JP2764170B2 - Manufacturing method of high ductility steel wire - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Object of the Invention" (Field of Industrial Application) The present invention relates to a method for producing a highly ductile steel wire.

(Prior Art) Extra fine wires used for tire cords or dot printers of about 0.2 mmφ are usually produced by drawing high-carbon steel wires of about 5.5 mmφ. However, in this method, since the material is hardened by wire drawing, it is necessary to apply a patenting process two or three times in the middle to recover ductility, which complicates the manufacturing process and increases the cost. there were.

Therefore, it has been attempted in recent years to use a low C-Si-Mn steel and use a wire composed of two phases of ferrite and martensite. This is one in which hard martensite is dispersed in a matrix of soft ferrite, and the workability is secured by ferrite, and the strength is secured by martensite. Typical examples of this manufacturing method, a wire from the two-phase temperature of ferrite + austenite of A _c1 ~A _c1 + 180 ℃, is intended to obtain a two-phase structure of ferrite + martensite by quenching. However, since the wire before heating usually has a relatively large grain structure of ferrite and pearlite, the resulting martensite structure is also massive and large. Cracks are easily generated from the interface of, and disconnection is easily generated. To prevent this, it is necessary to supercool the prestructure to bainite or martensite. Until now, intermediate heat treatment to obtain a fine prestructure offline and a two-phase structure of ferrite + martensite The finishing heat treatment to obtain it was indispensable.

FIG. 9 shows the thermal history of steel in the conventional method, together with the Fe-C phase diagram. That is, first, the wire is austenitized and quenched to obtain a supercooled structure mainly composed of bainite and martensite. After such an intermediate heat treatment, quenching is performed by further heating to a two-phase temperature range of ferrite + austenite.

Usually, this intermediate heat treatment is performed off-line.

(Problems to be Solved by the Invention) As described above, in the case of the conventional method, the complexity of the manufacturing process or the step of off-line intermediate heat treatment or the like cannot be omitted by any of the methods. efficiency,
High costs were inevitable. The present invention has been devised in view of such a situation, and a method of improving a conventional Stemmore method, omitting intermediate heat treatment offline, and producing a highly ductile wire excellent in drawability at a low cost. The purpose is to provide.

"Constitution of the Invention" (Means for solving the problem) In order to achieve the above-mentioned object, the present inventors have set C: 0.20 wt% or less, Si: 2.0 wt% or less, and Mn: 0.50 to 2.50 wt%. , P: 0.015 wt% or less, S: 0.015 wt% or less, Ti: 0.005 wt% or less, Al: 0.005 wt% or less, total nitrogen content: 50 ppm or less, total oxygen content: 10-40 ppm, and the balance substantially Fe In addition, a steel in which the size of oxide-based inclusions is 10 μm or less and the ratio of Al ₂ O _{3 in} the oxide is 40% or less is formed into a wire by hot rolling, and a non-concentric ring is formed. In the middle of the conveyance in the state, using a mist nozzle above the wire rod 0.5 to 1
0 m ³ / min water, water / water ratio of 100 Nm ³ / m ³ or less to generate fine water vapor mist, and the wire is cooled at a speed of 15 ° C./sec. , without a tissue mainly mixed structure of bainite or martensite or both, then a and該線material offline _c1 to a _c1 +
The present invention proposes a method for producing a highly ductile steel wire characterized by heating to 180 ° C. and quenching at a cooling rate of 50 ° C./sec. Or more. By adopting the method of the present invention, a highly ductile steel wire can be manufactured with high efficiency and at low cost.

(Function) A feature of the method of the present invention is that the Stellermore method is improved, and a fine water / water mist is sprayed from above on a hot-rolled wire conveyed in a non-concentric ring state, and blast from below. Spraying water to achieve uniform and effective rapid cooling. Under the strict regulation of chemical composition, generation condition of steam and water mist, and cooling rate as described in detail below, And the off-line intermediate heat treatment can be omitted.

To further describe the characteristics of the present invention, a cooling mist is different from a mist for cooling and atomizing simply by pressurizing and spraying feedwater, and a water / water mist which is sprayed under pressure using water and air having a specific air / water mixing ratio. Water droplets are miniaturized, and this fine air-water mist is blown from above the wire being transported, and a blast is blown from below. Therefore, it is forcibly cooled by the air-water mist from above and below. Usually, the wire conveyed in a non-concentric ring shape has more overlapping portions at the end than at the center, but the whole is cooled at a uniform speed by selecting the mounting position of the mist nozzle, the number, the air-water ratio, etc. .

The supercooled structure of bainite or martensite cannot be easily obtained by the stealmore method using a conventional conditioning cooling device.

Next, the reasons for the numerical limitations described in the claims of the present invention will be described.

A. Chemical composition ・ C: 0.20 wt% or less (hereinafter, wt is abbreviated to simply%).

C is an important element for securing strength, but 0.2
%, The martensite ductility is significantly reduced.

-Si: 2.0% or less.

Si is dissolved in ferrite and is an important element for strengthening ferrite. However, if it exceeds 2.0%, decarburization at a two-phase temperature of ferrite and austenite becomes remarkable, so that the content is set to 2.0% or less.

-Mn: 0.50-2.50%.

Mn enhances the hardenability of the steel and is an effective element for strengthening the steel. However, if the content is less than 0.5%, the effect of adding Mn is not exhibited. It was within this range.

・ P: 0.015% or less.

P segregates at the crystal grain boundaries, lowers the ductility of the steel, and lowers the drawability.

・ S: 0.015% or less.

S is combined with Mn, and as MnS, lowers the cleanliness of the steel and lowers the ductility of the steel.

・ Ti: 0.005% or less.

Ti has a strong bonding force with nitrogen, and TiN has a large size of 5 to 20 μm, and exists as an angular non-ductile inclusion, which significantly impairs the drawability of an ultrafine wire. It is necessary to sufficiently control the composition of the steel when it is melted. If 0.01% of Ti is contained, it is impossible to draw an ultrafine wire. 0.005% is the limit.

-Al: 0.005% or less.

Al has a strong bonding force with oxygen and easily forms an oxide. Also, similarly to nitrides, the wire drawing property is impaired, so it is necessary to minimize the drawability. In order to reduce the number of inclusions in the steel and make the inclusions themselves ductile, the content must be 0.005% or less.

・ Total nitrogen content: 50 ppm or less.

N forms a solid solution in ferrite and lowers its ductility, and at the same time, causes strain age hardening during drawing, thereby significantly lowering ductility. When N exceeds 50 ppm, disconnection frequently occurs.

・ Total oxygen content: 10-40 ppm.

O is an element serving as a base for oxide formation, and if it exceeds 40 ppm, the cleanliness of steel deteriorates. However, when the content is less than 10 ppm, the proportion of Al ₂ O _{3 in} the oxide-based inclusions increases, the inclusions become high-melting-point inclusions, and are less likely to be deformed during wire drawing.

・ About other elements.

In the present invention, C, Si, Mn and Fe are the main elements, but may contain a quenching improving element such as Cr, Mo, and the like, and a precipitation hardening element such as V. Nb, as long as the ductility is not impaired. Although not listed as elements to be regulated, Zr has the same effect as Ti, so care must be taken in its incorporation. In addition, unavoidable elements such as Cu and Ni are present in steel, but if they are trace amounts, they do not pose any particular problem.

B. oxide inclusions in the form for-oxide-based inclusions in Al ₂ O _3: total 40% or less.

Inclusions detected from the broken surface during wire drawing are Al ₂ O
₃ , MgO-Al ₂ O ₃ , etc., which has a high content of Al ₂ O ₃ and the high melting point (about 2000 ° C.) Al ₂ O ₃ -SiO ₂ -CaO
Low melting point (approximately 1200 ° C) as a multi-component inclusion of a refined slag system such as a system or Al ₂ O ₃ -SiO ₂ -CaO-MnO system
By doing so, disconnection can be prevented. For this purpose, the content of Al ₂ O ₃ in the entire oxide-based inclusion must be suppressed to 40% or less.

-Size of oxide inclusions: 10 µm or less.

If the size of the non-ductile oxide-based inclusions exceeds 10 μm, disconnection frequently occurs during wire drawing, so that the size was limited to 10 μm or less.

C. Conditions for rapid cooling ・ Air-water mist generation conditions: 0.5 to 10 m ³ / min water to air-water ratio of 10
Generated at 0 Nm ³ / m ³ or less.

The amount of cooling water was 0.5 to 10 m ³ / min is not sufficient cooling efficiency is obtained is less than 0.5 m ³ / min, is because the supercooled structure of bainite and martensite that this object can not be obtained, This is because the upper limit of 10 m ³ / min achieves the purpose and does not require any more water. The reason that the water / water ratio is set to 100 Nm ³ / m ³ or less is that if the water / water ratio exceeds 100 Nm ³ / m ³ , the number of water particles in the water / water mist decreases, the cooling capacity is deteriorated, and it is difficult to obtain a supercooled structure.

・ Cooling rate by air-water mist and blast: 15 ° C / sec or more.

At a cooling rate of less than 15 ° C./sec, it is necessary to add a quenching improving element such as Cr or Mo to obtain a supercooled structure of bainite or martensite.

D. Off-line processing conditions Heating temperature: A _{c1 to} A _c1 + 180 ° C.

Heating less than A _c1 does not provide the austenite necessary to obtain martensite, and exceeding A _c1 + 180 ° C.
This range was set because the amount of ferrite was reduced.

・ Quenching cooling rate: 50 ° C / sec or more.

If the temperature is lower than 50 ° C / sec., It becomes difficult to completely transform austenite into martensite by quenching. Water or oil is used as a quenching agent. By quenching, the ferrite structure remains as it is, austenite is transformed into martensite, and a two-phase structure of ferrite + martensite is obtained. In the present invention, the air-water mist is not a spray mist using only pressurized water, but refers to a water mist in which air is mixed and water is sprayed under pressure to make water droplets fine. The steam mist is sufficient only from the top,
The air-water mist may be blown upward from the lower surface as an auxiliary.

FIG. 3 is a conceptual diagram in which a cooling curve is superimposed on a continuous cooling transformation curve of steel, and a curve 10 is a Stellmore method and a curve 11 is a cooling curve of the present invention. In the Stellmore method, the cooling rate is low and the structure after transformation is ferrite + pearlite, whereas in the present invention, a supercooled structure of bainite + martensite is obtained. Depending on the composition of the steel, about 10% or less of ferrite may be mixed in the supercooled structure, but a small amount of ferrite does not pose any particular problem.

(Embodiment) FIG. 1 shows an example of an apparatus for carrying out the steam-water mist cooling step of the present invention. (A) is a front view, and (b) is a side view. Here, 1 is a wire rod, 2 is a conveyor, 3 is a blast,
Reference numeral 4 denotes an air supply pipe, 5 denotes a water supply pipe, 6 denotes a mist nozzle, 7 denotes a blast control plate, and 8 denotes a flow of a blast mist. That is, the air and water sent by the air supply pipe and the water supply pipe are mixed by the nozzle to form a steam mist, which cools the wire from the upper surface. A blast is blown from below to forcibly cool the wire from above and below simultaneously. It is necessary to reduce the amount of air-water mist at the center of the ring and increase it at the end according to the degree of overlap of the wires. Therefore, on the upper surface, the number of mist nozzles at the end of the ring was increased, and on the lower surface, the air hole of the plate was increased at the end to adjust the amount of blast and air-water mist.

FIG. 2 is a view showing the overlap of the non-concentric ring-shaped wires 1, where the overlap is large at the end of the conveyor 2, and especially at the extreme end, several rings of wires overlap in a substantially parallel state and have a layer thickness. I have. In the single-sided cooling of only the upper surface or the lower surface with respect to such a layer thickness portion, the wire located on the opposite surface is hardly cooled, resulting in a large non-uniform cooling rate, and as a result, the structure and strength vary greatly. . A wire rod obtained by performing a finishing heat treatment on a wire rod having such a structure and a strength variation also has a large variation in the structure and the strength. In order to obtain a supercooled wire having a uniform structure and strength, it is necessary to perform forced cooling from both the upper and lower surfaces particularly for the layer thickness portion. Therefore, in the present invention, the air-water mist is blown from the upper surface, and the blast is blown from the lower surface. (Note that a mist nozzle may be provided on the lower surface side to enhance the cooling by the impingement from the lower surface, or steam-water mist may be blown from the lateral direction to the layer thickness portion.) FIG. FIG. ₂ is a view showing a liquidus surface of a ternary phase diagram of —Al ₂ O ₃ —SiO ₂ . A low Al ₂ O ₃ side of ANORTHITE composition 1200 to 1300 ° C.
It can be seen that a low melting point composition region exists.

Table 1 shows the chemical components of the test materials used in the test and the _Ac1 temperature. Steel S is a steel according to the present invention having a low impurity element and excellent in the size and composition of oxide-based inclusions, and steel T is conversely rich in impurity elements and poor in the size and composition of the inclusions. It is steel. Table 2 shows the conditions of the intermediate heat treatment, and Table 3 shows the results.

That is, in the conventional Stemmore method under the cooling condition a, since the cooling rate is low, only the structure of ferrite + pearlite is obtained.

Fig. b shows the case where the impulse was stopped and air-water mist was sprayed only from the upper surface of the wire. However, the layer thickness was cooled on one side and the cooling rate varied, so that the microstructure was not only bainite but also ferrite + pearlite. Will remain and strength,
It has large ductility.

c is a case where the water amount of the steam-water mist is too small, and the cooling capacity is still insufficient and ferrite and pearlite remain.

d is the present invention in which the air-water mist having an appropriate amount of water and the air-water ratio and the blast are simultaneously blown. At this stage, there is no difference between the properties of the steels S and T.

e is the case where the amount of water of the steam mist is unnecessarily large,
The structure is bainite mixed with martensite and has high strength. Although there is no problem in characteristics, the purpose of the present invention is achieved with a tissue obtained with a smaller amount of water, so that useless cooling water is used.

f is a comparative example in which the sending of air was stopped and the spray water and the blast were blown at the same time. However, when the blowing of the air was stopped, the cooling rate became slightly lower and the operating conditions required careful attention. The resulting structure, strength and ductility are almost the same as d. It can be seen that the effect of the upper surface mist is large.

G is the case where the intermediate heat treatment was applied offline, and the structure is fine martensite.

Table 4 shows the conditions of the finishing heat treatment, that is, the condition h is the present invention of the proper heating temperature, cooling method and tempering temperature, i
Is the case where the heating temperature is too high, j is the case where it is too low, and k is the case where the cooling after heating is performed by air cooling.

The results are shown in Table 5 for each test No. That is, No. 1 requires a mist coolant having a baitnite structure in steel S.
The structure obtained is ferrite + fine martensite, and both strength and ductility are high.

No. 2 is a comparative example in which a mist coolant having a bainite structure was treated in steel T under condition h, but the obtained structure and characteristics are equivalent to those of steel S of No. 1 at this stage.

No. 3 is a case where a stellmore coolant having a ferrite + pearlite structure was treated under the condition h, but the obtained martensite was coarse and low in ductility.

Nos. 4 to 7 are each obtained by subjecting the mist coolant having the bainite structure shown in Table 3 to heat treatment under each condition, while No. 4 treated under the condition i becomes a single phase of austenite upon heating. The resulting structure is only coarse martensite and has very low ductility. No. 5 treated under the condition j has a too low heating temperature, is simply bainite in which the prestructure has been tempered, and has low strength. N processed under condition k
In the case of o.6, the cooling rate after heating was too low, the structure became ferrite + pearlite, and both the strength and ductility were low.

No. 7 obtained by treating a wire rod subjected to an appropriate intermediate heat treatment offline under the condition h has the same mechanical properties as No. 1 of the present invention.

FIG. 5 shows the electron microscope structures of a (Stellmore method) and d (Steel S of the present invention). It can be seen that the Stermore method has a coarse ferrite + pearlite structure, whereas the present invention has a fine undercooled structure. (In the structure a, the white part is ferrite, the black part is pearlite,
In the structure d, the whole is bainite in which fine carbides are dispersed in a ferrite ground. FIG. 6 shows the changes in the strength and the drawing of the wires of Nos. 1, 2, and 3 during drawing. The wire of No. 1 of the present invention had a small reduction in drawing and could be drawn to 0.2 mmφ, but it was No. 2 using non-clean steel T and No. 3 which was treated by the conventional Stellmore method.
Was disconnected on the way. 5.5mm before wire drawing of No.1 and 3
φ wire (No.1: (a), No.3: (b)) and during wire drawing
FIG. 7 shows the electron microscopic structure at the 1.0 mmφ line (No. 1: (c), No. 3: (d)). (The white part is martensite and the black part is ferrite in the structure of 5.5mmφ wire), while the structure of No.1 is fine before drawing and is uniformly deformed after drawing, In .3, the coarse structure was obtained before drawing, and the coarse martensite hindered uniform deformation even after drawing.

FIG. 8 shows an example of an oxide obtained by extracting a 5.5 mmφ wire rod of steel T by acid dissolution. In other words, it is a huge thing exceeding 10μm,
Further, as shown in Table 1, the composition is high Al ₂ O ₃ , high in melting point and non-ductile, and thus causes disconnection more easily than steel S.

In addition, in the apparatus used for carrying out the method of the present invention described above, an experiment was also conducted in which an upward mist nozzle was provided below the wire and air-water mist was blown to cool the wire. It was exactly the same as the example.

"Effects of the Invention" As described above in detail, the method of the present invention, during the transfer after the end of hot rolling, by remodeling the device of the Stellmore method, by efficiently utilizing the air-water mist and blast, Conventionally, it has succeeded in omitting those that required an intermediate heat treatment offline. It is an invention that makes it possible to produce high-ductility steel wires economically with high efficiency in combination with the limitation of the chemical composition, the specification of cooling conditions, etc., which is an invention that greatly contributes to the development of the industry. Can be said.

[Brief description of the drawings]

FIG. 1 is a drawing for explaining an apparatus for carrying out the air-water mist cooling step of the present invention, wherein (a) is a front view, (b) is a side view, and FIG. 2 is a non-concentric ring-shaped wire. FIG. 3 is a plan view showing the superposition, FIG. 3 is a table showing the continuous cooling transformation curve of steel superimposed with the cooling curve of the present invention and the Sterol method, and FIG. 4 is CaO-A.
FIG. 5 is a chart showing the temperature of the liquidus surface of the l ₂ O ₃ —SiO ₂ system phase diagram. FIG. 5 is an electron micrograph showing the respective metal structures obtained by the Stellmore method and the steam-water mist cooling according to the method of the present invention. FIG. 7 is a table showing the change in strength and drawing during drawing of the wire rod. FIG. 7 is an electron micrograph of the metal structures obtained by the method of the present invention and the comparative method at 1.0 mmφ before and during drawing. FIG. 8 shows an oxide-based inclusion contained in steel T extracted by dissolving a metal part in an acid and trapped on a sieve, and its particle structure is shown by an electron micrograph, and FIG.
The figure shows the conventional heat treatment process for producing a wire with a two-phase structure.
It is a chart shown in conjunction with the Fe-C system phase diagram. 1: wire rod, 2: conveyor, 3: blast, 4: air pipe, 5: water pipe: 6:
Mist nozzle, 7: plate for blast adjustment, 8: flow of blast mist, 9: steam mist.

Continuation of the front page (56) References JP-A-61-56264 (JP, A) JP-A-53-138917 (JP, A) JP-A-62-50414 (JP, A) JP-A-52-59015 (JP, A) JP-A-62-92719 (JP, A) JP-B-63-6289 (JP, B2)

Claims (1)

(57) [Claims] C: 0.20 wt% or less, Si: 2.0 wt% or less, Mn: 0.50 to 2.50 wt%, P: 0.015 wt% or less, S: 0.015 wt% or less, Ti: 0.005 wt% or less, Al: 0.005 wt% or less, total nitrogen content: 50 ppm or less, total oxygen content: 10 to 40 ppm, the balance is substantially Fe, and the size of oxide-based inclusions is 10 μm or less.
The steel in which the proportion of Al ₂ O ₃ is 40% or less is formed into a wire by hot rolling, and in the middle of being conveyed in a non-concentric ring state, using a mist nozzle above the wire by 0.5 to 10 m.
³ / min water, an air-water mist of fine particles with an air-water ratio of 100 Nm ³ / m ³ or less is generated, and the wire is cooled at a speed of 15 ° C./sec. None the tissue mainly mixed structure of bainite or martensite or both, then a _c1 to該線material offline to a _c1 +18
A method for producing a highly ductile steel wire, comprising heating to 0 ° C and quenching at a cooling rate of 50 ° C / sec. Or more.