Ultrahigh-strength steel sheet and method for producing same

Abstract

The present invention provides an ultra-high strength steel sheet comprising, in wt%, 0.1% to 0.30% of carbon (C), 1.0% to 2.0% of silicon (Si), 1.5% to 3.0% of manganese (Mn), 0% (excluding) to 0.05% of aluminum (Al), 0% (excluding) to 0.05% of a sum of one or more selected from niobium (Nb), titanium (Ti), and vanadium (V), 0% (excluding) to 0.02% of phosphorus (P), 0% (excluding) to 0.005% of sulfur (S), 0% (excluding) to 0.006% of nitrogen (N), and the balance of steel. And the balance of iron (Fe) and unavoidable impurities, and having a final microstructure consisting of tempered martensite, martensite, pearlite, and retained austenite, having a yield point (YP) of 850 MPa or more, a tensile strength (TS) of 1180 MPa or more, and an elongation index (El) of 14% or more.

Description

Technical Field

[0001] The present invention relates to a steel plate and a method for manufacturing the same, and more particularly to an ultra-high strength steel plate with excellent elongation and a method for manufacturing the same. Background Art

[0002] The focus of automotive steel sheet development is to improve strength to achieve user safety and vehicle weight reduction, as well as to ensure elongation to achieve processability. At present, commonly used ultra-high strength steels include dual-phase steels (which are composed of ferrite and martensite phases to ensure elongation) and transformation induced plasticity (TRIP) steels (which achieve strength and elongation through the phase transformation of residual austenite in the final structure during plastic deformation). However, the development based on dual-phase steels (which may not be able to overcome the limitations of the law of mixtures) and TRIP steels (which exhibit relatively low strength due to the main matrix being bainite) has reached its limit. Therefore, the development of the next generation of ultra-high strength automotive steel sheets that can ensure ultra-high strength and high formability by improving the microstructure of TRIP steels is attracting the attention of steel manufacturers.

[0003] Related art includes Japanese Patent Laid-Open No. 2005-105367. Summary of the invention

[0004] Technical issues

[0005] The present invention provides an ultra-high strength steel plate with excellent elongation and a method for manufacturing the same.

[0006] Technical Solution

[0007] According to one aspect of the present invention, there is provided an ultra-high strength steel plate, the ultra-high strength steel plate comprising: carbon (C): 0.1 wt% to 0.30 wt%, silicon (Si): 1.0 wt% to 2.0 wt%, manganese (Mn): 1.5 wt% to 3.0 wt%, aluminum (Al): greater than 0 wt% and not greater than 0.05 wt%, the sum of one or more elements selected from niobium (Nb), titanium (Ti) and vanadium (V): greater than 0 wt% and not greater than 0.05 wt%, phosphorus (P): greater than 0 wt% and not greater than % and not more than 0.006 wt %, and the balance is iron (Fe) and other inevitable impurities, wherein the final microstructure of the ultra-high strength steel plate consists of tempered martensite, martensite, ferrite and retained austenite, and the ultra-high strength steel plate has a yield strength (YP) of 850 MPa or more, a tensile strength (TS) of 1180 MPa or more, and an elongation (El) of 14% or more.

[0008] The final microstructure may have a martensitic matrix instead of bainite.

[0009] Ferrite may occupy 10% to 20% of the area fraction, retained austenite may occupy 10% to 20% of the area fraction, and tempered martensite and martensite may occupy the remaining area fraction.

[0010] Ferrite may contain carbides, and the density of carbides with an average size of 50 nm or more is 20 particles / μm 2 or smaller.

[0011] The product of TS and El may be no less than 15000 MPa·%.

[0012] According to another aspect of the present invention, there is provided a method for manufacturing an ultra-high strength steel plate, the method comprising producing a hot-rolled steel plate, the hot-rolled steel plate comprising: carbon (C): 0.1 wt% to 0.30 wt%, silicon (Si): 1.0 wt% to 2.0 wt%, manganese (Mn): 1.5 wt% to 3.0 wt%, aluminum (Al): greater than 0 wt% and not greater than 0.05 wt%, the sum of one or more elements selected from niobium (Nb), titanium (Ti) and vanadium (V): greater than 0 wt% and not greater than 0.05 wt%, phosphorus (P): greater than 0 wt% and not greater than 0.02 wt%. %, sulfur (S): greater than 0 weight % and not more than 0.005 weight %, nitrogen (N): greater than 0 weight % and not more than 0.006 weight %, and the balance of iron (Fe) and other inevitable impurities; softening the hot rolled steel sheet by continuously maintaining a temperature of 500°C to 650°C; producing a cold rolled steel sheet by cold rolling the softened steel sheet; annealing the cold rolled steel sheet at a temperature of 820°C to 860°C; cooling the cold rolled steel sheet in multiple stages to a temperature of 200°C to 260°C; and partitioning the cooled cold rolled steel sheet at a temperature of 370°C to 460°C.

[0013] The production of the hot rolled steel plate may include preparing a steel billet having the above-mentioned alloy composition; reheating the steel billet at 1150°C to 1250°C; producing the hot rolled steel plate by hot rolling the reheated steel billet at a finishing outlet temperature (FDT) of 850°C to 1000°C; and coiling the hot rolled steel plate at 500°C to 700°C.

[0014] Annealing of the cold rolled steel sheet may be performed by heating the cold rolled steel sheet at a heating rate of 1° C. / s to 10° C. / s and maintaining a temperature of 820° C. to 860° C. for 40 seconds to 120 seconds.

[0015] The multi-stage cooling of the cold rolled steel plate may include a primary cooling step of slowly cooling the cold rolled steel plate to a final temperature of 550°C to 750°C at a cooling rate of 1°C / s to 10°C / s; and a secondary cooling step of rapidly cooling the cold rolled steel plate to a final temperature of 200°C to 260°C at a cooling rate of 50°C / s or more.

[0016] Partitioning may be performed by heating the cooled cold-rolled steel sheet at a heating rate of 20° C. / s or more, and maintaining a temperature of 370° C. to 460° C. for 10 seconds to 240 seconds.

[0017] Beneficial effects

[0018] According to the embodiments of the present invention, an ultra-high strength steel sheet having excellent elongation and a method for manufacturing the same can be achieved.

[0019] However, the scope of the present invention is not limited to the above-mentioned effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 : is a microscopic image showing the final microstructure of an ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention.

[0021] Figure 2 1 is a microscopic image showing the carbide density in ferrite in the final microstructure of an ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention.

[0022] Figure 3 4 is a flow chart of a method for manufacturing an ultra-high strength steel plate having excellent elongation according to an embodiment of the present invention. DETAILED DESCRIPTION

[0023] An ultra-high strength steel sheet having excellent elongation and a method for manufacturing the same according to an embodiment of the present invention will now be described in detail. Terms used herein are selected based on their functions in the present invention, and their definitions should be combined with the context of the entire specification.

[0024] The focus of automotive steel sheet development is to improve strength to achieve user safety and vehicle weight reduction, as well as to ensure elongation to achieve processability. At present, commonly used ultra-high strength steels include dual-phase steels (which are composed of ferrite and martensite phases to ensure elongation) and transformation induced plasticity (TRIP) steels (which achieve strength and elongation through the phase transformation of residual austenite in the final structure during plastic deformation). However, the development based on dual-phase steels (which may not be able to overcome the limitations of the law of mixtures) and TRIP steels (which exhibit relatively low strength due to the main matrix being bainite) has reached its limit. Therefore, the development of the next generation of ultra-high strength automotive steel sheets that can ensure ultra-high strength and high formability by improving the microstructure of TRIP steels is attracting the attention of steel manufacturers.

[0025] The present invention provides a next generation ultra-high strength automotive coated steel sheet which can ensure high strength and suitable elongation by using martensite instead of bainite matrix in TRIP steel, thereby overcoming the mechanical property limitations of existing TRIP steel.

[0026] An ultra high strength steel sheet having excellent elongation according to an embodiment of the present invention will now be described in detail.

[0027] Steel Plate

[0028] An ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention includes a steel sheet composed of carbon (C): 0.1 wt% to 0.30 wt%, silicon (Si): 1.0 wt% to 2.0 wt%, manganese (Mn): 1.5 wt% to 3.0 wt%, aluminum (Al): greater than 0 wt% and not greater than 0.05 wt%, the sum of one or more elements selected from niobium (Nb), titanium (Ti) and vanadium (V): greater than 0 wt% and not greater than 0.05 wt%, phosphorus (P): greater than 0 wt% and not greater than 0.02 wt%, sulfur (S): greater than 0 wt% and not greater than 0.005 wt%, nitrogen (N): greater than 0 wt% and not greater than 0.006 wt%, and the balance of iron (Fe) and other inevitable impurities. The functions and contents of the components contained in the steel sheet will now be described.

[0029] Carbon (C)

[0030] C is the most important alloying element in steelmaking, and is mainly intended for basic strengthening and austenite stabilization in the present invention. A high concentration of C in austenite can improve the stability of austenite, so it is easy to ensure appropriate austenite for improving material properties. When the content of C is less than 0.1% by weight of the total weight, the above effect may not be achieved, and sufficient strength may not be ensured. On the other hand, when the content of C is greater than 0.3% by weight of the total weight, the carbon equivalent may increase, resulting in reduced weldability and reduced machinability.

[0031] Silicon (Si)

[0032] Si is an element used to inhibit the formation of carbides, in particular to prevent the deterioration of material properties caused by the formation of Fe3C. Si is a well-known ferrite stabilizing element, and thus can improve ductility by increasing the fraction of ferrite during cooling. Si is also known to promote the formation of martensite by carbon enrichment in austenite, thereby ensuring strength. At the same time, Si can be added together with Al as a deoxidizer to remove oxygen from steel in the steelmaking process, and has a solid solution strengthening effect.

[0033] Si can be added in a content ratio of 1.0 wt % to 2.0 wt % of the total weight of the steel plate for forming the ultra-high strength steel plate with excellent elongation according to an embodiment of the present invention. When the content of Si is less than 1.0 wt % of the total weight, ductility may not be ensured, and the above-mentioned effect of Si addition may not be properly achieved. On the other hand, when the content of Si is greater than 2.0 wt % of the total weight due to excessive addition, oxides (SiO2) may be formed on the surface of the steel plate, thereby deteriorating wettability and reducing coating performance, red oxide scale may be formed during reheating and hot rolling, thereby reducing surface quality, toughness and plasticity may be reduced, and the weldability of the steel may also be reduced.

[0034] Manganese (Mn)

[0035] Mn is a main element for stabilizing austenite. When Mn is added, the austenite start temperature (Ms) may gradually decrease, so the fraction of retained austenite may increase in the continuous annealing process. Mn is also an element for promoting low temperature transformation and improving strength by solid solution strengthening. Mn reduces the acid resistance and oxidation resistance of steel, but increases the yield strength by refining pearlite and solid solution strengthening ferrite.

[0036] Mn may be added in a content ratio of 1.5 wt % to 3.0 wt % of the total weight of the steel plate for forming the ultra-high strength steel plate with excellent elongation according to an embodiment of the present invention. When the content of Mn is less than 1.5 wt %, the above-mentioned strength improvement effect may not be fully achieved. When the content of Mn is greater than 3.0 wt %, the carbon equivalent may increase, thereby reducing weldability, oxides (MnO) may be formed on the surface of the steel plate during processing, thereby deteriorating weldability and reducing coating performance, and segregation areas may be formed inside and outside the continuously cast steel billet and steel plate, and may cause the formation and expansion of cracks, thereby reducing bendability.

[0037] Aluminum (Al)

[0038] Al functions similarly to Si, and is mainly used to strengthen the solid solution and suppress the formation of carbides. Al is an element generally used as a deoxidizer, and promotes the formation of ferrite, improves elongation, and stabilizes austenite by increasing the concentration of C in austenite. Al serves as a layer between Fe and zinc (Zn) coatings, thereby improving coating properties, and effectively suppresses the formation of Mn bands in hot rolled coils.

[0039] Al may be added at a content ratio of more than 0 wt% and not more than 0.05 wt% of the total weight of the steel sheet for forming the ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention. When the content of Al is more than 0.05 wt% due to excessive addition, Al inclusions may increase, thereby reducing continuous castability, Al enrichment may occur on the surface of the steel sheet, thereby deteriorating coating properties, and AlN may be formed in the slab, thereby causing hot rolling cracks.

[0040] One or more elements selected from niobium (Nb), vanadium (V) and titanium (Ti)

[0041] Nb, V and / or Ti are main elements precipitated in the form of carbides in steel, and in the present invention, they are intended to ensure the stability of retained austenite and improve the strength by forming precipitates to refine the initial austenite grains, and to achieve precipitation hardening through ferrite grain refinement and the presence of precipitates in ferrite.

[0042] The sum of one or more elements selected from Nb, V and Ti may be added in a content ratio of greater than 0 wt% and not greater than 0.05 wt% of the total weight of the steel sheet for forming the ultra-high strength steel sheet with excellent elongation according to an embodiment of the present invention. When the content of the sum of one or more elements selected from Nb, V and Ti is greater than 0.05 wt%, deterioration of material properties and increase in production costs may occur, grain coarsening may result in the formation of coarse precipitates, and the recrystallization temperature may be excessively increased, resulting in an uneven structure.

[0043] Phosphorus (P)

[0044] The function of P is similar to that of Si, and can be used to improve strength by solid solution strengthening and inhibit the formation of carbides. P can be added in a content ratio of greater than 0 wt % and not greater than 0.02 wt % of the total weight of a steel plate for forming an ultra-high strength steel plate with excellent elongation according to an embodiment of the present invention. When the content of P is greater than 0.02 wt %, problems such as embrittlement of welded joints, brittleness, reduced press formability and reduced impact resistance may occur. Therefore, in the present invention, it is necessary to control the content of P to be as low as possible.

[0045] Sulfur (S)

[0046] S is an element that combines with Mn or Ti to improve the machinability of steel and forms fine MnS precipitates to improve machinability, but generally inhibits ductility and weldability. S can be added in a content ratio of greater than 0 wt% and 0.005 wt% of the total weight of a steel plate for forming an ultra-high strength steel plate with excellent elongation according to an embodiment of the present invention. When the content of S is greater than 0.005 wt%, the number of MnS inclusions may increase, thereby reducing machinability, and segregation may occur during continuous casting solidification, resulting in high temperature cracks. Therefore, in the present invention, it is necessary to control the content of S to be as low as possible.

[0047] Nitrogen (N)

[0048] N is a solid solution strengthening element that can improve the strength of the steel plate and is generally introduced from the atmosphere. In the steelmaking process, the N content needs to be controlled by a degassing process. When the N content is greater than 0.006 wt%, problems such as weld embrittlement, low temperature brittleness, reduced press formability, and reduced impact resistance may occur. Therefore, in the present invention, the N content needs to be controlled to be as low as possible.

[0049] In the ultra-high strength steel sheet with excellent elongation having the above alloy element composition according to the embodiment of the present invention, the steel sheet may have a yield strength (YP) of 850MPa or more, a tensile strength (TS) of 1180MPa or more, and an elongation (El) of 14% or more. For example, the steel sheet may have a YP of 850MPa to 1070MPa, a TS of 1180MPa to 1250MPa, and an El of 14% to 20%. In addition, the product of TS and El may be not less than 15000MPa%. In the present invention, the product of TS and El is 15000MPa% or more, which is more than twice the value of 7000MPa% (generally indicating the product of TS and total El at similar strength levels), so it can be understood that at the same strength, more excellent formability can be achieved compared to existing ultra-high strength steels.

[0050] Figure 1 To show the final microstructure of the ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention, and Figure 2 1 is a microscopic image showing the carbide density in ferrite in the final microstructure of an ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention.

[0051] refer to Figure 1 and Figure 2 , the final microstructure of the steel plate consists of tempered martensite, martensite, ferrite and retained austenite. The final microstructure has a martensitic matrix instead of bainite. Ferrite may account for 10% to 20% of the area fraction, retained austenite may account for 10% to 20% of the area fraction, and tempered martensite and martensite may account for the remaining area fraction. In addition, ferrite may contain carbides, and the density of carbides with an average size of 50nm or more is 20 particles / μm 2 or smaller.

[0052] A method of manufacturing an ultra-high strength steel sheet having excellent elongation having the above-described composition, properties, and microstructure according to an embodiment of the present invention will now be described.

[0053] Method for manufacturing steel plate

[0054] Figure 3 4 is a flow chart of a method for manufacturing an ultra-high strength steel plate having excellent elongation according to an embodiment of the present invention.

[0055] refer to Figure 3, a method for manufacturing a steel sheet according to an embodiment of the present invention includes producing a hot-rolled steel sheet, wherein the hot-rolled steel sheet comprises: C: 0.1 wt% to 0.30 wt%, Si: 1.0 wt% to 2.0 wt%, Mn: 1.5 wt% to 3.0 wt%, Al: greater than 0 wt% and not greater than 0.05 wt%, the sum of one or more elements selected from Nb, Ti and V: greater than 0 wt% and not greater than 0.05 wt%, P: greater than 0 wt% and not greater than 0.02 wt%, S: greater than 0 wt% and not greater than 0.005 wt%, N: greater than 0 wt% and Not more than 0.006 weight %, and the balance is Fe and other inevitable impurities (S100); softening the hot rolled steel sheet by continuously maintaining a temperature of 500°C to 650°C (S200); producing a cold rolled steel sheet by cold rolling the softened steel sheet (S300); annealing the cold rolled steel sheet at a temperature of 820°C to 860°C (S400); cooling the cold rolled steel sheet to a temperature of 200°C to 260°C in multiple stages (S500); and partitioning the cooled cold rolled steel sheet at a temperature of 370°C to 460°C (S600).

[0056] The step (S100) of producing a hot-rolled steel plate may include preparing a steel billet having the above-mentioned alloy composition; reheating the steel billet at 1150°C to 1250°C; producing a hot-rolled steel plate by hot rolling the reheated steel billet at a finishing outlet temperature (FDT) of 850°C to 1000°C; and coiling the hot-rolled steel plate at 500°C to 700°C.

[0057] The step (S100) of producing a hot-rolled steel sheet may include reheating the steel material at 1150°C to 1250°C. The steel slab having the composition of the present invention is reheated to a temperature of Ac3 or higher to redissolve the cast components. When the reheating temperature is low, the hot rolling load may increase, and when the reheating temperature is high, the slab may warp, making it difficult to insert into the furnace or to remove from the furnace. When the steel material is reheated at the above temperature, the components segregated during the continuous casting process may be redissolved. In order to increase the strength by precipitation and solid solution strengthening, the strengthening elements need to be fully dissolved in austenite before hot rolling, so the steel material needs to be heated to 1150°C or higher. When the reheating temperature is lower than 1150°C, the hot rolling load may increase, various carbides may not be fully dissolved, and the segregated components may not be evenly distributed during the continuous casting process.

[0058] However, a reheating temperature higher than 1250°C may cause adverse effects such as austenite coarsening and decarburization, and the desired strength may not be achieved. That is, when the reheating temperature is higher than 1250°C, very coarse austenite grains may be formed, and thus strength may not be easily ensured. In addition, when the reheating temperature is higher than 1250°C, the slab may be warped, making it difficult to insert into or take out of the furnace, and the heating cost and process time may increase, thereby increasing production costs and reducing productivity.

[0059] The step (S100) of producing a hot-rolled steel plate may include hot rolling under the conditions of a FDT of 850°C to 1000°C, a cooling rate of 10°C / s to 30°C / s, and a coiling temperature (CT) of 500°C to 700°C. Since high alloy steel needs to minimize edge cracking and rolling load to ensure productivity, FDT and CT may be set within a high temperature range. FDT is an important factor affecting the final material properties, and rolling at 850°C to 1000°C may refine austenite.

[0060] However, when the hot rolling temperature is lower than 850°C, the rolling load may increase and a mixed grain structure may appear at the edge. Rolling at a temperature higher than 1000°C may cause grain coarsening and fail to achieve the desired mechanical properties. After hot rolling, cooling is performed at a cooling rate of 10°C / s to 30°C / s. The greater the cooling rate, the smaller the average grain size.

[0061] Meanwhile, when CT is lower than 500°C, the hot rolled coil may have an uneven shape and the cold rolling load may increase. When CT is higher than 700°C, an uneven microstructure may occur due to the difference in cooling rate between the center and edge of the steel plate, and the inside of the grain boundary may be oxidized.

[0062] In the ultra-high strength steel sheet manufacturing method, the step of annealing the cold rolled steel sheet may be performed by heating the cold rolled steel sheet at a heating rate of 1 to 10° C. / s and maintaining a temperature of 820 to 860° C. for 40 to 120 seconds.

[0063] In the method for manufacturing an ultra-high strength steel plate, the step of performing multi-stage cooling on the cold-rolled steel plate may include a primary cooling step of slowly cooling the cold-rolled steel plate to a final temperature of 550°C to 750°C at a cooling rate of 1°C / s to 10°C / s; and a secondary cooling step of rapidly cooling the cold-rolled steel plate to a final temperature of 200°C to 260°C at a cooling rate of 50°C / s or more.

[0064] In the ultra-high strength steel sheet manufacturing method, the partitioning may be performed by heating the cooled cold-rolled steel sheet at a heating rate of 20° C. / s or more and maintaining a temperature of 370° C. to 460° C. for 10 to 240 seconds.

[0065] After producing the hot rolled steel sheet ( S100 ) and before producing the cold rolled steel sheet by cold rolling the softened steel sheet ( S300 ), the hot rolled steel sheet may be softened by continuously maintaining a temperature of 500° C. to 650° C. ( S200 ).

[0066] The composition system used in the present invention corresponds to a high alloy steel with a high content of alloying elements (e.g., Mn), so a delay in material hardening occurs. When cooled to room temperature after hot rolling, a hard phase of martensite is formed, rather than soft phases such as ferrite and pearlite, so the strength of the hot-rolled material is very high. Therefore, softening is performed after hot rolling. Since problems such as thickness fluctuations and shape defects may occur during cold rolling when the strength of the hot-rolled sheet is high, in order to effectively cold roll, cold rolling properties can be ensured by softening to reduce the hardness of the hot-rolled material.

[0067] Table 1 shows the specific process conditions of the softening step (S200). Referring to Table 1, the softening step (S200) may include, for example, heating to 500°C to 650°C at a heating rate of 30°C / hr to 80°C / hr, maintaining for 2 hours to 16 hours, and then cooling to room temperature at a cooling rate of 30°C / hr to 60°C / hr.

[0068] [Table 1]

[0069]

[0070] For softening, heat treatment is performed for a sufficient time within a temperature range of 500°C to 650°C. When the softening temperature is lower than 500°C, the martensite formed after hot rolling is not recrystallized, but only tempered, so that supersaturated C is formed in the microstructure and cementite (θ) in the microstructure is spheroidized. In this case, embrittlement of martensite may occur, resulting in safety accidents such as plate breakage during cold rolling. When the softening temperature is higher than 650°C, excessive formation of martensite may occur during softening, and martensite may be formed during cooling, thereby preventing effective reduction of strength. Therefore, by controlling the temperature range to 500°C to 650°C, the strength of the hot-rolled material can be reduced. A step (S300) of producing a cold-rolled steel sheet by cold rolling the softened steel sheet is performed to reduce the hot-rolled material to the final thickness of the steel sheet. Before cold rolling, pickling may be performed. The reduction rate in the cold rolling process can be controlled according to the final product specifications, for example, controlled to 40% to 60%. Higher reduction ratios in cold rolling improve formability due to the refinement effect on the microstructure. When the reduction ratio in cold rolling is less than 40%, it is not easy to achieve a uniform microstructure, and when the reduction ratio in cold rolling is greater than 60%, the rolling force increases, thereby increasing the process load.

[0071] The step (S400) of annealing the cold rolled steel sheet may include heating the cold rolled steel sheet at a heating rate of 1°C / s to 10°C / s, and maintaining an annealing temperature of 820°C to 860°C for 40 seconds to 120 seconds. The annealing process is performed under the conditions of an annealing start temperature and an annealing holding time in the dual phase region of austenite and ferrite. In the current embodiment, the heat treatment is performed in the dual phase region, thereby achieving the final material properties required for the steel sheet by ensuring an appropriate fraction of ferrite in the ideal composition of ferrite, tempered martensite, and retained austenite in the final microstructure.

[0072] The annealed cold rolled steel sheet is multi-stage cooled to a temperature of 200°C to 260°C (S500). The step of multi-stage cooling the cold rolled steel sheet (S500) may include a primary cooling step of slowly cooling the cold rolled steel sheet to a final temperature of 550°C to 750°C at a cooling rate of 1°C / s to 10°C / s; and a secondary cooling step of rapidly cooling the cold rolled steel sheet to a final temperature of 200°C to 260°C at a cooling rate of 50°C / s or more.

[0073] The primary cooling step is a step of performing slow cooling after annealing, thereby achieving plasticity of the final microstructure by attempting to ensure a certain amount of ferrite in the final microstructure during the heat treatment process. The secondary cooling step may include a step of performing rapid cooling, for example, to a rapid cooling final temperature of 200°C to 260°C at a cooling rate of 70°C / s or more after the primary cooling step. The final material properties can be easily ensured by controlling the rapid cooling final temperature to transform austenite in the microstructure into martensite after slow cooling, and a cooling rate of 70°C / s or more is required to suppress phase transformation during rapid cooling.

[0074] The cold rolled steel sheet subjected to multi-stage cooling is partitioned at a temperature of 370°C to 460°C (S600). The partitioning step (S600) may be performed by heating the cooled cold rolled steel sheet at a heating rate of 20°C / s or more, and maintaining the temperature of 370°C to 460°C for 10 to 240 seconds. The partitioning step S600 is intended to ensure strength and elongation through carbon enrichment in the tempering of retained austenite and martensite, and finally maintain the final microstructure configuration. The partitioning step (S600) may include, for example, a process of maintaining a reheating temperature of 400°C to 460°C for 60 seconds.

[0075] After performing the partitioning step ( S600 ), the steel sheet may be immersed in a Zn—Mg—Al coating bath and then cooled to room temperature, thereby producing a final steel sheet.

[0076] The ultra-high strength steel plate of the present invention manufactured by performing the above steps may have a YP of 850 MPa or more, a TS of 1180 MPa or more, and an El of 14% or more. For example, the steel plate may have a YP of 850 MPa to 1070 MPa, a TS of 1180 MPa to 1250 MPa, and an El of 14% to 20%. In addition, the product of TS and El may be not less than 15000 MPa·%. In the present invention, the product of TS and El is 15000 MPa·% or more, which is more than twice the value of 7000 MPa·% (generally indicating the product of TS and total El at a similar strength level), so it can be understood that at the same strength, more excellent formability can be achieved compared to existing ultra-high strength steels.

[0077] In existing methods of forming vehicle body parts, the fracture of components made of ultra-high strength materials during the forming process can be explained by the evaluation criteria represented in the general forming limit diagram (such as drawability and biaxial stretchability) and the hole expansion ratio (not shown in the forming limit diagram). Generally, in the processing of vehicle body parts, steel materials with better formability evaluation results can be used for more complex forming structures. These formability indicators are important factors in the process of forming vehicle parts by pressing operations. Generally, ultra-high strength materials show a tendency to decrease in elongation as strength increases. In order to improve the formability of these ultra-high strength materials, materials are being developed by applying specialized forming processes or deformation mechanisms aimed at improving formability.

[0078] Compared with existing ultra-high strength steel, the present invention aims to improve formability by utilizing the microstructure of ferrite, retained austenite and martensite / tempered martensite, and seeks to develop a cold-rolled steel sheet that maintains the microstructure of ferrite, retained austenite and martensite / tempered martensite by adding a component system and heat treatment control while satisfying material properties. Based on the results of simulation tests, the present invention proposes a component system and a heat treatment range.

[0079] In the case of existing ultra-high strength steels with a dual-phase structure of ferrite and martensite, dislocations are formed and move in the microstructure when the steel material is plastically deformed. This movement of dislocations causes plastic deformation through a basic deformation mechanism (where fracture occurs as defects form and grow). In order to ensure strength under this deformation mechanism, hard phases such as martensite and bainite are formed. However, increasing the fraction of hard phases to ensure strength will inevitably lead to a decrease in elongation. In order to compensate for the decrease in elongation, soft phases such as ferrite are formed in the microstructure. In ultra-high strength steels with the above-mentioned final microstructure, strength and elongation follow the law of mixtures (ROM), so improving material properties outside the ROM is essentially limited.

[0080] Steels developed to improve ultra-high strength steels having a dual-phase structure of ferrite and martensite include TRIP steels, which are capable of ensuring strength and elongation by ensuring retained austenite in the final structure through phase transformation of retained austenite during plastic deformation. However, TRIP steels may not be able to significantly improve formability due to the low area fraction of retained austenite included in the final microstructure.

[0081] Therefore, the present invention aims to ensure the formability of ultra-high strength steel by ensuring retained austenite in the final microstructure, and the final microstructure of the produced steel plate consists of ferrite, retained austenite and tempered martensite (see Figure 1). Retained austenite is a structure that can easily ensure the strength and elongation of the steel sheet through the TRIP mechanism. However, when excessively contained, a large amount of alloying elements may be required to ensure the stability of the implementation of the TRIP mechanism. Therefore, the retained austenite may be limited to 10% to 20%, the ferrite may be 10% to 20%, and the balance may be composed of martensite / tempered martensite.

[0082] Therefore, the method proposed in the present invention for ensuring the yield strength, tensile strength and elongation by realizing the above-mentioned microstructure can be summarized as follows.

[0083] ① Method 1: Steelmaking, continuous casting, hot rolling and cold rolling are performed by using an optimally controlled composition system with austenite stabilizing elements (such as C and Mn) to ensure retained austenite in the final microstructure after annealing and coating.

[0084] ② Method 2: By using the obtained cold-rolled coil to control intercritical annealing, rapid cooling and reheating, the microstructure proposed by the present invention is ensured.

[0085] In the present invention, the final microstructure is set based on the following principles so as to ensure high formability, and process optimization for obtaining the final microstructure is performed through simulation tests.

[0086] ① Design direction 1: To ensure strength by using TRIP of martensite / tempered martensite and retained austenite, it may not be possible to ensure elongation and hole expansion ratio.

[0087] ② Design direction 2: Ensure improved elongation by simultaneously applying the existing method of ensuring elongation by ensuring a soft phase (ferrite) in the final microstructure of ultra-high strength steel and the method of ensuring more retained austenite for TRIP steel in the final microstructure.

[0088] ③ Design direction 3: In addition, in order to optimally ensure the stability of retained austenite, the density of carbides in the final microstructure is controlled to meet the desired material properties.

[0089] In order to construct the above-mentioned microstructure, in the present invention, heat treatment conditions for a cold rolled steel sheet having a final microstructure of ferrite, retained austenite and tempered martensite are established by applying intercritical annealing, rapid cooling, reheating and continuous coating processes.

[0090] Regarding design directions 1 and 2, intercritical annealing, rapid cooling, and reheating need to be applied to construct the final microstructure.

[0091] First, it is important to ensure that there is a sufficient amount of ferrite in the final microstructure by intercritical annealing. The ferrite in the final microstructure can be formed in a total of two steps of annealing and slow cooling in the heat treatment process. When the annealing zone is set to a single-phase zone, it enters the dual-phase zone during the slow cooling process and achieves a phase transformation from austenite to ferrite. However, since the time is very short and the amount is small, it may not be possible to form a sufficient amount of ferrite desired in the present invention. On the other hand, when the annealing zone is set in the dual-phase zone, in addition to the ferrite ensured in the dual-phase zone, ferrite can also be ensured during the slow cooling process, so the desired fraction of ferrite can be ensured and easily constructed in the final microstructure. In the present invention, considering the controllability of a general continuous annealing line, the annealing temperature can be set in the dual-phase zone, and the slow cooling final temperature can be set to 550°C to 750°C. More specifically, the annealing temperature can be set to 820°C to 860°C, and the slow cooling final temperature can be set to 600°C to 700°C.

[0092] Secondly, it is important to ensure retained austenite by rapid cooling after slow cooling. When the steel plate is cooled at a higher rate without hardenability interference after slow cooling, the austenite present in addition to ferrite is transformed into martensite and austenite due to rapid cooling. Therefore, before reheating, the fraction of the martensitic structure that can easily ensure strength and the retained austenite that can easily ensure formability in the final microstructure can be determined. Therefore, when determining the fraction, it is important to find an appropriate temperature range between the cooling rate and the martensite start and final temperature. In the present invention, considering the controllability of the continuous annealing line, the rapid cooling rate is set to 70°C / s or more, and the rapid cooling final temperature is set to 200°C to 260°C. More specifically, the rapid cooling final temperature can be set to 210°C to 250°C.

[0093] Third, carbon redistribution between phases in the formed microstructure is induced by reheating (partitioning), while causing stabilization of the formed austenite and softening of the martensite structure. After the reheating process, the stabilized austenite does not undergo further phase transformation even when cooled to room temperature, and this austenite is called retained austenite. Retained austenite serves as the main matrix to simultaneously ensure the formability and strength of the steel sheet by causing the TRIP phenomenon during subsequent plastic deformation. The amount of redistributed carbon generally increases with increasing reheating temperature and time, and the reheating process can be flexibly changed according to the desired final material properties. In the present invention, the reheating temperature can be set to 370°C to 460°C, more specifically 400°C to 430°C.

[0094] Finally, the effects of annealing heating rate and holding time, slow cooling rate, and reheating rate and holding time were examined. When the total annealing time is too long or too short, the phase transformation in the corresponding unit process may be excessive, resulting in recrystallization and grain growth, and preventing the formation of the desired microstructure, or it may not be sufficient to meet the desired material properties. In the present invention, the annealing heating rate and holding time are set to 1°C / s to 10°C / s and 40 seconds to 120 seconds, the slow cooling rate is set to 1°C / s to 10°C / s, and the reheating rate and holding time are set to 20°C / s or more and 10 seconds or more. More specifically, the annealing heating rate and holding time can be set to 1°C / s to 5°C / s and 50 seconds to 110 seconds, the slow cooling rate can be set to 5°C / s to 10°C / s, and the reheating rate and holding time can be set to 30°C / s or more and 10 seconds to 240 seconds.

[0095] Regarding design direction 3, the carbide formation in the final microstructure needs to be controlled to ensure optimal stability of the retained austenite.

[0096] The present invention includes ensuring the final microstructure of the concept of design directions 1 and 2, while aiming to control the phase transformation based on the plastic deformation behavior through the stability control of the retained austenite (which is the main structure for ensuring the material properties), thereby ensuring the desired material properties. In a series of heat treatment processes (especially annealing and reheating), an appropriate amount of austenite stabilizing elements needs to be included in the austenite structure. To this end, it is necessary to suppress the reduction of the diffusion of the austenite stabilizing elements caused by the formation of unnecessary inclusions, so as to finally form a sufficiently stable retained austenite in the microstructure after heat treatment.

[0097] In the present invention, C is used specifically as an austenite stabilizing element to form retained austenite in the final microstructure, and in this case, it is necessary to suppress the formation of unnecessary carbides so that C diffuses into austenite. Typically, carbides are mainly observed in martensite and ferrite phases. In particular, in the present invention, the formation and growth of carbides mainly occur during the reheating process after annealing and cooling. The formation of carbides is mainly driven by the following two factors: 1) the phase transformation of austenite to a portion of bainite after rapid cooling during reheating, and 2) the tempering of martensite during reheating. In this case, by adding Si to steel, the formation of carbides is minimized. However, although the above method effectively limits the growth of carbides previously formed during hot rolling and softening, it is ultimately necessary to control the amount of previously formed carbides to promote the diffusion behavior of C to austenite and the composition control effect.

[0098] Therefore, the present invention aims to ensure the stability of retained austenite by minimizing the amount of carbides formed in the final microstructure. The test results of the present invention show that when the density of carbides (average size of 50nm or more) in ferrite is 20 particles / μm 2 or less, the best material properties can be ensured. In addition, these carbides in ferrite (soft phase) may become crack initiation sites under deformation, thereby adversely affecting formability.

[0099] Based on the ultra-high strength steel plate and the manufacturing method thereof, according to the technical features of the present invention, strength and elongation are ensured by using a strengthening mechanism based on TRIP steel and carbide control. In order to ensure the desired strength and elongation, unlike the existing ultra-high strength steel that ensures elongation by ensuring a soft phase in the final microstructure, the present invention ensures improved elongation by ensuring that there is more retained austenite for TRIP steel in the final microstructure. In addition, the present invention establishes a carbide condition for controlling the diffusion of austenite stabilizing elements (such as C), thereby optimally controlling the stability of the retained austenite and satisfying the desired material properties.

[0100] Test Example

[0101] Test examples will now be described to better understand the present invention. However, the following test examples are only for facilitating understanding of the present invention, and the present invention is not limited thereto.

[0102] 1. Sample composition

[0103] This test example provides samples having the alloy element compositions shown in Table 2.

[0104] [Table 2]

[0105]

[0106] The ingredient system of Table 2 represents components for constructing an ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention, and satisfies the following components: C: 0.1 wt% to 0.30 wt%, Si: 1.0 wt% to 2.0 wt%, Mn: 1.5 wt% to 3.0 wt%, Al: greater than 0 wt% and not greater than 0.05 wt%, the total of one or more elements selected from Nb, Ti, and V: greater than 0 wt% and not greater than 0.05 wt%, P: greater than 0 wt% and not greater than 0.02 wt%, S: greater than 0 wt% and not greater than 0.005 wt%, N: greater than 0 wt% and not greater than 0.006 wt%, and the balance of Fe.

[0107] 2. Process conditions and property evaluation

[0108] Table 3 shows the process conditions applicable to this test example having the alloy composition of Table 2. In Table 3, softening corresponds to Figure 3 The softening step (S200) shown in FIG. 1 corresponds to the annealing step (S200). Figure 3 The annealing step (S400) shown, slow cooling and fast cooling correspond to Figure 3 The multi-stage cooling step (S500) shown in FIG. Figure 3 The partitioning step (S600) is shown. Within the process condition range of the above-mentioned method for manufacturing ultra-high strength steel plate of the present invention, other process conditions are applied in the same manner.

[0109] Table 4 shows the microstructure and properties achieved by the present test example according to Tables 2 and 3. In Table 4, the criteria for determining whether the material properties are achieved include YP of 850 MPa or more, TS of 1180 MPa or more, El of 14% or more, an area fraction of ferrite of 10% to 20%, an area fraction of retained austenite of 10% to 20%, an area fraction of remaining tempered martensite / martensite, and 20 particles / μm. 2 or smaller ferrite with an average size of 50nm or more.

[0110] [Table 3]

[0111]

[0112] [Table 4]

[0113]

[0114]

[0115] Referring to Table 3 and Table 4, Test Examples 1, 2, 4, 6 and 8 correspond to embodiments of the present invention and satisfy the following process conditions: for example, a softening temperature of 500°C to 650°C, an annealing heating rate of 1°C / s to 10°C / s, an annealing temperature of 820°C to 860°C, an annealing holding time of 40 seconds to 120 seconds, a slow cooling rate of 1°C / s to 10°C / s, a slow cooling final temperature of 550°C to 750°C, a rapid cooling rate of 50°C / s or more, a rapid cooling final temperature of 200°C to 260°C, a reheating rate of 20°C / s or more, a reheating temperature of 370°C to 460°C, and a reheating holding time of 10 seconds to 240 seconds. Therefore, the final microstructure consists of tempered martensite, martensite, ferrite and retained austenite, ferrite accounts for 10% to 20% area fraction, retained austenite accounts for 10% to 20% area fraction, and tempered martensite and martensite account for the remaining area fraction. Ferrite contains carbides, and the density of carbides with an average size of 50nm or more is 20 particles / μm.2 or less. All satisfy the YP of 850MPa to 1070MPa, the TS of 1180MPa to 1250MPa, the El of 14% to 20%, and the product of TS and El of 15000MPa·% or more. On the other hand, Test Example 3 corresponds to the comparative example of the present invention, and does not satisfy and exceeds the annealing temperature range of 820°C to 860°C. Therefore, the area fraction of ferrite does not satisfy and is less than the range of 10% to 20%, the YP does not satisfy and is greater than the range of 850MPa to 1070MPa, the El does not satisfy and is less than the range of 14% to 20%, and the product of TS and El does not satisfy and is less than the range of 15000MPa·% or more. That is, when the annealing region is set to the single-phase region, it enters the dual-phase region during the slow cooling process, and the phase transformation from austenite to ferrite is achieved. However, since the time is very short and the amount is small, it may not be possible to form a sufficient amount of ferrite desired by the present invention, thereby reducing the elongation.

[0116] Test Example 5 corresponds to a comparative example of the present invention, and does not satisfy and exceeds the slow cooling final temperature range of 550° C. to 750° C. Therefore, the area fraction of ferrite does not satisfy and is less than the range of 10% to 20%, YP does not satisfy and is greater than the range of 850 MPa to 1070 MPa, El does not satisfy and is less than the range of 14% to 20%, and the product of TS and El does not satisfy and is less than the range of 15000 MPa·% or more.

[0117] Test Example 7 corresponds to a comparative example of the present invention, and does not satisfy and exceeds the rapid cooling final temperature range of 200° C. to 260° C. Therefore, the area fraction of retained austenite does not satisfy and is less than the range of 10% to 20%, and YP does not satisfy and is less than the range of 850 MPa to 1070 MPa.

[0118] Test Example 9 corresponds to a comparative example of the present invention, and does not satisfy and exceeds the reheating temperature range of 370° C. to 460° C. Therefore, YP does not satisfy and is greater than the range of 850 MPa to 1070 MPa, El does not satisfy and is less than the range of 14% to 20%, and the product of TS and El does not satisfy and is less than the range of 15000 MPa·% or more.

[0119] Test Examples 10 and 11 correspond to comparative examples of the present invention, and do not satisfy and are less than the softening temperature range of 500° C. to 650° C. Therefore, the density of carbides having an average size of 50 nm or more in ferrite does not satisfy and is greater than 20 particles / μm. 2 or smaller range, and El does not satisfy and is less than the range of 14% to 20%.

[0120] While the invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined in the following claims.

Claims

1. An ultra-high strength steel sheet, comprising: carbon (C): 0.1 wt% to 0.30 wt%, silicon (Si): 1.0 wt% to 2.0 wt%, manganese (Mn): 1.5 wt% to 3.0 wt%, aluminum (Al): greater than 0 wt% and not greater than 0.05 wt%, the sum of one or more elements selected from niobium (Nb), titanium (Ti) and vanadium (V): greater than 0 wt% and not greater than 0.05 wt%, phosphorus (P): greater than 0 wt% and not greater than 0.02 wt%, sulfur (S): greater than 0 wt% and not greater than 0.005 wt%, nitrogen (N): greater than 0 wt% and not greater than 0.006 wt%, and the balance of iron (Fe) and other inevitable impurities, in, The final microstructure of the ultra-high strength steel plate consists of tempered martensite, martensite, ferrite and retained austenite, and The ultra-high strength steel plate has a yield strength (YP) of 850 MPa or more, a tensile strength (TS) of 1180 MPa or more, and an elongation (El) of 14% or more.

2. The ultra-high strength steel plate according to claim 1, wherein: The final microstructure has a martensitic matrix instead of bainite.

3. The ultra-high strength steel plate according to claim 1, wherein: Ferrite accounts for 10% to 20% area fraction, The retained austenite accounts for 10% to 20% area fraction, and Tempered martensite and martensite occupy the remaining area fraction.

4. The ultra-high strength steel plate according to claim 1, wherein: Ferrite contains carbides and The density of carbides with an average size of 50nm or more is 20 particles / μm 2 or smaller.

5. The ultra-high strength steel plate according to claim 1, wherein: The product of TS and El is not less than 15000 MPa·%.

6. A method for manufacturing an ultra-high strength steel plate, the method comprising: Producing a hot rolled steel sheet, the hot rolled steel sheet comprising: carbon (C): 0.1 wt% to 0.30 wt%, silicon (Si): 1.0 wt% to 2.0 wt%, manganese (Mn): 1.5 wt% to 3.0 wt%, aluminum (Al): greater than 0 wt% and not greater than 0.05 wt%, the sum of one or more elements selected from niobium (Nb), titanium (Ti) and vanadium (V): greater than 0 wt% and not greater than 0.05 wt%, phosphorus (P): greater than 0 wt% and not greater than 0.02 wt%, sulfur (S): greater than 0 wt% and not greater than 0.005 wt%, nitrogen (N): greater than 0 wt% and not greater than 0.006 wt%, and the balance of iron (Fe) and other inevitable impurities; Softening the hot rolled steel sheet by continuously maintaining a temperature of 500°C to 650°C; Producing a cold-rolled steel sheet by cold-rolling a softened steel sheet; annealing the cold rolled steel sheet at a temperature of 820°C to 860°C; Cooling the cold-rolled steel sheet in multiple stages to a temperature of 200° C. to 260° C.; and The cooled cold rolled steel sheet is partitioned at a temperature of 370°C to 460°C.

7. The method according to claim 6, wherein: Production of hot rolled steel plates includes: preparing a steel billet having the above alloy composition; reheating the steel billet at 1150°C to 1250°C; producing a hot rolled steel plate by hot rolling the reheated steel slab at a finishing outlet temperature (FDT) of 850° C. to 1000° C.; and The hot rolled steel sheet is coiled at 500°C to 700°C.

8. The method according to claim 6, wherein: Annealing of the cold rolled steel sheet is performed by heating the cold rolled steel sheet at a heating rate of 1° C. / s to 10° C. / s and maintaining a temperature of 820° C. to 860° C. for 40 seconds to 120 seconds.

9. The method according to claim 6, wherein: The multi-stage cooling of the cold rolled steel plate includes: a primary cooling step of slowly cooling the cold rolled steel sheet to a final temperature of 550° C. to 750° C. at a cooling rate of 1° C. / s to 10° C. / s; and A secondary cooling step of rapidly cooling the cold rolled steel sheet to a final temperature of 200°C to 260°C at a cooling rate of 50°C / s or more.

10. The method according to claim 6, wherein: Partitioning is performed by heating the cooled cold-rolled steel sheet at a heating rate of 20° C. / s or more, and maintaining a temperature of 370° C. to 460° C. for 10 seconds to 240 seconds.

Images