High ductility FB780 hole expansion steel and critical zone annealing manufacturing method thereof

Abstract

The present application relates to the technical field of steel material and its processing technology, and particularly relates to a high ductility FB780 hole expanding steel and a critical zone annealing manufacturing method thereof. The steel composition comprises 0.06-0.10% C, 0.2-0.5% Si, 1.5-2.0% Mn, 0.3-0.5% Cu, 0.002-0.004% B and the like. The manufacturing process constructs a volume fraction of greater than or equal to 65% ferrite matrix, island-shaped hard phase with a size of less than or equal to 5 microns and dispersed 5-12 nm grade epsilon-Cu nano particles in the structure through two-stage critical zone annealing, partition cooling and overaging treatment. The present application can utilize the synergistic effect of Cu and B and precise phase change regulation, and on the basis of ensuring 780 MPa grade strength, excellent performance of greater than or equal to 18% elongation after fracture and greater than or equal to 140% hole expansion rate is obtained, and the forming cracking problem of automobile chassis parts is solved.

Description

Technical Field

[0001] This invention belongs to the technical field of steel materials and their processing technology, specifically a high-ductility FB780 expanded hole steel and its critical zone annealing manufacturing method. Background Technology

[0002] As the automotive industry moves towards lightweighting, energy conservation, emission reduction, and enhanced safety, the application of high-strength steel in automotive bodies and structural components is increasing. Among them, 780MPa grade ferritic-bainitic (FB) steel is widely used in automotive chassis, suspension systems, and various complex-shaped stamped parts due to its high strength, good flanging performance, and excellent hole-expanding properties.

[0003] In actual production, to obtain the ideal microstructure of FB steel, it is usually necessary to precisely control the chemical composition of the steel and subsequent heat treatment processes. The microstructure of FB steel is mainly composed of soft ferrite and hard bainite, and its properties largely depend on the ratio, size, and uniformity of distribution of the two phases. Especially when performing complex hole-expansion processes, the ductility and hole expansion ratio of the material become key indicators for measuring its forming limit. However, how to further improve the ductility and hole expansion performance of steel while ensuring a strength grade of 780MPa has always been a key focus and challenge in the research and development of high-strength steel.

[0004] Patent CN113564456A discloses a hot-rolling manufacturing method for high-strength steel. This method controls the microstructure of the strip in a laminar flow cooling section, enabling the steel coil to undergo phase transformation before coiling, thereby improving its strength before coiling and reducing the subsequent rolling load. While this technical solution optimizes the production efficiency and raw material strength in the hot rolling stage, its focus is on phase transformation control during the hot rolling process. For FB780 grade expanded-hole steel, which requires cold rolling and annealing processes, relying solely on microstructure control during the hot rolling stage leaves room for optimization in precisely controlling the ferrite grain refinement and bainite phase dispersion of the final product. This, in turn, affects the consistency of the expanded-hole structure under extreme forming conditions.

[0005] Patent WO2024244040A1 discloses a multi-purpose high-strength steel for automobiles and its preparation method. This method proposes using steel plates with the same or similar compositions and different heat treatment processes to achieve coverage of multiple strength grades. While this "one steel, multiple functions" strategy helps simplify production management and reduce costs, when developing for specific grades such as FB780 steel, its universal composition system and heat treatment parameter settings present limitations in balancing extremely high strength and ultra-high porosity. Because the porosity of FB steel is extremely sensitive to the critical annealing temperature and cooling rate, the multi-purpose process design still has room for improvement in achieving high homogenization of the FB780 steel microstructure and suppressing microsegregation, resulting in the final product's ductility and porosity performance failing to meet top-tier performance requirements. Summary of the Invention

[0006] This invention provides a high-ductility FB780 expanded-hole steel and its critical zone annealing manufacturing method. The aim is to construct a high proportion of ferrite matrix and dispersed island-like hard phase in the microstructure of steel by using specific chemical composition ratios and precisely controlled two-stage critical zone annealing and zoned cooling processes. The strengthening effect of nanoscale copper precipitates is utilized to solve the technical problem of existing 780MPa grade high-strength steels that are difficult to balance ultra-high expanded-hole ratio and ductility while ensuring strength.

[0007] In a first aspect, the present invention provides a high-ductility FB780 expanded hole steel, the chemical composition of which, by mass percentage, comprises: carbon C 0.06% to 0.10%, silicon Si 0.2% to 0.5%, manganese Mn 1.5% to 2.0%, aluminum Al 0.02% to 0.06%, phosphorus P ≤ 0.015%, sulfur S ≤ 0.003%, copper Cu 0.3% to 0.5%, boron B 0.002% to 0.004%, with the balance being iron Fe and unavoidable impurities.

[0008] In the above-mentioned component system, the technical means of each element are as follows: Carbon, as the core element for stabilizing austenite and improving strength, has its content controlled in the range of 0.06% to 0.10%. During the critical zone annealing process, carbon is enriched in austenite, which makes the hard phase formed during the subsequent cooling process have sufficient hardness. At the same time, controlling the carbon content at a low level reduces the tendency for large-sized carbides to precipitate.

[0009] When the silicon content is in the range of 0.2% to 0.5%, it produces a solid solution strengthening effect in ferrite and inhibits the nucleation and growth of cementite during the decomposition of austenite. This promotes the high enrichment of carbon in the untransformed austenite, providing a compositional basis for the formation of isolated bainite or martensite.

[0010] The manganese content is in the range of 1.5% to 2.0%. By reducing the critical cooling rate, it increases the stability of supercooled austenite, delays the pearlite transformation in the partitioned cooling stage, and causes the hard phase structure to precipitate in an island-like form at a lower temperature.

[0011] When the copper content is in the range of 0.3% to 0.5%, it precipitates from the ferrite matrix in the form of nano-sized ε to Cu particles during the over-aging treatment stage, resulting in a significant precipitation strengthening effect that compensates for the strength loss caused by the increased proportion of ferrite.

[0012] When the boron content is in the range of 0.002% to 0.004%, it agglomerates at the original austenite grain boundaries, inhibiting the non-uniform nucleation of ferrite at the grain boundaries. In conjunction with the addition of copper, the phase transformation kinetics are adjusted, resulting in the hard phase structure exhibiting an island-like distribution rather than a continuous network distribution.

[0013] The high-ductility FB780 expanded hole steel provided by this invention has the following microstructure characteristics: ferrite forms a continuous matrix with a volume fraction greater than or equal to 65%; the hard phase structure includes martensite and bainite, which are distributed in an island-like pattern on the ferrite matrix, and the average size of the hard phase islands is less than or equal to 5 micrometers. ε-Cu nanoparticles with a size of 5 to 12 nanometers are dispersed within the ferrite matrix.

[0014] According to the present invention, through the above-mentioned composition design and microstructure control, the tensile strength of the steel is greater than or equal to 780 MPa, the elongation after fracture A50 is greater than or equal to 18%, and the porosity λ is greater than or equal to 140%.

[0015] Secondly, the present invention provides a method for manufacturing the above-mentioned high ductility FB780 expanded steel, comprising the following steps: S10: Smelting and casting. Steel is produced by converter steelmaking according to the above component ratio, followed by vacuum degassing and continuous casting into billets.

[0016] S20: Hot rolling treatment. The billet is heated to 1150°C to 1250°C and held for 1 to 2 hours; multiple passes of finishing rolling are performed, with the final rolling temperature controlled at 850°C to 920°C; after finishing rolling, it is cooled to 550°C to 620°C at a cooling rate of 30°C to 60°C per second and then coiled.

[0017] S30: Pickling and cold rolling. Hot-rolled plates are pickled to remove oxide scale, followed by cold rolling. The total reduction rate during cold rolling is controlled between 50% and 70% to obtain cold-rolled hard plates.

[0018] S40: Two-stage critical zone annealing. The cold-rolled hardened strip is fed into a continuous annealing furnace and subjected to two stages of heat treatment. The first stage is a high-temperature heat treatment, with a heating temperature of 790°C to 810°C and a holding time of 100 to 150 seconds; the second stage is a medium-temperature heat treatment, with the strip temperature adjusted to 770°C to 790°C and a holding time of 150 to 200 seconds.

[0019] S50: Zoned Cooling. After annealing, the strip enters the cooling section, which is carried out in two stages. The first stage of cooling rapidly cools the strip from 770 to 790 degrees Celsius to 650 degrees Celsius at a cooling rate of ≥50 degrees Celsius per second; the second stage of cooling slowly cools the strip from 650 degrees Celsius to 450 degrees Celsius at a cooling rate of ≤10 degrees Celsius per second.

[0020] S60: Over-aging treatment. The strip steel is held at a temperature range of 280°C to 320°C for 250 to 350 seconds, and then cooled to room temperature.

[0021] According to the present invention, in step S40, the use of two-stage critical zone annealing is a key technical means to construct a specific microstructure. During the first stage of high-temperature holding at 790°C to 810°C, the cold-rolled microstructure undergoes recovery and recrystallization, while ferrite partially transforms into austenite. Since this temperature is at a relatively high position in the critical zone, a larger proportion of austenite is formed. During the second stage of medium-temperature holding at 770°C to 790°C, as the temperature decreases, some austenite undergoes reverse transformation into ferrite. This process promotes further enrichment of alloying elements such as carbon and manganese in the remaining austenite, improving the thermodynamic stability of the residual austenite and causing the austenite to tend towards islanding in spatial distribution.

[0022] In step S50, the design logic of the zoned cooling lies in precisely controlling the phase transformation path. One stage of cooling employs a high-speed cooling rate of 50 degrees Celsius or higher per second, allowing the strip to quickly pass through the pearlite transformation zone and suppressing the precipitation of strip-shaped or network-shaped carbides. Once the temperature drops to 650 degrees Celsius, a slow cooling rate of 10 degrees Celsius or lower per second is adopted. This stage utilizes the interface movement between austenite and ferrite, causing the carbon-rich austenite to further evolve into an island-like morphology, and subsequently partially transform into bainite during the cooling process, ultimately forming an island-like hard phase structure surrounded by ferrite.

[0023] In step S60, the over-aging treatment temperature is set between 280°C and 320°C. This temperature range avoids the temper brittleness zone of conventional martensitic steel and is within the sensitive zone for the supersaturated precipitation of copper in ferrite. Holding at this temperature for 250 to 350 seconds promotes the segregation and precipitation of copper atoms dissolved in the ferrite matrix, forming ε-Cu nanoparticles with diameters between 5 and 12 nanometers. These nanoparticles maintain a coherent or semi-coherent relationship with the matrix, generating precipitation strengthening through pinning dislocation movement. Since precipitation strengthening mainly occurs in the ferrite phase, this effectively improves the yield strength of the soft phase matrix and reduces the hardness difference between ferrite and the hard phase (martensite / bainite) without significantly reducing the material's plasticity.

[0024] In some embodiments, in step S20, the finishing rolling process employs 7 consecutive passes. The reduction rate of the first 3 passes is greater than 25%, the reduction rate of the last 4 passes gradually decreases, and the reduction rate of the last pass is controlled between 10% and 15%. The large reduction rates in the first few passes allow for sufficient fragmentation and deformation of the austenite grains, increasing the number of phase transformation nucleation sites and refining the hot-rolled microstructure.

[0025] In some embodiments, in step S20, the coiling temperature is controlled between 580 degrees Celsius and 600 degrees Celsius. Coiling within this temperature range ensures that the microstructure of the hot-rolled sheet mainly consists of fine ferrite and pearlite, providing a uniform initial microstructure for subsequent cold rolling and annealing processes.

[0026] In some embodiments, in step S30, the pickling process employs a three-stage hydrochloric acid pickling process, with hydrochloric acid concentrations of 50 g / L to 80 g / L, 100 g / L to 150 g / L, and 180 g / L to 220 g / L, respectively, and the acid solution temperature controlled at 75°C to 85°C. This gradient concentration pickling thoroughly removes the surface oxide layer and avoids hydrogen embrittlement.

[0027] In some embodiments, in step S40, the first-stage annealing temperature is 800 degrees Celsius, and the holding time is 120 seconds; the second-stage annealing temperature is 780 degrees Celsius, and the holding time is 180 seconds. Under this precise combination of parameters, the ratio of ferrite to austenite reaches thermodynamic equilibrium, and the carbon concentration in the austenite reaches a critical value, which is conducive to the formation of islands with a size of less than 5 micrometers.

[0028] In some embodiments, in step S50, the cooling medium for the first stage of cooling is high-pressure water mist or jet cooling, and the second stage of cooling is achieved by adjusting the power of the radiant tubes or the fan speed of the cooling section of the continuous annealing furnace. By adjusting the flow rate and pressure of the cooling medium, the heat transfer coefficient of the strip surface is controlled within a specific range, ensuring that the cooling rate of the core and surface of the steel plate is consistent, thereby reducing residual stress.

[0029] In some embodiments, in step S60, the over-aging temperature is 300 degrees Celsius, and the holding time is 300 seconds. Under these conditions, the precipitation kinetics of ε-Cu nanoparticles are most active, the volume fraction of the precipitated phase is maximized, and the proportion of particles with a size distribution of 8 to 10 nanometers is the highest.

[0030] In some embodiments, after step S60, a leveling process is further included, with the leveling elongation controlled at 0.3% to 0.8%. The leveling process eliminates the yield plateau on the strip surface through slight plastic deformation and improves the strip shape and flatness.

[0031] The high-ductility FB780 expanded hole steel and its manufacturing method provided by this invention represent a significant technological advancement compared to existing technologies. Firstly, by introducing the synergistic effect of copper and boron, and combining it with a bi-critical zone annealing process, this invention achieves an "island-like" distribution of the hard phase microstructure at the microscale. This island-like microstructure (size ≤ 5 micrometers) is uniformly embedded in a ferrite matrix with a volume ratio exceeding 65%, effectively avoiding stress concentration caused by the chain or network distribution of bainite in traditional FB steel. During intensive forming operations such as hole expansion, cracks often preferentially initiate at the interface between the hard and soft phases. The island-like distribution and small size of the hard phase significantly increase the tortuosity of the crack propagation path, thereby increasing the hole expansion rate to an extremely high level of over 140%.

[0032] Secondly, this invention utilizes nanoscale ε-Cu particles precipitated during the over-aging stage to strengthen the ferrite matrix. In traditional high-strength steel, strength improvement often relies on increasing the proportion of hard phases (such as martensite), but this significantly reduces the overall ductility of the material. This invention strengthens the ferrite matrix itself through copper particle precipitation, thereby allowing for a higher proportion of ferrite phase (greater than or equal to 65%) while maintaining a strength level of 780 MPa. This directly increases the elongation after fracture of the material to over 18%.

[0033] Furthermore, the design of the two-stage critical zone annealing process (800 degrees Celsius followed by 780 degrees Celsius) is not only for controlling the phase transformation ratio, but more importantly, it utilizes the difference in the solubility of carbon atoms in austenite at different temperatures through temperature gradient changes to achieve precise control of the internal composition of austenite. The high temperature in the first stage ensures sufficient recrystallization and austenitization of the microstructure, while the cooling in the second stage is a "carbon removal" process, which causes ferrite to precipitate while squeezing carbon towards the center of the residual austenite. This compositional gradient distribution makes the hard phase formed by subsequent cooling hard at the center and has a certain degree of toughness at the edges, further optimizing the bonding force of the soft and hard phase interface.

[0034] Finally, the zoned cooling process precisely controls the transformation kinetics of supercooled austenite by combining rapid cooling (≥50°C / s) and slow cooling (≤10°C / s). The rapid cooling stage effectively avoids diffusion-type phase transformation regions (such as pearlite regions), while the slow cooling stage provides time for the growth of bainitic ferrite and further enrichment of carbon atoms, ensuring that the hard phase exists in an island-like morphology in the final microstructure, rather than forming a coarse lath structure. This microstructure plays a decisive role in improving the flanging performance of the material.

[0035] In summary, the high-ductility FB780 expanded hole steel and its manufacturing method provided by this invention, through the high synergy of composition, microstructure and process, achieves excellent ductility (A50 ≥ 18%) and outstanding expanded hole performance (λ ≥ 140%) while maintaining a high strength of 780MPa. This solves the cracking problem of automotive chassis parts and structural parts in complex stamping processes, and has extremely high practical value and promising industrial application prospects.

[0036] The manufacturing method of high-ductility FB780 expanded hole steel provided by the second aspect of this invention comprises steps S10 to S60, which constitute a complete and interconnected technical system. From the initial composition smelting, the addition of copper and boron lays the groundwork for subsequent microstructure evolution; temperature and cooling control during hot rolling establishes the foundation for a refined initial microstructure; the high dislocation density provided by the cold rolling process drives recrystallization and austenite nucleation during annealing; and the core double-stage annealing, zoned cooling, and over-aging treatment, through precise intervention in thermodynamic equilibrium and kinetic paths, ultimately construct a high-performance microscopic "composite structure" within the steel.

[0037] In actual production, this method exhibits good tolerance for fluctuations in process parameters. For example, the temperature window for two-stage annealing is set within a range of ±10 degrees Celsius, which can be stably achieved in modern advanced continuous annealing units. The rapid cooling stage of the partitioned cooling requires a temperature of at least 50 degrees Celsius per second, which can be achieved by adjusting the nozzle pressure and cooling medium ratio; the slow cooling stage is maintained through residual heat balancing in the furnace heating section or low-power radiant heating. The 280-320 degree Celsius range after aging treatment, located at the furnace outlet, is easily controlled through heat balance. These specific operational measures ensure the feasibility of the technical solution described in this invention and the consistency of product quality.

[0038] Furthermore, the precipitation of ε-Cu nanoparticles (size 5 to 12 nanometers) described in this invention is achieved through isothermal holding in the over-aging section. This process does not require additional complex equipment; only minor adjustments to the temperature parameters are needed in the over-aging section of the existing continuous annealing production line. This precipitation strengthening method, which combines microalloying elements with conventional heat treatment processes, offers a better cost-effectiveness ratio compared to adding expensive strong carbide-forming elements such as niobium, vanadium, and titanium. It also requires a lower maximum heating temperature in the continuous annealing furnace, which helps extend equipment life and reduce production energy consumption.

[0039] In terms of microstructure characterization, scanning electron microscopy (SEM) combined with electron backscatter diffraction (EBSD) clearly revealed island-like hard phases distributed within the ferrite matrix. Due to the use of two-stage annealing, the orientation relationship between the hard phase edges and the ferrite matrix was smoother, and the density of geometrically necessary dislocations (GNDs) at the interface was lower. This microscopically explains why this steel exhibited extremely high edge crack resistance in the hole expansion test. Transmission electron microscopy (TEM) revealed diffusely distributed copper nanoparticles within the ferrite. These particles were nearly spherical, uniformly distributed within the grains, and provided stable strengthening contributions.

[0040] The FB780 steel involved in this invention has a hole expansion ratio λ greater than or equal to 140%, a figure far exceeding the average level of commercially available FB steel of the same grade (typically between 80% and 100%). This means that when producing parts such as automotive chassis reinforcements and control arms, smaller pre-punched hole diameters or greater flanging deformation can be used without worrying about edge cracking. This not only increases the design freedom of parts but also reduces the overall manufacturing cost by decreasing the scrap rate.

[0041] The high-ductility FB780 expanded hole steel provided in the first aspect is prepared according to the method described in the second aspect. Since the steel strictly follows the above-mentioned composition design and process path, the ratio of ferrite to hard phase, the morphological distribution of hard phase, and the state of nano-precipitated phase in its internal structure all reach the preset optimal scheme. Therefore, the steel possesses all the aforementioned mechanical property advantages and processing and forming advantages.

[0042] In some embodiments, in step S10, the vacuum degassing process lasts for at least 15 minutes, and the vacuum level is controlled below 67 Pa. Through deep degassing, the content of gaseous impurities such as oxygen, nitrogen, and hydrogen in the steel is reduced to extremely low levels, thereby reducing the formation of non-metallic inclusions.

[0043] In some embodiments, in step S10, the casting speed of the continuously cast billet is controlled between 0.8 m / min and 1.2 m / min, and end-effector electromagnetic stirring technology is employed. By controlling the casting speed and electromagnetic stirring, center segregation of the continuously cast billet is reduced, ensuring the uniform distribution of elements such as copper and manganese in the thickness direction of the billet, thereby guaranteeing the isotropic properties of the final cold-rolled sheet.

[0044] In some embodiments, in step S20, the heating furnace employs a five-stage continuous heating system, with the following temperatures for each stage: preheating stage (800°C to 900°C), first heating stage (1050°C to 1150°C), second heating stage (1200°C to 1250°C), and soaking stage (1180°C to 1220°C). This stepped heating process allows for the full release of internal stress in the cast billet and ensures complete solid solution of the alloying elements.

[0045] In some embodiments, in step S30, the cold rolling mill adopts a five-stand, six-roll cold continuous rolling mill, with the reduction rate distribution for each stand as follows: 25% to 30% for the first stand, 20% to 25% for the second stand, 15% to 20% for the third stand, 10% to 15% for the fourth stand, and 2% to 5% for the fifth stand. By scientifically distributing the load of each stand, excellent surface quality and dimensional accuracy are achieved while ensuring the total reduction rate.

[0046] In some embodiments, in step S40, the strip steel undergoes alkaline degreasing, electrolytic cleaning, and three-stage rinsing before entering the annealing furnace. The alkaline solution concentration in the cleaning section is 1% to 3%, and the temperature is 70 to 80 degrees Celsius. This thoroughly removes residual oil and iron powder from the surface, preventing surface color differences or carburization during the high-temperature annealing process.

[0047] In some embodiments, during step S50, the strip tension is controlled between 10 MPa and 20 MPa during the zoned cooling process. Constant tension control prevents the strip from warping or becoming wavy during rapid cooling, ensuring the flatness of the strip shape.

[0048] The high-ductility FB780 expanded hole steel and its manufacturing method described in this invention construct a novel paradigm for high-performance steel manufacturing through meticulous management of the entire process, including composition design, hot rolling control, cold rolling, two-stage annealing, zoned cooling, and over-aging strengthening. Its core lies in breaking through the limitations of traditional FB steel microstructure control by utilizing the deep coupling of copper atom precipitation kinetics and critical region phase transformation kinetics to achieve dual optimization of microstructure in both spatial distribution and phase composition. This technique not only raises the performance ceiling of the product but also provides new technical insights for the development of high-strength automotive steel. Detailed Implementation

[0049] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0050] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with an embodiment or example that are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0051] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0052] As mentioned in the background section above, the automotive industry has an increasing demand for high-strength steel, especially in chassis and structural components. This requires materials with tensile strengths of 780 MPa and extremely high expansion rates to handle complex flanging processes. While traditional ferritic bainitic steel (FB steel) ensures a certain degree of ductility through its ferrite matrix, uneven distribution or excessively large size of the hard phase makes it prone to crack initiation at the interface between the soft and hard phases during the expansion process, making it difficult to exceed 120% expansion rate.

[0053] Based on this, the present invention provides a high-ductility FB780 expanded hole steel and its critical zone annealing manufacturing method. The aim is to construct a high proportion of ferrite matrix and dispersed island-like hard phase in the microstructure of steel by using specific chemical composition ratios and precisely controlled two-stage critical zone annealing and partitioned cooling processes, and to adjust the micromechanical response of the material by utilizing the strengthening effect of nanoscale copper precipitates.

[0054] In a first aspect, the present invention provides a high-ductility FB780 expanded hole steel, the chemical composition of which, by mass percentage, comprises: carbon C 0.06% to 0.10%, silicon Si 0.2% to 0.5%, manganese Mn 1.5% to 2.0%, aluminum Al 0.02% to 0.06%, phosphorus P ≤ 0.015%, sulfur S ≤ 0.003%, copper Cu 0.3% to 0.5%, boron B 0.002% to 0.004%, with the balance being iron Fe and unavoidable impurities.

[0055] In the composition system of this invention, the content control of each element follows a clear metallurgical logic. The carbon content is controlled between 0.06% and 0.10%, ensuring sufficient carbon enrichment in austenite during critical-zone annealing, resulting in a predetermined hardness value for the final quenched martensite or bainite hard phase. The silicon content is between 0.2% and 0.5%, inhibiting carbide precipitation during ferrite transformation and promoting carbon transfer into austenite. The manganese content is between 1.5% and 2.0%, primarily improving the stability of supercooled austenite to avoid the pearlite transformation zone during subsequent zone cooling. The copper content is between 0.3% and 0.5%, precipitating as nanoscale ε-Cu particles during subsequent over-aging treatment. The boron content is between 0.002% and 0.004%, segregating at grain boundaries and affecting phase transformation kinetics.

[0056] The high-ductility FB780 expanded hole steel provided by this invention has a microstructure consisting of a ferrite matrix with a volume fraction greater than or equal to 65%, and island-like hard phases dispersed on the matrix. The hard phases include martensite and bainite, and the average size of the hard phase islands is less than or equal to 5 micrometers. Within the ferrite matrix, ε-Cu nanoparticles with a particle size of 5 to 12 nanometers are distributed.

[0057] Secondly, the present invention provides a method for manufacturing the above-mentioned high ductility FB780 expanded steel, comprising the following steps: S10: Smelting and Casting. Steel is produced in a converter according to the above-mentioned composition ratio, followed by vacuum degassing treatment. The vacuum level is controlled below 67 Pa, and the treatment time is no less than 15 minutes. Finally, the steel is continuously cast into billets. During continuous casting, the casting speed is controlled between 0.8 m / min and 1.2 m / min, and the end electromagnetic stirring is activated.

[0058] S20: Hot rolling treatment. The billet is fed into a heating furnace and continuously heated in five stages. The preheating stage temperature is 800°C to 900°C, the first heating stage is 1050°C to 1150°C, the second heating stage is 1200°C to 1250°C, and the soaking stage is 1180°C to 1220°C. The total holding time is 1 to 2 hours. This is followed by 7 passes of finish rolling. The reduction rate in the first 3 passes is greater than 25%, the reduction rate in the next 4 passes gradually decreases, and the reduction rate in the last pass is 10% to 15%. The final rolling temperature is controlled at 850°C to 920°C. After finish rolling, the billet is cooled to 550°C to 620°C at a cooling rate of 30°C to 60°C per second before coiling.

[0059] S30: Pickling and Cold Rolling. A three-stage hydrochloric acid pickling process is used to remove oxide scale from the surface of the hot-rolled plate. The hydrochloric acid concentrations are 50 g / L to 80 g / L, 100 g / L to 150 g / L, and 180 g / L to 220 g / L, respectively, and the acid temperature is 75°C to 85°C. Cold rolling is then performed, with the total reduction rate controlled at 50% to 70%. A five-stand, six-roll continuous cold rolling mill is used, with the reduction rates distributed as follows: first stand 25% to 30%, second stand 20% to 25%, third stand 15% to 20%, fourth stand 10% to 15%, and fifth stand 2% to 5%.

[0060] S40: Two-stage critical zone annealing. After being degreased by alkaline washing (alkali concentration 1% to 3%, temperature 70°C to 80°C), the cold-rolled sheet enters a continuous annealing furnace. The first stage involves heating to 790°C to 810°C and holding for 100 to 150 seconds; the second stage involves adjusting the strip temperature to 770°C to 790°C and holding for 150 to 200 seconds.

[0061] S50: Sectional cooling. The first stage of cooling rapidly cools the strip from 770-790 degrees Celsius to 650 degrees Celsius at a rate greater than or equal to 50 degrees Celsius per second. The second stage of cooling slowly cools the strip from 650 degrees Celsius to 450 degrees Celsius at a rate less than or equal to 10 degrees Celsius per second. During the cooling process, the strip tension is maintained between 10 and 20 MPa.

[0062] S60: Over-aging treatment. The strip steel is held at a temperature between 280°C and 320°C for 250 to 350 seconds, and then cooled to room temperature. Finally, a leveling process is performed, with the leveling elongation controlled at 0.3% to 0.8%.

[0063] In step S40, the design of the two-stage critical zone annealing is based on the difference in phase transformation thermodynamics. The high temperature in the first stage (e.g., 800 degrees Celsius) allows the microstructure to recrystallize sufficiently and form a certain proportion of austenite; the cooling in the second stage (e.g., 780 degrees Celsius) induces a partial reverse transformation of austenite into ferrite. This process is accompanied by the displacement of carbon atoms into the remaining austenite core, which increases the stability of austenite and makes it tend to be isolated in space.

[0064] In step S50, the first stage of rapid cooling avoids the pearlite transformation zone and prevents the network precipitation of carbides; the second stage of slow cooling provides sufficient time for the carbon-rich austenite to be further refined and islanded during the interface movement.

[0065] In step S60, the over-aging temperature range of 280°C to 320°C is a sensitive region for the supersaturation precipitation of copper atoms in ferrite. At this temperature, copper atoms agglomerate and form ε-Cu nanoparticles. These particles are dispersed in the ferrite matrix and enhance the yield strength of ferrite through a dislocation pinning mechanism, thereby reducing the hardness difference between the soft phase matrix and the hard phase islands.

[0066] The technical solution of the present invention will be further described in detail below with reference to specific embodiments.

[0067] Example 1 The high-ductility FB780 expanded hole steel of this embodiment has the following chemical composition by mass percentage: C: 0.08%, Si: 0.35%, Mn: 1.8%, Al: 0.04%, P: 0.010%, S: 0.002%, Cu: 0.4%, B: 0.003%, with the balance being Fe and unavoidable impurities.

[0068] Its manufacturing methods include: S10: After converter steelmaking, vacuum degassing is performed at a vacuum degree of 60 Pa for 20 minutes, followed by continuous casting into billets at a casting speed of 1.0 m / min.

[0069] S20: The billet is heated to 1200 degrees Celsius and held for 1.5 hours. The finishing rolling temperature is 900 degrees Celsius, the cooling rate is 45 degrees Celsius per second, and the coiling temperature is 590 degrees Celsius.

[0070] S30: Cold rolled after pickling, with a total reduction rate of 60%.

[0071] S40: Two-stage annealing, first stage at 800 degrees Celsius for 120 seconds, second stage at 780 degrees Celsius for 180 seconds.

[0072] S50: Zoned cooling, rapidly cooling from 780 degrees Celsius to 650 degrees Celsius (cooling rate 55 degrees Celsius per second), then slowly cooling to 450 degrees Celsius (cooling rate 8 degrees Celsius per second).

[0073] S60: Over-aging treatment, temperature 300 degrees Celsius, heat treatment for 300 seconds. Elongation at break: 0.5%.

[0074] Example 2 The high-ductility FB780 expanded hole steel of this embodiment has the following chemical composition by mass percentage: C: 0.06%, Si: 0.5%, Mn: 2.0%, Al: 0.06%, P: 0.012%, S: 0.002%, Cu: 0.3%, B: 0.004%, with the balance being Fe and unavoidable impurities.

[0075] Its manufacturing methods include: S10: Vacuum degassing vacuum degree 50 Pa, treatment time 18 minutes, continuous casting speed 0.8 m / min.

[0076] S20: The billet is heated to 1150 degrees Celsius and held for 2 hours. The finishing rolling temperature is 860 degrees Celsius, the cooling rate is 35 degrees Celsius per second, and the coiling temperature is 560 degrees Celsius.

[0077] S30: Total cold rolling reduction rate of 50%.

[0078] S40: Two-stage annealing, first stage at 790 degrees Celsius for 150 seconds, second stage at 770 degrees Celsius for 200 seconds.

[0079] S50: Zoned cooling, rapidly cooling from 770 degrees Celsius to 650 degrees Celsius (cooling rate 50 degrees Celsius per second), then slowly cooling to 450 degrees Celsius (cooling rate 10 degrees Celsius per second).

[0080] S60: Over-aging treatment, temperature 280 degrees Celsius, heat treatment for 350 seconds. Elongation at break: 0.3%.

[0081] Example 3 The high-ductility FB780 expanded hole steel of this embodiment has the following chemical composition by mass percentage: C: 0.10%, Si: 0.2%, Mn: 1.5%, Al: 0.02%, P: 0.015%, S: 0.003%, Cu: 0.5%, B: 0.002%, with the balance being Fe and unavoidable impurities.

[0082] Its manufacturing methods include: S10: Vacuum degassing vacuum degree 65 Pa, treatment time 25 minutes, continuous casting speed 1.2 m / min.

[0083] S20: The billet is heated to 1250 degrees Celsius and held for 1 hour. The finishing rolling temperature is 920 degrees Celsius, the cooling rate is 60 degrees Celsius per second, and the coiling temperature is 610 degrees Celsius.

[0084] S30: Cold rolling total reduction rate of 70%.

[0085] S40: Two-stage annealing, first stage at 810 degrees Celsius for 100 seconds, second stage at 790 degrees Celsius for 150 seconds.

[0086] S50: Zoned cooling, rapidly cooling from 790 degrees Celsius to 650 degrees Celsius (cooling rate 60 degrees Celsius per second), then slowly cooling to 450 degrees Celsius (cooling rate 5 degrees Celsius per second).

[0087] S60: Over-aging treatment, temperature 320 degrees Celsius, heat treatment for 250 seconds. Elongation at break: 0.8%.

[0088] Example 4 The composition and process parameters of this embodiment are slightly adjusted based on Example 1: Cu content is 0.45% and B content is 0.0035%. The first stage of the two-stage annealing is at 805 degrees Celsius, and the second stage is at 785 degrees Celsius. The over-aging temperature is 310 degrees Celsius, and the holding time is 320 seconds. The remaining steps are the same as in Example 1.

[0089] Comparative Example 1 The chemical composition is the same as in Example 1, but copper (Cu) and boron (B) are not added; that is, Cu is less than 0.01% and B is less than 0.0005%. In the manufacturing process, S40 uses single-stage annealing, with the temperature held at 800 degrees Celsius for 300 seconds, and S50 uses conventional continuous cooling (cooling directly to 450 degrees Celsius at a cooling rate of 20 degrees Celsius per second). The remaining steps are the same as in Example 1.

[0090] Comparative Example 2 The chemical composition is the same as in Example 1. In the manufacturing process, S40 uses single-stage annealing (holding at 800 degrees Celsius for 300 seconds), and the partitioned cooling of S50 is changed to direct rapid cooling to room temperature, without the over-aging treatment of S60.

[0091] Comparative Example 3 The chemical composition is the same as in Example 1. In the manufacturing process, the two-stage annealing of S40 is changed to: a first stage at 750 degrees Celsius, and a second stage at 800 degrees Celsius (i.e., lower first, then higher). The remaining steps are the same as in Example 1.

[0092] Mechanical properties of the steels prepared in Examples 1 to 4 and Comparative Examples 1 to 3 were tested. Test items included tensile strength (Rm), elongation after fracture (A50), and porosity (λ). The microstructure was observed using metallographic microscopy and scanning electron microscopy, and the ferrite volume fraction and hard phase island size were statistically analyzed. The precipitation of ε-Cu nanoparticles was observed using transmission electron microscopy. The test results are shown in Table 1.

[0093] Table 1: Data on the microstructure and mechanical properties of the examples and comparative examples

[0094] Based on the data analysis in Table 1, we can conclude that: Examples 1 to 4 all achieved the targets of tensile strength greater than or equal to 780 MPa, elongation after fracture (A50) greater than or equal to 18%, and porosity (λ) greater than or equal to 140%. Examples 1 and 2, through precise annealing temperature control, obtained a ferrite matrix exceeding 70%, with the size of hard phase islands controlled at approximately 3 micrometers. Combined with reinforcement from fine ε-Cu nanoparticles, excellent porosity was achieved while maintaining high strength.

[0095] Due to the lack of Cu and B elements and the absence of two-stage annealing and partitioned cooling, the hard phase structure of Comparative Example 1 exhibits a distinct banded distribution with large dimensions, resulting in severe stress concentration during the hole expansion process. The hole expansion rate is only 75%, and the strength does not reach 780 MPa.

[0096] Although the composition of Comparative Example 2 is the same as that of Example 1, the copper element failed to precipitate in the form of nanophase because no aging treatment was performed. In addition, the proportion of ferrite was low due to direct rapid cooling. Although the material hardness was high, the ductility and porosity were greatly reduced.

[0097] Comparative Example 3 adopted a low-to-high annealing sequence, which is not conducive to the enrichment and islanding of carbon atoms in austenite. The resulting hard phase is larger and unevenly distributed. At the same time, due to the annealing logic error, the copper particles precipitated after over-aging were coarsened (from 15 nm to 25 nm), the strengthening effect was weakened, and the overall performance was poor.

[0098] Further observation of the sample from Example 1 using transmission electron microscopy (TEM) revealed a large number of nearly spherical ε-Cu nanoparticles dispersed within the ferrite matrix, maintaining a coherent relationship with the matrix. At the interface between the hard phase and ferrite, due to the carbon enrichment effect of the two-stage annealing, the interface transition was relatively smooth, and no obvious accumulation of brittle carbides was observed. This microstructural explanation explains the ultra-high porosity of the steel of this invention.

[0099] In step S10, the deep vacuum degassing treatment (less than 67 Pa) ensures the purity of the steel and reduces sulfide and oxide inclusions. These inclusions often act as crack initiation points during the pore-expanding process. By reducing the impurity content and further combining with the island-like structure, the edge forming performance of the material is improved.

[0100] In the S20 hot rolling process, the large reduction rate (greater than 25%) in the first few passes of finishing rolling causes severe deformation and fragmentation of austenite grains, significantly increasing the density of nucleation sites for deformation-induced precipitation and subsequent phase transformation. This results in a very fine and uniform microstructure before final cold rolling, laying the microstructural foundation for the formation of fine hard phase islands during the subsequent two-stage annealing process.

[0101] In the S30 cold rolling step, a total reduction of 50% to 70% provides sufficient deformation energy. In the first stage of the S40 two-stage annealing, this energy induces rapid recrystallization. Subsequently, during the cooling process in the second stage, as the temperature is in the lower-middle part of the critical region, newly formed ferrite grows inward from the original austenite grain boundaries, pushing carbon atoms towards the austenite center, thus naturally forming island-like austenite surrounded by ferrite.

[0102] In the S50's zone cooling process, the two-stage slow cooling (at least 10 degrees Celsius per second) to 450 degrees Celsius is the critical period for bainitic transformation. During this stage, the carbon-rich austenitic islands transform into bainite, while the remaining portion transforms into martensite during subsequent cooling. These mixed hard-phase islands, composed of bainite and martensite, exhibit better internal toughness compared to single-phase martensite, slowing the rate at which cracks penetrate the hard phase.

[0103] During the overaging stage of S60, a temperature of around 300 degrees Celsius ensures both the diffusion driving force of copper atoms and avoids coarsening of ferrite grains. Holding this temperature for about 300 seconds, the precipitation kinetics curve of copper atoms is in the peak region, and the resulting nanoparticles are 8 to 10 nanometers in size, providing a yield strength increment of approximately 100 to 150 MPa.

[0104] In summary, the high-ductility FB780 expanded hole steel provided by this invention, through Cu and B alloying design combined with dual-critical-zone annealing, zoned cooling, and over-aging strengthening processes, successfully resolves the contradiction between the strength, expanded hole rate, and ductility of high-strength steel. This steel has significant application advantages in the manufacturing of lightweight automotive chassis parts and can meet the integrated stamping forming requirements of complex-shaped parts.

[0105] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-ductility FB780 expanded hole steel, characterized in that, Its chemical composition by mass percentage includes: carbon C 0.06% to 0.10%, silicon Si 0.2% to 0.5%, manganese Mn 1.5% to 2.0%, aluminum Al 0.02% to 0.06%, phosphorus P ≤ 0.015%, sulfur S ≤ 0.003%, copper Cu 0.3% to 0.5%, boron B 0.002% to 0.004%, with the balance being iron Fe and unavoidable impurities; Its microstructure includes a ferrite matrix and a hard phase structure distributed on the ferrite matrix. The volume fraction of the ferrite matrix is ​​greater than or equal to 65%. The hard phase structure includes martensite and bainite. The hard phase structure is distributed in an island-like pattern, and the average size of the hard phase islands is less than or equal to 5 micrometers. ε to Cu nanoparticles with a size of 5 to 12 nanometers are dispersed in the ferrite matrix.

2. A method for manufacturing high-ductility FB780 expanded steel as described in claim 1, characterized in that, Includes the following steps: S10: Smelting and casting, steelmaking in a converter according to the stated composition ratio, vacuum degassing treatment, and continuous casting into billets; S20: Hot rolling treatment, heating the billet to 1150°C to 1250°C and holding it for 1 to 2 hours; performing multi-pass finishing rolling, with the final rolling temperature controlled at 850°C to 920°C; after finishing rolling, cooling to 550°C to 620°C at a cooling rate of 30°C to 60°C per second and then coiling. S30: Pickling and cold rolling. Hot-rolled plates are pickled and then cold-rolled. The total reduction rate of cold rolling is controlled at 50% to 70% to obtain cold-rolled hard plates. S40: Two-stage critical zone annealing, which involves heat treatment of cold-rolled hardened sheet in two stages. The first stage heating temperature is 790°C to 810°C, and the holding time is 100 to 150 seconds. The second stage adjusts the strip temperature to 770°C to 790°C, and the holding time is 150 to 200 seconds. S50: Zoned cooling. The first stage of cooling cools the strip from 770 to 790 degrees Celsius to 650 degrees Celsius at a cooling rate of 50 degrees Celsius per second or higher. The second stage of cooling cools the strip from 650 degrees Celsius to 450 degrees Celsius at a cooling rate of 10 degrees Celsius per second or lower. S60: Over-aging treatment, the strip steel is held at a temperature range of 280 degrees Celsius to 320 degrees Celsius for 250 to 350 seconds, and then cooled to room temperature.

3. The method for manufacturing high-ductility FB780 expanded hole steel according to claim 2, characterized in that, In step S20, the finishing rolling process adopts 7-pass continuous rolling, the reduction rate of the first 3 passes is greater than 25%, and the reduction rate of the last pass is controlled between 10% and 15%.

4. The method for manufacturing high-ductility FB780 expanded hole steel according to claim 2, characterized in that, In step S20, the winding temperature is controlled between 580 degrees Celsius and 600 degrees Celsius.

5. The method for manufacturing high-ductility FB780 expanded hole steel according to claim 2, characterized in that, In step S30, the pickling adopts a three-stage hydrochloric acid pickling process, with the hydrochloric acid concentrations of the three stages being 50 g / L to 80 g / L, 100 g / L to 150 g / L, and 180 g / L to 220 g / L, respectively, and the acid temperature is controlled at 75 degrees Celsius to 85 degrees Celsius.

6. The method for manufacturing high-ductility FB780 expanded hole steel according to claim 2, characterized in that, In step S40, the first stage annealing temperature is 800 degrees Celsius and the holding time is 120 seconds; the second stage annealing temperature is 780 degrees Celsius and the holding time is 180 seconds.

7. The method for manufacturing high-ductility FB780 expanded hole steel according to claim 2, characterized in that, In step S60, the over-aging temperature is 300 degrees Celsius, and the holding time is 300 seconds.

8. The method for manufacturing high-ductility FB780 expanded hole steel according to claim 2, characterized in that, After step S60, a leveling process is also included, with the leveling elongation controlled at 0.3% to 0.8%.

9. The method for manufacturing high-ductility FB780 expanded hole steel according to claim 2, characterized in that, In step S10, the vacuum degassing treatment time is not less than 15 minutes, and the vacuum degree is controlled below 67 Pa; the casting speed of the continuous casting billet is controlled between 0.8 m / min and 1.2 m / min.

10. The method for manufacturing high-ductility FB780 expanded hole steel according to claim 2, characterized in that, In step S50, the strip tension during the zoned cooling process is controlled between 10 MPa and 20 MPa.

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