JP6047515B2 - Surface treatment method of stainless steel and heat exchanger using the same - Google Patents

Description

  The present invention relates to a stainless steel surface processing method.

  Stainless steel is widely used as a material for building members, sanitary members, electrical equipment and the like because it is excellent in strength and corrosion resistance. In recent years, in order to further improve the corrosion resistance of stainless steel, it is often used by laminating a paint or a resin film on the surface. When laminating a paint or a resin film on stainless steel, it is necessary to roughen the surface of the stainless steel in order to improve adhesion to the stainless steel. As the surface roughening technology for stainless steel, there are a blasting process for physically forming unevenness, a roughening etching process for forming unevenness chemically or electrically, or a method of combining them. For example, Patent Document 1 proposes a method of chemically and uniformly roughening the surface of stainless steel with an etching solution mainly composed of sulfuric acid, chlorine ions, and cupric ions. Patent Document 2 proposes a method of chemically etching with hydrofluoric acid, nitric acid, hydrochloric acid, phosphoric acid aqueous solution or the like after sandblasting.

Japanese Patent Laid-Open No. 2001-11662 Japanese Patent Laid-Open No. 5-264045

  In the surface roughening technique using the etching process described in Patent Document 1 or the like, a concavo-convex structure is formed by preferentially etching crystal grain boundaries. Therefore, the size of the concavo-convex structure such as width and height depends on the crystal grain size of stainless steel. Therefore, since the crystal grain size of general stainless steel is 10 to 200 μm, it is difficult to form the size of the concavo-convex structure to about 10 μm or less. In addition, it is possible to reduce the etching amount to reduce the height of the convex portion, but the surface area decreases with a decrease in height. When a fine structure having a height of 5 μm is formed by a surface roughening technique using an etching process, the surface area is limited to about 14 times that of a smooth surface, and it is difficult to increase the surface area. .

  In Patent Document 2, a complex concavo-convex structure can be formed by combining sandblasting and etching, but a large concavo-convex structure as a base is formed by sandblasting using an abrasive having a diameter of about 100 μm. It is difficult to form a size of 10 μm or less.

  Therefore, the present invention provides a surface processing method for stainless steel that forms a fine and high surface area uneven structure (roughened surface) on the surface of stainless steel.

  The present invention includes a plurality of means for solving the above-described problems. To give an example, the surface processing method for stainless steel according to the present invention forms a microstructure on the surface of stainless steel. A first step of applying a crystal grain refining process for refining crystal grains inside the surface of the steel; and a second step of roughening and etching the surface of the stainless steel with an etching treatment liquid after the first step; It is characterized by having.

  ADVANTAGE OF THE INVENTION According to this invention, the surface processing method of the stainless steel which can form the fine concavo-convex structure (roughening surface) with a high surface area on stainless steel can be provided.

It is the schematic of a surface microstructure formation process. It is sectional drawing of the base material in a surface fine structure formation process. It is the result of the surface of a surface microstructure, a cross-sectional SEM image, and a surface area ratio. It is a result of cross-sectional crystal observation after crystal grain refinement processing. It is a result of a passive film evaluation before and after surface fine structure formation. It is a result of corrosion resistance evaluation before and after surface fine structure formation. 6 is a structural diagram of a shell and tube heat exchanger according to Embodiment 2. FIG.

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

  FIG. 1 is a procedural diagram showing a surface microstructure forming process of stainless steel according to the present invention.

  In the present invention, as shown in FIG. 1, the crystal grain refining process (first process) and the roughening etching process (second process) are performed in this order. In addition, you may perform the removal process (3rd process) which removes a substituted precipitation metal after a roughening etching process (2nd process) as needed.

  The feature of the surface processing method of the stainless steel for forming a fine structure on the surface of the stainless steel of the present invention is that the crystal grain boundary inside the surface of the stainless steel is refined and then the grain boundaries are preferentially etched. is there. In addition, as shown in FIG. 2, the concavo-convex structure formed by the crystal grain refining process in the first step is R1, the concavo-convex structure formed by the roughening etching process in the second step is R2, and the third step When the concavo-convex structure formed by the replacement deposited metal removal treatment is R3, the final surface microstructure obtained in the present invention is R2 or R3. That is, the surface fine structure obtained by the present invention does not include the uneven structure (R1) formed by the crystal grain refining treatment in the first step. The size of the concavo-convex structure is R3 <R2 << R1, but R2 and R3 are substantially the same, and a sufficiently fine concavo-convex structure can be obtained. With these features, a fine and high surface roughened surface can be formed on stainless steel.

  The crystal grain refining process (first step) is a process of refining crystal grains inside the surface of stainless steel to form a fine crystal layer on the surface layer. Since the final surface microstructure obtained in the present invention is highly dependent on the thickness and grain size of the fine crystal layer formed by the crystal grain refinement process, the fine crystal layer has a thickness of 1 μm or more. The particle size is preferably 1 μm or less. The crystal grain size of general stainless steel is 10 μm or more. The crystal grain refinement processing method is not particularly limited, and known peening, polishing, turning, or machining by a grinder can be used. Suitable is a peening process in which the thickness and crystal grain size of the fine crystal layer can be adjusted according to the processing conditions such as the abrasive size and pressure.

  The rough etching process (second process) is a process for preferentially etching the crystal grain boundaries of the fine crystal layer formed in the first process. Therefore, it is preferable that the liquid composition of the rough etching treatment is strongly acidic and contains chloride ions and transition metal ions that can be deposited by substitution on iron. The effect of chloride ions is to destroy the passive film mainly composed of chromium oxide formed on the stainless steel surface. The chloride ion is not particularly limited, and hydrochloric acid, sodium chloride, potassium chloride and the like can be used. The effect of transition metal ions that can be substituted and deposited on iron is to promote the etching of the crystal grain boundaries by the metal being deposited and deposited on iron, which is the main component of stainless steel, and suppressing the etching. Transition metal ions that can be deposited on iron are copper (+0.34 (V)) and silver (+0.80 (V) which are noble than the standard potential of iron -0.44 (V) and can obtain a high potential difference. )), Palladium (+0.99 (V)), platinum (+1.19 (V)), gold (+1.50 (V)). Particularly suitable are inexpensive copper ions.

The replacement deposited metal removal process (third process) is a process of selectively etching the metal deposited on the stainless steel surface in the second process. An etching process without selectivity is not preferable because the surface microstructure of stainless steel is also etched together. Therefore, it is important that the liquid composition used in the replacement deposited metal removal treatment does not contain chloride ions or nitric acid that promotes etching of stainless steel, or has a large dissolution rate ratio with respect to stainless steel. The liquid composition also varies depending on the type of substitutional deposited metal. When the substitution deposition metal is copper, the liquid composition containing persulfate or hydrogen peroxide is good. When the substituted deposited metal is gold, platinum, palladium, or silver, the liquid composition containing potassium cyanide or ammonium peroxosulfate is good.
Note that the third step is performed in the case where there is an adverse effect due to the presence of substitutional deposited metal in the microstructure of the stainless steel surface due to the characteristics of the product. For example, when the surface microstructure is required to have corrosion resistance, it is preferable to perform the replacement deposited metal removal treatment when the replacement deposited metal is copper that is easily oxidized. On the other hand, the third step is omitted when it is not necessary to remove the substituted deposited metal, such as when there is no adverse effect even if the substituted deposited metal is present in the surface microstructure.
(Surface and cross-section observation method)
A scanning electron microscope (SEM) was used to observe the surface and cross section of the surface microstructure. Note that the height and width of the surface microstructure were determined from a cross-sectional SEM image. Here, among the convex portions observed in the cross-sectional SEM image, the values of the five convex portions having a large height and width were measured, and the average values were taken as the height and width of the convex portions of the surface microstructure.
(Surface area evaluation method)
The krypton gas adsorption method was used for the surface area measurement. The measured surface area was evaluated as a surface area ratio based on a smooth test piece not subjected to the surface microstructure formation process.
(Passive film evaluation method)
For evaluation of the passive film, the surface chromium concentration was measured using Auger electron spectroscopy. The measured surface chromium concentration was evaluated as a surface chromium concentration ratio based on a smooth test piece not subjected to the surface microstructure formation process.
(Fine crystal layer observation method)
Electron beam backscatter diffraction (EBSD) was used for the observation of the fine crystal layer. The crystal orientation analysis software used was OIM-Analysis manufactured by TSL.
(Corrosion resistance evaluation method)
For the corrosion resistance evaluation, a combined cycle test based on JISK5600-7-9 “Neutral salt spray cycle test method” was used. The number of test cycles was 42 cycles. The determination of the corrosion state was performed by the rating number method of JISZ2371 Appendix 1 (normative).
(Adhesion evaluation method)
The adhesion evaluation was performed by applying polyimide (Hitachi Chemical Co., Ltd.) to a test piece to a thickness of 50 μm and peeling it off with an adhesive tape.

In Example 1, the transition metal ions that can be substituted and deposited on iron are used as the copper ions in the wet peening process and the rough etching process (second process) in the crystal grain refining process (first process) in FIG. The case where the substitutional precipitation metal removal treatment liquid containing persulfate is used for the roughening etching treatment liquid and the substitutional precipitation metal removal treatment (third step) will be described. In addition, SUS304 was used for the test piece.
(1) Grain refinement process (first step)
Wet peening processing conditions were as follows: spherical glass having a diameter of about 50 μm was used as an abrasive, air pressure was 0.33 MPa, and conveyance speed was 20 mm / sec. In addition, it is preferable that wet peening conditions appropriately correspond to the required crystal grain size and the thickness of the fine crystal layer.
(2) Roughening etching process (second step)
The composition of the rough etching solution was 500 g / l sulfuric acid, 90 g / l sodium chloride, and 30 g / l cupric chloride dihydrate. The liquid temperature was 40 ° C. and the treatment time was 5 min. In addition, about processing time, it is preferable to respond | correspond suitably according to the etching amount required.
(3) Displacement deposited metal removal treatment (third step)
The composition of the substitutional precipitation metal removal treatment liquid was 200 g / sodium persulfate that can selectively remove copper because copper was used as the transition metal ion capable of substitutional precipitation on iron in the roughening etching treatment liquid in the second step. 1 and sulfuric acid 50 ml / l. The liquid temperature was 30 ° C. and the treatment time was 10 minutes. In addition, about processing time, it is preferable to respond | correspond suitably according to the amount of substitutional precipitation metal.

  About the test piece of Example 1, surface and cross-sectional SEM observation, surface area evaluation, and fine crystal layer observation were performed.

  FIG. 3 shows the results of surface and cross-sectional SEM observation and surface area evaluation. From the surface of FIG. 3 and the cross-sectional SEM image, it can be confirmed that a fine structure having a width of 1 μm or less and a height of about 3 μm is formed. Moreover, the surface area by this fine structure was 21 times with respect to the smooth surface. Thus, a surface microstructure having a height of 5 μm or less and a surface area ratio of 15 times or more can be formed on stainless steel.

  FIG. 4 shows an observation result of the fine crystal layer after the wet peening process. The fine crystal layer was formed with a depth of about 7 μm from the surface of the stainless steel, and the crystal grain size was about 0.8 μm. Thus, a fine crystal layer having a crystal grain size of 1 μm or less and a thickness of 1 μm or more can be formed inside the surface of stainless steel by wet peening.

  In this example, a fine structure was formed on the surface of the test piece under the same conditions as in Example 1 except that the processing time of the rough etching process (second process) in Example 1 was changed to 3 min. About the test piece of Example 2, surface and cross-sectional SEM observation and surface area evaluation were performed. FIG. 3 shows the results of surface and cross-sectional SEM observation and surface area evaluation. From the surface of FIG. 3 and the cross-sectional SEM image, it can be confirmed that a fine structure having a width of 1 μm or less and a height of about 1 μm is formed. Moreover, the surface area by this fine structure was 15 times with respect to the smooth surface. As described above, it is possible to adjust the size and the surface area of the fine structure by changing the conditions of the wet peening process and the etching process. In addition, the surface area ratio is smaller because the height of the fine structure is lower than that of Example 1, but it is possible to realize a surface area ratio of 15 times even at a height of 1 μm of the fine structure. there were.

  In this example, the results of performing passive film evaluation, corrosion resistance evaluation, and adhesion evaluation on the test piece prepared in Example 1 will be described.

  FIG. 5 shows the evaluation of the passive film after forming the surface microstructure. It was confirmed that the chromium concentration on the surface of the stainless steel was 1.5 times higher than before forming the surface microstructure. The cause of this is thought to be that chromium dispersed in the stainless steel remains on the surface by the surface processing method of this example.

  FIG. 6 shows the corrosion resistance evaluation before and after the formation of the surface microstructure. The rating number was 10 (corrosion area 0.0%) after formation of the surface microstructure against 6 (corrosion area 0.5 to 1.0%) before formation of the surface microstructure. Thus, it was confirmed that the corrosion resistance was improved by forming the surface microstructure by the surface processing method of this example. The cause of this is thought to be due to the fact that the chromium concentration on the surface is increased by the surface microstructure formation process from the passive film evaluation result.

As a result of the adhesion evaluation after the surface microstructure formation process, peeling of the polyimide film did not occur. Thus, it is possible to obtain a high adhesion force by performing the surface fine structure forming process.
[Comparative Example 1]
In Comparative Example 1, a case where wet blasting is used as a method for forming the surface microstructure of stainless steel will be described. In addition, SUS304 was used for the test piece.

  The wet blasting conditions were an air pressure of 0.2 MPa using a polygonal alumina having a diameter of about 15 μm as the abrasive.

  The test piece of Comparative Example 1 was subjected to surface and cross-sectional SEM observation and surface area evaluation. FIG. 3 shows the results of surface and cross-sectional SEM observation and surface area evaluation. In Comparative Example 1, it can be confirmed from the surface of FIG. 3 and the cross-sectional SEM image that a fine structure having a width of 2 to 3 μm and a height of about 0.5 μm is formed. The surface area of Comparative Example 1 was 3 times that of the smooth surface.

Further, as a result of the adhesion evaluation after the surface microstructure forming process, the polyimide film was peeled off on the entire surface.
[Comparative Example 2]
In Comparative Example 2, a case where a rough etching process is used as a method for forming a surface microstructure of stainless steel will be described. In addition, SUS304 was used for the test piece.

  The roughening etching process conditions were the same as in Example 1. The liquid composition was 500 g / l sulfuric acid, 90 g / l sodium chloride, 30 g / l cupric chloride dihydrate, the liquid temperature was 40 ° C., and the treatment time was 5 min. I went there.

  The test piece of Comparative Example 2 was subjected to surface and cross-sectional SEM observation and surface area evaluation. FIG. 3 shows the results of surface and cross-sectional SEM observation and surface area evaluation. In Comparative Example 2, it can be confirmed from the surface of FIG. 3 and the cross-sectional SEM image that a fine structure having a width of 2 to 5 μm and a height of about 3 μm is formed. The surface area of Comparative Example 2 was 10 times that of the smooth surface.

  Further, as a result of the adhesion evaluation after the surface microstructure formation process, the polyimide film was partially peeled off.

  When Examples 1 and 2 are compared with Comparative Examples 1 and 2, the surface microstructure formed by wet blasting in Comparative Example 1 affects the abrasive size, but the distance between the microstructures is somewhat small. Large and dense. Further, the surface fine structure formed by the roughening etching process of Comparative Example 2 has a large width of the fine structure depending on the crystal grain size of the base material due to the grain boundary etching. As described above, it is difficult to form a surface microstructure having a height of 5 μm or less and a surface area ratio of 15 times or more in stainless steel by wet blasting and roughening etching, which are conventional surface structure forming methods.

  Next, an application example of stainless steel having a surface microstructure formed by the surface processing method of the present invention will be described. As a suitable example to which a stainless steel having a fine uneven structure with a high surface area can be applied, there is an air-cooled heat exchanger that performs heat exchange with gas. By applying the surface-treated stainless steel of the present invention to the heat transfer part of the air-cooled heat exchanger, the heat transfer performance is improved by increasing the surface area, and the uneven structure is very fine, resulting in pressure loss. Is suppressed, and a high heat transfer promoting effect can be obtained.

  In this example, heat transfer performance was evaluated by a shell-and-tube heat exchanger using a stainless steel heat transfer tube having a surface microstructure formed by the surface processing method of Example 1.

  FIG. 7 is a structural diagram of a shell-and-tube heat exchanger according to the fourth embodiment. Tube plates 202 are installed on both sides of the circular or polygonal shell 200 to support the heat transfer tubes 201 that are heat transfer portions. A large number of holes for passing the heat transfer tubes 201 are arranged in a staggered pattern in the tube plate 202, and the heat transfer tubes 201 are inserted into these tube holes and fixed to the tube plate 202 at both ends. The length of the heat transfer tube 201 is 25 times or more of the representative length D of the flow. In addition, the representative length D of the flow along the tube group of the present embodiment takes a hydraulic equivalent diameter. By setting the length of the heat transfer tube 201 to 25 times or more of the typical flow length D, the heat transfer performance can be further improved when the airflow state is turbulent.

  The fine structure 203 is formed on the outer surface of the heat transfer tube 201. Air 204, which is a low-temperature fluid, flows into the heat exchanger from a nozzle 205 provided on the side surface of the lower portion of the heat exchanger, rises outside the heat transfer tube 201, and steam 206, which is a high-temperature fluid, through the heat transfer tube wall. Exchange heat. Water vapor 206, which is a high-temperature fluid, flows into the heat exchanger from the nozzle 207 at the top of the heat exchanger, and flows down through the heat transfer tube 201 via the water chamber 208 at the top of the heat exchanger. The water vapor 206 is condensed by exchanging heat with the air 204, which is a low-temperature fluid, through the heat transfer tube wall and becomes compressed water. The compressed water flows out of the heat exchanger from the nozzle at the bottom of the heat exchanger via the water chamber at the bottom of the heat exchanger.

  As a result of using the stainless steel in which the surface microstructure is formed by the surface processing method of Example 1 for the heat transfer tube 201, the heat transfer performance is improved by about 6% compared to the shell and tube heat exchanger not subjected to the surface processing. I was able to. Thus, by applying the stainless steel having the surface microstructure formed by the surface processing method of the present invention to the heat transfer tube, the heat transfer performance can be improved without increasing the number of heat transfer tubes. That is, when obtaining the target heat transfer performance, the number of heat transfer tubes can be reduced, and the cost of the heat exchanger can be reduced.

  For comparison, as a result of using the stainless steel in which the surface microstructure is formed by the surface processing method of Comparative Example 2 for the heat transfer tube 201, the heat transfer performance is higher than that of the shell and tube heat exchanger not subjected to the surface processing. About 2.5% improvement. From this result, it was confirmed that the heat transfer performance is improved as the surface area ratio of the heat transfer tube 201 is increased.

  Further, regarding the height of the fine structure, it is preferable that the height is lower if the surface area of the fine structure is the same. This is because the pressure loss increases as the microstructure increases. When the height of the microstructure exceeds the boundary layer, a high heat transfer acceleration effect can be expected, but the pressure loss increases in a trade-off manner. As a result, the heat transfer performance may not improve as a whole due to an increase in pressure loss. Note that the boundary layer is a thin layer that exists in the vicinity of the contact surface with which the gas is in contact and cannot ignore the viscosity of the gas (it is strongly influenced by the viscosity of the gas). The thickness of the boundary layer varies depending on various requirements such as the heat exchanger specifications, that is, the flow rate and flow of gas, and the shape of the heat transfer section. Therefore, from the viewpoint of reliably reducing the pressure loss, it is desirable that the height of the fine structure is lower and the surface area ratio is large. According to the surface processing method of the present invention, since it is possible to form a surface microstructure having a height of 5 μm or less and a surface area ratio of 15 times or more, which has been difficult in the past, stainless steel having this microstructure on the surface is used as a heat exchanger. By adopting the heat transfer section, it is possible to obtain higher heat transfer performance than before.

  Moreover, as shown in FIGS. 5 and 6, the chromium concentration on the surface of the stainless steel is increased, and the corrosion resistance is excellent. Therefore, it is suitable for the heat transfer part of a heat exchanger that requires corrosion resistance.

  In addition, although Example 4 demonstrated the shell and tube type heat exchanger as an example as a heat exchanger, the heat exchanger applicable by this invention is not necessarily limited to this. For example, in heat exchangers that perform heat exchange in contact with gas, such as fin-type heat exchangers (heat sinks) for power semiconductors, cross fin-type heat exchangers for air conditioners and radiators of automobiles, etc. It can be implemented in a heat exchanger whose part is stainless steel.

DESCRIPTION OF SYMBOLS 100 Stainless steel 101 Fine crystal layer 102 Substitution precipitation metal R1 Concavity and convexity structure after crystal grain refinement processing R2 Concavity and convexity structure after roughening etching processing R3 Concavity and convexity structure after substitution precipitation metal removal processing 200 Shell 201 Heat transfer tube 202 Tube plate 203 Fine Structure 204 Air 205 Nozzle 206 Water vapor 207 Nozzle 208 Water chamber

Claims (12)

A surface processing method for stainless steel that forms a microstructure on the surface of stainless steel,
A first step of applying a crystal grain refining process for refining crystal grains inside the surface of stainless steel;
After said first step, it possesses a second step of roughening etching treatment of the surface of the stainless steel etching solution, and
The thickness of the fine crystal layer formed inside the surface of the stainless steel by the grain refinement treatment in the first step is 1 μm or more, and the grain size of the crystal grain of the fine crystal layer is 1 μm or less. Stainless steel surface processing method characterized.   The stainless steel surface processing method according to claim 1, wherein the crystal grain refining treatment is any one of peening, machining, and polishing.   2. The surface processing method for stainless steel according to claim 1, wherein the roughening etching treatment in the second step preferentially etches crystal grain boundaries on the surface of the stainless steel.   2. The method of surface processing stainless steel according to claim 1, wherein the etching solution is acidic and contains transition metal ions that are noble than a standard potential of chloride ions and iron. The surface treatment method for stainless steel according to claim 4 , wherein the transition metal ion is one or more of copper, silver, palladium, gold, and platinum. The stainless steel surface processing method according to claim 3 , further comprising a third step of removing substitutional deposited metal deposited on the surface of the stainless steel by etching treatment after the second step. 7. The surface processing method for stainless steel according to claim 6 , wherein in the third step, the substitutional precipitation metal is etched with a metal removal treatment solution that hardly dissolves the stainless steel and preferentially dissolves the substitutional precipitation metal. . The surface treatment method for stainless steel according to claim 7 , wherein the substitutional deposited metal is copper, and the metal removal treatment liquid contains one of persulfate and hydrogen peroxide. The surface treatment of stainless steel according to claim 7 , wherein the substitutional deposited metal is any one of silver, palladium, gold, and platinum, and the metal removal treatment liquid contains either potassium cyanide or ammonium peroxosulfate. Method.   A heat transfer portion that contacts the gas to perform heat exchange is provided, and the heat transfer portion that contacts the gas has a microstructure portion on the surface having a height of 5 μm or less and a surface area of 15 times or more of a smooth surface. A heat exchanger characterized by comprising stainless steel. The heat exchanger according to claim 10 , wherein an internal crystal grain size in the fine structure portion of the heat transfer portion is 1 μm or less. The heat | fever of Claim 10 characterized by the above-mentioned. The surface chromium density | concentration in the microstructure part of the said heat-transfer part is 1.5 times or more with respect to the surface which does not form the microstructure part of the said heat-transfer part. Exchanger.