WO2009104792A1 - オーステナイト系耐熱鋳鋼及びそれからなる排気系部品 - Google Patents
オーステナイト系耐熱鋳鋼及びそれからなる排気系部品 Download PDFInfo
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- WO2009104792A1 WO2009104792A1 PCT/JP2009/053195 JP2009053195W WO2009104792A1 WO 2009104792 A1 WO2009104792 A1 WO 2009104792A1 JP 2009053195 W JP2009053195 W JP 2009053195W WO 2009104792 A1 WO2009104792 A1 WO 2009104792A1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
- F01N13/16—Selection of particular materials
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
- F01N13/08—Other arrangements or adaptations of exhaust conduits
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
- F01N13/08—Other arrangements or adaptations of exhaust conduits
- F01N13/10—Other arrangements or adaptations of exhaust conduits of exhaust manifolds
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N2530/00—Selection of materials for tubes, chambers or housings
- F01N2530/02—Corrosion resistive metals
- F01N2530/04—Steel alloys, e.g. stainless steel
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
- F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
- F02B39/00—Component parts, details, or accessories relating to, driven charging or scavenging pumps, not provided for in groups F02B33/00 - F02B37/00
Definitions
- the present invention relates to a heat-resistant cast steel suitable for exhaust system parts of gasoline engines and diesel engines for automobiles, and in particular, austenitic heat-resistant cast steel excellent in heat resistance and weldability such as oxidation resistance and thermal fatigue life, and an exhaust system comprising the same. Regarding parts.
- technologies for improving the performance and fuel efficiency of the engine itself include direct fuel injection, high-pressure fuel injection, increased compression ratio, and boost pressure of turbochargers (superchargers).
- the engine is lighter and more compact (downsizing) due to the increase, engine displacement reduction, and supercharging.
- These technologies have been introduced not only in luxury cars but also in popular cars.
- the engine tends to burn at a higher temperature and pressure, and accordingly, the temperature of the exhaust gas discharged from the engine combustion chamber to the exhaust system parts also tends to increase.
- the temperature of exhaust gas may be 1000 ° C. or higher, which is the same level as that of a luxury sports car.
- exhaust system parts such as exhaust manifolds and turbine housings, which are components of automobile gasoline engines and diesel engines, have been manufactured from castings with a high degree of freedom due to their complicated shapes. Because it is harsh at high temperatures, it has excellent heat resistance and oxidation resistance, such as high-Si spheroidal graphite cast iron, Ni-resist cast iron (Ni-Cr austenitic cast iron) and other heat-resistant cast iron, ferritic heat-resistant cast steel, austenitic heat-resistant cast steel, etc. It is used.
- conventional heat-resistant cast irons such as high-Si spheroidal graphite cast iron and Ni-resist cast iron have a relatively high strength up to about 900 ° C as the exhaust gas temperature and about 850 ° C as the temperature of exhaust system parts. In an exposed environment, the strength decreases, and the heat resistance such as oxidation resistance and thermal fatigue life decreases.
- Ni-resist cast iron has a problem that it is expensive because it contains a large amount of Ni which is a rare metal (rare metal) at around 35%.
- Ferritic heat-resistant cast steel also has a problem that it is generally inferior in high-temperature strength at 900 ° C. or higher.
- Austenitic heat-resistant cast steel is a material that can withstand higher temperatures than heat-resistant cast iron and ferritic heat-resistant cast steel.
- Japanese Patent Application Laid-Open No. 7-228948 discloses an austenitic heat-resistant cast steel suitable for automobile engine exhaust system parts and the like in terms of mass ratio, C: 0.2 to 1.0%, C-Nb / 8: 0.05 to 0.6%, Si: 2 %, Mn: 2% or less, Cr: 15-30%, Ni: 8-20%, W: 1-6%, Nb: 0.5-6%, N: 0.01-0.3%, S: 0.01-0.5% An austenitic heat-resistant cast steel comprising the balance Fe and inevitable impurities is disclosed.
- JP-A-7-228948 discloses a heat-resistant cast steel obtained by adding an appropriate amount of Nb, W, N and S to a 20Cr-10Ni austenitic heat-resistant cast steel, which has an improved high-temperature strength of 900 ° C. or more, and castability and It describes that it is suitable for exhaust system parts because of excellent machinability.
- the 20Cr-10Ni austenitic heat-resistant cast steel described in JP-A-7-228948 has been proposed on the assumption that the temperature of exhaust system parts is about 900 to 950 ° C. At temperatures around 1000 ° C, The oxidation resistance and thermal fatigue life are not sufficient, and the heat resistance and durability are inferior. In particular, the thermal fatigue life is not satisfactory and there is room for improvement. Therefore, it cannot be used for exhaust system parts whose surface temperature reaches around 1000 ° C. (for example, a turbocharger turbine housing in which a high boost pressure is set).
- JP-A-2000-291430 describes an exhaust system part made of austenitic heat-resistant cast steel with improved durability under high temperature use conditions, by mass ratio, C: 0.2 to 1.0%, Si: 2% or less, Mn : 2% or less, P: 0.04% or less, S: 0.05 to 0.25%, Cr: 20 to 30%, Ni: 16 to 30%, the balance containing Fe and inevitable impurities, and W: 1 to 4 And / or Nb: Exhaust system parts made of high Cr high Ni austenitic heat-resistant cast steel containing more than 1% and less than 4% and a Cr / Ni mass ratio of 1.0 to 1.5 are disclosed.
- 2000-291430 is a 25Cr-20Ni austenitic heat-resistant cast steel in which the contents of Cr and Ni, which are the main alloy elements, are increased compared to the 20Cr-10Ni austenitic heat-resistant cast steel. Based on the above, the composition range and structure of the material are controlled to significantly improve not only the high-temperature strength but also the oxidation resistance. It is suitable for exhaust system parts.
- the 25Cr-20Ni austenitic heat-resistant cast steel described in JP-A No. 2000-291430 contains a large amount of expensive rare metals such as Cr and Ni in order to ensure high temperature characteristics and heat resistance. Since these rare metals are produced only in small amounts in uneven countries and regions, they are not only expensive but also susceptible to the global economic situation, and are concerned about stable supply. Have such problems. 25Cr-20Ni austenitic heat-resistant cast steel described in JP-A-2000-291430 contains about 25% by mass and 20% by mass of Cr and Ni, respectively. There are problems in terms of economy and supply stability.
- Exhaust system parts have various technical problems to be improved in addition to the heat resistance and durability described above in order to achieve exhaust gas purification and fuel efficiency improvement of automobiles.
- the catalyst is heated and activated early when the engine is started, It is necessary to improve the purification performance by uniformly supplying the catalyst and the entire filter.
- the temperature drop of the exhaust gas passing through the exhaust system parts that is, to prevent the exhaust gas from being deprived of heat as much as possible. Therefore, in order to reduce the heat capacity (heat mass) of the exhaust passage, the exhaust system parts are required to be thin.
- automobiles are also required to reduce the weight of vehicles for the purpose of reducing fuel consumption, reduce the air resistance of vehicle bodies, and improve safety.
- hood height directly above the engine room as a device for improving the aerodynamic characteristics
- shock absorbing (crash bull) zone in the engine room to ensure safety during a collision, etc.
- Measures are being taken.
- the degree of freedom of layout design in the engine room is decreasing, and the exhaust system parts are also required to be reduced in weight and volume and save space.
- the exhaust system parts it is necessary for the exhaust system parts to cope with weight reduction, compactness, smooth exhaust passage, and the like.
- the tubular portion of the branch pipe which is the exhaust passage in the exhaust manifold, is made of a thin sheet metal or pipe member, and a counterpart member such as a cylinder head or a turbine housing.
- a long exhaust manifold with a thin and light exhaust manifold with a small heat capacity in the exhaust passage. Is divided into a plurality of casting members, and the casting members are welded to each other with a bellows-like pipe member to form an exhaust manifold that prevents cracks due to thermal expansion.
- C Exhaust manifold and turbine housing When both are cast members, they are usually fastened with bolts. As unnecessary the thickness tool insertion space for the meat of the flange and the fastening work for fastening, proposed such a lightweight, compact exhaust system components to reduce the heat capacity have been made.
- sheet metal members, pipe members, and castings are required to cope with the high heat resistance and durability required for exhaust system parts, as well as thinning, lightening, compactness, and smoothing of the exhaust passage. It is effective to join members or cast members together by welding.
- Exhaust system parts that tend to be complex shapes include a cast member having a high degree of freedom in shape and can be molded by welding, so that the design freedom and ease of manufacture are improved, and fastening bolts and gaskets Etc. can be reduced.
- the object of the present invention is excellent in heat resistance and weldability such as oxidation resistance and thermal fatigue life at around 1000 ° C., and also has a low content of rare metals, economical efficiency, effective utilization of resources, stable supply capability, etc. Is to provide an austenitic heat-resistant cast steel having a good quality and an exhaust system component suitable as a component part of an automobile engine made of this austenitic heat-resistant cast steel.
- the Si content is increased, the heat resistance equivalent to that of the 25Cr-20Ni system can be obtained even in the 20Cr-10Ni system with low Cr and Ni, but a large amount of Si content significantly deteriorates the weldability. I understood. Therefore, as a result of further earnest research to find a composition range that can impart heat resistance and durability without deteriorating weldability even if Si is increased, (a) high temperature strength, oxidation resistance, etc.
- the content of main alloy elements such as C, Mn, Cr, Ni, W, Mo, Nb, N and S is limited to an appropriate range while increasing Si.
- the austenitic heat-resistant cast steel of the present invention is % By mass C: 0.3-0.6% Si: 1.1-2% Mn: 1.5% or less, Cr: 17.5-22.5%, Ni: 8-13% At least one of W and Mo: (W + 2Mo) 1.5-4%, Nb: 1-4% N: 0.01-0.3% S: 0.01-0.5% It consists of the remainder Fe and inevitable impurities, and satisfies the following formulas (1), (2), (3) and (4).
- the austenitic heat-resistant cast steel of the present invention preferably has an oxidation weight loss of 20 mg / cm 2 or less when held in the atmosphere at 1000 ° C. for 200 hours.
- the austenitic heat-resistant cast steel of the present invention preferably has a thermal fatigue life of 800 cycles or more as measured by a thermal fatigue test in which the heating upper limit temperature is 1000 ° C., the temperature amplitude is 850 ° C. or more, and the constraint is 0.25.
- the exhaust system component of the present invention is characterized by comprising the austenitic heat-resistant cast steel described above.
- the exhaust system component is preferably an exhaust manifold, a turbine housing, a turbine housing integrated exhaust manifold, a catalyst case, a catalyst case integrated exhaust manifold, or an exhaust outlet.
- the austenitic heat-resistant cast steel of the present invention has excellent weldability in addition to heat resistance such as oxidation resistance and thermal fatigue life in the vicinity of 1000 ° C., and relatively expensive rare metals such as Cr and Ni. Since heat resistance is given instead of the above, it contributes not only to the economic effect of reducing raw material costs but also to the effective use and stable supply of rare metal resources.
- the material made of the austenitic heat-resistant cast steel of the present invention is suitable as a material for exhaust system parts for automobiles.
- the exhaust system parts made of the austenitic heat-resistant cast steel of the present invention have high heat resistance and durability required for exhaust gas purification, fuel efficiency improvement and safety improvement of automobiles, and also have excellent weldability. It is possible to cope with thinning, weight reduction, compactness, smooth exhaust passage, and the like. Moreover, since rare metals can be manufactured at low cost, it can be applied to popular vehicles and is suitable as a component part of an automobile engine.
- FIG. 3 is a schematic diagram showing a thermal analysis result by differential scanning calorimetry (DSC) of austenitic heat-resistant cast steel. It is a graph which shows the relationship between the composition of Si and (W + 2Mo), and the thermal fatigue life of austenitic heat-resistant cast steel.
- DSC differential scanning calorimetry
- Austenitic heat-resistant cast steel The structure of the austenitic heat-resistant cast steel of the present invention will be described in detail below. In addition, content of each element which comprises an alloy is shown by the mass% unless there is particular notice.
- C forms (a) the fluidity of the molten metal, that is, the castability is improved, (b) the effect of solid-solution strengthening by dissolving in a part of the base, and (c) the formation of Cr crystallized carbides and precipitated carbides.
- the C content needs to be 0.3% or more.
- Cr crystallized carbides and precipitated carbides become too much and become brittle, ductility is lowered and workability is deteriorated. Further, if there is too much Cr crystallized carbide, weldability deteriorates. Therefore, the C content is specified to be 0.3 to 0.6%.
- a preferable content of C is 0.4 to 0.55%.
- Si silicon
- the oxidation resistance is closely related to the composition of the oxide layer near the surface of the casting.
- the Si content is low, the fastest growing Fe-rich Oxidation resistance is inferior because an oxide layer is formed, but when the Si content is high, a Cr oxide layer is formed on the outermost layer, and an Si oxide phase is formed in a lump on the inner side.
- the growth of the Cr and Si oxide layers is slow and shows good oxidation resistance.
- a Si content of at least 1.1% is required.
- the Si content is set to 2% or less. For this reason, the Si content is specified to be 1.1 to 2%.
- the Si content is preferably 1.25 to 1.8%, more preferably 1.3 to 1.6%.
- Mn manganese: 1.5% or less Mn is effective as a deoxidizer for molten metal, just like Si, but if it is contained excessively, the oxidation resistance deteriorates, so the Mn content is 1.5% or less. To do.
- Cr chromium
- 17.5-22.5% Cr is an extremely important element that increases the high temperature strength and oxidation resistance by austenitizing the structure of heat-resistant cast steel together with Ni described later, and also increases the high temperature strength by forming crystallized carbides and precipitated carbides.
- it is necessary to contain 17.5% or more of Cr.
- Cr exceeds 22.5%, ferrite crystallizes in the structure. Slightly crystallized ferrite of about several percent suppresses the occurrence of weld cracks and improves weldability, but when ferrite increases, the high-temperature strength decreases.
- the Cr content is specified to be 17.5 to 22.5%.
- Ni nickel
- Ni nickel
- the austenitic heat-resistant cast steel of the present invention has a Si content of 1.1% or more and imparts heat resistance in the vicinity of 1000 ° C. equivalent to 25Cr-20Ni austenitic heat-resistant cast steel, the Ni content is 13%. The following can be suppressed. Therefore, the Ni content is specified as 8-13%. The preferred Ni content is 9-12%.
- Nb (niobium) 1-4% Nb combines with C to form fine carbides and improves the high temperature strength and thermal fatigue life of heat-resistant cast steel. It also improves oxidation resistance and machinability by suppressing the formation of Cr crystallized carbides. Furthermore, since Nb produces eutectic carbide, it improves the castability, which is important when manufacturing thin-walled and complex-shaped castings such as exhaust system parts. For this purpose, the Nb content must be 1% or more. However, when Nb is contained in a large amount, eutectic carbides generated at the grain boundaries increase and become brittle, and the strength and ductility are significantly reduced. Therefore, the Nb content is 1 to 4%.
- N nitrogen
- N nitrogen
- 0.01-0.3% N is a strong austenite-forming element, stabilizes the austenite base of heat-resistant cast steel and improves high-temperature strength.
- the N content is set to 0.01 to 0.3%.
- the main inevitable impurity contained in the austenitic heat-resistant cast steel of the present invention is P mixed from raw materials.
- P is preferably as small as possible because it segregates at the grain boundaries and significantly reduces the toughness, and is preferably 0.04% or less.
- the austenitic heat-resisting cast steel of the present invention improves the castability by generating eutectic carbide of Nb, and obtains high strength by precipitating an appropriate amount of carbide.
- Eutectic carbide (NbC) is formed by mass ratio of C and Nb 8 times that of C.
- eutectic carbide can be obtained by forming eutectic carbide. It is necessary to secure an amount of C that exceeds the amount consumed.
- (C-Nb / 8) represented by the formula (1) is required to be 0.05 or more. However, if (C-Nb / 8) exceeds 0.6, the carbides become excessive and hard and brittle, and ductility and machinability deteriorate. Therefore, (C-Nb / 8) in equation (1) is set to 0.05 to 0.6. In particular, thin cast products require high castability, and the proportion of eutectic carbide is important. A preferable range of (C-Nb / 8) in the formula (1) is 0.1 to 0.3.
- Equation (3) It is necessary to satisfy both the formula (2) and the formula (3) in order to ensure 17.5 ⁇ 17.5Si ⁇ (W + 2Mo) represented by the formula (2) is a necessary condition for suppressing the increase of the precipitated carbide in the austenite base, and 5.6Si + (W + 2Mo) ⁇ 13.7 represented by the formula (3) This is a necessary condition for suppressing the formation of ferrite with low high-temperature strength. In order to improve the thermal fatigue life and provide heat resistance and durability, it is necessary to satisfy the formulas (2) and (3).
- the value on the left side of Equation (3) is preferably 12.7 or less.
- Equation (4) is a condition necessary to ensure weldability even if the amount of Si is increased.By satisfying Equation (4), the temperature range of a specific solidification temperature range is reduced, and weld cracking occurs. Can be effectively suppressed.
- the susceptibility to weld cracking has a correlation with the solidification temperature range ⁇ T from the start to the end of solidification of the material, and it is said that weld cracking is less likely to occur as ⁇ T is smaller.
- the weld crack susceptibility is higher than ⁇ T until solidification of about 70% is completed from the start of solidification. It was found that there is a correlation with the solidification temperature range of ⁇ T 0.7, and welding cracks can be suppressed by reducing ⁇ T 0.7 .
- FIG. 1 schematically shows the result of thermal analysis of the solidification process of austenitic heat-resistant cast steel by differential scanning calorimetry (DSC).
- the heat-resistant cast steel of the present invention starts solidification at point A, first austenite crystallizes (point B), then eutectic of Nb carbide (NbC) and austenite crystallizes (point C), then Nb carbide At the end of crystallization of austenite, MnS crystallizes (D point). Finally, a eutectic of Cr carbide and austenite crystallizes (E point), and solidification ends at F point.
- ⁇ T shown in FIG.
- ⁇ T 0.7 is 70% of solidification from the start of solidification (point A). Temperature range up to. Here, the temperature until the completion of 70% solidification is the area indicated by the diagonal line in FIG. Is the total (100%). On the other hand, the heat flow area is accumulated for each unit temperature starting from the start of solidification (point A), and the accumulated area reaches 70%.
- the present inventor investigated the relationship between the thermal analysis results of heat-resistant cast steels with various composition ranges and the occurrence of weld cracks, and found that heat-resistant cast steel with a small heat flow value at the peak (valley depth) at point E shown in FIG. Compared to several heat-resistant cast steels that have less cracking and that have different compositions but have the same ⁇ T and different peak heat flow values at point E, the heat-resistant cast steels with smaller heat flow values are more solidified. It was found that the range ⁇ T 0.7 was reduced and welding cracks were less likely to occur.
- welding cracks are generally caused by thermal stress acting on the remaining liquid phase in the late stage of solidification, but if the solidification temperature range is reduced, solidification proceeds rapidly after the start of solidification. It seems that weld cracking is reduced because solidification is completed before cracking occurs even if the amount is reduced and subjected to thermal stress. In addition, the rapid progress of solidification promotes the generation of a large number of solidified nuclei, while the growth of the generated solidified nuclei is suppressed, the solidified structure is refined and the strength is improved, and crystals of low melting point impurity elements such as P are crystallized.
- weld cracking such as the remaining amount of liquid phase described above are due to the composition of the cast steel, and the composition is the solidification temperature until the end of solidification when the last liquid phase disappears and solidification is completed. It is not reflected in the range ⁇ T, but is significantly reflected in the solidification temperature range ⁇ T 0.7 from the start of solidification to the end of 70% solidification. Therefore, if ⁇ T is almost the same, the smaller ⁇ T 0.7 is less likely to cause weld cracking. Guessed.
- Point E is the change in heat flow that occurs as the eutectic of Cr carbide and austenite crystallizes in the late stage of solidification. Therefore, if the crystallization amount of Cr carbide and austenite eutectic can be reduced, it is considered that ⁇ T 0.7 can be reduced by reducing the heat flow value at the peak of the E point.
- the present inventor further examined the content of the basic component in order to improve the weldability, and found the component parameter for controlling the crystallization amount of Cr carbide and austenite eutectic. It was. In other words, if the content of Si, Cr, Ni, W and Mo is reduced and (C-Nb / 8) in the above formula (1) is controlled to be small, the coexistence of Cr carbide and austenite generated in the latter stage of solidification. The amount of crystallized crystals decreases, the heat flow value at the peak of point E shown in FIG. 1 decreases, ⁇ T 0.7 decreases, and the susceptibility to weld cracking decreases.
- Equation (4) is a component parameter for controlling eutectic crystallization of Cr carbide and austenite, found from the above study, and is an index for improving the weldability by reducing the susceptibility to occurrence of weld cracks. It is. Specifically, if the value of the left side given by the formula (4) is 0.96 or less from the contents of C, Si, Cr, Ni, W, Mo and Nb, the sensitivity of the occurrence of weld cracks is reduced, and Si Even if the amount is increased, an austenitic heat-resistant cast steel having good weldability can be obtained.
- the content of the alloying element should be specified so that the value on the left side of the formula (4) is as small as possible so that the eutectic of Cr carbide and austenite is not crystallized.
- the eutectic crystallization of Cr carbide and austenite is extremely reduced, the high temperature strength and oxidation resistance are insufficient, and the heat resistance and durability, which are the original functions of the austenitic heat-resistant cast steel of the present invention, cannot be secured. Therefore, the lower limit of the value on the left side given by Equation (4) is limited according to the above-described contents of Si, Cr, Ni, W, and Mo and the value of (C-Nb / 8).
- the austenitic heat-resistant cast steel of the present invention preferably has an oxidation loss of 20 mg / cm 2 or less when held in an atmosphere at 1000 ° C. for 200 hours.
- An exhaust system part made of austenitic heat-resistant cast steel becomes high temperature due to exhaust gas from the engine, and is exposed to an oxidizing gas such as sulfur oxide and nitrogen oxide to form an oxide film on the surface of the member.
- an oxidizing gas such as sulfur oxide and nitrogen oxide
- the oxidation weight loss of the austenitic heat-resistant cast steel of the present invention is more preferably 15 mg / cm 2 or less, and most preferably 10 mg / cm 2 or less.
- Thermal fatigue life 800 cycles or more
- the austenitic heat-resistant cast steel of the present invention has a thermal fatigue life measured by a thermal fatigue test in which the heating upper limit temperature is 1000 ° C., the temperature amplitude is 850 ° C. or more, and the restraint ratio is 0.25. Is preferably 800 cycles or more. Exhaust system parts are required to have a long thermal fatigue life against repeated engine operation (heating) and stopping (cooling).
- the thermal fatigue life is one of the indexes representing superiority or inferiority of heat resistance and durability. As the number of cycles until thermal fatigue failure is increased due to cracks and deformation caused by repeated heating and cooling in the thermal fatigue test, the thermal fatigue life is longer and the heat resistance and durability are superior.
- the thermal fatigue life is, for example, a smooth round bar test piece with a distance between gauge points of 25 mm and a diameter of 10 mm, in which the upper heating limit temperature is 1000 ° C, the lower cooling limit temperature is 150 ° C, the temperature amplitude is 850 ° C or more Repeat the heating / cooling cycle with one cycle of heating time 2 minutes, holding time 1 minute, and cooling time 4 minutes in total 7 minutes, mechanically restraining expansion and contraction due to heating and cooling, and causing thermal fatigue failure Can be evaluated.
- the criteria for determining the thermal fatigue life is that each cycle with the maximum tensile load (generated at the cooling lower limit temperature) of the second cycle as the reference (100%) in the load-temperature diagram obtained from the load change with repeated heating and cooling.
- the degree of mechanical restraint is expressed by a restraint ratio defined by (free thermal expansion elongation ⁇ elongation under mechanical restraint) / (free thermal expansion elongation).
- a restraint factor of 1.0 refers to a mechanical restraint condition that does not allow elongation at all when a test piece is heated from 150 ° C. to 1000 ° C., for example.
- the restraint ratio of 0.5 means a mechanical restraint condition that allows only 1 mm of elongation where the free expansion and elongation extends, for example, 2 mm.
- the austenitic heat-resistant cast steel of the present invention is defined by a restraint rate of 0.25.
- Austenitic heat-resistant cast steel can be said to have excellent thermal fatigue life if the thermal fatigue life is 800 cycles or more under the conditions of heating upper limit temperature 1000 ° C, temperature amplitude 850 ° C or higher, and restraint ratio 0.25, As described above, it is suitable for exhaust system parts exposed to high-temperature exhaust gas.
- the exhaust system parts made of the austenitic heat-resistant cast steel of the present invention have excellent heat resistance and durability even in an environment exposed to exhaust gas of 1000 ° C. or higher, and have a long life until thermal fatigue failure.
- the thermal fatigue life measured by a thermal fatigue test under the same conditions as described above is more preferably 850 cycles or more, and most preferably 900 cycles or more.
- exhaust system parts of the present invention are manufactured using the 20Cr-10Ni system austenitic heat-resistant cast steel of the present invention.
- Preferred examples of the exhaust system parts include an exhaust manifold, a turbine housing, a turbine housing integrated exhaust manifold in which the turbine housing and the exhaust manifold are integrally cast, a catalyst case, and a catalyst case integrated exhaust manifold in which the catalyst case and the exhaust manifold are integrally cast.
- Any exhaust system part made of austenitic heat-resistant cast steel of the present invention including, but not limited to, a cast member that is welded to a sheet metal or pipe member. Is also targeted.
- the exhaust system parts of the present invention are excellent in heat resistance such as high oxidation resistance and thermal fatigue life even if the exhaust system parts themselves are exposed to high temperature exhaust gas of 1000 ° C or higher and the surface temperature of the exhaust system parts itself reaches around 950 to 1000 ° C. And demonstrates durability. Furthermore, since it also has excellent weldability, weld cracks do not occur in welded joints between sheet metal members, pipe members and cast members, between cast members, or in welding repair of casting defects. In addition, since the rare metal can be reduced and manufactured at a low cost, it is excellent in economy. In other words, the exhaust system parts of the present invention have high heat resistance and durability required for the parts, and can cope with weight reduction and downsizing, and can be easily applied to popular cars. It is expected to contribute to exhaust gas purification, fuel efficiency improvement and safety improvement.
- Tables 1 and 2 show the chemical compositions of the heat-resistant cast steel specimens of Examples 1-28 and Comparative Examples 1-22.
- the value of the formula (1) to the value of the formula (4) are the values of the formulas in the formulas (1) to (4) defined in the present invention, respectively.
- Equation (1) is the value of (C-Nb / 8)
- the value of equation (2) is the value of [17.5Si- (W + 2Mo)]
- the value of equation (3) is [5.6Si + (W + 2Mo) )]
- (4) are the values of [0.08Si + (C-Nb / 8) + 0.015Cr + 0.011Ni + 0.03W + 0.02Mo] The content (% by mass) of each element contained in is shown.)
- Examples 1 to 28 are austenitic heat-resistant cast steels within the composition range defined in the present invention.
- Comparative Examples 1, 2, 8 to 17 the composition range in which the content of one or more elements of C, Ni, Mn, Cr, W, Mo, (W + 2Mo) and Nb is defined in the present invention Out of these, the comparative examples 2 and 16 are cast steels in which the value of the formula (4) is too large.
- Comparative Examples 3 to 5 are cast steels in which the value of the formula (2) is too small.
- Comparative Example 4 is a cast steel having an excessively low Si content
- Comparative Example 5 is a 20Cr- described in JP-A-7-228948. This is an example of a 10Ni austenitic heat-resistant cast steel.
- Comparative Examples 6 and 7 are cast steels in which the value of the formula (3) is too large, and Comparative Example 7 is a cast steel in which the Si content is too large. Comparative Examples 18 to 21 are cast steels in which the value of formula (4) is too large. Comparative Example 22 is an example of a 25Cr-20Ni high Cr high Ni austenitic heat-resistant cast steel described in JP-A-2000-291430.
- the cast steels of Examples 1 to 28 and Comparative Examples 1 to 22 were melted in the air using a 100 kg kg high-frequency melting furnace (basic lining), then heated at 1550 to 1600 ° C and immediately JIS standard at 1500 to 1550 ° C.
- a test material was prepared by pouring the mold into a mold to be a Y-shaped B test material and a mold to be a cylindrical test piece for weldability evaluation. The following evaluation tests were performed on each sample material.
- Table 1 (continued) Value of formula (1): (C-Nb / 8) Value of formula (2): 17.5Si- (W + 2Mo) Value of formula (3): 5.6Si + (W + 2Mo) Value of formula (4): 0.08Si + (C-Nb / 8) + 0.015Cr + 0.011Ni + 0.03W + 0.02Mo
- the high temperature proof stress of the test pieces of Examples 1 to 28 of the present invention is 50 MPa or more, and particularly when the C content is 0.40% or more, the high temperature proof stress is stable and 60 MPa or more. It can be seen that the increase in the C content contributes to the improvement of the high temperature strength.
- the oxidation weight loss is 20 mg / kg, which is the preferred oxidation weight loss of the austenitic heat-resistant cast steel of the present invention. cm 2 or less and less, it can be seen that comparable oxidation resistance and Comparative example 22, high-Cr, high-Ni, austenitic cast steel of 25Cr-20Ni-based.
- the comparative material 16 with a small amount has a high oxidation weight loss exceeding 20 mg / cm 2 . From these results, it was confirmed that the heat-resistant cast steel of the present invention has sufficient oxidation resistance for exhaust system parts exposed to exhaust gas at 1000 ° C. or higher, though it is a 20Cr-10Ni system.
- Thermal fatigue life is the same electro-hydraulic servo-type material testing machine as the above high temperature proof stress test. After mounting with a restraint ratio of 0.25, each test piece in the atmosphere is cooled at a lower limit temperature of 150 ° C., heated upper limit temperature of 1000 ° C., temperature amplitude of 850 ° C. The heating and cooling cycle was repeatedly evaluated with a total cooling time of 4 minutes and 7 minutes. Based on the maximum tensile load in the load-temperature diagram of the second cycle as a reference (100%), the number of heating / cooling cycles when the maximum tensile load decreased to 75% was counted as the thermal fatigue life. The evaluation results are shown in Tables 3 and 4.
- Comparative Material 1 with a low C content Comparative Examples 3 to 5 in which the value of Formula (2) is too small, Comparative Examples 6 and 7 in which the value of Formula (3) is too large, Comparative Example 8 with a low Ni content
- the thermal fatigue life is short with less than 800 cycles.
- Comparative Example 5 corresponding to a conventional 20Cr-10Ni austenitic heat-resistant cast steel the value of the formula (2) was smaller than 17.5 defined in the present invention, and the thermal fatigue life was less than 800 cycles. From these results, it was confirmed that the heat-resistant cast steel of the present invention has a sufficient thermal fatigue life for exhaust system parts exposed to exhaust gas at 1000 ° C. or higher, although it is a 20Cr-10Ni system.
- Figure 2 shows the relationship between the composition of Si and (W + 2Mo) and the thermal fatigue life of austenitic heat-resistant cast steel.
- FIG. 2 shows the values of Examples 1 to 28, Si, W, Mo, (W + 2Mo), the values of Formula (2) and the values of Formula (3) and other compositions and relational expressions. Comparative examples 3 to 7 and Comparative examples 12 to 15 within the specified range are plotted.
- the shape of each point represents the thermal fatigue life (number of cycles), diamond marks ( ⁇ ) for those less than 800, triangle marks ( ⁇ ) for those less than 800 and less than 850, square marks for those that are 850 and less than 900 ( ⁇ ), 900 or more items are indicated by circles ( ⁇ ).
- the solid line bold frame indicates the Si: 1.1 to 2 range, (W + 2Mo): 1.5 to 4 range, 17.5 ⁇ 17.5 Si- (W + 2Mo) range represented by Formula (2), Formula (3 ),
- Each boundary line for the region of 5.6Si + (W + 2Mo) ⁇ 13.7 is shown, and the region surrounded by the solid thick frame is a region satisfying the composition range of Si and (W + 2Mo) defined in the present invention. is there.
- FIG. 2 shows that the austenitic heat-resistant cast steel of the present invention has a thermal fatigue life of 800 cycles or more if Si and (W + 2Mo) are in this region.
- the composition is not based on the composition range based on the individual contents of Si and W and / or Mo, but based on the relationship between Si and (W + 2Mo), which exhibits excellent thermal fatigue life. It means that a range exists.
- Weldability was determined by manufacturing a pair of cylindrical specimens with an outer diameter of 50 mm, a wall thickness of 5 mm, and a groove shape of the welded part I from each specimen. After butt welding under welding conditions, evaluation was performed by cutting the 7 places except the welding start part and welding end part and confirming the occurrence of cracks. Tables 3 and 4 show the weldability evaluation results.
- Solidification temperature range ⁇ T 0.7 is a temperature of up to 900 ° C in an argon atmosphere by using a differential scanning calorimeter (DSC (manufactured by SETARAM)) with a 2 mm diameter and 2 mm long test piece cut out from each specimen. From the thermal analysis curve obtained by heating at a rate of 15 ° C / min and a rate of temperature increase from 900 to 1600 ° C at 5 ° C / min, image analysis processing is performed as follows using an image analyzer (IP1000 type manufactured by Asahi Kasei). And asked. That is, as described above with reference to FIG. 1, the area indicated by the diagonal lines in FIG.
- Comparative Example 7 Since the crack occurrence location of Comparative Example 7 is not a bead but a base material, Comparative Example 7 has an excessive Si content alone, so low melting point Si concentrated at the crystal grain boundary of the cast steel base material is welded. It is considered that cracking occurred due to local melting due to heat input.
- Example 29 Using the austenitic heat-resistant cast steel of Example 15, an exhaust manifold (main wall thickness: 4.0 to 5.0 mm) for automobile exhaust system parts was cast and then machined as-cast. The obtained exhaust manifold was free from casting defects such as shrinkage cavities, poor hot water and gas defects, and there were no cutting defects in machining or abnormal wear or damage of cutting tools.
- the exhaust simulator of this example was assembled in an exhaust simulator equivalent to an inline 4-cylinder high-performance gasoline engine with a displacement of 2000 cc, and an endurance test was conducted to examine the life until cracks occurred and the occurrence of cracks and oxidation.
- the exhaust gas temperature at full load is about 1050 ° C at the outlet of the exhaust section downstream of the exhaust manifold
- the upper limit heating temperature of the exhaust manifold surface is about 1000 ° C
- the lower limit cooling temperature is the central section.
- a heating / cooling cycle consisting of heating for 10 minutes and cooling for 10 minutes was performed as one cycle.
- the target of the heating / cooling cycle was 1500 cycles.
- the exhaust manifold of this example cleared the endurance test of 1500 cycles without causing leakage or cracking of exhaust gas.
- visual and penetrant testing after the endurance test, it was confirmed in the penetrant test that a very small crack occurred in a part of the branch pipe, but it was confirmed by visual inspection as well as through cracks. There were no cracks that could occur, and there was less oxidation of the entire part. As a result, it was confirmed that the exhaust manifold of this example had excellent heat resistance and durability.
- Comparative Example 23 When an exhaust manifold having the same shape was manufactured under the same conditions as in Example 29 using the cast steel of Comparative Example 5, there were no casting defects or defects in machining. The obtained exhaust manifold was assembled in an exhaust simulator, and an endurance test was conducted under the same conditions as in Example 29 with a target of 1500 cycles. The surface temperature of the aggregate part of the exhaust manifold in the durability test was almost the same as in Example 29.
- the exhaust system parts manufactured using the austenitic heat-resistant cast steel of the present invention have high oxidation resistance and thermal fatigue life around 1000 ° C as the temperature of the exhaust system parts. It was confirmed to be excellent. Since the exhaust system parts of the present invention are made of austenitic heat-resistant cast steel that has a low content of rare metals and is economical in terms of cost and resource saving, it is suitable as a component part of an automobile engine.
- the austenitic heat-resistant cast steel of the present invention is, for example, a combustion engine such as a construction machine, a ship, an aircraft, High oxidation resistance, thermal fatigue life, etc. for various equipment such as furnaces, heat treatment furnaces, incinerators, kilns, boilers, cogeneration equipment, and other plant equipment such as petrochemical plants, gas plants, thermal power plants, nuclear power plants It can also be used for casting parts that require excellent heat resistance and durability as well as weldability.
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Abstract
Description
質量%で、
C:0.3~0.6%、
Si:1.1~2%、
Mn:1.5%以下、
Cr:17.5~22.5%、
Ni:8~13%、
W及びMoの少なくとも1種:(W+2Mo)で1.5~4%、
Nb:1~4%、
N:0.01~0.3%、
S:0.01~0.5%、
残部Fe及び不可避不純物からなり、かつ下記式(1)、(2)、(3)及び(4)を満足することを特徴とする。
0.05≦(C-Nb/8)≦0.6 ・・・(1)
17.5≦17.5Si-(W+2Mo) ・・・(2)
5.6Si+(W+2Mo)≦13.7 ・・・(3)
0.08Si+(C-Nb/8)+0.015Cr+0.011Ni+0.03W+0.02Mo≦0.96 ・・・(4)
ここで、各式中の元素記号は鋳鋼中に含まれる各元素の含有量(質量%)を示す。
本発明のオーステナイト系耐熱鋳鋼の構成について以下詳細に説明する。なお、合金を構成する各元素の含有量は、特に断りのない限り質量%で示す。
Cは、(a)溶湯の流動性、すなわち鋳造性を良くする作用、(b)一部基地に固溶して固溶強化する作用、(c)Crの晶出炭化物や析出炭化物を形成し、高温強度を高める作用、及び(d)Nbと共晶炭化物を形成し、鋳造性を高めるとともに高温強度を向上させる作用がある。このような作用を有効に発揮するために、Cの含有量は0.3%以上必要である。しかし、Cが0.6%を超えるとCrの晶出炭化物や析出炭化物が多くなり過ぎて脆化し、延性が低下するとともに加工性が劣化する。また、Crの晶出炭化物が多すぎると溶接性が劣化する。従って、Cの含有量は0.3~0.6%に規定する。Cの好ましい含有量は0.4~0.55%である。
Siは、溶湯の脱酸剤としての役割を有するほか、耐酸化性の向上と、これに起因する熱疲労寿命の改善に有効な元素である。耐酸化性は、鋳物の表面付近の酸化層の組成と密接に関係している。本発明の20Cr-10Ni系の耐熱鋳鋼において、1000℃付近に加熱されたときの表面付近の酸化層に着目すると、Si含有量が少ない場合は、表面直下の最表層に成長の早いFeリッチの酸化層が形成するため耐酸化性は劣るが、Si含有量が多いと最表層にはCrの酸化層が、その内側にはSiの酸化相が塊状に形成される。Cr及びSiの酸化層の成長は遅く、良好な耐酸化性を示す。最表層にCrの酸化層、その内側にSiの酸化相を形成するためには、少なくとも1.1%以上の含有量のSiが必要である。しかし、Siは過剰に加えるとオーステナイト組織が不安定になり、鋳造性の劣化を招く。また、ある程度のSiの増加は溶接性を改善するものの、Siが過剰になると溶接性が著しく悪化して溶接割れが発生し易くなるため、Siの含有量は2%以下とする。このため、Siの含有量は1.1~2%に規定する。Siの含有量は、好ましくは1.25~1.8%であり、より好ましくは1.3~1.6%である。
Mnは、Siと同様に溶湯の脱酸剤として有効であるが、過剰に含有すると耐酸化性が劣化するので、Mnの含有量は1.5%以下とする。
Crは、後述のNiとともに耐熱鋳鋼の組織をオーステナイト化することで高温強度や耐酸化性を高めるほか、晶出炭化物や析出炭化物を形成して高温強度を高める極めて重要な元素である。特に1000℃付近の高温域でこれらの効果を発揮させるためには、Crを17.5%以上含有する必要がある。しかし、Crは、22.5%を超えて含有すると組織中にフェライトが晶出する。数%程度の僅かな晶出フェライトは溶接割れの発生を抑制して溶接性を向上させるが、フェライトが増加すると高温強度が低下してしまう。また、Crが過剰に含有すると晶出炭化物が多くなり過ぎて脆化し、延性を低下させる。さらに、Crは希少金属のため経済性の観点から過剰な含有は抑制すべきである。このため、Cr含有量は17.5~22.5%に規定する。
Niは、前述のCrとともに耐熱鋳鋼をオーステナイト組織とし、その組織を安定にするとともに、一般に薄肉で複雑形状である排気系部品の鋳造性を高めるのに有効な元素である。このような作用を発揮するためには、Niは8%以上含有することが必要である。しかし、NiはCrと同様、希少金属のため価格のみならず資源の有効活用や安定供給等経済性の観点から、過剰な含有は避けるべきである。本発明のオーステナイト系耐熱鋳鋼は、Siの含有量を1.1%以上として、25Cr-20Niオーステナイト系耐熱鋳鋼と同等の1000℃付近での耐熱性を付与しているので、Niの含有量は13%以下に抑制できる。そのため、Ni含有量は8~13%に規定する。Niの好ましい含有量は9~12%である。
W及びMoは、いずれも耐熱鋳鋼の高温強度を改善する。この効果は少なくとも一方を含有させることにより得られるが、両者とも多量に含有すると耐酸化性を劣化させる。従って、Wを単独で添加する場合、Wの含有量は1.5~4%とし、好ましくは2~3.5%である。Moは、質量比でW = 2Moの割合でWとほぼ同様の効果を発揮するので、Wの一部又は全量をMoに置換することも可能である。Moを単独で含有する場合、Moの含有量は0.75~2%とし、好ましくは1~1.75%である。両者を複合添加する場合には、(W+2Mo)として1.5~4%とし、好ましくは2~3.5%である。
Nbは、Cと結合して微細な炭化物を形成し、耐熱鋳鋼の高温強度と熱疲労寿命を向上させる。また、Crの晶出炭化物の生成を抑制することによって耐酸化性と被削性を向上させる。さらに、Nbは共晶炭化物を生成するため、排気系部品のような薄肉で複雑形状の鋳物を製造する際に重要な鋳造性を向上させる。このような目的でNbの含有量は1%以上必要である。しかし、Nbが多量に含有すると、結晶粒界に生成する共晶炭化物が多くなって脆化し、強度と延性が著しく低下する。従って、Nbの含有量は、1~4%とする。
Nは、強力なオーステナイト生成元素であり、耐熱鋳鋼のオーステナイト基地を安定にして高温強度を向上させる。しかし、Nは多量に含有すると、室温付近の衝撃値を低下させ、また鋳造時にピンホールやブローホール等のガス欠陥の発生を助長して鋳造歩留りを悪化させる。そのため、Nの含有量は0.01~0.3%とする。
Sは、鋳鋼においては球状又は塊状の硫化物を生成し、この硫化物は潤滑効果を有するため被削性を向上させる。この効果を得るには、Sは0.01%以上必要である。しかし、Sが0.5%を超えて含有すると、室温付近の衝撃値が低下する。そのため、Sの含有量は0.01~0.5%とする。Sの好ましい含有量は0.05~0.2%である。
本発明のオーステナイト系耐熱鋳鋼に含有される不可避的不純物の主なものは、原材料から混入するPである。Pは結晶粒界に偏析して靭性を著しく低下させるので少ないほど好ましく、0.04%以下とするのが望ましい。
本発明のオーステナイト系耐熱鋳鋼は、Nbの共晶炭化物を生成させて鋳造性を高めるとともに、適当量の炭化物を析出させて高い強度を得ている。共晶炭化物(NbC)は、質量比率でCとCの8倍のNbとで形成されるが、共晶炭化物(NbC)のほかに析出炭化物を適当量得るには、共晶炭化物の形成により消費される量を超える量のCを確保することが必要となる。優れた鋳造性と高温強度とを得るためには、式(1)で表される(C-Nb/8)が0.05以上必要である。しかし、(C-Nb/8)が0.6を超えると、炭化物が過剰となって硬く脆くなり、延性と被削性が劣化する。従って、式(1)の(C-Nb/8)は0.05~0.6とする。特に薄肉鋳物では高い鋳造性を要し、共晶炭化物の割合は重要である。式(1)の(C-Nb/8)の好ましい範囲は0.1~0.3である。
前述したように、本発明者は、本発明のオーステナイト系耐熱鋳鋼において、SiとW及び/又はMoとの含有量の関係が、熱疲労寿命に影響を及ぼすことを見出した。本発明のオーステナイト系耐熱鋳鋼は、Si含有量を増加して良好な耐酸化性を付与しているが、本発明で規定する基本成分の範囲において、Siが少ない又は多い範囲で、W及び/又はMoを増量すると、耐酸化性には大きな影響はないものの、熱疲労寿命が悪化するという新たな知見を得た。すなわち、本発明の基本成分の範囲内で、Siを減量してW及び/又はMoを増量すると、オーステナイト基地中の析出炭化物が増加し、一方、Siを増量してW及び/又はMoを増量すると、高温強度の低いフェライトが生成する。オーステナイト基地中の析出炭化物が増加すると延性が低下するために、また高温強度の低いフェライトが生成すると基地中の強度の弱い相に応力が集中するために、いずれも熱疲労寿命が悪化する。
本発明の20Cr-10Ni系のオーステナイト系耐熱鋳鋼は、耐熱性を得るため単にSiを増量しただけでは溶接性が悪化する。そこで、本発明者は、C、Si、Cr、Ni、W、Mo及びNbの総量が、溶接性に影響を及ぼすとの知見を得て、溶接性を損なうことのないC、Si、Cr、Ni、W、Mo及びNbからなる上記式(4)で規定される成分パラメータを見出した。式(4)は、Siを増量しても溶接性を確保するために必要な条件で、式(4)を満足させることによって特定の凝固温度範囲の温度幅が縮小して、溶接割れの発生を効果的に抑制することができる。
(14)酸化減量:20 mg/cm2以下
本発明のオーステナイト系耐熱鋳鋼は、1000℃の大気中に200時間保持したときの酸化減量が20 mg/cm2以下であるのが好ましい。オーステナイト系耐熱鋳鋼からなる排気系部品は、エンジンからの排ガスにより高温となり、硫黄酸化物、窒素酸化物等の酸化性ガスに曝されて部材表面に酸化膜を生成する。さらに酸化が進行すると、生成した酸化膜を起点に亀裂が入り部材内部まで酸化が進展する。最終的には部材の表面から裏面まで亀裂が貫通して排ガスの漏洩や部材の割れを招く。
本発明のオーステナイト系耐熱鋳鋼は、加熱上限温度1000℃、温度振幅850℃以上、及び拘束率0.25の条件で加熱冷却する熱疲労試験により測定した熱疲労寿命が800サイクル以上であるのが好ましい。排気系部品には、エンジンの運転(加熱)と停止(冷却)の繰り返しに対する熱疲労寿命が長いことが要求される。熱疲労寿命は、耐熱性及び耐久性の優劣を表す指標の1つである。熱疲労試験での加熱冷却の繰り返しで生じる亀裂や変形により、熱疲労破壊に至るまでのサイクル数が多いほど熱疲労寿命が長く、耐熱性及び耐久性に優れている。
本発明の排気系部品は、上記20Cr-10Ni系の本発明のオーステナイト系耐熱鋳鋼を用いて製造される。排気系部品の好ましい例は、エキゾーストマニホルド、タービンハウジング、タービンハウジングとエキゾーストマニホルドとを一体に鋳造したタービンハウジング一体エキゾーストマニホルド、触媒ケース、触媒ケースとエキゾーストマニホルドとを一体に鋳造した触媒ケース一体エキゾーストマニホルド、又はエキゾーストアウトレットであるが、これに限定されず、板金製又はパイプ製の部材と溶接接合して使用される鋳物部材を含み、本発明のオーステナイト系耐熱鋳鋼からなる鋳造製のいかなる排気系部品も対象とされる。
実施例1~28及び比較例1~22の耐熱鋳鋼供試材の化学組成を表1及び表2に示す。表1及び表2において、式(1)の値~式(4)の値とは、それぞれ本発明で規定する式(1)~式(4)中の式の値であり、具体的には式(1)の値とは(C-Nb/8)の値、式(2)の値とは[17.5Si-(W+2Mo)]の値、式(3)の値とは[5.6Si+(W+2Mo)]の値、式(4)の値とは[0.08Si+(C-Nb/8)+0.015Cr+0.011Ni+0.03W+0.02Mo]の値である(ここで、各式中の元素記号は鋳鋼中に含まれる各元素の含有量(質量%)を示す。)。
式(1)の値:(C-Nb/8)
式(2)の値:17.5Si-(W+2Mo)
式(3)の値:5.6Si+(W+2Mo)
式(4)の値:0.08Si+(C-Nb/8)+0.015Cr+0.011Ni+0.03W+0.02Mo
式(1)の値:(C-Nb/8)
式(2)の値:17.5Si-(W+2Mo)
式(3)の値:5.6Si+(W+2Mo)
式(4)の値:0.08Si+(C-Nb/8)+0.015Cr+0.011Ni+0.03W+0.02Mo
排気系部品の高温強度の指標として1000℃における0.2%耐力(MPa)を評価した。各供試材から切り出した標点間距離50 mm、直径10 mmの平滑丸棒つばつき試験片を、電気-油圧サーボ式材料試験機(株式会社島津製作所製、サーボパルサーEHF-ED10T-20L)に取り付け、各試験片の高温耐力として、大気中1000℃で0.2%耐力(MPa)を測定した。評価結果を表3及び表4に示す。表3及び表4から明らかなように、本発明の実施例1~28の試験片の高温耐力は50 MPa以上であり、特にC含有量が0.40%以上では高温耐力が安定して60 MPa以上であり、C含有量の増加が高温強度の向上に寄与することがわかる。
排気系部品が1000℃付近の排ガスに曝されることを想定し、1000℃における耐酸化性を評価した。耐酸化性の評価は、各供試材から切り出した直径10 mm、長さ20 mmの丸棒試験片を作製し、これを大気中1000℃に200時間保持し、取り出した後ショットブラスト処理を施して酸化スケールを除去し、酸化試験前後の単位面積当たりの質量変化[酸化減量(mg/cm2)]を求めることにより行った。評価結果を表3及び表4に示す。
熱疲労寿命は、各供試材から切り出した標点間距離25 mm、直径10 mmの平滑丸棒試験片を、前記高温耐力試験と同じ電気-油圧サーボ式材料試験機に拘束率0.25で取り付けた後、各試験片に大気中で、冷却下限温度150℃、加熱上限温度1000℃、温度振幅850℃で、1サイクルを昇温時間2分、保持時間1分、及び冷却時間4分の合計7分として加熱冷却サイクルを繰り返し評価した。2サイクル目の荷重-温度線図における最大引張荷重を基準(100%)に、最大引張荷重が75%に低下したときの加熱冷却サイクルの数をカウントして熱疲労寿命とした。評価結果を表3及び表4に示す。
溶接性は、各供試材から外径50 mm、肉厚5 mm、溶接部の開先形状をI型とした1対の円筒状試験片を製作し、これを下記の溶接条件で突合せ溶接後、溶接開始部分及び溶接終了部分を除いた7箇所を切断して割れの発生状況を確認することで評価した。表3及び表4に溶接性の評価結果を示す。
溶接方法:MIGパルス溶接
ワイヤ:φ1.2 mm、JIS Z 3321 Y310ソリッドワイヤ
平均電流:200 A
電圧:20 V
送り速度:110 cm/min
ノズル-ワーク間距離:10 mm
シールドガスの種類:Ar-2%O2
シールドガスの流量:15 L/min
トーチ角度:10°(前進法)
予熱:なし
凝固温度範囲ΔT0.7は、各供試材から切り出した直径2 mm、長さ2 mmの試験片を示差走査熱量測定装置(DSC(SETARAM製))により、アルゴン雰囲気中で900℃までの昇温速度を15℃/分、900~1600℃までの昇温速度を5℃/分で昇温して得た熱分析曲線から、画像解析装置(旭化成製IP1000型)により以下のように画像解析処理して求めた。すなわち、図1を用いて前述したとおり、凝固開始から全ての凝固が終了するまでの凝固温度範囲ΔTでの温度と熱流の関係から図1の斜線で示された面積を総計(100%)として算出し、これに対して、凝固開始を起点として単位温度毎に熱流の面積を累積して、その累積面積が70%に達したときの温度を凝固温度範囲ΔT0.7とした。得られた凝固温度範囲ΔT0.7(℃)を表3及び表4に示す。
実施例15のオーステナイト系耐熱鋳鋼を用いて、自動車用排気系部品のエキゾーストマニホルド(主要肉厚4.0~5.0 mm)を鋳造した後、鋳放しのまま機械加工した。得られたエキゾーストマニホルドには引け巣、湯廻り不良、ガス欠陥等の鋳造欠陥は認められず、また機械加工での切削不具合や切削工具の異常摩耗、損傷等もなかった。
比較例5の鋳鋼を用いて、実施例29と同じ条件で同一形状のエキゾーストマニホルドを製造したところ、鋳造欠陥や機械加工での不具合はなかった。得られたエキゾーストマニホルドを排気シミュレータに組み付け、実施例29と同一条件で1500サイクルを目標に耐久試験を実施した。耐久試験でのエキゾーストマニホルドの集合部の表面温度は実施例29とほぼ同じであった。
Claims (5)
- 質量%で、
C:0.3~0.6%、
Si:1.1~2%、
Mn:1.5%以下、
Cr:17.5~22.5%、
Ni:8~13%、
W及びMoの少なくとも1種:(W+2Mo)で1.5~4%、
Nb:1~4%、
N:0.01~0.3%、
S:0.01~0.5%、
残部Fe及び不可避不純物からなり、かつ下記式(1)、(2)、(3)及び(4)を満足することを特徴とするオーステナイト系耐熱鋳鋼。
0.05≦(C-Nb/8)≦0.6 ・・・(1)
17.5≦17.5Si-(W+2Mo) ・・・(2)
5.6Si+(W+2Mo)≦13.7 ・・・(3)
0.08Si+(C-Nb/8)+0.015Cr+0.011Ni+0.03W+0.02Mo≦0.96 ・・・(4)
ここで、各式中の元素記号は鋳鋼中に含まれる各元素の含有量(質量%)を示す。 - 請求項1に記載のオーステナイト系耐熱鋳鋼において、1000℃において200時間大気中に保持したときの酸化減量が20 mg/cm2以下であることを特徴とするオーステナイト系耐熱鋳鋼。
- 請求項1又は2に記載のオーステナイト系耐熱鋳鋼において、加熱上限温度1000℃、温度振幅850℃以上、及び拘束率0.25の条件で加熱冷却する熱疲労試験により測定した熱疲労寿命が800サイクル以上であることを特徴とするオーステナイト系耐熱鋳鋼。
- 請求項1~3のいずれかに記載のオーステナイト系耐熱鋳鋼からなることを特徴とする排気系部品。
- 請求項4に記載の排気系部品において、エキゾーストマニホルド、タービンハウジング、タービンハウジング一体エキゾーストマニホルド、触媒ケース、触媒ケース一体エキゾーストマニホルド、又はエキゾーストアウトレットであることを特徴とする排気系部品。
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US8388889B2 (en) | 2013-03-05 |
KR20100113520A (ko) | 2010-10-21 |
JP5353716B2 (ja) | 2013-11-27 |
EP2258883A4 (en) | 2014-05-14 |
CN101946018A (zh) | 2011-01-12 |
EP2258883A1 (en) | 2010-12-08 |
US20110000200A1 (en) | 2011-01-06 |
KR101576069B1 (ko) | 2015-12-09 |
EP2258883B1 (en) | 2015-04-15 |
JPWO2009104792A1 (ja) | 2011-06-23 |
CN101946018B (zh) | 2013-01-16 |
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