980mpa grade hot-dip galvanizing multiphase steel with high hole expansion property and preparation method thereof

Abstract

The application provides a 980MPa-grade hot-dip galvanizing multiphase steel with high hole expansion property and a preparation method, and belongs to the field of steel preparation. The hot-dip galvanizing multiphase steel comprises a steel base and a galvanizing layer attached to at least part of the surface of the steel base, and the chemical composition of the steel base comprises, in mass fraction, C: 0.03% to 0.3%, Si: 0.1% to 1.0%, Mn: 0.2% to 3.0%, P: less than or equal to 0.01%, S: less than or equal to 0.006%, Cr: 0.05% to 1.0%, Nb: 0.01% to 0.1%, Ti: 0.01% to 0.1%, and the balance of Fe and inevitable impurities; and the metallographic structure of the steel base comprises, in volume fraction, bainite: 10% to 20%, martensite: 30% to 50%, and ferrite: 20% to 30%.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to a 980MPa grade hot-dip galvanized multiphase steel with high porosity and its preparation method. Background Technology

[0002] To meet the automotive industry's demands for greater safety, lighter weight, and environmental friendliness, the use of advanced high-strength steel is essential for achieving both lightweighting and safety. Among these advanced high-strength steels, multiphase steel has fine grains and high tensile strength. Compared to dual-phase steel with the same tensile strength, multiphase steel exhibits superior bending and flanging properties, as well as a higher yield strength ratio. Therefore, it can improve the resistance of critical components to intrusion, which is of great significance for enhancing safety performance.

[0003] Dual-phase steel is widely used due to its good comprehensive properties. However, it is undeniable that dual-phase steel has a high cracking rate when forming parts with requirements for hole expansion and flanging. The multiphase microstructure of dual-phase steel plays a positive role in improving edge ductility. However, in the existing technology, the excessive addition of alloying elements leads to increased alloy costs. At the same time, the insufficient local forming properties such as hole expansion of traditional dual-phase steel make it difficult to form complex parts. Summary of the Invention

[0004] This application provides a 980MPa grade hot-dip galvanized multiphase steel with high hole expansion capability and its preparation method, in order to solve the technical problem of insufficient hole expansion capability and other local forming properties of traditional multiphase steel.

[0005] In a first aspect, this application provides a 980MPa grade hot-dip galvanized multiphase steel with high porosity. The hot-dip galvanized multiphase steel includes a steel substrate and a zinc coating adhered to at least a portion of the surface of the steel substrate. The chemical composition of the steel substrate, by mass fraction, includes: C: 0.03%–0.3%, Si: 0.1%–1.0%, Mn: 0.2%–3.0%, P≤0.01%, S≤0.006%, Cr: 0.05%–1.0%, Nb: 0.01%–0.1%, Ti: 0.01%–0.1%, with the balance being Fe and unavoidable impurities.

[0006] The metallographic structure of the steel matrix, by volume fraction, comprises: bainite: 10%–20%, martensite: 30%–50%, and ferrite: 20%–30%.

[0007] Optionally, the grain diameter of the bainite is 1 μm to 2 μm, the grain diameter of the martensite is 1 μm to 4 μm, and the grain diameter of the ferrite is 3 μm to 6 μm.

[0008] Optionally, the chemical composition satisfies the following relationship: 0.001×[TS]-[Si] / 1.3<[Cr]+[Mn] / 5,

[0009] In the formula, [TS] represents the design value of the tensile strength of the hot-dip galvanized multiphase steel, [Si] represents the content of Si, [Cr] represents the content of Cr, and [Mn] represents the content of Mn.

[0010] Optionally, the hot-dip galvanized multiphase steel meets at least one of the following properties: yield strength ≥780MPa, tensile strength ≥980MPa, elongation ≥10%, and hole expansion rate ≥50%.

[0011] Secondly, this application provides a method for preparing hot-dip galvanized multiphase steel according to any embodiment of the first aspect, the method comprising:

[0012] A slab having the aforementioned chemical composition is obtained;

[0013] The slab is rolled and coiled to obtain a hot-rolled coil;

[0014] The hot-rolled coil is then cold-rolled to obtain a steel matrix;

[0015] The steel substrate is pre-plated to obtain a pre-plated steel plate; and

[0016] The pre-coated steel sheet is subjected to continuous hot-dip galvanizing annealing to obtain hot-dip galvanized multiphase steel.

[0017] Optionally, the step of continuously hot-dip galvanizing and annealing the pre-coated steel sheet to obtain hot-dip galvanized multiphase steel includes:

[0018] The pre-coated steel plate is first heated, and the following parameters of the first heating are controlled: first heating rate, first heating endpoint temperature and first holding time;

[0019] The pre-coated steel plate after the first heating is subjected to a second heating, and the following parameters of the second heating are controlled: second heating rate, second heating endpoint temperature and second holding time;

[0020] The pre-coated steel plate after the second heating is subjected to a first cooling, and the following parameters of the first cooling are controlled: first cooling rate and first cooling endpoint temperature;

[0021] The pre-coated steel plate after the first cooling is subjected to a second cooling, and the following parameters of the second cooling are controlled: the second cooling rate and the second cooling endpoint temperature;

[0022] The pre-coated steel sheet after the second cooling is hot-dip galvanized to obtain a galvanized steel sheet;

[0023] The galvanized steel sheet is subjected to a third cooling process, and the following parameters of the third cooling process are controlled: the third cooling rate and the third cooling endpoint temperature. After the cooling process is completed, the sheet is flattened to obtain hot-dip galvanized multiphase steel.

[0024] Optionally, the first heating rate is 2℃ / s to 5℃ / s, the first heating endpoint temperature is 640℃ to 680℃, and the first holding time is 0.5 min to 3 min; and / or,

[0025] The second heating rate is 3℃ / s to 8℃ / s, the second heating endpoint temperature is (Ac3+10)℃ to (Ac3+40)℃, and the second holding time is 3min to 6min.

[0026] Optionally, the first cooling rate is 1℃ / s to 15℃ / s, and the first cooling endpoint temperature is Ac1 to Ac3; and / or,

[0027] The second cooling rate is ≥45℃ / s, and the second cooling endpoint temperature is 440℃~540℃; and / or,

[0028] The third cooling rate is ≥30℃ / s, and the third cooling endpoint temperature is ≤Mf.

[0029] Optionally, the step of pre-plating the steel substrate to obtain a pre-plated steel plate includes:

[0030] The steel substrate is pretreated before plating.

[0031] The pretreated steel substrate is heated to (Ac3+10)℃~(Ac3+40)℃;

[0032] A pre-plated metal is applied to at least a portion of the surface of the steel substrate heated to (Ac3+10)℃~(Ac3+40)℃, followed by post-plating treatment to obtain a pre-plated steel sheet. The pre-plated metal includes one or more of Ni, Fe, Co, and Cu.

[0033] Optional, relative to an area of ​​1m 2 The amount of the pre-plated metal in the steel substrate is 50mg to 1200mg.

[0034] The technical solutions provided in this application have the following advantages compared with the prior art:

[0035] This application discloses a 980MPa grade hot-dip galvanized multiphase steel with high porosity. By rationally designing the steel's chemical composition, the target microstructure is optimized, resulting in a uniform and refined multiphase microstructure with a porosity exceeding 50%. Controlling the amount of Si added improves the coating performance during the hot-dip galvanizing process, while limiting the addition of Cr and Mn allows for a good balance between strength and elongation. The product of this invention has a yield strength ≥780MPa, tensile strength ≥980MPa, and elongation ≥10%, while also exhibiting excellent porosity expansion performance, with a porosity ≥50%. Attached Figure Description

[0036] 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.

[0037] 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.

[0038] Figure 1 A schematic flowchart illustrating a method for preparing hot-dip galvanized multiphase steel provided in this application embodiment;

[0039] Figure 2 Metallographic diagram of the steel matrix provided in Embodiment 1 of this application;

[0040] Figure 3 The images show hot-dip galvanized multiphase steel samples after hole enlargement, provided in Examples 1(a) and 4(b) of this application. Detailed Implementation

[0041] 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.

[0042] 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.

[0043] Furthermore, in the description of this application, the terms "comprising," "including," etc., 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" 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.

[0044] 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.

[0045] The technical solution of this application embodiment is to solve the above-mentioned technical problems, and the overall concept is as follows:

[0046] This application systematically studies hot-dip galvanized multiphase steel with excellent porosity and coatability, and its preparation method. The results show that, under conditions of high carbon and manganese content, the target microstructure can be optimized by increasing the annealing temperature during hot-dip galvanizing and simultaneously implementing a stepped heating process. During the stepped heating process, the strip steel is heated to below Ac1 for isothermal treatment. This allows for more complete recrystallization of ferrite grains in different locations, reducing grain size differences. Simultaneously, martensite undergoes more thorough tempering, transforming into equiaxed ferrite and granular cementite; pearlite also undergoes significant spheroidization. Since grain boundaries are rapid diffusion channels for carbon atoms, carbon atoms gradually diffuse into the originally carbon-depleted ferrite region during the isothermal process, resulting in a uniform distribution of carbide particles. After heating above Ac1, the reverse-transformed austenite is uniformly distributed in the matrix, thereby eliminating the inherited unevenness of the original microstructure. Subsequently, the temperature is heated to Ac3+10~40℃ for full austenitization. After cold rolling, the fibrous microstructure completes the recovery and recrystallization process, transforming entirely into austenite. Simultaneously, residual cementite is fully dissolved, resulting in complete homogenization of the austenite's composition and microstructure. Cooling is then performed at 1-15℃ / s to the Ac1-Ac3 temperature range to suppress the formation of new ferrite during the initial cooling process. This not only avoids the evolution of microstructure inhomogeneity but also prevents a decrease in austenite hardenability due to carbon enrichment. Finally, the temperature is cooled at a rate of 45℃ / s or higher to 440-540℃ for hot-dip galvanizing, completing the process. This temperature range falls within the bainitic transformation region. When the hardenability of austenite is insufficient, bainitic transformation inevitably occurs, resulting in the formation of a certain amount of bainite. After the experimental steel exits the zinc bath, it is cooled at a rate exceeding 30℃ / s to below the martensitic transformation temperature, completing the austenitic-to-martensite transformation and ultimately forming a multiphase microstructure of martensite, bainite, and ferrite. This yields a fine and uniform martensite and ferrite dual-phase microstructure, containing approximately 10-20% bainite, with a martensite volume fraction exceeding 50%, the remainder being ferrite. The martensite grain diameter is 1-4 μm, the ferrite grain diameter is 3-6 μm, and the bainite grain diameter is 1-2 μm. Furthermore, by optimizing the steel composition and controlling the amount of Si added, the coating performance during hot-dip galvanizing can be improved. Simultaneously, limiting the addition of Cr and Mn allows for a good balance between strength and elongation.

[0047] This application provides a 980MPa grade hot-dip galvanized multiphase steel with high porosity. The hot-dip galvanized multiphase steel includes a steel substrate and a zinc coating adhered to at least a portion of the surface of the steel substrate. The chemical composition of the steel substrate, by mass fraction, includes: C: 0.03%–0.3%, Si: 0.1%–1.0%, Mn: 0.2%–3.0%, P≤0.01%, S≤0.006%, Cr: 0.05%–1.0%, Nb: 0.01%–0.1%, Ti: 0.01%–0.1%, with the balance being Fe and unavoidable impurities.

[0048] The metallographic structure of the steel matrix, by volume fraction, comprises: bainite: 10%–20%, martensite: 30%–50%, and ferrite: 20%–30%.

[0049] In the embodiments of this application, the positive effects of controlling the C content to be 0.03% to 0.3% are as follows: C is the most important solid solution strengthening element and the element that improves the hardenability of austenite in hot-dip galvanized duplex steel. In order to obtain sufficient martensite during the cooling process to ensure a tensile strength of over 980 MPa, and at the same time, C forms carbonitrides with microalloyed Nb and Ti elements during heat treatment, which refines the grains and strengthens ferrite, improves the strength and hardness in ferrite, and helps to obtain a yield strength of over 780 MPa. At the same time, it reduces the hardness difference with martensite and improves the hole expansion performance. However, considering that the C content should not be too high and deteriorate the weldability, the C content is preferably controlled to be 0.15% to 0.20%. For example, the C content can be 0.03%, 0.08%, 0.10%, 0.15%, 0.20%, 0.25%, 0.28%, 0.3%, etc.

[0050] The positive effects of controlling the Si content to 0.1%–1.0% include: Si is an important solid solution strengthening element, and it can effectively promote the enrichment of C into austenite, improving the hardenability of austenite while purifying the ferrite phase and improving elongation. 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 color difference. To obtain good galvanized surface quality, this invention preferably controls the Si content to be between 0.2% and 0.5%. For example, the Si content can be 0.1%, 0.2%, 0.4%, 0.5%, 0.7%, 0.8%, 0.9%, 1.0%, etc.

[0051] The positive effects of controlling the Mn content to be between 0.2% and 3.0% include: Mn is an austenite stabilizing element; during annealing, it can diffuse from ferrite to austenite, improving austenite stability and hardenability. Additionally, Mn can enhance the strength of bainite and martensite through solid solution strengthening, ensuring that the multiphase steel achieves a high yield strength of over 780 MPa and a high tensile strength of over 980 MPa. However, excessive Mn content can easily cause microstructure segregation, leading to forming cracks and deteriorating the overall performance of the steel. Furthermore, Mn can accumulate on the surface during annealing; therefore, the Mn content should not be too high. For example, the Mn content can be 0.2%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, etc.

[0052] The positive effects of controlling the P content to ≤0.01% are: as an interstitial solid solution atom, P can appropriately improve the strength of steel plates, but it is also prone to segregation at grain boundaries, which can deteriorate plasticity and formability. For example, the P content can be 0.005%, 0.007%, 0.009%, 0.01%, etc.

[0053] The positive effects of controlling the sulfur content to ≤0.006% are that sulfur easily combines with manganese (Mn) to form coarse MnS inclusions, which deteriorates the forming properties of steel plates, such as hole expansion and flanging. For example, the sulfur content can be 0.003%, 0.004%, 0.005%, 0.006%, etc.

[0054] The positive effects of controlling the Cr content to 0.05%–1.0% are as follows: Cr can improve the hardenability of austenite in steel, thereby obtaining a sufficient amount of martensite to ensure strength. Adding a certain amount of Cr can replace the expensive Mo element to obtain a tensile strength of over 980 MPa while ensuring that the alloy manufacturing cost is sufficiently low. However, Cr is also a ferrite region expanding element; excessive Cr will lead to a shrinkage of the two-phase region and will also hinder bainite transformation. Therefore, limiting the addition of Cr is necessary to ensure the acquisition of a certain amount of bainite. For example, the Cr content is 0.05%, 0.10%, 0.20%, 0.40%, 0.50%, 0.70%, 0.90%, 1.0%, etc.

[0055] The positive effects of controlling the Nb content to 0.01%–0.1% include: Nb not only has a significant grain-refining effect but is also a strong carbide-forming element. It can improve the yield strength of multiphase steel through precipitation strengthening, reduce the hardness difference between ferrite and bainite / martensite, and improve local forming properties such as hole expansion and bending. However, Nb significantly inhibits recrystallization. Excessive Nb content can cause the annealing stage to enter the non-recrystallized zone, leading to the formation of deformation bands along the rolling direction and resulting in uneven properties. Examples of Nb contents include 0.01%, 0.03%, 0.04%, 0.06%, 0.08%, and 0.1%.

[0056] The positive effects of controlling the Ti content to 0.01%–0.1% include: Like Nb, Ti is a strong carbide-forming element, which can improve yield strength through precipitation strengthening. Simultaneously, Ti is beneficial for refining grains and obtaining a uniform equiaxed microstructure. Furthermore, Ti is less expensive than Nb. To achieve good precipitation strengthening while avoiding precipitation saturation, exemplary Ti contents of 0.01%, 0.03%, 0.04%, 0.06%, 0.08%, and 0.1% are used.

[0057] The positive effects of controlling the bainite volume fraction to 10%–20% include: achieving a higher yield strength ratio; reducing the hardness gradient between martensite and ferrite; and obtaining excellent porosity. However, if this volume fraction is too high, the corresponding martensite volume fraction decreases, which is detrimental to achieving the required tensile strength; conversely, if it is too low, the negative impact will not achieve the desired effect. Examples of bainite volume fractions include 10%, 12%, 14%, 16%, 18%, and 20%.

[0058] The positive effects of controlling the volume fraction of martensite to be between 30% and 50% are that it provides the required tensile strength for the product. An excessively high volume fraction results in excessively high strength, while an excessively low volume fraction results in excessively low strength. For example, the volume fraction of martensite can be 30%, 32%, 35%, 38%, 40%, 42%, 45%, 50%, etc.

[0059] The principle behind the synergistic effect (i.e., 1+1+1>3) of 10%–20% volume fraction bainite, 30%–50% volume fraction martensite, and the balance ferrite is as follows: By precisely controlling the volume fraction and size of each phase in the microstructure through appropriate multiphase microstructure regulation, effects that cannot be achieved by iron-martensite dual-phase or single-phase microstructures can be obtained. Introducing a certain amount of bainite phase into the ferrite and martensite phases in the multiphase microstructure is beneficial for obtaining a higher yield strength ratio and excellent porosity. For example, the volume fraction of ferrite can be 20%, 22%, 24%, 26%, 28%, 30%, etc.

[0060] In some embodiments, the grain diameter of the bainite is 1 μm to 2 μm, the grain diameter of the martensite is 1 μm to 4 μm, and the grain diameter of the ferrite is 3 μm to 6 μm.

[0061] The positive effects of controlling the bainite grain size to be 1μm to 2μm include: achieving the desired strengthening effect; a grain size that is too large results in a weak strengthening effect, while a grain size that is too small results in a strong strengthening effect; furthermore, the bainite size in this invention is already very small. For example, the bainite grain size can be 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2μm, etc.

[0062] The positive effects of controlling the martensite grain size to be between 1 μm and 4 μm include: fine and dispersed martensite facilitates uniform strain distribution during forming, resulting in better porosity. If the grain size is too large, martensite bands are easily formed, which can lead to initial cracks at the phase interface during forming and premature cracking. The martensite islands in this invention are already very fine; further minimization would significantly increase production difficulty and be of little practical use. For example, the martensite grain sizes are 1 μm, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, and 4 μm.

[0063] The positive effects of controlling the ferrite grain size to be 3μm to 6μm include providing the required plasticity. If the grain size is too large, the soft phase is too large, which is not conducive to uniform strain distribution and expansion during forming. If the grain size is too small, it increases excessive fine-grain strengthening, which is not conducive to improving the overall performance of the product. For example, the ferrite grain size is 3μm, 3.2μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, etc.

[0064] By adopting the above design, the final product can achieve a porosity of over 50%.

[0065] In some embodiments, the chemical composition satisfies the following relationship: 0.001×[TS]-[Si] / 1.3<[Cr]+[Mn] / 5

[0066] In the formula, [TS] represents the design value of the tensile strength of the hot-dip galvanized multiphase steel, [Si] represents the content of Si, [Cr] represents the content of Cr, and [Mn] represents the content of Mn.

[0067] The above formula defines the interrelationship between Cr, Mn, and Si elements, meaning that the influence on surface quality and strength is reflected in this formula. If the formula is not satisfied, it is impossible to obtain hot-dip galvanized multiphase steel with a surface quality of over 980 MPa.

[0068] In some embodiments, the hot-dip galvanized multiphase steel satisfies at least one of the following properties: yield strength ≥780MPa, tensile strength ≥980MPa, elongation ≥10%, and hole expansion rate ≥50%.

[0069] By optimizing and controlling the annealing process, a low-cost 980MPa grade hot-dip galvanized multiphase steel product with excellent surface quality was obtained. The product exhibits a yield strength ≥780MPa, tensile strength ≥980MPa, and elongation ≥10%. It also demonstrates excellent hole-expanding performance, with a hole-expanding rate ≥50%. Through optimized and controlled galvanizing annealing process, the banded distribution of carbon-rich manganese martensite islands was suppressed, resulting in a uniform and refined multiphase microstructure and achieving a hole-expanding rate of over 50%. For example, the yield strength of the hot-dip galvanized multiphase steel can be 780MPa, 800MPa, 820MPa, 840MPa, 880MPa, 900MPa, etc., the tensile strength can be 980MPa, 1000MPa, 1020MPa, 1040MPa, 1060MPa, etc., the elongation can be 10%, 10.2%, 10.5%, 10.8%, 11%, 11.5%, etc., and the hole expansion rate can be 50%, 52%, 54%, 56%, 58%, 60%, etc.

[0070] Figure 1 This is a schematic flowchart illustrating a method for preparing hot-dip galvanized multiphase steel, as provided in an embodiment of this application.

[0071] Please see Figure 1 This application provides a method for preparing hot-dip galvanized multiphase steel according to any embodiment of the first aspect, the method comprising:

[0072] S1. Obtain a slab having the aforementioned chemical composition;

[0073] S2. Roll and coil the slab to obtain a hot-rolled coil;

[0074] S3. The hot-rolled coil is cold-rolled to obtain a steel matrix;

[0075] S4. Pre-plat the steel substrate to obtain a pre-plated steel plate;

[0076] In some embodiments, the step of pre-plating the steel substrate to obtain a pre-plated steel sheet includes:

[0077] The steel substrate is pretreated before plating.

[0078] The pretreated steel substrate is heated to (Ac3+10)℃~(Ac3+40)℃;

[0079] A pre-plated metal is applied to at least a portion of the surface of the steel substrate heated to (Ac3+10)℃~(Ac3+40)℃, followed by post-plating treatment to obtain a pre-plated steel sheet. The pre-plated metal includes one or more of Ni, Fe, Co, and Cu.

[0080] The positive effects of heating the pretreated steel substrate to (Ac3+10)℃~(Ac3+40)℃ are as follows: Heating to between Ac3+10 and 40℃ to complete the pre-plating process prevents the external oxidation of alloying elements such as Mn and Si, which would otherwise accumulate on the steel surface, thus avoiding surface quality defects such as incomplete plating and zinc stripping during galvanizing. For example, the heating temperature can be (Ac3+10)℃, (Ac3+15)℃, (Ac3+20)℃, (Ac3+25)℃, (Ac3+30)℃, (Ac3+40)℃, etc.

[0081] In some embodiments, the pre-plated metal includes one or both of Ni and Cu.

[0082] Pre-plating with Ni and Cu metals has little impact on the steel plate and the coating, forming an iron-based alloy layer. This not only improves the adhesion of the coating but also further prevents the plating solution from entering the steel substrate during resistance spot welding.

[0083] In some implementations, relative to an area of ​​1m 2 The amount of the pre-plated metal in the steel substrate is 50mg to 1200mg.

[0084] Relative to an area of ​​1m 2 The positive effect of using 50mg to 1200mg of pre-plated metal on the steel substrate: if it is less than 50mg / m 2 If the concentration is higher than 1200 mg / m³, it will not be able to inhibit the accumulation of surface elements. 2 On the one hand, uneven pre-plating results in poor performance; on the other hand, it increases the cost of the plating solution. For example, relative to an area of ​​1m²... 2 The amount of the pre-plated metal used in the steel substrate can be 50mg, 100mg, 300mg, 400mg, 500mg, 800mg, 1000mg, 1200mg, etc.

[0085] In some embodiments, the pre-plating pretreatment includes pre-spray cleaning, pickling, and rinsing.

[0086] In some embodiments, the post-plating treatment includes surface extrusion, hot water rinsing, final spraying, and evaporation.

[0087] In some embodiments, the coating is performed using flash plating.

[0088] Specifically, the pre-plating process includes pre-spray cleaning, pickling unit, rinsing unit, flash plating unit, squeeze roller device, hot water rinsing device, final spray cleaning device, and evaporator unit.

[0089] S5. The pre-coated steel sheet is subjected to continuous hot-dip galvanizing annealing to obtain hot-dip galvanized multiphase steel.

[0090] In some embodiments, the step of continuously hot-dip galvanizing and annealing the pre-coated steel sheet to obtain hot-dip galvanized multiphase steel includes:

[0091] The pre-coated steel plate is first heated, and the following parameters of the first heating are controlled: first heating rate, first heating endpoint temperature and first holding time;

[0092] The pre-coated steel plate after the first heating is subjected to a second heating, and the following parameters of the second heating are controlled: second heating rate, second heating endpoint temperature and second holding time;

[0093] The pre-coated steel plate after the second heating is subjected to a first cooling, and the following parameters of the first cooling are controlled: first cooling rate and first cooling endpoint temperature;

[0094] The pre-coated steel plate after the first cooling is subjected to a second cooling, and the following parameters of the second cooling are controlled: the second cooling rate and the second cooling endpoint temperature;

[0095] The pre-coated steel sheet after the second cooling is hot-dip galvanized to obtain a galvanized steel sheet;

[0096] The galvanized steel sheet is subjected to a third cooling process, and the following parameters of the third cooling process are controlled: the third cooling rate and the third cooling endpoint temperature. After the cooling process is completed, the sheet is flattened to obtain hot-dip galvanized multiphase steel.

[0097] In some embodiments, the first heating rate is 2°C / s to 5°C / s, the first heating endpoint temperature is 640°C to 680°C, and the first holding time is 0.5 min to 3 min; and / or,

[0098] The second heating rate is 3℃ / s to 8℃ / s, the second heating endpoint temperature is (Ac3+10)℃ to (Ac3+40)℃, and the second holding time is 3min to 6min.

[0099] Ac3 refers to the actual phase transformation temperature of carbon steel during heating; it is relative to the solid-state equilibrium critical temperature line A3 of carbon steel. At the Ac3 point, the ferrite in the steel completely transforms into austenite.

[0100] The positive effects of controlling the heating rate of the first heating to 2℃ / s~5℃ / s, the first heating endpoint temperature to 640℃~680℃, and the first holding time to 0.5min~3min are as follows: When the heating rate is lower than 2℃ / s, the strip steel stays in the continuous annealing furnace for a long time, requiring a very long heating section to complete the heating process, resulting in excessive equipment investment; if the heating rate is higher than 5℃ / s, the C / Mn elements cannot diffuse sufficiently, and the recrystallization is insufficient, resulting in a fibrous microstructure that is not conducive to homogenization. Holding at 640~680℃ for 0.5min~3min further tempers the cold-rolled sheet at high temperature within this temperature range, achieving the purpose of microstructure homogenization. In this way, the ferrite grains in different parts will recrystallize more fully, reducing the grain size difference of ferrite; at the same time, the martensite will be tempered more fully, transforming into equiaxed ferrite and granular cementite; and the pearlite will also undergo more obvious spheroidization. Since grain boundaries are rapid diffusion channels for carbon atoms, during the isothermal process, carbon atoms gradually diffuse into the originally carbon-depleted ferrite region, achieving a uniform distribution of carbide particles. If the temperature or time is below this range, the desired uniformity of microstructure and properties will not be achieved; if the temperature or time is above this range, the production line layout will be longer, which is uneconomical. For example, the heating rate of the first heating can be 2℃ / s, 2.5℃ / s, 3℃ / s, 3.5℃ / s, 4℃ / s, 4.5℃ / s, 5℃ / s, etc., the first heating endpoint temperature can be 640℃, 650℃, 660℃, 670℃, 680℃, etc., and the first holding time can be 0.5min, 1.0min, 1.5min, 2min, 2.5min, 3min, etc.

[0101] The positive effects of controlling the heating rate of the second heating to 3℃ / s~8℃ / s, the endpoint temperature of the second heating to (Ac3+10)℃~(Ac3+40)℃, and the second holding time to 3min~6min are as follows: Heating at 3~8℃ / s to Ac3+10~40℃ and holding at this temperature for 3~6min is effective. The 3~8℃ / s heating rate is similar to 2~5℃ / s, but since this stage involves heating at a higher temperature, a further increase in the heating rate is needed to heat the steel plate to the annealing temperature in a short time. The annealing temperature is Ac3+10~40℃. Within this temperature range, after full austenitization, the fibrous microstructure after cold rolling completes the recovery and recrystallization process and is completely transformed into austenite. Simultaneously, residual cementite is fully dissolved, and the components and microstructure within the austenite are fully homogenized. If the temperature is lower than this, the desired effect will not be achieved; if the temperature is higher, energy consumption and costs will increase significantly. The soaking time should be 3–6 minutes. If the soaking time is less than 3 minutes, the carbides of Nb and Ti will not precipitate sufficiently during the soaking stage, which will not effectively improve the strength and hardness of the ferrite structure or reduce the hardness difference between the soft and hard phases. If the soaking time exceeds 6 minutes, the original austenite grain size will increase, which is not conducive to ensuring a tensile strength of over 980 MPa. For example, the heating rate of the second heating can be 3℃ / s, 4℃ / s, 5℃ / s, 6℃ / s, 7℃ / s, 8℃ / s, etc., the second heating endpoint temperature can be (Ac3+10)℃, (Ac3+15)℃, (Ac3+20)℃, (Ac3+25)℃, (Ac3+30)℃, (Ac3+40)℃, etc., and the second soaking time can be 3 min, 3.5 min, 4 min, 4.5 min, 5 min, 5.5 min, 6 min, etc.

[0102] In some embodiments, the first cooling rate is 1°C / s to 15°C / s, and the first cooling endpoint temperature is Ac1 to Ac3; and / or,

[0103] The second cooling rate is ≥45℃ / s, and the second cooling endpoint temperature is 440℃~540℃; and / or,

[0104] The third cooling rate is ≥30℃ / s, and the third cooling endpoint temperature is ≤Mf.

[0105] Ac1, also known as the "heating phase transformation point" or "lower critical point," is an important temperature at which phase transformation occurs in steel materials during heating. At this temperature, ferrite begins to transform into austenite.

[0106] Mf is the martensitic transformation completion temperature.

[0107] The positive effects of controlling the cooling rate of the first cooling to be 1℃ / s to 15℃ / s and the first cooling endpoint temperature to be Ac1 to Ac3 are as follows: Cooling to the Ac1 to Ac3 temperature range at a rate of 1 to 15℃ / s ensures that the formation of new ferrite can be suppressed during the initial cooling process. This not only avoids the evolution of microstructure inhomogeneity but also avoids the reduction in austenite hardenability caused by carbon enrichment into austenite. It also prevents further precipitation of proeutectoid ferrite, which would deteriorate the porosity. For example, the first cooling rate can be 1℃ / s, 3℃ / s, 5℃ / s, 7℃ / s, 9℃ / s, 12℃ / s, 15℃ / s, etc., and the first cooling endpoint temperature can be Ac1, (Ac1+10)℃, (Ac1+15)℃, (Ac1+20)℃, (Ac1+25)℃, (Ac1+30)℃, (Ac1+40)℃, Ac3, etc.

[0108] The positive effects of controlling the cooling rate of the second cooling stage to ≥45℃ / s and the endpoint temperature of the second cooling stage to be 440℃~540℃: Cooling to 440~540℃ at a cooling rate of ≥45℃ / s for hot-dip galvanizing occurs within the bainitic transformation zone. In cases of insufficient austenite hardenability, bainitic transformation is inevitable, resulting in the formation of a certain amount of bainite. Simultaneously, the transformation from austenite to martensite can be achieved. Cooling below this rate will hinder martensitic transformation and prevent the achievement of a tensile strength exceeding 980MPa. Cooling to 440~540℃ for hot-dip galvanizing is problematic. Below 440℃, the fluidity of the zinc liquid decreases, easily leading to zinc flow marks on the strip surface; above 540℃, the formation of zinc dross and zinc ash is accelerated, affecting the purity of the zinc liquid. For example, the cooling rate of the second cooling can be 45℃ / s, 46℃ / s, 47℃ / s, 48℃ / s, 49℃ / s, 50℃ / s, etc., and the second cooling endpoint temperature can be 440℃, 460℃, 480℃, 500℃, 520℃, 540℃, etc.

[0109] The positive effects of controlling the cooling rate of the third cooling to be ≥30℃ / s and the endpoint temperature of the third cooling to ≤Mf are as follows: Finally, cooling is carried out at a rate of ≥30℃ / s to below the martensitic transformation temperature, causing some of the untransformed austenite to form partial bainite, ultimately obtaining a multiphase steel with a multiphase structure of bainite, martensite, and a small amount of ferrite. For example, the cooling rate of the third cooling can be ≥30℃ / s, 31℃ / s, 32℃ / s, 33℃ / s, 34℃ / s, 35℃ / s, etc., and the endpoint temperature of the third cooling can be (Mf-100)℃, (Mf-80)℃, (Mf-60)℃, (Mf-40)℃, (Mf-20)℃, Mf, etc.

[0110] The product prepared by the method of preparing hot-dip galvanized multiphase steel is the hot-dip galvanized multiphase steel described above. The chemical composition and microstructure of the hot-dip galvanized multiphase steel prepared by the method can be referred to the above embodiments. Since the method of preparing hot-dip galvanized multiphase steel adopts some or all of the technical solutions of the hot-dip galvanized multiphase steel embodiments, it has at least all the beneficial effects brought about by the technical solutions of the hot-dip galvanized multiphase steel embodiments, which will not be elaborated here.

[0111] 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 industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0112] Examples 1-6 and Comparative Examples 1-4 were smelted and cast according to the chemical composition shown in Table 1 and the alloy composition ratio designed in Table 1.

[0113] Table 1. Chemical composition of the steel matrix of hot-dip galvanized multiphase steel, the remainder being Fe and unavoidable impurities.

[0114]

[0115]

[0116] Based on the chemical composition of the steel matrix of the hot-dip galvanized multiphase steel described above, Examples 1-6 and Comparative Examples 1-4 of this application provide a method for preparing hot-dip galvanized multiphase steel, the method comprising:

[0117] S11. Obtain a slab having the aforementioned chemical composition;

[0118] S21. The slab is rolled and coiled to obtain a hot-rolled coil;

[0119] S31. The hot-rolled coil is cold-rolled to obtain a steel matrix;

[0120] S41. The steel substrate is pre-plated to obtain a pre-plated steel sheet. Pre-plating includes: pre-treating the steel substrate before plating; heating the pre-treated steel substrate to (Ac3+10)℃~(Ac3+40)℃; applying the pre-plating metal to at least a portion of the surface of the heated steel substrate; and then performing post-plating treatment to obtain a pre-plated steel sheet. The pre-plating metal includes one or more of Ni, Fe, Co, and Cu.

[0121] S51. The pre-coated steel sheet is subjected to continuous hot-dip galvanizing annealing to obtain hot-dip galvanized multiphase steel. The continuous hot-dip galvanizing annealing includes: subjecting the pre-coated steel sheet to a first heating, the parameters of which include: a heating rate of 2℃ / s~5℃ / s, a heating endpoint temperature of 640℃~680℃, and a holding time of 0.5min~3min; subjecting the pre-coated steel sheet after the first heating to a second heating, the parameters of which include: a heating rate of 3℃ / s~8℃ / s, a heating endpoint temperature of (Ac3+10)℃~(Ac3+40)℃, and a holding time of 3min~6min; and subjecting the pre-coated steel sheet after the second heating to... The process involves a first cooling step, with parameters including a cooling rate of 1℃ / s to 15℃ / s and a cooling endpoint temperature of Ac1 to Ac3. The pre-coated steel sheet after the first cooling is then subjected to a second cooling step, with parameters including a cooling rate ≥ 45℃ / s and a cooling endpoint temperature of 440℃ to 540℃. The pre-coated steel sheet after the second cooling is then hot-dip galvanized to obtain a galvanized steel sheet. The galvanized steel sheet is then subjected to a third cooling step, with parameters including a cooling rate ≥ 30℃ / s and a cooling endpoint temperature ≤ Mf, followed by leveling. The process parameters for the preparation methods of hot-dip galvanized multiphase steel in Examples 1-6 and Comparative Examples 1-4 are shown in Tables 2 and 3, respectively.

[0122] Table 2. Process parameters for the preparation methods of hot-dip galvanized multiphase steel in Examples 1-6

[0123]

[0124] Table 3. Process parameters for the preparation methods of hot-dip galvanized multiphase steel in Comparative Examples 1-4

[0125]

[0126] The steels obtained in Examples 1-6 and Comparative Examples 1-4 were tested for performance and porosity. The test results are shown in Table 4.

[0127] Table 4. Results of performance and porosity testing of the steels obtained in Examples 1-6 and Comparative Examples 1-4.

[0128]

[0129] The tensile strength Rm, yield strength Rp0.2 and A80 in the table above are evaluated according to GBT 228.1-2021 Metallic materials, tensile testing - Part 1: Test method at room temperature; the surface quality of the steel plate can be visually inspected for phenomena such as uncoated or dezincified areas, or simply by bending and forming to observe whether the zinc layer has peeled off.

[0130] The hole expansion ratio in the table above is calculated according to the ISO 16630 standard for the test steel using the hole expansion test (HET). The hole expansion test is used to measure the material's ability to resist edge fracture during stamping processes such as flanging and hole turning. The sample size is 100mm × 100mm × 1.5mm, with an initial hole diameter of 10mm. Five sets of samples are used for each steel grade. To ensure that the sample is fully pressed and fixed during hole expansion, the distance between the center of the hole and the edge of the sample should not be less than 45mm. To examine the hole expansion ratio under worst-case conditions, the direction of the burr during hole expansion should be consistent with the direction of the punch movement. This hole expansion method can cause the tear band to deform first, leading to the propagation and cracking of microcracks, thereby obtaining the worst edge resistance of the material. The hole expansion ratio HER (%) is calculated using the following formula, where df is the hole diameter at the end of hole expansion, and d0 is the initial hole diameter.

[0131] HER = (df - d0) / d0 × 100%

[0132] Appendix Figure 2 and 3 Detailed explanation:

[0133] Figure 2 The metallographic structure of the steel substrate provided in Embodiment 1 of this application is shown.

[0134] Depend on Figure 2 It can be seen that the metallographic structure consists of bainite, martensite, and ferrite. The composition is 20% bainite, 50% martensite, and 30% ferrite.

[0135] Figure 3 The images show hot-dip galvanized multiphase steel samples after hole enlargement, provided in Examples 1(a) and 4(b) of this application.

[0136] Depend on Figure 3 It can be seen that the hole expansion rate of the hot-dip galvanized multiphase steel in Example 1 reaches more than 50%, while the hole expansion rate of the hot-dip galvanized multiphase steel in Comparative Example 4 is less than 30%.

[0137] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0138] (1) The low-cost, high-hole-expansion hot-dip galvanized multiphase steel provided in this embodiment of the invention does not contain the expensive element Mo. Each addition of this alloying element by 0.1% will increase the alloy cost per ton of steel by about RMB 500. Therefore, this product has a low alloy cost, and the product has a yield strength ≥780MPa, tensile strength ≥980MPa, elongation ≥10%, and hole expansion rate ≥50%. At the same time, the product has excellent surface quality.

[0139] (2) In the hot-dip galvanized multiphase steel provided in this embodiment of the invention, during the stepped heating process, the strip steel is heated to below Ac1 for isothermal treatment. This allows for more thorough recrystallization of ferrite grains in different locations, reducing the grain size difference of ferrite. Simultaneously, martensite undergoes more thorough tempering, transforming into equiaxed ferrite and granular cementite; pearlite also undergoes significant spheroidization. Since grain boundaries are rapid diffusion channels for carbon atoms, carbon atoms gradually diffuse into the originally carbon-depleted ferrite region during the isothermal process, achieving a uniform distribution of carbide particles. This method achieves homogenization of the microstructure before annealing without modifying the production line furnace area, providing a very good foundation for the high porosity of over 50% in multiphase steel.

[0140] (3) The annealing temperature used in the method provided in this embodiment of the invention is Ac3+10~40℃. After full austenitization, the fibrous structure after cold rolling is restored, recrystallized, and completely transformed into austenite. At the same time, the residual cementite is fully dissolved, and the composition and structure of the austenite are fully homogenized. Then, it is slowly cooled to a higher temperature to ensure that the formation of new ferrite during the initial cooling process is suppressed. This not only prevents the evolution of non-uniform structure, but also avoids carbon enrichment in austenite, which delays the transformation of bainite and increases the content of martensite. Minimizing carbon enrichment in austenite will increase the Ms point temperature, thereby promoting the formation of martensite during the final cooling process. This method further provides a precise control method for multiphase structure with high porosity, while also achieving the required strength.

[0141] (4) This invention provides a method for pre-plating metal onto the surface of a steel substrate before galvanizing, namely, plating both sides of the steel plate with a metal coating of 50-1200 mg / m². 2 A pre-plating layer is formed on one or more of Ni, Fe, Co, and Cu, with Ni and Cu being preferred. The pre-plating process includes pre-spray cleaning, pickling, rinsing, flash plating, squeeze roller assembly, hot water rinsing, final spray cleaning, and evaporator assembly. This ensures excellent surface quality for hot-dip galvanized multiphase steel.

[0142] (5) The embodiments of the present invention optimize the composition of steel and control the amount of Si added, which can improve the coating performance of the hot-dip galvanizing process. At the same time, by limiting the amount of Cr and Mn added, it can have a good balance of strength and elongation.

[0143] 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 980MPa grade hot-dip galvanized multiphase steel with high porosity, characterized in that, Hot-dip galvanized multiphase steel comprises a steel substrate and a zinc coating adhered to at least a portion of the surface of the steel substrate. The chemical composition of the steel substrate, by mass fraction, comprises: C: 0.03%~0.3%, Si: 0.1%~1.0%, Mn: 0.2%~3.0%, P≤0.01%, S≤0.006%, Cr: 0.05%~1.0%, Nb: 0.01%~0.1%, Ti: 0.01%~0.1%, with the balance being Fe and unavoidable impurities. The metallographic structure of the steel matrix, by volume fraction, comprises: bainite: 10%~20%, martensite: 30%~50%, and ferrite: 20%~30%. The chemical composition satisfies the following relationship: 0.001×[TS]-[Si] / 1.3<[Cr]+[Mn] / 5, In the formula, [TS] represents the design value of the tensile strength of the hot-dip galvanized multiphase steel, [Si] represents the content of Si, [Cr] represents the content of Cr, and [Mn] represents the content of Mn; The preparation method of the hot-dip galvanized multiphase steel includes: A slab having the aforementioned chemical composition is obtained; The slab is rolled and coiled to obtain a hot-rolled coil; The hot-rolled coil is then cold-rolled to obtain a steel matrix; The steel substrate is pre-plated to obtain a pre-plated steel plate; and The pre-coated steel sheet is subjected to continuous hot-dip galvanizing and annealing to obtain hot-dip galvanized multiphase steel; Pre-coated steel sheets are subjected to continuous hot-dip galvanizing and annealing to obtain hot-dip galvanized multiphase steel, including: The pre-coated steel plate is first heated, and the following parameters of the first heating are controlled: first heating rate, first heating endpoint temperature and first holding time; The pre-coated steel plate after the first heating is subjected to a second heating, and the following parameters of the second heating are controlled: second heating rate, second heating endpoint temperature and second holding time; The pre-coated steel plate after the second heating is subjected to a first cooling, and the following parameters of the first cooling are controlled: first cooling rate and first cooling endpoint temperature; The pre-coated steel plate after the first cooling is subjected to a second cooling, and the following parameters of the second cooling are controlled: the second cooling rate and the second cooling endpoint temperature; The pre-coated steel sheet after the second cooling is hot-dip galvanized to obtain a galvanized steel sheet; The galvanized steel sheet is subjected to a third cooling process, and the following parameters of the third cooling process are controlled: the third cooling rate and the third cooling endpoint temperature. After the cooling process is completed, the sheet is flattened to obtain hot-dip galvanized multiphase steel. The first heating rate is 2℃ / s to 5℃ / s, the first heating endpoint temperature is 640℃ to 680℃, and the first holding time is 0.5min to 3min; The second heating rate is 3℃ / s to 8℃ / s, the second heating endpoint temperature is (Ac3+10)℃ to (Ac3+40)℃, and the second holding time is 3min to 6min; the first cooling rate is 1℃ / s to 15℃ / s, and the first cooling endpoint temperature is Ac1 to Ac3. The second cooling rate is ≥45℃ / s, and the second cooling endpoint temperature is 440℃~540℃; The third cooling rate is ≥30℃ / s, and the third cooling endpoint temperature is ≤Mf.

2. The hot-dip galvanized multiphase steel according to claim 1, characterized in that, The grain diameter of the bainite is 1μm~2μm, the grain diameter of the martensite is 1μm~4μm, and the grain diameter of the ferrite is 3μm~6μm.

3. The hot-dip galvanized multiphase steel according to claim 1, characterized in that, The hot-dip galvanized multiphase steel meets at least one of the following properties: yield strength ≥780MPa, tensile strength ≥980MPa, elongation ≥10%, and hole expansion rate ≥50%.

4. A method for preparing hot-dip galvanized multiphase steel according to any one of claims 1 to 3, characterized in that, The method includes: A slab having the aforementioned chemical composition is obtained; The slab is rolled and coiled to obtain a hot-rolled coil; The hot-rolled coil is then cold-rolled to obtain a steel matrix; The steel substrate is pre-plated to obtain a pre-plated steel plate; and The pre-coated steel sheet is subjected to continuous hot-dip galvanizing annealing to obtain hot-dip galvanized multiphase steel.

5. The method according to claim 4, characterized in that, The step of pre-plating the steel substrate to obtain a pre-plated steel plate includes: The steel substrate is pretreated before plating. The pretreated steel substrate is heated to (Ac3+10)℃~(Ac3+40)℃; A pre-plated metal is applied to at least a portion of the surface of the steel substrate heated to (Ac3+10)℃~(Ac3+40)℃, followed by post-plating treatment to obtain a pre-plated steel sheet. The pre-plated metal includes one or more of Ni, Fe, Co, and Cu.

6. The method according to claim 5, characterized in that, Relative to an area of ​​1m 2 The amount of the pre-plated metal in the steel substrate is 50mg~1200mg.

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