JP5138504B2 - Ferritic stainless steel for exhaust gas flow path members - Google Patents

Description

  The present invention relates to a ferritic stainless steel that is used as an exhaust gas passage member for various internal combustion engines including automobiles, such as an exhaust manifold, a front pipe, a center pipe, and a catalytic converter outer cylinder, and has excellent heat resistance, low temperature toughness, and weldability. .

  The exhaust gas flow path member of an automobile is exposed to a high temperature atmosphere that the exhaust gas directly touches during operation, and is subjected to thermal stress and vibration of the engine during operation due to repeated operation and stop. In cold regions, mechanical stress at low temperatures is also applied at the start of winter. Therefore, the material used for the exhaust system member is required to have durability in a very severe environment.

  When stainless steel plates and pipes are used as exhaust gas flow path members, they are not only excellent in heat resistance, but are also required to have excellent weldability and workability because they are assembled into product shapes by welding and processing. It is. Toughness (low temperature toughness) that can withstand secondary mechanical processing during molding and mechanical loads at low temperatures during use is also required.

Ferritic stainless steel has smaller thermal expansion than austenitic stainless steel, and is excellent in thermal fatigue characteristics and scale peel resistance. Since the steel material cost is also low, ferritic stainless steel is often used for the exhaust gas flow path member.
Ferritic stainless steel has inherently lower high temperature strength than austenitic stainless steel, and therefore has been improved to improve high temperature strength. For example, SUS430J1l-based Nb-added steel, Nb, Si composite-added steel (JP-A-3-274245), Nb, Mo composite-added steel (JP-A-5-125491), and the like. Among these, Nb and Mo composite-added steel has the highest high-temperature strength and is used in parts that require excellent thermal fatigue characteristics. However, Nb and Mo composite-added steels tend to be inferior in workability and low temperature toughness as compared with other steel types. Although there are some cases where workability and low temperature toughness are improved, it is not necessarily sufficient. It is also a disadvantage that the steel material cost is high because it contains a large amount of expensive element Mo.

  By the way, looking at the correlation between high-temperature strength (heat-resistant fatigue failure) and the high-temperature oxidation characteristics (abnormal oxidation limit temperature), which are most important in an environment where the exhaust gas path member is exposed, the high-temperature strength and high-temperature oxidation characteristics are not necessarily high. Some parts do not need to be balanced at the level. Specifically, in areas where the structure is very complex even if the exhaust gas temperature is not very high, high-temperature strength is more important than high-temperature oxidation characteristics, and workability and low-temperature toughness that can handle the complexity of the structure are also important. . At present, Nb and Mo composite-added steel must be used for such parts, and there is room for improvement in terms of workability, low temperature toughness, and cost, even though the heat resistance is sufficient.

  The present invention has been devised to solve such problems, and without using expensive Mo as an alloy component, it has heat resistance comparable to that of Nb, Mo composite added steel, workability, An object of the present invention is to provide a ferritic stainless steel for exhaust gas passage members that is excellent in low temperature toughness and weldability.

In order to achieve the object of the ferritic stainless steel for exhaust gas flow path member of the present invention, C: 0.03% by mass or less, Si: 1.0% by mass or less, Mn: 1.5% by mass or less, Ni: 0.6 mass% or less, Cr: 10 to 20 mass%, Nb: 0.50 mass% or less, Ti: 0.05 to 0.30 mass%, Cu: 1.0 to 2.0 mass%, Al: 0.03% by mass or less, V: 0.03 to 0.20% by mass, N: 0.03% by mass or less, and satisfying Nb ≧ 8 (C + N) (however, Cr: 14 to 19% by mass, Si: 0.5% by mass or less, Mn: 0.5% by mass or less, Al: 0.1% by mass or less, Ti: 0.02 to 0.40% by mass, Nb: 0 -0.5 mass%, Cu: 1.0 mass%, V: 0-1.0 mass%, Ni: 0-1.0 mass%, Ca: 0-0.0 mass% 5% by mass, the balance being Fe and inevitable impurities, C as impurities: 0.015% by mass or less, N: 0.015% by mass or less, P: 0.04% by mass or less, and the following ( 1) to (4) except for ferritic stainless steels that satisfy the equations (4).
C (mass%) + N (mass%) ≦ 0.025 (mass%) (1)
Ti (mass%) / (C (mass%) + N (mass%)) ≧ 5 (2)
0.5 × C (mass%) ≦ S (mass%) ≦ 0.001 (mass%) (3)
Ti (mass%) / Mn (mass%) ≧ 0.5 (4)
This ferritic stainless steel does not contain Mo as an alloy component, and can further contain 0.0005 to 0.02 mass% B in order to further improve the secondary workability.

  Conventionally, SUH409L, SUS430J1l, and SUS429 stainless steels have been used as materials that satisfy the heat resistance characteristics in the environment to which the exhaust gas flow path member is exposed, but the conventional steel types have a maximum temperature of about 800 to 900 ° C. There are also parts that require much higher high-temperature strength. The site where high temperature strength is required is also a place where thermal fatigue failure is likely to occur because the structure is very complicated and thermal stress is repeatedly applied. In addition, workability and low-temperature toughness that cannot be obtained with Mo-added steel are required for the constituent material of the part.

The present inventors investigated the influence of various alloy elements in order to satisfy the characteristics required for the constituent materials of such parts. As a result, it was found that the combined addition of V and Cu improved the high-temperature strength at 900 ° C. or lower, workability, and low-temperature toughness, and the same level as the Nb and Mo-added steel was obtained.
With respect to Nb-containing ferritic stainless steel in which a small amount of V was added and the Cu content was changed, 0.2% yield strength was measured by a high-temperature tensile test at 700 ° C and 800 ° C. As a result, it has been found that the high temperature strength of 900 ° C. or lower is dramatically increased by the addition of a small amount of V and the Cu content, and a high temperature strength comparable to that of the Nb, Mo composite added steel can be obtained.

FIG. 1 shows the results of a tensile test of steel obtained by adding Cu at various contents to the basic composition of 17Cr-0.4Nb-0.1V steel. In FIG. 1, the strength level of SUS444 series steel which has 18Cr-2Mo-0.4Nb which is Nb and Mo compound addition steel as a basic component is also shown for the comparison.
As is clear from the results of FIG. 1, the 0.2% proof stress at 700 ° C. and 800 ° C. increases rapidly as the amount of Cu increases. When the Cu content is 0.8% by mass or more, 0.2% proof stress equivalent to or higher than that of the SUS444 system containing about 2% by mass of Mo is obtained. As for 0.2% proof stress at 900 ° C, it was confirmed in another experiment that the increase in V and Cu did not reach the level of SUS444 steel, but improved 0.2% proof stress over Nb-containing ferritic stainless steel. doing. That is, the combined addition of V and Cu is effective in improving the high-temperature strength in the temperature range of 900 ° C. or lower, and no serious adverse effect appears at 900 ° C. or higher.

  The reason why the combined addition of Nb, Cu, and V ensures a high level of high-temperature strength has not been fully elucidated, but Nb-based precipitates and Cu-based precipitates are observed in both short time and long time heating. A finely dispersed structure is observed as compared with the steel containing Nb alone. From this observation result, generation of Nb-based precipitates and Cu-based precipitates is suppressed by preferentially precipitating V in the annealed state or in the initial stage of heating. As a result, fine Nb-based precipitates and Cu-based precipitates are formed. It is presumed that the dispersion precipitates and contributes to precipitation strengthening. Presumably, the finely dispersed precipitates at the initial stage of heating do not aggregate even when heated for a long time, and the precipitation strengthening works effectively for a long time.

Further, since V, which is a strong carbonitride-forming element, is combined with C and N, when the amount of Nb added is the same, more solute Nb effective in improving the high temperature strength can be secured than in the steel without V addition. . In other words, it can be achieved with the Nb content in which the high temperature strength at the same level as that of the V-free steel is reduced, and as a result, it contributes to the improvement of workability and low temperature toughness.
In addition, in a system in which Nb and V coexist, the amount of Nb-based and V-based carbonitrides in an annealed state increases. As the Nb-based and V-based carbonitrides increase, the crystal grains in the weld heat-affected zone are less likely to be coarsened, and the toughness is improved. Since the formation of Cr-based carbide is also suppressed, the intergranular corrosion resistance is improved.

Hereinafter, the alloy components, contents, and the like of the ferritic stainless steel targeted by the present invention will be described.
C: 0.03 mass% or less
N: 0.03 mass% or less C and N are generally effective elements for improving high-temperature strength such as creep strength, but if included excessively, oxidation characteristics, workability, low-temperature toughness, weldability Decreases. In this component system in which V and Nb are added as elements for fixing C and N as carbonitrides, it is necessary to add V and Nb in amounts corresponding to the C and N concentrations. In order to suppress an increase in raw material cost due to the increase in V and Nb, both C and N were regulated to 0.03% by mass or less (preferably 0.015% by mass or less).

Si: 1.0% by mass or less Si is an element that is very effective for improving high-temperature oxidation characteristics, but is not so effective in increasing high-temperature strength at 900 ° C. or less. Conversely, when Si is excessively added, the hardness increases, and the workability and low temperature toughness decrease. Therefore, the Si content is regulated to 1.0% by mass or less (preferably 0.1 to 0.5% by mass).
Mn: 1.5% by mass or less Although Mn is an alloy element that improves high-temperature oxidation characteristics of ferritic stainless steel, particularly scale peelability, excessive addition of Mn deteriorates workability and weldability. Moreover, since it is an austenite phase stabilizing element, when Mn is excessively added to a steel type with a small amount of Cr, a martensite phase is likely to be generated, resulting in deterioration of thermal fatigue characteristics and workability. Therefore, the Mn content is regulated to 1.5% by mass or less (preferably 0.5% by mass or less).

Ni: 0.6 mass% or less An austenite phase stabilizing element. When Ni is added excessively to a steel type having a low Cr content, a martensite phase is generated in the same manner as Mn, and thermal fatigue characteristics and workability are lowered. The excessive addition of Ni should also be avoided due to the high raw material price. Therefore, the Ni content is regulated to 0.6% by mass or less (preferably 0.5% by mass or less).
Cr: 10 to 20% by mass
In addition to stabilizing the ferrite phase, it is an essential element for improving oxidation resistance, which is important for high-temperature materials. From the standpoint of oxidation resistance, the higher the Cr content, the better. However, if excessively added, the steel material becomes brittle, and the workability deteriorates due to the increase in hardness. Therefore, the Cr content is selected in the range of 10 to 20% by mass. The Cr content is preferably adjusted according to the use temperature of the material. For example, 16 to 19% by mass is preferable for high temperature oxidation resistance up to 950 ° C., and 12 to 16% by mass is sufficient for high temperature oxidation resistance at 900 ° C. or less.

Nb: 8 (C + N) to 0.50 mass%
C and N are fixed as carbonitride, and the remaining solid solution Nb to which carbonitride is fixed exhibits the effect of increasing the high-temperature strength. However, when an excessive amount of Nb is added, workability and low-temperature toughness deteriorate, and the weld hot cracking susceptibility increases. Nb content satisfying Nb ≧ 8 (C + N) is necessary for fixing C and N, but the upper limit of Nb content is set to 0.5% in order to suppress adverse effects on workability, low temperature toughness and weld hot cracking susceptibility. Set to mass%. Preferably, an Nb content satisfying 8 (C + N) + 0.10 ≦ Nb ≦ 0.45 is selected.
Cu: 0.8-2.0 mass%
In this component system, it is a very important element for improving high-temperature strength. In the temperature range investigated by the present inventors, almost the entire amount of Cu is dissolved in the annealed matrix and precipitates during heating. Precipitated Cu exhibits a strengthening action in the initial stage as in the case of Mo-added steel, but the strengthening action gradually disappears when heated for a long time. In order to obtain the necessary high-temperature strength, a Cu content of 0.8% by mass or more is necessary as is apparent from FIG. However, with increasing Cu content, workability, low temperature toughness, and weldability deteriorate. In order to suppress adverse effects on workability, low temperature toughness and weldability, the upper limit of Cu content is regulated to 2.0 mass%. A preferable Cu content is in the range of 1.0 to 1.7% by mass.

Al: 0.03% by mass or less Al is added as a deoxidizer during steelmaking, and also exhibits an effect of improving high-temperature oxidation resistance. However, excessive addition of Al deteriorates the surface properties and adversely affects workability, weldability, and low temperature toughness. Therefore, the lower the Al content, the better. The upper limit is regulated to 0.03% by mass (preferably 0.02% by mass).
V: 0.03-0.20 mass%
When combined with Nb and Cu, the high temperature strength of ferritic stainless steel is improved. In addition, coexistence with Nb improves workability, low-temperature toughness, and intergranular corrosion susceptibility, and also improves the toughness of the heat affected zone. Although these effects appear at a V content of 0.03% by mass or more, excessive addition exceeding 0.20% by mass leads to deterioration of workability and low temperature toughness. Therefore, the V content is selected in the range of 0.03 to 0.20 mass% (preferably 0.04 to 0.15 mass%).

Ti: 0.05-0.30 mass%
It is an element that improves the r value (Rankford value) of steel and improves formability, and the effect of addition becomes noticeable at 0.05% by mass or more. However, when an excessive amount of Ti is added, the surface properties of the steel material deteriorate due to the formation of TiN, and the weldability and low temperature toughness are also adversely affected. Therefore, it is desired to reduce the Ti content as much as possible even when adding Ti to improve the moldability. Therefore, the upper limit of the Ti content is regulated to 0.30% by mass (preferably 0.20% by mass).
B: 0.0005 to 0.02 mass%
It is an element that improves the secondary workability of steel and suppresses cracking during multi-stage forming, and the effect of adding B becomes remarkable at 0.0005% by mass or more. However, if a large amount of B is added, the productivity and weldability deteriorate. Therefore, the B content is selected in the range of 0.0005 to 0.02 mass% (preferably 0.001 to 0.01 mass%).
Mo: Less than 0.10% by mass The ferritic stainless steel of the present invention is premised on that expensive Mo is not added, but is an element that is easily mixed as an inevitable impurity during the production of stainless steel. If a large amount of Mo is mixed, there are harmful effects such as deterioration of workability, low temperature toughness, and weldability. Therefore, it is desirable to limit the mixing amount to less than 0.10% by mass.

  Although elements other than those listed above are not particularly restricted, it is preferable to reduce general impurities such as P, S, and O as much as possible. Considering hot workability, oxidation resistance and the like, it is preferable that the upper limit of P, S, and O is 0.04 mass%, 0.03% by mass, and 0.02 mass%, respectively. W, Zr, Y, REM (rare earth elements) effective for improving heat resistance, Ca, Mg, Co, etc. effective for improving hot workability can be added as necessary.

  The production conditions of the ferritic stainless steel are not particularly restricted, and as long as Cu is dissolved in advance, excellent heat resistance can be obtained with the hot-rolled annealed plate. When a steel plate having a desired plate thickness cannot be produced by hot rolling, a steel plate having heat resistance equivalent to that of the hot rolled annealing plate can be produced by repeating cold rolling and annealing one or more times. When Cu is finely dispersed at any stage of the manufacturing process as required, a better high temperature strength can be obtained. The excellent characteristics are maintained even after hot-rolled annealed plates, cold-rolled annealed plates, etc. are processed or welded (including tube forming) into a desired shape.

Next, the present invention will be described more specifically with reference to examples.
Various ferritic stainless steels having the compositions shown in Tables 1 and 2 were melted in a vacuum melting furnace and cast into a 30 kg ingot. The ingot was forged, and cold-rolled annealed plates having thicknesses of 2.0 mm and 1.2 mm were manufactured through hot rolling, annealing, cold rolling, and finish annealing. In the table, No. 1-6 is a reference steel, No. 7-10 are steels of the present invention , No. 11 to 19 are comparative steels. Among the comparative steels, No. 11 is SUS430J1l equivalent steel, No. 11; 15 is SUH409L equivalent steel, No.15. 16 is 14Cr—Si—Nb steel, No. 16; No. 17 is SUS444 equivalent steel, and all steel types have a track record of use for exhaust manifolds.

A cold-rolled annealed plate having a thickness of 2.0 mm was subjected to a high-temperature tensile test, a high-temperature oxidation test, a room temperature tensile test, and a Charpy impact test, and a cold-rolled annealed plate having a thickness of 1.2 mm was subjected to a welding hot crack test.
In the high-temperature tensile test, the test piece was pulled at 800 ° C. in accordance with JISG0567 and the 0.2% yield strength was measured.
In the high-temperature oxidation test, the test piece was continuously heated to 850 ° C., 900 ° C., 950 ° C., 1000 ° C., and 1100 ° C. for 200 hours in accordance with JISZ2281. The heated specimen was visually observed for the occurrence of abnormal oxidation (thick oxide penetrating in the thickness direction), and the limit temperature at which abnormal oxidation did not occur was determined.

In the room temperature tensile test, a cold-rolled annealed plate having a thickness of 2.0 mm was processed into a No. 13B test piece in accordance with JISZ2241, and the elongation at break after the tensile test was determined.
In the Charpy impact test, the test piece was impacted at −75 ° C., −50 ° C., −25 ° C., 0 ° C., and 25 ° C. using a sub-size test piece having a thickness of 2.0 mm in accordance with JISZ2242. In addition, the ductility-toughness transition temperature was determined.
In the welding hot cracking test, both ends of a 40 mm × 20 mm test piece were held and subjected to TIG welding in a state in which a tensile stress was applied in the longitudinal direction, and the minimum strain amount at which cracking started to occur was determined. The obtained critical strain amount was used as an index of the hot cracking sensitivity.

The above test results are shown in Table 3.
No. Inventive steels 1 to 10 and reference steels 1 to 10 each have a 0.2% proof stress of 800 ° C. significantly higher than Ti-added steel (No. 15), Nb, and Si-added steel (No. 16). It was a 0.2% proof stress value comparable or superior to that of Nb, Mo composite added steel (No. 17). Elongation at room temperature tensile test, ductile brittle transition temperature by Charpy impact test, critical strain by welding hot cracking test have characteristics equal to or better than Nb and Mo composite added steel (No. 17), no Mo added However, it was confirmed that the target performance was obtained. Regarding abnormal oxidation, no. 4, no. 5, no. As can be seen from the result of 12, the lower the Cr content, the lower the limit temperature. From the influence of the Cr content on the abnormal oxidation, it can be understood that it is necessary to set an appropriate amount of Cr according to the temperature of the application location.

Comparative steel No. lacking V and Cu. 11, no. 15, no. 16, no. Although No. 19 has sufficient workability, low temperature toughness and weldability, the high temperature strength at 800 ° C. is inferior. Comparative steel containing excessive Cu No. Although No. 12 was excellent in high temperature strength, the workability and weldability were inferior to those of the Nb and Mo composite added steel, which hindered the processing and welding into the product shape.
Even when the Cu content is within the specified range, the comparative steel No. 13 and comparative steel No. 1 with too much Nb content. No. 14 was inferior to the steel of the present invention in workability, low temperature toughness and weldability even though it was excellent in high temperature strength.

Comparative Steel No. 1 with low V content and high Al content. No. 18 has the same heat resistance and workability as the steel of the present invention, but is inferior in low temperature toughness, and troubles due to insufficient toughness during product processing and use are expected. Comparative steel No. No. 19 lacks high temperature strength.
Comparative steel No. containing Mo No. 17 has the same performance as the steel of the present invention, but the low-temperature toughness is slightly lower. And since Mo is contained about 2 mass%, it is inevitable that material cost will become higher than this invention steel.

  As explained above, by strictly regulating the content of various alloy elements contained in ferritic stainless steel, especially the range of V and Cu, expensive Mo is not required and excellent heat resistance is ensured. However, workability, low temperature toughness, and weldability are improved, and ferritic stainless steel suitable for exhaust gas path members can be obtained. This ferritic stainless steel is used for exhaust gas passage members such as automobile engines, exhaust manifolds, front pipes, center pipes, catalytic converter outer cylinders and the like by utilizing excellent characteristics.

FIG. 1 is a graph showing the effect of Cu on 0.2% yield strength at high temperatures.

Claims (3)

C: 0.03 mass% or less, Si: 1.0 mass% or less, Mn: 1.5 mass% or less, Ni: 0.6 mass% or less, Cr: 10 to 20 mass%, Nb: 0.50 mass %: Ti: 0.05 to 0.30 mass%, Cu: 1.0 to 2.0 mass%, Al: 0.03 mass% or less, V: 003 to 0.20 mass%, N: 0.00. Ferritic stainless steel for automobile exhaust gas flow path members (provided that Cr: 14-19), characterized in that it contains not more than 03% by mass and satisfies Nb ≧ 8 (C + N), and the balance consists of Fe and inevitable impurities % By mass, Si: 0.5% by mass or less, Mn: 0.5% by mass or less, Al: 0.1% by mass or less, Ti: 0.02 to 0.40% by mass, Nb: 0 to 0.5% by mass %, Cu: 1.0 mass%, V: 0 to 1.0 mass%, Ni: 0 to 1.0 mass%, Ca: 0 to 0.0 mass% 5% by mass, the balance being Fe and inevitable impurities, C as impurities: 0.015% by mass or less, N: 0.015% by mass or less, P: 0.04% by mass or less, and the following ( 1) to (4) except for ferritic stainless steels that satisfy the equations (4).
C (mass%) + N (mass%) ≦ 0.025 (mass%) (1)
Ti (mass%) / (C (mass%) + N (mass%)) ≧ 5 (2)
0.5 × C (mass%) ≦ S (mass%) ≦ 0.001 (mass%) (3)
Ti (mass%) / Mn (mass%) ≧ 0.5 (4)   The ferritic stainless steel for automobile exhaust gas passage members according to claim 1, wherein Mo contained as an inevitable impurity is regulated to less than 0.10% by mass.   The ferritic stainless steel for automobile exhaust gas passage members according to claim 1 or 2, further comprising 0.0005 to 0.02 mass% B.

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