Invention name: Austenitic heat-resistant steel
Technical field
[0001]
The present invention relates to austenitic heat resistant steel.
This application claims priority based on Japanese Patent Application No. 2019-156592 filed in Japan on August 29, 2019, and the contents thereof are incorporated herein by reference.
Background technology
[0002]
In recent years, from the viewpoint of reducing the environmental load, the operating conditions of boilers for power generation have been increasing in temperature and pressure on a global scale, and the materials used for superheater tubes and reheater tubes are superior. It is required to have high temperature strength and corrosion resistance.
As a material satisfying such a requirement, a large amount of N and Ni are contained in order to increase the high temperature strength, and more than 20% of Cr is contained in order to improve the corrosion resistance and the steam oxidation resistance at high temperature. Various austenitic heat resistant steels are disclosed.
[0003]
For example, Patent Document 1 contains 20% to 27% Cr, 22.5% to 32% Ni, and 0.1% to 0.3% N, thereby providing high temperature strength, steam oxidation resistance, and resistance to steam oxidation. Heat-resistant austenitic stainless steels with enhanced fireside corrosiveness and tissue stability have been proposed.
Patent Document 2 contains austenitic stainless steel having excellent high temperature strength and creep ductility containing Cr of more than 22% to less than 30%, Ni of more than 18% to less than 25%, and N of 0.1% to 0.35%. Steel has been proposed.
Patent Document 3 contains Cr of more than 22% to less than 30%, Ni of more than 18% to less than 25%, N of 0.1% to 0.35%, and the amount of impurity elements such as Sn and Sb. Austenitic heat-resistant steel, which is excellent in high-temperature strength and workability after long-term use by reducing the amount, has been proposed.
Patent Document 4 contains 15% to 30% Cr, 6% to 30% Ni, 0.03% to 0.35% N, and reduces impurity elements such as P, S, and Sn. , Austenitic stainless steels having excellent high temperature strength and embrittlement cracking resistance of welded portions during long-term use have been proposed.
[0004]
By the way, it is necessary to periodically stop the boiler for power generation to check the soundness, and at that time, the temperature of the members such as the pipes used will drop. The above-mentioned austenitic stainless steel and heat-resistant steel have excellent high-temperature strength, and although they have excellent performance for the problems to be solved by each, when the weldability is not sufficient and / or, It was found that when the temperature drops after long-term use at high temperature, sufficient toughness may not be stably obtained.
Prior art literature
Patent documents
[0005]
Patent Document 1: Japanese Patent Publication No. 2002-537486
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-250783
Patent Document 3: Japanese Patent Application Laid-Open No. 2009-84606
Patent Document 4: International Publication No. 2009/044796
Outline of the invention
Problems to be solved by the invention
[0006]
[Correction based on Rule 91 19.10.2021]
The present invention has been made in view of the above situation. An object of the present invention is to provide an austenitic heat-resistant steel having excellent weldability, excellent creep strength, and stable toughness after being held at a high temperature for a long time.
Means to solve problems
[0007]
[Correction based on Rule 91 19.10.2021]
In order to solve the above-mentioned problems, the present inventors have Cr as 20.0% to 25.0%, Ni as 22.0% to 28.0%, and Co as 0.90% in terms of creep strength. It contains ~ 2.40%, N is 0.16% ~ 0.30%, and from the viewpoint of weldability (ability to form back beads at the time of first layer welding), S: 0.0001% ~ 0. A detailed investigation was conducted on the toughness of austenitic heat-resistant steels containing 0020% and Sn: 0.0005% to 0.0230% at high temperature and after holding (heating) for a long time. As a result, the following findings were clarified.
(A) The toughness of steel held at high temperature for a long time decreases remarkably as the contents of S and Sn increase. As a result of observing the fracture surface after the impact test, when the content of S and Sn increased, the proportion of the region destroyed at the austenite grain boundary increased, and S and Sn were detected from the fracture surface. From this result, the reason why the toughness of the steel containing S and Sn decreases after long-term holding at high temperature is that S and Sn contained in the steel segregate at the austenite grain boundaries during long-term holding at high temperature. It is presumed that this is because the element of the above reduces the binding force of the grain boundaries.
(B) On the other hand, as a result of the studies by the present inventors, in order to secure the toughness after holding at high temperature for a long time, S and Sn should be reduced as much as possible within the range where the weldability is not impaired, and the total content thereof should be reduced. Therefore, it was found that it is effective to contain B in an appropriate range. The reason for this is that B contained in the steel has a high diffusion rate and segregates into austenite grain boundaries faster than S and Sn when held at high temperature for a long time, and as a result, B has a decrease in grain boundary bonding force due to S and Sn. It is considered that this is to suppress the decrease in toughness and reduce the decrease in toughness.
[0008]
The present invention has been completed based on the above findings, and the gist thereof is the austenitic heat-resistant steel shown below.
(1) The austenite-based heat-resistant steel according to one aspect of the present invention has a chemical composition of% by mass, C: 0.04% to 0.12%, Si: 0.10% to 0.30%, Mn: 0.20% to 0.80%, P: 0% to 0.030%, S: 0.0001% to 0.0020%, Sn: 0.0005% to 0.0230%, Cu: 2.3% ~ 3.8%, Co: 0.90% ~ 2.40%, Ni: 22.0% ~ 28.0%, Cr: 20.0% ~ 25.0%, Mo: 0.01% ~ 0 .40%, W: 2.8% to 4.2%, Nb: 0.20% to 0.80%, B: 0.0010% to 0.0050%, N: 0.16% to 0.30 %, Al: 0% to 0.030%, O: 0% to 0.030%, V: 0% to 0.08%, Ti: 0% to 0.08%, Ta: 0% to 0.08. %, Ca: 0% to 0.010%, Mg: 0% to 0.010%, REM: 0% to 0.080%, the balance is composed of Fe and impurities, and the formula (i) is satisfied. Austenite heat resistant steel.
0.0012% ≤ [% S] + [% Sn] ≤ 2.5 x [% B] + 0.0125% (i)
However, [% S], [% Sn], and [% B] in the formula (i) indicate the contents of S, Sn, and B in mass%, respectively.
(2) The austenitic heat-resistant steel according to (1) above has a chemical composition of V: ​​0.01% to 0.08%, Ti: 0.01% to 0.08%, Ta: 0.01. One or more selected from% to 0.08%, Ca: 0.001% to 0.010%, Mg: 0.001% to 0.010%, REM: 0.0005% to 0.080%. May be contained.
The invention's effect
[0009]
According to the above aspect of the present invention, an austenitic stainless steel having excellent weldability and having both stable toughness and excellent creep strength after being held at a high temperature for a long time (for example, at 450 to 800 ° C. for 500 hours or more). Heat resistant steel can be provided. The austenite-based heat-resistant steel according to the above aspect of the present invention is, for example, a boiler pipe for a coal-fired power plant, an oil-fired power plant, a waste incineration power plant, a biomass power plant, or a decomposition pipe in a petrochemical plant for a long time at high temperature. Suitable for the equipment used.
A brief description of the drawing
[0010]
[Fig. 1] Fig. 1 is a diagram showing a groove shape during a welding test.
Embodiment for carrying out the invention
[0011]
Hereinafter, the austenitic heat-resistant steel according to the embodiment of the present invention (austenitic heat-resistant steel according to the present embodiment) will be described. The austenitic heat-resistant steel according to the present embodiment is, for example, a steel compatible with the austenitic stainless steel and the austenitic heat-resistant steel described in JIS G0203: 2009.
[0012]

The austenitic heat resistant steel according to this embodiment has a predetermined chemical composition. The reasons for limiting its chemical composition are as follows.
In the following explanation, the "%" indication of the content of each element means "mass%". Further, in the present specification, 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.
[0013]
C: 0.04% to 0.12%
C is an element that stabilizes the austenite structure and combines with Cr to form carbides, improving creep strength at high temperatures. In order to obtain this effect sufficiently, the C content needs to be 0.04% or more. The C content is preferably 0.05% or more, more preferably 0.06% or more.
On the other hand, when C is excessively contained, a large amount of carbide is deposited and the toughness is lowered. Therefore, the C content is set to 0.12% or less. The C content is preferably 0.11% or less, more preferably 0.10% or less.
[0014]
Si: 0.10% to 0.30%
Si has a deoxidizing effect and is an element necessary for ensuring corrosion resistance and oxidation resistance at high temperatures. In order to obtain the effect, the Si content needs to be 0.10% or more. The Si content is preferably 0.12% or more, more preferably 0.15% or more.
On the other hand, when Si is excessively contained, the stability of the austenite structure is lowered and the creep strength is lowered. Therefore, the Si content is set to 0.30% or less. The Si content is preferably 0.28% or less, more preferably 0.25% or less.
[0015]
Mn: 0.20% to 0.80%
Mn is an element having a deoxidizing effect like Si. It is also an element that stabilizes the austenite structure and contributes to the improvement of creep strength. In order to obtain these effects, the Mn content needs to be 0.20% or more. The Mn content is preferably 0.25% or more, more preferably 0.30% or more.
On the other hand, if the Mn content becomes excessive, the creep ductility decreases. Therefore, the Mn content is set to 0.80% or less. The Mn content is preferably 0.75% or less, more preferably 0.70% or less.
[0016]
P: 0% to 0.030%
P is contained as an impurity and is an element that enhances the susceptibility to liquefaction cracking during welding. Further, when P is contained in a large amount, the creep ductility also decreases. Therefore, an upper limit is set for the P content, and the P content is set to 0.030% or less. The P content is preferably 0.028% or less, more preferably 0.025% or less. The P content is preferably reduced as much as possible, i.e., the content may be 0%. However, an extremely low P content leads to an increase in steelmaking costs. Therefore, the preferable lower limit of the P content is 0.001%, and the more preferable lower limit is 0.002%.
[0017]
S: 0.0001% to 0.0020%
S is an element that segregates at the austenite grain boundaries during holding at high temperatures and weakens its binding force. Therefore, when the S content is high, the toughness of the heat-resistant steel after being held at a high temperature for a long time is lowered. In the content range of other elements in the austenitic heat-resistant steel according to the present embodiment, in order to prevent a decrease in toughness, the S content must be 0.0020% or less, and the Sn content and B content. It is necessary to satisfy the relationship between the quantity and the following. The S content is preferably 0.0018% or less, more preferably 0.0015% or less. The S content is preferably reduced as much as possible from the viewpoint of toughness. However, S is also an element that affects the flow of hot water in the molten pool during welding, increases the penetration depth, and enhances welding workability, particularly back wave forming ability during first layer welding. Therefore, it is necessary to set the S content to 0.0001% or more and to satisfy the relationship with Sn, which will be described later. The S content is preferably 0.0002% or more, more preferably 0.0003% or more.
[0018]
Sn: 0.0005% -0.0230%
Sn is an element that evaporates from the molten pool during welding and contributes to the formation of the current-carrying path of the arc, and also has the effect of improving the weldability by increasing the penetration depth. In order to obtain this effect, the content range of other elements in the austenitic heat-resistant steel according to this embodiment In the box, it is necessary that the Sn content is 0.0005% or more and that the relationship with the S content described later is satisfied. The Sn content is preferably 0.0010% or more, more preferably 0.0015% or more.
On the other hand, when Sn is excessively contained, Sn segregates at the austenite grain boundaries during holding at a high temperature, weakening the binding force of the grain boundaries. As a result, the toughness of the steel after being held at high temperature for a long time is reduced. Therefore, in the content range of other elements in the austenitic heat-resistant steel according to the present embodiment, it is necessary to set the Sn content to 0.0230% or less and to satisfy the relationship between the S content and the B content described later. be. The Sn content is preferably 0.0220% or less, more preferably 0.0200% or less.
[0019]
Cu: 2.3% -3.8%
Cu is an element that enhances the stability of the austenite structure and finely precipitates during retention at high temperatures, contributing to the improvement of creep strength. In order to obtain this effect sufficiently, it is necessary to set the Cu content to 2.3% or more. The Cu content is preferably 2.5% or more, more preferably 2.7% or more.
On the other hand, if Cu is excessively contained, the hot workability is lowered. Therefore, the Cu content is set to 3.8% or less. The Cu content is preferably 3.5% or less, more preferably 3.3% or less.
[0020]
Co: 0.90% to 2.40%
Co is also an element that enhances the stability of the austenite structure and contributes to the improvement of creep strength. In order to obtain the effect sufficiently, the Co content needs to be 0.90% or more. The Co content is preferably 1.00% or more, more preferably 1.20% or more, and even more preferably 1.40% or more.
On the other hand, when Co is excessively contained, not only the effect is saturated, but also Co is a very expensive element, which causes an increase in cost. Therefore, the Co content is 2.40% or less. The Co content is preferably 2.20% or less, more preferably 2.00% or less.
[0021]
Ni: 22.0% to 28.0%
Ni is an element that enhances the stability of the austenite structure and contributes to the improvement of creep strength. In order to obtain the effect sufficiently, it is necessary to set the Ni content to 22.0% or more. The Ni content is preferably 22.2% or more, more preferably 22.5% or more.
On the other hand, Ni is a very expensive element, so if Ni is excessively contained, not only the effect will be saturated, but also the cost will increase. Therefore, Ni should be 28.0% or less. The Ni content is preferably 27.8% or less, more preferably 27.5% or less.
[0022]
Cr: 20.0% to 25.0%
Cr is an element effective for ensuring oxidation resistance and corrosion resistance at high temperatures. In addition, Cr is an element that forms fine carbides and contributes to the improvement of creep strength. In order to obtain these effects sufficiently, it is necessary to set the Cr content to 20.0% or more. The Cr content is preferably 20.5% or more, more preferably 21.0% or more.
On the other hand, when Cr is excessively contained, the stability of the austenite structure is lowered and the creep strength is lowered. Therefore, the Cr content is set to 25.0% or less. The Cr content is preferably 24.5% or less, more preferably 24.0% or less.
[0023]
Mo: 0.01% -0.40%
Mo is an element that dissolves in steel and contributes to the improvement of creep strength and tensile strength at high temperatures. In order to obtain the effect sufficiently, it is necessary to set the Mo content to 0.01% or more. The Mo content is preferably 0.02% or more, more preferably 0.03% or more.
On the other hand, when Mo is excessively contained, the stability of the austenite structure is remarkably lowered, and the creep strength is lowered. Furthermore, since Mo is an expensive element, excessive content leads to an increase in cost. Therefore, the Mo content is set to 0.40% or less. The Mo content is preferably 0.38% or less, more preferably 0.35% or less.
[0024]
W: 2.8% to 4.2%
W is an element that dissolves in steel and contributes to the improvement of creep strength and tensile strength at high temperatures. In order to obtain the effect sufficiently, it is necessary to set the W content to 2.8% or more. The W content is preferably 3.0% or more, more preferably 3.2% or more.
On the other hand, when W is excessively contained, the stability of the austenite structure is lowered, and the creep strength is rather lowered. Therefore, the W content is set to 4.2% or less. The W content is preferably 4.0% or less, more preferably 3.8% or less.
[0025]
Nb: 0.20% to 0.80%
Nb is an element that precipitates in the grains of austenite as fine carbides and nitrides and contributes to the improvement of creep strength and tensile strength at high temperatures. In order to obtain the effect sufficiently, it is necessary to set the Nb content to 0.20% or more. The Nb content is preferably 0.25% or more, more preferably 0.30% or more.
On the other hand, when Nb is excessively contained, a large amount of carbonitride is deposited and the creep ductility is lowered. Therefore, the Nb content is set to 0.80% or less. The Nb content is preferably 0.75% or less, more preferably 0.70% or less.
[0026]
B: 0.0010% -0.0050%
B improves the creep strength by finely dispersing the grain boundary carbides, and segregates at the grain boundaries during holding at high temperature to suppress the segregation of S and Sn at the grain boundaries after holding at high temperature. It is an element that contributes to improving the toughness of steel. In order to sufficiently obtain these effects, it is necessary to set the B content to 0.0010% or more and to satisfy the relationship between the S content and the Sn content described later. The B content is preferably 0.0012% or more, more preferably 0.0015% or more.
On the other hand, if B is excessively contained, the crack sensitivity of the heat-affected zone during welding increases. Therefore, the B content is set to 0.0050% or less. The B content is preferably 0.0048% or less, more preferably 0.0045% or less.
[0027]
N: 0.16% to 0.30%
N is an element that stabilizes the austenite structure and dissolves in steel or precipitates as a nitride, contributing to the improvement of high-temperature strength. In order to obtain the effect sufficiently, it is necessary to set the N content to 0.16% or more. The N content is preferably 0.18% or more, more preferably 0.20% or more.
On the other hand, if N is excessively contained, the ductility will decrease. Therefore, the N content is set to 0.30% or less. The N content is preferably 0.28% or less, more preferably 0.26% or less.
[0028]
Al: 0% to 0.030%
Al is an element added as a deoxidizing agent. However, when Al is excessively contained, the cleanliness of the steel deteriorates and the hot workability deteriorates. Therefore, it is necessary to set the Al content to 0.030% or less. The Al content is preferably 0.025% or less, more preferably 0.020% or less. The lower limit does not need to be set in particular, that is, the Al content may be 0%. However, an extreme reduction in Al content leads to an increase in manufacturing cost. Therefore, the Al content is preferably 0.001% or more, more preferably 0.002% or more.
[0029]
O: 0% to 0.030%
O (oxygen) is an element contained as an impurity. When O is excessively contained, the hot workability is lowered and the ductility is deteriorated. Therefore, it is necessary to set the O content to 0.030% or less. The O content is preferably 0.025% or less, more preferably 0.020% or less. The lower limit does not need to be set in particular, that is, the O content may be 0%. However, an extreme reduction in O content leads to an increase in manufacturing cost. Therefore, the O content is preferably 0.001% or more, more preferably 0.002% or more.
[0030]
0.0012% ≤ [% S] + [% Sn] ≤ 2.5 x [% B] + 0.0125% (1)
In the austenitic heat-resistant steel according to the present embodiment, it is necessary to control the content of each element as described above, and further, the S content, the Sn content, and the B content need to satisfy the above formula (1). be.
Here, [% S], [% Sn], and [% B] in the above formula (1) indicate the contents of S, Sn, and B in mass%, respectively.
S and Sn are elements that segregate at the austenite grain boundaries during holding at high temperatures and weaken their binding force. Therefore, in general, steel containing S and Sn has a reduced toughness after being held at a high temperature for a long time. However, as the present inventors have found, B has a high diffusion rate and segregates at the austenite grain boundaries faster than S and Sn, and suppresses the decrease in toughness due to the segregation of S and Sn at the grain boundaries. .. In order to sufficiently obtain the effect, the total content of S and Sn must be 2.5 × [% B] + 0.0125% or less with respect to the content of B.
On the other hand, the lower the content of S and Sn, the more advantageous it is to improve the toughness after holding at high temperature, but they affect the convection of the molten pool during welding and the arc phenomenon, respectively. It has the effect of increasing the penetration depth and improving the weldability (particularly, the back wave forming ability at the time of initial layer welding). In the austenitic heat-resistant steel according to the present embodiment, the total amount of S and Sn needs to be 0.0012% or more in order to obtain the effect. The total content of the preferred S and Sn is 0.0015% or more, and the more preferable total content is 0.0018% or more.
[0031]
The austenitic heat-resistant steel according to the present embodiment basically contains the above elements and the balance is Fe and impurities. However, in addition to the above, the following group is used instead of a part of Fe as an alloy component. It may contain at least one element in the above. However, since these elements do not necessarily have to be contained, the lower limit is 0%. The reasons for the limitation are described below.
[0032]
V: 0 to 0.08%
V is an element that combines with carbon (C) or nitrogen (N) to form fine carbides or carbonitrides and contributes to the improvement of creep strength. Therefore, it may be contained as needed. When this effect is obtained, the V content is preferably 0.01% or more, more preferably 0.02% or more.
However, when V is excessively contained, a large amount of carbonitride is deposited and the creep ductility is lowered. Therefore, even when it is contained, the V content needs to be 0.08% or less. The V content is preferably 0.07% or less, more preferably 0.06% or less. Even more preferably, it is 0.04% or less.
[0033]
Ti: 0 to 0.08%
Like V, Ti is an element that combines with carbon or nitrogen to form fine carbides or carbonitrides and contributes to the improvement of creep strength. Therefore, it may be contained as needed. When this effect is obtained, the Ti content is preferably 0.01% or more, more preferably 0.02% or more.
However, if Ti is excessively contained, a large amount of carbonitride is deposited and the creep ductility is lowered. Therefore, even when it is contained, the Ti content needs to be 0.08% or less. The Ti content is preferably 0.07% or less, more preferably 0.06% or less.
[0034]
Ta: 0-0.08%
Like V and Ti, Ta is an element that combines with carbon or nitrogen to form fine carbides or carbonitrides and contributes to the improvement of creep strength. Therefore, it may be contained as needed. When this effect is obtained, the Ta content is preferably 0.01% or more, more preferably 0.02% or more.
However, when Ta is excessively contained, a large amount of carbonitride is deposited and the creep ductility is lowered. Therefore, even when it is contained, the Ta content needs to be 0.08% or less. The Ta content is preferably 0.07% or less, more preferably 0.06% or less.
[0035]
Ca: 0 to 0.010%
Ca is an element that has the effect of improving hot workability during manufacturing. Therefore, it may be contained as needed. To obtain this effect, The a content is preferably 0.001% or more, more preferably 0.002% or more.
However, when Ca is excessively contained, it binds to oxygen (O) and the cleanliness is remarkably lowered, so that the hot workability is deteriorated. Therefore, even when it is contained, the Ca content is 0.010% or less. The Ca content is preferably 0.008% or less, more preferably 0.006% or less.
[0036]
Mg: 0 to 0.010%
Like Ca, Mg is an element that has the effect of improving hot workability during manufacturing. Therefore, it may be contained as needed. When this effect is obtained, the Mg content is preferably 0.001% or more, more preferably 0.002% or more.
However, when Mg is excessively contained, it binds to oxygen (O), the cleanliness is remarkably deteriorated, and the hot workability is deteriorated. Therefore, even when it is contained, the Mg content is 0.010% or less. The Mg content is preferably 0.008% or less, more preferably 0.006% or less.
[0037]
REM: 0 to 0.080%
REM, like Ca and Mg, is an element that has the effect of improving hot workability during manufacturing. Therefore, it may be contained as needed. When this effect is obtained, the REM content is preferably 0.0005% or more, more preferably 0.001% or more.
However, when REM is excessively contained, it is combined with oxygen, the cleanliness is remarkably deteriorated, and the hot workability is deteriorated. Therefore, even when it is contained, the REM content is 0.080% or less. The REM content is preferably 0.060% or less, more preferably 0.050% or less.
"REM" is a general term for a total of 17 elements of Sc, Y and lanthanoids, and the content of REM refers to the total content of one or more elements of REM. Further, REM is generally contained in mischmetal. Therefore, for example, it may be added in the form of misch metal so that the REM content is within the above range.
[0038]
[Production method]
The austenitic heat-resistant steel according to the present embodiment is made into a slab by casting, for example, molten steel having the above-mentioned predetermined chemical composition, and the slab is hot forged, then hot-worked and, if necessary. It is obtained by performing cold working, forming into a predetermined shape, and then performing a solution heat treatment (solution heat treatment) in which the product is held at 1050 to 1280 ° C. for 2 to 60 minutes and then cooled with water. The processing conditions such as hot forging, hot working, and cold working are not particularly limited and may be appropriately determined according to the shape.
[0039]
The austenitic heat-resistant steel according to this embodiment is used for equipment used at high temperatures, such as a boiler for power generation. Examples of equipment used at high temperatures include boiler pipes for coal-fired power plants, petroleum-fired power plants, waste incineration power plants and biomass power plants, decomposition pipes for petrochemical plants, and the like.
Here, "use at high temperature" includes, for example, an embodiment of use in an environment of 450 ° C. or higher and 800 ° C. or lower (further, 500 ° C. or higher and 750 ° C. or lower).
Example
[0040]
Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.
[0041]
Hot forging and hot rolling were performed on the ingot cast by melting and casting the materials of reference numerals A to N having the chemical compositions shown in Tables 1A and 1B (the balance is Fe and impurities: the unit is mass%). It was formed into a plate shape with a thickness of 18 mm.
This plate-shaped material was heated to 1180 ° C., held at that temperature for 30 minutes, and then water-cooled to obtain austenitic heat-resistant steel (Nos. 1 to 14).
[0042]
[Table 1A]

[0043]
[Table 1B]

[0044]
[Charpy impact test / evaluation of toughness]
From the austenitic heat-resistant steel after the solution treatment, the front and back surfaces were ground by machining, and a plurality of plate materials (base material for impact test) having a plate thickness of 15 mm, a width of 150 mm, and a length of 150 mm were collected. Further, a part of the impact test base material was subjected to an aging heat treatment at 700 ° C. for 1000 hours.
After that, for the impact test base material that was not subjected to aging heat treatment and the impact test base material that was subjected to aging heat treatment, three 2 mm V notch full-size Charpy impact test pieces with notches were processed from the center in the thickness direction of each plate. They were collected one by one and subjected to a Charpy impact test.
The Charpy impact test was conducted in accordance with JISZ2242: 2005. The test was carried out at 20 ° C., and those with an average value of absorbed energy of 3 test pieces of 27 J or more were regarded as "passed", and among them, the individual values ​​of absorbed energy of all 3 test pieces were 27 J or more. The ones that become "excellent" and the others are "acceptable". On the other hand, those in which the average value of absorbed energy of the three test pieces was less than 27 J were regarded as "failed".
[0045]
[Welding test / Evaluation of weldability]
In addition, the front and back surfaces were ground by machining from the austenitic heat-resistant steel after the solution treatment, and a plate material (base material for welding test) having a plate thickness of 15 mm, a width of 50 mm, and a length of 100 mm was collected. After performing the groove processing shown in FIG. 1 in the longitudinal direction of the base metal for welding test, the welding is performed by automatic gas tungsten arc welding using Ar as the shield gas, and "no filler material" and "welded material". The first layer was welded under the condition of "Yes".
For welding, JIS-Z3334 (2011) SNi6617 with an outer diameter of 1.2 mm is used as the filler material, and the heat input is 9 kJ / cm. Butt welding was performed as cm.
Welding workability is "passed" when the back bead is formed over the entire length of the weld line of the obtained welded joint, and "excellent" is when the width of the back bead is 2 mm or more over the entire length of the weld line. Is less than 2 mm, but the one with a back bead with a width of 1 mm or more is regarded as "OK", and there is a part where the back bead is not formed in some of the two joints, or the bead width is less than 1 mm in some parts. Welding workability was judged to be "failed".
[0046]
[Creep rupture test / evaluation of creep strength]
In addition, for austenitic heat-resistant steel that passed the above-mentioned impact test and welding test, a round bar creep test piece was collected from the impact test base material that had not been subjected to aging heat treatment, and a creep rupture test was conducted. At that time, a creep rupture test was conducted under the condition that the target rupture time of the base metal was 1000 hours at 700 ° C. × 167 MPa. The creep rupture test was performed in accordance with JISZ2271: 2010.
The break time exceeding the target break time (1000 hours) was regarded as "pass", and the break time shorter than that was regarded as "fail".
[0047]
[Table 2]

[0048]
From Table 2, No. 1 manufactured using the symbols A to E, M, and N satisfying the conditions specified in the present invention. It can be seen that Nos. 1 to 5, 13 and 14 stably obtain excellent toughness after being held at a high temperature for a long time, and also have weldability and creep strength.
[0049]
On the other hand, No. 1 using the codes F to H. In 6 to 8, the total content of S and Sn exceeded the range of the relational expression with the B content specified in the present invention. Therefore, the effect of suppressing the decrease in the grain boundary binding force due to the segregation of S and Sn grain boundaries by B could not be sufficiently obtained. As a result, the toughness after aging heat treatment (holding at high temperature for a long time) did not satisfy the target.
No. using the codes I and J. In 9 and 10, since the contents of S and Sn exceeded the upper limit, respectively, the decrease in grain boundary binding force due to the segregation of these elements became remarkable. As a result, the toughness after aging heat treatment (holding at high temperature for a long time) did not satisfy the target.
In addition, No. 1 using the reference numeral K. In No. 11, since the total content of S and Sn was below the range specified in the present invention, the effect of improving the back wave forming ability by these elements could not be obtained, and the welding workability was inferior.
No. using the symbol L. In No. 12, the Co content was below the range specified in the present invention. As a result, a sufficient creep strength improving effect could not be obtained.
[0050]
It can be seen that only when the requirements of the present invention are satisfied as described above, stable and excellent toughness can be obtained after long-term holding without impairing weldability, and sufficient creep strength can also be obtained.
Industrial applicability
[0051]
According to the present invention, it is possible to provide an austenitic heat-resistant steel having excellent weldability, stable toughness after being held at a high temperature for a long time, and excellent creep strength.
The scope of the claims
[Claim 1]
The chemical composition is mass%,
C: 0.04% to 0.12%,
Si: 0.10% to 0.30%,
Mn: 0.20% to 0.80%,
P: 0% to 0.030%,
S: 0.0001% to 0.0020%,
Sn: 0.0005% -0.0230%,
Cu: 2.3% to 3.8%,
Co: 0.90% to 2.40%,
Ni: 22.0% to 28.0%,
Cr: 20.0% to 25.0%,
Mo: 0.01% -0.40%,
W: 2.8% to 4.2%,
Nb: 0.20% to 0.80%,
B: 0.0010% to 0.0050%,
N: 0.16% to 0.30%,
Al: 0% to 0.030%,
O: 0% to 0.030%,
V: 0% to 0.08%,
Ti: 0% to 0.08%,
Ta: 0% -0.08%,
Ca: 0% to 0.010%,
Mg: 0% to 0.010%,
REM: 0% to 0.080%,
The balance consists of Fe and impurities.
And satisfying equation (1),
Austenitic heat-resistant steel characterized by this.
0.0012% ≤ [% S] + [% Sn] ≤ 2.5 x [% B] + 0.0125% (1)
However, [% S], [% Sn], and [% B] in the formula (1) indicate the contents of S, Sn, and B in mass%, respectively.
[Claim 2]
The chemical composition is
V: 0.01% -0.08%,
Ti: 0.01% -0.08%,
Ta: 0.01% -0.08%,
Ca: 0.001% -0.010%,
Mg: 0.001% -0.010%,
REM: 0.0005% -0.080%,
Contains one or more selected from
The austenitic heat-resistant steel according to claim 1.