A method for preventing hot flat rolling of a dual-phase steel coil

Abstract

This invention discloses a method for preventing hot duplex steel coils from flattening. Based on the latest flattening theory, namely the deformation theory caused by phase transformation expansion and phase transformation plasticity, for hot duplex steel coils with different carbon contents, the method selects a reasonable cooling mode, coiling temperature, coiling tension, and residence time by considering the contribution of phase transformation expansion and phase transformation plasticity to the deformation of the duplex steel coil, combined with phase transformation thermodynamics, phase transformation kinetics, and strip strength, thereby achieving the purpose of eliminating hot duplex steel coil flattening.

Description

Technical Field

[0001] This invention belongs to the field of steel rolling technology and relates to a method for preventing hot duplex steel coils from flattening. Background Technology

[0002] Dual-phase steel is a common type of steel composed of ferrite and martensite phases. It has a low yield strength ratio, a high work hardening index, and excellent ductility. Dual-phase steel is generally produced by hot rolling followed by cold rolling, and then heat treatment to obtain the desired properties. In the hot rolling process, duplex steel is rolled and cooled into coils. After being pulled out of the coil, it is transported and stored horizontally. The steel coil may collapse under its own weight [Wang Chang, Yu Yang, Wang Lin, et al. Analysis of the causes and mechanisms of flattened coils of bundled steel. Physical Testing, 2014, 32(6):40-44], phase transformation [Zhang Hanlong, Wang Li. The latest progress in the study of flattened defects of hot-rolled steel coils. Baosteel Technology, 2017, (4):56-67] and the combined effect of phase transformation plasticity [Cho HH, Cho YG, Im YR, et al. A finite element analysis for asymmetric contraction after coiling of hot-rolled steel. Journal of Materials Processing Technology, 2010, 210(6-7):907-913.], i.e., flattening occurs. This type of flat coil is extremely detrimental to subsequent cold rolling processes. At best, it will affect the uncoiling speed of downstream units; at worst, it will prevent downstream processes from opening the steel coil normally, thus affecting the yield.

[0003] To address the issue of flat rolls, various solutions have been proposed, including physical support and process optimization. While these methods can reduce the flat roll rate to some extent, they also bring other problems, such as poor surface quality and increased difficulty in subsequent processing, which affect production capacity.

[0004] Patent CN200940057Y discloses a manual coiling device for metal strip, including a pull rod, a cam handle, and related support sleeves. The support sleeve, through its inner diameter, rotates the cam handle to tighten the steel coil exhibiting flattening, preventing flattening and loosening of the inner ring. This technology, employing a physical method, can solve the flattening problem for all steel grades. However, it involves increased equipment, occupies production space, and has low operability and feasibility, severely impacting rolling rhythm and reducing production capacity.

[0005] Patent CN107812789A discloses a method to prevent hot-rolled coil flattening, which involves coiling duplex steel at 650-700℃ and high-strength low-alloy steel at 500-550℃, with a dwell time of 5-20 seconds and a coiling tension of 5-15 MPa. This technology reduces the flattening rate by controlling the process parameters during coiling, but it also has some problems. Excessive coiling temperature for duplex steel results in poor surface quality, affecting subsequent processing and reducing the yield.

[0006] Patent CN108754104A discloses a method for eliminating the flattening defect in hot-rolled coils of 590MPa grade duplex steel. Specifically, the method involves controlling the finishing rolling temperature at 890±20℃, rapidly cooling to 740±20℃, and then slowly cooling to 680±20℃ before coiling. The coiling tension is 30-37kN, and the residence time in the coiling machine is 10-15s. This technology addresses the issue from a phase transformation perspective, controlling the microstructure during coiling to be ferrite and pearlite with sufficient phase transformation, avoiding the formation of bainite, and solving the problem of flattening caused by phase transformation expansion. However, due to the excessively high coiling temperature, the surface quality is poor, resulting in red iron scale, which is very detrimental to subsequent cold rolling processes.

[0007] Patent CN102335681B discloses a coiling method to prevent hot-rolled strip steel from flattening. Specifically, the hot-rolled strip steel is coiled after laminar flow at a temperature of 500-600℃, with a coiling machine dwell time of 30-60 seconds. While this method can solve some coil flattening problems, it does not address the issue of hot-rolled duplex steel. Furthermore, the process parameters vary too much, making the selection of specific process parameters unclear. The excessively long dwell time in the coiling machine leads to low cooling water temperatures at the head and tail of the strip, resulting in poor temperature uniformity throughout the coil. This causes significant performance deviations at the head, middle, and tail, hindering subsequent processing, affecting rolling rhythm, and reducing production capacity.

[0008] Patent JP5594578B2 discloses a method for coiling hot-rolled strip steel, in which the coiling temperature decreases along the length of the strip, with a target coiling temperature of 400-700℃, and the head 1 / 10-1 / 3 of the strip being 50-200℃ higher. This method is ineffective and severely affects the uniformity of the hot-rolling temperature, resulting in significant performance differences along the length and impacting subsequent processing.

[0009] Patent WO2013137068A1 discloses a method for manufacturing hot-rolled strip steel, which addresses the issue of flattened coils. The method primarily involves controlling the coiling temperature: the inner coil temperature is 30-80°C higher than the target temperature, while the outer coil temperature is the target temperature. Along the length of the strip, the coiling temperature decreases, resulting in a higher ferrite content in the inner coil and reduced phase transformation expansion. However, this method still suffers from performance variations along the length of the coil, leading to quality issues and significantly impacting subsequent processing, resulting in reduced cold-rolled yield.

[0010] During the hot rolling process of duplex steel, after laminar cooling and coiling, the phase transformation expansion caused by bainitic phase transformation and the plastic deformation caused by phase transformation plasticity result in flattening of the steel coil. After laminar flow coiling, the strip still undergoes some phase transformation. Volume expansion occurs during the transformation from austenite to pearlite and bainite, leading to interlaminar slippage within the coil. When the slippage distance is too large, it disrupts the overall stability of the coil, resulting in flattening. Simultaneously, phase transformation plasticity also exists during the transformation from austenite to pearlite and bainite. Due to the weight of the steel coil, forces below the elastic limit of the weaker phase (such as austenite) exist during the phase transformation, causing irreversible deformation. This is unavoidable and can only be offset by increasing the coiling tension to reduce the deformation caused by phase transformation plasticity or extending the retention time in the coiler to complete the phase transformation.

[0011] In view of the above, the present invention, in order to prevent the problem of hot dual-phase steel coil flattening, selects reasonable coiling temperature, coiling tension and dwell time based on the contribution of phase transformation expansion and phase transformation plasticity to the deformation of hot steel coil during the phase transformation process, and combines three aspects: phase transformation thermodynamics, phase transformation kinetics and strip strength, so as to achieve the purpose of eliminating steel coil flattening. Summary of the Invention

[0012] To address the aforementioned deficiencies in existing technologies, the present invention aims to provide a method for preventing hot duplex steel coils from flattening. Based on the latest flattening theory, namely the deformation theory caused by phase transformation expansion and phase transformation plasticity, and considering the contribution of phase transformation expansion and phase transformation plasticity to the deformation of hot duplex steel coils with different carbon contents, and combining phase transformation thermodynamics, phase transformation kinetics, and strip strength, a reasonable cooling mode, coiling temperature, coiling tension, and residence time are selected to eliminate the flattening of hot duplex steel coils.

[0013] To achieve the above objectives, the present invention adopts the following technical solution:

[0014] This invention provides a method for preventing hot dual-phase steel coils from flattening, comprising:

[0015] S1. Develop a cooling method based on the phase transformation kinetics of pearlite and bainite to promote the transformation of austenite to pearlite and bainite before coiling and avoid ferrite transformation.

[0016] S2, set the coiling temperature. The coiling temperature is set according to the bainite transformation temperature and the phase transformation plasticity coefficient. The coiling temperature of the duplex steel is controlled to be lower than the bainite transformation temperature and within the temperature range of low phase transformation plasticity coefficient.

[0017] S3, set the winding tension according to the amount of phase transformation expansion and phase transformation plasticity during the transformation from austenite to bainite;

[0018] S4, set the dwell time of the steel coil after winding is completed. The dwell time of the duplex steel coil in the coiler is set according to the strip strength and phase change speed.

[0019] Preferably, in step S1, the cooling method for the hot dual-phase steel is to use dense cooling in the front section of the layer cooling zone, and to immediately water cool to the target temperature and complete the coiling after finishing rolling, with the cooling rate controlled at 50-80℃ / s.

[0020] Preferably, in step S2, the winding temperature is 10-20°C lower than the bainite transformation temperature.

[0021] Preferably, in step S2, the winding temperature is 520–590°C.

[0022] Preferably, in step S3, the winding tension is 38–48 kN.

[0023] Preferably, in step S4, the dwell time of the bidirectional steel coil in the coiler is 20 to 40 seconds.

[0024] Preferably, the dual-phase steel comprises the following chemical composition by mass percentage: C: 0.04-0.1%, Si: 0.1-0.5%, Mn: 1.2-2.2%, P: 0-0.02%, S: 0-0.02%, Al: 0-0.05%, Cr: 0-0.4%, with the balance being Fe and unavoidable impurities.

[0025] Preferably, the flattening rate of the dual-phase steel is 0%.

[0026] The method for preventing hot dual-phase steel coils from flattening provided by this invention has the following beneficial effects:

[0027] 1. The method for preventing hot duplex steel coil flattening of the present invention is based on the latest flattening theory, namely the deformation theory caused by phase transformation expansion and phase transformation plasticity. For hot duplex steel coils with different carbon contents, the method selects a reasonable cooling mode, coiling temperature, coiling tension and residence time by combining the contribution of phase transformation expansion and phase transformation plasticity to the deformation of the duplex steel coil, and considering the three aspects of phase transformation thermodynamics, phase transformation kinetics and strip strength, thereby achieving the purpose of eliminating hot duplex steel coil flattening.

[0028] 2. The method of the present invention for preventing hot dual-phase steel coil flattening requires selecting a bainite region with a low phase transformation plasticity coefficient when selecting the coiling temperature to reduce the degree of flattening caused by phase transformation plasticity. At the same time, it is necessary to consider the slippage phenomenon of the inner ring of the steel coil caused by bainite phase transformation expansion, appropriately increase the coiling temperature, control a certain phase transformation expansion, and solve the flattening problem. That is, the coiling temperature is controlled at slightly lower than the bainite transformation temperature and within the temperature range of low phase transformation plasticity coefficient.

[0029] 3. The method of the present invention for preventing hot dual-phase steel coil flattening involves controlling the coiling temperature, formulating a reasonable cooling mode based on phase transformation kinetics, determining the corresponding residence time in the coiler based on the strip strength and phase transformation speed, and setting the coiling tension based on the phase transformation expansion and phase transformation plasticity, thereby ultimately avoiding the phenomenon of hot dual-phase steel coil flattening. Attached Figure Description

[0030] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0031] Figure 1 This is a graph showing the phase transformation plasticity coefficient of the biaxial steel with different carbon contents according to the present invention.

[0032] Figure 2 This is a microstructure diagram of the duplex steel with a carbon content of 0.06 wt% after curling in Example 1 of this invention;

[0033] Figure 3 This is a microstructure diagram of the duplex steel with a carbon content of 0.08 wt% after curling in Example 2 of this invention;

[0034] Figure 4 This is a microstructure diagram of the duplex steel with a carbon content of 0.10 wt% after it has been rolled up in Example 3 of this invention. Detailed Implementation

[0035] To better understand the above-mentioned technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with embodiments.

[0036] The method for preventing hot-state duplex steel coil flattening provided by this invention is based on the theory of deformation caused by phase transformation expansion and phase transformation plasticity. Targeting hot-state duplex steel coils with different carbon contents, it is based on the mechanism of deformation caused by phase transformation expansion and phase transformation plasticity, and combines three aspects: phase transformation thermodynamics, phase transformation kinetics, and strip strength. Reasonable technical parameters are formulated, including cooling method, coiling temperature, coiling tension, and residence time of the duplex steel coil in the coiler, to achieve the purpose of eliminating hot-state duplex steel coil flattening. The method for preventing hot-state duplex steel coil flattening of this invention mainly targets duplex steel with the following chemical composition by mass percentage: C: 0.04-0.1%, Si: 0.1-0.5%, Mn: 1.2-2.2%, P: 0-0.02%, S: 0-0.02%, Al: 0-0.05%, Cr: 0-0.4%, with the balance being Fe and unavoidable impurities. The method for preventing hot-state duplex steel coil flattening of this invention specifically includes the following steps:

[0037] S1. Develop a cooling method based on the phase transformation kinetics of pearlite and bainite to promote the transformation of austenite to pearlite and bainite before coiling and avoid ferrite transformation.

[0038] Specifically, in the hot rolling process of duplex steel, the cooling method needs to be set according to the phase transformation kinetics of pearlite and bainite. Before coiling, the transformation of austenite to pearlite and bainite should be promoted to avoid the phenomenon of ferrite transformation, which would lead to low strength of the steel coil. The hot duplex steel adopts dense cooling in the front section of the layer cooling zone, and is immediately water-cooled to the target temperature after finishing rolling and coiling is completed. The cooling rate is controlled at 50-80℃ / s to achieve the effect of rapid cooling after rolling.

[0039] S2, set the coiling temperature. The coiling temperature is set according to the bainite transformation temperature and the phase transformation plasticity coefficient. The coiling temperature of the duplex steel is controlled to be lower than the bainite transformation temperature and within the temperature range of low phase transformation plasticity coefficient.

[0040] Specifically, for duplex steels with a carbon content of 0.04–0.1 wt%, the strip strength is lower at higher temperatures, increasing the likelihood of coil flattening. Furthermore, the phase transformation plasticity coefficient changes with increasing carbon content, such as… Figure 1 As shown, with the increase of carbon content, the phase transformation plasticity coefficient gradually decreases, and the amount of deformation caused by phase transformation plasticity gradually decreases. At the same time, the phase transformation plasticity coefficient of pearlite is higher than that of bainite. If the bainite transformation temperature is selected as the winding temperature, it will further promote the occurrence of flat winding.

[0041] Therefore, the coiling temperature needs to be set based on the theories of phase transformation expansion and phase transformation plasticity. The coiling temperature for duplex steel should be set slightly lower than the bainite transformation temperature, for example, 10-20°C lower, and within the temperature range of low phase transformation plasticity coefficients. Simultaneously, the surface quality of the steel coil should be ensured, with no obvious defects. In other words, the coiling temperature should be selected within the bainite region with a low phase transformation coefficient to reduce the flattening caused by phase transformation plasticity. At the same time, the slippage of the inner coil caused by bainite phase transformation expansion should be considered, and the coiling temperature should be appropriately increased to control a certain amount of phase transformation expansion and solve the flattening problem. In a specific embodiment, the coiling temperature can be set to 520-590°C.

[0042] S3, set the winding tension according to the amount of phase transformation expansion and phase transformation plasticity during the transformation from austenite to bainite;

[0043] Specifically, the setting of the coiling tension needs to compensate for the phase transformation expansion and plastic deformation caused by the phase transformation plasticity during the transformation of austenite to bainite, while also avoiding changes in the strip width caused by excessive coiling tension. Therefore, the coiling tension of duplex steel is set to 38-48 kN.

[0044] S4, set the dwell time of the steel coil after winding is completed. The dwell time of the duplex steel coil in the coiler is set according to the strip strength and phase change speed.

[0045] Specifically, after coiling, the dwell time of the duplex steel coil in the coiler needs to be considered to promote phase transformation, reduce phase transformation expansion and plastic deformation caused by phase transformation plasticity. Therefore, the dwell time of the duplex steel coil in the coiler is set according to the strip strength and phase transformation speed, and can be set to 20-40s.

[0046] By using the methods described above, the phenomenon of flattening of dual-phase steel coils can ultimately be avoided.

[0047] The method for preventing hot dual-phase steel coils from flattening according to the present invention will be further described below with specific examples.

[0048] Example 1

[0049] A certain duplex steel hot-rolled coil has the following chemical composition by mass percentage: C: 0.065%, Si: 0.10%, Mn: 1.2%, P: 0.015%, S: 0.01%, Al: 0.04%, Cr: 0.15%, with the balance being Fe and unavoidable impurities. After finishing rolling, it enters the pre-cooling section of the laminar flow cooling zone and undergoes rapid cooling at a rate of 80℃ / s to ensure sufficient phase transformation before coiling, promoting the transformation of austenite to pearlite and bainite. The coiling temperature is 570±20℃, slightly lower than the bainite transformation temperature and within the low phase transformation plasticity coefficient range, ensuring minimal phase transformation expansion and plastic deformation caused by phase transformation plasticity. The coiling tension is 43–48 kN to compensate for the amount of plastic deformation caused by phase transformation expansion and phase transformation plasticity during the austenite-to-bainite transformation, while avoiding strip width variations caused by excessive tension. The residence time is 40s, which further promotes phase transformation and reduces phase transformation expansion and plastic deformation caused by phase transformation plasticity.

[0050] Select hot-rolled steel coil 1-1 with the same composition as comparative example 1. No special treatment is used. The coil is wound according to the normal production process.

[0051] Table 1 compares the flattening and rolling results of the two schemes. Figure 2 This is the microstructure of the duplex steel 2 after winding in this embodiment. The bainite transformation temperature of the duplex steel 1 is 594℃. After winding at a temperature of 570±20℃, the microstructure mainly consists of bainite and a small amount of banded structure.

[0052] Example 2

[0053] A certain duplex steel hot-rolled coil has the following chemical composition by mass percentage: C: 0.079%, Si: 0.35%, Mn: 1.7%, P: 0.015%, S: 0.01%, Al: 0.04%, Cr: 0.05%, with the balance being Fe and unavoidable impurities. After finishing rolling, it enters the pre-cooling zone and undergoes rapid cooling at a rate of 65℃ / s to ensure sufficient phase transformation before coiling, promoting the transformation of austenite to pearlite and bainite. The coiling temperature is 550±20℃, slightly lower than the bainite transformation temperature and within the low phase transformation plasticity coefficient range, ensuring minimal phase transformation expansion and plastic deformation caused by phase transformation plasticity. The coiling tension is 40–45 kN to compensate for the amount of plastic deformation caused by phase transformation expansion and phase transformation plasticity during the austenite-to-bainite transformation, while avoiding strip width variations caused by excessive tension. The residence time is 30s, which further promotes phase transformation and reduces phase transformation expansion and plastic deformation caused by phase transformation plasticity.

[0054] Select hot-rolled steel coil 2-1 with the same composition as comparative example 2. No special treatment was used, and it was wound according to the normal production process.

[0055] Table 1 compares the flattening and rolling results of the two schemes. Figure 3 This is the microstructure of the duplex steel 2 after winding in this embodiment. The bainite transformation temperature of the duplex steel 2 is 572℃. After winding at a temperature of 550±20℃, the microstructure mainly consists of bainite and a very small amount of banded structure.

[0056] Example 3

[0057] A certain dual-phase steel hot-rolled coil has the following chemical composition by mass percentage: C: 0.1%, Si: 0.15%, Mn: 2.0%, P: 0.015%, S: 0.01%, Al: 0.04%, Cr: 0.25%, with the balance being Fe and unavoidable impurities. After finishing rolling, it enters the pre-cooling section of the laminar flow cooling zone and undergoes rapid cooling at a rate of 50℃ / s to ensure sufficient phase transformation before coiling, promoting the transformation of austenite to pearlite and bainite. The coiling temperature is 530±20℃, slightly lower than the bainite transformation temperature and within the low phase transformation plasticity coefficient range, ensuring minimal phase transformation expansion and plastic deformation caused by phase transformation plasticity. The coiling tension is 38–43 kN to compensate for the amount of plastic deformation caused by phase transformation expansion and phase transformation plasticity during the austenite-to-bainite transformation, while avoiding strip width variations caused by excessive tension. The residence time is 20s, which further promotes phase transformation and reduces phase transformation expansion and plastic deformation caused by phase transformation plasticity.

[0058] Hot-rolled steel coil 3-1 with the same composition was selected as comparative example 3. No special treatment was used, and it was wound according to the normal production process.

[0059] Table 1 compares the flattening and rolling results of the two schemes. Figure 3This is the microstructure of the duplex steel 3 after winding in this embodiment. The bainite transformation temperature of the duplex steel 3 is 556℃. After winding at a temperature of 550±20℃, the microstructure mainly consists of bainite and a very small amount of banded structure.

[0060] Table 1 shows the flatness of the double-eye steel coils produced in the examples and comparative examples.

[0061]

[0062]

[0063] Table 1 shows the statistics of flattening of duplex steel coils produced in the examples and comparative examples. The test results show that among duplex steels with different carbon contents, 30 coils were produced without the improved flattening method, and 8 coils exhibited flattening, resulting in an overall flattening rate of 26.7%. Using the method of this invention, there was no flattening problem. Therefore, the method of this invention can solve the problem of flattening in hot-rolled duplex steel.

[0064] Those skilled in the art should recognize that the above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Any variations or modifications to the above embodiments that are within the spirit and essence of the present invention will fall within the scope of the claims of the present invention.

Claims

1. A method of preventing hot flat rolling of a dual phase steel coil, characterized by, include: S1. Develop a cooling method based on the phase transformation kinetics of pearlite and bainite. This method promotes the transformation of austenite to pearlite and bainite before coiling, avoiding ferrite transformation. The cooling method for hot duplex steel is to use rapid cooling in the front section of the layer cooling zone, and immediately water cooling to the target temperature after finishing rolling and coiling is completed, with the cooling rate controlled at 50-80℃ / s; S2, Set the coiling temperature. The coiling temperature is set according to the bainite transformation temperature and the phase transformation plasticity coefficient. The coiling temperature of the duplex steel is controlled to be lower than the bainite transformation temperature and within the temperature range of low phase transformation plasticity coefficient. The winding temperature is 520-590°C, which is 10-20°C lower than the bainite transformation temperature. S3, Set the winding tension based on the phase transformation expansion and phase transformation plasticity during the austenite to bainite transformation. The winding tension is 38–48 kN; S4 sets the dwell time of the steel coil after winding. The dwell time of the duplex steel coil in the coiler is set according to the strip strength and phase transformation speed. The dwell time of the duplex steel coil in the coiler is 20–40 seconds. The dual-phase steel comprises the following chemical composition by mass percentage: C: 0.04-0.1%, Si: 0.1-0.5%, Mn: 1.2-2.2%, P: 0-0.02%, S: 0-0.02%, Al: 0-0.05%, Cr: 0-0.4%, with the balance being Fe and unavoidable impurities.

2. The method of preventing flat rolling of hot dual phase steel coils as claimed in claim 1 wherein, The flattening rate of the duplex steel is 0%.

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