20 mm. The steel plates were heated at 1050ºC for one hour and then air cooled. After that, the steel plates were heated for a temperature treatment at 780ºC for one hour and air cooled. The specimens for the creep rupture test and the Charpy impact test were prepared from the steel plates thus obtained and the creep rupture tests and the Charpy impact tests were carried out under the following conditions.

(1) Creep rupture test

Sample: diameter 6.0 mm, measuring length 30 mm

Test temperature: 650ºC

Applied pressure: 98 M Pa

(2) Charpy impact test

Sample: 10mm x 10mm x 55mm, 2mm V-notch

Test temperature: 0ºC

The creep rupture time and Charpy impact value (J/cm²) obtained by the above-mentioned tests are shown in Table 3 and Table 4. Table 3 (Examples of the present invention)

: The samples were made from steel in normal and tempered condition. Table 4 (comparative examples)

: The samples were made from steel in normal and tempered condition.

It is apparent that the creep rupture life of steels Nos. 1 to 3 and Nos. 4 to 6, which are the examples of this invention containing less than 0.02% N, is significantly improved in comparison with steels A and B, which are the comparative examples whose N content exceeds the amount defined by this invention. The improvement of the creep rupture life by reducing the N content is obvious in the case of steels Nos. 7 to 17, which are the examples of this invention, when compared with steels C to M, which are the comparative examples corresponding to steels Nos. 7 to 17.

In steels N, O, P and Q, which are comparative examples, the contents of Nd and N do not satisfy the following condition. In this case, in comparison with steels Nos. 17, 19, 21 and 29, which are examples of this invention, which satisfy the following condition, it is seen that there is hardly any difference in creep rupture time, but the impact value is smaller. Therefore, when the toughness of the steel is important, the contents of Nd and N are preferably set so as to satisfy the following condition:

Nd(%) ≤ 5 · N(%) + 0.1(%)

Industrial applicability

The heat-resistant high-Cr ferritic steel of the present invention has excellent properties in terms of long-term creep strength at high temperatures of 625ºC or more and toughness at room temperature, and is therefore excellently suited to be used as a material for a heat-resistant steel pipe and steel piping, a steel plate of a pressure vessel, and a steel member of a turbine used in nuclear power plants and chemical plants. Consequently, this steel has considerable advantages in industrial application.

Claims (3)

1. A high Cr heat-resistant ferritic steel having excellent long-term creep strength and toughness at high temperatures, comprising, in % by weight, C: 0.02 to 0.15% Mn: 0.05 to 1.5% P: 0.03% or less S: 0.015% or less Cr: 8 to 13% W: 1.5 to 4% Co: 2 to 6% V: 0.1 to 0.5% Ta: 0.01 to 0.15% Nb: 0.01 to 0.15% Nd: 0.001 to 0.2% N: less than 0.02% B: 0.0005 to 0.02% Al: 0.001 to 0.05% Mo: 0 to 1% Si: 0 to 1% Ca: 0 to 0.02% La: 0 to 0.2% Ce: 0 to 0.2% Y: 0 to 0.2% Hf: 0 to 0.2%, where the proportions of Nd and N meet the following condition and the remainder is iron and impurities: Nd(%) ? 5 · N(%) + 0.1(%). 2. Ferritic steel according to claim 1, characterized in that the W content is 2 to 3.5%. 3. Ferritic steel according to claim 1 or 2, characterized in that the V content is 0.15 to 0.35% and the Nd content is 0.005 to 0.15%.