JPS6032691B2 - Heat treatment method for metal materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat treatment method for cooling high temperature metal at a predetermined cooling rate, and particularly to a heat treatment method performed using hot water.

For example, in the case of carbon steel wire, in order to improve line workability, tensile strength, and baggability, the wire is austenitized at a temperature of 850 qo or higher, and then heated in a lead bath at 4000 to 600 qo;
Alternatively, a patenting process is performed in which the wire is immersed in a molten salt bath and cooled at a slower cooling rate than normal quenching to form a fine pearlite structure with good wire drawability.

When wire rods are immersed in cold water or cold oil, the cooling rate is fast and martensite is formed, resulting in a quenched state and making wire drawing impossible. However, heat loss is large in lead-rated or salt bath processes.
The materials and equipment for this are also expensive. On the other hand, for example, as described in Kosho Tsubaki 45-8536, a steel wire rod in an austenitic state is immersed in hot water of 70°C or higher, in other words, a film of water vapor that is easily generated is formed on the surface of the immersed wire rod. A heat treatment method is used in which the duration of the film boiling phenomenon is matched to the cooling rate of the patenting process by utilizing the fact that the heat transfer rate, that is, the cooling rate decreases by producing a film boiling phenomenon. However, when this method is actually used, it is difficult to maintain the hot water at a constant temperature other than the boiling point because the hot water is heated by a high-temperature copper wire. must be close to the boiling point temperature. This means that the heat transfer coefficient per surface area of the material, that is, the cooling condition, is fixed, so for steel wire, the wire diameter is 5
Optimal patenting conditions can be obtained with a medium wire of ~8 wire diameter, but with a thicker wire diameter, for example, 1 ratio? In this case, the cooling rate is slow and the required strength cannot be obtained. On the other hand, Unexamined Japanese Patent Publication No. 95915
A method using pressurized hot water has been proposed. In other words, the hot water tank is sealed with a lid and compressed air is sent into the upper space to pressurize the hot water, or a hot steel wire is immersed in the bottom of a deep hot water tank and water pressure is used to generate pressurized hot water. This method increases the heat transfer coefficient, that is, the cooling rate, by cooling the steel wire inside the steel wire, thereby optimally patenting wire rods with large wire diameters. However, conversely, there are many cases where a cooling rate lower than that of normal boiling point hot water is required. For example, in the case of a thin wire having a diameter of 4 ribs or less, even if hot water at the boiling point temperature is used, the cutting speed is too fast and the wire becomes hardened. In addition, even in the case of thick wire diameter, the desired transformation temperature can be adjusted from high temperature to 600~700℃.
Attempts have been made to rapidly cool the wire to 0 qo using a method such as jetting cold water, and then to carry out the overflow transformation at a much slower cooling rate to obtain a steel wire of good quality. In the various cases mentioned above, a cooling method with a slower cooling rate than immersing a high-temperature steel wire in hot water at a normal boiling point is required. In order to achieve this objective, the inventors of the present invention have conducted various experiments and discovered that the cooling rate decreases when hot water under low pressure is used, leading to the present invention. That is, the present invention is a heat treatment method in which the upper space of a closed hot water tank is depressurized using an exhaust device, and a high-temperature metal material is immersed in hot water whose temperature is close to the boiling point at that pressure.

When the pressure of hot water is reduced, the boiling point decreases, and although the heat of vaporization of water remains almost unchanged, the volume of steam generated on the surface of high-temperature materials in hot water under reduced pressure increases in inverse proportion to the pressure.

For example, the boiling point of hot water under a reduced pressure of 0.5 atm is approximately 800.
0, and the volume of the generated steam is doubled. Therefore, a thicker and more stable vapor film is generated on the material surface, and the phenomenon of film boiling continues for a long time, reducing the heat transfer coefficient and slowing down the cooling rate. In addition, according to this method, if the pressure applied to the hot water is kept constant, the boiling point of the overflow water, and therefore the cooling conditions, will be kept constant, so the cooling rate can be easily controlled by adjusting the pressure, which can be controlled easily and quickly. I can do it. In this method, the pressure inside the hot water tank is always negative, and all the generated steam is sucked into the exhaust device (blower), making post-treatment of the exhaust steam easy, and unlike a high-pressure hot water tank, high-temperature steam is ejected from gaps. It also has the advantage of being safe to work with. In this case, an aqueous solution or suspension of a surfactant or the like can be used instead of hot water. To explain one embodiment of the present invention using the drawings, as shown in FIG. 1, hot water 3 is placed in a sealed hot water tank 2 and heated by a heating device 8.

When the pressure in the upper space 3 of the hot water tank 2 is reduced using the exhaust device 4, the hot water starts to boil at a boiling point corresponding to the pressure in the upper space 3.
The wire 1 is guided by a guide rope 7 in the direction of the arrow in the figure, and is continuously immersed in the hot water 2 and drawn out. In this case, the inlet 6 for the wire into the hot water tank 2 is preferably a pipe made of refractory material, as the high-temperature wire passes therethrough. The upper space 3 can be maintained at a constant low pressure by using an exhaust system with a capacity large enough to handle the inflow of outside air from the wire inlet/outlet ports 6, 6' and the steam generated by boiling of hot water during heat treatment. Using the apparatus shown in Figure 1 above, carbon content is 0.
．． The results of patenting a wire rod with a diameter of 8% and 10 legs in hot water at 980° C. are as shown in FIGS. 2 and 3.

Figure 2 shows the temperature change at the center of the line village, and it can be seen that if the pressure in the upper space is reduced, the cooling rate becomes slower. Third
The figure shows the tensile strength of the line village obtained by this patenting process, and it can be seen that by reducing the pressure in the upper space, the tensile strength becomes smaller and a gradual quenching state can be obtained. As explained above, the method of the present invention reduces the pressure in the upper space of a sealed hot water tank using an exhaust device, and heat-treats the high-temperature metal material by immersing it in hot water with a temperature close to the boiling point under reduced pressure. It is very useful because a slower cooling rate can be obtained and the cooling rate can be easily and stably controlled by controlling the pressure.

Furthermore, since the pressure inside the hot water tank is always lower than atmospheric pressure, there is no ejection of steam, making it safe and making it easy to dispose of exhaust gas.

[Brief explanation of the drawing]

FIG. 1 is a drawing showing an example of an apparatus for carrying out the present invention, and FIG.
3 are graphs showing the results when carbon steel wire was treated by the method of the present invention. Figure l Figure 2 Figure 3

Claims (1)

[Claims] 1. In a heat treatment method in which a high-temperature metal material is cooled by immersing it in hot water, the upper space of a closed hot water tank is depressurized using an exhaust device, and hot water at a temperature close to the boiling point under the pressure is removed. A method for heat treatment of metal materials, characterized by immersing the metal materials at high temperature in the medium. 2. The method for heat treatment of metal materials according to claim 1, characterized in that an aqueous solution such as a surfactant is used.