A method for improving performance stability of ultra-high strength multiphase steel by controlling hot rolling initial microstructure features

Abstract

The present application relates to the technical field of ultrahigh-strength multiphase steel production, and particularly relates to a method for realizing performance stability improvement of ultrahigh-strength multiphase steel by controlling hot-rolled initial organization characteristics, which comprises a hot-rolled initial organization characteristic regulation step, an unrecrystallization zone annealing + pickling step and a continuous annealing step.The method provided by the present application is beneficial to improving the performance stability of ultrahigh-strength multiphase steel, and the prepared ultrahigh-strength multiphase steel has high performance stability, low cost, good hole expansion and other advantages, the standard deviation of the yield strength ratio of which is ≤±0.025, and is stably controlled within 0.625~0.650; the yield strength is stably controlled within 860±20MPa; the tensile strength is stably controlled within 1360±30MPa; the elongation after fracture is >15.0%, and is stably controlled within 17.5±2.5%; the hole expansion ratio λ is >40%, and is stably controlled within 45±5%; and the product of strength and plasticity is >20.5GPa·%, and is stably controlled within 23±3GPa·%.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength multiphase steel production technology, specifically to a method for improving the performance stability of ultra-high strength multiphase steel by controlling the initial microstructure characteristics of hot-rolled steel. Background Technology

[0002] Given the unique safety and lightweight requirements of new energy vehicles, reducing vehicle weight and improving collision safety have become key focuses for automakers. Therefore, ultra-high strength steel has enormous potential for application in new energy vehicles. Among them, ultra-high strength multiphase steel, with its unique microstructure and small strength differences between phases, not only possesses excellent cold forming properties but also superior hole expansion and flanging performance, enabling the manufacture of various complex parts. Simultaneously, multiphase steel has a high yield strength ratio, exhibiting higher deformation resistance compared to high-strength materials of the same strength level. In the event of a collision, it can absorb more energy, further enhancing vehicle safety. It is particularly suitable for manufacturing safety components and is therefore widely used in chassis suspension components, B-pillars, bumpers, and other parts of new energy vehicles, attracting significant attention from industry researchers.

[0003] The multiphase, multi-size microstructure design of ultra-high strength multiphase steel achieves a reasonable balance between material strength and plasticity, demonstrating great competitiveness and potential in the design and application of lightweight materials for new energy vehicles. However, it also faces numerous challenges in terms of composition improvement, process optimization, and microstructure control, mainly manifested in:

[0004] 1. Poor Performance Stability: Increasing the application ratio of ultra-high strength steel in automotive body-in-white and structural components to further achieve vehicle lightweighting and improve vehicle safety, while reducing fuel consumption and emissions, is a widely accepted consensus in the industry. However, for ultra-high strength multiphase steel, especially ultra-high strength multiphase steel above 1300MPa, automakers, in order to ensure vehicle safety and the stability of mass production of parts, place higher demands on steel companies regarding performance stability. While increasing strength, ultra-high strength multiphase steel products inevitably lose some plasticity, making it difficult to fully meet the dual requirements of improving strength and formability in automotive steel.

[0005] 2. High dependence on alloy design and process: Ultra-high strength multiphase steel has stringent requirements on alloy composition and process window, and cannot be well adapted to existing "long process" industrial production equipment, which seriously restricts its industrial application and promotion.

[0006] Current research on 1300MPa-grade ultra-high strength multiphase steel mainly focuses on laboratory studies. Many papers focus on the effects of process regimes and alloying elements on microstructure and properties, such as "Study on Microstructure Control and Deformation Mechanism of 1300MPa-grade Quenched Partition Steel" and "Microstructure and Properties of 1300MPa-grade Nb Microalloyed DH Steel". However, based on currently available patents, no production method, processing method, or manufacturing / preparation method has been provided to improve the performance stability of 1300MPa-grade ultra-high strength multiphase steel. Summary of the Invention

[0007] To address the technical problem of the lack of a method for improving the performance stability of 1300MPa-level ultra-high strength multiphase steel in existing technologies, this invention provides a method for improving the performance stability of ultra-high strength multiphase steel by controlling the initial microstructure characteristics of hot-rolled steel.

[0008] The technical solution of this invention is as follows:

[0009] A method for improving the performance stability of ultra-high strength multiphase steel by controlling the initial microstructure characteristics of hot-rolled steel includes a hot-rolled initial microstructure characteristic control step, an annealing + pickling rolling step in the non-recrystallization zone, and a continuous annealing step.

[0010] The initial microstructure characteristics control steps for hot rolling are as follows: control the slab homogenization temperature to 1220~1240℃, and ensure that the total furnace time is 210min ≤ total time in the furnace ≤ 240min. A r3 -10℃ < Finishing rolling temperature < A r3 +15℃, after finishing rolling, a pre-roll laminar flow cooling process is adopted to cool the strip to the coiling temperature at a cooling rate of ≥30℃ before coiling. B s +5℃ < Middle winding temperature < B s +15℃, followed by 48h slow cooling measures, to regulate the initial microstructure of hot rolling to acicular ferrite with a volume fraction ≥90%, and satisfy 3 < aspect ratio <9;

[0011] The non-recrystallization zone annealing + pickling process involves annealing the hot-rolled coil in a bell-type annealing furnace. A c1 -30℃ < Annealing homogenization temperature ≤ A c1 -10℃, heat treatment time is 6~8h; pickled and annealed steel coils are then cold rolled to the target thickness with a cumulative reduction of ≥50%;

[0012] The continuous annealing process is as follows: annealing homogenization temperature > A c3After the soaking zone, the strip steel is cooled to 380±10℃ at a cooling rate of >40℃ / s and subjected to over-aging treatment; the final cooling temperature is ≤170℃.

[0013] In the above steps, A r3 It refers to the temperature at which the ferrite phase transformation begins. B s It refers to the temperature at which the bainite phase transformation begins. A c1 It refers to the temperature at which the material begins to transform into austenite upon heating. A c3 It refers to the final temperature at which the material transforms into austenite upon heating.

[0014] Furthermore, A r3 , B s , A c1 , A c3 The calculations were performed using material property simulation software.

[0015] Furthermore, in the initial microstructure control step of hot rolling, the laminar flow cooling process in the front section after rolling adopts a "U-shaped cooling" method, with 80m each for the hot head and hot tail, ensuring the temperature in the middle while ensuring that the temperature at the head and tail is ±15℃ of the middle coiling temperature.

[0016] Furthermore, in the non-recrystallization zone annealing + pickling step, the target thickness for cold rolling is 1.0~2.5mm.

[0017] Furthermore, taking into account the requirements for environmental protection and low-carbon energy conservation, in the continuous annealing step, A c3 <annealing uniform heating temperature≤ A c3 +20℃.

[0018] Furthermore, in the continuous annealing step, the strip running speed is controlled according to the thickness specification, with the strip speed being 110±5m / min for 1.0~1.5mm thick strip, 95±5m / min for 1.6~1.9mm thick strip, and 80±5m / min for 2.0~2.5mm thick strip.

[0019] Furthermore, the continuous annealing step also includes leveling the strip, with the actual rolling force controlled between 7000 and 7300 kN during the leveling process.

[0020] Furthermore, the ultra-high strength multiphase steel adopts an economical and reduced composition design, with C-Mn-Si as the basic components. Its chemical composition by mass percentage is: C: 0.12%~0.18%, Mn: 2.00%~2.60%, Si: 0.50%~0.90%, Alt: 0.40%~0.80%, Cr: 0.20%~0.60%, Mo≤0.30%, Nb≤0.050%, Ti≤0.040%, V≤0.20%, B≤0.0030%, N≤0.030%, with the balance being Fe and other unavoidable impurities.

[0021] Furthermore, Cr / Mn≤0.3, 1.2%≤Si+Alt≤1.5%, Nb+Ti≤0.050%, Mo+V≤0.30%, and control the impurities to P≤0.005%, S≤0.005%, and O≤0.005%.

[0022] The beneficial effects of this invention are as follows:

[0023] The method provided by this invention is beneficial for improving the performance stability of ultra-high strength multiphase steel. The ultra-high strength multiphase steel prepared has high performance stability and advantages such as low cost and good hole expansion properties. The standard deviation of the yield strength ratio of the ultra-high strength multiphase steel is ≤±0.025 and is stably controlled within 0.625~0.650; the yield strength is stably controlled within 860±20MPa; the tensile strength is stably controlled within 1360±30MPa; the elongation after fracture is >15.0% and is stably controlled within 17.5±2.5%; the hole expansion rate λ is >40% and is stably controlled within 45±5%; the strength-ductility product is >20.5GPa·% and is stably controlled within 23±3GPa·%. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 The image shows the initial hot-rolled microstructure of the ultra-high strength multiphase steel produced by process 1-I in Example 1.

[0026] Figure 2 The image shows the initial hot-rolled microstructure of the ultra-high strength multiphase steel produced by process 1-II in Example 1.

[0027] Figure 3 This is a diagram of the initial hot-rolled microstructure of the ultra-high strength multiphase steel produced by process 2-I in Example 2.

[0028] Figure 4The image shows the initial hot-rolled microstructure of the ultra-high strength multiphase steel produced by process 2-II in Example 2.

[0029] Figure 5 This is a typical microstructure scan of the ultra-high strength multiphase steel product produced by process 1-II in Example 1. Detailed Implementation

[0030] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0031] Examples 1 to 8 are ultra-high strength multiphase steels with a chemical composition conforming to the following range: C: 0.12%~0.18%, Mn: 2.00%~2.60%, Si: 0.50%~0.90%, Alt: 0.40%~0.80%, Cr: 0.20%~0.60%, Mo≤0.30%, Nb≤0.050%, Ti≤0.040%, V≤0.20%, B≤0.0030%, N≤0.030%, with the balance being Fe and other unavoidable impurities. Among the impurities, P≤0.005%, S≤0.005%, and O≤0.005%. The actual smelting composition is shown in Table 1.

[0032] Table 1 Actual smelting composition of the embodiments (unit: wt%, balance is Fe and other unavoidable impurities)

[0033]

[0034] Examples 1 to 8 improve performance stability by controlling the initial microstructure characteristics of hot rolling. The steps include:

[0035] The steps for controlling the initial microstructure characteristics of hot rolling, the annealing and pickling process in the non-recrystallization zone, and the continuous annealing process;

[0036] The initial microstructure characteristics control steps for hot rolling are as follows: control the slab homogenization temperature to 1220~1240℃, and ensure that the total furnace time is 210min ≤ total time in the furnace ≤ 240min. A r3 -10℃ < Finishing rolling temperature < A r3 +15℃, after finishing rolling, a pre-roll laminar flow cooling process is adopted to cool the strip to the coiling temperature at a cooling rate of ≥30℃ before coiling. B s+5℃ < Middle winding temperature < B s +15℃, using the "U-shaped cooling" method, with 80m each for the hot head and hot tail, ensuring the temperature in the middle while ensuring the temperature at the head and tail is ±15℃ of the middle coiling temperature; then, a 48h slow cooling measure is taken to regulate the initial microstructure of the hot rolling to a acicular ferrite microstructure with a volume fraction ≥90%, and satisfying 3 < length-to-width ratio <9.

[0037] The non-recrystallization zone annealing + pickling process involves annealing the hot-rolled coil in a bell-type annealing furnace. A c1 -30℃ < Annealing homogenization temperature ≤ A c1 -10℃, holding time is 6~8h; pickled and annealed steel coils are then cold rolled to the target thickness of 1.0~2.5mm, with a cumulative reduction of ≥50%;

[0038] The continuous annealing process is as follows: annealing homogenization temperature > A c3 ℃, taking into account both environmental protection and energy-saving requirements for low carbon emissions, A c3 <annealing uniform heating temperature≤ A c3 After the soaking stage, the strip is cooled to the bainitic transformation temperature at a cooling rate of >40℃ / s at +20℃, and then subjected to over-aging treatment, i.e., the over-aging temperature is 380±10℃; the final cooling temperature is ≤170℃; the actual rolling force during the leveling process is controlled between 7000~7300kN; the strip running speed is controlled according to the thickness specification, with the strip speed of 1.0~1.5mm thickness strip being 110±5m / min, the strip speed of 1.6~1.9mm thickness strip being 95±5m / min, and the strip speed of 2.0~2.5mm thickness strip being 80±5m / min.

[0039] The specific process parameters of Examples 1 to 8 in actual production are shown in Tables 2 and 3.

[0040] Table 2 Specific process parameters for hot-rolled initial microstructure characteristic control steps in the embodiments

[0041]

[0042] Table 3. Specific process parameters for the non-recrystallization zone annealing + pickling and continuous annealing steps in the embodiments.

[0043]

[0044] After the initial microstructure characteristic control step of hot rolling was completed, samples were taken from each embodiment, and the initial microstructure of the samples was analyzed. The results showed that the initial microstructure characteristic control of hot rolling was acicular ferrite with a volume fraction ≥90%, and satisfied 3 < aspect ratio <9. Figures 1-4 The initial hot-rolled microstructure of the ultra-high strength multiphase steel produced by processes 1-I, 1-II, 2-I, and 2-II in Examples 1 and 2 are shown.

[0045] After continuous annealing, samples of the prepared multiphase steel were taken for microstructure analysis and mechanical property testing. The test and analysis results are shown in Table 4. The mechanical property testing method was prepared according to GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Test at room temperature" standard. 50 Tensile specimens were prepared and tested for yield strength, tensile strength, and elongation at break (A) on a Zwick / Roell Z100 tensile testing machine. 50 The test was conducted by preparing a 150×150mm sample according to the standard GB / T 15825.4-2008 "Forming properties and test methods of sheet metal - Part 4: Hole expansion test". Holes were made by drilling and then a hole expansion test was performed to detect the hole expansion rate λ.

[0046] Table 4 Mechanical property test results of the embodiments

[0047]

[0048] Analysis of the mechanical property test results shows that the ultra-high strength multiphase steel prepared in Examples 1-8 of this invention has high performance stability and advantages such as low cost and good hole expansion properties. Its yield strength ratio standard deviation is ≤±0.025 and is stably controlled between 0.625 and 0.650; yield strength is stably controlled between 860±20MPa; tensile strength is stably controlled between 1360±30MPa; elongation after fracture is >15.0% and is stably controlled between 17.5±2.5%; hole expansion rate λ is >40% and is stably controlled between 45±5%; strength-ductility product is >20.5GPa·% and is stably controlled between 23±3GPa·%.

[0049] Based on the microstructure analysis and the test and calculation results of the volume fraction of each phase, it can be seen that the matrix structure of the ultra-high strength multiphase steel products prepared in Examples 1 to 8 of this invention is bainite, and also contains retained austenite and a small amount of ferrite. The volume fraction of bainite is >80%, and the volume fraction of retained austenite is in the range of 8.0% to 20%. Figure 5 This is a typical microstructure scan of the ultra-high strength multiphase steel product produced in Examples 1-II.

[0050] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for improving the performance stability of ultra-high strength multiphase steel by controlling the initial microstructure characteristics of hot-rolled steel, characterized in that, This includes steps for controlling the initial microstructure characteristics of hot rolling, annealing and acid rolling in the non-recrystallization zone, and continuous annealing. The initial microstructure characteristics control steps for hot rolling are as follows: control the slab homogenization temperature to 1220~1240℃, and ensure that the total furnace time is 210min ≤ total time in the furnace ≤ 240min. A r3 -10℃ < Finishing rolling temperature < A r3 +15℃, after finishing rolling, a pre-roll laminar flow cooling process is adopted to cool the strip to the coiling temperature at a cooling rate of ≥30℃ before coiling. B s +5℃ < Middle winding temperature < B s +15℃, followed by 48h slow cooling measures, to regulate the initial microstructure of hot rolling to acicular ferrite with a volume fraction ≥90%, and satisfy 3 < aspect ratio <9; The non-recrystallization zone annealing + pickling process involves annealing the hot-rolled coil in a bell-type annealing furnace. A c1 -30℃ < Annealing homogenization temperature ≤ A c1 -10℃, heat treatment time is 6~8h; pickled and annealed steel coils are then cold rolled to the target thickness with a cumulative reduction of ≥50%; The continuous annealing process is as follows: annealing homogenization temperature > A c3 After the soaking zone, the strip steel is cooled to 380±10℃ at a cooling rate of >40℃ / s and subjected to over-aging treatment; the final cooling temperature is ≤170℃. Ultra-high strength multiphase steel adopts an economical and reduced composition design, with C-Mn-Si as the basic components. Its chemical composition by mass percentage is: C: 0.12%~0.18%, Mn: 2.00%~2.60%, Si: 0.50%~0.90%, Alt: 0.40%~0.80%, Cr: 0.20%~0.60%, Mo≤0.30%, Nb≤0.050%, Ti≤0.040%, V≤0.20%, B≤0.0030%, N≤0.030%, with the balance being Fe and other unavoidable impurities. Cr / Mn≤0.3, 1.2%≤Si+Alt≤1.5%, Nb+Ti≤0.050%, Mo+V≤0.30%, and control impurities to P≤0.005%, S≤0.005%, and O≤0.005%.

2. The method as described in claim 1, characterized in that, In the initial microstructure control step of hot rolling, the laminar flow cooling process in the front section after rolling adopts the "U-shaped cooling" method, with 80m each for the hot head and hot tail, ensuring the temperature in the middle while ensuring that the temperature of the head and tail is ±15℃ of the middle coiling temperature.

3. The method as described in claim 1, characterized in that, In the non-recrystallization zone annealing + pickling process, the target thickness for cold rolling is 1.0~2.5mm.

4. The method as described in claim 1, characterized in that, In the continuous annealing process, A c3 <annealing uniform heating temperature≤ A c3 +20℃.

5. The method as described in claim 1, characterized in that, During the continuous annealing process, the strip running speed is controlled according to the thickness specifications. The strip speed is 110±5m / min for 1.0~1.5mm thick strip, 95±5m / min for 1.6~1.9mm thick strip, and 80±5m / min for 2.0~2.5mm thick strip.

6. The method as described in claim 1, characterized in that, The continuous annealing step also includes leveling the strip, with the actual rolling force controlled between 7000 and 7300 kN during the leveling process.

7. The method as described in claim 1, characterized in that, The standard deviation of the yield strength ratio of ultra-high strength multiphase steel is ≤ ±0.025, and is stably controlled within 0.625~0.650; the yield strength is stably controlled within 860±20MPa; the tensile strength is stably controlled within 1360±30MPa; the elongation after fracture is >15.0%, and is stably controlled within 17.5±2.5%; the porosity λ is >40%, and is stably controlled within 45±5%; the strength-ductility product is >20.5GPa·%, and is stably controlled within 23±3GPa·%.

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