JP2015189990A - Austenitic stainless steel for exhaust gas flow passage member excellent in corrosion resistance, particularly having improved sensitization property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless steel having excellent corrosion resistance in environment in an exhaust gas flow passage using a fuel with high S concentration and exhaust gas at high temperature over 500°C.SOLUTION: The present invention provides an austenitic stainless steel material for an exhaust gas flow passage excellent in corrosion resistance having a chemical composition containing C:0.030 mass% or less, Si:0.10 to 0.70 mass%, Mn:0.10 to 2.00 mass%, Ni:10.00 to 40.00 mass%, Cr:17.00 to 30.00 mass%, P:0.005 to 0.40 mass%, S:0.0005 to 0.003 mass%, Cu:0.01 to 0.5 mass%, Mo:1.00 to 6.00 mass%, Al:0.1 mass% or less, N:0.005 to 0.050 mass%, Nb:2.00 mass% or less, further satisfying Nb/(C+N)≥20, and the balance Fe with inevitable impurities and having a solid solution C amount of 0.005 mass% or less and non-deposited laves phase. The present invention also provides a method of producing the austenitic stainless steel material.

Description

  The present invention is a stainless steel material mainly used for applications that require corrosion resistance, in particular, sensitization resistance, such as exhaust gas members in which poor fuel is used, and is an Nb-containing austenitic stainless steel with improved sensitization characteristics The present invention relates to a steel material and a manufacturing method thereof.

Environmental regulations in the automotive industry becomes increasingly stronger, NO _x reduction in exhaust gas, a situation where the fuel efficiency is required. As countermeasures, EGR (Exhaust Gas Recirculation) and exhaust heat recovery devices are being installed.

  The materials used for the EGR cooler and the exhaust heat recovery device are required to have corrosion resistance against snow melting salt, corrosion resistance against a solution used as circulating water, and corrosion resistance against exhaust gas condensate generated by condensation of exhaust gas.

Exhaust gas condensate is dissolved in the moisture contained in the exhaust gas flow path member by the salt content, SO _X , NO _X contained in the exhaust gas, and HCl, H ₂ SO ₄ , H ₂ SO ₃ , HNO ₃ , formic acid and acetic acid To form an acid environment. These are not all discharged out of the system, but gradually become concentrated in the exhaust gas flow path member. In addition, as the condensation of exhaust gas and evaporation of condensed water is repeated, the pH of the condensed water decreases and the corrosive ions are concentrated, so the corrosive environment becomes severe and promotes corrosion of the exhaust gas flow path member. .

  From the above required characteristics, as described in Patent Document 1, austenitic stainless steel represented by SUS304 and SUS316, or ferritic stainless steel disclosed in Patent Document 2 is used as the material of the EGR cooler. Yes. Further, ferritic stainless steel such as SUS436L and SUS444 is used for the exhaust heat recovery device.

JP 2007-46890 A JP 2009-174040 A Japanese Patent Laid-Open No. 2003-166039

  In emerging countries represented by China, fuels that are poorly refined and have a high sulfur (S) concentration are often used. Since the exhaust gas condensate when using a fuel having a high S concentration has a low pH and forms a severe corrosive environment in the exhaust gas flow path, ferritic stainless steels such as SUS444 did not exhibit sufficient corrosion resistance.

  Moreover, in order to improve the performance of the EGR cooler and the exhaust heat recovery device, it is preferable to use high-temperature exhaust gas. However, if the exhaust gas temperature exceeds 500 ° C., even if the amount of C is SUS316L with 0.010% by mass, Intergranular corrosion due to sensitization occurred.

  In view of the above-mentioned problems of the prior art, the present invention uses an austenitic stainless steel having excellent corrosion resistance in an environment in an exhaust gas flow path using a high S concentration fuel and using high temperature exhaust gas exceeding 500 ° C. The purpose is to provide.

  In order to achieve the above object, the inventors of the present invention focused on reducing the amount of dissolved C in order to prevent intergranular corrosion due to sensitization of an austenitic stainless steel material. And as a result of diligent examination, solution treatment at a specific temperature was performed on the steel material to which Nb was added, and then subjected to stabilization heat treatment in which the treatment temperature and time were strictly controlled, thereby suppressing the precipitation of the laves phase. It has been found that it is possible to efficiently precipitate NbC and reduce the amount of solid solution C of the steel material. In particular, it was found that the steel material obtained by the production method set in the present invention has a solid solution C content controlled to 0.005% by mass or less and exhibits excellent resistance to intergranular corrosion under a high temperature environment.

Specifically, the present invention includes C: 0.030 mass% or less, Si: 0.10 to 0.70 mass%, Mn: 0.10 to 2.00 mass%, Ni: 10.00 to 40.00. Mass%, Cr: 17.00 to 30.00 mass%, P: 0.005 to 0.40 mass%, S: 0.0005 to 0.003 mass%, Cu: 0.01 to 0.5 mass% , Mo: 1.00 to 6.00 mass%, Al: 0.10 mass% or less, N: 0.005 to 0.050 mass%, Nb: 2.00 mass% or less, and Nb / (C + N) ) Exhaust gas flow path member having a chemical composition satisfying ≧ 20 and having a balance of Fe and inevitable impurities, having a solid solution C content of 0.005% by mass or less, and having no laves phase precipitated, having excellent corrosion resistance An austenitic stainless steel material is provided.
As the exhaust gas channel member, an exhaust gas recirculation device or a member of an exhaust heat recovery device is preferable.

  Furthermore, C: 0.030 mass% or less, Si: 0.10-0.70, Mn: 0.10-2.00 mass%, Ni: 10.00-40.00 mass%, Cr: 17. 00 to 30.00 mass%, P: 0.005 to 0.40 mass%, S: 0.0005 to 0.003 mass%, Cu: 0.01 to 0.5 mass%, Mo: 1.00 6.00% by mass, Al: 0.10% by mass or less, N: 0.005 to 0.050% by mass, Nb: 2.00% by mass or less, and further satisfies Nb / (C + N) ≧ 20 A steel material having a chemical composition composed of Fe and inevitable impurities is subjected to solution treatment at 1050 to 1150 ° C., and then stabilized at 800 to 900 ° C. for 10 hours or more and in the time range satisfying the following formula (1). Austenating for exhaust gas flow path member having excellent corrosion resistance, including treatment To provide a method of manufacturing site stainless steel.

The austenitic steel according to the present invention has good corrosion resistance against exhaust gas condensate when a fuel having a high S concentration is used, and particularly exhibits improved sensitization characteristics in a high temperature environment exceeding 500 ° C.
Thus, the steel material of the present invention having excellent corrosion resistance and improved sensitization characteristics can be used as an exhaust gas flow path member, particularly as an exhaust gas recirculation device or exhaust heat recovery device member.

It is the figure which showed the relationship between the temperature and time in the stabilization heat processing of this invention, and the amount of solute C in steel materials using the invention steel which has the chemical composition of Table 1. A black circle indicates a steel material having a concentration in which the solid solution C amount exceeds 0.005 mass%, and a white circle indicates a steel material in which the solid solution C amount has a concentration of 0.005 mass% or less. It is the figure which showed the NbC precipitation condition on the steel material surface after carrying out the stabilization heat processing at 800 degreeC for 100 hours by the invention steel which has the chemical composition of Table 1 by high-resolution SEM. It is the figure which showed the precipitation condition of the laves phase of the steel materials which stabilized and heat-processed the invention steel which has the chemical composition of the composition of Table 1 on the conditions (900 degreeC for 100 hours) outside the prescription | regulation range of this invention. In an Example, it is a flowchart of the corrosion-resistance evaluation test method performed imitating the condensation environment of the exhaust gas at the time of using the fuel with inadequate refinement | purification. It is the photograph which observed the corrosion generation | occurrence | production location of the sample after the corrosion-resistance evaluation test in an Example with the optical microscope. (A) shows the inventive steel 1 and (b) shows the comparative steel 6.

The inventors have extensively studied the corrosion resistance of stainless steel in an environment with high H ₂ SO ₄ concentration and low pH, and the structure and corrosion resistance of stainless steel in a high temperature environment, and an exhaust gas flow path when using a fuel with high S concentration It was found that the corrosion of the members (EGR cooler, exhaust heat recovery device, etc.) constituting the material can be suppressed in the environment, and further the sensitization characteristics in the high temperature environment can be improved.

Hereinafter, the chemical composition of the steel material targeted by the present invention will be described.
C: 0.030 mass% or less C is an element inevitably contained in stainless steel. Reduction of the C content is effective in improving the sensitization characteristics. In the steel of the present invention, the amount of solute C is reduced to 0.005% by mass or less by adding Nb and heat treatment. Since an increase in the C content causes an increase in the Nb addition amount, the C content is 0.030% by mass.

Si: 0.10 to 0.70 mass%
Si is contained as a deoxidizer for stainless steel. However, if contained in a large amount, the steel is hardened and the workability is lowered, so the Si content is set to 0.10 to 0.70 mass%.

Mn: 0.10 to 0.70 mass%
Mn is necessary as a deoxidizer or an austenite stabilizing element, and is contained at least 0.10% by mass or more. On the other hand, Mn content is 0.10 to 2.00% by mass because it combines with S contained as an impurity in stainless steel to produce MnS which is a scientifically unstable sulfide and lowers the corrosion resistance. It was made the range.

Ni: 10.00-40.00 mass%
Ni is indispensable for obtaining an austenite phase, and is also effective for enhancing corrosion resistance. Accordingly, the Ni content must be 10.00% by mass or more. However, the Ni content is 40.00% by mass, preferably 30.00% by mass, and more preferably 25.00% by mass because a large amount causes an increase in cost.

Cr: 17.00 to 30.00 mass%
Cr is a main alloy element that forms a passive film on the surface of stainless steel, and improves pitting corrosion resistance, crevice corrosion resistance, and general corrosion resistance. As a result of investigations by the inventors, it was found that a Cr content of 17% by mass or more should be ensured in order to provide the corrosion resistance required in a high S condensed water environment. However, when the Cr content increases, the mechanical properties and toughness are impaired, which further increases the cost. Therefore, in the present invention, the upper limit is 30.00% by mass, preferably 25.00% by mass.

P: 0.004 to 0.040 mass%
P is segregated at the interface between the steel substrate and the corrosion product and at the grain boundaries in the base metal, and promotes corrosion by the molten salt and intergranular corrosion. However, an extreme decrease in content causes an increase in cost. Therefore, the P content is in the range of 0.005 to 0.040 mass%.

S: 0.0005 to 0.003 mass%
S generates Mn and sulfide and becomes the starting point of pitting corrosion. Further, in the austenitic stainless steel as in the present invention, S is segregated at the grain boundary, and the hot workability is lowered. Therefore, the lower the amount of S, the better. However, extremely reducing the S content causes an increase in production cost, so the S content is in the range of 0.0005 to 0.003 mass%.

Cu: 0.01-0.5 mass%
Cu is effective in improving the corrosion resistance and improving the toughness of the base metal, but if contained in a large amount, it causes a reduction in the toughness of the heat affected zone. Therefore, the Cu content is set to 0.0 to 0.10% by mass.

Mo: 1.00 to 6.00 mass%
Mo, like Cr, is a basic component for ensuring stable corrosion resistance. The effect is obtained when the content is 1% by mass or more, and the corrosion resistance improves as the content increases. However, when the content exceeds 6% by mass, the hot workability decreases. Therefore, the Mo content is in the range of 1.00 to 6.00 mass%.

Al: 0.10% by mass or less Al is added as a deoxidizer. However, if added excessively, Al ₂ O ₃ inclusions are generated and the workability is lowered, so the upper limit of the Al content is 0.10% by mass.

N: 0.005 to 0.050 mass%
N is effective as an austenite stable element, and further improves the corrosion resistance of stainless steel, particularly pitting corrosion resistance, together with Cr, Ni, and Mo. Therefore, N needs to be added in an amount of 0.005% by mass or more. On the other hand, if added excessively, productivity is lowered and Nb carbide formation is prevented, so the N content is made 0.050% by mass as the upper limit.

Nb: 2.00% by mass or less Since Nb is an element having a strong affinity with C and N, when added, it forms a carbonitride to reduce the amount of dissolved C and N. And reduction of the solid solution C and N by Nb addition can suppress precipitation of Cr carbide | carbonized_material in a high temperature environment, and can improve the sensitization characteristic. In the present invention, as described above, NbC and NbN are precipitated by heat treatment, so Nb / (C + N) needs to be 20 or more. However, if the amount of Nb added is too large, the effect of reducing the solid solution C and N is saturated, and cold workability and hot workability are deteriorated due to the influence of Nb itself. Furthermore, in order to promote precipitation of the laves phase having low corrosion resistance in a high temperature environment, the upper limit was made 2.00% by mass.

In the present invention, in order to improve the sensitization characteristics and the intergranular corrosion resistance of stainless steel, the amount of solute C is set to 0.005 mass% or less. In order to achieve this, the chemical component composition is specified as described above, and an optimum stabilization heat treatment for precipitating NbC is required.
The method for producing stainless steel of the present invention can use a conventional method for producing stainless steel until the step of melting the steel having the chemical composition described above and performing hot rolling. If necessary, the steel material obtained by hot rolling may be cold-rolled. In the present invention, the solution treatment is further performed at 1050 to 1150 ° C. for 5 to 15 minutes, and then the stabilization treatment is performed at 800 to 900 ° C. for 10 hours or more and in the time range satisfying the following formula (1). It is characterized by doing.

Below, the basis for setting the manufacturing conditions for the stainless steel material of the present invention will be described.
-Method for investigating the amount of dissolved C From the state of precipitation of NbC in the grains, the amount of dissolved C in the austenite phase that causes Cr carbide formation during heating was investigated.
Steel having the chemical composition and inevitable impurities shown in Table 1 below was melted, and a hot rolled sheet having a thickness of 3.0 mm was manufactured by hot rolling. This hot-rolled sheet was annealed and then cold-rolled to a sheet thickness of 1.0 mm and subjected to finish annealing at 1150 ° C. for 5 minutes. Furthermore, after stabilizing heat treatment was performed under the time and temperature conditions plotted in FIG. 1 and pickling, the amount of solute C in each steel material was investigated.

  NbC can be confirmed using a high resolution SEM. As an example, FIG. 2 shows the state of NbC precipitation after stabilization heat treatment at 800 ° C. for 100 hours for the inventive steels having the components shown in Table 1. A mesh (for example, 20 × 20) in which the cross-section of the steel sheet has an intersection of 400 at 100 nm intervals (for example, 20 × 20) is used, and the intersection of the mesh overlapping with NbC precipitated in the crystal grains is counted. This operation was performed for 10 or more fields of view, and the number of intersections overlapping NbC was divided by the number of all intersections to determine the area ratio of Nb carbide.

Stabilized C amount (% by mass) = NbC area ratio × NbC specific gravity (7.78 g / cm ³ ) ÷ Stainless steel specific gravity (7.93 g / cm ³ ) × C atomic weight (12) ÷ NbC molecular weight (92.9) × 100 = NbC area ratio x 12.7
And the amount of solid solution C was derived | led-out based on the following formula | equation (2).
Solid solution C amount (mass%) = Contained C amount (mass%) − NbC area ratio × 12.7 (2)

In the austenitic stainless steel material of the present invention, no laves phase is precipitated. Here, “the laves phase is not precipitated” means that the laves phase in the stainless steel material has an area ratio of 0.1% or less, preferably 0.01% or less. Below, the confirmation method of laves phase precipitation is demonstrated. The area ratio is obtained from the average of 50 random fields by measuring the ratio of the deposited area of the laves phase in the field with an optical microscope 400 times after the sample etching.
As an example, FIG. 3 shows the state of laves phase precipitation in a sample obtained by subjecting the inventive steel having the composition shown in Table 1 to heat treatment at 900 ° C. for 100 hours.

Stabilization heat treatment temperature: 800 ° C to 900 ° C
The amount of C that can be dissolved in the austenite phase increases with increasing temperature. Therefore, NbC cannot be sufficiently precipitated at a temperature higher than 900 ° C., and a sufficient effect of reducing the amount of dissolved C cannot be obtained. Therefore, the upper limit of the heat treatment temperature is set to 900 ° C. Further, at a temperature lower than 800 ° C., Nb combines with Fe and easily forms a laves phase which is an intermetallic compound. The formation of the laves phase inhibits the precipitation of NbC and the corrosion resistance of the laves phase itself is low, so the lower limit of the heat treatment temperature was set to 800 ° C.

  In order to precipitate a sufficient amount of NbC for reducing the amount of dissolved carbon, the lower limit of the heat treatment time was set to 10 hours. However, heat treatment for an excessively long time leads to precipitation of laves phase in the grains and at the grain boundaries even in the temperature range of 800 ° C. to 900 ° C., which reduces the corrosion resistance of the material.

It was time.
The range of appropriate stabilization heat treatment conditions derived by investigating the amount of dissolved C and the laves phase precipitation state for samples produced under various stabilization heat treatment conditions is shown in FIG.

  Using the austenitic stainless steel described above as a raw material, exhaust gas flow path members such as EGR coolers and exhaust heat recovery devices are manufactured. A known manufacturing method is adopted for the shape and structure of the apparatus. There is no restriction | limiting in a shaping | molding means, It manufactures by press work, Ni brazing, welding, etc.

  Stainless steel having chemical components shown in Table 2 was melted, and hot rolled sheets having a thickness of 3.0 mm were manufactured by hot rolling. This hot-rolled sheet was cold-rolled to a thickness of 1.0 mm, subjected to finish annealing at 1150 ° C. for 5 minutes, further subjected to stabilization heat treatment under the conditions shown in Table 2, pickled, and then subjected to the test.

Corrosion resistance evaluation test for austenitic stainless steel material for exhaust gas flow path member In order to simulate the internal environment of the exhaust gas flow path member in which exhaust gas condensation and evaporation are repeated, the corrosion resistance was evaluated by the test method shown in FIG.
A test piece of 50 mm × 50 mm was cut out from each stainless steel plate having a thickness of 1.0 mm and heated at 700 ° C. for 1000 hours in an atmospheric environment, and the surface scale was removed by polishing.
The test solution was prepared with reference to an example of analysis of condensed water collected from an EGR cooler of an actual vehicle using a fuel having a high S concentration. Table 3 shows the composition of the test solution. The pH was adjusted using aqueous ammonia. In the corrosion resistance evaluation test for austenitic stainless steel materials for exhaust gas flow path members, 100 ml of the test solution is dropped onto the test piece and dried in an environment of 80 ° C. and 40% relative humidity in a constant temperature / humidity bath for 30 minutes. After evaporation, it was kept for 3 hours in an environment at a temperature of 80 ° C. and a relative humidity of 85% to form condensed water 35 times the test solution on the surface. After repeating this cycle 50 times, rust was removed and the corrosion morphology was observed.
In addition, precipitation of the laves phase was confirmed by the above method. The maximum erosion depth was determined by the depth of focus method using an optical microscope.

-Result of corrosion resistance evaluation test FIG. 5 shows the surface appearance of the corrosion part after the test of invention steel 1 (a) and comparative steel 6 (b). Intergranular corrosion occurred deeply in the comparative steel 6 in which NbC was not precipitated and the solute C content was 0.015 mass%, but the invention steel 1 in which NbC was generated and the solute C content was 0.004 mass%. Then shallow pitting occurred.
Further, as can be seen from the results shown in Table 4, NbC is precipitated in Comparative Steel 5, and the amount of solute C is 0.005% by mass, but the laves phase is precipitated by long-time heat treatment outside the proper range. As a result, the corrosion resistance of the grain boundaries decreased and deep grain boundary corrosion occurred. In comparative steel 8, NbC is precipitated, but deep intergranular corrosion occurs because the amount of dissolved C is not reduced to 0.005% by mass or less by high-temperature heat treatment outside the proper range. However, in the steel of the present invention, NbC is precipitated, the amount of dissolved C is 0.005% by mass, and the laves phase is not precipitated. Since these corrosion forms were pitting corrosion and the maximum erosion depth was 50 μm or less, the steel of the present invention was not sensitized in an environment of 500 ° C. or higher and had high intergranular corrosion resistance. It was confirmed that

  When the austenitic stainless steel according to the present invention is used, an exhaust gas passage member having good corrosion resistance even when used for a long time in a high temperature environment of 500 ° C. or higher, for example, an EGR cooler or a member of an exhaust heat recovery device is obtained. I can do it.

Claims (3)

  C: 0.030 mass% or less, Si: 0.10-0.70 mass%, Mn: 0.10-2.00 mass%, Ni: 10.00-40.00 mass%, Cr: 17.00 -30.00 mass%, P: 0.005-0.40 mass%, S: 0.0005-0.003 mass%, Cu: 0.01-0.5 mass%, Mo: 1.00-6 0.000% by mass, Al: 0.10% by mass or less, N: 0.005 to 0.050% by mass, Nb: 2.00% by mass or less, and further satisfies Nb / (C + N) ≧ 20. And an austenitic stainless steel material for exhaust gas passage members having a chemical composition comprising inevitable impurities, having a solid solution C content of 0.005% by mass or less, and having no laves phase precipitated, and having excellent corrosion resistance.   The austenitic stainless steel material for an exhaust gas channel member excellent in corrosion resistance according to claim 1, wherein the exhaust gas channel member is a member of an exhaust gas recirculation device or an exhaust heat recovery device. C: 0.030 mass% or less, Si: 0.10-0.70 mass%, Mn: 0.10-2.00 mass%, Ni: 10.00-40.00 mass%, Cr: 17.00 -30.00 mass%, P: 0.005-0.40 mass%, S: 0.0005-0.003 mass%, Cu: 0.01-0.5 mass%, Mo: 1.00-6 0.000% by mass, Al: 0.10% by mass or less, N: 0.005 to 0.050% by mass, Nb: 2.00% by mass or less, and further satisfies Nb / (C + N) ≧ 20. And a steel material having a chemical composition composed of unavoidable impurities is subjected to a solution treatment at 1050 to 1150 ° C., and then stabilized at 800 to 900 ° C. for 10 hours or more and in the time range satisfying the following formula (1). The exhaust gas flow having excellent corrosion resistance according to claim 1 or 2, Method of manufacturing a member for austenitic stainless steel.

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