JPH0147524B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for heat treating metals. Annealing, spheroidizing, sintering, brazing, malleable,
Heat treatments of metals, such as hardening and hardening, are usually preferably carried out in a reducing or non-oxidizing atmosphere in the furnace. Heat treatment can be carried out batchwise or continuously. Continuous heat treatment furnaces are usually open ended. These furnaces include an inlet for the metal or workpiece to be heat treated, a heat treatment zone, a cooling zone, and a workpiece outlet. The workpiece is passed through the furnace at a certain speed, heated in a heat treatment zone to the desired treatment temperature, and then cooled in a cooling zone to a temperature at which it will not oxidize when exposed to ambient air. The required atmosphere is usually created, for example, by an exothermic or endothermic gas generator or an ammonia gas generator. The atmosphere is supplied to the heat treatment zone and expands, filling the gas space within the furnace. Since the atmosphere is particularly flammable, combustion must normally occur at both the inlet and the outlet of the furnace. There are therefore two directions of flow inside the furnace, and gas emanates from the heat treatment area in both directions. The gas must be supplied at a flow rate sufficient to maintain a positive pressure within the furnace. Otherwise, there is a risk that air will enter the furnace from the furnace inlet or outlet and create oxidizing conditions within the furnace. It is therefore practical to feed at a flow rate in excess of what is theoretically necessary to create the required reducing or non-oxidizing conditions within the heat treatment zone. An object of the present invention is to provide a heat treatment method that improves the drawbacks of such conventional methods. According to the invention, in a method for heat treating a metal strip or for sintering another metal onto a metal strip in a continuous furnace having successively provided inlets, heat treatment zones, cooling zones and outlets, substantially preventing air from entering the furnace; introducing non-reacting gases and hydrogen gas and optionally other reducing gases other than hydrogen into the heat treatment zone; cooling the non-reacting gases and hydrogen; A method is provided comprising the steps of introducing the metal strip into the furnace, the concentration of hydrogen in the cooling zone being higher than the hydrogen concentration in the heat treatment zone, and passing the metal strip through the furnace from the inlet to the outlet. The term "non-reactive gas" as used herein refers to a gas that does not react with other components of the atmosphere and with the workpiece or metal being heat treated. Nitrogen is generally used as the non-reactive gas, but in some cases, when nitrogen has some deleterious effect on the metal being heat treated, argon may be used. Furthermore, the term "heat treatment of metal" as used herein includes annealing (including spheroidization), sintering (for example, of powders including non-metallic powders), brazing, quenching, and malleable.
The method of the invention is particularly suitable for annealing and sintering. The method of the invention is suitable for continuous strip wire furnaces. Typically, non-reacting gases and reducing gases, such as hydrogen gas, are introduced directly into the heat treatment zone of the horizontal furnace. Alternatively, the gases are introduced into one or more points in the cooling zone proximate to the heat treatment zone, or further away from the heat treatment zone, or both, where a substantial portion of the reducing gas is introduced into the heat treatment zone. Allowed to flow inward. Normally, non-reacting gases and reducing gases are introduced into the furnace at ambient temperature, but the heat treatment zone can reach temperatures as high as 1100°C, depending on the type of heat treatment being performed, so there is a substantial Expansion occurs. Gas is thus allowed to flow from the heat treatment zone both in the direction of the furnace inlet and in the direction of the cooling zone. In the case of a horizontal furnace, it is preferred that the amount of gas exiting the heat treatment zone is substantially greater at one end of the zone than at the other end. It is also desirable that the gas leaving both ends of the furnace be non-flaming, but the flaming gas is only burned out near one end of the furnace. Typically, the end where the gas is burnt out is the inlet of the furnace, and the flow of gas from the heat treatment zone to the cooling zone is blocked. A device may be provided in the cooling zone of the furnace to physically obstruct the gas flow to prevent the flow of gas from exiting the cooling zone through the furnace outlet. Preferably, a non-reacting gas having a horizontal velocity component in the direction of the heat treatment zone is supplied to one or more points in the cooling zone to further inhibit the flow of gas from the heat treatment zone to the cooling zone. Ru. This makes it possible to considerably reduce the amount of gas flowing from the heat treatment zone into the part of the cooling zone opposite this zone, thus reducing the overall supply of non-reacting gases to the heat treatment zone. Can be done. These devices substantially reduce the amount of reducing gas that reaches the furnace outlet and generally eliminate the need to combust the gas at the furnace outlet. Therefore, in order to prevent air from flowing into the furnace, it is desirable to seal the flue, etc. at or near the outlet. To create a positive flow of gas through the outlet of a horizontal furnace that prevents or substantially restricts air from entering the furnace from the outlet, the non-reacting gases have a velocity component in the direction of the outlet. Therefore, it is desirable to introduce the material into the horizontal furnace from one or more points at or near the outlet. Typically, such non-reactive gas also serves the function described above to reduce the proportion of non-reactive gas in the atmosphere exiting the furnace outlet. Preferably, to help prevent the ingress of air from the furnace outlet, a flow restriction device is provided that prevents gas from entering the furnace without preventing the metal being processed from exiting the furnace. Typically, the device that physically obstructs the exit of gas from the cooling zone also serves to prevent gas from entering the furnace through the outlet. A suitable flow restriction device is one comprised of one or more curtains comprising a plurality of generally vertically depending fibers or filaments. When these fibers or filaments are not moved, they close off substantially the entire cross-sectional area of the furnace at or near the outlet, but their lower ends are moved laterally to the metal passing through the furnace. pass. The combination of such a flow restriction device and a device for introducing non-reactive gas into the furnace at or near the device generally substantially prevents air from entering the furnace through the outlet. It is particularly effective. In a preferred embodiment of the method of the invention, spaced partitions and/or curtains or the like define at least one chamber into which nitrogen or other non-reacting gases are introduced in the part of the cooling zone close to the furnace outlet. will be provided. The partitions are configured to allow the metal to be processed to pass through the chamber. Such techniques would be commonly applied in heat treatment furnaces to reduce the overall consumption of gas.
For example, in sintering, it is known to provide a preheating zone between the sintering (or heat treatment) zone and the furnace inlet. Lubricants etc. are oxidized in the preheating zone. Accordingly, it is permissible for a limited amount of air to enter through the inlet to create an oxidizing atmosphere.The invention therefore also provides for successive inlets, heat treatment zones,
both spaced partitions and curtains (or the like) comprising a cooling area and an outlet and defining at least one chamber at or near at least one end through which the metal to be processed can pass; or a method of heat treating metal in a continuous furnace comprising one or the other, the introduction of a suitable gas or gases into the heat treatment zone to create a suitable atmosphere for the heat treatment; supplying a non-reactive gas to the chamber to substantially prevent it from entering through the chamber into a section of the furnace between the chamber and the heat treatment zone; Provided is a method comprising the steps of threading metal. Non-reactive gases are preferably introduced directly into the chamber. The rate of introduction of non-reactive gas into the chamber and the physical obstruction of the partition to the gas flow are preferably such that the following conditions hold. Pc>Pu and Pu>Pa where Pc is the pressure inside the chamber, Pu is the pressure of the furnace atmosphere in the vicinity of the chamber and the area between the chamber and the heat treatment area, and Pa is the atmospheric pressure. . More preferably, the horizontal furnace is equipped with at least two chambers and as follows. Pc＞Pu＞Pt＞Pa where Pc is the pressure in the chamber closest to the heat treatment zone, Pu and Pa are the same as before, and Pt is the pressure inside the heat treatment zone. If the chamber is in the cooling zone near the outlet end of the furnace and the pressure within the chamber is greater than ambient pressure, less air will enter the cooling zone of the furnace from the furnace outlet. If the pressure in the chamber is greater than the pressure in the rest of the cooling zone and in the heat treatment zone, the formation of the required flow in the furnace by the non-flaming gas mixture leaving the furnace outlet is facilitated, and at the same time The total demand for non-reactive and reducing gases can be reduced. If two or more such chambers are used, they will typically be interconnected at substantially the same pressure. Preferably at least two, and usually three or more, chambers are provided, each of which is directly supplied with non-reactive gas. Especially in such a room configuration,
If necessary, maintain a relatively high hydrogen (or other reactant gas) concentration (e.g., 50% or more by volume) in the portion of the cooling zone between the heat treatment zone and those chambers, and (The hydrogen concentration in the outermost chamber is such that the atmosphere there is non-flammable). Since the specific heat of hydrogen is relatively high, increasing the concentration of hydrogen in the cooling zone aids in cooling the metal. Such high hydrogen concentrations are desirable if the furnace processes particularly large amounts of metal per hour. However, in other cases it may be unnecessary and instead the hydrogen within the heat treatment zone may be reduced by introducing most of the hydrogen directly into the heat treatment zone or into a portion of the cooling zone that is close to the heat treatment zone. It is desirable to maximize the hydrogen concentration. The partition is usually hinged or pivoted to a suitable support at or near the ceiling of the furnace. If the metal to be processed is an elongated strip, each partition will be in the form of a flap that seals or engages the metal to be processed and the sides of the furnace in a substantially fluid-tight manner. If the metal being treated is a bulkier item (e.g. a gear), each partition may contain one
It may consist of a row of overlapping "finger" members pivoted or hinged to the top of the furnace and depending down to the workpiece transport surface of the furnace. Such a configuration allows the metal being processed to pass through, but creates a physical barrier to gas flow. Various materials may be used for the partitions. For example, it can be a steel or ceramic material, or a suitable composite or laminate. The reducing gas can be hydrogen, typically supplied from a high pressure vessel of commercially pure hydrogen, ammonia, or a hydrocarbon such as methane or propane. The reducing gas can also be a mixture of hydrogen and carbon monoxide created in situ by thermal decomposition of volatile organic compounds such as methanol. At temperatures above 700°C, 1 mole of methanol decomposes to form 1 mole of carbon monoxide and 2 moles of hydrogen. Methanol is a good source of hydrogen if the workpiece being heat treated is oily. If there is grease on the workpiece,
Water may be introduced into the processing or preheating zone to remove soot on the workpiece. This may be done by passing the non-reacting gas through a humidifier before entering the workpiece into the processing zone. The metal itself is not oxidized and it is therefore important that non-oxidizing and reducing conditions are maintained substantially throughout the furnace as far as the metal is concerned. If necessary, a non-reacting gas, preferably with a horizontal velocity component in the direction of the inlet, is passed between the heat treatment zone and the inlet to prevent or restrict air from entering the furnace through the inlet. It may be introduced into the furnace at one or more points. The inlet end of the furnace could be provided with one or more baffles or chambers supplied with non-reacting gases such as those used in the cooling zone. The reducing gas in the gas mixture passing through the inlet is burned off as in normal operation of a continuous heat treatment furnace. The composition of the atmosphere in the heat treatment area of the furnace depends, among other things, on the type of treatment being carried out, the composition of the metal being treated, the dew point of the fluid forming the atmosphere, and the presence of oil on the surface of the metal being treated entering the furnace. are selected according to their efficiency in regulating or excluding air from the furnace. By substantially restricting the ingress of air into the furnace, it is possible to reduce the concentration of reducing gas in the furnace atmosphere for a given process, if necessary, without substantially reducing the concentration of reducing gas available in the heat treatment zone. Operations can be performed at lower average concentrations of reducing agent than in conventional furnace operations. Furthermore, by using commercially pure hydrogen and nitrogen rather than, for example, cracked ammonia, the dew point of the furnace atmosphere can be lowered, and therefore lower reducing gas levels than would be possible if cracked ammonia was used as the reducing gas source. Able to work at the level of By blocking the flow of gas from the heat treatment zone to the cooling zone, the amount of reducing gas that must be supplied to the heat treatment zone can be reduced. This means that in many cases the gas feed rate to conventional horizontal processing furnaces can be significantly reduced, thereby lowering their operating costs. Typical atmospheres that can be created in the heat treatment area for various heat treatments of different materials are shown in Tables 1 and 2 below (in some cases high hydrogen concentrations may be used). The method of the invention relates to heat treatment of metal strips. A metal strip is an elongated piece of metal. To heat treat a metal strip in this way, it must be pulled just outside the exit of the device. Rubber rolls are usually used for such tensioning.
In conventional heat treatment of metals, it is sufficient to cool the metal in a cooling zone to a temperature at which it is no longer oxidized. However, when processing a metal strip by pulling it over a rubber roll located just outside the exit of the apparatus, the processed metal strip must be cooled to a temperature that does not damage the rubber of the rubber roll. The temperature that does not damage the rubber is significantly lower than the temperature that does not oxidize the metal. A longer cooling zone would eliminate such problems since the metal strip would be cooled to a temperature that would not damage the rubber until it reached the exit of the device. However, increasing the length of the cooling zone increases the overall size of the device, makes the device expensive, and requires a large amount of space to install the device. Furthermore, the inventor tried to inject nitrogen gas into the cooling zone to increase the cooling effect. However, even with this, the desired cooling effect could not be achieved. The present inventor conducted a wide range of experiments and unexpectedly discovered that hydrogen gas is a thermally conductive gas, leading to the present invention. Therefore, the present invention is characterized in that the cooling efficiency in the cooling zone is increased by making hydrogen exist at a higher concentration in the cooling zone than in the heat treatment zone. In the present invention, hydrogen is introduced into the cooling zone to cool the metal strip to a temperature that does not damage the rubber. Tables 1 and 2 below show the atmosphere composition for each treatment purpose for various metals.

[Table] Carbon steel section

Claims (1)

[Claims] 1. A method of heat treating metal strip or sintering metal strip with other metals in a continuous furnace having successive inlets, heat treatment zones, cooling zones, and outlets, through which air flows into the furnace. introducing non-reacting gases and hydrogen gas and optionally other reducing gases other than hydrogen into the heat treatment zone; introducing non-reacting gases and hydrogen into the cooling zone; , the concentration of hydrogen in the cooling zone is higher than the hydrogen concentration in the heat treatment zone, and the method comprises the steps of passing the metal strip through the furnace from the inlet to the outlet.