Heat-resistant austenitic stainless steel

Abstract

A steel containing, by mass%, 0.35-0.6% carbon, 0.35-2.0% silicon, 6.0-21.0% manganese, 7.0-16.0% nickel, 21.5-26.5% chromium, 3.5% or less tungsten, 2.2% or less niobium, 0.1-0.75% nitrogen, 4.0% or less copper, 4.0% or less molybdenum, 0.1% or less acid-soluble Al (sol.Al), 0.05% or less phosphorus, 0.3% or less sulfur, and 0.04% or less calcium, the total of the rare earth elements, hafnium, zirconium, boron, yttrium, titanium, calcium, magnesium, and vanadium 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.

Description

[Technical field]

[0001] The present invention relates to a heat-resistant stainless steel. [Background technology]

[0002] Heat-resistant stainless steels are suitable for casting. One application for stainless steels is in turbocharger components, such as the turbocharger housing.

[0003] In vehicles with turbocharged combustion engines, there is a trend to increase the operating temperature of the turbocharger to improve engine efficiency. At the time of writing, operating temperatures often exceed 1050 °C and are expected to increase further. Currently, traditional cast stainless steel grades such as 1.4837, 1.4848 and 1.4849 are often used to manufacture the housings of the high-temperature sections of turbochargers. These steels must withstand oxidation as well as repeated heating and cooling cycles that can result in large temperature gradients within the component.

[0004] Therefore, turbocharger housing materials need to meet several suitable properties, such as adequate yield strength maintained up to very high temperatures, good oxidation resistance to prevent the initiation and eventual propagation of cracks, and a stable microstructure maintained over a wide temperature range, all at a relatively low cost.

[0005] Cast austenitic stainless steels strengthened with eutectic carbides are one of the few classes of materials that meet all these constraints. Such steels are expected to resist thermomechanical fatigue, which promotes crack initiation and formation (failure in thermomechanical fatigue is caused not only by cyclic strain as in conventional fatigue, but also by temperature changes and surface oxidation). Such materials are also resistant to creep. The present invention describes such types of austenitic stainless steels.

[0006] It would be desirable to provide a steel that has adequate yield strength maintained up to very high temperatures, good oxidation resistance to prevent the initiation and ultimate propagation of cracks, and a stable microstructure maintained over a wide temperature range, all at a relatively low cost. Summary of the Invention [Means for solving the problem]

[0007] The present invention provides a steel containing, by mass%, 0.35 to 0.6% carbon, 0.35 to 2.0% silicon, 6.0 to 21.0% manganese, 7.0 to 16.0% nickel, 17.0 to 26.5% chromium, 3.5% or less tungsten, 2.2% or less niobium, 0.1 to 0.75% nitrogen, 4.0% or less copper, 4.0% or less molybdenum, 0.1% or less acid-soluble Al (sol.Al), 0.05% or less phosphorus, and 0.3% or less sulfur, with the total content of rare earth elements, hafnium, zirconium, boron, yttrium, titanium, calcium, magnesium and vanadium being 1.0% or less, and the balance being iron and unavoidable impurities.

[0008] The steel exhibits improved oxidation resistance compared to conventional steels of this type and offers a good balance of high temperature microstructural stability, strength and cost. Such alloys are suitable for use as cast austenitic stainless steels for turbocharger applications at temperatures up to 1050°C and above.

[0009] In one embodiment, the steel satisfies the following formula: W Ni +4W Nb >10 Such steels offer reduced cost for a given microstructural stability.

[0010] In one embodiment, the amounts of chromium, manganese, molybdenum, nickel, silicon, tungsten, niobium, vanadium, and copper in the steel are each set to W Cr , W Mn , W Mo , W Ni , W Si , WW , W Nb , W V and W Cu Then, the steel satisfies the following equation: 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 +0.325W V ≦4.6, preferably ≦4.5, more preferably ≦4.0, even more preferably ≦3.5 Such steels are less expensive.

[0011] In one embodiment, the steel satisfies the following formula: -0.3180W C -0.0415W Si +0.0111W Mn -0.0081W Ni +0.0313W Cr +0.0252W W +0.0222W Nb -0.1470W N -0.0009W Cu +0.0230W Mo -0.537≦0.10, preferably ≦0.05, most preferably ≦0.03 Such steels have reduced sigma stability.

[0012] In one embodiment, the steel satisfies the following formula: 0.836W C +0.063W Cr +0.181W Cu +0.509W Mn +0.194W Mo +20.0W N -0.606W Nb -0.143W Ni -0.437W Si +0.06W W +51.429≧58.8 Preferably, the following formula is satisfied: 0.836W C +0.063W Cr +0.181W Cu +0.509W Mn +0.194W Mo +20.0W N -0.606W Nb -0.143W Ni -0.437W Si +0.06W W +51.429≧59.0 More preferably, the following formula is satisfied. 0.836W C +0.063W Cr +0.181W Cu +0.509W Mn +0.194W Mo +20.0W N -0.606W Nb -0.143W Ni -0.437W Si +0.06W W +51.429≧60.0 Such steel has higher strength.

[0013] In one embodiment, the steel satisfies the following formula: 10 -18 (0.0263W Cr +0.1770W Cu +0.0399W Mn +0.0278W Mo +0.1070W Ni -0.0817W Si +0.0342W W +0.0051W Nb ) ≧2.1e-18, preferably ≧2.2e-18, even more preferably ≧3.0e-18 Such steels have a higher oxidation resistance.

[0014] In one embodiment, the steel satisfies the following formula: 10 -4 (-5.35W C +0.635W Si -0.231W Mn +0.583WNi +1.5W Cr -0.452W W -11.4W N +1.1W Cu +12.3) ≥ 0.0050, preferably ≥ 0.0055, preferably ≥ 0.0060 Such steels have an enhanced ability to form a protective chromia scale.

[0015] In one embodiment, the steel contains niobium in a percentage by weight of not more than 0.2%, preferably not more than 0.1%, which reduces costs.

[0016] In one embodiment, the steel contains, by mass, 7.5% or more of nickel, preferably 9.0% or more, more preferably 10.0% or more, and most preferably 12.0% or more, which improves the oxidation resistance.

[0017] In one embodiment, the steel contains, by weight, nickel up to 14.5%, preferably up to 14.0%, more preferably up to 13.5%, even more preferably up to 13.0%, and optionally up to 10.0%, thereby reducing the cost of the steel.

[0018] In one embodiment, the steel contains, by weight, at most 1.75% silicon, preferably at most 1.6%, more preferably at most 1.0%, even more preferably at most 0.7%, even more preferably at most 0.6%, and most preferably at most 0.5%, which improves creep resistance by reducing the precipitation of deleterious phases.

[0019] In one embodiment, the steel contains manganese in an amount, by weight, of at least 6.5%, preferably at least 7.0%, more preferably at least 8.0%, preferably at least 9.0%, more preferably at least 9.5%, even more preferably at least 10.5%, such steel having improved strength.

[0020] In one embodiment, the steel contains, by weight, at most 17.5% manganese, preferably at most 15.5%, more preferably at most 15.0%, even more preferably at most 13.0%, even more preferably at most 9.0%, and most preferably at most 8.0%, such steel has improved creep and oxidation resistance.

[0021] In one embodiment, the steel contains, by mass, 19.0% or more of chromium, preferably 19.5% or more, and more preferably 22.0% or more of chromium, which improves the oxidation resistance of such a steel.

[0022] In one embodiment, the steel contains chromium in mass percentages of not more than 25.5%, preferably not more than 21.0%, such a steel has reduced costs.

[0023] In one embodiment, the steel contains tungsten in mass percent not greater than 3.0%, preferably not greater than 0.5%, more preferably not greater than 0.2%, and most preferably not greater than 0.1%, such steels have reduced cost and fewer brittle phases.

[0024] In one embodiment, the steel contains at least 0.5% molybdenum, by weight, which provides high strength and creep resistance.

[0025] In one embodiment, the steel contains, by weight, 3.0% or less copper, preferably 2.2% or less copper, more preferably 0.5% or less copper, which provides improved high temperature strength and ductility.

[0026] In one embodiment, the steel contains, by weight, 3.0% or less of molybdenum, preferably 2.0% or less, more preferably 1.25% or less, even more preferably 1.1% or less, even more preferably 0.5% or less, and most preferably 0.1% or less. Such steels are low in cost and have low levels of brittle phases.

[0027] In one embodiment, the steel contains, by weight, niobium in an amount not greater than 2.0%, preferably not greater than 1.5%, more preferably not greater than 1.0%, which reduces the cost of the steel and improves resistance to the formation of intermetallic compounds.

[0028] In one embodiment, the steel contains niobium in a percentage by weight of at least 0.5%, preferably at least 0.75%, which improves the long-term microstructural stability of such steel.

[0029] In one embodiment, the steel contains, by weight, at most 0.7% nitrogen, preferably at most 0.65%, more preferably at most 0.6%, even more preferably at most 0.5%, even more preferably at most 0.4%, more preferably at most 0.35%, and most preferably at most 0.3%, which significantly reduces the risk of porosity and reduces the risk of brittle chromium and molybdenum nitrides forming.

[0030] In one embodiment, the steel contains, by mass, 0.15% or more of nitrogen, preferably 0.20% or more, more preferably 0.25% or more, even more preferably 0.30% or more. Such steel has high strength.

[0031] In one embodiment, the steel contains, by mass, 0.4% or more of silicon, preferably 0.45% or more, most preferably 0.5% or more of silicon. Such steel has excellent forming and filling properties and deoxidization properties.

[0032] In one embodiment, the steel contains, by weight, 0.25%, preferably at least 0.5%, more preferably at least 1.0%, even more preferably at least 1.5%, and most preferably at least 1.75% copper, which enhances the ability of such steel to form a protective chromia scale.

[0033] In one embodiment, the steel contains, by weight, at least 0.4% carbon, preferably at least 0.45%, which improves high temperature strength and creep resistance.

[0034] In one embodiment, the steel contains, by weight, 0.55% or less, preferably 0.51% or less, more preferably 0.5% or less of carbon, which improves creep resistance and ductility of such steel.

[0035] In one embodiment, the steel satisfies the following formula: W Ni +6W Cu ≧10.1 Preferably, the following formula is satisfied: W Ni +6W Cu ≧10.5 Such steels have even greater oxidation resistance.

[0036] In one embodiment, the steel satisfies the following formula: 3.43e-04W Fe -3.45e-03W Ni -1.01e-04W Cr -9.97e-03W Si +4.04e-04W Mn +8.60e-02W N +6.61e-04W Cu -1.26e-03W Mo -1.44e-02W Nb +6e-02W V +0.046 ≦0.058, preferably ≦0.055, even more preferably ≦0.050, most preferably ≦0.040 In such steels the volume fraction of nitrides is further reduced.

[0037] In one embodiment, the steel contains vanadium in a percentage by mass of up to 0.3%, preferably up to 0.2%, which improves the ductility.

[0038] In one embodiment, the cast product is composed of the above mentioned steel, which is optimized for casting parts for use in high temperature applications.

[0039] In one embodiment, the turbocharger housing is constructed from the steel described above. This steel is well suited for such purposes. The turbocharger can be conveniently cast.

[0040] The term "comprising" is used herein to indicate that a composition is 100%, excluding the presence of additional components to make up the percentage to 100%.

[0041] 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]

[0042] [Figure 1] 1 shows the improvement in oxidation resistance of the present invention over two prior art steel compositions. [Diagram 2] 1 shows the tensile strength improvement of the present invention over three prior art steels. [Diagram 3] This shows the improved microstructural stability of the steel of the present invention compared to the prior art steels. [Figure 4] 1 shows Vickers hardness values ​​of examples of the present invention and comparative examples in the as-cast state. [Diagram 5] 4 shows the microstructure of the example after casting and stress relief treatment, and after ageing at 800° C. for 140 hours. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] Composition domain and metric selection Aiming to address at least some of the above issues, a modeling-based approach used to identify alloys according to the present invention is described herein. This approach combines a computational materials model framework with machine learning to estimate design-relevant properties over a very wide compositional range. In principle, this alloy design tool allows the so-called inverse problem to be solved; i.e., the optimal alloy composition that best satisfies specified design constraints can be identified.

[0044] The first step in the design process is to define the table of elements and the upper and lower compositional limits associated with the table of elements. In the present invention, the compositional limits for each element's addition, called the "alloy design space", are considered. The compositional limits are detailed in Table 1. These limits were selected by the inventors based on the following explanation. Some knowledge comes from metallurgical experience, while others (such as the impact on strength, chromium diffusivity, nitride fraction, cost, and the presence of intermetallic phases) have been established by the inventors based on the following thermodynamic calculations for a wider range of compositions than those shown below. [Table 1]

[0045] The elements and their ranges in Table 1 were selected for the following reasons.

[0046] Nickel: Its slow diffusion rate in the austenite matrix phase stabilizes the austenite phase and improves creep resistance. It also positively influences the activity and diffusivity of chromium, thereby improving oxidation resistance. On the other hand, high nickel additions are prohibitively expensive and greatly reduce nitrogen solubility in the melt, severely limiting the achievable yield strength. High levels of nickel also stabilize brittle nitrides. Nickel is present at least 7.0 wt. % to achieve a good balance between oxidation, creep, chromium activity and diffusivity, and microstructural stability, while being limited to 16.0 wt. % or less to meet the criteria of cost, strength, and microstructural stability. When the balance of desired properties is more inclined towards lower cost, the maximum amount of nickel is limited to 14.5 wt. %, preferably 14.0 wt. %, more preferably 13.5 wt. %, and even more preferably 13.0 wt. % or less. Nickel can be present in even lower concentrations. On the other hand, the inventors have found that when the amount of nickel is less than or equal to 10.0% by weight (the upper possible limit), it is preferable to add copper to maintain oxidation resistance (see below). If the main objective of the alloy is oxidation resistance, the amount of nickel added is increased accordingly, for example the nickel content is at least 7.5% by weight or more, or 9.0% by weight or more, or even 10.0% by weight or more. In such steels, the oxidation resistance is improved at the expense of increased cost. In embodiments where nickel is present in an amount of 12.0% by weight or more, even higher oxidation resistance is achieved.

[0047] Chromium: Provides solid solution strengthening and improves creep resistance. It is also the primary source of oxidation resistance, forming a protective chromia scale that acts as a barrier against further oxidation at high temperatures. The lower limit is restricted to at least 21.5 wt. % to maintain adequate oxidation resistance. Higher levels of chromium provide even better oxidation resistance, so where high oxidation resistance is important, the preferred minimum chromium content is at least 22.0 wt. %, preferably at least 22.5 wt. %, more preferably at least 24.0 wt. %, and even more preferably at least 24.5 wt. %. On the other hand, large chromium additions stabilize ferrite and also promote the formation of detrimental sigma phases, which make ferrite stable at very high temperatures and precipitate more rapidly. The presence of sigma phases significantly reduces the ductility and oxidation resistance of the alloy. High chromium additions also promote the formation of chromium nitride precipitates at high temperatures, the presence of which also reduces the ductility and oxidation resistance of the alloy. Therefore, in applications where the ductility of the alloy is particularly important, chromium is limited to 26.5%, preferably to no more than 25.5%, and more preferably to no more than 21.0%, by weight.

[0048] Silicon: provides solid solution strengthening, improves the castability of the alloy, and deoxidizes the melt. It also improves oxidation resistance through a "third element effect", since, like chromium, it is an element capable of forming protective oxide scales. Silicon's presence also improves fluidity, which, combined with its deoxidizing properties, improves the quality of the casting. This latter effect is not given an associated figure of merit, but it contributes to the requirement (see below) that a minimum amount of silicon must be at least 0.35%. Studies of high-temperature oxidation have shown that small additions of elements that form protective oxide scales, such as aluminum and silicon, to alloys containing chromium, significantly improve oxidation resistance, even when their concentrations are too low to form continuous alumina or silica scales. On the other hand, large additions of silicon stabilize several potentially harmful phases, such as nitrides, G-phases, and Laves phases. Therefore, the silicon content is limited to 2.0% by weight, more preferably 1.75% by weight or less or 1.6% by weight or less, even more preferably 1.0% by weight or less, optionally 0.7% by weight or less or 0.6% by weight or less, most preferably 0.5% by weight or less. Silicon is necessary for better forming, filling and deoxidizing the melt, so the lower limit is 0.35% by weight or more, preferably 0.4% by weight or more, more preferably 0.45% by weight or more, even more preferably 0.5% by weight or more.

[0049] Aluminum: Aluminum (Al) acts similarly to silicon, deoxidizing steel and forming a protective oxide scale in chromium-containing alloys, thus enhancing oxidation resistance. On the other hand, high concentrations of aluminum in the presence of nitrogen in the melt form hard and coarse aluminum nitrides, which adversely affect ductility and fatigue limit. Therefore, the upper limit for Al content is 0.1%, more preferably 0.050%. In this specification, the "Al" content refers to the "acid-soluble Al", i.e. "sol.Al" content.

[0050] Carbon: provides solid solution strengthening, stabilizes the austenite phase, and forms a characteristic dendritic eutectic carbide network, which imparts the characteristic creep resistance and high temperature strength to the alloy of the present invention. Taking into account the above considerations and the creep figure of merit calculated during the alloy development, its minimum content is limited to 0.35% by weight or more, preferably 0.4% by weight or more, and more preferably 0.45% by weight or more. On the other hand, excessive addition of carbon leads to an undesirably large volume fraction of dendritic carbides, which adversely affects creep resistance and ductility. Therefore, the carbon content is limited to 0.6% by weight or less, preferably 0.55% by weight or less, more preferably 0.51% by weight or less, and even more preferably 0.5% by weight or less. In one embodiment, carbon is 0.45% by weight or less or 0.4% by weight or less.

[0051] Manganese: It provides solid solution strengthening, stabilizes the austenite phase, and increases nitrogen solubility without excessively stabilizing the various brittle nitride phases. Manganese also increases the work hardening rate of austenitic steels, which has a beneficial effect on extending fatigue life, especially in the low cycle range. Since these steels can be fatigued by thermal cycles, a relatively higher Mn content than that normally used is specified. For this reason, the minimum manganese content is limited to 6.0% by weight or more, preferably 7.0% by weight or more, more preferably 8.0% by weight or more, and even more preferably 9.0% by weight or more, 9.5% by weight or more, and 10.5% by weight or more, in order to increase strength. On the other hand, excessive addition stabilizes primary delta ferrite, which remains in the cast material and reduces the creep resistance of the material and acts as a nucleation site for harmful sigma phases. Furthermore, manganese is also known to have a negative effect on oxidation resistance by promoting the formation of manganese-rich oxide phases that are permeable to oxygen and prone to spalling. Therefore, the manganese content is limited to 21.0 mass% or less, more preferably 17.5 mass% or less, even more preferably 15.5 mass% or less, even more preferably 15.0 mass% or less, even more preferably 13.0 mass% or less, even more preferably 9.0 mass% or less, and most preferably 8.0 mass% or less.

[0052] Molybdenum and tungsten: Provides solid solution strengthening and creep resistance, and increases the solubility of nitrogen. However, excessive additions increase the cost of the alloy and tend to stabilize various brittle phases, such as sigma and Laves phases. Therefore, the contents are limited to 4.0% or less, by weight, for molybdenum, and 3.5% or less, by weight, for tungsten. Preferably, molybdenum and tungsten are each limited to 3.0% or less, by weight. In one embodiment, the sum of these elements is 3.0% or less, by weight. In certain embodiments where cost and the absence of brittle phases are paramount, tungsten is limited to 0.5% or less, or even 0.2% or less, or even 0.1% or less, by weight. Molybdenum may be limited to 2.0% or less, 1.25% or less, 1.1% or less, or 0.5% or less, by weight, to reduce cost and the incidence of brittle phases. When high strength and creep resistance are required, small essential additions of molybdenum (e.g., 0.5% by weight or more) can be advantageous. Molybdenum is an optional element. In one embodiment, the alloy contains 0.1% by weight or less of molybdenum. Tungsten is an optional element.

[0053] Nitrogen: provides solid solution strengthening and stabilizes the austenite phase. Therefore, the nitrogen content is limited to 0.1% by weight or more, more preferably 0.15% by weight or more. In some embodiments, to further increase the strength, the amount of nitrogen is at least 0.20% or 0.25% by weight, or even 0.3% or 0.4% by weight or more. On the other hand, excessive addition stabilizes the formation of brittle chromium and molybdenum nitrides at high temperatures. It can also cause processing problems by exceeding the nitrogen solubility in the melt. This can lead to nitrogen evaporation from the melt, resulting in a lower than expected nitrogen content. In castings, more serious problems occur when there is not enough nitrogen solubility in the solid matrix phase that is initially formed upon cooling, and nitrogen is not returned to the melt, resulting in nitrogen gas bubbles that are trapped in the semi-solid casting, leading to significant porosity. Therefore, the upper limit of the nitrogen content is limited to 0.75% by weight or less, more preferably 0.6% by weight or less, and even more preferably 0.5% by weight or less. Even lower levels (0.4% by weight or less, or even 0.35% by weight or less or 0.3% by weight or less) are possible.

[0054] Copper: It stabilizes the austenite phase somewhat, imparts creep resistance, and improves oxidation resistance by increasing the activity and diffusivity of Cr. On the other hand, excessive addition significantly reduces the solubility of nitrogen, causes Cu segregation to grain boundaries, making the material brittle, and there may even be a Cu-rich liquid film at temperatures close to 1000 °C, where the material is completely solid. The liquid film significantly reduces ductility and strength at high temperatures. Cu stabilizes various carbide and nitride phases and promotes their formation from the melt, resulting in a coarse microstructure, which adversely affects the mechanical properties. The copper content is therefore limited to 4.0% by weight or less, more preferably 3.0% by weight or less, even more preferably 2.5% by weight or less or 2.2% by weight or less, and even more preferably 0.5% by weight or less. On the other hand, when the nickel content of the alloy is low (nickel content less than 10.0% by weight), where cost is the main consideration, copper becomes mandatory so that the steel achieves the required oxidation resistance and the following formula is satisfied: W Ni +6W Cu ≧10 In a particular embodiment, to ensure high oxidation resistance, W Ni +6W Cu ≧10.1 Furthermore, W Ni +6W Cu ≧10.5 In certain embodiments, copper is present in an amount of 0.25% by weight or more, preferably 0.5% by weight or more, or 1.0% by weight or more to improve oxidation resistance. In embodiments containing 1.5% by weight or more copper, the oxidation resistance is further improved, and in embodiments containing 1.75% by weight or more copper, the oxidation resistance is even further improved.

[0055] Niobium: significantly improves creep resistance and high temperature strength by forming hard and stable dendritic carbides, and promotes the formation of Z-phase at the expense of chromium molybdenum nitrides. Z-phase generally has a finely dispersed morphology and coarsens slowly compared to the rapidly coarsening chromium molybdenum nitrides, which often precipitate as brittle pearlite-like cellular colonies. On the other hand, excessive addition is costly, stabilizes the ferrite phase, and promotes the formation of brittle intermetallic compounds (sigma phase, G phase). Therefore, the niobium content is limited to 2.2% by weight or less, preferably 2.0% by weight or less, more preferably 1.5% by weight or less, and even more preferably 1.0% by weight or less. In some embodiments, for example when long-term microstructural stability is desired, niobium is essential, in which case niobium is present in an amount of 0.5% by weight or more, preferably 0.75% by weight or more. In some embodiments, in order to keep the cost of the steel low, niobium is not present at all or is present only in an amount of 0.2% or less by weight, or 0.1% or less by weight, or even 0.02% or less by weight. Niobium is an optional element.

[0056] Phosphorus: Phosphorus is an impurity. It segregates at grain boundaries and reduces the SSC resistance of the steel. The ductility and strength of the material can also be significantly reduced due to reduced cohesive strength of the grain boundaries. Therefore, the phosphorus content should be less than 0.05%. The preferred phosphorus content is less than 0.02%. It is desirable that the phosphorus content be as low as possible.

[0057] Sulfur: Sulfur is an impurity. It segregates at grain boundaries and reduces the SSC resistance of steel. The ductility and strength of the material can also be significantly reduced due to the reduced cohesive strength of the grain boundaries. On the other hand, sulfur is also known to improve machinability. The sulfur content is not more than 0.3%. The preferred sulfur content is not more than 0.05%, more preferably not more than 0.01%. The preferred sulfur content is not more than 0.005%, more preferably not more than 0.003%. It is desirable to have the sulfur content as low as possible.

[0058] Selenium, tellurium, antimony, bismuth, lead: these elements are impurities that segregate at grain boundaries. These elements reduce the cohesive strength of the grain boundaries, which can significantly reduce the ductility, strength and creep resistance of the material. Therefore, their content is not more than 0.005%, preferably not more than 0.001%, and even more preferably not more than 0.0003%. It is desirable to keep the content of these elements as low as possible. On the other hand, in some cases, it is desirable to have a higher content of these elements in order to make the alloy easier to machine. In this case, their total content is preferably 0.005% or more and not more than 0.04%.

[0059] Calcium: added to the melt, it 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 modifies sulfide inclusions well. Both mechanisms contribute to improving the machinability of the alloy. The calcium content is preferably 0.005% to 0.04%, by weight.

[0060] Additionally, the alloys of the present invention may contain any combination of trace alloying elements not to exceed a total of 1.0 wt.%. Trace alloying elements include rare earth elements (REM), particularly lanthanides (atomic numbers 57-71, including lanthanum) for more favorable carbide morphology, hafnium, zirconium and boron for grain boundary strengthening effects, yttrium for improved oxidation resistance, and titanium (preferably 0.1 wt.% or less) and vanadium (0.3 wt.% or less, preferably 0.2 wt.% or less) for improved resistance to coarsening of MC carbides (wherein M is primarily Nb, partially substituted by Ti, V, or Zr). Boron may aid in grain boundary strengthening and may optionally be present in amounts up to 0.1 wt.%. Trace amounts of calcium and magnesium may also be present (up to 0.1 wt.% each) as by-products of slag processing or deoxidation.

[0061] Additionally, the alloy may contain small amounts of unavoidable impurities of elements not listed in the sections above.

[0062] In some cases, the nickel content needs to be reduced because the alloy of the present invention has a low cost. Nickel plays an important role in improving oxidation resistance, so it needs to be replaced by other elements with similar roles. The most effective element among these is copper, and according to the diffusion index of Cr, even a small amount can significantly improve oxidation resistance. Therefore, it is preferable to use W. Ni +6W Cu >10% by mass, more preferably W Ni +6W Cu >12% by weight, most preferably W Ni +6W Cu >15% by mass.

[0063] In some cases, the nickel content of the alloys of the present invention may need to be reduced due to their lower cost. Nickel reduces the nitride fraction index, so it may need to be replaced with niobium. Thus, W Ni +4W Nb >10% by mass.

[0064] The microstructure of the steel is predominantly austenite reinforced with eutectic carbides.

[0065] The second step relies on thermodynamic calculations that are used to calculate the phase diagrams and thermodynamic properties of specific alloy compositions, often referred to as the CALculation of PHAse Diagrams (CALPHAD) method.

[0066] The third step is to identify alloy compositions that have the desired properties calculated in the second step. Candidate alloys within the investigated compositional region were selected based on various indices of merit detailed below. The alloys of the present invention are specifically designed to achieve a good balance of the indices of merit: strength, Cr diffusivity, nitride fraction, Cr activity, cost, and sigma fraction, all of which are described below. Some alloys in this region achieve a slightly different balance than others, e.g., some achieve very high oxidation resistance at the expense of higher than optimal cost. As can be seen from the examples in Table 1, the steels within the scope of the present invention have better indices of merit for strength, Cr diffusivity, nitride fraction, Cr activity, cost, and sigma fraction compared to the comparative steels that do not meet the compositional requirements. The relatively high manganese content also provides improved fatigue resistance, and the relatively high silicon content also provides improved castability.

[0067] Strength Merit Index: It reflects the yield strength of an alloy at room temperature. The higher the value, the less likely plastic deformation occurs due to thermal strain during repeated heating and cooling, and the longer the service life of parts made of stronger materials. It is based on two assumptions. First, the grain size changes as cast with composition are small, so the grain boundary strength is constant throughout the compositional range. Second, the precipitation strength depends only on the volume fraction of carbides and nitrides. Due to the complex solidification conditions, the variation in size distribution is ignored and assumed to be constant. Therefore, the change in yield strength is dominated by the solid solution strength. The formula for the strength merit index is:

number

[0068] An approximation of the strength index based on the alloy composition was discovered. Strength index = 0.836W C +0.063W Cr +0.181W Cu +0.509W Mn +0.194W Mo +20.0W N -0.606W Nb -0.143W Ni -0.437W Si +0.06W W +51.429 (1)

[0069] Compositions that result in a strength figure of merit of 58.8 or greater are preferred, as this ensures that the alloy has sufficient strength for the tasks it is designed for. More preferably, the alloy has a strength figure of merit of 59.0 or greater, or even 60.0 or greater, as this would indicate even greater strength.

[0070] Chromium Diffusion Index: It reflects the ability of an alloy to form a protective chromia scale or to reform a protective chromia scale after spalling. 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 diffusivity of chromium is low. The rate of oxidation after spalling depends on the composition of this depleted layer. If the 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 diffusivity of chromium results in a low degree of chromium depletion. Thus, the chromia scale reforms rapidly after the spalling event, slowing down the overall oxidation rate.

[0071] The diffusivity of chromium can be adjusted because it depends on the composition of the austenite. Nickel and copper in particular are known to increase the diffusivity of chromium. The diffusivity of chromium in austenite can be determined by thermodynamic calculations. A fairly accurate approximation has been found using a linear combination of the alloying elements. Chromium diffusion index = 10 -18 (0.0263W Cr +0.1770W Cu +0.0399W Mn +0.0278W Mo +0.1070W Ni -0.0817W Si +0.0342W W +0.0051W Nb ) (2)

[0072] Preferred steels achieve Chromium Diffusion Index values ​​of at least 2.1e-18 or greater, preferably 2.2e-18 or greater, and more preferably 3.0e-18 or greater.

[0073] Nitride fraction index: is a measure of the volume fraction of M2N nitrides stable at high temperatures. The volume fraction is determined using thermodynamic calculations. A low nitride fraction index is an indication of microstructural stability and ductility. A satisfactory and simple approximation of the nitride volume fraction derived from regression analysis of the thermodynamic calculations is: Nitride fraction exponent = 3.43e-04W Fe -3.45e-03W Ni -1.01e-04W Cr -9.97e-03W Si +4.04e-04W Mn +8.60e-02W N +6.61e-04W Cu -1.26e-03W Mo -1.44e-02W Nb +6e-02W V +0.046

[0074] The steel according to the invention preferably has a nitride fraction index of less than or equal to 0.058, more preferably less than or equal to 0.055, even more preferably less than or equal to 0.050, and most preferably less than or equal to 0.040.

[0075] Cost Index: Reflects the cost (GBP / kg) of the raw materials required to produce the alloy. In each case a master alloy containing iron and the specified alloying elements is used, assuming different percentages of the alloying elements. The cost is a weighted sum of the master alloy and the remaining pure iron, taking into account the iron contained in the master alloy.

number

[0076] The index i is the alloying element, n is the number of alloying elements, p i is the mass fraction of alloying element i in the master alloy (the "purity" of the master alloy - the remainder is assumed to be iron), w i is the mass fraction of alloying element i in the alloy, c i is the cost of the master alloy of element i. The index Fe represents iron. Estimated master alloy purities (mass fractions) and costs are given in the table below. [Table 2]

[0077] Assuming one starts with the pure elements, the alloy cost has a simpler formula: 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 +0.363W W +0.325W V

[0078] The cost of steel is preferably 4.6 GBP kg -1 Less than or equal to 4.5 GBP kg -1 Less than or equal to 4.2 GBP kg, preferably -1 Less than or equal to 4.1 GBP kg, preferably -1 Less than or equal to 4.0 GBP kg, preferably -1 Below 3.5GBP kg -1 The following is the result.

[0079] Sigma fraction index: It is a measure of the volume fraction of sigma phase at 800 °C when the alloy is in thermodynamic equilibrium. A low sigma fraction exponent is required to prevent excessive loss of ductility. The formula estimates the sigma fraction as a function of composition and is derived from a regression analysis of thermodynamic calculations (so it does not predict absolute volume fractions but is an indication of trends). Sigma phase fraction exponent = -0.3180W C -0.0415W Si +0.0111W Mn -0.0081W Ni +0.0313W Cr +0.0252W W +0.0222W Nb -0.1470W N -0.0009W Cu +0.0230WMo -0.537

[0080] A low sigma phase fraction value (including negative values) indicates poor sigma stability at 800° C. The sigma phase fraction index is desirably 0.1 or less, preferably 0.05 or less, more preferably 0.04 or less, and most preferably 0.03 or less.

[0081] Creep Figure of Merit: As with nickel superalloys, this reflects the assumption that the microstructure remains constant during creep and creep-type deformation is confined to the matrix (austenite) phase. Dislocation segments are rapidly pinned at carbide interfaces. The rate-limiting step is the breakaway of the pinned dislocation configurations from the austenite-carbide interfaces. This depends on the local chemistry (in this case the composition of the austenite phase), so alloy composition has a large effect on creep properties. The governing equation is:

number

[0082] Chromium Activity Index: It reflects the ability of an alloy to maintain a protective chromia scale and to resist oxidation at high temperatures. According to Wagner's oxidation theory, the rate of oxidation of a chromia-forming alloy is inversely proportional to the chromium activity in the alloy under given conditions. Thus, the higher the activity of chromium, the slower the oxidation. Chromia scales usually spall off when they reach a certain thickness. This is because the stress at the metal-oxide interface increases with oxide thickness and usually does not vary with the alloy composition. This means that a slower growing scale will last longer before spalling, minimizing mass loss. Chromium activity usually varies with the chromium concentration in austenite, but is also very sensitive to the presence of other elements. For example, nickel and copper are known to significantly increase chromium activity. Chromium activity in austenite can be obtained by thermodynamic calculations. A fairly accurate approximation using a linear combination of alloying elements has been found. Chromium Activity Index = 10 -4 (-5.35W C +0.635W Si -0.231W Mn +0.583W Ni +1.5W Cr -0.452W W -11.4W N +1.1W Cu +12.3)

[0083] A chromium activity index of 0.0050 or greater is particularly desirable as it indicates an alloy with a good ability to form a protective chromia scale, more preferably a chromium activity index of 0.0055 or greater, and most preferably a chromium activity index of 0.0060 or greater.

[0084] Examples and Comparative Examples Experimental testing was carried out on the alloys in the table below: Comparative Example 1 is an experimental austenitic stainless steel, while Comparative Examples 3, 4 and 28 are the well-known austenitic heat resistant DIN standard grade 1.4848, known for its good oxidation resistance but relatively low yield strength. [Table 3] [Table 4] [Table 5]

[0085] Alloys satisfying the main claim (Example 1) were experimentally tested for oxidation resistance and yield strength. Several further example alloys satisfying the main claim were tested for oxidation resistance, hardness, yield strength and thermal stability.

[0086] FIG. 1 shows a comparison of the oxidation resistance of the inventive and comparative examples. The data was obtained from a cyclic oxidation test in which semicircular specimens of the material were exposed to air at 1000° C. in a box furnace. After the exposure step, they were air-cooled to room temperature and the masses were measured and recorded. Examples 27, 33 and Comparative Example 4 show a steady mass gain, indicating the formation of a protective chromia scale. Comparative Example 1 shows a rapid mass loss, indicating a lack of chromia scale formation, and therefore the alloy is not suitable for use at 1000° C. Examples 27 and 33 oxidize more rapidly than Comparative Example 4, but have sufficient oxidation resistance for use at 1000° C. Example 34 also shows a parabolic curve, but with a mass gain higher than is desirable at this temperature.

[0087] Figure 2 shows a comparison of yield strength over a range of temperatures for an inventive example and three comparative examples. The room temperature tensile tests are in accordance with ASTM E8-16a, and the high temperature tensile tests are in accordance with ASTM E21-17. Example 1 is significantly better than all the comparative examples. The inventive example has the highest yield strength.

[0088] FIG. 3 shows a comparison of the microstructures of Example 27 (left) and Comparative Example 1 in the as-cast state and after 25.5 hours at 750°C. In the as-cast state, both alloys show lamellar nitrides. In Comparative Example 1, these are mainly isolated regions with sparsely populated larger nitride colonies (~200 μm diameter). In Example 27, the nitride colonies are interconnected and more uniformly distributed, with no large colonies present. After 25.5 hours at 750°C, the size and volume fraction of the colonies in Comparative Example 1 have increased dramatically, adversely affecting ductility and creep resistance. In Example 27, they have only grown slightly, indicating excellent microstructural stability.

[0089] Figure 4 shows the Vickers hardness values ​​(load 5 kg, holding time 10 seconds) in the as-cast state for the examples of the present invention and the comparative examples. The examples significantly outperformed Comparative Examples 1 and 4. Comparative Example 1 has a similar hardness but a lower oxidation resistance (Figure 1), and is therefore not suitable for high temperature applications despite its high hardness.

[0090] Figure 5 shows the microstructures of Examples 33 and 34 after casting and stress relief (first row) and after aging at 800°C for 140 hours. The microstructures of both examples are relatively unaffected with no signs of deleterious phases (cellular nitrides, coarse intermetallic phases), indicating good thermal stability.

Claims

1. In mass%, Carbon: 0.35-0.6%, Silicon: 0.35 to 2.0%, Manganese: 6.0 to 21.0%, Nickel: 7.0 to 16.0%, Chromium: 21.5-26.5%, Tungsten: 3.5% or less, Niobium: 2.2% or less, Nitrogen: 0.1 to 0.75%, Copper: 4.0% or less, Molybdenum: 4.0% or less, Acid-soluble Al (sol.Al): 0.1% or less, Phosphorus: 0.05% or less, Sulfur: 0.3% or less, Calcium: 0.04% or less Including, the total content of rare earth elements, hafnium, zirconium, boron, yttrium, titanium, calcium, magnesium, and vanadium 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 being iron and unavoidable impurities.

2. 2. The steel according to claim 1, which satisfies the following formula: W Ｎｉ +4W Ｎｂ >10

3. The mass percentages of chromium, manganese, molybdenum, nickel, silicon, tungsten, niobium, vanadium, and copper in steel are respectively expressed as W Cr , W Mn , W Mo , W Ni , W Si , W W , W Nb , W V and W Cu 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 +0.325W Ｖ ≦4.6

4. The mass percentages of carbon, chromium, manganese, molybdenum, nickel, silicon, tungsten, niobium, nitrogen, and copper in steel are defined as W, C , W Cr , W Mn , W Mo , W Ni , W Si , W W , W Nb , W N and W Cu The steel according to claim 1, which satisfies the following formula: -0.3180W Ｃ -0.0415W Ｓｉ +0.0111W Ｍｎ -0.0081W Ｎｉ +0.0313W Ｃｒ +0.0252W Ｗ +0.0222W Ｎｂ -0.1470W Ｎ -0.0009W Ｃｕ +0.0230W Ｍｏ -0.537≦0.10

5. The mass percentages of nickel and copper in the steel are W Ni and W Cu The steel according to claim 1, which satisfies the following formula: W Ｎｉ +6W Ｃｕ >10

6. The mass percentages of carbon, chromium, manganese, molybdenum, nickel, silicon, tungsten, niobium, nitrogen, and copper in steel are respectively represented by W C , W Cr , W Mn , W Mo , W Ni , W Si , W W , W Nb , W N and W Cu The steel according to claim 1, which satisfies the following formula: 0.836W Ｃ +0.063W Ｃｒ +0.181W Ｃｕ +0.509W Ｍｎ +0.194W Ｍｏ +20.0W Ｎ -0.606W Ｎｂ -0.143W Ｎｉ -0.437W Ｓｉ +0.06W Ｗ +51.429≧58.8

7. The mass percentages of chromium, manganese, molybdenum, nickel, silicon, tungsten, niobium, and copper in steel are respectively represented by W Cr , W Mn , W Mo , W Ni , W Si , W W , W Nb and W Cu The steel according to claim 1, which satisfies the following formula: 10 －１８ (0.0263W Ｃｒ +0.1770W Ｃｕ +0.0399W Ｍｎ +0.0278W Ｍｏ +0.1070W Ｎｉ -0.0817W Ｓｉ +0.0342W Ｗ +0.0051W Ｎｂ )≧2.1e-18

8. The mass percentages of carbon, chromium, manganese, nickel, silicon, tungsten, nitrogen, copper, and vanadium in steel are respectively expressed as W C , W Cr , W Mn , W Ni , W Si , W W , W N , W Cu and W V The steel according to claim 1, which satisfies the following formula: 10 －４ (-5.35W Ｃ +0.635W Ｓｉ -0.231W Ｍｎ +0.583W Ｎｉ +1.5W Ｃｒ -0.452W Ｗ -11.4W Ｎ +1.1W Ｃｕ +12.3)≧0.0050

9. 2. The steel according to claim 1, containing, by mass, niobium in an amount of up to 0.2%.

10. 2. The steel according to claim 1, containing, by mass %, at least 7.5% nickel.

11. 2. The steel according to claim 1, containing up to 14.5% by weight of nickel.

12. 2. The steel according to claim 1, containing up to 1.75% silicon, in mass percent.

13. 2. The steel according to claim 1, containing, by mass %, at least 6.5% manganese.

14. 2. The steel according to claim 1, containing up to 17.5% manganese, in mass %.

15. 2. The steel according to claim 1, containing, by mass %, 22.0% or more of chromium.

16. 2. The steel according to claim 1, containing up to 25.5% chromium, in mass %.

17. 2. The steel according to claim 1, containing, by mass, up to 3.0% tungsten.

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

19. 2. The steel according to claim 1, containing, by mass, up to 3.0% copper.

20. 2. The steel according to claim 1, containing, by mass, up to 3.0% molybdenum.

21. 2. The steel according to claim 1, containing, in mass %, up to 2.0% niobium.

22. 2. The steel according to claim 1, containing, by mass %, niobium in an amount of at least 0.5%.

23. 2. The steel according to claim 1, containing, by mass, up to 0.7% nitrogen.

24. 2. The steel according to claim 1, containing, by mass %, at least 0.15% nitrogen.

25. 2. The steel according to claim 1, containing, by mass %, at least 0.4% silicon.

26. 2. The steel according to claim 1, containing, by mass, at least 0.25% copper.

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 0.55% carbon.

29. 2. The steel according to claim 1, which satisfies the following formula: W Ｎｉ +6W Ｃｕ ≧10.1

30. The mass percentages of iron, chromium, manganese, molybdenum, nickel, silicon, niobium, nitrogen, copper, and vanadium in the steel are expressed as W, Fe , W Cr , W Mn , W Mo , W Ni , W Si , W Nb , W N , W Cu and W V The steel according to claim 1, which satisfies the following formula: 3.43e-04W Fe -3.45e-03W Ni -1.01e-04W Cr -9.97e-03W Si +4.04e-04W Mn +8.60e-02W N +6.61e-04W Cu -1.26e-03W Mo -1.44e-02W Nb + 6e-02W V +0.046 ≦0.058

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

32. A steel comprising, by weight, about 0.50% carbon, about 0.5% silicon, about 7.4% manganese, about 12.5% ​​nickel, about 25.0% chromium, about 0.20% nitrogen, about 2.0% copper, and the balance being iron and unavoidable impurities.

33. A cast product made from a steel according to any one of claims 1 to 32.

34. A turbocharger housing constructed from the steel of any one of claims 1 to 32.

35. 35. The turbocharger of claim 34, wherein the turbocharger housing is a casting.

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