Ultra-high strength steel sheet excellent in ductility and method for producing the same

Abstract

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

Description

Technical Field

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

Background Art

[0002] Recently, in the automotive industry, due to various environmental regulations and energy use regulations, etc., the use of high-strength steel sheets has been demanded in order to improve fuel efficiency or durability.

[0003] However, when increasing the strength of the steel sheet, a problem has been found that the ductility relatively decreases. Therefore, many studies have been conducted to improve the relationship between strength and ductility. As a result, transformation-induced steel that utilizes the retained austenite phase, along with low-temperature structures such as martensite and bainite, has been developed and applied.

[0004] Transformation-induced steel is classified into ferrite-martensite two-phase (Dual Phase, DP) steel in which a hard martensite phase is formed in a ferrite matrix, TRIP (Tranformation Induced Plasticity) steel that uses transformation-induced plasticity of retained austenite, and CP (Complexed Phase) steel composed of ferrite and a hard bainite or martensite structure. Each of these steels has different mechanical properties, that is, levels of tensile strength and elongation, depending on the types and fractions of the parent phase and the second phase.

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

[0006] As an example, Patent Document 1 discloses a steel that contains about 10% of retained austenite phase in addition to ferrite and martensite, has a product of tensile strength and elongation rate of 21,000 MPa% or more, and can ensure a tensile strength of 780 MPa or more. However, since the content of carbon (C) in this steel is about 0.2% and the content of silicon (Si) is added in a large amount of about 1.5% or more, there is a risk of inferior spot weldability and hot dip galvanizing property. In addition, in order to achieve high physical properties, annealing is performed twice, resulting in a problem of increased manufacturing cost of the steel sheet.

[0007] On the other hand, in Patent Document 2, in order to ensure good plating property and spot weldability, the content of Si is reduced to the 1% level, and it is composed of martensite, bainite, and ferrite without containing a retained austenite phase as a fine structure, and a technology capable of ensuring a tensile strength of 980 MPa or more and an elongation rate of 15% or more is disclosed. However, recently, regulations on the impact stability of automobiles have been expanded, and although high-strength steels with excellent yield strength are being adopted for structural members such as members, seat rails, and pillars in order to improve the impact resistance of the vehicle body, the yield strength of this steel is 700 MPa or less, and there is a limit to the applicable objects.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] One aspect of the present invention is to provide a steel sheet suitable for automobile structural members and the like, which is excellent not only in tensile strength but also in yield strength and has improved ductility, and a method for manufacturing the same.

[0010] The problems of the present invention are not limited to the above-described content. The problems of the present invention can be understood from the entire content of this specification, and there is no difficulty in understanding further problems of the present invention for those having ordinary knowledge in the technical field to which the present invention belongs.

Means for Solving the Problems

[0011] One aspect of the present invention provides a high-strength steel sheet with excellent ductility, which contains, by weight%, carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, manganese (Mn): 2.0 to 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 being Fe and other inevitable impurities, and satisfies the following relational expressions 1 to 3.

[0012] [Relational Expression 1] 1110[C]+41.5[Si]+575[Mn]-1092[Al]-3590[Nb]-5181[Ti]+258[Cr]+664[Mo]≧1380

[0013] [Relational Expression 2] 2853[C]+95[Si]+309[Mn]-153[Al]+4661[Nb]-780[Ti]+210[Cr]+457[Mo]≧1300

[0014] [Relational Expression 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 Expressions 1 to 3, each element means the weight content.)

[0015] Another aspect of the present invention includes the steps of preparing a steel slab satisfying the alloy composition and relational expressions 1 to 3 described above; heating the steel slab in a temperature range of 1050 to 1300°C; hot rolling the heated steel slab in a temperature range of 800 to 1000°C to produce a hot-rolled steel sheet; winding up the hot-rolled steel sheet in a temperature range of 400 to 700°C; cold rolling the wound hot-rolled steel sheet with a total reduction rate of 20 to 70% to produce a cold-rolled steel sheet; annealing the cold-rolled steel sheet in a temperature range of 800 to 900°C; cooling the continuously annealed cold-rolled steel sheet in a temperature range of 250 to 400°C; and reheating and maintaining the cooled cold-rolled steel sheet. The reheating and maintaining step is performed in a temperature range of the cooled temperature + 50°C or more to the cooled temperature + 200°C or less for 0.1 to 60 minutes, thereby providing a method for manufacturing an ultra-high strength steel sheet with excellent ductility.

Advantages of the Invention

[0016] According to the present invention, it is possible to provide a steel sheet having excellent yield strength as well as tensile strength and improved ductility. Such a steel sheet of the present invention has the advantage that formability and collision stability required for a steel sheet for cold forming are ensured.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0018] The inventors of the present invention have earnestly studied to provide a steel sheet that is excellent in yield strength as well as tensile strength and ductility as a material for automobiles, and is applicable to structural members that require processing into complex shapes because formability and collision stability are ensured.

[0019] As a result, by optimizing the alloy composition system and manufacturing conditions, it was confirmed that a steel sheet having a structure advantageous for ensuring the target physical properties can be provided, and the present invention was completed.

[0020] In particular, the present invention is characterized by providing a steel sheet having a composite structure in which a soft phase and a hard phase are appropriately dispersed by controlling the content relationship of specific elements among the alloy components and optimizing the process conditions of the steel sheet manufactured through a series of processes.

[0021] Hereinafter, the present invention will be described in detail.

[0022] The ultra-high strength steel sheet excellent in ductility according to one aspect of the present invention can contain, by weight%, carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, manganese (Mn): 2.0 to 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, and nitrogen (N): 0.02% or less.

[0023] Hereinafter, the reasons for restricting the alloy composition of the steel sheet provided by the present invention as described above will be described in detail.

[0024] On the other hand, unless otherwise specified in the present invention, the content of each element is based on weight, and the ratio of the structure is based on area.

[0025] Carbon (C): 0.1 to 0.2% Carbon (C) is an element that greatly contributes to the strengthening of the steel plate. The above C precipitates in the crystal grains of the steel plate to induce solid solution strengthening, promotes the formation of martensite in the steel, and strengthens the steel. Also, the above C is an austenite stabilizing element and plays an important role in the formation of retained austenite. Specifically, the more the amount of carbon (C) dissolved in austenite increases, the higher the austenite stability and the higher the fraction of austenite in the steel. This induces an increase in the fraction of martensite formed by the transformation of the above austenite, resulting in the effect of improving the strength of the steel plate, and a part of the austenite remains at room temperature as retained austenite.

[0026] To fully obtain the above effects, C can be added in an amount of 0.1% or more. However, if its content exceeds 0.2%, the fraction of the martensite phase increases excessively, and the fraction of the ferrite phase, which is excellent in elongation and impact absorption energy relatively, decreases. This causes a decrease in the ductility of the steel plate and an increased possibility of brittleness.

[0027] Therefore, the above C can be contained in an amount of 0.1 to 0.2%, more preferably 0.12% or more and 0.18% or less.

[0028] Silicon (Si): 0.1 - 1.0% Silicon (Si) is an element that suppresses the precipitation of carbides in ferrite, induces the diffusion of carbon in ferrite into austenite, and contributes to the stabilization of retained austenite.

[0029] To obtain the above effects, it is advantageous to contain Si in an amount of 0.1% or more. However, if its content exceeds 1.0%, there is a possibility of inhibiting the effects of hot dip galvanizing and chemical conversion coating because SiO₂ is formed on the surface of the steel.

[0030] Therefore, the above Si can be contained in an amount of 0.1 to 1.0%, more preferably 0.2% or more, and even more preferably 0.4% or more. On the other hand, the above Si can be contained in an amount of more preferably 0.9% or less.

[0031] Manganese (Mn): 2.0 to 3.0% Manganese (Mn), similar to the above C, can act as an austenite stabilizing element. Specifically, the above Mn can contribute to reducing the critical cooling rate at which martensite is formed in the duplex steel and increasing the fraction of martensite in the steel.

[0032] To sufficiently obtain the above-described effects, it is advantageous to contain 2.0% or more of Mn. However, if the content exceeds 3.0%, the weldability of the steel sheet may decrease, and the hot workability may be reduced. In addition, there is a problem of forming a streak-like band called Mn-Band, which inhibits formability and increases the risk of occurrence of processing cracks.

[0033] Therefore, the above Mn can be contained in an amount of 2.0 to 3.0%, more preferably 2.2% or more and 2.8% or less.

[0034] Aluminum (Al): 1.0% or less Aluminum (Al) is an element added for deoxidation of steel and is a ferrite stabilizing element similar to the above Si. The above Al is effective in distributing carbon in ferrite to austenite to improve the hardening ability of martensite, and is useful for effectively suppressing the precipitation of carbides in bainite during maintenance in the bainite region and improving the ductility of the steel sheet.

[0035] If the content of such Al exceeds 1.0%, the continuous castability during steelmaking may decrease during continuous casting operation, and inclusions may be excessively formed, increasing the possibility of material defects in the annealed material.

[0036] Therefore, the above Al can be contained in an amount of 1.0% or less, excluding 0%. More preferably, the above Al can be contained in an amount of 0.01% or more.

[0037] In the present invention, Al means soluble aluminum (Sol.Al).

[0038] Chromium (Cr): 1.0% or less Chromium (Cr) is an element added to improve the hardenability of steel and ensure high strength, and plays an important role in the formation of martensite. Also, it minimizes the decrease in elongation rate with the increase in strength, which is advantageous for the production of duplex stainless steel with high ductility.

[0039] When the content of Cr exceeds 1.0%, not only do the above-described effects saturate, but there are problems such as excessive increase in hot-rolled strength and poor cold-rolling properties, and a large increase in the martensite fraction after annealing, leading to a decrease in elongation rate.

[0040] Therefore, it is clarified that the above Cr can be contained at 1.0% or less, and it is not difficult to ensure the intended physical properties even without intentionally adding the above Cr.

[0041] Molybdenum (Mo): 0.5% or less Molybdenum (Mo) is an element that forms carbides in steel, and can contribute to the improvement of yield strength and tensile strength by combining with Ti, Nb, etc. in the steel to form fine carbides in the steel. When the content of such Mo exceeds 0.5%, there is a problem that the elongation rate of the steel decreases and the manufacturing cost increases.

[0042] Therefore, it is clarified that the above Mo can be contained at 0.5% or less, and it is not difficult to ensure the intended physical properties even without intentionally adding the above Mo.

[0043] Titanium (Ti): 0.1% or less Titanium (Ti) can form fine carbides in steel, similar to the above-mentioned Mo, and contribute to ensuring the yield strength and tensile strength of the steel. In addition, by forming nitrides, Ti can precipitate the N contained in the steel as TiN, suppressing the precipitation of the above N combined with Al as AlN, which has the effect of reducing the risk of crack generation in the continuous casting process.

[0044] When the content of such Ti exceeds 0.1%, coarse carbides may precipitate, and since C is reduced in the steel, there is a risk of a decrease in the strength of the steel plate. Furthermore, such coarse carbides may induce clogging of the nozzle in the continuous casting process.

[0045] Therefore, it is clarified that the above Ti can be contained at 0.1% or less, and it is not difficult to ensure the intended physical properties even without intentionally adding the above Ti.

[0046] Niobium (Nb): 0.1% or less Niobium (Nb) segregates at the austenite grain boundaries, suppresses the coarsening of austenite grains during annealing heat treatment, and can contribute to an increase in the strength of the steel plate by precipitating fine carbides in the above grains.

[0047] When the content of such Nb exceeds 0.1%, there are problems such as a reduction in the C content in the steel due to the formation of coarse carbides, a decrease in the strength and elongation rate of the steel plate, and an increase in the manufacturing cost of the steel.

[0048] Therefore, it is clarified that the above Nb can be contained at 0.1% or less, and it is not difficult to ensure the intended physical properties even without intentionally adding the above Nb.

[0049] Antimony (Sb): 0.1% or less Antimony (Sb) is distributed at the grain boundaries and has the beneficial effect of suppressing the surface enrichment of oxides by retarding the diffusion of oxidizing elements such as Mn, Si, and Al in the steel through the grain boundaries, and suppressing the coarsening of the surface enrichment due to temperature rise and changes in the hot rolling process.

[0050] When the content of such Sb exceeds 0.1%, there are problems such as not only poor workability but also an increase in manufacturing cost.

[0051] Therefore, the above Sb can be contained at 0.1% or less, excluding 0%. More preferably, the above Sb can be contained at 0.01% or more.

[0052] Phosphorus (P): 0.05% or less Phosphorus (P) segregates at the grain boundaries and is the main cause of the occurrence of temper brittlement, and there is a problem of inhibiting weldability and toughness. Therefore, it is advantageous to control the content of the above P to be as close to 0% as possible. However, due to the steel manufacturing process, it is inevitably contained, and it is difficult to have a process to reduce the content of such P, and the production cost increases due to additional processes. Therefore, it is effective to manage its upper limit.

[0053] Therefore, the above P can be limited to 0.05% or less, and more preferably to 0.03% or less. However, it is clarified that 0% can be excluded considering the inevitably added level.

[0054] Sulfur (S): 0.02% or less Sulfur S is an impurity inevitably contained in the steel together with the above-mentioned P, and there is a problem of inhibiting the ductility and weldability of the steel plate. Therefore, it is also advantageous to control the content of the above S to be as close to 0% as possible. However, considering the cost and time consumed in the process to reduce the content of S, it is effective to manage its upper limit.

[0055] Therefore, S can be limited to 0.02% or less, more preferably 0.01% or less. However, it is clarified that 0% can be excluded considering the inevitably added level.

[0056] Nitrogen (N): 0.02% or less Nitrogen (N) can combine with Al in the steel to form alumina-based non-metallic inclusions of AlN. Since the above AlN reduces the continuous casting quality and increases the brittleness of the steel plate, it increases the risk of fracture defects occurring.

[0057] Therefore, the above N can be limited to 0.02% or less, more preferably 0.01% or less. However, 0% can be excluded considering the inevitably flowing-in level.

[0058] The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be mixed in from the raw materials or the surrounding environment, and thus they cannot be excluded. Since these impurities are understandable to any technician in the normal manufacturing process, all of their contents are not particularly mentioned in this specification.

[0059] The steel plate of the present invention having the above alloy composition preferably satisfies all of the following relational expressions 1 to 3 for the content relationship between specific elements in the steel.

[0060] [Relational Expression 1] 1110[C] + 41.5[Si] + 575[Mn] - 1092[Al] - 3590[Nb] - 5181[Ti] + 258[Cr] + 664[Mo] ≧ 1380

[0061] [Relational Expression 2] 2853[C] + 95[Si] + 309[Mn] - 153[Al] + 4661[Nb] - 780[Ti] + 210[Cr] + 457[Mo] ≧ 1300

[0062] [Relational Expression 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 expressions 1 to 3, each element represents a weight content.)

[0063] The above relational expressions 1 and 2 are component relational expressions derived by quantifying the degree of contribution to the strengthening of the yield strength and tensile strength of the steel sheet by controlling the phase fraction of the microstructure constituting the steel sheet and improving the solid solution strengthening effect.

[0064] In the above relational expressions 1 and 2, the above C has a relatively large coefficient compared to the above Si and Mn. This is because the above C dissolves in the crystal grains of the steel sheet and greatly contributes to the improvement of strength. On the other hand, the above Si has a relatively small coefficient compared to the above C. This is because the effect of contributing to solid solution strengthening is smaller than that of the above C. Furthermore, although the above Al has a negative coefficient value, although it contributes to solid solution strengthening, the effect of leaving dual phase region ferrite during annealing or promoting ferrite transformation during subsequent cooling, leading to a decrease in strength, is greater. On the other hand, the above Cr and Mo are typical hardening ability elements, and have the effect of improving strength in order to suppress ferrite transformation during cooling after annealing, and are represented by positive values.

[0065] Since Ti and Nb are elements that form fine carbides and contribute to the improvement of strength, they can have positive coefficient values in the strength relational expression by component elements. However, when forming fine carbides, the amount of solid solution carbon decreases at the same time, and the solid solution strengthening effect of carbon decreases. Therefore, Ti and Nb can have positive coefficient values when the precipitation strengthening effect is dominant due to their addition, while they can be represented by negative coefficient values when the solid solution strengthening effect of carbon due to carbide precipitation is dominant.

[0066] The above relational expression 3 is a component relational expression derived by quantifying the degree of contribution to the improvement of the elongation rate of the steel sheet while improving the solid solution strengthening effect by specific elements.

[0067] Generally, considering that the elongation rate tends to decrease as the strength of the steel plate increases, the coefficients of the respective elements in the above relational expression 3 tend to be contrary to those in the above relational expressions 1 and 2.

[0068] Specifically, C and Mn above are advantageous for improving strength due to the solid solution strengthening effect. However, since such an increase in strength tends to decrease the elongation rate, they come to have negative coefficient values. On the other hand, since Al is effective in increasing the elongation rate, it has a positive coefficient value. In the case of Si, on the other hand, while contributing to improving the strength by solid solution strengthening, it also contributes to ensuring retained austenite. Therefore, it also has a positive coefficient value in relational expression 3.

[0069] If any one of the above relational expressions 1 to 3 proposed by the present invention is not satisfied, there is a problem that the physical properties of the steel plate, particularly, any one or more of the tensile strength, yield strength, and elongation rate are inferior. This will be clarified by being demonstrated from the examples described later.

[0070] The steel plate of the present invention having the above-described alloy component system contains a soft phase and a hard phase appropriately dispersed as a microstructure, and particularly has a ferrite with an area fraction of 3 to 20%, a retained austenite with an area fraction of 1 to 10%, a bainite with an area fraction of 1 to 30%, a tempered martensite with an area fraction of 30 to 70%, and the balance being fresh martensite.

[0071] The above ferrite is an allotrope of iron (Fe) having a body-centered cubic structure (BCC), and is a soft structure different from martensite and bainite. Therefore, it has an advantage of having a higher elongation rate and excellent impact absorption energy compared to the above bainite and martensite phases.

[0072] If the fraction of such ferrite exceeds 20%, a soft structure in the steel plate may be excessively formed and plastic deformation may be promoted, which causes a decrease in the yield strength of the steel plate. On the other hand, if the fraction of the above ferrite is less than 3%, there is a problem that the elongation rate of the steel plate decreases and the formability deteriorates.

[0073] Therefore, the ferrite can be included at an area fraction of 3 to 20%, more preferably 5 to 15%.

[0074] The retained austenite means an austenite structure that remains in the steel and cannot transform into martensite or bainite during a series of heat treatment processes (corresponding to the [annealing - cooling - reheating and holding] process in the present invention) during the manufacturing process of the steel sheet, and plays a role in adjusting the balance between the strength and elongation of the steel sheet.

[0075] Generally, when the strength of the steel sheet increases, the elongation decreases and the formability deteriorates, and when the elongation of the steel sheet increases, the strength decreases and it is difficult to ensure the physical properties required for a structural member. However, the retained austenite phase is useful for improving the balance between strength and elongation because it increases the value of the tensile strength (TS) × elongation (El) of the steel sheet.

[0076] In order to sufficiently obtain the above - mentioned effects, the retained austenite phase can contain an area fraction of 1% or more. However, if the fraction exceeds 10%, there is a problem that the sensitivity to liquid metal embrittlement increases and the spot weldability deteriorates.

[0077] Therefore, the retained austenite can be included at an area fraction of 1 to 10%, more preferably 3 to 9%.

[0078] The bainite in the steel can reduce the strength difference between the structures and contribute to the improvement of workability. That is, it plays a role in preventing cracks, defects, and fractures from occurring in the steel sheet due to the hardness difference between the relatively low - hardness ferrite and retained austenite phases and the relatively high - hardness tempered martensite and fresh martensite.

[0079] In order to fully obtain the above-described effects, it can be included at an area fraction of 1% or more, more preferably 5% or more. However, when the fraction exceeds 30%, the fraction of fresh martensite decreases and it is difficult to secure the strength at the target level.

[0080] Therefore, the above bainite can be included at an area fraction of 1 to 30%.

[0081] The above tempered martensite means a structure obtained by tempering the martensite phase obtained by quenching austenite at a temperature of about 500°C to soften it. Since such a tempered martensite phase has a higher strength than the above-described structures, it greatly contributes to the improvement of the yield strength and tensile strength of the steel sheet. In addition, carbon in the martensite obtained by quenching is distributed to the surrounding austenite during the tempering process, enhancing the thermal stability of the austenite so that it can remain at room temperature, thereby having the effect of improving the elongation rate of the steel sheet.

[0082] In order to fully obtain the above-described effects, it is preferable to include the above tempered martensite phase at an area fraction of 30% or more. However, when the fraction exceeds 70%, there is a problem that the fraction of the retained austenite phase relatively decreases.

[0083] Therefore, the above tempered martensite can be included at an area fraction of 30 to 70%.

[0084] As the remaining structure excluding the above ferrite, retained austenite, bainite and tempered martensite phases, it can include a fresh martensite phase.

[0085] The above-mentioned fresh martensite phase is a structure obtained during the process of final cooling to room temperature. Since it has the highest strength, it greatly contributes to the improvement of the yield strength and tensile strength of the steel plate. Although there is no particular limitation on the fraction of such a fresh martensite phase, for example, it is clarified that it can be included at an area fraction of 3% or more.

[0086] As described above, the steel plate of the present invention is characterized by excellent tensile strength, yield strength, and elongation rate due to the appropriate formation of the soft phase and the hard phase. Specifically, it can have a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation rate of 13% or more.

[0087] On the other hand, the steel plate of the present invention may be a cold-rolled steel plate, and may be a hot-dip galvanized steel plate including a zinc-based plating layer on at least one surface of the cold-rolled steel plate, or an alloyed hot-dip galvanized steel plate obtained by subjecting the hot-dip galvanized steel plate to an alloying treatment.

[0088] Although not particularly limited, the zinc-based plating layer may be a zinc plating layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc.

[0089] Hereinafter, a method for manufacturing a super high-strength steel plate with excellent ductility provided by the present invention, which is another aspect of the present invention, will be described in detail.

[0090] Briefly speaking, the present invention can manufacture a target steel plate through the steps of [steel slab reheating - hot rolling - coiling - cold rolling - continuous annealing - cooling - reheating and maintaining], and then, the steps of "hot-dip galvanizing - alloying heat treatment" can be further performed.

[0091] The conditions for each stage will be described in detail below.

[0092] [Steel slab heating] First, after preparing a steel slab that satisfies all of the above-described alloy component systems, it can be heated. This step is performed to smoothly carry out the subsequent hot rolling process and sufficiently obtain the physical properties of the target steel plate.

[0093] The above heating step can be performed in a temperature range of 1050 to 1300°C. If the above heating temperature is less than 1050°C, there is a problem that the friction between the steel plate and the rolling mill increases, and the load applied to the roller during hot rolling rapidly increases. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required for the temperature rise increase, but the amount of surface scale may increase, leading to material loss.

[0094] Therefore, the above heating step can be performed in a temperature range of 1050 to 1300°C, and more preferably in a temperature range of 1090 to 1250°C.

[0095] [Hot Rolling] The steel slab heated as described above can be hot rolled into a hot-rolled steel plate, and at this time, finish hot rolling can be performed in a temperature range of 800 to 1000°C.

[0096] By performing finish hot rolling in the above-described temperature range, the effect of simultaneously improving the rigidity and formability of the steel plate can be obtained. However, if the temperature is less than 800°C, rolling in the ferrite region causes an increase in friction between the steel plate and the rolling mill, and a large increase in the load due to rolling. This forms excessive dislocations and induces the formation of coarse crystal grains on the surface of the steel plate during subsequent coiling or cold rolling, resulting in a decrease in strength. On the other hand, if the temperature exceeds 1000°C, there is a problem that the size of the ferrite crystal grains increases, and the strength also decreases. Furthermore, scale may occur on the surface of the hot-rolled steel plate, inducing surface defects and shortening the life of the rolling rolls.

[0097] Therefore, the finish hot rolling during the above hot rolling can be carried out in the temperature range of 800 to 1000 °C, more preferably in the temperature range of 850 to 950 °C.

[0098] [Coiling] The hot-rolled steel sheet manufactured as described above can be coiled, and at this time, it can be carried out in the temperature range of 400 to 700 °C.

[0099] If the coiling temperature is less than 400 °C, the strength of the hot-rolled steel sheet becomes excessively high, which may induce a rolling load during subsequent cold rolling. In addition, the cost and time for cooling the hot-rolled steel sheet to the coiling temperature are excessively high, which causes an increase in the process cost. On the other hand, if the temperature exceeds 700 °C, scale is likely to excessively form on the surface of the hot-rolled steel sheet, inducing surface defects and causing weak plating properties.

[0100] Therefore, the above coiling process can be carried out in the temperature range of 400 to 700 °C, more preferably in the temperature range of 500 to 700 °C.

[0101] [Cooling] The coiled hot-rolled steel sheet can be cooled to room temperature. At this time, the cooling rate is not particularly limited, but it can be carried out by air cooling.

[0102] [Cold Rolling] Thereafter, the above hot-rolled steel sheet can be cold-rolled into a cold-rolled steel sheet, and at this time, it can be carried out with a cold rolling reduction rate of 20 to 70%.

[0103] If the cold rolling reduction rate is less than 20% during the above cold rolling, it is difficult to obtain a steel sheet with the target thickness, and there is a drawback that it is difficult to correct the shape of the steel sheet. On the other hand, if it exceeds 70%, cracks are likely to occur at the edge portion of the steel sheet, causing a problem of bringing a cold rolling load. Furthermore, there is a possibility that coarse ferrite is formed during subsequent continuous annealing on the surface of the steel sheet due to excessive load.

[0104] Therefore, the cold rolling can be carried out at a cold rolling reduction rate of 20 to 70%, more preferably at a cold rolling reduction rate of 30 to 60%.

[0105] On the other hand, before performing the cold rolling, a pickling treatment can be performed on the hot-rolled steel sheet. The pickling treatment is a process of removing the scale formed on the surface of the hot-rolled steel sheet using hydrochloric acid (HCl) or the like, and since it can be carried out under normal conditions, the conditions are not particularly limited.

[0106] [Annealing] The cold-rolled steel sheet manufactured as described above can be subjected to an annealing treatment. As an example, a Continuous Annealing Process can be carried out, but it is not limited thereto, and any known annealing method may be used.

[0107] In the present invention, the ferrite formed in the cold-rolled steel sheet by the annealing process can be recrystallized, and the fractions of ferrite and austenite in the steel can be adjusted. The strength of the steel sheet manufactured after the final heat treatment (referring to the reheating process described later) is determined by the fraction of each phase formed at this time. Generally, the higher the fraction of austenite, the more the fraction of martensite or bainite transformed from austenite increases, and the strength of the steel sheet tends to improve. However, the strength of the present invention can be further controlled by a series of heat treatment conditions described later.

[0108] Also, the carbon (C) in the steel can be distributed by the annealing process, thereby increasing the amount of carbon (C) contained in austenite, and it is possible to have an austenite phase of up to 10 area% even at room temperature.

[0109] The annealing process can be carried out in a temperature range of 800 to 900°C.

[0110] If the temperature during the annealing is less than 800°C, the fraction of austenite formed by the annealing process decreases, and the fractions of tempered martensite, bainite, and fresh martensite formed during the heat treatment described below may not be sufficient. This can cause a decrease in the yield strength and tensile strength of the final steel plate. On the other hand, if the temperature exceeds 900°C, the fraction of austenite in the steel plate becomes excessively high, and there is a problem that part of the austenite transforms into ferrite during the heat treatment process described below. Also, there is a possibility that the carbon enrichment of the retained austenite becomes low and the mechanical stability decreases. In this case, it causes a decrease in the elongation rate of the steel plate. Furthermore, the moisture generated while Fe in the steel oxidizes during the annealing process reacts with Si, Mn, and Al in the steel, increasing the possibility of forming an oxide film on the steel plate. The oxide film may inhibit the wettability of Zn during hot-dip galvanizing and may deteriorate the surface quality of the steel plate.

[0111] Therefore, the annealing process can be carried out in the temperature range of 800 to 900°C, more preferably in the temperature range of 820 to 870°C.

[0112] [Cooling] The cold-rolled steel plate that has completed the annealing process as described above can be cooled.

[0113] The present invention can form quenched martensite by cooling the annealed cold-rolled steel plate described above. For this purpose, the cooling is preferably carried out below the martensite transformation start temperature (Ms). More preferably, it can be carried out down to the temperature range of 250 to 400°C.

[0114] During the cooling, the lower the temperature, the higher the fraction of quenched martensite, and it can induce an improvement in the strength of the steel plate. Also, the carbon supersaturated in the martensite is distributed to the surrounding austenite during the subsequent heat treatment process, enhancing the stability of the retained austenite. As a result, an improvement in the elongation rate can be achieved.

[0115] However, if the above cooling temperature is less than 250°C, there is a problem that the fraction of quenched martensite increases excessively, rather the fraction of retained austenite decreases, and the shape of the steel sheet deteriorates. On the other hand, if the temperature exceeds 400°C, quenched martensite is not sufficiently formed, and it becomes difficult to expect the above-described effects.

[0116] When cooling to the above-described temperature range, it can be carried out at an average cooling rate of 2 to 50°C / s. If the rate during the above cooling is less than 2°C / s, ferrite further transforms during cooling, inducing a decrease in strength. On the other hand, if the rate exceeds 50°C / s and rapid cooling occurs, there is a problem that the shape of the steel sheet deteriorates due to the occurrence of uneven cooling by position of the steel sheet. When performing cooling at the above-described cooling rate, the cooling method is not particularly limited. As an example, the above cooling may be a single cooling method of cooling to the cooling end temperature at the initially set cooling rate. As another example, it may be a step-by-step cooling method in which slow cooling is performed up to a certain section and then rapid cooling is performed up to the cooling end temperature, but it is clarified that it is not limited thereto.

[0117] On the other hand, it is possible to go through a process of maintaining at the above-cooled temperature for a certain period of time. In this process, an isothermal transformation phase is further introduced, and the effect of promoting the transformation of bainite can be obtained in subsequent processes. For this purpose, the above maintenance process can be carried out for 0.1 to 60 minutes.

[0118] [Reheating and Maintenance] The above-cooled cold-rolled steel sheet, and further, the cold-rolled steel sheet cooled and maintained can be subjected to tempering treatment by reheating it to a temperature range about 50 to 200°C higher than the above cooling temperature and then maintaining it for a certain period of time.

[0119] By reheating the above-mentioned cooled cold-rolled steel sheet, the quenched martensite phase formed in the above cooling process is tempered and transformed into tempered martensite. The above-mentioned tempered martensite has the advantage that carbon is fixed to dislocations and the yield strength is high. Also, during the above tempering process, carbon (C) supersaturated in the quenched martensite is redistributed to the surrounding austenite, or bainite transformation is induced to improve the stability of the retained austenite, and the effect of improving the elongation rate can be obtained.

[0120] The fixation of the above dislocations and the carbon distribution to austenite occur more smoothly as the tempering temperature is higher. Therefore, it is necessary to reheat at a temperature 50°C or more higher than the above cooling temperature (cooled temperature + 50°C or more). However, if the temperature is excessively high, cementite is generated in the quenched martensite, coarsens, the strength of the steel sheet decreases, the carbon redistribution effect to austenite decreases, and it becomes difficult to expect an improvement in the elongation rate. Considering this, the above reheating can be restricted to be carried out at a temperature of the above cooled temperature + 200°C or less.

[0121] After reheating the cold-rolled steel sheet cooled in the above-mentioned temperature range, it is preferable to maintain it at that temperature for 0.1 to 60 minutes to fully realize the above-mentioned effects.

[0122] When maintaining, if the time is excessive and exceeds 60 minutes, there is a problem that ferrite and cementite, which are equilibrium phases at the maintenance temperature, are formed and the strength of the steel sheet decreases. If it is less than 0.1 minute, the intended effect cannot be obtained.

[0123] After completing the steps of reheating and maintaining the cold-rolled steel sheet cooled as described above, it can be cooled to room temperature under normal conditions, and finally, a steel sheet having a structure in which a certain fraction of soft phase and hard phase are appropriately distributed can be obtained.

[0124] Specifically, a steel sheet having a microstructure composed of ferrite with an area fraction of 3 to 20%, retained austenite with an area fraction of 1 to 10%, bainite with an area fraction of 1 to 30%, tempered martensite with an area fraction of 30 to 70%, and the balance being fresh martensite can be obtained. Such a steel sheet of the present invention has excellent yield strength and tensile strength and can have the effect of improved ductility.

[0125] The process of cooling to the above room temperature is not particularly limited. As an example, it can be carried out by air cooling. However, it is obvious that it can be replaced by known cooling methods such as water cooling, oil cooling, and furnace cooling.

[0126] On the other hand, by plating the cold-rolled steel sheet that has completed the above series of heat treatment steps as described below, a plated steel sheet having a plating layer on at least one surface can be manufactured.

[0127] [Hot-dip galvanizing] The steel sheet manufactured through the above-described series of steps can be immersed in a hot-dip zinc-based plating bath to manufacture a hot-dip galvanized steel sheet.

[0128] At this time, the hot-dip galvanizing can be carried out under normal conditions. As an example, it can be carried out in a temperature range of 430 to 490°C. Further, regarding the composition of the hot-dip zinc-based plating bath during the above hot-dip galvanizing, it is not particularly limited, and it may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0129] [Alloying heat treatment] If necessary, by subjecting the above hot-dip galvanized steel sheet to an alloying heat treatment, an alloyed hot-dip galvanized steel sheet can be obtained.

[0130] In the present invention, the conditions of the above alloying heat treatment step are not particularly limited, and normal conditions may be used. As an example, the alloying heat treatment step can be carried out in a temperature range of 480 to 600°C.

[0131] Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are for illustrating the present invention in more detail and are not intended to limit the scope of the rights of the present invention. The scope of the rights of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

Example

[0132] (Example) Thirty kilograms of slabs having the alloy compositions shown in Table 1 below were heated at a temperature of 1200°C for 1 hour, and then the heated slabs were finish hot-rolled at 900°C to produce hot-rolled steel sheets. Thereafter, each hot-rolled steel sheet was charged into a furnace preheated to 600°C and maintained for 1 hour, and then simulated hot-rolled coiling with furnace cooling was performed. Thereafter, after cooling to room temperature (air cooling), cold rolling was performed at a cold reduction rate of 45% to produce cold-rolled steel sheets.

[0133] For each of the cold-rolled steel sheets produced as described above, after continuous annealing treatment at the temperature T1 (°C) shown in Table 2 below for 1 minute, cooling to the temperature T2 (°C) and maintaining for 10 seconds, reheating to the temperature T3 (°C) and maintaining for 1 minute, and then cooling to room temperature (air cooling) to produce final steel sheets. After the annealing treatment, the cooling to the temperature T2 was uniformly performed at a cooling rate of 15°C / s.

[0134] For each of the steel sheets produced through all the steps described above, mechanical properties and internal structures were measured, and the results are shown in Table 3 below.

[0135] As the above mechanical properties, the yield strength (YS), tensile strength (TS), and elongation rate (El) were measured using an ASTM tensile test piece and a universal tensile testing machine.

[0136] For the above internal structure, after polishing the test piece and then nital etching, the area of each phase was calculated using a scanning electron microscope (SEM).

[0137]

Table 1

[0138] [Table 2] (In Table 2, Steels 9, 10, and 11 are classified as comparative examples because their alloy component systems deviate from the present invention.)

[0139] [Table 3]

[0140] As shown in Tables 1 to 3 above, Invention Examples 1 to 11 that satisfy all of the alloy component systems and manufacturing conditions proposed in the present invention formed the intended tissue structure, thereby ensuring the target physical properties.

[0141] On the other hand, it can be seen that in Comparative Examples 1 and 2 that do not satisfy at least one of Relational Expressions 1 and 2 of the relational expressions of the component relational expressions proposed in the present invention, one or more physical properties of the yield strength and the tensile strength are not ensured at the target level. Further, it can be confirmed that in Comparative Example 7 that does not satisfy Relational Expression 3 among the relational expressions, the elongation rate is significantly inferior.

[0142] Thereby, it was proved that Relational Expression 1 characterized in the present invention contributes to the fraction of the fine structure of the steel sheet and the strengthening of the yield strength by the solid solution strengthening effect, Relational Expression 2 contributes to the improvement of the tensile strength of the steel sheet, and Relational Expression 3 contributes to the improvement of the ductility of the steel sheet.

[0143] That is, when Relational Expressions 1 and 2 of the present invention are not satisfied, the strength of the steel sheet is inferior, and when Relational Expression 3 is not satisfied, the ductility of the steel sheet is inferior.

[0144] On the other hand, although the alloy component system proposed in the present invention is satisfied, in Comparative Examples 3 to 6 in which the heat treatment conditions deviate from the present invention, the soft phase and the hard phase are not appropriately formed as intended. As a result, it was not possible to ensure the coexistence of excellent strength and ductility in all examples.

[0145] FIG. 1 shows the micrograph of Invention Example 1, and it can be confirmed that ferrite, retained austenite, tempered martensite, and bainite are formed within the range of the target fraction, and a fresh martensite phase is formed as the remaining structure of the other part.

[0146] FIG. 2 shows the micrograph of Comparative Example 6, and it can be confirmed that the tempered martensite phase is not formed at the target fraction, the retained austenite phase cannot be sufficiently ensured, and the fraction of the fresh martensite phase is formed relatively high.

Claims

1. By weight, carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, manganese (Mn): 2.0 to 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, consisting of the balance of Fe and other inevitable impurities, satisfying the following relational expressions 1 to 3, As the microstructure, it consists of ferrite with an area fraction of 3 to 20% and the balance of hard phases (including retained austenite, bainite, and tempered martensite), An ultra-high strength steel sheet with excellent ductility, having a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 13% or more. [Relational Expression 1] 1110[C] + 41.5[Si] + 575[Mn] - 1092[Al] - 3590[Nb] - 5181[Ti] + 258[Cr] + 664[Mo] ≧ 1380 [Relational Expression 2] 2853[C] + 95[Si] + 309[Mn] - 153[Al] + 4661[Nb] - 780[Ti] + 210[Cr] + 457[Mo] ≧ 1300 [Relational Expression 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 Expressions 1 to 3, each element means the weight content.)

2. The hard phase of the steel sheet consists of retained austenite with an area fraction of 1 to 10%, bainite with an area fraction of 1 to 30%, tempered martensite with an area fraction of 30 to 70%, and the balance of fresh martensite, the ultra-high strength steel sheet with excellent ductility according to Claim 1.

3. The steel sheet contains a fresh martensite phase with an area fraction of 3% or more, the ultra-high strength steel sheet with excellent ductility according to Claim 1.

4. The steel sheet is any one of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet, the ultra-high strength steel sheet with excellent ductility according to Claim 1.

5. By weight, carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, manganese (Mn): 2.0 to 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, consisting of the balance Fe and other inevitable impurities, and preparing a steel slab satisfying the following relational expressions 1 to 3; heating the steel slab in a temperature range of 1050 to 1300°C; performing finish hot rolling on the heated steel slab in a temperature range of 800 to 1000°C to produce a hot-rolled steel sheet; coiling the hot-rolled steel sheet in a temperature range of 400 to 700°C; performing cold rolling on the coiled hot-rolled steel sheet with a total reduction rate of 20 to 70% to produce a cold-rolled steel sheet; annealing the cold-rolled steel sheet in a temperature range of 800 to 900°C; cooling the annealed cold-rolled steel sheet in a temperature range of 250 to 400°C; including reheating and maintaining the cooled cold-rolled steel sheet; The reheating and maintaining step is performed for 0.1 to 60 minutes in a temperature range of the cooled temperature + 50°C or more to the cooled temperature + 200°C or less. A method for manufacturing an ultra-high strength steel sheet with excellent ductility according to claim 1. [Relational Expression 1] 1110[C] + 41.5[Si] + 575[Mn] - 1092[Al] - 3590[Nb] - 5181[Ti] + 258[Cr] + 664[Mo] ≧ 1380 [Relational Expression 2] 2853[C] + 95[Si] + 309[Mn] - 153[Al] + 4661[Nb] - 780[Ti] + 210[Cr] + 457[Mo] ≧ 1300 [Relational Expression 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 Expressions 1 to 3, each element means a weight content.)

6. The cooling of the cold-rolled steel sheet is performed at a cooling rate of 2 to 50°C / s. A method for manufacturing an ultra-high strength steel sheet with excellent ductility according to claim 5.

7. The method for manufacturing an ultra-high strength steel sheet with excellent ductility according to claim 5 further includes a step of maintaining for 0.1 to 60 minutes in the temperature range of the cooled temperature before reheating the cooled cold-rolled steel sheet.

8. The method for manufacturing a high-strength steel sheet with excellent ductility according to claim 5, further comprising a step of performing hot-dip galvanizing after the reheating and maintaining steps.

9. The method for manufacturing a high-strength steel sheet with excellent ductility according to claim 8, further comprising a step of performing alloying heat treatment after the hot-dip galvanizing step.

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