LU83353A1 - PROCESS FOR MANUFACTURING IMPROVED QUALITY GALVANIZED SHEET - Google Patents

Description

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I · BL-3146 / EM / BM

. "C. 2131/8105.

“METALLURGICAL RESEARCH CENTER -CENTRUM V00R RESEARCH IN DE METALLURGIE,

Non-profit association - Vereniging zonder winstoogmerk in BRUXELLES, (Belgium)

Process for manufacturing improved quality galvanized sheets.

The present invention relates to a process for manufacturing galvanized sheets of improved quality.

* This is particularly applicable to the production of galvanized sheets * on one or both sides which must have good drawing and / or mechanical strength properties.

It is known to those skilled in the art that obtaining sheets having good drawing properties is linked to the fact that the sheet undergoes a thermal cycle comprising (after maintenance in the recrystallization field) rapid cooling followed by overaging treatment.

On the other hand, high resistance sheets 2 of the "dual phase" type are obtained by subjecting the sheet to heating up to the intercritical range, followed by rapid cooling. Depending on the composition of the steel and the intensity of quenching, tempering may be necessary after quenching.

By modulating the values of the heating and tempering temperatures, as well as the composition of the steel, it is also possible to produce sheets with high elastic limit.

The abovementioned techniques have resulted in the development of various rapid cooling technologies, namely: - blowing of cooled atmosphere gas in a gas-water exchanger (jet cooling); - water spray; - projection of a water mist; - immersion in boiling water.

The evolution of the technique and especially the desire to obtain galvanized sheets with good stamping and / or resistance properties has had the effect that we are trying to achieve the above-mentioned theoretical cycles by integrating them into a line continuous galvanizing.

> »This is how it is known that the high * heat exchange coefficient of molten zinc makes it possible to carry out heat treatment of the sheet, during the hot-dip galvanizing operation.

In the same context, it has also been proposed to take advantage of the passage of the sheet in the hot-dip galvanizing tank to achieve rapid cooling of the said sheet by passing it through a layer this molten salt supernatant on the bath of zinc.

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It therefore follows from the above that it is possible to carry out in a continuous hot-dip galvanizing line the optimum thermal cycles capable of giving the sheet the desired stamping and / or resistance properties.

However, it is also known that depending on the cooling method chosen, the technical problems change.

! ï In the case of the "jet coolina", the refrôiâis-? This is very slow and requires a very long aging period.

; The water jet method requires gold.

4 heats the sheet before galvanizing and has the effect of dete | improvement of the surface of the cylinder yes must undergo a Dréoai srior.

additional before being hot dip galvanized.

Cooling by spraying with water mist or by immersion in boiling water allows cooling to be interrupted and therefore prevents reheating, but requires the reduction of the oxide formed before galvanizing.

Regarding the use of a tin of molten salt supernatant on the zinc bath to cool the sheet, there is a chemical stability of the salt in the dark state and a certain difficulty in ensuring compatibility with the hot-dip galvanizing operation.

Finally, concerning the cooling method using the galvanizing bath itself, a major problem to solve is the evacuation of the heat provided by the sheet in the molten zinc bath and this if possible at the lowest cost. or at least by reusing it as much as possible.

The process, object of the invention, using the galvanizing bath in order to operate the cooling of the sheet, solves in an original way the evacuation of the calories brought by the sheet in the bath and makes it possible to accomplish the thermal cycles necessary for give said sheet good drawing properties and / or high yield strength.

The description which follows is essentially centered on hot galvanizing of steel sheets, however the principle of the process of the invention could be transposed with! slight modifications to the hot coating using other metals, for example tin, copper, aluminum or zinc-aluminum alloys, ",., and this without much trouble.

| The process, object of the present invention, consisting of hot-dip galvanizing a sheet by means of a zinc bath or alloy based on molten zinc, is essentially characterized in that said sheet undergoes a thermal cycle comprising at least cooling rapid obtained by bringing it into contact with the zinc bath or molten zinc alloy and in that the said bath is maintained at an adequate temperature by the use of an appropriate heat exchanger device.

η * According to a first embodiment of the process of the invention, the heat exchanger device uses water vapor as a secondary fluid, which can be wet at the inlet of the exchanger and superheated at exit.

According to a second embodiment of the method of the present invention, an overaging section is provided downstream from the galvanizing-cooling operation; 5 I: fast.

According to another implementation of the method of the invention, an additional cooling device is interposed between the rapid galvanizing-cooling operation and the above-mentioned overaging section.

This arrangement makes it possible to rapidly cool "dual phase" steels after the quenching-galvanizing operation following the passage of the sheet through the galvanizing tank.

I According to a preferred implementation i | of the process, object of the present invention, the sheet passes successfully! The sections listed below on the ΙοΙ gic diagram (Figure 1) which can be supplemented by other treatments

Accessory items consisting essentially of a section I

(NOF) for the preparation of the surface of the sheet and for preheating of the latter; a section 2 (RTF) where we operate the heating! sheet metal end; a temperature maintenance section 3 (SF)! determined; a section 4 (QGP) for rapid cooling and gal vanization; to which is annexed a section 5 (HES) for regulating the evacuation of the heat transmitted by the sheet to zinc tin or zinc alloy and also for modulating the temperature thereof; a section 6 (SEC C) of secondary cooling * to lower the temperature of the sheet at its exit from the zinc or zinc alloy bath to the aging temperature in the case of mild steels or to a lower temperature or equal to 250 ° C in the case of "dual phase" steels; an overaging section 7 (OAS); and finally a section £ (FINC) of final cooling by conventional means for lowering the temperature of the sheet from the aging temperature to the final temperature of exit from the sheet in galvanized form and having the sealing properties. weaving and / or elastic limit desired.

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According to yet another preferred embodiment of the process of the invention, the preheating in section 1 (NOF) is carried out by an open flame oven.

According to a sixth embodiment of the process of the invention, the preparation of the surface of the sheet as well as the preheating of the latter are carried out in two separate installations, one comprising an electrolytic degreasing and the other a tube oven. radiant.

In another mode of application of the method of the invention, the final heating of section 2 (RTF) is carried out in an oven with radiant tubes, possibly electric.

; According to an eighth mode of application of the method of the present invention, the survival treatment; The smoothing operation carried out in section 7 (OAS) is carried out at a temperature of around 400 ° C. with a minimum duration of 45 to 60 seconds and possibly by a multi-pass device.

According to a preferred embodiment of section 7 (OAS) as described in the previous method, the rollers of section 7 (OAS) are covered with a coating comprising at least one surface layer formed by means of one or several constituents of the four groups below, taken individually or in combination: 1. group of oxides, comprising · oxides of Mg, Ca, Sr,

Ba, Ti, Zr, Cr, W, Fe, Co, Ni, Zn, Ce, B, Al, Si, Ge, Sr.,

Pb; 2. group of silicates, comprising the silicates of Li, Na, K, Mg, Ca, Sr, Ba, Zr, V, Cr, Mn, Fe, Ce, Ni, B, Al, Sn, i Pb; 3. group of zirconates, comprising Mg zirconates,

Ca, Sr, Ba; T f 7 4. so-called "mixed" group, comprising serpentines, amphiboles, silicon carbides; and which has an appropriate roughness so as to eliminate flowering and / or deterioration of the zinc or alloy coating! zinc age when in contact with the section 7 (OAS) overaging rollers.

According to a preferred embodiment of section 5 (HRS) of the process of the present invention, this may include, simultaneously or not, the different elements described on the logic diagram (Fig. 2) / namely: an exchanger heat (9) zinc or zinc alloy - water vapor, a reheater (10) of the heat transfer liquid, which allows, by reversing the flow in the exchanger (9) / to maintain the zinc or the alloy zinc at an adequate temperature even when the galvanizing line is stopped, the additional heat can be supplied by resistors supplied by an alternator (11) or by means of another additional exchanger (12); said heat transfer fluid is partially vaporized in an economizer-vaporizer exchanger (13) using the sensible heat of the fumes from sections (1) (NOF) and (2) (RTF), these fumes being at approximately 600 ° C. after recoveries conventional energy sources such as the preheating of the sheet metal and the combustion air of the ovens used in the installation implementing the method y • of the invention; a device for regulating the temperature of the zinc or liquid zinc alloy (17) preferably controlled by the line control computer (22) which acts mainly on the flow of the heat-transfer fluid by modifying for example the speed of the pump supply (14) by action on a variable speed motor (15); these bodies for measuring the flow rate and the temperature of the zinc or zinc alloy (16); a condenser recovery device (18); and finally a turbine (19) coupled to an alternator (20) allowing the production of electrical energy which can be used in the stop heater (10). line or elsewhere (21).

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It should be noted that the method of the invention has a number of advantages which are: - a reduction in the length of the galvanizing line, and consequently in investment costs, by eliminating the device conventional cooling before gal- *. vanization and also a shortening of the aging section following rapid quenching in the molten zinc bath, - a decrease in the energy consumed per tonne of steel by the elimination of blowing fans from conventional cooling devices, the reduction in the number of motorized rollers and the recovery of part of the energy contained in the sheet, - the possibility of treating steels with high elastic limit and "dual phase" steels under economic conditions, namely with low contents of alloying elements, - obtaining a galvanized sheet free from flowering.

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Claims (8)

1. Method for manufacturing galvanized sheets [of improved quality, consisting of hot-dip galvanizing a sheet f | , by means of a bath of zinc or alloy based on molten zinc, es- | _ essentially characterized in that the said sheet undergoes a cycle | thermal comprising at least rapid cooling obtained by bringing it into contact with the zinc bath or molten zinc alloy and in that said bath is maintained at an adequate temperature by the use of an appropriate heat exchanger device. 2. Method according to claim 1, characterized in that, in the heat exchanger device, the secondary fluid is water vapor, which can be humid at 1 '! exchanger inlet and superheated at the outlet. 3. method according to claims 1 or 2, | characterized in that an overaging section is provided .. downstream of the rapid galvanizing-cooling operation. 4. Method according to one or more of the resales; dications 1 to 3, characterized in that an additional cooling device is interposed between the galva- operation; s t. fast cooling and the aforementioned overaging section. 1 i "Process according to one or more of the claims 1 to 4, characterized in that the sheet successively collects by the sections listed below: - a section for preparing the surface of the sheet and for pre-heating of the latter; - a section in which the final heating of the sheet is operated; - a section for maintaining at a fixed temperature; r ~ a section for rapid cooling and galvanizing; to which is annexed a section for regulating the evacuation of the heat transmitted by the sheet to the zinc or zinc alloy bath and also to modulate the temperature thereof; - a secondary cooling section to lower the temperature of the sheet when it comes out of the zinc or zinc alloy allies up to the aging temperature in *, in the case of mild steels or up to a temperature less than or equal to 250 ° C in the case of "dual-phase" steels; - an overaging section; - finally a ref section final stiffening by conventional means to lower the temperature of the sheet from the aging temperature to the final exit temperature of the sheet in galvanized form and having the desired stamping and / or yield strength properties. 6. Method according to claim 5, characterized in that the preheating is carried out by an open flame oven. 7. Method according to claims 5 or 6, characterized in that the preparation of the surface of the sheet as well as the preheating thereof are carried out in two separate installations comprising one electrolytic degreasing and the other a furnace radiant tubes. & v 8. Method according to one or more of claims 5 to 7, characterized in that the final heating is | carried out in an oven with radiant tubes, possibly electric. | \ 9. Process according to one or more of claims 1 to 8, characterized in that in the over-aging treatment; ment is carried out at a temperature of about 400 ° C with a minimum duration j of 45 to 60 seconds and optionally by an i - device; multi-pass. I jr- i it 5: p »‘ 10. Method according to claim 9, characterized in that the rollers of the aforementioned overaging section are covered with a coating, comprising at least one surface layer formed by means of one or more constituents of the following four groups, taken individually or in combination: 1. group of oxides, comprising the oxides of Mg, Ca, Sr, Ba, Ti, Zr, Cr, W, Fe, Co, Ni, Zn, Cd, B, Al, Si, Ge, Sn, Pb; 2. group of silicates, comprising the silicates of Li, Na, K, ’Mg, Ca, Sr, Ba, Zr, V, Cr, Mn, Fe, co, Ni, B, Al, Sn, Pb; 3. group of zirconates, comprising the zirconates of Mg, Ca, Sr, Ba; 4. so-called "mixed" group comprising serpentines, amphiboles, silicon carbides; and which has an appropriate roughness so as to eliminate flowering and / or deterioration of the coating of zinc or of zinc alloy when in contact with the rollers of the overaging section. 11. Method according to one or more of claims 1 to 10, characterized in that the regulation and discharge section of the heat transmitted to the bair. of zinc or a zinc alloy may comprise simultaneously or not, the various ♦ following elements: - a zinc heat exchanger - or zinc alloy - water vapor, a heater for the heat transfer liquid, the said heat transfer fluid being partially vaporized in an exchanger-economiser-vaporizer using the sensible heat of the fumes from the preparation sections of this surface and this preheating, these fumes being at approximately 60 ° C. after conventional energy recoveries such as the preheating of the sheet metal and the combustion air of the ovens used in the installation implementing the method of the invention; - a device for regulating the temperature of the zinc or alloy v ("t ij of zinc licruide controlled by Dereference by the computer of"! the speed of the feed pump by action on a variable speed motor; - flow and temperature measurement devices for zinc or zinc alloy; - a condenser recovery device; and finally a turbine coupled to an alternator allowing the production of electrical energy which can be used in the line stop heater or elsewhere. ___ / i ^ τ '] ^ I v "