Method for continuous thermal treatment of a steel strip

Abstract

Disclosed is a method for continuous thermal treatment of a steel strip. The strip passes through consecutive thermal treatment chambers, is quickly cooled in at least one of the chambers by spraying liquid onto the strip, or by spraying a fluid made up of gas and liquid or spraying a combination of gas and liquid forming a mist. After quick cooling, a protective metal layer is deposited on the strip by dip coating. The cooling fluid strips iron oxides or other alloy elements contained in the steel to be treated, minimizing oxidation and reducing the oxides on the strip. Spray pressure and distance are chosen to facilitate the stripping property and the mechanical action of the sprayed fluid, reducing the layer of oxides on the strip. The temperature of the strip at the end of the cooling step is the temperature necessary for carrying out the desired treatment cycle.

Claims

1. A process for depositing a metal coating on a steel strip on a continuous processing line, to produce at least a high yield strength steel, wherein: the strip passes through successive heat treatment chambers, a rapid cooling of the strip, at a rate greater than 200 C./sec, is carried out in at least one of the chambers by spraying liquid onto the strip, or spraying a fluid composed of gas and liquid or spraying a combination of gas and liquid in a mist form, the fluid sprayed for the rapid cooling of the strip is a fluid having a pickling property with regard to iron oxides or oxides of other alloying elements contained in the steel to be treated, in order to limit oxidation of the strip and reduce the oxides that have been able to be formed on the strip in order to eliminate surface defects during a hot-dip metal coating operation, the fluid is sprayed under a pressure and at a distance from the strip such that the combined effect of the pickling property and of the mechanical action of the sprayed fluid reduces the layer of oxides at the surface of the strip, and wherein a temperature of the strip at the end of the rapid cooling is above 200 C. and is that needed for a subsequent treatment cycle; and after the end of the rapid cooling or the subsequent treatment cycle, a protective metal layer is deposited on the strip by hot-dip coating.

2. The process as claimed in claim 1, further comprising use of a system for controlling the parameters of the reduction process, in which the fluid is sprayed onto the strip in order to achieve the amounts of residual oxides that are compatible with a subsequent process or product quality.

3. The process as claimed in claim 1, wherein the cooling fluid is sprayed by nozzles onto the strip, further comprising the adjustment of the parameters for the cooling of the strip by adjusting the amounts of liquid injected onto the strip by each nozzle and for each section of the nozzle width in order to produce a theoretical cooling curve as a function of the metallurgical process to achieve.

4. The process as claimed in claim 1, wherein the liquid having a pickling property that is sprayed onto the strip is an acid solution having a pH of less than 5, the acid solution including formic or boric acid.

5. The process as claimed in claim 1, wherein the liquid sprayed onto the strip comprises additives, including one or more of surfactants or wetting agents that serve as corrosion inhibitors, including benzotriazole.

6. The process as claimed in claim 1, wherein the liquid is sprayed under a pressure of less than 1 bar and at a distance from the strip of between 40 and 250 mm.

7. The process as claimed in claim 1, wherein the liquid is sprayed under a pressure of greater than 5 bar and at a distance from the strip of between 40 and 250 mm.

8. The process as claimed in claim 1, wherein the heating zones located upstream of the rapid cooling zone are in an atmosphere that is not very reducing, with a hydrogen content of less than 5%, or in air, so that the formation of oxides is facilitated, the layer of oxides improving the efficiency of the heat exchanges in the heating chamber(s), and these oxides formed then being reduced by the spraying of the cooling fluid in order to achieve the amounts of residual oxides that are compatible with a subsequent process or product quality.

9. The process as claimed in claim 1, wherein the strip length cooled by the cooling fluid is adjusted as a function of the speed of the line or of the characteristics of the strip or of the inlet and outlet temperatures of the strip.

10. The process as claimed in claim 2, further comprising use of an algorithm for calculating the risk of formation of wrinkles at the surface of the strip in order to adjust the longitudinal and transverse cooling slopes.

11. The process as claimed in claim 2, wherein the cooling fluid is sprayed by nozzles onto the strip, further comprising the adjustment of the parameters for the cooling of the strip by adjusting the amounts of liquid injected onto the strip by each nozzle and for each section of the nozzle width in order to produce a theoretical cooling curve as a function of the metallurgical process to achieve.

12. The process as claimed in claim 2, wherein the liquid having a pickling property that is sprayed onto the strip is an acid solution having a pH of less than 5, the acid solution including formic or boric acid.

13. The process as claimed in claim 3, wherein the liquid having a pickling property that is sprayed onto the strip is an acid solution having a pH of less than 5, the acid solution including formic or boric acid.

Description

(1) The invention consists, apart from the arrangements set out above, of a certain number of other arrangements that will be mentioned more explicitly hereinbelow with respect to exemplary embodiments described with reference to the appended drawings, but which are in no way limiting. In these drawings:

(2) FIG. 1 is a schematic view of a continuous line, according to the prior art, for the heat treatment of a steel strip;

(3) FIG. 2 is a view similar to FIG. 1 of a continuous line, according to the invention, for the heat treatment of a steel strip;

(4) FIG. 3 is a front view of a vertical portion of the steel strip with checkerboard-type zones for a control of the spraying nozzles provided by a control algorithm; and

(5) FIG. 4 is a graphical representation of various cooling curves of the strip, the time being given on the abscissa and the strip temperature on the ordinate.

(6) FIG. 1 presents a vertical annealing-galvanizing line according to the prior art. It is understood that the same process may be carried out in a horizontal line.

(7) The steel strip 1 passes successively through a preheating chamber 2 then a heating chamber 3 on sets of rollers 4. In this example, the strip then passes through the chamber 5 which corresponds to a slow cooling, the chamber 6 which corresponds to a conventional or rapid cooling by jets of gas on the strip from cooling boxes 7, and the chamber 8 which is a hold chamber. The strip is conveyed by an atmosphere sheath 9 and immersed at one of its ends into a bath of molten zinc or metals 11 via a roller 10.

(8) The chambers for rapid cooling by spraying liquid onto the strip are isolated from the upstream and downstream chambers of the furnace by atmosphere separation seals. For the implementation of the process according to the invention, this tightness is reinforced in order to avoid the release of vapors, for example water and acid vapors present in the rapid cooling chamber, in particular by the use of seals 14, 17 (FIG. 2) as described in FR 2 903 122 or comparable technologies. The function of these seals is to separate the atmosphere of the wet cooling chamber from the upstream and downstream chambers and to limit the passage of an atmosphere containing vapors of acids or of chemical compounds used for reducing the oxides present at the surface of the strip. Atmosphere outlets 13, 16 (FIG. 2) make it possible to discharge the acid vapors to a retreatment system external to the cooling zone.

(9) It is also understood that the line implementing the process according to the invention is equipped with a circuit (not represented) for treating the cooling liquid of the type known for the cooling, and the separation of the chemical products formed by the reduction of the oxides and also of the optional foreign substances, but also with specific equipment (not represented) for controlling the composition of the cooling liquid, especially the pH value as a function of the condition of the strip and its degree of oxidation at the inlet of the cooling zone.

(10) The wet rapid cooling zone with acid or corrosive solutions present is made from materials that are resistant to these chemical compounds, in the liquid phase or in the vapor phase, especially stainless steels or synthetic materials for the feed and return pipework of the cooling products.

(11) Rapid cooling operations such as those carried out in the invention cause significant stresses that may lead to permanent deformations being produced at the surface of the product, these deformations possibly being unacceptable for the production of products of commercial quality.

(12) According to the invention, the portion of the strip present in the cooling zone is partitioned (FIG. 3) by the calculation along the length of the strip and its width, each of the boxes thus obtained is the subject of a determination of the stresses in the material caused by the cooling in order to verify whether these stresses are below the limit permissible by the material. On this subject, reference may be made to EP 1 994 188/WO 2007/096502 in the name of the applicant company. The result of this calculation is delivered to the computer (not represented) of the line in order to adjust the cooling parameters such as the speed of the cooling gas and the amount of water or liquid sprayed onto the strip. By this means, each portion of the strip is the subject of a cooling optimization calculation in order to meet the metallurgical objectives without causing to permanent deformation at the surface of the strip.

(13) FIG. 2 presents a vertical galvanizing line according to the invention. The chambers upstream and downstream of the rapid cooling zone 6 are unchanged, with respect to FIG. 1.

(14) The rapid cooling zone 6 is isolated from the upstream chamber 5 and downstream chamber 8 by seal 14 and 16 according to known technologies, in particular according to FR 2 809 418 with a gas outlet 13 and 15 intended to guarantee the absence of communication between the atmospheres of the wet cooling chamber 6 and the upstream and downstream chambers.

(15) A communication tunnel 17 between the chambers 5 and 8 upstream and downstream of the rapid cooling chamber 6 makes it possible to prevent communications of atmospheres between these chambers in the case where there is a pressure difference between the chambers 5 and 8.

(16) The rapid cooling of the strip 1 is obtained by spraying a liquid onto the strip, by a combination of spraying liquid through a series of nozzles (not visible) and atmosphere through an independent series of nozzles or by creating a mixture of atmosphere and of liquid through a series of combined nozzles. This apparatus is represented by the boxes 12 positioned along the strip over a vertical line, the strip preferably running vertically from top to bottom so that the gravity flow of the cooling liquid can take place at the coldest strip temperatures.

(17) Each of the cooling processes listed above is equipped with means for regulating their effectiveness which make it possible to control the coefficient of heat exchange with the strip as a function of its temperature, of the type of cooling curve to be achieved in order to obtain the desired metallurgical structure and to avoid the formation of surface defects such as wrinkles or buckles.

(18) FIG. 3 presents the operating principle of this system for controlling the cooling of the strip. Seen in front view is the portion of the strip 1 present in the rapid cooling zone 6 with the upper roller 18 and lower roller 19. On this strip section, a portion denoted by L corresponds to the zone of the cooling boxes. This length L is divided vertically into a plurality of segments L1, L2 . . . L7 in this example and horizontally into three portions: O for the operator side, C for the center and M for the motor side. This gives, in this example, the zones L4O, L4C and L4M. The number of horizontal and vertical zones is not limited, each zone may have a dimension different from the other zones in order to correspond to the arrangement of the cooling boxes, of irregularities such as in particular the presence of stabilizing rollers, or for enabling a greater precision of control, especially in the zones where the risk of formation of wrinkles or buckles on the surface of the strip is high.

(19) The cooling means are designed so as to correspond to the cutting into zones of the cooled portion of the strip, especially with control valves controlled by the control system of the line in order to adjust the pressure or the flow rate of the fluid as a function of the exchange coefficient to be obtained.

(20) The system for controlling the line comprises a set of algorithms for calculating the stresses induced in the material of the strip as a function of the desired cooling, for example for passing a strip from a temperature of 850 C. to 470 C. in around 1.5 seconds, and will optimize the cooling curve in order to limit the stresses in the strip during this cooling.

(21) FIG. 4 presents this type of cooling between 850 C. and 470 C. over a time t: the curve C1 shows small cooling slopes for the high temperatures close to 850 C. and larger slopes for temperatures close to 470 C.; the curve C2 shows a linear cooling slope between the starting temperature of 850 C. and the final temperature of 450 C.; N.B. or less if the thermal cycle makes it necessary; the curve C3 presents larger cooling slopes for the highest temperatures close to 850 C. and smaller slopes close to 470 C.

(22) The longitudinal cooling curve may thus be optimized in order to control the actuators, and the liquid spray nozzles, equipping the zones L1 to L7 in order to obtain the final result without causing to surface defects on the strip.

(23) Similarly, the transverse temperature profile of the strip, for example at the furnace inlet or cooling section inlet, may be integrated into the calculation in order to control the actuators and the nozzles of the transverse zones in order to compensate for a pre-existing profile or to deliberately create a desired temperature profile on the strip.

(24) Temperature measurement means (not represented) may be used upstream or downstream of the cooling zone by the control system of the furnace in order, especially, to compensate for a temperature level or profile existing at the inlet of the cooling zone or, by measurement at the outlet of this cooling zone, to modify the setpoints of the actuators for obtaining the required effect.

(25) According to one variant of implementation of the invention, the effectiveness of the pickling and of the reduction of the oxides obtained owing to the implementation of the process is taken into account. It becomes possible to let the heating zones, corresponding to the chambers 3 and 5, with atmospheres that are less developed, for example with a smaller content of hydrogen typically of less than 5%, and that are therefore less reducing, optionally even in air. The surface oxidation of the strip obtained during the heating is facilitated in these less reducing atmospheres, and has the effect of increasing the emissivity coefficient of the strip which increases the effectiveness of the radiant heating and makes it possible to reduce the size and the cost of the apparatus. Such a line will be more compact and therefore have a lower investment cost and a lower operating cost while enabling the production of improved steels with respect to the prior art.

(26) The invention may be used on an annealing line, even if the constraint of galvanization is not present. The advantages of the in-line pickling, and the possibilities of atmospheres that are less developed in the heating zones will however remain present in this type of apparatus.