JPH0368087B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat treatment method for imparting strength and wire drawability to steel wire rods (hereinafter simply referred to as wire rods).
The present invention relates to an improvement in a method of immersing a high-temperature wire rod in hot water or a hot aqueous solution to provide a desired cooling rate and subjecting it to a predetermined heat treatment. It is known to heat-treat a high-temperature steel wire by cooling it in hot water (e.g., Japanese Patent Publication No. 45-8536), and it is also known to heat-treat it by continuously immersing it in hot water (e.g., Japanese Patent Publication No. 46-8089). No.). With these conventional methods, it is possible to select an appropriate temperature for the hot water, but unlike oil, the heat transfer coefficient of water varies greatly depending on the temperature of the water, so the temperature of the hot water must be determined very strictly. that it is necessary to manage
Furthermore, when processing hot-rolled wire rods, for example, the amount of heat that the wire rods bring into the hot water is extremely large, which poses the big problem of requiring large-scale equipment to maintain the temperature of the hot water at a desired temperature. Therefore, it is common practice to set the water temperature to the boiling point and use the heat of evaporation to cool the water. When using boiling water in this way, there is an advantage that the cooling conditions are stable, but on the other hand, if it is used as it is,
It is difficult to change the cooling conditions, and in some cases the cooling rate may be too slow. For example, with large diameter wires, there is a problem that the desired heat treatment cannot be performed. This problem can be solved by, for example, applying a coating on the surface of the wire that can become the nucleus of nucleate boiling and then immersing it in hot water (Japanese Patent Application Laid-Open No. 48-34727), or by cooling the wire at the initial stage of cooling. In order to increase the speed, there is a method of pre-cooling by spraying cold water, etc., or a method of first immersing it in a bath of water below the boiling point for rapid cooling, and then cooling it in boiling water (Special Publication No. 16217-1981). has already been proposed. Among these known methods, Japanese Patent Application Laid-Open No. 48-34727
In the method of No. 1, the coating condition is not stable, resulting in poor uniformity of cooling conditions.In the method of Japanese Patent Publication No. 55-16217, in order to increase the cooling rate to 50°C/sec or more, hot water of 70°C or less is used. Since the cooling conditions are unstable and sometimes partial supercooling occurs in the evaluation circle, it is difficult to manage the end point of precooling. Furthermore, in the case of hot-rolled wire rods, there is a problem in that it is extremely difficult to control the end point of pre-cooling due to variations in finishing temperature and variations in scale adhesion. Furthermore, as mentioned above, special equipment for keeping the water temperature constant also poses a major problem. The present invention was achieved as a result of various studies in order to solve the above-mentioned problems, and by using a water tank with a special temperature gradient, it is possible to prevent the formation of supercooled structures on the surface of the wire. Furthermore, the present invention provides a method for heat treating steel wire rods in which cooling conditions can be easily controlled. Taking the patenting treatment of high carbon steel wire as an example, the austenitizing temperature of the wire is generally 900°~
1000℃, let's say 950℃, and convert it to 50℃
Consider a patenting process in which the material is pre-cooled with hot water, cooled to a temperature near the transformation point, for example 650°C, and then cooled in boiling water. In this case, the reason why the preliminary cooling process is difficult to stabilize is
Since the water temperature is low, even when cooling by film boiling, the cooling rate of the core of the wire is 50℃/second or more, and the surface is even more rapidly cooled, so a slight difference in immersion time can easily appear in the surface temperature. Moreover, since the stability of film boiling is low, there is a risk that nucleate boiling will occur locally and the cooling rate will increase significantly. As a result, local temperature unevenness occurs particularly on the surface, and depending on the location, there is a risk of forming martensite or bainite in supercooled areas. Therefore, even if the immersion time and the water temperature are controlled, the resulting end point temperature varies greatly, which poses a problem. The present inventors have conducted various studies on increasing the cooling rate only in the temperature range near the transformation point, which is important for cooling, without making the preliminary cooling rate unnecessarily high. It was discovered that a directly heat-treated wire rod with high tensile strength can be obtained even in a wire rod having a large diameter, and the present invention was completed based on this finding. That is, the present invention provides a method for heat-treating a steel wire rod at high temperature by continuously immersing it in high water or in an aqueous solution enriched with additives, in which the wire rod is cooled from just above the _A1 transformation point of the wire rod. The wire rod is placed in a bath of hot water or a hot aqueous solution having a temperature gradient at least in the middle of the traveling direction, and the temperature on the inlet side is lower than the boiling point and the temperature on the outlet side is higher than the inlet side. This is a method for heat treatment of steel wire, characterized in that the heat treatment is carried out by passing the wire through the wire. In the present invention, the steel wire rod is a steel wire rod made of carbon steel containing 0.01 to 1.0% C, steel containing unavoidable impurities, other elements for improving strength, etc. Further, the term "steel wire rod at high temperature" means a hot-rolled high-temperature wire rod or a reheated high-temperature wire rod (a wire rod in the process of being drawn). Hereinafter, the present invention will be explained by examples using drawings. FIG. 1 is a longitudinal sectional view and a hot water temperature distribution diagram showing an example of a continuous heat treatment apparatus used in an embodiment of the method of the present invention. In the figure, a steel wire 1 at a high temperature passes continuously through a pre-cooling tank section 2, a temperature gradient tank section 3, and a post-cooling tank section 4 and is wound up. Each tank part 2, 3, 4
Hot water or a hot aqueous solution containing additives is housed inside. As the additive, for example, a surfactant such as PVA (polyvinyl alcohol), a substance having an effect of preventing rust or forming a lubricating film, etc. are used. As shown in the figure below, the water temperature in the pre-cooling tank section 2 and the post-cooling tank section 4 is maintained near the boiling point, and the water temperature in the temperature gradient tank section 3 is maintained at a low temperature (e.g., below the boiling point) at the inlet side. , 80℃), the temperature on the outlet side is higher than the temperature on the inlet side (e.g., boiling point 100℃), and all or part of the section of this tank is lower than the boiling point from the inlet side to the outlet side. A temperature gradient is provided in the direction of travel from a low temperature to a high temperature approaching the boiling point. The temperature on the outlet side needs to be higher than the temperature on the inlet side, and may be considerably lower than the boiling point. Reference numerals 16 and 17 are wire thermometers for measuring the temperature of the wire rod 1 exiting the tank sections 2 and 3, respectively. The steel wire 1, which is at a high temperature (eg, 950°C), is first precooled by boiling water in the tank 2, and the wire temperature is brought to a level just above the _A1 transformation point (eg, 750°C).
In this case, the wire is cooled by film boiling, so the cooling rate is relatively slow and very stable, and the temperature of the wire can be easily controlled. Note that this preliminary cooling is not necessarily necessary in the method of the present invention, and depending on the initial linear temperature (eg, 800° C. or less), it may be omitted and immersed in the temperature gradient bath section 3 from the beginning. Next, the wire 1 near the A ₁ transformation point is guided to the temperature gradient bath section 3 and cooled. In this cooling process, the cooling rate is the highest in the low-temperature part of the inlet side, after which the cooling rate gradually decreases, and the cooling rate approaches the cooling rate of boiling water in the outlet side to prevent the occurrence of supercooling phenomena. , it is possible to perform the desired heat treatment even on large-diameter wire rods. The cooling conditions in this case are adjusted by adjusting the cooling rate of the wire, the temperature distribution in the tank, the immersion time, or the immersion length.Furthermore, in creating the temperature gradient, the evaporation of the hot water is controlled as described later. Form using make-up water to make up for the amount. Regarding the mechanism by heat treatment according to the method of the present invention,
I will explain in more detail. For example, when patenting a high-temperature high-carbon steel wire by continuous cooling (this also applies to the case where a hot-rolled high-temperature wire is cooled by immersion in boiling water), cooling that substantially affects the tensile strength Note that the rate is the cooling rate below the so-called transformation point, and that the cooling rate becomes irrelevant after the transformation is complete, so it is sufficient to use ordinary boiling water immersion as described above as pre-cooling. Moreover, it is possible to suppress scale formation, and if the cooling rate in this case is used, the temperature near the transformation point (e.g.
It takes, for example, 10 to 15 seconds to reach 750℃.
It can be seen that wire temperature control is easy by adjusting this immersion time. At the cooling rate of boiling water cooling, the temperature difference between the surface and core of the wire is approximately 20°C.
(Wire diameter: 10mm), approximately 60℃ for hot water at 50℃
℃ (or higher), the probability of overcooling occurring is greatly reduced. The wire is then immersed in hot water of, for example, around 80°C to be cooled. If the relationship between the moving speed of the wire and the flow rate of the hot water is constant, the hot water will be heated by the amount of heat transferred from the wire being treated. The temperature gradually rises, and there is a constant temperature gradient in which the temperature of the hot water gradually increases along the direction of movement of the wire. The temperature of the wire when it leaves the tank is determined by the flow rate of hot water and the temperature of the hot water at that time, but this means that if these conditions are selected appropriately, the temperature of the wire can be set to the desired value. It shows that. The water supply device 5 shown in FIG. 1 is provided to form a desired temperature gradient in the temperature gradient tank 3 by utilizing this principle. In the figure, the water replenishment device 5 is provided with a hot water tank 6 and a mixing tank 8, and 100°C hot water 7 from the hot water tank 6 and replenishment water 9 are poured into the mixing tank 8 and mixed by an agitator 10. 80° C. hot water is created, and the hot water 11 is supplied to the inlet side of the temperature gradient tank section 3 through a valve 12. Hot water at 100° C. in the hot water tank 6 is pumped up from the temperature gradient tank section 3 by a pump 13. Matayu tank 6
From then on, hot water is supplied to the post-cooling tank section 4 through a valve 14, and surplus hot water is discharged through a valve 15.
18 to 23 are thermometers for hot water. By appropriately operating the water replenishing device 5 configured in this way, a desired temperature gradient can be created in the temperature gradient tank section 3 in a steady state, as will be described in the next embodiment. Note that T ₁ , T ₂ , and T ₃ are examples of the hot water temperatures at each part, and T ₁ = 80°C, T ₂ = 100°C, and T ₃ = 90°C. Hereinafter, more details will be described with reference to Examples. Example Using the continuous heat treatment equipment shown in Figure 1, a 10 mm
A 0.8% C steel wire was patented. First, the wire 1 at a temperature of 950°C was immersed in boiling water at 100°C in the tank 2 to pre-cool it. The cooling rate at this time was approximately 18°C/sec, and the linear temperature reached 750°C in about 11 seconds. The temperature difference between the core and the surface of the wire is approximately 20℃.
The temperature was approximately 740℃ at the surface and approximately 760℃ at the core. The temperature of the wire is measured by a wire thermometer 16 on the outlet side,
The immersion time or immersion length can be adjusted. The film boiling cooling rate in boiling water at 100℃ is very stable, and the outlet temperature can be controlled by changing the immersion time. Next, the wire 1 is introduced into hot water having a temperature gradient that changes in the direction of travel from 80°C to 100°C in the tank section 3. At temperatures above 80°C, the cooling rate is lower than that of boiling water cooling. Although the size is large, the film boiling is sufficiently stable, and the cooling conditions can be controlled by the water temperature and immersion time. At this stage, the wire temperature is lowered, for example, from 750°C to 625°C. Finally, the wire 1 was cooled by immersion in boiling water at 100°C in the tank 4 to complete the transformation, and the wire at about 100°C was pulled out of the tank 4 to obtain a patented wire. Wire rods treated in this way have a higher level of so-called austenite and pseudo-stable region and cooling rate during transformation from just above the _A1 transformation point until the actual transformation is completed, compared to conventional treatment in boiling water alone. Since it is possible to increase the temperature, the performance of the obtained wire rod is improved, and favorable cooling conditions are also created for wire rods with a large diameter. Note that the temperature of the hot water below the boiling point (called a "subcooled" state) in the temperature gradient tank 3 increases when it comes into contact with the wire rod 1, so in order to obtain the desired conditions, the flow rate is adjusted as follows. , was adjusted by changing the initial water temperature. As mentioned above, the wire rod is heated from 950℃ to 750℃ with pre-cooling.
During this time, the amount of heat lost from 1 kg of wire is 30 Kcal, and 0.056 of the hot water is lost by evaporation. Next, when cooling in a subcooled state, the wire rod has a temperature of 750
It is cooled from ℃ to 625℃, and the amount of heat lost during this time is 18.8Kcal, but there is no evaporation. Finally, the wire is cooled in boiling water to 625℃.
When cooled from to 100℃, 96Kcal is lost and the amount of evaporation is 0.178. Therefore, the total evaporation amount is 0.234, and 0.234 of hot water needs to be replenished per 1 kg of wire. If we were to mix this make-up water with hot water at 100℃ to create sub-cooled hot water, in order to make hot water at 80℃, we would need three times this amount to make the make-up water at room temperature (20℃). It will be fine if you mix it with hot water.
That is, in the method of the present invention, 100°C from the temperature gradient tank section
If 0.936 of 80°C hot water made by mixing 0.702 of hot water with 0.234 of 20°C water in the mixing tank 8 is brought into contact with 1 kg of wire in the temperature gradient tank 3, the evaporated content can be replenished. The temperature distribution shown in the figure can be maintained, and cooling in a subcooled state can be inserted. However, in this case, the temperature of the hot water is 80°C at the inlet of the wire, but rises to 100°C at the outlet. The amount of heat lost in the wire at this stage is per 1 kg.
It is 18.8Kcal and the amount of hot water is 0.936, so it is 20℃
It is calculated that it will rise. (You can also think that water 0.234 at 20°C rises by 80°C to 100°C.) In this way, using make-up water to create a steady temperature gradient in the direction of movement of the wire is another aspect of the present invention. This is one feature. In this case, the speed difference between the hot water and the wire is irrelevant, which is advantageous for controlling the cooling conditions. As in this example, when hot water rises to 100℃, the amount of hot water supplied will uniquely determine the amount of temperature reduction of the wire in this section, including the prevention of overcooling. , which is convenient for controlling cooling conditions. Furthermore, when the temperature rises to 100℃,
The partition between the subcooling tank section 3 and the next post-cooling tank section 4 may be removed. As mentioned above, the amount of heat lost in the subcool tank 3 is 18.8 Kcal per 1 kg of wire rod, so if we try to suppress the rise in temperature of the hot water from the subcool to 1℃ when it comes into contact with the wire rod, per 1kg of wire rod
Requires 18.8. If the temperature rises by 2℃, it will be 9.4 (the same applies below). If a 20°C rise is allowed as in this example, 18.8/20=0.936 would be sufficient. It is sufficient to supply this amount of hot water at 80°C per 1 kg of wire, and the flow rate is irrelevant. However, in this case, for example, if we allow a 1°C increase from 80°C to 81°C, the hot water at 81°C is 18.5 and the water at 20°C is
Approximately 0.3 is mixed, but since the flow of 20°C water is slightly larger than the amount to replenish the evaporated water, it is necessary to drain the excess amount (0.3 - 0.234 = 0.016). In addition, if make-up water exceeding the amount of evaporated water is required in this way, the amount of water to be added at 20°C should be limited to the amount of water that has evaporated, and dry air should be blown into the mixing tank 8 in the water replenishing device 5. Alternatively, a method may be adopted in which the hot water is auxiliary cooled mainly based on the principle of removing latent heat of vaporization. In this case, if the amount of air blown is appropriate (approximately 13 in the case of the above example), there is no need to drain excess water. Similar calculations can be made for other temperatures where the temperature rise is less than 20°C. If the temperature rises by about 1° C., it can be said that there is no temperature gradient, but this condition is also within the scope of the present invention. In addition, the decrease in wire temperature in subcooled hot water,
For example, by increasing the amount of hot water supplied to 750℃
The temperature can be increased arbitrarily from 550°C to 550°C, and it is easily understood that in this case, it is effective in suppressing heat generation during transformation. It is also possible to implement a method that does not involve pre-cooling in boiling water at 100°C. As described above, in the method of the present invention, when the water replenishing device 5 as described above is used, the amount of water at room temperature that is approximately equivalent to the amount of hot water that is lost due to evaporation or being carried out by adhering to the wire is used. Then, create hot water at any temperature below the boiling point and stable film boiling, and flow it in the same direction as the traveling direction of the wire in at least a part of the way until the transformation is completed to form a wire image. By changing the temperature of hot water that is brought into contact and supplied, and the amount of hot water supplied per unit weight of the wire rod, a desired temperature gradient of hot water is created in this section in the traveling direction in a steady state. The effect of increasing the cooling rate in this section can be obtained. As an example of the present invention, the temperature of the hot water supplied from the water replenishment device 5 is set to 80°C and 60°C, and the replenishment amount is changed as shown in Table 1.
Examples of cooling curves for %C steel wire rods are shown in FIGS. 2 and 3. Figure 2 shows the temperature on the outlet side in the temperature gradient tank section 3.
In the case of 100°C, Figure 3 shows the case where the temperature increase is 1°C. Additionally, as a comparative example, a case where the sample was continuously immersed in boiling water at 100°C is also shown. From the figure, compared to the comparative example, the method according to the present invention
It can be seen that the cooling rate in the subcooling section increases, and this cooling rate increases as the supplied water temperature decreases and as the temperature rise in the tank decreases. Further, the performance of the obtained patented steel wire rod is as shown in Table 1.

[Table] From Table 1, it can be seen that No. 1 to No. 4 according to the present invention have improved strength compared to the comparative example. The method of the present invention configured as described above has the following effects. (b) In the above-mentioned method for heat treatment of steel wire, the cooling treatment from just above the _A1 transformation point of the wire is performed with a temperature gradient at least in the middle in the direction of movement of the wire, and the temperature on the inlet side is set to the boiling point. Since the wire rod is passed through a bath of hot water or a hot aqueous solution whose temperature at the outlet side is higher than that at the inlet side and the temperature at the outlet side is lower than that at the inlet side. , the cooling rate is initially high in the hot water section below the boiling point, but gradually becomes slower, and the amount of decrease in the temperature of the wire in this section, for example, by keeping the amount of hot water supplied constant. As a result, supercooling is naturally prevented, and bainite and bainite are uniformly maintained over the entire length.
It is possible to obtain a steel wire rod with high strength and good wire drawability without defects such as martensite, and it is easy to control cooling conditions, and a patented wire rod with high tensile strength can be obtained even with large diameter wire rods. It has the advantage of being (b) Since boiling water can be used as pre-cooling at the beginning of cooling the wire, the cooling rate of the pre-cooling does not become too fast, so the pre-cooling conditions can be easily managed. (c) The temperature gradient in the temperature gradient tank can be created using make-up water to replenish the amount lost due to evaporation, etc., so the equipment is simple. In addition, although the above explanation mainly describes the case where the method of the present invention is applied to steel wire rods, it is clear that this method can be applied not only to the case of steel but also to heat treatment of other general metal wire rods. .

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view and a hot water temperature distribution diagram showing an example of a continuous heat treatment apparatus used in an embodiment of the method of the present invention. FIG. 2 and FIG. 3 are diagrams showing cooling curves of an example of the present invention and a comparative example, respectively. DESCRIPTION OF SYMBOLS 1...Steel wire rod, 2...Preliminary cooling tank section, 3...Temperature gradient tank, 4...Post cooling tank section, 5...Water supply device, 6...Hot water tank, 7...Hot water, 8... Mixing tank, 9... Water for replenishment, 10... Stirrer, 11...
...Hot water, 12,14,15...Valve, 13...
Pump, 16, 17... Line thermometer, 18-23
...Thermometer for hot water, T ₁ , T ₂ , T ₃ ... Water temperature.

Claims (1)

[Claims] 1. A method in which a steel wire rod at a high temperature is heat-treated by continuously immersing it in hot water or a hot water solution containing additives, in which a cooling treatment is performed from just above the _A1 transformation point of the wire rod. , in the bath of hot water or hot aqueous solution, which has a temperature gradient at least in the middle of the traveling direction of the wire, and has a temperature at the inlet side that is lower than the boiling point and a temperature at the outlet side that is higher than the temperature at the inlet side. A method for heat treatment of a steel wire, characterized in that the heat treatment is carried out by passing the wire through the wire. 2. The method for heat treating a steel wire rod according to claim 1, wherein preliminary cooling before the cooling treatment from just above the _A1 transformation point is performed in boiling water or an aqueous solution. 3. The method for heat treating a steel wire rod according to claim 1 or 2, wherein the cooling after the cooling treatment from just above the _A1 transformation point is performed in boiling point water or an aqueous solution. 4. The method for heat treating a steel wire according to claim 1, 2 or 3, wherein the temperature on the outlet side is a boiling point. 5. The steel according to claim 1, 2, 3 or 4, in which the temperature gradient in the tank is created using make-up water to replenish what is lost through evaporation. Heat treatment method for wire rods. 6. The method for heat treating a steel wire rod according to claim 1, 2, 3, 4 or 5, wherein the steel wire rod at high temperature is a hot rolled high temperature steel wire rod. 7 Claims 1, 2, and 3 in which the steel wire at high temperature is a reheated high-temperature steel wire
The method for heat treatment of steel wire according to item 4, item 5, or item 5.