Steel sheet having excellent bendability and method for producing same

Abstract

The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a steel sheet having excellent bendability and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a steel plate and a method for manufacturing the same, and more particularly, to a steel plate having excellent bendability and a method for manufacturing the same. Background Art

[0002] In order to ensure the safety of passengers in the event of a car collision, the safety regulations for cars are being strengthened. For this reason, the steel plates used in cars need to have high strength or thick thickness. However, due to environmental issues, car manufacturers are constantly demanding lightweight bodies to improve the fuel efficiency of cars. Therefore, in order to ensure both the collision stability and lightweight of cars, the high strength of steel plates is inevitable.

[0003] Generally, the method of strengthening steel includes solid solution strengthening, precipitation strengthening, strengthening by grain refinement, phase transformation strengthening, etc. Among them, precipitation strengthening type high-strength steel using precipitation strengthening is to precipitate carbides and nitrides by adding carbide and nitride forming elements such as Cu, Nb, Ti, V, so as to strengthen the steel plate, or to suppress grain growth by fine precipitates to ensure strength by refining grains. This technology has the advantage of being easy to obtain high strength compared to low manufacturing cost, but the disadvantage is that due to fine precipitates, the recrystallization temperature rises rapidly, so in order to ensure ductility by causing sufficient recrystallization, high temperature annealing must be performed. In addition, precipitation strengthening steel that is strengthened by precipitating carbides and nitrides in the ferrite matrix has the problem of being difficult to obtain high-strength steel of grades above 600MPa.

[0004] In addition, with regard to phase transformation strengthened high-strength steel, a variety of steels have been developed, including ferrite-martensite dual phase steel (Dual Phase) containing hard martensite in a ferrite matrix, transformation induced plasticity (TRIP) steel utilizing the phase transformation induced plasticity of retained austenite, or complexed phase (CP) steel consisting of ferrite and hard bainite or martensite structures.

[0005] In recent years, higher strength steel sheets are required for automobiles to improve fuel efficiency and durability, and in terms of collision safety and passenger protection, the amount of high-strength steel sheets with a tensile strength of 780 MPa or more used as body structures or reinforcements is increasing.

[0006] However, as the strength gradually increases, cracks or wrinkles are generated during the stamping process of automobile parts, reaching the limit of manufacturing complex parts. In particular, if the ductility (El) and bendability of DP steel, the most widely used among phase transformation strengthened high-strength steels, can be improved, the application of high-strength steel in complex parts can be expanded by preventing processing defects such as cracks or wrinkles generated during stamping.

[0007] As a prior art for such a high-tensile steel sheet, the invention disclosed in Patent Document 1 can be cited. The prior art describes a method for manufacturing a cold-rolled steel sheet composed of a composite structure including ferrite, bainite, martensite, and retained austenite, wherein Si is added to the steel and retained austenite is introduced into the final annealed steel sheet through bainite transformation to ensure the ductility of the steel sheet. However, with the addition of Si, dents may be generated in the furnace during continuous annealing or liquid metal embrittlement may occur in the plated steel sheet during spot welding at the customer company.

[0008] [Prior art literature]

[0009] [Patent Document]

[0010] (Patent Document 1) Korean Patent Publication No. 2019-0076258 Summary of the invention

[0011] 1. Technical issues to be resolved

[0012] An object according to one aspect of the present invention is to provide a steel sheet having excellent bendability and a method for producing the same.

[0013] The technical problems of the present invention are not limited to the above contents. A person skilled in the art can easily understand the additional technical problems of the present invention based on the overall contents of this specification.

[0014] (II) Technical solution

[0015] According to one embodiment of the present invention, a steel plate can be provided, which contains, by weight%, carbon (C): 0.05-0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0-3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1-1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, and the balance is iron (Fe) and other inevitable impurities, the T value defined in the following relationship 1 is 1648 or more, and the fine structure contains, by area%, 50-80% of ferrite, 5-25% of bainite, 10-30% of fresh martensite, and less than 5% of retained austenite.

[0016] [Equation 1]

[0017]

[0018] (In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element.)

[0019] The RT value of the steel plate defined in the following Relationship 2 may be 0.01 or more.

[0020] [Equation 2]

[0021] RT=[Si]+[Nb]+[Ti]

[0022] (In the formula, [Si], [Nb], and [Ti] are the weight % of each element.)

[0023] The steel plate may have a tensile strength (TS) of 780 MPa or more, and an elongation (El) of 14.0% or more.

[0024] The value of the bending angle (°) / thickness (mm) of the steel plate in a 180° bending test may be 50° / mm or more (wherein the bending angle (°) refers to a bending angle at which no cracks are generated in the bent portion during the 180° bending test).

[0025] The steel plate may further include a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface.

[0026] According to another embodiment of the present invention, a method for manufacturing a steel plate may be provided, comprising the following steps: reheating a steel billet, wherein the steel billet comprises, by weight%, carbon (C): 0.05-0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0-3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1-1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, and the remainder of iron (Fe) and other inevitable impurities, the T value defined in the following relationship 1 of the steel billet is greater than 1648; hot rolling the reheated steel billet; coiling the hot rolled steel plate and then cooling it; cold rolling the cooled steel plate; heating the cold rolled steel plate to a T1 temperature of 800-850°C, cooling it to a T2 temperature of 400-600°C at an average cooling rate of less than 20°C / second, and then maintaining it for more than 50 seconds for continuous annealing; and cooling the continuously annealed steel plate to room temperature, wherein the R value defined in the following relationship 3 is 1797 to 1850.

[0027] [Equation 1]

[0028]

[0029] (In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element.)

[0030] [Equation 3]

[0031]

[0032] (In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element, and T1 and T2 are the heating temperature (°C) and cooling end temperature (°C) during continuous annealing, respectively.)

[0033] The RT value of the steel billet defined in the following Relationship 2 may be 0.01 or more.

[0034] [Equation 2]

[0035] RT=[Si]+[Nb]+[Ti]

[0036] (In the formula, [Si], [Nb], and [Ti] are the weight % of each element.)

[0037] The reheating can be carried out in a temperature range of 1100-1300°C, the hot rolling can be carried out at a finishing temperature of 800-950°C, the cooling step after coiling can be carried out in a temperature range of 400-700°C and then cooled to room temperature at an average cooling rate of less than 0.10°C / second, and the cold rolling can be carried out at a reduction rate of 40-70%.

[0038] The cold rolling step may further include a step of pickling the steel plate.

[0039] After the continuous annealing step and before the cooling step, the method may further include hot-dip galvanizing the steel sheet at a temperature ranging from 430° C. to 490° C.

[0040] After the hot-dip galvanizing step and before cooling, the method may further include subjecting the steel sheet to an alloying heat treatment at a temperature ranging from 460° C. to 530° C.

[0041] (III) Beneficial effects

[0042] According to one aspect of the present invention, a steel sheet having excellent bendability and a method for manufacturing the same can be provided.

[0043] According to one aspect of the present invention, a steel plate and a method for manufacturing the same can be provided, wherein the steel plate can be used as a steel plate for automobile structural parts, has excellent workability, and can be used in a complex shape when stamped. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a photograph of the microstructure of Invention Example 13 according to one embodiment of the present invention observed using an electron microscope.

[0045] Figure 2This is a photograph of the microstructure of Comparative Example 6 according to one embodiment of the present invention observed with an electron microscope. Implementation

[0046] The preferred specific embodiments of the present invention are described below. The specific embodiments of the present invention can be modified into various forms, and it should not be interpreted that the scope of the present invention is limited to the specific embodiments described below. This specific embodiment is provided to explain the present invention in more detail to those skilled in the art.

[0047] According to one embodiment of the present invention, it was confirmed that the alloy composition can be optimized by adding a minimum amount of Si or no Si to meet the physical properties of existing DP steel while reducing the occurrence of furnace dents and liquid metal embrittlement during spot welding, and excellent bendability can be obtained, thereby completing the present invention.

[0048] Hereinafter, the present invention will be described in detail.

[0049] Hereinafter, the steel composition of the present invention will be described in detail.

[0050] Unless otherwise specified, the % representing the content of each element in the present invention is based on weight.

[0051] The steel sheet according to one embodiment of the present invention may contain, by weight%, carbon (C): 0.05-0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0-3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1-1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, and the balance of iron (Fe) and other inevitable impurities.

[0052] Carbon (C): 0.05-0.20%

[0053] Carbon (C) is a very important element added for phase transformation organization strengthening. Carbon (C) realizes high strength and promotes the formation of martensite in composite organization steel. When the content of carbon (C) increases, the amount of martensite in the steel increases. However, when the content of carbon (C) exceeds 0.20%, the strength of martensite increases, but the strength difference with ferrite with low carbon concentration may increase. This strength difference is easy to cause fracture at the phase interface when stress is applied, so the bendability may be reduced. According to one embodiment, the content of carbon (C) can be less than 0.17%. In addition, due to poor weldability, welding defects may occur when the customer company processes the parts. In addition, when the content of carbon (C) is less than 0.05%, it may be difficult to ensure the desired level of strength. As an embodiment of the present invention, the content of carbon (C) can be more than 0.07%.

[0054] Silicon (Si): 0.10% or less

[0055] Silicon (Si) is a ferrite stabilizing element that can promote ferrite phase transformation, and is an element that contributes to the formation of martensite by promoting C enrichment to untransformed austenite. In addition, silicon (Si) has excellent solid solution strengthening energy, effectively reduces the interphase hardness difference by increasing the strength of ferrite, and is a useful element that can ensure strength without reducing the ductility of the steel sheet. However, when the content of silicon (Si) exceeds 0.10%, surface scale defects are induced, resulting in deterioration of the quality of the plated surface, and liquid metal embrittlement may be induced during spot welding of the plated material. According to one embodiment, the content of silicon (Si) may be less than 0.05%.

[0056] Manganese (Mn): 1.0-3.0%

[0057] Manganese (Mn) is an element that refines particles without damaging ductility, and completely precipitates S in steel as MnS, thereby preventing the hot brittleness caused by the generation of FeS, and strengthening steel. In addition, in composite structure steel, the manganese (Mn) plays a role in reducing the critical cooling rate of obtaining martensite, so that martensite can be formed more easily. When the content of manganese (Mn) is less than 1.0%, it is difficult to ensure the strength desired by the present invention. According to one embodiment, the content of manganese (Mn) can be more than 1.6%. On the other hand, when the content of manganese (Mn) exceeds 3.0%, the possibility of problems such as weldability and hot rolling is high, excessive martensite is formed and the material is unstable, and a manganese band (Mn-Band) (band of Mn oxide) is formed in the organization, so that there may be a problem of increased risk of processing cracks and plate fractures. In addition, Mn oxide is dissolved to the surface during annealing and there may be a problem of significantly hindering plating. According to one embodiment, the content of manganese (Mn) can be less than 2.5%.

[0058] Aluminum (sol.Al): less than 1.00%

[0059] Aluminum (sol.Al) is an element added for grain refinement and deoxidation of steel, and similar to Si, aluminum (sol.Al) is a ferrite stabilizing element. In addition, the aluminum (sol.Al) is an effective component for improving the hardenability of martensite by distributing C in ferrite to austenite, and is a useful element for improving the ductility of the steel sheet by effectively suppressing the precipitation of carbides in bainite when maintained in the bainite region. However, when the content of aluminum (sol.Al) exceeds 1.00%, it is beneficial to improve the high strength due to the grain refinement effect, but during the steelmaking continuous casting operation, due to the formation of excessive inclusions, the possibility of surface defects of the plated steel sheet is high, and there is a problem of increasing the manufacturing cost. According to one embodiment of the present invention, the content of aluminum (sol.Al) can be less than 0.50%.

[0060] Chromium (Cr): 0.1-1.0%

[0061] Chromium (Cr) is an element that can be added to improve the hardenability of steel and ensure high strength. In addition, chromium (Cr) is an element that plays a very important role in the formation of martensite, and minimizes the decrease in elongation relative to the increase in strength, thereby also facilitating the production of composite structure steel with high ductility. In particular, chromium (Cr) is an element that forms Cr during hot rolling. 23 Cr-based carbides such as C6, which are partially melted during annealing and partially unmelted and remain, can control the amount of solid solution C in martensite below an appropriate level after cooling, so by suppressing the occurrence of yield point elongation (YP-El), it is beneficial to manufacture composite structure steel with low yield strength ratio. Therefore, in the present invention, the content of chromium (Cr) can be limited to more than 0.1%. According to one embodiment of the present invention, the content of chromium (Cr) can be less than 0.8%. However, when the content of chromium (Cr) exceeds 1.0%, not only the above effect is saturated, but also due to excessive increase in hot rolling strength, there may be a problem of poor cold rolling performance. With the increase in the fraction of Cr-based carbides and the coarsening of Cr-based carbides, the size of martensite after annealing becomes coarsened, which may cause a decrease in elongation. According to one embodiment, the content of chromium (Cr) can be more than 0.2%.

[0062] Niobium (Nb): 0.05% or less

[0063] Niobium (Nb) is an element that segregates at austenite grain boundaries and suppresses the coarsening of austenite grains during annealing heat treatment, and forms fine carbides, thereby contributing to increasing strength. However, when the content of niobium (Nb) exceeds 0.05%, coarse carbides are precipitated, and the strength and elongation may be reduced due to the reduction in carbon content in steel, and there is also a problem of increased manufacturing cost. According to one embodiment of the present invention, the content of niobium (Nb) may be 0.04% or less.

[0064] Titanium (Ti): less than 0.05%

[0065] Titanium (Ti) is a fine carbide-forming element, which can help ensure yield strength and tensile strength. In addition, titanium (Ti) is a nitride-forming element, which has the effect of precipitating N in steel into TiN to inhibit the precipitation of AlN, and therefore has the advantage of reducing the risk of cracks during continuous casting. However, when the content of titanium (Ti) exceeds 0.05%, coarse carbides are precipitated, and due to the reduction of carbon content in steel, strength and elongation may be reduced, which may cause nozzle clogging during continuous casting. As an embodiment of the present invention, the content of titanium (Ti) may be less than 0.03%.

[0066] Phosphorus (P): 0.100% or less

[0067] Phosphorus (P) is a substitutional element with the greatest solid solution strengthening effect. It is an element that improves in-plane anisotropy and is the most favorable element for ensuring strength without significantly damaging formability. However, when excessive phosphorus (P) is added, the possibility of brittle fracture increases significantly, there is a possibility of plate fracture in the slab during hot rolling, and phosphorus (P) acts as an element that hinders the surface characteristics of the coating, so in the present invention, the content of phosphorus (P) can be limited to less than 0.100%. However, considering the level that is inevitably added during the manufacturing process, 0% is excluded.

[0068] Sulfur (S): 0.010% or less

[0069] Sulfur (S) is an impurity element inevitably added to steel and is an element that reduces ductility and weldability, so it is important to control the content of sulfur (S) to the lowest possible level. In particular, there is a problem of increasing the possibility of causing red-hot brittleness, so it is preferred to control the content of sulfur (S) to 0.010% or less. However, considering the level that is inevitably added during the manufacturing process, 0% is excluded.

[0070] Nitrogen (N): 0.010% or less

[0071] Nitrogen (N) is a component that effectively stabilizes austenite, but when the content of nitrogen (N) exceeds 0.010%, there may be a problem that the refining cost of steel increases rapidly. In addition, during continuous casting, the risk of cracking due to the formation of AlN, etc. increases significantly, so it is preferred to limit the upper limit of nitrogen (N) to 0.010%. However, considering the level that is inevitably added during the manufacturing process, 0% is excluded.

[0072] The steel material of the present invention may contain a balance of iron (Fe) and inevitable impurities in addition to the above composition. Inevitable impurities may be mixed in unexpectedly during the usual manufacturing process, and therefore these impurities cannot be completely eliminated. Such impurities are well known to those skilled in the art of ordinary steel manufacturing, and therefore all of them are not particularly mentioned in this specification.

[0073] The steel sheet according to one embodiment of the present invention may have a T value defined in the following Relationship 1 of 1648 or more.

[0074] [Equation 1]

[0075]

[0076] (In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element.)

[0077] The above-mentioned relational expression 1 is a formula which quantitatively expresses the contribution of the additive elements in the steel sheet to the strength and bendability of the steel sheet. In the present invention, the contents of C, Mn, Nb, Ti and Cr as representative component systems in the steel components constituting the steel sheet can be limited so as to satisfy the above-mentioned relational expression 1.

[0078] Specifically, C and Mn have the effect of improving the strength of the steel plate due to the solid solution strengthening effect of steel. However, the contribution of each element to the strength of the steel plate is different, and the constant value multiplied by each component in this relationship relatively represents the contribution of each element to the strength. In addition, Nb and Ti have a precipitation strengthening effect and contribute to improving the strength, and precipitate in the ferrite matrix in DP steel, thereby having a ferrite strengthening effect, reducing the phase hardness difference between ferrite and martensite, thereby having the effect of improving the bending of the steel plate, so the multiplied constant value is represented by a positive value. On the other hand, in the case of Cr, it is the element with the smallest solid solution strengthening effect among the elements and is an element that greatly increases hardenability. When Cr is added in large quantities, a large amount of martensite is generated, and the bending may be reduced, so the value of the constant can have a negative value.

[0079] When the T value defined in the above relational expression 1 is less than 1648, there is a problem that the strength and bendability characteristics of the steel sheet desired by the present invention cannot be ensured. According to one embodiment of the present invention, the T value defined in relational expression 1 may be 1650 or more. In the present invention, although not particularly limited, when an excessive amount of components is added, the strength is excessively increased and there may be a problem that the elongation is reduced to a level below the desired level. Therefore, in consideration of this situation, the upper limit of the T value may be effectively limited to 1800 or less.

[0080] The steel sheet according to one embodiment of the present invention may have an RT value defined in the following Relationship 2 of 0.01 or more.

[0081] [Equation 2]

[0082] RT=[Si]+[Nb]+[Ti]

[0083] (In the formula, [Si], [Nb], and [Ti] are the weight % of each element.)

[0084] When Si is added in large amounts in the steel sheet according to one embodiment of the present invention, there may be problems such as causing pit defects of the steel sheet in the annealing furnace, phosphate treatment properties of the cold-rolled steel sheet, liquid metal embrittlement of the plated steel sheet, and deterioration of the plating properties, so it is desirable to minimize Si. In addition, when the amount of Si added is reduced, mechanical properties may deteriorate. In order to overcome this problem, Nb and Ti may be added to prevent deterioration of mechanical properties due to precipitation of carbides.

[0085] When the RT value defined in the relational expression 2 is less than 0.01, the physical properties of the desired level of the present invention may not be ensured. According to one embodiment of the present invention, the RT value defined in the relational expression 2 may be 0.02 or more. In addition, although not particularly limited in the present invention, the upper limit of the RT value may be limited to 0.2% in the same manner as the maximum addition within the limit range of each component.

[0086] Hereinafter, the steel microstructure of the present invention will be described in detail.

[0087] Unless otherwise specified, the percentage indicating the fraction of fine structures in the present invention is based on area.

[0088] The fine structure of the steel plate according to one embodiment of the present invention may include, in terms of area%, 50-80% of ferrite, 5-25% of bainite, 10-30% of fresh martensite, and less than 5% of retained austenite.

[0089] The ferrite is a soft structure and can contribute to the ductility of the steel sheet. When the area fraction of the ferrite is less than 50% of the entire microstructure contained in the steel sheet, it may be difficult to ensure the desired bendability. On the other hand, when the area fraction of the ferrite exceeds 80%, it may be difficult to ensure the strength at the level desired by the present invention.

[0090] The bainite is a phase having an intermediate hardness between ferrite and martensite, and the bainite may be appropriately contained. When the area fraction of the bainite is less than 5%, ferrite and martensite dominate, and the bendability may be deteriorated. On the other hand, when the area fraction of the bainite exceeds 25%, there may be a problem of reduced strength.

[0091] The fresh martensite is a phase that helps increase strength. When the area fraction of fresh martensite is less than 10%, the desired strength may not be ensured. On the other hand, when the area fraction of fresh martensite exceeds 30%, the area fraction of bainite is relatively reduced, so there may be a problem of poor bendability.

[0092] The retained austenite may be generated in a small amount of 5% or less during the final cooling process. In the case of a plated steel sheet with a high area fraction of retained austenite, liquid metal embrittlement tends to occur easily during spot welding of automobile parts assembly. Therefore, the retained austenite in the steel sheet is preferably controlled to be less than 5%.

[0093] According to one embodiment of the present invention, the fine structure fraction may be analyzed by analyzing the matrix structure at ¼ of the plate thickness of the continuously annealed steel plate. Specifically, the area fraction of the fine structure may be measured using FE-SEM, an image analyzer, and XRD.

[0094] Hereinafter, the method for producing the steel sheet of the present invention will be described in detail.

[0095] The steel sheet according to one embodiment of the present invention may be manufactured by reheating, hot rolling, coiling, cooling, cold rolling, continuous annealing, and cooling a steel billet satisfying the above-mentioned alloy composition.

[0096] Reheating

[0097] The steel billet satisfying the alloy composition of the present invention may be reheated within a temperature range of 1100-1300°C.

[0098] Reheating may be performed to smoothly perform the subsequent rolling process and fully obtain the desired physical properties of the steel plate. The present invention is not particularly limited to such reheating conditions as long as they are common reheating conditions. However, the preferred reheating temperature range may be 1100-1300°C.

[0099] When the reheating temperature is lower than 1100°C, the resolubility of precipitated elements such as Nb and Ti is reduced, so the effect of adding the corresponding elements may be reduced. On the other hand, when the reheating temperature exceeds 1300°C, the process cost increases and a large amount of hot rolling oxide is generated, so there is a problem that the surface quality of the steel plate deteriorates.

[0100] Hot Rolling

[0101] The reheated steel slab may be hot rolled at a finishing temperature of 800-950°C.

[0102] In the present invention, the reheated steel slab can be hot rolled at a normal hot rolling temperature. By hot rolling, a hot rolled steel sheet in which carbides serving as nucleation sites for austenite are finely dispersed can be manufactured. By uniformly dispersing fine carbides during such hot rolling, the effect of dissolving carbides during annealing and finely dispersing the generated austenite is achieved. As a result, martensite generated during cooling after annealing can be finely and uniformly dispersed, which can contribute to improving the strength and elongation of the final steel sheet.

[0103] During hot rolling, when the finishing temperature is lower than 800°C, the hot rolling load may increase due to the low hot rolling temperature. On the other hand, when the finishing temperature exceeds 950°C, the grains become coarse, the strength of the steel plate decreases, and the hot rolling oxides in the surface layer increase, which may cause the surface quality of the steel plate to deteriorate.

[0104] Winding and cooling

[0105] The hot-rolled steel sheet may be coiled at a temperature in the range of 400-700° C. and then cooled to room temperature at an average cooling rate of 0.10° C. / sec or less.

[0106] When the coiling temperature is lower than 400°C, a large amount of low-temperature structures such as martensite or bainite are generated, the strength of the hot-rolled steel sheet increases significantly, and there may be a problem of rolling load during cold rolling. On the other hand, when the coiling temperature exceeds 700°C, the hot-rolled fine structure becomes coarse, the strength of the final annealed steel sheet decreases, and the surface quality and plating properties of the steel sheet may deteriorate due to the increase of oxides on the surface of the steel sheet.

[0107] Furthermore, when the average cooling rate after coiling exceeds 0.10°C / sec, the cold rolling load increases due to the formation of low temperature structures, and the shape of the hot rolled steel sheet deteriorates due to the fast cooling rate, and sheet breakage may occur during cold rolling.

[0108] Cold Rolling

[0109] The cooled steel sheet may be cold rolled at a reduction ratio of 40-70%.

[0110] When the reduction rate during cold rolling is less than 40%, it may be difficult to ensure the desired thickness and it may be difficult to correct the shape of the steel plate. On the other hand, when the reduction rate during cold rolling exceeds 70%, the possibility of cracks on the edge of the steel plate is high, and there may be a problem of causing cold rolling load. Therefore, in the present invention, the reduction rate is preferably limited to 40-70%. As an embodiment of the present invention, a pickling process of pickling the steel plate may be further included before cold rolling.

[0111] Continuous annealing

[0112] The cold-rolled steel sheet may be heated to a T1 temperature of 800-850° C., cooled to a T2 temperature of 400-600° C. at an average cooling rate of 20° C. / sec or less, and then maintained for more than 50 seconds for continuous annealing.

[0113] In the present invention, continuous annealing may be performed to perform recrystallization while forming ferrite and austenite and distributing carbon.

[0114] During continuous annealing, when the heating temperature (T1) is lower than 800°C, sufficient recrystallization cannot be achieved, and it is difficult to form sufficient two-phase austenite, and the desired fractions of martensite and bainite may not be ensured after annealing. On the other hand, when the heating temperature (T1) exceeds 850°C, productivity decreases, and too much austenite is formed, and the fractions of bainite and martensite increase significantly after cooling, so the yield strength may increase and the ductility may decrease. In addition, the surface enrichment of elements such as Si, Mn and B that reduce the wettability of hot-dip galvanizing becomes serious, so the coating surface quality may also be reduced. In view of this situation, in the present invention, the heating temperature is preferably limited to 800-850°C during the continuous annealing. Within the above temperature range, the fractions of austenite and ferrite in the two-phase region in the steel sheet are determined, and the strength of the final steel sheet is different according to the fractions. Generally, as the fraction of austenite in the two-phase region increases, the strength of the final annealed steel sheet tends to increase, but the subsequent processes may also affect the final microstructure, so the physical properties of the steel sheet may change.

[0115] The two-phase region austenite in the heated steel sheet may be transformed into ferrite of different fractions according to the cooling end temperature (T2). During continuous annealing, when the cooling end temperature (T2) exceeds 600°C, a large amount of ferrite transformation occurs during heat treatment, so there may be a problem of strength reduction. On the other hand, when the cooling end temperature (T2) is lower than 400°C, the fraction of bainite is too high and the formation of martensite is reduced in the process of holding for more than 50 seconds, so there may be a problem of strength deterioration.

[0116] Thereafter, the annealed steel sheet may be cooled to room temperature. When cooling to room temperature, cooling conditions are not particularly limited, but as an example, air cooling may be performed.

[0117] The steel sheet according to one embodiment of the present invention may have an R value defined in the following Relationship 3 during continuous annealing of 1797 to 1850.

[0118] [Equation 3]

[0119]

[0120] (In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element, and T1 and T2 are the heating temperature (°C) and cooling end temperature (°C) during continuous annealing, respectively.)

[0121] In the present invention, in order to simultaneously satisfy the desired strength and bendability of the steel sheet during continuous annealing, the composition and annealing conditions are specified, and the contents of C, Si, Mn, and Al components and the conditions T1 and T2 are optimized.

[0122] The temperature of T1 refers to the heating temperature in the continuous annealing process, which determines the fraction of austenite and ferrite in the two-phase region in the steel plate, and the strength of the final annealed steel plate may be different by this fraction. Generally, as the fraction of austenite in the two-phase region increases, the strength of the final annealed steel plate tends to increase, but the subsequent process may also affect the final microstructure and change the physical properties of the steel plate, so it is difficult to describe the effect of a single annealing temperature. The two-phase region austenite can further transform into ferrite with different fractions according to the temperature of T2 as the cooling termination temperature during the subsequent cooling process, so T2 is one of the important factors affecting the physical properties of the steel plate. In addition, according to the T2 temperature, the fractions of bainite, residual austenite and martensite in the final annealing structure may be different.

[0123] When the T2 temperature is higher than the bainite transformation start temperature and lower than the martensite transformation temperature, bainite cannot be introduced into the structure of the steel sheet, so the T2 temperature must be set to a temperature between the bainite transformation start temperature and the martensite transformation start temperature. The above T1 temperature and T2 temperature together with the composition of the steel sheet affect the final annealing steel sheet microstructure, which ultimately affects the physical properties of the steel sheet, and in order to ensure the target physical properties, the optimized relationship 3 needs to be satisfied. Therefore, even if the amount of Si added is minimized, a high-tensile steel sheet with excellent bendability and target strength can be obtained.

[0124] As described above, the final material of the steel sheet is affected by the composition and the temperature and time of each important heat treatment process. Therefore, when the following relationship conditions are satisfied, a high-tensile steel sheet with an optimal combination of physical properties and excellent bendability can be manufactured. In addition, when the R value defined in the following relationship 3 is less than 1797, there may be a problem of insufficient strength of the steel sheet. In addition, in order to ensure the desired bendability, the upper limit of the R value may be limited to 1850.

[0125] Hot dip galvanizing

[0126] According to one embodiment of the present invention, the continuously annealed steel sheet may be hot-dip galvanized at a temperature ranging from 430°C to 490°C.

[0127] The steel sheet manufactured in the present invention can be plated by a plating method in which the steel sheet is immersed in a molten zinc plating bath. In the present invention, the hot-dip galvanizing conditions are not particularly limited, and hot-dip galvanizing can be performed under the usual conditions applicable in the same technical field. By hot-dip galvanizing, the steel sheet according to one embodiment of the present invention may include a hot-dip galvanized layer on the surface. In addition, as required, after the hot-dip galvanizing step, the steel sheet may be subjected to an alloying heat treatment. As an embodiment, the hot-dip galvanized steel sheet may be subjected to an alloying heat treatment in a temperature range of 460-530°C and then cooled to room temperature. By the alloying heat treatment, the steel sheet may also include an alloyed hot-dip galvanized layer on the surface.

[0128] The steel plate of the present invention manufactured as described above can have a tensile strength (TS) of 780 MPa or more, an elongation (El) of 14.0% or more, and a value of a bending angle (°) / thickness (mm) of 50° / mm or more during a 180° bending test (wherein the bending angle (°) refers to a bending angle at which no cracks are generated in the bent portion during a 180° bending test), thereby ensuring excellent properties of strength and bendability. DETAILED DESCRIPTION

[0129] The present invention is described in more detail below by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention in more detail and are not intended to limit the scope of the present invention.

[0130] (Example)

[0131] A steel slab having a composition disclosed in Table 1 below was manufactured, and then reheated under the conditions of Table 2 below and hot finish rolled. The steel plate hot-rolled as described above was coiled under the conditions of Table 2 below and then cooled to room temperature to manufacture a steel plate. Thereafter, the steel plate was pickled, and then cold rolled at a reduction ratio of 50%, as shown in Table 2 below, heated to a T1 temperature, cooled to a T2 temperature, held for more than 50 seconds, and then hot-dip coated at 460° C. as a hot-dip coating temperature and finally cooled to room temperature.

[0132] [Table 1]

[0133]

[0134] [Equation 1]

[0135]

[0136] (In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element.)

[0137] [Equation 2]

[0138] RT=[Si]+[Nb]+[Ti]

[0139] (In the formula, [Si], [Nb], and [Ti] are the weight % of each element.)

[0140] [Table 2]

[0141]

[0142] [Equation 3]

[0143]

[0144] (In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element, and T1 and T2 are the heating temperature (°C) and cooling end temperature (°C) during continuous annealing, respectively.)

[0145] The measurement results of the mechanical and physical properties of each steel plate manufactured as described above are shown in Table 3 below. At this time, each test piece was subjected to a tensile test in the L direction using the ASTM standard to evaluate the tensile physical properties at room temperature. In particular, the bendability was measured by conducting a 180° bending test and dividing the bending radius at which no cracks are generated in the bending portion by the thickness (mm) of the test piece to represent the value. Among them, the bending portion may refer to the portion of the steel plate where the bending angle is applied, and may generally refer to the portion where the bending is applied. The fine structure fraction is an analysis of the matrix structure at the 1 / 4 position of the plate thickness of the continuously annealed steel plate and the results thereof are utilized. Specifically, the fractions of ferrite (F), bainite (B), fresh martensite (M) and retained austenite (A) were measured using FE-SEM, an image analyzer and XRD.

[0146] [Table 3]

[0147]

[0148] As shown in Table 3, in the case of the inventive examples satisfying the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed by the present invention are satisfied, and the physical properties expected by the present invention can also be ensured. Figure 1 This is a photograph of the microstructure of Invention Example 13 according to one embodiment of the present invention observed using an electron microscope.

[0149] On the other hand, Comparative Examples 1 and 2 are examples in which the T1 temperature during continuous annealing does not satisfy the conditions of the present invention, and Relational Expression 3 also does not satisfy the conditions of the present invention. As a result, the desired elongation and bendability are not achieved.

[0150] Although Comparative Example 3 satisfied Relational Expression 3, the T1 temperature did not satisfy the conditions of the present invention, and thus the elongation was poor.

[0151] Comparative Example 4 satisfies the T1 temperature and the T2 temperature during continuous annealing, but does not satisfy the condition of Relationship 3 proposed by the present invention. Therefore, an excessive amount of bainite is formed compared to the area fraction expected by the present invention, and the expected elongation and bendability characteristics cannot be ensured.

[0152] Comparative Examples 5 and 6 are examples in which Relational Expressions 1 and 2 do not satisfy the conditions of the present invention, and excessive martensite is formed compared to the area fraction expected by the present invention, so the bendability is poor. Figure 2 This is a photograph of the microstructure of Comparative Example 6 according to one embodiment of the present invention observed with an electron microscope, and it can be confirmed that excessive martensite is formed.

[0153] Comparative Examples 7 and 8 are alloy compositions that meet the conditions of the present invention, but the T1 temperature does not meet the conditions of the present invention. Compared with the area fraction expected by the present invention, too much ferrite is formed and insufficient bainite, so the elongation is reduced.

[0154] T2 of Comparative Example 9 did not meet the conditions of the present invention, and too much bainite was formed compared to the proposed level, and martensite was reduced, so that the desired strength could not be ensured.

[0155] The present invention has been described in detail above through the embodiments, but other forms of embodiments may also be included. Therefore, the technical concept and scope of the claims are not limited to the embodiments.

Claims

1. A steel plate, comprising, by weight%, carbon (C): 0.05-0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0-3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1-1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.0100% or less, nitrogen (N): 0.010% or less, and the balance of iron (Fe) and other inevitable impurities, The T value defined in the following relationship 1 is 1648 or more, In terms of area%, the microstructure contains: 50-80% ferrite, 5-25% bainite, 10-30% new martensite and less than 5% retained austenite. [Equation 1] In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element.

2. The steel plate according to claim 1, wherein: The RT value of the steel plate defined in the following relational expression 2 is 0.01 or more, [Equation 2] RT=[Si]+[Nb]+[Ti] In the formula, [Si], [Nb] and [Ti] are the weight % of each element.

3. The steel plate according to claim 1, wherein: The steel plate has a tensile strength (TS) of 780 MPa or more and an elongation (El) of 14.0% or more.

4. The steel plate according to claim 1, wherein: The bending angle / thickness value of the steel plate in a 180° bending test is greater than 50° / mm, wherein the bending angle refers to the bending angle at which no cracks are generated in the bending portion during the 180° bending test, the unit of the bending angle is °, and the unit of the thickness is mm.

5. The steel plate according to claim 1, wherein: The steel plate further comprises a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface.

6. A method for manufacturing a steel plate, comprising the following steps: The steel billet is reheated, wherein the steel billet contains, in weight %, carbon (C): 0.05-0.20%, silicon (Si): 0.10% or less, manganese (Mn): 1.0-3.0%, aluminum (sol.Al): 1.00% or less, chromium (Cr): 0.1-1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.100% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, and the balance is iron (Fe) and other inevitable impurities, and the T value defined in the following relational formula 1 of the steel billet is 1648 or more; hot rolling the reheated steel billet; The hot-rolled steel plate is coiled and then cooled; cold rolling the cooled steel plate; The cold-rolled steel sheet is heated to a T1 temperature of 800-850° C., cooled to a T2 temperature of 400-600° C. at an average cooling rate of less than 20° C. / s, and then maintained for more than 50 seconds for continuous annealing; and The continuously annealed steel sheet is cooled to room temperature. Wherein, the R value defined in the following relation 3 is 1797 to 1850, [Equation 1] In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element. [Equation 3] In the formula, [C], [Mn], [Nb], [Ti] and [Cr] are the weight % of each element, T1 and T2 are the heating temperature and cooling end temperature during continuous annealing, respectively, and the units of the heating temperature and the cooling end temperature are ° C.

7. The method for manufacturing a steel plate according to claim 6, wherein: The RT value of the steel billet defined in the following relational expression 2 is greater than or equal to 0.01, [Equation 2] RT=[Si]+[Nb]+[Ti] In the formula, [Si], [Nb] and [Ti] are the weight % of each element.

8. The method for manufacturing a steel plate according to claim 6, wherein: The reheating is carried out in the temperature range of 1100-1300°C, the hot rolling is carried out at a finishing temperature of 800-950°C, the cooling step after coiling is carried out in the temperature range of 400-700°C and then cooled to room temperature at an average cooling rate of less than 0.10°C / second, and the cold rolling is carried out at a reduction rate of 40-70%.

9. The method for manufacturing a steel plate according to claim 6, wherein: The cold rolling step further includes a step of pickling the steel plate.

10. The method for manufacturing a steel plate according to claim 6, wherein: After the continuous annealing step and before the cooling step, the method further comprises a step of hot-dip galvanizing the steel sheet at a temperature range of 430-490°C.

11. The method for manufacturing a steel plate according to claim 10, wherein: After the hot-dip galvanizing step and before cooling, the method further comprises subjecting the steel sheet to an alloying heat treatment at a temperature range of 460-530°C.

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