JP5661001B2 - High strength austenitic heat resistant steel with excellent post-aging toughness - Google Patents

Description

  The present invention relates to a high-strength austenitic heat-resistant steel applied to steel pipes for high-strength boilers used for ultra-supercritical coal-fired power generation and coal gasification combined power generation, and in particular, high-strength austenitic heat-resistant steel with excellent post-aging toughness. Related to steel.

  In recent years, reduction of carbon dioxide emissions has been demanded as a measure against global warming. Coal-fired power generation systems are widely used as a major power source all over the world because of their high economic efficiency and safety. However, since it is the power generation method that emits the most carbon dioxide, there is a strong demand for higher efficiency in power generation.

  The high-strength austenitic heat-resistant steels that have been put into practical use are mainly 18% Cr-8% Ni and (22-25%) Cr- (15-20%) Ni. It is used properly according to the environment. According to the “Technical Standards for Thermal Power Facilities for Thermal Power Generation”, 18% Cr-8% Ni materials include fire SUS304J1HTB steel and fire SUS347J1TB steel (22-25%) Cr -(15-20%) Ni-based materials include fire SUS310J1TB steel and fire SUS310J2TB steel. Although these heat resistant steels all have good post-aging toughness at 700 ° C., the creep rupture strength at 100,000 hours is less than 100 MPa.

  As a prior art, a heat resistant steel having a composition in which a creep rupture strength at 700 ° C. to 100,000 hours exceeds 100 MPa is proposed mainly by a combined addition of C, Mo, W, Nb, Ti and B (for example, patents) Reference 1). However, since the composition contains a large amount of Mo and W, a large amount of σ phase and Laves phase are precipitated, and there is a problem that excellent post-aging toughness cannot be obtained.

  Furthermore, high strength austenitic heat resistant steels having excellent weldability and good high temperature corrosion resistance have been proposed by extremely reducing C and appropriately mixing Mo, W, Nb, Ti and N (for example, , See Patent Document 2). Among these, although examples showing excellent creep rupture strength have been presented, those having high strength have a problem that they cannot obtain excellent post-aging toughness because they have a large amount of Mo and W.

  Furthermore, the addition of Cu, B and Mg to the N-containing steel further improves the creep rupture strength, suppresses the decrease in strength and toughness by reducing the amount of Si and Al, and further uses Nb, Mo or W alone. Or it has proposed the austenitic heat-resistant steel which is excellent in high temperature strength and structure | tissue stability by adding in composite (for example, refer patent document 3). However, in this invention, since the structural stability when Mo and W are added has not been studied in detail, there are many cases in which the post-aging toughness is significantly deteriorated, leaving room for improvement.

  Furthermore, an austenitic heat-resistant steel with good high-temperature strength has been proposed by adding Cu, B and Mg to N-containing steel, reducing the Si and Al contents, and further suppressing the Mn content ( For example, see Patent Document 4.) However, even in this invention, since the structural stability when Mo and W are added has not been studied in detail, there are many cases where the post-aging toughness is remarkably lowered, and there remains room for improvement.

  By the way, recent coal-fired power plants are operated at a maximum main steam temperature of 600 ° C. High-strength austenitic heat-resistant steel is used for superheater tubes that heat main steam and reheater tubes that reheat main steam that has passed through a steam turbine. In order to increase the efficiency of power generation, heat-resistant steel having higher strength than conventional materials is required. This requires a high temperature strength with a creep rupture strength at 700 ° C. to 100,000 hours of 100 MPa or more. At the same time, it is required that the impact value after aging at a high temperature for a long time (hereinafter, this property is referred to as “post-age toughness”) maintains a good value. So far, several heat-resistant steels satisfying the strength have been found, but no heat-resistant steel having good post-aging toughness has been found yet.

JP 63-183155 A JP-A-6-322488 JP-A-62-133048 JP-A-8-13102

  The problem to be solved by the present invention is to optimize the chemical composition of the Mo and W-added austenitic heat-resisting steels containing N, so that the creep rupture strength at 700 ° C to 100,000 hours is 100 MPa or higher. It is to provide an austenitic heat resistant steel having strength and excellent post-aging toughness.

In order to combine the high temperature strength and post-aging toughness of austenitic steel, the content range of N and Mo in the chemical composition is limited to a narrow range at the same time, and after aging by grain boundary precipitation of σ phase In order to suppress the decrease in toughness, the inventor found out that the value of the formula that defines the austenite balance is defined as a high value, and obtained the invention of the present application.
To explain the principle, N in the austenitic heat-resisting steel is generally widely used as an element that stabilizes the austenite structure and at the same time improves the high-temperature strength by solid solution strengthening and carbonitride precipitation strengthening. However, when N is added at the same time as W or Mo, N acts to promote the precipitation of the Laves phase at the grain boundary and causes the early deterioration of toughness after aging. The present inventors have found that the formation and growth of the Laves phase is remarkably promoted, which further reduces the toughness after aging, and the heat-resistant steel of the present invention is obtained by utilizing this principle.

Therefore, the means of the present invention for solving the above-mentioned problem is that, in the invention of claim 1, in mass%, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: 0.04% or less, S: 0.010% or less, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4. 2%, Mo: 0.5% or less, Nb: 0.2-0.5%, Al: 0.001-0.040%, N: 0.07-0.13%, the balance Fe and It is a high-strength austenitic heat-resistant steel made of inevitable impurities and satisfying the following formulas (1) and (2) and excellent in post-aging toughness.
here,
W + 2Mo = 2.8 to 4.25% (1)
Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo + 0.5W) -0.5Si-0.3Nb + 10 ≧ 9.5% (2)
It is.

In the invention of claim 2, by mass, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: 0.04 %: S: 0.010% or less, Ni: more than 15 to 26%, Cr: 18-23%, W: 1.8-4.2%, Mo: 0.5% or less, Nb: 0.2 -0.5%, Al: 0.001-0.040%, N: 0.07-0.13%, further Cu: 2.0-3.2%, the remainder Fe and unavoidable It is a high-strength austenitic heat-resistant steel made of impurities and satisfying the following formulas (1) and (2) and excellent in post-aging toughness.
here,
W + 2Mo = 2.8 to 4.25% (1)
Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo + 0.5W) -0.5Si-0.3Nb + 10 ≧ 9.5% (2)
It is.

In the invention of claim 3, by mass, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: 0.04 %: S: 0.010% or less, Ni: more than 15 to 26%, Cr: 18-23%, W: 1.8-4.2%, Mo: 0.5% or less, Nb: 0.2 -0.5%, Al: 0.001-0.040%, N: 0.07-0.13%, Cu: 2.0-3.2%, and further B: 0.001- 0.004%, Ca: containing any one or two of 0.001 to 0.007, consisting of remaining Fe and inevitable impurities, satisfying the following formulas (1) and (2), post-aging toughness It is a high-strength austenitic heat-resistant steel with excellent resistance.
here,
W + 2Mo = 2.8 to 4.25% (1)
Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo + 0.5W) -0.5Si-0.3Nb + 10 ≧ 9.5% (2)
It is.

  The reasons for limiting the chemical components of the high-strength austenitic heat-resistant steel in the above means of the present invention and the reasons for limiting the formulas (1) and (2) will be described. In addition,% shows the mass%.

C: 0.02 to 0.08%
C is an element necessary for improving high-temperature strength, and C is added by 0.02% or more. However, if C exceeds 0.08%, a large amount of M ₂₃ C ₆ type carbide precipitates and the toughness after aging is lowered. Therefore, C is set to 0.02 to 0.08%.

Si: more than 0.3% to 0.8%
Si needs to be added in excess of 0.3% for deoxidation. However, Si makes it easy to precipitate the σ phase at the grain boundaries and lowers the toughness after aging. Therefore, Si is more than 0.3% to 0.8%, preferably more than 0.3% to 0.6%.

Mn: 0.6 to 2.0%
Mn is added for deoxidation. However, if Mn is added excessively, the cost becomes high. Therefore, Mn is set to 0.6 to 2.0%.

P: 0.040% or less P is contained as an unavoidable impurity, but when P exceeds 0.040%, weldability deteriorates. Therefore, P is set to 0.040% or less.

S: 0.010% or less S is contained as an unavoidable impurity, but when S exceeds 0.010%, hot workability deteriorates. Therefore, S is set to 0.010% or less.

Ni: Over 15% to 26%
Ni needs to be contained more than 15% in order to stabilize the austenite structure. However, since a large amount of Ni is expensive, the upper limit is made 26%.

Cr: 18-23%
Cr needs to be added in an amount of 18% or more in order to improve high temperature corrosion resistance and steam oxidation resistance. However, when an excessive amount of Cr is added, the σ phase is precipitated at the grain boundaries, and the toughness after aging is significantly reduced. Therefore, Cr is 18 to 23%, preferably 19 to 22%.

W: 1.8-4.2%
W needs to be added by 1.8% or more in order to improve the high temperature strength. However, if added over 4.2%, a large amount of the Laves phase precipitates at the grain boundary, and the toughness after aging is significantly reduced. Therefore, W is set to 1.8 to 4.2%.

Mo: 0.5% or less Mo is appropriately added as necessary for improving the high-temperature strength. However, if Mo is added in excess of 0.5%, the post-aging toughness is remarkably lowered. Therefore, Mo is 0.5% or less, preferably 0.3% or less.

Nb: 0.2-0.5%
Nb needs to be added in an amount of 0.2% in order to improve the high temperature strength. However, if Nb is added in excess of 0.5%, weldability deteriorates. Therefore, Nb is set to 0.2 to 0.5%.

Al: 0.001 to 0.040%
Al needs to be added in an amount of 0.001% or more for deoxidation. However, if the content exceeds 0.040%, AlN is generated at the grain boundary and the post-aging toughness is deteriorated. Therefore, Al is made 0.001 to 0.040%.

N: 0.07 to 0.13%
N needs to be added by 0.07% or more in order to improve the high temperature strength. However, if N exceeds 0.13%, a Laves phase is generated at the grain boundary, and the post-aging toughness is deteriorated. Therefore, N is set to 0.07 to 0.13%.

Cu: 2.0 to 3.2%
Cu is added in an amount of 2.0% or more in order to obtain the steel of claim 2 which is further improved in high-temperature strength as compared with the steel of claim 1. However, if Cu exceeds 3.2%, the post-aging toughness decreases. Therefore, Cu is set to 2.0 to 3.2%.

Any one or two of B: 0.001 to 0.004% and Ca: 0.001 to 0.007% B or Ca is an element that improves hot workability. If it is 001% or more, the hot ductility is improved by grain boundary strengthening, but if it exceeds 0.004%, the hot workability is deteriorated. Ca fixes S at 0.001% or more to improve hot ductility, but if it exceeds 0.007%, the effect is saturated and the cost becomes high. Therefore, B is 0.001 to 0.004%, and Ca is any one or two of 0.001 to 0.007%.

W + 2Mo = 2.8 to 4.25% (1)
The reason why the lower limit of the formula (1) is 2.8% is to improve the high temperature strength of the steel of the present invention. The reason why the upper limit value of the formula (1) is 4.25% is that the Laves phase is precipitated at the grain boundary, and the post-aging toughness is lowered. Therefore, the value of W + 2Mo in the formula (1) is set to 2.8 to 4.25%.

Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo + 0.5W) -0.5Si-0.3Nb + 10 ≧ 9.5% (2)
The formula (2) is a Ni balance formula, and the reason why the lower limit of the value of (2) is set to 9.5% is to suppress the precipitation of the σ phase and the decrease in post-aging toughness. Therefore, the value of Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo + 0.5W) -0.5Si-0.3Nb + 10 in the formula (2) is set to 9.5% or more.

  The present invention is a high-strength austenitic heat-resistant steel having excellent post-aging toughness at 700 ° C., comprising the chemical components of the above means and satisfying the formulas (1) and (2). It has an effect applicable to steel pipes for high-strength boilers used for thermal power generation and coal gasification combined power generation.

  No. of the invention example containing the chemical component shown in Table 1. 1-18 and Comparative Example No. Each of 19 to 32 steel types was melted into a 100 kg steel ingot in a vacuum melting furnace. This steel ingot was hot forged into a steel bar having a diameter of 20 mm. Furthermore, this steel bar was subjected to a solution heat treatment at 1170-1230 ° C. to obtain an experimental material. In addition, No. of invention example. Nos. 1 to 11 are Nos. 12 and no. No. 13 is an embodiment of claim 2 and No. 14 to 18 are embodiments of claim 3.

網掛けは本発明の範囲を外れることを示す。

Shading indicates that it is outside the scope of the present invention.

  Creep rupture test pieces having a parallel part diameter of 6 mm and a gauge distance of 30 mm were prepared from the experimental materials of the above invention examples and comparative examples, and creeped at temperatures of 700 ° C., 750 ° C., and 800 ° C. A break test was performed. The result of the creep rupture test was arranged with the Larson-Miller parameter, and the estimated creep rupture strength at 700 ° C. and 100,000 hours was obtained. A material having a breaking strength of 100 MPa or more was evaluated as “◯”, and a material having a breaking strength as “×” was evaluated as shown in Table 1.

Further, the above experimental materials were subjected to aging heat treatment at temperatures of 700 ° C., 750 ° C. and 800 ° C. for a maximum of 10,000 hours, and then processed into Charpy impact test pieces having a width of 10 mm and a 2 mm-V notch at room temperature. A Charpy impact test was performed. The result of this impact test was arranged with the Orr-Sherby-Dorn parameter, and the estimated Charpy impact value at 700 ° C. and 100,000 hours was obtained. A material having an impact value of 30 J / cm ² or more was evaluated as “◯”, and a material having an impact value of less than 30 J / cm ² was evaluated as “x”.

No. In Examples 1 to 18, the creep rupture strength at 700 ° C. and 100,000 hours was 100 MPa or more, and the Charpy impact value at 700 ° C. and 100,000 hours was 30 J / cm ² or more. Therefore, these examples are austenitic heat-resisting steels having excellent creep rupture strength characteristics and post-aging toughness at 700 ° C.

In contrast, no. Comparative Examples 19 to 28 of the present invention are No. 29 and No. With the exception of No. 30, any of the chemical components of steel is out of the scope of the present invention. 29 and No. Although the chemical composition of steel 30 is within the scope of the present invention, the value of formula (1) or (2) is out of the scope of the present invention. As a result, No. of the comparative example of the present invention. 19 to 28, the creep rupture strength at 700 ° C. and 100,000 hours is less than 1 million MPa, or the Charpy impact value at 700 ° C. and 100,000 hours is less than 30 J / cm ^2. I was not satisfied.

Claims (3)

By mass%, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: 0.04% or less, S: 0.0. 010% or less, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: 0.5% or less, Nb: 0.2 to 0.5%, Al : 0.001-0.040%, N: 0.07-0.13%, balance Fe and inevitable impurities, satisfying the following formulas (1) and (2) High strength austenitic heat resistant steel with excellent aftertoughness.
W + 2Mo = 2.8-4.2% (1)
Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo + 0.5W) -0.5Si-0.3Nb + 10 ≧ 9.5% (2) By mass%, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: 0.04% or less, S: 0.0. 010% or less, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: 0.5% or less, Nb: 0.2 to 0.5%, Al : 0.001 to 0.040%, N: 0.07 to 0.13%, further Cu: 2.0 to 3.2%, balance Fe and unavoidable impurities, the following (1 ) And high-strength austenitic heat-resistant steel excellent in post-aging toughness characterized by satisfying the formula (2).
W + 2Mo = 2.8-4.2% (1)
Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo + 0.5W) -0.5Si-0.3Nb + 10 ≧ 9.5% (2) By mass%, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: 0.04% or less, S: 0.0. 010% or less, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: 0.5% or less, Nb: 0.2 to 0.5%, Al : 0.001 to 0.040%, N: 0.07 to 0.13%, Cu: 2.0 to 3.2%, B: 0.001 to 0.004%, Ca: High in excellent post-aging toughness characterized by containing any one or two of 0.001 to 0.007, consisting of the balance Fe and inevitable impurities and satisfying the following formulas (1) and (2) High strength austenitic heat resistant steel.
W + 2Mo = 2.8-4.2% (1)
Ni + 27C + 23N + 0.2Mn + 0.3Cu-1.2 (Cr + Mo + 0.5W) -0.5Si-0.3Nb + 10 ≧ 9.5% (2)