CA1247470A - Differentially coated galvanized steel strip and method and apparatus for producing same - Google Patents
Differentially coated galvanized steel strip and method and apparatus for producing sameInfo
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- CA1247470A CA1247470A CA000558428A CA558428A CA1247470A CA 1247470 A CA1247470 A CA 1247470A CA 000558428 A CA000558428 A CA 000558428A CA 558428 A CA558428 A CA 558428A CA 1247470 A CA1247470 A CA 1247470A
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- strip
- heavy
- coated
- galvanized steel
- iron
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Abstract
ABSTRACT
A differentially coated, galvanized steel strip is produced by hot dip coating both sides of a steel strip and adjusting the weight of coating metal on opposite sides of the strip to produce a light coated side and a heavy coated side. The strip is then precooled to fully solidify the heavy coated side following which the galvanized strip is subjected to a treating step in which simultaneously the light coated side is heated and the heavy coated side is cooled. This fully transforms the light coated side to iron-zinc alloy throughout while only partially transform-ing an inner layer on the heavy coated side to iron-zinc alloy.
The outer layer on the heavy coated side consists entirely of coating metal, and there are no intermittent bleed-throughs of iron-zinc alloy to the outer surface of the heavy coated side.
A differentially coated, galvanized steel strip is produced by hot dip coating both sides of a steel strip and adjusting the weight of coating metal on opposite sides of the strip to produce a light coated side and a heavy coated side. The strip is then precooled to fully solidify the heavy coated side following which the galvanized strip is subjected to a treating step in which simultaneously the light coated side is heated and the heavy coated side is cooled. This fully transforms the light coated side to iron-zinc alloy throughout while only partially transform-ing an inner layer on the heavy coated side to iron-zinc alloy.
The outer layer on the heavy coated side consists entirely of coating metal, and there are no intermittent bleed-throughs of iron-zinc alloy to the outer surface of the heavy coated side.
Description
7~
DIFFERENTI~LLY COATED GALVANIZED STEEL STRIP
, BACKGROUND OF THE INVENTION
The present invention relates generally to hot dip coated galvanized steel strip and more particularly to a differentially coated, galvanized steel strip wherein one of two opposite strip sides has a relatively light coat composed of iron-zinc alloy and the other strip s;de has a relatively heavy coat at least the outer part of which consists essentially of ~inc. The present invention also relates particularly to methods and apparatuses for producing such a differentially coated, galvanized steel strip~
~ differentially coated, galvanized steel strip of the general type described in the preceding paragraph, and a method and apparatus for producing such a galvanized steel strip is described in Patil et al., U.S. Patent No. 4,1~1,394 issued October 16, 1979, and entitled "Process of ~ot-dip Galvanizing and Alloying".
A differentially coated~ gal~anized steel strip of the general type described in said Patil et al. patent is 2Q produced by passing a continuous steel strip through a bath of molten coating metal consisting essentially of zinc to coat ~oth sides of the strip with the molten coating metal.
Immediately upon withdrawal of the coated metal strip from the bath of molten coating metal, the weight of the molten coating metal on opposîte sides of the strip is adjusted by impingement against opposite sldes of the strip of jets of gas or steam. The jets on respective opposite sides of the strip are adjusted to provide one strip side with a relatively light coat of the coating metal and the other strip 3Q side with a relatively heavy coat of the coating metal.
~ ~2~7~7~
Typically, the weight of the coating metal on the light coated side is in the range 0.05-0.25 oz./ft.2.
~0.015-0.075 kg/m2~, and the weigh.t of the coating metal on the heavy coated side is in the range 0~35-1.0 oz./ft.2 (:0.105-0.30 kg/m21.
~: Immediately following th.e weight adjusting step, the strip is passed through a treating zone wherein simultaneously the ligh.t coated strip side undergoes heating and th.e heavy coated strip side under~oes cooling.
As a result, the coating metal on th.e light coated side is transformed to iron-zinc alloy throughout. It is esired~ as a result of th.e treating step, that the coating metal on the heavy coated side be only partially transformed to iron-zinc alloy, and that the outer part of the coating on the heavy coated side consists essentially of zinc. In a method in accordance with.the Patil et al.
patent, the heavy coated side is at least partially molten at the time the simultaneous hRating and cooling step is performed.
: 2Q In a typical commercial product employing the subject matter of the Patil et al. patent, the weight of the coating metal on the heavy coated side is about 0.50 oz./ft.2 (:0 15 kg~m2~. In such a product, the iron-zinc alloy on the heavy coated side constitutes an inner layer wh.ich ranges from 30 to 50~ of the coating thick-ness on that side. Occasionally, ho~ever, as a result of the simultaneous heating and cooling step, in some spots the iron-zinc alloy may extend all the ~ay through to the:outer surface on th.e heavy coated side causing a defect known as 3Q "bleed-through". Th.ese bleed-throughs occur intermittently along the outer surface of the heavy coated strip side.
o In cross-section, a bleed-through resem~les the vertical cross-section of a mushroom Bleed-throughs are undesirable because the iron-zinc alloy in the bleed-through area at the strip's outer surface tends to powder when the galvanized steel str;p is subjected to a stamping operation, and this is undesirable. Although the light coated side of the strip consists entirely of iron-zinc alloy throughout, that coating is relatively so t~in that it can undergo a stamping operation without powdering. The fully alloyed light coated side is readily paintable.
Galvanized steel strip with bleed~through on the heavy coated s;de is not acceptable to those who fabricate the galvanized steel strip into products, and such a strip is not saleable. A typical customer for differentially coated galvanized steel strip is a stamping shop making parts for the automotive industry.
Recently there has been a demand, particularly from the automotive industry, for a differentially coated, galvanized steel strip in which the heavy coated side has a thinner coating, e.g., a weight substantially less than 0.50 oz./ft.2 (0.15 kg/m21, typically in the range 0.25--0.45 oz./
ft.2 C0~075-0.135 kg~m21. ~ thinner coating on the heavy coated side makes the strip more weldable. However, the thinner the coating on the heavy coated side, the more likely there is to be a bleed-through when the differentially coated strip is subjected to a simultaneous heating and cooling treatment of the type described in the Patil et al. patent.
When the heavy coated side has a coating weight of 0.50 oz./ft.2 ~0.15 kg/m2~, a strip with a bleed-~through occurs only occasionally, and the strip rejection rate for this defect is about 3~4%. When the coating on the heavy coated side has a weight su~stantially less than 0.50 oz./ft.2 (0.15 kg/m2~, it is virtually impossible to prevent bleed--~ 3 _~
~2~N~75~
, ..~
throughs when employing a process in accordance with the Patil et al. patent, and the iron-zinc alloy may constitute .
100% of the coating thickness on the heavy coated side.
Summary of the Invention I~ accordance with the present invention, the heavy coated strip side has a coating we;ght less than 0.50 oz./ft.2 (0.15 kg/m22, but intermittent bleed-throughs of iron-zinc alloy at the outer surface of the heavy coated side of the differentially coated, galvanized steel strip are prevented.
~his is accomplished by precooling the heavy coated side of the strip between the weight adjusting step and the simul-taneous heating and cooling step. The precooling step sub-stantially fully solidifies the molten coating metal on the heavy coated side before the start of the simultaneous heating and cooling step~
The weight adjusting step which precedes the precool-ing step provides a coating metal thickness on the heavy coated side of the strip ~hlch, absent the precooling step, is thick enough to be at least partially molten at the start 2Q of the simultaneous heating and cooling step but not thick enough to avoid intermittent bleed~throughs as a result of the simultaneous heating and cooling step.
Iron diffuses in zinc less rapidly ~hen the zinc is solid than when the zinc is molten~ Because the precool-ing step fully solidifies the coatîng metal on the heavy coated side, there is reduced diffusion of iron in the coating on the heavy coated side during the simultaneous heatlng and cooling step, compared to the diffusion which would occur if the coating on the hea~y coated side was not solidified before 3Q the start of the simultaneous heating and cooling step.
Because of the reduced diffusion, bleed~through is prevented.
`` ~247~
The precooling step is accomplished by impinging a fluid-cooling medium against the heavy coated strip side.
This fluid-cooling medium is preferably steam but may also comprise a;r, nitrogen, or inert gases.
~hen the heavy coated side is imp;nged ~ith a fluid-cooling medium such as steam, during the precooling step, the impinging cooling medium causes waves or 'rsag lines" on the solidified coating metal surface. Such minor surface irregularities are undesirable because they "print through"
on the reverse side of the galvanlzed steel strip during stamping. Ilowever, during the simultaneous heating and cool-ing treatment, following precGoling, there ;s superficial melting of a heavy coated side having a thickness in accord-ance with the present invention, there is relatively rapid solidification oF the heavy coated side following the heating and cooling step, and the sag lines or other minor surface irregularities are eliminated.
On a differentially coated, galvanized steel strip produced in accordance with the present invention, there are no sag lines, the spangle boundaries are flat (i.e., level with the surface of the coating on the heavy coated side~
and the outer surface on the heavy coated strip side is rela-tively smooth compared to the heavy coated side on a dif.
ferentially coated strip in accordance with prior art pro-cedures. Because that surface is so smooth, it is un-necessary to skin roll the strip as heavy as was necessary with a less smooth surface~ Because the strip is subjected to less skin rolling, it is more ductile and has better formàbility.
Other features and advantages are inherent in the subject matter claimed and dlsclosed or will become apparent to those skilled in the art from the following detailed des-cription in conjunction w;th the accompanying diagrammatic drawings.
DIFFERENTI~LLY COATED GALVANIZED STEEL STRIP
, BACKGROUND OF THE INVENTION
The present invention relates generally to hot dip coated galvanized steel strip and more particularly to a differentially coated, galvanized steel strip wherein one of two opposite strip sides has a relatively light coat composed of iron-zinc alloy and the other strip s;de has a relatively heavy coat at least the outer part of which consists essentially of ~inc. The present invention also relates particularly to methods and apparatuses for producing such a differentially coated, galvanized steel strip~
~ differentially coated, galvanized steel strip of the general type described in the preceding paragraph, and a method and apparatus for producing such a galvanized steel strip is described in Patil et al., U.S. Patent No. 4,1~1,394 issued October 16, 1979, and entitled "Process of ~ot-dip Galvanizing and Alloying".
A differentially coated~ gal~anized steel strip of the general type described in said Patil et al. patent is 2Q produced by passing a continuous steel strip through a bath of molten coating metal consisting essentially of zinc to coat ~oth sides of the strip with the molten coating metal.
Immediately upon withdrawal of the coated metal strip from the bath of molten coating metal, the weight of the molten coating metal on opposîte sides of the strip is adjusted by impingement against opposite sldes of the strip of jets of gas or steam. The jets on respective opposite sides of the strip are adjusted to provide one strip side with a relatively light coat of the coating metal and the other strip 3Q side with a relatively heavy coat of the coating metal.
~ ~2~7~7~
Typically, the weight of the coating metal on the light coated side is in the range 0.05-0.25 oz./ft.2.
~0.015-0.075 kg/m2~, and the weigh.t of the coating metal on the heavy coated side is in the range 0~35-1.0 oz./ft.2 (:0.105-0.30 kg/m21.
~: Immediately following th.e weight adjusting step, the strip is passed through a treating zone wherein simultaneously the ligh.t coated strip side undergoes heating and th.e heavy coated strip side under~oes cooling.
As a result, the coating metal on th.e light coated side is transformed to iron-zinc alloy throughout. It is esired~ as a result of th.e treating step, that the coating metal on the heavy coated side be only partially transformed to iron-zinc alloy, and that the outer part of the coating on the heavy coated side consists essentially of zinc. In a method in accordance with.the Patil et al.
patent, the heavy coated side is at least partially molten at the time the simultaneous hRating and cooling step is performed.
: 2Q In a typical commercial product employing the subject matter of the Patil et al. patent, the weight of the coating metal on the heavy coated side is about 0.50 oz./ft.2 (:0 15 kg~m2~. In such a product, the iron-zinc alloy on the heavy coated side constitutes an inner layer wh.ich ranges from 30 to 50~ of the coating thick-ness on that side. Occasionally, ho~ever, as a result of the simultaneous heating and cooling step, in some spots the iron-zinc alloy may extend all the ~ay through to the:outer surface on th.e heavy coated side causing a defect known as 3Q "bleed-through". Th.ese bleed-throughs occur intermittently along the outer surface of the heavy coated strip side.
o In cross-section, a bleed-through resem~les the vertical cross-section of a mushroom Bleed-throughs are undesirable because the iron-zinc alloy in the bleed-through area at the strip's outer surface tends to powder when the galvanized steel str;p is subjected to a stamping operation, and this is undesirable. Although the light coated side of the strip consists entirely of iron-zinc alloy throughout, that coating is relatively so t~in that it can undergo a stamping operation without powdering. The fully alloyed light coated side is readily paintable.
Galvanized steel strip with bleed~through on the heavy coated s;de is not acceptable to those who fabricate the galvanized steel strip into products, and such a strip is not saleable. A typical customer for differentially coated galvanized steel strip is a stamping shop making parts for the automotive industry.
Recently there has been a demand, particularly from the automotive industry, for a differentially coated, galvanized steel strip in which the heavy coated side has a thinner coating, e.g., a weight substantially less than 0.50 oz./ft.2 (0.15 kg/m21, typically in the range 0.25--0.45 oz./
ft.2 C0~075-0.135 kg~m21. ~ thinner coating on the heavy coated side makes the strip more weldable. However, the thinner the coating on the heavy coated side, the more likely there is to be a bleed-through when the differentially coated strip is subjected to a simultaneous heating and cooling treatment of the type described in the Patil et al. patent.
When the heavy coated side has a coating weight of 0.50 oz./ft.2 ~0.15 kg/m2~, a strip with a bleed-~through occurs only occasionally, and the strip rejection rate for this defect is about 3~4%. When the coating on the heavy coated side has a weight su~stantially less than 0.50 oz./ft.2 (0.15 kg/m2~, it is virtually impossible to prevent bleed--~ 3 _~
~2~N~75~
, ..~
throughs when employing a process in accordance with the Patil et al. patent, and the iron-zinc alloy may constitute .
100% of the coating thickness on the heavy coated side.
Summary of the Invention I~ accordance with the present invention, the heavy coated strip side has a coating we;ght less than 0.50 oz./ft.2 (0.15 kg/m22, but intermittent bleed-throughs of iron-zinc alloy at the outer surface of the heavy coated side of the differentially coated, galvanized steel strip are prevented.
~his is accomplished by precooling the heavy coated side of the strip between the weight adjusting step and the simul-taneous heating and cooling step. The precooling step sub-stantially fully solidifies the molten coating metal on the heavy coated side before the start of the simultaneous heating and cooling step~
The weight adjusting step which precedes the precool-ing step provides a coating metal thickness on the heavy coated side of the strip ~hlch, absent the precooling step, is thick enough to be at least partially molten at the start 2Q of the simultaneous heating and cooling step but not thick enough to avoid intermittent bleed~throughs as a result of the simultaneous heating and cooling step.
Iron diffuses in zinc less rapidly ~hen the zinc is solid than when the zinc is molten~ Because the precool-ing step fully solidifies the coatîng metal on the heavy coated side, there is reduced diffusion of iron in the coating on the heavy coated side during the simultaneous heatlng and cooling step, compared to the diffusion which would occur if the coating on the hea~y coated side was not solidified before 3Q the start of the simultaneous heating and cooling step.
Because of the reduced diffusion, bleed~through is prevented.
`` ~247~
The precooling step is accomplished by impinging a fluid-cooling medium against the heavy coated strip side.
This fluid-cooling medium is preferably steam but may also comprise a;r, nitrogen, or inert gases.
~hen the heavy coated side is imp;nged ~ith a fluid-cooling medium such as steam, during the precooling step, the impinging cooling medium causes waves or 'rsag lines" on the solidified coating metal surface. Such minor surface irregularities are undesirable because they "print through"
on the reverse side of the galvanlzed steel strip during stamping. Ilowever, during the simultaneous heating and cool-ing treatment, following precGoling, there ;s superficial melting of a heavy coated side having a thickness in accord-ance with the present invention, there is relatively rapid solidification oF the heavy coated side following the heating and cooling step, and the sag lines or other minor surface irregularities are eliminated.
On a differentially coated, galvanized steel strip produced in accordance with the present invention, there are no sag lines, the spangle boundaries are flat (i.e., level with the surface of the coating on the heavy coated side~
and the outer surface on the heavy coated strip side is rela-tively smooth compared to the heavy coated side on a dif.
ferentially coated strip in accordance with prior art pro-cedures. Because that surface is so smooth, it is un-necessary to skin roll the strip as heavy as was necessary with a less smooth surface~ Because the strip is subjected to less skin rolling, it is more ductile and has better formàbility.
Other features and advantages are inherent in the subject matter claimed and dlsclosed or will become apparent to those skilled in the art from the following detailed des-cription in conjunction w;th the accompanying diagrammatic drawings.
2~
Brief Description of the Drawings Fig. l is a diagrammatic view, partially in section, illustrating a method and apparatus in accordance with an embodiment of the present invention; and Fig. 2 is an enlarged, fragmentary, sectional view illustrating a differentially coated, galvanized steel strip in accordance with an em~odiment of the present invention.
Detailed Description .
Referring initially to Fig~ l there is illustrated an embodiment of a method and apparatus for producing a dif-ferentially coated, galvanized steel strip in accordance with the present invention. Indicated generally at lO is an uncoated steel strip having a composition conventionally utilized for a continuous, hot-dip galvanizing process, such as the conventional Sendz;mir~type process. The strip is moved in the direction of the arro~s along a processing path illustrated in Fig. l, employing conventional equipment for moving the strîp.
In the course of moving along this path, strip 10 passes over a turn-down roller 12 located within a hood 11 containing a reducing atmosphere. Hood ll extends into a bath 13 of molten coating metal consisting essentially of zinc and having a bath temperature in the range 850-880 F
(454-471C~ preferably 865F ~463C~. 5trip lO has been preheated and enters bath 13 at essentially the same ; temperature as the bath. Bath 13 may contain other elements conventionally employed in galvan~zing compositions, and an example thereof is disclosed in the a~orementioned Patil et al.
4rl71~394. Preferably, the bath contains 0.14-0.16 wt.%
aluminum.
- 6 -~
7C~
Strip 10 passes around a roller 14 located within bath 13 and then passes up~ardly out of the bath as a gal-vanized strip 15 containing substantially equal weights of coatin~ metal on opposite sides of the strip. Galvanized strip 15 passes upwardly between a pair of steam jet nozzles 17, 17 adjustable to control the weight of coating on op-posite sides of strip 15. The galvanized strip is at a temperature typically in the range 850 ~88Q F (454 -471 C~.
The steam jets from nozzles 17, 17 are at a temperature in the range 300-350F (148-176C), so that the jets partially cool strip 15 as well as control the ~eight of the metal on opposite sides of the strip.
Conventionally, the steam jets are adjusted to provide one strip side 27 with a relatively light coat of the coating metal and the other strip side 28 with a relatively heavy coat of the coating metal. As a result there is produced a differentially coated, galvanized steel strip indicated generally at 18.
Typically, the coating metal weight on light coated side 27 is in the range 0.05-0.15 oz./ft.2 (Q.015-0.045 kg/m2~, and the coating metal weight on heavy coated strip side 28 i5 typically in the range Q.25-0.45 oz./ft.2 (0.075-0.135 kg/m ~, in accordance with the present invention. In other words the coating metal thickness on strip side 28 is less than 0.50 oz~/ft~ (Q.15 kg/m2).
In one embodiment, strip side 28 has a coating thickness less than 0.35 oz./ft.2 (Q.105 kg/m21, e.g., 0.32 oz./ft.2 (0.096 kg/m21 minimum.
The coating on side 27 is thin enough so that the coating is fully solidified when strip 18 enters a treating chamber 22 located above weight-controlling nozzles 17, 17.
7~
On the other hand, the coating on side 28 is thick enough to be at least partially molten when the stxip enters chamber 22, absent a precooling step ~etween nozzles 17, 17 and treating chamber 22. The cooling effect imparted by steam jet nozzles 17, 17 is not enough to fully solid;fy the coating on side 28.
In treating chamber 22, differentially coated steel strip 18 is subjected to s;multaneous heating of light coated strip side 27 and cooling of ~eavy coated s~trip side 28.
~his transforms the coating metal on t~e light coated side to iron-zinc alloy throughout wh~le desirably only partially transforming the coating metal on strip side 28 to iron-zinc alloy.
The heating operation is performed ~y a plurality of gas jet nozzles at 23, 23 which provide heating flames directed toward light coated strip side 27, and the galvanized strip is heated thereby to a temperature in the range 860-930F (460-499C~ or higher. The determining factor as regards strip temperature is to heat the strip to a tempera-ture which will fully alloy the light coated side. The cool-ing operation is provided by a plurality of air jet nozzlesat 24, 24 directed toward heavy coated strip side 28. The air jets are at ambient temperature, e.g., 60F (16 C~.
In chamber 22, the gas~jet nozzles at 23, 23 com-municate with a mani~old 36 into which gas is supplied via a conduit 37. The air jet nozzles at 24, 24 communicate with a manifold 38 into which air is supplied via a conduit 39.
When heavy coated strip side 28 has the coating weight descri~ed five paragraphs above, there can be a problem with intermittent ~leed~throughs of iron-zinc alloy to the surface of strip side 28 as a result of the simul-taneous heating and cooling step. To avoid such ~leed~oughs there is provided, in accordance ~ith the present invention, ~2~7~
,.
a precooling step between the weigh.t adjusting step and the simultaneous heating and cooling step. This precooling step is performed with one or more banks of steam nozzles 20 con-nected by a conduit 21 to the same steam source as provides steam to nozzles 17, 17. Steam at a temperature in the range 300-350F ~148C-176Cl is directed toward heavy coated strip side 28 to cool strip side 28 and fully solidify the coating metal on that strip side.
As previously noted, strip side 28 is still at least partially molten after passing between we;`gh:t adjusting nozzles 17, 17, Because the precooling step fully solidifies the relatively heavy coating on strip side. 28 before strip 18 enters chamber 22, th.e diffusion of iron in the heavy coat during the simul.taneous heating and cooling step is substan-tially reduced compared to the diffusion which ~ould occur if the heavy coat were not fully solidi-.'ied ~efore the start of the. simultaneous h.eating and caoling step.
When the coating metal consists essentially of zinc, it has a melting point typically about 78QF ~416C), and the coating metal on the heavy coated side should be cooled to a temperature at least 18 F Cll C~ belo~ that melting point to fully solidify the coating.
The precooling step not only fully solidifies heavy coated side 28 but probably also l~ght coated side 27.
However, during the treating step, because the coating on side 27 is so lightl it melts immediately upon subjection to the heating flames in ch.amber 22, allo~ing rapid dif-fusion of iron in the light coating so as to fully alloy the latter. The heavy coated side, however, undergoes only superficial melting during the treating step, and there is insufficient di.ffusion of iron to fully alloy or cause bleed-throughs in the heavy coated side.
~ ~2~
The precooling medium is prefe.rably steam (either wet or dry~. Steam is conveniently availa~le at the site of the precooling step because the same fluid medium is utilized at weight-controlling nozzles 17/ 1~. H.owever, other fluid cooling media may he employed, such as air, nitrogen and inert gases.
Whatever its composition, sufficient fluid cooling medium must ~e employed to cool the coating metal on heavy coated strip side 28 to a temperature belo~ its melting point and fully solidify the coating. Although only one ~ank 2Q
of precooling nozzles is illustrated in Fig. 1, two or more banks may be employed when necessar~ to cool heavy coated side 28 to the temperature required to obtain a fully solidified coating on side 28. ~hen two or more banks are employed, they are arranged at spaced locations between nozzles 17, 17 and chamber 22.
Staam nozzles 20 should be positioned upstream of chamber 22. The steam nozzles should not be located within substantially closed chamber 22 because of possible corrosion 2Q problems.
As noted above, during the precooling step, the jets of fluid cooling medium from nozzles 20 are directed against a coating on strip side 28 which.is at least partially molten. This causes minor surface i.rregularities at the outer surface on strip side 28. These minor surface irregularities are in the form of "sag lines'r or ~aves on the solidified outer surface on heavy coated strip side 28.
T~hen strip 18 enters treating cham~er 22, heavy coated strip side 28 is in a fully solidified condition.
However, during the simultaneous heating and cooling step in chamber 22, there is superficial.melting of the coating metal on the heavy coated strip side at the outer surface 7~
thereof. Immediately follo~ing the heating and cooling step there is relatively rap~d solidification of the heavy coated side, compared to a heavy coated side with a thicker coating deposit in accordance with prior practices. A process in accordance with the present invention smooths out the minor surface irregularities resulting from the precooling step, and it also flattens spangle ~o~mdaries so that they are level with the outer surface of the heavy coated strip side.
It is desira~le to eliminate these minor surface irregularities because they can "print throughll on the reverse side of the galvanized steel strip during a su~sequent stamping operation performed on that strip.
The differentially coated, galvanized steel strip exiting chamber 22 is indicated at 26. This strip is con-ventionally subjected to a "skin rollingl' step at an in-line skin rolling station 41 downstream of cham~er 22, employing conventional skin rolls. Alternatively~ skin rolling may be performed out of line, after the strip has ~een otherwise processed and coiled. ~hen skin rolling is performed out of 2a linel the cooled strip is uncoiled, skin rolled and then re-coiled.
For a differentially coated strip produced in accord-ance with the present invention, the outer surface of the heavy coated strip side is relatively so smooth before skin rolling that, in order to obtain the desired surface texture on the outer surface of the heavy coated strip side, it is not necessary to skin roll so heavily as before in order to obtain that desired surface texture. More specifically~ a skin roll producing a deformation below 1~, e.g., in the 3a range 0. 5-a. 8~, is employed in accordance with, the present invention compared to a deformation i`n the range 1.0-1.4~
on differenti,ally coated strlp produced in accordance ~ith ~ %~7~7~
prior practices. Because deformat;on is comparatively less when the strip is produced in accordance with the present invention, the strip is comparat~vely more ductile and has better forma~ilit~.
Except for Cll the precooling step employing nozzles 20, ~2~ the adjustment of the weight controlling step to reduce th.e t~icknes~ of the coating on the heavy coated side and C3~ th.e decrease ;`n deformation during skin roll;ng, all in accordance with. the present invention, the process conditions employed herein are essentially the same as those described ln Patil et al., U.S. Patent No.
4,171,394. Nevertheless, the resulting differentially coated, galvanized steel strip of the present in~ention has a ductility about 2~ higher on a scale of 100% than the same strip not produced with the processing differences described in the preceding sentence. More particularly, where a strip produced in accordance with the processing conditions of said Patil et al. patent would h.ave a typical ductility after skin rolling in the range 40~43%, the same strip produced in accordance with the present invention would have a ductility after skin rolling in th.e range 42-45~. The figures in the preceding sentence would be representative of an in-line heat treated, drawing quality, aluminum killed steel.
Referring again to Fig. 1, ind~cated at 40 is a drive roll located between chamber 22 and skin rolling station 41, and located at the downstream end of the processing path is a coiler 42.
The differentially coated, galvanized steel strip, produced in accordance with the present invention, is indicated generally at 2~ in Fig~ 2. Strip 26 comprises a steel substrate 30 havin~ a pai`.r of opposite sides coated ~2~ 7¢~
with a coating metal consisting essentially of zinc. One of the strip sides has a relatlvely light coat composed of iron-zinc alloy t~roughout, and th;s is indîcated at 31 in Fig. 2. Th.e other strip slde has a relatively heavy coat, at least the outer part of the heavy coat consisting es-sentially of z~nc ~ndicated at 32 ~n Fig. 2. Located ~e-tween zinc outer part 32 and steel su~strate 3Q is iron-zinc alloy indicated at 33. The thickness of iron~zinc alloy 33 on the heavy coated side of the strip varies, but it is always less than 25~ of the thickness of th.e heavy coat, as a maximum, and the average thickness ;s a~out 10%~
AS shown in Fig. 2, there are no interm;ttent iron-zinc alloy bleed-throughs at the outer surface 34 of the heavy coated strip side. The coating weights on the res-pective ligh.t and heavy coated sides are as descrihed above for the present invention. When the heavy coated strip side : has a coating no greater than that described for the present inventionl the a~oidance of intermittent bleed-throughs of iron-zinc alloy at the outer surface 34 on the heavy coated side would be virtually impossible a~sent the precooling step performed by nozzles 2Q.
: ~ Moreover, in a strip made in accordance with the : present invention, the outer surface 34 on the heavy coated side is krighter than on conventionally produced strips, so that on the rare occasion when a bleed-through may occur Ce.g., ~ecause of some malfunction of the process or ap~
paratus~ such a bleed~throug~.can be readily spotted by an operator and appropriate steps can be taken.
A bleed-through is to ~e distînguished from the 3a average thickness of the iron~zinc alloy layer on the heavy coated side. In conven~ional differentially coatecl gal~
vanized steel strip, the average thickne.ss of the iron-zinc alloy can be substantially less than the full thickness of the metal coating on that side~ but a ~leed~through may still ~e present. In a differentially coated, galvanized steel strip produced in accordance with the present invention, the iron-zinc alloy layer on the heavy coated side is less than 25% of the thickness of the coat on that side, as a maximum, with the average thickness ~eing la%, and there are absolutely no bleed-throughs.
The foregoing detailed descripti~on has ~een given for clearness of understanding only, and no unnecessary limita-tions sh.ould be understood therefrom, as modificat.ions will be obvious to those skilled in the art.
Brief Description of the Drawings Fig. l is a diagrammatic view, partially in section, illustrating a method and apparatus in accordance with an embodiment of the present invention; and Fig. 2 is an enlarged, fragmentary, sectional view illustrating a differentially coated, galvanized steel strip in accordance with an em~odiment of the present invention.
Detailed Description .
Referring initially to Fig~ l there is illustrated an embodiment of a method and apparatus for producing a dif-ferentially coated, galvanized steel strip in accordance with the present invention. Indicated generally at lO is an uncoated steel strip having a composition conventionally utilized for a continuous, hot-dip galvanizing process, such as the conventional Sendz;mir~type process. The strip is moved in the direction of the arro~s along a processing path illustrated in Fig. l, employing conventional equipment for moving the strîp.
In the course of moving along this path, strip 10 passes over a turn-down roller 12 located within a hood 11 containing a reducing atmosphere. Hood ll extends into a bath 13 of molten coating metal consisting essentially of zinc and having a bath temperature in the range 850-880 F
(454-471C~ preferably 865F ~463C~. 5trip lO has been preheated and enters bath 13 at essentially the same ; temperature as the bath. Bath 13 may contain other elements conventionally employed in galvan~zing compositions, and an example thereof is disclosed in the a~orementioned Patil et al.
4rl71~394. Preferably, the bath contains 0.14-0.16 wt.%
aluminum.
- 6 -~
7C~
Strip 10 passes around a roller 14 located within bath 13 and then passes up~ardly out of the bath as a gal-vanized strip 15 containing substantially equal weights of coatin~ metal on opposite sides of the strip. Galvanized strip 15 passes upwardly between a pair of steam jet nozzles 17, 17 adjustable to control the weight of coating on op-posite sides of strip 15. The galvanized strip is at a temperature typically in the range 850 ~88Q F (454 -471 C~.
The steam jets from nozzles 17, 17 are at a temperature in the range 300-350F (148-176C), so that the jets partially cool strip 15 as well as control the ~eight of the metal on opposite sides of the strip.
Conventionally, the steam jets are adjusted to provide one strip side 27 with a relatively light coat of the coating metal and the other strip side 28 with a relatively heavy coat of the coating metal. As a result there is produced a differentially coated, galvanized steel strip indicated generally at 18.
Typically, the coating metal weight on light coated side 27 is in the range 0.05-0.15 oz./ft.2 (Q.015-0.045 kg/m2~, and the coating metal weight on heavy coated strip side 28 i5 typically in the range Q.25-0.45 oz./ft.2 (0.075-0.135 kg/m ~, in accordance with the present invention. In other words the coating metal thickness on strip side 28 is less than 0.50 oz~/ft~ (Q.15 kg/m2).
In one embodiment, strip side 28 has a coating thickness less than 0.35 oz./ft.2 (Q.105 kg/m21, e.g., 0.32 oz./ft.2 (0.096 kg/m21 minimum.
The coating on side 27 is thin enough so that the coating is fully solidified when strip 18 enters a treating chamber 22 located above weight-controlling nozzles 17, 17.
7~
On the other hand, the coating on side 28 is thick enough to be at least partially molten when the stxip enters chamber 22, absent a precooling step ~etween nozzles 17, 17 and treating chamber 22. The cooling effect imparted by steam jet nozzles 17, 17 is not enough to fully solid;fy the coating on side 28.
In treating chamber 22, differentially coated steel strip 18 is subjected to s;multaneous heating of light coated strip side 27 and cooling of ~eavy coated s~trip side 28.
~his transforms the coating metal on t~e light coated side to iron-zinc alloy throughout wh~le desirably only partially transforming the coating metal on strip side 28 to iron-zinc alloy.
The heating operation is performed ~y a plurality of gas jet nozzles at 23, 23 which provide heating flames directed toward light coated strip side 27, and the galvanized strip is heated thereby to a temperature in the range 860-930F (460-499C~ or higher. The determining factor as regards strip temperature is to heat the strip to a tempera-ture which will fully alloy the light coated side. The cool-ing operation is provided by a plurality of air jet nozzlesat 24, 24 directed toward heavy coated strip side 28. The air jets are at ambient temperature, e.g., 60F (16 C~.
In chamber 22, the gas~jet nozzles at 23, 23 com-municate with a mani~old 36 into which gas is supplied via a conduit 37. The air jet nozzles at 24, 24 communicate with a manifold 38 into which air is supplied via a conduit 39.
When heavy coated strip side 28 has the coating weight descri~ed five paragraphs above, there can be a problem with intermittent ~leed~throughs of iron-zinc alloy to the surface of strip side 28 as a result of the simul-taneous heating and cooling step. To avoid such ~leed~oughs there is provided, in accordance ~ith the present invention, ~2~7~
,.
a precooling step between the weigh.t adjusting step and the simultaneous heating and cooling step. This precooling step is performed with one or more banks of steam nozzles 20 con-nected by a conduit 21 to the same steam source as provides steam to nozzles 17, 17. Steam at a temperature in the range 300-350F ~148C-176Cl is directed toward heavy coated strip side 28 to cool strip side 28 and fully solidify the coating metal on that strip side.
As previously noted, strip side 28 is still at least partially molten after passing between we;`gh:t adjusting nozzles 17, 17, Because the precooling step fully solidifies the relatively heavy coating on strip side. 28 before strip 18 enters chamber 22, th.e diffusion of iron in the heavy coat during the simul.taneous heating and cooling step is substan-tially reduced compared to the diffusion which ~ould occur if the heavy coat were not fully solidi-.'ied ~efore the start of the. simultaneous h.eating and caoling step.
When the coating metal consists essentially of zinc, it has a melting point typically about 78QF ~416C), and the coating metal on the heavy coated side should be cooled to a temperature at least 18 F Cll C~ belo~ that melting point to fully solidify the coating.
The precooling step not only fully solidifies heavy coated side 28 but probably also l~ght coated side 27.
However, during the treating step, because the coating on side 27 is so lightl it melts immediately upon subjection to the heating flames in ch.amber 22, allo~ing rapid dif-fusion of iron in the light coating so as to fully alloy the latter. The heavy coated side, however, undergoes only superficial melting during the treating step, and there is insufficient di.ffusion of iron to fully alloy or cause bleed-throughs in the heavy coated side.
~ ~2~
The precooling medium is prefe.rably steam (either wet or dry~. Steam is conveniently availa~le at the site of the precooling step because the same fluid medium is utilized at weight-controlling nozzles 17/ 1~. H.owever, other fluid cooling media may he employed, such as air, nitrogen and inert gases.
Whatever its composition, sufficient fluid cooling medium must ~e employed to cool the coating metal on heavy coated strip side 28 to a temperature belo~ its melting point and fully solidify the coating. Although only one ~ank 2Q
of precooling nozzles is illustrated in Fig. 1, two or more banks may be employed when necessar~ to cool heavy coated side 28 to the temperature required to obtain a fully solidified coating on side 28. ~hen two or more banks are employed, they are arranged at spaced locations between nozzles 17, 17 and chamber 22.
Staam nozzles 20 should be positioned upstream of chamber 22. The steam nozzles should not be located within substantially closed chamber 22 because of possible corrosion 2Q problems.
As noted above, during the precooling step, the jets of fluid cooling medium from nozzles 20 are directed against a coating on strip side 28 which.is at least partially molten. This causes minor surface i.rregularities at the outer surface on strip side 28. These minor surface irregularities are in the form of "sag lines'r or ~aves on the solidified outer surface on heavy coated strip side 28.
T~hen strip 18 enters treating cham~er 22, heavy coated strip side 28 is in a fully solidified condition.
However, during the simultaneous heating and cooling step in chamber 22, there is superficial.melting of the coating metal on the heavy coated strip side at the outer surface 7~
thereof. Immediately follo~ing the heating and cooling step there is relatively rap~d solidification of the heavy coated side, compared to a heavy coated side with a thicker coating deposit in accordance with prior practices. A process in accordance with the present invention smooths out the minor surface irregularities resulting from the precooling step, and it also flattens spangle ~o~mdaries so that they are level with the outer surface of the heavy coated strip side.
It is desira~le to eliminate these minor surface irregularities because they can "print throughll on the reverse side of the galvanized steel strip during a su~sequent stamping operation performed on that strip.
The differentially coated, galvanized steel strip exiting chamber 22 is indicated at 26. This strip is con-ventionally subjected to a "skin rollingl' step at an in-line skin rolling station 41 downstream of cham~er 22, employing conventional skin rolls. Alternatively~ skin rolling may be performed out of line, after the strip has ~een otherwise processed and coiled. ~hen skin rolling is performed out of 2a linel the cooled strip is uncoiled, skin rolled and then re-coiled.
For a differentially coated strip produced in accord-ance with the present invention, the outer surface of the heavy coated strip side is relatively so smooth before skin rolling that, in order to obtain the desired surface texture on the outer surface of the heavy coated strip side, it is not necessary to skin roll so heavily as before in order to obtain that desired surface texture. More specifically~ a skin roll producing a deformation below 1~, e.g., in the 3a range 0. 5-a. 8~, is employed in accordance with, the present invention compared to a deformation i`n the range 1.0-1.4~
on differenti,ally coated strlp produced in accordance ~ith ~ %~7~7~
prior practices. Because deformat;on is comparatively less when the strip is produced in accordance with the present invention, the strip is comparat~vely more ductile and has better forma~ilit~.
Except for Cll the precooling step employing nozzles 20, ~2~ the adjustment of the weight controlling step to reduce th.e t~icknes~ of the coating on the heavy coated side and C3~ th.e decrease ;`n deformation during skin roll;ng, all in accordance with. the present invention, the process conditions employed herein are essentially the same as those described ln Patil et al., U.S. Patent No.
4,171,394. Nevertheless, the resulting differentially coated, galvanized steel strip of the present in~ention has a ductility about 2~ higher on a scale of 100% than the same strip not produced with the processing differences described in the preceding sentence. More particularly, where a strip produced in accordance with the processing conditions of said Patil et al. patent would h.ave a typical ductility after skin rolling in the range 40~43%, the same strip produced in accordance with the present invention would have a ductility after skin rolling in th.e range 42-45~. The figures in the preceding sentence would be representative of an in-line heat treated, drawing quality, aluminum killed steel.
Referring again to Fig. 1, ind~cated at 40 is a drive roll located between chamber 22 and skin rolling station 41, and located at the downstream end of the processing path is a coiler 42.
The differentially coated, galvanized steel strip, produced in accordance with the present invention, is indicated generally at 2~ in Fig~ 2. Strip 26 comprises a steel substrate 30 havin~ a pai`.r of opposite sides coated ~2~ 7¢~
with a coating metal consisting essentially of zinc. One of the strip sides has a relatlvely light coat composed of iron-zinc alloy t~roughout, and th;s is indîcated at 31 in Fig. 2. Th.e other strip slde has a relatively heavy coat, at least the outer part of the heavy coat consisting es-sentially of z~nc ~ndicated at 32 ~n Fig. 2. Located ~e-tween zinc outer part 32 and steel su~strate 3Q is iron-zinc alloy indicated at 33. The thickness of iron~zinc alloy 33 on the heavy coated side of the strip varies, but it is always less than 25~ of the thickness of th.e heavy coat, as a maximum, and the average thickness ;s a~out 10%~
AS shown in Fig. 2, there are no interm;ttent iron-zinc alloy bleed-throughs at the outer surface 34 of the heavy coated strip side. The coating weights on the res-pective ligh.t and heavy coated sides are as descrihed above for the present invention. When the heavy coated strip side : has a coating no greater than that described for the present inventionl the a~oidance of intermittent bleed-throughs of iron-zinc alloy at the outer surface 34 on the heavy coated side would be virtually impossible a~sent the precooling step performed by nozzles 2Q.
: ~ Moreover, in a strip made in accordance with the : present invention, the outer surface 34 on the heavy coated side is krighter than on conventionally produced strips, so that on the rare occasion when a bleed-through may occur Ce.g., ~ecause of some malfunction of the process or ap~
paratus~ such a bleed~throug~.can be readily spotted by an operator and appropriate steps can be taken.
A bleed-through is to ~e distînguished from the 3a average thickness of the iron~zinc alloy layer on the heavy coated side. In conven~ional differentially coatecl gal~
vanized steel strip, the average thickne.ss of the iron-zinc alloy can be substantially less than the full thickness of the metal coating on that side~ but a ~leed~through may still ~e present. In a differentially coated, galvanized steel strip produced in accordance with the present invention, the iron-zinc alloy layer on the heavy coated side is less than 25% of the thickness of the coat on that side, as a maximum, with the average thickness ~eing la%, and there are absolutely no bleed-throughs.
The foregoing detailed descripti~on has ~een given for clearness of understanding only, and no unnecessary limita-tions sh.ould be understood therefrom, as modificat.ions will be obvious to those skilled in the art.
Claims (7)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A differentially coated, galvanized steel strip having a pair of opposite sides and comprising:
a steel substrate;
one of said strip sides having a relatively light coat composed of iron-zinc alloy throughout;
the other strip side having a relatively heavy coat, at least the outer part of said heavy coat consisting essentially of zinc;
iron-zinc alloy between said zinc outer part of the heavy coat and said steel substrate;
said other strip side having an outer surface;
there being no intermittent iron-zinc alloy bleed-throughs at the outer surface of said other strip side;
the light coat on said one strip side having a coating weight in the range 0.05-0.15 oz./ft.2 (0.015-0.045 kg/m2);
the heavy coat on said other strip side having a coating weight less than 0.50 oz./ft.2 (0.15 kg/m2).
a steel substrate;
one of said strip sides having a relatively light coat composed of iron-zinc alloy throughout;
the other strip side having a relatively heavy coat, at least the outer part of said heavy coat consisting essentially of zinc;
iron-zinc alloy between said zinc outer part of the heavy coat and said steel substrate;
said other strip side having an outer surface;
there being no intermittent iron-zinc alloy bleed-throughs at the outer surface of said other strip side;
the light coat on said one strip side having a coating weight in the range 0.05-0.15 oz./ft.2 (0.015-0.045 kg/m2);
the heavy coat on said other strip side having a coating weight less than 0.50 oz./ft.2 (0.15 kg/m2).
2, A differentially coated, galvanized steel strip as recited in claim 1 wherein:
the coating weight on said other strip side is in the range 0.25-0.45 oz./ft.2 (0.075-0.135 kg/m2).
the coating weight on said other strip side is in the range 0.25-0.45 oz./ft.2 (0.075-0.135 kg/m2).
3. A differentially coated, galvanized steel strip as recited in claim 2 wherein:
the coating weight on said other strip side is less than 0.35 oz./ft.2 (0.105 kg/m2).
the coating weight on said other strip side is less than 0.35 oz./ft.2 (0.105 kg/m2).
4. A differentially coated, galvanized steel strip as recited in claim 1 wherein:
the thickness of the iron-zinc alloy in said heavy coat is less than about 25% of the thickness of the heavy coat, as a maximum.
the thickness of the iron-zinc alloy in said heavy coat is less than about 25% of the thickness of the heavy coat, as a maximum.
5. A differentially coated, galvanized steel strip as recited in claim 4 wherein:
the average thickness of the iron-zinc alloy in said heavy coat is about 10% of the thickness of the heavy coat.
the average thickness of the iron-zinc alloy in said heavy coat is about 10% of the thickness of the heavy coat.
6. A differentially coated, galvanized steel strip as recited in claim 1 and comprising:
a smooth, skin rolled surface on both strip sides;
and a ductility greater than that of what is other-wise the same differentially coated, galvanized steel strip except for a coating weight on said other strip side greater than 0.50 oz./ft.2 (0.15 kg/m2).
a smooth, skin rolled surface on both strip sides;
and a ductility greater than that of what is other-wise the same differentially coated, galvanized steel strip except for a coating weight on said other strip side greater than 0.50 oz./ft.2 (0.15 kg/m2).
7. A differentially coated, galvanized steel strip as recited in claim 6 wherein;
said ductility is about 2% higher on a scale of 100% than that of said otherwise same strip having said greater coating weight.
said ductility is about 2% higher on a scale of 100% than that of said otherwise same strip having said greater coating weight.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000558428A CA1247470A (en) | 1984-01-20 | 1988-02-08 | Differentially coated galvanized steel strip and method and apparatus for producing same |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/572,386 US4513033A (en) | 1984-01-20 | 1984-01-20 | Differentially coated galvanized steel strip and method and apparatus for producing same |
| US572,386 | 1984-01-20 | ||
| CA000462669A CA1240570A (en) | 1984-01-20 | 1984-09-07 | Differentially coated galvanized steel strip and method and apparatus for producing same |
| CA000558428A CA1247470A (en) | 1984-01-20 | 1988-02-08 | Differentially coated galvanized steel strip and method and apparatus for producing same |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000462669A Division CA1240570A (en) | 1984-01-20 | 1984-09-07 | Differentially coated galvanized steel strip and method and apparatus for producing same |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1247470A true CA1247470A (en) | 1988-12-28 |
Family
ID=25670484
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000558428A Expired CA1247470A (en) | 1984-01-20 | 1988-02-08 | Differentially coated galvanized steel strip and method and apparatus for producing same |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1247470A (en) |
-
1988
- 1988-02-08 CA CA000558428A patent/CA1247470A/en not_active Expired
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