JP4946758B2 - High temperature austenitic stainless steel with excellent workability after long-term use - Google Patents

Description

  The present invention relates to a high-temperature austenitic stainless steel excellent in workability after long-term use, and more specifically, a steel pipe used as a boiler superheater tube or reheater tube, a chemical reactor reactor tube, and the like, and The present invention relates to a high-temperature austenitic stainless steel excellent in workability after long-term use suitable as a material such as a steel plate, a bar steel, and a forged steel product used as a heat and pressure resistant member.

In recent years, new super-critical pressure boilers with higher steam temperature and pressure have been developed all over the world for higher efficiency. Specifically, it is also planned to increase the steam temperature, which has been around 600 ° C. until now, to 650 ° C. or higher, and further to 700 ° C. or higher. This is based on the fact that energy conservation, effective utilization of resources, and reduction of CO ₂ gas emissions for environmental conservation are one of the challenges for solving energy problems and are important industrial policies. In the case of a power generation boiler for burning fossil fuel or a reaction furnace for the chemical industry, a highly efficient ultra super critical pressure boiler or reaction furnace is advantageous.

  The high temperature and high pressure of steam raises the temperature in actual operation of a superheater tube of a boiler, a reaction furnace tube for the chemical industry, and a steel plate, a bar steel, a forged steel product, etc. as a heat and pressure resistant member to 700 ° C. or higher. For this reason, steels used in such harsh environments are required to have not only high-temperature strength and high-temperature corrosion resistance, but also good long-term stability of the microstructure, creep rupture ductility and creep fatigue resistance. .

  Furthermore, in maintenance such as repair after long-term use, it becomes necessary to perform work such as cutting, processing, welding, etc. on materials that have changed over time, and not only the characteristics as a new material but also the soundness as an aging material has recently been It was strongly demanded.

  Austenitic stainless steel is superior in high-temperature strength and high-temperature corrosion resistance compared to ferritic steel. For this reason, austenitic stainless steel is used in a high temperature range of 650 ° C. or higher where ferritic steel cannot be used in terms of strength and corrosion resistance. Typical examples include 18Cr-8Ni steel (hereinafter referred to as “18-8 system”) represented by SUS347H and SUS316H, and 25Cr steel represented by SUS310. However, even with austenitic stainless steel, there is a limit to the operating temperature in terms of high temperature strength and corrosion resistance. Further, the conventional 25Cr SUS310 steel is superior in corrosion resistance to the 18-8 steel, but has a low high-temperature strength at 600 ° C. or higher.

  In view of this, various attempts have been made to increase both high-temperature strength and corrosion resistance, and austenitic stainless steels as described below have been proposed.

  (1) Patent Document 1 discloses a steel whose high-temperature creep strength is increased by adding Al and Mg in addition to adding a large amount of N.

  (2) In Patent Document 2, in addition to adding an appropriate amount of B, a combination of Al and N is added, and further, O (oxygen) is limited to 0.004% or less to increase the high temperature strength and hot workability. Enhanced steel is disclosed.

  (3) Patent Document 3 discloses a steel in which hot workability is improved by adding Al, N, Mg and Ca in combination and further limiting O (oxygen) to 0.007% or less. Yes.

  (4) Patent Document 4 includes precipitation strengthening and solid solution strengthening by nitride by adding N, and contents of Cr, Mn, Mo, W, V, Si, Ti, Nb, Ta, Ni and Co. A steel is disclosed in which the toughness after use for a long time is improved without impairing the high temperature strength by limiting the precipitation of the σ phase by limiting it to a specific amount or less in correlation with each other.

  (5) In Patent Document 5, one or more of Ti, Nb, Zr and Ta are added in a range of 1 to 10 times the C content, and 1 to 13 times the C content in total. A steel whose high-temperature strength is increased by making the metal structure a structure of 3 to 5 in terms of JIS austenite grain size number is disclosed.

  (6) Patent Documents 6 to 9 disclose austenite that is excellent in creep rupture properties at high temperatures, particularly strength, ductility, and brittleness resistance by devising special components and metal structures based on 18Cr-8Ni chemical components. Stainless steel is disclosed.

  (7) Patent Document 10 discloses an austenitic stainless steel excellent in high temperature corrosion due to addition of a special element such as REM and toughness after prolonged aging.

  (8) Patent Document 11 discloses an austenitic stainless steel having excellent weld crack resistance.

  (9) Patent Document 12 discloses an austenitic stainless steel excellent in high temperature strength and creep rupture ductility by controlling the metal structure by precipitation of a complex oxide composed of a small amount of Ti and oxygen proposed by the present inventors. ing.

  However, the conventional steels represented by the above (1) to (9) have the following problems. That is, even a material having excellent high-temperature strength and ductility is actually used for a long period of time (for example, 30 years for heat exchanger tubes and pipes used for thermal power generation targeted by the present invention). In many cases, it will cause a change over time and the characteristics of the new material will change (deteriorate) and become a practical problem.

  Specifically, in periodic inspections after long-term use and maintenance work due to accidents and malfunctions during use, it is necessary to cut out some defective materials and replace them with new materials. Must be welded with aged material. Further, depending on the situation, it may be necessary to partially perform bending work.

  At this time, the materials used over time tend to cause weld cracks and work cracks, which can easily lead to construction problems. If you try to continue using them, it may cause a serious accident such as a burst during plant operation.

  In fact, the above-mentioned prior art does not disclose anything about tackling from the material side the problem of how to suppress the deterioration of the material with such aging. And in recent large-scale plants that are not in the past, high temperatures and high pressures, the major issue is how to suppress material deterioration over the characteristics of new materials and guarantee safe and reliable materials. I came.

JP 57-164971 A JP 11-61345 A Japanese Patent Laid-Open No. 11-293212 JP 2001-11583 A JP 59-23855 A JP-A 61-143562 Japanese Patent Laid-Open No. 2-99295 Japanese Patent Laid-Open No. 3-153846 JP-A-3-169497 JP 7-278757 A JP-A-55-28307 JP 2004-250783 A

  The present invention has been made in view of the above circumstances, and the first object is suitable as a material for equipment used in next-generation high-temperature and high-pressure boilers and the like that can surely obtain the second object steel. Is to provide an austenitic stainless steel.

The second object is excellent in strength under high temperature and high pressure of 700 ° C. or higher, and even when used for over 100,000 hours under such high temperature and high pressure, it has characteristics such as strength, ductility, toughness and weldability. It is an object of the present invention to provide austenitic stainless steel that is unlikely to deteriorate over time and that can perform operations such as welding and processing in an indispensable maintenance after long-term use. More specifically, the creep rupture elongation exceeding 10,000 h at a temperature of 750 ° C. is 15% or more, and the impact value at 0 ° C. (specifically, the width when subjected to long-term heating at 750 ° C. for 10000 h) Provides an austenitic stainless steel with an impact value using a 10 mm V-notch Charpy impact test piece) of 50 J / cm ² or more, which is less likely to deteriorate over time and has excellent workability after long-term use, such as weldability and cutability. There is to do.

  The present inventors hardly aged deterioration in properties such as strength, ductility, toughness and weldability even when used at a high temperature of 700 ° C. or higher and a high pressure of 100,000 hours or longer, which is indispensable after long-term use. In the maintenance, various studies were conducted on austenitic stainless steel suitable as a material for equipment used in next-generation high-temperature and high-pressure boilers and the like that are required to perform operations such as welding and processing soundly.

  The pressure in the “next-generation high-temperature high-pressure boiler” is assumed to be a super supercritical pressure of 20 MPa or more, although it depends on the environment and design concept. For this reason, the “next-generation high-temperature high-pressure boiler” is generally referred to as “super-supercritical boiler”.

  And as a result of the above examination, first, the austenitic stainless steel as a material of equipment used for the next generation high-temperature high-pressure boiler exposed to the harsh environment is a so-called “22-30Cr austenitic stainless steel”. Was found to be suitable.

  And when it turned out that the aged characteristic requested | required of the said austenitic stainless steel is the following (a)-(c), and satisfy | fills the characteristic of (a) and (b), it is after long-term use. It has been clarified that workability such as welding and bending of long-term aged materials is remarkably improved when defects such as welding and bending do not occur, and in addition to these, the characteristics of (c) are also satisfied.

  (A) The creep rupture strength at a temperature of 700 ° C. or higher is not stably lowered even when used for 10,000 h or more, that is, 100,000 h or more.

  (B) The creep rupture elongation exceeding 10000 h at 750 ° C. is 15% or more.

(C) When subjected to long-term heating at 750 ° C. for 10000 hours, the impact value at 0 ° C. using a V-notch Charpy impact test piece having a width of 10 mm is 50 J / cm ² or more, and it is used as a structural material even after long-term use. Has sufficient impact performance.

  Therefore, the present inventors further used various “22-30Cr austenitic stainless steels” for a high temperature long time creep rupture test and an aging test. As a result, the following important findings (d) to (f) were obtained.

  (D) Since 22-30Cr austenitic stainless steel contains commercial scrap in its raw materials, Sn, Pb, Sb, Zn, and As cannot be managed as impurities and are inevitably mixed. These impurities remain in the product because they cannot be refined and removed at the time of melting, which accounts for most of the electric furnace, except for Zn, which is easily sublimated at high temperatures.

  (E) In a long-time creep rupture test material of 700 ° C. or higher and 10,000 h or longer and a long-time aging test material equivalent thereto, the above-mentioned impurities have an important influence on long-term aging and workability of the aging material.

  (F) In order to satisfy the aging characteristics already described in (a) to (c) and to provide long-term aging materials with good workability such as weldability and cutability, “22-30Cr austenite In addition to optimizing the contents of the main elements constituting the "system stainless steel", the contents of P and S in the impurities and the above-mentioned Sn, Pb, Sb, Zn and As and "Sn (%) + Pb ( %) ”And“ Sb (%) + Zn (%) + As (%) ”need to be regulated.

  The above-described 22-30Cr austenitic stainless steel has been investigated in detail for the prior art, but there is no specific suggestion that the above-mentioned impurity elements are controlled and controlled. No technology has been found that suggests the suppression of aging as already described.

  This invention is completed based on said knowledge, The summary exists in the high temperature austenitic stainless steel excellent in the workability after long-term use shown to following (1)-(5).

  (1) By mass%, C: 0.03-0.12%, Si: 0.2-2%, Mn: 0.1-3%, Ni: more than 18% and less than 25%, Cr: 22% And less than 30%, Nb: 0.1 to 1%, V: 0.01 to 1%, B: more than 0.0005% and 0.2% or less, sol. Al: not less than 0.0005% and less than 0.03%, N: 0.1 to 0.35%, O (oxygen): 0.001 to 0.008%, the balance being Fe and impurities, impurities P, S, Sn, Pb, Sb, Zn and As in each of P: 0.03% or less, S: 0.01% or less, Sn: 0.020% or less, Pb: 0.010% or less, Sb: 0.005% or less, Zn: 0.005% or less and As: 0.005% or less, and Sn (%) + Pb (%) ≦ 0.025% and Sb (%) + Zn (%) + As ( %) ≦ 0.010% A high temperature austenitic stainless steel excellent in workability after long-term use.

  (2) Instead of a part of Fe, in mass%, one or more selected from Mo: 5% or less, W: 5% or less, and Cu: 5% or less are contained alone or in total of 5% or less The high-temperature austenitic stainless steel having excellent workability after long-term use as described in (1) above.

  (3) The above (1) or characterized in that, instead of a part of Fe, by mass%, it contains one or two of Co: 0.8% or less and Ti: 0.1% or less A high temperature austenitic stainless steel excellent in workability after long-term use as described in (2).

  (4) Instead of a part of Fe, by mass%, Mg: 0.02% or less, Ta: 0.2% or less, Zr: 0.2% or less, Ca: 0.05% or less, REM: 0 One of or more than 2%, Pd: 0.2% or less, and Hf: 0.2% or less are included, any one of (1) to (3) above High-temperature austenitic stainless steel with excellent workability after long-term use.

  (5) Processing after long-term use characterized by having the chemical composition according to any one of (1) to (4) above and having a creep rupture elongation at 750 ° C. exceeding 10,000 h of 15% or more. High-temperature austenitic stainless steel with excellent properties.

  In the present invention, REM, that is, a rare earth element refers to 17 elements of Sc, Y and lanthanoid.

  Hereinafter, the inventions related to high-temperature austenitic stainless steels having excellent workability after long-term use as shown in the above (1) to (5) are referred to as “present invention (1)” to “present invention (5)”, respectively. . Also, it may be collectively referred to as “the present invention”.

  According to the present invention, since it has excellent creep rupture strength at 700 ° C. or higher, creep rupture ductility and toughness after long-term use compared to conventional 18-8 and 25Cr steels, it is excellent in workability after long-term use. In addition, it is possible to reliably provide high-temperature austenitic stainless steel. For this reason, the great effect is acquired with respect to acceleration | stimulation of high temperature / high pressure, such as a power generation boiler in recent years.

  Hereinafter, the reason for limitation of the component elements in the high temperature austenitic stainless steel excellent in workability after long-term use of the present invention will be described in detail. In the following description, “%” display of the content of each element means “mass%”.

C: 0.03-0.12%
C is a main element that generates carbides. The content of C that is necessary for securing the appropriate tensile strength and high temperature creep rupture strength as a high temperature austenitic stainless steel is 0.03%. On the other hand, excessive C forms a large amount of undissolved carbides during processing, increases the total amount of carbides in the product, and decreases weldability. In particular, when the C content exceeds 0.12%, the weldability is significantly lowered. Therefore, the content of C is set to 0.03 to 0.12%. Note that the lower limit of the C content is preferably 0.04%, and more preferably 0.05%. Moreover, 0.08% is preferable as the upper limit of the C content, and 0.07% is more preferable.

Si: 0.2-2%
Si is added as a deoxidizing element. Si is also an important element for improving the steam oxidation resistance. In order to obtain these effects, a Si content of 0.2% or more is required. On the other hand, if it exceeds 2%, not only the workability is impaired, but also the stability of the structure at high temperature is deteriorated. Therefore, the Si content is set to 0.2 to 2%. Note that the lower limit of the Si content is preferably 0.25%, and more preferably 0.3%. Further, the upper limit of the Si content is preferably 0.6%, and more preferably 0.5%.

Mn: 0.1 to 3%
Mn forms sulfur and sulfide (MnS) and improves hot workability. However, if the content is less than 0.1%, the above effect cannot be obtained. On the other hand, excessive Mn increases the hardness and makes the steel brittle, and on the other hand, the workability and weldability are impaired. In particular, when the content of Mn exceeds 3%, workability and weldability are significantly deteriorated. Therefore, the Mn content is set to 0.1 to 3%. Note that the lower limit of the Mn content is preferably 0.2%, and more preferably 0.5%. The upper limit of the Mn content is preferably 1.5%, and more preferably 1.3%.

Ni: more than 18% and less than 25% Ni is an element that stabilizes the austenite structure, and is also an important element for ensuring corrosion resistance. From the balance with the amount of Cr described below, a content exceeding 18% is required. On the other hand, Ni of 25% or more not only causes an increase in cost but also decreases the creep strength. Therefore, the Ni content is more than 18% and less than 25%. The lower limit of the Ni content is preferably 18.5%. The upper limit of the Ni content is preferably 23%.

Cr: more than 22% and less than 30% Cr is an important element in securing oxidation resistance, steam oxidation resistance and corrosion resistance. In addition, it produces Cr-based carbonitrides and contributes to improvement of strength. In particular, in order to increase the high temperature corrosion resistance at 700 ° C. or higher to 18-8 steel or higher, a Cr content exceeding 22% is required. On the other hand, excessive Cr decreases the stability of the structure, facilitates the formation of intermetallic compounds such as the σ phase, and decreases the creep strength. Further, an increase in Cr causes an increase in expensive Ni for stabilizing the austenite structure, leading to an increase in cost. In particular, when the Cr content is 30% or more, the creep strength decreases and the cost increases remarkably. Therefore, the Cr content is more than 22% and less than 30%. Note that the lower limit of the Cr content is preferably 23%, and more preferably 24%. Further, the upper limit of the Cr content is preferably 28%, and more preferably 26%.

Nb: 0.1 to 1%
Nb finely disperses and precipitates as carbonitride and contributes to creep strengthening. For this purpose, an Nb content of at least 0.1% is required. However, the addition of a large amount of Nb impairs the weldability. In particular, when the content exceeds 1%, the weldability is significantly reduced. Therefore, the Nb content is set to 0.1 to 1%. Note that the lower limit of the Nb content is preferably 0.3%, and more preferably 0.4%. Further, the upper limit of the Nb content is preferably 0.6%, and more preferably 0.5%.

V: 0.01 to 1%
V precipitates as carbonitride and improves the creep strength. However, if the content is less than 0.01%, the above effect cannot be obtained. On the other hand, if the content exceeds 1%, an embrittled phase is generated. Therefore, the content of V is set to 0.01 to 1%. Note that the lower limit of the V content is preferably 0.03%, and more preferably 0.04%. Moreover, 0.5% is preferable as the upper limit of the V content, and 0.2% is more preferable.

B: More than 0.0005% and 0.2% or less B is present in carbonitride by replacing part of C (carbon) forming carbonitride, or B is present at grain boundaries by itself. There is an effect of suppressing grain boundary sliding creep occurring at a high temperature of 700 ° C. or higher. However, if the content is 0.0005% or less, there is no effect, while if it exceeds 0.2%, the weldability is impaired. Therefore, the content of B is set to exceed 0.0005% and not more than 0.2%. Note that 0.001% is preferable as the lower limit of the B content, and 0.0013% is more preferable. Further, the upper limit of the B content is preferably 0.005%, and more preferably 0.003%.

sol. Al: 0.0005% or more and less than 0.03% Al is added as a deoxidizing element. To obtain a deoxidizing effect, the sol. A content of 0.0005% or more is required for Al. On the other hand, the addition of a large amount of Al deteriorates the stability of the structure and causes σ phase embrittlement. If the Al content exceeds 0.03%, σ phase embrittlement becomes significant. Therefore, the content of Al is sol. Al content is set to 0.0005% or more and less than 0.03%. Note that sol. The lower limit value of the Al content in Al is preferably 0.005%. The upper limit is preferably 0.02% and more preferably 0.015%.

N: 0.1 to 0.35%
N is added in order to secure the high-temperature stability of the austenite structure in place of precipitation strengthening by carbonitride and a part of expensive Ni. In order to increase the tensile strength and the high temperature creep strength, the N content needs to be 0.1% or more. However, addition of a large amount of N impairs ductility, weldability, and toughness. In particular, when the content exceeds 0.35%, the ductility, weldability, and toughness are significantly reduced. Therefore, the N content is set to 0.1 to 0.35%. Note that the lower limit of the N content is preferably 0.15%, and more preferably 0.2%. Moreover, 0.3% is preferable as the upper limit of the N content, and 0.27% is more preferable.

O (oxygen): 0.001 to 0.008%
O (oxygen) contributes to creep strength and ductility when finely dispersed as non-metallic inclusions. In order to obtain this effect, the O (oxygen) content needs to be 0.001% or more. However, if the content of O becomes excessive, especially exceeding 0.008%, the amount of non-metallic inclusions becomes too large, and the creep rupture ductility and creep fatigue characteristics are impaired. Therefore, the content of O (oxygen) is set to 0.001 to 0.008%. The lower limit value of O (oxygen) content is preferably 0.004%, more preferably 0.005%, and the upper limit value is preferably 0.007%.

  In the present invention, first, it is necessary to limit the contents of P and S in impurities to a specific value or less. This will be described below.

P: 0.03% or less P is inevitably mixed as an impurity, and excessive P significantly impairs weldability and workability. Therefore, the upper limit of the content is set to 0.03%. The preferable P content is 0.02% or less, and it is preferable to reduce it as much as possible.

S: 0.01% or less S is inevitably mixed as an impurity, and excessive S impairs weldability and workability, so the upper limit of its content was set to 0.01%. The preferable S content is 0.005% or less, and S should be reduced as much as possible.

  In the present invention, the contents of Sn, Pb, Sb, Zn and As in the impurities, the value of “Sn (%) + Pb (%)” and “Sb (%) + Zn (%) + As (% ) "Must be limited to a certain value. This will be described below.

  Any of the above Sn, Pb, Sb, Zn, and As is contained in scrap as a raw material for 22-30Cr austenitic stainless steel, and because it is a low melting point metal, it segregates at grain boundaries during solidification and cracks. Induces and inhibits ductility and toughness. Moreover, as a result of detailed investigations by the present inventors, the contents of the above elements are 0.020% for Sn, 0.010% for Pb, 0.005% for Sb, 0.005% for Zn, and As. And the values of “Sn (%) + Pb (%)” and “Sb (%) + Zn (%) + As (%)” are 0.025% and 0.010%, respectively. If it exceeds 1, the creep properties of creep rupture elongation, squeezing, and long-term aging material deteriorate significantly in response to creep of 700 ° C. or more and 10,000 h or more of precipitates formed during aging. It has been found that it causes deterioration of workability of aged materials.

  Therefore, Sn, Pb, Sb in impurities are used in order to satisfy the aging characteristics (a) to (c) described above and to provide long-term aging materials with good workability such as weldability and cutability. The Zn and As contents are Sn: 0.020% or less, Pb: 0.010% or less, Sb: 0.005% or less, Zn: 0.005% or less, and As: 0.005% or less, respectively. Further, it was defined that Sn (%) + Pb (%) ≦ 0.025% and Sb (%) + Zn (%) + As (%) ≦ 0.010% were satisfied.

  For the above reasons, the high-temperature austenitic stainless steel excellent in workability after long-term use according to the present invention (1) contains elements from C to O (oxygen) in the above-mentioned range, with the balance being Fe and impurities. P, S, Sn, Pb, Sb, Zn and As in the impurities are in the above-mentioned ranges, and Sn (%) + Pb (%) ≦ 0.025% and Sb (%) + Zn (% ) + As (%) ≦ 0.010%.

In addition, the high temperature austenitic stainless steel excellent in workability after long-term use according to the present invention (1) is replaced with a part of the Fe, if necessary,
First group: Mo: 5% or less, W: 5% or less, and Cu: 5% or less, alone or in total of 5% or less,
Second group: one or two of Co: 0.8% or less and Ti: 0.1% or less,
Third group: Mg: 0.02% or less, Ta: 0.2% or less, Zr: 0.2% or less, Ca: 0.05% or less, REM: 0.2% or less, Pd: 0.2% And Hf: one or more of 0.2% or less,
One or more elements of each group can be selectively contained.

  That is, one or more elements of any group from the first group to the third group may be added and contained as optional elements.

  Hereinafter, the above optional elements will be described.

Group 1: Mo: 5% or less, W: 5% or less, and Cu: 5% or less, alone or in total 5% or less Mo, W and Cu as elements of the first group Is an element effective for improving the high temperature creep strength. For this reason, when it is desired to obtain this effect, one or more of Mo, W and Cu may be positively added, and the effect can be obtained when the total content is 0.1% or more. On the other hand, addition of a large amount of Mo, W and Cu impairs toughness, strength and ductility, so the upper limit of their content is preferably 5%. The lower limit values of Mo, W, and Cu are each preferably 0.5%, and more preferably 1%. The upper limit is preferably 3% and more preferably 2%.

Group 2: Co: 0.8% or less and Ti: 0.1% or less One or two of the following: Group 2 elements, Co and Ti, increase the creep rupture strength at 700 ° C. or higher. Therefore, in order to obtain this effect, the above elements may be added and contained. Hereinafter, the second group of elements will be individually described.

Co: 0.8% or less Co has the effect of improving the creep rupture strength at 700 ° C. or higher. Co also has the effect of helping Ni to stabilize the austenite structure. For this reason, when it is desired to obtain such an effect, Co may be positively added, and the above effect can be obtained when the Co content is 0.04% or more. On the other hand, the upper limit of the content was set to 0.8% so as not to contaminate the melting furnace as a radioactive element. Note that the lower limit of the Co content is preferably 0.05%, and more preferably 0.1%. Moreover, 0.5% is preferable as the upper limit of the Co content, and 0.45% is more preferable.

Ti: 0.1% or less Ti has the effect of improving the creep rupture strength at 700 ° C. or higher. In order to obtain this effect, the Ti content is preferably 0.002% or more. However, when a large amount of Ti is added, undissolved Ti carbonitrides are formed, causing crystal grains to be mixed, causing uneven creep deformation and ductility deterioration, and in particular, the Ti content If it exceeds 0.1%, unnecessary carbonitrides are produced, and the creep rupture ductility and creep fatigue properties are impaired. Therefore, when Ti is added, the content of Ti is set to 0.1% or less. The lower limit of the Ti content when added is preferably 0.002%, more preferably 0.004%, and still more preferably 0.005%. Further, the upper limit value of the Ti content when added is preferably 0.05%, and more preferably less than 0.01%.

  In addition, said Co and Ti can be contained only in any 1 type in them, or 2 types of composites.

Third group: Mg: 0.02% or less, Ta: 0.2% or less, Zr: 0.2% or less, Ca: 0.05% or less, REM: 0.2% or less, Pd: 0.2% 1 or 2 or more of Hf: 0.2% or less The elements of the third group, Mg, Ta, Zr, Ca, REM, Pd, and Hf are all hot while fixing S. It is an effective element for improving workability. Further, Mg has a deoxidizing effect when added in a very small amount.

  For this reason, when it is desired to obtain the effect, one or more kinds may be positively added, and the above effect can be obtained with a content of 0.0005% or more for any element. On the other hand, a Mg content exceeding 0.02% impairs steel quality and impairs creep strength, creep fatigue properties, and ductility. Ta and Zr whose content exceeds 0.2% not only forms oxides and nitrides and causes mixed grains, but also harms the steel quality and impairs creep strength, creep fatigue properties, and ductility. If the Ca content exceeds 0.05%, ductility and workability are impaired. REM, Pd, and Hf with a content exceeding 0.2% increase not only the oxide and other inclusions, but also deteriorate the workability and weldability, and also increase the cost.

  Therefore, when added, the content is Mg: 0.02% or less, Ta: 0.2% or less, Zr: 0.2% or less, Ca: 0.05% or less, REM: 0.2% or less, Pd: 0.2% or less and Hf: 0.2% or less. The content of the above elements when added is as follows: Mg is 0.0005 to 0.02%, Ta, Zr, REM, Pd and Hf are all 0.0005 to 0.2%, and Ca is 0.0005. It should be 0.05%.

  Preferred as the lower limit of the content is as follows.

  Mg, Ta, Zr and Ca: All are 0.001%, more preferably 0.002%. REM, Pd and Hf: All are 0.01%, more preferably 0.02%.

  Further, the upper limit of the content is preferably as follows.

  Mg: 0.008%, more preferably 0.006%. Ta and Zr: 0.1%, more preferably 0.05%. Ca: 0.03%, more preferably 0.01%. REM, Pd and Hf: All are 0.15%, more preferably 0.1%.

  Here, as described above, REM in the present invention, that is, the rare earth element indicates 17 elements of Sc, Y and lanthanoid.

  In addition, said Mg, Ta, Zr, Ca, REM, Pd, and Hf can contain only any 1 type in them, or 2 or more types of composites.

  For the above reasons, the high-temperature austenitic stainless steel excellent in workability after long-term use according to the present invention (2) is the high-temperature austenitic stainless steel excellent in workability after long-term use according to the present invention (1). Instead of a part of Fe of steel, it was specified that one or more of Mo, W and Cu, which are the elements of the first group, were contained alone or 5% or less in total.

  Moreover, the high temperature austenitic stainless steel excellent in workability after long-term use according to the present invention (3) is used for high temperature excellent in workability after long-term use according to the present invention (1) or the present invention (2). Instead of a part of Fe of the austenitic stainless steel, it is defined that one or two of Co and Ti as elements of the second group are included.

  Furthermore, the high-temperature austenitic stainless steel excellent in workability after long-term use according to the present invention (4) has high workability after long-term use according to any of the present invention (1) to the present invention (3). Instead of a part of Fe of excellent high-temperature austenitic stainless steel, one or more of Mg, Ta, Zr, Ca, REM, Pd and Hf, which are the elements of the third group, are included. Stipulated.

  The austenitic stainless steel having the chemical composition according to any one of the present invention (1) to the present invention (4) can be easily obtained as long as the creep rupture elongation exceeding 10,000 h at 750 ° C. is 15% or more. ), That is, the creep rupture strength at a temperature of 700 ° C. or higher is 10000 h or more, that is, the property that it does not decrease stably even when used for 100,000 h or more. There is no problem in processing such as welding and bending.

  Therefore, the high temperature austenitic stainless steel excellent in workability after long-term use according to the present invention (5) has the chemical composition according to any one of the present invention (1) to the present invention (4), It was stipulated that the creep rupture elongation exceeding 10,000 h at 750 ° C. was 15% or more.

  The high temperature austenitic stainless steel excellent in workability after long-term use according to the present invention (1) to the present invention (4) is subjected to a thorough detailed analysis on the raw materials used for melting, and particularly Sn in impurities. , Pb, Sb, Zn, and As, respectively, Sn: 0.020% or less, Pb: 0.010% or less, Sb: 0.005% or less, Zn: 0.005% or less, and As: After selecting 0.005% or less and satisfying Sn (%) + Pb (%) ≦ 0.025% and Sb (%) + Zn (%) + As (%) ≦ 0.010%, an electric furnace, It can be manufactured by melting using an AOD furnace or a VOD furnace.

  Next, when the molten metal is made into a slab, bloom or billet by hot forging or continuous casting after casting into an ingot by the so-called “ingot-making method”, and using these as raw materials, In the case of processing into a plate or coil by hot rolling and processing into a tube material, for example, it is hot processed into a tube by a hot extrusion tube manufacturing method or a Mannesmann tube manufacturing method.

  That is, any type of hot working may be used. For example, when the final product is a steel pipe, the hot extrusion pipe making method represented by the Eugene Sejurne method, the Mannesmann plug mill method, the Mannesmann mandrel mill, etc. Examples thereof include a roll rolling pipe manufacturing method (Mannesmann pipe manufacturing method) typified by a method, and in the case where the final product is a steel plate, a normal method for manufacturing a thick steel plate or a hot-rolled steel strip can be mentioned.

  The processing end temperature of the hot processing is not particularly specified, but is preferably 1150 ° C. or higher. This is because when the processing end temperature is less than 1150 ° C., the solid solution of Nb, Ti and V carbonitrides becomes insufficient, and the creep strength and ductility are impaired.

  The cold working may be performed after the hot working. For example, when the final product is a steel pipe, the cold drawing is performed by drawing the raw pipe manufactured by the hot working described above. Examples thereof include a pipe manufacturing method and a cold rolling pipe manufacturing method using a cold pilger mill. When the final product is a steel plate, a normal method for manufacturing a cold-rolled steel strip can be used. In addition, in order to make the structure uniform and to further stabilize the strength, it is better to recrystallize and size-size during heat treatment by giving processing strain. It is desirable to apply distortion by performing the reduction rate of 10% or more.

  Further, the heating temperature of the final heat treatment after the hot working described above or the heating temperature of the final heat treatment after further cold working after the hot working may be performed at 1150 ° C. or higher. The upper limit of the heating temperature is not particularly specified, but if it exceeds 1350 ° C., not only is it likely to cause high-temperature grain boundary cracking or a decrease in ductility, but the crystal grains become extremely large, and the workability is significantly reduced. For this reason, the upper limit of the heating temperature is preferably 1350 ° C.

  By the method as described above, the high-temperature austenitic stainless steel excellent in workability after long-term use according to the present invention (5) can be produced.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Twenty-seven kinds of steels having the chemical compositions shown in Tables 1 and 2 were melted using a vacuum melting furnace with a capacity of 50 kg, and the obtained steel ingot was hot forged into a 15 mm thick plate.

  Steels 1 to 18 in Tables 1 and 2 are steels whose chemical compositions are within the range defined by the present invention (hereinafter referred to as “the present invention steel”). On the other hand, each of the steels 19 to 27 has a component element content, either “Sn (%) + Pb (%)” or “Sb (%) + Zn (%) + As (%)”. It is a comparative steel (hereinafter referred to as “comparative steel”) that deviates from the specified conditions.

  The plate material having a thickness of 15 mm was held at 1220 ° C. for 1 h and then subjected to a water-cooling heat treatment. Hereinafter, the plate material subjected to the heat treatment is referred to as a “base material”.

  In addition, about the steel 1-3 and the steel 25-27, as a raw material which simulated the welding heat affected zone (henceforth "HAZ"), it is 20 mm width, 20 mm thickness, and 100 mm length from the heat-treated base material. A small piece was cut out, held at 1300 ° C. for 2 minutes, and then subjected to a water-cooled heat treatment. Hereinafter, this heat-treated piece is referred to as “coarse-grained HAZ material”.

  Next, a creep rupture test and a Charpy impact test were performed.

  That is, the creep rupture test was performed by taking a round bar creep test piece having an outer diameter of 6 mm and a gauge distance of 30 mm from the base material and the coarse HAZ material obtained as described above, a test temperature of 750 ° C., and a load stress. The creep rupture time (h) and creep rupture elongation (%) were measured under the condition of 70 MPa.

  In addition, the Charpy impact test was performed using a V-notch Charpy impact test piece having a width of 10 mm from each of the base material obtained as described above and a long-term aging treatment material obtained by heating the base material at 750 ° C. for 10000 hours and then air-cooling. (A V-notch test piece having a length of 55 mm, a height of 10 mm, a width of 10 mm, and a height of 8 mm under the notch) was collected, and the impact value at 0 ° C. was measured.

  Table 3 summarizes the above test results. In addition, in the “0 ° C. Charpy impact value” column of Table 3, the above-mentioned base material that has not been subjected to long-term aging treatment is referred to as “new material”, and the base material that has been subjected to long-term aging treatment is referred to as “aging material”. Indicated.

As is apparent from Table 3, the Charpy impact value at 0 ° C. is a high value of about 200 J / cm ² or more without any difference between the inventive steels 1 to 27 and the comparative steels 28 to 36 in the case of the new material. On the other hand, the impact value after aging at 750 ° C. for 10,000 h is that the steels of the present invention 1 to 27 have practically sufficient toughness of 50 J / cm ² or more, whereas the impact values of the comparative steels 28 to 36 are 24 J / It was below cm ² and a significant decrease was observed.

  That is, it was found that the comparative steel (conventional steel) heated at a high temperature for a long time is significantly deteriorated by long-term use, while the steel of the present invention is greatly improved in toughness.

  Next, regarding the creep rupture characteristics, the creep rupture time of the base material is rupture time of 10000 h or more for all of the steels of the present invention, whereas the comparative steel is broken at about 8000 h or less. In particular, the difference in creep rupture elongation was large, and the steel of the present invention showed a rupture elongation of 15% or more even when ruptured at 10000 h or more, but the comparative steel had a small rupture elongation of less than 15% and deteriorated creep rupture elongation. Was found to be large.

  Further, as a tendency of the coarse-grained HAZ material, in the case of the steel of the present invention, there is not much difference in creep rupture time from the base material, and the creep rupture elongation is 15% or more, whereas in the case of the comparative steel, the creep rupture time It itself was significantly shorter than the base metal, and the creep rupture elongation was deteriorated by an order of magnitude.

  That is, it was found that the aging degradation of the rupture strength and creep rupture elongation of the steel of the present invention over a long period of time is significantly improved compared to the comparative steel.

  According to the present invention, since it has excellent creep rupture strength at 700 ° C. or higher, creep rupture ductility and toughness after long-term use compared to conventional 18-8 and 25Cr steels, it is excellent in workability after long-term use. In addition, it is possible to reliably provide high-temperature austenitic stainless steel. For this reason, the great effect is acquired with respect to acceleration | stimulation of high temperature / high pressure, such as a power generation boiler in recent years.

Claims (5)

  In mass%, C: 0.03-0.12%, Si: 0.2-2%, Mn: 0.1-3%, Ni: more than 18% and less than 25%, Cr: more than 22% and more than 30 %, Nb: 0.1 to 1%, V: 0.01 to 1%, B: more than 0.0005% and 0.2% or less, sol. Al: not less than 0.0005% and less than 0.03%, N: 0.1 to 0.35%, O (oxygen): 0.001 to 0.008%, the balance being Fe and impurities, impurities P, S, Sn, Pb, Sb, Zn and As in each of P: 0.03% or less, S: 0.01% or less, Sn: 0.020% or less, Pb: 0.010% or less, Sb: 0.005% or less, Zn: 0.005% or less and As: 0.005% or less, and Sn (%) + Pb (%) ≦ 0.025% and Sb (%) + Zn (%) + As ( %) ≦ 0.010% A high temperature austenitic stainless steel excellent in workability after long-term use.   Instead of a part of Fe, it contains one or more selected from Mo: 5% or less, W: 5% or less, and Cu: 5% or less, alone or in total 5% or less in mass%. The high-temperature austenitic stainless steel excellent in workability after long-term use according to claim 1.   It replaces with a part of Fe and contains 1 type or 2 types of Co: 0.8% or less and Ti: 0.1% or less in the mass%, The Claim 1 or 2 characterized by the above-mentioned. High-temperature austenitic stainless steel with excellent workability after long-term use.   Instead of a part of Fe, by mass%, Mg: 0.02% or less, Ta: 0.2% or less, Zr: 0.2% or less, Ca: 0.05% or less, REM: 0.2% The processing after long-term use according to any one of claims 1 to 3, wherein one or more of Pd: 0.2% or less and Hf: 0.2% or less are included. High-temperature austenitic stainless steel with excellent properties.   It has the chemical composition according to any one of claims 1 to 4 and has a creep rupture elongation exceeding 10000h at 750 ° C of 15% or more, and is excellent in workability after long-term use. Austenitic stainless steel.