Galvanized steel sheet and manufacturing method thereof

Abstract

The invention belongs to the technical field of metal manufacturing, and particularly relates to a galvanized steel plate and a manufacturing method thereof.The galvanized steel plate comprises a base steel plate and an alloying hot-dip galvanizing layer, and the surface of the base steel plate is plated with the alloying hot-dip galvanizing layer; the matrix steel plate comprises 0.35-0.45% of C, 0.10-0.55% of Si, 4.0-7.5% of Mn, less than or equal to 0.02% of P, less than or equal to 0.01% of S, less than or equal to 0.008% of N and the balance of Fe; an alloyed hot-dip galvanized layer having an average Fe content in the range of 8-11%, the balance being Zn, and a surface phase comprising a columnar phase and a polygonal phase, in which: the proportion of the columnar phase does not exceed 30%, and the ratio R of length to width satisfies: 1 lt; rlt; 5. Compared with the prior art, the problem that the galvanized steel sheet cannot be manufactured at low cost in the prior art is solved. According to the scheme, the galvanized steel sheet is low in cost and high in quality, the friction coefficient of the coating surface is small, and good anti-stripping performance is achieved.

Description

Technical Field

[0001] This invention belongs to the field of metal manufacturing technology, and specifically relates to a galvanized steel sheet and its manufacturing method. Background Technology

[0002] In recent years, the demand for ultra-high strength steel for automobiles has been increasing. Ultra-high strength steel has good mechanical and performance properties, making it suitable for the manufacture of automotive structural components. Its widespread application can effectively reduce vehicle weight.

[0003] With increasing demands for corrosion resistance in the automotive industry, alloyed hot-dip galvanized steel sheets are widely used. After hot-dip galvanizing, the coating is heated to its alloying temperature, causing Fe from the substrate to diffuse into the coating, forming different zinc-iron alloy phases. Compared to pure zinc coatings, this results in better appearance and corrosion resistance. However, the performance of alloyed coatings is closely related to their phase structure. Poor alloying leads to the formation of an excessively thick ζ phase on the coating surface, increasing the coefficient of friction, while over-alloying results in the formation of an excessively thick Γ phase at the interface between the coating and the substrate. Therefore, controlling the alloying process to achieve a better phase structure is crucial to the performance of hot-dip galvanized steel sheets.

[0004] International patent WO2017102982A1 discloses a method for producing high-strength hot-dip galvanized steel sheets. Its typical composition is 0.19C-2.0Mn-0.6Si-0.45Al-0.12V-0.024B. By controlling the hot rolling and continuous annealing processes, a multiphase microstructure of ferrite, martensite, retained austenite, and a large number of precipitated phases is obtained. Continuous work hardening is provided through nanoprecipitation strengthening and the TRIP effect during deformation, resulting in good strength and plasticity. Its yield strength, tensile strength, and elongation can reach 550MPa, 1080MPa, and 15%, respectively. However, the addition of large amounts of Al and V increases the manufacturing difficulty and cost.

[0005] US Patent US20180044750A1 invented an ultra-high-strength steel with high yield strength. Its typical composition is 0.18C-2.0Mn-1.5Si-0.6Al-0.15Mo. By adjusting the coiling temperature and hot-dip galvanizing process parameters, a microstructure with a small amount of ferrite, martensite, and austenite is formed, enabling the material to achieve an elongation of 14% at the 980MPa strength level and an elongation of 13% at the 1180MPa strength level. However, the high Si composition design is not conducive to improving the surface quality of the galvanized sheet; at the same time, the addition of a large amount of the precious metal element Mo also significantly increases the alloy cost.

[0006] Chinese patent CN108486501A discloses a 1000MPa grade cold-rolled hot-dip galvanized duplex steel with enhanced plasticity and its manufacturing method. Its typical composition is 0.19C-2.0Mn-0.3Si-0.7Al-0.45Cr-0.02Ti. Using conventional DP steel manufacturing processes (coiling temperature up to 650℃), a duplex microstructure of ferrite, martensite, retained austenite, and bainite can be obtained. Through TRIP reinforcement and plasticization during deformation, its yield strength, tensile strength, and elongation can reach 700MPa, 980MPa, and 14%, respectively. However, the high Al content increases the difficulty of smelting; the high Cr content not only increases the alloy cost but also negatively impacts the surface quality of the galvanized sheet. Furthermore, the high coiling temperature significantly increases the risk of surface defects.

[0007] US Patent 20180023155A1 discloses an ultra-high strength cold-rolled steel sheet with excellent elongation and porosity exceeding 980 MPa and its manufacturing method. The composition includes: C: 0.1~0.5%, Si: 0.8~4.0%, Mn: 1.0~4.0%, P: less than 0.015%, S: less than 0.005%, Al: 0~2%, Ti: 0.02~0.15%, and other elements may be added. The final microstructure must contain ferrite, bainite, and martensite phases, and must contain 10~25% retained austenite. Its uniqueness mainly relies on the addition of Si to obtain retained austenite, thereby achieving better elongation and porosity; and the porosity at 980 MPa can only reach 30% or more.

[0008] Analysis of the existing publicly available patents reveals that current steel plates primarily utilize the composite addition of large amounts of Cr / Mo to improve performance, while Paraaugural high-formability steel mainly relies on the addition of retained austenite to the microstructure to enhance the plasticity of hot-dip galvanized duplex steel. However, the addition of large amounts of Si is detrimental to improving the coating phase structure of current hot-dip galvanized products; while reducing the Si content leads to the addition of large amounts of Al (>0.5wt.%), increasing manufacturing difficulty; and the addition of precious alloying elements Cr, Mo, and V will significantly increase alloy costs.

[0009] Furthermore, Chinese patent CN121228148A discloses an alloyed hot-dip galvanized steel sheet. The substrate of the alloyed hot-dip galvanized steel sheet contains 0-1.0% Si by mass, and the average Fe content in the alloyed hot-dip galvanized layer is 7-10% by mass. The surface phase structure of the alloyed hot-dip galvanized layer contains columnar zinc-iron alloy phases, and the length-to-width ratio R of the columnar zinc-iron alloy phases satisfies: 2 ≤ R < 7. Chinese patent CN121228147A discloses an alloyed hot-dip galvanized steel sheet. The substrate of the alloyed hot-dip galvanized steel sheet contains 0-1.0% Si by mass, and the average Fe content in the alloyed hot-dip galvanized layer is 10-12% by mass. The surface phase structure of the alloyed hot-dip galvanized layer contains polygonal zinc-iron alloy phases, and the length-to-width ratio R of the polygonal zinc-iron alloy phases satisfies: 1 ≤ R < 2. However, these two patents mainly target steel plates with low C (0.05~0.20%) and low Mn series (1.5~2.8%) content, whose tensile strength is only >780MPa. Furthermore, patent CN121228148A's solution involves forming an alloyed coating with low Fe content to improve the steel plate's resistance to pulverization, but this increases its coefficient of friction. In contrast, patent CN121228147A's solution involves forming an alloyed coating with high Fe content to reduce the steel plate's coefficient of friction, but this weakens its resistance to pulverization.

[0010] CN114990431A proposes an alloyed hot-dip galvanized steel sheet. The surface of the steel sheet has an alloyed hot-dip galvanized layer. The chemical composition of the steel sheet, by mass%, is C: 0.25~0.70%, Si: 0.25~2.50%, Mn: 1.00~5.00%, Al: 0.005~3.50%, P: less than 0.15%, S: less than 0.03%, N≤0.02%, and O≤0.01%. Its tensile strength is above 1470 MPa, and the product of tensile strength and uniform elongation is above 13000 MPa%. However, this invention has a high Si content and does not take into account the plating issues caused by the high Mn content, which will increase manufacturing costs. Summary of the Invention

[0011] The purpose of this invention is to provide a galvanized steel sheet and its manufacturing method to solve at least one of the above-mentioned problems, thereby addressing the issues of plating applicability and high cost in the prior art for manufacturing galvanized steel sheets from high-strength steel sheets. The galvanized steel sheet of this solution has low cost, high quality, excellent overall performance, a low coefficient of friction on the coating surface, and good anti-peeling properties.

[0012] The objective of this invention is achieved through the following technical solution: The first aspect of this invention discloses a galvanized steel sheet, comprising a base steel sheet and an alloyed hot-dip galvanized layer, wherein the alloyed hot-dip galvanized layer is coated on the surface of the base steel sheet; The base steel plate, by mass percentage, contains: C: 0.35~0.45%, Si: 0.10~0.55%, Mn: 4.0~7.5%, P≤0.02%, S≤0.01%, N≤0.008%, the balance includes Fe and unavoidable impurities; The alloyed hot-dip galvanized layer, by mass percentage, has an average Fe content in the range of 8-11%, with the remainder containing Zn and unavoidable impurities; The surface phase of the alloyed hot-dip galvanized layer consists of two zinc-iron alloy phases: columnar phase and polygonal phase, wherein: The proportion of columnar phase does not exceed 30%, and, The ratio R of the length to width of the columnar phase satisfies: 1 <R<5。

[0013] Preferably, the base steel plate further comprises, by weight percentage: One or more of the following: Al: 0.1~1.0%, Ti: 0.005~0.1%, Nb: 0.005~0.1%, Cr: 0.01~0.7%, Mo: 0.05~0.2%, V: 0.005~0.2%, B: 0.0001~0.010%.

[0014] Preferably, the alloyed hot-dip galvanized layer further comprises, by weight percentage: Al: 0.01~1.0%.

[0015] Preferably, the tensile strength of the galvanized steel sheet is ≥1000MPa.

[0016] A second aspect of the present invention discloses a method for manufacturing a galvanized steel sheet as described in any of the above descriptions, comprising the following steps: The base steel plate is subjected to a series of processes including preheating, homogenization, galvanizing, and alloying to obtain a galvanized steel plate with an alloyed hot-dip galvanized layer on the surface.

[0017] Preferably, the base steel plate is obtained through smelting, hot rolling, pickling and cold rolling processes.

[0018] Preferably, the preheating conditions are as follows: The oxygen content in the preheated atmosphere is in the range of 0.01% to 0.1%. The preheating temperature is 600~850℃.

[0019] Preferably, the conditions for heat equalization are: The hydrogen volume concentration in the homogenized atmosphere is 3~25%, the dew point DP is -50~0℃, and the balance is N2 and unavoidable impurities. The temperature for uniform heating is 700~900℃.

[0020] Preferably, the galvanizing is as follows: After the steel plate is cooled, it is hot-dip galvanized in a zinc bath at 450-465°C.

[0021] Preferably, the conditions for alloying are as follows: The temperature range for alloying is controlled at 450-500°C; The time range for alloying is controlled at 10-30 s.

[0022] The working principle of the present invention is as follows: In the solution of the present invention, the core mechanism for achieving a columnar phase ratio of ≤30% and an aspect ratio R satisfying 1 < R < 5 in the alloyed hot-dip galvanized layer on the surface of the galvanized steel plate depends on the combined control of the composition design of medium-high C, high Mn, and low Si in the base steel plate and the dew point (DP) in the soaking section during annealing. The specific principle is as follows: 1. Regulation of selective oxidation by each component of the base steel plate In the base steel plate of the present invention, the C content is 0.35-0.45%, the Mn content is 4.0-7.5%, and at the same time, the Si content is strictly limited to 0.10-0.55%. This design of high Mn and low Si: during preheating (600-850°C) and soaking (700-900°C), Mn therein will preferentially undergo internal oxidation and form fine and dispersed Mn oxide particles near the surface of the steel plate; while Si, due to its low content, the SiO2 film formed by its external oxidation will be significantly inhibited. The design of medium-high C can reduce the Ac3 temperature of the steel, make austenitization more complete during soaking, and at the same time promote the diffusion of Mn to the surface, further strengthening the effect of internal oxidation.

[0023] Therefore, this surface state dominated by internal oxidation avoids the damage to the wetting property of galvanizing caused by a continuous and dense Si-Mn composite oxide layer, and provides a uniform and active base surface for the subsequent Fe-Zn alloying reaction.

[0024] 2. Precise regulation of the type and distribution of surface oxides by dew point (DP) Controlling the dew point DP in the soaking atmosphere at -50 to 0°C (preferably -30 to -10°C) is one of the key process parameters of the present invention: DP too low (< -50 °C): If the soaking atmosphere is too dry, the internal oxidation of Mn will be inhibited, and slight external oxidation will occur instead, resulting in a decrease in local surface activity, uneven Fe-Zn alloying, and easy formation of excessive columnar ζ phase (manifested as R > 5, proportion > 30%). In this solution, by controlling DP within the range of -50 to 0 °C: an appropriate amount of water vapor (H2O) and H2 form a suitable P(H2O) / P(H2) ratio, which can effectively promote the internal oxidation of Mn at grain boundaries, thereby forming nano-scale Mn oxide particles on the surface layer of the steel sheet. These nano-scale Mn oxide particles will further serve as "blocking sites" for subsequent Fe diffusion, controlling the Fe outward diffusion rate to be moderate: the Fe content in the coating can be stabilized at 8 - 11%, avoiding being too low (manifested as too much ζ phase) or too high (manifested as too thick Γ phase).

[0025] In addition, this solution also controls the alloying reaction to proceed under mild conditions: at an alloying temperature of 450 - 500 °C, polygon δ phase is preferentially formed, and only short and thick columnar ζ phase (1 < R < 5) may be formed in local areas where Fe diffusion is slightly faster, and its area proportion is effectively limited to ≤ 30%.

[0026] In summary, the galvanized steel sheet manufactured by this solution: 1) In its alloying hot-dip galvanized coating, polygon δ phase is the main component (≥ 70%): originating from the equilibrium reaction between Fe and Zn under controlled diffusion, endowing the coating with a low friction coefficient and good peel resistance; columnar ζ phase is the auxiliary component (≤ 30%, 1 < R < 5): its existence provides a mechanical locking effect, enhancing the bonding force between the galvanized coating and the base steel sheet (improving anti-powdering property), and at the same time, due to its low proportion and short and thick morphology (small R), it will not significantly increase the surface friction coefficient.

[0027] 2) If DP is too high (> 0 °C), the external oxidation of Mn intensifies, there are no blocking sites locally, Fe diffuses too fast, and the proportion of columnar ζ phase increases and R > 5; if DP is too low (< -50 °C), the internal oxidation is insufficient, which also leads to out-of-control alloying. That is, only within the DP range defined in this invention can the dual requirements of the columnar phase proportion and aspect ratio be satisfied simultaneously, so that the galvanized steel sheet manufactured by this solution has excellent comprehensive performance.

[0028] Compared with the prior art, this invention has the following beneficial effects: The present invention provides a steel plate with excellent comprehensive properties, and its surface has a phase structure composed of columnar zinc-iron alloy phase and polygonal zinc-iron alloy phase. Among them, the columnar zinc-iron alloy phase can wrap the coating and will not form an overly thick Γ phase due to over-alloying, and has good anti-powdering performance; and the proportion of the columnar zinc-iron alloy phase does not exceed 30%, which can ensure that the friction coefficient on the surface of the coating is small and has good anti-peeling performance; and this formulation does not require the introduction of a large amount of precious alloy elements, nor the use of a large amount of difficult-to-manufacture and process Al elements, making the overall cost of this solution more economical.

[0029] Compared with the prior arts CN121228148A (low C, low Mn, high proportion of columnar phase, large friction coefficient) and CN121228147A (high Fe content, poor anti-powdering), the present invention, through a unique composition combination of high Mn + low Si + medium-high C, in combination with medium-low dew point (-50~0°C), for the first time realizes a coating structure with a columnar ζ phase proportion ≤ 30% and 1 < R < 5 in high-strength steel with a tensile strength ≥ 1000 MPa, solving the technical problem that it is impossible to simultaneously achieve low friction coefficient and high anti-powdering performance under high-strength conditions. Description of the Drawings

[0030] Figure 1 It is the surface SEM image of the galvanized steel plate manufactured in Example 2 of the present invention.

[0031] Figure 2 It is the surface SEM image of the galvanized steel plate manufactured in Comparative Example 2 of the present invention. Detailed Embodiments

[0032] The present invention will be described in detail below in conjunction with the drawings and specific embodiments. It should be noted that the following specific embodiments and drawings do not unduly limit the technical solution of the present invention.

[0033] If not specifically stated in the following description, the reagents used are conventional commercially available products, the methods used are well-known means in the art, and for the rest of the matters not covered, existing technologies can be selected.

[0034] The purpose of the present invention is to provide a galvanized steel plate with excellent comprehensive properties and its manufacturing method, and this steel plate has excellent comprehensive mechanical properties.

[0035] The steel plate described in the present invention is composed of a base steel plate and a zinc layer on the surface (coating). Specifically, its surface layer is an alloyed hot-dip galvanized layer, the average Fe content in this zinc layer is within the range of 8~11% (by mass percentage), and in this zinc layer, the surface phase is composed of two zinc-iron alloy phases, namely columnar phase and polygonal phase, among which the proportion of the columnar zinc-iron alloy phase does not exceed 30%, and the ratio R of the length to the width of the columnar zinc-iron alloy phase satisfies: 1 < R < 5.

[0036] Furthermore, the zinc layer also contains more than 0.01% and less than 1.0% (0.01~1.0%) Al by mass percentage.

[0037] The base steel plate of the present invention contains, by mass percentage, C: 0.35~0.45%, Si<0.55%, Mn: 4.0~7.5%, P≤0.02%, S≤0.01%, N≤0.008%, with the remainder including Fe and unavoidable impurities.

[0038] Furthermore, the base steel plate of the present invention, by mass percentage, also contains one or more of the following: Al: 0.1~1.0%, Ti: 0.005~0.1%, Nb: 0.005~0.1%, Cr: 0.01~0.7%, Mo: 0.05~0.2%, V: 0.005~0.2%, B: 0.0001~0.010%.

[0039] Furthermore, the steel plate described in this invention has a tensile strength ≥1000MPa.

[0040] Next, the compositional characteristics of the base steel plate and the galvanized layer constituting the steel plate of the present invention will be explained.

[0041] 1) Average Fe content in the coating: 8~11% The alloyed hot-dip galvanized steel sheet (galvanized steel sheet) of this invention has an Fe content in the coating range of 8-11%. If the Fe content in the coating is less than 8%, the coating surface is prone to incomplete alloying, resulting in low iron content and easy adhesion to the mold. During stamping, this increases the friction between the steel sheet and the mold, and the high frictional shear stress generated by the ζ phase on the surface causes strain concentration at the interface, leading to peeling of the coating at the interface and even stamping cracks. If the Fe content in the coating is higher than 11%, a Γ phase will form at the interface between the steel sheet and the coating. This phase is hard and brittle, which is detrimental to the powdering control of this invention. Therefore, the Fe content of the coating is controlled at 8-11%.

[0042] 2) Area of ​​columnar zinc-iron alloy phase: ≤30% The surface phase structure of the alloyed coating directly affects its anti-powdering and anti-peeling properties. To achieve a better balance between preventing powdering and preventing peeling, a reasonable alloying process is needed to control the composition of the coating surface to obtain a two-phase mixture. In this scheme, the coating surface is mainly composed of polygonal δ phases, supplemented by a small amount of columnar ζ phases, with the maximum area of ​​the ζ phase set at 30%.

[0043] 3) The aspect ratio R of the columnar zinc-iron alloy phase: 1 <R<5 This invention also defines the aspect ratio of the columnar phase. If the aspect ratio R < 1, a higher alloying temperature is required, and an excessively thick Γ phase is easily formed at the interface; if the aspect ratio > 5, it is easy to cause an excessive amount of ζ phase to form on the coating surface.

[0044] 4) Al content in the coating: 0.01~1.0% Diffusion occurs during alloying. The Fe2Al5 barrier layer initially formed at the interface between the coating and the substrate during hot-dip galvanizing decomposes and diffuses into the coating during heating. Therefore, the alloyed coating contains not only Fe and Zn, but also a certain amount of Al. If the Al content (Al concentration) in the coating is less than 0.01%, it indicates excessive Zn-Fe alloying diffusion reaction in the coating; while if the Al content exceeds 1.0%, it indicates that Al inhibits the Zn-Fe alloying reaction rate.

[0045] Furthermore, the zinc plating layer proposed in this invention is an alloyed plating layer formed by zinc-iron diffusion with Zn as the main component. In addition to Zn, Fe, and Al, the diffusion plating layer may also contain one or more of Si, Mn, Ti, Nb, Cr, Mo, V, and B. The introduction of these elements does not impair the effect of this invention.

[0046] Specifically, the design principles of the steel plate and the various chemical elements introduced into the steel plate are as follows: 1) C: 0.35~0.45% Carbon (C) is the most basic and economical solid solution strengthening element in steel, and also an austenite stabilizing element. During the rapid cooling process after continuous annealing, high C content significantly improves the hardenability of steel, promoting the transformation of austenite formed during critical zone annealing into high-strength and high-hardness martensite during cooling; for every 0.1% increase in C content, the martensitic strength can be increased by approximately 100-150 MPa. When the C content is below 0.35%, the martensite content is insufficient or its carbon supersaturation is too low, making it difficult to stably reach the 1000 MPa level; however, when the C content is above 0.45%, although the strength can be further improved, the weldability and toughness deteriorate significantly, and quenching cracks are easily generated.

[0047] 2) Si: 0.10 <Si<0.55% Si is a substitutional solid solution alloying element that promotes the enrichment of carbon in austenite, increasing austenite stability, improving the strength of the steel sheet, and to some extent enhancing its toughness. When the Si content is below 0.10%, coarse iron-based carbides are generated during the alloying process of the coating, deteriorating the formability. However, when the Si content exceeds 0.55%, it directly affects the plating applicability of the substrate. Therefore, this invention specifies an upper limit of 0.55% for the Si content.

[0048] 3) Mn: 4.0~7.5% Mn is another core strengthening element in this invention, and its content in this solution is much higher than that of conventional high-strength steel (usually ≤3.0%). It is well known that increased Mn content often leads to plating problems; therefore, existing processes typically control the Mn content at a low level. This solution improves the platingability of the base steel plate by adding a pre-oxidation (preheating) process, overcoming the plating problems caused by high Mn content and overcoming the bias of existing technologies. In the base steel plate, Mn is dissolved in the ferrite / martensite matrix in the form of substitutional atoms, producing lattice distortion and hindering dislocation movement. The solid solution strengthening coefficient of Mn is approximately 30~40 MPa / wt.%, therefore, compared to the conventional 2.0~3.0% Mn content, the 4.0~7.5% Mn content of this invention can contribute an additional strength increment of approximately 60~150 MPa. The addition of Mn significantly reduces the martensitic transformation initiation temperature (Ms) and termination temperature (Mf), allowing for more complete enrichment of C in austenite, thus obtaining stable retained austenite (volume fraction up to 5-15%) at room temperature, providing a basis for the TRIP effect. Mn can also inhibit austenite grain growth, improving strength through a grain refinement strengthening mechanism (Hall-Petch relationship). Increasing the Mn content from the conventional 3.0% to 4.0-7.5% can refine the effective grain size from 5-10 μm to 2-5 μm.

[0049] 4) P: ≤0.02% P in steel significantly reduces its plasticity and toughness, especially at low temperatures, leading to "cold brittleness." Therefore, it must be strictly controlled and limited to below 0.02%.

[0050] 5) S: ≤0.01% S, as an impurity element, must be kept at a low level. S has a detrimental effect on weldability and manufacturability during casting and hot rolling. S forms FeS, which causes hot brittleness. In addition, S combines with Mn to form coarse MnS, which reduces the ductility of the material. Therefore, the S content is limited to below 0.01%.

[0051] 6) N: ≤0.008% Nitrogen readily forms large nitrides, which deteriorates ductility. This tendency becomes significant if the nitrogen content exceeds 0.0100%, so its upper limit is set at 0.0100%.

[0052] 7) Al: 0.1~1.0% When Al exists in solid solution, it can increase stacking fault energy, inhibit cementite precipitation and γ-to-martensite transformation, and improve austenite stability. Furthermore, Al forms fine, dispersed, insoluble particles with C and N to pin grain boundaries, refining the grain size. However, the amount of Al added should not be too high, as it easily forms a large number of oxide inclusions, which is detrimental to continuous casting in steelmaking and can easily cause problems such as nozzle blockage during the continuous casting process. Therefore, this invention controls the Al content to be 0.1~1.0%.

[0053] 8) Ti: 0.005~0.1% Ti strengthens steel sheets by combining with C and N to form Ti(C,N), TiN, and TiC precipitates, and by inhibiting ferrite grain growth. When the Ti content is below 0.005%, the refining and strengthening effects are not significant; however, adding more than 0.1% Ti significantly increases costs and also increases the content and size of the aforementioned precipitates, deteriorating the formability of the steel sheet. Therefore, this invention controls the Ti content to be between 0.005% and 0.1%.

[0054] 9) Nb: 0.005~0.1% Like Ti, Nb can combine with C and N to form Nb(C,N), which can improve the strength of steel plates through precipitation strengthening, inhibiting ferrite grain growth, and fine grain strengthening. When the Nb content is below 0.005%, the strengthening effect is not obvious; while when the Nb content exceeds 0.1%, more carbide precipitation occurs, and the formability deteriorates. Therefore, this invention controls the Nb content to be 0.005~0.1%.

[0055] 10) Cr: 0.01~0.7% Cr is a ferrite-forming element that promotes carbon diffusion into austenite, improves austenite stability, and reduces the critical cooling rate during annealing. It can replace a portion of C and / or Mn, but its effect is not significant when the addition amount is below 0.01%. However, a Cr content exceeding 0.7% will impair the ductility of the steel. Therefore, this invention controls the Cr content to be between 0.01% and 0.7%.

[0056] 11) Mo: 0.05~0.2% Mo acts similarly to Cr, shifting the C-curve to the right and improving the hardenability of steel. Simultaneously, Mo significantly increases strength without affecting the surface quality of the coating. However, it is expensive, and adding large amounts of Mo increases manufacturing costs. Therefore, this invention controls the Mo content to 0.05~0.2%.

[0057] 12) V: 0.005~0.2% In hot-dip galvanized duplex steel, vanadium (V) mainly exists in the form of volatile organic compounds (VC). It improves the steel's strength and toughness by refining grains through pinning grain boundaries and dispersing ferrite within the ferrite. However, its effect is not significant when the addition amount is below 0.005%. Adding V increases manufacturing costs, and when the addition amount exceeds 0.2%, the increased precipitation of carbonitrides deteriorates the formability. Therefore, this invention controls the V content to be between 0.005% and 0.2%.

[0058] 13) B: 0.0001~0.010% The main function of boron (B) is to improve the hardenability of steel. B tends to segregate at austenite grain boundaries, delaying the transformation of austenite to ferrite. Adding a small amount to steel can have a significant effect, but excessive B content will increase the strength of the steel, which is not conducive to obtaining good plasticity. Therefore, the B content is specified to be below 0.01%, more preferably below 0.005%, and to ensure its effect, the lower limit of B content is preferably above 0.0001%. Therefore, in this invention, the B content is controlled in the range of 0.0001~0.01%.

[0059] Taking into account both performance and cost control, at least one of the above-mentioned elements can preferably be added to the technical solution described in this invention.

[0060] Another object of the present invention is to provide a method for manufacturing the alloyed hot-dip galvanized steel sheet with excellent anti-powdering properties, comprising the following steps: The steel with the above chemical composition is smelted and hot-rolled, as well as pickled and cold-rolled, to obtain the base steel sheet. Then, it is fed into a continuous annealing and hot-dip galvanizing production line and the following processes are performed in sequence: (1) Preheating The base steel plate is heat-treated, and the oxygen content in the preheating process is in the range of 0.01-0.1%, and the steel plate is heated to the range of 600~850℃.

[0061] (2) Heat equalization The steel plate is heated and held at a homogenization temperature of 700-900°C in an atmosphere with hydrogen concentration limited to 3-25% by volume, dew point DP limited to -50-0°C, and the balance being N2 and unavoidable impurities.

[0062] (3) Galvanizing The steel plate is cooled to a zinc bath at a temperature of 450-465°C and then hot-dip galvanized.

[0063] (4) Alloying After the galvanizing process, the steel sheet is then alloyed in an alloying furnace. The alloying temperature is controlled within the range of 450~500℃ to obtain the desired alloyed hot-dip galvanized steel sheet with excellent surface anti-powdering properties.

[0064] Preferably, the hydrogen concentration in step (2) is further limited to 3-10%.

[0065] Preferably, the dew point DP in step (2) is further limited to -30~-10℃.

[0066] Preferably, the alloying temperature in step (4) is further limited to 470~490℃.

[0067] Preferably, the alloying time in step (4) is limited to 10~30s.

[0068] In the manufacturing method of this invention, steel with the chemical composition defined in this invention is smelted and hot-rolled, pickled and cold-rolled respectively. There are no particular limitations on the processes preceding annealing of the cold-rolled steel sheet. During the annealing process, a continuous hot-dip galvanizing annealing line with a preheating section, a reduction section and a plating section is preferentially used. The steel is annealed while passing through the preheating and reduction sections, and the annealing process ends before reaching the plating section, where the plating process is carried out.

[0069] Through extensive experimental research, the inventors have summarized the process conditions for obtaining the alloyed coated steel sheet with excellent pulverization performance as described in the above claims: 1) Oxygen content in the preheating process: 0.01~0.1% If the oxygen content is below 0.01%, a pre-oxidized layer cannot be formed. If the oxygen content is above 0.1%, an excess iron oxide layer will be generated on the steel plate surface. This layer cannot be fully reduced during the subsequent reduction annealing process, resulting in a residual iron oxide layer beneath the reduced iron layer, ultimately reducing the adhesion of the coating.

[0070] 2) Heating temperature for the preheating process: 600~850℃ If the temperature is below 600℃ during the pre-oxidation process, a pre-oxidation layer cannot be formed; if the temperature of the pre-oxidation layer exceeds 850℃, an excessively thick oxide layer will also be formed, and the Si / Mn oxides will form external oxidation, which will reduce the adhesion of the coating.

[0071] The heating temperature for pre-oxidation in the preheating section is set to 850°C or below, preferably 750°C or below.

[0072] 3) H2 content in the soaking process, DP: -50~0℃ The H2 concentration needs to be sufficient to suppress Fe oxidation, so it should be adjusted to 3 vol% (corresponding to a DP of -50℃) or higher. If the H2 concentration exceeds 25 vol% (corresponding to a DP of 0℃), the cost will increase rapidly.

[0073] 4) Zinc bath temperature: 450~465℃ When the temperature of the zinc bath is below 450℃, the viscosity of the zinc bath is too high, making it difficult to control the thickness of the coating and affecting the appearance and alloying of the steel sheet. It is preferable to have a temperature above 455℃. On the other hand, if the temperature of the zinc bath exceeds 465℃, the zinc bath will evaporate, and the vaporized Zn will adhere to the grate, causing zinc ash to fall off and adhere to the surface of the strip steel, forming defects.

[0074] 5) Alloying temperature: 450~500℃ To ensure that Zn and Fe can diffuse into each other, a portion of polygonal alloy phase is obtained on the surface of the alloyed coating. The alloying temperature cannot be too low, and the minimum temperature needs to be 450°C. On the other hand, in order to obtain a surface columnar zinc-iron alloy phase ratio of no more than 30% in this invention, the alloying temperature needs to be controlled below 500°C.

[0075] 6) Alloying time: 10~30s On different continuous hot-dip galvanizing production lines, the alloying time varies depending on the length of the alloying furnace, the thickness of the strip, and the production speed of the strip. The shortest alloying time needs to be at least 10 seconds. To obtain the columnar zinc-iron alloy phase ratio of the surface layer of this invention, it must not exceed 30%, and the longest holding time must not exceed 30 seconds.

[0076] Examples 1-10 The steel plate of this invention is obtained by the following steps: Table 1 lists the chemical element weight percentages (%) of the galvanized steel sheets of Examples 1-10, with the remainder being Fe. Table 2 summarizes the main process parameters of the manufacturing methods for the galvanized steel sheets of Examples 1-10.

[0077] The steels with the chemical compositions listed in Table 1 are smelted and hot-rolled, pickled and cold-rolled respectively; then, they are fed into a continuous annealing and hot-dip galvanizing production line and the following processes are performed in sequence: (1) Preheating The base steel plate is heat-treated with an oxygen content in the heating furnace ranging from 0.01% to 0.1%, and the steel plate is heated to a temperature range of 600 to 850°C.

[0078] (2) Heat equalization The steel plate is heated and held at a homogenization temperature of 700-900°C in an atmosphere with a hydrogen concentration limited to 3-25% (volume)% and a dew point (DP) limited to -30-0°C, with the balance being N2 and unavoidable impurities. Preferably, the hydrogen concentration is further limited to 3-10%.

[0079] (3) Galvanizing The steel plate is cooled to a zinc bath at a temperature of 450-465°C and then hot-dip galvanized.

[0080] (4) Alloying After the galvanizing process, the steel sheet is then alloyed in an alloying furnace at a temperature controlled between 470 and 490°C to obtain the desired alloyed hot-dip galvanized steel sheet with excellent surface anti-powdering properties.

[0081] Comparative Examples 1-3 Table 1 lists the chemical element weight percentages (%) of the galvanized steel sheets of Comparative Examples 1-3, with the remainder being Fe. Table 2 summarizes the main process parameters for the manufacturing methods of the galvanized steel sheets of Comparative Examples 1-3.

[0082] The formulations of Comparative Examples 1-3 are the same as those of Examples 1-10, with only some differences in the amount of ingredients, as summarized in Tables 1 and 2.

[0083] The manufacturing processes of Comparative Examples 1 to 3 are the same as those of Examples 1 to 10, with only some differences in process parameters, as summarized in Table 2.

[0084] Table 1. Content of each element under different formulations (in mass percentage, balance is Fe). Table 2 Summary of formulations and main process parameters used in Examples 1-10 and Comparative Examples 1-3 Table 3 Summary of performance results of Examples 1-10 and Comparative Examples 1-3 Tables 1-3 list the specific process parameters, surface phase structure characteristics, and related properties of the coatings in the above-mentioned examples and comparative examples. It can be seen that within the composition and process range of this invention, Examples 1-10 can obtain the galvanized steel sheet required by this invention. However, Comparative Example 1, under pretreatment conditions without oxidation, has a low alloying temperature, resulting in an excessively high proportion of the ζ phase in the final coating, leading to a high surface friction coefficient and a risk of cracking during part stamping. Comparative Example 2, under pretreatment conditions without oxidation, has an increased alloying temperature, but its annealing dew point is too low, resulting in a high aspect ratio of the columnar phase, which also increases the surface friction coefficient. Comparative Example 3, although under pretreatment conditions with oxidation, has an excessively high oxygen content, ultimately resulting in a surface ζ phase proportion that does not meet requirements, and its powdering level also fails to meet requirements.

[0085] Figures 1-2 SEM images of Example 2 and Comparative Example 2 are provided. Figure 1 The proportion of the ζ phase in it is less than 10%, while Figure 2 The proportion of the ζ phase is much higher than 30%, and the coating exhibits a typical over-alloyed morphology.

[0086] In this invention, the coefficient of friction is tested according to ISO 22462:2020 standard, using the flat plate sliding method to test the dynamic coefficient of friction of the coating surface. The chalking performance is tested using the JIS C 2550 standard, employing an 0T bending tape peel test to observe the amount of powder adhering to the tape and evaluate its anti-chalking performance. Generally, the chalking performance requirement for automotive parts is no more than level 4. The mechanical property test method is a room temperature tensile test, primarily based on GB / T 228.1-2021 standard to determine tensile strength.

[0087] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A galvanized steel sheet, characterized in that, It includes a base steel plate and an alloyed hot-dip galvanized layer, wherein the alloyed hot-dip galvanized layer is coated on the surface of the base steel plate; The base steel plate, by mass percentage, contains: C: 0.35~0.45%, Si: 0.10~0.55%, Mn: 4.0~7.5%, P≤0.02%, S≤0.01%, N≤0.008%, the balance includes Fe and unavoidable impurities; The alloyed hot-dip galvanized layer, by mass percentage, has an average Fe content in the range of 8-11%, with the remainder containing Zn and unavoidable impurities; The surface phase of the alloyed hot-dip galvanized layer consists of two zinc-iron alloy phases: columnar phase and polygonal phase, wherein: The proportion of columnar phase does not exceed 30%, and, The ratio R of the length to width of the columnar phase satisfies: 1 <R<5。 2. The galvanized steel sheet according to claim 1, characterized in that, The base steel plate, by mass percentage, also contains: One or more of the following: Al: 0.1~1.0%, Ti: 0.005~0.1%, Nb: 0.005~0.1%, Cr: 0.01~0.7%, Mo: 0.05~0.2%, V: 0.005~0.2%, B: 0.0001~0.010%.

3. The galvanized steel sheet according to claim 1, characterized in that, The alloyed hot-dip galvanized layer, by weight percentage, further comprises: Al：0.01~1.0%。 4. A galvanized steel sheet according to claim 1, characterized in that, The galvanized steel sheet has a tensile strength ≥1000MPa.

5. A method for manufacturing a galvanized steel sheet as described in any one of claims 1 to 4, characterized in that, Includes the following steps: The base steel plate is subjected to a series of processes including preheating, homogenization, galvanizing, and alloying to obtain a galvanized steel plate with an alloyed hot-dip galvanized layer on the surface.

6. The method for manufacturing a galvanized steel sheet according to claim 5, characterized in that, The base steel plate is obtained through smelting, hot rolling, pickling and cold rolling processes.

7. The method for manufacturing a galvanized steel sheet according to claim 5, characterized in that, The preheating conditions are as follows: The oxygen content in the preheated atmosphere is in the range of 0.01% to 0.1%. The preheating temperature is 600~850℃.

8. The method for manufacturing a galvanized steel sheet according to claim 5, characterized in that, The conditions for heat equalization are as follows: The hydrogen volume concentration in the homogenized atmosphere is 3~25%, the dew point DP is -50~0℃, and the balance is N2 and unavoidable impurities. The temperature for uniform heating is 700~900℃.

9. A method for manufacturing a galvanized steel sheet according to claim 5, characterized in that, The zinc plating mentioned is: After the steel plate is cooled, it is immersed in molten zinc at 450~465℃ for hot-dip galvanizing.

10. A method for manufacturing a galvanized steel sheet according to claim 5, characterized in that, The alloying conditions are as follows: The alloying temperature range is controlled between 450 and 500℃; The alloying time is controlled within the range of 10~30s.

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