Method for manufacturing hot-dip galvanized steel sheets, steel sheets and automotive parts

Abstract

The present invention provides a method for manufacturing a hot-dip galvanized steel sheet comprising performing a heat treatment step, a hot-dip plating step, and an alloying treatment step on a steel sheet having high Si and Mn content, wherein the heat treatment step includes a first heating step and a soaking step, the first heating atmosphere of the first heating step comprises O2 in a volume fraction of 0.01 to 0.5% and the remainder is N2 and unavoidable impurities, the soaking atmosphere of the soaking step comprises H2 in a volume fraction of 0.5% or more and the remainder is N2 and unavoidable impurities, and the dew point of the first heating atmosphere and the soaking atmosphere is controlled to ≥-20℃. The present invention also provides a hot-dip galvanized steel sheet and a vehicle part. The present invention controls the heat treatment atmosphere to prevent Si and Mn alloy elements from concentrating on the steel plate surface, the interface between iron oxide and the base steel plate, and the interface between iron reduction and the base steel plate. Therefore, when a steel plate with high Si and Mn content is selected as the base material for a hot-dip galvanized steel plate, a sufficient plating layer Fe content and high fracture elongation can be obtained after undergoing the alloying treatment step.

Description

Technology Field

[0001] The present invention relates to a metal processing method and the product thereof, and in particular to a method for manufacturing a hot-dip galvanized steel sheet, a steel sheet, and a vehicle part. Background Technology

[0002] Recently, the demand for hot-dip galvanized steel sheets has significantly increased in fields such as automobiles, home appliances, and construction materials. The manufacturing method involves sequentially performing reduction annealing, hot-dip plating, and alloying treatments on the steel sheets. In the reduction annealing stage, individual oxidation points of the steel sheet are reduced to allow the sheet to form an austenite structure at high temperatures. Even after annealing is completed, partial austenite (i.e., residual austenite) is preserved to increase the elongation at break of the steel sheet. In the hot-dip plating stage, a pure zinc plating layer is formed on the steel sheet, and in the alloying treatment stage, the Zn in the plating layer and the Fe in the steel sheet diffuse into each other to ensure the plating layer obtains a sufficient Fe content, thereby forming a zinc-iron alloy plating layer. After undergoing alloying treatment, hot-dip galvanized steel sheets secure reliable and excellent corrosion resistance as a plating layer that does not peel off easily, leading to their widespread application in large quantities.

[0003] To improve the tensile strength of hot-dip galvanized steel sheets, a method using steel sheets with high Si and Mn content as the base material has been proposed. However, Si and Mn alloying elements become concentrated on the steel sheet surface in the form of continuous oxides during the reduction annealing process, hindering the diffusion of zinc and iron and thus affecting the alloying rate. For example, if the alloying temperature and time remain unchanged, the plating layer of the steel sheet after alloying will have an insufficient Fe content. For instance, if the alloying temperature is increased or the alloying time is extended to ensure the Fe content of the plating layer, the stability of the residual austenite may be compromised. Consequently, the proportion of the residual austenite phase in the microstructure of the hot-dip galvanized steel sheet decreases, leading to a reduction in the elongation at break of the hot-dip galvanized steel sheet.

[0004] Patent document CN101809182B provides a method for manufacturing a steel sheet with an annealed hot-dip galvanized layer, the method comprising the following steps: oxidizing the steel sheet to form an iron oxide layer and an internal oxide of one or more oxides selected from the group consisting of Si oxide, Mn oxide, Al oxide, a composite oxide containing Si and Mn, a composite oxide containing Si and Al, a composite oxide containing Al and Mn, and a composite oxide containing Si, Mn and Al on the surface of the steel sheet; reducing the oxidized steel sheet to reduce the iron oxide layer; performing hot-dip galvanizing on the reduced steel sheet to form a steel sheet coated with a zinc-based coating; and alloying the steel sheet coated with a zinc-based coating to form a steel sheet with an annealed galvanized layer. The plating layer of such a steel sheet has insufficient Fe content or the fracture elongation of the steel sheet is too low.

[0005] To summarize the above, in existing technology, when selecting a steel sheet with high Si and Mn content as the base material for hot-dip galvanized steel, there is a problem in that it is difficult to simultaneously obtain a sufficient Fe content in the plating layer and a high fracture elongation after alloying treatment.

[0006] The inventors have discovered that while the iron oxide layer formed by the conventional method has a certain effect in suppressing the concentration of Si and Mn alloy elements on the surface of the steel plate, this method causes a new problem in which the iron oxide layer and the reduced iron layer each form an interface with the base steel plate, and the internal oxide formed by Si and Mn alloy elements is concentrated at the interface between the iron oxide layer and the base steel plate and at the interface between the reduced iron layer and the base steel plate, and the diffusion range of zinc and iron during the alloying process includes not only the surface of the steel plate but also the interface between the reduced iron layer and the base steel plate, so the alloying rate is affected, resulting in insufficient Fe content in the plating layer of the steel plate or too low the fracture elongation of the steel plate.

[0007] Based on these problems, the present invention aims to provide a method for manufacturing a hot-dip galvanized steel sheet that not only prevents Si and Mn alloy elements from concentrating on the surface of the steel sheet, but also prevents Si and Mn alloy elements from concentrating at the interface between iron oxide and the base steel sheet, and at the interface between iron reduction and the base steel sheet, thereby enabling a sufficient plating layer Fe content and high fracture elongation after an alloying treatment step when a steel sheet with high Si and Mn content is selected as the base material for the hot-dip galvanized steel sheet.

[0008] In order to solve the aforementioned problem, the inventors conducted repeated research and discovered that the level of the dew point affects the degree of enrichment of Si and Mn alloy elements at the interface between iron oxide and the base steel sheet and at the interface between reduced iron and the base steel sheet, and that increasing the dew point can suppress such enrichment. Accordingly, the inventors have completed the present invention by presenting that, in the process of oxidizing the steel sheet with oxygen and then reducing it with hydrogen, by simultaneously controlling the dew point to ≥-20℃, not only can the enrichment on the surface of the steel sheet be effectively suppressed, but the enrichment at the interface between iron oxide and the base steel sheet and at the interface between reduced iron and the base steel sheet can also be effectively suppressed.

[0009] The present invention provides a method for manufacturing a hot-dip galvanized steel sheet comprising performing a heat treatment step, a hot-dip plating step, and an alloying treatment step on a base steel sheet containing 0.5% or more of Si and 0.5% or more of Mn by mass, wherein the heat treatment step includes a first heating step and a soaking step, wherein the first heating atmosphere of the first heating step contains O2 in a volume fraction of 0.01 to 0.5% and the remainder is N2 and unavoidable impurities, and the soaking atmosphere of the soaking step contains H2 in a volume fraction of 0.5% or more and the remainder is N2 and unavoidable impurities, and the dew point of the first heating atmosphere and the soaking atmosphere is controlled to ≥-20℃.

[0010] Optionally, the heat treatment step further includes a second heating step, the second heating atmosphere of the second heating step contains H2 in a volume fraction of 0.5% or more, the remainder being N2 and unavoidable impurities, and the dew point of the second heating atmosphere is controlled to ≥-20℃.

[0011] Optionally, the base steel sheet comprises 0.1 to 0.3% C, 0.5 to 3.0% Si, and 0.5 to 4.0% Mn by mass, with the remainder being Fe and unavoidable impurities. In some embodiments, the base steel sheet comprises 0.1 to 0.3% C, 0.5 to 1.8% Si, and 2.0 to 4.0% Mn by mass, with the remainder being Fe and unavoidable impurities.

[0012] Optionally, in the hot-dip galvanizing step, the temperature of the steel sheet when immersed in the zinc pot is 450 to 520°C, the temperature of the plating solution is 450 to 500°C, the plating solution contains 0.10 to 0.15% Al by mass, the remainder being Zn and unavoidable impurities, and the zinc plating adhesion amount per side of the steel sheet after completing the hot-dip galvanizing step is 30 to 90 g / m² 2 This is. In some implementation methods, the temperature of the steel plate during the zinc pot immersion stage is 450 to 500°C.

[0013] Optionally, the steel plate temperature of the alloying treatment step is controlled to ≤500℃, and the alloying treatment time is controlled to ≤25 seconds.

[0014] Optionally, the steel plate temperature of the alloying treatment step is controlled to ≥460℃, and the alloying treatment time is controlled to ≥5 seconds.

[0015] In some implementation methods, the temperature of the steel plate during the alloying treatment step is controlled to 460 to 500°C, and the alloying treatment time is controlled to 5 to 25 seconds.

[0016] Optionally, the outlet temperature of the first heating step is controlled to 570 to 750°C.

[0017] Optionally, the steel plate temperature during the soaking step is controlled to 750 to 930°C, and the soaking time is controlled to 30 to 300 seconds.

[0018] Optionally, the volume fraction of O2 in the first heating atmosphere is 0.03 to 0.3%,

[0019] Optionally, the volume fraction of H2 in the second heating atmosphere and soaking atmosphere is ≤10%.

[0020] Optionally, the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is controlled to ≤30℃.

[0021] Optionally, the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is controlled to ≥-10℃.

[0022] Optionally, the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is controlled to ≥0℃.

[0023] The second objective of the present invention is to provide a hot-dip galvanized steel sheet having a sufficient plating layer Fe content and a high fracture elongation after undergoing an alloying treatment step, using a steel sheet with high Si and Mn content as the base material.

[0024] To achieve the above-described objective, the present invention also provides a hot-dip galvanized steel sheet manufactured by the above-described manufacturing method.

[0025] Optionally, the steel sheet comprises a zinc-iron alloy plating layer and an internal oxide layer in order from the outside to the inside, the zinc-iron alloy plating layer contains 7 to 13% Fe by mass, the proportion of the remaining austenite phase of the structure is ≥5%, the tensile strength of the steel sheet is ≥980 MPa, and the elongation at break is ≥20%.

[0026] The third objective of the present invention is to provide a vehicle component that satisfies the performance requirements of the vehicle component.

[0027] To achieve the above-described objective, the present invention also provides a vehicle part made of the above-described hot-dip galvanized steel sheet.

[0028] The method for manufacturing a hot-dip galvanized steel sheet provided by the present invention controls the heat treatment atmosphere to suppress the concentration of Si and Mn alloy elements on the surface of the steel sheet, as well as the concentration of Si and Mn alloy elements at the interface between the iron oxide and the base steel sheet and at the interface between the reduced iron and the base steel sheet. Therefore, when a steel sheet with a high Si and Mn content is selected as the base material for the hot-dip galvanized steel sheet, it is advantageous to select an appropriate steel sheet temperature and time during the alloying treatment step to obtain a sufficient Fe content in the plating layer and a high fracture elongation. Brief explanation of the drawing

[0029] Figure 1 is a schematic diagram of the temperature control curve of the manufacturing method provided in the present invention. FIG. 2 is a schematic cross-sectional view of a steel plate after each step of the manufacturing method provided in the present invention. Figure 3 is a cross-sectional metal structure photograph of a hot-dip galvanized steel sheet obtained in Example 1. Figure 4 is a cross-sectional metal structure photograph of a hot-dip galvanized steel sheet obtained in Comparative Example 7. Specific details for implementing the invention

[0030] The specific embodiments below describe embodiments of the present invention, and those skilled in the art can easily understand other advantages and effects of the present invention through the contents disclosed herein. The description of the present invention will be introduced with reference to preferred embodiments, but this does not mean that the features of the present invention are limited to these embodiments. Conversely, the purpose of introducing the invention with reference to embodiments is to encompass other options or modifications that are extendable based on the claims of the present invention. To provide a deeper understanding of the present invention, the description below includes various specific details. The present invention may be implemented without these details. Furthermore, it should be noted that specific descriptions have been omitted where it is deemed that they might unnecessarily obscure the essence of the present invention.

[0031] The present invention provides a method for manufacturing a hot-dip galvanized steel sheet comprising a heat treatment step, a hot-dip plating step, and an alloying treatment step for a base steel sheet containing 0.5% or more of Si and 0.5% or more of Mn by mass. Heat treatment is a metal thermal processing process in which a material is heated, maintained at a temperature, and cooled in a solid state to obtain the expected structure and performance. Hot-dip plating is a process method in which a workpiece is immersed in a molten metal liquid to form a metal plating layer on its surface; in the present technical solution, it specifically refers to immersing a steel sheet in a zinc liquid to form a pure zinc plating layer on its surface. In the present technical solution, alloying treatment refers to maintaining the steel sheet at a specific temperature, wherein that temperature is the temperature of the steel sheet during the alloying treatment step and the temperature holding time is the time during the alloying treatment step, so that the iron element of the steel sheet and the zinc element of the pure zinc plating layer diffuse into each other, thereby causing the pure zinc plating layer to become a zinc-iron alloy plating layer. The heat treatment step includes a first heating step and a soaking step, the first heating step is a step of heating a steel plate from an ambient temperature to a specific temperature, the temperature being the outlet temperature of the first heating step, the heating atmosphere of the first heating step being called the first heating atmosphere, the soaking step is a step of maintaining the steel plate at a specific temperature, the temperature being the temperature of the steel plate in the soaking step, and the temperature holding time being the time of the soaking step. Here, the first heating atmosphere of the first heating step contains O2 in a volume fraction of 0.01 to 0.5%, the remainder being N2 and unavoidable impurities, and the soaking atmosphere of the soaking step contains H2 in a volume fraction of 0.5% or more, the remainder being N2 and unavoidable impurities, and the dew point of the first heating atmosphere and the soaking atmosphere is controlled to ≥-20℃.

[0032] The dew point is the temperature at which air reaches saturation when cooled while the water vapor content of the atmosphere remains unchanged and the air pressure is kept constant; in other words, it is the temperature at which water vapor and water reach a state of equilibrium.

[0033] After undergoing the first heating step, the O2 of the first heating atmosphere oxidizes the base steel sheet to form a layer of iron oxide on its surface, and since the internal oxide layer formed by the Si and Mn alloy elements essentially appears only below the iron oxide, the concentration of Si and Mn alloy elements on the steel sheet surface during the first heating step and the soaking step is suppressed. At the same time, the high dew point causes the internal oxide layer formed by the Si and Mn alloy elements to essentially appear only on the base steel sheet below the iron oxide and not at the interface between the iron oxide and the base steel sheet, thereby suppressing the concentration of Si and Mn alloy elements at the interface between the iron oxide and the base steel sheet during the first heating step, and the high dew point also has a certain promoting effect on suppressing the concentration on the steel sheet surface.

[0034] After the soaking step, iron oxide is reduced to reduced iron by H2 in the soaking atmosphere. During the soaking process, the internal oxide layer formed by Si and Mn alloy elements essentially appears only beneath the unreduced iron oxide and the reduced iron obtained by reducing the iron oxide; thus, it suppresses the concentration of Si and Mn alloy elements on the steel sheet surface during the soaking step, thereby maintaining the effect of suppressing concentration on the steel sheet surface during the first heating step and preparing it for subsequent hot-dip galvanizing. At the same time, the high dew point prevents the thickness of the internal oxide layer of the base steel sheet from increasing in the inward direction of the base steel sheet and not in the direction of the interface (where the interface includes the interface between the iron oxide and the base steel sheet and the interface between the reduced iron and the base steel sheet), that is, it suppresses concentration at the interface portion, thereby maintaining the effect of suppressing concentration at the interface between the iron oxide and the base steel sheet during the first heating step, and the high dew point also has a certain promoting effect on suppressing concentration on the steel sheet surface.

[0035] If the volume fraction of O2 is <0.01%, the degree of oxidation of the base steel sheet is too low, so it cannot sufficiently suppress the concentration on the surface of the steel sheet; if the volume fraction of O2 is >0.5%, the degree of oxidation of the base steel sheet is too high, which is disadvantageous for subsequent reduction and adhesion of the plating layer. Therefore, the volume fraction of O2 in the first heating atmosphere is limited to 0.01 to 0.5%.

[0036] If the volume fraction of H2 is <0.5%, the reduction effect is insufficient, which is disadvantageous for subsequent molten plating. Therefore, the volume fraction of H2 in the soaking atmosphere is limited to ≥0.5%, and since the reduction effect is superior when the volume fraction of H2 is ≥2%, it is desirable for the volume fraction of H2 in the soaking atmosphere to be ≥2%.

[0037] If the dew point of the atmosphere is <-20℃, the concentration inhibition effect at the interface between iron oxide and the base steel sheet, and between iron reduction and the base steel sheet, is almost non-existent during the first heating step and the soaking step, so the dew point of the first heating atmosphere and the soaking atmosphere is limited to ≥-20℃.

[0038] Optionally, as illustrated in FIG. 1, the heat treatment step further includes a second heating step, the second heating step is a step of heating the steel plate from the outlet temperature of the first heating step to the temperature of the steel plate of the soaking step, the heating atmosphere of the second heating step is called the second heating atmosphere, the second heating atmosphere of the second heating step contains H2 with a volume fraction of 0.5% or more, the remainder being N2 and unavoidable impurities, and the dew point of the second heating atmosphere is controlled to ≥-20℃.

[0039] A method for manufacturing a hot-dip galvanized steel sheet further comprises a first cooling step and a second cooling step, wherein the first cooling step cools the steel sheet from the steel sheet temperature of the soaking step to the steel sheet temperature when immersed in a zinc pot, and the second cooling step cools the steel sheet from the steel sheet temperature of the alloying treatment step to an ambient temperature.

[0040] As shown in FIG. 2, after undergoing the first heating step, the cross-section of the steel plate changes from that shown in FIG. 2(a) to that shown in FIG. 2(b), and the O2 of the first heating atmosphere oxidizes the base steel plate (1) to form a layer of iron oxide (3) on its surface, so that the internal oxide layer (2) formed by the Si and Mn alloy elements essentially appears only below the iron oxide (3), thereby suppressing the concentration of Si and Mn alloy elements on the surface of the steel plate during the first heating step, the second heating step, and the soaking step. At the same time, the high dew point causes the internal oxide layer (2) formed by the Si and Mn alloy elements to essentially appear only on the base steel plate (1') below the iron oxide (3) and not on the interface between the iron oxide (3) and the base steel plate (1'), thereby suppressing the concentration of the Si and Mn alloy elements in the interface portion between the iron oxide (3) and the base steel plate (1') during the first heating step, and the high dew point also has a certain promoting effect on suppressing the concentration on the steel plate surface.

[0041] After undergoing the second heating and soaking steps, the cross-section of the steel plate changes from that shown in FIG. 2(b) to that shown in FIG. 2(c), and the iron oxide (3) is reduced to reduced iron (4) by the H2 of the second heating and soaking atmosphere. During the second heating and soaking steps, the internal oxide layer (2) formed by the Si and Mn alloy elements appears only below the iron oxide (3) that is not essentially reduced and the reduced iron (4) obtained by reducing the iron oxide (3), thereby suppressing the concentration of the Si and Mn alloy elements on the surface of the steel plate during the second heating and soaking steps, maintaining the effect of suppressing concentration on the surface of the steel plate during the first heating step, and preparing for the subsequent molten plating. At the same time, the high dew point prevents the thickness of the internal oxide layer (2) in the base steel plate (1') from increasing in the internal direction of the base steel plate (1') and from increasing in the direction of the interface (wherein the interface includes the interface between the iron oxide (3) and the base steel plate (1'), and the interface between the reduced iron (4) and the base steel plate (1'), that is, by suppressing the concentration at the interface portion, the effect of suppressing the concentration at the interface between the iron oxide (3) and the base steel plate (1') is maintained in the first heating step, and the high dew point also has a certain promoting effect on suppressing the concentration on the steel plate surface.

[0042] If the volume fraction of O2 is <0.01%, the degree of oxidation of the base steel plate (1) is too low, so it cannot sufficiently suppress the concentration of the steel plate surface; if the volume fraction of O2 is >0.5%, the degree of oxidation of the base steel plate (1) is too high, which is disadvantageous for subsequent reduction and adhesion of the plating layer. Therefore, the volume fraction of O2 in the first heating atmosphere is limited to 0.01 to 0.5%.

[0043] If the volume fraction of H2 is <0.5%, the reduction effect is insufficient and is disadvantageous for subsequent molten plating. Therefore, the volume fraction of H2 in the second heating atmosphere and the soaking atmosphere is limited to ≥0.5%, and since the reduction effect is superior when the volume fraction of H2 is ≥2%, it is preferable that the volume fraction of H2 in the second heating atmosphere and the soaking atmosphere be ≥2%.

[0044] If the dew point of the atmosphere is <-20℃, the concentration inhibition effect at the interface between iron oxide (3) and base steel plate (1') and reduced iron (4) and base steel plate (1') is almost non-existent during the first heating step, second heating step, and soaking step, so the dew point of the first heating atmosphere, second heating atmosphere, and soaking atmosphere is limited to ≥-20℃.

[0045] Optionally, the base steel sheet (1) contains 0.1 to 0.3% C, 0.5 to 3.0% Si, and 0.5 to 4.0% Mn by mass, with the remainder being Fe and unavoidable impurities.

[0046] If the mass fraction of C in the base steel plate (1) is <0.1%, the tensile strength of the steel plate is insufficient, and if the mass fraction of C in the base steel plate (1) is >0.3%, the welding performance of the steel plate is deteriorated, so the mass fraction of C is limited to 0.1 to 0.3%. If the mass fraction of Si in the base steel plate (1) is <0.5%, the tensile strength and fracture elongation of the steel plate are insufficient, and if the mass fraction of Si in the base steel plate (1) is >3.0%, the high-temperature plasticity of the steel plate is insufficient and the defect occurrence rate during processing increases, so the mass fraction of Si is limited to 0.5 to 3.0%. In some implementation methods, the mass fraction of Si in the base steel plate is 0.5 to 1.8%. If the mass fraction of Mn in the base steel plate (1) is <0.5%, the tensile strength and fracture elongation of the steel plate are insufficient, and if the mass fraction of Mn in the base steel plate (1) is >4.0%, the hardenability of the steel plate is too high and is disadvantageous for fine control of the structure, so the mass fraction of Mn is limited to 0.5 to 4.0%. In some implementation methods, the mass fraction of Mn in the base steel plate is 2.0 to 4.0%.

[0047] Optionally, in the hot-dip galvanizing step, the temperature of the steel sheet when immersed in the zinc pot is 450 to 520°C, the temperature of the plating solution is 450 to 500°C, the plating solution contains 0.10 to 0.15% Al by mass, the remainder being Zn and unavoidable impurities, and the zinc plating adhesion amount per side of the steel sheet after completing the hot-dip galvanizing step is 30 to 90 g / m² 2 am.

[0048] After undergoing molten plating, the cross-section of the steel plate changes from that shown in FIG. 2(c) to that shown in FIG. 2(d), and a pure zinc plating layer (5) is formed on the steel plate. Since the temperature of the steel plate and the temperature of the plating solution when immersed in the zinc pot affect the reaction between the steel plate and the plating solution, if the temperature of the steel plate and the temperature of the plating solution when immersed in the zinc pot are too low, the reaction rate is slow and efficiency is reduced, and if the temperature of the steel plate and the temperature of the plating solution when immersed in the zinc pot are too high, the reaction rate is too fast and a "burst structure" is formed, which is disadvantageous for controlling the Fe content of the plating layer in the subsequent alloying process. Accordingly, in hot-dip plating, the temperature of the steel plate when immersed in the zinc pot is limited to 450 to 520°C, and the temperature of the plating solution is limited to 450 to 500°C. Since the reaction rate is appropriate when the temperature of the steel plate when immersed in the zinc pot is 470 to 500°C and the temperature of the plating solution is 460 to 480°C, it is preferable that the temperature of the steel plate when immersed in the zinc pot be 470 to 500°C and the temperature of the plating solution be 460 to 480°C. The plating solution contains 0.10 to 0.15% Al by mass, preferably 0.10 to 0.12% Al. The zinc plating adhesion amount per side of the steel plate after completing the hot-dip plating step is <30 g / m² 2 On this side, corrosion resistance is poor, and the zinc plating thickness on each surface is >90 g / m² 2 On the other hand, the zinc-iron alloy plating layer (6) has low formability and easily pulverizes and falls off, affecting the alloying process. Since this is disadvantageous for the steel plate to obtain a sufficient plating layer Fe content and high fracture elongation, the zinc plating adhesion amount per side of the steel plate after completing the hot-dip plating step is 30 to 90 g / m² 2 It is limited to.

[0049] Optionally, the temperature of the steel plate during the alloying treatment step is controlled to ≤500℃ and the time to ≤25 seconds.

[0050] After undergoing the alloying treatment, the cross-section of the steel plate changes from the state shown in FIG. 2(d) to the state shown in FIG. 2(e), and during the alloying treatment process, Zn of the pure zinc plating layer (5) and Fe of the base steel plate (1') diffuse into each other to form a zinc-iron alloy plating layer (6) on the steel plate. In the alloying treatment step, the temperature and time of the steel plate both affect the stability of the residual austenite and the Fe content of the plating layer. If the temperature of the steel plate is >500℃ or the time is >25 seconds, the stability of the residual austenite of the steel plate decreases, and the proportion of the residual austenite phase decreases, which lowers the fracture elongation of the steel plate. Also, the degree of diffusion of zinc and iron is too high, which can worsen the powdering prevention performance of the zinc-iron alloy plating layer (6). Therefore, the temperature of the steel plate in the alloying treatment step is limited to ≤500℃ and the time to ≤25 seconds. If the temperature of the steel plate is ≤480℃ and the time is ≤20 seconds, the proportion of the residual austenite phase of the steel plate increases and the fracture elongation of the steel plate also increases. Therefore, it is preferable that the temperature of the steel plate in the alloying treatment step be ≤480℃ and the time be ≤20 seconds.

[0051] Optionally, the temperature of the steel plate during the alloying treatment step is controlled to ≥460℃, and the time to ≥5 seconds.

[0052] Both the temperature and time of the steel sheet during the alloying treatment step affect the Fe content of the plating layer, and if the temperature is <460℃ or the time is <5 seconds, the diffusion of zinc and iron is insufficient and the Fe content of the plating layer is insufficient, so the temperature of the steel sheet during the alloying treatment step is limited to ≥460℃ and the time to ≥5 seconds.

[0053] Optionally, the outlet temperature of the first heating step is controlled to 570 to 750°C.

[0054] The outlet temperature affects the degree of oxidation of the base steel plate (1). If the temperature is <570°C, the degree of oxidation of the base steel plate (1) is too low to sufficiently suppress the concentration of the steel plate surface. If the temperature is >750°C, the degree of oxidation of the base steel plate (1) is too high, which is disadvantageous for subsequent reduction and adhesion of the plating layer. Therefore, the outlet temperature of the first heating step is limited to 570 to 750°C. When the outlet temperature is 600 to 720°C, the degree of oxidation is appropriate, so the outlet temperature of the first heating step is preferably 600 to 720°C.

[0055] Optionally, the temperature of the steel plate during the soaking step is controlled to 750 to 930°C, and the time is controlled to 30 to 300 seconds.

[0056] The temperature of the steel sheet during the soaking stage affects the formation of austenite. If the temperature of the steel sheet is <750°C, the austenitization of the steel sheet is insufficient, and if the temperature of the steel sheet is >930°C, coarse austenite grains are formed in the steel sheet, which is detrimental to the performance of the steel sheet; therefore, the temperature of the steel sheet during the soaking stage is limited to 750 to 930°C. If the time of the soaking stage is <30 seconds, the austenitization of the steel sheet is insufficient, and if the time of the soaking stage is >300 seconds, it affects production efficiency; therefore, the time of the soaking stage is limited to 30 to 300 seconds.

[0057] Optionally, the volume fraction of O2 in the first heating atmosphere is 0.03 to 0.3%.

[0058] If the volume fraction of O2 in the first heating atmosphere is 0.03 to 0.3%, the degree of oxidation of the base steel plate (1) is more suitable, so the volume fraction of O2 in the first heating atmosphere is preferably 0.03 to 0.3%.

[0059] Optionally, the volume fraction of H2 in the second heating atmosphere and soaking atmosphere is ≤10%.

[0060] After the volume fraction of H2 in the second heating atmosphere and soaking atmosphere is >10%, the reduction action on iron oxide (3) is no longer increased, so the volume fraction of H2 in the second heating atmosphere and soaking atmosphere is limited to ≤10% for economic reasons.

[0061] In some implementations, the volume fraction of H2 in the second heating atmosphere and the soaking atmosphere is 0.5 to 10%, preferably 2 to 10%.

[0062] Optionally, the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is controlled to ≤30℃.

[0063] After the dew point is >30℃, the concentration inhibition effect on the interface portion between iron oxide (3) and the base steel plate (1') and the interface portion between iron reduction (4) and the base steel plate (1') no longer increases, so the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is limited to ≤30℃.

[0064] Optionally, the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is controlled to ≥-10℃.

[0065] If the dew point of the atmosphere is ≥-10℃, the concentration inhibition effect on the interface between iron oxide (3) and base steel plate (1') and iron reduction (4) and base steel plate (1') is distinct, so the dew point of the first heating atmosphere, second heating atmosphere, and soaking atmosphere is preferably ≥-10℃.

[0066] Optionally, the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is controlled to ≥0℃.

[0067] If the dew point of the atmosphere is ≥0℃, the concentration inhibition effect on the interface between the iron oxide (3) and the base steel plate (1'), and between the iron reduction (4) and the base steel plate (1') is more pronounced, so it is more preferable that the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere be ≥0℃.

[0068] In some embodiments, the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is -20°C to 30°C. In some embodiments, the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is -10°C to 30°C. In some embodiments, the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is 0°C to 30°C.

[0069] The present invention also provides a hot-dip galvanized steel sheet manufactured by the manufacturing method described above.

[0070] Optionally, the steel plate comprises a zinc-iron alloy plating layer (6) and an internal oxide layer (2) in order from the outside to the inside, the zinc-iron alloy plating layer (6) contains 7 to 13% Fe by mass, the proportion of the remaining austenite phase of the structure is ≥5%, the tensile strength of the steel plate is ≥980 MPa, and the elongation at break is ≥20%.

[0071] The present invention also provides a vehicle part made of the aforementioned hot-dip galvanized steel sheet.

[0072] Examples

[0073] Examples 1 to 11 and Comparative Examples 1 to 6

[0074] A base steel sheet having the composition shown in Table 1 was selected, and hot-dip galvanized steel sheets of Examples 1 to 11 and Comparative Examples 1 to 6 were manufactured using the following manufacturing method.

[0075] (1) First heating step: The base steel plate was heated from ambient temperature to the outlet temperature, the atmosphere was O2 atmosphere, the remainder was N2 and unavoidable impurities, and the dew point was controlled.

[0076] (2) Second heating stage: The steel plate was heated from the outlet temperature to the temperature of the steel plate in the soaking stage, the atmosphere was H2 atmosphere, the remainder was N2 and unavoidable impurities, and the dew point was controlled.

[0077] (3) Soaking stage: The temperature of the steel plate was maintained, the atmosphere was H2, the remainder was N2 and unavoidable impurities, and the dew point was controlled.

[0078] (4) First cooling step: The steel plate was cooled from the temperature of the steel plate at the soaking step to the temperature of the steel plate when immersed in the zinc pot, and the cooling temperature was ≥10℃ / s.

[0079] (5) Hot-dip galvanizing: A steel sheet is immersed in a plating solution, the plating solution is maintained at the temperature of the plating solution, contains Al, and the remainder is Zn and unavoidable impurities, and the amount of zinc plating applied per side of the steel sheet after hot-dip galvanizing is 30 to 90 g / m² 2 am.

[0080] (6) Alloying treatment: The steel plate after molten plating was controlled to the steel plate temperature of the alloying treatment step and maintained to form a zinc-iron alloy plating layer.

[0081] (7) Second cooling step: The steel plate was cooled from the temperature of the steel plate in the alloying treatment step to ambient temperature.

[0082] Comparative Example 7

[0083] A base steel sheet having the same composition as in Table 1 was selected, and the hot-dip galvanized steel sheet of Comparative Example 7 was manufactured using the following manufacturing method.

[0084] (1) First heating step and second heating step: The base steel plate was heated from the ambient temperature to the steel plate temperature of the soaking step, the atmosphere was H2 atmosphere, the remainder was N2 and unavoidable impurities, and the dew point was controlled.

[0085] (2) to (6) are identical to (3) to (7) of the manufacturing methods of Examples 1 to 11 and Comparative Examples 1 to 6.

[0086] Performance test method:

[0087] Performance tests were performed on hot-dip galvanized steel sheets in accordance with the test methods described below, and the test results for each item are shown in Table 1:

[0088] (1) Tensile strength (MPa)

[0089] Tests were conducted using a general-purpose tensile testing machine according to the method described in GB / T 228.1-2010.

[0090] (2) Elongation at break (%)

[0091] Tests were conducted using a general-purpose tensile testing machine according to the method described in GB / T 228.1-2010.

[0092] (3) Fe content of plating layer (%)

[0093] A 50 mm diameter plate obtained by punching a steel plate was used as a test sample, and the plating layer was dissolved using a 10% hydrochloric acid solution by volume containing hexamethylenetetramine corrosion inhibitor, and the Fe content was measured by the ICP-AES method.

[0094] (4) Proportion of austenite phase (%)

[0095] After preparing the cross-sectional metallography, differentiation and quantitative analysis of each phase were performed using a scanning electron microscope backscatter diffraction (EBSD) to calculate the proportion of the remaining austenite phase.

[0096] Method of photographing metal tissue:

[0097] The metal structure of a hot-dip galvanized steel sheet was photographed according to the photographing method described below, and the photographs are shown in Figures 3 and 4.

[0098] After preparing the cross-sectional metal structure, it was photographed using a metal structure microscope.

[0099] Table 1: Base material composition, manufacturing method, and performance of hot-dip galvanized steel sheets

[0100]

[0101] Note: The chemical composition of the base steel sheet is all by mass fraction, and the remainder is Fe and unavoidable impurities; the gas content is all by volume fraction; the Al content of the plating solution is by mass fraction; and the Fe content of the plating layer is by mass fraction.

[0102] The results are as shown in Table 1. As can be seen from Examples 1 to 11, the tensile strength of the hot-dip galvanized steel sheets obtained by the manufacturing method of the present invention is ≥980 MPa, the Fe content of the plating layer is within the range of 7 to 13% by mass, and the elongation at break is ≥20%. By using steel sheets with high Si and Mn content as the base material, high tensile strength was obtained, along with sufficient Fe content in the plating layer and high elongation at break. Figure 3 is a cross-sectional metallographic image of the hot-dip galvanized steel sheet obtained in Example 1, which shows that the alloying of the zinc-iron alloy plating layer is sufficient and the internal oxide layer formed by Si and Mn alloying elements is relatively thick, and explains that the degree of enrichment at the steel sheet surface, the interface between the iron oxide and the base steel sheet, and the interface between the reduced iron and the base steel sheet is low.

[0103] In the case of Comparative Example 1, the O2 content in the first heating step is insufficient, so the degree of oxidation of the base steel sheet is insufficient, and the effect of iron oxide on Si and Mn alloying elements on the steel sheet surface to inhibit enrichment is weakened, and since Si and Mn alloying elements form continuous oxides on the steel sheet surface and affect the alloying rate, the alloying temperature is raised to 550℃ and the alloying time is extended to 30s, so the austenite content is reduced and the fracture elongation of the steel sheet is only 17%.

[0104] In the case of Comparative Example 2, the dew point of the first heating step is too low at -30°C, so the dew point concentration inhibition effect for Si and Mn alloying elements is reduced, and in particular, the concentration inhibition effect for the interface between iron oxide and the base steel sheet is reduced, so Si and Mn alloying elements form a continuous oxide at the interface and affect the alloying rate, so the alloying temperature is raised to 530°C, the austenite content is reduced, the fracture elongation of the steel sheet is 14.7%, and the iron content of the plating layer is still only 3.7%.

[0105] In the case of Comparative Example 3, the exit temperature of the first heating step is too low at 550°C, so the degree of oxidation of the base steel sheet is insufficient, and the effect of inhibiting the enrichment of iron oxide on Si and Mn alloy elements on the surface of the steel sheet is weakened, and Si and Mn alloy elements form continuous oxides on the surface of the steel sheet, affecting the alloying rate, so the iron content of the plating layer is only 5.2%.

[0106] In the case of Comparative Example 4, the O2 content in the first heating step is too high, so the degree of oxidation of the base steel sheet is too high, the subsequent reduction is not thorough, and the adhesion of the plating layer is weak, and the temperature of the plating solution in the molten plating step is too low at 360℃, so the reaction between the plating solution and the steel sheet is not excellent, and the alloying temperature is raised to 510℃, so the austenite content is reduced, the fracture elongation of the steel sheet is 18.8%, and the iron content of the plating layer is still only 2.5%.

[0107] In the case of Comparative Example 5, the dew point of the second heating step is too low at -25°C, so the dew point concentration inhibition effect for Si and Mn alloy elements is reduced, and in particular, the concentration inhibition effect for the interface (where the interface includes the interface between iron oxide and the base steel sheet and the interface between iron reduction and the base steel sheet) is reduced, so Si and Mn alloy elements form continuous oxides at the interface and affect the alloying rate, so the iron content of the plating layer is only 3.4%.

[0108] In the case of Comparative Example 6, the dew point of the soaking step is too low at -25℃, so the dew point concentration inhibition effect for Si and Mn alloy elements is reduced, and in particular, the concentration inhibition effect for the interface (where the interface includes the interface between iron oxide and the base steel sheet and the interface between iron reduction and the base steel sheet) is reduced, so Si and Mn alloy elements form continuous oxides at the interface and affect the alloying rate, so the iron content of the plating layer is only 2.8%.

[0109] In the case of Comparative Example 7, since the first heating step is in an H2 atmosphere and not an O2 atmosphere, an iron oxide layer is not formed, and since there is no interface between the base steel sheet and iron oxide or reduced iron, the enrichment of Si and Mn alloying elements is not suppressed and affects the alloying rate. Therefore, the alloying temperature is raised to 550°C, the austenite content is reduced, the fracture elongation of the steel sheet is 16%, and the iron content of the plating layer is still only 2.3%. Figure 4 is a cross-sectional metallographic image of the hot-dip galvanized steel sheet obtained in Comparative Example 7. It can be seen that the alloying of the zinc-iron alloy plating layer is insufficient and the internal oxide layer formed by Si and Mn alloying elements is relatively thin, which explains that the degree of enrichment on the surface of the steel sheet is severe.

[0110] In summary, the method for manufacturing a hot-dip galvanized steel sheet provided by the present invention controls the atmosphere during the heating and soaking steps to suppress the concentration of Si and Mn alloy elements on the surface of the hot-dip galvanized steel sheet, as well as the concentration of Si and Mn alloy elements at the interface between iron oxide and the base steel sheet, and at the interface between reduced iron and the base steel sheet. Consequently, when a steel sheet with high Si and Mn content is selected as the base material for the hot-dip galvanized steel sheet, an appropriate steel sheet temperature and time can be selected during the alloying treatment step. This is advantageous for obtaining a sufficient Fe content in the plating layer and a high fracture elongation, making it suitable for use in automotive parts.

[0111] The above-described embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Anyone with ordinary knowledge familiar with the art may modify or vary the above-described embodiments without departing from the spirit and scope of the present invention. Accordingly, all equivalent modifications and variations made by persons with ordinary knowledge in the art within the scope of the spirit and technical concept disclosed in the present invention shall be included within the claims of the present invention. Explanation of the symbols

[0112] (a): No steps are taken (b): After the first heating step (c): After the second heating step and soaking step (d): After the molten plating step (e) After the alloying treatment step 1: Base steel plate 1': Base steel plate 2: Internal oxide layer 3: Iron oxide 4: Reduced iron 5: Pure zinc plating layer 6: Zinc-iron alloy plating layer

Claims

Claim 1 A method for manufacturing a hot-dip galvanized steel sheet comprising sequentially performing a heat treatment step, a hot-dip plating step, and an alloying treatment step on a base steel sheet comprising, by mass, 0.1 to 0.3% C, 0.5 to 3.0% Si, 0.5 to 4.0% Mn, and the remainder (balance) of Fe and unavoidable impurities, wherein the heat treatment step sequentially comprises a first heating step, a second heating step, and a soaking step, wherein the first heating atmosphere of the first heating step comprises O2 in a volume fraction of 0.01 to 0.5% and the remainder is N2 and unavoidable impurities, and the outlet temperature of the first heating step is controlled to 570 to 750°C, the second heating atmosphere of the second heating step comprises H2 in a volume fraction of 0.5 to 10% and the remainder is N2 and unavoidable impurities, and the soaking atmosphere of the soaking step comprises H2 in a volume fraction of 0.5 to 10% and the remainder is A manufacturing method characterized by N2 and unavoidable impurities, wherein the dew points of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere are controlled to ≥-20℃ and ≤30℃. Claim 2 A manufacturing method according to claim 1, characterized in that the first heating atmosphere of the first heating step comprises 0.03 to 0.3% by volume fraction of O2, and the remainder is N2 and unavoidable impurities. Claim 3 In claim 1, in the molten plating step, the temperature of the steel plate when immersed in the zinc pot is 450 to 520°C, the temperature of the plating solution is 450 to 500°C, the plating solution contains 0.10 to 0.15% Al by mass, the remainder being Zn and unavoidable impurities, and the zinc plating adhesion amount per side of the steel plate after completing the molten plating step is 30 to 90 g / m² 2 A manufacturing method characterized by being. Claim 4 A manufacturing method according to claim 1, characterized in that the temperature of the steel plate in the alloying treatment step is controlled to 460 to 500℃ and the alloying treatment time is controlled to 5 to 25 seconds. Claim 5 A manufacturing method according to claim 1, characterized in that the steel plate temperature of the soaking step is controlled to 750 to 930℃ and the time is controlled to 30 to 300 seconds. Claim 6 A manufacturing method according to claim 1, characterized in that the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is controlled to ≥-10℃. Claim 7 A manufacturing method according to claim 1, characterized in that the dew point of the first heating atmosphere, the second heating atmosphere, and the soaking atmosphere is controlled to ≥0℃. Claim 8 A hot-dip galvanized steel sheet, manufactured by a manufacturing method according to any one of claims 1 to 7, wherein the steel sheet comprises a zinc-iron alloy plating layer and an internal oxide layer in order from the outside to the inside, and the zinc-iron alloy plating layer comprises 7 to 13% Fe by mass. Claim 9 A hot-dip galvanized steel sheet according to claim 8, characterized in that the tensile strength of the steel sheet is ≥980 MPa and the elongation at break is ≥20%. Claim 10 As a vehicle part, a vehicle part made of hot-dip galvanized steel sheet according to paragraph 8. Claim 11 delete Claim 12 delete Claim 13 delete Claim 14 delete Claim 15 delete

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