JP2006183084A - Batch type annealing method for ferritic stainless steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an annealing method for a ferritic stainless steel sheet, which prevents the outer part of a wound coil from being overheated and the ferritic stainless steel sheet from being nitrided without degrading production efficiency, in a batch type annealing treatment. <P>SOLUTION: The method for annealing the ferritic stainless steel sheet in a batch process by setting a target temperature of a bell and heating the ferritic stainless steel sheet comprises the steps of: setting the target temperature of the bell in between (soaking target temperature -50°C) and (soaking target temperature -20°C); heating the bell to the target temperature and keeping it at the temperature; subsequently setting the target temperature of the bell to a target temperature of a soaking step; and restarting heating for the bell toward the target temperature. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for annealing ferritic stainless steel sheets in a batch annealing furnace.

In general, heating of a steel sheet in a batch annealing furnace is performed by setting a bell temperature so as to reach a heat cycle (soaking time, soaking temperature) determined for each material.
When heating is performed, the heating pattern (heating rate and heating time) is determined in advance so that the coldest point of the coil reaches a predetermined temperature in consideration of the weight loaded in the furnace, the number of loaded coils, and the like. In accordance with this heating pattern, the heating control up to the soaking temperature determined in the heat cycle is carried out. During heating, the actual coil temperature or ambient gas temperature is measured, and control is performed so that heating can be performed at the heating rate determined by the heating pattern.

 At this time, for example, as the control temperature, the furnace temperature, that is, the bell temperature that is the temperature between the outer cover and the inner cover is adopted, the heating pattern of the control temperature is determined, and the furnace temperature is controlled, as described above. The actual coil temperature or ambient gas temperature is measured, the bell temperature is maintained at a predetermined temperature until the actual coil temperature or ambient temperature reaches the target temperature, and then the soaking time required by the coil material is maintained. Is done.

  Alternatively, as a method for controlling the temperature of the coil itself, which is an object to be annealed, and performing more appropriate temperature control, for example, Patent Document 1 describes the interior of the object to be annealed using a heat transfer model equation related to the object to be annealed. A method is described in which the temperature is predicted and the combustion control is performed so that the prediction result follows a predetermined target value.

  However, with the method described in Patent Document 1, although the coil coldest point can reach a predetermined temperature, it is not possible to prevent excessive overheating of the coil outer winding that tends to increase in temperature. On the other hand, by changing the heating pattern and reducing the heating rate, overheating of the coil outer winding temperature during heating can be prevented, but the production efficiency is lowered.

  Here, when annealing a ferritic stainless steel plate in a batch-type annealing furnace, annealing is performed by setting the soaking temperature to a temperature just below the γ transformation point in order to prevent deterioration of the material due to coarsening of the crystal grain size. In addition, in annealing ferritic stainless steel sheets, annealing is performed in a reducing atmosphere gas, but in order to reduce the amount of expensive hydrogen used, normally 3-50% of the hydrogen concentration is used as the atmosphere of the remaining nitrogen, and the nitrogen concentration is 50% or more. In such an atmosphere, batch-type annealing is performed, and if heated excessively in such an atmosphere, nitriding occurs and reacts with Cr in the steel to produce Cr compounds, resulting in a decrease in corrosion resistance. .

  Since annealing of ferritic stainless steel sheets is set as described above, when Patent Document 1 is applied to annealing of ferritic stainless steel sheets, the decrease in production efficiency is of course, and the steel sheet is nitrided due to excessive overheating of the coil outer winding. Is a problem.

  Patent Document 2 discloses a method of performing an intermediate soaking process at an intermediate soaking temperature that is 70 to 200 ° C. lower than the heating temperature during the heating process when annealing the steel strip coil after cold rolling using a bell-type annealing furnace. Are listed.

  However, although the technique described in Patent Document 2 can prevent crimping flaws, it cannot prevent nitriding of the coil outer winding, which is a problem when batch annealing is performed on a ferritic stainless steel sheet.

  In Patent Document 3, in the temperature control of the bell-type annealing, the upper limit of the bell temperature is specified and the temperature is raised by the base temperature control. When the bell temperature reaches the specified temperature, the base temperature control is started. An annealing temperature control method for switching to bell temperature control is described.

However, there is a case where the overheating of the coil outer winding that occurs when the soaking temperature is reached cannot be sufficiently prevented only by switching from the base temperature control to the bell temperature control.
Japanese Patent Laid-Open No. 5-21792 Japanese Patent Laid-Open No. 10-251764 JP-A-6-299254

  As described above, in annealing a ferritic stainless steel sheet with a batch annealing furnace, the conventional method causes nitriding of the ferritic stainless steel sheet due to overheating of the coil outer winding or a large decrease in production efficiency. There are problems such as.

  In view of such circumstances, the present invention provides a ferritic stainless steel sheet annealing method that prevents overheating of the coil outer winding and prevents nitriding of the ferritic stainless steel sheet without causing a significant decrease in production efficiency in batch annealing. The purpose is to provide.

  The present inventors have intensively studied to solve the above problems. As a result, by first heating and holding before the soaking target temperature, and then heating again to the soaking target temperature and performing batch-type annealing treatment, overheating of the outer winding of the coil without causing a significant decrease in production efficiency Has been found to be preventable. Furthermore, in the present invention, by first defining the temperature to maintain soaking within the range of (soaking target temperature −50 ° C.) to (soaking target temperature −20 ° C.) It was also found that nitriding of ferritic stainless steel sheet can be prevented.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

  In the invention, the ferritic stainless steel sheet is heated by setting a target bell temperature and batch annealing is performed. The target bell temperature is set from (soaking target temperature -50 ° C) to (soaking target temperature -20 ° C). The temperature of the bell temperature is raised and maintained at the target bell temperature, and then the temperature of the bell temperature is restarted with the setting of the target bell temperature as the soaking target temperature. It is a type annealing method.

  According to the annealing method of the present invention, it is possible to perform annealing in which batch coil annealing prevents overheating of the coil outer winding and prevents nitriding of the ferritic stainless steel sheet. Furthermore, in the present invention, annealing can be performed without reducing the heating rate, so that a reduction in production efficiency can be minimized.

  As a result of studies by the present inventors, it has been found that when the coil temperature exceeds the γ transformation point, not only the crystal grain size becomes coarse as described above, but also the problem of nitriding becomes remarkably large. It has been found that reliably preventing heating is important in preventing nitriding.

  In order to reliably prevent overheating of the coil outer winding, the batch-type annealing method of the ferritic stainless steel sheet of the present invention is first heated and held before the soaking target temperature, and then heated again to the soaking target temperature. It is characterized by. Further, it is further characterized in that the temperature to be initially heated and held is (soaking target temperature −50 ° C.) to (soaking target temperature −20 ° C.). Specifically, first, the target bell temperature is set to (soaking target temperature −50 ° C.) to (soaking target temperature −20 ° C.), the bell temperature is raised to the target bell temperature, and then, The temperature of the bell temperature is restarted by setting the target bell temperature as the soaking target temperature. With such characteristics, 1) annealing can be performed when the coil outer winding temperature is equal to or lower than the γ transformation point, and nitriding of the coil outer winding can be prevented. 2) Since annealing is performed without reducing the heating rate until the temperature reaches the range of -50 ° C to -20 ° C, the reduction in production efficiency can be minimized.

  Next, the batch annealing method for the ferritic stainless steel sheet of the present invention will be described in detail. The following shows one embodiment of the present invention, and the present invention is not limited to this.

  First, set the target bell temperature to (soaking target temperature -50 ° C) to (soaking target temperature -20 ° C) (for example, 800 ° C below) until the bell temperature reaches the target bell temperature 800 ° C. Raise the temperature. The temperature rise at this time can be performed with the maximum capacity of the furnace, and although the maximum capacity varies depending on the equipment specifications, the rate of temperature rise to the target bell temperature may be about 30 ° C / h to 100 ° C / h. it can. On the other hand, the base temperature is measured simultaneously with the bell temperature. The bell temperature is kept at 800 ° C. until the base temperature reaches a predetermined temperature (for example, 650 ° C. below). Next, when the base temperature reaches 650 ° C., the bell target temperature is reset to the soaking target temperature (for example, 830 ° C. below), and the temperature rise of the bell temperature is resumed. After the bell temperature reaches the target bell temperature 830 ° C, the bell temperature is maintained at 830 ° C until the base temperature reaches 830 ° C. When the base temperature reaches 830 ° C., assuming that the entire coil has reached the soaking temperature, the soaking time is started and held for a predetermined soaking time.

  Here, as described above, the target bell temperature is first set to (soaking target temperature −50 ° C.) to (soaking target temperature −20 ° C.), and the bell temperature is raised to this target bell temperature and temporarily held. As mentioned above, the temperature of the coil outer winding part where the temperature becomes highest exceeds the target temperature by overheating to the target soaking temperature at a fast heating rate to ensure production efficiency, Or it is for preventing that a production efficiency falls by reducing a heating rate.

  The target bell temperature is first set within the range of (soaking target temperature −50 ° C.) to (soaking target temperature −20 ° C.). If the temperature is lower than (soaking target temperature -50 ° C), the production efficiency is lowered by regulating the heating capacity more than necessary, which is not preferable. On the other hand, if the temperature exceeds (soaking target temperature -20 ° C), the coil outer winding temperature is excessively heated, and the risk of nitriding beyond the γ transformation point increases, which is not preferable. The temperature is more preferably (soaking target temperature -30 ° C) or higher.

  The annealing atmosphere preferably has a nitrogen concentration of 50 vol% or more in order to reduce the hydrogen gas intensity.

  The predetermined value of the base temperature is determined in advance through experiments to find the temperature at which the coil outer winding temperature reaches the target bell temperature set in the range of (soaking target temperature -50 ° C) to (soaking target temperature -20 ° C). Just keep it. Here, the coil outer winding temperature may be the highest coil outer winding temperature closest to the burner. In addition, in the batch type annealing furnace whose temperature increase rate to the target bell temperature is about 30 ° C./h to 100 ° C./h, the predetermined value of the base temperature may be about 650 ° C.

  The bell temperature holding time within the range of (soaking target temperature -50 ° C) to (soaking target temperature -20 ° C) is that the base temperature reaches the predetermined temperature (650 ° C) after the bell temperature reaches the target bell temperature. For example, if it is a batch annealing furnace having a capacity of about 30 ° C./h to 100 ° C./h, the holding time is about 10 hours to 20 hours. Good.

  The target soaking temperature is generally in the range of γ transformation point -80 ° C to γ transformation point -40 ° C. If the temperature is lower than (γ transformation point -80 ° C), it cannot be recrystallized sufficiently. If the temperature exceeds (γ transformation point -40 ° C), the coil outer winding temperature is overheated, and the location exceeds the γ transformation point locally. Is generated and is nitrided.

  In the above description, the base temperature was measured as the furnace temperature setting position. In consideration of the uniformity of the temperature of the entire coil, it is preferable to measure the base temperature, but the measurement position of the furnace temperature is not particularly limited.

  As described above, by setting the target bell temperature to a temperature just before the soaking target temperature, overheating of the coil outer winding does not occur even if heating is performed at a high heating rate. Further, by heating and holding at a temperature before the soaking target temperature, annealing is performed when the coil outer winding temperature is equal to or lower than the γ transformation point. As a result, nitriding of the coil outer winding is prevented. Further, the temperature difference between the inside and outside of the coil can be reduced by holding at the above temperature and waiting for the base temperature to rise to a predetermined temperature.

  Furthermore, since the heating rate is not reduced until the temperature reaches the range of the soaking target temperature -50 ° C to the soaking target temperature -20 ° C, a reduction in production efficiency can be minimized.

  The ferritic stainless steel sheet was subjected to batch annealing in accordance with the temperature rise / soaking pattern during heating of the ferritic stainless steel sheet shown in FIG. The soaking target temperature was set to 830 ° C. (in the range of −40 ° C. immediately below the γ transformation point of 870 ° C.), and heating and holding were performed once at an annealing soaking target temperature of 30 ° C. (800 ° C.). The configuration of the batch annealing furnace used for annealing is shown in FIG. According to FIG. 3, a coil 2 of products A to D is placed on a base 1, covered with an inner cover 3, and further covered with a bell 4 for heating. A bell thermometer 5 is installed in the furnace wall of the bell 4, and a base thermometer 6 is installed in the base 1.

  Specifically, the target bell temperature is first set to 800 ° C, the temperature is raised with the heating capacity increased, and after the bell temperature ((1) in Fig. 1) reaches 800 ° C, the base temperature (Fig. 1 The bell temperature was maintained at 800 ° C. until medium (2)) reached 650 ° C. The average heating rate up to the target bell temperature after ignition was 80 ° C./hr. As a result, the holding time at 800 ° C. was 12 hours. Next, after the base temperature reached 650 ° C., the target bell temperature was reset to 830 ° C., which is the soaking target temperature, and the temperature increase of the bell temperature was resumed. After the bell temperature reached 830 ° C, the temperature was maintained at 830 ° C. In the example, the A product coil outer winding temperature ((3) in FIG. 1) closest to the burner 7 was also measured simultaneously with the bell temperature and the base temperature. The obtained results are also shown in FIG.

  Further, as a comparative example, the ferritic stainless steel sheet was annealed in accordance with the temperature rise / soaking pattern during heating of the conventional ferritic stainless steel sheet shown in FIG. The configuration of the batch annealing furnace used for annealing is the same as in the example.

  As shown in FIG. 2, in the comparative example (conventional example), annealing was performed with only the soaking temperature (830 ° C.) set as the target bell temperature. Further, similarly to the example, the A coil outer winding temperature ((3) in FIG. 1) was measured simultaneously with the bell temperature and the base temperature. The obtained results are also shown in FIG.

  From the above, in the example of the present invention, the highest temperature of the A product coil outer winding temperature at which the temperature tends to be the highest is 830 ° C. or less, which is the soaking target temperature, and the coil outer winding temperature is less than the γ transformation point. Annealing annealing became possible.

  On the other hand, in the comparative example, the A-coil outer winding was overheated exceeding the soaking target temperature, and the maximum temperature reached at the A-coil outer winding temperature was 850 ° C.

  As a result, by using the batch annealing method of the present invention, it was possible to reduce the nonconformity rate, that is, the occurrence rate of nitriding defects from 1.0% to 0.3% as compared with the conventional method.

  By using the batch annealing method of the present invention, it is possible to perform annealing for the purpose of preventing overheating of the coil outer winding without causing a significant decrease in production efficiency.

It is a figure which shows one Example of the temperature curve and temperature rising / soaking pattern during the heating of the ferritic stainless steel plate by the method of this invention. It is a figure which shows an example of the temperature curve and temperature rising / soaking pattern during the heating of the ferritic stainless steel plate by the conventional annealing method. It is a figure which shows the structure of a batch type annealing furnace.

Explanation of symbols

1 base
2 coils
3 Inner cover
4 bell
5 bell thermometer
6 Base thermometer
7 Burner

Claims (1)

When ferritic stainless steel sheet is heated and batch-type annealed with the target bell temperature set,
The target bell temperature is set to (soaking target temperature −50 ° C.) to (soaking target temperature −20 ° C.), the bell temperature is raised and held at the target bell temperature, and then the target bell temperature is set. A batch-type annealing method for ferritic stainless steel sheets, characterized in that the temperature of the bell temperature is restarted with the soaking target temperature as the temperature.

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