Title of Invention

LOW ALLOY STEEL

Abstract

A low alloy steel for a heat-resistant structure member being improved in high-temperature long-time creep ductility and temper softening resistance, characterized in that it comprises 0.03 to 0.10 mass% of C, 0.30 mass% or less of Si, 1.0 mass% or less of Mn, more than 1.5 mass% and not more than 2.5 mass% of Cr, 0.01 to 1.0 mass% of Mo, 0.04 to 0.30 mass% of V, 0.001 to 0.10 mass% of Nb, 0.001 to 0.020 mass% of Ti, 0.0001 to 0.020 mass% of B, 0.001 to 0.01 mass% of Al, 0.0001 to 0.05 mass% of Nd and the balance comprising Fe and impurities, the impurities include 0.020 mass% or less of P, 0.03 mass% or less of S, less than 0.0050 mass% of N and 0.0050 mass% or less of O(oxygen), and a value of BSO represented by the following formula (1) is 0.0001 to 0.010; BSO = B - (11/14)N - (11/32)S - (11/16)O ---- (1) where the symbol of an element in the formula (1) represents the content (mass %) of the element. The steel may further contain one or more of W, Cu, Ni, Co, Mg, Ca, La, Ce, Y, Sm and Pr.

Full Text

LOW ALLOY STEEL TECHNICAL FIELD [0001] The present invention relates to a low alloy steel excellent in high temperature creep characteristics and toughness. The low alloy steel of the present invention is suitable for heat-resisting structural members such as electric power plant boilers, turbines, nuclear power plant facilities, chemical industry facilities and other facihties or apparatus, which are used at high temperatiures, BACKGROUND ART [0002] Electric power plant boilers, turbines, nuclear power plant facilities, chemical industry facilities and the lil:e are used at high temperature and high pressure conditions for a long time. Therefore, the heat-resisting materials to be used in such facihties and the like are generally required to be excellent in strength, corrosion resistance and oxidation resistance at high temperatures as well as toughness and the like at room temperature. In those fields of application, austenitic stainless steels (for example, JIS SUS321H and SUS347H steels), low aUoy steels (for example, JIS STBA24 steel, namely 2.25Cr-lMo steel) and further, 9-12Cr t3Tpe high-Cr ferritic steels (for example, JIS STBA26 steel, namely 9Cr-lMo steel, and JIS STBA28 steel, namely improved 9Cr-lMo steel) have been used in the past. [0003] Recently, in the thermal power plants, attempts have been made to reduce the discharge of CO2 and so forth for the prevention of global warming. Therefore, it is essential to improve the thermal efficiency and, regarding the boilers, new tj^e plants have been built to be operated under high temperature and high pressure steam conditions (for example, 300 atmospheres at temperatures over 600'C). On the other hand, a number of existing plants which were built during the period of rapid economic growth, are each nearing the end of their scheduled life span one after another and it is becoming a great social problem whether they should be replaced with new advanced plants or their Hves shoxdd be prolonged by partial mending. This is also a problem which involves the energy pohcy of Japan. [0004] On the other hand, advanced Hberahzation in the electric power business field in compliance with the request for deregulation inside and outside the country, has now made it possible for an enterprise outside the electric power industry to go into that field. This has resulted in increased price competition and therefore economical efficiency in electric power plants has become more important as well. [0005] Therefore due to a need to reduce the cost of new power plants, a trend is growing toward improving the strength of the heat-resisting structiural materials used in the power plants and also reduce the steel consumption in order to meet this requirement, new high strength materials are under development. [0006] In the relatively low temperature region up to about 500°C, Cr-Mo type low alloy steels such as JIS STBA22 steel (lCr0.5Mo steel), STBA23 steel (l.25Cr"0.5Mo steel) and the above-mentioned STBA24 steel (2.25Cr-lMo steel) and the hke, have so far been used. Also a steel containing W in substitution for a part of Mo in order to increase the high temperature strength more is disclosed in the Patent Document 1. Further, a steel improved in hardenabihty by adding Co is also disclosed in the Patent Document 2. [0007] In such new steels as referred to above, the high temperature softening resistance is improved by W or Co, and especially the creep strength at 500°C or above is markedly improved as compared with the conventional multipurpose steels. However, there is a problem in that increases in strength result in a deterioration in toughness and marked decreases in long time creep ductihty (that is, elongation and reduction of area). [0008] In the Patent Documents 3 and 4, as steels prevented from deterioration in toughness and also improved in reheat cracking resistance, steels resulting from the addition of a very small amount of Ti to the Cr-Mo steels, with a nitrogen content suppressed to a very low level, are disclosed. The steels are definitely improved in toughness but fail to simultaneously attain high creep strength and creep ductility. Further, in regions subjected to repeated SR treatment following welding, reheat cracking may occur and, in addition, marked decreases in creep strength may be sometimes encountered due to reheat softening. [0009] In the Patent Document 5, a low and medium Cr type heat-resisting steel characterized by a regulated density of occurrence of precipitates within a specific range of size is disclosed. This steel is high in creep strength but the composition design is not always made in consideration of long time creep ductility or reheat softening resistance characteristics. [0010] In the Patent Document 6, a low alloy steel with a Cr content of 0.40 to 1.50% is disclosed. However, the Cr content is too low, therefore the resistance to high temperature corrosion in the temperature range above 500^C is not always sufficient and the temperature range for its use is restricted. Patent Document 1- Japanese Laid-open Patent Publication No. 08-134584, Patent Document % Japanese Laid-open Patent Publication No. 09-268343, Patent Document 3^ Japanese Laid-open Patent Publication No. 08-144010, Patent Document 4: Japanese Laid-open Patent Publication No. 2001-234276 Patent Document 5- Japanese Laid-open Patent Publication No, 2001-342549 Patent Document 6* Japanese Laid-open Patent Publication No. 2004-107719 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0011] The objective of the present invention is to provide a steel which is markedly improved in long time creep ductility at high temperatures and in temper softening resistance and is capable of being used even in a temperature range up to about 550°C by improving low alloy steels for heat-resisting structural members which have so far been in a temperature range of up to about 500°C in electric power plants and so forth. MEANS FOR SOLVING THE PROBLEM [0012] The present inventors made detailed investigations concerning the effects of the chemical composition of each material and the metallurgical structure (that is, micro structure) on the creep deformation properties, namely creep strength, creep ductility and reheat softening characteristics, in various heat-resisting low alloy steels. As a result, the following new findings were obtained. [0013] (a) When, in the Cr-Mo steels, a part of Mo is substituted with W, the carbides become stable for a longer time and the creep strength increases but, contrariwise, the toughness and creep ductihty markedly decrease. For example the said carbides occurred in the form of M3C, M7C3, M23C6 and MeC. M mainly comprises Fe and Cr and some quantities of Mo, W and the like are dissolved in the said M, With the increase in the amount of Cr, M3C changes into M7C3, M23C6 and MeC. [0014] (b) When Co is added to the Cr-Mo steels, the hardenability is markedly improved with the increase of the amount of Co. However, the addition of Co in large amounts, like W, leads to increases in susceptibility to creep embrittlement. [0015] (c) On the other hand, when V, Nb and the like are added to the Cr-Mo steels, MC carbides (M comprising mainly V and Nb, and a part of Mo is dissolved in the said M) precipitate out finely and dispersedly and a more significant precipitation hardening effect is obtained compared with the single addition of Mo; the high temperature creep strength is thus improved. However, the susceptibility to creep embrittlement also increases markedly. [0016] (d) The addition of B to the Cr-Mo steels is effective in increasing the hardenability, and so the strength and toughness are improved. Although such a phenomenon is already known, the investigations made by the present inventors revealed that when excessive B is added, the toughness markedly decreases. [0017] (e) Further research works made by the present inventors revealed that the creep ductihty and reheat softening resistance are both markedly improved when the contents of B, N, S and 0 (oxygen) are respectively optimized and the value of BSO represented by the formula (l) given below is adjusted to 0.0001 to 0.010. It was also revealed that it becomes possible to add W and B, for instance, in large amounts. [0018] The present invention has been accompKshed on the basis of the above-mentioned findings. The gists of the present invention are the following low alloy steels. [0019] (1) A low alloy steel, which comprises by mass percent, C- 0.03 to 0.10%, Si: not more than 0.30%, Mn: not more than 1.0%, Cr: more than 1.5% to not more than 2.5%, Mo: 0.01 to 1.0%, V: 0.04 to 0.30%, Nb: 0.001 to 0.10%, Ti: 0.001 to 0.020%, B: 0.0001 to 0.020%, Al: 0.001 to 0.01% and Nd: 0.0001 to 0.050%, with the balance being Fe and impurities, wherein the content of P is not more than 0.020%, the content of S is not more than 0.003%, the content of N is less than 0.0050% and the content of 0 (oxygen) is not more than 0.0050% among the impurities, in which the value of BSO represented by the following formula (l) is 0.0001 to O.OIO: BSO = B - (11/14)N - (11/32)S - (11/16)0 (l), wherein each element symbol in the formula (l) represents the content (by mass %) of the element concerned. [0020] (2) Alow alloy steel according to the above (l), which further contains W: not more than 2.0% by mass in heu of part of Fe. [0021] (3) A low alloy steel according to the above (l) or (2), which further contains one or more elements selected from among Cu, Ni and Co each at a level not more than 0.50% by mass in lieu of part of Fe. [0022] (4) Alow alloy steel according to any one of the above (l) to (3), which further contains one or more elements selected from among Mg^ not more than 0.005% by mass, Ca^ not more than 0.005% by mass. La- not more than 0.02% by mass, Ce^ not more than 0.02% by mass, Y- not more than 0.05% by mass, Sm^ not more than 0.05% by mass and Pr: not more than 0.05% by mass. [BEST MODE FOR CARRYING OUT THE INVENTION [0023] The working-effect of the each component constituting the low alloy steel of the present invention and the reasons for restricting the contents thereof will next be explained. In the following description, the symbol "%" for the content of each component represents "% by mass", [0024] C: 0.03 to 0.10% C serves as an austenite-stabilizing element and stabilizes the bainite phase (lower bainite phase) or martensite phase, which is the basic parent phase of the Cr-Mo steels. It also forms various carbides and contributes toward increasing strength. If the content of C is less than 0,03%, however, the extent of carbide precipitation is small; hence a sufficient level of strength cannot be obtained. On the other hand, if the content of C exceeds 0,10%, the steel is markedly hardened and the weldability and workability are deteriorated. A more preferable lower and the upper limit of C are 0.04% and 0.08%, respectively. [0025] Si: not more than 0.30% Si is used as a deoxidizer in the steelmaking process and inevitably remains in the steel. Conventionally, Si is positively added as an element necessary for securing the oxidation resistance in the steels used for heat-resisting structural members. However, according to the study made by the present inventors, it was revealed that reductions in the amount of Si contained as an impurity can produce the effects of reducing not only the creep embrittlement but also the reheat embrittlement and the reheat cracking susceptibility. When the content of Si is suppressed to 0.30% or less, the effects become significant. Even when the content of Si is suppressed to 0.30% or below, the Cr captures oxygen and therefore causes no harmful effect on the oxidation resistance. From the reasons mentioned above, the content of Si is set to not more than 0.30%. A more preferable content of Si is not more than 0.15%. [0026] Mn^ not more than 1.0% Like C, Mn is an austenite-stabilizing element and important for the stabiUzation of the bainite phase. However, higher levels of the addition of Mn cause a lower Aci transformation point of the steel and further, cause reheat embrittlement. Therefore, the content of Mn is set to not more than 1.0%. If the content of Mn is not more than 0.30%, the creep ductility is further improved. The lower limit content of Mn may be an ordinary impiurity level. [0027] Cr: more than 1.5% to not more than 2.5% Cr is essential for the stabUization of the low carbon type bainitic parent phase. In order to obtain the said effect, the content of Cr is set to more than 1.5%. A more preferable lower limit content of Cr is more than 1.6%. On the other hand, if the content of Cr exceeds 2.5%, the precipitation of M7C3 and M23C6 type carbides increases remarkably, and it causes a decrease in creep strength. [0028] Mo: 0,01 to 1.0% Mo is an element which produces solid solution hardening and contributes to the stabilization of M3C, M7C3 and M23C6 type carbides and further, it forms M02C and, in addition, contributes to the stabilization of MC type carbides and improves the creep strength. In order to obtain the said effects, the content of Mo is set to not less than 0.01%. However, if there is an excessive addition of Mo, the bainitic or martensitic parent phase becomes unstable, and therefore, the upper limit content of Mo is set to 1.0%. [0029] V: 0.04 to 0.30% V, together with Nb which will be mentioned later herein, forms MC type carbides and remarkably contributes to improvement in creep strength. In order to obtain the said effect, the content of not less than 0.04% of V is needed. Since, at excessive additional levels, it markedly reduces the long time creep ductility, the upper Hmit content of V is set to 0.30%. [0030] Nb: 0.001 to 0.10% Like V, Nb forms fine carbides which contribute toward increasing the creep strength. In order to obtain the said effect, the content of not less than 0.001% of Nb is needed. However, if the content of Nb exceeds 0.10%, the toughness deteriorates due to the excessive formation of carbonitrides. A more preferable lower and the upper limit of Nb are 0.020% and 0.060%, respectively. [0031] Ti: 0.001 to 0.020% Ti forms fine carbides and contributes toward increasing the strength. Therefore, the content of not less than 0.001% of Ti is needed. In particular, it is effective in improving the creep ductility and in preventing embrittlement and cracking dturing reheating, so that the content of not less than 0.010% of Ti is more preferable. Excessive addition, however, adversely affects the toughness; hence the upper limit content of Ti is set to 0.020%. B: 0.0001 to 0.020% B is effective in increasing the hardenabihty. The said effect is obtained if the content of B is not less than 0.0001%. On the other hand, at excessive additional levels, it adversely affects the toughness and therefore, the upper limit content of B should be set to 0.020%. It is noted that the upper limit content of B is preferably 0.015% and more preferably 0,012%. It is necessary that the content of B is set so that the value of BSO represented by the formula (l) given above may fall within the range of 0.0001 to 0.010. [0033] Nd: 0.0001 to 0.050% Nd is an element which improves long time creep ductility. In order to obtain the said effect, the content of not less than 0.0001% of Nd is needed. Excessive Nd, however, forms coarse inclusions unfavorable to the toughness and therefore, the upper limit content of Nd is set to 0.050%. A more preferable content of Nd is more than 0.010% and not more than 0.050%. [0034] Al: 0.001 to 0.01% Al is an element which is important as a steel deoxidizer for steels. In order to obtain the deoxidizing effect, the content of not less than 0.001% of Al is needed. On the other hand, the content of Al levels exceeding 0.01% is unfavorable to simultaneously securing both the strength and toughness which is an aim of the present invention. [0035] One of low alloy steels according to the present invention comprises the components mentioned above with the balance being Fe and impurities. It is necessary, however, to suppress the contents of P, S, N and 0 (oxygen) among the impurities in the following manner. P; not more than 0.020%, S^ not more than 0.003%, O^ not more than 0.0050% These elements are unfavorable impurities which deteriorate the toughness of the steel. The contents thereof should be not more than the respective upper limits given above and are preferably as low as possible. [0037] N: less than 0.0050% N is an element which produces solid solution hardening and forms carbonitrides therefore sometimes contributes to the high temperature strength. However, in the practice of the present invention, the content of N is suppressed to less than 0.0050% in order to obtain both the creep strength and toughness, and also to obtain the improved creep ductility. Further, it is necessary that the content of N is adjusted so that the value of BSO represented by the formula (l) given above may fall within the range of 0.0001 to 0.010. [0038] BSO: 0.0001 to 0.010 As mentioned hereinabove, the BSO is expressed in terms of the formula (l) given below. In order to improve the creep ductility and the reheat softening resistance, it is necessary that the value of BSO is 0.0001 to O.OIO: BSO = B - (11/14)N - (11/32)S - (11/16)0 (l). [0039] The technical meaning of the BSO is to secxu-e an amount of B which is effective in preventing the carbonitrides from becoming coarse and also effective in preventing grain boundary embrittlement when the steel of the present invention is used at high temperatures. If the value of BSO is smaller than 0.0001, no effective amount of B is secured. And when it is greater than 0.010, coarse inclusions which are harmful to the toughness are formed. Therefore, the value of BSO is set to 0.0001 to 0.010. A more preferable lower hmit value of BSO is 0.001. [0040] The low alloy steel according to the present invention also contains the components mentioned above and further one or more components selected fi^om among W, Cu, Ni, Co, Mg, Ca, La, Ce, Y, Sm and Pr. The working-effects of these components and the reasons for restricting the contents thereof are described below. [0041] W: not more than 2.0% W is added when a further improvement in the long time creep strength at high temperatures is desired. As mentioned hereinabove, high addition levels of W have so far been regarded as causing reheat embrittlement and also increase cracking susceptibility. However, in the steels having a value of BSO within the range of 0.0001 to 0.010, the content of 2.0% or less of W does not produce these above harmful effects. W also contributes to improvement in creep ductility. In order to definitely obtain these effects, a content of W of not less than 0.20% is preferable. [0042] Cu, Ni, Co: each not more than 0.50% All of these elements are austenite-stabilizing elements and contribute to the stabilization of the bainite phase or martensite phase. In order to definitely obtain this effect, the content of each element is preferably not less than 0.01%. However, if the content of each element is above 0.50%, the steel sometimes becomes too high in strength, making it necessary for example to carry out excessive softening heat treatment and the like. Therefore, when these components are added, the content of each should be suppressed to not more than 0.50%. Mg, Ca: each not more than 0.005%, La, Ce: each not more than 0.02%, Y, Sm, Pr: each not more than 0.05% All of these elements have an effect of preventing solidification cracking dinring steel casting, and therefore they are added according to necessity. At levels exceeding the respective upper limit values described above, they adversely affect the toughness. In order to secure the effect of their addition, the content of each is preferably not less than 0.0001%. [0044] The steel of the present invention, after processing into pipes, plates and so forth, is subjected to "normahzing-tempering" heat treatment and then used. The microstructure after the said heat treatment is mainly composed of tempered bainite or tempered martensite, EXAMPLES [0045] In the following, the effects of the present invention are explained concretely in reference to examples. Steels having chemical compositions shown in Table 1 were melted by use of a 150 kg vacuum induction melting furnace, followed by ingot making and hot forging, to give plate-like test materials, 25 mm in thickness X 100 mm in width. Then, they were subjected to normalizing treatment, namely they were maintained at 950X for 1 hour and water cooled, and then reheated and maintained at a tempering temperature varying within the range of 720 to 760°C for 1 hour and then air cooled. The microstructure of each steel was tempered bainite or tempered martensite. [0046] A portion of the said test materials after the above heat treatment, was further reheated at 730°C for 10 hours in order to examine the changes in hardness and for reheat softening resistance evaluation. Creep rupture test specimens, 6 mm in diameter and 30 mm in GL and Charpy test specimens 10 mm x 10 mm x 5 mm in size, having a 2 mm V notch were cut out from each test material obtained. The creep rupture test was carried out under the condition of a temperature of 550^C and an apphed stress of 200 MPa, and the Charpy impact test was carried out within a temperature range of-60^C to 60°C. The results of these tests are shown in Table 2. [0048] In Table 2, in the column "toughness", the mark "oo" indicates that the vTrs was lower than -40°C; the mark "o" indicates that the vTrs was within the range of-40°C to "20°C; the mark "A" indicates that the vTrs was within the range exceeding -20°C and not higher than 0°C; and the mark "x" indicates that the vTrs was higher than O^'C. In the column "reheat softening resistance", the mark "o" indicates that the decrease in Vickers hardness (Hv) after the reheating mentioned above (that is, 10 hours of heating at TSO^'C) was smaller than 20% and the mark "x" indicates 20% or larger. [0049] [Table 1] As shown in Table 2, all of the inventive steels Nos. 1 to 22 showed a creep rupture time exceeding 10000 hours and were superior to the conventional steels (Nos. 51 to 54) in this respect. As regards the toughness as well, the vTrs was not higher than -20'*C and was very excellent. [0052] On the other hand, the comparative steels Nos. 30 to 37 had a r composition outside the range specified in accordance with the present invention or had a value of BSO represented by the formula (l) outside the range of 0.0001 to 0.010. These were inferior in the reduction of area in the said creep rupture test and reheat softening resistance to the inventive steels and further, they were unsatisfactory in the toughness as well. INDUSTRIAL APPLICABILITY [0053] The steel of the present invention is a low alloy steel usable in a high temperature range up to about 550°C and excellent in long time creep ductility, reheat softening resistance and toughness. This steel is useful as a structural material for high temperature, high pressure operation-aimed electric power plants and the like. WHAT IS CLAIMED IS: 1. A low alloy steel, which comprises by mass percent, C: 0.03 to 0.10%, Si: not more than 0.30%, Mn: not more than 1.0%, Cr: more than 1.5% to not more than 2.5%, Mo: o.Ol to 1.0%, V: 0.04 to 0.30%, Nb: 0.001 to 0.10%, Ti: 0.001 to 0.020%, B: 0.0001 to 0.020%, Al: 0.001 to 0.01% and Nd: 0.0001 to 0.050%, with the balance being Fe and impurities, wherein the content of P is not more than 0.020%, the content of S is not more than 0.003%, the content of N is less than 0.0050% and the content of O (oxygen) is not more than 0.0050% among the impurities, in which the value of BSO represented by the following formula (l) is 0.0001 to O.OIO: BSO = B " (11/14)N - (11/32)S - (11/16)0 (l), wherein each element symbol in the formula (l) represents the content (by mass %) of the element concerned. 2. A low alloy steel according to claim 1, which further contains W: not more than 2.0% by mass in lieu of part of Fe. 3. A low alloy steel according to claim 1 or 2, which further contains one or more elements selected from among Cu, Ni and Co each at a level not more than 0.50% by mass in Ueu of part of Fe. 4. A low alloy steel according to any one of claims 1 to 3, which further contains one or more elements selected from among Mg: not more than 0.005% by mass, Ca: not more than 0.005% by mass, La: not more than 0.02% by mass, Ce: not more than 0.02% by mass, Y not more than 0.05% by mass, Sm: not more than 0.05% by mass and Pr: not more than 0.05% by mass.

Documents:

5199-CHENP-2007 AMENDED PAGES OF SPECIFICATION 09-02-2012.pdf

5199-CHENP-2007 AMENDED PAGES OF SPECIFICATION 18-10-2012.pdf

5199-CHENP-2007 AMENDED CLAIMS 09-02-2012.pdf

5199-CHENP-2007 AMENDED CLAIMS 18-10-2012.pdf

5199-CHENP-2007 EXAMINATION REPORT REPLY RECEIVED 09-02-2012.pdf

5199-CHENP-2007 FORM-3 09-02-2012.pdf

5199-CHENP-2007 FORM-3 18-10-2012.pdf

5199-CHENP-2007 POWER OF ATTORNEY 09-02-2012.pdf

5199-CHENP-2007 CORRESPONDENCE OTHERS 29-11-2011.pdf

5199-CHENP-2007 EXAMINATION REPORT REPLY RECEIVED 18-10-2012.pdf

5199-chenp-2007-abstract.pdf

5199-chenp-2007-claims.pdf

5199-chenp-2007-correspondnece-others.pdf

5199-chenp-2007-description(complete).pdf

5199-chenp-2007-form 1.pdf

5199-chenp-2007-form 18.pdf

5199-chenp-2007-form 3.pdf

5199-chenp-2007-form 5.pdf

5199-chenp-2007-pct.pdf