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
  • C22C38/002—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
  • C22C38/18—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
  • C22C38/18—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
  • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
• • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys

Definitions

• the present invention relates to a heat resistant high Cr ferritic steel, more particularly, a high Cr ferritic steel having excellent high temperature long term creep strength and toughness and being suitable for uses such as a heat resistant steel tube and pipe, a pressure vessel steel plate or a turbine steel member used under high temperature and high pressure conditions in a power boiler, in a nuclear power plant and in a chemical plant.
• a heat resistant steel used under high temperature and high pressure condition such as those in a power boiler, a nuclear power plant and a chemical plant, is generally required to be excellent in high temperature strength, corrosion resistant property, oxidation resistant property and toughness.
• An austenitic stainless steel such as JIS-SUS321H steel (Equivalent to AISI-TP321H), JIS-SUS347H steel (Equivalent to AISI-TP347H), a low alloy steel such as 2.1/4Cr-1Mo steel and a high Cr ferritic steel such as 9-12 Cr steel have been used in the past for these applications.
• a high Cr ferritic steel is superior to a low alloy steel in high temperature strength and corrosion resistant property at a temperature of 500 to 650°C.
• a high Cr ferritic steel has many advantages such that it is less expensive, high in thermal conductivity and excellent in thermal fatigue property because of low thermal expansion coefficient and also it does not cause stress corrosion cracking.
• a steam condition of a power boiler has recently became higher and higher in temperature and pressure in order to improve a thermal efficiency further in a thermal power plant.
• An example of such steam condition is 538°C in temperature and 246atm in pressure, which is so-called a super critical condition.
• a steam condition such as 625°C or higher in temperature and 300atm or higher in pressure, so-called a ultra super critical condition, has been studied for a future operation. According to these changes in the steam condition, the properties required for a steel tube and pipe used in power boiler become severer.
• a ordinary high Cr ferritic steel therefore, has faced the situation that it can not satisfy fully the requirement of a long term creep strength at a high temperature required for a higher steam condition.
• An austenitic stainless steel has the properties which can satisfy the above-mentioned servere condition but it is expensive. Accordingly, many studies on an improvement of the properties of a high Cr ferritic steel has been carried out in order to utilize a high Cr ferritic steel which is less expensive compared with an austenitic stainless steel.
• the object of this invention is to provide a heat resistant high Cr ferritic steel which is excellent in long term creep strength at a high temperature and toughness and can be used under the condition of high temperature and high pressure such as 625°C or more and 300atm or more.
• the gist of the present invention is a heat resistant high Cr ferritic steel as described below.
• a heat resistant high Cr ferritic steel having excellent high temperature long term creep strength and toughness which comprises, by mass %, C : 0.02 to 0.15% Mn: 0.05 to 1.5% P : 0.03% or less S : 0.015% or less Cr: 8 to 13% W : 1.5 to 4% Co: 2 to 6% V : 0.1 to 0.5% Ta: 0.01 to 0.15% Nb: 0.01 to 0.15% Nd: 0.001 to 0.2% N : less than 0.02% B : 0.0005 to 0.02% Al: 0.001 to 0.05% Mo: 0 to 1% Si: 0 to 1% Ca: 0 to 0.02% La: 0 to 0.2% Ce: 0 to 0.2% Y : 0 to 0.2% Hf: 0 to 0.2%, the contents of Nd and N satisfying the following formula, and the balance being Fe and incidental impurities.
• the inventors have studied in detail the effect of various alloy elements on high temperature long term creep strength and toughness of a heat resistant high Cr ferritic steel containing Nd, which was previously developed by the inventors, for its further improvement. As a result, the inventors have obtained the following findings and accomplished the present invention.
• C forms MC type carbides (M represents an alloying element), such as M 7 C 3 and M 23 C 6 type carbides. These carbides contribute to increase creep strength and also C itself stabilizes a metallurgical structure by serving as an austenite-stabilizing element.
• M represents an alloying element
• M 7 C 3 and M 23 C 6 type carbides contribute to increase creep strength and also C itself stabilizes a metallurgical structure by serving as an austenite-stabilizing element.
• the C content is less than 0.02%, the precipitation of carbides is insufficient and the amount of ⁇ -ferrite increases, which results in the failure to obtain the satisfactory creep strength and toughness.
• the upper limit of the C content is set to 0.15%.
• the C content is preferably set to 0.05 to 0.13%.
• Mn is an element effective in serving as a deoxidizer and fixing S, and also serves as an austenite-stabilizing element.
• the Mn content must be 0.05% or more.
• the Mn content exceeding 1.5% deteriorates tougness of the steel. Therefore the Mn content is set to 0.05 to 1.5%, preferably 0.05 to 0.7%.
• P and S which are impurity elements, are detrimental to hot workability, weldability and toughness. Therefore, it is preferable that the P and S contents are as low as possible but the P content of 0.03% or less and the S content of 0.015% or less do not directly affect the properties of the steel of this invention. Accordingly, the upper limits of P and S contents are set to 0.03% and 0.015%, respectively.
• Cr is an essential element for the steel of this invention to ensure corrosion resistance and oxidation resistance, particularly steam oxidation resistance, at a high temperature. Besides, Cr forms carbides and thereby increases creep strength. Cr also forms tight oxide-film to thereby improve corrosion resistance and oxidation resistance. In order to obtain these effects, the Cr content must be 8% or more. However, an excessive Cr content promotes the formation of ⁇ -ferrite to thereby deteriorate toughness of the steel. Therefore, the upper limit of Cr content is set to 13%. The Cr content is preferably set to 9 to 12%.
• W is one of the main elements serving as a strengthening element of the steel of this invention.
• W precipitates finely and dispersively in a form of inter-metallic compounds such as the Fe 7 W 6 type ⁇ -phase and the Fe 2 W type Laves phase during high temperature service, thereby contributing to the improvement of long term creep strength.
• W is also dissolved partially into Cr-carbides and prevents the carbides from gathering and coarsening, thereby contributing to maintaining strength of the steel.
• an excessive amount of W promotes the formation of ⁇ -ferrite. Therefore, the W content is set to 1.5 to 4%, preferably 2 to 3.5%.
• Co is an austenite-stabilizing element and essential for the steel of this invention into which W is intentionally added. Co does not impair creep strength and rather increase creep strength. In this respect, Co differs from Ni which is also an austenite-stabilizing element. In order to attain these effects, Co must be added in an amount of 2% or more but an excessive Co content exceeding 6% lowers remarkably Ac 1 transformation temperature of the steel and impairs creep strength of the steel. Therefore, the Co content is preferably set to 2 to 4%.
• V is an important element for the steel of this invention in order to form the fine carbonitrides, thereby contributing to the improvement of creep strength.
• the V content In order to obtain this effect, the V content must be 0.1% or more, but the effect saturates when the V content is more than 0.5%. Therefore, the V content is set to 0.1 to 0.5%, preferably 0.15 to 0.35%.
• each of the contents of Ta and Nb must be 0.01% or more, respectively, but the effect saturates when each content is more than 0.15%. Therefore, each of the contents of Ta and Nb is set to 0.01 to 0.15%, preferably 0.01 to 0.1%.
• Nd is effective to precipitate the carbides such as NdC 2 finely and stably even after used for a long term at a high temperature, thereby contributing greatly to prevent the recovery and softening of a martensite structure and to increase creep strength.
• the Nd content must be 0.001% or more but an excessive Nd content exceeding 0.2% deteriorates toughness of the steel. Therefore, the Nd content is set to 0.001 to 0.2%, preferably 0.005 to 0.15%.
• N serves as an austenite-stabilizing element.
• a coarse NdN remains as an inclusion in the steel, thereby impairing the improvement effect of creep strength by Nd and also toughness of the steel. Therefore, in order to secure the effect of Nd fully, the upper limit of the N content in the steel must be less than 0.02%.
• toughness of the steel it is preferable that the balance of the Nd content and the N content is controlled to satisfy the following formula.
• the N content is preferably not more than 0.017%. Nd(%) ⁇ 5 ⁇ N(%) + 0.1(%)
• B has an effect of precipitating the M 23 C 6 type carbide finely and dispersively when B is added in a small amount.
• B contributes to the improvement of the high temperature long term creep strength.
• B is also effective in increasing the quench-hardening property, especially for a steel product having a heavy wall whose cooling speed after a heat-treatment is slow, thereby attaining an important role in securing high temperature strength. This effect becomes remarkable when the B content is 0.0005% or more, but the B content exceeding 0.02% forms the coarse precipitations, thereby lowering toughness of the steel. Therefore, the B content is set to 0.0005 to 0.02%, preferably 0.002 to 0.01%.
• Al must be contained in an amount of 0.001% or more for serving as a deoxidizer in the molten steel.
• an excessive Al content exceeding 0.05% lowers creep strength. Therefore, the Al content is set to 0.001 to 0.05%, preferably 0.001 to 0.03%.
• Si is optionally added to serve as a deoxidizer in the molten steel.
• Si is effective to increase steam oxidation resistance at a high temperature.
• the Si content is set to 0 to 1%.
• steam oxidation resistance is particularly important, it is preferable that the Si content is 0.1% or more.
• Mo is an optional element which contributes to the improvement of creep strength by serving as a solution strengthening element.
• the Mo content exceeding 1% accelerates to precipitate an inter-metallic compound such as the Laves phase.
• the inter-metallic compound precipitates very coarsely, which thereby does not contribute to the improvement of creep strength and also lowers toughness of the steel after aging. Therefore, the Mo content is set to 0 to 1%.
• At least one element selected from Ca, La, Ce, Y and Hf is optionally added. These elements are effective in improving creep strength and also hot workability of the steel by strengthening the grain boundaries even if the amount of the addition is very little. However, excessive amount of these elements deteriorates hot workability of the steel. Therefore, the upper limit of the Ca content is set to 0.02% and each of the upper limits of La, Ce, Y and Hf is set to 0.2%, respectively.
• the steels having the chemical composition shown in Table 1 and Table 2 were melted in a vacuum induction furnace and cast into the 50Kg ingots with a diameter of 144mm.
• the steel No.1 to No.23 are the examples of the steel of this invention and the steel A to Q are the comparative examples.
• the heat resistant high Cr ferritic steel according to the present invention is excellent in high temperature long term creep strength at a high temperature of 625°C or more and in toughness at room temperature and thus can achieve superior performance as a material for a heat resistant steel tube and pipe, a pressure vessel steel plate and a turbine steel member used in a field of a nuclear power plant and a chemical plant. Therefore, this steel has a great advantage for an industrial application.

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• Engineering & Computer Science (AREA)
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• Organic Chemistry (AREA)
• Heat Treatment Of Steel (AREA)

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• 1998
  • 1998-07-08 JP JP19308498A patent/JP3982069B2/ja not_active Expired - Lifetime
• 1999
  • 1999-06-16 EP EP99925355A patent/EP1103626B1/fr not_active Expired - Lifetime
  • 1999-06-16 WO PCT/JP1999/003231 patent/WO2000003050A1/fr active IP Right Grant
  • 1999-06-16 DE DE69904336T patent/DE69904336T2/de not_active Expired - Lifetime
• 2001
  • 2001-01-05 US US09/754,050 patent/US20020020473A1/en not_active Abandoned

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