JPS6157374B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing coated steel strips with high embossing properties and/or ductility.
The invention is sometimes of interest when steel strips are coated in a bath consisting of a mixture of zinc and aluminum by the dip coating process. The main properties required by users of coated steel strips are the highest possible embossability, especially for the type of steel used, and, depending on the intended use and case of the steel strip, satisfactory embossability. fatigue resistance, ductility and weldability. The applicant has recognized that the cooling conditions applied to the strip at the outlet of the coating bath are of great importance in this respect and has developed a new process of this type, which is precisely the object of the present invention. The method of the invention, in which the steel strip is immersed in a metal bath at a temperature above 500° C., is characterized in that at the exit of the bath the steel strip undergoes a primary cooling to a temperature at which the metal coating it hardens; It is characterized by being subjected to intense cooling to temperatures below 475°C, but without damaging the flatness of the steel strip. Before the immersion treatment, it is preheated to a certain temperature above its recrystallization temperature and kept at this temperature for 30 seconds or more,
It is then cooled to a temperature no lower than the temperature of the metal bath. According to the method of the invention, the time between the moment when the steel strip begins to cool down at a temperature above the recrystallization temperature and the moment when the coated metal layer is hardened after leaving the metal bath is not less than 20 seconds, but Not more than 100 seconds. This method makes it possible to obtain coated steel strips with particularly interesting properties. The temperature of the metal bath is kept above 500°C to ensure that it is well above the melting point of the coating alloy or metal, so as to avoid the risk of localized solidification in the bath. . In the method of the invention, the steel strip immersed in a metal bath is first cooled at its outlet to a temperature that causes hardening of the coating, without impairing the smoothness of the steel strip. and to ensure good quality of the coating, and in the process of the invention is then followed by rapid cooling to a temperature below 475°C. This is done to obtain the desired smoothness and ductility of the steel strip, and in the case of mild steel, which will be discussed later, is also the upper limit of the temperature range used for overaging the coated steel strip. In addition, the reason why the steel strip is kept at a temperature higher than the recrystallization temperature for 30 seconds or more before the immersion treatment is to ensure that the steel strip has a uniform temperature.
This is to ensure complete recrystallization. It is then cooled to a temperature that is not lower than the temperature of the metal bath; if the temperature is lower than the metal bath, local solidification of the molten metal may occur when the steel strip is introduced into the bath and immersion may occur, resulting in loss of smoothness. This is to avoid In addition, the time between the moment when the steel strip starts to be cooled at a temperature above the recrystallization temperature mentioned above and the moment when quenching starts is kept to 20 seconds or less, as this is necessary to obtain the desired softening of the steel. In addition, the time should not be longer than 100 seconds because in this method, taking longer time will not bring any further advantage to the properties of the obtained steel strip, and it will be difficult to shorten the process time. I found that it was a good idea to do so. According to one aspect of the invention, in the case of a strip made of mild steel for embossing, intense and rapid cooling is
C. to a temperature between 375.degree. C. and 475.degree. C., the strip is then held at the temperature at the end of the cooling for more than 30 seconds and then subjected to a second, less rapid cooling to room temperature in a known manner. When processing extremely soft steel strips, the final temperature of intensive cooling is below 350℃, and the strip steel is then
It is subjected to a reheating operation in a suitable oven to a temperature of the order of 400°C to 500°C and held at this temperature for at least 30 seconds before being slowly cooled to room temperature. Furthermore, according to another variant of the invention, in the case of thin steel plates with high strength, intensive rapid cooling is carried out to a certain temperature below 300 °C, preferably below 250 °C, and immediately to room temperature, no faster. A second cooling is performed in a known manner. According to the invention, the bath in which the steel strip to be coated is immersed is composed of several metals or alloys having a melting point below 700°C. Among these metals, light metals such as aluminum and their alloys are especially praised. One particularly advantageous application is a bath consisting of aluminum and zinc. Also according to the invention, intense rapid cooling of the strip at the outlet of the coating bath is carried out by quenching in a water bath maintained at a temperature above 75° C., preferably at its boiling point. The applicant has already been able to show the benefits of such quenching in an aqueous medium in other applications, such as for example continuous annealing of steel sheets, electroplated or not. In other words, it was possible to provide these steel sheets with an excellent combination of elastic limit and elongation. It was therefore possible to provide them with a great suitability for embossing and a great uniformity of properties over all their widths. The water bath in which the steel plate is immersed may consist solely of water in order to rapidly cool the steel plate. or substances capable of modifying the conductivity of heat as suspensions and/or solutions, for example salts (especially calcium chloride or borax) or surface tension-active substances such as sodium or potassium salts of palmitic, stearic, oleic acids. It may also optionally contain corrosion inhibitors. According to one variant of the invention, the intense rapid cooling of the steel strip at the outlet of the coating bath is achieved by injecting a fluid consisting of a homogeneous mist of water suspended in a gas, preferably air. It is carried out with The inventor has completed a spray tube capable of providing such cooling as described in Belgian Patent No. 853,821. This spray tube consists of a central conduit having the shape of a Laval tube and at least one conduit extending from the central conduit at an angle of 30° or more, preferably about 45°, to the exit area of the part that widens from the central conduit. The central duct is
At the inlet end of the converging section, it is supplied with a comminution gas (preferably air, but optionally nitrogen, etc.), and the side tube is supplied with water to be comminuted. Such spray tubes have a specific cooling power which can exceed 3 MW per m ² and which can vary over a very wide range (of the order of 1 to 10) without loss of uniform cooling. These spray tubes furthermore exhibit the advantage of very good efficiency and therefore low water consumption for the same cooling power. The following examples are provided by way of illustration and not by way of limitation. Example 1 Steel strip made of mild steel of normal quality Cold rolled steel strip 0.8 mm thick was processed in a continuous coating process line by immersion in a bath consisting mainly of aluminum and zinc. The treatment is carried out in a non-oxidizing open flame oven for 700 min.
It consists of heating the strip to a temperature of 0.degree. C. and then holding it at this temperature for a short period of about 35 seconds. The strip was then cooled to 600°C for 10 seconds and then immediately immersed in an Al-Zn bath at a temperature of 590°C. From the outlet of the bath, the strip was rapidly cooled for about 5 seconds to a temperature of 510°C to harden the coating, then very quickly cooled to 425°C and held at this temperature for 45 seconds. This final cooling was accomplished by immersing the strip in a water bath containing a passivating element such as potassium dichromate or chromic anhydride. The bath was maintained at 95°C. The process was terminated by gradual cooling to 80°C for 45 seconds. For comparison, the same strip was treated with the conventional method of coating by hot dipping and a single cooling step at the exit of the bath, where the strip was brought to 590 °C (temperature of the coating bath) for 1 minute or once to 80 °C. . The results obtained are as follows.

[Table] It can be seen that the elongation of the steel strip treated according to the present invention was clearly improved. This is a significant advantage for embossing. Example 2 High strength coated thin steel plate The following composition (c=0.06%, Mn=0.8%, Si=0.1
%, Si = 0.01%, Al = 0.05%, the rest iron containing ordinary impurities) was similarly treated in a continuous coating process line by immersion in a bath consisting mainly of aluminum and zinc. It was done. The process similarly involves heating the steel plate in a non-oxidizing open-flame furnace to a temperature of 800°C, which is then heated to a temperature of approx.
It consists of keeping at this temperature for a short period of 30 seconds. The steel plate was then cooled to 650°C for 20 seconds and then immediately immersed into an Al-Zn bath at a temperature of 620°C. From the outlet of the bath, the strip is rapidly cooled for 10 seconds or more to a temperature of 530°C to harden the coating, and then exposed to a mist of water suspended in the air as described in Belgian Patent No. 853,821. It was once very rapidly cooled down to 20°C in 2.5 seconds by the injecting spray tube. For comparison, the same steel sheets were treated according to the conventional method of coating by hot dipping and cooling to 20° C. for 1 minute or more at the exit of the bath. The results obtained are as follows.

It can be seen that the failure load of the steel plate treated according to the invention is clearly superior to that of the same steel plate treated according to the conventional method. Example 3 Ultra-mild steel plate coating bath properties for deep embossing: Zinc-aluminum alloy, Bath temperature: 600°C. (Hardening degree of coating) Temperature at the beginning of rapid cooling: 530°C Method of rapid cooling: The steel immersed in a boiling water bath is made of ultra-mild steel with the following composition (%): C = 0.067, Mn =0.29, Si=−−, Al−
-, S = 0.015, P = 0.010, N ₂ = 0.0045 The steel plate was hot rolled to a thickness of 2.5 mm. (The final rolling temperature is 880℃, the winding temperature is 700℃)
Cold rolled to a thickness of 0.8mm. The steel plate was subjected to various heat treatment steps shown in Figure 1 during the coating process, namely: - rapid heating to 700°C, - maintenance at this temperature for more than 30 seconds, and - atmospheric gas up to a temperature of around 610°C. - The coating is dehydrated at the exit of the bath and cooled to 530℃, the cooling period varies; ● Immersed in a boiling water bath, the steel plate is heated to 150℃ or more from the bath.
Served at a temperature of 350°C, ● Rapid reheating to 450°C, maintained at this temperature for various periods, ● Final cooling in air to 80°C for 45 seconds or more. As a comparison, the same steel was treated by conventional methods with continuous cooling to 80° C. for one minute or one time at the exit of the coating bath. Result: (1)

[Table] (2) Method of the invention (a) Effect of the entire period of slow cooling between tempering and the beginning of the rapid cooling after coating. This effect is illustrated by Figure 2, where only this one parameter is varied and the others are fixed as follows: Temperature of the steel plate at the outlet of the water bath used for quenching: 150°C Period maintained at 450°C: 45 seconds During the entire slow cooling period of 20 seconds or more, it is recognized that the steel clearly softened. (b) Effect of temperature at the exit of the quench (before reheating to 450 °C). The following results were obtained with other parameters fixed as follows: ● Total duration of slow cooling: 27 seconds ● Duration of holding at 450 °C: 45 seconds

[Table] It is observed that the quality for deep embossing is achieved when the exit temperature of the quench is below or equal to 350°C. Similarly, the aging tendency is significantly reduced below this temperature. (c) Effect of holding period at 450°C, ie overaging period. This parameter was studied with other parameters fixed as follows. ● Total period of gradual cooling: 30 seconds ● Temperature at the end of rapid cooling: 150 seconds

[Table] The best ductility and aging strength are the least
It can be seen that what is obtained for an overage period of 30 seconds. In the present invention, the expression "ordinary soft steel" corresponds to German standard DIN.U.St.12, and the expression "ultra mild steel (for embossing)" corresponds to German standard DIN.U.St.12.
It should be understood as equivalent to St.13.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of the various heat treatment steps that a steel strip undergoes in the method of the present invention. FIG. 2 is a diagram showing the effect of the present invention in FIG. 1.

Claims (1)

[Claims] 1. In a method for manufacturing a coated steel strip by immersion in a suitable metal bath maintained at a temperature of 500°C or higher, prior to the immersion, the steel strip is heated to a temperature higher than its recrystallization temperature. Heating to a temperature, holding at this temperature for a period of not less than 30 seconds, and then cooling to a temperature not lower than that of the metal bath, the strip is heated at the outlet of the bath to a temperature at which the metal layer covering it is hardened. The moment when the steel strip at a temperature above the recrystallization temperature starts to cool, it undergoes primary cooling and then strong and rapid cooling to a certain temperature below 475°C, but does not impair the flatness of the steel strip. , the period between leaving the metal bath and the moment when the coated metal layer hardens and quenching begins is not less than 20 seconds.
A method for producing coated steel strip, characterized in that the process takes no more than 100 seconds. 2 In the case of mild steel strip, strong and rapid cooling is 375
℃ and 475℃, then keeping the strip at the last temperature of cooling for at least 30 seconds, and then in a known manner to room temperature the strip undergoes a second cooling, less rapid than the previous one. A method according to claim 1, characterized in that the method comprises: 3. The steel strip is made of ultra-soft steel, the final temperature of intense rapid cooling is above 350°C, and then the steel strip is reheated in a suitable furnace to a temperature of about 400°C to 500°C. Heated, this strip steel has at least
A method according to claim 1, characterized in that it is held at this temperature for 30 seconds and then allowed to cool gradually to room temperature. 4. The steel strip is thin and has high strength and is subjected to intense and rapid cooling to a temperature below 300°C, preferably below 250°C, and then immediately thereafter subjected to a second, no faster cooling than before, to room temperature by known methods. A method according to claim 1, characterized in that it is subjected to cooling. 5. Any one of claims 1 to 4, characterized in that the bath in which the steel strip to be coated is immersed is composed of several metals or alloys having a melting point of 800°C or less. The method described in one. 6. The method according to claim 5, characterized in that the coating bath is composed of an alloy of aluminum and zinc. 7. The strong and rapid cooling of the strip at the outlet of the coating bath is carried out by quenching in a water bath maintained at a temperature above 75° C., preferably at its boiling point. The method described in any one of Items 1 to 6. 8. The strong and rapid cooling at the outlet of the coating bath is characterized in that it is carried out by injecting a fluid constituted by a homogeneous mist of water suspended in a gas, preferably air. A method according to any one of claims 1 to 6.