Steel plate and manufacturing method thereof

Abstract

According to exemplary embodiments of the present invention, provided is an ultra-high strength steel plate having improved formability. According to other exemplary embodiments of the present invention, provided is a method for manufacturing the ultra-high strength steel plate having improved formability.

Description

Steel plate and method for manufacturing the same

[0001] The present invention relates to a steel plate and a method for manufacturing the same.

[0002] More specifically, it relates to steel plates used in automobile crash and structural members and a method for manufacturing the same.

[0003] Recently, automotive steel sheets have been required to have higher strength to improve fuel efficiency and durability due to various environmental and energy consumption regulations. In particular, with the recent expansion of automobile impact safety regulations, high-strength steels with superior yield strength are being adopted for structural components such as members, seat rails, and pillars to enhance the impact resistance of the body. These structural components have the characteristic of having a higher yield strength relative to the tensile strength, i.e., a higher yield ratio (tensile strength / yield strength), which is advantageous for impact energy absorption. However, as the strength of steel sheets generally increases, the elongation decreases, resulting in a decrease in formability. Therefore, the development of materials that can compensate for this problem is urgently needed.

[0004] Conventional methods for strengthening steel include solid solution strengthening, precipitation strengthening, grain refinement strengthening, and transformation strengthening. However, among the above methods, solid solution strengthening and grain refinement strengthening have the disadvantage of making it very difficult to produce high-strength steel with a tensile strength of 490 MPa or higher.

[0005] Precipitation-strengthened high-strength steel is a technology that secures strength by precipitating carbon and nitride-forming elements such as Cu, Nb, Ti, and V to strengthen the steel sheet, or by refining the grains by suppressing grain growth due to fine precipitates. This technology has the advantage of easily obtaining high strength at a low manufacturing cost, but has the disadvantage of requiring high-temperature annealing to ensure sufficient recrystallization and ductility because the recrystallization temperature rises rapidly due to the fine precipitates. In addition, precipitation-strengthened steel, which strengthens by precipitating carbon and nitrides in a ferrite matrix, has the problem that it is difficult to obtain high-strength steel of the 600 MPa class or higher.

[0006] Various transformation-strengthened high-strength steels have been developed, including dual-phase ferrite-martensite steels that incorporate hard martensite into a ferrite matrix, TRIP (Transformation Induced Plasticity) steels that utilize the transformation-induced plasticity of retained austenite, and CP (Complexed Phase) steels composed of ferrite and hard bainite or martensite structures. However, the tensile strength of these advanced high-strength steels is limited to about 8% when the tensile strength is 1500 MPa. In addition, hot press forming steels that secure the final strength through rapid cooling through direct contact with a die that is formed at high temperatures and then cooled by water are attracting attention for application to structural members to ensure crashworthiness. However, the high investment in facilities and the high costs of heat treatment and processing have hindered their widespread application.

[0007] Recently, automotive seat components are being manufactured with both high strength and lightweight construction to further enhance passenger safety in the event of a collision. These components are manufactured using both roll forming and press forming. Seat components, as the connecting element between the passenger and the vehicle body, must support passengers under high stress to prevent them from being ejected during a collision. This requires high yield strength and yield ratio. Furthermore, most machined components require stretch flangeability, necessitating the use of steels with excellent hole expandability.

[0008] Meanwhile, Patent Document 1 discloses a technology for a high-strength cold-rolled steel sheet having a tensile strength of 880 to 1170 MPa, which is controlled to a martensite single-phase structure by optimizing the composition and heat treatment conditions of the steel sheet. Patent Document 2 discloses a technology for a manufacturing method of a high-strength steel sheet, which comprises heating and maintaining a steel sheet in which a volume ratio of a low-temperature transformation phase composed of martensite and retained austenite accounts for 90% or more of the total metal structure, to a structure of fine ferrite and austenite including laths of the low-temperature transformation phase, and then cooling to form a metal structure in which ferrite and the low-temperature transformation phase are finely dispersed in the lath phase.

[0009] Patent Documents 1 and 2 claim that high yield strength can be achieved without water cooling. However, without proper water cooling, there are disadvantages such as very low ductility and poor stretch flangeability due to the large amount of austenite generated in the steel.

[0010] Therefore, it is necessary to develop ultra-high strength steel plates with excellent formability.

[0011] (Patent Document 1) Japanese Patent Publication No. 3729108

[0012] (Patent Document 2) Japanese Patent Publication No. 2005-272954

[0013] The technical idea of ​​the present invention is to provide a steel plate and a method for manufacturing the same.

[0014] More specifically, the problem to be solved by the technical idea of ​​the present invention is to provide an ultra-high strength steel plate with excellent formability and a method for manufacturing the same.

[0015] The objectives of the present invention are not limited to the above-described content. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall description of the present invention.

[0016] According to exemplary embodiments of the present invention, a steel plate is provided. The above steel plate contains, in wt%, carbon (C): 0.10 to 0.30%, silicon (Si): 2.5% or less (excluding 0%), manganese (Mn): 1.0 to 3.0%, chromium (Cr): 1.20% or less (excluding 0%), phosphorus (P): 0.0010 to 0.10%, sulfur (S): 0.010% or less (excluding 0%), soluble aluminum (Sol.Al): 0.010 to 0.10%, molybdenum (Mo): 0.001 to 0.2%, boron (B): 0.0010 to 0.0050%, titanium (Ti): 0.010 to 0.080%, the remainder being Fe and other unavoidable impurities, and satisfying the following relational expression 1, and the microstructure is, in area%, the sum of tempered martensite and bainite: 60 to 90%, Retained austenite: 3-15%, and the remainder contains at least one of ferrite and fresh martensite.

[0017] [Relationship 1]

[0018] 2.1 ≤ 7*[C]+(1.3*[Si]+[Mn]) / 6+([Cr]+1.2*[Mo]) / 5+100*[B]+2*[Ti]≤3.0

[0019] (In the above equation 1, [C], [Si], [Mn], [Cr], [Mo], [B], and [Ti] represent the contents (weight %) of C, Si, Mn, Cr, Mo, B, and Ti, respectively.)

[0020] The average grain size of the above tempered martensite may be 3㎛ or less (excluding 0㎛).

[0021] The average particle size of the above bainite may be 3㎛ or less (excluding 0㎛).

[0022] The above steel plate can have a tensile strength of 1180 MPa or more, a hole expandability of 30% or more, and an elongation of 14% or more.

[0023] The above steel plate can have a yield strength of 800 MPa or more.

[0024] According to other exemplary embodiments of the present invention, a method for manufacturing a steel plate is provided. The method for manufacturing the steel plate comprises the steps of: heating a steel slab comprising, in wt%, carbon (C): 0.10 to 0.30%, silicon (Si): 2.5% or less (excluding 0%), manganese (Mn): 1.0 to 3.0%, chromium (Cr): 1.20% or less (excluding 0%), phosphorus (P): 0.0010 to 0.10%, sulfur (S): 0.010% or less (excluding 0%), soluble aluminum (Sol.Al): 0.010 to 0.10%, molybdenum (Mo): 0.001 to 0.20%, boron (B): 0.0010 to 0.0050%, titanium (Ti): 0.010 to 0.080%, the remainder being iron and other unavoidable impurities, and satisfying the following relational expression 1; It includes a step of finishing hot rolling the heated steel slab at Ar3 to 1000°C to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet; a step of pickling and cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; a step of annealing the cold-rolled steel sheet at 780 to 870°C; a step of first cooling the annealed cold-rolled steel sheet to 550 to 700°C; a step of second cooling the first-cooled cold-rolled steel sheet to 250 to 350°C; and a step of overaging the second-cooled cold-rolled steel sheet at 200 to 450°C.

[0025] [Relationship 1]

[0026] 2.1 ≤ 7*[C]+(1.3*[Si]+[Mn]) / 6+([Cr]+1.2*[Mo]) / 5+100*[B]+2*[Ti]≤3.0

[0027] [Relationship 2]

[0028] 80.0 ≤ 0.3*[SS-Ac1]+0.85*[Ms-Tq]-1.22(Tp-Tq)+65*[Relationship 1]≤110.0

[0029] (In the above relational expression 2, SS means continuous annealing temperature, Ac1 means eutectoid transformation temperature, Ms means martensite transformation initiation temperature, Tq means secondary cooling end temperature, Tp means overaging treatment temperature, and [relational expression 1] means a value calculated by the above-described relational expression 1.)

[0030] In the step of heating the above steel slab, the heating temperature may be 1000 to 1350°C.

[0031] In the step of coiling the above hot-rolled steel sheet, the coiling temperature may be 500 to 750°C.

[0032] In the above first cooling step, the cooling rate may be 1 to 10°C / s.

[0033] In the above secondary cooling step, the cooling rate may be 5 to 40°C / s.

[0034] The method for manufacturing the above steel plate may further include a step of subjecting the over-aged steel plate to temper rolling at a reduction ratio in the range of 0.1 to 1.0%.

[0035] According to exemplary embodiments of the present invention, the formability of a steel sheet can be improved by controlling the alloy composition and microstructure of the steel sheet.

[0036] According to other exemplary embodiments of the present invention, a method for manufacturing a steel plate can be provided that can provide an ultra-high strength steel plate with excellent formability by optimizing the manufacturing conditions of the steel plate.

[0037] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0038] As mentioned above, microstructural control is crucial for producing steel sheets with excellent formability. Martensite generally possesses the highest strength among low-temperature structures. As is well known, the easiest way to produce martensite is to maintain the annealing process for a sufficient period of time for austenite to form, followed by water cooling and tempering. However, this well-known water-cooling method can cause material deviations and shape defects, thereby degrading the productivity of the steel sheet.

[0039] The inventors of the present invention attempted to secure uniform martensite by controlling the composition of alloying elements. Specifically, they attempted to secure martensite even at low cooling rates by adding a certain amount of hardenable elements such as manganese and chromium. However, this method can cause problems such as deterioration in weldability due to the high addition of alloying elements. Therefore, the present invention sought to minimize the carbon content, which has the greatest impact on weldability.

[0040] Furthermore, to ensure a high yield ratio under cooling conditions, such as in the steel plates according to exemplary embodiments of the present invention, alloying elements must be added in as large a quantity as possible. However, such attempts additionally cause problems such as deterioration in weldability and increased hot-rolled strength, and thus, solutions are necessary.

[0041] Accordingly, the inventors of the present invention have confirmed through various studies that the formability of a steel sheet can be improved and the yield ratio can be increased by controlling the size of martensite and the nano-precipitates of the microstructure without adding excessive alloying elements, and have thus completed the present invention.

[0042] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Prior to this, it should be noted that the terms and words used in this specification and claims should not be construed as limited to their conventional or dictionary meanings. Based on the principle that the inventor can appropriately define the concepts of terms to best explain his or her invention, they should be interpreted in a way that aligns with the technical spirit of the present invention.

[0043] Hereinafter, steel sheets and methods for manufacturing steel sheets according to exemplary embodiments will be described in detail. Prior to this, it should be noted that the terms and words used in this specification and claims should not be construed as limited to their conventional or dictionary meanings. Based on the principle that inventors can appropriately define the concepts of terms to best explain their inventions, they should be interpreted in a way that aligns with the technical spirit of the present invention.

[0044] Hereinafter, when describing with reference to drawings, identical or corresponding components are given the same drawing reference numerals and redundant descriptions thereof are omitted.

[0045] In the embodiments below, terms such as first, second, etc. are not used in a limiting sense, but are used for the purpose of distinguishing one component from another.

[0046] In the following embodiments, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0047] In the embodiments below, terms such as “include” or “have” mean that a feature or component described in the specification is present, and do not preclude the possibility that one or more other features or components may be added.

[0048] For convenience of explanation, the sizes of components in the drawings may be exaggerated or reduced. For example, the sizes and thicknesses of each component shown in the drawings are arbitrarily indicated for convenience of explanation, and thus the present invention is not necessarily limited to what is shown.

[0049] If a particular embodiment is capable of being implemented differently, a particular process sequence may be performed in a different order than described. For example, two processes described in succession may be performed substantially simultaneously, or in a reverse order from the described order.

[0050] In addition, when describing the present invention, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present invention, the detailed description is omitted.

[0051] Additionally, unless otherwise defined, the contents of the alloy composition described below refer to weight percent.

[0052] According to exemplary embodiments, carbon (C) may be included in an amount of 0.10 to 0.30%. More specifically, C may be included in an amount of 0.15 to 0.30%. C may be included in an amount of 0.2 to 0.3%. More specifically, C may be included in an amount of 0.25 to 0.30%.

[0053] Carbon (C) is a very important element added to strengthen the transformation structure. C promotes the high strength of steel sheets and promotes the formation of martensite in transformed structure steel. As the C content increases, the amount of martensite in the steel increases. However, when the C content exceeds 0.30%, the strength of martensite increases, but the strength difference with ferrite, which has a low carbon concentration, may increase. This strength difference between microstructures can easily cause fracture at the interphase interface when stress is applied, which can reduce the stretch flangeability. In addition, the weldability is deteriorated, which can cause welding defects during steel sheet processing. When the C content is less than 0.10%, it can be very difficult to secure the strength of martensite proposed in the present invention.

[0054] According to exemplary embodiments, silicon (Si) may be included in an amount of 2.50% or less (excluding 0%). More specifically, Si may be included in an amount of 2.30% or less. Si may be included in an amount of 2.0% or less. Si may be included in an amount greater than 1.0%. Si may be included in an amount greater than 1.0% and less than or equal to 2.3%.

[0055] Silicon in steel promotes ferrite transformation and increases the carbon content in untransformed austenite, forming a composite structure of ferrite and martensite, which can hinder martensite strength gains. Furthermore, it can cause surface scale defects and reduce chemical processability, thus limiting its addition whenever possible.

[0056] According to exemplary embodiments, manganese (Mn) may be included in an amount of 1.0 to 3.0%. More specifically, Mn may be included in an amount of 1.5 to 3.0%. Mn may be included in an amount of 2.0 to 3.0%. Mn may be included in an amount of 2.3 to 2.9%.

[0057] Manganese (Mn) in steel can refine particles without damaging ductility and completely precipitate sulfur (S) in the steel as MnS, thereby preventing hot embrittlement caused by the formation of FeS and strengthening the steel. At the same time, Mn lowers the critical cooling rate at which martensite is formed, making it easier to form martensite. If the Mn content is less than 1.0%, it may be difficult to secure the strength targeted in the present invention. If the Mn content exceeds 3.0%, it may cause problems such as weldability and hot-rollability of the steel sheet.

[0058] According to exemplary embodiments, chromium (Cr) may be included at 1.2% (excluding 0%). More specifically, Cr may be included at 1.0% or less. Cr may be included at 0.01 to 1.0%.

[0059] Chromium (Cr) is added to steel to enhance its hardenability and ensure high strength. Cr, in particular, plays a crucial role in the formation of martensite, a low-temperature transformation phase. Therefore, higher Cr contents further enhance the aforementioned effects. However, if Cr content exceeds 1.2%, not only will the effects become saturated, but excessive increases in hot-rolled strength can also lead to deterioration of cold-rollability.

[0060] According to exemplary embodiments, molybdenum (Mo) may be included in an amount of 0.001 to 0.20%. More specifically, Mo may be included in an amount of 0.04 to 0.2%. Mo may be included in an amount of 0.04 to 0.1%.

[0061] Molybdenum (Mo) is an element added to secure strength and hardenability, and when added together with Ti, it forms carbides together with Ti. In order to obtain the tissue strengthening effect resulting from the formation of such carbides, the content of Mo must be added at least 0.001%. However, as it is an expensive element, it may reduce the economic feasibility of the steel sheet and may excessively delay phase transformation, thereby inducing the formation of fresh martensite. Therefore, the content of Mo may not exceed 0.2%.

[0062] According to exemplary embodiments, boron (B) may be included in an amount of 0.0010 to 0.0050%. More specifically, B may be included in an amount of 0.0010 to 0.0025%. B may be included in an amount of 0.0010 to 0.0021%. B may be included in an amount of 0.0014 to 0.0018%.

[0063] In steel, B is a component that delays the transformation of austenite into pearlite during the cooling process during annealing, thereby inhibiting ferrite formation and promoting martensite formation. However, when the B content is less than 0.0010%, it is difficult to achieve the above effect, and when it exceeds 0.0050%, cost deterioration due to excessive alloying iron may occur.

[0064] According to exemplary embodiments, titanium (Ti) may be included in an amount of 0.010 to 0.080%. More specifically, Ti may be included in an amount of 0.010 to 0.050%. Ti may be included in an amount of 0.010 to 0.030%. Ti may be included in an amount of 0.020 to 0.080%. Ti may be included in an amount of 0.030 to 0.060%.

[0065] Ti in steel can contribute to increased strength of steel sheets and grain refinement by precipitating nano-precipitates through bonding with carbon. These nano-precipitates can strengthen the matrix structure and reduce the hardness difference between phases. To achieve this, Ti can be included at a level of 0.01% or more. However, if the Ti content exceeds 0.08%, coarsening precipitates can reduce elongation.

[0066] According to exemplary embodiments, available aluminum (Sol.Al) may be included in an amount of 0.010 to 0.10%.

[0067] Sol.Al can contribute to improving martensitic hardening by combining with oxygen in steel, acting as a deoxidizer and distributing carbon within ferrite to austenite, similar to Si. For this purpose, Sol.Al can be included in an amount of 0.01% or more. However, if the Sol.Al content exceeds 0.10%, the aforementioned effects are saturated, and manufacturing costs may increase.

[0068] According to exemplary embodiments, sulfur (S) may be included in an amount of 0.010% or less (excluding 0%).

[0069] S, as an impurity element in steel, can impair the ductility and weldability of steel sheets. More specifically, if the S content exceeds 0.01%, the ductility and weldability of the steel sheet are likely to be impaired. Therefore, a lower S content is preferable. However, since it may inevitably be included during the manufacturing process, a 0% S content is excluded.

[0070] According to exemplary embodiments, phosphorus (P) may be included in an amount of 0.0010 to 0.10%. More specifically, P may be included in an amount of 0.0010 to 0.050%. P may be included in an amount of 0.0010 to 0.030%. P may be included in an amount of 0.0010 to 0.011%.

[0071] Among the substitutional alloying elements with the greatest strengthening effect, phosphorus (P) can improve in-plane anisotropy and contribute to the strength of steel sheets. However, if the P content is less than 0.001%, the effect cannot be achieved and can lead to manufacturing costs. Excessive addition of P can deteriorate the press formability of the steel sheet and cause brittleness.

[0072] The remaining component is iron (Fe). However, during the normal 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 normal manufacturing process, their full details are not specifically mentioned in this specification.

[0073] According to exemplary embodiments, the steel plate can satisfy the alloy composition described above and at the same time satisfy the following relationship 1.

[0074] [Relationship 1]

[0075] 2.1≤7*[C]+(1.3*[Si]+[Mn]) / 6+([Cr]+1.2*[Mo]) / 5+100*[B]+2*[Ti]≤3.0

[0076] In the above relational expression 1, [C], [Si], [Mn], [Cr], [Mo], [B], and [Ti] represent the contents (weight %) of C, Si, Mn, Cr, Mo, B, and Ti, respectively.

[0077] In order to improve formability while maintaining the strength of the steel plate, numerous experiments were conducted on steel plates that satisfy the alloy composition of the steel plate described above. Through these experiments, it was confirmed that it is important to satisfy the above-described relationship 1 in order to secure a hole expansion ratio (HER) of at least 25% and an elongation of 10% or more under the condition of securing ductility of the steel plate. Therefore, even if the alloy composition satisfies the above-described range, if the above-described relationship 1 is not satisfied, it may be difficult to achieve the steel plate according to the exemplary embodiments of the present invention and the effects thereof.

[0078] According to exemplary embodiments, the microstructure of the steel sheet may include, in area %, a sum of tempered martensite and bainite: 60 to 90%, retained austenite: 3 to 15%, and the remainder being at least one of ferrite and fresh martensite.

[0079] In order to achieve an ultra-high strength steel sheet capable of exhibiting a tensile strength (TS) of 1180 MPa or higher, the microstructure of the steel sheet may contain tempered martensite and bainite in a combined fraction of 60% or more, in terms of area %. However, in consideration of improving the elongation of the steel sheet, the sum of tempered martensite and bainite may be 90% or less. As a non-limiting example, tempered martensite may be contained in an amount of 50 to 80%. Bainite may be contained in an amount of 10 to 30%.

[0080] The purpose of the steel plate of the present invention is to secure excellent formability at a tensile strength (TS) of 1180 MPa or higher. In particular, in order to obtain high local formability, the difference in hardness between the microstructure phases constituting the steel plate must be reduced. Retained austenite is a structure that increases the elongation of steel through the TRIP effect, and a higher fraction of retained austenite allows for a higher elongation to be obtained. In order to obtain a required level of elongation, the fraction of retained austenite may be 3% or more. However, in order to obtain a retained austenite exceeding 15%, a phase transformation must be obtained at a high temperature, and it may be difficult to achieve a high strength of 1180 MPa or higher.

[0081] Fresh martensite is formed during the final cooling and has a high contribution to strength, but since it significantly impairs formability in an untempered state, its fraction can be kept low to obtain the high elongation of the present invention. Thus, controlling the fraction of fresh martensite to a low level can be advantageous in securing the formability of the steel sheet, and thus the final steel sheet may contain substantially no fresh martensite. As a non-limiting example, fresh martensite may be contained at 10% or less (including 0%).

[0082] Meanwhile, if the ferrite fraction exceeds 5%, the yield strength may decrease and formability, such as hole expandability, may deteriorate. In this way, ferrite, as a soft structure, can contribute to improving the formability of steel sheets, and its lower limit may be 0.5% or more.

[0083] The average grain size of tempered martensite may be 3 ㎛ or less (excluding 0 ㎛). More specifically, the average grain size of tempered martensite may be 2 ㎛ or less. The average grain size of tempered martensite may be 1.2 to 2.0 ㎛.

[0084] The average grain size of bainite may be 3 ㎛ or less (excluding 0 ㎛). The average grain size of bainite may be 2.5 ㎛ or less. The average grain size of bainite may be 2.0 to 3.0 ㎛.

[0085] The grain size of the hard tissue is related to the formability of the steel sheet. In other words, the formability of the steel sheet can be improved by finely controlling the grain size of the hard tissue.

[0086] The steel sheet according to exemplary embodiments including the above-described alloy composition and microstructure can have a tensile strength of 1180 MPa or more and a yield strength of 800 MPa or more. At the same time, it can have a hole expandability (HER) of 30% or more and an elongation of 14% or more. Thus, according to exemplary embodiments, an ultra-high strength steel sheet having a tensile strength of 1180 MPa or more and excellent formability can be provided.

[0087] Hereinafter, a method for manufacturing a steel plate according to exemplary embodiments will be described in detail. According to exemplary embodiments, the steel plate can be manufactured through the process of [steel slab heating - hot rolling - cold rolling - continuous annealing - cooling - overaging treatment]. However, the present invention is not limited thereto, and those skilled in the art can appropriately modify the method without departing from the technical spirit of the present invention.

[0088] The steel slab heating can be performed by heating a steel slab that satisfies the above-described alloy composition and the relationship 1. According to exemplary embodiments, the steel slab heating can be performed by heating at 1000 to 1350°C. More specifically, the steel slab heating can be performed at 1100 to 1350°C. The steel slab heating can be performed at 1130 to 1350°C. The steel slab heating can be performed at 1100 to 1300°C. The steel slab heating can be performed at 1100 to 1250°C.

[0089] When the steel slab's heating temperature is below 1000℃, the steel slab can be hot-rolled within the range below the finishing hot-rolling temperature range. However, when the steel slab's heating temperature exceeds 1350℃, it may reach the steel's melting point and melt.

[0090] Hot rolling can be performed on a heated steel slab, thereby producing a hot-rolled steel sheet.

[0091] According to exemplary embodiments, the hot rolling may be performed such that the exit temperature of the finishing mill is Ar3 to 1000°C. More specifically, the hot rolling may be performed such that the exit temperature of the finishing mill is 800°C or higher during the finishing hot rolling. The hot rolling may be performed such that the exit temperature of the finishing mill is 850°C or higher during the finishing hot rolling. The hot rolling may be performed such that the exit temperature of the finishing mill is 980°C or lower during the finishing hot rolling. The hot rolling may be performed such that the exit temperature of the finishing mill is 960°C or lower during the finishing hot rolling.

[0092] If the exit temperature of the finishing mill is below the Ar3 point, the hot deformation resistance is likely to increase rapidly, and after coiling, the top, bottom (tail), and edges of the hot-rolled coil may become single-phase regions, which may increase in-plane anisotropy and deteriorate formability. However, if the exit temperature of the finishing mill exceeds 1000°C, not only will an excessively thick oxide scale be generated, but the microstructure of the steel sheet is likely to coarsen.

[0093] Ar3 can be obtained through the following [Formula 1].

[0094] [Formula 1]

[0095] Ar3(℃) = 910 - 203√v[C] - 30[Mn] + 44.7[Si] - 11[Cr] + 31.5[Mo] - 15.2[Ni]

[0096] In the above formula 1, [C], [Mn], [Si], [Cr], [Mo], and [Ni] represent the contents of alloy components, respectively.

[0097] According to exemplary embodiments, hot-rolled steel sheets can be coiled at a temperature of 500 to 750°C. This allows for the production of hot-rolled coils. More specifically, hot-rolled steel sheets can be coiled at a temperature of 530 to 750°C. Hot-rolled steel sheets can be coiled at a temperature of 600 to 750°C. Hot-rolled steel sheets can be coiled at a temperature of 500 to 700°C.

[0098] If the coiling temperature is below 500℃, excessive martensite or bainite may form, resulting in excessive strength gains in hot-rolled steel sheets. This can lead to problems such as shape defects due to load during subsequent cold rolling. Conversely, if the temperature exceeds 750℃, surface scale may increase, reducing pickling properties.

[0099] Cold rolling can be performed on hot rolling. More specifically, cold rolling can be performed using hot-rolled coils. This allows for the production of cold-rolled steel sheets.

[0100] As a non-limiting example, pickling may be performed on hot rolled steel prior to cold rolling.

[0101] In the present invention, there are no particular limitations on the pickling and cold rolling conditions, and all conditions used in the relevant technical field can be utilized. However, as an example, the cold rolling may be performed at a cold reduction ratio of 20 to 90%. If the cold reduction ratio is less than 20%, it may be difficult to secure the target thickness precision and shape correction of the steel sheet may also become difficult. If the cold reduction ratio exceeds 90%, cracks may occur at the edge of the steel sheet, and the cold rolling load may become excessively large in terms of productivity.

[0102] Continuous annealing can be performed on cold rolled steel sheets.

[0103] According to exemplary embodiments, the annealing temperature may be 780 to 870°C.

[0104] At low annealing temperatures, large amounts of ferrite are formed, making it difficult to secure yield and tensile strength. However, at high annealing temperatures, the austenite grain size increases due to high-temperature annealing, which can increase the martensite packet size produced upon cooling. This can make it difficult to control the average grain size of the transformed structures.

[0105] Cooling can be performed on a continuously annealed cold rolled steel sheet. According to exemplary embodiments, the cooling can include primary cooling and secondary cooling.

[0106] According to exemplary embodiments, the first cooling may be performed by cooling the continuously annealed cold-rolled steel sheet to 550 to 700°C. More specifically, the cooling rate during the first cooling may be 1 to 10°C / s. The first cooling step is to suppress the ferrite transformation of austenite and transform most of the austenite into martensite. To this end, it is important to control the cooling rate and the cooling end temperature within the above-described range. If the first cooling end temperature is less than 550°C or exceeds 700°C, productivity may decrease. If the first cooling rate is less than 1°C / s, a ferrite phase may be formed during cooling, making it difficult to secure high strength. If the first cooling rate exceeds 10°C / s, the cooling amount in the second cooling may increase, which may have the disadvantage of increasing the final temperature deviation and material deviation.

[0107] Secondary cooling may be performed after primary cooling. According to exemplary embodiments, secondary cooling may be performed by cooling the primary cooled steel plate to 250 to 350°C. More specifically, the cooling rate during secondary cooling may be 5 to 40°C / s.

[0108] This secondary cooling end temperature is to secure the shape of the coil in the width and length directions, as well as to secure a high yield ratio and hole expandability. If the secondary cooling end temperature is low, the yield strength and tensile strength may increase simultaneously due to an excessive increase in the amount of martensite during the overaging treatment, and the ductility may deteriorate significantly. In particular, shape deterioration due to rapid cooling is expected to occur, which may deteriorate workability when processing automobile parts. If the secondary cooling end temperature is high, the austenite generated during annealing cannot transform into martensite, and high-temperature transformation phases such as bainite and granular bainite are excessively generated, which causes a rapid deterioration in the yield strength. The excessive generation of such structures can cause a decrease in the yield ratio and a deterioration in hole expandability.

[0109] Overaging treatment can be performed by reheating and overaging the cold-rolled steel sheet obtained after cooling. According to exemplary embodiments, the overaging treatment can be performed at 200 to 450°C.

[0110] The above overaging treatment is intended to stabilize retained austenite and promote additional bainite phase transformation. If the overaging temperature is below 200°C, the strength may be excessively high and formability may deteriorate. If the overaging temperature exceeds 450°C, it may be difficult to achieve the strength desired in the present invention.

[0111] According to exemplary embodiments, the method for manufacturing a steel sheet may further include a step of temper rolling the over-aged steel sheet.

[0112] Temper rolling can be performed by skin pass rolling the over-aged steel sheet at a reduction ratio in the range of 0.1 to 1.0%.

[0113] Typically, when skin-pass rolling a transformed structure steel, yield strength increases by at least 50 MPa with minimal increase in tensile strength. If the reduction ratio is less than 0.10%, shape control can become very difficult in ultra-high-strength steels such as the steel of the present invention. If the reduction ratio exceeds 1.0%, high-elongation operations can significantly destabilize operability.

[0114] According to exemplary embodiments, a method for manufacturing a steel plate can satisfy the following relationship 2.

[0115] [Relationship 2]

[0116] 80.0 ≤ 0.3 * [SS-A1] + 0.85 * [Ms-Tq] - 1.22(Tp-Tq) + 65 * [Relationship 1] ≤ 110.0

[0117] In the above relational expression 2, SS represents the continuous annealing temperature, Ac1 represents the eutectoid transformation temperature, Ms represents the martensite transformation initiation temperature, Tq represents the secondary cooling end temperature, and Tp represents the overaging treatment temperature.

[0118] In the above relational expression 2, [relational expression 1] means the value calculated by the above-described relational expression 1.

[0119] In the above equation 2, Ac1=723-10.7[Mn]-16.9[Ni]+29.1[Si]+16.9[Cr], Ms=539-423[C]-30.4[Mn]-12.1[Cr]-7.5[Mo]. [Mn], [Ni], [Si], [Cr], [C], and [Mo] represent the contents (weight %) of Mn, Ni, Si, Cr, C, and Mo, respectively.

[0120] The above relational expression 2 is intended to simultaneously secure strength, elongation, and hole expandability. If the value of the above relational expression 2 is less than 80.0, the target tensile strength cannot be achieved. If the value of the above relational expression 2 exceeds 110.0, the target elongation cannot be achieved. Therefore, the value of the above relational expression 2 is preferably in the range of 80.0 to 110.0.

[0121] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0122] (Example)

[0123] The steel slabs prepared as shown in Table 1 were heated at 1200°C for 1 hour. Thereafter, hot rolling was performed on the heated steel slabs and then coiled. The temperature conditions during the hot rolling operation were 880 to 920°C, which is higher than Ar3 as shown in Table 2, and the coiling temperature was controlled at 500 to 680°C. The hot-rolled steel sheets were pickled and then cold rolled. The cold-rolled steel sheets were subjected to continuous annealing, cooling, and overaging treatments under the conditions shown in Table 2, and temper rolling was performed at a reduction ratio of 0.2%.

[0124] Steel grade CSiMnCrTiMoBSol.AlPSRelationship 110.1811.452.250.320.0210.0480.00140.0210.0090.0032.2120.1511.712.430.40.0320.10.00210.0210.0110.0042.2130.221.32.120.050.040.05 0.00180.0180.0110.0042.4640.310.762.690.530.090.120.00220.0250.0110.0033.3250.180.622.340.210.040.070.00170.0250.0120.0032.09 [Relationship 1] 2.1 ≤7*[C]+(1.3*[Si]+[Mn]) / 6+([Cr]+1.2*[Mo]) / 5+100*[B]+2*[Ti]≤3.0

[0125] Classification Steel grade No. Ar3 (℃) Ac1 (℃) Ms (℃) Finishing rolling mill exit temperature (℃) Coiling temperature (℃) Continuous annealing temperature (SS) (℃) First cooling end temperature (℃) Second cooling end temperature (Tq) (℃) Overaging treatment temperature (Tp) (℃) Relationship equation 2 Invention example 11819747.65 390 90 55 22 8 5 5 6 12 3 0 4 18 110.0 Invention example 21819747.65 390 90 55 22 8 49 6 23 3 18 4 3 4 9 4.0 Comparative example 11819747.65 390 90 55 22 8 5 26 13 3 11 4 9 5 18.0 Comparative example 21819747.6539090552285365120042068.0 Comparative Example 31819747.6539090552282161429843577.0 Invention Example 32833754.74396897543851611311423108.0 Invention Example 42833754.74396897543853611298426100.0 Comparative Example 42833754.7439689754385262319842366.0 Invention Example 53810740.05381910532855614300425110.0 Invention Example 63810740.05381910532851614300428105.0Invention example 73810740.0538191053284761429943595.0Comparative example 53810740.05381910532855623298375170.0Comparative example 64748726.64319898524850614300420122.0Comparative example 74748726.64319898524857624300420124.0Comparative example 84748726.64319898524855614300420124.0Comparative example 94748726.64319898524852603305420125.0Comparative example 105781720.72389903512853617302307243.0[Relationship 2] 80.0 ≤0.3*[SS-Ac1]+0.85*[Ms-Tq]-1.22(Tp-Tq)+65*[Relationship 1]≤110.0

[0126] The microstructure and mechanical properties of the steel plates manufactured in this manner were measured, and the results are shown in Table 3 below.

[0127] The types, fractions, and average particle sizes of the microstructures were measured using EBSD and XRD analysis techniques.

[0128] The hole expandability (HER) was measured by forming a 10 mmΨ punching hole (die inner diameter 10.3 mm, clearance 12.5%) by pressing and expanding a conical punch with a 60° apex angle at 20 mm / min in the direction in which the burr of the punching hole becomes outer.

[0129] Hole expandability (HER) (%) = {(D - D0) / D0} Х 100

[0130] (Note that D: refers to the hole diameter (mm) when the crack penetrates the steel plate, and D0: refers to the initial hole diameter (mm).)

[0131] The yield strength (YS), tensile strength (TS), and elongation (El) of the steel plate were measured by producing JIS No. 5 tensile test specimens and performing a tensile test.

[0132] Classification Microstructure Steel Plate Properties TM+B(Area%) Residual γ(Area%) FM + F(Area%) TM Grain Size(㎛) B Grain Size(㎛) Yield Strength(MPa) Tensile Strength(MPa) Elongation(%) Hole Expandability(%) Invention Example 1877.45.61.72.4995121014.941 Invention Example 2878.34.71.42.2956119814.635 Comparative Example 1842.313.71.53.41050112512.638 Comparative Example 2951.93.11.22.1923132510.534 Comparative Example 3557.337.71.32.41211134210.629 Invention Example 3888.43.61.62.3945123415.946 Invention example 485781.32.5995121014.935 Comparative example 4962.21.81.22.1126513858.121 Invention example 5878.84.21.32.31011122016.136 Invention example 68110.18.91.52.51026119815.232 Invention example 78211.86.21.42.31033123415.429 Comparative example 58311.85.21.52.11105130210.338 Comparative example 6843.412.61.52.5112513029.823Comparative example 7863.110.91.42.3113512879.532Comparative example 8843.212.81.62.4116512958.528Comparative example 9844.211.81.52.31221131210.221Comparative example 1089831.61.61052132411.236TM: Tempered martensite, B: Bainite, γ: Austenite, FM: Fresh martensite, F: Ferrite

[0133] Referring to Table 3, it was confirmed that invention examples 1 to 7, which satisfy both the alloy composition and manufacturing conditions according to exemplary embodiments, achieved a tensile strength of 1180 MPa or more, an HER of 30% or more, and an elongation of 14% or more. That is, according to exemplary embodiments of the present invention, an ultra-high strength steel sheet having a tensile strength of 1180 MPa or more and excellent formability can be provided.

[0134] Comparative Example 1 satisfied the alloy composition proposed by the present invention, but exhibited poor elongation due to inadequate formation of retained austenite. This is believed to be due to the excessively high overaging temperature and failure to satisfy the range of Equation 2 of the manufacturing process conditions.

[0135] Comparative Examples 2 and 4 satisfied the alloy composition proposed by the present invention, but the sum of tempered martensite and bainite was excessively high, and retained austenite was not formed properly. As a result, the elongation of the steel sheet was inferior. This is believed to be due to the excessively low secondary cooling end temperature, which led to the rapid formation of a transformed structure, thereby reducing the formability of the steel sheet.

[0136] Comparative Example 3 satisfied the alloy composition and individual process conditions proposed by the present invention, but exhibited poor elongation and hole expandability. This is believed to be due to the manufacturing process conditions not satisfying the range of Equation 2, resulting in excessive formation of fresh martensite.

[0137] Comparative Examples 5 to 10 satisfied the individual process conditions proposed by the present invention, but the formability of the steel sheets was inferior. This is believed to be because the alloy composition range was not satisfied and the manufacturing process conditions did not satisfy the range of Equation 2.

[0138] The present invention has been described in more detail through drawings and examples. However, the configurations described in the drawings or examples described in this specification are merely embodiments of the present invention and do not represent all of the technical ideas of the present invention. Therefore, it should be understood that various equivalents and modified examples may exist as of the time of this application.

Claims

1. Contains, in wt%, carbon (C): 0.10 to 0.30%, silicon (Si): 2.5% or less (excluding 0%), manganese (Mn): 1.0 to 3.0%, chromium (Cr): 1.20% or less (excluding 0%), phosphorus (P): 0.0010 to 0.10%, sulfur (S): 0.010% or less (excluding 0%), available aluminum (Sol.Al): 0.010 to 0.10%, molybdenum (Mo): 0.001 to 0.20%, boron (B): 0.0010 to 0.0050%, titanium (Ti): 0.010 to 0.080%, and the remainder is made up of iron and other unavoidable impurities. Satisfies the following relation 1, A steel sheet having a microstructure, in area %, comprising the sum of tempered martensite and bainite: 60 to 90%, retained austenite: 3 to 15%, and the remainder being at least one of ferrite and fresh martensite. [Relationship 1] 2.1 ≤7*[C]+(1.3*[Si]+[Mn]) / 6+([Cr]+1.2*[Mo]) / 5+100*[B]+2*[Ti]≤3.0 (In the above equation 1, [C], [Si], [Mn], [Cr], [Mo], [B], and [Ti] represent the contents (weight %) of C, Si, Mn, Cr, Mo, B, and Ti, respectively.) 2. In paragraph 1, Steel plate having an average grain size of the above tempered martensite of 3㎛ or less (excluding 0㎛).

3. In paragraph 1, Steel plate having an average grain size of the above bainite of 3㎛ or less (excluding 0㎛).

4. In paragraph 1, The above steel plate, Steel plate having a tensile strength of 1180 MPa or more, a hole expandability of 30% or more, and an elongation of 14% or more.

5. In paragraph 1, The above steel plate, Steel plate having a yield strength of 800 MPa or more.

6. A step of heating a steel slab comprising, by weight%, carbon (C): 0.10 to 0.30%, silicon (Si): 2.5% or less (excluding 0%), manganese (Mn): 1.0 to 3.0%, chromium (Cr): 1.20% or less (excluding 0%), phosphorus (P): 0.0010 to 0.10%, sulfur (S): 0.010% or less (excluding 0%), available aluminum (Sol.Al): 0.010 to 0.10%, molybdenum (Mo): 0.001 to 0.20%, boron (B): 0.0010 to 0.0050%, and titanium (Ti): 0.010 to 0.080%, with the remainder being iron and other unavoidable impurities, and satisfying the following relational expression 1; A step of obtaining a hot rolled steel sheet by final hot rolling the above heated steel slab at Ar3~1000℃; A step of coiling the above hot-rolled steel plate; A step of obtaining a cold rolled steel sheet by pickling and cold rolling the above-mentioned hot rolled steel sheet; A step of continuously annealing the above cold rolled steel sheet at 780 to 870°C; A step of first cooling the above-mentioned annealed cold rolled steel sheet to 550 to 700°C; A step of secondarily cooling the first-cooled cold-rolled steel sheet to 250 to 350°C; and A method for manufacturing a steel sheet, comprising: a step of overaging the secondarily cooled cold rolled steel sheet at 200 to 450°C. [Relationship 1] 2.1 ≤7*[C]+(1.3*[Si]+[Mn]) / 6+([Cr]+1.2*[Mo]) / 5+100*[B]+2*[Ti]≤3.0 [Relationship 2] 80.0 ≤0.3*[SS-Ac1]+0.85*[Ms-Tq]-1.22(Tp-Tq)+65*[Relationship 1]≤110.0 (In the above relational expression 2, SS represents the continuous annealing temperature, Ac1 represents the eutectoid transformation temperature, Ms represents the martensite transformation initiation temperature, Tq represents the secondary cooling end temperature, Tp represents the overaging treatment temperature, and [relational expression 1] represents the value calculated by the above-described relational expression 1.) 7. In paragraph 6, A method for manufacturing a steel plate, wherein in the step of heating the steel slab above, the heating temperature is 1000 to 1350°C.

8. In paragraph 6, A method for manufacturing a steel plate, wherein in the step of coiling the hot-rolled steel plate, the coiling temperature is 500 to 750°C.

9. In paragraph 6, A method for manufacturing a steel plate, wherein in the first cooling step, the cooling rate is 1 to 10°C / s.

10. In paragraph 6, A method for manufacturing a steel plate, wherein in the second cooling step, the cooling rate is 5 to 40°C / s.

11. In paragraph 6, The manufacturing method of the above steel plate is: A method for manufacturing a steel sheet further comprising the step of subjecting the over-aging-treated steel sheet to temper rolling at a reduction ratio in the range of 0.1 to 1.0%.