JP3659542B2 - Copper-plated steel sheet for double-wound pipes with excellent copper permeation resistance, and method for producing the same - Google Patents

Description

【００３３】
【表２】

【００３４】
【発明の効果】
本発明の二重巻きパイプ用銅めっき鋼板は、セルフ・ブレージング処理における耐銅浸透性が高く、銅の侵入およびそれに起因する粒界脆化を抑制防止し、またブレージング処理後のフェライト組織の粗大化や針状化とそれに付随するパイプの硬質化も抑制防止される。従って得られる二重巻きパイプは、高い延性を有し、拡管加工やフレア加工等における加工割れが抑制防止され、製造歩留りの向上，パイプ品質の向上安定化等の効果が得られる。
【図面の簡単な説明】
【図１】二重巻きパイプの伸び値と母材鋼板のＣ量との関係を示すグラフである。
【図２】二重巻きパイプの伸び値と母材鋼板のＡｌ，Ｎ量との関係を示すグラフである。
【図３】二重巻き成形加工前の銅めっき鋼板フープのフェライト組織を示す図面代用顕微鏡写真（倍率×１００）である。
【図４】二重巻きパイプにおける鋼板のフェライト組織を示す図面代用顕微鏡写真（倍率×１００）である。
【図５】二重巻きパイプを示す模式的断面図である。
【符号の説明】
１: 母材鋼板
２: 銅めっき層
３: 融着層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a copper-plated steel sheet having excellent copper permeation resistance and the like and a method for producing the same for producing a double-pipe pipe used as a brake tube for an automobile, a cooling tube for a refrigerator, or the like.
[0002]
[Prior art]
A double-pipe pipe made of copper-plated steel sheet is a DX gas heated to a copper melting point or higher (eg 1130 ° C) after a copper-plated steel sheet hoop cut into a predetermined width is rolled into a pipe shape with a pipe-making roll. It is manufactured by holding a suitable time (about 1 to 2 minutes), melting the copper plating layer, and performing a so-called self-brazing treatment in which the wound surfaces are fusion bonded. FIG. 5 shows a cross section of a double-wound pipe. Reference numeral 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 winding surfaces is determined by eddy current testing or bending test, etc. Inspected by
Copper-plated steel sheets used for the production of double-pipe pipes are self-brazing heat treatment, the crystal structure of the base steel sheet does not become too coarse (resistance to coarsening), and the penetration of molten copper into the base steel sheet It is required that the steel plate is less likely to cause embrittlement (copper permeation resistance), and has good ductility to withstand tube expansion and flare processing performed after double-winding pipe forming.
[0003]
Conventionally, low carbon aluminum killed steel has been used as the base steel for the copper-plated steel sheet. As an improved material, 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 to 0.1%, P: 0.015% or less, B: 0.0004 to 0.004%, and the balance of the base steel plate made of Fe and inevitable impurities is disclosed. As described therein, this copper-plated steel sheet has effects such as refinement of crystal structure and improvement of weld crack resistance as the B element-containing effect of the base steel sheet.
[0004]
[Problems to be solved by the invention]
Conventional copper-plated steel sheets, even if the base steel sheets belong to the category of low-carbon aluminum killed steel sheets, are not self-adhesive after double-winding pipe forming due to lack of copper permeation resistance, coarse grain resistance, etc.・ In brazing heat treatment, there are many cases where cracks occur during subsequent tube expansion or flare processing, which tends to cause grain boundary embrittlement due to the intrusion of molten copper and decrease in ductility due to acicular or coarsening of the ferrite structure. It is done. As described in the above publication, the addition of element B is effective for refining the ferrite structure. However, in a low-carbon aluminum killed steel sheet having an elemental content of B of about 20 ppm or more, a needle shape is formed during the cooling process after brazing. This produces a ferrite structure and impairs the workability of the pipe.
The present invention is provided with improved coarse grain resistance, copper permeation resistance, etc. to solve the conventional problems related to double-winding pipes and to ensure good workability that can withstand tube expansion processing, flare processing, etc. A copper-plated steel sheet and a method for producing the same are provided.
[0005]
[Means for Solving the Problems]
The copper-plated steel sheet for double-wound pipes of the present invention is in weight%,
C: 0.03 to 0.08%,
Si: 0.1% or less,
Mn: 0.05 to 0.5%,
P: 0.015% or less,
S: 0.015% or less,
sol.Al: 0.03-0.08%,
N: 0.003 to 0.008%,
The balance is composed of a base steel plate substantially composed of Fe and inevitable impurities and a plating layer covering the surface, and the base steel plate has a surface layer portion having a layer thickness of 50 to 100 μm in which fine AlN precipitates are densely dispersed. It is characterized by that.
[0006]
The copper-plated steel sheet for double-wound pipes of the present invention is obtained by hot rolling a steel slab having the above chemical composition and cold rolling it at a reduction ratio of 50 to 90%, and then subjecting the cold rolled steel sheet to a hydrogen concentration of 2 vol%. In the above-mentioned N ₂ —H ₂ mixed gas, it is manufactured by a step of annealing in a temperature range of a recrystallization temperature to 850 ° C. and then subjecting the steel plate to a copper plating treatment.
[0007]
According to the chemical composition of the base steel sheet, particularly the amount of C, Al, and N, the copper-plated steel sheet of the present invention suppresses and prevents the acicularization and coarsening of the ferrite structure in the self-brazing heat treatment of the double-winding pipe, In addition, AlN precipitates (about 50 to 500 mm) densely dispersed in the steel sheet surface layer (layer thickness of about 50 to 100 μm) serve as a pinning effect in the recrystallization of ferrite generated by self-brazing heat treatment and A fine ferrite structure is formed. The crystal grain size FGS.No (JIS G 0552) of the surface layer is remarkably fine, about 10 or more, and by refinement of the surface layer structure, the copper penetration resistance in brazing heat treatment is enhanced and the molten copper enters the grain boundary. And the embrittlement of the grain boundary due to this is suppressed and prevented.
[0008]
The AlN grains in the surface layer portion that enable the refinement of the ferrite structure in the surface layer portion are formed by a nitriding reaction in an annealing process in which an N ₂ —H ₂ mixed gas is used as the atmosphere. As a specified effect such as the amount of Al and N, coarsening is suppressed and prevented, and a relatively fine ferrite structure (FGS. No .: about 6 or more) is provided. Accordingly, in the present invention, unlike the copper-plated steel sheet disclosed in the above publication (Japanese Patent Publication No. 8-14013), it is not necessary to use a refining element such as B element. In addition, since the layer thickness in which the precipitates of AlN in the steel plate surface layer are dispersed abundantly is 50 to 100 μm, the workability of the steel plate is not impaired by the AlN rich layer.
The double-winding pipe produced using the copper-plated steel sheet of the present invention is given good ductility by preventing and preventing coarse graining and intergranular penetration of molten copper and embrittlement caused thereby, The ductility required for tube expansion and flare processing (elongation of about 25% or more is required) can be sufficiently satisfied.
[0009]
The reasons for limiting the chemical composition of the base steel sheet in the present invention are as follows.
C: 0.03-0.08%
From the point of increasing the ductility of the cold-rolled steel sheet, the smaller the amount of C, the better. However, when the amount is too small, the coarse grain resistance is insufficient. Further, the copper permeation resistance also decreases due to the decrease in grain boundary strength. On the other hand, when the amount of C increases, ductility decreases due to an increase in the amount of precipitated carbide. Especially when it exceeds 0.08%, a needle-like ferrite structure is easily formed in the cooling process after the self-brazing treatment, and the ductility is reduced. The decrease becomes noticeable. For this reason, it is 0.03 to 0.08%.
[0010]
Si: 0.1% or less Si is added as a deoxidizing element in the steel melting process. For this purpose, it is sufficient to add up to 0.1%. Moreover, since addition of a larger amount reduces ductility, this is made the upper limit.
Mn: 0.05-0.5%
Mn is added for the purpose of preventing hot brittleness of the steel. If it is less than 0.05%, the effect is insufficient, while if it exceeds 0.5%, ductility is lowered.
[0011]
P: 0.015% or less,
P has the effect of increasing the yield strength and tensile strength, but if added in a large amount, it causes a decrease in ductility, and segregates at the crystal grain boundary, thereby lowering the grain boundary strength. For this reason, it is made 0.015% or less. Preferably it is 0.008 to 0.015%.
S: 0.015%
S forms non-metallic inclusions such as MnS to lower the workability of the steel sheet. If it is 0.015% or less, the actual damage is avoided, so this is the upper limit.
[0012]
sol.Al: 0.03-0.08%
Al is an element added as a deoxidizer in the steel melting process. However, in the present invention, not only that, but a precipitate of AlN is formed, and the coarsening resistance and copper penetration resistance in brazing are improved. It is added for the purpose of enhancing. In order to obtain these effects, at least 0.03% is required as the sol.Al amount (soluble Al amount). However, if added in a large amount, the ductility is lowered due to excessive precipitation of AlN, and the steel sheet surface quality is lowered (increase in surface flaws) due to the increase of nonmetallic inclusions. For this reason, the upper limit of the amount of sol.Al is 0.08%.
[0013]
N: 0.003 to 0.008%,
N reacts with Al to form fine precipitates of AlN to enhance the coarse grain resistance of the base steel sheet and prevent the ferrite structure from becoming coarse in the brazing treatment. If the content is less than 0.003%, the precipitation amount of AlN is insufficient, and the coarsening resistance cannot be ensured. On the other hand, if it is added too much, ductility is lowered due to excessive precipitation of AlN, and the pipe is hardened. For this reason, 0.008% is made the upper limit.
[0014]
Next, the manufacturing process of the present invention will be described.
First, steel melted to a predetermined chemical composition in a steelmaking furnace is made into a slab by ingot-making / decomposition rolling or continuous casting, and the slab surface is appropriately treated, followed by hot rolling. Following the continuous casting, the hot slab may be inserted into a heating furnace as it is and hot rolled. Hot rolling is performed by a conventional method. From the viewpoint of hot rolled steel sheet quality, hot rolling efficiency, etc., the finishing temperature is adjusted to a temperature just above the _Ar3 transformation point, and the coiling temperature is suitably in the range of about 500 to 700 ° C.
[0015]
The hot-rolled steel sheet is subjected to cold rolling after the pickling treatment. In cold rolling, in order to suppress the coarsening of crystal grains and obtain a cold-rolled steel sheet having good ductility, the rolling reduction needs to be 50% or more. When the rolling reduction exceeds 90%, the effect of refining the crystal grains is saturated, and the rolling reduction beyond that only causes a disadvantage of operation due to an increase in rolling load, so 90% is the upper limit.
[0016]
The surface of the cold-rolled steel sheet is subjected to annealing treatment after being purified. In the annealing treatment, the steel sheet is recrystallized, and a surface layer portion (AlN rich layer) in which fine AlN grains are densely dispersed is formed by nitriding reaction. This annealing process is performed by using a N ₂ —H ₂ mixed gas having a hydrogen concentration of 2% by volume or more as an atmosphere and heating in a temperature range of a recrystallization temperature (about 600 ° C.) to 850 ° C. The N ₂ -H ₂ gas mixture is NX gas (H ₂ : 2 vol%, CO: 3 vol%, balance N ₂ ), DX gas (H ₂ : 10 vol%, CO: 10 vol%, (CO ₂ : 7 vol%, balance N ₂ ) may be used (these mixed gases contain CO gas or CO ₂ gas, but the annealing effect is not impaired thereby).
[0017]
The upper limit temperature of the annealing treatment is set to 850 ° C., not only the higher temperature is not required, but also the growth of crystal grains is promoted as the temperature is increased, and it becomes difficult to secure a fine structure. Because. 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 about 650 ° C. to 720 ° C., and in the case of continuous annealing with a short processing time, about It may be adjusted to 750 to 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 due to the reducing action and to form an AlN rich layer by the nitriding reaction. The reason why the H ₂ concentration is specified to be 2% by volume or more is that, if the concentration is lower than that, the reduction action is not sufficient, and it is difficult to ensure the metallic luster. However, if the H ₂ concentration is too high, the nitriding reaction efficiency decreases due to the relative decrease in the N ₂ concentration, so it is appropriate to set the upper limit at about 90% by volume. In order to achieve the annealing treatment more efficiently, it is preferable to perform the annealing treatment under the conditions of H ₂ concentration of N ₂ —H ₂ mixed gas: 10 to 80% by volume and dew point of annealing atmosphere: −10 ° C. or less.
[0018]
Precipitates of AlN on the steel sheet surface layer produced by the nitriding reaction (having a size of about 50 to 500 mm) should be recrystallized into a very fine ferrite structure in the brazing process of the double-pipe pipe. As a refinement effect, high copper permeation resistance is imparted to the steel sheet. In order to ensure sufficient copper permeation resistance, the layer thickness of the AlN-rich surface layer is preferably about 50 μm or more. Moreover, if the layer thickness of the surface layer part does not exceed 100 μm, the workability of the steel sheet is not adversely affected. The thickness of the surface layer portion can be controlled by the atmospheric gas composition of the annealing treatment, the treatment temperature / time, and the like.
[0019]
The annealed steel sheet is subjected to copper plating (plating layer thickness: for example, 1 to 5 μm / per side) by temper rolling and continuous electroplating according to a conventional method to finish a copper-plated steel sheet for a double-winding pipe. . The obtained copper-plated steel sheet is subjected to a brazing heat treatment (processing temperature: about 1100 to 1150 ° C.) that is formed into a double-winding pipe and is fused between the wound surfaces.
[0020]
In the brazing heat treatment, recrystallization (about 900 to 950 ° C.) of the base steel sheet occurs in the initial stage. In the surface layer portion (AlN grain aggregation layer), fine AlN precipitates have a pinning effect in recrystallization of ferrite, and an extremely fine ferrite structure (crystal grain size FGS No. of about 10 or more) is formed. When the temperature of the pipe reaches or exceeds the melting point of copper (about 1100 ° C.), fusion bonding between the wound surfaces due to melting of the copper plating layer occurs. At this time, the base steel plate has already been recrystallized, and a very fine ferrite structure has been formed in the surface layer. As a result of the refinement, the fusion between the winding surfaces is suppressed while preventing the grain boundary penetration of the molten copper. Wear can be achieved. In addition, as described above, as the effect of defining the chemical composition (C, Al, N amount, etc.) of the base steel plate, acicular and coarsening of the ferrite structure in the cooling process after brazing heat treatment is suppressed and prevented, and the base steel plate The ferrite structure at the center of the plate thickness also has a relatively fine ferrite structure (FGS No. about 6 or more).
[0021]
Next, the influence of the chemical composition of the ground steel plate on the ductility and workability of the double-wound pipe will be specifically described.
FIG. 1 shows the influence of the C content of the base steel sheet on the ductility of the double-winding pipe. The manufacturing conditions of the test steel plates and pipes (tube diameter: 4.76 mm) are as follows.
(1) Chemical composition of base steel sheet (wt%)
C: 0.01-0.12, Si: 0.008, Mn: 0.25. P: 0.013, S: 0.006, sol Al: 0.035, N: 0.0040, Fe: Bal.
(2) Cold rolling: reduction ratio 83%, sheet thickness: 0.335 mm.
(3) Annealing treatment (batch annealing)
Atmosphere: N ₂ -10vol% H ₂ gas mixture Processing temperature / time: 660 ° C x 12 hr
(4) Temper rolling: Reduction ratio 1%
(5) Copper plating: Continuous electroplating, layer thickness 5 μm / per side
(6) double-wrap forming after the self-brazing treatment atmosphere: DX gas _{(10vol% H 2 -10vol% CO-} 6vol% CO 2 -N 2, dew point: + 5 ° C.)
Processing temperature / time: 1130 ℃ × 1min
[0022]
As shown in FIG. 1, the double-winding pipe is given high ductility with an elongation of 25% or more in the range of the C content of the base steel sheet of 0.03 to 0.08%. The low ductility in the region where the C content is less than 0.03% is due to excessive coarsening of the ferrite structure of the steel sheet, while the ductility reduction in the region where the C content exceeds 0.08% This is because the pipe was hardened due to the increase in the amount of carbide (Fe ₃ C) and the formation of a needle-like ferrite structure. Ductility with an elongation of about 25% or more is required for pipe expansion and flare workability of double-wound pipes. FIG. 1 shows that the C content of the base steel sheet is 0.03 to satisfy the requirement. It shows that it is necessary to adjust to the range of ˜0.08%.
[0023]
FIG. 2 shows the relationship between the Al amount and N amount of the base steel sheet and the elongation value of the double-winding pipe. The manufacturing conditions of the test steel plate and pipe (tube diameter: 4.76 mm) are as follows.
(1) Chemical composition of base steel sheet (wt%)
C: 0.05, Si: 0.009, Mn: 0.35. P: 0.013, S: 0.006, sol Al: 0.010 to 0.090, N: 0.0010 to 0.0090, Fe: Bal
(2) Cold rolling: 83% reduction, sheet thickness: 0.335 mm
(3) Annealing treatment (batch annealing)
Atmosphere: N ₂ -12 vol% H ₂ gas mixtureTemperature and time: 670 ° C x 10 hr
(4) Temper rolling: Reduction ratio 1%
(5) Copper plating: Continuous electroplating, layer thickness 5μm (per side)
(6) double-wrap forming after the self-brazing treatment atmosphere: DX gas _{(10vol% H 2 -10vol% CO-} 6vol% CO 2 -N 2, dew point: + 5 ° C.)
Processing temperature / time: 1130 ℃ × 1min
[0024]
Each symbol in FIG. 2 is as follows.
○ ... Elongation rate of 25% or more △ ... Elongation rate of AlN is less than 25% × ... AlN precipitation rate is insufficient (ferrite grain coarsening), and elongation rate is less than 25%・ In order to satisfy the ductility required for flare processing (elongation rate of about 25% or more), the Al content is adjusted to 0.03 to 0.08% and the N content is adjusted to a range of 0.003 to 0.008%. I know what to do.
[0025]
FIG. 3 shows a hoop of a copper-plated steel sheet according to the present invention (a flat plate material cut to a required plate width before double-winding forming), and FIG. 4 shows a double-wound pipe (brazing treatment) formed using the hoop. (Completed) shows the ferrite structure of each (both magnification × 100).
The production conditions of the specimens are as follows.
(1) Chemical composition of base steel sheet (wt%)
C: 0.06, Si: 0.010, Mn: 0.45. P: 0.015, S: 0.010, sol Al: 0.060, N: 0.0060, Fe: Bal
(2) Cold rolling: reduction ratio: 83%, sheet thickness: 0.335 mm
(3) Annealing treatment (batch annealing)
Atmosphere gas: N ₂ -15 vol% H ₂ gas mixture Processing temperature and time: 660 ° C x 8 hr
(4) Temper rolling: Reduction ratio 1%
(5) Copper plating: Continuous electroplating, layer thickness 5μm (per side)
(6) Self-brazing treatment atmosphere after double winding molding: DX gas (10 vol% H ₂ -10 vol% CO-6 vol% CO ₂ -N _2, dew point: + 5 ° C)
Processing temperature / time: 1130 ℃ × 1 min
[0026]
FIG. 3 (a hoop before the double winding forming process) exhibits a pancake-like ferrite structure from the surface layer of the steel plate to the entire internal cross section. The ferrite crystal grain size (FGS No.) is 8.3.
On the other hand, the ferrite structure of the steel plate surface layer portion in FIG. 4 (after the brazing treatment of the double-winding pipe) is remarkably finer than the plate thickness center portion. The layer thickness of the surface layer is about 80 μm, and the crystal grain size FGS No is 10-12. This refinement of the surface layer portion is an effect due to recrystallization performed by using fine AlN in the surface layer portion as a pinning for ferrite formation in the brazing process as described above. Further, the refinement of the surface layer portion occurs prior to the melting of the copper plating layer, and suppresses and prevents the penetration of grain boundaries of the molten copper.
[0027]
【Example】
[1] Production of test material Molten steel that has been melted and adjusted by degassing equipment using a converter and degassing equipment is subjected to continuous casting to form a slab, which is hot-rolled → hot-rolled sheet pickled → cold-rolled → Electrolytic cleaning treatment of cold-rolled sheet → annealing treatment → temper rolling → copper plating → double winding forming process and brazing treatment to obtain a 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 62 are comparative examples in which the content of any element (in the table, underlined) is not within the scope of the present invention.
(2) Hot rolling heating temperature: 1230 ° C, hot rolling finishing temperature: 890 ° C, hot rolling coiling temperature: 520 ° C
(3) Cold rolling reduction: 83%, cold rolled sheet thickness: 0.335 mm
[0028]
(4) Annealing treatment (batch annealing)
Atmosphere: N ₂ —H ₂ gas mixture (H ₂ concentration 2-20% by volume)
Processing temperature: 650 ~ 700 ° C, Processing time: 8 ~ 15 hr
(5) Temper rolling: Reduction ratio 1%
(6) Copper plating (continuous electroplating): Plating layer thickness 5μm (per side)
(7) Double winding forming and forming method: Roll pipe making (hoop width 27.4 mm)
・ Self-brazing process:
Atmosphere: DX gas (10 vol% H ₂ -10 vol% CO-6 vol% CO ₂ -N _2, dew point: + 5 ° C)
Processing temperature / time: 1130 ℃ × 1 min
[0029]
[2] Pipe characteristic evaluation
(a) Tensile test: According to JIS Z 2241 (No. 11 test piece used).
(b) Ferrite grain size: The pipe cross-section is corroded with 5% nital, and the grain size number (FGS No) is determined by the cutting method (JIS G 0552) (magnification: × 200).
(c) Copper penetration depth: After the pipe cross-section was corroded with 5% nital, a Cu characteristic X-ray image of the copper welded part (magnification: × 500) was taken with an XMA analyzer and the penetration depth ( μm).
[0030]
Tables 1 and 2 show the product pipe test results together with the chemical composition of the base steel sheet, the production conditions of the copper-plated steel sheet and the double-winding pipe.
The pipes of Invention Examples Nos. 1 to 14 have an elongation of 25% or more required for pipe expansion processing and flare processing. In addition, the ferrite structure of the surface layer portion (layer thickness of about 50 to 100 μm) is remarkably fine as FGS No. 10 to 12, so that the copper penetration resistance is high and the penetration depth of copper is about 3 to 11 μm. And few. The center of the wall also has a relatively fine ferrite structure of FGS No. 8 or more. As described above, the penetration of copper is suppressed, and since the fine ferrite structure is formed over the entire thickness, the workability in tube expansion processing and flare processing is also good.
[0031]
On the other hand, in Comparative Examples Nos. 51 to 62, the ductility of the pipes No. 51 and No. 52 is low because of the insufficient amount of C in the steel sheet, the ferrite grains are coarsened, and the copper penetration depth is large. This is because the grain boundaries become brittle.
The reason why the ductility of the No. 53 and No. 54 pipes is inferior is that the amount of carbon (Fe ₃ C) in the steel is excessively precipitated due to the excessive amount of C in the base steel sheet, and the needle-like ferrite structure is It is hardened by being formed.
The ductility of pipes No. 55 and No. 56 (base steel sheet Al amount shortage) and No. 59 and No. 60 (base steel sheet N shortage amount) is inferior. This is because the ferrite grains are coarsened and the copper penetration depth becomes large, and the grain boundaries become brittle.
In addition, the ductility of pipes No. 57 and No. 58 (excessive amount of base steel sheet Al) and No. 61 and No. 62 (excessive amount of base steel sheet N) is too low. This is because AlN is excessively precipitated and hardened.
[0032]
[Table 1]

[0033]
[Table 2]

[0034]
【The invention's effect】
The copper-plated steel sheet for double-wound pipes of the present invention has high copper permeation resistance in self-brazing treatment, suppresses and prevents copper intrusion and grain boundary embrittlement resulting therefrom, and the ferrite structure after brazing treatment is coarse It is also possible to suppress or prevent the formation of needles and needles and the accompanying hardening of pipes. Therefore, the obtained double-wound pipe has high ductility, and prevents cracking in pipe expansion processing, flare processing, and the like, and effects such as improvement in manufacturing yield and improvement and stabilization of pipe quality can be obtained.
[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 amount of a base steel plate.
FIG. 2 is a graph showing the relationship between the elongation value of a double-winding pipe and the amounts of Al and N of a base steel plate.
FIG. 3 is a drawing-substituting micrograph (magnification × 100) showing a ferrite structure of a copper-plated steel sheet hoop before the double winding forming process.
FIG. 4 is a drawing-substituting micrograph (magnification × 100) showing a ferrite structure of a steel plate in a double-winding pipe.
FIG. 5 is a schematic cross-sectional view showing a double-winding pipe.
[Explanation of symbols]
1: Base steel plate 2: Copper plating layer 3: Fusion layer

Claims (2)

% By weight
C: 0.03 to 0.08%,
Si: 0.1% or less,
Mn: 0.05 to 0.5%,
P: 0.015% or less,
S: 0.015% or less,
sol.Al: 0.03-0.08%,
N: 0.003 to 0.008%,
The balance is composed of a base steel plate substantially composed of Fe and inevitable impurities and a plating layer covering the surface, and the base steel plate has a surface layer portion with a layer thickness of 50 to 100 μm in which fine AlN precipitates are densely dispersed. A copper-plated steel sheet for double-winding pipes with excellent copper permeation resistance and other characteristics. % By weight
C: 0.03 to 0.08%,
Si: 0.1% or less,
Mn: 0.05 to 0.5%,
P: 0.015% or less,
S: 0.015% or less,
sol.Al: 0.03-0.08%,
N: 0.003 to 0.008%,
The slab composed of the remaining Fe and inevitable impurities is hot-rolled and cold-rolled at a reduction rate of 50 to 90%, and then the cold-rolled steel sheet is recrystallized in an N ₂ —H ₂ mixed gas having a hydrogen concentration of ₂ vol% or more. A method for producing a copper-plated steel sheet for double-wound pipes, which is excellent in copper permeation resistance and the like, characterized by annealing in a temperature range of ˜850 ° C. and then subjecting the steel sheet to copper plating.

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