Method for controlling microstructure uniformity of low carbon equivalent 355mpa grade normalized rolled thick plate

Abstract

The application discloses a low-carbon-equivalent 355MPa grade normalized rolled thick plate organization uniformity control method, wherein the temperature of the billet before charging is less than or equal to 400 DEG C, the temperature of the soaking furnace is 1130-1150 DEG C, the time of the soaking section is 45-55 minutes, the total furnace time is 170-185 minutes, the second opening temperature of the billet is 880-940 DEG C, the finishing rolling is 6-8 passes, the cumulative reduction is 60-70%, and the final rolling temperature is 860-900 DEG C; the steel plate cooling is realized by adopting a laminar flow cooling system, the full-automatic water pouring control cooling mode is adopted, the water inlet temperature is 720-740 DEG C, the red temperature is 640-660 DEG C, and the surface temperature of the steel plate after cooling is monitored. The head and tail and the full-thickness organization type of the normalized rolled steel plate are all 'ferrite + pearlite' organizations, the grain size level is all above 9.0, there is no obvious banded structure and no bainite structure, and the problem of the organization difference of the 355MPa grade thick plate product caused by the normalized rolling is solved.

Description

Technical Field

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

[0002] The requirements for carbon equivalent in 355MPa grade low-alloy structural steel delivered in the normalized state are becoming increasingly stringent. As the carbon equivalent decreases, the Ar3 and Ar1 temperatures of the steel plate increase accordingly, leading to premature phase transformation. In this case, normalizing rolling results in coarse ferrite grains during the post-rolling cooling process, causing a decrease in the yield strength of the steel plate under the same conditions. However, if appropriate accelerated cooling is used after normalizing rolling, higher strength and toughness can be obtained compared to the air-cooled state. The steel plate obtained using the "normalizing + appropriate rapid cooling" process has very fine grains and microstructure. A higher cooling rate can increase the degree of undercooling, lower the transformation temperature of austenite to ferrite and pearlite, effectively improve the nucleation rate and grain growth, and obtain a fine ferrite and fine lamellar pearlite microstructure.

[0003] To improve production efficiency and reduce energy consumption, continuous casting billets generally adopt a hot-feed direct loading method. The surface temperature of the billet is often above 750℃, significantly shortening the total time the billet spends in the furnace. While this significantly improves heating efficiency, it also brings new problems: 1. The billet lacks the phase transformation process of "ferrite + pearlite" re-austenitization in the furnace, and the NbC "pinning" effect on grain boundaries is not achieved. Direct heating from the austenitic grains causes abnormal growth of the original austenitic grains, resulting in correspondingly larger grains after normalizing and rolling of the steel plate; 2. The billet directly contacts the pad above the fixed beam in the walking beam furnace. The pad is welded to the cooling water pipes inside the fixed beam and walking beam, and the cooling water pipes continuously circulate water for cooling, resulting in a low temperature of the pad. During the process, the fixed beam itself has a shielding effect on the radiative heat transfer of the billet. These reasons cause the area near the contact point between the billet and the pad block to be not heated well, thus forming a water beam mark. The water beam mark can cause the temperature difference in a local area on the lower surface of the billet to reach more than 100℃, which seriously affects the accuracy of the roughing mill inlet temperature acquisition, thereby affecting the accuracy of rolling force control. This in turn affects the dynamic recrystallization ratio in the thickness direction of the steel plate during hot rolling, and can also cause problems such as thickness difference and plate shape. 3. After hot delivery and direct loading, the total time of the billet in the furnace is greatly shortened. The original C and Mn segregation and MnS inclusions in the billet do not have time to be homogenized and diffused, resulting in severe banded structure in the rolled steel plate. Large MnS inclusions are elongated after plastic deformation. All of the above situations seriously affect the elongation and impact toughness of the normalized rolled steel plate.

[0004] Currently, the relevant patents regarding microstructure control in the normalizing rolling process of medium and heavy plates below 355MPa are as follows: Wuhan Iron and Steel Group's E'gang Steel's application, "A Method for Obtaining Steel Plates with Uniform Longitudinal Properties by Normalizing Rolling and Its Manufacturing Method" (application number: 201910709706.8), mainly involves a low-alloy steel with the following mass percentage composition: C: 0.12~0.20%, Mn: 1.20~1.50%, Ti: 0.008~0.022%, Si: 0.2~0.4%, N: 0.010~0.02%, Als: 0.020~0.05%, S≤0.008%, P≤0.015%, with the balance being Fe and unavoidable impurities, while simultaneously satisfying: 83%Si + 354%N > 23. This invention employs a two-stage controlled rolling, relaxation, and post-rolling water cooling process for the normalized rolled steel plate. The carbon equivalent of this steel is 0.35~0.52%. When the carbon equivalent exceeds 0.43%, it will significantly improve the stability of austenite, leading to the transformation of upper bainite during phase transformation. This structure is no longer a normalized structure, and steel plates with upper bainite have relatively poor plasticity and toughness.

[0005] The application filed by Shandong Iron & Steel Rizhao Co., Ltd., entitled "A Normalized Rolled Medium-Thick Ship Plate Steel and Its Production Method for Eliminating Banded Structure" (application number: 202010300243.2), mainly relates to a ship plate steel with the following mass percentage composition: C: 0.10~0.15%, Si: 0.25~0.40%, Mn: 0.90~1.10%, P≤0.015%, S≤0.005%, Als: 0.025~0.06%, Nb: 0.025~0.035%, V: 0.020~0.030%, Ti: 0.008~0.014%, H≤2PPm, N≤60PPm, O≤30PPm, Cep≤0.40%, with the remainder being Fe and unavoidable impurities. This invention employs a two-stage controlled rolling process, with a second rolling temperature of 780~820℃ and a final rolling temperature of 720~770℃. Cooling is not controlled, and deformation is carried out according to the finishing rolling temperature of this composition system. After the proeutectoid ferrite is elongated, during the cooling process of the steel plate, part of the austenite transforms into pearlite, and the other part transforms into bainite and twinned martensite, which greatly increases the yield strength and results in an increased yield strength ratio.

[0006] The application filed by Wuhan Iron and Steel Group (hereinafter referred to as "Normalized Rolled Structural Steel for Bridges with Low Yield-to-Tear Ratio and its Production Method") (application number: CN201610591133.X) mainly involves a normalized rolled bridge structural steel. It adopts normalized rolling and ACC controlled cooling, with a final rolling temperature of 870~920℃, an initial cooling temperature of 780~850℃, and a final cooling temperature of 550~700℃. This invention improves low-temperature toughness by feeding Ca to induce sulfide spheroidization; it also improves plasticity and toughness by adding Ti to control Ti / N ≥ 3.4 and Al ≥ 0.015% to avoid the formation of a large amount of TiN. The rolling process belongs to the conventional TMCP process. The cooling rate is controlled at the upper limit of the range of 3.6~15℃ / s. Bainite is easily generated on the upper and lower surfaces of thinner (below 20mm) steel plates. Summary of the Invention

[0007] The purpose of this invention is to provide a method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent, thereby solving the problem of microstructure differences within the same plate caused by normalizing and rolling of 355MPa grade thick plate products.

[0008] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0009] A method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent includes billet heating, billet rolling, plate cooling, and plate surface temperature monitoring, specifically:

[0010] The billet heating furnace is divided into a preheating section, heating section 1, heating section 2, and soaking section. The furnace temperatures are set as follows: heating section 1: 1080~1100℃; heating section 2: 1140~1160℃; soaking section: 1130~1150℃. The billet temperature before loading into the furnace is ≤400℃. The billet spends 25~30 minutes in the preheating section, 35~40 minutes in heating section 1, 55~60 minutes in heating section 2, and 45~55 minutes in the soaking section, for a total furnace time of 170~185 minutes.

[0011] The billet is rolled in 6 to 8 passes with a cumulative reduction of 60 to 70% and a final rolling temperature of 860 to 900℃.

[0012] The steel plate is cooled using a laminar flow cooling system. The controlled cooling mode uses fully automatic water spraying with an inlet water temperature of 720~740℃ and a return temperature of 640~660℃.

[0013] The furnace temperature and in-furnace time range for each heating section are designed based on the thickness of the continuous casting, the rolling force and torque of the rolled product, and the thermal conductivity of the steel billet involved in this invention. The furnace temperature and in-furnace time can ensure the uniformity of steel burning of the cast billet and meet the rolling force and torque requirements of wide-specification rolled products, ensuring uniform deformation in the width and thickness directions of the steel plate.

[0014] The design of the second rolling temperature, cumulative reduction rate of finishing rolling, and final rolling temperature range aims to ensure that the billet can be fully deformed in the non-recrystallization temperature range, and the final rolling temperature is limited to 860~900℃ to ensure that a fully austenitic structure is obtained after rolling.

[0015] The design water inlet temperature is 720~740℃, and the red-hot temperature is 640~660℃. The higher red-hot temperature can ensure that the steel plate does not undergo the intermediate temperature transformation of bainite, while obtaining a fine ferrite and fine lamellar pearlite structure.

[0016] By creatively combining a "hybrid cold-charging furnace + walking beam process" to achieve the following: 1. Improved heating efficiency of continuously cast billets with cold charging furnace input without affecting rolling rhythm; 2. Allowed NbC to fully precipitate during cooling of the billet through cold charging furnace input, and refined the original austenitic grain size through the "pinning" effect of NbC, which is beneficial for further grain refinement and uniform microstructure during rolling; 3. Effectively solved the "water beam mark" problem of the billet by limiting the translational and lifting motion speeds, thereby improving the deformation uniformity of the rolling process.

[0017] 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%~1.15%, 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%.

[0018] The design using a low manganese content of 0.90~1.15% can reduce manganese segregation in the core of the billet, avoid the formation of supercooled austenite and subsequent bainitic or martensitic phase transformation due to manganese segregation in the core of the rolled steel plate. By reducing manganese, supercooled austenite can be eliminated, thereby eliminating or reducing banded structure and improving the uniformity of steel plate structure.

[0019] Specifically, a matching process of "low carbon equivalent + controlled cooling normalizing rolling" was developed. In particular, the manganese content of 355MPa grade low-alloy steel is limited to 0.90~1.15%, combined with controlled cooling normalizing rolling, replacing traditional non-controlled cooling normalizing rolling or offline normalizing heat treatment. This process: 1. Significantly improves the banded microstructure problem caused by manganese segregation in 355MPa grade low-alloy steel with a "ferrite + pearlite" structure; 2. Controlled cooling can mitigate the strength loss problem caused by offline normalizing; 3. The low-manganese design avoids the formation of bainite microstructure in controlled cooling normalizing rolling, solving the abnormal microstructure problem caused by controlled cooling normalizing rolling. This low-manganese composition design also provides technical support for the development of low-cost, uniformly microstructured high heat input welding medium and heavy plates.

[0020] Furthermore, the billet is a continuously cast billet with a thickness of 200~360mm, which is rolled to obtain a steel plate with a thickness of 16~80mm and a width of 1300~4500mm.

[0021] The purpose of using 200~360mm cross-section continuous casting billets is to ensure that the uniformity requirements of steel plate structure and properties can be met within the designed roughing compression ratio range.

[0022] Furthermore, the steel billet is not hot-fed and directly loaded; instead, the steel billet and the hot-fed and directly loaded casting billet are inserted into the furnace.

[0023] The requirement is to eliminate the hot-feeding direct loading of the billet in the composition system of this invention, and instead mix it with hot-feeding direct-loading billets of other composition systems in the furnace. This serves two purposes: first, it ensures that the billet of this invention fully precipitates NbC during the cooling process at the stockyard stack. The "pinning" effect of NbC inhibits the migration of the original austenite grain boundaries during the billet reheating process, thereby controlling the austenite grain size in the billet and facilitating further grain refinement and uniform microstructure during rolling; second, mixing it with hot-feeding direct-loading billets improves the preheating efficiency of the continuously cast billet in the preheating section of the furnace without increasing the total furnace time.

[0024] Furthermore, the heating furnace adopts a walking beam furnace. The transfer of steel billets in the heating furnace is accomplished by the horizontal and vertical movements of the walking beam. The movement trajectory is rectangular, and one movement cycle is 40~75s. The vertical movement is controlled at 8~15s, and the horizontal movement is controlled at 30~60s.

[0025] The horizontal movement time of the walking beam in one motion cycle is increased to 30-60 seconds, and the vertical movement time of the walking beam in one motion cycle is reduced to 8-15 seconds. The purpose is to minimize the water beam marks caused by the water-cooled fixed beam being in contact with the steel billet for a long time without affecting the speed at which the billet moves in the heating furnace.

[0026] Furthermore, the billet maintains the lifting steps of the walking beam during the non-transfer stage in the heating furnace.

[0027] The speed at which the continuously cast billet travels in each heating section of the furnace is set to match the time it spends in the furnace, the rolling rhythm, and the furnace temperature of each heating section, taking into account both the billet heating efficiency and the rolling rhythm.

[0028] Limiting the upper limit of the billet walking speed is intended to prevent the walking beam from generating impact vibrations due to inertia during the steel picking (placing) process; limiting the lower limit of the billet walking speed is intended to ensure that the billet is thoroughly heated while matching the rolling rhythm, thus avoiding the phenomenon of the rolling mill "waiting for steel".

[0029] Furthermore, the billet's movement speed in the heating and holding sections of the heating furnace is controlled by a steel demand signal sent from the downstream rolling mill PLC to the heating furnace PLC. The heating furnace PLC determines whether the steel tapping conditions are met based on interlocking conditions and controls the billet's transport rhythm within the heating furnace, maintaining a speed of 8-10 mm / s in the preheating section. -1 During the heating phase, the walking speed is controlled at 6~7.5 mm·s. -1 During the heating phase, the walking speed is controlled at 5.5~6.5 mm·s. -1 The walking speed in the heat exchange zone is controlled at 5~6 mm·s. -1 .

[0030] Furthermore, for steel plates with a width of 1300~2490mm, a "full longitudinal rolling" mode is adopted, with 4~6 passes in roughing rolling, a pass reduction rate of 30~35%, and a cumulative reduction rate of 75~80%. For steel plates with a width of 2500~3199mm, a "transverse-longitudinal rolling" mode is adopted, with 6~8 passes in roughing rolling, using two passes for widening rolling, a pass reduction rate of 15~20%, and four passes for widening rolling, a pass reduction rate of 10~15%, and a cumulative reduction rate of 70~75%. For steel plates with a width of 3200~4500mm, a "longitudinal-transverse-longitudinal rolling" mode is adopted, with 8~10 passes in roughing rolling, using two passes for longitudinal widening rolling, a pass reduction rate of 10~15%, and two passes for transverse widening rolling, a pass reduction rate of 8~12%, and a cumulative reduction rate of 60~65%.

[0031] The design of billet rolling strategies, as well as corresponding roughing passes and pass reduction rates, aims to maximize the release of rolling force based on different billet thicknesses and widths, ensuring complete recrystallization of the workpiece during roughing and improving the uniformity of the final steel plate structure.

[0032] The rolling strategy is designed according to the width of the steel plate. For different width and thickness spacings, the rolling force is released to the maximum extent to ensure that the workpiece undergoes complete recrystallization during rough rolling and improve the uniformity of the final steel plate structure.

[0033] Furthermore, the surface temperature monitoring of the steel plate is achieved by monitoring the surface temperature of the steel plate after cooling using an infrared thermal imager installed after heat straightening. This allows for the acquisition of a two-dimensional temperature image of the steel plate surface. The plate is maintained according to the red-hot temperature range specified in the process, and the system automatically determines whether the red-hot temperature of the entire steel plate has been reached. Steel plates with abnormal temperature points are treated as downgraded products.

[0034] In particular, an online monitoring system for the surface temperature of the steel plate after controlled cooling was designed for the "normalizing rolling + controlled cooling" process. This system can automatically determine whether the red temperature at any position on the entire steel plate has been reached, thereby effectively solving the problems of local non-normalized structure and uneven grain size that are common in normalized rolled steel plates.

[0035] Compared with the prior art, the beneficial effects of the present invention are:

[0036] This invention, based on the Q355NB(C / D) composition in GB / T 1591-2018 standard, reduces the carbon and manganese content without adding additional microalloying elements. It effectively solves the common problems of non-normalized microstructure and uneven grain size in normalized rolled steel plates by optimizing the billet charging rules, designing the furnace temperature and walking beam speed of each section of the heating furnace, controlling the billet's residence time in each section based on the walking speed, and designing corresponding rolling passes and reduction rates according to the batch rolling strategy, as well as designing reasonable second-stage rolling temperature, final rolling temperature, immersion temperature, and reheating temperature. The normalized rolled steel plate obtained by this invention, through the above process, has a similar microstructure to offline normalized steel plates. The microstructure at the beginning and end of the steel plate and throughout its thickness is "ferrite + pearlite," with a grain size level of 9.0 or higher, and no obvious banded or bainite microstructure. Attached Figure Description

[0037] Figure 1 Metallographic structure diagram of the first 1 / 4 thickness of the steel plate;

[0038] Figure 2 Metallographic structure diagram of the last 1 / 4 thickness of the steel plate;

[0039] Figure 3 Infrared thermal imagers monitor the surface temperature of steel plates after controlled cooling.

[0040] The metallographic structure of the 16-80mm thick 355MPa grade normalized rolled extra-thick plate of this invention is "ferrite + pearlite". Detailed Implementation

[0041] 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.

[0042] This invention discloses a method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent, belonging to the technical field of steel plate rolling process control. The method is applicable to a 355MPa grade extra-thick plate. Its chemical composition (by weight percentage) is: C: 0.08%~0.15%, Si: 0.20%~0.35%, Mn: 0.90%~1.15%, Nb: 0.005%~0.010%, P < 0.030%, S < 0.025%, with the remainder being Fe and unavoidable inclusions.

[0043] The steel billet described in this invention is a continuously cast billet with a thickness of 200~360mm.

[0044] The steel plate described in this invention has a thickness of 16~80mm and a width of 1300~4500mm.

[0045] The continuous casting billet described in this invention is not hot-charged directly; it needs to be cooled to below 400°C in the raw material warehouse of the thick plate production line before being inserted into the furnace along with the hot-charged billet.

[0046] The continuous casting billet of the present invention adopts a walking beam furnace. The steel billet is transferred in the heating furnace by the horizontal and vertical movement of the walking beam. The vertical stroke of the walking beam is 200mm and the horizontal stroke is 300mm, which is a rectangular movement trajectory. One movement cycle is 40~75s. Depending on the different thickness specifications of the steel billet, the vertical movement is controlled at 8~15s and the horizontal movement is controlled at 30~60s.

[0047] The present invention requires that the steel billet maintain the lifting steps of the walking beam during the non-transfer stage in the heating furnace to prevent the black marks on the billet from aggravating at the fixed beam.

[0048] The heating furnace described in this invention is divided into a preheating section, a heating section 1, a heating section 2, and a soaking section. Within the heating furnace, the billet's movement speed in the heating and holding sections is controlled by a steel demand signal sent from the downstream rolling mill PLC to the heating furnace PLC. The heating furnace PLC determines whether the steel tapping conditions are met based on interlocking conditions. The heating furnace PLC system is designed to control the billet's movement speed within the furnace, maintaining a speed of 8-10 mm / s in the preheating section. -1 During the heating phase, the walking speed is controlled at 6~7.5 mm·s. -1During the heating phase, the walking speed is controlled at 5.5~6.5 mm·s. -1 The walking speed in the heat exchange zone is controlled at 5~6 mm·s. -1 .

[0049] The furnace temperatures of the continuously cast billet in the walking beam furnace of the present invention are set as follows: furnace temperature of heating section 1: 1080~1100℃, furnace temperature of heating section 2: 1140~1160℃, and furnace temperature of soaking section: 1130~1150℃.

[0050] The steel billet described in this invention spends 25-30 minutes in the preheating section, 35-40 minutes in the first heating section, 55-60 minutes in the second heating section, and 45-55 minutes in the soaking section, for a total furnace time of 170-185 minutes.

[0051] The steel billet of this invention is designed with corresponding rolling strategies, roughing passes and pass reduction rates according to the width of the finished steel plate: (1) For steel plate widths of 1300~2490mm, a "full longitudinal rolling" mode is adopted, with 4~6 roughing passes, a pass reduction rate of 30~35%, and a cumulative reduction rate of 75~80%; (2) For steel plate widths of 2500~3199mm, a "transverse-longitudinal" mode is adopted, with 6~8 roughing passes, a two-pass widening rolling process with a pass reduction rate of 15~20%, and a four-pass widening rolling process with a pass reduction rate of 10~15% and a cumulative reduction rate of 70~75%; (3) For steel plate widths of 3200~4500mm, a rolling process of 1300~2490mm is adopted, with a "transverse-longitudinal" mode, a roughing pass of 1300~2490mm, a two-pass widening rolling process with a pass reduction rate of 15~20%, and a four-pass widening rolling process with a pass reduction rate of 10~15% and a cumulative reduction rate of 70~75%; mm, using "longitudinal-transverse-longitudinal" rolling, rough rolling 8~10 passes, using two passes of longitudinal rolling for widening, with a pass reduction rate of 10~15%, using two passes of transverse rolling for secondary widening, with a pass reduction rate of 8~12%, and a cumulative reduction rate of 60~65%.

[0052] The billet of the present invention has a second rolling temperature of 880~940℃, a finishing rolling of 6~8 passes, a cumulative reduction rate of 60~70%, and a final rolling temperature of 860~900℃.

[0053] The weak cooling of the steel plate described in this invention is achieved by using a laminar flow cooling system. The controlled cooling mode adopts fully automatic water pouring, with an inlet water temperature of 720~740℃ and a return-to-red temperature of 640~660℃.

[0054] The steel plate described in this invention uses an infrared thermal imager installed after hot straightening to monitor the surface temperature of the steel plate after controlled cooling, thereby acquiring a two-dimensional temperature image of the steel plate surface. Maintenance is performed according to the red-hot temperature range specified in the process, and the system automatically determines whether the red-hot temperature of the entire steel plate has been reached. Steel plates with abnormal temperature points are treated as downgraded products. These measures effectively solve the problems commonly found in normalized rolled steel plates, such as localized non-normalized microstructures and uneven grain size.

[0055] According to the production process requirements of the steel of this invention, without increasing the content of microalloying elements such as niobium, carbon and manganese are reduced, cold charging is replaced by hot charging and the furnace temperature of the billet is limited, the movement parameters of the walking beam of the heating furnace are controlled, the temperature of each section of the furnace is controlled, the time of the billet in each heating section is controlled, the reduction rate is formulated according to different rolling strategies, and the rolling and cooling processes are controlled. Finally, through online normalizing rolling combined with weak water cooling process, a normalized steel plate with a slight banded structure and a full thickness structure of "ferrite + pearlite" is obtained, with a maximum thickness of up to 80mm. It also has the characteristics of high strength and toughness, small performance fluctuation and low alloy cost.

[0056] The present invention will be further described in detail below with reference to specific embodiments.

[0057] A method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent is described in the following specific implementation:

[0058] The chemical composition of the continuously cast billet and the nominal thickness of the rolled steel plate in each embodiment are shown in Table 1.

[0059] Table 1 Chemical composition of continuously cast billets and nominal specifications of steel plates

[0060] Example C / % Si / % Mn / % P / % S / % Nb / % Carbon equivalent / % Steel plate thickness / mm Steel plate width / mm 1 0.09 0.30 1.02 0.017 0.012 0.021 0.260 16 2380 2 0.11 0.25 1.11 0.015 0.010 0.025 0.295 35 2950 3 0.10 0.29 1.08 0.015 0.011 0.023 0.28 60 3370 4 0.10 0.31 1.15 0.016 0.009 0.020 0.29 80 4150

[0061] The heating process for the continuously cast billet in each embodiment is shown in Table 2.

[0062] Table 2 Heating process for continuously cast billets

[0063] Example Thickness / mm Furnace temperature / °C Preheating time / min Heating stage 1 temperature / ℃ Heating time / min Heating stage 2 temperature / ℃ Heating time 2 minutes / min Temperature of the soaking zone / ℃ Soaking time / min 1 200 311 25 1085 37 1141 55 1137 46 2 250 324 28 1092 36 1153 58 1132 51 3 250 355 26 1098 36 1149 57 1140 54 4 360 389 30 1095 38 1158 59 1147 52

[0064] The step-by-step process of the heating furnace in each embodiment is shown in Table 3.

[0065] Table 3 Stepper process

[0066] Example Walking beam lifting time / s Horizontal movement time of the walking beam / s <![CDATA[Walking speed of continuous casting billet in preheating section / mm·s -1 > <![CDATA[Continuous casting billet heating section 1 walking speed / mm·s -1 > <![CDATA[Continuous casting billet heating 2-stage walking speed / mm·s -1 > <![CDATA[Walking speed in soaking section of continuous casting billet / mm·s -1 > 1 8 37 9.9 7.4 6.3 5.9 2 11 42 9.1 7.0 6.1 5.4 3 13 53 9.5 6.9 6.0 5.7 4 15 58 8.7 6.2 5.8 5.0

[0067] The rolling strategies, roughing passes, pass reduction rates, and cumulative reduction rates of roughing and finishing rolling for each embodiment are shown in Table 4.

[0068] Table 4 Rolling process parameters

[0069] Example Rolling strategy Number of roughing passes Roughing reduction rate / % Total reduction rate in roughing rolling / % Finishing mill second opening / ℃ Number of finishing rolling passes Total reduction rate in finishing rolling / % Finishing rolling temperature / ℃ 1 Full longitudinal rolling 6 32 77 925 6 70 861 2 Horizontal-Vertical 8 13 74 911 6 68 899 3 Vertical-Horizontal-Vertical 9 13 65 895 8 65 870 4 Vertical-Horizontal-Vertical 10 9 60 882 8 62 877

[0070] The controlled cooling process parameters for each embodiment are shown in Table 5.

[0071] Table 5 Cooling process parameters

[0072] Example Inlet water temperature / ℃ Final cooling temperature / ℃ 1 721 643 2 733 648 3 736 655 4 739 659

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; 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; and these 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 controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent, comprising billet heating, billet rolling, and plate cooling, characterized in that... The billet heating furnace is divided into a preheating section, heating section 1, heating section 2, and soaking section. The furnace temperatures are set as follows: heating section 1: 1080~1100℃; heating section 2: 1140~1160℃; soaking section: 1130~1150℃. The billet temperature before loading into the furnace is ≤400℃. The billet spends 25~30 minutes in the preheating section, 35~40 minutes in heating section 1, 55~60 minutes in heating section 2, and 45~55 minutes in the soaking section, for a total furnace time of 170~185 minutes. The billet is rolled in 6 to 8 passes with a cumulative reduction of 60 to 70% and a final rolling temperature of 860 to 900℃. The steel plate is cooled by a laminar flow cooling system. The controlled cooling mode uses fully automatic water pouring with an inlet water temperature of 720~740℃ and a return temperature of 640~660℃. Steel plate surface temperature monitoring: The surface temperature of the steel plate is monitored after it has cooled. 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%~1.15%, 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 controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent as described in claim 1, characterized in that, The billet is a continuously cast billet with a thickness of 200~360mm. After rolling, it yields a steel plate with a thickness of 16~80mm and a width of 1300~4500mm.

3. The method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent as described in claim 1, characterized in that, The steel billet is not hot-fed and directly loaded; instead, the steel billet and the hot-fed and directly loaded casting billet are inserted into the furnace.

4. The method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent as described in claim 1, characterized in that, The heating furnace adopts a walking beam furnace. The steel billet is transferred in the heating furnace by the horizontal and vertical movement of the walking beam. The movement trajectory is rectangular, and one movement cycle is 40~75s. The vertical movement is controlled in 8~15s, and the horizontal movement is controlled in 30~60s.

5. The method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent as described in claim 4, characterized in that, The steel billet maintains the lifting steps of the walking beam during the non-transfer stage in the heating furnace.

6. The method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent as described in claim 1, characterized in that, In the heating and holding section of the heating furnace, the billet travel speed is controlled by a steel demand signal sent from the downstream rolling mill PLC to the heating furnace PLC. The heating furnace PLC determines whether the steel tapping conditions are met based on interlocking conditions and controls the transfer rhythm of the billet within the heating furnace. The travel speed in the preheating section is controlled at 8~10 mm·s. -1 During the heating phase, the walking speed is controlled at 6~7.5 mm·s. -1 During the heating phase, the walking speed is controlled at 5.5~6.5 mm·s. -1 The walking speed in the heat exchange zone is controlled at 5~6 mm·s. -1 .

7. A method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent as described in claim 1 or 2, characterized in that, The steel plate width is 1300~2490mm, and the "full longitudinal rolling" mode is adopted. The rough rolling is carried out in 4~6 passes, with a pass reduction rate of 30~35% and a cumulative reduction rate of 75~80%.

8. A method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent as described in claim 1 or 2, characterized in that, The steel plate width is in the range of 2500~3199mm. It adopts the "transverse-longitudinal rolling" mode, with 6~8 passes for rough rolling, two-pass widening rolling with a pass reduction rate of 15~20%, and four-pass widening rolling with a pass reduction rate of 10~15%, and a cumulative reduction rate of 70~75%.

9. A method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent as described in claim 1 or 2, characterized in that, The steel plate width is 3200~4500 mm. It is rough rolled in 8~10 passes using the "longitudinal-transverse-longitudinal rolling" mode, then widened by two passes of longitudinal rolling with a pass reduction rate of 10~15%, and widened again by two passes of transverse rolling with a pass reduction rate of 8~12%, for a cumulative reduction rate of 60~65%.

10. The method for controlling the microstructure uniformity of 355MPa grade normalized rolled thick plates with low carbon equivalent according to claim 1, characterized in that, The surface temperature monitoring of the steel plate is achieved by using an infrared thermal imager installed after heat straightening to monitor the surface temperature of the steel plate after controlled cooling. This allows for the acquisition of a two-dimensional temperature image of the steel plate surface. The plate is maintained according to the red-hot temperature range specified in the process. The system automatically determines whether the red-hot temperature of the entire steel plate has been reached. Steel plates with abnormal temperature points are treated as downgraded products.

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