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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • 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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/18—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
• • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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

Definitions

• the present invention relates to a ferritic stainless steel for use as exhaust gas path members such as typically exhaust manifolds, catalyst converter casings (outer casings), front pipes and center pipes, and to an automobile exhaust gas path member using the same.
• Patent References 1 and 2 disclose a ferritic stainless steel with approximately from 1 to 2 % by mass of Cu added thereto. Cu in the steel precipitates as a Cu phase when heated, and acts to enhance the high-temperature strength and the thermal fatigue resistance of the steel.
• the Cu-containing steel of the type is suitable to exhaust gas path members to be connected to engines of a type where the exhaust gas temperature is high.
• Patent Reference 1 WO03/004714
• Patent Reference 2 JP-A 2006-117985
• Patent References 1 and 2 For enhancing the high-temperature strength and the thermal fatigue resistance of ferritic stainless steel, there is known a method of adding a suitable amount of Cu to the steel, as in the above-mentioned Patent References 1 and 2; and in Patent Reference 2, in particular, there is employed a method of adding Nb to the steel in an amount of at most 0.6 % by mass for the purpose of enhancing the high-temperature strength of the steel in a high-temperature range over 700°C.
• Patent References 1 and 2 may have good thermal fatigue resistance when the maximum ultimate temperature thereof is high (for example, from 200 to 900°C), but when the maximum ultimate temperature thereof is low (for example, from 200 to 750°C), its thermal fatigue resistance is somewhat inferior to that of SUS444-type materials. Accordingly, the steel of Patent References 1 and 2 is suitable to automobiles with high-power engines mounted therein where the exhaust gas temperature is high, but is not suitable so much for application to automobiles with small-sized engines mounted therein where the exhaust gas temperature is relatively low. Depending on the mode of its use, the exhaust gas temperature in a high-power engine may vary, and therefore for the exhaust gas path members, it is desired to use a material having good thermal fatigue resistance even in a case where the maximum ultimate temperature is low.
• An object of the present invention is to provide a ferritic stainless steel capable of exhibiting excellent thermal fatigue resistance when applied to exhaust gas path members of both cases where the maximum ultimate temperature is high and low, and having excellent low-temperature toughness.
• the thermal fatigue resistance of steel can be enhanced through precipitation of a Cu phase therein.
• the thermal fatigue resistance of steel in a case where the maximum ultimate temperature is low for example, at around 750°C or lower can be enhanced through control of the precipitation morphology of Nb in steel.
• a ferritic stainless steel can be realized capable of being well applicable to both cases where the maximum ultimate temperature is high and low.
• the present invention provides a stainless steel for exhaust gas path members, which has a composition comprising, in terms of % by mass, at most 0.03 % of C, at most 1 % of Si, at most 1.5 % of Mn, at most 0.6 % of Ni, from 10 to 20 % of Cr, from more than 0.5 to 0.7 % of Nb, from 0.05 to 0.3 % of Ti, from more than 1 to 2 % of Cu, at most 0.2 % of V, at most 0.03 % of N, from 0.0005 to 0.02 % of B, and optionally at most 0.1 % of Al, and further optionally at least one of Mo, W, Zr and Co in an amount of at most 4 % in total, with a balance of Fe and inevitable impurities, and having an [Nb] value, as defined by the following formula (2) or (3) in accordance with the [Ti] value thereof as defined by the following formula (1), falling within a range of from 0.5 to 0.65, and which has a texture where the Cu phase having
• Preferred embodiments of the exhaust gas path members include, for example, automobile exhaust manifolds, catalyst converters, front pipes and center pipes. Needless-to-say, the members may be applicable to any other various exhaust gas path members than those of automobiles.
• the present invention has realized a ferritic stainless steel improved in point of both the thermal fatigue resistance thereof in a case where the maximum ultimate temperature is high (for example, 200 to 900°C) and the thermal fatigue resistance thereof in a case where the maximum ultimate temperature is low (for example, 200 to 750°C). Accordingly, the ferritic stainless steel of the invention is widely applicable to a case where the exhaust gas path members comprising it are used at a high exhaust gas temperature and to a case where they are used at a low exhaust gas temperature.
• the steel material satisfies the fundamental heat resistance (high-temperature oxidation resistance, high-temperature strength) required for automobile exhaust gas path members, and has excellent low-temperature toughness, and is therefore extremely useful for the recent exhaust gas path members that are required to be worked under severe conditions.
• the steel of the invention contains Cu and Nb, in which different types of precipitation phases of a Cu phase and an Nb compound phase are formed in actual service environments, and therefore, the steel exhibits excellent thermal fatigue resistance in both cases where the maximum ultimate temperature is high and low.
• the Cu phase is a so-called ⁇ -Cu precipitation phase, and this tends to readily grow in one direction and is generally in the form of rods.
• the Nb compound phase is a precipitation mainly comprising Fe 2 Nb, and when containing Mo, it generally has a morphology of Fe 2 (Mo,Nb).
• the Nb compound phase also tends to readily grow in one direction, and is generally in the form of rods. Accordingly, it is reasonable to evaluate the size of the precipitation phase as the major diameter thereof. Concretely, the major diameter of the precipitation appearing in the image projected through a transmission electronic microscope (TEM) (corresponding to the projection length in the image picture) may be taken as the major diameter of the phase.
• TEM transmission electronic microscope
• the projected phase may be identified with an analyzer (e.g., EDX) attached to TEM.
• an analyzer e.g., EDX
• Nb carbides and Nb nitrides are excluded from the Nb compound phase as referred to herein.
• the carbides and the nitrides are often in the form of masses or spheres, and from their forms, they may be relatively readily differentiated from the Fe 2 Nb-type precipitation phase.
• the phases may be identified with the above-mentioned analyzer (e.g., EDX).
• the maximum ultimate temperature in use of the steel is around 900°C or higher than it, Cu therein may sufficiently re-dissolve as a solid solution therein by that heating, therefore precipitating a fine Cu phase essentially at 500 to 700°C. Accordingly, the fatigue resistance in repeated heating (that is, the thermal fatigue resistance) of the steel can be thereby enhanced.
• the maximum ultimate temperature is at most around 750°C and is low, Cu could not sufficiently re-dissolve in the steel. Accordingly, the steel could not take a sufficient effect of enhancing the thermal fatigue resistance thereof owing to the fine precipitation of the Cu phase therein.
• the thermal fatigue resistance of the steel in the case where the maximum ultimate temperature is low and the Cu phase could not sufficiently improve the resistance by itself is compensated by the fine precipitation of the Nb compound phase in the steel.
• the Nb compound phase brings about precipitation reinforcement by heating at 700 to 750°C though within an extremely short period of time. It has been found that the precipitation reinforcement phenomenon within the short period of time remarkably enhances the thermal fatigue resistance of the steel in a temperature range of from 200 to 750°C.
• the steel could be free from balging to be caused by Ratchetting deformation or compression stress in the initial stage of repeated heating, and this could be advantageous for the thermal fatigue resistance of the steel where the maximum ultimate temperature is low.
• C and N are generally said to be effective for enhancing the creep strength and other high-temperature strength of steel; however, when steel contains them excessively, then the oxidation resistance, the workability, the low-temperature toughness and the weldability of the steel worsen.
• both C and N are limited to a content of at most 0.03 % by mass each.
• Si is effective for enhancing the high-temperature oxidation resistance of steel. In addition, it bonds to oxygen in the atmosphere in welding, therefore having the effect of preventing oxygen from penetrating into steel. However, when the Si content is excessive, then the hardness of the steel may increase and the workability and the low-temperature toughness thereof may worsen.
• the Si content is limited to at most 1 % by mass, and for example, it may be limited to a range of from 0.1 to 0.6 % by mass.
• Mn enhances the high-temperature oxidation resistance, especially the scale peeling resistance of steel.
• Mn is an austenite-stabilizing element, a martensite phase may be readily formed when it is added too much, therefore causing a factor of reducing the workability of steel.
• the Mn content is limited to at most 1.5 % by mass, more preferably at most 1.3 % by mass.
• the element may be defined to be from 0.1 to less than 1 % by mass.
• Ni is an austenite-stabilizing element, and when added excessively, it causes the formation of a martensite phase like Mn, therefore bringing about a factor of reducing the workability of steel.
• the Ni content is defined to be up to at most 0.6 % by mass.
• the Cr content is to be from 10 to 20 % by mass.
• the Cr content is controlled in accordance with the temperature in use of the material. For example, in case where high-temperature oxidation resistance up to 950°C is desired, the Cr content is preferably at least 16 % by mass; and when the resistance up to 900°C is desired, the Cr content may be from 12 to 16 % by mass.
• Nb is an element extremely effective for securing the high-temperature strength of steel in a high-temperature range over 700°C. It is believed that the Nb solid solution reinforcement in this composition system may greatly contribute toward the high-temperature strength enhancement. Nb fixes C and N and is therefore effective for preventing toughness reduction in steel. These effects of Nb are heretofore general ones; and further in the invention, the steel takes advantage of the fine precipitation of the Nb compound phase therein, therefore having another advantage of thermal fatigue resistance enhancement in a case where the maximum ultimate temperature is around at most 750°C and is low (as described in the above). For sufficiently securing the effect of Nb, the Nb content must be more than 0.5 % by mass, more effectively more than 0.6 % by mass. However, too much addition of Nb worsens the workability and the low-temperature toughness of steel, and increases the high-temperature cracking sensitivity in welding; and therefore, the Nb content is limited up to at most 0.7 % by mass.
• Nb readily bonds to C and N.
• the [Nb] value of the composition as defined by the following formula (2) or (3) in accordance with the [Ti] value thereof as defined by the following formula (1), or that is, the effective Nb amount in the composition is specifically defined.
• the composition secures the Ti content over the amount capable of bonding to C and N, or that is, when the effective Ti amount [Ti] is not less than 0, then the Nb content value can be directly taken as the effective Nb amount [Nb] as in the formula (2).
• the composition when the effective Ti amount [Ti] is less than 0, then the composition must secure an Nb content enough to compensate the effective Ti amount, and therefore, as in the formula (3), a value smaller than the Nb content is taken as the effective Nb amount [Nb].
• the Nb content is defined to fall within a range of from more than 0.5 to 0.7 % by mass, and in addition, the effective Nb amount [Nb] is defined to fall within a range of from 0.5 to 0. 65.
• the Nb content is severely defined within an extremely narrow range, and this is important to enhance the thermal fatigue resistance of the steel in a case where the maximum ultimate temperature is low, in addition to enhancing the high-temperature strength and the low-temperature toughness thereof.
• Ti generally fixes C and N and is effective for enhancing the shapability of steel and preventing the toughness reduction in steel.
• the Ti content must also be severely controlled from the viewpoint of securing the effective Nb amount as so mentioned in the above.
• the Ti content must be at least 0.05 % by mass.
• addition of excessive Ti may worsen the surface property of steel owing to the formation of a large quantity of TiN, and may further have some negative influences on the weldability and the low-temperature toughness of steel. Accordingly, the Ti content is defined to be from 0.05 to 0.3 % by mass.
• Al is a deoxidizer, and is an element that acts for enhancing the high-temperature oxidation resistance of steel.
• Al may be in the steel in a range of at most 0.1 % by mass. Excessive Al, if any in steel, may form a large quantity of oxides in welding, and may act as a starting point of working cracks.
• the invention takes advantage of the fine dispersion precipitation phenomenon of the Cu phase of steel to thereby enhance the strength thereof at from 500 to 700°C especially when the maximum ultimate temperature is not lower than around 900°C and is high. Accordingly, the Cu content must be more than 1 % by mass. However, too much Cu, if any in steel, may worsen the workability, the low-temperature toughness and the weldability of steel, and therefore, the Cu content is limited to be at most 2 % by mass.
• V contributes to enhancing the high-temperature strength of steel when added in combination with Nb and Cu.
• V improves the workability, the low-temperature toughness, the resistance to grain boundary corrosion susceptibility, and the toughness of weld heat affected regions of steel.
• excessive addition of V rather impairs the workability and the low-temperature toughness of steel.
• the V content is limited to at most 0.2 % by mass.
• the V content is from 0.01 to 0.2 % by mass, more preferably from 0.03 to 0.15 % by mass.
• the B is effective in enhancing the resistance to secondary working brittleness of steel.
• the mechanism thereof is assumed to be due to the decrease in the solid solution C in grain boundaries or due to the strengthening of the grain boundaries.
• the addition of excessive B worsens the producibility and the weldability of steel.
• the B content is from 0.0005 to 0.02 % by mass.
• Mo, W, Zr and Co are effective for enhancing the high-temperature strength of the ferritic stainless steel comprising the composition of the invention. If desired, at least one of these may be added to steel. However, too much addition may make the steel brittle, and therefore, when these elements are added, their total content is defined to be at most 4 % by mass. More effectively, the total content is from 0.5 to 4 % by mass.
• the ferritic stainless steel having the above-mentioned composition can be produced according to an ordinary stainless steel smelting and casting process, and then, for example, this may be processed according to a cycle of "hot rolling ⁇ annealing ⁇ washing with acid” and optionally further according to one or more cycles of "cold rolling ⁇ annealing ⁇ washing with acid", thereby giving an annealed steel sheet having a thickness of, for example, from 1 to 2.5 mm or so.
• the steel material is heated at from 950 to 1100°C, preferably at from 1000 to 1100°C, and thereafter the mean cooling speed thereof from 1000 to 700°C as a temperature range for Nb compound phase precipitation (when the heating temperature is lower than 1000°C, the mean cooling speed is from that heating temperature to 700°C) is from more than 30 to 100°C/sec, and the mean cooling speed from 700 to 400°C as a temperature range for Cu phase precipitation is from 5 to 50°C/sec.
• a steel material (annealed steel sheet) can be obtained, having a texture condition where the Cu phase having a major diameter of at least 0.5 ⁇ m is controlled to be in an amount of at most 10 grains/25 ⁇ m 2 and the Nb compound phase having a major diameter of at least 0. 5 ⁇ m is in an amount of at most 10 grains/25 ⁇ m 2 .
• "Final annealing” as referred to herein is the final annealing to be carried out in a process of producing a steel material.
• an exhaust gas path member is constructed.
• the tubular member is constructed as follows: The annealed steel sheet is roll-formed into a predetermined tubular form, then the butting portions of the two forms are welded to give a tube.
• employable is any known tube-forming welding method of TIG welding, laser welding, high-frequency welding or the like.
• the steel tube is, if desired, processed for heat treatment and washing with acid, and then worked into an exhaust gas path member.
• a ferritic stainless steel having the composition shown in Table 1 was produced through smelting and casting, and then processed according to a process of "hot rolling ⁇ annealing/washing with acid ⁇ cold rolling ⁇ final annealing/washing with acid" to give an annealed steel sheet having a thickness of 2 mm.
• a part of the cast slab is hot-forged into a round rod having a diameter of about 25 mm, and this was processed for final annealing.
• the final annealing of the sheet and the final annealing of the rod were as follows, except for Steel No. 19.
• the sheet or the rod was soaked at 1050°C for 1 minute, and then cooled from 1000°C to 700°C at a mean cooling speed of from more than 30 to 100°C/sec, and further from 700°C to 400°C at a mean cooling speed of from 5 to 50°C/sec.
• the final annealing of Steel No. 19 was as follows: From 1000°C to 700°C, the mean cooling speed was controlled to be from 10 to 20°C/sec, and the other condition was the same as that of the other samples (common to both the sheet material and the rod material).
• the rolling direction of the sheet material and the longitudinal direction of the rod material are referred to as L direction.
• the sheet material and the rod material were analyzed for the metal texture in the cross direction thereof cut perpendicularly to the L direction.
• TEM transmission electronic microscope
• the Cu phase and the Nb compound phase were analyzed for their size, and the number of the Cu phase grains and the Nb compound phase grains having a major diameter of at least 0. 5 ⁇ m seen per 25 ⁇ m 2 was counted. At least 10 viewing sites were analyzed in one sample, and the data were averaged.
• the type of the precipitation phase was identified by quantifying Fe, Nb, Mo and Cu with EDX (energy dispersion fluorescent X-ray analyzer) attached to TEM.
• the constitutive elements of the steel base were detected together with the phase-constituting elements. Therefore, of the found data of the above-mentioned four elements to which the precipitation phase was targeted, the value of at least 50 % by mass for Cu was identified as the Cu phase; and the value of at least 30 % by mass of Nb was identified as the Nb compound phase.
• the samples where the Cu phase having a major diameter of at least 0.5 ⁇ m was in an amount of at most 10 grains/25 ⁇ m 2 are O (good), and the others are x (not good), as in the column of the Cu phase in Table 2.
• the sheet material was tested in an impact test for the low-temperature toughness thereof.
• a V-notched impact test piece was so collected that the direction in which the impact is to be given to the piece could be the rolling direction of the sheet.
• the samples of which the transition temperature was lower than -25°C (the samples showing ductile fracture even at -25°C) were evaluated as O (good), and the others were evaluated as ⁇ (not good).
• a heat cycle of "200°C ⁇ 0.5 minutes soaking ⁇ heating up to 750°C at a heating speed of about 3°C/sec ⁇ 750°C ⁇ 2.0 minutes soaking ⁇ cooling to 200°C at a cooling speed of about 3°C/sec” with a restraint ratio (ratio of imparted strain to thermal expansion) of 25 % was repeated in every sample.
• the samples having a thermal fatigue life of at least 1800 cycles were evaluated as O (good) ; those having a thermal fatigue life of from 1500 cycles to less than 1800 cycles were evaluated as ⁇ (relatively not good) ; and those having a thermal fatigue life of less than 1500 cycles were evaluated as ⁇ (not good).
• the samples with the evaluation "O" passed the test.
• a heat cycle of "200°C ⁇ 0.5 minutes soaking ⁇ heating up to 900°C at a heating speed of about 3°C/sec ⁇ 900°C x 0.5 minutes soaking ⁇ cooling to 200°C at a cooling speed of about 3°C/sec” with a restraint ratio (ratio of imparted strain to thermal expansion) of 20 % was repeated in every sample.
• the samples having a thermal fatigue life of at least 900 cycles were evaluated as ⁇ (good) ; and those having a thermal fatigue life of less than 900 cycles were evaluated as ⁇ (not good).
• the samples with the evaluation " ⁇ " passed the test.
• Table 2 confirms that the samples of the invention, having the chemical composition as defined in the invention and satisfying the precipitation morphology of the Cu phase and the Nb compound phase, were improved in point of both the thermal fatigue resistance under the high maximum ultimate temperature condition (200 to 900°C) and the thermal fatigue resistance under the low maximum ultimate temperature condition (200 to 750°C), and had good low-temperature toughness.
• No. 19 is a steel having the composition defined in the invention; however, in its final annealing, the cooling speed within the temperature range for Nb compound phase precipitation was too late, and coarse Nb compound phase grains were formed. As a result, in the following heating process, fine Nb compound phase grains could not be sufficiently precipitated in this, and therefore the thermal fatigue resistance at 200 to 750°C of this sample was poor. In addition, owing to the influence of the coarse Nb compound phase grains thereon, the low-temperature toughness of the sample was also poor.

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• 2007
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