JPH11199972A - Copper plated steel sheet for doubly wound pipe excellent in copper interstition resistance or the like and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a copper plated steel sheet excellent in copper interstition resistance or the like and used for producing a doubly wound pipe and to provide a method for producing it. SOLUTION: This copper plated steel sheet is composed of a base steel sheet composed of 0.03 to 0.08%, C, <=0.1% Si, 0.05 to 0.5% Mn, <=0.020% P, <=0.015% S, 0.03 to 0.08% sol.Al, 0.003 to 0.008%. N, 0.0008 to 0.002% B, and the balance substantial Fe with inevitable impurities and a plating layer covering the surface. This copper plated steel sheet can be produced by subjecting a slab having the above chemical compsn. to hot rolling, subjecting it to cold rolling at 40 to 90% draft, thereafter subjecting the cold rolled steel sheet to annealing treatment in the temp. range of the recrystallization temp. to 850 deg.C in a gaseous mixture of N2 -H2 , with >=2 vol.% hydrogen concn. and subsequently executing copper plating treatment.

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 the like for producing double-wound pipes used as brake tubes of automobiles and cooling tubes of refrigerators, and a method for producing the same. About.

[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. 6 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 if the base steel sheet belongs to the category of low-carbon aluminum-killed steel sheet, has a lack of copper penetration resistance, coarse-graining resistance and the like. In the self-brazing heat treatment after forming the double-wound pipe, it is easy to cause grain boundary embrittlement due to infiltration of molten copper and a decrease in ductility due to needle-like and coarsening of the ferrite structure. In many cases, cracks occur. As described in the above-mentioned publication, the addition of the element B to the low-carbon aluminum-killed steel is effective in refining the ferrite structure.
Depending on the content of sol. 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]

Means for Solving the Problems The copper-plated steel sheet for a double-wound pipe according to the present invention has a C content of 0.03-0.08% by weight.
%, Si: 0.1% or less, Mn: 0.05-0.5%,
P: 0.020% or less, S: 0.015% or less, sol.A
l: 0.03 to 0.08%, N: 0.003 to 0.00
8%, B: 0.0008 to 0.002%, and the balance is substantially composed of a base steel sheet substantially composed of Fe and unavoidable impurities and a plating layer covering the surface thereof.

[0006] The copper-plated steel sheet of the present invention has the above chemical composition.
Hot-rolled steel slab having a rolling reduction of 40-90%
After cold-rolling, the cold-rolled steel sheet is subjected to a hydrogen concentration of 2 vol% or more.
N _Two-H_TwoRecrystallization temperature to 850 ° C in mixed gas
In the zone where the steel sheet is annealed, and then the steel sheet is plated with copper.
It is manufactured by the process. Using the copper plated steel sheet of the present invention
The manufactured double-wound pipe has the chemical composition of the base steel sheet,
According to the provisions of C, Al, N and B contents,
Infiltration of molten copper during graining heat treatment and grain boundary brittleness
Can be effectively prevented and can be used for brazing heat treatment.
In the subsequent cooling process, the ferrite structure becomes finer and needle-like.
In addition to the reduction in the amount of solute N in steel.
Thus, the effect of softening the pipe is obtained. With these effects
To provide improved workability, pipe expansion and flare processing
Required ductility (elongation of about 25% or more is required)
) Can be sufficiently satisfied.

In the improvement of the above-mentioned properties of the double-wound pipe made of the copper-plated steel sheet of the present invention, the effect of adding an appropriate amount of B is important. In the brazing heat treatment of double-wound pipes, it contributes to the prevention of coarsening of ferrite crystal grains and the suppression of infiltration of molten copper at grain boundaries, and in the cooling process following brazing, the ferrite structure becomes acicular and excessively fine. It is effective in preventing suppression, softening the pipe, and preventing aging deterioration. Furthermore, the effect of improving the formability in the double-winding pipe forming process before the brazing heat treatment is provided.

[0008] That is, since solid solution B in steel has a stronger affinity for N than solid solution Al, the solid solution B in steel in the winding stage of a hot-rolled steel plate and the annealing stage of a cold-rolled steel plate is increased. To form a precipitate of boron nitride (BN).
Aluminum nitride (AlN) forms fine precipitates, whereas BN forms larger precipitates than AlN,
Therefore, compared to AlN precipitates as nitrides generated by precipitation,
This is advantageous for softening the base steel sheet. Therefore, the yield point (Y
The effect of softening the base steel sheet by reducing P) and improving the ductility (El) is obtained, the formability of the double-wound pipe performed before the brazing heat treatment is improved, and the productivity of pipe forming is enhanced.

In the brazing heat treatment (1100 to 1150 ° C.) of a double-wound pipe, in the initial stage (about 950 to 1050 ° C.), a part of the BN precipitate is decomposed, and solute B and solute N are dissolved. Generate The solute B precipitates at the ferrite crystal grain boundaries as the temperature of the pipe rises. At the stage where the pipe temperature is equal to or higher than the melting point of copper (about 1100 ° C.), the copper plating layer is melted, and fusion bonding of the winding surface occurs, solute B is already precipitated at the ferrite crystal grain boundary, As an effect of strengthening the grain boundary by the precipitation, fusion bonding of the wrapped surface is achieved while suppressing and preventing penetration of the molten copper into the grain boundary. In addition, B precipitated at the grain boundaries suppresses abnormal growth of crystal grains and is effective in preventing ferrite structure from becoming coarse.

Further, in the cooling process following the brazing heat treatment, the solute B in the steel combines with the solute N to form BN precipitates. Note that AlN is also decomposed into Al and N in the brazing heat treatment process and is dissolved in steel, respectively. However, since the cooling rate in this cooling process is relatively high, Al and N (The formation of AlN precipitates) hardly occurs. On the other hand, B forms BN precipitates because it has a stronger affinity for N than Al. The BN precipitate formation reaction reduces the amount of solute N in the base steel, and as a result, the pipe is softened. In addition, the BN precipitates are relatively large, which advantageously acts to soften the base steel. As these effects, processing of double wound pipes (expansion processing, flare processing, etc.)
It is possible to secure a high degree of workability (elongation rate of about 25% or more) required for (1).

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 amount of C, the better. However, if the amount is too small, the coarse graining resistance will be insufficient. Further, the copper penetration resistance is also reduced due to the decrease in the grain boundary strength. On the other hand, when the C content is increased, the ductility is reduced due to an increase in the amount of carbide precipitation. In particular, 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 drop is remarkable. Therefore, the content is set to 0.03 to 0.08%.

Si: 0.1% or less Si is added as a deoxidizing element in the steel melting 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, when 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. For this reason,
0.020% or less. Preferably 0.008-0.
015%. S: 0.015% S forms nonmetallic inclusions such as MnS and causes work cracking. 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 the present invention is not limited to this. 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 during the brazing heat treatment. Precipitates at the grain boundaries to increase the coarse-graining resistance, and suppresses and prevents the penetration of the molten copper into the grain boundaries and the embrittlement of the grain boundaries. Further, in the cooling process following the brazing heat treatment, the steel is combined with the solute N in the base steel to reduce the amount of solute N, soften the pipe, and effectively prevent aging deterioration. To obtain these effects, a content of at least 0.0008% is required. However, 0.00
If the content exceeds 2%, the microstructure of the base steel structure in the cooling process after brazing becomes excessively fine, and the base steel becomes hardened due to acicularization.
The workability of the pipe is impaired. 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%.

The cold rolled steel sheet is subjected to an annealing treatment after its surface is purified. In the annealing process, the steel sheet recrystallizes. In this annealing treatment, an N ₂ -H ₂ mixed gas having a hydrogen concentration of 2% by volume or more is used as an atmosphere, and a recrystallization temperature (about 600 ° C.) to 850 ° C.
This is performed by heating in the temperature range described above. The upper limit of the processing temperature is set to 850 ° C. because not only does not require a higher temperature, but also a higher temperature causes a coarsening of the crystal structure. 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 annealing temperature is set to about 650 ° C to 720 ° C.
° C and in the case of continuous annealing with a short treatment time,
Preferably, the temperature is adjusted to 850 ° C.

The reason why the N ₂ -H ₂ mixed gas is used as the annealing atmosphere is to secure the metallic luster of the steel sheet by the reducing action, and the H ₂ concentration is set to 2% by volume or more. If the concentration is lower than this, the reducing action is not sufficient and it is difficult to secure the metallic luster. Its N ₂
As an H ₂ mixed gas, for example, NX gas (H ₂ : 2 vol
%, CO: 3 vol%, balance N ₂ ), DX gas (H ₂ : 10 vol%, C
_{O: 10 vol%, CO 2} : 7 vol%, may be used, balance N _2), etc. (These mixed gas contains CO and CO _2, thereby to impair the annealing effect Absent).
The H ₂ concentration of the atmospheric gas is increased, and the reducing action is enhanced. However, when the concentration exceeds about 80% by volume, the effect is saturated, and a further increase in the concentration is accompanied by a disadvantage in operation such as an increase in cost. . Therefore, the upper limit is preferably about 80% by volume.
From the viewpoints of processing efficiency and cost, the concentration of H ₂ : 10 to 7
It is appropriate to perform the annealing treatment under the condition of 5% by volume and the dew point of the annealing atmosphere: -10 ° C or less.

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 the copper-plated steel sheet for a double-wound pipe. Finished. The obtained copper-plated steel sheet is formed into a double wound pipe,
Brazing heat treatment for fusing between wound surfaces (processing temperature: about 1100 to 1150 ° C, processing time: about 1 to 2 minutes)
Attached to

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: 0.25. P: 0.013, S: 0.00
6, sol Al: 0.035, N: 0.0040, B: 0.0015, Fe: Bal. (2) Cold rolling: reduction of 83%, thickness: 0.335 mm.

(3) Annealing treatment (batch annealing) Atmosphere: N ₂ -12 vol% H ₂ Treatment temperature / time: 660 ° C. × 12 hr (4) Temper rolling: Reduction rate 1% (5) Copper plating: continuous Electroplating, layer thickness 3.5μm (per side) (6) Self-brazing treatment after double winding molding 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,
High ductility with an elongation of 25% or more is given. The low ductility in the region where the C content is less than 0.03% is due to the ferrite structure of the steel sheet being excessively coarsened. On the other hand, the decrease in ductility in the region where the C content exceeds 0.08% is due to an increase in the amount of carbide (Fe ₃ C) in the steel and the hardening of the pipe due to the formation of a needle-like ferrite structure. The expansion and flare workability of a double wound pipe requires an elongation of about 2
FIG. 1 shows that the C content of the base steel sheet is set to 0.03 to 0.03% in order to satisfy the requirement.
This indicates that it is necessary to adjust to the range of 8%.

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: 0.05, Si: 0.009, Mn: 0.35.P: 0.013, S: 0.006, sol A
l: 0.010 to 0.090, N: 0.0010 to 0.0090, B: 0.0014, 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: 670 ° C. × 10 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

Each symbol in FIG. 2 is as follows. ○: Elongation rate of 25% or more △: Elongation of less than 25% due to excessive precipitation of AlN ×: Elongation of less than 25% due to insufficient precipitation of AlN (coarse ferrite grains) -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.
It can be seen that it should be adjusted to the range of 0.008%.

FIG. 3 shows the relationship between the B content of the base steel plate and the elongation value of the double-wound pipe, and FIG. 4 shows the relationship between the B content of the base steel plate and the ferrite grain size number (FGS.NO) of the pipe. FIG. 5 shows the relationship between the B content of the base steel sheet and the copper penetration depth of the pipe into the 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: 0.45.P: 0.018, S: 0.005, sol A
l: 0.040, N: 0.0050, B: 0.0004 to 0.004, Fe: Bal (2) Cold rolling: 80% reduction, thickness: 0.335 mm

(3) Annealing treatment (batch annealing) Atmosphere: N ₂ -12 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,
The infiltration of the molten copper into the grain boundaries cannot be sufficiently suppressed and prevented. On the other hand, if the B content exceeds 0.002%, FGS.N0 8.5
The fine grain structure described above is obtained, and the ferrite structure becomes acicular in the cooling process after the brazing 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 grain boundary embrittlement and a decrease in pipe ductility due to the embrittlement can be prevented. Thus, FIGS. 3 to 5 show that the B content is 0.0008% to 0.002%, so that the ferrite crystal grains are coarsened, the molten copper is infiltrated, the grain boundary is embrittled, and the ferrite structure is increased. It can be seen that excessive miniaturization can be prevented.

[0032]

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: 1230 ° C, hot rolling finishing temperature: 890 ° C, hot rolling winding temperature: 510 ° C (3) Cold rolling reduction: 80%, 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%) Processing temperature: 650 to 700 ° C., Processing time: 8 to 15 hr (5) Tempering Rolling: rolling reduction 1% (6) Copper plating (continuous electroplating): plating layer thickness 3.5μm
(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 characteristics of pipe (a) Tensile test: According to JIS Z 2241 (using No. 11 test piece). (b) Ferrite particle size: The cross section of the pipe was corroded with 5% nital, and the particle size number (FGS.N0) was determined by the cutting method (JIS G 0552) (magnification: × 200). (c) Copper penetration depth into the base steel of the pipe:
% Nital, and by XMA analyzer,
A copper characteristic X-ray image of a copper welded portion (magnification: × 500) 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 high copper penetration resistance, and the copper penetration depth is as small as about 3 to 9 μm. The ferrite structure also has a moderate grain size of about 7 to 8 FGS.NO. As described above, the infiltration of copper is suppressed, and the ferrite structure has an elongation of 25% or more, which is required for pipe expansion and flaring as an effect of having an appropriate grain size.

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 the ferrite grains being coarsened, the copper penetration depth being large, and the grain boundaries being embrittled due to the lack of C content in the steel sheet. No.53 and
The poor ductility of No. 54 pipe is due to the excessive amount of carbide (Fe ₃ C) in the steel due to the excessive C content of the base steel sheet and the formation of acicular ferrite structure. Is hardened. The poor ductility of the pipes of No. 55 and No. 56 (insufficient Al content of the base steel plate) and No. 59 and No. 60 (insufficient N content of the base steel plate) is due to the insufficient amount of AlN precipitation and coarse ferrite grains. This is because the penetration depth of copper is increased and the grain boundaries are embrittled.

The inferior ductility of the pipes of No. 57 and No. 58 (base steel sheet with excessive amount of Al) and No. 61 and No. 62 (base steel sheet with excessive amount of N) are due to excessive precipitation of AlN in the steel. Due to hardening. The low ductility of the No. 63 and No. 64 pipes (insufficient B content of the base steel plate) is because the B content of the base steel plate is small, ferrite grains are coarsened, copper penetration depth is increased, and grain boundary Due to embrittlement. No.65 and No.66
The reason why the pipe ductility of (base steel sheet B content is excessive) is low is that the base steel sheet has too much B content, so that the solute B content precipitates excessively,
Hardening is caused by excessively fine ferrite grains and the formation of an acicular ferrite structure.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

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, suppresses copper penetration and grain boundary embrittlement resulting from the penetration, and furthermore, after 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, processing cracks in pipe expansion processing, flare processing, and the like are suppressed, and effects such as improvement in production yield, improvement in pipe quality, and stabilization of pipe quality are 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 content of a base steel sheet.

Fig. 2 Elongation value of double wound pipe and Al, N of base steel sheet
It is a graph which shows the relationship with quantity.

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 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, sol.
0.03 to 0.08%, N: 0.003 to 0.008
%, B: 0.0008 to 0.002%, with the balance being a base steel sheet substantially composed of Fe and unavoidable impurities and a plating layer covering the surface thereof, and having excellent copper penetration resistance and the like. Copper plated steel sheet for heavy wound pipe. 2. In% by weight, C: 0.03 to 0.08%, Si: 0.1% or less, M
n: 0.05-0.5%, P: 0.020% or less, S:
0.015% or less, sol.Al: 0.03-0.08%,
N: 0.003-0.008%, B: 0.0008-
A slab consisting of 0.002%, balance Fe and unavoidable impurities is hot-rolled and cold-rolled at a rolling reduction of 40 to 90%, and then a cold-rolled steel sheet is mixed with 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 a double-wound pipe having excellent copper penetration resistance and the like, wherein the steel sheet is subjected to an annealing treatment in a temperature range of a recrystallization temperature to 850 ° C. and then to a copper plating treatment.