FR2520386A1 - METHOD AND APPARATUS FOR TREATING STEEL SHEET STRUCTURES - Google Patents

Abstract

For the phosphate-dipping treatment of a steel sheet structure such as a motor vehicle body, a process is provided which is carried out with the use of a treatment tank having a flat bottom. An object to be treated, such as a motor vehicle body, is dipped into the treatment solution. The object to be treated is inclined at least once in the upward direction and in the downward direction, in order to remove oil, reaction gases and air which have collected in the interior of the object. In addition, equipment for carrying out the phosphate-dipping treatment of the object is provided, in which the treatment solution is sprayed in on the feed side of the treatment tank, in order to generate small ripples, so that the formation of a boundary line marking on the surfaces of the object due to the temporary interruption of the feeding operation can be prevented. A wavy curvature is formed in a conveying device provided above the treatment tank or a pair of lifting and lowering elements, which can be operated individually, is connected to the conveying device, so that the object can be inclined in the upward direction or downward direction.

Description

 The present invention relates to an apparatus and a method for treating steel sheet structures and, in particular for treating steel sheets in automobile bodies or the like.

 A chemical phosphate conversion treatment is generally carried out to treat sheet steel structures such as car bodies. Typical known phosphate chemical conversion treatments are the immersion process and the spray process. These methods have advantages and disadvantages. To process objects which are produced on automatic production lines, such as car bodies, the spray method is widely adopted due to its stable productivity and quality.

In recent years, in snow-covered regions in winter, such as North America and European countries, a de-icing agent, such as rock salt or a mixture of rock salt and sand, has been spreading on road surfaces. . As a result, the exterior panels of automobile bodies, the wheel arches, the stretchers and the interior of the doors are strongly corroded by the salty components and it would be desirable to make improvements as regards their resistance. to tarry corrosion and perforating corrosion
With regard to the exterior panels of the automobile body, these resistances have been significantly improved by improving the composition of the phosphate treatment solution essentially so as to obtain a lower zinc content (Zn / PO4 = 1 * or less) and a suitable total thickness of paint film.

However, the treatment solution cannot be directly sprayed onto the interior panel of an automobile body having complicated profiles. Even in the case where the treatment solution is sprayed directly on the interior panel, it is not possible to achieve satisfactory degreasing during the cleaning step and satisfactory formation of the coating resulting from the chemical conversion, although it is unclear whether this is due to the short treatment time or an insufficient amount of treatment solution.

 In the case of the immersion treatment method, the treatment time is 300 seconds or more compared to the time of 90 to 150 seconds used in the case of the spray treatment method. The weight per unit area of the coating resulting from the conversion can also reach 5 g / m2 as opposed to a weight of 1.5 to 2.5 g / m2. The adhesion to the coating of the paint film which is to be formed in the following process is also less good.

 In order to shorten the processing time in the immersion process, reduce the weight of the coating resulting from the conversion to only 2.0 to 3.5 g / m2 and obtain a coating resulting from the chemical conversion to phosphate which allows good adhesion of the paint film to be formed on it, it is necessary to precisely control the composition of the phosphate treatment solution and the conditions of the dip treatment. For example, it is reported in the literature that the characteristics of the excellent quality zinc phosphate coating formed by the dip treatment process are within the range shown in Table 1 below.

Table 1 - Characteristics of a zinc phosphate coating
Item Specifications.

coating weight 2.5 to 3.8 g / m2
P / (P + H) 0.9 to 1.0
Crystal size 3.5 to 10 fi
Form of columnar or granular crystals
Notes
(1) P / (P + H): P represents phosphophyllite (Zn2Fe (P04) 2.4H20) and H represents hopeite (Zn3 (P04) 2.4H20) which are included in the zinc phosphate coating.

The characteristic indicated in table I above indicates the ratio of the phosphophyllite content.

 (2) crystal size: it indicates the maximum length of the crystals.

 (3) form of crystals: the coating of zinc phosphate formed by the spraying method and by the conventional immersion method comprises sharp needle-like crystals.

 Even if the composition and the temperature of the treatment solution during the immersion treatment are controlled, the characteristics of the coating obtained vary according to the immersion conditions.

In the coating treatment by phosphate conversion of a structure into steel sheets by immersion,
in order to form a correct and uniform phosphate coating suitable for constituting a base layer for painting by electrodeposition, it is desirable to drain
previously a liquid layer at a given depth below the surface of the solution, in the vicinity of the parts where the steel sheet structure enters a tank of a coating treatment solution obtained by conversion, until '' a flow speed of 20 m / min, preferably 10 m / min, is reached, and to maintain the passage time for a given point on the body of the sheet steel structure at 15 sec or less, preferably 10 sec or less.

 Even when the flow rate of the liquid layer is 20 m / min or more and the passage time of 15 sec or more, the coating obtained by phosphate conversion is formed in the same way. The coating of the phosphate thus formed, however, is of lower quality and is not adopted to constitute a base layer for painting by electrodeposition, because of the characteristics of the coating, the size of the crystals which has become greater than 15 p and the shape of the crystals, acicular, and on the other hand, of the P / P + H ratio become equal to 0.8 or less.

 In addition, in the treatment of a steel sheet structure by coating obtained by phosphate conversion, air pockets tend to occur, mainly on the internal surface of the parts of the steel sheet structure. or gases, formed by chemical reactions, which are stagnant or move slowly. Thus the formation of phosphate coating in these parts is largely hampered by such a tendency, and this obstacle constitutes a factor for forming a rust and blue color (iron phosphate). Consequently, the parts where the formation of the coating obtained by conversion is insufficient, clearly exhibits poor adhesion of the paint film and poor corrosion resistance after the painting operation.

 Consequently, in order to form an excellent coating obtained by conversion to phosphate, it is necessary to facilitate the dispersion and displacement of the gases produced by the chemical reactions, away from the steel sheet structure.

 In addition to this, in the chemical conversion to phosphate treatment, the sludge formed by the reaction is deposited on the horizontal part or on the body part of the object to be treated, and it is retained in the bites which exist between the crystals. of the coating or it deposits on the surface of the coating. Even if the object to be sprayed during the next rinsing step, the mud cannot be easily removed. This leads to making the plating which must be carried out subsequently, defective, and degrades the qualities of the paint film, such as the adhesion of the paint, the water resistance and the resistance to blistering.

In order to solve these problems, it is necessary to change the shape of the bottom of the treatment bath tank or to reduce the amount of sludge contained in the treatment bath.

 The present invention has been made to solve the various problems of the prior art and one of its main aims is to provide a method and an apparatus for the treatment by immersion of a steel sheet structure according to which and by means of which the interior and exterior surfaces of the sheet steel structure which has a complex shape are brought into uniform contact with a treatment solution to form thereon a coating having a uniform thickness.

 Another object of the present invention is to provide a method and an apparatus for the immersion treatment of a steel sheet structure according to which and by means of which the mud concentration of the treatment solution is adjusted to prevent fixation mud to the coating resulting from the conversion.

Other characteristics of the invention will appear on reading the description which follows and on examining the appended drawings in which
Figure 1 is a schematic view showing the main part of an immersion treatment apparatus according to an embodiment of the present invention;
Figure 2 is a schematic view showing the main part of an immersion treatment apparatus according to another embodiment of the present invention;
Figure 3 is a sectional view along the line
III-III of Figure 2;
Figure 4 is a schematic view showing the main part of a diving type apparatus according to yet another embodiment of the present invention;
FIG. 5 is a graph which shows the relationship between the characteristics of the coating resulting from the chemical conversion formed on the surface of the object to be treated and the speed of flow of the phosphate treatment solution; and
Figure 6 is a graph which shows the relationship between the characteristics (crystal shape and crystal size) of the coating resulting from the chemical conversion and the flow rate of the phosphate treatment solution.

 In FIG. 1 to which reference will be made, the reference 1 designates a treatment tank whose general shape is that of a bowl and whose bottom surface is flat. A steel sheet structure which must be treated in this tank 1 is an automobile body 3. The automobile body 3 is carried by a conveyor 4 so as to pass without interruption through the interior of the tank 1. A corrugation 5 is formed in the conveyor 4 so that the automobile body 3 is inclined towards the top then downwards approximately at the center of the tank 1. This corrugation 5 comprises an ascending part 5a, a descending part 5b and a straight part 5c. The automobile body 3 is introduced at a predetermined speed into the solution 7 of phosphate treatment contained in the tank 1 so as to be completely submerged therein for the treatment while it is suspended from two suspension members 6a and 6b connected to the conveyor 4. The automobile body 3 is moved in the direction indicated by an arrow 8. Any quantity of treatment solution which overflows as a result of the immersion of the automobile body 3 flows outside via an overflow tank 9 or else it is recycled. A roof 10 is formed for each cabin above the conveyor 4 to the shape of which it adapts. Although only part of the chemical conversion treatment apparatus has been shown in FIG. the device comprises in part, upstream of the immersion tank, cleaning stages of the spray type and of the immersion type, several rinsing stages and a surface treatment tank of the immersion type. The apparatus also comprises, downstream of the immersion tank 1, rinsing stages of the spray type and of the immersion type of the drying stages and of the painting stages which are connected to it. Therefore, the series of cleaning, rinsing, surface conditioning and painting operations can be carried out continuously.

 In this embodiment, the steel sheet structure displaced by the conveyor 4, namely the automobile body 3, is inclined and immersed in the phosphate treatment solution. The angle of inclination of the automobile body 3 is, in this case, preferably between 20 and 300. Although the automobile body is treated while it is immersed in the treatment solution, in the Chemical conversion to phosphate treatment, hydrogen gas and other gases are generated by the reaction of the solution with the surface of the steel sheets. In addition to this, air bubbles introduced with the automobile body 3 accumulate inside the top of the automobile body and cannot be exhausted. In places where gas or air bubbles accumulate, the surface of the automobile body cannot be contacted with the treatment solution and cannot be treated, which results in non-uniform treatment. To solve this problem, when the suspension members 6a and 6b cross the corrugation 5 after a predetermined period of time has elapsed since the immersion of the automobile body 3 in the phosphate treatment solution, the automobile body 3 is inclined upwards then downwards. When the suspension members 6a and 6b are in the ascending part 5a, the hydrogen gas and the other gases are evacuated through the front opening of the bodywork. When the suspension members 6a and 6b pass through the descending part 5b, l 'gaseous hydrogen and other gases' are evacuated through the rear opening of the bodywork, thus completely evacuating the gases accumulated below the top of the bodywork and ensuring uniform contact of the treatment solution with the bodywork. After that, when the suspension members 6a and 6b run along the straight part 5c, the automobile body is finally treated as immersed.

 The conveyor 4 advances the automobile body 3 at a speed of 2 to 8 m / min and the automobile body 3 is gradually withdrawn from the treatment tank 1.

 In the embodiment shown in FIG. 2, means for circulating the treatment solution and means for removing the sludge are added to the apparatus of the embodiment shown in FIG. 1. An outlet orifice 11 for discharging the treatment solution is formed in the bottom of the treatment tank 1. A circulation conduit, which comprises a tap 12 and a pump 13, is connected to this outlet orifice 11. A device 15 eliminating sludge communicates with the circulation duct 14. The treatment solution from which the sludge has been extracted by this device 15 for sludge removal flows to the overflow tank 9 through a supply conduit 16.

Another circulation duct 18 is connected to the overflow tank 9 by means of a pump 17,
The circulation duct 18 divides into two branches after a heat exchanger 19; a circulation conduit 18a is connected to a lower injection conduit 23 with interposition of a tap 20 and the other circulation conduit 18b extends to the inlet side of the treatment tank 1 which receives the bodywork automobile to be treated, the conduit 18b which contains a tap 21 communicating with an upper injection conduit 22, the circulation conduit 18 thereby supplying the upper and lower injection conduits 22 and 23 of the treatment tank 1.

 The upper injection conduit 22 extends below the level of the surface of the treatment solution and is connected to nozzles arranged in the vicinity of the level of the surface. The treatment solution is ejected from the injection ports of the nozzles arranged face to face, thereby causing a predetermined flow of solution at the surface and the formation of small waves.

The lower injection conduit 23 extends along the treatment bottom 1 to project the treatment solution downwards so that the deposition of the mud on the bottom of the tank is prevented. In this way, the sludge formed in the treatment tank 1 does not remain in the tank 1 but is entrained as far as the sludge removing device 15 with the treatment solution by the outlet orifice 11 and the circulation duct 14 In the sludge removing device 15, the sludge is deposited; the supernatant liquid is transferred to the overflow tank 9. The treatment solution is then heated in the heat exchanger 19 to then be introduced into the treatment tank 1 via the upper injection pipe 22 and the 23 lower injection pipe. The heat exchanger 19 is of a known configuration of a type generally used for heating liquids. A heating medium, such as hot water, is supplied to the heat exchanger 19 by a conduit 24 for circulation of the heating medium.

 When the automobile body 3 to be treated is introduced into the treatment tank 1 of this embodiment, the treatment solution is sprayed onto it by the nozzles of the upper injection duct 22. In this way, the body 3 d The automobile is immersed in the treatment solution while a solution flow occurs at a predetermined speed and small waves are formed at the surface of the treatment solution. When this measurement is taken, even if there is an interruption of approximately 10 seconds in the advance of the conveyor 4, the level of the surface of the treatment solution cannot be marked on the surface of the object to be treat, namely the automobile body 3.

When the automobile body is immersed in the treatment solution, the speed of advance of the conveyor 4 is approximately 2 to 8 m / min. The automobile body 3 is immersed in the treatment solution at approximately the same speed as that mentioned above, which ensures continuous contact of the automobile body with the treatment solution. The reaction between the steel surface and the treatment solution progresses more rapidly in the initial stage. Consequently, if there is an interruption in the conveyor advance, the dividing line between the part of the body which is submerged and the part of the body which is not submerged, i.e. the level from the surface, is distinctly marked on the surface of the body, however, since in this embodiment, small waves are formed on the surface of the treatment solution, the separation line is blurred. In addition, because the treatment solution is ejected through the injection pipe 22, the local flow velocity of the treatment solution on the inlet side of the tank is kept below 20 m / min and preferably lower at 10 m / min so that a given point on the automobile body 3 can cross the surface in less than 15 seconds and preferably in less than 10 seconds. When this is achieved, a treatment producing a coating resulting from the non-uniform conversion is prevented and the thickness of the electro-deposited coating is made uniform. The chemical conversion reaction is slowed down at the initial stage during the continuous treatment since the quality of the finished automobile body depends, in large part, on the formation of the coating resulting from the chemical conversion by uniform crystallization. The automobile body is tilted up and down at least once when it is in the submerged state to discharge the gas which accumulates below the top of the automobile body 3.

 In the embodiment shown in Figure 4, a diving type treatment apparatus is used. The treatment tank 31 has an approximately rectangular cross section. A conveyor 32 serving to move the object to be treated, namely a bodywork 3 of an automobile, is arranged above the treatment tank 31, as in the case of the embodiments described below. Two lifting winches 33 and 34 are mounted on the conveyor 32 at a predetermined spacing from one another. Suspension cables 35 and 36 are mounted on the winches 33 and 34 in a suitable manner so that these suspension cables can be wound and unwound. A chassis 37 is fixed to the lower ends of the suspension cables 35 and 36. The automobile body 3 is placed on the chassis 37. With this treatment apparatus of the diving type, as in the case of the previous embodiments, cleaning and rinsing stages are provided upstream of the treatment tank and, in front of the tank, rinsing, drying and painting stages are also provided.

 In this embodiment, after the automobile body has been immersed in the treatment solution, the targets 35 and 36 are alternately wound and unwound from the winches 33 and, respectively, 34, to tilt the automobile body 3 in order evacuate gases, oil, etc. accumulated below the top of the automobile body so that its interior surface can be brought into uniform contact with the treatment solution. Therefore, even with a diving-type treatment apparatus, by tilting the automobile body in the submerged state, it can be obtained that the interior surface of the automobile body is uniformly treated.

To obtain the phosphate coating having the characteristics indicated in Table 1, the treatment solution is a zinc phosphate treatment solution, having the following composition
Zn from 0.5 to 2.0 g / l
Ni from O to 1.5 g / l NO3 from 3 to 25 g / l
P04 from 10 to 18 g / l
F from O to 1.3 g / l
C103 from O to 2.0 g / l N02 from 0.06 to 0.14 g / l
The zinc phosphate treatment solution having this composition is neutralized with caustic soda (NaOH) or with sodium carbonate (Na2CO3) so that the total acid number of the treatment solution can reach a value included between 15 and 20 and that the acidity ratio of the total acid index to the free index can reach -a value between 12 and 26. The treatment temperature is controlled with a tolerance of + 30C. Even when treatment solutions with the same composition are used, phosphate coatings with different characteristics are obtained depending on the immersion conditions,
Figures 5 and 6 show the characteristics of the phosphate coating as a function of the processing conditions. It can be seen in these figures that the characteristics of the coating vary depending on the speed of contact between the treatment solution and the surface of the object to be treated, namely the automobile body.
Although various reasons for this are conceivable, it is believed that the most important factor is the difference in the rate of increase in pH of the layer at the reaction interface. In particular in the treatment by the immersion process, a coating having excellent characteristics can be obtained by controlling the relative speed between the object to be treated and the solution in the range of 2 to 10 m / min, preferably 2 to 5 m / min.

(Although in practice the object to be treated is moved, this speed is represented in the figures as being the flow speed of the solution, since there is no need to take into account that the relative speed between the object and the solution). When the flow rate of the solution is 10 m / min or less, crystals having a columnar or granular shape which is preferable are obtained. When the flow speed of the solution exceeds 20 m / min, pointed needle-like crystals are obtained which do not provide a coating having advantageous characteristics, in other words, when the flow speed of the solution exceeds 20 m / min, the size of the crystals, which is one of the characteristics of the coating, becomes equal to 15 / solid or more (see Figure 6) and the ratio P / (W / H) becomes equal to 0.8 or less (see Figure 5), which produces a lower quality coating.

 However, in the case of the immersion process according to which the automobile body is constantly moved in the treatment tank, if there is a stop even of about 0.5 to 1 second only during the movement of the automobile body. automobile during the initial reaction step, a separation line is formed at the surface of the solution on the surface of the automobile body and a non-uniform coating is formed. This non-uniform thickness results in producing irregular potential or other irregular conditions of the surface of the automobile body during the subsequent electroplating. This in turn causes non-uniform deposition of the paint which is electrodeposited and the formation of a non-uniform paint coating. In order to prevent the formation of a non-uniform phosphate coating during the initial reaction step , by keeping the flow rate of the solution at the surface level at about 20 m / min, the abrupt increase in pH at the reaction interface and the precipitation of the crystals are considerably reduced, thanks to the supply of new processing at the reaction interface.

 Therefore, even if the automobile body is introduced into the tank with stops or interruptions in advance, a dividing line is not marked on the surface of the automobile body and a coating of phosphate uniform thickness is formed by the flow effect of the treatment solution from the injection pipe.

 With the immersion process as described above, automobile bodies can be processed at a rate of 20 to 60 automobile bodies per hour. If it is assumed that the surface to be treated by automobile bodywork is 80 m2, the surface of sheet metal treated is from 1600 to 4800 m2 per hour. In this case, the amount of mud formed by the reaction is 3 to 4 g per square meter.

Therefore, if auto bodies are processed at a rate of 20 auto bodies per hour, there is 4.8 to 6.4 kg of mud per hour. If one processes automobile bodies at a rate of 60 automobile bodies per hour, it is formed from 14.4 to 19.2 kg of mud per hour. The mud thus formed is fine and it floats. As time passes, the mud particles develop into particles having a size of 50 to 500 µm and having a sedimentation rate of 0.08 to 0.13 m / min.

 However, in accordance with the present invention, the treatment solution is continuously drawn off from the tank to remove the sludge in the sludge removing device. Consequently, the sludge does not settle on the bottom surface of the treatment tank. By giving the surface of the bottom of the treatment tank a flat shape, it is possible to facilitate the evacuation of the mud and a uniform agitation of the treatment solution so that the deposit or the fixing of the mud on the object to be treat is prevented.

  In summary, in accordance with the present invention.

the immersion process is adopted in the chemical conversion to phosphate treatment of automobile bodies. Although oil and gases, such as hydrogen, accumulate inside the automobile body and, in particular, below the top of the automobile body, it is discharges these products by tilting the automobile body up and down, so that the entire surface of the body can be brought into uniform contact with the treatment solution.

Therefore, the problem of non-uniform treatment is solved and a treatment of the steel sheets can be carried out which provides excellent resistance to darting corrosion and to perforating corrosion.

Claims (8)

 Process for phosphate treatment by immersion of a steel sheet structure, such as an automobile body (3), characterized in that it consists in immersing an automobile body in a treatment solution (7 ) contained in a treatment tank (1; 31) having a flat bottom (2) and to be inclined at least once the automobile body up and down to evacuate the oil, the reaction gases and l accumulated inside the automobile body. 2) Process according to claim 1, characterized in that the relative speed between the bodywork (3) of the automobile and the phosphate treatment solution (7) during their contact is between 2 and 10 m / min. 3) Method according to claim 1, characterized in that the treatment solution is ejected below the surface level of the treatment solution (7) on the inlet side of the phosphate treatment tank to form small waves and cause the solution to flow at a predetermined speed. 4) Process according to claim 1, characterized in that the treatment solution flows at a speed of 2 to 5 m / min in the phosphate treatment tank (1) to produce a uniform distribution of the reaction mud in the treatment solution (7) and in that a fixed quantity of the treatment solution is continuously withdrawn from the treatment tank and sent into a sludge removal device (15) thus eliminating the sludge and returning the supernatant liquid to the phosphate treatment tank. 5) An apparatus for phosphate treatment by immersion of a surface of a structure (3) made of steel sheets, characterized in that it comprises a conveyor (4; 32) used to transport a bodywork (3) automobile disposed above a flat-bottomed phosphate treatment tank (1; 31) (2) and in that a corrugation (5) is formed in the conveyor or a pair of winches (33, 34 ) are mounted on the conveyor so that the automobile body can be tilted up or down. 6) Apparatus according to claim 5, characterized in that an injection pipe (22) for ejecting the treatment solution is arranged on one side of the phosphate treatment tank (1) so as to cause the formation of small waves and the flow of the treatment solution at a predetermined speed.  7) Apparatus according to claim 5, characterized in that an injection conduit (23) for agitating the treatment solution is arranged in the vicinity of the bottom of the phosphate treatment solution in the tank (1) phosphate treatment to cause the treatment solution to flow at a predetermined speed and to prevent the deposition of mud on the bottom of the phosphate treatment tank. 8) Apparatus according to claim 5, characterized in that a circulation conduit (14, 16, 18) is connected to the tank (1) for phosphate treatment by means of a pump (13) and a tap (12) for continuously withdrawing treatment solution from the phosphate treatment tank and in that a sludge removal device (15) and a heat exchanger (19) are connected to the circulation duct.