JP6169863B2 - Ferritic stainless steel for exhaust system members with excellent corrosion resistance - Google Patents

Description

  The present invention relates to a ferritic stainless steel excellent in corrosion resistance for exhaust system members of automobiles and motorcycles.

  Ferritic stainless steel plates and pipes have been widely used for exhaust system parts of automobiles and motorcycles. Among them, SUH409L is a steel type containing 11% Cr, fixing C and N with Ti to prevent sensitization of the welded portion and having excellent workability, and has a sufficiently high temperature characteristic at 700 ° C. or less. It exhibits some resistance to exhaust gas condensate corrosion and salt damage corrosion. In addition, the amount of alloying elements is small and inexpensive. For this reason, it has been most frequently used among various ferritic steel types.

  On the other hand, recently, there is an increasing demand for extending the life of exhaust system parts. One factor that governs the life of exhaust system parts is corrosion resistance. In particular, in the SUH409L steel, the alloy element contributing to corrosion resistance is only Cr and the content is as low as 11%, so the life as an exhaust system part greatly depends on the corrosion resistance. For this reason, if it is intended to ensure the long life of the exhaust system parts to which SUH409L has been applied, a high-grade material such as SUS439 steel (17Cr steel) will be applied, resulting in an increase in cost.

  Therefore, a technique for improving the corrosion resistance at the lowest possible cost while utilizing the low price of SUH409L steel has been demanded. 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.

  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, expensive alloy elements such as Ni and V are used here, and Nb is also contained for securing high-temperature strength. Therefore, it is difficult to obtain excellent cost performance as the present invention aims. There's a problem.

  In Patent Document 3, C and N are fixed with Ti and Cr is contained in an amount of 10 to 14%, and an appropriate amount of S (0.5 times the C content, 0.010% or less and 0.0030% or more). Steel that contains both corrosion resistance and workability is 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 do not discuss post-heat corrosion resistance handled in the present invention.

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

  The object of the present invention is to provide 11Cr stainless steel that improves the corrosion resistance at the lowest possible cost while utilizing the low price of SUH409L steel.

  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 was found that the corrosion resistance improvement effect of Sn is amplified by coexistence with Mo, and the degree of this effect depends on the Mo 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: 0.20 to 0.70 %, Mn: 0.20 to 0.60 %, P: 0.050% or less, S: 0.0100 % or less, N: 0.0150% or less, Al: 0.010~0.20%, Cr: 10.50 ~ 11.50%, Mo: 0.02~0.20%, and Sn: 0.005 A containing 0.5 to 0.050%, further containing one or two of Ti: 0.030 to 0.30% and Nb: 0.030 to 0.50%, and defined by the formula (1) A ferritic stainless steel for exhaust system members excellent in corrosion resistance, characterized in that the value is 0.00065% ² or more, and the balance consists of Fe and inevitable impurities.
A = [Mo] × [Sn] ------- (1)
Here, [Mo] and [Sn] are contents as mass% of Mo and Sn, respectively.
(2) In mass%, Cu: 0.01 to 0.10%, Ni: 0.02 to 0.20%, V: 0.01 to 0.10%, or one or more The ferritic stainless steel for exhaust system members having excellent corrosion resistance as described in (1) above.
(3) Ferritic stainless steel for exhaust system members having excellent corrosion resistance as described in (1) or (2) above, further containing B: 0.0002 to 0.0050% by mass%.

  According to the present invention, 11% Cr stainless steel having improved corrosion resistance while utilizing the low cost of SUH409L steel can be obtained, so that the industrial effect is great.

The influence of Mo and Sn on corrosion resistance is shown.

  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 the coexistence of Mo significantly improves the corrosion resistance with a very small amount of 0.005 to 0.050%.

An example of the survey results is shown in FIG. FIG. 1 shows the corrosion weight loss of a candidate material in which the Mo and Sn contents are changed based on 11% Cr as a ratio to the corrosion weight loss of SUH409L steel, and 2/3 of the corrosion weight loss of SUH409L steel (that is, corrosion resistance). Is a value obtained by plotting the corrosion resistance of candidate materials by layering them with “◯” and “×” marks. From this, it was found that Mo and Sn interact, and that the corrosion resistance after heating depends on the product of the contents of Mo and Sn in the region where the Mo content is 0.02% or more. This was quantified with the A value defined by equation (1).
A = [Mo] × [Sn] ------- (1)
Here, [Mo] and [Sn] are contents in terms of mass% of Mo and Sn, respectively, the curve in FIG. 1 is a curve showing A = 0.00065, and the A value is 0.00065% ² The above results in excellent corrosion resistance.

  It was also found that the corrosion resistance after heating was saturated even if Si, Mn, Al, Mo, and Sn were excessively contained, and it became clear that the content of these alloy elements had an appropriate range.

  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 Si, Mn, and Al on the condensed water corrosion resistance after heating and the interaction between Sn and Mo were the same as in the case of the salt corrosion resistance after heating described above.

  From the above, by adding a very small amount of Sn in the presence of a small amount of Mo and adjusting the Si, Mn, and Al contents, the corrosion resistance after heating without increasing the Cr content that dominates the corrosion resistance of SUH409L steel. Can be greatly improved. In addition, since the quantity of Sn and Mo contained here is very small amount, it will not impair a low price significantly. Moreover, Cu, Ni, and V can be contained in a very small range, and further improvement in corrosion resistance can be pursued.

  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 0.0150%, preferably 0.0100%. 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 the corrosion resistance after heating, so 0.20% or more is contained, but even if contained in a large amount, the effect is saturated, so the upper limit is limited to 0.70%. In consideration of workability, the Si content is desirably 0.60% or less.

  Mn: Since Mn also has an effect of improving the corrosion resistance after heating, it is contained in an amount of 0.20% or more. However, even if contained in a large amount, the effect is saturated, so the upper limit is limited to 0.60%. Considering workability, the Mn content is desirably 0.40% or less.

  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 0.0100%. 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 after heating. An appropriate amount is essential and is contained in an amount of 10.50% or more, but even if it is contained in a large amount, the effect is saturated, so the upper limit is limited to 11.50%. . Considering the cost, the Cr content is preferably 11.40% or less.

  Al: Al is useful as a deoxidizing element and has the effect of improving the corrosion resistance after heating, so it is contained in an amount of 0.010% or more. % Should be limited.

  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. However, since the effect is saturated even if it is contained excessively, 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 has the effect of fixing C and N as carbonitrides to suppress intergranular corrosion, so 0.03% is included as the lower limit, but the effect is saturated even if included excessively Therefore, the upper limit of the content is 0.50%.

  Mo: Mo contains 0.02% as the lower limit in order to improve the corrosion resistance after heating in the presence of a small amount in the coexistence with Sn. Limit to 20%.

  Sn: Sn is useful as an element that improves the corrosion resistance after heating in an extremely small amount in the coexistence with Mo, and the lower limit when it is contained is 0.005%. On the other hand, there is a concern that Sn segregates at grain boundaries and degrades hot workability. Even if a large amount is contained, the effect is saturated, so the upper limit of the content is limited to 0.050%. Preferably, it exceeds 0.010%. The upper limit is preferably less than 0.050%.

A value: Calculated from the contents of Ni and Sn, and the A value defined by the above formula (1) is an index of oxidation resistance. To obtain the target oxidation resistance, 0.00065% ² or more is secured. It is necessary to.

  Cu, Ni, V: Reversed from the viewpoints of alloy saving and low cost, but in search of the ultimate improvement in corrosion resistance after heating, containing trace amounts of one or more of Cu, Ni, V useful for improving corrosion resistance You may let them. The lower limit of the Ni content is preferably 0.02%, and the upper limit is preferably 0.20%. The lower limit of the Cu and V contents is preferably 0.01%, and the upper limit is preferably 0.10%.

  B: B is an element useful as a measure for reducing the grain boundary strength due to the addition of Sn, and is an element that does not affect the corrosion resistance after heating. For this reason, 0.0002% may be contained as a lower limit, but if it exceeds 0.0050%, problems such as deterioration of hot workability occur, so the upper limit is preferably made 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 for the cold-rolled sheet were a material thickness after grinding the hot-rolled sheet: 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.

  Corrosion test pieces were collected from this steel plate, and the test surface was polished by # 600 emery to simulate a combined cycle corrosion test simulating a salt damage environment (salt spray: 5% NaCl spray 35 ° C. × 2 Hr, dry: relative humidity 20%, 60 ° C. × 4Hr, wet: 90% relative humidity, 50 ° C. × 2Hr repetition) and a condensed water corrosion test specified in JASO M911-A. In any test, a heat treatment of 400 ° C. × 8 Hr was performed in an atmospheric furnace before the test. The sample after the corrosion test was subjected to derusting treatment, and then the weight loss was determined by the gravimetric method.

  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 SUH409L steel, which is a reference for comparison. Corrosion resistance after heating is determined by comparing the weight loss of the candidate material with that of Comparative Example No. It was also expressed as the ratio of 101 weight loss. In the present invention, with the aim of improving the corrosion resistance by at least 1.5 times that of SUH409L steel, the aim was to obtain a corrosion weight loss ratio of less than 0.66.

  Comparative Example No. No. 102 has a Mo content and an A value that are outside the lower limits of the range of the present invention. In 103-105, since the A value is outside the lower limit of the range of the present invention, a satisfactory corrosion resistance improvement effect is not obtained. Comparative Example No. Nos. 106 to 108 have a Sn content and an A value that are outside the lower limit of the range of the present invention, so that the corrosion resistance does not reach a satisfactory level.

  Comparative Example No. No. 109, the Mo content exceeds the upper limit of the range of the present invention, and the corrosion resistance is sufficient, but the present invention No. 109 having a lower Mo content. It is only a corrosive weight loss ratio equivalent to 7, and it is not necessary to dare to contain a large amount.

  Comparative Example No. No. 110, the Sn content exceeds the upper limit of the range of the present invention, and the corrosion resistance is sufficient, but the present invention No. 110 having a smaller Sn content. It is only a corrosion weight loss ratio of the same level as 2, and it is not necessary to intentionally contain a large amount.

  Comparative Example No. No. 111 has a Cr content outside the lower limit of the range of the present invention, and the corrosion resistance is inferior to SUH409L.

  Comparative Example No. No. 112 has a Cr content exceeding the upper limit of the range of the present invention, and the corrosion resistance is sufficient, but the present invention No. 112 having a lower Cr content. It is only a corrosion weight loss ratio of the same level as 33 and 34, and it is not necessary to dare to contain a large amount.

  Comparative Example No. No. 113 has a Si content outside the lower limit of the range of the present invention. No. 115 has a Mn content outside the lower limit of the range of the present invention. In 117, the Al content is outside the lower limit of the range of the present invention, and the corrosion resistance does not reach a satisfactory level. On the other hand, Comparative Example No. No. 114 has a Si content exceeding the upper limit of the range of the present invention. No. 116 has a Mn content exceeding the upper limit of the range of the present invention. No. 118, although the Al content exceeds the upper limit of the range of the present invention, the corrosion resistance is saturated and the Si, Mn, Al content is less than these. It stays at the same level as 14.

  In Comparative Examples No. 119 and 120, the contents of Ti and Nb are out of the scope of the present invention, so that satisfactory corrosion resistance cannot be obtained.

  On the other hand, the present invention No. In Nos. 1 to 34, the contents of Si, Mn, Al, Mo, and Sn and the A value were within the scope of the present invention, and satisfactory corrosion resistance after heating was obtained.

Claims (3)

In mass%, C: 0.0150% or less, Si: 0.20 to 0.70 %, Mn: 0.20 to 0.60 %, P: 0.050% or less, S: 0.0100% or less, N: 0.0150% or less, Al: 0.010~0.20%, Cr: 10.50 ~ 11.50%, Mo: 0.02~0.20%, and Sn: 0.005 to 0. 050% is contained, Ti: 0.030 to 0.30% and Nb: 0.030 to 0.50% are contained, or A value defined by the formula (1) is 0 000665% A ferritic stainless steel for exhaust system members excellent in corrosion resistance, characterized in that it is ² or more and the balance is made of Fe and inevitable impurities.
A = [Mo] × [Sn] ------- (1)
Here, [Mo] and [Sn] are contents as mass% of Mo and Sn, respectively.   It is characterized by containing one or more of Cu: 0.01 to 0.10%, Ni: 0.02 to 0.20%, and V: 0.01 to 0.10%. The ferritic stainless steel for exhaust system members having excellent corrosion resistance according to claim 1.   The ferritic stainless steel for exhaust system members having excellent corrosion resistance according to claim 1 or 2, further comprising B: 0.0002 to 0.0050% in terms of mass%.

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