260mpa grade galvannealed outer panel for new energy vehicle and preparation method thereof

Abstract

The application provides a 260MPa-grade alloyed hot-dip galvanizing outer plate for new energy vehicles and a manufacturing method, and the chemical components and mass percentages are as follows: C: 0.0008%-0.0012%, Si: 0.01%-0.015%, Mn: 0.3%-0.5%, P: 0.02%-0.03%, Cu: 0.1%-0.4%, N: 0.003% or less, S: 0.003% or less, Al: 0.015%-0.02%, and the rest is Fe and inevitable impurities, and wherein the nano Cu precipitation size is between 5-10nm, the critical zone ferrite is 10%-14%, and the epitaxial ferrite is 85% or more. The manufacturing method comprises smelting, refining, casting, hot rolling, pickling, cold rolling, and alloyed hot-dip galvanizing. The application solves the natural aging problem of bake-hardening steel plates and ensures high strength.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of galvanized sheet manufacturing for automobile outer plate, in particular to a 260MPa grade alloyed hot-dip galvanized outer plate for new energy automobile and a preparation method thereof. BACKGROUND

[0002] Automobile lightweighting is the most important measure for automobile industry to deal with resource, energy and environmental problems, therefore, the application of high-strength and high-surface quality automobile steel on the vehicle body is gradually increasing. As the largest outer plate for automobile, high-strengthening and high-surfacing is particularly important. BH steel is a kind of commonly used high-strength outer plate for automobile, its good deep drawing performance and significant strength increase during paint baking stage ensure its dominant position in automobile outer plate. However, BH steel has a serious natural aging during storage, that is, after storage for 1-3 months in a natural state, the strength increases significantly and the plasticity decreases by 3%-6%, leading to stamping cracking. The main reason is that during storage, the saturation solid solubility of C in ferrite decreases due to elastic deformation of ferrite, and the elastic interaction between free C and dislocation leads to increase of strength and decrease of plasticity.

[0003] Patent No. 201310463769.2 discloses a Nb-treated hot-dip galvanized ultra-low carbon bake-hardening steel sheet and a manufacturing method thereof, wherein a steel sheet blank is produced, the weight percentage of C in the steel sheet blank is controlled to be 0.001-0.0025%, the weight percentage of Nb is controlled to be 0.003-0.012%, and the excess content of C relative to Nb is in the range of 5-15ppm; the purpose is to "capture" free C in the steel by adding Nb, thereby inhibiting natural aging. The yield strength is 220-240MPa, the tensile strength is 350-380, the elongation after fracture is 35-40%, the plastic strain ratio is 1.9-2.3, and the bake hardening value BH2 is 42-49, and the plasticity decay is not studied.

[0004] Patent No. 201811219364.3 discloses a low-carbon bake-hardening steel and a production method thereof, wherein the composition of the low-carbon bake-hardening steel is as follows: C: 0.015-0.021wt%, Mn: 0.2-0.3wt%, Si: ≤0.03wt%, P: 0.02-0.03wt%, S: 0.005-0.015wt%, Al: 0.03-0.06wt%, N: ≤0.003wt%, Cu: 0.002-0.015wt%, B: 0.0005-0.001wt%, Co: 0.001-0.002wt%, and the rest is Fe and inevitable impurities. The purpose is to solve the aging problem, but the mechanical properties are poor, the tensile strength is only 360, and the elongation after fracture is 30%.

[0005] Therefore, it is particularly important to effectively solve the natural aging problem while ensuring the comprehensive performance of the steel plate for BH steel. SUMMARY

[0006] According to the above technical problems, a 260MPa grade alloyed hot-dip galvanized outer plate for new energy vehicles and a preparation method thereof are provided.

[0007] A 260MPa grade alloyed hot-dip galvanized outer plate for new energy vehicles, the chemical composition and mass percentage of the alloyed hot-dip galvanized outer plate are as follows:

[0008] C: 0.0008% to 0.0012%, Si: 0.01% to 0.015%, Mn: 0.3% to 0.5%, P: 0.02% to 0.03%, Cu: 0.1% to 0.4%, N≤0.003%, S≤0.003%, Al: 0.015% to 0.02%, the balance being Fe and unavoidable impurities, and wherein the nano-Cu precipitation size is between 5 and 10 nm. And the ferrite is divided into critical zone ferrite (IF) 10% to 14%, epitaxial ferrite (EF) ≧85%, and the rest is unidentified phase. The yield strength is above 260 to 290 MPa, the tensile strength is above 390 MPa, the elongation after fracture is above 40%, the plastic strain ratio is above 2.2, the bake hardening value BH2 is between 30 and 35 MPa, and the plasticity attenuation value within 3 months is ≤1%.

[0009] The reasons for the alloy design of the present application are as follows:

[0010] C: Strict control of C is crucial in the present application. It plays a crucial role in the bake hardening value and aging resistance of BH steel. The main reason for natural aging is the interaction of C and dislocations, therefore the C content is strictly controlled to be 0.0008% to 0.0012%. If C is too low, it cannot achieve the BH value of the paint baking stage of the steel plate, and if C is too high, natural aging is obvious.

[0011] Mn: Mn element is an important element in the present application, which plays a role in solid solution strengthening. However, when the addition amount of Mn element is too high, the strength of the steel plate is too high to affect its forming performance, therefore the content is limited to 0.3% to 0.5%.

[0012] Si: Si element is one of the important elements in the present application, which plays a role in solid solution strengthening. However, too high content of Si will seriously affect the surface quality of the galvanized plate, therefore the content is limited to 0.01% to 0.015%.

[0013] Cu: Cu is the main element of the present application, in the present application, the addition of ultra-low C, the elemental precipitation of Cu is beneficial to realize the performance of the steel plate, therefore its content is limited to 0.1%~0.4%. Cu can be added in various forms in steel to provide strength supplement to the steel plate, in the interval of 500-680℃, in the form of nano-elemental precipitation, and not combined with C, improve the grain boundary strength. This is crucial in the present application, which shows that Cu does not consume a small amount of C, and improves the strength of the steel plate alone, without affecting the influence of other elements on the aging performance.

[0014] P: P element is one of the important elements in the present application, which plays a role in solid solution strengthening. However, P is prone to segregation at the grain boundary, and when the P content is too high, it will cause the brittleness of the steel plate to decrease during secondary processing. At the same time, excessive P element makes the strength of the steel plate too high to affect its forming performance, therefore its content is limited to 0.02%~0.03%.

[0015] N: N is an impurity element in steel, which affects the aging resistance of the steel plate and is not conducive to the improvement of the r value, therefore its content is limited to ≤0.003%.

[0016] S: S is an impurity element in steel, which is easy to react with Mn to form MnS, causing the performance of the steel plate to deteriorate, so the lower the content is the better, therefore its content is limited to ≤0.003%.

[0017] Al: The content of Al is strictly controlled in the present application, because the content of Al is too low, the content of O in the steel increases, which leads to the quality of the casting blank, and the content of Al is too high, which leads to the decrease of the strength of the steel plate, therefore its content is limited to 0.015%~0.02%.

[0018] The present application also provides a preparation method of a 260MPa grade alloyed hot-dip galvanized outer plate for new energy vehicles, which comprises the following steps:

[0019] (1) Smelting: smelting, refining and casting into a casting blank according to the above chemical composition.

[0020] In the casting process, the tundish molten steel casting temperature is 1500~1600℃, which ensures the uniformity of the structure and composition.

[0021] (2) Hot rolling: the heating temperature is between 1180~1220℃, the furnace time is 120~180min, the heating stage ensures the homogenization of the casting blank organization, prevents the austenite grain from coarsening due to too high temperature, and prevents the austenite grain from coarsening due to too long isothermal time.

[0022] The opening rolling temperature is between 1120~1150℃, and the final rolling temperature is between 950~980℃, which promotes the rolling of the austenite recrystallization zone and effectively refines the austenite grain;

[0023] The winding temperature is between 500 and 590°C to promote the precipitation of elemental Cu at this stage and prevent the precipitation from becoming coarse.

[0024] (3) Pickling: Pickling is used to remove the iron oxides present on the surface of the hot-rolled steel.

[0025] (4) Cold rolling: The cold-rolled product specifications are maintained at a thickness of 0.6-0.8 mm, corresponding to the target automotive panel thickness of the product. The rolling reduction is controlled at 80-85%. The energy stored in the cold rolling deformation is the driving force for recrystallization. Sufficient reduction can ensure the effect of ferrite recrystallization and form a strong {111} / / ND(γ) texture that is beneficial to deep drawing performance. The γ texture ratio is controlled to be above 80%. However, excessive rolling reduction increases the load on the cold rolling mill and cannot guarantee the achievement of the target thickness.

[0026] (5) Alloyed hot-dip galvanizing: First, heat the steel plate to 500-680℃ at a rate of 1-2.5℃ / s, ensuring Cu precipitation behavior during this stage. Then, heat the plate to 860-920℃ and hold it at the same temperature for 10-60s. Heat the steel plate above the critical zone to introduce a certain amount of critical zone ferrite, balancing the plasticity of the steel plate. Effectively control the temperature and time range to prevent excessive ferrite coarsening. Slow cooling stage: Cool the steel plate slowly to 620-650℃ at a cooling rate of 2-4℃ / s to promote the formation of epitaxial ferrite during this stage. Prevent excessive cooling rate from causing insufficient nucleation driving force. Ensure Cu precipitation during this stage, as excessive temperature will result in insufficient undercooling. Next, cool the steel plate to 450-470℃ at a cooling rate of ≥30℃ / s and hold it at the same temperature for 10-20s as one aging step. Then, place the steel plate in a zinc pot at 450-460℃. The steel plate is then placed in an alloying furnace for alloying treatment at a temperature of 460–490℃ for 10–24 seconds; finally, the steel plate is cooled to room temperature. Key parameters are explained below:

[0027] The alloying hot-dip galvanizing process involves a continuous increase in the iron content of the coating under constant temperature conditions. Studies have shown that the alloying process is complete when the iron content on the coating surface reaches 8%-10%, at which point the coating contains either the η+δ phase or the δ phase (FeZn). 10 Of which 9%-10% is the ideal dense δ phase (FeZn) 10 The iron content on the coating surface is ≥8%, and <9% is the η+δ phase. When the alloying temperature is too high, Fe3Zn is easily formed in the coating. 10 (Γ phase) or Fe5Zn 21 (Γ1 phase) and Cu precipitation coarsens.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] This invention provides a 260MPa grade alloyed hot-dip galvanized outer sheet for new energy vehicles with a yield strength of 260-290MPa or higher, a tensile strength of 390MPa or higher, an elongation after fracture of 40% or higher, a plastic strain ratio of 2.2 or higher, a bake-hardening value (BH2) of 30-35MPa, and a plasticity decay of ≤1% within 3 months. It solves the problem of natural aging in bake-hardened steel sheets, cleverly balancing resistance to natural aging and ensuring high strength, thus opening up a new direction in the design of bake-hardened steel.

[0030] Based on the above reasons, this invention can be widely applied in fields such as baking-hardening steel. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a typical metallographic image of a 260MPa grade alloyed hot-dip galvanized outer sheet for new energy vehicles, according to a specific embodiment of the present invention. Detailed Implementation

[0033] It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of this invention can be combined with each other. To make the objectives, technical solutions, and advantages of the embodiments of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0034] A 260MPa grade alloyed hot-dip galvanized outer sheet for new energy vehicles, wherein the chemical composition and mass percentage of the alloyed hot-dip galvanized outer sheet are as follows:

[0035] C: 0.0008%–0.0012%, Si: 0.01%–0.015%, Mn: 0.3%–0.5%, P: 0.02%–0.03%, Cu: 0.1%–0.4%, N≤0.003%, S≤0.003%, Al: 0.015%–0.02%, with the balance being Fe and unavoidable impurities. The nano-Cu precipitates are 5–10 nm in size. The ferrite is divided into critical ferrite (IF) 10%–14%, epitaxial ferrite (EF) ≥85%, and the remainder being unidentified phases. Its yield strength is above 260–290 MPa, tensile strength is above 390 MPa, elongation after fracture is above 40%, plastic strain ratio is above 2.2, bake hardening value (BH2) is between 30–35 MPa, and plasticity decay within 3 months is ≤1%. Typical metallographic images are available in [link to metallographic diagram]. Figure 1 .

[0036] Its preparation method includes the following steps:

[0037] (1) Smelting: Smelting, refining and casting into billets according to the above chemical composition.

[0038] During the casting process, the molten steel in the ladle is poured at a temperature of 1500–1600℃ to ensure uniform microstructure and composition.

[0039] (2) Hot rolling: The heating temperature is between 1180 and 1220℃, and the furnace time is 120 to 180 minutes. During the heating stage, the billet structure is ensured to be homogeneous, and the austenite grains are prevented from coarsening due to excessive temperature and excessive isothermal time.

[0040] The initial rolling temperature is between 1120 and 1150℃, and the final rolling temperature is between 950 and 980℃, which promotes the rolling of the austenite recrystallization zone and effectively refines the austenite grains.

[0041] The winding temperature is between 500 and 590°C to promote the precipitation of elemental Cu at this stage and prevent the precipitation from becoming coarse.

[0042] (3) Pickling: Pickling is used to remove the iron oxides present on the surface of the hot-rolled steel.

[0043] (4) Cold rolling: The cold-rolled product specifications are maintained at a thickness of 0.6-0.8 mm, corresponding to the target automotive panel thickness of the product. The rolling reduction is controlled at 80-85%. The energy stored in the cold rolling deformation is the driving force for recrystallization. Sufficient reduction can ensure the effect of ferrite recrystallization and form a strong {111} / / ND(γ) texture that is beneficial to deep drawing performance. The γ texture ratio is controlled to be above 80%. However, excessive rolling reduction increases the load on the cold rolling mill and cannot guarantee the achievement of the target thickness.

[0044] (5) Alloyed hot-dip galvanizing: First, heat the steel plate to 500-680℃ at a rate of 1-2.5℃ / s, ensuring Cu precipitation behavior during this stage. Then, heat the plate to 860-920℃ and hold it at the same temperature for 10-60s. Heat the steel plate above the critical zone to introduce a certain amount of critical zone ferrite, balancing the plasticity of the steel plate. Effectively control the temperature and time range to prevent excessive ferrite coarsening. Slow cooling stage: Cool the steel plate slowly to 620-650℃ at a cooling rate of 2-4℃ / s to promote the formation of epitaxial ferrite during this stage. Prevent excessive cooling rate from causing insufficient nucleation driving force. Ensure Cu precipitation during this stage, as excessive temperature will result in insufficient undercooling. Next, cool the steel plate to 450-470℃ at a cooling rate of ≥30℃ / s and hold it at the same temperature for 10-20s as one aging step. Then, place the steel plate in a zinc pot at 450-460℃. The steel plate is then placed in an alloying furnace for alloying treatment at a temperature of 460–490°C for 10–24 seconds. Finally, the steel plate is cooled to room temperature.

[0045] The composition, preparation process, and performance of the present invention will be described in detail below with reference to the following fifteen examples.

[0046] The components of the fifteen embodiments are shown in Table 1:

[0047] Table 1. Main chemical composition of the steel in the examples, wt%

[0048] Examples C Mn Si P Cu N S Al IF content / % EF / % Cu precipitation size / nm 1 0.0009 0.42 0.012 0.024 0.18 0.0015 0.002 0.016 13.6 85.6 7.8 2 0.001 0.47 0.011 0.023 0.26 0.0024 0.003 0.018 12.5 85.1 5.7 3 0.0012 0.48 0.013 0.028 0.33 0.003 0.002 0.015 10.6 86.4 6.4 4 0.0009 0.38 0.015 0.025 0.34 0.0028 0.003 0.02 10.1 88.7 6.8 5 0.001 0.44 0.014 0.024 0.26 0.0014 0.001 0.017 12.4 85.4 6.9 6 0.0012 0.32 0.012 0.026 0.35 0.0016 0.001 0.019 11.2 85.6 7.2 7 0.0009 0.31 0.011 0.027 0.4 0.0024 0.003 0.018 11.6 86.2 6.5 8 0.0009 0.35 0.013 0.025 0.29 0.0028 0.002 0.015 13.8 86.4 6.8 9 0.001 0.42 0.015 0.024 0.24 0.0018 0.003 0.015 12.5 86.4 7.5 10 0.0009 0.38 0.014 0.026 0.28 0.0025 0.002 0.02 11.7 87.2 6.4 11 0.001 0.42 0.012 0.028 0.19 0.0014 0.003 0.02 10.5 85.2 8.2 12 0.0012 0.5 0.014 0.024 0.12 0.0016 0.003 0.02 11.6 85.3 8.9 13 0.0009 0.31 0.012 0.026 0.15 0.0028 0.002 0.017 10.4 85.7 9.1 14 0.001 0.33 0.011 0.027 0.25 0.0014 0.003 0.019 10.9 88.1 9.5 15 0.0012 0.36 0.013 0.025 0.12 0.0016 0.001 0.018 11.2 85.6 9.8

[0049] The casting, hot rolling, and cold rolling processes of the fifteen embodiments are shown in Table 2:

[0050] Table 2. Casting, hot rolling, and cold rolling processes of the steel in the examples.

[0051]

[0052] The alloying hot-dip galvanizing processes of fifteen embodiments are shown in Table 3:

[0053] Table 3. Alloying and hot-dip galvanizing process of steel in the examples.

[0054]

[0055] The performance of the fifteen embodiments is shown in Table 4:

[0056] Table 4 Properties of the Steel in Examples

[0057] Examples Fe content / % Zn-Fe phase Rp0.2 / MPa Rm / MPa A80 / ％ r BH value / MPa 3 month plasticity decay value / % 1 8.6 η+δ 267 405 41.5 2.3 32 0.92 2 9.2 δ 272 408 40.8 2.35 34 0.88 3 8.5 η+δ 265 396 41.2 2.24 35 0.85 4 8.4 η+δ 266 399 41.2 2.28 30 0.95 5 9.2 δ 263 402 41.1 2.21 34 0.92 6 9.3 δ 266 401 40.9 2.25 32 0.98 7 9.4 δ 272 413 40.8 2.22 31 0.99 8 9.8 δ 274 405 40.5 2.35 34 0.92 9 9.4 δ 268 408 42.6 2.26 31 0.88 10 9.4 δ 273 402 43.2 2.24 35 0.91 11 8.8 η+δ 277 398 40.2 2.28 35 0.93 12 8.9 η+δ 263 397 40.5 2.21 35 0.94 13 9.4 δ 264 405 41.5 2.22 34 0.92 14 8.5 η+δ 271 402 41.8 2.24 31 0.95 15 8.8 η+δ 265 406 40.9 2.23 32 0.95

[0058] As shown in Tables 1 to 4, the 260MPa grade alloyed hot-dip galvanized outer sheet for new energy vehicles prepared with the above-mentioned components and parameters solves the problem of natural aging of bake-hardened steel sheets while giving the steel excellent mechanical properties.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A 260 MPa grade galvannealed outer panel for a new energy vehicle, characterized by, The chemical composition of the alloyed hot-dip galvanized outer plate is as follows in percentage by mass: C: 0.0008%-0.0012%, Si: 0.01%-0.015%, Mn: 0.3%-0.5%, P: 0.02%-0.03%, Cu: 0.1%-0.4%, N≤0.003%, S≤0.003%, Al: 0.015%-0.02%, the balance being Fe and inevitable impurities, and wherein the nano-Cu has a precipitation size of 5-10 nm, is precipitated in elemental form in the range of 500-680 ℃ and does not combine with C.

2. The 260 MPa grade galvannealed outer panel for a new energy vehicle according to claim 1, characterized in that, The ferrite in the critical zone of the alloyed hot-dip galvanized outer plate is 10%-14%, and the epitaxial ferrite is ≥85%.

3. The 260 MPa grade galvannealed outer panel for a new energy vehicle according to claim 1, characterized in that, The iron content of the coating surface of the alloyed hot-dip galvanized outer plate is 8%-10%.

4. The 260 MPa grade galvannealed outer panel for a new energy vehicle according to claim 1, characterized in that, The coating of the alloyed hot-dip galvanized outer plate is η+δ phase or δ phase.

5. The 260 MPa grade galvannealed outer panel for a new energy vehicle according to claim 1, characterized in that, The yield strength of the alloyed hot-dip galvanized outer plate is ≥260 MPa, the tensile strength is ≥390 MPa, the elongation after fracture is ≥40%, and the plastic strain ratio is ≥2.

2.

6. The 260 MPa grade galvannealed outer panel for a new energy vehicle according to claim 1, characterized in that, The bake hardening value BH2 of the alloyed hot-dip galvanized outer plate is between 30-35 MPa.

7. The 260 MPa grade galvannealed outer panel for a new energy vehicle according to claim 1, characterized in that, The plasticity attenuation value of the alloyed hot-dip galvanized outer plate within 3 months is ≤1%.

8. The preparation method of the 260 MPa grade galvannealed outer panel for new energy vehicles according to any one of claims 1-7, characterized in that, The method comprises the following steps: Smelting, refining and casting into a casting blank are performed according to the composition of claim 1; Hot rolling: the heating temperature is between 1180-1220 ℃, the furnace time is 120-180 min, the opening rolling temperature is 1120-1150 ℃, the final rolling temperature is 950-980 ℃, and the coiling temperature is 500-590 ℃; Pickling: the iron oxide on the surface of the hot-rolled coiled steel is removed by pickling; Cold rolling; Galvannealing: first, the temperature is raised to 500-680 ℃ at a speed of 1-2.5 ℃ / s, then the temperature is raised to 860-920 ℃, the isothermal time is 10-60 s, the steel plate is slowly cooled to 620-650 ℃ at a cooling speed of 2-4 ℃ / s, the steel plate is cooled to 450-470 ℃ at a cooling speed of ≥30 ℃ / s, then the steel plate is placed in a zinc pot at 450-460 ℃, then the steel plate is subjected to galvannealing treatment in a galvannealing furnace at a galvannealing temperature of 460-490 ℃ for a galvannealing time of 10-24 s, and finally the steel plate is cooled to room temperature to obtain the alloyed hot-dip galvanized outer plate.

9. The preparation method of the 260 MPa grade galvannealed outer panel for new energy vehicles according to claim 8, characterized in that, In the casting process, the tundish molten steel casting temperature is 1500-1600 ℃.

10. The preparation method of the 260 MPa grade galvannealed outer panel for new energy vehicles according to claim 8, characterized in that, In the cold rolling process, the rolling reduction rate is controlled at 80-85%.

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