WO2015174079A1 - フェライト系ステンレス鋼 - Google Patents
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- WO2015174079A1 WO2015174079A1 PCT/JP2015/002407 JP2015002407W WO2015174079A1 WO 2015174079 A1 WO2015174079 A1 WO 2015174079A1 JP 2015002407 W JP2015002407 W JP 2015002407W WO 2015174079 A1 WO2015174079 A1 WO 2015174079A1
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- 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
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- C21D2211/005—Ferrite
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- 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
Definitions
- the present invention relates to a ferritic stainless steel having excellent thermal fatigue properties, oxidation resistance, and high temperature fatigue properties.
- the ferritic stainless steel of the present invention is particularly applicable to exhaust system members used at high temperatures such as exhaust pipes and converter cases of automobiles and motorcycles, and exhaust ducts of thermal power plants.
- Exhaust system members such as automobile exhaust manifolds, exhaust pipes, converter cases, and mufflers are required to have excellent oxidation resistance, thermal fatigue characteristics, and high temperature fatigue characteristics (hereinafter collectively referred to as "heat resistance”).
- heat resistance high temperature fatigue characteristics
- thermal fatigue and the high temperature fatigue are specifically as follows. In the description of the component composition below, “%” means “% by mass”.
- thermal fatigue When the exhaust system member is repeatedly heated and cooled as the engine is started and stopped, it is in a state of being restrained in relation to surrounding parts. For this reason, thermal expansion and contraction of the exhaust system member are limited, and thermal distortion occurs in the material itself. The fatigue phenomenon resulting from this thermal strain is called thermal fatigue.
- high temperature fatigue is a phenomenon that leads to destruction, such as cracking when it continues to receive vibrations while being heated by exhaust gas from the engine.
- Type 429 (15% Cr-0.9% Si-0.4% Nb, for example, JFE 429EX) to which Nb and Si are added.
- JFE 429EX JFE 429EX
- Type 429 cannot be said to satisfy the required characteristics, but in particular, the thermal fatigue characteristics cannot be sufficiently satisfied.
- Patent Document 2 discloses a Cr-containing steel containing 10 to 20% of Cr, Nb: 0.50% or less, Cu: 0.8 to A ferritic stainless steel for an exhaust gas passage member of an automobile to which 2.0% and V: 0.03 to 0.20% are added is disclosed.
- Patent Document 3 describes steel containing 10 to 20% Cr, Ti: 0.05 to 0.30%, Nb: 0.10 to 0.60%, Cu: 0.8 to 2.0%, B: Ferritic stainless steel excellent in thermal fatigue properties with 0.0005 to 0.02% added is disclosed.
- Patent Document 4 discloses a ferritic stainless steel for automobile exhaust system parts in which Cu: 1 to 3% is added to Cr-containing steel containing 15 to 25% Cr. These steels are characterized by adding Cu to improve thermal fatigue characteristics.
- Patent Document 5 describes thermal fatigue characteristics by adding Al: 0.2 to 2.5%, Nb: more than 0.5 to 1.0%, and Ti: 3 ⁇ (C + N) to 0.25%.
- An enhanced ferritic stainless steel is disclosed.
- Patent Document 6 discloses that an Al 2 O 3 film is formed on a steel surface by adding Al to a Cr-containing steel containing 10 to 25% Cr and Ti: 3 ⁇ (C + N) to 20 ⁇ (C + N).
- a ferritic stainless steel having improved oxidation resistance is disclosed.
- Patent Document 7 C and N are fixed to a Cr-containing steel containing 6 to 25% of Cr by adding Ti, Nb, V and Al, thereby improving crack resistance after hydroforming.
- Ferritic stainless steel is disclosed.
- Patent Document 8 a Cr-containing steel containing 16 to 23% of Cr, Nb: 0.3 to 0.65%, an appropriate amount of Cu: 1.0 to 2.5%, and Al: 0.00%.
- a steel having excellent thermal fatigue properties, oxidation resistance and high temperature fatigue properties by adding 2 to 1.0% in combination is disclosed.
- JP 2004-018921 A International Publication Number WO03 / 004714 JP 2006-117985 A JP 2000-297355 A JP 2008-285693 A JP 2001-316773 A JP 2005-187857 A JP 2011-140709 A
- Patent Documents 5 and 6 have high high-temperature strength and excellent oxidation resistance due to the addition of Al.
- the effect cannot be sufficiently obtained only by adding Al.
- Al preferentially forms an oxide or nitride even when Al is added.
- the solid solution amount of Al decreases, and the desired high-temperature strength cannot be obtained.
- the steel of Patent Document 6 to which a large amount of Al exceeding 1.0% is added not only the workability at room temperature is remarkably lowered but also the oxidation resistance is lowered because Al is easily combined with O (oxygen). Resulting in.
- an object of the present invention is to provide a ferritic stainless steel that realizes extremely excellent high temperature fatigue characteristics in a Cu and Al composite-added steel and is excellent in heat resistance.
- the “excellent high temperature fatigue characteristics” in the present invention means that no fracture occurs even when a plane bending stress of 75 MPa at 850 ° C. is repeatedly applied 100 ⁇ 10 5 times.
- the “excellent thermal fatigue characteristics” in the present invention specifically means that the thermal fatigue life when repeated at a constraint rate of 0.35 between 100 ° C. and 850 ° C. is 1120 cycles or more.
- excellent oxidation resistance as used in the present invention means that the increase in oxidation after being kept in the atmosphere at 950 ° C. for 300 hours is 27 g / m 2 or less.
- the inventors have conducted intensive studies on the effects of various additive elements on the high temperature fatigue properties of Cu and Al combined addition steel in addition to Nb, and found that the amount of O (oxygen) in the steel affects the high temperature fatigue properties.
- the present invention has been completed. More specifically, the present invention provides the following.
- the component composition further includes B: 0.0030% or less, REM: 0.080% or less, Zr: 0.50% or less, V: 0.50% or less, Co: 0.50% or less, and Ni: Ferritic stainless steel according to [1], containing one or more selected from 0.50% or less.
- the component composition further contains one or two selected from Ca: 0.0050% or less and Mg: 0.0050% or less [1] or [2] Ferritic stainless steel described in 1.
- ferritic stainless steel having high-temperature fatigue characteristics exceeding SUS444 can be provided at low cost. Therefore, the steel of the present invention can be suitably used particularly for exhaust system members such as automobiles.
- FIG. 4 is a diagram for explaining the influence of the Al content and the O content on the high temperature fatigue test characteristics.
- C 0.015% or less C is an element effective for increasing the strength of steel. However, when the C content exceeds 0.015%, the toughness and moldability are significantly reduced. Therefore, in the present invention, the C content is set to 0.015% or less.
- the C content is preferably 0.010% or less from the viewpoint of securing moldability. Further, the C content is preferably 0.001% or more from the viewpoint of securing the strength as the exhaust system member. More preferably, it is in the range of 0.003 to 0.008%.
- Si 1.0% or less Si is an element that improves oxidation resistance.
- the Si content is preferably 0.02% or more.
- the Si content is set to 1.0% or less. Preferably they are 0.20% or more and 1.0% or less.
- Si is an element that contributes to improving the oxidation resistance in an atmosphere containing water vapor, such as automobile exhaust gas.
- the Si content is preferably 0.40% or more.
- a more preferable Si content is 0.50 to 0.90%.
- Si is an important element in order to effectively utilize the solid solution strengthening ability of Al described later.
- Al is an element having a solid solution strengthening action even at a high temperature and an effect of increasing the strength in the entire temperature range from room temperature to a high temperature.
- Al content is higher than the Si content, Al preferentially forms oxides and nitrides at a high temperature, and the solid solution Al content decreases. For this reason, Al cannot fully contribute to solid solution strengthening.
- Si content is higher than the Al content, Si is preferentially oxidized and a dense oxide layer is continuously formed on the steel sheet surface.
- This oxide layer has an effect of suppressing inward diffusion of oxygen and nitrogen from the outside, so that oxidation and nitridation of Al can be minimized.
- the Si content and the Al content satisfy the relationship of Si ⁇ Al. It is more preferable to adjust the Si content and the Al content so as to satisfy Si ⁇ 1.4 ⁇ Al.
- Si and Al mean the content (% by mass) of each element.
- Mn 1.0% or less
- Mn is an element added as a deoxidizer and to increase the strength of steel. Mn also has an effect of suppressing exfoliation of oxide scale. In order to obtain these effects, the Mn content is preferably 0.02% or more. However, when Mn is contained excessively, a ⁇ phase is easily generated at a high temperature, and heat resistance is lowered. Therefore, the Mn content is 1.0% or less.
- a preferable Mn content is 0.05 to 0.80%. More preferably, it is 0.10 to 0.50%.
- P 0.040% or less
- P is a harmful element that lowers the toughness of steel, and is desirably reduced as much as possible. Therefore, in the present invention, the P content is 0.040% or less. Preferably, it is 0.030% or less.
- S 0.010% or less
- S is a harmful element that lowers elongation and r value, adversely affects formability, and lowers corrosion resistance, which is a basic characteristic of stainless steel. Therefore, it is desirable to reduce the S content as much as possible. Therefore, in the present invention, the S content is set to 0.010% or less. Preferably, it is 0.005% or less.
- Cr 10.0-23.0% Cr is an important element effective for improving the corrosion resistance and oxidation resistance, which are the characteristics of stainless steel. If the Cr content is less than 10.0%, sufficient oxidation resistance cannot be obtained. On the other hand, Cr is an element that solidifies and strengthens steel at room temperature to make it harder and lower ductility. In particular, when the Cr content exceeds 23.0%, the adverse effect becomes significant. Therefore, the Cr content is in the range of 10.0 to 23.0%. Preferably, it is in the range of 12.0 to 20.0%. More preferably, it is 14.0 to 18.0%.
- Al 0.2 to 1.0%
- Al is an element indispensable for improving the oxidation resistance of the Cu-added steel.
- the Al content needs to be 0.2% or more.
- the Al content is in the range of 0.2 to 1.0%.
- it is in the range of 0.25 to 0.80%. More preferably, it is in the range of 0.30 to 0.50%.
- Al is an element that has the effect of dissolving as a solid solution in steel and acting as a solid solution strengthening element.
- Al is an important element in the present invention because it contributes to an increase in high-temperature strength at temperatures exceeding 700 ° C. Further, Al exerts a stronger solid solution strengthening effect when the strain rate is low as in the thermal fatigue test.
- the Al content is higher than the Si content, Al preferentially forms oxides and nitrides at high temperatures. As a result, the solid solution amount of Al decreases, and Al hardly contributes to solid solution strengthening.
- Si is preferentially oxidized, and a dense oxide layer is continuously formed on the steel sheet surface. This oxide layer serves as a barrier for inward diffusion of oxygen and nitrogen, and can stably maintain Al in a solid solution state. As a result, the high temperature strength can be increased by solid solution strengthening of Al.
- N 0.015% or less
- N is an element that lowers the toughness and formability of steel. This disadvantage is noticeable when the N content exceeds 0.015%. Therefore, the N content is set to 0.015% or less.
- the N content is preferably 0.004% or more and less than 0.012%.
- Cu 1.0 to 2.0% Cu is an element that is very effective in improving thermal fatigue characteristics.
- the Cu content needs to be 1.0% or more in order to obtain thermal fatigue characteristics equivalent to or better than SUS444.
- the Cu content exceeds 2.0%, the steel is remarkably hardened, the workability at room temperature is remarkably lowered, and embrittlement is likely to occur during hot working.
- the inclusion of Cu improves the thermal fatigue properties but reduces the oxidation resistance of the steel itself. That is, the heat resistance may be reduced overall due to the Cu content.
- the cause of the overall decrease in heat resistance is thought to be due to the fact that Cu is concentrated in the generated Cr-depleted layer directly under the scale and suppresses re-diffusion of Cr, which is an element that improves the original oxidation resistance of stainless steel. Therefore, the Cu content is in the range of 1.0 to 2.0%. Preferably it is 1.0 to 1.8% of range. More preferably, it is 1.2 to 1.6%.
- Nb 0.30 to 0.65%
- Nb forms carbonitride with C and N to fix C and N, and has the effect of improving corrosion resistance, formability and intergranular corrosion resistance of welds, and also increases the high temperature strength and thermal fatigue characteristics. It has the effect
- the Nb content is preferably in the range of 0.40 to 0.50%. More preferably, it is in the range of 0.43 to 0.48%.
- Ti 0.50% or less Ti, like Nb, is an element that fixes C and N, improves corrosion resistance and formability, and prevents intergranular corrosion of welds.
- Ti is an extremely effective element for improving the oxidation resistance.
- Ti when used in a high temperature range exceeding 1000 ° C., Ti is an effective additive element in order to obtain excellent oxidation resistance.
- the Ti content is preferably 0.005% or more.
- the Ti content exceeds 0.50%, not only the effect of improving the oxidation resistance is saturated, but also the toughness is reduced due to the formation of coarse nitrides.
- the upper limit of the Ti content is 0.50%.
- the reason why the oxidation resistance of the Al-containing steel is improved by the inclusion of Ti is that Ti added to the steel is preferentially bonded to N at a high temperature, and Al is combined with N to precipitate as AlN. It is because it suppresses doing.
- free Al increases in the steel, and O (oxygen) that penetrates and penetrates without being stopped by the dense Si oxide layer formed on the surface of the steel plate described above, enters the interface between the base material and the Si oxide layer.
- Al oxide Al 2 O 3
- O oxygen
- oxygen oxygen
- O is an important element in the Al-containing steel as in the present invention.
- O present in the steel is preferentially combined with Al in the steel when exposed to a high temperature, and the solid solution amount of Al is reduced.
- the solid solution amount of Al decreases, the high temperature strength decreases.
- the Al oxide coarsely precipitated in the steel becomes a starting point of crack generation in the high temperature fatigue test, and deteriorates the high temperature fatigue characteristics of the steel.
- a large amount of O is present in the steel, not only is the amount of solid solution of Al reduced as a result of being combined with a large amount of Al, but it is also easy for O to enter from the outside.
- the O content is limited to 0.0030% or less.
- the content is limited to 0.0020% or less. More preferably, it is 0.0015% or less.
- Al / O ⁇ 100 As described above, in the steel to which Al is added as in the present invention, it is important to reduce the O content in order to improve the high temperature fatigue characteristics utilizing the solid solution strengthening of Al. Furthermore, the inventors have also examined the influence of the content ratio of Al and O on the high temperature fatigue properties, and after satisfying Al: 0.2 to 1.0% and O: 0.0030% or less, Al It has been found that by satisfying / O ⁇ 100, extremely high temperature fatigue properties are imparted to the steel. The reason why this effect can be obtained is that the Al oxide produced in association with O present in steel is inferior in density to the Al oxide in association with O that has entered from the outside air when exposed to high temperature. This is considered to be because it hardly contributes to the improvement of the oxidation resistance, allows further O intrusion from the outside air, and promotes the generation of Al oxide that becomes the starting point of the crack.
- Component composition C: 0.010%, Si: 0.8%, Mn: 0.2%, P: 0.030%, S: 0.002%, Cr: 17%, N: 0.010% , Cu: 1.3%, Nb: 0.5%, Ti: 0.1%, and Al and O are 0.1 to 0.5% and 0.001 to 0.006%, respectively.
- a steel with various contents in the range of 1 to 30 kg was melted in the laboratory to obtain a 30 kg steel ingot. The steel ingot was heated to 1170 ° C. and then hot rolled to form a sheet bar having a thickness of 35 mm ⁇ width of 150 mm. The sheet bar was heated to 1050 ° C.
- the hot-rolled annealed plate annealed at 900 to 1050 ° C. and pickled was cold-rolled to a thickness of 2 mm, and finish-annealed at 850 to 1050 ° C. to obtain a cold-rolled annealed plate. This was subjected to the following high temperature fatigue test.
- a high temperature fatigue test piece having a shape as shown in FIG. 1 was produced from the cold-rolled annealed plate obtained as described above, and was subjected to the following high temperature fatigue test.
- O (%) on the horizontal axis means O content
- Al (%) on the vertical axis means Al content
- the ferritic stainless steel of the present invention further contains one or more selected from B, REM, Zr, V, Co, Ni, Ca, Mg, and Mo. It can contain in the range of.
- B 0.0030% or less
- B is an element effective for improving the workability of steel, particularly the secondary workability.
- B also has an effect of preventing Al from being nitrided by being combined with N in the steel. These effects can be obtained by making the B content 0.0003% or more. If the B content exceeds 0.0030%, BN is excessively generated, and BN is easily coarsened, so that workability is deteriorated. Therefore, when adding B, B content shall be 0.0030% or less. Preferably it is 0.0005 to 0.0020% of range. More preferably, it is 0.0008 to 0.0015%.
- REM 0.080% or less
- Zr 0.50% or less
- REM rare earth element
- Zr 0.50% or less
- REM rare earth element
- Zr are both elements that improve oxidation resistance.
- the content is preferably 0.005% or more for REM and 0.005% or more for Zr. If the REM content exceeds 0.080%, the steel becomes brittle. On the other hand, if the Zr content exceeds 0.50%, the Zr intermetallic compound precipitates and the steel becomes brittle. Therefore, when it contains REM and Zr, they are 0.080% or less and 0.50% or less, respectively.
- V 0.50% or less
- V is an element effective for improving the workability of steel and an element effective for improving oxidation resistance. These effects become significant when the V content is 0.01% or more. However, if the V content exceeds 0.50%, coarse V (C, N) precipitation is caused and the surface properties of the steel are lowered. Therefore, when adding V, the content is made 0.50% or less.
- the content is preferably in the range of 0.01 to 0.50%. More preferably, it is 0.03 to 0.40% of range. More preferably, it is 0.05 to less than 0.20%.
- V is an element effective for improving the toughness of steel.
- oxidation resistance of 1000 ° C. or higher is required, adding V to Ti-containing steel containing Ti is extremely effective from the viewpoint of improving toughness. This effect can be obtained by setting the V content to 0.01% or more. If the V content exceeds 0.50%, the toughness decreases. Therefore, in the Ti-containing steel used for applications requiring toughness, the V content is preferably in the range of 0.01 to 0.50%.
- the effect of improving the toughness of V in Ti-containing steel is considered to be caused by replacing a part of Ti of TiN precipitated in the steel with V. This is because (Ti, V) N, which has a slower growth rate than TiN, precipitates, and it is considered that precipitation of coarse nitrides that cause toughness reduction is suppressed.
- Co 0.50% or less
- Co is an element effective for improving the toughness of steel.
- Co also has the effect of reducing the thermal expansion coefficient of steel and improving thermal fatigue properties.
- the Co content is preferably 0.005% or more.
- Co is an expensive element, and even if the Co content exceeds 0.50%, the above effect is only saturated. Therefore, when Co is added, the Co content is preferably 0.50% or less. More preferably, it is in the range of 0.01 to 0.20%. When excellent toughness of the cold-rolled sheet is required, the Co content is preferably 0.02 to 0.20%.
- Ni 0.50% or less
- Ni is an element that improves the toughness of steel.
- Ni also has the effect of improving the oxidation resistance of steel. In order to acquire the effect, it is preferable to make Ni content 0.05% or more.
- Ni is a strong ⁇ -phase-forming element, and the inclusion of Ni facilitates the formation of ⁇ -phase at high temperatures.
- the Ni content is 0.50% or less.
- the Ni content is preferably in the range of 0.05 to 0.40%. More preferably, it is 0.10 to 0.25%.
- Ca 0.0050% or less
- Ca is an effective component for preventing nozzle clogging due to precipitation of Ti-based inclusions that are likely to occur during continuous casting. The effect is acquired by making Ca content 0.0005% or more. In order to obtain good surface properties without generating surface defects, the Ca content needs to be 0.0050% or less. Therefore, when Ca is added, the Ca content is preferably in the range of 0.0005 to 0.0050%. More preferably, it is 0.0005% or more and 0.0030% or less of range. More preferably, it is 0.0005% or more and 0.0015% or less of range.
- Mg 0.0050% or less
- Mg is an element effective for improving the equiaxed crystal ratio of the slab and improving workability and toughness. Furthermore, Mg is an effective element for suppressing the coarsening of Nb and Ti carbonitrides. When the Ti carbonitride becomes coarse, it becomes a starting point for brittle cracks, so that toughness decreases. Further, when the Nb carbonitride is coarsened, the amount of Nb solid solution in the steel is reduced, which leads to a decrease in thermal fatigue characteristics. These effects are acquired by making Mg content 0.0010% or more. On the other hand, when the Mg content exceeds 0.0050%, the surface properties of the steel are deteriorated. Therefore, when adding Mg, it is preferable to make the content into the range of 0.0010% or more and 0.0050% or less. More preferably, it is 0.0010% or more and 0.0020% or less of range.
- Mo 0.1 to 1.0% or less Mo is an element that can improve heat resistance by increasing high-temperature strength. Moreover, since Mo is an expensive element, it does not tend to be actively added. If excellent heat resistance is required without considering the cost, Mo may be contained in the range of 0.1 to 1.0%.
- the balance other than the above essential elements and selective elements is Fe and inevitable impurities.
- the method for producing the stainless steel of the present invention is not particularly limited, and basically any method for producing ferritic stainless steel can be suitably used.
- the production conditions are controlled in the refining process as described later.
- the example of a manufacturing method is shown below. Steel is produced in a known melting furnace such as a converter or an electric furnace, or further subjected to secondary refining such as ladle refining or vacuum refining to obtain steel having the above-described component composition of the present invention.
- secondary refining such as ladle refining or vacuum refining to obtain steel having the above-described component composition of the present invention.
- the addition of Al may not sufficiently reduce the O content in the steel.
- the basicity (CaO / Al 2 O 3 ) of the slag to be generated is small, the equilibrium oxygen concentration increases and the O content in the steel increases. Moreover, when the air release time after vacuum refining becomes long, oxygen from the atmosphere may enter the steel. Therefore, when manufacturing this developed steel, it controls so that the basicity of slag becomes large, and shortens the time for which the molten steel after vacuum refining is kept in the atmosphere as much as possible.
- it is made into a steel slab (slab) by the continuous casting method or the ingot-bundling rolling method, and then cooled through processes such as hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, pickling.
- a fire-annealed plate can be manufactured.
- the cold rolling may be one or two or more cold rolling sandwiching intermediate annealing. Moreover, you may perform repeatedly each process of cold rolling, finish annealing, and pickling. Furthermore, the hot-rolled sheet annealing may be omitted. Further, when the surface gloss or roughness adjustment of the steel sheet is required, skin pass rolling may be applied to the cold rolled sheet after cold rolling or the annealed sheet after finish annealing.
- the steel melted in a converter or an electric furnace is secondarily refined by a VOD method or the like, and the steel contains the above essential components and components added as necessary.
- the molten steel can be made into a steel material (slab) by a known method, it is preferable to use a continuous casting method in terms of productivity and quality. Thereafter, the steel material is heated to 1000 to 1250 ° C. and is hot rolled into a hot rolled sheet having a desired thickness. Of course, you may hot-process in shapes other than a board
- the annealing may not be performed, and in this case, a hot-rolled sheet after hot rolling is used as a hot-rolled product.
- the cooling rate after annealing is not particularly limited, but it is desirable to cool in as short a time as possible. If necessary, the scale may be removed by shot blasting before pickling.
- the hot-rolled annealed plate or hot-rolled plate may be a cold-rolled product through a process such as cold rolling.
- the cold rolling may be performed once, but may be performed twice or more with intermediate annealing in view of productivity and required quality.
- the total rolling reduction of the cold rolling process once or twice is preferably 60% or more, more preferably 70% or more.
- the cold-rolled steel sheet is preferably subjected to continuous annealing (finish annealing) at a temperature of preferably 900 to 1150 ° C., more preferably 950 to 1120 ° C., pickling, and forming a cold-rolled product.
- finish annealing continuous annealing
- the cooling rate after annealing is not particularly limited, but is desirably as large as possible.
- skin pass rolling or the like may be performed to adjust the shape, surface roughness, and material quality of the steel sheet.
- the hot-rolled product or cold-rolled product obtained as described above is then subjected to processing such as cutting, bending processing, overhanging processing and / or drawing processing according to each application, and exhaust pipes for automobiles and motorcycles.
- the catalyst outer cylinder material and the exhaust duct of a thermal power plant or a fuel cell related member such as a separator, an interconnector and a reformer are formed.
- the method for welding these members is not particularly limited, and normal arc welding such as MIG (Metal Inert Gas), MAG (Metal Active Gas) and TIG (Tungsten Inert Gas), spot welding, and seam welding. Examples thereof include resistance welding such as high frequency resistance welding such as electric resistance welding, high frequency induction welding, and the like.
- Table 1 Steel having the composition shown in Table 1 (Table 1-1, Table 1-2, and Table 1-3 are combined into Table 1) is melted in a vacuum melting furnace and cast into a 50 kg steel ingot. Forged and divided into two parts. Thereafter, one of the two steel ingots was heated to 1170 ° C. and hot-rolled to obtain a hot-rolled sheet having a thickness of 5 mm. Thereafter, the structure was confirmed within a range of 1000 to 1100 ° C., and hot-rolled sheet annealing was performed at a temperature determined for each steel, and pickling was performed.
- cold rolling with a rolling reduction of 60% confirming the structure at a temperature in the range of 1000 to 1100 ° C, finish annealing at a temperature determined for each steel, pickling and cold rolling annealing with a sheet thickness of 2 mm A board was used.
- the cold rolled annealed plate was used for the following high temperature fatigue test.
- a test piece having the shape shown in FIG. 1 was prepared from the cold-rolled annealed plate obtained as described above and subjected to a high-temperature plane bending fatigue test.
- the test temperature was 850 ° C.
- the bending of both swings was repeated so that the plane stress was 75 MPa
- the number of cycles in which cracks occurred was measured as the life, and evaluated as follows.
- Table 1 summarizes the results obtained from less than breakage.
- ⁇ Atmospheric continuous oxidation test> Cut out a 30mm x 20mm sample from the various cold-rolled annealed plates obtained as described above, drill a 4mm ⁇ hole at the top of the sample, polish the surface and end face with # 320 emery paper, degrease, and heat to 950 ° C The sample was suspended in a furnace in a maintained atmospheric atmosphere and held for 300 hours. After the test, the mass of the sample was measured, the difference from the pre-measured mass before the test was determined, and the increase in oxidation (g / m 2 ) was calculated.
- the remaining steel ingot of the 50 kg steel ingot divided into two parts is heated to 1170 ° C. and hot-rolled into a sheet bar having a thickness of 30 mm ⁇ width of 150 mm, and then this sheet bar is forged and each 35 mm square bar Then, after annealing at a temperature of 1030 ° C., it was machined, processed into a thermal fatigue test piece having the shape and dimensions shown in FIG. 2, and subjected to the following thermal fatigue test.
- the thermal fatigue test was performed under the condition that the temperature rise / fall was repeated between 100 ° C. and 850 ° C. while restraining the test piece with a restraint rate of 0.35.
- the temperature increase rate and temperature decrease rate at this time were 10 ° C./sec, the retention time at 100 ° C. was 2 min, and the retention time at 850 ° C. was 5 min.
- the thermal fatigue life is calculated by dividing the load detected at 100 ° C. by the cross-sectional area of the test piece soaking parallel part (see FIG. 2), and the stress is 75 for the initial stress (5th cycle). The number of cycles when the stress was reduced to%.
- the thermal fatigue characteristics were evaluated as “ ⁇ ” (passed) when 1120 cycles or more and “x” (failed) when less than 1120 cycles.
- Table 1 summarizes the results of the high temperature fatigue test, the atmospheric continuous oxidation test, and the thermal fatigue test of the above examples.
- the steel of the inventive example adapted to the component composition of the present invention has excellent high temperature fatigue properties in addition to excellent thermal fatigue properties and oxidation resistance. Meet the goal.
- none of the comparative steels outside the scope of the present invention has obtained extremely excellent high temperature fatigue characteristics, and the target of the present invention has not been achieved.
- the ferritic stainless steel of the present invention is not only suitable for high-temperature exhaust system members such as automobiles, but also as exhaust system members for thermal power generation systems and solid oxide type fuel cell members that require similar characteristics. Can also be suitably used.
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Abstract
Description
Cは、鋼の強度を高めるのに有効な元素である。しかし、C含有量が0.015%を超えると、靭性および成形性の低下が顕著となる。よって、本発明では、C含有量は0.015%以下とする。なお、C含有量は、成形性を確保する観点からは0.010%以下が好ましい。また、C含有量は、排気系部材としての強度を確保する観点からは0.001%以上が好ましい。より好ましくは0.003~0.008%の範囲である。
Siは、耐酸化性を向上させる元素である。その効果を得るためにはSi含有量を0.02%以上にすることが好ましい。一方、Si含有量が1.0%を超えると、鋼が硬質化し加工性が低下するので、本発明では、Si含有量を1.0%以下とする。好ましくは0.20%以上1.0%以下である。
さらに、Siは、後述するAlの固溶強化能を有効に活用するためにも重要な元素である。Alは、高温においても固溶強化作用を有し、室温から高温までの全温度域で強度を増加させる効果を有する元素である。しかし、Al含有量がSi含有量より多い場合には、Alは高温で優先的に酸化物や窒化物を形成し、固溶Al量が減少する。このため、Alは固溶強化に十分寄与することができなくなる。一方、Si含有量がAl含有量以上の場合には、Siが優先的に酸化して鋼板表面に緻密な酸化物層が連続的に形成される。この酸化物層は、外部からの酸素や窒素の内方拡散を抑制する効果があるため、Alの酸化や窒化を最小限に抑えることができる。その結果、Alの固溶状態が安定して確保されるので、高温強度を向上させることができる。したがって、本発明では、Si含有量とAl含有量とがSi≧Alの関係を満たすようにする。Si≧1.4×Alを満たすようにSi含有量、Al含有量を調整すればより好ましい。なお、上記不等式におけるSi、Alは各元素の含有量(質量%)を意味する。
Mnは、脱酸剤として、また、鋼の強度を高めるために添加される元素である。また、Mnは酸化スケールの剥離を抑制する効果も有する。これらの効果を得るためには、Mn含有量を0.02%以上とすることが好ましい。しかし、Mnを過剰に含有すると、高温でγ相が生成しやすくなり、耐熱性が低下する。よって、Mn含有量は1.0%以下とする。好ましいMn含有量は0.05~0.80%である。さらに好ましくは0.10~0.50%である。
Pは、鋼の靭性を低下させる有害な元素であり、可能な限り低減するのが望ましい。よって、本発明では、P含有量は0.040%以下とする。好ましくは、0.030%以下である。
Sは、伸びやr値を低下させ、成形性に悪影響を及ぼすとともに、ステンレス鋼の基本特性である耐食性を低下させる有害元素である。したがって、S含有量はできる限り低減することが望ましい。よって、本発明では、S含有量を0.010%以下とする。好ましくは、0.005%以下である。
Crは、ステンレス鋼の特徴である耐食性、耐酸化性を向上させるのに有効な重要元素である。Cr含有量が10.0%未満では、十分な耐酸化性が得られない。一方、Crは、室温において鋼を固溶強化し、硬質化、低延性化する元素である。特にCr含有量が23.0%を超えると、その弊害が顕著となる。よって、Cr含有量は、10.0~23.0%の範囲とする。好ましくは、12.0~20.0%の範囲である。さらに好ましくは14.0~18.0%である。
Alは、Cu添加鋼の耐酸化性を向上するのに必要不可欠な元素である。特に、Cu添加鋼でSUS444と同等以上の耐酸化性を得るにはAl含有量を0.2%以上にすることが必要である。一方、Al含有量が1.0%を超えると、鋼が硬質化して加工性が低下してしまう。よって、Al含有量は0.2~1.0%の範囲とする。好ましくは、0.25~0.80%の範囲である。より好ましくは0.30~0.50%の範囲である。
Nは、鋼の靭性および成形性を低下させる元素である。N含有量が0.015%を超えるとこの不利益が顕著に現れる。よって、N含有量は0.015%以下とする。なお、N含有量は、靭性および成形性を確保する観点からは、できるだけ低減するのが望ましく、0.012%未満とするのが好ましい。このように、Nを積極添加しないことが好ましい。ただし、N含有量を0.004%未満まで低減するには脱窒に時間がかかり製造コストが高くなってしまう。そこで、特性とコストのバランスを考慮し、N含有量は0.004%以上0.012%未満が望ましい。
Cuは、熱疲労特性の向上に非常に有効な元素である。本発明のようなNb添加鋼において、SUS444と同等以上の熱疲労特性を得るには、Cu含有量を1.0%以上とする必要がある。しかし、Cu含有量が2.0%を超えると、鋼を著しく硬質化し室温での加工性が著しく低下するとともに、熱間加工時に脆化を起こしやすくする。さらに重要なことは、Cuの含有は、熱疲労特性を向上させるものの、鋼自身の耐酸化性を低下させる。つまり、Cuの含有により、総合的には耐熱性が低下する場合がある。総合的に耐熱性が減少する原因は、生成したスケール直下の脱Cr層にCuが濃化し、ステンレス鋼本来の耐酸化性を向上する元素であるCrの再拡散を抑制するためと考えられる。よって、Cu含有量は、1.0~2.0%の範囲とする。好ましくは1.0~1.8%の範囲である。より好ましくは1.2~1.6%である。
Nbは、CおよびNと炭窒化物を形成してCやNを固定し、耐食性や成形性および溶接部の耐粒界腐食性を高める作用を有するとともに、高温強度を上昇させて熱疲労特性を向上させる作用を有する。したがって、Nbは本発明において重要な元素である。このような効果は、Nb含有量を0.30%以上にすることで得られる。しかし、Nb含有量が0.65%を超えると、Laves相(Fe2Nb)が析出しやすくなり、脆化が促進される。さらに、Nb固溶量が減少すると高温強度向上効果が失われてしまう。よって、Nb含有量は0.30~0.65%の範囲とする。好ましくは、0.35~0.55%の範囲である。なお、高温強度と靭性のバランスを考慮するとNb含有量は0.40~0.50%の範囲が好ましい。より好ましくは0.43~0.48%の範囲である。
Tiは、Nbと同様、CおよびNを固定して、耐食性や成形性を向上し、溶接部の粒界腐食を防止する元素である。また、本発明のようなAl含有鋼において、Tiは耐酸化性の向上に極めて有効な元素である。特に1000℃を超える高温域で使用される場合には、優れた耐酸化性を得るために、Tiは有効な添加元素である。そのような高温での耐酸化性を得るために、Ti含有量は0.005%以上とするのが好ましい。しかし、Ti含有量が0.50%を超えると、耐酸化性向上効果が飽和するのみならず、粗大な窒化物の生成により靭性の低下を招く。例えば、熱延板焼鈍ラインで繰り返し受ける曲げ-曲げ戻しによって破断を起こしたりする等、製造性に悪影響を及ぼすようになる。さらには、粗大なTiNは高温疲労試験の際にも亀裂の起点となりやすいため、優れた高温疲労特性が得られなくなる。よって、Ti含有量の上限は0.50%とする。
Oは本発明のようなAl含有鋼において重要な元素である。鋼中に存在するOは、高温に曝された際に、鋼中のAlと優先的に結びつき、Alの固溶量を減少させる。Alの固溶量が減少すると高温強度が低下する。また、鋼中で粗大に析出したAl酸化物は、高温疲労試験において亀裂発生の起点となり、鋼の高温疲労特性を低下させる。Oが鋼中に多く存在すると、それだけ多くのAlと結びついてAlの固溶量が減少してしまうのみならず、外部からOを侵入させやすくする。このため、Oが鋼中に多く存在すると、鋼中O含有量以上にAl酸化物を形成しやすくなってしまう。従って、O含有量はなるべく低減するのが好ましく、その含有量は0.0030%以下に限定する。好ましくは0.0020%以下である。さらに好ましくは0.0015%以下である。
上述したように、本発明のようにAlを添加した鋼においては、Alの固溶強化を利用した高温疲労特性向上のためにO含有量の低減が重要となる。さらに、発明者らは、高温疲労特性に及ぼすAlとOの含有量比の影響も精査し、Al:0.2~1.0%かつO:0.0030%以下を満たした上で、Al/O≧100を満たすことによって、極めて優れた高温疲労特性が鋼に付与されることを見出した。この効果が得られる理由としては、鋼中に存在するOと結びついて生成したAl酸化物は、高温に曝された際に外気から侵入したOと結びついたAl酸化物に比べ緻密性に劣るため、耐酸化性の向上に寄与しにくく、外気からのさらなるOの侵入を許し、亀裂の起点となるAl酸化物の生成を促進してしまうためと考えられる。
以下、鋼の成分組成を規定する成分%は、全て質量%を意味する。
成分組成は、C:0.010%、Si:0.8%、Mn:0.2%、P:0.030%、S:0.002%、Cr:17%、N:0.010%、Cu:1.3%、Nb:0.5%、Ti:0.1%、をベースとし、これにAl、Oをそれぞれ0.1~0.5%、0.001~0.006%の範囲で含有量を種々に変化させた鋼を実験室的に溶製して30kg鋼塊とした。鋼塊を1170℃に加熱後、熱間圧延して厚さ35mm×幅150mmのシートバーとした。このシートバーを1050℃に加熱後、熱間圧延して板厚5mmの熱延板とした。その後900~1050℃で熱延板焼鈍し酸洗した熱延焼鈍板を冷間圧延により板厚を2mmとし、850~1050℃で仕上げ焼鈍して冷延焼鈍板とした。これを下記の高温疲労試験に供した。
上記のようにして得た冷延焼鈍板から図1に示すような形状の高温疲労試験片を作製し、下記の高温疲労試験に供した。
○(合格):繰り返し数100×105回で破断無し
△(不合格):繰り返し数15×105回以上100×105回以下で破断
×(不合格):繰り返し数15×105回未満で破断
図4に高温疲労試験の結果を示す。図4から、O含有量を0.0030%以下、Al含有量を0.2%以上さらにAl/O≧100とすることにより、極めて優れた高温疲労寿命が得られることがわかる。なお、横軸のO(%)はO含有量を意味し、縦軸のAl(%)はAl含有量を意味する。
Bは、鋼の加工性、特に二次加工性を向上させるのに有効な元素である。また、Bは鋼中のNと結びつくことでAlが窒化してしまうのを防ぐ効果も有する。これらの効果は、B含有量を0.0003%以上にすることで得られる。B含有量が0.0030%を超えると、BNが過剰に生成し、また、BNが粗大化しやすくなるため、加工性が低下する。よって、Bを添加する場合は、B含有量は0.0030%以下とする。好ましくは0.0005~0.0020%の範囲である。さらに好ましくは0.0008~0.0015%である。
REM(希土類元素)およびZrはいずれも、耐酸化性を向上する元素である。その効果を得るためには、REMであればその含有量を0.005%以上、Zrであればその含有量を0.005%以上にすることが好ましい。REM含有量が0.080%を超えると、鋼が脆化する。また、Zr含有量が0.50%を超えると、Zr金属間化合物が析出して、鋼が脆化する。よって、REMおよびZrを含有する場合は、それぞれ0.080%以下、0.50%以下とする。
Vは、鋼の加工性向上に有効な元素であるとともに、耐酸化性の向上にも有効な元素である。それらの効果は、V含有量を0.01%以上にすることで顕著となる。しかし、V含有量が0.50%を超えると、粗大なV(C、N)の析出を招き、鋼の表面性状が低下する。よって、Vを添加する場合、その含有量は0.50%以下とする。また、その含有量は0.01~0.50%の範囲とするのが好ましい。より好ましくは、0.03~0.40%の範囲である。さらに好ましくは0.05~0.20%未満である。
Coは、鋼の靭性向上に有効な元素である。また、Coは、鋼の熱膨張係数を低減し、熱疲労特性を向上させる効果も有する。その効果を得るためには、Co含有量を0.005%以上にすることが好ましい。しかし、Coは、高価な元素であり、また、Co含有量が0.50%を超えても、上記効果は飽和するだけである。よって、Coを添加する場合、Co含有量は0.50%以下とするのが好ましい。より好ましくは、0.01~0.20%の範囲である。なお、優れた冷延板の靭性が必要とされる場合には、Co含有量を0.02~0.20%とするのが好ましい。
Niは、鋼の靭性を向上させる元素である。また、Niは、鋼の耐酸化性を向上させる効果も有する。その効果を得るためには、Ni含有量を0.05%以上にすることが好ましい。一方、Niは高価であることに加えて、強力なγ相形成元素でありNiの含有により高温でγ相が生成しやすくなる。γ相が生成すると、耐酸化性が低下するのみならず、熱膨張係数が増加し、熱疲労特性も低下する。よって、Niを含有する場合は、Ni含有量を0.50%以下とする。Ni含有量は、好ましくは、0.05~0.40%の範囲である。より好ましくは0.10~0.25%である。
Caは、連続鋳造の際に発生しやすいTi系介在物析出によるノズルの閉塞を防止するのに有効な成分である。その効果はCa含有量を0.0005%以上にすることで得られる。表面欠陥を発生させず良好な表面性状を得るためには、Ca含有量を0.0050%以下とする必要がある。従って、Caを添加する場合、Ca含有量は0.0005~0.0050%の範囲が好ましい。より好ましくは0.0005%以上0.0030%以下の範囲である。さらに好ましくは0.0005%以上0.0015%以下の範囲である。
Mgはスラブの等軸晶率を向上させ、加工性や靱性を向上させるのに有効な元素である。さらに、Mgは、NbやTiの炭窒化物の粗大化を抑制するのに有効な元素である。Ti炭窒化物が粗大化すると、脆性割れの起点となるため靱性が低下する。また、Nb炭窒化物が粗大化すると、Nbの鋼中の固溶量が低下するため、熱疲労特性の低下につながる。Mg含有量を0.0010%以上にすることで、それらの効果が得られる。一方で、Mg含有量が0.0050%超えとなると、鋼の表面性状を悪化させてしまう。従って、Mgを添加する場合、その含有量を0.0010%以上0.0050%以下の範囲とするのが好ましい。より好ましくは0.0010%以上0.0020%以下の範囲である。
Moは、高温強度を増加させることで耐熱性を向上させることができる元素である。また、Moは高価な元素なため積極的に添加されない傾向にある。コストを考慮せず優れた耐熱性が必要な場合には、Moを0.1~1.0%の範囲で含有してもよい。
上記のようにして得た冷延焼鈍板から図1に示す形状の試験片を作製し、高温平面曲げ疲労試験に供した。試験温度は850℃、周波数22Hz(=1,300rpm)とし、平面応力が75MPaとなるように両振りの曲げを繰り返し、亀裂が生じたサイクル数を寿命として測定して、下記のように評価した。
○(合格):繰り返し数100×105回で破断無し
△(不合格):繰り返し数15×105回以上100×105回以下で破断
×(不合格):繰り返し数15×105回未満で破断
以上より得られた結果を表1にまとめて示す。
上記のようにして得た各種冷延焼鈍板から30mm×20mmのサンプルを切り出し、サンプル上部に4mmφの穴をあけ、表面および端面を#320のエメリー紙で研磨し、脱脂後、950℃に加熱保持した大気雰囲気の炉内にサンプルを吊り下げて、300時間保持した。試験後、サンプルの質量を測定し、予め測定しておいた試験前の質量との差を求め、酸化増量(g/m2)を算出した。なお、試験は各2回実施し、酸化増量の平均値が27g/m2以下の場合を「○」(合格)、27g/m2を超えた場合は「×」(不合格)として耐酸化性を評価した。
二分割した上記50kg鋼塊の残りの鋼塊を、1170℃に加熱後、熱間圧延して厚さ30mm×幅150mmのシートバーとした後、このシートバーを鍛造し、35mm角の各棒とし、1030℃の温度で焼鈍後、機械加工し、図2に示した形状、寸法の熱疲労試験片に加工し、下記の熱疲労試験に供した。
Claims (4)
- 質量%で、C:0.015%以下、Si:1.0%以下、Mn:1.0%以下、P:0.040%以下、S:0.010%以下、Cr:10.0~23.0%、Al:0.2~1.0%、N:0.015%以下、Cu:1.0~2.0%、Nb:0.30~0.65%、Ti:0.50%以下、O:0.0030%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、
Si含有量とAl含有量とがSi≧Alの関係を満たし、
Al含有量とO含有量とがAl/O≧100の関係を満たすことを特徴とするフェライト系ステンレス鋼。 - 前記成分組成は、さらに、B:0.0030%以下、REM:0.080%以下、Zr:0.50%以下、V:0.50%以下、Co:0.50%以下およびNi:0.50%以下のうちから選ばれる1種または2種以上を含有することを特徴とする請求項1に記載のフェライト系ステンレス鋼。
- 前記成分組成は、さらに、Ca:0.0050%以下およびMg:0.0050%以下のうちから選ばれる1種または2種を含有することを特徴とする請求項1または2に記載のフェライト系ステンレス鋼。
- 前記成分組成は、さらに、質量%で、Mo:0.1~1.0%以下を含有することを特徴とする請求項1~3のいずれか1項に記載のフェライト系ステンレス鋼。
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