JPS647139B2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) The present invention is applicable to springs, PCs (prestressed
When manufacturing steel wire rods used for concrete (for concrete) steel wires, prestressed steel stranded wires, etc., hot-rolled steel wire rods in a high temperature state are adjusted and cooled with a refrigerant to produce wire rods with excellent wire drawability. This invention relates to improvements in the so-called direct heat treatment method.

(Background Art) The purpose of the direct heat treatment method for steel wire is, for example, to cool the steel wire immediately after hot rolling (hereinafter simply referred to as wire) at an appropriate cooling rate and almost uniformly over the entire length of the coil, thereby improving its metallographic structure. The purpose is to make the wire mainly composed of fine pearlite, and to make the strength and wire drawability similar to that of lead patenting treatment. By using this wire, the patenting process can be omitted depending on the wire diameter and product quality specifications. However, with the conventional direct heat treatment method, for example, when the wire diameter is large and high strength is required, such as for PC,
The tensile strength is about 10 kg/mm ² lower than that of lead patenting, and the variation in strength is also inferior, so lead patenting cannot be omitted at present.

Conventionally, various methods have been proposed as direct heat treatment methods for wire rods, but each method has the following losses.

Stelmer method in which ring-shaped coils are forcedly air-cooled while being rolled out on a horizontal conveyor (Special Publication No. 42-15463)
No.), there is no localized quenching section and a fairly homogeneous wire can be obtained, but the cooling power is weak and the strength is insufficient. Although this strength increases to some extent due to strong winds, the overlapping portions of the rings are ineffective and therefore induce non-uniformity.

Alternatively, the wire rod is formed into a ring-shaped coil and wound up in hot water (Special Publication No. 85336, 1972), or immersed in hot water while being transferred on a horizontal conveyor (Special Publication No. 1977-85336).
No. 8089) With the hot water cooling method, a homogeneous wire can be obtained by cooling in boiling water, but it lacks strength (tensile strength is 10 kg/ ^mm2 lower than lead patenting), and the tensile strength remains low even after strong agitation by blowing air. 5~
7Kg/mm ² low. In addition, with supercooled boiling cooling (water temperature, 95℃ or less), the strength increases, but film boiling is unstable, nucleate boiling is induced even at high temperatures, localized rapid cooling occurs, and a martensitic structure is generated. It becomes defective.

(Disclosure of the Invention) The present invention has been made to solve the above-mentioned problems, and does not induce nucleate boiling even in supercooled boiling cooling, obtains a necessary and sufficient cooling rate only by film boiling,
It is an object of the present invention to provide a direct heat treatment method capable of producing a steel wire rod having strength equivalent to that obtained by lead patenting, less variation, homogeneity, and good wire drawability.

The present invention involves continuously transporting a ring-shaped coil of hot-rolled steel wire at a high temperature with an austenitic metallographic structure in a horizontally expanded form.
In the method of controlled cooling and direct heat treatment to transform into a pearlite structure, a ring-shaped coil of steel wire spread horizontally is left to cool in the air for 3 to 20 seconds to oxidize the surface, and then immediately The speed is 3 to 20 cm/sec, the gas mixed phase ratio is 0.1 to 0.35,
When the oxygen concentration Y (%) in the bubbles is the refrigerant temperature X°C, the refrigerant is maintained at a predetermined temperature between 70 and 95°C, which is Y - 1/3 This is a direct heat treatment method for steel wire, which is characterized by immersing it in and passing it through.

In the present invention, the steel wire rod at high temperature refers to a wire rod that is hot-rolled and is made of carbon steel or alloy steel to which a small amount of alloying elements such as Ni, Cr, V, Mo, or W is added. This means that the alloy structure exhibits an austenate structure.

The present inventors investigated various surface treatment conditions and refrigerant conditions in order to obtain a predetermined cooling rate to obtain a strength comparable to that of lead patenting, prevent the induction of nucleate boiling, and provide uniform cooling. result,
The desired purpose can be achieved by oxidizing the surface of the wire and immersing it in a refrigerant consisting of an air-water multiphase fluid containing oxidizing bubbles at 95°C or below, that is, by performing surface chemical treatment and cooling treatment at the same time. This is what I discovered.

Hereinafter, the present invention will be explained by examples using the drawings.

1 and 2 are diagrams showing an example of a direct heat treatment apparatus used in an embodiment of the method of the present invention, with FIG. 1 being a longitudinal cross-sectional view and FIG. 2 being a cross-sectional view. In the figure,
Reference numeral 1 denotes a ring-shaped coil (hereinafter referred to as a coil) formed by forming high-temperature rolled steel wire into a predetermined ring diameter using a laying head (not shown). It is left to cool inside.

On this conveyor 2, the surface of the coil 1 is air oxidized. This is to prevent the induction of nucleate boiling due to the surface oxide film during the next immersion in the refrigerant, and to improve the cooling rate at the film boiling stage.
~20 seconds is required.

After surface oxidation by preliminary air cooling, the coil 1 falls onto the horizontal conveyor 3 in the heat treatment tank 4 and is transferred in a horizontally developed form. The heat treatment tank 4 contains a refrigerant 5 made of air-water multiphase fluid, and a conveyor 3 is connected to the refrigerant 5.
The upper coil 1 is immersed for a predetermined time.

The refrigerant 5 is in a state of strong stirring, contains a large amount of oxidizing bubbles 6 in hot water to form a mixed state of air and water, and is maintained at a temperature of 95°C to 70°C. The oxidizing bubbles 6 may be made of a gas containing oxygen, such as oxygen, oxygen-enriched air, or air.

In order to contain a large amount of oxidizing bubbles 6 in hot water, for example, air 8 is supplied by a gas supply system 7 in the figure.
Blow into the top of the warm water to create bubbles.

Note that this gas may be blown in from the bottom or side of the hot water. Alternatively, a gas/water mixed phase fluid containing a large amount of oxidizing bubbles may be prepared outside the heat treatment tank 4 and supplied into the tank from the top, side, or bottom of the tank 4.

Such refrigerant 5 is strongly agitated throughout the tank by a plurality of agitators 9, and the coil 1 is cooled with the air-water multiphase fluid in a strongly agitated state, thereby receiving a predetermined controlled cooling. . The refrigerant and disturbance conditions in this case will be described later.

In addition, the horizontally deployed coil 1 has dense overlapping at both ends in the direction perpendicular to the running direction, and in order to equalize the cooling rate, the cooling of this part is preferentially strengthened. measures will be taken. For example, increase the agitation in this part or increase the inclusion of air bubbles.

The coil 1 that has been cooled for a predetermined period of time is lifted out of the refrigerant 5 by, for example, an inclined conveyor 10 for carrying out, and is collected into a collection machine (not shown).

Next, the following experiments were conducted under various conditions in the method of the present invention.

(Experiment example) The steel wire sample is 11.0mmφ, C0.8%, Si0.2
%, Mn0.7% SWRH82B (JIS standard) wire rod, heated to 950℃ in a non-oxidizing atmosphere, left to cool in air for various times to air oxidation, then immersed in various refrigerants, A controlled cooling experiment was conducted.

(1) As a refrigerant, (a) hot water (conventional method) and (b) air-water multiphase fluid (invention method if the refrigerant temperature is 95°C or lower, air blown into hot water. However, the refrigerant temperature is 100°C) (No blowing in.) Air oxidation time before refrigerant immersion is 0.5 seconds or less, 3
It was immersed in a refrigerant maintained at 70 to 100°C for 100 seconds for 5 seconds, 10 seconds, and 15 seconds, and then pulled out.

The relationship between the refrigerant temperature and the tensile strength of the treated wire at each air oxidation time is shown in FIG.

The following can be seen from Figure 3.

(a) The material using the air-water multiphase fluid of the present invention has greater strength than the material using hot water.

(b) In air-water mixed-phase fluids, when the air oxidation time is 5 seconds or less, nucleate boiling does not occur before the transformation is completed at temperatures above 75°C, stable film boiling is obtained, and a wire with high strength can be obtained. Tensile strength around ℃ 125
The strength of Kg/mm ² is comparable to that of lead patenting, and the lower the temperature, the higher the strength.
Its increase rate is greater than hot water cooling. On the other hand, with hot water cooling, if the air oxidation time is 3 seconds or more, nucleate boiling will be induced before the transformation is completed at temperatures below 90°C, a martensitic structure will occur due to local rapid cooling, the strength will deteriorate, and the air oxidation time 0.5
In seconds or less, it's a little better. Martensitic structure does not occur even at 80℃.

(c) In air-water mixed phase fluids, the longer the air oxidation time before immersion, the greater the increase in strength.

Based on these results, the temperature of the refrigerant is 70 to 95℃.
is suitable, preferably 75 to 90°C. 70℃
If it is less than 95°C, nucleate boiling tends to occur, martensitic structure is formed and the strength is deteriorated, and if it exceeds 95°C, the strength becomes insufficient. Also, if it is below 75°C, there is a risk of nucleate boiling, and if it exceeds 90°C, it will not be possible to obtain the same strength as lead patenting.

Further, the appropriate time for air oxidation before immersion in the refrigerant is 3 to 20 seconds. Note that this cooling in the air is normally performed after exiting the rolling mill until coil forming and refrigerant immersion, so it is not necessarily necessary to provide a device for cooling (such as a conveyor). If the time exceeds 20 seconds, the increase in strength will be saturated, and it will be time consuming and uneconomical.

(2) A sample with an air oxidation time of 4 seconds was immersed in the same refrigerants as in item 1 held at 80°C: (a) hot water (conventional method) and (b) air-water multiphase fluid (invention method), and the cooling rate was measured. investigated. The temperature of the sample before immersion was 950°C.

The relationship between the elapsed immersion time and the temperature of the wire is shown in FIG.

From FIG. 4, it can be seen that with the air-water mixed phase fluid, cooling is extremely stable, the desired cooling rate can be obtained, and nucleate boiling always occurs below 500°C, so there is no problem. In contrast, with hot water cooling, the cooling rate varies greatly from test to test, and reproducibility is low. It is observed that nucleate boiling is likely to occur, and the temperature at which it occurs is not constant.

(3) The feature of the air-water multiphase fluid in the present invention is that the air bubbles escape from the fluid due to buoyancy from time to time.
Since its physical and chemical properties are affected by superficial velocity (volume of gas passing per unit time and unit area), we investigated the influence of superficial velocity on the strength of the wire.

As a refrigerant, (b) an oxidizing gas-entrained fluid and (c)
A nitrogen gas-containing fluid (nitrogen gas blown into hot water) was used (no gas blown at superficial velocity 0) and maintained at 82°C. A sample with an air oxidation time of 4 seconds was processed at a superficial velocity of 0 to 10 cm/sec.
The specimens were immersed and treated in each refrigerant for 100 seconds.

The relationship between the superficial velocity and the tensile strength of the wire is as shown in FIG.

From FIG. 5, it can be seen that (b) the fluid-based method of the present invention is
When the superficial velocity increases to 1 cm/gsec or more, film boiling becomes stable, and (c) the tensile strength is much higher than that using fluid, and it can be seen that the tensile strength increases as the superficial velocity increases. . This is because increasing superficial velocity increases disturbance, increases the heat transfer coefficient, and increases the cooling rate. That is, when the superficial velocity is sufficiently high, the water temperature around the wire is always maintained at the set temperature, and a wire with a high tensile strength corresponding to the set temperature can be obtained. In contrast,
When the superficial velocity decreases, the circulation of hot water around the wire worsens and stagnation occurs, and the water temperature increases due to the heat flux supplied from the wire. It is thought that for this reason, the cooling rate of the wire rod decreases, and the tensile strength of the obtained wire rod also decreases.

On the other hand, (c) one using fluid has extremely low tensile strength. This is because nucleate boiling is likely to occur and the cooling rate becomes abnormally high, resulting in the formation of a martensitic structure.

From these results, it is appropriate that the superficial velocity of the air-water mixed phase fluid is 3 to 20 cm/sec. If it is less than 3 cm/sec, the strength improvement effect will be insufficient, and if it exceeds 20 cm/sec, "blow-through" (single phase formation due to bubble coalescence) will occur, which is not good.

(4) Using a refrigerant consisting of air-water multiphase fluid (b), the relationship between superficial velocity, gas hold, up (air-water mixed phase ratio), and approximate disturbance strength with and without mechanical stirring is as follows. As shown in Figure 6.

From Figure 6, the above-mentioned required superficial velocity is 3 to 20 cm/
In seconds, the air-water mixed phase ratio is 0.1 to 0.35, and the approximate disturbance intensity is 5 to 7×10 ³ erg/cm ³ .

If it is less than these ranges, the strength improvement effect will be insufficient, and if it exceeds these ranges, "blow-through" will occur.

(5) Using a refrigerant made of air-water multiphase fluid (b), the oxygen concentration and bubble expansion coefficient at a refrigerant temperature of 70 to 100°C were investigated, and the results are shown in FIG.

From FIG. 7, an appropriate oxygen concentration in the oxidizing bubbles is 10% or more at a refrigerant temperature of 75°C, and 5% or more at a refrigerant temperature of 90°C. Expressing this relationship using a formula, if the oxygen concentration is Y% and the refrigerant temperature is X°C, it is approximated as Y-1/3X+35.

The above formula was obtained as follows.

(1) The saturated water vapor partial pressure in the air is expressed by the following equation (1), where the temperature is T(K).

P = ^8.071 It is expressed as

V=T/To(1-P)・Vo (2) where Vo is the volume of blown air (3) The dilution of O ₂ due to volume expansion is calculated as follows, assuming the air oxygen concentration Y is 21%.

Y = 21 / (V / _{Vo )} X ₁ = 70℃ Y ₁ = 21 / 1.807 = 11.6% =0.334, B=34.98 From this, the above formula can be found.

When air is mixed into hot water to produce a refrigerant made of a gas-water multiphase fluid, water vapor immediately evaporates into unsaturated water vapor bubbles and saturates them. the result,
While the effective superficial velocity, ie the disturbance force, increases, the oxygen concentration becomes diluted. This is advantageous for stirring power and bubble mixing ratio, but is disadvantageous for oxidizing power, so when mixing air into hot water, to increase the oxygen concentration if necessary, you can mix oxygen gas using the above formula. . In actual results, a predetermined film boiling was stably obtained within the above range.

Next, the cooling rate of the wire rod is determined by the above experimental examples (1) and (3).
By appropriately combining the necessary conditions obtained in ~(5), the wire temperature is 15~25℃ in the range of 900~650℃ as shown in Figure 4.
10-15 seconds, in the range of 630-500℃ after metamorphosis is finished
Preferably, the temperature is controlled at ℃/second. 900~650℃
If the temperature is less than 15℃/sec, the transformation temperature will shift to a high temperature, resulting in insufficient strength; if it exceeds 25℃/sec, the transformation temperature will shift to a low temperature, and in some cases, some martensitic transformation will occur instead of pearlite transformation. not good.
In addition, if the temperature is less than 10℃/sec in the range of 630 to 500℃, untransformed austenite transforms into a pearlite structure that is not very fine, resulting in insufficient strength.If the temperature exceeds 20℃/sec, there is usually no problem, but in materials with segregation, martensite Tissues tend to form, which is not good. In addition, the hardenability of steel increases with wire rods containing alloying elements,
The above conditions are shifted to the lower cooling rate side.

Also, pearlite transformation begins at around 600°C, and at this time it must be cooled at a rate of 2 to 3 Kcal/Kgsec. If it is less than 2Kcal/Kgsec, the transformation temperature will rise to a high temperature, resulting in insufficient strength, and if it exceeds 3Kcal/Kgsec, the transformation temperature will shift to a low temperature, easily inducing martensitic transformation.

The preferred conditions for the method of the present invention obtained from the above experimental examples depend on the steel type of the wire rod, size, coil size, wire speed, refrigerant capacity, type of oxidizing gas, tank length, etc. be selected accordingly.

(Example) Hot rolled 11.0mmφ C0.82%, Mn0.72
%, Si0.22% SWRH82B (JIS standard) steel wire was directly heat treated by the method of the present invention using the apparatus shown in FIGS. 1 and 2.

The rolling speed of the wire was 9 m/sec, the wire temperature immediately after rolling was 920°C, and the wire was formed into a coil with a ring diameter of 1050 mm. A steam-water multiphase fluid, which is made by blowing air into hot water, is used as a refrigerant, and the temperature is maintained at 82℃, and the superficial velocity is 10.
cm/sec, and the gas mixing ratio was approximately 0.2.

The wire temperature was pre-cooled uniformly over the entire length from 920℃ to 850℃ using a water cooling nozzle, air oxidized for about 10 seconds, then immersed in heat treatment tank 4, treated for about 25 seconds, and then transferred to tank 4.
It was then pulled up and directly heat treated.

For comparison, the same hot-rolled wire rod was cooled by immersion in hot water maintained at 98°C and then directly heat-treated using the conventional method. The resulting coil was sampled continuously at intervals of 40 cm, and the tensile strength was measured.

The distribution of tensile strength according to the method of the present invention and the conventional method is as shown in FIG.

From FIG. 8, the product according to the present invention has an average tensile strength of 125.9 Kg/mm ² , which is comparable to lead patenting.
It can be seen that it shows a normal distribution.

In contrast, the conventional method has a tensile strength of approximately
11Kg/mm ² low.

(Effects of the Invention) The method for direct heat treatment of steel wire of the present invention configured as described above has the following effects.

(a) Adjusted cooling is performed by immersing and passing the steel wire through a refrigerant consisting of a gas-water multiphase fluid that is under strong stirring and that is maintained at a temperature of 95°C or lower and contains a large amount of oxidizing bubbles. Therefore, the wire surface is oxidized by exposure to the air immediately after rolling or cooling in the air and oxidizing bubbles in the refrigerant to form an oxide film, and then the wire is immersed in an air-water multiphase fluid containing oxidizing bubbles. As a result, nucleate boiling does not occur even during subcooled boiling cooling, and the desired cooling rate can be achieved extremely stably, resulting in a steel wire with strength comparable to lead patenting, less variation, and excellent wire drawability. can be manufactured.

(b) When a gas/water multiphase fluid containing a large amount of oxidizing bubbles is used and a large amount of gas unsaturated with water vapor is mixed in, a large amount of water vapor evaporates into the bubbles toward equilibrium vapor pressure, and as a result, the refrigerant Since the temperature of the refrigerant decreases, the refrigerant has self-cooling properties, and this can be effectively used to control the refrigerant temperature, so that the temperature of the refrigerant can be maintained economically. Note that this cooling capacity can be easily calculated from the ratio of the processing capacity of the wire (T/hour) and the temperature of the refrigerant.

[Brief explanation of drawings]

1 and 2 are diagrams showing an example of a direct heat treatment apparatus used in an embodiment of the method of the present invention, with FIG. 1 being a longitudinal cross-sectional view and FIG. 2 being a cross-sectional view. Figure 3~
Figure 7 is a diagram showing the results obtained in the experimental examples of the present invention, Figure 3 shows the relationship between the refrigerant temperature and the tensile strength of the wire rod after treatment for each air oxidation time, and Figure 4 shows the relationship between various refrigerant Figure 5 shows the relationship between the superficial velocity of various refrigerants and the tensile strength of the wire, Figure 6 shows the relationship between the superficial velocity of the refrigerant and the gas mixed phase ratio, and Figure 7 shows the relationship between the superficial velocity of the refrigerant and the gas mixed phase ratio. Oxygen concentration and bubble expansion rate at 100°C are shown. FIG. 8 is a diagram showing the tensile strength distribution of steel wire rods manufactured by an example of the method of the present invention and a conventional method. DESCRIPTION OF SYMBOLS 1... Ring-shaped coil, 2, 3, 10... Conveyor, 4... Heat treatment tank, 5... Refrigerant, 6... Oxidizing bubbles,
7... Gas supply system, 8... Air, 9... Stirrer.

Claims (1)

[Claims] 1. A ring-shaped coil of hot-rolled steel wire at a high temperature with an austenite metal structure is continuously transported in a horizontally expanded form and cooled in an adjusted manner so as to transform it into a pearlite structure. In the direct heat treatment method, a horizontally developed ring-shaped coil of steel wire is left to cool in the air for 3 to 20 seconds to oxidize the surface, and then immediately after the superficial velocity reaches 3 to 20 seconds.
When the temperature is 20cm/sec, the gas mixed phase ratio is 0.1 to 0.35, and the oxygen concentration Y (%) in the bubbles is the refrigerant temperature of X℃, it is maintained at a predetermined temperature between 70 and 95℃, which is Y - 1/3X + 35. immersed in a refrigerant consisting of a disturbed air-water multiphase fluid,
A method for direct heat treatment of steel wire, characterized by passing it through. 2 Adjusted cooling increases the cooling rate of steel wire from 900 to 650
15-25℃/sec in the range of ℃, after the metamorphosis is almost completed
A method for direct heat treatment of steel wire according to claim 1, characterized in that the heat treatment is carried out at a temperature of 630 to 500°C and controlled at a rate of 10 to 15°C/sec.