Gas sensor

Abstract

Gas sensor (1), comprising: a housing (2) with a receiving hole (21); a sensor element (3) comprising a solid electrolyte (31) and electrodes (32A, 32B) located on both sides of the solid electrolyte (31), wherein the sensor element (3) is inserted into the receiving hole (21) alone or via an insulator (4); and a sealing element (51) formed from a ceramic powder, which fills a gap (S1) between the receiving hole (21) and the sensor element (3) or the insulator (4), and wherein the sealing element (51) is compressed by a part of the housing (2) so that the gap (S1) is sealed, and wherein The material of which the casing (2) is made is ferritic stainless steel which, based on mass, contains 15 to 25% Cr, 0.01 to 1.0% Nb, 0.5 to 4% of at least one of the elements W and Mo alone or in combination and the following remainder: Fe and unavoidable impurities including C, N, Mn and Si, where: after heating the material forming the casing (2) to a heat treatment temperature of 850°C or more, the Laves phase of Fe2W or Fe2W and Fe2Mo is formed as intermetallic compounds in a mother phase of the casing (2), and a quantity of Laves phase excreted into the mother phase of the shell (2) is less than 0.1 mass percent.

Description

[Technical field]

[0001] The present disclosure relates to a gas sensor with a sensor element in which the electrodes are mounted on a solid electrolyte. [State of the art]

[0002] DE 10 2006 044 430 A1 discloses a gas sensor for determining a physical property of a measuring gas, in particular the temperature or concentration of a gas component of a gas mixture, comprising a sensor element with an element section exposed to the measuring gas and a protective casing enclosing the element section made of a metal wire composite porous for gas passage. To extend the service life of the gas sensor by preventing corrosive decomposition of the metallic protective casing, additives are incorporated into the wire material of the metal wire composite, which cause the formation of a stable oxide layer on the wire surface.

[0003] DE 697 33 048 T2 discloses an air-fuel ratio sensor comprising: a sensor element with a long measuring section which is closed at one end and is capable of detecting an air-fuel ratio in a gas to be measured, a flange section which is formed in the middle region of the element, and a hollow section which extends from the flange section to the other end of the element and has an open end, wherein a housing 14 is provided with a housing opening which extends along an axial direction therefrom and is capable of housing the sensor element therein, such that the measuring section extends through the housing opening from one side of the housing, wherein the housing opening has a chamfered inner wall which serves as a stopper of the flange section of the sensor element and the housing has a hot crimped section on the other side therefrom.to hold the hollow section of the sensor element fixed in the housing opening, characterized in that the housing is made entirely of ferritic stainless steel, consisting essentially of 16.0 to 18.0 wt.% Cr, 0.75 wt.% or less Si, 1.0 wt.% or less Mn, 0.04 wt.% or less P, 0.030 wt.% or less S, the remainder being Fe and unavoidable impurities, provided that the total amount of C and N is 0.03 wt.% or less.

[0004] DE 10 2009 010 590 A1 discloses a gas sensor comprising the following: a gas detection element; a metal sheath extending in an axial direction; and a metal cylinder extending in an axial direction, wherein a front end section of the metal cylinder surrounds a foot end section of the metal sheath and is attached to the metal sheath by means of a circumferentially welded section. The metal sheath comprises: Fe in an amount equal to or greater than 50.0% by mass; C in an amount of 0.02% to 0.15% by mass; Cr in an amount of 11.5% to 18.0% by mass; and Nb in an amount equal to or greater than twice the amount of C by mass.

[0005] Examples of gas sensors include air-fuel ratio sensors, oxygen sensors and NOx sensors, which detect the air-fuel ratio, the oxygen concentration and the concentration of a specific gas component such as NOx in the exhaust gas of an internal combustion engine.

[0006] In a gas sensor, a sensor element, either alone or via an insulator, is positioned in a receiving hole of a housing. A sealing element, such as talc, which fills the gap between the receiving hole and the sensor element or the insulator, is compressed by a crimped section of the housing. This secures the sensor element to the housing and ensures a tight seal in the gap where the sealing element is located.

[0007] In this gas sensor, which uses air as a reference gas, the exhaust gas flowing through the exhaust pipe of an internal combustion engine is introduced into a sensing section of the sensor element that protrudes from the housing, while air drawn in from outside the exhaust pipe is directed into the interior of the sensor element. Since the pressure of the exhaust gas is higher than atmospheric pressure, the tightness of the gap containing the sealing element prevents the exhaust gas from mixing with the air inside the sensor element.

[0008] JP 2009-198 422 A discloses a technique that, for example, modifies the composition of the casing. In JP 2009-198 422 A, the casing contains iron as its main component, at least 0.02 to 0.15% carbon by mass, 11.5 to 18.0% chromium by mass, and nitrogen with twice the mass of carbon or more. [List of citations][Patent literature] JP 2009-198 422 A DE 10 2006 044 430 A1 DE 697 33 048 T2 DE 10 2009 010 590 A1 [Summary of the invention]

[0009] It is an object of the present invention to further improve the topic explained above.

[0010] This problem is solved by the gas sensor with the features of claim 1 and by the gas sensor with the features of claim 2. Advantageous embodiments are found in the respective dependent claims.

[0011] The temperature of the installation environment of a gas sensor used as an exhaust gas sensor in an exhaust-carrying environment has increased due to factors such as downsizing to improve vehicle fuel efficiency and the close mounting of an exhaust aftertreatment catalyst, which leads to early temperature rises. To match the coefficient of thermal expansion of the housing and the exhaust pipe, which is made of ferritic stainless steel, a ferritic stainless steel such as grade 430 is generally used for the housing. While the housing made of grade 430 stainless steel offers excellent machinability, it has the disadvantage of exhibiting significantly reduced strength at temperatures of 550 °C and above.

[0012] For this reason, for example, in an environment where the temperature of the crimped housing section reaches 650 °C, the pressure on the sealing element, such as talc, is reduced by permanent deformation of the housing, e.g., at the crimped section. Depending on the circumstances, the exhaust gas in the exhaust pipe may mix with the air that is introduced into the interior of the sensor element through the gap in which the sealing element is located.

[0013] Some air-fuel ratio sensors have an air channel that directs air into the interior of the sensor element. In such an air-fuel ratio sensor, unburned gas, when the air-fuel ratio is high, causes a chemical reaction at the electrode exposed to the exhaust gas. Accordingly, the oxide ions (O₂) move 2-) from the electrode exposed to the air through the solid electrolyte to the electrode exposed to the exhaust gas, so that the air-fuel ratio is measured in the fuel-rich state.

[0014] When the exhaust gas mixes with the air introduced into the sensor element within the air-fuel ratio sensor, including the air duct, during a fuel-rich state, the oxygen concentration in the air decreases. This may prevent the oxide ions (O₂) from reacting with the air. 2- The electrolyte is transferred from the electrode exposed to the air to the electrode exposed to the exhaust gas via the solid electrolyte. In this case, the detection range in which the air-fuel ratio can be measured under fuel-rich conditions may be limited.

[0015] Additionally, the gas sensor contains internal contact terminals. These terminals electrically connect the sensor element and a heater, which heats the sensor element, to the outside of the gas sensor. If the exhaust gas mixes with the air introduced into the sensor element, the exhaust gas may reach the contact terminals. In this case, the contact terminals could potentially corrode, for example, due to moisture and nitrogen compounds in the exhaust gas.

[0016] To ensure the detection range for measuring the air-fuel ratio under fuel-rich conditions, or the corrosion resistance of the contact connections, it is therefore important to guarantee the airtightness of the gap containing the sealing element, even at high ambient temperatures of 550°C or more. It was found that a further modification of the housing material composition is necessary to suppress a decrease in the strength of the crimped housing section.

[0017] The present disclosure aims to provide a gas sensor that suppresses the permanent deformation of a housing and ensures the airtightness of the gas sensor under high temperature conditions.

[0018] One aspect of the present disclosure is a gas sensor comprising a housing, a sensing element, and a sealing element. The housing 2 contains a receiving hole. The sensing element contains a solid electrolyte and electrodes located on both sides of the solid electrolyte. The sensing element is inserted into the receiving hole alone or via an insulator. The sealing element consists of a ceramic powder that fills a gap between the receiving hole and the sensing element or the insulator. The sealing element is compressed by a portion of the housing, thus sealing the gap. The housing is made of ferritic stainless steel with a 0.2% yield strength of 80 MPa or more at 650°C.

[0019] Another aspect of the present disclosure is a gas sensor comprising a housing, a sensing element, and a sealing element. The housing has a receiving hole. The sensing element contains a solid electrolyte and electrodes located on both sides of the solid electrolyte. The sensing element is inserted into the receiving hole alone or via an insulator. The sealing element consists of a ceramic powder that fills a gap between the receiving hole and the sensing element or the insulator. The sealing element is compressed by a portion of the housing, thus sealing the gap. The housing is made of ferritic stainless steel containing, by mass, 15 to 25% Cr, 0.01 to 1.0% Nb, 0.5 to 4% of at least one of the elements W and Mo alone or in combination, and the remainder being Fe and unavoidable impurities including C, N, Mn, and Si. [Advantageous effects of the invention]

[0020] According to the gas sensor of the first aspect, since the housing is made of ferritic stainless steel, which has a 0.2% yield strength (which can be referred to simply as the yield strength hereafter) of 80 MPa or more at 650°C, a decrease in the housing's strength under high-temperature conditions of 550°C or more is suppressed. With this housing configuration, even under high ambient temperatures of 550°C or more, the force with which a part of the housing compresses the sealing element is maintained, and the airtightness of the gap between the housing's receiving hole and the sensor element or the insulator, achieved by the sealing element, is preserved.

[0021] Thus, the gas sensor of the first aspect suppresses the permanent deformation of the housing and ensures the airtightness of the gas sensor under high temperature conditions.

[0022] The gas sensor of the other aspect counteracts the decrease in the strength of the housing under high-temperature conditions of 550°C or more by modifying the housing composition. The material from which the housing is made increases its yield strength at high temperatures of 550°C or more while maintaining the low thermal expansion characteristic of ferritic stainless steel, which contains 15 to 25 percent by mass of Cr (chromium) in Fe (iron) to resist expansion even when heated.

[0023] More precisely, to increase the yield strength of the material at high temperatures of 550°C or above, the housing material contains, in addition to Cr, 0.01 to 1.0 wt% Nb (niobium) and 0.5 to 4 wt% of at least one of the elements W (tungsten) and Mo (molybdenum), either alone or in combination with Fe. This suppresses permanent deformation of the housing at high temperatures of 550°C and above. As a result, even at high ambient temperatures of 550°C and above, the force with which a part of the housing compresses the sealing element is maintained, and the tightness of the gap between the housing's receiving hole and the sensor element, or the insulator formed by the sealing element, is preserved.

[0024] Thus, the gas sensor on the other side also reduces the permanent deformation of the housing and ensures the airtightness of the gas sensor under high temperature conditions.

[0025] Reference numerals in parentheses used for components in an aspect of the present disclosure indicate correspondence with reference numerals in the drawing of the embodiment and do not restrict the components to the content of the embodiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The task, properties and advantages described above are made clearer by the following detailed description with reference to the accompanying figures, which show the following: Fig. Figure 1 is an explanatory diagram showing a cross-section of a gas sensor according to one embodiment; Fig. Figure 2 is a partially enlarged explanatory diagram showing the cross-section of the gas sensor according to the embodiment; Fig. Figure 3 is an explanatory diagram showing the cross-section of a sensor element of the gas sensor according to the embodiment; Fig. Figure 4 is an explanatory diagram showing the cross-section of another gas sensor according to one embodiment; Fig. Figure 5 is a graph showing the relationship between the material from which a housing is made and the yield strength after test 1 of a verification trial or test; Fig. Figure 6 shows the relationship between the case temperature and the yield point after test 3 of the verification test; Fig. Figure 7 shows the relationship between the heat treatment temperature of the housing and the yield strength at room temperature according to test 4 of the verification test; Fig. Figure 8 is a diagram showing the relationship between the ignition temperature and the amount of precipitation or precipitate of the Laves phase according to test 5 of the verification test; and Fig. Figure 9 is a graph showing the amount of leakage that occurred in the housing according to test 7 of the verification tests. [Description of the embodiments]

[0027] A gas sensor according to a preferred design is described.

[0028] Precipitation strengthening and substitution solid solution strengthening are generally known to be effective methods for increasing the yield strength of a material at high temperatures, i.e., its high-temperature strength. It is well known that the precipitation strengthening process strengthens the material through the precipitation of carbide or nitride by adding elements such as Nb, Mo, W, Si, and Cu. Since the precipitation strengthening method significantly increases high-temperature strength, it is effective in improving airtightness under high-temperature conditions.

[0029] However, in the precipitation strengthening method, the housing material can precipitate undesirably in the high-temperature environment where the gas sensor used as an exhaust gas sensor is located, making the material brittle. Additionally, in the precipitation strengthening method, the precipitate can dissolve during a crimping process due to electrical heating of the housing, thus negating the intended improvement in the material's high-temperature strength. Furthermore, while the precipitation strengthening process increases the material's yield strength at high temperatures, its machinability, such as resistance to deformation, elongation, and toughness at room temperature, deteriorates significantly. For these reasons, cold forging of the housing becomes difficult, and the housing's production costs may increase.

[0030] The substitution method for solid solution strengthening pays little attention to the embrittlement of the material under high-temperature conditions and the loss of the improved high-temperature strength. Furthermore, the process suppresses the deterioration of the material's machinability. This, in turn, prevents the cold forging machinability necessary for the housing.

[0031] The cold forging workability index encompasses resistance to deformation, elongation, and toughness at room temperature. Elements used in substitutional solid solution strengthening can include Nb, W, Mo, Ta, and V. In addition to reducing carbon and nitrogen content, room-temperature workability can be further improved by annealing.

[0032] Manufacturing the housing is facilitated by methods such as hot forging and cutting. However, for a gas sensor intended for mass production, these processes are unsuitable due to manufacturing costs. Cold forging is a more cost-effective option. Furthermore, cold forging increases the housing's hardness, preventing damage to the shape of threaded or hexagonal sections under clamping forces when the gas sensor is mounted, for example, on an exhaust pipe. After cold forging, at least part of the housing exhibits a hardness of Hv220 or higher due to work hardening of the housing material. Therefore, ensuring machinability at room temperature is crucial.

[0033] In the case of the gas sensor of one aspect, the material from which the housing is made may contain 15 to 25 mass percent Cr, 0.01 to 1.0 percent Nb and 0.5 to 2 percent of at least one of the elements W and Mo alone or in combination, and the remainder may consist of Fe and unavoidable impurities including C, N, Mn and Si.

[0034] In this case, modifying the housing composition results in a material with a yield strength of 80 MPa or more at 650°C. The housing material increases the yield strength at high temperatures of 550°C or more while maintaining low thermal expansion. This is a characteristic of ferritic stainless steel, which contains 15 to 25% by mass of Cr (chromium) in Fe (iron) to resist expansion even when heated.

[0035] More precisely, to increase the yield strength of the material at high temperatures of 550°C or above, the housing material contains, in addition to Cr, 0.01 to 1.0 wt% Nb (niobium) and 0.5 to 2 wt% of at least one of the elements W (tungsten) and Mo (molybdenum), either alone or in combination with Fe. This improves the yield strength at high temperatures of 550°C and above. The increased yield strength, or relaxation resistance (stress relaxation and wear resistance), at higher temperatures suppresses permanent deformation of the housing. As a result, the crimped section of the housing retains the force required to compress the sealing element, even at high ambient temperatures of 550°C or above, ensuring that the sealing element maintains an airtight seal between the housing's receiving hole and the sensor element or insulator.

[0036] The housing material composition described above thus suppresses permanent deformation of the housing and ensures the gas sensor's airtightness under high-temperature conditions. Ensuring airtightness is guaranteed, for example, by covering the detection area where the air-fuel ratio can be measured in fuel-rich conditions, and by the corrosion resistance of the contact connections.

[0037] Even if steel to which the solid solution strengthening element is added undergoes solid solution treatment, a deterioration in workability compared to the original material is unavoidable. Regarding resistance to deformation and elongation, it is generally known that intermediate annealing during cold forging facilitates processing. However, it is contradictory that intermediate annealing increases the energy required for processing, thus increasing processing costs, and that the component can be deformed by external forces during installation because its hardness is insufficient.

[0038] It is known that repeatedly subjecting an unprocessed housing material to a wire drawing process to finer crystals is an effective means of improving toughness. However, this also contradicts the fact that it increases processing costs. It is also known that preheating before forging is an effective means of improving toughness. However, this too is contradictory, as it increases processing costs and temperature control is associated with expenses.

[0039] Additionally, toughness can be improved by adding 0.15 to 0.6% nickel by mass to the housing material. However, since this increases the resistance to deformation, the processing speed during the forging process cannot be increased, leading to higher manufacturing costs.

[0040] Which of the above-mentioned methods should be chosen therefore depends on the design.

[0041] The chemical composition is described below. (Contents of Cr)

[0042] The chromium content in the entire material from which the housing is made is 15 to 25% by mass. This ensures, for example, the oxidation resistance, corrosion resistance, and low thermal expansion that ferritic stainless steel achieves. With a chromium content of less than 15% by mass, the housing material may not exhibit sufficient properties such as oxidation and corrosion resistance. If the chromium content exceeds 25% by mass, toughness decreases with increasing resistance to deformation, and machinability may be impaired. Considering that the housing is formed by cold forging, the chromium content is preferably 21% or less by mass, better 18% or less. The chromium content is a design consideration, adjusted as needed within the range that ensures properties such as oxidation resistance and machinability. (Content of Nb)

[0043] Since the housing material contains Nb, its yield strength is increased at high temperatures of 550°C and above. Because the housing material contains Nb, sensitization is further suppressed. Stoichiometrically, the Nb content is equal to the C and N content necessary for sensitization resistance. However, since the chemical bonding between Nb and C and N is a stochastic event, the Nb must be present in a certain excess. For example, it is generally known that the Nb content is preferably three times the total C and N content, as required for stainless steel 430LX.

[0044] Since the material from which the casing is made contains Nb, fine NbC crystals are also formed. These fine crystals serve as a starting point to suppress the coarsening of the microstructure during heat treatment and the deterioration of toughness.

[0045] It is known that the improvement in the material's yield strength at high temperatures of 550°C or more, achieved through the addition of nitrogen (Nb), is saturated at approximately 1.0% by mass. The higher the Nb content, the greater the resistance to deformation and the poorer the machinability of the housing. Therefore, it is preferable to contain no more Nb than necessary. Considering that the housing is cold-formed, the Nb content is preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less.

[0046] If the Nb content is less than 0.01% by mass, there is a possibility that the benefit of the Nb content will not be achieved. (Content of W and Mo)

[0047] Since the material from which the housing is made contains at least one of W and Mo, the yield strength of the material is increased at high temperatures of 550°C or more.

[0048] If the content of at least one of the elements, W and Mo, alone or in total, is below 0.3% by mass, the benefit of an increased yield strength of the material at high temperatures of 550°C or above is not sufficiently achieved. If the content of at least one of the two elements, W and Mo, alone or in total, exceeds 2% by mass, the material's resistance to deformation increases, potentially impairing the processability of the housing.

[0049] While the sublimation temperature of molybdenum oxide (Mo3O) is approximately 700°C, the sublimation temperature of tungsten oxide (WO3) is approximately 1000°C. Therefore, it is preferable to use tungsten oxide (W) with a higher sublimation temperature as a housing material. Furthermore, the atomic weight of tungsten oxide (W) is greater than that of molybdenum oxide (Mo), and tungsten oxide (W) is less likely to diffuse than Mo. If the material contains tungsten oxide (W), its creep resistance is expected to improve, as is its relaxation resistance.

[0050] Elements such as Ta (tantalum) and V (vanadium) are also known as elements that increase the yield strength of the housing material at high temperatures. Note that, for reasons of availability and economy, the housing material preferably contains Nb, W, and Mo, either alone or in combination. (Mn and Si content)

[0051] Mn (manganese) and Si (silicon) suppress oxide layer flaking and improve high-temperature oxidation resistance. Particularly when high-temperature oxidation resistance is a primary concern, it is effective to adjust the Mn and Si content in the housing material to 0.05 wt% or more. It is known that increasing the Mn and Si content worsens brittleness. Therefore, for the housing material, which must retain its cold workability, a low Mn and Si content is desirable. The total Mn and Si content is preferably 2.0 wt% or less, and more preferably 1.5 wt% or less. (Content of P and S)

[0052] Although sulfur (S) is known to be a cutting agent, it is an unavoidable and difficult-to-reduce impurity. High levels of phosphorus (P) and sulfur can reduce corrosion resistance and cause porosity during welding. Therefore, low levels of phosphorus and sulfur are preferred. The P and S content in the housing material should be controlled to 0.07% by mass or less, preferably 0.05% by mass or less. (Contents of C and N)

[0053] Carbon (C) is a typical solid solution element. Additionally, C forms a carbide with elements such as boron (Nb) and titanium (Ti), thus suppressing crystal grain growth. To achieve this effect, the C content in the housing material must be at least 0.001% by mass. Carbon (C) and nitrogen (N) are unavoidable impurities that are difficult to reduce and impair cold workability, toughness, and corrosion resistance. Therefore, the combined content of C and N is preferably 0.12% by mass or less, and even more preferably 0.03% by mass or less for each element. (Ni content)

[0054] Like copper, nickel is an element that improves low-temperature toughness. In other words, nickel lowers the ductile brittleness transition temperature of the housing material to facilitate cutting and cold forging of the housing.

[0055] To achieve such an effect, the Ni content should preferably be 0.1 percent by mass or more.

[0056] Increasing the nickel content increases the resistance to deformation, thus impairing workability. Since nickel is an austenite-stabilizing element, excessively high levels can potentially lead to the formation of an austenitic microstructure in some parts of the material. This can undesirably increase the coefficient of thermal expansion and result in the undesirable development of a two-phase stainless steel, in which an austenitic microstructure is mixed with a ferrite microstructure. This can significantly impair the material's workability. As mentioned above, the material used for the casing may contain 0.1 to 0.6 percent nickel by mass. (Content of Al)

[0057] The material from which the casing is made may also contain 0.15 to 0.6 wt% of at least one of the elements Al and Ti alone or in combination.

[0058] Since the housing material contains at least one of each of Al (aluminum) and Ti (titanium), its oxidation resistance is improved. If the housing material also contains Mo, the diffusion of Mo into the material is further suppressed, as the housing material contains at least one of each of Al and Ti. This improves the material's creep resistance. embodiment

[0059] A gas sensor 1 of the present embodiment contains, as described in the Fig. , Fig. until Fig.The figure shows a housing 2 having a receiving hole 21, a sensor element 3 containing a solid electrolyte 31, electrodes 32A and 32B provided on both sides of the solid electrolyte 31, an insulator 4 holding the sensor element 3 and located in the receiving hole 21, and a sealing element 51 formed from a ceramic powder filling a gap S1 between the receiving hole 21 and the insulator 4. In the gas sensor 1, a crimped section 24 of the housing 2 compresses the sealing element 51, thus sealing the gap S1. (Internal combustion engine)

[0060] The gas sensor 1 is located in an exhaust pipe 7 of an internal combustion engine of a vehicle and detects the exhaust gas G flowing in the exhaust pipe 7. The gas sensor 1 of this embodiment is used as an air / fuel ratio sensor, which detects the air / fuel ratio of the internal combustion engine calculated from the composition of the exhaust gas G. The gas sensor 1 can be arranged upstream of a section in the exhaust pipe 7 containing a catalytic converter.

[0061] As in Fig.As shown in Figure 3, a predetermined voltage is applied in the air / fuel sensor to generate the limiting current characteristic between a detection electrode 32A, located on one side of the solid electrolyte 31 and exposed to the exhaust gas G, and a reference electrode 32B, located on the other side of the solid electrolyte 31 and exposed to air A. When the oxygen concentration of the exhaust gas G changes, the amount and direction of migration of the oxide ions (O₂) change. 2- ) between the detection electrode 32A and the reference electrode 32B, and the air-fuel ratio is detected in the fuel-rich and fuel-poor state within a predetermined detection range.

[0062] Since a voltage is applied between the detection electrode 32A and the reference electrode 32B in the air / fuel sensor, the oxide ions (O₂) move when the air-fuel ratio is low. 2-) from the detection electrode 32A to the reference electrode 32B through the solid electrolyte 31. If the air-fuel ratio is in the fuel-rich state, because the unburned gas causes a chemical reaction in the detection electrode 32A, the oxide ions (O 2- ) from the reference electrode 32B through the solid electrolyte 31 to the detection electrode 32A.

[0063] The pressure of the exhaust gas G, which is directed into the gas sensor 1, is often higher than the air pressure directed into the gas sensor 1. For this reason, the gap S1 between the receiving hole 21 of the housing 2 and the insulator 4 is sealed by the sealing element 51 so that the exhaust gas G drawn into the gas sensor 1 does not mix with the air A drawn into the gas sensor 1.

[0064] The gas sensor 1 can be an oxygen probe that determines whether the air-fuel ratio obtained from the composition of the exhaust gas G is in a fuel-rich state or a fuel-poor state with respect to the stoichiometric air-fuel ratio by ON and OFF. (Sensor element 3)

[0065] The direction in which the section of the gas sensor 1 of the present embodiment is located in the exhaust pipe 7 is referred to as the distal direction L1 and the direction opposite to the distal direction L1 as the proximal direction L2.

[0066] The solid electrolyte 31 of the in Fig.The sensor element 3 shown in Figure 3 contains zirconium dioxide as its main component and consists of stabilized zirconium dioxide or partially stabilized zirconium dioxide, in which rare earth metal elements or alkaline earth metal elements replace some of the zirconium dioxide. The solid electrolyte 31 can, for example, consist of yttrium oxide-stabilized zirconium dioxide or yttrium oxide-partially stabilized zirconium dioxide. The solid electrolyte 31 has an ionic conductivity that, at a predetermined activation temperature, allows the formation of oxide ions (O₂). 2- ) conducts. Electrodes 32A and 32B contain platinum, which exhibits catalytic activity for oxygen, and material that has the same properties as the material of which the solid electrolyte 31 is composed.

[0067] The sensor element 3 of the present embodiment is a laminated sensor element in which the electrodes 32A, 32B are located on both sides of the plate-shaped solid electrolyte 31, and a heater 35 is laminated onto the solid electrolyte 31. The sensor element 3 is held by the housing 2 in a state in which the sensor element 3 is inserted into the insulator 4. The heater 35 consists of a heating element 352, which is located in a ceramic substrate 351. The heating element 352 is heated by the application of an electric current.

[0068] As shown in Figures 1 and 2, the sealing element 51 fills the gap S1 between the receiving hole 21 of the housing 2 of the present embodiment and the insulator 4. The sealing element 51 is a ceramic powder made of talc. Furthermore, in the proximal direction L2 of the sealing element 51, there is an insulating body 52, also made of ceramic, and in the proximal direction L2 of the insulating body 52, there is a metal ring 53. The sealing element 51, the insulating element 52, and the metal ring 53 are securely compressed by the crimped section 24, which is formed by bending inwards a proximal end section 240 of the housing 2. In this state, the sealing element 51, the insulating element 52, and the metal ring 53 are pressed from the proximal direction L2 towards the distal direction L1.

[0069] Furthermore, sensor element 3, as shown in Fig.As shown in Figure 4, the solid electrolyte 31 is cup-shaped. That is, electrodes 32A and 32B are located on the outside and inside of the solid electrolyte 31, which is tubular and has a closed end, and the heater 35 is located inside the solid electrolyte 31. In this case, the insulator 4 is not used, and the sensor element 3 is held directly by the receiving hole 21 of the housing 2. The gap S1 between the receiving hole 21 and the sensor element 3 is sealed by the sealing element 51, which absorbs the pressure force from the compression fitting 24 of the housing 2. The other structures of the gas sensor 1 are shown in Figure 4. Fig. 4 are the same as those of gas sensor 1 of the Fig. 1. (Shape of the case 2)

[0070] As in Fig.As shown in Figure 1, the housing 2 forms the housing of the gas sensor 1 and is a component for attaching the gas sensor 1 to the exhaust pipe 7. The housing 2 has the shape of a cylinder with the receiving hole 21 in the central part and comprises a threaded section 22, a hexagonal flange section 23, and the crimped section 24. The threaded section 22 is screwed into a threaded bore 711 in a mounting boss 71 of the exhaust pipe 7. The hexagonal flange section 23 is formed next to the threaded section 22 in the proximal direction L2 and forms the outer circumferential surface that projects furthest outwards. The crimped section 24 is formed next to the flange section 23 in the proximal direction L2.

[0071] As in Fig.As shown in Figure 2, the receiving hole 21 of the housing 2 includes a small-diameter bore 211, a large-diameter bore 212 which is formed in the proximal direction L2 of the small-diameter bore 211 and is larger than the small-diameter bore 211, and a step 213 formed between the small-diameter bore 211 and the large-diameter bore 212. The crimped section 24 forms the large-diameter bore 212. The sealing element 51, the insulating element 52, and the metal ring 53 are located in the large-diameter bore 212. (Insulator 4)

[0072] The insulator 4 includes an insertion bore 41 that receives the sensor element 3, a recess 42 that extends proximal to the insertion bore 41 L2, and a projection 43 that forms the outermost, outwardly projecting circumferential surface. When the insulator 4 is located in the receiving hole 21 of the housing 2, the projection 43 is located in the large-diameter bore 212 and faces, for example, a metal part 431 of the step 213. The sealing element 51, the insulating element 52, and the metal ring 53 are located in the large-diameter bore 212. The sealing element 51, the insulating element 52, and the metal ring 53 are compressed between the projection 43 and the crimped part 24 by bending the crimped part 24 inwards. Furthermore, when the sensor element 3 is inserted into the insertion bore 41, insulating particles 44, such as a ceramic powder, are introduced into the recess 42.The sensor element 3 is held back by the insulator 4 by the insulating particles 44.

[0073] As in Fig. As shown in Figure 2, in the gas sensor 1, a gap S2 between the sensor element 3 and the insertion bore 41 of the insulator 4 is sealed by the insulating particles 44, and the gap S1 between the insulator 4 and the receiving hole 21 of the housing 2 is sealed by the sealing element 51. The position of the insulating particles 44 and the sealing element 51 prevents the exhaust gas G, which flows distally towards the section of the insulator 4 in the direction L1, from flowing from the distal direction L1 of the insulator 4 in the proximal direction L2 through the gaps S1 and S2.

[0074] As in Fig. 1 and Fig.As shown in Figure 3, the electrode pair 32A and 32B is located at a distal end section 36 of the sensor element 3, thus forming a detection section 361 for gas detection. The detection section 361 contains a diffusion resistance element 331, which introduces the exhaust gas G into the detector electrode 32A at a predetermined diffusion rate. The detection electrode 32A is located in a gas chamber 33, which is connected to the diffusion resistance element 331. Although not shown, a protective layer of porous ceramic forms around the detection section 361. The distal end section 36 of the sensor element 3 is exposed to the exhaust gas G.

[0075] As in Fig. 2 and Fig.As shown in Figure 3, the conductor sections 321, each connected to the electrode pair 32A and 32B, and a conductor section 353 of the heating element 352 of the heater 35 are extended to a proximal end section 37 of the sensor element 3. The distal end section 36 of the sensor element 3 projects from the insulator 4 and the housing 2 in a distal direction L1, and the proximal end section 37 of the sensor element 3 projects from the insulator 4 and the housing 2 in a proximal direction L2. (Contact point 54)

[0076] Another insulator 4A is located proximal to insulator 4 (L2). On this other insulator 4A are the contact terminals 54 for the electrical connection of the sensor element 3 and the heater 35. The conductor sections 321 of electrodes 32A and 32B of sensor element 3 and the conductor section 353 of heating element 352 of heater 35 extend from the distal end section 36 of sensor element 3 and are drawn out from the proximal end section 37 of sensor element 3. The contact terminals 54 include one that is in contact with conductor sections 321 of electrodes 32A and 32B and one that is in contact with conductor section 353 of heating element 352.

[0077] The contact terminals 54 are made of conductive metal and contact the sensor element 3 by means of a contact force generated by elastic deformation. Inside the sensor element 3, a channel 34 is formed for the air supply A to the reference electrode 32B. The channel 34 is open in the proximal end section 37 of the sensor element 3, and the air A is directed from the proximal end section 37 of the sensor element 3 to the reference electrode 32B. (Protective cover 61 and proximal end cover 62)

[0078] As in Fig.As shown in Figure 1, a protective cover 61 is mounted on the housing part 2 in the distal direction L1, covering the distal end section 36 of the sensor element 3 to protect the sensor element 3. A proximal end cover 62, which accommodates components such as the contact terminals 54, the other insulator 4A, and connecting wires 55 attached to the contact terminals 54, is mounted on the housing part 2 in the proximal direction L2. The protective cover 61 contains the exhaust gas passage bores 611 through which the exhaust gas G flows. The exhaust gas G flows through the exhaust gas passage bores 611 into the protective cover 61, which is inserted into the detection electrode 32A of the sensor element 3, and flows out through the exhaust gas passage bores 611 from the protective cover 61.

[0079] An air inlet opening 621 is formed in the proximal end cover 62. The air inlet opening 621 is provided with a filter 622 that allows air A to pass through while simultaneously preventing water from passing through. The air A introduced into the proximal end cover 62 is drawn into the channel 34 by the proximal end section 37 of the sensor element 3 and supplied to the reference electrode 32B in the channel 34. The proximal end cover 62 is mounted on the outer circumference of the proximal end section 240 of the housing 2, on which the crimped section 24 is formed. A socket 56, which receives the connecting wires 55, is located in the proximal end section of the proximal end cover 62. (Composition of the housing 2)

[0080] The housing 2 of the present embodiment is made of ferritic stainless steel with a 0.2% yield strength of 80 MPa or more at 650°C. The housing 2 increases the yield strength of the material at high temperatures of 550°C or more, while maintaining the low thermal expansion of the ferritic stainless steel with 15 to 25% mass fraction of Cr in Fe.

[0081] The housing 2 of the present embodiment contains Fe (iron), Cr (chromium), Nb (niobium), Ni (nickel) and Al (aluminium) as components and Mn (manganese), Si (silicon), C (carbon) and N (nitrogen) as unavoidable impurities.

[0082] The material from which housing 2 is made has the following composition, based on mass: 15 to 25% Cr, 0.01 to 1.0% Nb, 0.5 to 4% W, 1.5% or less Mn and Si, 0.1 to 0.6% Ni, 0.15 to 0.6% Al, 0.03% or less C and N total, and the remainder Fe. C, N, Mn, and Si are treated as unavoidable impurities. Mo may be used instead of W, and W and Mo may be used in a mixture.

[0083] The crystal structure of the material from which the housing 2 is made is a body-centered cubic lattice structure with a ferrite structure. The ferrite structure has the property of resisting thermal expansion compared to the austenite structure. The threaded section 22 of the housing 2 is screwed into the threaded bore 711 of the mounting fitting 71 of the exhaust pipe 7, so that the gas sensor 1 is mounted on the exhaust pipe 7. Since the exhaust gas G flowing through the exhaust pipe 7 has a high temperature of 550°C or more, the threaded section 22 and the threaded bore 711 are heated to a high temperature of 550°C or more.

[0084] The mounting fitting 71 of the exhaust pipe 7 is often made of ferritic stainless steel. Since the crystal structure of the housing 2 is ferrite, the metal structure forming the threaded section 22 and the threaded bore 711 is also ferrite. Therefore, the coefficients of thermal expansion of the threaded section 22 and the threaded bore 711 are very close. This prevents the threaded section 22 and the threaded bore 711 from seizing or thermally seizing due to heat.

[0085] The housing 2 of the present embodiment is formed in its raw material state by a solid solution heat treatment. During solid solution heat treatment, a precipitate such as carbide, which can consist of Nb, W, Mn, Si, Ni, and Al, dissolves into the base material, i.e., Fe. The solid solution heat treatment is carried out by heating the housing material 2 to a predetermined heat treatment temperature and subsequently cooling it. If the heat treatment temperature is low, the precipitate that forms during the slow cooling process does not dissolve sufficiently into Fe. If the heat treatment temperature is too high, the ferrite crystal may become coarsened, which reduces the elongation and toughness of the material.

[0086] The Laves phase, known as an intermetallic compound such as Fe₂W, Fe₂Mo, and Fe₂Nb, is formed in the mother phase of the housing 2. Although the Laves phase improves the yield strength at room temperature and at elevated temperatures, it increases the resistance to deformation and reduces the toughness. Therefore, it is desirable for the Laves phase content to be low. The heat treatment for dissolving the Laves phase in the base material of the housing 2 is preferably 850°C or more, more preferably 850 to 1000°C. The inventors' study found that heating the housing 2 material to a heat treatment temperature of 850°C or more reduces the Laves phase content and improves the workability of the housing 2 material at room temperature.The temperature of the heat treatment is predicted by calculating the equilibrium state between the metals in housing 2, and the Laves component is adjusted according to the composition of the additive in housing 2.

[0087] The amount of precipitate of the Laves phase in the mother phase of casing 2 is preferably less than 0.1% by mass. If the amount of precipitate is 0.1% by mass or more, the toughness of the material may decrease significantly.

[0088] If the temperature of the heat treatment process, which heats the housing 2 material, is too low, the Laves component will not dissolve sufficiently. This can undesirably reduce the toughness. However, if the heat treatment temperature is too high, the NbC precipitation and the ferrite crystal grains will become coarser, further reducing the material's toughness. In this case, undesirable foreign matter such as scale may form during the heat treatment. This could potentially increase the input energy required for the heat treatment and thus increase manufacturing costs.

[0089] If the heat treatment temperature is set to a higher temperature of 1250°C or more, NbC dissolves in the material of the housing 2. Apart from the coarsening of the ferrite crystal, which is a problem, it is difficult to carry out the heat treatment at 1250°C or more on the material of the housing 2 that has been subjected to wire drawing. (Manufacturing process)

[0090] The following briefly describes a method for manufacturing the housing 2 and the gas sensor 1.

[0091] The manufacture of the housing 2 of the present embodiment involves melting the metal material, such as Fe, Nb, W, Mn, Si, Ni, and Al; drawing the metal material into an elongated element with a predetermined cross-sectional shape; subjecting the metal material to solid solution heat treatment; shearing the elongated metal material to form individual metal workpieces; cold forging each metal workpiece to shape it into the housing 2; and cutting the metal workpiece into the shape of the housing 2 to form the final shape of the housing 2 before assembly. In particular, since Fe contains Ni, the toughness of the metal material is improved, which facilitates the shearing and cold forging of the metal material.

[0092] During the manufacture of the gas sensor 1, the crimped section 24 of the housing 2 is deformed by crimping for securing it. During the assembly of the housing 2 in the manufacture of the gas sensor 1, the insulator 4, in which the sensor element 3 is held, is positioned as shown in Fig. The sealing element 51, the insulating element 52, and the metal ring 53 are inserted into the receiving hole 21 of the housing 2, as shown in Figure 2. The sealing element 51, the insulating element 52, and the metal ring 53 are placed in the gap S1 between the insulating body 4 and the receiving hole 21 of the housing 2. The proximal end piece 240 of the housing 2 is bent inwards around its entire circumference to secure it by crimping. The crimping can be carried out by thermal crimping, in which the proximal end section 240 is heated to a high temperature to make it easily deformable.

[0093] The heating of the proximal end section 240 is achieved by applying an electric current to the proximal end section 240 of the housing 2 in order to heat a thick, small-diameter section 241 of the proximal end section 240 to a temperature of 550°C or more, down to 1000°C or less. Since the material of the housing 2 contains a suitable amount of Nb and a limited amount of C and N, the reduction of the Cr concentration in Fe and the sensitization of the material of the proximal end section 240 are suppressed at this time. This preserves the corrosion resistance of the housing 2 material.

[0094] Furthermore, after the proximal end cover 62 has been mounted on the outer circumference of the crimped section 24 of the housing 2, a mounting section 623 (see Fig.2) the proximal end cover 62 is partially welded to the housing 2. In such a case, the crimped section 24 is heated to 550°C or more and 1000°C or less by the heat generated during welding. Since the material of which the housing 2 is made contains a suitable amount of Nb and a limited amount of C and N, the reduction of the concentration of Cr in Fe and the sensitization of the material of which the proximal end section 240 is made are also suppressed at this time. This preserves the corrosion resistance of the housing 2 material. (Case hardness 2)

[0095] The hardness of the crimped section 24 of the housing 2 of the present embodiment is, at least in the delivered condition of the gas sensor 1, in the range of Hv220 to Hv400 Vickers hardness. This results in a high yield strength of the material from which the housing 2 is made, and permanent deformation of the housing 2 is suppressed. The Vickers hardness is a value determined according to the "Vickers hardness test" of the Japanese Industrial Standard (JIS) Z 2244. JIS Z 2244 corresponds to the ISO standard ISO 6507.

[0096] If the hardness of the cold-forged housing 2 is less than Hv220, the yield strength will be low even at room temperature. Therefore, when mounting the gas sensor 1 on the exhaust pipe, the threaded section 22 and the flanged section (hexagonal profile) 23 may be undesirably damaged. Furthermore, if the hardness of the crimped section 24 is below Hv220, sections other than the crimped section 24 may be unintentionally deformed during crimping. It is undesirable for the hardness of the crimped section 24 to exceed Hv400, as manufacturing is difficult and cracking due to deformation may be undesirable.

[0097] If the metal material used to form the housing 2 is subjected to an annealing treatment in which it is heated to a temperature of approximately 780°C, the achievable Vickers hardness is approximately Hv160 to Hv180. In contrast, the metal material used to form the housing 2 in the present embodiment is heated to 850 to 1000°C to perform the solid solution heat treatment. Thus, the housing 2 achieves a Vickers hardness of Hv220 or higher.

[0098] Since the material from which housing 2 is made contains the aforementioned dissolved elements, such as Nb, W, and Ni, its high-temperature strength is improved. Furthermore, because housing 2 is formed by cold forging, a grain flow (fiber orientation) occurs in the metal structure of the material from which housing 2 is made. This ensures that the hardness of housing 2 remains high. (Operational advantage)

[0099] In the gas sensor 1 of the present embodiment, the strength of the crimped section 24 of the housing 2 is suppressed under high-temperature conditions of 550°C or higher because the material of the housing 2 has the aforementioned composition. The material of the housing 2 contains, in addition to Cr, 0.01 to 1.0% by mass of Nb and 0.5 to 4% by mass of W in Fe. This suppresses permanent deformation of the housing 2 at high temperatures of 550°C and higher. As a result, the crimped section 24 of the housing 2 retains the force to compress the sealing element 51 even at high temperatures of 550°C and higher, so that the sealing element 51 maintains the airtightness of the gap S1 between the receiving hole 21 of the housing 2 and the sensor element 3 or the insulator 4.

[0100] The gas sensor 1 of the present design thus suppresses the permanent deformation of the housing 2 and ensures the airtightness of the gas sensor 1 under high temperature conditions.

[0101] Since the gas sensor 1 of the current version is used as an air / climate sensor, the following advantages result from maintaining the airtightness of the gas sensor 1.

[0102] Since the high-temperature strength of the crimped section 24 of the housing 2 is maintained in the air cushion sensor, the exhaust gas G is prevented from mixing with the air A drawn into the sensor element 3. This prevents the interior of the channel 34 of the sensor element 3 from being filled with the exhaust gas G instead of the air A. In particular, this prevents the oxide ions (O₂) from forming in the air-fuel mixture of the combustion engine when the exhaust gas G is rich in fuel. 2-) cannot be transferred from the reference electrode 32B via the solid electrolyte 31 to the detection electrode 32A. When the air / fuel sensor detects the air / fuel ratio in the fuel-rich state, the large detection range for the fuel-rich state is maintained. The detection range refers to the area (scale) within which the air / fuel ratio in the fuel-rich state can be detected within a specified error range.

[0103] Even if the gas sensor 1 is not used as an air / fuel sensor, since the airtightness of the gas sensor 1 is maintained, the following advantage results.

[0104] In the gas sensor 1, because the high-temperature strength of the crimped section 24 of the housing 2 is maintained, mixing of the exhaust gas G with the air A drawn in by the sensor element 3 is prevented. This prevents the exhaust gas G from coming into direct contact with the metal contact terminals 54 that connect to the sensor element 3. This, in turn, prevents the contact terminals 54 from corroding, for example, due to moisture or nitrogen compounds in the exhaust gas G. <verifikationstest>(Test 1)

[0105] In test 1, the ratio between the material from which housing 2 is made and the yield strength was measured. Fig. Figure 5 shows the changes in the yield strength (MPa) of alloy steel with 17 wt% Cr and 0.35% Nb in Fe at 650°C when the whey content was changed to 0, 1, 2, and 4 wt%. The figure shows that the yield strength increases with increasing whey content.

[0106] As used here, the yield strength refers to the elastic limit (yield strength). Since the material also contains material that does not clearly indicate the yield strength, the 0.2% proof strength is used instead. The 0.2% proof strength was measured according to JIS Z 2241 (corresponding international standard: ISO 6892-1) or JIS G 0567 (corresponding international standard: ISO 6892-2).

[0107] However, if the tungsten content exceeds 2% by mass, the yield strength no longer increases, and at 2% by mass, the increase in the yield strength is saturated. If the tungsten content increases, processability, such as ductility, deteriorates. It has therefore been found that the tungsten content in the material from which the housing 2 is made is preferably 2% or less by mass. If the tungsten content is too low, the yield strength also decreases. The tungsten content is therefore preferably 0.3% by mass or more.

[0108] Note that Mo has the same property as W. If the material from which the housing 2 is made contains Mo instead of W, the Mo content is also preferably 0.3 to 2% by mass. (Test 2)

[0109] In Test 2, the airtightness was examined using housing 2 of a test product made of alloy steel with 17% Cr, 0.35% Nb, and 2% W in Fe, and housing 2 of a comparison product made of stainless steel (grade 430) with 17% Cr in Fe. In Test 2, the gas sensor 1 was connected to each housing 2. It was determined whether a leakage of exhaust gas G occurred in the gap S1 between the receiving hole 21 of the housing 2 and the insulator 4 in each gas sensor 1.

[0110] In test 2, 3000 heating and cooling cycles of housing 2 were performed. In each cycle, the hexagonal section of housing 2 (the section with the largest outer diameter) was heated to 650°C and then cooled with air to 50°C or less. While the hexagonal part of housing 2 was heated to 650°C and the pressure at sensor element 3 was set to 0.4 MPa, the leakage rate at gap S1 between the receiving hole 21 of housing 2 and the insulator 4 was measured. If a leakage of 1 cc / min or more occurred at gap S1, housing 2 was found to be not airtight. If the exhaust gas leakage G at gap S1 was less than 1 cm 3 / min, it was determined that housing 2 was airtight.

[0111] As a result of the tests, housing 2 of the comparison product was found not to be airtight, while housing 2 of the test product was found to be airtight. The results showed that housing 2 of the test specimen reliably maintains the airtightness of the gap S1 between the receiving hole 21 of housing 2 and the insulator 4. (Test 3)

[0112] In test 3, the changes in yield strength under temperature variations were investigated. The housing 2 of the test product was made of alloy steel with a mass fraction of 17% Cr, 0.35% Nb, and 2% W in Fe, while the housing 2 of the comparison product was made of stainless steel (grade 430) with a mass fraction of 17% Cr in Fe. Test product 1 and test product 2 were prepared as housing 2 components. Test product 1 was obtained by subjecting the housing 2 material to an annealing process, in which the material was heated to approximately 780°C and then cooled. Test product 2 was obtained by a solid solution heat treatment of the housing 2 material, in which the material was heated to approximately 950°C and then cooled. The housing 2 of the comparison product was subjected to the annealing process of heating to approximately 780°C and subsequent cooling.The graph of the yield strength of test products 1 and 2 and the comparison product shows the result in the temperature range from room temperature to 700°C.

[0113] As in Fig. As shown in Figure 6, the yield strength of housing 2 of test product 1, which underwent annealing, is higher than that of housing 2 of the comparison product over a wide temperature range. However, since the yield strength is also high at room temperature, its processability at room temperature is poor. In contrast, the yield strength of housing 2 of test product 2, which underwent solid solution heat treatment, is only higher than that of housing 2 of the comparison product when the temperature is in a high range. In the case of housing 2 of test product 2, the processability at room temperature is good because the yield strength at room temperature is kept low. Thus, it was found that using housing 2, which underwent solid solution heat treatment, ensures the airtightness of gas sensor 1 under high-temperature conditions and improves the processability of housing 2, for example, when...cold forging is carried out at room temperature. (Test 4)

[0114] In test 4, the changes in yield strength (MPa) at room temperature during temperature changes were investigated for the heat treatment of the housing material 2, where the housing 2 of the test product was made of alloy steel with a mass fraction of 17.1% Cr, 0.35% Nb and 2.00% W in Fe. As in Fig. As shown in Figure 7, the yield strength at room temperature is high at a heat treatment temperature of approximately 750°C and decreases at a heat treatment temperature of approximately 900°C. The yield strength does not change at heat treatment temperatures above 900°C.

[0115] It can be said that the lower the yield strength at room temperature, the better the machinability when housing 2 is cold-forged at room temperature. For comparison, a case is also shown in which the yield strength at approximately 750°C was investigated for housing 2 of the reference product made of stainless steel (grade 430) with 16.8 mass percent Cr in Fe. Since Nb and W are not added to the reference product, the yield strength at room temperature is initially low.

[0116] Using the maximum cold forging stress of housing 2 of the reference product as a reference, the extent to which the maximum cold forging stress of housing 2 of the test product could be reduced was investigated. It was found that when the heat treatment temperature was set to the annealing temperature of 780 °C, the maximum cold forging stress increased by a factor of 1.1. Conversely, when the heat treatment temperature was set to the solid solution heat treatment temperature of 900 °C, the maximum cold forging stress was reduced to a level close to that of the reference product.

[0117] It can thus be said that carrying out the solid solution heat treatment at a temperature of 850°C or more, preferably 900°C or more, improves the workability of the material for forming the housing 2 during cold forging. This is because the high-temperature heat treatment dissolves the Laves phase, which is a type of intermetallic compound such as Fe₂W and Fe₂Mo, in the core phase of the housing 2. It is known that while the formation of the Laves phase contributes to improved hot strength, it significantly reduces toughness. Therefore, the amount of Laves phase deposited in the housing 2 material is preferably less than 0.1% by mass. (Test 5)

[0118] In Test 5, a materials test, the dissolved state of the Laves phase was examined by solid solution heat treatment (annealing). The composition of the material under evaluation, by mass, contains 17% Cr, 0.35% Nb, 2% W, 0.02% C + N, 0.02% P + S, 0.9% other unavoidable impurities such as Si and Mn, and the remainder: Fe. The grain size of the material under evaluation was adjusted by hot forging to a grain size index of No. 5 to No. 9, corresponding to the grain size of the wire drawing material. The material, with its adjusted grain size, was subjected to a second heat treatment (annealing), and after a 4-hour dwell time at a predetermined temperature, a quantitative analysis of the dissolved amount of the Laves phase was performed. The grain size index is specified in JIS G 0551. JIS G 0551 corresponds to ISO 643 of the ISO standards.

[0119] Fig. Figure 8 shows the extent to which the Laves phase precipitated from the mother phase when the heat treatment temperature was changed from 700 to 900°C. As illustrated in the graph, the amount of Laves phase precipitated (by mass) decreases with increasing heat treatment temperature, resulting in a greater dissolution of the Laves phase in the mother phase. Specifically, it was found that at a heat treatment temperature of 850°C or higher, the amount of Laves phase precipitated is less than 0.1% by mass. It is therefore inferred that setting the heat treatment temperature to 850°C or higher dissolves a greater proportion of the Laves phase in the mother phase and improves processability at room temperature.

[0120] The temperature at which the Laves phase dissolves in the parent phase can be predicted from the calculation of the equilibrium state between two metals. Since the heat treatment temperature varies according to the composition of the material from which housing 2 is made, the heat treatment temperature can be set to a temperature of more than 850°C if required.

[0121] Several quantitative analysis methods are known for the Laves phase. An example of these methods is shown below.

[0122] One of the quantitative analytical methods for the Laves phase is the extraction residue analysis method. In this method, precipitates are extracted and separated from samples of the input material and the aged material. The precipitates are further separated into the Laves phase and other precipitates (such as carbide and nitride) for quantitative analysis. Extract residue analysis employs electrolysis extraction using a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution as the electrolyte, employing constant-current electrolysis at a current density of 20 mA / cm². Following electrolysis, filtration is performed using a 0.2 µm pore-bore filter to separate the filtrate and residue.The excretions, such as NbC and the Laves phase, were separated by weight analysis and X-ray diffraction analysis (XRD analysis) of the residue. (Test 6)

[0123] In test 6, a compositional test, the compositions of samples 1 to 7 were varied as needed, and the relationship between the composition, the 0.2% yield strength, and the workability at room temperature was investigated. The composition of the material under evaluation and the method of heat treatment were identical to those used in test 5.

[0124] The basic composition of samples 1 to 7 is one that, based on mass, contains 16.8 to 17.1% Cr, 0 or 0.35% Nb, 0 to 4% W, 0.02% C + N, 0.02% P + S, 0.9% other unavoidable impurities such as Si and Mn, and the remainder: Fe. In samples 1 to 7, the W content was modified, and Mo or Ni was included as required.

[0125] The composition of samples 1 to 7 and the test results are shown in Table 1. [Table 1] Composition [mass %] Heat treatment temperature [°C] 0.2% yield strength [MPa] at 650°C Verdict Room temperature processing Verdict Cr Note W Mon Ni Deformation resistance [MPa] Elongation [%] Ductile-brittle fracture transition temperature [°C] Sample 1 16,8 - - - - 780 60 Bad 620 44 10 Excellent Sample 2 17,1 0,35 2,00 - - 780 114 Excellent 880 30 40 Bad Sample 3 17,0 0,35 1,02 - - 900 85 Excellent 720 42 20 Excellent Sample 4 17,1 0,35 2,00 - - 900 102 Excellent 740 41 20 Excellent Sample 5 17,0 0,35 3,99 - - 900 108 Excellent 820 38 40 Bad Sample 6 17,0 0,35 - 2,01 - 900 100 Excellent 740 42 20 Excellent Sample 7 17,0 0,35 2,00 - 1,12 900 104 Excellent 790 42 10 Excellent

[0126] The 0.2% yield strength at 650°C is determined and shown as a value obtained through a static tensile test on a JIS No. 4 test specimen. If the 0.2% yield strength was 80 MPa or more, which is the yield strength necessary to maintain airtightness, the product was rated as good (Excellent), while others were rated as not good (Poor). The criterion for determining the 0.2% yield strength depends on the product shape and is not absolute.

[0127] The processability at room temperature was measured as the resistance to deformation at room temperature (20°C), the elongation at room temperature, and the ductile brittle transition temperature.

[0128] The deformation resistance at room temperature is specified as a value at 70% compression achieved through a cylindrical compression test (deformation rate 6.0 m / sec) simulating cold forging. Products with a deformation resistance of less than 800 MPa were rated as good (Excellent), while those with a lower value were rated as poor (Poor). The determination criterion for deformation resistance is dependent on the forging process and is not absolute.

[0129] The elongation at room temperature is specified as a value obtained by performing a static tensile test on a JIS No. 4 specimen. If the elongation was sufficient to prevent fracture during forging, the product was considered good. The elongation criterion is dependent on the forging process and is not absolute.

[0130] The toughness transition temperature is specified as a value determined by a Charpy impact test (2 mm V-notch, evaluated every 10°C). Based on the criterion that no fracture occurs during cutting and forging of the wire drawing material when the toughness transition temperature is lower than room temperature (25°C), it was rated as a good product (Excellent) in some cases and as a poor product in others. The ductile brittleness transition temperature refers to a predetermined temperature below which the material loses toughness and becomes impact-resistant. An energy of 50 J / cm² was applied in the Charpy impact test.

[0131] In Table 1, which shows the results of test 6, the assessment of the 0.2% yield strength for specimen 1, which has the composition of the current housing 2 of gas sensor 1 and contains no Nb or W, was poor. Although the Nb and W content is adequate, the assessment of the elongation at room temperature for specimen 2, which was heat-treated at a temperature of 780°C, was poor. Although Nb and W are present, the assessment of the elongation at room temperature for specimen 5, in which the W content is greater than 2% by mass, was poor.

[0132] Samples 3, 4, and 7, containing 1 or 2 wt% W and treated at a heat treatment temperature of 900°C, exhibited excellent 0.2% yield strength and room-temperature workability. Sample 6, containing Mo instead of W, and sample 7, containing Ni along with W, also exhibited excellent 0.2% yield strength and room-temperature workability.

[0133] As a result of test 6, it was found that the 0.2% yield strength at 650°C is improved with specimen 2, which contains the corresponding Nb and W, compared to specimen 1, whose composition is frequently used in the housing 2 of the present gas sensor 1. However, since the heat treatment temperature is only 780°C, so that the Laves phase remains in the microstructure of the material, the workability at room temperature and, in particular, the toughness of specimen 2 deteriorate significantly.

[0134] Compared to sample 2, after samples 3 and 4, which were heat-treated at a temperature of 900°C, an improvement in room-temperature workability was observed, although the 0.2% yield strength at 650°C decreased. This improvement was due to a decrease in room-temperature deformation resistance, an improvement in elongation, and a decrease in the toughness transition temperature. After samples 3 to 5, the W content was varied. It was found that when the W content reached 2% by mass, the 0.2% yield strength at 650°C was saturated, and when the W content exceeded 2% by mass, the deterioration in room-temperature workability became significant.

[0135] Since sample 6 contains 2% molybdenum instead of tungsten carbide (W), it was found that the 0.2% yield strength and the room-temperature workability are the same as those of sample 4, which contains W. Since sample 7 contains 2% W and 1% Ni by mass, it was found that while the deformation resistance at room temperature is increased, the toughness transition temperature is improved. (Test 7)

[0136] In test 7 for product evaluation, a test was performed to examine the airtightness of housing 2 with the compositions of samples 1, 3, and 4 from test 6. Housing 2 with each composition was formed by cold forging. Gas sensor 1, with housing 2 containing the respective composition, was mounted on the pipe, and gas at 650°C and 0.4 MPa (gauge pressure) was passed through the pipe. At this point, the amount of gas leaking from the crimped section 24 of housing 2 of gas sensor 1 was measured.

[0137] Fig. Figure 9 shows the results of the leakage rate measurement after gas sensors 1 with the compositions of samples 1, 3, and 4. The leakage rate is given as a value under normal conditions. As shown in the graph, the leakage rate of samples 3 and 4 was found to be less than 1.0 mL / min, ensuring the airtightness of housing 2. For sample 1, the leakage rate was found to exceed 1.0 mL / min, and the airtightness of housing 2 was poor. Thus, it was determined that if the material of housing 2 contains 1.02% or 2.00% of mass W, as in samples 3 and 4, the 0.2% yield strength at 650°C is maintained, and the airtightness of housing 2 remains high. If the material of housing 2 contains 4% mass W, its workability at room temperature is poor. The W content in the material of the housing 2 is therefore preferably 2% by mass or less.< / verifikationstest>

Claims

Gas sensor (1) comprising: a housing (2) with a receiving hole (21); a sensor element (3) with a solid electrolyte (31) and electrodes (32A, 32B) located on both sides of the solid electrolyte (31), wherein the sensor element (3) is inserted into the receiving hole (21) alone or via an insulator (4); and a sealing element (51) formed from a ceramic powder, which fills a gap (S1) between the receiving hole (21) and the sensor element (3) or the insulator (4), wherein the sealing element (51) is compressed by a part of the housing (2) so that the gap (S1) is sealed, and wherein the material of which the housing (2) is made is formed of ferritic stainless steel which, based on mass, contains 15 to 25% Cr, 0.01 to 1.0% Nb, 0.5 to 4% of at least one of the elements W and Mo alone or in combination and the following remainder: Fe and unavoidable impurities including C, N, Mn and Si, wherein: after heating the material,that forms the case (2), is heated to a temperature of 850°C or more, the Laves phase of Fe2W or Fe2W and Fe2Mo is formed as intermetallic compounds in a mother phase of the case (2), and the amount of precipitate of the Laves phase in the mother phase of the case (2) is less than 0.1% by mass. Gas sensor (1) according to claim 1, wherein the housing (2) has a 0.2% yield strength of 80 MPa or more at 650°C. Gas sensor (1) according to one of claims 1 to 2, wherein the material of which the housing (2) is made contains 0.05% by mass or less C. Gas sensor (1) according to one of claims 1 to 3, wherein the material of which the housing (2) is made further contains 0.1 to 0.6 wt% Ni. Gas sensor (1) according to one of claims 1 to 4, wherein the hardness of a crimped section of the housing (2) is in a range of Hv220 to Hv400 in its Vickers hardness. Gas sensor (1) according to one of claims 1 to 5, further comprising: a heater (35) with a heating element (352) that heats the sensor element (3); and a contact connection (54) that contacts a conductor section (321) of the electrodes (32A, 32B) of the sensor element (3) or a conductor section (353) of the heating element (352).

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