Rapid cooling of high yield strength steel plates

Abstract

A method for reducing non-uniformity defects of a strip cooled by spraying a liquid, or a mixture of gas and liquid, along a cooling zone of a continuous heat treatment line, wherein the cooling intensity is adjusted in the direction of movement of the strip such that the cooling intensity achieves a relative position between at least one temperature and the Leidenfrost temperature at which the metallurgical structure changes to minimize internal stress of the strip, and an apparatus for implementing the method are provided.

Description

Technology Field

[0001] The present invention relates to a continuous line for producing metal strips. The present invention is particularly related to a rapid cooling section of an annealing or galvanizing line for steel strips in which the strips are cooled at a cooling rate of more than 100 °C / s by spraying a liquid, or a mixture of liquid and gas.

[0002] In order to develop new steels with very high yield strengths, generally exceeding 500 MPa, which are required by automobile manufacturers to develop a lighter structure with high mechanical strength due to environmental constraints, heat treatment at a cooling rate exceeding 100 ℃ / s is necessary to establish a complex structure with a diverse distribution of various metallurgical phases among austenite, ferrite, pearlite, bainite, and martensite phases.

[0003] In particular, AHSS and UHSS steels with very high yield strength can be produced from a fully austenitic or ferrite-austenite mixed metallurgical structure by controlling the cooling rate of a continuous annealing line.

[0004] The heat treatment applied to the continuous line depends on the chemical composition of the steel, the state at the start of the line, and the expected mechanical properties at the end of the treatment. This includes a heating step to a heating end temperature of 750 to 950°C, heating to ambient or intermediate temperatures at a specific cooling rate for each metallurgical grade, and holding time at the temperature reached at the end of cooling.

[0005] For example, to obtain conventional steel, an annealing temperature higher than the austenitizing temperature may be required, followed by a holding time at this temperature, slow cooling for partial transformation from austenite to ferrite, and finally rapid cooling for transformation from austenite to martensite.

[0006] The mechanical properties achieved at the end of the process depend on the chemical composition of the steel and the control of the thermal process to obtain a specific microstructure. The quality of the product at the end of the process, namely the uniformity of mechanical properties and the flatness of the strip, depends on the type of cooling commonly used in such heat treatment methods, namely cooling by a gas jet, cooling by spraying a liquid or gas-liquid mixture, and cooling by quenching. After cooling, a tempering or aging step may optionally follow. Background Technology

[0007] Technical problems to be solved by the present invention

[0008] The rapid cooling step can cause non-uniformity defects in strips of variable type and amplitude depending on the applied cooling method.

[0009] A common non-uniformity defect that occurs during the heat treatment of metal strips is a wave type in the direction of travel for moderate cooling by a gas jet on thin strips, i.e., a cooling rate of less than 100 °C / s.

[0010] The above defect is of the long edge or long center type for rapid cooling by a gas jet, liquid spray, or a spray of a gas and liquid mixture, i.e., a cooling rate of about 100 to 500 °C / s. The above defect is of the dispersed blister type for providing a very rapid cooling rate by spraying a liquid or a gas and liquid mixture or by cooling by quenching, i.e., a cooling rate exceeding 500 °C / s. The observed non-uniformity defect is caused by the occurrence of internal strip stress during cooling.

[0011] The stress causing non-uniformity defects is due to the following:

[0012] ㆍ The geometry of the product, i.e., width, thickness, and initial non-uniformity,

[0013] ㆍ Product temperature distribution, direction of movement, product width, and product thickness for thick strips if possible,

[0014] Transformation of metallurgical phases

[0015] Changes in the thermophysical properties of a product as a function of temperature change during heat treatment.

[0016] The distribution and amplitude of stress controlling the type and amplitude of observed defects depend on the grade of steel and the distribution of metallurgical phases during the rapid cooling phase of the heat treatment method.

[0017] For example, in the case of steel grades known to those skilled in the art as a double-phase of mixed austenite and ferrite structure having varying proportions of austenite prior to the rapid cooling stage, the risk of non-uniformity defects caused by thermal heterogeneity and heterogeneity of transformation from the austenite phase to the martensite phase increases with the proportion of austenite available at the start of rapid cooling.

[0018] Technical Background

[0019] According to the latest technology, various techniques can be used to rapidly cool steel strips in a continuous line. Cooling with gas jets having varying hydrogen contents can achieve a cooling gradient of up to 200 ℃ / s. When cooling in contact with a liquid, a gradient exceeding 200 ℃ / s can be achieved depending on three types of techniques.

[0020] ㆍ Cooling by atomizing a gas and liquid mixture using a dual-fluid nozzle,

[0021] ㆍ Cooling by atomizing liquid using a single-fluid nozzle,

[0022] ㆍ Quenching by immersion in liquid, combined with liquid spraying if possible.

[0023] According to the latest technology, regardless of gas jet cooling technology, various methods can be applied to reduce non-uniformity defects caused by stress resulting from thermal gradient discontinuity observed in the direction of product movement by spraying liquid or a mixture of gas and liquid.

[0024] Regardless of the fluid used, interruption in the product's direction of movement is attributed to the following various causes:

[0025] ㆍ Increase in cooling gradient at the inlet for the rapid cooling section,

[0026] Cooling discontinuity between two continuous cooling zones,

[0027] ㆍ Contact with driving or stabilizing rollers.

[0028] When cooling by spraying a liquid or a mixture of liquid and gas, the Leidenfrost phenomenon, which consists of a sharp increase in the heat transfer coefficient due to convection during the transition between the vapor phase cooling regime and the liquid phase cooling regime, also creates a cooling discontinuity in the direction of product movement.

[0029] Therefore, during the rapid cooling step of the heat treatment method, the strip undergoes continuous internal tensile or compressive stress, which can cause irreversible deformation of the product if the stress is sufficiently high.

[0030] The type of deformation resulting from such discontinuity is a wave type in the direction of the product's movement.

[0031] EP1108795 by the present applicant describes a method for reducing the risk of non-uniformity in gas jet cooling sections by controlling cooling between continuous cooling boxes to reduce slope discontinuity. Such a method is applied to cooling by gas jets that are considered to be perfectly homogenized across the width of the product, and is also applied to grades that do not undergo metallurgical phase transformation in the temperature range associated with the cooling method, such as, for example, conventional steels having a whole ferrite structure.

[0032] According to the latest technology, various methods can be applied to reduce non-uniformity defects caused by temperature differences observed along the width of the product, for example, where the edges are colder than the center or the center is colder than the edges. Regardless of the fluid used, differences in thermal efficiency across the width of the product are attributed to the following various causes:

[0033] Differential fluid flow between the center and the edge,

[0034] ㆍ Initial non-uniformity defects,

[0035] ㆍ Imbalance in gas or fluid distribution,

[0036] ㆍ Vibration of the strip.

[0037] In particular, FR2940978 by the applicant, having the corresponding publication US20110270433A1, describes a method for controlling the homogeneity of cooling by spraying a liquid or a gas and liquid mixture along the width and / or length of a metal strip, determining a zone where the vapor film disappears, and controlling cooling parameters such as the temperature, velocity, flow rate or droplet size of the liquid, and the gas flow rate to cool the gas and liquid mixture by spraying it to maintain the vapor phase at all points. This patent does not consider the metallurgical transformation during cooling and the effect on the flatness of the strip. Therefore, there is no mention of modifications to the metallurgical structure (characteristics and proportions of phases) caused by changes in the cooling parameters.

[0038] Maintaining spray cooling over a vapor film and such a method, which is inefficient in terms of heat exchange, does not make it possible to achieve the cooling gradient required to produce steel with very high yield strength.

[0039] A solution based on the latest technology has been proposed to improve product flatness by reducing internal stresses caused by discontinuous temperature distribution in the direction of movement and non-uniform width of the products. However, such a solution is insufficient for the heat treatment of steels with very high yield strength, particularly those undergoing metallurgical phase changes during rapid cooling methods, such as dual-phase steel, TRIP steel, or martensitic steel.

[0040] These steels must be rapidly cooled at a gradient of more than 200°C / s by spraying a liquid or a liquid and gas mixture from a temperature of 780 to 850°C at the end of holding after heating or from a temperature of about 650 to 750°C after a first slow cooling to a temperature of about 400 to 50°C at the end of cooling depending on the grade.

[0041] The specificity of liquid spray cooling is based on observations of three continuous cooling methods:

[0042] In general, for high-temperature cooling surfaces exceeding about 600°C, vapor film cooling occurs. The vapor layer is completely insulated so that the strip does not come into contact with the liquid, leading to a stable and low heat exchange coefficient.

[0043] In general, for intermediate temperatures that limit the transition range, such as about 600°C to 200°C, the cooling method is unstable and a large change in the heat exchange coefficient is observed.

[0044] ㆍ In general, at lower temperatures of about 200°C to 100°C, cooling occurs in the nucleate boiling method, and as the surface temperature decreases, the heat exchange coefficient decreases rapidly.

[0045] ㆍ Generally, for low temperatures below 100℃, the above cooling method is a convection method.

[0046] The Leidenfrost temperature, indicated by point L on the graph in the attached Figure 2, is the temperature that separates the stable vapor film cooling range from the unstable transition range. The temperature indicated by the letter N on the graph separates the transition range and the cooling range in the nucleation boiling method.

[0047] Local thermal instability in the transition zone between gas phase cooling and liquid phase cooling, which is generally observed at 200 to 600°C, can cause non-uniformity defects in response to local thermal heterogeneity and desynchronization of phase transformation in different parts of the strip.

[0048] Metallurgical phase transformation stress resulting from the discontinuity of the product's physical properties is superimposed on differential expansion stress arising from the thermal gradient in the product's direction of movement and width.

[0049] In particular, the discontinuity in the gradient observed in the development curve of the expansion coefficient related to the volume change of metallurgical phases over the temperature range according to the rapid cooling method generates stress at the origin of non-uniformity defects observed in very high yield strength steels with complex metallurgical structures.

[0050] Methods to reduce thermal stress at the origin caused by non-uniform temperature distribution in the product's movement and width directions are not sufficient to guarantee the flatness required for use in products processed, particularly for automobile manufacturing.

[0051] Desynchronization of metallurgical phase transformation associated with low-amplitude temperature changes induces internal stresses high enough to observe local irreversible deformation. The problem to be solved

[0052] The present invention proposes improving the flatness of ultra-high yield strength steel by rapidly cooling it through spraying a liquid or gas and liquid mixture by optimizing the relative positions of thermal and metallurgical critical points to reduce internal stress caused by combined thermal and metallurgical phenomena. means of solving the problem

[0053] To this end, according to a first aspect of the present invention, a method is proposed for reducing non-uniformity defects of a cooled strip by spraying a liquid or gas and liquid mixture along a cooling zone of a continuous heat treatment line, wherein the cooling zone has means for controlling the cooling intensity along the cooling zone, and the method comprises the step of determining a thermal profile to be applied to the strip by the cooling zone in the cooling direction, i.e., a thermal development as a function of distance, wherein the thermal profile having a critical strip temperature called the Leidenfrost temperature is substantially reached simultaneously with a first metallurgical transformation temperature, or is reached after the onset of the first metallurgical transformation temperature and before the onset of the second metallurgical transformation temperature, and the method also comprises the step of applying the thermal profile determined by the means for controlling the cooling zone.

[0054] The above thermal profile is determined by computational means. It may also include experimental means.

[0055] In this specification, the expression "metallurgical transformation" refers to a change in metallurgical structure.

[0056] In the method according to the present invention, the cooling intensity is adjusted in the direction of movement of the strip to obtain a relative position between the Leidenfrost temperature and at least one metallurgical transformation temperature in order to minimize internal stress of the strip.

[0057] Therefore, it has a compensating effect due to the superposition of thermal stress and microstructural stress.

[0058] According to the present invention, the cooling intensity can be adjusted along the cooling zone as follows:

[0059] ■ Depending on the minimum cooling rate to obtain the selected metallurgical transformation, and / or

[0060] ■ So that the metallurgical transformation substantially begins at a selected temperature, and / or

[0061] ■ So that the Leidenfrost temperature is substantially the same as the selected value, and / or

[0062] ■ The Leidenfrost temperature is made to be substantially the same as the onset temperature of the metallurgical transformation.

[0063] When the target metallurgical transformation is a transformation from austenite to martensite, the cooling intensity can be controlled so that the Leidenfrost temperature is within a temperature range of ±50°C of the martensite transformation start temperature.

[0064] When at least two metallurgical transformations are targeted, the cooling intensity can be adjusted so that the Leidenfrost temperature becomes an intermediate temperature between the initiation temperature of the first metallurgical transformation and the initiation temperature of the last metallurgical transformation.

[0065] According to the present invention, the cooling intensity can be controlled by controlling the cooling length and / or controlling the flow rate and pressure of the cooling liquid, or a mixture of gas and liquid.

[0066] The proposed method to reduce the risk of internal stress and non-uniformity defects can be described as having four steps.

[0067] The first step is to calculate the minimum cooling rate to obtain the desired metallurgical transformation(s). If only the transformation from austenite to martensite is sought, the minimum cooling rate is calculated to avoid transformation to other phases, such as bainite.

[0068] The cooling rate required to obtain the desired metallurgical microstructure depends on the composition of the steel, the annealing temperature, and the holding time at said annealing temperature. According to the latest technology, the type of metallurgical transformation observed from the whole or partial austenite structure obtained at the end of the holding zone depends on the applied cooling rate.

[0069] For slow cooling rates of less than 1 ℃ / s, transformation from austenite to ferrite and pearlite structures is observed. For average cooling rates of less than 25 ℃ / s, transformation from austenite to ferrite and bainite structures is observed. For rapid cooling rates of 100 ℃ / s or more, transformation from austenite to martensite is mainly observed when the temperature at the end of cooling is lower than the temperature at which martensite transformation begins. The content of alloying elements such as manganese, chromium, or molybdenum affects these cooling rate critical values.

[0070] The second step consists of calculating the metallurgical transformation temperature. Metallurgical transformation is accompanied by significant changes in mechanical and thermophysical properties, particularly in the coefficient of thermal expansion. In fact, the transformation from the austenite phase to the ferrite, pearlite, bainite, or martensite phase is accompanied by changes in crystal structure and an increase in volume, resulting in a decrease in the coefficient of thermal expansion.

[0071] Such discontinuity observed in the development curve of the coefficient of thermal expansion during cooling methods generates internal stress due to volume changes accompanied by phase changes.

[0072] The combination of such metallurgical phenomena and thermal non-uniformity caused by the aforementioned cooling method results in a non-uniform stress distribution in which tensile and compressive stresses alternate at different points of the product.

[0073] The development of the metallurgical transformation temperature and expansion coefficient depends on the product composition and the annealing thermal cycle, which determines the proportion of austenite that can be transformed and the cooling rate.

[0074] The development curves of the metallurgical transformation temperature and thermophysical properties during rapid cooling can be accurately determined from operational data, such as the chemical composition of the product and the associated thermal cycle, by general laws available according to the latest technology. Typical values ​​for the bainite transformation temperature are 600 to 400°C. Typical values ​​for the temperature at which the martensite transformation begins are 250 to 450°C.

[0075] The third step consists of determining thermal critical points, particularly in the case of rapid cooling methods by liquid spraying, which entails the Leidenfrost phenomenon in which the exchange coefficient in the transition zone between the vapor and liquid phases increases significantly due to strong changes in internal strip stress.

[0076] The Leidenfrost temperature, which is the critical point of increase in the exchange coefficient between the strip and the fluid at the moment the insulating vapor layer ruptures, depends on many parameters, particularly spray characteristics such as the velocity and diameter of the droplets, the mesh size of the nozzle, the distance from the nozzle to the strip, temperature, and the characteristics of the fluid. These parameters can be experimentally determined for various types of spray nozzles to construct a table applicable to industrial production.

[0077] Typical Leidenfrost temperature values ​​are between 200°C for the lowest sputtering rate and 1000°C for the highest rate.

[0078] The fourth step consists of controlling cooling parameters. From the identification of a limited cooling gradient in Step 1, the identification of a limited metallurgical transformation critical temperature in Step 2, and the identification of a strong discontinuity in the limited cooling gradient in Step 3, optimal control of cooling parameters to minimize compressive stress at any point of the strip is proposed. Such control consists of modulating the thermal profile of the strip to obtain the relative positions of the Leidenfrost transition and metallurgical transformation critical points that minimize internal stress of the strip.

[0079] In particular, displacement to a temperature below the most critical transformation temperature relative to the Leidenfrost transition temperature—that is, the temperature that induces the largest change in volume during the phase transformation—allows for the limitation of internal stress due to the non-synchronization of the phase transformation, which is particularly important in the thermally unstable Leidenfrost transition zone.

[0080] Such optimal control is achieved by controlling the cooling length, controlling the flow rate and liquid pressure in the case of liquid spray cooling, and controlling the flow rate and pressure of the gas in the case of cooling by spraying a liquid and gas mixture, within the range of flow rates and pressures available for each zone of the cooling section.

[0081] According to the possibilities provided by the present invention, a liquid, or a mixture of gas and liquid, may be selected to be non-oxidizing with respect to the strip. The liquid is, for example, an aqueous solution containing 0.1 mass% and 6 mass% of formic acid. The gas is, for example, nitrogen, or a mixture of nitrogen and hydrogen.

[0082] According to a second aspect of the present invention, a cooling zone of a continuous processing line for a metal strip is proposed, said strip is configured to cool said strip by spraying a liquid or a mixture of gas and liquid by means of nozzles arranged on both sides thereof with respect to the plane of movement of said strip, said cooling zone having means for controlling a cooling intensity suitable for performing the steps of the method according to the present invention.

[0083] According to one possibility, the cooling zone comprises at least two rows of nozzles arranged transversely to the plane of movement of the strip, and the second row of nozzles in the direction of movement of the strip has a spraying speed equal to or greater than the spraying speed of the first row of nozzles.

[0084] The above-mentioned adjustment means may include means for adjusting the supply pressure and flow rate of the nozzle in the length and / or width of the product.

[0085] According to a third aspect of the present invention, a continuous heat treatment line comprising one or more of the cooling zone according to the second aspect of the present invention, or improvements thereof, is also proposed.

[0086] The line according to the present invention may include means for calculating the minimum cooling rate to obtain a desired metallurgical transformation.

[0087] The line according to the present invention may include means for calculating the metallurgical transformation temperature of the strip as a function of chemical composition and applied thermal cycle.

[0088] A line according to the present invention may include means for calculating the thermal profile of a strip along a cooling zone to determine an optimal cooling distribution along the cooling zone. Means for controlling the supply pressure and flow rate of a nozzle in the length and width of a product enable such a distribution to be obtained.

[0089] The line according to the present invention may include an experimental database for determining the Leidenfrost temperature associated with each production case.

[0090] According to another aspect of the present invention, a computer program product comprising instructions for guiding a cooling zone according to the present invention is proposed to execute the steps of the method according to the present invention.

[0091] According to another aspect of the present invention, a computer-readable medium on which a computer program product according to the present invention is recorded is proposed. Brief explanation of the drawing

[0092] In addition to the devices described above, the present invention comprises, but is by no means limited to, a certain number of other devices which will be discussed more clearly below in relation to the embodiments described with reference to the accompanying drawings. In these drawings: FIG. 1 is a schematic diagram of a longitudinal section of a strip in a cooling section according to one embodiment of the present invention. FIG. 2 is a graph of heat flux development as a function of surface temperature of a typical liquid spray cooling method. FIG. 3 is a graph of the coefficient of thermal expansion development as a function of temperature according to a first embodiment. FIG. 4 is a graph of strip temperature development in the direction of travel for a uniform pressure distribution of a spray nozzle. FIG. 5 is a graph of heat exchange coefficient development corresponding to the thermal profile of FIG. 3. FIG. 6 is a graph of longitudinal stress development at the edge of the strip for a thermal profile corresponding to the thermal profile of FIG. 3. FIG. 7 is a graph of strip temperature development in the direction of travel for a uniform pressure distribution at a spray nozzle and an optimized pressure distribution at a spray nozzle according to a first embodiment. FIG. 8 is a graph of heat exchange coefficient development as a function of strip temperature corresponding to the thermal profile of FIG. 6. FIG. 9 is a graph of longitudinal stress development at the edge of the strip for a longitudinal thermal profile corresponding to the thermal profile of FIG. 6. FIG. 10 is FIG. 11 is a graph showing the development of the coefficient of thermal expansion as a function of temperature according to the second embodiment. FIG. 12 is a graph showing the development of the strip temperature in the direction of movement for a uniform pressure distribution at the spray nozzle and an optimized pressure distribution at the spray nozzle according to the second embodiment. FIG. 12 is a graph showing the development of the heat exchange coefficient as a function of the strip temperature corresponding to the thermal profile of FIG. 9. FIG. 13 is a graph showing the development of longitudinal stress at the edge of the strip for a longitudinal thermal profile corresponding to the thermal profile of FIG. 9. Specific details for implementing the invention

[0093] According to the first embodiment, the rapid cooling zone of the continuous processing line metal strip shown in FIG. 1 is arranged to cool the strip (1) by spraying a liquid, or a mixture of gas and liquid, by nozzles arranged on both sides of the strip with respect to the plane of movement of the strip, and includes three rows (2) of single-fluid or dual-fluid nozzles in the direction of movement (F) of the strip, followed by rows of single-fluid or dual-fluid nozzles having a spray flow rate faster than the previous nozzles, and said rows of nozzles are arranged transversely with respect to the plane of movement of the strip.

[0094] In the case of carbon steel composed of 0.1% carbon, 1% manganese, and 1% silicon, it is annealed at a temperature above 850°C for complete austenitization and cooled from 650°C to 100°C by spraying with an average cooling gradient of about 500°C / s in a rapid cooling section, and a single gradient break can be observed at 450±15°C on the development curve of the coefficient of thermal expansion schematically described by curve (D2) of the attached Fig. 3, which corresponds to the complete martensitic transformation of the austenite phase.

[0095] The minimum cooling rate for complete martensitic transformation, that is, when austenite does not transform into other phases such as bainite or pearlite, can be determined from the transformation curve established for the composition of the steel, i.e., 200 °C / s in the case of the example being considered.

[0096] The thermal profile indicated by the curve in the attached FIG. 4 is obtained in a cooling section comprising four rows of nozzles for spraying liquid at a constant flow rate and pressure along the entire length of the cooling section to cool the strip from 650°C to 100°C at an average cooling rate of 480°C / s for a strip moving at 70 m / min. The critical points are located at 550°C, indicated by point A corresponding to the Leidenfrost temperature, and at 450°C, indicated by point B corresponding to the martensite transformation start temperature.

[0097] Points A and B also appear on the development curve of the heat exchange coefficient between the strip and the sprayed liquid, which is indicated by the curve in the attached Fig. 5. The curve shows an increase in the heat exchange coefficient between the two critical points. Thus, at point B of the martensitic transformation, the low initial thermal heterogeneity is amplified by the increase in the heat exchange coefficient, which will result in a significant risk regarding the desynchronization of the martensitic transformation and the related heterogeneity of the internal stress distribution.

[0098] Without considering the metallurgical transformation at point B, the development of longitudinal stress along the edge of the strip, indicated by curve C1 in the attached Fig. 6, shows a compressive stress peak at the start of cooling to 650°C, which corresponds to a decrease in the heat exchange coefficient at the end of the Leidenfrost transition zone at the thermal gradient discontinuity at 150°C on the cooling curve.

[0099] Considering the metallurgical transformation at 450°C due to the disturbance of the coefficient of thermal expansion at the point indicated by curve D2 in Fig. 3, it causes a compressive stress peak at the martensite transformation point and an increase in tensile stress upstream of the martensite transformation point, as illustrated by curve C2 in the attached Fig. 6. These results highlight the opposite effects of shrinkage of the strip during cooling, which causes tensile stress, and a relative increase in volume accompanying the metallurgical transformation, which causes compressive stress.

[0100] From the identification of critical points, it is proposed that the second thermal profile has an average cooling rate equal to 480 °C / s, indicated by curve T2 in the attached Fig. 7, and the critical point C corresponds to the simultaneous occurrence of the martensite transformation temperature and the Leidenfrost temperature at 450 °C.

[0101] The thermal profile optimized in this way can be obtained by differently controlling the following two consecutive zones of the cooling section:

[0102] ㆍ To delay the appearance of the Leidenfrost point, a first zone having a length equal to 3 / 4 of the total length of the cooling section where the pressure is limited to 1 bar,

[0103] A second zone having a length equal to 1 / 4 of the total length of the cooling section, to which a maximum pressure of 8 bar is applied.

[0104] Points A, B, and C are also shown on the development curve of the heat exchange coefficient between the strip and the sprayed liquid, indicated by curves H1 and H2 in the attached Fig. 8, and compared with the slope observed at point B on curve H1, curve H2 highlights the reduction in the amplitude of the change in the heat exchange coefficient for small temperature changes of the strip in the martensite transformation zone.

[0105] Similarly, the development of longitudinal stress at the edge of the strip, indicated by curve C2 in the attached Fig. 9, highlights a more favorable distribution of stress along the strip, with an increase in tensile stress upstream of the non-critical martensite transformation point and a decrease in compressive stress at the onset of buckling and irreversible deformation downstream of the martensite transformation point.

[0106] Therefore, the coexistence of the Leidenfrost temperature and the martensite transformation temperature, indicated by point C in the attached Fig. 8, induces internal stresses of opposite signs, namely shrinkage at the Leidenfrost point and expansion at the metallurgical transformation point, thereby reducing the amplitude of internal stress and the risk of associated non-uniformity defects.

[0107] Furthermore, the location of the martensite transformation point at a more favorable point on the heat exchange coefficient development curve reduces the risk of amplification of local coupling phenomena of thermal and metallurgical heterogeneity.

[0108] According to the second embodiment, in the case of carbon steel composed of 0.25% carbon, 1% manganese, and 1% silicon, it is annealed at a temperature above 850°C for complete austenitization and cooled from 650°C to 100°C in a metal cooling section by spraying at an average cooling rate of about 100°C / s for a strip moving at 15 m / min, and two inclined discontinuities can be observed on the development curve of the coefficient of thermal expansion, the first discontinuity at 550°C corresponding to the bainite transformation and the second discontinuity at 400°C corresponding to the martensite transformation of the residual austenite are schematically illustrated in the curve of the attached FIG. 10 corresponding to the total martensite transformation of the austenite phase.

[0109] The thermal profile obtained for uniform spraying along the length of the product, indicated by curve T1 in the attached Fig. 11, highlights three critical points: the Leidenfrost transition point indicated by point A at 600°C, the first metallurgical transformation point indicated by point B at 550°C, and the second metallurgical transformation point indicated by point C at 400°C.

[0110] The thermal profile obtained for the optimized control of cooling distribution follows:

[0111] ㆍ A first zone having a length equal to 3 / 4 of the total length of the cooling section where the pressure is limited to 0.5 bar,

[0112] ㆍ A second zone having a length equal to 1 / 4 of the total length of the cooling section to which a maximum pressure of 8 bar is applied.

[0113] Curve T2 of the attached Fig. 11 illustrates three critical points, namely the first metallurgical transformation point indicated by point D, which shifts to 560°C considering the reduction in cooling rate, the second metallurgical transformation point indicated by point F, which shifts to 410°C, and the Leidenfrost transition point indicated by point E at an intermediate temperature of 500°C between the two metallurgical transformation points.

[0114] Points (A, B, C, D, E, and F) are also shown on the development curve of the heat exchange coefficient between the strip and the sprayed liquid, described by the curves (H1 and H2) of the attached Fig. 12, which correspond respectively to the thermal profiles (T1 and T2) of the attached Fig. 11, highlighting the amplitude of change in the heat exchange coefficient for small changes in the strip temperature at the metallurgical transformation points.

[0115] A comparison of longitudinal stresses at the edge of the strip calculated under such assumptions, described by curves (C1 and C2) on the graph of the attached Fig. 13 corresponding respectively to the thermal profiles (T1 and T2) of Fig. 11, highlights the optimization of the stress distribution along the strip appearing at the two metallurgical transformation points and downstream of the last transformation point, along with a reduction in compressive stress.

[0116] The present invention is not limited to the examples described so far, and many modifications to such examples may be made without departing from the scope of the invention. Furthermore, various features, forms, variations, and embodiments of the present invention may be grouped together in various combinations, provided they are not incompatible or mutually exclusive.

Claims

Claim 1 A method for reducing non-uniformity defects of a strip being cooled by spraying a liquid or a mixture of gas and liquid along a cooling zone of a continuous heat treatment line, wherein the cooling zone has means for controlling the cooling intensity along the cooling zone, the method comprising: a step of determining, by means of calculation, a thermal profile to be applied to the strip by the cooling zone in the cooling direction, wherein the thermal profile has a critical strip temperature called the Leidenfrost temperature, wherein the critical strip temperature is reached simultaneously with a first temperature at which the metallurgical structure changes, or is reached after the onset of the first metallurgical transformation temperature and before the onset of the second metallurgical transformation temperature; and a step of applying the thermal profile determined by means of controlling the cooling zone, wherein the change in the metallurgical structure is a change from austenite to martensite, and the cooling intensity is controlled such that the Leidenfrost temperature is within a temperature range of ± 50°C from the temperature at which the martensite structure change begins. Claim 2 A method according to claim 1, wherein the cooling intensity is controlled along the cooling zone according to the minimum cooling rate to achieve a selected metallurgical structural change. Claim 3 A method according to claim 1, wherein the cooling intensity is controlled along the cooling zone such that a change in the metallurgical structure begins at the first temperature. Claim 4 A method according to claim 1, wherein the cooling intensity is adjusted along the cooling zone so that the Leidenfrost temperature is equal to a predetermined value. Claim 5 A method according to claim 1, wherein the cooling intensity is controlled along the cooling zone such that the Leidenfrost temperature is equal to the metallurgical structural change initiation temperature. Claim 6 delete Claim 7 A method according to claim 1, wherein the cooling intensity is controlled along the cooling zone so that the Leidenfrost temperature is at an intermediate temperature between the initiation temperature of the first metallurgical structural change and the initiation temperature of the last metallurgical structural change. Claim 8 A method according to claim 1, wherein the cooling intensity is controlled by controlling the cooling length along the cooling zone and / or controlling the flow rate and pressure of the cooling liquid, or the mixture of gas and liquid. Claim 9 A method according to any one of claims 1 to 5, 7 and 8, wherein the liquid, or the mixture of gas and liquid, is non-oxidized with respect to the strip. Claim 10 A cooling zone of a continuous processing line for a metal strip, wherein the cooling zone is arranged to cool the strip (1) by spraying a liquid, or a mixture of gas and liquid, by means of nozzles arranged on both sides of the strip with respect to the plane of movement of the strip, and the cooling zone has means for controlling the cooling intensity to perform the steps of the method according to claim 1. Claim 11 In claim 10, a cooling zone comprising at least two rows of nozzles arranged transversely in the movement plane of the strip, wherein the nozzles of the second row (3) have a spraying speed equal to or greater than the spraying speed of the nozzles of the first row in the movement direction (F) of the strip. Claim 12 A cooling zone according to claim 10 or 11, wherein the adjusting means comprises means for adjusting the supply pressure and flow rate of the nozzles in the length and width of the product. Claim 13 A continuous heat treatment line comprising a cooling zone according to claim 10 or 11. Claim 14 A continuous heat treatment line according to claim 13, further comprising means for calculating the temperature at which the metallurgical structure of the strip changes as a function of the chemical composition of the metallurgical structure and the applied thermal cycle. Claim 15 A continuous heat treatment line according to claim 13, further comprising an experimental database for determining the Leidenfrost temperature associated with each production case. Claim 16 A continuous heat treatment line according to claim 13, further comprising means for calculating the thermal profile of the strip along the cooling zone to determine the cooling distribution according to the cooling zone. Claim 17 A computer program stored on a computer-readable medium, wherein the computer program comprises instructions that cause a cooling zone according to claim 10 to perform the steps of the method according to claim 1. Claim 18 A computer-readable medium on which a computer program product according to paragraph 17 is recorded.

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