JP2015014022A - Corrosion resistance imparting method of cylindrical body made of stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corrosion resistance imparting method capable of improving corrosion resistance by suppressing generation of chromium nitride on the inner surface part side, even in the case of a cylindrical body made of stainless steel.SOLUTION: A corrosion resistance imparting method of a cylindrical body made of stainless steel includes: a nitrogen absorption step in which the cylindrical body made of stainless steel is heated under a gaseous nitrogen atmosphere so as to allow the stainless steel to absorb nitrogen, and then cooled by gas so as to be austenitized; and a nitrogen solid solution step in which the cylindrical body having passed through the nitrogen absorption step is reheated under the gaseous nitrogen atmosphere, and then chilled quickly.

Description

  The present invention relates to a method for imparting corrosion resistance to a stainless steel cylinder.

  Conventionally, as a corrosion resistance imparting method for improving the corrosion resistance of stainless steel, after ferritic stainless steel is heated in a nitrogen gas atmosphere at 1100 to 1250 ° C. in a heating furnace, and after performing a nitrogen absorption step in which the ferritic stainless steel absorbs nitrogen A method including a step of quenching to austenite part or all of the ferritic stainless steel is known (for example, see Patent Document 1).

  Stainless steel manufactured by such a method for imparting corrosion resistance is used, for example, as a separator for a polymer electrolyte fuel cell that requires high corrosion resistance.

JP 2012-92413 A

However, the conventional method for imparting corrosion resistance to stainless steel is effective for a plate-like member such as a separator of a solid polymer fuel cell, for example, but a hollow shape such as an exhaust system component mounted on a vehicle (for example) For example, when the above-described nitrogen absorption process is applied to a cylindrical body having a box shape or the like, a difference occurs in the cooling rate between the inner surface portion side and the outer surface portion side of the cylindrical body. In particular, the inner surface portion side of the cylinder has a slower cooling rate than the outer surface portion side, so that chromium nitride (Cr ₂ N, CrN) is formed, which may deteriorate the corrosion resistance.

  The present invention has been made in view of the above circumstances, and a corrosion resistance imparting method capable of improving the corrosion resistance by suppressing the generation of chromium nitride on the inner surface side even in the case of a stainless steel cylinder. The purpose is to provide.

  In order to achieve the above object, the method for imparting corrosion resistance to a stainless steel cylinder according to the present invention is as follows: (1) after the stainless steel cylinder is heated in a nitrogen gas atmosphere to allow the stainless steel to absorb nitrogen And a nitrogen absorption step of austenitizing by gas cooling, and a nitrogen solid solution step of rapidly cooling the cylindrical body that has undergone the nitrogen absorption step after reheating in a nitrogen gas atmosphere.

  According to the method for imparting corrosion resistance of a stainless steel cylinder according to the present invention, after the nitrogen absorption process, the austenitized cylinder is reheated in a nitrogen gas atmosphere, and then a nitrogen solid solution process is performed in which it is rapidly cooled. The chromium nitride generated on the inner surface side of the cylindrical body by reheating in the same process can be re-dissolved, and a high concentration nitrogen solid solution layer can be obtained by rapid cooling in the same process. Thereby, by implementing a nitrogen solid solution process after a nitrogen absorption process, even if it is a stainless steel cylinder, generation | occurrence | production of chromium nitride can be suppressed by the inner surface part side, and corrosion resistance can be improved.

  ADVANTAGE OF THE INVENTION According to this invention, even if it is a stainless steel cylinder, the corrosion-resistance provision method which can suppress generation | occurrence | production of chromium nitride by the inner surface part side and can improve corrosion resistance can be provided.

It is a figure which shows the corrosion resistance provision process of the stainless steel cylinder which concerns on embodiment of this invention, Comprising: (a) shows a nitrogen absorption process, (b) shows a nitrogen solid solution process. It is a graph which shows the heat cycle of the corrosion resistance provision process which concerns on embodiment of this invention, Comprising: (a) shows the heat cycle of a nitrogen absorption process, (b) shows the heat cycle of a nitrogen solid solution process. It is a figure which shows the structure | tissue change of the inner surface part of the cylinder which concerns on embodiment of this invention, Comprising: (a) is an inner surface part structure | tissue after a nitrogen absorption process, (b) is an inner surface part structure | tissue after a nitrogen solid solution process. Show. It is a figure which shows the outline | summary of the conventional nitrogen absorption process, (a) is a figure which shows the process process of the nitrogen absorption process, (b) is a graph which shows the heat cycle of the nitrogen absorption process.

  Embodiments of the present invention will be described below with reference to the drawings.

  Examples of the stainless steel cylinder subjected to the corrosion resistance imparting process according to the present embodiment include exhaust system parts mounted on vehicles and the like, and in particular, EGR coolers that require corrosion resistance against condensed water. Exhaust system parts are listed. Here, the cylindrical body has an open end at least in part. Further, the above-described cylinder includes, for example, a square cylinder shape (for example, a box shape) in addition to the cylindrical shape.

In the present embodiment, a stainless steel material (for example, SUS447J1 etc.) having excellent corrosion resistance is used as the stainless steel material used as the material of the cylindrical body. Specifically, as the chemical component composition (unit: mass%), for example, the following (1) to (3) can be used, and the balance is composed of iron (Fe) and inevitable impurities.
(1) 15.0% to 30.0% of chromium (Cr)
(2) 0.0% to 4.0% molybdenum (Mo)
(3) Nitrogen (N) is 0.4% to 2.0%

  Nitrogen (N) is added during the treatment. Moreover, as said ferritic stainless steel, you may contain manganese (Mn) and niobium (Nb) in addition to said (1)-(3) as needed.

  Next, with reference to FIGS. 1-3, the corrosion resistance provision process of the cylinder processed using the above-mentioned ferritic stainless steel as a raw material is demonstrated.

  As shown in FIGS. 1A and 1B, the corrosion resistance imparting process according to the present embodiment includes a nitrogen absorption process and a nitrogen solid solution process.

In the nitrogen absorption process, as shown in FIG. 1A, a batch process is used in which a large number of cylinders 2 are simultaneously processed in a furnace 1. Here, the furnace 1 is a heating chamber in which a stainless steel cylinder 2 is heated in, for example, a nitrogen gas (N ₂ ) atmosphere pressurized to 50 to 400 kPa to perform nitrogen absorption treatment in which the stainless steel absorbs nitrogen. 3 and a cooling chamber 4 for performing a gas cooling process in which the stainless steel in which nitrogen has been absorbed in the heating chamber 3 is gas-cooled with pressurized nitrogen gas (N ₂ ) to form austenite.

Therefore, in the nitrogen absorption process, first, a plurality of cylinders 2 are put into the heating chamber 3 in the furnace 1, and the inside of the heating chamber 3 is filled with nitrogen gas (N ₂ ) pressurized to, for example, 50 to 400 kPa. . Next, nitrogen is heated to a predetermined temperature (for example, a range of 1100 ° C. to 1260 ° C.) by a heating device (not shown) in a nitrogen gas (N ₂ ) atmosphere with respect to the plurality of cylinders 2 put into the heating chamber 3. Perform absorption treatment. By this nitrogen absorption treatment, the stainless steel of each cylinder 2 is austenitized. Thereafter, the plurality of cylinders 2 that have been heated in the heating chamber 3 are kept soaked at a predetermined temperature for a predetermined time (for example, 30 min to 720 min). Thereby, nitrogen (N) is absorbed with respect to the stainless steel of each cylinder 2.

Thereafter, in the nitrogen absorption process, when a certain period of time elapses with the temperature maintained, a heat insulating door (not shown) between the heating chamber 3 and the cooling chamber 4 is opened, and a plurality of cylinders are conveyed by a conveying means (not shown). The body 2 is transferred from the heating chamber 3 to the cooling chamber 4. When the plurality of cylinders 2 are conveyed to the cooling chamber 4, the heat insulating door is closed, nitrogen gas (N ₂ ) pressurized to 600 kPa, for example, is blown into the cooling chamber 4, and a predetermined cooling rate (for example, 6 Gas cooling treatment is performed in which the gas is cooled to about room temperature, for example.

Various inert gases can be used as the cooling gas for gas cooling. In this embodiment, nitrogen gas (N ₂ ) is used as the cooling gas. Moreover, in this Embodiment, the nitrogen absorption process is comprised by these two processes (nitrogen absorption process, gas cooling process).

  Here, the structure change on the inner surface side of the cylindrical body 2 in the nitrogen absorption process will be described together with the heat cycle shown in FIG.

As shown in FIG. 2A, when the temperature of the cylindrical body 2 is first raised to a predetermined temperature, the structures on the inner surface side and the outer surface side of the cylindrical body 2 are both ferrite (α) phases. Thereafter, the structures on the inner surface side and the outer surface side of the cylindrical body 2 both transform from the ferrite (α) phase to the austenite (γ) phase as the temperature of the nitrogen absorption treatment increases. And while maintaining soaking | uniform-heating at predetermined temperature (predetermined time), the structure | tissue of the inner surface part side and the outer surface part side of the cylinder 2 is maintained in an austenite (gamma) phase. Here, the above-mentioned fixed time is adjusted within a range of 30 min to 720 min, for example, so that an austenite (γ) phase having a required depth is obtained. In addition, since the N concentration in a range in which the austenite (γ) phase and chromium nitride (Cr ₂ N) are not formed in the nitrogen solid solution process is ensured, the austenite (γ) phase is not necessarily obtained at the time of soaking. Good.

  Thereafter, the structures on the inner surface side and the outer surface side of the cylindrical body 2 are gas cooled so as to maintain the austenite (γ) phase in the gas cooling process. At this time, a difference occurs in the cooling rate between the inner surface portion side and the outer surface portion side of the cylindrical body 2. In particular, in FIG. 2A, the cooling rate of the inner surface side of the cylindrical body 2 indicated by the solid line is slower than that of the outer surface side indicated by the broken line in FIG. In the member having an internal space such as the cylindrical body 2, it is difficult for the cooling gas to go around to the inner surface side, and radiant heat is also present, so that the cooling rate is slower than that on the outer surface side. For example, when the cylinder 2 is an EGR cooler or the like, the cooling rate is particularly slow because fins are attached to the inner surface of the cooler to form a complicated shape.

For this reason, the structure on the inner surface side of the cylindrical body 2 having a slow cooling rate is changed from austenite (γ) phase to a two-phase state of ferrite (α) phase and austenite (γ) phase in the course of gas cooling treatment. It is in a state (γ → α + γ + Cr ₂ N) in which a nitride (Cr ₂ N, CrN: hereinafter, only Cr ₂ N is described) is generated. That is, the structure on the inner surface side of the cylindrical body 2 was formed with a compound such as chromium nitride (Cr ₂ N) (shown by a black portion in FIG. 3A) as shown in FIG. It becomes an organizational state. If the cooling rate is high, the structure is frozen and the austenite (γ) phase is maintained.

Although such a nitrogen absorption process has been conventionally performed, chromium nitride (Cr ₂ N) is generated as described above only in the nitrogen absorption process for a cylinder 2 having a slow cooling rate on the inner surface side. As a result, this chromium nitride (Cr ₂ N) causes a decrease in corrosion resistance.

  Further, as a method for suppressing such deterioration in corrosion resistance, for example, in the cooling process of the nitrogen absorption process, it is conceivable to use water cooling or oil cooling with a coolant instead of gas cooling as shown in FIG.

However, even when such water cooling or oil cooling is used, cooling by water cooling or oil cooling is not yet performed while transferring the plurality of cylinders after nitrogen absorption treatment to the cooling chamber, but a constant cooling rate Cooled by. The cooling rate during this conveyance is extremely slow. Therefore, nitrogen (N) dissolved in the stainless steel of the cylindrical body is lost during the period from the soaking to the start of cooling by water cooling or oil cooling (during conveyance), and the nitrogen solidification occurs due to the temperature drop. The solubility limit decreases and Cr ₂ N is generated. Note that nitrogen (N) escapes because the energy of solute nitrogen (N) is not as stable as the compound and is easy to move, so at high temperatures (for example, a temperature drop occurs during transportation but still high). This is because it has the property of diffusing inside the stainless steel and coming out of the surface.

Thus, even when water cooling or oil cooling is used in the cooling process of the nitrogen absorption process, chromium nitride (Cr ₂ N) is still generated, and this chromium nitride (Cr ₂ N) is corrosion resistant. It will cause the decrease.

Therefore, in the present embodiment, in order to obtain the cylindrical body 2 having excellent corrosion resistance that does not contain chromium nitride (Cr ₂ N) as described above, a nitrogen solid solution process described below is performed after the nitrogen absorption process. Newly added.

In this nitrogen solid solution process, as shown in FIG. 1B, a single process for processing one cylindrical body 2 in the cooling device 5 is used. Here, the cooling device 5 includes a high-frequency heating device 6 and a cooling jacket 7. Further, the cooling device 5 is configured so that when the cylindrical body 2 is heated by the high-frequency heating device 6, the inside of the device is filled with nitrogen gas (N ₂ ) pressurized to, for example, 50 to 400 kPa, that is, an N ₂ atmosphere. It is comprised so that it may become. For example, the cooling device 5 depressurizes the inside of the device with a vacuum pump (not shown) and purges the air inside the device, and then circulates the pressurized nitrogen gas (N ₂ ) inside the device to create an N ₂ atmosphere.

  The high-frequency heating device 6 performs heating using an induction heating phenomenon. Specifically, the high-frequency heating device 6 generates an AC magnetic field by energizing the induction coil 6a with an AC current, generates an AC current in a heat generating body (cylinder 2) disposed in the generated AC magnetic field, The heat generating body is caused to generate heat by the alternating current. By using this high-frequency heating device 6, the cylinder 2 can be rapidly heated to a predetermined temperature (for example, 1100 ° C. to 1260 ° C.). The apparatus for heating the cylindrical body 2 is not limited to the high-frequency heating apparatus 6, and other apparatuses may be used as long as rapid heating is possible.

  The cooling jacket 7 has a plurality of injection holes (not shown) for injecting the cooling liquid into the cooling device 5. By injecting the cooling liquid from the plurality of injection holes, the high-frequency heating device 6. The cylindrical body 2 rapidly heated by the above is rapidly cooled to, for example, the room temperature.

In the cooling device 5 configured as described above, the above-described nitrogen solid solution step is performed. Specifically, prior to the nitrogen solid solution step, first, one of the plurality of cylinders 2 that has undergone the above-described nitrogen absorption step is disposed inside the induction coil 6 a inside the cooling device 5. Thereafter, the inside of the cooling device 5 is made an N ₂ atmosphere.

Next, a nitrogen solid solution step is performed in which the cylindrical body 2 disposed inside the cooling device 5 is reheated by the high-frequency heating device 6 in an N ₂ atmosphere and then rapidly cooled by the cooling jacket 7. Specifically, in the nitrogen solid solution step, the cylinder 2 is rapidly heated to a predetermined temperature (for example, 1100 ° C. to 1260 ° C.) by the high frequency heating device 6. Each time, the cylinder 2 rapidly heated in the cooling device 5 is kept soaked at a predetermined time (for example, 1100 ° C. to 1260 ° C.) for a certain time (for example, 1 min to 15 min). Here, the above-mentioned fixed time is, for example, in the range of 1 min to 15 min, and is longer than the time necessary for dissolving the chromium nitride (Cr ₂ N) formed in the nitrogen absorption process.

  Thereafter, in the nitrogen solid solution step, after a certain period of time during which the soaking is maintained, the cylinder 2 that has been soaked is cooled by the cooling jacket 7 at a predetermined cooling rate (for example, a cooling rate of 50 ° C./sec or more). For example, it is rapidly cooled to about the room temperature. Thereby, a series of nitrogen solid solution process is complete | finished.

  Here, the structure change on the inner surface side of the cylindrical body 2 in the nitrogen solid solution step will be described together with the heat cycle shown in FIG.

As shown in FIG. 2B, when the cylinder 2 that has undergone the nitrogen absorption process is first rapidly heated to a predetermined temperature, the structure on the inner surface side of the cylinder 2 is composed of ferrite (α) phase and austenite (γ ) Phase in which chromium nitride (Cr ₂ N) is generated in a two-phase state (γ → α + γ + Cr ₂ N).

Thereafter, the structure on the inner surface side of the cylindrical body 2 is such that the chromium nitride (Cr ₂ N) is re-dissolved with rapid heating by the high-frequency heating device 6, whereby the ferrite (α) phase and the austenite (γ) phase. The state in which chromium nitride (Cr ₂ N) is generated in the two-phase state is transformed into the austenite (γ) phase. And while maintaining soaking | uniform-heating at predetermined temperature (a fixed time), the structure | tissue of the inner surface part side of the cylinder 2 is maintained in an austenite (gamma) phase.

  Thereafter, the structure on the inner surface side of the cylindrical body 2 is rapidly cooled by the cooling jacket 7 so as to maintain the austenite (γ) phase. The cooling rate at this time is faster than the cooling rate of gas cooling in the nitrogen absorption process. Moreover, in this Embodiment, since the reheating by the high frequency heating apparatus 6 and the rapid cooling by the cooling jacket 7 are performed inside the cooling device 5, the cylindrical body does not take time such as transporting the cylindrical body 2 after the rapid heating. 2 can be cooled.

  As a result, as shown in FIG. 3 (b), the structure on the inner surface side of the cylindrical body 2 has no compound formation as in the nitrogen absorption step and is uniformly austenite (γ) phase γ single phase (nitrogen). Solid solution structure, high concentration nitrogen solid solution layer) can be obtained. That is, a structure excellent in corrosion resistance can be obtained as the structure on the inner surface side of the cylindrical body 2.

As described above, according to the method for imparting corrosion resistance to the stainless steel cylinder according to the present embodiment, after the nitrogen absorption step, the austenitized cylinder 2 is reheated in an N ₂ atmosphere and then rapidly cooled. Since the nitrogen solid solution step is performed, the chromium nitride (Cr ₂ N) generated on the inner surface side of the cylinder 2 is re-dissolved by reheating in the same step, and the high concentration nitrogen solid solution layer is rapidly cooled in the same step. Can be obtained. Thereby, by carrying out the nitrogen solid solution process after the nitrogen absorption process, even in the case of the stainless steel cylinder 2, the generation of chromium nitride (Cr ₂ N) is suppressed on the inner surface side, thereby improving the corrosion resistance. Can be made.

  As described above, according to the method for imparting corrosion resistance of a stainless steel cylinder according to the present invention, the corrosion resistance is improved by suppressing the generation of chromium nitride on the inner surface side even in the case of a stainless steel cylinder. It is useful as a method for imparting corrosion resistance to a stainless steel cylinder.

DESCRIPTION OF SYMBOLS 1 ... Furnace, 2 ... Cylindrical body, 3 ... Heating chamber, 4 ... Cooling chamber, 5 ... Cooling device, 6 ... High frequency heating device, 6a ... Induction coil, 7 ... Cooling jacket

Claims (1)

After the stainless steel cylinder is heated in a nitrogen gas atmosphere to absorb the nitrogen in the stainless steel, the nitrogen absorption step of gas cooling to austenite,
A method for imparting corrosion resistance to a stainless steel cylinder, comprising: a nitrogen solid solution process in which the cylinder having undergone the nitrogen absorption process is reheated in a nitrogen gas atmosphere and then rapidly cooled.