Preparation method of 390MPa grade aluminized silicon coated automotive steel and its steel sheet

Abstract

This invention relates to a method for preparing 390MPa grade aluminized silicon coated automotive steel and the steel sheet thereof, comprising smelting, hot rolling, pickling, cold rolling, continuous annealing aluminized silicon coating, and finishing processes; the continuous annealing aluminized silicon coating process includes the following steps: 1) Annealing isothermal temperature is 810–870℃, annealing time is 40–80s. 2) Slow cooling rate is 2–10℃ / s, slow cooling temperature is 650–720℃, isothermal time is 0–20s. 3) Aluminized silicon coating: After isothermal treatment at the slow cooling temperature, the steel sheet enters the aluminized silicon coating bath, requiring the temperature difference between the aluminized silicon coating bath and the slow cooling temperature of the steel sheet to be ≤10℃, and the aluminized silicon coating time is 3–10s. 4) After aluminized silicon coating, the steel sheet continues to be rapidly cooled at a rate ≥20℃ / s to room temperature. This invention, through reasonable composition design and the innovative combination of continuous annealing and aluminized silicon coating processes, achieves the preparation of a 390MPa grade automotive outer panel steel with both excellent mechanical properties and high surface quality.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel manufacturing, and particularly relates to a method for preparing 390MPa grade aluminum-silicon coated automotive steel and its steel sheet. Background Technology

[0002] The automotive industry is a vital component of the national economy and a major steel-consuming sector, making its development crucial to the steel industry. Currently, steel materials account for over 60% of automotive materials; however, with the increasing development of aluminum, magnesium, titanium alloys, and non-metallic composite materials, competition for steel in the automotive sector is intensifying. Furthermore, with the advancement of lightweighting in automobiles, high-strength steel materials have become a key development direction for automotive steel, both now and in the future.

[0003] As one of the main types of steel used in automobiles, automotive outer panels are primarily used to manufacture parts such as car doors, engine hoods, and trunk lids. Therefore, this material requires excellent formability, dent resistance, weldability, and corrosion resistance. Regarding corrosion resistance, regions like Europe have introduced even more stringent requirements, demanding that the corrosion resistance of the outer panels match the lifespan of the automobile. Currently, automotive outer panel steel is mainly produced using hot-dip galvanizing, electro-galvanizing, and alloy galvanizing. However, with the decreasing availability and rising price of zinc resources, a wider range of suitable coating methods are needed to meet the demands of the automotive industry, and aluminum-silicon coating is one of the most promising options. Hot-dip aluminum-silicon steel sheets possess excellent heat resistance and oxidation resistance, allowing for long-term use at high temperatures and maintaining excellent corrosion resistance even in complex working environments (such as sulfur-containing industrial atmospheres, marine atmospheres, and humid environments). Therefore, developing high-strength aluminum-silicon coated automotive outer panels will help the steel industry cope with the increasingly competitive automotive materials market in the future. Summary of the Invention

[0004] This invention provides a method for preparing 390MPa grade aluminum-silicon coated automotive steel and its steel sheet. By combining reasonable composition design with innovative continuous annealing and aluminum-silicon coating processes, a 390MPa grade automotive outer panel steel with excellent mechanical properties and high surface quality is prepared.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] The preparation method of 390MPa grade aluminized silicon coated automotive steel includes smelting, hot rolling, pickling, cold rolling, continuous annealing aluminized silicon coating, and finishing processes.

[0007] Smelting process: The alloy composition within the above range is obtained through smelting, and then cast into slabs.

[0008] The hot rolling process includes the following methods:

[0009] 1) The heating temperature should be between 1230 and 1320℃, with a furnace time of ≥100 min. A heating temperature of 1230–1320℃ ensures uniform distribution of the added alloying elements and prevents the microstructure of the billet from becoming excessively coarse, thus affecting the final steel plate properties. Simultaneously, this temperature range ensures the formation of TiN precipitates. A heating time of ≥100 min is also to ensure uniform distribution of the alloying elements.

[0010] 2) The initial rolling temperature is between 1100 and 1200℃, and the final rolling temperature is required to be ≥900℃; thus, hot rolling is carried out in the single-phase region of austenite, thereby reducing the phenomenon of abnormal grain growth.

[0011] 3) The coiling temperature should be controlled between 650 and 720℃. Above 720℃, uneven grains are likely to appear at the edges of the steel plate, and the excessively large precipitates are not conducive to the mechanical properties of the final steel plate; below 650℃, the carbonitride precipitates are fine and dispersed, which is not conducive to the complete recrystallization of grains, thereby reducing the plasticity and formability of the material.

[0012] The cold rolling process requires a reduction rate of 75% to 85%. A larger reduction provides higher deformation energy storage, allowing the steel sheet to recrystallize during continuous annealing, thus resulting in strong... <111> / / ND texture gives the final steel sheet good mechanical and formability. Excessive reduction has little impact on the annealing recrystallization behavior of the steel sheet and places excessive demands on the capacity of the cold rolling mill, thus hindering the achievement of the corresponding reduction.

[0013] Pickling process: Removes surface oxides from hot-rolled plates to ensure the smooth progress of subsequent cold rolling and the surface properties of the steel plates.

[0014] The continuous annealing aluminum-silicon plating process includes the following methods:

[0015] 1) The isothermal annealing temperature is 810–870℃, and the annealing time is 40–80 seconds; this isothermal temperature and time ensure complete recrystallization of the deformed structure within the steel plate, thereby forming a strong... <111> / / ND texture, while avoiding excessively large grain size that would affect the strength and surface properties of the steel plate.

[0016] 2) The slow cooling rate is 2–10℃ / s, the slow cooling temperature is 650–720℃, and the slow cooling isothermal time is 0–20s. This limited slow cooling rate and temperature ensures the precipitation of Nb-containing carbides, effectively removes interstitial solid solution atoms from the steel plate, and also promotes a favorable texture. <111> / / Further enhancements to ND.

[0017] 3) Aluminosilicate plating: After the steel sheet has been slow-cooled to an isothermal temperature, it enters the aluminosilicate plating bath at a temperature of 650–720℃. The temperature difference between the aluminosilicate plating bath and the slow-cooling temperature of the steel sheet must be ≤10℃, and the plating time is 3–10 seconds. This small temperature difference between the aluminosilicate plating bath and the steel sheet minimizes temperature fluctuations in the plating bath, thus preventing a large number of Fe-Al alloy particles from adhering to the steel sheet surface and affecting its surface quality.

[0018] 4) After aluminosilicate coating is completed, the steel plate continues to be rapidly cooled to room temperature at a rate of ≥20℃ / s. This rapid cooling rate can suppress the growth of dendritic crystals in the steel plate coating and prevent the formation of obvious crystal flowers on the coating surface, which would affect the surface quality.

[0019] The finishing process requires a finishing elongation rate of 0.5% to 1.5%. This effectively eliminates the yield plateau of the steel plate and ensures surface quality.

[0020] The composition of the aluminum-silicon plating bath, by mass percentage, is: Si 7%–12%, Fe ≤2%, with the remainder being Al and unavoidable impurity elements. This range of Si content in the aluminum-silicon plating bath helps improve its fluidity and, by first forming a Fe-Si phase with the Fe on the steel substrate, inhibits the growth of a brittle Fe-Al alloy layer, thus improving the processability of the coating. Fe in the plating bath typically enters from the substrate during the aluminum-silicon plating process. Excessive Fe content causes the formation of Fe-Al mesophase particles in the plating bath, affecting the quality of the steel plate coating; therefore, the Fe content in the empty plating bath should be ≤2%.

[0021] The steel sheet prepared using the method for preparing 390MPa grade aluminum-silicon coated automotive steel has the following chemical composition by weight percentage: C 0.002%–0.006%, Mn 0.5%–0.8%, P 0.04%–0.08%, Si 0.01%–0.04%, Ti 0.02%–0.03%, Nb 0.02%–0.03%, N≤0.002%, S≤0.004%, with the balance being Fe and unavoidable impurities.

[0022] The steel plate has a yield strength of 240-290 MPa, a tensile strength of 390 MPa or more, an elongation after fracture of 35% or more, a plastic strain ratio r of 2.0 or more, and a strain hardening index n of 0.2 or more.

[0023] The microstructure of the steel plate includes ferrite and (Ti,Nb)(N,C) precipitates.

[0024] C: C is an important additive element in this invention. When the C content increases, excessive amounts of alloying elements Ti and Nb are needed to fix the C, resulting in excessive carbides or cementite in the steel. This severely affects the mechanical properties of the steel, such as elongation after fracture and r-value. Simultaneously, excessive C dissolved in the matrix phase causes a decrease in the product's plasticity, i.e., aging problems occur. However, if the C content in the steel is too low, it will cause difficulties in steelmaking and excessively increase product costs. It will also weaken precipitation strengthening, affecting the product's strength. Therefore, to ensure the comprehensive mechanical properties of the steel plate, this invention requires a C content of 0.002% to 0.006%.

[0025] Mn: Mn is one of the key elements in this invention, playing a role in solid solution strengthening and thus improving the strength of the steel plate. However, excessive Mn addition can easily lead to the formation of inclusions such as MnS during the smelting process, severely impairing the mechanical and formability properties of the steel plate, while also reducing the solid solution Mn content and lowering the steel plate's strength. Conversely, insufficient Mn addition results in insignificant solid solution strengthening and excessively low steel plate strength. Therefore, this invention requires an Mn content of 0.5% to 0.8%.

[0026] P: P is one of the important elements in this invention. It is also a solid solution strengthening element and has a significant impact on the strength of the steel plate. Excessive addition of P will result in excessively high strength and decreased plasticity of the steel plate, and will also cause segregation at grain boundaries, which will worsen the plasticity. Insufficient addition will result in insufficient improvement in the strength of the steel plate. Therefore, this invention requires the P content to be 0.04% to 0.08%.

[0027] Si: Si is also a solid solution strengthening element. An appropriate amount added ensures the strength of the steel plate, but excessive Si content severely reduces the plasticity, formability, and weldability of the steel plate. Therefore, this invention requires a Si content of 0.01% to 0.04%.

[0028] Ti: Ti is one of the key elements in this invention. Adding an appropriate amount of Ti allows it to react with interstitial C and N atoms to form carbonitrides Ti(N,C), ensuring the steel sheet has good plasticity and formability. Excessive Ti increases cost and, moreover, dissolves in the ferrite matrix, thus increasing strength but decreasing plasticity. Therefore, this invention requires a Ti content of 0.02% to 0.03%.

[0029] Nb: Nb is one of the key elements in this invention. Nb plays two main roles in steel: first, it forms precipitates with interstitial elements such as C, thereby eliminating interstitial atoms; second, its solid solution atoms exert a dragging effect on dislocations and grain boundaries, thus achieving grain refinement and strengthening. Furthermore, compared to Ti, the precipitation temperature of Nb is lower than that of Ti carbonitrides, so Nb primarily functions as a grain refiner in this invention. Excessive addition of Nb can lead to excessively high strength and decreased plasticity in the steel sheet, and it can also hinder the recrystallization of ferrite. <111> The reduced texture strength of Nb significantly decreases the formability of the steel sheet. Therefore, this invention requires an Nb content of 0.02% to 0.03%.

[0030] Nitrogen (N): Nitrogen is an impurity element in steel, and its presence affects the plasticity and toughness of the steel plate. However, excessively low levels of nitrogen increase the difficulty and cost of steelmaking. Therefore, this invention requires that the nitrogen content be ≤0.002%.

[0031] S: S is an impurity element in steel, which readily reacts with elements such as Mn to form inclusions such as MnS, severely impairing the mechanical properties of the steel plate. Similarly, excessively low S content increases the difficulty and cost of smelting. Therefore, this invention requires an S content ≤ 0.004%.

[0032] The final microstructure of the 390-grade steel plate of this invention consists of ferrite and (Ti,Nb)(N,C) precipitates, and the steel plate exhibits a strong texture. <111> / / ND has a γ texture, and the steel plate surface is coated with an aluminum-silicon coating. This invention employs a novel production process that combines the aluminum-silicon coating process with a slow cooling stage of continuous annealing. This process yields an aluminum-silicon coated automotive outer panel with a yield strength between 240-290 MPa, a tensile strength of over 390 MPa, an elongation after fracture greater than 35%, a plastic strain ratio r greater than 2.0, and a strain hardening index n greater than 0.2. Simultaneously, it achieves excellent mechanical properties and high surface quality for the automotive outer panel.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1) The steel plate of the present invention adopts the composition design concept of Ti+Nb composite fixed interstitial atoms and makes full use of the atomic drag effect of Nb to achieve the purpose of grain refinement. At the same time, a small amount of Mn and P elements are added, thereby controlling the cost of the steel plate in the composition design.

[0035] 2) Based on the temperature of the aluminum-silicon plating solution, this invention novelly combines the process with the slow cooling stage, cleverly combining hot-dip aluminum-silicon plating with the heat treatment of the steel plate. Placing the plating process in the slow cooling stage not only reduces production steps and improves efficiency and energy consumption, but also ensures the mechanical properties and high surface quality of the steel plate.

[0036] 3) The aluminum-silicon coated automotive outer panel provided by this invention gives the steel sheet a superior surface quality. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0038] Table 1 lists the chemical composition of the steels in the examples, Table 2 lists the hot rolling and cold rolling process parameters of the steels in the examples, Table 3 lists the process parameters of continuous annealing and aluminized silicon plating of the steels in the examples, and Table 4 gives the mechanical properties of the steels in the examples.

[0039] Table 1 Chemical composition of the steel in the examples, wt%.

[0040] No. C Mn P Si Ti Nb N S 1 0.0024 0.61 0.062 0.026 0.025 0.025 0.0015 0.0030 2 0.0027 0.53 0.056 0.018 0.022 0.023 0.0012 0.0026 3 0.0052 0.55 0.060 0.030 0.028 0.026 0.0014 0.0035 4 0.0036 0.68 0.048 0.035 0.020 0.028 0.0010 0.0031 5 0.0042 0.72 0.055 0.028 0.025 0.024 0.0009 0.0026 6 0.0038 0.75 0.050 0.035 0.029 0.021 0.0010 0.0020 7 0.0053 0.62 0.065 0.029 0.030 0.022 0.0016 0.0017 8 0.0047 0.58 0.072 0.025 0.021 0.027 0.0013 0.0032

[0041] Table 2. Hot rolling and cold rolling processes for steel in the examples.

[0042] Example Heating temperature / ℃ Furnace duration / min Rolling temperature / ℃ Final rolling temperature / ℃ Winding temperature / ℃ Cold rolling reduction rate / % 1 1255 110 1120 905 690 76 2 1305 108 1180 912 712 78 3 1283 120 1108 905 705 81 4 1315 125 1105 908 686 79 5 1278 118 1130 920 703 83 6 1305 115 1130 917 716 82 7 1260 125 1137 906 698 76 8 1258 130 1142 917 687 77

[0043] Table 3 Examples of Steel Annealing and Aluminized Silicon Plating Processes

[0044]

[0045] Table 4 Mechanical properties of the steel in the examples

[0046] Example <![CDATA[R p0.2 / MPa]]> <![CDATA[R m / MPa]]> <![CDATA[A 50 / %]]> Plastic strain ratio r strain hardening index n steel plate surface 1 255 405 37 2.2 0.22 Crystal Flower 2 262 398 38 2.1 0.24 Crystal Flower 3 248 415 36 2.3 0.22 Crystal Flower 4 260 410 36 2.2 0.23 Crystal Flower 5 258 407 38 2.5 0.25 Crystal Flower 6 267 423 35 2.3 0.21 Crystal Flower 7 272 435 36 2.1 0.25 Crystal Flower 8 263 408 38 2.4 0.22 Crystal Flower

[0047] As can be seen from the above embodiments, by designing the Nb+Ti composite addition, and by innovatively adopting a production method that combines the aluminized silicon process with a continuous annealing and slow cooling stage, aluminized silicon steel for automotive outer panels with both excellent mechanical properties and high surface quality is prepared, with a yield strength of 240-290MPa, tensile strength >390MPa, and elongation >35%.

Claims

A method for preparing 1.390MPa grade aluminized silicon coated automotive steel, characterized in that, This includes smelting, hot rolling, pickling, cold rolling, continuous annealing, aluminizing and silicon plating, and finishing processes; The chemical composition of the substrate, by weight percentage, is as follows: C 0.0042%~0.006%, Mn 0.5%~0.68%, P 0.04%~0.08%, Si 0.01%~0.035%, Ti 0.02%~0.03%, Nb 0.02%~0.03%, N≤0.002%, S≤0.004%, with the balance being Fe and unavoidable impurities; The continuous annealing aluminum-silicon plating process includes the following methods: 1) The isothermal annealing temperature is 810–870℃, and the annealing time is 40–58s; 2) The slow cooling rate is 2-10℃ / s, the slow cooling temperature is 650-720℃, and the slow cooling isothermal time is 8-20s; 3) Aluminosilicate plating: After the steel plate is cooled to a slow temperature, it enters the aluminum-silicon plating bath. The temperature difference between the aluminum-silicon plating bath and the slow cooling temperature of the steel plate is ≤10℃. The aluminum-silicon plating time is 3~10s and the temperature of the aluminum-silicon plating bath is 650~662℃. 4) After the aluminized silicon coating is completed, the steel plate continues to be rapidly cooled at a rate of ≥20 ℃ / s to room temperature.

2. The method for preparing a 390MPa grade aluminized silicon coated automotive outer panel according to claim 1, characterized in that, The hot rolling process includes the following methods: 1) Heating temperature between 1230 and 1320℃, furnace time ≥ 100 min; 2) The initial rolling temperature should be between 1100 and 1200℃, and the final rolling temperature should be ≥900℃; 3) The winding temperature should be controlled between 650 and 720℃.

3. The method for preparing a 390MPa grade aluminized silicon coated automotive outer panel according to claim 1, characterized in that, The cold rolling process has a cold rolling reduction rate of 75% to 85%.

4. The method for preparing a 390MPa grade aluminized silicon coated automotive outer panel according to claim 1, characterized in that, The finishing process described above has a finishing elongation rate of 0.5% to 1.5%.

5. The method for preparing a 390MPa grade aluminized silicon coated automotive outer panel according to claim 1, characterized in that, The composition of the aluminized silicon liquid, by mass percentage, is: Si 7%–12%, Fe ≤2%, with the remainder being Al and unavoidable impurity elements.

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