Nitrogen-dominated bake-hardened ultralow-carbon steel for automobile outer plate and manufacturing method of nitrogen-dominated bake-hardened ultralow-carbon steel

Abstract

The invention relates to nitrogen-dominated bake-hardened ultra-low carbon steel for an automobile outer plate and a manufacturing method of the nitrogen-dominated bake-hardened ultra-low carbon steel. The steel comprises the following chemical components: 0.0005-0.0015% of C; n: 0.003% to 0.006%; ti: 0.005% to 0.015%; 0.03% to 0.06% of Al; b: 0.0002% to 0.0005%; 0.001% to 0.003% of Nb; 0.25% to 0.5% of Mn; 0.03% to 0.05% of P; and the balance of Fe and impurities. The composition design that nitrogen serves as a main bake hardening element and carbon serves as an auxiliary strengthening element is adopted, and N-Al-Ti-B multi-element synergy is combined, so that balance of bake hardening and ageing resistance is achieved, and the problem of performance fluctuation of the carbon-based bake hardening steel is solved; by utilizing the characteristics that nitrogen is easy to control in the steelmaking production process and no nitrogen increasing risk exists in the follow-up process, accurate control over the nitrogen content is achieved, and fluctuation of the BH2 value is reduced.

Description

Technical Field

[0001] This invention relates to the field of automotive steel production technology, and in particular to an ultra-low carbon automotive outer panel steel with nitrogen-dominant bake hardening and its manufacturing method. Background Technology

[0002] Ultra-low carbon automotive steel has a carbon content of less than 0.005%, or 50 ppm, far lower than traditional low-carbon steel (typically with a carbon content of 0.05%–0.25%). It belongs to the category of advanced high-strength steels, mainly including interstitial steel, bake-hardening steel, dual-phase steel, transformation-induced plasticity steel, quenched ductile steel, and medium-manganese steel. Ultra-low carbon automotive steel possesses excellent deep-drawing properties, superior ductility and toughness, good weldability, and a bake-hardening effect, making it a primary material for modern automotive body-in-white. It is mainly used to manufacture external body panels (such as doors), internal structural components (such as chassis components), and safety structural components (such as A-pillars).

[0003] Baking hardening is a special material phenomenon that can be simply understood as the automatic increase in strength of a steel after undergoing a low-temperature heating process. This characteristic perfectly meets the needs of automotive manufacturing processes. The initial strength of bake-hardened steel is not necessarily very high; its high strength is achieved during user use (through the manufacturing process). It solves the problem of difficult forming of traditional high-strength steel and is particularly suitable for manufacturing automotive outer panels and some inner panels with high requirements for anti-concavity and formability. It achieves a balance between excellent formability and high service strength, making it one of the key materials for lightweighting automobiles and improving vehicle body quality.

[0004] H220BD is a classic and well-balanced grade of bake-hardening steel for automobiles. It achieves a good balance between strength, formability, and final dent resistance, making it a primary material for manufacturing high-end exterior body panels for mainstream passenger vehicles. To ensure stable application, the industry generally adopts an ultra-low carbon design approach, adding stabilizing elements such as Ti, Nb, and V to fix the dissolved carbon content in the steel, thereby achieving a balance between bake-hardening characteristics and aging resistance. However, the carbon gain problem in the current steelmaking process is difficult to solve through composition design and process optimization. Carbon atoms are highly susceptible to fluctuations during refining and continuous casting, and uncontrollable carbon gain occurs in subsequent processes, leading to large fluctuations in the bake-hardening value (BH2) and unstable aging resistance. This is an inherent defect of carbon-based bake-hardening steel, and new solutions are urgently needed to address this issue.

[0005] Chinese patent application CN105603158A discloses a method for controlling the carbon content in ultra-low carbon bake-hardening steel. After the RH (heated water) system arrives at the station, the carbon, oxygen, and temperature are adjusted to meet the following conditions: at 1610℃~1620℃, 0.015wt%≤C<0.025wt%, 0.045wt%≤O<0.055wt%; at 1620℃~1630℃, 0.025wt%≤C<0.035wt%, 0.055wt%≤O<0.065wt%; at 1630℃~1640℃, 0.035wt%≤C<0.045wt%, 0.065wt%≤O<0.075wt%. This method can control the carbon content to within 0.0020wt%≤C≤0.0030wt%, with a carbon content accuracy of ±0.0003wt%, offering advantages such as high control precision and simple operation. It focuses on the solution carbon control of ultra-low carbon bake-hardening steel. It achieves carbon control through complex processes such as multi-stage decarburization and precise carbon addition in RH vacuum refining. However, it suffers from narrow process window and poor production stability, resulting in large performance fluctuations.

[0006] Chinese patent application CN119242883A discloses a "smelting system and method for ultra-low carbon and nitrogen steel," and Chinese patent application CN118345300A discloses a "smelting method and ultra-low carbon and ultra-low nitrogen steel," both involving ultra-low nitrogen control, but the smelting objective is to reduce gaseous impurities, without utilizing the strengthening effect of nitrogen. Chinese invention patent CN115927788B discloses "an ultra-low carbon and nitrogen steel and its smelting method," which uses electric furnace powder injection denitrification (injecting inert gas + metal oxide powder) combined with RH deep vacuum treatment (vacuum degree ≤0.3mbar) to reduce nitrogen content, belonging to a "passive denitrification" process.

[0007] Chinese invention patent CN116219323B discloses a "hot-dip galvanized alloyed H220BD steel sheet with uniform porosity and low surface waviness and its preparation method." The H220BD steel sheet composition has C≤0.003% and N≤0.005%, but it does not establish a correlation between nitrogen and bake hardening, still relying on the traditional solid solution carbon mechanism (requiring C-Nb×12 / 93=10~18ppm). Surface waviness is mainly improved through modifications to the hot rolling / galvanizing process (e.g., alloying temperature 450~550℃).

[0008] In summary, the current production of H220BD automotive bake-hardening steel has the following shortcomings: (1) The carbon content is difficult to control: the vacuum decarbonization kinetics are unstable and the carbon increase problem in the production line cannot be avoided, which leads to fluctuations in carbon content and causes fluctuations in BH2 value and aging resistance.

[0009] (2) Waste of nitrogen function: Existing public literature (such as CN116219323B) mostly restricts the N content, but only uses Ti to fix the residual nitrogen to prevent aging, without exploring the potential role of nitrogen in baking hardening.

[0010] (3) Complex process: Conventional deep decarburization (≤0.6mbar) requires more than 45 minutes, which consumes a lot of energy. In addition, the carbon increase problem makes hot-dip galvanizing production difficult and cannot guarantee the stability of product performance. Summary of the Invention

[0011] This invention provides an ultra-low carbon automotive outer panel steel with nitrogen-dominant bake hardening and its manufacturing method. It adopts a composition design with nitrogen as the dominant bake hardening element and carbon as the auxiliary strengthening element, combined with the N-Al-Ti-B multi-element synergistic fixation and release mechanism, to achieve a balance between bake hardening and anti-aging properties, and solve the problem of performance fluctuation in carbon-based bake hardening steel. Taking advantage of the ease of nitrogen control during steelmaking and the absence of nitrogen increase risk in subsequent processes, it achieves precise control of nitrogen content and reduces BH2 value fluctuation.

[0012] To achieve the above objectives, the present invention employs the following technical solution: A nitrogen-dominant bake-hardening ultra-low carbon steel for automotive exterior panels, wherein the chemical composition of the steel, by mass percentage, is: C: 0.0005%–0.0015%; N: 0.003%–0.006%; Ti: 0.005%–0.015%; Al: 0.03%–0.06%; B: 0.0002%–0.0005%; Nb: 0.001%–0.003%; Mn: 0.25%–0.5%; P: 0.03%–0.05%; with the remainder being Fe and unavoidable impurities; wherein, [N] / [C] = 2.5–5.0, and satisfies the following relationships: -0.10%≤[Al]-3.93×[Ti]≤0.15%, [C]-[Nb]×12 / 93≤0.0005%.

[0013] The baked hardening value of the finished steel plate is BH2=38±3MPa, and the yield strength deviation after natural aging for 180 days is ≤5MPa; the surface waviness Wsa≤0.28μm.

[0014] A method for manufacturing ultra-low carbon automotive outer panel steel with nitrogen-dominant bake hardening includes steelmaking, continuous casting, hot rolling, cold rolling, and alloying continuous hot-dip galvanizing processes; the following processes are controlled: 1) Steelmaking: A nitrogen-preserving and decarburizing refining method is adopted, as detailed below: Oxygen retention during steel tapping in converter: The final oxygen content is controlled at 500-600 ppm; RH vacuum light treatment: vacuum degree is 50-80 mbar, cycle time is 8-12 min, carbon content is controlled at 0.0005%-0.0015%, and more than 90% of the original nitrogen is retained; Nitrogen microalloying: Before RH vacuum breaking, manganese nitride was added to perform nitrogen microalloying according to the target nitrogen increment Δ[N]; bottom blowing argon weak stirring, argon flow rate was 30-50 NL / min; 2) Hot rolling: The final rolling temperature is 900-920℃ to ensure that the deformation in the non-recrystallization zone is ≥65%; the coiling temperature is 520-560℃; 3) Alloyed continuous hot-dip galvanizing: The dew point of the heating section is controlled at -25 to -35℃; the slow cooling section is cooled to 520 to 560℃ at a cooling rate of 10 to 15℃ / s; the fast cooling section is cooled to 450 to 480℃ at a cooling rate of 20 to 30℃ / s; the alloying temperature is 490 to 520℃, and the alloying holding time is 15 to 22s.

[0015] During the nitrogen microalloying process, the amount of manganese nitride added is calculated as follows: (Δ[N] × weight of molten steel × 10) / (mass fraction of nitrogen in manganese nitride × nitrogen recovery rate). In this formula, the mass fraction of nitrogen in manganese nitride is 1.5% to 3.0%, and the nitrogen recovery rate is 80% to 95%.

[0016] Compared with the prior art, the beneficial effects of the present invention are: (1) Stable product performance: BH2 = 38 ± 3 MPa (conventional carbon-based steel can only be controlled within ± 11 MPa, such as...). Figure 2 As shown), the natural aging stabilization period is extended to more than 180 days, and the yield strength shift after aging is ≤5MPa (≤15MPa for conventional carbon-based steel, such as...). Figure 3 As shown); the principle of nitrogen-dominated baking hardening is that the diffusion activation energy of nitrogen atoms in α-Fe (76 kJ / mol) is higher than that of carbon (84 kJ / mol), and the migration rate at room temperature is relatively low; however, at baking temperatures (170–190 °C), the migration rate of nitrogen atoms increases significantly, allowing them to quickly migrate to their designated locations and form Cottrell atmospheres (as shown). Figure 1 (As shown). This dual characteristic of low-temperature inertness and high-temperature activity is the basis for achieving both room temperature stability and baking-enhanced properties in this invention.

[0017] (2) Cost reduction: RH refining time is shortened by 40%, and deep vacuum equipment is not required.

[0018] (3) Improved surface quality: Surface waviness Wsa≤0.28μm, which is better than the standard value (≤0.35μm). Attached Figure Description

[0019] Figure 1 This is a schematic diagram of solid solution nitrogen pinning dislocations as described in this invention.

[0020] Figure 2 This is a comparison chart of the BH value fluctuations of nitrogen-based steel and carbon-based steel described in this invention.

[0021] Figure 3 This is a comparison chart of the yield strength changes of nitrogen-based steel and carbon-based steel after 180 days of natural aging, as described in this invention. Detailed Implementation

[0022] The present invention discloses a nitrogen-dominant bake-hardening ultra-low carbon automotive outer panel steel, characterized in that the chemical composition of the steel, by mass percentage, is: C: 0.0005%–0.0015%; N: 0.003%–0.006%; Ti: 0.005%–0.015%; Al: 0.03%–0.06%; B: 0.0002%–0.0005%; Nb: 0.001%–0.003%; Mn: 0.25%–0.5%; P: 0.03%–0.05%; the remainder being Fe and unavoidable impurities; wherein, [N] / [C] = 2.5–5.0, and satisfies the following relationships: -0.10%≤[Al]-3.93×[Ti]≤0.15%, [C]-[Nb]×12 / 93≤0.0005%.

[0023] The reasons for selecting each element in steel and their functions are as follows: Carbon (C): The C content in the steel of this invention is significantly reduced to 0.0005% to 0.0015%, retaining only the carbon content necessary to maintain basic strength, thus eliminating the dominant influence of carbon fluctuations on bake hardening.

[0024] Nitrogen (N): In this invention, the nitrogen content is precisely controlled between 0.003% and 0.006%. As the core solute atom for baking and hardening, its content must be higher than that of carbon, but lower than the aging-sensitive threshold (0.008 wt%).

[0025] Nitrogen-to-carbon ratio ([N] / [C]): A key parameter of this invention, the range is limited to 2.5 to 5.0, to ensure that when the bake hardening value (BH2) is ≥30MPa, the natural aging effect is weaker than that of conventional bake hardening steel.

[0026] Aluminum (Al): The content is limited to 0.03% to 0.06%. It forms AlN pinning points with nitrogen to inhibit excessive coarsening of nitrides during hot rolling. The following relationship must be met: -0.10%≤[Al]-3.93×[Ti]≤0.15% to ensure a reasonable Al / Ti ratio.

[0027] Titanium (Ti): The content is limited to 0.005% to 0.015%. It preferentially combines with nitrogen to form TiN, which can reduce the aging embrittlement caused by free nitrogen.

[0028] Boron (B): The content is limited to 0.0002% to 0.0005%. It blocks the diffusion channels of nitrogen atoms through the grain boundary segregation characteristics, and extends the natural aging stability period to more than 180 days.

[0029] Manganese (Mn): The content is limited to 0.25% to 0.50% (lower than the upper limit of 0.65% for conventional bake-hardening steel) to reduce the promoting effect of Mn on carbon activity and avoid residual carbon interfering with the nitrogen-dominated hardening effect.

[0030] Phosphorus (P): The content is limited to 0.03% to 0.05%. It is a solid solution strengthening element that can compensate for the strength loss due to carbon reduction.

[0031] Niobium (Nb): The content is limited to 0.003% to 0.006%. It is coupled with residual carbon to control the content and meet the condition that [C]-[Nb]×12 / 93≤0.0005%. This reduces the solid solution carbon content and achieves fine grain strengthening, which is beneficial to the improvement of strength.

[0032] The present invention discloses a method for manufacturing ultra-low carbon automotive outer panel steel with nitrogen-dominant bake hardening, comprising steelmaking, continuous casting, hot rolling, cold rolling, and alloying continuous hot-dip galvanizing processes; the following processes are controlled: 1) Steelmaking: A nitrogen-preserving and decarburizing refining method is adopted, as detailed below: Oxygen retention during steel tapping in converter: The final oxygen content is controlled at 500-600 ppm; natural decarburization is achieved through carbon-oxygen reaction, avoiding nitrogen intake caused by forced oxygen blowing.

[0033] RH vacuum light treatment: vacuum degree is 50-80 mbar (traditional deep decarbonization requires ≤1 mbar), cycle time is 8-12 min, carbon content is controlled at 0.0005%-0.0015%, and more than 90% of the original nitrogen is retained; Nitrogen microalloying: Before RH vacuum breaking, manganese nitride (MnN) is added according to the target nitrogen increment Δ[N] for nitrogen microalloying. Bottom blowing argon with weak stirring, argon flow rate is 30-50 NL / min to promote uniform dissolution.

[0034] 2) Hot rolling: The final rolling temperature is 900–920℃ to ensure that the deformation in the non-recrystallization zone is ≥65%; this promotes the precipitation of nano-sized TiN (≤20nm in size), serving as the "seeds" for nitrogen atom segregation during subsequent baking. The coiling temperature is 520–560℃; low-temperature coiling (conventional coiling temperature is 530–620℃) is used to suppress AlN / TiN coarsening. Experiments show that for every 50℃ decrease in coiling temperature, the average size of the nitride decreases by approximately 15nm, and the solid solution nitrogen retention rate increases by 20%.

[0035] 3) Alloyed continuous hot-dip galvanizing: The dew point of the heating section is controlled at -25 to -35℃ (normally -15℃) to reduce the formation of oxide layer and ensure the pathway for nitrogen atoms to diffuse to the surface.

[0036] The cooling strategy involves a zoned cooling section that cools the material at a rate of 10–15 °C / s to 520–560 °C, inducing nitrogen atoms to pre-segregate towards dislocation lines. A rapid cooling section cools the material at a rate of 20–30 °C / s (higher than the conventional 9–25 °C / s) to 450–480 °C to freeze the dissolved nitrogen. The alloying temperature is 490–520 °C, and the alloying holding time is 15–22 s. This aims to promote Fe diffusion in the coating while avoiding excessive nitrogen atom precipitation due to over-tempering.

[0037] The bake hardening value of the finished steel plate is BH2=38±3MPa, and the yield strength deviation after natural aging for 180 days is ≤5MPa; the product waviness Wsa≤0.28μm.

[0038] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0039]

Example 1

[0040] The chemical composition (wt%) of the steel is C: 0.001%, N: 0.0042%, Ti: 0.008%, B: 0.0003%, Al: 0.045%, Nb: 0.004%, where [N] / [C] = 4.2, [Al] - 3.93 × [Ti] = 0.014%, and [C] - [Nb] × 12 / 93 = 0.0005%.

[0041] Main process and parameters: RH light decarburization (vacuum degree 60mbar, 10min cycle) → MnN addition amount is 0.8kg / t steel; hot rolling final rolling temperature 910℃ → coiling temperature 540℃; alloying continuous hot-dip galvanizing rapid cooling rate 25℃ / s → alloying temperature 500℃, holding for 18s.

[0042] Performance: BH2 = 35–40 MPa, Wsa = 0.22–0.28 μm; ΔRp0.2 = 1–5 MPa after 180 days (no yield plateau). Measured performance is shown in Table 1. Table 1 Properties of Finished Steel Plates

Example 2

[0043] 1. The chemical composition of nitrogen-based steel is C: 0.0006%, N: 0.0030%, Ti: 0.010%, B: 0.0004%, Al: 0.050%, Nb: 0.0010%, Mn: 0.35%, P: 0.040%, with the balance being Fe and unavoidable impurities; where [N] / [C] = 0.0030% / 0.0006% = 5.0; Al-Ti balance: [Al] - 3.93 × [Ti] = 0.050% - 0.0393% = 0.0107%; solid solution carbon control: [C] - [Nb] × 12 / 93 = 0.00047%, which meets the design requirements.

[0044] 2. Production process and parameters of nitrogen-based steel: 1) Steelmaking: The oxygen content at the converter endpoint is 550ppm; RH vacuum light treatment, vacuum degree is 70mbar, circulation time is 9min; carbon content is controlled to 0.0006%, retaining more than 90% of the original nitrogen; before RH vacuum breaking, add 240kg of manganese nitride (220 tons of molten steel, the mass fraction of nitrogen in manganese nitride is 3%, and the nitrogen recovery rate is 92%) according to the target nitrogen increment (Δ[N]=0.0030%), bottom blow argon stirring, argon flow rate is 50NL / min.

[0045] 3) Alloyed continuous hot-dip galvanizing: the dew point of the heating section is -30℃; the slow cooling section cools to 540℃ at a cooling rate of 12℃ / s; the fast cooling section cools to 460℃ at a cooling rate of 25℃ / s; the alloying temperature is 510℃ and the alloying holding time is 20s.

[0046] 3. The chemical composition of carbon-based steel is C: 0.0025%, N: 0.0020%, Ti: 0.008%, Al: 0.045%, Nb: 0.003%, Mn: 0.30%, P: 0.035%; the balance is Fe and unavoidable impurities.

[0047] 4. Carbon-based steel production process and parameters: 1) Steelmaking: The oxygen content at the converter endpoint is controlled at 550ppm; RH deep decarburization treatment (vacuum degree ≤1.0mbar, cycle time ≥25min) is carried out, and the carbon content is precisely controlled to 0.0025%; no nitrogen microalloying is performed.

[0048] 2) Hot rolling: The final rolling temperature is 890℃, and the coiling temperature is 600℃.

[0049] 3) Alloyed continuous hot-dip galvanizing: heating section dew point -15℃; slow cooling section cools to 680℃ at 8℃ / s; fast cooling section cools to 520℃ at 15℃ / s; alloying temperature 480℃, holding time 25s.

[0050] The properties of nitrogen-based steel and carbon-based steel are shown in Table 2: Table 2 Comparison of properties of finished steel plates As can be seen from Table 2, the BH2 fluctuation range of nitrogen-based steel is 35-40 MPa, while that of carbon-based steel is 28-43 MPa. The nitrogen-dominated bake-hardening ultra-low carbon automotive outer panel steel described in this invention is significantly superior to traditional carbon-based steel in terms of BH2 value stability, aging resistance, and surface quality, and is particularly suitable for automotive outer panels with extremely high requirements for performance consistency.

[0051] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A nitrogen-dominant bake-hardening ultra-low carbon steel for automotive exterior panels, characterized in that, The chemical composition of the steel, by mass percentage, is C: 0.0005%~0.0015%; N：0.003%～0.006%； Ti: 0.005%~0.015%; Al：0.03%～0.06%； B：0.0002%～0.0005%； Nb :0.001%~0.003%; Mn: 0.25%~0.5%; P: 0.03%~0.05%; the remainder is Fe and unavoidable impurities; among which, [N] / [C]=2.5~5.0, and satisfies the following relationships: -0.10%≤[Al]-3.93×[Ti]≤0.15%, [C]-[Nb]×12 / 93≤0.0005%.

2. The nitrogen-dominant bake-hardening ultra-low carbon automotive exterior steel according to claim 1, characterized in that, The baked hardening value of the finished steel plate is BH2=38±3MPa, and the yield strength deviation after natural aging for 180 days is ≤5MPa; the surface waviness Wsa≤0.28μm.

3. A method for manufacturing ultra-low carbon automotive exterior steel with nitrogen-dominated bake hardening as described in claim 1 or 2, comprising steelmaking, continuous casting, hot rolling, cold rolling, and alloying continuous hot-dip galvanizing processes; characterized in that, Control the following process: 1) Steelmaking: A nitrogen-preserving and decarburizing refining method is adopted, as detailed below: Oxygen retention during steel tapping in converter: The final oxygen content is controlled at 500-600 ppm; RH vacuum light treatment: vacuum degree is 50-80 mbar, cycle time is 8-12 min, carbon content is controlled at 0.0005%-0.0015%, and more than 90% of the original nitrogen is retained; Nitrogen microalloying: Before RH vacuum breaking, manganese nitride was added to perform nitrogen microalloying according to the target nitrogen increment Δ[N]; bottom blowing argon weak stirring, argon flow rate was 30-50 NL / min; 2) Hot rolling: The final rolling temperature is 900-920℃ to ensure that the deformation in the non-recrystallization zone is ≥65%; the coiling temperature is 520-560℃; 3) Alloyed continuous hot-dip galvanizing: The dew point of the heating section is controlled at -25 to -35℃; the slow cooling section is cooled to 520 to 560℃ at a cooling rate of 10 to 15℃ / s; the fast cooling section is cooled to 450 to 480℃ at a cooling rate of 20 to 30℃ / s; the alloying temperature is 490 to 520℃, and the alloying holding time is 15 to 22s.

4. The method for manufacturing ultra-low carbon automotive outer panel steel with nitrogen-dominated bake hardening according to claim 3, characterized in that, During the nitrogen microalloying process, the amount of manganese nitride added is calculated as follows: (Δ[N] × weight of molten steel × 10) / (mass fraction of nitrogen in manganese nitride × nitrogen recovery rate). In this formula, the mass fraction of nitrogen in manganese nitride is 1.5% to 3.0%, and the nitrogen recovery rate is 80% to 95%.

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