Steel sheet and method for manufacturing same

Abstract

The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet having excellent ductility and hole expandability, and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly to a high-strength cold-rolled steel sheet having excellent ductility and hole expansion properties and a method for manufacturing the same. Background Art

[0002] In recent years, with the increasing attention to carbon dioxide emissions reduction and the consequent continuous improvement of regulatory standards, automobile manufacturers are working on improving fuel efficiency by lightweighting the vehicle body. To effectively achieve such vehicle body lightweighting, it is necessary to use high-strength steels that reduce the body weight by decreasing the steel sheet thickness while ensuring passenger safety.

[0003] In recent years, for improving the impact resistance of the vehicle body, high-strength steels have been increasingly widely used in structural components such as members, seat rails, and A / B / C pillars. The characteristics of automotive structural components are that the higher the yield strength ratio (yield strength / tensile strength) with respect to the tensile strength, the more beneficial it is for absorbing impact energy.

[0004] However, with the increase in strength, the ductility decreases, resulting in reduced formability. Therefore, it is necessary to develop materials with improved formability while having high yield strength ratio characteristics. Thus, to meet such technical requirements, it is necessary to develop an automotive steel sheet that not only has strength suitable for automotive materials but also has excellent ductility and hole expansion properties at the same time.

[0005] Patent Document 1 discloses a technique for manufacturing a steel sheet having a martensite phase with an area fraction of 80% or more by using water cooling and overaging treatment during continuous annealing. In the annealing process, when tempering is performed by immersing in water after soaking, a steel sheet with a tempered martensite structure can be manufactured. At this time, although the tempering effect of martensite improves the yield strength ratio, due to the temperature deviation in the width and length directions of the steel sheet, the shape quality of the coiled sheet deteriorates, or due to material deviation problems, such as soaking during forming, the workability may be reduced.

[0006] In addition, Patent Document 2 discloses a technique for manufacturing a composite structure steel sheet with excellent workability by using a retained austenite phase. This technique can simultaneously ensure the strength and ductility required by automobile manufacturers by using transformation-induced plasticity. However, due to the large amounts of Si and Al added to form the retained austenite, it is difficult to ensure the quality of steelmaking and continuous casting, and there is a problem of difficulty in obtaining a surface quality at the outer panel level.

[0007] Therefore, it is necessary to develop a steel sheet that can solve the problems of the above-mentioned prior art while simultaneously ensuring strength, ductility, and hole expansion properties.

[0008] [Prior Art Documents]

[0009] [Patent Documents]

[0010] (Patent Document 1) Japanese Published Patent Gazette No. 1992-289120

[0011] (Patent Document 2) Japanese Published Patent Gazette No. 2015-113504 Summary of the Invention

[0012] (1) Technical Problem to be Solved

[0013] According to one embodiment of the present invention, an object is to provide a steel sheet and a method for manufacturing the same.

[0014] According to one embodiment of the present invention, an object is to provide a high-strength cold-rolled steel sheet having excellent ductility and hole expansion property and a method for manufacturing the same.

[0015] The technical problems of the present invention are not limited to the above. Those skilled in the art can easily understand additional technical problems of the present invention from the entire content of this specification.

[0016] (2) Technical Solution

[0017] According to one embodiment of the present invention, a steel sheet can be provided which, by weight%, contains: carbon (C): 0.05 - 0.20%, manganese (Mn): 2.3 - 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, the balance being iron (Fe) and unavoidable impurities, the R value defined in the following relational expression 1 being 5.0 to 6.0, and the microstructure containing 40% or more tempered martensite by area%, the balance of the microstructure containing ferrite, fresh martensite, bainite, and retained austenite, and the MR defined in the following relational expression 2 being 0.60 to 0.80.

[0018] [Relational Expression 1]

[0019] R = [C] + 1.3[Si] + 1.5[Mn] + 1.8[Nb]

[0020] (In the formula, [C], [Si], [Mn], and [Nb] are the weight% of each element.)

[0021] [Relational Expression 2]

[0022] MR = TM / (FM + TM)

[0023] (In the formula, TM and FM are the area% of tempered martensite and fresh martensite, respectively.)

[0024] The fine microstructure may contain 30 - 50% ferrite and 5 - 25% fresh martensite.

[0025] The steel sheet may contain more than 10 2 per unit area (m 12 of one or more of Nb-based fine precipitates and Ti-based fine precipitates having an average diameter (circular equivalent diameter) of 50 nm or less.

[0026] The yield strength of the steel sheet may be 700 MPa or more, the tensile strength may be 1000 MPa or more, the yield ratio may be 0.70 or more, and the elongation may be 13% or more.

[0027] The product of the yield strength and the elongation of the steel sheet may be 9100 MPa·% or more, the product of the tensile strength and the elongation may be 13000 MPa·% or more, and the hole expansion rate (HER) may be 50% or more.

[0028] According to an embodiment of the present invention, a method for manufacturing a steel sheet can be provided. The manufacturing method includes the following steps: reheating a steel slab, which, by weight%, contains: carbon (C): 0.05 - 0.20%, manganese (Mn): 2.3 - 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, the balance being iron (Fe) and unavoidable impurities, and the R value defined in the following relational expression 1 is 5.0 to 6.0; hot rolling the reheated steel slab; cooling and coiling the hot-rolled steel sheet; cold rolling the coiled steel sheet; heating the cold-rolled steel sheet to a temperature range of 760 - 830 °C and holding for 30 - 300 seconds for continuous annealing; performing a first cooling on the continuously annealed steel sheet at an average cooling rate of 2.0 - 6.0 °C / second to cool to a temperature range of 500 - 700 °C; performing a second cooling at an average cooling rate of 35.0 - 60.0 °C / second after the first cooling to cool to a temperature range of 100 - 300 °C; and performing an overaging treatment by heating to a temperature range of 200 - 450 °C and holding for 100 - 500 seconds after the second cooling.

[0029] [Relational Expression 1]

[0030] R = [C] + 1.3[Si] + 1.5[Mn] + 1.8[Nb]

[0031] (In the formula, [C], [Si], [Mn], and [Nb] are the weight % of each element.)

[0032] The reheating step can be carried out within a temperature range of 1000 - 1350°C, the hot rolling step can be carried out at a finish rolling temperature of Ar3 to Ar3 + 50°C, the coiling step can be carried out within a temperature range of 450 - 700°C, and the cold rolling step can be carried out with a reduction ratio of 35 - 65%.

[0033] It may further include a step of pickling the steel plate after the coiling step.

[0034] It may further include a step of skin pass rolling with a reduction ratio of 0.1 - 1.0% after the over - aging treatment step.

[0035] (III) Advantageous Effects

[0036] According to an embodiment of the present invention, a steel plate and a manufacturing method thereof can be provided.

[0037] According to an embodiment of the present invention, a high - strength cold - rolled steel plate with excellent ductility and hole - expansion property and a manufacturing method thereof can be provided.

[0038] According to an embodiment of the present invention, a cold - rolled steel plate that can be used as an automotive material, especially as a material for components that require high formability, etc., and has excellent strength while having excellent ductility and hole - expansion property, and a manufacturing method thereof can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a micro - structure photograph of Invention Example 1 according to an embodiment of the present invention.

[0040] Figure 2 It is a micro - structure photograph of Comparative Example 4 deviating from an embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

[0041] The following describes the preferred specific embodiments of the present invention. The specific embodiments of the present invention can be deformed into various forms, and should not be construed as limiting the scope of the present invention to the specific embodiments described below. These specific embodiments are provided to more specifically explain the present invention to those skilled in the art.

[0042] The inventors of the present invention have conducted in - depth research to provide a steel plate with improved ductility and hole - expansion property, such that the steel plate has a strength suitable for automotive materials and at the same time has a formability that can be processed into components requiring complex shapes.

[0043] As a result, it was confirmed that by optimizing the alloy composition system and microstructure of the steel, the desired physical properties can be ensured, and thus a steel sheet suitable for automotive structural components and the like that need to be processed into complex shapes can be provided, thereby completing the present invention.

[0044] Hereinafter, the present invention will be described in detail.

[0045] Hereinafter, the steel sheet composition of the present invention will be described in detail.

[0046] Unless otherwise specifically stated, the % indicating the content of each element in the present invention is based on weight.

[0047] According to an embodiment of the present invention, by weight %, the steel sheet may contain: carbon (C): 0.05 - 0.20%, manganese (Mn): 2.3 - 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, the balance being iron (Fe) and inevitable impurities.

[0048] Carbon (C): 0.05 - 0.20%

[0049] Carbon (C) is an element that is beneficial to ensuring strength by forming martensite in the steel and is an essential element for manufacturing high-strength steel. Generally, the higher the content of the carbon (C), the easier it is to form martensite, which is beneficial to forming the composite tissue phase required for manufacturing high-strength steel. Therefore, in the present invention, in order to ensure the desired strength and form an appropriate level of martensite, the content of the carbon (C) may be 0.05% or more. As an embodiment of the present invention, the content of the carbon (C) may be 0.07% or more. However, in order to control both the desired strength and elongation at the same time, the carbon (C) content must be controlled at an appropriate level. When the carbon (C) content exceeds 0.20%, there are problems of poor weldability and formability. According to an embodiment of the present invention, the carbon (C) content may be 0.18% or less.

[0050] Manganese (Mn): 2.3 - 3.0%

[0051] Manganese (Mn) is an element that improves the hardenability of steel. In particular, it plays an important role in the formation of martensite. In addition, the manganese (Mn) contributes to increasing the strength of the steel through solid solution strengthening, and precipitates the inevitably added S in the steel in the form of MnS, thereby playing a role in suppressing the occurrence of plate fracture caused by S during hot rolling and the phenomenon of hot brittleness. In order to fully obtain the above effects, 2.3% or more of the manganese (Mn) can be added. As an embodiment of the present invention, the content of manganese (Mn) can be 2.5% or more. However, when the content of manganese (Mn) exceeds 3.0%, not only does the weldability deteriorate, but also an excessive amount of martensite is formed, resulting in unstable material properties, the formation of oxide bands in a band shape, and an increased risk of processing cracks and plate fracture. In addition, during annealing, manganese oxide dissolves out on the surface of the steel plate, which may hinder the plating property. As an embodiment of the present invention, the content of manganese (Mn) can be 2.8% or less.

[0052] Silicon (Si): 1.5% or less

[0053] Silicon (Si) is a useful element that can ensure strength without reducing the ductility of the steel plate. This silicon (Si) promotes the formation of ferrite and, by promoting the enrichment of C into the untransformed austenite, is conducive to promoting the formation of martensite. However, when the content of silicon (Si) exceeds 1.5%, it may cause hydrogen embrittlement and deterioration of weldability. Therefore, in the present invention, the content of silicon (Si) can be 1.5% or less. However, considering the inevitably added level in the steel, 0% can be excluded.

[0054] Aluminum (Al): 0.10% or less

[0055] Aluminum (Al) is an element added for the deoxidation and grain size refinement of steel. When the content of aluminum (Al) exceeds 0.10%, it not only causes a decrease in castability during the continuous casting process, but also there is an increased possibility of material defects in the annealed material and surface defects in the plated material due to excessive formation of inclusions. Therefore, in the present invention, the content of aluminum (Al) can be 0.10% or less. However, considering the inevitably added level in the steel, 0% can be excluded. As an embodiment of the present invention, the content of aluminum (Al) can be 0.01% or more.

[0056] Phosphorus (P): 0.05% or less

[0057] Phosphorus (P) is the most favorable element for ensuring the strength of steel while not significantly impairing formability. However, when added in excess, it greatly increases the possibility of brittle fracture and is also an element that increases the occurrence of slab breakage during the hot rolling process. Therefore, in the present invention, the content of phosphorus (P) can be limited to 0.05% or less. However, considering the level inevitably added in the steel, 0% can be excluded.

[0058] Sulfur (S): 0.010% or less

[0059] Sulfur (S) is an impurity element inevitably added in steel, and its content is preferably controlled at as low a level as possible. In particular, sulfur (S) in the steel increases the possibility of red hot embrittlement. In the present invention, the sulfur (S) content can be limited to 0.010% or less. However, when manufacturing steel, considering the inevitably added level, 0% can be excluded.

[0060] Nitrogen (N): 0.010% or less

[0061] Nitrogen (N) is an impurity element inevitably added in steel, and its content is preferably controlled at as low a level as possible. However, since this causes a problem of rapid increase in the refining cost of steel, the content of nitrogen (N) can be controlled at 0.010% or less within the range of operable conditions. However, when manufacturing steel, considering the inevitably added level, 0% can be excluded.

[0062] Niobium (Nb): 0.10% or less

[0063] Niobium (Nb) is an element that segregates at the austenite grain boundaries, inhibits the coarsening of austenite grains during annealing heat treatment, and forms fine carbides to contribute to the increase in yield strength and tensile strength. However, when the content of the niobium (Nb) is too high, due to the precipitation of coarse carbides and the reduction of the carbon content in the steel, the strength and elongation may decrease, the manufacturing cost increases, and thus the economy may deteriorate. Therefore, in the present invention, when adding the niobium (Nb), its upper limit can be limited to 0.10%. As an embodiment of the present invention, the upper limit of niobium (Nb) can be limited to 0.05%. However, when manufacturing steel, considering the inevitably added level, 0% can be excluded.

[0064] Titanium (Ti): 0.10% or less

[0065] Titanium (Ti) not only helps to ensure the yield strength and tensile strength by forming fine carbides, but also inhibits the precipitation of AlN by precipitating N in the form of TiN in the steel, thereby effectively reducing the risk of crack generation during continuous casting. However, when the titanium (Ti) content is too high, due to the precipitation of coarse carbides and the reduction of carbon content in the steel, the strength and elongation may decrease, and nozzle blockage may occur during continuous casting. Therefore, in the present invention, when adding the said titanium (Ti), its upper limit can be restricted to 0.10%. As an embodiment of the present invention, the upper limit of the titanium (Ti) content can be restricted to 0.05%. However, when manufacturing steel, 0% can be excluded considering the inevitable addition level.

[0066] Boron (B): 0.003% or less

[0067] Boron (B) can effectively delay the transformation of austenite to pearlite during the cooling process after continuous annealing. However, when the content of the said boron (B) is too high, boron (B) may accumulate on the steel plate surface, resulting in surface defects. Therefore, in the present invention, when adding the said boron (B), its upper limit can be restricted to 0.003%. However, when manufacturing steel, 0% can be excluded considering the inevitable addition level.

[0068] In the steel of the present invention, in addition to the above components, it may contain the balance of iron (Fe) and inevitable impurities. Inevitable impurities may be inadvertently mixed in during the conventional manufacturing process, so these impurities cannot be excluded. These impurities are well known to those skilled in the field of conventional steel manufacturing, so no special description is made of all of their contents in this specification.

[0069] According to an embodiment of the present invention, the R value defined in the following relational expression 1 of the steel plate can be 5.0 to 6.0.

[0070] [Relational expression 1]

[0071] R = [C] + 1.3[Si] + 1.5[Mn] + 1.8[Nb]

[0072] (In the formula, [C], [Si], [Mn] and [Nb] are the weight percentages of each element.)

[0073] The said relational expression 1 is an empirical value obtained based on the content relationship of specific components, aiming to ensure the basic material of the steel plate desired in the present invention.

[0074] When the R value defined in the relational expression 1 is less than 5.0, the hardenability of the steel decreases, and the phase fraction of low-temperature transformation structures such as martensite decreases, resulting in a problem that the strength desired in the present invention cannot be ensured. As an embodiment of the present invention, the R value can be 5.2 or more. As an embodiment of the present invention, the R value can be 5.3 or more. On the other hand, when the R value exceeds 6.0, the hardenability of the steel is too high, and a material with the formability desired in the present invention cannot be obtained. As an embodiment of the present invention, the upper limit of the R value can be 5.8.

[0075] Hereinafter, the fine structure of the steel plate of the present invention will be described in detail.

[0076] Unless otherwise specifically stated in the present invention, the % representing the fraction of the fine structure is based on the area.

[0077] According to an embodiment of the present invention, the fine structure of the steel plate may contain 40% or more of tempered martensite by area%. In addition, the balance structure may contain ferrite, fresh martensite, bainite, and retained austenite.

[0078] In the present invention, martensite can be divided into tempered martensite and fresh martensite. The martensite can be formed by the transformation of partial austenite during secondary cooling below the martensite transformation start temperature (Ms) after passing through a primary cooling range.

[0079] The tempered martensite may refer to a structure formed by tempering the martensite that transforms during secondary cooling while passing through an overaging treatment range. During the overaging treatment, the carbon (C) released during the tempering of martensite is fixed to the surrounding dislocations, resulting in the Cottrell atmosphere phenomenon, and at this time, the yield strength of the steel plate may increase.

[0080] The fresh martensite may refer to a structure formed by non-diffusion transformation when the remaining austenite cools to room temperature before final cooling after the overaging treatment.

[0081] When the tempered martensite is less than 40%, there is a problem that the strength and ductility desired in the present invention cannot be properly ensured. According to an embodiment of the present invention, its upper limit can be 65%.

[0082] Generally, when phase transformation occurs in ferrite, bainite, etc., the enrichment degree of carbon in the remaining austenite around increases. Therefore, due to the martensite with increased solid-solved carbon in the final structure, the strength of the steel increases. At this time, if the stabilization of retained austenite is accompanied by the TRIP effect, an increase in elongation can be expected simultaneously. However, according to the present invention, although the fraction of retained austenite is relatively low, it has a high elongation rate, which can be considered to benefit from the uniform structure of tempered martensite and the significant reduction in the hardness difference between phases. In other words, due to the high fraction of tempered martensite, it has a high strength with a tensile strength of 1000 MPa or more, and due to the uniformity of the structure and the reduction in the hardness difference between phases, ductility and hole expansion property can be ensured.

[0083] According to an embodiment of the present invention, in order to more effectively ensure the strength and ductility proposed by the present invention, it contains 30 - 50% ferrite, 5 - 25% fresh martensite, and the balance structure may contain bainite and retained austenite. According to an embodiment of the present invention, it may contain 30 - 45% ferrite.

[0084] According to an embodiment of the present invention, the MR defined in the following relational expression 2 for the steel plate may be 0.60 to 0.80.

[0085] [Relational expression 2]

[0086] MR = TM / (FM + TM)

[0087] (In the formula, TM and FM are the area percentages of tempered martensite and fresh martensite respectively.)

[0088] Relational expression 2 may refer to the fraction of tempered martensite relative to the total fraction of martensite. When the MR value defined in relational expression 2 is 0.60 to 0.80, the desired strength can be ensured, and the yield ratio is 0.70 or more to ensure formability.

[0089] When the MR value defined in relational expression 2 is less than 0.60, the fraction of fresh martensite in martensite increases, so the tensile strength can increase, but it is difficult to meet the desired yield ratio and hole expansion property. On the other hand, when the MR value exceeds 0.80, an excessive amount of tempered martensite is formed in martensite, and thus it may be difficult to ensure the target tensile strength.

[0090] According to an embodiment of the present invention, the steel plate may contain 2 ) 10 12 or more of one or more of Nb-based fine precipitates and Ti-based fine precipitates having an average diameter (circular equivalent diameter) of 50 nm or less per unit area (m

[0091] In the present invention, by including the above-mentioned fine precipitates, it is possible to have the effect of more favorably ensuring the desired high-strength characteristics. More specifically, in the present invention, through the tempering of martensite and precipitates, the hardness difference between phases is effectively reduced, and not only the bendability can be effectively ensured, but also the hole expansion ratio (HER) can be effectively ensured.

[0092] According to an embodiment of the present invention, the yield strength of the steel plate is 700 MPa or more, the tensile strength is 1000 MPa or more, the yield ratio is 0.70 or more, the elongation is 13% or more, the product of the yield strength and the elongation is 9100 MPa·% or more, the product of the tensile strength and the elongation is 13000 MPa·% or more, and the hole expansion ratio (HER) is 50% or more, so that excellent strength can be achieved while having excellent ductility and hole expansion properties.

[0093] Hereinafter, the manufacturing method of the steel plate of the present invention will be described in detail.

[0094] According to an embodiment of the present invention, the steel plate can be manufactured by reheating, hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, and overaging treatment of a steel slab that satisfies the above alloy composition.

[0095] Reheating

[0096] The steel slab that satisfies the alloy composition of the present invention can be reheated in the temperature range of 1000 - 1350 °C.

[0097] When the reheating temperature is lower than 1000 °C, hot rolling may be performed in a temperature range below the desired finish rolling temperature. On the other hand, when the reheating temperature exceeds 1350 °C, there is a risk of melting due to reaching the melting point of the steel.

[0098] Hot rolling

[0099] The reheated steel slab can be hot rolled at a finish rolling temperature of Ar3 to Ar3 + 50 °C.

[0100] When the finish rolling temperature is lower than Ar3, the hot deformation resistance is likely to increase rapidly. On the other hand, when the finish rolling temperature exceeds Ar3 + 50 °C, not only too thick scale is generated, but also the grains of the hot rolled steel plate are formed coarsely, and ultimately the physical properties of the steel plate may be reduced. Among them, the finish rolling temperature may refer to the temperature at the exit side of the finishing mill.

[0101] [Formula]

[0102] Ar3 = 910 - 95 [C] - 15.2 [Ni] + 44.7 [Si] + 104 [V] + 31.5 [Mo] - (15 [Mn] + 11 [Cr] + 20 [Cu] - 700 [P] - 400 [Al] - 400 [Ti])

[0103] (In the formula, [C], [Ni], [Si], [V], [Mo], [Mn], [Cr], [Cu], [P], [Al], and [Ti] are the weight percentages of the respective elements.)

[0104] Coiling

[0105] The hot-rolled steel sheet can be cooled and coiled within a temperature range of 450 - 700 °C.

[0106] When the coiling temperature is lower than 450 °C, excessive martensite or bainite is generated, and during subsequent cold rolling processes, manufacturing problems such as shape defects caused by load may occur. On the other hand, when the coiling temperature exceeds 700 °C, the surface scale increases, and thus there is a problem of poor pickling performance. Additionally, the cooling conditions from after hot rolling until the coiling temperature are not particularly limited, but can be cooling conditions applicable in the same technical field. As an embodiment of the present invention, air cooling can be performed.

[0107] Cold Rolling

[0108] The coiled steel sheet can be cold rolled with a reduction ratio of 35 - 65%.

[0109] During cold rolling, when the reduction ratio is less than 35%, the recrystallization driving force weakens, it is difficult to ensure good recrystallized grains, and shape correction may be difficult. On the other hand, when the reduction ratio exceeds 65%, the possibility of cracks occurring in the edge part of the steel sheet increases, and the rolling load may increase sharply.

[0110] According to an embodiment of the present invention, before cold rolling, a pickling process can be performed to remove the scale generated on the surface of the steel sheet. This pickling process can be carried out under conventional conditions, and its conditions do not need to be particularly limited.

[0111] Continuous Annealing

[0112] The cold-rolled steel sheet can be heated to a temperature range of 760 - 830 °C and subjected to continuous annealing for 30 - 300 seconds.

[0113] In the present invention, the desired fine microstructure basis in the present invention can be prepared through the continuous annealing.

[0114] During the continuous annealing, when the heating temperature is lower than 760 °C, there is a fraction of unrecrystallized ferrite, which may cause the defect that the material deviation in the length or width direction of the coil plate becomes larger. According to an embodiment of the present invention, it can be heated to above 770 °C. In addition, when the heating temperature exceeds 830 °C, annealing oxides may be generated on the surface of the steel plate. Furthermore, during the continuous annealing process, since ferrite is not generated, the target elongation may not be ensured. According to an embodiment of the present invention, it can be carried out at a temperature below 810 °C.

[0115] During the continuous annealing, when the holding time is less than 30 seconds, full recrystallization cannot be achieved, it is difficult to ensure the elongation, and the possibility of material deviation in the length or width direction of the steel plate increases. On the other hand, when the holding time exceeds 300 seconds, the annealing effect is saturated, and there is a problem of reduced productivity.

[0116] Primary cooling

[0117] The steel plate after the continuous annealing can be subjected to primary cooling at an average cooling rate of 2.0 - 6.0 °C / second and cooled to a temperature range of 500 - 700 °C.

[0118] In the present invention, during primary cooling, compared with the subsequent secondary cooling process, it can be cooled slowly, and the deterioration of the plate shape caused by the temperature drop during the relatively fast cooling interval of the secondary cooling can be suppressed.

[0119] During primary cooling, when the cooling termination temperature is lower than 500 °C or exceeds 700 °C, it deviates from the appropriate temperature gradient range of the subsequent secondary cooling, so it is difficult to ensure stable cooling capacity.

[0120] During primary cooling, when the average cooling rate exceeds 6.0 °C / second, the enrichment of C and Mn in the austenite may be insufficient. In the present invention, during the primary cooling, no special limitation is imposed on the lower limit of the average cooling rate, but it can be cooled at the cooling rate of conventional slow cooling. In the present invention, its lower limit can be 2.0 °C / second.

[0121] Secondary cooling

[0122] After the primary cooling, it can be subjected to secondary cooling at an average cooling rate of 35.0 - 60.0 °C / second and cooled to a temperature range of 100 - 300 °C.

[0123] In the present invention, during secondary cooling, according to the width and thickness of the steel plate expected to be obtained, the cooling rate and the cooling termination temperature can be appropriately adjusted, so that the best plate shape can be ensured.

[0124] During the secondary cooling, when the cooling termination temperature is lower than 100 °C or the average cooling rate exceeds 60.0 °C / second, these become conditions that are difficult to achieve in ordinary manufacturing processes, and problems such as reduced productivity may occur. According to an embodiment of the present invention, the upper limit of the average cooling rate can be 58.0 °C / second.

[0125] On the other hand, when the cooling termination temperature exceeds 300 °C or the average cooling rate is less than 35.0 °C / second, during the secondary cooling, the martensite fraction of the phase transformation decreases, and as a result, the tempered martensite fraction decreases, so the yield strength may decrease.

[0126] Overaging treatment

[0127] An overaging treatment step of heating to a temperature range of 200 - 450 °C and holding for 100 - 500 seconds can be performed after the secondary cooling.

[0128] During the overaging treatment, when the overaging temperature is lower than 200 °C, during the secondary cooling, the amount of carbon (C) escaping during the overaging treatment in the martensite of the phase transformation decreases. Therefore, the degree of tempering decreases, and the yield strength of the steel plate may decrease. On the other hand, when the overaging temperature exceeds 450 °C, during the secondary cooling, the amount of carbon (C) escaping during the overaging treatment in the martensite of the phase transformation increases excessively. Therefore, the degree of tempering deviates from the appropriate range, and the tensile strength of the steel plate may decrease.

[0129] During the overaging treatment, if the holding time is too long and exceeds 500 seconds, excessive bainite transformation occurs during the holding process, and the fraction of bainite as the final structure may become high. This may lead to a decrease in the fraction of martensite, and thus the desired strength may not be effectively ensured. On the other hand, when the holding time is less than 100 seconds, since the formation of the bainite nose is avoided, the final structure may not contain bainite, which may lead to a decrease in the ductility of the steel.

[0130] In summary, the steel plate after the overaging treatment can be cooled to room temperature under normal conditions, and no special limitation is imposed on this cooling process. However, it is obvious that known cooling methods such as water cooling, oil cooling, and furnace cooling can be used instead.

[0131] Skin pass rolling

[0132] Skin pass rolling can be performed at a reduction rate of 0.1 - 1.0% after the overaging treatment.

[0133] Generally, when skin pass rolling a steel sheet, an effect of increasing the yield strength can be obtained even without increasing the tensile strength. However, when the reduction ratio of the skin pass rolling is less than 0.1%, the effect of increasing the yield strength may be negligible, and shape control may also become difficult. On the other hand, when the reduction ratio exceeds 1.0%, there is a risk that the workability may be significantly reduced due to the high elongation operation. Detailed Description of the Invention

[0134] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention for more detailed description and do not limit the scope of the rights of the present invention.

[0135] (Example)

[0136] A steel billet having the alloy composition shown in Table 1 below (the balance being Fe and inevitable impurities) was subjected to vacuum melting, reheated in the temperature range of 1200 °C, then hot-rolled at a finish rolling temperature of 880 - 920 °C which is above Ar3, and coiled at 600 °C. Thereafter, pickling was carried out to remove the surface scale, and then cold rolling was carried out at a cold reduction ratio of 50% to manufacture a cold-rolled steel sheet. Thereafter, the cold-rolled steel sheet was subjected to continuous annealing, step cooling, and overaging treatment under the conditions shown in Table 2 below.

[0137] [Table 1]

[0138]

[0139] [Relationship 1]

[0140] R = [C] + 1.3[Si] + 1.5[Mn] + 1.8[Nb]

[0141] (In the formula, [C], [Si], [Mn], and [Nb] are the weight percentages of the respective elements.)

[0142] [Table 2]

[0143]

[0144] The results of observing the microstructure of the manufactured steel sheet and measuring the physical properties are shown in Table 3 below.

[0145] In the microstructure, for tempered martensite (TM), fresh martensite (FM), ferrite (F), and bainite (B), the polished cross-section of the specimen was etched with a nital solution and observed by a scanning electron microscope (SEM), and retained austenite (R-γ) was measured by XRD analysis.

[0146] The precipitates in the fine microstructure were observed using a transmission electron microscope (TEM). At this time, the image was observed at a magnification of 30,000, and then for the precipitates having an average diameter (circular equivalent diameter) of 50 nm or less, the number of precipitates per unit area was measured and shown, and the average diameter of the precipitates was shown. The fine precipitates refer to one or more fine precipitates selected from the group consisting of Nb-based and Ti-based.

[0147] In addition, the physical property values of each test piece were measured, and the results are shown in Table 3 below. The yield strength (YS), tensile strength (TS), and elongation (El) were evaluated by a tensile test, and based on the direction of 90° with respect to the rolling direction, the test pieces collected according to the JIS No. 5 standard were used for evaluation and each mechanical physical property was measured. After that, the yield ratio (YR), the product of the yield strength and the elongation (YS×El), and the product of the tensile strength and the elongation (TS×El) were calculated and shown.

[0148] In addition, the hole expansion rate (HER) was evaluated by a hole expansion test. A punching hole with a diameter of 10 mm (the inner diameter of the die is 10.3 mm, and the gap is 12.5%) was formed, and then a conical punch with a vertex angle of 60° was inserted into the punching hole with the punching burr facing outward, and the periphery of the punching hole was compressed and expanded at a moving speed of 12 mm / min, and then calculated using the following [formula].

[0149] [Formula]

[0150] Hole expansion rate (HER, %) = {(D - D0) / D0} × 100

[0151] (In the formula, D represents the hole diameter (mm) when the crack penetrates the steel plate in the thickness direction, and D0 represents the initial hole diameter (mm).)

[0152] [Table 3]

[0153]

[0154] TM: Tempered martensite, FM: Fresh martensite, F: Ferrite, B: Bainite, R-γ: Retained austenite

[0155] [Relationship 2]

[0156] MR = TM / (FM + TM)

[0157] (In the formula, TM and FM are the area percentages of tempered martensite and fresh martensite, respectively.)

[0158] As shown in Table 3, in the case of the inventive examples that satisfy the conditions of the present invention, the fine microstructure characteristics proposed by the present invention are satisfied, and the physical properties desired in the present invention can be ensured.

[0159] Figure 1 Microstructure photograph of Invention Example 1 according to an embodiment of the present invention. As Figure 1 shown, it can be confirmed that a large amount of tempered martensite is formed, and in addition, a large amount of ferrite structure is formed.

[0160] On the other hand, Comparative Example 1 is a case where the continuous annealing temperature exceeds the temperature range of the present invention. During continuous annealing and cooling, the formation of ferrite as a soft phase is insufficient, and as a final microstructure, an excessive amount of tempered martensite is formed. As a result, the desired elongation cannot be ensured.

[0161] Comparative Example 2 is a case where the termination temperature of the first cooling does not reach the temperature range of the present invention. Since the average cooling rate exceeds the range proposed in the present invention, it is difficult to ensure a stable cooling capacity. Therefore, the fraction of bainite increases during cooling, and then the formation of tempered martensite is insufficient, and the desired strength cannot be ensured.

[0162] Comparative Example 3 is a case where the termination temperature of the second cooling does not reach the temperature range of the present invention. The average cooling rate is higher than the range proposed in the present invention. As a result, most of the structure transforms into martensite during cooling, and the martensite is tempered during the overaging treatment, so the desired tensile strength cannot be ensured.

[0163] In Comparative Example 4, the termination temperature of the second cooling is the same as that of the invention example, corresponding to a range below the martensite transformation start temperature (Ms). However, as this temperature approaches the martensite transformation start temperature (Ms), the fraction of martensite transformed during the second cooling decreases. As a result, the fraction of tempered martensite in the generated martensite decreases, while the fraction of fresh martensite increases. As a result, the desired yield strength cannot be obtained. Figure 2 Microstructure photograph of Comparative Example 4 deviating from an embodiment of the present invention. As Figure 2 shown, it can be confirmed that a large amount of fresh martensite is formed, and the formation of tempered martensite is insufficient.

[0164] In Comparative Example 5, the termination temperature of the second cooling corresponds to a range above the martensite transformation start temperature (Ms). At this time, only the fresh martensite transformation is induced without tempered martensite, so it exhibits a low yield ratio and thus low hole expansion property.

[0165] Comparative Example 6 is a case where the overaging treatment temperature does not reach the temperature range of the present invention. During the second cooling, the amount of carbon (C) escaping during the overaging treatment in the transformed martensite decreases. Therefore, the degree of tempering decreases, and the desired yield strength cannot be ensured.

[0166] In Comparative Example 7, due to the excessive addition of C in the steel, the formed martensite phase becomes relatively harder, and the hardness difference from the surrounding phase increases. Therefore, the ductility and hole expansion property are poor.

[0167] Comparative Example 8 and Comparative Example 9 are cases where the value of relational expression 1 does not satisfy the scope of the present invention. Specifically, in Comparative Example 8, Si was not added, and thus the solution strengthening effect brought about by the addition of Si was not obtained. Therefore, the value of relational expression 1 was 4.1. On the other hand, in Comparative Example 9, the Si content in the steel was too high, and it was found that the value of relational expression 1 was 6.3 and exceeded 6.0. That is, it was found that Comparative Example 8 and Comparative Example 9 could not ensure the desired ductility and hole expansion property in the present invention.

[0168] Comparative Example 10 and Comparative Example 11 are examples where the Mn content in the steel deviated from the scope of the present invention. In Comparative Example 10, the Mn content in the steel was insufficient, and the desired strength and hole expansion property could not be ensured. In particular, in Comparative Example 11, the value of relational expression 1 was 4.5. Therefore, Comparative Example 10 can be understood as being unable to ensure the desired strength in the present invention due to the reduced hardenability of the steel. Comparative Example 11 is a case where the Mn content in the steel is too high. In particular, it was found that the value of relational expression 1 was 6.4 and exceeded 6.0. That is, in Comparative Example 11, due to the excessive increase in the hardenability of the steel, the desired ductility and hole expansion property in the present invention could not be ensured.

[0169] Comparative Example 12 and Comparative Example 13 are examples where the Nb content deviated from the scope of the present invention. In Comparative Example 12, Nb was not added. This was because the target level of precipitates could not be ensured in ferrite, resulting in a poor interphase hardness difference, and thus the target hole expansion property could not be ensured. Comparative Example 13 is a case where the Nb content in the steel is too high. Due to the excessive formation of precipitates in ferrite, the ductility is reduced, and thus the desired ductility in the present invention cannot be ensured.

[0170] As described above, the present invention has been described in detail by way of examples, but it may also be implemented in other forms of examples. Therefore, the technical idea and scope of the claims are not limited to the examples.

Claims

1. A steel plate, by weight %, the steel plate comprises: carbon (C): 0.05 - 0.20%, manganese (Mn): 2.3 - 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, the balance being iron (Fe) and inevitable impurities, the R value defined in the following relational expression 1 is 5.0 to 6.0, by area %, the microstructure comprises 40% or more of tempered martensite, and the balance of the microstructure comprises ferrite, fresh martensite, bainite and retained austenite, the MR defined in the following relational expression 2 is 0.60 to 0.80, [Relational expression 1] R = [C] + 1.3[Si] + 1.5[Mn] + 1.8[Nb] In the formula, [C], [Si], [Mn] and [Nb] are the weight % of each element, [Relational expression 2] MR = TM / (FM + TM) In the formula, TM and FM are the area % of tempered martensite and fresh martensite respectively.

2. The steel plate according to claim 1, wherein, The microstructure comprises 30 - 50% of ferrite and 5 - 25% of fresh martensite.

3. The steel plate according to claim 1, wherein, The steel plate contains one or more of Nb-based fine precipitates and Ti-based fine precipitates having an average diameter, i.e., a circular equivalent diameter, of 50 nm or less, at 10 2 per unit area (m 12 ²).

4. The steel plate according to claim 1, wherein, The yield strength of the steel plate is 700 MPa or more, the tensile strength is 1000 MPa or more, the yield ratio is 0.70 or more, and the elongation is 13% or more.

5. The steel plate according to claim 1, wherein, The product of the yield strength and the elongation of the steel plate is 9100 MPa·% or more, the product of the tensile strength and the elongation is 13000 MPa·% or more, and the hole expansion rate (HER) is 50% or more.

6. A method for manufacturing a steel plate, which comprises the following steps: Reheating a steel billet, by weight %, the steel billet comprises: carbon (C): 0.05 - 0.20%, manganese (Mn): 2.3 - 3.0%, silicon (Si): 1.5% or less, aluminum (Al): 0.10% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, niobium (Nb): 0.10% or less, titanium (Ti): 0.10% or less, boron (B): 0.003% or less, the balance being iron (Fe) and inevitable impurities, and the R value defined in the following relational expression 1 is 5.0 to 6.0; Hot rolling the reheated steel billet; Cooling and coiling the hot-rolled steel plate; Cold rolling the coiled steel plate; Heating the cold-rolled steel plate to a temperature range of 760 - 830 °C and performing continuous annealing for 30 - 300 seconds; Performing primary cooling on the continuously annealed steel plate at an average cooling rate of 2.0 - 6.0 °C / second, cooling to a temperature range of 500 - 700 °C; Performing secondary cooling at an average cooling rate of 35.0 - 60.0 °C / second after the primary cooling, cooling to a temperature range of 100 - 300 °C; and Performing overaging treatment by heating to a temperature range of 200 - 450 °C and holding for 100 - 500 seconds after the secondary cooling, [Relationship 1] R = [C] + 1.3[Si] + 1.5[Mn] + 1.8[Nb] In the formula, [C], [Si], [Mn] and [Nb] are the weight percentages of the respective elements.

7. The manufacturing method of the steel plate according to claim 6, wherein, The reheating step is carried out in the temperature range of 1000 - 1350 °C, the hot rolling step is carried out at a finish rolling temperature of Ar3 to Ar3 + 50 °C, the coiling step is carried out in the temperature range of 450 - 700 °C, and the cold rolling step is carried out at a reduction rate of 35 - 65%.

8. The manufacturing method of the steel plate according to claim 6, wherein, The manufacturing method further includes a step of pickling the steel sheet after the coiling step.

9. The manufacturing method of the steel plate according to claim 6, wherein, The manufacturing method further includes a step of skin pass rolling at a reduction rate of 0.1 - 1.0% after the overaging treatment step.

Images