Steel sheet and manufacturing method therefor

Abstract

An aspect of the present invention provides a steel sheet. The steel sheet comprises, in weight%, C: 0.25–0.55%, Si: 3.0% or less, Mn: 0.9–3.4%, Al: 3.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0015–0.005%, and the balance being Fe and inevitable impurities, wherein the microstructure includes tempered martensite, bainite, and retained austenite, and the value of W may be 0.08 or more as defined by Relational Expression 1.

Description

Steel plate and method for manufacturing the same

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

[0002] The automotive industry has recently been focusing on lightweight materials to protect the environment and ways to ensure passenger safety. To meet these demands for safety and weight reduction, demand for high-strength steel plates is increasing.

[0003] In general, as the strength of steel plates increases, their ductility and workability decrease. Therefore, steel plates for automobile components are required to have not only high strength but also excellent ductility and workability.

[0004] As a technology for improving the ductility of steel plates, methods utilizing tempered martensite are disclosed in Patent Documents 1 and 2. Tempered martensite, created by tempering hard martensite, is a softened form of martensite and exhibits a difference in strength from conventional untempered martensite (fresh martensite). Suppressing fresh martensite and forming tempered martensite increases ductility and workability.

[0005] However, the technology disclosed in patent documents 1 and 2 does not satisfy the product of tensile strength and elongation (TSΥEl) of 22,000 MPa% or more, which means that it is difficult to secure a steel plate having both excellent strength and ductility.

[0006] Meanwhile, TRIP (Transformation Induced Plasticity) steel, which utilizes the transformation-induced plasticity of retained austenite, has been developed to achieve high strength, ductility, and excellent workability for automotive steel sheets. Patent Documents 3 and 4 disclose TRIP steel with excellent ductility and workability.

[0007] Patent Document 3 discloses a technology that improves ductility and workability by including polygonal ferrite, retained austenite, and martensite. However, Patent Document 3 uses bainite as the main phase, failing to secure high strength, and the product of tensile strength and elongation (TSYEl) does not exceed 22,000 MPa%.

[0008] Patent Document 4 discloses a technique for promoting the formation of ferrite and refinement of retained austenite, thereby forming a composite structure including tempered martensite, thereby improving ductility and workability. However, Patent Document 4 contains a large amount of soft ferrite, making it difficult to achieve high strength.

[0009] Accordingly, the demand for steel plates with high strength and excellent ductility is not being met.

[0010] (Patent Document 1) Korean Patent Publication No. 10-2006-0118602

[0011] (Patent Document 2) Japanese Patent Publication No. 2009-019258

[0012] (Patent Document 3) Korean Patent Publication No. 10-2014-0012167

[0013] (Patent Document 4) Korean Patent Publication No. 10-2010-0092503

[0014] One aspect of the present invention is to provide a steel plate having excellent elongation and hole expandability and a method for manufacturing the same.

[0015] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0016] One aspect of the present invention provides a steel sheet. The steel sheet contains, in wt%, C: 0.25 to 0.55%, Si: 3.0% or less (excluding 0%), Mn: 0.9 to 3.4%, Al: 3.0% or less (excluding 0%), P: 0.15% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), B: 0.0015 to 0.005%, the remainder being Fe and unavoidable impurities, and the microstructure includes tempered martensite, bainite and retained austenite, and the value of W defined by the following equation 1 may be 0.08 or more.

[0017] [Relationship 1]

[0018] W = TTS Х 100 Х B

[0019] (In relational expression 1, TTS represents a value calculated based on relational expression 2 below after measuring the thickness before fracture and the thickness after fracture of a specimen tested using the tensile test JIS 5 standard test method, and B represents the content (weight%) of boron contained in the steel plate.)

[0020] [Relationship 2]

[0021] TTS = ln(t0 / t1)

[0022] (In equation 2, t0 represents the thickness before fracture of the specimen tested using the JIS 5 standard tensile test method, and t1 represents the thickness after fracture of the specimen.)

[0023] Additionally, in the steel plate described above, the sum of the contents of Si and Al may be 1.0 to 6.0%.

[0024] In addition, in one of the aforementioned steel plates, the steel plate may further include one or more selected from the following (1) to (3).

[0025] (1) At least one of Ti: 0~0.5%, Nb: 0~0.5%, and V: 0~0.5%

[0026] (2) Cr: 0~3.0% and Mo: 0~3.0%, at least one of these

[0027] (3) Cu: 0~4.5% and Ni: 0~4.5%, at least one of these

[0028] In addition, in one of the aforementioned steel plates, the area fraction of the tempered martensite among the total area fraction of the microstructure may be 30 to 85%, the area fraction of the bainite may be 10 to 50%, and the area fraction of the retained austenite may be 10 to 40%.

[0029] Additionally, in one of the aforementioned steel plates, the microstructure may further include ferrite, and the area fraction of the ferrite among the total area fraction of the microstructure may be 5% or less.

[0030] In addition, in one of the aforementioned steel plates, the steel plate may have a product of tensile strength and elongation (TSХEl) of 22,000 MPa% or more and a value of HER of 20% or more.

[0031] Another aspect of the present invention provides a method for manufacturing a steel plate. The method comprises the steps of: reheating a steel slab containing, in wt%, C: 0.25 to 0.55%, Si: 3.0% or less (excluding 0%), Mn: 0.9 to 3.4%, Al: 3.0% or less (excluding 0%), P: 0.15% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), B: 0.0015 to 0.005%, the remainder being Fe and unavoidable impurities; subjecting the reheated steel slab to a finish hot rolling; coiling the finish hot rolled steel plate; subjecting the coiled steel plate to a hot rolling annealing heat treatment at a temperature range of 500 to 850°C for 600 to 85,000 seconds; The method may include a step of cold rolling the hot-rolled and annealed steel sheet; a step of first heating the cold-rolled steel sheet to a first heating temperature of Ac1 or higher and first maintaining the temperature for 120 seconds or longer; a step of first cooling the first-heated steel sheet to a first cooling temperature of 100 to 300°C at an average cooling rate of 20°C / s or higher; a step of second heating the first-cooled steel sheet to a second heating temperature of 300 to 500°C and second maintaining the temperature for 300 to 90,000 seconds; and a step of second cooling the second-heated steel sheet to room temperature.

[0032] Additionally, in the method described above, in the reheating step, the steel slab may have a sum of the contents of Si and Al of 1.0 to 6.0%.

[0033] In addition, in one of the above-described methods, in the reheating step, the steel slab may further include one or more selected from the following (1) to (3).

[0034] (1) At least one of Ti: 0~0.5%, Nb: 0~0.5%, and V: 0~0.5%

[0035] (2) Cr: 0~3.0% and Mo: 0~3.0%, at least one of these

[0036] (3) Cu: 0~4.5% and Ni: 0~4.5%, at least one of these

[0037] Additionally, in one of the aforementioned methods, the reheating step may be performed at 1000 to 1350°C, and the finishing hot rolling step may be performed at 800 to 1000°C.

[0038] In addition, in one of the above-described methods, in the coiling step, the finished hot-rolled steel sheet may be cooled to 300 to 700°C at an average cooling rate of 10°C / s or more and then coiled.

[0039] Additionally, in one of the aforementioned methods, the cold rolling step may be performed at a reduction ratio of 20 to 90%.

[0040] In addition, in one of the above-described methods, in the first maintaining step, the cold-rolled steel sheet can be heated to the first heating temperature at an average heating rate of 10°C / s or more.

[0041] In addition, in one of the aforementioned methods, in the second maintaining step, the first-cooled steel plate can be heated to the second heating temperature at an average heating rate of 15°C / s or more.

[0042] In addition, in one of the aforementioned methods, in the secondary cooling step, the secondary heated steel plate can be cooled to room temperature at an average cooling rate of 1°C / s or more.

[0043] According to the present invention, a high-strength steel plate having excellent elongation and hole expandability and a method for manufacturing the same can be provided.

[0044] In addition, according to the present invention, since excellent strength, ductility and processing characteristics can be secured, it can be suitably used as a steel sheet for automobile structures that require both weight reduction and stability.

[0045] The various advantageous and beneficial advantages and effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0046] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified in various other forms, and the terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include plural forms, unless the relevant definition clearly indicates a contrary meaning.

[0047] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0048] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0049] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0050] The inventors of the present invention have discovered that in transformation-induced plasticity (TRIP) steels containing bainite, tempered martensite, and retained austenite, the strength, ductility, and workability can be controlled by stabilizing the retained austenite and controlling the content, size, etc. of the retained austenite. Accordingly, the inventors of the present invention have experimentally confirmed a method for improving the ductility and workability of high-strength steel sheets, and have completed the present invention.

[0051] First, the alloy composition of the steel plate according to one embodiment of the present invention will be described. The content of the alloy composition refers to weight percent unless otherwise specified.

[0052] Carbon (C): 0.25 to 0.55%

[0053] Carbon (C) is an element that provides strength to steel plates and can stabilize retained austenite, which increases the ductility of the steel plates. If the C content is less than 0.25%, it may be difficult to secure the desired tensile strength. Furthermore, if the C content exceeds 0.55%, cold rolling may be difficult during the production of the steel plates. That is, the C content may be 0.25 to 0.55%, more specifically 0.28 to 0.45%, and even more specifically 0.30 to 0.40%.

[0054] Silicon (Si): 3.0% or less (excluding 0%)

[0055] Silicon (Si) is an element that has the effect of improving strength through solid solution strengthening, and can strengthen ferrite, homogenize the structure, and improve workability. In addition, silicon (Si) can contribute to the formation of retained austenite by suppressing cementite precipitation. If the Si content exceeds 3.0%, problems such as plating defects such as underplating may occur during the plating process when manufacturing a steel sheet, or the weldability of the steel sheet may deteriorate. That is, the Si content may be 3.0% or less, more specifically, 2.8% or less, and even more specifically, 2.5% or less. There is no need to specifically limit the lower limit of the Si content, but as a non-limiting example, the Si content may be 0.5% or more, 0.7% or more, or 1.0% or more.

[0056] Manganese (Mn): 0.9 to 3.5%

[0057] Manganese (Mn) can improve both strength and ductility. However, if the Mn content is less than 0.9%, it may be difficult to fully secure the aforementioned effects. Furthermore, if the Mn content exceeds 3.5%, the bainite transformation time increases, so the carbon enrichment in austenite may not be sufficient, making it difficult to secure a sufficient fraction of retained austenite. That is, the Mn content may be 0.9 to 3.5%, more specifically 1.0 to 3.3%, and even more specifically 1.5 to 3.0%.

[0058] Aluminum (Al): 3.0% or less (excluding 0%)

[0059] Aluminum (Al) can combine with oxygen in steel to perform a deoxidizing effect. In addition, aluminum (Al) can stabilize residual austenite by suppressing cementite precipitation. If the Al content exceeds 3.0%, the workability of the steel sheet may deteriorate and inclusions may increase. There is no need to specifically limit the lower limit of the Al content, but as a non-limiting example, the Al content may be 0.015% or more, 0.020% or more, or 0.050% or more.

[0060] Phosphorus (P): 0.15% or less (excluding 0%)

[0061] Phosphorus (P) may be contained as an impurity and deteriorate impact toughness. That is, the P content may be 0.15% or less, more specifically 0.10% or less, and even more specifically 0.08% or less. There is no need to specifically limit the lower limit of the P content, but as a non-limiting example, the P content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0062] Sulfur (S): 0.03% or less (excluding 0%)

[0063] Sulfur (S) is contained as an impurity and can form MnS in the steel sheet and deteriorate ductility. That is, the S content may be 0.03% or less, more specifically 0.01% or less, and even more specifically 0.008% or less. There is no need to specifically limit the lower limit of the S content, but as a non-limiting example, the S content may be 0.001% or more, 0.002% or more, or 0.003% or more.

[0064] Nitrogen (N): 0.03% or less (excluding 0%)

[0065] Nitrogen (N) is contained as an impurity and can cause cracks in the slab by forming nitrides during continuous casting. That is, the N content may be 0.03% or less, more specifically 0.01% or less, and even more specifically 0.008% or less. There is no need to specifically limit the lower limit of the N content, but as a non-limiting example, the N content may be 0.001% or more, 0.002% or more, or 0.003% or more.

[0066] Boron (B): 0.0015 to 0.005%

[0067] Boron (B) not only strengthens grain boundaries to provide resistance to LME and hydrogen embrittlement cracking, but also suppresses ferrite transformation during cooling after annealing. In addition, boron (B) can improve the HER (Hole Expansion Ratio) value through resistance to grain boundary destruction. If the B content is less than 0.0015%, it may be difficult to sufficiently secure the above-described effect. In addition, if the B content exceeds 0.005%, hot rollability may deteriorate, and excessive B may accumulate on the surface, which may hinder plating properties. That is, the B content may be 0.0015 to 0.005%, specifically 0.0015 to 0.0050%, more specifically 0.0016 to 0.0040%, and even more specifically 0.0018 to 0.0035%.

[0068] A steel sheet for hot forming according to one embodiment of the present invention may include the above components, as well as iron (Fe) as a remaining component. Furthermore, during typical manufacturing processes, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus, this cannot be excluded. Since these impurities are readily apparent to anyone skilled in typical manufacturing processes, their full content is not specifically discussed herein.

[0069] For example, the steel plate may have a combined Si and Al content of 1.0 to 6.0%. Si and Al are components that influence microstructure formation, thereby affecting ductility and bending workability. When the combined Si and Al content is within the above range, excellent ductility and bending workability can be secured.

[0070] If the sum of the contents of Si and Al is less than 1.0%, it may be difficult to sufficiently secure the above-described effects. In addition, if the sum of the contents of Si and Al exceeds 6.0%, excessive oxide formation may occur during hot rolling, resulting in poor operability and deterioration in playability. That is, the sum of the contents of Si and Al may be 1.0 to 6.0%, more specifically 1.2 to 5.0%, and even more specifically 1.5 to 4.0%.

[0071] For example, the steel plate may further include one or more selected from (1) to (3).

[0072] (1) At least one of titanium (Ti): 0~0.5%, niobium (Nb): 0~0.5%, and vanadium (V): 0~0.5%

[0073] Titanium (Ti), niobium (Nb), and / or vanadium (V) can form precipitates and refine grains. In addition, titanium (Ti), niobium (Nb), and / or vanadium (V) can improve the strength and impact toughness of the steel sheet. If the content of Ti, Nb, or V exceeds 0.5%, excessive precipitates are formed, which not only reduces the impact toughness but also may cause an increase in manufacturing cost. That is, the content of Ti, Nb, or V may be 0.5% or less, more specifically 0.4% or less, and even more specifically 0.3% or less.

[0074] (2) At least one of chromium (Cr): 0~3.0% and molybdenum (Mo): 0~3.0%

[0075] Chromium (Cr) and / or molybdenum (Mo) can suppress austenite decomposition and stabilize austenite during the alloying process in the production of steel plates. When the content of Cr or Mo exceeds 3.0%, the bainite transformation time increases, so that the carbon enrichment in austenite may not be sufficient, making it difficult to secure a sufficient fraction of retained austenite. That is, the content of Cr or Mo may be 3.0% or less, more specifically 2.5% or less, and even more specifically 2.0% or less.

[0076] (3) Copper (Cu): 0~4.5% and nickel (Ni): 0~4.5%, at least one of these

[0077] Copper (Cu) and / or nickel (Ni) can stabilize austenite and inhibit corrosion. In addition, copper (Cu) and / or nickel (Ni) can be concentrated on the surface of the steel plate and prevent the intrusion of hydrogen moving into the steel plate, thereby inhibiting hydrogen-delayed fracture. When the content of Cu or Ni exceeds 4.5%, not only the above-mentioned characteristic effects may be excessive, but it may also cause an increase in manufacturing costs. That is, the content of Cu or Ni may be 4.5% or less, more specifically 3.0% or less, and even more specifically 2.0% or less.

[0078] Hereinafter, the microstructure of a steel sheet according to one embodiment of the present invention will be described. The microstructure fraction refers to area % unless otherwise specified. In addition, the microstructure fraction can be measured by analyzing the matrix structure at a point 1 / 4 the thickness of the steel sheet, and specifically, the microstructure fraction can be measured using FE-SEM, an image analyzer, and XRD.

[0079] The microstructure of the steel plate may include tempered martensite, bainite, and retained austenite.

[0080] Tempered martensite can improve the strength of steel sheets. Specifically, both untempered martensite (fresh martensite) and tempered martensite can improve the strength of steel sheets. Among these, because tempered martensite is not softened by tempering heat treatment, fresh martensite can reduce the ductility of steel sheets compared to tempered martensite. In other words, utilizing tempered martensite can provide steel sheets with a better balance of strength and ductility and superior workability.

[0081] For example, the area fraction of tempered martensite among the total area fraction of the microstructure may be 30 to 85%. If the area fraction of tempered martensite is less than 30%, it may be difficult to secure a product of tensile strength and elongation (TSХEl) of 22,000 MPa% or more. In addition, if the area fraction of tempered martensite exceeds 85%, ductility and workability may deteriorate, and the product of tensile strength and elongation (TSХEl) may be less than 22,000 MPa% or the HER value may be less than 20%. That is, the area fraction of tempered martensite may be 30 to 85%, more specifically, 35 to 80%, and even more specifically, 40 to 75%.

[0082] Bainite can improve the balance of strength and ductility of steel sheets and enhance the workability of the steel sheets. For example, the area fraction of bainite among the total area fraction of the microstructure can be 10 to 50%. When the area fraction of bainite is less than 10%, the product of tensile strength and elongation (TSХEl) can be less than 22,000 MPa% or the HER value can be less than 20%. In addition, when the area fraction of bainite exceeds 50%, it can be difficult to sufficiently secure the product of tensile strength and elongation (TSХEl) because the area fraction of tempered martensite is relatively reduced. That is, the area fraction of bainite can be 10 to 50%, more specifically 15 to 45%, and even more specifically 20 to 40%.

[0083] Retained austenite can induce transformation-induced plasticity during the transformation of austenite to martensite during the processing of steel plates, thereby improving ductility and workability. For example, the area fraction of retained austenite among the total area fraction of the microstructure can be 10 to 40%. When the area fraction of retained austenite is within the above range, the balance between the strength and ductility of the steel plate is improved, and the workability of the steel plate can also be improved.

[0084] When the area fraction of retained austenite is less than 10%, the product of tensile strength and elongation (TSХEl) may be less than 22,000 MPa% or the HER value may be less than 20%. In addition, when the area fraction of retained austenite exceeds 40%, the local elongation may decrease. That is, the area fraction of retained austenite may be 10 to 40%, more specifically 12 to 35%, and even more specifically 15 to 30%.

[0085] For example, the microstructure of a steel plate may contain additional ferrite. For example, the area fraction of ferrite may be 5% or less of the total area fraction of the microstructure. If the area fraction of ferrite exceeds 5%, a soft phase is introduced, increasing the interphase hardness difference, making it difficult to ensure sufficient hole expandability.

[0086] For example, the microstructure of the steel sheet may further include an inevitable structure. In one embodiment of the present invention, the inevitable structure may include fresh martensite, pearlite, martensite austenite constituent (MA), etc. For example, the area fraction of the inevitable structure among the total area fraction of the microstructure may be 20% or less. If the area fraction of the inevitable structure exceeds 20%, the ductility and workability of the steel sheet may be deteriorated, or the area fraction of retained austenite may be reduced.

[0087] A steel sheet according to one embodiment of the present invention can stabilize retained austenite, thereby ensuring not only high strength but also excellent ductility and workability. Typically, retained austenite can be stabilized by enriching C and Mn in austenite within the ferrite, bainite, and tempered martensite of the steel sheet. However, utilizing ferrite to enrich C within austenite can result in a reduction in the strength of the steel sheet due to the low strength properties of ferrite.

[0088] In one embodiment of the present invention, the above-described problem can be improved by stabilizing retained austenite by enriching C and Mn in austenite in bainite and tempered martensite of the steel plate. If the enrichment of C and Mn in the retained austenite is insufficient, the retained austenite becomes unstable in tensile deformation, resulting in a decrease in ductility and workability, and the product of tensile strength and elongation (TSХEl) may be less than 22,000 MPa% or the HER value may be less than 20%.

[0089] According to one embodiment of the present invention, a steel plate may have a product of tensile strength and elongation (TSХEl) of 22,000 MPa% or more. Additionally, the steel plate may have a HER value of 20% or more. Here, the HER value can be calculated by punching a hole with an initial diameter of 10 mm at the center of a 120 mm Х 120 mm rolled plate, pushing it up in a conical shape, and measuring the maximum diameter at which no crack is observed with the naked eye, using the formula [(maximum diameter - initial diameter) / initial diameter Х 100]. Since the product of tensile strength and elongation (TSХEl) and the HER of the steel plate satisfy the above ranges, a steel plate having excellent balance of strength and ductility can be provided.

[0090] According to one embodiment of the present invention, the steel plate may have a value of W defined by the following relational expression 1 of 0.08 or more.

[0091] [Relationship 1]

[0092] W = TTS Х 100 Х B

[0093] In the above relational expression 1, TTS represents a value calculated based on the following relational expression 2 after measuring the thickness before fracture and the thickness after fracture of a specimen tested using the tensile test JIS 5 standard test method. In addition, in relational expression 1, B represents the content (weight%) of boron contained in the steel plate.

[0094] [Relationship 2]

[0095] TTS = ln(t0 / t1)

[0096] In the above relational expression 2, t0 represents the thickness before fracture of the specimen tested using the tensile test method of JIS 5 standard, and t1 represents the thickness after fracture of the specimen.

[0097] The value of W defined by the above relational expression 1 is a quantitative expression that indicates that the strength and B content of the steel plate can contribute to the physical properties of the steel plate (e.g., ductility, hole expandability, and workability, etc.). When the value of W is less than 0.08, the strength and / or B content of the steel plate are low, so that it may be difficult to sufficiently secure hole expandability due to insufficient resistance to intergranular fracture. That is, the value of W may be 0.08 or more, specifically 0.080 or more, more specifically 0.085 or more, and even more specifically 0.090 or more.

[0098] Hereinafter, a method for manufacturing a steel plate according to an embodiment of the present invention will be described in detail. However, this does not necessarily mean that the steel plate according to an embodiment of the present invention must be manufactured using the manufacturing method described below.

[0099] [River slab reheating stage]

[0100] A steel slab having the aforementioned composition can be reheated. At this time, the composition of the steel slab is identical to that of the steel plate described above, and the explanation for the steel plate described above can be applied equally to the reasons for adding each component and limiting the content of the component in the slab.

[0101] For example, the reheating step may be performed at 1000 to 1350°C. If the reheating temperature is lower than 1000°C, a problem may arise in which hot rolling is performed below the finishing hot rolling temperature range. If the reheating temperature exceeds 1350°C, a problem may arise in which the steel reaches its melting point and melts. That is, the reheating temperature may be 1000 to 1350°C, more specifically 1050 to 1300°C, and even more specifically 1100 to 1280°C.

[0102] [Finishing hot rolling stage]

[0103] The reheated steel slab can be finished hot rolled. For example, the finishing hot rolling step can be performed at 800 to 1000°C. If the finishing hot rolling temperature is lower than 800°C, the high strength of the steel may cause a load on the rolling mill. In addition, if the finishing hot rolling temperature exceeds 1000°C, the grain size of the steel sheet after hot rolling may become coarse, which may deteriorate the physical properties of the steel sheet. That is, the finishing hot rolling temperature may be 800 to 1000°C, more specifically 830 to 980°C, and even more specifically 850 to 960°C.

[0104] [Cooling and winding stage]

[0105] The hot-rolled steel sheet can be coiled. For example, the coiling step can be performed at a temperature of 300 to 700°C. If the coiling temperature is lower than 300°C, the coiling process may not be easily performed. In addition, if the coiling temperature exceeds 700°C, scale formed on the surface of the hot-rolled steel sheet may form inside the steel sheet, which may cause a problem in that the pickling process becomes difficult. That is, the coiling temperature may be 300 to 700°C, more specifically 350 to 680°C, and even more specifically 400 to 650°C.

[0106] For example, the coiling step may involve cooling the finished hot-rolled steel sheet to the coiling temperature range at an average cooling rate of 10°C / s or higher, and then coiling the steel sheet. If the average cooling rate is less than 10°C / s, there may be problems such as reduced productivity of hot rolling or the need to use a cooling medium with poor cooling capacity during production.

[0107] [Hot rolling annealing heat treatment stage]

[0108] The coiled steel sheet can be subjected to hot rolling annealing heat treatment. Through the hot rolling annealing heat treatment step, pickling and cold rolling after coiling can be easily performed. For example, the hot rolling annealing heat treatment temperature can be 500 to 850°C. If the hot rolling annealing heat treatment temperature is lower than 500°C, the strength of the hot rolling annealed steel sheet may increase excessively, making cold rolling difficult. In addition, if the hot rolling annealing heat treatment temperature exceeds 850°C, scale may be formed deep inside the steel sheet, which may cause a problem of difficulty in the pickling process. That is, the hot rolling annealing heat treatment temperature can be 500 to 850°C, more specifically 530 to 800°C, and even more specifically 550 to 750°C.

[0109] For example, the hot-rolling annealing heat treatment step may be performed for 600 to 85,000 seconds. If the hot-rolling annealing heat treatment time is less than 600 seconds, the strength of the hot-rolling annealing heat treatment steel sheet may increase excessively, making cold rolling difficult. In addition, if the hot-rolling annealing heat treatment time exceeds 85,000 seconds, scale may be formed deep inside the steel sheet, making the pickling process difficult. That is, the hot-rolling annealing heat treatment time may be 600 to 85,000 seconds, more specifically, 3,000 to 85,000 seconds, and even more specifically, 6,000 to 85,000 seconds.

[0110] [Cold rolling stage]

[0111] A hot-rolled and annealed steel sheet can be cold rolled. For example, the cold rolling step can be performed at a reduction ratio of 20 to 90%. If the reduction ratio is less than 20%, it is difficult to secure the target thickness, and shape correction of the steel sheet can be difficult. Furthermore, if the reduction ratio exceeds 90%, the strength of the steel sheet becomes excessively high, making it difficult to perform cold rolling in a short period of time. That is, the reduction ratio can be 20 to 90%, more specifically 25 to 80%, and even more specifically 30 to 70%.

[0112] For example, a pickling process may be further included prior to the cold rolling process, whereby the hot-rolled and annealed steel sheet undergoes pickling. Pickling can remove scale formed on the surface of the steel sheet. The pickling process is not particularly limited, and any method commonly used in the art (e.g., immersing the steel sheet in a hydrochloric acid bath) can be applied.

[0113] A method for manufacturing a steel sheet according to one embodiment of the present invention can secure excellent strength, ductility, and workability by performing a continuous annealing process after a cold rolling step. The continuous annealing process is described in detail below.

[0114] [1st heating and maintenance stage]

[0115] The cold-rolled steel sheet can be first heated to a first heating temperature of Ac1 or higher, and then first held at the first heating temperature. If the first heating temperature (or first holding temperature) is lower than Ac1, bainite, retained austenite, and tempered martensite are not sufficiently formed, and excessive ferrite is formed, making it difficult to secure the strength, ductility, hole expandability, and workability of the steel sheet. That is, the first heating temperature (or first holding temperature) may be Ac1 or higher, more specifically, Ac1+10℃ or higher, and even more specifically, Ac1+20℃ or higher. In addition, the upper limit of the first heating temperature (or first holding temperature) is not particularly limited, but in order to minimize the decrease in toughness due to grain coarsening, the first heating temperature (or first holding temperature) may be 950℃ or lower.

[0116] For example, a cold-rolled steel sheet may be first heated at an average heating rate of 10°C / s or more to the first heating temperature. If the average heating rate is less than 10°C / s, a problem of reduced productivity may occur. The upper limit of the average heating rate is not particularly limited, but the average heating rate may be 500°C / s or less.

[0117] For example, the cold-rolled steel sheet may be held at the first heating temperature for 120 seconds or longer. If the first holding time is less than 120 seconds, the structure may not be sufficiently homogenized, which may result in deterioration of the physical properties of the steel sheet. That is, the first holding time may be 120 seconds or longer, more specifically, 150 seconds or longer, and even more specifically, 180 seconds or longer. The upper limit of the first holding time is not particularly limited, but in order to minimize the decrease in toughness due to grain coarsening, the first holding time may be 1200 seconds or shorter.

[0118] [1st cooling stage]

[0119] The steel sheet that has been first heated and maintained can be first cooled to a first cooling temperature of 100 to 300°C. If the first cooling temperature is less than 100°C, tempered martensite may be formed excessively and retained austenite may be insufficient, so that the product of tensile strength and elongation (TSХEl) and the HER of the steel sheet may decrease. In addition, if the first cooling temperature exceeds 300°C, bainite may be formed excessively and tempered martensite may be insufficient, so that the product of tensile strength and elongation (TSХEl) of the steel sheet may decrease. That is, the first cooling temperature may be 100 to 300°C, more specifically, 110 to 290°C, and even more specifically, 120 to 280°C.

[0120] For example, a steel sheet that has been first heated and maintained at an average cooling rate of 20°C / s or more to the first cooling temperature can be first cooled. If the average cooling rate is less than 20°C / s, productivity may decline. The upper limit of the average cooling rate is not particularly limited, but may be 100°C / s or less.

[0121] [Secondary heating and maintenance stage]

[0122] The steel sheet that has been first cooled can be secondarily heated to a second heating temperature of 300 to 500°C and then secondarily maintained at the second heating temperature. If the second heating temperature (or second holding temperature) is less than 300°C, tempered martensite may be excessively formed and the control of Si and Al contents in the retained austenite may be insufficient, making it difficult to secure the area fraction of retained austenite, and thus the product of tensile strength and elongation (TSХEl) and HER of the steel sheet may decrease. In addition, if the second heating temperature (or second holding temperature) exceeds 500°C, the control of Si content in the retained austenite may be insufficient, making it difficult to secure the area fraction of retained austenite. That is, the secondary heating temperature (or secondary holding temperature) may be 300 to 500°C, more specifically 310 to 480°C, and even more specifically 320 to 460°C.

[0123] For example, a steel sheet that has been first cooled can be secondarily heated at an average heating rate of 15°C / s or more to the secondary heating temperature. If the average heating rate is less than 15°C / s, a problem of reduced productivity may occur. The upper limit of the average heating rate is not particularly limited, but the average heating rate may be 100°C / s or less.

[0124] For example, the first-cooled steel plate may be secondarily maintained at the second heating temperature for 300 seconds or longer. If the second holding time is less than 300 seconds, tempered martensite may be excessively formed and the control of Si and Al contents in the retained austenite may be insufficient, making it difficult to secure the area fraction of retained austenite. Accordingly, the product of tensile strength and elongation (TSХEl) and the HER of the steel plate may decrease. That is, the second holding time may be 300 seconds or longer, more specifically, 350 seconds or longer, and even more specifically, 400 seconds or longer. In addition, if the second holding time is excessively long, the control of Si content in the retained austenite may be insufficient, making it difficult to secure the area fraction of retained austenite. As a non-limiting example, the second holding time may be 90,000 seconds or less.

[0125] [Second cooling stage]

[0126] The secondarily heated and maintained steel plate can be secondarily cooled to room temperature. Here, room temperature can mean a temperature range of 15 to 25°C. For example, the secondarily heated and maintained steel plate can be secondarily cooled to room temperature at an average cooling rate of 1°C / s or more. If the average cooling rate is less than 1°C / s, a problem of reduced productivity may occur. The upper limit of the average cooling rate is not particularly limited, but may be 100°C / s or less.

[0127] For example, in a continuous annealing process including primary heating and holding, primary cooling, secondary heating and holding, and secondary cooling, internal oxidation of the steel sheet can be performed by controlling the gas atmosphere and dew point within the continuous annealing furnace. Internal oxidation can secure plating properties and improve liquid metal embrittlement (LME). In addition, internal oxidation can introduce a soft ferrite phase to the extreme surface layer of the steel sheet, thereby improving hole expandability. In addition, internal oxidation forms micro-voids in the extreme surface layer of the steel sheet, and the micro-voids act as hydrogen trap sites, thereby improving hydrogen embrittlement resistance.

[0128] For example, the gas atmosphere within a continuous annealing furnace can be controlled to be a gas composed of nitrogen (80 to 99%) or more by volume, with the remainder being hydrogen. In addition, for example, the dew point within the continuous annealing furnace can be controlled to be 5°C or more.

[0129] For example, after the secondary cooling step, a step of plating the secondary cooled steel sheet may be further included. For example, the plating step may employ a plating method commonly used in the relevant technical field, such as hot-dip galvanizing, electrolytic galvanizing, or hot-dip aluminum plating, and the method and conditions thereof are not particularly limited.

[0130] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0131] (Example)

[0132] A steel slab (thickness: 100 mm) having the composition described in Table 1 below (remaining components of the alloy composition: Fe and unavoidable impurities, where P is expressed in ppm and the remaining components in wt%) was manufactured, and then the steel slab was reheated at 1200°C. Thereafter, the reheated steel slab was finish hot-rolled at 900°C, cooled at an average cooling rate of 30°C / s, and coiled under the conditions described in Table 2 to manufacture a hot-rolled steel sheet having a thickness of 3 mm. The hot-rolled steel sheet was hot-annealed under the conditions described in Table 2, pickled to remove surface scale, and then cold-rolled to a thickness of 1.5 mm. Thereafter, the cold-rolled steel sheet was first heated and held under the conditions described in Table 2, and then first cooled, second heated and held, and second cooled under the conditions described in Table 3.

[0133] Steel alloy composition CSiMnPSAlNMoNbBA0.392.042.132.090.00080.020.0032--0.0026B0.382.092.212.170.00130.020.0028--0.0038C0.372.011.881.840.0010.020 .00290.11-0.003D0.332.383.353.870.00120.030.0030.23-0.0021E0.411. 912.062.020.00090.030.0031-0.030.0018F0.521.732.332.280.00080.020 .0027-0.050.0041G0.721.692.412.360.00110.020.0034--0.0006H0.380.9 02.112.070.0011.930.00330.1-0.0002I0.361.112.072.030.00132.350.00 31-0.040.0004J0.350.021.951.910.0014.670.003--0.0001K0.360.892.15 2.120.0011.920.0032--0.0031L0.330.021.941.920.0012.230.004--0.0027

[0134] Classification Steel grade Coiling temperature (℃) Hot rolling annealing heat treatment temperature (℃) Hot rolling annealing heat treatment time (s) 1st average heating speed (℃) 1st heating temperature (℃) 1st holding time (s) Invention example 1A500750120010880120 Comparative example 1A5009001000 Pickling failure comparison example 2A5006001300 Cold rolling fracture occurrence comparison example 3A45075092000 Pickling failure comparison example 4A500750500 Cold rolling fracture occurrence comparison example 5A500750150010730120 Comparative example 6A5507501200108801 Comparative example 7A500750120010880120 Invention example 2B500700130010880120 Invention example 3B500750100010880120 Invention example 4B55075080010880120 Invention example 5C500800100010880120 Comparative example 8C500750120010880120 Comparative example 9C450750110010880120 Comparative example 10C500700110010880120 Comparative example 11C550750100010880120 Comparative example 12C500800130010880120 Comparative example 13C500750150010880120 Invention example 6D500750160010880120Invention example 7E50065090010880120Invention example 8F550850100010880120Comparative example 14G450750170010880120Comparative example 15H500800120010880120Comparative example 16I45075060010880120Comparative example 17J500750140010880120Invention example 9K500750100010880120Invention example 10L500750120010880120

[0135] Classification Steel grade 1st average cooling speed (℃) 1st cooling temperature (℃) 2nd average heating speed (℃) 2nd heating temperature (℃) 2nd holding time (s) 2nd average cooling speed (℃) Invention example 1A 20 18 0 15 40 0 ​​30 0 10 Comparative example 1A Pickling defect Comparative example 2A Fracture occurrence during cold rolling Comparative example 3A Pickling defect Comparative example 4A Fracture occurrence during cold rolling Comparative example 5A 20 2 0 15 40 0 ​​30 0 10 Comparative example 6A 20 20 0 15 40 0 ​​30 0 10 Comparative example 7A 0.5 20 0 15 40 0 ​​30 0 10 Invention example 2B 20 25 0 15 40 0 ​​30 0 10 Invention example 3B 20 13 0 15 35 0 60 0 10 Invention example 4B202701545030010Invention example 5C202201540030010Comparative example 8C20701540030010Comparative example 9C203301540030010Comparative example 10C202101527030010Comparative example 11C202101553030010Comparative example 12C20180154004010Comparative example 13C2018015400172,80010Invention example 6D201801540030010Invention example 7E201801540030010Invention example 8F202001540030010Comparative example 14G202001535030010Comparative Example 15H202001540060010Comparative Example 16I202001540030010Comparative Example 17J202201540030010Invention Example 9K202201540030010Invention Example 10L202201545030010

[0136] For each steel plate, the fraction of microstructure, the product of tensile strength and elongation (TSХEl), the value of TTS, the value of W, and the value of HER were measured, and the results are shown in Table 4 below. Among the microstructures, ferrite (F), bainite (B), tempered martensite (TM), and pearlite (P) were observed through SEM after nital-etching the polished specimen cross-section. Among these, the fraction of bainite and tempered martensite, which are difficult to distinguish, was calculated using the dilatation curve after the dilatation evaluation. Meanwhile, since fresh martensite (FM) and retained austenite (retained γ) are also not easy to distinguish, the fraction of retained austenite calculated by X-ray diffraction was determined as the fraction of fresh martensite by subtracting the fraction of martensite and retained austenite observed by SEM.

[0137] The product of tensile strength and elongation (TSХEl) was measured by tensile testing. The tensile test was conducted using test specimens taken in a direction 90° to the rolling direction of the rolled plate in accordance with JIS No. 5.

[0138] The value of TTS was calculated based on the following relationship 2 after measuring the thickness before fracture (t0) and the thickness after fracture (t1) of the specimen tested using the tensile test JIS 5 standard test method.

[0139] [Relationship 2]

[0140] TTS = ln(t0 / t1)

[0141] The value of W was calculated according to the following equation 1 based on the value of the calculated TTS (TTS) and the content of B (B) added during the manufacture of the steel plate.

[0142] [Relationship 1]

[0143] W = TTS Х 100 Х B

[0144] The HER value was calculated by punching a hole with an initial diameter of 10 mm in the center of a 120 mm Х 120 mm rolled plate, pushing it up in a conical shape, and measuring the maximum diameter at which no cracks were observed with the naked eye, using the formula [(maximum diameter - initial diameter) / initial diameter Х 100].

[0145] Classification Microstructure (area %)TSХEL (MPa %)TTS valueW valueHER (%)FBTMFM Residue γP Invention example 1021561220308610.3860.1036 Comparative example 1 Defective pickling Comparative example 2 Fracture during cold rolling Comparative example 3 Defective pickling Comparative example 4 Fracture during cold rolling Comparative example 533410161138090.3480.0934 Comparative example 621857950286660.3890.1012 Comparative example 71411581313218910.4240.1124 Invention example 2021610180304060.4150.1632 Invention example 3016630210332040.3160.1231 Invention example 4025551190276700.3490.1332 Invention example 5029512180321720.3410.1028 Comparative example 80293050183020.3930.129 Comparative example 907641190201170.3710.1122 Comparative example 1001578250119030.4050.127 Comparative example 1102467180194430.3850.1224 Comparative example 1201477270186250.3910.127 Comparative example 1302962450201440.4050.1221 Invention example 6022540240252510.5920.1225 Invention example 7014680180329590.4490.0833 Invention example 8025531210315490.5620.2334 Comparative example 14041352220283140.4220.0318 Comparative example 15023511250253450.4720.019 Comparative example 16019561240293740.5110.0213 Comparative example 17021580210262070.4820.0011 Invention example Invention example 9024590170362550.6150.1931 10015661180347030.5720.1535

[0146] As shown in Tables 1 to 4 above, in the case of Invention Examples 1 to 10 satisfying the alloy composition and manufacturing conditions proposed in the present invention, TSХEl was measured to be 22,000 MPa% or more, the HER value was measured to be 20% or more, and the W value was measured to be 0.08 or more. That is, it was found that Invention Examples 1 to 10 had excellent strength, as well as excellent ductility, hole expandability, and workability. Meanwhile, in the case of Comparative Examples 1 to 4 in which the temperature conditions and processing time conditions in the hot-rolling annealing heat treatment process were adjusted outside the range of the examples of the present invention, it was confirmed that defects occurred in the pickling process or fractures occurred in the cold rolling process.

[0147] In Comparative Example 5, where the primary heating temperature (or primary holding temperature) was low, excessive ferrite was formed and the area fractions of bainite and tempered martensite were insufficient. Accordingly, in Comparative Example 5, TSХEl was measured to be less than 22,000 MPa%, and the strength was reduced.

[0148] In Comparative Example 6, which had a short primary holding time, the microstructure was formed unevenly, ferrite was formed excessively, and the area fractions of bainite and retained austenite were insufficient. Accordingly, Comparative Example 6 measured a HER value of less than 20%, and the hole expandability was poor.

[0149] In Comparative Example 7, where the average cooling rate in the first cooling was low, excessive ferrite was formed and the area fraction of retained austenite was insufficient. Accordingly, Comparative Example 7 was measured to have a TSХEl of less than 22,000 MPa%, and the strength was poor.

[0150] In Comparative Example 8, where the primary cooling temperature was low, excessive tempered martensite was formed, and the bainite and retained austenite fractions were insufficient. Accordingly, Comparative Example 8 measured a HER value of less than 20%, and the hole expandability was reduced.

[0151] In Comparative Example 9, where the primary cooling temperature was high, excessive bainite was formed and insufficient tempered martensite was formed. Accordingly, Comparative Example 9 was measured to have a TSХEl of less than 22,000 MPa%, and its strength was poor.

[0152] In Comparative Examples 10 and 11, where the secondary heating temperature (or secondary holding temperature) was low or high, the area fraction of retained austenite was insufficient. Accordingly, Comparative Examples 10 and 11 were measured to have a TSХEl of less than 22,000 MPa%, resulting in reduced strength. In addition, Comparative Example 10 had excessive formation of tempered martensite, resulting in a HER value of less than 20%.

[0153] In the case of Comparative Example 12, which had a low secondary holding time, the formation of retained austenite was insufficient. Accordingly, in Comparative Example 12, the TSХEl was measured to be less than 22,000 MPa%, and the HER value was measured to be less than 20%.

[0154] In the case of Comparative Example 13, which had a high secondary holding time, the formation of retained austenite was insufficient, so TSХEl was measured to be less than 22,000 MPa%, and the strength was poor.

[0155] In Comparative Examples 14 to 17 with low B content, the value of W defined by Equation 1 was measured to be less than 0.08. Accordingly, the value of HER was measured to be less than 20%, and the workability and hole expandability of the steel plate were poor.

[0156] The above embodiments are merely examples and the present invention is not limited thereto. Anything that has substantially the same configuration and achieves the same operational effects as the technical concepts described in the claims of the present invention is included within the technical scope of the present invention.

Claims

1. In weight%, C: 0.25 to 0.55%, Si: 3.0% or less (excluding 0%), Mn: 0.9 to 3.4%, Al: 3.0% or less (excluding 0%), P: 0.15% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), B: 0.0015 to 0.005%, the remainder including Fe and unavoidable impurities. The microstructure includes tempered martensite, bainite and retained austenite. Steel plate having a value of W defined by the following relational expression 1 of 0.08 or greater. [Relationship 1] W = TTS X 100 X B (In relational expression 1, TTS represents the value calculated based on relational expression 2 below after measuring the thickness before fracture and the thickness after fracture of a specimen tested using the JIS 5 standard tensile test method, and B represents the content (weight%) of boron contained in the steel plate.) [Relationship 2] TTS = ln(t0 / t1) (In equation 2, t0 represents the thickness before fracture of the specimen tested using the JIS 5 standard tensile test method, and t1 represents the thickness after fracture of the specimen.) 2. In paragraph 1, A steel plate wherein the sum of the contents of Si and Al is 1.0 to 6.0%.

3. In paragraph 1, The steel plate further comprises at least one selected from the following (1) to (3). (1) At least one of Ti: 0~0.5%, Nb: 0~0.5%, and V: 0~0.5% (2) Cr: 0~3.0% and Mo: 0~3.0%, at least one of these (3) Cu: 0~4.5% and Ni: 0~4.5%, or more 4. In paragraph 1, A steel plate, wherein the area fraction of the tempered martensite among the total area fraction of the microstructure is 30 to 85%, the area fraction of the bainite is 10 to 50%, and the area fraction of the retained austenite is 10 to 40%.

5. In paragraph 1, The above microstructure further contains ferrite, A steel plate, wherein the area fraction of the ferrite among the total area fraction of the above microstructure is 5% or less.

6. In paragraph 1, The above steel plate is a steel plate having a product of tensile strength and elongation (TSХEl) of 22,000 MPa% or more and a HER value of 20% or more.

7. A step of reheating a steel slab containing, by weight%, C: 0.25 to 0.55%, Si: 3.0% or less (excluding 0%), Mn: 0.9 to 3.4%, Al: 3.0% or less (excluding 0%), P: 0.15% or less (excluding 0%), S: 0.03% or less (excluding 0%), N: 0.03% or less (excluding 0%), B: 0.0015 to 0.005%, the remainder being Fe and unavoidable impurities; A step of final hot rolling the above reheated steel slab; A step of coiling the above-mentioned hot-rolled steel sheet; A step of hot-rolling and annealing the above-mentioned rolled steel plate at a temperature range of 500 to 850℃ for 600 to 85,000 seconds; A step of cold rolling the above hot-rolled and annealed steel sheet; A step of first heating the above cold-rolled steel sheet to a first heating temperature of Ac1 or higher and maintaining it for 120 seconds or longer; A step of first cooling the above-mentioned first heated steel plate to a first cooling temperature of 100 to 300°C at an average cooling rate of 20°C / s or more; A step of reheating the first-cooled steel plate to a second heating temperature of 300 to 500°C and maintaining it for 300 to 90,000 seconds; and A method for manufacturing a steel plate, comprising the step of secondarily cooling the secondarily heated steel plate to room temperature.

8. In paragraph 7, A method for manufacturing a steel plate, wherein in the reheating step, the steel slab has a sum of the contents of Si and Al of 1.0 to 6.0%.

9. In paragraph 7, A method for manufacturing a steel plate, wherein in the reheating step, the steel slab further includes at least one selected from the following (1) to (3). (1) At least one of Ti: 0~0.5%, Nb: 0~0.5%, and V: 0~0.5% (2) Cr: 0~3.0% and Mo: 0~3.0%, at least one of these (3) Cu: 0~4.5% and Ni: 0~4.5%, or more 10. In paragraph 7, A method for manufacturing a steel plate, wherein the reheating step is performed at 1000 to 1350°C, and the finishing hot rolling step is performed at 800 to 1000°C.

11. In paragraph 7, A method for manufacturing a steel plate, wherein in the coiling step, the finished hot-rolled steel plate is cooled to 300 to 700°C at an average cooling rate of 10°C / s or more and then coiled.

12. In paragraph 7, A method for manufacturing a steel plate, wherein the cold rolling step is performed at a reduction ratio of 20 to 90%.

13. In paragraph 7, A method for manufacturing a steel plate, wherein in the first maintaining step, the cold-rolled steel plate is heated to the first heating temperature at an average heating rate of 10°C / s or more.

14. In paragraph 7, A method for manufacturing a steel plate, wherein in the second maintaining step, the first cooled steel plate is heated to the second heating temperature at an average heating rate of 15°C / s or more.

15. In paragraph 7, A method for manufacturing a steel plate, wherein the secondarily heated steel plate is cooled to room temperature at an average cooling rate of 1°C / s or more in the second cooling step.