EP0903421B1 - Ferritischer,wärmebeständiger Stahl und Verfahren zur Herstellung - Google Patents

Ferritischer,wärmebeständiger Stahl und Verfahren zur Herstellung Download PDF

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Publication number
EP0903421B1
EP0903421B1 EP98307629A EP98307629A EP0903421B1 EP 0903421 B1 EP0903421 B1 EP 0903421B1 EP 98307629 A EP98307629 A EP 98307629A EP 98307629 A EP98307629 A EP 98307629A EP 0903421 B1 EP0903421 B1 EP 0903421B1
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Prior art keywords
steel
added
ferritic heat
resistant steel
amount
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EP98307629A
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English (en)
French (fr)
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EP0903421A1 (de
Inventor
Nobuyuki Fujitsuna
Fujio Abe
Takehiko Itagaki
Masaaki Igarashi
Masakazu Muneki
Kazuhiro Kimura
Hideaki Kushima
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National Institute for Materials Science
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National Research Institute for Metals
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Priority claimed from JP25648197A external-priority patent/JPH1192880A/ja
Priority claimed from JP25647997A external-priority patent/JP3752523B2/ja
Priority claimed from JP25648097A external-priority patent/JP3752524B2/ja
Application filed by National Research Institute for Metals filed Critical National Research Institute for Metals
Priority to EP03007333A priority Critical patent/EP1329532B8/de
Priority to EP03007332A priority patent/EP1329531B8/de
Publication of EP0903421A1 publication Critical patent/EP0903421A1/de
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    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • 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
    • C22C38/04—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
    • C22C38/22—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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
    • F28F19/00—Preventing the formation of deposits or corrosion, e.g. by using filters or scrapers
    • F28F19/02—Preventing the formation of deposits or corrosion, e.g. by using filters or scrapers by using coatings, e.g. vitreous or enamel coatings
    • F28F19/06—Preventing the formation of deposits or corrosion, e.g. by using filters or scrapers by using coatings, e.g. vitreous or enamel coatings of metal
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C21D1/28—Normalising
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips

Definitions

  • the present invention relates to ferritic heat-resistant steel and to a method for producing it. More precisely, it relates to ferritic heat-resistant steel suitable for materials for apparatus that are used under high-temperature and high-pressure conditions, such as boilers, apparatus in chemical industry, etc., and to a method for producing it. Specifically, it relates to ferritic heat-resistant steel having excellent oxidation-resistance at high temperatures, especially steam oxidation-resistance which are not worsened even at high temperatures higher than 630°C, and having high creep strength which is comparable to that of ordinary steel, and relates to a method for producing it.
  • heat-resistant steel for use for high-temperature heat-resistant and pressure-resistant parts of boilers, atomic powered apparatus and other apparatus in chemical industry is required to have high-temperature strength, toughness, high-temperature erosion resistance, oxidation resistance, etc.
  • austenitic stainless steel such as JIS-SUS321H, JIS-SUS347H, etc.
  • low-alloy steel such as JIS-STBA24 (2-1/4Cr-1Mo steel), etc.
  • 9 to 12 Cr-type, high-ferrite steel such as JIS-STBA26 (9Cr-1Mo steel) have heretofore been used.
  • high-Cr ferritic steel is widely used in the art, as having various advantages. Specifically, it has higher strength and higher erosion resistance at temperatures falling between 500 and 650°C than low-alloy steel, and is more inexpensive than austenitic stainless steel. Further, as its thermal conductivity is high and its thermal expansion is small, high-Cr ferrite steel has good thermal fatigue-resistance while hardly causing scale peeling and stress erosion cracking.
  • boilers are being driven under higher temperature and higher pressure conditions for the purpose of improving the thermal efficiency therein.
  • boilers in those plants are driven under a supercritical pressure condition at 538°C and 246 atmospheres, but will be driven under an ultra-supercritical pressure condition at 650°C and 350 atmospheres in future.
  • steel for boilers is being required to have extremely high performance, and conventional high-Cr ferrite steel could no more satisfy the requirements of high oxidation resistance and long-term creep strength, especially steam oxidation-resistance. If the steam oxidation-resistance of boilers are poor, oxide films will be formed on the inner surfaces of steel pipes of boilers through which high-temperature steam passes.
  • the oxide films peel off due to thermal stress that may be caused by the temperature change in boilers, for example, when boilers being driven are stopped, by which pipes will be clogged. Therefore, the prevention of steam oxidation of steel pipes, especially the prevention of peeling of oxide films is an important theme.
  • austenitic stainless steel As one material capable of satisfying the requirements noted above, known is austenitic stainless steel.
  • austenitic stainless steel is expensive, and its use in commercial plants is limited because of the economic reasons.
  • austenitic stainless steel has a large thermal expansion coefficient, its thermal stress to be caused by the temperature change in drive stopping or the like is large.
  • the use of austenitic stainless steel in plants is problematic because of the difficulties in designing and driving the plants using it. In view of these, it is desired to improve the performance of ferritic steel which has a smaller thermal expansion coefficient and is more inexpensive.
  • JP-A Hei-3-097832 Cu-containing, high-cr heat-resistant steel has been proposed, of which the W content is higher than that of conventional steel. Cu is added to this for improving its high-temperature oxidation resistance.
  • JP-A Hei-4-371551 and Hei-4-371552 high-Cr heat-resistant steel has been proposed. In this, the ratio of Mo/W is optimized, and Co and B are both added (thereto to) thereby increase the high-temperature strength and toughness of the steel.
  • JP-A Hei-5-263195 reducing the amount of Cr to be added to steel has been proposed in JP-A Hei-5-263195, etc.; and adding a large amount of austenite-forming elements such as Ni, Cu, Co and the like to steel has been proposed in JP-A Hei-5-311342, Hei-5-311343, Hei-5-311344, Hei-5-3111345, Hei-5-311-346, etc. These are to improve the toughness of steel by the proposed techniques.
  • JP-A Hei-5-263196 could not have a sound scale structure since Mo enters the scale consisting essentially of Cr. Therefore, this has poor steam oxidation resistance.
  • JP-A Hei-8-85847 another proposal has been proposed in JP-A Hei-85847, in which no Mo or only a small amount of Mo is added to W-containing steel.
  • W is an essential element added thereto for reinforcing it.
  • this steel is still defective, like the steel disclosed in JP-A Hei-5-311342, in that it changes the structure of oxides consisting essentially of Cr 2 O 3 and that its steam oxidation resistance is poor.
  • the high-Cr ferrite steel disclosed in JP-A-5-311342 and others has a low A 1 transformation point and a low A 3 transformation point, as it contains a large amount of Ni, Cu, etc.
  • the temper softening resistance of the steel is poor, and, in addition, carbides and nitrides in the steel rapidly aggregate to form large coarse grains therein. Therefore, the long-term creep strength of the steel is low.
  • Ni, Cu and other elements added to the steel change the steel layer formed to make it have a brittle structure, like in the heat-resistant steel disclosed in JP-A Hei-5-263196, whereby the steam oxidation resistance of the steel is worsened.
  • a ferritic heat-resistant steel is disclosed with enhanced toughness, improved resistivity to creep and satisfactory weldability and formability at reduced temperature.
  • the ferritic steel has a greatly reduced carbon content with yttrium or a rare earth element, niobium, chromium, molybdenum and vanadium being present to reduce embrittlement, also silicon and manganese being present to act as deoxidising agents for enhanced resistance to oxidation of the steel.
  • the present invention has been made in consideration of the current situation noted above, and its subject matter is to provide ferritic steel which is free from the drawbacks of conventional ferritic steel.
  • the object of the invention is to provide ferritic steel, of which the steam oxidation resistance is not lowered even at high temperatures higher than 630°C, and which has excellent long-term creep strength.
  • ferritic heat-resistant steel capable of forming an oxide film on its surface during use and having good steam oxidation-resistance, which has a composition by weight comprising from 8.0 to 13.0% of Cr; at least one of from 0.06 to 0.18% of C, from 0.01 to 1.0% of Si, from 0.05 to 1.5% of Mn, from 0 to 1% of Ni, from 0 to 4.0% of W, from 0 to 2.0% of Mo, provided that W + 2Mo ⁇ 4%, from 0.10 to 0.50% of V, from 0.02 to 0.14% of Nb, from 0 to 0.1% of N, from 0 to 0.010% of B and not greater than 0.010% of O; at least one of Ti and Y in an amount of 0.01% ⁇ Ti + Y ⁇ 0.30%; and a balance of Fe and inevitable impurities, which is characterised in that ultra-fine oxide particles having a diameter of not larger than 1 micron are formed from said added
  • the invention further provides the following:
  • 1 is an outer scale layer
  • 2 is an inner scale layer
  • 3 is a steel base
  • 4 is an oxide particle
  • 5 is a void.
  • the present invention is characterized by the features mentioned hereinabove.
  • the problems with steel having poor oxidation resistance are that the oxide film formed on the inner surfaces of steel pipes peels off and deposits in the pipes to clog them, and that the peeled oxide film scatters in steel pipes and erodes the apparatus disposed in the later zone.
  • the present invention has been made, and its subject matter is, as so mentioned hereinabove, to homogeneously form ultra-fine oxide particles having a size of not larger than 1 ⁇ m in and/or around the interface between the oxide film formed on the surface of a steel base and the steel base just below the oxide film, thereby improving the adhesiveness between the oxide film and the steel base.
  • the invention provides ferritic heat-resistant steel having both good oxidation resistance and high creep strength even at high temperatures of 600°C or higher.
  • the essential reason for oxide film peeling is thermal stress to be caused by the temperature change in steel.
  • the thermal stress shall be greater with the growth of the oxide film on steel (that is, with the increase in the thickness of the film).
  • the thermal stress exceeds the adhesiveness (adhesion strength) between the film and the underlying steel base, the film peels from the steel base. Therefore, increasing the adhesiveness of the film to the steel base is effective for preventing the film peeling.
  • the film adhesiveness is generally increased by densifying the oxide film itself to produce the condition in which pores or voids are difficult to form in the interface between the film and the steel base.
  • fine particles are formed in the interface between the oxide film and the steel base, so that they act as a barrier to the film peeling propagation in the film/base interface while preventing the film from swelling up.
  • Ti or Y oxides are formed through internal oxidation in steel, while the steel is to have a scale structure composed of an outer scale layer (of Fe oxides) (1) and an inner scale layer (of Fe-Cr oxides) (2) as formed on the surface of the steel base (3), as in Fig. 1, in which fine oxide particles (4) exist around the scale/base interface.
  • Existing oxide particles having a size of not larger than 1 micron, but preferably not larger than 0.5 microns in and/or around the interface between the oxide film and the steel base prevents the film from peeling, and is effective to attain the intended purpose.
  • large particles having a size of 3 microns or larger, if existing in the interface are not effective for the intended purpose, but rather promote the film peeling.
  • the oxide film formed on ferritic heat-resistant steel is composed of an outer layer consisting essentially of Fe oxides and an inner layer consisting essentially of Cr oxides or Fe-Cr oxides. Stabilizing the sound Cr 2 O 3 film without peeling it is effective for improving the oxidation resistance of the steel.
  • Cr is one essential alloying element in the invention. Regarding its amount to be added, Cr must be added to steel in an amount of not smaller than 8.0 % in order to form a sound oxide film. However, if the amount of Cr added is larger than 13.0 %, much Cr will promote the formation of d-ferrite, whereby the properties of the steel, including the toughness thereof, are much worsened. For these reasons, the Cr content of the steel of the invention preferably falls between 8.0 and 13.0 %.
  • Ti has high affinity for oxygen. When added to steel in a small amount, Ti forms fine oxide particles just below the oxide film formed on steel. Ti easily bonds to not only oxygen but also carbon and nitrogen. Therefore, Ti added in steel alloys well bonds to those elements to form its carbides, etc. If the amount of Ti added is smaller than 0.01 %, all Ti will bond to carbon and other elements existing in steel alloys, and could no more form its oxides while the alloys are used. Therefore, it is desirable to add Ti to steel in an amount not smaller than 0.01 %. On the other hand, however, if the amount of Ti added is too large, Ti oxides formed will be in the form of coarse and large particles, and have some negative influences on steel. For these reasons, the uppermost limit of the amount of Ti to be added may be 0.3 %.
  • Ti traps oxygen. Therefore, adding Ti to steel prevents oxygen from diffusing into the inside of steel, whereby the oxidation speed in steel is greatly reduced.
  • Ti added to steel forms coarse and large carbides, nitrides and carbonitrides particles (inclusions) in the steel, whereby the amount of carbides, nitrides and carbonitrides of V and Nb that contributes to strengthen of the steel is greatly reduced, resulting in that the creep strength of the steel is lowered.
  • Ti is not added to ferritic heat-resistance steel.
  • Ti-adding steel is heated at temperatures of 1250°C or higher, the Ti carbides formed therein will be re-dissolved to form solid solution.
  • Ti-added steel is subjected to predetermined plastic working, such as forging, rolling, extrusion or the like, at temperatures falling within that range, and then immediately cooled to and kept at temperatures falling between 1000 and 1150°C, and thereafter further cooled to temperatures not higher than its martensitic transformation-finishing point, it may have a martensitic structure with no large and coarse Ti carbides. After this, the steel is tempered at temperatures falling between 650 and 800°C, whereby fine M23C6 and MC particles are precipitated in the tempered martensite phase.
  • the creep strength of the thus-worked, Ti-added steel may be the same as that of the non-worked, Ti-free basic steel.
  • the hot working is to promote the dissolution of Ti carbides in the steel, and therefore the hot working temperature is preferably higher.
  • the Ti carbides in the steel could be dissolved to form solid solution.
  • the steel is heated at temperatures not lower than 1300°C.
  • Y is an element having high affinity for oxygen, and this is effective for positively exhibiting the effect of the invention.
  • the lowermost limit of the amount of Y to be added is 0.01 %, while the uppermost limit thereof may be 0.3 % for the same reasons as those for Ti.
  • Y traps oxygen. Therefore, adding Ti to steel prevents oxygen from diffusing into the inside of steel, whereby the oxidation speed in steel is greatly reduced.
  • the total amount of the two is suitably from 0.01 to 0.3 %. If smaller than 0.01 %, they could not sufficiently exhibit the intended effect of the invention. However, if larger than 0.3 %, they will form coarse and large particles. Anyhow, the amount overstepping the range is unfavorable.
  • C is an element that forms carbides of various types, MC [as the case may be, in the form of carbonitrides, M(C,N), in which M indicates an alloying element, and the same shall apply hereunder], M 7 C 3 , M 4 C and M 23 C 6 , and this has great influences on the properties of steel.
  • fine carbide particles of VC, NbC and the like are precipitated in steel while the steel is used, and they contribute to the increase in the long-term creep strength of steel. In order that such fine carbide particles are effectively precipitated to strengthen steel, the amount of C to be in steel must not be smaller than 0.06 %.
  • the C content of steel is defined to fall between 0.06 and 0.18 %.
  • Si is an element effective for deoxidizing steel melt and for improving the high-temperature steam oxidation resistance of steel.
  • too much Si lowers the toughness of steel. Therefore, in general, the Si content of steel is defined to fall between 0.01 and 1.0 % in the prior art. Accordingly, also in the invention, the uppermost limit of the Si content is 1.0 %.
  • Mn is an element to be added to steel for the purpose of deoxidizing and desulfurizing steel melt, and this is effective for increasing the short-term creep strength of steel under high stress. In order to attain its effect, Mn must be added in an amount not smaller than 0.05 %. On the other hand, however, if larger than 1.6 %, it is known that too much Mn lowers the toughness of steel. For these reasons, it is suitable that the amount of Mn to be added falls between 0.05 and 1.5 %.
  • Mo is effective for solution strengthening of steel. In addition, it stabilizes M 23 C 6 and increases the high-temperature strength of steel. However, if its amount is larger than 2 %, Mo promotes the formation of d-ferrite, while promoting the precipitation and aggregation of M 6 C and Laves phases to give coarse and large particles. Therefore, its uppermost limit is defined to be 2 %. Like Mo, W is also suitable for solution strengthening of steel. In addition, this contributes the precipitation of fine particles of M 23 C 6 , while preventing carbides from being aggregated to give coarse and large particles. Owing to those effects, W greatly increases the high-temperature and long-term creep strength of steel.
  • V is an element that forms fine carbides, nitrides and carbonitride particles to contribute to the increase in the creep strength of steel. In order to attain its effect, V must be added to steel in an amount not less than 0.10%. However, even if added in an amount greater than 0.50%, too much V is no more effective, since the effect of V is saturated when its amount is up to 0.50 %. Therefore, it is suitable that the V content falls between 0.10 and 0.50%.
  • Nb precipitates in steel in the form of its carbides, nitrides and carbonitrides to thereby increase the high-temperature strength of steel. In addition, it acts to make the microstructure of steel fine, thereby increasing the toughness of steel. Therefore, it is said that the lowermost limit of Nb to be in steel is 0.02%. However, it is believed that, if Nb is added in an amount of 0.15% or more, it could not completely penetrate into the matrix of steel to form solid solution at normalizing temperatures, and therefore could not sufficiently exhibit its effect to strengthen steel. Accordingly, it is suitable that the Nb content falls between 0.02 and 0.14%.
  • N is an element to form nitrides and carbonitrides to thereby increase the creep strength of steel. In general, however, if the N content is greater than 0.1%, the nitrides formed grow to give coarse and large particles, which rather greatly lower the toughness of steel. Therefore, the uppermost limit of the N content is preferably 0.1 %.
  • Ni is an austenite-stabilizing element. It is known that this is effective for retarding the formation of ⁇ -ferrite and increasing the toughness of steel. However, if added in an amount greater than 1%, too much Ni lowers the creep strength of steel. Therefore, the uppermost limit of Ni is preferably 1%.
  • B It is known that B is effective for strengthening the intergranular strength of steel and for finely dispersing M 23 C 6 carbides in steel, and that this contributes to the increase in the high-temperature strength of steel and is effective for improving the quenchability of steel. It is also known that too much B greater than 0.01% forms coarse and large B-containing precipitates thereby embrittling steel. Therefore, it is suitable that the uppermost limit of B is 0.01 %.
  • Co, Rh, Ir Apart from those mentioned hereinabove, Co is known as an element effective for retarding the formation of ⁇ -ferrite. The recent studies in the prior art are toward the addition of Co to steel. However, it is known that too much Co lowers the strength of steel and even embrittles steel. In general, it is said that the uppermost limit of Co is 5 %. Like Co, Rh and Ir are both effective. Co, Rh and Ir may be added to steel in an amount of from 0.3 to 5.0 % each. Where two or more of these are added, the total amount is suitably from 0.3 to 5.0 %
  • Sol. Al added to steel essentially acts as a deoxidizer for steel melt.
  • Al added exists in the form of its oxides and in any other form. In analysis, the latter is referred to as HCI-soluble Al (sol. Al). So far as steel could be deoxidized by any other elements added thereto, sol. Al is not specifically needed. If added in an amount greater than 0.050% by weight, too much Al will lower the creep strength of steel.
  • the sol. Al content of steel is suitably from 0 to 0.050 % by weight.
  • P and S are both inevitable impurities in steel. These elements have some negative influences on the hot workability of steel, the toughness of welded parts of steel, etc. Therefore, their content is preferably as small as possible. Specifically, P shall not be greater than 0.030% by weight, and S not greater than 0.015% by weight.
  • O is also an inevitable impurity in steel. If it locally exists in steel in the form of coarse and large oxide particles, the particles have some negative influences on the toughness and other properties of steel. In order to ensure the toughness of steel, it is desirable that the O content of steel is minimized as much as possible. When the O content of not larger than 0.010 % by weight, its influence on the toughness of steel is satisfactorily small. Therefore, the O content shall not be larger than 0.010 %.
  • the subject matter of the present invention is to form fine oxide particles having a size of not larger than 1 micron just below the film formed on steel, whereby the film is prevented from peeling off owing to the bridging effect of the oxide particles.
  • the components constituting the steel of the invention are not whatsoever limited to those specifically referred to hereinabove, so far as the steel attains the object of the invention.
  • ferritic heat-resistant steel of the invention which is characterized by the matters specifically mentioned hereinabove, has been completed on the basis of the following findings that have resulted from the data of the detailed studies, which the present inventors have made relative to the relationship between the property of the steel including its high, long-term creep strength and steam oxidation resistance, and the chemical components constituting the steel and the metallic structure (microstructure) of the steel.
  • Rh and Ir and also Co are all in the same Group of the Periodic Table, and they are austenite-forming elements. It has heretofore been believed that, when existing in steel, they greatly lower the Al transformation point of steel thereby lowering the temper softening resistance of steel.
  • Rh and Ir are added to high-Cr ferritic steel containing Mo and W, the A 1 transformation point of the steel is not so much lowered.
  • Rh and Ir added to the steel do not promote the aggregation and growth of carbides, nitrides and carbonitrides into coarse and large particles.
  • Adding Rh and Ir to the steel makes the martensitic lath structure of the steel fine, while strengthening the martensite phase in the steel. This phenomenon is confirmed in ordinary heat treatment of the steel. There is found no significant difference in the degree of hardness between the high-Cr ferritic steel and conventional steel after they are quenched, but the temper softening resistance of the high-Cr ferritic steel is much higher than that of conventional steel.
  • the high-Cr ferritic steel After having been normalized and tempered, the high-Cr ferritic steel shall have a martensitic texture that contains carbides, nitrides and carbonitrides precipitated therein.
  • the martensitic structure in the steel tends to recover and soften with the lapse of time at high temperatures higher than 630°C, which could be prevented by Rh and Ir added to the steel.
  • the long-term creep strength of the steel at high temperatures higher than 630°C is greatly increased, and the steel shall have excellent long-term creep strength.
  • Rh and Ir are added to high-cr ferritic steel containing much Mo and W, they do not convert the sound, corundum-type scale layer consisting essentially of Cr 2 O 3 and formed on the steel into a spinel-type structure. Therefore, the scale layer formed on the steel is not broken, and the steam oxidation resistance of the steel is not lowered even at high temperatures higher than 630°C.
  • Rh and Ir is noticeable when at least any one of the two is added to the steel in an amount of from 0.3 to 5 % by weight, but preferably when Rh is added thereto in an amount not smaller than 0.3 % by weight and/or Ir is added in an amount not smaller than 0.6 % by weight.
  • Rh and Ir larger than 5 % by weight each, even if added to the steel, will saturate their effect without augmenting it any more.
  • the amount of Rh and Ir to be added is from 0.3 to 5.0 % by weight and that of Ir is from 0.6 to 5.0 % by weight.
  • Rh and Ir are both added to the steel.
  • the amount of the two shall be 0.3 % ⁇ Rh + (1/2)Ir ⁇ 5.0 %, in which % being by weight, in view of their ability to exhibit and saturate the effect.
  • the ferritic heat-resistant steel of the invention can be produced in any ordinary equipment and process generally employed in the prior art.
  • steel is melted in a furnace such as an electric furnace, a converter or the like, and deoxidizers and alloying elements are added thereto to control the steel composition.
  • a furnace such as an electric furnace, a converter or the like
  • deoxidizers and alloying elements are added thereto to control the steel composition.
  • the steel melt may be subjected to vacuum treatment prior to adding alloying elements thereto.
  • the steel melt having been specifically modulated to have a predetermined chemical composition is then cast into slabs, billets or ingots in a continuous casting method or a slab-making method, and which are thereafter shaped into pipes, sheets, etc.
  • seamless steel pipes are produced, for example, billets are extruded or forged into them.
  • slabs are hot-rolled into hot-rolled sheets.
  • the resulting hot-rolled sheets may be cold-rolled into cold-rolled sheets.
  • the hot-working is followed by the cold-working such as cold-rolling, it is desirable that the hot-worked sheets are annealed and washed with acids prior to being subjected to ordinary cold-working.
  • the thus-produced steel pipes and sheets may be optionally subjected to heat treatment such as annealing or the like, to thereby make them have predetermined characteristics.
  • Comparative Samples 1, 2 and 3 are samples of standard steel of ASTM T91, T92 and T122, respectively.
  • test pieces Prior to being subjected to steam oxidation tests for evaluating their steam oxidation resistance, all test pieces were pre-treated for AC normalization at 1050°C for hours followed by AC tempering at 780°C for 1 hour. In one steam oxidation test, each test piece was kept heated in a steam atmosphere at 700°C for 1000 hours, and the thickness of the scale layer formed was measured. In another heat-cycle test, each test piece was heated at the same temperature of 700°C for 96 hours, and then cooled to room temperature, and the heat cycle was repeated for a total of 10 times. After the heat-cycle test, the amount of scale peeled off was measured.
  • Sample 2 of the invention in Table 1 was forged at different temperatures falling between 1100 and 1400°C, then immediately inserted into a furnace at 1050°C and kept therein for 1 hour, and thereafter cooled with water. After this, the thus-processed samples were post-treated for AC tempering at 780°C for 1 hour. Then, these were subjected to a creep rupture test at 650°C and under 100 MPa. The data obtained are shown in Table 3.
  • Each steel melt was cast into ingots having a diameter of 70 mm, which were then hot-forged at a temperature varying from 1250°C to 1000°C into sheets having a square of 45 mm x 45 mm and a length of 400 mm. Then, these were cold-rolled at a temperature varying from 1100°C to 900°C into sheets having a square of 15 mm x 15 mm.
  • Samples Nos. 1 to 5 of the invention in Table 4 were thereafter kept at 1100°C for 1 hour and then normalized by air cooling, or were kept at 800°C for 1 hour and then tempered by air cooling.
  • Comparative Samples 1 and 2 in Table 4 were subjected ordinary post-heat-treatment. Briefly, these were kept at 950°C for 1 hour and then normalized by air cooling, or were kept at 750°C and then tempered by air cooling. Comparative Samples 1 and 2 had a chemical composition of ASTM-A213-T91 and DIN-X20CrMoWV121, respectively.
  • Test pieces were sampled out of those eight samples, and tested for the high-temperature creep strength and the steam oxidation resistance.
  • Test Piece diameter 8.0 mm gauge length 40 mm Test Temperature (1) 650°C, (2) 700°C Stress (1) 140 MPa, (2) 120 MPa Measured Matter Time before Rupture
  • test pieces were subjected to a steam oxidation test, for which the test condition is mentioned below.
  • the time for creep rupture of all Samples 1 to 5 of the invention at 650°C and under 140 MPa was longer than 3000 hours, and that at 700°C and under 120 MPa was longer than 100 hours.
  • the mean thickness of the scale layer formed in the steam oxidation test at 700°C for 1000 hours was not longer than 73 ⁇ m.
  • Comparative Samples 1 and 2 were much inferior to that of Samples 1 to 5 of the invention, as in Table 5.
  • the thickness of the scale layer formed in Comparative Sample 1 was about 2 times that of Samples 1 to 5 of the invention. This means that the steam oxidation resistance of Comparative Sample 1 is poor.
  • Each steel melt was cast into ingots having a diameter of 70 mm, which were then hot-forged at a temperature varying from 1250°C to 1000°C into sheets having a square of 45 mm x 45 mm and a length of 400 mm. Then, these were cold-rolled at a temperature varying from 1100°C to 900°C into sheets having a square of 15 mm x 15 mm.
  • Samples Nos. 1 to 6 of the invention in Table 6 were thereafter kept at 1100°C for 1 hour and then normalized by air cooling, or were kept at 800°C for 1 hour and then tempered by air cooling.
  • Comparative Samples 1 and 2 in Table 6 were subjected ordinary post-heat-treatment. Briefly, these were kept at 950°C for 1 hour and then normalized by air cooling, or were kept at 750°C and then tempered by air cooling. Comparative Samples 1 and 2 had a chemical composition of ASTM-A213-T91 and DIN-X20CrMoWV121, respectively.
  • Test pieces were sampled out of those eight samples, and tested for the high-temperature creep strength and the steam oxidation resistance.
  • thickness of the scale layer formed is less than 36 ⁇ m (625 °C ⁇ 1000h), less than 48 ⁇ m (650 °C ⁇ 1000h) and less than 57 ⁇ m (700 °C ⁇ 1000h). It was found that each steel of the samples 1 ⁇ 6 has superior steam oxidation-resistance at the high temperature of over 630°C and is extremely stable.
  • the present invention provides ferritic heat-resistant steel having excellent steam oxidation resistance and creep strength characteristics.
  • the creep strength of the steel of the invention is at least comparable to or higher than that of conventional steel.
  • the steel of the invention is useful for high-temperature heat-resistant and pressure resistant parts capable of being widely used in various industrial fields, for example, for those of boilers, atomic powered apparatus and other apparatus in chemical industry.
  • the steel may be used for pipes, sheets for pressure containers, turbines, etc.

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Claims (7)

  1. Ferritischer wärmebeständiger Stahl, der fähig ist, bei der Verwendung einen Oxidfilm auf seiner Oberfläche zu bilden, und der einen guten Dampfoxidationswiderstand hat, der eine Zusammensetzung bezogen auf das Gewicht umfassend von 8,0 bis 13,0% Cr, wenigstens eines von von 0,06 bis 0,18% C, von 0,01 bis 1,0% Si, von 0,05 bis 1,5% Mn, von 0 bis 1% Ni, von 0 bis 4,0% W, von 0 bis 2,0% Mo mit der Maßgabe, dass W + 2Mo ≤ 4% ist, von 0,10 bis 0,50% V, von 0,02 bis 0,14% Nb, von 0 bis 0,1% N, von 0 bis 0,010% B und nicht mehr als 0,010% O, wenigstens eines von Ti und Y in einer Menge von 0,01% ≤ Ti + Y ≤ 0,30% und als Rest Fe und unvermeidbare Verunreinigungen hat, dadurch gekennzeichnet, dass ultrafeine Oxidpartikel mit einem Durchmesser von nicht mehr als 1 µm aus dem hinzugefügten Ti und/oder Y in der und/oder um die Grenzfläche zwischen der Stahlbasis und dem darauf gebildeten Oxidfilm gebildet sind und dadurch das Adhäsionsvermögen zwischen dem Oxidfilm und der Stahlbasis erhöhen.
  2. Ferritischer wärmebeständiger Stahl nach Anspruch 1, der wenigstens eines von Co, Rh, Ir, Pd und Pt in einer Gesamtmenge von nicht mehr als 5,0 Gew.-% enthält.
  3. Ferritischer wärmebeständiger Stahl nach Anspruch 1, der wenigstens eines von Rh und Ir in einer Gesamtmenge von 0,3 bis 5,0 Gew.-% enthält.
  4. Ferritischer wärmebeständiger Stahl nach Anspruch 3, der bezogen auf das Gewicht wenigstens eines von Rh und Ir in einer Menge von 0,3 bis 5,0% Rh und von 0,6 bis 5,0% Ir und in einem Verhältnis von 0,3% ≤ Rh + (1/2) Ir ≤ 5,0% enthält.
  5. Ferritischer wärmebeständiger Stahl nach Anspruch 3 oder 4, bei dem eine feine Lattenstruktur gemacht ist und die Martensitphase durch wenigstens eines von Rh und Ir, das hinzugefügt ist, verstärkt ist.
  6. Ferritischer wärmebeständiger Stahl nach einem der Ansprüche 3 bis 5, der bezogen auf das Gewicht von 0,06 bis 0,18% C, von 0,01 bis 1,0% Si, von 0,05 bis 1,5% Mn, nicht mehr als 0,030% P, nicht mehr als 0,05% S, von 8,0 bis 13,0% Cr, von 0 bis 4,0% W, von 0 bis 2,0% Mo mit der Maßgabe, dass W + 2 Mo ≤ 4,0% ist, von 0,030 bis 0,14% Nb, von 0,10 bis 0,50% V, von 0 bis 0,10% N, von 0 bis 0,010% B, nicht mehr als 0,010% O und von 0 bis 0,050% gelöstes Al, wenigstens eines von Rh und Ir in einer Gesamtmenge von 0,3 bis 5,0% und als Rest Fe und unvermeidbare Verunreinigungen umfasst.
  7. Verfahren zur Herstellung eines ferritischen wärmebeständigen Stahls nach einem der Ansprüche 1 bis 6, das Erwärmen von Stahl auf eine Temperatur nicht niedriger als 1250 °C, dessen Unterziehen einer plastischen Bearbeitung, dann dessen sofortiges Halten für eine Stunde oder länger bei einer Temperatur, die zwischen 1000 und 1150 °C liegt, danach dessen schnelles Abkühlen auf eine Temperatur, die nicht höher als sein martensitischer Transformationsendpunkt ist, wobei bewirkt wird, dass er ein martensitisches Gefüge hat, und danach dessen Erwärmen und Anlassen bei einer Temperatur, die zwischen 650 und 800 °C liegt, umfasst.
EP98307629A 1997-09-22 1998-09-21 Ferritischer,wärmebeständiger Stahl und Verfahren zur Herstellung Expired - Lifetime EP0903421B1 (de)

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JP25648197A JPH1192880A (ja) 1997-09-22 1997-09-22 耐酸化性・耐水蒸気酸化特性に優れた 高Ｃｒフェライト系耐熱鋼とその製造方法
JP25647997 1997-09-22
JP256480/97 1997-09-22
JP25647997A JP3752523B2 (ja) 1997-09-22 1997-09-22 フェライト系耐熱鋼
JP25648097A JP3752524B2 (ja) 1997-09-22 1997-09-22 フェライト系耐熱鋼
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DE69827729D1 (de) 2004-12-30
US20020011285A1 (en) 2002-01-31
US20060054253A1 (en) 2006-03-16
DE69837055T2 (de) 2007-11-08
DE69827729T2 (de) 2005-04-28
DE69837055D1 (de) 2007-03-22
EP1329531A3 (de) 2003-07-30
EP1329531A2 (de) 2003-07-23
US20030127163A1 (en) 2003-07-10
EP1329531B1 (de) 2007-02-07
EP1329531B8 (de) 2007-09-19
DE69829012D1 (de) 2005-03-17
EP1329532B1 (de) 2005-02-09
EP1329532A3 (de) 2003-07-30
EP1329532A2 (de) 2003-07-23
DE69829012T2 (de) 2005-07-07
EP0903421A1 (de) 1999-03-24
EP1329532B8 (de) 2007-09-19

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