FERRITIC STAINLESS STEEL AND STEEL SHEET FOR HEAT PIPES, AND HEAT PIPE AND HIGH-TEMPERATURE EXHAUST HEAT RECOVERY SYSTEM

Abstract

Provided is a ferritic stainless steel for heat pipes of high-temperature exhaust heat recovery systems, which comprises, in terms of % by mass, from 16 to 32% of Cr, at most 0.03% of C, at most 0.03% of N, at most 3% of Si, at most 2% of Mn, at most 0.008% of S, from 0 to 0.3% of Al, and at least one of at most 0.7% of Nb, at most 0.3% of Ti, at most 0.5% of Zr and at most 1% of V, and optionally at least one of at most 3% of Mo, at most 3% of W, at most 3% of Cu, at most 0.1% of Y, at most 0.1% of REM (rare earth metal) and at most 0.01% of Ca, with a balance of Fe and inevitable impurities, and which satisfies at least the following formula (1), formula (2) and formula (5): Cr+3(Mo+Cu)≧20  (1) Cr+3(Si+Mn+Al−Ti)≧20  (2) 0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5).

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to stainless steel and steel sheets for use for heat pipes for heat exchange that harness the latent heat of evaporation of water, as well as to the heat pipe thereof and a high-temperature exhaust heat recovery system comprising it.

2. Background Art

Recently, a high-temperature exhaust heat recovery system is being put into practical use for the purpose of recovering the heat of engine exhaust gas to be discharged at high temperatures in vehicle driving and recycling it as an energy source for vehicles for the purpose of enhancing fuel efficiency in vehicles. In general, a heat exchanger is applied to recovery of exhaust heat, which is for heat exchange between exhaust heat and cooling water or any other heat medium; and as a method of realizing efficient heat exchange, a heat exchanger with a heat-transferring means that may be referred to as a heat pipe is being specifically noted for use in high-temperature exhaust heat recovery systems for vehicles, etc.

FIG. 1 shows an example of an ordinary exhaust gas passageway constitution that comprises a high-temperature exhaust heat recovery system built in the exhaust gas passageway of a vehicle. In many cases, the exhaust heat recovery system is disposed backward of the underfloor converter as in this drawing. The heat recovered by the high-temperature exhaust heat recovery system is effectively used for heating the engine cooling water at the start of vehicle driving and for space heating in winter; and this contributes toward enhancing the fuel efficiency in gasoline vehicles, diesel vehicles, hybrid vehicles, etc., and saving the batteries therein.

FIG. 2 schematically shows the principle of a heat pipe.

[0. Initial State]

The heat pipe 10 is a metallic closed vessel with pure water sealed therein in vacuum (at most 100 Pa); and the inside of the vessel forms a liquid phase part 11 of liquid water and a space part 12.

[1. Heating/Cooling State]

When the site including the liquid phase part 11 (heating zone) of the heat pipe 10 is heated with an exhaust gas, then water in the liquid phase part 11 is actively evaporated. The evaporation is an endothermic reaction, and therefore the site can efficiently absorb the heat of exhaust gas. Specifically, most of the heat energy of exhaust heat is transferred to the water vapor as a latent heat of evaporation (heating). When a part of the space part 12 of the heat pipe 10 (cooling zone) is cooled with cooling water or the like, then the water vapor condenses on the inner surface of the cooled vessel of the heat pipe, and the condensed water returns back to the liquid phase part 11. The condensation is an exothermic reaction, and therefore the heat energy corresponding to the latent heat of the water vapor is released and transferred to the cooling water (cooling). The cooling water thus having received the heat energy is heated, and utilized as hot water. The cycle of heating and cooling occurs continuously, thereby realizing efficient heat exchange that harnesses the latent heat of evaporation of water.

Patent Reference 1 discloses a heat pipe-type high-temperature exhaust heat recovery system. FIG. 3 schematically shows the part of the system. This comprises a heating zone 22 where exhaust gas runs and a cooling zone 32 where cooling water 31 runs; and cups 23 are disposed in parallel in the heating zone 22. Between the neighboring cups 23, disposed is a heat-collecting fin 24 as brazed to the cups 23. The exhaust gas is led to pass through the heat-collecting fin 24. The both ends of each cup 23 are connected to the upper header (vapor flow path) and bottom header (liquid reflux path), through which the heating zone 22 is connected to the cooling zone 32; and the cups 23 and headers are charged with water and sealed up after suction in vacuum as so mentioned in the above. A mode switch valve 33 capable of opening and closing the liquid reflux path is disposed at the lower part of the cooling zone 32. When water in the cup 23 is heated by an exhaust gas in an open state of the mode switch valve 33 (heat recovery mode), then the water circulates in a cycle of boiling (vapor)→condensation (condensed liquid), and recovers the exhaust heat of the exhaust gas. In many cases, the cup 23 is formed to have a flattened shape so as to increase the specific surface area thereof and to minimize as much as possible the emission resistance of the exhaust gas; and in general, the cup is formed by pressing a stainless steel sheet having good heat resistance and good corrosion resistance. In many cases, the temperature of the exhaust gas to be fed to the heating zone 22 is elevated owing to the catalytic effect of the converter, and is often 800° C. or higher. The material temperature of the heating zone 22 is the highest when the mode switch valve 33 is in a closed state (heat shutoff mode), and is presumed to reach in a temperature range of from 600 to 900° C.

In that manner, the heat pipe is heated up to a temperature range of 900° C. or so, and therefore must be formed of a material excellent in high-temperature strength (creeping resistance, high-temperature fatigue resistance, heat fatigue resistance) and also excellent in high-temperature oxidation resistance. In addition, it requires excellent workability, weldability and brazability. Importantly, in addition, it must be inexpensive. Totaling these requirements, at present, it is considered that stainless steel is the most suitable for heat pipe material. Patent Reference 2 discloses a heat pipe formed of a stainless steel which contains 14-27% by weight of Cr.

Patent Reference 1: JP-A 2007-327719

Patent Reference 2: JP-A 7-243784

However, a heat pipe formed of stainless steel may have a large quantity of hydrogen generated inside it in the early stage of driving. As a result of various investigations, it has been clarified that the inner pressure may be often over 1 MPa owing to the generated hydrogen. In such a case, the heat pipe shall be a pressure vessel to be under severe legal controls. For handy utilization as an exhaust heat recovery system of high popularity, it is desirable to construct a high-temperature exhaust heat recovery system in which the inner pressure of the heat pipe is not over 1 MPa.

In addition, the hydrogen generation inside the heat pipe is a risk factor of lowering the heat transfer efficiency to cooling water and is additionally a risk factor of imparting excessive stress to the system; and therefore this may be a remote cause of system damage and heavy disasters.

A ferritic stainless steel has a smaller thermal expansion coefficient as compared with austenitic steels, and is therefore advantageous in point of the thermal fatigue resistance in cycles of heating and cooling. In addition, a ferritic stainless steel has a smaller hydrogen diffusion coefficient as compared with austenitic steels, and is therefore advantageous for discharging hydrogen generated inside a vessel out of the system. Further, a ferritic stainless steel is generally more inexpensive than austenitic steels. On the negative side, a ferritic stainless steel is generally inferior to austenitic steels in point of high-temperature oxidation resistance, high-temperature strength, corrosion resistance, etc.

SUMMARY OF THE INVENTION

An object of the present invention is to develop a ferritic stainless steel of a type of ferritic steels naturally having the above-mentioned advantages, which has the property of stably preventing the pressure increase to be caused by hydrogen generation in use thereof for vessels constituting a heat pipe, which has high-temperature oxidation resistance and high-temperature strength enough for use thereof in a temperature range of from 600 to 900° C., and which has corrosion resistance and Ni brazability suitable for heat pipes; and to provide a technique capable of constructing a heat pipe-type exhaust heat recovery system of high popularity.

To attain the above-mentioned object, the invention provides a ferritic stainless steel for heat pipes of high-temperature exhaust heat recovery systems, which comprises, in terms of % by mass, from 16 to 32% of Cr, at most 0.03% of C, at most 0.03% of N, at most 3% of Si, at most 2% of Mn, at most 0.008% of S, from 0 (no addition) to 0.3% of Al, and at least one of at most 0.7% of Nb, at most 0.3% of Ti, at most 0.5% of Zr and at most 1% of V, with a balance of Fe and inevitable impurities, and which satisfies all of the following formula (1), formula (2) and formula (5) and satisfies at least one of the following formula (3) and formula (4). The invention also provides the ferritic stainless steel further containing at least one of at most 3% of Mo, at most 3% of W and at most 3% of Cu. The ferritic stainless steel may further contain at least one of at most 0.1% of Y, at most 0.1% of REM (rare earth metal) and at most 0.01% of Ca.

Cr+3(Mo+Cu)≧20  (1)

Cr+3(Si+Mn+Al−Ti)≧20  (2)

Ti+Al≦0.5  (3)

Nb≧Ti+Al  (4)

0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5)

In formulae (1) to (5), the site of the atomic symbol is substituted with the value of the content of the corresponding element in terms of % by mass, and the site of the element not added to the steel is substituted with 0 (zero).

The invention also provides a ferritic stainless steel sheet for heat pipes, which is formed of the above-mentioned steel and which has a polish-finished surface at a polish count of from #100 to #800 as defined in JIS R6001:1998, or has a polish-finished surface at HL as defined in Table 14 in JIS G4305:2005.

The invention also provides a heat pipe constructed by vacuuming a vessel formed of a steel material of the above-mentioned steel having a thickness of from 0.5 to 1 mm, followed by introducing water thereinto and sealing it up to thereby form a liquid phase part and a space part therein. Preferably, the vessel has, as the inner surface thereof, a polish-finished surface at a polish count of from #100 to #800 as defined in JIS R6001:1998, or a polish-finished surface at HL as defined in Table 14 in JIS G4305:2005. The invention further provides a high-temperature exhaust heat recovery system having the above-mentioned heat pipe disposed in a site where the material temperature reaches in a range of from 600 to 900° C.

According to the invention, the increase in the inner pressure of a stainless steel-made heat pipe, which is caused by hydrogen generation in the heat pipe and is problematic therein, can be significantly relieved. Using the heat pipe can realize a high-temperature exhaust heat recovery system of high popularity at a low cost, therefore contributing toward further popularization of exhaust heat recovery systems in vehicles, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of an exhaust gas passageway constitution that comprises a high-temperature exhaust heat recovery system built in the exhaust gas passageway of a vehicle.

FIG. 2 is a conceptual view schematically showing the principle of a heat pipe.

FIG. 3 is a schematic view showing an example of a conventional high-temperature exhaust heat recovery system.

FIG. 4 is a schematic view showing the constitution of a hydrogen generation test apparatus.

FIG. 5 is a view schematically showing the member constitution of a test body for hydrogen permeation test.

FIG. 6 is a view schematically showing the constitution of a hydrogen permeation test apparatus.

FIG. 7 is a view explaining a Corrosion test method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For preventing the pressure increase to be caused by hydrogen generation in a heat pipe, first the mechanism thereof must be known. After vacuumed, a heat pipe is charged with pure water and sealed up, and therefore, only pure water and metal of the heat pipe (vessel) itself exist inside it. The hydrogen generation inside the heat pipe is caused by oxidation of the metal constituting the heat pipe (vessel) with water (water vapor). The reaction may be considered essentially as follows:

xM+yH₂O→M_xO_y+yH₂↑

where M means a metal, and x and y each are a coefficient.

The present inventor's investigations have clarified that the hydrogen generated inside a heat pipe penetrates through the heat pipe material (vessel) to go outside in some degree. Accordingly, for reducing the pressure increase to be caused by the hydrogen generation inside the heat pipe, it is effective to use any of the following two materials in constituting the heat pipe.

(i) Material which hardly generates hydrogen.
(ii) Material through which hydrogen readily penetrates.

Different frost austenitic steels, ferritic steels are naturally almost good in point of (ii). Accordingly, it is important to improve ferritic steels in point of (i).

Regarding the above (i), it is necessary to enhance the water vapor oxidation resistance of stainless steel that constitutes a heat pipe. When coated with a dense Cr₂O₃ film on the surface thereof, stainless steel may have enhanced water vapor oxidation resistance. As the means for it, the present inventors have found that employing the following method [1] is extremely effective for a ferritic stainless steel, and employing the following method [2] or [3] is more preferred. In the invention, the following [1] is employed, and if desired, any one or both of the following [2] and [3] are employed.

[1] The steel composition comprises Cr, Si and Mn each in the range mentioned below, and satisfies the following formula (2):

Cr+3(Si+Mn+Al−Ti)≧20  (2)

For preventing the hydrogen generation in use of heat pipes, it is effective to positively add to a ferritic stainless steel, Cr, Si and Mn having the effect of inhibiting the growth of oxidation scale at 600 to 900° C., each in the range mentioned below. Addition of Al is also effective. However, when the steel contains Ti, it has been found that the hydrogen generation increases owing to oxidation of Ti. Ti may be added as an element for fixing C and N in steel, as mentioned below, and therefore in the invention, it is especially important to control the Ti content of the ferritic stainless steel of the invention. As a result of various investigations, the inventors have found that the above-mentioned formula (2) must be satisfied for reducing hydrogen generation.

[2] The steel composition contains Y, REM (rare earth metal) and Ca each in the range mentioned below.

These elements have the effect of enhancing the high-temperature oxidation resistance of a ferritic stainless steel, and are effective for reducing hydrogen generation.

[3] The steel sheet has a polish-finished surface.

A ferritic stainless steel may readily form a poorly protective oxidation scale (red scale) that comprises Fe₂O₃ as the main ingredient thereof, in a low-oxygen high-moisture environment at 500 to 600° C. A heat pipe is often exposed to that environment in a cycle of heating it up to the highest service temperature thereof and cooling it to ordinary temperatures, in which the oxidation reaction of forming red scale may be often problematic as generating a large quantity of hydrogen. The present inventors have found that, for this problem, it is extremely effective to previously impart working strain to the surface layer of the ferritic stainless steel sheet that is exposed to the above-mentioned environment. According to this, Cr, Si, Mn and Al could readily diffuse into the surface layer of the steel sheet in a middle temperature range of from 500 to 600° C., whereby a protective dense oxidation film could be rapidly formed to prevent the formation of red scale on the steel sheet.

For forming working strain in the surface layer, there may be employed a method of shot blasting, polishing or the like; but in the invention, polishing is employed suitable for large-scale mass-production. The present inventors' investigations have clarified that strain introduction to a depth of 50 μm or so from the surface layer may be enough for the purpose. For this, the steel sheet may be so processed as to have a polish-finished surface at a polish count of from #100 to #800 as defined in JIS R6001:1998, or a polish-finished surface at HL as defined in Table 14 in JIS G4305:2005.

The steel to which the invention is directed is preferably a steel (ferritic single-phase steel) of which the composition is so designed as not to form as much as possible an austenitic phase at a high temperature. A steel having a composition that may readily form an austenitic phase at a high temperature is undesirable in that, when a Cr-deficient layer is formed in the matrix just below the oxidation film that covers the steel, then the Cr-deficient part may form locally an austenitic phase at a high temperature and may be therefore a bar to diffusion of hydrogen. In the steel of the type, in addition, when the austenitic phase formed at a high temperature is transformed into a martensitic phase in cooling, then the steel may have still another problem of fatigue strength reduction at ordinary temperature and at high temperatures owing to hydrogen embrittlement.

The alloying elements are described below. In this description, “%” for the alloying elements is “by mass” unless otherwise specifically indicated.

Cr is an important alloying ingredient for imparting the necessary corrosion resistance and oxidation resistance to the stainless steel. In order that the steel can secure water vapor oxidation resistance at 600 to 900° C., it requires at least 16% of Cr, and must satisfy the following formula (1):

Cr+3(Mo+Cu)≧20  (1)

For example, when Mo and Cu are not added to the steel, then 0 (zero) is substituted into the sites of Mo and Cu, and Cr alone must account for at least 20% in the formula (1).

When the Cr content is higher, then the ferrite stability is higher, therefore retarding Cr-deficient layer formation and retarding austenitic transformation in the Cr-deficient layer, if formed. More preferably, the Cr content is at least 13%, and even more preferably at least 20% especially when the above-mentioned polishing or preliminary oxidation treatment is not applied to the steel. On the other hand, however, the Cr content of more than 32%, if any, may greatly worsen the workability and the embrittlement resistance at 475° C. of the steel. Accordingly, the Cr content is within a range of at most 32%.

C and N are elements of enhancing the high-temperature strength, especially the creeping resistance of the steel; however, when they are in the steel as solid solution elements therein, then they may trap hydrogen in the steel to form methane and ammonia, thereby causing a risk factor of reducing the high-temperature strength, the toughness and the hydrogen permeability of the steel. In addition, since C and N are austenite-forming elements, they may also cause a risk factor of forming an austenitic phase at a high temperature. In this case, the steel may have a problem of hydrogen permeability reduction in the Cr-deficient layer and fatigue strength reduction owing to hydrogen embrittlement, as so mentioned in the above. Accordingly, the steel preferably has a lower C content and a lower N content as much as possible. As a result of various investigations, C and N are both acceptable in an amount of up to 0.03% each. However, for sufficiently lowering the sum total of solid solution C and solid solution N (hereinafter referred to as “amount of solid solution C+N”) in the steel, it is important that the steel satisfies the following formula (5) in relation to the content of Nb, Ti, Zr and V therein that readily bond to C and N:

0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5)

In the formula (5), the left-hand member is an index of indicating the amount of solid solution C+N in the ferritic stainless steel having the composition of the invention. When the steel does not satisfy the formula (5), the durability of the heat pipe formed of the steel may greatly lower since the thermal fatigue resistance at high temperatures of the steel is poor.

Si is an element of enhancing the oxidation resistance and the red scale resistance of the steel, therefore acting to prevent hydrogen generation. In addition, this is an element of having an effect of ferrite stabilization. Especially effectively, the Si content is at least 0.1%; however, too much Si, if any, may worsen the hot ductility of the steel and may cause formation of faults in the steel surface, therefore causing a risk factor of greatly worsening the producibility and the weldability of the steel. Accordingly; the Si content is limited to fall in a range of at most 3%.

Mn is an element of enhancing the scale peeling resistance of the stainless steel, and is effective for preventing the pressure increase to be caused by the hydrogen generation inside the heat pipe formed of the steel. However, too much Mn, if any, may detract from the toughness of the steel; and therefore, the Mn content is limited to be at most 2%.

S is an ingredient that may have some negative influences on the hot workability and the welding-resistant high-temperature cracking resistance of the steel, and may be a starting point for abnormal oxidation in the steel. Therefore, the S content is preferably as low as possible; and its uppermost limit is defined to be at most 0.008% by mass. More preferably, the S content is at most 0.005% by mass.

Nb, Ti, Zr and V act to fix C and N to thereby reduce the amount of solid solution C and solid solution N in the steel. In addition, they also act to enhance the high-temperature strength of the ferritic stainless steel through precipitation enhancement by fine dispersion of their carbides and nitrides in the steel. Al has an effect of enhancing the oxidation resistance of the steel and an effect of reducing the hydrogen generation by the steel. To attain these effects in the invention, at least one of at most 0.7% of Nb, at most 0.3% of Ti, at most 0.5% of Zr and at most 1% of V is added to the steel. In particular, more preferably, at least one of from 0.001 to 0.7% of Nb, from 0.005 to 0.3% of Ti, from 0.001 to 0.5% of Zr and from 0.05 to 1% of V is added. Al is optionally added to the steel in an amount of at most 0.3%. In case where Al is added, then its content is more effectively in a range of from 0.03 to 0.3%. However, special attention should be paid to adding Ti and Al. One reason is that Ti may be a risk factor of hydrogen generation as so mentioned in the above. This problem may be solved by composition control to satisfy the above formula (2). Another reason is that Ti and Al may be a risk factor of worsening the brazability of the steel. Most heat pipe members are put into practical use by brazing; however, when the content of Ti and Al in the ferritic stainless steel is high, Ti and Al may form an oxidation film under heat in brazing even under a low oxygen partial pressure condition, therefore detracting from the brazability of the steel (especially the wettability of the steel to a brazing material). As a result of detailed investigations, this problem can be solved by composition control to satisfy at least one of the following formula (3) and formula (4):

Ti+Al≦0.05  (3)

Nb≧Ti+Al  (4)

As so mentioned in the above, the steel must contain at least one of Nb, Ti, Zr and V satisfying the formula (5) in order to fully reduce the amount of solid solution C+N therein.

Mo, W and Cu are all elements of enhancing the high-temperature strength of the stainless steel through solid solution reinforcement. In the invention, a method of adding a large amount of C and N for enhancing the strength of the steel could not be employed, and therefore, in case where the strength of the steel is emphasized, preferably, at least one of Mo, W and Cu is added to the steel. In addition, Mo and Cu especially have an effect of retarding the generation and growth of pitting corrosion of the dew point corrosion to be caused by the dew condensation in exhaust gas. Adding at least one of Mo and Cu may broaden the range of the Cr content (that is, may lower the lowermost limit thereof) necessary for satisfying the formula (1). In case where at least one of Mo, W and Cu is added, preferably, they are so controlled that the Mo content is at most 3%, the W content is at most 3% and the Cu content is at most 3%. More effectively, the content of Mo, W and Cu is at least 0.1% each.

Y, REM (rare earth metal) and Ca have an effect of fixing S that is harmful to the scale peeling resistance of the steel, thereby to prohibit thickening S in the scale/base interface, and have an effect of reducing the defect density in the oxidation film to thereby reinforce the oxidation film, whereby the oxidation resistance of the steel can be greatly enhanced. Accordingly, in the invention, at least one of these may be optionally added to the steel. However, too much addition of those elements may not only harden the steel material too much but also cause surface faults in production of the steel, therefore resulting in the increase in the production cost. As a result of various investigations, in case where these elements are added, they are preferably so controlled that the Y content is at most 0.1%, the REM content is at most 0.1%, and the Ca content is at most 0.01%. More effectively, at least one of from 0.0005 to 0.1% of Y, from 0.0005 to 0.1% of REM and from 0.0005 to 0.01% of Ca is added to the steel.

In melt-producing the stainless steel, Ni, P, B, Mg and others may mix therein, for example, from the starting materials and from the deposits adhering to the wall surface of the smelting equipment. The acceptable range of Ni is up to 0.6%, that of P is up to 0.05%, that of B is up to 0.01%, and that of Mg is up to 0.005%.

In producing a heat pipe by the use of the ferritic stainless steel, first a stainless steel sheet having the above-mentioned predetermined composition is prepared according to a production process for an ordinary ferritic stainless steel sheet; then a sheet piece cut out of the steel sheet is shaped and welded into a vessel, and thereafter the vessel is vacuumed and then charged with pure water. Another method is also employable wherein a welded steel pipe is formed from the steel sheet, or a seamless steel pipe is formed from the steel billet and the steel pipes are worked into a vessel. The technique of “charging with pure water” means that the vessel into which pure water has been put is sealed up by welding or meltdown fusing. When the wall thickness of the vessel is too thin, the vessel could hardly have sufficient durability. On the other hand, when too thick, then the hydrogen diffusion distance through the vessel wall may be long since hydrogen must penetrate through the thick wall, and this is disadvantageous for reducing the inner pressure of the vessel. With the increase in the heat transfer distance through the steel material, the thermal conductivity resistance of the steel material may increase, which, however, is disadvantageous for efficient thermal exchange based on the evaporation latent heat of steam. As a result of various investigations, the wall thickness of the heat pipe is preferably within a range of from 0.5 to 1 mm.

Example 1

Steels shown in Table 1 (30 kg each) were melt-produced in a vacuum smelting furnace, and hot-rolled, annealed, pickled, cold-rolled and finish-annealed according to an ordinary ferritic stainless steel sheet production method, thereby producing cold-rolled annealed pickled steel materials (sample materials) having a thickness of 1.0 mm, for which the pickling finish was No. 2D as defined in Table 14 in JIS G4305:2005.

	TABLE 1
	Chemical Composition (mas. %)
Classification	No.	C	Si	Mn	P	S	Ni	Cr	Mo	Cu	N	Nb	Ti	Al	others
Samples of	A1	0.02	0.30	1.00	0.025	0.002	0.11	18.3	2.05	0.18	0.01	0.65	—	—	—
the invention	A2	0.01	0.19	0.22	0.033	0.001	0.18	21.9	0.98	0.12	0.01	0.27	0.17	0.08	—
	A3	0.01	0.25	0.35	0.030	0.002	0.15	18.2	1.07	0.08	0.01	0.35	—	—	—
	A4	0.01	0.22	0.31	0.027	0.002	0.18	18.5	1.01	0.13	0.01	—	—	—	Zr: 0.24
	A5	0.01	0.27	0.42	0.031	0.001	0.08	18.4	0.95	—	0.01	0.10	—	—	V: 0.23
	A6	0.01	0.55	0.26	0.028	0.001	0.09	18.5	—	0.49	0.02	0.45	0.04	—	Ca: 0.005
	A7	0.01	0.31	0.22	0.021	0.003	0.11	20.2	2.01	1.52	0.01	0.45	—	—	W: 1.24
	A8	0.02	0.22	0.31	0.022	0.001	0.09	20.5	0.23	—	0.01	0.42	—	—	REM: 0.02
	A9	0.01	0.18	0.25	0.029	0.001	0.07	23.9	—	—	0.01	0.05	—	—	Y: 0.02
	A10	0.01	0.22	0.28	0.028	0.001	0.11	29.9	1.95	0.08	0.01	0.35	0.19	0.11	—
Comparative	B1	0.01	0.45	0.35	0.030	0.001	0.15	17.9	1.01	—	0.01	0.04	0.35	—	—
Samples	B2	0.01	0.29	0.22	0.024	0.002	0.03	17.3	0.05	0.07	0.01	—	0.41	—	—
	B3	0.01	1.52	0.31	0.022	0.001	0.12	12.2	—	—	0.01	—	0.15	0.98	—
	B4	0.02	0.22	0.25	0.025	0.001	0.15	18.1	0.05	0.07	0.01	—	0.13	2.97	—
	B5	0.03	0.95	1.10	0.033	0.001	0.15	14.0	—	0.07	0.01	0.44	—	—	—
	B6	0.06	0.55	0.25	0.033	0.003	0.15	16.9	0.09	—	0.02	0.01	—	—	—
	B7	0.04	0.22	0.45	0.035	0.002	0.22	19.2	—	—	0.02	0.05	—	—	—
			left-hand member	left-hand member		left-hand member
	Classification	No.	of formula (1)	of formula (2)	Ti + Al	of formula (5)
	Samples of	A1	24.99	22.20	0.0	0.004
	the invention	A2	25.20	22.86	0.25	0.003
		A3	21.65	20.00	0.00	0.005
		A4	21.92	20.09	0.00	0.007
		A5	21.25	20.47	0.00	0.004
		A6	19.97	20.81	0.04	0.005
		A7	30.79	21.79	0.00	0.004
		A8	21.19	22.09	0.00	0.006
		A9	23.90	25.19	0.00	0.031
		A10	35.99	31.16	0.30	0.003
	Comparative	B1	20.93	19.25	0.35	0.003
	Samples	B2	17.66	17.60	0.41	0.003
		B3	12.20	20.18	1.13	0.006
		B4	18.46	28.03	3.10	0.010
		B5	14.21	20.15	0.00	0.008
		B6	17.17	19.30	0.00	0.593
		B7	19.20	21.21	0.00	0.090

<<Hydrogen Generation Test>>

Hydrogen generation test pieces of 10 mm×50 mm×1.0 mm each were cut out of each sample material. These were grouped into two; and the sheet surface of those in one group was the No. 2D pickle-finished surface as such, while that in the other group was a #400 dry polish-finished surface. The cut edges of all the test pieces were all #600 dry polish-finished ones.

FIG. 4 schematically shows the constitution of the hydrogen generation test apparatus used herein. A test piece is put into the quartz tube, and this is set in an electric furnace. The quartz tube is insulated from the outside world by a quartz cover fitting thereto. However, a sheathed thermocouple housed in a quartz protective tube is inserted into the quartz tube so as to measure the temperature around the sample. A Pyrex® tube is connected to the quartz tube, and the Pyrex tube is connected to a vacuum pump and a pure water tank. The quartz tube, the quartz cover, the Pyrex tube and the quartz protective tube are all connected to each other by lapping, and the airtightness of the lapped part is suitably increased by a vacuum grease. At the beginning, all the valves (A to D) are shut.

First, only the valve D is opened at normal temperature. In this step, the tube between the valves C and D is filled with about 30 mL of pure water. Next, the valve D is again shut. Next, the vacuum pump is driven, the valve B is opened and the quartz tube is vacuumed, and then the valve A is opened and the pressure is measured. After vacuuming to 1 Pa, the valve B is shut. Next, the valve C is opened. In this stage, the pure water remaining between the valves C and D is led into the quartz tube via the Pyrex tube. As vigorously sucked into the vacuum, almost all the water having remained in the tube C-D reaches near the test piece.

Next, the inside of the quartz tube is heated by the electric furnace. Monitored with the sheathed thermocouple, the temperature inside the quartz tube is kept at 600° C. or 800° C. The pressure change during the test where the temperature is kept at 600° C. or 800° C. for 10 hours is monitored, and from the pressure change at the time after 10 hours, the hydrogen generation rate from the sample surface per the unit time and the unit area thereof is computed. The samples from which the hydrogen generation rate at the time after 10 hours is at most 1.0×10⁻⁶ mol/(h·cm²) are determined as those having the property of reducing the inner pressure increase to at most 10 kPa in a real heat pipe. Accordingly, the samples from which the hydrogen generation rate at the time after 10 hours is at most 1.0×10⁻⁶ mol/(h·cm²) are evaluated as good (as effective for reducing hydrogen generation), while those from which the hydrogen generation rate is over the range are as bad (as ineffective for reducing hydrogen generation). The results are shown in Table 2.

For reference, in Table 2, the data of the hydrogen generation rate of each sample at the time after kept at 600° C. for 1 hour are also shown under the same evaluation standard as above.

<<Hydrogen Permeation Test, Brazability Evaluation>>

FIG. 5 schematically shows the member constitution of a test body for hydrogen permeation test. The members formed of a test material are plates (two) and cups (four). The cups are formed by pressing; and the outer shape dimension thereof is about 100 mm (length)×30 mm (width)×5 mm (height). In FIG. 5, the width and the height of the cup are drawn as exaggerated, as compared with the length thereof. Two holes are made in the bottom of each cup, and the inner space of one cup communicates with that of the adjacent cup via linking pipes attached to the holes. The linking pipe is made of a ½-inch pipe of SUS310S by flattening it. The distance between the two cups linked via the linking pipes is 8 mm. The center two cups are integrated to be a bag. The side cup is covered with the plate. A hole is made in the center of the plate to be fitted to one end of the test body, and hydrogen gas is introduced into the test body through the hole. These members are braded and soldered with a Ni-based brazing material (BNi-5), and the test body is thus constructed. The braded part is airtightly sealed up to prevent gas leakage through it. The brazing is attained in a vacuum furnace, and the vacuum brazing condition is as follows: The pressure is 1 Pa, the brazing temperature is 1175° C., the heating time from ordinary temperature up to the brazing temperature is 2 hours, and the soaking time at the brazing temperature is 30 minutes.

FIG. 6 schematically shows the constitution of a hydrogen permeation test apparatus. The test body in FIG. 6 is drawn as exaggerated in the height direction (in the lamination direction) thereof. The test body is set in an electric furnace. Via the ¼ inch tube of SUS316 braded to the test body, hydrogen gas is introduced into the inside of the test body from a hydrogen generation unit. First, the inside of the test body is vacuumed to 1 Pa by the vacuum pump. While the valve Y is shut, the valve X is opened and hydrogen gas is introduced into the test body from the hydrogen generation unit, and when the pressure inside the test body becomes higher than 120 kPa, the valve X is shut. Accordingly, hydrogen gas having a hydrogen partial pressure of more than about 120 kPa is sealed up inside the test body. Next, the electric furnace is heated and the test body therein is kept at 800° C. While kept at 800° C., the pressure change inside the test body is monitored; and from the pressure change at the time at which the pressure is 100 kPa (or that is, at the time at which the hydrogen partial pressure is about 100 kPa), the hydrogen permeation rate is computed. Then, the value of the hydrogen permeation rate is divided by the surface area of the part exposed to the inner atmosphere of the four cups and the two plates composed of the test material, thereby giving the hydrogen permeation rate of the test material per the unit time and per the unit area thereof. Hydrogen may permeate into the other members than those of the test material existing inside the surface (part of ¼ inch tube and linking pipe); however, the surface area of the other members than those of the test material is sufficiently smaller than the surface area of the test material, and the hydrogen permeation through the other members than the test material can be ignored.

The samples of which the hydrogen permeation rate thus measured in the manner as above is at least 1.0×10⁻⁹ mol/(h·cm²·Pa^1/2) are determined as those having the property of reducing the inner pressure increase to at most 10 kPa in a real heat pipe, based on the condition that the hydrogen generation rate from the samples, as measured according to the above-mentioned test method, is at most 1.0×10⁻⁶ mol/(h·cm²). Accordingly, the samples of which the hydrogen permeation rate is at least 1.0×10⁻⁹ mol/(h·cm²·Pa^1/2) are evaluated as good (as effective for increasing hydrogen permeation), while those of which the hydrogen permeation rate is lower than the range are as bad (as ineffective for increasing hydrogen permeation).

The samples with brazing failure could not have a predetermined vacuum degree owing to vapor leakage through the braded part in vacuuming, and therefore could not be tested in the hydrogen permeation test. Accordingly, the brazability of the samples is evaluated as follows: The samples that could have a predetermined vacuum degree are as good (as having good brazability); and the others are evaluated as bad (as having poor brazability).

The results are shown in Table 2.

<<Corrosion Test>>

The corrosion test is attained according to a test in which an exhaust gas dew condensation environment is simulated; and an outline of the test method is shown in FIG. 7. Specifically, a test piece of 25 mm×70 mm is cut out of each test material; its surface is #400 dry polish-finished and its cut edges are #600 dry polish-finished; and this is heat-treated at 800° C. for 10 hours to prepare a corrosion test piece. The composition of simulated condensed water is shown in the appendix table in FIG. 7. The corrosion test piece is set between a vertical heating furnace and a water tank filled with simulated condensed water put below it, in such a manner that it can be reciprocated up and down between the two, and exposed to 50 heating and dipping cycles, in which one cycle comprises “keeping in furnace at 350° C. for 6 minutes (including soaking time of about 1 minute)→cooling outside the furnace for 7 minutes (the temperature of the test piece, not higher than 100° C.)→dipping the lower part, 20 mm of the test piece in the simulated condensed water for 1 minute→drying in air for 10 minutes (for condensation of the ingredients in the liquid)”. Then, the test piece is kept in a thermo-hygrostat at 30° C. and 80% RH for 2000 hours, and after the soaking therein, the maximum corrosion depth of the corroded pores formed in the test piece is measured according to a focal depth method using an optical microscope. In case where the maximum corrosion depth in this test method is more than 0.8 mm, there may be a possibility that the cup part of the heat pipe formed of the test piece may be corroded to have a through-hole after exposed to a vehicle exhaust gas dew condensation atmosphere for 15 years, even though a plate thickness of 1 mm that is the largest thickness planned for heat pipes for vehicles is assumed. Accordingly, the samples of which the maximum corrosion depth in this test is at most 0.8 mm are evaluated as good (as having good corrosion resistance), and the others are as bad (as having poor corrosion resistance).

The results are shown in Table 2.

	TABLE 2
	Hydrogen Generation Rate		(reference) Hydrogen
	800° C.,	600° C.,		Hydrogen	Corrosion	Generation Rate
	after 10 hr	after 10 hr		Permeation	Test	600° C., after 1 hr
Classification	No.	#400	2D	#400	2D	Brazability	Rate 800° C.	2000 hr	#400	2D
Samples of	A1	good	good	good	good	good	good	good	good	bad
the invention	A2	good	good	good	good	good	good	good	good	bad
	A3	good	good	good	good	good	good	good	good	bad
	A4	good	good	good	good	good	good	good	good	bad
	A5	good	good	good	good	good	good	good	good	bad
	A6	good	good	good	good	good	good	good	good	bad
	A7	good	good	good	good	good	good	good	good	bad
	A8	good	good	good	good	good	good	good	good	bad
	A9	good	good	good	good	good	good	good	good	bad
	A10	good	good	good	good	good	good	good	good	bad
Comparative	B1	bad	bad	good	good	good	(immeasurable)	good	good	bad
Samples	82	bad	bad	good	good	good	(immeasurable)	bad	good	bad
	B3	good	good	good	good	good	(immeasurable)	bad	good	bad
	B4	good	good	good	good	good	(immeasurable)	bad	good	bad
	B5	good	good	good	good	good	good	bad	good	bad
	B6	bad	bad	good	good	good	good	bad	good	bad
	B7	good	good	good	good	good	good	bad	good	bad

As known from Table 2, all the samples of the invention had good results in all the test items. In particular, it is considered that the samples with strain introduced into the surface layer thereof by polishing could rapidly form a protective oxidation scale in the initial stage of heating in a middle temperature range of around 600° C., and the hydrogen generation rate from them decreased at the time after 1 hour at 600° C. Accordingly, these samples could exhibit a more stable hydrogen generation-preventing effect in the initial stage of starting the use of heat pipes.

As opposed to these, the samples B2 to B7 not satisfying the formula (1) were poor in the corrosion resistance; and the samples B1, B2 and B6 not satisfying the formula (2) had a large hydrogen generation rate at 800° C. falling within the maximum ultimate temperature range of heat pipes. The samples B1 to B4 not satisfying the formula (3) and the formula (4) were poor in brazability.

Example 2

Heating/Cooling Cycle Durability Test

Using a cold-rolled annealed steel sheet (#400 dry polish-finished steel sheet) having a thickness of 0.8 mm of the samples A1 to A3, B6 and B7 in Table 1, the heat pipe (cup 23) of a high-temperature exhaust heat recovery system as in FIG. 3 was constructed. The heating method employed herein comprises introducing a high-temperature gas into the heat-collecting fin 24 from an external gas burner. The system was exposed to a test of heating/cooling 2000 cycles, in which one cycle comprises “1. heating with coolant (water) circulation for 5 minutes→2. further heating for 5 minutes with coolant circulation stopped→3. coolant circulation and heating stopped for 5 minutes”. The temperature of the heat pipe was about 400° C. in the step 1 of heating with coolant circulation, about 800° C. in the step 2 of heating with coolant circulation stopped, and from 100 to 200° C. in the cooling step 3; and in this cycle, the system is not corroded by dew condensation. After the heating/cooling cycle test, the system was dismantled, and the members of the cup 23 were color-checked for the presence or absence of damages. As a result, no damage was detected in the cup 23 formed of the sample A1 to A3 of the invention. As opposed to this, the cup 23 formed of the sample B6 or B7 not satisfying the formula (5) had defects running through the sheet owing to the large content of solid solution C+N therein.

Claims

1. A ferritic stainless steel for heat pipes of high-temperature exhaust heat recovery systems, which comprises, in terms of % by mass, from 16 to 32% of Cr, at most 0.03% of C, at most 0.03% of N, at most 3% of Si, at most 2% of Mn, at most 0.008% of S, from 0 (no addition) to 0.3% of Al, and at least one of at most 0.7% of Nb, at most 0.3% of Ti, at most 0.5% of Zr and at most 1% of V, with a balance of Fe and inevitable impurities, and which satisfies all of the following formula (1), formula (2) and formula (5) and satisfies at least one of the following formula (3) and formula (4): wherein the site of the atomic symbol is substituted with the value of the content of the corresponding element in terms of % by mass, and the site of the element not added to the steel is substituted with 0 (zero) in formulae (1) to (5).

Cr+3(Mo+Cu)≧20  (1)

Cr+3(Si+Mn+Al−Ti)≧20  (2)

Ti+Al≦0.05  (3)

Nb≧Ti+Al  (4)

0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5),

2. The ferritic stainless steel for heat pipes as claimed in claim 1, which further contains at least one of at most 3% of Mo, at most 3% of W and at most 3% of Cu.

3. The ferritic stainless steel for heat pipes as claimed in claim 1, which further contains at least one of at most 0.1% of Y, at most 0.1% of REM (rare earth metal) and at most 0.01% of Ca.

4. A ferritic stainless steel sheet for heat pipes, which is formed of the steel of claim 1 and which has a polish-finished surface at a polish count of from #100 to #800 as defined in JIS R6001:1998, or has a polish-finished surface at HL as defined in Table 14 in JIS G4305:2005.

5. A heat pipe for high-temperature exhaust heat recovery systems, as constructed by vacuuming a vessel formed of a steel material of the steel of claim 1 and having a thickness of from 0.5 to 1 mm, followed by introducing water thereinto and sealing it up to thereby form a liquid phase part and a space part therein.

6. A heat pipe for high-temperature exhaust heat recovery systems, as constructed by vacuuming a vessel formed of a steel material of the steel of claim 1 and having a thickness of from 0.5 to 1 mm and having, as the inner surface thereof, a polish-finished surface at a polish count of from #100 to #800 as defined in JIS R6001:1998 or a polish-finished surface at HL as defined in Table 14 in JIS G4305:2005, followed by introducing water thereinto and sealing it up to thereby form a liquid phase part and a space part therein.

7. A high-temperature exhaust heat recovery system having the heat pipe of claim 5 as disposed in a site where the material temperature reaches from 600 to 900° C.