Hot-rolled steel sheet and hot-rolled plated steel sheet with high-strength, and method of manufacturing same

Abstract

Disclosed are hot-rolled steel sheet and hot-rolled plated steel sheet with high-strength, and a manufacturing method thereof, the hot-rolled steel sheet and hot-rolled plated steel sheet being high strength to be suitable as material for high-strength structural supports, and allowing reduction in the rolling load of hot-rolling finish. A hot-rolled steel sheet according to the present invention contains, in wt%, 0.03-0.1% of C, 0.1% or less of Si, 0.8-1.4% of Mn, 0.03-0.08% of Ti, 0.02% or less of P, 0.02% or less of S, and 0.01% or less of N, with the remainder comprising Fe and inevitable impurities, and satisfies relational expression 1. Relational expression 1: 3 ⅹ ([Mn] - 1.2)2 + 1200 ⅹ ([Ti] - 0.06)2 + 0.0004 ⅹ (coiling temperature (°C) - 630)2 ≤ 1.0 (In relational expression 1, [ ] indicates the wt% of each element.)

Description

High-strength hot-rolled steel sheet, high-strength hot-rolled galvanized steel sheet, and manufacturing method thereof

[0001] The present invention relates to a high-strength hot-rolled steel sheet, a high-strength hot-rolled plated steel sheet, and a method for manufacturing the same, which have high strength suitable as a high-strength support structural material and can reduce hot-rolling finishing rolling load.

[0002] High-strength hot-rolled steel sheets and hot-rolled galvanized steel sheets are mainly used as supporting structural materials.

[0003] In particular, high-strength hot-rolled galvanized steel sheets have excellent deformation resistance and corrosion resistance, and are more economical than cold-rolled galvanized steel sheets, so they are used in various ways as materials for strong construction materials such as construction scaffolding, vinyl house structural materials, and solar power supports.

[0004] In the case of steel used as solar power structures, the demand for variable support structures that follow the sun in all directions, up, down, left, and right, rather than being fixed, is rapidly increasing to secure maximum power production for the installation cost, and thus the market demand for high-strength steel is increasing.

[0005] Meanwhile, finish rolling is the final hot rolling process that applies pressure to the steel sheet to adjust the desired thickness. It is crucial to ensure even thickness at the center and both edges of the sheet. The material's inherent high-temperature deformation resistance significantly influences the rolling load during finish hot rolling, and subsequent rolling loads significantly impact the product's dimensional and shape quality. Generally, the rolling load increases as the strength of the steel increases. However, high rolling loads can lead to decreased productivity due to frequent rolling roll replacement and poor surface finish.

[0006] Therefore, there is an urgent need to develop hot-rolled steel sheets and hot-rolled galvanized steel sheets that have high strength properties suitable as support structural materials while reducing the finishing rolling load and increasing productivity.

[0007] The purpose of the present invention is to provide a high-strength hot-rolled steel sheet, a high-strength hot-rolled plated steel sheet, and a method for manufacturing the same, which have suitable high strength as a high-strength support structural material and can reduce hot-rolling finishing rolling load.

[0008] The purpose of the present invention is to provide a high-strength hot-rolled steel sheet having excellent yield strength, tensile strength and elongation, a high-strength hot-rolled galvanized steel sheet and a method for manufacturing the same.

[0009] The objectives of the present invention are not limited to those mentioned above. Other objectives and advantages of the present invention not mentioned above can be understood through the following description and will be more clearly understood through the embodiments of the present invention. Furthermore, it will be readily apparent that the objectives and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims.

[0010] The hot-rolled steel sheet according to the present invention contains, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.8 to 1.4%, Ti: 0.03 to 0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder Fe, and other unavoidable impurities, and is characterized by satisfying the following relational expression 1.

[0011] Relationship 1: 3 x ([Mn] - 1.2) 2 + 1200 ⅹ ([Ti] - 0.06) 2 + 0.0004 x (coiling temperature (℃) - 630) 2 ≤ 1.0 (In the above equation 1, [ ] represents the weight% of each element.)

[0012] For the total 100 area%, it may contain 90 area% or more of ferrite, and the remainder may contain one or more of pearlite, bainite, and martensite.

[0013] The average size of the above ferrite crystal grains may be 15㎛ or less.

[0014] The above hot-rolled steel sheet contains carbides, the average size of the carbides is 8 nm or less, and the carbide distribution is 10,000 nm. 2 There may be more than 4.0 per area.

[0015] The yield strength may be 450 MPa or more, the tensile strength may be 540 MPa or more, and the elongation may be 16% or more.

[0016]

[0017] A hot-rolled steel sheet according to another embodiment of the present invention is characterized by containing, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.8 to 1.4%, Ti: 0.03 to 0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder being Fe and other unavoidable impurities.

[0018]

[0019] A hot-rolled galvanized steel sheet according to the present invention comprises a base steel sheet; and a plating layer disposed on at least one surface of the base steel sheet; wherein the base steel sheet comprises the hot-rolled steel sheet described above.

[0020] The above plating layer may include any one of zinc, aluminum, zinc alloy, and aluminum alloy.

[0021]

[0022] The method for manufacturing a hot-rolled steel sheet according to the present invention comprises the steps of (a) reheating a steel slab containing, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.8 to 1.4%, Ti: 0.03 to 0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder being Fe and other unavoidable impurities; (b) hot-rolling the steel slab; and (c) cooling and coiling the hot-rolled hot-rolled steel sheet; and is characterized in that it satisfies the following relational expression 1.

[0023] Relationship 1: 3 x ([Mn] - 1.2)2 + 1200 ⅹ ([Ti] - 0.06) 2 + 0.0004 x (coiling temperature (℃) - 630) 2 ≤ 1.0 (In the above equation 1, [ ] represents the weight% of each element.)

[0024] In the above step (a), reheating can be performed at 1150 to 1300°C.

[0025] In the above step (b), the steel slab can be hot rolled at a finishing rolling temperature of 800 to 1000°C.

[0026] In the above step (c), the hot-rolled steel sheet can be cooled to 570 to 680°C and then coiled.

[0027]

[0028] A method for manufacturing a hot-rolled steel sheet according to another embodiment of the present invention comprises the steps of: (a) reheating a steel slab containing, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.8 to 1.4%, Ti: 0.03 to 0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder being Fe and other unavoidable impurities; (b) hot-rolling the steel slab; and (c) cooling and coiling the hot-rolled hot-rolled steel sheet.

[0029]

[0030] The method for manufacturing a hot-rolled galvanized steel sheet according to the present invention includes the method for manufacturing the hot-rolled steel sheet, and is characterized by including, after the coiling step, a step of heat-treating the coiled hot-rolled steel sheet; and a step of forming a plating layer on at least one surface of the hot-rolled steel sheet.

[0031] The above heat treatment step can be performed at 450 to 750°C.

[0032] The step of forming the above plating layer can be performed by any one of a hot-dip plating method, an electroplating method, and a plasma method.

[0033] The high-strength hot-rolled steel sheet, the high-strength hot-rolled galvanized steel sheet and the manufacturing method thereof according to the present invention are high-strength support structural materials, and have the effect of reducing the hot-rolling finishing rolling load while having suitable high strength.

[0034] In terms of high strength, it has excellent effects on yield strength, tensile strength and elongation.

[0035] In addition to the effects described above, specific effects of the present invention are described below while explaining specific details for carrying out the invention.

[0036] Figure 1 is a TEM carbide analysis photograph of Invention Example 1 according to the present invention.

[0037] The above-described objects, features, and advantages will be described in detail below with reference to the accompanying drawings, so that those skilled in the art can easily practice the technical idea of ​​the present invention. In describing the present invention, if it is determined that a detailed description of known technologies related to the present invention may unnecessarily obscure the gist of the present invention, a detailed description thereof will be omitted. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to indicate the same or similar components.

[0038] Hereinafter, the phrase "any configuration is placed on (or below)" a component or "on (or below)" a component may mean that any configuration is placed in contact with the upper surface (or lower surface) of said component, and that other configurations may be interposed between said component and any configuration placed on (or below) said component.

[0039] Additionally, when it is described that a component is "connected," "coupled," or "connected" to another component, it should be understood that the components may be directly connected or connected to one another, but that other components may also be "interposed" between the components, or that each component may be "connected," "coupled," or "connected" through another component.

[0040] Hereinafter, high-strength hot-rolled steel sheets, high-strength hot-rolled plated steel sheets, and manufacturing methods thereof according to some embodiments of the present invention will be described.

[0041] The hot-rolled steel sheet according to the present invention can exhibit high strength suitable for use as a high-strength support structural material such as a building scaffold, a vinyl house structural material, a solar power support, etc.

[0042] The hot-rolled steel sheet according to the present invention contains, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.8 to 1.4%, Ti: 0.03 to 0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder Fe, and other unavoidable impurities, and is characterized by satisfying the following relational expression 1.

[0043] Relationship 1: 3 x ([Mn] - 1.2) 2 + 1200 ⅹ ([Ti] - 0.06) 2 + 0.0004 x (coiling temperature (℃) - 630) 2 ≤ 1.0 (In the above equation 1, [ ] represents the weight% of each element.)

[0044] Hereinafter, the steel composition and composition ratio included in the hot-rolled steel sheet with excellent strength according to an embodiment of the present invention will be described in more detail. Hereinafter, unless otherwise specified, the percentage indicating the content of each element is based on weight.

[0045] In the composition ratio of the present invention, 0.x0% is written as 0.x%, and 0.0x0% is written as 0.0x%.

[0046] Carbon (C): 0.03 to 0.1%

[0047] Carbon (C) is added not only for the effect of strengthening the solid solution but also to form carbides with titanium, and is an element for securing tensile strength. To achieve these effects, it is desirable to add at least 0.03 wt%.

[0048] However, if the amount of carbon (C) added is excessive, exceeding 0.1 wt%, carbide coarsening occurs, making it impossible to sufficiently secure the precipitation strengthening effect, and the pearlite fraction in the microstructure increases, making it impossible to secure a ferrite structure with an area ratio of 90% or more, which is intended for the present invention. In addition, during continuous casting, surface cracks may occur due to the hypo-peritectic region, which typically has a carbon content of 0.07 to 0.13 wt%, so it is necessary to limit the carbon content to 0.1 wt% or less for the soundness of the material. In the hypo-peritectic region, peritectic refers to a phenomenon in which the original crystal is surrounded by another crystal.

[0049] On the other hand, when the carbon content is less than 0.03%, there is a problem of insufficient strength as there is less carbon to form TiC carbide.

[0050] Therefore, carbon (C) can be added in a content ratio of 0.03 to 0.1 wt% of the total weight of the hot-rolled steel sheet according to the present invention, and preferably can be added in a content ratio of 0.04 to 0.08 wt%. When carbon (C) satisfies a content ratio of 0.04 to 0.08 wt%, TiC precipitation is sufficiently induced, and cementite formation of the remaining carbon (C) is further reduced, so that the elongation of the material can be further improved.

[0051] Silicon (Si): 0.1% or less

[0052] Silicon (Si) is not only useful for deoxidizing steel, but also effectively enhances strength through solid solution strengthening. However, when the silicon (Si) content exceeds 0.1 wt%, it forms silicon oxide, making plating difficult.

[0053] Therefore, the content of silicon (Si) may be 0.1 wt% or less, preferably may be added at a content ratio of 0.001 to 0.1 wt%, and more preferably may be added at a content ratio of 0.04 to 0.09 wt%. When silicon (Si) satisfies a content ratio of 0.04 to 0.09 wt%, there is an effect of improving surface quality by suppressing the generation of blisters on the surface of the steel during hot rolling.

[0054] Manganese (Mn): 0.8 ~ 1.4%

[0055] Manganese (Mn) is added to enhance the hardening effect and ensure the hardenability of the weld during cooling after welding. To achieve these effects, manganese (Mn) may be added in amounts exceeding 0.8 wt%. However, if manganese (Mn) is added in large amounts exceeding 1.4 wt%, the hardenability due to Mn increases, making it difficult to obtain the desired ferrite structure of 90% or more due to the appearance of low-temperature structures.

[0056] Therefore, manganese (Mn) can be added in a content ratio of 0.8 to 1.4 wt% of the total weight of the hot-rolled steel sheet according to the present invention, and preferably can be added in a content ratio of 1.05 to 1.35 wt%. When manganese (Mn) satisfies a content ratio of 1.05 to 1.35 wt%, there is a sufficient solid solution strengthening effect, and the appearance of low-temperature structures is suppressed, which is advantageous in securing the material.

[0057] Titanium (Ti): 0.03 to 0.08%

[0058] Titanium (Ti) is added to enhance precipitation strengthening and suppress grain coarsening.

[0059] If the amount of titanium (Ti) added is less than 0.03 wt%, it is difficult to obtain the high strength targeted in the present invention, and if it exceeds 0.08 wt%, coarse carbides are formed, making precipitation strengthening ineffective.

[0060] Therefore, titanium (Ti) can be added in a content ratio of 0.03 to 0.08 wt% of the total weight of the hot-rolled steel sheet according to the present invention, and preferably can be added in a content ratio of 0.045 to 0.075 wt%. When titanium (Ti) satisfies a content ratio of 0.045 to 0.075 wt%, higher strength and elongation can be secured through well-distributed TiC precipitation in a smaller size.

[0061] Phosphorus (P): 0.02% or less

[0062] Phosphorus (P) is an impurity element that segregates at grain boundaries and reduces toughness, so it is desirable to avoid including it if possible.

[0063] Therefore, phosphorus (P) can be limited to 0.02 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention, and preferably limited to 0.018 wt% or less. When phosphorus (P) satisfies a content ratio of 0.018 wt% or less, it is advantageous for securing impact properties.

[0064] Sulfur (S): 0.02% or less

[0065] Sulfur (S) is an impurity element and is the main element that forms MnS, and the formation of coarse MnS can reduce toughness.

[0066] Therefore, sulfur (S) can be limited to 0.02 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention, and preferably limited to 0.01 wt% or less. When sulfur (S) satisfies a content ratio of 0.01 wt% or less, it is advantageous for securing impact properties.

[0067] Nitrogen (N): 0.01% or less

[0068] Nitrogen (N) is an impurity element, and when its content exceeds 0.01 wt%, it reacts with Ti at high temperatures to form nitrides, which reduces the content of Ti that actually contributes to precipitation strengthening, thus lowering the strength of the steel sheet.

[0069] Therefore, nitrogen (N) can be limited to 0.01 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention, and preferably limited to 0.0070 wt% or less. When nitrogen (N) satisfies a content ratio of 0.0070 wt% or less, coarse TiN formation is slightly suppressed, which is advantageous for improving strength.

[0070] Additionally, the high-strength hot-rolled steel sheet according to an embodiment of the present invention may contain iron and other unavoidable impurities in addition to the aforementioned steel composition. Unavoidable impurities can be unintentionally introduced during the typical steelmaking process, and cannot be completely excluded. Those skilled in the field of conventional steelmaking will readily understand their significance.

[0071] For example, unavoidable impurities may include Nb, etc., in amounts of 0.002 wt% or less.

[0072] In addition, the hot-rolled steel sheet having excellent strength according to an embodiment of the present invention does not completely exclude the addition of other compositions other than the steel composition described above.

[0073] It is preferable that the hot-rolled steel sheet of the present invention satisfies the following relational expression 1.

[0074] Relationship 1: 3 x ([Mn] - 1.2) 2 + 1200 ⅹ ([Ti] - 0.06) 2 + 0.0004 x (coiling temperature (℃) - 630) 2 ≤ 1.0 (In the above equation 1, [ ] represents the weight% of each element.)

[0075] Relationship 1 shows that the content and coiling temperature for the effect of solid solution by Mn and TiC precipitation are key parameters for achieving the desired high-strength steel, and when Relationship 1 has a value of 1 or less, excellent high-strength steel can be secured.

[0076] In equation 1, Mn is an element that affects the material by lowering the Ar3 temperature, which is the phase transformation temperature, to refine the size of carbides generated during phase transformation and by strengthening the material through solid solution, so its content must be limited.

[0077] Additionally, by controlling the Ti content in equation 1, the strength due to TiC precipitation can be increased. The coiling temperature is a major factor inducing the growth of carbides generated during phase transformation and additional precipitation of the dissolved Ti.

[0078] If the relationship 1:3 ⅹ ([Mn] - 1.2) 2 + 1200 ⅹ ([Ti] - 0.06) 2 + 0.0004 x (coiling temperature - 630) 2 If the value exceeds 1.0, the number of precipitates is small or too large, resulting in insufficient material quality of the steel plate.

[0079] Therefore, the relationship 1:3 ⅹ ([Mn] - 1.2) 2 + 1200 ⅹ ([Ti] - 0.06) 2 + 0.0004 x (coiling temperature - 630) 2 It is desirable that the value of is 1.0 or less, and more desirable that it is 0.1 to 0.8. By satisfying the value of relational expression 1 to 0.1 to 0.8, the rolling load is lowered, which improves productivity or secures superior material, and the material deviation in the length and width directions within the coil can be reduced, thereby securing the quality of hot-rolled steel sheets.

[0080] In order to ensure excellent formability, it is important that the hot-rolled steel sheet of the present invention contain at least 90% by area of ​​ferrite relative to the total 100% by area. From this perspective, the high-strength hot-rolled steel sheet according to an embodiment of the present invention preferably has a ferrite fraction of at least 91% by area, more preferably at least 92%, and even more preferably at least 98%.

[0081] The remainder may contain one or more of pearlite, bainite and martensite.

[0082] The microstructure of the present invention is theoretically preferably a single ferrite phase, but the manufacturing process may inevitably result in the formation of one or more of pearlite, bainite, and martensite. However, if a large amount of low-temperature transformation phases such as bainite or martensite are present, formability deteriorates. Furthermore, if pearlite is present in the hot-rolled steel sheet, cracks are likely to occur during processing due to the hard phase, cementite.

[0083] Therefore, the less residual tissue there is, the more desirable it is.

[0084] The ferrite, which is the main structure of the present invention, may be one or more of polygonal ferrite (PF), bainitic ferrite (BF), and acicular ferrite (AF).

[0085] Polygonal ferrite is a structure formed when austenite transformed at high temperatures undergoes phase transformation in the range of 700℃ or higher, and refers to ferrite that is completely polygonal in shape.

[0086] Bainitic ferrite is a structure created by displacive phase transformation that does not involve diffusion. Since it is created by shear transformation, spiral dislocations are regularly arranged within the structure to reduce the amount of shear strain created during the transformation.

[0087] Aesicular ferrite is a structure formed during the cooling or coiling process, with an internal structure composed of laths, similar to wooden sticks. Consequently, it exhibits grain refinement and offers an excellent combination of strength and ductility. Aesicular ferrite is also called acicular ferrite.

[0088] There are two types of bainite, the residual structure of the present invention. The one obtained through a relatively high-temperature isothermal transformation is called upper bainite, and the one formed at a low temperature is called lower bainite. Upper bainite is feather-shaped, while lower bainite has a needle-like, thin, and sharp acicular shape.

[0089] Pearlite and martensite are high-hardness structures formed in areas where C, Si, and Mn are locally concentrated when polygonal ferrite is formed. Since the concentration of these elements is unavoidable when polygonal ferrite is formed, the formation of these two structures cannot be completely prevented in the invention material. However, from the perspective of ductility, it is desirable to minimize the size of the localized area.

[0090] In the present invention, the average size of the ferrite crystal grains may be 15 µm or less, preferably 13 µm or less, and more preferably 7 to 9 µm.

[0091] If the average size of the ferrite grains exceeds 15㎛, sufficient strength may not be obtained due to grain coarsening.

[0092] One important method for securing strength in the present invention is to refine the precipitate size. Fine precipitates help secure not only strength but also ductility. From this perspective, the high-strength hot-rolled steel sheet according to the present invention may include (Ti, Nb)(C) precipitates. This means that it includes carbide precipitates containing at least one of Ti and Nb. The carbides may include carbonitrides. Among the precipitates, nano-sized carbides significantly contribute to improving strength, and therefore, it is preferable to include carbides.

[0093] The average size of carbides present at a point 1 / 4 in the thickness direction from the surface of the hot-rolled steel sheet may be 8 nm or less, and preferably 3 to 5 nm.

[0094] Here, the average size of carbides is calculated as the sum of the average values ​​of the major and minor axes of the carbides ÷ the total number of carbides.

[0095] In addition, the above carbide distribution is 10,000 nm 2 There may be 4.0 or more per area, preferably 5.0 to 10.0, and more preferably 5.0 to 8.0.

[0096] Regarding the distribution of carbides, since TEM measures with magnification, the number of carbides in the entire area of ​​the photograph is counted as in Figure 1 and the area is converted to 100 nm X 100 nm.

[0097] For example, the total area of ​​the photograph in Fig. 1 is 240 nm X 240 nm.

[0098] In this way, by forming a large amount of fine carbides having an average size of 8 nm or less, an excellent strength enhancement effect can be obtained without destruction of the carbides.

[0099] A high-strength hot-rolled steel sheet according to an embodiment of the present invention satisfies the proposed alloy composition, microstructure fraction, and relational expression 1, thereby having excellent mechanical properties such as a yield strength (YS): 450 MPa or more, a tensile strength (TS): 540 MPa or more, and an elongation: 16% or more.

[0100] Regarding mechanical properties, the yield strength can be greater than 450 MPa and can reach a maximum of 700 MPa. The tensile strength can be greater than 540 MPa and can reach a maximum of 800 MPa. The elongation can be greater than 16% and can reach a maximum of 40%.

[0101] As a result, the hot-rolled steel sheet according to the embodiment of the present invention is suitable for use as a high-strength support structural material such as a building scaffold, a vinyl house structural material, or a solar power support structure that requires high strength and excellent formability.

[0102] The present invention can provide a hot-rolled galvanized steel sheet manufactured using the high-strength hot-rolled steel sheet.

[0103] A hot-rolled galvanized steel sheet according to the present invention comprises a base steel sheet; and a plating layer disposed on at least one surface of the base steel sheet; wherein the base steel sheet is characterized in that it includes the hot-rolled steel sheet.

[0104] The term "base steel sheet" is not particularly limited and can be interpreted as encompassing all steel sheets that can be typically used in the production of galvanized steel sheets. For example, the base steel sheet can be interpreted as encompassing not only cold-rolled steel sheets, hot-rolled steel sheets, and heat-treated steel sheets, but also wire rods and steel wires. Preferably, it can include the aforementioned high-strength hot-rolled steel sheets.

[0105] The above plating layer may include any one of zinc, aluminum, zinc alloy, and aluminum alloy.

[0106] Hot-rolled galvanized steel sheets can exhibit high-strength properties similar to those of the hot-rolled steel sheets, such as mechanical properties such as yield strength (YS): 450 MPa or more, tensile strength (TS): 540 MPa or more, and elongation: 16% or more, and are suitable for use as high-strength support structural materials such as building scaffolding, vinyl house structural materials, and solar power supports.

[0107] A method for manufacturing a hot-rolled steel sheet according to the present invention comprises the steps of (a) reheating a steel slab containing, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.8 to 1.4%, Ti: 0.03 to 0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder Fe, and other unavoidable impurities, (b) hot-rolling the steel slab, and (c) cooling and coiling the hot-rolled hot-rolled steel sheet, and is characterized in that it satisfies the following relational expression 1.

[0108] Relationship 1: 3 x ([Mn] - 1.2) 2 + 1200 ⅹ ([Ti] - 0.06) 2 + 0.0004 x (coiling temperature (℃) - 630) 2 ≤ 1.0 (In the above equation 1, [ ] represents the weight% of each element.)

[0109] Reheating step

[0110] In the reheating step, a steel slab containing, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.8 to 1.4%, Ti: 0.03 to 0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, and the remainder being Fe and other unavoidable impurities, can be reheated at 1,150 to 1,300°C.

[0111] In this step, the steel slab is reheated at a temperature ranging from 1,150 to 1,300°C to ensure uniform alloy composition and microstructure. If the reheating temperature is below 1,150°C, the precipitates formed in the steel slab will not be dissolved, preventing sufficient precipitation strengthening in subsequent processes. Conversely, if the reheating temperature exceeds 1,300°C, excessive grain growth will occur, making it difficult to achieve the desired material quality and performance.

[0112] Therefore, the reheating temperature of the steel slab is preferably 1,150 to 1,300°C, and more preferably 1,180 to 1,270°C.

[0113] Hot rolling stage

[0114] In the hot rolling stage, the reheated steel slab can be hot rolled at a finishing rolling temperature of 800 to 1000℃.

[0115] During hot rolling, the finish delivery temperature (FDT) affects the microstructure fraction and grain size. At this stage, if the finish delivery temperature is below 800°C, some of the austenite may transform into ferrite, potentially resulting in an uneven grain size.

[0116] On the other hand, when the finishing hot rolling temperature exceeds 1000℃, the grain refinement effect may be reduced due to scale defects and reduced controlled rolling effect.

[0117] Therefore, the finishing rolling temperature may be 800 to 1000°C, and preferably 850 to 950°C.

[0118] The finishing rolling temperature refers to the measured value of the steel plate surface at the exit side of the finishing rolling mill.

[0119] Cooling and winding stage

[0120] In the cooling and coiling step, the hot-rolled hot-rolled steel sheet can be cooled to 570 to 680°C and coiled.

[0121] In this way, the hot-rolled hot-rolled steel sheet can be coiled at a coiling temperature of 570 to 680°C after water cooling. If the coiling temperature is lower than 570°C, not only is the microstructure desired by the present invention, such as the appearance of a low-temperature structure, not obtained, but carbide formation is insufficient, making it impossible to obtain a sufficient precipitation strengthening effect.

[0122] On the other hand, if the coiling temperature exceeds 680℃, coarsening of carbide occurs and the target strength cannot be obtained.

[0123] Therefore, the hot-rolled hot-rolled steel sheet can be coiled by cooling it to 570 to 680°C, and preferably, by cooling it to 590 to 650°C.

[0124] A method for manufacturing a hot-rolled steel sheet according to the present invention includes a method for manufacturing the hot-rolled steel sheet, and is characterized by including, after the coiling step, a step of heat-treating the coiled hot-rolled steel sheet, and a step of forming a plating layer on at least one surface of the hot-rolled steel sheet.

[0125] The heat treatment process can be performed at 450 to 750°C, and preferably at 450 to 720°C, to secure the temperature of the steel sheet and to stably secure carbides in the subsequent plating process.

[0126] After the heat treatment process, at least one surface of the heat-treated hot-rolled steel sheet may be plated to form a plated layer. The components and formation method of the plated layer of the present invention are not particularly limited, and can be interpreted as a concept that includes the components and formation method of the plated layer typically provided to hot-rolled plated steel sheets.

[0127] For example, the step of forming a plating layer may be performed using any one of a hot dip plating method, an electroplating method, and a plasma method. The plating layer may be formed of any one material selected from the group consisting of zinc, aluminum, a zinc-based alloy, and an aluminum-based alloy.

[0128] Here, specific examples of high-strength hot-rolled steel sheets, high-strength hot-rolled galvanized steel sheets, and their manufacturing methods are as follows.

[0129] 1. Preparation of the sample

[0130] After preparing a steel slab having the alloy composition described in Table 1 below, a hot-rolled galvanized steel sheet was manufactured by applying the process conditions described in Table 2 below. The remainder in Table 1 is iron (Fe) and other unavoidable impurities, and a hot-rolled galvanized steel sheet having a thickness of 1.6 mm was manufactured by applying the process conditions in Table 2.

[0131] [Table 1] (Unit: wt%)

[0132]

[0133] [Table 2]

[0134]

[0135] 2. Physical property evaluation method and results

[0136] Table 3 shows the results of property evaluation for specimens according to invention examples 1 to 6 and comparative examples 1 to 9.

[0137] 1) Microstructure measurement

[0138] The types and fractions (area (%)) of microstructures were measured using an optical microscope (OM). In addition, the average size (㎛) of ferrite grains was measured using the circular intercept method of ASTM E 112 after photographing them using a scanning electron microscope (SEM).

[0139] 2) Measurement of average precipitate size

[0140] The average size (nm) of the precipitates was measured by collecting the precipitates from the specimens using the carbon replica method and using a transmission electron microscope (TEM).

[0141] 3) Tensile test

[0142] Yield strength (YS, MPa), tensile strength (TS, MPa), and elongation (EL, %) were measured by performing a tensile test using the tensile test method of KS B 0802. In the case of hot-rolled steel sheets, tensile specimens were processed with the length direction aligned with the hot rolling direction using the No. 5 test piece of KS B 0801 standard.

[0143] [Table 3]

[0144]

[0145] As shown in Tables 1 to 3, in the case of invention examples 1 to 6 that satisfy the alloy composition, relational expression 1, and process conditions proposed by the present invention, it can be confirmed that the invention has high strength mechanical properties by securing the microstructure and precipitates that the present invention seeks to obtain, and exhibits a low finishing rolling load factor of 100% or less.

[0146] Figure 1 is a TEM carbide analysis photograph of Invention Example 1 according to the present invention.

[0147] As shown in Figure 1, many carbides smaller than 10 nm are observed, and most of the carbides are observed to be single TiC.

[0148] On the other hand, Comparative Example 1 exceeded the C content, and in the case of Comparative Examples 1 to 3, the Nb component was added in an amount of 0.01 to 0.03 wt% instead of the Ti component, thereby showing excellent mechanical properties due to the strengthening effect caused by grain refinement and NbC precipitation.

[0149] However, in Comparative Examples 1 to 3, the finishing rolling load increased to more than 100% due to the effect of increasing the temperature of the non-recrystallized region due to the Nb component during hot finishing rolling, which increased the high-temperature deformation resistance of the material itself.

[0150] In addition, in the case of Comparative Example 4, the solid solution strengthening effect was low due to the low Mn content, and a sufficient precipitation strengthening effect was not obtained due to changes in phase transformation conditions during cooling, so the high strength targeted by the present invention was not secured.

[0151] In addition, in the case of Comparative Example 5, as the content of Mn increased, the hardenability increased, and the strength increased significantly due to the appearance of a low-temperature structure, but the elongation did not exceed 16%.

[0152] In addition, comparative examples 6 and 7 are outside the range of Ti content.

[0153] When the Ti content was low, as in Comparative Example 6, precipitation did not occur sufficiently, resulting in insufficient strength. When the Ti content was high, as in Comparative Example 7, an increase in strength occurred due to excessive precipitation strengthening effect by TiC, but the elongation did not exceed 16%.

[0154] In the case of Comparative Example 8, the coiling temperature was lower than the limited range, so precipitation did not occur sufficiently and a low-temperature microstructure appeared, resulting in low yield strength and insufficient elongation.

[0155] In the case of Comparative Example 9, the coiling temperature was higher than the limited range, so the precipitate grew significantly, resulting in insufficient strength. In addition, in Comparative Example 9, the average size of the carbide exceeded 8 nm and 10,000 nm. 2 Since the number of sugar carbides was less than 4, the yield strength and tensile strength were significantly low.

[0156] In addition, it can be confirmed through Table 3 that comparative examples 1 to 9 do not satisfy [Relationship 1] ≤ 1.

[0157] With regard to the finishing rolling load in Table 2, generally, the lower the FDT, the more deformed γ is generated by rolling in the non-recrystallization region (below the Tnr temperature), and the more α nucleation occurs during cooling in the deformed band in the shape of a pancake, resulting in a finer final grain size.

[0158] Nb expands the non-recrystallized region (increases Tnr), enabling rolling in the non-recrystallized region even at high temperatures, and has the effect of making the grain size finer than that of general C-Si steel, and securing a high-strength material through some NbC precipitation.

[0159] However, in Comparative Examples 1 to 3 containing Nb of 0.01% or more, recrystallization does not occur during FM rolling, so the rolling load is maintained high.

[0160] On the other hand, in the case of the invention Ti steel (invention examples 1 to 6), the rolling load decreases due to recrystallization of the material during the finishing hot rolling, and the effect becomes greater as the finishing rolling progresses.

[0161] Table 4 shows the mechanical properties of hot-rolled galvanized steel sheets plated with molten zinc according to Tables 1 and 2, as a molten galvanized material.

[0162] [Table 4]

[0163]

[0164] Except for the mechanical properties in Table 4, the remaining items showed the same values ​​as in Table 3.

[0165] Compared to the hot-rolled material in Table 3, the yield strength and tensile strength of the molten galvanized material increased slightly, and the elongation was similar.

[0166] Although the present invention has been described with reference to the drawings exemplified above, it is to be understood that the present invention is not limited to the embodiments and drawings disclosed herein, and that various modifications may be made by those skilled in the art within the scope of the technical idea of ​​the present invention. Furthermore, even if the operational effects according to the configuration of the present invention have not been explicitly described while describing the embodiments of the present invention, it is natural that the effects predictable by the corresponding configuration should also be acknowledged.

Claims

1. A hot-rolled steel sheet containing, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.8 to 1.4%, Ti: 0.03 to 0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder being Fe and other unavoidable impurities, and satisfying the following relationship 1. Relationship 1: 3 x ([Mn] - 1.2) 2 + 1200 x ([Ti] - 0.06) 2 + 0.0004 x (coiling temperature (℃) - 630) 2 ≤ 1.0 (In the above equation 1, [ ] represents the weight% of each element.) 2. In paragraph 1, A hot-rolled steel sheet comprising 90% or more of ferrite for a total area of ​​100%, and the remainder comprising at least one of pearlite, bainite, and martensite.

3. In paragraph 1, Hot-rolled steel sheet having an average size of the above ferrite grains of 15㎛ or less.

4. In paragraph 1, The above hot rolled steel plate contains carbides, The average size of the above carbide is less than 8 nm, and the carbide distribution is 10,000 nm. 2 Hot rolled steel plate having 4.0 or more per area.

5. In paragraph 1, Hot rolled steel sheet with a yield strength of 450 MPa or more, a tensile strength of 540 MPa or more, and an elongation of 16% or more.

6. A steel plate; and a plating layer disposed on at least one surface of the steel plate; The above-mentioned steel sheet is a hot-rolled galvanized steel sheet including a hot-rolled steel sheet according to claim 1.

7. In paragraph 6, A hot-rolled galvanized steel sheet wherein the above-mentioned plating layer contains one of zinc, aluminum, a zinc alloy, and an aluminum alloy. 8.(a) A step of reheating a steel slab containing, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.8 to 1.4%, Ti: 0.03 to 0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder being Fe and other unavoidable impurities; (b) a step of hot rolling the above steel slab; and (c) a step of cooling and coiling the hot-rolled hot-rolled steel sheet; A method for manufacturing a hot-rolled steel sheet satisfying the following relational expression 1. Relationship 1: 3 x ([Mn] - 1.2) 2 + 1200 x ([Ti] - 0.06) 2 + 0.0004 x (coiling temperature (℃) - 630) 2 ≤ 1.0 (In the above equation 1, [ ] represents the weight% of each element.) 9. In paragraph 8, A method for manufacturing a hot-rolled steel sheet by reheating at 1150 to 1300°C in the step (a) above.

10. In paragraph 8, A method for manufacturing a hot-rolled steel sheet, wherein, in the step (b) above, a steel slab is hot-rolled at a finishing rolling temperature of 800 to 1000°C.

11. In paragraph 8, A method for manufacturing a hot-rolled steel sheet, comprising: in the step (c) above, cooling the hot-rolled steel sheet to 570 to 680°C and coiling it.

12. Includes a method for manufacturing a hot-rolled steel plate according to Article 8, After the above coiling step, a step of heat treating the coiled hot-rolled steel sheet; and A method for manufacturing a hot-rolled galvanized steel sheet, comprising: a step of forming a galvanized layer on at least one surface of the hot-rolled steel sheet.

13. In paragraph 12, A method for manufacturing a hot-rolled galvanized steel sheet, wherein the above heat treatment step is performed at 450 to 750°C.

14. In paragraph 12, A method for manufacturing a hot-rolled galvanized steel sheet, wherein the step of forming the above-mentioned plating layer is performed by any one of a hot-dip galvanizing method, an electroplating method, and a plasma method.

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