Annealing method

Abstract

The invention relates to a method for manufacturing a steel sheet in a device comprising a pre-heating section, a heating section having a maximum heating rate, and a soaking section, the method comprising a calibration step, a recrystallization anneal and soaking, and a coating step. The calibration step allows to define a lower dew point which allows to achieve predetermined quality targets.

Description

Technical Field

[0001] This invention relates to a method for manufacturing steel plates. Background Technology

[0002] During the manufacture of hardened steel, it is annealed before coating to improve its strength-ductility balance. During annealing, the steel sheet is heated in a controlled atmosphere and held above its recrystallization temperature. The steel strip is then cooled and coated, typically by hot-dip galvanizing in a galvanizing bath.

[0003] For example, a common practice is to heat the fully hardened steel sheet from ambient temperature to a temperature above the recrystallization point of the steel (heating step), and then hold the steel at that temperature (soaking step). Both steps are carried out in the same atmosphere and at the same dew point, such as an atmosphere containing 5% by volume H2 and an inert gas with a dew point between -40°C and +10°C.

[0004] During the heating process, the gradual increase in temperature and the presence of oxygen cause oxygen to diffuse into the steel, leading to two types of reactions. First, oxygen reacts with carbon to form gases such as CO2 and CO, resulting in a reduction of carbon atoms beneath the steel surface. However, simultaneously, carbon atoms diffuse from the bulk to the carbon-reduced region. The subsurface layer will decarburize as long as more carbon atoms leave the subsurface layer than enter it. Second, oxygen reacts with steel alloying elements such as manganese (Mn), aluminum (Al), silicon (Si), or chromium (Cr), which have a higher affinity for oxygen than iron. This results in the formation of oxides beneath the steel surface. The formation of these oxides beneath the surface reduces the amount of alloying elements available for forming surface oxides.

[0005] Compared to the heating step, the temperature is higher in the soaking step. Due to the higher temperature in the soaking section, alloying elements that have not formed any internal oxides can diffuse from the bulk to the steel surface and may form external selective oxides, which is considered to have a negative impact on the wettability of the steel.

[0006] In subsequent processing steps, these steels are typically coated with a metallic alloy, such as a zinc-based coating, to improve their properties, such as corrosion resistance and / or phosphating susceptibility. The metallic coating can be deposited using hot-dip galvanizing or electroplating.

[0007] In the prior art, it is believed that the external selective oxides formed by steel alloying elements on the steel plate surface during the annealing step prevent reactive wetting between the substrate, i.e., the steel, and the coating, i.e., the aluminum-based coating or the zinc-based coating. This results in a discontinuous and uneven inhibitory layer. This can lead to problems such as areas on the final product that do not contain the coating (e.g., exposed spots) or delamination of the coating, which is detrimental to product quality.

[0008] EP 3 378 965 A1 describes a method for manufacturing high-strength hot-dip galvanized steel sheets with excellent impact resistance and corrosion resistance in the processed parts. The heating step is carried out in an atmosphere containing 0.1 to 20% H2 by volume up to 650°C, and this atmosphere satisfies the following conditions: -1.7 ≤ log (PH20 / PH2) ≤ -0.6. For an H2 concentration of 5%, these parameters correspond to a dew point between -20°C and +10°C. The temperature rise rate is 0.5°C·s⁻¹ to 5°C·s⁻¹. The temperature rise of the steel sheet is limited to 5°C·s⁻¹ because otherwise, recrystallization at the surface layer of the steel sheet base material would occur before the formation of internal oxide particles, and also because a decarburized layer cannot be obtained in time. Summary of the Invention

[0009] The purpose of this invention is to increase the reliability of annealing treatment and improve the wettability of steel substrates and the quality of coatings.

[0010] This objective is achieved by providing the following methods: A method for manufacturing a steel plate having a thickness t in an apparatus comprising a preheating section, a heating section, and a soaking section, wherein the heating section has a maximum heating rate for the steel plate having a thickness t, the method comprising: A) Calibration steps, in which: i) A steel plate having a thickness t and the following chemical composition by weight percentage is heated from room temperature to a temperature T1 below 600°C: 0.05% ≤ C ≤ 0.50%, 0.3% ≤ Mn ≤ 8.0%, 0.01% ≤ Si ≤ 5%, and optionally at least one of the following elements by weight percentage: 0.01% ≤ Al ≤ 1.5%, B ≤ 0.004%, Co ≤ 0.1%, 0.001% ≤ Cr ≤ 1.00%, Cu ≤ 0.5%, 0.001% ≤ Mo ≤ 0.5%, Nb ≤ 0.1%, Ni ≤ 1.0%, Ti ≤ 0.1%, N ≤ 0.01%, P < 0.1%, S ≤ 0.01%, V < 0.2%, the remainder of said composition consisting of iron and unavoidable impurities. ii) The steel plate is heated in atmosphere A1 at the maximum heating rate from T1 to a recrystallization temperature T2 in the range of 720°C to 1000°C, wherein atmosphere A1 contains 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and atmosphere A1 has a dew point DPCAL. iii) The steel plate is then held in atmosphere A2 at temperature T2, atmosphere A2 comprising 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and the dew point of atmosphere A2 is at least -40°C. iv) The steel sheet is then hot-dip coated in a coating bath, and the quality of the coating is evaluated. v) a) If the coating quality meets the predetermined quality target, repeat the calibration steps i) to iv) at a lower dew point DPCAL until the coating quality no longer meets the target, and the penultimate dew point DPCAL is defined as DP1; b) If the coating quality is unsatisfactory, repeat calibration steps i) to v) at a higher dew point DPCAL until the coating quality meets the target, and the final DPCAL is defined as DP1; B) Production step, in which the steel plate having the thickness t and the chemical composition is subjected to: - Recrystallization annealing, wherein the recrystallization annealing sequentially includes a preheating step, a heating step, a homogenization step, and a cooling step, wherein: i) The preheating step includes heating from room temperature to a temperature T1 below 600°C. ii) The heating step includes heating from T1 to a recrystallization temperature T2 in the range of 720°C to 1000°C in an atmosphere A1 at a heating rate less than or equal to the maximum heating rate, wherein the atmosphere A1 contains 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and the dew point of the atmosphere A1 is set to at least the DP1 value determined during the calibration step. iii) The homogenization step includes maintaining a temperature in atmosphere A2 within the range of T2 - 30°C to T2 + 30°C, wherein atmosphere A2 contains 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and the dew point DP2 of atmosphere A2 is set to -40°C or higher. - Coating step, in which the steel plate is hot-dip coated in the coating bath.

[0011] The method may also include any of the following features: Optionally, the weight percentage ratio of manganese to silicon in the chemical composition of the steel body follows the rule: Mn / Si < 4.

[0012] Optionally, the weight percentage ratio of aluminum to magnesium in the chemical composition of the steel body follows the rule: Mn / Al < 1.

[0013] Optionally, the weight percentage ratio of manganese, aluminum and silicon in the chemical composition of the steel body follows the rule: Mn / (Al+(4×Si))< 1.

[0014] Optionally, in the preheating step i), the temperature T1 is below 550°C, preferably below 500°C.

[0015] Optionally, in the preheating step i), the heating rate is higher than 50°Cs⁻¹.

[0016] Optionally, in the heating step ii), the atmosphere A1 comprises 1% to 20% by volume of H2, at least an inert gas, and unavoidable impurities.

[0017] Optionally, the heating step ii) lasts from 10 seconds to 1000 seconds.

[0018] Optionally, in the heat equalization step iii), the steel plate is maintained at a temperature from (T2-10°C) to (T2+10°C).

[0019] Optionally, the heat equalization step iii) lasts from 10 seconds to 1000 seconds.

[0020] Optionally, in the homogenization step iii), the atmosphere A2 comprises between 1% and 20% by volume of H2, at least an inert gas, and unavoidable impurities.

[0021] Optionally, the coating bath is a zinc-based coating bath containing 0.1% to 0.3% by weight of aluminum and optionally magnesium.

[0022] Optionally, the coating bath is an aluminum-based bath containing 5% to 15% silicon by weight.

[0023] Optionally, in step ii) of the calibration step A), the minimum value of DPCAL is -40°C, and in step v)a), if the coating quality is satisfied at DPCAL at -40°C, then -40°C is defined as DP1.

[0024] Optionally, the coating bath of step iv) in calibration step A) and the coating bath of the coating step in production step B) have the same basic elements.

[0025] Other features and advantages of the invention will become apparent from the following detailed description. Attached Figure Description

[0026] To illustrate the present invention, various embodiments and experiments of non-limiting examples will be described with particular reference to the following drawings: Figure 1 The illustration shows an embodiment of the annealing furnace and the hot-dip coating equipment.

[0027] Figure 2 The illustration shows an embodiment of a coating (A) that meets the quality objectives and an embodiment of a coating (B) that does not meet the quality objectives.

[0028] Figure 3 This reflects step v)a) of the calibration procedure.

[0029] Figure 4 This reflects step v)b) of the calibration procedure. Detailed Implementation

[0030] This invention relates to a method for manufacturing a steel plate having a thickness t in an apparatus comprising a preheating section, a heating section, and a soaking section, wherein the heating section has a maximum heating rate for the steel plate having a thickness t, the method comprising: A) Calibration steps, in which: i) A steel plate having a thickness t and the following chemical composition by weight percentage is heated from room temperature to a temperature T1 below 600°C: 0.05% ≤ C ≤ 0.50%, 0.3% ≤ Mn ≤ 8.0%, 0.01% ≤ Si ≤ 5%, and optionally at least one of the following elements by weight percentage: 0.01% ≤ Al ≤ 1.5%, B ≤ 0.004%, Co ≤ 0.1%, 0.001% ≤ Cr ≤ 1.00%, Cu ≤ 0.5%, 0.001% ≤ Mo ≤ 0.5%, Nb ≤ 0.1%, Ni ≤ 1.0%, Ti ≤ 0.1%, N ≤ 0.01%, P < 0.1%, S ≤ 0.01%, V < 0.2%, the remainder of said composition consisting of iron and unavoidable impurities. ii) The steel sheet is heated from T1 to a recrystallization temperature T2 in the range of 720°C to 1000°C at the maximum heating rate in atmosphere A1, atmosphere A1 containing 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and atmosphere A1 having a dew point DPCAL. iii) The steel plate is then held in atmosphere A2 at temperature T2, atmosphere A2 containing 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and the dew point of atmosphere A2 is at least -40°C. iv) The steel plate is then hot-dip coated, and the quality of the coating is evaluated. v) a) If the coating quality meets the predetermined quality target, repeat the calibration steps i) to iv) at a lower dew point DPCAL until the coating quality no longer meets the target, and the penultimate dew point DPCAL is defined as DP1; b) If the coating quality is unsatisfactory, repeat calibration steps i) to v) at a higher dew point DPCAL until the coating quality meets the target, and the final DPCAL is defined as DP1; B) Production steps, in which the steel plate having the said thickness t and the said chemical composition is subjected to: - Recrystallization annealing, which sequentially includes a preheating step, a heating step, a homogenization step, and a cooling step, wherein: i) The preheating step includes heating from room temperature to a temperature T1 below 600°C. ii) The heating step includes heating from T1 to a recrystallization temperature T2 in the range of 720°C to 1000°C in an atmosphere A1 at a heating rate less than or equal to the maximum heating rate, the atmosphere A1 containing 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and the dew point of the atmosphere A1 being set to a DP1 value determined at least during the calibration step. iii) The homogenization step includes maintaining an atmosphere A2 at a temperature ranging from T2 - 30°C to T2 + 30°C, the atmosphere A2 comprising 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and the dew point DP2 of the atmosphere A2 being set to -40°C or higher. - Coating step, in which the steel plate is hot-dip coated.

[0031] like Figure 1 As illustrated, device 1' is a device capable of performing heat treatment, such as an annealing oven or annealing furnace. This device includes a preheating section 2', a heating section 3', a soaking section 4', and a cooling section 5', wherein the temperature, heating rate, and atmosphere of these sections can be set and controlled.

[0032] The temperature of the steel plate increases in the preheating section and the heating section. The maximum heating rate of the heating section is the highest heating rate at which the manufactured steel plate with thickness t can be heated during the heating section in production step ii). The maximum heating rate refers to the inherent maximum heating rate of the heating section for a steel plate with thickness t. The maximum heating rate involves an average value.

[0033] The carbon content is between 0.05 wt% and 0.50 wt%. If the carbon content is below 0.05 wt%, there is a risk of insufficient tensile strength. Furthermore, if the steel's microstructure contains retained austenite, the stability necessary to achieve sufficient elongation cannot be obtained. If the carbon content is greater than 0.5 wt%, the hardenability of the weld increases.

[0034] The manganese content is from 0.3 wt% to 8.0 wt%. Manganese is a solid solution hardening element that contributes to achieving high tensile strength. This effect is achieved when the Mn content is at least 0.3 wt%. However, when the Mn content is greater than 8.0 wt%, this can lead to the formation of structures with excessively significant segregation regions, which may adversely affect the mechanical properties of the weld. Preferably, the manganese content is from 1.5 wt% to 5.0 wt%. This allows for satisfactory mechanical strength without increasing the difficulty of industrial manufacturing of the steel or the hardenability of the weld.

[0035] The silicon content ranges from 0.01% to 5% by weight. Silicon delays carbide formation and stabilizes austenite. When the silicon content exceeds 5% by weight, the plasticity and toughness of the steel decrease significantly.

[0036] Steel may optionally contain elements such as Al, B, Co, Cr, Cu, Mo, N, Nb, Ni, P, S, Ti, and V for the following reasons: Aluminum may optionally be included in the steel sheet in an amount from 0.01 wt% to 1.5 wt%. Al increases the Ms temperature and thus destabilizes the retained austenite. Furthermore, as the Al content increases above 1.5 wt%, the Ac3 temperature increases, leading to difficulties in industrial production. Preferably, the aluminum content is from 0.01 wt% to 1.0 wt%.

[0037] Boron may optionally be included in the steel sheet at a content of less than or equal to 0.004% by weight. By segregating at grain boundaries, B lowers the grain boundary energy and thus contributes to improved resistance to liquid metal embrittlement.

[0038] Chromium may optionally be included in the steel sheet at a content of less than or equal to 1.00% by weight. Chromium allows for the delay of proeutectoid ferrite formation during the cooling step after maintaining the highest temperature during the annealing cycle, making it possible to obtain higher strength levels. For cost reasons and to prevent over-hardening, the chromium content is limited to 1.00% by weight.

[0039] Copper may optionally be included in the steel plate at a content of less than or equal to 0.5% by weight for the purpose of hardening the steel by precipitation of copper metal.

[0040] Molybdenum may optionally be included in the steel sheet at a content of less than or equal to 0.5% by weight. Molybdenum is effective in increasing hardenability and stabilizing retained austenite because this element delays the decomposition of austenite.

[0041] Nickel may optionally be included in the steel plate at a content of less than or equal to 1.0% by weight to improve toughness.

[0042] Titanium may optionally be included in the steel sheet at a content of less than or equal to 0.1% by weight. Niobium may optionally be included in the steel sheet at a content of less than or equal to 0.1% by weight. Titanium and niobium harden and strengthen the steel by forming precipitates. However, when the Nb content is greater than 0.1% by weight and / or the Ti content is greater than 0.1% by weight, there is a risk that excessive precipitation may lead to a decrease in toughness, which must be avoided.

[0043] Vanadium may optionally be included in the steel sheet at a content of less than or equal to 0.2% by weight. Vanadium forms precipitates, thereby hardening and strengthening the steel.

[0044] Phosphorus and sulfur are considered residual elements produced during steelmaking. P may be present in amounts less than or equal to 0.04% by weight. S may be present in amounts less than or equal to 0.01% by weight.

[0045] Preferably, the chemical composition of the steel does not include bismuth (Bi). In practice, it is not desirable to be bound by any theory that if the steel sheet contains Bi, the wettability is reduced, and therefore the coating adhesion is reduced.

[0046] The calibration step is preferably performed using the same apparatus as the production step. However, different apparatus can be used for the calibration step and the production step. Preferably, the apparatus for the calibration step is an annealing furnace.

[0047] In calibration step iv), the assessment of coating quality can be performed by visual inspection and / or by inspection instruments such as scanning electron microscopes (SEM) or field emission gun scanning electron microscopes (FEG-SEM).

[0048] In calibration step v), the predetermined quality objective is preferably associated with the coating uniformity on the steel plate. Preferably, the predetermined quality objective takes into account the absence of uncoated areas and / or the average coating thickness and / or the percentage of coated areas.

[0049] For example, Figure 2Two coating qualities are shown: a satisfactory coating quality on the left (A) and an unsatisfactory coating quality on the right (B). The coating quality on the left is satisfactory because it is coated with a uniform thickness throughout the sample. The coating quality on the right is unsatisfactory because some areas of the sample are not coated.

[0050] Figure 3 The diagram illustrates the iteration of the process described in v)a). Following steps i) through iv), a first sample "A1" has been produced at a dew point DPCAL1 and a maximum heating rate (MaxHR). The coating quality meets the predetermined quality target because, as illustrated on sample "A1", the entire surface is coated (shown in gray). Then, following steps i) through iv), a second sample "A2" has been produced at a dew point DPCAL2 below DPCAL1 and a maximum heating rate (MaxHR). The coating quality meets the predetermined quality target because, as illustrated on sample "A2", the entire surface is coated (shown in gray). Then, following steps i) through iv), a third sample "A3" has been produced at a dew point DPCAL3 below DPCAL2 and a maximum heating rate (MaxHR). The coating quality does not meet the predetermined quality target because, as illustrated on sample "A3", some areas of the surface of sample "A3" are not coated (shown in white). Therefore, DPCAL2 is defined as DP1.

[0051] Preferably, in step ii), DPCAL has a minimum value of -40°C. Even more preferably, in step ii) of calibration step A), DPCAL has a minimum value of -40°C, and in step v)a), if the coating quality is met at DPCAL at -40°C, then -40°C is defined as DP1. Therefore, if the sample is produced at a dew point of -40°C using the maximum heating rate and meets the predetermined coating quality, then -40°C can be defined as DP1.

[0052] Figure 4The diagram illustrates the iteration of the process described in v) and b). Following steps i) to iv), a first sample "B1" has been produced at a dew point DPCAL1 and a maximum heating rate (MaxHR). Because some areas were not coated, the coating quality did not meet the predetermined quality target. Then, following steps i) to iv), a second sample "B2" has been produced at a dew point DPCAL2 (greater than DPCAL1) and a maximum heating rate (MaxHR). Because some areas were not coated, the coating quality did not meet the predetermined quality target. Then, following steps i) to iv), a third sample "B3" has been produced at a dew point DPCAL3 (greater than DPCAL2) and a maximum heating rate (MaxHR). The coating quality of this sample met the predetermined quality target. Therefore, DPCAL3 is defined as DP1.

[0053] It has been surprisingly observed that, due to the formation of external oxides, the key driver of the wettability of the metal coating is the relationship between the heating rate and the dew point at the heating rate, rather than the process parameters of the soaking section, which is contrary to what has been believed in the prior art.

[0054] Therefore, due to the calibration step, the limiting dew point (DP1) in the heating section can be defined, and according to this limiting dew point, the predetermined coating quality can be achieved for any heating rate in the heating zone.

[0055] The preheating step typically occurs after the steel has been cold-rolled. During this preheating, the steel sheet is heated from room temperature to a temperature T1 below 600°C. This step can be carried out, for example, in an RTF (Radiant Tube Furnace) with an atmosphere consisting of N2, H2, and unavoidable impurities, or by means of an induction device, or in a DFF (Direct-Fired Furnace) with an air / gas ratio < 1. However, in a DFF comprising several zones, such as five zones, the air / gas ratio can be > 1 in the last one or the last two zones.

[0056] Limiting the preheating temperature to below 600°C is advantageous because it reduces oxidation on the steel sheet. Furthermore, this is particularly advantageous in radiant tube furnaces (RTF) because it allows for the avoidance of potentially harmful selective oxidation.

[0057] During the heating step, the steel plate is heated from temperature T1 to recrystallization temperature T2 between 720°C and 1000°C in atmosphere A1 at a heating rate lower than or equal to the maximum heating rate. Atmosphere A1 contains 0.1 vol% to 90 vol% H2, at least inert gases, and unavoidable impurities. Atmosphere A1 has a dew point DP1 defined during the calibration step.

[0058] During the homogenization step, the steel plate is maintained in an atmosphere A2 within a temperature range of (T2-30°C) to (T2+30°C), the atmosphere A2 containing 0.1 vol% to 90 vol% H2, at least inert gases, and unavoidable impurities, and the dew point DP2 of the atmosphere A2 is at least -40°C. For example, if T2 is 950°C, then in the homogenization step iii), the steel is maintained in a temperature range of 920°C to 980°C.

[0059] The atmosphere in the heating and homogenization steps can be achieved by using preheated steam and mixing N2-H2 gas in a furnace equipped with H2 detectors in different sections to monitor the atmosphere dew point temperature.

[0060] Preferably, the weight percentage ratio of manganese to silicon in the chemical composition of the steel body follows the rule: Mn / Si < 4.

[0061] Preferably, the weight percentage ratio of aluminum to magnesium in the chemical composition of the steel body follows the rule: Mn / Al < 1.

[0062] Preferably, the weight percentage ratio of manganese, aluminum and silicon in the chemical composition of the steel body follows the rule: Mn / (Al+(4×Si))< 1.

[0063] All three of the aforementioned compositions allow for a reduction in the formation of FeO-MnO on the steel surface, and thus improve coating adhesion and uniformity.

[0064] Preferably, in the preheating step i), the temperature T1 is below 550°C. Even more preferably, in the preheating step i), the temperature T1 is below 500°C. Clearly, further limiting the temperature at the end of the preheating (T1) allows for further reduction of oxidation of the steel sheet. Furthermore, this reduces the risk of potentially harmful selective oxidation in the RTF.

[0065] Preferably, in the preheating step i), the heating rate is higher than 50°C s⁻¹. Increasing the heating rate in the preheating section can reduce the length of the section and / or increase productivity.

[0066] Preferably, in the heating step ii), the atmosphere A1 comprises 1% to 20% by volume of H2, as well as at least an inert gas and unavoidable impurities. Even more preferably, in the heating step ii), the atmosphere A1 comprises between 3% and 8% by volume of H2, as well as at least an inert gas and unavoidable impurities.

[0067] Preferably, the heating step ii) lasts for 10 to 1000 seconds.

[0068] Preferably, in the heat homogenization step iii), the steel plate is maintained at a temperature of T2-10°C to T2+10°C.

[0069] Preferably, the heat equalization step iii) lasts for 10 to 1000 seconds.

[0070] Preferably, in the heat homogenization step iii), the atmosphere A1 comprises between 1% and 20% by volume of H2, as well as at least an inert gas and unavoidable impurities. Even more preferably, in the heat homogenization step iii), the atmosphere A1 comprises between 3% and 8% by volume of H2, as well as at least an inert gas and unavoidable impurities.

[0071] Preferably, the coating bath is a zinc-based coating bath, also known as hot-dip galvanizing, which contains 0.1% to 0.3% by weight of aluminum and optionally magnesium.

[0072] Preferably, the coating bath is an aluminum-based bath containing 5% to 15% by weight of silicon.

[0073] Preferably, in the coating step C), the steel plate is set to a temperature between 0°C and 10°C higher than the temperature of the hot-dip coating bath, and the temperature of the hot-dip coating bath is maintained between 420°C and 470°C.

[0074] Preferably, the coating bath in step iv) of calibration step A) and the coating bath in the coating step of production step B) have the same base elements.

[0075] Preferably, the coating bath in step iv) of calibration step A) and the coating bath in the coating step of production step B) is a zinc-based coating bath containing 0.1% to 0.3% by weight of aluminum and optionally magnesium.

[0076] Preferably, the coating bath in step iv) of calibration step A) and the coating bath in the coating step of production step B) are aluminum-based baths containing 5% to 15% by weight of silicon.

[0077] Experimental results

[0078] The following section presents experimental results showing that the wettability of steel is related to parameters in the heated section.

[0079] The first set of experiments was conducted to evaluate the effect of T1 temperature on coating quality. In this first set of experiments, cold-rolled FeSi steel containing 0.03 wt% carbon, 3 wt% silicon, 0.2 wt% manganese, and 0.01 wt% aluminum was subjected to annealing cycles. In this first set of experiments, all parameters except T1 were kept constant. The heating rate was 4.4°C s⁻¹, the dew point in the heating and soaking zones was -20°C, the H₂ concentration in the heating and soaking zones was 5 vol%, and the soaking duration was 37.5 seconds. The coating quality was visually evaluated. Three experiments were conducted for T1 values ​​of 500°C, 600°C, and 700°C. These parameters are summarized in Table 1.

[0080] As can be seen, when T1 is above 600°C, with all other parameters remaining constant, the coating quality is unsatisfactory. Furthermore, with all other parameters constant, it appears that the smaller T1 is, the better the coating quality.

[0081]

[0082] Table 1

[0083] A second set of experiments was conducted to reproduce the calibration steps of the claimed process, and thus DP1 was determined. In this second set of experiments, an annealing cycle was applied to cold-rolled FeSi steel containing 0.03 wt% carbon, 3 wt% silicon, 0.2 wt% manganese, and 0.01 wt% aluminum. In this second set of experiments, all parameters were constant except for the dew point in the heated section. T1 was 600°C, the heating rate was 20°C s⁻¹, the dew point in the soaking zone was -20°C, the H₂ concentration in both the heated and soaking zones was 5 vol%, and the soaking duration was 37.5 seconds. The coating quality was visually assessed. Four experiments were conducted for the dew point DPCAL values ​​at -10°C, -20°C, -25°C, and -30°C, according to steps A)i) to A)v)a) of the claimed process. Then -25°C was defined as DP1, because -25°C is the penultimate dew point in the calibration step, which is the last dew point at which the coating quality is satisfactory. These parameters are summarized in Table 2.

[0084]

[0085] Table 2

[0086] A third set of experiments was conducted to evaluate the reliability of the process for which protection is claimed. In this third set of experiments, cold-rolled FeSi steel of the same grade as in the second set of experiments was subjected to annealing cycles. In this third set of experiments, the calibration procedure limited DP1 to -30°C and the maximum heating rate to 10°Cs⁻¹. In this third set of experiments, all parameters were constant except for the heating rate in the heated section. T1 was 600°C, the dew point in the heated zone was -30°C, the dew point in the soaking zone was -20°C, the H₂ concentration in both the heated and soaking zones was 5% by volume, and the soaking duration was 37.5 seconds. The coating quality was visually assessed. Three experiments were conducted according to steps B)i) to B)iii) of the process for which protection is claimed, for heating rates of 10°Cs⁻¹, 9.5°Cs⁻¹, and 4.5°Cs⁻¹. These parameters are summarized in Table 3.

[0087] It can be observed that the coating quality is satisfactory for any heating rate lower than the maximum heating rate in the heating zone.

[0088]

[0089] Table 3

Claims

1. A method for manufacturing a steel plate having a thickness t in an apparatus comprising a preheating section, a heating section, and a soaking section, wherein the heating section has a maximum heating rate for the steel plate having a thickness t, the method comprising: A) Calibration steps, in which: i) A steel plate having a thickness t and the following chemical composition by weight percentage is heated from room temperature to a temperature T1 below 600°C: 0.05% ≤ C ≤ 0.50%, 0.3% ≤ Mn ≤ 8.0%, 0.01% ≤ Si ≤ 5%, and optionally at least one of the following elements by weight percentage: 0.01% ≤ Al ≤ 1.5%, B ≤ 0.004%, Co ≤ 0.1%, 0.001% ≤ Cr ≤ 1.00%, Cu ≤ 0.5%, 0.001% ≤ Mo ≤ 0.5%, Nb ≤ 0.1%, Ni ≤ 1.0%, Ti ≤ 0.1%, N ≤ 0.01%, P < 0.1%, S ≤ 0.01%, V < 0.2%, the remainder of said composition consisting of iron and unavoidable impurities. ii) The steel sheet is heated in atmosphere A1 at the maximum heating rate from T1 to a recrystallization temperature T2 in the range of 720°C to 1000°C, wherein atmosphere A1 contains 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and atmosphere A1 has a dew point DP. CAL , iii) The steel plate is then held in atmosphere A2 at temperature T2, atmosphere A2 comprising 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and the dew point of atmosphere A2 is at least -40°C. iv) The steel sheet is then hot-dip coated in a coating bath, and the quality of the coating is evaluated. v) a) If the coating quality meets the predetermined quality target, then at a lower dew point DP CAL Repeat calibration steps i) to iv) until the coating quality no longer meets the target, at the penultimate dew point DP. CAL Limited to DP1; b) If the coating quality is unsatisfactory, then at a higher dew point DP CAL Repeat calibration steps i) to v) until the coating quality meets the target, resulting in the final DP. CAL Limited to DP1; B) Production step, in which the steel plate having the thickness t and the chemical composition is subjected to: - Recrystallization annealing, wherein the recrystallization annealing sequentially includes a preheating step, a heating step, a homogenization step, and a cooling step, wherein: i) The preheating step includes heating from room temperature to a temperature T1 below 600°C. ii) The heating step includes heating from T1 to a recrystallization temperature T2 in the range of 720°C to 1000°C in an atmosphere A1 at a heating rate less than or equal to the maximum heating rate, wherein the atmosphere A1 contains 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and the dew point of the atmosphere A1 is set to at least the DP1 value determined during the calibration step. iii) The homogenization step includes maintaining a temperature in atmosphere A2 within the range of T2 - 30°C to T2 + 30°C, wherein atmosphere A2 contains 0.1 vol% to 90 vol% H2, the balance being inert gases and unavoidable impurities, and the dew point DP2 of atmosphere A2 is set to -40°C or higher. - Coating step, in which the steel plate is hot-dip coated in the coating bath.

2. The method according to claim 1, wherein, The weight percentage ratio of manganese to silicon in the bulk chemical composition of the steel plate follows the rule: Mn / Si < 4.

3. The method according to claim 1 or 2, wherein, The weight percentage ratio of aluminum to manganese in the bulk chemical composition of the steel plate follows the rule: Mn / Al < 1.

4. The method according to claim 1, wherein, The weight percentage ratio of manganese, aluminum and silicon in the main chemical composition of the steel plate follows the rule: Mn / (Al+(4×Si))< 1.

5. The method according to claim 1 or 2, wherein, In the preheating step i), the temperature T1 is below 550°C.

6. The method according to claim 5, wherein, In the preheating step i), the temperature T1 is below 500°C.

7. The method according to claim 1 or 2, wherein, In the preheating step i), the heating rate is higher than 50°C·s. -1 .

8. The method according to claim 1 or 2, wherein, In the heating step ii), the atmosphere A1 contains 1% to 20% by volume of H2, with the balance being inert gases and unavoidable impurities.

9. The method according to claim 1 or 2, wherein, The heating step ii) lasted from 10 seconds to 1000 seconds.

10. The method according to claim 1 or 2, wherein, In the heat homogenization step iii), the steel plate is maintained at a temperature ranging from (T2-10°C) to (T2+10°C).

11. The method according to claim 1 or 2, wherein, The heat equalization step iii) lasted from 10 seconds to 1000 seconds.

12. The method according to claim 1 or 2, wherein, In the homogenization step iii), the atmosphere A2 contains between 1% and 20% by volume of H2, with the balance being inert gases and unavoidable impurities.

13. The method according to claim 12, wherein, The coating bath is a zinc-based coating bath containing 0.1% to 0.3% by weight of aluminum.

14. The method according to claim 13, wherein, The coating bath is a zinc-based coating bath containing 0.1% to 0.3% by weight of aluminum and magnesium.

15. The method according to claim 12, wherein, The coating bath is an aluminum-based bath containing 5% to 15% by weight of silicon.

16. The method according to claim 1 or 2, wherein, In step ii) of the calibration step A), DP CAL The minimum value is -40°C, and in step v)a), if DP is at -40°C... CAL If the coating quality is satisfied, then -40°C is defined as DP1.

17. The method according to claim 1 or 2, wherein, The coating bath in step iv) of calibration step A) and the coating bath in the coating step of production step B) have the same base elements.

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