Ferrite bainite plated steel sheet and method for manufacturing the same

Abstract

The application relates to the field of manufacturing technology of advanced high-strength steel for automobiles, in particular to a ferrite-bainite plated steel plate and a preparation method thereof. The chemical components of a steel base body of the plated steel plate are C, Si, Mn, Al, Cr, Mo, Nb, V, Ti, P, S and Fe; wherein, the content of C is 0.12-0.25% in mass fraction, the content of Si is 0.1%-0.3%, the content of Mn is 1.4%-2.5%, the content of Al is 0.5%-0.9%, the content of Cr is 0.05%-0.5%, the content of Mo is 0.02%-0.3%, the content of Nb is 0.02%-0.06%, the content of V is 0-0.2%, the content of Ti is 0.06%-0.15%, the content of P is 0-0.01% and the content of S is 0-0.003%. The application solves the technical problem of low tensile strength of the existing ferrite-bainite plated steel plate.

Description

Technical Field

[0001] This application relates to the field of advanced high-strength steel manufacturing technology for automobiles, and in particular to a ferritic bainitic coated steel sheet and its preparation method. Background Technology

[0002] The automotive industry faces immense pressure to reduce carbon emissions. Among the various emission reduction methods, lightweighting plays a crucial role. The vehicle chassis, a vital component accounting for nearly 20% of the total vehicle weight, is a key pathway and breakthrough point for automotive lightweighting. For passenger car chassis systems, every 1 kg reduction in weight results in a lightweighting effect 5 to 10 times that of other system components. Chassis parts have complex shapes, requiring not only stamping but also frequent hole expansion and flanging. Therefore, ferritic bainitic steel, which possesses excellent ductility and hole expansion / bending properties, is used extensively.

[0003] Currently, the strength of ferritic bainitic steels used is mainly in the 450-580 MPa range, with 780 MPa grade applications being relatively rare, and ultra-high strength grades of 980 MPa being even less common. Therefore, there is an urgent need to develop and apply 980 MPa grade ferritic bainitic steel to improve the lightweighting of chassis systems. Furthermore, for chassis components such as control arms that require particularly high corrosion resistance, the demand for continuously hot-dip galvanized ferritic bainitic steel with high-strength hot-rolled ferritic bainitic steel as the base material is continuously increasing. The application of ferritic bainitic coated steel sheets can improve component lifespan and reduce energy consumption and carbon emissions associated with component repair, replacement, and remanufacturing. Summary of the Invention

[0004] This application provides a ferritic bainitic coated steel sheet and its preparation method to solve the technical problem of low tensile strength of existing ferritic bainitic coated steel sheets.

[0005] In a first aspect, this application provides a ferritic bainitic coated steel sheet, wherein the chemical composition of the steel substrate of the coated steel sheet includes:

[0006] C, Si, Mn, Al, Cr, Mo, Nb, V, Ti, P, S, and Fe; wherein, by mass fraction,

[0007] The content of C is 0.12%–0.25%, the content of Si is 0.1%–0.3%, the content of Mn is 1.4%–2.5%, the content of Al is 0.5%–0.9%, the content of Cr is 0.05%–0.5%, the content of Mo is 0.02%–0.3%, the content of Nb is 0.02%–0.06%, the content of V is 0%–0.2%, the content of Ti is 0.06%–0.15%, the content of P is 0%–0.01%, and the content of S is 0%–0.003%.

[0008] Optionally, the microstructure of the coated steel sheet includes: ferrite, bainite, and carbides; wherein,

[0009] The area fraction of ferrite is 30%–50%, the area fraction of bainite is 50%–70%, and the area fraction of carbides is 0%–3%.

[0010] Optionally, the coated steel sheet has a yield strength ≥750MPa, a tensile strength ≥980MPa, an elongation after fracture ≥15%, and a hole expansion rate ≥35%.

[0011] Secondly, this application provides a method for preparing a ferritic bainitic coated steel sheet, used to prepare the coated steel sheet described in any embodiment of the first aspect, the method comprising:

[0012] The slab is heated under the conditions of a first set time and a set temperature;

[0013] The heated slab is rolled, then cooled in stages and coiled after cooling to obtain a hot-rolled coil; wherein the rolling includes finishing rolling, and the starting rolling temperature and the finishing rolling temperature are controlled.

[0014] Under a second set time condition, the hot-rolled coil is kept at a constant temperature, then cooled and pickled after cooling to obtain a steel matrix;

[0015] The steel substrate is subjected to heat treatment; wherein the heat treatment includes homogenization, and the temperature and time of homogenization are controlled.

[0016] The heat-treated steel substrate is hot-dip galvanized with metal to obtain a ferritic bainitic coated steel sheet; wherein the hot-dip galvanized metal comprises:

[0017] The heat-treated steel substrate is then hot-dip galvanized with aluminum-magnesium alloy; or,

[0018] The heat-treated steel substrate is hot-dip galvanized and then alloyed.

[0019] Optionally, the first set time is 200 min to 300 min, and / or the set temperature is 1240℃ to 1270℃.

[0020] Optionally, the initial rolling temperature of the finishing mill is 980℃~1040℃, and / or the final rolling temperature of the finishing mill is 860℃~920℃.

[0021] Optionally, the step of rolling the heated slab, followed by staged cooling and cooling coiling to obtain a hot-rolled coil includes:

[0022] The heated slab is rolled to obtain a hot-rolled plate;

[0023] The hot-rolled sheet is subjected to a first water cooling, an air cooling, and a second water cooling, and then coiled at the final temperature of the second water cooling to obtain a hot-rolled coil; wherein,

[0024] The cooling rate of the first water cooling is 40℃ / s to 60℃ / s, the final temperature of the first water cooling is 620℃ to 720℃, and the final temperature of the second water cooling is 350℃ to 450℃.

[0025] Optionally, the second set time is ≥12h.

[0026] Optionally, the temperature for heat equalization is 640℃~680℃, and / or the heat equalization time is 30s~70s.

[0027] Optionally, the alloying temperature is 500℃~540℃.

[0028] The technical solutions provided in this application have the following advantages compared with the prior art:

[0029] The ferritic bainitic coated steel sheet provided in this application utilizes the grain refinement strengthening of Nb and Ti during hot rolling and the precipitation strengthening of Nb, V, and Ti during galvanizing to increase the tensile strength of ferritic bainitic steel to 980 MPa and above, which is in line with the trend of lightweight development. The low-Si and high-Al composition design ensures the proportion of ferrite and that the ferritic bainitic steel still achieves an elongation of more than 15% when the tensile strength reaches the 980 MPa level. Attached Figure Description

[0030] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

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

[0032] Figure 1 A schematic flowchart illustrating a method for preparing a ferritic bainitic coated steel sheet according to an embodiment of this application;

[0033] Figure 2 The microstructure of a ferritic bainitic coated steel sheet provided in the embodiments of this application;

[0034] Figure 3 This is a flowchart of an alloying zinc plating process provided for an embodiment of this application. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0036] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0037] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0038] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0039] In a first aspect, this application provides a ferritic bainitic coated steel sheet, wherein the chemical composition of the steel substrate of the coated steel sheet includes:

[0040] C, Si, Mn, Al, Cr, Mo, Nb, V, Ti, P, S, and Fe; wherein, by mass fraction,

[0041] The content of C is 0.12%–0.25%, the content of Si is 0.1%–0.3%, the content of Mn is 1.4%–2.5%, the content of Al is 0.5%–0.9%, the content of Cr is 0.05%–0.5%, the content of Mo is 0.02%–0.3%, the content of Nb is 0.02%–0.06%, the content of V is 0%–0.2%, the content of Ti is 0.06%–0.15%, the content of P is 0%–0.01%, and the content of S is 0%–0.003%.

[0042] In the embodiments of this application, the positive effects of controlling the C content to be 0.12%–0.25% are as follows: C is a highly effective interstitial solid solution atom, which can significantly improve the strength of the matrix. Furthermore, C combines with Nb, V, and Ti to form microalloyed carbide precipitation, resulting in a significant precipitation strengthening effect. To obtain good solid solution strengthening and precipitation strengthening effects and ensure a tensile strength of not less than 980 MPa, the C content should not be less than 0.12%. However, excessively high C content impairs weldability. Specifically, the C content can be 0.12%, 0.20%, 0.25%, etc.

[0043] The positive effects of controlling the Si content to 0.1%–0.3% are as follows: To obtain a tensile strength of over 980 MPa, the steel plate contains relatively high amounts of Mn, Cr, and Mo, elements that improve hardenability and austenite stability. This results in difficulty in forming the desired proportion of ferrite to improve plasticity during the short air cooling section after rolling. To address this defect, an appropriate amount of Si is added to broaden the ferrite formation process window and promote ferrite formation. However, Si is a significant element that causes iron oxide scale to form on the surface of hot-rolled plates. Residual iron oxide scale after pickling can cause surface defects such as incomplete plating and poor coating adhesion. Therefore, the Si content should not be too high. Specifically, the Si content can be 0.1%, 0.2%, 0.3%, etc.

[0044] The positive effects of controlling the Mn content to 1.4%–2.5% include: Mn can improve the strength of the matrix structure through solid solution strengthening and is an important component element for ensuring the tensile strength of ferritic bainitic steel. To ensure a tensile strength of over 980 MPa after galvanizing, it is also necessary to avoid excessive Mn content leading to elemental segregation. Specifically, the Mn content can be 1.4%, 2.0%, 2.5%, etc.

[0045] The positive effects of controlling the Al content to 0.5%–0.9% are as follows: Al is an important ferrite-forming element. To ensure the surface quality of the steel plate, the Si content is controlled below 0.3%, which is insufficient to obtain the desired ferrite ratio. Therefore, the insufficient Si content must be compensated for by increasing the Al content. However, excessive Al content increases the viscosity of molten steel, causing continuous casting nozzle blockage and significantly increasing costs. Specifically, the Al content can be 0.5%, 0.7%, 0.9%, etc.

[0046] The positive effects of controlling the Cr content to 0.05%–0.5% are as follows: Cr is an effective strengthening element in steel, promoting bainite formation and improving tensile strength, without causing elemental segregation like Mn. However, excessively high Cr content can increase hardenability, hindering ferrite formation, and during annealing and galvanizing, Cr easily forms oxides on the steel plate surface, deteriorating plating applicability. Specifically, the Cr content can be 0.05%, 0.1%, 0.3%, 0.5%, etc.

[0047] The positive effects of controlling the Mo content to 0.02%–0.3% include: Mo is an element that improves hardenability, promotes bainite formation, and increases tensile strength; however, Mo is an expensive alloying element, and excessively high Mo content leads to increased costs. Specifically, the Mo content can be 0.02%, 0.1%, 0.2%, 0.3%, etc.

[0048] The positive effects of controlling the Nb content to 0.02%–0.06% include: during slab heating, the dragging effect of dissolved Nb can prevent excessive austenite grain growth; during hot rolling, Nb carbonitrides pinning grain boundaries can further refine the grains, which is extremely beneficial for improving the strength and plasticity of ferritic bainitic steel. Furthermore, during annealing and galvanizing, Nb carbonitrides further precipitate on ferrite, which can improve the strength of ferrite and reduce the hardness difference between ferrite and bainite, thus improving local forming properties such as hole expansion and bending. However, Nb is relatively expensive and should not be added in excessive amounts. Specifically, the Nb content can be 0.02%, 0.04%, 0.06%, etc.

[0049] The positive effects of controlling the V content to 0–0.2% include: V is a strong carbide-forming element; during the annealing and homogenization process of galvanizing, the V precipitates can significantly improve yield strength, reduce the hardness difference between ferrite and bainite, and improve local forming properties such as hole expansion and bending. However, when the V content exceeds 0.2%, the precipitation effect will reach saturation. Specifically, the V content can be 0.1%, 0.2%, etc.

[0050] The positive effects of controlling the Ti content to be between 0.06% and 0.15% are as follows: Ti plays a similar role to Nb in the embodiments of this application, refining grains and providing precipitation strengthening. Furthermore, Ti is relatively inexpensive and does not cause microstructural banding like Nb; therefore, this invention tends to add a higher Ti content. However, when the Ti content reaches 0.15% or more, the strengthening effect reaches saturation. Specifically, the Ti content can be 0.06%, 0.09%, 0.15%, etc.

[0051] The positive effects of controlling the phosphorus (P) content to 0–0.01% include: as an interstitial solid solution atom, P can appropriately improve the strength of steel plates; however, excessively high P content can easily lead to segregation at grain boundaries, thus deteriorating plasticity and formability. Specifically, the P content can be 0.01%, 0.009%, etc.

[0052] The positive effects of controlling the sulfur (S) content to 0–0.003% are that S readily combines with Mn to form coarse MnS inclusions, which deteriorates the forming properties of steel plates, such as hole expansion and flanging. Specifically, the S content can be 0.003%, 0.002%, etc.

[0053] In some embodiments, the microstructure of the coated steel sheet includes ferrite, bainite, and carbides; wherein,

[0054] The area fraction of ferrite is 30%–50%, the area fraction of bainite is 50%–70%, and the area fraction of carbides is 0%–3%.

[0055] The positive effects of controlling the ferrite area fraction to 30%–50% and the bainite area fraction to 50%–70% are that a tensile strength of not less than 980 MPa and an elongation after fracture of not less than 15% can be obtained at this microstructure ratio. If the ferrite area fraction is too high or the bainite area fraction is too low, the tensile strength will be lower than 980 MPa to some extent; if the ferrite area fraction is too low or the bainite area fraction is too high, the elongation after fracture will be lower than 15% to some extent. Specifically, the ferrite area fraction can be 30%, 40%, 50%, etc., and the bainite area fraction can be 50%, 60%, 70%, etc. The microstructure of this steel can be found in [reference needed]. Figure 2 .

[0056] The positive effects of controlling the area fraction of carbides to 0-3% include: ensuring a yield strength of not less than 750 MPa and a porosity of not less than 35%. Specifically, the area fraction of bainite can be 1%, 2%, 3%, etc. This carbide is distributed in the ferrite matrix or at grain boundaries, specifically including one or more of cementite, NbC, VC, TiC, and (Nb, Ti)C.

[0057] In some embodiments, the yield strength of the coated steel sheet is ≥750 MPa, and the tensile strength of the coated steel sheet is...

[0058] The strength is ≥980MPa, the elongation after fracture of the coated steel sheet is ≥15%, and the hole expansion rate of the coated steel sheet is ≥35%.

[0059] In the embodiments of this application, the above-mentioned ferritic bainitic coated steel sheet has excellent mechanical properties.

[0060] Secondly, this application provides a method for preparing a ferritic bainitic coated steel sheet; please refer to [link to method]. Figure 1 The method for preparing the coated steel sheet according to any embodiment of the first aspect comprises:

[0061] S1. Under the conditions of a first set time and a set temperature, the slab is heated;

[0062] In some embodiments, the first set time is 200 min to 300 min, and / or the set temperature is 1240 °C to 1270 °C.

[0063] "First set time" indicates the heating time, and "set temperature" indicates the heating temperature;

[0064] The positive effects of controlling the heating time to 200-300 minutes include: ensuring sufficient solid solution of microalloying elements and reasonable dimensional structure of the cast billet. If the heating time is too short, the microalloying elements may not be fully dissolved, resulting in excessive deformation resistance during steel plate rolling; if the heating time is too long, the microstructure may grow and coarsen, leading to insufficient tensile strength of the steel plate and making the surface prone to overheating. Specifically, the heating time can be 200 minutes, 250 minutes, 300 minutes, etc.

[0065] The positive effects of controlling the heating temperature to 1240℃~1270℃ include ensuring sufficient solid solution of microalloying elements and reasonable microstructure dimensions of the cast billet. In the embodiments of this application, a relatively high content of microalloying elements such as Nb, V, and Ti is added. If the heating temperature is too low, the microalloying elements may not dissolve sufficiently; if the heating temperature is too high, it may lead to coarse grains, which is detrimental to the local forming performance of the steel plate. Specifically, the heating temperature can be 1240℃, 1250℃, 1260℃, 1270℃, etc.

[0066] S2. The heated slab is rolled, then cooled in stages and coiled after cooling to obtain a hot-rolled coil; wherein the rolling includes finishing rolling, and the starting rolling temperature and the finishing rolling temperature are controlled.

[0067] In some embodiments, the initial rolling temperature of the finishing mill is 980°C to 1040°C, and / or the final rolling temperature of the finishing mill is 860°C to 920°C.

[0068] The positive effects of controlling the initial rolling temperature of the finishing mill to 980℃~1040℃ include: good surface quality and uniform microstructure of the hot-rolled coil. If the initial rolling temperature is too high, it can lead to an increase in iron oxide scale on the surface of the hot-rolled steel sheet, which is detrimental to subsequent annealing and galvanizing; if the initial rolling temperature is too low, it can increase the temperature drop at the edges of the intermediate billet, resulting in uneven temperature distribution across the sheet width and potentially causing differences in microstructure and properties. Specifically, the initial rolling temperature for the finishing mill can be 980℃, 1000℃, 1020℃, 1040℃, etc.

[0069] The positive effects of controlling the final rolling temperature of 860℃~920℃ are: high tensile strength and expansion rate, as well as good rolling stability, can be obtained at this temperature. If the final rolling temperature is too high, the austenite grains will be relatively large to some extent. The ferrite and bainite formed after cooling and coiling will inherit the relatively coarse grain size of austenite, which can easily lead to insufficient tensile strength of the steel plate. If the final rolling temperature is too low, the non-recrystallization zone will be rolled to some extent, resulting in obvious banded microstructure. The uneven microstructure will lead to a decrease in the expansion performance of the steel plate. In addition, since the alloy content is high in the embodiments of this application, a low final rolling temperature may also increase the rolling force and increase the mill load. Specifically, the final rolling temperature of the finishing mill can be 860℃, 880℃, 900℃, 920℃, etc.

[0070] The above rolling process also includes roughing, with a roughing start temperature of 1100℃~1150℃ and a roughing finish temperature of 1040℃~1090℃.

[0071] In some embodiments, the process of rolling the heated slab, followed by staged cooling and subsequent coiling to obtain a hot-rolled coil includes:

[0072] The heated slab is rolled to obtain a hot-rolled plate;

[0073] The hot-rolled sheet is subjected to a first water cooling, an air cooling, and a second water cooling, and then coiled at the final temperature of the second water cooling to obtain a hot-rolled coil; wherein,

[0074] The cooling rate of the first water cooling is 40℃ / s to 60℃ / s, the final temperature of the first water cooling is 620℃ to 720℃, and the final temperature of the second water cooling is 350℃ to 450℃.

[0075] The positive effects of controlling the cooling rate of the first water cooling stage to be 40℃ / s to 60℃ / s include: at this cooling rate, the final cooling temperature and grain size can be reasonably controlled. If the cooling rate of the first water cooling stage is too high, it will be difficult to accurately control the final water cooling temperature and overcooling may occur, leading to poor strip shape; if the cooling rate of the first water cooling stage is too low, the strip will remain in the high-temperature zone for a longer period, making the grains prone to growth and coarsening. Specifically, the cooling rate of the first water cooling stage can be 40℃ / s, 50℃ / s, 60℃ / s, etc.

[0076] The positive effects of controlling the final temperature of the first water cooling to 620℃~720℃ are: this temperature achieves a reasonable ferrite ratio, thus resulting in ideal tensile strength. If the final temperature of the first water cooling is too high, excessive ferrite will form and grow, leading to a decrease in tensile strength; if the final temperature is too low, the ferrite ratio will be insufficient during a short air cooling time. Specifically, the final temperature of the first water cooling can be 620℃, 680℃, 720℃, etc.

[0077] The endpoint temperature of the second water cooling, which is also the coiling temperature and the subsequent holding temperature after coiling, has the following positive effects when the coiling temperature is controlled between 350℃ and 450℃: it can avoid premature precipitation of microalloying elements and the formation of unnecessary martensite. If the coiling temperature is too high, microalloying carbides such as Nb, V, and Ti are more likely to precipitate during the coiling process, thus weakening their precipitation effect in the subsequent annealing and galvanizing process. Furthermore, the precipitated phases during the coiling process will grow and coarsen during the subsequent annealing and homogenization process, which is not conducive to improving yield strength and tensile strength. If the coiling temperature is too low, it will be unfavorable for bainitic phase transformation to some extent, resulting in the formation of undesirable martensite, leading to excessively high tensile strength and insufficient plasticity in the steel plate. Specifically, the endpoint temperature of the second water cooling can be 620℃, 680℃, 720℃, etc.

[0078] The air cooling time is 2s to 5s, and the cooling rate of the second water cooling is 20℃ / s to 40℃ / s.

[0079] S3. Under the condition of a second set time, the hot-rolled coil is kept at a constant temperature, then cooled and pickled after cooling to obtain a steel matrix.

[0080] In some implementations, the second set time is ≥12h.

[0081] "Second set time" refers to the heat preservation time, which means that the steel plate is immediately placed in a constant temperature environment after being coiled, and the heat preservation temperature is the same as the coiling temperature. The positive effects of controlling the heat preservation time to ≥12h are as follows: Unlike the traditional ferritic bainitic steel production process, this embodiment adds a heat preservation process after coiling. This is because the coiling temperature in this embodiment is relatively low, reducing the driving force of the bainitic phase transformation. Therefore, it is necessary to add a constant temperature heat preservation process to provide sufficient time for the bainitic phase transformation, avoiding excessively rapid cooling after coiling to form martensite, and effectively improving the uniformity of microstructure and mechanical properties. Specifically, the heat preservation time can be 12h, 13h, 14h, etc.

[0082] S4. The steel substrate is subjected to heat treatment; wherein the heat treatment includes homogenization, and the temperature and time of homogenization are controlled.

[0083] In some embodiments, the temperature for heat equalization is 640°C to 680°C, and / or the heat equalization time is 30s to 70s.

[0084] The positive effects of controlling the soaking temperature to 640℃~680℃ include achieving higher tensile strength and better galvanized surface quality. If the soaking temperature is too high, it can lead to significant tempering of the microstructure and easier growth and coarsening of microalloyed precipitates, resulting in a significant decrease in the tensile strength of the steel plate. If the soaking temperature is too low, it can lead to insufficient reduction of the steel plate at high temperatures, poor surface wettability, weaker chemical interaction between the plating bath and the steel plate surface, and a higher likelihood of zinc flow defects. Specifically, the soaking temperature can be 640℃, 660℃, 680℃, etc.

[0085] The positive effects of controlling the soaking time to 30s–70s include achieving higher tensile strength and porosity. If the soaking time is too long, it can lead to severe bainite decomposition during tempering, making it difficult to guarantee a tensile strength above 980MPa. If the soaking time is too short, it can result in insufficient precipitation of Nb, V, and Ti carbides during the soaking stage, failing to effectively improve the strength and hardness of the ferrite structure and reduce the hardness difference between the soft and hard phases. Specifically, the soaking time can be 30s, 50s, 70s, etc.

[0086] The heat treatment also includes: preheating the steel substrate to 210°C to 230°C, and then heating it to the above-mentioned homogenization temperature at a rate of 15°C / s to 25°C / s.

[0087] S5. The heat-treated steel substrate is hot-dip galvanized with metal to obtain a ferritic bainitic coated steel sheet; wherein the hot-dip galvanized metal comprises:

[0088] The heat-treated steel substrate is then hot-dip galvanized with aluminum-magnesium alloy; or,

[0089] The heat-treated steel substrate is hot-dip galvanized and then alloyed.

[0090] In some embodiments, the alloying temperature is 500°C to 540°C.

[0091] Alloying refers to the diffusion and reaction between the steel substrate and the zinc layer to form a Zn-Fe alloy coating with a suitable phase structure and optimal iron content. Controlling the alloying temperature to 500℃~540℃ has the advantage of achieving ideal alloyed coating quality. If the alloying temperature is too high, the coating's resistance to powdering will decrease to some extent; if the alloying temperature is too low, the diffusion reaction will be insufficient, resulting in inadequate alloying. Specifically, the alloying temperature can be 500℃, 520℃, 540℃, etc.

[0092] The steps for hot-dip galvanizing the heat-treated steel substrate are as follows: the heat-treated steel substrate is cooled to 430℃~450℃ at a rate of 8℃ / s~16℃ / s and then placed in a zinc pot for galvanizing.

[0093] The hot-dip galvanizing process for the heat-treated steel substrate involves: cooling the heat-treated steel substrate to 450°C–470°C at a rate of 8°C / s–16°C / s, immersing it in a zinc bath for galvanizing, removing it from the zinc bath and purging it with an air knife to 420°C–440°C, followed by alloying treatment at the alloying temperature for 10–30 seconds. The above heat treatment process and the alloying galvanizing process curve can be found in [reference needed]. Figure 3 .

[0094] Through continuous hot-dip galvanizing of aluminum-magnesium alloys or alloy galvanizing, the corrosion resistance is significantly improved compared to traditional hot-rolled ferritic bainitic steel, which significantly extends the service life of chassis parts and reduces the increased costs, energy consumption, and carbon emissions caused by parts repair, replacement, and remanufacturing.

[0095] The preparation method of the ferritic bainitic coated steel sheet is based on the above-mentioned ferritic bainitic coated steel sheet. The chemical composition of the ferritic bainitic coated steel sheet can be referred to the above embodiments. Since the preparation method of the ferritic bainitic coated steel sheet adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0096] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed. Molten steel is cast into slabs, the chemical composition of which is shown in Table 1.

[0097] Table 1. Chemical composition (wt%) of ferritic bainitic coated steel sheets

[0098]

[0099]

[0100] Specific process steps:

[0101] S1. Under the conditions of a first set time and a set temperature, the slab is heated;

[0102] S2. The heated slab is rolled, then cooled in stages and coiled after cooling to obtain a hot-rolled coil.

[0103] The rolling process includes finishing rolling, and the starting rolling temperature and the finishing rolling temperature are controlled.

[0104] S3. Under the condition of a second set time, the hot-rolled coil is kept at a constant temperature, then cooled and pickled after cooling to obtain a steel matrix.

[0105] S4. The steel substrate is subjected to heat treatment; wherein the heat treatment includes homogenization, and the temperature and time of homogenization are controlled.

[0106] S5. The heat-treated steel substrate is hot-dip galvanized with metal to obtain a ferritic bainitic coated steel sheet; wherein the hot-dip galvanized metal comprises:

[0107] The heat-treated steel substrate is then hot-dip galvanized with aluminum-magnesium alloy; or,

[0108] The heat-treated steel substrate is hot-dip galvanized and then alloyed.

[0109] For specific process parameters, please refer to Tables 2 and 3.

[0110] Table 2. Process parameters for hot rolling and cooling of ferritic bainitic coated steel sheets

[0111]

[0112]

[0113] Table 3 Hot-dip galvanizing process parameters for ferritic bainitic coated steel sheets

[0114]

[0115] Microstructure analysis: Samples for microstructure observation were prepared and etched with a 4% (v / v) nitric acid alcohol solution. Images were obtained under a metallographic microscope, where the gray-white structure was ferrite and the dark structure was bainite. The precipitation of cementite and carbides of Nb, V, and Ti in the microalloys was observed using a transmission electron microscope, and the precipitation ratio was statistically analyzed.

[0116] Mechanical property testing: The yield strength, tensile strength and elongation after fracture of the ferritic bainitic coated steel sheet were tested using a ZWICK / Roell Z100 tensile testing machine, and the hole expansion rate was tested using a ZWICK BUP1000 forming testing machine.

[0117] According to GB / T 10125 standard, the weight of a single-sided coating is 50g / m². 2 The samples were subjected to a neutral salt spray test, and the time it took for red rust to appear on the sample surface was measured to evaluate the corrosion resistance.

[0118] The microstructure and mechanical properties of ferritic bainitic coated steel sheets can be found in Table 4.

[0119] Table 4 Microstructure and Mechanical Properties of Ferritic-Bainitic Coated Steel Sheets

[0120]

[0121] As shown in Table 4, the ferritic bainitic coated steel sheet in this embodiment has a yield strength ≥750MPa and a tensile strength ≥750MPa.

[0122] ≥980MPa, elongation after fracture ≥15%, porosity ≥35%, corrosion resistance is superior in Examples 2 and 3;

[0123] In Comparative Example 1, the mass fraction of Mn is outside the range of the embodiments of the present invention, and the yield strength of the steel plate is only 738 MPa and the tensile strength is only 935 MPa, which does not reach the level of 980 MPa and above.

[0124] In Comparative Example 2, the soaking temperature was outside the range of the embodiments of the present invention, and the yield strength of the steel plate was only 680 MPa and the tensile strength was only 853 MPa, which did not reach the level of 980 MPa and above.

[0125] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A ferritic bainitic plated steel sheet characterized by, The chemical composition of the steel substrate of the plated steel sheet comprises: C, Si, Mn, Al, Cr, Mo, Nb, V, Ti, P, S, and the balance being Fe; wherein, in mass fraction, the content of C is 0.12-0.25%, the content of Si is 0.1%-0.3%, the content of Mn is 1.4%-2.5%, the content of Al is 0.5%-0.9%, the content of Cr is 0.05%-0.5%, the content of Mo is 0.02%-0.3%, the content of Nb is 0.02%-0.06%, the content of V is 0-0.2%, the content of Ti is 0.06%-0.15%, the content of P is 0-0.01%, and the content of S is 0-0.003%; The preparation method of the plated steel sheet comprises: heating the slab under the condition of a first set time and a set temperature; rolling the heated slab, then carrying out stage cooling and coiling after cooling to obtain a hot-rolled coil; wherein, the rolling comprises finish rolling, and the opening temperature of the finish rolling and the final rolling temperature of the finish rolling are controlled; insulating the hot-rolled coil under the condition of a second set time, then carrying out cooling and pickling after cooling to obtain a steel substrate; carrying out heat treatment on the steel substrate; wherein, the heat treatment comprises soaking, and the temperature and time of the soaking are controlled; carrying out hot-dip metal plating on the steel substrate after heat treatment to obtain a ferrite-bainite plated steel sheet; wherein, the hot-dip metal plating comprises: carrying out hot-dip zinc-aluminum-magnesium plating on the steel substrate after heat treatment; or, carrying out hot-dip zinc plating on the steel substrate after heat treatment, then carrying out alloying; the opening temperature of the finish rolling is 980-1040℃, and the final rolling temperature of the finish rolling is 860-920℃; the rolling of the heated slab, then carrying out stage cooling and coiling after cooling to obtain a hot-rolled coil, comprises: rolling the heated slab to obtain a hot-rolled plate; carrying out first water cooling, air cooling and second water cooling on the hot-rolled plate, then coiling under the condition of the terminal temperature of the second water cooling ; wherein, the cooling speed of the first water cooling is 40-60℃ / s, the terminal temperature of the first water cooling is 620-720℃, and the terminal temperature of the second water cooling is 350-450℃; the second set time is ≥12h; the temperature of the soaking is 640-680℃, and the soaking time is 30-70s; the time of the air cooling is 2-5s, and the cooling speed of the second water cooling is 20-40℃ / s.

2. The plated steel sheet according to claim 1, characterized by, The microstructure of the plated steel sheet comprises: ferrite, bainite and carbide; wherein, the area fraction of the ferrite is 30%-50%, the area fraction of the bainite is 50%-70%, and the area fraction of the carbide is 0-3%. The yield strength of the plated steel sheet is ≥750MPa, the tensile strength of the plated steel sheet is ≥980MPa, the elongation after fracture of the plated steel sheet is ≥15%, and the reaming rate of the plated steel sheet is ≥35%.

3. The plated steel sheet according to claim 1 or 2, characterized by, ​ 4. The plated steel sheet according to claim 1 or 2, characterized by, The first set time is 200 min to 300 min, and or the set temperature is 1240°C to 1270°C.

5. The method of claim 1, wherein, The alloying temperature is 500°C to 540°C.

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