KR20010055298A - A Low Alloy Steel of High Toughness for A Compressor and Turbine Wheel of A Combined cycle Power Plant - Google Patents

Abstract

PURPOSE: A high toughness low alloy steel for a compressor wheel and a turbine wheel of a combined cycle power plant is provided which can be used even at a temperature of -20 deg.C or less by optimizing contents of carbon and niobium and optimizing elements such as chromium, nickel, molybdenum and vanadium for developing a toughness improved material. CONSTITUTION: The high toughness low alloy steel for a compressor wheel and a turbine wheel of a combined cycle power plant comprises 0.2 to 0.4 wt.% of carbon, 0.05 to 1.5 wt.% of manganese, 0.01 to 0.5 wt.% of silicon, 0.1 to 2.0 wt.% of nickel, 0.5 to 3.0 wt.% of chromium, 0.05 to 2.0 wt.% of molybdenum, 0.1 to 0.5 wt.% of vanadium and a balance of Fe and other inevitable impurities containing 0.01 wt.% or less of sulphur, 0.01 wt.% or less of phosphorus, 0.02 wt.% or less of aluminum and 0.01 to 0.03 wt.% of niobium, wherein a multiply of carbon and niobium is 0.12 wt.% or less within the range of chemical constituents of the low alloy steel.

Description

A Low Alloy Steel of High Toughness for A Compressor and Turbine Wheel of A Combined Cycle Power Plant}

The present invention relates to a material having a ductile-brittle transition temperature of -30 ° C. or less by improving the impact toughness of a high toughness low alloy steel for a compressor wheel and a turbine wheel used in a combined cycle power plant, and in particular, a steel containing 0.5 to 3 wt% of chromium. The present invention relates to a low alloy steel in which an element such as nickel (Ni), molybdenum (Mo), vanadium (V), and niobium (Nb) is added to further improve impact properties.

Since conventional materials for compressor wheels and turbine wheels are ferritic low alloy steels, the initial operating temperature is determined according to the ductile-brittle transition temperature, FATT (Fracture Appearance Transition Temperature). Combined cycle power plants repeat start and stop frequently according to electric power demand. Therefore, if the ambient temperature is below the ductile-brittle transition temperature, there is a possibility of sudden brittle fracture, so the ductile-brittle transition temperature should be as low as possible.

In general, the required properties of materials for compressor wheels and turbine wheels should be excellent in tensile strength and toughness, but conventional steels containing 1% of chromium have low toughness when the ductile-brittle transition temperature is -20 ℃ or less. It cannot be used below -20 ℃.

This is because the alloy elements contained in the conventional steel have low temperature impact characteristics and thus cannot maintain toughness. Therefore, the initial operating temperature of the compressor wheel and turbine wheel materials of the combined cycle power plant is stably operated even at -20 ° C or lower. In order to achieve this, it is required to develop a material of low alloy steel containing 0.5 to 3 wt% of improved chromium.

As described above, the low-alloy steel for the conventional compressor wheel and the turbine wheel is limited in its initial use temperature such that the ductile-brittle transition temperature is -20 ° C, and thus cannot be used at lower temperatures.

The most influential factors for this soft-brittle transition temperature are grain size and carbide. If the grain size is large, the grain size is more easily broken at a lower temperature than the material having a smaller grain size. The grain size is a key factor in determining the toughness of the grain, and since the carbide plays a role in fixing the grain boundary, the grain size depends on the alloying element. Is adjusted.

The present invention has been made to improve the problems of the conventional materials for compressor wheels and turbine wheels of the combined cycle power plant as described above, carbon and nickel to improve the tensile strength and toughness of the low alloy steel containing 0.5 to 3wt% chromium Niobium is added to control the content and improve the toughness to improve the ductile-brittle transition temperature from -20 ° C to -30 ° C.

The characteristics of the alloy components of the steel grade according to the present invention are as follows.

To improve toughness, alloy composition is determined by adding niobium in an appropriate amount to increase nickel content and reduce grain size. Smaller niobium does not affect the grain size, and in many cases, the optimum content should be set because it adversely affects the formation of new compounds that are harmful to toughness.

The steel grade according to the present invention optimizes the content of carbon and niobium, and develops materials with improved toughness by optimizing elements such as chromium, nickel, molybdenum and vanadium, so that the compressor wheel and turbine of the combined cycle power plant can be used even below -20 ° C. Its purpose is to provide high toughness low alloy steel for wheels.

1 is a process carbide formation relationship graph according to Nb content and C content,

2 is a graph comparing the size of the grains according to the Nb content,

3 is a comparative graph of ductility-brittle transition temperature according to the Nb content.

The chemical composition of the steel according to the present invention is 0.2 to 0.4wt% carbon, 0.05 to 1.5wt% manganese, 0.01 to 0.5wt% silicon, 0.1 to 2.0wt% nickel, 0.5 to 3.0wt% chromium, 0.05 to 2.0wt% molybdenum, Vanadium 0.1 to 0.5wt%, niobium 0.01 to 0.03wt%, residuals are inevitable incorporation of iron and ferroalloy or during the manufacturing process, and are composed of 0.01 wt% or less of sulfur, 0.01 wt% or less of phosphorus, and 0.02 wt% or less of aluminum. .

Hereinafter, the 0.5-3 wt% chromium low alloy steel and its manufacturing method which concern on this invention are demonstrated in detail.

Conventional alloy steels are excellent in strength and toughness at -20 ° C or higher, but are low at -20 ° C or lower. Therefore, it is necessary to reduce the size of crystal grains so as to maintain toughness even at low temperatures. The present invention is an alloy steel in which fine grains are added to 0.5 to 3 wt% chromium low alloy steel excellent in strength and toughness to refine grains.

Next, the reason for the component addition and the reason for limiting the composition of the 0.5 to 3 wt% chromium low alloy steel according to the present invention will be described.

Carbon (C)

Carbon is an austenite forming element, and carbon supersaturated in steel precipitates as carbide during quenching or tempering, causing precipitation strengthening, grain growth inhibition, and, if a large amount of carbon, many carbides are formed in the tempering step. Is easily coarsened during use and loses the grain boundary fixing effect.

In addition, elements contributing to solid solution strengthening such as Nb, V, Mo, W, Cr, etc. are also consumed by the formation of carbides early, and lose the solid solution strengthening effect. In order to improve the ductile-brittle transition temperature, the carbon content must be low in order to suppress the tempering process or temporary carbide precipitation on the short-term side, and to precipitate fine carbides that are effective in reducing the grain size.

However, if the carbon is too low, the mechanical strength may be less than room temperature and the toughness is poor. If the carbon is too high, coarse carbides are formed to reduce grain refining effect, and when the product of niobium and carbon content is 0.12 or more, coarse carbides are formed. Since it is formed, as shown in Figure 1, since the appropriate content should be present, the carbon content is 0.2 to 0.4wt%.

Molybdenum (Mo)

Molybdenum is a ferrite-forming element, which is effective in improving high temperature strength through solid solution strengthening and precipitation strengthening, and improves corrosion resistance in stainless steel. Molybdenum contributes to the improvement of strength by forming stable carbides, which improves creep strength at high temperatures for a long time in connection with maintaining the solid solution and microprecipitation effects of molybdenum for a long time. If the amount is large, the precipitate tends to be coarsened, so the toughness may decrease, so the content of molybdenum is made 0.05 to 2.0 wt%.

Vanadium (V)

Vanadium is a ferrite-forming element, which is effective in improving high temperature strength through solid solution strengthening and precipitation strengthening, and many carbides are precipitated during tempering. If the content of vanadium is large, the vanadium consumes all of the carbon in the matrix, making it difficult to uniformly distribute M ₂₃ C ₆ carbide during use, resulting in easy coarsening. Vanadium delays the recovery of matrix structure even after using the material for a long time and suppresses coarsening of grains, so the vanadium content is 0.1 to 0.5 wt%.

Niobium (Nb)

Niobium is a ferrite-forming element, and since most of the solid solution is not known, most of the added amount is present as carbides. Fine niobium carbides reduce the size of the crystalline-brittle transition by miniaturizing the grain size. When niobium is added in a small amount, the low temperature toughness and creep strength are improved by synergy with VN dispersion precipitation strengthening, but when a large amount is added, coarse Nb (C) eutectic carbide is precipitated to reduce toughness. In particular, care must be taken to increase niobium in large ingots, since process niobium carbides that are very detrimental to ductility in segregation can be produced. Therefore, since an appropriate amount of niobium is required, the content of niobium is set to 0.01 to 0.03 wt%.

Chrome (Cr)

Chromium is substituted by iron and contributes to the improvement of high temperature strength through solid solution strengthening and precipitation strengthening. M ₂₃ C ₆ As the main element of carbide formation, it improves strength and toughness. The addition of chromium increases the corrosion resistance and oxidation resistance and contributes to the improvement of high temperature strength. However, in the case of low alloy steel, the chromium content is 3% in consideration of economic efficiency. If the amount of chromium is small, the amount of carbide required to maintain the high temperature strength should be at least 0.5%, so the chromium content is 0.5 to 3wt% to maintain strength and toughness.

Nickel (Ni)

Nickel is a substitutional solid solution for iron and is an austenite forming element. The addition of an appropriate amount of nickel improves the toughness, but the addition of a large amount of nickel lowers the austenite starting temperature, which accelerates the recovery of the matrix structure in the high temperature creep test, thereby reducing the creep rupture strength. If the maximum creep strength is exceeded from the viewpoint of creep strength, the sharp creep strength may decrease, so the nickel content is 0.1 to 2.0 wt% in order to increase the toughness by adding the creep strength in the maximum range. .

Aluminum (Al)

Aluminum is substituted with iron and is a ferrite forming element. Aluminum is added for deoxidation in steelmaking and combines with nitrogen and oxygen to inhibit grain growth. In addition, there is little effect on creep rupture strength and ductility, but if an appropriate amount is present, all of the solid solution nitrogen is consumed by AlN (aluminum nitride), and thus the creep strength decreases drastically. Therefore, the maximum amount of Al remaining as the deoxidizer should be less than the value required for the stoichiometric coupling of N as AlN, so it is limited to a maximum of 0.02%.

Phosphorus (P) and Sulfur (S)

Phosphorus and sulfur are limited in their maximum content because they have a significant effect on the temper embrittlement, which degrades strength and impact energy in materials used at high temperatures. Since it should be kept as low as possible as an impurity element in steel, phosphorus and sulfur are limited to about 0.01% at maximum in the current steelmaking method.

Hereinafter, the test results of various characteristics of the high toughness low alloy steel manufactured according to the method of the present invention and the alloy steel produced by the conventional method will be described through comparative examples.

In the following table, the present invention steel is 1 to 3, and the conventional steel is 4 to 8 to represent each alloy composition.

The alloy having the chemical composition shown in Table 1 was dissolved in an electric furnace, and the steel ingot was forged into a square bar shape having a width of 100 mm, a thickness of 50 mm, and a length of 300 mm in the range of 950 to 1200 ° C., followed by annealing treatment at 850 ° C. for 70 hours.

Quenching heat treatment was performed by heating to 900-1000 ° C., immersing in water and quenching, and then cooling the tempering heat treatment to 500-700 ° C. in order to impart ductility and toughness of the material. Tensile test, impact test and hardness test specimens were taken to investigate the mechanical properties after heat treatment.

Table 2 below shows the test results of the alloy steel composed of conventional steel and the present invention steel.

The size of the double grains is shown in FIG. 2 is a graph showing the relationship between the two by showing the size of the grains on the vertical axis and the content of niobium on the horizontal axis. As shown in FIG. 2, the size of the grains decreases according to the content of niobium, and the diameter of the grains of the present invention is 40 to 70 micrometers in average, and in the case of conventional alloy steels, the diameter is 100 micrometers in average.

Steels 1 to 3 of the present invention is the niobium content of 0.01, 0.02, 0.03wt%, the niobium content of conventional steels 4 and 5 is 0.04, 0.05wt%, the size of the crystal grains from 100 micrometers to 40 micrometers according to the niobium content It is reduced to meta, and at 0.03 wt% or more, it is no longer reduced but saturated.

An embodiment of the ductile-brittle transition temperature is shown in FIG. 3, which shows the ductility between the ductile-brittle transition temperature and the content of niobium on the horizontal axis. As shown in FIG. 3, the soft-brittle transition temperature of the inventive steel has a value that is at least three times lower than that of conventional low alloy steels. The steel produced by the conventional method is made of niobium-free steel and niobium added 0.04 to 0.05 wt%. The tensile strength is 120 to 125 ksi, the yield strength is 102 to 106 ksi, and the elongation and the section shrinkage are 15 to 18% and 47 to 47, respectively. 50%, and ductile-brittle transition temperature is 0-21 ° C. Steels 1 to 3 of the present invention have tensile strength, yield strength, elongation and cross-sectional shrinkage similar to those of conventional steel, and the ductile-brittle transition temperature is -30 to -68 ° C.

As can be seen from Table 2 and FIGS. 1 and 2, the toughness of the inventive steel was very excellent compared to the conventional steel.

Therefore, the low alloy steel produced by the method according to the present invention, the optimum addition of niobium to the low alloy steel that has been used in the prior art to refine the crystal grains, which was the biggest problem in the prior art, to form a stable carbide to suppress the growth and ductility of the grains- By reducing the brittle transition temperature, it is possible to produce 0.5 to 3 wt% chromium steel with excellent toughness.

According to the present invention having the above objects and configurations, 0.5-3wt% chromium alloy steel having excellent tensile strength and low temperature toughness characteristics is obtained, so that the compressor wheel and turbine wheel materials of the combined cycle power plant have an initial operating temperature of -20 ° C or lower. It becomes possible to use.

Claims (2)

0.2 to 0.4 wt% carbon, 0.05 to 1.5 wt% manganese, 0.01 to 0.5 wt% silicon, 0.1 to 2.0 wt% nickel, 0.5 to 3.0 wt% chromium, 0.05 to 2.0 wt% molybdenum, 0.1 to 0.5 wt% vanadium, iron Compressor wheel and turbine of a combined cycle power plant, characterized in that it contains 0.01 wt% or less of sulfur, 0.01 wt% or less of phosphorus, 0.02 wt% or less of aluminum, and 0.01 to 0.03 wt% of niobium, as a residual portion and unavoidable impurities. High toughness low alloy steel for wheels. The method of claim 1, High toughness low alloy steel for compressor wheel and turbine wheel of a combined cycle power plant, characterized in that the product of carbon and niobium content within the chemical composition of the low alloy steel is 0.12wt% or less.