Hot-dip galvanized steel sheet and method for manufacturing the same

Abstract

A manufacturing method of a hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet are provided, the method includes hot-rolling a slab to obtain a steel sheet, coiling the steel sheet, pickling and cold-rolling the steel sheet, performing continuous annealing by setting the annealing temperature at 840-870°C, setting the annealing dew point at -10-0°C, cooling to 710-730°C at a cooling rate of ≦10°C / s, further cooling to 220-320°C at a cooling rate of ≧50°C / s, reheating to 410-460°C, and maintaining the temperature for 20-100 s, and galvanizing the steel sheet to obtain a hot-dip galvanized steel sheet having a chemical element composition of C: 0.17-0.21wt%, Si: 1.2-1.7wt%, Al: 0.02-0.05%, Mn: 1.60-2.1wt%, N: ≦0.008wt%, and the balance being Fe and unavoidable impurities. The hot-dip galvanized steel sheet of the present invention has a yield strength of 400-600 MPa, a tensile strength of 730-900 MPa, an elongation of 25-35%, and a hole expansion ratio of 35-60%.

Description

[Technical Field]

[0001] The present invention relates to metal materials and methods for processing the same, and more particularly to hot-dip galvanized steel sheets and methods for manufacturing the same. [Background technology]

[0002] A 10% reduction in vehicle weight can save 5-8% on fuel consumption, while simultaneously reducing CO2 greenhouse gases and NOx. x It is estimated that the emission of pollutants such as SO2 can be reduced accordingly. Steel sheets are the main material of the car body, accounting for about 60-70% of the car's weight. Therefore, increasing the strength of steel sheets to reduce their thickness has become one of the directions of steel sheet development in recent years. However, due to the metallurgical mechanism, as the strength of conventional high-strength steel increases, its plasticity generally decreases, thus limiting the application of high-strength steel in automotive structural components with complex shapes. High-formability TRIP steel, which uses metastable austenite phase transition strengthening as its main strengthening mechanism, overcomes the contradiction that conventional steels could not achieve both ultra-high strength and high formability simultaneously, demonstrating good applicability in automotive body structural materials, and its development and application are becoming the focus of research for major steel companies and automotive companies worldwide.

[0003] TRIP steel (Transformation-Induced Plasticity Steel) is a material in which a certain amount of metastable austenite is introduced into the martensite or bainite structure, achieving high strength and high plasticity through the dynamic phase transition of the metastable austenite. However, conventional TRIP steel has a complex microstructure and high work hardness. In particular, with conventional material designs and methods, there is a large hardness difference between the soft and hard phases, resulting in poor local deformability when pre-cutting high-strength TRIP steel. In particular, flange workability and hole expansion are significantly worse compared to ordinary ultra-high-strength steel. For example, the hole expansion rate of duplex steel (DP) at the 780 MPa level is 30% or more, but that of TRIP steel at the same level is only 10-15%. Typically, multiple forming methods are used in the forming process of automotive parts. In addition to conventional drawing and stretching which relate to overall formability, there are also forming methods that relate to local formability, such as flange processing, hole expansion, and bending. Due to its low hole expansion ratio, TRIP steel has limited use in parts where flange processing and hole expansion forming are frequently performed, preventing it from fully utilizing its high plasticity and thus limiting its application in automotive parts manufacturing.

[0004] While numerous patents exist for manufacturing high-strength TRIP steel in conventional technology, these inventions tend to employ a microstructure design of soft phase + hard phase + metastable austenite to maintain the overall formability of the steel sheet. Although this significantly increases the elongation compared to steel of the same level, in a multiphase composite structure, there is a large difference in hardness between different phases. When deformation occurs locally, the deformation of the soft and hard phases does not match, causing cracks to form at the phase interface, impairing the local formability of the material, such as flange workability, hole expansion ability, and cold bending ability.

[0005] Furthermore, compared to ordinary cold-rolled steel sheets, hot-dip galvanized products have far superior corrosion resistance, leading to their widespread use in automobiles. On average, their usage exceeds 80%, and in some models, it reaches 100%. However, to achieve stability and a sufficient volume proportion of metastable austenite, TRIP steel contains many alloying elements such as Si, Al, and Mn. These elements have active chemical properties, making them prone to surface oxidation during heat treatment, which reduces plating performance and makes it difficult to consistently produce hot-dip galvanized products with high plating layer quality. Therefore, designs with low Si and Mn content are usually adopted to improve the plating performance of steel materials. However, Si and Mn are the most effective low-cost strengthening elements in steel, and designs with low Si and Mn content reduce the performance of the steel, necessitating the addition of expensive alloying elements such as Cr, Mo, and Nb, which increases the cost of the steel material and potentially reduces the manufacturability of the product.

[0006] The following patents were found in the search: JP 2010255097 discloses a high-strength hot-dip galvanized steel sheet with excellent workability and a method for manufacturing the same. The composition, by mass%, contains C: 0.04~0.15%, Si: 0.7~2.3%, Mn: 0.8~2.2%, P: <0.1%, S: less than 0.01%, Al: <0.1%, and N: less than 0.008%, with the remainder being iron and unavoidable impurities. The microstructure consists of 70% or more ferrite phase, 2% or more and 10% or less bainite phase, 0% or more and 12% or less pearlite phase, and 1% or more and 8% or less retained austenite phase. The average grain size of ferrite is 18 μm or less, and the average grain size of retained austenite is 2 μm or less. The steel of this invention has a tensile strength of 590 MPa or more and also exhibits excellent workability (elongation and hole-expanding properties). However, this invention achieves a tensile strength of only 600-700 MPa, which is insufficient for ultra-high-strength steel.

[0007] WO 2020151856 A1 discloses cold-rolled ultra-high-strength steel at a level of 1380 MPa and a method for producing the same. The mass percentages of the components are: C: 0.15~0.25%, Si: 0.7~1.6%, Mn: 2.2~3.2%, Mo: ≤0.2%, Cr: ≤0.8%, Al: 0.03~1.0%, Nb / V: ≤0.04%, Ti: 0.01~0.04%, B: 0.001~0.005%, Cu: ≤0.15%, Ni: ≤0.15%, Ca: ≤0.01%, with the remainder being Fe and unavoidable impurities. This invention has a multiphase structure containing 40% or more tempered martensite, 40% or less bainite, 20% or less fresh martensite, and 2-20% retained austenite. In this invention, the hole expansion ratio reaches 40% or more, but the elongation ratio is only 5%, which does not meet the high formability requirements for complex parts.

[0008] WO 2020128574 A1 discloses a hot-dip galvanized ultra-high-strength steel having a tensile strength of 1470 MPa or higher, and a method for manufacturing the same. The mass percentages of the components are: C: 0.3~0.4%, Si: 0.8~1.60%, Mn: 2.0~4.0%, Al: 0.01~0.6%, Mo: 0.15~0.50%, Cr: 0.3~1.0%, Ti: ≤0.06%, Nb: ≤0.06%, V: ≤0.2%, Ni: ≤0.8%, B: 0.0003~0.0005%, with the remainder being Fe and unavoidable impurities. The microstructure of this steel consists of 15~30% retained austenite with a carbon content of 0.7% or more, 70~85% tempered martensite, and 5% fresh martensite. This invention achieves a tensile strength of 1470 MPa or higher, an elongation of 13% or higher, a hole expansion ratio of 15% or higher, and an LME index of 0.7 or lower. However, because this invention uses steel with a high carbon content and to which large amounts of Nb, V, and Ti alloying elements are added, the material cost is significantly increased, and manufacturing processes such as casting, hot rolling, and welding become more difficult. At the same time, the hole expansion ratio of the material is not high, limiting the applications of these products.

[0009] CN 109023053 B discloses a 600 MPa level multiphase steel sheet with good flange processing performance, the composition of which is: C: 0.060~0.100%, Si: 0.060~0.400%, Mn: 1.20~2.00%, P: 0.020% or less, S: 0.010% or less, Al: 0.015~0.070%, Cr: 0.15~0.35%, Ti: 0.010~0.035%, Nb: 0.010~0.035%, N: 0.006% or less. The production process of this steel includes the following: casting after normal smelting according to the composition; hot rolling process; cold rolling process; natural cooling to room temperature and use. In this invention, the yield strength of the steel reaches 360-440 MPa, the tensile strength is 600-700 MPa, the elongation is 19% or more, and the hole expansion ratio is 45% or more. The microstructure contains pearlite, bainite, ferrite, and small amounts of martensite and retained austenite, thus maintaining high strength while also maintaining good formability, flange workability, and impact energy absorption. In this invention, both the hole expansion ratio and elongation ratio of the steel are good, but the steel level is low and the tensile strength is only at the 600 MPa level, so it does not meet the demand for high strength and thin walls in automobiles.

[0010] WO 2013144376 A1 discloses a cold-rolled ultra-high-strength steel for automotive applications, with a composition of C: 0.1-0.3%, Si: 0.4-1.0%, Mn: 2.0-3.0%, and Nb: ≤0.01. This steel contains a multiphase structure with 5-20% retained austenite, 80% or more bainite / bainite ferrite / tempered martensite, and 10% or less polygonal ferrite. It has a tensile strength of 980 MPa or more, an elongation of 4% or more, a hole expansion ratio of 20% or more, a strength-to-elongation product of 13000% MPa or more, and a hole expansion ratio-to-strength product of 40000% MPa or more. In this invention, although the steel has high strength, its elongation and hole expansion ratio are not high, resulting in poor formability and failing to meet the demand for forming complex parts. [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] Conventional high-strength hot-dip galvanized steel sheets have a problem in that the quality of the plating layer is poor, and when localized deformation occurs, the deformation of the soft and hard phases does not match, causing cracks to form at the phase interface, which impairs the local formability of the material, such as flange workability, hole expansion ability, and cold bending ability. To overcome this problem, the present invention provides a method for manufacturing hot-dip galvanized steel sheets, and this method makes it possible to manufacture hot-dip galvanized steel sheets with high strength, good plating layer quality, and excellent local formability that can be applied to automotive structural parts and safety parts that require high corrosion resistance. [Means for solving the problem]

[0012] The method for manufacturing a hot-dip galvanized steel sheet according to the present invention includes the following steps: S1: The slab is hot-rolled to obtain a steel sheet, which is then coiled and subjected to pickling and cold rolling; S2: Continuous annealing is performed by setting the annealing temperature to 840-870°C, the annealing dew point to -10-0°C, slowly cooling to the rapid cooling start temperature of 710-730°C at a cooling rate of ≤10°C / s, then rapidly cooling to the rapid cooling end temperature of 220-320°C at a cooling rate of ≥50°C / s, heating to a reheating temperature of 410-460°C, and holding the temperature for 20-100 seconds; S3: Perform zinc plating; after zinc plating is complete, cool to room temperature to obtain hot-dip galvanized steel sheet; However, this hot-dip galvanized steel sheet consists of the following mass percentages of chemical elements: C: 0.17~0.21 wt%; Si: 1.2~1.7 wt%; Al: 0.02~0.05%; Mn: 1.60~2.1 wt%; N: ≤0.008 wt%; the remainder being Fe and unavoidable impurities.

[0013] In the above proposed technology, continuous annealing is employed in the annealing process, and a weakly oxidizing atmosphere with an annealing dew point of -10 to 0°C is used. This prevents internal oxidation from occurring on the subsurface of the steel sheet, thus preventing the accumulation of elements such as Si and Mn on the surface and suppressing the formation of Si / Mn oxide films on the surface. This solves the problem of deterioration of plating quality due to surface oxidation. Experiments have shown that when the annealing dew point is -10 to 0°C, the plating layer quality of hot-dip galvanized steel sheets is good. Setting the annealing temperature to a relatively high 840 to 870°C allows for the formation of a uniform austenite structure, which is advantageous for improving the strength of the steel. Slow cooling at a cooling rate of ≤10°C / s to a rapid cooling start temperature of 710 to 730°C forms localized ferrite, reducing the temperature difference during rapid cooling and improving the shape of the sheet. Rapid cooling at ≥50°C / s to a rapid cooling end temperature between 220 and 320°C partially converts the austenite into distributed martensite. Subsequently, the material is reheated to a temperature of 410-460°C and held for 20-100 seconds. During this time, carbon enters the austenite from the partitioned martensite, reducing the carbon content in the martensite and lowering its hardness. Simultaneously, the carbon content in the austenite increases, stabilizing it, and the ferrite recovers, increasing its hardness. Finally, zinc plating is performed to obtain a hot-dip galvanized product with high plating layer quality.

[0014] In the continuous annealing and galvanizing processes, carbon is redistributed between the distributed martensite and austenite, increasing the carbon content in the austenite and thus increasing stability, resulting in more metastable austenite and improved plasticity. More importantly, the carbon content in the distributed martensite decreases, effectively lowering the hardness of the distributed martensite unless tempering occurs. In the microstructure, ferrite recovers during the annealing and galvanizing processes, significantly reducing the high-density mobile dislocations in the ferrite caused by volume expansion during the phase transition of the distributed martensite, thus increasing the hardness of the ferrite. The decrease in hardness of the distributed martensite and the increase in hardness of the ferrite effectively reduce the interphase hardness difference between the distributed martensite and ferrite, improving the hole-expanding and flange-workability of the material. Conventional methods require lowering the hardness of the distributed martensite by generating tempered distributed martensite. That is, supersaturated carbon in the distributed martensite dissolves at the tempering temperature, forming carbides. In such methods, a large amount of carbides are generated, and since this carbon cannot contribute to the stabilization of retained austenite, the effective carbon content in the material decreases. In the manufacturing method of the present invention, not only is the hardness of the distribution-treated martensite reduced, but tempering of the distribution-treated martensite is also avoided, no carbides are generated, and the alloying elements in the material are fully utilized, resulting in a design that achieves high efficiency at low cost. Due to the high Si content, the metastable austenite in the steel does not decompose in the zinc plating process, and the desired microstructure is ultimately obtained.

[0015] The hot-dip galvanized steel sheet produced by the method for producing hot-dip galvanized steel sheets according to the present invention has a final microstructure consisting of ferrite, partitioned martensite, and metastable austenite. The dynamic phase transition of metastable austenite, combined with the soft phase ferrite and the hard phase partitioned martensite, provides the hot-dip galvanized steel sheet with the advantages of high strength and high plasticity. According to another embodiment of the present invention, the microstructure of the hot-dip galvanized steel sheet produced by the method for producing hot-dip galvanized steel sheets according to the present invention consists of ferrite, partitioned martensite, and metastable austenite. By volume ratio, the proportion of the ferrite phase is 30-50%, the proportion of the partitioned martensite phase is 40-60%, and the proportion of the metastable austenite phase is 10-20%.

[0016] According to another embodiment of the present invention, in the method for manufacturing a hot-dip galvanized steel sheet according to the present invention, the statistically stored dislocation (SSD) density of ferrite is 5.0 × 10 13 / m 2 ~1 × 10 14 / m 2 The hardness of the ferrite is 180-230 HV, the hardness of the partitioned martensite is 315-380 HV, preferably 320-380 HV, and the hardness ratio of the partitioned martensite to the ferrite is ≤ 1.8. In one embodiment, the hardness ratio of the partitioned martensite to the ferrite is 1.4-1.8.

[0017] In the above embodiment, the hot-dip galvanized steel sheet produced by the method for producing hot-dip galvanized steel sheets according to the present invention has high strength and high hole-expanding properties. In one embodiment of the present invention, the yield strength of the hot-dip galvanized steel sheet obtained by the method for producing hot-dip galvanized steel sheets according to the present invention is 400 to 600 MPa, the tensile strength is 730 to 900 MPa, preferably 780 to 900 MPa, the elongation is 25 to 35%, and the hole-expanding properties are 35 to 60%.

[0018] According to another embodiment of the present invention, in the method for manufacturing a hot-dip galvanized steel sheet according to the present invention, before hot rolling, the slab is heated and held at a temperature of 1230 to 1260 °C.

[0019] In one embodiment, it is slowly cooled at a slow cooling rate of 2 to 10 °C / s until the rapid cooling start temperature reaches 710 to 730 °C. In one embodiment, it is rapidly cooled at a rapid cooling rate of 50 to 100 °C / s until the rapid cooling end temperature reaches 220 to 320 °C.

[0020] Preferably, it is held in a high-temperature heating furnace. By doing so, C and N compounds are easily dissolved sufficiently, and the formation of spinel-based oxide scale that is difficult to remove is avoided.

[0021] According to another embodiment of the present invention, in step S1 of the method for manufacturing a hot-dip galvanized steel sheet according to the present invention, the rolling end temperature of hot rolling is 920 ± 30 °C.

[0022] Here, at a high rolling end temperature, the steel sheet before cooling tends to remain in a completely austenite state, and no phase transition occurs.

[0023] According to another embodiment of the present invention, in step S1 of the method for manufacturing a hot-dip galvanized steel sheet according to the present invention, at the time of coiling, the temperature is 450 to 550 °C, and at the time of cold rolling, the amount of cold rolling strain is 20 to 60%.

[0024] Here, at a low coiling temperature, the eutectoid reaction due to oxide scale is likely to decrease, and problems such as a decrease in pickling efficiency and deterioration of surface quality are prevented.

[0025] In the method for manufacturing a hot-dip galvanized steel sheet according to the present invention, based on a high Si and Mn content, the strength of the hot-dip galvanized steel sheet is increased in the steelmaking, hot rolling, cold rolling, continuous annealing, and galvanizing processes, and it has a good elongation rate. Also, a microstructure having appropriate ferrite and distributed martensite hardness is formed to enhance the hole expansion property. At the same time, the plating layer of the steel sheet is good, meeting the requirements of hot-dip galvanized ultra-high strength steel for automobiles.

[0026] The present invention further provides a hot-dip galvanized steel sheet manufactured by the method for manufacturing a hot-dip galvanized steel sheet according to the present invention, wherein the hot-dip galvanized steel sheet consists of the following mass percent of chemical elements: C: 0.17~0.21 wt%; Si: 1.2~1.7 wt%; Al: 0.02~0.05%; Mn: 1.60~2.1 wt%; N: ≤0.008 wt%; the remainder being Fe and unavoidable impurities. The microstructure of this hot-dip galvanized steel sheet consists of ferrite, partitioned martensite, and metastable austenite, with the proportion of the ferrite phase being 30~50%, the proportion of the partitioned martensite phase being 40~60%, and the proportion of the metastable austenite phase being 10~20%.

[0027] The hot-dip galvanized steel sheet of the present invention employs the above-mentioned range of chemical components, for the following reasons: C is the most fundamental strengthening element in steel and also a stabilizing element of austenite. A high C content in austenite tends to increase the proportion of metastable austenite and improve the performance of the material. However, a high C content worsens the weldability of the steel material. Therefore, in order to achieve the desired effect, the C content in this invention is set within the range of 0.17 to 0.21 wt%.

[0028] Si is an element that inhibits carbide formation. Its solubility in carbides is very low, so it can effectively suppress or delay carbide formation. In the hot-dip galvanizing process, it is easy to suppress the decomposition of austenite, and in the partitioning process, it is possible to form austenite with a high carbon content, which can be retained as metastable austenite until room temperature. However, high Si content deteriorates the plating properties of the material. Therefore, the Si content in this invention is set within the range of 1.2 to 1.7 wt%. Hot-dip galvanized steel sheets have improved plating properties during manufacturing, and the quality of the zinc plating is ensured.

[0029] Mn: It is an austenite stabilizing element. Due to the presence of Mn, the transformation temperature of the partitioned martensite decreases, and the content of metastable austenite increases. Also, Mn is a solid solution strengthening element, which is beneficial for improving the strength of the steel plate. However, if the Mn content is too high, the hardenability of the steel material is too high, which is disadvantageous for the precise control of the microstructure of the material. Also, it is similar to Si, and when there is a lot of Mn, the plating property of the steel plate deteriorates similarly. Therefore, the Si content of the present invention is within the range of 1.2 to 1.7 wt%. The hot-dip galvanized steel plate has improved plating property during manufacturing, ensuring the quality of the zinc plating.

[0030] Al: It is similar to Si, mainly enabling solid solution strengthening, suppressing the formation of carbides, and having the effect of improving the stability of metastable austenite. However, the strengthening effect of Al is weaker than that of Si. In the present invention, the Al content is 0.02 to 0.05%.

[0031] N: In order to reduce the adverse effect of N on the control of inclusions, during steelmaking, N should be made as low as possible, that is, ≤ 0.008 wt%.

[0032] The final microstructure of the hot-dip galvanized steel plate of the present invention consists of 30 - 50% ferrite, 40 - 60% partitioned martensite, and 10 - 20% metastable austenite. The hot-dip galvanized steel plate has the advantages of high strength and high plasticity.

[0033] According to another embodiment of the present invention, in the hot-dip galvanized steel plate according to the present invention, the SSD density of ferrite is 5.0×10 13 / m 2 ~1×10 14 / m 2 and the hardness of ferrite is 180 - 230 HV, the hardness of the partitioned martensite is 315 - 380 HV, preferably 320 - 380 HV, and the hardness ratio of the partitioned martensite to ferrite ≤ 1.8.

[0034] According to another embodiment of the present invention, the yield strength of the hot-dip galvanized steel sheet according to the present invention is 400 to 600 MPa, the tensile strength is 730 to 900 MPa, preferably 780 to 900 MPa, the elongation is 25 to 35%, and the hole expansion ratio is 35 to 60%. [Effects of the Invention]

[0035] The method for manufacturing hot-dip galvanized steel sheets according to the present invention has advantages such as a high Si and Mn content, good plating layer quality, good local formability, and high strength, and has good applicability in automotive safety structural components. For example, it is suitable for manufacturing vehicle structural components and safety components that have particularly complex shapes and require high standards for overall formability, local formability, and corrosion resistance, such as reinforced frames, energy absorption boxes, and A and B columns. [Brief explanation of the drawing]

[0036] [Figure 1] Figure 1 shows the results of processing a hot-dip galvanized steel sheet product manufactured by the hot-dip galvanized steel sheet manufacturing method of Example 1 of the present invention in the flange processing step. [Figure 2] Figure 2 shows the results of processing a hot-dip galvanized steel sheet product manufactured by the hot-dip galvanized steel sheet manufacturing method of Comparative Example 1 in the flange processing step. [Figure 3] Figure 3 shows the results of measuring the adhesive strength of the zinc layer in a hot-dip galvanized steel sheet product manufactured by the hot-dip galvanized steel sheet manufacturing method of Example 1 of the present invention. [Figure 4] Figure 4 shows the results of measuring the adhesive strength of the zinc layer in a hot-dip galvanized steel sheet product manufactured by the hot-dip galvanized steel sheet manufacturing method of Comparative Example 1. [Modes for carrying out the invention]

[0037] While a detailed explanation follows, it should be understood that in this specification and in the claims, all numerical values ​​indicating the amounts of components used are, in all cases, modified by the term "about," unless otherwise indicated in any example or otherwise. Therefore, unless otherwise taught, the numerical parameters described below in the specification and in the claims are approximations that vary depending on the desired properties to be obtained in this application. At the very least, without attempting to apply the principle of equivalents only to the claims, each numerical parameter should be interpreted with the usual rounding down applied, at least according to the disclosed number of significant figures.

[0038] The terms used herein are used solely for the purpose of describing specific embodiments and should not be interpreted restrictively. The singular forms “one” and “the foregoing” as used herein should be understood to include the plural form unless the context clearly indicates otherwise. Expressions such as “at least one of…” when preceding or following an element list refer to the entire element list, not to individual elements of the list.

[0039] Furthermore, the terms “including” or “consisting of” as used in this specification indicate the presence of the features, areas, wholes, styles, operations, elements, and / or components described herein, but do not preclude the presence or addition of one or more additional features, areas, wholes, styles, operations, elements, components, and / or sets thereof.

[0040] As used in this application, “approximately” or “about” means within the range of deviation from a specific value determined by a person skilled in the art, taking into account the stated value and, for example, the measured value discussed and the error associated with the measurement of a particular quantity (i.e., the limits of the measuring system). Unless otherwise taught, all parameter ranges disclosed include the endpoints and all values ​​between them.

[0041] In this description of the present invention, terms have the same meanings as those generally understood by those skilled in the art unless otherwise taught; however, in the event of any difference, the definitions provided in this invention shall prevail. Test methods are conventional methods unless otherwise taught. The raw materials and test materials used in this invention are readily available unless otherwise taught.

[0042] To further clarify the objectives, technical proposals, and advantages of the present invention, the method for manufacturing hot-dip galvanized steel sheets of the present invention will be further described in the following preferred examples 1 to 5, however, the present invention is not limited to these examples. Furthermore, the technical effects of the present invention will be explained by comparative examples 1 to 3, which differ from the method for manufacturing hot-dip galvanized steel sheets of the present invention.

[0043] Example 1 S1: Produced on a standard steel production line or a thin slab continuous casting and rolling production line, with slabs obtained after continuous casting. The slabs were heated and kept warm at 1250°C. Then hot rolling was performed to obtain steel plates of a certain thickness. The thickness was determined by the required thickness for the final product, and the rolling completion temperature was set to 920°C. Winding was performed at 500°C. Pickling and cold rolling were performed, with the cold rolling strain amount set to 40%.

[0044] S2: Continuous annealing was performed. The annealing temperature was controlled, and the annealing dew point was used during the annealing stage. The material was cooled to the rapid cooling start temperature at a cooling rate of ≤10°C / s, and then further cooled to the rapid cooling end temperature at a cooling rate of ≥50°C / s. After that, it was heated to the reheating temperature and held at that temperature for a certain period of time. Refer to Table 1 for specific parameters.

[0045] S3: The steel plates were placed in a zinc plating oven and the zinc plating process was completed. Finally, they were cooled to room temperature. The chemical elemental composition of hot-dip galvanized steel sheets shows the content of C, Si, Mn, Al, and N in Table 2, with the remainder being Fe and unavoidable impurities.

[0046] Example 2 S1: Produced using a standard steel production line or a thin slab continuous casting and rolling production line, with slabs obtained after continuous casting. The slabs were heated and kept warm at 1260°C. Then hot rolling was performed to obtain steel plates of a certain thickness. The thickness was the same as in Example 1, and the rolling completion temperature was set to 930°C. Winding was performed at 450°C. Pickling and cold rolling were performed, with the cold rolling strain amount set to 20%.

[0047] S2: Continuous annealing was performed. See Table 1 for specific parameters. S3: The steel plates were placed in a zinc plating oven and the zinc plating process was completed. Finally, they were cooled to room temperature.

[0048] The chemical elemental composition of hot-dip galvanized steel sheets shows the content of C, Si, Mn, Al, and N in Table 2, with the remainder being Fe and unavoidable impurities.

[0049] Example 3 S1: Produced on a standard steel production line or a thin slab continuous casting and rolling production line, with slabs obtained after continuous casting. The slabs were heated and kept warm at 1230°C. Then hot rolling was performed to obtain steel plates of a certain thickness. The thickness was the same as in Example 1, and the rolling completion temperature was set to 950°C. Winding was performed at 550°C. Pickling and cold rolling were performed, and the cold rolling strain was set to 60%.

[0050] S2: Continuous annealing was performed. See Table 1 for specific parameters. S3: The steel plates were placed in a zinc plating oven and the zinc plating process was completed. Finally, they were cooled to room temperature.

[0051] The chemical elemental composition of hot-dip galvanized steel sheets shows the content of C, Si, Mn, Al, and N in Table 2, with the remainder being Fe and unavoidable impurities.

[0052] Example 4 S1: Produced on a standard steel production line or a thin slab continuous casting and rolling production line, with slabs obtained after continuous casting. The slabs were heated and kept warm at 1240°C. Then hot rolling was performed to obtain steel plates of a certain thickness. The thickness was the same as in Example 1, and the rolling completion temperature was set to 890°C. Winding was performed at 470°C. Pickling and cold rolling were performed, with the cold rolling strain amount set to 50%.

[0053] S2: Continuous annealing was performed. See Table 1 for specific parameters. S3: The steel plates were placed in a zinc plating oven and the zinc plating process was completed. Finally, they were cooled to room temperature.

[0054] The chemical elemental composition of hot-dip galvanized steel sheets shows the content of C, Si, Mn, Al, and N in Table 2, with the remainder being Fe and unavoidable impurities.

[0055] Example 5 S1: Produced using a standard steel production line or a thin slab continuous casting and rolling production line, with slabs obtained after continuous casting. The slabs were heated and kept warm at 1250°C. Then hot rolling was performed to obtain steel plates of a certain thickness. The thickness was the same as in Example 1, and the rolling completion temperature was set to 900°C. Winding was performed at 520°C. Pickling and cold rolling were performed, with the cold rolling strain amount set to 30%.

[0056] S2: Continuous annealing was performed. See Table 1 for specific parameters. S3: The steel plates were placed in a zinc plating oven and the zinc plating process was completed. Finally, they were cooled to room temperature.

[0057] The chemical elemental composition of hot-dip galvanized steel sheets shows the content of C, Si, Mn, Al, and N in Table 2, with the remainder being Fe and unavoidable impurities.

[0058] Comparative Example 1 S1: Produced using a standard steel production line or a thin slab continuous casting and rolling production line, with slabs obtained after continuous casting. The slabs were hot-rolled to obtain steel plates. The rolling was completed at a temperature of 850°C. After winding at 400°C, pickling and cold rolling were performed.

[0059] S2: Annealing was performed. See Table 1 for specific parameters. S3: Zinc plating was completed. It was cooled to room temperature.

[0060] In terms of chemical elemental composition, the content is shown in Table 2, and the content of the chemical components is within the range of chemical content according to the manufacturing method of the present invention, with high Si and Mn content.

[0061] Comparative Example 2 S1: Produced using a standard steel production line or a thin slab continuous casting and rolling production line, with slabs obtained after continuous casting. The slabs were hot-rolled to obtain steel plates. The rolling was completed at a temperature of 850°C. After winding at 400°C, pickling and cold rolling were performed.

[0062] S2: Annealing was performed. See Table 1 for specific parameters. S3: Zinc plating was completed. It was cooled to room temperature.

[0063] In terms of chemical elemental composition, the content is shown in Table 2, and the content of the chemical components differs from the chemical content obtained by the manufacturing method of the present invention, with lower C and Mn content.

[0064] Comparative Example 3 S1: Produced using a standard steel production line or a thin slab continuous casting and rolling production line, with slabs obtained after continuous casting. The slabs were heated and kept warm at 1162-1189°C. The slabs were hot-rolled to obtain steel plates. The rolling was completed at a temperature of 862-882°C. After winding at 571-590°C, pickling and cold rolling were performed.

[0065] S2: Continuous annealing was performed. During this time, the annealing temperature was set to 835-847°C, the rapid cooling start temperature to 626-639°C, and the rapid cooling end temperature to 385-395°C. The reheating temperature was set to 310-346°C.

[0066] S3: Zinc plating was completed. It was allowed to cool naturally to room temperature. In terms of chemical elemental composition, the content is shown in Table 2. The content of the chemical components differs from that of the manufacturing method of the present invention, with low content of C, Si, Mn, and Al, but with the addition of Cr, Nb, and Ti.

[0067] The parameters for the annealing process in Examples 1-5 and Comparative Examples 1-3 are shown in Table 1, and the chemical component content in Examples 1-5 and Comparative Examples 1-3 is shown in Table 2.

[0068] [Table 1]

[0069] [Table 2]

[0070] Performance measurement: In this invention, performance measurements were performed on the hot-dip galvanized steel sheets obtained in Examples 1 to 5 and Comparative Examples 1 to 3. The performance measurements for Examples 1 to 5 included the proportion of each phase in the microstructure, dynamic performance (yield strength, tensile strength, elongation, hole expansion rate), statistically accumulated dislocation density, hardness of each phase in the microstructure, and adhesive strength of the zinc layer. The performance measurements for Comparative Examples 1 to 3 included dynamic performance, and further measurements were performed on the statistically accumulated dislocation density, hardness of each phase in the microstructure, and adhesive strength of the zinc layer compared to Comparative Example 1.

[0071] The method for measuring dynamic performance follows the American Society for Testing and Materials standard ASTM E8 / E8M-13 "Standard Test Methods For Tension Testing of Metallic Materials," and the elongation test uses 50 mm pitch elongation samples according to the ASTM standard, with the elongation direction perpendicular to the rolling direction.

[0072] For a method of measuring the statistically accumulated dislocation density, refer to Y. Zhong, F. Yin, T. Sakaguchi, K. Nagai, K. Yang, "Dislocation structure evolution and characterization in the compression deformed Mn-Cu alloy," Acta Materialia, Volume 55, Issue 8, 2007, Pages 2747-2756. Specifically, a 10×20mm sample is cut from a steel plate, and after surface polishing, the XRD (X-ray diffraction) spectrum is measured. The spectrum is then subjected to full pattern fitting and calculation using the MWAA (Modified Warren-Averbach Analysis) method to obtain the statistically accumulated dislocation density value in the sample.

[0073] The method for measuring the adhesive strength of the zinc layer includes the following: A 300 x 70 mm sheet sample is cut from a steel plate, and it is cold-bent on a bending machine to a bending diameter of 180° with a thickness three times the original thickness. After that, cellophane tape is applied to the outside of the bent angle after washing, and after removing the tape, it is checked whether there is any transfer of delamination material on the tape. If no delamination material is found, the adhesive strength of the zinc layer is judged to be acceptable (OK); otherwise, it is judged to be unacceptable (NG).

[0074] The proportion of each phase in the microstructure is measured using X-ray diffraction quantitative phase analysis. Method for measuring hardness: GB / T 4340.1-2012 Vickers hardness test for metallic materials, Part 1: Test method.

[0075] The measurement results are as follows. The measurement results for the proportion of the microstructure phase and dynamic performance of Examples 1-5 and Comparative Examples 1-3 are shown in Table 3, and the measurement results for the statistically accumulated dislocation density, microstructure hardness, and zinc layer adhesion strength of Examples 1-5 and Comparative Example 1 are shown in Table 4.

[0076] [Table 3]

[0077] [Table 4]

[0078] As can be seen from Table 3, in terms of volume ratio, the hot-dip galvanized steel sheets according to Examples 1 to 5 of the present invention have a ferrite phase of 30-50%, a distribution-treated martensite phase of 40-60%, and a metastable austenite phase of 10-20%. In terms of dynamic performance, the yield strength is 400-600 MPa, the tensile strength is 730-900 MPa, the elongation (ASTM 50 mm) is 25-35%, and the hole expansion ratio is 35-60%. The resulting product is a highly formable, ultra-high-strength hot-dip galvanized product with high hole expansion ratio and high tensile strength. In Comparative Example 1, due to the high content of Si and Mn, the resulting hot-dip galvanized product has high yield strength and tensile strength, and is a high-strength hot-dip galvanized product, but it has the problem of low elongation ratio and hole expansion ratio, and poor local formability. In Comparative Examples 2 and 3, the Si and Mn content is reduced, resulting in hot-dip galvanized products that have high elongation and hole expansion rates, but low yield strength and tensile strength, and are not high-strength hot-dip galvanized products. In other words, the hot-dip galvanized products manufactured according to the present invention have advantages such as high strength and excellent local formability.

[0079] Figure 1 shows the result of processing a hot-dip galvanized steel sheet product manufactured in Example 1 of the present invention during the flange processing process, and Figure 2 shows the result of processing a hot-dip galvanized steel sheet product manufactured in Comparative Example 1 during the flange processing process. As can be seen, cracks occur at the flange location in Comparative Example 1, while no cracks occur in Example 1. Therefore, it can be seen that the hot-dip galvanized steel sheet product manufactured in the present invention has significantly improved flange performance and a high level of overall formability, and cracks can be effectively avoided on similar parts.

[0080] As can be seen from Table 4, in the hot-dip galvanized steel sheets according to Examples 1 to 5 of the present invention, the dislocation density statistically accumulated in ferrite is 5.0 × 10⁻⁶.13 / m 2 ~1 × 10 14 / m 2 The hardness of ferrite is within the range of 180-230 HV, and the hardness of distributed martensite is within the range of 315-380 HV. The ratio of the hardness of distributed martensite to ferrite is ≤1.8. In Comparative Example 1, the statistically accumulated dislocation density of ferrite exceeds the range of the present invention, and the ratio of the hardness of distributed martensite to ferrite is >1.8. In the embodiment of the present invention, the plating properties are good and the quality of the plating layer is good, but in Comparative Example 1, the quality of the plating layer is poor. Figure 3 shows the measurement results of the adhesive strength of the zinc layer in Embodiment 1 of the present invention, and Figure 4 shows the measurement results of the adhesive strength of the zinc layer in Comparative Example 1. As shown, in the hot-dip galvanized steel sheet with high hole expansion ratio and ultra-high strength of the present invention, the quality of the plating layer and adhesive strength are significantly improved, and no peeling defects of the zinc layer occurred during measurement.

[0081] The above is a further detailed description of the present invention by specific embodiments, but this does not mean that the specific embodiments of the present invention are limited to these descriptions. A person ordinary of the art in which the present invention pertains can make simple inferences or substitutions without departing from the concept of the present invention, and these should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for manufacturing a hot-dip galvanized steel sheet, the method comprising the following steps: S1: A slab is hot-rolled to obtain a steel sheet, and after the steel sheet is wound up, it is pickled and cold-rolled; S2: Continuous annealing is performed by setting the annealing temperature to 840-870°C, the annealing dew point to -10-0°C, cooling to a rapid cooling start temperature of 710-730°C at a cooling rate of ≤10°C / s, further cooling to a rapid cooling end temperature of 220-320°C at a cooling rate of ≥50°C / s, heating to a reheating temperature of 410-460°C, and holding the temperature for 20-100 seconds; S3: Perform zinc plating; after the zinc plating is completed, cool to room temperature to obtain the hot-dip galvanized steel sheet; However, the hot-dip galvanized steel sheet consists of the following mass percentages of chemical elements: C: 0.17-0.21 wt%; Si: 1.2-1.7 wt%; Al: 0.02-0.05%; Mn: 1.60-2.1 wt%; N: ≤0.008 wt%; the remainder being Fe and unavoidable impurities; The microstructure of the aforementioned hot-dip galvanized steel sheet consists of ferrite, partitioned martensite, and metastable austenite, with the ferrite phase accounting for 30-50% by volume, the partitioned martensite phase for 40-60%, and the metastable austenite phase for 10-20%. The statistically accumulated dislocation density of the ferrite is 5.0 × 10⁻⁶. １３ / m ２ ~1 x 10 １４ / m ２ The hardness of the ferrite is 180 to 230 HV, the hardness of the distributed martensite is 315 to 380 HV, and the ratio of the hardness of the distributed martensite to the hardness of the ferrite is ≤ 1.

8. A method for manufacturing hot-dip galvanized steel sheets.

2. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein the hardness of the distributed martensite is 320 to 380 HV.

3. The method for manufacturing a hot-dip galvanized steel sheet according to claim 2, wherein the yield strength of the hot-dip galvanized steel sheet is 400 to 600 MPa, the tensile strength is 730 to 900 MPa, the elongation is 25 to 35%, and the hole expansion ratio is 35 to 60%.

4. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein in step S1, the slab is heated and kept warm at a temperature of 1230 to 1260°C before the hot rolling is performed on the slab.

5. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein in step S1, the rolling completion temperature of the hot rolling is 920 ± 30°C.

6. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein in step S1, the winding temperature is 450 to 550°C, and in the cold rolling, the amount of cold rolling strain is 20 to 60%.

7. In step S2, the material is cooled to a rapid cooling start temperature of 710 to 730°C at a cooling rate of 2 to 10°C / s, according to the method for manufacturing a hot-dip galvanized steel sheet as described in claim 1.

8. In step S2, the material is cooled from the rapid cooling start temperature to the rapid cooling end temperature of 220 to 320°C at a cooling rate of 50 to 100°C / s, as described in claim 1, for manufacturing a hot-dip galvanized steel sheet.

9. The method for manufacturing a hot-dip galvanized steel sheet according to claim 2, wherein the ratio of the hardness of the distributed martensite to the ferrite is 1.4 to 1.

8.

10. A hot-dip galvanized steel sheet, wherein the hot-dip galvanized steel sheet consists of the following mass percent of chemical elements: C: 0.17 to 0.21 wt%; Si: 1.2 to 1.7 wt%; Al: 0.02 to 0.05%; Mn: 1.60 to 2.1 wt%; N: ≤0.008 wt%; the remainder being Fe and unavoidable impurities; The microstructure of the aforementioned hot-dip galvanized steel sheet consists of ferrite, partitioned martensite, and metastable austenite, with the ferrite phase accounting for 30-50% by volume, the partitioned martensite phase for 40-60%, and the metastable austenite phase for 10-20%. The statistically accumulated dislocation density of the ferrite is 5.0 × 10⁻⁶. １３ / m ２ ~1 x 10 １４ / m ２ A hot-dip galvanized steel sheet wherein the hardness of the ferrite is 180 to 230 HV, the hardness of the distributed martensite is 315 to 380 HV, and the ratio of the hardness of the distributed martensite to the hardness of the ferrite is ≤ 1.

8.

11. The hot-dip galvanized steel sheet according to claim 10, wherein the hardness of the distributed martensite is 320 to 380 HV.

12. The hot-dip galvanized steel sheet according to claim 10, wherein the yield strength of the hot-dip galvanized steel sheet is 400 to 600 MPa, the tensile strength is 730 to 900 MPa, the elongation is 25 to 35%, and the hole expansion ratio is 35 to 60%.

13. The hot-dip galvanized steel sheet according to claim 11, wherein the ratio of the hardness of the distributed martensite to the ferrite is 1.4 to 1.8.

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