Method for a continuous heat treatment of a steel strip, and installation for dip coating a steel strip

Abstract

The invention relates to a method and a system (10) for hot-dip coating a high-strength steel strip (102) moving in a transport direction (T), in particular of oxidation-sensitive AHSS grades, comprising a hot-dip bath (104) into which the steel strip (102) can be immersed for coating, wherein - viewed in the transport direction (T) of the steel strip (102) - upstream of the hot-dip bath (104) at least a first heating chamber (107) with at least one inductor (108, 109), a rapid cooling chamber (116) and a holding chamber (117) for partitioning the steel strip (102) are arranged.

Description

[0001] The invention relates to a method for the continuous heat treatment of a steel strip, and a system for hot-dip coating a high-strength steel strip moving in a transport direction according to the preamble of claim 7.

[0002] Conventional high-strength strip steels contain manganese, silicon, and / or aluminum as alloying elements. During recrystallization annealing prior to hot-dip coating, these alloying elements diffuse towards the surface. Since these alloying elements are highly oxygen-reactive, they are almost inevitably oxidized if they are located at shallow depths within the strip or on its surface. The base material, iron, is not oxidized in this process. This is therefore referred to as selective oxidation. The oxides formed on the surface or at shallow depths impair the wettability of the steel strip with a coating metal, e.g., in molten form, resulting in defects (bare spots) or poor adhesion of the metallic coating.

[0003] In view of the aforementioned problem of selective oxidation, a known countermeasure according to the prior art is so-called pre-oxidation, in which these oxides are covered by an FeO layer and subsequently reduced to iron (Fe). This creates a pure Fe layer on the surface of a steel strip to be coated, to which a metallic coating adheres well. However, with some materials, there is a tendency for adhesion to fail at greater depths, as the selectively formed oxides, such as MnO, create a passive layer to which the adhesion of the pure Fe layer is poor.

[0004] EP 1 819 840 B1 and EP 2 732 062 B1 each disclose a method and a plant for hot-dip coating a strip of high-strength steel, as well as a method for continuous heat treatment of a steel strip according to the preamble of claim 1 of the present patent application.

[0005] From WO 2016 / 001888 A2, a system for hot-dip coating a steel strip moving in a transport direction is known according to the preamble of claim 7.

[0006] EP 3 170 913 A1 discloses a method and a device for treating a steel strip, wherein the steel strip is heated in the area of ​​a first section by indirect heating means in the form of resistance heating, induction heating or radiant tube heating.

[0007] From EP 1 936 000 A1 a method for a continuous heat treatment of a steel strip is known, in which the steel strip is heated at temperatures above 300 °C by indirect heating means and under a predetermined protective atmosphere.

[0008] The invention is based on the objective of suppressing selective oxidation in preparation for coating steel strips to such an extent that these oxides no longer interfere with the subsequent application of a metallic coating to a surface of the steel strip, and in the course of this, heating the steel strip in an energy-optimized manner.

[0009] This problem is solved by a method having the features of claim 1, as well as by a system having the features specified in claim 7. Advantageous embodiments of the invention are defined in the dependent claims.

[0010] A method according to the invention serves for the continuous heat treatment of a high-strength steel strip, in particular of oxidation-sensitive AHSS grades, in which the steel strip is moved through at least one furnace. The method comprises the following steps: i) Heating the steel strip (102) to a temperature of at least 600 °C by a direct fired furnace (DFF) (106) in an exhaust gas atmosphere with a lack of air, ii) Heating the steel strip (102) to a temperature between 700 °C and 750 °C by an inductor in a hydrogen-containing atmosphere, iii) Heat treatment of the steel strip in an oxidizing atmosphere with an oxygen content of 2–5% O₂ to form iron oxide layers on the surfaces of the steel strip, wherein this heat treatment has a duration of 5–20 seconds, iv) Heating the steel strip to a temperature of up to 950 °C in an atmosphere containing hydrogen (H₂), water vapor, and residual nitrogen (N₂), wherein the steel strip is held at a temperature of up to 950 °C for a duration of ≥ 40 seconds. v) Rapid cooling of the steel strip to a temperature in the range between 200 °C and 450 °C,under a hydrogen-containing atmosphere, wherein the steel strip is subsequently heated to a partitioning temperature of at least 300 °C, preferably 320 °C, in an atmosphere containing ≥ 20% hydrogen (H₂) and the remainder nitrogen (N₂), the steel strip remaining in this atmosphere for a duration of ≥ 30 seconds, and vi) applying a metallic coating to at least one surface of the steel strip.

[0011] In the aforementioned method according to the invention, a coating device can be provided for step vi) which – viewed in the transport direction of the steel strip – is arranged downstream of a furnace device. Such a coating device can be a hot-dip coating bath or in the form of a PVD (= P physical V apor DThe coating unit (eposition) is designed to apply a metallic coating to at least one surface of the steel strip, preferably to the surfaces of the steel strip on its top and bottom sides. If the coating unit is designed as a hot-dip bath, it is advantageous if the steel strip is immersed in it, particularly with zinc.

[0012] In an advantageous further development of the method according to the invention, it can be provided that in step iv) the steel strip is heated by an RTF furnace section (RTF = Radiant Tube Furnace), preferably that the steel strip is additionally heated at the beginning of step iv) by a transverse field inductor with a heating rate of at least 50 K / s to at least 820 °C. Heating the steel strip by means of the transverse field inductor at the beginning of step iv) has the advantage that, due to the aforementioned high heating rate, the steel strip is brought to the desired holding temperature more quickly.

[0013] The invention also provides a system for hot-dip coating a high-strength steel strip, particularly of oxidation-sensitive AHSS grades, moving in a transport direction. Such a system comprises a hot-dip bath into which the steel strip can be immersed for coating. Viewed in the transport direction of the steel strip, upstream of the hot-dip bath are arranged at least a first heating chamber with at least one inductor, preferably in the form of a transverse field inductor, a rapid cooling chamber, and a holding chamber for partitioning the steel strip. Advantageously, an inductor can be provided in the inlet area of ​​the holding chamber or upstream thereof, and / or a further inductor can be provided in the outlet area of ​​the holding chamber.In any case, upstream of the first heating chamber, a preheating chamber with a directly heated preheater (DFF = Direct Fired Furnace) is arranged in the transport direction of the steel strip.

[0014] The invention is based on the essential insight that the heating of the steel strip to a temperature of up to 950 °C should be carried out as quickly as possible, with the subsequent holding time or residence time for the steel strip at a predetermined temperature being short. This has the advantage that selective oxidation can be (largely) suppressed during heating. For this purpose, it is generally advantageous to set the atmosphere in which the steel strip is heated to be as reducing as possible, namely with the highest possible or maximum hydrogen content and a minimum dew point, while simultaneously setting a heating rate of > 50 K / s. This applies to step iv) of the process according to the invention.

[0015] In an advantageous embodiment of the invention, the temperature to which the steel strip is heated and preferably maintained in step iv) is below 950 °C and, for example, assumes a value of 945 °C, 940 °C, 935 °C, 930 °C, 925 °C, or 920 °C. It is also possible for this temperature, to which the steel strip is heated and preferably maintained in step iv), to assume a value between the aforementioned example values, for example, a value of 942 °C or other intermediate values.

[0016] To effectively suppress oxidation on the surfaces of the steel strip above temperatures of approximately 700 °C, the residence time of the steel strip above 750 °C must be as short as possible. According to the invention, when heating the steel strip to a temperature of up to 950 °C, it has been shown that, with an atmosphere containing at least 20% hydrogen, preferably the remainder nitrogen, and a dew point of less than or below -40 °C, residence times of up to 180 seconds are permissible. Depending on the properties of the steel strip to be coated, this residence time can also be shorter than 180 seconds. In any case, during the holding or residence time, the material of the steel strip is partially or completely converted into austenite.

[0017] In the process according to the invention, the steel strip is rapidly cooled to < 500 °C under a hydrogen-containing atmosphere, as defined in step v). For such rapid cooling, the cooling rate can be at least 40 K / s, for which a high hydrogen content is advantageous. It is advantageous to use a hydrogen-rich protective gas with a hydrogen content of, for example, 50% in the heating section and / or in the slow cooling section to prevent oxidation.

[0018] In an advantageous embodiment of the invention, it can be provided that the steel strip is cooled to a temperature between 200 °C and 450 °C during the rapid cooling process in step v). In this case, it is further advantageous that, before step c) or following step v), the steel strip is heated to a partitioning temperature of at least 300 °C, preferably 320 °C, in an atmosphere containing ≥ 20% hydrogen (H₂) and the remainder nitrogen (N₂), and that the steel strip remains in this atmosphere for a duration of ≥ 30 seconds.

[0019] In an advantageous embodiment of the invention, slow cooling of the steel strip can be carried out. In the process according to the invention, such slow cooling takes place between steps iv) and v). In any case, it is advantageous if such slow cooling is carried out under a hydrogen-containing atmosphere, which, for example, contains at least 20% hydrogen and has a dew point of < -40 °C. Furthermore, it is advantageous if this atmosphere contains residual nitrogen in addition to the hydrogen. In any case, it is important or advantageous for the slow cooling that the ferrite + austenite mixed-phase region is traversed with slow cooling, depending on the alloy, down to 750 °C, in order to establish a defined austenite content. Therefore, the time of slow cooling with regard to the oxidation of Si is part of the aforementioned residence time.

[0020] In an advantageous embodiment of the invention, further process steps can be provided for the continuous heat treatment of the steel strip, which may, for example, involve reheating and / or holding the steel strip at a specific temperature. These possible further process steps are carried out at temperatures above 600 °C and are therefore irrelevant with regard to the oxidation of silicon. While a high hydrogen content is not required, it is also not detrimental, so that these further process steps can generally be carried out in the same atmosphere as the preceding rapid cooling.

[0021] Preferred embodiments of the invention are described in detail below with reference to a schematically simplified drawing. The drawing shows: Fig. 1 a general side view of a system according to the invention, Fig. 2 a general side view of a system according to a further embodiment, Fig. 3 a general side view of a system according to a further embodiment, Fig. 4 the temperature profile for a steel strip during treatment in the system of Fig. 3 Fig. 5 shows a tabular overview of parameters for a possible operating mode of a method according to the invention, Fig. 6 shows the temperature profile for a steel strip during the operating mode of Fig. 5 Fig. 7 shows a tabular overview of parameters of a possible operating mode of a method according to the invention in a further embodiment, Fig. 8 shows the temperature profile for a steel strip during the operating mode of Fig. 7 Fig. 9 shows a tabular overview of parameters of a possible operating mode of a method according to the invention in a further embodiment, Fig. 10 shows the temperature profile for a steel strip during the operating mode of Fig. 9 Fig. 11 shows a tabular overview of parameters of a possible operating mode of a method according to the invention in a further embodiment, and Fig. 12 shows the temperature profile for a steel strip during the operating mode of Fig. 11 .

[0022] The following are, with reference to the Fig. 1 bis 12 A preferred embodiment of a method according to the invention for the continuous heat treatment of a steel strip 102 and an apparatus 10 according to the invention is explained. Identical features in the drawing are identified by the same reference numerals. It should be noted that the drawing is simplified and, in particular, not to scale.

[0023] Fig. 1 Annex 10 shows a simplified side view of this process. This Annex 10 is a continuous hot-dip galvanizing line (CGL) in which a steel strip 102 undergoes heat treatment in various steps or chambers, followed by the application of a hot-dip galvanizing treatment to at least one surface of the steel strip, preferably to all surfaces, in a hot-dip galvanizing bath 104. Fig. 1 Designated as a "zinc pot", a metallic coating is applied, preferably in the form of a zinc layer. Accordingly, the hot-dip bath 104 is filled with liquid zinc.

[0024] For the heat treatment of the steel strip 102, the system 10 comprises several chambers through which the steel strip 102 is successively passed for heating and cooling before being immersed in the hot-dip bath 104 for the application of the zinc coating. The individual chambers of the system 10 are as follows: - Kammer 1: Preheating chamber, directly fired; in Fig. 1 labelled "105"; - Kammer 2: First heating chamber for rapid heating, optionally equipped with pre-oxidation; in Fig. 1 labelled "107"; - Kammer 3: Second heating chamber, heated by radiant tubes, serves for slow heating and temperature retention; in Fig. 1 labelled "112"; - Kammer 4: Slow cooling chamber; in Fig. 1 labelled "115"; - Kammer 5: Rapid cooling chamber; in Fig. 1 labelled "116"; - Kammer 6: Chamber for partitioning or over-aging; in Fig. 1 labelled "117".

[0025] Chambers 1-6 will subsequently be referred to as the first to sixth chamber, corresponding to these numbers.

[0026] The simplified side view according to Fig. 1 This illustrates that the steel strip 102 is guided in a transport direction T along individual strip paths 1-24 through the aforementioned chambers 1-6. The steel strip 102 is fed into the first chamber 105 through an inlet, then through the second to fifth chambers, and finally exited through an outlet at the end of the sixth chamber 117 for subsequent immersion in the melt bath 102. Openings or passages are formed between the adjacent chambers, through which the steel strip 102 is guided (further) in the transport direction T.

[0027] The individual chambers according to the embodiment of Fig. 1 are explained separately below: The first chamber, or preheating chamber 105, is equipped with at least one directly fired preheater or furnace section (DFF) with which the steel strip 102 can be heated to a temperature of at least 600 °C. In this respect, the first chamber 105 fulfills the function of a preheating chamber. The first chamber 105 comprises strip paths 1 and 2. The first chamber 105 serves in particular for the cost-effective heating of less oxidation-sensitive products or steel strips 102. The second chamber 107 forms a first heating chamber for the rapid heating of the steel strip 102 and is equipped for this purpose with a first inductor 108 (in Fig. 1 (also referred to as "inductor 1") and with a second inductor 109 (in Fig. 1 (also referred to as "inductor 2"). Viewed in the transport direction T of the steel strip 102, the second inductor 109 is arranged downstream of the first inductor 108. The first inductor 108 is designed as a longitudinal field inductor. The second inductor 109 is designed as a transverse field inductor. The second chamber 107 comprises the strip paths 3, 4, and 5. Optionally, the second chamber 107 can be equipped with a pre-oxidation chamber 110, which is arranged between the first inductor 108 and the second inductor 109. In this case, the steel strip 102, after being heated by the first inductor 108, first passes through the pre-oxidation chamber 110 before being heated by the second inductor 109. The third chamber 112 forms a second heating chamber and is heated by a radiant tube, and serves to slowly heat the steel band 102 to a specific temperature and then to hold it at this temperature.The third chamber 112 forms an RTF furnace section (RTF = Radiant Tube Furnace) 113 and is equipped with a plurality of radiant tubes 114 arranged along the strip paths 6-13. Longer holding times can also be set for the steel strip 102 within the third chamber 112 if required. A further inductor, e.g., in the form of a transverse field inductor, can be provided at the end of the third chamber 112. Fig. 1 The third chamber 112 is designated as "Inductor 3". Inductor 3 heats the steel strip 102 to a temperature of at least 820 °C, for example, at heating rates of at least 50 K / s, before it leaves the third chamber 112. The third chamber 112 comprises strip paths 6-13. The fourth chamber 115 serves for the slow cooling of the steel strip 102 and comprises strip paths 14 and 15. The fifth chamber 116 serves as a rapid cooling chamber and is equipped for this purpose with cooling devices in the form of "Rapid Cooling 1" and "Rapid Cooling 2", which are arranged one after the other along the strip path 16. The sixth chamber 117 serves to heat the steel strip 102 to a partitioning temperature of at least 300 °C, preferably 320 °C. An inductor 4 may be provided in the inlet area of ​​the sixth chamber 117, and an inductor 5 may be provided in the outlet area or at the end of the sixth chamber 117.With these inductors 4, 5, which are preferably designed as longitudinal field inductors, the steel strip 102 can be heated to a predetermined temperature at a high heating rate. The sixth chamber 117 comprises the strip paths 17-24.

[0028] In the respective chambers of Plant 10, where heat treatment (heating or cooling) is carried out on the steel strip 102, specific atmospheres are maintained to which the steel strip 102 is exposed as it passes through the individual chambers. It should be noted that the openings or passages between the individual chambers are equipped with seals so that the designated atmosphere is maintained in each chamber. The following explanations apply to the atmospheres in the individual chambers: The first chamber 105 contains a slightly reducing atmosphere with exhaust gas and a (slight) lack of air. The atmosphere in the second chamber 107 ("heating chamber") consists of at least 20% hydrogen (H₂), preferably > 50% H₂, and has a dew point of < -40 °C. The remaining portion of this atmosphere consists of nitrogen (N₂). The fourth chamber 115 contains an atmosphere for the slow cooling of the steel strip, containing at least 20% hydrogen (H₂) and the remainder nitrogen (N₂), with a dew point of < -40 °C. The fifth chamber 116, which functions as a rapid cooling chamber, contains the same atmosphere as the second chamber 107. For increased cooling capacity and performance, the hydrogen content is preferably > 50%.

[0029] The representation of Fig. 1 Figure 1 further illustrates a belt bypass, whereby the steel belt 102 – if required – can be introduced directly into the fifth chamber 116 for rapid cooling after exiting the second chamber 109 (or the heating chamber). This means that, in this case, the third chamber 112 and the fourth chamber 115 are not traversed by the steel belt 102.

[0030] In the individual chambers 1-6 of the system 10, the steel strip 102 undergoes heat treatment with different heating and cooling rates. This is explained in detail below using various embodiments of the invention as examples:

[0031] The embodiment of Fig. 2 represents a simplified modification of the system of Fig. 1 and is used for the treatment of oxidation-sensitive AHSS steels. In view of the fact that the embodiment of Fig. 2 a reduction in the size of the facility Fig. 1 represents, in the embodiment of Fig. 2 Only 10 band paths are provided and named accordingly.

[0032] In the embodiment of Fig. 2 Both the first chamber 105 and the pre-oxidation chamber 110 are out of operation. Instead, the steel strip 102 is heated directly inductively in the second chamber 107 ("heating chamber") in two stages to, for example, 950 °C (strip path 1). As explained, the first inductor 108 is configured with a longitudinal field and the second inductor 109 with a transverse field. The heating rate for heating the steel strip 102 by means of the inductors 108 and 109 is at least 50 K / s and can be further reduced in the case of the embodiment of Fig. 2 The temperature must be > 85 K / s. As explained above, the atmosphere inside the second chamber 107 contains hydrogen. Due to the short residence time within the second chamber 107, the high hydrogen content, and the low water content, the selective oxidation and diffusion of Si and Mn to the surface(s) of the steel strip 102 are largely suppressed. Due to the high temperature, very rapid complete austenitization occurs, with the required holding time for the example shown here being approximately 5 seconds, e.g., exactly 7 seconds. Such a short holding time is helpful to prevent further oxidation of the steel strip 102's surfaces.

[0033] In the embodiment of Fig. 2 After leaving the second chamber 107, the steel belt 102 enters directly into the fifth chamber 116 (or the "rapid cooling chamber"). In this embodiment, the belt bypass described above is thus implemented.

[0034] The atmosphere in the fifth chamber, or rapid cooling chamber 116, is the same as in the heating chamber 107. For optimal cooling performance, the hydrogen content in this atmosphere is preferably > 50%. Within rapid cooling chamber 116, the steel strip 102 is cooled down to approximately 250 °C at a cooling rate of, for example, 70 K / s.

[0035] After the steel strip 102 enters the sixth chamber 117 following the rapid cooling chamber 116, it is first heated to the partitioning temperature of, for example, 320 °C by the inductor 3 in the inlet area of ​​the sixth chamber 117. The steel strip 102 is then maintained at this partitioning temperature in the individual strip paths 5-10 of the sixth chamber 117 before being heated to the "zinc pot temperature" by the inductor 4 in the outlet area of ​​the sixth chamber 117 and then fed into the hot-dip molten bath 104 ("zinc pot") at this temperature.

[0036] Fig. 3 shows a further embodiment of a modified system 10, which is also based on a simplification or shortening of the system of Fig. 1 based and used for the treatment of oxidation-sensitive AHSS steels. In this respect, the embodiment of Fig. 3 also based on the aforementioned possibility of a band bypass. In contrast to the embodiment of Fig. 2 The first chamber 105 is in operation, functioning as a directly fired preheating chamber. Because the first chamber 105 has two belt paths, the embodiment of Fig. 3 compared to that of Fig. 2 Two more tape paths, namely a total of 12 tape paths, which are shown in the representation of Fig. 3 are named accordingly.

[0037] In connection with the design of Fig. 3 It is noted that in most cases the oxidation tendency of a steel strip is negligible up to a temperature of approximately 700 °C. Therefore, the heating rate for the steel strip 102 is insignificant up to a temperature of about 700 °C, and exhaust gas from a combustion process with a slight air deficiency is sufficient for the atmosphere within the first chamber 105. Thus, the first inductor 108 in the second chamber 106 can be omitted and replaced by a directly heated preheater or furnace section (DFF) 106, with which the first chamber 105 is equipped. In other words, the second chamber 109 is then only equipped with the second inductor 109 (designed as a transverse field inductor).

[0038] In comparison to the embodiment of Fig. 2 and the electrical energy consumption for the first inductor 108, the embodiment of Fig. 3 the advantage of significantly lower heating energy costs, which are incurred due to gas heating for the DFF oven section.

[0039] The temperature profile of the steel strip 102 over time is for the embodiment of Fig. 3 in the diagram of Fig. 4 shown. In this diagram, paths 1-12 are shown, which, as explained, are also represented in the diagram of Fig. 3 The images shown are taken at different times during the band treatment.

[0040] The atmosphere in the second chamber, or heating chamber 106, at least from the point of entry into the second inductor 109, consists of at least 20% hydrogen (H₂), preferably > 50% H₂, and has a dew point of < -40 °C. A sufficiently short residence time of the steel strip 102 above a temperature of 700 °C is relevant for suppressing the selective oxidation of Si and Mn. In the embodiment according to Fig. 3 In any case, this dwell time should be less than 60 seconds, with the dwell time shown in the diagram by Fig. 4 for example, 15 seconds.

[0041] With regard to Annex 10 according to the embodiment of Fig. 1 It should be noted that a hot-dip galvanizing line (CGL) is only utilized to its full capacity with AHSS steels in a few cases. In practice, there is often a need to produce conventional grades of steel strip, such as deep-drawing grades, at competitive production costs, and these conventional grades are generally less susceptible to oxidation. In light of this, a multi-purpose CGL equipped with Annex 10 according to [relevant standard / regulation] is recommended. Fig. 1 is being implemented.

[0042] Further driving modes for a method according to the invention are explained below, with which a plant 10 according to Fig. 1 can be operated. In this regard, it should be noted that the conditions for these driving modes are set out in tables (see below). Fig. 5 , Fig. 7 , Fig. 9 , Fig. 11 ) are shown and the information mentioned herein for the oven sections refers to the designations of Fig. 1 The resulting temperature profiles of the steel strip 102 over time are shown in diagrams (see...). Fig. 6 , Fig. 8 , Fig. 10 , Fig. 12 ) shown, whereby the band paths 1-24 mentioned herein also show the in Fig. 1 The band paths shown correspond to those shown. In detail: The first operating mode of a method according to the invention, with its associated parameters, is shown in the table of Fig. 5 entered or named, whereby the resulting temperature profile in Fig. 6 This operating method is used for machining AHSS steels, whereby selective oxidation is (largely) suppressed.

[0043] According to the explanations in the table of Fig. 5 In the first operating mode, chamber 1 (or the first chamber 105) can be switched off, meaning no pre-oxidation takes place in chamber 2 (or the second chamber 107). Accordingly, the second chamber 107, which in this case functions as a heating chamber, contains only the first inductor 108 and the second inductor 109, arranged one behind the other in the transport direction T of the steel strip 102. In the second chamber, or heating chamber 107, the steel strip 102 is heated to a holding temperature of up to 950 °C at a heating rate of > 50 K / s. The exact holding temperature, which may also be below 950 °C, e.g., at 920 °C, depends on the desired degree of austenitization. For example, the holding temperature can be between 840 °C and 920 °C, or possibly even above 920 °C. The first inductor 108 with longitudinal field heats the steel strip 102 to approximately700 °C, with the second inductor 109, operating with a transverse field, subsequently heating the steel strip to a holding temperature of, for example, 920 °C. The required holding time, during which the steel strip 102 remains at this holding temperature, is a maximum of 180 seconds, possibly even less than 180 seconds, and is carried out in the third chamber 112 and the fourth chamber 115. The atmosphere consists of > 20% hydrogen, with a dew point of < -40 °C. Due to the sufficiently short holding time (< 180 seconds) and the strongly reducing atmosphere, the selective oxidation of the elements Si and Mn is suppressed, which is advantageous for very oxidation-sensitive steel grades.

[0044] Regarding further details for the respective temperatures and atmospheres that occurred during the initial operating mode in the individual chambers of Plant 10 of Fig. 1 The entries in the table of [table name] are selected or configured at this point. Fig. 5 and in the diagram of Fig. 6 be referred.

[0045] A possible second driving mode for a method according to the invention is described below with reference to the table of Fig. 7 explained, with the resulting progression of the band temperature over time shown in the diagram of Fig. 8 As shown. In the same way as the first operating mode mentioned above, the second operating mode also serves to treat or process AHSS steels, whereby selective oxidation is (largely) suppressed.

[0046] In the second operating mode, the steel strip is heated to a temperature of up to 600 °C in the first chamber 105 ("preheating chamber") under an atmosphere containing exhaust gas with a lack of air. Such heating is still uncritical for selective oxidation. Following heating in the preheating chamber 105, the steel strip 102 is heated to a maximum temperature of 700 °C in the second chamber 107 (or "first heating chamber") by the first inductor 108. Considering that the steel strip 102 has already been heated in the preheating chamber 105 before entering the second chamber 107 and therefore has a higher temperature compared to the first operating mode, the first inductor 108 can now operate at a lower load or with reduced power in the second operating mode, resulting in the advantage of lower energy costs compared to the first operating mode.

[0047] The remaining process steps of the second driving mode can correspond to the process steps of the first driving mode, so that, to avoid repetition, the above explanation regarding the Fig. 5 +6 may be referred.

[0048] For further details of the respective temperatures and atmospheres that occur during the second driving mode in the individual chambers of plant 10 of Fig. 1 The entries in the table of [table name] are selected or configured at this point. Fig. 7 and in the diagram of Fig. 8 be referred.

[0049] It should be specifically noted here that the first and second operating modes are identical in that the pre-oxidation chamber 110 is in operation in both cases. Consequently, the steel strip 102 in the second chamber 107 ("first heating chamber") is only heated by the inductors 108, 109.

[0050] A possible third driving mode for a method according to the invention is described below with reference to the table of Fig. 9 explained, with the resulting progression of the band temperature over time shown in the diagram of Fig. 10 The third operating mode is used for processing AHSS steels, in which pre-oxidation is carried out. Specifically: In the first chamber 105, the steel strip 102 is heated to a temperature of at least 600 °C by means of the directly heated furnace section 106 under open heating. When the steel strip 102 then enters the second chamber 107, it is heated precisely to a temperature in the range of 650–700 °C by the first inductor 108 to prepare for pre-oxidation. After heating by the first inductor 108, the steel strip 102 passes through the pre-oxidation chamber 110 under a hydrogen-containing atmosphere. Subsequently, the steel strip 102 is heated in the second chamber 107 by the second inductor 109 to just below austenitization (e.g. approx. 820 °C) with a heating rate > 50 K / s.The conversion from ferrite to austenite is traversed slowly in the third chamber 112 ("second heating chamber"). Depending on the operating mode or control of the second inductor 109 and the selected heating rate by the radiant tube furnace 113 in the third chamber 112, the holding time or residence time for the steel strip 102 can vary. This is advantageous for steel grades that, due to their microstructure, require slow austenitization and a longer holding time. In any case, the desired reduction of the steel strip 102 takes place in the third chamber 112 and in the fourth chamber 115.

[0051] For the third driving mode, it is additionally pointed out that the second inductor 109 is operated under partial load, which advantageously leads to lower energy costs.

[0052] Upon entering the third chamber 112 (= belt path 6), the steel belt 102 has a temperature of 820 °C. The heating rate in the third chamber 112 ("second heating chamber") is approximately 2 K / s. Thus, a holding temperature of 920 °C is reached for the steel belt 102 at the end of path 8 (cf. Fig. 10 ). For maintaining a temperature of 920 °C, paths 9-13 are available in the third chamber 112, with a holding time of approximately 84 seconds.

[0053] For further details of the respective temperatures and atmospheres that occur during the third operating mode in the individual chambers of plant 10 of Fig. 1 The entries in the table of [table name] are selected or configured at this point. Fig. 9 and in the diagram of Fig. 10 be referred.

[0054] A possible fourth operating mode for a process according to the invention also serves for AHSS steels with controlled pre-oxidation and is described below with reference to the table of Fig. 11 explained, with the resulting progression of the band temperature over time shown in the diagram of Fig. 12 shown.

[0055] In the fourth operating mode, the steel strip 102 is heated in the first chamber 105, then heated in the second chamber 107 ("first heating chamber") by the first inductor 108, and treated in the pre-oxidation chamber 110 in the same way as in the third operating mode. It should be noted that in the fourth operating mode, the second inductor 109 remains switched off at the end of the second chamber 107. Thus, the steel strip 102 has a temperature of only 700 °C upon entering the third chamber 112 ("second heating chamber"). Consequently, the steel strip 102 is heated conventionally in the third chamber 112 by the radiant tubes 114 at a lower heating rate. Compared to the third operating mode, this is reflected in the fact that the holding temperature of 920 °C for the steel strip 102 in the third chamber 112 is only reached at the end of the strip path 10.For maintaining the holding temperature of 920 °C, only paths 11-13 are available in the third chamber 112, with a holding time or dwell time of approximately 47 seconds.

[0056] For the implementation of the present invention, it is advantageous to carry out all transition processes during the operation of plant 10 with fill grades in order to pre-set the conditions for the AHSS grades. This includes both the timely adjustment of the respective atmospheres in the individual chambers with the necessary purging processes and the start-up of the inductive rapid heating. When switching from AHSS to fill grades, this is carried out in reverse order, so that the AHSS product encounters the necessary conditions from the beginning to the end of the conveyor belt.

[0057] Another advantageous aspect of operation with inductive rapid heating is that the transferred heat flux can be known with good accuracy from electrical parameters. Using the heat flux and strip data, the temperature of the steel strip 102 can be determined. A radiation pyrometer downstream of an inductor can be used to determine the emissivity with a known strip temperature. During pre-oxidation, the temperature of the steel strip 102 remains constant, although the surface area, and thus the emissivity, can change significantly. By using a radiation pyrometer downstream of the pre-oxidation chamber 110, it is possible to detect this surface change caused by pre-oxidation. This "online" determined emissivity of the steel strip 102 can be used via the thermal furnace model for precise control of the subsequent heating in the radiant tube furnace.

[0058] Finally, it should be noted that, in view of the various driving modes of a procedure explained above, with which an Annex 10 of Fig. 1 according to the present invention, such a plant 10 represents a multi-purpose plant or multi-purpose CGL with which both a heat treatment of a steel strip with suppressed selective oxidation and a conventional treatment with pre-oxidation can be carried out, and in addition the cost-effective production of comparatively undemanding fill grades of steel strips is also possible. Bezugszeichenliste

[0059] 10 Hot-dip coating system 102 Steel strip 104 Hot-dip bath 105 Preheating chamber 106 Direct fired furnace (DFF) 107 First heating chamber 108 (First) inductor 109 (Second) inductor 110 Pre-oxidation chamber 112 Second heating chamber 113 RTF furnace section (RTF = Radiant Tube Furnace) 114 Radiant tube(s) 115 Slow cooling chamber 116 Rapid cooling chamber 117 (Partitioning) holding chamber 118 Inductor (in the discharge area of ​​holding chamber 117) 1-24 Strip paths of system 10, in the embodiment of Fig. 1 1-10 Band paths of Annex 10, in the embodiment of Fig. 2 1-12 band paths of Annex 10, in the embodiment of Fig. 3 TTransport direction for moving the steel band 102

Claims

1. Method for continuous heat treatment of a steel strip (102) of high-strength quality, particularly of oxidisation-sensitive AHSS qualities, in which the steel strip (102 is moved through at least one furnace device, comprising the following steps: i) heating the steel strip (102) to a temperature of at least 600° C by a directly heated preheater (DFF = Direct Fired Furnace) (106) in a waste gas atmosphere with absence of air, ii) heating the steel strip (102) to 700° C to 750° C by an inductor (108) in an atmosphere with hydrogen content, iii) heat treatment of the steel strip (102) in an oxidising atmosphere (110) with an oxygen content of 2 to 5% O2 in order to thereby form iron oxide layers at the surfaces of the steel strip (102), wherein this heat treatment has a duration of 5 to 20 seconds, iv) heating the steel strip (102) to a temperature of up to 960° C in an atmosphere containing hydrogen (H2), water vapour and the remainder nitrogen (N2), wherein the steel strip (102) is kept at a temperature of up to 950° C for a time period of ≥ 40 seconds, v) rapid cooling of the steel strip (102) to a temperature, which lies in a range between 200° C and 450°, under an atmosphere with hydrogen content, wherein subsequently thereto heating of the steel strip (102) to a partitioning temperature of at least 300° C, preferably 320° C, in an atmosphere containing ≥ 20% hydrogen (H2) and the remainder nitrogen (N2) is carried out, wherein the steel strip (102) has a dwell time in this atmosphere of ≥ 30 seconds, and vi) applying a metallic coating to at least one surface of the steel strip.

2. Method according to claim 1, characterised in that in step iv) the steel strip (102) is heated by an RTF furnace part (RTF = Radiant Tube Furnace) (113), preferably in that the steel strip (102) at the start of the step iv) is additionally heated by a cross-field inductor (109) at a heating rate of at least 50 K / s to at least 820° C.

3. Method according to claim 1 or 2, characterised in that a slow cooling of the steel strip (102) to a temperature of < 850° C in an atmosphere with hydrogen content is carried out between the steps iv) and v), preferably in that this atmosphere contains ≥ 20% hydrogen (H2) and has a dew point of < -40° C, more preferably that the atmosphere for this slow cooling contains apart from hydrogen (H2) a remainder of nitrogen (N2).

4. Method according to any one of claims 1 to 3, characterised in that provided subsequently to the step v) are further process steps in which heating, maintenance at a specific temperature and / or cooling are carried out for the steel strip.

5. Method according to any one of claims 1 to 4, characterised in that the steel strip (102) at the end of the partitioning heating is heated by an inductor (119), preferably in the form of a longitudinal-field inductor, to the necessary temperature for entry into a melt dip bath (104), preferably to a temperature of 460° C.

6. Method according to any one of the preceding claims, characterised in that in the step vi) the steel strip (102) is metallically coated at at least one surface thereof by means of a coating device, preferably in that the coating device is a hot-dip bath (104) in which the steel strip is dip-coated with, in particular, zinc.

7. Plant (10) for hot-dip coating of a steel strip (102), which is moved in a transport direction (T), of high-strength quality, particularly of oxidisation-sensitive AHSS qualities, comprising a hot-dip bath (104) into which the steel strip (102) can be dipped for coating, wherein at least one heating chamber (107) with at least one inductor (108, 109), a fast cooling chamber (116) and a holding chamber (117) for partitioning of the steel strip (102) are arranged upstream of the melt dip bath (104) as seen in the transport direction (T) of the steel strip (102), characterised in that a preheating chamber (105) with a directly heated preheater (DFF = Direct Fired Furnace) (106) is arranged upstream of the first heating chamber (107) as seen in the transport direction (T) of the steel strip (102).

8. Plant (10) according to claim 7, characterised in that an inductor (118) is provided in the inlet region of the holding chamber (117).

9. Plant (10) according to claim 7 or 8, characterised in that an inductor (119) is provided in the outlet region of the holding chamber (117).

10. Plant (10) according to any one of claims 7 to 9, characterised in that a cross-field inductor (109) is provided in the first heating chamber (107).

11. Plant (20) according to claim 10, characterised in that a longitudinal-field inductor (108) is provided upstream of the cross-field inductor (109).

12. Plant (10) according to claim 11, characterised in that a pre-oxidisation chamber (110) containing an atmosphere with an oxygen content of 2 to 5% O2 is provided between the longitudinal-field inductor (108) and the cross-field inductor (109).

13. Plant (10) according to any one of claims 7 to 12, characterised in that a second heating chamber (112) comprising at least one RTF furnace part (RTF = Radiant Tube Furnace) (113) is arranged downstream of the first heating chamber (107) as seen in the transport direction (T) of the steel strip (102).

14. Plant (10) according to any one of claims 7 to 13, characterised in that a slow cooling chamber (115) is arranged upstream of the fast cooling chamber (116) as seen in the transport direction (T) of the steel strip (102).

15. Use of a plant (10) according to any one of claims 12 to 14 for performing a method according to any one of claims 1 to 6.

Images