JP2842297B2 - Heating method of steel sheet in continuous annealing furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for heating a steel sheet in a continuous annealing furnace, and more particularly to a method for heating a steel sheet for energy saving.

[0002]

2. Description of the Related Art When a steel sheet is annealed in a continuous annealing furnace, the steel sheet is heated at a temperature equal to or higher than a predetermined temperature T _S ° C for a predetermined time based on metallurgical processing conditions for imparting predetermined mechanical properties to the steel sheet. It is necessary to maintain at least t _S seconds.

[0003] In a continuous annealing furnace, generally, carbon gas fuel, usually coke oven gas (C gas) is used as fuel for heating by direct heat in a heating zone by a burner and / or indirect heating using a radiant tube. Heat in soaking zone with electric heater, then cool and quench by cooling zone.

Conventionally, the heat pattern required for this heat treatment has been practiced on the principle that the steel sheet is heated in such a manner that the heat pattern set for a certain material is not disturbed as much as possible.

FIG. 13 shows an example of such a heat pattern. That is, first, by heating in the heating zone, the temperature T
The temperature of the steel sheet is raised to around _S ° C. and then to a temperature above the temperature T _S ° C in the solitary tropics. Then, lowering to a temperature below T _S ° C. in a cooling zone. At this time,
The heat pattern is determined so that a predetermined time t _S seconds elapses while passing through the solitary zone and the cooling zone, thereby giving the steel sheet predetermined mechanical properties.

In addition, as shown by a dashed line in FIG. 1, a steel sheet is heated to T _s ° C. or more in a heating zone, and this temperature is maintained in a soaking zone (hereinafter referred to as “isothermal soaking pattern”). Call) has also been widely adopted.

[0007] If the predetermined time t _S second for maintaining the predetermined temperature T _S ° C exceeds this, the properties of the steel sheet will not be improved. Therefore, from the viewpoint of energy saving, the predetermined time t _S second should not exceed t _S second as much as possible. Is desirable.

However, conventionally, since the predetermined time t _S for maintaining the predetermined temperature T _S ° C changes according to the line speed of the steel sheet, the steel sheet is kept at the predetermined temperature T _S even at the highest line speed. The heat pattern was set so that the temperature was maintained for a predetermined time t _S seconds. Therefore, when the line speed is decreased due to an increase in the cross section of the steel sheet or the like, the time for maintaining the temperature equal to or higher than the predetermined temperature T _s ° C greatly exceeds the predetermined time t _s seconds, so that energy is wasted correspondingly. What happened.

To solve this problem, Japanese Patent Application Laid-Open No. 55-1191
In Japanese Patent Publication No. 35, the strip temperature set value at the outlet of each furnace is calculated so that the heat input of the entire furnace is minimized under different metal strip input conditions within a range satisfying the metallurgical processing conditions. A method is disclosed in which a set value of a result is given to a strip temperature calculating device corresponding to each part of the furnace to control a heat pattern, thereby saving energy in the entire furnace.

[0010]

However, the method described in the above-mentioned publication requires a device for calculating the strip set temperature and a device for controlling the strip temperature, which increases the equipment cost and necessitates maintenance. .

It is an object of the present invention to provide a method that does not require new capital investment and that can reduce the current operating cost itself.

[0012]

A method for heating a steel sheet in a continuous annealing furnace according to the present invention which has solved the above-mentioned problems has a heating zone in which heating is performed by a gas generated by a steel mill and a soaking zone in which heating is performed by an electric heater. In the method for heating a steel sheet continuously passed by a continuous heating furnace, a target sheet temperature of the steel sheet on the heating strip exit side is set higher than a target sheet temperature of the steel sheet on the soaking zone exit side; A temperature gradient from the target sheet temperature of the steel sheet in the soaking zone to the target sheet temperature of the steel sheet on the outlet side as a substantially continuous downward slope, performing heat input to the steel sheet in the heating zone and the soaking zone. Is what you do.

[0013]

The present inventor pays attention to the difference between the cost of gas generated at a steelworks, for example, coke oven gas, and the cost of electricity required for heating by an electric heater in a heating zone, and the former is much more cost-effective. We paid attention to low. However, in accordance with the present invention, the outlet temperature of the heating zone is increased by the steel mill generated gas in the heating zone, and then the calorific value of the steel sheet itself is used as it is, and in the solitary zone, the outlet temperature is lower than in the past. If heating is carried out, or if the heating is omitted in some cases, the amount of gas generated by the steelworks increases, but the electricity bill is reduced or eliminated. Heat costs can be reduced.

On the other hand, at first, there was a concern that the mechanical properties of the steel sheet might be deteriorated by the so-called inclined soaking method.
The slope temperature difference shown in the figure also depends on the material of the steel sheet.
If the temperature difference was within 0 ° C., it was found that the temperature difference was substantially the same as the conventional example, as shown in the examples described later, and the effectiveness of the present invention was confirmed.

Thus, the objective can be achieved by using only the existing equipment as it is and changing only the amount of heat input in each zone, so that no equipment investment is required and the apparatus is extremely practical.

Further, as a result, as shown in FIG. 1, the difference between the outlet plate temperature in the solitary zone and the inlet temperature in the overaged zone is:
Since ΔT ₁ is smaller than ΔT _{0 of the} conventional example, the Samar's crown of the entrance side hearth roll in the overage zone is reduced, and as a result, the unstable region of the cooling buckle is reduced, and more stable operation is performed. Being able to do it was unexpected.

[0017]

Embodiments of the present invention will be described below in detail. As shown in FIG. 1 illustrated in comparison with the conventional example, in a continuous annealing furnace, a steel sheet is heated by direct heating using gas generated from a steel mill, and / or a heating zone that performs indirect heating using a radiant tube. 1 through the tropics to soak
It is led to the next cooling zone and then through the overaging zone.

In the present invention, the target sheet temperature of the steel sheet on the exit side of the heating zone is set higher than the target sheet temperature of the steel sheet on the exit side of the soaking zone. The heat input to the steel sheet is performed in the heating zone and the soaking zone by setting the temperature gradient up to the target sheet temperature as a substantially continuous downward slope (hereinafter, referred to as “gradient soaking pattern”). Therefore, in the soaking tropics, a slope soaking mode having a downward temperature gradient is provided.

The inventor of the present invention has conducted various laboratory tests on the effect of the inclined soaking on the mechanical properties of the steel sheet, and has confirmed the effect on the actual machine.

<Experiment 1> Using Ti-Nb ultra-low carbon steel,
The outlet temperature of the heating zone (HF) shown in FIG.
According to the outlet side temperature of A), annealing was performed in the inclined soaking pattern and the isothermal soaking pattern, and the elongation (EL) and the yield point (YP) showing the mechanical properties of the 0.7 mm thick steel sheet were obtained. The respective results are shown in FIGS. From these results, it was found that the YP value and the EL value were also substantially the same even with the inclined soaking method according to the present invention, as compared with the case where heat was input according to the isothermal soaking pattern.

<Experiment 2> With respect to general low-carbon steel, the conventional heating strip side sheet temperature of 740 ° C. and the soaking zone exit side sheet temperature of 740 ° C.
In addition to the above case, in order to perform inclined soaking, the heating strip temperature is set to + 10 ° C, + 20 ° C, and + 30 ° C, while the soaking layer temperature is set to -40 ° C, -20 ° C, and -10 ° C. Elongation and yield point changes are distinguished by the presence or absence of aging,
Upon examination, the results shown in FIGS. 7 to 10 were obtained.

From these results, it is found that the heating outlet side sheet temperature is +1.
At 0 ° C., it was found that the desired mechanical properties could be obtained even when the soaking outboard temperature was set to −40 ° C.

<Experiment 3> By the way, the most energy-saving method in actual operation involves heating only in the heating zone, and turning off the heater in the soaking zone to perform soaking.
Under what conditions this can be achieved was simulated by the following equation (1).

[0024] _{(T RA -T HF) × C} × (T / Hr) = heat input + heat output (furnace zone diffusion + other) ‥‥‥ (1) T _RA; soaking delivery side temperature T _HF; heating home use Side plate temperature C; specific heat input; electric heater output; furnace zone dissipation + other In equation (1), heat input is the amount of heat input by the electric heater in the solitary zone, and the heat output is furnace zone dissipation and other (atmosphere gas , Taking out to the cooling zone). In the equation (1), assuming that the production amount is 75 T / Hr,
As a result of calculating the furnace zone diffusion on the right and other numerical values, 160.4 Mcal / Hr and 22.4 Mcal / Hr, respectively, were obtained.
Met. However, even if the heat input is set to zero, the heating outlet side sheet temperature and the soaking zone exit side sheet temperature are each set to 740 ° C. Within the range, the above (1)
Was found to hold. Similarly, in the actual operation, when the heating outlet sheet temperature was set to + 10 ° C., the solitary tropical exit side sheet temperature became −40 ° C., so it could be analyzed that this was a valid matter.

As described above, since the heat output is constant,
The solitary tropical exit side sheet temperature depends on the heating zone exit side sheet temperature (heat import) and the production amount Ton / Hr, and can be expressed by the following equation (2).

T _RA = heat output / ((T / Hr) × C) + T _HF ‥‥‥ (2) FIG. 11 shows the relationship by changing T _HF based on equation (2). It is. From the result of FIG. 11, it was found that the region of the amount of annealing in which the electric heater in the solitary zone could be turned off was 28 T / Hr or more.

<Experiment 4> Based on the above results, for the low-carbon steel, the target sheet temperature on the heating outlet side was set at 750 ° C., and the target sheet temperature on the soaking zone was set at 700 ° C.
The actual machine test was performed with the electric heater in the solitary zone turned off. Table 1 shows the energy consumption of the actual machine at this time in comparison with the conventional example. In the case of the inclined soaking pattern according to the present invention, although the coke oven gas consumption in the heating zone increases, the electric heat required for heating in the soaking tropics is increased.
The power input to the motor became zero and the total cost was reduced by about 10%.

[0028]

[Table 1]

Under the same conditions, the microstructure of the steel sheet annealed in each of the inclined soaking pattern and the isothermal soaking pattern was observed by an optical photograph, and there was no substantial difference. The relationship with the average particle size is shown in FIG.
And no decrease in physical properties was observed from this point.

(Others) As a result of various actual machine operations including Experiment 4 described above, the power-free inclined soaking pattern method according to the present invention can be used under the condition that the mechanical properties are not impaired, and the sheet temperature of the heating strip and the soaking zone are controlled. It has been found that when the exit side sheet temperature is in the range of 650 ° C. to 850 ° C., the inclined soaking pattern system of the present invention can be adopted.

Further, since the difference between the outlet plate temperature in the solitary zone and the inlet plate temperature in the overaged zone becomes smaller, the Samar crown of the inlet hearth roll in the overaged zone becomes smaller, and as a result, the cooling buckle becomes unstable. The area is reduced, and more stable operation can be performed.

[0032]

As is apparent from the above description, according to the present invention, there is no need for new capital investment, and the present operating cost can be reduced, and the present invention is extremely practical. Also, there is no decrease in mechanical properties,
Conversely, there are advantages such as a reduction in the Samar's crown of the entrance hearth roll in the overaging zone and a reduction in the unstable region of the cooling buckle.

[Brief description of the drawings]

FIG. 1 is a graph showing a gradient soaking pattern according to the present invention and an isothermal soaking pattern according to a conventional example.

FIG. 2 is a graph showing the temperature at the exit side of the heated strip and the temperature at the exit side of the tropical zone in the isothermal soaking pattern of the present invention in an experiment and the inclined soaking pattern according to the conventional example.

FIG. 3 is a graph showing elongation in the case of inclined soaking.

FIG. 4 is a graph showing elongation in the case of isothermal soaking;

FIG. 5 is a graph showing a yield point in the case of inclined soaking.

FIG. 6 is a graph showing a yield point in the case of isothermal soaking.

FIG. 7 is a graph showing the elongation in the case of changing the heating outlet side sheet temperature and the soaking layer exit side sheet temperature in the inclined soaking (without aging).

FIG. 8 is a graph showing the elongation when the heating strip side temperature and the soaking zone exit side sheet temperature are changed (with aging) in inclined soaking.

FIG. 9 is a graph showing a yield point in the case where the heating strip side sheet temperature and the soaking zone exit side sheet temperature in the inclined soaking are changed (without aging).

FIG. 10 is a graph showing the yield point in the case of changing the heating outlet side sheet temperature and the soaking zone exit side sheet temperature in the inclined soaking (with aging).

FIG. 11 is an explanatory graph showing a non-heated region in a solitary zone under the correlation of a heating outlet side plate temperature, a production amount, and a solitary outlet side plate temperature.

FIG. 12 is a correlation diagram between elongation and average particle size in a conventional example and an example of the present invention.

FIG. 13 is an explanatory diagram of an example of a conventional heat pattern.

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Claims (1)

(57) [Claims] 1. A method for heating a steel sheet which is continuously passed through a continuous heating furnace having a heating zone for heating by gas generated from a steel mill and a soaking zone for uniform heating by an electric heater. The target sheet temperature of the steel sheet at the side of the soaking zone is set higher than the target sheet temperature of the steel sheet at the soaking zone, and the temperature gradient from the target sheet temperature of the steel plate in the heating zone to the target sheet temperature of the steel plate at the soaking zone. A heat input to the steel sheet in the heating zone and the soaking zone, as a substantially continuous downward slope, wherein the steel sheet is heated in a continuous annealing furnace.