Austenitic stainless steel

Abstract

The steel contains, by mass%, 16 to 50% nickel, 10 to 27% chromium, 0.1 to 0.75% carbon, 2.0 to 6.5% acid-soluble Al (sol.Al), 2.5% or less silicon, 0.75% or less manganese, 4.0% or less copper, 4.0% or less molybdenum, 3.0% or less tungsten, 2.0% or less niobium, 2.5% or less vanadium, 0.15% or less boron, 0.04% or less calcium, 2.0% or less zinc, 5.0% or less cobalt, 0.05% or less phosphorus, and 0.1% or less sulfur, the total of the lanthanide elements, hafnium, zirconium, yttrium, cerium and titanium being 1.0% or less by mass, the total of the selenium, tellurium, antimony, bismuth and lead being 0.04% or less by mass, and the balance being iron and unavoidable impurities, and satisfies the following formula (1): 10 5 (-0.0689W Al + 1.41W C - 0.0248W Cr - 0.0662W Cu - 0.0205W Mn + 0.0333W Mo - 0.182W Nb + 0.0499W Ni - 0.219W Si -0.97)≧-7000 (1) Here, W Al , W C , W Cr , W Mn , W Mo , W Ni , W Si , W Nb and W Cu are the amounts of aluminum, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel.

Description

[Technical field]

[0001] The present invention relates to an austenitic stainless steel. [Background technology]

[0002] In vehicles using turbocharged combustion engines, there is a trend to increase the operating temperature of the turbocharger to improve engine efficiency. Currently, traditional cast stainless steel grades such as 1.4837, 1.4848, and 1.4849 are often used to manufacture the housings of the hot sections of the turbocharger. These steels must withstand repeated heating and cooling cycles, which can result in large temperature gradients within the component, as well as oxidation. These conditions lead to thermomechanical fatigue, where failure is caused not only by cyclic strains, as in normal fatigue, but also by temperature changes and surface oxidation, which promotes the initiation and propagation of cracks. Therefore, the material for the turbocharger housing must meet several requirements, including a moderate yield strength that is maintained up to very high temperatures, good resistance to creep and oxidation, and a stable microstructure that is maintained over a wide temperature range, all at a relatively low cost. Cast austenitic stainless steels strengthened with eutectic carbides are one of the few classes of materials that meet all these constraints. This invention describes exactly such a type of austenitic stainless steel. The austenitic stainless steel of the present invention is suitable for casting and / or turbocharger (particularly turbocharger housing) applications. Summary of the Invention [Means for solving the problem]

[0003] The present invention provides a steel containing, by mass%, 16 to 50% nickel, 10 to 27% chromium, 0.1 to 0.75% carbon, 2.0 to 6.5% acid-soluble Al (sol.Al), 2.5% or less silicon, 0.75% or less manganese, 4.0% or less copper, 4.0% or less molybdenum, 3.0% or less tungsten, 2.0% or less niobium, 2.5% or less vanadium, 0.15% or less boron, 0.04% or less calcium, 2.0% or less zinc, 5.0% or less cobalt, 0.05% or less phosphorus, and 0.1% or less sulfur, the total of the lanthanide elements, hafnium, zirconium, yttrium, cerium and titanium being 1.0% or less by mass, the total of the selenium, tellurium, antimony, bismuth and lead being 0.04% or less by mass, and the balance being iron and unavoidable impurities, and satisfying the following formula (1). 10 5 (-0.0689W Al + 1.41W C - 0.0248W Cr - 0.0662W Cu - 0.0205W Mn + 0.0333W Mo - 0.182W Nb + 0.0499W Ni - 0.219W Si -0.97)≧-7000 (1) Here, W Al , W C , W Cr , W Mn , W Mo , W Ni , W Si , W Nb and W Cu are the amounts of aluminum, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel.

[0004] This alloy offers reasonable yield strength maintained up to very high temperatures, excellent resistance to creep and oxidation, and a stable microstructure maintained over a wide temperature range, all at a relatively low cost.

[0005] In some embodiments, the steel contains, by weight, 30% or more of nickel, preferably 35% or more, more preferably 37% or more, even more preferably 40% or more, and most preferably 42% or more. Such steels in particular enjoy improved oxidation and creep resistance.

[0006] In some embodiments, the steel contains, by weight, 45% or less of nickel, preferably 30% or less, preferably 26% or less. Such steels have even lower cost.

[0007] In some embodiments, the steel contains, by weight, 0.3% or less vanadium, preferably 0.2% or less, more preferably 0.1% or less vanadium, such steels being less expensive.

[0008] In some embodiments, the steel contains, by weight, at least 0.2%, preferably at least 1.0%, more preferably at least 1.5% silicon, which improves oxidation resistance and strength of such steel.

[0009] In some embodiments, the steel contains silicon in a percentage by weight of up to 1.5%, preferably up to 1.0%, which improves the ductility of such steel.

[0010] In some embodiments, the steel contains manganese in an amount, by weight, of at least 0.1%, preferably at least 0.2%, which reduces sulfur embrittlement in such steel.

[0011] In some embodiments, the steel contains, by weight, chromium at least 11.0%, preferably at least 12.0%, more preferably at least 16%, more preferably at least 22.0%. A high chromium content helps maintain a chromia scale at intermediate temperatures.

[0012] In some embodiments, the steel contains, by weight, chromium in an amount not greater than 20.0%, preferably not greater than 16%, more preferably not greater than 13%, which improves the formation of alumina scale and, as a result, improves high temperature oxidation resistance.

[0013] In some embodiments, the steel contains tungsten in a percentage by weight of 1.5% or less, preferably 1.0% or less, more preferably 0.5% or less, which reduces the cost and provides a lower intermetallic content.

[0014] In some embodiments, the steel contains, by mass%, 0.1% or more, preferably 1.5% or more, more preferably 2.5% or more of molybdenum. In such steel, the solid solution strength is increased. The increase in the solid solution strength contributes to the creep strength.

[0015] In some embodiments, the steel contains, by weight, 0.5% or less, preferably 0.3% or less, of molybdenum, which reduces the likelihood of intermetallic phases forming.

[0016] In some embodiments, the steel contains, by weight, niobium in an amount of not more than 1.0%, preferably not more than 0.2%, more preferably not more than 0.01%, which improves the machinability of the steel.

[0017] In some embodiments, the steel contains, by weight, niobium in an amount of at least 0.6%, preferably at least 1.0%, more preferably at least 1.5%, such steel exhibits improved high temperature creep.

[0018] In some embodiments, the steel contains, by weight, 0.03% or more, preferably 0.07% or more, of boron. Such steels are creep resistant.

[0019] In some embodiments, the steel contains, by weight, vanadium at least 0.1%, preferably at least 0.3%, more preferably at least 1.0%, even more preferably at least 1.5%, and most preferably at least 1.9%, which improves the high temperature creep resistance of such steel.

[0020] In some embodiments, the steel contains tungsten in an amount of at least 0.5% by weight, which improves creep resistance.

[0021] In some embodiments, the steel contains, by weight, 0.4% or more of carbon, preferably 0.5% or more, more preferably 0.475% or more, which improves the creep resistance of such steel.

[0022] In some embodiments, the steel contains, by weight, 0.6% or less of carbon, preferably 0.4% or less, more preferably 0.3% or less, which improves the ductility of such steel.

[0023] In some embodiments, the steel contains aluminum in mass percent of 3.5% or more, preferably 4.0% or more, even more preferably 4.3% or more, and most preferably 4.5% or more. Such steel is oxidation resistant.

[0024] In some embodiments, the steel contains aluminum in an amount, by weight, of up to 5.5%, preferably up to 5.0%, more preferably up to 3.5%, which improves the ductility and creep resistance of such steel.

[0025] In some embodiments, the steel contains, by weight, 0.2% or more of copper, preferably 0.5% or more, more preferably 0.75% or more, more preferably 1.5% or more, and most preferably 2.2% or more of copper, which improves castability.

[0026] In some embodiments, the steel contains copper in an amount by weight of up to 1.5%, preferably up to 0.5%, which improves the oxidation resistance of such steel.

[0027] In some embodiments, the steel contains boron in an amount of up to 0.01%, by weight, which improves the ductility of such steel.

[0028] The term "comprising" is used herein to indicate that a composition is 100% and to the exclusion of the presence of additional components making the percentage 100%.

[0029] The present invention will now be more fully described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0030] [Figure 1] The dependence of the oxidation index, chromium activity index and cost index on nickel and chromium levels is shown. [Diagram 2] 1 shows the results of oxidation tests for three examples of the present invention and a comparative example. [Diagram 3] 4 shows experimental results of Vickers hardness tests in two comparative examples and an embodiment of the present invention. [Figure 4] 1 shows experimentally determined Larson-Miller parameter values ​​for Example 10 (two points corresponding to two separate creep tests) and Comparative Example 1 (two points corresponding to two separate creep tests). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Typical heat-resistant steel grades form a chromia scale that grows continuously inward during high-temperature oxidation. The chromia scale is adherent and impermeable to oxygen and nitrogen, thus acting as a protective barrier and limiting the rate of oxidation. However, the maximum service temperature of chromia-forming steels is limited to 1050°C. The scale has a low density compared to the base metal, which exerts stresses at the oxide-metal interface. This stress increases with scale thickness and eventually leads to spalling of the scale. This exposes the bare chromium-depleted metal surface to the environment, which oxidizes more rapidly. The rapid thickening of the scale above 1050°C reduces the time required for spalling and the resulting rate of mass loss becomes unacceptably high. Spalling of the scale and the resulting mass loss are promoted by the rapid heating and cooling conditions typical of turbocharger housings. Under such conditions, stresses on the metal-oxide interface increase due to the mismatch in the thermal expansion coefficients of the two sides.

[0032] Therefore, for components exposed to repeated oxidation at temperatures above 1050°C, chromia-forming materials may not provide sufficient oxidation protection. The solution to this is the alumina-forming austenitic steels (AFA). Alumina scales are much less permeable to oxygen, thicken at a much slower rate, and provide adequate oxidation resistance at temperatures above 1050°C. They also have better resistance to spalling under repeated oxidation conditions than chromia scales.

[0033] This patent describes a class of austenitic stainless steels that provides relatively high yield strength over a range of temperatures and creep resistance comparable to traditional chromia-forming austenitic grades at comparable cost, and forms part of a completely continuous protective alumina scale.

[0034] Table 1 shows the boundaries of the main alloying elements of the steel of the present invention. Below, the effect of each element is explained and some figures of merit are listed. The figures of merit are developed based on experimental and / or theoretical modeling and are used to narrow the compositional range of the present invention to identify compositions with superior properties in heat resistant steels, including adequate yield strength maintained at very high temperatures, excellent resistance to creep and oxidation, and stable microstructure maintained over a wide temperature range, all at a relatively low cost. Based on the figures of merit, complex trade-offs between different properties are made, resulting in the restriction of elements within the compositional range. Figure 1 below shows how the cost index, chromium activity index, and oxidation figure of merit are affected by changes in nickel and chromium content. Although the complex interrelationship between the figures of merit and the composition is difficult to visualize, Table 2 below shows the average effect of each element on the various figures of merit over the compositional range. [Table 1]

[0035] To achieve a high level of creep resistance, the steels of the present invention also achieve a certain minimum creep index. This means that a certain amount of an element must be present in the steel to achieve a minimum creep index. Such a creep index is not achieved with compositions within the ranges of Table 1. High alloy steels with higher amounts of carbon and nickel and lower amounts of aluminum and chromium achieve the desired creep index of merit. The addition of molybdenum increases the likelihood of achieving a creep index of merit, while the addition of copper, manganese, niobium, and silicon decreases the likelihood of achieving a creep index of merit.

[0036] <Merit Index>

[0037] Nickel Aluminide Index: It is a measure of the volume fraction of the nickel aluminide phase that is stable at high temperatures. This volume fraction is determined using thermodynamic calculations. A low nickel aluminide index is an indication of microstructural stability and ductility. The equation that is proportional to the nickel aluminide volume fraction is: Nickel aluminide index = (34.487W Al + 0.209W Cr - 0.263W Cu - 1.538W Fe + 1.009W Mn + 0.171W Mo - 1.406W Ni + 16.880W Si + 0.949W W ) / 100 Here, W Cr , W Mn , W Cu , W Mo , W Ni , W Si , W W , W Al and W Fe are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminum and iron in the steel.

[0038] Preferably, the nickel aluminide index is 0.05983 or less, since this results in a low equilibrium volume fraction of nickel aluminide of about 10-15%. More preferably, the nickel aluminide index is 0.05128 or less, or even 0.04274 or less, or even 0.03419 or less, such that the equilibrium volume fraction of nickel aluminide is less than 10%. Nickel aluminide indexes of 0.01282 or less are also achievable, and most preferred.

[0039] Strength Merit Index: It reflects the yield strength of an alloy at room temperature. A high value is often, but not always, desirable because it indicates that thermal strains during repeated heating and cooling are less likely to cause plastic deformation, resulting in a longer service life for components made from high-strength alloys. The strength figure of merit is based on two assumptions. First, the as-cast grain size varies only slightly with composition, so the grain boundary strength is constant throughout the composition range. Second, the precipitation strength depends only on the volume fraction of precipitates. Variations in size distribution are ignored due to the complex solidification conditions and are assumed to be constant. Thus, the variation in yield strength is dominated by the solid solution strength. The formula for the strength figure of merit is:

number

[0040] where x i is the mole fraction of element i in austenite predicted by thermodynamic calculation, S i is its reinforcement factor, τ0 is the Taylor modulus (value 3.06), G is the shear modulus of steel (74 GPa), b is the length of the Burgers vector (2.5 nm), r p is the radius of the precipitate (assumed to be 1 μm), φ p is the volume fraction. The constant 300 MPa is due to grain boundaries and other strength contributions.

[0041] A formula was discovered that is proportional to strength based on the alloy composition. Strength index = 0.923W Al + 2.752W C - 0.037W Cr + 2.162W Cu + 1.342W Mn + 2.137W Mo - 0.224W Nb - 0.339W Ni + 5.684W Si + 0.667W W + 128.038 Here, W Cr , W Mn , W Cu , WMo , W Ni , W Si , W W , W Al , W Nb and W C are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminum, niobium and carbon in the steel.

[0042] Preferably, the strength figure of merit is 115.385 or greater so that a strength of at least about 260-280 MPa is achieved, and 119.658 or greater or 121.795 or greater so that even higher strengths of up to 285 MPa or greater are possible. Most preferably, the strength figure of merit is 128.205 or greater.

[0043] Creep Merit Index: In high carbon austenitic stainless steels, creep resistance is governed by two mechanisms: the inherent creep resistance of austenite due to its solid solution strength, and increased creep resistance due to the precipitation of secondary carbides at high temperatures. Both of these mechanisms are composition dependent and can be derived from thermodynamic calculations. It has been found in accordance with the present invention that the combined effect of the two mechanisms is approximated by the following equation: Creep Index = 10 5 (-0.0689W Al + 1.41W C - 0.0248W Cr - 0.0662W Cu - 0.0205W Mn + 0.0333W Mo - 0.182W Nb + 0.0499W Ni - 0.219W Si - 0.97) Here, W Al , W C , W Cr , W Mn , W Mo , W Ni , W Si , W Nb and W Cuare the amounts of aluminum, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel.

[0044] In the present invention, a steel with better creep resistance than the alloys of the prior art is desired. Thus, the minimum creep index is -7,000, resulting in a steel with not only good oxidation properties but also good creep resistance. It is possible to achieve even higher creep index values, preferably creep indexes of 0 or more, more preferably 10,000 or more, more preferably 30,000 or more. It is particularly preferred that many of the best performing alloys with high oxidation resistance achieve creep indexes of 50,000 or more. Some alloys achieve even higher creep indexes, more preferably creep indexes of 60,000 or more, more preferably 90,000 or more, most preferably 1,100,000 or more.

[0045] Chromium Activity Index: It reflects the ability of an alloy to form a protective chromia scale. Chromia forms significantly faster than alumina. The ability to form a continuous chromia scale therefore provides protection during the early stages of oxidation, allowing time for the alumina scale to grow at the chromia / metal interface. If this condition is not met, the formation of a continuous alumina scale may be prevented. Thus, a high chromium activity index improves oxidation resistance. In addition, chromia scale is more resistant to high temperature corrosion, which may be desirable in applications where sulfur or chloride ions are present. Chromium activity is obtained by thermodynamic calculations. The chromium activity index is a function of the alloying elements. Chromium Activity Index = 10 -5 (25.5W Al + 23.3W Cr + 13.8W Cu - 0.493W Fe + 1.11W Mn - 1.0W Mo + 22.8W Nb + 4.34W Ni + 11.4W Si ) Here, W Cr , W Mn , W Cu , W Mo , W Ni , W Si , W Al , W Nb and W Fe are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, aluminum, niobium, and iron in the steel.

[0046] Preferably, the chromium activity index is greater than or equal to 0.00455. The chromium activity index can be even greater, preferably having a chromium index of at least 0.0050, even at least 0.0060, or even at least 0.0070.

[0047] Chromium Diffusion Index: It reflects the ability of an alloy to reform a protective chromia scale or to reform a protective chromia scale after spalling. Rapid chromia formation promotes the formation of an alumina scale, improving oxidation resistance. The growth of the chromium scale reduces the chromium concentration directly beneath it somewhat compared to the bulk metal. The extent of chromium depletion is particularly severe when the chromium diffusion rate is low. The rate of oxidation after spalling depends on the composition of this depleted layer. If the degree of depletion is high, the chromia scale cannot form and various porous, non-adherent oxides may form instead. It is only when oxygen passes through this depleted layer that the local chromium concentration becomes high enough that chromia formation begins again. Conversely, a high chromium diffusion rate results in a low degree of chromium depletion. Thus, the chromia scale reforms rapidly after the spalling event, slowing the overall oxidation rate.

[0048] The diffusivity of chromium can be adjusted because the interdiffusion of elements in austenite is not constant but depends on the composition of the austenite. Nickel and copper in particular are known to increase it. The interdiffusion coefficient of chromium in austenite can be determined by thermodynamic calculations. An approximation using a linear combination of the alloying elements has been found. Chromium diffusion index = 10 -20 (171.0W Al + 12.0W Cr + 39.2W Cu + 0.964W Fe - 8.56W Mn + 3.09W Mo + 6.72W Nb + 6.87W Ni - 76.9W Si + 4.96W W ) Here, W Cr , W Mn , W Cu , W Mo , W Ni , W Si , W W , W Al , W Nb and W Fe are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminum, niobium, and iron in the steel.

[0049] The chromium diffusion index is preferably 8.7E-18 or greater, although greater values ​​are possible and it is desirable to achieve at least 9.0E-18, or at least 9.2E-18. It is also possible and desirable to achieve 9.5E-18 or at least 1E-17. The most preferred alloys achieve a chromium diffusion index of 1.2E-17 or greater.

[0050] Cost Index: Reflects the cost (in 2021 GBP / kg) of the raw materials required to produce the alloy. Assuming we start with pure elements, a simple formula can be used for the cost of the alloy: Cost index = 0.065W Cr + 0.047W Cu + 0.008W Fe + 0.017W Mn + 0.327W Mo + 0.484W Nb + 0.12W Ni + 0.013W Si + 0.363WW + 0.015W Al Here, W Cr , W Mn , W Cu , W Mo , W Ni , W Si , W W , W Nb , W Al and W Fe are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, niobium, aluminum, and iron in the steel.

[0051] The cheaper the alloy, the lower the cost index. Desirably the cost index is 7.5 or less. However, even cheaper alloys are possible within the scope of the present invention, so it is more desirable to have a cost index of 6.5 or less, 6.0 or less, 5.0 or less, or 4.6 or less.

[0052] Oxidation Index: According to Sato, A., YL. Chiu and RC Reed., "Oxidation of nickel-based single-crystal superalloys for industrial gas turbine applications," Acta Materialia 59.1 (2011): 225-240, the requirement for internal oxidation-limited continuous alumina scale formation depends on the activity of aluminum in the alloy, the Gibbs free energy of alumina scale formation (ΔG AL2O3 ) and the effective valence of cations in the alumina scale (V eff ) and Oxidation Index (OIF), both quantities being highly dependent on the alloy composition. Alloys that form a continuous alumina scale have high Oxidation Index values, which are the weighted sum of the effective valence of the alumina scale and the Gibbs free energy of alumina formation.

number

[0053] The constants k1 and k2 are phenomenological and derived from numerous oxidation experiments of aluminum-containing austenitic steels described in the literature. Thermodynamic calculations are required to obtain accurate values ​​of the oxidation exponents. Based on the alloy composition, the following relationships have been found: Oxidation Index = 10 -3 (4.67W Al + 16.9W C - 1.45W Cr + 5.81W Cu - 11.6W Mn - 4.8W Mo - 2.19W Nb + 0.768W Ni + 1.23W Si + 5.14) Here, W Cr , W Mn , W Cu , W Mo , W Ni , W Si , W Al , W Nb and W C are the amounts of chromium, manganese, copper, molybdenum, nickel, silicon, aluminum, niobium and carbon in the steel.

[0054] It is preferred that the oxidation index is 0.01 or greater. Higher oxidation indices are achievable, preferably 0.02 or greater, or even 0.03 or greater. Values ​​up to 0.045% or greater are achievable and are desirable. High oxidation indices distinguish the alloys of the present invention because they allow high oxidation resistance to be achieved without sacrificing other desirable properties. Alloys having an oxidation index of 0.03 or greater are most preferred, and the examples in Tables 3 and 4 show that this can be achieved while also having a creep index of 50,000 or greater (e.g., Examples 6, 27, and 28). The combination of an oxidation index of 0.03 or greater and a creep index of 50,000 or greater is especially preferred.

[0055] The oxidation index only indicates the possibility of the formation of a continuous alumina scale due to thermodynamic effects. However, since the formation of alumina is kinetically slow, it is preferable for the alloy to be able to form a protective chromia scale in the early stages of oxidation. This promotes the formation of alumina scale and inhibits the internal oxidation of aluminum in solid solution. To achieve optimal oxidation resistance, the alloy should exhibit sufficiently high Cr activity index and Cr diffusion index values.

[0056] The approximate effect of the evaluated alloying elements on the individual figures of merit are shown in Table 2 below. The scores in the table were derived as the product of the coefficient of the element for a given figure of merit multiplied by the average value of the element range within the claimed composition range. These values ​​were then divided by the average of the weighted coefficient values ​​to allow comparison of the effect of the element across the various figures of merit (columns). Large negative values ​​represent a strong negative effect, and large positive values ​​represent a strong positive effect. The table summarizes how an element can affect multiple key properties simultaneously. The table also shows that some elements are generally more important than others. For example, a change in Mn does not have a significant effect on the figures of merit, except for a decrease in the oxidation index. On the other hand, Al has a strong positive effect on the nickel aluminide index, strength index, Cr diffusion index, and oxidation index, and a strong negative effect on the creep index. [Table 2]

[0057] Using the figure of merit, they were able to explore complex trade-offs between desirable properties and discover alloys with the optimal combination of these properties.

[0058] The elements and their ranges in Table 1 were selected for the following reasons.

[0059] nickel: Due to its slow rate of diffusion in the austenite matrix phase, it stabilizes the austenite phase and also greatly improves creep resistance. In addition, it greatly improves the oxidation resistance of alumina-forming steels by lowering the effective valence of the scale and increasing the activity of aluminum, thereby promoting the formation of a continuous alumina scale. On the other hand, high nickel additions are prohibitively expensive. Nickel contributes significantly to precipitation strengthening by forming nickel aluminide and gamma prime phases. On the other hand, nickel aluminide is an undesirable phase, and gamma prime is unlikely to form unless both nickel and aluminum contents are very high. Therefore, the strength model places more emphasis on solid solution strength. Literature on solid solution strength indicates that in Ni-Fe solid solutions Ni has little or even a negative effect on yield strength. Nickel is therefore considered to have a net negative effect on strength. The best balance of properties was found to be achieved with a minimum nickel content of 16.0% by weight and a maximum nickel concentration of 50.0% by weight. The presence of nickel is beneficial for all merit indices except cost and strength. Thus, a minimum amount of nickel is required, particularly to achieve the desired creep strength, oxidation resistance, and the desired volume fraction of nickel aluminides, all of which improve with increasing nickel concentration. Thus, it is desirable to have a nickel concentration of 30.0% or more, by weight, and more preferably 35.0% or more. Increasing the nickel concentration to 37.0% or more, by weight, or more, or 40% or more, increases cost and reduces strength to some extent, but further improves the alloy's properties. In one embodiment, nickel is present in an amount of 42.0% or more, by weight, which further improves oxidation and creep resistance. If desired, the loss of strength at higher nickel concentrations can be at least partially offset by the use of preferred higher silicon and aluminum concentrations, and possibly the addition of copper and molybdenum as alloying elements. In some applications, cost is a key factor.Thus, in some embodiments, the amount of nickel is limited to not more than 45.0 wt.%, or even not more than 30.0 wt.%, and preferably not more than 26.0 wt.%.

[0060] chromium: It provides solid solution strengthening and improves creep resistance. It also forms a protective chromia scale that is a source of oxidation resistance and acts as a barrier against further oxidation at high temperatures. The fast-growing chromia scale provides oxidation resistance during the initial stages of oxidation and then promotes the formation of a slower-growing and more protective alumina scale. Therefore, a sufficient chromium content is necessary for oxidation resistance. However, high chromium additions stabilize ferrite and various harmful intermetallic compounds, including sigma phases. Their presence significantly reduces the ductility and oxidation resistance of the alloy. Therefore, the chromium content should be greater than or equal to 10.0% and less than or equal to 27.0%, by weight. Increasing the chromium content within this range helps to maintain the chromium scale at intermediate temperatures. Therefore, it is preferred that the chromium content is at least 11.0%, by weight. In some embodiments where oxidation resistance is paramount, the chromium content is preferably greater than or equal to 12.0%, even more preferably greater than or equal to 16.0%, and most preferably greater than or equal to 22.0%, by weight. On the other hand, higher chromium concentrations may reduce oxidation resistance due to insufficient formation of alumina scale at high temperatures. Thus, in some embodiments, chromium is limited to 20.0% by weight or less, which distinguishes the alloys of the present invention from other similar alloys and provides significantly better oxidation resistance. Preferably, the alloys have 16.0% by weight or less chromium, more preferably 13.0% by weight or less chromium.

[0061] Silicon: It strengthens the solid solution, improves the castability of the alloy, and deoxidizes the melt. On the other hand, the presence of silicon is optional, since the other elements of the present invention can also fulfill the above roles. Furthermore, the addition of large amounts of silicon stabilizes the G and Laves phases, which are intermetallic compounds that are detrimental to ductility. Therefore, silicon is an optional element. Therefore, the maximum allowable amount of silicon is specified to be 2.5% by weight or less. Nevertheless, small amounts of silicon (e.g., 0.2% by weight or more) are particularly beneficial for improving the strength of the alloy and improving its oxidation resistance at low temperatures. Conversely, reducing the amount of silicon (e.g., 1.5% by weight or less) reduces the precipitation of intermetallic phases and improves ductility. The amount of silicon is preferably at a level of 1.0% by weight or less. This further improves ductility.

[0062] carbon: Carbon provides solid solution strengthening, stabilizes the austenite phase, and forms a characteristic network of dendritic eutectic carbides that impart the alloys of the present invention with their characteristic creep resistance and high temperature strength. The addition of carbon improves secondary precipitation and contributes to creep resistance. On the other hand, excessive carbon addition leads to an undesirably large volume fraction of dendritic carbides, which adversely affects the ductility of the alloy. Thus, the carbon range is 0.1% to 0.75%, by weight. By increasing the carbon content, for example, to 0.4% or more, or even 0.45% or more, more preferably 0.475% or more, improvement in creep resistance can be achieved. However, if the carbon content is increased too much, ductility may decrease, so in one embodiment, the carbon content of the steel is 0.6% or less, by weight. In some embodiments, the carbon is further reduced, for example, to 0.4% or less, or even 0.3% or less, by weight. This completely improves the ductility of the alloy.

[0063] manganese: It provides solid solution strengthening, stabilizes the austenite phase, and neutralizes the embrittlement effects of sulfur impurities by forming manganese sulfides. On the other hand, excessive additions stabilize primary delta ferrite that may precipitate in the as-cast material, reducing creep resistance and acting as nucleation sites for detrimental sigma phases. Manganese is also known to adversely affect oxidation resistance by promoting the formation of manganese-rich oxide phases that are oxygen permeable and prone to spalling. The inventors have discovered that in many, but not all, cases the disadvantages of manganese outweigh the advantages, and the required physical properties can often be achieved without manganese present. Thus, manganese is an optional element, and manganese is limited to 0.75% or less, preferably 0.5% or less, or even 0.3% or less, by weight. In one embodiment, the addition of manganese in an amount of 0.1% or more, by weight, reduces the risk of sulfur embrittlement. In one embodiment, the addition of manganese in an amount of 0.2% or more, by weight, further reduces the possibility of sulfur embrittlement.

[0064] molybdenum: It provides solid solution strengthening and creep resistance. However, excessive addition increases the cost of the alloy and tends to stabilize various brittle intermetallic compounds such as sigma and Laves phases. Therefore, molybdenum is an optional element and the amount of molybdenum is limited to 0.4% by weight or less. The addition of small amounts of molybdenum, 0.1% by weight or more, is mandatory to achieve solid solution strengthening without the risk of harmful intermetallic compounds. Further additions of molybdenum to 1.5% by weight or more, or even 2.5% by weight or more, can significantly increase the solid solution strength, which is an optional feature of the present invention. However, in one embodiment, the molybdenum content of the steel is limited to 0.5% by weight or less (or 0.3% by weight or less) to reduce the possibility of the formation of sigma and Laves phases.

[0065] tungsten: Like molybdenum, tungsten provides solid solution strengthening, but the benefits to creep resistance are reduced. On the other hand, excessive additions increase the cost of the alloy and tend to stabilize various brittle intermetallic compounds such as sigma and Laves phases. In some embodiments of the present invention, tungsten can be present optionally, but is limited to 3.0% by weight or less. Preferably, tungsten is present at even lower levels (1.5% by weight or less, or even 1.0% by weight or less, or 0.5% by weight or less). On the other hand, when a better creep-resistant alloy is required, especially in steels with low molybdenum content, tungsten can be present in an amount of 0.5% by weight or more.

[0066] copper: It improves oxidation resistance by stabilizing the austenite phase and by increasing Cr activity and Cr diffusivity in chromia-forming alloys and by lowering the effective valence of the alumina scale in alumina-forming alloys. On the other hand, high Cu content can lead to poor castability due to Cu segregation at grain boundaries, embrittling the material, and even the presence of a Cu-rich liquid film around 1000°C. The liquid film devastatingly reduces ductility and strength at high temperatures. Cu also stabilizes various carbide phases and promotes carbide precipitation in the melt. This creates large primary carbides that adversely affect ductility and fatigue life. Thus, copper additions are beneficial and copper is optional, but in any case is limited to 4.0% by weight or less. Due to the risks of copper, in some embodiments copper is limited to 2.5% by weight or less, preferably 2.0% by weight or less, more preferably 1.5% by weight or less, or even 0.5% by weight or less. However, in one embodiment, copper is added in an amount of 0.2 wt % or more due to the potential benefits of adding copper, particularly to improve oxidation resistance, and since increasing the amount of copper further improves oxidation resistance, an amount of copper of 0.5 wt % or more, preferably 0.75 wt % or more, more preferably 1.5 wt % or more, or even 2.2 wt % or more may be beneficial for certain applications.

[0067] niobium: By forming hard and stable dendritic carbides, it significantly improves creep resistance and high temperature strength. On the other hand, excessive additions are expensive and stabilize the ferrite phase, promoting the formation of brittle intermetallic compounds such as sigma and G phases. Niobium is therefore an optional element, limited in any case to 2.0% by weight or less. Niobium is preferably present in an amount of 1.0% by weight or less, more preferably 0.2% by weight or less, or even 0.01% by weight or less, in order to keep the cost of the alloy low and ensure better machinability. On the other hand, small additions of niobium (0.6% by weight or more, or even 1.0% by weight or more) form a network of very hard MC carbides in the interdendritic regions, improving the strength of the material at very high temperatures, and therefore this level of niobium is preferred. In one embodiment, niobium is present in an amount of 1.5% by weight or more.

[0068] aluminum: It improves oxidation resistance by forming a protective oxide scale on the surface of the alloy. In some cases, it can also significantly improve the yield strength of the alloy up to moderate temperatures by forming gamma prime (Ni3Al) precipitates with nickel. The activity of aluminum in the alloy must be high enough to form a continuous aluminia scale. However, excessive additions can lead to reduced ductility and reduced creep resistance due to stabilization of NiAl precipitates. Aluminum is therefore present in an amount of 2.0% to 6.5%, by weight. Increasing the aluminum content (e.g., 3.5% or more, or even 4.0% or more, by weight) improves oxidation resistance. For this reason, it is even more preferred to increase the aluminum content to 4.3% or even 4.5%, by weight. Reducing the maximum amount of aluminum to 5.5% or less, or even 5.0% or less, by weight, can be beneficial due to increased ductility and increased creep resistance. In one embodiment, aluminum is limited to an amount of 3.5% or less, by weight.

[0069] vanadium: During coagulation, M 23It promotes the formation of hard M7C3 carbides on the C6 carbides, resulting in improved high temperature strength. It also reduces the M 23 It also promotes the precipitation of secondary MC carbides on the C6 carbides. 23 It is more effective at improving creep resistance than C6 carbides. It also coarsens slower, which means that the improvement in mechanical properties (yield strength and creep resistance) due to secondary precipitation decreases very slowly over time. However, excessive addition significantly increases the cost of the alloy. In some embodiments of the present invention, vanadium can be optionally present. Vanadium is therefore an optional element, limited in any case to 2.5% by weight or less. Due to the drawbacks of including vanadium, it is preferred that vanadium is present in an amount of 0.3% by weight or less, or even 0.2% by weight or less, or even 0.1% by weight or less. Nevertheless, vanadium can be included as an essential element in an amount of 0.1% by weight or more if an alloy with maximum high-temperature creep resistance is desired. Increasing vanadium further increases high-temperature creep resistance, so the minimum amount can be 0.3%, 1.0%, or even 1.5% or 1.9% by weight.

[0070] Boron: Boron segregates at grain boundaries to strengthen the grain boundaries and improve the high temperature strength and creep resistance of the alloy. However, high additions of boron can reduce ductility. In some embodiments of the invention, boron can be present as an optional element. Although boron is an optional element, it may be present in amounts up to 0.15% by weight. Due to the benefits of adding boron, boron may be added in amounts of 0.03% by weight or more, or even 0.07% by weight or more, particularly when improved creep resistance is desired. When boron is present in amounts up to 0.01% by weight, a more ductile alloy is obtained.

[0071] In addition, the alloy of the present invention may contain any combination of trace alloying elements not exceeding 1.0% by weight. Trace alloying elements include lanthanides (atomic elements 57 (lanthanum) to 71 (lutetium)) for better carbide morphology and improved oxidation resistance, hafnium, zirconium and boron for grain boundary strengthening effects, yttrium for improved oxidation resistance, and titanium for improved resistance to coarsening of MC carbides (where M is primarily Nb with some substitution by Ti, V, or Zr). The amount of titanium is preferably limited to 0.15% by weight or less. The presence of titanium may be beneficial since it can partially replace low levels or the absence of niobium. Yttrium is preferably 0.01% by weight or less, and cerium is preferably 0.01% by weight or less. Yttrium and rare earths are known to improve oxide scale adhesion and resistance to high temperature corrosion. On the other hand, if yttrium and rare earths are contained in large amounts, segregation and deterioration of mechanical properties may occur in the casting. In addition, due to their reactivity, processing becomes difficult. Hafnium is expensive, so it is preferable to limit its content to 0.01% by mass or less.

[0072] Lynn: Phosphorus is an impurity. Phosphorus segregates at grain boundaries and reduces the SSC resistance of steel. Therefore, the phosphorus content is 0.05 mass% or less. The phosphorus content is preferably 0.02 mass% or less. The phosphorus content is preferably as low as possible.

[0073] sulfur: Sulfur is an impurity that segregates at grain boundaries. The cohesive forces are reduced, which can significantly reduce the ductility, strength and creep resistance of the material. Therefore, the sulfur content is not more than 0.01% by weight. The preferred sulfur content is not more than 0.005% by weight, more preferably not more than 0.003% by weight. It is preferable that the sulfur content is as low as possible. However, in some cases it may be desirable to increase the sulfur content, as this makes the alloy easier to machine. In this case, the sulfur content is preferably 0.01% to 0.1% by weight.

[0074] Selenium, tellurium, antimony, bismuth, lead: These elements are impurities that segregate at grain boundaries. They reduce the cohesive force of the grain boundaries, which can significantly reduce the ductility, strength, and creep resistance of the material. Therefore, their content is 0.005% by mass or less, preferably 0.001% by mass or less, and more preferably 0.0003% by mass or less. It is preferable that these contents are as low as possible. However, it may be desirable to increase the content of these elements in order to make the alloy easier to machine. In this case, their total content is preferably 0.005% by mass or more and 0.04% by mass or less.

[0075] calcium: It is added to the melt and tends to promote the formation of softer oxide inclusions than the harder oxide inclusions normally formed by Si and Al that may be present in the melt. It also favorably modifies sulfide inclusions. Both mechanisms contribute to improving the machinability of the alloy. The calcium content is preferably 0.005% to 0.04%, by weight.

[0076] Up to 2.0% by weight of zinc can be added, which improves oxidation resistance. In one embodiment, at least 0.5% by weight of zinc is added. Zinc is an optional element. In one embodiment, zinc is present in an amount of 0.1 weight percent or less.

[0077] Cobalt behaves similarly to nickel, so the cobalt content is up to 5.0 wt.%. Cobalt is a slow diffusing element and improves creep resistance. In one embodiment, the sum of cobalt and nickel is in the range of 16-50 wt.%, or other ranges given for nickel alone elsewhere herein. However, cobalt is even more expensive than nickel, so its content is limited to 0.5 wt.% or less. Cobalt is an optional addition. In one embodiment, the alloy is substantially free of cobalt.

[0078] Additionally, the alloy may contain small amounts of other unavoidable impurity elements not listed in the sections above.

[0079] FIG. 1 shows the dependence of the oxidation index (black solid contour lines), cost index (dark grey dotted contour lines) and chromium activity index (light grey contour lines) plotted against the Cr and Ni contents in the compositional range of Table 1. As can be seen from FIG. 1, an increase in nickel content increases the chromium activity merit index and the oxidation activity merit index, both of which are favorable. On the other hand, an increase in nickel content also leads to an increase in cost, which is unfavorable. As can be seen from FIG. 1, an increase in chromium content increases the cost slightly. But most importantly, an increase in chromium content increases the activity of chromium. On the other hand, an increase in chromium content too much has a negative effect on oxidation resistance. These trade-offs are shown in Table 2. In Table 2, a low cost merit index and nickel aluminide merit index are desired, while a high strength merit index, creep merit index, chromium activity index, chromium diffusion index and oxidation merit index are advantageous. Based on such trade-offs, the compositional range of the alloys listed in Table 1 was determined.

[0080] Examples and Comparative Examples

[0081] Experiments were carried out on some of the examples and comparative examples in Table 3. Comparative examples 3, 4, 5, and 6 are experimental austenitic stainless steels, while comparative examples 1 and 2 are well-known austenitic heat-resistant steels corresponding to DIN standards 1.4848 and 1.4849, respectively. Both are known for their good oxidation resistance but relatively low yield strength, and are widely used as heat-resistant materials. However, comparative examples 1 and 2 are chromia formers rather than alumina formers. Comparative examples 7 to 11 are prior art alloys not falling within the scope of the present invention. [Table 3]

[0082] For comparison, the figure of merit values ​​for the alloys in Table 3 are given in Table 4.

[0083] [Table 4]

[0084] As can be seen from Table 4, all of the inventive examples achieve most of the desired figures of merit mentioned above. Only Example 2 does not fully achieve the desired minimum chromium diffusion index (8.7E-18) or oxidation index (>0.01). Example 3 also does not fully achieve the desired minimum oxidation index (0.01) (and is rather expensive). Examples 27 and 28 do not fully achieve the desired nickel aluminide index (0.0598 or less). Nevertheless, the improvement in properties of the inventive examples over the well-known comparative examples 1 and 2 is evident in both the chromium diffusion index and oxidation index achieved by these alloys. Comparative example 3 has a low oxidation index due to too high a level of manganese, and comparative examples 3-8 and 11 all have low creep figures of merit. Comparative examples 7-10 show that the proportion of nickel aluminide is too high. Comparative examples 9 and 10 are very expensive, and comparative example 10 has poor oxidation resistance. Comparative Examples 9 and 10 achieve good creep resistance, but at the expense of excessively high cost and an excessively high volume fraction of nickel aluminide. Comparative Example 9 also has relatively low strength and a low strength index.

[0085] FIG. 2 shows the results of comparing the mass gain during cyclic oxidation testing of alloy samples of the present invention and Comparative Example 2. The oxidation tests were carried out in a box furnace at 1100° C. for various periods of time (as shown in the figure). The samples were placed in an open alumina boat, which was then inserted into a preheated furnace. After exposure, the boat was removed from the furnace, air cooled for 1 hour, and weighed. Examples 4 and 10 showed stable and moderate mass gain, indicating that a protective alumina scale was formed. Comparative Example 2, being a chromia-forming alloy, showed rapid and significant mass loss, which was likely due to the evaporation and spalling of the scale. Thus, the experimental results indeed show that the composition of Comparative Example 2 has poor oxidation resistance due to its low chromium diffusion index and oxidation index.

[0086] 3 shows the Vickers hardness (load 5 kg, dwell time 10 seconds) of an example of the present invention. Compared to the comparative example, the Vickers hardness value is equivalent, which indicates that the yield strength is equivalent.

[0087] FIG. 4 shows the Larson-Miller parameter values ​​(constant term 20) for creep tests performed at 950°C and 60 MPa for Example 10 of the present invention and Comparative Example 1. For each alloy, two samples were tested, so a total of four creep tests were performed. Compared to the Comparative Example, Example 10 of the present invention shows a larger Larson-Miller parameter value due to its longer creep rupture life.

[0088] Example 6 is a particularly promising composition having very high creep resistance (aided by the high nickel content) and very good oxidation resistance. Examples 27 and 28 also achieve a good balance of properties while reducing cost compared to Example 6.

[0089] Table 5 shows the elemental ranges for the steel of the present invention, with preferred ranges for the major alloying elements, with other elements being substantially absent, for example at or near the minimum levels shown herein. [Table 5]

[0090] A preferred steel having particularly high creep resistance has the following element ranges: [Table 6]

[0091] A preferred steel having particularly good oxidation resistance has the following element ranges: [Table 7]

[0092] Other preferred steels having particularly good oxidation resistance have the following element ranges: [Table 8]

Claims

1. In mass%, 16-50% nickel, 10-27% chromium, 0.1 to 0.75% carbon, 2.0 to 6.5% acid-soluble Al; not more than 2.5% silicon; not more than 0.75% manganese; not more than 4.0% copper; not more than 4.0% molybdenum; not more than 3.0% tungsten, not more than 2.0% niobium, not more than 2.5% vanadium, not more than 0.15% boron; 0.04% or less calcium, not more than 2.0% zinc; not more than 5.0% cobalt; not more than 0.05% phosphorus; Less than 0.1% sulfur Including, the total content of lanthanide elements, hafnium, zirconium, yttrium, cerium, and titanium is 1.0% by mass or less, The total content of selenium, tellurium, antimony, bismuth, and lead is 0.04% by mass or less, The balance is iron and unavoidable impurities, A steel that satisfies the following formula (1): 10 5 (-0.0689W Al + 1.41W C - 0.0248W Cr - 0.0662W Cu - 0.0205W Mn + 0.0333W Mo - 0.182W Nb + 0.0499W Ni - 0.219W Si - 0.97)≧-7000 (1) where W Al , W C , W Cr , W Mn , W Mo , W Ni , W Si , W Nb and W Cu are the amounts of aluminum, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel.

2. The amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, niobium, aluminum, and iron in the steel are expressed as W, Cr , W Mn , W Cu , W Mo , W Ni , W Si , W W , W Nb , W Al and W Fe The steel according to claim 1, which satisfies the following formula: 0.065W Cr + 0.047W Cu + 0.008W Fe + 0.017W Mn + 0.327W Mo + 0.484W Nb + 0.12W Ni + 0.013W Si + 0.363W W + 0.015W Al ≦ 7.5 (2)

3. The amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminum, and iron in the steel are expressed as W, Cr , W Mn , W Cu , W Mo , W Ni , W Si , W W , W Al and W Fe The steel according to claim 1, which satisfies the following formula: (34.487W Al + 0.209W Cr - 0.263W Cu - 1.538W Fe + 1.009W Mn + 0.171W Mo - 1.406W Ni + 16.880W Si + 0.949W W ) / 100 ≦ 0.05983 (3)

4. The amounts of chromium, manganese, copper, molybdenum, nickel, silicon, aluminum, niobium, and carbon in the steel are expressed as W, ... Cr , W Mn , W Cu , W Mo , W Ni , W Si , W Al W Nb and W C The steel according to claim 1, which satisfies the following formula: 10 -3 (4.67W Al + 16.9W C - 1.45W Cr + 5.81W Cu - 11.6W Mn - 4.8W Mo - 2.19W Nb + 0.768W Ni + 1.23W Si + 5.14) ≧ 0.01 (4)

5. The amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminum, niobium, and carbon in the steel are expressed as W, W, and W, respectively. Cr , W Mn , W Cu , W Mo , W Ni , W Si , W W , W Al , W Nb and W C The steel according to claim 1, which satisfies the following formula: <h2 style=";text-align:left;direction:ltr">0.923W<h2 style=";text-align:left;direction:ltr"> Al <h2 style=";text-align:left;direction:ltr"> + 2.752W<h2 style=";text-align:left;direction:ltr"> C <h2 style=";text-align:left;direction:ltr"> - 0.037W<h2 style=";text-align:left;direction:ltr"> Cr <h2 style=";text-align:left;direction:ltr"> + 2.162W<h2 style=";text-align:left;direction:ltr"> Cu <h2 style=";text-align:left;direction:ltr"> + 1.342W<h2 style=";text-align:left;direction:ltr"> Mn <h2 style=";text-align:left;direction:ltr"> + 2.137W<h2 style=";text-align:left;direction:ltr"> Mo <h2 style=";text-align:left;direction:ltr"> - 0.224W<h2 style=";text-align:left;direction:ltr"> Nb <h2 style=";text-align:left;direction:ltr"> - 0.339W<h2 style=";text-align:left;direction:ltr"> Ni <h2 style=";text-align:left;direction:ltr"> + 5.684W<h2 style=";text-align:left;direction:ltr"> Si <h2 style=";text-align:left;direction:ltr"> + 0.667W<h2 style=";text-align:left;direction:ltr"> W <h2 style=";text-align:left;direction:ltr"> + 128.038 ≧ 115.385 (5)

6. The amounts of chromium, manganese, copper, molybdenum, nickel, silicon, aluminum, niobium, and iron in the steel are expressed as W, ... Cr , W Mn , W Cu , W Mo , W Ni , W Si , W Al , W Nb and W Fe The steel according to claim 1, which satisfies the following formula: 10 -5 (25.5W Al + 23.3W Cr + 13.8W Cu - 0.493W Fe + 1.11W Mn - 1.0W Mo + 22.8W Nb + 4.34W Ni + 11.4W Si ) ≧0.00455 (6)

7. The amounts of chromium, manganese, copper, molybdenum, nickel, silicon, tungsten, aluminum, niobium, and iron in the steel are expressed as W, Cr , W Mn , W Cu , W Mo , W Ni , W Si , W W , W Al , W Nb and W Fe The steel according to claim 1, which satisfies the following formula: <h2 style=";text-align:left;direction:ltr">10<h2 style=";text-align:left;direction:ltr"> -20 <h2 style=";text-align:left;direction:ltr"> (171.0W<h2 style=";text-align:left;direction:ltr"> Al <h2 style=";text-align:left;direction:ltr"> + 12.0W<h2 style=";text-align:left;direction:ltr"> Cr <h2 style=";text-align:left;direction:ltr"> + 39.2W<h2 style=";text-align:left;direction:ltr"> Cu <h2 style=";text-align:left;direction:ltr"> + 0.964W<h2 style=";text-align:left;direction:ltr"> Fe <h2 style=";text-align:left;direction:ltr"> - 8.56W<h2 style=";text-align:left;direction:ltr"> Mn <h2 style=";text-align:left;direction:ltr"> + 3.09W<h2 style=";text-align:left;direction:ltr"> Mo <h2 style=";text-align:left;direction:ltr"> + 6.72W<h2 style=";text-align:left;direction:ltr"> Nb <h2 style=";text-align:left;direction:ltr"> + 6.87W<h2 style=";text-align:left;direction:ltr"> Ni <h2 style=";text-align:left;direction:ltr"> - 76.9W<h2 style=";text-align:left;direction:ltr"> Si <h2 style=";text-align:left;direction:ltr"> + 4.96W<h2 style=";text-align:left;direction:ltr"> W <h2 style=";text-align:left;direction:ltr"> ) ≧ 8.7 E-18

8. The amounts of aluminum, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel are respectively represented by W Al , W C , W Cr , W Mn , W Mo , W Ni , W Si , W Nb and W Cu The steel according to claim 1, which satisfies the following formula: 10 5 (-0.0689W Al + 1.41W C - 0.0248W Cr - 0.0662W Cu - 0.0205W Mn + 0.0333W Mo - 0.182W Nb + 0.0499W Ni - 0.219W Si - 0.97) ≧ 0

9. 2. The steel according to claim 1, containing, by mass %, 30% or more of nickel.

10. 2. The steel according to claim 1, containing up to 45% by mass of nickel.

11. 2. The steel according to claim 1, containing, by mass, up to 0.3% vanadium.

12. 2. The steel according to claim 1, containing, by mass, at least 0.2% silicon.

13. 2. The steel according to claim 1, containing up to 1.5% silicon, in mass percent.

14. 2. The steel according to claim 1, containing, by mass %, at least 0.1% manganese.

15. 2. The steel according to claim 1, containing, by mass %, 11.0% or more of chromium.

16. 2. The steel according to claim 1, containing, in mass %, up to 20.0% chromium.

17. 2. The steel according to claim 1, containing up to 1.5% by mass of tungsten.

18. 2. The steel according to claim 1, containing, by mass, at least 0.1% molybdenum.

19. 2. The steel according to claim 1, containing, by mass, up to 0.5% molybdenum.

20. 2. The steel according to claim 1, containing titanium in an amount of up to 0.15% by mass.

21. 2. The steel according to claim 1, containing niobium in an amount of not more than 1.0% by mass.

22. 2. The steel according to claim 1, containing, by mass %, 0.6% or more of niobium.

23. 2. The steel according to claim 1, containing, by mass %, at least 0.03% boron.

24. 2. The steel according to claim 1, containing, by mass %, 0.1% or more of vanadium.

25. 2. The steel according to claim 1, containing, by mass %, at least 0.5% zinc.

26. 2. The steel according to claim 1, containing, by mass, 0.5% or more of tungsten.

27. 2. The steel according to claim 1, containing, by mass%, 0.4% or more of carbon.

28. 2. The steel according to claim 1, containing, by mass, not more than 0.6% carbon.

29. 2. The steel according to claim 1, containing, by mass %, at least 3.5% aluminum.

30. 2. The steel according to claim 1, containing up to 5.5% by mass of aluminum.

31. 2. The steel according to claim 1, containing, by mass, at least 0.2% copper.

32. 2. The steel according to claim 1, containing, by mass, up to 2.5% copper.

33. 2. The steel according to claim 1, containing yttrium in an amount of not more than 0.01% by mass.

34. 2. The steel according to claim 1, containing cerium in an amount of not more than 0.01% by mass.

35. 2. The steel according to claim 1, containing, by mass, up to 0.5% manganese.

36. 2. The steel according to claim 1, containing, by mass, not more than 0.01% boron.

37. 2. The steel according to claim 1, containing, by mass, up to 0.1% zinc.

38. 2. The steel according to claim 1, containing hafnium in an amount of not more than 0.1% by mass.

39. The amounts of aluminum, carbon, chromium, manganese, molybdenum, nickel, silicon, niobium, and copper in the steel are respectively represented by W Al , W C , W Cr , W Mn , W Mo , W Ni , W Si , W Nb and W Cu The steel according to claim 1, which satisfies the following formula: 10 -3 (4.67W Al + 16.9W C - 1.45W Cr + 5.81W Cu - 11.6W Mn - 4.8W Mo - 2.19W Nb + 0.768W Ni + 1.23W Si + 5.14) ≧ 0.03 and 10 5 (-0.0689W Al + 1.41W C - 0.0248W Cr - 0.0662W Cu - 0.0205W Mn + 0.0333W Mo - 0.182W Nb + 0.0499W Ni - 0.219W Si - 0.97) ≧ 50000

40. A steel comprising, by weight, about 43% nickel, about 12% chromium, about 0.5% carbon, about 0.3% silicon, about 0.22% manganese, about 4.8% aluminum, about 2.0% copper, and the balance being iron and unavoidable impurities.

41. A cast product made from a steel according to any one of claims 1 to 40.

42. A turbocharger housing made from a steel according to any one of claims 1 to 40.

43. 42. The turbocharger of claim 41, wherein the turbocharger housing is a cast product.

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