KR20000001321A - BOILER TUBE, BLADE AND HIGH HARDNESS STAINLESS FOR ROTOR HEAT RESISTANCE MATERIAL OF HEAT power PLANT - Google Patents

Abstract

PURPOSE: A material having a high tensile strength and creep strength is manufactured by mixing steel consisted of 9-13 weight% of chrome with a chemical element such as tungsten, boron, cobalt, and others CONSTITUTION: The material comprises 0.07-0.2 weight% of carbon, 0.005-1.5 weight% of manganese, 0.01-0.1 weight% of silicone, 0.1-2.0 weight% of nickle, 9-13 weight% of chrome, 1-5.5 weight% of tungsten, 0.05-2,0 weight% of molybdenum, 0.1-0.5 weight% of vanadium, 0.02-0.08 weight% of niobium, 0.001-0.03 weight% of boron, 0.01-0.08 weight% of nitrogen, 0-6.0 weight% of cobalt, and residue of iron and alloy or unavoidably contained impurities such as below 0.01 weight% of sulfur, below 0.01 weight% of phosphorus, and 0.02 weight% of aluminium. The material is heat-treated at 1,02000-1,150°C for 1.5 hr per inch and then at 52000-800°C for 1-1.5 hr per inch.

Description

High strength stainless heat resistant materials for thermal power plant boiler tubes, blades and rotors

The present invention relates to a high-strength stainless heat-resistant steel material that can be used above 590 ° C by improving the creep properties of high-strength stainless heat-resistant steel for boiler tubes, blades and rotors used in thermal power plants. (W), boron (B), cobalt (Co) and the like to add an element to further improve the high temperature creep properties.

Since boilers, blades, and rotor materials have to be excellent in high temperature strength, fatigue properties, and creep properties, the materials for boiler tubes, blades, and rotor materials have been determined according to the use temperature.

Conventional steels containing 1% of chromium have poor creep properties when they are above 550 ° C and cannot be used above 550 ° C. Conventional steels containing 9 to 13% of chromium can be used above 550 ° C but above 590 ° C. Also, the creep properties are poor, so the maximum operating temperature does not exceed 590 ℃. This is because the alloying elements contained in the conventional steel alone do not maintain their strength due to the coarsening of carbides during use at high temperatures. Therefore, as the use temperature of the boiler tube, the blade and the rota material is increased to 590 ℃ or more, it is required to develop a new material to replace the conventional steel.

An object of the present invention is to solve the conventional problems as described above, 590 by adding elements such as tungsten (W), boron (B), cobalt (Co) to the steel containing 9 to 13% by weight of chromium It is to provide a high strength stainless heat resistant material for thermal power plant boiler tube, blade and rotor having excellent tensile strength and high temperature creep strength even above ℃.

1 is a chemical composition in a tissue state diagram represented by chromium and nickel equivalents

It is a figure which shows a creation area,

2 shows the creep rupture strength and Larson-Miller parameters at 100,000 hours at 650 ° C.

Graph showing the relationship.

That is, the present invention is to reduce the delta ferrite content as much as possible for the fatigue strength, so that the cobalt is added in the graph of Fig. 1 represented by chromium equivalent and nickel equivalent, 100% martensite tissue region or Martens containing less than 5% deltaferrite The alloy composition was determined to be located in the site region. Also, the material was improved in high temperature creep strength by lowering carbon, optimizing molybdenum and tungsten, and optimizing invasive elements such as boron or nitrogen. A high strength stainless heat resistant material for boiler tubes, blades and rotors.

Chemical composition of the high-strength stainless heat-resistant material for boiler tubes, blades and rotors of the present invention is 0.07 to 0.2% by weight of carbon (C), 0.05 to 1.5% by weight of manganese (Mn), 0.01 to 0.1% by weight of silicon (Si), Nickel (Ni) is 0.1 to 2.0 wt%, chromium (Cr) is 9 to 13 wt%, tungsten (W) is 1 to 5.5 wt%, molybdenum (Mo) is 0.05 to 2.0 wt%, vanadium (V) is 0.1 To 0.5% by weight, Navinium (Nb) is 0.02 to 0.08% by weight, boron (B) is 0.001 to 0.03% by weight, nitrogen (N) is 0.01 to 0.08% by weight, cobalt (Co) is 0 to 6.0% by weight, Residuals are impurities that are inevitably mixed in the iron and ferroalloy or during the manufacturing process, sulfur (S) is 0.01% by weight or less, phosphorus (P) is 0.01% by weight or less, aluminum (Al) is characterized in that 0.02% by weight or less.

In the present invention, carbon is an austenite forming element, and the content is preferably 0.07 to 0.2% by weight. If the amount of carbon in the steel is excessive, carbides are formed in the quenching or tempering step and coarsened to precipitate, thereby not exerting a strengthening effect or Nb, V. Elements contributing to solid solution strengthening such as Mo, W, Cr, etc. are also initially consumed by the formation of carbides and lose solid solution strengthening. In addition, if the amount of carbon is too low, the mechanical strength at room temperature may be lowered, the toughness is worsened, and the net chromium equivalent may be increased to form delta ferrite. Therefore, in order to improve the creep strength for a long time, it is preferable to suppress the tempering process or the temporary carbide precipitation on the short time side, and to control the carbon content in order to continuously precipitate the carbide during use.

Nitrogen is an austenite forming element, and its content is preferably 0.01 to 0.08% by weight. Even at a small amount, nitrogen inhibits delta ferrite formation, improves creep strength, and forms nitrides by combining with V, Nb, B, Al, and Si, or It is solid-dissolved in an invasive type to increase the creep strength, and as the amount added increases, toughness and ductility decrease. When the sum of carbon and nitrogen in the chromium steel of 9 to 13% by weight is 0.17%, the creep strength is the highest at 593 ° C for 10 hours, and the content of carbon, nitrogen and boron tends to be lowered. Therefore, the developed steel grade is in the range of up to 0.08%, higher than 0.05% of solid iron solubility. This value is close to the maximum value that can be added in the air, and this value is set high for a long time such as VN, NbN, and BN. The purpose is to increase the creep strength through the formation of stable nitrides.

Boron is an effective element to improve the hardenability and high temperature creep strength. It can be dissolved in iron or substituted with iron, and its content is preferably 0.001 to 0.03% by weight. The maximum creep strength is shown at 0.012% and the highest creep ductility at 0.028%. In this steel grade, it exists as boron carbide or boride or segregates at the grain boundary to strengthen grain boundary.

Tungsten is a ferrite-forming element substituted with iron, and the content is preferably 1.0 to 5.5% by weight. When the addition amount is increased, the Laves phase is formed on the short time side, and the total amount of precipitates is increased on the long time side, and the toughness is sharply decreased. Delta ferrite, which is harmful to fatigue strength, is formed and segregation tends to be strong, making it difficult to control segregation in the manufacture of large ingots. However, some of the added tungsten is dissolved in the matrix and delays the recovery of the matrix, and some forms precipitates such as W ₂ C, Fe ₂ W, and is partially dissolved in M ₂₃ C ₆ and M ₆ C carbides. By delaying the coarsening of carbides, tungsten makes a significant contribution to improving creep strength.

If molybdenum is lectures A high volume of the added amount is a coarse precipitates at a high temperature for a long time side, but may occur in the creep strength is lowered, the composite of molybdenum and tungsten is added together as ferrite forming element content is preferably 0.05 to 2.0 wt% M ₆ The tendency of formation of C is reduced, and this increases the creep strength at high temperatures for a long time in connection with maintaining the solid solution effect and the fine precipitation effect of molybdenum or tungsten for a long time. In addition, molybdenum is effective in improving high temperature strength through solid solution strengthening and precipitation strengthening, and improves corrosion resistance in stainless steel.

Vanadium is a ferrite forming element, and the content is preferably 0.1 to 0.5% by weight, but when the amount is large, vanadium and Naobium consume all the carbon in the matrix, and M ₂₃ C ₆ carbide or other forms of carbides are continuously precipitated during use. This makes it difficult, and a small amount reduces the place of precipitation, making it difficult to distribute the M ₂₃ C ₆ carbide uniformly, resulting in coarsening. However, if the content is properly adjusted as described above, vanadium is effective in improving the high temperature strength through solid solution strengthening and precipitation strengthening, and precipitates many carbides during tempering to delay the recovery of the matrix on the long-term side and suppress the coarsening of precipitates. It works.

Naobium is a ferrite forming element, and its content is preferably 0.02 to 0.08% by weight, but when a large amount is added, Nb (C, N) and VN are precipitated in combination to increase the unemployed Nb (C, N) during normalization. When the amount of VN effective in the deposition is reduced, and excessively added, it consumes the carbon in the matrix to reduce the formation of M ₂₃ C ₆ carbides, thereby lowering the creep strength for a long time, and Naobium carbide is not used even at the normalizing temperature of 1050 ° C. This unemployed Naobium carbide improves the short-term creep strength, but after 300 hours at 650 ° C, it promotes coarsening of precipitates and lowers the long-term creep strength. However, when a small amount is added, creep strength is improved by strengthening VN dispersion precipitation and synergistic effect, and most of the added amount is present as carbide because there is little solid solution at the base.

Chromium is a ferrite-forming element substituted with iron, and its content is preferably 9 to 13% by weight. This is for obtaining oxidation resistance and optimum creep strength, and obtaining sufficient oxidation resistance and corrosion resistance in the temperature range of 600 to 650 ° C. The minimum required chromium content is 9%, and the iron-chromium-carbon diagram shows that the maximum solid solution of chromium must be in the range of 11-14%, depending on the carbon content, in order to become single phase austenite at high temperatures. Therefore, in order to obtain oxidation resistance and optimum creep strength of the ingot, it is preferable to set the chromium content to 9 to 13% by weight.

Cobalt is an austenite forming element that increases the starting temperature of martensite. The content of cobalt is preferably 0 to 6% by weight. The cobalt addition is expected to increase the amount of tungsten and increase the carbon content rather than increase the strength directly by cobalt. It is added to suppress delta ferrite production when there is a possibility of delta ferrite, and when dissolved in iron, it inhibits softening of matrix structure with temperature and does not form carbide and affects nucleation and growth rate of other carbides. Crazy

Nickel is preferably an austenite forming element substituted with iron, and its content is preferably 0.1 to 2.0% by weight. The higher the added amount, the higher the toughness. However, when the nickel content exceeds 2% in terms of creep strength, a sudden decrease in creep strength may occur. In addition, an appropriate amount of nickel is required to suppress delta ferrite generation due to the decrease in toughness and the increase in net chromium equivalent due to the addition of tungsten.

Aluminum is a ferrite-forming element substituted and substituted with iron, and the maximum content is preferably 0.02% by weight. If an appropriate amount is present, Al is consumed as AlN and the creep strength rapidly decreases. The maximum residual content is AlN, which should be less than or equal to the value required for the stoichiometric bonding of N, aluminum is added for deoxidation in steelmaking, and combined with nitrogen or oxygen to inhibit grain growth.

Phosphorus and sulfur are limited in their maximum content because they make a significant contribution to the temper embrittlement of strength and impact energy in materials used at high temperatures. It should be kept as low as possible as an impurity element in the steel, and in the current steelmaking method, phosphorus is limited to at most 0.01% by weight or less and sulfur at most to 0.01% by weight.

The results of testing various properties by manufacturing a high strength stainless alloy according to the embodiment of the present invention and the test results of the comparative alloy deviating from the claims of the present invention are as follows.

Table 1 shows each alloy composition of the development materials 1 to 6 and the comparative materials 7 to 8. The alloy having the chemical composition shown in Table 1 was dissolved in an electric furnace, and the steel ingot was forged into a bar shape having a width of 100 mm, a thickness of 50 mm, and a length of 300 mm at a temperature range of 950-1200 ° C., followed by annealing treatment at 700 ° C. for 70 hours. Quality heat treatment was heated at 1000 ℃ to 1150 ℃, immersed in oil and quenched, and then tempered heat treatment was heated to 500 to 800 ℃ and then slowly cooled to give ductility and toughness of the material. Tensile test, creep test and hardness test piece were taken to investigate the mechanical properties after heat treatment.

Table 2 shows the results of each test of the alloys prepared with the development and comparative materials, and the results of double creep rupture strength are shown in FIG. FIG. 2 is a graph showing the relationship between the two, showing the creep rupture strength at the longitudinal side and the temperature and time at the horizontal axis as one parameter. In FIG. 2, the solid line shows the creep rupture strength curves for the 2nd and 5th alloys of the development material, and the dotted line shows the comparative material alloy. As shown in FIG. 2, the creep rupture strength of the Larson-Miller parameter of 41.5 corresponding to 650 ° C. and 100,000 hours is 12.8 kg / mm 2 for the developing alloy and 5.2 kg / mm 2 for the comparative alloy. The development materials alloys 1 to 6 can withstand 10 kg / mm2 of creep rupture strength requirement at 650 ° C. required for rota and boiler materials and exhibit tensile strengths of 89.4 to 97.7 kg / mm2 or more. This embodiment has a creep rupture strength value twice that of the conventional 9-13% chromium steel shown in FIG. The steel produced as a comparative material is a tungsten-free steel with a tensile strength of 88.8 to 89.6 kg / mm2, a yield strength of 69.4 to 75.4 kg / mm2, and an elongation and section shrinkage of 15 to 18% and 40 to 50%, respectively. The developed steel grade has a creep rupture strength of 11.4 to 12.8kg / mm2 for 100,000 hours at 650 ° C and exceeds 10kg / mm2. As shown in Table 2 and FIG. 2, the creep properties were very excellent compared to the conventional comparative steel.

Therefore, the development alloy manufactured according to the embodiment method of the present invention suppressed carbide coarsening, which was the biggest problem by adding a small amount of tungsten, boron, and cobalt to a comparative material that had been used in the past, and formed a stable carbide at high temperature. It is possible to produce 9-13% chromium steel having excellent high temperature strength and high temperature creep rupture characteristics.

Table 1

TABLE 2

As described above, the high-strength stainless heat-resistant material for the thermal power plant boiler tube, blade and rotor of the present invention, in particular, 9 to 13% chromium alloy steel excellent in tensile strength and high temperature creep rupture characteristics is obtained, the operating temperature is 590 ℃ or more It can be used as a boiler tube, blade and rota material, and the developed steel grade is cheaper than the superalloy material used at a temperature of 590 ° C. or higher, so it is economically effective.

Claims (3)

High strength stainless steel for boiler plants, blades and rotors, characterized in that 9 to 13% by weight of chromium, 1 to 5.5% by weight of tungsten, 0.001 to 0.03% by weight of boron, 0 to 6.0% by weight of cobalt, the remainder comprising iron Heat-resistant materials. The thermal power plant boiler tube according to claim 1, wherein 9 to 13% by weight of chromium, 1 to 5.5% by weight of tungsten, 0.001 to 0.03% by weight of boron, 0 to 6.0% by weight of cobalt, and the remainder of iron, High strength stainless heat resistant materials for blades and rotors are carbon 0.07 to 0.2 wt%, manganese 0.05 to 1.5 wt%, silicon 0.01 to 0.1 wt%, nickel 0.1 to 2.0 wt%, molybdenum 0.05 to 2.0 wt%, vanadium 0.1 to 0.5 wt% , 0.02 to 0.08% by weight of Naobinium, 0.01 to 0.08% by weight of nitrogen, high strength stainless heat-resistant material for the boiler tube, blades and rotors, characterized in that it comprises an impurity. The high-strength stainless heat resistant material for a thermal power plant boiler tube, blade and rotor, characterized in that the impurity is composed of less than 0.01% by weight of sulfur, less than 0.01% by weight of phosphorus, and less than 0.02% by weight of aluminum.