JP2018145498A - FERRITIC STAINLESS STEEL EXCELLENT IN Ni BRAZABILITY - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel excellent in brazability when brazing is conducted by using Ni solder under high temperature and low oxygen partial pressure, and excellent in corrosion resistance after brazing joint.SOLUTION: There is provided a ferritic stainless steel having a composition containing by mass%, C:0.01 to 0.03%, Si:0.10 to 1.00%, Mn:1.50% or less, P:0.04% or less, S:0.03% or less, Ni:0.6% or less, Cr:17 to 25%, Mo:2.50% or less, Cu:0.1 to 0.50%, N:0.030% or less, Nb:7×(C+N)% or more, 0.80% or less, Ti:0.1% or less, Al:0.01% or less and the balance Fe with inevitable impurities, satisfying the following formula (1), and excellent in Ni brazability. 3C+Cu-Ti-40Al>0 (1) Formula, wherein C, Cu, Ti and Al mean mass% of each element.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a ferritic stainless steel used for members connected by Ni brazing joint.

  The members to which the ferritic stainless steel of the present invention is applied include automobile EGR cooler parts, waste heat recovery equipment, plate heat exchangers, hot water supply equipment, fuel cells, etc., and generally have a complicated shape. It is applied to applications that emphasize pressure resistance, sealing properties and fatigue strength and require higher corrosion resistance. As the brazing method, brazing in a furnace under an atmosphere of high temperature and low oxygen partial pressure or brazing by high frequency heating using hydrogen gas and nitrogen gas is used.

  In recent years, the issue of global warming due to environmental pollution has been regarded as important, and the exhaust gas from automobiles has been recognized as the main factor. Harmful emission regulations in countries around the world, including Japan, are becoming stricter year by year, and in particular, immediate measures against nitrogen oxides (NOx) generated from gasoline engines and diesel engines are required. At present, next-generation vehicles such as electric vehicles and hydrogen vehicles have been developed and are becoming popular, but the number of next-generation vehicles produced is still less than 10% of the total number of vehicles. Therefore, there is a continuing demand for the development of gasoline and diesel engine technologies, and automakers are working to improve the performance of direct-injection engines, lean burns, and EGR.

  Against this background, the mounting of EGR systems is rapidly increasing in order to reduce automobile exhaust gas. The EGR cooler is a heat exchanger for cooling the exhaust gas to an appropriate temperature in order to return a part of the high-temperature exhaust gas discharged from the engine to the engine as a part of this system. Conventionally, austenitic stainless steel having high high-temperature strength and excellent workability has been used. In recent years, there has been a demand for higher inflowing gas for the purpose of increasing the efficiency of EGR of exhaust gas temperature. Along with this, ferritic stainless steel that is less susceptible to thermal fatigue degradation and excellent in high-temperature oxidation resistance has been used. Yes.

  As described above, ferritic stainless steel is inferior in workability and corrosion resistance while being excellent in thermal fatigue characteristics and high-temperature oxidation resistance. Further, brazing joints are used in such EGR cooler members to ensure hermeticity and pressure resistance. However, ferritic stainless steel has a drawback that the wettability of Ni brazing is poor compared to austenitic stainless steel. In general, when a material with poor wettability is applied, the incidence of brazing defects increases. In addition, the amount of Ni brazing used is increased to obtain a sufficient wet spreading area. In recent years, the price of Ni bullion has continued to soar, and this point greatly increases the cost.

  Therefore, when brazing ferritic stainless steel to Ni, it is common to use Ni—Cr—P—Si brazing having high corrosion resistance and strength and having a low melting point. P and Si that lower the liquidus temperature of Ni are added to the Ni brazing. However, Ni brazing containing Si has a drawback that since it has a large temperature difference between the liquidus and solidus, melting and separation occur and brazing is liable to occur.

  Conventionally, for example, there are the following as knowledge about stainless steel excellent in brazing ability. Patent Document 1 discloses a ferritic stainless steel with improved atmosphere brazing by reducing Ti and Al, which are elements that form oxides even under a low oxygen partial pressure. Patent Document 2 discloses a ferritic stainless steel excellent in heat exchange characteristics, corrosion resistance, and brazing, a manufacturing method thereof, and a heat exchanger using the stainless steel as a member.

  Further, in both Patent Documents 1 and 2, when Ni solder starts melting, the low melting point phase first wets and spreads, and the high melting point phase remains in the vicinity of the position where the brazing is provided. The phenomenon of “parting” may occur. When this detachment occurs, the joint strength and corrosion resistance are greatly reduced at that portion, so that the reliability as an industrial product is greatly impaired. Thus, in the ferritic stainless steel having sufficient workability and corrosion resistance, there is a problem that it is difficult to achieve both ensuring of wettability and suppression of melting separation.

JP 2009-174046 A International Publication No. 2006-013482

  In Patent Document 1, since Al forms an oxide film and inhibits brazing, the amount of Al is limited to 0.1% or less. However, stainless steel containing Al has a wetting spread coefficient of about 3, and in recent years the shape of the joint has become complicated, and the amount of Ni brazing used has been reduced as much as possible, so further wettability is required. Has been.

  Patent Document 2 proposes a method of controlling the oxidizing atmosphere after melting the brazing material after raising the temperature in a reducing atmosphere for the purpose of positively generating Al or Ti oxides on the surface layer. However, in order to obtain sufficient characteristics, it is necessary to perform special heat treatment a plurality of times, and there is a problem of cost increase due to an increase in the number of processes.

  An object of the present invention is to provide a ferritic stainless steel having excellent brazing properties when brazed at high temperature and low oxygen partial pressure using Ni brazing and excellent in corrosion resistance after brazing joining. And

  In order to achieve the above-mentioned problems, the present inventors have conducted a detailed study on the mechanism of Ni brazing melting on stainless steel and the influence of alloy elements and surface conditions on the mechanism. As a result, the adverse effect of Ti and Al on the brazeability of ferritic stainless steel is reduced, and while securing good Ni brazeability, the conditions on the material side that can further prevent melting and segregation are found. It came to complete. Specifically, the present invention provides the following.

(1) The present invention is mass%,
C: 0.01 to 0.03%
Si: 0.10 to 1.00%
Mn: 1.50% or less P: 0.04% or less S: 0.03% or less Ni: 0.6% or less Cr: 17 to 25%
Mo: 2.50% or less Cu: 0.10 to 0.50%
N: 0.030% or less Nb: 7 × (C + N)% or more, 0.80% or less Ti: 0.1% or less Al: 0.01% or less, with the balance being Fe and inevitable impurities Have
It is a ferritic stainless steel that satisfies the following formula (1) and has excellent Ni brazing properties.
3C + Cu-Ti-40Al> 0 ... (1) Formula Here, C, Cu, Ti, and Al in Formula (1) mean the mass% of each element.

(2) The present invention further includes one or more of V: 0.20% or less and W: 1.00% or less in terms of mass%, and the Ni brazeability according to (1) Excellent ferritic stainless steel.

(3) The present invention is a ferritic stainless steel having excellent Ni brazeability according to (1) or (2), further containing B: 0.007% or less by mass%.

  According to the present invention, Ni brazing is excellent in brazing when brazing at high temperature and low oxygen partial pressure, and excellent workability capable of being processed into a complicated shape of a brazed joint. Ferritic stainless steel can be provided. Even when the ferritic stainless steel of the present invention is made into a thin and complicated part, Ni brazing can be easily performed, and deterioration of the quality of the joint due to melting and separation can be suppressed. Therefore, it is suitable for applications involving Ni brazing, such as EGR cooler members and fuel cell members.

It is a figure which shows the relationship between the value of (1) Formula and the wetting spread coefficient in an Example.

  The good brazing property means that the brazing material penetrates the entire joint by a heat treatment at a low temperature for a short time and a well-shaped fillet having a small contact angle is formed. Furthermore, in the case of Ni brazing, it is desirable that no brazing failure occurs due to melting and separation.

Ni brazing is performed in a vacuum of 1000 to 1120 ° C. or in a hydrogen atmosphere. Here, the most important factor affecting the brazeability is the oxygen partial pressure, which is considered to decrease to about 10 ⁻²⁰ atm, which is the reduction region of Cr, in both vacuum and hydrogen atmosphere. It has been. However, from an industrial point of view, in a large-scale mass production facility, the atmosphere may be non-uniform due to gaps between products, and an atmosphere having a high oxygen partial pressure may be generated.

[Wet spread on stainless steel by molten Ni]
In general, the equilibrium contact angle between an Fe-based alloy and molten Ni is small, whereas the equilibrium contact angle between an oxide such as Cr ₂ O ₃ and molten Ni is 90 ° or more. Therefore, in order for the Ni solder melted in such an atmosphere to spread on the stainless steel, the oxide film covering the surface of the stainless steel must be removed and the matrix exposed when the Ni braze is melted. There is.

  As described above, brazing is performed in an atmosphere in which Cr serves as a reduction region. However, in large-scale mass production facilities, the atmosphere may change from the set conditions. In addition, even if the set atmosphere is a reduction region for Fe, Cr, and Ni, which are main components of stainless steel, it corresponds to an oxidation region for elements that are more easily oxidized than Cr, such as Si, Ti, and Al. Therefore, Si, Ti, or Al may form an oxide on the surface of stainless steel and inhibit the wetting of Ni brazing.

[Removal of oxide film by C]
C is an element that plays an important role in removing oxides on the surface of stainless steel under such a low oxygen partial pressure. When the oxygen partial pressure in the atmosphere is less than 10 ⁻¹⁸ atm, the solid solution C in the stainless steel shifts to the state of the reduction zone, and the oxide on the stainless steel surface and C react to form a metal carbide. To do. Then, this metal carbide reacts with oxygen in the atmosphere to form CO or CO ₂ that is released into the atmosphere. At this time, the metal carbide is reduced to a simple metal. Furthermore, because the CO ₂ partial pressure in the atmosphere tends to gradually decrease during brazing due to the influence of evacuation or gas flow, the above reaction in which metal carbide is reduced to metal and releases CO _2. Proceeds continuously without reaching equilibrium during brazing. From this, it can be said that solid solution C in stainless steel shows the effect of reducing and removing oxides on the surface of stainless steel in a brazing atmosphere. In addition, not only Fe, Cr, Ni but also Al and Ti located in the oxidation region in the Ellingham diagram are reduced by passing through metal carbides in the presence of C. Causes a reaction.

Similarly, when C is contained in members such as brazing furnace parts, jigs, and heating elements, the effect of releasing CO gas and promoting the reduction reaction of oxides on the surface of stainless steel is exhibited. However, since the reaction from CO to CO ₂ proceeds only under a low oxygen partial pressure when the CO ₂ partial pressure is extremely small, as a means for removing oxides on the surface of stainless steel, reduction by solute C is used. Is the most effective.

  On the other hand, the solid solution C of stainless steel reduces workability (ductility) by solid solution strengthening. Further, when the cooling rate is slow, Cr carbide is formed during cooling, causing sensitization and adversely affecting corrosion resistance. Therefore, when applying the removal mechanism of the surface oxide using the solid solution C to the brazing joint of the EGR member where workability and corrosion resistance are important, it is necessary to devise.

[Improvement of wax flow by using carbides]
Thus, the present inventors have found that brazing can be improved while suppressing adverse effects of C on workability and corrosion resistance by utilizing NbC finely dispersed in a stainless steel matrix. That is, C exists as NbC at room temperature and is rendered harmless. When the temperature is raised to the vicinity of the brazing temperature, a part of NbC is dissolved, and solid solution C and solid solution Nb are generated. Thereafter, the surface oxide film is removed by the solid solution C, and the wetting and spreading of the Cu brazing is improved. Since the C remaining without being used for removing the oxide film then forms NbC again during cooling, the workability and corrosion resistance do not deteriorate even after brazing heat treatment.

[Improvement of Ni solder wetting and spreading speed by utilizing Cu]
The above-described mechanism for removing the surface oxide film of stainless steel using carbide is a phenomenon that always occurs in a steady state. However, in a large-scale mass production facility, it is necessary to further devise in order to carry out within the actual brazing heat treatment time. As a result of detailed investigations by the present inventors, regarding the Ti, Al, C, and Cu in stainless steel, by optimizing the composition ratio within a certain range, the carbides can be reduced under practical atmosphere brazing conditions. It was found that the brazing performance improvement effect that was utilized can be obtained.

  Cu has the effect of reducing the surface tension of the molten Ni braze. By the action of Cu, Marangoni convection is generated in the vicinity of the molten Ni brazing / stainless steel / atmosphere three-phase interface, and the stirring is promoted to improve the Ni brazing flow. Cu in ferritic stainless steel is generally less than 0.5%, but is concentrated in the vicinity of the interface, and it is possible to greatly improve the wetting and spreading rate of Ni brazing. By such an effect, wetting and spreading can be completed within a practical processing time of mass production equipment.

[Reduction of melting separation]
However, as described above, when a brazing material having a large temperature difference between the liquidus and the solidus is used, melting and separation may occur. This is a phenomenon caused when the low melting point phase melts during temperature rise and quickly spreads and the high melting point phase remains undissolved. Therefore, it can be said that a material having better wax flow is more likely to occur. It is possible to suppress the occurrence of melting separation by increasing the heating rate. However, since it is difficult to maintain a constant heating state while processing a large amount of products in an industrial mass production facility, an increase in the heating rate causes defective products.

  Here, by applying the oxide film removing method utilizing the above-described carbide, it is possible to suppress melting and segregation while maintaining good wax flowability. Ni-Cr-Si-P brazing is used for stainless steel for general corrosion resistance applications. The solidus temperature of the Ni brazing is about 950 ° C. Moreover, the surface film removal by the solid solution C occurs in a temperature range of about 800 to 900 ° C. or more. If the surface coating is removed when the brazing material reaches the solidus temperature due to the temperature rise of brazing and melting of the low melting point phase occurs, melting and segregation occur as a result of rapid wetting and spreading. However, the temperature range is about 1000 ° C. when NbC in the stainless steel is decomposed and free C solid solution starts in the base material. By the time the temperature rise reaches that temperature range, an oxide film is formed on the surface of the stainless steel, and the wetting and spreading of the brazing filler metal due to the molten low melting point phase is prevented by the oxide film. Furthermore, even if the temperature rise exceeds 1000 ° C., the start of wetting spread is delayed by the time required for the solute C to diffuse and the oxide film to be removed by reduction. During this time, the temperature inside the furnace continues to increase, and when wetting and spreading actually occur, the temperature of the brazing material rises to the vicinity of the liquidus where the melting of the high melting point phase ends. As a result, it is considered that melting and separation are prevented.

  However, when using the oxide film removal method using carbide, the brazed stainless steel is better because the time during which wetting and spreading of the wax occurs is shortened compared to the case where the method is not used. It is necessary to have high wettability.

  The ferritic stainless steel according to the present invention has been achieved based on the above findings, and has the following chemical composition.

In mass%, C: 0.01 to 0.03%, Si: 0.10 to 1.00%, Mn: 1.50% or less, P: 0.04% or less, S: 0.03% or less, Ni: 0.6% or less, Cr: 17 to 25%, Mo: 2.50% or less, Cu: 0.10 to 0.50%, N: 0.030% or less, Nb: 7 × (C + N)% Or more, 0.80% or less, Ti: 0.1% or less, Al: 0.01% or less, the balance is composed of Fe and inevitable impurities, and satisfies the following formula (1). Ferritic stainless steel with excellent Ni brazing.
3C + Cu-Ti-40Al> 0 ... (1) Formula Here, C, Cu, Ti, and Al in Formula (1) mean the mass% of each element.

[Chemical composition]
The reason for selecting the composition constituting the ferritic stainless steel of the present invention will be described. In the present specification, “%” related to the chemical composition of steel means “% by mass” unless otherwise specified.

[Chemical composition]
C is one of the most important elements in the present invention, and is fixed as NbC at room temperature. A part of C dissolves at the brazing temperature and contributes to the removal of the oxide film. C content can contain 0.01% or more. However, excessive C content causes sensitization and causes a decrease in workability due to solid solution strengthening. Therefore, the C content is preferably 0.03% or less.

  Si is one of the elements generally used as a deoxidizer for stainless steel, and can contain 0.10% or more. However, excessive Si addition hardens the stainless steel and causes a decrease in workability. Therefore, the Si content is preferably 1.00% or less.

  Mn is used as a deoxidizer for stainless steel. Since stainless steel made from scrap is unavoidably mixed with Mn to some extent, Mn can contain 1.50% or less. On the other hand, when Mn is contained in the passive film, the corrosion resistance is lowered, so that the Mn content is desirably low. 1.20% or less is preferable.

  Since P impairs the toughness of the base material and the brazed part, a lower value is desirable. However, since dephosphorization by refining is difficult in the production of Cr-containing steel, excessive reduction in the P content is accompanied by an excessive cost increase such as careful selection of raw materials. Therefore, in the present invention, the P content is allowed to be 0.04% or less, as in general ferritic stainless steel.

  S is an element that forms MnS that tends to be a starting point of pitting corrosion and inhibits corrosion resistance. Moreover, when S content is high, it becomes easy to produce the high temperature crack of a brazing part. Therefore, the S content is preferably 0.03% or less.

  Ni is an element effective for suppressing a decrease in toughness when ferrite crystal grains are coarsened during brazing, and also has an effect of improving corrosion resistance. Ni can contain 0.01% or more. It is also effective in reducing the erosion caused by Ni brazing. On the other hand, excessive addition of Ni leads to the formation of an austenite phase at a high temperature range, which adversely affects hot workability. Therefore, the Ni content is preferably 0.60% or less, and more preferably 0.30% or less.

Cr is a main constituent element of the passive film, and improves local corrosion properties such as pitting corrosion resistance and crevice corrosion resistance. In the Ni brazing application, the plate thickness is thin, and it is used in a state where it is affected by heat due to brazing. Furthermore, since Cu is contained as an additive element, it is assumed that a severe environment exists in which Cu ²⁺ that promotes corrosion exists. Therefore, in order to ensure crevice corrosion resistance when Cu is contained, the Cr content is preferably 17% or more, and more preferably 18% or more. On the other hand, if the Cr content is excessive, it is difficult to reduce C and N, which deteriorates mechanical properties and toughness, and increases costs. Therefore, the Cr content is preferably 25% or less, and more preferably 23% or less.

  Mo is an element that improves the corrosion resistance. In addition, it is an element effective for suppressing the coarsening of crystal grains due to the drag effect of solute Mo on the crystal grain growth during brazing heat treatment. Mo can contain 0.01% or more. On the other hand, since Mo is expensive, it causes an increase in cost. Therefore, the Mo content is preferably 2.50% or less, more preferably 2.00% or less and 1.00% or less.

  Cu is an element that improves the brazeability of stainless steel by reducing the surface tension of molten Ni and generating Marangoni convection, thereby increasing the wetting and spreading rate of the brazing. In order to improve wettability, Cu preferably contains 0.10% or more. On the other hand, when Cu is added excessively, it precipitates as an ε-Cu phase at room temperature and lowers the corrosion resistance. Therefore, the Cu content is preferably 0.50% or less, and more preferably 0.40% or less.

  N is an element that improves the corrosion resistance, but causes a decrease in workability due to solid solution strengthening, and further reduces the effective Nb in the steel by forming NbN, so the N content is reduced to 0.03% or less. It is preferable to do.

  Al is an element widely used as a deoxidizer, but as described above, it forms an oxide on the surface of stainless steel and lowers the brazeability. In order to utilize the oxide film removal mechanism utilizing carbide, the Al content needs to be reduced to 0.01% or less.

  Nb forms NbC to detoxify C at room temperature, and contributes to suppression of grain coarsening by the drag effect of solid solution Nb near the brazing temperature. Furthermore, NbC is also a source of solid solution C at the time of brazing. As described in [Reduction of Melting Separation], the start of removal of the oxide film by solid solution C is delayed, so that the rate of wetting and spreading is moderately adjusted, thereby contributing to suppression of melting separation. Therefore, the upper limit of the Nb content is preferably 7 × (C + N)% or more, and more preferably 8 × (C + N)% or more. The said (C + N) means the sum total of each content (mass%) of C and N. On the other hand, if Nb is added excessively, the workability is lowered, so the upper limit of the Nb content is preferably 0.80% or less, and more preferably 0.60% or less.

Furthermore, in the case where an element that adversely affects brazing such as Al or Ti is included, in order to achieve the oxide removal mechanism utilizing the above-described carbide under the brazing conditions of industrial mass production equipment, C, Si It is preferable that the content (% by mass) of Al, Ti satisfies the following formula (1).
3C + Cu-Ti-40Al> 0 (1) Formula

  When Al and Ti are contained excessively, the time required for the reduction and removal of the oxide becomes longer, leading to a decrease in productivity and an increase in manufacturing cost. There is a risk of coarsening and reducing the toughness of the product.

  Examples according to the present invention will be described below. The present invention is not limited to the following description.

Stainless steel having the chemical composition shown in Table 1 was melted, and a cold-rolled sheet having a thickness of 1 mm was manufactured through hot rolling, cold rolling, and annealing processes. The surface is 2D (nitric hydrofluoric acid immersion finish). The cold-rolled plate was cut out to 15 mm × 15 mm and then ultrasonically washed with acetone to obtain a test piece. A 0.05 g hemispherical Ni solder containing a binder was placed in the center of the test piece, and heat treatment was performed at 1120 ° C. and soaking for 10 minutes in a vacuum atmosphere with an oxygen partial pressure of 10 ⁻²⁰ atm. Then, it cooled to normal temperature and measured the wetting and spreading area of Ni brazing. The chemical composition shown in Table 1 is mass%, with the balance being Fe and inevitable impurities.

(Evaluation for brazing)
Brazing property was evaluated by wettability (wetting spread coefficient) and the degree of separation. The value obtained by dividing the wet spreading area after brazing heat treatment by the projected area of the brazing material installed was defined as the “wetting spread coefficient”. When the wetting spread coefficient was 5 or more, the wettability was good (◯), and when it was less than 5, the wettability was poor (x).

  The test piece having good wettability (◯) was further evaluated for melting and separation, and the presence or absence of melting and separation was determined as follows. The sample after the brazing heat treatment was observed from the horizontal direction, and the height of the residue at the brazing material installation portion was measured. When the residual height is less than one-fourth of the brazing filler metal height, the degree of melting is small and good (○), and when the brazing filler metal height exceeds one-fourth the brazing filler metal height Was determined to be largely unsuitable (x).

(Evaluation on corrosion resistance)
In order to evaluate the corrosion resistance of the Ni brazing member, the salt-wet repetition test of the test piece after brazing was performed up to 10 cycles, and the glazing area ratio was measured. The salt dry / wet repeat test was carried out by spraying 5% NaCl aqueous solution (35 ° C. × 15 min) → drying (RH 30%, 60 ° C. × 1 h) → wetting (RH 95%, 50 ° C. × 3 h) ” Each treatment was repeated as one cycle. When 10 cycles passed, the test piece was taken out, washed with water and dried, and then the test piece was observed.

  The measurement of the area ratio of sprout was performed according to the following procedure. The appearance of the test piece after the salt dry and wet repeated test was photographed, and the area of the sprung portion was determined for the exposed portion of the stainless steel excluding the end face and the portion covered with the wax. The area was measured by binarizing the appearance photograph of the test piece by image analysis, calculating the area per pixel, and then counting the number of pixels in the wax part. In the present specification, a value obtained by dividing the area of the rusted portion by the area of the entire exposed portion is defined as “fogging area ratio”, and the rusting area ratio is obtained from the above measured value. Those having a sprout area ratio of 1% or less were evaluated as “◯” (corrosion resistance was good) and those exceeding 1% were evaluated as “×” (corrosion resistance was poor).

  The test results are shown in Table 2. As a reference, the value of equation (1) and the wetting spread coefficient of each example are also shown. When the value of the formula (1) is zero or minus, it is displayed as zero (0.00).

  Invention Example No. having the component composition of the present invention. 1-No. All of No. 10 had good wettability, a small degree of melting and separation, excellent corrosion resistance, and exhibited good characteristics as a Ni brazing member.

  On the other hand, Comparative Example No. 1, no. As for No. 2, the state of big melting and separation was recognized in the brazing material installation location. This is presumably because the Nb content is less than 7 × (C + N)%, so that sufficient solute C to remove the oxide film could not be supplied, and rapid wetting spread occurred. Comparative Example No. No. 2 was inferior in corrosion resistance because the Cr content was less than 17%. Comparative Example No. In No. 3, the C content was less than 0.01% and the Al content exceeded 0.01%, so the wettability was poor. Comparative Example No. 3-No. 6, no. In No. 8, since the component did not satisfy the formula (1), the oxides of Al and Ti formed on the stainless steel surface could not be sufficiently removed, and sufficient wettability could not be obtained. Comparative Example No. No. 7 was inferior in corrosion resistance because Cr was less than 17%.

  FIG. 1 shows the relationship between the value of equation (1) shown in Table 2 and the wetting spread coefficient. It has been confirmed that the wetting spread coefficient starts to rise in the range where the value of the expression (1) exceeds 0.

  Since the ferritic stainless steel according to the present invention has excellent Ni brazeability and good corrosion resistance and workability, Ni brazeability and workability of EGR cooler parts, heat exchangers, fuel cells, etc. are required. In addition, it can be suitably used for applications requiring excellent corrosion resistance.

Claims (3)

% By mass
C: 0.01 to 0.03%
Si: 0.10 to 1.00%
Mn: 1.50% or less P: 0.04% or less S: 0.03% or less Ni: 0.6% or less Cr: 17 to 25%
Mo: 2.50% or less Cu: 0.10 to 0.50%
N: 0.030% or less Nb: 7 × (C + N)% or more, 0.80% or less Ti: 0.1% or less Al: 0.01% or less, with the balance being Fe and inevitable impurities Have
Ferritic stainless steel that satisfies the following formula (1) and has excellent Ni brazing properties.
3C + Cu-Ti-40Al> 0 (1) Formula Here, C, Cu, Ti, and Al in the formula (1) mean mass% of each element.   Furthermore, the ferritic stainless steel excellent in Ni brazing property of Claim 1 containing the 1 type (s) or 2 or more types of V: 0.20% or less and W: 1.00% or less in the mass%.   Furthermore, the ferritic stainless steel excellent in Ni brazing property of Claim 1 or Claim 2 which contains B: 0.007% or less by mass%.

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