Classifications

• • 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
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • 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
• • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • 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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • 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

Definitions

• the present invention relates to a ferritic stainless steel material for use as automobile exhaust gas passage components, in particular to a ferritic stainless steel material for use as automobile exhaust gas passage components, which has excellent heat resistance and low-temperature toughness favorable for exhaust gas upstream passage components where the material temperature may be over 900°C or further over 950°C, for example, exhaust manifolds, catalyst converters, front pipes and the like.
• two typical ferritic steel species are used properly for automobile exhaust gas passage components, depending on the service temperature range of the components.
• One is a steel species such as typically SUS429 steel mainly applied to the components of which the maximum ultimate temperature of the material may be on a level of 750°C; and the other is a steel species such as typically SUS444 steel mainly applied to the components of which the maximum ultimate temperature of the material may be on a level of 850°C.
• exhaust gas passage upstream components are required to have excellent workability into various shapes.
• the components are required to have excellent workability durable to severe working into complicated shapes.
• exhaust gas passage components are also required to have good low-temperature toughness.
• Patent Reference 1 shows a ferritic stainless steel of which the composition and the texture are so controlled that it may surely have a sufficient amount of solid solution Nb so as to be durable to use in a temperature range over 900°C and may have a tensile strength of 20 MPa at 950°C.
• this has no description relating to 0.2 % yield strength, and the durability of the steel in a case where the material temperature has actually risen up to about 1000°C is not confirmed. In this, any special consideration is not taken for thermal fatigue resistance and low-temperature toughness.
• Patent Reference 2 shows a ferritic stainless steel having excellent high-temperature strength at 900°C and having excellent low-temperature toughness. However, this has no description relating to 0.2 % yield strength, and in this, the measures for sufficiently securing the durability in a case where the material temperature has actually risen up to 1000°C or so could not be said to be always satisfactory.
• Patent Reference 3 describes a ferritic stainless steel having good high-temperature strength at 950°C and good workability. However, this shows nothing relating to 0.2 % yield strength, and in this, it is not certain as to whether or not the material could be actually durable to exposure to about 1000°C or so. No special consideration is taken for low-temperature toughness.
• Patent Reference 4 shows an Fe-Cr alloy of which the thermal expansion coefficient is lowered. However, there is taken no intension of improving the high-temperature strength of the material in a temperature range of about 1000°C or so.
• Patent Reference 5 describes a ferritic stainless steel having excellent thermal fatigue resistance and good low-temperature toughness. In this, however, the material was evaluated for the high-temperature strength in terms of the 0.2 % yield strength thereof at 600°C, and its durability is not clear in a case where the material temperature has actually risen up to about 1000°C.
• Patent Reference 6 shows a ferritic stainless steel for exhaust gas system components to be used at a temperature of not lower than 700°C. Regarding high-temperature strength, however, this shows only the tensile strength data of the material at 600°C and 850°C, and it is not clear as to whether or not the material could be resistant to exposure to temperatures of 1000°C or so. In addition, this has no description relating to low-temperature toughness.
• a method capable of stably realizing a material that exhibits excellent durability when used at a temperature over 900°C and satisfies both good low-temperature toughness and good workability is not as yet established (see the above Patent References).
• An object of the present invention is to provide a ferritic stainless steel material for automobile exhaust gas passage components, which simultaneously satisfies 0.2 % yield strength at high temperature of 1000°C, thermal fatigue resistance, low-temperature toughness and workability all on a high level and which, even when used under the condition where the material temperature actually reaches a high-temperature range of higher than 900°C and even higher than 950°C, still exhibits excellent durability.
• the invention provides a ferritic stainless steel material having excellent heat resistance and low-temperature toughness for automobile exhaust gas passage components, which has a composition essentially containing, in terms of % by mass, at most 0.03 % of C, at most 1 % of Si, from 0.6 to 2 % of Mn, at most 3 % of Ni, from 10 to 25 % of Cr, from 0.3 to 0.7 % of Nb, from more than 1 to 2 % of Cu, from 1 to 2.5 % of Mo, from 1 to 2.5 % of W, at most 0.15 % of Al, from 0.03 to 0.2 % of V, and at most 0.03 % of N, and optionally containing at least one of Ti and Zr in an amount of less than 1 % in total, or further containing at least one of B in an amount of at most 0.02 % and Co in an amount of at most 2 %, or further containing at least one of REM (rare earth element) and Ca in an amount of at most 0.1 % in total
• the element code is substituted with the content of the corresponding element expressed in terms of % by mass.
• “Ferritic stainless steel material for automobile exhaust gas passage components” means a steel material processed for final annealing under heat at a temperature higher than 1000°C (for example, from 1050 to 1100°C) (this may be simply referred to as “final annealing") in a process of producing automobile exhaust gas passage components.
• final annealing a steel material processed for final annealing under heat at a temperature higher than 1000°C (for example, from 1050 to 1100°C) (this may be simply referred to as "final annealing”) in a process of producing automobile exhaust gas passage components.
• final annealing a steel material processed for final annealing under heat at a temperature higher than 1000°C (for example, from 1050 to 1100°C) (this may be simply referred to as "final annealing”) in a process of producing automobile exhaust gas passage components.
• the pipe after the final annealing corresponds to the ferritic stainless steel material for automobile exhaust gas passage components as referred to herein.
• the steel sheet after the final annealing, and the pipe, cylindrical casing or the like obtained by further working the final annealed sheet correspond to the ferritic stainless steel material for automobile exhaust gas passage components.
• a ferritic stainless steel material for automobile exhaust gas passage components which satisfies all the requirements of high-temperature strength durable to exposure to high temperature of 1000°C, good thermal fatigue resistance, good workability and good low-temperature toughness.
• the material meets the recent tendency in the art toward elevated exhaust gas temperatures and brings about an broadened latitude in planning exhaust gas passage upstream components.
• the high-temperature strength (0.2 % yield strength) of the steel material at a level of 1000°C with keeping high the high-temperature strength (0.2 % yield strength) thereof at a level of 600°C. It is extremely effective to make the steel material have high strength both in the two temperature ranges for keeping high the thermal fatigue resistance thereof.
• the 0.2 % yield strength at 600°C and the 0.2 % yield strength at 1000°C of the steel material are both at least 1.5 times higher than the yield strength at the same temperatures of SUS444 steel.
• the 0.2 % yield strength at 600°C of the steel material is at least 200 MPa and the 0.2 % yield strength at 1000°C thereof is at least 15 MPa. It has been found that the material having such high-temperature strength characteristics has good high-temperature fatigue resistance satisfactory for practical use when it receives repeated temperature change between ordinary temperature and 1000°C or so as automobile exhaust gas passage components.
• Cu is used for improving the high-temperature strength of the steel material in a temperature range including 600°C (range of from about 500 to 800°C).
• a temperature range including 600°C range of from about 500 to 800°C.
• an ⁇ -Cu phase is precipitated at a temperature of around 600°C, and this finely disperses in the matrix of the material to thereby express a precipitation-reinforcing phenomenon.
• the steel material can keep the high-temperature strength (0.2 % yield strength) in the temperature range higher by at least about 1.5 times than that of SUS444 steel, it is necessary to take advantage of Nb and Mo solid solution reinforcement in addition to the precipitation of the ⁇ -Cu phase.
• the coefficient of Nb in formula (2) corresponds to the increase in the 0.2 % yield strength (MPa) at 1000°C per 0.1 % by mass of Nb; and the coefficient of Mo and Cu each correspond to the increase in the 0.2 % yield strength (MPa) at 1000°C per 1 % by mass of Nb and Cu, respectively.
• the composition that satisfies the above formula (2) is not enough. More detailed investigations have confirmed that, in particular, it is extremely important to make the steel material have a metal texture in which the Nb and Mo precipitates are reduced as much as possible. Concretely, after final annealing, the steel material must have a texture condition in which the total amount of Nb and Mo existing as a precipitation phase therein is at most 0.2 % by mass.
• the low-temperature toughness and the weldability thereof it is extremely effective to make the steel material have the above-mentioned texture condition after final annealing.
• the amount of Nb or Mo added is considerably large, the amount of the solid solution Mo or the solid solution Nb can be sufficiently secured even when the total amount of Nb and Mo existing as a precipitation phase is more than 0.2 % by mass, and the high-temperature strength of the steel material at 1000°C could be increased owing to their solid solution reinforcement. In this case, however, it is difficult to enhance both the low-temperature toughness and the workability of the steel material.
• Total amount (% by mass) of Nb and Mo existing as a precipitation phase can be determined as follows: The residue of the precipitation phase as extracted out through constant potential electrolysis in an water-free solvent electrolytic solution (SPEED method) is analyzed for elementary quantification, and the total mass of Nb and Mo in the residue is divided by the total mass of the dissolved matrix and the extracted precipitation phase in electrolysis, and this is expressed as percentage.
• the cooling rate from 1050°C to 500°C in the cooling step in the final annealing must be controlled to at least 5°C/sec.
• a steel sheet before formed into a pipe, or after formed into a pipe but before used as the component may be processed at least once for final annealing that comprises soaking under heat at 1050 to 1100°C for from 0 to 10 minutes followed by cooling from 1050°C to 500°C at a cooling rate of at least 5°C/sec.
• any superfluous precipitation phase of Nb and Mo would not form when the automobile exhaust gas passage component formed of the steel material is used under heat at a temperature of 1000°C or so, and practically, therefore, the high-temperature strength and the low-temperature toughness of the steel material would not worsen.
• the alloying ingredients are described below.
• C and N are generally effective for improving creep strength and other high-temperature strength properties but degrade oxidation resistant property, workability, low-temperature toughness and weldability when contained in excess.
• both C and N are limited to a content of at most 0.03 % by mass.
• Si is effective for improving high-temperature oxidation resistance. However, when added in excess, it increases hardness and thus degrades workability and low-temperature toughness.
• the Si content is limited to at most 1 % by mass.
• Mn improves high-temperature oxidation resistance, especially scale peeling resistance.
• the Mn content In order to sufficiently secure high-temperature oxidation resistance on a level of 1000°C, the Mn content must be at least 0.6 % by mass. However, Mn impairs workability and weldability when added in excess. Further, Mn is an austenite-stabilizing element that when added in a large amount facilitates martensite phase formation and thus causes a decline in thermal fatigue resistance and workability.
• the Mn content is therefore limited to at most 2 % by mass, preferably at most 1.5 % by mass, more preferably less than 1.5 % by mass.
• Ni contributes to improvement of low-temperature toughness, but when added too much, it may lower cold elongation.
• the acceptable Ni content is up to 3 % by mass, but more preferably, the Ni content is at most 0.6 % by mass.
• the Cr stabilizes ferrite phase and contributes to improvement of oxidation resistance, an important property of high-temperature materials.
• the Cr content is secured to be at least 15 % by mass for sufficiently exhibiting its effect.
• too much Cr makes the steel material brittle and worsens the workability thereof, and therefore the Cr content is not more than 25 % by mass.
• Nb is effective for increasing high-temperature strength in a temperature range of around 600°C or so by solid solution reinforcement, but the invention takes advantage of the solid solution reinforcing effect of Nb for securing high-temperature strength in a high temperature range of higher than 900°C.
• the Nb content must be at least 0.3 % by mass, and it must satisfy the above-mentioned formula (2).
• the invention must secure the steel texture condition where the total amount of Nb and Mo existing as a precipitation phase is at most 0.2 % by mass.
• Nb has a strong affinity for C and N, therefore readily forming precipitates that may lower high-temperature strength, low-temperature toughness, workability and other properties. Accordingly, the Nb content is limited to at most 0.7 % by mass.
• Cu is an important element in the invention. Specifically, as so mentioned in the above, the invention takes advantage of the fine dispersion precipitation phenomenon of the ⁇ -Cu phase of the steel material to thereby enhance the strength thereof at around 600°C (from about 500 to 850°C) and to improve the thermal fatigue resistance thereof. In a high temperature range over 850°C, Cu further plays a role of assisting the high-temperature strength-enhancing effect of Nb and Mo, based on the solid solution enhancement with Cu. As a result of various studies, the Cu content must be at least more than 1 % by mass for satisfactorily attaining these effects. However, too much Cu worsens workability, low-temperature toughness and weldability, and therefore the uppermost limit of the Cu content is limited to 2 % by mass.
• Mo like Nb
• the high temperature strength in a high temperature range over 900°C must be increased, and Mo addition in an amount of at least 1 % by mass is indispensable.
• the invention must secure the steel texture condition where the total amount of Nb and Mo existing as a precipitation phase is at most 0.2 % by mass. Excess Mo addition may result in formation of carbide and Laves phase (Fe 2 Mo), thereby impairing high temperature strength and low-temperature toughness. Accordingly, the Mo content is limited to at most 2.5 % by mass.
• W is an element effective for increasing high temperature strength in a high temperature range over 900°C, and in the invention, the W content must be at least 1 % by mass. However, excess W addition impairs workability, and therefore, the W content must be at most 2.5 % by mass, more preferably at most 2 % by mass.
• Al is used as a deoxidizer in a steel making, and acts for improving high temperature oxidation resistance.
• too much Al addition has negative influences on surface properties, workability, weldability and low-temperature toughness. Accordingly, Al is added within a range of at most 0.15 % by mass.
• V contributes to improvement of high-temperature strength when added in combination with Nb and Cu.
• V improves workability, low-temperature toughness, resistance to grain boundary corrosion susceptibility, and toughness of weld heat affected zone
• V is added in the invention in an amount of at least 0.03 % by mass.
• excessive addition of V impairs workability and low-temperature toughness. Accordingly, the V content is limited to at most 0.2 % by mass.
• Ti and Zr are elements effective for improving high-temperature strength; and if desired, at least one of these may be added. However, excessive addition impairs toughness. In case where at least one of Ti and Zr is added, the total content thereof must be less than 1 % by mass.
• B and Co like Ni, are elements contributing to low-temperature toughness. If desired, one or two of B and Co may be added. However, excessive addition lowers cold elongation; and therefore, the B content is at most 0.02 % by mass and the Co content is at most 2 % by mass. More effectively, the B content is from 0.0005 to 0.02 % by mass.
• REM rare earth element
• Ca are elements that contribute to high-temperature oxidation resistance. If desired, at least one of these may be added. More effectively, the total content of REM and Ca is at least 0.001 % by mass. However, excessive addition thereof may have some negative influences on producibility, and therefore, the total content of REM and Ca is limited to at most 0.1 % by mass.
• the stainless steel material of the invention may be produced by preparing a stainless steel having a controlled composition as above according to an ordinary steel melting method, then working it into a steel sheet having a predetermined thickness according to an ordinary stainless steel sheet producing method, thereafter welding it into a pipe, or shaping and further working it.
• a controlled composition as above according to an ordinary steel melting method
• a steel sheet having a predetermined thickness according to an ordinary stainless steel sheet producing method
• welding it into a pipe or shaping and further working it.
• the steel could hardly have a texture condition where the total amount of Nb and Mo existing as a precipitation phase is at most 0.2 % by mass, and it may be difficult to enhance the high-temperature strength (0.2 % yield strength) of the steel material at 1000°C stably on a level of at least about 1.5 times that of SUS444. Under the condition, in addition, the low-temperature toughness of the steel material may also be lowered.
• Ferritic stainless steels shown in Table 1 were produced according to a steel melting method, and then worked into cold-rolled annealed steel sheets having a thickness of 2 mm according to a process of hot rolling, annealing of hot-rolled sheets, cold rolling and final annealing.
• the final annealing was attained under the condition as simulated for final annealing of steel materials for exhaust gas passage components.
• the final annealing condition was as follows: After heated at 1050°C with soaking for 1 minute, the steels except some comparative samples (such as No. 21) were cooled from 1000°C to 500°C at a mean cooling rate of at least 5°C/sec. The cooling rate was monitored with a thermocouple attached to the surface of each sample.
• the samples (after final annealing) were tested and analyzed for the total amount of Nb and Mo existing as a precipitation phase therein (this is expressed as "amount of precipitated Nb + precipitated Mo"), and the 0.2 % yield strength at 600°C, the 0.2 % yield strength at 1000°C, the low-temperature toughness and the cold workability thereof in the manner mentioned below.
• a sample is tested though constant potential electrolysis at a potential at which the matrix of the sample dissolves but the precipitation phase thereof does not dissolve, and the residue of the extracted precipitation phase is analyzed for elementary determination.
• the total mass of Nb and Mo in the residue is divided by the total mass of the dissolved matrix and the extracted precipitation phase in electrolysis, and this is expressed as percentage of the amount of precipitated Nb + precipitated Mo.
• used is 10 % acetylacetone + 1 % tetramethylammonium chloride + methyl alcohol solution as a water-free solvent.
• a test piece for tensile strength having a thickness of 2 mm (the pulling direction of the sample is the same as the rolling direction thereof) is tested for tensile strength at 600°C and tensile strength at 1000°C according to JIS G0567.
• a V-notch Charpy impact test piece is cut out of a sample having a thickness of 2 mm (the direction in which the test piece is hit with a hammer is in parallel to the rolling direction of the sample), and tested in a Charpy impact test at a pitch of 25°C within a range of from -75°C to 25°C according to JIS Z2242, thereby determining the ductility-toughness transition temperature of the sample.
• Samples having EL A of at least 30 % are rated G (good in point of the cold workability); and those having EL A of smaller than 30 % are rated NG (not good in point of the cold workability).
• Cooling rate in final annealing means the mean cooling rate from 1050°C to 500°C.
• Table 2 Steel No. Cooling Rate in Final Annealing (°C/sec) Amount of Precipitated Nb + Precipitated Mo (mass%) 0.2 % Yield Strength at 600°C (MPa) 0.2 % Yield Strength at 1000°C (MPa)
• No. 21 is not good, though its composition falls within the scope of the invention. This is because the cooling rate from 1000°C to 500°C in the final annealing was lower than 5°C/sec, and therefore a large amount of Nb and Mo precipitates formed during the cooling step thereby giving a texture condition in which the amount of precipitated Nb + precipitated Mo was too much.
• This comparative sample was poor in the high-temperature strength at 1000°C, the low-temperature toughness and the cold workability.
• the content of Mo and Nb was small; and in No. 23, the Cu content was additionally small. Since these do not satisfy the formulas (1) and (2), their high-temperature strength at 600°C and 1000°C was poor.
• No. 22 the content of Mo and Nb was small.
• the Nb content was high, and in the texture thereof, the amount of precipitated Nb + precipitated Mo was too much, and its cold workability was poor.
• the content of Mo and Nb was high, and in the texture thereof, the amount of precipitated Nb + precipitated Mo was too much, and its low-temperature toughness was poor.
• the Cu content was small, its high-temperature strength at 600°C was low.
• the Mo content was too high, and in the texture of thereof, the amount of precipitated Nb + precipitated Mo was too much.
• the high-temperature strength at 1000°C of the comparative sample was high, but the low-temperature toughness and the cold workability thereof were poor.

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• Mechanical Engineering (AREA)
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