780mpa grade hot-rolled dual phase steel with low yield ratio and method for producing the same

Abstract

This invention discloses a 780MPa grade hot-rolled duplex steel with a low yield strength ratio and its production method. The production method includes: obtaining a steel billet with a chemical composition matching that of the duplex steel after smelting and continuous casting; sequentially heating and rolling the steel billet to form a steel plate; sequentially laminar cooling and coiling the steel plate, followed by pickling or EPS (expanded polystyrene) treatment. The duplex steel of this invention possesses a low yield strength ratio, which can meet the requirements of complex automotive parts. The production method of this invention, through precise control of the laminar cooling process, can achieve regulation of the martensite ratio, thereby improving the material's performance and forming stability. The production method of this invention takes into account both material performance and surface quality control, better meeting the actual needs of users.

Description

Technical Field

[0001] This invention belongs to the field of high-strength hot-rolled steel technology, and particularly relates to 780MPa grade hot-rolled duplex steel with low yield strength ratio and its production method. Background Technology

[0002] In recent years, the automotive industry has seen a widespread trend of upgrading products with high strength, which is achieved by increasing the strength of materials to reduce their thickness. However, reducing the thickness of materials will reduce their stiffness. Therefore, the industry will compensate by increasing the structural stiffness by adding reinforcing ribs or increasing the complexity of the structure. This places higher demands on the formability of materials.

[0003] Currently, hot-rolled automotive structural steel mainly falls into two categories: microalloyed steel and ferritic steel. Microalloyed steel employs fine-grain strengthening and precipitation strengthening, resulting in a high yield strength ratio. However, as the strength level increases, springback during forming is significant, and the material's fluidity is poor. Ferritic bainitic steel, also known as high-expansion steel, exhibits a small hardness difference between ferrite and bainite phases, and the bainite contains a high dislocation density, thus possessing high flanging and expansion performance. However, this type of steel still has a high yield strength ratio, leading to high springback and forming cracks during complex forming processes. Therefore, there is an urgent need to develop a hot-rolled high-strength steel material with a low yield strength ratio and good formability to meet the customized requirements of complex parts.

[0004] In addition, existing patents with publication numbers CN109321827A, CN109694985A, CN105925905A, CN105779874A, CN105838997A, and CN108588568A involve various microalloying methods, but all of them use a three-stage laminar flow cooling combined with low-temperature coiling process to trial-produce 780MPa grade hot-rolled dual-phase steel. In some patents, the coiling temperature is as low as 20℃. However, the above-mentioned low-temperature process is prone to problems such as surface quality, steel plate shape, and forming cracks, and these existing patents do not explain or solve these problems. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a 780MPa grade hot-rolled duplex steel with a low yield strength ratio and its production method, which can effectively solve problems such as surface quality, steel plate shape and forming cracking.

[0006] The objective of this invention is achieved through the following technical solution: This invention provides a 780MPa grade hot-rolled duplex steel with a low yield strength ratio. The composition of the duplex steel, by mass fraction, includes: C: 0.05%~0.1%, Mn: 1.5%~2%, P≤0.015%, S≤0.005%, N≤0.004%, Cr: 0.3%~0.5%, Ti: 0.06%~0.08%; The chemical composition of duplex steel also includes: Mo: 0.15%~0.25%, Si: 0.05%~0.1%, Als: 0.02%~0.04%; or Nb: 0.03%~0.04%, Si: 0.4%~0.8%, Als: 0.02%~0.04%; or Nb: 0.03%~0.04%, Si: 0.05%~0.1%, Als: 0.3%~0.4%; The remaining chemical composition of duplex steel consists of Fe and unavoidable impurities.

[0007] Furthermore, the microstructure of the dual-phase steel consists of 75%–85% ferrite and 15%–25% martensite, with a grain size ≤8 μm.

[0008] Furthermore, the yield strength of duplex steel is 500MPa~650MPa, the tensile strength of duplex steel is 780MPa~880MPa, the yield-to-tensile ratio of duplex steel is 0.65~0.75, and the elongation of duplex steel is ≥22%.

[0009] Secondly, a production method is provided for producing 780MPa grade hot-rolled duplex steel with a low yield strength ratio, comprising the following steps: After smelting and continuous casting, a steel billet with a chemical composition matching that of duplex steel is obtained; The steel billet is sequentially heated and rolled into a slab to form a steel plate; After the steel plate is subjected to laminar flow cooling and coiling in sequence, it is then pickled or EPS processed.

[0010] Furthermore, the slab heating of the steel casting includes: Steel billets with a thickness of 200mm to 230mm are cold-charged into a slab heating furnace. The furnace entry temperature of the steel billets is ≤400℃, the furnace exit temperature is 1180℃ to 1230℃, and the furnace time is 200min to 300min.

[0011] Furthermore, rolling the steel billet includes: The steel billet is rough rolled to obtain an intermediate billet; The intermediate billet is precision rolled to obtain steel plate.

[0012] Furthermore, the roughing process involves 5 to 6 rolling passes, with 4 to 5 passes for removing the slag and a deformation of ≥18% per pass. The final rolling temperature is 1000℃ to 1100℃, and the thickness of the intermediate billet after roughing is 40mm to 50mm. The deformation amount in the final pass of finishing rolling is 7%~9%, the initial rolling temperature of finishing rolling is 1020℃~1060℃, the final rolling temperature of finishing rolling is 800℃~840℃, and the thickness of the steel plate obtained by finishing rolling is 2mm~6mm; among which, when the steel plate thickness is 2mm~4.49mm, the exit speed of finishing rolling is 5m / s~5.5m / s; when the steel plate thickness is 4.5mm~6mm, the exit speed of finishing rolling is 4.5m / s~4.99m / s.

[0013] Furthermore, the laminar flow cooling of the steel plate includes: The steel plate is cooled in the first stage by water cooling, wherein the cooling rate of the first stage is 15℃ / s~25℃ / s; The steel plate is cooled in a second stage by air cooling, wherein the air cooling temperature of the second stage is 680℃~720℃ and the air cooling time of the second stage is 6~10s. The steel plate undergoes a third stage of cooling via water cooling. Specifically, when the Mo content in the chemical composition is 0.15%~0.25%, the cooling rate of the third stage is 20℃ / s~40℃ / s, and the final cooling temperature of the third stage is 420℃~480℃. When the Nb content in the chemical composition is 0.03%~0.04% and the Si content is 0.4%~0.8%, the cooling rate of the third stage is 50℃ / s~80℃ / s, and the final cooling temperature of the third stage is 160℃~240℃. When the Nb content in the chemical composition is 0.03%~0.04% and the Al content is 0.3%~0.4%, the cooling rate of the third stage is 50℃ / s~80℃ / s, and the final cooling temperature of the third stage is 160℃~240℃.

[0014] Furthermore, the chloride ion content in the cooling water during water cooling is 40 mg / L to 80 mg / L.

[0015] Furthermore, after the steel plate is coiled and cooled to room temperature, it is sent to a pickling coil unit or an EPS unit to remove the iron oxide scale. When the roll is sent to the pickling machine, the pickling temperature is 40℃~60℃, and the acid solution is HCl with a concentration of 50g / L~80g / L; When the material is sent to the EPS unit, the sandblasting rate is 10m / s to 20m / s.

[0016] The beneficial effects of this invention are as follows: The duplex steel in this invention has a low yield strength ratio, which can meet the requirements of complex automotive parts. The production method in this invention can achieve the regulation of martensite ratio through precise control of laminar flow cooling process, thereby improving the material's performance and forming stability. The production method in this invention takes into account both material performance and surface quality control, and is more in line with the actual needs of users. Attached Figure Description

[0017] The invention will now be described in more detail with reference to embodiments and the accompanying drawings. Figure 1 A schematic flowchart of the production method in this invention is shown; Figure 2 The microstructure diagram of Embodiment 1 of the present invention is shown; In the accompanying drawings, the same parts use the same reference numerals. The drawings are not to scale. Detailed Implementation

[0018] The invention will now be further described with reference to the accompanying drawings.

[0019] This invention provides a 780MPa grade hot-rolled duplex steel with a low yield strength ratio. The composition of the duplex steel, by mass fraction, includes: C: 0.05%~0.1%, Mn: 1.5%~2%, P≤0.015%, S≤0.005%, N≤0.004%, Cr: 0.3%~0.5%, Ti: 0.06%~0.08%; The chemical composition of duplex steel also includes: Mo: 0.15%~0.25%, Si: 0.05%~0.1%, Als: 0.02%~0.04%; or Nb: 0.03%~0.04%, Si: 0.4%~0.8%, Als: 0.02%~0.04%; or Nb: 0.03%~0.04%, Si: 0.05%~0.1%, Als: 0.3%~0.4%; The remaining chemical composition of duplex steel is Fe and unavoidable impurities; The microstructure of duplex steel consists of 75%~85% ferrite and 15%~25% martensite, with a grain size ≤8µm.

[0020] Understandably, the microstructure of dual-phase steel is designed to consist of 75%–85% ferrite and 15%–25% martensite. Ferrite is a soft phase, while martensite is a hard phase. Ferrite can improve the elongation properties of the material through dislocation slip, while martensite can improve the strength of the steel through a higher dislocation density. During the elongation process, dislocation slip occurs first in the ferrite microstructure of the dual-phase steel, resulting in a lower yield strength. Meanwhile, a certain proportion of martensite improves the tensile strength of the material. In summary, this dual-phase steel possesses a low yield strength ratio and excellent forming elongation properties.

[0021] It should be noted that carbon (C) is an important component of martensite in steel. When the C content is too low, the martensite content will decrease, which will reduce the strength of the material. However, the C content should not be too high either, as excessive C content will lead to excessive martensite content, which will reduce the formability of the material. Therefore, this invention limits the C content to the range of 0.05% to 0.1%. Mn in steel can enhance the strength of the material through solid solution strengthening. Simultaneously, Mn can form a complete solid solution with Fe in austenite, thus improving the toughness and plasticity of the material. Furthermore, increasing the Mn content can increase the stability of supercooled austenite, promoting the transformation of more supercooled austenite into martensite during the third cooling stage; that is, increasing the Mn content can increase the martensite content. However, excessively high Mn content not only easily leads to the formation of segregation bands in the center of the steel plate thickness but also results in excessively high martensite content, thereby reducing the formability and toughness of the material. Therefore, this invention limits the Mn content to the range of 1.5% to 2%. P, S, and N are unavoidable impurity elements in steel. Excessive P content increases the material's cold brittleness, excessive S content leads to excessive MnS inclusions, and excessive N content results in the formation of liquid-precipitated TiN inclusions. Therefore, this invention requires the use of low P, S, and N contents, specifically ≤0.015%, ≤0.005%, and ≤0.004%, respectively. Cr can significantly improve the hardenability of materials, and its alloy cost is relatively low; therefore, a certain amount of Cr is added to duplex steel to promote martensitic transformation. In this invention, the Cr content is limited to the range of 0.3% to 0.5%. Ti is a significant precipitation strengthening element in steel. It can react with carbon to form nano-sized TiC precipitation, thereby improving the strength of the material. The steel in this invention has a strength of over 780 MPa, indicating a high steel grade. By adding Ti, a large number of nano-sized TiC particles are formed during rolling and air cooling in the second stage of laminar cooling. This strengthens the ferrite structure, thereby improving the strength of the material. However, the Ti content should not be too low, otherwise Ti will react with elements such as N, S, and O in the molten steel during smelting to form inclusions, resulting in a lower content of TiC precipitation. Furthermore, the Ti content should not be too high, otherwise it will increase the content of liquid-precipitated TiN inclusions, reducing the formability and toughness of the material. Therefore, this invention limits the Ti content to 0.06%~0.08%. Mo can not only improve the hardenability of materials, but also significantly increase the martensitic transformation temperature of materials to suppress the pearlite transformation. Therefore, when Cr and Mo are added at the same time, ferrite-martensite dual-phase steel can be prepared at a higher coiling temperature, thereby reducing the production difficulty and improving the plate shape of the steel plate. In this invention, 0.15% to 0.25% of Mo element can be added. Nb plays the following roles in steel: it combines with C to form NbC nanoprecipitates, thereby improving the strength of the ferrite structure; the Nb elements and NbC precipitates dissolved in the steel during the rolling process can pin the austenite grain boundaries, thereby refining the grains; and it can suppress the transformation of the high-temperature pearlite phase and promote the transformation of the low-temperature martensite phase. Si can improve the stability of supercooled austenite and increase the phase transformation temperature. Therefore, this invention can add 0.4% to 0.8% Si to promote the transformation of martensite. However, the Si content should not be too high. This is because Si is easy to react with Fe and O to generate Fe2SiO4 olivine phase when the slab is heated. This substance has a low melting point and easily causes the steel matrix and surface iron oxide scale to adhere, resulting in insufficient removal of iron oxide scale during hot rolling descaling, which leads to the deterioration of the surface quality of the finished steel plate. Al in steel can play a similar role to Si in raising the phase transformation temperature, thus promoting the martensitic transformation to occur at a more suitable temperature, thereby reducing the difficulty of industrial production and the difficulty of steel plate shape control; the present invention can add 0.3%~0.4% Als (acid-soluble aluminum).

[0022] In one embodiment, the microstructure of the duplex steel comprises 75% to 85% ferrite and 15% to 25% martensite, with a grain size ≤ 8 μm.

[0023] In one embodiment, the yield strength of the duplex steel is 500MPa~650MPa, the tensile strength of the duplex steel is 780MPa~880MPa, the yield-to-tensile ratio of the duplex steel is 0.65~0.75, and the elongation of the duplex steel is ≥22%.

[0024] This invention also provides a production method for producing 780MPa grade hot-rolled duplex steel with a low yield strength ratio, such as... Figure 1 As shown, it includes the following steps: After smelting and continuous casting, a steel billet with a chemical composition matching that of duplex steel is obtained; The steel billet is sequentially heated and rolled into a slab to form a steel plate; After the steel plate is subjected to laminar flow cooling and coiling in sequence, it is then pickled or EPS processed.

[0025] In one embodiment, slab heating of a steel casting includes: Steel billets with a thickness of 200mm to 230mm are cold-charged into a slab heating furnace. The furnace entry temperature of the steel billets is ≤400℃, the furnace exit temperature is 1180℃ to 1230℃, and the furnace time is 200min to 300min.

[0026] It should be noted that the main function of the slab heating process is to promote the complete dissolution of microalloying elements. However, if the slab heating temperature is too low or the furnace time is too short, the microalloying elements will not be completely dissolved. Conversely, if the slab heating temperature is too high or the furnace time is too long, the original austenite structure will coarsen, leading to a coarser microstructure in the finished steel. Therefore, this invention requires an initial furnace temperature ≤400℃, an exit furnace temperature of 1180℃~1230℃, and a furnace time of 200min~300min. Furthermore, the cold-charging method used in this invention helps refine the slab microstructure, thereby refining the microstructure of the finished steel.

[0027] In one embodiment, rolling a steel billet includes: The steel billet is rough rolled to obtain an intermediate billet; The intermediate billet is precision rolled to obtain steel plate.

[0028] In one embodiment, the roughing process is performed in 5 to 6 passes, the number of passes for removing the slag is 4 to 5, and the deformation amount per pass is ≥18%. The final rolling temperature of the roughing process is 1000℃ to 1100℃, and the thickness of the intermediate billet after roughing is 40mm to 50mm. The deformation amount in the final pass of finishing rolling is 7%~9%, the initial rolling temperature of finishing rolling is 1020℃~1060℃, the final rolling temperature of finishing rolling is 800℃~840℃, and the thickness of the steel plate obtained by finishing rolling is 2mm~6mm; among which, when the steel plate thickness is 2mm~4.49mm, the exit speed of finishing rolling is 5m / s~5.5m / s; when the steel plate thickness is 4.5mm~6mm, the exit speed of finishing rolling is 4.5m / s~4.99m / s.

[0029] It should be noted that the main function of the rough rolling process is to promote the dynamic recrystallization of austenite, thereby achieving dynamic refinement of austenite grains. Therefore, the rough rolling process imposes limitations on the reduction per pass and the rolling temperature. Specifically, the deformation per pass is required to be ≥18%. This is because if the deformation per pass is too low, the reduction in the core of the steel plate will be small, which will not only fail to induce dynamic recrystallization but will also lead to uneven austenite structure, i.e., some austenite will recrystallize while some austenite will revert to coarsening. At the same time, this invention also requires the rough rolling exit temperature to be 1000℃~1100℃. This is because, in order to ensure the occurrence of dynamic recrystallization in the rough rolling process, the rolling temperature must be higher than the critical austenite recrystallization temperature (~950℃). In addition, this invention also limits the number of descaling passes in the rough rolling process, which is to remove iron oxide scale and improve the surface quality of the steel plate.

[0030] It should also be noted that the main function of the finishing rolling process is to promote the flattening of the austenite structure, providing more nucleation sites for subsequent phase transformations, thereby refining the grains of the finished steel. Therefore, the finishing rolling inlet temperature needs to be controlled at a relatively low level (1020℃~1060℃) to increase the rolling resistance of the steel plate during finishing rolling, promote deformation of the steel plate, and flatten the austenite structure. Simultaneously, the cumulative deformation during finishing rolling affects the degree of austenite flattening. This invention limits the intermediate billet thickness and the finished steel plate thickness to 40mm~50mm and 2mm~6mm respectively, that is, limiting the cumulative compression ratio of finishing rolling to 8.3~20. Furthermore, this invention sets requirements for the finishing rolling exit speed, specifically: "When the finished steel plate thickness is 2.0mm~4.49mm, the finishing rolling exit speed is 5m / s~5.5m / s; when the thickness is 4.5mm~6.0mm, the finishing rolling exit speed is 4.5m / s~4.99m / s." It is evident that this invention requires constant speed rolling for finishing rolling, and the steel plate... The finishing rolling speed is higher for thinner plates and lower for thicker plates. This is because constant-speed rolling helps solidify the laminar cooling model, reducing fluctuations in the production process and the laminar cooling model, thus stabilizing the martensite structure and the mechanical properties of the material. The finishing rolling speed varies with plate thickness because thinner plates cool faster in air, so a higher finishing rolling speed is used to maintain stable rolling temperature. In addition, this invention requires a lower final rolling temperature, specifically 800~840℃. This is because lowering the final rolling temperature reduces austenite stability and the martensite proportion, so a lower final rolling temperature is preferable. However, the final rolling temperature should not be too low, otherwise it is easy to enter the two-phase region during rolling, thus forming a mixed-grain structure. Finally, this invention also requires controlling the deformation of the final finishing pass to 7%~9%. The use of a lower final passing deformation is to avoid excessive rolling force affecting the plate shape and causing the iron oxide scale on the steel plate surface to break or be pressed in, thus affecting the surface quality of the finished steel.

[0031] In one embodiment, sequential laminar flow cooling of the steel plate includes: The steel plate is cooled in the first stage by water cooling, wherein the cooling rate of the first stage is 15℃ / s~25℃ / s; The steel plate is cooled in a second stage by air cooling, wherein the air cooling temperature of the second stage is 680℃~720℃ and the air cooling time of the second stage is 6~10s. The steel plate undergoes a third stage of cooling via water cooling. Specifically, when the Mo content in the chemical composition is 0.15%~0.25%, the cooling rate of the third stage is 20℃ / s~40℃ / s, and the final cooling temperature of the third stage is 420℃~480℃. When the Nb content in the chemical composition is 0.03%~0.04% and the Si content is 0.4%~0.8%, the cooling rate of the third stage is 50℃ / s~80℃ / s, and the final cooling temperature of the third stage is 160℃~240℃. When the Nb content in the chemical composition is 0.03%~0.04% and the Al content is 0.3%~0.4%, the cooling rate of the third stage is 50℃ / s~80℃ / s, and the final cooling temperature of the third stage is 160℃~240℃.

[0032] It should be noted that laminar flow cooling adopts a three-stage cooling process of "rapid cooling-air cooling-rapid cooling". The first stage of water cooling is to cool the steel plate to the two-phase region of ferrite and austenite. The second stage of air cooling is to promote ferrite nucleation and set the temperature in the two-phase region. The third stage of water cooling is to promote the transformation of supercooled austenite in the steel into martensite. For Cr-Mo steel, this invention employs a medium-temperature coiling route, requiring a first-stage cooling rate of 15℃ / s to 25℃ / s; a second-stage air-cooling temperature of 680℃ to 720℃ and an air-cooling time of 6s to 10s; and a third-stage cooling rate of 20℃ / s to 40℃ / s, with a final cooling temperature of 420℃ to 480℃. For Si-Cr-Nb or Al-Cr-Nb steel, this invention employs a low-temperature coiling route, requiring a first-stage cooling rate of 15℃ / s to 25℃ / s; a second-stage air-cooling temperature of 680℃ to 720℃ and an air-cooling time of 6s to 10s; and a third-stage cooling rate of 50℃ / s to 80℃ / s, with a final cooling temperature of 160℃ to 240℃. Here, the first-stage cooling rate... The cooling rate cannot be too low, otherwise it will easily lead to coarsening of the microstructure and the risk of forming abnormal pearlite microstructure. The air cooling temperature is set within the ferrite nucleation temperature range to facilitate ferrite nucleation. At the same time, the air cooling temperature and air cooling time have a significant impact on the martensite content of the finished steel. When the air cooling temperature is too high and the air cooling time is too long, the ferrite content in the finished steel increases and the martensite content decreases. When the air cooling temperature is too low and the air cooling time is too short, the ferrite content in the finished steel decreases and the martensite content increases. This invention limits the air cooling temperature and air cooling time, which is beneficial to stably control the martensite ratio in the finished steel to stabilize the mechanical properties of the finished steel. The cooling rate of the third stage cooling is controlled at a high level to promote martensitic transformation. When the cooling rate is too low, bainite microstructure may be formed. The coiling temperature is set within the martensitic transformation temperature range. When using a Cr-Mo composition system, the martensitic transformation temperature range is 420℃~480℃. When using a Si-Cr-Nb or Al-Cr-Nb composition system, the martensitic transformation temperature range is 160℃~240℃. Here, a coiling temperature that is too low will lead to an increase in phase transformation undercooling and an increase in martensite content. The martensite morphology will transform from island-like to chain-like, which will affect the forming performance of the material. Furthermore, a coiling temperature that is too low is not conducive to the shape control of the steel sheet. A coiling temperature that is too high may form abnormal structures such as bainite or pearlite.

[0033] In one embodiment, the chloride ion content in the cooling water during water cooling is 40 mg / L to 80 mg / L.

[0034] It should be noted that this invention requires controlling the chloride ion content in the cooling water during laminar flow cooling. This is because chloride salts (such as NaCl) in the cooling water can remain on the surface of the steel plate, causing iron oxide to appear on the surface of the steel plate through electrochemical corrosion, thereby affecting the surface quality of the material. Therefore, this invention limits the chloride ion content in the cooling water to the range of 40 mg / L to 80 mg / L.

[0035] In one embodiment, after the steel plate is coiled and cooled to room temperature, it is sent to a pickling coil unit or an EPS unit to remove iron oxide scale. When the roll is sent to the pickling machine, the pickling temperature is 40℃~60℃, and the acid solution is HCl with a concentration of 50g / L~80g / L; When the material is sent to the EPS unit, the sandblasting rate is 10m / s to 20m / s.

[0036] It should be noted that after the steel coil is cooled and then subjected to pickling or EPS treatment, the pickling temperature and acid concentration are limited to 40℃~60℃ and 50g / L~80g / L respectively in order to improve the pickling effect of iron oxide on the steel plate surface; the EPS sandblasting rate is limited to 10m / s~20m / s in order to improve the physical removal effect of iron oxide on the steel plate surface.

[0037] The specific embodiments of the present invention will be further described below with reference to examples, but the present invention is not limited to the scope of the embodiments described herein.

[0038] Table 1 shows the composition of Examples 1-4 and Comparative Examples 1-4, Table 2 and Table 2 Continued show the production process parameters of Examples 1-4 and Comparative Examples 1-4, and Table 3 shows the microstructure and mechanical properties of the finished steel in Examples 1-4 and Comparative Examples 1-4.

[0039] In Examples 1-4, finished steel plates were obtained through slab heating, rolling, laminar flow cooling, coiling, cooling, and pickling or EPS. Specific compositions, processes, and microstructures are shown in Tables 1, 2, 2 (continued), and 3, respectively. In Table 3, F, P, and M represent ferrite, pearlite, and martensite, respectively. As can be seen from the tables, the mechanical properties of the duplex steel in the examples all meet the requirements. The yield strength of the duplex steel is 500 MPa~650 MPa, the tensile strength is 780 MPa~880 MPa, the yield strength ratio is 0.65~0.75, and the elongation is ≥22%. The high elongation and low yield strength ratio of the duplex steel in the examples are due to the fact that the microstructure of the duplex steel is a combination of ferrite and martensite, with a martensite proportion of 15%~25%. This combination of hard and soft phases increases the elongation of the material and reduces the yield strength ratio. Specifically, the microstructure of Example 1 is as follows: Figure 2 As shown.

[0040] The chemical composition of the steel in Comparative Example 1 does not meet the requirements of this invention, but the hot rolling process does. Specifically, Comparative Example 1 did not use any of the following three addition methods: 1. Adding 0.15%-0.25% Mo; 2. Adding 0.03%~0.04% Nb, increasing the Si content to 0.4%~0.8%; 3. Adding 0.03%~0.04% Nb, increasing the Als content to 0.3%~0.4%. Here, both Mo and Nb can inhibit pearlite nucleation and promote martensite nucleation, while Si and Al can increase the phase transformation temperature and promote martensitic phase transformation. Therefore, the microstructure of the steel in Comparative Example 1 after phase transformation did not obtain a sufficient proportion of martensite, specifically 12%. Since the martensite content has a significant impact on tensile strength, and the yield strength ratio increases with the increase of martensite content, the tensile strength of the steel in Comparative Example 1 is relatively low, specifically 769 MPa, which is lower than the required value of 780 MPa to 880 MPa. The yield strength ratio of the steel in Comparative Example 1 is relatively high, specifically 0.79, which exceeds the required value of 0.65 to 0.75.

[0041] The chemical composition of the steels in Comparative Examples 2-4 meets the requirements of this invention, but the hot rolling process does not meet the requirements of this invention.

[0042] In Comparative Example 2, the furnace entry and exit temperatures of the steel billet were too high, at 682℃ and 1247℃ respectively, exceeding the requirements of ≤400℃ and 1180℃~1230℃. The first pass reduction in roughing and the entry temperature in finishing rolling were 16% and 1073℃ respectively, failing to meet the requirements of ≥18% and 1020℃~1060℃. These factors resulted in a coarsened microstructure in the finished steel, with a grain size of 8.7μm, exceeding the requirement of ≤8μm. Furthermore, the steel in Comparative Example 2 did not employ the three-stage laminar flow cooling mode of "rapid cooling-air cooling-rapid cooling," but only a conventional one-stage rapid cooling method. The coiling temperature was also high at 618℃, exceeding the requirements of this invention. Therefore, the microstructure of the steel in Comparative Example 2 was ferrite plus pearlite, rather than ferrite plus martensite, and its yield strength ratio obviously could not reach the level of dual-phase steel, with an actual value of 0.90, far exceeding the requirement of 0.65~0.75 in this invention.

[0043] In Comparative Example 3, the final rolling temperature of the steel was too high, specifically 866℃, exceeding the required range of 800℃~840℃. The cooling rate of the third-stage cooling was also too high, and the coiling temperature was too low, at 84℃ / s and 80℃ respectively, failing to meet the requirements of 50℃ / s~80℃ / s and 160℃~240℃. The higher final rolling temperature increases the stability of supercooled austenite, promoting the transformation of more supercooled austenite into martensite during the third-stage cooling. Furthermore, the faster cooling rate and lower coiling temperature increase the degree of undercooling in this transformation, leading to more martensite nucleation and increasing the risk of martensite segregation. Therefore, the martensite content in the microstructure of Comparative Example 3 steel was too high, specifically 31%, exceeding the required range of 15~25%. The high content of brittle martensite phase also resulted in a lower elongation of 20.5%, below the required range of ≥22%.

[0044] In Comparative Example 4, the number of descaling water openings in the roughing mill was 2, which is less than the 4-5 passes required by this invention. The finishing mill's final rolling temperature was too low, and the final reduction in the finishing mill was too high, at 792℃ and 11% respectively, failing to meet the requirements of 800℃~840℃ and 7~9%. The lower rolling temperature and higher rolling deformation will cause the iron oxide scale on the steel plate surface to break or even be pressed into the steel plate. At the same time, the chloride ion content in the cooling water during laminar cooling was too high, specifically 144mg / L, which is far higher than the required value of 40mg / L~80mg / L. Since chloride ions have a significant corrosive effect on the steel matrix, and the corrosion products are mainly iron oxide, this will further lead to a deterioration in the surface quality of the steel plate, resulting in defects such as pitting and horizontal lines after pickling.

[0045] Table 1

[0046] Table 2

[0047] Table 2 (continued)

[0048] Table 3

[0049] In the description of this invention, it should be understood that the terms "upper", "lower", "bottom", "top", "front", "rear", "inner", "outer", "left", "right", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0050] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A 780MPa grade hot-rolled duplex steel with a low yield strength ratio, characterized in that, The formation composition of the dual-phase steel, by mass fraction, includes: C: 0.05%~0.1%, Mn: 1.5%~2%, P≤0.015%, S≤0.005%, N≤0.004%, Cr: 0.3%~0.5%, Ti: 0.06%~0.08%; The chemical composition of the duplex steel also includes: Mo: 0.15%~0.25%, Si: 0.05%~0.1%, Als: 0.02%~0.04%; or Nb: 0.03%~0.04%, Si: 0.4%~0.8%, Als: 0.02%~0.04%; or Nb: 0.03%~0.04%, Si: 0.05%~0.1%, Als: 0.3%~0.4%; The remaining chemical composition of the duplex steel is Fe and unavoidable impurities.

2. The 780MPa grade hot-rolled duplex steel with low yield strength ratio according to claim 1, characterized in that, The microstructure of the duplex steel comprises 75% to 85% ferrite and 15% to 25% martensite, with a grain size ≤ 8 μm.

3. The 780MPa grade hot-rolled duplex steel with low yield strength ratio according to claim 1, characterized in that, The yield strength of the duplex steel is 500MPa~650MPa, the tensile strength of the duplex steel is 780MPa~880MPa, the yield strength ratio of the duplex steel is 0.65~0.75, and the elongation of the duplex steel is ≥22%.

4. A production method for producing the 780MPa grade hot-rolled duplex steel with a low yield strength ratio as described in any one of claims 1-3, characterized in that, Includes the following steps: After smelting and continuous casting, a steel billet with a chemical composition matching that of duplex steel is obtained; The steel billet is sequentially heated and rolled into a slab to form a steel plate; After the steel plate is subjected to laminar flow cooling and coiling in sequence, it is then pickled or EPS processed.

5. The production method according to claim 4, characterized in that, Slab heating of steel castings includes: Steel billets with a thickness of 200mm to 230mm are cold-charged into a slab heating furnace. The furnace entry temperature of the steel billets is ≤400℃, the furnace exit temperature is 1180℃ to 1230℃, and the furnace time is 200min to 300min.

6. The production method according to claim 4 or 5, characterized in that, Rolling steel billets includes: The steel billet is rough rolled to obtain an intermediate billet; The intermediate billet is precision rolled to obtain steel plate.

7. The production method according to claim 6, characterized in that, The roughing process involves 5 to 6 rolling passes, with 4 to 5 passes for removing the slag and a deformation of ≥18% per pass. The final rolling temperature is 1000℃ to 1100℃, and the thickness of the intermediate billet after roughing is 40mm to 50mm. The deformation amount in the final pass of finishing rolling is 7%~9%, the initial rolling temperature of finishing rolling is 1020℃~1060℃, the final rolling temperature of finishing rolling is 800℃~840℃, and the thickness of the steel plate obtained by finishing rolling is 2mm~6mm; among which, when the steel plate thickness is 2mm~4.49mm, the exit speed of finishing rolling is 5m / s~5.5m / s; when the steel plate thickness is 4.5mm~6mm, the exit speed of finishing rolling is 4.5m / s~4.99m / s.

8. The production method according to claim 4, characterized in that, The laminar flow cooling of the steel plate includes: The steel plate is cooled in the first stage by water cooling, wherein the cooling rate of the first stage is 15℃ / s~25℃ / s; The steel plate is cooled in a second stage by air cooling, wherein the air cooling temperature of the second stage is 680℃~720℃ and the air cooling time of the second stage is 6~10s. The steel plate undergoes a third stage of cooling via water cooling. Specifically, when the Mo content in the chemical composition is 0.15%~0.25%, the cooling rate of the third stage is 20℃ / s~40℃ / s, and the final cooling temperature of the third stage is 420℃~480℃. When the Nb content in the chemical composition is 0.03%~0.04% and the Si content is 0.4%~0.8%, the cooling rate of the third stage is 50℃ / s~80℃ / s, and the final cooling temperature of the third stage is 160℃~240℃. When the Nb content in the chemical composition is 0.03%~0.04% and the Al content is 0.3%~0.4%, the cooling rate of the third stage is 50℃ / s~80℃ / s, and the final cooling temperature of the third stage is 160℃~240℃.

9. The production method according to claim 8, characterized in that, When water is used for cooling, the chloride ion content in the cooling water is 40 mg / L to 80 mg / L.

10. The production method according to claim 4 or 9, characterized in that, After the steel plate is coiled and cooled to room temperature, it is sent to the pickling coil unit or EPS unit to remove iron oxide scale. When the roll is sent to the pickling machine, the pickling temperature is 40℃~60℃, and the acid solution is HCl with a concentration of 50g / L~80g / L; When the material is sent to the EPS unit, the sandblasting rate is 10m / s to 20m / s.

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