High elongation flange steel with tensile strength ≥ 780 mpa and method for manufacturing the same

Abstract

The application discloses a high-elongation flange steel with a tensile strength of greater than or equal to 780 MPa and a preparation method thereof. The method comprises the following steps: converter smelting of molten iron to obtain converter molten steel; deoxidation and alloying treatment of the converter molten steel in an argon station to obtain initially alloyed molten steel; LF refining treatment of the initially alloyed molten steel to obtain LF refined molten steel; RH refining treatment of the LF refined molten steel to obtain RH refined molten steel; continuous casting of the RH refined molten steel to obtain a continuous casting billet; and hot rolling treatment of the continuous casting billet to obtain a hot-rolled steel coil. The preparation method of the high-elongation flange steel with a tensile strength of greater than or equal to 780 MPa provides a hot-rolled steel plate with stable forming performance and stable hole expansion performance, and the production process is simple and controllable.

Description

Technical Field

[0001] This application belongs to the field of flange steel technology, and particularly relates to a high elongation flange steel with a tensile strength ≥780MPa and its preparation method. Background Technology

[0002] Ferritic / martensitic duplex steel possesses excellent strength and plasticity matching, a high initial work hardening rate, and good bake hardening properties. However, the presence of a ferrite-martensitic interface with significantly different deformability makes it prone to cracking at openings during stamping and forming, resulting in poor hole-expanding performance. Particularly after flash welding, the martensitic phase is susceptible to tempering and softening in the heat-affected zone. Furthermore, the steel exhibits low fatigue strength, making it unsuitable for the production of wheel spokes, rims, and chassis. Improvements are urgently needed. Summary of the Invention

[0003] This application provides a high-elongation flange steel with a tensile strength ≥780MPa and its preparation method, and provides a hot-rolled steel plate with stable forming performance and stable hole expansion performance, with a simple and controllable production process.

[0004] In a first aspect, this application provides a method for preparing high-elongation flange steel with a tensile strength ≥780MPa, comprising: smelting molten iron in a converter to obtain converter steel; subjecting the converter steel to deoxidation and alloying treatment in an argon station to obtain pre-alloyed steel; subjecting the pre-alloyed steel to LF refining treatment to obtain LF refined steel; subjecting the LF refined steel to RH refining treatment to obtain RH refined steel; continuously casting the RH refined steel to obtain a continuously cast billet; and hot-rolling the continuously cast billet to obtain a hot-rolled steel coil.

[0005] According to an embodiment of the first aspect of this application, the LF refining treatment of pre-alloyed molten steel includes: adding lime and Al to the pre-alloyed molten steel to form CaS solid at the slag / steel interface, controlling the S content of the molten steel to within 0.003 wt.% and the Ca / S ratio to between 1.0 and 3.0, thereby obtaining LF refined molten steel.

[0006] According to an embodiment of the first aspect of this application, RH refining treatment is performed on LF refined molten steel, including: Si-Ca treatment of LF refined molten steel to deform the sulfides in the molten steel into spherical shapes.

[0007] According to an embodiment of the first aspect of this application, the main components of RH refined molten steel, by mass percentage, are: C: 0.05wt.%~0.07wt.%, Si: 0.05wt.%~0.15wt.%, Mn: 1.5wt.%~1.7wt.%, P: ≤0.013wt.%, S: ≤0.0010wt.%, Nb: 0.05wt.%~0.07wt.%, Mo: 0.10wt.%~0.20wt.%, Ti: ≤0.001wt.%, N: ≤0.003wt.%, Alt: 0.010wt.%~0.030wt.%, O: ≤0.002wt.%, with the balance being Fe and unavoidable impurities.

[0008] According to the embodiments of the first aspect of this application, the continuously cast billet has a uniform composition and a segregation level ≤ C1.0.

[0009] According to the embodiment of the first aspect of this application, in the step of continuous casting of RH refined steel, the thickness of the continuous casting billet is 210mm to 220mm, the electromagnetic stirring adopts an alternating stirring mode, the alternation time is reversed every 20 to 40 seconds, the current is 200A to 300A, the frequency is 5Hz to 10Hz, and the cooling mode is strong cooling.

[0010] According to an embodiment of the first aspect of this application, hot rolling of a continuously cast billet includes: heating the continuously cast billet to obtain a heated billet heated to 1220°C to 1280°C; rough rolling the heated billet to obtain an intermediate billet; finishing rolling the intermediate billet to obtain a finished strip; laminar cooling the finished strip to 820°C to 880°C to obtain a cooled finished strip; and coiling the cooled finished strip to obtain a hot-rolled steel coil.

[0011] According to the embodiment of the first aspect of this application, in the step of laminar flow cooling of the finished strip, the cooling process adopts water cooling and air cooling. The cooling rate of water cooling is 60℃ / s to 100℃ / s, and the air cooling is achieved by air cooling during the transportation of the finished strip, with an air cooling rate of 5℃ / s to 8℃ / s, so that the final cooling temperature of the finished strip is controlled at 640℃ to 660℃.

[0012] According to an embodiment of the first aspect of this application, in the step of coiling the cooled precision-rolled strip, the optimal precipitation temperature for Nb precipitation-strengthened steel is 580°C to 620°C.

[0013] Secondly, this application provides a high-elongation flange steel with a tensile strength ≥780MPa. The composition of the high-elongation flange steel with a tensile strength ≥780MPa, by mass percentage, includes: C: 0.05wt.%~0.07wt.%, Si: 0.05wt.%~0.15wt.%, Mn: 1.5wt.%~1.7wt.%, P: ≤0.013wt.%, S: ≤0.0010wt.%, Nb: 0.05wt.%~0.07wt.%, Mo: 0.10wt.%~0.20wt.%, Ti: ≤0.001wt.%, N: ≤0.003wt.%, Alt: 0.010wt.%~0.030wt.%, O: ≤0.002wt.%, with the balance being Fe and unavoidable impurities.

[0014] According to an embodiment of the second aspect of this application, the hot-rolled steel coil produced by this application comprises a hot-rolled steel plate with a thickness of 2.0 mm to 6.0 mm, and its mechanical properties reach: Rel ≥ 700 MPa, R m ≥750MPa, A≥14%, expansion rate≥70%, and performance fluctuation of the steel coil head and tail ≤30MPa.

[0015] According to an embodiment of the second aspect of this application, the metallographic structure of the hot-rolled steel coil is a quasi-polygonal ferrite and bainite structure, wherein it further comprises dispersed nanoscale second-phase precipitates; the content of the quasi-polygonal ferrite structure is 60% to 70%, and the average particle size of the quasi-polygonal ferrite structure is 3 μm to 6 μm; the content of the bainite structure is 30% to 40%, and the average particle size of the bainite structure is 5 μm to 8 μm; the content of the nanoscale second-phase precipitates is 0.1% to 0.13%, and the average particle size of the nanoscale second-phase precipitates is 8 nm to 12 nm.

[0016] The high elongation flange steel with a tensile strength ≥780MPa and its preparation method in this application involve deoxidizing and alloying the molten steel in the rotary kiln before it enters the argon station, and then refining the initially alloyed molten steel using LF and RH refining. The LF-refined molten steel undergoes Si-Ca refining, which, on the one hand, further purifies the molten steel, and on the other hand, modifies the sulfides in the molten steel, transforming them into non-deformable, stable, fine spherical sulfides. This suppresses the hot brittleness of sulfur, improves the low-temperature toughness of the steel plate, enhances the fatigue crack propagation resistance, elongation, and anisotropy of the steel plate's toughness. This provides a hot-rolled steel plate with stable forming performance and stable hole expansion performance, and the production process is simple and controllable. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic flowchart of the preparation method of high elongation flange steel with tensile strength ≥780MPa provided in the embodiments of this application.

[0019] Figure 2 This is a metallographic diagram of a high-elongation flange steel with a tensile strength ≥780MPa provided in Embodiment 1 of this application.

[0020] Figure 3 This is a metallographic diagram of the high elongation flange steel with a tensile strength ≥780MPa provided in Embodiment 1 of this application, containing second phase precipitates.

[0021] Figure 4 This is a metallographic diagram of a high-elongation flange steel with a tensile strength ≥780MPa provided in Embodiment 2 of this application.

[0022] Figure 5 This is a metallographic diagram of the high elongation flange steel with a tensile strength ≥780MPa provided in Embodiment 2 of this application, containing spherical calcium-treated sulfides.

[0023] Figure 6 This is a metallographic diagram of the second phase precipitates of high-elongation flange steel with a tensile strength ≥780MPa provided in Embodiment 3 of this application.

[0024] Figure 7 This is a metallographic diagram of the ferrite / martensitic dual-phase steel containing slender strip-shaped sulfide inclusions provided in Comparative Example 1.

[0025] Figure 8 These are comparative images of the product samples of the high elongation flange steel with a tensile strength ≥780MPa provided in Example 2 of this application and the flange steel provided in Comparative Example 2 after hole enlargement; wherein, 4-2 on the left is an actual picture of the product sample of the flange steel provided in Comparative Example 2 after hole enlargement, and 4-1 on the right is an actual picture of the product sample of the high elongation flange steel with a tensile strength ≥780MPa provided in Example 2 after hole enlargement. Detailed Implementation

[0026] The features and exemplary embodiments of various aspects of this application will be described in detail below. To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain this application and not to limit it. For those skilled in the art, this application can be implemented without some of these specific details. The following description of the embodiments is merely to provide a better understanding of this application by illustrating examples.

[0027] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes said element.

[0028] Based on the prior art disclosure, this application develops ferritic / bainitic duplex steel, also known as stretch-flangeable steel. When the bainite content is 10%~20%, ferritic / bainitic steel exhibits excellent formability, particularly stretch-flangeable properties. Compared to low-alloy high-strength steel (HSLA steel) and ferritic / martensitic duplex steel, the main advantages of ferritic / bainitic duplex steel are improved shear edge ductility, higher strain hardening index, and higher total elongation. Therefore, ferritic / bainitic duplex steel is more suitable for stamping parts requiring thicker thickness and good elongation, such as automotive chassis. Furthermore, during the solidification process of ferritic / bainitic duplex steel, due to varying degrees of element segregation, elemental agglomeration occurs at the center of the billet, such as C, Mn, P, and S. This agglomeration increases the concentration of these elements at the billet center to more than ten times the average concentration, resulting in highly unstable properties.

[0029] To address the problems existing in the prior art, this application provides a high-elongation flange steel with a tensile strength ≥780MPa and a method for preparing the same. The method for preparing the high-elongation flange steel with a tensile strength ≥780MPa provided in this application will be described below.

[0030] Figure 1A schematic flowchart illustrating the preparation method of high-elongation flange steel with a tensile strength ≥780MPa provided in this application embodiment is shown. Figure 1 As shown, the method for preparing high-elongation flange steel with a tensile strength ≥780MPa provided in the first aspect of this application includes: smelting molten iron in a converter to obtain converter steel; subjecting the converter steel to deoxidation and alloying treatment in an argon station to obtain pre-alloyed steel; subjecting the pre-alloyed steel to LF refining treatment to obtain LF refined steel; subjecting the LF refined steel to RH refining treatment to obtain RH refined steel; continuously casting the RH refined steel to obtain a continuously cast billet; and hot-rolling the continuously cast billet to obtain a hot-rolled steel coil.

[0031] The method for preparing high-elongation flange steel with a tensile strength ≥780MPa according to the embodiments of this application involves deoxidizing and alloying molten steel in a rotary kiln argon station, and then refining the initially alloyed molten steel using LF and RH refining. The LF-refined molten steel undergoes Si-Ca refining, which, on the one hand, further purifies the molten steel, and on the other hand, modifies the sulfides in the molten steel, transforming them into non-deformable, stable, fine spherical sulfides. This suppresses the hot brittleness of sulfur, improves the low-temperature toughness of the steel plate, enhances the fatigue crack propagation resistance, elongation, and anisotropy of the steel plate's toughness. This provides a hot-rolled steel plate with stable forming performance and stable hole expansion performance, and the production process is simple and controllable.

[0032] In some embodiments, the LF refining treatment of pre-alloyed molten steel includes: adding lime and Al to the pre-alloyed molten steel to form CaS solid at the slag / steel interface, controlling the S content of the molten steel to within 0.003 wt.% and the Ca / S ratio to between 1.0 and 3.0, thereby obtaining LF refined molten steel.

[0033] In some embodiments, RH refining treatment of LF refined steel includes: Si-Ca treatment of LF refined steel to deform the sulfides in the steel into spherical shapes.

[0034] The inventors of this application have discovered that silicon-calcium treatment can deform sulfides in LF refined steel, transforming them from softer strips into smaller, harder spheres, thereby improving the formability of steel products.

[0035] In the preparation method of high elongation flange steel with tensile strength ≥780MPa provided in the embodiments of this application, Si-Ca refining treatment is performed on LF refined steel. On the one hand, it can further obtain purified and clean steel liquid. On the other hand, it can modify the sulfides in the steel liquid to make them into non-deformable, stable and fine spherical sulfides, so as to suppress the hot brittleness of S, improve the low temperature toughness of the steel plate, improve the fatigue crack propagation characteristics, elongation and anisotropy of the steel plate toughness.

[0036] In some embodiments, the main components of RH refined molten steel, by mass percentage, are: C: 0.05wt.%~0.07wt.%, Si: 0.05wt.%~0.15wt.%, Mn: 1.5wt.%~1.7wt.%, P: ≤0.013wt.%, S: ≤0.0010wt.%, Nb: 0.05wt.%~0.07wt.%, Mo: 0.10wt.%~0.20wt.%, Ti: ≤0.001wt.%, N: ≤0.003wt.%, Alt: 0.010wt.%~0.030wt.%, O: ≤0.002wt.%, with the balance being Fe and unavoidable impurities.

[0037] The method for preparing high-elongation flange steel with a tensile strength ≥780MPa in this application embodiment yields a quasi-polygonal ferrite structure, which is beneficial for improving the coordination of the microstructure during deformation, limiting the content of S, N, and Ti in the steel, and controlling the number, morphology, and size of inclusions such as MnS and TiN, which is beneficial for improving formability and fatigue performance. Furthermore, the cooling method combining water cooling and air cooling, along with a coiling temperature of 580℃~620℃, makes the production process controllable and achieves a high success rate.

[0038] In some embodiments, the casting speed for producing continuous casting billets is 1.2 m / min to 1.4 m / min.

[0039] In some embodiments, in the step of continuously casting RH refined steel, the thickness of the continuously cast billet is 210mm to 220mm, the electromagnetic stirring adopts an alternating stirring mode, the alternation time is reversed every 20 to 40 seconds, the current is 200A to 300A, the frequency is 5Hz to 10Hz, and a strong cooling mode is used for cooling.

[0040] In some embodiments, the continuously cast billet has a uniform composition and a segregation grade ≤ C1.0.

[0041] In the method for preparing high-elongation flange steel with a tensile strength ≥780MPa provided in this application embodiment, the thickness of the continuously cast billet is 210mm~220mm, and electromagnetic stirring is used, with a forced cooling mode. In the forced cooling mode, the surface cooling rate of the continuously cast billet is as high as 600℃ / min~700℃ / min, but the core cooling rate is only 10℃ / min~15℃ / min, resulting in a huge difference between the internal and external cooling rates. Elements such as Ti and N will segregate during solidification, meaning that the element content in the remaining molten steel will increase as solidification progresses. Therefore, using a thinner continuously cast billet can increase the cooling rate of the billet core while ensuring production efficiency, increasing the core cooling rate to 40℃ / min~50℃ / min. Furthermore, electromagnetic stirring can ensure uniform mixing of the molten steel in the continuous casting machine, reducing the content of elements prone to segregation at the end of solidification, thereby reducing center segregation and the formation of large-sized inclusions.

[0042] In the preparation method of high elongation flange steel with tensile strength ≥780MPa provided in the embodiments of this application, electromagnetic stirring is used in the continuous casting process to alleviate element segregation. Through the dual effects of Nb grain refinement and precipitation strengthening, the matrix strength of ferrite is improved, thereby achieving its mechanical strength. At the same time, the content of S, N, Ti and O in the steel is limited, and the quantity, morphology and size of inclusions such as MnS and TiN are controlled, thereby improving the formability and fatigue performance of the steel.

[0043] In some embodiments, hot rolling of the continuously cast billet includes: heating the continuously cast billet to obtain a heated billet heated to 1220°C to 1280°C; rough rolling the heated billet to obtain an intermediate billet; finishing rolling the intermediate billet to obtain a finished strip; laminar cooling the finished strip to 820°C to 880°C to obtain a cooled finished strip; and coiling the cooled finished strip to obtain a hot-rolled steel coil.

[0044] In the preparation method of high-elongation flange steel with a tensile strength ≥780MPa provided in the embodiments of this application, the continuously cast billet is heated to 1220℃~1280℃. The inventors of this application have experimentally demonstrated that when the steel contains only Nb, the complete solution temperature of Nb is only 1200℃. After all Nb is dissolved, austenite grains will grow abnormally. However, the addition of Ti can significantly increase the complete solution temperature to over 1300℃, and under the coupling effect of Nb and Ti, the complete solution temperature of Nb can be increased by about 50℃. Therefore, selecting a heating temperature of 1220℃~1280℃ ensures the solution of Nb while preventing excessive austenite grain growth. If the heating temperature is too low, insufficient dissolution of microalloying elements will affect their precipitation strengthening. Therefore, controlling the heating temperature is crucial.

[0045] In some embodiments, the finishing rolling temperature is 820°C to 880°C. Using a lower rolling temperature increases the deformation of the material in the non-recrystallized austenite region, increases the number of dislocations in the deformed austenite, promotes the formation of a fine-grained transformation structure, and enhances fine-grain strengthening. Therefore, the finishing rolling temperature is 820°C to 880°C.

[0046] In some embodiments, in the step of laminar flow cooling of the finished strip, the cooling process employs water cooling and air cooling. The cooling rate of water cooling is 60°C / s to 100°C / s, and air cooling is achieved by air cooling during the transportation of the finished strip at a rate of 5°C / s to 8°C / s. The final cooling temperature of the finished strip is controlled at 640°C to 660°C.

[0047] In some embodiments, the cooling process employs a front-stage cooling method, namely water cooling, with a cooling rate of 60°C / s to 100°C / s. The subsequent cooling stage involves air cooling during steel plate transport at a rate of 5°C / s to 8°C / s, controlling the final cooling temperature of the finished strip at 640°C to 660°C. This process involves a short dwell time in the high-temperature zone, suppressing the precipitation of the second phase of Nb. Ultra-fast cooling is not used because rapid cooling rates and large temperature drops would significantly increase the internal stress of the steel plate, leading to deformation and warping during processing.

[0048] In the preparation method of high elongation flange steel with tensile strength ≥780MPa provided in the embodiments of this application, water cooling and air cooling methods and a coiling temperature of about 600°C are adopted, making the production process simple and controllable with a high success rate.

[0049] In some embodiments, during the step of coiling the cooled finished strip, the optimal precipitation temperature for Nb precipitation-strengthened steel is 580°C to 620°C.

[0050] The method for preparing high-elongation flange steel with a tensile strength ≥780MPa in this application embodiment uses an optimal precipitation temperature of approximately 600℃ for Nb precipitation-strengthened steel. Considering the specifications of this product, the coiling temperature (CT) is controlled between 580℃ and 620℃ to induce the precipitation of a large amount of Nb and Ti second phases, increasing the matrix strength of the ferrite. The dual effect of grain refinement and precipitation allows the mechanical properties of the steel in this application to meet user requirements. It should be noted that the precipitation of Nb and Ti second phases here refers to the grain refinement of these two metals and their precipitation within the metallographic structure, which is not the same as the precipitation of second-phase precipitates from inclusions.

[0051] Secondly, embodiments of this application provide a high-elongation flange steel with a tensile strength ≥780MPa. The composition of this high-elongation flange steel, by mass percentage, includes: C: 0.05wt.%~0.07wt.%, Si: 0.05wt.%~0.15wt.%, Mn: 1.5wt.%~1.7wt.%, P: ≤0.013wt.%, S: ≤0.0010wt.%, Nb: 0.05wt.%~0.07wt.%, Mo: 0.10wt.%~0.20wt.%, Ti: ≤0.001wt.%, N: ≤0.003wt.%, Alt: 0.010wt.%~0.030wt.%, O: ≤0.002wt.%, with the balance being Fe and unavoidable impurities.

[0052] In the high-elongation flange steel with a tensile strength ≥780MPa provided in this application embodiment, carbon, as an important alloying element, plays a significant role in improving the strength of the steel plate and promoting the precipitation of the second phase. However, excessive carbon content can affect weldability and formability. Low-carbon steel for cold forming is currently widely used. This application adopts a relatively low carbon content range, which can ensure the full precipitation of Nb and Mo while avoiding the formation of pearlite. Therefore, a carbon content of 0.05wt.%~0.07wt.% is used.

[0053] In some embodiments, silicon offers advantages such as increasing steel plate strength, expanding the intermediate-temperature phase transformation zone, and suppressing carbide precipitation. However, excessively high silicon content negatively impacts the surface quality of the steel plate. Furthermore, as a deoxidizer, excessively low silicon content hinders control of the oxygen content in the steel. Therefore, the silicon content is typically set within the range of 0.05 wt.% to 0.15 wt.%.

[0054] In some embodiments, manganese is the main solid solution strengthening element in steel, a typical austenite stabilizing element, and plays a role in refining ferrite grains and delaying the pearlite transformation. Therefore, manganese is used in the range of 1.5 wt.% to 1.7 wt.%.

[0055] The single quasi-polygonal ferrite structure in steel also helps to improve the coordination of the microstructure during deformation, avoids cracks at the two-phase interface due to the incoordination of the two-phase structure in the steel during deformation, and thus has good hole expansion performance.

[0056] In some embodiments, phosphorus, as a harmful inclusion in steel, has a significant detrimental effect on the steel’s low-temperature impact toughness, elongation, weldability, and fatigue crack propagation resistance. Theoretically, the lower the phosphorus content, the better. However, considering the operability and cost of steelmaking, the phosphorus content is controlled at ≤0.013wt.%.

[0057] In some embodiments, sulfur, as a harmful inclusion element in steel, mainly manifests as long, linear sulfides, which significantly impair the low-temperature toughness and fatigue crack propagation resistance of steel. More importantly, sulfur combines with manganese (Mn) in steel to form MnS inclusions. During hot rolling, the plasticity of MnS inclusions causes MnS to extend along the rolling direction, forming MnS inclusion bands along the rolling direction, which severely damages the fatigue crack propagation resistance of the steel plate. Theoretically, the lower the sulfur content, the better; however, considering the operability of steelmaking, steelmaking costs, and the principle of smooth logistics, the sulfur content is controlled at ≤0.001 wt.%.

[0058] In some embodiments, the microalloying element niobium exhibits significant grain refinement strengthening and moderate precipitation strengthening effects, which are beneficial for improving the strength of steel plates. Furthermore, the products formed by Nb with C and N will re-dissolve during heating and precipitate during cooling. Nb does not react with elements such as S and O, resulting in high yield and stable strengthening effect. Considering both cost and strength, Nb is controlled at 0.05 wt.%~0.07 wt.%.

[0059] In some embodiments, the addition of titanium also contributes to strength. However, Ti has a strong affinity for N, and the presence of Ti will greatly increase the risk of TiN inclusions. Therefore, the suitable Ti content range is ≤0.001 wt.%, and the N content is controlled at ≤0.003 wt.%.

[0060] In some embodiments, aluminum, as the main deoxidizing element, determines the oxygen (O) content in steel. However, excessively high Al content will generate Al₂O₃, which is the core for TiN heterogeneous nucleation, leading to excessive TiN inclusions. Therefore, it is advisable to control Al content between 0.010 wt.% and 0.030 wt.%.

[0061] In some embodiments, oxygen is an element removed from steel through deoxidation, and it is a major component of oxide inclusions in steel. However, for Ti, trace amounts of O can actually fix Ti, because O reacts with Ti to form Ti₂O₃ more easily than TiN. Furthermore, spherical Ti₂O₃ poses a far less detrimental effect on fatigue than square TiN. Therefore, the O content is specified as 0.001 wt.%~0.002 wt.%, and [%Ti] - 2[%O] < 0. This prevents Ti from reacting with O and then not reacting with N. Similarly, the reaction of Al with O can also avoid the formation of impurities such as AlN.

[0062] In some embodiments, the hot-rolled steel coils produced in this application comprise hot-rolled steel plates with a thickness of 2.0 mm to 6.0 mm, and mechanical properties reaching: Rel ≥ 700 MPa, R mThe steel has a strength of ≥750MPa, an alumina (A) of ≥14%, a hole expansion rate of ≥70%, and a performance fluctuation of ≤30MPa between the beginning and end of the steel coil. When the steel is processed into parts, the forming qualification rate is 100%, fully meeting user requirements.

[0063] In some embodiments, the metallographic structure of the hot-rolled steel coil is a quasi-polygonal ferrite and bainite structure, wherein it further comprises dispersed nanoscale second-phase precipitates; the content of the quasi-polygonal ferrite structure is 60% to 70%, and the average grain size of the quasi-polygonal ferrite structure is 3 μm to 6 μm; the content of the bainite structure is 30% to 40%, and the average grain size of the bainite structure is 5 μm to 8 μm; the content of the nanoscale second-phase precipitates is 0.1% to 0.13%, and the average grain size of the nanoscale second-phase precipitates is 8 nm to 12 nm.

[0064] The technical solutions and effects of this application will be described in detail below through specific embodiments and comparative examples.

[0065] Examples 1-4 The preparation method of high-elongation flange steel with tensile strength ≥780MPa in Examples 1-4 includes: smelting molten iron in a converter to obtain converter steel; deoxidizing and alloying the converter steel in an argon station to obtain pre-alloyed steel; performing LF refining on the pre-alloyed steel, including: adding lime and Al to the pre-alloyed steel to form CaS solid at the slag / steel interface, controlling the S content of the steel to within 0.003wt.% and the Ca / S ratio to between 1.0 and 3.0 to obtain LF refined steel; and performing RH refining on the LF refined steel, including... LF refined steel was subjected to Si-Ca treatment to obtain RH refined steel. The main components of RH refined steel, by mass percentage, are: C: 0.05wt.%~0.07wt.%, Si: 0.05wt.%~0.15wt.%, Mn: 1.5wt.%~1.7wt.%, P: ≤0.013wt.%, S: ≤0.0010wt.%, Nb: 0.05wt.%~0.07wt.%, Mo: 0.10wt.%~0.20wt.%, Ti: ≤0.001wt.%, N: ≤0.003wt. %, Alt: 0.010wt.%~0.030wt.%, O: ≤0.002wt.%, balance is Fe and unavoidable impurities; RH refined steel is continuously cast, in which the thickness of the continuously cast billet is 210mm, the electromagnetic stirring adopts an alternating stirring mode, the alternation time changes every 20-40 seconds, the current is 200A-300A, the frequency is 5Hz-10Hz, and a strong cooling mode is used for cooling to obtain the continuously cast billet; the continuously cast billet is hot-rolled, including: heating the continuously cast billet to 1220℃. The process involves heating a billet to approximately 1280℃; roughing the heated billet to obtain an intermediate billet; finishing the intermediate billet with a final rolling temperature of 820℃ to 880℃ to obtain a finished strip; and subjecting the finished strip to laminar flow cooling using a combination of water and air cooling. The water cooling rate is 60℃ / s to 100℃ / s, while the air cooling is achieved by air cooling during transport at a rate of 5℃ / s to 8℃ / s. The final cooling temperature of the finished strip is controlled at 640℃ to 660℃. The finished strip is then cooled to 640℃ to 660℃ to obtain the cooled finished strip. The cooled, finely rolled strip is coiled to obtain hot-rolled steel coils. The optimal precipitation temperature for Nb precipitation-strengthened steel is 580℃~620℃.

[0066] Comparative Example 1, Comparative Example 2 and Comparative Example 3 Comparative Examples 1-3 respectively provide existing methods for preparing ferritic / martensitic dual-phase steel, whose preparation processes are similar to those of Example 1. The differences are as follows: Compared with the composition of Example 1, the content of C, Ti, and N in Comparative Example 1 is relatively high; the Si content in the molten steel of Comparative Example 2 is relatively low, and the Si-Ca refining treatment and electromagnetic stirring steps are missing; Comparative Example 3 adopts a stronger strong cooling mode, a lower hot rolling heating temperature, a lower first-stage laminar cooling rate, a higher air cooling endpoint temperature, and a lower coiling temperature.

[0067] The steel composition, preparation process parameters, and other details of Examples 1-4 are recorded in Tables 1-3 below to provide a more effective comparison of the schemes and technical effects of this application and existing preparation methods.

[0068] Table 1. Chemical composition of Examples 1-4 and Comparative Examples 1-3

[0069] Table 2 Main process parameters of each embodiment of this application

[0070] Comparing the preparation methods of Examples 1-4 and Comparative Examples 1-3, the high elongation flange steel with tensile strength ≥780MPa was prepared. The mechanical properties were measured using a Zwick Z600E electronic tensile testing machine according to GB / T 228.1-2010, and the elongation flange properties were measured using a Zwick / roell BUP400 sheet metal forming testing machine according to GB / T 24524-2009. The test results are shown in Table 3.

[0071] Table 3. Test results of mechanical properties of steel in various embodiments of this application.

[0072] Comparative Examples 1-4 and Comparative Examples 1-3 yield high-elongation flange steel with tensile strength ≥780MPa. In the examples employing the preparation method for high-elongation flange steel with tensile strength ≥780MPa provided in this application, as shown... Figure 2 As shown in the metallographic diagram of the high-elongation flange steel with a tensile strength ≥780MPa provided in Example 1, the microstructure consists of quasi-polygonal ferrite and bainite; Figure 3 As shown, the metallographic structure also contains black, diffusely distributed nanoscale second-phase precipitates. The mechanical properties of the high-elongation flange steel in Example 1 meet the standard requirements, and its hole expansion rate is 100% qualified.

[0073] like Figure 4As shown in the metallographic diagram of the high-elongation flange steel with a tensile strength ≥780MPa provided in Example 2, the microstructure consists of quasi-polygonal ferrite and bainite; Figure 5 As shown, the metallographic structure contains spherical calcium-treated sulfides. The mechanical properties of the high-elongation flange steel in Example 2 meet the standard requirements, and its hole expansion rate is 100% qualified.

[0074] The microstructure of the high-elongation flange steel with a tensile strength ≥780MPa provided in Example 3 is quasi-polygonal ferrite and bainite; such as Figure 6 As shown, the metallographic structure also includes black, diffusely distributed nanoscale second-phase precipitates. The mechanical properties of the high-elongation flange steel in Example 3 meet the standard requirements, and its hole expansion rate is 100% qualified.

[0075] However, the steel composition of Example 1 does not meet the requirements of this application, such as... Figure 7 As shown, its metallographic structure is black and consists of chain-like TiN inclusions. After the strip-shaped MnS peels off, strip-shaped pits are formed, indicating that the morphology of the inclusions in its metallographic structure affects the stamping and hole-expanding forming effect of the product. Its hole-expanding forming rate is only 86% qualified.

[0076] Comparative Example 2's steelmaking process does not conform to the process of this application. It lacks the Si-Ca refining treatment and electromagnetic stirring steps. The MnS inclusions in the metallographic structure of the steel are coarse, resulting in obvious anisotropy of the finished steel product and severe central segregation. The expanded hole has crack defects, indicating that the morphology of the inclusions in its metallographic structure affects the stamping forming effect of the product. Its expanded hole forming rate is only 78% qualified.

[0077] The heating process of Comparative Example 3 did not meet the specified parameters of this application. Although its hole expansion rate was only 100% qualified, it resulted in relatively low mechanical strength of its product, including yield strength and tensile strength.

[0078] like Figure 8 As shown, the hole-expansion forming effect of the high-elongation flange steel prepared by the method of preparing high-elongation flange steel with tensile strength ≥780MPa provided in the embodiments of this application (see the hole-expansion forming physical picture represented by 4-1) is significantly better than the hole-expansion forming effect of the flange steel product sample provided in Comparative Example 2 (see the hole-expansion forming physical picture represented by 4-2).

[0079] The above description is merely a specific implementation of this application. Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working process described above can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here. It should be understood that the protection scope of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the protection scope of this application.

Claims

1. A method for preparing high-elongation flanged steel with a tensile strength ≥780MPa, characterized in that, include: Molten iron is smelted in a converter to obtain converter steel; The molten steel from the converter is deoxidized and alloyed at the argon station to obtain pre-alloyed molten steel. The pre-alloyed molten steel is subjected to LF refining treatment to obtain LF refined molten steel; LF refined molten steel is subjected to RH refining treatment to obtain RH refined molten steel; RH refined steel is continuously cast to obtain a continuously cast billet; The continuously cast billet is hot-rolled to obtain hot-rolled steel coils.

2. The method for preparing high-elongation flanged steel with a tensile strength ≥780MPa according to claim 1, characterized in that, The LF refining treatment of the pre-alloyed molten steel includes: adding lime and Al to the pre-alloyed molten steel to form CaS solid at the slag / steel interface, controlling the S content of the molten steel to within 0.003 wt.% and the Ca / S ratio to between 1.0 and 3.0, to obtain LF refined molten steel; Optionally, the RH refining treatment of the LF refined steel includes: Si-Ca treatment of the LF refined steel to deform the sulfides in the steel into spherical shapes.

3. The method for preparing high-elongation flanged steel with a tensile strength ≥780MPa according to claim 1, characterized in that, The main components of the RH refined steel, by mass percentage, are: C: 0.05wt.%~0.07wt.%, Si: 0.05wt.%~0.15wt.%, Mn: 1.5wt.%~1.7wt.%, P: ≤0.013wt.%, S: ≤0.0010wt.%, Nb: 0.05wt.%~0.07wt.%, Mo: 0.10wt.%~0.20wt.%, Ti: ≤0.001wt.%, N: ≤0.003wt.%, Alt: 0.010wt.%~0.030wt.%, O: ≤0.002wt.%, with the balance being Fe and unavoidable impurities.

4. The method for preparing high-elongation flange steel with a tensile strength ≥780MPa according to claim 1, characterized in that, In the step of continuously casting RH refined steel, the thickness of the continuously cast billet is 210mm to 220mm, the electromagnetic stirring adopts an alternating stirring mode, the alternation time is reversed every 20 to 40 seconds, the current is 200A to 300A, the frequency is 5Hz to 10Hz, and the cooling mode is strong cooling. Optionally, the continuously cast billet has a uniform composition and a segregation level ≤ C1.

0.

5. The method for preparing high-elongation flanged steel with a tensile strength ≥780MPa according to claim 1, characterized in that, The hot rolling process of the continuously cast billet includes: The continuously cast billet is heated to obtain a heated billet heated to 1220℃~1280℃; The heated billet is rough rolled to obtain an intermediate billet; The intermediate billet is then finished by precision rolling to obtain precision rolled strip steel; Laminar flow cooling treatment is applied to the finished strip steel to cool it to 820℃~880℃ to obtain cooled finished strip steel. The cooled precision-rolled strip is coiled to obtain hot-rolled steel coils.

6. The method for preparing high-elongation flange steel with a tensile strength ≥780MPa according to claim 5, characterized in that, In the step of laminar flow cooling of the finished strip, the cooling process adopts water cooling and air cooling. The cooling rate of water cooling is 60℃ / s to 100℃ / s, and the air cooling is carried out by air cooling during the transportation of the finished strip, with an air cooling rate of 5℃ / s to 8℃ / s. The final cooling temperature of the finished strip is controlled at 640℃ to 660℃.

7. The method for preparing high-elongation flanged steel with a tensile strength ≥780MPa according to claim 1, characterized in that, In the step of curling the cooled precision-rolled strip, the optimal precipitation temperature for Nb precipitation-strengthened steel is 580℃~620℃.

8. A high-elongation flange steel with a tensile strength ≥780MPa, characterized in that, The high-elongation flange steel with a tensile strength ≥780MPa is prepared by the method according to any one of claims 1-7. The composition of the high-elongation flange steel with a tensile strength ≥780MPa includes, by mass percentage: C: 0.05wt.%~0.07wt.%, Si: 0.05wt.%~0.15wt.%, Mn: 1.5wt.%~1.7wt.%, P: ≤0.013wt.%, S: ≤0.0010wt.%, Nb: 0.05wt.%~0.07wt.%, Mo: 0.10wt.%~0.20wt.%, Ti: ≤0.001wt.%, N: ≤0.003wt.%, Alt: 0.010wt.%~0.030wt.%, O: ≤0.002wt.%, with the balance being Fe and unavoidable impurities.

9. The high-elongation flange steel with a tensile strength ≥780MPa according to claim 8, characterized in that, The hot-rolled steel coil comprises a hot-rolled steel plate with a thickness of 2.0 mm to 6.0 mm, and its mechanical properties reach: Rel ≥ 700 MPa, R m ≥750MPa, A≥14%, expansion rate≥70%, and performance fluctuation of the steel coil head and tail ≤30MPa.

10. The high-elongation flange steel with a tensile strength ≥780MPa according to claim 8, characterized in that, The metallographic structure of the hot-rolled steel coil is a quasi-polygonal ferrite structure and a bainite structure, wherein it also contains dispersed nanoscale second-phase precipitates; the content of the quasi-polygonal ferrite structure is 60% to 70%, and the average grain size of the quasi-polygonal ferrite structure is 3 μm to 6 μm; the content of the bainite structure is 30% to 40%, and the average grain size of the bainite structure is 5 μm to 8 μm; the content of the nanoscale second-phase precipitates is 0.1% to 0.13%, and the average grain size of the nanoscale second-phase precipitates is 8 nm to 12 nm.

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