High-performance cold-rolled interstitial-free steel HC180Y and preparation method thereof

Abstract

The invention belongs to the field of metal materials, and particularly relates to high-performance cold-rolled interstitial-free steel HC180Y and a preparation method of the high-performance cold-rolled interstitial-free steel HC180Y. 0.01% to 0.05% of Si; 0.35% to 0.40% of Mn; p is less than or equal to 0.02%; s is less than or equal to 0.01%; 0.01% to 0.02% of V; b: 0.005% to 0.01%; 0.01% to 0.015% of Nb; and the balance of Fe and inevitable impurities. The method has the advantages that the comprehensive performance such as strength, toughness, forming performance and corrosion resistance of the steel is remarkably improved, meanwhile, production energy consumption is reduced, production efficiency is improved, and application of the steel to the fields such as high-end automobile manufacturing, aerospace and precision machinery which have strict requirements for material performance is expanded.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, specifically relating to a method for preparing high-performance cold-rolled interstitial steel HC180Y. Background Technology

[0002] In traditional processes, HC180 steel is mainly prepared through single titanium (Ti) microalloying and conventional hot rolling, cold rolling, and annealing. For example, patent application CN202180011706.6 discloses an ultra-low carbon interstitial atomless steel, employing an ultra-low carbon (C≤0.005%) and single Ti (≤0.10%) alloying design. While this improves deep-drawing performance to some extent, it suffers from the following problems: ① Single composition design: Relying solely on Ti solid solution strengthening, it is limited by the solid solubility of Ti, making it difficult to achieve a synergistic improvement in both strength and toughness. ② Poor billet quality: Low equiaxed crystal ratio (≤60%) and severe central segregation (level 1.5-2.0) lead to uneven microstructure during subsequent rolling, affecting formability and mechanical properties. ③ Coarse grains: Ferrite grain size reaches 10-15μm, limiting further improvements in strength and toughness. ④ Insufficient molding performance: Although the plastic strain ratio (r value) and strain hardening index (n value) are at a certain level (r≥2.0, n≥0.22), they fluctuate greatly and are difficult to meet the high-precision molding requirements of complex parts.

[0003] Patent application number CN201310226162.2 discloses a cold-rolled steel sheet with interstitial atomic structure and its production method. It attempts to optimize microstructure and properties by adjusting the final hot rolling temperature (900-950℃) and cooling process, but fails to quantify the cooling rate and lacks advanced continuous casting control methods, resulting in insufficient optimization of billet quality and limited grain refinement. Furthermore, this technology does not incorporate advanced continuous casting techniques such as electromagnetic stirring and dynamic light reduction, making it difficult to effectively solve the problems of center segregation and grain coarsening. Similarly, patent application number CN201910208564.7 discloses a Ti microalloyed interstitial atomic steel and its manufacturing method. While using single Ti microalloying simplifies the process, it is limited by the solid solution strengthening effect of Ti, making it difficult to overcome the bottleneck of balancing strength and toughness. Its wide hot rolling temperature range (1050-1250℃) and lack of refined annealing process lead to coarse microstructure control and poor performance consistency.

[0004] Under traditional processes, the strength improvement of HC180Y is limited: the yield strength is typically below 280MPa, and the tensile strength is less than 450MPa, failing to meet the high strength requirements of high-end structural components. Simultaneously, its forming performance indicators (such as r-value and n-value) are unstable, making it prone to defects such as cracking and wrinkling during complex cold forming processes. Furthermore, traditional processes are energy-intensive (approximately 600kWh per ton of steel for hot rolling and approximately 440kWh for cold rolling and annealing), resulting in low production efficiency and insufficient corrosion resistance (only 200-300 hours of red rust time in neutral salt spray tests), limiting its application in humid and corrosive environments.

[0005] Existing technologies face challenges in the development of high-performance HC180Y steel, such as limited composition design, poor billet quality, coarse grains, and low energy efficiency. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a high-performance cold-rolled interstitial steel HC180Y and its preparation method. Through composition optimization and innovation of the entire process of continuous casting-hot rolling-cold rolling, the strength, toughness, formability and corrosion resistance of the steel are synergistically improved, while reducing production energy consumption, so as to meet the requirements for the comprehensive performance of the material.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A high-performance cold-rolled interstitial steel HC180Y has the following chemical composition by mass percentage: C: 0.0010%~0.0015%; Si: 0.01%~0.05%; Mn: 0.35%~0.40%; P≤0.02%; S≤0.01%; V: 0.01%~0.02%; B: 0.005%~0.01%; Nb: 0.01%~0.015%; the remainder is Fe and unavoidable impurities.

[0008] The aforementioned interstitial atomic steel HC180Y has a yield strength of 280–320 MPa and a tensile strength of 450–500 MPa; a plastic strain ratio r of 1.8–2.2 and a strain hardening index n of 0.22–0.25, meeting the cold working requirements of complex parts; a neutral salt spray test red rust time of 600–850 h, expanding its application scenarios in harsh environments; and a metallographic structure consisting of a single ferrite grain with a grain size of 5–8 μm, an equiaxed grain ratio ≥85%, and a center segregation level ≤1.0.

[0009] This invention, based on the traditional chemical composition of HC180Y steel, strictly controls the carbon content (below 0.0015%) to reduce the adverse effects of interstitial atoms on the steel's properties. The addition of trace alloying elements V, B, and Nb achieves grain refinement and precipitation strengthening through microalloying, thereby improving the steel's strength and toughness. Simultaneously, the strict control of impurity elements phosphorus (P) and sulfur (S) reduces the negative impact of impurities on the steel's properties.

[0010] The rationale for the chemical composition design of the high-performance cold-rolled interstitial steel HC180Y described in this invention is as follows: By strictly controlling the carbon content to an extremely low carbon level of less than 0.0015%, the adverse effects of interstitial atoms on the steel's properties (such as strain aging and reduced toughness) are almost eliminated, laying the foundation for subsequent performance improvements.

[0011] Addition of trace alloying elements: V: By forming fine carbonitrides, grain growth is inhibited and precipitation strengthening is achieved, thereby improving strength and toughness.

[0012] B: Improves the hardenability of steel, enhances grain boundary strength, and optimizes work hardening ability.

[0013] Nb combines with carbon and nitrogen to form a dispersed precipitate phase, which refines the austenite grains and improves the strength and formability of steel.

[0014] Strictly limit impurity elements: Controlling the content of impurities such as P and S reduces the risk of grain boundary segregation and brittle fracture, thereby improving the purity and corrosion resistance of steel.

[0015] A method for preparing high-performance cold-rolled interstitial steel HC180Y includes smelting, continuous casting, hot rolling, cold rolling, continuous annealing, and leveling, wherein: Smelting: A converter smelting + RH refining dual process is adopted. The RH refining vacuum degree is ≤67Pa, the processing time is ≥15min, and the argon blowing flow rate is 200~300L / min (standard state) to ensure that the molten steel [H]≤0.0003% and [N]≤0.0070%; Continuous casting: High-frequency pulsed electromagnetic stirring is employed, with a pulse frequency of 10–20 Hz and a pulse current of 50–100 A; adaptive dynamic light reduction technology is used, with the reduction amount controlled between 2 and 6 mm; the reduction rate is ≤1 mm / s, reducing the carbon segregation index at the center of the billet to ≤1.05. This effectively reduces center segregation and porosity defects in the billet, improves internal quality uniformity, and provides high-quality billets for subsequent rolling processes.

[0016] Hot-rolled: The furnace exit temperature is 1180–1230℃ to ensure full solid solution of alloying elements, creating conditions for subsequent strengthening mechanisms. In the rough rolling stage, low-temperature, high-reduction rolling is employed at 1000–1050℃, with a total reduction of 40%–50%, promoting full deformation of austenite grains, increasing dislocation density, and providing impetus for grain refinement. In the finish rolling stage, the final rolling temperature is 880–920℃, using low-reduction, multi-pass rolling (5–7 passes) with a total reduction of 10%–20%; precise control of plate shape and further grain refinement are achieved. Online air-mist cooling is used during hot rolling at a rate of 10–30℃ / s to inhibit grain growth and improve the strength and toughness of the steel; the cooling rate is dynamically adjusted according to the steel thickness and performance requirements: for steel plate thickness ≤ 1.5mm, the cooling rate is 20–30℃ / s; for steel plate thickness > 1.5mm, the cooling rate is 10–20℃ / s. Air mist cooling nozzle parameters: A fan-shaped nozzle is used, with a nozzle spacing of 150–250 mm and a spray angle of 42°–50°. The cooling medium is compressed air + deionized water, with a water pressure of 5–8 MPa and an air pressure of 0.3–0.5 MPa. Function: To inhibit grain growth during hot rolling, retain a high dislocation density structure, and simultaneously form a favorable phase transformation structure through rapid cooling, thereby improving strength and toughness.

[0017] Cold rolling: The total cold rolling reduction is 75%–85%, using multi-roll mills (such as six-roll or eight-roll mills) and ester-based rolling oil lubrication technology. The thickness accuracy of the sheet is within ±0.02mm, while improving surface quality. Continuous annealing employs a two-stage heating and three-stage cooling process. The material is rapidly heated to 780–820°C at a rate of 100–150°C / s for recrystallization annealing, and held for 30–60s to quickly eliminate cold rolling work hardening and form uniform recrystallized grains. Then, it is cooled to 650–700°C at a rate of 10–20°C / s for tempering, and held for 120–180s to promote uniform precipitation of the second phase and further optimize the balance between strength and formability. Finally, it is cooled to room temperature at a rate of 5–10°C / s to reduce residual stress and improve dimensional stability and resistance to deformation.

[0018] Compared with the prior art, the beneficial effects of the present invention are: This invention significantly improves the overall performance of steel, including strength, toughness, formability, and corrosion resistance, while reducing production energy consumption and increasing production efficiency, expanding its application in demanding fields such as high-end automobile manufacturing, aerospace, and precision machinery. It employs multi-element microalloying elements such as V, Nb, and B to form a composite strengthening mechanism, overcoming the performance bottleneck of single Ti alloying. The yield strength of the steel plate reaches 280–320 MPa, and the tensile strength reaches 450–500 MPa, meeting the high-strength requirements of high-end fields. The elongation is maintained at 30%–35%, balancing strength and toughness and resolving traditional contradictions. The plastic strain ratio (r value) is 1.8–2.2, and the strain hardening index (n value) is 0.22–0.25, significantly improving cold forming performance and adapting to high-precision machining of complex parts. The time for red rust to appear in the neutral salt spray test is significantly prolonged, and the optimized composition and microstructure significantly enhance durability.

[0019] This invention improves the equiaxed grain ratio (≥85%) and reduces center segregation (level ≤1.0) by employing high-frequency pulsed electromagnetic stirring and adaptive dynamic light pressing techniques. Combined with segmented temperature-controlled rolling, online atomized cooling, and multi-stage annealing, it precisely controls grain size (ferrite grains 5-8μm) and phase transformation microstructure. This solves the problem of balancing strength, toughness, formability, and production efficiency in traditional processes, providing a new direction for the research and application of high-performance cold-rolled steel. The steel sheets can be used in critical automotive safety / appearance components, high-precision mechanical parts, aerospace secondary structural components, and lightweight components for rail transportation. Attached Figure Description

[0020] Figure 1 This is a 200x metallographic microstructure image of Example 1. Detailed Implementation

[0021] 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.

[0022] Example 1: 1. Raw material preparation: High-quality steelmaking raw materials are selected. The smelting process employs converter smelting + RH refining, with an RH vacuum of 50 Pa and an argon blowing flow rate of 250 L / min. After refining, the chemical composition of the steel, by mass percentage, is as follows: C: 0.0012%, Si: 0.03%, Mn: 0.35%, P: 0.018%, S: 0.008%, V: 0.012%, B: 0.006%, Nb: 0.01%, with the remainder being Fe and unavoidable impurities.

[0023] 2. Continuous casting: High-frequency pulsed electromagnetic stirring is activated at a frequency of 12Hz and a pulse current of 60A. The adaptive dynamic light reduction system sets the initial reduction to 3mm based on monitoring data and makes real-time fine adjustments to obtain the continuously cast billet.

[0024] 3. Hot rolling: Furnace exit temperature 1200℃, roughing temperature 1020℃, reduction rate 45%; finishing temperature 900℃, reduction rate 15%. Steel plate thickness 1.2mm, online atomized cooling rate controlled at 22℃ / s, nozzle spacing 200mm, spray angle 45°, water pressure 6MPa, air pressure 0.4MPa, coiling temperature 700℃.

[0025] 4. Cold rolling and annealing: The cold rolling reduction rate is 80%, using a high-precision six-roll mill. Continuous annealing is first heated to 800℃ at a rate of 120℃ / s and held for 40s; then cooled to 680℃ at a rate of 15℃ / s and held for 150s; finally cooled to room temperature at a rate of 7℃ / s.

[0026] Performance testing: Yield strength 292MPa, tensile strength 475MPa, elongation 33.5%, r value 2.1, n value 0.23, and red rust appeared in 720h of neutral salt spray test.

[0027] Finished product inspection results: Mechanical properties: Sample number Yield strength MPa Tensile strength (MPa) <![CDATA[Elongation A 80 %]]> r value n value Salt spray red rust time (h) HC180Y-1# 292 475 33.5 2.1 0.23 720 HC180Y-2# 286 470 34.0 2.2 0.23 800 Corrosion resistance test standard: The neutral salt spray test follows GB / T10125-2021, with a sample size of 100mm×50mm×1mm, a NaCl salt solution concentration of 5%, a pH value of 6.5~7.2, and a test cycle of 1000h.

[0028] Metallographic structure: Transverse specimen, perpendicular to the rolling direction. Figure 1This is a 200x metallographic microstructure image of the high-performance cold-rolled interstitial steel HC180Y from Example 1. The microstructure exhibits uniform and fine single ferrite grains. This is due to the high-frequency pulsed electromagnetic stirring (10-20 Hz, 50-100 A) during the continuous casting stage, which breaks up dendrites to form equiaxed grains. Combined with segmented temperature control during hot rolling (880-920°C for finishing) and air mist cooling (10-30°C / s), grain growth is suppressed. The carbonitrides formed by microalloying elements V (0.01%-0.02%) and Nb (0.01%-0.015%) are distributed as fine and dispersed particles at grain boundaries or in the matrix (faint dot-like aggregates can be seen under an optical microscope). The ultra-low carbon content (≤0.0015%) ensures that the microstructure is free of pearlite or cementite, consisting only of single ferrite. After cold rolling and annealing, it exhibits complete recrystallization characteristics with no obvious work hardening traces. Metallographic transmission electron microscopy (TEM) analysis revealed a significant refinement in the size of precipitates in the steel, primarily distributed in the range of 10–50 nm, and exhibiting uniform dispersion. The clean grain boundaries (impurities P ≤ 0.02%, S ≤ 0.01%) directly explain the breakthroughs in formability (r value 1.8–2.2, n value 0.22–0.25) and corrosion resistance (salt spray red rust time 600–850 h). The fine-grained and dispersed precipitation characteristics observed under 200x metallographic magnification provide direct experimental evidence for the innovative approach of "ultra-low carbon microalloying + full-process control."

[0029] Example 2: 1. Raw material preparation: High-quality steelmaking raw materials are selected. The smelting process employs converter smelting + RH refining, with an RH vacuum of 50 Pa and an argon blowing flow rate of 250 L / min. After refining, the chemical composition of the steel, by mass percentage, is as follows: C: 0.001%, Si: 0.04%, Mn: 0.38%, P: 0.015%, S: 0.006%, V: 0.015%, B: 0.008%, Nb: 0.012%, with the remainder being Fe and unavoidable impurities.

[0030] 2. Continuous casting: High-frequency pulsed electromagnetic stirring frequency 15Hz, pulse current 70A. The adaptive dynamic light reduction system sets the initial reduction to 4mm based on monitoring data and makes real-time fine adjustments.

[0031] 3. Hot rolling: furnace exit temperature 1220℃, roughing temperature 1030℃, reduction rate 48%; finishing temperature 910℃, reduction rate 18%. Cooling rate 20℃ / s, nozzle spacing 220mm, spray angle 48°, water pressure 7MPa, air pressure 0.45MPa, coiling temperature 710℃.

[0032] 4. Cold rolling and annealing: Cold rolling reduction rate of 82%, rolled on an eight-roll mill. Annealing: heating to 810℃ at a rate of 130℃ / s, holding for 35s; cooling to 690℃ at a rate of 18℃ / s, holding for 130s; cooling to room temperature at a rate of 8℃ / s.

[0033] Performance testing: Yield strength 305MPa, tensile strength 475MPa, elongation 33%, r value 2.0, n value 0.24, red rust appeared in neutral salt spray test in 800h.

[0034] Example 3: 1. Raw material preparation: High-quality steelmaking raw materials are selected. The smelting process employs converter smelting + RH refining, with an RH vacuum of 50 Pa and an argon blowing flow rate of 250 L / min. After refining, the chemical composition of the steel, by mass percentage, is as follows: C: 0.0010%, Si: 0.04%, Mn: 0.4%, P: 0.012%, S: 0.005%, V: 0.018%, B: 0.01%, Nb: 0.015%, with the remainder being Fe and unavoidable impurities.

[0035] 2. Continuous casting: High-frequency pulsed electromagnetic stirring frequency 18Hz, pulse current 80A. The adaptive dynamic light reduction system sets the initial reduction to 5mm based on monitoring data and makes real-time fine adjustments.

[0036] 3. Hot rolling: furnace exit temperature 1230℃, rough rolling temperature 1040℃, reduction rate 50%; finish rolling temperature 920℃, reduction rate 20%. Cooling rate 18℃ / s (steel plate thickness 2.0mm), nozzle spacing 250mm, spray angle 50°, water pressure 8MPa, air pressure 0.5MPa, coiling temperature 720℃.

[0037] 4. Cold rolling and annealing: Cold rolling reduction rate of 85%, rolled on an eight-roll mill. Annealing: heating to 820℃ at a rate of 140℃ / s, holding for 30s; cooling to 700℃ at a rate of 20℃ / s, holding for 120s; cooling to room temperature at a rate of 9℃ / s.

[0038] Performance testing: Yield strength 320MPa, tensile strength 490MPa, elongation 34%, r value 2.1, n value 0.25, red rust appeared in neutral salt spray test in 850h.

[0039] Comparative Example 1: 1. Raw material preparation: High-quality steelmaking raw materials are selected. The smelting process employs converter smelting + RH refining, with an RH vacuum of 50 Pa and an argon blowing flow rate of 250 L / min. After refining, the chemical composition of the steel, by mass percentage, is as follows: C: 0.0014%, Si: 0.02%, Mn: 0.32%, P: 0.02%, S: 0.01%, V: 0.01%, B: 0.005%, Nb: 0.008%, with the remainder being Fe and unavoidable impurities.

[0040] 2. Continuous casting: High-frequency pulsed electromagnetic stirring frequency 10Hz, pulse current 50A. Initial reduction under dynamic light pressure is 2mm, fine-tuned as needed.

[0041] 3. Hot rolling: furnace exit temperature 1180℃, rough rolling temperature 1000℃, reduction rate 40%; finish rolling temperature 880℃, reduction rate 10%. Cooling rate 10℃ / s (steel plate thickness 1.5mm), coiling temperature 680℃.

[0042] 4. Cold rolling and annealing: Cold rolling reduction rate of 75%, rolled on a four-high mill. Annealing: heating to 780℃ at a rate of 100℃ / s, holding for 50s; cooling to 650℃ at a rate of 10℃ / s, holding for 180s; cooling to room temperature at a rate of 5℃ / s.

[0043] Performance testing: Yield strength 270MPa, tensile strength 440MPa, elongation 30%, r value 1.8, n value 0.22, red rust appeared in neutral salt spray test in 600h.

[0044] Examples 1-3 progressively increased the amount of alloying elements, enhanced the electromagnetic stirring intensity and dynamic light reduction during continuous casting, and optimized the hot rolling and cold rolling annealing process parameters. This resulted in a gradual improvement in the steel's strength, toughness, formability, and corrosion resistance, demonstrating the positive correlation between process optimization and performance enhancement. Comparative Example 1, using relatively conservative process parameters, served as a control example, contrasting with the first three examples to highlight the significant effect of the optimized process parameters in improving the overall performance of the steel. Together, these examples validate the scientific validity and effectiveness of the preparation process of this invention, as well as the importance of parameter optimization.

Claims

1. A high-performance cold-rolled interstitial steel HC180Y, characterized in that, The chemical composition of the steel, expressed as a percentage by mass, is as follows: C: 0.0010%~0.0015%; Si: 0.01%~0.05%; Mn: 0.35%~0.40%; P≤0.02%; S≤0.01%; V: 0.01%~0.02%; B: 0.005%~0.01%; Nb: 0.01%~0.015%; the remainder is Fe and unavoidable impurities.

2. The high-performance cold-rolled interstitial steel HC180Y according to claim 1, characterized in that, The interstitial atomic steel HC180Y plate has a yield strength of 280-320 MPa and a tensile strength of 450-500 MPa; a plastic strain ratio r of 1.8-2.2 and a strain hardening index n of 0.22-0.25; a neutral salt spray test red rust time of 600-850 h; an equiaxed crystal ratio ≥85% and a center segregation level ≤1.

0.

3. The high-performance cold-rolled interstitial steel HC180Y according to claim 1, characterized in that, The metallographic structure of the interstitial atom-free steel HC180Y is a single ferrite with a ferrite grain size of 5-8 μm.

4. A method for preparing high-performance cold-rolled interstitial steel HC180Y as described in any one of claims 1-3, characterized in that, This includes smelting, continuous casting, hot rolling, cold rolling, continuous annealing, and leveling, among which: Continuous casting: High-frequency pulse electromagnetic stirring is adopted, with a pulse frequency of 10-20Hz and a pulse current of 50-100A; the reduction is controlled at 2-6mm; the reduction rate is ≤1mm / s; Hot-rolled: The furnace exit temperature is 1180–1230℃; in the roughing stage, low-temperature, high-reduction rolling is used, with a rolling temperature of 1000–1050℃ and a total reduction rate of 40%–50%; in the finishing stage, the final rolling temperature is 880–920℃, with small reduction in 5–7 passes and a total reduction rate of 10%–20%; online air mist cooling is used during hot rolling, with a cooling rate of 10–30℃ / s; Cold rolling: The total cold rolling reduction rate is 75%–85%, and the sheet thickness accuracy is within ±0.02mm; continuous annealing adopts a two-stage heating and three-stage cooling process. Recrystallization annealing is performed by rapidly heating to 780–820°C at a rate of 100–150°C / s and holding for 30–60 s; then tempering is performed by cooling to 650–700°C at a rate of 10–20°C / s and holding for 120–180 s; finally, cooling to room temperature is performed by cooling to 5–10°C / s.

5. The method for preparing a high-performance cold-rolled interstitial steel HC180Y according to claim 4, characterized in that, In the hot rolling process, the cooling rate is dynamically adjusted according to the thickness of the steel plate. When the thickness of the steel plate is ≤1.5mm, the cooling rate is 20~30℃ / s; when the thickness of the steel plate is >1.5mm, the cooling rate is 10~20℃ / s.

6. The method for preparing a high-performance cold-rolled interstitial steel HC180Y according to claim 4, characterized in that, In the hot rolling process, the online aerosol cooling uses fan-shaped nozzles with a nozzle spacing of 150-250 mm and a spray angle of 42°-50°. The cooling medium is compressed air + deionized water with a water pressure of 5-8 MPa and an air pressure of 0.3-0.5 MPa.

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