JPH08311566A - Method for heating steel sheet in continuous annealing furnace - Google Patents

Abstract

PURPOSE: To reduce a present driving cost itself without needing new equipped investment. CONSTITUTION: The target temp. of a steel sheet at the outlet side of a heating zone is set higher than the target temp. of the steel sheet at the outlet side of a soaking zone and the input of heat is executed to the steel sheet at the heating zone and the soaking zone so that temp. gradient from the target temp. of the steel sheet at the heating zone to the target temp. of the steel sheet at the outlet side of the soaking zone becomes substantially continuous falling. By this method, the unstable range of cooling buckle is reduced.

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 annealed steel sheet in a continuous annealing furnace, based on the metallurgical process conditions for applying a predetermined mechanical properties of the steel sheet, the steel sheet at a predetermined temperature T _S ° C. or higher temperature for a predetermined time It is necessary to maintain t _S seconds or more.

In a continuous annealing furnace, generally, a carbon gas fuel, usually a coke oven gas (C gas) is used as a fuel for heating by a direct flame heating in a heating zone and / or an indirect heating using a radiant tube, followed by heating. , Heating in a soaking zone equipped with an electric heater, followed by cooling and quenching by a cooling zone.

Conventionally, the heat pattern required for this heat treatment has been performed in practice by heating the steel sheet so that the heat pattern set for a certain material is not disturbed as much as possible.

An example of such a heat pattern is shown in FIG. That is, first, the temperature T is set by heating in the heating zone.
The temperature of the steel sheet is raised to around _S ° C., and then raised to a temperature above the temperature T _S ° C. in the soaking zone. Then, the temperature is lowered to a temperature below T _S ° C in the cooling zone. At this time,
The heat pattern is determined so that a predetermined time t _S seconds elapses while passing through the soaking zone and the cooling zone to give the steel sheet predetermined mechanical characteristics.

Further, as shown by the alternate long and short dash line in FIG. 1, a heat pattern (hereinafter referred to as "isothermal soaking pattern") for raising the temperature of the steel sheet to T _S ℃ or more in the heating zone and maintaining this temperature even in the soaking zone Called) is also often adopted.

The predetermined time t _S second for maintaining the predetermined temperature T _S ° C does not improve the properties of the steel sheet even if the predetermined time t _S second is exceeded. Therefore, from the viewpoint of energy saving, it should not exceed t _S second as much as possible. Is desirable.

[0008] However, conventionally, a predetermined temperature in accordance with the line speed of the steel sheet T for the reason that the predetermined time t _S to maintain the _S ° C. is varied, the predetermined temperature T _S even steel sheet during the fastest line speeds The heat pattern was set so that the temperature was kept at a predetermined time t _S seconds. Therefore, when the line speed is reduced due to a large steel plate cross section or the like, the time for maintaining the temperature at or above the predetermined temperature T _S ° C greatly exceeds the predetermined time t _S seconds, and energy is wasted accordingly. It happened.

With respect to this problem, Japanese Patent Laid-Open No. 55-1191
In Japanese Patent Publication No. 35-35, a strip temperature set value at each outlet of the furnace is calculated and calculated so that the heat input amount 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 the resulting set value is given to a strip temperature calculation device corresponding to each part of the furnace to control the heat pattern to save energy in the entire furnace.

[0010]

However, the method described in the above publication requires a device for calculating the set temperature of the strip and a device for controlling the strip temperature, resulting in high equipment cost and maintenance. .

Therefore, an object of the present invention is to provide a method capable of reducing the current operating cost itself without requiring new capital investment.

[0012]

A method of 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 for heating by a gas generated from a steel mill and a soaking zone for soaking and heating by an electric heater. In a method of heating a steel sheet continuously passed by a continuous heating furnace, a target sheet temperature of the steel sheet on the heating zone exit side is set higher than a target sheet temperature of the steel sheet on the soaking zone exit side, and the heating zone The temperature gradient from the target plate temperature of the steel plate to the target plate temperature of the steel plate on the soaking zone is set as a substantially continuous downward gradient, and heat is applied to the steel plate in the heating zone and the soaking zone. To do.

[0013]

The present inventor pays attention to the difference between the cost of the gas generated in the steel mill, for example, the gas of the coke oven and the cost of electricity required for heating by the electric heater in the heating zone, and the former is far more cost effective. Focused on the low. However, according to the present invention, the steelmaking gas is used to heat the outlet side temperature in the heating zone higher than before, and then the calorific value of the steel sheet itself is used as it is, in the soaking zone, to a lower outlet side temperature. If it is heated so that it is heated, or if heating is omitted in some cases, the amount of gas generated by the steel mill will increase, but the electricity bill will be reduced or eliminated. The heat cost can be reduced.

On the other hand, at first, it was feared that the so-called inclined soaking method would deteriorate the mechanical properties of the steel sheet.
The gradient temperature difference shown in 6 depends on the material of the steel plate.
If the temperature difference was within 0 ° C., it was found to be substantially the same as the conventional example, as shown in the examples described later, and the effectiveness of the present invention could be confirmed.

Thus, the existing equipment can be used as it is, and the object can be achieved by changing only the amount of heat input in each zone, so that the equipment investment becomes unnecessary and it becomes extremely practical.

Further, as a result, as shown in FIG. 1, the difference between the outlet plate temperature in the soaking zone and the inlet side temperature in the overaging zone is
Since ΔT ₁ is smaller than ΔT _{0 of the} conventional example, the samar crown of the entrance side hearth roll in the overaging zone is small, and as a result, the unstable region of the cooling buckle is reduced, and more stable operation is performed. It was unexpected to be able to do.

[0017]

EXAMPLES Examples of the present invention will be described in detail below. As shown in FIG. 1 illustrated in comparison with the conventional example, in the continuous annealing furnace, the steel plate is heated by a steel mill-generated gas in a direct flame and / or indirectly heated by a radiant tube. 1 through the soaking tropics for soaking
It is guided to the next cooling zone and then passed through the overaging zone.

In the present invention, the target plate temperature of the steel plate on the heating zone exit side is set higher than the target plate temperature of the steel sheet on the soaking zone, and the steel plate on the soaking zone is set from the target plate temperature of the steel sheet in the heating zone. With the temperature gradient up to the target plate temperature of 1 as a substantially continuous downward gradient, heat is applied to the steel plate in the heating zone and the soaking zone (hereinafter referred to as "gradient soaking pattern"). Therefore, in the soaking zone, there is a slope soaking mode having a downward temperature gradient.

The inventor of the present invention confirmed the influence on the mechanical properties of the steel sheet due to the gradient soaking by various laboratory tests and then by an actual machine.

<Experiment 1> Using Ti-Nb ultra-low carbon steel,
The temperature at the outlet side of the heating zone (HF) shown in FIG.
The elongation (EL) and the yield point (YP) showing the mechanical properties of the steel sheet having a thickness of 0.7 mm were obtained by performing annealing in the inclined soaking pattern and the isothermal soaking pattern according to the exit side temperature of A). The respective results are shown in FIGS. From this result, it was found that the YP value and the EL value were almost the same as in the case of heat input by the isothermal soaking pattern even by the gradient soaking method according to the present invention.

<Experiment 2> Regarding general low carbon steel, the conventional heating zone outlet side plate temperature of 740 ° C. and the soaking zone outlet side plate temperature of 740 ° C.
In addition to the above case, the heating zone outlet side plate temperature is set to + 10 ° C, + 20 ° C, + 30 ° C in order to perform the gradient soaking, while the soaking zone outlet side plate temperature is set to −40 ° C, −20 ° C, −10 ° C. Distinguishing changes in elongation and yield point with or without aging,
When examined, the results shown in FIGS. 7 to 10 were obtained.

From these results, the heating zone exit side plate temperature is +1.
It was found that when the temperature was 0 ° C., the desired mechanical characteristics could be obtained even when the temperature distribution side plate temperature was set to −40 ° C.

<Experiment 3> By the way, the most energy-saving method in actual operation is to heat only in the heating zone, and in the soaking zone, the heater is turned off to carry out soaking.
The conditions under which this can be achieved were simulated by the following formula (1).

(T _RA −T _HF ) × C × (T / Hr) = heat input + heat output (furnace zone diffusion + others) (1) T _RA ; temperature distribution side plate temperature T _HF ; heating zone output Side plate temperature C; specific heat heat input; electric heater heat output; furnace zone heat dissipation + others In (1), the heat input is the heat input by the electric heater in the soaking zone, and the heat output is the furnace zone heat dissipation, other (atmosphere gas It can be assumed to be constant because it is brought out to the cooling zone. In the formula (1), when the production amount is 75 T / Hr,
As a result of calculating the furnace zone diffusion and other numerical values in the right section, 160.4 Mcal / Hr and 22.4 Mcal / Hr, respectively
Met. However, even if the heat input is zero, when the heating zone outlet side plate temperature and the heating zone outlet side plate temperature are 740 ° C. respectively, and the heating zone outlet side plate temperature is + 10 ° C., the soaking zone outlet side plate temperature is up to −40 ° C. Within the range, the above (1)
It was found that Similarly, even in actual operation, when the heating zone outlet side plate temperature is + 10 ° C., the soaking zone outlet side plate temperature becomes −40 ° C., so it was possible to analyze that this is a valid item.

As mentioned above, since the heat output is constant,
The soaking zone outlet side plate temperature depends on the heating zone outlet side plate temperature (carrying in heat) 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) Based on the equation (2), T _HF was changed and the relationship was shown in FIG. Is. From the results shown in FIG. 11, it was found that the range of annealing amount at which the soaking zone electric heater can be turned off is 28 T / Hr or more.

<Experiment 4> Based on the above results, the low carbon steel was subjected to the actual machine test with the heating zone outlet side target plate temperature of 750 ° C., the soaking zone target side sheet temperature of 700 ° C., and the soaking zone electric heater turned off. It was The energy consumption of the actual machine at this time is shown in Table 1 in comparison with the conventional example. In the case of the gradient soaking pattern according to the present invention, although the consumption amount of the coke oven gas in the heating zone increases, the amount of electric power required for the radiant tube becomes zero, and the total cost can be reduced by about 10%. .

[0028]

[Table 1]

Further, under the same conditions, the microstructures of the steel sheets annealed in the inclined uniform heating pattern and the isothermal uniform heating pattern were observed by optical photographs, and there was no substantial difference. Figure 12 shows the relationship with the average grain size.
The physical properties were not deteriorated from this point as well.

(Others) As a result of various actual machine operations including Experiment 4 described above, the gradient soaking pattern method of the present invention, which does not use any electric power, does not impair the mechanical characteristics, and the heating zone exit side plate temperature and the soaking zone are equalized. It was found that the gradient soaking pattern method of the present invention can be adopted when the outlet plate temperature is in the range of 650 ° C to 850 ° C.

Further, since the difference between the outgoing side plate temperature in the soaking zone and the incoming side plate temperature in the overaging zone becomes small, the Samart crown of the incoming side hearth roll in the overaging zone becomes small, 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, no new facility investment is required and the current operating cost itself can be reduced. Also, there is no deterioration in mechanical properties,
On the contrary, the samar crown of the entrance-side hearth roll in the overaging zone is reduced, and the unstable region of the cooling buckle is reduced.

[Brief description of drawings]

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

FIG. 2 is a graph showing the heating zone outlet side plate temperature and the tropical zone outlet side plate temperature in the isothermal soaking pattern of the present invention and the inclined soaking pattern according to the conventional example in the experiment.

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 when the heating zone outlet side plate temperature and the soaking zone outlet side plate temperature in inclined soaking are changed (without aging).

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

FIG. 9 is a graph showing the yield point when the heating zone outlet side plate temperature and the soaking zone outlet side plate temperature in inclined soaking are changed (without aging).

FIG. 10 is a graph showing the yield point when the heating zone outlet side plate temperature and the soaking zone outlet side plate temperature in inclined soaking are changed (with aging).

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

FIG. 12 is a correlation diagram between the extensibility and the average grain size in the conventional example and the example of the present invention.

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

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[Procedure amendment]

[Submission date] May 30, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0027

[Correction method] Change

[Correction content]

<Experiment 4> Based on the above results, the low carbon steel was subjected to the actual machine test with the heating zone outlet side target plate temperature of 750 ° C., the soaking zone target side sheet temperature of 700 ° C., and the soaking zone electric heater turned off. It was The energy consumption of the actual machine at this time is shown in Table 1 in comparison with the conventional example. For inclined soaking pattern according to the present invention, although the consumption of coke oven gas in the heating zone is increase, pressure at the soaking zone
The electric power input to the electric heater, which was required for heat, was reduced to zero, and the total cost could be reduced by about 10%.

Claims (1)

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