WO1988006636A1 - A method for controlling the thickness of an intermetallic layer on a continuous steel product in a continuous hot-dip galvanizing process - Google Patents

A method for controlling the thickness of an intermetallic layer on a continuous steel product in a continuous hot-dip galvanizing process Download PDF

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Publication number
WO1988006636A1
WO1988006636A1 PCT/FI1988/000026 FI8800026W WO8806636A1 WO 1988006636 A1 WO1988006636 A1 WO 1988006636A1 FI 8800026 W FI8800026 W FI 8800026W WO 8806636 A1 WO8806636 A1 WO 8806636A1
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WO
WIPO (PCT)
Prior art keywords
zinc
steel product
flow
steel
temperature
Prior art date
Application number
PCT/FI1988/000026
Other languages
French (fr)
Inventor
Pertti Juhani Sippola
Original Assignee
Rasmet Ky
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Rasmet Ky filed Critical Rasmet Ky
Priority to BR888805642A priority Critical patent/BR8805642A/en
Priority to AT88901847T priority patent/ATE71987T1/en
Priority to DE8888901847T priority patent/DE3867988D1/en
Priority to KR1019880701350A priority patent/KR930001781B1/en
Publication of WO1988006636A1 publication Critical patent/WO1988006636A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/003—Apparatus
    • C23C2/0034—Details related to elements immersed in bath
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/003—Apparatus
    • C23C2/0034—Details related to elements immersed in bath
    • C23C2/00342—Moving elements, e.g. pumps or mixers
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/003—Apparatus
    • C23C2/0034—Details related to elements immersed in bath
    • C23C2/00342—Moving elements, e.g. pumps or mixers
    • C23C2/00344—Means for moving substrates, e.g. immersed rollers or immersed bearings
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/003—Apparatus
    • C23C2/0038—Apparatus characterised by the pre-treatment chambers located immediately upstream of the bath or occurring locally before the dipping process
    • C23C2/004—Snouts
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/14—Removing excess of molten coatings; Controlling or regulating the coating thickness

Definitions

  • the present invention relates to a method for controlling the thickness of an intermetallic layer on a continuous steel product in a continuous hot-dip galva ⁇ nizing process.
  • the continuous steel product is gene ⁇ rally either a strip or a wire.
  • a cold-rolled steel strip can be given a good formability by means of a heat treatment disclosed in my earlier U.S. Patent 4,361,448.
  • After annealing at a tem ⁇ perature - j _ (720 to 850°C) the steel strip is slowly cooled to a temperature 2 (600 to 650°C), from which temperature it is rapidly quenched in a zinc bath to a temperature T3.
  • the time interval between T 2 and Tg is ab ut 0.5 seconds.
  • a steel strip travelling through a zinc bath causes a laminar zinc flow following the surface of the steel strip.
  • the heat from inside the steel strip raises the temperature of the laminar zinc flow (layer) to a value higher than the operating temperature of the zinc bath. Since iron and zinc react strongly in a conven ⁇ tional zinc bath (containing 0.15 to 0.25 % aluminium) at temperature above 480°C, the result is that a thick intermetallic layer is formed on the zinc coating.
  • the intermetallic layer should be as thin as possible.
  • the thickness of the intermetallic layer is controlled by rapidly cooling the steel product by quenching it in a bath of molten zinc, and controlling the structure of the coating to be formed on the steel product by re ⁇ gulating the end temperature of the steel product in the quenching by directing a flow of molten zinc, cooled :o a temperature below the operating temperature of the zinc bath, towards the steel product as it moves through the zinc bath.
  • a first flow of molten zinc is di ⁇ rected towards the steel product close to the immersion point thereof and obliquely against the movement direc ⁇ tion of the steel product, by means of first nozzles, and a second flow of cooled molten zinc is directed at least essentially perpendicularly towards the steel pro ⁇ duct at a point after said obliquely directed flow, by means of second nozzles.
  • the flow of molten zinc directed towards the steel product is cooled e.g. by means of a heat exhanger cooler, preferably to a temperature 1° to 15°C below ' the operating temperature of the zinc bath, the flow of zinc through the cooler to said nozzles being separated from the rest of the zinc bath.
  • the essential feature of locally cooling the zinc bath brings about the additional important advantage that the iron content of the zinc bath is lowered.
  • the iron content in a zinc bath, in a continuous hot-dip galvanizing process of a thin steel sheet is ge ⁇ nerally at saturation, according to the respective tem ⁇ perature. Even a small change in the temperature causes a precipitation of iron and zinc, i.e. either at the bottom of the bath or as a drift of precipitates onto the surface of the steel strip to be galvanized, which impairs the quality of the coating.
  • the solubility of iron in molten zinc is general- ly a linear function of the temperature; at a normal galvanizing temperature of approximately 455°C, the iron content is about 0.06 %, and at a temperature of about 420°C, the iron content is about 0,01 %.
  • Fe-Zn precipitates (slag particles) on the zinc coating should be avoided.
  • the iron content in the zinc bath is lowered to about 0.025 % when the temperature of the zinc bath is about 450°C and the tem ⁇ perature of the zinc after the cooler about 5°C lower.
  • the iron content is at a level about 50 % of the saturated value and corresponding to the iron content in a zinc bath at about 430°C.
  • the extra iron precipitates as very small Fe-Al-Zn particles from the molten zinc.
  • small Fe-Al-Zn particles adhere as an even layer to the surface of the steel product and leave the zinc bath as a part of the zinc coating.
  • the temperature and the rate of the zinc flow should preferably be at con- stant value.
  • the heat loss caused by the zinc cooler can be compensated by adjusting the speed of the steel pro ⁇ duct the temperature of which is higher than the tempe ⁇ rature of the zinc bath.
  • Figure 1 is a thermal diagram illustrating the heat treatment disclosed in the U.S. patent 4,361,448.
  • Figure 2 is a diagram illustrating the cooling (quenching) step in a zinc bath, in the treatment of figure 1, for a steel strip having a thickness of 1 mm.
  • Figure 3 shows schematically the zinc bath arran ⁇ gement of the invention, in a longitudinal section.
  • Figure 4 is a diagram illustrating the cooling (quenching) step according to the invention.
  • Figures 1 and 2 are shown to facilitate the un ⁇ derstanding of the prior art such as discussed in the beginning of the specification and to by comparision il- lustrate the advantages which are achieved by the pre ⁇ sent invention.
  • Figure 3 shows the new zinc bath arrangement.
  • Reference numeral 1 indicates a continuous step strip, with a thickness of e.g. 1 mm
  • 2 indicates a pot for a bath 3 of molten zinc with an aluminium content up to about 5 %.
  • 4 indicates an end chute of the last zone of a soaking furnace wherein the temperature of the steel is controlled to the temperature 2 (fig. 1), 5 indi ⁇ cates a snout which may be water cooled, 6 and 7 indi- cate quide rolls within the zinc bath which rolls can be used for regulating the galvanizing time in a known man ⁇ ner, e.g. by adjusting the roll 6 vertically.
  • Reference numeral 8 indicates gas jet nozzles.
  • the novelty of the zinc bath arrangement shown in figure 3, by means of which the present method is car ⁇ ried out, is a specific apparatus for circulating cooled molten zinc towards the steel strip 1 at its immersion into the zinc bath, this apparatus being generally de ⁇ signated by the reference numeral 10.
  • 11 indicates a cooler
  • 12 indicates a duct surrounding the cooler 11
  • 13 indicates a circulation pump after the cooler 11.
  • a bottom part 17 is mounted adjustably to the unit 14 (vertical arrows); a similar arrangement may be provided at the upper nozzles 15.
  • the zinc bath cooler 11, the zinc pump 13 and the nozzles 15, 16 form an integral unit, so that the tem ⁇ perature of the zinc flowing through the cooler can be lowered 1° to 15°C below the operating temperature of the zinc bath.
  • the nozzles 15 direct the zinc flow obli ⁇ quely towards the steel strip, preferably against the travel direction thereof, preventing the warming of the zinc within the snout 5 and the formation of. zinc vapors in the furnace 4.
  • the nozzles 16 direct the zinc flow e.g. perpendicularly towards the steel strip.
  • the nozzles are preferably adjustable so that the volume flows of the different nozzles can be varied. The total amount of the zinc flow can be controlled by means of the speed of rotation of the pump 13.
  • the cooler 11 preferably comprises a number of cooler tubes interspaced in such a manner that the zinc flow nowhere stops in a "dead position" and that the surface temperature of the cooler tubes remains approxi ⁇ mately the same across the duct 12. Said surface tempe- rature of the cooler tubes should be kept at a value preventing the zinc from solidifying on the tubes; such a solidification could cause defects in the zinc coat ⁇ ing.
  • the temperature Tg of the steel strip i.e. the end temperature of the rapid cooling can be reduced and/or controlled by means of the method according to the invention in a manner illustrated in Figure 4.
  • T is as close as possible to the operating temperature of the zinc bath, e.g. 450°C
  • the formation of an intermetallic layer disadvantageous to the form ⁇ ing operation on the zinc coating, is prevented nearly completely in a conventional zinc bath (having an alumi ⁇ nium content of 0.15 fo 0.25 %).
  • the thick ⁇ ness of an intermetallic layer on the zinc coating of a steel strip can be controlled by varying the temperature of the zinc bath between 440°C and 465°C and by adjust ⁇ ing the difference between the temperature Tg and the temperature of the zinc bath.
  • the temperature of the steel strip preferably exceeds 550°C before entering the zinc bath.
  • the operating temperature can be kept between 415°C and 425°C, so that the method according to the invention makes it possible to reduce the end tempe- rature of the rapid cooling of the steel strip to a value considerably below 450°C. This improves the quality of the coating, because the rapid cooling makes the eutectic alloyed coating fine-granular. In addition, the formation of uncoated spots is prevented by the high steel strip temperature in spite of the high surface tension of the zinc alloy.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Coating With Molten Metal (AREA)
  • Physical Vapour Deposition (AREA)

Abstract

The present invention relates to a method for controlling the thickness of an intermetallic layer on a continuous steel product in a continuous hot-dip galvanizing line. The steel product is rapidly cooled by quenching in a zinc bath and the structure of the coating to be formed on the steel product is controlled by directing a flow of molten zinc, cooled to a temperature 1 to 15C below the operating temperature of the zinc bath, towards the steel strip. At least a part of said flow is preferably directed towards the steel product close to its immersion point into the zinc bath, obliquely against the movement direction of the steel product.

Description

A method for controlling the thickness of an intermetal- lic layer on a continuous steel product in a continuous hot-dip galvanizing process
The present invention relates to a method for controlling the thickness of an intermetallic layer on a continuous steel product in a continuous hot-dip galva¬ nizing process. The continuous steel product is gene¬ rally either a strip or a wire. A cold-rolled steel strip can be given a good formability by means of a heat treatment disclosed in my earlier U.S. Patent 4,361,448. After annealing at a tem¬ perature -j_ (720 to 850°C) the steel strip is slowly cooled to a temperature 2 (600 to 650°C), from which temperature it is rapidly quenched in a zinc bath to a temperature T3. The time interval between T2 and Tg is ab ut 0.5 seconds.
In the arrangement of the U.S. Patent 4,361,448 a zinc bath cooler and a zinc pump, with nozzles, are sep- arate units. Molten metal having the same temperature as the zinc bath is pumped through a snout to the immersion point of the steel strip. Therefore the end temperature T3 of the rapid cooling is rather high, and the steel strip does not reach the temperature of the zinc bath during the entire immersion time (about two seconds).
A steel strip travelling through a zinc bath causes a laminar zinc flow following the surface of the steel strip. The heat from inside the steel strip raises the temperature of the laminar zinc flow (layer) to a value higher than the operating temperature of the zinc bath. Since iron and zinc react strongly in a conven¬ tional zinc bath (containing 0.15 to 0.25 % aluminium) at temperature above 480°C, the result is that a thick intermetallic layer is formed on the zinc coating. In order to achieve a good formability of the zinc coating, the intermetallic layer should be as thin as possible. In the method according to the invention, the thickness of the intermetallic layer is controlled by rapidly cooling the steel product by quenching it in a bath of molten zinc, and controlling the structure of the coating to be formed on the steel product by re¬ gulating the end temperature of the steel product in the quenching by directing a flow of molten zinc, cooled :o a temperature below the operating temperature of the zinc bath, towards the steel product as it moves through the zinc bath.
Preferably a first flow of molten zinc is di¬ rected towards the steel product close to the immersion point thereof and obliquely against the movement direc¬ tion of the steel product, by means of first nozzles, and a second flow of cooled molten zinc is directed at least essentially perpendicularly towards the steel pro¬ duct at a point after said obliquely directed flow, by means of second nozzles.
The flow of molten zinc directed towards the steel product is cooled e.g. by means of a heat exhanger cooler, preferably to a temperature 1° to 15°C below' the operating temperature of the zinc bath, the flow of zinc through the cooler to said nozzles being separated from the rest of the zinc bath.
The essential feature of locally cooling the zinc bath brings about the additional important advantage that the iron content of the zinc bath is lowered. The iron content in a zinc bath, in a continuous hot-dip galvanizing process of a thin steel sheet is ge¬ nerally at saturation, according to the respective tem¬ perature. Even a small change in the temperature causes a precipitation of iron and zinc, i.e. either at the bottom of the bath or as a drift of precipitates onto the surface of the steel strip to be galvanized, which impairs the quality of the coating.
Thus, to maintain a good quality, variations in the temperature of the zinc bath should be avoided. Therefore, some galvanizing lines are provided with se¬ parate pots for preliminary melting of zinc so that e.g. the melting temperature of the zinc to be added would not change the temperature of the zinc bath.
The solubility of iron in molten zinc is general- ly a linear function of the temperature; at a normal galvanizing temperature of approximately 455°C, the iron content is about 0.06 %, and at a temperature of about 420°C, the iron content is about 0,01 %. To improve the quality of a hot-dip galvanized thin steel sheet, Fe-Zn precipitates (slag particles) on the zinc coating should be avoided. Thus, it is of advantage to lower the iron content in the zinc bath from the saturated area, where¬ by a use of different galvanizing temperatures is pos¬ sible without precipitation of such particles. By means of the present method, the iron content in the zinc bath is lowered to about 0.025 % when the temperature of the zinc bath is about 450°C and the tem¬ perature of the zinc after the cooler about 5°C lower. Thus, the iron content is at a level about 50 % of the saturated value and corresponding to the iron content in a zinc bath at about 430°C.
During the local cooling of the zinc bath, the extra iron precipitates as very small Fe-Al-Zn particles from the molten zinc. When the zinc flows towards the steel strip small Fe-Al-Zn particles adhere as an even layer to the surface of the steel product and leave the zinc bath as a part of the zinc coating.
To keep the Fe-Al-Zn particles as small as pos¬ sible and homogeneously distributed, the temperature and the rate of the zinc flow should preferably be at con- stant value. The heat loss caused by the zinc cooler can be compensated by adjusting the speed of the steel pro¬ duct the temperature of which is higher than the tempe¬ rature of the zinc bath. Specific features of the invention are stated in the claims and appear likewise from the following de¬ scription with reference to the enclosed drawing.
Figure 1 is a thermal diagram illustrating the heat treatment disclosed in the U.S. patent 4,361,448.^ Figure 2 is a diagram illustrating the cooling (quenching) step in a zinc bath, in the treatment of figure 1, for a steel strip having a thickness of 1 mm.
Figure 3 shows schematically the zinc bath arran¬ gement of the invention, in a longitudinal section. Figure 4 is a diagram illustrating the cooling (quenching) step according to the invention.
Figures 1 and 2 are shown to facilitate the un¬ derstanding of the prior art such as discussed in the beginning of the specification and to by comparision il- lustrate the advantages which are achieved by the pre¬ sent invention.
Figure 3 shows the new zinc bath arrangement. Reference numeral 1 indicates a continuous step strip, with a thickness of e.g. 1 mm, 2 indicates a pot for a bath 3 of molten zinc with an aluminium content up to about 5 %. 4 indicates an end chute of the last zone of a soaking furnace wherein the temperature of the steel is controlled to the temperature 2 (fig. 1), 5 indi¬ cates a snout which may be water cooled, 6 and 7 indi- cate quide rolls within the zinc bath which rolls can be used for regulating the galvanizing time in a known man¬ ner, e.g. by adjusting the roll 6 vertically. Reference numeral 8 indicates gas jet nozzles.
So far the arrangement of figure 3 corresponds to figure 2 of the U.S. patent 4,361,448. The treatment steps before the chute 4 and after the gas jet nozzles 18 belong likewise to the prior art, reference can again be made e.g. to figure 2 of the U.S. patent 4,361,448.
The novelty of the zinc bath arrangement shown in figure 3, by means of which the present method is car¬ ried out, is a specific apparatus for circulating cooled molten zinc towards the steel strip 1 at its immersion into the zinc bath, this apparatus being generally de¬ signated by the reference numeral 10. 11 indicates a cooler, 12 indicates a duct surrounding the cooler 11 and 13 indicates a circulation pump after the cooler 11.
14 indicates a nozzle unit with upper nozzles 15 and lower nozzles 16. A bottom part 17 is mounted adjustably to the unit 14 (vertical arrows); a similar arrangement may be provided at the upper nozzles 15.
The zinc bath cooler 11, the zinc pump 13 and the nozzles 15, 16 form an integral unit, so that the tem¬ perature of the zinc flowing through the cooler can be lowered 1° to 15°C below the operating temperature of the zinc bath. The nozzles 15 direct the zinc flow obli¬ quely towards the steel strip, preferably against the travel direction thereof, preventing the warming of the zinc within the snout 5 and the formation of. zinc vapors in the furnace 4. The nozzles 16 direct the zinc flow e.g. perpendicularly towards the steel strip. The nozzles are preferably adjustable so that the volume flows of the different nozzles can be varied. The total amount of the zinc flow can be controlled by means of the speed of rotation of the pump 13. The cooler 11 preferably comprises a number of cooler tubes interspaced in such a manner that the zinc flow nowhere stops in a "dead position" and that the surface temperature of the cooler tubes remains approxi¬ mately the same across the duct 12. Said surface tempe- rature of the cooler tubes should be kept at a value preventing the zinc from solidifying on the tubes; such a solidification could cause defects in the zinc coat¬ ing.
The temperature Tg of the steel strip i.e. the end temperature of the rapid cooling can be reduced and/or controlled by means of the method according to the invention in a manner illustrated in Figure 4. Pro¬ vided that T is as close as possible to the operating temperature of the zinc bath, e.g. 450°C, the formation of an intermetallic layer, disadvantageous to the form¬ ing operation on the zinc coating, is prevented nearly completely in a conventional zinc bath (having an alumi¬ nium content of 0.15 fo 0.25 %). Accordingly, the thick¬ ness of an intermetallic layer on the zinc coating of a steel strip can be controlled by varying the temperature of the zinc bath between 440°C and 465°C and by adjust¬ ing the difference between the temperature Tg and the temperature of the zinc bath. The temperature of the steel strip preferably exceeds 550°C before entering the zinc bath.
When the aluminium content of the zinc-aluminium bath is about 5 %, the operating temperature can be kept between 415°C and 425°C, so that the method according to the invention makes it possible to reduce the end tempe- rature of the rapid cooling of the steel strip to a value considerably below 450°C. This improves the quality of the coating, because the rapid cooling makes the eutectic alloyed coating fine-granular. In addition, the formation of uncoated spots is prevented by the high steel strip temperature in spite of the high surface tension of the zinc alloy.

Claims

Claims :
1. A method for controlling the thickness of an intermetallic layer on a continuous steel product in a continuous hot-dip galvanizing process, comprising the steps of rapidly cooling the steel product by quenching it in a bath of molten zinc, and controlling the structure of the coating to be formed on the steel pro- • duct by regulating the end temperature of the steel pro- duct in the quenching by directing a flow of molten zinc, cooled to a temperature below the operating tempe¬ rature of the zinc bath, towards the steel product as it moves through the zinc bath.
2. A method according to claim 1, wherein the flow of molten zinc is directed towards the steel pro¬ duct close to the immersion point thereof and obliquely against the movement direction of the steel product,- by means of first nozzles.
3. A method according to claim 2, wherein a se- cond flow of cooled molten zinc is directed at least es¬ sentially perpendicularly towards the steel product at a point after said obliquely directed flow, by means of second nozzles.
4. A method according to claim 1, wherein the temperature of the cooled zinc flow towards the steel product is 1° to 15°C below the operating temperature of the zinc bath.
5. A method according to claim 1, wherein the flow of cooled zinc is directed towards the steel pro- duct evenly over the width thereof and from both sides.
6. A method according to claim 2 and claim 3, wherein the said first and second nozzles directing the flow of cooled zinc towards the steel product are indi¬ vidually adjustable.
7. A method according to claim 1, wherein the flow of molten zinc directed towards the steel product is cooled by means of a heat exhanger cooler, and flow of zinc through the cooler being separated from the rest of the zinc bath.
PCT/FI1988/000026 1987-02-27 1988-02-23 A method for controlling the thickness of an intermetallic layer on a continuous steel product in a continuous hot-dip galvanizing process WO1988006636A1 (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
BR888805642A BR8805642A (en) 1987-02-27 1988-02-23 PROCESS TO CONTROL THE THICKNESS OF AN INTERMETAL LAYER IN A CONTINUOUS STEEL PRODUCT IN A CONTINUOUS HOT IMMERSION GALVANIZATION PROCESS
AT88901847T ATE71987T1 (en) 1987-02-27 1988-02-23 METHOD OF CONTROLLING THE THICKNESS OF AN INTERMETALLIC LAYER ON A CONTINUOUS STEEL PRODUCT DURING A HOT-DIP GALVANIZING PROCESS.
DE8888901847T DE3867988D1 (en) 1987-02-27 1988-02-23 METHOD FOR CONTROLLING THE THICKNESS OF AN INTERMETALLIC LAYER ON A CONTINUAL STEEL PRODUCT WITHIN A FIRE GALVANIZING PROCESS.
KR1019880701350A KR930001781B1 (en) 1987-02-27 1988-02-23 Method for controlling the thickness of an intermetallic layer on a steel strip in a continuous hot-dip galvanzing process

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US07/020,106 US4752508A (en) 1987-02-27 1987-02-27 Method for controlling the thickness of an intermetallic (Fe-Zn phase) layer on a steel strip in a continuous hot-dip galvanizing process
US020,106 1987-02-27

Publications (1)

Publication Number Publication Date
WO1988006636A1 true WO1988006636A1 (en) 1988-09-07

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US (1) US4752508A (en)
EP (1) EP0308435B1 (en)
JP (1) JPH01502915A (en)
KR (1) KR930001781B1 (en)
AT (1) ATE71987T1 (en)
AU (1) AU604862B2 (en)
BR (1) BR8805642A (en)
CA (1) CA1328785C (en)
DE (1) DE3867988D1 (en)
SU (1) SU1706393A3 (en)
WO (1) WO1988006636A1 (en)

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AU1369888A (en) 1988-09-26
KR890700692A (en) 1989-04-26
EP0308435B1 (en) 1992-01-22
AU604862B2 (en) 1991-01-03
DE3867988D1 (en) 1992-03-05
US4752508A (en) 1988-06-21
SU1706393A3 (en) 1992-01-15
KR930001781B1 (en) 1993-03-13
JPH0521977B2 (en) 1993-03-26
EP0308435A1 (en) 1989-03-29
CA1328785C (en) 1994-04-26

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