Patent Application
20150114528
This disclosure relates to advantageous production of a steel strip that can lower the dew point of an ambient gas in a continuous annealing furnace and has high wettability and, in particular, relates to a method of lowering the dew point of an ambient gas in an annealing furnace, a device for the method, and a method of producing a cold-rolled and annealed steel sheet.
It is known that when the dew point of an ambient gas in a continuous annealing furnace is −45° C. or less, surface segregation of Mn during annealing can be suppressed, and the adhesion of zinc or zinc alloy plating after annealing is improved (see Tetsu-to-Hagané (Bulletin of the Iron and Steel Institute of Japan), 96-1 (2010), pp. 11-20).
The following are examples of a known method to lower the dew point of an ambient gas in a continuous annealing furnace:
In accordance with JP '953, the low-temperature gas is directly introduced into the high-temperature furnace. Thus, a large amount of thermal energy is required to maintain the steel strip temperature in the furnace, the gas temperature cannot be controlled, and the energy efficiency is very low.
In accordance with JP '830, even when the low-temperature gas has a low dew point, the low-temperature gas is mixed with a large amount of ambient gas having a high dew point in the furnace. Thus, the dew point of the ambient gas in the furnace cannot be sufficiently lowered.
In accordance with JP '622, the temperature of a gas to be returned to the furnace is not sufficiently increased and, as described in JP '622, the water adsorption filter having a low dehumidification capacity lowers the dew point only to approximately −30° C. and cannot lower the dew point to −45° C. or less. Thus, known techniques to lower the dew point of the atmosphere of a continuous annealing furnace have problems that they cannot achieve a low dew point of −45° C. or less and that they have very low energy efficiency.
We provide a means of installing a dryer, for example, of a desiccant method or a compressor method that allows a dew point of −45° C. or less to lower the dew point of an annealing furnace ambient gas and a circulator to lower the dew point to −45° C., installing a heat exchanger in the circulator to increase or decrease the temperature of the gas, and modifying a gas inflow (gas introduction) into a heating zone and a cooling zone of the furnace to improve energy efficiency.
We provide in particular:
Part of an ambient gas in the heating zone and/or the soaking zone is sucked out and is cooled through a high-temperature gas passage of the heat exchanger by heat exchange with a gas in a low-temperature gas passage, is then further cooled by mixing with part of an ambient gas of the cooling zone, is then further cooled through the gas cooler, is then dehumidified to a dew point of −45° C. or less in the dryer, is then heated through the low-temperature gas passage of the heat exchanger by heat exchange with a gas in the high-temperature gas passage, and is returned to the heating zone and/or the soaking zone. More preferably, part of gas flowing from the dryer toward the low-temperature gas passage of the heat exchanger is returned directly to the cooling zone without passing through the heat exchanger. These can achieve a very low dew point of −45° C. or less in the annealing furnace and significantly improve energy efficiency.
When a cold-rolled steel strip is continuously annealed and subsequently plated with zinc or a zinc alloy, adhesion of the plating depends greatly on the dew point in an annealing furnace. It is known that this results from the amount of Mn oxide on the surface of the steel strip. At a dew point in the vicinity of −10° C., Mn oxide is present within an oxide film on the surface of the steel strip and is rarely found on the surface of the steel strip. At a dew point of −45° C. or less, Mn oxide is negligibly produced. At an intermediate dew point in the vicinity of −35° C. (−15° C. to −40° C.), a large amount of Mn oxide is produced on the surface of the steel strip and inhibits the adhesion of plating. Thus, we provide the annealing furnace with a circulator equipped with a dryer that allows a dew point of −45° C. or less to achieve a very low dew point to prevent concentration of Mn oxide on the surface of the steel strip.
Attention is now focused on the temperatures of an ambient gas sucked from the furnace into the circulator (hereinafter referred to as a sucked gas) and an ambient gas introduced from the circulator into the furnace (hereinafter referred to as an introduced gas). The desired ambient gas temperature in the annealing furnace is different in a heating zone, a soaking zone, and a cooling zone. More specifically, the sucked gas is cooled to approximately room temperature in a gas cooler before entering the dryer, dehumidified in the dryer, and returned to the furnace. Thus, if a low-temperature gas is directly introduced into a high-temperature region such as the heating zone or the soaking zone, a high temperature required to anneal the steel strip cannot be maintained. For this reason, the temperature of the introduced gas from the circulator must be increased.
We employed a method of installing a heat exchanger between the furnace and the gas cooler. More specifically, a high-temperature gas sucked from the heating zone or the soaking zone of the furnace (sucked gas) is cooled in the cooler before entering the dryer. Utilizing thermal energy resulting from the temperature difference, therefore, the gas cooled in the gas cooler and dehumidified in the dryer can be heated. Thus, thermal energy discharged from the gas cooler can be effectively utilized. A high-temperature gas sucked from the heating zone or the soaking zone of the furnace is passed through the heat exchanger, cooled in the gas cooler, dehumidified in the dryer, heated in the heat exchanger, and then returned to the heating zone or the soaking zone of the furnace.
The temperature of the high-temperature gas sucked from the heating zone or the soaking zone after the heat exchange is sometimes higher than the gas temperature in the cooling zone. Thus, the gas after the heat exchange can advantageously be mixed with a low-temperature gas sucked from the cooling zone to lower energy required to further cool the gas in the downstream gas cooler.
Furthermore, since the gas temperature after cooling with the gas cooler is lower than the temperature of the cooling zone of the furnace, part of gas cooled in the gas cooler, dehumidified in the dryer, and returned directly to the cooling zone without passing through the heat exchanger can lower the temperature and the dew point of the cooling zone, thus further improving energy efficiency.
Unlike a water adsorption filter made of activated alumina, alternately operated and stopped, and having a low dehumidification capacity as described in JP '622, a dryer for use herein preferably has a high dehumidification capacity, for example, of a desiccant method for continuous dehumidification using calcium oxide, zeolite, silica gel, or calcium chloride or a compressor method using an alternative chlorofluorocarbon.
The conditions of these sucked gases and introduced gases were changed. Table 1 shows the dew points of the sucked gases and the dew points of the introduced gases passing through the gas passages and exhausted heat energy during the passage in Examples and Conventional Examples. Table 1 shows that the dew points of the gases introduced into the annealing furnaces in No. 1 to No. 3 of Example 1 and No. 4 to No. 6 of Example 2 are satisfactorily lower than the target temperature of −45° C., as compared with Conventional Examples No. 7 to No. 10. Furthermore, the dew points in the furnaces measured upstream from an annealing furnace outlet 21 are also satisfactorily lower than −45° C. Furthermore, No. 1 to No. 3 of Example 1 and No. 4 to No. 6 of Example 2 exhausted less heat energy and have very high energy efficiency.
Adhesion of zinc alloy plating was examined in zinc alloy plating of a steel strip after continuous annealing in accordance with a JIS-H8504(g) tape test method (a chipping test method). As a result, Examples No. 1 to No. 6 had satisfactorily strong adhesion, but Conventional Examples No. 7 to No. 10 had coating defects.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-088089 | Apr 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/002353 | 4/5/2013 | WO | 00 |
