Stainless steel alloy, turbocharger component formed from stainless steel alloy, and method for manufacturing same

Abstract

The invention discloses an austenitic stainless steel alloy. The present invention relates to a cast iron alloy comprising or consisting of, by weight, from about 22.0% to about 23.0% chromium, from about 4.0% to about 5.0% nickel, from about 4.0% to about 5.0% manganese, from about 0.2% to about 0.4% molybdenum, from about 0.2% to about 0.4% niobium, from about 0.5% to about 1.5% silicon, from about 0.35% to about 0.45% carbon, from about 0.2% to about 0.28% nitrogen, the balance iron and other inevitable / inevitable impurities present in trace amounts. Elemental phosphorus and sulfur exceeding impurity levels are excluded from the alloy. A turbocharger turbine housing made of the stainless steel alloy and a method of manufacturing the same are also disclosed. The stainless steel alloy is suitable for use in turbocharger turbine applications at temperatures up to about 1000 DEG C.

Description

Technical Field

[0001] The present disclosure relates generally to stainless steel alloys. More particularly, the present disclosure relates to stainless steel alloys that exhibit oxidation resistance at elevated temperatures for use in casting applications such as turbine and turbocharger housings, exhaust manifolds, and combustion chambers. Background Art

[0002] During operation, automotive or aircraft turbocharger components are subject to elevated operating temperatures. These components must be able to contain turbine wheels that generate extremely high rotational speeds. At high temperatures, the exhaust gases from the automotive or aircraft engine first contact the metal sections of the turbocharger, such as the intake area of ​​the turbocharger. As high speed performance is improved by increasing exhaust temperatures, attempts have been made to gradually increase the exhaust temperatures of the engine. Due to these high temperatures, the thermal loads on components such as the exhaust manifold and the turbine housing become significant.

[0003] Many problems are encountered as these elevated exhaust gas temperatures contact the metal sections of the turbocharger. For example, one problem caused by elevated exhaust gas temperatures is corrosion or oxidation problems. At temperatures above about 800°C, for example, depending on the specific alloy employed, oxygen may begin to attack the metal elements in the alloy, causing them to oxidize or corrode and thus lose their beneficial physical and material properties. Over repeated operating cycles, corrosion or oxidation may eventually lead to complete failure of the component.

[0004] To overcome the challenges associated with higher operating temperatures, alloys used in turbocharger applications in the prior art have included stainless steel alloys with higher chromium and nickel contents, such as high chromium and / or nickel stainless steels available on the market. As used herein, the term operating temperature refers to the maximum temperature of the exhaust gas that the turbine housing and the impeller components of the turbocharger are designed to withstand (excluding occasional higher transient temperatures). These higher chromium and / or nickel stainless steels are primarily austenitic, with a stable austenite phase that exists well above the operating temperature, and minimal to no delta ferrite phase that promotes adverse effects on material properties. Stainless steel alloys of the 1.48XX series, such as 1.4837 stainless steel, are well known in the art. With specifications of chromium at 24% to 27% and nickel at 11% to 14% (all percentages by weight), they are exemplary prior art materials for turbine housing applications up to 1000°C. Although meeting the high temperature property requirements for turbocharger housings, 1.4837 stainless steel is quite expensive due to its high chromium and nickel content. Since the turbocharger housing is typically the most expensive component in a turbocharger, the overall cost of the machine is greatly affected by the choice of material used for this component.

[0005] Table 1, shown below, provides the specifications for 1.4837 stainless steel, with percentages by weight.

[0006] Table 1.1. Composition of 4837 stainless steel

[0007]

[0008] In addition, some lower chromium and nickel alloys have been investigated for shell temperatures up to 1000°C, however such alloys are generally associated with various manufacturing problems, such as problems associated with the casting process. For example, if the tapping temperature is as high as 1650°C to 1700°C, parts formed from such lower chromium and nickel alloys may have porosity problems due to outgassing of nitrogen during the casting process.

[0009] Therefore, materials that are less expensive and have fewer manufacturing problems, and also have better oxidation resistance, would be suitable alternatives to viable options. These materials should have a stable austenite phase that exists above the operating temperature, and minimal to no delta ferrite phase. Therefore, there is a need for stainless steel alloys that can be used in turbocharger applications that can withstand the higher operating temperatures generated by modern engines, while minimizing the expensive nickel content. Moreover, other desirable features and properties of the subject matter disclosed herein will become clear from the subsequent detailed description and the appended claims in conjunction with the accompanying drawings and this background technology. Summary of the invention

[0010] A stainless steel alloy, a turbocharger turbine component, and a method for manufacturing a turbocharger turbine component are provided.

[0011] In an embodiment, an austenitic stainless steel alloy comprises or consists of, by weight, about 22.0% to about 23.0% chromium, about 4.0% to about 5.0% nickel, about 4.0% to about 5.0% manganese, about 0.2% to about 0.4% molybdenum, about 0.2% to about 0.4% niobium, about 0.5% to about 1.5% silicon, about 0.35% to about 0.45% carbon, about 0.2% to about 0.28% nitrogen, and the balance iron, and other unavoidable / inevitable impurities present in trace amounts. Phosphorus and sulfur above impurity levels are excluded from the alloy.

[0012] According to an alternative embodiment, a turbocharger turbine component includes an austenitic stainless steel alloy. The austenitic stainless steel alloy includes or consists of, by weight, about 22.0% to about 23.0% chromium, about 4.0% to about 5.0% nickel, about 4.0% to about 5.0% manganese, about 0.2% to about 0.4% molybdenum, about 0.2% to about 0.4% niobium, about 0.5% to about 1.5% silicon, about 0.35% to about 0.45% carbon, about 0.2% to about 0.28% nitrogen and the balance iron, and other unavoidable / inevitable impurities present in trace amounts. Phosphorus and sulfur exceeding impurity levels are excluded from the alloy.

[0013] According to an alternative embodiment, a method for manufacturing a turbocharger turbine component is provided. The method includes forming a turbocharger turbine component using an austenitic stainless steel alloy. The austenitic stainless steel alloy includes or consists of the following items by weight: about 22.0% to about 23.0% chromium, about 4.0% to about 5.0% nickel, about 4.0% to about 5.0% manganese, about 0.2% to about 0.4% molybdenum, about 0.2% to about 0.4% niobium, about 0.5% to about 1.5% silicon, about 0.35% to about 0.45% carbon, about 0.2% to about 0.28% nitrogen and the balance iron, and other unavoidable / unavoidable impurities present in trace amounts. Phosphorus and sulfur exceeding the impurity level are excluded from the alloy.

[0014] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0016] Figure 1 is a system view of an embodiment of a turbocharger for a turbocharged internal combustion engine according to the present disclosure.

[0017] Figure 2 is a graphical representation of the nitrogen solubility limit at 1700° C. of stainless steel alloys having various chromium (Cr) contents as a function of manganese (Mn) content in accordance with the present disclosure.

[0018] Figure 3 is a uniaxial plot of phase fractions for a stainless steel alloy according to the present disclosure over a temperature range from 600°C to 1600°C.

[0019] Figure 4is a uniaxial plot of phase fractions for a stainless steel alloy according to the present disclosure over a temperature range from 600°C to 1600°C.

[0020] Figure 5 is a uniaxial plot of phase fractions for a stainless steel alloy according to the present disclosure over a temperature range from 600°C to 1600°C.

[0021] FIG. 6A to FIG. 6C Graphs of creep properties of a stainless steel alloy according to the present disclosure at temperatures of 800° C., 900° C., and 1000° C., respectively.

[0022] Figure 7 is a graphical representation of the surface tension of stainless steel alloys having various manganese (Mn) contents and nitrogen (N) contents at a temperature range from about 1200° C. to 1600° C. according to the present disclosure.

[0023] Figure 8 is a graphical representation of the oxidation of a stainless steel alloy at 1000° C. as a function of time in accordance with the present disclosure. DETAILED DESCRIPTION

[0024] As required, detailed embodiments of the present disclosure are disclosed herein; however, it should be understood that the disclosed embodiments are merely examples of the present disclosure that may be implemented in various forms and alternative forms. The drawings are not necessarily drawn to scale; some features may be exaggerated or reduced to show details of particular components. Therefore, the specific structural and functional details disclosed herein should not be interpreted as limiting, but merely as a representative basis for teaching those skilled in the art to adopt the present disclosure in various ways.

[0025] Unless otherwise specified in the context, the term "about" as used herein should be understood to be within the normal tolerance range in the art, such as within 2 standard deviations of the mean. "About" can be understood to be within 10%, 5%, 1%, 0.5%, 0.1%, 0.05% or 0.01% of the value. "About" can alternatively be understood to imply the exact value stated. Unless otherwise clear from the context, the numerical values ​​provided herein are modified by the term "about".

[0026] The present disclosure generally relates to austenitic stainless steel alloys suitable for use in various turbocharger turbine and exhaust applications. Exemplary turbocharger turbine components according to the present disclosure include turbine housing components and turbine exhaust components, which withstand operating temperatures of up to about 1000°C in some applications. The turbocharger turbine housing, which is typically cast stainless steel or cast iron, is often the most expensive component in a turbocharger. Reducing the cost of the housing will have a direct impact on the cost of the turbocharger. In order to withstand the high operating temperatures typically generated by the exhaust gas impinging on the turbine housing, the turbine housing material is typically alloyed with elements such as chromium and nickel in addition to other carbide-forming elements, resulting in increased costs. Reducing the content and / or eliminating these expensive alloying elements will have a direct impact on the cost of the turbine housing.

[0027] Typical embodiments of the present disclosure are directed to vehicles, such as land-based, air-based, or water-based vehicles, equipped with a powered internal combustion engine ("ICE") and a turbocharger. The turbocharger is equipped with a unique combination of features that, in various embodiments, can provide efficiency gains by relatively limiting the amount (and kinetic energy) of secondary flow in the turbine and / or compressor compared to comparable unmodified systems.

[0028] refer to Figure 1 , an exemplary embodiment of a turbocharger 101 having a radial turbine and a radial compressor includes a turbocharger housing and a rotor configured to rotate within the turbocharger housing about a rotor rotation axis 103 during operation of the turbocharger on thrust bearings or two sets of journal bearings (one set for each respective rotor wheel) or alternatively on other similar support bearings. The turbocharger housing includes a turbine housing 105, a compressor housing 107, and a bearing housing 109 (i.e., a center housing containing bearings) connecting the turbine housing to the compressor housing. The rotor includes a radial turbine wheel 111 substantially located within the turbine housing 105, a radial compressor wheel 113 substantially located within the compressor housing 107, and a shaft 115 extending through the bearing housing 109 along the rotor rotation axis 103 to connect the turbine wheel 111 to the compressor wheel 113.

[0029] The turbine housing 105 and the turbine wheel 111 form a turbine that is configured to circumferentially receive a high-pressure, high-temperature exhaust gas flow 121 from an engine, such as an exhaust manifold 123 of an internal combustion engine 125. The turbine wheel 111 (and therefore the rotor) is driven by the high-pressure, high-temperature exhaust gas flow to rotate about the rotor rotation axis 103, which becomes a low-temperature, low-pressure exhaust gas flow 127 and is released axially into an exhaust system (not shown).

[0030] The compressor housing 107 and the compressor impeller 113 form a compressor stage. The compressor impeller, which is driven to rotate by the turbine impeller 111 driven by the exhaust gas, is configured to compress the axially received input air (e.g., ambient air 131, or pressurized air from the previous stage of the multi-stage compressor) into a pressurized air flow 133, which is ejected circumferentially from the compressor. Due to the compression process, the pressurized air flow is characterized by an elevated temperature higher than the input air.

[0031] Optionally, the pressurized air flow may be directed through a convection-cooled charge air cooler 135 configured to remove heat from the pressurized air flow thereby increasing the density of the pressurized air flow. The resulting cooled and pressurized output air flow 137 is directed through into an intake manifold 139 on the internal combustion engine, or alternatively into a subsequent stage of a series compressor. Operation of the system is controlled by an ECU (engine control unit) 151, which is connected to the rest of the system via a communication connection 153.

[0032] Embodiments of the present disclosure are directed to improvements over currently available stainless steel alloys for use in turbochargers operating at temperatures up to about 1000°C. Specifically, embodiments of the present disclosure are directed to austenitic stainless steel alloys having lower chromium and nickel contents than 1.4837 stainless steel for cost considerations and to improve formability and / or manufacturability, including castability / machinability. The stainless steel alloys described herein include iron alloyed with various alloying elements, as described in more detail below in weight percentages based on the total weight of the alloy. Moreover, the discussion of the effects of containing specific percentages of elements is specific to the exemplary alloys described herein.

[0033] In an embodiment, the austenitic stainless steel alloy of the present disclosure includes or consists of from about 22.0% to about 23.0% chromium (Cr), for example, about 22.3% to about 22.7% Cr, such as about 22.5% Cr. Chromium is provided, for example, to achieve a desired austenite phase in the alloy for oxidation resistance / corrosion resistance when operating at relatively high temperatures, such as up to about 1000°C. However, as initially stated, it is desirable to minimize the Cr content to reduce cost. In addition, as the Cr content increases, the content of Ni, which is almost as expensive, is also increased to maintain the volume fraction, resulting in further cost increases. In addition, if Cr is added too much, coarse primary carbides of Cr are formed, resulting in extreme brittleness. Therefore, it has been found herein that a balance between sufficient austenite phase stability and prevention of delta ferrite phase formation (along with cost reduction) is achieved when Cr is provided in the above range, for example, from about 22.0% to about 23.0%.

[0034] In an embodiment, the austenitic stainless steel alloy of the present disclosure includes nickel (Ni) from about 4.0% to about 5.0%, for example, about 4.3% to about 4.7% Ni, such as about 4.5% Ni or consisting of the above-mentioned content of nickel (Ni). Ni, as well as manganese and nitrogen (as described in more detail below, included in the alloy of the present disclosure) are elements for stabilizing the austenite phase, and together with the aforementioned Cr, it is expected to achieve oxidation resistance / corrosion resistance at high temperatures as described above. To reduce production costs, if the content of relatively expensive Ni is reduced, the reduction in Ni can be replaced by increasing the content of manganese and nitrogen that form the austenite phase. However, it has been found that if the content of Ni is excessively reduced, manganese and nitrogen are excessively required, so that corrosion resistance / oxidation resistance and hot formability characteristics are deteriorated. Therefore, it has been found herein that when Ni is provided in the above-mentioned range, for example, from about 4.0% to about 5.0%, a balance between sufficient austenite phase stability and casting considerations (along with cost reduction) is achieved.

[0035] In an embodiment, the austenitic stainless steel alloy of the present disclosure includes from about 4.0% to about 5.0% manganese (Mn), for example, about 4.3% to about 4.7% Mn, such as about 4.5% Mn or consists of the above content of manganese (Mn). As initially described above, Mn is provided for the stability of the austenite phase. Mn, like silicon (Si) (as described in more detail below, is included in the alloy of the present disclosure) is an effective element as a deoxidizer for the melt and has the function of improving fluidity during the casting operation. To effectively perform this function, the amount of Mn is about 6% or less, for example, about 5% or less. Higher Mn contents can combine with sulfur in the steel and form excessive levels of manganese sulfide, thereby degrading the corrosion resistance / oxidation resistance and hot formability characteristics. Mn is also an austenite stabilizer (as described above) and promotes the solubility of N in the melt without N escaping as a gas. A minimum amount of 4% Mn is used for a given Ni to promote austenite stability of the material, and for a given Cr and N to promote efficient solubility of N in the melt, as shown in the formula below. Thus, having Mn in the range of about 4% to about 5% is beneficial for achieving desired properties as well as melt processability.

[0036] Nitrogen solubility limit

[0037] 19.628*Cr+9.009*Mn>467.51

[0038] In an embodiment, the austenitic stainless steel alloy of the present disclosure includes from about 0.2% to about 0.4% Mo, for example, about 0.25% to about 0.35% Mo, such as about 0.3% Mo or consists of the above content of Mo. For reduced Nb and Ni content, the stainless steel of the present disclosure is provided with high temperature strength by adding Mo provided in the above range.

[0039] In an embodiment, the austenitic stainless steel alloy of the present disclosure includes from about 0.2% to about 0.4% niobium (Nb), for example, about 0.25% to about 0.35% Nb, such as about 0.3% Nb or consisting of the above content of niobium (Nb). The stainless steel alloy of the present disclosure is provided with high castability and high strength by forming eutectic carbides of Nb.

[0040] In an embodiment, the austenitic stainless steel alloy of the present disclosure includes from about 0.5% to about 1.5% silicon (Si), such as about 0.8% to about 1.2% Si or consists of silicon (Si) in the above content. Si has the effect of improving the stability of its metal structure and its oxidation resistance. In addition, it has the function of a deoxidizer and is also effective in improving castability and reducing pinholes in the resulting cast product. If the content of Si is too much, Si deteriorates the mechanical properties of the alloy, such as the impact toughness of the steel. Therefore, it has been found herein that when Si is provided in the above range, for example, from about 0.5% to about 1.5%, a balance between sufficient mechanical properties, deoxidation properties and casting considerations is achieved.

[0041] In an embodiment, the austenitic stainless steel alloy of the present disclosure includes from about 0.35% to about 0.45% carbon (C), such as about 0.37% to about 0.43% C or consists of C in the above content. C generally provides hardness and strength to stainless steel, and can form carbides with metallic elements. In addition, C has the function of improving melt fluidity and castability. However, when provided too much, C may embrittle the stainless steel, making it more susceptible to cracking during use in turbocharger applications. Therefore, it has been found herein that when C is provided in the above range, for example, from about 0.35% to about 0.45%, a balance between sufficient mechanical properties and casting considerations is achieved.

[0042] In an embodiment, the austenitic stainless steel alloy of the present disclosure includes nitrogen (N) from about 0.2% to about 0.28%, such as from about 0.22% to about 0.26% N or consists of N in the above content. Together with Ni, N is one of the elements that contribute to the stability of the austenite phase. As the N content increases, corrosion resistance / oxidation resistance and high strengthening are achieved. However, when the N content is too high, the hot formability and machinability of the steel will deteriorate, thereby reducing its production yield. In addition, N is an element that can improve high temperature strength and thermal fatigue resistance like C, however, when the N content is too much, embrittlement due to the precipitation of Cr nitrides may be encountered. Therefore, it has been found herein that when N is provided in the above range, for example, from about 0.2% to about 0.28%, a balance between austenite phase stability and corrosion resistance / oxidation resistance, sufficient mechanical properties, and casting / machining considerations is achieved.

[0043] In an embodiment, the austenitic stainless steel alloy of the present disclosure may also include from about 0.3% to about 0.5% copper (Cu), for example, about 0.35% to about 0.45% Cu, such as about 0.4% Cu or consisting of the above content of copper (Cu). When present in the above range, Cu also provides additional stability to the austenite phase, for example, similar to Mn.

[0044] Certain unavoidable / inevitable impurities may also be present in the austenitic stainless steel alloys of the present disclosure. The amount of such impurities is minimized as far as practicable. In an embodiment, phosphorus (P) may be present in the alloy, but is minimized to about 0.04% or less, such as about 0.03% or less, such as about 0.02% or less. P is seeded in grain boundaries or interfaces and tends to degrade corrosion resistance and toughness. Therefore, the content of P is reduced as much as possible. In an exemplary embodiment, P in excess of impurity levels is excluded from the alloy.

[0045] In an embodiment, certain relatively expensive carbide-forming elements exceeding the impurity level may be excluded. These include, for example, tungsten, etc., and any combination of two or more thereof may be excluded. It has been found that without including these elements exceeding the level that cannot be avoided as impurities (such as levels less than about 0.3%, less than about 0.1%, or less than about 0.05%), austenite phase stability, delta ferrite phase minimization, and adequate mechanical and casting properties may be achieved. Additional specific elements that may be excluded from the alloy (in amounts greater than impurities) include one or more of zirconium, phosphorus, sulfur, aluminum, titanium, vanadium, yttrium, boron, and / or cobalt, and any combination of two or more thereof exceeding the level that cannot be avoided as impurities (such as levels less than about 0.3%, less than about 0.1%, or less than about 0.05%) may be excluded, the percentage depending on the specific element under consideration. In some embodiments, sulfur exceeding the impurity level is excluded from the alloy, for example, if present, in an amount of about 0.04% or less, such as about 0.02% or less, for example about 0.01% or less. S in steel deteriorates hot working properties and may form sulfide inclusions (e.g., MnS) that adversely affect pitting corrosion resistance. Therefore, the S content is reduced as much as possible. In some embodiments, zirconium and / or phosphorus are excluded from the alloy in excess of impurity levels, for example, if present, in an amount of about 0.02% or less, such as about 0.01% or less, such as about 0.005% or less, for example about 0.002% or less.

[0046] Iron constitutes the balance of the alloys described herein. The disclosed alloys may include the aforementioned elements, wherein other elements may be included in the alloy composition within the scope of the present disclosure. However, preferably, the disclosed alloys consist of the aforementioned elements, wherein other elements other than those described above are not included in the alloy (in amounts greater than unavoidable / inevitable impurities).

[0047] Table 2 lists composition ranges (all elements in wt. %) for exemplary austenitic stainless steel alloys of the present disclosure according to the described embodiments provided above.

[0048] Table 2. Composition of exemplary stainless steel alloys

[0049]

[0050] Illustrative examples

[0051] The present disclosure is now illustrated by the following non-limiting examples. It should be noted that various changes and modifications may be applied to the following examples and processes without departing from the scope of the present disclosure defined in the appended claims. Therefore, it should be noted that the following examples should be interpreted only as illustrative and not limiting in any sense.

[0052] refer to Figure 2 , illustrates a graph 200 including a vertical axis showing nitrogen solubility at 1700° C. for stainless steel alloys having various chromium (Cr) contents as a function of manganese (Mn) content represented by the horizontal axis. Lines 202, 204, and 206 represent corresponding stainless steel alloys having compositions corresponding to nominal values ​​between the upper and lower limits shown in Table 2 above, but having Cr contents adjusted as follows: line 202 has a Cr content of 22%, line 204 has a Cr content of 22.5%, and line 206 has a Cr content of 23%. In the case of N in an exemplary range from 0.2% to 0.28%, the minimum amount of Mn is 4% to ensure that nitrogen (N) is dissolved into the melt for the casting process. If the Mn content is below 4%, N will escape in gaseous form if the N content is at the upper limit of 0.28%. It is therefore apparent that an upper limit of 0.28% for N and a lower limit of 4% for Mn are critical to ensuring that nitrogen remains soluble in the stainless steel alloy and does not escape in gaseous form during the casting process.

[0053] In addition, the following regression equation for nitrogen solubility was established as a function of Cr content and Mn content to ensure that nitrogen remains soluble in the stainless steel alloy and does not escape in gaseous form during the casting process:

[0054] Nitrogen solubility limit

[0055] 19.628*Cr+9.009*Mn>467.51,

[0056] Wherein Cr is the weight percentage of Cr in the stainless steel alloy, and Mn is the weight percentage of Mn in the stainless steel alloy.

[0057] In one example, for an exemplary stainless steel alloy with a Cr content at a lower limit of 22% and a Mn content at a lower limit of 4% (from Table 2), the regression equation for nitrogen solubility is solved as follows:

[0058] 19.628*(22)+9.009*(4)=467.852>467.51,

[0059] This indicates that nitrogen will remain soluble in the stainless steel alloy during the casting process.

[0060] In an alternative example, for an exemplary stainless steel alloy having a Cr content at the lower limit of 22% but unlike an Mn content just below the lower limit of 3.5% (from Table 2), the regression equation for nitrogen solubility is solved as follows:

[0061] 19.628*(22)+9.009*(3.5)=463.3475<467.51,

[0062] It is thus evident that the nitrogen will not remain completely soluble in the stainless steel alloy and therefore at least some of the nitrogen will escape in gaseous form during the casting process. It is therefore apparent that the lower limit of the Mn content of 4.0% is critical to ensuring that the nitrogen remains soluble in the stainless steel alloy and does not escape in gaseous form during the casting process.

[0063] In yet another alternative example, for an exemplary stainless steel alloy having a Mn content at the lower limit of 4% but having a Cr content just below the lower limit of 21.5% (from Table 2), the regression equation for nitrogen solubility is solved as follows:

[0064] 19.628*(21.5)+9.009*(4)=458.038<467.51,

[0065] It is thus evident that the nitrogen will not remain completely soluble in the stainless steel alloy and therefore at least some of the nitrogen will escape in gaseous form during the casting process. It is therefore apparent that the lower limit of Cr content of 22.0% is critical to ensuring that the nitrogen remains soluble in the stainless steel alloy and does not escape in gaseous form during the casting process.

[0066] refer to Figure 3 , shows a uniaxial plot of phase fractions of a stainless steel alloy over a temperature range from 600° C. to 1600° C. The stainless steel alloy has a composition corresponding to the nominal values ​​between the upper and lower limits shown in Table 2 above. As shown, the alpha ferrite phase and the delta ferrite phase are absent over a wide temperature range from about 700° C. to about 1110° C., thereby allowing very high austenite phase fractions, such as about 0.9 or greater, over the desired operating temperature range.

[0067] refer to Figure 4 , shows a uniaxial diagram of the phase fraction of a stainless steel alloy in the temperature range of 600°C to 1600°C. The stainless steel alloy has a composition corresponding to the nominal values ​​between the upper and lower limits shown in Table 2 above, but the difference is that the N content is adjusted to 0.15%, which is just outside the lower limit of 0.2%. As shown in the figure, the delta ferrite phase exists from about 940°C to a higher temperature, thereby narrowing the range in which the alpha ferrite phase and the delta ferrite phase do not exist, specifically from about 750°C to about 940°C, and the resulting austenite phase fraction is low, such as about 0.9 or less. Therefore, the reduction of the N content of the stainless steel alloy to below the lower limit of 0.2% results in the alloy having a more unstable austenite material phase at an expected operating temperature such as 1000°C, and the presence of a harmful delta ferrite phase. It is therefore apparent that the lower limit of the N content of 0.2% is critical to ensuring that the stainless steel alloy has a very high austenite phase fraction, for example about 0.9 or more, without the presence of a deleterious delta ferrite material phase over the desired operating temperature range.

[0068] refer to Figure 5 , shows a uniaxial diagram of the phase fraction of a stainless steel alloy in the temperature range of 600°C to 1600°C. The stainless steel alloy has a composition corresponding to the nominal values ​​between the upper and lower limits shown in Table 2 above, but the difference is that the Mn content is adjusted to 3.5%, which is just outside the lower limit of 4.0%. As shown in the figure, the α ferrite phase exists from about 820°C to a lower temperature, thereby narrowing the range in which the α ferrite phase and the δ ferrite phase do not exist, specifically from about 820°C to about 1110°C, and the phase fraction of austenite obtained at a temperature of about 820°C to about 900°C is low, for example, about 0.9 or less. Therefore, the reduction of the Mn content of the stainless steel alloy to below the lower limit of 4.0% results in an alloy having a more unstable austenitic material phase at the lower end of the expected temperature range.

[0069] refer to FIG. 6A to FIG. 6C , graphs 600, 602, and 604 show creep properties of two stainless steel alloys, specifically, an exemplary stainless steel alloy 610 having a composition shown in Table 2 above, and a 1.4837 stainless steel 620 having a composition shown in Table 1 above. The vertical axis of each of the graphs 600, 602, and 604 shows the stress measured in MPa on the stainless steel alloy, and the horizontal axis shows the rupture time measured in hours. Graph 600 shows the results of creep testing the stainless steel alloy at 800°C, graph 602 shows the results of creep testing the stainless steel alloy at 900°C, and graph 604 shows the results of creep testing the stainless steel alloy at 1000°C. As shown, the exemplary stainless steel alloy 610 shows better creep properties than the 1.4837 stainless steel 620 at both 800°C and 900°C, and is only slightly lower than the 1.4837 stainless steel 620 at 1000°C. However, as will be discussed in further detail below, increasing the Mn content of the exemplary stainless steel alloy 610 above the upper limit of 5.0% may result in a significant increase in oxidation of the steel alloy, which may adversely affect the creep properties of the exemplary stainless steel alloy 610, thereby further reducing the creep properties of the exemplary stainless steel alloy 610 at 1000° C. to be significantly lower than that of the 1.4837 stainless steel 620.

[0070] refer to Figure 7, graph 700 illustrates the surface tension of exemplary stainless steel alloys having various manganese (Mn) contents and nitrogen (N) contents over a temperature range from about 1200° C. to 1600° C. As shown, the stainless steel alloy having a Mn content of 5.5%, just outside the upper limit of 5.0%, has a significantly lower surface tension than all other stainless steel alloys having lower Mn contents or varying N contents. For parts produced from the stainless steel alloy having a Mn content of 5.5%, the higher Mn content will translate into higher scrap rates due to a significant increase in "sand burn" during the casting process. In particular, when casting relatively high manganese steel alloys, silicon dioxide from the sand casting mold can react with the steel alloy to form low melting point manganese silicates, resulting in the "sand burn" problem. An example of a sand reaction is the reaction between silicon dioxide (SiO2) and iron oxide (Wustite, FeO) to produce various iron silicates. This can cause the grains of the sand mold to fuse and collapse as they melt into each other, since some silicates have melting points below the casting temperature. The reacted grains adhere to the surface of the casting due to the presence of the low melting point liquid "glue". This is known as "sand burning". It is therefore obvious that the upper limit of Mn content to 5.0% is critical to ensuring that the stainless steel alloy has good castability without significant "sand burning" problems.

[0071] refer to Figure 8 , graph 800 shows the oxidation of a stainless steel alloy at 1000° C. as a function of time. The vertical axis is in mg / mm 3 The mass gain is plotted, and the horizontal axis plots the time in minutes. Three stainless steel alloys were evaluated, specifically, 1.4837 stainless steel 802 having the composition shown in Table 1 above, exemplary stainless steel alloy 804 having the composition shown in Table 2 above, and stainless steel alloy 806 having the composition shown in Table 2 above, but with the Mn content adjusted to 8%. As shown, both 1.4837 stainless steel 802 and exemplary stainless steel alloy 804 have very comparable and excellent oxidation resistance when exposed to 1000°C for a long time. However, compared to the other two stainless steels 802 and 804, stainless steel alloy 806 shows a significant increase in oxidation when exposed to 1000°C for a long time. Therefore, it is obvious that the upper limit of the Mn content is 5.0% to ensure that the stainless steel alloy has excellent oxidation resistance in its expected operating temperature range to ensure enhanced performance.

[0072] Thus, embodiments of the present disclosure provide numerous benefits over the prior art, including minimizing expensive nickel content, minimizing or eliminating porosity formed during the casting process, while maintaining desirable material properties for use as turbocharger turbine components such as housing components or exhaust components. In addition, the disclosed alloys maintain a stable austenitic material phase above expected operating temperatures such as 1000° C., while substantially minimizing the deleterious delta ferrite phase. Thus, embodiments of the present disclosure are suitable for use as low-cost alloys for turbocharger turbine components such as turbocharger turbine housings, for design operation up to about 1000° C.

[0073] While the best modes for carrying out the disclosure have been described in detail, those familiar with the art to which this disclosure relates will recognize various alternative designs and embodiments for practicing the disclosure within the scope of the appended claims.

Claims

1. An austenitic stainless steel alloy, comprising by weight: about 22.0% to about 23.0% chromium; from about 4.0% to about 5.0% nickel; from about 4.0% to about 5.0% manganese; about 0.2% to about 0.4% molybdenum; from about 0.2% to about 0.4% niobium; about 0.5% to about 1.5% silicon; from about 0.35% to about 0.45% carbon; from about 0.2% to about 0.28% nitrogen; and The balance of iron and other inevitable or unavoidable impurities present in trace amounts, with phosphorus and sulfur exceeding impurity levels being excluded from the austenitic stainless steel alloy.

2. The austenitic stainless steel alloy of claim 1, comprising from about 22.3% to about 22.7% chromium.

3. The austenitic stainless steel alloy of claim 1 comprising from about 4.3% to about 4.7% nickel.

4. The austenitic stainless steel alloy of claim 1 comprising about 4.3% to about 4.7% manganese.

5. The austenitic stainless steel alloy of claim 1 comprising from about 0.25% to about 0.35% molybdenum.

6. The austenitic stainless steel alloy of claim 1 comprising from about 0.8% to about 1.2% silicon.

7. The austenitic stainless steel alloy of claim 1 comprising from about 0.37% to about 0.43% carbon.

8. The austenitic stainless steel alloy of claim 1 comprising from about 0.22% to about 0.26% nitrogen.

9. The austenitic stainless steel alloy of claim 1 comprising from about 0.25% to about 0.35% niobium.

10. The austenitic stainless steel alloy of claim 1, consisting by weight of: about 22.0% to about 23.0% chromium; from about 4.0% to about 5.0% nickel; from about 4.0% to about 5.0% manganese; about 0.2% to about 0.4% molybdenum; from about 0.2% to about 0.4% niobium; about 0.5% to about 1.5% silicon; from about 0.35% to about 0.45% carbon; from about 0.2% to about 0.28% nitrogen; and The balance of iron and other inevitable or unavoidable impurities present in trace amounts, with phosphorus and sulfur exceeding impurity levels being excluded from the austenitic stainless steel alloy.

11. The austenitic stainless steel alloy of claim 1 comprising from about 0.3% to about 0.5% copper.

12. The austenitic stainless steel alloy of claim 11, consisting by weight of: about 22.0% to about 23.0% chromium; from about 4.0% to about 5.0% nickel; from about 4.0% to about 5.0% manganese; about 0.2% to about 0.4% molybdenum; from about 0.2% to about 0.4% niobium; about 0.5% to about 1.5% silicon; from about 0.35% to about 0.45% carbon; about 0.3% to about 0.5% copper; from about 0.2% to about 0.28% nitrogen; and The balance of iron and other inevitable or unavoidable impurities present in trace amounts, with phosphorus and sulfur exceeding impurity levels being excluded from the austenitic stainless steel alloy.

13. A turbocharger turbine component, the turbocharger turbine component comprising: Austenitic stainless steel alloy, wherein the austenitic stainless steel alloy comprises by weight: about 22.0% to about 23.0% chromium; from about 4.0% to about 5.0% nickel; from about 4.0% to about 5.0% manganese; about 0.2% to about 0.4% molybdenum; from about 0.2% to about 0.4% niobium; about 0.5% to about 1.5% silicon; from about 0.35% to about 0.45% carbon; from about 0.2% to about 0.28% nitrogen; and The balance of iron and other inevitable or unavoidable impurities present in trace amounts, with phosphorus and sulfur exceeding impurity levels being excluded from the austenitic stainless steel alloy.

14. The turbocharger turbine component according to claim 13, wherein: The austenitic stainless steel alloy consists by weight of: about 22.0% to about 23.0% chromium; from about 4.0% to about 5.0% nickel; from about 4.0% to about 5.0% manganese; about 0.2% to about 0.4% molybdenum; from about 0.2% to about 0.4% niobium; about 0.5% to about 1.5% silicon; from about 0.35% to about 0.45% carbon; from about 0.2% to about 0.28% nitrogen; and The balance of iron and other inevitable or unavoidable impurities present in trace amounts, with phosphorus and sulfur exceeding impurity levels being excluded from the austenitic stainless steel alloy.

15. The turbocharger turbine component of claim 13, wherein: The austenitic stainless steel alloy includes about 0.3% to about 0.5% copper.

16. The turbocharger turbine component according to claim 15, wherein: The austenitic stainless steel alloy consists by weight of: about 22.0% to about 23.0% chromium; from about 4.0% to about 5.0% nickel; from about 4.0% to about 5.0% manganese; about 0.2% to about 0.4% molybdenum; from about 0.2% to about 0.4% niobium; about 0.5% to about 1.5% silicon; from about 0.35% to about 0.45% carbon; about 0.3% to about 0.5% copper; from about 0.2% to about 0.28% nitrogen; and The balance of iron and other inevitable or unavoidable impurities present in trace amounts, with phosphorus and sulfur exceeding impurity levels being excluded from the austenitic stainless steel alloy.

17. The turbocharger turbine component of claim 13, wherein: The turbocharger turbine component includes a turbocharger turbine housing.

18. A vehicle comprising the turbocharger turbine component according to claim 13.

19. A method of manufacturing a turbocharger turbine component, the method comprising forming the turbocharger turbine component using an austenitic stainless steel alloy, wherein: The austenitic stainless steel alloy comprises, by weight: about 22.0% to about 23.0% chromium; from about 4.0% to about 5.0% nickel; from about 4.0% to about 5.0% manganese; about 0.2% to about 0.4% molybdenum; from about 0.2% to about 0.4% niobium; about 0.5% to about 1.5% silicon; from about 0.35% to about 0.45% carbon; from about 0.2% to about 0.28% nitrogen; and The balance of iron and other inevitable or unavoidable impurities present in trace amounts, with phosphorus and sulfur exceeding impurity levels being excluded from the austenitic stainless steel alloy.

20. The method according to claim 19, wherein: The austenitic stainless steel alloy includes about 0.3% to about 0.5% copper.

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