WO2003004714A1 - Ferritic stainless steel for member of exhaust gas flow passage - Google Patents

Abstract

A ferritic stainless steel for use in a member of an exhaust gas flow passage, which has a chemical composition, in mass %: C: 0.03 % or less, Si: 1.0 % or less, Mn: 1.5 % or less, Ni: 0.6 % or less, Cr: 10 to 20 %, Nb: 0.50 % or less, Cu: 0.8 to 2.0 %, Al: 0.03 % or less, V: 0.03 to 0.20 %, N: 0.03 % or less, provided that Nb ≥ 8(C+N) is satisfied, and balance: Fe and inevitable impurities. The Mo content of the ferritic stainless steel as an impurity is preferably controlled to 0.10 mass % or less, and Ti and B may be optionally contained in amounts of 0.05 to 0.30 and 0.0005 to 0.02 mass %, respectively. The ferritic stainless steel exhibits a thermal resistance comparable to that of a ferritic stainless steel containing Nb and Mo added thereto, and also is excellent in formability, the toughness at a low temperature and weldability.

Description

 Description Ferrite stainless steel for exhaust gas flow passage members Technical field

 INDUSTRIAL APPLICABILITY The present invention is used as an exhaust gas flow path member of various internal combustion engines such as automobiles, such as an exhaust manifold, a front pipe, a center pipe, a catalytic converter outer cylinder, etc., and has heat resistance, low temperature toughness, Related to ferritic stainless steel with excellent weldability. Background art

 Exhaust gas flow path members of automobiles are exposed to a high-temperature atmosphere that is in direct contact with the exhaust gas during operation, and are subjected to thermal stress due to repeated operation and shutdown and vibration of the engine during operation. In cold climates, cold winters are also subject to mechanical stress at low temperatures. Therefore, the materials used for exhaust system components must be durable in extremely harsh environments. When a stainless steel plate or pipe is used as an exhaust gas flow path member, it is important to have excellent heat resistance, as well as excellent weldability and workability because it can be assembled into a product shape by welding and processing. It is a characteristic. Toughness (low-temperature toughness) that can withstand secondary processing during molding and mechanical load at low temperature during use is also required.

Ferritic stainless steel has a smaller thermal expansion than austenitic stainless steel, and is superior in thermal fatigue properties and scale peel resistance. Since the cost of steel is low, ferrite stainless steel is often used for exhaust gas flow passage members. Ferritic stainless steels have inherently lower high-temperature strength than austenitic stainless steels, and improvements have been made to improve high-temperature strength. For example, there are SUS430J11-based Nb-added steel, Nb / Si composite-added steel (JP-A-3-274245), and Nb / Mo composite-added steel (JP-A-5-125491). Of these, Nb and Mo complex addition steels have the highest high-temperature strength and are used in places where excellent thermal fatigue properties are required. However, Nb and Mo composite additive steels tend to have lower workability and lower temperature toughness than other steel types. Cases of improved workability and low-temperature toughness are occasionally seen, Is not enough. Another disadvantage is the high cost of steel because it contains a large amount of expensive elemental Mo.

 By the way, the correlation between high-temperature strength (thermal fatigue fracture) and high-temperature oxidation characteristics (abnormal oxidation limit temperature), which is most important in an environment where the exhaust gas path members are exposed, shows that high-temperature strength and high-temperature oxidation characteristics do not necessarily match. Some sites do not need to be balanced at a high level. Specifically, in areas where the structure is very complex even if the exhaust gas temperature is not too high, high-temperature strength is more important than high-temperature oxidation characteristics, and workability and low-temperature toughness that can cope with the complexity of the structure are also important. You. At present, Nb and Mo composite-added steel must be used for such parts, and there is still room for improvement in workability, low-temperature toughness, and cost, even if the heat resistance is sufficient. Disclosure of the invention

 The present invention has been devised to solve such a problem, and does not use expensive Mo as an alloy component, has heat resistance comparable to that of Nb and Mo composite added steel, An object of the present invention is to provide a ferritic stainless steel having excellent low-temperature toughness and weldability for exhaust gas flow path members.

 In order to achieve the object, the ferritic stainless steel for exhaust gas flow passage members of the present invention has the following characteristics: C: 0.03% by mass, Si: 1.0% by mass, Mn: 1.5% by mass, Ni: 0.6% by mass, Cr: : 10 to 20% by mass, Nb: 0.50% by mass or less, Cu: 0.8 to 2.0% by mass, A1: 0.03% by mass or less, V: 0.03 to 0.20% by mass, N: 0.03% by mass or less, and Nb ≥8 (C + N).

 This ferritic stainless steel does not contain Mo as an alloying component and contains 0.05 to 0.30% by mass of i and Z to further enhance formability or 0.0005 to 0.02% by mass β to further enhance secondary workability. You can also. BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a graph showing the effect of Cu on 0.2% proof stress at high temperatures. Conventionally, SUH409L, SUS430J11, and SUS429 stainless steels have been used as materials that satisfy the heat resistance characteristics in the environment where the exhaust gas flow path members are exposed, but although the maximum temperature remains at about 800 to 900 ° C. Some parts require significantly higher high-temperature strength than conventional steel grades. Areas requiring high-temperature strength are also places where the structure is extremely complex and thermal fatigue is likely to occur due to repeated application of thermal stress. In addition, the constituent material of the relevant part is required to have workability and low-temperature toughness that cannot be obtained with Mo-added steel.

 The present inventors have investigated the effects of various alloying elements in order to satisfy the characteristics required for the constituent material of such a portion. As a result, it was found that the combined addition of V and Cu improved the high-temperature strength below 900 ° C, workability, and low-temperature toughness, and achieved the same level as Nb and Mo-added steel.

 For Nb-containing ferritic stainless steels with a small amount of V added and varied Cu content, 0.2% resistance was measured in 700 and 800 high temperature tensile tests. As a result, it was found that the high-temperature strength of 900 ° C or less was dramatically increased by the addition of a small amount of V and the regulation of the Cu content, and a high-temperature strength comparable to that of Nb and Mo composite-added steel was obtained.

 Figure 1 shows the results of a tensile test of steels with various contents of Cu added to the basic composition of 17Cr-0.4Nb-0.1V steel. Fig. 1 also shows the strength level of SUS444-based steel containing 18Cr-2Mo-0.4Nb, which is a Nb and Mo composite added steel, as a basic component for comparison.

 As is evident from the results in Fig. 1, the 0.2% resistance at 700 and 80 CTC increases sharply with increasing Cu content. When the Cu content is 0.8% by mass or more, a 0.2% proof stress equivalent to or higher than the SUS444 system containing about 2% by mass of Mo is obtained. Regarding the 0.2% resistance to heat at 900 ° C, it was confirmed by another experiment that the increase in V and Cu did not reach the level of SUS444 steel, but the 0.2% resistance was improved more than the Nb-containing ferritic stainless steel. ing. In other words, the combined addition of V and Cu is effective in improving the high-temperature strength in a temperature range of 900 or less, and no significant adverse effect appears at 900 or more.

The reason why the high-temperature strength is maintained at a high level by the combined addition of Nb, Cu, and V is not fully understood, but Nb-based precipitates, A structure in which Cu-based precipitates are finely dispersed compared to the steel with Nb added alone is observed. From these observation results, it is found that V preferentially precipitates in the as-annealed state or in the early stage of heating, thereby suppressing the formation of Nb-based precipitates and Cu-based precipitates. As a result, fine Nb-based precipitates and Cu-based precipitates It is presumed that they are dispersed and contribute to precipitation strengthening. Presumably, this is because the finely dispersed precipitates do not agglomerate even after heating for a long time in the early stage of heating, and the precipitation strengthening works effectively for a long time.

 Furthermore, since V, which is a strong carbonitride forming element, bonds with N, more solid-solution Nb effective for improving high-temperature strength can be secured than V-free steel when the amount of Nb added is the same. In other words, the same high-temperature strength as V-free steel can be achieved with a reduced Nb content, and as a result, 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 and V carbonitrides in the as-annealed state increases. With the increase in Nb-based and V-based carbonitrides, the grain size of the weld heat affected zone becomes less likely to become coarse and the toughness is improved. Since the generation of Cr-based carbides is also suppressed, the intergranular corrosion resistance is also improved.

 Hereinafter, the alloy components and contents 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 considered to be effective elements for improving high-temperature strength such as creep strength, but if they are contained excessively, oxidation properties, workability, low-temperature toughness, and weldability are reduced. In this component system in which V and Nb are added as elements that fix C and N as carbonitrides, it is necessary to add V and Nb in amounts corresponding to the C and Ν concentrations. In order to suppress a rise in raw material costs due to an 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 mass% or less

It is a very effective element for improving high temperature oxidation characteristics, but is not so effective for increasing the high temperature strength below 900. Conversely, when Si is excessively added, the hardness increases, and the workability and low-temperature toughness decrease. Therefore, the Si content was regulated to 1.0% by mass or less (preferably, 0 :! to 0.5% by mass). Mn: 1.5 mass% or less

 Although it is an alloy element that improves the high-temperature oxidation properties of ferritic stainless steel, especially scale exfoliation, excessive addition of Mn deteriorates workability and weldability. In addition, since it is an austenite phase stabilizing element, if Mn is excessively added to a steel type containing a small amount of Cr, a martensitic phase is likely to be formed, leading to deterioration of thermal fatigue characteristics and workability. Therefore, the Mn content was 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 excessively added to a steel type with a low Cr content, a martensitic phase is formed similarly to Mn, and the thermal fatigue properties and workability are reduced. Due to the high price of raw materials, excessive addition of Ni should be avoided. Therefore, the Ni content was regulated to 0.6% by mass or less (preferably 0.5% by mass or less). Cr: 10-20% by mass

 It is an indispensable element that stabilizes the ferrite phase and improves oxidation resistance, which is important for high-temperature materials. From the standpoint of oxidation resistance, the higher the Cr content, the better, but if added excessively, the steel material becomes brittle and the workability is degraded 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 to the working temperature of the material. For example, for high-temperature oxidation resistance up to 950 ° C, 16 to 19% by mass is preferable, and for high-temperature oxidation resistance at 900 ° C or lower, 12 to 16% by mass is sufficient.

 Nb: 8 (C + N to 0.50 mass%

 C and N are fixed as carbonitrides, and the remaining solid solution Nb to which the carbonitrides are fixed has 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 susceptibility to welding hot cracking increases. The Nb content that satisfies Nb ≥ 8 (C + N) is required for fixing C and N, but the upper limit of the Nb content is set to suppress adverse effects on workability, low-temperature toughness, and susceptibility to welding hot cracking. Is set to 0.5% by mass. Preferably, an Nb content satisfying 8 (C + N) + 0.10≤Nb≤0.45 is selected. Cu: 0.8 to 2.0 mass%

In this component system, it is a very important element for improving the high-temperature strength. Departure In the temperature range studied by the authors, almost all of Cu is dissolved in the annealed matrix and precipitates during heating. The precipitated Cu exerts a strengthening effect at the initial stage, similar to the Mo-added steel, but the strengthening effect gradually disappears with prolonged heating. In order to obtain the required high-temperature strength, as is clear from Fig. 1, a Cu content of 0.8% by mass or more is required. However, as the Cu content increases, the workability, low-temperature toughness, and weldability decrease. The upper limit of the Cu content is regulated to 2.0% by mass in order to suppress adverse effects on workability, low-temperature toughness, and weldability. The preferred Cu content is in the range of 1.0 to 1.7% by mass. A1: 0.03 mass% or less

 It is added as a deoxidizer during steelmaking and also has the effect of improving high-temperature oxidation resistance. However, excessive addition of A1 deteriorates the surface properties and adversely affects workability, weldability, and low-temperature toughness. Therefore, the smaller the A1 content, the better, and the upper limit is 0.03% by mass.

(Preferably 0.02% by mass).

V: 0.03 to 0.20 mass%

 When combined with Nb and Cu, the high-temperature strength of ferritic stainless steel improves. In addition, coexistence with Nb improves workability, low-temperature toughness, intergranular corrosion susceptibility, and toughness of the heat affected zone. These effects appear at a V content of 0.03% by mass or more, but an excessive addition exceeding 0.20% by mass causes deterioration in 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 to 0.30 mass%

 It is an element that improves the r-value (Rankford value) of steel to improve formability. The effect of addition becomes significant 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 adversely affected. Therefore, it is desired to reduce the Ti content as much as possible even when Ti is added for improving formability. Therefore, the upper limit of the Ti content is restricted 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. At 0.0005% by mass or more, the effect of adding B becomes remarkable. However, add a large amount of B As a result, the manufacturability and weldability deteriorate. Therefore, the B content is selected in the range of 0.0005 to 0.02% by mass (preferably 0.001 to 0.01% by mass).

Mo: less than 0.10% by mass

 The ferritic stainless steel of the present invention is based on the premise 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 in, it will adversely affect workability, low-temperature toughness, and weldability, etc. Therefore, it is desirable to limit the amount of Mo mixed to less than 0.10% by mass.

 There is no particular restriction on the elements other than those listed above, but it is preferable to reduce P, S, 0, etc., which are common impurities, as much as possible. In consideration of hot workability, oxidation resistance, and the like, it is preferable that the upper limits of P, S, and 0 be 0.04 mass%, 0.03 mass%, and 0.02 mass%, respectively. W, Zr, Y, REM (rare earth element), which is effective for improving heat resistance, and Ca, Mg, Co, which is effective for improving hot workability, can be added as needed.

 There are no particular restrictions on the manufacturing conditions for ferritic stainless steel, and as long as Cu is dissolved in advance, excellent heat resistance can be obtained as a hot-rolled annealed sheet. When a steel sheet having a desired thickness cannot be produced by hot rolling, cold rolling and annealing are repeated once or more times to produce a steel sheet having heat resistance equivalent to that of a hot rolled annealed sheet. If necessary, when Cu is finely dispersed at any stage of the manufacturing process, better high-temperature strength can be obtained. Excellent properties are maintained even after hot-rolled annealed sheets, cold-rolled annealed sheets, etc. are added or welded to the desired shape (including tube forming). 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 made into 30k ingots. The ingot was forged and subjected to hot rolling, annealing, cold rolling, and finish annealing to produce 2.0 mm and 1.2 mm cold-rolled annealed sheets. In the table, Nos.! To 10 are the steels of the present invention, and Nos. 11 to 19 are comparative steels. Among the comparative steels, No.ll was SUS430JU equivalent steel, No.15 was SUH409L equivalent steel, No.16 was 14Cr—Si—Nb steel, No.17 was SUS444 equivalent steel, and all steel types were exhaust manifolds. It has been used for holding. Table 1: Composition of test material 'Composition (Steel of the present invention)

B: ρυπι single [Nb] = Nb-8 [C + N] One: below detection limit

Table 2: Composition and composition of test material (comparative steel)

 B: ppm unit [Nb〗 = Nb-8 [C + N] i: Below the detection limit

An underline indicates that the value is out of the range defined in the present invention.

A 2.0-mm-thick cold-rolled annealed plate 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 1.2-mm-thick cold-rolled annealed plate was subjected to a weld hot cracking test.

 In the high-temperature tensile test, the test piece was pulled at 800 ° C in accordance with JISG0567, and 0.2% resistance was measured.

 In the high temperature oxidation test, the test piece was continuously heated at 850 ° C, 900 ° C, 950 ° C, 1000 ° C, and 1100 ° C for 200 hours in accordance with JISZ2281. The heated test specimens were visually observed for the occurrence of abnormal oxidation (thick oxide penetrating in the plate thickness direction), and the critical 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 breaking elongation after the tensile test was determined.

 In the Charpy impact test, a test piece was used at a temperature of -75 ° C, -50t, -25, 0 ° C, and 25 ° C using a subsize test piece with a thickness of 2.0 nm in accordance with JISZ2242. A shock was applied to the steel to determine the ductility-toughness transition temperature.

 In the welding hot cracking test, TIG welding was performed with both ends of a 40 mm X 20 mm test piece held and tensile stress applied in the longitudinal direction, and the minimum strain at which cracking began to occur was determined. The obtained critical strain amount was used as an index of the hot cracking susceptibility.

 Table 3 shows the test results.

 Each of the steels Nos. 1 to 10 of the present invention has a much higher 0.2% proof stress at 800 compared to the Ti-added steel (No. 15), Nb, and Si-added steel (No. 16). It had a 0.2% power resistance comparable to or surpassed that of the Mo-added steel (No. 17). Elongation by room-temperature tensile test, ductile-brittle transition temperature by Charpy impact test, and critical strain by welding hot cracking test have properties equal to or higher than those of Nb and Mo composite-added steel (No. 17). It was confirmed that the target performance could be obtained without doing so. As for abnormal oxidation, as can be seen from the results of Nos. 4, 5, and 12, the lower the Cr content, the lower the critical temperature. From the effect of Cr content on abnormal oxidation, it can be understood that it is necessary to set an appropriate amount of Cr content according to the temperature of the application site.

Comparative steels No.11, No.15, No.16 and No.19, which lack V and Cu, have sufficient workability, low-temperature toughness and weldability, but inferior high-temperature strength at 800 ° C. ing. Cu Although comparative steel No. 12, which contains an excessive amount of steel, has excellent high-temperature strength, its workability and weldability are inferior to those of Nb and Mo composite-added steels, and it hinders processing into product shapes and welding.

 Even though the Cu content is within the specified range, the comparative steel No. 13 containing too much Si and the comparative steel No. 14 containing too much Nb have excellent workability and low-temperature toughness even if they have excellent high-temperature strength. However, the weldability was inferior to the steel of the present invention.

 Comparative steel No. 18 with low V content and high A1 content, although having similar heat resistance and workability to the steel of the present invention, is inferior in low-temperature toughness, resulting from insufficient toughness during product processing and use. Trouble is expected to occur. Comparative steel No. 19, which lacks V, lacks high-temperature strength.

Comparative steel No. 17 containing Mo has the same performance as the steel of the present invention, but has a slightly lower low-temperature toughness. Moreover, since Mo is contained at about 2% by mass, it is inevitable that the material cost will be higher than that of the steel of the present invention.

Table 3: Evaluation test results

Critical strain 3% by mass or more is indicated by 〇, and less than 3% by mass is indicated by X. The underline indicates that the property does not satisfy the purpose of the present invention. Industrial applicability

 As described above, by strictly controlling the content of various alloying elements contained in ferritic stainless steel, especially the range of V and Cu, high heat resistance is secured without the need for expensive Mo. While improving workability, low-temperature toughness, and weldability, ferritic stainless steel suitable for exhaust gas path members can be obtained. This ferritic stainless steel is used in exhaust gas flow passage members, such as automobile engines, exhaust pipes, front pipes, center pipes, and outer tubes of catalytic converters, utilizing its excellent properties.

Claims

The scope of the claims 1. C: 0.03% by mass or less, Si: 1.0% by mass or less, Mn: 1.5% by mass or less, Ni: 0.6% by mass or less, Cr: 10 to 20% by mass, Nb: 0.50% by mass or less, Cu: 0.8 to 2.0% % By mass, A1: 0.03% by mass or less, V: 0.03 to 0.20% by mass, N: 0.03% by mass or less, and satisfies Nb≥8 (C + N), with the balance being Fe and unavoidable impurities A ferritic stainless steel for a vehicle exhaust gas flow passage member.  2. The ferrite stainless steel for automobile exhaust gas flow path members according to claim 1, wherein Mo contained as an inevitable impurity is regulated to less than 0.10% by mass. 3. The ferritic stainless steel for an automobile exhaust gas flow passage member according to claim 1, further comprising 0.05 to 0.30% by mass of Ti. 4. The ferritic stainless steel for a vehicle exhaust gas flow path member according to claim 1, further comprising 0.0005 to 0.02% by mass of B.