JP6376218B2 - Austenitic stainless steel sheet that is difficult to diffuse and bond - Google Patents

Description

  The present invention relates to an austenitic stainless steel sheet that is difficult to be diffusion bonded.

  Austenitic stainless steel is used in heat-resistant parts such as exhaust gaskets for automobiles and motorcycles that require high heat resistance. From the viewpoint of improving fuel consumption, exhaust temperatures are increasing year by year, and these heat-resistant components may be exposed to high temperatures of 700 ° C. or higher. At such a high temperature, in addition to softening of the material, there is a problem that it joins with surrounding parts. This is a phenomenon called diffusion junction in which atoms in contact with each other diffuse.

For example, Patent Document 1 proposes a ferritic stainless steel that is difficult to be diffusion bonded by adding 3 to 10% Al and generating an Al ₂ O ₃ film.

JP 2011-032524 A

  As in the technique of Patent Document 1, ferritic stainless steel cannot obtain sufficient high-temperature strength as an exhaust gasket. Further, as disclosed in Patent Document 1, when a large amount of Al is added, inclusions made of AlN are easily generated, and a thin part such as a gasket has a remarkable adverse effect on fatigue characteristics.

  An object of the present invention is to provide an industrially stable austenitic stainless steel that is difficult to be diffusion bonded even at high temperatures in order to solve the problems of the prior art.

The present inventors considered that the diffusion bondability is greatly influenced by the surface film of the steel sheet, and investigated and examined the relationship between the composition and thickness of the film and the structure of the film constituent material and the diffusion bondability. As a result, it is effective to suppress diffusion bonding by reducing the amount of Fe in the film and concentrating the amount of Si present as SiO ₂ in the film and increasing the thickness of the Si-concentrated layer. I found out.

Since SiO _{2 in} the film on the surface of the austenitic stainless steel is hard to disappear even at a high temperature as compared with Cr ₂ O _{3 which} is a general stainless steel film, it has an effect of suppressing bonding. Since Fe is present in a large amount in the base material of austenitic stainless steel, it can be present in the vicinity of the bonding interface as an Fe oxide. However, the Fe oxide easily disappears in the diffusion bonding process as compared with the above-described SiO ₂ and Cr ₂ O ₃ . For this reason, when a large amount of Fe is present in the vicinity of the bonding interface, even if SiO ₂ in the surface film is concentrated, it is difficult to suppress the diffusion of Fe, and the effect of suppressing the bonding is insufficient. It becomes. Therefore, it is important to reduce Fe in the surface coating and concentrate Si present as SiO ₂ .

Further, as a result of detailed examination of the production method, the present inventors have found heat treatment conditions effective for forming a film having a high Si amount as described above. For example, FIG. 1 shows the relationship between the heat treatment temperature (° C.), the maximum Si amount (mass%) in the coating on the stainless steel surface after the heat treatment, and the Si concentrated layer thickness (nm). This experiment was performed using a heat treatment furnace in a mixed gas atmosphere (dew point: -50 ° C.) containing N ₂ : 90 vol% and H ₂ : 10 vol% and changing various temperatures. As shown in FIG. 1, it was found that the maximum amount of Si in the surface film becomes extremely large and the Si concentrated layer becomes thick at a specific heat treatment temperature. The heat treatment conditions are specifically the treatment temperature, the atmosphere, and the dew point. The treatment temperature is a temperature after soaking for a certain time in a heat treatment furnace set to a predetermined temperature, and specifically, is the same as the set temperature of the heat treatment furnace. With this knowledge, a Si-enriched layer can be formed on the surface, but the Fe on the surface cannot always be reduced, and the suppression of bonding is insufficient.

Therefore, as a result of studying a method capable of industrially stably forming a Si concentrated layer after reducing Fe in the film, it is preferable to perform electrolytic treatment at a high current density. It has been found. FIG. 2 shows the relationship between the current density during electrolytic treatment (mA / cm ² ) and the maximum amount of Si (mass%) in the coating on the stainless steel surface. As shown in FIG. 2, the inventors have found that an austenitic stainless steel sheet capable of achieving the object of the present invention can be provided industrially stably at a specific current density because the amount of Si in the film is very large.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) An austenitic stainless steel sheet having an oxide film formed on at least a part of its surface,
The chemical composition of the austenitic stainless steel sheet is mass%,
C: 0.01% or more and 0.10% or less,
Si: 0.2% or more and 2.0% or less,
Mn: 1.5% or less,
Mo: 1.0% or less,
Cr: 15.0% to 22.0%,
Ni: 4.5% or more and 10.0% or less,
Cu: 1.0% or less,
Nb: 0.30% or less,
N: 0.01% or more and 0.15% or less,
The balance is Fe and inevitable impurities,
The oxide film includes a Si concentrated layer having a maximum Si amount in a range from the surface layer to 10 nm of 10.0% to 30.0% and a maximum Fe amount of 8.5% or less,
An austenitic stainless steel sheet in which the thickness of the Si-concentrated layer is 5 nm or more and is difficult to be diffusion bonded.
(2) The austenitic stainless steel sheet which is difficult to be diffusion-bonded according to (1), wherein the austenitic stainless steel sheet is a cold-rolled steel sheet.

  According to the present invention, austenitic stainless steel that is difficult to be diffusion-bonded even at high temperatures can be provided industrially stably.

It is a figure which shows the relationship between heat processing temperature (degreeC), the maximum amount of Si (mass%) in the film | membrane of the stainless steel surface after heat processing, and Si concentrated layer thickness (nm). It is a figure which shows the relationship between the current density (mA / cm < ² >) at the time of electrolytic treatment, and the maximum amount of Si (mass%) in the film | membrane of the stainless steel surface after heat processing. It is a figure which shows the definition of the maximum Si amount (mass%) and Si concentration layer thickness (nm) based on the relationship between the distance (nm) from the surface, and Si quantity (mass%). It is a figure which shows the relationship between the ratio (%) of the grain boundary over a joining interface, and the maximum Si amount (mass%) in Si concentration layer. It is a figure which shows the relationship between the ratio (%) of the grain boundary over a joining interface, and the maximum amount of Fe (mass%) in Si concentration layer. It is a figure which shows the relationship between the ratio (%) of the grain boundary over a joining interface, and Si concentration layer thickness (nm).

1. Chemical composition of austenitic stainless steel sheet The chemical composition of the steel sheet of the present invention is the chemical composition necessary for obtaining an austenitic stainless steel sheet, the heat resistance such as high temperature strength, and the chemical composition necessary for obtaining a high Si film. In addition, it is prescribed. Specifically, it is as follows. However,% means the mass%.

C: 0.01% or more and 0.10% or less C is an element that contributes to high strength at high temperatures by solid solution strengthening or precipitation strengthening. Therefore, C is contained in an amount of 0.01% or more. Preferably it is 0.03% or more. If it is contained in a large amount, coarse Cr carbide precipitates at the grain boundaries during heat treatment, and the oxidation resistance at high temperature is lowered, so the content is made 0.10% or less. Preferably it is 0.08% or less.

Si: 0.2% or more and 2.0% or less Si is one of the most important elements in the steel sheet of the present invention. Si is an element that forms a high Si amount film composed of SiO _{2 on the} surface of the steel sheet and makes diffusion bonding difficult. Therefore, Si is contained by 0.2% or more. Preferably, it is 0.31% or more, more preferably 0.5% or more. If contained in a large amount, the toughness is reduced and the manufacturability of the plate is deteriorated, so the content is made 2.0% or less. Preferably it is 1.8% or less, More preferably, it is 1.20%.

Mn: 1.5% or less Mn is an element that contributes to preventing brittle fracture during hot working and strengthening steel. However, if contained in a large amount, the corrosion resistance deteriorates, so the content is made 1.5% or less. Preferably it is 1.35% or less, More preferably, it is 1.2% or less. The lower limit includes 0%, but unavoidably about 0.001% is mixed from the iron raw material and usually remains in the steel sheet. Therefore, 0.001% is a practical lower limit. In order to surely obtain the above effect, 0.21% or more is preferable, and 0.5% or more is more preferable.

Mo: 1.0% or less Mo is an element contributing to the improvement of corrosion resistance. However, even if contained in a large amount, the cost is significantly increased, so the content is made 1.0% or less. Preferably, it is 0.80% or less, more preferably 0.7% or less. The lower limit includes 0%, but it is unavoidably mixed from the iron raw material and about 0.001% and remains in the steel sheet. Therefore, 0.001% is a practical lower limit. In order to surely obtain the above effect, 0.02% or more is preferable, and 0.5% or more is more preferable.

Cr: 15.0% or more and 22.0% or less Cr is a basic element of stainless steel, and is an element that forms a metal oxide layer Cr ₂ O ₃ on the surface of the steel sheet and enhances corrosion resistance. Therefore, Cr is contained 15.0% or more. Preferably it is 16.1% or more, more preferably 17.0% or more. However, Cr is also a strong ferrite stabilizing element, and if it is contained in a large amount, δ ferrite that inhibits the hot workability of the material is generated, so its content is made 22.0% or less. Preferably it is 21.0%, More preferably, it is 20.0% or less.

Ni: 4.5% or more and 10.0% or less Ni is an austenite-generating element and is an element necessary for stabilizing the austenite phase at room temperature. Ni is also an element effective for improving high-temperature strength. Therefore, Ni is contained 4.5% or more. Preferably it is 4.9% or more, more preferably 5.0% or more. However, if contained in a large amount, the processing-induced martensitic transformation during cold rolling is suppressed. Furthermore, Ni is an expensive element, and a large amount of addition causes a significant increase in cost. Therefore, the Ni content is 10.0% or less. Preferably it is 9.5% or less, More preferably, it is 8.0% or less.

Cu: 1.0% or less Cu is an austenite-forming element and is an element capable of adjusting the stability of the austenite phase. However, if it is contained in a large amount, it segregates at the grain boundary during the production process, remarkably hinders hot workability and makes production difficult. Preferably it is 0.8% or less, More preferably, it is 0.70% or less. The lower limit includes 0%, but it is unavoidably mixed from the iron raw material and about 0.001% and remains in the steel sheet. Therefore, 0.001% is a practical lower limit. In order to surely obtain the above effect, 0.02% or more is preferable, and 0.5% or more is more preferable.

Nb: 0.30% or less Nb is an element that forms fine carbides or nitrides, contributes to high strength, and suppresses softening due to recrystallization at high temperatures. However, even if contained in a large amount, the cost is increased, so the content is made 0.30% or less. Preferably it is 0.20% or less, More preferably, it is 0.079% or less. The lower limit includes 0%, but it is unavoidably mixed from the iron raw material and about 0.001% and remains in the steel sheet. Therefore, 0.001% is a practical lower limit. In order to surely obtain the above effect, 0.01% or more is preferable.

N: 0.01% or more and 0.15% or less N, like C, is a solid solution strengthening element, and is an element contributing to the improvement of high temperature strength. Therefore, N is contained by 0.01% or more. Preferably it is 0.03% or more, More preferably, it is 0.04% or more. On the other hand, if it is contained in a large amount, a large number of coarse nitrides that become the starting point of fracture are produced in the production process of the steel sheet, hot workability is deteriorated, and production may be difficult. The following. Preferably it is 0.13% or less, More preferably, it is 0.12% or less.

  In the chemical composition of the austenitic stainless steel sheet according to the present invention, the balance is Fe and inevitable impurities.

2. Si Concentrated Layer The austenitic stainless steel sheet of the present invention has a film formed on at least a part of the surface, and after reducing Fe in the film, the maximum Si content in the range from the surface layer to 10 nm is 10%. The Si-enriched layer having the maximum Fe content of 8.5% or less is provided. The film is an oxide film mainly composed of an oxide.

Maximum amount of Si in the range from the surface layer to 10 nm: 10.0% or more In order to suppress diffusion bonding at high temperatures, it is effective to maintain a coating on the surface of the steel sheet that suppresses diffusion even at high temperatures. Si in the film in the present invention exists mainly as Si oxide (SiO ₂ ). Si oxide exists stably at high temperature compared with Cr oxide which is a general film composition of stainless steel.

  Therefore, by increasing the amount of Si within the range of 10 nm from the surface layer (film outermost surface), diffusion bonding between stainless steel parts can be suppressed even at high temperatures. In order to obtain this effect, the Si content on the outermost surface of the coating is set to 10.0% or more. Preferably it is 12.5% or more, More preferably, it is 14.0% or more.

  Since the Si amount on the outermost surface of the film varies depending on the Si amount of the steel sheet and the film reforming heat treatment conditions, the upper limit is not particularly defined, but 30.0% is the upper limit on the practical steel sheet.

Maximum Fe content in the range from the surface layer to 10 nm: 8.5% or less Since Fe is present in a large amount in the base material of austenitic stainless steel, it can be present in the vicinity of the bonding interface as Fe oxide. However, Fe oxide disappears more easily in the diffusion bonding process than SiO ₂ and Cr ₂ O ₃ . For this reason, when a large amount of Fe is present in the vicinity of the bonding interface, even if SiO ₂ in the surface film is concentrated, it is difficult to suppress the diffusion of Fe, and the effect of suppressing the bonding is insufficient. It becomes. Therefore, the maximum Fe content in the range from the surface layer to 10 nm is set to 8.5% or less.

Si-enriched layer thickness: 5 nm or more As described above, diffusion bonding is suppressed when the amount of Si in the film is large. However, when the Si-enriched layer is thin, the film is exposed to high temperatures for a long time. It is gradually decomposed into metal and oxygen gas, and the stainless steel parts are joined together. Therefore, the thickness of the Si concentrated layer is set to 5 nm or more. Preferably it is 8 nm or more.

  Here, the definition of the thickness of the Si concentrated layer will be described. FIG. 3 shows the definition of the maximum Si amount (% by mass) and the Si concentrated layer thickness (nm) based on the relationship between the distance from the surface (nm) and the Si amount (% by mass). As shown in FIG. 3, the thickness of the Si concentrated layer is a thickness until the Si amount becomes 1/2 of the maximum Si amount (1/2 Si amount in the figure).

3. Manufacturing Method Next, a method suitable for manufacturing the austenitic stainless steel sheet of the present invention will be described. Melting, hot rolling and the like may be performed by the same method as in the past. In the following, conditions for the film modification heat treatment and the film modification electrolytic treatment as the final finishing treatment are shown, but there are no special conditions for treatments other than these treatments.

3-1. Film Modification Heat Treatment As described above, it is important that the Si amount on the outermost surface of the film (in the range of 10 nm from the surface layer) is 10% or more as a surface state in which diffusion bonding is difficult. In general, finish annealing of austenitic stainless steel is performed in a mixed atmosphere of H ₂ and N ₂ in order to maintain surface gloss, and the temperature is about 1100 to 1150 ° C.

  However, at the above temperature, it is difficult to obtain a film having a large amount of Si as defined by the steel sheet of the present invention (see FIG. 1). In addition, when cold rolling or the like is performed after finish annealing, the coating may be partially broken or divided, and a new Cr oxide coating may be generated on the new surface, thereby reducing the amount of Si in the coating.

The finish annealing (film reforming heat treatment) is preferably performed by maintaining at 750 to 1000 ° C. in a mixed atmosphere of H ₂ and N ₂ . This is because a predetermined Si-enriched layer is formed on the surface layer of the film (see FIG. 1). A preferable lower limit of the treatment temperature is 800 ° C, and a preferable upper limit is 950 ° C. Further, the in-furnace time is not particularly defined as long as the steel plate can be soaked at the above processing temperature, but if the time is too short, Si concentration in the film may be insufficient, so the in-furnace time is 10 seconds or more is desirable.

If the dew point of the mixed atmosphere of H ₂ and N ₂ is high, the film formed during the heat treatment becomes a film mainly composed of Cr oxide, so the dew point is preferably −45 ° C. or lower. Preferably it is -60 degrees C or less. On the other hand, in order to obtain an excessively low dew point, a large cost is required. Therefore, in practice, the dew point is set to −70 ° C. or higher. Preferably it is -65 degreeC or more.

The mixing ratio of H ₂ and N ₂ (H ₂ / N ₂ ) is not particularly limited, but the mixing ratio is preferably 1/19 or more in order to obtain an atmosphere exhibiting sufficient reducing properties. On the other hand, increasing the proportion of expensive hydrogen gas has a problem in economy, so it is preferable to set it to 1/2 or less.

3-2. Film Modification Electrolytic Treatment Usually, an electrolytic cleaning process is performed in which the steel sheet after heat treatment is subjected to electrolytic treatment in a predetermined liquid to remove the film generated by the heat treatment. The film-modified electrolytic treatment has a part in common with the conventional electrolytic cleaning treatment in that the electrolytic treatment is performed in a predetermined solution. However, the film-modified electrolytic treatment is performed after reducing Fe in the film and then in the film. This is greatly different in that it is performed to enrich Si. Specifically, it is preferable to pass the steel plate while applying a voltage so that the steel plate becomes positive in a nitric acid aqueous solution having a liquid temperature of 30 to 50 ° C. and a concentration of about 5 to 10%. If the liquid temperature or concentration is too low, a sufficient reforming effect cannot be obtained, and if the liquid temperature or concentration is too high, the surface roughness of the steel sheet may be increased or the electrolytic cell may be damaged.

This electrolytic treatment is preferably performed so that the current density is 100 mA / cm ² or more with respect to the plate area. Thereby, Si is concentrated after reducing Fe in the film on the surface of the steel sheet (see FIG. 2). Preferably it is 150 mA / cm ² or more. In the surface concentration of Si by electrolysis, in the electrolysis process, Fe, Cr, etc. are eluted and removed from the surface by oxidation reaction, but Si present as SiO ₂ is not oxidized any more and remains on the surface. It is to do.

When the current density is less than 100 mA / cm ² , Si does not concentrate on the steel sheet surface, and particularly when the current density is about 20 mA / cm ^2, which is the current density during general electrolytic cleaning treatment, the amount of Si is reduced. (See FIG. 2).

On the other hand, if the current density is excessive, the steel plate is excessively shaved, yield decreases, and the surface of the steel plate becomes rough. Therefore, the current density is preferably 300 mA / cm ² or less (see FIG. 2). Preferably it is 250 mA / cm ² or less.

  If the energization time is short, the concentration level of Si is small, so the energization time is preferably 10 seconds or more. Preferably it is 15 seconds or more. The upper limit of the energization time is not particularly defined, but is practically about 60 seconds.

  In an electrolytic treatment tank, a voltage may be applied by using a steel plate as a positive electrode or a negative electrode, or a voltage may be applied by alternately repeating positive and negative. To do. Also in this case, the time for energizing the steel plate as positive is set to 10 seconds or more.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
Table 1 shows the chemical composition of the test steel.

  Small ingots having chemical compositions A to G shown in Table 1 were melted, and after cutting, hot rolling, annealing, and descaling, cold rolling and annealing were repeated three times. Thereafter, the plate thickness was set to 0.2 mm by finish rolling, and film reforming heat treatment and film reforming electrolytic treatment were then performed under the conditions shown in Table 2. A test piece was collected from the obtained steel plate, and the characteristics were investigated in the following manner. The results are also shown in Table 2.

Maximum amount of Si, maximum amount of Fe and Si concentrated layer thickness While sputtering the coating formed on the steel sheet surface with Ar ions, GD-OES was used to make Si from the outermost surface of the coating to a depth of about 100 nm or less. The amount and the amount of Fe were measured (see FIG. 3). The maximum Si amount is the Si amount (% by mass) at which the Si amount is maximum, the maximum Fe amount is the Fe amount (% by mass) at which the Fe amount is maximum, and the Si concentrated layer thickness is the outermost surface To a position where the Si amount is a half of the maximum Si amount.

Identification of the main constituents of surface oxides Cut out from steel plates to include the surface oxides by FIB processing, and use TEM-EDS to analyze the crystal structure and composition of the surface oxides. Was identified.

Bondability The steel plate was processed into two φ8 mm disk-shaped test pieces. Two test pieces were overlapped, and a load of 20 MPa was applied in a vacuum chamber at 750 ° C., followed by pressurization for 30 seconds.

  After pressurization, the stacked specimens are taken out of the chamber, and the case where the two specimens are not joined is marked as ◯. It appears that they are joined, but after polishing the resin-filled specimen, The cross section was observed with an optical microscope, and the case where the ratio of crystal grains across the bonding interface was less than 10% was evaluated as Δ, and the case where the ratio of crystal grains across the bonding interface was 10% or more was evaluated as x. These results are shown in Table 2 and plotted in FIGS.

  Steel plates 1 to 7 (invention examples) shown in Table 2 are steel plates that satisfy the provisions of the present invention and are difficult to be diffusion bonded. The steel plates 8 to 13 of the comparative examples are steel plates that are easy to be diffusion bonded. The steel sheet 8 has a very low maximum Si content in the film. This is due to the low temperature of the film modification heat treatment. The steel sheet 9 is a steel sheet that has a high dew point at the time of film reforming heat treatment and is easy to be diffusion bonded because it is a film mainly composed of Cr oxide.

  The steel plate 10 is SUS304 which is made on a trial basis under relatively general manufacturing conditions. Although the heat treatment temperature and the current density during the electrolytic treatment have general conditions, the maximum amount of Si in the film is low and the Si concentrated layer depth is also small, so that the steel plate 10 is a steel plate that is easily joined. Since the maximum amount of Si is extremely low, the steel plate 11 is a steel plate that does not concentrate Si on the surface of the steel plate even by a film reforming heat treatment under appropriate conditions and is easily diffusion bonded.

  The steel plate 12 is also a steel plate that is easy to be diffusion bonded because the maximum amount of Si in the film is low and the Si concentrated layer depth is also small. This is because the film modification heat treatment and the film modification electrolytic treatment are not performed. The steel plate 13 is a steel plate that is easily diffusion-bonded because the maximum amount of Si in the film is in the range defined by the present invention, but the maximum Fe amount is excessive. This is because the energization time as a negative electrode is long.

  As shown in FIGS. 4 to 5, the maximum Si amount in the Si concentrated layer is 10 mass% or more, the Si concentrated layer thickness is 5 nm or more, and the maximum Fe amount in the Si concentrated layer is 8.5% or less. By doing so, the ratio of the grain boundary straddling the bonding interface is rapidly reduced, and the effect of suppressing diffusion bonding is exhibited. Note that “▲” in the figure is the point of Comparative Example 13. In this example, the maximum amount of Si in the film is in the range specified by the present invention, but the maximum Fe amount is excessive, so that the ratio of grain boundaries straddling the bonding interface was high.

As described above, according to the present invention, it is possible to provide industrially stable austenitic stainless steel that is difficult to be diffusion bonded even at high temperatures. Therefore, the present invention has high applicability in the stainless steel manufacturing / utilizing industry.

Claims (2)

An austenitic stainless steel sheet having an oxide film formed on at least a part of the surface,
The chemical composition of the austenitic stainless steel sheet is mass%,
C: 0.01% or more and 0.10% or less,
Si: 0.2% or more and 2.0% or less,
Mn: 1.5% or less,
Mo: 1.0% or less,
Cr: 15.0% to 22.0%,
Ni: 4.5% or more and 10.0% or less,
Cu: 1.0% or less,
Nb: 0.30% or less,
N: 0.01% or more and 0.15% or less,
The balance is Fe and inevitable impurities,
The oxide film includes a Si concentrated layer having a maximum Si amount in a range from the surface layer to 10 nm of 10.0% to 30.0% and a maximum Fe amount of 8.5% or less,
An austenitic stainless steel sheet in which the thickness of the Si-concentrated layer is 5 nm or more and is difficult to be diffusion bonded.   The austenitic stainless steel sheet according to claim 1, wherein the austenitic stainless steel sheet is a cold-rolled steel sheet, which is difficult to be diffusion bonded.

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