High-strength steel sheet excellent in bendability and formability and method for manufacturing the same

Abstract

The present invention relates to a steel suitable for use in automotive materials, and more particularly to a high-strength steel sheet having excellent bendability and formability and a method of manufacturing the same.

Description

Technical Field

[0001] This invention relates to a steel suitable for automotive materials, and more specifically to a high-strength steel sheet with excellent bending and formability, and a method for manufacturing the same. Background Technology

[0002] In recent years, the automotive industry has needed to use high-strength steel to improve fuel efficiency or durability due to environmental and energy regulations related to CO2 emissions.

[0003] In particular, with the expansion of regulations on the impact stability of automobiles, high-strength steel with excellent strength is being used as the material for structural components such as members, seat rails, and pillars to improve the impact resistance of the vehicle body.

[0004] These automotive parts have complex shapes for stability and design purposes, and are mainly formed and manufactured using stamping dies, thus requiring high strength and a high level of formability.

[0005] As the strength of steel increases, it gains the advantage of absorbing impact energy. However, as strength increases, elongation typically decreases, leading to reduced processability. Furthermore, when the yield strength is too high, the formability deteriorates due to the reduction of material introduced into the mold during forming, and manufacturing costs increase.

[0006] Furthermore, because automotive parts have numerous expanded forming areas after hole machining, bendability is required for successful forming. However, high-strength steel has low bendability, leading to defects such as cracks during the forming process. As mentioned above, poor bendability can cause cracks in the formed parts of the component during a car collision, making the component easily damaged and potentially threatening passenger safety.

[0007] In addition, representative high-strength steels used in automotive materials include dual-phase steel (DP steel), transformation-induced plasticity steel (TRIP steel), complex-phase steel (CP steel), and ferrite-bainite steel (FB steel).

[0008] DP steel, as an ultra-high tensile steel, has a low yield strength ratio of approximately 0.5 to 0.6, making it easy to process. It also boasts a high elongation rate, second only to TRIP steel. Therefore, it is primarily used in automotive door exterior trim, seat rails, seat belts, suspension systems, armrests, and wheel hubs.

[0009] Because TRIP steel has a yield strength ratio in the range of 0.57 to 0.67, it exhibits excellent formability (high ductility), making it suitable for components, roofs, seat belts, bumpers, and other parts requiring high formability.

[0010] CP steel has a low yield strength ratio, high elongation and bending workability, so it is used in side panels, body bottom reinforcements, etc. FB steel has excellent hole expansion properties, so it is mainly used in suspension lower arms or wheel discs, etc.

[0011] DP steel is primarily composed of ferrite with excellent ductility and hard phases (martensite and bainite), and may contain trace amounts of retained austenite. This type of DP steel exhibits low yield strength and high tensile strength, resulting in a low yield ratio (YR). It also possesses excellent properties such as high work hardening rate, high ductility, continuous yield behavior, resistance to aging at room temperature, and bake hardening. Furthermore, by controlling the fraction of each phase, the degree of recrystallization, and the uniformity of distribution, high-strength steel with high bending flexibility can be produced.

[0012] However, in order to ensure ultra-high strength with a tensile strength of over 980 MPa, it is necessary to increase the fraction of hard phases such as martensite, which are beneficial to improving strength. In this case, due to the increase in yield strength, there is a problem of defects such as cracks generated during the stamping process.

[0013] Typically, automotive DP steel is manufactured into slabs through steelmaking and continuous casting processes. The slabs are then subjected to [heating-rough rolling-hot finishing rolling] to obtain hot-rolled coils, which are then annealed to produce the final product.

[0014] Annealing is a process mainly carried out during the manufacturing of cold-rolled steel sheets. Cold-rolled steel sheets are manufactured by pickling hot-rolled coils to remove surface oxide scale, cold-rolling them at room temperature with a certain reduction rate, and then annealing and further leveling rolling processes as needed.

[0015] Cold-rolled steel sheets (cold-rolled materials) obtained by cold rolling are in a very solidified state and are not suitable for manufacturing parts that require machinability. Therefore, they can be softened by heat treatment in a continuous annealing furnace as a subsequent process to improve machinability.

[0016] As an example, in the annealing process, steel plates (cold-rolled materials) in a heating furnace can be heated to about 650-850°C and held for a certain period of time to reduce hardness and improve processability through recrystallization and phase transformation.

[0017] Steel plates that have not undergone annealing have high hardness, especially high surface hardness, and poor machinability. In contrast, steel plates that have undergone annealing have recrystallized structures, and due to the reduction in hardness, yield point, and tensile strength, their machinability can be improved.

[0018] As a representative method for reducing the yield strength of DP steel, continuous annealing allows ferrite to be fully recrystallized and manufactured in an equiaxed crystal shape during the heating process. In subsequent processes, austenite is formed and grown in an equiaxed crystal shape, which is beneficial for forming austenite phase with small and uniform particle size.

[0019] Furthermore, as a prior art for improving the workability of high-strength steel, Patent Document 1 proposes a method for microstructure refinement, specifically disclosing a method for dispersing finely precipitated copper particles with a particle size of 1-100 nm within the microstructure of a composite microstructure steel plate with martensite as the main component. However, this technique requires the addition of 2-5% Cu to obtain good finely precipitated phase particles, which may lead to red-hot brittleness caused by a large amount of Cu, and also results in an excessive increase in manufacturing costs.

[0020] Patent Document 2 discloses a steel sheet with ferrite as the matrix structure, having a structure containing 2-10% by area pearlite, and improving strength through precipitation strengthening and grain refinement brought about by the addition of carbide-nitride forming elements (e.g., Ti, etc.). This steel sheet exhibits good porosity; however, it has limitations in further improving tensile strength due to its high yield strength and low ductility, leading to cracking during new forming.

[0021] Patent document 3 discloses a manufacturing technology for cold-rolled steel sheets that simultaneously achieve high strength and high ductility using tempered martensitic phase and have excellent sheet shape after continuous annealing. However, the carbon (C) content in the steel is as high as 0.2% or more, which leads to poor weldability and the addition of a large amount of Si, resulting in furnace indentation defects.

[0022] Based on the existing technologies mentioned above, in order to improve the formability and bending properties of high-strength steel that meet physical properties such as weldability, it is necessary to develop a method that can reduce yield strength while improving ductility.

[0023] (Patent Document 1) Japanese Patent Publication No. 2005-264176

[0024] (Patent Document 2) Korean Patent Publication No. 2015-0073844

[0025] (Patent Document 3) Japanese Patent Publication No. 2010-090432 Summary of the Invention

[0026] Technical problems to be solved

[0027] One aspect of the present invention is to provide a high-strength steel sheet and a method for manufacturing the same, wherein the high-strength steel sheet is a material suitable for use in automotive structural components, etc., and has a low yield strength ratio, high strength, and excellent formability such as bending properties by improving ductility.

[0028] The technical problem of this invention is not limited to the above description. The technical problem of this invention can be understood from the entire contents of this specification, and those skilled in the art can readily understand the additional technical problems of this invention.

[0029] Technical solution

[0030] One aspect of the present invention provides a high-strength steel sheet with excellent bending and formability, comprising, by weight percent: carbon (C): 0.05-0.12%, manganese (Mn): 2.0-3.0%, silicon (Si): less than 0.5% (except 0%), chromium (Cr): less than 1.0% (except 0%), niobium (Nb): less than 0.1% (except 0%), titanium (Ti): less than 0.1% (except 0%), boron (B): less than 0.0025% (except 0%), aluminum (… 0.02-0.05% of sol.Al, less than 0.05% of phosphorus (P), less than 0.05% of sulfur (S), less than 0.01% of nitrogen (N), less than 0.01% of iron (Fe), and other unavoidable impurities. The microstructure comprises 35-50% ferrite, 35-45% bainite, and the balance martensite in an area fraction of 8-15% unrecrystallized ferrite and 27-35% recrystallized ferrite in an area fraction of 27-35%.

[0031] Another aspect of the present invention provides a method for manufacturing a high-strength steel sheet with excellent bending and formability, characterized by comprising the following steps: preparing a steel billet having the above-mentioned alloy composition; heating the steel billet in a temperature range of 1100-1300°C; hot-rolling the heated steel billet to manufacture a hot-rolled steel sheet; coiling the hot-rolled steel sheet in a temperature range of 400-700°C; cooling the hot-rolled steel sheet to room temperature after coiling; cold-rolling the cooled hot-rolled steel sheet to manufacture a cold-rolled steel sheet; subjecting the cold-rolled steel sheet to continuous annealing; after the continuous annealing, performing a first cooling at an average cooling rate of 1-10°C / second to cool to a temperature range of 650-700°C; and after the first cooling, performing a second cooling at an average cooling rate of 5-50°C / second to cool to a temperature range of 300-580°C, wherein the cold rolling is performed in 7 passes or less, and the total reduction rate is 55-70%.

[0032] Beneficial effects

[0033] According to the present invention, a steel plate can be provided that has high strength and excellent bending properties (three-point bending), thereby improving formability and impact resistance.

[0034] As described above, the improved formability of the steel sheet of the present invention prevents processing defects such as cracks or wrinkles during stamping, thus making it suitable for use in structural components and other parts that require processing into complex shapes. Furthermore, it is also effective in manufacturing materials with improved crash resistance that are less prone to cracking and other defects when a car using such a component inevitably collides with another vehicle. Attached Figure Description

[0035] Figure 1 This is a photograph showing the microstructure of the inventive steel according to an embodiment of the present invention.

[0036] Figure 2 This is a photograph illustrating the microstructure of a comparative steel according to an embodiment of the present invention.

[0037] Figure 3 This is a graph showing the change in physical properties during cold rolling according to the reduction rate in one embodiment of the present invention.

[0038] Figure 4 This is a graph showing the change in physical properties according to annealing temperature in one embodiment of the present invention.

[0039] Best practice

[0040] The inventors of this invention conducted in-depth research in order to develop a material with formability suitable for use as a component in automotive materials that requires processing into complex shapes.

[0041] In particular, the inventors have confirmed that the objective can be achieved by inducing sufficient recrystallization of the soft phase that affects the ductility of steel, thus completing the present invention.

[0042] The present invention will now be described in detail.

[0043] According to one aspect of the invention, a high-strength steel sheet with excellent bending and formability may contain, by weight percent: carbon (C): 0.05-0.12%, manganese (Mn): 2.0-3.0%, silicon (Si): less than 0.5% (except 0%), chromium (Cr): less than 1.0% (except 0%), niobium (Nb): less than 0.1% (except 0%), titanium (Ti): less than 0.1% (except 0%), boron (B): less than 0.0025% (except 0%), aluminum (sol.Al): 0.02-0.05%, phosphorus (P): less than 0.05% (except 0%), sulfur (S): less than 0.01% (except 0%), and nitrogen (N): less than 0.01% (except 0%).

[0044] The reasons for limiting the alloy composition of the steel plate provided in this invention will be explained in detail below.

[0045] Furthermore, in this invention, unless otherwise stated, the content of each element is based on weight, and the proportion of the tissue is based on area.

[0046] Carbon (C): 0.05-0.12%

[0047] Carbon (C) is an important element added for solid solution strengthening. This C combines with precipitating elements to form fine precipitates, thus helping to improve the strength of steel.

[0048] When the carbon content exceeds 0.12%, hardenability increases, and as martensite forms during cooling in the steel manufacturing process, the strength rises excessively, leading to a decrease in elongation. Furthermore, weld defects may occur when the weldability deteriorates and the steel is machined into parts. Additionally, when the carbon content is less than 0.05%, it is difficult to ensure the target strength level.

[0049] Therefore, it may contain 0.05-0.12% of the C. More preferably, it may contain more than 0.06% of the C, and may contain less than 0.10% of the C.

[0050] Manganese (Mn): 2.0-3.0%

[0051] Manganese (Mn) is an element that causes sulfur (S) in steel to precipitate as MnS, prevents hot brittleness caused by the formation of FeS, and is beneficial to the solid solution strengthening of steel.

[0052] When the Mn content is less than 2.0%, the aforementioned effects cannot be achieved, and it is difficult to ensure the target strength level. On the other hand, when the Mn content exceeds 3.0%, the likelihood of weldability and hot-rollability problems increases, and due to increased hardenability, martensite is easily formed, thus potentially reducing ductility. Furthermore, the formation of excessive Mn-bands (Mn oxide bands) in the microstructure increases the risk of defects such as processing cracks. Additionally, the dissolution of Mn oxides on the surface during annealing significantly reduces plating properties.

[0053] Therefore, it may contain 2.0-3.0% of the Mn, more preferably 2.2-2.8% of the Mn.

[0054] Silicon (Si): Less than 0.5% (except 0%)

[0055] Silicon (Si) is a ferrite stabilizing element, which helps ensure a target level of ferrite fraction by promoting ferrite phase transformation. Furthermore, silicon (Si) effectively improves the strength of ferrite due to its excellent solid solution strengthening ability, and is a useful element that ensures strength without reducing the ductility of steel.

[0056] When the Si content exceeds 0.5%, the solid solution strengthening effect is too great, which reduces ductility and causes surface oxide scale defects, thus adversely affecting the quality of the plated surface. In addition, it hinders the formation process.

[0057] Therefore, the Si may be contained in less than 0.5% and may be excluded at 0%. More preferably, the Si may be contained in more than 0.1%.

[0058] Chromium (Cr): Less than 1.0% (except 0%)

[0059] Chromium (Cr) is an element that readily forms the bainitic phase, inhibits the formation of the martensitic phase during annealing heat treatment, and is also an element that helps to improve strength by forming fine carbides.

[0060] When the Cr content exceeds 1.0%, the elongation decreases due to the formation of excessive bainite phase, and the strength and elongation may deteriorate when carbides form at grain boundaries. Furthermore, there is an increase in manufacturing costs.

[0061] Therefore, the Cr may be less than 1.0%, and 0% may be excluded.

[0062] Niobium (Nb): Less than 0.1% (except 0%)

[0063] Niobium (Nb) is an element that helps to suppress the coarsening of austenite grains during annealing heat treatment and improves strength by forming fine carbides, thus preventing segregation at austenite grain boundaries.

[0064] When the Nb content exceeds 0.1%, coarse carbides precipitate, and due to the reduced carbon content in the steel, the strength and elongation may deteriorate, and there is an issue of increased manufacturing costs.

[0065] Therefore, less than 0.1% of the Nb may be included, and 0% may be excluded.

[0066] Titanium (Ti): Less than 0.1% (except 0%)

[0067] Titanium (Ti) is an element that forms fine carbides, helping to ensure yield strength and tensile strength. Furthermore, Ti causes nitrogen in steel to precipitate as TiN, effectively suppressing the formation of AlN from the inevitably present Al in steel, thus reducing the likelihood of cracking during continuous casting.

[0068] When the Ti content exceeds 0.1%, coarse carbides precipitate, potentially reducing strength and elongation due to the decreased carbon content in the steel. Furthermore, it can cause nozzle clogging during continuous casting and increases manufacturing costs.

[0069] Therefore, the Ti may be less than 0.1% and may be excluded at 0%.

[0070] Boron (B): less than 0.0025% (except 0%)

[0071] Boron (B) is an element that delays the transformation from austenite to pearlite during the cooling process after annealing heat treatment. However, when the boron (B) content exceeds 0.0025%, excessive B accumulates on the surface, which may lead to poor coating adhesion.

[0072] Therefore, less than 0.0025% of the aforementioned B may be included, and 0% may be excluded.

[0073] Aluminum (acid-soluble aluminum (sol.Al)): 0.02-0.05%

[0074] Aluminum (sol.Al) is added to refine the grain size of steel and for deoxidation. When the aluminum (sol.Al) content is less than 0.02%, aluminum-killed steel cannot be manufactured in a stable state. On the other hand, when the aluminum (sol.Al) content exceeds 0.05%, the grains are refined, resulting in increased strength. However, this also leads to the formation of excessive inclusions during continuous casting operations, increasing the likelihood of surface defects in the coated steel sheet.

[0075] Therefore, it may contain 0.02-0.05% of the acid-soluble aluminum.

[0076] Phosphorus (P): less than 0.05% (except 0%)

[0077] Phosphorus (P) is the substitution element with the greatest solid solution strengthening effect and improves in-plane anisotropy, helping to ensure strength without significantly reducing formability. However, when too much P is added, the possibility of brittle fracture increases significantly, the possibility of slab breakage during hot rolling increases, and it also hinders the properties of the coated surface.

[0078] Therefore, in this invention, the content of P can be controlled below 0.05%, and considering the unavoidable level of addition, 0% can be excluded.

[0079] Sulfur (S): less than 0.01% (except 0%)

[0080] Sulfur (S) is an impurity element in steel and is inevitably added. It hinders ductility, therefore, it is preferable to control the sulfur (S) content to the lowest possible level. In particular, S increases the likelihood of developing red-hot brittleness, so it is preferable to control the sulfur (S) content to below 0.01%. However, considering the unavoidable addition during manufacturing, 0% can be excluded.

[0081] Nitrogen (N): less than 0.01% (except 0%)

[0082] Nitrogen (N) is a solid solution strengthening element, but when the nitrogen (N) content exceeds 0.01%, the risk of brittleness increases, and it combines with Al in the steel to precipitate excessive AlN, which may reduce the quality of continuous casting.

[0083] Therefore, the N may be less than 0.01%, and 0% may be excluded considering the unavoidable level of addition.

[0084] The remaining component of this invention is iron (Fe). However, unwanted impurities inevitably mix in from raw materials or the surrounding environment during normal manufacturing processes, and therefore these impurities cannot be completely eliminated. These impurities are well known to those skilled in the art, and therefore their contents are not specifically mentioned in this specification.

[0085] The microstructure of the steel plate of the present invention having the above alloy composition can be composed of ferrite and bainite and martensite phases as hard phases.

[0086] Specifically, the steel plate of the present invention comprises 35-50% ferrite phase and 35-45% bainite phase by area. Furthermore, martensite phase may be included as a balance, and trace amounts of retained austenite phase may also be included.

[0087] The ferrite phase consists of unrecrystallized ferrite and recrystallized ferrite, and may contain 8-15% of the unrecrystallized ferrite and 27-35% of the recrystallized ferrite in terms of area.

[0088] As the degree of non-recrystallization of ferrite increases, the inhomogeneity in the microstructure increases, which may worsen the workability. Therefore, it is preferable to induce the formation of a uniform microstructure in the steel through appropriate recrystallization.

[0089] When the fraction of unrecrystallized ferrite is less than 8%, excessive recrystallization occurs, which may lead to a decrease in strength. On the other hand, when the fraction of unrecrystallized ferrite exceeds 15%, the yield strength increases excessively due to the biased distribution of the elongated hard phase in the microstructure, making it difficult to ensure workability.

[0090] When the bainite phase fraction is too high, the soft phase fraction is relatively low, thus the target level of formability cannot be guaranteed. However, when the bainite phase fraction is less than 35%, the bending performance may deteriorate.

[0091] In the microstructure other than the ferrite and bainite phases, the fraction of martensite phase is not specifically limited, but to ensure ultra-high strength of 980 MPa or more, it is advantageous to include martensite phase with an area fraction of less than 20% (excluding 0%). When the fraction of martensite phase exceeds 20%, ductility decreases, making it difficult to ensure the target level of machinability.

[0092] Furthermore, it is advantageous for the residual austenite phase fraction to be no more than 3%, and even if it is 0%, there is no difficulty in ensuring the desired physical properties.

[0093] The steel plate of the present invention having the above-mentioned fine structure has a thickness of 0.5-2.5 mm, a tensile strength of 980 MPa or more, a yield strength of 550-650 MPa, an elongation (total elongation) of 12% or more, and can have the characteristics of high strength and high ductility.

[0094] Furthermore, the steel plate, with a three-point bending angle of over 90 degrees, can achieve excellent bending performance.

[0095] The following describes in detail a method for manufacturing a high-strength steel sheet with excellent bending and formability according to another aspect of the present invention.

[0096] In short, the present invention can manufacture the desired steel plate through the process of [steel billet heating-hot rolling-coiling-cold rolling-continuous annealing], and the following is a detailed description of each process.

[0097] [Bill Heating]

[0098] First, after preparing a steel billet that meets the above alloy composition, the steel billet can be heated.

[0099] This process is performed to facilitate subsequent hot rolling processes and to fully obtain the desired physical properties of the steel sheet. In this invention, there are no particular limitations on the conditions of this heating process; any normal conditions are acceptable. As an example, the heating process can be carried out in a temperature range of 1100-1300°C.

[0100] [Hot Rolled]

[0101] The steel billet heated as described above can be hot-rolled to produce hot-rolled steel sheet, wherein hot finishing rolling can be performed at an exit temperature above Ar3 and below 1000°C.

[0102] When the exit temperature during hot finishing rolling is below Ar3, the resistance to hot deformation increases rapidly, and the top, bottom, and edge portions of the hot-rolled coil become single-phase regions with increased in-plane anisotropy, which may worsen formability. Conversely, when the exit temperature exceeds 1000°C, the relatively reduced rolling load is beneficial for productivity but may result in thick oxide scale.

[0103] More specifically, the hot finishing rolling can be carried out in a temperature range of 760-940°C.

[0104] [Collection]

[0105] The hot-rolled steel sheet manufactured as described above can be rolled into a coil shape.

[0106] The coiling process can be carried out within a temperature range of 400-700℃. When the coiling temperature is below 400℃, excessive martensite or bainite phases are formed, leading to an excessive increase in the strength of the hot-rolled steel sheet. This may cause problems such as shape defects due to load during subsequent cold rolling. On the other hand, when the coiling temperature exceeds 700℃, the surface oxide scale increases, resulting in poor pickling properties.

[0107] [cool down]

[0108] Preferably, the coiled hot-rolled steel sheet is cooled to room temperature at an average cooling rate of 0.1°C / second or less (except 0°C / second). In this case, the coiled hot-rolled steel sheet can be cooled after processes such as conveying and stacking, and the processes prior to cooling are not limited to this.

[0109] As described above, by cooling the coiled hot-rolled steel sheet at a certain speed, a hot-rolled steel sheet with finely dispersed carbides serving as austenite nucleation sites can be obtained.

[0110] [Cold Rolled]

[0111] Cold-rolled steel sheets can be manufactured by cold rolling hot-rolled steel sheets that have been coiled as described above.

[0112] The inventors of this invention have confirmed that in the technical field of this invention, when using conventional continuous rolling mills (e.g., multi-stand processes with five or more stands) to manufacture cold-rolled steel sheets, there are no problems in rolling to the desired thickness, but there are limitations in ensuring material uniformity and in terms of productivity. Therefore, as a method to overcome the aforementioned limitations of cold rolling processes, this invention is characterized by providing a method for manufacturing cold-rolled steel sheets using an ultra-thin cold rolling mill (ZRM). For example, it can be a rolling mill with a pair of work rolls and multiple (e.g., about 17-19) back rolls connected to the work rolls, without limitation, as long as the rolling load can be achieved.

[0113] Specifically, the cold rolling using the ultra-thin cold rolling mill (ZRM) can be carried out in fewer than 7 passes, preferably in 5-7 passes, which is characterized by the use of fewer passes compared to existing continuous rolling mills (8-14 passes).

[0114] Furthermore, in this invention, the number of passes of 7 or less can be set as one stand, and strong pressing can be performed with a total pressing rate of 55% or more, preferably 55-70%, thus having an economically advantageous effect.

[0115] When the total reduction during cold rolling is less than 55%, ferrite recrystallization is delayed, making it difficult to obtain a fine and uniform austenitic phase. On the other hand, when the total reduction exceeds 70%, the yield strength increases excessively due to excessive recrystallization and the formation of fine grains, leading to a decrease in workability. During annealing, excessive recrystallization and recovery simultaneously suppress phase transformation, making it difficult to form a low-temperature transformation phase. Therefore, it may be difficult to ensure the target strength level.

[0116] In this invention, the target thickness can be achieved even with a low number of passes during cold rolling using the ultra-thin cold rolling mill. However, for hot-rolled steel sheets with a thickness of 4.0 mm or more, the target reduction rate can be achieved by repeatedly cold rolling 15-20 times (passes) using a reversing mill. In this case, 15-20 passes can be defined as one stand. A reversing mill is a mill used for rolling thin materials, specifically a mill that rolls materials while they reciprocate between two rolls, where each reciprocating motion can be defined as one pass.

[0117] As described above, the present invention performs cold rolling under high pressure, thereby further improving the material uniformity of the manufactured cold-rolled steel sheet and ensuring a thinner thickness compared to existing cold-rolled steel sheets.

[0118] Preferably, the cold-rolled steel sheet of the present invention may have a thickness of 0.5-2.5 mm.

[0119] The present invention allows for pickling of hot-rolled steel sheets before cold rolling, and the pickling process can be carried out using conventional methods.

[0120] [Continuous annealing]

[0121] Preferably, the cold-rolled steel sheet manufactured as described above is subjected to continuous annealing. As an example of the continuous annealing process, it can be carried out in a continuous annealing furnace (CAL).

[0122] Typically, a continuous annealing furnace (CAL) can consist of a heating zone, a soaking zone, a cooling zone (slow cooling zone and rapid cooling zone) and (an over-aging zone if necessary). After the cold-rolled steel sheet is loaded into the continuous annealing furnace as described above, it can be heated to a specific temperature in the heating zone and then held in the soaking zone for a certain period of time after reaching the target temperature.

[0123] In this invention, the temperatures of the heating zone and the soaking zone during continuous annealing can be controlled to be the same, which means that the end temperature of the heating zone and the start temperature of the soaking zone are controlled to be the same.

[0124] Specifically, the temperature of the heating zone and the soaking zone can be controlled between 770-810°C. When the temperature is below 770°C, sufficient heat input for recrystallization cannot be applied, but when the temperature exceeds 810°C, productivity decreases and excessive austenite phase is formed. After subsequent cooling, the fraction of hard phase increases significantly, and therefore the ductility of the steel may deteriorate.

[0125] [Step-by-step cooling]

[0126] By cooling the cold-rolled steel sheet subjected to the continuous annealing process described above, the desired microstructure can be formed, wherein, preferably, the cooling is performed stepwise.

[0127] In this invention, the step-by-step cooling can consist of a primary cooling and a secondary cooling. Specifically, after the continuous annealing, a primary cooling can be performed at an average cooling rate of 1-10°C / second to cool to a temperature range of 650-700°C, and then a secondary cooling can be performed at an average cooling rate of 5-50°C / second to cool to a temperature range of 300-580°C.

[0128] At this point, by performing primary cooling more slowly compared to secondary cooling, the rapid temperature drop during the subsequent secondary cooling, which is a relatively rapid cooling zone, can be suppressed, thus preventing poor plate shape.

[0129] When the termination temperature during the first cooling is below 650°C, the low temperature results in low carbon diffusion activity. As the carbon concentration in ferrite increases, the carbon concentration in austenite decreases, leading to an excessively high fraction of hard phases and an increased yield strength ratio. Consequently, the tendency for cracks to form during processing increases. Furthermore, the excessively rapid cooling rates in the soaking zone and cooling zone (slow cooling zone) cause the plate shape to become uneven. When the termination temperature exceeds 700°C, it has the disadvantage of requiring excessively high cooling rates for subsequent cooling (secondary cooling).

[0130] Furthermore, sufficient carbon diffusion cannot occur when the average cooling rate during the first cooling step exceeds 10°C / second. Additionally, considering productivity, a single cooling process can be performed at an average cooling rate of 1°C / second or higher.

[0131] As mentioned above, after the first cooling is completed, rapid cooling (secondary cooling) can be performed at a certain or higher cooling rate. At this time, when the secondary cooling termination temperature is below 300°C, cooling deviations occur in the width and length directions of the steel plate, so the shape of the plate may deteriorate. On the other hand, when the secondary cooling termination temperature exceeds 580°C, the hard phase cannot be sufficiently secured, so the strength will decrease.

[0132] Furthermore, when the average cooling rate during the secondary cooling is less than 5°C / second, the fraction of the hard phase may be too high, but when the average cooling rate during the secondary cooling exceeds 50°C / second, the hard phase may be insufficient.

[0133] Additionally, if necessary, an aging process can be performed after the step-by-step cooling is completed.

[0134] The over-aging treatment is a process that involves holding the material at the secondary cooling termination temperature for a certain period of time, performing uniform heat treatment in both the width and length directions of the roll material, thereby improving its shape quality. Therefore, the over-aging treatment can be performed for 200-800 seconds.

[0135] The over-aging treatment can be performed immediately after the termination of the secondary cooling, so its temperature is the same as the termination temperature of the secondary cooling, or it can be performed within the range of the termination temperature of the secondary cooling.

[0136] The high-strength steel sheet of the present invention, manufactured as described above, has a microstructure composed of hard and soft phases. In particular, by optimizing the cold rolling and annealing process, the recrystallization of ferrite is maximized, ultimately resulting in a microstructure in which bainite and martensite phases, which are hard phases, are uniformly distributed on the matrix of recrystallized ferrite.

[0137] Therefore, even though the steel plate of the present invention has a high strength of 980 MPa or more, it can ensure a low yield strength ratio and high ductility, thereby ensuring excellent bending and formability.

[0138] The present invention will now be described in more detail through embodiments. However, these embodiments are merely illustrative of implementation of the invention, and the invention is not limited to these embodiments. This is because the scope of the invention is determined by the matters set forth in the claims and the content reasonably inferred therefrom. Detailed Implementation

[0139] (Example)

[0140] After manufacturing steel billets with the alloy compositions shown in Table 1, each billet is heated at 1200°C for 1 hour, and then hot-rolled at a finishing temperature of 880-920°C to produce hot-rolled steel sheets. At this time, the thickness of each hot-rolled steel sheet is 2.1-3.5 mm, and in the case of steel with a cold-rolled material thickness of 0.8 mm (refer to Table 2), the thickness of the hot-rolled steel sheet is 8 mm.

[0141] Subsequently, each hot-rolled steel sheet was coiled at 650°C and then cooled to room temperature at a cooling rate of 0.1°C / second. The coiled hot-rolled steel sheets were then cold-rolled and continuously annealed under the conditions shown in Table 2 below, followed by step cooling (one-two cycles) and then over-aging at 360°C for 520 seconds to produce the final steel sheet.

[0142] At this point, during the step-cooling process, the first cooling is carried out at an average cooling rate of 3℃ / second, and the second cooling is carried out at an average cooling rate of 20℃ / second.

[0143] The microstructure of each steel plate manufactured as described above was observed, and the tensile and processing properties were evaluated. The results are shown in Table 3 below.

[0144] At this point, in the tensile test of each specimen, a tensile specimen of size JIS5 can be taken in the direction perpendicular to the rolling direction and the tensile test can be carried out at a strain rate of 0.01 / s.

[0145] In addition, the three-point bending test used to evaluate bending performance is conducted according to the VDA standard (VDA238-100) specified by the German Association of Automotive Manufacturers. The displacement measured at the maximum load in the bending test is converted into an angle in the VDA standard to measure the bending angle. The test piece size is 60mm × 60mm, the diameter of the bending roller is 30mm, the roller spacing is 2.9mm, the punch radius (R) is 0.4mm, and the punch insertion speed is 20mm / min.

[0146] In addition, the bainite and martensite phases, corresponding to the hard phases, were observed by SEM at 5000x magnification after etching with nital solution. The fraction of the observed hard phase was then measured. The fractions of the remaining phases were also measured using SEM and an image analyzer after etching with nital solution. The unrecrystallized ferrite was represented by the image analyzer as the fraction of ferrite remaining in the deformed structure within the total ferrite fraction.

[0147] In addition, to confirm whether the weldability standards are met after the automotive structural components are machined, the carbon equivalent (C) is measured. eq The value is calculated according to the following formula.

[0148] Equation (1)...C eq (%) = C + (Si / 30) + (Mn / 20) + 2P + 4S (where each element represents its weight content (%)). [Table 1]

[0149]

[0150] [Table 2]

[0151]

[0152] [Table 3]

[0153]

[0154]

[0155] As shown in Tables 1 to 3, it can be confirmed that the alloy composition and manufacturing conditions of the steel, especially the cold rolling and continuous annealing processes, meet the conditions proposed in this invention. In Invention Examples 1 to 6, ferrite recrystallization occurs fully during the annealing process after cold rolling. Therefore, the steel has high strength, yield strength that is conducive to plate processing, and excellent elongation and three-point bending properties, thereby ensuring the target level of formability.

[0156] In particular, the recrystallized ferrite formed in the aforementioned example contains 27% or more, thus exhibiting the characteristic of improving the material uniformity of the steel sheet. Regarding the recrystallization of steel, due to the rearrangement of ferrite atoms during annealing, austenitic phase transformation occurs in various directions as the degree of recrystallization increases, and the overall material uniformity of the steel increases, thereby improving machinability.

[0157] On the other hand, in Comparative Examples 1 and 2, where the continuous annealing process in the steel sheet manufacturing process resulted in low homogenization temperatures and low cold rolling reduction rates, there was an excessive amount of ferrite phase that had not undergone sufficient recrystallization, leading to excessively high yield strength and tensile strength, as well as small elongation and three-point bending angle, resulting in poor processability. Furthermore, in Comparative Example 3, it was confirmed that the low homogenization temperature and low cold rolling reduction rate during continuous annealing resulted in the formation of excessive non-recrystallized ferrite phase, thus causing a poor three-point bending angle.

[0158] In Comparative Examples 6, 7, 11 to 13, the annealing temperature used for recrystallization drive meets the requirements of the present invention. However, since the total reduction rate during cold rolling is controlled to be less than 55%, the elongated hard phase is well developed, resulting in excessively high yield strength and tensile strength, thus leading to poor processability.

[0159] In Comparative Example 8, the total reduction during cold rolling was less than 55%, but it was higher than that in Comparative Example 6 or Comparative Example 7. Therefore, it showed the level of the present invention in terms of processability, but it showed poor ductility.

[0160] In Comparative Examples 4 to 5, 9 to 10, and 14 to 15, the total reduction during cold rolling was 90%, which was too high.

[0161] In Comparative Examples 4 to 5 and 10, excessive recrystallization occurred during the annealing process after cold rolling, resulting in decreased strength as the inverse austenite transformation was suppressed. Inverse austenite transformation is not easily achieved in recrystallized ferrite, and in environments with very high recrystallization driving forces, the inverse austenite transformation can be suppressed. This leads to a decrease in the martensite fraction during cooling or an excessively high ferrite fraction in the final microstructure.

[0162] In Comparative Example 9, the yield strength became too high due to the grain refinement effect caused by the excessively high reduction rate, making it difficult to form and resulting in increased processing costs.

[0163] In Comparative Examples 14 and 15, in addition to the rolling of the steel, excessive austenite was formed during annealing at relatively high temperatures, resulting in an increase in the fraction of hard phases upon cooling, which exceeded the yield strength.

[0164] Figure 1 Photographs of the fine structures of Examples 3 and 4 of the Invention are shown. Figure 2 Photographs of the fine tissues of Comparative Example 6 and Comparative Example 7 are shown.

[0165] like Figure 1 As shown, it can be confirmed that the steel plate according to the present invention forms a fine bainite phase and a certain percentage of martensite phase while homogenizing on a sufficient fraction of recrystallized ferrite matrix.

[0166] On the other hand, such as Figure 2 As shown in Comparative Examples 6 and 7, it can be confirmed that ferrite elongates along the rolling direction, and that bainite forms in the same shape due to insufficient recrystallization. It is evident that due to the high fraction of bainite, the yield strength and yield ratio increase excessively while the formability is poor.

[0167] Figure 3 It is a graph showing the change in processability based on the reduction rate during cold rolling. Figure 4 It is a graph showing the change in processability according to the annealing temperature.

[0168] like Figure 3 As shown, when the reduction rate during cold rolling under the annealing conditions proposed in this invention is 55% or more, both elongation and three-point bending angle can be satisfied simultaneously.

[0169] Furthermore, applying a reduction rate of 45% or more during cold rolling can improve elongation and three-point bend angle; however, it is recognized that in order to ensure the workability desired in this invention, it is necessary to control the alloy composition of phase transformation and recrystallization, as well as the annealing conditions, etc. Figure 4 ).

Claims

1. A high-strength steel sheet with excellent bending and formability, comprising, by weight percent: carbon (C): 0.05-0.12%, manganese (Mn): 2.0-3.0%, silicon (Si): less than 0.5% and excluding 0%, chromium (Cr): less than 1.0% and excluding 0%, niobium (Nb): less than 0.1% and excluding 0%, titanium (Ti): less than 0.1% and excluding 0%, boron (B): less than 0.0025% and excluding 0%, aluminum (sol.Al): 0.02-0.05%, phosphorus (P): less than 0.05% and excluding 0%, sulfur (S): less than 0.01% and excluding 0%, nitrogen (N): less than 0.01% and excluding 0%, iron (Fe), and other unavoidable impurities. The fine microstructure consists of 35-50% ferrite, 35-45% bainite, and the balance martensite. The steel plate contains martensitic phase with an area fraction of less than 19% and excluding 0%. The ferrite consists of 8-15% unrecrystallized ferrite and 27-35% recrystallized ferrite in terms of area fraction.

2. The high-strength steel plate with excellent bending and formability according to claim 1, wherein, The steel plate further comprises an area fraction of less than 3% and includes 0% residual austenite phase.

3. The high-strength steel plate with excellent bending and formability according to claim 1, wherein, The steel plate has a tensile strength of 980 MPa or higher, a yield strength of 550-650 MPa, and a total elongation of 12% or higher.

4. The high-strength steel plate with excellent bending and formability according to claim 1, wherein, The steel plate has a three-point bending angle of 90 degrees or more.

5. The high-strength steel plate with excellent bending and formability according to claim 1, wherein, The steel plate has a thickness of 0.5-2.5 mm.

6. A method for manufacturing high-strength steel plates with excellent bending and formability, characterized in that, Includes the following steps: Prepare a steel billet, which, by weight percent, comprises: carbon (C): 0.05-0.12%, manganese (Mn): 2.0-3.0%, silicon (Si): less than 0.5% and excluding 0%, chromium (Cr): less than 1.0% and excluding 0%, niobium (Nb): less than 0.1% and excluding 0%, titanium (Ti): less than 0.1% and excluding 0%, boron (B): less than 0.0025% and excluding 0%, aluminum (sol.Al): 0.02-0.05%, phosphorus (P): less than 0.05% and excluding 0%, sulfur (S): less than 0.01% and excluding 0%, nitrogen (N): less than 0.01% and excluding 0%, iron (Fe), and other unavoidable impurities; The steel billet is heated within a temperature range of 1100-1300℃; The heated steel billet is hot-rolled to produce hot-rolled steel sheet; The hot-rolled steel sheet is coiled within a temperature range of 400-700℃; After the coiling, the hot-rolled steel sheet is cooled to room temperature at a cooling rate of less than 0.1°C / second and except 0°C / second. The cooled hot-rolled steel sheet is cold-rolled to produce cold-rolled steel sheet; The cold-rolled steel sheet is subjected to continuous annealing treatment; Following the continuous annealing, a single cooling process is performed at an average cooling rate of 1-10°C / second, cooling to a temperature range of 650-700°C; and After the first cooling, a second cooling process is performed at an average cooling rate of 5-50°C / second, cooling the temperature to a range of 300-580°C. The cold rolling process is carried out in 7 passes or less, with a total reduction rate of 55-70%.

7. The method for manufacturing a high-strength steel sheet with excellent bendability and formability according to claim 6, wherein, The hot rolling is carried out at an exit temperature above Ar3 and below 1000°C.

8. The method for manufacturing a high-strength steel sheet with excellent bendability and formability according to claim 6, wherein, The continuous annealing is carried out in a device equipped with a heating zone, a soaking zone, and a cooling zone, wherein the heating zone and the soaking zone are controlled within a temperature range of 770-810°C.

9. The method for manufacturing a high-strength steel sheet with excellent bendability and formability according to claim 6, wherein, After the secondary cooling, the method further includes an over-aging process, which is performed for 200-800 seconds.

10. The method for manufacturing a high-strength steel sheet with excellent bendability and formability according to claim 6, wherein, When the thickness of the hot-rolled steel plate is 4 mm or more, the cold rolling is carried out in 15-20 passes using a reversible rolling mill.

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