Ultra-high strength steel sheet with excellent ductility and method for manufacturing the same

Abstract

The present invention relates to a steel sheet suitable for use as an automobile material, and more particularly to an ultra-high strength steel sheet having excellent ductility.

Description

Technical Field

[0001] The present invention relates to a steel sheet suitable for use as an automobile material, and more particularly to an ultra-high strength steel sheet having excellent ductility. Background Art

[0002] In recent years, in the automotive industry, the use of high-strength steel sheets has become necessary to improve fuel efficiency and durability in response to various environmental regulations and energy usage regulations.

[0003] However, increasing the strength of steel sheets leads to a relative decrease in ductility. Consequently, extensive research has been conducted to improve the relationship between strength and ductility. This has led to the development and application of steels that utilize a transformation structure consisting of martensite, bainite, and retained austenite, which are low-temperature microstructures.

[0004] Phase transformation steels are divided into ferrite-martensite dual phase (DP) steels in which a hard martensite phase is formed on a ferrite matrix; transformation-induced plasticity (TRIP) steels that utilize transformation-induced plasticity of retained austenite; and complexed phase (CP) steels consisting of ferrite and hard bainite or martensite structures. The mechanical properties of each of these steels, namely the levels of tensile strength and elongation, vary depending on the type and fraction of the parent phase and the secondary phase.

[0005] In particular, the TRIP steel containing a large amount of retained austenite phase has the highest balanced value of tensile strength and elongation (TS×E1).

[0006] As an example, Patent Document 1 discloses a steel that contains approximately 10% retained austenite in addition to ferrite and martensite, achieves a tensile strength / elongation product of 21,000 MPa% or greater, and maintains a tensile strength of 780 MPa or greater. However, due to the large amounts of carbon (C) (approximately 0.2%) and silicon (Si) (approximately 1.5%) added to this steel, spot weldability and hot-dip galvanizing properties may be poor. Furthermore, to achieve high physical properties, annealing is performed in two stages, increasing the production cost of the steel sheet.

[0007] Patent Document 2 discloses a technology that reduces the Si content to 1% to ensure excellent plateability and spot weldability. This technology achieves a tensile strength of 980 MPa or higher and an elongation of 15% or higher, even when the microstructure consists of martensite, bainite, and ferrite, without any retained austenite. However, in recent years, with the expansion of regulations on automobile impact resistance, high-strength steels with excellent yield strength are being used in structural components such as frames, seat rails, and pillars to improve vehicle body impact resistance. However, the yield strength of these steels is below 700 MPa, limiting their application.

[0008] (Patent Document 1) Korean Patent Publication No. 2015-0130612

[0009] (Patent Document 2) Korean Patent Publication No. 2013-0106142 Summary of the Invention

[0010] Technical problems to be solved

[0011] One aspect of the present invention aims to provide a steel sheet suitable for use as a structural member of an automobile, etc., having excellent tensile strength and yield strength and improved ductility, and a method for producing the same.

[0012] The technical problem of the present invention is not limited to the above content. The technical problem of the present invention can be understood from the overall content of this specification, and those skilled in the art of the technical field to which the present invention belongs can easily understand the additional technical problems of the present invention.

[0013] Technical Solution

[0014] One aspect of the present invention provides an ultra-high-strength steel plate with excellent ductility, characterized in that, in terms of weight %, the steel plate contains: carbon (C): 0.1-0.2%, silicon (Si): 0.1-1.0%, manganese (Mn): 2.0-3.0%, aluminum (Al): 1.0% or less (excluding 0%), chromium (Cr): 1.0% or less, molybdenum (Mo): 0.5% or less, titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, antimony (Sb): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, and the balance is Fe and other inevitable impurities, and satisfies the following Relationships 1 to 3.

[0015] [Equation 1]

[0016] 1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]-5181[Ti]+258[Cr]+664[Mo]≥1380

[0017] [Equation 2]

[0018] 2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]-780[Ti]+210[Cr]+457[Mo]≥1300

[0019] [Equation 3]

[0020] -29[C]+0.6[Si]-7.3[Mn]+7.8[Al]-145.2[Nb]+62.6[Ti]-3.3[Cr]-2.2[Mo]≥-24

[0021] (In equations 1 to 3, each element represents the weight content.)

[0022] Another aspect of the present invention provides a method for manufacturing an ultra-high strength steel plate with excellent ductility, which comprises the following steps: preparing a steel billet satisfying the above-mentioned alloy composition and Relationships 1 to 3; heating the steel billet in a temperature range of 1050-1300°C; hot rolling the heated steel billet in a temperature range of 800-1000°C to manufacture a hot-rolled steel plate; coiling the hot-rolled steel plate in a temperature range of 400-700°C; cold rolling the coiled hot-rolled steel plate at a total reduction ratio of 20-70% to manufacture a cold-rolled steel plate; annealing the cold-rolled steel plate in a temperature range of 800-900°C; cooling the continuously annealed cold-rolled steel plate to a temperature range of 250-400°C; and reheating and holding the cooled cold-rolled steel plate, wherein the reheating and holding step is performed in a temperature range of from above the cooling temperature + 50°C to below the cooling temperature + 200°C for 0.1-60 minutes.

[0023] Beneficial effects

[0024] According to the present invention, a steel sheet having excellent tensile strength and yield strength and improved ductility can be provided, and this steel sheet has the advantage of ensuring the formability and collision stability required for a steel sheet for cold forming. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a photograph showing the microstructure of the invention steel according to one embodiment of the present invention measured by SEM.

[0026] Figure 21 is a photograph showing the microstructure of a comparative steel according to one embodiment of the present invention measured by SEM.

[0027] Best Practice

[0028] The inventors of the present invention have conducted intensive research to provide a steel sheet that has excellent tensile strength, ductility, and yield strength as an automotive material while ensuring formability and collision stability, and can therefore be used in structural parts that need to be processed into complex shapes.

[0029] As a result, they confirmed that by optimizing the alloy composition system and production conditions, a steel sheet having a structure that is advantageous for ensuring desired physical properties can be provided, and thus completed the present invention.

[0030] In particular, the present invention is characterized in that a steel sheet having a composite phase in which a soft phase and a hard phase are appropriately dispersed is provided by controlling the content relationship of specific elements in the alloy composition and optimizing the process conditions of the steel sheet manufactured through a series of processes.

[0031] Hereinafter, the present invention will be described in detail.

[0032] The ultra-high-strength steel sheet having excellent ductility according to one aspect of the present invention may contain, in terms of weight percentage, carbon (C): 0.1-0.2%, silicon (Si): 0.1-1.0%, manganese (Mn): 2.0-3.0%, aluminum (Al): 1.0% or less and excluding 0%, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.5% or less, titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, antimony (Sb): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, and nitrogen (N): 0.02% or less.

[0033] Hereinafter, the reasons for limiting the alloy composition of the steel sheet provided by the present invention as described above will be described in detail.

[0034] In addition, unless otherwise specified, the content of each element in the present invention is based on weight, and the proportion of the structure is based on area.

[0035] Carbon (C): 0.1-0.2%

[0036] Carbon (C) is an element that significantly contributes to the strength of steel sheets. It precipitates within the grains of the steel sheet, inducing solid solution strengthening and strengthening the steel by promoting the formation of martensite within the steel. Furthermore, C is an austenite-stabilizing element and plays a significant role in the formation of retained austenite. Specifically, as the amount of carbon (C) dissolved in austenite increases, the stability of the austenite increases, leading to an increase in the fraction of austenite in the steel. This increases the fraction of martensite formed by the phase transformation of the austenite, thereby improving the strength of the steel sheet. Furthermore, some austenite remains at room temperature as retained austenite.

[0037] To fully achieve the above effects, 0.1% or more of C can be added. However, if the C content exceeds 0.2%, the martensite phase fraction increases excessively, resulting in a decrease in the ferrite phase fraction, which is relatively excellent in elongation and impact absorption energy. This results in a decrease in the ductility of the steel sheet and an increased likelihood of brittleness.

[0038] Therefore, the C content may be 0.1-0.2%, and more preferably, the C content may be 0.12% or more and 0.18% or less.

[0039] Silicon (Si): 0.1-1.0%

[0040] Silicon (Si) is an element that contributes to stabilization of retained austenite by suppressing the precipitation of carbides in ferrite and inducing diffusion of carbon in ferrite into austenite.

[0041] In order to obtain the above effects, it is advantageous to contain Si in an amount of 0.1% or more. However, when the Si content exceeds 1.0%, Si oxides are formed on the steel surface, which may hinder the effects of hot-dip plating and chemical conversion coating.

[0042] Therefore, the Si content may be 0.1-1.0%, more preferably 0.2% or more, further preferably 0.4% or more, and more preferably 0.9% or less.

[0043] Manganese (Mn): 2.0-3.0%

[0044] Manganese (Mn) can serve as an austenite stabilizing element, similar to C. Specifically, Mn can contribute to increasing the fraction of martensite in the steel by reducing the critical cooling rate for forming martensite in the composite structure steel.

[0045] To fully achieve the above effects, a Mn content of 2.0% or more is advantageous. However, if the Mn content exceeds 3.0%, the weldability of the steel sheet decreases, and hot rolling properties may also be reduced. Furthermore, the formation of bands called Mn bands impairs formability and increases the risk of work cracking.

[0046] Therefore, the Mn content may be 2.0-3.0%, more preferably 2.2% or more and 2.8% or less.

[0047] Aluminum (Al): 1.0% or less

[0048] Aluminum (Al) is an element added for deoxidation of steel and, like Si, is an element that stabilizes ferrite. Al effectively improves the hardenability of martensite by distributing carbon from ferrite into austenite. Furthermore, Al effectively suppresses the precipitation of carbides in bainite while remaining in the bainite region, contributing to improved ductility of the steel sheet.

[0049] When the Al content exceeds 1.0%, the continuous castability is reduced during the steelmaking continuous casting operation, and excessive inclusions are formed, thereby increasing the possibility of quality defects in the annealed material.

[0050] Therefore, the Al content may be 1.0% or less, excluding 0%. More advantageously, the Al content may be 0.01% or more.

[0051] In the present invention, Al refers to acid-soluble aluminum (Sol.Al).

[0052] Chromium (Cr): 1.0% or less

[0053] Chromium (Cr) is an element added to improve the hardenability of steel and ensure high strength. It plays an important role in the formation of martensite. In addition, it minimizes the reduction in elongation compared to the increase in strength, thus facilitating the production of composite steel with high ductility.

[0054] When the Cr content exceeds 1.0%, not only the above-mentioned effect is saturated, but also the hot rolling strength is excessively increased, thereby deteriorating the cold rolling property. Furthermore, the martensite fraction after annealing is greatly increased, thereby reducing the elongation.

[0055] Therefore, the Cr content can be 1.0% or less, and this indicates that desired physical properties can be ensured even without intentionally adding Cr.

[0056] Molybdenum (Mo): 0.5% or less

[0057] Molybdenum (Mo) is an element that forms carbides in steel. Molybdenum combines with Ti, Nb, and other elements in steel to form fine carbides, contributing to improved yield strength and tensile strength. However, if the Mo content exceeds 0.5%, the elongation of the steel decreases, increasing manufacturing costs.

[0058] Therefore, the Mo content can be 0.5% or less, and this indicates that desired physical properties can be ensured even without intentionally adding Mo.

[0059] Titanium (Ti): 0.1% or less

[0060] Titanium (Ti), like Mo, forms fine carbides in steel, contributing to the steel's yield strength and tensile strength. Furthermore, Ti forms nitrides, causing nitrogen contained in the steel to precipitate as TiN. This prevents nitrogen from combining with aluminum and precipitating as AlN, thus reducing the risk of cracking during the continuous casting process.

[0061] When the Ti content exceeds 0.1%, coarse carbides precipitate, and the strength of the steel sheet may be reduced due to the reduction of C in the steel. In addition, the coarse carbides may cause nozzle clogging during the continuous casting process.

[0062] Therefore, the Ti content can be 0.1% or less, and this indicates that desired physical properties can be ensured even without intentionally adding Ti.

[0063] Niobium (Nb): 0.1% or less

[0064] Niobium (Nb) segregates at austenite grain boundaries to suppress the coarsening of austenite grains during annealing heat treatment, and precipitates fine carbides on the grains, thereby contributing to increasing the strength of the steel sheet.

[0065] When the Nb content exceeds 0.1%, the C content in the steel decreases due to the formation of coarse carbides, thereby causing problems of lowering the strength and elongation of the steel sheet and increasing the production cost of the steel.

[0066] Therefore, the Nb content can be 0.1% or less, which shows that desired physical properties can be ensured even without intentionally adding Nb.

[0067] Antimony (Sb): 0.1% or less

[0068] Antimony (Sb) is distributed in the grain boundaries, delaying the diffusion of oxidizing elements such as Mn, Si, and Al in the steel through the grain boundaries, thereby inhibiting the surface enrichment of oxides and having the effect of inhibiting the coarsening of surface enrichments caused by temperature rise and changes in hot rolling process.

[0069] When the Sb content exceeds 0.1%, there is a problem that not only the workability is deteriorated but also the manufacturing cost is increased.

[0070] Therefore, the Sb content may be 0.1% or less, except for 0%. More advantageously, the Sb content may be 0.01% or more.

[0071] Phosphorus (P): 0.05% or less

[0072] Phosphorus (P) segregates at grain boundaries, causing temper brittleness and hindering weldability and toughness. Therefore, it is advantageous to control the P content to as low as possible, close to 0%. However, since P is inevitably present in the steelmaking process, reducing its content is complex and increases production costs due to additional processing. Therefore, controlling the upper limit of P is effective.

[0073] Therefore, the P content may be controlled to be 0.05% or less, more preferably 0.03% or less. However, considering the level that is inevitably added, 0% may be excluded.

[0074] Sulfur (S): 0.02% or less

[0075] Sulfur (S) is an impurity inevitably contained in steel along with the aforementioned P, and poses a problem of impairing the ductility and weldability of the steel plate. Therefore, it is advantageous to control the S content to a low level as close to 0% as possible. However, considering the cost and time required to reduce the S content, it is effective to control the upper limit of S.

[0076] Therefore, the S content can be limited to 0.02% or less, more preferably 0.01% or less. However, it is indicated here that 0% may be excluded in consideration of the level that is inevitably added.

[0077] Nitrogen (N): 0.02% or less

[0078] Nitrogen (N) can combine with Al in steel to form alumina-based non-metallic inclusions called AlN, which reduces the quality of continuous casting and increases the brittleness of the steel plate, thereby increasing the risk of damaging defects.

[0079] Therefore, the N content may be controlled to be 0.02% or less, more preferably 0.01% or less, but 0% may be excluded in consideration of the level that is inevitably introduced.

[0080] The remaining component of the present invention is iron (Fe). However, undesirable impurities may inevitably be introduced from the raw materials or the surrounding environment during conventional manufacturing processes, and such impurities cannot be eliminated. These impurities are well known to those skilled in conventional manufacturing processes, and therefore, all of them are not specifically described in this specification.

[0081] The content relationship between the specific elements in the steel of the steel sheet of the present invention having the above-mentioned alloy composition preferably satisfies all of Relational Formulas 1 to 3.

[0082] [Equation 1]

[0083] 1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]-5181[Ti]+258[Cr]+664[Mo]≥1380

[0084] [Equation 2]

[0085] 2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]-780[Ti]+210[Cr]+457[Mo]≥1300

[0086] [Equation 3]

[0087] -29[C]+0.6[Si]-7.3[Mn]+7.8[Al]-145.2[Nb]+62.6[Ti]-3.3[Cr]-2.2[Mo]≥-24

[0088] (In equations 1 to 3, each element represents the weight content.)

[0089] The above-mentioned Relational Expressions 1 and 2 are compositional relations derived by numerically expressing the degree of contribution to strengthening the yield strength and tensile strength of the steel plate by controlling the fraction of the fine structural phase constituting the steel plate and improving the solid solution strengthening effect.

[0090] In Relationship 1 and Relationship 2, the coefficient of C is relatively large compared to Si and Mn. This is because C dissolves into the grains of the steel plate and contributes to a significant increase in strength. On the other hand, the coefficient of Si is relatively small compared to C because its effect on solid solution strengthening is smaller than that of C. In addition, Al has a negative coefficient value because, although Al contributes to solid solution strengthening, it has a greater effect of reducing strength by leaving ferrite in the dual phase region during annealing or promoting ferrite transformation during the subsequent cooling process. In addition, Cr and Mo, as representative hardenability elements, suppress ferrite transformation during the cooling process after annealing, thereby having the effect of improving strength, and are therefore expressed as positive values.

[0091] Furthermore, Ti and Nb contribute to strength by forming fine carbides, and therefore can have positive coefficient values ​​in the strength relationship equation based on the component elements. However, the formation of fine carbides reduces the amount of dissolved carbon, which reduces the solid solution strengthening effect of carbon. Therefore, when the precipitation strengthening effect is dominant due to the addition of Ti and Nb, Ti and Nb have positive coefficient values. On the other hand, when the solid solution strengthening effect of carbon due to the precipitation of carbides is dominant, they can be expressed as negative coefficient values.

[0092] The above-mentioned relational expression 3 is a composition relational expression derived by numerically expressing the effect of enhancing solid solution strengthening and the degree to which a specific element contributes to improving the elongation of a steel sheet.

[0093] Generally, considering that the elongation of a steel plate tends to decrease as the strength increases, the coefficients of the elements of the relational expression 3 tend to be opposite to those of the relational expressions 1 and 2.

[0094] Specifically, C and Mn contribute to increased strength due to their solid solution strengthening effects, but since this increased strength tends to reduce elongation, they have negative coefficient values. On the other hand, Al is effective in increasing elongation and therefore has a positive coefficient value. Furthermore, Si, while having the effect of increasing strength due to solid solution strengthening, also helps to secure retained austenite, and therefore also has a positive coefficient value in Relational Equation 3.

[0095] If any of the above equations 1 to 3 proposed in the present invention are not satisfied, the physical properties of the steel sheet, particularly any one or more of the tensile strength, yield strength, and elongation, may deteriorate. This is demonstrated in the examples described below.

[0096] The steel sheet of the present invention having the above alloy composition system is characterized in that the microstructure of the steel sheet is appropriately distributed and contains soft phases and hard phases, particularly ferrite with an area fraction of 3-20%, retained austenite of 1-10%, bainite of 1-30%, tempered martensite of 30-70%, and the balance fresh martensite.

[0097] Ferrite is an allotrope of iron (Fe) with a body-centered cubic (BCC) structure. Unlike martensite and bainite, it is a soft structure. Therefore, compared with bainite and martensite, it has the advantages of high elongation and excellent impact absorption energy.

[0098] When the ferrite fraction exceeds 20%, excessive soft tissue forms in the steel sheet, which may promote plastic deformation and reduce the yield strength of the steel sheet. On the other hand, when the ferrite fraction is less than 3%, there is a problem of reduced elongation and reduced formability of the steel sheet.

[0099] Therefore, the area fraction of ferrite may be 3-20%, more preferably 5-15%.

[0100] Retained austenite refers to the austenite structure that remains in the steel without being transformed into martensite or bainite during the series of heat treatment processes (equivalent to the [annealing-cooling-reheating and holding] process in the present invention) during the manufacturing process of the steel plate. The retained austenite plays a role in adjusting the balance between the strength and elongation of the steel plate.

[0101] Generally, when the strength of a steel sheet increases, the elongation decreases and the formability deteriorates, and when the elongation of a steel sheet increases, the strength decreases and it becomes difficult to ensure the physical properties required as a structural component. However, the retained austenite phase increases the tensile strength (TS) × elongation (El) value of the steel sheet and is therefore useful for improving the balance between strength and elongation.

[0102] In order to fully obtain the above-mentioned effects, the retained austenite phase may be contained in an area fraction of 1% or more. However, when the area fraction exceeds 10%, the susceptibility to embrittlement of the liquid metal increases, which may lead to a problem of deterioration in spot weldability.

[0103] Therefore, the area fraction of the retained austenite may be 1-10%, more preferably 3-9%.

[0104] Bainite can help improve workability by reducing strength differences between different structures in the steel. Specifically, bainite prevents cracks, defects, and damage in the steel sheet caused by the hardness difference between the relatively low-hardness ferrite and retained austenite phases and the relatively high-hardness tempered martensite and fresh martensite.

[0105] To fully achieve the above effects, the area fraction of bainite may be 1% or more, more preferably 5% or more. However, when the area fraction of bainite exceeds 30%, the area fraction of newly formed martensite decreases, making it difficult to ensure the target strength level.

[0106] Therefore, the area fraction of bainite may be 1-30%.

[0107] Tempered martensite refers to a structure formed by tempering and softening the martensite phase obtained by quenching austenite at approximately 500°C. Compared to the aforementioned structures, this tempered martensite phase possesses higher strength, significantly contributing to improved yield strength and tensile strength of the steel sheet. Furthermore, the carbon in the quenched martensite is distributed into the surrounding austenite during the tempering process, increasing the thermal stability of the austenite and allowing it to remain at room temperature, thereby contributing to improved elongation of the steel sheet.

[0108] In order to fully achieve the above-mentioned effects, the area fraction of the tempered martensite phase is preferably 30% or more. However, when the area fraction of the tempered martensite phase exceeds 70%, there is a problem that the fraction of the retained austenite phase is relatively reduced.

[0109] Therefore, the area fraction of the tempered martensite may be 30-70%.

[0110] The balance of the structure other than the ferrite, retained austenite, bainite, and tempered martensite phases may include a fresh martensite phase.

[0111] The freshly formed martensite phase is obtained during the final cooling process at room temperature and has the highest strength, thus contributing to improved yield strength and tensile strength of the steel sheet. While the fraction of the freshly formed martensite phase is not particularly limited, as an example, the area fraction of the freshly formed martensite phase can be 3% or greater.

[0112] As described above, the steel sheet of the present invention has excellent tensile strength, yield strength and elongation by appropriately forming soft phases and hard phases. Specifically, it can have a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 13% or more.

[0113] In addition, the steel sheet of the present invention may be a cold-rolled steel sheet, a hot-dip galvanized steel sheet including a zinc-based coating on at least one side of the cold-rolled steel sheet, or an alloyed hot-dip galvanized steel sheet obtained by alloying the hot-dip galvanized steel sheet.

[0114] Although not particularly limited, the zinc-based plating layer may be a galvanized layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc.

[0115] Hereinafter, a method for producing an ultra-high strength steel sheet having excellent ductility provided by the present invention, which is another aspect of the present invention, will be described in detail.

[0116] In short, the present invention can produce the desired steel plate through the process of [slab reheating - hot rolling - coiling - cold rolling - continuous annealing - cooling - reheating and holding], and then can further perform the process of [hot dip galvanizing - alloying heat treatment].

[0117] The conditions of each step are described in detail below.

[0118] [Heating of steel billet]

[0119] First, a steel slab that meets all of the above alloy composition systems can be prepared and then heated. This process is performed to smoothly carry out the subsequent hot rolling process and to fully obtain the desired physical properties of the steel plate.

[0120] The heating process can be performed within a temperature range of 1050-1300°C. When the heating temperature is lower than 1050°C, friction between the steel plate and the rolling mill increases, leading to a sharp increase in the load on the rolls during hot rolling. On the other hand, when the heating temperature exceeds 1300°C, not only does the energy cost required for the temperature increase increase, but the amount of surface oxide scale also increases, potentially leading to material loss.

[0121] Therefore, the heating process may be performed within a temperature range of 1050-1300°C, more advantageously within a temperature range of 1090-1250°C.

[0122] [Hot Rolling]

[0123] The steel slab heated as described above may be hot rolled to produce a hot-rolled steel plate, and at this time, hot finish rolling may be performed at a temperature ranging from 800 to 1000°C.

[0124] By performing hot finish rolling within the above-mentioned temperature range, the effect of simultaneously improving the rigidity and formability of the steel sheet can be achieved. However, when the temperature of hot finish rolling is lower than 800°C, since rolling is performed in the ferrite region, the friction between the steel sheet and the rolling mill increases, and therefore there is a problem of a significant increase in the load caused by rolling. This leads to the formation of excessive dislocations, and coarse grains are formed on the surface of the steel sheet during the subsequent coiling or cold rolling process, thereby reducing the strength. On the other hand, when the temperature of hot finish rolling exceeds 1000°C, the size of the ferrite grains increases, and there is also a problem of reduced strength. In addition, scale is generated on the surface of the hot-rolled steel sheet, which may cause surface defects and shorten the life of the rolls.

[0125] Therefore, the hot finish rolling during the hot rolling may be performed at a temperature range of 800-1000°C, more advantageously at a temperature range of 850-950°C.

[0126] [Collect]

[0127] The hot rolled steel sheet manufactured as described above may be coiled, wherein the coiling may be performed at a temperature ranging from 400°C to 700°C.

[0128] If the coiling temperature is below 400°C, the strength of the hot-rolled steel sheet increases excessively, potentially causing rolling loads during subsequent cold rolling. Furthermore, excessive cost and time are required to cool the hot-rolled steel sheet to the coiling temperature, increasing process costs. On the other hand, if the coiling temperature exceeds 700°C, excessive scale forms on the surface of the hot-rolled steel sheet, increasing the likelihood of surface defects and deteriorating coating properties.

[0129] Therefore, the winding process may be performed within a temperature range of 400-700°C, more advantageously within a temperature range of 500-700°C.

[0130] [cool down]

[0131] The coiled hot-rolled steel sheet may be cooled to room temperature. In this case, the cooling rate is not particularly limited, but air cooling may be used.

[0132] [Cold Rolling]

[0133] Thereafter, the hot-rolled steel sheet may be cold-rolled to produce a cold-rolled steel sheet, wherein the cold-rolling reduction ratio may be 20-70%.

[0134] When the cold rolling reduction is less than 20%, it is difficult to achieve the target thickness of the steel sheet and the shape of the steel sheet is difficult to correct. On the other hand, when the cold rolling reduction exceeds 70%, the likelihood of cracks forming at the steel sheet edges increases and there are problems associated with the cold rolling load. In addition, the excessive load on the steel sheet surface may lead to the formation of coarse ferrite during subsequent continuous annealing.

[0135] Therefore, the cold rolling may be performed at a cold rolling reduction ratio of 20-70%, more preferably 30-60%.

[0136] In addition, the hot-rolled steel sheet may be subjected to pickling treatment before the cold rolling. The pickling treatment is a process of removing oxide scale formed on the surface of the hot-rolled steel sheet using hydrochloric acid (HCl) or the like, and can be performed under conventional conditions, so the conditions are not particularly limited.

[0137] [annealing]

[0138] The cold-rolled steel sheet manufactured as described above may be subjected to an annealing treatment. As an example, a continuous annealing process may be performed, but the present invention is not limited thereto and any known annealing method may be used.

[0139] In the present invention, the ferrite formed in the cold-rolled steel sheet can be recrystallized through the annealing process, thereby adjusting the ferrite and austenite fractions in the steel. The strength of the steel sheet produced after the final heat treatment (referred to as the reheating process described below) is determined by the fractions of each phase formed. Generally, a higher austenite fraction increases the fraction of martensite or bainite transformed from austenite, which tends to improve the strength of the steel sheet. However, the present invention further allows for strength control through a series of heat treatment conditions described below.

[0140] Furthermore, carbon (C) in the steel can be distributed through the annealing process, thereby increasing the carbon (C) content contained in the austenite, and having an austenite phase of up to 10 area % even at room temperature.

[0141] The annealing process may be performed at a temperature ranging from 800° C. to 900° C.

[0142] When the temperature during the annealing is lower than 800°C, the fraction of austenite formed by the annealing process is reduced, resulting in insufficient fractions of tempered martensite, bainite, and newly formed martensite during the subsequent heat treatment. This may be the reason for the reduction in the yield strength and tensile strength of the final steel plate. On the other hand, when the temperature during the annealing exceeds 900°C, the fraction of austenite in the steel plate increases excessively, and there is a problem that part of the austenite is transformed into ferrite during the subsequent heat treatment. In addition, the carbon enrichment of the retained austenite is reduced, and the mechanical stability may be reduced, in which case, the elongation of the steel plate is reduced. In addition, during the annealing process, the moisture generated while the Fe in the steel is oxidized reacts with the Si, Mn, and Al in the steel, so there is a high possibility of forming an oxide film on the steel plate. The oxide film hinders the wettability of Zn during hot-dip galvanizing, resulting in the possible deterioration of the surface quality of the steel plate.

[0143] Therefore, the annealing process may be performed in a temperature range of 800-900°C, more advantageously in a temperature range of 820-870°C.

[0144] [cool down]

[0145] The cold-rolled steel sheet having completed the annealing process as described above may be cooled.

[0146] In the present invention, quenched martensite can be formed by cooling the annealed cold-rolled steel sheet. To this end, the cooling is preferably performed below the martensite transformation start temperature (Ms), more preferably to a temperature range of 250-400°C.

[0147] The lower the cooling temperature, the higher the fraction of quenched martensite, which can induce an increase in the strength of the steel sheet. In addition, the supersaturated carbon in the martensite is distributed into the surrounding austenite during the subsequent heat treatment, thereby improving the stability of the retained austenite, which can contribute to an increase in elongation.

[0148] However, if the cooling temperature is lower than 250°C, the fraction of quenched martensite increases excessively, causing the fraction of retained austenite to decrease, and there is a problem of deterioration in the shape of the steel sheet. On the other hand, if the cooling temperature exceeds 400°C, quenched martensite cannot be fully formed, making it difficult to expect the above-mentioned effect.

[0149] When cooling is performed in the above-mentioned temperature range, it can be performed at an average cooling rate of 2-50°C / second (s). When the cooling rate is lower than 2°C / second, the ferrite is further transformed during the cooling process, causing a decrease in strength. On the other hand, when the cooling rate exceeds 50°C / second and the cooling is rapid, cooling deviations occur at different positions of the steel plate, so there is a problem of deterioration in the shape of the steel plate. When cooling is performed at the above-mentioned cooling rate, the cooling method is not particularly limited. As an example, the cooling may be a single cooling method of cooling to the cooling end temperature according to the initially set cooling rate. As another example, the cooling may be a step-by-step cooling method of slowly cooling to a certain interval and then strongly cooling to the cooling end temperature, but it is not limited thereto.

[0150] In addition, the cooling temperature may be maintained for a certain period of time to further introduce an isothermal transformation phase, thereby promoting the phase transformation of bainite in subsequent processes. To this end, the holding process may be performed for 0.1-60 minutes.

[0151] [Reheat and keep]

[0152] The cooled cold-rolled steel sheet may be further cooled and maintained by reheating the cooled and maintained cold-rolled steel sheet to a temperature range of about 50-200° C. higher than the cooling temperature and then maintaining the temperature for a certain period of time, thereby performing a tempering treatment.

[0153] By reheating the cooled cold-rolled steel sheet, the quenched martensite phase formed during the cooling process is tempered and transformed into tempered martensite. Tempered martensite has the advantage of high yield strength due to carbon being fixed in dislocations. Furthermore, during the tempering process, supersaturated carbon (C) in the quenched martensite is redistributed into the surrounding austenite, or bainite transformation is induced, thereby increasing the stability of the retained austenite, thereby achieving an effect of improving elongation.

[0154] The higher the tempering temperature, the more smoothly the dislocation fixation and carbon distribution to austenite occur. Therefore, reheating should be performed at a temperature 50°C or higher than the cooling temperature (cooling temperature + 50°C or higher). However, if the reheating temperature is too high, cementite forms and coarsens itself within the quenched martensite, reducing the strength of the steel sheet and diminishing the effect of carbon redistribution to austenite, making it difficult to expect an increase in elongation. For this reason, reheating can be limited to a temperature below the cooling temperature + 200°C.

[0155] Preferably, the cold-rolled steel sheet cooled to the above temperature range is reheated and then maintained at the temperature for 0.1-60 minutes, so as to fully achieve the above effect.

[0156] During the holding, if the holding time is too long, exceeding 60 minutes, ferrite and cementite are formed as equilibrium phases at the holding temperature, causing a problem of reduced strength of the steel sheet. On the other hand, if the holding time is less than 0.1 minutes, the desired effect cannot be obtained.

[0157] After the process of reheating and holding the cooled cold-rolled steel sheet as described above is completed, it can be cooled to room temperature under conventional conditions, and finally a steel sheet with a structure having a certain proportion of soft phase and hard phase properly distributed can be obtained.

[0158] Specifically, a steel plate having a fine structure composed of 3-20% by area of ​​ferrite, 1-10% by area of ​​retained austenite, 1-30% by area of ​​bainite, 30-70% by area of ​​tempered martensite, and the balance of fresh martensite can be obtained. Such a steel plate of the present invention can have excellent yield strength and tensile strength and improved ductility.

[0159] Although the cooling process to the room temperature is not particularly limited, as an example, it can be performed by air cooling. However, it is also obvious that it can be replaced by a well-known cooling method such as water cooling, oil cooling, furnace cooling, etc.

[0160] In addition, a plated steel sheet having a plated layer on at least one side can be manufactured by subjecting a cold-rolled steel sheet that has undergone a series of heat treatment processes as described above to a plating treatment to be described later.

[0161] [Hot-dip galvanizing]

[0162] The steel sheet manufactured through the above series of processes may be immersed in a molten zinc-based plating bath to manufacture a hot-dip galvanized steel sheet.

[0163] At this time, hot-dip galvanizing can be performed under conventional conditions, and as an example, can be performed at a temperature range of 430-490° C. Furthermore, during the hot-dip galvanizing, the composition of the molten zinc-based plating bath is not particularly limited, and can be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0164] [Alloying heat treatment]

[0165] If necessary, the hot-dip galvanized steel sheet may be subjected to alloying heat treatment to obtain an alloyed hot-dip galvanized steel sheet.

[0166] In the present invention, there is no particular limitation on the conditions of the alloying heat treatment process, as long as they are conventional conditions. As an example, the alloying heat treatment process can be performed within a temperature range of 480-600°C.

[0167] The present invention is described in more detail below using examples. However, it should be noted that the following examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the contents of the claims and any reasonable deductions therefrom. DETAILED DESCRIPTION

[0168] (Example)

[0169] 30 kg of slabs having the alloy composition shown in Table 1 below were heated at 1200°C for 1 hour and then hot finish rolled at 900°C to produce hot-rolled steel sheets. Each hot-rolled steel sheet was then placed in a furnace preheated to 600°C, held for 1 hour, and then furnace-cooled to simulate hot-rolled coiling. The steel sheets were then cooled (in air) to room temperature and then cold rolled at a 45% cold reduction to produce cold-rolled steel sheets.

[0170] Each of the cold-rolled steel sheets produced as described above was subjected to continuous annealing treatment at temperature T1 (°C) shown in Table 2 below for 1 minute, then cooled to temperature T2 (°C), held for 10 seconds, then heated to temperature T3 (°C) and held for 1 minute, and then cooled (air-cooled) to room temperature to produce a final steel sheet. The cooling after annealing to temperature T2 was performed at a cooling rate of 15°C / second.

[0171] For each steel plate manufactured through all the above processes, mechanical physical properties and internal structure were measured, and the results are shown in Table 3 below.

[0172] As the mechanical physical properties, yield strength (YS), tensile strength (TS), and elongation (El) were measured, and were measured using an ASTM tensile test piece and a universal tensile testing machine.

[0173] Regarding the internal structure, the specimen was ground and then etched with nitric acid, and then the area of ​​each phase was calculated using a scanning electron microscope (SEM).

[0174] [Table 1]

[0175]

[0176] [Table 2]

[0177]

[0178]

[0179] (In Table 2, the alloy compositions of Steels 9, 10, and 11 are outside the scope of the present invention, and therefore they are classified as comparative examples.)

[0180] [Table 3]

[0181]

[0182]

[0183] As shown in Tables 1 to 3, in Invention Examples 1 to 11, which satisfy all the alloy composition systems and production conditions proposed in the present invention, desired microstructures are formed, thereby ensuring desired physical properties.

[0184] On the other hand, in Comparative Examples 1 and 2, which do not satisfy at least one of Relationship 1 and Relationship 2 of the composition relationship proposed in the present invention, it is found that one or more physical properties, namely, yield strength and tensile strength, cannot be maintained at the target level. Furthermore, in Comparative Example 7, which does not satisfy Relationship 3 of the composition relationship, it is confirmed that the elongation is significantly deteriorated.

[0185] This demonstrates that Relationship 1, a characteristic feature of the present invention, contributes to an improvement in the yield strength of the steel plate due to the fine structure fraction and solid solution strengthening effect, Relationship 2 contributes to an improvement in the tensile strength of the steel plate, and Relationship 3 contributes to an improvement in the ductility of the steel plate.

[0186] That is, this means that when Relational Expressions 1 and 2 of the present invention are not satisfied, the strength of the steel sheet is poor, and when Relational Expression 3 is not satisfied, the ductility of the steel sheet is poor.

[0187] In addition, in Comparative Examples 3 to 6, which satisfy the alloy composition system proposed in the present invention but have heat treatment conditions outside the scope of the present invention, the desired soft phase and hard phase are not properly formed. As a result, excellent strength and ductility cannot be simultaneously ensured in all examples.

[0188] Figure 1 ] shows a microstructure photograph of Inventive Example 1, and it can be confirmed that ferrite, retained austenite, tempered martensite, and bainite are formed within the desired fraction range, and in addition, a fresh martensite phase is formed as the remainder of the microstructure.

[0189] Figure 2 The microstructure photograph of Comparative Example 6 is shown in FIG. 3 , and it can be confirmed that the tempered martensite phase is not formed at a desired fraction, the retained austenite phase cannot be sufficiently secured, and a relatively high fraction of the newly formed martensite phase is formed.

Claims

1. An ultra-high strength steel plate with excellent ductility, characterized in that: In terms of weight %, the steel plate comprises: carbon (C): 0.1-0.2%, silicon (Si): 0.1-1.0%, manganese (Mn): 2.0-3.0%, aluminum (Al): 1.0% or less and excluding 0%, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.5% or less, titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, antimony (Sb): 0.1% or less and excluding 0%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, the balance being Fe and other unavoidable impurities, and satisfying the following equations 1 to 3, The microstructure contains 3-20% ferrite, 1-10% retained austenite, 1-30% bainite, 30-70% tempered martensite and the balance fresh martensite. [Equation 1] 1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]- 5181[Ti]+258[Cr]+664[Mo]≥1380 [Equation 2] 2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]- 780[Ti]+210[Cr]+457[Mo]≥1300 [Equation 3] -29[C]+0.6[Si]-7.3[Mn]+7.8[Al]-145.2[Nb]+62.6[Ti]-3.3[Cr]-2.2[Mo]≥-24 In Relational Formulas 1 to 3, each element represents the weight content.

2. The ultra-high strength steel plate having excellent ductility according to claim 1, wherein: The steel sheet includes a freshly formed martensite phase having an area fraction of 3% or more.

3. The ultra-high strength steel plate having excellent ductility according to claim 1, wherein: The steel plate has a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 13% or more.

4. The ultra-high strength steel plate having excellent ductility according to claim 1, wherein The steel plate is any one of a cold-rolled steel plate, a hot-dip galvanized steel plate and an alloyed hot-dip galvanized steel plate.

5. A method for manufacturing an ultra-high strength steel plate having excellent ductility, comprising the following steps: A steel billet is prepared, wherein the steel billet comprises, by weight %, carbon (C): 0.1-0.2%, silicon (Si): 0.1-1.0%, manganese (Mn): 2.0-3.0%, aluminum (Al): 1.0% or less and excluding 0%, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.5% or less, titanium (Ti): 0.1% or less, niobium (Nb): 0.1% or less, antimony (Sb): 0.1% or less and excluding 0%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, the balance being Fe and other unavoidable impurities, and satisfying the following equations 1 to 3; heating the steel billet at a temperature ranging from 1050° C. to 1300° C.; hot rolling the heated steel billet at a temperature ranging from 800° C. to 1000° C. to produce a hot-rolled steel plate; Coiling the hot-rolled steel sheet at a temperature ranging from 400° C. to 700° C. cold rolling the coiled hot-rolled steel sheet at a total reduction ratio of 20-70% to produce a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature ranging from 800° C. to 900° C. Cooling the continuously annealed cold-rolled steel sheet to a temperature range of 250-400° C.; and reheating and maintaining the cooled cold-rolled steel sheet, wherein the reheating and holding step is carried out for 0.1-60 minutes in a temperature range of above the cooling temperature + 50°C to below the cooling temperature + 200°C, The microstructure of the ultra-high strength steel plate with excellent ductility includes 3-20% by area fraction of ferrite, 1-10% by area fraction of retained austenite, 1-30% by area fraction of bainite, 30-70% by area fraction of tempered martensite and the balance of fresh martensite. [Equation 1] 1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]- 5181[Ti]+258[Cr]+664[Mo]≥1380 [Equation 2] 2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]- 780[Ti]+210[Cr]+457[Mo]≥1300 [Equation 3] -29[C]+0.6[Si]-7.3[Mn]+7.8[Al]-145.2[Nb]+62.6[Ti]-3.3[Cr]-2.2[Mo]≥-24 In Relational Formulas 1 to 3, each element represents the weight content.

6. The method for producing an ultra-high strength steel plate having excellent ductility according to claim 5, wherein: The cooling of the cold-rolled steel sheet is performed at a cooling rate of 2-50° C. / second.

7. The method for producing an ultra-high strength steel plate having excellent ductility according to claim 5, wherein: Before reheating the cooled cold-rolled steel sheet, the method further comprises the step of maintaining the steel sheet within the cooling temperature range for 0.1 to 60 minutes.

8. The method for producing an ultra-high strength steel plate having excellent ductility according to claim 5, wherein: After the reheating and holding, the step of hot-dip galvanizing is further included.

9. The method for producing an ultra-high strength steel plate having excellent ductility according to claim 8, wherein: After the hot-dip galvanizing, the method further includes the step of performing alloying heat treatment.

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