JP3288620B2 - Steel sheet for double-wound pipe and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method of plating copper or a metal having a self-brazing property similar to copper on a surface thereof,
The present invention relates to a cold-rolled steel sheet suitable for use in a double-wound pipe manufactured by heating to a melting point of a plated metal for a short time after being formed into a tubular shape, and a method for manufacturing the same.

[0002]

2. Description of the Related Art In the fields of connecting pipes for various compressors, brake tubes for automobiles, etc., it has the same appearance and excellent thermal characteristics and aesthetics as copper pipes, while providing the high strength and toughness of iron. Also, a so-called double wound pipe is used. For double wound pipes, for example, iron and steel
There is a detailed explanation in 1980, p. 130, 1980. A general method of manufacturing a double wound pipe is performed as follows. First, electrolytic copper plating is applied to the front and back surfaces of a cold-rolled steel sheet having a thickness of about 0.30 mm. Thereafter, the steel sheet is rounded for two turns so that the rolling direction of the steel sheet is in the axial direction of the pipe and the wall of the pipe is a double plate. Thereafter, by heating to a temperature equal to or higher than the melting point of copper to melt the copper, the gap between the plates is filled, and the steel plates are joined to each other, so-called “self-brazing” is performed. Thereafter, this is subjected to shape correction, dimensional adjustment and the like in the cold to obtain a product. 2 manufactured in this way
The heavy wound pipe is generally required to have reliability such as airtightness for the above-mentioned applications.

[0003] The steel sheet used for the double wound pipe is as follows:
Since it is an ultra-thin cold-rolled steel sheet having a thickness of 0.35 mm or less and requiring extremely high formability, a box-annealed material of low-carbon steel has been generally used conventionally. This box-annealed material is relatively soft in material and also has good formability, so that it can be sufficiently used as a material for a double-wound pipe, but the production process requires several days, resulting in poor production efficiency. In addition, there is a problem that the material is largely non-uniform in the longitudinal direction and the width direction of the coil. In addition, the basic characteristics of the steel sheet for double-wound pipes are to reduce the consumption of the mold for forming pipes and to improve the shape freezing property in the pipe-making (tube winding) process, while ensuring strength. There is a need for a softer material having excellent moldability.

[0004]

In recent years, ultra-low carbon steels (less than 0.020%) with significantly reduced carbon content have attracted attention in the field of general cold-rolled steel sheets. The ultra-low carbon steel is suitable for a continuous annealing method excellent in production efficiency and material uniformity, and is further characterized by being soft and excellent in formability. Therefore,
In order to solve the above-mentioned problems, it is considered that application of a continuously annealed material using ultra-low carbon steel is promising. But,
In the manufacturing process of the double-wound pipe, after being wound around the pipe, a cold strain of about 7 to 8% is applied by drawing, and a high temperature of not less than the melting point of copper (1083 ° C.) for a short time. A heat treatment for self-brazing is applied. For this reason, there is a concern about the coarsening of the steel structure due to the processing and heat treatment.
In fact, it has been found that the production of double-wound pipes from ultra-low carbon steel often results in the generation of coarse grains which significantly affect strength and toughness.

[0005] Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to achieve both high production efficiency and uniformity of the material while significantly improving the material as compared with the conventional material. An object of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the cold-rolled steel sheet, which are suitable for manufacturing a double-wound pipe utilizing self-brazing properties. A specific object of the present invention is to provide a cold-rolled steel sheet suitable for use in manufacturing a double-wound pipe having the following characteristics and a method for manufacturing the same. 1) The heat treatment for self-brazing does not cause deterioration in characteristics, particularly, strength and toughness due to coarse grains. 2) Low deformation resistance during pipe production, minimizing mold wear, and extending the service life. 3) Being soft at the time of pipe production and having excellent shape freezing properties. 4) Ultimately have sufficient strength, ductility and toughness. Or 5) an ultra-thin steel sheet having a thickness of 0.35 mm or less, and having excellent material uniformity in the longitudinal direction and the width direction of the steel sheet (steel strip) and no variation in shape.

[0006]

As a result of repeated experiments and studies for solving the above-mentioned problems, the inventors have contradicted conventional knowledge that control of precipitates is effective in preventing grain growth. Rather, it has been found that it is effective to secure a certain amount of Nb or Ti in a non-precipitated state. Then, in addition to the regulation of the steel components, the finish rolling finish temperature, and the hot rolling conditions such as the winding temperature, by controlling the annealing conditions in an appropriate range, Nb and Ti in the predetermined amount or more are not precipitated (ie, (Solid solution state), the crystal grains can be controlled to the optimum range, and it has been found that stable mechanical properties can be ensured even after heat treatment at the time of pipe production, and the present invention has been completed.

That is, the gist of the present invention is as follows. (1) C: 0.0005 to 0.020 wt%, Nb: 0.003 to 0.040 wt%
%, Ti: one or two kinds of 0.005 to 0.060 wt%, and at least one of Nb and Ti is present in a solid solution state in an amount of 0.005 wt% or more, and the crystal grain size of the ferrite structure is 5-10 μm, excellent in moldability,
A steel sheet for double-wound pipe with excellent pipe strength and toughness after forming and heat treatment.

(2) C: 0.0005 to 0.020 wt%, S: 0.02
wt% or less, N: 0.0050 wt% or less, Nb: 0.003 to 0.040
wt%, Ti: one or two of 0.005 to 0.060 wt% and surplus Nb, Ti calculated assuming that TiN, TiS, TiC and NbC are formed as much as possible in this order.
Amount is less than 0.005 wt%, and Nb, Ti
At least one of which is present in a solid solution state in an amount of 0.005 wt% or more, and has a ferrite structure having a crystal grain size of 5 to 10 μm, and has excellent formability, and pipe strength and toughness after forming and heat treatment. Excellent for double-wound pipes.

(3) C: 0.0005 to 0.020 wt%, Si: 0.10 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.02 wt% or less, S: 0.02 wt% or less, Al: 0.100 wt% or less, N: 0.0050% by weight or less, Nb: 0.003 to 0.040% by weight, Ti: 0.005 to 0.060% by weight, the balance being Fe and unavoidable impurities. ) Or the steel sheet for a double-wound pipe according to (2).

(4) C: 0.0005 to 0.020 wt%, Si: 0.10 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.02 wt% or less, S: 0.02 wt% or less, Al: 0.100 wt% or less, N: not more than 0.0050 wt%, Nb: 0.003 to 0.040 wt%, Ti: one or two of 0.005 to 0.060 wt%, B: 0.0005 to 0.0020 wt%, Cu: 0.5 wt% In the following, Ni: 0.5 wt% or less, Cr: 0.5 wt% or less, Mo: 0.5 wt% or less selected from one or two groups, and the balance is Fe and The steel sheet for a double wound pipe according to the above (1) or (2), which has a steel composition of unavoidable impurities.

(5) C: 0.0005 to 0.020 wt%, Nb: 0.00
A steel material containing 3 to 0.040 wt% and one or two of Ti: 0.005 to 0.060 wt% is hot-finished and rolled at an end temperature of 1000 to 850 ° C, wound up at 750 ° C or less, and then cooled. Rolled, and continuously annealed at 650 ° C to 850 ° C for 20 seconds or less,
Nb, Ti by secondary cold rolling at a rolling reduction of 20% or less
At least one of which is present in a solid solution state in an amount of 0.005 wt% or more, and is adjusted so that the crystal grain size of the ferrite structure is 5 to 10 μm. A method for manufacturing a steel sheet for a double-wound pipe having excellent pipe strength and toughness.

(6) C: 0.0005 to 0.020 wt%, S: 0.02 wt%
%, N: 0.0050 wt% or less, Nb: 0.003 to 0.040 wt%
%, Ti: one or two of 0.005 to 0.060 wt%, and the amount of excess Nb and Ti calculated assuming that TiN, TiS, TiC and NbC are formed as much as possible in this order. However, steel materials with less than 0.005 wt%
Hot finish rolling at 1000 to 850 ° C, winding at 750 ° C or less, then cold rolling, continuous annealing at 650 ° C to 850 ° C for 20 seconds or less, secondary cooling at a rolling reduction of 20% or less By cold rolling, at least one of Nb and Ti is present in a solid solution state in an amount of 0.005 wt% or more, and is adjusted so that the crystal grain size of the ferrite structure is 5 to 10 μm. A method for producing a steel sheet for a double-wound pipe having excellent formability and excellent pipe strength and toughness after forming and heat treatment.

(7) C: 0.0005 to 0.020 wt%, Si: 0.10 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.02 wt% or less, S: 0.02 wt% or less, Al: 0.100 wt% or less, N: 0.0050 wt% or less, Nb: 0.003 to 0.040 wt%, Ti: one or two of 0.005 to 0.060 wt%, the balance being Fe and unavoidable impurities. ) Or the method for producing a steel sheet for a double-wound pipe according to (6).

(8) C: 0.0005 to 0.020 wt%, Si: 0.10 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.02 wt% or less, S: 0.02 wt% or less, Al: 0.100 wt% or less, N: not more than 0.0050 wt%, Nb: 0.003 to 0.040 wt%, Ti: one or two of 0.005 to 0.060 wt%, and as Group A, B:
0.0005 to 0.0020 wt%, selected from Group 1 or 2 of Group B: Cu: 0.5 wt% or less, Ni: 0.5 wt% or less, Cr: 0.5 wt% or less, Mo: 0.5 wt% or less The method for producing a steel sheet for a double-wound pipe according to the above (5) or (6), wherein the steel composition contains one or more kinds, and the balance is a steel composition of Fe and inevitable impurities.

(9) Within one second after finishing the hot finish rolling,
The method for producing a steel sheet for a double-wound pipe according to any one of the above (5) to (8), wherein the steel sheet is rapidly cooled at a rate of 30 ° C./sec or more.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. (1) Regarding steel components: C: 0.0005 to 0.020 wt% C contributes to the improvement of formability (reduction of deformation stress and improvement of shape freezing property) at the time of pipe making due to its extremely low content. But 0.000
If the content is less than 5 wt%, the crystal grains become remarkably coarse, making it difficult to secure the required strength and toughness. In addition, the risk of causing skin roughness similar to the so-called orange peel phenomenon increases. On the other hand, if it exceeds 0.020 wt%, the ductility and shape freezing property of the steel sheet are remarkably deteriorated, and the tendency of the workability to deteriorate due to the thinning of the steel sheet is further increased. In addition, an excessive amount of C also lowers cold rollability. Therefore, the amount of C is 0.00
The range is from 05 to 0.020 wt%. In addition, when more advanced material stability and excellent ductility are required, 0.0010-0.
It is desirable to be in the range of 015 wt%.

Si: 0.10 wt% or less When a large amount of Si is added, it causes a decrease in surface treatment properties and a decrease in corrosion resistance. In addition, since the steel is remarkably solid-solution strengthened, the deformation resistance during molding is increased. , The upper limit is 0.10wt%
And If particularly high corrosion resistance is required, 0.
It is preferable to limit it to 02 wt% or less.

Mn: 0.1 to 1.5 wt% Mn is an effective element for preventing hot cracking caused by S. In particular, in the case of steel without Ti, it is necessary to add Mn according to the amount of S contained. desirable. Further, Mn is desirably added because it has an effect of refining the crystal grains, particularly an effect of suppressing the coarsening of the crystal grains when kept at a high temperature. In order to exert these effects, it is necessary to add at least 0.1 wt%. However, excessive addition deteriorates corrosion resistance and deteriorates cold rollability due to hardening of the steel sheet, so the upper limit is made 1.5 wt%. In the case where better corrosion resistance and moldability are required, it is desirable to add the compound in a range of 0.60% by weight or less.

P: not more than 0.02 wt% P is a harmful element that hardens steel, deteriorates flanging workability and shape freezing property, and deteriorates corrosion resistance.
The upper limit is set to 0.02 wt%. If these characteristics are particularly important, the content is preferably 0.01 wt% or less.

S: 0.02 wt% or less S is an element that exists as an inclusion in steel and reduces the ductility of a steel sheet and deteriorates corrosion resistance. Therefore, the upper limit is set to 0.02 wt%. For applications requiring particularly good workability, the content is desirably 0.01 wt% or less.

Al: 0.100 wt% or less Al is an element useful for deoxidizing steel, but if its content is too large, it deteriorates the surface properties, so the upper limit is 0.100 wt%.
And From the viewpoint of material stability, 0.008 to
It is desirable to add in the range of 0.060 wt%.

N: 0.0050 wt% or less N increases the content of N, which promotes the generation of internal defects in the steel sheet, causes slab cracking during continuous casting, and hardens the steel excessively. wt%. In addition, from the viewpoints of material stability and yield improvement in consideration of the entire manufacturing process, the content is preferably in the range of 0.0030 wt% or less.

Nb: 0.003 to 0.040 wt% Nb is an element effective for refining the structure of a steel sheet, and its effect is maintained even after heat treatment after pipe forming. Due to such refinement of the steel structure, 2
The secondary formability (that is, formability such as bending and tension in a pipe state) is remarkably improved, and the impact resistance is also improved. Such an effect of Nb is exhibited by adding 0.003 wt% or more. However, if it is added more than 0.040 wt%, the steel is hardened, slab cracks are easily generated, and hot,
Cold rollability deteriorates. Therefore, the amount of Nb added is 0.003 to
The range is 0.040 wt%. Note that a more preferable range in terms of the material is 0.020 wt% or less.

Ti: 0.005 to 0.060 wt% Ti has an effect of making the structure finer, almost similarly to Nb. To obtain this effect, 0.005 wt% or more is necessary, but if it exceeds 0.060 wt%, the generation of surface defects increases. Therefore, the content of Ti is set in the range of 0.005 to 0.060 wt%. In addition, a more preferable range on the material is 0.015
% Or less. The effects of Nb and Ti are not canceled out even if they are added alone or in combination.

Nb and Ti in a solid solution state are one of the very important components in the present invention. Although the detailed mechanism is not always clear, at least one of Nb and Ti in a solid solution state is
The presence of 0.005 wt% or more can significantly prevent the coarsening of the structure after the forming and heat treatment of the double-wound pipe as shown in FIG. The steel composition used for the experiment in FIG. 1 was 0.0025 wt% C-0.02 wt% Si-0.5 wt% Mn-0.
01wt% P-0.010wt% S-0.040wt% Al-0.0020wt% N
-Nb or Ti, Nb is 0.018% and 0.015%, Ti is 0.
Two levels of 040% and 0.060% respectively. The hot rolling conditions and heat treatment conditions were as follows: hot rolling end temperature 950-870 ° C, winding temperature 720-540 ° C, annealing condition 750 ° C-20sec.
After the annealing, 2% secondary cold rolling was performed. As a result,
Nb or solid solution Ti could be changed in the range of 0 to 0.015%. It is necessary that at least one of these elements be present in the form of a solid solution, and the above effect cannot be obtained even if both elements are present in a total amount of 0.005 wt% or more. Further, even when the solid solution amounts of Nb and Ti are both 0.005 wt% or more, their effects appear as a synergistic effect,
They do not cancel each other out. Therefore, at least one of Nb and Ti needs to be present in a solid solution state in an amount of 0.005 wt% or more. Note that Nb and Ti in the solid solution state described here are the precipitates quantified by electrolytic extraction analysis from the total amount of Nb and Ti contained in the steel, respectively.
Defined as the amount after subtracting Nb and Ti. Here, the electrolytic extraction analysis method is an analysis method using a non-aqueous solvent-based electrolytic solution constant potential electrolysis method, in which a sample is electrolyzed with 10% acetylacetone-1% tetramethylammonium chloride-methyl alcohol electrolytic solution, and 0.2% The residue is extracted with a μm Newpore filter, and each element is quantified by an absorption spectrophotometry.

Excess Ti and Nb As described above, Ti and Nb are important elements in the present invention, while excessive addition has undesirable aspects for the following reasons. That is, Ti and Nb are, in a general cold-rolled steel sheet,
It is a preferable element for improving formability, especially softening, r value and ductility. However, in the case of an ultra-thin steel sheet as in the present invention, an extremely high cold rolling reduction rate in the manufacturing process (at least 70% or more, usually 80
%), The load of cold rolling is large,
Excessive addition of Ti and Nb is not preferable because it has the disadvantage of significantly increasing the deformation resistance during rolling and deteriorating the surface properties.
In addition, there is a disadvantage that the difference in each property such as strength, r value, and ductility depending on the processing direction, that is, anisotropy is increased. To prevent this, it is necessary to avoid excessive addition of Ti and Nb.
In addition, it is desirable that both Ti and Nb have the necessary minimum amount in view of the addition cost. For the reasons described above, the inventors have studied the necessary minimum amount of addition of Ti and Nb from the precipitation process, and as a result, it has become clear that the following addition amount may be set as the upper limit. That is, using the component values of steel, TiN, Ti
The surplus Ti and Nb are calculated assuming that S, TiC and NbC are formed as much as possible in this order.
It must be less than wt%.

[0027] Specifically, the excess of Ti (hereinafter, represented by Ti _ex) is, TiN, TiS, because it is Ti remaining after the formation of the TiC, with each% by weight, stoichiometrically by the following formula Can be calculated. Ti _ex = Ti- (48/14) N- (48/32) S- (48/1
2) · C excess Nb (hereinafter calculations represented by Nb _ex) is calculated separately in the following cases. 1) When Ti is not added, since TiN, TiS, and TiC are not formed, it can be obtained by the following equation in consideration of only NbC. Nb _ex = Nb− (93/12) · C 2) Since Ti is added and Ti _ex ≧ 0, C to form NbC does not remain, so it is determined by the following equation. Nb _ex = Nb 3) When Ti is added and Ti _ex ≦ 0 First, Ti formed as TiN and TiS (hereinafter referred to as Ti _NS ) is calculated, and Ti _NS = Ti− (48/14 ) · N− (48/32) · S, depending on the value of Ti _NS , 3a) If Ti _NS ≦ 0, all C forms NbC, so Nb _ex = Nb− (93 / 12) · C ······ 1) 3b) If Ti _NS > 0, after forming TiC by the amount of Ti _NS ,
Since the remaining C to form NbC, Nb _ex = Nb- determined by (93/12) · (C- (12/48 ) · Ti NS). As described above, setting an upper limit on the amount of Ti and Nb to be added has a surface that makes it difficult to secure the amount of solid solution. However, under such restrictions, the required amount of solid solution Ti and Nb is still secured, The present invention is significant in that both the material properties and the strength and toughness after forming the double wound pipe are compatible.

B: 0.0005 to 0.0020 wt% (Group A)
One or two of the following groups: Cu: 0.5 wt% or less, Ni: 0.5 wt% or less, Cr: 0.5 wt% or less, Mo: 0.5 wt% or less (above B group).
Any one or more selected from the group can be contained. B: 0.0005 to 0.0020 wt% B is an element effective for ensuring the strength by making the structure finer after the pipe production. Such an effect is exhibited by the addition of 0.0005 wt% or more. However, if the addition exceeds 0.0020 wt%, the hot rolling property of the steel sheet is undesirably deteriorated. Therefore,
The amount of B is 0.0005 to 0.0020 wt%, preferably 0.0005 to 0.0020%.
Add in the range of 10 wt%.

Cu: 0.5 wt% or less, Ni: 0.5 wt% or less, C
r: 0.5 wt% or less, Mo: 0.5 wt% or less All of these elements have the effect of increasing the strength of the steel sheet, especially after the heat treatment applied during brazing of pipe making, and are added as necessary. I do. However, if it is added in excess of 0.5 wt%, the cold rollability deteriorates.
Add in the range of wt% or less. Each element belonging to the group of B, the group of Cu, Ni, Cr and Mo, which is the above-mentioned selective addition element, may be added alone or more alone in each group, or two or more kinds over both groups. You may add multiple.

(2) Regarding the crystal structure and the like; The crystal grain size of the ferrite is 5 to 10 μm. If the crystal grain size is less than 5 μm, the steel is hardened, and defects such as poor shape and increased tool wear during pipe forming become remarkable. On the other hand, when the crystal grain size is 10
If it exceeds μm, it will be difficult to keep the structure after forming-heat treatment uniform and fine, and the strength and toughness as use characteristics of the product will decrease. Therefore, the crystal grain size of the steel sheet is 5 to 10 μm.

The hardness (tempering degree) of the steel sheet is from T1 to T
It is desirable to set it to 3. If the degree of tempering exceeds T3, the deterioration of the moldability becomes apparent, and the shortening of the tool life becomes remarkable. If sufficient strength can be ensured after the tube forming and heat treatment, the lower the material strength, the better. In addition, the toughness as well as the strength after forming and heat treatment of the steel sheet for a double-wound pipe described above is one of the important characteristics. As an evaluation method, there is a test by pulling a notch in a pipe state or a high-speed pull.

(3) Manufacturing conditions: Hot finish rolling; When the finish temperature of the hot finish rolling is lower than 850 ° C., the uniformity of the structure after hot rolling is reduced, which is inherited even after cold rolling annealing. Therefore, the dispersion of the material is increased, and the reliability of the mechanical characteristics is reduced, which is not preferable. On the other hand, 10
When the temperature exceeds 00 ° C., the generation of surface flaws due to the scale becomes apparent. Therefore, the finish temperature of hot finish rolling is 1000
The temperature is preferably in the range of ~ 850 ° C. The range of 950 to 850 ° C. is preferable in consideration of hot rolling property. Further, in order to reduce the chance of precipitation of Ti and Nb after completion of hot finish rolling, it is preferable to rapidly cool at a rate of 30 ° C./sec or more within one second after finish rolling. Furthermore, when the finish rolling of the sheet bar after the hot rough rolling is performed, the continuous rolling (endless rolling) for joining the sheet bar on the side including the finish rolling mill is applied to the end of the steel strip, This is preferable because it stabilizes the threading at the rear end and facilitates the rapid cooling immediately after finish rolling over the entire length of the steel strip.

Winding after hot rolling; winding temperature after hot rolling is 75
If the temperature exceeds 0 ° C, Nb and Ti in the added steel hardly remain in the solid solution state, and the effect of suppressing the coarsening of the crystal grains during pipe production by Nb and Ti in the solid solution state cannot be sufficiently exhibited. . In this case, it is also difficult to obtain a uniform material in the longitudinal direction. Therefore, the coiling temperature after hot rolling is set to 750 ° C or lower, preferably 650 ° C or lower. The conditions for pickling and cold rolling to be performed thereafter do not need to be particularly determined, and may be in accordance with a normal method for manufacturing a very thin steel sheet.

Annealing after cold rolling: When the annealing temperature is lower than 650 ° C., most of the structure becomes an unrecrystallized structure, the steel sheet cannot be softened, and the goal of reducing the load during pipe production has been achieved. Will not be. Annealing at 650 ° C. or higher does not provide a complete recrystallized structure, but achieves sufficient softening in the present invention. If the annealing temperature is 750 ° C. or higher, a recrystallized structure is almost obtained, and extremely excellent workability is secured. However, when annealing is performed at a high temperature exceeding 850 ° C, as is the case with ordinary cold-rolled steel sheets for ultra-low carbon processing, the steel structure becomes coarse and uneven, and during annealing, Precipitation of Ti or Nb is promoted, and uniform and fine structure of the structure after pipe making-heat treatment cannot be achieved. Therefore, the annealing temperature is preferably in the range of 650 to 850 ° C, and preferably in the range of 700 to 800 ° C in consideration of the stability of the material. Further, considering the economy and the stability of the material after the heat treatment, the temperature is preferably 780 ° C. or lower.

The soaking time for annealing is also one of the important components. In conventional annealing, it was usual to perform annealing for at least about 30 seconds in order to obtain a stable recrystallized structure.However, in this case, precipitation of Ti or Nb during annealing causes solid solution required by the present invention. It becomes difficult to secure Ti and Nb. As described above, by setting the annealing temperature to 850 ° C or less and the soaking time to 20 seconds or less,
Ti and Nb can be secured. Conventionally, in such a short-time annealing, it has been considered that the r value and ductility are insufficient in the ultra-low carbon steel presumed to be used for deep drawing, but the use of the present invention is not sufficient. Applicable without problems.

Secondary Cold Rolling after Annealing: Secondary cold rolling performed after annealing not only adjusts the surface roughness but also reduces the thickness of the sheet. The secondary cold rolling reduction for this purpose is 1.0
% Or more is desirable. However, when the secondary cold rolling is performed at more than 20%, particularly the yield stress among the mechanical properties increases, so that the pipe formability deteriorates. Therefore, after annealing,
The rolling reduction of the next cold rolling shall be 20% or less. Preferably 1.0
~ 10%.

Surface treatment: The steel sheet according to the present invention can be manufactured through the above-described steps. The final thickness of the steel sheet is not particularly limited, but the advantage of applying the present invention is more effectively exhibited in the range of 0.35 mm or less. The steel plate described above is plated with a metal having a self-brazing action, such as copper, and is subjected to a brazing treatment by a heat treatment after pipe production. Therefore,
Although further plating treatment is basically unnecessary, it is possible to perform chemical and electrochemical treatments as necessary to supplement the action of the metal plating.

[0038]

EXAMPLES Example 1 A steel having the composition shown in Table 1 and the balance substantially consisting of Fe was smelted in a converter, and this steel slab was hot-rolled (after hot rolling was completed) under the conditions shown in Table 2. Rapid cooling at 50 ° C / sec within 0.5 seconds).
In hot rolling, a 260 mm thick slab is roughly rolled in four passes,
As a 30 mm-thick sheet bar, a 2.6 mm hot rolled mother plate was manufactured by a tandem rolling mill of 7 stands. Next, it was pickled, cold rolled by a tandem rolling mill, and subjected to annealing and secondary cold rolling. This steel plate is electroplated with a thickness of 30 μm,
It was formed into a 3.45 mmφ double wound pipe by a usual method, and after 5% drawing, heat treatment was performed at 1120 ° C. for 20 seconds to melt and braze the copper plating layer.

[0039]

[Table 1]

[0040]

[Table 2]

The following investigation was carried out on the steel sheet and the self-brazing double-wound pipe manufactured as described above. 1) Ferrite grain size in cross section 2) Tensile strength by static tensile test 3) Drawing (evaluation of toughness) by low temperature (-40 ° C) tensile test: equivalent to shocking high speed tensile 4) Bending test (180 °) Bending) In each of the tests, the method was the same as that used for investigating ordinary mechanical characteristics, except that a double-wound pipe was carried out as it was.

Table 3 shows the obtained test results. The present invention example, in which the amounts of Nb and Ti in the solid solution state are within the appropriate range, has sufficient strength and ductility, and good toughness at low temperature (drawing by a tensile test) without causing the crystal grains to become coarse even in a high-temperature heat treatment. ), Bending workability and good shape freezing property. In addition, steel 12, 13 and 14 have a hard steel plate,
A good shape could not be secured at the stage of the final cold rolled steel sheet, and the bending workability was also poor.

[0043]

[Table 3]

Example 2 A slab having the composition of No. 1 in Table 1 was hot-rolled under the conditions shown in Table 4 (cooling conditions were the same as in Example 1), pickled, cold-rolled, and then continuously annealed. And secondary cold rolling was performed to produce an ultra-thin cold-rolled steel sheet. A box-annealed low carbon aluminum killed steel, which is a conventional steel, was used as a comparative example. Next, the surface of this steel plate was subjected to the same copper plating as in Example 1 to produce a double-wound pipe. As test items for evaluation, in addition to the test performed in Example 1, the amount of wear (die life) of the die used for pipe making was added. The evaluation of the mold life was represented by a relative ratio where the life of the comparative example (box-annealed low carbon aluminum killed steel) was set to 1.

[0045]

[Table 4]

The test results obtained are shown in Table 4. As can be seen from Table 4, the examples of the present invention are excellent in mold life, which is about 1.5 times higher than that of the comparative example because they are soft. Further, Nb and Ti in a solid solution state within the scope of the present invention.
In the case of containing, it is apparent that the coarsening of the tissue after the tube production is effectively suppressed.

[0047]

As described above, according to the present invention,
Since it is soft during pipe production, its deformation resistance is low, and the life of the mold can be extended by reducing the wear of the mold. Further, according to the present invention, not only the excellent formability but also the coarsening of the ferrite grain size is suppressed even after the tube forming-heat treatment step, so that the double winding having excellent properties such as strength and toughness is provided. It becomes possible to manufacture pipes. Further, according to the present invention, since the continuous annealing method is employed, high production efficiency and uniform material can be achieved. Therefore, according to the present invention, a high quality, highly airtight double wound pipe
It can be manufactured efficiently and economically.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of Nb in solid solution or the amount of Ti in solid solution and the crystal grain size of ferrite.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI C22C 38/54 C22C 38/54 (56) References JP-A-6-81080 (JP, A) JP-A-7-228921 (JP) JP-A-8-170148 (JP, A) JP-A-4-325656 (JP, A) JP-A-5-230595 (JP, A) JP-A-4-3111519 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (8)

(57) [Claims] (1) C: 0.0005 to 0.020 wt%, Nb: 0.003 to 0.040 wt%, and Ti: 0.005 to 0.060 wt%, and at least one of Nb and Ti Is present in a solid solution state in an amount of 0.005 wt% or more,
A steel sheet for a double-wound pipe having excellent formability and excellent pipe strength and toughness after forming and heat treatment, wherein the ferrite structure has a crystal grain size of 5 to 10 μm. 2. One or two of C: 0.0005 to 0.020 wt%, S: 0.02 wt% or less, N: 0.0050 wt% or less, Nb: 0.003 to 0.040 wt%, Ti: 0.005 to 0.060 wt%. Containing TiN, TiS, TiC
And NbC are calculated as having been formed in this order as much as possible. The amount of excess Nb and Ti is less than 0.005 wt%, and at least one of Nb and Ti is in a solid solution state. A steel sheet for a double-wound pipe which is present in an amount of 0.005 wt% or more and has a ferrite structure having a crystal grain size of 5 to 10 μm, and has excellent formability, and excellent pipe strength and toughness after forming and heat treatment. C: 0.0005 to 0.020 wt%, Si: 0.10 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.02 wt% or less, S: 0.02 wt% or less, Al: 0.100 wt% or less, N : Nb: 0.003 to 0.040 wt%, one or two of Ti: 0.005 to 0.060 wt%, the balance being steel composition of Fe and unavoidable impurities. Or the steel sheet for a double wound pipe according to 2. 4. C: 0.0005 to 0.020 wt%, Si: 0.10 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.02 wt% or less, S: 0.02 wt% or less, Al: 0.100 wt% or less, N : 0.005 wt% or less, Nb: 0.003 to 0.040 wt%, Ti: one or two of 0.005 to 0.060 wt%, B: 0.0005 to 0.0020 wt%, Cu: 0.5 wt% or less , Ni: 0.5 wt% or less, Cr: 0.5 wt% or less, Mo: 0.5 wt% or less, the balance being a steel composition of Fe and unavoidable impurities. Item 1
Or the steel sheet for a double wound pipe according to 2. 5. A steel material containing 0.0005 to 0.020 wt% of C: 0.003 to 0.040 wt% of Nb, and 0.005 to 0.060 wt% of Ti: an end temperature of 1000%.
Hot finish rolling at ~ 850 ° C, winding at 750 ° C or less, then cold rolling, continuous annealing at 650 ° C to 850 ° C for 20 seconds or less, secondary cold rolling at a rolling reduction of 20% or less A method for producing a steel sheet for a double-wound pipe having excellent formability, and excellent in pipe strength and toughness after forming and heat treatment, characterized by rolling. 6. One or two of C: 0.0005 to 0.020 wt%, S: 0.02 wt% or less, N: 0.0050 wt% or less, Nb: 0.003 to 0.040 wt%, Ti: 0.005 to 0.060 wt%. Containing TiN, TiS, TiC
And NbC are calculated as formed in this order as much as possible. A surplus of Nb and Ti, each of which is less than 0.005 wt%, is subjected to hot finish rolling at an end temperature of 1000 to 850 ° C. Winding at 750 ° C or less, then cold rolling, 650
Continuous annealing at ℃ ~ 850 ℃, 20 seconds or less, rolling reduction 20%
A method for producing a steel sheet for a double-wound pipe having excellent formability, and excellent pipe strength and toughness after forming and heat treatment, characterized by performing secondary cold rolling below. 7. C: 0.0005 to 0.020 wt%, Si: 0.10 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.02 wt% or less, S: 0.02 wt% or less, Al: 0.100 wt% or less, N And Nb: 0.003 to 0.040 wt%, and Ti: 0.005 to 0.060 wt%, and the balance is steel composition of Fe and inevitable impurities. 7. The method for producing a steel sheet for a double-wound pipe according to 6. 8. C: 0.0005 to 0.020 wt%, Si: 0.10 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.02 wt% or less, S: 0.02 wt% or less, Al: 0.100 wt% or less, N : 0.005 wt% or less, Nb: 0.003 to 0.040 wt%, Ti: one or two of 0.005 to 0.060 wt%, B: 0.0005 to 0.0020 wt%, Cu: 0.5 wt% or less , Ni: 0.5 wt% or less, Cr: 0.5 wt% or less, Mo: 0.5 wt% or less, the balance being a steel composition of Fe and unavoidable impurities. 7. The method for producing a steel sheet for a double-wound pipe according to 5 or 6.