Method for manufacturing hot-dip galvanized steel sheets and method for manufacturing alloyed hot-dip galvanized steel sheets

Abstract

To provide a method for manufacturing a hot-dip galvanized steel sheet which has a high Si content and has good plating adhesion.SOLUTION: This method for manufacturing a hot-dip galvanized steel sheet includes the steps of: hot rolling a steel raw material containing 1.0 mass% or more of Si and then winding the same at 500°C-700°C; subjecting the surface of the steel sheet after the winding to an oxidation treatment at a heating temperature under 750°C in terms of steel sheet temperature and then subjecting this surface to a reduction treatment; and subjecting the steel sheet after the reduction treatment to a hot-dip galvanizing treatment to form a galvanized layer on the surface of the steel sheet.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet with high Si content, high strength and high workability, and a method for manufacturing an alloyed hot-dip galvanized steel sheet.

Background Art

[0002] In the automotive industry, from the viewpoints of improving fuel efficiency for CO2 reduction and improving collision safety performance, weight reduction and high strength of automotive components such as automobile bodies are required. Therefore, ultra-high strength steel sheets with a tensile strength of 980 MPa or more are applied to automotive components such as automobile bodies. In order to improve the workability of such high-strength steel sheets, a method of containing inexpensive Si in the chemical composition of the steel sheet is known. By containing Si in the chemical composition of the steel sheet, not only the strength of the steel sheet but also the workability can be improved.

[0003] Generally, when applying Si-added steel to automotive components, hot-dip galvanized steel sheets (GI steel sheets) and alloyed hot-dip galvanized steel sheets (GA steel sheets) obtained by alloying the hot-dip galvanized steel sheets are used from the viewpoints of ensuring corrosion resistance and weldability. However, in the manufacturing process of a hot-dip galvanized steel sheet with Si added, since a SiO2 layer covers the steel sheet surface, finally, problems such as non-galvanizing, reduction in galvanizing adhesion, and uneven alloying in the alloying treatment are likely to occur. Furthermore, problems such as peeling of the plating during processing of the alloyed hot-dip galvanized steel sheet may also occur. In order to suppress such problems caused by Si addition, hot-dip galvanized steel sheets containing Si in the steel material are often manufactured using an oxidation-reduction method using an annealing furnace having an oxidation heating zone and a reduction heating zone. According to the oxidation-reduction method, since iron oxide generated in the oxidation heating zone generates a reduced Fe layer during reduction annealing, the plating wettability during plating can be improved. Furthermore, a method of forming an internal oxidation layer containing SiO2 or the like necessary for plating in advance in the steel sheet by increasing the coiling temperature in hot rolling is also used.

[0004] Furthermore, in recent years, various developments have been underway to further improve the strength and workability of hot-dip galvanized steel sheets, including increasing the Si content of the steel sheet to 1% by mass or more, and developing methods for forming a good internal oxide layer.

[0005] Specifically, for example, Patent Document 1 describes a high-strength alloyed hot-dip galvanized steel sheet with a good appearance, characterized in that oxides containing Si are present at the grain boundaries and within the grains on the steel sheet side, within a distance of 5 μm from the interface between the high-strength steel sheet and the plating layer, with an average content of 0.6 to 10 mass%, and oxides containing Si are present in the plating layer with an average content of 0.05 to 1.5 mass%, with an average content of 0.05 to 1.5 mass%, with an average content of 0.05 to 1.5 mass%, with an alloyed hot-dip galvanized layer on the steel sheet side, with an average content of 0.05 to 1.5 mass%, with an average content of 0.05 to 1.5 mass%, with an alloyed hot-dip galvanized layer on the steel sheet side, with an alloyed hot-dip galvanized layer containing Fe, with the remainder being Zn and unavoidable impurities, with an average content of 0.6 to 10 mass%, with an alloyed hot-dip galvanized layer on the steel sheet side, within a distance of 5 μm from the interface between the high-strength steel sheet and the plating layer.

[0006] Furthermore, for example, Patent Document 2 describes a method for manufacturing a high-strength hot-dip galvanized steel sheet with excellent plating adhesion, workability, and appearance, comprising hot-rolling a slab containing, by mass%, C: 0.05~0.30%, Si: 0.1~2.0%, and Mn: 1.0~4.0%, followed by heating at a specific temperature T C A method for manufacturing a high-strength hot-dip galvanized steel sheet is described, comprising: a hot-rolling step of winding the sheet into a coil and pickling it; a cold-rolling step of cold-rolling the hot-rolled sheet obtained in the hot-rolling step; an annealing step of annealing the cold-rolled sheet obtained in the cold-rolling step under specific conditions; and a hot-dip galvanizing step of hot-dip galvanizing the annealed sheet after the annealing step using a hot-dip galvanizing bath containing 0.12 to 0.22 mass% of Al.

[0007] Furthermore, for example, Patent Document 3 describes a cold-rolled steel sheet characterized in that, after hot-rolling a raw steel billet, with the mill scale still attached, it is heat-treated at a temperature range of 650 to 950°C in an atmosphere where substantially no reduction occurs to form an internal oxide layer on the surface layer of the steel sheet, and then pickling, cold-rolling, and recrystallization annealing are performed according to conventional methods. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2006-233333 [Patent Document 2] International Publication No. 2016 / 038801 [Patent Document 3] Japanese Patent Publication No. 2000-309824 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, when increasing the Si content to 1% by mass or more in order to obtain a hot-dip galvanized steel sheet with a tensile strength of 980 MPa or more, it is difficult to obtain a hot-dip galvanized steel sheet with good plating adhesion by simply applying conventional methods. In particular, it is difficult to ensure good plating adhesion near the coil width direction edge of the steel sheet (hereinafter simply referred to as the "width direction edge") compared to near the coil width direction center of the steel sheet (hereinafter simply referred to as the "width direction center").

[0010] Specifically, when using high-Si-added steel, the cooling of the coil after winding in hot rolling is steep near the widthwise edge of the steel sheet. As a result, the internal oxide layer does not grow easily near the widthwise edge of the steel sheet, and a thin layer is formed. On the other hand, near the widthwise center of the steel sheet, the internal oxide layer grows sufficiently, and a thick layer is formed. Furthermore, in the internal oxide layer near the widthwise edge of the steel sheet, the grain boundary oxidation, which is difficult to dissolve by pickling, is thin. Therefore, in the subsequent pickling process, the internal oxide layer near the widthwise edge of the steel sheet dissolves preferentially more than the layer near the widthwise center of the steel sheet, and a large amount of intragranular oxidized SiO2 remains on the surface of the steel sheet. As a result, the SiO2 on the surface of the steel sheet reacts with FeO produced by the oxidation of the steel sheet surface during the subsequent oxidation treatment in the redox method, forming a thick firelight layer ((FeO)2·SiO2 layer) between the steel sheet and the plating after annealing. When a firelight layer is formed, the adhesion of the plating deteriorates in the subsequent plating process, making the plating more prone to peeling after alloying. Figure 1 shows a schematic diagram of an alloyed hot-dip galvanized steel sheet when a firelight layer is formed. As shown in Figure 1, in the alloyed hot-dip galvanized steel sheet 1, the plating layer 4 peels off from the steel sheet 2 due to the firelight layer 3 formed on the surface of the steel sheet 2.

[0011] These problems cannot be solved even by using the technologies described in the aforementioned patent documents. For example, in the steel sheet manufacturing method described in Patent Document 1, rapid cooling of the coil near the widthwise edge is not considered, making it difficult to make the thickness of the internal oxide layer uniform in the widthwise direction of the steel sheet, and a large amount of SiO2 tends to remain on the surface near the widthwise edge of the steel sheet. Also, in the manufacturing method described in Patent Document 2, the winding temperature needs to be lowered as the Si and Mn content increases, making it difficult to generate a predetermined amount of oxide near the widthwise edge, and it is thought that SiO2 tends to remain on the surface of the steel sheet. As a result, even if the technologies disclosed in Patent Documents 1 and 2 are used, there is a risk of firelight layer formation, and it is difficult to obtain uniform and good plating adhesion in the widthwise direction of the steel sheet. On the other hand, in the manufacturing method described in Patent Document 3, the heat treatment temperature is high, so the reduced Fe layer covering the surface cannot be removed by pickling. As a result, contamination of the steel sheet and decarburization near the surface of the steel sheet progress, making it difficult to obtain a steel sheet with a predetermined strength, for example, a tensile strength of 980 MPa.

[0012] Therefore, the present invention aims to provide a method for manufacturing a hot-dip galvanized steel sheet that has a high Si content and good plating adhesion. [Means for solving the problem]

[0013] The present inventors have conducted diligent studies to solve the above problems and have arrived at the present invention. That is, the present invention encompasses the following preferred embodiments.

[0014] A method for manufacturing a hot-dip galvanized steel sheet according to the first aspect of the present invention comprises the steps of hot-rolling a steel material having a Si content of 1.0% by mass or more and winding it at 500°C to 700°C, The process involves applying an oxidation treatment to the surface of the steel sheet after winding at a heating temperature of 750°C or lower, followed by a reduction treatment. The process includes a step of applying a hot-dip galvanizing treatment to the steel sheet after the reduction treatment to form a galvanized layer on the surface of the steel sheet.

[0015] In the method for manufacturing the hot-dip galvanized steel sheet described above, it is preferable to further include a step of annealing the steel sheet at a soaking temperature of 540°C to 620°C in a non-reducing atmosphere between the coiling and the oxidation treatment.

[0016] The method for manufacturing an alloyed hot-dip galvanized steel sheet according to the second aspect of the present invention further includes a step of alloying the galvanized layer formed on the hot-dip galvanized steel sheet obtained by the method for manufacturing a hot-dip galvanized steel sheet according to the first aspect described above.

Effects of the Invention

[0017] According to the present invention, it is possible to provide a method for manufacturing a hot-dip galvanized steel sheet that contains a high Si content and has good plating adhesion.

Brief Description of the Drawings

[0018] [[ID= eighteen]] [Figure 1] FIG. 1 is a schematic view of an alloyed hot-dip galvanized steel sheet when a firelite layer is formed.

Embodiments for Carrying Out the Invention

[0019] The present inventors have conducted various studies on a method for manufacturing a hot-dip galvanized steel sheet having good plating adhesion using a steel sheet with a high Si content. Then, paying attention to suppressing the formation of the firelite layer during the heating of the oxidation treatment, the present invention was completed. Specifically, it has been found that the formation of the firelite layer can be suppressed by setting the heating temperature of the oxidation treatment in the oxidation-reduction method to 750°C or lower in terms of the steel sheet temperature during the manufacture of the hot-dip galvanized steel sheet. As a result, it has been found that a high-strength and high-workability hot-dip galvanized steel sheet having good plating adhesion can be obtained even in the vicinity of the edge in the width direction of the steel sheet. The hot-dip galvanized steel sheet thus obtained and the alloyed hot-dip galvanized steel sheet in which the plating layer is alloyed can have a tensile strength of, for example, 980 MPa or more.

[0020] Hereinafter, embodiments of the present invention will be described in detail. The scope of the present invention is not limited to the embodiments described here, and various modifications can be made without departing from the spirit of the present invention.

[0021] In this specification, the "internal oxide layer" means an internal oxide layer containing SiO2 (including both the oxidized parts of grain boundary oxidation and intragranular oxidation) that can be formed inside the steel sheet during heating in annealing before hot rolling and pickling (in other words, "annealing after hot rolling and before annealing by the oxidation-reduction method"; the same applies hereinafter). Further, the internal oxide layer exists between the surface layer of the steel sheet to be subjected to hot-dip galvanizing treatment and the steel sheet base part which is the inner part of the steel sheet not containing oxides such as SiO2.

[0022] In this specification, the "coil width direction edge (of the steel sheet)" or "width direction edge" basically refers to both edges in the coil width direction, that is, both ends in the sheet width direction, unless a specific position is indicated. Also, in this specification, the "vicinity of the coil width direction edge (of the steel sheet)" or "vicinity of the width direction edge" means the peripheral part of the position of the coil width direction edge. When indicating a specific position from the coil width direction edge, the distance from the said width direction edge (in other words, the position of 0 mm in the width direction) is also described.

[0023] In this specification, the "coil width direction center (of the steel sheet)" or "width direction center" refers to the center in the sheet width direction of the steel sheet. Also, in this specification, the "vicinity of the coil width direction center (of the steel sheet)" or "vicinity of the width direction center" means the peripheral part of the position of the coil width direction center.

[0024] In this specification, the "rear end in the rolling direction (of the steel sheet)" refers to the position of the rear end in the direction parallel to the rolling direction of the steel sheet for hot-dip galvanizing, that is, the position of 0 mm at the very rear end. Also, in this specification, the "vicinity of the rear end in the rolling direction (of the steel sheet)" means the peripheral part of the position of the rear end in the rolling direction.

[0025] In this specification, "(the leading edge in the rolling direction of the steel sheet)" refers to the position of the leading edge of the hot-dip galvanized steel sheet in a direction parallel to the rolling direction, i.e., the position of the foremost edge (0 mm). Furthermore, in this specification, "the vicinity of the leading edge in the rolling direction of the steel sheet" refers to the area surrounding the position of the leading edge in the rolling direction.

[0026] 1. Method for manufacturing hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets The method for manufacturing a hot-dip galvanized steel sheet in this embodiment includes the steps of: hot-rolling a steel material (steel or steel sheet) having a Si content of 1.0% by mass or more and winding it at 500°C to 700°C; applying an oxidation treatment to the surface of the winded steel sheet at a heating temperature of 750°C or lower, followed by a reduction treatment; and applying a hot-dip galvanizing treatment to the steel sheet after the reduction treatment to form a zinc plating layer on the surface of the steel sheet.

[0027] The following provides a detailed explanation of each process and any processes before or after each process.

[0028] (Preparation of steel material for rolling) First, a steel material such as a rolling slab having a chemical composition with a Si content of 1.0 mass% or more is prepared. The chemical composition of the steel material will be described in detail later. The steel material such as a slab can be prepared by any known method. One method for preparing the slab is to melt steel having the chemical composition described later and produce the slab by ingot formation or continuous casting. If necessary, the cast material obtained by ingot formation or continuous casting may be rolled in a bloc to obtain a slab.

[0029] (Hot rolling) Next, the obtained steel material, such as slabs, is hot-rolled to obtain hot-rolled steel sheets.

[0030] Hot rolling may be carried out by any known method, but the winding temperature should be set to 500°C to 700°C. Setting the winding temperature to 500°C or higher allows for sufficient growth of the internal oxide layer, preventing the exposure and dissolution of SiO2 on the surface of the steel sheet near the widthwise edge after subsequent processes, which could lead to the formation of a firelight layer. The winding temperature is preferably 520°C or higher, more preferably 530°C or higher. Furthermore, setting the winding temperature to 700°C or lower avoids an increase in the amount of reduced iron generated during cooling after hot rolling, which would make subsequent pickling difficult. The winding temperature is preferably 680°C or lower, more preferably 660°C or lower.

[0031] Other conditions during hot rolling are not particularly limited. For example, in hot rolling, the slab before hot rolling can be uniformly heated to a temperature of 1000°C to 1300°C or lower according to conventional methods, the finish rolling temperature can be set to 800°C or higher, and then it can be wound into a coiled steel sheet. Furthermore, the wound hot-rolled steel sheet after hot rolling may be naturally cooled to room temperature.

[0032] (Annealing) Furthermore, it is preferable to anneal the steel sheet after winding in a non-reducing atmosphere at a uniform heat holding temperature of 540°C to 620°C.

[0033] In this specification, "non-reducing atmosphere" means a gas atmosphere in which the surface of the steel sheet is not substantially reduced. While not limited to these, preferred gas atmospheres include, for example, N2-1.0 volume% or less H2, argon, or air.

[0034] By setting the soaking temperature during annealing to 540°C or higher, the internal oxide layer can be grown and retained well up to the vicinity of the edge in the width direction of the steel sheet. As a result, a hot-dip galvanized steel sheet that can be alloyed uniformly can be obtained. Furthermore, it is even more preferable to grow and retain the internal oxide layer well not only from the vicinity of the center in the width direction of the steel sheet to the vicinity of the edge in the width direction, but also from the vicinity of the front end in the rolling direction of the steel sheet to the vicinity of the rear end in the rolling direction of the steel sheet. As a result, a hot-dip galvanized steel sheet that can be alloyed uniformly and reliably over almost the entire surface of the steel sheet can be obtained. Note that it is difficult to grow the internal oxide layer sufficiently up to the vicinity of the edge in the width direction by heating during winding during hot rolling as described above. Also, by setting the soaking temperature during annealing to 620°C or lower, it is possible to avoid the difficulty of scale removal by subsequent pickling. The soaking temperature during annealing is more preferably 550°C or higher. Furthermore, the soaking temperature during annealing is more preferably 610°C or lower.

[0035] The soaking time during annealing is not particularly limited and should be controlled to a suitable time for obtaining the desired internal oxide layer, taking into account the hot rolling conditions (especially the winding temperature), the soaking temperature in this process, and the pickling conditions in subsequent processes. For example, by ensuring a soaking time of 30 hours or more during annealing, a plating base sheet with the desired internal oxide layer can be obtained. There is no particular upper limit to the soaking time during annealing, but for example, if the soaking time is not excessively longer than 30 hours, a decrease in productivity can be prevented.

[0036] (pickling) Next, it is preferable to pickle the annealed steel sheet. The pickling method is not particularly limited, and any known method may be applied. For example, scale can be removed by immersion in hydrochloric acid or the like.

[0037] Pickling should preferably be carried out while adjusting the degree to which the scale adhering to the steel sheet is removed. Specifically, for example, the amount of weight loss per unit area of ​​the steel sheet after pickling (i.e., the amount of weight loss per unit area of ​​the steel sheet) after scale removal should be 31 g / m². 2It is preferable to perform pickling so that the amount of pickling loss is less than 31 g / m². 2 By keeping the value below a certain level, it is possible to prevent some of the base steel sheet crystal grains in the internal oxide layer on the surface of the steel sheet from dissolving and peeling off, particularly near the edges in the width direction of the steel sheet, and preventing SiO2 from being exposed and dispersed on the surface of the steel sheet. This reliably suppresses the formation of the firelight layer after plating, and makes it possible to obtain a hot-dip galvanized steel sheet with superior plating adhesion.

[0038] The amount of material lost during pickling can be increased by appropriately controlling the type of pickling solution (e.g., hydrochloric acid solution), the concentration of the pickling solution, the temperature of the pickling solution, and the pickling time. For example, the hydrochloric acid concentration should preferably be set to 3% by mass or more, more preferably to 5% by mass or more. Alternatively, the hydrochloric acid concentration should preferably be set to 20% by mass or less, more preferably to 15% by mass or less. Furthermore, for example, the temperature of the pickling solution should preferably be set to 60°C or higher, more preferably to 70°C or higher. Alternatively, the temperature of the pickling solution should preferably be set to 90°C or lower, more preferably to 80°C or lower. The pickling time should, for example, be set to achieve a pickling loss of 31 g / m³. 2 The concentration and temperature of the pickling solution should be adjusted as needed to keep the result below a certain level.

[0039] (Cold rolling) Furthermore, the steel sheet may be cold-rolled after pickling. The method of cold rolling is not particularly limited, and any known method may be applied. For example, to achieve the desired sheet thickness, the cold rolling ratio can be set to a range of 10% to 70%. The thickness of the steel sheet is not particularly limited.

[0040] (Oxidation and reduction treatments) Next, redox annealing is applied to the surface of the obtained steel sheet. First, an oxide Fe layer is formed on the surface of the steel sheet by applying an oxidation treatment. Furthermore, a reduction treatment (also referred to as "reduction annealing treatment" in this specification) is applied to the oxide Fe layer in a reducing atmosphere to form a reduced Fe layer. At this time, the oxygen supplied from the oxide Fe layer by reduction oxidizes Si and Mn inside the steel sheet. In other words, by applying such redox annealing, the oxide Fe layer acts as a barrier layer, retaining Si oxide inside the steel sheet and suppressing the oxidation of Si near the surface of the steel sheet. As a result, the wettability for hot-dip galvanizing can be improved, and ultimately the adhesion of the plating can also be improved.

[0041] Oxidation and reduction treatments can be carried out using any known single or multiple pieces of equipment. Preferably, from the viewpoint of manufacturing efficiency, cost, and quality preservation, equipment for a continuous galvanizing line (CGL) is used. By using a continuous galvanizing line, oxidation and reduction treatments by oxidation-reduction methods, and the galvanizing and alloying treatments described later, can be carried out continuously in a series of manufacturing lines. More specifically, oxidation and reduction treatments by oxidation-reduction methods are more preferably carried out using, for example, an annealing furnace in a continuous galvanizing line of the type of non-oxidizing furnace (NOF) or direct-fired furnace (DFF).

[0042] The oxidation treatment is performed on the surface of the steel sheet at a heating temperature of 750°C or lower, for example, in an oxidation heating zone within an NOF or DFF type annealing furnace. By keeping the steel sheet temperature below 750°C, the reaction between SiO2 and FeO produced by the oxidation treatment, particularly near the edges in the width direction of the steel sheet, can be suppressed. As a result, the formation of a firelight layer after annealing can be prevented, and a hot-dip galvanized steel sheet with good plating adhesion can be obtained.

[0043] In this specification, "steel plate temperature" during heating in oxidation treatment means the highest plate temperature reached by the steel plate under heating control in the oxidation heating zone.

[0044] The steel sheet temperature during the oxidation treatment is preferably 730°C or lower, more preferably 720°C or lower, and even more preferably 700°C or lower. The lower limit of the steel sheet temperature during the oxidation treatment is not particularly limited and should be any temperature at which an oxidized Fe layer is formed on the surface of the steel sheet under the gas atmosphere described later. For example, the steel sheet temperature during the oxidation treatment is preferably 650°C or higher, more preferably 670°C or higher.

[0045] The heating time in the oxidation treatment is not particularly limited and should be adjusted so as not to be excessively long, which would cause the formation of a firelight layer during the oxidation treatment. Specifically, the heating time in the oxidation treatment should be appropriately adjusted considering the hot rolling conditions (especially the winding temperature), the annealing conditions before pickling, the pickling conditions, and the steel sheet temperature during heating in the oxidation treatment. For example, the heating time in the oxidation treatment is preferably 10 seconds or more, more preferably 15 seconds or more. Alternatively, for example, the heating time in the oxidation treatment is preferably 120 seconds or less, more preferably 90 seconds or less.

[0046] The oxidation treatment is not particularly limited, but can be carried out in a gas atmosphere containing, for example, O2, CO2, N2, and H2O. More specifically, the oxidation treatment can be carried out in a gas atmosphere in which the unburned O2 concentration is controlled, for example, in an NOF type or DFF type annealing furnace, in a combustion gas such as coke oven gas (COG) or liquefied petroleum gas (LPG). The O2 concentration is preferably controlled in the range of 100 ppm to 17000 ppm. The O2 concentration is more preferably controlled to 500 ppm or more, and even more preferably to 2000 ppm or more. Furthermore, the O2 concentration is more preferably controlled to 15000 ppm or less, and even more preferably to 13000 ppm or less.

[0047] The heating temperature (stable temperature) of the steel sheet in the reduction annealing treatment is not particularly limited and should be set to a temperature at which the oxide Fe layer formed by the oxidation treatment becomes a reduced Fe layer. Specifically, it is preferable to perform reduction annealing at a stable temperature of Ac3 or higher. The Ac3 point can be calculated using the following formula (i) (from "Leslie's Materials Science" (published by Maruzen Co., Ltd., by William C. Leslie, p. 273)). The elemental symbols enclosed in [ ] in formula (i) represent the content (mass %) of the element.

[0048] Ac3(°C) = 910 - 203 × [C] 1 / 2 -15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W]-{30×[Mn]+11×[Cr]+20×[Cu]-700×[P]-400×[Al]-120×[As]-400×[Ti]} …(i)

[0049] Furthermore, the heating time (stable heating time) in the reduction treatment is not particularly limited and should be appropriately adjusted so that the oxidized Fe layer formed by the oxidation treatment becomes a reduced Fe layer. For example, the heating time in the reduction treatment is preferably 30 seconds or more, more preferably 45 seconds or more. Also, the heating time in the reduction treatment is preferably 600 seconds or less, more preferably 500 seconds or less.

[0050] Reduction annealing can be performed by any known method, for example, in a reduction heating zone within an NOF or DFF type annealing furnace. Specifically, it can be performed by heating the surface of a steel sheet in a reducing atmosphere mainly containing H2 gas and an inert gas such as N2. When using a mixed gas containing H2 gas and an inert gas such as N2, for example, H2 gas may be included in a proportion of 3% to 25% by volume, with the remainder being an inert gas such as N2.

[0051] (Hot-dip galvanizing treatment) Furthermore, by applying a hot-dip galvanizing treatment to the steel sheet after the reduction treatment, a galvanized layer can be formed on the surface of the steel sheet, thereby manufacturing the hot-dip galvanized steel sheet according to this embodiment.

[0052] The method of hot-dip galvanizing is not particularly limited, and any known method may be applied. For example, a galvanized layer can be formed on the surface of a steel sheet by immersing it in a galvanizing bath at a steel sheet temperature of approximately 400°C to 500°C. Furthermore, the immersion time of the steel sheet in the galvanizing bath can be adjusted according to the desired amount of galvanized coating.

[0053] (Alloying treatment) The method for manufacturing alloyed hot-dip galvanized steel sheets according to this embodiment further includes a step of alloying the zinc plating layer formed on the hot-dip galvanized steel sheet obtained by the method described above.

[0054] Specifically, by heating a hot-dip galvanized steel sheet to a predetermined alloying temperature, Fe atoms contained in the steel sheet diffuse into the zinc plating layer, thereby alloying the zinc plating layer. The alloying method is not particularly limited, and any known method can be applied. The alloying temperature is not particularly limited, but for example, it can preferably be set to 460°C to 650°C. The heating time at the alloying temperature is also not particularly limited, but for example, it can preferably be set to 10 seconds to 40 seconds. Furthermore, the heating for alloying can be carried out, for example, in an atmospheric environment.

[0055] 2. Chemical composition of steel material The chemical composition of the steel material used in the manufacturing method of hot-dip galvanized steel sheet and the manufacturing method of alloyed hot-dip galvanized steel sheet in this embodiment is not particularly limited other than Si. An example of the chemical composition of the steel material is described below.

[0056] [Si: 1% by mass or more] Si is an inexpensive steel strengthening element and has little effect on the workability of steel sheets. Furthermore, Si is an element that can suppress the decomposition of retained austenite, which is useful for improving the workability of steel sheets, and the formation of carbides. In order to effectively exert these effects, the Si content is 1.0 mass% or more, preferably 1.1 mass% or more, and more preferably 1.2 mass% or more. There is no particular upper limit to the Si content, but if the Si content is too high, the solid solution strengthening effect by Si may become significant, which may increase the rolling load, and Si scale may be generated during hot rolling, potentially causing surface defects in the steel sheet. For this reason, for example, from the viewpoint of manufacturing stability, the Si content is preferably 3.0 mass% or less, more preferably 2.7 mass% or less, and even more preferably 2.5 mass% or less.

[0057] [Mn: Preferably 1.5% by mass or more and 3.0% by mass or less] Like Si, Mn is an inexpensive steel strengthening element and is effective in improving the strength of steel sheets. Mn, when added to steel together with Si and, if necessary, with C, is a particularly effective strengthening element for ensuring a tensile strength of 980 MPa or higher for hot-dip galvanized steel sheets. Furthermore, Mn stabilizes austenite and contributes to improved workability of steel sheets by the formation of retained austenite. To effectively exert these effects, the Mn content is preferably 1.5% by mass or more, more preferably 1.8% by mass or more, and even more preferably 2.0% by mass or more. However, if the Mn content is too high, the ductility of the steel sheet decreases, adversely affecting the workability and potentially reducing the weldability of the steel sheet. From this viewpoint, the Mn content is preferably 3.0% by mass or less, more preferably 2.8% by mass or less, and even more preferably 2.7% by mass or less.

[0058] [C: Preferably 0.08% by mass or more and 0.30% by mass or less] Carbon (C) is an effective element for improving the strength of steel sheets. When added to steel together with Si, and optionally with Mn, it is a particularly effective reinforcing element for ultimately ensuring a tensile strength of 980 MPa or higher for hot-dip galvanized steel sheets. Furthermore, carbon is an element necessary to ensure retained austenite and improve workability. To effectively exert these effects, the carbon content is preferably 0.08% by mass or more, more preferably 0.11% by mass or more, and even more preferably 0.13% by mass or more. From the viewpoint of ensuring the strength of the steel sheet, a higher carbon content is preferable, but if the carbon content is too high, corrosion resistance, spot weldability, and workability may deteriorate. Therefore, the carbon content is preferably 0.30% by mass or less, more preferably 0.25% by mass or less, and even more preferably 0.20% by mass or less.

[0059] [P: Preferably more than 0% by mass and 0.1% by mass or less] P is an element that is inevitably present as an impurity element. If the P content is excessive, it may degrade the weldability of the steel plate. Therefore, the P content is preferably suppressed to 0.1% by mass or less, more preferably 0.08% by mass or less, and even more preferably 0.05% by mass or less.

[0060] [S: Preferably more than 0% by mass and 0.05% by mass or less] S is an element that is inevitably present as an impurity element. Normally, steel inevitably contains about 0.0005% by mass of S. If the S content is excessive, it can form sulfide inclusions, promote hydrogen absorption in corrosive environments, degrade the delayed fracture resistance of the steel sheet, and may degrade the weldability and workability of the steel sheet. For this reason, the S content is preferably suppressed to 0.05% by mass or less, more preferably 0.01% by mass or less, and even more preferably 0.005% by mass or less.

[0061] [Al: Preferably more than 0% by mass and 1.0% by mass or less] Al is an element that has a deoxidizing effect. To effectively exert this effect, the Al content is preferably more than 0% by mass, more preferably 0.005% by mass or more, and even more preferably 0.02% by mass or more. If the Al content is too high, the number of inclusions such as alumina will increase, which may deteriorate the workability of the steel sheet. Therefore, the Al content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and even more preferably 0.5% by mass or less.

[0062] [Cr: Preferably more than 0% by mass and 1.0% by mass or less] Cr is an effective element for improving the strength of steel sheets. Furthermore, Cr is an element that improves the corrosion resistance of steel sheets and has the effect of suppressing the generation of hydrogen due to corrosion of steel sheets. Specifically, Cr has the effect of promoting the formation of iron oxide (α-FeOOH). Iron oxide is said to be thermodynamically stable and protective among the rust that forms in the atmosphere. By promoting the formation of such rust, it is possible to suppress the penetration of generated hydrogen into the steel sheet, and even when steel sheets are used in harsh corrosive environments, for example in the presence of chlorides, hydrogen-induced cracking can be sufficiently suppressed. In addition, since Cr, like B and Ti, is an effective element for the delayed fracture resistance of steel sheets, it can be added in an amount that does not affect the strength and workability such as elongation of the steel sheet. To effectively exert these effects, the Cr content is preferably more than 0% by mass, more preferably 0.003% by mass or more, and even more preferably 0.01% by mass or more. On the other hand, if the Cr content is excessive, the workability such as elongation of the steel sheet may deteriorate. Therefore, the Cr content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and even more preferably 0.6% by mass or less.

[0063] [Cu: Preferably more than 0% by mass and 1.0% by mass or less] Like Cr, Cu is effective in improving the strength of steel sheets and also has the effect of suppressing the generation of hydrogen due to corrosion of steel sheets, thereby improving the corrosion resistance of steel sheets. Like Cr, Cu also has the effect of promoting the formation of iron oxide. To effectively exert these effects, the Cu content is preferably greater than 0% by mass, more preferably 0.003% by mass or more, and even more preferably 0.05% by mass or more. Furthermore, from the viewpoint of the workability of the steel sheet, the Cu content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and even more preferably 0.5% by mass or less.

[0064] [Nitrile: Preferably more than 0% by mass and 1.0% by mass or less] Ni, like Cr and Cu, is effective in improving the strength of steel sheets and also has the effect of suppressing the generation of hydrogen due to corrosion of steel sheets, thereby improving the corrosion resistance of steel sheets. Ni, like Cr and Cu, also has the effect of promoting the formation of iron oxide. To effectively exert these effects, the Ni content is preferably greater than 0% by mass, more preferably 0.003% by mass or more, and even more preferably 0.05% by mass or more. Furthermore, from the viewpoint of the workability of the steel sheet, the Ni content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and even more preferably 0.5% by mass or less.

[0065] [Ti: Preferably more than 0% by mass and 0.15% by mass or less] Like Cr, Cu, and Ni, Ti is effective in improving the strength of steel sheets and also has the effect of suppressing the generation of hydrogen due to corrosion of steel sheets, thereby improving the corrosion resistance of steel sheets. Like Cr, Cu, and Ni, Ti also has the effect of promoting the formation of iron oxide. Furthermore, like B and Cr, Ti is an element that is effective in improving the delayed fracture resistance of steel sheets, and can therefore be added in an amount that does not affect the strength and workability such as elongation of the steel sheet. To effectively exert these effects, the Ti content is preferably greater than 0% by mass, more preferably 0.003% by mass or more, and even more preferably 0.05% by mass or more. Also, from the viewpoint of the workability of the steel sheet, the Ti content is preferably 0.15% by mass or less, more preferably 0.12% by mass or less, and even more preferably 0.10% by mass or less.

[0066] [Nb: preferably more than 0% by mass and 0.15% by mass or less] Nb is an element that is effective in improving the strength of steel sheets and also acts to improve the toughness of steel sheets by refining the austenite grains after quenching. To effectively exert these effects, the Nb content is preferably greater than 0% by mass, more preferably 0.03% by mass or more, and even more preferably 0.005% by mass or more. On the other hand, if the Nb content is excessive, a large amount of carbides, nitrides, or carbonitrides may be generated, which may deteriorate the workability or delayed fracture resistance of the steel sheet. Therefore, the Nb content is preferably 0.15% by mass or less, more preferably 0.12% by mass or less, and even more preferably 0.10% by mass or less.

[0067] [V: Preferably more than 0% by mass and 0.15% by mass or less] Like Nb, V is effective in improving the strength of steel sheets and also acts to improve the toughness of steel sheets by refining the austenite grains after quenching. To effectively exert these effects, the V content is preferably greater than 0% by mass, more preferably 0.03% by mass or more, and even more preferably 0.005% by mass or more. On the other hand, if the V content is excessive, like Nb, it can generate a large amount of carbides, nitrides, or carbonitrides, which may deteriorate the workability or delayed fracture resistance of the steel sheet. Therefore, the V content is preferably 0.15% by mass or less, more preferably 0.12% by mass or less, and even more preferably 0.10% by mass or less.

[0068] [B: Preferably more than 0% by mass and 0.005% by mass or less] B is an element useful for improving the hardenability and weldability of steel sheets. Furthermore, like Ti and Cr, B is also effective in improving the delayed fracture resistance of steel sheets, and can therefore be added in amounts that do not affect the strength and workability such as elongation of the steel sheet. To effectively exert these effects, the B content is preferably greater than 0% by mass, more preferably 0.0002% by mass or more, even more preferably 0.0003% by mass or more, and particularly preferably 0.0004% by mass or more. On the other hand, if the B content is excessive, these effects may saturate, and ductility may decrease, potentially leading to poor workability. Therefore, the B content is preferably 0.005% by mass or less, even more preferably 0.004% by mass or less, and even more preferably 0.003% by mass or less.

[0069] [N: Preferably more than 0% by mass and 0.01% by mass or less] N is an element that is inevitably present as an impurity element. If the N content is excessive, it may form nitrides, which may degrade the workability of the steel sheet. In particular, when the steel sheet contains B to improve hardenability, N combines with B to form BN precipitates, which inhibits the hardenability-improving effect of B. Therefore, the N content is preferably suppressed to 0.01% by mass or less, more preferably 0.008% by mass or less, and even more preferably 0.005% by mass or less.

[0070] Furthermore, the chemical composition of the steel material in this embodiment may also contain other well-known optional components in addition to the above-mentioned components, as long as they do not impair strength or sufficient workability.

[0071] [Remainder] The remainder consists of Fe and unavoidable impurities. As unavoidable impurities, the inclusion of trace elements (e.g., As, Sb, Sn, etc.) introduced depending on the raw materials, materials, and manufacturing equipment is acceptable. As mentioned above, P, S, and N are generally preferable in lower amounts and can therefore also be considered unavoidable impurities. However, the present invention can exert its effects by limiting the content of these elements to a specific range, hence the above definition. For this reason, in this specification, "unavoidable impurities" constituting the remainder is a concept that excludes elements whose composition range is defined.

[0072] According to the manufacturing method of this embodiment, since a high-Si content steel material with a Si content of 1% by mass or more is used, high-strength, highly workable hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets can be manufactured inexpensively, and good plating adhesion can be achieved in the coil width direction. As a result, in alloyed hot-dip galvanized steel sheets, the plating does not peel off even near the edges in the width direction of the steel sheet. More specifically, the manufactured hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets can have a tensile strength of 980 MPa or more.

[0073] Furthermore, as mentioned above, by using a continuous hot-dip galvanizing line to perform oxidation treatment, reduction treatment, hot-dip galvanizing treatment, and alloying treatment in a continuous production line, it is possible to manufacture high-strength, highly workable alloyed hot-dip galvanized steel sheets with good plating adhesion at low cost and efficiently while maintaining product quality. [Examples]

[0074] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way by these examples.

[0075] (Example 1) In Example 1, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet, which are examples of the present invention, were manufactured, and the plating adhesion of the manufactured hot-dip galvanized steel sheet was evaluated.

[0076] [Manufacturing of hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets] Steel material with the chemical composition of steel type A shown in Table 1 below was melted in a converter, and then slabs were produced by continuous casting. The obtained slabs were hot-rolled to a thickness of 2.0 mm with a finish rolling end temperature of 900°C, and as shown in Table 2 below, 640 At ℃ The resulting hot-rolled steel sheet was wound and cooled to room temperature. Then, the hot-rolled steel sheet was placed in an annealing furnace and annealed. The annealing conditions were as follows: under a non-reducing atmosphere of less than 0.9% H2 by volume, the hot-rolled steel sheet was heated to 580°C over approximately 8.5 hours, maintained at 580°C for 30 hours, and then cooled to below 200°C over approximately 5 hours. Afterward, the resulting annealed steel sheet was pickled by immersion in 8% hydrochloric acid at 85°C for 40 seconds. Finally, the annealed steel sheet was cold-rolled until its thickness decreased from 2.0 mm to 1.4 mm. Next, the steel sheet thus produced was subjected to oxidation treatment, reduction treatment, hot-dip galvanizing treatment, and alloying treatment using a continuous hot-dip galvanizing line with an NOF-type annealing furnace. In the oxidation treatment, the steel plates were heated to a temperature of 716°C over a heating time of 45 seconds in a combustion exhaust gas atmosphere containing less than 17,000 ppm of O2, CO2, N2, and H2O.

[0077] Here, "steel plate temperature" refers to the highest plate temperature reached by the steel plate under heating control in the oxidation heating zone of an NOF type (or DFF type in Examples 2 and 3 described later) annealing furnace.

[0078] In the reduction treatment, the oxidized steel sheet was heated in an N2-H2 gas atmosphere at a uniform temperature of approximately 800°C (800°C to 900°C) for approximately 60 seconds (50 seconds to 60 seconds). In the hot-dip galvanizing treatment, the reduced steel sheet was immersed in a zinc plating bath at 430°C to form a hot-dip galvanized layer. A hot-dip galvanized steel sheet was obtained in this way, and subsequently, an alloying treatment was performed to obtain an alloyed hot-dip galvanized steel sheet.

[0079] [Evaluation of Plating Adhesion of Hot-Dip Galvanized Steel Sheets] The plating adhesion of the obtained alloyed hot-dip galvanized steel sheet was evaluated. Specifically, a hat-shaped molded member was formed using a crank press under the following conditions using the obtained alloyed hot-dip galvanized steel sheet, and the degree of plating peeling on the sliding surface of the side wall of the molded member was visually determined. The evaluation sample was cut from the obtained alloyed hot-dip galvanized steel sheet so as to include a position 50 mm to 100 mm from the widthwise edge near the trailing end in the rolling direction. The specific evaluation criteria are also shown below. The evaluation results of the plating adhesion of the hot-dip galvanized steel sheet of Example 1, along with the manufacturing conditions of the steel sheet, are summarized in Table 2 below.

[0080] (Conditions for a crank press) Size of evaluation sample: 40mm wide x 250mm long Die width: 52mm Die shoulder radius: 2mm Punch width: 48mm Punch shoulder radius: 5mm Bead: Yes Bead tip radius: 2mm Bead height: 3mm Molding height: 60mm

[0081] (Evaluation criteria for plating adhesion) ○: No clear delamination was observed in the plating layer. ×: Flake-like lifting is observed in the plating layer.

[0082] (Example 2) In Example 2, steel materials with the chemical composition of steel grade C shown in Table 1 below were used, the hot rolling and winding temperature was set to 660°C, and a continuous hot-dip galvanizing line with a DFF-type annealing furnace was applied to perform oxidation treatment, reduction treatment, hot-dip galvanizing treatment, and alloying treatment. The steel sheet temperature in the oxidation heating zone of the DFF-type annealing furnace was set to 671°C, the soaking temperature for the reduction treatment was set to approximately 900°C (900°C to 950°C), and the soaking time was set to approximately 300 seconds (240 seconds to 360 seconds). Except for these differences, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets were manufactured in the same manner as in Example 1 described above. Furthermore, the plating adhesion was evaluated in the same manner. The evaluation results of the plating adhesion of the hot-dip galvanized steel sheets of Example 2, along with the steel sheet manufacturing conditions, are summarized in Table 2 below.

[0083] (Example 3) In Example 3, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets were manufactured using the same method as in Example 2, except that steel materials with the chemical composition of steel grade D shown in Table 1 below were used, and the steel sheet temperature in the oxidation heating zone of the DFF type annealing furnace was set to 679°C. Furthermore, the plating adhesion was evaluated in the same manner. The evaluation results of the plating adhesion of the hot-dip galvanized steel sheets of Example 3, along with the steel sheet manufacturing conditions, are summarized in Table 2 below.

[0084] (Comparative Example 1) In Comparative Example 1, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets were manufactured using the same method as in Example 1, except that the hot-rolling and winding temperature was set to 550°C and the steel sheet temperature in the oxidation heating zone of the NOF-type annealing furnace was set to 906°C, using steel material with the chemical composition of steel grade B shown in Table 1 below. Furthermore, the plating adhesion was evaluated in the same manner. The evaluation results of the plating adhesion of the hot-dip galvanized steel sheet of Comparative Example 1, along with the steel sheet manufacturing conditions, are summarized in Table 2 below.

[0085] [Table 1]

[0086] [Table 2]

[0087] (Consideration) As shown in Table 2 above, the hot-dip galvanized steel sheets of Example 1 (heating temperature of 716°C), Example 2 (671°C), and Example 3 (679°C) in the hot-dip galvanizing line exhibited excellent plating adhesion. On the other hand, the hot-dip galvanized steel sheet of Comparative Example 1, even though the annealing conditions before pickling were the same as those of Examples 1, 2, and 3, had a higher heating temperature in the hot-dip galvanizing line, deviating from the specified temperature in this embodiment, resulting in inferior plating adhesion. This is thought to be because, in Comparative Example 1, the steel sheet temperature during the oxidation treatment heating was too high, causing a thick firelight layer to form between the steel sheet and the plating.

[0088] This application is based on Japanese Patent Application No. 2021-036226, filed on March 8, 2021, and its contents are included in this application.

[0089] The embodiments and examples disclosed herein should be understood in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended. [Explanation of Symbols]

[0090] 1. Alloyed hot-dip galvanized steel sheet 2 steel plate 3 Firelight Layer 4 Plating layer

Claims

1. A method for manufacturing a hot-dip galvanized steel sheet, A process of hot-rolling a steel material having a Si content of 1.0% by mass or more and winding it at 500°C to 700°C, The process involves annealing the steel sheet after winding in a non-reducing atmosphere at a uniform heat holding temperature of 540°C to 620°C, The process involves pickling the steel sheet after annealing, such that the amount of pickling loss in the steel sheet after scale removal is less than 31 g / m², The process involves applying an oxidation treatment to the surface of the pickled steel sheet for a heating time of 10 seconds to 120 seconds while controlling the heating temperature in the oxidation heating zone so that the steel sheet temperature is 750°C or lower, followed by a reduction treatment. A method for manufacturing a hot-dip galvanized steel sheet, comprising the step of applying a hot-dip galvanizing treatment to the steel sheet after the reduction treatment to form a galvanized layer on the surface of the steel sheet.

2. A method for manufacturing alloyed hot-dip galvanized steel sheets, A method for manufacturing an alloyed hot-dip galvanized steel sheet, further comprising the step of alloying the zinc plating layer formed on the hot-dip galvanized steel sheet obtained by the method for manufacturing a hot-dip galvanized steel sheet according to claim 1.

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