US20230074936A1 - Heat-resistant cast steel, and preparation method and use thereof - Google Patents

Heat-resistant cast steel, and preparation method and use thereof Download PDF

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US20230074936A1
US20230074936A1 US17/793,482 US202117793482A US2023074936A1 US 20230074936 A1 US20230074936 A1 US 20230074936A1 US 202117793482 A US202117793482 A US 202117793482A US 2023074936 A1 US2023074936 A1 US 2023074936A1
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heat
cast steel
resistant cast
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steel
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Shuai Shi
Linbo MEI
Lingen Sun
Chunxiang An
Yu Wang
Xia Liu
Hongwei SHEN
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Shanghai Electric Power Generation Equipment Co Ltd
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D1/26—Methods of annealing
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt

Definitions

  • the present invention relates to the field of metal materials, in particular to a heat-resistant cast steel, and a preparation method and use thereof.
  • a steam turbine in turbomachinery also known as a steam turbine engine, is a rotary steam power unit.
  • high-temperature high-pressure steam passes through a fixed nozzle to become an accelerated steam flow which is then directed onto blades, so that the rotor with a row of blades rotates and does external work.
  • the steam turbine is the main equipment of a modern thermal power plant.
  • Improving the steam temperature parameters of a coal-fired unit in the thermal power plant may increase the unit efficiency, reduce the consumption of fossil fuels and achieve energy conservation and emission reduction.
  • the operating temperature of the steam turbine is limited by the maximum operating temperatures of the materials of key components (cylinders, valves, rotors, blades, etc.).
  • High-temperature casting materials used for steam turbine cylinders and valve casings have been developed from Cr-Mo steel to various 9%-12% Cr ferritic steels.
  • ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB and the like are currently available options.
  • the maximum working temperature of steel grade ZG12Cr10Mo1W1VNbN cannot exceed 610° C.
  • the maximum working temperature of steel grade ZG13Cr9Mo2Co1NiVNbNB cannot exceed 625° C.
  • there is currently no heat-resistant cast steel material for steam turbine castings that can satisfy the working temperature of 635° C.
  • an objective of the present invention is to provide a heat-resistant cast steel, and a preparation method and use thereof in order to solve the problems in the prior art.
  • the present invention provides a heat-resistant cast steel. Based on the total mass of the heat-resistant cast steel, the heat-resistant cast steel includes the following elements in mass percentage:
  • 0.08 wt %-0.18 wt % of C 0.10 wt %-0.40 wt % of Si, 0.30 wt %-0.70 wt % of Mn, 9.80 wt %-10.70 wt % of Cr, 3.00 wt %-3.50 wt % of Co, 1.60 wt %-2.00 wt % of W, 0.45 wt %-0.85 wt % of Mo, 0.10 wt %-0.30 wt % of V, 0.02 wt %-0.08 wt % of Nb, 0.010 wt %-0.035 wt % of N, 0.001 wt %-0.010 wt % of B, ⁇ 0.20 wt % of Ni and 79 wt %-85.5 wt % of Fe.
  • the mass percentage of Fe is 80.5 wt %-84.7 wt %.
  • the mass percentage of Fe is 81 wt %-83.8 wt %.
  • the corresponding mass percentages of Al, P, S, Cu, Ti and Sn are: ⁇ 0.030 wt % of Al, ⁇ 0.030 wt % of P, ⁇ 0.020 wt % of S, ⁇ 0.25 wt % of Cu, ⁇ 0.030 wt % of Ti and ⁇ 0.030 wt % of Sn, more preferably, ⁇ 0.020 wt % of Al, ⁇ 0.020 wt % of P, ⁇ 0.015 wt % of S, ⁇ 0.15 wt % of Cu, ⁇ 0.020 wt % of Ti and ⁇ 0.020 wt % of Sn.
  • the heat-resistant cast steel includes the following elements in mass percentage: 0.10 wt %-0.16 wt % of C, 0.20 wt %-0.30 wt % of Si, 0.40 wt %-0.60 wt % of Mn, 10.00 wt %-10.50 wt % of Cr, 3.10 wt %-3.40 wt % of Co, 1.65 wt %-1.90 wt % of W, 0.55 wt %-0.75 wt % of Mo, 0.15 wt %-0.25 wt % of V, 0.03 wt %-0.07 wt % of Nb, 0.015 wt %-0.030 wt % of N, 0.002 wt %-0.008 wt % of B, ⁇ 0.10 w
  • the present invention further discloses use of the above-described heat-resistant cast steel in turbomachinery, especially use as a casting material in the field of steam turbines.
  • Carbon (C) ensures hardenability.
  • M23C6 carbides are formed at grain boundaries and martensite boundaries, and after C is combined with Nb, V and other elements, MX carbonitrides are formed in the martensite.
  • the high-temperature strength can be increased.
  • C is also an indispensable element to inhibit the formation of harmful phases ⁇ -ferrite and BN.
  • the C content should be 0.08% or above.
  • the C content is limited to 0.08%-0.18%. Further, the optimal content of C should be limited to 0.10%-0.16%.
  • Si is an effective element for deoxidation of molten steel, and together with Cr, can improve the oxidation resistance of the steel.
  • Si promotes the precipitation of Laves phases, which is not conducive to the toughness of the steel and has an adverse effect on creep strength. Therefore, the Si content is limited to 0.10%-0.40%. Further, the optimal content of Si should be limited to
  • Mn Manganese (Mn): Mn can remove oxygen and sulfur in molten steel and improve the hardenability of steel. However, with the increase of the Mn content, the creep rupture strength decreases. Therefore, the Mn content is limited to 0.30%-0.70%. Further, the optimal content of Mn should be limited to 0.40%-0.60%.
  • Chromium (Cr) in steel mainly functions to improve oxidation resistance and corrosion resistance, and it is an indispensable element as a constituent element of M 23 C 6 carbides that improve high-temperature strength through precipitation strengthening.
  • the minimum Cr content in the heat-resistant cast steel of the present invention is 9.80%.
  • the Cr content exceeds 10.70%, it is easy to form ⁇ -ferrite, which decreases the high-temperature strength and toughness. Therefore, the Cr content is limited to 9.80%-10.70%. Further, the optimal content of Cr should be limited to 10.00%-10.50%.
  • Mo Molybdenum
  • the addition of Mo mainly increases the tempering stability of the steel and strengthens the secondary hardening effect.
  • the grain boundary segregation of Mo improves the bonding force of the grain boundaries, which increases the strength of the steel and reduces the loss of toughness.
  • an excess of Mo leads to the formation of ⁇ -ferrite and the precipitation of the intermetallic compound Laves phase, which obviously decreases the toughness. Therefore, the Mo content is limited to 0.45%-0.85%. Further, the optimal content of Mo should be limited to 0.55%-0.75%.
  • Tungsten (W) W is effective in inhibiting the coarsening of M 23 C 6 carbides, and its effect exceeds that of Mo.
  • the addition of W to replace part of Mo ensures that the Mo equivalent (Mo+1 ⁇ 2W) is about 1.5%, the creep strength is the best, and there is no excess of ⁇ -ferrite formed. If the amount of W added exceeds 2%, the castings are prone to segregation. Therefore, the W content is limited to 1.60%-2.00%. Further, the optimal content of W should be limited to 1.65%-1.90%.
  • Co Co
  • Mo Mo
  • W are important elements that are distinctive from others in the present invention. Co can inhibit the formation of ⁇ -ferrite after high-chromium ferritic steel is normalized or quenched at high temperature, fully exert the solid solution strengthening effect of Mo and W and increase the toughness of steel, which is critical for the heat-resistant cast steel of the present invention with high W content.
  • the Co content is limited to 3.00%-3.50%. Further, the optimal content of Co should be limited to 3.10%-3.40%.
  • V and Nb are easily combined with C and N in the martensite to form MX carbonitrides.
  • an excess of V and Nb will fix the carbon content excessively and reduce the precipitation of the M 23 C 6 carbides, which leads to the decrease of the high-temperature strength.
  • Nb is easy to segregate in castings. Therefore, the V content is limited to 0.10-0.30%, and the Nb content is limited to 0.02%-0.08%. Further, the optimal content of V should be limited to 0.15%-0.25%, and the optimal content of Nb should be limited to 0.03%-0.07%.
  • Nickel (Ni) An appropriate amount of Ni can increase the hardenability of the steel, inhibit the formation of ⁇ -ferrite and BN, and improve the strength and toughness at room temperature. However, an excessive addition is not conducive to the high-temperature creep properties of the steel. Therefore, the Ni content should be as low as possible. The Ni content is expected not to exceed 0.20%, and optimally not to exceed 0.10%.
  • B has the grain boundary strengthening effect, can form a solid solution in the M 23 C 6 carbides, has the effect of inhibiting the coarsening of the M 23 C 6 carbides, and can improve the high-temperature strength.
  • the minimum content of B should be 0.001%. However, if the B content is0.010% or above, castability and weldability will be impaired. Therefore, the B content is limited to 0.001%-0.010%. Further, the optimal content of B should be limited to 0.002%-0.008%.
  • N may precipitate VN (vanadium nitride) with V, which combines in the solid solution state with Mo and W to improve the high-temperature strength.
  • the minimum content of N should be 0.01%. However, if the N content exceeds 0.04%, N may easily combine with B to precipitate BN, which impairs the creep properties of the steel. Therefore, the N content is limited to 0.010%-0.035%. Further, the optimal content of N should be limited to 0.015%-0.030%.
  • the impurities include P and/or S and/or Al and/or Cu and/or Ti and/or Sn.
  • S is a harmful impurity element in steel, which will reduce the thermoplasticity of the steel, affect hot workability and reduce corrosion resistance. The segregation of S at the grain boundaries will reduce the bonding force of the grain boundaries, resulting in a decrease of the high-temperature strength.
  • P is also a harmful impurity element in steel, and if the P content is high, the steel will be brittle. Al may easily precipitate an AlN phase with N, which has adverse effects on the plasticity and toughness and the long-term creep properties of the steel.
  • Sn may easily segregate at the grain boundaries, which significantly reduces the high-temperature strength of the alloy.
  • impurity elements P, S, Al, Cu, Ti and Sn have adverse effects on the mechanical properties of the heat-resistant cast steel and alloy, and their contents should be reduced as much as possible.
  • Table 1 shows a comparison of composition range medians of the heat-resistant cast steel for castings in the present invention and ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB specified in the industry standard JB/T 11018-2010.
  • the present invention further discloses a preparation method of the heat-resistant cast steel described above, including the following steps:
  • Proportioning of raw materials is determined according to proportioning of components in the formula; the raw materials are melted, refined and poured into a mold; and then, quenching or normalizing is carried out, and finally tempering is carried out.
  • the quenching is carried out at 1080-1180° C.
  • the tempering is carried out at a temperature of 700-780° C.
  • the tempering is carried out one or more times.
  • the present invention further discloses use of the above-described heat-resistant cast steel in preparation of turbomachinery.
  • Co and B are added to the composition of the present invention, the ratio of the solid solution strengthening elements Mo and W is adjusted, the contents of Mn, N and Ni are reduced, and the high-temperature creep strength is improved.
  • W is added to the composition of the present invention, the ratio of B and N is adjusted, the contents of Cr and Co are increased, the contents of Mn, Mo and Ni are reduced, and the high-temperature creep strength and oxidation resistance are improved. This will increase the operating temperature of the casting material, thereby improving the thermal efficiency of the generating unit and reducing the coal consumption and carbon dioxide emissions.
  • the material designation of this novel heat-resistant cast steel is ZG12Cr10Co3W2MoVNbNB, referred to as CW2 for short.
  • FIG. 1 is a graph showing results of an oxidation weight gain test of materials in embodiments of the present invention at 635° C.
  • proportioning of raw materials is determined according to proportioning of components in the formula; the raw materials are melted, refined and poured into a mold; and then, quenching or normalizing is carried out, and finally tempering is carried out.
  • Industrial pure iron is used as the raw material to serve as the source of Fe.
  • Elemental carbon is used as the raw material to serve as the source of C.
  • Industrial silicon is used as the raw material to serve as the source of Si.
  • Electrolytic manganese is used as the raw material to serve as the source of Mn.
  • Metallic chromium and chromium nitride are used as the raw materials to serve as the source of Cr.
  • Electrolytic cobalt is used as the raw material to serve as the source of Co.
  • Tungsten bars are used as the raw material to serve as the source of W.
  • Metallic vanadium is used as the raw material to serve as the source of V.
  • Niobium bars are used as the raw material to serve as the source of Nb.
  • Chromium nitride is used as the raw material to serve as the source of N.
  • Elemental boron is used as the raw material to serve as the source of B.
  • Electrolytic nickel is used as the raw material to serve as the source of Ni.
  • the mechanical property indicators of the existing casting materials ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB are listed in Table 3.
  • the heat-resistant cast steel materials obtained in Embodiments 1 and 2 were subjected to the room temperature tensile test according to the standard GB/T 228.1, and subjected to the creep rupture strength test according to the standard GB/T 2039. Then, according to the extrapolation method specified in the standard GB/T 2039, the creep rupture strength limit R u 100,000 h/635° C.
  • the extrapolated value of the creep rupture strength of the material of the present invention is higher than 80 MPa, which is increased by 30% or above as compared with the extrapolated value of the creep rupture strength of the casting material ZG12Cr10Mo1W1VNbN, and by 20% or above as compared with the extrapolated value of the creep rupture strength of ZG13Cr9Mo2Co1NiVNbNB.
  • the material of the present invention has an obvious strengthening effect, and can satisfy the use requirements of the steam turbine cylinder and valve casing with a working temperature of 635° C.
  • Embodiments 1 and 2 ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB were subjected to the oxidation weight gain test at 635° C. Samples of the four materials were placed in a flowing steam environment of 635° C. and 27 MPa for a maximum time of 2000 h. The weight gain of each sample was measured in this time period. The smaller the oxidation weight gain, the better the oxidation resistance of the material. The test results are shown in FIG. 1 . As can be seen from FIG. 1 , the oxidation resistance of Embodiments 1 and 2 is significantly better than that of ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB.

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