JPH11229084A - Copper plated steel sheet for double wound pipe, excellent in copper penetration resistance, workability, and the like, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a copper plated steel sheet used for manufacture of a double roll pipe and excellent in copper penetration resistance, grain coarsening resistance, workability, etc., and its production. SOLUTION: This copper plated steel sheet is constituted by applying copper plating to a steel sheet which has a chemical composition consisting of 0.03-0.08% C, <=0.1% Si, 0.05-0.5% Mn, <=0.020% P, <=0.015% S, 0.03-0.08% sol. Al, 0.003-0.008% N, 0.0008-0.002% B, and the balance essentially Fe and where fine AlN precipitates are densely dispersed in the surface layer (20 to 100 μm layer thickness). This copper plated steel sheet can be produced by applying cold rolling to a hot rolled steel plate of a slab having the above chemical composition at 40 to 90% draft, subjecting the resultant cold rolled steel sheet to annealing treatment in the temp. region between the recrystallization temp. and 850 deg.C in an N2 -H2 gaseous mixture of >=2 vol.% hydrogen concentration, and then applying copper plating to the steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copper-plated steel sheet having excellent copper penetration resistance and workability for producing a double-wound pipe used as a brake tube of an automobile or a cooling tube of a refrigerator. It relates to the manufacturing method.

[0002]

2. Description of the Related Art A double-wound pipe made of a copper-plated steel sheet is obtained by winding a hoop of a copper-plated steel sheet cut to a predetermined width into a pipe shape using a pipe-forming roll, and then melting the copper to a temperature equal to or higher than the melting point of copper (eg, 1130 ° C.) For a suitable time (about 1 to
It is manufactured by performing so-called self-brazing treatment in which the copper plating layer is melted while being held for about 2 minutes, and the wrapped surfaces are fused and bonded. FIG. 8 shows a cross section of a double wound pipe. 1 is a base steel plate (cold rolled steel plate), 2 is a copper plating layer, 3 is a copper fusion layer formed by self-brazing, and the quality of the bonding between the wrapped surfaces is determined by an eddy current flaw detection method, a bending test, or the like. Inspected by

[0003] The copper-plated steel sheet used in the production of the double-wound pipe must not be excessively coarsened by the heat treatment of self-brazing (coarse-graining resistance). Infiltration of copper (causing embrittlement of steel sheet)
(Copper infiltration resistance), and good ductility to withstand expansion and flaring performed after forming a double wound pipe. Low carbon aluminum killed steel is used as the base steel of the copper-plated steel sheet. Japanese Patent Publication No. 8-14013 discloses that C: 0.01 to 0.15%, Si: 0.
1% or less, Mn: 0.05-0.6%, Al: 0.003-0.1%,
P: 0.015% or less, B: 0.0004 to 0.004%, and the balance is F
It discloses that a base steel plate made of e and unavoidable impurities is subjected to copper plating. It describes that as a B-containing effect of the base steel sheet, the crystal structure is refined and the weld crack resistance is improved.

[0004]

The conventional copper-plated steel sheet, even though the base steel sheet belongs to the category of low-carbon aluminum-killed steel sheet, has a problem in that it lacks copper penetration resistance and coarse-graining resistance. In self-brazing heat treatment after forming a heavy wound pipe, it is easy to cause grain boundary embrittlement due to infiltration of molten copper and decrease in ductility due to needle-like and coarsening of ferrite structure, and in subsequent expanding and flare processing, There are many cases where cracks occur. Although the addition of the B element to the low-carbon aluminum-killed steel is effective in refining the ferrite structure as described in the above-mentioned publication, the B addition amount, C, sol.
Depending on the content of Al, N, etc., the ferrite structure becomes coarse in the brazing heat treatment, grain boundary embrittlement due to copper intrusion, or the ferrite structure becomes excessively fine or acicular in the cooling process after the brazing treatment. Hardening easily occurs. For this reason, it is difficult to guarantee sufficient formability required for pipe expansion and flare processing after pipe formation. The present invention solves the conventional problems related to double-wound pipes, and has improved coarse-graining resistance, copper penetration resistance, and the like for ensuring good workability that can withstand tube expansion and flaring. And a method for producing the same.

[0005]

The copper-plated steel sheet for a double-wound pipe according to the present invention has a C content of 0.03 to 0.0% by weight.
8%, Si: 0.1% or less, Mn: 0.05 to 0.5
%, P: 0.020% or less, S: 0.015% or less, sol. Al: 0.03 to 0.08%, N: 0.00
3 to 0.008%, B: 0.0008 to 0.002
%, And the balance is substantially composed of Fe, and the base steel sheet having a surface layer having a layer thickness of 20 to 100 μm in which fine AlN precipitates are dispersed is plated with copper.

[0006] The copper-plated steel sheet for a double-wound pipe of the present invention is obtained by hot rolling a slab of steel having the chemical composition described above,
After cold rolling at a rolling reduction of 40 to 90%, the cold-rolled steel sheet is annealed in an N ₂ -H ₂ mixed gas having a hydrogen concentration of ₂ vol% or more in a temperature range of a recrystallization temperature to 850 ° C. It is manufactured by a process of applying copper plating to the metal. The double-wound pipe manufactured by using the copper-plated steel sheet of the present invention has a needle of a ferrite structure in a self-brazing heat treatment as an effect of an appropriate content of the chemical composition of the base steel sheet, particularly, C, Al, N and B. In addition to preventing the formation and coarsening from being suppressed,
Due to the high copper penetration resistance, copper penetration and the resulting grain boundary embrittlement are avoided, and the aging properties of the base steel sheet are also improved. As these effects, high ductility (an elongation of about 25% or more is required) required for pipe expansion and flare processing is ensured.

[0007] In the above-mentioned improvement of the quality of the copper-plated steel sheet of the present invention, the effects of the B content of the base steel sheet and the precipitation and dispersion of fine AlN in the surface layer are important. Fine AlN on steel sheet surface
Precipitates are formed by nitriding reaction in the annealing process of cold rolled steel sheet to atmosphere N ₂ -H ₂ mixture gas. This A
The 1N precipitates are remarkably fine particles having a particle size of about 50 to 500 °, and have a pinning effect in the recrystallization process of ferrite during the brazing heat treatment of the double-wound pipe, resulting in a remarkably fine ferrite structure on the surface layer of the base steel sheet. Is formed. Its surface layer structure has a ferrite grain size number FGS.
NO (JIS G 0552) It is extremely fine, about 10 or more, and has high resistance to penetration of molten copper at grain boundaries.

On the other hand, B in the steel added to the base steel sheet is
It has a stronger affinity for N than Al, and is preferentially bonded to solute N in steel during the winding step of a hot-rolled steel sheet and the annealing treatment step of a cold-rolled steel sheet to form BN precipitates. This BN precipitate is subjected to a brazing heat treatment (1100 to 115) for a double wound pipe.
In the initial stage (approximately 950 to 1050 ° C.), a part thereof is decomposed to form solid solution B and solid solution N. The solid solution B precipitates at the ferrite grain boundaries until the temperature of the double-wound pipe reaches or exceeds the melting point of copper (about 1100 ° C.), and strengthens the ferrite grain boundaries. By this grain boundary strengthening, penetration of the molten copper into the grain boundary is suppressed and prevented. Further, B precipitated at the grain boundaries suppresses abnormal growth of ferrite crystals, and is effective in preventing the coarsening of the structure.

Further, in the cooling step (cooling rate: about 20 to 40 ° C./sec) following the brazing heat treatment, solute B in the steel combines with N to form BN precipitates. AlN precipitates are also treated with the brazing heat treatment (about 1130).
C), a part thereof is decomposed into Al and N and forms a solid solution in the steel. However, since the cooling rate after the brazing heat treatment is relatively large, the reaction between Al and N (precipitation of AlN) during this cooling process Almost no occurrence. On the other hand, since B has a higher affinity for N than Al, it combines with solute N to form BN precipitates even at a relatively high cooling rate. This BN precipitate formation reaction reduces the amount of solute N (which causes hardening), improves aging of the pipe, and increases ductility. In addition, since the BN precipitate is larger than the AlN precipitate, it advantageously acts to lower the yield point (YP) of the pipe and improve the ductility. In addition, due to the precipitation of BN, a remarkable effect of improving the workability of the double-wound pipe performed before the brazing heat treatment can be obtained.

The reasons for limiting the chemical composition of the base steel sheet in the present invention are as follows. C: 0.03 to 0.08% From the viewpoint of increasing the ductility of the cold-rolled steel sheet, the smaller the C content is, the better. However, if the C content is too small, the steel sheet becomes insufficient in coarse-graining resistance in self-brazing heat treatment. . Further, the copper penetration resistance is also reduced due to the decrease in the grain boundary strength. Meanwhile C
If the amount increases, the ductility decreases due to an increase in the amount of carbide precipitation. In particular, if the amount exceeds 0.08%, a needle-like ferrite structure is easily formed in the cooling process after the self-brazing treatment, and the ductility decreases. Become noticeable. Therefore, 0.0
3 to 0.08%.

Si: 0.1% or less Si is added as a deoxidizing element in the steel smelting process. The addition amount for that purpose is sufficient up to 0.1%. Further, addition of a large amount more than this lowers the ductility, so this is made the upper limit. Mn: 0.05-0.5% Mn is added for the purpose of preventing hot brittleness of steel.
If it is less than 0.05%, the effect will be insufficient, while on the other hand it will be less than 0.1%.
If it exceeds 5%, the ductility decreases.

P: 0.020% or less, P has an effect of increasing the yield strength and tensile strength. However, if added in a large amount, it causes a decrease in ductility and segregates at the crystal grain boundaries, and the strength of the grain boundaries is increased. Lower. Therefore, the content is set to 0.020% or less. Preferably it is 0.008 to 0.015%. S: 0.015% S forms nonmetallic inclusions such as MnS and reduces the workability of the steel sheet. If it is 0.015% or less, the actual harm is avoided, so this is set as the upper limit.

Sol. Al: 0.03 to 0.08% Al is an element added as a deoxidizing agent in the steel smelting process, but in the present invention, not only that, but also a precipitate of AlN is formed. It is added for the purpose of increasing the resistance to coarsening and the penetration of copper during brazing. To obtain these effects, at least 0.03% is required as the sol.Al content (soluble Al content). However, when added in large amounts,
Since the excessive precipitation of AlN lowers the ductility, and the increase in nonmetallic inclusions lowers the steel sheet surface quality (increases in surface flaws), the upper limit is made 0.08%.

N: 0.003-0.008%, N is A
reacts with 1 to form fine precipitates of AlN to increase the coarse-graining resistance of the base steel sheet and prevent the ferrite structure from becoming coarse during the brazing treatment. If the content is less than 0.003%, the precipitation amount of AlN is insufficient, and it is not possible to secure the coarse graining resistance. On the other hand, if you add too much,
This leads to a decrease in ductility due to excessive precipitation of AlN and a hardening of the pipe. Therefore, the upper limit is 0.008%.

B: 0.0008 to 0.002% B, as described above, enhances the workability of the double-wound pipe before the brazing heat treatment, and increases the ferrite crystal grains in the brazing heat treatment. In addition to increasing the coarsening resistance by precipitation at the boundaries, it also prevents molten copper from entering the grain boundaries and suppressing grain boundary embrittlement. Further, in the cooling process following the brazing heat treatment, the solid solution is combined with solid solution N in the base steel to form solid solution N.
It is effective in reducing the amount, softening the pipe, and preventing aging deterioration. To achieve these effects, at least 0.0
008% content is required. If the content exceeds 0.002%, the microstructure of the base steel in the cooling process after brazing is excessively refined and hardened due to needle-like formation, thereby impairing the workability of the pipe. Therefore, the upper limit is 0.002%.

Next, the manufacturing process of the present invention will be described.
First, steel melted to a specified chemical composition in a steelmaking furnace is cast into
The slab is formed by decomposition rolling or continuous casting, and after appropriately treating the slab surface, hot rolling is performed. After the continuous casting, the hot slab may be directly charged into a heating furnace and hot rolled. Hot rolling is performed by a conventional method. In view of the quality of hot-rolled steel sheet and hot-rolling efficiency, the finishing temperature is adjusted to the temperature just above the _Ar3 transformation point, and the winding temperature is about 5
The range of 00 to 700 ° C is appropriate.

After the pickling treatment, the hot-rolled steel sheet is subjected to cold rolling. In cold rolling, it is necessary to reduce the rolling reduction to 40% or more in order to suppress the coarsening of crystal grains and obtain a cold-rolled steel sheet having good ductility. If the rolling reduction exceeds 90%, the effect of refining the crystal grains is saturated, and if the rolling reduction is higher than that, the operation surface becomes disadvantageous due to an increase in the rolling load, so the upper limit is 90%.

[0018] The cold-rolled steel sheet is subjected to a surface cleaning treatment and then subjected to an annealing treatment. In the annealing process, the steel sheet recrystallizes,
In addition, a surface layer portion (AlN-rich layer) in which fine AlN particles are densely dispersed by the nitridation reaction is formed. This annealing treatment is performed by heating in a temperature range of a recrystallization temperature (about 600 ° C.) to 850 ° C. in an atmosphere of a N ₂ —H ₂ mixed gas having a hydrogen concentration of 2% by volume or more. N to form atmosphere
_{The 2-} H ₂ mixed gas is, for example, NX gas (H ₂ : 2vol%, CO: 3
vol%, the balance: N _2), DX gas _{(H 2: 10vol%, CO} : 10vol%, CO
₂ : 7vol%, balance: N ₂ ) etc. can be applied.

The reason that the upper limit temperature of the annealing treatment is set to 850 ° C. is that not only does not require a higher temperature, but also the growth and coarsening of crystal grains are promoted with the higher temperature, and an appropriate ferrite structure is secured. This is because it becomes difficult. The annealing method may be either batch annealing or continuous annealing, but in the case of batch annealing in which a relatively long processing time is given, the ambient temperature is set to about 650 ° C to 720 ° C, and in the case of continuous annealing in which the processing time is short, about It is good to adjust and set it to 750-850 ° C.

The reason why the annealing atmosphere is the N ₂ -H ₂ mixed gas is to secure the metallic luster of the steel sheet by the reducing action and to form an AlN-rich layer on the surface of the steel sheet by the nitriding reaction. The reason why the H ₂ concentration is set to 2% by volume or more is that if the concentration is lower than that, the reducing action is not sufficient, and it is difficult to secure metallic luster. However, if the H ₂ concentration is too high, the nitridation reaction efficiency is reduced due to a relative decrease in the N ₂ concentration, so it is appropriate to set the upper limit to about 95% by volume. In order to achieve the annealing process more efficiently, N ₂
The annealing treatment is preferably performed under the following conditions: the H ₂ concentration of the —H ₂ mixed gas: 10 to 75% by volume, and the dew point of the annealing atmosphere: −10 ° C. or less.

The fine AlN precipitates (particle diameter: about 50 to 100 °) formed on the surface layer of the steel sheet during the annealing process serve as a pinning effect of ferrite recrystallization during the brazing heat treatment of the pipe as described above. A fine ferrite structure (FGS.NO: about 10 or more) is formed on the surface layer to enhance copper penetration resistance. To make this effect sufficient, Al
The layer thickness of the N-rich surface layer portion needs to be 20 μm or more. Further, if the layer thickness is 100 μm or less, there is no substantial adverse effect on the workability of the steel sheet. The thickness of the surface layer is controlled by the composition of the atmosphere gas, the temperature and time of the annealing.

The annealed steel sheet is subjected to copper plating (plating layer thickness: for example, 1 to 5 μm / per side) by temper rolling, continuous electroplating, etc., according to a conventional method, and is a copper-plated steel sheet for a double wound pipe. Finished. The obtained copper-plated steel sheet is formed into a double wound pipe,
It is subjected to a brazing heat treatment (processing temperature: about 1100 to 1150 ° C.) for fusing between the winding surfaces.

In the brazing heat treatment, recrystallization of the base steel sheet (about 900 to 950 ° C.) occurs in the initial stage, and when the pipe temperature reaches the melting point of copper (about 1100 ° C.),
The fusion bonding between the winding surfaces occurs due to the melting of the copper plating layer. As described above, at this point, recrystallization of the base steel sheet has already been completed, and an extremely fine ferrite structure (FGS.NO of about 10 or more) has been formed in the surface layer due to the pinning effect of the fine AlN grains. Part of the solid solution B in the steel sheet is precipitated at the ferrite grain boundary. As the effects of the refinement of the surface layer structure and the precipitation of the grain boundary of B, fusion bonding of the winding surface of the double-wound pipe is achieved while suppressing intrusion of the copper grain boundary and accompanying grain boundary embrittlement.

Further, in the cooling process after the brazing heat treatment, as described above, the reaction between the solid solution B and the solid solution N in the steel (B
N precipitates), the amount of solute N causing hardening is reduced, the effect of improving the aging and ductility of the pipe is brought about, and the BN precipitates are different from the AlN precipitates. Is relatively large, and the yield point (
YP) is advantageously reduced and ductility is improved. Further, as a specified effect of the chemical composition (amount of C, Al, N, B, etc.) of the base steel sheet, needle-like or coarse-grained ferrite structures in the cooling process after the brazing treatment are also suppressed and prevented. The part also has a relatively fine ferrite structure (FGS No. about 6 or more). As these effects, a high degree of workability (elongation rate of about 25% or more) required for expanding processing, flare processing, and the like of the double wound pipe is secured.

Next, the effect of the chemical composition of the base steel sheet on the ductility and workability of the double wound pipe will be specifically described. Fig. 1 shows the effect of C on the base steel sheet on the ductility of the double-wound pipe.
The effect of content is shown. The manufacturing conditions for the test steel plate and pipe (tube diameter: 4.76 mm) are as follows. (1) Chemical composition of base steel sheet (wt%) C: 0.01 to 0.12, Si: 0.008, Mn: 0.30. P: 0.010, S: 0.00
8, sol Al: 0.040, N: 0.0045, B: 0.0015, Fe: Bal. (2) Cold rolling: reduction rate 85%, thickness: 0.335 mm.

(3) Annealing treatment (batch annealing) Atmosphere: N ₂ -30 vol% H ₂ Treatment temperature / time: 670 ° C. × 10 hr (4) Temper rolling: Reduction rate 1% (5) Copper plating: continuous Electroplating, layer thickness 4.5μm (per side) (6) Self-brazing treatment after double winding forming Atmosphere: DX gas (10vol% H ₂ -10vol% CO-6vol% CO ₂ -N
_(2, dew point: + 5 ℃) Processing temperature / time: 1130 ℃ × 1min

As shown in FIG. 1, the double wound pipe is
In the range of 0.03-0.08% of C amount of the base steel sheet,
It has high ductility with an elongation of 25% or more. C amount 0.0
The low ductility in the region of less than 3% is due to the ferrite structure of the steel sheet being excessively coarse. On the other hand, C
The decrease in ductility in the region exceeding 0.08% is due to the increase in the amount of carbide (Fe ₃ C) in the steel and the accompanying excessive refinement, and the hardening of the pipe due to the formation of acicular ferrite structure. by. The expansion and flare workability of a double-wound pipe requires ductility of at least about 25% elongation.
FIG. 1 shows that it is necessary to adjust the C content of the base steel sheet to a range of 0.03 to 0.08% in order to satisfy the requirement.

FIG. 2 shows the relationship between the Al content and the N content of the base steel sheet and the elongation value of the double wound pipe. The manufacturing conditions for the test steel plate and pipe (tube diameter: 4.76 mm) are as follows. (1) Chemical composition of base steel sheet (wt%) C: 0.06, Si: 0.009, Mn: 0.45.P: 0.013, S: 0.008, sol A
l: 0.010 to 0.090, N: 0.0010 to 0.0090, B: 0.0016, Fe: Ba
l (2) Cold rolling: reduction rate 83%, thickness: 0.335 mm

(3) Annealing treatment (batch annealing) Atmosphere: N ₂ -12 vol% H ₂ mixed gas Treatment temperature / time: 650 ° C. × 15 hr (4) Temper rolling: Reduction rate 1% (5) Copper plating: Continuous electroplating, layer thickness 3.0μm (per side) (6) Self-brazing treatment after double winding forming Atmosphere: DX gas (10vol% H ₂ -10vol% CO-6vol% CO ₂ -N
_(2, dew point: + 5 ℃) Processing temperature / time: 1130 ℃ × 1min

Each symbol in FIG. 2 is as follows. ○: Elongation rate of 25% or more △: Elongation rate of less than 25% due to excessive precipitation of AlN ×: Elongation rate of less than 25% due to insufficient precipitation of AlN (coarse ferrite grains) In order to satisfy the ductility (elongation rate of about 25% or more) required for flare processing, the Al content is 0.03 to 0.08%, and the N content is 0.003.
This indicates that the value should be adjusted to the range of 0.008%.

FIG. 3 shows the relationship between the B content of the base steel sheet and the elongation value of the double-wound pipe (tube diameter: 4.76 mm). FIG. 4 shows the B content of the base steel sheet and the ferrite grain size number of the pipe ( FG
FIG. 5 shows the relationship between the B content of the base steel sheet and the copper penetration depth in the pipe base steel. The manufacturing conditions for these test steel plates and double-wound pipes (tube diameter: 4.76 mm) are as follows. (1) Chemical composition of base steel sheet (wt%) C: 0.07, Si: 0.010, Mn: 0.50.P: 0.018, S: 0.005, sol A
l: 0.050, N: 0.0050, B: 0.0004 to 0.0040, Fe: Bal (2) Cold rolling: reduction rate 89%, thickness: 0.335 mm

(3) Annealing treatment (batch annealing) Atmosphere: N ₂ -75 vol% H ₂ mixed gas Treatment temperature / time: 660 ° C. × 12 hr (4) Temper rolling: Reduction rate 1% (5) Copper plating: Continuous electroplating, layer thickness 4.0μm (per side) (6) Self-brazing treatment after double winding forming Atmosphere: DX gas (10vol% H ₂ -10vol% CO-6vol% CO ₂ -N
_(2, dew point: + 5 ℃) Processing temperature / time: 1130 ℃ × 1min

As shown in FIG. 3, the double wound pipe is
When the B content of the base steel sheet is in the range of 0.0008 to 0.002%, the base steel sheet has a high ductility of at least 25%. FIG. 4 shows that the ferrite structure becomes finer as the B content of the base steel sheet increases. B amount is 0.000
If it is less than 8%, a coarse grain structure having a fallite grain size number (FGS.NO) of 6.0 or less, which results in insufficient grain boundary strength,
It becomes impossible to sufficiently suppress and prevent the penetration of the molten copper into the grain boundaries. On the other hand, if the B content exceeds 0.002%, FGS.N0
It is excessively refined to 8.5 or more, and as described above, the ferrite structure becomes acicular in the cooling process after the brazing heat treatment.

Further, as shown in FIG. 5, as a result of the addition of B, the penetration depth of copper into the base steel of the pipe is reduced. When the copper penetration depth exceeds about 20 μm, actual harm such as insufficient ductility due to embrittlement of grain boundaries occurs. B amount 0.000
When the content is 8% or more, the penetration depth of copper can be suppressed to 20 μm or less, and it becomes possible to prevent grain boundary embrittlement and a decrease in pipe ductility due to the embrittlement. As described above, FIGS.
As an effect of adding B, in order to obtain the effect of coarsening of ferrite crystal grains and accompanying penetration of molten copper, grain boundary embrittlement, and prevention of excessive refinement of ferrite structure, B content is set to 0.0
This indicates that the adjustment should be made in the range of 008% to 0.002%.

FIG. 6 shows a hoop of the copper-plated steel sheet of the present invention (a flat sheet cut to a required sheet width before double winding forming),
FIG. 7 shows the ferrite structure of each of the double-wound pipes (brazed heat-treated) manufactured using the hoop (all magnifications of × 10).
0). The manufacturing conditions for the test material are as follows. (1) Chemical composition of base steel sheet (wt%) C: 0.04, Si: 0.007, Mn: 0.25. P: 0.012, S: 0.007, sol A
l: 0.035, N: 0.0035, B: 0.0016, Fe: Bal (2) Cold rolling: Reduction: 80%, Thickness: 0.335 mm

(3) Annealing treatment (batch annealing) Atmosphere gas: N ₂ -15 vol% H ₂ mixed gas Treatment temperature / time: 660 ° C. × 12 hr (4) Temper rolling: Reduction rate 1% (5) Copper plating : Continuous electroplating, layer thickness 2.5μm (per side) (6) Self-brazing treatment after double winding forming Atmosphere: DX gas (10vol% H ₂ -10vol% CO-6vol% CO ₂ -N
_(2, dew point: + 5 ° C) Processing temperature / time: 1130 ° C x 1 min

FIG. 6 (hoop before the double winding forming process)
A pancake-like ferrite structure is exhibited from the surface layer of the steel sheet to the entire internal cross section. Its ferrite grain size (FGS.NO) is 8.3. On the other hand, the ferrite structure of the surface layer of the steel sheet in FIG. 7 (after the brazing treatment of the double-wound pipe) is remarkably finer than the central part of the sheet thickness.
The thickness of the surface layer is about 50 μm, and the grain size FGS.
Is 10-12. The miniaturization of the surface layer portion occurs prior to the melting of the copper plating layer as described above, and is effective in preventing molten copper from entering the grain boundary.

[0038]

Example [1] Production of test material Molten steel that has been melted and component-adjusted by a converter and degassing equipment is subjected to continuous casting to form a slab, and then hot-rolled → pickling of hot-rolled sheet → Cold rolling → Electrolytic cleaning of cold rolled sheet → Annealing treatment → Temper rolling → Copper plating → Double winding forming / brazing process Double winding pipe (tube diameter 4.
76 mm). (1) Steel composition: See Tables 1 and 2 Nos. 1 to 14 are invention examples, and Nos. 51 to 66 are contents of any element (underlined in the table) out of the range of the present invention. It is a comparative example. (2) Hot rolling Heating temperature: 1200 ° C, hot rolling finishing temperature: 895 ° C, hot rolling winding temperature: 500 ° C (3) Cold rolling reduction: 85%, cold rolled sheet thickness: 0.335 mm

(4) Annealing treatment (batch annealing) Atmosphere: N ₂ -H ₂ mixed gas (H ₂ concentration: _{2 to} 75 vol%) Treatment temperature: 650 to 700 ° C., Treatment time: 8 to 15 hr Quality rolling: Reduction rate 1% (6) Copper plating (continuous electroplating): Plating layer thickness 3.0μm
(Per side) (7) Double winding forming process-Forming method: Roll tube forming (hoop width 27.5 mm)-Self-brazing process: Atmosphere: DX gas (10vol% H ₂ -10vol% CO-6vol% CO ₂ -N
_(2, dew point: + 5 ° C) Processing temperature / time: 1130 ° C x 1 min

[2] Evaluation of pipe characteristics (a) Tensile test: According to JIS Z 2241 (using a No. 11 test piece). (b) Ferrite grain size: The cross section of the pipe was corroded with 5% nital, and the grain size number (FGS.NO) was determined by the cutting method (JIS G 0552) (magnification: × 200). (c) Copper penetration depth: After corroding the cross section of the pipe with 5% nital, the XMA analyzer was used to weld the copper (magnification: ×
500) X-ray image of Cu characteristic was taken to measure the penetration depth (μm).

Tables 1 and 2 show the chemical composition of the base steel sheet,
The test results of the product pipe are shown together with the production conditions of the copper plated steel sheet and the double wound pipe. The pipes of Invention Examples Nos. 1 to 14 have an elongation of 25% or more, which is required for pipe expansion and flare processing. The surface layer (layer thickness of about 20 to 10)
The ferrite structure (0 μm) is extremely fine as FGS.NO 10-12. Due to the refinement of the surface layer structure and the effect of adding B, the copper penetration depth is extremely small, about 1 to 5 μm. The thick center also has a relatively fine ferrite structure with FGS.NO of about 7-8. As described above, since the penetration of copper is suppressed and the ferrite structure has an appropriate grain size over the entire thickness, the workability in the pipe expanding process and the flare process is also good.

On the other hand, in Comparative Examples Nos. 51 to 66, No. 51
The low ductility of the No. 52 and No. 52 pipes is due to coarse ferrite grains, large copper penetration depth, and embrittlement of grain boundaries due to insufficient C content of the steel sheet. No.53 and
The poor ductility of the No. 54 pipe is due to the excessive precipitation of carbides (Fe ₃ C) in the steel, excessive refinement of ferrite grains, and the needle-like ferrite structure due to the excessive C content of the base steel sheet. Due to hardening caused by the formation of No.55 and
The poor ductility of No.56 (insufficient base steel sheet AL) and No.59 and No.60 (insufficient base steel sheet N) are due to Al, N
This is because the amount of AlN was small and the amount of AlN precipitated was insufficient, and the ferrite grains were coarsened, so that the penetration depth of copper became large and the grain boundaries became embrittled.

The poor ductility of the pipes of No. 57 and No. 58 (excess of the base steel sheet AL) and No. 61 and No. 62 (excess of the base steel sheet N) are due to the excessive Al and N contents. To be
This is due to excessive precipitation of AlN and excessive refinement of ferrite grains. The low pipe ductility of No. 63 and No. 64 (base steel plate lacking B content) is due to the lack of B content of the base steel plate, the ferrite grains are coarsened, the penetration depth of the molten copper increases, and the grain boundaries become brittle. Due to the occurrence of No.65 and No.6
6 The inferior pipe ductility of the (base steel sheet B excess) is because the excess B content of the base steel sheet caused excessive precipitation of the solute B and excessively fine ferrite grains, and The needle-like ferrite structure is formed and hardened.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

The copper-plated steel sheet for a double-wound pipe according to the present invention has a high resistance to copper penetration in self-brazing treatment, prevents copper intrusion and grain boundary embrittlement caused by the copper penetration, and prevents copper intrusion after the brazing treatment. The ferrite structure is also prevented from becoming coarse or needle-like and the accompanying hardening of the pipe is prevented. Therefore, the obtained double-wound pipe has high ductility, which can prevent processing cracks during pipe expansion and flare processing, and can improve the production yield, simplify the pipe processing process, and streamline and reduce costs. At the same time, effects such as improvement and stabilization of pipe quality are brought about. In addition, it is possible to cope with complicated processing shapes of pipes, and it is possible to expand and diversify applications of double-wound pipes.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the elongation value of a double-wound pipe and the C content of a base steel sheet.

FIG. 2 is a graph showing the relationship between the elongation value of a double-wound pipe and the amounts of Al and N of a base steel sheet.

FIG. 3 is a graph showing the relationship between the elongation value of a double-wound pipe and the B content of a base steel sheet.

FIG. 4 is a graph showing a relationship between a ferrite grain size number (FGS. NO) and a B content of a base steel sheet of a double-wound pipe.

FIG. 5 is a graph showing the relationship between the penetration depth of copper into the base steel of the double-wound pipe and the B content of the base steel sheet.

FIG. 6 is a drawing-substituting micrograph (magnification ×) showing the ferrite structure of a copper-plated steel sheet hoop before forming a double-wound pipe.
100).

FIG. 7 is a micrograph (magnification × 100) showing a ferrite structure of a base steel sheet in a double-wound pipe.

FIG. 8 is a schematic sectional view showing a double wound pipe.

[Explanation of symbols]

 1: Base steel sheet 2: Copper plating layer 3: Copper fusion layer

Claims (2)

[Claims] 1. C: 0.03 to 0.08 by weight%
%, Si: 0.1% or less, Mn: 0.05-0.5%,
P: 0.020% or less, S: 0.015% or less, so
l. Al: 0.03-0.08%, N: 0.003-
0.008%, B: 0.0008% to 0.002%, the balance being substantially composed of Fe, and copper plating is applied to a base steel sheet having a surface layer having a thickness of 20 to 100 μm in which fine AlN precipitates are dispersed. A copper-plated steel sheet for double-wound pipes, which is excellent in copper penetration resistance and workability. 2. In% by weight, C: 0.03 to 0.08%, Si: 0.1% or less, M
n: 0.05 to 0.5%, P: 0.020% or less, S
: 0.015% or less, sol. Al: 0.03 to 0.08
%, N: 0.003 to 0.008%, B: 0.00
After hot rolling a slab consisting of 08 to 0.002% and the remainder substantially made of Fe and cold rolling at a rolling reduction of 40 to 90%, the cold rolled steel sheet is N ₂ -H having a hydrogen concentration of ₂ vol% or more. _{2. The} steel sheet according to claim 1, wherein the steel sheet is annealed in a temperature range from a recrystallization temperature to 850 ° C. in a mixed gas, and then the steel sheet is plated with copper. Manufacturing method of copper plated steel sheet for heavy wound pipe.