Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron

Definitions

• 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 facilities or apparatus, which are used at high temperatures.
• Electric power plant boilers, turbines, nuclear power plant facilities, chemical industry facilities and the like are used at high temperature and high pressure conditions for a long time. Therefore, the heat-resisting materials to be used in such facilities 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.
• austenitic stainless steels for example, JIS SUS321H and SUS347H steels
• low alloy steels for example, JIS STBA24 steel, namely 2.25Cr-1Mo steel
• 9-12Cr type high-Cr ferritic steels for example, JIS STBA26 steel, namely 9Cr-1Mo steel, and JIS STBA28 steel, namely improved 9Cr-1Mo steel
• Cr—Mo type low alloy steels such as JIS STBA22 steel (1Cr-0.5Mo steel), STBA23 steel (1.25Cr-0.5Mo steel) and the above-mentioned STBA24 steel (2.25Cr-1Mo steel) and the like, have so far been used.
• 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.
• a steel improved in hardenability by adding Co is also disclosed in the Patent Document 2.
• 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.
• W or Co the high temperature softening resistance
• the creep strength at 500° C. or above is markedly improved as compared with the conventional multipurpose steels.
• increases in strength result in a deterioration in toughness and marked decreases in long time creep ductility (that is, elongation and reduction of area).
• 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.
• Patent Document 6 a low alloy steel with a Cr content of 0.40 to 1.50% is disclosed.
• 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 2 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
• 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.
• the present inventors made detailed investigations concerning the effects of the chemical composition of each material and the metallurgical structure (that is, microstructure) 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.
• the present invention has been accomplished on the basis of the above-mentioned findings.
• the gists of the present invention are the following low alloy steels.
• a low alloy steel according to any one of the above (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.
• 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.
• Si is used as a deoxidizer in the steelmaking process and inevitably remains in the steel.
• Si is positively added as an element necessary for securing the oxidation resistance in the steels used for heat-resisting structural members.
• 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.
• 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%.
• Mn is an austenite-stabilizing element and important for the stabilization of the bainite phase.
• higher levels of the addition of Mn cause a lower Ac 1 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 impurity level.
• Cr more than 1.5% to not more than 2.5% Cr is essential for the stabilization of the low carbon type bainitic parent phase.
• the content of Cr is set to more than 1.5%.
• a more preferable lower limit content of Cr is more than 1.6%.
• the content of Cr exceeds 2.5%, the precipitation of M 7 C 3 and M 23 C 6 type carbides increases remarkably, and it causes a decrease in creep strength.
• Mo is an element which produces solid solution hardening and contributes to the stabilization of M 3 C, M 7 C 3 and M 23 C 6 type carbides and further, it forms Mo 2 C and, in addition, contributes to the stabilization of MC type carbides and improves the creep strength.
• 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%.
• V forms MC type carbides and remarkably contributes to improvement in creep strength.
• 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 limit content of V is set to 0.30%.
• Nb forms fine carbides which contribute toward increasing the creep strength.
• the content of not less than 0.001% of Nb is needed.
• 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.
• 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 during 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 is effective in increasing the hardenability. 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 (I) given above may fall within the range of 0.0001 to 0.010.
• 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%.
• Al is an element which is important as a steel deoxidizer for steels.
• the content of not less than 0.001% of Al is needed.
• 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.
• 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 O (oxygen) among the impurities in the following manner.
• N is an element which produces solid solution hardening and forms carbonitrides therefore sometimes contributes to the high temperature strength.
• 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 (I) given above may fall within the range of 0.0001 to 0.010.
• BSO is expressed in terms of the formula (I) given below.
• BSO B ⁇ (11/14)N ⁇ (11/32)S ⁇ (11/16)O (1).
• the technical meaning of the BSO is to secure 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 limit value of BSO is 0.001.
• the low alloy steel according to the present invention also contains the components mentioned above and further one or more components selected from 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.
• W is added when a further improvement in the long time creep strength at high temperatures is desired.
• high addition levels of W have so far been regarded as causing reheat embrittlement and also increase cracking susceptibility.
• 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.
• 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%.
• the content of each is preferably not less than 0.0001%.
• the steel of the present invention after processing into pipes, plates and so forth, is subjected to “normalizing-tempering” heat treatment and then used.
• the microstructure after the said heat treatment is mainly composed of tempered bainite or tempered martensite.
• 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 ⁇ 10 mm ⁇ 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 applied 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.
• the mark “ ⁇ ” indicates that the vTrs was lower than ⁇ 40° C.; the mark “ ⁇ ” indicates that the vTrs was within the range of ⁇ 40° C. to ⁇ 20° C.; the mark “ ⁇ ” 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 0° C.
• the mark “o” indicates that the decrease in Vickers hardness (Hv) after the reheating mentioned above (that is, 10 hours of heating at 730° C.) was smaller than 20% and the mark “x” indicates 20% or larger.
• the comparative steels Nos. 30 to 37 had a composition outside the range specified in accordance with the present invention or had a value of BSO represented by the formula (I) 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.
• 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.

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• 2006
  • 2006-04-17 EP EP06745409A patent/EP1873270B1/de not_active Ceased
  • 2006-04-17 KR KR1020077023015A patent/KR100915489B1/ko not_active Expired - Fee Related
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