JP3720185B2 - Copper-plated steel sheet for double-wound pipes excellent in copper penetration resistance and workability, etc. and method for producing the same - Google Patents

Description

【００４５】
【表２】

【００４６】
【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入性が高く、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，パイプ加工工程の簡素化などの合理化・低コスト化を可能とすると共に、パイプ品質の向上安定化等の効果をもたらす。またパイプの加工形状の複雑化への対応が可能となり、二重巻きパイプの用途の拡大・多様化を可能とするものである。
【図面の簡単な説明】
【図１】二重巻きパイプの伸び値と素地鋼板の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素地鋼板のAl,N量との関係を示すグラフである。
【図３】二重巻きパイプの伸び値と素地鋼板のＢ量との関係を示すグラフである。
【図４】二重巻きパイプの素地鋼板のフェライト粒度番号（FGS.NO）とＢ量との関係を示すグラフである。
【図５】二重巻きパイプの素地鋼への銅の浸入深さと素地鋼板のＢ量との関係を示すグラフである。
【図６】二重巻きパイプ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７】二重巻きパイプにおける素地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８】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: 素地鋼板
２: 銅めっき層
３: 銅の融着層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a copper-plated steel sheet excellent in copper penetration resistance and workability for manufacturing a double-winding pipe used as a brake tube for an automobile, a cooling tube for a refrigerator, and the like, and a method for manufacturing the same.
[0002]
[Prior art]
A double-winding pipe made of copper-plated steel sheet is made of DX that is heated to a copper melting point or higher (for example, 1130 ° C) after a copper-plated steel sheet hoop cut into a predetermined width is rolled into a pipe shape by a pipe-making roll. It is manufactured by holding a suitable time (about 1 to 2 minutes) in a gas, melting the copper plating layer, and performing a so-called self-brazing treatment in which the wound surfaces are fusion bonded. FIG. 8 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
[0003]
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 (coarse grain resistance), and the penetration of molten copper into the base steel sheet It is required that the steel sheet is less likely to cause embrittlement (copper penetration resistance) and has good ductility to withstand tube expansion processing and flare processing performed after double-winding pipe forming.
Low carbon aluminum killed steel is used as the base steel for the above copper-plated steel sheet. In Japanese Patent Publication No. 8-14013, C: 0.01 to 0.15%, Si: 0.1% or less, Mn: 0.05 to 0.6%, Al: 0.003 ˜0.1%, P: 0.015% or less, B: 0.0004 to 0.004%, and the balance is disclosed by subjecting a base steel plate made of Fe and inevitable impurities 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]
[Problems to be solved by the invention]
Conventional copper-plated steel sheets, even if the base steel sheet belongs to the category of low-carbon aluminum killed steel sheet, due to the lack of copper penetration resistance, coarse grain resistance, etc., self- In brazing heat treatment, it tends to cause grain boundary embrittlement due to the intrusion of molten copper and decrease in ductility due to acicular and coarsening of the ferrite structure, and there are many cases where cracks are generated in subsequent tube expansion and flare processing. . As described in the above publication, the addition of B element in low carbon aluminum killed steel is effective for refinement of the ferrite structure, but the amount of B addition and the content of C, sol. As a result, the ferrite structure becomes coarse due to brazing heat treatment, grain boundary embrittlement due to copper intrusion, or hardening due to excessive refinement or acicularization of the ferrite structure during the cooling process after brazing treatment. For this reason, it is difficult to guarantee sufficient formability required for pipe expansion processing and flare processing after pipe making.
The present invention is provided with improved coarse grain resistance, copper penetration resistance, etc. to eliminate 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% by weight,
C: 0.03 to 0.08%,
Si: 0.1% or less,
Mn: 0.05-0.5%,
P: 0.020% or less,
S: 0.015% or less,
Sol.Al: 0.03 to 0.08%,
N: 0.003 to 0.008%,
B: 0.0008 to 0.002%,
The balance consists of Fe and inevitable impurities , and a copper base plate is applied to a base steel plate having a surface layer with a layer thickness of 20 to 100 μm in which fine AlN precipitates are dispersed.
[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 rate of 40 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 annealing at a recrystallization temperature to 850 ° C. and then copper plating the steel sheet.
The double-winding pipe manufactured using the copper-plated steel sheet of the present invention has a ferrite structure needle in the self-brazing heat treatment as an effect of containing an appropriate amount of C, Al, N and B as a chemical composition of the base steel sheet. In addition to being prevented from being formed and coarsened, high copper penetration resistance prevents copper penetration and resulting grain boundary embrittlement, and improves the aging of the base steel sheet. As these effects, high ductility (an elongation rate of about 25% or more is required) necessary for tube expansion processing and flare processing is ensured.
[0007]
In the material improvement 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 precipitates on the surface layer of the steel sheet are formed by a nitriding reaction in an annealing process of a cold-rolled steel sheet using an N ₂ —H ₂ mixed gas as an atmosphere. This AlN precipitate is a remarkably fine particle having a particle size of about 50 to 500 mm, and becomes a pinning effect in the recrystallization process of ferrite in the brazing heat treatment of the double-wound pipe, and the remarkably fine ferrite on the surface of the base steel sheet. Form an organization. Its surface structure is ferrite grain size number FGS. NO (JIS G 0552) It is remarkably fine, about 10 or more, and has high resistance to intergranular penetration of molten copper.
[0008]
On the other hand, B in steel added to the base steel plate has a stronger affinity with N than Al, and it has priority over solute N in steel at the winding stage of hot-rolled steel sheet and annealing process of cold-rolled steel sheet. Bond together to form BN precipitates. This BN precipitate is partly decomposed to form solute B and solute N at the initial stage (about 950 to 1050 ° C.) of the brazing heat treatment (1100 to 1150 ° C.) of the double-wound pipe. The solid solution B precipitates at the ferrite crystal grain boundary until the double-winding pipe temperature reaches the melting point of copper (about 1100 ° C.) or higher, and strengthens the ferrite crystal grain boundary. By this grain boundary strengthening, the grain boundary penetration of molten copper is suppressed and prevented. Further, B precipitated at the grain boundary suppresses the abnormal growth of the ferrite crystal and is effective in preventing the coarsening of the structure.
[0009]
Further, in the cooling process following the brazing heat treatment (cooling rate: about 20 to 40 ° C./second), the solid solution B in the steel combines with N to form BN precipitates. AlN precipitates are also partly decomposed into Al and N by the brazing heat treatment (about 1130 ° C.) and are dissolved in the steel, but the cooling rate after the brazing heat treatment is relatively high. Almost no reaction between Al and N (precipitation of AlN) occurs. On the other hand, since B has a stronger affinity for N than Al, it binds to solute N and produces BN precipitates even at a relatively high cooling rate. By the formation reaction of this BN precipitate, the amount of dissolved N (causing hardening) is reduced, the aging property of the pipe is improved, and the ductility is increased. 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, the precipitation of BN can provide a remarkable improvement effect on the tube-forming workability of the double-winding pipe performed before the brazing heat treatment.
[0010]
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, if the amount is too small, the coarsening resistance of the steel sheet is insufficient in the self-brazing heat treatment. In addition, the resistance to copper penetration also decreases due to a 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%.
[0011]
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.
[0012]
P: 0.020% 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 into 0.020% 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.
[0013]
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 resistance to coarsening and copper penetration in brazing is achieved. 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 decreases due to excessive precipitation of AlN, and the steel sheet surface quality decreases (increases in surface flaws) due to an increase in non-metallic inclusions, so 0.08% is made the upper limit.
[0014]
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.
[0015]
B: 0.0008 to 0.002%
As described above, B improves the tube-forming workability of the double-winding pipe in the stage before the brazing heat treatment, and in the brazing heat treatment, it precipitates at the grain boundaries of the ferrite crystal grains and improves the resistance to coarsening. In addition, it suppresses and prevents grain boundary penetration and grain boundary embrittlement of molten copper. Further, in the cooling process subsequent to the brazing heat treatment, it combines with the solid solution N in the base steel to reduce the amount of the solid solution N, soften the pipe, and to prevent aging deterioration. In order to obtain these effects, a content of at least 0.0008% is required. If it exceeds 0.002%, excessive refinement of the base steel structure in the cooling process after brazing and hardening due to acicularization will be caused, and the workability of the pipe will be impaired. For this reason, the upper limit is made 0.002%.
[0016]
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.
[0017]
The hot-rolled steel sheet is subjected to cold rolling after the pickling treatment. In cold rolling, in order to suppress coarsening of crystal grains and obtain a cold-rolled steel sheet having good ductility, it is necessary to set the rolling reduction to 40% or more. If the rolling reduction exceeds 90%, the effect of refining the crystal grains is saturated, and a rolling reduction beyond that causes a disadvantage in operation due to an increase in rolling load, so 90% is made the upper limit.
[0018]
The cold-rolled steel sheet is subjected to a surface purification treatment and subjected to an annealing treatment. 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 a 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 mixed gas forming the atmosphere is, for example, NX gas (H ₂ : ₂ vol%, CO: 3 vol%, balance: N ₂ ), DX gas (H ₂ : 10 vol%, CO: 10 vol%, CO ₂ : 7vol%, balance: N ₂ ) etc. can be applied.
[0019]
The upper limit temperature of the annealing process is set to 850 ° C., not only does not require a higher temperature, but also the growth of crystal grains is promoted as the temperature rises, and it is difficult to secure an appropriate ferrite structure. Because it becomes. 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.
[0020]
The reason why the annealing atmosphere is 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 nitriding reaction on the steel sheet surface layer. 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 reducing 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 95% 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 75% by volume and dew point of annealing atmosphere: −10 ° C. or less.
[0021]
The fine AlN precipitates (grain size: about 50 to 100 mm) formed on the steel sheet surface layer during the annealing process become the pinning effect of ferrite recrystallization in the brazing heat treatment process of the pipe as described above and are fine on the steel sheet surface layer. A strong ferrite structure (FGS.NO: about 10 or more) to improve copper penetration resistance. In order to make this effect sufficient, the layer thickness of the AlN-rich surface layer portion needs to be 20 μm or more. Moreover, 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 atmosphere gas composition of the annealing process, the processing temperature / time, and the like.
[0022]
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.
[0023]
In the brazing heat treatment, recrystallization (about 900 to 950 ° C.) of the base steel plate occurs at the initial stage, and when the pipe temperature reaches the melting point of copper (about 1100 ° C.), the space between the winding surfaces due to the melting of the copper plating layer Fusion bonding occurs. As mentioned above, recrystallization of the base steel plate has already been completed at this point, and the surface layer has a very fine ferrite structure (FGS.NO of about 10 or more) 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 boundaries. As an effect of the refinement of the surface layer structure and the grain boundary precipitation of B, fusion bonding of the lap surfaces of the double-winding pipe is achieved while preventing the grain boundary penetration of copper and the accompanying grain boundary embrittlement.
[0024]
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 (generation of BN precipitates) occurs, and the amount of the solid solution N that causes the hardening is reduced. However, the aging of the pipe is improved and the ductility is improved. The BN precipitates are relatively large in size unlike the AlN precipitates, and the yield point (YP) of the base steel sheet is reduced. This is advantageous for improving ductility. In addition, as a result of the regulation of the chemical composition (C, Al, N, B amount, etc.) of the base steel plate, it is possible to prevent and prevent the ferrite structure from becoming acicular and coarse in the cooling process after brazing treatment, and to center the plate thickness. The part is also given a relatively fine ferrite structure (FGS No. approx. 6 or more).
As these effects, a high degree of workability (elongation rate of about 25% or more) required for pipe expansion processing and flare processing of double-winding pipes is ensured.
[0025]
Next, the influence 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 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: 0.30. P: 0.010, S: 0.008, sol Al: 0.040, N: 0.0045, B: 0.0015, Fe: Bal.
(2) Cold rolling: reduction ratio of 85%, sheet thickness: 0.335 mm.
[0026]
(3) Annealing treatment (batch annealing)
Atmosphere: N ₂ −30 vol% H ₂
Processing temperature / time: 670 ℃ × 10 hr
(4) Temper rolling: Reduction ratio 1%
(5) Copper plating: Continuous electroplating, layer thickness 4.5μm (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
[0027]
As shown in FIG. 1, the double-wound pipe has high ductility with an elongation rate of 25% or more in the range of the C content of the base steel sheet of 0.03 to 0.08%. The reason why the ductility is low in the region where the C content is less than 0.03% is that the ferrite structure of the steel sheet is excessively coarsened. On the other hand, the ductility drop in the region where the C content exceeds 0.08% is due to the increase in carbide (Fe ₃ C) in the steel, excessive refinement, and the formation of acicular ferrite structure. Because it became. 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%.
[0028]
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: 0.06, Si: 0.009, Mn: 0.45.P: 0.013, S: 0.008, sol Al: 0.010 to 0.090, N: 0.0010 to 0.0090, B: 0.0016, Fe: Bal
(2) Cold rolling: 83% reduction, sheet thickness: 0.335 mm
[0029]
(3) Annealing treatment (batch annealing)
Atmosphere: N ₂ -12 vol% H ₂ gas mixture Processing temperature / time: 650 ° C x 15 hr
(4) Temper rolling: Reduction ratio 1%
(5) Copper plating: Continuous electroplating, layer thickness 3.0μm (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
[0030]
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 amount of Al is adjusted to the range of 0.03 to 0.08% and the amount of N: 0.003 to 0.008%. Indicates what should be done.
[0031]
Fig. 3 shows the relationship between the B content of the base steel plate and the elongation value of the double-wound pipe (tube diameter: 4.76 mm). Fig. 4 shows the B content of the base steel plate and the ferrite grain size number of the pipe (FGS.NO 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 of these test steel plates and double-winding 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 Al: 0.050, N: 0.0050, B: 0.0004 ~ 0.0040, Fe: Bal
(2) Cold rolling: reduction ratio 89%, sheet thickness: 0.335 mm
[0032]
(3) Annealing treatment (batch annealing)
Atmosphere: N ₂ -75 vol% H ₂ gas mixtureTemperature and time: 660 ° C x 12 hr
(4) Temper rolling: Reduction ratio 1%
(5) Copper plating: Continuous electroplating, layer thickness 4.0μm (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
[0033]
As shown in FIG. 3, the double-winding pipe has high ductility with an elongation of 25% or more when the B content of the base steel plate is in the range of 0.0008 to 0.002%.
FIG. 4 shows that the ferrite structure becomes finer as the B content of the base steel sheet increases. When the amount of B is less than 0.0008%, a coarse grain structure having a farite grain size number (FGS.NO) of 6.0 or less is obtained, which causes insufficient grain boundary strength and sufficiently suppresses and prevents the penetration of grain boundary of molten copper. Can not be. On the other hand, when the amount of B exceeds 0.002%, it becomes excessively fine as FGS.N0 8.5 or more, and as described above, the ferrite structure becomes acicular in the cooling process after the brazing heat treatment.
[0034]
Furthermore, as shown in FIG. 5, as an effect of 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 damage such as lack of ductility due to grain boundary embrittlement occurs. By setting the amount of B to 0.0008% or more, it is possible to suppress the penetration depth of copper to 20 μm or less, and to prevent grain boundary embrittlement and the resulting decrease in pipe ductility.
As described above, FIGS. 3 to 5 are intended to obtain the effect of preventing the coarsening of the ferrite crystal grains and the accompanying intrusion of molten copper and the grain boundary embrittlement, and excessive refinement of the ferrite structure as an effect of adding B. Indicates that the amount of B should be adjusted to a range of 0.0008% to 0.002%.
[0035]
FIG. 6 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. 7 shows a double-wound pipe (brazing) formed using the hoop. Each of the ferrite structures is shown for (heat-treated) (both magnification × 100).
The production conditions of the specimens 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 Al: 0.035, N: 0.0035, B: 0.0016, Fe: Bal
(2) Cold rolling: reduction ratio: 80%, sheet thickness: 0.335 mm
[0036]
(3) Annealing treatment (batch annealing)
Atmosphere gas: N ₂ -15 vol% H ₂ gas mixture Processing temperature and time: 660 ° C x 12 hr
(4) Temper rolling: Reduction ratio 1%
(5) Copper plating: Continuous electroplating, layer thickness 2.5μm (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
[0037]
FIG. 6 (hoop before double winding forming) exhibits a pancake-like ferrite structure 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 steel plate surface layer portion in FIG. 7 (after the brazing treatment of the double-winding pipe) is remarkably finer than the plate thickness center portion. The surface layer has a layer thickness of about 50 μm and a crystal grain size FGS.NO of 10-12. This miniaturization of the surface layer portion occurs prior to the melting of the copper plating layer as described above, and is effective in preventing the grain boundary penetration of the molten copper.
[0038]
【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 66 are comparative examples in which the content of any element (in the table, underlined) is out of the definition of the present invention.
(2) Hot rolling heating temperature: 1200 ° C, hot rolling finishing temperature: 895 ° C, hot rolling coiling temperature: 500 ° C
(3) Cold rolling reduction: 85%, cold rolled sheet thickness: 0.335 mm
[0039]
(4) Annealing treatment (batch annealing)
Atmosphere: N ₂ -H ₂ mixture gas (H ₂ concentration from 2 to 75 vol%)
Processing temperature: 650 ~ 700 ° C, Processing time: 8 ~ 15 hr
(5) Temper rolling: Reduction ratio 1%
(6) Copper plating (continuous electroplating): Plating layer thickness 3.0μm (per side)
(7) Double winding forming and forming method: Roll pipe making (hoop width 27.5 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
[0040]
[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 particle 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 to measure the penetration depth (μm).
[0041]
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. Further, the ferrite structure of the surface layer portion (layer thickness of about 20 to 100 μm) is remarkably fine as FGS.NO 10 to 12. Due to the refinement of the surface layer structure and the B addition effect, the penetration depth of copper is remarkably as small as about 1 to 5 μm. The center of the wall also has a relatively fine ferrite structure with FGS.NO of about 7-8. Thus, since the penetration | invasion of copper is suppressed and it has the ferrite structure | tissue of a suitable particle size over the whole thickness, the workability in a pipe expansion process or a flare process is also favorable.
[0042]
On the other hand, in Comparative Examples Nos. 51 to 66, 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 ductility of No.53 and No.54 pipes is inferior because the amount of C in the base steel sheet is too large, so excessive precipitation of carbide (Fe ₃ C) in steel, excessive refinement of ferrite grains, and needles This is due to the hardening caused by the formation of a ferrite structure.
The ductility of pipes No. 55 and No. 56 (base steel sheet AL amount shortage) and No. 59 and No. 60 (base steel plate N shortage amount) are inferior. This is because the ferrite grains are coarsened and the copper penetration depth becomes large, and the grain boundaries become brittle.
[0043]
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 inferior because of the excessive amounts of Al and N. This is due to excessive precipitation of AlN and excessive fineness of ferrite grains.
The pipe ductility of No. 63 and No. 64 (insufficient base steel sheet B amount) is low because the ferrite grain becomes coarse due to the insufficient B amount of the base steel sheet, the penetration depth of the molten copper increases, and the grain boundary becomes brittle. This is due to the fact that
The pipe ductility of No. 65 and No. 66 (excessive amount of base steel plate B) is inferior because the amount of B in the base steel plate is excessive, so that the amount of solid solution B is excessively precipitated and the ferrite crystal grains are excessive. It is refined and a needle-like ferrite structure is formed and hardened.
[0044]
[Table 1]

[0045]
[Table 2]

[0046]
【The invention's effect】
The copper-plated steel sheet for double-wound pipes of the present invention has high copper penetration resistance in self-brazing treatment, suppresses and prevents copper intrusion and grain boundary embrittlement caused by it, 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, prevents cracking in pipe expansion processing and flare processing, etc., and enables rationalization and cost reduction such as improvement of manufacturing yield and simplification of pipe processing process. At the same time, it brings about effects such as improving and stabilizing pipe quality. In addition, it is possible to cope with the complex shape of pipes and to expand and diversify the applications of double-winding pipes.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the elongation value of a double-winding 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 graph showing the relationship between the elongation value of a double-winding pipe and the B amount of a base steel plate.
FIG. 4 is a graph showing the relationship between the ferrite grain size number (FGS.NO) and the B amount 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-pipe pipe and the B amount of the base steel plate.
FIG. 6 is a drawing-substituting micrograph (magnification × 100) showing a ferrite structure of a copper-plated steel sheet hoop before forming a double-wound pipe.
FIG. 7 is a drawing-substituting micrograph (magnification × 100) showing a ferrite structure of a base steel sheet in a double-winding pipe.
FIG. 8 is a schematic cross-sectional view showing a double-winding pipe.
[Explanation of symbols]
1: Base steel plate 2: Copper plating layer 3: Copper fusion layer

Claims (2)

% By weight
C: 0.03 to 0.08%,
Si: 0.1% or less,
Mn: 0.05-0.5%,
P: 0.020% or less,
S: 0.015% or less,
Sol.Al: 0.03 to 0.08%,
N: 0.003 to 0.008%,
B: 0.0008 to 0.002%,
The remainder consists of Fe and inevitable impurities , and copper plating is applied to the base steel sheet having a surface layer with a layer thickness of 20 to 100 μm in which fine AlN precipitates are dispersed. Excellent copper-plated steel sheet for double-wound pipes. % By weight
C: 0.03 to 0.08%,
Si: 0.1% or less,
Mn: 0.05-0.5%,
P: 0.020% or less,
S: 0.015% or less,
Sol.Al: 0.03 to 0.08%,
N: 0.003 to 0.008%,
B: 0.0008 to 0.002%,
The remainder is a hot slab made of Fe and inevitable impurities, and after cold rolling at a rolling reduction of 40 to 90%, the cold rolled steel sheet is mixed with N ₂ —H ₂ with a hydrogen concentration of ₂ vol% or more. The copper for double-winding pipes according to claim 1, wherein the steel plate is annealed in a temperature range of recrystallization temperature to 850 ° C, and then plated with copper on the steel sheet. Manufacturing method of plated steel sheet.

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