420mpa grade galvanized low alloy high strength steel and method of production thereof

Abstract

This invention belongs to the field of metallurgical technology, and particularly relates to a 420MPa grade galvanized low-alloy high-strength steel and its production method. The steel composition by mass percentage is as follows: C: 0.06%~0.08%, Si: 0.05%~0.08%, Mn: 0.95%~1.15%, P≤0.025%, S≤0.010%, Al: 0.015%~0.060%, O≤0.0060%, N≤0.0060%, V: 0.01%~0.03%, Ti: 0.01%~0.03%, with the balance being Fe and unavoidable impurities. The advantages are: through V-Ti composite microalloying and controlled hot and cold rolling processes, synergistic effects of fine grain strengthening and precipitation strengthening are achieved. The ferrite grain size is 2~5μm, the (Ti,V)C precipitate size is 20~30nm, the steel plate yield strength is ≥420MPa, and the elongation is maintained above 20%.

Description

Technical Field

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

[0002] With the automotive industry's increasing demands for lightweighting, energy conservation, emission reduction, and safety, steel used in body-in-white is gradually developing towards higher strength and higher corrosion resistance. Low-alloy high-strength steel, due to its excellent strength, toughness, weldability, and formability, has become one of the core materials for automotive structural components; while hot-dip galvanizing, as an effective anti-corrosion method, can significantly extend the service life of the body-in-white. Currently, 420MPa grade hot-dip galvanized low-alloy high-strength steel has become the main material used in the automotive industry because it can simultaneously meet the requirements for strength and corrosion resistance.

[0003] In the prior art, patent application number CN202510232416.4 discloses a low-cost cold-rolled low-alloy high-strength steel production method. For a 420MPa grade product, the chemical composition is: C: 0.07%–0.10%, Si: 0.10%–0.20%, Mn: 0.75%–0.90%, P≤0.025%, S≤0.020%, Alt: 0.020%–0.060%, Ti: 0.060%–0.075%, with the remainder being Fe and unavoidable impurities. This chemical composition has a high Ti content, and the content of the N precipitate formed by combining with Ti is not clearly defined. When the N content fluctuates significantly, the finished product's performance stability is easily compromised, failing to meet the forming requirements of complex vehicle body structural parts.

[0004] Patent application CN202510013543.5 discloses a low-alloy high-strength steel with a yield strength ratio of 420MPa and its preparation method. The chemical composition includes: C: 0.07%–0.12%, Si: 0.10%–0.40%, Mn: 1.60%–1.80%, P: ≤0.015%, S: ≤0.010%, Alt: 0.020%–0.060%, Nb: 0.03%–0.05%, Ti: 0.01%–0.03%, N ≤0.005%, and the matrix element Fe. While the high Si and Mn content in this patented chemical composition can improve strength, these elements easily form an oxide layer on the steel surface during annealing, significantly reducing the wettability of the steel with the zinc bath. This leads to defects in the coating, affecting corrosion resistance and failing to meet the surface quality standards for automotive steel.

[0005] Patent application number CN202410999068.9 discloses a galvanized low-alloy high-strength steel strip with a yield strength of 420MPa for automobiles and its production method. The process involves calcium treatment during smelting, which increases costs.

[0006] Patent application CN202310987746.5 discloses a galvanized low-alloy high-strength steel and its production method. In the hot rolling process, the continuously cast billet is heated at 1200℃~1250℃ for 200 minutes~400 minutes, and the finishing rolling temperature is 820℃~900℃. The long heating time leads to significant growth of austenite grains in the billet. Due to the heritability of microstructure properties, this will affect the finished product grain size, preventing it from meeting the target of "grain size of 12.5~14 for 420MPa grade galvanized low-alloy high-strength steel," thus affecting the strength level. The lower finishing rolling temperature significantly slows down the rolling pace, increases cooling water consumption, and increases the mill load, resulting in reduced production efficiency and increased production costs.

[0007] Patent application number CN202011575408.3 discloses a hot-dip galvanized 420MPa grade low alloy high-strength steel for automobiles and its production method. The smelting and continuous casting production process includes: KR desulfurization - converter - LF refining - RH vacuum treatment - slab continuous casting - slow cooling; the use of LF+RH dual-path production leads to increased costs. 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 and its production method. Through precise composition design and multi-stage heat treatment process control, the synergistic effect of fine grain strengthening and precipitation strengthening is achieved. While ensuring the mechanical properties of the steel, the coating is guaranteed to be free of defects and have excellent adhesion, thus meeting the stringent requirements of automotive body-in-white structural components.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A 420MPa grade galvanized low-alloy high-strength steel, the composition of which, by mass percentage, is as follows:

[0011] C: 0.06%~0.08%, Si: 0.05%~0.08%, Mn: 0.95%~1.15%, P≤0.025%, S≤0.010%, Al: 0.015%~0.060%, O≤0.0060%, N≤0.0060%, V: 0.01%~0.03%, Ti: 0.01%~0.03%, balance being Fe and unavoidable impurities.

[0012] Considering production costs and overall performance, the chemical composition and main functions of the steel described in this invention are as follows:

[0013] C: Carbon is an important solid solution strengthening element with a significant strengthening effect. However, excessive carbon content can deteriorate the weldability and toughness of steel plates, and it can also form carbide precipitation with strong carbide-forming elements such as V and Ti. Therefore, this invention controls the carbon content to be between 0.06% and 0.08% to maintain good weldability and toughness while ensuring strength.

[0014] Si: Si acts as a solid solution strengthening element in steel, increasing its strength. It also acts as a deoxidizer, helping to remove oxygen from the molten steel. However, excessively high Si content can affect the surface quality and coating adhesion during hot-dip galvanizing, leading to galvanizing defects. Therefore, this invention controls the Si content to be between 0.05% and 0.08% to balance strength improvement and galvanizing process stability.

[0015] Mn: Mn is a major solid solution strengthening element that can significantly improve the strength and hardness of steel. It also combines with sulfur (S) to form MnS, preventing S-induced hot brittleness and improving hot workability. Therefore, this invention controls the Mn content to be between 0.95% and 1.15% to provide sufficient strength without compromising toughness and hot rolling processability.

[0016] P: P element has a solid solution strengthening effect in steel, but excessive content will significantly reduce the toughness and plasticity of steel, causing cold brittleness, especially at low temperatures. Therefore, this invention limits the P content to ≤0.025% to minimize its adverse effect on toughness and ensure the low-temperature performance of the steel.

[0017] S: S element is generally considered a harmful impurity, forming sulfide inclusions such as MnS. These inclusions reduce the toughness and plasticity of steel, promote crack initiation, and affect fatigue life. Therefore, this invention strictly controls the S content to ≤0.010% to improve the purity and mechanical properties of the steel.

[0018] Al: Al is mainly used as a deoxidizer, combining with oxygen to form Al₂O₃ and with nitrogen to form AlN, refining grains and improving strength and toughness. However, excessive Al content can lead to too many inclusions, affecting the cleanliness of the steel. Therefore, this invention controls the Al content between 0.015% and 0.060% to optimize deoxidation and grain refinement effects.

[0019] O: O is a harmful element in steel, forming oxide inclusions such as Al2O3. These inclusions can become stress concentration points, reducing fatigue life and toughness. Therefore, this invention limits the O content to ≤0.0060% to ensure the cleanliness and high toughness of the steel.

[0020] Nitrogen (N): Nitrogen elements can form nitrides such as AlN or VN, which can refine grains. However, excessively high N content can cause aging brittleness and reduced toughness. Therefore, this invention controls the N content to ≤0.0060% to take advantage of its grain-refining benefits while avoiding the risk of brittleness.

[0021] V: V is a strong carbonitride forming element. It enhances the strength of steel through precipitation strengthening, while refining the grain size and improving toughness and weldability. Therefore, this invention controls the V content to be between 0.01% and 0.03% to achieve effective precipitation strengthening without excessively impairing plasticity.

[0022] Ti: Ti is a very strong carbonitride forming element, forming TiC or TiN. These precipitates refine the grains, increase strength, and fix C and N, reducing their harmful effects. Therefore, this invention controls the Ti content to be between 0.01% and 0.03% to optimize precipitation strengthening and grain refinement, while improving the overall stability of the steel.

[0023] The microstructure of the steel is ferrite and a small amount of pearlite, with the ferrite area percentage content being 90-95% and the ferrite grain size being 2-5 μm; the (Ti,V)C precipitate size is 20-30 nm.

[0024] The steel has a yield strength ≥420MPa and an elongation of ≥20%.

[0025] A method for producing 420MPa grade galvanized low-alloy high-strength steel includes hot metal pretreatment, converter smelting, LF refining, continuous casting, slab heating, hot continuous rolling, laminar flow cooling and coiling, pickling, cold rolling, continuous annealing and hot-dip galvanizing, finishing and tension leveling, wherein:

[0026] The continuous annealing and hot-dip galvanizing process described above:

[0027] Heating and holding section: The strip steel is heated to 780-800℃ at a rate of 2-5℃ / s and held for 50-140s;

[0028] Slow cooling section: After heating and heat preservation, the strip steel is cooled to 600-750℃ at a rate of 2-8℃ / s;

[0029] Rapid cooling section: After the slow cooling is completed, the strip steel is cooled to 440-500℃ at a rate of 10-35℃ / s;

[0030] Equalization section: After rapid cooling, the strip is held at 455-465℃ for 30-80 seconds;

[0031] Hot-dip galvanizing: The zinc bath contains 0.1% to 0.5% Mg by mass and 0.15% to 0.18% Al by mass, the zinc bath temperature is 455 to 465℃, and the galvanizing time is 2 to 5 seconds;

[0032] The aforementioned finishing and straightening:

[0033] For strip steel produced by continuous annealing and hot-dip galvanizing, the finishing elongation is controlled according to the strip steel thickness t.

[0034] When 0.40mm ≤ t < 0.50mm, the finishing elongation is 0.6% to 0.8%;

[0035] When 0.50mm ≤ t < 0.70mm, the finishing elongation is 0.8%~1.0%;

[0036] When 0.70mm ≤ t < 0.90mm, the finishing elongation is 1.0% to 1.2%;

[0037] When 0.90mm ≤ t < 1.10mm, the finishing elongation is 1.2% to 1.4%;

[0038] When 1.10mm ≤ t < 1.50mm, the finishing elongation is 1.4% to 1.6%;

[0039] When 1.50mm≤t≤2.50mm, the finishing elongation is 1.5%~1.7%;

[0040] After finishing, the strip is tension-straightened, and the elongation rate is controlled according to the strip thickness t and width w.

[0041] When 0.40mm≤t<0.50mm and 800mm≤w≤1250mm, the tensile elongation is 0.3%~0.8%;

[0042] When 0.50mm≤t<0.70mm and 1250mm<w≤1600mm, the tensile elongation is 0.4%~1.2%;

[0043] When 0.70mm≤t<0.90mm and 800mm≤w≤1250mm, the tensile elongation is 0.5%~1.4%;

[0044] When 0.90mm≤t<1.10mm and 800≤w≤1250mm, the tensile elongation is 0.6%~1.6%;

[0045] When 1.10mm≤t<1.50mm and 800mm≤w≤1250mm, the tensile elongation is 0.7%~1.6%;

[0046] When 1.50mm≤t≤2.50mm and 800mm≤w≤1250mm, the tensile elongation is 0.8%~1.6%;

[0047] When 0.70mm≤t<0.90mm and 1250mm<w≤1870mm, the tensile elongation is 0.5%~1.8%;

[0048] When 0.90mm≤t<1.10mm and 1250mm<w≤1870mm, the tensile elongation is 0.6%~1.8%;

[0049] When 1.10mm ≤ t < 1.50mm and 1250mm < w ≤ 1870mm, the tensile elongation is 0.7% to 1.8%.

[0050] When 1.50mm≤t≤2.50mm and 1250mm<w≤1870mm, the tensile elongation is 0.8%~2.0%.

[0051] The aforementioned molten iron pretreatment involves using desulfurization powder and magnesium powder to desulfurize the molten iron to S≤0.015%, with slag thickness≤20mm and molten iron outlet temperature of 1240~1450℃;

[0052] The converter smelting process involves first adding refined scrap steel to an oxygen top and bottom blowing converter, then adding molten iron, with a tapping temperature of 1630–1670°C.

[0053] The LF refining process is as follows: slag thickness 50-100mm, net clearance 250-700mm, processing time ≥30min, and final oxygen content 300-600ppm.

[0054] The continuous casting process involves controlling nitrogen absorption at the sprue, with an increase of ≤5ppm; maintaining a constant casting speed of 1.0–1.6 m / min during the casting process; and employing light reduction of 3–8 mm.

[0055] The slab heating process involves heating the slab to 1150–1200°C in a walking beam furnace and holding it at that temperature for 2–3 hours.

[0056] The hot continuous rolling process is as follows: the roughing rolling adopts three passes of R1 and three or five passes of R2, and the final rolling temperature is 850-900℃;

[0057] The laminar flow cooling and winding process is as follows: the cooling rate is 20-30℃ / s, and the winding is performed after cooling to 500-550℃;

[0058] The pickling process involves a tensile elongation of 1.4–1.8% and a pickling speed of 60–270 m / min.

[0059] The cold rolling process involves a reduction rate of 55% to 75%.

[0060] Compared with the prior art, the beneficial effects of the present invention are:

[0061] 1. This invention achieves synergistic strengthening of fine grains and precipitation strengthening through V-Ti composite microalloying, hot rolling and cold rolling process control. The ferrite grain size is 2-5μm and the (Ti,V)C precipitate size is 20-30nm, so that the yield strength of the steel is ≥420MPa while the elongation is maintained above 20%.

[0062] 2. The process of this invention has high stability. Slow cooling has no significant impact on performance over a wide temperature range. Only precise control of heating and holding temperatures is required to promote recrystallization while inhibiting premature precipitation of Ti and V. Holding in the equilibrium section after rapid cooling can fix the size of the precipitates and stabilize mechanical properties.

[0063] 3. In the zinc plating process, the present invention adds Mg to the zinc bath to improve the toughness and adhesion of the coating. The lower Al content reduces the raw material cost while ensuring performance. It can form a continuous and uniform inhibition layer on the steel substrate surface and improve the adhesion of the coating.

[0064] 4. By controlling the elongation rate of the strip through finishing and tension straightening for width and thickness specifications, the flatness of the strip can be improved and stable mechanical properties can be obtained, making it suitable for the stamping of subsequent parts and ensuring good product quality. Attached Figure Description

[0065] Figure 1 This is a metallographic diagram of a 420MPa grade galvanized low-alloy high-strength steel plate.

[0066] Figure 2 This is a diagram of the inhibition layer morphology of a 420MPa grade galvanized low-alloy high-strength steel plate. Detailed Implementation

[0067] The present invention will now be described in detail with reference to the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0068] Example:

[0069] 420MPa grade galvanized low-alloy high-strength steel, see Figure 1 , Figure 2 The finished product has a microstructure consisting of ferrite and a small amount of pearlite. The ferrite area percentage content is 90-95%, and the ferrite grain size is 2-5 μm. The (Ti,V)C precipitate size is 20-30 nm, the yield strength is ≥420 MPa, and the elongation is ≥20%.

[0070] The production method of 420MPa grade galvanized low-alloy high-strength steel includes hot metal pretreatment, converter smelting, LF refining, continuous casting, slab heating, hot continuous rolling, laminar flow cooling and coiling, pickling, cold rolling, continuous annealing and hot-dip galvanizing, finishing and tension leveling. The hot metal pretreatment parameters for each embodiment are shown in Table 1, the converter smelting parameters 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 slab heating parameters, hot continuous rolling parameters, laminar flow cooling and coiling parameters are shown in Table 5, the pickling parameters and cold rolling parameters are shown in Table 6, the continuous annealing parameters are shown in Table 7, the hot-dip galvanizing parameters are shown in Table 8, the finishing and tension leveling parameters are shown in Table 9, and the mechanical property test results of the finished steel plate are shown in Table 10.

[0071] Table 1. Pretreatment parameters of molten iron for each embodiment.

[0072]

[0073] Table 2 shows the converter smelting parameters and LF refining parameters for each embodiment.

[0074]

[0075] Table 3. Continuous casting parameters for each embodiment.

[0076]

[0077] Table 4 Chemical composition (wt%) of each example.

[0078]

[0079] Table 5 shows the slab heating parameters, hot rolling parameters, laminar cooling and coiling parameters for each embodiment.

[0080]

[0081] Table 6 shows the pickling and cold rolling parameters for each embodiment.

[0082]

[0083] Table 7 shows the continuous annealing parameters for each embodiment.

[0084]

[0085] Table 8. Hot-dip galvanizing parameters for each embodiment.

[0086]

[0087] Table 9 shows the smoothing and straightening parameters for each embodiment.

[0088]

[0089] Table 10 shows the test results of the mechanical properties of the finished steel plates in each embodiment.

[0090]

[0091] Typical tissue and inhibition layer morphologies are as follows: Figure 1 and Figure 2 As shown, by Figure 1 and Figure 2 It can be seen that the ferrite grain size in the microstructure is about 3 μm, the (Ti,V)C precipitate size is about 25 nm, and they are evenly distributed. The inhibition layer is completely covered, the coating has no exposed spots, the coating has excellent adhesion, and the strip has good flatness.

Claims

1. A 420MPa grade galvanized low-alloy high-strength steel, characterized in that, The composition of the steel, by mass percentage, is as follows: C: 0.06%~0.08%, Si: 0.05%~0.08%, Mn: 0.95%~1.15%, P≤0.025%, S≤0.010%, Al: 0.015%~0.060%, O≤0.0060%, N≤0.0060%, V: 0.01%~0.03%, Ti: 0.01%~0.03%, balance being Fe and unavoidable impurities; The microstructure of the steel is ferrite and a small amount of pearlite, with the ferrite area percentage content being 90-95% and the ferrite grain size being 2-5 μm; the (Ti,V)C precipitate phase size is 20-30 nm. The steel has a yield strength ≥420MPa and an elongation of ≥20%. The production method of the 420MPa grade galvanized low-alloy high-strength steel includes hot metal pretreatment, converter smelting, LF refining, continuous casting, slab heating, hot continuous rolling, laminar flow cooling and coiling, pickling, cold rolling, continuous annealing and hot-dip galvanizing, finishing and tension leveling, wherein: The continuous annealing and hot-dip galvanizing process described above: Heating and holding section: The strip steel is heated to 780-800℃ at a rate of 2-5℃ / s and held for 50-140s; Slow cooling section: After heating and heat preservation, the strip steel is cooled to 600-750℃ at a rate of 2-8℃ / s; Rapid cooling section: After the slow cooling is completed, the strip steel is cooled to 440-500℃ at a rate of 10-35℃ / s; Equalization section: After rapid cooling, the strip is held at 455-465℃ for 30-80 seconds; Hot-dip galvanizing: The zinc bath contains 0.1% to 0.5% Mg by mass and 0.15% to 0.18% Al by mass, the zinc bath temperature is 455 to 465℃, and the galvanizing time is 2 to 5 seconds; The aforementioned finishing and straightening: For strip steel produced by continuous annealing and hot-dip galvanizing, the finishing elongation is controlled according to the strip steel thickness t. When 0.40mm ≤ t < 0.50mm, the finishing elongation is 0.6% to 0.8%; When 0.50mm ≤ t < 0.70mm, the finishing elongation is 0.8%~1.0%; When 0.70mm ≤ t < 0.90mm, the finishing elongation is 1.0% to 1.2%; When 0.90mm ≤ t < 1.10mm, the finishing elongation is 1.2% to 1.4%; When 1.10mm ≤ t < 1.50mm, the finishing elongation is 1.4% to 1.6%; When 1.50mm≤t≤2.50mm, the finishing elongation is 1.5%~1.7%; After finishing, the strip is tension-straightened, and the elongation rate is controlled according to the strip thickness t and width w. When 0.40mm≤t<0.50mm and 800mm≤w≤1250mm, the tensile elongation is 0.3%~0.8%; When 0.50mm≤t<0.70mm and 1250mm<w≤1600mm, the tensile elongation is 0.4%~1.2%; When 0.70mm≤t<0.90mm and 800mm≤w≤1250mm, the tensile elongation is 0.5%~1.4%; When 0.90mm≤t<1.10mm and 800≤w≤1250mm, the tensile elongation is 0.6%~1.6%; When 1.10mm≤t<1.50mm and 800mm≤w≤1250mm, the tensile elongation is 0.7%~1.6%; When 1.50mm≤t≤2.50mm and 800mm≤w≤1250mm, the tensile elongation is 0.8%~1.6%; When 0.70mm≤t<0.90mm and 1250mm<w≤1870mm, the tensile elongation is 0.5%~1.8%; When 0.90mm≤t<1.10mm and 1250mm<w≤1870mm, the tensile elongation is 0.6%~1.8%; When 1.10mm ≤ t < 1.50mm and 1250mm < w ≤ 1870mm, the tensile elongation is 0.7% to 1.8%. When 1.50mm≤t≤2.50mm and 1250mm<w≤1870mm, the tensile elongation is 0.8%~2.0%.

2. The 420MPa grade galvanized low-alloy high-strength steel according to claim 1, characterized in that, The aforementioned molten iron pretreatment involves using desulfurization powder and magnesium powder to desulfurize the molten iron to S≤0.015%, with slag thickness≤20mm and molten iron outlet temperature of 1240~1450℃; The converter smelting process involves first adding refined scrap steel to an oxygen top and bottom blowing converter, then adding molten iron, with a tapping temperature of 1630–1670°C. The LF refining process is as follows: slag thickness 50-100mm, net clearance 250-700mm, processing time ≥30min, and final oxygen content 300-600ppm.

3. The 420MPa grade galvanized low-alloy high-strength steel according to claim 1, characterized in that, The continuous casting process involves controlling nitrogen absorption at the sprue, with an increase of ≤5ppm; maintaining a constant casting speed of 1.0–1.6 m / min during the casting process; and employing light reduction of 3–8 mm. The slab heating process involves heating the slab to 1150–1200°C in a walking beam furnace and holding it at that temperature for 2–3 hours. The hot continuous rolling process is as follows: the roughing rolling adopts three passes of R1 and three or five passes of R2, and the final rolling temperature is 850-900℃; The laminar flow cooling and winding process is as follows: the cooling rate is 20-30℃ / s, and the winding is performed after cooling to 500-550℃; The pickling process involves a tensile elongation of 1.4–1.8% and a pickling speed of 60–270 m / min. The cold rolling process involves a reduction rate of 55% to 75%.

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