A low internal stress weathering steel sheet and a method for manufacturing the same

Abstract

This invention relates to the field of steel rolling technology, specifically to a low-stress weathering steel plate and its preparation method. The chemical composition is as follows: C 0.054~0.074%, Si 0.51~0.62%, Mn 0.77~0.87%, P 0.104~0.116%, S≤0.006%, Ti 0.102~0.122%, Cr 1.04~1.24%, Cu 0.43~0.54%, V 0.042~0.064%, Als 0.026~0.056%, N≤0.0040%, satisfying Mn / C = 10.4~16.2, Ti / N = 25~61, with the remainder being Fe and impurities. This solution provides a steel strip with uniform or no internal stress distribution and good plate shape, improving the plate shape quality of hot-rolled strip steel, reducing additional processes caused by deformation during strip steel application, and increasing yield and production efficiency.

Description

Technical Field

[0001] This invention belongs to the field of steel rolling technology, specifically relating to a low internal stress weathering steel plate and its preparation method. Background Technology

[0002] In recent years, with the development of railway vehicles towards lower cost, heavier loads, and higher speeds, the demand for upgrading weathering steel plates used in railway vehicles has also increased. Improving the strength of weathering steel plates is one of the most direct means to achieve lower cost, heavier loads, and lighter weight in railway vehicles. Currently, the strength of steel used in railway vehicles is mostly improved in the following two ways: 1. By adding a large amount of microalloying elements such as Nb, V, Ti, Cr, Ni, Mo, and Cu. The disadvantages of this method are high alloy cost, high carbon equivalent, increased sensitivity to welding cracks, loss of weldability, and insufficient market competitiveness; 2. Using "water instead of alloying," employing water-cooling or air-cooling equipment to reduce the final cooling temperature during the rolling process. This method causes a sharp increase in the internal stress of the steel plate, affecting the material's processing and forming performance, and frequently resulting in uneven cooling and uneven hardness of the steel plate. In addition, as the strength grade continues to increase, the difficulty of controlling the uniformity and stability of cooling during the rolling process becomes increasingly greater, which directly affects the internal stress and the shape of the finished plate. Even if a better plate shape quality can be obtained after straightening with high-strength straightening equipment, once the user performs flame or plasma cutting, the steel plate will experience secondary warping and deformation caused by the release of internal stress due to uneven temperature, making it difficult to continue using.

[0003] Internal stress in steel plates refers to the stress that exists within a component and remains balanced throughout the component under no working load. The presence of internal stress affects the formability and product quality of the steel plate, and the relaxation of internal stress also affects the dimensional stability of the material. When cutting steel plates, deformation occurs because the internal stress of the metal sheet cannot be released in time. The deformation characteristics of different plates are detailed in [link to relevant documentation]. Figure 1 .

[0004] Due to the characteristics of hot-rolled strip steel production, local residual stress remains inside the steel plate and on its upper and lower surfaces. If this residual stress is unevenly distributed along the width of the strip, this unbalanced residual stress will generate a moment on the cross-section of the steel plate, causing the steel plate to bend laterally after slitting.

[0005] Users desire steel strips with uniform or no internal stress distribution and good plate shape. Improving the plate shape quality of hot-rolled strip steel, reducing additional processes caused by deformation during strip steel application, increasing yield and production efficiency, and solving the internal stress problem in strip production are urgent issues for researchers to address. Summary of the Invention

[0006] The purpose of this invention is to provide a low-stress weathering steel plate and its preparation method that can improve the plate quality of hot-rolled strip steel, reduce the additional processes caused by deformation during strip steel application, and improve yield and production efficiency.

[0007] According to one aspect of the present invention, a low-stress weathering steel plate is provided, the chemical composition of which, by mass percentage, is as follows: C 0.054%~0.074%, Si 0.51%~0.62%, Mn 0.77%~0.87%, P 0.104%~0.116%, S≤0.006%, Ti 0.102%~0.122%, Cr 1.04%~1.24%, Cu 0.43%~0.54%, V 0.042%~0.064%, Als 0.026%~0.056%, N≤0.004%, and satisfies Mn / C=10.4~16.2, Ti / N=25~61, with the remainder being Fe and impurities.

[0008] Based on the above technical solution, the metallographic structure of the low internal stress weathering steel plate is composed of fine and uniform ferrite and pearlite, wherein the ferrite accounts for 76~85% and the pearlite accounts for 15~24%, with a total proportion of 100%.

[0009] Based on the above technical solution, the thickness of the low internal stress weathering steel plate is 1.5~16 mm, Rp 0.2 For 750~805MPa, R m The strength is 790~860MPa, A is 28~36%, KV2 at -40℃ is 200~260J, and the grain size is 10.5~12.0 grade.

[0010] The purpose of the content design of each chemical element in this invention is as follows:

[0011] C: One of the most effective strengthening elements for increasing strength. In this invention, it exists in the form of V (C, N) and Ti (C, N). It dissolves into the matrix to play a role in solid solution strengthening and precipitation strengthening. Excessive C forms more carbides in steel, which acts as a galvanic cell, promotes corrosion and thus increases the corrosion rate of steel. It is also detrimental to welding and forming performance. Therefore, the C content is limited to between 0.054% and 0.074%.

[0012] Si is a deoxidizer in steel. The SiO2 formed plays a role in precipitation strengthening. It is also a corrosion-resistant element and reduces the density of steel plates, thus achieving the goal of lightweighting. However, excessive Si content will deteriorate the toughness of the weld heat-affected zone. Therefore, Si is limited to 0.51%~0.62%.

[0013] Mn is an important strengthening and toughening element in steel, playing a role in solid solution strengthening, expanding the austenite phase region, lowering the supercooled austenite transformation temperature, and accelerating the formation of medium and low temperature strengthening elements in steel. However, excessive Mn content will increase hardenability, deteriorate the weldability of steel plates, and increase the cost of alloys. Therefore, this invention limits the Mn content to between 0.77% and 0.87%, while satisfying Mn / C = 10.4 to 16.2.

[0014] P: Improves the strength of steel plate, promotes the formation of protective rust layer on the surface of steel plate, and improves the corrosion resistance of steel plate. However, P is prone to segregation at the boundary, which reduces the plasticity, toughness and grain boundary bonding energy of steel, and increases the cold brittleness of steel plate during forming. Therefore, this invention limits P to 0.104%~0.116%.

[0015] Sulfide (S) is a harmful element in steel, which not only reduces the low-temperature toughness of steel and significantly increases the anisotropy of steel plates, thus negatively impacting cold forming performance, but also significantly reduces the weather resistance of steel plates due to sulfide inclusions. Therefore, this invention limits S to ≤ 0.006%.

[0016] Ti: An inexpensive alloying element, it exists in steel as TiC and Ti(C,N) in the form of carbonitrides and strong ferrite forming elements, readily forming complexes with O, S, C, and N. TiN in steel can also inhibit austenite grain growth, playing a role in precipitation strengthening and refining the microstructure. Furthermore, Ti can prevent the recrystallization of deformed austenite and promote the formation of granular bainite. The precipitated Ti(C,N) particles can suppress grain coarsening in the weld heat-affected zone, further improving weldability. This invention limits the Ti content to 0.102%~0.122%, while satisfying Ti / N = 25~61.

[0017] Cr: It can form a continuous solid solution with Fe, resulting in solid solution strengthening and secondary strengthening effects. In steel, it forms M3C, M2C3, and M... 23 C6 and other carbides. The enrichment of Cr in the rust layer can effectively improve the selective permeability of the rust layer to corrosive media, promote the formation of a dense passivation film or protective rust layer on the steel surface, thereby improving the passivation ability of the steel; at the same time, Cr can also increase the self-corrosion potential of steel and improve the atmospheric corrosion resistance of steel. Therefore, the present invention limits the Cr content to 1.04%~1.24%.

[0018] Cu plays a role in solid solution and precipitation strengthening in steel. Furthermore, Cu has a higher electrochemical potential than Fe, promoting the formation of a dense rust layer on the steel surface, which improves corrosion resistance. Appropriate amounts of Cu combine with residual S in the steel to form a Cu₂S protective film, and can also combine with P to form a dense Cu-P compound protective rust layer on the substrate surface, further inhibiting the penetration of corrosive media. Cu is also often soluble in Fe to form substitution solid solutions, providing solid solution strengthening and thus increasing the strength of the steel plate. However, excessive Cu content can easily cause "copper embrittlement" defects and increase costs. Therefore, this invention limits the Cu content to 0.43%~0.54%.

[0019] V: Ferrite readily nucleates and grows with VN as the nucleus. Before the austenite-to-ferrite transformation, if there are many dispersed VN or V(C,N) particles of a certain size at the austenite grain boundaries or within the grains, the ferrite transformation is promoted. Solid solution V also has a certain delaying effect on the pearlite transformation, which objectively expands the temperature range of the proeutectoid ferrite transformation, thereby increasing the area fraction of ferrite transformation, promoting the precipitation of the passivation film, and improving acid corrosion resistance; therefore, the V content in this invention is controlled at 0.042%~0.064%.

[0020] Als: Als refers to acid-soluble aluminum, which is added to steel during the steelmaking process as a strong deoxidizer. Trace amounts of Al help refine grains and improve the strength and toughness of steel. Adding appropriate amounts of Al increases the corrosion potential of steel, which helps inhibit corrosion. Simultaneously, the formation and aggregation of complex nanoscale oxides containing Al and Si in the inner rust layer can increase charge-mass resistance, thereby inhibiting the corrosion process. However, Al reduces austenite stability and austenite undercooling, causing rapid growth of new phase nuclei, thus reducing hardenability and increasing the critical quenching rate. Furthermore, as a ferrite-forming element, excessive Al reduces steel plate strength and increases the brittleness of ferrite, leading to a decrease in steel toughness. Therefore, this invention limits the Al content to 0.026%-0.056%.

[0021] Nitrogen (N) contributes to the formation of TiN, VN, Ti(C, N), and V(C, N) precipitates in steel. These fine precipitates help to refine austenite grains by pinning grain boundaries. Furthermore, the formation of AlN nonmetallic inclusions disrupts the continuity of the steel matrix. Higher N concentrations tend to accumulate at defects, worsening low-temperature impact toughness. Therefore, this invention limits N to ≤ 0.0040%.

[0022] According to another aspect of the present invention, a method for preparing low internal stress weathering steel plate is provided, comprising the following steps: hot metal pretreatment - combined blowing converter smelting - LF ladle furnace refining - continuous casting - hot charging and hot delivery - heating - rolling - laminar flow cooling - coiling - slow cooling - annealing - leveling and finishing. Rolling: Two-stage controlled rolling is adopted. The temperature of the workpiece entering the roughing mill is 1132~1154℃. The workpiece is kept in a heat preservation hood throughout the roughing and finishing milling process. Before entering the finishing mill, the workpiece is moved to wait for the temperature to rise. The temperature of the workpiece entering the finishing mill is 965~985℃. During the finishing milling process, the edge heater is put into place to compensate the edge temperature of the rolled piece. The edge temperature compensation is 68~78℃ to reduce the difference in microstructure between the edge and the middle of the steel plate, improve the uniformity of steel plate performance and plate shape quality. The finishing milling temperature is 856~886℃. Laminar flow cooling: Water cooling is applied immediately after exiting the finishing mill, using a front rapid cooling process and an edge shielding process; Coiling: Divide the steel strip into three parts: head, middle and tail. The head of the strip is fed into the coil at 55~65℃, and the tail is fed into the coil at 50~60℃ to improve the temperature uniformity along the length of the strip. The coiling temperature is controlled at 552~574℃, which is conducive to promoting the uniformity of internal stress along the length.

[0023] Based on the above technical solution, the molten iron pretreatment specifically involves: using a spraying method to inject passivated magnesium particles and lime into the molten iron through a spray gun, and after pretreatment, S≤0.005%, wherein magnesium:lime = 1:3.5; The specific process of the combined blowing converter smelting is as follows: In the early stage of blowing, 7000-8500 kg of lime and 300-350 kg of fluorite are rapidly added at a position of 1.91 m to 2.0 m above the molten steel to form a highly oxidizing and highly alkaline foamy slag; in the middle stage of blowing, the lance position is lowered to 1.7-1.9 m above the molten steel, entering a period of intense decarburization, with a nitrogen flow rate of 200-300 Nm³. 3 Bottom-blown nitrogen stirring for 7-10 minutes per hour to rapidly raise the temperature of the molten steel; at the end of the blowing process, the carbon content in the molten steel is tested to be 0.04-0.05%, and the temperature reaches 1686-1696℃ before tapping the steel and lifting the lance, blocking slag and tapping the steel. At the same time, alloying is carried out by adding 1910-2210 kg of ferromanganese alloy, 1310-1600 kg of ferrosilicon alloy, 130-215 kg of ferrovanadium alloy, 765-960 kg of copper plate, and 3400-4000 kg of ferrochrome alloy. The LF ladle furnace refining process specifically involves adding 880-1040 kg of ferrotitanium for alloying during the refining process, controlling the refining outlet temperature at 1592-1604℃, and refining time at 46-58 min.

[0024] Based on the above technical solution, the continuous casting is specifically as follows: the continuous casting temperature is 1543~1552℃, the casting time is 23~32min, and a continuous casting billet with a size of 250mm×(1910~2060)mm is obtained.

[0025] Based on the above technical solution, the hot charging and hot delivery specifically involves cutting the continuous casting billet into fixed lengths of 10.5~10.8m and then sending it to a heating furnace for heating, with the continuous casting billet entering the furnace at a temperature ≥620℃. The heating process specifically involves using a walking beam furnace with an outlet temperature of 1256~1276℃ and a heating time of ≥118min. The high heating temperature and sufficient heating time are intended to ensure complete solid solution of the added Cr, Cu, and V alloying elements and titanium carbonitride, thereby eliminating the adverse effects of insufficient floating of large precipitate phases into inclusions and increased internal stress.

[0026] Based on the above technical solution, the slow cooling specifically involves: in order to eliminate internal stress, the coiled steel coil is hoisted into a slow cooling pit or a hot coil surrounding cooling process is used to avoid uneven cooling caused by direct cold air blowing, and the coil is slowly cooled from 552~574℃ to room temperature at a slow cooling rate of 6~12℃ / h for ≥48 hours.

[0027] Based on the above technical solution, the annealing specifically involves: hoisting the steel coil into a bell-type annealing furnace for tempering treatment, with a heating rate of 50~80℃ / h, a holding temperature of 492~518℃, a target temperature of 500℃, a holding time of 6.5h~7.5h, and after the holding time, slowly cooling the coil with the furnace at a cooling rate of ≤50℃ / h until it reaches below 200℃ before being removed from the furnace.

[0028] Based on the above technical solution, the leveling and finishing process specifically involves: using a leveling and straightening unit with strong straightening capacity, requiring a straightening force ≥650kN in actual production, and performing repeated bending and straightening through reasonable setting of the roughing and fine straightening roller gaps to eliminate the original curvature, homogenize the residual stress on the strip surface, and ensure that the absolute value of the camber is not greater than 1.5mm.

[0029] Beneficial effects (1) The technical solution disclosed in this invention optimizes the alloy design of Mn / C and Ti / N, resulting in low cost and applicability to components with high safety requirements such as railway vehicles, containers, and photovoltaic brackets. It has high internal stress uniformity and low absolute value; excellent formability, good cold bending performance in both longitudinal and transverse directions, and meets the GB / T 232 test method for bending of metallic materials, achieving 180° and d=0.5a (d is the diameter of the bending mandrel and a is the thickness of the steel plate) when tested. It does not warp when cold bent along the entire length and has good straightness.

[0030] (2) To eliminate residual stress in the steel plate, it is necessary to strictly control the plate shape quality. The plate shape quality depends on the accuracy of the plate shape control parameter setting calculation. To ensure the accuracy of the plate shape control parameter setting calculation, the rolling of corrosion-resistant steel should be arranged under the conditions that the hot roll shape of the finishing work roll has been formed and stabilized, the wear of the finishing work roll is small, and irregular wear roll shape has not yet appeared. In addition, when rolling corrosion-resistant steel, the difference between the rolling force and the rolling force of the previous few coils should be minimized to ensure the accuracy of the rolling force setting, thereby ensuring the accuracy of the plate shape control parameter setting.

[0031] (3) In response to the large difference between the final rolling temperature and the coiling temperature of weathering steel products, and the rapid temperature drop of strip steel during laminar cooling, a head micro-edge wave target control strategy is adopted in production. Operators pre-adjust the bending roll force to prevent the strip head from generating a medium wave that seriously affects the temperature and shape control during laminar cooling, thereby effectively improving the strip shape quality and temperature control accuracy after laminar cooling. Attached Figure Description

[0032] Figure 1 The deformation of different steel plates after cutting into strips (left: flat plate, right: longitudinally cut plate); Figure 2 Microstructure diagram of the steel plate prepared in Example 1; Figure 3 This is a diagram showing the locations of internal stress detection. Figure 4 This is a trend chart of internal stress detection. Figure 5 This is a flowchart of the preparation method. Detailed Implementation

[0033] To make the objectives and technical solutions of this invention clearer, the following embodiments are provided for further explanation. However, the scope of protection of this invention is not limited to these embodiments; the embodiments are merely for illustrative purposes. Those skilled in the art should understand that any changes or equivalent substitutions that do not depart from the concept of this invention are included within the scope of protection of this invention.

[0034] Unless otherwise specified, all reagents and raw materials used in this invention are obtained through purchase.

[0035] The present invention provides a low-stress weathering steel plate in the specific embodiments section. Its chemical composition by mass percentage is as follows: C 0.054%~0.074%, Si 0.51%~0.62%, Mn 0.77%~0.87%, P 0.104%~0.116%, S≤0.006%, Ti 0.102%~0.122%, Cr 1.04%~1.24%, Cu 0.43%~0.54%, V 0.042%~0.064%, Als 0.026%~0.056%, N≤0.004%, and satisfies Mn / C=10.4~16.2, Ti / N=25~61, with the remainder being Fe and impurities.

[0036] Based on the above technical solution, the metallographic structure of the low internal stress weathering steel plate is fine and uniform ferrite + pearlite, wherein the ferrite accounts for 76~85% and the pearlite accounts for 15~24%, calculated as a total percentage of 100%.

[0037] Based on the above technical solution, the thickness of the low internal stress weathering steel plate is 1.5~16 mm, Rp0.2 For 750~805MPa, R m It has a strength of 790~860MPa, an A value of 28~36%, and a KV2 value of 200~260J at -40℃. It exhibits good cold bending performance in both longitudinal and transverse directions. According to the GB / T 232 test method for bending of metallic materials, it can reach 180° and d=0.5a, which is qualified. It does not warp when cold bent along the entire length, has good straightness, and a grain size of 10.5~12.0.

[0038] The present invention also provides a method for preparing low internal stress weathering steel plate in the specific embodiments section, including the following steps: hot metal pretreatment - combined blowing converter smelting - LF ladle furnace refining - continuous casting - hot charging and hot delivery - heating - rolling - laminar flow cooling - coiling - slow cooling - annealing - leveling and finishing. Hot metal pretreatment: Passivated magnesium particles and lime are sprayed into the hot metal through a spray gun using a blowing method. After pretreatment, S≤0.005%, wherein magnesium:lime =1:3.5; Re-blown converter smelting; LF ladle furnace refining: 80~1040kg of ferrotitanium is added during the refining process for alloying, the refining outlet temperature is controlled at 1592~1604℃, and the refining time is 46~58min; Continuous casting: The continuous casting temperature is 1543~1552℃, the casting time is 23~32min, and the resulting continuous casting billet has a size of 250mm×1910~2060mm. Hot charging and hot delivery: After the continuous casting billet is cut into fixed lengths, it is sent to the heating furnace for heating. The fixed length is 10.5~10.8m, and the temperature of the continuous casting billet entering the furnace is ≥620℃. Heating: A walking beam furnace is used for heating. The furnace exit temperature is 1256~1276℃ and the heating time is ≥118min. The high heating temperature and sufficient heating time are intended to ensure that the alloying elements Cr, Cu and V and titanium carbonitride are completely dissolved, thereby eliminating the adverse effects of large particle precipitates not floating up sufficiently and becoming inclusions, which would increase internal stress. Rolling: Two-stage controlled rolling is adopted. The temperature of the roughing mill is 1132~1154℃. The entire process between the roughing and finishing mills is covered with an insulation cover. Before entering the finishing mill, the workpiece is moved to wait for the temperature to rise. The temperature of the workpiece entering the finishing mill is required to be between 965~985℃. During the finishing mill, an edge heater is used to compensate for the edge temperature of the rolled piece. The edge compensation temperature is 68~78℃ to reduce the difference in microstructure between the edge and the middle of the steel plate, improve the uniformity of the steel plate properties and the quality of the plate shape. The finishing mill final rolling temperature is 856~886℃. Laminar flow cooling: Water cooling is applied immediately after exiting the finishing mill, using a front rapid cooling process and an edge shielding process; Coiling: Divide the steel strip into three parts: head, middle and tail. The head of the strip is fed into the coil at 55~65℃, and the tail is fed into the coil at 50~60℃ to improve the temperature uniformity along the length of the strip. The coiling temperature is controlled at 552~574℃, which is conducive to promoting the uniformity of internal stress along the length. Slow cooling: In order to eliminate internal stress, the coiled steel coil is hoisted into a slow cooling pit or hot coil is surrounded by a cooling process to avoid uneven cooling caused by direct cold air blowing. The coil is slowly cooled from 552~574℃ to room temperature for ≥48 hours at a slow cooling rate of 6~12℃ / h. Annealing: The steel coil is hoisted into a bell-type annealing furnace for annealing treatment. The heating rate is 50~80℃ / h, the holding temperature is 492℃~518℃, the target temperature is 500℃, and the holding time is 6.5h~7.5h. After the holding time is completed, the coil is slowly cooled in the furnace at a cooling rate of ≤50℃ / h until it reaches below 200℃ before being taken out of the furnace. Leveling and finishing: A leveling and straightening unit with strong straightening capacity is adopted. In actual production, the straightening force is required to be ≥650kN. Through reasonable setting of the gap between the coarse and fine straightening rollers, repeated bending and straightening are performed to eliminate the original curvature, homogenize the residual stress on the surface of the strip, and ensure that the absolute value of the camber is not greater than 1.5mm.

[0039] Examples and Comparative Examples The thickness and chemical composition of the steels in Examples 1-5 and Comparative Examples 1-2 are shown in Table 1. Parameters related to continuous casting, hot charging, heating, rolling, and slow cooling are shown in Table 2. Parameters related to annealing and leveling are shown in Table 3. According to national standards GB / T228 and GB / T231, the performance test results and metallographic structure of the steels obtained in Examples 1-5 and Comparative Examples 1-2 are shown in Table 3. As can be seen from Table 3, the tensile properties of the examples and comparative examples are at the same level. However, the low-temperature impact toughness of the present invention is significantly higher than that of the comparative examples. This is related to the more uniform and lower internal stress control of the steel plate. The metallographic structure of the steel prepared in Example 1 of the present invention is shown in Table 2. Figure 2 It can be seen that the metallographic structure of the steel prepared in Example 1 is ferrite + pearlite, and the grain size is grade 12.0. Figure 3 The diagrams show the locations of internal stress tests on the steel in Examples 1-5, and the trend diagrams of internal stress tests on the steel prepared in Examples 1-5 are shown below. Figure 4 (Positions 1-5 and) Figure 3 Correspondingly), from Figure 4 As can be seen, the internal stress in the embodiment of the present invention is smaller and the uniformity is higher, while the internal stress uniformity in the comparative example is poor; see the flowchart of the specific preparation method. Figure 5 .

[0040] Table 1. Thickness and Chemical Composition of Steel

[0041] Table 2. Relevant parameters for continuous casting, hot charging and heating, and rolling.

[0042] Table 3. Parameters related to annealing and leveling.

[0043] Table 4. Performance test results and metallographic structure of the steel

[0044] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A low-stress weathering steel plate, characterized in that, The chemical composition of the steel plate by mass percentage is as follows: C 0.054%~0.074%, Si 0.51%~0.62%, Mn 0.77%~0.87%, P 0.104%~0.116%, S≤0.006%, Ti 0.102%~0.122%, Cr 1.04%~1.24%, Cu 0.43%~0.54%, V 0.042%~0.064%, Als 0.026%~0.056%, N≤0.0040%, and satisfying Mn / C=10.4~16.2, Ti / N=25~61, with the remainder being Fe and impurities.

2. The low internal stress weathering steel plate according to claim 1, characterized in that, The metallographic structure of the low internal stress weathering steel plate consists of fine and uniform ferrite and pearlite, with ferrite accounting for 76-85% and pearlite accounting for 15-24%, based on a total proportion of 100%.

3. The low internal stress weathering steel plate according to claim 1, characterized in that, The thickness of the low internal stress weathering steel plate is 1.5~16 mm, Rp 0.2 For 750~805MPa, R m The strength is 790~860MPa, A is 28~36%, KV2 at -40℃ is 200~260J, and the grain size is 10.5~12.0 grade.

4. A method for preparing a low-stress weathering steel plate according to any one of claims 1 to 3, characterized in that, The process includes the following steps: hot metal pretreatment - combined blowing converter smelting - LF ladle furnace refining - continuous casting - hot charging and hot delivery - heating - rolling - laminar flow cooling - coiling - slow cooling - annealing - leveling and finishing; Rolling: Two-stage controlled rolling is adopted. The temperature of the workpiece entering the roughing mill is 1132~1154℃. The workpiece is kept in a heat preservation cover throughout the roughing and finishing milling process. Before entering the finishing mill, the workpiece is moved to wait for the temperature to rise. The temperature of the workpiece entering the finishing mill is 965~985℃. During the finishing milling process, the edge heater is put into the mill to compensate for the edge temperature of the workpiece. The edge compensation temperature is 68~78℃. The finishing milling temperature is 856~886℃. Laminar flow cooling: Water cooling is applied immediately after exiting the finishing mill, using a front rapid cooling process and an edge shielding process; Coiling: Divide the steel strip into three parts: head, middle and tail. The head of the strip is fed into the coil at 55~65℃ for 100 meters and the tail at 50~60℃ for 80 meters. The coiling temperature is controlled at 552~574℃.

5. The preparation method according to claim 4, characterized in that, The molten iron pretreatment specifically involves: using a spraying method to inject passivated magnesium particles and lime into the molten iron through a spray gun, resulting in a pretreatment S≤0.005%, wherein the magnesium:lime ratio is 1:3.5; The LF ladle furnace refining process specifically involves adding 880-1040 kg of ferrotitanium for alloying during the refining process, controlling the refining outlet temperature at 1592-1604℃, and refining time at 46-58 min.

6. The preparation method according to claim 4, characterized in that, The continuous casting process specifically involves a casting temperature of 1543~1552℃ and a casting time of 23~32min, resulting in a continuously cast billet with dimensions of 250mm×(1910~2060)mm.

7. The preparation method according to claim 4, characterized in that, The hot charging and hot delivery process specifically involves cutting the continuous casting billet into fixed lengths and then sending it to a heating furnace for heating. The fixed length is 10.5~10.8m, and the temperature of the continuous casting billet entering the furnace is ≥620℃. The heating process specifically involves using a walking beam furnace, with an outlet temperature of 1256~1276℃ and a heating time of ≥118min.

8. The preparation method according to claim 4, characterized in that, The slow cooling process specifically involves: the coiled steel coil is suspended in a slow cooling pit or a hot coil cooling process is used to slowly cool it from 552~574℃ to room temperature at a slow cooling rate of 6~12℃ / h for ≥48h.

9. The preparation method according to claim 4, characterized in that, The annealing process specifically involves: hoisting the steel coil into a bell-type annealing furnace for annealing treatment, with a heating rate of 50~80℃ / h, a holding temperature of 492℃~518℃, a holding time of 6.5h~7.5h, and after the holding time, slowly cooling the coil with the furnace at a cooling rate of ≤50℃ / h until it reaches below 200℃ before being removed from the furnace.

10. The preparation method according to claim 4, characterized in that, The leveling and finishing process specifically includes: a straightening force ≥ 650kN and an absolute value of the sickle bend not greater than 1.5mm.

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