Hot-rolled steel sheet and galvanized steel sheet comprising same, and method for manufacturing hot-rolled steel sheet and method for manufacturing galvanized steel sheet comprising same

Abstract

The present application relates to a hot-rolled steel sheet and a galvanized steel sheet comprising same, and a method for manufacturing a hot-rolled steel sheet and a method for manufacturing a galvanized steel sheet comprising same. According to the present application, it is possible to provide a hot-rolled steel sheet having high strength and excellent ductility without performing rapid cooling, a galvanized steel sheet comprising same, and a method for manufacturing a hot-rolled steel sheet and a method for manufacturing a galvanized steel sheet comprising same.

Description

Hot-rolled steel sheet and galvanized steel sheet including the same, and method for manufacturing a hot-rolled steel sheet and method for manufacturing a galvanized steel sheet including the same

[0001] The present application relates to a hot-rolled steel sheet, a galvanized steel sheet including the same, a method for manufacturing a hot-rolled steel sheet, and a method for manufacturing a galvanized steel sheet including the same.

[0002] The demand for high-strength hot-rolled steel sheets is increasing every year as a material that can improve automobile fuel efficiency.

[0003] Patent documents 1 (Korean Registered Patent No. 10-2090884) and 2 (Korean Published Patent No. 10-2021-0047334) propose hot-rolled steel sheets having a tensile strength (TS) of 940 MPa or more, an elongation (EL) of 8% or more, and a hole expansion rate of 40% or more. However, the technology proposed in patent documents 1 and 2 has limitations in that the tensile strength is at the level of 1.0 GPa and the lower limit of the elongation is managed at 8%, which makes it difficult to apply to various component materials.

[0004] Patent Document 3 (Korean Published Patent No. 10-2017-0027745) proposes a method for manufacturing a steel sheet having a yield strength (YS) of over 1000 MPa, a tensile strength of over 1150 MPa, and a total elongation of over 8%. However, in the technology proposed in Patent Document 3, to produce such a steel sheet, the rolled steel sheet is annealed and then cooled to a quenching temperature (TQ) of 220°C or higher and 330°C or lower. During cooling, a cooling rate of at least 15°C / s is required from an initial cooling temperature (TC) of at least 500°C to the quenching temperature (TQ). This results in the problem of incurring additional process costs, such as high-speed cooling equipment.

[0005] Therefore, there is a need for a hot-rolled steel sheet having high strength and excellent ductility without performing rapid cooling after hot rolling, a galvanized steel sheet containing the same, a method for manufacturing a hot-rolled steel sheet, and a method for manufacturing a galvanized steel sheet containing the same.

[0006] The objective of the present application is to provide a hot-rolled steel sheet having high strength and excellent ductility without performing rapid cooling, a galvanized steel sheet containing the same, a method for manufacturing a hot-rolled steel sheet, and a method for manufacturing a galvanized steel sheet containing the same.

[0007] To solve the above problem, the hot-rolled steel sheet of the present application comprises, in weight percent, C: greater than 0.3% and less than or equal to 0.5%, Si: greater than or equal to 1.5% and less than or equal to 2.0%, Mn: greater than or equal to 1.5% and less than or equal to 3.0%, Al: greater than or equal to 0.01% and less than or equal to 0.5%, Mo: greater than or equal to 0.001% and less than or equal to 0.15%, Nb: greater than or equal to 0.001% and less than or equal to 0.05%, and the remainder consists of Fe and other unavoidable impurities, and a martensite structure having an area fraction of 60% and less than or equal to 80% and a retained austenite structure having an area fraction of 20% and less than or equal to 40% are formed inside.

[0008] In addition, the hot-rolled steel sheet may further include one or more selected from, in weight%, P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0100%, Cr: greater than 0% and less than or equal to 1.0%, B: greater than 0% and less than or equal to 0.0030%, and Ti: greater than 0% and less than or equal to 0.01%.

[0009] In addition, the hot-rolled steel sheet may have a tensile strength of 1470 MPa or more and an elongation of 14% or more.

[0010] In addition, the galvanized steel sheet of the present application comprises the hot-rolled steel sheet; and a galvanized layer formed on the hot-rolled steel sheet.

[0011] In addition, the method for manufacturing a hot-rolled steel sheet according to the present application comprises the steps of: reheating and hot-rolling a slab comprising, in weight percent, C: greater than 0.3% and less than or equal to 0.5%, Si: greater than or equal to 1.5% and less than or equal to 2.0%, Mn: greater than or equal to 1.5% and less than or equal to 3.0%, Al: greater than or equal to 0.01% and less than or equal to 0.5%, Mo: greater than or equal to 0.001% and less than or equal to 0.15%, and Nb: greater than or equal to 0.001% and less than or equal to 0.05%, and the remainder being Fe and other unavoidable impurities; and multi-stage cooling of the hot-rolled steel sheet through a first cooling step and a second cooling step. The method includes the step of winding the cooled hot-rolled steel sheet, wherein the first cooling step is performed by water cooling and air cooling at an average cooling rate of 13 ℃ / s or more and 45 ℃ / s or less from the rolling finish temperature in the hot rolling step to 650℃, and the average cooling rate is faster than the average cooling rate of the second cooling step.

[0012] In addition, the above slab may further include one or more selected from, in weight%, P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0100%, Cr: greater than 0% and less than or equal to 1.0%, B: greater than 0% and less than or equal to 0.0030%, and Ti: greater than 0% and less than or equal to 0.01%.

[0013] In addition, the second cooling step can be performed by water cooling and air cooling at an average cooling rate of 5 ℃ / s or more and 25 ℃ / s or less from 650℃ to the winding temperature in the winding step.

[0014] In addition, in the above cooling step, the total air cooling time may be 1 second or more and 10 seconds or less.

[0015] In addition, the hot rolling step can be performed at a rolling finish temperature of 850°C or higher and 950°C or lower.

[0016] In addition, the above-mentioned winding step can be performed at a winding temperature of 500°C or higher and 600°C or lower.

[0017] In addition, the method for manufacturing a galvanized steel sheet according to the present application includes the step of, after unwinding a hot-rolled steel sheet manufactured according to the method for manufacturing a hot-rolled steel sheet, applying molten zinc plating on the hot-rolled steel sheet.

[0018] In addition, the method for manufacturing the above-mentioned galvanized steel sheet may further include a step of heat-treating the steel sheet, through which a molten zinc plating layer is formed after the above-mentioned molten zinc plating step, to form a molten zinc alloy.

[0019] According to the present application, a hot-rolled steel sheet having high strength and excellent ductility without performing rapid cooling, a galvanized steel sheet including the same, a method for manufacturing a hot-rolled steel sheet, and a method for manufacturing a galvanized steel sheet including the same can be provided.

[0020] In the description of numerical ranges in this specification, the notation “X~Y” indicates X or greater and Y or less, unless otherwise specifically stated. Additionally, “greater than or equal to” may be replaced with “greater than,” and “less than or equal to” may be replaced with “less than.”

[0021] In addition, regarding the numerical ranges described stepwise in this specification, any upper or lower limit value described in any numerical range may be substituted with an upper or lower limit value of another numerical range described stepwise, or may be substituted with a value shown in the examples.

[0022] The present application relates to hot-rolled steel sheets. Specifically, the present application relates to hot-rolled steel sheets for application to automotive parts such as structural members or frame members of an automobile. The hot-rolled steel sheets comprise, in weight percent, C: greater than 0.3% and less than or equal to 0.5%, Si: greater than or equal to 1.5% and less than or equal to 2.0%, Mn: greater than or equal to 1.5% and less than or equal to 3.0%, Al: greater than or equal to 0.01% and less than or equal to 0.5%, Mo: greater than or equal to 0.001% and less than or equal to 0.15%, and Nb: greater than or equal to 0.001% and less than or equal to 0.05%, and the remainder consists of Fe and other unavoidable impurities, and have a martensite structure having an area fraction of 60% and less than or equal to 80% and a retained austenite structure having an area fraction of 20% and less than or equal to 40% formed therein. According to the hot-rolled steel sheet of the present application, by including the aforementioned components in the aforementioned component ratios and forming a structure that satisfies the aforementioned area fraction range, it can have high strength and excellent ductility.

[0023] Specifically, the area fraction of the martensite structure formed inside the hot-rolled steel sheet may be 70% or more and 79% or less. In addition, the area fraction of the retained austenite structure formed inside the hot-rolled steel sheet may be 21% or more and 30% or less.

[0024] At this time, the martensite structure is a structure formed by the residual austenite structure in the hot-rolled steel sheet undergoing the first cooling step and the second cooling step described later, and may have a structure similar to tempered martensite due to the auto-tempering phenomenon during cooling.

[0025] The above hot-rolled steel sheet can have excellent strength as the martensite structure formed inside satisfies the aforementioned area fraction range.

[0026] The alloy composition of the above-mentioned rolled steel sheet is described below.

[0027] C: Exceeding 0.3 wt% and up to 0.5 wt%

[0028] Carbon (C) is an element that ensures sufficient strength, suppresses phase transformation of the bainite structure upon cooling, and improves the stability of the retained austenite structure. If the carbon is included in the hot-rolled steel sheet in an amount less than the lower limit of the aforementioned range, it may reduce the strength of the hot-rolled steel sheet. Additionally, if the carbon is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may exceed the desired strength. Accordingly, the carbon may be included in the hot-rolled steel sheet in an amount greater than 0.3 weight% and less than or equal to 0.5 weight%, and specifically, in an amount greater than or equal to 0.33 weight% and less than or equal to 0.4 weight%.

[0029] Si: 1.5 wt% or more, 2.0 wt% or less

[0030] Silicon (Si) is an element that suppresses the formation of carbides, thereby refining and stabilizing the austenite structure through carbon homogenization and homogenizing the structure. If the silicon is included in the hot-rolled steel sheet in an amount less than the lower limit of the aforementioned range, it fails to adequately perform the role of controlling carbides, resulting in a decrease in the austenite fraction, which may consequently lower the strength and elongation of the hot-rolled steel sheet. Furthermore, if the silicon is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may form silicon (Si)-based oxides on the surface of the hot-rolled steel sheet, thereby degrading the surface and plating properties of the steel sheet. Accordingly, the silicon may be included in the hot-rolled steel sheet in an amount of 1.5 wt% or more and 2.0 wt% or less, specifically, in an amount of 1.5 wt% or more and less than 1.8 wt%.

[0031] Mn: 1.5 wt% or more, 3.0 wt% or less

[0032] Manganese (Mn) is an element that possesses hardenability to secure the area fraction of a martensite structure, has excellent tensile strength, and avoids segregation issues that degrade ductility. In addition, the manganese is an element that stabilizes the austenite structure and strengthens solid solution; if it is included in the hot-rolled steel sheet in an amount less than the lower limit of the aforementioned range, it may be difficult to secure the area fraction of the austenite structure and strength. Furthermore, if the manganese is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may be difficult to apply it to parts due to material deterioration caused by the formation of inclusions. Therefore, the manganese may be included in the hot-rolled steel sheet in an amount of 1.5 wt% or more and 3.0 wt% or less, and specifically, in an amount of 1.8 wt% or more and 2.5 wt% or less.

[0033] Al: 0.01 wt% or more, 0.5 wt% or less

[0034] Aluminum (Al) is an element that improves the stability of the austenite structure by suppressing the formation of carbides. If the aluminum is included in the hot-rolled steel sheet in an amount less than the lower limit of the aforementioned range, it may be disadvantageous in terms of process costs. In addition, if the aluminum is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may form coarse AlN nitrides, thereby reducing the elongation of the steel sheet. Accordingly, the aluminum may be included in the hot-rolled steel sheet in an amount of 0.01 wt% or more and 0.5 wt% or less, and specifically, in an amount of 0.02 wt% or more and 0.03 wt% or less.

[0035] Mo: 0.001 wt% or more, 0.15 wt% or less

[0036] Molybdenum (Mo) is a carbide-forming element that can improve strength through precipitation strengthening. If the molybdenum is included in the hot-rolled steel sheet in an amount less than the lower limit of the aforementioned range, the effect of adding molybdenum may not be fully exerted. Furthermore, if the molybdenum is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, strength is improved, but an excessive amount of carbides is formed, which may impair the stability of the austenite structure. Accordingly, the molybdenum may be included in the hot-rolled steel sheet in an amount of 0.001 wt% or more and 0.15 wt% or less, and specifically, in an amount of 0.002 wt% or more and 0.10 wt% or less.

[0037] Nb: 0.001 wt% or more, 0.05 wt% or less

[0038] Niobium (Mo) is a carbide-forming element that can improve strength through precipitation strengthening. If the niobium is included in the hot-rolled steel sheet in an amount less than the lower limit of the aforementioned range, the effect of adding the niobium may not be fully exerted. Furthermore, if the niobium is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, strength is improved, but an excessive amount of carbides is formed, which may impair the stability of the austenite structure. Therefore, the niobium may be included in the hot-rolled steel sheet in an amount of 0.001 wt% or more and 0.05 wt% or less, and specifically, in an amount of 0.01 wt% or more and 0.04 wt% or less.

[0039] Remaining Fe and other unavoidable impurities

[0040] The aforementioned unavoidable impurities are impurities introduced during the steelmaking and manufacturing processes of hot-rolled steel sheets. Since this is widely known in the industry, a detailed description is omitted. In one embodiment of this application, the addition of elements other than the components of the hot-rolled steel sheet described above is not excluded, and various elements may be included within a scope that does not impair the technical concept of this application. If additional elements are included, they may be included to replace the remainder, which is iron (Fe).

[0041] In one example, the hot-rolled steel sheet may further include, in weight%, one or more selected from P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0100%, Cr: greater than 0% and less than or equal to 1.0%, B: greater than 0% and less than or equal to 0.0030%, and Ti: greater than 0% and less than or equal to 0.01%.

[0042] P: Greater than 0 wt% and less than or equal to 0.0200 wt%

[0043] Phosphorus (P) is an element that segregates during the manufacturing process of hot-rolled steel sheets and causes a decrease in toughness and delayed fracture. Accordingly, the phosphorus may be included in the hot-rolled steel sheet in an amount greater than 0 wt% and less than or equal to 0.0200 wt%, specifically in an amount greater than 0 wt% and less than or equal to

[0044] S: Greater than 0 wt% and less than or equal to 0.0100 wt%

[0045] Sulfur (S) is an element that reduces the toughness of hot-rolled steel sheets. Therefore, the sulfur may be included in the hot-rolled steel sheet in an amount greater than 0 wt% and less than or equal to 0.0100 wt%, specifically in an amount greater than 0 wt% and less than or equal to, respectively. On the other hand, if the sulfur is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may cause the aforementioned problems.

[0046] Cr: Greater than 0 wt% and less than or equal to 1.0 wt%

[0047] Chromium (Cr) is a carbide-forming element that can improve strength through precipitation strengthening. If the chromium is included in the hot-rolled steel sheet in an amount less than the lower limit of the aforementioned range, the effect of adding the chromium may not be fully exerted. Furthermore, if the chromium is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, strength is improved, but an excessive amount of carbides may be formed, which may impair the stability of the austenite structure. Accordingly, the chromium may be included in the hot-rolled steel sheet in an amount greater than 0 wt% and less than or equal to 1.0 wt%, and specifically, in an amount greater than 0 wt% and less than or equal to 0.8 wt%.

[0048] B: Greater than 0 wt% and less than or equal to 0.0030 wt%

[0049] Boron (B) is a carbide-forming element and can improve strength through precipitation strengthening. If the boron is included in the hot-rolled steel sheet in an amount less than the lower limit of the aforementioned range, the effect of adding boron may not be fully exerted. In addition, if the boron is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, strength is improved, but an excessive amount of carbides is formed, which may impair the stability of the austenite structure. Therefore, the boron may be included in the hot-rolled steel sheet in an amount greater than 0 wt% and less than or equal to 0.0030 wt%, and specifically, in an amount greater than 0 wt% and less than or equal to 0.0020 wt%.

[0050] Ti: Greater than 0 wt% and less than or equal to 0.01 wt%

[0051] Titanium (Ti) is a carbide-forming element that can improve strength through precipitation strengthening. If the titanium is included in the hot-rolled steel sheet in an amount less than the lower limit of the aforementioned range, the effect of adding titanium may not be fully exerted. Furthermore, if the titanium is included in the hot-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, strength is improved, but an excessive amount of carbides may be formed, which may impair the stability of the austenite structure. Therefore, the titanium may be included in the hot-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.01 weight%.

[0052] In one example, the hot-rolled steel sheet may have a tensile strength (TS) of 1,470 MPa or more and an elongation (EL) of 14% or more. Additionally, the hot-rolled steel sheet may have an upper limit of tensile strength (TS) of 2,000 MPa or less and an upper limit of elongation (EL) of 20% or less. By satisfying the aforementioned ranges for tensile strength and elongation, the hot-rolled steel sheet can improve fuel efficiency when applied to automotive parts. At this time, the tensile strength and elongation were measured through a tensile test described below.

[0053] In addition, the hot-rolled steel sheet may have a yield strength (YP) of 600 MPa or more. Since the yield strength of the hot-rolled steel sheet satisfies the aforementioned range, fuel efficiency can be improved when applied to automotive parts. At this time, the yield strength was measured through a tensile test described later.

[0054] This application also relates to galvanized steel sheets. The galvanized steel sheets relate to steel sheets including the aforementioned hot-rolled steel sheets. Since specific details regarding the hot-rolled steel sheets described below can be applied in the same way as those described for the hot-rolled steel sheets, they will be omitted.

[0055] The above galvanized steel sheet includes the above hot-rolled steel sheet and a galvanized layer formed on the above hot-rolled steel sheet.

[0056] The above zinc plating layer may be a hot-dip galvanized layer formed through galbanized iron (GI) as described below, or an alloyed hot-dip galvanized layer formed through galva-annealed iron (GA) as described below. Specifically, the above zinc plating layer may comprise iron (Fe) and aluminum (Al), and the remainder may consist of zinc (Zn) and other unavoidable impurities. More specifically, the above zinc plating layer may comprise iron (Fe) at least 0.02 wt% and 1.0 wt% and aluminum (Al) at least 0.1 wt% and 1.0 wt%, and the remainder may consist of zinc (Zn) and other unavoidable impurities. Corrosion resistance can be secured by forming the above zinc plating layer with the aforementioned composition.

[0057] The above-mentioned galvanized layer may have a thickness of 5 μm or more and 20 μm or less from the surface of the hot-rolled steel sheet. By having the aforementioned thickness, the above-mentioned galvanized layer may be advantageous in terms of corrosion resistance and surface roughness. Conversely, if the thickness of the above-mentioned galvanized layer is less than the lower limit of the aforementioned range, it may be disadvantageous in terms of corrosion resistance and surface roughness. Additionally, if the thickness of the above-mentioned galvanized layer exceeds the upper limit of the aforementioned range, it may be disadvantageous in terms of paintability and weldability.

[0058] This application also relates to a method for manufacturing a hot-rolled steel sheet. The method for manufacturing the hot-rolled steel sheet relates to a method for manufacturing the aforementioned hot-rolled steel sheet. Since specific details regarding the hot-rolled steel sheet described below can be applied in the same way as those described for the hot-rolled steel sheet, they will be omitted.

[0059] The method for manufacturing the above hot-rolled steel sheet comprises: a step of hot rolling; a step of multi-stage cooling including a first cooling step and a second cooling step; and a step of coiling. In the multi-stage cooling step, the first cooling step is performed by water cooling and air cooling at an average cooling rate of 13 ℃ / s or more and 45 ℃ / s or less from the rolling finish temperature mentioned above in the hot rolling step to 650℃.

[0060] In one example, a hot-rolled steel sheet manufactured according to the method for manufacturing the hot-rolled steel sheet described above may homogeneously form a martensite structure having an area fraction of 60% or more and 80% or less and a retained austenite structure having an area fraction of 20% or more and 40% or less within the sheet, and may suppress the formation of ferrite, pearlite, and bainite structures within the hot-rolled steel sheet, thereby possessing high strength and excellent ductility. On the other hand, if the first cooling step is performed with water cooling below the lower limit of the average cooling rate range described above from the aforementioned temperature to the aforementioned temperature, the aforementioned effect cannot be achieved. Furthermore, if the first cooling step is performed with water cooling exceeding the upper limit of the average cooling rate range described above from the aforementioned temperature to the aforementioned temperature, a problem of increased process costs arises. A detailed explanation regarding the area fraction of each structure formed in the hot-rolled steel sheet is the same as that described in the hot-rolled steel sheet, so it will be omitted.

[0061] The above hot rolling step is a step for manufacturing a slab into a hot-rolled steel sheet, and is performed by reheating and then hot-rolling a slab comprising, in weight percent, C: greater than 0.3% and less than or equal to 0.5%, Si: greater than or equal to 1.5% and less than or equal to 2.0%, Mn: greater than or equal to 1.5% and less than or equal to 3.0%, Al: greater than or equal to 0.01% and less than or equal to 0.5%, Mo: greater than or equal to 0.001% and less than or equal to 0.15%, Nb: greater than or equal to 0.001% and less than or equal to 0.05%, and the remainder being Fe and other unavoidable impurities. Since a specific description of the composition of the above slab is the same as that described in the above hot-rolled steel sheet, it will be omitted.

[0062] In one example, the slab may further include, in weight percent, one or more selected from P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0100%, Cr: greater than 0% and less than or equal to 1.0%, B: greater than 0% and less than or equal to 0.0030%, and Ti: greater than 0% and less than or equal to 0.01%. Since a specific description of the additional components further included in the slab is the same as that described in the hot-rolled steel sheet above, it will be omitted.

[0063] In the above hot rolling step, reheating the slab brings the slab to a temperature at which it can be rolled, and this is performed before hot rolling the slab. The reheating temperature of the slab is not particularly limited, but, for example, it may be between 1200°C and 1250°C. By reheating the slab within the aforementioned temperature range, the casting structure is destroyed, and alloy elements and precipitates segregated during casting can be re-dissolved. Conversely, if the reheating temperature of the slab is below the lower limit of the aforementioned range, it is not easy to perform hot rolling, and if it exceeds the upper limit of the aforementioned range, severe oxidation of the slab surface may occur, causing rolling defects.

[0064] As described above, the hot rolling is performed to manufacture the reheated slab into a hot-rolled steel sheet. For example, the rolling finish temperature during the hot rolling may be 800°C or higher and 950°C or lower. If the rolling finish temperature during the hot rolling is below the lower limit of the aforementioned range, the rolling load increases, which may lead to a decrease in productivity. Furthermore, if the rolling finish temperature during the hot rolling exceeds the upper limit of the aforementioned range, it may cause grain coarsening, which may lead to a decrease in strength. Therefore, the rolling finish temperature during the hot rolling may satisfy the aforementioned range.

[0065] In addition, the reduction rate during the hot rolling above may be 93% or more. The upper limit of the reduction rate may be less than 100%. At this time, the thickness of the hot-rolled steel sheet may be 1 mm or more and 30 mm or less. If the reduction rate during the hot rolling above is less than the lower limit of the aforementioned range, the homogeneity of the microstructure may decrease. In addition, if the reduction rate during the hot rolling above exceeds the upper limit of the aforementioned range, the equipment load may increase. Therefore, the reduction rate during the hot rolling above may satisfy the aforementioned range.

[0066] The above multi-stage cooling step is a step of cooling a hot-rolled steel sheet through a first cooling step and a second cooling step. Specifically, the above multi-stage cooling step can be performed by sequentially carrying out water cooling and air cooling using a laminar flow, which is a cooling facility, on a run-out table while the hot-rolled steel sheet is being transferred to a coiler for winding, in order to comply with cooling history control to secure the desired material of the transferred hot-rolled steel sheet strip. At this time, since the specific description of the above first cooling step is the same as previously described, it will be omitted.

[0067] The above secondary cooling step is a step of cooling the hot-rolled steel sheet, which has been cooled in the first step, a second time through water cooling and air cooling. For example, the above secondary cooling step may be performed by water cooling and air cooling at an average cooling rate of 5 ℃ / s or more and 25 ℃ / s or less from 650℃ to the coiling temperature in the coiling step. By performing water cooling from the aforementioned temperature to the aforementioned temperature within the aforementioned average cooling rate range in the above secondary cooling step, the temperature rise of the hot-rolled steel sheet caused by hot rolling can be suppressed, thereby controlling the phase transformation. If the above secondary cooling step is performed at a rate below the lower limit of the aforementioned average cooling rate range from the aforementioned temperature to the aforementioned temperature, the aforementioned effect cannot be achieved. Furthermore, if the above secondary cooling step is performed at a rate exceeding the upper limit of the aforementioned average cooling rate range from the aforementioned temperature to the aforementioned temperature, a problem of increased process costs arises.

[0068] In one example, the total air cooling time in the cooling step may be 1 second or more and 10 seconds or less, specifically 2 seconds or more and 8 seconds or less. By satisfying the aforementioned time range for the total air cooling time in the cooling step, the formation of ferrite structure, pearlite structure, and bainite structure in the hot-rolled steel sheet can be suppressed.

[0069] The above-mentioned winding step is a step of winding the cooled hot-rolled steel sheet into a coil shape after going through a cooling step.

[0070] In one example, the coiling step may be performed by coiling the hot-rolled steel sheet obtained by the hot rolling at a coiling temperature of 500°C or higher and 600°C or lower. If the coiling temperature of the hot-rolled steel sheet is below the lower limit of the aforementioned range, the hot-rolled steel sheet can secure excellent elongation through the control of the hardness of the martensite structure. In addition, if the coiling temperature of the hot-rolled steel sheet exceeds the upper limit of the aforementioned range, the strength of the hot-rolled steel sheet may be reduced. Therefore, the coiling temperature of the hot-rolled steel sheet may satisfy the aforementioned range.

[0071] The present application also relates to a method for manufacturing a galvanized steel sheet. The method for manufacturing the galvanized steel sheet relates to a method for manufacturing a galvanized steel sheet comprising the aforementioned method for manufacturing a hot-rolled steel sheet. Since specific details regarding the method for manufacturing the hot-rolled steel sheet described below can be applied identically to the content described in the method for manufacturing the hot-rolled steel sheet, such details will be omitted.

[0072] The method for manufacturing the above-mentioned galvanized steel sheet includes a step of hot-dip galvanizing.

[0073] The above-mentioned hot-dip galvanizing step is a step of forming a galvanized layer on a hot-rolled steel sheet, and after unrolling the hot-rolled steel sheet manufactured according to the above-mentioned method for manufacturing a hot-rolled steel sheet, hot-dip galvanizing can be performed on the hot-rolled steel sheet.

[0074] Specifically, the hot-dip galvanizing step can be performed by immersing the aforementioned hot-rolled steel sheet in a hot-dip galvanizing bath containing a zinc-based plating composition to form a hot-dip galvanizing layer on the hot-rolled steel sheet, specifically on each of the two sides of the hot-rolled steel sheet. At this time, the hot-dip galvanizing step can be performed at a temperature of 450°C or higher and 550°C or lower. By performing the hot-dip galvanizing step within the aforementioned temperature range, the corrosion resistance of the manufactured galvanized steel sheet can be secured.

[0075] The zinc-based plating composition contained in the above molten zinc plating bath may consist of iron (Fe) and aluminum (Al), and the remainder being zinc (Zn) and other unavoidable impurities. Specifically, it may consist of iron (Fe) at least 0.02 wt% and aluminum (Al) at least 0.1 wt% and 1.0 wt%, and the remainder being zinc (Zn) and other unavoidable impurities.

[0076] In one example, the method for manufacturing the above-mentioned galvanized steel sheet may further include a step of molten zinc alloying. The molten zinc alloying step is a step of alloying the molten zinc plating layer, and can be performed by heat-treating the steel sheet in which the molten zinc plating layer is formed through the molten zinc plating step, thereby alloying the steel sheet and zinc by thermal diffusion. For example, the molten zinc alloying step may be performed at a temperature of 470°C or higher and 650°C or lower. By performing the molten zinc alloying step within the aforementioned temperature range, a galvanized steel sheet having excellent corrosion resistance and excellent adhesion to paint can be manufactured.

[0077] Subsequently, the alloyed hot-dip galvanized layer formed through the above-mentioned molten zinc alloying step can be formed as an alloyed layer of iron and zinc with a thickness of 3 μm or more and 30 μm or less. Subsequently, the alloyed hot-dip galvanized steel sheet formed through the above-mentioned molten zinc alloying step can be cooled to room temperature, specifically from 15°C to 25°C.

[0078]

[0079] The present application will be described in more detail below through embodiments according to the present application and comparative examples not according to the present application, but the scope of the present application is not limited by the embodiments presented below.

[0080]

[0081] Preparation Examples 1 to 8

[0082] Preparation of the slab

[0083] A slab was prepared consisting of the components shown in Table 1 below, the remainder being Fe and other unavoidable impurities.

[0084] Alloy composition of slab (weight%) CSI Mn PSCr AlMo Nb BTI Preparation Example 10.35 1.76 2.00.00 950.00 300.50.04 0.10.01 0.00 50 Preparation Example 20.34 1.75 2.00.00 94 0.00 200.50.03 0.10.04 0.00 50 Preparation Example 30.35 1.50 2.00.01 000.00 300.50.05 0.10.04 0.00 50 Preparation Example 40.36 1.69 1.50.01 200.00 160.00 70.02 0.00 40.02 0.00 200 Preparation Example 50.381.502.00.01040.00300.80.030.0040.010.00100Preparation Yes 60.351.801.20.00940.00100.40.050.0040.020.00180Preparation Yes 70.181.752.70.01040.001200.04000.00220Preparation Yes 80.251.842.230.01500.001900.04000.00050Preparation Yes 90.300.241.50.01360.001200.0500.030.00220.05Preparation Yes 100.191.73.00.01200.00300.50.040.0300.00250.02

[0085]

[0086] Example 1

[0087] manufacturing of hot-rolled steel sheets

[0088] As shown in Table 2 below, the slab of Preparation Example 1 was reheated at a temperature of 1200°C, and then hot-rolled at a rolling start temperature of 1200°C and a rolling finish temperature of 900°C with a reduction rate of 93.3% to produce a hot-rolled steel sheet with a thickness of 2.4 mm.

[0089] Subsequently, a first cooling step was performed in which the hot-rolled steel sheet was water-cooled and air-cooled from the rolling finish temperature (FDT) to 650°C at an average cooling rate of 14.7°C / s, and a second cooling step was performed in which the sheet was water-cooled and air-cooled from 650°C to the coiling temperature at an average cooling rate of 12.0°C / s. At this time, the total air cooling time in the first cooling step and the second cooling step is 2.22 seconds.

[0090] Afterwards, the hot-rolled steel sheet was manufactured by winding it at a winding temperature of 500℃.

[0091]

[0092] Examples 2 to 13 and Comparative Examples 1 to 9

[0093] manufacturing of hot-rolled steel sheets

[0094] A hot-rolled steel sheet was manufactured in the same manner as in Example 1, except that the type of slab and manufacturing conditions were changed to the conditions shown in Table 2 below.

[0095] Slab Hot Rolling Cooling Coiling Temperature (°C) Reheating Temperature (°C) Rolling Finish Temperature (°C) Reduction Rate (%) Average Cooling Rate of 1st Cooling Stage (°C / s) Average Cooling Rate of 2nd Cooling Stage (°C / s) Total Air Cooling Time in Cooling Stage (Seconds) Example 1 Preparatory Example 1 1 200 900 9 3.3% 14.7 12.0 2.2 2500 Example 2 Preparatory Example 1 1 200 900 9 3.3% 14.7 6.3 2.2 2600 Example 3 Preparatory Example 2 1 200 900 9 3.3% 14.7 12.0 3.1 3500 Example 4 Preparatory Example 2 1 200 900 9 3.3% 14.7 6.3 3.1 3600 Example 5 Preparatory Example 3120095094.2%25.812.55.73530 Example 6 Pre-example 3125085094.2%25.87.55.73550 Example 7 Pre-example 3125093094.2%25.86.35.73600 Example 8 Pre-example 4120087094.7%42.06.66.68560 Example 9 Pre-example 4125087094.2%42.012.56.80530 Example 10 Pre-example 4120087094.7%42.05.16.95560 Example 11 Pre-example 4120087094.7%42.05.17.14560 Example 12 Preparation Example 4 12008709 4.7% 42.05.17.35560 Example 13 Preparation Example 5 12008709 4.7% 42.05.17.35560 Comparative Example 1 Preparation Example 6 12008709 4.7% 14.7 12.02.22500 Comparative Example 2 Preparation Example 7 12008709 4.7% 14.7 12.02.22500 Comparative Example 3 Preparation Example 8 12008709 4.7% 14.7 6.32.22600 Comparative Example 4 Preparation Example 9 12008709 4.7% 14.7 9.02.22560 Comparative Example 5 Preparation Example 10120087094.7%14.712.52.22530Comparative Example 6 Pre-Example 1120090094.7%14.76.32.22630Comparative Example 7 Pre-Example 2120090094.7%15.015.015.00550Comparative Example 8 Pre-Example 3120090094.7%5.03.02.22550Comparative Example 9 Pre-Example 4120090094.7%5.03.015.00550

[0096]

[0097] Evaluation Example 1. Evaluation of tissue area fraction

[0098] For the hot-rolled steel sheets produced in each of the examples and comparative examples, the area fraction of the internal structure was evaluated using a scanning electron microscope (SEM), and the results are shown in Table 3 below. At this time, the evaluation using the scanning electron microscope was performed under an acceleration voltage of 20 kV.

[0099]

[0100] Evaluation Example 2. Tensile Test Evaluation

[0101] Tensile tests were performed on the hot-rolled steel sheets manufactured in each of the examples and comparative examples according to ASTM-E8, and the yield strength (YP), tensile strength (TS), and elongation (EL) were measured, and the results are shown in Table 3 below.

[0102] Area fraction of structure (%) Tensile properties MFRABP Yield strength (MPa) Tensile strength (MPa) Elongation (%) Example 1 79.50 21.50 80 2150 814.8 Example 2 77.60 22.40 863 150 217.0 Example 3 78.00 22.00 866 147 215.3 Example 4 78.30 21.70 794 148 316.9 Example 5 79.40 21.60 800 1530 18.4 Example 6 77.80 22.200 843 1500 17.4 Example 7 77.70 22.300 863 147 715.8 Example 875.8024.200697147218.2 Example 976.6023.400789150218.2 Example 1077.0023.000800153018.4 Example 1178.5021.500729150015.6 Example 1278.8021.200908147818.3 Example 1378.8021.2001018148017.2 Comparative Example 186.9013.100766114711.8 Comparative Example 2023.5076.50121814609.6 Comparative Example 320.235.80045.051997417.3 Comparative Example 4045.80054.251968024.5 Comparative Example 585.79.011.300702125010.2 Comparative Example 665.304.210.220.3778146411.3 Comparative Example 773.600026.4786116912.9 Comparative Example 865.6010.1024.3794110510.8 Comparative Example 953.35.23.1038.4881106110.2 M: Martensitic structure F: Ferritic structure RA: Retained austenitic structure B: Bainitic structure P: Pearlitic structure

[0103] As shown in Tables 1 to 3 above, it was confirmed that the hot-rolled steel sheets produced in each of Examples 1 to 13 satisfy a specific composition ratio and specific manufacturing conditions, and unlike the hot-rolled steel sheets produced in each of Comparative Examples 1 to 9, they consist of a martensite structure and a retained austenite structure with an area fraction satisfying a specific range.

[0104] Specifically, it was confirmed that the hot-rolled steel sheet produced in Comparative Example 1 had a Mn content below the lower limit of a specific range, so the austenite structure was unstable and the area fraction of the residual austenite structure in the final structure was below the lower limit of a specific range, and as a result, the elongation and tensile strength did not satisfy the specific range.

[0105] In addition, it was confirmed that the hot-rolled steel sheets produced in Comparative Examples 2, 3, and 5 each had an austenite structure that was unstable because C was below the lower limit of a specific range, so the area fraction of the residual austenite structure in the final structure was below the lower limit of a specific range, and the area fraction of the martensite structure in the final structure was below the lower limit of a specific range or above the upper limit due to insufficient hardenability, and as a result, the elongation and tensile strength did not satisfy the specific range.

[0106] In addition, it was confirmed that the hot-rolled steel sheet produced in Comparative Example 4 had a Si content below the lower limit of a specific range, so an excessive amount of pearlite structure was generated within the structure of the hot-rolled steel sheet, and as a result, the elongation and tensile strength did not satisfy a specific range.

[0107] In addition, it was confirmed that the hot-rolled steel sheet produced in Comparative Example 6 did not satisfy the specific range because, during production, the coiling temperature exceeded the upper limit of a specific range, causing the residual austenite structure to decompose and the area fraction of the bainite structure and pearlite structure to increase.

[0108] In addition, it was confirmed that the hot-rolled steel sheet produced in Comparative Example 7 did not satisfy the specific range because, during production, the average cooling rate of the first cooling stage was the same as the average cooling rate of the second cooling stage and the total air cooling time during the cooling stage exceeded the upper limit of a specific range, so the area fraction of the residual austenite structure in the hot-rolled steel sheet was below the lower limit of a specific range and the pearlite structure was excessively generated.

[0109] In addition, it was confirmed that the hot-rolled steel sheet produced in Comparative Example 8 did not satisfy the specific range because the average cooling rate in the first cooling stage during production was below the lower limit of a specific range, the area fraction of the residual austenite structure in the hot-rolled steel sheet was below the lower limit of a specific range, and the pearlite structure was excessively generated.

[0110] In addition, it was confirmed that the hot-rolled steel sheet produced in Comparative Example 9 did not satisfy the specific range because, during production, the average cooling rate in the first cooling stage was below the lower limit of a specific range and the total air cooling time in the cooling stage exceeded a specific range, so the area fraction of the martensite structure in the hot-rolled steel sheet was below the lower limit of a specific range, the area fraction of the retained austenite structure was below the lower limit of a specific range, and the pearlite structure was excessively generated.

[0111] As a result, it was confirmed that the hot-rolled steel sheets produced in each of Examples 1 to 13, unlike the hot-rolled steel sheets produced in each of Comparative Examples 1 to 9, have excellent physical properties in which yield strength, tensile strength, and elongation simultaneously satisfy specific ranges.

Claims

1. In wt%, it comprises C: greater than 0.3% and less than or equal to 0.5%, Si: greater than or equal to 1.5% and less than or equal to 2.0%, Mn: greater than or equal to 1.5% and less than or equal to 3.0%, Al: greater than or equal to 0.01% and less than or equal to 0.5%, Mo: greater than or equal to 0.001% and less than or equal to 0.15%, and Nb: greater than or equal to 0.001% and less than or equal to 0.05%, and the remainder consists of Fe and other unavoidable impurities, and Hot-rolled steel sheet having a martensitic structure with an area fraction of 60% or more and 80% or less and a retained austenitic structure with an area fraction of 20% or more and 40% or less formed inside.

2. In Paragraph 1, A hot-rolled steel sheet further comprising, in weight%, one or more selected from P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0100%, Cr: greater than 0% and less than or equal to 1.0%, B: greater than 0% and less than or equal to 0.0030%, and Ti: greater than 0% and less than or equal to 0.01%.

3. In Paragraph 1, Hot-rolled steel sheet having a tensile strength of 1470 MPa or higher and an elongation of 14% or higher.

4. Hot-rolled steel sheet according to any one of claims 1 to 3; and A galvanized steel sheet comprising a galvanized layer formed on the above hot-rolled steel sheet.

5. A step of reheating and then hot-rolling a slab comprising, in weight%, C: greater than 0.3% and less than or equal to 0.5%, Si: greater than or equal to 1.5% and less than or equal to 2.0%, Mn: greater than or equal to 1.5% and less than or equal to 3.0%, Al: greater than or equal to 0.01% and less than or equal to 0.5%, Mo: greater than or equal to 0.001% and less than or equal to 0.15%, Nb: greater than or equal to 0.001% and less than or equal to 0.05%, and the remainder being Fe and other unavoidable impurities; A step of multi-stage cooling of a hot-rolled steel sheet through a first cooling step and a second cooling step; and It includes the step of winding the cooled hot-rolled steel sheet, A method for manufacturing a hot-rolled steel sheet, wherein the first cooling step has an average cooling rate faster than the average cooling rate of the second cooling step, and is performed by water cooling and air cooling at an average cooling rate of 13 ℃ / s or more and 45 ℃ / s or less from the rolling finish temperature in the hot rolling step to 650℃.

6. In Paragraph 5, A method for manufacturing a hot-rolled steel sheet, wherein the above slab further comprises, in weight%, one or more selected from P: greater than 0% and less than or equal to 0.0200%, S: greater than 0% and less than or equal to 0.0100%, Cr: greater than 0% and less than or equal to 1.0%, B: greater than 0% and less than or equal to 0.0030%, and Ti: greater than 0% and less than or equal to 0.01%.

7. In Paragraph 5, A method for manufacturing a hot-rolled steel sheet, wherein the above second cooling step is performed by water cooling and air cooling at an average cooling rate of 5 ℃ / s or more and 25 ℃ / s or less from 650℃ to the coiling temperature in the above coiling step.

8. In Paragraph 7, A method for manufacturing a hot-rolled steel sheet in which the total air cooling time in the above cooling step is 1 second or more and 10 seconds or less.

9. In Paragraph 5, A method for manufacturing a hot-rolled steel sheet in which the above hot-rolling step is performed at a rolling finishing temperature of 850°C or higher and 950°C or lower.

10. In Paragraph 5, A method for manufacturing hot-rolled steel sheets in which the above-mentioned coiling step is performed at a coiling temperature of 500°C or higher and 600°C or lower.

11. A method for manufacturing a galvanized steel sheet comprising the step of, after unwinding a hot-rolled steel sheet manufactured according to any one of claims 5 to 10, applying hot-dip galvanizing to the hot-rolled steel sheet.

12. In Paragraph 11, A method for manufacturing a galvanized steel sheet, further comprising the step of heat-treating a steel sheet in which a molten zinc plating layer is formed through the above-mentioned molten zinc plating step to form a molten zinc alloy.