Process for producing a coated steel strip

Abstract

The invention relates to a process for producing a coated steel strip comprising the steps of: (a) producing a steel strip, (b) surface cleaning the steel strip, (c) heat treating the steel strip to prepare for coating thereof, (d) hot dip coating the steel strip, wherein in step (c) the steel strip is heated to a target temperature which is below the recrystallization temperature and above the melt bath temperature of step (d), characterized in that in step (c) the steel strip is heated at a heating rate of at least 10 K / second.

Description

[0001] Method for producing a coated steel strip

[0002] The invention relates to a method for producing a coated steel strip according to the preamble of claim 1.

[0003] According to the current state of the art, strip material is typically produced from cast slabs that are hot-rolled and cooled in coils. During the hot rolling process, not only is the thickness reduced, but the cast structure is also converted into a rolled structure. In conjunction with targeted temperature control, the structure, or rather its composition and grain size, are adjusted. Hot rolling may be followed by a pickling process, which is usually followed by a cold rolling process. Following cold rolling, a recrystallization annealing process usually takes place to relieve the strain hardening caused by the forming process. A coating process can also be performed as a supplement or alternative.

[0004] WO 2020 / 079200 A1 discloses a method for producing a hot-formable steel flat product, comprising: a production step for producing a cold-rolled quenched and tempered steel flat product; a heating step for heating the quenched and tempered steel flat product; and a coating step for coating the heated quenched and tempered steel flat product using a hot-dip process. To reduce energy consumption for producing hot-formable steel flat products, the quenched and tempered steel flat product is heated in the heating step to a maximum temperature that is greater than or equal to a temperature of a molten bath used in the hot-dip process and lower than an austenitizing temperature of the cold-rolled quenched and tempered steel flat product.In this case, the heat-treated flat steel product heated in the heating step is fed to the coating step without an intermediate step or after adjusting the temperature of the heat-treated flat steel product to a coating temperature.

[0005] From the aforementioned WO 2020 / 079200 A1, it is known to perform a heat treatment / annealing of a cold-rolled steel prior to hot-dip coating, wherein the heat treatment temperature is below the austenitizing temperature but greater than or equal to the melt bath temperature. This process is intended to produce a hot-formable steel with the lowest possible energy consumption. The technology according to

[0006] WO 2020 / 079200 A1 is based on the finding that the recrystallization of the heated quench and temper steel flat product fed to the coating step and the associated adjustment of the mechanical properties of the quench and temper steel flat product are unnecessary for hot forming, since the properties of the finished quench and temper steel flat product are adjusted at the end of hot forming. Accordingly, in the production of cold-rolled (e.g., manganese-boron alloyed) quench and temper steel flat products, any recrystallization annealing at a temperature greater than or equal to the austenitizing temperature of the cold-rolled quench and temper steel flat product (i.e., T > Ac3) can be dispensed with.

[0007] With increasing process reliability in the production of thin hot-rolled strip from 3 mm to 0.5 mm thick, the aim is generally to eliminate the cold rolling step. However, it must be ensured that the material properties, in particular the strength values ​​such as yield strength and tensile strength, are maintained. However, the aforementioned process according to WO 2020 / 079200 A1 does not take into account the increase in strength that occurs during work hardening during cold rolling and thus deviates from the material properties achieved by hot rolling. Thus, a disadvantage associated with the process according to WO 2020 / 079200 A1 is that the end product produced thereby does not have the anticipated material properties.In detail, when using the process according to WO 2020 / 079200 A1, the material strength achieved after production of the coated strip is not adjusted with sufficient accuracy, since the forming influences are not taken into account or eliminated by cold rolling and any necessary toughness is not present.

[0008] EP 2 843 077 B1 discloses a method for producing a galvanized steel sheet for hot stamping. This method comprises heat-treating a hot-rolled pickled steel sheet or cold-rolled steel sheet in a reducing atmosphere, followed by galvanizing the steel sheet. The heat treatment of the steel sheet is carried out at a temperature of 500°C to 700°C for a duration of 30 to 270 seconds, with the heating rate consistently being 8°C / second.

[0009] The invention is based on the object of achieving an improvement in the material properties of a coated steel strip in a simple and inexpensive manner when producing it.

[0010] This object is achieved by a method for producing a coated steel strip having the features specified in claim 1. Advantageous developments of the invention are defined in the dependent claims.

[0011] A method according to the present invention is for producing a coated steel strip, and comprises the steps:

[0012] (a) Manufacturing a steel strip,

[0013] (b) Surface cleaning of the steel strip,

[0014] (c) heat treatment of the steel strip in preparation for its coating,

[0015] (d) hot-dip coating the steel strip, wherein in step (c) the steel strip is heated to a target temperature which is below the recrystallization temperature and above the melt bath temperature of step (d), and wherein in step (c) the steel strip is heated at a heating rate of at least 10 K / second.

[0016] The method according to the invention is based on the essential finding that, thanks to the heating rate, which during the heat treatment of the steel strip in step (c) assumes a value of at least 10 K per second, the steel strip's yield point and tensile strength are increased. What is important for the present invention is that such an increase in yield point and tensile strength for the steel strip can be achieved without the addition of usually expensive alloying components. Instead, this increase in yield point and tensile strength according to the present invention is based, among other things, on the fact that precipitates of microalloying elements, such as vanadium (V) and / or niobium (Nb), are not fully converted during the heat treatment of the steel strip in step (c), thus leading to a percentage increase in the content of these microalloying elements in the structure of the steel strip.It should also be noted that the influence on the breaking strength of the steel strip is variable.

[0017] In connection with the aforementioned increase in yield strength, it should be noted that these increases in yield strength can occur in the range of 5 to 20%, namely for both steels with a pronounced yield strength and steels without a pronounced yield strength. For steels without a pronounced yield strength, a smaller increase occurred than for steels with a pronounced yield strength.

[0018] At this point, it is specifically pointed out that it is important for the method according to the present invention that a cold rolling process is omitted before step (d), in which a hot-dip coating of the steel strip is carried out.

[0019] For example, in step (a) of the method according to the invention, no cold rolling of the steel strip is carried out.

[0020] In an advantageous development of the method according to the invention, the heating rate for the steel strip in step (c) can be between 10 and 40 K / second. Advantageously, the heating rate for the steel strip can be between 15 and 35 K / second.

[0021] In an advantageous development of the method according to the invention, it can be provided that in step (c), the target temperature to which the steel strip is heated is between 400°C and 650°C. This target temperature can preferably be between 520°C and 620°C. In any case, it is advantageous for the heat treatment of the steel strip according to step (c) if the target temperature to which the steel strip is heated is maintained for a predetermined holding time. For example, this predetermined holding time can be between 3 and 30 seconds, or between 5 and 20 seconds.

[0022] In an advantageous development of the method according to the invention, it can be provided that the heat treatment of the steel strip according to step (c) takes place entirely in a protective atmosphere. The heat treatment zone is expediently sealed against air by a lock passage as the steel strip enters this area, where the protective atmosphere prevails. Such a lock passage can be equipped with contact brushes for the purpose of sealing against air.

[0023] The heat treatment takes place entirely in a protective atmosphere. The heat treatment zone is sealed against air by a lock passage (with contact brushes) as the strip enters the area.

[0024] In an advantageous development of the method according to the invention, it can be provided that a protective atmosphere present during step (c) contains at least nitrogen. Additionally or alternatively, the aforementioned protective atmosphere can be provided with a hydrogen content of between 10 and 50% and a dew point between -50°C and -10°C.

[0025] In an advantageous development of the method according to the invention, the heat treatment of the steel strip according to step (c) can be carried out in a protective atmosphere, and a slight overpressure is set for this purpose. Such an overpressure can expediently be 2 mbar compared to the ambient pressure.

[0026] With regard to all of the aforementioned developments of the method according to the invention in which a protective atmosphere is provided for carrying out step (c), it should be specifically noted at this point that, in the course of the heat treatment after this step (c), at least the cooling of the steel strip to the bath temperature also takes place. In other words, in step (c), at least the cooling of the steel strip to the bath temperature takes place in a protective atmosphere.

[0027] Furthermore, with regard to the aforementioned protective atmosphere, it should be noted that, according to an advantageous development of the method according to the invention, this atmosphere is maintained in interaction with the steel strip until the steel strip enters or dips into a molten bath in step (d), in order to then carry out the hot-dip coating of the steel strip therein. According to an advantageous development of the method according to the invention, the sealing of a reduction chamber in which the heat treatment according to step (c) is carried out is achieved by entering a coating bath provided downstream of this.

[0028] In an advantageous development of the method according to the invention, it can be provided that in step (d) the bath temperature is in the range from 440 to 460 °C when coating with zinc, in the range from 360 to 460 °C when coating with zinc-magnesium or 580 °C when coating with Galvalum.

[0029] According to a further advantageous development of the method according to the invention, it can be provided that in step (b) the surface cleaning of the steel strip takes place at a temperature which is below 100°C.

[0030] In an advantageous development of the method according to the invention, it can be provided that in step (a) the steel strip is produced by hot rolling, wherein following step (a) the steel strip is cooled to a temperature which is above the ambient temperature before carrying out steps (b) and (c).

[0031] According to a further advantageous development of the method according to the invention, it can be provided that when performing step (c), at least one parameter is set to a predetermined value, wherein this parameter is selected from a group consisting of the throughput speed of the steel strip, the target temperature of the steel strip according to step (c), and / or the holding time of the steel strip. In this development of the method according to the invention, it can be provided that the at least one parameter is set or maintained at a predetermined value in a controlled manner using a control loop.

[0032] In an advantageous development of the method according to the invention, it can be provided that no forming of the steel strip takes place between steps (b) and (c). In the context of the present invention, this means that the steel strip, after its surfaces have been cleaned in step (b), is then subjected to the heat treatment according to step (c). As already explained elsewhere above, such a heat treatment may also include cooling of the steel strip.

[0033] Furthermore, it is emphasized again at this point that the method according to the present invention is characterized, among other things, in that in step (a), in which the steel strip is produced, no cold rolling of the steel strip takes place.

[0034] The present invention relates to the production of rolled steel strips, eliminating the need for thickness reduction through cold rolling and adjusting the final thickness of the steel strip during hot rolling. Fine calibration of the final thickness can be achieved in the subsequent coating process, with a reduction range of up to 15% (preferably up to 10%).

[0035] Further advantages that can be achieved by means of a method according to the present invention result from the following features:

[0036] The material properties of a hot-rolled steel strip can be further improved after hot rolling, particularly thanks to the rapid heating rate of at least 10 K / s; additional rolling steps with a thickness reduction are not required.

[0037] Optimizing the material properties of the steel strip produced using the method according to the invention while simultaneously shortening the manufacturing process.

[0038] Economic and energy savings.

[0039] Reduction of CO2 footprint, enabling the production of “green” steel.

[0040] In the production of strip material, not only geometric properties and qualities, but also material properties such as hardness, tensile strength, formability, etc. are essential parameters that define the further use of the strip.

[0041] The invention will be explained below by way of example with reference to the attached figure using preferred embodiments, wherein the features explained below can represent an advantageous or further developing aspect of the invention, both individually and in combination of at least two of these features. They show:

[0042] Fig. 1 is a flowchart of an embodiment of a method according to the invention, and

[0043] Fig. 2 is a flowchart of a further embodiment of the method according to the invention.

[0044] Preferred embodiments of a method according to the invention for producing a coated steel strip are explained below with reference to Figs. 1 and 2.

[0045] A method according to one embodiment of the present invention comprises several steps that are carried out sequentially to produce a coated steel strip. Specifically, this method comprises the following steps:

[0046] (a) Manufacturing a steel strip,

[0047] (b) Surface cleaning of the steel strip,

[0048] (c) heat treatment of the steel strip in preparation for its coating,

[0049] (d) Hot-dip coating of the steel strip.

[0050] With regard to step (a) of the above-mentioned process according to the invention, it is pointed out that the production of a hot-rolled strip or

[0051] Steel strip can be produced by continuous casting and subsequent hot rolling of the continuous cast product.

[0052] Furthermore, step (a) according to the method according to the invention can also provide for a targeted cooling of the steel strip in order to achieve a defined microstructure.

[0053] With regard to step (b) of the aforementioned method according to the invention, it should be noted that the surface cleaning of the steel strip provided for in this case can preferably be carried out by pickling. Alternatively, other methods for cleaning the steel strip are also possible. In any case, the surface temperature of the steel strip during the surface cleaning according to step (b) is below 100°C.

[0054] With regard to the above-mentioned inventive method, it should be emphasized that in step (c), the steel strip is heated to a target temperature that is below the recrystallization temperature and at least equal to the melt bath temperature of step (d). Preferably, the target temperature to which the steel strip is heated in step (c) is above the melt bath temperature of step (d). Furthermore, it should be emphasized that in step (c), the steel strip is heated at a heating rate of at least 10 K / second.

[0055] With regard to the heat treatment according to step (c) of the process according to the invention, the following further aspects are specifically mentioned at this point:

[0056] The heating rate depends on the alloy and the presets. This heating rate can be in the range of 10 to 50 K / second, preferably in the range of 10 to 40 K / second, and most preferably in the range of 15 to 35 K / second. And / or:

[0057] The temperature window (i.e., the target temperature to which the steel strip is heated) is in the range between 400°C and 650°C, preferably in the range between 520°C and 620°C. And / or:

[0058] The target temperature to which the steel strip is heated (also referred to as "holding time" in the context of the present invention) is between 3 and 30 seconds, preferably between 5 and 20 seconds. And / or:

[0059] After heating to the target temperature and a predetermined holding time, the steel strip can be cooled, in particular moderately, to a bath temperature, for example, to a temperature of up to approximately 460 °C. And / or: After the steel strip has reached such a bath temperature, step (d) is then carried out to coat the steel strip. And / or:

[0060] During the cooling process of the steel strip, it may be provided that the steel strip is heated again in a targeted manner, particularly moderately, up to a maximum of 600 °C. To achieve such reheating, a radiant heater may be used, which is provided or installed along at least part of the path along which the steel strip undergoes heat treatment. And / or:

[0061] The cooling of the steel strip to the bath temperature can take place within a period of between 10 and 100 seconds. This cooling of the steel strip to the bath temperature can preferably take place within a period of between 20 and 60 seconds. And / or:

[0062] At least the cooling of the steel strip to the bath temperature can be done in a protective atmosphere. And / or:

[0063] The heat treatment of the steel strip according to step (c) can be carried out entirely in a protective atmosphere. To avoid repetition, reference should be made to the aspects of the protective atmosphere already explained elsewhere above. And / or:

[0064] In connection with maintaining a target temperature for the steel strip in a protective atmosphere, it should be emphasized according to the invention that the creation of a protective atmosphere following the heating zone leads to the removal / reduction of residual oxides in the near-surface area of ​​the steel strip passing through such an area or zone with a protective atmosphere. Such a reduction then improves the adhesion of the coating to the surface of the steel strip, which is subsequently applied in step (d). And / or:

[0065] The protective atmosphere is maintained until the steel strip enters the molten bath (see step d)

[0066] With regard to step (d) of the above-mentioned process according to the invention, it is pointed out that the bath temperature is in the range of 440 to 460 °C when coating with zinc, in the range of 360 to 460 °C when coating with zinc-magnesium, or 580 °C when coating with Galvalum.

[0067] According to a further embodiment of the method according to the invention, the aforementioned steps (c) and (d) can be combined into a continuous process. For the purposes of the present invention, this means that when these steps (c) and (d) are combined, any type of forming between strip cleaning and heating is dispensed with. An optimal throughput speed for the two continuously running steps (c) and (d) is between 30 and 180 m / min. The residence time in a zone in which steps (c) and (d) are carried out is between 1 and 100 seconds, preferably between 1 and 60 seconds.

[0068] According to yet another embodiment of the method according to the invention, the aforementioned steps (b), (c), and (d) can be combined into a continuous process. In the context of the present invention, this means that after the steel strip has been heated up and the (target) temperature has been maintained, the steel strip is then fed into the coating chamber as quickly as possible. The preferred distance from the end of the heating zone, in which step (c) is carried out, to the entry of the steel strip into the melting pot or molten bath for carrying out step (d) is between 10 and 50 m. If the steel strip is also reduced in size in step (c), the distance is somewhat different depending on the protective atmosphere and its settings.

[0069] In the latter embodiment of the method according to the invention, the throughput speed of the steel strip is set to between 30 and 180 m / min. This is due to the continuous process through which the steel strip undergoes.

[0070] Fig. 2 shows a flowchart for another preferred embodiment of the method according to the invention. In contrast to the method of Fig. 1, a further step (e) is performed following step (d), in which the steel strip is temper-passed and / or stretch-straightened. In the case of temper-passing, this can be performed close to the surface, for example, in the range of 0.2 to 10%.

[0071] With regard to the last-mentioned embodiment of the inventive method according to Fig. 2, it is specifically noted that in step (e), the degree of skin-passing can be <10%. Preferably, the degree of skin-passing can also be <5%. With regard to the last-mentioned embodiment of the inventive method according to Fig. 2, the following additional aspects are specifically noted at this point:

[0072] If in step (e) the deformation of the steel strip is > 3%, then the temperature of the steel strip is < 60°C. In other words, for deformation steps of > 3%, the inlet temperature of the steel strip into the stand should generally not be > 60°C.

[0073] Steps (b), (c), (d), and (e) can be combined into a continuous process. In the context of the present invention, this means that after coating the steel strip, a defined surface structure is applied. A skin-pass and / or stretch-leveling process advantageously improves the flatness of the steel strip and / or eliminates pronounced yield points.

[0074] In connection with a skin-pass and / or stretch-leveling process according to step (e), there is a possibility that such forming may result in an increase in the temperature of the steel strip. In view of this, it is advantageous if the temperature at the coater is <45°C. This can also be achieved through additional cooling steps, for example, through the use of cooling rolls.

[0075] According to yet another embodiment, the method according to the invention can be provided for incorporating the hot rolling of the steel strip into one of the aforementioned endless processes, in which steps (c) and (d), or steps (b), (c) and (d), or steps (b), (c), (d) and (e) are suitably combined with one another as explained. The residual temperature of the hot strip is utilized to cool the steel strip to a level greater than the ambient temperature in order to keep the heating processes prior to surface cleaning in step (b) and for heating in step (c) as energy-efficient as possible. In this context, it is specifically noted that coupling with a strip casting plant is particularly suitable for this purpose, due to similar process speeds.

[0076] Finally, it is pointed out that within the scope of the present invention, it is possible to preset or regulate the following parameters as variables in order to adapt to individual process parameters and / or to set desired strength increases for the steel strip: o Throughput speed of the steel strip, and / or o Temperature level in step (c), and / or o Holding and reduction times in step (c):

Claims

Patent claims 1. Method for producing a coated steel strip , comprising the steps: (a) Manufacturing a steel strip, (b) Surface cleaning of the steel strip, (c) heat treatment of the steel strip in preparation for its coating, (d) hot-dip coating the steel strip, wherein in step (c) the steel strip is heated to a target temperature which is below the recrystallization temperature and above the melt bath temperature of step (d), characterized in that in step (c) the steel strip is heated at a heating rate of at least 10 K / second.

2. A method according to claim 1, characterized in that in step (c) the heating rate for the steel strip is between 10 and 40 K / second.

3. A method according to claim 1, characterized in that in step (c) the heating rate for the steel strip is between 15 and 35 K / second.

4. Method according to one of the preceding claims, characterized in that in step (c) the target temperature to which the steel strip is heated is between 400°C and 650°C, preferably between 520°C and 620°C.

5. Method according to claim 4, characterized in that the target temperature to which the steel strip is heated is maintained for a predetermined holding time which is between 3 and 30 seconds.

6. Method according to claim 4, characterized in that the target temperature to which the steel strip is heated is maintained for a predetermined holding time which is between 5 and 20 seconds.

7. Method according to one of the preceding claims, characterized in that in step (c) the steel strip is cooled to a bath temperature after heating to the target temperature and expiry of a predetermined holding time, wherein after reaching the bath temperature for the steel strip, step (d) is carried out in order to coat the steel strip.

8. Method according to claim 7, characterized in that the steel strip is heated again in the course of its cooling, preferably using radiant heating, to a temperature of up to a maximum of 600 °C.

9. A method according to claim 7 or 8, characterized in that the cooling of the steel strip to the bath temperature takes place within a period of between 10 and 100 seconds.

10. A method according to claim 7 or 8, characterized in that the cooling of the steel strip to the bath temperature takes place within a period of between 20 and 60 seconds.

11. The method according to claim 9 or 10, characterized in that in step (c) at least the cooling of the steel strip to the bath temperature takes place under a protective atmosphere.

12. Method according to one of the preceding claims, characterized in that the heat treatment of the steel strip according to step (c) takes place entirely under a protective atmosphere.

13. A method according to claim 11 or 12, characterized in that the protective atmosphere contains at least nitrogen.

14. Method according to one of claims 11 to 13, characterized in that the protective atmosphere has a hydrogen content of between 10 and 50% and a dew point of between -50°C and -10°C.

15. Method according to one of claims 11 to 14, characterized in that an overpressure is set for the protective atmosphere, preferably that the overpressure is 2 mbar compared to the ambient pressure.

16. The method according to any one of the preceding claims, characterized in that in step (d) the bath temperature is in the range of 440 to 460 °C when coating with zinc, in the range of 360 to 460 °C when coating with zinc-magnesium or 580 °C when coating with Galvalum.

17. Method according to one of the preceding claims, characterized in that following step (d) a further step (e) is carried out: skin-passing and / or stretch-leveling of the steel strip.

18. The method according to claim 17, characterized in that in step (e) the degree of tempering is < 10%, preferably < 5%.

19. The method according to claim 17 or 18, characterized in that, if in step (e) a deformation of the steel strip is > 3%, then a temperature of the steel strip is < 60°C.

20. Method according to one of the preceding claims, characterized in that steps (c) and (d) are combined to form a continuous process.

21. A method according to claim 20, characterized in that the steel strip passes through steps (c) and (d) at a speed between 30 and 180 m / min.

22. Method according to claim 20 or 21, characterized in that a residence time of the steel strip in a zone in which steps (c) and (d) are carried out is between 1 second and 100 seconds, preferably between 1 and 60 seconds.

23. Method according to one of claims 1 to 19, characterized in that steps (b), (c) and (d) are combined to form a continuous process.

24. Method according to claim 23, characterized in that a throughput speed of between 30 and 180 m / min is set for the steel strip.

25. A method according to claim 23 or 24, characterized in that a distance from the end of a heating zone in which step (c) is carried out to the entry of the steel strip into a bath device in or with which step (d) is carried out is between 10 and 50 meters.

26. Method according to one of claims 17 to 19, characterized in that steps (b), (c), (d) and (e) are combined to form a continuous process.

27. Method according to one of claims 17 to 26, characterized in that after the coating of the steel strip according to step (d), the application of a defined surface structure, the improvement of the flatness and / or the elimination of pronounced yield points of the steel strip takes place in step (e).

28. Method according to one of the preceding claims, characterized in that in step (b) the surface cleaning of the steel strip takes place at a temperature which is below 100°C.

29. Method according to one of the preceding claims, characterized in that in step (a) the steel strip is produced by hot rolling, wherein following step (a) the steel strip is cooled to a temperature which is above the ambient temperature before carrying out steps (b) and (c).

30. Method according to one of the preceding claims, characterized in that when carrying out step (c) at least one parameter is set to a predetermined value, this parameter being selected from a group formed by the passage speed of the steel strip, the target temperature of the steel strip according to step (c) and / or the holding time of the steel strip.

31. Method according to claim 30, characterized in that the at least one parameter is adjusted or maintained at a predetermined value using a control loop.

32. Method according to one of the preceding claims, characterized in that no deformation of the steel strip takes place between steps (b) and (c).

33. Method according to one of the preceding claims, characterized in that in step (a) no cold rolling of the steel strip takes place.

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