High-elongation hot-rolled steel sheet with tensile strength of 800mpa and method for manufacturing the same

Abstract

This invention discloses a hot-rolled steel sheet with high elongation and a tensile strength of 800MPa, and its manufacturing method, mainly solving the technical problems of low elongation and high production cost of existing hot-rolled steel sheets with a tensile strength of 800MPa. The technical solution is a hot-rolled steel sheet with a tensile strength of 800MPa and high elongation, whose chemical composition by weight percentage is: C: 0.03%–0.06%, Si: 0.20%–0.40%, Mn: 1.00%–1.50%, P≤0.015%, S≤0.003%, Alt: 0.02%–0.05%, Nb: 0.01%–0.02%, Ti: 0.14%–0.20%, Cr: 0.10%–0.30%, N≤0.0040%, with the balance being Fe and unavoidable impurity elements; the elongation at break (A) of the 2.0–6.0 mm thick hot-rolled steel sheet is 20%–30%. It is used to manufacture structural parts for automobiles and other applications.

Description

Technical Field

[0001] This invention relates to a hot-rolled steel plate, and more particularly to a hot-rolled steel plate with a tensile strength of 800 MPa and a high elongation rate, and its manufacturing method, belonging to the field of iron-based alloy technology. Background Technology

[0002] Microalloyed high-strength steel is mainly used in high-strength structures and automotive structures, employing one or more microalloying elements such as Nb, V, and Ti to enhance strength. In high-strength steels with tensile strengths of 750 MPa and above, in addition to Nb, V, and Ti, further alloying elements such as Cr, Ni, Mo, and B are required. Combined with controlled rolling and cooling, or controlled rolling and cooling plus heat treatment, the manufacturing process is relatively easy to implement, resulting in high yield strength and strong resistance to deformation, but the alloy cost is relatively high. Simultaneously, the elongation is relatively low (generally only around 18%), often making it difficult to meet the requirements in applications with high elongation requirements.

[0003] Chinese patent application CN107699791A discloses a 900MPa grade high-cold-bending-performance low-alloy high-strength steel plate and its preparation method. It employs a low-carbon Nb and Ti micro-alloying composition design, using C (0.05–0.1%), Si (0.2–0.6%), Mn (0.5–1.0%), Nb (0.03–0.1%), and Ti (0.05–0.1%). The final rolling temperature is 880–910℃, and the coiling temperature is 620–650℃. The composition and process are simple, requiring cold rolling and annealing. The steel plate achieves a tensile strength of over 900MPa, but due to the use of a semi-annealing process, the elongation is only 5%.

[0004] Chinese patent application CN103725973A discloses a low-composition, low-Pcm value 800MPa grade high-strength steel and its production method. It employs low C (0.06–0.085%), Mn (1.70–1.80%), Cr (0.10–0.20%), and Nb (0.03–0.06%), V (0.07–0.09%), Ti (0.01–0.02%), and B (0.0010–0.0018%) compositions to achieve alloy reduction in production, saving resources and energy. However, it requires tempering treatment after rolling (600–660℃, 10–60 min), increasing process costs. The tensile strength of the steel plate is between 810 and 885 MPa, with an elongation of only 14–19%.

[0005] Chinese patent application CN105154769A discloses a 780MPa grade hot-rolled high-strength, high-expansion steel and its manufacturing method. The steel employs a composition system of C (0.03–0.07%), Mn (1.0–2.0%), Ti (0.08–0.20%), and Mo (0.15–0.25%), simultaneously satisfying the following conditions: 0.25% ≤ Ti / Mo ≤ 1.5%, and 0.03% ≤ (Ti-3.42N) / 4 + Mo / 8 ≤ 0.07%. The final rolling temperature is 800–900℃. After final rolling, the steel plate is cooled to 600–700℃ at a rate of ≥100℃ / s, and then air-cooled to room temperature at a rate of ≤20℃ / h after coiling. The example shows a tensile strength of 800MPa and an elongation of 20.5–26.0%. However, the addition of approximately 0.2% Mo results in a relatively high alloy cost.

[0006] Existing hot-rolled steel plates with a tensile strength of 800 MPa have an elongation of less than 20%. Some hot-rolled steel plates with elongation exceeding 20% ​​have higher alloy costs, leading to higher production costs. Therefore, current technology struggles to simultaneously meet the requirements of low cost and high elongation. Summary of the Invention

[0007] The purpose of this invention is to provide a hot-rolled steel plate with high elongation and a tensile strength of 800MPa and its manufacturing method, mainly to solve the technical problems of low elongation and high production cost of existing hot-rolled steel plates with a tensile strength of 800MPa.

[0008] The design concept of this invention is to achieve high strength and high elongation through low C, Mn, Nb, Ti, and Cr composition design and matching hot rolling process. The metallographic structure of the hot-rolled steel sheet includes 80%–90% ferrite (by area fraction), 10%–20% bainite, and a large number of TiC precipitates. The uniform and fine ferrite matrix ensures that the material has both good elongation and high strength, while the uniform and dispersed precipitation of TiC in the ferrite matrix further enhances the strength of the ferrite matrix. Relying solely on ferrite grain refinement and TiC precipitation reinforcement is insufficient to achieve higher strength, and plasticity and toughness will also be further reduced. By introducing low-temperature structures such as bainite, the material strength can be further improved while maintaining high elongation and toughness. The low P and ultra-low S design ensures that the steel sheet has good surface quality and a low number of inclusions, while also possessing good cold bending performance. This invention's hot-rolled steel sheet achieves an ideal microstructure match through appropriate composition and process design, satisfying both high strength and high elongation.

[0009] The technical solution adopted in this invention is a hot-rolled steel plate with a tensile strength of 800MPa and a high elongation, the chemical composition by weight percentage of which is: C: 0.03%~0.06%, Si: 0.20%~0.40%, Mn: 1.00%~1.50%, P≤0.015%, S≤0.003%, Alt: 0.02%~0.05%, Nb: 0.01%~0.02%, Ti: 0.14%~0.20%, Cr: 0.10%~0.30%, N≤0.0040%, with the balance being Fe and unavoidable impurity elements.

[0010] The metallographic structure of the hot-rolled steel sheet of this invention comprises 80%–90% ferrite by area fraction + 10%–20% bainite + a large amount of TiC precipitate particles, with the ferrite grain size being 11.0–13.0 grade; the yield strength R of the 2.0–6.0 mm thick hot-rolled steel sheet is... p0.2 The tensile strength is 700-800 MPa, and the tensile strength R is... m The strength is 800-900 MPa, the elongation after fracture (A) is 20%-30%, and the 180° bending test (d=0a) is qualified.

[0011] The hot-rolled steel sheet of this invention has high strength and high elongation, and can be used to manufacture structural components for automobiles, buildings, bridges and other applications.

[0012] The reasons for limiting the chemical composition of the hot-rolled steel sheet with a tensile strength of 800 MPa and high elongation according to the present invention to the above-mentioned range are as follows:

[0013] Carbon: While high carbon content is beneficial for increasing strength, excessive carbon content can lead to the formation of numerous large and brittle carbide particles in the steel, which is detrimental to plasticity and toughness. Excessive carbon content can also cause segregation bands in the center of the steel plate, negatively impacting bending performance. Furthermore, excessive carbon content increases the welding carbon equivalent, which is unfavorable for welding processes. Therefore, this invention adopts a low-carbon design approach, with the C content set at 0.03% to 0.06%.

[0014] Silicon: Although silicon promotes the formation of proeutectoid ferrite and expands the ferrite formation window, silicon solid solution has a significant strengthening effect on the steel plate matrix. However, high silicon content will impair plasticity and easily produce red iron scale on the surface, affecting surface quality. The Si content set in this invention is 0.20% to 0.40%.

[0015] Manganese: Manganese is a deoxidizing element that can remove large inclusions in steel to ensure its purity. Simultaneously, manganese reacts with sulfur to form manganese sulfide, avoiding the adverse effects of FeS on plasticity. Manganese plays a role in solid solution strengthening and improves the hardenability of materials, making it an important element for increasing material strength. It also expands the γ-region, lowers the γ→α transformation temperature, and widens the rolling process window. However, high Mn content easily leads to segregation, reduces material toughness, deteriorates performance, and increases carbon equivalent, which is detrimental to welding. This invention limits the Mn content to 1.00%–1.50%.

[0016] Sulfur and phosphorus: Excessive levels of sulfur and phosphorus can negatively impact the toughness and plasticity of materials. Phosphorus (P) tends to segregate at grain boundaries, reducing the toughness and plasticity of steel plates. Sulfur (S) readily forms sulfide inclusions and microstructure segregation with elements such as manganese (Mn) in steel, reducing the strength and toughness of the steel. Therefore, this invention limits S to ≤0.003% and P to ≤0.015%.

[0017] Niobium: Niobium can stabilize austenite grains, increase the austenite recrystallization temperature, refine grains, and promote ferrite formation. It can also form NbC precipitates, effectively improving the material's strength. Due to the relatively high cost of Nb, the Nb content in this invention is controlled at 0.01%–0.02%.

[0018] Titanium: Titanium is an important microalloying element in this invention. Ti is a strong carbide-nitride forming element. At high temperatures, it forms TiN precipitates, effectively refining austenite grains. At low temperatures, it forms TiC precipitates, readily producing fine, dispersed precipitates that effectively improve strength. Even a small amount of Ti can achieve a significant strengthening effect. In particular, the precipitation of carbides such as TiC in ferrite effectively improves the strength of the ferrite matrix. When the Ti content is below 0.04%, the precipitation strengthening effect is insufficient. Furthermore, compared to Nb, V, Mo, etc., Ti alloys have the lowest cost. To achieve the strength required for this invention, the Ti content is limited to 0.14%–0.20%.

[0019] Chromium: Chromium is an element that improves hardenability, inhibits the formation of polygonal ferrite and pearlite, and promotes the transformation of bainite or martensite, thereby increasing the hardness of steel. Compared with alloying elements such as Mo, Nb, and V, Cr is relatively inexpensive. However, higher Cr content will affect the toughness of steel and cause temper brittleness. This invention utilizes Cr to improve the hardenability of the material, inhibits the formation of pearlite in the high-temperature zone during cooling, and makes it easier to obtain a low-temperature structure. In addition, Cr can also form carbides and oxides, enhancing the precipitation strengthening effect. Therefore, this invention limits the Cr content to 0.10% to 0.30%.

[0020] Aluminum: In this invention, aluminum acts as a deoxidizer. Aluminum is a strong oxidizing element, reacting with oxygen in steel to form Al2O3, which is removed during steelmaking. Excessive aluminum content leads to excessive Al2O3 inclusions, which can easily clog the casting nozzle during continuous casting. Furthermore, Al2O3 inclusions severely impair the fatigue properties of hot-rolled steel plates, necessitating strict control of Al2O3 inclusions. This invention limits the aluminum content to 0.02%–0.05%.

[0021] Nitrogen: Excessive nitrogen content severely deteriorates the plasticity and toughness of materials, especially for Ti microalloyed high-strength steel. Because N combines with Ti at high temperatures to produce larger TiN particles, it affects the material's toughness and reduces the effective titanium content when the steel combines with C to produce fine TiC particles, thus leading to a decrease in strength. Therefore, this invention limits N to ≤ 0.0040%.

[0022] A method for manufacturing a hot-rolled steel sheet with a tensile strength of 800 MPa and high elongation, the method comprising the following steps:

[0023] Molten steel is continuously cast to obtain continuously cast slabs. The chemical composition of the molten steel is as follows (weight percentage): C: 0.03%–0.06%, Si: 0.20%–0.40%, Mn: 1.00%–1.50%, P≤0.015%, S≤0.003%, Alt: 0.02%–0.05%, Nb: 0.01%–0.02%, Ti: 0.14%–0.20%, Cr: 0.10%–0.30%, N≤0.0040%, with the balance being Fe and unavoidable impurity elements. During the continuous casting process, the superheat of the molten steel in the tundish is ≤25℃.

[0024] The continuously cast slab is heated in a heating furnace and then hot-rolled. The heating temperature of the continuously cast slab is 1230–1280℃, and the heating time is 150–240 min. The time of the continuously cast slab in the soaking zone of the heating furnace is ≥30 min. The hot rolling is a two-stage rolling process. The roughing rolling is a 6-pass continuous rolling process, rolled above the austenite recrystallization temperature, and the roughing rolling end temperature is 1050–1150℃. The finishing rolling is a 7-pass continuous rolling process, rolled in the non-recrystallization zone of austenite, and the finishing rolling end temperature is 860–900℃. The total reduction rate of the finishing rolling is 85%–90%. After finishing rolling, the temperature is controlled... The steel plate thickness is 2.0–6.0 mm; laminar flow cooling adopts a three-stage cooling method. The first stage is water cooling, with a cooling rate of 40–80℃ / s and a final temperature of 640–680℃; the second stage is air cooling, with a cooling rate of 5–20℃ / s and a cooling time of 8–12s; the third stage is water cooling, with a cooling rate of 80–120℃ / s and a final temperature of 430–470℃; the coiling temperature is 410–450℃, and the hot-rolled steel plate is coiled into hot-rolled steel coils using a hot-rolling coiler.

[0025] The rationale for the production process adopted in this invention is as follows:

[0026] 1. Setting the superheat of molten steel during continuous casting.

[0027] During continuous casting, the superheat of molten steel significantly affects the compositional segregation of the slab. The composition of this invention contains alloys such as Mn, Nb, Ti, and Cr, which tend to diffuse towards the center of the slab during casting, forming central segregation. This can easily lead to uneven material properties and is detrimental to stamping and load-bearing capacity. Therefore, during continuous casting, the superheat of the molten steel in the tundish should be controlled to ≤25℃.

[0028] 2. Setting the heating temperature of continuously cast slabs

[0029] After being heated in a furnace, the continuously cast slab is hot-rolled. This composition design incorporates a certain amount of Ti to achieve dispersed, fine TiC precipitation for strengthening. However, TiN and other particles, due to their high melting points, are generated and aggregate into large particles during continuous casting. It is crucial to fully dissolve these coarse TiN particles to ensure sufficient, dispersed, fine TiC particles precipitate during the cooling process after rolling. This is essential for the technical solution of this invention. If the temperature is too low or the heating time is too short, the original coarse TiN and TiC particles in the continuously cast slab cannot be fully dissolved. Conversely, if the temperature is too high or the heating time is too long, the original slab structure becomes coarse, and severe surface oxidation and decarburization negatively impact the final performance and surface quality of the steel plate, while also consuming energy. This invention sets the heating temperature of the continuously cast slab at 1230–1280℃ and the heating time at 150–240 min. To ensure sufficient dissolution of TiN in the slab, the soaking time of the continuously cast slab in the furnace is ≥30 min.

[0030] 3. Setting the finishing temperature of rough rolling

[0031] During rough rolling, it is essential to ensure that the austenite undergoes repeated deformation and recrystallization to obtain uniform and fine austenite grains. The rough rolling end temperature should not be too low, but it should also not be too high, as this can easily lead to recrystallization and growth of austenite. Therefore, the rough rolling end temperature is set at 1050–1150℃.

[0032] 4. Setting the finishing temperature

[0033] The finishing rolling temperature setting of this invention serves two purposes. Firstly, rolling in the non-recrystallized austenite region yields flattened austenite grains with internal deformation bands, which transform into fine ferrite grains during subsequent laminar cooling, thus providing grain refinement strengthening. Secondly, the finishing rolling temperature setting must prevent most second-phase particles such as TiC from precipitating prematurely within the deformed austenite. If fine, dispersed TiC particles cannot precipitate in the ferrite, sufficient strength cannot be achieved. If the finishing rolling temperature is too low, deformation-induced precipitation occurs, causing TiC to precipitate prematurely in the austenite region, failing to provide precipitation strengthening. In this invention, the Ar3 temperature is designed to be 854℃, and the finishing rolling end temperature is set to 860–900℃.

[0034] 5. Setting of laminar flow cooling method, cooling rate and cooling time after finishing rolling.

[0035] Controlling the cooling process after finishing rolling is crucial for obtaining the desired microstructure and properties. For high strength and high elongation requirements, a three-stage cooling method of water cooling + air cooling + water cooling is adopted. The cooling rate of the first stage should be as fast as possible. If the cooling rate is too slow, ferrite phase transformation will occur during the cooling process, forming coarse ferrite microstructure, resulting in insufficient material strength and elongation. The cooling endpoint temperature should not be too high or too low. Too high a cooling endpoint temperature will also result in coarse ferrite microstructure, which is detrimental to material strength; too low a cooling endpoint temperature will result in insufficient impetus for TiC second-phase particle precipitation during the second stage of air cooling, which is detrimental to precipitation strengthening and will also reduce material strength. Considering that the Ar1 of the material in this invention is 686℃, and taking into account the final ferrite microstructure ratio of this invention, the first stage water cooling rate is set to 40–80℃ / s, and the cooling endpoint temperature is set to 640–680℃.

[0036] The second stage of cooling employs air cooling. The purpose of air cooling is to obtain a certain proportion of ferrite structure. Simultaneously, within this temperature range, a large number of second-phase particles such as TiC precipitate within the ferrite, forming a fine-grained ferrite structure with dispersed fine TiC precipitates, thus fully utilizing the grain refinement strengthening and precipitation strengthening effects. Therefore, the cooling rate of the second stage is set to 5–20 °C / s, and the air cooling time is set to 8–12 s.

[0037] The third stage involves water cooling to obtain a bainitic structure. The cooling rate should be as fast as possible; if the cooling rate is too slow, the supercooled austenite will undergo a pearlite transformation, preventing the formation of a bainitic structure. Similarly, the final cooling temperature must be above the martensitic transformation initiation temperature of the composition of this invention to avoid the formation of martensite. Therefore, the cooling rate for the third stage is set at 80–120 °C / s, and the final cooling temperature is 430–470 °C.

[0038] 6. Setting the hot rolling coiling temperature

[0039] After laminar flow cooling, the steel plate enters the coiler on the rolling line for coiling. Based on the cooling termination temperature, the hot rolling coiling temperature is set to 410-450℃ in this invention.

[0040] The metallographic structure of the hot-rolled steel sheet produced by the method of this invention comprises 80%–90% ferrite by area fraction + 10%–20% bainite + a large amount of TiC precipitate particles, with the ferrite grain size being 11.0–13.0 grade; the yield strength R of the 2.0–6.0 mm thick hot-rolled steel sheet is... p0.2 The tensile strength is 700-800 MPa, and the tensile strength R is... m The strength is 800-900 MPa, the elongation after fracture (A) is 20%-30%, and the 180° bending test (d=0a) is qualified.

[0041] Compared with the prior art, the present invention has the following positive effects: 1. The present invention achieves an effective match between fine grain strengthening, microstructure strengthening and precipitation strengthening through reasonable composition and process design, especially cooling process design. The metallographic structure of the hot-rolled steel plate includes 80% to 90% ferrite + 10% to 20% bainite + a large number of TiC precipitate particles by area fraction. The grain size of the ferrite in the metallographic structure is 11.0 to 13.0 grade; the yield strength R of the 2.0 to 6.0 mm thick hot-rolled steel plate is... p0.2 The tensile strength is 700-800 MPa, and the tensile strength R is... m 1. The tensile strength is 800-900 MPa, the elongation after fracture (A) is 20%-30%, and the 180° bending test shows d=0a as acceptable. 2. Compared with existing hot-rolled high-strength steel, the 800 MPa grade hot-rolled steel sheet of this invention achieves high strength while maintaining high elongation through a reasonable match of microstructure and hot rolling process. It does not contain alloying elements such as Mo and Ni, resulting in lower production costs. 3. The hot-rolled steel sheet of this invention adopts a low P and ultra-low S design, ensuring good cold bending performance, and is used to manufacture structural components for automobiles, buildings, bridges, etc. Attached Figure Description

[0042] Figure 1 This is a metallographic photograph of the hot-rolled steel plate of Embodiment 1 of the present invention. Detailed Implementation

[0043] The present invention will be further described below with reference to Examples 1 to 5, as shown in Tables 1 to 4.

[0044] Table 1 shows the chemical composition (by weight percentage) of the steel in the embodiments of the present invention, with the balance being Fe and unavoidable impurities.

[0045] Table 1 Chemical composition of steel in the embodiments of the present invention, unit: weight percentage.

[0046]

[0047] The steel is smelted in a converter, refined in an LF ladle refining furnace with Ar blowing treatment and vacuum circulation degassing in an RH furnace, and its composition is finely adjusted to obtain molten steel that meets the composition requirements. This molten steel is then continuously cast into slabs. The thickness of the continuously cast slabs is 210–250 mm, the width is 900–1600 mm, and the length is 5000–10000 mm.

[0048] The slabs produced in steelmaking are sent to a reheating furnace for reheating, and after descaling, they are sent to a hot continuous rolling mill for rolling. Rolling is controlled by a roughing and finishing continuous rolling mill, followed by laminar flow cooling and coiling. The laminar flow cooling uses a three-stage cooling method to produce qualified hot-rolled steel coils. The thickness of the hot-rolled steel plate is 2.0–6.0 mm, and the hot rolling process control parameters are shown in Tables 2–3.

[0049] Table 2 Hot rolling process control parameters of the present invention (I)

[0050]

[0051]

[0052] Table 3 Hot rolling process control parameters (II) of this invention embodiment

[0053]

[0054] For hot-rolled steel sheets obtained using the above method, see [link to relevant documentation]. Figure 1 The microstructure of hot-rolled steel sheets consists of 80%–90% ferrite (by area fraction), 10%–20% bainite, and a large amount of TiC precipitates. The grain size of the ferrite in the microstructure is 11.0–13.0 grade. The yield strength R of 2.0–6.0 mm thick hot-rolled steel sheets is... p0.2 The tensile strength is 700-800 MPa, and the tensile strength R is... m The strength is 800-900 MPa, the elongation after fracture (A) is 20%-30%, and the 180° bending test (d=0a) is qualified.

[0055] The hot-rolled steel plate obtained by this invention was subjected to tensile testing according to the method for tensile testing of metallic materials (GB / T 228.1), microstructure detection according to the method for evaluating the microstructure of steel (GB / T 13299), and bending testing according to GB / T 232-2010 Method for bending testing of metallic materials. Its mechanical properties are shown in Table 4.

[0056] Table 4 Mechanical properties of hot-rolled steel plates according to embodiments of the present invention

[0057]

[0058] As shown in Table 4, the hot-rolled steel sheet obtained by the present invention has the advantages of high strength, high elongation, and low cost.

[0059] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A hot-rolled steel plate with a tensile strength of 800 MPa and high elongation, wherein the chemical composition by weight percentage is: C: 0.03%–0.06%, Si: 0.20%–0.40%, Mn: 1.00%–1.50%, P≤0.015%, S≤0.003%, Alt: 0.02%–0.05%, Nb: 0.01%–0.02%, Ti: 0.14%–0.20%, Cr: 0.10%–0.30%, N≤0.0040%, with the balance being Fe and unavoidable impurity elements; the yield strength R of the hot-rolled steel plate with a thickness of 2.0–6.0 mm is... p0.2 The tensile strength is 700-800 MPa, and the tensile strength R is... m The strength is 800-900 MPa, the elongation after fracture (A) is 20%-30%, and the 180° bending test (d=0a) is qualified.

2. The high elongation hot-rolled steel plate with a tensile strength of 800 MPa as described in claim 1, characterized in that, The metallographic structure of the hot-rolled steel plate comprises 80%–90% ferrite, 10%–20% bainite, and a large number of TiC precipitate particles by area fraction, with the ferrite grain size in the metallographic structure being 11.0–13.0 grade.

3. The method for manufacturing a high-elongation hot-rolled steel plate with a tensile strength of 800 MPa as described in claim 1, characterized in that, Includes the following steps: Molten steel is continuously cast to obtain continuously cast slabs. The chemical composition of the molten steel is as follows (weight percentage): C: 0.03%–0.06%, Si: 0.20%–0.40%, Mn: 1.00%–1.50%, P≤0.015%, S≤0.003%, Alt: 0.02%–0.05%, Nb: 0.01%–0.02%, Ti: 0.14%–0.20%, Cr: 0.10%–0.30%, N≤0.0040%, with the balance being Fe and unavoidable impurity elements. During the continuous casting process, the superheat of the molten steel in the tundish is ≤25℃. The continuously cast slab is heated in a heating furnace and then hot-rolled. The heating temperature of the continuously cast slab is 1230–1280℃, and the heating time is 150–240 min. The time of the continuously cast slab in the soaking zone of the heating furnace is ≥30 min. The hot rolling is a two-stage rolling process. The roughing rolling is a 6-pass continuous rolling process, rolled above the austenite recrystallization temperature, and the roughing rolling end temperature is 1050–1150℃. The finishing rolling is a 7-pass continuous rolling process, rolled in the non-recrystallization zone of austenite, and the finishing rolling end temperature is 860–900℃. The total reduction rate of the finishing rolling is 85%–90%. After finishing rolling, the temperature is controlled... The steel plate thickness is 2.0–6.0 mm; laminar flow cooling adopts a three-stage cooling method. The first stage is water cooling, with a cooling rate of 40–80℃ / s and a final temperature of 640–680℃; the second stage is air cooling, with a cooling rate of 5–20℃ / s and a cooling time of 8–12s; the third stage is water cooling, with a cooling rate of 80–120℃ / s and a final temperature of 430–470℃; the coiling temperature is 410–450℃, and the hot-rolled steel plate is coiled into hot-rolled steel coils using a hot-rolling coiler.

4. The method for manufacturing a high-elongation hot-rolled steel plate with a tensile strength of 800 MPa as described in claim 3, characterized in that: The metallographic structure of the hot-rolled steel plate comprises 80% to 90% ferrite, 10% to 20% bainite, and a large number of TiC precipitate particles by area fraction. The grain size of the ferrite in the metallographic structure is 11.0 to 13.0 grade.

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