JP2021195602A - Low-alloy heat-resistant steel - Google Patents

Abstract

To provide a low-alloy heat-resistant steel that is excellent in toughness at low temperature and creep strength at high temperature.SOLUTION: Provided is a low-alloy heat-resistant steel that contains, in mass%, C: 0.04 to 0.10%, Si: 0.01 to 0.50%, Mn: 0.10 to 0.60%, Cr: 1.75 to 3.00%, Mo: 0.01 to 0.30%, W: 1.20 to 2.00%, V: 0.10 to 0.50%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.020%, Al: 0.005 to 0.030%, B: 0.0005 to 0.0060%, N: 0.0010 to 0.0050%, P, S, selectively contains Ni, Cu, Ta, Co, La, Ce, Y, Ca, Zr, and Mg, the balance consists of Fe and impurities, and that satisfies the following equations (1) and (2), contains TiN, and the average circle-equivalent diameter of TiN is 3.0 μm or less. Ti(%)/48-N(%)/14≥0 -- equation (1) Ti(%)×N(%)≤9.0×10-5 -- equation (2).SELECTED DRAWING: None

Description

The present disclosure relates to low alloy heat resistant steels.

As high temperature heat resistant pressure resistant members for boilers, chemical industry, nuclear power, etc., austenite stainless steel, high Cr ferritic steel with Cr content of 9 to 12%, low alloy steel with Cr content of 3% or less, and carbon steel are used. It is used. These are appropriately selected in consideration of the usage environment and economy such as the operating temperature and pressure of the target component.

Among the above materials, low alloy steels with a Cr content of 3% or less are characterized by being superior in oxidation resistance, high temperature corrosion resistance and high temperature strength compared to carbon steel because they contain Cr, and ferritic stainless steel. It is much cheaper than the above, has a small thermal expansion coefficient, does not cause stress corrosion cracking, is cheaper than high Cr ferritic steel, and is excellent in toughness, thermal conductivity and weldability.

As representatives of low alloy steels, SCMV4 or STBA24 (2.25Cr-1Mo steel), SCMV3 or STBA22 (1.25Cr-0.5Mo steel) are in the JIS standard and are usually collectively referred to as Cr-Mo steel.
Patent Document 1 proposes steel to which a precipitation strengthening element or the like is added for the purpose of improving high temperature strength.

Further, in Patent Document 2, by adding W instead of Mo, high temperature oxidation resistance, corrosion resistance and high temperature strength are improved as compared with Cr-Mo steel, and the high Cr ferrite steel and austenite stainless steel are replaced. Techniques for low Cr heat resistant steels that can be used have been proposed.

Patent Document 3 and Patent Document 4 disclose low alloy heat-resistant steels having improved creep strength and toughness by controlling the amounts of Ti and N.

Patent Document 5 proposes a low alloy heat-resistant steel having high creep strength, tempering embrittlement resistance, and SR crack resistance.

Japanese Unexamined Patent Publication No. 57-131349 Japanese Unexamined Patent Publication No. 2-217438 Japanese Unexamined Patent Publication No. 4-268040 Japanese Unexamined Patent Publication No. 8-225884 Japanese Unexamined Patent Publication No. 2001-262268

Low alloy heat resistant steels used as high temperature heat resistant pressure resistant members for boilers, chemical industry, nuclear power, etc. are required to have more excellent creep strength and toughness. However, as a result of verification by the present inventors, it became clear that there is room for improvement in any of Patent Documents 1 to 5.

Specifically, low alloy heat-resistant steel used as a high-temperature heat-resistant pressure-resistant member is required to have improved creep strength at high temperatures in order to withstand high-temperature conditions during the reaction process. Further, considering the case where the above-mentioned low alloy heat-resistant steel is used in a cold region, it is required to exhibit further excellent toughness in a low temperature environment, that is, to improve the toughness at a low temperature.

The object of the present disclosure is to solve the above problems and to take advantage of the low alloy steel having a Cr content of 3% or less, and to take advantage of the creep strength at a high temperature of 500 ° C. or higher (hereinafter, also simply referred to as "creep strength"). ) And low alloy heat resistant steel having excellent toughness at low temperature (hereinafter, also simply referred to as "toughness").

Means for solving the problem include the following aspects.

<1>
By mass%,
C: 0.04 to 0.10%,
Si: 0.01-0.50%,
Mn: 0.10 to 0.60%,
Cr: 1.75 to 3.00%,
Mo: 0.01-0.30%,
W: 1.20 to 2.00%,
V: 0.10 to 0.50%,
Nb: 0.01 to 0.10%,
Ti: 0.005-0.020%,
Al: 0.005-0.030%,
B: 0.0005 to 0.0060%,
N: 0.0010 to 0.0050%,
P: 0.030% or less,
S: 0.010% or less,
Ni: 0-0.40%,
Cu: 0-0.40%,
Ta: 0-0.10%,
Co: 0-1.0%,
La: 0-0.20%,
Ce: 0 to 0.20%,
Y: 0-0.20%,
Ca: 0-0.20%,
Zr: 0 to 0.20%, and Mg: 0 to 0.050%
The balance is composed of Fe and impurities, and has a chemical composition satisfying the following formulas (1) and (2).
A low alloy heat-resistant steel containing TiN and having an average circle-equivalent diameter of the TiN of 3.0 μm or less.
Ti (%) / 48-N (%) / 14 ≧ 0 ... Equation (1)
Ti (%) x N (%) ≤ 9.0 x ^10-5 ... Equation (2)
<2>
The chemical composition satisfies the following formula (3).
The low alloy heat-resistant steel according to <1>, which contains an Al—Ti composite oxide and has an average circular equivalent diameter of the Al—Ti composite oxide of 3.0 μm or less.
Al (%) / Ti (%) ≧ 0.50 ・ ・ ・ Equation (3)
<3>
Ni: 0.01-0.40%,
Cu: 0.01-0.40%,
Ta: 0.001 to 0.10%, and Co: 0.01 to 1.0%,
The low alloy heat resistant steel according to <1> or <2>, which contains at least one element selected from.
<4>
La: 0.01 to 0.20%,
Ce: 0.01 to 0.20%,
Y: 0.01 to 0.20%,
Ca: 0.01 to 0.20%,
Zr: 0.01 to 0.20%, and Mg: 0.0005 to 0.050%,
The low alloy heat resistant steel according to any one of <1> to <3>, which contains at least one element selected from.

According to one aspect of the present disclosure, there is provided a low alloy heat resistant steel having excellent creep strength at high temperature and toughness at low temperature.

Hereinafter, the low alloy heat resistant steel of the present disclosure will be described.
In the present disclosure, the "%" indication of the content of each element means "mass%". Further, in the present disclosure, the numerical range represented by using "~" means a range including the numerical values before and after "~" as the lower limit value and the upper limit value unless otherwise specified. In addition, the numerical range when "greater than" or "less than" is added to the numerical values before and after "~" means a range in which these numerical values are not included as the lower limit value or the upper limit value.
In the numerical range described in the present disclosure in stages, the upper limit of the numerical range in one stage may be replaced with the upper limit of the numerical range described in another stage, and is also shown in Examples. It may be replaced with the value of. In addition, the lower limit of the numerical range in one step may be replaced with the lower limit of the numerical range described in another step, or may be replaced with the value shown in the embodiment.
Further, the term "process" is included in this term not only as an independent process but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes.
Further, the "low alloy" in the present disclosure does not limit the total content of alloying elements, and is included in the low alloy heat-resistant steel according to the present disclosure as long as the content of each element is within the above range. ..

<Low alloy heat resistant steel>
The low alloy heat resistant steel according to the present disclosure is based on mass%.
C: 0.04 to 0.10%,
Si: 0.01-0.50%,
Mn: 0.10 to 0.60%,
Cr: 1.75 to 3.00%,
Mo: 0.01-0.30%,
W: 1.20 to 2.00%,
V: 0.10 to 0.50%,
Nb: 0.01 to 0.10%,
Ti: 0.005-0.020%,
Al: 0.005-0.030%,
B: 0.0005 to 0.0060%,
N: 0.0010 to 0.0050%,
P: 0.030% or less,
S: 0.010% or less,
Ni: 0-0.40%,
Cu: 0-0.40%,
Ta: 0-0.10%,
Co: 0-1.0%,
La: 0-0.20%,
Ce: 0 to 0.20%,
Y: 0-0.20%,
Ca: 0-0.20%,
Zr: 0 to 0.20%, and Mg: 0 to 0.050%
The balance is composed of Fe and impurities, has a chemical composition satisfying the following formulas (1) and (2), contains TiN, and has an average circle equivalent diameter of TiN of 3.0 μm or less.
Ti (%) / 48-N (%) / 14 ≧ 0 ... Equation (1)
Ti (%) x N (%) ≤ 9.0 x ^10-5 ... Equation (2)

By using the low-alloy heat-resistant steel of the present disclosure as described above, a low-alloy heat-resistant steel having excellent creep strength at high temperature and toughness at low temperature can be obtained. The reason is considered to be based on the following findings.

Since the low alloy heat-resistant steel of the present disclosure contains 1.20 to 2.00% of W, it is considered that the creep strength is high. Further, the low alloy heat resistant steel of the present disclosure contains 0.0010 to 0.0050% of N (nitrogen). When N (nitrogen) is contained in an amount of 0.0010% or more, TiN is produced preferentially over the production of BN, so that B (boron) tends to remain alone. In the presence of a single B (boron), the tissue tends to form bainite, which tends to increase creep strength. In particular, the low alloy heat-resistant steel of the present disclosure has a chemical composition satisfying the above formula (1), and by satisfying the formula (1), the creep strength is independent of the cooling rate at the time of casting. We have found that it is maintained.
Further, in the low alloy heat resistant steel of the present disclosure, by setting the N (nitrogen) content to 0.0050% or less, the crystallization temperature of TiN can be controlled to the low temperature side, which causes a decrease in toughness. It is considered that the production of TiN is suppressed. In particular, the present inventors have found that the chemical composition of the low alloy heat-resistant steel of the present disclosure easily improves toughness by satisfying the above formula (2).
Further, the low alloy heat resistant steel of the present disclosure is a steel having performance equal to or higher than that of the existing low alloy steel in terms of workability and weldability, and austenite is conventionally used in a field where the use of the low alloy steel is restricted. It is a low alloy heat resistant steel that can be used as a low cost high temperature heat resistant pressure resistant member in place of stainless steel or high Cr ferrite steel.

[composition]
The reasons for limiting the chemical composition of the low alloy heat resistant steel according to the present disclosure are as follows.

C: 0.04 to 0.10%
C stabilizes the structure as an austenite stabilizing element. Further, the low alloy heat-resistant steel of the present disclosure is preferably a single-phase structure of martensite or bainite, or a mixed structure of these two phases, and more preferably a single-phase structure of bainite, but the C content is these. It is also important for organizational balance control. Further, when it contains precipitation-enhanced elements such as V, Nb, and Ti, it forms MX-type fine carbides with these elements and contributes to the improvement of high-temperature strength. However, if the C content is less than 0.04%, the above effect cannot be obtained, and the hardenability is lowered, which impairs creep strength and toughness. On the other hand, if it exceeds 0.10%, the precipitated carbide becomes coarse and the creep strength and toughness are impaired. Therefore, the C content is 0.04 to 0.10%. It is preferably 0.05 to 0.08%.

Si: 0.01-0.50%
Si is an element to be contained as necessary, and there is an element effective as a deoxidizing agent, and when it is contained, it is also an element that enhances the water vapor oxidation resistance property of steel. This effect is obtained when the Si content is 0.01% or more. However, if it is contained in an amount of more than 0.50%, the toughness is remarkably lowered and it is also harmful to the creep strength. Therefore, the Si content is set to 0.01 to 0.50%. It is preferably 0.10 to 0.30%.

Mn: 0.10 to 0.60%
Mn is an element that improves hot workability by desulfurization and deoxidizing effects during melting, and also improves hardenability to form a bainite or martensite structure. These effects are obtained at 0.10% or more. However, if it is contained in excess of 0.60%, the stability of fine carbides effective for creep strengthening is impaired, and the creep strength at high temperature for a long time is lowered. Therefore, the Mn content is set to 0.10 to 0.60%. The Mn content is preferably 0.30 to 0.50%.

Cr: 1.75 to 3.00%
Cr is an essential element for improving oxidation resistance and high temperature corrosion resistance. Further, when Cr is contained, the hardenability is improved, so that the structure can be bainite or martensite. If the Cr content is less than 1.75%, these effects cannot be obtained. On the other hand, when the Cr content exceeds 3.00%, coarse carbides are deposited and the creep strength and toughness are lowered. Therefore, the Cr content is 1.75 to 3.00%. The Cr content is preferably 2.00% to 2.75%.

Mo: 0.01-0.30%
Mo has the effect of strengthening solid solution and contributes to the improvement of strength. It also has a precipitation strengthening effect because it forms fine precipitates such as _{Mo 2 C carbide or MX type carbide.} These effects are obtained when the Mo content is 0.01% or more. On the other hand, if the Mo content exceeds 0.30% and is excessively contained _{, the precipitation amount of coarse carbides such as M 23} C ₆ or M ₆ C increases, which adversely affects toughness and creep strength. Therefore, the Mo content is set to 0.01 to 0.30%. The preferred Mo content is 0.05-0.25%.

W: 1.20 to 2.00%
Like Mo, W contributes to solid solution strengthening and is effective in improving creep strength at higher temperatures. Moreover, this effect is larger than that of Mo. In addition, in order to improve hardenability, the structure may be bainite or martensite. This effect is obtained when the W content is 1.20% or more. However, W content exceeds 2.00%, it forms coarse M _{6 C-type} precipitates during long-time use, impairing the creep strength and toughness. Therefore, the W content is 1.20 to 2.00%. The preferred W content is 1.40 to 1.80%.

V: 0.10 to 0.50%
As for V, V forms MX-type fine carbonitride and contributes to high strength. However, if the V content is less than 0.10%, these effects cannot be obtained. On the other hand, when the V content exceeds 0.50%, the MX-type carbonitride becomes coarse, and the creep strength and toughness are rather impaired. Therefore, the V content is set to 0.10 to 0.50%. The preferred V content is 0.15-0.25%.

Nb: 0.01 to 0.10%
Like V, Nb forms MX-type fine carbonitrides and contributes to higher strength. Further, if Nb is contained in combination with Ti, (Nb, Ti) (N, C) is compound-precipitated. The composite precipitates (Nb, Ti) (N, C) are fine and stable over a wide temperature range, and are therefore effective in preventing grain grain coarsening. However, if the Nb content is less than 0.01%, these effects cannot be obtained. On the other hand, when the Nb content exceeds 0.10%, coarse carbonitride is formed and the toughness is deteriorated. Therefore, the Nb content is set to 0.01 to 0.10%. The preferred Nb content is 0.03 to 0.08%.

Ti: 0.005-0.020%
Ti combines with C and N to form fine carbonitrides to prevent coarse graining of crystal grains, and is effective in improving toughness, tempering embrittlement and suppressing SR cracking. This effect is obtained when the Ti content is 0.005% or more. However, when the Ti content exceeds 0.020%, coarse carbonitride is formed and the toughness is deteriorated. Therefore, the Ti content is 0.005 to 0.020%. The preferred Ti content is 0.008 to 0.018%.

Al: 0.005-0.030%
Al is an effective element as a deoxidizing agent. In addition, by adding a small amount, it contributes to strengthening the solid solution and contributes to the improvement of creep strength. This effect is obtained when the Al content is 0.005% or more. However, if the Al content exceeds 0.030%, the creep strength and workability are impaired. Therefore, the Al content is 0.005 to 0.030%. The preferred Al content is 0.010 to 0.025%. The Al referred to in the present disclosure is an acid-soluble Al (sol.Al).

B: 0.0005 to 0.0060%
B is an element effective for ensuring stable strength by improving hardenability. Further, by segregating at the grain boundaries, the grain boundary strength is increased and the creep strength is improved. This effect is obtained when the B content is 0.0005% or more. However, if the B content exceeds 0.0060%, the boron compound is coarsened, which causes a decrease in strength and a decrease in toughness. Therefore, the B content is set to 0.0005 to 0.0060%. The preferred range of B content is 0.0010 to 0.0045%.

N: 0.0010 to 0.0050%
N forms a nitride. N contributes to the improvement of creep strength when fine nitrides are formed and the improvement of toughness by grain refinement. This effect is obtained when the N content is 0.0010% or more. On the other hand, when the N content exceeds 0.0050%, the nitride becomes coarse and the toughness deteriorates. Further, when it cannot be fixed by Ti or Al, BN is formed, hardenability is lowered, and strength and toughness are impaired. Therefore, the content of N is 0.0010 to 0.0050%. The preferred N content is 0.0015 to 0.0045%.

P: 0.030% or less P is an impurity element and segregates at grain boundaries to reduce toughness and creep ductility. Therefore, it is preferable to reduce it as much as possible, and the P content is 0.030% or less.
The smaller the content of P, the better, that is, the closer the content is to 0%, but since the extreme reduction leads to an increase in manufacturing cost, the preferable lower limit is 0.003% or more, and the more preferable lower limit is 0. It is 005% or more.

S: 0.010% or less S is an impurity element like P, and segregates at grain boundaries to reduce toughness and creep ductility. Therefore, it is preferable to reduce it as much as possible, and the amount of S is 0.010% or less.
The smaller the content of S, the better, that is, the closer the content is to 0%, but the extreme reduction significantly increases the manufacturing cost. Therefore, the preferable lower limit of the S content is 0.0001%, and the more preferable lower limit is 0.003%.

The low alloy heat resistant steel of the present disclosure can selectively contain the following alloying elements in addition to the above alloy components.
The low alloy heat resistant steels of the present disclosure may contain at least one element selected from Ni, Cu, Ta, and Co.

Cu: 0-0.40%
Cu is an element to be contained if necessary, and if it is contained, it is an element that contributes to the stabilization of austenite and has a solid solution strengthening action. Therefore, it is effective in improving the creep strength and preventing a decrease in the creep strength after long-term use. It also has the effect of improving toughness. In order to obtain these effects more reliably, the Cu content is preferably 0.01% or more. However, if the Cu content exceeds 0.40%, the creep strength decreases. In addition, excessive content is not preferable from the viewpoint of economy. Therefore, the Cu content when contained is 0.40% or less. The preferable Cu content is 0.01 to 0.20%, and the more preferable Cu content is 0.03 to 0.10.

Ni: 0 to 0.40%
Ni is an element to be contained as necessary, and if it is contained, it is an element that contributes to the stabilization of austenite and has a solid solution strengthening action. Therefore, it is effective in improving the creep strength and preventing a decrease in the creep strength after long-term use. It also has the effect of improving toughness. In order to obtain these effects more reliably, the Ni content is preferably 0.01% or more. However, if the Ni content exceeds 0.40%, the creep strength decreases. In addition, excessive content is not preferable from the viewpoint of economy. Therefore, the amount of elements to be contained is 0.40% or less for Ni. The preferable Ni content is 0.01 to 0.20, and the more preferable Ni content is 0.03 to 0.10%.

Ta: 0 to 0.10%
Ta is an element to be contained if necessary, and if it is contained, MX-type fine carbonitride is formed like V and Nb, which contributes to high strength. In order to obtain this effect more reliably, the Ta content is preferably 0.001% or more. On the other hand, when the Ta content exceeds 0.10%, coarse carbonitride is formed and the toughness and creep strength are deteriorated. Therefore, when Ta is contained, it should be 0.10% or less. The preferred Ta content is 0.001 to 0.08%, and the more preferable Ta content is 0.002 to 0.04%.

Co: 0-1.0%
Co is an element to be contained as necessary, and if contained, it is an element that contributes to the stabilization of austenite and has a solid solution strengthening action. Therefore, it is effective in improving the creep strength and preventing a decrease in the creep strength after long-term use. It also has the effect of improving toughness. In order to obtain these effects more reliably, the Co content is preferably 0.01% or more. However, if the Co content exceeds 1.0%, the creep strength decreases. In addition, excessive content is not preferable from the viewpoint of economy. Therefore, the amount of Co element when contained is 1.0% or less. The preferable Co content is 0.01 to 0.50%, and the more preferable Co content is 0.05 to 0.30%.

The low alloy heat resistant steel of the present disclosure can selectively contain the following alloying elements in addition to the above alloy components.
The low alloy heat resistant steel of the present disclosure may contain at least one element selected from La, Ce, Y, Ca, Zr, and Mg.
La, Ce, Y, Ca, Zr, and Mg combine with impurities S and O to increase the cleanliness in the steel, thereby improving toughness, strength, and ductility.
La, Ce, Y, Ca, and Zr are each preferably contained in the range of 0 to 0.20%, more preferably in the range of 0.01 to 0.20%. Mg is preferably contained in the range of 0 to 0.050%, more preferably in the range of 0.0005 to 0.050%.
The lower limit of the content of La, Ce, Y, Ca, and Zr is preferably 0.01% or more, respectively, in order to obtain the above-mentioned effect more reliably. Further, the lower limit of the Mg content is preferably 0.0005% or more in order to obtain the above effect more reliably.
On the other hand, when at least one of the La, Ce, Y, Ca, and Zr contents exceeds 0.20%, or the Mg content exceeds 0.050%, inclusions increase and the toughness is rather impaired.

The low alloy heat-resistant steel of the present disclosure has a chemical composition containing each of the above-mentioned elements and the balance of Fe and impurities.
In addition, "impurities" refer to those mixed by various factors in the manufacturing process, including raw materials such as ore or scrap, when steel materials are industrially manufactured. Examples of impurities include H, Zn, Pb, Cd, As and the like. The content of these elements is, for example, 0.01% or less.

The chemical composition of the low alloy heat resistant steel of the present disclosure satisfies the following formulas (1) and (2). Further, it is preferable that the chemical composition satisfies the following formula (3).

Equation (1): Ti (%) / 48-N (%) / 14 ≧ 0
In order for the solid solution amount of B in the matrix to be 0.0005% or more and BN not to precipitate, the contents of Ti and N must satisfy the formula (1). From the viewpoint of obtaining a low alloy heat-resistant steel having excellent creep strength at high temperature and toughness at low temperature, the value on the left side of the formula (1) ^{is preferably 5.0 × 10-5} or more, preferably 1.0 ×. It is more preferably 10 ^{-4 or more.} The upper limit of the value on the left side of the equation (1) is not particularly limited, but may be, for example, 3.5 × ^10-4 or less.
Here, "Ti (%)" and "N (%)" represent the content of Ti (% by mass) and the content of N (% by mass), respectively.

When the left side of the formula (1) is less than 0, BN is formed and the creep strength and toughness are deteriorated due to the decrease in hardenability.

Equation (2): Ti (%) × N (%) ≦ 9.0 × ^10-5
In order not to crystallize coarse TiN, the contents of Ti and N need to satisfy the formula (2). From the viewpoint of obtaining a low alloy heat-resistant steel having excellent creep strength at high temperature and toughness at low temperature, the value on the left side of the formula (2) ^{is preferably 8.0 × 10-5} or less, preferably 7.0 ×. It is more preferably ^{10-5 or less.} The lower limit of the value on the left side of the equation (2) is not particularly limited, but may be, for example, 5.0 × ^10-6 or more.

When the left side of the formula (2) ^{exceeds 9.0 × 10-5} , TiN crystallizes from the liquid phase and grows coarsely. This coarse TiN deteriorates toughness.

Equation (3): Al (%) / Ti (%) ≧ 0.50
In order to suppress the crystallization of the coarse Al—Ti composite oxide, it is preferable that the contents of Ti and Al satisfy the formula (3). When the contents of Ti and Al satisfy the formula (3), the crystallization of the coarse Al—Ti composite oxide can be suppressed and the toughness can be further improved.
Here, "Al (%)" represents the content (mass%) of Al.

By setting the left side of the formula (3) to 0.50 or more, deoxidation by Ti in addition to Al can be suppressed. Since the Al—Ti composite oxide produced by deoxidation of Al and Ti is _{produced at a temperature lower than that of Al 2} O ₃ , it is difficult to remove it from the steel and it may grow coarsely. Therefore, in order to suppress the deterioration of toughness due to the coarse Al—Ti composite oxide, it is preferable that the left side of the formula (3) is 0.50 or more.
Further, from the viewpoint of workability, the left side of the formula (3) is preferably 5.00 or less, more preferably 2.00 or less, and further preferably 1.50 or less.

The low alloy heat resistant steels of the present disclosure contain TiN. The average circle equivalent diameter of TiN is 3.0 μm or less. TiN is generated in the low alloy heat resistant steel of the present disclosure in the manufacturing process.
Here, TiN means an inclusion (also simply referred to as “TiN”) containing a compound consisting only of Ti and N. Specifically, TiN refers to a compound having a composition of mass%, Ti: 20 to 80%, N: 20 to 80%, and the total amount of Ti and N exceeds 50% with respect to the total TiN. .. Further, examples of the element that can be contained in the TiN other than Ti and N include O (oxygen), Ca, Mg and the like.

In the low alloy heat resistant steels of the present disclosure, TiN exists as an inclusion. Since the average circle equivalent diameter of TiN is 3.0 μm or less, the low alloy heat-resistant steel of the present disclosure is excellent in creep strength at high temperature and toughness at low temperature. Further, from the viewpoint of improving creep strength and toughness, the average circle equivalent diameter of TiN is preferably 2.5 μm or less. The lower limit of the average circle equivalent diameter of TiN is not particularly limited, but may be, for example, 0.1 μm or more.
The average circle equivalent diameter is measured by the method described later.

In the low alloy heat-resistant steel of the present disclosure, the number of TiNs having an average circle equivalent diameter of 3.0 μm or less per unit area is preferably ^{3 to 30 pieces / mm 2.}
By setting the number of TiNs having an average circle equivalent diameter of 3.0 μm or less per unit area to 3 pieces / mm ² or more, the coarseness of crystal grains can be suppressed and the toughness can be further improved. The number of TiNs per unit area is more preferably 10 pieces / mm ² or more.
Further, when the number of TiNs having an average circle equivalent diameter of 3.0 μm or less per unit area is 30 pieces / mm or less, the increase of inclusions which are the starting points of fracture can be suppressed and the toughness can be further improved. The number of TiNs per unit area is more preferably 20 pieces / mm ² or less.

The low alloy heat-resistant steel of the present disclosure contains an Al—Ti composite oxide, and the average circle equivalent diameter of the Al—Ti composite oxide is preferably 3.0 μm or less. The Al—Ti composite oxide is produced in the steel during the manufacturing process of the low alloy heat resistant steel of the present disclosure.
Here, the Al—Ti composite oxide has a composition of% by mass, Al: 10 to 50%, Ti: 10 to 80%, O: 5 to 80%, N: less than 20%, and the amount of Al, Ti. A compound having a total amount and O amount of more than 60% of the total amount of the Al—Ti composite oxide. In addition, examples of the elements that can be contained in the Al—Ti composite oxide other than Al, Ti, O, and N include Fe, Ca, and Mg.

In the low alloy heat resistant steels of the present disclosure, the Al—Ti composite oxide is present as an inclusion. Further, from the viewpoint of improving creep strength and toughness, the average circle equivalent diameter of the Al—Ti composite oxide is preferably 2.5 μm or less. The lower limit of the average circle equivalent diameter of the Al—Ti composite oxide is not particularly limited, but may be, for example, 0.1 μm or more.

In the low alloy heat-resistant steel of the present disclosure, the number of Al—Ti composite oxides having an average circle equivalent diameter of 3.0 μm or less per unit area ^{is preferably 50 pieces / mm 2} or less.
When the number of Al—Ti composite oxides having an average circle equivalent diameter of 3.0 μm or less per unit area is 50 pieces / mm ² or less, the increase of inclusions that are the starting points of fracture is suppressed and the toughness is further improved. be able to. Therefore, the number of Al—Ti composite oxides per unit area is more preferably 30 pieces / mm ² or less.

-Measuring method of equivalent circle diameter-
The average circle-equivalent diameter of inclusions contained in the low alloy heat-resistant steel of the present disclosure can be obtained by using an automatic inclusion analyzer (Metal Quality Analyzer).
Specifically, the cross section cut along the rolling direction at the center of the plate width of the steel sheet is paper-polished with at least # 1000 or more according to the JIS standard, and then finished buffed with diamond particles having an abrasive grain size of 1 μm. Then, the cross section is subjected to elemental analysis of inclusions by an inclusions automatic analyzer (MQA). The observation of the above cross section is performed by taking a photograph of a metal structure of 10 μm × 10 μm in the central portion of the plate thickness of the cut cross section in a total of one field of view. Then, for the inclusions identified as TiN or Al—Ti composite oxide by the above elemental analysis, the average circle equivalent diameter of the inclusions contained in the low alloy heat-resistant steel is obtained. Furthermore, the number of inclusions per unit area is calculated.
The average number of circle-equivalent diameters of inclusions of 0.1 μm or more is defined as the average circle-equivalent diameter. The circle-equivalent diameter of the inclusions is the diameter of a circle having an area equal to the area of the inclusions. The number average is calculated after excluding inclusions with a circle equivalent diameter of less than 0.1 μm.

[Production method]
The method for producing the low alloy heat-resistant steel of the present disclosure is not limited, but an example will be described.

(1) Molding process In the production of the low alloy heat resistant steel of the present disclosure, a material (steel material) having the above-mentioned chemical composition (chemical component) is molded into the final shape of the low alloy heat resistant steel. The forming step includes all steps involving deformation to obtain the final shape, and includes, for example, steps such as casting, forging, and rolling.
As a forming process, for example, an ingot in which a material is melted and cast is formed by hot forging and hot rolling, or by hot forging, hot rolling, and cold working. , The process of finalizing the low alloy heat resistant steel.

(2) Normalizing heat treatment step After the molding step, for example, normalizing heat treatment may be performed. For example, it is preferable to perform a normalizing heat treatment at 1000 ° C. to 1100 ° C. for 0.1 hour to 1.5 hours.

(3) Tempering heat treatment step Further, after the normalizing heat treatment step, for example, a tempering heat treatment may be performed. For example, it is preferable to perform tempering heat treatment at 750 ° C. to 790 ° C. for 0.2 hours to 5 hours.
The above method is an example. For example, regarding the normalizing heat treatment and the tempering heat treatment, the low alloy heat resistant steel of the present disclosure may be produced even if the above preferable conditions are not satisfied depending on the steel composition and other conditions in the process. May be possible.

[Use]
The application of the low alloy heat-resistant steel of the present disclosure is not limited, but it is suitably used for equipment used at high temperature such as a boiler for power generation.
Examples of equipment used at high temperatures include pipes for waste heat recovery boilers; pipes for boilers such as coal-fired power plants, oil-fired power plants, waste incineration power plants, and biomass power plants; decomposition in petrochemical plants. Tube; etc.
Here, the term "used at high temperature" in the present disclosure includes, for example, an embodiment of use in an environment of 350 ° C. or higher and 700 ° C. or lower (further, 400 ° C. or higher and 650 ° C. or lower).
For example, in the case of producing a steel pipe formed of the low alloy heat resistant steel of the present disclosure, known pipe making techniques can be applied except that the low alloy heat resistant steel of the present disclosure is used for the steel pipe body. Specifically, it may be a seamless steel pipe formed of the low-alloy heat-resistant steel of the present disclosure, or may be a welded pipe in which the low-alloy heat-resistant steel of the present disclosure is formed into a tubular shape and welded.

Hereinafter, the low alloy heat resistant steel according to the present disclosure will be described more specifically by way of examples. The low alloy heat resistant steel according to the present disclosure is not limited to these examples.

Forty-two kinds of steels having the chemical compositions shown in Table 1 were melted in a 30 kg vacuum induction melting furnace. Underlined values in Table 1 mean outside the scope of this disclosure. Further, in Table 1, the blank portion means that the component (element) is not contained (not intentionally added). The obtained ingot was hot forged and then hot rolled to obtain a steel plate having a thickness of 15 mm. This steel sheet was normalized at 1050 ° C. for 30 minutes and tempered at 750 to 770 ° C. for 30 minutes. As for the tempering conditions, the tempering temperature was adjusted so that the Vickers hardness of the steel sheet was within the range of 200 ± 10.
Charpy impact test pieces and creep test pieces were collected from the tempered steel sheet, and the ductility-brittle transition temperature was measured by the Charpy impact test under the following conditions, and the creep test was performed under the conditions of 500 ° C. and 270 MPa. In addition, inclusions were observed using a scanning electron microscope (SEM).

(1) Charpy Impact Test / Toughness A 10 mm × 10 mm × 55 mm test piece was cut out from a Charpy impact test piece, and 10 2 mm V notch full-size Charpy impact test pieces with a machined notch were collected and subjected to a Charpy impact test. The Charpy impact test was conducted in accordance with JIS-Z2242: 2005. The test was carried out at −80 ° C. to + 60 ° C., and those having a fracture surface transition temperature of −20 ° C. or lower were regarded as “passed (acceptable)”, and those having a fracture surface transition temperature higher than −20 ° C. were regarded as “failed”. In addition, those having a fracture surface transition temperature of -30 ° C or lower and having particularly excellent impact toughness were designated as "passed (excellent)".

(2) Creep rupture test Using a creep test piece having a diameter of 6 mm, a creep rupture test was conducted under the conditions of 500 ° C. and 270 MPa with a distance between reference points of 30 mm. The creep rupture test was carried out in accordance with JIS-Z2271: 2010. Those having a creep rupture time of 3000 hours or more were regarded as "pass (excellent)", and those having a creep rupture time of less than 3000 hours were regarded as "failed". Further, those having a creep rupture time of 5000 hours or more and having particularly excellent creep strength were regarded as "passed (excellent)".

(3) Observation of inclusions Observation of inclusions was carried out by the method described above. As a result of elemental analysis, inclusions composed of chemical components having Ti: 20 to 80% by mass, N: 20 to 80%, and the total amount of Ti and N exceeding 50% are referred to as "inclusions 1" (that is, inclusions 1). "TiN"). Further, the chemical composition in terms of mass% is Al: 10 to 50%, Ti: 10 to 80%, O: 5 to 80%, N: less than 20%, and the total amount of Al, Ti and O is more than 60%. The inclusions composed of were designated as "inclusions 2" (that is, Al—Ti composite oxide). Then, the average circle-equivalent diameter of these inclusions 1 and 2 was measured by the method described above. In addition, the number of inclusions 1 and inclusions 2 was calculated per unit area and evaluated according to the following evaluation criteria.

-Evaluation criteria-
(About inclusion 1)
A (excellent): 10 pieces / mm ² or more and 20 pieces / mm ² or less B (possible): 3 pieces / mm ² or more and 10 pieces / mm ^{less than 2} or 20 pieces / mm ² or more and 30 pieces / mm ² or less (intervention) About thing 2)
A (excellent): 30 pieces / mm ² or less B (possible): 30 pieces / mm ^{2 or} more 50 pieces / mm ² or less

Regarding inclusions 1 (TiN), those having an average circle equivalent diameter of more than 3.0 μm were not evaluated as described above and were evaluated as “unevaluated”. Further, regarding the inclusions 2 (Al—Ti composite oxide), those having an average circle equivalent diameter of more than 3.0 μm were not evaluated as described above and were evaluated as “unevaluated”.

(4) Observation of metallographic structure The metallographic structure is determined by polishing the cross section of the test piece in the same manner as in the above-mentioned inclusion analysis, and then performing nital corrosion (mixture of nitric acid and ethanol, nitric acid: ethanol = 2:98 in this experiment). This was carried out, and the metallographic structure of each steel plate was determined by observing at a magnification of 500 times using an optical microscope.
“B” in Table 2 indicates that the metal structure is a bainite single-phase structure, and “B + F” indicates that the metal structure is a mixed structure of bainite and ferrite. "B + M" indicates that the metallographic structure is a mixed structure of bainite and martensite.

The results of these tests are shown in Table 2.

Symbols in Table 2 A steel has V content, B steel has Al content, C steel has formula (2), D steel has Ti, E steel has Mn content, F steel has Nb content and formula (1). G steel contains P content and Mo content, H steel contains formula (2), I steel contains B content, J steel contains Cu content, K steel contains N content and formula (1), and L steel contains Si. Amount, M is C content and S content, N is Co content, O is Al content, P is Ta content, Q is W content, R is Cr content and formula (1), S is formula. (1), T is C content, U is Mn content, V is Cr content, W is Mo content, X is W content, Y is B content, and Z is the formula (2) of the present disclosure. Comparative steel that is out of range. As is clear from Table 2, at least one of the toughness and the creep strength of the comparative steel is not superior to that of the example (the present disclosed steel).

On the other hand, in the disclosed steels, all of the formulas (1) and (2) were satisfied, and the size of the inclusions 1 was 3.0 μm or less. As a result, the transition temperature was −20 ° C. or lower, showing good toughness.
Further, in particular, the disclosed steels 1 to 4, 6 to 11, 13 to 16 also satisfy the formula (3), the size of the inclusions 2 is 3.0 μm or less, and the transition temperature is −30 ° C. or less. It showed particularly good toughness.
The disclosed steel had a breaking time of 3000 hours or more and good creep strength in a creep test at 500 ° C. × 270 MPa. Further, the disclosed steels 2 and 15 satisfy a preferable component range, and in a creep test of 500 ° C. × 270 MPa, the breaking time was 5000 hours or more and the creep strength was particularly good.

The low alloy heat resistant alloy member of the present disclosure is excellent in creep strength at high temperature and toughness at low temperature. Therefore, the low alloy heat resistant alloy member of the present disclosure is suitable for use as a pressure resistant member for heat exchanger pipes, piping pipes, heat resistant valves, connection joints, reaction vessels, etc. in fields such as boilers, chemical industries, and nuclear power plants. Is.

Claims (4)

By mass%,
C: 0.04 to 0.10%,
Si: 0.01-0.50%,
Mn: 0.10 to 0.60%,
Cr: 1.75 to 3.00%,
Mo: 0.01-0.30%,
W: 1.20 to 2.00%,
V: 0.10 to 0.50%,
Nb: 0.01 to 0.10%,
Ti: 0.005-0.020%,
Al: 0.005-0.030%,
B: 0.0005 to 0.0060%,
N: 0.0010 to 0.0050%,
P: 0.030% or less,
S: 0.010% or less,
Ni: 0-0.40%,
Cu: 0-0.40%,
Ta: 0-0.10%,
Co: 0-1.0%,
La: 0-0.20%,
Ce: 0 to 0.20%,
Y: 0-0.20%,
Ca: 0-0.20%,
Zr: 0 to 0.20%, and Mg: 0 to 0.050%
The balance is composed of Fe and impurities, and has a chemical composition satisfying the following formulas (1) and (2).
A low alloy heat-resistant steel containing TiN and having an average circle-equivalent diameter of the TiN of 3.0 μm or less.
Ti (%) / 48-N (%) / 14 ≧ 0 ... Equation (1)
Ti (%) x N (%) ≤ 9.0 x ^10-5 ... Equation (2) The chemical composition satisfies the following formula (3).
The low alloy heat-resistant steel according to claim 1, which contains an Al—Ti composite oxide and has an average circle equivalent diameter of the Al—Ti composite oxide of 3.0 μm or less.
Al (%) / Ti (%) ≧ 0.50 ・ ・ ・ Equation (3) Ni: 0.01-0.40%,
Cu: 0.01-0.40%,
Ta: 0.001 to 0.10%, and Co: 0.01 to 1.0%,
The low alloy heat resistant steel according to claim 1 or 2, which contains at least one element selected from. La: 0.01 to 0.20%,
Ce: 0.01 to 0.20%,
Y: 0.01 to 0.20%,
Ca: 0.01 to 0.20%,
Zr: 0.01 to 0.20%, and Mg: 0.0005 to 0.050%,
The low alloy heat resistant steel according to any one of claims 1 to 3, which contains at least one element selected from.