JP2002194485A - Low alloy heat resistant steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide heat resistant steel as inexpensive low alloy steel having a Cr content of 1.5 to 3% which has high temperature strength and perticularly excellent long time creep characteristics or/and low temperature toughness. SOLUTION: The low alloy heat resistant steel is provided with one or more among the various characteristics described in the PURPOSE by controlling any of (1) the lath width of bainite and martensite and the average value of the grain size of ferritic subgrains, (2) the content of solid solution [W+Mo] and the average grain size thereof, and (3) the content of solid solution [W+Mo] on the grain boundaries and the average grain size thereof as well as the control of alloy components including 1.5 to 3% Cr, 0.01 to 0.5% Mo, 1 to 3% W, 0.05 to 0.35% V, 0.01 to 0.1% Nb, 0.002 to 0.03% N, 0.0001 to 0.02% B and 1 to 3% W.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a boiler, a nuclear power plant,
The present invention relates to a heat-resistant steel that is particularly suitable for use at a high temperature of 500 ° C. or more in fields such as the chemical industry. The heat-resisting steel of the present invention has all the properties required for the above-mentioned applications, but is characterized in that at least one of high-temperature strength, long-time creep strength and low-temperature toughness is remarkably excellent. The heat-resistant steel of the present invention is particularly suitable for use in, for example, heat exchanger tubes, piping tubes, and the like.

[0002]

2. Description of the Related Art Austenitic stainless steel, Cr
High Cr ferritic steel with 9-12% content, 3.5% Cr content
% Or less of low Cr-Mo ferritic steel and carbon steel. Among them, low-alloy heat-resistant steels represented by 2.25% Cr-1% Mo steels are inexpensive and are used in large amounts according to the use environment (in this specification, the percentage of the component content
Are all% by mass).

A low-alloy heat-resistant steel containing several percent of Cr generally has a so-called ferritic structure such as tempered bainite and tempered martensite. In recent years, a low-alloy heat-resistant steel having improved high-temperature strength and improved toughness by adding Mo, W, V, Nb, etc. to this low-alloy steel as a base has been disclosed in, for example, Japanese Patent Application Laid-Open No. 4-268040 and Japanese Patent Publication No. No. 2926,
Many proposals have been made in JP-A-6-6771, JP-A-8-325669, and JP-A-10-46290.

In boiler design standards, for example, in a temperature range where the creep phenomenon hardly causes a problem even in a high temperature range, tensile strength (tensile strength at use temperature: referred to as high temperature strength) is emphasized, and a temperature at which the creep phenomenon becomes a problem is considered. In a temperature range of, for example, 520 ° C. or higher, long-term creep strength is emphasized.

[0005] The addition of a precipitation strengthening element such as V, Nb, etc., leads to an improvement in high-temperature strength and long-term creep strength.
Conventionally known. However, since the high-temperature strength and the creep strength have different strengthening mechanisms, these two properties are not always improved at the same time. Further, since these precipitation strengthening elements cause lowering of the low-temperature toughness, the use is restricted depending on the shape and thickness of the product.

In a ferritic heat-resistant steel, even if the high-temperature strength and the short-time creep strength are high, the creep strength is greatly reduced when exposed to a high temperature under a low load stress for a long time (for example, a period exceeding 10,000 hours). is there. Such a long-term decrease in creep strength cannot be predicted from the results of a short-term creep test (for example, a test under conditions where the rupture time is about 3,000 hours). That is, even if the material has sufficient strength in an unused state or a state used for a short time, the strength may be reduced in a long-time use. Therefore, unless the long-term creep strength exceeding 10,000 hours is high, the application as a heat-resistant structural material is limited.

The conventional low-alloy heat-resistant steels proposed in the above-mentioned publications are insufficient in the stability of the long-term creep strength, and there is still room for improvement in the high-temperature strength and low-temperature toughness. There is. In particular, a low-alloy heat-resistant steel having all of high-temperature strength, long-time creep strength and low-temperature toughness has not yet been developed.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide an inexpensive heat-resistant steel which can be used in place of austenitic stainless steel and high Cr ferritic steel in the field where the use of low-alloy heat-resistant steel has been restricted. The objective was to take advantage of low-alloy steel with a Cr content of 3% or less and to provide basic characteristics as a heat-resistant material used in the temperature range of 500 ° C to 600 ° C. Another object of the present invention is to provide a heat-resistant steel having one or more of high-temperature strength, long-term creep strength and low-temperature toughness, and a method for producing the same.

[0009]

Means for Solving the Problems The inventors of the present invention have conducted various experiments for the purpose of clarifying factors governing various properties of low alloy steel including high temperature strength, and as a result, have obtained the following findings. Was. (1) High-temperature strength (1) -1. The tensile strength of ferritic heat-resistant steel composed of bainite and martensite decreases significantly at high temperatures. However, the high-temperature strength can be improved by increasing the tempering softening resistance.

[0010] (1) -2. V, Nb,
It is effective to add a precipitation strengthening element such as Mo, W, or B or a solid solution strengthening element.

(1) -3. Of these elements, Mo and W have conventionally been considered to have substantially the same effect. However, the effect of W is greater in improving the temper softening resistance, and the higher the amount of W added, the higher the high-temperature strength.

(1) -4. In the structure of heat-resistant ferritic steel composed of bainite and martensite, small units such as laths or sub-crystal grains are further present in the prior austenite grains. Temper softening resistance is improved by lath or sub-crystal grain refinement. Here, the lath is a small unit constituting martensite or bainite, and the sub-crystal grains are sub-crystal grains of ferrite formed by tempering (see FIG. 10 described later). (2) Long-term creep strength (2) -1. For example, in heat-resistant steel used for a long time at high temperature as a boiler tube, it is necessary to design a material that further enhances the long-term creep strength. However, even if a large amount of a precipitation strengthening element or a solid solution strengthening element is added, the creep strength may decrease during long-term use.

(2) -2. The decrease in the long-term creep strength suppresses the precipitation of W and Mo, and the total amount of solid solution W and solid solution Mo (hereinafter, referred to as solid solution [W + Mo] amount). It can be prevented by holding. However, even if the amount of solid solution [W + Mo] is sufficient, if the crystal grains are fine, the creep strength is reduced. That is, in order to improve the creep strength, it is necessary to design a component that suppresses the precipitation of W and Mo at a high temperature for a long time and makes the crystal grain size fine.

(2) -3. Since W has a lower deposition rate than Mo and can exist in a solid solution state for a longer time, W addition is more effective than Mo.

(2) -4. Precipitation of W and Mo can be suppressed by reducing the Mn content and adding V. (3) Regarding low-temperature toughness (3) -1. For example, in the case of a steel material used for a thick-walled member or a part having a complicated shape, it is necessary to design a material in consideration of low-temperature toughness.

(3) -2. In the material having deteriorated low-temperature toughness, the total concentration of solid solution W and solid solution Mo at the grain boundary is reduced, that is, the total amount of solid solution W and solid solution Mo on the grain boundary ( That is, a deficiency in the amount of solid solution [W + Mo]) is observed. Preventing a decrease in the amount of solid solution [W + Mo] on the grain boundaries can improve low-temperature toughness. The amount of solid solution [W + Mo] on the grain boundaries effective for improving low-temperature toughness is 0.4% or more. In addition, the amount of solid solution [W + Mo] on the grain boundary is a concentration not containing a precipitate, and means the amount of solid solution [W + Mo] within a range of ± 0.5 μm across the grain boundary.

(3) -3. The low-temperature toughness can be further improved by increasing the nitrogen content (higher N content). This is because the precipitation amount of NbN increases and the coarsening of crystal grains is suppressed.

(3) -4. The effective particle size for improving the low-temperature toughness is 150 μm or less in average particle size.

The present invention has been made based on the above findings, and the features of the heat-resistant steel of the present invention are as follows.

(A) Chemical composition (% with respect to the component content is
% By mass. ) C: 0.03-0.15%, Si: 0.7% or less, Mn: 0.7% or less, C
r: 1.5-3%, Mo: 0.01-0.5%, W: 1-3%, V: 0.
05-0.35%, Nb: 0.01-0.1%, N: 0.002-0.03%,
B: 0.0001 to 0.02%, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, Mg: 0.05% or less,
Ca: 0.05% or less, Zr: 0.05% or less, Fe and impurities: balance.

Of these components, Si, Mn, Al, Ti, N
i, Cu, Mg, Ca and Zr do not always need to be positively added. The content when not added is the usual impurity level. However, since each has a specific function and effect, it may be contained in the range of the above upper limit or less depending on the purpose of use of the steel. The effect and desired content in that case will be described later. (B) Metal structure and others (B) -1. In steels characterized by particularly high high-temperature strength, the average lath width of bainite and martensite and the average grain size of subcrystal grains are 3 μm or less.

(B) -2. A steel characterized by particularly high long-term creep strength has a solid solution [W + Mo] of more than 0.9% and an average crystal grain size of 5 μm or more. (B) -3. In a steel characterized by particularly excellent low-temperature toughness, the solid solution [W + Mo] on the grain boundary is 0.4% or more and the average crystal grain size is 150 μm or less.

The steel of the present invention has the above characteristics (B) -1 to (B) -3
May be provided independently, or a plurality of these may be provided in combination.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the chemical composition and metal structure of the steel of the present invention will be described in detail. (A) Chemical composition C: 0.03 to 0.15% C combines with Cr, Fe, W, Mo, V, Nb and Ti added as necessary and contributes to high-temperature strength, and itself stabilizes austenite. Since it is an element, it is important for forming a martensite and / or bainite structure. If the C content is less than 0.03%, these effects cannot be obtained, and the crystal grains are coarsened to deteriorate the toughness. On the other hand, when the C content exceeds 0.15%, carbides are excessively precipitated, and the steel is hardened to impair workability and weldability. Further, precipitation of W and Mo added as a solid solution strengthening element is promoted, and the amount of these solid solutions is reduced. Therefore, the proper content of C is
0.03 to 0.15%. Desirable is 0.04 to 0.12%, and more preferable is 0.05 to 0.08%.

Si: 0.7% or less Si is effective as a deoxidizing agent for steel and has an effect of increasing steam oxidation resistance. However, when the content of Si exceeds 0.7%, toughness and workability are reduced, and strength is reduced. Further, it promotes tempering embrittlement and impairs weldability.

Since Si need not always be added, the lower limit of the content may be the impurity level. However, when the steam oxidation resistance is intended to be improved, the content is preferably 0.2% or more. Even in that case, the upper limit of the content is
0.7%. The desirable content is 0.5% or less, and the most desirable content range is 0.2-0.3%.

Mn: 0.7% or less Mn is an element effective for improving hardenability, and is added as necessary. However, when the Mn content exceeds 0.7%, W and M
Since the precipitation of o is promoted and the amount of these solid solutions is reduced, the tempering softening resistance is reduced and the high-temperature strength is significantly reduced. Further, like Si, it promotes tempering embrittlement and deterioration of weldability. Therefore, the appropriate content of Mn is 0.7% or less.

In order to improve the long-term creep strength, Mn
It is desirable to keep the content below 0.5%. This is because the amount of solid solution of W and Mo increases. In order to make the most of this effect, the Mn content is more desirably less than 0.35%.

FIGS. 3 and 4 show a part of the test data of the examples described later (the steel No. is the steel No. in Table 1 described later). As is clear from FIG. 3, the creep strength of steel with a higher Mn content decreases. on the other hand,
According to FIG. 4, the higher the Mn content of the steel, the more the solid solution [W + Mo]
The amount is small. 3 and 4, it can be seen that the amount of solid solution [W + Mo] can be increased by reducing the Mn content, and thus the long-term creep strength can be increased.

Cr: 1.5-3% Cr is an essential element for improving the oxidation resistance and high temperature corrosion resistance of low alloy steel. To increase the tensile strength or creep strength at a high temperature such as 500 to 600 ° C, a Cr content of 1.5% or more is required. On the other hand, if the Cr content exceeds 3%,
In addition to deteriorating toughness and weldability, the material cost increases, and the characteristics of low alloy steel are impaired. Therefore, the proper content of Cr is 1.5 to 3%. The desirable Cr content is 2-3%,
The most desirable is 2.2-2.7%.

Mo: 0.01 to 0.5% Mo is an element having the functions of solid solution strengthening and precipitation strengthening like W. Although its effect is smaller than that of W, even a small amount prevents grain boundary embrittlement, and does not impair ductility even with a high-strength material. It is also effective in improving low-temperature toughness and weldability.
However, the effect cannot be expected if it is less than 0.01%. If it exceeds 0.5%, toughness and workability are impaired.
Therefore, the appropriate content of Mo is 0.01 to 0.5%. More desirable is 0.03-0.3%, most desirable
It is 0.05 to 0.15%.

W: 1 to 3% W is an element having the functions of solid solution strengthening and precipitation strengthening like Mo. However, since diffusion is slower than that of Mo, the solid solution state lasts longer and the solid solution strengthening action is obtained. Is maintained for a long time. For this reason, it is effective in improving temper softening resistance and improving long-time creep strength. Further, coarsening of carbides containing Cr, Fe, Mo, W and the like is suppressed. This is also because the diffusion of W is slow. However, if the W content is less than 1%, the above effects cannot be obtained. On the other hand, if the W content exceeds 3%, the steel is hardened significantly, and toughness, workability, and weldability are impaired. Therefore, the appropriate content of W is 1-3%, preferably 1.3-2%, and more preferably 1.4-1.8%.

The above contents of Mo and W are the average contents of the steel of the present invention. 0.9% of solid solution [W + Mo]
If it exceeds, the creep strength is greatly improved. The presence of more than 1% solid solution [W + Mo] is even more desirable. This will be described in detail later.

V: 0.05 to 0.35% V is mainly bonded to C and MX (M represents W + Mo + V;
X forms a fine carbide of the type C, that is to say carbon),
Contributes to improvement in creep strength. Further, V bonds with C in preference to W and Mo, so that the formation of W and Mo carbides is suppressed, and the decrease in the amount of solid solution [W + Mo] is suppressed. 0.05%
If it is less than 0.35%, this effect is not sufficient, and if it exceeds 0.35%, high temperature strength and creep strength are impaired, and toughness,
The weldability also decreases. Therefore, the proper content of V is 0.05-0.
35%. Desirable is 0.1-0.28%, and more desirable is 0.18-0.25%.

Nb: 0.01-0.1% Nb binds to N and C to form MX (M is Nb, X is C + N
) Type fine carbonitride is formed, grain growth is suppressed, high temperature strength is improved, and low temperature toughness is improved. If the Nb content is less than 0.01%, this effect cannot be expected. On the other hand, if the Nb content exceeds 0.1%, the strength is reduced due to abnormal grain growth, and coarse nitride is formed, and the toughness is rather deteriorated. Therefore, the proper content of Nb is 0.01 to 0.1%. Desirable is 0.02 to 0.07%, and more preferable is 0.04 to 0.06%.

N: 0.002-0.03% N is bonded to Nb and C to form MX (M is Nb, X is C + N
) -Type fine carbonitride is formed, grain growth is suppressed, and together with solid solution strengthening by solid solution N, contributes to improvement of high temperature strength and low temperature toughness. However, when the N content is less than 0.002%, the precipitation amount of MX is small, and the effects of suppressing grain growth, improving toughness, and improving high-temperature strength cannot be sufficiently obtained. On the other hand, 0.03
% Of N forms coarse nitrides and rather deteriorates toughness. Therefore, the proper content of N is 0.002
~ 0.03%.

In order to increase the low temperature toughness, it is necessary to suppress the coarsening of the crystal grains by NbN. In order to effectively precipitate NbN, it is desirable to contain 0.004% or more and up to 0.01% of N. Even more desirable is more than 0.005
The range is up to 0.01%.

FIG. 7 is a plot of the relationship between the N content and the average crystal grain size of the steels of the examples described later (steel No. in Table 1 is shown in the figure). It can be seen that if the N content is 0.004% or more, a desirable average crystal grain size of 150 μm or less can be obtained.

B: 0.0001 to 0.02% B improves the hardenability of steel and improves high-temperature strength and low-temperature toughness. Further, it stabilizes carbides for a long time and contributes to improvement of long-term creep strength. However, if the B content is less than 0.0001%, the above effects cannot be obtained. On the other hand, B
If the content exceeds 0.02%, coarse carbides are formed on the grain boundaries, and the lack of W and Mo at the grain boundaries is promoted, and the toughness and weldability are rather deteriorated. Therefore,
The appropriate content of B is 0.0001 to 0.02%. A more desirable content is 0.001 to 0.01%. 0.0 is more desirable
03-0.004%.

Al: 0.03% or less Al is added as a deoxidizing agent. In addition, since Al combines with N to form a nitride, it is effective in preventing coarsening of steel.
However, when the Al content exceeds 0.03%, creep strength and workability are impaired. Therefore, the appropriate content of Al is 0.
03% or less, desirably 0.02% or less. More desirable is less than 0.01%. Note that the Al content may be at a normal impurity level.

The steel of the present invention can optionally contain the following components in addition to the above-described components, if necessary. When these components are not positively added, the lower limits of the contents are all the usual impurity levels.

Ti: 0.05% or less (a desirable content when added is 0.002 to 0.05%) Ti combines with Nb and N to form MX (M is Ti + Nb, X is N, ie, nitrogen) type To increase the high-temperature strength of the steel. 0.002 to ensure this effect
% Is desirable. However, when the Ti content exceeds 0.05%, the nitride coarsens and the segregation of TiC induces abnormal grain growth and formation of a mixed grain structure. Therefore,
When adding Ti, the appropriate range of the content is 0.002-0.0
5%. More preferably, it is 0.003 to 0.02%, and still more preferably 0.005 to 0.01%.

Ni: 0.8% or less (desirable content when added is 0.1 to 0.5%) Ni is an austenite stabilizing element and contributes to improvement in toughness, and may be added as necessary. However, if the Ni content exceeds 0.8%, the long-term creep strength decreases. Therefore, even when Ni is added, the content should be kept at 0.8% or less. Desirable is 0.5% or less, and more desirable is 0.3% or less. In order to obtain the above effects, Ni
Is added, the lower limit of the content is preferably 0.1%.

Cu: 0.5% or less (a desirable content when added is 0.1 to 0.3%) Cu is also an austenite stabilizing element like Ni and contributes to improvement in toughness. Each can be added alone as needed. However, when the Cu content exceeds 0.5%, tempering embrittlement is likely to occur. Therefore, even if it is added positively, its content should be 0.5% or less. Desirable is 0.3% or less. When Cu is added to obtain the above effects, the lower limit of the content is preferably 0.1%. It is desirable that Cu be used in combination with Ni.

Mg, Ca, Zr: 0.05% or less, respectively (desirable contents when added are 0.0005-0.05%, respectively) These elements combine with impurities P, S, and O (oxygen), and are not preferable. Reduces the effect and improves the toughness of the steel. In any case, the effect is not remarkable when the content is less than 0.0005%. Therefore, when each of them is added, the content is desirably 0.0005% or more. However, if the content of each exceeds 0.05%, the number of inclusions increases, and on the contrary, the toughness is impaired. Therefore, even when added, each content is 0.05%
When two or more of these elements are used, the total content is preferably 0.05% or less.
Desirable is 0.01% or less. If no addition is made, the lower limit is the impurity level.

The essential components and optional components of the steel of the present invention are as described above. The balance is Fe and impurities, and among the impurities, P and S are particularly desirable to be as low as possible because they impair the toughness and creep ductility of steel. The allowable upper limit of P is 0.03%, and that of S is 0.015%.

(B) Metal structure and others (B) -1. (Lath width and / or particle size of sub-crystal grains: 3 μm
Hereinafter, the structure of the low-alloy heat-resistant steel of the present invention basically consists of bainite or martensite or a mixed structure thereof. It may also contain a small amount of ferrite generated during tempering.

FIG. 10 shows the steel of the present invention (N shown in Table 1 below).
FIG. 3 is a diagram showing an example of the metallographic structure of o.1 steel). As shown in the figure, lath or ferrite subcrystal is the minimum unit constituting bainite, martensite and their tempered structures. If the average of these is greater than 3 μm, the tempering softening resistance is reduced and the high temperature strength is significantly reduced. Therefore, when high-temperature strength is particularly important, the average lath width in the tempered structure of bainite and / or martensite is 3 μm or less, and when ferrite is included, the average of the subcrystal grain size is 3 μm.
Below, it is necessary to make it as small as possible. Desirably, each is 2 μm or less, and more preferably, 1 μm or less.

FIG. 1 is a diagram showing the relationship between the average value of the lath width (and subcrystal grain size) and the high-temperature strength of the steels of the examples described below (the steels shown in Table 1). As is clear from this figure,
The smaller the lath width (and subcrystal grain size), the higher the high-temperature strength.

The following methods can be used to reduce the lath width of martensite and bainite and the grain size of sub-crystal grains of ferrite.

A method in which the content of W and Mo is increased to make dislocations less likely to move, thereby suppressing the movement of lath interfaces and grain boundaries.

MX carbonitride (M is Ti, Nb, X is C,
A method of suppressing grain growth using the pinning effect of N).

Specifically, the steel of the present invention can be manufactured by the following manufacturing method. That is, materials (billets, slabs, etc.)
Is heated to, for example, 1,100 ° C. or more, then processed at an austenite temperature of 20% or more, cooled to 300 ° C. or less, and then heated at a rate of 1,000 ° C./h or less to a predetermined austenitizing temperature (Ac ₁ point). Above, 1,150 ° C or less is desirable)
Heat until normalizing. Next, the steel sheet is heated to a tempering temperature from “Ac ₁ point-50 ° C.” to Ac ₁ point at a heating rate of 1,000 ° C./h or less to perform a predetermined tempering.

In order to obtain the structure specified in the present invention, it is particularly important to adjust the temperature raising rate during normalizing and tempering. This is due in part to the temperature rise during normalizing.
Fine MX carbonitride mainly composed of i and Nb (M is Ti, N
b, X precipitates C, N) and suppresses austenite coarsening. Two of them are carbonitrides MX mainly composed of V (M is V, X is C, N) during tempering during tempering. ) To prevent coarsening of martensite and / or bainite lath and ferrite sub-crystal grains. The above effects cannot be obtained at a high temperature rising rate exceeding 1,000 ° C./h in both normalizing and tempering. A desirable heating rate is 500 to 700 ° C / h.

The coarsening of laths and sub-crystal grains occurs particularly during tempering. Therefore, in order to obtain the above effects, it is important to minimize the precipitation of W and Mo during tempering, increase the amount of solid solution [W + Mo], and reduce the size of MX carbonitride. . For this purpose, it is desirable to adjust the temperature increase rate during the heat treatment as described above together with the adjustment of the components.

(3) Amount of solid solution [W + Mo]: an amount exceeding 0.9% This is a condition to be provided especially when a long-term creep strength is required. In low alloy steels with a Cr content of 3% or less, W and Mo precipitate as carbides during long-term use. However, long-term creep strength is maintained by keeping as much W and Mo in a solid solution state as possible for as long as possible. And the creep ductility is improved. Specifically, within the normal life of a boiler (for example, 20 years), the amount of solid solution [W + Mo] can be reduced during use.
It is necessary to secure about 0.5% or more. In order to secure the amount of solid solution [W + Mo] of steel in use to 0.5% or more, the solid solution [W + Mo] must be obtained in the state of the product (for example, a boiler tube) before use.
The amount must exceed 0.9% (preferably 1%). At 0.9% or less, the amount of solid solution [W + Mo] during use is 0.5%
And the creep strength is greatly reduced.

FIG. 2 shows the relationship between the solid solution [W + Mo] before use and the creep strength at 575 ° C. × 100,000 hours before use of the steels of the examples described later (the steels shown in Table 1).

The amount of solid solution [W + Mo] can be determined, for example, by extracting only precipitates by the extraction residue method, performing quantitative chemical analysis, and subtracting the extraction residue amount from the total amount of W and Mo contained in the steel. .

An amount of solid solution [W + Mo] of more than 0.9% can be ensured by the above manufacturing conditions. It is particularly important that the tempering temperature be in the range from “Ac ₁ point-50 ° C.” to Ac ₁ point. The solid solution [W
+ Mo "amount decreases.

(4) Average crystal grain size: 5 μm or more, or 150
μm or less Particularly when long-term creep strength is required, it is necessary to secure the above-mentioned solid solution [W + Mo] amount and to make the average crystal grain size 5 μm or more. In the low alloy heat resistant steel, when the average crystal grain size is less than 5 μm, the creep strength decreases. Therefore, the average crystal grain size needs to be 5 μm or more. It is preferably at least 20 μm, more preferably at least 40 μm.

FIG. 5 is a diagram in which test data of Examples described later are arranged in relation to the average particle size and the creep strength. As shown in the drawing, steel having a crystal grain size of 5 μm or more, particularly 40 μm or more shows high creep strength (for details, see Table 2 in Examples).
reference).

On the other hand, to improve the low temperature toughness, a fine grain structure is better. If the average crystal grain size exceeds 150 μm, the toughness is remarkably deteriorated.

FIG. 6 is a diagram in which the data of the examples described later are arranged in relation to the average crystal grain size and the impact value. Particle size 150μ
It is clear that the impact value is significantly increased below m, especially below 80 μm. Therefore, especially when low temperature toughness is emphasized, the average crystal grain size should be 150 μm or less, preferably 100 μm or less. It is more preferable that the thickness be 80 μm or less.

As described above, the average crystal grain size is desirably 5 μm or more in order to increase the long-term creep strength. Therefore, in order to combine long-time creep strength and low-temperature toughness, the average crystal grain size is 5 to 150 μm, preferably 20 to 100 μm.
μm, and more preferably 40-80 μm.

(5) The amount of solid solution [W + Mo] on the grain boundary: 0.4% or more This is a condition to be prepared especially when low-temperature toughness is emphasized.

As schematically shown in FIG. 9, in the case of a Cr-containing low alloy, a deficient layer in which the concentration of Cr, W, Mo, V, etc. is locally reduced may be formed at the grain boundary. Such a depletion layer causes deterioration of low-temperature toughness.

FIG. 8 shows the data of the examples described below arranged according to the relationship between the amount of solid solution [W + Mo] on the grain boundaries and the impact value.
When the solid solution [W + Mo] on the grain boundaries is less than 0.4%, the impact value is remarkably low. Therefore, when low-temperature toughness is emphasized, the amount of solid solution [W + Mo] on the grain boundary is preferably set to 0.4% or more.

The amount of solid solution [W + Mo] on the grain boundary means an average value of the amount of solid solution [W + Mo] within a range of ± 0.5 μm across the grain boundary. It can be measured using EDX.

The control of the amount of solid solution [W + Mo] on the grain boundaries as described above can be realized by adjusting the tempering time and temperature in accordance with the steel composition. Desirable tempering conditions are
In the range from `` Ac ₁ point -50 ° C '' mentioned above to Ac ₁ point, 0.
Hold for 5 to 1 hour.

[0070]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Test method Ingots obtained by melting and casting each steel having the chemical composition shown in Table 1 in a 50 kg vacuum melting furnace were forged at 1150 to 950 ° C, and processed by 75 to 85% to form a 20 mm thick plate. did.

The heat treatment conditions for each steel are as follows.

Steel Nos. 1 to 18: Normalized: heated to 1,000 to 1,100 ° C. at a heating rate of 500 ° C./h, held for 30 minutes, and air-cooled to room temperature.

Tempering: 750 to 790 ° C. at a heating rate of 500 ° C./h
After heating for a predetermined time determined by the following tempering parameters, air-cool to room temperature.

Steel 7-1: Normalizing: heating to 1,050 ° C at a heating rate of 1,000 ° C / h
After holding for 30 minutes, air-cool to room temperature. Tempering: heating to 800 ° C. ( ₁ point of Ac + 5 ° C.) at a heating rate of 500 ° C./h, holding for a predetermined time determined by the following tempering parameters, and then air cooling to room temperature.

Steels 2-2 and 16-1: Normalization: Heated at a heating rate of 10,000 ° C./h to 1,050 ° C., held for 30 minutes, and air-cooled to room temperature.

Tempering: 650 ° C. (Ac ₁
(−145 ° C.) and maintained for a predetermined time determined by the following tempering parameters, and then air-cooled to room temperature.

The holding time of the tempering process is a tempering parameter, TP [TP = (T + 273) logt + 20). Where T
Is temperature (° C.) and t is time (h)] is 21,000 to 21,3
It was adjusted to be in the range of 00. From each of the steel sheets thus obtained, a test piece for extracting extraction residue, a creep rupture test piece, a Charpy impact test piece, and a constant load tensile test piece were sampled and subjected to the following tests.

(1) Measurement of solid solution [W + Mo] amount: Electrolyte solution: 10% acetylacetone-methanol solution Current density: 20 mA / cm ² Dissolution amount: 0.4 g W content in extraction residue collected under the above conditions And the Mo content as the amount of precipitation, and the difference from the total amount of W and Mo in the steel was defined as the amount of solid solution W and the amount of solid solution Mo.

(2) High temperature tensile test: JIS G0567 test piece: φ6 mm × GL 30 mm Test temperature: 575 ° C. (3) Creep rupture test: JIS Z2272 test piece: φ6 mm × GL 30 mm Test temperature: 500 to 650 ° C. Load stress : Under the condition of 50-350 MPa or more, data is collected up to 15,000 hours,
The average value of the creep rupture strength at 575 ° C. × 100,000 hours was extrapolated. In addition, the average strength of 10,000 hours at 575 ° C and 100
The difference in average strength over time was determined, and the stability of long-term creep strength was evaluated.

(4) Charpy impact test: Specimen: 10 mm × 10 mm × 55 mm, 2 mm V notch Test temperature: 0 ° C. (5) Measurement of solid solution [W + Mo] concentration at grain boundaries: EDX analysis by transmission electron microscope (energy dispersion) X-ray analysis). Ten points were analyzed at a pitch of 0.1 μm in a range of ± 0.5 μm across the grain boundary, and the minimum value of the amount of [W + Mo] therein was defined as the solid solution [W + Mo] concentration.

(6) Measurement of grain size: The grain size of 100 prior austenite grains was measured for each steel and evaluated by the average value. 2. Test results Table 2 shows the test results. The test results are summarized below.

(1) The chemical compositions of all the steels shown in Table 1 are within the range defined by the present invention. Some of them satisfy one or more of the following conditions.

Conditions: Average of lath width of bainite, lath width of martensite, and grain size of ferrite subcrystal are 3 μm or less.

Conditions: Solid solution [W + Mo] exceeds 0.9%,
And the average particle size is 5 μm or more.

Conditions: 0.4% of solid solution [W + Mo] on grain boundaries
And the average particle size is 150 μm or less.

(2) Since steels other than steel Nos. 2-2 and 16-1 all satisfy the conditions, they have particularly high high-temperature strength exceeding 400 MPa at 575 ° C.

(3) Steel satisfying the conditions (Steel Nos. 1, 2, 3 to
7,8-11,14,16 and 17) have an average strength at 575 ° C x 100,000 hours exceeding 100 MPa. That is, it has a particularly high creep strength. Further, the difference between the creep rupture strength at 575 ° C. × 100 hours and the creep rupture strength at 575 ° C. × 10,000 hours is 90 MPa or less, and the long-term creep strength is extremely stable.

(4) Steel satisfying the conditions (Nos. 14, 16, and 1)
Steels other than 7) are No. 14, 16 and 1
Impact properties superior to steel No. 7 (impact value of 250 J / cm ² or more)
Having.

(5) Steel No. 1, which satisfies all of conditions 1 to
2,3-7,8-12 and 15 are remarkably excellent in all of high-temperature strength, long-term creep strength and low-temperature toughness.

[0090]

[Table 1]

[Table 2]

As is clear from the test results of the examples,
The steel of the present invention has excellent overall properties as a heat-resistant material due to the proper content of alloy components. In addition, high-temperature strength, long-term creep strength and / or low-temperature toughness are particularly greatly improved.

Since the steel of the present invention is an inexpensive low alloy steel having a Cr content of 1.5 to 3%, it has the above-mentioned excellent properties.
It can be used in place of austenitic heat resistant steel or high Cr heat resistant steel.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the average value of lath width (and subcrystal grain size) and high-temperature strength.

FIG. 2 is a diagram showing the relationship between the amount of solid solution [W + Mo] and creep strength.

FIG. 3 is a diagram showing the relationship between Mn content and creep strength.

FIG. 4 is a diagram showing the relationship between the Mn content and the amount of solid solution [W + Mo].

FIG. 5 is a diagram showing a relationship between an average crystal grain size and creep strength.

FIG. 6 is a diagram showing the relationship between the average crystal grain size and the Charpy impact characteristics.

FIG. 7 is a diagram showing a relationship between N content and average crystal grain size.

FIG. 8 is a graph showing a relationship between a solid solution [W + Mo] concentration at a grain boundary and an impact value.

FIG. 9 is a diagram schematically showing the distribution of W and Mo concentrations at the crystal grain boundaries.

FIG. 10 is a microstructure diagram illustrating an example of a metal structure of the steel of the present invention.

Claims (8)

[Claims] C .: 0.03 to 0.15%, Si: 0.7% by mass%
Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5
%, W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1
%, N: 0.002 to 0.03%, B: 0.0001 to 0.02%, Al: 0.0
3% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5%
Hereinafter, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the balance being Fe and impurities, the average of lath width of bainite, lath width of martensite, and grain size of ferrite subcrystal grains. Is a low alloy heat-resistant steel excellent in high-temperature strength, having a diameter of 3 μm or less. 2. C: 0.03 to 0.15%, Si: 0.7% by mass%
Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5
%, W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1
%, N: 0.002 to 0.03%, B: 0.0001 to 0.02%, Al: 0.0
3% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5%
Hereinafter, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the balance is composed of Fe and impurities, and solid solution [W + M
o] is more than 0.9% and the average crystal grain size is 5 μm or more. 3. C .: 0.03 to 0.15%, Si: 0.7% by mass%
Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5
%, W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1
%, N: 0.002 to 0.03%, B: 0.0001 to 0.02%, Al: 0.0
3% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5%
Hereinafter, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the balance is composed of Fe and impurities, the solid solution [W + Mo] on the grain boundary is 0.4% by mass or more, and the average crystal grain Diameter 150
Low alloy heat resistant steel with excellent low temperature toughness of less than μm. 4. In mass%, C: 0.03 to 0.15%, Si: 0.7%
Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5
%, W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1
%, N: 0.002 to 0.03%, B: 0.0001 to 0.02%, Al: 0.0
3% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5%
Hereinafter, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the balance is composed of Fe and impurities, and the average of the lath width of bainite, the lath width of martensite, and the grain size of ferrite subcrystal grains. Is not more than 3 μm, the solid solution [W + Mo] exceeds 0.9%, and the average crystal grain size is 5 μm.
m, which is excellent in high-temperature strength and long-term creep strength. 5. C: 0.03 to 0.15%, Si: 0.7% by mass%
Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5
%, W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1
%, N: 0.002 to 0.03%, B: 0.0001 to 0.02%, Al: 0.0
3% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5%
Hereinafter, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the balance is composed of Fe and impurities, and the average of the lath width of bainite, the lath width of martensite, and the grain size of ferrite subcrystal grains. Low-alloy heat-resistant steel excellent in high-temperature strength and low-temperature toughness, having a solid solution [W + Mo] of 0.4% by mass or more and an average crystal grain size of 150 μm or less. 6. In mass%, C: 0.03 to 0.15%, Si: 0.7%
Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5
%, W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1
%, N: 0.002 to 0.03%, B: 0.0001 to 0.02%, Al: 0.0
3% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5%
Hereinafter, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the balance is composed of Fe and impurities, and solid solution [W + M
o] is more than 0.9% and the average crystal grain size is
0 μm or less, and solid solution on grain boundaries [W + Mo]
Is a low alloy heat-resistant steel excellent in long-term creep strength and low-temperature toughness characterized by having a content of 0.4 mass% or more. 7. In mass%, C: 0.03 to 0.15%, Si: 0.7%
Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5
%, W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1
%, N: 0.002 to 0.03%, B: 0.0001 to 0.02%, Al: 0.0
3% or less, Ti: 0.05% or less, Ni: 0.8% or less, and Cu: 0.
One or two kinds of 5% or less, Mg: 0.05% or less, Ca: 0.05
%, Zr: 0.05% or less, the balance being Fe and impurities, the average of the lath width of bainite, the lath width of martensite, and the grain size of ferrite sub-crystal grains being 3 μm or less, solid solution [W + Mo ] Is more than 0.9%, the average crystal grain size is 5 μm or more and 150 μm or less, the solid solution [W + Mo] on the grain boundary is 0.4 mass% or more, and the average crystal grain size is Low-alloy heat-resistant steel with excellent high-temperature strength, long-term creep strength and low-temperature toughness. 8. In mass%, C: 0.03 to 0.15%, Si: 0.7%
Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5
%, W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1
%, N: 0.002 to 0.03%, B: 0.0001 to 0.02%, Al: 0.0
3% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5%
Hereinafter, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the remainder is made of steel composed of Fe and impurities in an austenite temperature range, and then cooled to 300 ° C or less, and then
Heating is performed at a heating rate of 1,000 ° C / h or less to the austenite temperature range, and then, at a heating rate of 1,000 ° C / h or less, a temperature range from “Ac ₁ point-50 ° C” to Ac ₁ point. The method for producing a heat-resistant steel according to any one of claims 1 to 7, wherein the tempering is performed by heating the steel.