JP6418175B2 - Dew point control method and method for producing hot dip galvanized steel sheet - Google Patents

Description

  The present invention relates to a dew point control method using atmospheric gas in a furnace when a hot dip galvanized steel sheet is manufactured using a continuous hot dip galvanizing facility. Moreover, this invention relates to the manufacturing method of the hot dip galvanized steel plate using this dew point control method.

  In recent years, in the fields of automobiles, home appliances, building materials, etc., there is an increasing demand for high-tensile steel sheets that can contribute to weight reduction of structures. As such a high-strength steel plate, it has been found that when Si is added to the steel, a high-strength steel plate with good hole expanding property can be produced.

  A hot dip galvanized steel sheet is manufactured by heat-annealing a base steel sheet at a temperature of about 600 to 900 ° C. in a reducing atmosphere or a non-oxidizing atmosphere, and then subjecting the steel sheet to a hot dip galvanizing process. Moreover, the alloyed hot-dip galvanized steel sheet is manufactured by heat-alloying galvanizing after hot-dip galvanizing treatment. However, when manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet containing a large amount of Si (particularly 0.2% by mass or more) as a base material, there are the following problems.

  Since Si in steel is an easily oxidizable element, it is selectively oxidized even in a generally used reducing atmosphere or non-oxidizing atmosphere, and is concentrated on the surface of the steel sheet to form an oxide. This oxide reduces wettability with molten zinc during the plating process and causes non-plating. Therefore, as the Si concentration in the steel increases, the wettability decreases sharply and non-plating occurs frequently. In addition, even when non-plating does not occur, there is a problem that the plating adhesion is poor. Further, when Si in the steel is selectively oxidized and concentrated on the surface of the steel sheet, there is a problem that a remarkable alloying delay occurs in the alloying process after hot dip galvanizing, and the productivity is remarkably hindered.

  In order to cope with such problems, by controlling the dew point of the atmosphere gas in the furnace during the production of hot dip galvanized steel sheets containing a large amount of easily oxidizable elements such as Si, the plating adhesion is improved and the appearance of the plating is good. The method of obtaining has been studied.

  For example, in Patent Document 1, while measuring the dew point of the gas in the furnace, the dew point of the gas in the reducing furnace is controlled by changing the position of supply and discharge of the gas in the furnace according to the measured value. A technique for suppressing the concentration of Si on the surface of a steel sheet is described.

  Patent Document 2 describes a technique for suppressing the surface concentration of a steel sheet in a relatively low temperature range until internal oxidation sufficiently occurs. In Patent Document 2, a pipe for heating and humidifying a water tank installed in a nitrogen atmosphere is separately installed in advance, and hydrogen gas is introduced into the humidified nitrogen gas and mixed, and this is put into the furnace. The dew point of the atmosphere is controlled by introducing it.

JP 2009-209397 A JP 2011-219779 A

  However, in the method described in Patent Document 1, since the atmosphere gas in the furnace supplied to the annealing furnace does not use a humidifier, the dew point in the furnace is not constant. For this reason, it is difficult to stably control the dew point so that the dew point region can increase the plating adhesion. Also, if the dew point rises, the dew point at the top of the furnace tends to increase. For example, when the dew point meter at the bottom of the furnace reaches 0 ° C., there may be a high dew point atmosphere of + 10 ° C. or higher at the top of the furnace. If the operation is continued for a long period of time, the oxide scale adheres to the roll in the annealing furnace, and there is a possibility that a so-called pick-up defect occurs in the steel plate.

  Further, in the method described in Patent Document 2, when controlling the furnace atmosphere dew point, the furnace atmosphere dew point is measured, hydrogen gas is introduced into and mixed with humidified nitrogen gas, and this is introduced into the furnace. By controlling the dew point. Since the dew point in the furnace changes depending on the steel plate size and the plate passing speed, in order to control the dew point stably, it is necessary to supply humidified nitrogen gas in consideration of the steel plate size and the plate passing speed to control the dew point. There is. However, Patent Document 2 does not take into consideration the steel plate size or the plate passing speed. In addition, it is difficult to control stably only by feedback control that measures the dew point in the furnace and adjusts the humidified gas flow rate accordingly, because the response of the dew point control is slow.

  In view of such circumstances, an object of the present invention is to produce a hot-dip galvanized steel sheet that has a more precise dew point and is excellent in plating properties.

  As a result of intensive studies, the present inventors have considered that it is necessary to consider the steel plate size and plate passing speed. As a result, when controlling the dew point of the atmospheric gas in the furnace during the production of hot dip galvanized steel sheets using a continuous hot dip galvanizing facility, the calculated flow rate of the humidified gas supplied to the furnace will By adjusting the flow rate of the humidified gas supplied into the furnace with the product of the correction value A [mm] and the flow rate correction value C determined based on the plate feed speed correction value B [mpm] as the target flow rate of the humidified gas. The present inventors have found that a hot-dip galvanized steel sheet with excellent dew point is produced by controlling the dew point more precisely.

The gist of the present invention for solving the above problems is as follows.
[1] In an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order,
Adjusting the flow rate of the humidified gas supplied into the furnace by multiplying the calculated flow rate of the humidified gas supplied into the annealing furnace by the flow rate correction value C calculated by the following equation (1) as the target flow rate of the humidified gas The dew point control method characterized by performing.
C = A × B (1)
In equation (1),
A: Plate width correction value set according to the plate width of the steel plate in the furnace managed by the host computer B: Feed plate which is the ratio of the reference plate speed in the furnace and the actual plate speed in the furnace This is the speed correction value.
[2] Manufacture of a hot dip galvanized steel sheet, wherein the steel sheet is annealed using the dew point control method according to [1] and then hot dip galvanized in a hot dip galvanizing facility adjacent to the annealing furnace. Method.

  ADVANTAGE OF THE INVENTION According to this invention, the dew point of the atmospheric gas in a furnace at the time of hot-dip galvanized steel plate manufacture using a continuous hot-dip galvanization equipment can be controlled more precisely. Therefore, by using the dew point control method of the present invention, it is possible to obtain a hot dip galvanized steel sheet having high plating adhesion and a good plating appearance.

  In addition, the hot dip galvanized steel sheet in this invention means both the hot dip galvanized steel sheet which is not alloyed and the alloyed hot dip galvanized steel sheet.

FIG. 1 is a schematic diagram showing a configuration of an annealing furnace and a hot dip galvanizing apparatus of a continuous hot dip galvanizing facility according to the present invention. FIG. 2 is a flow diagram of gas supplied into the annealing furnace in the present invention. FIG. 3 is a control block diagram of the mixed gas supplied into the annealing furnace. FIG. 4 is a simulation result comparing changes in the dew point in the furnace when controlled by the dew point control method of the present invention and when controlled by the conventional dew point control method.

  Hereinafter, the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic diagram showing a configuration of an annealing furnace and a hot dip galvanizing apparatus of a continuous hot dip galvanizing facility according to the present invention. In the annealing furnace 1, a heating zone 2, a soaking zone 3, and a cooling zone 4 are arranged in this order. The hot dip galvanizing apparatus 5 is adjacent to the cooling zone 4 and has a hot dip galvanizing bath 6 and an alloying furnace 7 for alloying the hot dip galvanized steel sheet.

FIG. 2 is a flow diagram of the gas supplied into the annealing furnace in the present invention. In the present invention, the dry gas and the mixed gas are supplied to the soaking zone 3 of the annealing furnace 1 (hereinafter, the annealing furnace or soaking zone may be simply referred to as “furnace”). Specifically, the dry gas supply pipe 9 and the mixed gas supply pipe 10 branch from the gas supply pipe 8, and the dry gas and the mixed gas are supplied into the furnace. The flow rate of the dry gas is adjusted by the flow rate control valve 11 for dry gas. Then, a predetermined amount of dry gas is supplied into the furnace through the dry gas inlet 12. The dry gas is H ₂ —N ₂ gas or N ₂ gas having a dew point of about −60 ° C. to −50 ° C.

  As for the mixed gas supply pipe 10, a part of the dry gas supplied from the gas supply pipe 8 is humidified by the humidifier 13. Then, the dry gas and the gas humidified by the humidifier 13 (hereinafter sometimes simply referred to as humidified gas) are mixed to form a mixed gas. The mixed gas is mixed at a predetermined mixing ratio according to line specifications such as the volume in the furnace, and a predetermined dew point is obtained.

  The dew point of the mixed gas is monitored by a mixed gas dew point meter 14. The flow rate of the mixed gas is adjusted by the mixed gas flow control valve 15, and a predetermined amount of the mixed gas is supplied into the furnace through the mixed gas inlet 16. Further, the amount of the mixed gas input is adjusted by the mixed gas flow control valve 15 based on the value of the in-furnace gas dew point meter 17 installed in the furnace. The flow rate of the dry gas supplied from the dry gas supply pipe 9 into the furnace is determined in advance from line specifications such as the volume in the furnace, and the opening degree of the dry gas flow control valve 11 is set so as to be this flow rate. Adjusted. Then, by supplying the mixed gas from the mixed gas supply pipe 10 into the furnace, the dry gas and the mixed gas are mixed in the furnace, and a predetermined dew point is obtained.

  FIG. 3 is a control block diagram of the mixed gas supplied into the furnace. In the present invention, a dew point adjuster and a flow rate adjuster are provided to control the dew point in the furnace. Since the amount of gas mixture supplied into the furnace is affected by disturbances such as fluctuations in furnace pressure, the dew point cannot be precisely controlled only by the dew point controller. Therefore, the influence of disturbance can be minimized by performing control using a dew point controller and a flow controller. In addition, when controlling the dew point in a furnace, the amount of mixed gas supplied into a furnace is adjusted by adjusting the valve opening degree of the flow control valve 15 suitably.

  What is input to the dew point adjuster is the difference between the target in-furnace dew point (target dew point) and the current in-furnace dew point feedback value measured by the dew point meter 17. The output from the dew point adjuster is the calculated flow rate of the humidified gas to be supplied so as to achieve the target furnace dew point. In the present invention, a flow rate obtained by multiplying the calculated flow rate by a flow rate correction value C is set as a target flow rate. Specifically, the difference between the target flow rate and the actual humidified gas flow rate measured by a flow meter (not shown) is taken as the input of the flow rate regulator. And the dew point in the furnace (control object 2) is controlled by adjusting the valve opening degree of the flow control valve 15 and adjusting the humidified gas flow rate (control object 1).

The flow rate correction value C is calculated by the following equation (1).
C = A × B (1)
In equation (1),
A: Plate width correction value set according to the plate width of the steel plate in the furnace managed by the host computer B: Feed plate which is the ratio of the reference plate speed in the furnace and the actual plate speed in the furnace This is the speed correction value.

  The plate width correction value A is managed by a host computer. The host computer has a setting table for the plate width correction value A corresponding to the plate width, and the plate width correction value A corresponding to the plate width of the steel plate currently in the furnace is selected from the setting table. The setting table of the plate width correction value A is set in advance for each line specification in consideration of the plate width information inserted into the furnace, the furnace volume, operating conditions, and the like.

  Further, the plate passing speed correction value B is a ratio of the reference plate passing speed in the furnace and the current plate passing speed in the furnace. The current plate-passing speed in the furnace is the actual plate-passing speed measured by a PLG (pulse generator) installed on the transport roll. A plate width correction value A and a plate speed correction value B are calculated, and the flow rate correction value C is calculated by multiplying these values.

In order to stably control the dew point, it is necessary to control the dew point by supplying humidified nitrogen gas or H ₂ —N ₂ gas in consideration of the steel plate size and the plate passing speed. As in the present invention, the flow rate correction value C represented by the plate width correction value A based on the plate width information and the plate passing speed correction value B based on the plate passing speed information (speed record) is calculated as the humidified gas. By multiplying the flow rate, the steel plate size and the plate passing speed can be taken into consideration. As a result, the dew point in the annealing furnace can be controlled more precisely.

  As described above, in the present invention, the dew point in the annealing furnace can be controlled more precisely by multiplying the calculated flow rate of the humidified gas by the flow rate correction value C and adjusting the flow rate of the humidified gas supplied into the furnace. In addition, by producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet using the method for controlling the dew point of the atmospheric gas in the furnace according to the present invention, the plating adhesion is high, the plating appearance is good, and the plating property is excellent. A hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet can be obtained.

  In addition, there is no restriction | limiting in particular about the manufacturing conditions of a galvannealed steel plate, It can manufacture by a conventional method.

  In producing a high strength steel sheet containing 0.2 mass% or more of Si, the change in the dew point in the furnace when controlled by the dew point control method of the present invention, and the conventional dew point control method (the calculated flow rate of the humidified gas as the target flow rate). In other words, a simulation experiment was performed to compare changes in the dew point in the furnace when the calculated flow rate was controlled by multiplying the calculated flow rate by not multiplying the flow rate correction value C.). Here, the target dew point in the furnace was −20 ° C. In the simulation, it was assumed that the passage speed in the furnace decreased from the reference speed after 30 minutes and the passage speed in the furnace returned to the reference speed when the elapsed time was 60 minutes. Moreover, the steel plate to be passed was set to a plate width of 1200 mm and a plate passing speed of 90 mpm. The reference plate width was 1350 mm and the reference speed was 90 mpm. The flow rate correction value C was calculated with the rate of change in plate width when the reference plate width was 1 as A, and the rate of change in plate speed when the reference speed was 1 as B, and dew point control was performed ( A = 0.89).

  FIG. 4 shows a simulation result. In both the dew point control method of the present invention and the conventional dew point control method, the in-furnace dew point began to increase after 30 minutes when the in-furnace plate speed decreased. The dew point control method of the present invention immediately returns to around the target dew point of −20 ° C., indicating that the dew point is controlled. On the other hand, it can be seen that the conventional dew point control method takes time to return to the target dew point of −20 ° C.

  Moreover, when a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet were produced by a conventional method using the dew point control method of the present invention, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet excellent in plating adhesion and plating appearance were obtained. Could get.

DESCRIPTION OF SYMBOLS 1 Annealing furnace 2 Heating zone 3 Soaking zone 4 Cooling zone 5 Hot dip galvanizing apparatus 6 Hot dip galvanizing bath 7 Alloying furnace 8 Nitrogen gas supply pipe 9 Drying gas supply pipe 10 Mixed gas supply pipe 11 Drying gas flow control valve 12 Drying Gas inlet 13 Humidifier 14 Dew point meter for mixed gas 15 Flow control valve for mixed gas 16 Mixed gas inlet 17 Gas dew point meter in furnace

Claims (2)

In an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order,
Adjusting the flow rate of the humidified gas supplied into the furnace by multiplying the calculated flow rate of the humidified gas supplied into the annealing furnace by the flow rate correction value C calculated by the following equation (1) as the target flow rate of the humidified gas The dew point control method characterized by performing.
C = A × B (1)
In equation (1),
A: Plate width correction value set according to the plate width of the steel plate in the furnace managed by the host computer B: Feed plate which is the ratio of the reference plate speed in the furnace and the actual plate speed in the furnace This is the speed correction value.   A method for producing a hot dip galvanized steel sheet, comprising subjecting a steel sheet to annealing using the dew point control method according to claim 1 and then performing hot dip galvanizing in a hot dip galvanizing facility adjacent to the annealing furnace.

Images