JP2011190468A - Ferritic stainless steel sheet superior in heat resistance, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel sheet which has high heat resistance even at a high temperature exceeding 850°C, and to provide a method for manufacturing the same. <P>SOLUTION: The ferritic stainless steel sheet superior in heat resistance includes, by mass%, 0.015% or less C, 0.020% or less N, more than 0.10 to 0.40% Si, 0.10-1.00% Mn, 16.5-25.0% Cr, 0.30-0.80% Nb, 1.00-4.00% Mo, 0.05-0.50% Ti, 0.0003-0.0030% B and 1.0-2.5% Cu; and has a structure in which carbonitrides having particle diameters of 0.2 μm or less among carbonitrides containing Nb in the steel as a main phase occupy 95% or more by a number ratio. Thereby, the stainless steel sheet can remarkably enhance its thermal fatigue strength at a temperature exceeding 850°C. A cooling rate from a final annealing temperature to 750°C is preferably 7°C/sec or higher. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a ferritic stainless steel sheet having excellent heat resistance, which is optimal for use in an exhaust system member that requires heat resistance, particularly thermal fatigue characteristics.

  Exhaust system members such as automobile exhaust manifolds pass high-temperature exhaust gas exhausted from the engine, so the materials that make up the exhaust members are required to have various characteristics such as high-temperature strength, oxidation resistance, and thermal fatigue properties. Ferritic stainless steel with excellent heat resistance is used.

  Although the exhaust gas temperature varies depending on the vehicle type, in recent years, the exhaust gas temperature is often about 750 to 850 ° C., and the temperature of the exhaust manifold through which the high-temperature exhaust gas discharged from the engine passes is about the same. However, due to the recent increase in environmental problems, exhaust gas regulations have been further strengthened and fuel efficiency has been improved, and the exhaust gas temperature is considered to further increase to nearly 1000 ° C.

  Ferritic stainless steels used in recent years include SUS429 (Nb-Si-added steel) and SUS444 (Nb-Mo-added steel). Based on Nb addition, high temperature strength is improved by adding Si and Mo. It is. Among them, SUS444 has the highest strength because about 2% of Mo is added. However, SUS444 cannot cope with an exhaust gas temperature exceeding 850 ° C., and a ferritic stainless steel having heat resistance higher than SUS444 is demanded.

In response to such demands, various exhaust system member materials have been developed. For example, in Patent Document 1, in order to improve thermal fatigue characteristics, the number of Cu phases having a major axis of 0.5 μm or more is controlled to 10/25 μm ² or less, and the number of Nb compound phases having a major axis of 0.5 μm or more is controlled to 10/25 μm ² or less. However, only coarse precipitates of Laves phase and ε-Cu phase are defined, and no other fine precipitates are disclosed. In Patent Documents 2 and 3, by defining the amount of precipitates, in addition to solid solution strengthening of Nb and Mo, solid solution strengthening of Cu and precipitation strengthening by Cu precipitates (ε-Cu phase), high temperature of SUS444 or higher. Although a method for increasing the strength is disclosed, the relationship between the size of the precipitate and the high temperature strength is not disclosed. Patent Documents 4 and 5 disclose techniques for adding W in addition to adding Nb, Mo, and Cu. Patent Document 4 discloses a relationship between a Laves phase or an ε-Cu phase as a precipitate and high-temperature strength, but does not disclose a relationship with carbonitride. In Patent Document 5, B is further added, but it is for improving workability, not from the viewpoint of control of precipitates and heat resistance.

  Inventors have recently disclosed a technique in which a Laves phase and an ε-Cu phase are finely dispersed by composite addition of Nb—Mo—Cu—Ti—B to obtain excellent high temperature strength at 850 ° C. in Patent Document 6. is doing.

  From the above, the conventional knowledge about the precipitate control for improving the heat resistance of the exhaust system member is mainly related to the Laves phase and the ε-Cu phase, and there is no disclosure of the knowledge of the precipitate control of the Nb carbonitride. On the other hand, as disclosed in Non-Patent Document 1, it is known that when Ti is added to Nb—Mo steel, the precipitation form of carbonitride and Laves phases changes.

JP 2008-189974 A JP 2009-120893 A JP 2009-120894 A JP 2009-197307 A JP 2009-197306 A JP 2009-215648 A

ISIJ International, 43 (2003), 1999.

  In conventional ferritic stainless steel, when used in a temperature range higher than 850 ° C., for example, even the technique of Patent Document 6 of the prior art can stably obtain sufficient heat resistance, particularly thermal fatigue characteristics. It turns out that there may not be.

  Therefore, the inventors of the present invention focused on the Nb carbonitride precipitate form in addition to the Laves phase, and as a result of intensive studies, they have obtained the following new findings as problems to be solved.

The Laves phase generally precipitates as Fe ₂ (Nb, Mo), resulting in a reduction in the amount of solid solution Nb and Mo. Patent Document 6 describes that fine precipitation is caused by the effect of B and contributes to precipitation strengthening. The present inventors have found that when a coarse Nb carbonitride exists, a large number of Laves phases are precipitated starting from the Nb carbonitride. To find out that coarse Nb carbonitrides not only reduce the solid solution amount of Nb and Mo, but also become coarse Laves phases starting from Nb carbonitride and do not contribute to precipitation strengthening. It came.

  The present invention has been made in view of such knowledge, and provides a ferritic stainless steel having heat resistance exceeding 850 ° C. higher than that of the prior art by controlling the precipitate form of Nb carbonitride. It is an object to do.

  In order to solve the above-mentioned problems, the present inventors consider the operating temperature range in the exhaust manifold, and in the temperature range of 750 to 950 ° C., a large amount of precipitates precipitate and grow. Various studies were conducted with the aim of maximizing the effects of solid solution and precipitation strengthening by controlling the precipitates of the Laves phase and carbonitride containing Nb as the main phase more precisely than the conventional technology. It was. As a result, it has been found that fine precipitation of carbonitrides containing Nb as the main phase is effective for maintaining solid solution strengthening in Nb—Mo—Cu—Ti—B composite added steel. Here, the carbonitride having Nb as the main phase is (Nb, X) (C, N) having Nb as the main phase, and is hereinafter referred to as Nb carbonitride. X contains other elements (such as Ti). Further, Nb as the main phase means that the mass of Nb is more than 50% when the mass ratios of Nb and X are compared. Specifically, Fe, Nb, Mo, Ti is quantified with an EDS apparatus (energy dispersive X-ray fluorescence spectrometer) attached to TEM, and when Fe and Mo are less than 5 mass% in carbonitride, It can be judged to be a carbonitride having Nb as the main phase.

  FIG. 1 shows 16.7% Cr-0.007% C-0.38% Si-0.70% Mn-1.7% Mo-1.3% Cu-0.64% Nb-0.15% Ti -0.010% N-0.0003% B steel particle size and the ratio of the Laves phase precipitated on the Nb carbonitride in the case of aging heat treatment at 950 ° C for 5 minutes are shown. It can be seen that as the particle size increases, the proportion of the Laves phase that precipitates on the Nb carbonitride increases, and increases rapidly when the particle size exceeds 0.2 μm. Also, FIG. 2 shows that 19.2% Cr-0.004% C-0.15% Si-0.33% Mn-2.1% Mo-1.2% Cu-0.40% Nb-0.11. % Ti-0.012% N-0.0026% B of Nb carbonitride with an average cooling rate from 1050 ° C. to 750 ° C., which is the final annealing temperature, of 0.2 μm or less It is the result which showed the relationship with existence probability (number ratio). It can be seen that the cooling rate is 7 ° C./sec or more, and among Nb carbonitrides, those having a particle size of 0.2 μm or less are 95% or more in terms of the number ratio. Further, in FIG. 3, 19.2% Cr-0.004% C-0.15% Si-0.33% Mn-2.1% Mo-1.2% Cu-0.40% Nb-0.11. Results showing the relationship between the probability of existence of Nb carbonitrides of 0.2 μm or less in% Ti-0.012% N-0.0026% B steel and the thermal fatigue life (constraint rate 20%) at a maximum temperature of 950 ° C. It is. It can be seen that, among Nb carbonitrides, those having a particle size of 0.2 μm or less are 95% or more in terms of the number ratio, so that the thermal fatigue life is remarkably improved.

  The mechanism by which a large number of Laves phases precipitate starting from Nb carbonitrides of a certain size or larger is not clear, but the interface becomes inconsistent due to the coarsening of Nb carbonitrides, and the energy of the interface increases to increase the nucleus of the Laves phase It is presumed that it will become a generation site.

  Further, in order to prevent precipitation of Nb carbonitride exceeding 0.2 μm in Nb—Mo—Cu—Ti—B composite added steel, the final annealing temperature is set to 1000 to 1200 ° C. in a normal stainless steel manufacturing method, It has been found that precipitation and coarsening of Nb carbonitride can be suppressed by controlling the cooling rate from the final annealing temperature to 750 ° C. to 7 ° C./sec or more.

  From these results, the solid solution strengthening ability of Nb and Mo can be maintained by controlling the cooling rate at the time of final annealing and having a structure in which the particle diameter of Nb carbonitride is 0.2 μm or less. Furthermore, it has been found that for precipitation of the Laves phase and the ε-Cu phase, the effect of fine precipitation by B disclosed in Patent Document 6 can be exhibited even at higher temperatures exceeding 850 ° C.

  As described above, the present invention has found an effect different from that of the conventional invention in the effect of finely depositing Nb carbonitride, and has led to the present invention that improves the thermal fatigue life. Then, by controlling the cooling rate from the final annealing temperature to 750 ° C., the Nb carbonitride is refined to refine the precipitation of the Laves phase, and to maximize the solid solution strengthening and precipitation strengthening. The present invention provides a ferritic stainless steel sheet having excellent properties.

The gist of the present invention is as follows.
(1) In mass%, C: 0.015% or less, N: 0.020% or less, Si: more than 0.10 to 0.40%, Mn: 0.10 to 1.00%, Cr: 16 0.5 to 25.0%, Nb: 0.30 to 0.80%, Mo: 1.00 to 4.00%, Ti: 0.05 to 0.50%, B: 0.0003 to 0.0030 %, Cu: 1.0 to 2.5%, the balance being Fe and inevitable impurities, among the carbonitrides whose main phase is Nb in steel, the particle diameter is 0.2 μm or less Is a ferritic stainless steel sheet having a structure in which the number ratio is 95% or more and excellent in heat resistance.
(2) The ferritic stainless steel sheet having excellent heat resistance according to claim 1, characterized by containing, in mass%, W: 3.00% or less.
(3) One or more of Al: 3.00% or less, Sn: 1.00% or less, and V: 0.10 to 1.00% by mass%, 2. A ferritic stainless steel sheet having excellent heat resistance according to 2.
(4) It is characterized by containing one or more of Zr: 1.00% or less, Hf: 1.00% or less, Ta: 3.00% or less, Mg: 0.0100% or less in mass%. The ferritic stainless steel sheet excellent in heat resistance according to any one of claims 1 to 3.
(5) The final annealing temperature is 1000 to 1200 ° C., and the cooling rate from the final annealing temperature to 750 ° C. is 7 ° C./sec or more. Of ferritic stainless steel sheet with excellent properties.

  Here, for the case where the lower limit is not specified, it indicates that an inevitable impurity level is included.

  According to the present invention, it is possible to provide a ferritic stainless steel having a high temperature characteristic of SUS444 or higher, that is, a thermal fatigue characteristic at 950 ° C. equal to or higher than that of SUS444. In particular, by applying it to exhaust system members such as automobiles, it is possible to cope with the exhaust gas temperature exceeding 850 ° C., that is, 950 ° C.

Results showing the particle size of Nb carbonitride of the aging material at 950 ° C. × 5 min and the ratio of the Laves phase deposited on the Nb carbonitride The result which showed the relationship between the average cooling rate to 1050-750 degreeC and the existence probability of Nb carbonitride of 0.2 micrometer or less Results showing the relationship between the probability of existence of Nb carbonitrides of 0.2 μm or less and the thermal fatigue life (restraint rate 20%) with a maximum temperature of 950 ° C.

  Hereinafter, the present invention will be described in detail. First, the reasons for limiting the present invention will be described.

  C deteriorates formability and corrosion resistance, promotes precipitation of Nb carbonitride and causes a decrease in high-temperature strength. Therefore, the lower the content, the better. Therefore, the content was made 0.015% or less. However, excessive reduction leads to an increase in refining costs, so 0.003% to 0.015% is made a preferable range.

  N, like C, deteriorates formability and corrosion resistance, promotes precipitation of Nb carbonitride and causes a decrease in high-temperature strength. Therefore, the smaller the content, the better. Therefore, the content was made 0.020% or less. However, excessive reduction leads to an increase in refining cost, so 0.005 to 0.020% is made a preferable range.

  Si is an element useful as a deoxidizer, but is an extremely important element for improving oxidation resistance. However, with regard to the high temperature strength, Si promotes precipitation of intermetallic compounds mainly composed of Fe, Nb, and Mo called the Laves phase at high temperatures. Further, regarding oxidation resistance, when the Si addition amount is 0.10% or less, abnormal oxidation tends to occur, and when it exceeds 0.40%, scale peeling tends to occur, so that it exceeds 0.10% to 0 40%. However, assuming that a factor that deteriorates the oxidation resistance such as generation of surface flaws is added, it is preferable that there is a margin in the oxidation resistance, and in this case, more than 0.10% to 0.30% is desirable.

  Mn is an element added as a deoxidizer, and forms a Mn-based oxide on the surface layer during long-time use, contributing to scale adhesion and suppression of abnormal oxidation. The effect is manifested at 0.10% or more. On the other hand, excessive addition exceeding 1.00% lowers the uniform elongation at normal temperature, forms MnS, lowers the corrosion resistance, and brings about deterioration of oxidation resistance. From these viewpoints, the upper limit was made 1.00%. Furthermore, considering high temperature ductility and scale adhesion, 0.10 to 0.70% is desirable.

  Cr is an essential element for ensuring oxidation resistance in the present invention. If it is less than 16.5%, the effect is not exhibited, and if it exceeds 25.0%, the workability is deteriorated or the toughness is deteriorated, so the content was made 16.5 to 25.0%. Furthermore, if considering high temperature ductility and manufacturing cost, 17.0 to 19.0% is desirable.

  Nb is an element necessary for improving high-temperature strength by solid solution strengthening and precipitation strengthening by fine precipitation of the Laves phase (an effect is remarkably exhibited by refining Nb carbonitride). In addition, C and N are fixed as carbonitrides, contributing to the development of the recrystallization texture that affects the corrosion resistance and r value of the product plate. In the Nb—Mo—Ti—B added steel of the present invention, the solid solution Nb increase and precipitation strengthening by the addition of B can be obtained by adding 0.30% or more of Nb, so the lower limit was made 0.30%. Moreover, excessive addition exceeding 0.80% promotes the coarsening of the Laves phase, does not contribute to high temperature strength and thermal fatigue life, and increases costs, so the upper limit was made 0.80%. Furthermore, if manufacturability and cost are taken into consideration, 0.40 to 0.70% is desirable.

  Mo improves corrosion resistance, suppresses high-temperature oxidation, and is effective for precipitation strengthening by fine precipitation of the Laves phase and high-temperature strength improvement by solid solution strengthening. However, excessive addition promotes coarse precipitation of the Laves phase, lowers precipitation strengthening ability, and degrades workability. In the present invention, in the Nb—Mo—B added steel, an increase in solid solution Mo due to the addition of B can be obtained by adding 1.00% or more of Mo, so the lower limit was made 1.00%. Excessive addition over 4.00% promotes coarsening of the Laves phase, does not contribute to high temperature strength and thermal fatigue life, and increases costs, so the upper limit was made 4.00%. Furthermore, if considering the manufacturability and cost, 1.50 to 3.00% is desirable.

  By adding an appropriate amount of Ti to Nb-Mo-Ti-B steel, Ti increases the amount of Nb and Mo during cold-rolled annealing, improves high-temperature strength, and improves high-temperature ductility, and improves thermal fatigue properties. It is an important element. This effect is manifested from 0.05% or more, but addition of more than 0.50% increases the amount of solid solution Ti and lowers uniform elongation, and forms coarse Ti-based precipitates during processing. And it becomes the starting point of the crack at the time of the thermal fatigue test, and deteriorates the workability and thermal fatigue characteristics, so the upper limit was made 0.50%. Furthermore, considering the occurrence of surface flaws and toughness, 0.08 to 0.15% is desirable.

  In the present invention, B is an important element that contributes to the stability of high-temperature strength and thermal fatigue life by reducing the amount of Nb and Mo-based precipitates by adding Nb—Mo—Ti—B as described above. Furthermore, it is also an element that improves the secondary workability during product press working. These effects are manifested at 0.0003% or more, but excessive addition degrades hardening and intergranular corrosion, and also causes weld cracks and deteriorates thermal fatigue properties. 0030%. Furthermore, if considering moldability and manufacturing cost, 0.0003 to 0.0020% is desirable.

  Cu is an element effective for improving high-temperature strength. This is a precipitation hardening effect caused by the precipitation of ε-Cu, and is remarkably exhibited by addition of 1.0% or more. On the other hand, excessive addition causes a reduction in uniform elongation and excessively high room temperature proof stress, which impairs press formability. Further, when added in an amount of 2.5% or more, an austenite phase is formed in a high temperature range, abnormal oxidation occurs on the surface, and the upper limit is set to 2.5% in order to further deteriorate the thermal fatigue characteristics. Considering manufacturability and scale adhesion, 1.2 to 2.0% is desirable.

  Nb carbonitride has a number of Laves phases precipitated at the interface of Nb carbonitride when the particle size exceeds 0.2 μm, which causes a decrease in the solid solution strengthening amount of Nb and Mo and a decrease in the precipitation strengthening amount of the Laves phase. Therefore, among the Nb carbonitrides, those having a particle size of 0.2 μm or less need to be 95% or more by number ratio. In this state, the Laves phase in the grains is precipitated mainly from a place other than Nb carbonitride and contributes as precipitation strengthening. The particle diameter of Nb carbonitride was determined by quantifying Fe, Nb, Mo, and Ti with an EDS apparatus (energy dispersive X-ray fluorescence spectrometer) attached to TEM, and Fe and Mo were less than 5 mass% in carbonitride. In some cases, Nb carbonitrides are judged to be Nb carbonitrides, and the area of 300 Nb carbonitrides is determined by image analysis, and the equivalent circle diameter can be calculated from the area.

  In order to further improve various properties such as high-temperature strength, the following elements may be added.

  W is an element having the same effect as Mo and improving the high temperature strength. This effect is stably manifested from 0.10% or more, but if added excessively, it dissolves in the Laves phase, coarsening precipitates and degrading manufacturability and workability. -3.00% is preferable. Furthermore, considering cost, oxidation resistance, etc., 1.00 to 1.80% is desirable.

  In addition to being added as a deoxidizing element, Al is an element that improves oxidation resistance. It is also useful for improving the strength as a solid solution strengthening element. Although its action is stably manifested from 0.10%, excessive addition hardens it to significantly reduce uniform elongation and toughness to remarkably decrease, so the upper limit was made 3.00%. Furthermore, considering the occurrence of surface defects, weldability, and manufacturability, 0.10 to 2.00% is desirable. When Al is added for the purpose of deoxidation, less than 0.10% of Al remains in the steel as an inevitable impurity.

  Sn is an element having a large atomic radius and effective for solid solution strengthening, and does not greatly deteriorate the mechanical properties at room temperature. The contribution to high-temperature strength is stable at 0.05% or more, but if added over 1.00%, manufacturability and workability deteriorate significantly, so the content was made 1.00% or less. Furthermore, if considering oxidation resistance and the like, 0.05 to 0.50% is desirable.

  V is combined with Nb to form fine carbonitrides, and a precipitation strengthening effect is generated, contributing to an improvement in high temperature strength. This effect is stable when 0.10% or more is added, but if added over 1.00%, Nb carbonitride (Nb, V) (C, N) becomes coarse and the high-temperature strength decreases. Since the thermal fatigue life and workability are lowered, the upper limit was made 1.00%. Furthermore, considering the manufacturing cost and manufacturability, 0.10 to 0.50% is desirable.

  Zr is an element that improves oxidation resistance, and exhibits an effect stably when added in an amount of 0.05% or more. However, the addition of more than 1.00% causes a coarse Laves phase to precipitate, resulting in significant deterioration in manufacturability and workability. Furthermore, if considering cost and surface quality, 0.05 to 0.50% is desirable.

  Hf, like Zr, is an element that improves oxidation resistance, and exhibits an effect stably when added in an amount of 0.05% or more. However, the addition of more than 1.00% causes a coarse Laves phase to precipitate, resulting in significant deterioration in manufacturability and workability. Furthermore, if considering cost and surface quality, 0.05 to 0.50% is desirable.

  Ta, like Zr and Hf, is an element that improves oxidation resistance, and exhibits an effect stably when added in an amount of 0.05% or more. However, the addition of more than 3.00% causes a coarse Laves phase to precipitate, resulting in significant deterioration in manufacturability and workability. Furthermore, if considering cost and surface quality, 0.05 to 1.00% is desirable.

  Mg is an element that improves secondary workability, and exhibits an effect stably by adding 0.0003% or more. However, if adding over 0.0100%, the workability is remarkably deteriorated, so 0.0003 to 0.0100% is preferable. Furthermore, if considering cost and surface quality, 0.0003 to 0.0020% is desirable.

  After producing a steel ingot by melting, a hot rolled sheet is produced by hot rolling, pickling, cold rolling and annealing, and in a normal ferritic stainless steel manufacturing method, among Nb carbonitrides, the particle size In order to obtain a structure having a number ratio of not more than 0.2 μm and a number ratio of 95% or more, the final annealing temperature is 1000 to 1200 ° C. and heating is performed in a soaking period of 0 to 20 minutes, and then from the final annealing temperature to 750 ° C. It is necessary to control the average cooling rate to 7 ° C./sec or more. Here, the particle diameter of the Nb carbonitride is obtained by calculating the area of 300 intragranular carbonitrides by image analysis from the photograph taken by TEM and calculating the equivalent circle diameter from the area. If the average cooling rate is controlled to 7 ° C./sec or more, Nb carbonitrides having a particle size of 0.2 μm or less will be 95% or more in terms of the number ratio, maintaining the solid solution strengthening of Nb and Mo, Even when Laves is precipitated, precipitation strengthening due to fine precipitation of Laves acts and the thermal fatigue life is improved. The larger the cooling rate, the smaller the particle size of Nb carbonitride. However, the cooling rate is preferably 7 to 25 ° C./sec in consideration of surface quality, steel plate shape and production cost.

  Moreover, since the solid solution of Nb carbonitride is promoted as the final annealing temperature is higher, the precipitation amount and particle diameter of Nb carbonitride on the cold-rolled annealing plate can be reduced. However, if the annealing temperature exceeds 1200 ° C, the crystal grains become coarse and cause toughness deterioration, so 1200 ° C is set as the upper limit of the final annealing temperature. Considering the surface quality, steel plate shape and manufacturing cost, the final annealing temperature is preferably 1000 to 1150 ° C.

  The manufacturing method of the steel sheet is not particularly specified except that the final annealing temperature of the cold rolled sheet is 1000 to 1200 ° C., and the cooling rate from the final annealing temperature to 750 ° C. is 7 ° C./sec or more. Conditions, hot-rolled sheet thickness, presence / absence of hot-rolled sheet annealing, cold-rolled conditions, hot-rolled sheet and annealing temperature, atmosphere, etc. may be selected as appropriate. Further, cold rolling / annealing may be repeated a plurality of times, or temper rolling or tension leveler may be applied after cold rolling / annealing. Further, the product plate thickness may be selected according to the required member thickness.

<Sample creation method>
Steels having the composition shown in Tables 1 and 2 were melted and cast into slabs, and the slabs were hot-rolled to form hot rolled coils having a thickness of 5 mm. Thereafter, the hot-rolled coil was annealed at 1000 to 1200 ° C. and then pickled, cold-rolled to a thickness of 2 mm, annealed and pickled to obtain a product plate. The annealing temperature of the cold rolled sheet was 1000 to 1200 ° C. No. in Table 1 1 to 21 are examples of the present invention, No. 1 in Table 2. 22 to 50 are comparative examples.

<Thermal fatigue test method>
The product plate thus obtained was wound into a pipe shape, and the ends of the plate were welded by TIG welding to produce a 30 mmφ pipe. Furthermore, this pipe was cut into a length of 300 mm, and a thermal fatigue test piece having a score of 20 mm was produced. Using a servo pulser type thermal fatigue test apparatus (heating method is a high-frequency induction heating apparatus), the test piece is restrained in the atmosphere at 20%, “temperature rising from 200 ° C. to 950 ° C. in 150 sec → holding at 950 ° C. for 120 sec → The thermal fatigue life was evaluated by repeating a pattern in which “falling down from 950 ° C. to 200 ° C. in 150 seconds” was 1 cycle. The number of repetitions when the crack penetrates the plate thickness was defined as the thermal fatigue life. The penetration was confirmed visually. The evaluation was evaluated as “good” when 1500 cycles or more passed and “bad” when less than 1500 cycles.

<Measurement method of Nb carbonitride>
Precipitates were collected from the cold-rolled annealed plate sample with a thickness of 1/2 so that the normal direction of the rolled surface could be observed by the extraction replica method, and observed with a transmission electron microscope (TEM). An arbitrary portion was observed with a TEM at a magnification of 50000 times, and several tens of images were photographed so that 300 pieces of Nb carbonitride precipitated in the grains could be measured. After taking the photograph with a scanner and performing monochrome image processing, the area of each particle is obtained using the image analysis software “Scion Image” made by Scion Corporation, and converted from the area to the equivalent circle diameter, Nb carbonitride The particle size of was measured. The types of precipitates were classified by quantifying Fe, Nb, Mo, and Ti with an EDS apparatus (energy dispersive X-ray fluorescence analyzer) attached to TEM. Since Nb carbonitride hardly contains Fe and Mo, the case where Fe and Mo were each less than 5 mass% was designated as Nb carbonitride. The evaluation of Nb carbonitride was evaluated as “good” when 95% or more of the Nb carbonitrides having a particle diameter of 0.2 μm or less passed as the number ratio, and “poor” when less than 95% was rejected.

<Oxidation resistance test>
An oxidation test piece with a thickness of 20 mm × 20 mm was produced from the product plate, and a continuous oxidation test was performed at 950 ° C. for 200 hours in the atmosphere to evaluate the presence or absence of abnormal oxidation and scale peeling (conforming to JIS Z 2281). . If the increase in oxidation was less than 10 mg / cm ² and the amount of scale peeling was less than 5 mg, it was rated as “no” for abnormal oxidation, and “x” for other abnormal oxidation.

<Method for evaluating workability at room temperature>
A JIS No. 13B test piece having a longitudinal direction parallel to the rolling direction was prepared, a tensile test was performed, and elongation at break was measured. Here, if the elongation at break at room temperature is 30% or more, it can be processed into a general exhaust part. Therefore, if it has a break elongation of 30% or more, it is ○, and if it is less than 30%, it is ×. .

<Evaluation results>
As apparent from Tables 1 and 2, the cooling rate from the final annealing temperature to 750 ° C. is produced at 7 ° C./sec or more, and the particle diameter of the Nb carbonitride of steel having the component composition defined in the present invention The example of the present invention in which the number ratio is less than 0.2 μm is 95% or more has a higher thermal fatigue life at 950 ° C. than the comparative example, and is excellent in oxidation resistance without abnormal oxidation and scale peeling. I understand that. Further, it can be seen that the fracture ductility is good in the mechanical properties at room temperature, and the processability is equal to or higher than that of the comparative example. No. In Steel Nos. 22 and 23, C and N are out of the upper limit, respectively, so the size of Nb carbonitride is out of the upper limit, and the thermal fatigue life and oxidation resistance at 950 ° C. are lower than those of the examples of the present invention. No. In Steel Nos. 24 and 26, Si and Mn are out of the lower limits, respectively, and the oxidation resistance is lower than that of the examples of the present invention. No. In Steel No. 25, Si exceeds the upper limit, and the oxidation resistance and thermal fatigue life are lower than those of the examples of the present invention. No. In Steel No. 27, Mn is added excessively, resulting in poor oxidation resistance and low ductility at room temperature. No. In the 28 and 32 steels, Cr and Mo are out of the lower limits, respectively, and the thermal fatigue life and oxidation resistance are lower than those of the examples of the present invention. No. In Steel No. 29, Cr is out of the upper limit and the thermal fatigue life and oxidation resistance are high, but the room temperature ductility is low. No. In Steels 30 and 34, Nb and Cu are outside the lower limits, respectively, and the thermal fatigue life at 950 ° C. is low. No. In Steels 31 and 33, Nb and Mo are outside the upper limits, respectively, and the thermal fatigue life is high, but the room temperature ductility is low. No. In 35 steel, Cu is out of the upper limit, thermal fatigue life and cold ductility are low, and oxidation resistance is also inferior. No. In Steel No. 36, Ti is outside the lower limit, and the room temperature ductility is equivalent to that of the present invention example, but the thermal fatigue life at 950 ° C. is low. No. In Steel No. 37, Ti exceeds the upper limit, the thermal fatigue life at 950 ° C. is low, and the room temperature ductility is lower than that of the examples of the present invention. No. In Steels 38 and 39, B is outside the lower limit and the upper limit, respectively, and the thermal fatigue life is lower than that of the examples of the present invention.

  No. In Steels 40 and 41, W and Al are out of the upper limits, respectively, and although the thermal fatigue life is high, the room temperature ductility is low. No. In the 42, 44 to 47 steels, Sn, Zr, Hf, Ta, and Mg are out of the upper limit, and the thermal fatigue life is high, but the room temperature ductility is low. No. In Steel No. 43, V exceeds the upper limit, so the size of Nb carbonitride deviates from the upper limit, and the thermal fatigue life at 950 ° C. and the normal temperature ductility are lower than those of the examples of the present invention.

  No. The 48,49 steel is a steel having the component composition defined in the present invention. Among Nb carbonitrides, those having a particle size of 0.2 μm or less are less than 95% in number ratio, and compared with the present invention examples. As a result, the thermal fatigue life and elongation at break are low. This is because the cooling rate from the final annealing temperature to 750 ° C. was produced at less than 7 ° C./sec, resulting in coarsening of Nb carbonitride. No. Steel No. 50 is SUS444, Cu is out of the lower limit, and the thermal fatigue life is low.

  Since the ferritic stainless steel of the present invention is excellent in heat resistance, it can be used as an exhaust gas path member of a power plant as well as an automobile exhaust system member. Furthermore, since Mo which is effective for improving corrosion resistance is added, it can be used for applications where corrosion resistance is required.

Claims (5)

  In mass%, C: 0.015% or less, N: 0.020% or less, Si: more than 0.10 to 0.40%, Mn: 0.10 to 1.00%, Cr: 16.5 25.0%, Nb: 0.30 to 0.80%, Mo: 1.00 to 4.00%, Ti: 0.05 to 0.50%, B: 0.0003 to 0.0030%, Cu : 1.0% to 2.5% carbon nitride having a balance of Fe and inevitable impurities and having Nb in steel as the main phase, and having a particle size of 0.2 μm or less A ferritic stainless steel sheet having a structure of 95% or more and excellent in heat resistance.   The ferritic stainless steel sheet excellent in heat resistance according to claim 1, characterized by containing, in mass%, W: 3.00% or less.   3. The composition according to claim 1, comprising at least one of Al: 3.00% or less, Sn: 1.00% or less, and V: 0.10 to 1.00% by mass%. Ferritic stainless steel sheet with excellent heat resistance.   It contains at least one of Zr: 1.00% or less, Hf: 1.00% or less, Ta: 3.00% or less, Mg: 0.0100% or less in mass%. The ferritic stainless steel sheet excellent in heat resistance according to any one of claims 1 to 3.   The final annealing temperature is 1000 to 1200 ° C, and the cooling rate from the final annealing temperature to 750 ° C is 7 ° C / sec or more, and the heat resistance according to any one of claims 1 to 4 is excellent. A method for producing a ferritic stainless steel sheet.

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