JP5207288B2 - Manufacturing method of high-strength copper-plated steel sheet for double-wound pipes - Google Patents

Description

  The present invention is excellent in copper penetration resistance during the self-brazing process when manufacturing a double-winding pipe used as a brake tube of an automobile or a cooling tube of a refrigerator. High strength copper that has high strength to show excellent pressure resistance, excellent ductility that can withstand pipe expansion processing, flare processing, bending processing, etc., and excellent low temperature toughness The present invention relates to a method for producing a plated steel sheet.

Double-winding pipes made of copper-plated steel plates are used for automobile brake tubes and refrigerator cooling tubes.
The double-winding pipe is obtained by rolling a plated steel sheet cut into a predetermined width into a pipe shape with a pipe-making roll, and then heating it to a temperature equal to or higher than the melting point of the plating layer (for example, heated to about 1130 ° C. when the plating layer is Cu). Hold in a mixed gas atmosphere of nitrogen and hydrogen for 1 to 2 minutes to melt the Cu plating layer, and melt and bond the wound surfaces to each other with the molten plating layer, so-called self-brazing (self-brazing) It is manufactured by.

  And it has a cross-sectional structure as shown in FIG. That is, 1 is a base steel plate (cold rolled steel plate), 2 and 3 are copper plating layers, and 4 is a copper fusion layer formed by self brazing. The bonding quality between the wound surfaces is inspected by an eddy current inspection method, a bending test or the like. The copper-plated steel sheet used for the production of double-pipe pipes is a self-brazing heat treatment in which 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 ( Steel plate embrittlement) (copper penetration resistance), good ductility to withstand tube expansion and flaring performed after double-winding pipe forming, and excellent low temperature toughness It is requested.

For this reason, various improved techniques have been proposed for low carbon aluminum killed steel that has been used in the past in order to meet the above requirements.
For example, the present applicant has also proposed a technique for dispersing fine AlN precipitates on a steel sheet surface layer for the purpose of improving copper penetration resistance and workability (see Patent Document 1).
In addition, another applicant has also proposed to use a ferrite structure having a finer average grain size in order to have both workability and strength (see Patent Document 2).
Japanese Patent No. 3720185 JP-A-2005-240101

The techniques proposed in Patent Documents 1 and 2 have also been useful because they have reasonable effects. However, as seen in recent automobiles, lighter weight and higher functionality are being developed. In order to satisfy these requirements, it is also required to reduce the thickness of the double-wound pipe. The techniques proposed in Patent Documents 1 and 2 do not always fully meet the above requirements. Conventional copper-plated steel sheets for double-wound pipes focus on ensuring workability rather than strength. For this reason, low carbon aluminum killed steel having a yield stress of 200 MPa class is used. For this reason, if the thickness is reduced for weight reduction, the desired pressure resistance cannot be ensured.
In addition, there is no literature that introduces an example of producing a double-winding pipe from a copper-plated steel sheet having a yield stress of 300 MPa or more.

The present invention has been devised to solve such a problem, even in the case of a thinned steel plate, it exhibits a yield stress of 300 MPa or more in order to exhibit excellent pressure resistance after production of a double-winding pipe, Moreover, it has excellent ductility that can withstand pipe expansion, flare processing, bending, etc. of double-winding pipes, excellent low-temperature toughness, and high resistance for double-winding pipes that have higher copper penetration resistance than conventional steel. It aims at providing the manufacturing method of a strength copper plating steel plate.
In addition, in this application, with respect to the conventional copper plating steel plate with a yield strength of 200 MPa class as disclosed in Patent Documents 1 and 2, a copper plating steel plate with a yield strength of 300 MPa class according to the present invention is a high strength material. Sometimes called.

  In order to achieve the object, the method for producing a high-strength copper-plated steel sheet for double-wound pipes according to the present invention includes C: 0.01 to 0.10% by mass, Mn: 0.05 to 1.0% by mass, P : 0.02-0.2 mass% or less, sol.Al: 0.03-0.10 mass%, N: 0.002-0.008 mass%, B: 0.002-0.005 mass% Containing, the remainder slab consisting of Fe and inevitable impurities is hot-rolled, wound at a coiling temperature of 450-600 ° C., cold-rolled at a rolling reduction of 50-90%, and then heated at 100-200 ° C./h Heat to 450 to 600 ° C. at a temperature and hold at that temperature for 1 h or more, then heat to 700 to 800 ° C. at a heating rate of 100 to 200 ° C./h and keep soaked at that temperature for 5 h or more, and then cool in the furnace After performing box annealing consisting of a process of performing copper plating on the steel sheet, To do.

According to the present invention, the contents of P, Al, N and B of the steel sheet to be subjected to copper plating are finely defined. For this reason, the effect of each component is fully exhibited, and the steel plate with high ductility and high strength is provided. In addition, it is possible to effectively suppress the intergranular penetration of the molten copper even when double winding and self-brazing after copper plating.
With these integrations, it is possible to provide a high-strength double-pipe pipe that can withstand tube expansion, flaring, bending, and the like at low cost.

The inventors of the present invention have made extensive studies on the optimal steel component composition and production conditions as a copper-plated steel sheet for double-winding pipes.
The following knowledge was obtained in the process.
(1) Increase the yield strength of the steel sheet by increasing the content of P, which is a solid solution strengthening element, based on low carbon aluminum killed steel. As a result, even if the thickness of the double-winding pipe is reduced, a predetermined pressure resistance performance can be obtained, so that a product manufactured using the double-winding pipe can be reduced in weight. Further, the content P has the effect of suppressing segregation at the crystal grain boundaries during the self-brazing treatment and intrusion of the molten copper into the grain boundaries to cause embrittlement cracks, as in B described later.
However, when the P content is increased, the low temperature toughness is lowered, so the addition of B suppresses the low temperature toughness.

(2) Solid annealing by step annealing at 450 to 600 ° C. and 700 to 800 ° C., which is a special heat treatment, causes precipitation of solute Al and solute N as AlN, thereby reducing the solute N in the steel sheet. The ductility is improved. Since the steel sheet is plated with copper to produce a double-winding pipe, the ductility of the steel sheet can be improved to increase the ductility of the double-winding pipe.

(3) B exhibits the following effects in addition to suppressing the above-described decrease in low-temperature toughness. At 950 to 1100 ° C. in the stage of heating for performing the self-brazing treatment, a part of the precipitated BN is decomposed to produce a solid solution B. The solid solution B segregates at the crystal grain boundaries and suppresses the penetration of the molten copper grain boundaries. On the other hand, the remaining BN suppresses the growth of crystal grains and prevents coarsening. And this coarse-grain-proof property will also suppress the grain boundary penetration | invasion of said molten copper. Furthermore, due to the difference in solubility product between AlN and BN, BN is less likely to decompose than AlN at about 1130 ° C. of self-brazing, and many remain as BN. Thereby, the action of fixing the solid solution N as BN is stronger than that of AlN, and the solid solution N can be reduced even after self-brazing. Therefore, the ductility of the double-winding pipe is improved.

As a result of integrating the above findings, the present invention has been achieved. In particular, the P content is increased to increase the amount of segregation and the segregation amount to the grain boundary is increased to utilize the effect of suppressing grain boundary embrittlement with respect to molten copper.
Hereinafter, the present invention will be described in detail. The present invention is characterized in that the contents of C, Mn, P, sol. Al, N, and B in the carbon steel sheet to be the base are strictly defined. .
The reasons for limiting the chemical composition of the base steel sheet in the present invention are as follows.

C: 0.01-0.10 mass%
The smaller the amount of C, the higher the ductility and the better the workability. However, decarburization to less than 0.01% by mass increases the decarburization time in the steel making process and increases the production cost. Moreover, it becomes easy to become a coarse grain and copper permeation resistance falls. On the other hand, when the amount exceeds 0.10% by mass, a needle-like ferrite structure is easily formed in the cooling process after the self-brazing treatment, and the ductility of the double-wound pipe is significantly reduced.

Mn: 0.05 to 1.0% by mass
Mn is added for the purpose of preventing hot brittleness of the steel. If the amount is less than 0.05% by mass, the object cannot be achieved. On the other hand, when the amount exceeds 1.0% by mass, a needle-like ferrite structure is easily formed in the cooling process after the self-brazing treatment, and the ductility of the double-wound pipe is significantly reduced.

P: 0.02-0.2 mass%
P is an element that contributes to increasing the strength of a steel plate and a double-winding pipe without deteriorating ductility. P, like B described later, segregates at the crystal grain boundary and suppresses the penetration of the molten copper grain boundary in the double-winding pipe. Moreover, 0.02 mass% or more is required in order to express these effects. However, if it exceeds 0.2% by mass, it segregates at the crystal grain boundaries, lowers the grain boundary strength, and significantly deteriorates the ductility of the double-winding pipe.

sol.Al: 0.03-0.10 mass%
Al is an element added as a deoxidizer in the steel melting process, but in the present invention, it is not limited to this, and has good ductility that can withstand pipe expansion processing, flare processing, bending processing, etc. It is an essential element for securing. In the step annealing process of box annealing, which is a special heat treatment, Al and N in a solid solution state in a hot-rolled sheet are precipitated as AlN and the solid solution N in the steel is reduced, and (111) texture is developed. Has the effect of improving the ductility of the steel sheet. Improving the ductility of the steel sheet leads to an improvement in the ductility of the double-pipe pipe. If the amount of sol.Al is less than 0.03% by mass, the effect cannot be obtained. Conversely, the effect is saturated even if the content exceeds 0.10% by mass.

N: 0.002-0.008 mass%
N is an essential element in order to ensure good ductility that can withstand tube expansion processing, flare processing, bending processing, and the like of a double-winding pipe, as with Al described above. In the step annealing process of box annealing, Al and N in a solid solution state in a hot-rolled sheet are precipitated as AlN, reducing the solid solution N in the steel, and developing a (111) texture to improve the ductility of the steel sheet. Have the effect of Improving the ductility of the steel sheet leads to an improvement in the ductility of the double-pipe pipe. The effect cannot be obtained unless N is less than 0.002% by mass. On the other hand, when the content is more than 0.008% by mass, the amount of solid solution N after the self-brazing treatment increases rapidly even when B described later is added. Ductility is extremely deteriorated.

B: 0.002 to 0.005%
B precipitates as solute N and BN, and prevents ductile deterioration due to solute N in steel plates and double-pipe pipes. Moreover, the solid solution B segregated at the grain boundaries improves the low temperature toughness of the double-pipe pipe. In order to exhibit this action, it is necessary to contain 0.002% by mass or more. However, the effect is saturated by containing 0.005 mass%. Moreover, when 0.005 mass% or more B is contained, in the above-described step annealing process of box annealing, the amount of N precipitated as AlN is insufficient, and conversely, the ductility of the steel plate and the double-winding pipe is reduced.

Precipitated BN is partially decomposed during the self-brazing process of heating at a temperature of 950 to 1100 ° C. to produce solid solution B. The solid solution B segregates at the crystal grain boundary and exhibits the effect of suppressing the penetration of the molten copper into the grain boundary. Moreover, the solid solution B segregated at the grain boundaries improves the low temperature toughness.
Furthermore, the remaining BN suppresses crystal grain growth and prevents coarsening during the self-brazing process. This coarse grain resistance also leads to the suppression of the above-mentioned intergranular penetration of molten copper. The effect of suppressing the growth of crystal grains during the self-brazing process is observed not only in BN but also in AlN. However, BN, which has a smaller solubility product than AlN, works more effectively.
In order to exert these actions, 0.001% by mass or more of B is required. In addition, when the amount of B exceeds 0.005% by mass, it acts on the latter coarse grain resistance, which acts as a precipitate of BN, but no solid solution B is generated during the self-brazing treatment, and copper penetration resistance. Becomes smaller.

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 preferably adjusted to a temperature just above the Ar ₃ transformation point.

The coiling temperature is in the range of 450 to 600 ° C.
When the coiling temperature exceeds 600 ° C., the crystal grains become coarse and all the AlN precipitates, and the surface appearance of the steel plate and the double-winding pipe is remarkably deteriorated. Also, ductility is reduced. On the other hand, if the coiling temperature is less than 450 ° C., the plate shape of the hot-rolled steel sheet is deteriorated, and the tube-forming property of the double-winding pipe and the shape of the double-winding pipe are poor.
The hot-rolled steel sheet is subjected to cold rolling after the pickling treatment. In order to obtain a steel sheet with high ductility by suppressing the coarsening of crystal grains by cold rolling, it is necessary to set the rolling reduction to 50% or more. When the rolling reduction exceeds 90%, the effect of improving the ductility is saturated, and a rolling reduction higher than that causes only an operational disadvantage due to an increase in rolling load. Therefore, the upper limit is 90%.

The cold-rolled cold-rolled steel sheet is subjected to an annealing treatment after the surface is purified. The present invention is characterized in that the annealing process is stepped in a two-step temperature range.
That is, it is heated to 450 to 600 ° C. at a heating rate of 100 to 200 ° C./h and held at that temperature for 1 hour or more, and subsequently heated to 700 to 800 ° C. at a heating rate of 100 to 200 ° C./h. The greatest feature is to perform box annealing consisting of a process of holding soaking for 5 hours or more and then cooling the furnace.

By special annealing, step annealing, which is performed at 450 to 600 ° C. and 700 to 800 ° C. of box annealing, Al and N dissolved in the cold-rolled steel sheet are precipitated as AlN to reduce the solid solution N. Thus, this condition is particularly necessary for further improving the ductility of the steel sheet and increasing the ductility of the double-winding pipe.
If the rate of temperature rise in the first step is less than 100 ° C / h, the crystal grains become coarse, yield stress and workability decrease, and conversely if it is too fast to exceed 200 ° C / h, the recrystallization temperature rises. As a result, an unrecrystallized structure remains. Moreover, when heating temperature becomes 450 degrees C or less and 600 degrees C or more, since solid solution Al and solid solution N do not precipitate as AlN, ductility will fall.

The conditions for the second step of annealing also need to be specified in detail.
If the rate of temperature rise is less than 100 ° C / h, the crystal grains become coarse, yield stress and workability decrease, and conversely if it is too fast to exceed 200 ° C / h, the recrystallization temperature rises and unrecrystallization occurs. The organization remains. If the heating temperature is less than 700 ° C., an unrecrystallized structure remains. On the other hand, if the temperature is too high to exceed 800 ° C., the crystal grains become coarse, and the yield stress and workability deteriorate.

The annealed steel sheet is subjected to temper rolling and the like according to a conventional method, and then subjected to copper plating by continuous electroplating or the like, and finished into a copper-plated steel sheet for double-winding pipes.
The obtained copper-plated steel sheet is subjected to a self-brazing heat treatment (processing temperature: about 1130 ° C.) for fusing between the wound surfaces after being formed into a double-wound pipe.
In the self-brazing heat treatment, recrystallization (about 900 to 950 ° C.) of the base steel sheet occurs in the initial stage. The fine AlN precipitates on the surface layer have a pinning effect in ferrite recrystallization, and an extremely fine ferrite structure (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 sheet has already been recrystallized, and a very fine ferrite structure has been formed in the surface layer. As a result of the refinement, fusion between the rolled surfaces is suppressed while suppressing the ingress of the grain boundary of the molten copper. Can be achieved.

Example 1:
A slab having the composition shown in Table 1 is hot-rolled to a sheet thickness of 2.0 mm under conditions of a hot-rolling finishing temperature of 900 ° C. and a winding temperature of 550 ° C., and after pickling, the slab has a cold rolling rate of 85%. Cold rolled to a thickness of 0.25 mm. Then, step annealing was performed. In step annealing, heating was performed at a heating rate of 150 ° C./h up to 500 ° C., and maintained at that temperature for 2 hours soaking, and then heated to 750 ° C. at the same heating rate for 8 h soaking, and then cooled in the furnace.

All the test materials were then passed through an electrolytic copper plating apparatus and subjected to copper plating with an adhesion amount of 10 μm on one side to obtain a copper-plated steel sheet for a double-wound pipe.
Subsequently, the copper-plated steel sheet for double-winding pipes is slit into hoops, subjected to double-winding processing, and further subjected to a heat treatment of 1130 ° C. × 60 s corresponding to self-brazing to melt the copper plating layer and double-winding It was a pipe. Thereafter, the tube was drawn to finish the outer diameter to 4.8 mm. The double-wound pipe thus manufactured was subjected to mechanical characteristics to evaluate yield stress and total elongation, penetration depth of molten copper, and low temperature toughness.

(1) Mechanical properties of the pipe; the pipe was cut into a length of 300 mm, a tensile test was performed according to JIS Z2241, and the yield stress (YS) and ductility (T.El) were measured. The gauge distance was 50 mm. A ductility of 25% or less was rejected.
(2) Penetration depth of molten copper: After corroding the cross section of the pipe with 5% nital, a Cu characteristic X-ray image of the welded portion of copper was photographed with an XMA analyzer, and the penetration depth was measured. A penetration depth of 15 μm or more was regarded as unacceptable.
(3) Evaluation of low-temperature toughness: The pipe was cut into a length of 300 mm, cooled to −40 ° C., a tensile test was performed, and the tensile strength (TS) was measured. What showed the tensile strength of 90% or more compared with the tensile strength at room temperature was set as the pass (symbol o), and less than 90% was set as the failure (symbol x).

Table 2 shows the evaluation results of the yield stress and the low temperature toughness of the double wound pipe. Double wound pipes manufactured from materials of any composition showed a yield stress of 300 N / mm 2 or more. However, no. 4 and 5 did not satisfy the standard in the evaluation of low temperature toughness because the amount of B was less than the range defined by the present invention.
FIG. 2 shows the relationship of the B amount on the copper penetration depth of the double-pipe pipe. The copper penetration depth has a strong relationship with the B content, and the penetration depth reaches 15 μm or more even if the B content is less than or greater than the range defined by the present invention. FIG. 3 shows the relationship between the B amount and the total elongation of the double-winding pipe. The total elongation is also strongly related to the amount of B. If the amount of B is not within the range specified by the present invention, the elongation of 25% or more required for tube expansion processing, flare processing, and bending processing cannot be obtained.

Example 2:
A slab having the composition shown in Table 3 was used as a raw material, and a raw steel plate was manufactured by changing the hot rolling coiling temperature, the heating rate of step annealing, and the heating temperature shown in Table 4. The conditions other than those shown in Table 4 are the same as in Example 1. The hot rolling finishing temperature is 900 ° C., the cold rolling rate is 85%, and the plate thickness is 0.25 mm. The manufactured steel sheet was subjected to either step annealing or normal annealing, and subsequently a double-pipe pipe was manufactured in the same process as in Example 1. Table 4 also shows the annealing conditions. Test No. 7-16 and No. 22-31 is step annealing. No. 17-21 are normal annealing, The conditions are as having described in the column of the 1st step.
And the following test and evaluation were performed about the manufactured double wound pipe. Table 5 shows the results.

(1) Mechanical properties of the material: According to JIS Z2241, a tensile test was performed with a JIS No. 5 test piece, and yield stress (YS), tensile strength (TS), and ductility (T.El) were measured.
(2) Mechanical properties of double-wound pipe: The pipe was cut into a length of 300 mm, and subjected to a tensile test according to JIS Z2241, and the yield stress (YS), tensile strength (TS), and ductility (T.El) were measured. The gauge distance was 50 mm. A ductility of 25% or less was rejected.
(3) Penetration depth of molten copper: After corroding the cross section of the pipe with 5% nital, a Cu characteristic X-ray image of the welded portion of copper was photographed with an XMA analyzer, and the penetration depth was measured. A penetration depth of 15 μm or more was regarded as unacceptable.
(4) Evaluation of low-temperature toughness: The pipe was cut out to a length of 300 mm, cooled to −40 ° C., a tensile test was performed, and the tensile strength (TS) was measured. Those exhibiting a tensile strength of 90% or more compared with the tensile strength at room temperature were evaluated as acceptable (symbol ◯), and less than 90% were rejected (symbol x).

  From the results of Table 5, No. having an appropriate component composition. Nos. 7 to 13 have a yield stress of a double-winding pipe of 300 MPa or more and a ductility of 25.8 to 28.0%, and have a ductility of 25% or more required for tube expansion, flaring, and bending. Yes. Further, the copper penetration depth of the double-winding pipe is as small as about 2 to 5 μm, and the copper penetration resistance is high. Furthermore, no. Nos. 14 to 16 have good ductility of the double-wound pipe of 29.6 to 30.9%, and the copper intrusion depth of the double-wound pipe is as small as about 2 to 3 μm and has high resistance to copper infiltration. .

In contrast, no. 17-21 are normally annealed and the ductility of a double-winding pipe is low. No. Since No. 22 has a large C content and a small B content, the ductility of the double-pipe pipe is low and the penetration depth of copper is large. No. Since No. 23 has a large content of Mn and a small content of B, the ductility of the double-wound pipe is low and the low-temperature toughness is poor. Furthermore, the penetration depth of copper is increased. No. Since No. 24 has a large content of P, the ductility of the double-wound pipe is low and the low-temperature toughness is poor. No. Since No. 25 has a large N content, the ductility of the double-wound pipe is low. Furthermore, no. No. 26 has a low content of P, so that not only the yield stress of the double-winding pipe is low, but also the penetration depth of copper is large. Furthermore, no. Since No. 27 has a small content of C, the penetration depth of copper is large.
No. Since the heating temperature of the second step 28 is high, the yield stress of the double-winding pipe is low. No. No. 29 has a fast heating rate in the first step and the second step. No. 30 has a low heating temperature in the first step. Since the hot rolling coiling temperature 31 is high, the ductility of the double-winding pipe is low.

Sectional drawing explaining the structure of a double wound pipe Graph showing the relationship between the B content and the properties of double-wound pipes (copper penetration depth) Graph showing the relationship between the B content and the properties (ductility) of double-wound pipes

Explanation of symbols

1 Base steel plate (cold rolled steel plate)
2 Copper plating layer 3 Copper plating layer 4 Copper fusion layer

Claims (1)

C: 0.01-0.10 mass%, Mn: 0.05-1.0 mass%, P: 0.02-0.2 mass% or less, sol.Al: 0.03-0.10 mass% , N: 0.002 to 0.008% by mass, B: 0.002 to 0.005% by mass, and the remaining slab consisting of Fe and inevitable impurities is hot-rolled, and the coiling temperature is 450 to 600 ° C. And then cold rolled at a reduction rate of 50 to 90%, heated to 450 to 600 ° C. at a heating rate of 100 to 200 ° C./h and held at that temperature for 1 h or more, and subsequently 100 to 200 ° C./h It is heated to 700 to 800 ° C. at a heating rate of 5 ° C., soaked at that temperature for 5 hours or more, and then subjected to box annealing consisting of a furnace cooling step, and then the steel plate is subjected to copper plating. A method for producing a copper-plated steel sheet for a wound pipe.

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