Steel sheet and method for manufacturing same

Abstract

The present invention relates to a steel sheet suitable as a material for a vehicle structural member, a reinforcing material and the like, and, more specifically, to a steel sheet with excellent hole expandability and surface layer hardness, and a method for manufacturing same.

Description

Steel plate and method for manufacturing the same

[0001] The present invention relates to a steel sheet suitable as a material for structural members, reinforcing materials, etc. of automobiles, and more specifically, to a steel sheet having excellent hole expandability and surface hardness and a method for manufacturing the same.

[0002] Recently, with the goal of reducing global greenhouse gas emissions, automotive powertrains are shifting from internal combustion engines to electric motors. In the electric vehicle market, competition to reduce vehicle weight to increase driving range per charge is intensifying. As part of this effort, aluminum alloys, which have a lower density than steel, are being actively adopted to minimize the weight of battery pack housing components.

[0003] Meanwhile, automakers and parts manufacturers are expanding their adoption of steel materials, which have relatively low material costs due to falling battery prices, low carbon emissions in terms of life cycle assessment, and excellent thermal runaway resistance, in addition to mass production for the popularization of electric vehicles.

[0004] The following documents can be cited as related technologies.

[0005] Patent Document 1 describes the application of steel to the lower plate of a battery housing to protect battery cells. This technology utilizes ultra-high-strength steel with a tensile strength of 980-1800 MPa to protect battery cells from impacts generated from the underbody during operation, thereby ensuring crashworthiness. This ultra-high-strength steel not only enhances crashworthiness but also supports heavy battery packs. However, securing high strength requires the use of large quantities of ferroalloy, and the complex heat treatment process also results in high manufacturing costs.

[0006] Patent Document 2 discloses a steel sheet having a tensile strength of 490 MPa or more, a relationship between tensile strength and elongation (TS × El) of 15,000 MPa% or more, and excellent plating properties, for use as a structural member or reinforcing material of an automobile body. In this technology, by adding Sb to the alloy composition, a uniform concentrated layer is formed on the extreme surface of the steel sheet, thereby suppressing the formation of oxidative inclusions on the surface, thereby securing plating properties. However, the suggested range of Sb content for forming the Sb concentrated layer on the surface of the steel sheet has limitations in securing crash resistance against external factors such as stone chipping that may occur during automobile operation. In other words, in order to secure the aforementioned crash resistance, the hardness of the surface of the steel sheet must be uniform, but the solid solution strengthening and transformation strengthening effects required to achieve this may not be sufficient. In addition, the addition of a large amount of Sb can not only act as a factor that reduces plating adhesion, but also increases manufacturing costs as it is an expensive element, which is economically disadvantageous.

[0007] Meanwhile, Patent Document 3 discloses a technology for manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet with a tensile strength of 590 MPa and excellent fatigue properties and crash resistance. In this technology, in order to suppress softening of the surface layer that can generally occur during the steel sheet manufacturing process, temper rolling is performed after hot rolling to increase the precipitation generation sites within the surface layer structure, thereby additionally implementing a precipitation strengthening effect, thereby securing surface hardness. However, the addition of expensive alloying elements such as Nb, Ti, Mo, and V is essential for the precipitation strengthening effect, and if these elements are unevenly distributed in the surface layer, there is a problem that the plating property is poor, such as the occurrence of unplating. Moreover, if the surface hardness of the steel sheet is solely dependent on the precipitation strengthening effect, defects such as transverse cracks may occur during the processing of parts.

[0008] (Patent Document 1) Korean Patent Publication No. 2023-0032302

[0009] (Patent Document 2) Korean Patent No. 10-0711358

[0010] (Patent Document 3) Korean Patent No. 10-1313957

[0011] One aspect of the present invention provides a steel plate suitable as a material for structural members, reinforcing materials, etc. of an automobile, having uniform surface hardness and excellent hole expandability to improve resistance to impacts that may occur during use of the automobile, and a method for manufacturing the same.

[0012] The objectives of the present invention are not limited to the above-described content. The objectives of the present invention can be understood from the overall content of this specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention.

[0013] According to one aspect of the present invention, a steel sheet is provided which contains, in wt%, carbon (C): 0.050 to 0.150%, silicon (Si): 0.010 to 1.000%, manganese (Mn): 1.000 to 3.000%, aluminum (Sol.Al): 0.01 to 0.10%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.020%, nitrogen (N): 0.0010 to 0.0100%, chromium (Cr): 0.020 to 0.300%, nickel (Ni): 0.010 to 0.100%, copper (Cu): 0.010 to 0.400%, the remainder being Fe and other unavoidable impurities.

[0014] In one embodiment of the present invention, the steel plate may have Si, Cr, and Ni in the alloy composition so as to satisfy the following relationship 1.

[0015] [Relationship 1]

[0016] 2.000 ≤ 17(Cr+Ni) / Si ≤ 4.000

[0017] (In equation 1, Cr, Ni, and Si represent the weight contents of each element.)

[0018] A steel sheet according to an embodiment of the present invention that satisfies Equation 1, which represents the relationship between the alloy composition and the content of specific elements, can control the enrichment behavior of Cr and Ni in the surface layer. In particular, the relationship between the content of Cr and Ni and the enrichment value of Cr and Ni in the surface layer can satisfy Equation 2 below.

[0019] [Relationship 2]

[0020] 0.150 ≤ Δ(Cr+Ni) ≤ 0.500

[0021] Δ(Cr+Ni) = [Cr (0.5㎛) +Ni (0.5㎛) ] - [Cr (1 / 4t) +Ni (1 / 4t) ]

[0022] (In relation 2, Cr (0.5㎛) and Ni (0.5㎛) It means the maximum concentration value (in weight%) of Cr and Ni in the surface layer region from the surface of the steel plate to a depth of 500 nm in the thickness direction, and Cr (1 / 4t) and Ni (1 / 4t) refers to the weight average content of Cr and Ni, respectively, at the point t / 4 in the thickness direction of the steel plate (where t means the thickness (mm) of the steel plate).

[0023] In one embodiment of the present invention, the steel plate may further include at least one selected from molybdenum (Mo), niobium (Nb), and antimony (Sb) in a total content of 1.0% or less, and may further include boron (B) in a content of 0.0030% or less.

[0024] In one embodiment of the present invention, the microstructure may include at least one of ferrite having an area fraction of 90% or more and residual bainite, martensite, and retained austenite.

[0025] The steel plate according to one embodiment of the present invention has high strength, excellent hole expandability, and can ensure uniform surface hardness.

[0026] As an example, the steel plate may have a tensile strength (TS) of 590 MPa or more, and the relationship between the tensile strength (TS) and hole expandability (HER) may satisfy the following equation 3. In addition, the steel plate may have a surface hardness (Vickers hardness) of 170 or more.

[0027] In one embodiment of the present invention, the steel sheet may include a zinc-based plating layer on at least one surface.

[0028] According to another aspect of the present invention, a method for manufacturing a steel sheet includes the steps of preparing a steel slab; heating the steel slab at a temperature range of 1000 to 1350°C; finishing hot-rolling the heated steel slab at a temperature range of 800 to 1000°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature range of 400 to 650°C; cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 30% or more to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet by heating and maintaining it at a temperature of Ac1 or higher; and cooling the annealed cold-rolled steel sheet to a temperature range of 450 to 650°C at a cooling rate of 35°C / s or less.

[0029] In one embodiment of the present invention, the steel slab may have the above-described alloy composition and may satisfy the above-described relational expression 1.

[0030] When a steel slab satisfying the alloy composition and relational expression 1 according to one embodiment of the present invention is used, the steel plate manufactured may satisfy the relational expression 2 described above.

[0031] In one embodiment of the present invention, a step of obtaining a hot-dip galvanized steel sheet by subjecting a cooled cold-rolled steel sheet to hot-dip galvanizing treatment in a temperature range of 440 to 480°C may be further included, and optionally, a step of subjecting the hot-dip galvanized steel sheet to alloying heat treatment may be further included.

[0032] According to the present invention, a steel plate having excellent hardness uniformity in the surface layer and improved hole expandability can be provided compared to a high-strength steel plate utilizing a conventional transformation structure.

[0033] The steel plate of the present invention is easy to form into parts, and thus has the effect of being suitably applicable to structural members or reinforcing materials of automobiles.

[0034] FIG. 1 shows a part of the results of measuring Cr and Ni concentration values ​​from the surface to a depth of 1 ㎛ in the thickness direction using a glow discharge spectrometer (GDS) for a steel plate according to one embodiment of the present invention.

[0035] The inventors of the present invention recognized the problems with ultra-high-strength steel, which has been used in existing automotive structural components, reinforcing materials, and other materials. Therefore, they conducted in-depth research into methods for ensuring resistance to the continuous impacts inflicted by automotive environments and improving the hole expandability of steel sheets to facilitate forming into parts and other components.

[0036] As a result, the alloy composition and manufacturing conditions of the steel plate were optimized, and it was confirmed that the content relationship between specific elements affects the surface hardness of the steel plate, thereby completing the present invention.

[0037] Hereinafter, the present invention will be described in detail.

[0038] A steel sheet according to one aspect of the present invention may contain, in wt%, carbon (C): 0.050 to 0.150%, silicon (Si): 0.010 to 1.000%, manganese (Mn): 1.000 to 3.000%, aluminum (Sol.Al): 0.01 to 0.10%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.020%, nitrogen (N): 0.0010 to 0.0100%, chromium (Cr): 0.020 to 0.300%, nickel (Ni): 0.010 to 0.100%, and copper (Cu): 0.010 to 0.400%.

[0039] Below, the reasons for limiting the alloy composition of the steel plate according to one aspect of the present invention are described in detail. Unless otherwise specified, the content of each element is based on weight, and the ratio of the structure is based on area.

[0040] Carbon (C): 0.050~0.150%

[0041] Carbon (C) is the most economical and effective element for strengthening steel. As its content increases, the fraction of hard phases (martensite, bainite, etc.) that make up the steel increases, thereby increasing the tensile strength.

[0042] In one embodiment of the present invention, if the C content is less than 0.050%, the fraction of hard phase in the steel structure becomes insufficient, making it impossible to secure the target level of strength. On the other hand, if the C content exceeds 0.150%, the formation of coarse carbides or pearlite in the steel becomes easy, resulting in poor formability and reduced weldability.

[0043] Therefore, in one embodiment of the present invention, C may be included in an amount of 0.050 to 0.150%. According to another embodiment of the present invention, C may be included in an amount of 0.060% or more, or 0.070% or more. According to yet another embodiment of the present invention, C may be 0.145% or less, or 0.140% or less.

[0044] Silicon (Si): 0.010~1.000%

[0045] Silicon (Si) is an element added to deoxidize molten steel and can exert a solid solution strengthening effect.

[0046] In one embodiment of the present invention, if the Si content is less than 0.010%, the aforementioned effects cannot be sufficiently obtained. On the other hand, if the Si content exceeds 1.000%, there is a problem of forming oxides on the steel surface, thereby hindering the plating properties.

[0047] Therefore, in one embodiment of the present invention, Si may be included in an amount of 0.010 to 1.000%. According to another embodiment of the present invention, Si may be included in an amount of 0.020% or more, or 0.030% or more. According to yet another embodiment of the present invention, Si may be included in an amount of 0.900% or less, or 0.850% or less.

[0048] Manganese (Mn): 1.000~3.000%

[0049] Manganese (Mn), like Si, can be added for the purpose of strengthening the steel. In addition, Mn increases the hardenability of steel, facilitating the formation of a hard phase in the steel structure.

[0050] In one embodiment of the present invention, if the manganese content is less than 1.000%, the aforementioned effects cannot be sufficiently achieved. On the other hand, if the manganese content exceeds 3.000%, the hardenability of the steel increases significantly, and as the fraction of the hard phase becomes excessively high, there is a concern that securing the intended ductility may become difficult.

[0051] Therefore, in one embodiment of the present invention, the Mn may be included in an amount of 1.000 to 3.000%. According to another embodiment of the present invention, the Mn may be included in an amount of 1.100% or more, or 1.200% or more. According to yet another embodiment of the present invention, the Mn may be 2.900% or less, or 2.850% or less.

[0052] Aluminum (Sol.Al): 0.01~0.10%

[0053] Aluminum (Sol.Al) is an element added to deoxidize molten steel.

[0054] In one embodiment of the present invention, if the Sol.Al content is less than 0.01%, the effect of addition is insufficient. On the other hand, if the content exceeds 0.10%, it easily causes corner cracks in the slab during the continuous casting process by combining with nitrogen (N) to form AlN. In addition, there is a risk of defects due to the formation of Al-based inclusions.

[0055] Therefore, in one embodiment of the present invention, the Sol.Al may be included in an amount of 0.01 to 0.10%.

[0056] Phosphorus (P): 0.001~0.050%

[0057] Phosphorus (P) is an element that inevitably enters steel. Excessive phosphorus content can impair the steel's weldability and increase the risk of brittleness. Meanwhile, controlling P to trace levels dramatically increases refining costs.

[0058] Accordingly, in one embodiment of the present invention, the content of P can be limited to 0.001 to 0.050%.

[0059] Sulfur (S): 0.001~0.020%

[0060] Sulfur (S) is an element that inevitably enters steel. Excessive levels can impair the steel's weldability and potentially reduce its ductility. However, controlling the S content to trace amounts can lead to a sharp increase in refining costs.

[0061] Accordingly, in one embodiment of the present invention, the content of S can be limited to 0.001 to 0.020%.

[0062] Nitrogen (N): 0.0010~0.0100%

[0063] Nitrogen (N) is an element that inevitably enters steel. Excessive levels increase the risk of steel brittleness. Furthermore, excessive precipitation of AlN, which reacts with aluminum, can degrade the steel's performance. Furthermore, controlling N to trace levels dramatically increases refining costs.

[0064] Accordingly, in one embodiment of the present invention, the content of N can be limited to 0.0010 to 0.0100%.

[0065] Chromium (Cr): 0.020~0.300%

[0066] Chromium (Cr) is an element that delays the formation of ferrite during the cooling process during the steel manufacturing process and promotes the formation of a hard phase.

[0067] In one embodiment of the present invention, if the Cr content is less than 0.020%, a hard phase is not sufficiently formed, making it impossible to secure the target level of strength. On the other hand, if the Cr content exceeds 0.300%, there is a problem in that a martensite phase is excessively formed in the steel structure, resulting in reduced ductility.

[0068] Therefore, in one embodiment of the present invention, Cr may be included in an amount of 0.020 to 0.300%. According to another embodiment of the present invention, Cr may be included in an amount of 0.025% or more, or 0.030% or more. According to yet another embodiment of the present invention, Cr may be included in an amount of 0.295% or less, or 0.290% or less.

[0069] Nickel (Ni): 0.010~0.100%

[0070] Nickel (Ni), similar to the above Cr, is an element that delays the formation of ferrite during the cooling process during the steel manufacturing process and promotes the formation of a hard phase.

[0071] In one embodiment of the present invention, if the Ni content is less than 0.010%, a hard phase is not sufficiently formed, making it impossible to secure the target level of strength. On the other hand, if the Ni content exceeds 0.100%, there is a problem in that an excessive martensite phase is formed in the steel structure, resulting in reduced ductility.

[0072] Therefore, in one embodiment of the present invention, Ni may be included in an amount of 0.010 to 0.100%. According to another embodiment of the present invention, Ni may be included in an amount of 0.015% or more, or 0.020% or more. According to yet another embodiment of the present invention, Ni may be included in an amount of 0.098% or less.

[0073] Copper (Cu): 0.010~0.400%

[0074] Copper (Cu) is a useful element for ensuring the weather resistance of steel and preventing hydrogen-induced embrittlement.

[0075] In one embodiment of the present invention, if the Cu content is less than 0.010%, the aforementioned effects cannot be sufficiently obtained. On the other hand, if the Cu content exceeds 0.400%, there is a risk that a large number of scab defects due to grain boundary liquefaction may occur on the slab surface.

[0076] Therefore, in one embodiment of the present invention, Cu may be included in an amount of 0.010 to 0.400%. In another embodiment of the present invention, Cu may be included in an amount of 0.012% or more. In yet another embodiment of the present invention, Cu may be included in an amount of 0.380% or less.

[0077] In addition to the alloy composition described above, a steel sheet according to one embodiment of the present invention may further include at least one selected from molybdenum (Mo), niobium (Nb), and antimony (Sb), and / or boron (B): 0.0030% or less.

[0078] Molybdenum (Mo), niobium (Nb), and boron (B) are elements that enhance the hardenability of steel and contribute to the formation of a hard phase in the steel structure. Antimony (Sb) is an element that is advantageous in securing plating properties by forming a uniform concentrated layer on the surface of the steel sheet.

[0079] In one embodiment of the present invention, when adding at least one of Mo, Nb, and Sb, if the content exceeds 1.0%, there is a problem that manufacturing costs rapidly increase. In particular, in the case of Sb, there is a concern that the plating adhesion may be deteriorated due to excessive surface concentration.

[0080] In one embodiment of the present invention, if the content of B exceeds 0.003%, there is a concern that the ductility of the steel may deteriorate.

[0081] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0082] In one embodiment of the present invention, in order to ensure uniform surface hardness of the steel plate while ensuring ductility, the content of Cr and Ni, which are hardenable elements, can be limited in relation to Si. Specifically, Cr, Ni, and Si can satisfy the following relationship 1.

[0083] [Relationship 1]

[0084] 2.000 ≤ 17(Cr+Ni) / Si ≤ 4.000

[0085] (In equation 1, Cr, Ni, and Si represent the weight contents of each element.)

[0086] Si present in steel is an element that is advantageous in forming oxides on the steel surface. According to one embodiment of the present invention, by increasing the content of Cr and Ni relative to Si, the concentration of Cr and Ni can be increased instead of forming oxides by Si on the steel surface.

[0087] In particular, according to one embodiment of the present invention, by optimizing the relationship between alloy compositions as described above and simultaneously controlling the manufacturing conditions of the steel plate, which will be described in detail later, the concentration values ​​of Cr and Ni in the steel surface layer can be secured as follows.

[0088] [Relationship 2]

[0089] 0.150 ≤ Δ(Cr+Ni) ≤ 0.500

[0090] Δ(Cr+Ni) = [Cr (0.5㎛) +Ni (0.5㎛) ] - [Cr (1 / 4t) +Ni (1 / 4t) ]

[0091] (In relation 2, Cr (0.5㎛) and Ni (0.5㎛) It means the maximum concentration value (in weight%) of Cr and Ni in the surface layer region from the surface of the steel plate to a depth of 500 nm in the thickness direction, and Cr (1 / 4t) and Ni (1 / 4t) refers to the weight average content of Cr and Ni, respectively, at the point t / 4 in the thickness direction of the steel plate (where t means the thickness (mm) of the steel plate).

[0092] That is, in the steel sheet according to one embodiment of the present invention, Cr and Ni, each added in a certain amount, can be mainly concentrated on the surface side of the steel sheet rather than the inner side, more specifically, directly under the scale layer formed on the surface of the steel sheet. By concentrating these hardenable elements on the surface side of the steel sheet (directly under the scale layer), it is possible to introduce solid solution and transformation strengthening effects on the surface side of the steel sheet. As a result, it is possible to obtain the effect of preventing a decrease in hardness in the surface layer of the steel sheet.

[0093] From this, when a steel plate according to one embodiment of the present invention is applied to a structural member, reinforcing member, etc. of an automobile, there is an advantage in that resistance to continuous impact occurring in an actual use environment can be enhanced.

[0094] Meanwhile, in relational expression 2, Cr and Ni measured at the point t / 4 in the thickness direction of the steel plate indicate the tempering component of each element added to the steel plate, and it should be noted that the content measured at the point t / 4 is based on this to represent it. In other words, Cr in relational expression 2 (1 / 4t) and Ni (1 / 4t) Each represents the content of Cr and Ni contained in the steel plate, and is set as a representative value measured at the t / 4 point in the thickness direction.

[0095] A steel sheet according to one embodiment of the present invention may include ferrite as a main phase in its microstructure and at least one of bainite, martensite, and retained austenite as a residual structure. Here, the residual structure is referred to as a second phase.

[0096] In one embodiment of the present invention, the ferrite formed as a main body may have an area fraction of 90% or more. If the area fraction of the ferrite is less than 90%, there is a concern that the ductility of the steel sheet may decrease.

[0097] Meanwhile, a steel plate according to one embodiment of the present invention can secure a target strength by including a hard phase, bainite (B) phase and martensite (M) phase, as a second phase. As an example, the hard phase (B+M) may be included at an area fraction of 3% or more, and does not exceed 10% at most.

[0098] In addition, the second phase may include a residual austenite phase, and this residual austenite phase is a structure remaining after forming the aforementioned ferrite and hard phase, and may be included at 0%. However, the residual austenite phase does not exceed 2% at most.

[0099] Meanwhile, it is not excluded that the second phase includes a trace amount of pearlite (P) phase, and the second phase may be composed of a retained austenite phase and pearlite in addition to a hard phase.

[0100] A steel sheet according to one embodiment of the present invention may have properties suitable for use as a material for automobiles, particularly as a material for structural members, reinforcing materials, etc. As one example, a steel sheet according to one embodiment of the present invention may have a tensile strength of 590 MPa or more.

[0101] In addition, the steel plate according to one embodiment of the present invention can have the characteristics of uniform surface hardness and excellent hole expandability.

[0102] As an example, the steel plate may have a relationship between tensile strength (TS) and hole expandability (HER) that satisfies the following equation 3, and may have a surface hardness (Vickers hardness) of 170 or more. Here, the surface hardness refers to a hardness value measured at a depth of up to 2 ㎛ in the thickness direction from the surface of the steel plate.

[0103] [Relationship 3]

[0104] TS×HER ≥ 30,000MPa%

[0105] The steel sheet according to one embodiment of the present invention means a cold-rolled steel sheet, but may also include a plated steel sheet including a plated layer on at least one surface of the cold-rolled steel sheet.

[0106] At this time, it should be noted that the plating layer may be a zinc-based plating layer containing zinc (Zn) as a main component, and addition of Mg, Al, Si, etc. in addition to the zinc may not be excluded. That is, the plating layer may be a pure zinc plating layer or an alloy plating layer.

[0107] Hereinafter, a method for manufacturing a steel plate according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method is an example for manufacturing a steel plate according to one embodiment of the present invention, particularly a cold-rolled steel plate, and a plated steel plate using the cold-rolled steel plate.

[0108] A steel plate according to one embodiment of the present invention can be manufactured by subjecting a prepared steel slab to the processes of [heating - hot rolling - coiling - cold rolling - annealing - cooling], and each process step is specifically described below.

[0109] [Heating of steel slabs]

[0110] First, a steel slab is prepared. In one embodiment of the present invention, the steel slab can be manufactured using a furnace or electric furnace method. The prepared steel slab can then be heated. This heating process facilitates the smooth performance of the hot rolling process described below and sufficiently achieves the desired properties of the steel plate.

[0111] As an example, the steel slab may have the same alloy composition and alloy composition relationship (relationship 1) as the steel plate according to one embodiment of the present invention, and the description of each alloy element and the description of the composition relationship are replaced with the above-mentioned matters.

[0112] In one embodiment of the present invention, the steel slab may be heated at a temperature range of 1000 to 1350°C. If the heating temperature is less than 1000°C, the formation of a scale layer by selective oxidation of Fe is not easy, and thus Cr and Ni cannot be sufficiently concentrated directly beneath the scale layer. On the other hand, if the temperature exceeds 1350°C, there is a problem of excessive scale formation.

[0113] [Hot rolling]

[0114] The above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.

[0115] In one embodiment of the present invention, a hot-rolled steel sheet can be manufactured by performing finishing hot rolling at a temperature range of 800 to 1000°C during the hot rolling. In one embodiment of the present invention, if the temperature during the finishing hot rolling is less than 800°C, there is a problem that the rolling load increases significantly. On the other hand, if the temperature exceeds 1000°C, there is a problem that the thermal fatigue of the rolling rolls increases significantly.

[0116] [Winding]

[0117] The hot-rolled steel sheet manufactured above can be coiled.

[0118] In one embodiment of the present invention, the coiling process may be performed at a temperature range of 400 to 650°C. If the coiling temperature is lower than 400°C, excessive bainite and martensite may form within the hot-rolled steel sheet, which may increase the rolling load during subsequent cold rolling. On the other hand, if the temperature exceeds 650°C, excessive scale may form on the surface of the hot-rolled steel sheet, which may impair the plating properties.

[0119] [Cold rolling]

[0120] The above-mentioned coiled hot-rolled steel sheet can be cold-rolled to produce a cold-rolled steel sheet. The cold rolling is a process performed to control the shape and thickness of the steel sheet.

[0121] In one embodiment of the present invention, the reduction ratio during cold rolling is not particularly limited. However, in order to suppress the formation of coarse ferrite during the subsequent recrystallization process during annealing, cold rolling may be performed at a reduction ratio of 30% or more. Furthermore, the upper limit of the reduction ratio is not particularly limited, and it should be noted that a person skilled in the art may appropriately select it depending on the desired thickness.

[0122] Meanwhile, prior to performing the cold rolling, the hot-rolled steel sheet may be pickled to remove the oxide layer formed on the surface. The pickling treatment may be performed under normal conditions, and there are no particular limitations on the conditions.

[0123] [Annealing and cooling]

[0124] The cold rolled steel sheet manufactured above can be cooled after annealing.

[0125] In one embodiment of the present invention, the annealing treatment may be performed by heating the cold-rolled steel sheet to a temperature Ac1 or higher. If the temperature during the annealing treatment is lower than Ac1, the austenite phase may not be sufficiently formed during the annealing treatment, and thus the final microstructure may not be formed as intended, resulting in reduced strength and aggravated anisotropy of physical properties.

[0126] In one embodiment of the present invention, the cold-rolled steel sheet may be heated and then maintained for a certain period of time. The maintaining process may be performed for at least 10 seconds to induce sufficient recrystallization.

[0127] After performing the above annealing and maintenance processes, the cold rolled steel sheet can be cooled. In one embodiment of the present invention, cooling can be performed at a cooling rate of 35°C / s or less (excluding 0°C / s) to a temperature range of 450 to 650°C.

[0128] If the cooling rate exceeds 35℃ / s during the above cooling, the non-uniformity of the ferrite transformation behavior increases, which may deteriorate the shape of the steel plate, such as the wave phenomenon, and there is a concern that the plate may break due to plate drift. In addition, if the cooling end temperature during the above cooling is less than 450℃, there is a problem that bainite and martensite phases are excessively formed, which deteriorates the hole expandability of the steel plate. On the other hand, if the temperature exceeds 650℃, there is a concern that pearlite is excessively formed, which also deteriorates the hole expandability of the steel plate. Meanwhile, the lower limit of the cooling rate during the above cooling is not particularly limited, but it may be performed at 3℃ / s or more in terms of securing the intended microstructure and physical properties.

[0129] Meanwhile, when performing annealing and cooling as described above, the conveying speed of the steel sheet, i.e., the line speed (mpm), may be 80 to 200 mpm. As an example, the annealing treatment of a cold-rolled steel sheet obtained by cold rolling may be performed by charging the cold-rolled steel sheet into an annealing furnace. At this time, if the time for the cold-rolled steel sheet to pass through the furnace is excessive, there is a concern that excessive oxides may be formed on the surface of the steel sheet during the annealing process, and therefore, the line speed may be set to 80 mpm or more in consideration of this. However, if the speed exceeds 200 mpm, there is a concern that recrystallization may not be sufficiently achieved during the annealing process. In another embodiment of the present invention, the line speed of the steel sheet during the annealing treatment may be 100 mpm or more, and also may be 180 mpm or less.

[0130] According to one embodiment of the present invention, by controlling the alloy composition and manufacturing conditions, a steel sheet (cold rolled steel sheet) having soft ferrite and a certain proportion of hard phase can be obtained. In particular, by increasing the concentration of Cr and Ni on the surface side of the steel sheet, a uniform surface hardness can be secured, and excellent hole expandability can be achieved.

[0131] In one embodiment of the present invention, a plated steel sheet can be obtained by plating a cold-rolled steel sheet, and as one example, hot-dip galvanizing can be performed on the cold-rolled steel sheet.

[0132] [Hot-dip galvanizing]

[0133] A hot-dip galvanized steel sheet can be manufactured by immersing a cold-rolled steel sheet according to one embodiment of the present invention in a hot-dip galvanized bath.

[0134] In one embodiment of the present invention, the hot-dip galvanizing process may be performed under normal conditions, but as an example, may be performed at a temperature range of 440 to 480°C. In addition, the composition of the hot-dip galvanizing bath is not particularly limited, and may be a pure zinc plating bath or a zinc alloy plating bath containing Si, Al, Mg, etc.

[0135] [Alloying heat treatment]

[0136] In addition, if necessary, an alloyed zinc-plated steel sheet can be obtained by subjecting a hot-dip galvanized steel sheet manufactured by hot-dip galvanizing according to one embodiment of the present invention to an alloying heat treatment.

[0137] In one embodiment of the present invention, the alloying heat treatment process conditions are not particularly limited, and any conventional conditions may be used. As an example, the alloying heat treatment process may be performed at a temperature range of 480 to 600°C.

[0138] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0139] (Example)

[0140] After preparing steel slabs having the alloy compositions shown in Table 1 below, each steel slab was heated in the temperature range of 1000 to 1350°C. Thereafter, cold-rolled steel sheets were manufactured through a series of processes under the conditions shown in Table 2 below.

[0141] The microstructure and mechanical properties of each manufactured cold-rolled steel sheet were measured, and the results are shown in Table 3 below.

[0142] First, the Cr and Ni concentration values ​​were measured from the surface of each cold-rolled steel sheet to a depth of 1 ㎛ in the thickness direction using a glow discharge spectrometer (GDS), and the (Cr+Ni) content at a depth of 500 nm from the surface was derived from the results. In addition, the (Cr+Ni) content at the point t / 4 in the thickness direction of each cold-rolled steel sheet (where t represents the thickness (mm) of the cold-rolled steel sheet) was measured at 50 locations at the corresponding point using the SEM-EDS technique, and then the average value was derived. The Δ(Cr+Ni) value was calculated from this, and the results are shown in Table 3 below.

[0143] The type and fraction of microstructure of each cold-rolled steel sheet were measured after etching using the Nital etching method and observing at 1000 to 5000 times magnification using a scanning electron microscope (SEM) and an image analyzer.

[0144] Among the mechanical properties of each cold-rolled steel sheet, tensile strength (TS) and elongation at break (El) were measured using ASTM standard test specimens taken in a direction parallel to the rolling direction. In addition, hardness was measured on the surface using a Micro-Vickers tester. At this time, a load of 500 g was applied, and measurements were made at 2 mm intervals for a total of 50 points with the analysis area set to 20 mm × 10 mm (width × length), and the minimum hardness value was expressed.

[0145] And, the hole expansion ratio (HER) of each cold-rolled steel plate was measured for a 90 mm × 120 mm (width × length) specimen according to the JSF T1001-1996 standard.

[0146] Classification Alloy composition (weight%) CSiMnSol.AlPSNCrNiCu pipe 1 Invention steel 10.0850.3701.7500.030.0110.0020.00260.0390.0250.0602.941 Invention steel 20.0860.3811.7310.030.0110.0060.00340.0310.0250.0802.499 Invention steel 30.0710.9202.3000.050.0100.0060.00300.0520.0820.3702.476 Invention steel 40.0710.9202.3000.060.0100.0060.00300.1120.0330.2202.679 Invention steel 50.0710.9202.3000.050.0100.0060.00300.1120.0970.1153.862 Comparison steel 10.0850.3601.7100.060.0120.0070.00340.1720.1310.08514.308 Comparison steel 20.0870.3501.7100.030.0100.0110.00310.0010.0020.2830.146 Comparison steel 30.0860.3811.7310.040.0110.0060.00340.0410.0230.0852.856Comparative steel 40.0860.3811.7310.060.0110.0060.00340.0410.0250.1102.945 In Table 1, Comparative steel 3 and Comparative steel 4 are classified as comparative steels because the manufacturing conditions in Table 2 below deviate from one embodiment of the present invention.

[0147] ClassificationHot finishing temperature (℃)Coiling temperature (℃)Cold rolling reduction ratio (%)Annealing coating thickness (mm)Line speed (mpm)Ac1Annealing temperature (℃)Cooling speed (℃ / s)Cooling temperature (℃)Hot-dip galvanizing (℃)Alloying heat treatment (℃)Inventive steel 1890580501307158006550--1.2Inventive steel 2980580501207168055550--1.2Inventive steel 3980580551207258054500450-1.3Inventive steel 49056005015072681510550--1.7Inventive steel 59055804515072581554804505001.4 Comparative steel 1898560501307168054490--1.2 Comparative steel 2905590501507158156550--1.2 Comparative steel 3900570501307168009400--1.2 Comparative steel 490057050130716800-700--1.2 Comparative steel 4 is the case where a separate cooling process is not applied after annealing and the steel is left in the annealed state up to 700℃.

[0148] Classification relationship 2 Microstructure (area fraction) Mechanical properties F Second phase (B + M + RA) TS (MPa) El (%) HER (%) TS × HER (MPa%) Surface hardness Inventive steel 10.20 69 55 65 32 65 9 38 5 2 7 1 8 Inventive steel 20.19 29 46 6 18 2 8 6 6 40 7 8 8 Inventive steel 30.20 69 46 7 8 5 2 1 5 1 4 0 0 3 5 2 3 Inventive steel 40.32 79 6 4 7 7 9 2 1 5 5 4 2 8 4 5 2 3 Inventive steel 50.36 29 37 7 9 32 0 4 0 3 1 7 2 0 2 3 Comparative steel 10.74 9 8 5 1 5 7 2 2 1 7 3 5 2 5 2 7 0 2 1 1 Comparative steel 20.037955599307041930147Comparative steel 30.2647030734183122754194Comparative steel 40.2428515668192718036159F: Ferrite, B: Bainite, M: Martensite, RA: Retained austenite phase. Regarding the residual structure (second phase) excluding F among the microstructures, the case of including a trace amount of pearlite (P) phase is not excluded, and the fraction of (B+M+RA) representing the residual structure is the fraction including the P phase.

[0149] As shown in Tables 1 to 3 above, the inventive steels 1 to 5, which satisfy the alloy composition and manufacturing conditions according to one embodiment of the present invention, have a microstructure formed primarily of ferrite and an appropriate hard phase. As a result, high strength and excellent hole expandability were secured. In addition, by controlling the enrichment behavior of Cr and Ni on the steel plate surface, the surface hardness value was excellent, reaching 170 or higher.

[0150] On the other hand, comparative steels 1 to 4, which did not satisfy at least one of the alloy composition system and manufacturing conditions according to one embodiment of the present invention, could not secure the intended properties.

[0151] Among these, Comparative Steel 1 did not satisfy Equation 2 because the amount of Cr and Ni enrichment in the surface layer was too excessive. As a result, the ferrite phase was not sufficiently formed and the residual structure was excessively formed, resulting in poor hole expandability. Comparative Steel 2 had significantly poor surface hardness due to insufficient Cr and Ni enrichment in the surface layer. Comparative Steels 3 and 4 are cases where the annealing conditions among the manufacturing conditions were not satisfied. Comparative Steel 3 had poor hole expandability because the fraction of the hard phase was excessive due to the end temperature in the rapid cooling zone being controlled too low. Comparative Steel 4 had poor hole expandability and low surface hardness because the fraction of pearlite in the structure was excessive due to the end temperature in the rapid cooling zone being controlled too high.

[0152] Figure 1 shows the GDS profile measurement results of invention steel 1 and comparative steels 1 and 2.

[0153] That is, based on the results of the (Cr+Ni) content measured in the thickness direction from the surface of the steel plate, it can be confirmed that the (Cr+Ni) content of Comparative Steel 1 is excessive, the (Cr+Ni) content of Comparative Steel 2 is considerably low, and the (Cr+Ni) content of Inventive Steel 5 is present at an appropriate level of Cr and Ni, based on the thickness direction of around 0.1㎛.

Claims

1. Contains, in wt%, carbon (C): 0.050 to 0.150%, silicon (Si): 0.010 to 1.000%, manganese (Mn): 1.000 to 3.000%, aluminum (Sol.Al): 0.01 to 0.10%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.020%, nitrogen (N): 0.0010 to 0.0100%, chromium (Cr): 0.020 to 0.300%, nickel (Ni): 0.010 to 0.100%, copper (Cu): 0.010 to 0.400%, the remainder being Fe and other unavoidable impurities. The above Si, Cr and Ni satisfy the following relationship 1: A steel plate in which the relationship between the content of Cr and Ni and the concentration value of Cr and Ni in the surface layer satisfies the following relational expression 2. [Relationship 1] 2.000 ≤ 17(Cr+Ni) / Si ≤ 4.000 (In equation 1, Cr, Ni, and Si represent the weight contents of each element.) [Relationship 2] 0.150 ≤ Δ(Cr+Ni) ≤ 0.500 Δ(Cr+Ni) = [Cr (0.5㎛) +Ni (0.5㎛) ] - [Cr (1 / 4t) +Ni (1 / 4t) ] (Cr in relation 2 (0.5㎛) and Ni (0.5㎛) It means the maximum concentration value (in weight%) of Cr and Ni in the surface layer region from the surface of the steel plate to a depth of 500 nm in the thickness direction, and Cr (1 / 4t) and Ni (1 / 4t) refers to the weight average content of Cr and Ni, respectively, at the point t / 4 in the thickness direction of the steel plate (where t refers to the thickness (mm) of the steel plate).

2. In paragraph 1, The above steel plate is a steel plate further containing at least one selected from molybdenum (Mo), niobium (Nb), and antimony (Sb) in a total content of 1.0% or less.

3. In paragraph 1, The above steel plate is a steel plate further containing boron (B): 0.0030% or less.

4. In paragraph 1, The above steel plate is a steel plate having a microstructure comprising at least one of ferrite having an area fraction of 90% or more and residual bainite, martensite, and retained austenite.

5. In paragraph 1, The above steel plate has a tensile strength (TS) of 590 MPa or more, A steel plate in which the relationship between the tensile strength (TS) and hole expandability (HER) satisfies the following relationship 3. [Relationship 3] TS×HER ≥ 30,000MPa% 6. In paragraph 1, The above steel plate is a steel plate having a surface hardness (Vickers hardness) of 170 or more. (Here, surface hardness refers to the hardness value measured at a depth of 2㎛ from the surface of the steel plate in the thickness direction.) 7. In any one of paragraphs 1 to 6, The above steel plate is a steel plate including a zinc-based plating layer on at least one surface.

8. A step for preparing a steel slab, which contains, by weight%, carbon (C): 0.050 to 0.150%, silicon (Si): 0.010 to 1.000%, manganese (Mn): 1.000 to 3.000%, aluminum (Sol.Al): 0.01 to 0.10%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.020%, nitrogen (N): 0.0010 to 0.0100%, chromium (Cr): 0.020 to 0.300%, nickel (Ni): 0.010 to 0.100%, copper (Cu): 0.010 to 0.400%, the remainder being Fe and other unavoidable impurities, wherein the Si, Cr and Ni satisfy the following relationship 1; A step of heating the above steel slab in a temperature range of 1000 to 1350℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab in a temperature range of 800 to 1000℃; A step of coiling the above hot-rolled steel plate at a temperature range of 400 to 650°C; A step of cold rolling the above-mentioned hot rolled steel sheet at a reduction ratio of 30% or more to obtain a cold rolled steel sheet; A step of annealing the cold rolled steel sheet by heating and maintaining it at a temperature of Ac1 or higher; and A method for manufacturing a steel sheet, comprising the step of cooling the cold rolled steel sheet subjected to the above annealing treatment to a temperature range of 450 to 650°C at a cooling rate of 35°C / s or less. [Relationship 1] 2.000 ≤ 17(Cr+Ni) / Si ≤ 4.000 (In equation 1, Cr, Ni, and Si represent the weight contents of each element.) 9. In paragraph 8, The above steel plate is a method for manufacturing a steel plate, wherein the relationship between the content of Cr and Ni and the concentration value of Cr and Ni in the surface layer satisfies the following relational expression 2. [Relationship 2] 0.150 ≤ Δ(Cr+Ni) ≤ 0.500 Δ(Cr+Ni) = [Cr (0.5㎛) +Ni (0.5㎛) ] - [Cr (1 / 4t) +Ni (1 / 4t) ] (Cr in relation 2 (0.5㎛) and Ni (0.5㎛) It means the maximum concentration value (in weight%) of Cr and Ni in the surface layer region from the surface of the steel plate to a depth of 500 nm in the thickness direction, and Cr (1 / 4t) and Ni (1 / 4t) refers to the weight average content of Cr and Ni, respectively, at the point t / 4 in the thickness direction of the steel plate (where t refers to the thickness (mm) of the steel plate).

10. In paragraph 8, A method for manufacturing a steel sheet further comprising the step of subjecting the cooled cold rolled steel sheet to hot-dip galvanizing at a temperature range of 440 to 480°C to obtain a hot-dip galvanized steel sheet.

11. In paragraph 10, A method for manufacturing a steel sheet further comprising the step of alloying heat treating the above-mentioned hot-dip galvanized steel sheet.

12. In paragraph 8, A method for manufacturing a steel plate, wherein the steel slab further contains at least one selected from molybdenum (Mo), niobium (Nb), and antimony (Sb) in a total content of 1.0% or less.

13. In paragraph 8, The above steel slab is a method for manufacturing a steel plate further containing boron (B) of 0.0030% or less.

Images