JP5338087B2 - Method for producing hot-dip galvanized steel sheet with excellent plating properties and continuous hot-dip galvanizing equipment - Google Patents

Description

  The present invention relates to a method for producing a hot dip galvanized steel sheet and a continuous hot dip galvanizing facility. More specifically, the present invention relates to a hot dip galvanized steel sheet capable of producing a hot dip galvanized steel sheet having excellent plating properties at a low cost regardless of the Si content in the steel. The present invention relates to a method for producing a plated steel sheet and a continuous hot dip galvanizing facility.

  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. With this high-strength steel sheet, adding Si to the steel may produce a high-strength steel sheet with good hole-expandability, and if it contains Si or Al, it will easily form residual γ and produce a steel sheet with good ductility. There is a possibility.

  In general, a steel sheet for hot dip galvanizing is manufactured by performing hot dip galvanization with a plating apparatus after annealing the steel sheet in an annealing furnace. The steel sheet is immersed in the plating bath through an annealing process in which it is preheated to about 300 ° C. in the pre-tropical zone, heated to about 800 ° C. in the reduction zone, and rapidly cooled to 500 ° C. in the quench zone.

  However, Si, Mn, etc. are easily oxidizable elements, and steel sheets containing these elements cause surface concentration of Si, etc. in the reduction zone, adversely affecting plating properties, and appearance such as non-plating and plating unevenness. It is known to cause defects and poor plating adhesion. This problem becomes significant when the Si content in the steel is 0.2 mass% or more.

As a method for producing a steel sheet containing a large amount of an easily oxidizable element such as Si, after performing a pre-plating treatment as described in Patent Document 1, for example, a method of hot dip galvanizing by performing heating / reduction and cooling treatment, A method of improving plating properties by controlling the atmosphere in the furnace as described in Patent Document 2 and simultaneously introducing CO ₂ into the furnace and oxidizing Si internally is known. However, the method as disclosed in Patent Document 1 has a problem that the pre-plating cost is high, and the method as disclosed in Patent Document 2 is likely to cause in-furnace contamination by CO ₂ or decarburization on the surface of the steel sheet to change the mechanical characteristics. It is thought that there is.

Similarly, there is a report of improving the plating property and chemical conversion property of low Si-high Al TRIP steel by increasing the dew point in the furnace and internally oxidizing easily oxidizable elements such as Si and Al (non-patent document). 1). This method is easy to implement because there are few problems of furnace contamination, but there is a problem that equipment costs and manufacturing costs increase because it is necessary to add equipment such as a moisture adding device for dew point management. It was.
JP-A-2-38549 JP 2005-60743 A Mahieu, Galvatech 01, p644 (2001)

  The present invention has been made in view of such circumstances, and when hot dip galvanizing is performed on a steel sheet containing 0.2 mass% or more of Si in the steel, poor appearance such as non-plating and plating unevenness and poor plating adhesion. It is an object of the present invention to provide a hot dip galvanized steel sheet manufacturing method and a continuous hot dip galvanizing facility that can prevent hot dip galvanized steel sheets from being produced at low cost.

In a steel sheet containing a large amount of Si, if the dew point of the reducing furnace atmosphere of the annealing furnace is increased, Si is internally oxidized, so the ratio of the H ₂ O concentration to the H ₂ concentration (hereinafter referred to as an index of the oxygen potential of the reducing furnace atmosphere) In this specification, it is described as “H ₂ O / H ₂ ratio.”) Is preferably higher. On the other hand, a steel sheet that does not contain a large amount of Si preferably has a low H ₂ O / H ₂ ratio in order to improve the reduction.

From this, in a continuous hot dip galvanizing facility that applies hot dip galvanizing to steel sheets with a wide range of Si content, it has good plating properties for hot dip galvanized steel sheets containing 0.2 mass% or more of Si in the steel. In order to ensure, it is necessary to be able to appropriately control the H ₂ O / H ₂ ratio of the gas in the reduction furnace in consideration of the Si content of the steel sheet. The inventors pay attention to effectively using H ₂ O released from the steel sheet, and by optimizing the supply / discharge position of the in-furnace gas, the steel contains 0.2% by mass or more of Si. It has been found that a hot dip galvanized steel sheet can be manufactured at a low cost while ensuring good plating properties. The present invention is based on this finding.

  Means of the present invention for solving the above-mentioned problems are as follows.

[1] When the steel sheet containing 0.2% by mass or more of Si in steel is heated and annealed in a reduction furnace and then hot-dip galvanized, the lower 1/3 of the steel plate passage area in the height direction of the reduction furnace In the manufacturing method of the hot dip galvanized steel sheet, the dew point of the gas in the furnace is controlled to be in the range of more than −30 ° C. and not more than 0 ° C., (1) When the measured value of the dew point in the lower 1/3 region is -30 ° C. or lower, the supply of the in-furnace gas to the reduction furnace is lower than 1/2 of the height direction steel plate passage area of the reduction furnace. And the discharge of the gas in the furnace of the reduction furnace is performed from a position lower than 1/2 of the height direction steel plate passage area of the reduction furnace, and the lower one third of the height direction steel plate passage area of the reduction furnace The dew point of the furnace gas in the region is over -30 ° C and below 0 ° C,
(2) When the measured value of the dew point in the lower 1/3 region of the steel plate passage region in the height direction of the reduction furnace exceeds 0 ° C, (i) exhausting the gas in the furnace of the reduction furnace, From a position higher than 1/2 of the steel plate passage area in the height direction of the reduction furnace, discharge an amount of 1/3 or more of the supply amount of the in-furnace gas supplied to the reduction furnace (including the case of discharging the entire amount), or (B) Supplying the in-furnace gas to the reduction furnace from a position higher than 1/2 of the height direction steel plate passage area of the reduction furnace, and discharging the in-furnace gas of the reduction furnace to the height direction steel plate of the reduction furnace It is performed from a position lower than 1/2 of the sheet passing region, and the dew point of the gas in the furnace in the lower 1/3 region of the steel plate passing region in the height direction of the reduction furnace is set to more than −30 ° C. and not more than 0 ° C.,
A method for producing a hot-dip galvanized steel sheet having excellent plating properties.

  [2] In a continuous hot dip galvanizing facility equipped with a plating apparatus and an annealing furnace upstream of the plating apparatus, the reduction furnace of the annealing furnace is located in the region of the lower third of the sheet passage area in the height direction of the reduction furnace. A dew point meter for measuring the dew point of the in-furnace gas is provided, and the discharge part of the in-furnace gas is located at a position higher than 1/2 of the height direction steel plate passage area of the reduction furnace and the height direction steel plate passage area of the reduction furnace. Provided at a position lower than 1/2, and provided a gas supply section in the furnace at a position lower than 1/2 of the height direction steel plate passage area of the reduction furnace, or further provided in the height direction steel plate passage area of the reduction furnace. A continuous hot-dip galvanizing facility characterized by being provided at a position higher than 1/2.

  ADVANTAGE OF THE INVENTION According to this invention, the hot dip galvanized steel plate which contains 0.2 mass% or more of Si in steel can be manufactured at low cost, ensuring favorable plating property.

  In order to improve the plating property, it is best that the steel plate is non-oxidized, and then the internal oxidation is good. Plating properties are poor when externally oxidized. In a steel sheet containing 0.2% by mass or more of Si, it is difficult to make it non-oxidized, and in a normal furnace atmosphere, Si is easily oxidized externally, and the plating property is hindered. Therefore, the manufacturing method of the hot dip galvanized steel sheet of the present invention is intended for the steel containing 0.2% by mass or more of Si.

In the continuous hot dip galvanizing facility, not only a steel plate containing a large amount of Si but also a steel plate not containing a lot of Si is manufactured. In a steel sheet containing 0.2% by mass or more of Si in steel, when the dew point of the reducing furnace atmosphere of the annealing furnace is increased, Si is internally oxidized, and thus H ₂ O / H, which is an index of the oxygen potential of the reducing furnace atmosphere. _{The 2} ratio is preferably high. A steel sheet having a Si content of less than 0.2% by mass in steel is a condition for internal oxidation under normal operating conditions, and since the absolute amount of Si is small, H ₂ O / H ₂ The ratio is preferably low.

From this, in continuous galvanizing equipment that performs hot dip galvanizing on steel sheets with a wide range of Si concentrations, good plating properties are achieved for hot dip galvanized steel sheets containing 0.2 mass% or more of Si in the steel. In order to ensure, it is necessary to be able to appropriately control the H ₂ O / H ₂ ratio of the gas in the reduction furnace in consideration of the Si content of the steel sheet. According to the present invention, by optimizing the supply / discharge positions of the in-furnace gas, H ₂ O released from the steel sheet is effectively used, and hot dip galvanizing is performed on a wide range of Si concentration steel sheets. In a plating facility, a hot-dip galvanized steel sheet containing 0.2% by mass or more of Si in steel can be manufactured at low cost while ensuring good plating properties.

  Hereinafter, the present invention will be specifically described.

  As a type of annealing furnace for continuous galvanizing equipment, the heating furnace for heating and heating the steel sheet is DFF (direct flame type) or NOF (non-oxidizing type), and the soaking furnace for soaking the heated steel sheet is a radiant tube ( RTF) type, all radiant tube type (RTF) from the heating furnace to the soaking furnace is available. Unlike DFF (direct flame type) and NOF (non-oxidation type), all-radiant tube type continuous hot dip galvanizing equipment has no oxidation process in the heating furnace immediately before annealing, and the iron itself is not oxidized. A steel sheet containing a large amount of an oxidizing element is disadvantageous in terms of securing plating properties because an internal oxide of Si is not formed.

  The effect of the present invention can be manifested in a continuous hot dip galvanizing facility equipped with an annealing furnace equipped with DFF and RTF, and NOF and RTF, but in terms of securing the plating property of a steel plate containing a large amount of an easily oxidizable element such as Si. In the case of an all-radiant tube type annealing furnace, which is disadvantageous, the effects of the present invention can be exhibited particularly significantly, which is preferable.

  In the present specification, the reduction furnace is a furnace part provided with a radiant tube, and in the annealing furnace provided with DFF and RTF, and in the annealing furnace provided with NOF and RTF, the reduction furnace indicates a soaking furnace. In an all radiant tube type annealing furnace, the reducing furnace is from a heating furnace to a soaking furnace.

  FIG. 1 is a view for explaining a structural example of a continuous hot dip galvanizing facility including an annealing furnace and a plating apparatus, wherein 1 is a steel plate, 2 is an all radiant tube type annealing furnace, and 7 is a plating apparatus. The annealing furnace 2 includes a preheating furnace 3, a radiant tube type reduction furnace 4, a quenching zone 5, and a slow cooling zone 6. The former stage of the reduction furnace 4 is a heating furnace, and the latter stage is a soaking furnace.

  FIG. 2 shows an embodiment of the supply route and discharge route of the in-furnace gas in the reduction furnace 4 of the facility shown in FIG. In FIG. 2, 11 is a reduction furnace, 12 is a lower roll for passing a steel plate, 13 is an upper roll for passing a steel plate, 14 is a dew point meter, and 15 is a steel plate. The steel plate passage region in the height direction of the reduction furnace 11 is from the top of the upper roll 13 to the lower end of the lower roll 12.

  The reducing furnace 11 is provided with two routes for supplying and discharging the gas in the furnace. Reference numerals 16 and 17 denote furnace gas supply routes, reference numeral 16 denotes a lower supply route, and reference numeral 17 denotes an upper supply route. Reference numerals 18 and 19 denote furnace gas discharge routes, reference numeral 18 denotes a lower discharge route, and reference numeral 19 denotes an upper discharge route.

  The lower supply route 16, the lower discharge route 18, and the upper discharge route 19 are essential. The lower supply route 16 arranges the gas supply part in the furnace at a position lower than ½ of the steel plate passage area in the height direction of the reduction furnace 11, and the lower discharge route 18 sets the gas supply part in the furnace to the reduction furnace. 11 is disposed at a position lower than ½ of the height direction steel plate passage area, and the upper discharge route 19 is configured so that the discharge portion of the in-furnace gas is higher than ½ of the height direction steel sheet passage area of the reduction furnace 11. Place in position.

  The upper supply route 17 is not essential, but when it is provided, it is preferable to arrange the supply portion of the in-furnace gas at a position higher than ½ of the height direction steel plate passage region of the reduction furnace 11. By providing such an upper supply route 17, it becomes possible to effectively purge the gas containing water vapor in the upper part of the reduction furnace in a short time.

  The in-furnace gas supply part to the reduction furnace of the lower supply route 16, the upper supply route 17, the discharge part of the in-furnace gas of the reduction furnace 11 of the lower discharge route 18, and the upper discharge route 19 are in the longitudinal direction of the line of the reduction furnace 11 ( It is preferable to provide it at a plurality of locations in the left and right direction of the drawing).

  The in-furnace gas supply part of the lower supply route 16 and the in-furnace gas discharge part of the lower discharge route 18 may be provided at the bottom of the reduction furnace 11. The furnace gas supply part of the upper discharge route 19 and the furnace gas supply part of the upper supply route 17 may be provided at the top of the reduction furnace 11.

As the in-furnace gas supplied into the reduction furnace, H ₂ —N ₂ gas generally used as a reducing atmosphere gas of the reduction furnace can be used.

  In the present invention, the dew point of the gas in the furnace of the reduction furnace 11 is measured, and the supply position of the gas in the furnace of the reduction furnace 11 according to the measured value of the dew point, or the measured value of the dew point and the Si content of the steel plate to be passed through. And the discharge position. The dew point to be referred to is preferably a dew point measured in the region of the lower third of the height direction steel plate passage region of the reduction furnace 11. Since steam has a lighter specific gravity than nitrogen, which occupies most of the gas in the furnace, the dew point tends to be higher and the dew point tends to fluctuate in the upper part of the furnace. On the other hand, this is because there is no such problem in the lower third region of the steel plate passage region. Therefore, in the present invention, the dew point is measured by the dew point meter 14 in the lower third region of the steel plate passage region in the height direction of the reduction furnace 11.

  When the Si concentration of the steel plate to be passed through and the dew point in the lower third region of the steel plate passage region in the height direction of the reduction furnace 11 is −30 ° C. when the Si concentration is 0.2 mass% or more. When the temperature is in the range of 0 ° C. or lower, good plating properties can be obtained. However, if the Si concentration and the dew point in the region of the lower third of the plate passing through the height direction steel plate of the reduction furnace 11 are out of the above range, the plateability becomes inferior.

  In the present invention, with a steel plate having a Si concentration in the steel of 0.2% by mass or more, the dew point in the lower third region of the steel plate passage region in the height direction of the reduction furnace 11 is measured by the dew point meter 14, Depending on the measured dew point, the supply / discharge of the gas in the reduction furnace 11 is performed as follows.

  (1) Supplying the gas in the furnace to the reduction furnace from a position lower than 1/2 of the height direction steel plate passage area of the reduction furnace, and discharging the gas in the furnace of the reduction furnace in the height direction steel plate It is performed from a position lower than 1/2 of the sheet passing region, and the dew point of the gas in the furnace in the lower 1/3 region of the steel plate passing region in the height direction of the reduction furnace is more than −30 ° C. and less than 0 ° C. To do.

  Under conditions where the dew point is −30 ° C. or lower, a steel sheet having a Si concentration of 0.2% by mass or more does not generate a sufficient internal oxide layer on the steel sheet in the reduction furnace, so Si is concentrated on the surface without being internally oxidized. Therefore, the plating property is hindered. Therefore, in order to generate a sufficient internal oxide layer on the steel sheet, it is necessary to raise the dew point to above -30 ° C. By setting both the gas discharge position and the supply position in the furnace to a position lower than 1/2 of the height direction steel plate passage area, the moisture discharged from the steel sheet stays in the furnace, so the dew point at the top of the furnace is The temperature rises above -30 ° C and an internal oxide layer is formed on the steel sheet. When this internal oxide layer is reduced, an internal oxide layer of Si is formed and external oxidation is suppressed, so that the plating property is not hindered. If the dew point becomes too high, the plating property is hindered, so the dew point should be 0 ° C. or less (see (2) below).

  (2) When the measured dew point exceeds 0 ° C, perform (b) or (b) below, and the in-furnace gas in the lower 1/3 region of the steel plate passage region in the height direction of the reduction furnace So that the dew point is over -30 ° C and below 0 ° C.

  (A) Exhaust gas in the furnace of the reduction furnace is discharged from a position higher than 1/2 of the steel plate passage area in the height direction of the reduction furnace to 1/3 or more of the in-furnace gas supply amount to the reduction furnace (total discharge) Including the case).

  When the dew point is higher than 0 ° C, a thick Fe oxide layer is formed on the steel sheet surface regardless of the Si concentration of the steel sheet, and this Fe oxide layer remains on the steel sheet surface without being reduced. Inhibition problems may occur. Under such conditions, it is necessary to lower the dew point to 0 ° C. or lower. If the gas in the furnace is discharged from a position higher than 1/2 of the steel plate passage area of the reduction furnace, more than 1/3 of the amount of gas supplied in the furnace can be prevented so that steam stays in the upper part of the furnace. Therefore, external oxidation can be prevented. If the dew point becomes too low, the plating property is hindered, so the dew point should be over -30 ° C.

  The furnace gas may be discharged from the height of the reduction furnace at a position higher than ½ of the steel plate passage area. In the case where the entire amount is not discharged, the remaining furnace gas is discharged from a position lower than 1/2 of the height direction steel plate passage area of the reduction furnace.

  The supply position of the furnace gas is not particularly limited. It may be supplied from either one of the positions higher than 1/2 of the steel plate passage area in the height direction of the reduction furnace and the position lower than 1/2 of the steel sheet passage area in the height direction of the reduction furnace. May be.

  (B) Supplying the in-furnace gas to the reduction furnace from a position higher than 1/2 of the height direction steel plate passage area of the reduction furnace, and discharging the in-furnace gas of the reduction furnace through the height direction steel plate of the reduction furnace It starts from a position lower than 1/2 of the plate area.

  Water vapor staying in the upper part of the furnace by supplying the gas in the furnace from a position higher than 1/2 of the height direction steel plate passage area and discharging from the position lower than 1/2 of the height direction steel sheet passage area. The dew point can be quickly lowered to 0 ° C. or lower. A steel plate having a Si concentration in the steel of 0.2% by mass or more can prevent the problem of inhibition of plating property due to the surface concentration of Si.

  In the above (1) and (2), with respect to the supply position of the furnace gas and the discharge position of the furnace gas, the position higher than 1/2 of the steel sheet passage area is higher than 3/4 of the steel sheet passage area. More preferably, the position lower than 1/2 of the steel plate passage area is more preferably lower than 1/4 of the steel sheet passage area.

  (3) In cases other than the above (1) and (2), that is, when the measured dew point is more than −30 ° C. and not more than 0 ° C., the risk of Si surface concentration is low, and the external oxidation of the steel sheet surface There is no problem that the plating property is hindered. Therefore, it is not necessary to change the furnace gas supply position and the furnace gas discharge position. If the measured value of the dew point is maintained above -30 ° C. and below 0 ° C., the gas supply position and gas discharge position may be selected as appropriate in consideration of thermal efficiency, gas cost, and the like.

  The dew point measurement frequency may be appropriately determined in consideration of the atmospheric gas supply amount, the dew point change trend, and the like.

  In the case of a steel sheet having a Si concentration of less than 0.2% by mass in steel, the dew point in the region of the lower third of the steel plate passage area in the height direction of the reduction furnace 11 is in the range of 0 ° C. or less. Good plating properties can be obtained, and when the dew point exceeds 0 ° C., the plating properties become poor. When the dew point exceeds 0 ° C., the above method (2) is performed, and by setting the dew point to 0 ° C. or less, good plating properties can be secured.

  A method for producing the hot dip galvanized steel sheet will be described.

  About the manufacturing method of a hot dip galvanized steel plate, it does not specifically limit except performing the supply and discharge | emission of gas in a reduction furnace as mentioned above.

  The steel sheet for plating may be a hot-rolled steel sheet pickled and descaled, or a descaled steel sheet cold-rolled. The upper limit of the Si concentration of the steel sheet is 2.5% by mass. This is because a steel sheet having a Si concentration of more than 2.5% by mass cannot secure good plating properties by the method of the present invention.

  A steel sheet for plating is charged into a continuous hot dip galvanizing facility, annealed in an annealing furnace, and then hot dip galvanized with a plating apparatus or further alloyed to produce a hot dip galvanized steel sheet.

Supply and discharge of the in-furnace gas to the reduction furnace of the annealing furnace are performed by the method described above, and the steel sheet is subjected to reduction annealing. The in-furnace gas supplied to the annealing furnace can be a commonly used H ₂ —N ₂ gas. The gas supply amount and the hydrogen concentration may be the conventional methods. The annealing conditions are not particularly limited. Appropriate conditions are adopted according to the steel type and material of the plated steel sheet.

  Cool after annealing, hot dip galvanize with hot dipping equipment, adjust zinc adhesion to required amount with gas wiping equipment, and if necessary, continue with alloying treatment to obtain the required hot dip galvanized steel sheet obtain. The hot dip galvanizing conditions and alloying treatment conditions are not particularly limited. Ordinary methods are acceptable. The amount of zinc plating is not limited.

  According to the method of the present invention, a hot-dip galvanized plated steel sheet has no unplating or the like, has a good appearance, and is excellent in plating adhesion. In a plated steel sheet that has been subjected to alloying after hot dip galvanization, a good appearance without uneven alloying is obtained, and the powdering resistance is also good.

  According to the present invention, the conventionally used furnace gas can be used as the furnace gas, and the apparatus for supplying and discharging the furnace gas is inexpensive, so that the molten zinc has a good appearance and excellent plating properties. A plated steel sheet can be manufactured at low cost.

  As described above, the annealing furnace including the all-radiant tube type reduction furnace has been described. However, the annealing furnace may be an annealing furnace provided with DFF and RTF, or an annealing furnace provided with NOF and RTF. In this case, the supply and discharge of the RTF in-furnace gas may be performed as described above. Other manufacturing conditions may be conventional methods.

  Hereinafter, the present invention will be specifically described based on examples.

  The annealing furnace of the continuous hot dip galvanizing equipment used in this example is an all radiant tube type annealing furnace. The reduction furnace of the annealing furnace includes a gas supply route, a gas discharge route, and a dew point meter as shown in FIG.

  The supply route of the gas in the furnace is that the upper supply route 17 is five positions (equally spaced) in the longitudinal direction of the line at the height 3/4 of the height direction steel plate passage region, and the lower supply route 16 is the height direction steel plate. Five positions (equal intervals) are provided in the longitudinal direction of the line at a position of a height of ¼ of the through plate area. Since the reduction furnace includes the upper supply route 17, when the dew point is higher than 0 ° C., it is possible to purge water vapor in the upper part of the furnace with a smaller amount of gas.

  As for the discharge route of the gas in the furnace, the lower discharge route 18 is located at approximately the same height as the lower plate 12 for feeding plates at three locations (equally spaced) in the longitudinal direction of the line, and the upper discharge route 19 is disposed in the longitudinal direction of the line at the top of the furnace. Three locations (equal intervals) are provided.

  The dew point detector of the dew point meter 14 is provided 1 m above the axis of the lower roll 12 and at the center in the longitudinal direction of the reduction furnace.

In order to prevent the air from flowing back into the furnace, it is usually necessary to set the positive pressure in the furnace. In this example, the furnace pressure of the reduction furnace was controlled to be about +0.2 kPa. At this time, the gas supply amount into the reduction furnace was 800 Nm ³ / hr, and the discharge amount was almost the same amount. The supply gas is H ₂ —N ₂ gas (H ₂ concentration: 10% by volume, dew point; −60 ° C.).

Low carbon cold-rolled steel sheets (thickness: 1.0 mm) having different Si concentrations are passed at a line speed of 90 mpm, heated in a reducing furnace at a temperature of 850 ° C. for 150 seconds, annealed, cooled, and hot-dip galvanized (plating amount; 50 g / m ² ) per one side and further alloyed to obtain an alloyed hot-dip galvanized steel sheet. At that time, the combination of the furnace gas supply position and the discharge position was changed, and the dew point was measured every 10 minutes.

  Observe the appearance of the resulting alloyed hot-dip galvanized steel sheet. If there is no appearance failure such as non-plating or uneven plating due to poor plating performance, the plating performance is good (symbol ○). It was determined to be defective (symbol x).

  Table 1 shows the Si concentration of the steel sheet, the measured value of the dew point, the atmospheric gas supply position, the discharge position, and the evaluation results of the plating properties at that time. FIG. 3 shows changes in the Si concentration and dew point of the steel sheet.

  Plating properties are good when the dew point is more than −30 ° C. and less than 0 ° C. When the measured dew point was less than −30 ° C. or exceeded 0 ° C., the gas supply and exhaust remained poor when the gas supply and exhaust were not performed according to the method of the present invention. When this method is used, good plating properties can be obtained.

  The manufacturing method of the hot dip galvanized steel sheet of this invention can be utilized as a method of manufacturing the hot dip galvanized steel sheet excellent in plating property at low cost. The apparatus of the present invention can be used as production equipment for the hot-dip galvanized steel sheet.

It is a figure explaining one structural example of the continuous hot dip galvanization equipment provided with an annealing furnace and a plating apparatus. It is a figure which shows one Embodiment of the supply route and discharge route of the gas in the reduction furnace of the continuous hot-dip galvanization equipment of FIG. It is a figure which shows the change of Si density | concentration in steel of an Example, and a dew point.

Explanation of symbols

1 Steel plate 2 Annealing furnace 3 Preheating furnace 4 Reduction furnace (heating furnace / soaking furnace)
5 Rapid Zone 6 Gradual Cooling Zone 7 Plating Apparatus 11 Reduction Furnace 12 Lower Roll 13 Upper Roll 14 Dew Point Meter 15 Steel Plate 16 Furnace Gas Lower Supply Route 17 Furnace Gas Upper Supply Route 18 Furnace Gas Lower Discharge Funnel 19 Upper discharge funnel for furnace gas

Claims (2)

When hot-dip galvanizing after heating and annealing a steel plate containing 0.2 mass% or more of Si in steel in a reduction furnace, within the lower 1/3 region of the steel plate passage region in the height direction of the reduction furnace A method for producing a hot-dip galvanized steel sheet in which the dew point of the gas in the furnace is controlled to be within a range of more than −30 ° C. and not more than 0 ° C., (1) When the measured value of the dew point in the region 3 is -30 ° C. or lower, supply the in-furnace gas to the reduction furnace from a position lower than 1/2 of the steel plate passage area in the height direction of the reduction furnace. In addition, the gas in the furnace of the reduction furnace is discharged from a position lower than 1/2 of the height direction steel plate passage area of the reduction furnace, and within the lower third of the height direction steel plate passage area of the reduction furnace. The dew point of the furnace gas of -30 ° C to 0 ° C or less,
(2) When the measured value of the dew point in the lower 1/3 region of the steel plate passage region in the height direction of the reduction furnace exceeds 0 ° C, (i) exhausting the gas in the furnace of the reduction furnace, From a position higher than 1/2 of the steel plate passage area in the height direction of the reduction furnace, discharge an amount of 1/3 or more of the supply amount of the in-furnace gas supplied to the reduction furnace (including the case of discharging the entire amount), or (B) Supplying the in-furnace gas to the reduction furnace from a position higher than 1/2 of the height direction steel plate passage area of the reduction furnace, and discharging the in-furnace gas of the reduction furnace to the height direction steel plate of the reduction furnace It is performed from a position lower than 1/2 of the sheet passing region, and the dew point of the gas in the furnace in the lower 1/3 region of the steel plate passing region in the height direction of the reduction furnace is set to more than −30 ° C. and not more than 0 ° C.,
A method for producing a hot-dip galvanized steel sheet having excellent plating properties. In a continuous hot-dip galvanizing facility equipped with a plating apparatus and an annealing furnace upstream of the plating apparatus, the reduction furnace of the annealing furnace is an in-furnace gas in the region of the lower third of the steel plate passage region in the height direction of the reduction furnace. Provided with a dew point meter for measuring the dew point of the furnace, and the discharge part of the gas in the furnace is located at a position higher than 1/2 of the height direction steel plate passage area of the reduction furnace and 1/2 of the height direction steel plate passage area of the reduction furnace It is provided at a lower position, and the gas supply section in the furnace is provided at a position lower than 1/2 of the height direction steel plate passage area of the reduction furnace, or further 1 / of the height direction steel plate passage area of the reduction furnace. A continuous hot-dip galvanizing facility provided at a position higher than 2.

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