420mpa grade galvanized low alloy high strength steel for automobiles and method for producing the same

Abstract

The present application belongs to the field of metallurgy, and particularly relates to a 420MPa grade galvanized low-alloy high-strength steel for automobiles and a production method thereof. The composition of the steel is as follows in terms of mass percentage: C: 0.04%-0.10%, Si: 0.01%-0.50%, Mn: 0.90%-1.30%, P≤0.025%, S≤0.010%, Al: 0.060%-0.120%, Nb: 0.025%-0.040%, N: 0.0120%-0.0160%, O≤0.0060%, the mass ratio of Al to Nb is 2.0-4.0, the mass ratio of N to (Al+Nb) is 0.10-0.25, the mass ratio of Al to N is 4.0-8.0, and the rest is Fe and inevitable impurities. The steel has the advantages of being capable of inhibiting uneven slab heating structure, obtaining uniform and fine austenite grains, and ensuring uniform and stable product performance.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a 420MPa grade galvanized low-alloy high-strength steel for automobiles and its production method. Background Technology

[0002] As the automotive industry moves towards lightweighting and enhanced safety, body-in-white structural components demand materials that simultaneously meet the requirements of "high strength, high toughness, and corrosion resistance." These materials must satisfy the mechanical requirements for weight reduction and collision safety while also resisting the corrosion risks of outdoor service. Traditional carbon steel, due to its difficulty in balancing strength and toughness and its poor surface corrosion resistance, is no longer suitable for this need. Low-alloy high-strength steel has become a key solution, with 420MPa-grade galvanized low-alloy high-strength steel becoming the mainstream material choice for critical body-in-white structural components, meeting the requirement of "integrated strength, toughness, and corrosion resistance."

[0003] Low-alloy high-strength steel often incorporates the microalloying element nitrogen (Nb) to enhance strength and toughness. However, Nb strongly segregates in the interdendritic region during solidification. This results in a large accumulation of precipitates (such as Nb(C,N)) in the interdendritic region, while the amount of precipitates in the dendritic stem region is extremely small. When the slab is heated to a specific temperature, the dendritic stem region, due to insufficient precipitates and weak pinning force, will experience abnormal grain growth first; while the interdendritic region, due to the accumulation of precipitates and strong pinning force, will retain fine grains. This non-uniform structure is hereditary and will continue into subsequent processing, affecting the uniformity and stability of the final product's performance.

[0004] Patent application number CN201910198653.8 discloses a multi-grade galvanized low-alloy high-strength steel strip and its production method. The mass percentage composition of the strip is: C: 0.06%–0.09%, Si≤0.10%, Mn: 0.70%–0.90%, P≤0.020%, S≤0.010%, Als: 0.020%–0.050%, Nb: 0.020%–0.035%, Ti: 0.015%–0.030%, N≤0.0060%, with the balance being Fe and unavoidable impurities. The addition of Ti to the chemical composition impairs the material's toughness. Controlling the furnace temperature at 1170–1230℃ leads to uneven microstructure in the cast billet during heating, affecting the uniformity and stability of the final product's performance.

[0005] Patent application CN202410999068.9 discloses a galvanized low-alloy high-strength steel strip with a yield strength of 420MPa for automobiles and its production method. The chemical composition of the steel strip is as follows (mass percentage): C: 0.05–0.08%, Si ≤ 0.06%, Mn: 0.8–1.1%, P ≤ 0.020%, S ≤ 0.015%, Alt: 0.015–0.050%, Nb: 0.03–0.06%, Ti: 0.01–0.025%, Ca: 0.0008–0.0020%, N ≤ 0.0070%, with the remainder being Fe and unavoidable impurities. The addition of Ti to this chemical composition impairs the material's toughness. Continuous casting of the slab at overheated temperatures of 15–40℃ and casting speeds controlled at 1.0–1.8 m / min is detrimental to the stable control of the initial microstructure, affecting the uniformity and stability of the final product's performance.

[0006] Patent application CN202111218345.0 discloses a low-alloy high-strength steel for automobiles and its preparation method. The low-alloy high-strength steel comprises C, Si, Mn, P, S, Alt, Nb, with the remainder being Fe and unavoidable impurities. The preparation method includes: obtaining a finished strip containing the aforementioned chemical composition; subjecting the finished strip to ultra-fast cooling and high-pressure water cooling, followed by coiling to obtain coiled strip; and continuously annealing the coiled strip to obtain the low-alloy high-strength steel. By controlling the composition of the low-alloy high-strength steel, the amount of niobium alloy carbon and nitrogen precipitates can be controlled, while manganese refines the size of the precipitates. Through the preparation method, controlling the ultra-fast cooling and high-pressure process parameters and coiling temperature, a good balance between grain refinement strengthening and precipitation strengthening effects is achieved, resulting in a uniform microstructure after forming, thereby improving the mechanical properties of the high-strength steel. However, this method involves ultra-fast cooling and high-pressure water cooling of the finished strip, increasing the difficulty of controlling the strip shape and resulting in high energy consumption.

[0007] Patent application CN201910016210.2 discloses a method for controlling the uniformity of the coiling performance of Nb-containing low-alloy high-strength steel. The high-strength steel has an Nb content of 0.03–0.10% in its chemical composition. The steel is produced by steelmaking refining to obtain a slab; the slab is then heated, rough-rolled, finish-rolled, and laminar cooled, before being coiled into a hot-rolled coil. The hot-rolled coil is then pickled, rolled, annealed, and leveled to obtain the finished product. The hot rolling process uses a low-temperature coiling process with a coiling temperature of 300–550℃. The addition of a relatively high Nb content to the chemical composition of this high-strength steel increases the alloy cost. The finishing rolling temperature of 920±20℃ and the coiling temperature of 300–550℃ increase the difficulty of controlling the strip shape, while also resulting in higher energy consumption. Summary of the Invention

[0008] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a 420MPa grade galvanized low-alloy high-strength steel for automobiles and its production method. By combining compositional synergistic design, initial microstructure control, and precise setting of the heating process window, the non-uniform microstructure phenomenon during the heating process of the low-alloy high-strength steel slab is effectively suppressed, and uniform and fine original austenite grains are obtained. This microstructure provides an ideal starting point for subsequent hot-rolled controlled rolling, and by virtue of the hereditary effect of microstructure properties, the uniformity and stability of the final product performance are guaranteed.

[0009] To achieve the above objectives, the present invention provides the following technical solution: A 420MPa grade galvanized low-alloy high-strength steel for automobiles, wherein the steel's composition by mass percentage is as follows: C: 0.04%~0.10%, Si: 0.01%~0.50%, Mn: 0.90%~1.30%, P≤0.025%, S≤0.010%, Al: 0.060%~0.120%, Nb: 0.025%~0.040%, N: 0.0120%~0.0160%, O≤0.0060%, and the mass ratio of Al to Nb is 2.0~4.0, the mass ratio of N to (Al+Nb) is 0.10~0.25, the mass ratio of Al to N is 4.0~8.0, and the remainder is Fe and unavoidable impurities.

[0010] The metallographic structure of the steel is ferrite + pearlite, wherein the area fraction of ferrite is 89-95%, and the balance is pearlite.

[0011] The steel has a yield strength of 428–487 MPa, a tensile strength of 503–572 MPa, and an elongation of A. 80 : 22.0%~27.0%, n value: 0.12~0.15.

[0012] The chemical composition and main functions of the galvanized low-alloy high-strength steel described in this invention are as follows: C: A key element for ensuring the strength of steel, and also promotes the formation of pearlite. To balance strength and weldability, this invention adopts a low-carbon design, controlling the carbon content to 0.04% to 0.10%. Silicon (Si) plays a role in deoxidation and solid solution strengthening in steel, but excessive addition can adversely affect toughness. Therefore, the silicon content in this invention is controlled at 0.01–0.50%. Mn: It can play a solid solution strengthening role in steel and stabilize austenite. According to the target strength level requirements, the manganese content is controlled at 0.90-1.30% in this invention. P and S are harmful elements in steel. To improve the purity of steel, their contents must be strictly controlled and kept at low levels. Therefore, this invention controls the phosphorus content to ≤0.025% and the sulfur content to ≤0.010%. Al is the core element for forming AlN in the dendritic stem region. By forming AlN particles, it effectively prevents the growth of austenite grains during slab heating. Furthermore, by utilizing the slight enrichment of Al in the dendritic stem region, this region can become the core area for AlN formation. Therefore, in this invention, the Al content is controlled at 0.060–0.120%. Nb is the core element for forming Nb(C,N) in the interdendritic region. By forming Nb(C,N) particles, it effectively prevents the growth of austenite grains during slab heating. Furthermore, utilizing the strong segregation characteristics of Nb in the interdendritic region, it can make the interdendritic region a core area for Nb(C,N) formation. Therefore, in this invention, the Nb content is controlled at 0.025–0.040%. Nitrogen (N) is a key "connecting" element in steel, capable of combining with Nb, Al, and C to form nitrides or carbonitrides (e.g., AlN and Nb(C,N)). These compounds play a crucial role in refining the grain structure. Therefore, this invention controls the nitrogen content to 0.0120–0.0160%. O: Oxide inclusions are easily formed in steel. To ensure the purity of steel and control the total amount of oxide inclusions, the present invention controls the oxygen content to ≤0.0060%.

[0013] Among them, Al, Nb and N must simultaneously satisfy the following relationship: The Al to Nb mass ratio is 2.0–4.0 to ensure sufficient Al to form adequate AlN in the dendritic stem region, balancing the pinning force provided by Nb(C,N) in the interdendritic region. If the Al / Nb ratio is too low, the pinning in the dendritic stem region is insufficient, leading to severe microstructural inhomogeneity; if it is too high, the AlN may be too coarse or the formation temperature too low, resulting in poor pinning effect and increased alloy cost.

[0014] The mass ratio of N to (Al+Nb) is 0.10 to 0.25 to ensure that there is an optimal amount of N to combine with Al and Nb to form an effective precipitate phase, while avoiding excessive free N atoms dissolved in the matrix, which would impair toughness.

[0015] The mass ratio of Al to N is 4.0 to 8.0, ensuring that under the "low-temperature pinning and controlled rolling process", there is a sufficient amount of undissolved AlN existing in the form of fine particles, thereby providing effective grain boundary pinning force.

[0016] A method for producing 420MPa grade galvanized low-alloy high-strength steel for automobiles includes: hot metal pretreatment, converter smelting, LF refining, continuous casting, slab heating, hot continuous rolling, laminar flow cooling and coiling, pickling and cold rolling, continuous annealing and hot-dip galvanizing, and finishing, wherein: The aforementioned molten iron pretreatment: Desulfurization powder and magnesium powder are used to desulfurize molten iron to S≤0.015%, iron slag thickness≤20mm, and molten iron outlet temperature 1240~1450℃; The aforementioned converter smelting: First, add refined scrap steel to the oxygen top and bottom blowing converter, then add molten iron, and tap the steel at a temperature of 1630-1670℃.

[0017] The aforementioned LF refining: Slag thickness 50-100mm, net space 250-700mm, processing time ≥30min, final oxygen content 300-600ppm.

[0018] The aforementioned continuous casting: The superheat is 15-25℃, the electromagnetic stirring current of the crystallizer is 300-500A, the frequency is 2-5Hz, the secondary cooling zone adopts strong cooling mode, and the water volume of the foot roller section and spray zone is 1.2-1.5L / kg; the casting process maintains a constant casting speed of 1.4-1.8m / min; light reduction is adopted, with a reduction of 3-8mm.

[0019] The slab heating described above: Slab heating: The slab is heated in a walking beam furnace at an initial temperature of 200–550°C. The heating temperature and holding time of the slab are determined according to the following process: 1) Low-Temperature Pinning Controlled Rolling Process: This process aims to obtain a uniform, fine-grained microstructure. It refines the original austenite grains by precisely controlling heating parameters and utilizing the pinning effect of precipitated phases. The heating temperature is 1120–1150℃, and the holding time is 1.0–3.0 h. Within this temperature range, AlN and Nb(C,N) precipitates can be completely retained, generating strong pinning forces that effectively inhibit the growth of the original austenite grains, thereby obtaining a fine and uniform austenite microstructure, laying the foundation for subsequent material performance improvements.

[0020] 2) Medium-temperature suppression rolling process: This process focuses on efficient production and basic performance assurance. Its core objective is to suppress mixed crystal formation, making it suitable for urgent orders with extremely short delivery times and low performance requirements. The heating temperature is 1160–1190℃, and the holding time is 0.5–0.7 hours. By shortening the holding time, rapid production and timely delivery can be achieved while ensuring the absence of significant mixed crystal defects in the microstructure and meeting basic performance requirements.

[0021] 3) High-temperature solution rolling process: This process aims to obtain a uniform coarse-grained structure while solving the compositional segregation problem of continuously cast billets, ensuring the hardenability of the material. The heating temperature is 1200–1250℃, and the holding time is 0.7–2.0 h. Within this temperature range, the AlN and Nb(C,N) precipitates in the material completely dissolve, which helps to homogenize the composition and creates conditions for fully utilizing hardenability during subsequent heat treatment. For billets with excessive alloy composition or severe central segregation, the high-temperature solution process reduces segregation by dissolving harmful precipitates and promoting atomic diffusion, transforming defective billets into qualified products.

[0022] The aforementioned hot continuous rolling: The heated slab is rough rolled in a 3+3 or 3+5 pattern with a total reduction of ≥70%, followed by finish rolling of the intermediate slab. The specific finish rolling process is as follows: Low-temperature pinning controlled rolling process: finishing rolling start temperature 970~1000℃, finishing rolling temperature 830~860℃; Medium-temperature suppressed rolling process: finishing rolling start temperature 1000~1030℃, finishing rolling temperature 860~890℃; High-temperature solution rolling process: finishing rolling start temperature 1030~1080℃, finishing rolling temperature 890~930℃; The laminar flow cooling and winding process described above: After final rolling, the hot-rolled steel strip is cooled to 550-600°C at a laminar flow cooling rate of 40°C / s or higher, and then air-cooled to 500-550°C for 3-6 seconds before being coiled. The pickling and cold rolling processes mentioned above: Hot-rolled steel strip is pickled to remove iron oxide scale and then cold-rolled. The cold rolling reduction rate is controlled as follows: Low-temperature pinning controlled rolling process: cold rolling reduction rate 35%–50%; Medium-temperature controlled rolling process: cold rolling reduction rate 50%–65%; High-temperature solution rolling process: cold rolling reduction rate of 65% to 80%.

[0023] The continuous annealing and hot-dip galvanizing process described above: The preheating section heats the strip steel to 130–200°C at a rate of 2–6°C / s. The heating section heats the strip steel to 760–800℃ at a rate of 2–10℃ / s. The soaking zone holds the strip at 760–800℃ for 60–150 seconds. The slow cooling section cools the strip steel to 660-700℃, with a slow cooling rate of 1-15℃ / s. The rapid cooling section cools the strip steel to 455-465℃, with a rapid cooling rate of 15-60℃ / s. The equalization section holds the strip steel at 455–465℃ for 30–150 seconds. The temperature of the zinc pot is 455-465℃, and the galvanizing time is 2-5 seconds.

[0024] The aforementioned finishing: The strip steel produced by continuous annealing and hot-dip galvanizing is finished with a finishing elongation of 0.6 to 1.8%.

[0025] Low-temperature pinning and controlled rolling is the key and main production process to ensure the core performance of products; while medium-temperature suppressed rolling and high-temperature solution rolling are flexible supplementary solutions adopted for specific situations such as urgent orders or defective slabs.

[0026] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention optimizes the composition design: Through Al-N-Nb composite microalloying, the content ratio of Al, Nb, and N elements is precisely controlled. It fully utilizes the compositional distribution characteristics of Nb segregation in the interdendritic region and Al slight enrichment in the dendritic trunk region during continuous casting, spontaneously forming a "spatial partitioning pinning" structure during slab heating. Specifically, Nb is enriched in the interdendritic region, forming thermally stable Nb(C,N) precipitates as pinning points; Al is enriched in the dendritic trunk region, forming uniformly distributed AlN precipitates as pinning points. This structure fundamentally solves the problem of weak pinning force caused by Nb segregation, ensuring that grain boundary pinning covers the entire area, thereby effectively suppressing non-uniform grain growth in the initial stage and providing a uniform and fine original austenite structure for subsequent processes.

[0027] 2. The production method of this invention ensures that the initial microstructure control is matched with the heating process: by synergistically regulating the continuous casting and heating processes, a foundation for microstructure uniformity is laid. In the continuous casting stage, a combination of processes such as high casting speed, strong secondary cooling, and low superheat is adopted, combined with a set slab heating process window, to precisely control temperature and time parameters, ensuring that the AlN and Nb(C,N) precipitates synergistically exert their pinning effect within the optimal range.

[0028] 3. This invention employs multi-pass high-reduction rolling: Based on a uniform initial microstructure, a multi-pass high-reduction rolling process is further implemented. This process significantly compresses the segregation band spacing, enhancing the "band pinning" effect and increasing the resistance to grain boundary migration from the upper limit of abnormal grain growth in physical space. The resulting narrow and continuous segregation band structure, in conjunction with the previously formed partitioned pinning, jointly ensures that the microstructure maintains uniformity and stability during subsequent heat treatment, providing a reliable guarantee for obtaining a final microstructure with uniform dimensions and consistent properties. Attached Figure Description

[0029] Figure 1This is a metallographic diagram of Example 1 in the low-temperature pinning and controlled rolling process.

[0030] Figure 2 This is a metallographic diagram of Example 6 in the medium-temperature suppressed rolling process.

[0031] Figure 3 This is a metallographic diagram of Example 11 in the high-temperature solution rolling process. Detailed Implementation

[0032] The present invention will now be described in detail with reference to the accompanying drawings. However, it should be noted that the implementation of the present invention is not limited to the following embodiments. In the following embodiments, the low-temperature pinning controlled rolling process is the key and main production process to ensure the core performance of the product; while the medium-temperature suppressed rolling and high-temperature solution rolling processes are flexible supplementary solutions adopted for specific situations such as urgent orders or defective slabs.

[0033] In Examples 1-5, the low-temperature pinning and rolling process is the key and main production process for ensuring the core performance of the product in this technical solution.

[0034] The metallographic structure of Example 1 of the low-temperature pinning controlled rolling process for 420MPa grade galvanized low-alloy high-strength steel used in automobiles is as follows: Figure 1 As shown, the metallographic structure is ferrite + pearlite, with ferrite area fraction of 95% and the balance of pearlite of 5%. Yield strength is 428-480 MPa, tensile strength is 503-565 MPa, elongation is 23-27%, and n value is 0.12-0.15.

[0035] The production method of 420MPa grade galvanized low-alloy high-strength steel for automobiles includes hot metal pretreatment, converter smelting, LF refining, continuous casting, slab heating, hot continuous rolling, laminar flow cooling and coiling, pickling and cold rolling, continuous annealing and hot-dip galvanizing, and finishing. The hot metal pretreatment parameters for each embodiment are shown in Table 1; the converter smelting and LF refining parameters are shown in Table 2; the continuous casting parameters are shown in Table 3; the chemical composition is shown in Table 4; the mass ratio of Al, Nb, and N is shown in Table 5; the slab heating and hot continuous rolling parameters are shown in Table 6; the laminar flow cooling and coiling parameters are shown in Table 7; the pickling and cold rolling parameters and continuous annealing parameters are shown in Table 8; the hot-dip galvanizing parameters and finishing parameters are shown in Table 9; and the mechanical property test results of the finished steel plates are shown in Table 10.

[0036] Table 1. Pretreatment parameters of molten iron for each embodiment.

[0037] Table 2 shows the converter smelting parameters and LF refining parameters for each embodiment.

[0038] Table 3. Continuous casting parameters for each embodiment.

[0039] Table 4 Chemical composition (wt%) of each example.

[0040] Table 5 shows the mass ratios of Al, Nb, and N in each embodiment.

[0041] Table 6 shows the slab heating parameters and hot rolling parameters for each embodiment.

[0042] Table 7. Laminar flow cooling and winding parameters for each embodiment.

[0043] Table 8 shows the pickling and cold rolling parameters and continuous annealing parameters for each embodiment.

[0044] Table 9 shows the hot-dip galvanizing parameters and finishing parameters for each embodiment.

[0045] Table 10 shows the test results of the mechanical properties of the finished steel plates in each embodiment.

[0046] Examples 6-10: The medium-temperature suppressed rolling of this technical solution is a flexible supplementary solution adopted for specific situations such as urgent orders or defective slabs.

[0047] The metallographic structure of Example 6 of 420MPa grade galvanized low-alloy high-strength steel for automobiles, produced using a medium-temperature suppressed rolling process, is as follows: Figure 2 As shown, the metallographic structure is ferrite + pearlite, with ferrite area fraction of 94% and the balance of pearlite of 6%. Yield strength is 438-479 MPa, tensile strength is 522-566 MPa, elongation is 22-26%, and n value is 0.12-0.15.

[0048] The production method of 420MPa grade galvanized low-alloy high-strength steel for automobiles includes hot metal pretreatment, converter smelting, LF refining, continuous casting, slab heating, hot continuous rolling, laminar flow cooling and coiling, pickling and cold rolling, continuous annealing and hot-dip galvanizing, and finishing. The hot metal pretreatment parameters for each embodiment are shown in Table 11; the converter smelting and LF refining parameters are shown in Table 12; the continuous casting parameters are shown in Table 13; the chemical composition is shown in Table 14; the mass ratio of Al, Nb, and N is shown in Table 15; the slab heating and hot continuous rolling parameters are shown in Table 16; the laminar flow cooling and coiling parameters are shown in Table 17; the pickling and cold rolling parameters and continuous annealing parameters are shown in Table 18; the hot-dip galvanizing parameters and finishing parameters are shown in Table 19; and the mechanical property test results of the finished steel plates are shown in Table 20.

[0049] Table 11 shows the molten iron pretreatment parameters for each embodiment.

[0050] Table 12 shows the converter smelting parameters and LF refining parameters for each embodiment.

[0051] Table 13 Continuous casting parameters for each embodiment.

[0052] Table 14 Chemical composition (wt%) of each example.

[0053] Table 15 shows the mass ratios of Al, Nb, and N in each embodiment.

[0054] Table 16 shows the slab heating parameters and hot rolling parameters for each embodiment.

[0055] Table 17 Laminar flow cooling and winding parameters for each embodiment.

[0056] Table 18 shows the pickling and cold rolling parameters and continuous annealing parameters for each embodiment.

[0057] Table 19 shows the hot-dip galvanizing parameters and finishing parameters for each embodiment.

[0058] Table 20 shows the test results of the mechanical properties of the finished steel plates in each embodiment.

[0059] Examples 11-15: The high-temperature solution rolling process of this technical solution is a flexible supplementary solution adopted for specific situations such as urgent orders or defective slabs.

[0060] The metallographic structure of Example 11 of 420MPa grade galvanized low-alloy high-strength steel for automobiles, produced by high-temperature solution rolling process, is as follows: Figure 3 As shown, the metallographic structure is ferrite + pearlite, with ferrite area fraction of 93% and the balance of pearlite of 7%. Yield strength is 445-487 MPa, tensile strength is 530-572 MPa, elongation is 22-25.5%, and n value is 0.12-0.14.

[0061] The production method of 420MPa grade galvanized low-alloy high-strength steel for automobiles includes hot metal pretreatment, converter smelting, LF refining, continuous casting, slab heating, hot continuous rolling, laminar flow cooling and coiling, pickling and cold rolling, continuous annealing and hot-dip galvanizing, and finishing. The hot metal pretreatment parameters for each embodiment are shown in Table 21; the converter smelting and LF refining parameters are shown in Table 22; the continuous casting parameters are shown in Table 23; the chemical composition is shown in Table 24; the mass ratio of Al, Nb, and N is shown in Table 25; the slab heating and hot continuous rolling parameters are shown in Table 26; the laminar flow cooling and coiling parameters are shown in Table 27; the pickling and cold rolling parameters and continuous annealing parameters are shown in Table 28; the hot-dip galvanizing parameters and finishing parameters are shown in Table 29; and the mechanical property test results of the finished steel plates are shown in Table 30.

[0062] Table 21 shows the molten iron pretreatment parameters for each embodiment.

[0063] Table 22 shows the converter smelting parameters and LF refining parameters for each embodiment.

[0064] Table 23 Continuous casting parameters for each embodiment.

[0065] Table 24 Chemical composition (wt%) of each example.

[0066] Table 25 shows the mass ratios of Al, Nb, and N in each embodiment.

[0067] Table 26 shows the slab heating parameters and hot rolling parameters for each embodiment.

[0068] Table 27 Laminar flow cooling and winding parameters for each embodiment.

[0069] Table 28 shows the pickling and cold rolling parameters and continuous annealing parameters for each embodiment.

[0070] Table 29 shows the hot-dip galvanizing parameters and finishing parameters for each embodiment.

[0071] Table 30 shows the test results of the mechanical properties of the finished steel plates in each embodiment.

[0072] Mechanical property tests were conducted on the finished steel plates from Examples 1 to 15, focusing on the transverse properties of the head, middle, and tail sections. The results are shown in Tables 10, 20, and 30. Typical microstructures are as follows: Figures 1-3 As shown.

[0073] From Tables 10, 20 and 30 and Figures 1-3 As can be seen, this application, through the organic combination of compositional synergistic design, initial microstructure control and heating process matching, and multi-pass high-reduction rolling, firstly systematically suppresses the non-uniform grain growth behavior of low-alloy high-strength steel slabs during heating, significantly improving the microstructure uniformity and stability. Based on this, the obtained fine and uniform austenite grains provide an ideal starting point for subsequent processes, thereby effectively improving the strength, toughness, and overall performance consistency of the final product. Furthermore, this technical solution also possesses good process adaptability and robustness, effectively addressing the demands of market diversification and production uncertainties while ensuring the quality of mainstream products.

Claims

1. A 420MPa grade galvanized low-alloy high-strength steel for automobiles, characterized in that, The composition of the steel, by mass percentage, is as follows: C: 0.04%~0.10%, Si: 0.01%~0.50%, Mn: 0.90%~1.30%, P≤0.025%, S≤0.010%, Al: 0.060%~0.120%, Nb: 0.025%~0.040%, N: 0.0120%~0.0160%, O≤0.0060%, and the mass ratio of Al to Nb is 2.0~4.0, the mass ratio of N to (Al+Nb) is 0.10~0.25, the mass ratio of Al to N is 4.0~8.0, and the remainder is Fe and unavoidable impurities; The metallographic structure of the steel is ferrite + pearlite, wherein the area fraction of ferrite is 89-95%, and the balance is pearlite. The steel has a yield strength of 428–487 MPa, a tensile strength of 503–572 MPa, and an elongation of A. 80 : 22.0%~27.0%, n value: 0.12~0.15; The production method of the 420MPa grade galvanized low-alloy high-strength steel for automobiles includes: hot metal pretreatment, converter smelting, LF refining, continuous casting, slab heating, hot continuous rolling, laminar flow cooling and coiling, pickling and cold rolling, continuous annealing and hot-dip galvanizing, and finishing, wherein: The slab heating described above: Slab heating: The slab is heated in a walking beam furnace at an initial temperature of 200–550°C. The heating temperature and holding time of the slab are determined according to the following process: 1) Low-temperature stapling and controlled rolling process: heating temperature 1120~1150℃, holding time 1.0~3.0h; 2) Medium-temperature suppressed rolling process: heating temperature 1160~1190℃, holding time 0.5~0.7h; 3) High-temperature solution rolling process: heating temperature 1200~1250℃, holding time 0.7~2.0h; The aforementioned hot continuous rolling: The heated slab is rough rolled in a 3+3 or 3+5 pattern with a total reduction of ≥70%, followed by finish rolling of the intermediate slab. The specific finish rolling process is as follows: Low-temperature pinning controlled rolling process: finishing rolling start temperature 970~1000℃, finishing rolling temperature 830~860℃; Medium-temperature suppressed rolling process: finishing rolling start temperature 1000~1030℃, finishing rolling temperature 860~890℃; High-temperature solution rolling process: finishing rolling start temperature 1030~1080℃, finishing rolling temperature 890~930℃; The laminar flow cooling and winding process described above: After final rolling, the hot-rolled steel strip is cooled to 550-600°C at a laminar flow cooling rate of 40°C / s or higher, and then air-cooled to 500-550°C for 3-6 seconds before being coiled. The pickling and cold rolling processes mentioned above: Hot-rolled steel strip is pickled to remove iron oxide scale and then cold-rolled. The cold rolling reduction rate is controlled as follows: Low-temperature pinning controlled rolling process: cold rolling reduction rate 35%–50%; Medium-temperature controlled rolling process: cold rolling reduction rate 50%–65%; High-temperature solution rolling process: cold rolling reduction rate 65%–80%; The continuous annealing and hot-dip galvanizing process described above: The preheating section heats the strip steel to 130–200°C at a rate of 2–6°C / s. The heating section heats the strip steel to 760–800℃ at a rate of 2–10℃ / s. The soaking zone holds the strip at 760–800℃ for 60–150 seconds. The slow cooling section cools the strip steel to 660-700℃, with a slow cooling rate of 1-15℃ / s. The rapid cooling section cools the strip steel to 455-465℃, with a rapid cooling rate of 15-60℃ / s. The equalization section holds the strip steel at 455–465℃ for 30–150 seconds. The temperature of the zinc pot is 455-465℃, and the galvanizing time is 2-5 seconds.

2. A method for producing 420MPa grade galvanized low-alloy high-strength steel for automobiles as described in claim 1, characterized in that, This includes hot metal pretreatment, converter smelting, LF refining, continuous casting, slab heating, hot rolling, laminar flow cooling and coiling, pickling and cold rolling, continuous annealing and hot-dip galvanizing, and finishing, among which: The slab heating described above: Slab heating: The slab is heated in a walking beam furnace at an initial temperature of 200–550°C. The heating temperature and holding time of the slab are determined according to the following process: 1) Low-temperature stapling and controlled rolling process: heating temperature 1120~1150℃, holding time 1.0~3.0h; 2) Medium-temperature suppressed rolling process: heating temperature 1160~1190℃, holding time 0.5~0.7h; 3) High-temperature solution rolling process: heating temperature 1200~1250℃, holding time 0.7~2.0h; The aforementioned hot continuous rolling: The heated slab is rough rolled in a 3+3 or 3+5 pattern with a total reduction of ≥70%, followed by finish rolling of the intermediate slab. The specific finish rolling process is as follows: Low-temperature pinning controlled rolling process: finishing rolling start temperature 970~1000℃, finishing rolling temperature 830~860℃; Medium-temperature suppressed rolling process: finishing rolling start temperature 1000~1030℃, finishing rolling temperature 860~890℃; High-temperature solution rolling process: finishing rolling start temperature 1030~1080℃, finishing rolling temperature 890~930℃; The laminar flow cooling and winding process described above: After final rolling, the hot-rolled steel strip is cooled to 550-600°C at a laminar flow cooling rate of 40°C / s or higher, and then air-cooled to 500-550°C for 3-6 seconds before being coiled. The pickling and cold rolling processes mentioned above: Hot-rolled steel strip is pickled to remove iron oxide scale and then cold-rolled. The cold rolling reduction rate is controlled as follows: Low-temperature pinning controlled rolling process: cold rolling reduction rate 35%–50%; Medium-temperature controlled rolling process: cold rolling reduction rate 50%–65%; High-temperature solution rolling process: cold rolling reduction rate 65%–80%; The continuous annealing and hot-dip galvanizing process described above: The preheating section heats the strip steel to 130–200°C at a rate of 2–6°C / s. The heating section heats the strip steel to 760–800℃ at a rate of 2–10℃ / s. The soaking zone holds the strip at 760–800℃ for 60–150 seconds. The slow cooling section cools the strip steel to 660-700℃, with a slow cooling rate of 1-15℃ / s. The rapid cooling section cools the strip steel to 455-465℃, with a rapid cooling rate of 15-60℃ / s. The equalization section holds the strip steel at 455–465℃ for 30–150 seconds. The temperature of the zinc pot is 455-465℃, and the galvanizing time is 2-5 seconds.

3. The method for producing 420MPa grade galvanized low-alloy high-strength steel for automobiles according to claim 2, characterized in that, The aforementioned molten iron pretreatment: Desulfurization powder and magnesium powder are used to desulfurize molten iron to S≤0.015%, iron slag thickness≤20mm, and molten iron outlet temperature 1240~1450℃; The aforementioned converter smelting: First, add refined scrap steel to the oxygen top and bottom blowing converter, then add molten iron, and tap the steel at a temperature of 1630-1670℃. The aforementioned LF refining: Slag thickness 50-100mm, net space 250-700mm, processing time ≥30min, final oxygen content 300-600ppm.

4. The method for producing 420MPa grade galvanized low-alloy high-strength steel for automobiles according to claim 2, characterized in that, The aforementioned continuous casting: The superheat is 15-25℃, the electromagnetic stirring current of the crystallizer is 300-500A, the frequency is 2-5Hz, the secondary cooling zone adopts strong cooling mode, and the water volume of the foot roller section and spray zone is 1.2-1.5L / kg; the casting process maintains a constant casting speed of 1.4-1.8m / min; light reduction is adopted, with a reduction of 3-8mm.

5. The method for producing 420MPa grade galvanized low-alloy high-strength steel for automobiles according to claim 2, characterized in that, The finishing process involves finishing the strip steel produced by continuous annealing and hot-dip galvanizing, with a finishing elongation of 0.6 to 1.8%.

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