A method for improving rolling efficiency of a low carbon equivalent 355mpa grade normalized rolled thick plate

Abstract

The application discloses a method for improving rolling efficiency of a low-carbon-equivalent 355MPa grade normalized rolled thick plate, and the method comprises the following steps: after a billet is heated by each heating section of a heating furnace, the billet surface temperature is reduced to 1000-1050 DEG C, and when the rolling rhythm is ensured and the billet surface temperature in the soaking section is in the range of 1000-1050 DEG C, the time when the billet is discharged is determined; the billet is rolled by one-stage rolling, the rolling pass number is controlled to be 2-4 passes in the cross rolling and spreading stage, the pass reduction rate is 8%-15%, and the rolling speed is controlled to be 2.0-2.2 m / s ‑1 ; the rolling pass number is controlled to be 4-8 passes in the longitudinal rolling stage, the pass reduction rate is 15%-36%, and the rolling speed is controlled to be 2.2-3.8 m / s ‑1 ; and the finish rolling temperature is controlled to be 860-900 DEG C. According to the method, the low-temperature characteristics of the billet after being discharged are utilized, the "temperature difference" characteristics of the billet in the thickness direction in each deformation stage are fully utilized, the double grain refining effects of recrystallization and large reduction grain crushing are realized, the grain size is more than 9 levels, the waiting time is saved, and the rolling efficiency is obviously improved.

Description

Technical Field

[0001] This invention relates to a normalizing rolling method for low-carbon equivalent low-alloy steel, and particularly to a method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent, belonging to the field of steel plate rolling process control technology. Background Technology

[0002] Conventional normalizing rolling involves controlling the final rolling temperature of the steel plate slightly above the Ar3 temperature, followed by air cooling, resulting in a microstructure of ferrite + pearlite. However, with increasingly stringent requirements for carbon equivalent in 355MPa grade low-alloy structural steel, a lower carbon equivalent reduces the stability of austenite. This leads to a higher phase transformation temperature during air cooling, resulting in relatively coarse ferrite grains in the conventionally normalized steel plate, causing a lower yield strength under the same conditions. As carbon equivalent requirements become more stringent, the final rolling temperature of the normalizing process is decreasing, forcing the steel plate to remain at the temperature for extended periods in batch rolling, severely impacting rolling efficiency and hindering shape control. In recent years, a "normalizing + weak water cooling" process has been developed, with a slightly faster cooling rate after normalizing than air cooling. This process significantly improves the yield strength of normalized steel plates while allowing for a slightly higher final rolling temperature, thereby increasing batch rolling efficiency.

[0003] In actual production, after exiting the heating furnace, rolled pieces must sequentially pass through descaling box, rolling mill, pre-straightener, laminar flow control cooling device, hot straightener, cooling bed, and cold straightener. The existing rolling material flow signal control program requires that the next rolled piece can only enter the pre-straightener process after the previous one has completely exited the laminar flow control cooling device. This means that while the head of the previous steel plate enters the pre-straightener, the tail of the steel plate exits the laminar flow control cooling device, the next steel plate must wait on the post-rolling roller table, occupying roller table space and severely impacting batch rolling efficiency. Therefore, for normalizing rolling using weak water cooling, the batch rolling strategy must be adjusted from batch rolling to single rolling mode, resulting in a slow rolling pace, decreased mill utilization, and a reduction in the number of pieces rolled per hour.

[0004] Currently, the relevant patents regarding improving the rolling efficiency of 355MPa grade medium and heavy plates are as follows:

[0005] The application submitted by Nanjing Iron & Steel Group, entitled "A Method for Improving the Rolling Efficiency of Class B Marine Medium-Thick Plates" (application number: CN202211165119), mainly relates to a Class B marine medium-thick plate with the following mass percentage composition: C: 0.16-0.20%, Mn: 0.60-0.80%, Si: 0.10-0.30%, P: ≤0.02%, S: ≤0.01%, Nb: ≤0.05%, with the balance being Fe and unavoidable impurities. The method employs low-temperature heating of the slab and local optimization of the rolling process to improve efficiency. High rolling efficiency: furnace exit temperature 1120~1160℃; using non-TMCP process, the first, second and sixth rolling passes are used to perform fine descaling of the rolled piece, and the final rolling temperature is 810℃±20℃. This invention uses uncontrolled rolling, and its main problem is that the rolled piece lacks the necessary warming process after rough rolling. The rolled piece deforms in part of the recrystallization zone, which will cause the rolled piece to continue recrystallizing inside during the finishing rolling, resulting in asynchronous recrystallization of the rolled piece. Ultimately, this will lead to mixed crystals in the steel plate, affecting the uniformity of the final product's structure and properties.

[0006] The application "A Method for Optimizing the Rolling Rhythm of a Single Stand" (application number: 200910092664.4) filed by Beijing Institute of Technology mainly involves a method to improve the batch rolling efficiency of a single-stand four-high rolling mill with variable rolling process. Specifically, it designs different roughing and finishing rolling times and sets the waiting thickness according to different steel plate specifications to find the optimal rolling strategy. This invention only considers the rolling efficiency and does not take into account the deformation temperature of the recrystallization zone and the non-recrystallization zone, which may cause the rolled piece to fall into part of the recrystallization zone and deform.

[0007] The application filed by Shagang Group, entitled "A DH36 Offshore Wind Power Steel Plate and Its Preparation Method" (application number: CN202110603321.0), mainly relates to an offshore wind power steel plate with the following mass percentage composition: C: 0.07~0.11%, Mn: ≤1.60%, Si: ≤0.25%, P: ≤0.015%, S: ≤0.005%, Al: 0.02~0.05%, Nb: 0.01~0.03%; Ti: 0.01~0.02%, N: ≤0.06%, with the balance being Fe and unavoidable impurities. The slab is heated in a soaking zone of 1120~1150℃. The invention uses low-temperature control to reduce the initial rolling temperature and adopts a conventional rolling process without limiting the final rolling temperature. The steel plate is water-cooled after rolling, with a final cooling temperature of 600-630℃. This invention uses a rolling temperature slightly lower than TMCP and combines it with a conventional rolling process to avoid excessively high final rolling temperatures. However, the steel plate produced by this invention lacks the necessary compression deformation process in the non-recrystallized region, resulting in insufficient dislocation energy accumulation. It relies solely on post-rolling water pouring for phase transformation strengthening to compensate for the insufficient dislocation energy. The grain size of the steel plate rolled in this way is larger than that of TMCP steel plate, and it is also prone to mixed grain structure. The steel plate is prone to problems such as large strength fluctuations and poor plasticity and toughness. Summary of the Invention

[0008] The purpose of this invention is to provide a method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent, and to solve the problem of low rolling efficiency caused by the large number of roughing and finishing passes and long waiting time in the controlled rolling mode of normalized medium and thick plates in a single-stand four-high reversible medium and thick plate mill.

[0009] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0010] A method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent involves the following steps: After the billet is heated in each heating section of the furnace, when the surface temperature of the billet in the soaking section drops to 1000–1050℃, the method determines when to remove it from the furnace based on the rolling rhythm and ensuring that the surface temperature of the billet in the soaking section remains within the range of 1000–1050℃. The billet undergoes a one-stage rolling process: in the transverse rolling widening stage, the number of rolling passes is controlled to 2–4, the pass reduction rate is 8%–15%, and the rolling speed is controlled to 2.0–2.2 m / s. -1 In the longitudinal rolling stage, the number of rolling passes is controlled to 4–8, the pass reduction rate is 15%–36%, and the rolling speed is controlled to 2.2–3.8 m / s. -1 The final rolling temperature is controlled at 860–900℃.

[0011] The number of rolling passes and the reduction rate of each pass are limited in the widening stage and the longitudinal rolling stage. Based on the principle of a certain steel plate crown, the purpose is to limit the boundary conditions of the rolling model, so as to ensure that each deformation pass can achieve full-load rolling, and to ensure good plate shape and avoid serious curling.

[0012] The lower rolling speed and pass reduction rate are set in the cross rolling widening stage, mainly because the workpiece width is large at this stage. Excessive reduction will lead to limited bite conditions. A lower rolling speed can ensure that the workpiece bites in smoothly.

[0013] The final rolling temperature is limited to 860-900℃, which can obtain the ideal microstructure and grain size level under the normalizing rolling process. At the same time, the automatic rolling mill model can significantly improve the operating efficiency of the transfer roller table between passes and increase the rolling rhythm based on this final rolling temperature range.

[0014] Furthermore, the surface temperature drop of the billet in the soaking zone is obtained by using a pre-calculation model of the billet surface temperature to determine the surface temperature of the billet at any given time within the soaking zone. The pre-calculation of the surface temperature of the billet at any given time within the soaking zone is as follows:

[0015] The heat flux density of any surface of a steel billet is expressed as:

[0016]

[0017] Where, ε 炉气 and ε钢坯 Indicates the emissivity of furnace gas and steel billets;

[0018] η 炉钢 This represents the angle coefficient of the furnace wall relative to the steel billet;

[0019] θ represents the forced convection heat transfer coefficient.

[0020] T 钢坯 (t) represents the temperature of any surface of the steel billet at time t.

[0021] T 炉 This indicates the temperature of the soaking zone, obtained from the real-time furnace temperature curve of the heating furnace.

[0022] The average predicted temperature of the steel billet at time t is expressed as:

[0023]

[0024] Where ρ represents the density of the steel billet.

[0025] c represents the specific heat of the steel billet.

[0026] q x ,q y ,q z Represent the heat flux densities of the x, y, and z planes, respectively.

[0027] V 钢坯 Indicates the volume of the steel billet

[0028] The average pre-calculated temperature of the billet at time t in the soaking zone can be obtained from the above equation.

[0029] This invention provides a pre-calculation model for the surface temperature of steel billets in a heating furnace, which is significantly different from the existing model that sets the tapping conditions based on the furnace temperature and the total heating time. The model calculates the surface temperature of the steel billet at any time in the soaking section, which can greatly improve the accuracy of steel heating in the heating furnace and perfectly match the subsequent high-speed rolling with large reduction, thus significantly improving heating efficiency and rolling efficiency.

[0030] Furthermore, the heating furnace is a walking beam furnace, and the residence time of the steel billet in the soaking zone is controlled by changing the billet distance and adjusting the walking cycle.

[0031] Furthermore, the steel billet is rolled using a semi-automatic model, meaning that the maximum reduction and speed regime for each pass are calculated manually, while the roller table speed and roll gap design during the stable rolling stage are given by the mill calculation model.

[0032] The algorithm for the maximum reduction in each pass is based on the principle that the steel plate crown is constant. Under the condition of ensuring full load in each pass, the control of plate crown is transformed into the control of rolling force, with rolling force as the target:

[0033] Based on the known steel plate crown δ, roll profile crown W, and roll thermal crown y t and bending roller force P W The effect is used to calculate the rolling force P in the nth pass. n :

[0034] δ=a P P n -a W Wa t y t -a PW P W

[0035] lnP n = 1.25 + 0.045lnε + 0.015(lnε) 2

[0036] Δh=[ε / (1-ε)]h

[0037] H = Δh / ε

[0038] Among them, a t a W a P a PW These represent the maximum difference in flattening of the work rolls, the deflection of the work rolls, and the diameters of the upper and lower work rolls, respectively; H, h, and Δh represent the inlet and outlet thicknesses and the single-pass pressing thickness, respectively; ε represents the pressing rate.

[0039] Calculation method for rolling speed (ν) in each pass under the condition of maximum reduction in each pass:

[0040] v = πa P P max / M

[0041] P max =λ P ·P e

[0042] M = M F / i+M f +M K +M D

[0043] M F =2·θ·F·l′ C

[0044]

[0045] ε Σ =aε0+(1-a)ε i

[0046] F = B(R′-R) / R 1 / 2

[0047] l′ C =(R′·Δh) 1 / 2

[0048] Among them, a P P represents the diameter of the work roll, in mm. max Maximum power of the motor shaft during rolling, kW; λ P Power surplus factor; P e Rated power of motor, kW; M; Torque of main motor, kN·m; M F Rolling torque, kN·m; M f Additional frictional torque, kN·m; M k Idle torque, kN·m; M D , driving torque, kN·m; i, main drive ratio; θ, lever arm coefficient; ε ∑ εi represents the total reduction rate; a is a weighting coefficient, taken as 0.4; ε0 represents the cumulative reduction rate before the i-th pass; εi represents the cumulative reduction rate before the i-th pass. i R is the cumulative reduction rate of the i-th pass; R is the radius of the work roll (mm); R′ is the radius of the flattened work roll (mm); B is the width of the steel plate (mm); F is the tension of the steel plate; l C The length of the contact arc after the work roll is flattened, in mm.

[0049] Based on the principle of fixed steel plate crown, this invention aims to maximize the reduction per pass while ensuring full load in each pass. It transforms the control of plate crown and shape into the control of rolling force and designs a calculation model for the number of rolling passes, the reduction rate per pass, and the corresponding rolling speed in a single stage. This model can maximize the potential of the rolling mill and greatly improve rolling efficiency under the premise of good rolled plate shape and uniform internal structure.

[0050] Furthermore, the furnace temperature in heating section 1 is 930–970℃, and the furnace temperature in heating section 2 is 1200–1240℃. After the billet enters the soaking section, the burners in the soaking section are closed. The billet can achieve full core burning in the preheating section, heating section 1, and heating section 2. The purpose of closing the burners in the soaking section is to allow the billet to cool down naturally in the soaking section. Since the surface cooling rate is greater than that of the core, the billet cools down naturally in the soaking section, making the surface temperature of the rolled piece lower than that of the core. The rolled piece can form an "outer cold, inner hot" effect in the early stage of deformation, which is more conducive to the penetration of rolling force into the core, promotes the flow of metal in the core of the rolled piece, improves the recrystallization softening effect, and reduces the overall deformation resistance of the rolled piece, creating conditions for large reduction deformation, thereby reducing the number of rolling passes. Compared with the conventional normalizing rolling process for steel plates of the same size and specification, the pure rolling time is reduced by more than 46%, significantly improving rolling efficiency.

[0051] Furthermore, the billet rolling strategy adopts "transverse-longitudinal" rolling, and the rolling model adopts the ordinary rolling mode. The model pre-calculates the number of rolling passes and the reduction per pass. The number of running groups of the pre-roll and post-rolling rollers is automatically selected by the mill model according to the length of the steel plate. The rolling process is automatically tracked by the steel plate transmission signal, and a bite signal is provided. By adopting the "transverse-longitudinal" rolling strategy, a large amount of metal inside the rolled piece can be extended laterally during the transverse rolling stage. Sulfide inclusions in the longitudinal segregation zone can be spread and dispersed laterally. The sulfides are no longer thin strips as in full longitudinal rolling, but coarse flakes or dots, thereby reducing the lamellar sulfide inclusions and making the grains more equiaxed, thus improving the transverse properties of the steel plate and reducing the anisotropy of the steel plate. After transverse rolling, the steel is transferred. The 8-15s rolling interval during the transfer process can be used to fully release the latent heat of rolling deformation to the surface of the steel plate, providing conditions for recrystallization and softening in the subsequent longitudinal rolling process.

[0052] Furthermore, the chemical composition of the steel plate by weight percentage is as follows: C: 0.08%–0.15%, Si: 0.20%–0.35%, Mn: 0.90%–1.65%, Nb: 0.005%–0.010%, P < 0.030%, S < 0.025%, with the remainder being Fe and unavoidable inclusions. The carbon equivalent of the steel plate is below 0.30%.

[0053] Furthermore, the steel billet is a continuously cast billet with a thickness of 200-300 mm, which is rolled to obtain a steel plate with a thickness of 16-60 mm.

[0054] Furthermore, the walking beam adopts a stepping mode when the steel billet stays in the soaking zone.

[0055] Furthermore, the steel billet stays in the preheating section for 20-25 minutes, in the first heating section for 45-70 minutes, and in the second heating section for 60-110 minutes.

[0056] Compared with the prior art, the beneficial effects of the present invention are:

[0057] The steel plate rolled by this invention can be roughly divided into three deformation stages according to the recrystallization law: the first 2-4 passes (including 1-3 passes of cross rolling + 1 pass of longitudinal rolling) utilize the initial "outer cold, inner hot" effect of the billet to allow the rolling force to penetrate to the core, ensuring recrystallization deformation in the core and refining the austenite grains in the core; the second 5-8 passes of the middle deformation stage, as the latent heat in the core of the workpiece is released, the plastic deformation range continuously moves towards the surface, and partial recrystallization deformation is the main process in the area from 1 / 4 of the workpiece to the surface, but the single-pass reduction rate of more than 15% can break the non-recrystallized grains, ensuring that the grains of the workpiece are fully refined in this stage; the last 2 passes of the deformation stage, as the latent heat is continuously released, the internal and external temperatures of the workpiece tend to be consistent, which is conducive to coordinating the overall deformation. This stage belongs to the non-recrystallization temperature range of deformation, compressing the deformed grains and improving the deformation energy storage of the workpiece.

[0058] This invention utilizes the low temperature of the surface layer of the rolled piece after it exits the furnace and makes full use of the "temperature difference" characteristics of the rolled piece in the thickness direction at each stage of deformation. It achieves a dual grain refinement effect of recrystallization and grain breakage under high pressure. Compared with the traditional "two-stage controlled rolling" normalizing rolling, the grain size exceeds level 9, but the waiting time is saved, which significantly improves the rolling efficiency. Attached Figure Description

[0059] Figure 1 Metallographic structure at 1 / 4 thickness of steel plate obtained under existing normalizing rolling process;

[0060] Figure 2 Metallographic structure at 1 / 4 thickness of steel plate obtained by normalizing rolling process of the present invention;

[0061] Both types of steel plates have nominal dimensions of 60×3200×20000mm. Detailed Implementation

[0062] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The described embodiments are merely some, not all, of the embodiments of this invention. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0063] A method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent is disclosed, belonging to the field of steel plate rolling process control technology. The method is applicable to medium-thick plates of low alloy steel with a thickness of 16-60mm, a carbon equivalent of less than 0.30%, and a strength grade of 355MPa.

[0064] The chemical composition of the steel plate is (by weight percentage): C: 0.08%–0.15%, Si: 0.20%–0.35%, Mn: 0.90%–1.65%, Nb: 0.005%–0.010%, P < 0.030%, S < 0.025%, with the remainder being Fe and unavoidable inclusions.

[0065] The steel plate described in this invention has a thickness of 16–60 mm.

[0066] The steel billet used in this invention is a continuously cast billet with a thickness of 200-300 mm.

[0067] The steel billet temperature control in each section of the walking beam furnace according to the present invention is as follows: the furnace temperature of heating section 1 is 930-970℃, the furnace temperature of heating section 2 is 1200-1240℃, the burner in the soaking section is closed, and when the surface temperature of the steel billet in the soaking section drops to 1000-1050℃, the surface temperature drop of the steel billet at any time in the soaking section is obtained through the pre-calculation model of the steel billet surface temperature, so as to ensure that the steel billet surface temperature in the soaking section is within the range of 1000-1050℃ and the tapping time is determined according to the rolling rhythm.

[0068] The following is a model for predicting the surface temperature of the steel billet at any time within the soaking zone:

[0069] The heat flux density of any surface of a steel billet is expressed as:

[0070]

[0071] Where, ε 炉气 and ε 钢坯 Indicates the emissivity of furnace gas and steel billets;

[0072] η 炉钢 This represents the angle coefficient of the furnace wall relative to the steel billet;

[0073] θ represents the forced convection heat transfer coefficient.

[0074] T 钢坯 (t) represents the temperature of any surface of the steel billet at time t.

[0075] T 炉 This indicates the temperature of the soaking zone, obtained from the real-time furnace temperature curve of the heating furnace.

[0076] The average predicted temperature of the steel billet at time t is expressed as:

[0077]

[0078] Where ρ represents the density of the steel billet.

[0079] c represents the specific heat of the steel billet.

[0080] q x ,q y ,q z Represent the heat flux densities of the x, y, and z planes, respectively.

[0081] V 钢坯 Indicates the volume of the steel billet

[0082] The average predicted temperature of the billet at time t in the soaking zone can be obtained from the above equation.

[0083] This invention can control the dwell time of the billet in the soaking zone by changing the billet distance and adjusting the stepping cycle. When the billet is in the soaking zone, the stepping beam adopts a stepping mode.

[0084] The steel billet described in this invention has a residence time of 20-25 minutes in the preheating section, 45-70 minutes in the first heating section, and 60-110 minutes in the second heating section.

[0085] The steel billet described in this invention is rolled using a semi-automatic model, meaning that the reduction amount and speed regime of each pass are calculated manually, while the roller table running speed and roll gap design during the stable rolling stage are given by the mill calculation model.

[0086] This invention provides an algorithm for the maximum reduction in each pass. Firstly, based on the principle of a fixed steel plate crown, and ensuring full load in each pass, the control of plate crown and shape is transformed into control of rolling force, with rolling force as the target.

[0087] Based on the known steel plate crown δ, roll profile crown W, and roll thermal crown y t and bending roller force P W The effect is used to calculate the rolling force P in the nth pass. n :

[0088] δ=a P P n -a W Wa t y t -a PW P W

[0089] lnP n = 1.25 + 0.045lnε + 0.015(lnε) 2

[0090] Δh=[ε / (1-ε)]h

[0091] H = Δh / ε

[0092] Among them, a t a W a P a PW These represent the maximum difference in flattening of the work rolls, the deflection of the work rolls, and the diameters of the upper and lower work rolls, respectively; H, h, and Δh represent the inlet and outlet thicknesses and the single-pass pressing thickness, respectively; ε represents the pressing rate.

[0093] Calculation method for rolling speed (ν) in each pass under the condition of maximum reduction in each pass:

[0094] v = πa P P max / M

[0095] P max =λ P ·P e

[0096] M = M F / i+M f +M K +M D

[0097] M F =2·θ·F·l′ C

[0098]

[0099] ε Σ =aε0+(1-a)ε i

[0100] F = B(R′-R) / R 1 / 2

[0101] l′ C =(R′·Δh) 1 / 2

[0102] Among them, a P P represents the diameter of the work roll, in mm. max Maximum power of the motor shaft during rolling, kW; λ P Power surplus factor; P e Rated power of motor, kW; M; Torque of main motor, kN·m; M F Rolling torque, kN·m; M f Additional frictional torque, kN·m; M k Idle torque, kN·m; M D , driving torque, kN·m; i, main drive ratio; θ, lever arm coefficient; ε ∑ εi represents the total reduction rate; a is a weighting coefficient, taken as 0.4; ε0 represents the cumulative reduction rate before the i-th pass; εi represents the cumulative reduction rate before the i-th pass. i R is the cumulative reduction rate of the i-th pass; R is the radius of the work roll (mm); R′ is the radius of the flattened work roll (mm); B is the width of the steel plate (mm); F is the tension of the steel plate; l C The length of the contact arc after the work roll is flattened, in mm.

[0103] The billet rolling strategy described in this invention selects "transverse-longitudinal" rolling, and the rolling model adopts the ordinary rolling mode. The model pre-calculates the number of rolling passes and the reduction per pass. The number of sets of rollers to run before and after rolling is automatically selected by the rolling mill model according to the length of the steel plate. The rolling process is automatically tracked by the steel plate transmission signal and a bite signal is provided.

[0104] The steel billet described in this invention adopts a one-stage rolling process: in the transverse rolling widening stage, the number of rolling passes is controlled to be 2 to 4, the pass reduction rate is 8% to 15%, and the rolling speed is controlled to be 2.0 to 2.2 m / s. -1In the longitudinal rolling stage, the number of rolling passes is controlled to be 4 to 8, the pass reduction rate is 15% to 36%, and the rolling speed is controlled to be 2.2 to 3.8 m / s. -1 The final rolling temperature is controlled between 860 and 900℃.

[0105] This invention provides a method for improving the rolling efficiency of 16-60mm thick medium-thick plates in a single-stand four-high reversible medium-thick plate mill during normalizing. Specifically, it provides a method for improving the rolling efficiency of 355MPa grade hot-rolled low-alloy steel with a carbon equivalent of less than 0.30%. Through differential temperature design in the billet heating process and a "transverse-longitudinal" one-stage rolling design, the rolled piece maintains a low rolling force even under large reduction in a single pass, ensuring smooth rolling. Compared to two-stage controlled rolling, one-stage continuous rolling of the rolled piece with a differential temperature gradient significantly reduces the work hardening rate during rolling, thereby increasing the reduction per pass, reducing the total number of rolling passes, fully utilizing the equipment's potential, significantly shortening the total rolling time, and improving rolling efficiency.

[0106] The present invention will be further described in detail below with reference to specific embodiments.

[0107] A method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent is described in the following specific implementation:

[0108] The carbon equivalent, billet size, and rolled steel plate size for each embodiment are shown in Table 1.

[0109] Table 1. Blank dimensions and finished steel plate dimensions for each embodiment.

[0110]

[0111] The heating conditions inside the furnace and the relevant physical properties of the billet heating in each embodiment are shown in Tables 2 and 3.

[0112] Table 2. Steel billet heating process for each embodiment.

[0113]

[0114] Table 3. Heating-related physical properties of steel billets and cast billets in each embodiment.

[0115]

[0116] The working roller conditions of each embodiment are shown in Table 4.

[0117] Table 4. Working roll status in each embodiment

[0118]

[0119] The rolling pass data for each embodiment are shown in Table 5.

[0120] Table 5 Rolling pass data for each embodiment

[0121]

[0122]

[0123] In Examples 1-3, the first four passes are transverse rolling, and the remaining passes are longitudinal rolling; in Example 4, the first two passes are transverse rolling, and the remaining passes are longitudinal rolling.

[0124] Table 6 shows a comparison of the rolling data of each embodiment with that of conventional controlled-rolled steel plates of the same specifications.

[0125] Table 6 Comparison of efficiency and final rolling temperature between one-stage and two-stage rolling

[0126]

[0127] The present invention provides a method for improving the rolling efficiency of 16-60mm thick medium-thick plates in normalizing rolling using a single-stand four-roll reversible medium-thick plate mill. Compared with conventional controlled rolling steel plates using the same equipment and rolling specifications, the final rolling temperature is basically the same, the rolling time is shortened by more than 46%, and the rolling efficiency is significantly improved.

[0128] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent, characterized in that, After the steel billet is heated in each heating section of the heating furnace, when the surface temperature of the billet in the soaking section drops to 1000~1050℃, the timing of unloading from the furnace is determined according to the rolling rhythm, ensuring that the surface temperature of the billet in the soaking section remains within the range of 1000~1050℃. The billet undergoes one-stage rolling: in the horizontal rolling widening stage, the number of rolling passes is controlled to 2~4, the pass reduction rate is 8%~15%, and the rolling speed is controlled to 2.0~2.2 m·s. -1 In the longitudinal rolling stage, the number of rolling passes is controlled to 4-8, the pass reduction rate is 15%-36%, and the rolling speed is controlled to 2.2-3.8 m / s. -1 The final rolling temperature is controlled at 860~900℃, and after rolling, a steel plate with a thickness of 16~60mm is obtained. The chemical composition of the steel plate by weight percentage is: C: 0.08%~0.15%, Si: 0.20%~0.35%, Mn: 0.90%~1.65%, Nb: 0.005%~0.010%, P<0.030%, S<0.025%, with the remainder being Fe and unavoidable inclusions. The carbon equivalent of the steel plate is below 0.30%.

2. The method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent as described in claim 1, characterized in that, The surface temperature drop of the billet in the soaking zone is obtained by using a pre-calculation model of the billet surface temperature. The pre-calculation of the surface temperature of the billet at any time in the soaking zone is as follows: The heat flux density of any surface of a steel billet is expressed as: ； in, and Indicates the emissivity of furnace gas and steel billets; This represents the angle coefficient of the furnace wall relative to the steel billet; Indicates the forced convection heat transfer coefficient; This represents the temperature of any surface of the steel billet at time t; This indicates the furnace temperature in the soaking zone, which is obtained from the real-time furnace temperature curve of the heating furnace. The average predicted temperature of the steel billet at time t is expressed as: ； in, Indicates the density of the steel billet; Indicates the specific heat of the steel billet; Let x, y, and z represent the heat flux densities of the x, y, and z planes, respectively. Indicates the volume of the steel billet; The average pre-calculated temperature of the billet at time t in the soaking zone can be obtained from the above equation.

3. The method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent as described in claim 1, characterized in that, The heating furnace is a walking beam furnace, and the residence time of the billet in the soaking zone is controlled by changing the billet distance and adjusting the walking cycle.

4. The method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent as described in claim 1, characterized in that, The steel billet is rolled using a semi-automatic model, meaning that the maximum reduction and speed of each pass are calculated manually, while the roller table speed and roll gap design during the stable rolling stage are given by the mill calculation model. The algorithm for the maximum reduction in each pass is based on the principle that the steel plate crown is constant. Under the condition of ensuring full load in each pass, the control of plate crown is transformed into the control of rolling force, with rolling force as the target: Based on the known convexity of the steel plate Roll crown Roll thermal crown and bending roller force The effect is used to calculate the rolling force of the nth pass. : ； ； ； ； in, , , , These are the maximum difference in flattening of the work rolls, the deflection of the work rolls, and the diameters of the upper and lower work rolls, respectively. , Δh and Δh represent the inlet thickness, outlet thickness, and single-pass reduction thickness, respectively. The reduction rate; Calculation method for rolling speed ν of each pass under the condition of maximum reduction in each pass: ； ； ； ； ； ； ； ； in, The diameter of the work roll is in mm; The maximum power of the motor shaft during rolling, kW; Power surplus coefficient; Rated power of the motor, kW; Main motor torque, kN·m; Rolling torque, kN·m; Additional frictional torque, kN·m; Idle torque, kN·m; Dynamic torque, kN·m; Main transmission ratio; ε is the lever arm coefficient; ∑ Total reduction rate; The weighting factor is set to 0.4; The cumulative reduction rate before the i-th pass; R is the cumulative reduction rate of the i-th pass; R is the radius of the work roll, mm; B is the radius of the flattened work roller (mm); F is the width of the steel plate (mm); F is the tension of the steel plate. The length of the contact arc after the work roll is flattened, in mm.

5. The method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent as described in claim 1, characterized in that, The furnace temperature in heating section 1 is 930~970℃, and the furnace temperature in heating section 2 is 1200~1240℃. After the billet enters the soaking section, the burner in the soaking section is turned off.

6. The method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent as described in claim 1, characterized in that, The rolling strategy for the steel billet is "horizontal-vertical" rolling. The rolling model adopts the ordinary rolling mode. The model pre-calculates the number of rolling passes and the reduction per pass. The number of sets of rollers to run before and after rolling is automatically selected by the rolling mill model according to the length of the steel plate. The rolling process is automatically tracked by the steel plate transmission signal, and a bite signal is provided.

7. The method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent as described in claim 1, characterized in that, The steel billet is a continuously cast billet with a thickness of 200~300mm.

8. The method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent as described in claim 3, characterized in that, When the steel billet is in the soaking zone, the walking beam adopts a stepping mode.

9. A method for improving the rolling efficiency of 355MPa grade normalized thick plates with low carbon equivalent as described in claim 6, characterized in that, The steel billet stays in the preheating section for 20-25 minutes, in the first heating section for 45-70 minutes, and in the second heating section for 60-110 minutes.

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