EP0835946B1 - Verwendung eines schweissbaren ferritischen Gussstahls mit niedrigem Chromgehalt und mit sehr gute Warmfestigkeit - Google Patents

Verwendung eines schweissbaren ferritischen Gussstahls mit niedrigem Chromgehalt und mit sehr gute Warmfestigkeit Download PDF

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Publication number
EP0835946B1
EP0835946B1 EP19970306432 EP97306432A EP0835946B1 EP 0835946 B1 EP0835946 B1 EP 0835946B1 EP 19970306432 EP19970306432 EP 19970306432 EP 97306432 A EP97306432 A EP 97306432A EP 0835946 B1 EP0835946 B1 EP 0835946B1
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Prior art keywords
content
steels
strength
low
temperature
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EP19970306432
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French (fr)
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EP0835946A1 (de
Inventor
Nobuyoshi Mitsubishi Heavy Ind. Ltd. Komai
Tomomitsu Mitsubishi Heavy Ind. Ltd. Yokoyama
Fujimitsu Mitsubishi Heavy Ind. Ltd. Masuyama
Kaori C/O Sumitomo Metal Ind. Ltd. Miyata
Masaaki C/O Sumitomo Metal Ind. Ltd. Igarashi
Yoshiatsu c/o Sumitomo Metal Ind. Ltd. Sawaragi
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Mitsubishi Heavy Industries Ltd
Nippon Steel Corp
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Mitsubishi Heavy Industries Ltd
Sumitomo Metal Industries Ltd
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    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

  • This invention relates to the use as cast materials of low-Cr ferritic cast steels which have excellent high-temperature strength, weldability, oxidation resistance and high-temperature corrosion resistance, especially for use in a high-temperature environment at or above 450°C, in the fields of boilers, nuclear power industry, chemical industry and the like.
  • Materials for use as heat-resistant and pressure-tight members in various types of equipment in the fields of boilers, nuclear power industry, chemical industry and the like include austenitic steels, high-Cr ferritic steels having a Cr content of 9 to 12%, low-Cr ferritic steels having a Cr content of 3.5% or less (e.g., 2 ⁇ 1/4Cr-1Mo steel), and carbon steel. These materials are suitably selected according to the service temperature, pressure and atmosphere for the particular member and with consideration for economic efficiency. Among others, high-Cr ferritic steels having a Cr content of 9 to 12% and low-Cr ferritic steels having a Cr content of 3.5% or less have been extensively investigated with respect to alloy systems containing various trace elements.
  • ferritic steels having high-temperature strength equal to or greater than that of austenitic steels have been developed.
  • most of them are intended to be used after being worked by forging, rolling or the like, and there are very few materials (such as cast steels) which are used without requiring forging and rolling.
  • the reason for this seems to be that it has been difficult to develop a material having excellent overall performance from the viewpoint of high-temperature strength, weldability, impact resistance, economic efficiency and the like.
  • cast steels As compared with forged steels, cast steels have the advantage that they can be easily formed into articles of complicated shapes without requiring a forging step and hence involve a less working cost. With the recent progress of casting techniques, the reliability of cast steels which was apprehended in the past has made a marked improvement. Accordingly, there is a need for an inexpensive cast steel having excellent high-temperature strength and weldability.
  • the existing Cr-containing ferritic cast steels have the following problems: (1) Low-Cr ferritic cast steels tend to develop a material deterioration due to the production of porosity and high-temperature cracking, especially in thick-walled members. (2) Their high-temperature creep strength at 450°C or above is low. (3) They have poor impact resistance. (4) They require preheating prior to welding.
  • EP-A-0505732, JP-A-02 217439, JP-A-02 217438 and EP-A-0560375 all describe low-chromium ferritic steels.
  • these references are silent on the inter relationship of magnesium, oxygen, sulphur and aluminium and furthermore, all require post-casting mechanical working such as by forging or rolling to provide adequate properties.
  • an object of the present invention is to provide low-Cr ferritic cast steels which develops no casting defect even in thick-walled members, show a marked improvement in high-temperature strength (particularly high-temperature creep strength) at 450°C or above as compared with conventional cast steels, have performance equal to or higher than that of the existing forged steels with respect to toughness and weldability, and achieve high economic efficiency.
  • the present inventors have made an effort to solve the above-described problems on the basis of the fundamental conceptions that (1) internal defects should be minimised even in thick-walled cast steels, (2) creep strength at 450°C or above should be improved as a result of precipitation hardening by V and Nb and solid solution strengthening by W, Mo and Cu, and (3) weldability should be improved by controlling the contents of C, Mn and B. As a result, the following facts have been found.
  • Low-Cr ferritic cast steels most probably tend to suffer from the macrosegregation of S, and this tendency becomes more pronounced in large ingots and weakly deoxidized materials. Even if sufficient deoxidation is effected, porosity tends to be concentrated in the parts where the macrosegregation of S occurs. Consequently, the macrosegregation of S also needs to be suppressed for the purpose of minimizing material deterioration due to porosity. Moreover, the macrosegregation of S causes the following problems: (1) the promotion of high-temperature cracking, for example, during welding, (2) a reduction in oxidation resistance and high-temperature corrosion resistance due to the destabilisation of Cr 2 O 3 film, and (3) a reduction in grain boundary strength.
  • the present inventors have investigated various methods for suppressing the segregation of S in low-Cr ferritic cast steels, and have discovered the following solution.
  • S can be stabilised by effecting sufficient deoxidation with Al and, at the same time, adding Mg having a strong affinity for S.
  • Mg having a strong affinity for S.
  • Mg also has the effect of stabilising scale of, e.g, Cr 2 O 3
  • Mg it is desirable to add Mg for the purpose of stabilising S.
  • Mg When Mg is added, its effect is governed by the balance between the Mg content and the S, O and Al contents.
  • Mg content must satisfy the following inequality: (Mg content) > (24/32) (S content) + (24/16) [(O content) - (8/9) (Al content)] That is, Mg has not only the effect of stabilising S in the form of MgS, but also the effect of stabilising scales in itself.
  • the present inventors have completed the present invention on the basis of the synergistic effect of a measure for suppressing the segregation of S and an optimisation of the contents of other alloying elements.
  • the present invention relates to the use according to claim 1.
  • C combines with Cr, Fe, W, V and Nb and with optionally added Mo and Cu to form carbides and thereby contributes to the improvement of high-temperature strength.
  • C itself acts as an austenite-stabilizing element to stabilize the structure. If its content is less than 0.03%, the precipitation of carbides will be insufficient to achieve adequate high-temperature strength. If its content is greater than 0.12%, excessive amounts of carbides will precipitate, resulting in marked hardening of the steel. Accordingly, the proper content of C is in the range of 0.03 to 0.12%. In this range, lower C contents provide better weldability. Consequently, the content of C should preferably be in the range of 0.05 to 0.08%.
  • Si is an element which acts as a deoxidizer and improves steam oxidation resistance. If its content is greater than 0.7%, Si will cause a marked reduction in toughness and will be detrimental to creep strength. If its content is less than 0.03%, the melt flowability during casting will become poor. Accordingly, the content of Si should be in the range of 0.03 to 0.7% by weight. Where greater importance is attached to creep strength than to melt flowability, the content of Si should preferably be in the range of 0.03 to 0.30% by weight.
  • Mn has desulfurizing and deoxidizing effects, and is effective in stabilizing the structure. If its content is less than 0.02%, no sufficient effect will be produced. If its content is greater than 1%, Mn will harden the steel and enhance sensitivity to temper embrittlement. When the content of S is particularly low, the content of Mn may be reduced. Accordingly, the content of Mn should be in the range of 0.02 to 1%. When the content of S is particularly low, the content of Mn may be in the range of 0.02 to 0.30%.
  • Co may be present as a steel impurity in an amount of up to 0.3%.
  • Co will exert no appreciable harmful effect at a content of up to 0.3%. Accordingly, the content of Co as an inevitable impurity should be up to 0.3%. Thus, Co need not be positively added during compositional adjustment.
  • Both P and S are elements which are detrimental to toughness. Since even a very slight amount of S destabilizes grain boundaries and Cr 2 O 3 scale film and thereby causes a reduction in high-temperature strength and toughness, its content should preferably be as low as possible within the aforesaid limit. Accordingly, the contents of P and S as inevitable impurities should be up to 0.025% and up to 0.015%, respectively.
  • Cr is an element which is indispensable from the viewpoint of the oxidation resistance and high-temperature corrosion resistance of low-alloy steels. If its content is less than 0.8%, Cr will fail to produce sufficient oxidation resistance and high-temperature corrosion resistance. On the other hand, Cr added in an amount of greater than 3% will detract from strength and toughness. Accordingly, the content of Cr should be in the range of 0.8 to 3% by weight.
  • Ni is an austenite-stabilizing element and contributes to the improvement of toughness. However, if its content is less than 0.01%, no sufficient effect will be produced. If its content is greater than 1%, Ni will detract from high-temperature creep strength. Moreover, the addition of large amounts of Ni is also disadvantageous from an economic point of view. Accordingly, the content of Ni should be in the range of 0.01 to 1% by weight.
  • V combines with C and N to form a fine precipitate comprising V(C,N) and the like.
  • This precipitate contributes greatly to the improvement of long-time creep strength at high temperatures. However, if its content is less than 0.01%, no sufficient effect will be produced. If its content is greater than 0.5%, the precipitation of V(C,N) will become excessive and, on the contrary, detract from creep strength and toughness. Accordingly, the proper content of V is in the range of 0.01 to 0.5%.
  • W acts as a solid solution strengthening and fine carbide precipitation strengthening element and is effective for the improvement of creep strength.
  • Mo has a similar effect
  • W has a lower diffusion rate in Fe and is hence more excellent in the high-temperature stability of its fine carbide which contributes to the improvement of creep strength.
  • W brings about a greater improvement in strength, particularly in high-temperature creep strength, than when added alone. If its content is less than 0.1%, no effect will be produced, and if its content is greater than 3%, W will harden the steel and detract from its toughness. Accordingly, the content of W should be in the range of 0.1 to 3%. In this range, the content of W should preferably be in the range of 1.0 to 2.0%.
  • Nb like V, combines with C and N to form Nb(C,N) and thereby contributes to the improvement of creep strength.
  • Nb shows a marked strength-improving effect at relatively low temperatures of 600°C or below. If its content is less than 0.01%, the above-described effect will not be produced. If its content is greater than 0.2%, Nb will harden the steel significantly and detract from its toughness and weldability. Accordingly, the content of Nb should suitably be in the range of 0.01 to 0.2%. In order to achieve a satisfactory combination of weldability and creep strength, the content of Nb should desirably be in the range of 0.03 to 0.15%.
  • Al is an indispensable deoxidizing element and forms a carbonitride. Moreover, Al also has the effect of making the structure finer. If its content is less than 0.001%, no effect will be produced, and if its content is greater than 0.05% by weight, Al will detract from creep strength and workability. Accordingly, the content of Al should be in the range of 0.001 to 0.05% by weight.
  • B has the effect of dispersing and stabilizing carbides and thereby contributes to the improvement of long-time creep strength. If its content is less than 0.0001%, no sufficient effect will be produced, and if its content is greater than 0.02%, B will detract from workability. Accordingly, B should be added so as to give a B content in the range of 0.0001 to 0.02%. In this range, the addition of B is also effective for the improvement of hardenability. Consequently, it is necessary from the viewpoint of structure control to regulate the amount of B added as required.
  • N is necessary for the formation of carbonitrides by combination with V and Nb. If its content is less than 0.001%, no effect will be produced. However, as its content becomes higher, N in solid solution will increase and the nitrides will become coarse, resulting in a reduction in creep strength. Moreover, if its content is greater than 0.05%, N may be responsible for the formation of blow-holes during casting. Accordingly, the content of N should be in the range of 0.001 to 0.05%.
  • O increases casting defects such as pipe flaws and blow-holes, and also exerts an adverse influence on toughness and hot workability. Accordingly, the content of O should be up to 0.03% and preferably up to 0.02%.
  • Mg is an element which stabilizes S and is effective for the suppression of porosity resulting from the segregation of S during casting, the suppression of weld defects, and the strengthening of grain boundaries. Moreover, Mg is also an important element which stabilizes Cr 2 O 3 film and, in the case of Cu addition as will be described later, Cu-O film. However, if its content is less than 0.0005% or does not satisfy the following inequality as expressed on a weight percentage basis: (Mg content) > (24/32)(S content) + (24/16)[(O content) - (8/9)(Al content)], the desired effect will not be produced. On the other hand, even if Mg is added in an amount greater than 0.05%, its effect will become saturated.
  • the content of Mg should be in the range of 0.0005 to 0.05% and, at the same time, should satisfy the following inequality: (Mg content) > (24/32)(S content) + (24/16)[(O content) - (8/9)(Al content)]
  • the above inequality means that it is necessary to secure a certain amount of Mg which is not fixed by S or O but exists in solid solution as a free metal.
  • This inequality has been formulated by considering the atomic weights of Mg, S, O and Al to be 24, 32, 16 and 27, respectively.
  • Ca, Ti, Zr, Y, La, Ce and Ta combine with P, O and S that are impurities.
  • one of more of these elements are added in very small amounts.
  • the addition of 0.01% of each element makes it possible to free the steel of such impurities as P, O and S, and thereby improve its strength and toughness. This is particularly effective for the improvement of creep strength.
  • the content of each element is greater than 0.2%, the resulting inclusion will increase and, on the contrary, detract from toughness. Accordingly, the content of each of these elements should be in the range of 0.01 to 0.2%.
  • Mo like W
  • Mo is effective for the improvement of creep strength.
  • Mo need not necessarily be added to the steels of the present invention which contain a large amount of W. Nevertheless, Mo produces a strength-improving effect when added in combination with W, and is also effective for the improvement of toughness when added in small amounts. If the content of Mo is less than 0.01%, the above-described effects will not be produced. If its content is greater than 3%, intermetallic compounds will precipitate at high temperatures, resulting in not only a reduction in toughness but also the loss of its effect on strength. Accordingly, when Mo is added, its content should be in the range of 0.01 to 3%.
  • Cu not only improves the strength of the steel owing to solid solution strengthening and precipitation hardening, but also contributes to the improvement of oxidation resistance. Moreover, Cu changes the structure into martensite or bainite and is hence effective for the improvement of toughness. However, the addition of excessive amounts of Cu will harden the steel to an undue extent.
  • the content of Cu should be up to 2.5% and its lower limit is 0.1%.
  • Steels A and B are typical conventional cast steel materials which have chemical compositions corresponding to SCPH 21 and SCPH 32, respectively, of JIS (Japanese Industrial Standards).
  • Steels C and D have chemical compositions corresponding to those of heat-resisting steels for small-diameter pipes which are used in boilers and the like.
  • Steels E to M are comparative steels in which the contents of some alloy components are modified so as to be outside the scope of the present invention.
  • Steels 1 to 24 are steels used in accordance with the present invention.
  • steels A to D were normalized by heating at 950°C for 2 hours and air cooling, and then tempered by heating at 730°C for 2 hours and air cooling.
  • Steels E to M and the inventive steels 1 to 24 were normalized by heating at 1,050°C for 2 hours and air cooling, and then tempered by heating at 770°C for 1.5 hours and air cooling.
  • the Charpy impact tests were performed according to JIS Z2202. That is, using No. 4 test pieces, the impact value at 0°C was measured three times, and the average of the three impact values was obtained.
  • the y-type weld cracking tests were performed according to JIS Z3158 by using a plate thickness of 20 mm and without preheating (i.e., at 20°C). The weldability was evaluated in terms of longitudinal section cracking rate.
  • the steels used in the present invention exhibit a tensile strength in the range of 600 to 700 MPa and an elongation of 20% or greater.
  • the comparative steels including conventional steels have a value of at most 84 MPa.
  • the steels used in the present invention have a value of 130 MPa or greater, indicating a marked improvement in high-temperature strength by a factor of more than 1.5 times.
  • steels 4 and 5 containing Mo have a higher creep rupture strength than steels 1-3, and steel 11 additionally containing Cu shows a further increase in creep rupture strength.
  • Steels 16-24 which contain one or more of Ca, Ti, Zr, Y, La, Ce, Ta and Mg, shows no reduction in creep rupture strength and hence have excellent high-temperature strength, even in the presence of relatively large amounts of impurities such as P and S.
  • the y-type weld cracking tests have revealed that the occurrence of full cracking or partial cracking was observed in all of the comparative steels, but the steels used in the present invention undergo no cracking even at 20°C. Thus, it can be seen that the steels used in the present invention have very excellent weldability and their preheating during welding may be omitted.
  • the low-Cr ferritic steels used in the present invention are materials which show a marked improvement in high-temperature strength over conventional low-Cr ferritic steels and also have excellent impact resistance and weldability. Consequently, the steels used in the present invention having such excellent properties may be substituted for forged steels in parts which have conventionally required the use of forged steels, resulting in a reduction in cost and an increase in reliability.
  • the steels used in the present invention can be widely used for cast steel articles of various shapes which are used as heat-resistant and pressure-tight members in the industrial fields of boilers, chemical industry, nuclear power industry and the like.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Claims (1)

  1. Verwendung eines ferritischen Gußstahls mit niedrigem Cr-Gehalt, der im wesentlichen aus folgendem auf Gew.-%-Basis besteht: 0,03 bis 0,12% C, 0,03 bis 0,7% Si, 0,02 bis 1% Mn, bis zu 0,3% Co, bis zu 0,025% P, bis zu 0,015% S, 0,8 bis 3% Cr, 0,01 bis 1% Ni, 0,01 bis 0,5% V, 0,1 bis 3% W, 0,01 bis 0,2% Nb, 0,001 bis 0,05% Al, 0,0001 bis 0,02% B, 0,001 bis 0,05% N, bis zu 0,03% O, 0,0005 bis 0,05% Mg, gegebenenfalls unter Zugabe von 0,01 bis 0,2 Gew.-% eines oder mehrerer Elemente, die ausgewählt sind aus der Gruppe Ca, Ti, Zr, Y, La, Ce und Ta; gegebenenfalls unter Zugabe von 0,01 bis 3 Gew.-% Mo; gegebenenfalls unter Zugabe von 0,1 bis 2,5 Gew.-% Cu; wobei der Rest aus Eisen und zufälligen Verunreinigungen besteht und der Mg-Gehalt des Gußstahls die folgende in Gew.-% ausgedrückte Ungleichung erfüllt: (Mg-Gehalt) > (24/32) (S-Gehalt) + (24/16) [(O-Gehalt) - (8/9) (Al-Gehalt)] in gegossenem Zustand für Gußstahlartikel.
EP19970306432 1996-10-09 1997-08-22 Verwendung eines schweissbaren ferritischen Gussstahls mit niedrigem Chromgehalt und mit sehr gute Warmfestigkeit Expired - Lifetime EP0835946B1 (de)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
JP268529/96 1996-10-09
JP26852996 1996-10-09
JP26852996A JP3572152B2 (ja) 1996-10-09 1996-10-09 高温強度と溶接性に優れた低Ｃｒフェライト鋳鋼

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EP0835946A1 EP0835946A1 (de) 1998-04-15
EP0835946B1 true EP0835946B1 (de) 2001-11-28

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JP3483493B2 (ja) 1999-03-19 2004-01-06 日本鋳鍛鋼株式会社 圧力容器用鋳鋼材及びそれを用いる圧力容器の製造方法
GB2364715B (en) * 2000-07-13 2004-06-30 Toshiba Kk Heat resistant steel casting and method of manufacturing the same
CN101111618B (zh) * 2006-02-01 2014-06-25 布哈拉特强电有限公司 用于汽轮机外壳应用的CrMo1/4V钢铸件中的铌添加
FR3014114B1 (fr) * 2013-12-04 2017-05-12 C T I F - Centre Technique Des Ind De La Fond Acier micro-allie
CN114480978A (zh) * 2022-01-11 2022-05-13 中国船舶重工集团公司第七二五研究所 一种非淬火和回火的微合金铸钢及其热处理方法

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DE977847C (de) * 1965-02-07 1971-09-02 Huettenwerk Oberhausen Ag Verwendung von Staehlen als Werkstoff fuer Panzerbleche oder -platten
JPH062927B2 (ja) * 1989-02-20 1994-01-12 住友金属工業株式会社 耐食、耐酸化性に優れた高強度低合金鋼
JPH062926B2 (ja) * 1989-02-20 1994-01-12 住友金属工業株式会社 高温クリープ強度の高い耐熱綱
DE69003202T2 (de) * 1989-07-31 1994-03-31 Mitsubishi Heavy Ind Ltd Hochfeste, hitzebeständige, niedrig legierte Stähle.
JP2659814B2 (ja) * 1989-08-30 1997-09-30 三菱重工業株式会社 高強度低合金耐熱鋼の製造方法
JP2967886B2 (ja) * 1991-02-22 1999-10-25 住友金属工業 株式会社 クリープ強度と靭性に優れた低合金耐熱鋼
JP2590657B2 (ja) * 1991-12-12 1997-03-12 日本鋼管株式会社 焼鈍時の密着焼付防止性およびガス放散性に優れたＦｅ−Ｎｉ合金およびその製造方法
JP3334217B2 (ja) * 1992-03-12 2002-10-15 住友金属工業株式会社 靱性とクリープ強度に優れた低Ｃｒフェライト系耐熱鋼
JP2624224B2 (ja) * 1995-11-27 1997-06-25 株式会社日立製作所 蒸気タービン

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DE69708574T2 (de) 2002-07-04
DE69708574D1 (de) 2002-01-10
JPH10121188A (ja) 1998-05-12
EP0835946A1 (de) 1998-04-15
JP3572152B2 (ja) 2004-09-29

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