JP6006660B2 - Alloy-saving ferritic stainless steel with excellent oxidation resistance and corrosion resistance for automotive exhaust system parts - Google Patents

Description

  The present invention relates to an alloy-saving ferritic stainless steel excellent in oxidation resistance and corrosion resistance for automobile exhaust system members.

  Ferritic stainless steel plates and steel pipes have been frequently used for exhaust system parts. For example, SUH409L is a steel type containing 11% Cr and fixing C and N with Ti to prevent sensitization of the welded part and has excellent workability, and has a sufficiently high temperature characteristic at 700 ° C. or less. It is most often used because it exhibits some resistance to condensed water corrosion. In addition, SUS439 steel, in which C and N are fixed with Ti and 17% of Cr, and SUS436L further containing Mo, and other steel types with improved resistance to condensed water corrosion and salt damage corrosion are also used.

  By the way, exhaust system parts are generally constructed in the order of the exhaust manifold, front pipe, catalytic converter, center pipe, muffler, and tail pipe in the order from the part close to the engine. descend. In other words, high temperature strength and oxidation resistance are important for upstream parts such as exhaust manifolds and front pipes, while the center pipe, muffler, and tail pipe downstream are relatively cold, so high temperature strength and oxidation resistance are not considered important. Since the moisture in the exhaust gas is easily condensed, the corrosion resistance of condensed water is required. Corrosion resistance to snow melting salt is also important throughout the exhaust system parts, and a phenomenon called high-temperature salt damage corrosion in which oxidation and wet corrosion are superimposed in a part exposed to high temperature, and a part mainly composed of wet corrosion in a part at a relatively low temperature. It becomes a phenomenon. In this way, the phenomenon that occurs depends on the temperature conditions to which the component is exposed, and the required characteristics change. Therefore, it is not possible to aim at a material excellent in all properties, and it is a common practice to apply an appropriate material in an appropriate place according to the severity of the environment to which the component is exposed.

  On the other hand, cost is important in material selection. There is a constant demand for steel types that can maintain practical performance while saving expensive alloy elements such as Mo. The SUS439 steel (17Cr series) is an intermediate position between SUH409L (11Cr) and SUS436L (17Cr-1Mo series), and is a steel type balanced in terms of both cost and practical performance, but still further cost reduction. It is requested.

  A steel type having a practical performance equivalent to that of SUS439 steel at a lower cost than SUS439 steel (17Cr series) is desired.

  Several similar techniques have been proposed for such problems.

  For example, in Patent Document 1, C and N are fixed with Ti, Cr is contained in 9.0 to 15.0%, and Ni and Cu are contained in 0.10 to 0.80% to achieve both corrosion resistance and workability. Disclosed steel is disclosed. However, since Ni is an expensive alloy element, it should be kept in a very small amount even if it is used.

  In Patent Document 2, C and N are fixed with Nb and Ti, Cr is contained at 11.0 to 15.0%, Ni is 0.6% or less, and V is 1.0% or less. Steels that ensure pipe properties, intergranular corrosion resistance, and high-temperature strength are disclosed. However, here too, expensive alloy elements such as Ni and V are used, and Nb is also contained for securing high temperature strength. Therefore, it is difficult to obtain excellent workability and cost performance as the present invention aims. There is a problem with it.

  In Patent Document 3, C and N are fixed with Ti, Cr is contained in an amount of 10 to 14%, and an appropriate amount of S (0.5 times greater than or equal to C content and less than or equal to 0.010%) is contained to improve corrosion resistance. Steels having both workability are disclosed. Compared to the above two technologies, it is excellent in cost performance. However, since S is contained in a larger amount than the current refining level, there is a problem that corrosion resistance deterioration due to S-based inclusions is a concern.

  These similar techniques do not sufficiently evaluate the corrosion resistance after heating handled in the present invention.

  On the other hand, a technique that improves the properties of steel materials by containing only a very small amount of Sn and Sb, which has not been attracting much attention as an alloy element, is disclosed in an interesting point close to the purpose of the present invention. ing.

  For example, Patent Document 4 proposes a ferritic stainless steel in which oxidation resistance is improved by containing 0.02 to 0.2% of Sb. In Patent Document 5, ferrite containing no surface scratches caused by intergranular corrosion during sulfuric acid pickling is prevented by containing at least one of 0.005 to 0.10% of Sn and Sb to prevent P grain boundary segregation. Stainless steel sheet is presented. Patent Document 6 proposes steel containing 0.05 to 2% Sn for the purpose of improving the high temperature strength of ferritic stainless steel.

  However, these techniques are not disclosed for post-heating corrosion resistance.

  Patent Document 7 discloses an alloy-saving ferritic stainless steel excellent in corrosion resistance after heating. Steels that have oxidation resistance equivalent to that of SUS439 steel have been studied.

Japanese Patent No. 3999141 Japanese Patent No. 2562740 Japanese Patent No. 3285179 JP 2005-146345 A Japanese Patent Laid-Open No. 11-92872 JP 2000-169943 A JP 2010-31315 A

  The technology described in the background art does not disclose steel having post-heating corrosion resistance equivalent to that of SUS439 steel. Therefore, the present invention aims to provide an alloy-saving ferritic stainless steel for automobile exhaust system members that has oxidation resistance and corrosion resistance equivalent to those of SUS439 steel (17Cr series) and is superior in cost to SUS439 (17Cr series). It is what.

  Note that the corrosion resistance after heating in the present invention should be emphasized as corrosion resistance and oxidation resistance as practical performance. Further, the corrosion resistance referred to here is the corrosion resistance in the environment of condensed water or salt damage after the heat treatment corresponding to the actual part, and is different from the corrosion resistance of the material itself when not heated.

  The present inventors have conducted many salt corrosion tests and condensed water corrosion tests on various stainless steel materials. As a result, the heat treatment before the corrosion test transforms the passive film into an Fe-rich oxide film, so that if it is not heat treated, it will not affect the corrosion resistance. It has been found that Sn has a function of improving the corrosion resistance, and that Sn is a powerful element for improving corrosion resistance after heating. Furthermore, it has been found that Sn has an effect of improving oxidation resistance in a small amount, and in addition, the degree of this effect depends on the Si content.

The present invention is configured based on the above findings, and the gist thereof is as follows.
(1) By mass%, C: 0.0150% or less, Si: 1.0 to 1.5%, Mn: 0.15 to 1.0%, P: 0.050% or less, S: 0.0100 %: N: 0.0150% or less, Al: 0.010-0.200%, Cr: 13.0-16.0%, and Sn: 0.002-0.050%, : 0.03 to 0.30% and Nb: 0.03 to 0.50% are included, and the A value defined by the formula (1) is 0.024 or more A ferritic stainless steel for automobile exhaust system members excellent in oxidation resistance and corrosion resistance, characterized in that the balance consists of Fe and inevitable impurities.
A = [Si] × [Sn] +0.014 [Si] ------- (1)
Here, [Si] and [Sn] are contents as mass% of Si and Sn, respectively.
(2) The composition according to (1), wherein the composition further contains one or two of Cu: 0.05 to 0.50% and Ni: 0.05 to 0.50% by mass%. Ferritic stainless steel with excellent oxidation resistance and corrosion resistance for automotive exhaust system members.
(3) The automobile exhaust system member having excellent oxidation resistance and corrosion resistance according to the above (1) 1 or (2), further comprising B: 0.0003 to 0.0050% by mass% Ferritic stainless steel.

  According to the present invention, an alloy-saving ferritic stainless steel for automobile exhaust system members having corrosion resistance and oxidation resistance equivalent to SUS439 steel (17Cr series) and lower cost than SUS439 steel can be obtained. .

It is a figure which shows the influence of Si and Sn which exerts on corrosion resistance.

  The embodiment of the present invention will be described more specifically.

  First, the present inventors have conducted a combined cycle corrosion test that simulates an actual salt damage environment (salt spray: 5% NaCl spray 35 ° C. × 2 Hr, dry: relative humidity 20%, 60 ° C. × 4 Hr, wet: relative humidity 90%. , 50 ° C. × 2 Hr repetition), the influence on the corrosion resistance due to the presence or absence of the heat treatment at 400 ° C. × 8 Hr was investigated using the steel in which the Cr content governing the corrosion resistance was changed.

  As a result, Cr is an element for improving corrosion resistance regardless of the presence or absence of heat treatment, but the effect of reducing Cr corrosion was greater when heat treatment was performed than when heat treatment was not performed. This means that an oxide layer is formed on the surface by the heat treatment, and the corrosion characteristics are affected by this oxide film. That is, it can be said that the corrosion resistance after heating after the formation of the oxide film is different from the corrosion resistance of the base material in the passive film state.

  By the same method, the influence on the corrosion resistance by the presence or absence of heat treatment was investigated for the steel in which the contents of Si, Mn, and Al were changed. As a result, it was found that Si, Mn, and Al are elements that do not affect corrosion without heat treatment, but are elements that improve corrosion resistance when heat treatment is performed. From this, it can be said that the corrosion resistance after heating after the formation of the oxide film is different from the corrosion resistance of the base material in the passive film state. The reason why Si, Mn, and Al affect the corrosion resistance after heating is presumed to be that Si, Mn, and Al contribute to the denseness and protection of the surface oxide film.

  Moreover, as a result of investigating Sn, it was found that Sn improves the corrosion resistance after heating with a very small amount of about 0.002 to 0.050%.

  Further, these Si, Mn, Al, and Sn are useful for improving the corrosion resistance after heating, but it has also been found that if the content is too large, the corrosion resistance is lowered.

  Next, the condensed water corrosion test by JASO M911-A was performed about the steel used for the salt damage corrosion test, and the influence of 400 degreeC x 8 Hr heat processing presence or absence was investigated. As a result, the influence of Cr, Si, Mn, Al, and Sn on the condensed water corrosion resistance after heating was the same as in the case of the above-mentioned salt corrosion resistance after heating.

  Subsequently, the steel used in the corrosion resistance test was subjected to an oxidation test in an air atmosphere to investigate the influence of alloying elements on the oxidation resistance. Although Cr, Si, Mn, and Al contribute to oxidation resistance in a known range, it is interesting that a small amount of Sn has an effect of improving oxidation resistance when superimposed with Si.

As an example, FIG. 1 shows the results of an oxidation test conducted on a steel of 14% Cr base steel with different contents of Si and Sn in an air atmosphere at 950 ° C. When the Si content is low, the effect of Sn is not exhibited, but when the Si content is large, a small amount of Sn significantly improves the oxidation resistance. That is, it has been found that Si and Sn interact, and the oxidation resistance depends on the product of Si and Sn content in the region where the Si content is 1.0% or more. This was quantified with the A value defined by equation (1).
A = [Si] × [Sn] +0.014 [Si] ------- (1)
Here, [Si] and [Sn] are the contents as mass% of Si and Sn, respectively, the curve in FIG. 1 is a curve showing A = 0.024, and the A value is 0.024 or more. Thus, excellent oxidation resistance can be obtained.

  Moreover, it was also found that if Sn is excessively contained, the oxidation resistance is deteriorated rather than that, and it has been clarified that the Sn content has an appropriate range.

  As described above, by adding a small amount of Sn and adjusting the Si, Mn, and Al contents, it is possible to ensure post-heating corrosion resistance and oxidation resistance equivalent to SUS439 steel even if the Cr content is reduced. .

  In addition, although it goes backward from the viewpoint of alloy saving and low cost, it is also possible to pursue a further improvement in corrosion resistance by containing a small amount of Ni and Cu.

  Hereinafter, the effect | action of the alloy element in this invention and the reason for limitation of the content are explained in full detail. Unless otherwise indicated,% means mass%.

  C, N: C and N are elements that cause intergranular corrosion in the weld heat affected zone, and deteriorate the corrosion resistance after heating. Moreover, workability is deteriorated. For this reason, the content of C and N should be limited to the lowest possible level, and the upper limit of C and N needs to be 0.015%, preferably 0.010%. On the other hand, the lower limit of the content is preferably 0.0020% for both C and N as a level that can be reached industrially in the current refining technology.

  Si: Si has an effect of improving corrosion resistance and oxidation resistance after heating. In order to maximize the synergistic effect with a small amount of Sn in oxidation resistance, 1.0% or more is contained. However, if contained in a large amount, the corrosion resistance after heating deteriorates, so the upper limit is limited to 1.5%. Desirably, the Si content is 1.2% or less.

  Mn: Since Mn also has the effect of improving the corrosion resistance and oxidation resistance after heating, it is contained in an amount of 0.15% or more. However, if it is contained in a large amount, the oxidation resistance and the corrosion resistance after heating deteriorate, so the upper limit is set to 1.0. % Should be limited. Preferably, it is 0.20 to 0.50%.

  P: An element that deteriorates workability. For this reason, the P content is desirably as low as possible. The upper limit of the allowable content is 0.050%. A desirable upper limit of P is 0.030%. On the other hand, the lower limit of the content is preferably 0.010% as a level that can be industrially reached in the current refining technology.

  S: Since it is an element that deteriorates corrosion resistance, the S content is desirably as low as possible. The upper limit of the allowable content is 0.010%. A desirable upper limit of the S content is 0.0050%, and more desirably 0.0030%. On the other hand, the lower limit of the content is preferably 0.0005% as a level that can be achieved industrially in the current refining technology.

  Cr: A basic element that ensures corrosion resistance and oxidation resistance after heating, and an appropriate amount is essential. In order to ensure post-heating corrosion resistance and oxidation resistance equivalent to SUS439 steel, the lower limit of the Cr content needs to be 13.0%. Desirably, the lower limit is 13.5%. On the other hand, it is necessary to set the upper limit content to 16.0% in order to suppress deterioration of workability and to be lower in cost than SUS439 steel. It is preferably less than 16.0%. The upper limit is desirably 15.5%.

  Al: Al is useful as a deoxidizing element and has an effect of improving the corrosion resistance after heating, so 0.010% or more is contained. However, if it is contained in a large amount, the corrosion resistance after heating deteriorates, so the upper limit is 0.200%. It is good to limit to. Preferably, it is 0.04 to 0.08%.

  The present invention contains one or two of Ti and Nb as follows.

  Ti: Ti has the action of fixing C and N as carbonitrides and suppressing intergranular corrosion. For this reason, 0.03% is contained as the lower limit, but even if it is contained excessively, the effect is saturated and the workability is impaired, so the upper limit of the content is made 0.30%. In addition, the proper content of Ti is preferably 5 times or more and 30 times or less the total content of C and N.

  Similar to Nb: Ti, Nb fixes C and N as carbonitrides and suppresses intergranular corrosion, so 0.03% is included as the lower limit. However, if excessively contained, workability is impaired. Therefore, the upper limit of the content is 0.50%. Preferably it is 0.015-0.30%.

  Sn: Sn is extremely useful as an element that improves the corrosion resistance and oxidation resistance after heating in a small amount, and the lower limit when contained is 0.002%. Desirably, the lower limit is 0.010%. On the other hand, when Sn is excessively contained, the oxidation resistance and the corrosion resistance after heating are deteriorated. In addition, Sn may segregate at the grain boundaries and degrade hot workability. For this reason, the upper limit of the content is made 0.050%. Preferably it is less than 0.050%. More preferably, it is good to contain 0.010 to 0.030%. Since the effect is exhibited even in a very small amount, 0.002 to 0.01% may be used.

  A value: Calculated from the contents of Si and Sn, and the A value defined by the above formula (1) is an index of oxidation resistance. To obtain the target oxidation resistance, ensure 0.024 or more. is necessary. 0.096 is the upper limit.

  Ni, Cu: In addition to these main elements, it is retrograde from the viewpoints of alloy saving and low cost, but in order to improve ultimate corrosion resistance after heating, it contains trace amounts of one or two kinds of Ni and Cu useful for improving corrosion resistance. You may let them. In that case, the lower limit of the content is preferably 0.05%. On the other hand, since these elements deteriorate the workability, the upper limit of the content is preferably set to 0.50%.

  B: An element useful for suppressing the segregation of Sn grain boundaries to prevent deterioration of hot workability due to a decrease in grain boundary strength, and does not affect the corrosion resistance and oxidation resistance after heating. For this reason, 0.0003% may be included as a lower limit. However, if it exceeds 0.0050%, hot workability is deteriorated, so the upper limit is preferably set to 0.0050%.

  Ferritic stainless steel of these compositions is a stainless steel for ordinary exhaust system members that is subjected to hot rolling, pickling, cold rolling, annealing, finishing pickling, etc. on steel pieces that have been melted and refined in converters, electric furnaces, etc. It is manufactured as a steel plate by the manufacturing method of a steel plate. Moreover, it manufactures as a welded pipe by the manufacturing method of the normal stainless steel pipe for exhaust system members, such as electrical resistance welding, TIG welding, and laser welding, using this steel plate as a raw material.

  The invention is explained in more detail on the basis of examples.

  A stainless steel having the composition shown in Tables 1 and 2 was melted in a 150 kg vacuum melting furnace and cast into a 50 kg steel ingot, and then a sheet thickness of 1.2 mm was obtained through the steps of hot rolling, grinding, cold rolling, annealing and finishing pickling. A steel plate was produced. The production conditions for the hot-rolled sheet were as follows: material thickness: 90 mm, heating temperature: 1160 ° C., rolling to 9 mm in 9 passes, finishing temperature: 850 ° C., winding temperature: 600 ° C. The production conditions of the cold-rolled plate were a material thickness: 2.8 mm and a finished thickness: 1.2 mm. The annealing conditions were 880 ° C. × 60 seconds and air cooling. The finish pickling was nitric hydrofluoric acid pickling.

  A corrosion test piece was taken from this steel plate, and the test surface was polished by # 600 emery, and a combined cycle corrosion test simulating a salt damage environment ((salt spray: 5% NaCl spray 35 ° C. × 2 Hr, dry: 20% relative humidity, 60% And a condensed water corrosion test specified in JASO M911-A was conducted in an atmospheric furnace before the test in 400 atmospheres. The sample after the corrosion test was subjected to the derusting treatment, and then the maximum corrosion depth was determined by the microscope depth of focus method. An oxidation test was conducted at a test time of 200 hr and a test temperature of 950 ° C.

  The test results are shown in Tables 1 and 2. In Table 2, numerical values outside the scope of the present invention are underlined. Comparative Example No. 2 in Table 2 101 is SUS439 steel and is a comparison standard. Regarding the corrosion resistance after heating, the maximum corrosion depth of the candidate material and the comparative example No. The ratio of the maximum corrosion depth of 101 was expressed. Considering the variation in the corrosion test, if this value is 0.9 to 1.1, it can be evaluated as equivalent. Moreover, oxidation resistance represented the said oxidation increase.

  Comparative Example No. No. 102 to No. No. 129 is not equivalent to SUS439 steel in either corrosion resistance or oxidation resistance or both because the content of Sn, Si, Mn, Cr, Al and the A value are outside the scope of the present invention. On the other hand, the present invention No. 1 to 31, the alloying element content was in an appropriate range, and both oxidation resistance and corrosion resistance were at the same level as SUS439 steel, and sufficiently satisfactory values were obtained.

Claims (3)

In mass%, C: 0.0150% or less, Si: 1.0 to 1.5%, Mn: 0.15 to 1.0%, P: 0.050% or less, S: 0.0100% or less, N: 0.0150% or less, Al: 0.010-0.200%, Cr: 13.0-16.0%, and Sn: 0.002-0.050%, and Ti: 0.000%. 3 or 0.30% and Nb: 0.03 to 0.50% or 1 type, and satisfying that the A value defined by the formula (1) is 0.024 or more, the balance being A ferritic stainless steel for automobile exhaust system members excellent in oxidation resistance and corrosion resistance, characterized by comprising Fe and inevitable impurities.
A = [Si] × [Sn] +0.014 [Si] ------- (1)
Here, [Si] and [Sn] are contents as mass% of Si and Sn, respectively.   The oxidation resistance according to claim 1, further comprising one or two of Cu: 0.05 to 0.50% and Ni: 0.05 to 0.50% in mass%. Ferritic stainless steel for automotive exhaust system with excellent corrosion resistance.   The ferritic stainless steel for automobile exhaust system members having excellent oxidation resistance and corrosion resistance according to claim 1 or 2, further comprising B: 0.0003 to 0.0050% by mass%. .

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