Hot-rolled steel sheet and hot-rolled plated steel sheet, both with high strength, and manufacturing method thereof

Abstract

The present invention relates to a hot-rolled steel sheet and a hot-rolled plated steel sheet, both with high strength, and a manufacturing method thereof, wherein the steel sheet comprises, in weight%, C: 0.03-0.1%, Si: 0.1% or less, Mn: 0.7-1.4%, Ti: 0.03-0.09%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, and the balance being Fe and other inevitable impurities, and satisfies relational expression 1 below. Relational expression 1) 3 * ([Mn]-0.9)2 + 1200 * ([Ti] - 0.06)2 + 0.0004 * (coiling temperature - 630)2 ≤ 1 (in relational expression 1, [ ] indicates the weight% of each element, and the coiling temperature is given in degrees Celsius (℃)).

Description

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

[0001] The present invention relates to a hot-rolled steel sheet having excellent strength and a hot-rolled plated steel sheet obtained by plating the hot-rolled steel sheet, and a method for manufacturing the same. The present invention relates to a hot-rolled steel sheet and a hot-rolled plated steel sheet having high strength suitable as a high-strength support structural material while simultaneously reducing the hot-rolling finishing rolling load, and a method for manufacturing the same.

[0002] High-strength hot-rolled steel sheets and high-strength hot-rolled plated steel sheets obtained by plating the high-strength hot-rolled steel sheets above not only have the excellent deformation resistance of high-strength hot-rolled steel sheets, but also can simultaneously secure corrosion resistance due to plating treatment.

[0003] Furthermore, hot-rolled steel sheets and hot-rolled galvanized steel sheets have superior economic efficiency compared to cold-rolled steel sheets and cold-rolled galvanized steel sheets.

[0004] High-strength hot-rolled steel sheets and hot-rolled galvanized steel sheets can be used as structural materials, especially as support structural materials, due to their excellent properties and economic efficiency as described above.

[0005] As a specific example, high-strength hot-rolled steel sheets and hot-rolled galvanized steel sheets are used in various ways as materials for reinforced construction materials such as construction scaffolding, vinyl house structural materials, and solar power supports.

[0006] Especially in the case of solar power structures, the demand for variable support structures that follow the sun up, down, left, and right rather than being fixed is rapidly increasing in order to secure maximum power production for the installation cost.

[0007] Accordingly, market demand for high-strength steel is increasing.

[0008] In general, hot-rolled steel sheets and hot-rolled galvanized steel sheets do not require a cold rolling process compared to cold-rolled steel sheets.

[0009] However, hot-rolled steel sheets and hot-rolled galvanized steel sheets are also essentially manufactured through a hot rolling process, and the rolling load in the hot rolling process is a very important factor in the quality, productivity, and economic efficiency of the steel sheets.

[0010] If the rolling load in the hot rolling process, especially the rolling load in the finishing rolling, is high, it is difficult to provide sufficient rolling force to the steel sheet, and as a result, there is a problem that it is difficult to control the microstructure.

[0011] In addition, if the rolling load is high, the rolling must be performed multiple times or the rolling temperature must be increased, which causes problems in microstructure control and productivity.

[0012] Furthermore, if the rolling load in the rolling mill is high, the load applied to the rolling mill increases or a rolling mill with a larger capacity is required, which reduces economic feasibility.

[0013] 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 and can increase productivity.

[0014] The purpose of the present invention is to provide a hot-rolled steel sheet having high strength suitable as a structural material and capable of reducing hot-rolling finishing rolling load, and a method for manufacturing the same.

[0015] In addition, the present invention provides a hot-rolled galvanized steel sheet having high strength and corrosion resistance suitable as a structural material while reducing the hot-rolled finishing rolling load, and a method for manufacturing the same.

[0016] 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.

[0017] A hot-rolled steel sheet according to one embodiment of the present invention for achieving the above purpose may be a hot-rolled steel sheet that contains, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.7 to 1.4%, Ti: 0.03 to 0.09%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, and the remainder being Fe and other unavoidable impurities, and satisfies the following relational expression 1.

[0018] Relationship 1) 3 * ([Mn]-0.9) 2 + 1200 * ([Ti]-0.06) 2 + 0.0004 * (coiling temperature - 630) 2 ≤ 1

[0019] (In the above relational expression 1, [ ] means the weight% of each element and the coiling temperature means Celsius (℃).)

[0020] At this time, the hot-rolled steel sheet may be a hot-rolled steel sheet that contains ferrite at an area fraction of 90% or more, the remainder containing a microstructure of one or more of pearlite, bainite, and martensite, and the average crystal grain size of the ferrite is 15 ㎛ or less.

[0021] Meanwhile, the ferrite may be one or more of polygonal ferrite, bainitic ferrite, and acicular ferrite.

[0022] At this time, the hot-rolled steel sheet contains carbide, and the average size of the carbide is 8 nm or less and 10,000 nm. 2 It may be a hot-rolled steel sheet having a carbide distribution configuration of 3.5 or more per area.

[0023] At this time, the hot-rolled steel sheet may have a yield strength of 440 MPa or more, a tensile strength of 490 MPa or more, and an elongation of 16% or more.

[0024] A hot-rolled galvanized steel sheet according to one embodiment of the present invention for achieving the above purpose may be a hot-rolled galvanized steel sheet including at least one of the above hot-rolled steel sheets as a base steel sheet.

[0025] At this time, the hot-rolled galvanized steel sheet may be a hot-rolled galvanized steel sheet including a galvanized layer located on both sides or at least one side of the hot-rolled steel sheet.

[0026] At this time, the plating layer may be a method for manufacturing a hot-rolled galvanized steel sheet, which includes any one component selected from zinc, aluminum, zinc alloy, and aluminum alloy.

[0027] According to one embodiment of the present invention for achieving the above object, a method for manufacturing a hot-rolled steel sheet includes the steps of: reheating a steel slab containing, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.7 to 1.4%, Ti: 0.03 to 0.09%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, and the remainder being Fe and other unavoidable impurities; hot-rolling the steel slab; and cooling the hot-rolled steel sheet to 570 to 680°C and coiling it; and may be a method for manufacturing a hot-rolled steel sheet that satisfies the following relational expression 1.

[0028] Relationship 1) 3 * ([Mn]-0.9) 2 + 1200 * ([Ti]-0.06) 2 + 0.0004 * (coiling temperature - 630) 2 ≤ 1

[0029] (In the above relational expression 1, [ ] means the weight% of each element and the coiling temperature means Celsius (℃).)

[0030] At this time, the reheating step may be a method for manufacturing a hot-rolled steel sheet, which is a step of reheating the steel slab at 1,150 to 1,300°C.

[0031] A method for manufacturing a hot-rolled galvanized steel sheet according to one embodiment of the present invention for achieving the above purpose may be a method for manufacturing a hot-rolled galvanized steel sheet, including at least one of the above hot-rolled steel sheet manufacturing methods, and including a step of forming a plating layer after the coiling step.

[0032] At this time, the method for manufacturing a hot-rolled plated steel sheet may further include a heat treatment step between the coiling step and the plating layer forming step, and the heat treatment step is a step of heat treating the coiled hot-rolled steel sheet at 450 to 750°C.

[0033] At this time, the step of forming the plating layer may be a method for manufacturing a hot-rolled plated steel sheet, which is a step of plating at least one surface of the hot-rolled steel sheet.

[0034] At this time, the step of forming the plating layer may be a method for manufacturing a hot-rolled plated steel sheet using any one process selected from a hot-dip galvanizing process, an electroplating process, and a plasma process.

[0035] At this time, the plating layer may be a method for manufacturing a hot-rolled galvanized steel sheet, which includes any one component selected from zinc, aluminum, zinc alloy, and aluminum alloy.

[0036] The hot-rolled steel sheet and hot-rolled galvanized steel sheet according to the present invention can have excellent mechanical properties such as a yield strength (YS) of 400 MPa or more, a tensile strength (TS) of 490 MPa or more, and an elongation of 16% or more through a controlled composition range and microstructure.

[0037] As a result, the hot-rolled steel sheet and the hot-rolled galvanized steel sheet having excellent mechanical properties according to the present invention are suitable for use as a supporting structural material requiring high strength properties.

[0038] The method for manufacturing a hot-rolled steel sheet and a hot-rolled plated steel sheet according to the present invention can provide a hot-rolled steel sheet and a hot-rolled plated steel sheet having high productivity, mechanical properties, and excellent corrosion resistance by reducing the hot-rolling finishing rolling load.

[0039] The effects of the present invention are not limited to the above-described matters, and can be interpreted to include effects that can be inferred from the description described below by a person skilled in the art.

[0040] Figure 1 is a photograph of the microstructure of carbide observed using a transmission electron microscope (TEM) of Invention Example 1 among the embodiments of the present invention.

[0041] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings so that those skilled in the art can easily implement the present invention. The present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0042] In order to clearly explain the present invention, parts that are not related to the description have been omitted, and the same or similar components are designated by the same reference numerals throughout the specification. In addition, some embodiments of the present invention will be described in detail with reference to exemplary drawings. When adding reference numerals to components in each drawing, the same components may have the same numerals as much as possible even if they are shown in different drawings. In addition, when explaining the present invention, if it is determined that a specific description of a related known configuration or function may obscure the gist of the present invention, the detailed description may be omitted.

[0043] When describing components of the present invention, terms such as first, second, A, B, (a), (b), etc. may be used. These terms are only intended to distinguish the components from other components, and the nature, order, sequence, or number of the components are not limited by the terms. When it is described that a component is "connected," "coupled," or "connected" to another component, it should be understood that the component may be directly connected or connected to the other component, but that other components may also be "interposed" between each component, or that each component may be "connected," "coupled," or "connected" through another component.

[0044]

[0045] Hereinafter, with reference to the attached drawings, a detailed description will be given of a hot-rolled steel sheet and a hot-rolled galvanized steel sheet having excellent strength according to preferred embodiments of the present invention and a method for manufacturing the same.

[0046]

[0047] The hot-rolled steel sheet having excellent strength according to an embodiment of the present invention can be used as a structural material due to its high strength characteristics.

[0048] Non-limiting and specific examples of structural materials include scaffolding for construction, greenhouse structural materials, and support structures such as solar panels.

[0049] To this end, a hot-rolled steel sheet having excellent strength according to one embodiment of the present invention may contain, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.7 to 1.4%, Ti: 0.03 to 0.09%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, and the remainder being Fe and other unavoidable impurities.

[0050] In addition, a hot-rolled steel sheet having excellent strength according to one embodiment of the present invention may have a microstructure including at least one of pearlite, bainite, and martensite, and including 90% or more of ferrite in terms of area ratio, and an average crystal grain size of the ferrite of 15 ㎛ or less (the average crystal grain size means the diameter equivalent to a circle).

[0051] At this time, the hot-rolled steel sheet having excellent strength according to one embodiment of the present invention has an average size of 8 nm or less and 10,000 nm 2 It may contain carbides having a distribution of 3.5 or more per area.

[0052] A hot-rolled steel sheet having excellent strength according to one embodiment of the present invention can achieve mechanical properties such as a yield strength of 400 MPa or more, a tensile strength of 490 MPa or more, and an elongation of 16% or more by having the components, composition range, and microstructure as described above.

[0053] Furthermore, a hot-rolled steel sheet having excellent strength according to one embodiment of the present invention is a hot-rolled steel sheet having excellent strength that satisfies the following relational expression 1.

[0054] Relationship 1: 3 * ([Mn]-0.9) 2 + 1200 * ([Ti]-0.06) 2 + 0.0004 * (coiling temperature - 630) 2 ≤ 1

[0055] (In the above relational expression 1, [ ] represents the weight% of each element, and the coiling temperature is based on Celsius temperature (℃).)

[0056]

[0057] In addition, a hot-rolled galvanized steel sheet having excellent strength and corrosion resistance according to another embodiment of the present invention comprises a hot-rolled steel sheet and a galvanized layer formed on at least one surface of the hot-rolled steel sheet.

[0058] At this time, the hot-rolled steel sheet in the hot-rolled galvanized steel sheet means any one of the hot-rolled steel sheets described above.

[0059] At this time, the plating layer is formed of one material selected from among zinc, aluminum, zinc alloy, and aluminum alloy.

[0060] A method for manufacturing a hot-rolled steel sheet having excellent strength according to another embodiment of the present invention is a manufacturing method comprising the steps of: reheating a steel slab at 1,150 to 1,300°C; hot-rolling the reheated steel slab at a final hot rolling temperature of 800 to 1,000°C; and cooling the hot-rolled hot-rolled steel sheet to 570 to 680°C and coiling it.

[0061] According to another embodiment of the present invention, a method for manufacturing a hot-rolled steel sheet having excellent strength and corrosion resistance includes the steps of: reheating a steel slab at 1,150 to 1,300°C; hot-rolling the reheated steel slab at a final hot rolling temperature of 800 to 1,000°C; cooling the hot-rolled hot-rolled steel sheet to 570 to 680°C and coiling it; and heat-treating the coiled hot-rolled steel sheet at 450 to 750°C, and then coating at least one surface of the hot-rolled steel sheet to form a plated layer.

[0062]

[0063] Hot rolled steel sheets and coated steel sheets

[0064]

[0065] Hereinafter, the steel composition and its composition ratio included in the hot-rolled steel sheet having excellent strength according to an embodiment of the present invention will be described in more detail.

[0066] Hereinafter, unless otherwise specified, the percentages indicating the content of each element are based on weight.

[0067]

[0068] Carbon (C): 0.03 to 0.1%

[0069] Carbon (C) is added not only for the effect of reinforcing the solid solution but also for forming carbides with Ti, and is an element for securing tensile strength.

[0070] To obtain the above effect, it is preferable to add carbon (C) in an amount of 0.03 wt% or more.

[0071] However, if the amount of carbon (C) added exceeds 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 to be obtained in the present invention.

[0072] In addition, surface cracks may occur due to the hypo-peritectic region during continuous casting, so a carbon content limit of 0.1 wt% or less is necessary to ensure the soundness of the material.

[0073] On the other hand, when the carbon content is less than 0.03%, there is a problem that the strength of the steel plate is insufficient because there is less carbon to form a Ti-based composition including a TiC composition.

[0074] Therefore, it is preferable that carbon (C) 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.

[0075]

[0076] Silicon (Si): 0.1% or less

[0077] Silicon (Si) is not only useful for deoxidation of steel, but is also effective in securing strength through solid solution strengthening.

[0078] However, when the silicon (Si) content exceeds 0.1 wt%, there is a disadvantage in that silicon oxide is formed, making plating difficult.

[0079] Therefore, it is preferable that the content of silicon (Si) be 0.1 wt% or less, and more specifically, it can be added at a content ratio of 0.001 to 0.1 wt%.

[0080]

[0081] Manganese (Mn): 0.7 to 1.4%

[0082] Manganese (Mn) is added to ensure the effect of strengthening the weld and the hardenability of the weld when cooling after welding.

[0083] In particular, manganese (Mn) is an element that affects the material through solid solution strengthening and refines the size of carbides produced during phase transformation by lowering the Ar3 temperature, which is the phase transformation temperature in the relational expression 1 described below, so its content must be limited.

[0084] To obtain the above effect, it is preferable to add manganese (Mn) in an amount of 0.7 wt% or more.

[0085] However, when manganese (Mn) is added in a large amount 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 structure.

[0086] Therefore, it is preferable that manganese (Mn) be added in a content ratio of 0.7 to 1.4 wt% of the total weight of the hot-rolled steel sheet according to the present invention.

[0087]

[0088] Titanium (Ti): 0.03 to 0.09%

[0089] Titanium (Ti) is added to enhance precipitation strengthening and suppress grain coarsening.

[0090] In particular, titanium (Ti) is one of the main elements that can increase the strength by precipitation of a Ti-based composition including a TiC composition by controlling the content in the following relational expression 1.

[0091] When 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.

[0092] However, when titanium (Ti) exceeds 0.09 wt%, coarse carbides are formed, making precipitation strengthening ineffective.

[0093] Therefore, it is preferable that titanium (Ti) be added in a content ratio of 0.03 to 0.09 wt% of the total weight of the hot-rolled steel sheet according to the present invention.

[0094]

[0095] Phosphorus (P): 0.02% or less

[0096] Phosphorus (P) is an impurity element that segregates at grain boundaries and reduces toughness, so it is desirable to avoid including it if possible.

[0097] Therefore, it is preferable to limit phosphorus (P) to 0.02 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention.

[0098]

[0099] Sulfur (S): 0.02% or less

[0100] Sulfur (S) is an impurity element and is the main element that forms MnS, and the formation of coarse MnS can reduce toughness.

[0101] Therefore, it is preferable to limit sulfur (S) to 0.02 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention.

[0102]

[0103] Nitrogen (N): 0.01% or less

[0104] 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.

[0105] Therefore, it is preferable to limit nitrogen (N) to 0.01 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention.

[0106]

[0107] In addition, the hot-rolled steel sheet having excellent strength according to an embodiment of the present invention may include Fe and other unavoidable impurities in addition to the steel composition described above.

[0108] Unavoidable impurities are those that can be unintentionally mixed in during the normal steel manufacturing process, and cannot be completely eliminated. Their meaning is easily understood by those skilled in the field of normal steel manufacturing.

[0109] 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.

[0110]

[0111] As described above, it is preferable that the hot-rolled steel sheet having excellent strength according to an embodiment of the present invention have a microstructure including ferrite at an area ratio of 90% or more.

[0112] In this way, in the present invention, it is important to secure a ferrite structure of 90% or more in terms of area ratio in order to secure excellent formability.

[0113] The microstructure of the present invention is theoretically preferably a single phase of ferrite, but one or more of pearlite, bainite, and martensite may inevitably be formed during the manufacturing process.

[0114] However, if there are many low-temperature transformation phases such as bainite or martensite, formability decreases.

[0115] In addition, if pearlite exists in the hot-rolled steel sheet, cracks are likely to occur during processing due to the hard phase, cementite.

[0116] Therefore, in the hot-rolled steel sheet having excellent strength according to an embodiment of the present invention, it is preferable that the residual structure excluding ferrite be as small as possible.

[0117] As a result, it is more preferable that the hot-rolled steel sheet having excellent strength according to the embodiment of the present invention has a ferrite fraction of 91% or more in terms of area ratio, and even more preferably 92% or more.

[0118] Meanwhile, the ferrite may be one or more of polygonal ferrite, bainitic ferrite, and acicular ferrite.

[0119] At this time, it is preferable that the average crystal grain size of the ferrite be 15㎛ or less, and more preferably 13㎛ or less.

[0120] If the average grain size of ferrite exceeds 15㎛, sufficient strength may not be obtained due to grain coarsening.

[0121] It is obvious to those skilled in the art that the above average grain size can be obtained statistically through microstructural observation, etc.

[0122]

[0123] In addition, it is preferable that the hot-rolled steel sheet having excellent strength according to an embodiment of the present invention includes carbides containing only Ti (even if alloying elements such as Nb and V are detected, the detected amount refers to carbides contained only at the level of impurities), and that the average diameter of the carbides has a size of 8 nm or less.

[0124] In this way, the hot-rolled steel sheet having excellent strength according to an embodiment of the present invention can obtain an excellent strength improvement effect without causing destruction of the carbide by forming a large amount of fine carbide having an average diameter of 8 nm or less.

[0125]

[0126] As described above, the hot-rolled steel sheet having excellent strength according to the embodiment of the present invention has excellent mechanical properties such as a yield strength (YS): 400 MPa or more, a tensile strength (TS): 490 MPa or more, and an elongation: 16% or more.

[0127] As a result, the hot-rolled steel sheet having excellent strength according to an 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.

[0128]

[0129] Meanwhile, the present invention can provide a hot-rolled galvanized steel sheet manufactured using the hot-rolled steel sheet described above.

[0130] In addition, a hot-rolled galvanized steel sheet having excellent strength and corrosion resistance according to another embodiment of the present invention comprises a hot-rolled steel sheet and a galvanized layer formed on at least one surface of the hot-rolled steel sheet.

[0131] At this time, the hot-rolled steel sheet means any one of the hot-rolled steel sheets described above.

[0132] At this time, the plating layer is formed of one material selected from among zinc, aluminum, zinc alloy, and aluminum alloy.

[0133]

[0134] Method for manufacturing hot-rolled steel sheets and coated steel sheets

[0135]

[0136] Reheating

[0137] The reheating step is a step of reheating a steel slab containing, in wt%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.7 to 1.4%, Ti: 0.03 to 0.09%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, and the remainder being Fe and other unavoidable impurities, at 1,150 to 1,300°C.

[0138] In this step, the steel slab is reheated in the temperature range of 1,150 to 1,300℃ to make the alloy composition and microstructure uniform.

[0139] At this time, if the reheating temperature is lower than 1,150℃, the precipitates formed on the steel slab are not dissolved, and the optimal precipitation strengthening effect cannot be obtained in the subsequent process.

[0140] On the other hand, if the reheating temperature exceeds 1,300℃, excessive grain growth occurs, making it difficult to secure the target material and quality.

[0141] Therefore, it is desirable that the reheating temperature of the steel slab be in the range of 1,150 to 1,300℃, and a more desirable range can be suggested as 1,180 to 1,270℃.

[0142]

[0143] hot rolling

[0144] The hot rolling stage is a stage in which the reheated steel slab is hot rolled at a final hot rolling temperature of 800 to 1000℃.

[0145] At this stage, if the finishing hot rolling (sand rolling) temperature is less than 800℃, there is a risk that some of the austenite will transform into ferrite, resulting in an uneven grain size of the final obtained crystals.

[0146] On the other hand, when the finishing hot rolling temperature exceeds 1000℃, the grain refinement effect may be reduced due to scale defects and a decrease in the controlled rolling effect.

[0147]

[0148] Cooling and winding

[0149] The cooling and coiling step is the step of cooling the hot-rolled hot-rolled steel sheet to 570 to 680℃ and coiling it.

[0150] The above hot-rolled hot-rolled steel sheet can be coiled at a coiling temperature of 570 to 680°C after water cooling.

[0151] Furthermore, the coiling temperature is a major factor that induces the growth of carbides generated during phase transformation and the additional precipitation of dissolved Ti in the equation 1 described later.

[0152] At this time, if the coiling temperature is less than 570°C, not only is it impossible to obtain the microstructure desired by the present invention, such as the appearance of a low-temperature structure, but also carbide formation is insufficient, making it impossible to obtain a sufficient precipitation strengthening effect.

[0153] On the other hand, if the coiling temperature exceeds 680℃, coarsening of carbide occurs and the target strength cannot be obtained.

[0154]

[0155] At this time, the hot-rolled steel plate satisfies the following relationship 1.

[0156] Relationship 1: 3 * ([Mn]-0.9) 2 + 1200 * ([Ti]-0.06) 2+ 0.0004 * (coiling temperature - 630) 2 ≤ 1

[0157] (In the above relational expression 1, [ ] represents the weight% of each element, and the coiling temperature is based on Celsius temperature (℃).)

[0158] In the above relational expression 1, Mn is an element that lowers the Ar3 temperature, which is the phase transformation temperature, to refine the size of carbides generated during phase transformation and affects the material through solid solution strengthening, so its content must be limited.

[0159] Additionally, it is one of the main elements that can increase strength by TiC precipitation by controlling the Ti content.

[0160] The coiling temperature is a major factor inducing the growth of carbides formed during phase transformation and the additional precipitation of dissolved Ti.

[0161] If, 3 * ([Mn]-0.9) 2 + 1200 * ([Ti]-0.06) 2 + 0.0004 * (coiling temperature - 630) 2 If the value is greater than 1, the number of precipitates is small or too many have grown, resulting in a lack of material.

[0162] So 3 * ([Mn]-0.9) 2 + 1200 * ([Ti]-0.06) 2 + 0.0004 * (coiling temperature - 630) 2 It is desirable that the value of is less than or equal to 1, as in relational expression 1.

[0163]

[0164] Heat treatment and plating

[0165] In the heat treatment and plating treatment step, the coiled hot-rolled steel sheet is heat treated, and then at least one side of the hot-rolled steel sheet is plated to form a plating layer.

[0166] At this stage, it is desirable to carry out the heat treatment process in a temperature range of 450 to 720℃ to secure the temperature of the steel plate and to stably secure carbides in the subsequent plating process.

[0167] After this heat treatment process, a plating layer can be formed on at least one side of the heat-treated hot-rolled steel sheet.

[0168] The components and forming method of the plating layer of the present invention are not particularly limited, and can be interpreted as a concept including the components and forming method of the plating layer typically provided to hot-rolled galvanized steel sheets.

[0169] As a non-limiting and specific example of a plating method, the plating layer can be formed by any one method selected from among a hot dip plating method, an electroplating method, and a plasma method.

[0170] Additionally, the plating layer may be formed of any one material selected from, for example, zinc, aluminum, zinc-based alloy, aluminum-based alloy, etc., as a non-limiting and specific example.

[0171]

[0172] The hot-rolled galvanized steel sheet manufactured by the manufacturing method according to the embodiment of the present invention described above has excellent mechanical properties such as a yield strength of 400 MPa or more, a tensile strength of 490 MPa or more, and an elongation of 16% or more.

[0173] As a result, the hot-rolled galvanized steel sheet manufactured by the manufacturing method 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.

[0174]

[0175] Example

[0176] Hereinafter, the configuration and operation of the present invention will be described in more detail through preferred embodiments of the present invention.

[0177] However, this is presented as a preferred example of the present invention and cannot be construed as limiting the present invention in any way.

[0178] Anything not described here will be omitted as it is technically feasible for those skilled in this field to infer.

[0179]

[0180] 1. Preparation of specimens

[0181] 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.

[0182] The relative comparison of the load of the upper 3 rolls of the finishing rolling and the relative comparison of the load of the lower 3 rolls of the finishing rolling in Table 2 are both values ​​calculated relatively by setting the load of the upper 3 rolls of the finishing rolling and the load of the lower 3 rolls of the finishing rolling of Invention Example 1 as 100, and then calculating the load values ​​measured in each experimental example.

[0183]

[0184] [Table 1]

[0185]

[0186] (Unit: wt%, while tr means that the measured composition range corresponds to an impurity level of trace amounts or less than several tens of ppm.)

[0187]

[0188] [Table 2]

[0189]

[0190]

[0191] 2. Physical property evaluation

[0192] Table 3 shows the results of property evaluation for specimens according to Examples 1 to 6 and Comparative Examples 1 to 9.

[0193]

[0194] 1) Microstructure measurement

[0195] The types and fractions of microstructures were measured using an optical microscope (OM). In addition, the ferrite grain size was measured using the circular intercept method of ASTM E 112 after photographing the grains using a scanning electron microscope (SEM).

[0196]

[0197] 2) Measurement of precipitate size

[0198] The size of the precipitate was measured by collecting the precipitate from the specimen using the carbon replica method and using a transmission electron microscope (TEM).

[0199]

[0200] 3) Tensile test

[0201] Yield strength (YS), tensile strength (TS), and elongation (EL) were measured by conducting a tensile test using the tensile test method of KS B 0802 standard.

[0202] In the case of hot-rolled steel sheets, a tensile test piece was processed with the longitudinal direction aligned with the hot-rolling direction using a No. 5 test piece of the KS B 0801 standard.

[0203]

[0204] [Table 3]

[0205]

[0206] As shown in Tables 1 to 3, in the case of Examples 1 to 6 that satisfy the components and composition ranges proposed by the present invention, equation 1, and process conditions, it can be confirmed that the present invention has high-strength mechanical properties and exhibits a low finishing rolling load ratio by securing the microstructure and precipitates that the present invention seeks to obtain.

[0207]

[0208] On the other hand, Comparative Examples 1, 2, and 3 showed excellent mechanical properties due to the strengthening effect caused by grain refinement and NbC precipitation by adding Nb instead of Ti.

[0209] However, in Comparative Examples 1, 2, and 3, the finish rolling load, especially the load of the three rolls at the end of the finish rolling, increased relatively more than the load of the three rolls at the top of the finish rolling due to the effect of the increase in temperature of the non-recrystallized region due to the Nb component during hot finish rolling increasing the high-temperature deformation resistance of the material itself.

[0210] Meanwhile, 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.

[0211] In addition, in the case of Comparative Example 5, as the content of Mn increased, the hardenability increased, and as a result, the strength increased significantly due to the appearance of a low-temperature structure, but the elongation was measured to have decreased significantly (16% or less).

[0212] In addition, Comparative Examples 6 and 7 are cases where the Ti content is outside the range of the hot-rolled steel sheet of the present invention.

[0213] When the Ti content was low, as in Comparative Example 6, TiC precipitation did not occur sufficiently, resulting in insufficient strength.

[0214] On the other hand, when the Ti content was high as in Comparative Example 7, the strength increased due to excessive precipitation strengthening effect by TiC, but the elongation was measured to be significantly reduced (16% or less).

[0215] Meanwhile, Comparative Examples 8 and 9 are cases where the coiling temperature deviates from the manufacturing method of the hot-rolled steel sheet according to the embodiment of the present invention, even though the components and composition ranges of the hot-rolled steel sheet according to the embodiment of the present invention are satisfied.

[0216] Specifically, as in Comparative Example 8, when the coiling temperature is lower than the range limited in the manufacturing method of the present invention, TiC precipitation does not occur sufficiently in terms of microstructure, and a low-temperature structure appears, resulting in low yield strength and insufficient elongation.

[0217] On the other hand, when it was higher than the limited range as in Comparative Example 9, the TiC precipitates grew too large, resulting in insufficient strength.

[0218]

[0219] Furthermore, it can be confirmed through Table 3 that the above comparative examples 1 to 9 do not satisfy [Relationship 1] ≤ 1 due to at least one factor among the factors of Mn, Ti and / or coiling temperature.

[0220]

[0221] 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 were not 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: 0.7 to 1.4%, Ti: 0.03 to 0.09%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, and the remainder being Fe and other unavoidable impurities, and satisfying the following relationship 1. Relationship 1) 3 * ([Mn]-0.9) 2 + 1200 * ([Ti]-0.06) 2 + 0.0004 * (coiling temperature - 630) 2 ≤ 1 (In the above equation 1, [ ] represents the weight% of each element and the coiling temperature represents Celsius (℃).) 2. In paragraph 1, The above hot-rolled steel sheet contains ferrite in an area fraction of 90% or more, The remainder contains one or more microstructures of pearlite, bainite and martensite. The average crystal grain size of the above ferrite is 15㎛ or less. Hot rolled steel plate.

3. In paragraph 1, The above hot rolled steel plate contains carbide, The average size of the above carbide is less than 8 nm, 10,000 nm 2 Having a carbon distribution composition of 3.5 or more per area, Hot rolled steel plate.

4. In paragraph 1, The yield strength of the above hot-rolled steel plate is 440 MPa or more, The tensile strength is 490 MPa or more. The elongation rate is 16% or more, Hot rolled steel plate.

5. Including the hot-rolled steel sheet of any one of clauses 1 to 4 as a base steel sheet, Hot rolled galvanized steel sheet.

6. In paragraph 5, Comprising a plating layer located on both sides or at least one side of the hot-rolled steel sheet, Hot rolled galvanized steel sheet.

7. In paragraph 6, The above plating layer comprises one component selected from zinc, aluminum, zinc alloy and aluminum alloy. Hot rolled galvanized steel sheet.

8. A step of reheating a steel slab containing, by weight%, C: 0.03 to 0.1%, Si: 0.1% or less, Mn: 0.7 to 1.4%, Ti: 0.03 to 0.09%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, and the remainder being Fe and other unavoidable impurities; A step of hot rolling the above steel slab; A step of cooling a hot-rolled steel sheet to 570 to 680℃ and coiling it; A method for manufacturing a hot rolled steel sheet satisfying the following relational expression 1. Relationship 1) 3 * ([Mn]-0.9) 2 + 1200 * ([Ti]-0.06) 2 + 0.0004 * (coiling temperature - 630) 2 ≤ 1 (In the above equation 1, [ ] represents the weight% of each element and the coiling temperature represents Celsius (℃).) 9. In paragraph 8, The above reheating step is a step of reheating the steel slab at 1,150 to 1,300℃. Method for manufacturing hot rolled steel sheet.

10. In paragraph 8, The above hot rolling step is a step of finishing hot rolling the steel slab at 800 to 1000℃. Method for manufacturing hot rolled steel sheet.

11. Including a method for manufacturing a hot-rolled steel plate according to any one of clauses 8 to 10, Comprising a step of forming a plating layer after the above-mentioned winding step, Method for manufacturing hot-rolled galvanized steel sheet.

12. In paragraph 11, Additionally comprising a heat treatment step between the above-mentioned winding step and the step of forming a plating layer, The above heat treatment step is a step of heat treating the coiled hot-rolled steel sheet at 450 to 750°C. Method for manufacturing hot-rolled galvanized steel sheet.

13. In paragraph 11, The step of forming the above plating layer is a step of plating at least one surface of the hot-rolled steel sheet. Method for manufacturing hot-rolled galvanized steel sheet.

14. In paragraph 11, The step of forming the above plating layer uses one process selected from among a molten plating process, an electroplating process, and a plasma process. Method for manufacturing hot-rolled galvanized steel sheet.

15. In paragraph 11, The above plating layer comprises one component selected from zinc, aluminum, zinc alloy and aluminum alloy. Method for manufacturing hot-rolled galvanized steel sheet.

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