Alloyed galvanized hot-rolled high-strength steel and preparation method thereof

Abstract

The invention relates to alloyed galvanized hot-rolled high-strength steel and a preparation method, and belongs to the technical field of steel preparation. The high-strength steel comprises the following chemical components in percentage by mass: 0.05%-0.12% of C, 0.1%-0.4% of Si, 1.5%-2.0% of Mn, 0.01%-0.06% of Al, less than or equal to 0.01% of P, less than or equal to 0.003% of S, less than or equal to 0.2% of Cr, less than or equal to 0.03% of Mo, less than or equal to 0.02% of Cu, less than or equal to 0.03% of Ni, at least one of 0.01%-0.12% of Ti, 0.01%-0.04% of Nb and 0.01%-0.08% of V, and the balance of Fe and inevitable impurities. The microstructure of the high-strength steel comprises, by area ratio, 35%-65% of ferrite, 35%-50% of martensite and smaller than or equal to 15% of martensite, the grain size of the ferrite ranges from 2 micrometers to 6 micrometers, the grain size of the martensite ranges from 0.5 micrometer to 2 micrometers, and the grain size of the martensite ranges from 1 micrometer to 3 micrometers. Through chemical component design, microstructure regulation and control and optimization of the proportion and size of a precipitated phase, the strength and hardness of a ferrite matrix are improved, the hardness difference between the soft phase and the hard phase is reduced, and the reaming performance is obviously improved.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to an alloyed galvanized hot-rolled high-strength steel and its preparation method. Background Technology

[0002] Hot-rolled pickled high-strength steel is widely used in the automotive manufacturing industry, especially in chassis and body structural components, due to its excellent formability. Among them, 800MPa high-expansion steel has become the preferred material for key components such as chassis control arms due to its superior hole-expansion and fatigue properties. In recent years, with the increase in exported models and the increasing demands for corrosion resistance in complex service environments, galvanized hot-rolled high-expansion steel has attracted significant attention from the industry, and major steel companies have begun to develop related products.

[0003] However, in practical applications, hot-rolled high-expansion steel generally faces the problem of decreased formability after galvanizing, with the expansion rate typically below 45%, which severely limits its application in high-end automotive chassis parts. Furthermore, traditional hot-dip galvanized steel sheets, due to the low melting point of the zinc layer, experience rapid zinc evaporation during arc welding, leading to an unstable molten pool, prone to welding spatter and porosity defects, and in severe cases, even cracks, thus affecting the coating, corrosion resistance, and safety performance of the parts. Summary of the Invention

[0004] This application provides an alloyed galvanized hot-rolled high-strength steel and its preparation method to solve the following technical problem: how to improve the hole expansion performance of existing galvanized hot-rolled high-strength steel.

[0005] In a first aspect, embodiments of this application provide an alloyed galvanized hot-rolled high-strength steel, wherein the chemical composition of the high-strength steel, by mass fraction, is: C: 0.05%–0.12%, Si: 0.1%–0.4%, Mn: 1.5%–2.0%, Al: 0.01%–0.06%, P≤0.01%, S≤0.003%, Cr≤0.2%, Mo≤0.03%, Cu≤0.02%, Ni≤0.03%, and at least one of Ti: 0.01%–0.12%, Nb: 0.01%–0.04%, and V: 0.01%–0.08%, with the balance being Fe and unavoidable impurities;

[0006] Based on area ratio, the microstructure of the high-strength steel comprises: ferrite: 35%–65%, Mao islands: 35%–50%, and island martensite ≤15%, wherein the grain size of ferrite is 2μm–6μm, the grain size of Mao islands is 0.5μm–2μm, and the grain size of island martensite is 1μm–3μm.

[0007] Optionally, precipitates are distributed on the ferrite matrix, the precipitates including one or more of TiC, NbC, VC and (Nb, Ti)C, the average equivalent diameter of the precipitates being ≤50nm.

[0008] Optionally, the total precipitation ratio of Nb, V and Ti is ≥70%.

[0009] Optionally, the high-strength steel meets at least one of the following properties: yield strength ≥ 650 MPa, tensile strength ≥ 780 MPa, elongation after fracture A80 ≥ 17%, and hole expansion rate ≥ 60%.

[0010] Optionally, the zinc layer thickness of the high-strength steel is 5μm to 16μm.

[0011] Secondly, embodiments of this application provide a method for preparing the high-strength steel described in the first aspect, the method comprising:

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

[0013] The slab is sequentially heated, rough rolled, finish rolled, cooled, and coiled to obtain a hot-rolled coil.

[0014] The hot-rolled coil is sequentially pickled, leveled, and subjected to secondary tension straightening to obtain a pickled plate.

[0015] The pickled plate is subjected to continuous hot-dip galvanizing and alloying treatments in sequence to obtain finished alloyed galvanized hot-rolled high-strength steel.

[0016] Optionally, the heating temperature is 1230℃~1270℃.

[0017] Optionally, the final rolling temperature of the roughing roll is 1070℃~1120℃.

[0018] Optionally, the entry temperature of the finishing mill is 950℃~1050℃, and the final rolling temperature of the finishing mill is 890℃~920℃.

[0019] Optionally, the thickness of the hot-rolled coil is 1.5mm to 6.0mm.

[0020] Optionally, the cooling rate is ≥20℃ / s, and the winding temperature is 350℃~500℃.

[0021] Optionally, the scaling and stretching elongation of the pickling is 0.3% to 0.6%, and the acid concentration of the pickling solution is 28 g / L to 35 g / L.

[0022] Optionally, the leveling rolling force is 2500kN to 4000kN.

[0023] Optionally, the elongation rate of the secondary tension straightening is 0.1% to 0.3%.

[0024] Optionally, the continuous hot-dip galvanizing includes: a preheating section, a heating section 1, a pre-oxidation section, a heating section 2, a soaking section, a slow cooling section, and a rapid cooling section.

[0025] Optionally, the temperature of the preheating section is 210℃~230℃.

[0026] Optionally, the heating rate of the heating section 1 is 1.5℃ / s to 5℃ / s.

[0027] Optionally, the temperature of the pre-oxidation section is 550℃~650℃.

[0028] Optionally, the heating rate of the second heating stage is 0.1℃ / s to 1℃ / s.

[0029] Optionally, the temperature of the heat-equalizing zone is 700℃~740℃, and the heat-equalizing zone time is 80s~150s.

[0030] Optionally, the cooling rate of the slow cooling section is 0.5℃ / s to 2℃ / s, and the final temperature of the slow cooling section is 660℃ to 680℃.

[0031] Optionally, the cooling rate of the rapid cooling section is 10℃ / s to 30℃ / s, and the final temperature of the rapid cooling section is 450℃ to 470℃.

[0032] Optionally, the alloying temperature is 500℃~550℃, and the alloying time is 9s~30s.

[0033] The technical solutions provided in this application have the following advantages compared with the prior art:

[0034] This application provides an alloyed galvanized hot-rolled high-strength steel. The chemical composition of the high-strength steel, by mass fraction, is: C: 0.05%–0.12%, Si: 0.1%–0.4%, Mn: 1.5%–2.0%, Al: 0.01%–0.06%, P≤0.01%, S≤0.003%, Cr≤0.2%, Mo≤0.03%, Cu≤0.02%, Ni≤0.03%, and Ti: 0.01%–0.1%. The high-strength steel comprises at least one of the following: 2% Fe, 0.01%–0.04% Nb, and 0.01%–0.08% V, with the balance being Fe and unavoidable impurities. Based on area ratio, the microstructure of the high-strength steel includes: ferrite: 35%–65%, Mao islands: 35%–50%, and island martensite ≤15%, wherein the grain size of ferrite is 2μm–6μm, the grain size of Mao islands is 0.5μm–2μm, and the grain size of island martensite is 1μm–3μm. Through chemical composition design, the addition of Si improves the strength of the ferrite matrix. The use of Nb, Ti, and V microalloying optimizes the proportion and size of precipitated phases, thereby improving the strength and hardness of the ferrite matrix, reducing the hardness difference between soft and hard phases, and significantly improving the pore-expanding performance. This improved pore-expanding performance greatly reduces the risk of material forming cracks. Attached Figure Description

[0035] 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.

[0036] 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.

[0037] Figure 1 A schematic flowchart illustrating a method for preparing alloyed galvanized hot-rolled high-strength steel according to an embodiment of this application;

[0038] Figure 2 This is a microstructure diagram of the alloyed galvanized hot-rolled high-strength steel provided in Example 1 of this application. Detailed Implementation

[0039] 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.

[0040] 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; in addition, whenever a numerical range is indicated herein, it means including any referenced number (fraction or integer) within the indicated range.

[0041] In this document, terms such as “comprising” mean “including but not limited to”. Relational terms such as “first” and “second” are used only 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. “And / or” describes the relationship between related objects, indicating that there can be three relationships, for example, A and / or B can mean: A exists alone, A and B exist simultaneously, or B exists alone; where A and B can be singular or plural. “At least one” means one or more, “more” 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 single or multiple. "Parts representation" such as parts by weight or parts by mass indicates the proportional relationship between components. In the proportional relationships discussed in this article, the parameters that need to be described by proportion should be understood as the first term of the proportion in the order of description, and the proportion figures should be understood as the second term of the proportion. For example, if the mass ratio of substance A, substance B, and substance C is 1:2:3, then substances A, B, and C should correspond one-to-one with the proportion figures in the proportion in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0042] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0043] In a first aspect, embodiments of this application provide an alloyed galvanized hot-rolled high-strength steel, wherein the chemical composition of the high-strength steel, by mass fraction, is: C: 0.05%–0.12%, Si: 0.1%–0.4%, Mn: 1.5%–2.0%, Al: 0.01%–0.06%, P≤0.01%, S≤0.003%, Cr≤0.2%, Mo≤0.03%, Cu≤0.02%, Ni≤0.03%, and at least one of Ti: 0.01%–0.12%, Nb: 0.01%–0.04%, and V: 0.01%–0.08%, with the balance being Fe and unavoidable impurities;

[0044] The positive effects of limiting the C mass fraction to 0.05% to 0.12% are as follows: C is an interstitial solid solution atom and an element that improves hardenability. It can promote the formation of hard phase structures such as bainite and martensite, and improve strength mainly through solid solution strengthening and phase transformation strengthening. In the embodiments of this application, in order to obtain a tensile strength of not less than 780 MPa, the C mass fraction cannot be less than 0.05%; however, the C mass fraction should not exceed 0.12%, otherwise the plasticity and hole expansion properties will deteriorate, and the weldability will also be impaired.

[0045] The positive effects of limiting the Si mass fraction to 0.1% to 0.4% include: Si has a strong solid solution strengthening effect, which can strengthen the ferrite structure, reduce the hardness difference between ferrite and bainite / martensite, and is beneficial to improving the hole expansion performance. Therefore, the Si mass fraction should not be lower than 0.1%. However, when the Si mass fraction is too high, strip-shaped red rust is easily formed on the surface of the hot-rolled coil, which forms white strip-shaped color difference after pickling and cannot be covered after galvanizing. Therefore, the embodiments of this application control the upper limit of the Si mass fraction to 0.4%.

[0046] The positive effects of limiting the mass fraction of manganese (Mn) to 1.5%–2.0% include: Mn has strong solid solution strengthening properties and is an important component element for ensuring high yield strength and tensile strength. To obtain the desired strength, the mass fraction of Mn is controlled at 1.5% or higher. However, excessively high Mn mass fractions not only easily lead to Mn segregation but also cause a significant decrease in plasticity and porosity. Therefore, the mass fraction of Mn in steel is limited to no more than 2.0%.

[0047] The positive effects of limiting the Al mass fraction to 0.01%–0.06% are that Al is a deoxidizing element, which can reduce oxide inclusions in steel. However, an excessively high Al mass fraction increases the difficulty of continuous casting and the cost of alloys; therefore, the Al mass fraction is controlled between 0.01% and 0.06%.

[0048] The positive effects of limiting P to ≤ 0.01%: 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. Therefore, the upper limit of the mass fraction of P is set at 0.01%.

[0049] The positive effects of limiting S to ≤ 0.003%: S easily combines with Mn to form coarse MnS inclusions, which deteriorates the forming performance of steel plates such as hole expansion and flanging. Therefore, the upper limit of the mass fraction of S is set at 0.003%.

[0050] The positive effects of limiting Cr to ≤0.2%: Cr is an element that improves hardenability, delays the pearlite phase transformation, and promotes bainite formation, which is beneficial for increasing strength. However, when the mass fraction of Cr exceeds 0.2%, the effect is no longer significant and it increases costs; therefore, the upper limit of the mass fraction of Cr is controlled at 0.2%.

[0051] The positive effects of limiting Mo to ≤0.03%: Mo is also an element that improves hardenability, but Mo is an expensive alloying element. In order to reduce the cost of alloys, the mass fraction of Mo is controlled to not exceed 0.03%.

[0052] The positive effects of limiting Cu to ≤0.02%: Cu improves the strength and corrosion resistance of steel and can significantly eliminate the harmful effects of sulfur in steel. However, Cu alloys are expensive; therefore, to reduce costs, a maximum Cu mass fraction of 0.02% is set.

[0053] The positive effects of limiting Ni to ≤0.03%: Ni is also an element that improves the atmospheric corrosion resistance of steel. Due to the high cost of Ni alloys, it is necessary to limit the upper limit to 0.03%.

[0054] The positive effects of limiting the Ti mass fraction to 0.01%–0.12% include: Ti is a strong carbonitride forming element, which can enhance the hardness and strength of the ferrite microstructure through precipitation strengthening. This not only increases the yield strength of the steel plate but also reduces the hardness difference between ferrite and bainite / martensite, which is beneficial for improving local forming properties such as hole expansion and bending. To obtain excellent strength and forming properties, an addition of not less than 0.01% Ti is selected. However, when the Ti mass fraction exceeds 0.12%, the strengthening effect no longer increases.

[0055] The positive effects of limiting the Nb mass fraction to 0.01%–0.04% include: Nb can refine grains through solute dragging and, as a strong carbide-forming element, can improve the hardness and strength of ferrite structures through precipitation strengthening, thus enhancing porosity. However, Nb increases the recrystallization temperature, leading to banded structures formed during hot rolling, which conversely worsens porosity. Furthermore, Nb alloys are relatively expensive. Therefore, the Nb mass fraction is set at 0.01%–0.04%.

[0056] The positive effect of limiting the mass fraction of V to 0.01% to 0.08% is that V can also improve strength and porosity through precipitation strengthening. However, in the process path of the embodiments of this application, the strengthening effect reaches saturation when the mass fraction of V exceeds 0.08%. Therefore, the mass fraction of V is limited to between 0.01% and 0.08%.

[0057] Fe is a matrix element, and the specific content / range of Fe can be obtained through the upper and lower limit formulas of the component, that is:

[0058] The sum of the percentages of all components in a composition shall be equal to 100%, and the content ranges of several components shall meet the following conditions: the upper limit of a certain component + the lower limit of other components ≤ 100; the lower limit of a certain component + the upper limit of other components ≥ 100.

[0059] Based on area ratio, the microstructure of the high-strength steel comprises: ferrite: 35%–65%, Mao islands: 35%–50%, and island martensite ≤15%, wherein the grain size of ferrite is 2μm–6μm, the grain size of Mao islands is 0.5μm–2μm, and the grain size of island martensite is 1μm–3μm.

[0060] Ferrite has low strength and hardness, making it easily deformable. It bears most of the strain during deformation and is a crucial component phase for ensuring the plasticity and formability of steel plates. When the ferrite proportion is less than 35%, the strength of the steel plate increases, but its plasticity and formability deteriorate. When the ferrite proportion exceeds 65%, it is difficult to guarantee a tensile strength above 780 MPa. Furthermore, ferrite is the microstructure that coordinates deformation in high-hole-expansion steel. When the ferrite grain size is less than 2 μm, the ability to coordinate deformation is poor, resulting in insufficient elongation after fracture. When the ferrite grain size is greater than 6 μm, it easily leads to insufficient tensile strength.

[0061] The Mao island microstructure is a special microstructure in metallic materials, exhibiting an island-like morphology. It contains two distinct phases: martensite and austenite. Mao islands are the hard phase structure that ensures the strength of high-expansion steel. When the Mao island content is less than 35%, a tensile strength above 780 MPa cannot be guaranteed. Furthermore, due to the low proportion of the hard phase, the hard phase microstructure becomes highly carbon-rich, leading to a significant increase in microhardness and widening the hardness difference between the soft and hard phases. This increases the likelihood of crack initiation during deformation, reducing the hole-expansion performance. When the Mao island content is greater than 50%, the elongation after fracture of the steel plate decreases, resulting in insufficient overall formability. When the Mao island grain size is less than 0.5 μm, it is difficult to guarantee a tensile strength above 780 MPa in high-expansion steel. When the Mao island grain size is greater than 2 μm, cracks easily initiate at grain boundaries, resulting in insufficient hole expansion.

[0062] Island martensite is a hard phase structure that can improve the strength of steel plates. However, its high strength and hardness create a significant hardness difference with ferrite, which is detrimental to hole expansion performance. Therefore, the upper limit for island martensite is limited to 15%. When the island martensite grain size is less than 1 μm, it is difficult to guarantee a tensile strength of over 780 MPa in high-hole-expansion steel. When the island martensite grain size is greater than 3 μm, cracks are prone to initiation at grain boundaries, resulting in insufficient hole expansion rate.

[0063] In some embodiments, precipitates are distributed on the ferrite matrix, the precipitates including one or more of TiC, NbC, VC and (Nb, Ti)C, the average equivalent diameter of the precipitates being ≤50nm.

[0064] To achieve higher yield strength and a relatively low hardness difference between the soft and hard phases, embodiments of this application employ precipitation strengthening by adding at least one of the microalloying elements V, Ti, and Nb. By controlling post-rolling cooling, coiling temperature, and soaking temperature during continuous hot-dip galvanizing, the precipitated phases are made fine, uniform, and dispersed in the ferrite matrix.

[0065] In this embodiment, the precipitated phases mainly include TiC, NbC, VC, and (Nb, Ti)C. These carbides are formed by the combination of microalloying elements (such as Ti, Nb, V) and carbon (C) in the steel, possessing high hardness and high melting point, and can play a strengthening role in the steel matrix. This embodiment also requires that the average equivalent diameter of the microalloyed carbides be ≤50 nm. The size of the microalloyed carbides has a significant impact on their strengthening effect. When the carbide size is small (≤50 nm), they can be more uniformly distributed in the ferrite matrix, forming a dispersion strengthening effect, effectively improving the yield strength and tensile strength of the steel. Simultaneously, the fine carbides can also reduce the hardness difference between the soft and hard phases, which is beneficial to the hole-expanding performance of the steel plate. When the carbide size is greater than 50 nm, the strengthening effect deteriorates, potentially leading to insufficient hole expansion.

[0066] In some implementations, the total precipitation ratio of Nb, V and Ti is ≥70%.

[0067] The total precipitation ratio of elements Nb, V and Ti is ≥70%. The precipitation ratio refers to the percentage of the mass fraction of these microalloying elements precipitated in the form of carbides to the total amount added. A precipitation ratio of ≥70% means that more microalloying elements exist in the steel matrix in the form of carbides, thereby more effectively improving the strength and hardness of the steel. When the precipitation ratio is less than 70%, the strengthening effect is insufficient, which may lead to a large hardness difference between the soft and hard phases, thus affecting the hole expansion performance of the steel plate.

[0068] In some embodiments, the high-strength steel satisfies at least one of the following properties: yield strength ≥ 650 MPa, tensile strength ≥ 780 MPa, elongation after fracture A80 ≥ 17%, and hole expansion rate ≥ 60%.

[0069] In some embodiments, the zinc layer thickness of the high-strength steel is 5 μm to 16 μm.

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

[0071] Please see Figure 1 Secondly, embodiments of this application provide a method for preparing the high-strength steel described in the first aspect, the method comprising:

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

[0073] S2. The slab is sequentially heated, rough rolled, finish rolled, cooled and coiled to obtain a hot-rolled coil;

[0074] In some embodiments, the heating temperature is 1230°C to 1270°C.

[0075] When the heating temperature is below 1230℃, the microalloying elements are not fully dissolved and the hot rolling load is too high; when the heating temperature is above 1270℃, it will lead to coarse grains and increase the iron oxide scale on the slab surface.

[0076] In some embodiments, the final rolling temperature of the roughing roll is 1070°C to 1120°C.

[0077] When the roughing and finishing rolling temperature is below 1070℃, it is difficult to control the formation of iron oxide scale on the edges; when it is above 1120℃, it will lead to coarse microstructure.

[0078] In some embodiments, the entry temperature of the finishing mill is 950°C to 1050°C, and the final rolling temperature of the finishing mill is 890°C to 920°C.

[0079] When the finishing mill inlet temperature is below 950℃, the finishing mill stage is prone to entering the non-recrystallization zone, leading to more obvious deformation bands in the hot-rolled microstructure and deteriorating the hole expansion performance. When the finishing mill inlet temperature is above 1050℃, iron oxide scale is prone to appear on the surface of the hot-rolled plate, affecting the quality of the galvanized surface. When the finishing mill temperature is below 890℃, the anisotropy of the hot-rolled steel plate increases, resulting in a decrease in the local forming performance of the final galvanized steel plate. When the finishing mill temperature is above 920℃, the hot-rolled grain size is larger, and the risk of grain growth in the high-temperature zone increases, which is not conducive to obtaining a fine microstructure after cooling.

[0080] In some embodiments, the cooling rate is ≥20℃ / s, and the winding temperature is 350℃~500℃.

[0081] When the cooling rate is less than 20℃ / s, the ability to suppress ferrite phase transformation and microalloying precipitation during the cooling process is poor, which is not conducive to the control of phase transformation and precipitation during subsequent annealing. When the coiling temperature is below 350℃, the non-uniformity of the microstructure and properties of the hot-rolled plate increases and the plate shape deteriorates; when the coiling temperature is above 500℃, the proportion of precipitated phases is high during the hot rolling stage, and they further grow and coarsen during subsequent annealing, resulting in insufficient strength and expansion rate of the galvanized coil.

[0082] In some embodiments, the thickness of the hot-rolled coil is 1.5 mm to 6.0 mm.

[0083] S3. The hot-rolled coil is sequentially pickled, leveled, and subjected to secondary tension straightening to obtain a pickled plate;

[0084] In some embodiments, the scaling and stretching elongation of the pickling is 0.3% to 0.6%, and the acid concentration of the pickling solution is 28 g / L to 35 g / L.

[0085] When the descaling elongation is below 0.3%, the iron oxide scale on the hot-rolled plate surface is not effectively broken down, resulting in residual iron oxide scale after pickling, which affects the quality of the galvanized surface. When the descaling elongation is above 0.6%, the iron oxide scale on the hot-rolled plate surface is excessively broken down. Although the iron oxide scale removal effect is better during pickling, the acid can easily penetrate the iron oxide scale and seep into the steel substrate, which will also affect the subsequent galvanizing quality, such as the formation of zinc flow marks. The acid concentration directly affects the pickling effect on the hot-rolled plate surface. When the acid concentration is below 28 g / L, it is difficult to clean the hot-rolled iron oxide scale; when the acid concentration is above 35 g / L, it is easy to cause over-pickling of the steel plate surface, which in turn affects the quality of the galvanized surface.

[0086] In some embodiments, the leveling rolling force is 2500kN to 4000kN.

[0087] When the leveling rolling force is below 2500kN, the surface morphology uniformity of the pickled strip is poor. By increasing the leveling rolling force, the transfer rate of the surface roughness of the leveling roll on the pickled strip can be improved, thereby making the surface roughness morphology of the pickled strip uniform and obtaining good micro-area coating uniformity after galvanizing. In addition, increasing the leveling rolling force will also improve the straightness of the strip and improve the strip shape. When the leveling rolling force is above 4000kN, it is easy to cause wavy and transverse roll marks on the pickled strip.

[0088] In some embodiments, the elongation rate of the secondary tension straightening is 0.1% to 0.3%.

[0089] The purpose of secondary straightening after pickling is to improve the strip shape and reduce the risk of deviation during subsequent annealing. When the elongation rate of secondary straightening is less than 0.1%, the strip shape improvement effect is not good; when the elongation rate of secondary straightening is greater than 0.3%, the strip thinning is too large and straightening marks are prone to appear.

[0090] S4. The pickled plate is subjected to continuous hot-dip galvanizing and alloying treatment in sequence to obtain finished alloyed galvanized hot-rolled high-strength steel.

[0091] In some embodiments, the continuous hot-dip galvanizing includes: a preheating section, a heating section 1, a pre-oxidation section, a heating section 2, a soaking section, a slow cooling section, and a rapid cooling section.

[0092] In some embodiments, the temperature of the preheating section is 210°C to 230°C.

[0093] Preheating is to gradually raise the temperature of the pickling plate, reduce thermal stress caused by sudden temperature changes, and prepare for the subsequent heating and homogenization process.

[0094] In some embodiments, the heating rate of the heating section 1 is 1.5℃ / s to 5℃ / s.

[0095] When the heating rate of the first heating section is less than 1.5℃ / s, the hot-rolled structure remains in the low-temperature zone for a long time, exhibiting obvious recovery characteristics, which is not conducive to phase transformation and recrystallization. When the heating rate is greater than 5℃ / s, uneven steel plate temperature easily generates internal stress and poor plate shape, leading to furnace zone deviation.

[0096] In some embodiments, the temperature of the pre-oxidation section is 550°C to 650°C.

[0097] When the temperature of the pre-oxidation section is below 550℃, the oxidation effect on the strip surface is poor, and surface defects such as incomplete plating and dezincification are prone to occur; when the temperature of the pre-oxidation section is above 650℃, over-oxidation is prone to occur, making subsequent surface reduction difficult.

[0098] In some embodiments, the heating rate of the second heating section is 0.1℃ / s to 1℃ / s.

[0099] When the heating rate of the second heating stage is less than 0.1℃ / s, the strip steel stays in the furnace for a long time, which easily leads to surface decarburization; when the heating rate is greater than 1℃ / s, the strip steel heats up quickly and the effect of homogenization of microstructure and properties is poor.

[0100] In some embodiments, the temperature of the heat-equalizing zone is 700°C to 740°C, and the heat-equalizing period lasts for 80s to 150s.

[0101] When the soaking temperature is below 700℃, the steel plate has not entered the two-phase region and is in a high-temperature tempered state, forming a large amount of cementite, which leads to a decrease in elongation after fracture and porosity. When the soaking temperature is above 740℃, the precipitated phases grow larger and coarsen significantly, and the content and size of the Mao islands will increase, resulting in a decrease in the porosity of the material. When the soaking time is less than 80s, the homogenization of the microstructure in the high-temperature zone is low, which easily leads to uneven performance. When the soaking time exceeds 150s, the grains grow excessively, which easily leads to insufficient tensile strength.

[0102] In some embodiments, the cooling rate of the slow cooling section is 0.5℃ / s to 2℃ / s, and the final temperature of the slow cooling section is 660℃ to 680℃.

[0103] When the cooling rate in the slow cooling section is below 0.5℃ / s, the strip remains in the high-temperature zone for a long time, which easily leads to grain and precipitate growth. When the cooling rate in the slow cooling section is above 2℃ / s, the ferrite content in the strip is low, resulting in a decrease in elongation after fracture. When the end temperature of the slow cooling section is below 660℃, a large amount of ferrite is easily formed during the slow cooling process, leading to insufficient strength of the steel plate. When the end temperature of the slow cooling section is above 680℃, the amount of ferrite generated is too low, which will result in a low elongation after fracture.

[0104] In some embodiments, the cooling rate of the rapid cooling section is 10°C / s to 30°C / s, and the final temperature of the rapid cooling section is 450°C to 470°C.

[0105] When the cooling rate in the rapid cooling section is below 10℃ / s, unnecessary phase transformations such as ferrite and pearlite are prone to occur during the cooling process. When the cooling rate in the rapid cooling section is above 30℃ / s, the strip steel is prone to shaking in the furnace, resulting in surface scratches. When the final temperature of the rapid cooling section is below 450℃, the bonding force between the strip steel and the zinc liquid weakens, making it prone to incomplete plating and dezincification defects. When the final temperature of the rapid cooling section is above 470℃, the temperature of the zinc liquid is prone to rise, increasing the risk of zinc ash and zinc dross.

[0106] In some embodiments, the alloying treatment temperature is 500°C to 550°C, and the alloying treatment time is 9s to 30s.

[0107] When the alloying temperature is below 500℃, the diffusion reaction is insufficient, resulting in inadequate alloying. When the alloying temperature is above 550℃, the coating's resistance to powdering decreases, making it prone to damage or powdering during forming. When the alloying time is below 9s, the alloying effect is poor; when the alloying time exceeds 30s, it easily leads to softening of the hard phase in the microstructure, resulting in insufficient tensile strength.

[0108] The method for preparing alloyed galvanized hot-rolled high-strength steel provided in this application adds Si to improve the strength of the ferrite matrix, while using Nb, Ti, and V microalloying design. By synergistically controlling key parameters such as post-rolling cooling, coiling temperature, and homogenization temperature during continuous hot-dip galvanizing, the proportion and size of precipitated phases are optimized, thereby improving the strength and hardness of the ferrite matrix, reducing the hardness difference between soft and hard phases, and significantly improving the hole-expanding performance. The improved hole-expanding performance greatly reduces the risk of material forming cracks.

[0109] The product prepared by the preparation method of this alloyed galvanized hot-rolled high-strength steel is the aforementioned alloyed galvanized hot-rolled high-strength steel. Since the preparation method of this alloyed galvanized hot-rolled high-strength steel adopts some or all of the technical solutions of the alloyed galvanized hot-rolled high-strength steel embodiment, it has at least all the beneficial effects brought about by the technical solutions of the alloyed galvanized hot-rolled high-strength steel embodiment, which will not be elaborated here.

[0110] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0111] The chemical composition of the slab is shown in Table 1.

[0112] Table 1. Chemical composition of the slab (wt%), balance being Fe and unavoidable impurities.

[0113] Group C Si Mn Al P S Cr Mo Cu Ni Ti Nb V Example 1 0.07 0.3 1.7 0.03 0.008 0.002 0.05 0.01 0.01 0.02 0.08 0.03 - Example 2 0.05 0.1 2.0 0.04 0.007 0.002 - 0.01 0.02 0.02 0.05 0.01 0.05 Example 3 0.06 0.2 1.9 0.02 0.01 0.003 0.02 - - 0.01 0.12 0.02 - Example 4 0.08 0.2 1.8 0.01 0.008 0.002 0.1 0.03 - - 0.01 0.01 0.08 Example 5 0.09 0.3 1.7 0.05 0.007 0.001 0.1 0.02 0.02 0.03 0.04 0.02 0.04 Example 6 0.1 0.4 1.6 0.06 0.009 0.002 0.2 0.01 0.02 0.02 0.07 0.04 0.01 Example 7 0.11 0.3 1.5 0.03 0.007 0.002 0.2 0.03 0.01 0.03 0.06 0.03 0.07 Example 8 0.12 0.4 1.6 0.04 0.009 0.001 0.1 0.02 - - 0.1 0.02 0.06 Comparative Example 1 0.07 0.2 1.4 0.04 0.008 0.002 0.02 - - - 0.08 0.035 - Comparative Example 2 0.08 0.2 1.8 0.03 0.007 0.001 0.02 - - 0.02 0.11 0.03 - Comparative Example 3 0.06 0.1 1.7 0.03 0.006 0.002 0.1 0.01 - - 0.06 0.04 0.04 Comparative Example 4 0.07 0.3 1.7 0.04 0.007 0.001 0.05 - - - 0.02 0.03 0.06

[0114] Based on the chemical composition of the slabs in the embodiments and comparative examples, this embodiment also provides a method for preparing alloyed galvanized hot-rolled high-strength steel, including the following steps:

[0115] A slab having the aforementioned chemical composition is obtained;

[0116] The slab is sequentially heated, rough rolled, finish rolled, cooled, and coiled to obtain a hot-rolled coil.

[0117] The hot-rolled coil is sequentially pickled, leveled, and subjected to secondary tension straightening to obtain a pickled plate.

[0118] The pickled sheet is subjected to continuous hot-dip galvanizing and alloying treatments to obtain finished alloyed galvanized hot-rolled high-strength steel. Please refer to Tables 2 and 3 for the main process parameters.

[0119] Table 2. Main process parameters of the examples and comparative examples

[0120]

[0121] Table 3. Main process parameters of the examples and comparative examples

[0122]

[0123]

[0124] The microstructure of the steel plate was detected and analyzed using metallographic microscopy, scanning electron microscopy, and electron backscatter diffraction (EBSD); the precipitation ratio of the precipitated phase was detected using small-sample electrolysis experiments; the size of the precipitated phase was analyzed using transmission electron microscopy; and the mechanical properties and porosity of the steel plate were determined using tensile testing machine and forming testing machine.

[0125] The microstructure and properties of each embodiment and comparative example are shown in Table 4.

[0126] Table 4. Microstructure and properties of the examples and comparative examples.

[0127]

[0128]

[0129] As shown in Tables 1-4, the embodiments of this application, through specific chemical composition design and process combination, obtained alloyed galvanized hot-rolled high-strength steel with yield strength ≥650MPa, tensile strength ≥780MPa, elongation after fracture A80 ≥17%, and hole expansion rate ≥60%.

[0130] Appendix Figure 2 Detailed explanation:

[0131] Figure 2 The image shows the microstructure of the alloyed galvanized hot-rolled high-strength steel provided in Example 1 of this application. Figure 2 As shown, the microstructure is fine and uniform, mainly consisting of ferrite, Mao islands and island martensite.

[0132] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0133] The embodiments of the present invention improve the hole expansion performance by increasing the strength of the ferrite matrix rather than reducing the proportion of ferrite. The proportion of ferrite in the embodiments of the present invention is maintained at 35% or more, thereby ensuring that the elongation after fracture reaches 17% or more. This can significantly improve the overall forming ability of the material and reduce the risk of edge cracking.

[0134] The alloyed galvanized hot-rolled high-strength steel provided in this invention, compared to pickled steel or pure zinc-coated steel of the same strength level, while ensuring comparable or superior hole-expanding performance, achieves a denser coating structure and higher melting point through surface alloying treatment, resulting in less zinc evaporation and significantly reduced weld porosity and spatter. Furthermore, the alloyed coating bonds more firmly to the substrate and more densely to the electrophoretic coating, thus exhibiting higher resistance to stone impact and corrosion.

[0135] 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 in this application.

Claims

1. An alloyed galvanized hot-rolled high-strength steel, wherein the chemical composition of the high-strength steel, by mass fraction, is: C: 0.05%–0.12%, Si: 0.1%–0.4%, Mn: 1.5%–2.0%, Al: 0.01%–0.06%, P≤0.01%, S≤0.003%, Cr≤0.2%, Mo≤0.03%, Cu≤0.02%, Ni≤0.03%, and at least one of Ti: 0.01%–0.12%, Nb: 0.01%–0.04%, and V: 0.01%–0.08%, with the balance being Fe and unavoidable impurities; The microstructure of the high-strength steel, based on area ratio, comprises: Ferrite: 35%–65%, Mao islands: 35%–50%, island martensite ≤15%, wherein the grain size of ferrite is 2μm–6μm, the grain size of Mao islands is 0.5μm–2μm, and the grain size of island martensite is 1μm–3μm.

2. The high-strength steel according to claim 1, characterized in that, Precipitated phases are distributed on the ferrite matrix, the precipitates comprising one or more of TiC, NbC, VC, and (Nb, Ti)C, wherein the average equivalent diameter of the precipitates is ≤50 nm; and / or, The total precipitation ratio of Nb, V and Ti is ≥70%.

3. The high-strength steel according to claim 1, characterized in that, The high-strength steel meets at least one of the following properties: yield strength ≥ 650 MPa, tensile strength ≥ 780 MPa, elongation after fracture A80 ≥ 17%, and hole expansion rate ≥ 60%.

4. The high-strength steel according to claim 1, characterized in that, The zinc layer thickness of the high-strength steel is 5μm to 16μm.

5. A method for preparing high-strength steel according to any one of claims 1 to 4, the method comprising: A slab having the aforementioned chemical composition is obtained; The slab is sequentially heated, rough rolled, finish rolled, cooled, and coiled to obtain a hot-rolled coil. The hot-rolled coil is sequentially pickled, leveled, and subjected to secondary tension straightening to obtain a pickled plate. The pickled plate is subjected to continuous hot-dip galvanizing and alloying treatments in sequence to obtain finished alloyed galvanized hot-rolled high-strength steel.

6. The method according to claim 5, characterized in that, The heating temperature is 1230℃~1270℃; and / or, The final rolling temperature of the roughing mill is 1070℃~1120℃; and / or, The entry temperature of the finishing mill is 950℃~1050℃, and the finishing mill's final rolling temperature is 890℃~920℃; and / or, The thickness of the hot-rolled coil is 1.5mm to 6.0mm.

7. The method according to claim 5, characterized in that, The cooling rate is ≥20℃ / s, and the winding temperature is 350℃~500℃.

8. The method according to claim 5, characterized in that, The scaling and stretching elongation rate of the pickling is 0.3% to 0.6%, and the acid concentration of the pickling solution is 28 g / L to 35 g / L; and / or, The leveling rolling force is 2500kN to 4000kN; and / or, The elongation rate of the secondary straightening is 0.1% to 0.3%.

9. The method according to claim 5, characterized in that, The continuous hot-dip galvanizing process includes: a preheating section, a heating section 1, a pre-oxidation section, a heating section 2, a soaking section, a slow cooling section, and a rapid cooling section; wherein... The temperature of the preheating section is 210℃~230℃; and / or, The heating rate of the first heating section is 1.5℃ / s to 5℃ / s; and / or, The temperature of the pre-oxidation section is 550℃~650℃; and / or, The heating rate of the second heating section is 0.1℃ / s to 1℃ / s; and / or, The temperature of the heat spreader is 700℃~740℃, and the heat spreader duration is 80s~150s; and / or, The cooling rate of the slow cooling section is 0.5℃ / s to 2℃ / s, and the final temperature of the slow cooling section is 660℃ to 680℃; and / or, The cooling rate of the rapid cooling section is 10℃ / s to 30℃ / s, and the final temperature of the rapid cooling section is 450℃ to 470℃.

10. The method according to claim 5, characterized in that, The alloying treatment temperature is 500℃~550℃, and the alloying treatment time is 9s~30s.

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