JP2011190524A - Ferritic stainless steel having excellent oxidation resistance, secondary processing brittleness resistance and weld zone toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel which has excellent oxidation resistance, secondary processing brittleness resistance and weld zone toughness. <P>SOLUTION: The ferritic stainless steel having excellent oxidation resistance, secondary processing brittleness resistance and weld zone toughness has a composition comprising, by mass, ≤0.03% C, ≤1.2% (preferably 0.3 to 1.2%) Si, ≤1.0% Mn, ≤0.04% P, ≤0.01% S, ≤2% Ni, 19 to 26% Cr, ≤0.03% N, 0.5 to 1.5% Al, ≤0.3% Ti and 0.0005 to <0.003% B, and, if required, comprising one or more kinds selected from ≤0.5% Nb, 0.05 to 0.5% Mo, 0.05 to 0.5% Cu, 0.05 to 0.5% V, 0.05 to 0.5% Zr, 0.001 to 0.05% REM (rare earth metals) and 0.001 to 0.01% Ca, and the balance Fe with inevitable impurities. and in which increased volume by oxidation after 1,000°C×800 h in a sheet thickness of 0.5 mm is ≤5 mg/cm<SP>2</SP>, transition temperature in a drop hammer test satisfies ≤-50°C, transition temperature in a Charpy impact test in a TIG toe weld zone in a sheet thickness of 1.5 mm satisfies <0°C, and also, inequality (1): 1.5≤Si%+2Al%+100(B%)≤3.0 is satisfied. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention is suitable for automobile exhaust gas path members such as exhaust gas sensors, heating elements of electric heaters, combustion cylinders of stoves, fuel cell reformers, solid oxide fuel cells, and other members used in high temperature environments. The present invention relates to a ferritic stainless steel excellent in high-temperature oxidation resistance, secondary work brittleness, and weld toughness.

  Ferritic stainless steel containing Al, Cr, and Si is characterized by excellent high-temperature oxidation resistance. Automotive exhaust path members such as exhaust gas sensors, heating elements of electric heaters, combustion cylinders of stoves, fuel cell reformers It is used as a material for parts exposed to high temperatures such as solid oxide fuel cells. The reason why these ferritic stainless steels exhibit excellent high-temperature oxidation resistance is that Al-based, Cr-based, and Si-based oxides are mainly formed on the material surface at high temperatures. This oxide forms a strong and dense oxide film, which serves as a protective layer against oxidation.

  When used in exhaust gas path members of automobiles such as exhaust gas sensors, heating elements of electric heaters and combustion cylinders of stoves, fuel cell reformers, solid oxide fuel cells, etc., by starting and stopping the engine or equipment, Exposure to repeated heating and cooling. The oxide film formed on the surface of the material by this repeated heating / cooling causes peeling due to the difference in thermal expansion coefficient from the metal base material. When the oxide film is peeled off, the metal base material under the peeled oxide film is directly exposed to the outside air, and the oxidation proceeds again at that portion. The higher the temperature, the higher the oxidation rate, and the more the oxide film peels off during cooling, so that the oxidation of the material is accelerated. Further, when the temperature is increased, the material is greatly deformed, which is likely to cause appearance problems and functional troubles. This is because the material strength generally decreases at high temperatures, and stress is generated on the material surface due to the growth of the oxide film.

  In addition, since these products often have very complicated product shapes, it is important that these products are excellent in elongation, stretchability, secondary workability, and the like. However, ferritic stainless steel, which has a high degree of processing difficulty, is often used from the viewpoint of suppressing thermal fatigue and reducing costs. Ferritic stainless steel has a problem that it tends to crack when subjected to severe processing or processing at low temperatures in winter, because it is elongated, inferior, and secondary workability is inferior to austenitic stainless steel.

  B is known as an element that improves the secondary work brittleness of ferritic stainless steel. Fe-Cr-Al ferritic stainless steels with improved workability by adding B are disclosed in JP-A Nos. 2001-316773 and 2004-307918. This is disclosed that the secondary workability is improved by the addition of B, but all are evaluated only in the primary work and considered to be insufficient. Exhaust gas path members and various combustors of automobiles are often complicated in structure, and it is important not to cause cracking even when processing at low temperatures in winter, so it is necessary to grasp secondary work brittleness and toughness deterioration of welds It is. Japanese Patent Application Laid-Open No. 10-158791 describes the improvement of oxidation resistance by containing B. However, since the optimum amount of B added is as large as 40 ppm to 200 ppm, there is a problem in manufacturability and toughness of the welded portion. It becomes. In addition, since the amount of Al added is large, the oxidation resistance is excellent, but the degree of processing difficulty is increased, and processing by the user and weld cracking are likely to occur. Japanese Patent Application Laid-Open No. 5-202449 has improved high-temperature oxidation resistance by the addition of B. However, since Zr and La, which are very expensive elements, are essential elements, there is a problem from the viewpoint of economy. Moreover, the improvement effect of secondary work embrittlement and weldability by addition of B is not described.

JP 2001-316773 A JP 2004-307918 A JP 10-158791 A JP-A-5-202449

  As described above, any of the above known techniques improves oxidation resistance by adding a large amount of an element effective for oxidation. Therefore, while improving oxidation resistance, it has the problem of degrading workability, and has the common problem of not satisfying secondary work brittleness and oxidation resistance simultaneously.

  The present invention provides a Fe-Cr-Al-Si ferritic stainless steel that is not easily oxidized abnormally even when used at a site exposed to an exhaust gas of about 1000 ° C, and is excellent in secondary work brittleness and weld toughness. It is characterized by providing.

Al forms very dense Al ₂ O ₃ and is effective in improving high-temperature oxidation resistance. Moreover, in addition to the effects of suppressing the occurrence of abnormal oxidation and preventing the peeling of the oxide film, it is also effective for improving the redness. However, the addition of excess Al or Si must be limited to an amount that does not cause deterioration from the viewpoints of manufacturability and workability because the toughness of the material is greatly deteriorated.

  In the present invention, it was confirmed that by adding B, not only the secondary work brittleness can be improved, but also the oxidation resistance and the toughness of the welded portion can be improved. The reason for this has not been clarified yet, but B is segregated to the grain boundaries preferentially over P and S, so that P and S that are harmful to oxidation resistance and secondary workability are segregated to the grain boundaries. It is presumed that this can be suppressed. For this reason, not only secondary workability but also oxidation resistance can be greatly improved. It has also been found that the toughness of the welded portion is greatly improved by addition of B. For that purpose, it is 0.0005 to less than 0.003%. If 0.003% or more is added, a boride is formed at the grain boundary, and the weldability and secondary workability are reduced.

The configuration of the present invention is as follows.
The invention according to claim 1 is, in mass%, C: 0.03% or less, Si: 1.2% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01 %: Ni: 2% or less, Cr: 19-26%, N: 0.03% or less, Al: 0.5-1.5%, Ti: 0.3% or less, B: 0.0005-0 Less than .003, consisting of the balance Fe and inevitable impurities, a plate thickness of 0.5 mm, an increase in oxidation after 1000 ° C. × 800 h is ≦ 5 mg / cm ² , a transition temperature in a drop weight test is ≦ −50 ° C., and The transition temperature in the Charpy impact test of TIG tanned welds with a plate thickness of 1.5 mm satisfies <0 ° C and is excellent in oxidation resistance, secondary work brittleness and weld toughness satisfying the following formula (1) Ferritic stainless steel. Here, each term is the content (mass%) of each alloy element.
1.5 ≦ Si% + 2Al% + 100 (B%) ≦ 3.0 (1)

  According to the second aspect of the present invention, Nb is 0.5% or less, Mo is 0.05 to 0.5%, Cu is 0.05 to 0.5%, and V is 0.05 to 0.5. The ferritic stainless steel having excellent oxidation resistance, secondary work brittleness and toughness of the welded portion according to claim 1, comprising one or more of 5%, Zr: 0.05 to 0.5%. .

  The invention according to claim 3 further includes one or more of REM (rare earth element): 0.001 to 0.05% and Ca: 0.0005 to 0.01%. 2. Ferritic stainless steel excellent in oxidation resistance, secondary work brittleness and toughness of welded portion described in 2.

  The invention according to claim 4 is a ferrite excellent in oxidation resistance, secondary work brittleness and toughness of a welded portion according to claims 1-3, characterized in that the lower limit of Si is 0.3%. Stainless steel.

  A fifth aspect of the present invention is the stainless steel according to the first to fourth aspects, which is for an exhaust gas sensor of an automobile.

  The ferritic stainless steel according to the present invention is unlikely to be abnormally oxidized even when used in a portion exposed to an exhaust gas of about 1000 ° C., and is excellent in secondary work brittleness.

The reasons for limiting each element are described below.
C: 0.03 mass% or less When the C content is high, abnormal oxidation tends to occur. Moreover, in high Al content ferritic stainless steel, when C content becomes high, the toughness of a slab and a hot coil will deteriorate, and manufacturability will deteriorate. Therefore, the upper limit of the C content is limited to 0.03% or less.

Si: 1.2 mass% or less Si has an effect of suppressing red scale formation of ferritic stainless steel. However, excessive addition deteriorates the secondary workability and the toughness of the weld. Therefore, the Si addition amount is limited to 1.2% or less. Preferably, it is 0.3 to 1.2%, more preferably 0.3 to 1.0%, and further preferably 0.5 to 0.8% for improving red scale resistance.

Mn: 1.0% by mass or less Mn generates a Mn-based oxide, inhibits the formation of a dense Al oxide layer, and adversely affects high-temperature oxidation resistance. Therefore, in order to maintain high temperature oxidation resistance, the Mn content is limited to 1.0% or less. Preferably, it is 0.5% or less.

P: 0.04 mass% or less P has an adverse effect on high-temperature oxidation resistance and toughness of hot-rolled sheet, so its content is limited to 0.04% or less.

S: 0.005 mass% or less S is a component inevitably contained in steel, and remarkably inhibits the formation of an Al ₂ O ₃ film. Therefore, the S content is limited to 0.005% or less.

Ni: 2% by mass or less Ni is an element that improves the hot-rolled sheet toughness and low-temperature toughness of ferritic stainless steel, but a large amount of addition impairs the stability of the phase and is accompanied by hardening of the steel. Therefore, set the upper limit to 2%.

Cr: 19 to 26% by mass
Cr is a basic and effective element as an element for improving high-temperature oxidation resistance. Al and / or Si must be added to ensure the necessary red scale resistance and oxidation resistance. However, these elements simultaneously reduce secondary workability and weld toughness, and will be described later. In addition, it is necessary to minimize the addition amount of Al and Si as much as possible. In order to obtain good high-temperature oxidation resistance and red scale resistance with the added amount, addition of 19% or more is necessary. However, excessive addition degrades 475 ° C embrittlement susceptibility and slab and hot coil toughness. Therefore, the Cr content is limited to 19 to 26%. Preferably it is 19.5-22%, More preferably, it is 20-21%.

N: 0.03 mass% or less N combines with Al in steel to form AlN, and becomes a starting point for abnormal oxidation. Therefore, the N content is limited to 0.03% or less in order to improve the high temperature oxidation resistance.

Al: 0.5 to 1.5% by mass
Al, like Cr, is the most important element for obtaining high-temperature oxidation resistance. When 19% or more of Cr is added, excellent high-temperature oxidation resistance is obtained by a dense Al oxide formed on the surface of the steel by adding 0.5% or more of Al, but excessively contains Al. If so, the upper limit is limited to 1.5% in order to raise the toughness deterioration of the slab and hot coil and the secondary processing brittle temperature during product processing.

B: 0.0005 or more and less than 0.003% by mass B is an important element that not only improves secondary work brittleness, but also improves oxidation resistance and weld toughness, as described above. For that purpose, addition of 0.0005 to less than 0.003% is required. If 0.003% or more is added, a boride is formed at the grain boundary, and the weldability and secondary workability are reduced. Preferably it is 0.0005 to 0.002%.

Ti: more than 0.05 to 0.3% by mass
Ti has an effect of significantly improving toughness by combining with C and N in steel. Further, when Ti is added, the high temperature strength of the steel is increased, and stress generated in the process of growing the oxide film is relieved to prevent deformation of the material. In order to obtain these effects of addition of Ti, it is necessary to add over 0.05%. However, if added too much, the toughness of the steel is lowered and the 475 ° C. embrittlement susceptibility increases. Limited to 3%.

Mo, Cu, V, Zr: 0.05 to 0.5 mass%
Addition of 0.05% or more of these elements improves the high temperature strength and heat fatigue resistance of ferritic stainless steel by solid solution strengthening or precipitation strengthening. Therefore, it is added as necessary, particularly in a region where thermal fatigue characteristics are considered. However, since excessive addition of the steel material hardens excessively, the upper limit of each was set to 0.5%.

Nb: 0.5% by mass or less Nb is an element that improves the ductility and workability by fixing solute C and N in steel as carbides. Moreover, the effect which suppresses the grain boundary precipitation of Cr carbide and improves corrosion resistance can also be expected. In order to obtain these effects, Nb may be added at 0.5% or less as necessary.

Rare earth element: 0.01-0.10 mass%
It is an important element that improves high temperature oxidation resistance. Rare earth elements such as La and Ce are thought to improve high temperature oxidation resistance by stabilizing the Al oxide film formed on the surface and improving the adhesion between the matrix and the oxide film. . This effect is effective when 0.01% or more of at least one rare earth element is added. However, if added excessively, hot workability and toughness are deteriorated, and inclusions that become the starting point of abnormal oxidation are likely to be generated, and the high-temperature oxidation resistance is lowered. Therefore, the upper limit of addition amount is 0.10%.

Ca: 0.0005 to 0.01% by mass
It is a component added as necessary, and by fixing S, the oxidation resistance of the ferritic stainless steel is further improved. The effect of addition becomes remarkable at 0.0005% or more. However, if an excessive amount of Ca is contained, the steel material is excessively hardened, and surface flaws are likely to occur during production, leading to an increase in production cost. Therefore, the upper limit was set to 0.01%.

Limiting formula: 1.5 ≦ Si% + 2Al% + 100 (B%) ≦ 3.0 (1)
This range is a steel to which 19 to 22% Cr and a predetermined amount of Al and Si are added, and satisfies both oxidation resistance, red scale resistance, secondary workability, and low temperature toughness of the welded portion. Each term is the content (% by mass) of each alloy element.
When this value is less than 1.5, oxidation resistance and red scale resistance cannot be satisfied. On the other hand, if this value exceeds 3.0, the toughness of the weld cannot satisfy the target. The upper limit is preferably less than 3.0.

Table 1 shows the chemical component values of the test materials. The steel shown in Table 1 was melted in a vacuum, and hot-rolled steel plate having a thickness of 3.5 mm was annealed, cold-rolled and pickled to produce a 0.5 mm cold-rolled annealed pickled material. . A test piece having a size of 25 mm × 35 mm was prepared from the obtained plate material, and a continuous oxidation test of 1000 ° C. × 800 h in the furnace was performed in an air atmosphere in an electric furnace, and as a red scale resistance test, 600 After each 300-h continuous oxidation test was performed in a nitrogen atmosphere containing 50% water vapor at 0 ° C., the weight of the test piece was measured. The measurement results of the increase in oxidation were good when the weight change was 5 mg / cm ² or less compared to the weight before the test (O: the same applies below), and poor when the weight change exceeded 5 mg / cm ² (× : The same applies below). As the evaluation of red scale resistance, a case where the weight change before and after the test was 0.2 mg / cm ² or less was evaluated as “◯”, and a case where the weight change exceeded 0.2 mg / cm ² was evaluated as “X”. In addition, the evaluation sample for secondary processing brittleness was prepared by dissolving and hot rolling in the same manner as the preparation of the oxidation test piece, and then repeatedly annealing and cold rolling to produce a plate material of 0.6 mm and a crystal grain size of 7.5. did. From the obtained plate material, a blank size was cut out with a diameter of 40 mm and subjected to drawing with a drawing ratio of 2.25. Thereafter, a drop weight test was performed by changing various temperatures using the primary drawn product. The drop weight test was performed with a weight of 3 kg, a weight height of 100 mm, and each temperature n = 3, and the material temperature when cracking occurred was defined as the transition temperature. A sample having a temperature of −50 ° C. or lower was evaluated as “◯”, and a sample cracked at a temperature higher than −50 ° C. was evaluated as “X”. These required characteristics are necessary and sufficient conditions that can be applied as a material for a high-temperature sensor (temperature sensor, oxygen sensor, or air ratio sensor) in an automobile exhaust gas part.

The embrittlement of the welded portion is as follows: 105A, 10V DC current, welding speed 300mm / min, electrode diameter φ2.4mm, argon seals for both front and back seal Charpy test piece that was subjected to TIG tanning welding at 10 L / min, and V-notched on the tanning weld (JIS excluding plate thickness)
Z2202), and a Charpy impact test was performed at a material temperature of 0 ° C. at n = 3 by a method according to JIS Z2242, and an average value indicating an impact value of 20 J / cm ² or more was good. A sample having an average impact value of less than 20 J / cm ² was evaluated as x.
As can be seen from the test results in Table 2, all of the steel types A1 to A16 of the examples of the present invention are excellent in oxidation resistance, secondary workability, and weld toughness. Further, all the red scale resistances are satisfied except for A1 (Si <0.3%).

  In the case of the comparative examples, neither oxidation resistance, secondary work brittleness and toughness of the welded portion are compatible and insufficient. With regard to oxidation resistance, when either of the Al addition amount and the formula (1) is not achieved, or when the B addition amount is not reached with respect to the secondary workability, the toughness of the welded portion is Si excess, the formula (1 ) Exceeding the upper limit, or the formula (1) is 2 or more and the B addition amount is not reached (B1 to B5).

The ferritic stainless steel according to the present invention is excellent in high-temperature oxidation resistance, secondary work brittleness, and weld toughness, and is used in automobile exhaust gas path members such as exhaust gas sensors, heating elements of electric heaters, combustion cylinders of stoves, fuel cells It is suitable for a member used in a high-temperature environment such as a reformer or a solid oxide fuel cell.

Claims (5)

In mass%, C: 0.03% or less, Si: 1.2% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% or less, Ni: 2% or less, Cr: 19-26%, N: 0.03% or less, Al: 0.5-1.5%, Ti: 0.3% or less, B: 0.0005 or more and less than 0.003%, the balance Fe And TIG at a plate thickness of 0.5 mm after 1000 ° C. × 800 h with an increase in oxidation of ≦ 5 mg / cm ² , a transition temperature in a drop weight test of ≦ −50 ° C., and a plate thickness of 1.5 mm A ferritic stainless steel having a transition temperature of <0 ° C. in the Charpy impact test of a tanned weld and excellent oxidation resistance, secondary work brittleness, and weld toughness satisfying the following formula (1).
1.5 ≦ Si% + 2Al% + 100 (B%) ≦ 3.0 (1)
(Each item is the content (mass%) of each alloy element.)   Further, by mass, Nb: 0.5% or less, Mo: 0.05-0.5%, Cu: 0.05-0.5%, V: 0.05-0.5%, Zr: 0.5. The ferritic stainless steel excellent in oxidation resistance, secondary work brittleness and toughness of a welded portion according to claim 1, comprising one or more of 0.5 to 0.5%.   The oxidation resistance according to claim 1, further comprising one or more of REM (rare earth element): 0.001 to 0.05% and Ca: 0.0005 to 0.01%. Ferritic stainless steel with excellent secondary work brittleness and weld toughness.   The ferritic stainless steel excellent in oxidation resistance, secondary work brittleness and toughness of a welded portion according to claims 1 to 3, wherein Si is 0.3% or more.   The stainless steel according to claim 1, wherein the stainless steel is used for an automobile exhaust gas sensor.